hydro thermal technology for nanotechnology

Progress in Crystal Growth and Characterization of Materials

53 (2007) 117e166www.elsevier.com/locate/pcrysgrow

Hydrothermal technology for nanotechnology

K. Byrappa a,*, T. Adschiri b

a University of Mysore, DOS in Geology, P.B. 21, Manasagangotri P.O., Mysore-570 006, Indiab Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira,

Aoba-ku, Sendai 980 8577, Japan

Abstract

The importance of hydrothermal technology in the preparation of nanomaterials has been discussed indetail with reference to the processing of advanced materials for nanotechnology. Hydrothermal technol-ogy in the 21st century is not just confined to the crystal growth or leaching of metals, but it is going totake a very broad shape covering several interdisciplinary branches of science. The role of supercriticalwater and supercritical fluids has been discussed with appropriate examples. The physical chemistry ofhydrothermal processing of advanced materials and the instrumentation used in their preparation with re-spect to nanomaterials have been discussed. The synthesis of monodispersed nanoparticles of variousmetal oxides, metal sulphides, carbon nanoforms (including the carbon nanotubes), biomaterials, andsome selected composites has been discussed. Recycling, waste treatment and alteration under hydrother-mal supercritical conditions have been highlighted. The authors have discussed the perspectives of hydro-thermal technology for the processing of advanced nanomaterials and composites.� 2007 Elsevier Ltd. All rights reserved.

PACS: 82.Rx; 61.46.þw; 81.40.�z; 81.10.Dn; 82.60.Lf; 35.Rh

Keywords: A1. Nanostructures; A1. Morphology control; A2. Hydrothermal technology; A2. Solvothermal; A2. Super-

critical fluid technology; A2. Nanoparticles fabrication

1. Introduction

The hydrothermal technique is becoming one of the most important tools for advancedmaterials processing, particularly owing to its advantages in the processing of nanostructural

* Corresponding author. Tel.: þ91 821 2419720; fax: þ91 821 2515346.

E-mail addresses: [email protected] (K. Byrappa), [email protected] (T. Adschiri).

0960-8974/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pcrysgrow.2007.04.001

mailto:[email protected]

mailto:[email protected]

http://www.elsevier.com/locate/pcrysgrow

118 K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials53 (2007) 117e166

materials for a wide variety of technological applications such as electronics, optoelectronics,catalysis, ceramics, magnetic data storage, biomedical, biophotonics, etc. The hydrothermaltechnique not only helps in processing monodispersed and highly homogeneous nanoparticles,but also acts as one of the most attractive techniques for processing nano-hybrid and nanocom-posite materials. The term ‘hydrothermal’ is purely of geological origin. It was first used by theBritish geologist, Sir Roderick Murchison (1792e1871) to describe the action of water at ele-vated temperature and pressure, in bringing about changes in the earth’s crust leading to theformation of various rocks and minerals. It is well known that the largest single crystal formedin nature (beryl crystal of >1000 g) and some of the large quantity of single crystals created byman in one experimental run (quartz crystals of several 1000s of g) are both of hydrothermalorigin.

Hydrothermal processing can be defined as any heterogeneous reaction in the presence ofaqueous solvents or mineralizers under high pressure and temperature conditions to dissolveand recrystallize (recover) materials that are relatively insoluble under ordinary conditions.Definition for the word hydrothermal has undergone several changes from the original Greekmeaning of the words ‘hydros’ meaning water and ‘thermos’ meaning heat. Recently, Byrappaand Yoshimura define hydrothermal as any heterogeneous chemical reaction in the presence ofa solvent (whether aqueous or non-aqueous) above the room temperature and at pressure greaterthan 1 atm in a closed system [1]. However, there is still some confusion with regard to the veryusage of the term hydrothermal. For example, chemists prefer to use a term, viz. solvothermal,meaning any chemical reaction in the presence of a non-aqueous solvent or solvent in super-critical or near supercritical conditions. Similarly there are several other terms like glycother-mal, alcothermal, ammonothermal, and so on. Further, the chemists working in the supercriticalregion dealing with the materials synthesis, extraction, degradation, treatment, alteration, phaseequilibria study, etc., prefer to use the term supercritical fluid technology. However, if welook into the history of hydrothermal research, the supercritical fluids were used tosynthesize a variety of crystals and mineral species in the late 19th century and the early20th century itself [1]. So, a majority of researchers now firmly believe that supercritical fluidtechnology is nothing but an extension of the hydrothermal technique. Hence, here the authorsuse only the term hydrothermal throughout the text to describe all the heterogeneous chemicalreactions taking place in a closed system in the presence of a solvent, whether it is aqueous ornon-aqueous.

The term advanced material is referred to a chemical substance whether organic or inorganicor mixed in composition possessing desired physical and chemical properties. In the currentcontext the term materials processing is used in a very broad sense to cover all sets of technol-ogies and processes for a wide range of industrial sectors. Obviously, it refers to the preparationof materials with a desired application potential. Among various technologies available today inadvanced materials processing, the hydrothermal technique occupies a unique place owing toits advantages over conventional technologies. It covers processes like hydrothermal synthesis,hydrothermal crystal growth leading to the preparation of fine to ultra fine crystals, bulk singlecrystals, hydrothermal transformation, hydrothermal sintering, hydrothermal decomposition,hydrothermal stabilization of structures, hydrothermal dehydration, hydrothermal extraction,hydrothermal treatment, hydrothermal phase equilibria, hydrothermal electrochemical reac-tions, hydrothermal recycling, hydrothermal microwave supported reactions, hydrothermalmechanochemical, hydrothermal sonochemical, hydrothermal electrochemical processes, hy-drothermal fabrication, hot pressing, hydrothermal metal reduction, hydrothermal leaching,hydrothermal corrosion, and so on. The hydrothermal processing of advanced materials has

119K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials

53 (2007) 117e166

lots of advantages and can be used to give high product purity and homogeneity, crystal sym-metry, metastable compounds with unique properties, narrow particle size distributions, a lowersintering temperature, a wide range of chemical compositions, single-step processes, dense sin-tered powders, sub-micron to nanoparticles with a narrow size distribution using simple equip-ment, lower energy requirements, fast reaction times, lowest residence time, as well as for thegrowth of crystals with polymorphic modifications, the growth of crystals with low to ultra lowsolubility, and a host of other applications.

In the 21st century, hydrothermal technology, on the whole, will not be just limited to thecrystal growth, or leaching of metals, but it is going to take a very broad shape covering severalinterdisciplinary branches of science. Therefore, it has to be viewed from a different perspec-tive. Further, the growing interest in enhancing the hydrothermal reaction kinetics using micro-wave, ultrasonic, mechanical, and electrochemical reactions will be distinct [2]. Also, theduration of experiments is being reduced at least by 3e4 orders of magnitude, which will inturn, make the technique more economic. With an ever-increasing demand for composite nano-structures, the hydrothermal technique offers a unique method for coating of various com-pounds on metals, polymers and ceramics as well as for the fabrication of powders or bulkceramic bodies. It has now emerged as a frontline technology for the processing of advancedmaterials for nanotechnology. On the whole, hydrothermal technology in the 21st centuryhas altogether offered a new perspective which is illustrated in Fig. 1. It links all the importanttechnologies like geotechnology, biotechnology, nanotechnology and advanced materials tech-nology. Thus it is clear that the hydrothermal processing of advanced materials is a highly in-terdisciplinary subject and the technique is popularly used by physicists, chemists, ceramists,hydrometallurgists, materials scientists, engineers, biologists, geologists, technologists, and

Bio-Technology

Advanced Materials

Technology

Ge

o-T

ec

hn

olo

gy

Na

no

-

Tech

no

lo

gy

Hydrothermal

Technology

Fig. 1. Hydrothermal technology in the 21st century.


so on. Fig. 2 shows various branches of science either emerging from the hydrothermal tech-nique or closely linked with the hydrothermal technique. One could firmly say that this familytree will keep expanding its branches and roots in the years to come.

The hydrothermal processing of materials is a part of solution processing and it can bedescribed as super heated aqueous solution processing. Fig. 3 shows the PT map of various ma-terials processing techniques [3]. According to this, the hydrothermal processing of advancedmaterials can be considered as environmentally benign. Besides, for processing nanomaterials,the hydrothermal technique offers special advantages because of the highly controlled diffusiv-ity in a strong solvent media in a closed system. Nanomaterials require control over their phys-ico-chemical characteristics, if they are to be used as functional materials. As the size isreduced to the nanometer range, the materials exhibit peculiar and interesting mechanicaland physical properties: increased mechanical strength, enhanced diffusivity, higher specificheat and electrical resistivity compared to their conventional coarse grained counter-partsdue to a quantization effect [4].

Hydrothermal technology as mentioned earlier in a strict sense also covers supercritical wa-ter or supercritical fluid technology, which is gaining momentum in the last 1½ decades owingto its enormous advantages in the yield and speed of production of nanoparticles and also in thedisintegration, transformation, recycling and treatment of various substances including toxic or-ganics, wastes, etc. In case of supercritical water technology, water is used as the solvent in the

Fig. 2. Hydrothermal tree showing different branches of science and technology.


system, whereas supercritical fluid technology is a general term when solvents like CO2 andseveral other organic solvents are used, and because these solvents have lower critical temper-ature and pressure compared to water this greatly helps in processing the materials at muchlower temperature and pressure conditions. Hence, chemists use the term green chemistryfor materials processing using supercritical fluid technology. K. Arai, T. Adschiri, M. Goto(all from Japan) and V.J. Krukonis, J. Watkins, P. Savage, T. Brill (USA), M. Poliakoff(UK), M. Perrut, F. Cansell (France), Buxing Han (China), K.P. Yoo and Y.W. Lee (South Ko-rea), etc., have done extensive studies in the area of supercritical fluid technology.

Supercritical water (SCW) and supercritical fluids (SCF) provide an excellent reaction me-dium for hydrothermal processing of nanoparticles, since they allow varying the reaction rateand equilibrium by shifting the dielectric constant and solvent density with respect to pressureand temperature, thus giving higher reaction rates and smaller particles. The reaction productsare to be stable in SCF leading to fine particle formation. The hydrothermal technique is idealfor the processing of very fine powders having high purity, controlled stoichiometry, high qual-ity, narrow particle size distribution, controlled morphology, uniformity, less defects, dense par-ticles, high crystallinity, excellent reproducibility, controlled microstructure, high reactivitywith ease of sintering and so on.

Further, the technique facilitates issues like energy saving, the use of larger volume equip-ment, better nucleation control, avoidance of pollution, higher dispersion, higher rates of reac-tion, better shape control, and lower temperature operations in the presence of the solvent. Innanotechnology, the hydrothermal technique has an edge over other materials processing tech-niques, since it is an ideal one for the processing of designer particulates. The term designerparticulates refers to particles with high purity, high crystallinity, high quality, monodispersedand with controlled physical and chemical characteristics. Today such particles are in great de-mand in the industry. Fig. 4 shows the major differences in the products obtained by ball

Fig. 3. Pressure temperature map of materials processing techniques [3].

53 (2007) 117e166


milling or sintering or firing and by the hydrothermal method [5]. In this respect hydrothermaltechnology has witnessed a seminal progress in the last decade in processing a great variety ofnanomaterials ranging from microelectronics to micro-ceramics and composites. Here the au-thors discuss the progress made in the area of hydrothermal technology for the past one decadein the processing of advanced nanomaterials. These materials, when put into proper use, willhave a profound impact on our economy and society at least in the early part of 21st century,comparable to that of semiconductor technology, information technology or cellular and molec-ular biology. It is widely speculated that the nanotechnology will lead to the next industrial rev-olution [6]. Though it is widely believed that commercial nanotechnology is still in its infancy,the rate of technology enablement is increasing in no small part, as substantial governmentmandated funds have been directed toward nanotechnology [7,8]. It is strongly believed thathydrothermal technology has a great prospect especially with respect to nanotechnologyresearch.

2. History of nanomaterial processing using hydrothermal technology

Gold nanoparticles have been around since Roman times. As per the literature data, MichaelFaraday was the first scientist to seriously experiment with gold nanoparticles starting in the1850s. They have recently become the focus of researchers interested in their electrical and op-tical properties. Similarly, the history of hydrothermal processing of nanomaterials is very inter-esting. It must have begun in 1845, when Schafthaul prepared fine powders of sub-microscopic

Fig. 4. Difference in particle processing by hydrothermal and conventional techniques [5].


53 (2007) 117e166

to nanosize quartz particles using a papin’s digester containing freshly precipitated silicic acid[9]. Majority of the early hydrothermal experiments carried out during the 1840s to the early1900s mainly dealt with the nanocrystalline products, which were discarded as failures due tothe lack of sophisticated electron microscopic techniques available during that time to observesuch small sized products.

Thus the whole focus was on the processing of bulk crystals or bulk materials. Many timeswhen bulk crystals or single crystals were not obtained as products of several millimeter sizethe experiments were considered failures and the materials were washed away. Prior to X-ray techniques, chemical techniques were mainly employed in identifying the products. Itwas only after the application of X-rays for crystal studies that the researchers slowly beganto study the powder diffraction patterns of the resultant products and by the 1920s a systematicunderstanding of the products began. Before that the experiments were considered as failures.The experiments were concluded by stating that the solubility was not suitable for growingcrystals. Until the works of Giorgio Spezia in 1900, hydrothermal technology did not gainmuch importance in the growth of bulk crystals, as the products in majority of the caseswere very fine grained without any X-ray data [10]. Even the use of seeded growth was initiatedby Spezia during that time. Morey [11] quotes in his classical work that the early hydrothermalexperimenters used to have horrible experiences since sometimes experiments lasted for 3e6months without any bearing on petrogenesis and phase equilibria, and ended up with veryfine product whose status was not clear. The experiments were simply discarded as failures[11]. Gradually, from the late 1920s to the late 1950s, the products were being analyzed asfine crystalline materials. During this period a great variety of phosphates, silicates, germinates,sulphates, carbonates, oxides, etc., even without natural analogues, were prepared. However, nospecial significance was attached to such fine crystalline products except for the phase equilib-ria studies. In fact, the experimental duration was also enhanced in several cases to transformthese fine crystalline products into small or bulk single crystals, whenever it was possible. Thusthe interest on the growth of bulk crystals was revived during the 1960s and it survived until the1990s. However, such attempts failed again because of the lack of knowledge on the hydrother-mal solution chemistry. It was only during the 1950s and 1960s; some attempts were made tounderstand the hydrothermal solution chemistry and kinetics of the hydrothermal reactions. Itwas during the 1970s that some attempts were made to observe the hydrothermal reactions us-ing sapphire windows in the autoclaves. However, owing to the extreme PT conditions theseworks were not encouraging and the in situ observation of the growth processes was later aban-doned. But today, it has become one of the most attractive aspects of hydrothermal researchtechnology. Combination of advanced hydrothermal reactor design with the new sophisticatedanalytical techniques like Laser Raman, FTIR, synchrotron, HR-SEM, etc. has greatly aided theobservation of nucleation and materials processing in situ. With the availability of high resolu-tion SEM from 1980 onwards hydrothermal researchers started observing such fine productswhich were earlier discarded as failures. The hydrothermal research in the 1990s marks the be-ginning of the work on the processing of fine to ultra fine particles with a controlled size andmorphology. The advanced ceramic materials prepared during that time justify this statement.In the last two decades these sub-micron to nanosized crystalline products have created a rev-olution in science and technology under a new terminology, ‘Nanotechnology’. Today hydro-thermal researchers are able to understand such nanosized materials and control theirformation process, which in turn, give the desired properties to such nanomaterials. Thus hy-drothermal technology and nanotechnology have a very close link ever since this hydrothermaltechnology was proposed.

124 K. Byrappa, T. Adschiri / Progress in Crystal Growth and Characterization of Materials

The recent advances in the hydrothermal solution chemistry through the principles of ther-modynamics, kinetics, and chemical energy have created a new trend in materials processing.For example, the materials synthesized under extreme PT conditions in the earlier days could bewell crystallized presently under much lower PT conditions. Table 1 gives the recent trends inhydrothermal research. Such trends have greatly helped in processing advanced materials at rel-atively lower PT conditions and at a much faster rate, thus having a great bearing on nanotech-nology of the 21st century.

Also, the trends shown in Table 1 take hydrothermal technology towards green technologyfor sustained human development since it consumes less energy with no or little solid waste/orwaste liquid/gases and involves no recovery treatment, no hazardous process materials, highselectivities, a closed system of processing, etc. The important subjects of technology in the21st century are predicted to be the balance of environmental and resource and/or energy prob-lems. This has led to the development of a new concept related to the processing of advancedmaterials in the 21st century, viz. industrial ecology or science of sustainability [12]. Severalresearchers have already used the terms green hydrothermal process, green hydrothermal tech-nology, green hydrothermal route, etc., since the last one decade [13,14].

3. Physical chemistry of hydrothermal processing of advanced materialsfor nanotechnology

Physical chemistry of hydrothermal processing of materials is perhaps the least known as-pect in the literature. The Nobel Symposium organized by the Royal Swedish Academy of Sci-ences during 1978, followed by the First International Symposium on hydrothermal reactionsorganized by the Tokyo Institute of Technology in 1982, helped in setting a new trend in hy-drothermal technology by attracting physical chemists in large number [15,16]. The hydrother-mal physical chemistry today has enriched our knowledge greatly through a properunderstanding of hydrothermal solution chemistry. The behaviour of the solvent under hydro-thermal conditions dealing with aspects like structure at critical, supercritical and sub-criticalconditions, dielectric constant, pH variation, viscosity, coefficient of expansion, density, etc.is to be understood with respect to pressure and temperature. Similarly, the thermodynamicstudies yield rich information on the behaviour of solutions with varying pressure temperatureconditions. Some of the commonly studied aspects are solubility, stability, yield, dissolutioneprecipitation reactions and so on, under hydrothermal conditions. Hydrothermal crystallization

Table 1

Current trends in hydrothermal technology [5]

Compound Earlier work Authora

Li2B4O7 T¼ 500e700 �C T¼ 240 �C

P¼ 500e1500 bars P¼<100 bars

Li3B5O8(OH)2 T¼ 450 �C T¼ 240 �C

P¼ 1000 bars P¼ 80 bars

NaR(WO4)2, R¼ La, Ce, Nd T¼ 700e900 �C T¼ 200 �C

P¼ 2000e3000 bars P¼<100 bars

R:MVO4, R¼Nd, Eu, Tm; M¼Y, Gd Melting point>1800 �C T¼ 100 �C

P¼<30 bars

LaPO4 Synthesized at >1200 �C T< 120 �C

P< 40 bars

a From the works of Prof. K. Byrappa.

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is only one of the areas where our fundamental understanding of hydrothermal kinetics is lackingdue to the absence of data related to the intermediate phases forming in solution. Thus our fun-damental understanding of hydrothermal crystallization kinetics is in the early stage although theimportance of kinetics of crystallization studies was realized with the commercialization of thesynthesis of zeolites during the 1950s and the 1960s itself. In the absence of predictive models,we must empirically define the fundamental role of temperature, pressure, precursor, and time oncrystallization kinetics of various compounds. Insight into this would enable us to understandhow to control the formation of solution species, solid phases and the rate of their formation.In recent years, the thermochemical modeling of the chemical reactions under hydrothermalconditions is becoming very popular. The thermochemcial computation data help in the intelli-gent engineering of the hydrothermal processing of advanced materials. The modeling can besuccessfully applied to very complex aqueous electrolyte and non-aqueous systems over wideranges of temperature and concentration and is widely used in both industry and academy.For example, OLI Systems Inc., USA provides the software for such thermochemical modeling,and using such a package aqueous systems can be studied within the temperature range �50 to300 �C, pressure ranging from 0 to 1500 bar and concentration 0e30 m in molal ionic strength;for the non-aqueous systems the temperature range covered is from 0 to 1200 �C and pressurefrom 0 to 1500 bar with species concentration from 0 to 1.0 mole fraction.

A key limitation to the conventional hydrothermal method has been the need for time-consuming empirical trial and error methods as a mean for process development. Currently,research is being focused on the development of an overall rational engineering-based approachthat will speed up process development. The rational approach involves the following four steps:

1. Compute thermodynamic equilibria as a function of chemical processing variables.2. Generate equilibrium diagrams to map the process variable space for the phases of interest.3. Design hydrothermal experiments to test and validate the computed diagrams.4. Utilize the processing variables to explore opportunities for controlling reactions and crys-

tallization kinetics.

Such a rational approach has been used quite successfully to predict the optimal synthesisconditions for controlling phase purity, particle size, size distribution, and particle morphologyof lead zirconium titanates (PZT), hydroxyapatite (HAp) and other related systems [17e19].The software algorithm considers the standard state properties of all system species as wellas a comprehensive activity coefficient model for the solute species. Table 2 gives an exampleof thermodynamic calculations and the yield of solid and liquid species outflows at T¼ 298 K,P¼ 1 atm., I¼ 0.049 m, and pH¼ 12.4.

Using such a modeling approach, theoretical stability field diagrams (also popularly knownas the yield diagrams) are constructed to get 100% yield. Assuming the product is phase-pure,the yield Y can be expressed as:

Yi ¼ 100�mip

i �meqi

��mip

i %

where mip and meq are the input and equilibrium molal concentrations, respectively, and sub-script i the designated atom. Figs. 5 and 6 show the stability field diagrams for the PZT andHA systems.

From Fig. 5 it is observed that the region with vertical solid lines represents the 99% yield ofPZT although the PZT forms within a wide range of KOH and Ti concentrations. The figure


illustrates clearly the region where all the solute species transform towards 100% product yield.Similarly from Fig. 6, it is observed that all the Ca species participate in the reaction to formHA and thus leading to 100% yield of HA in the region denoted by a black square. Thick dottedlines indicate the boundary above which 99% Ca precipitates as HA. The other regions markthe mixed phase precipitation like hydroxyapatite, monatite and other calcium phosphatephases.

Such thermodynamic studies help to intelligently engineer the hydrothermal processing andalso to obtain a maximum yield for a given system. This area of research has a great potentialapplication in advanced materials processing including nanomaterials.

4. Instrumentation in hydrothermal processing of nanomaterials

Material processing under hydrothermal conditions requires a pressure vessel capable ofcontaining a highly corrosive solvent at high temperature and pressure. Hydrothermal

Table 2

Thermodynamic calculations for HAp system

Species name Inflows moles Outflows

Liquid/mol Solid/mol

H2O 55.51 55.51 8.10� 10�2

Ca(OH)2 0.1 7.2� 10�6

CaO

Ca2+ 1.5� 10�2

Ca(OH)+ 4.0� 10�3

H+ 4.45� 10�13

OH� 3.41� 10�2

Total 55.61 55.56 8.10� 10�2

0

-0.8

-1.6

-2.4

-3.20 2 4 6 8 10 12 14

[KOH] (mol/kg H2O)

Lo

g [T

i]

No PZT

PbO

0% < Yield < 99%

180°C

•• •

PZT 70/30, yield > 99%

Fig. 5. Calculated stability field diagram for the PZT system at 180 �C with KOH as the mineralizer [17].


experimental investigators require facilities that must operate routinely and reliably under ex-treme pressure temperature conditions. Often they face a variety of difficulties, and some pe-culiar problems pertaining to the design, procedure and analysis. Designing a suitable orideal hydrothermal apparatus popularly known as an autoclave, or reactor, or pressure vessel,or high pressure bomb is the most difficult task and perhaps impossible to define, becauseeach project has different objectives and tolerances. However, an ideal hydrothermal autoclaveshould have the following characteristics:

i. Inertness to acids, bases and oxidizing agents.ii. Ease of assembly and dissembly.

iii. Sufficient length to obtain a desired temperature gradient.iv. Leak-proof with unlimited capabilities to the required temperature and pressure range.v. Rugged enough to bear high pressure and temperature experiments for long periods with

no damage so that no machining or treatment is needed after each experimental run.

Keeping in mind the above requirements, autoclave fabrication is carried out using a thickglass cylinder, a thick quartz cylinder and high strength alloys, such as 300 series (austenitic)stainless steel, iron, nickel, cobalt-based super alloys, and titanium and its alloys. It is inappro-priate to describe all the autoclave designs and working principles here. Instead, the authorsprefer to describe only a few selected and commonly used autoclaves in the hydrothermal pro-cessing of nanomaterials. The first and foremost parameters to be considered in selecting

Fig. 6. The calculated stability field diagram for the HAp system at 200 �C and 25 bars with Ca:P ratio at 1.24 [19].

53 (2007) 117e166


a suitable reactor are the experimental temperature and pressure conditions and its corrosionresistance in the pressure temperature range in a given solvent or hydrothermal fluid. If the re-action takes place directly in the vessel, the corrosion resistance is of course a prime factor inthe choice of reactor material. In some of the experiments, the reactors need not contain anylining or liners or cans. For example, the growth of quartz can be carried out in low carbon steelreactors. The low carbon steel is corrosion resistant in systems containing silica and NaOH, be-cause, relatively insoluble NaFe-silicate forms and protectively coats the ground vessel. In con-trast, the materials processing from aqueous phosphoric acid media or other highly corrosivemedia like extreme pH conditions require a Teflon lining or beakers or platinum, gold, silvertubes or lining to protect the autoclave body from the highly corrosive media. Also in somecases hastealloy metal reactors are used to protect from the solvent medium. Therefore, the cor-rosion resistance of any metal under hydrothermal conditions is very important. For example,turbine engineers have long known that boiler water with pH> 7 is less corrosive than slightlyacidic water, especially for alloys containing silicon. The commonly used reactors in the hydro-thermal processing of advanced nanomaterials are listed below:

� General purpose autoclaves.� Morey type e flat plate seal.� Stirred reactors.� Cold-cone seal TuttleeRoy type.� TZM autoclaves.� Batch reactors.� Flow reactors.� Microwave hydrothermal reactors.� Mechanochemicalehydrothermal.� Piston cylinder apparatus.� Belt apparatus.� Opposed anvil.� Opposed diamond anvil.

Figs. 7 and 8 show the most popular autoclave designs such as general purpose autoclaves,Morey autoclaves, modified Bridgman autoclaves and TuttleeRoy autoclaves. In most of these

Fig. 7. General purpose autoclave popularly used for hydrothermal treatment and hydrothermal synthesis [5].


53 (2007) 117e166

autoclaves, pressure can be either directly measured using the Bourdon gauge fixed to theautoclaves, or it can be calculated using the PVT relations for water proposed by Kennedy[20]. Fig. 9 shows the PVT relations in the SiO2eH2O system.

These hydrothermal reactors can be used for a variety of applications like materials synthesis,crystal growth, phase equilibrium studies, hydrothermal alteration, reduction, structure stabiliza-tion, and so on. There are several new reactor designs commercially available, which are popu-larly known as the stirred reactors. Fig. 10 shows the popular make of a stirred reactor commonlyused in the hydrothermal materials processing. These reactors have special features: the reactor

Fig. 8. Commonly used reactors in hydrothermal processing of materials: (a) Morey autoclave and (b) TuttleeRoy

autoclave [1].

Fig. 9. Kennedy’s PVT diagram for the SiO2eH2O system [20].


contents can be continuously stirred at different rates, the fluids can be withdrawn while runningthe hydrothermal experiment, and also the desired gas can be supplied externally into the reac-tors. Such features readily enable the withdrawal of fluids from time to time in order to carry outvarious analytical techniques so as to determine the intermediate phases, which can facilitate anunderstanding of the hydrothermal reaction mechanism for a given material preparation.

There are several other reactors popularly used for materials processing under hydrothermalconditions with special provisions for microwave, mechanochemical, electrochemical or sono-chemical energies, flow reactors, rocking autoclaves, and so on, which greatly help in providingenhanced kinetics for hydrothermal reactions. Figs. 11e14 show the photographs of these fourspecial reactors. For the laboratory scale as well as the pilot scale production of advanced nano-materials, however, only general purpose reactors, stirred reactors of larger volume, flow reactors,

Fig. 10. Commercially available stirred reactors with facilities to withdraw the fluids and externally pump the desired

gas into the autoclave, coupled magnetic stirrer assembly, and autoclave quenching facility with the circulation of

chilled water through the cooling coils running inside the autoclave [1].

Fig. 11. A commercially available microwave reactor.

53 (2007) 117e166


batch reactors, microwave reactors and mechanochemicalehydrothermal reactors are commonlyused. The rest of the reactors are only for small scale or laboratory scale processing only.

Whatever the type of reactor/equipment, it is the safety and maintenance which are of utmostimportance in hydrothermal research whether it is the synthesis of bulk materials or nanomate-rials. It is estimated that for a 100 cm3 vessel at 20,000 psi, the stored energy is about15,000 ft-lb. The hydrothermal solutions e either acidic or alkaline e at high temperaturesare hazardous to human beings, if the reactor explodes. Therefore, the vessels should have

Fig. 12. A commercially available mechanochemicalehydrothermal reactor (MICROS:MIC-0, Japan).

Fig. 13. Flow reactor available in Prof. Tadafumi Adschiri’s laboratory.

53 (2007) 117e166


rupture discs calibrated to burst above a given pressure. Such rupture discs are commerciallyavailable for various ranges of bursting pressure. The most important arrangement is that provi-sion should be made for venting the live volatiles out in the event of rupture. Proper shielding ofthe reactor should be given to divert the corrosive volatiles away from the personnel workingnearby.

5. Hydrothermal processing of advanced materials and nanotechnology

There are hundreds of nanomaterials processed using hydrothermal technologies with over8000 publications dealing with various aspects of advanced nanomaterials processing in the last8 years. The trend is in increasing order and it covers all the groups of advanced materials likemetals, metal oxides, and semiconductors including the IIeVI and IIIeV compounds: silicates,sulphides, hydroxides, tungstates, titanates, carbon, zeolites, ceramics, and a variety of compos-ites. It is not possible to discuss the processing of all these nanocrystalline materials using hy-drothermal technology. Instead, the authors will deal with the processing of some representativeand technologically most important nanomaterials including a variety of nanotubes. The em-phasis is on the nanocrystalline compounds prepared in the present authors’ laboratories.

5.1. Hydrothermal processing of nanoforms of metals

In recent years noble metal particles (like Au, Ag, Pt, etc.), magnetic metals (like Co, Ni andFe), metal alloys (like FePt, CoPt) and multilayers (like Cu/Co, Co/Pt), etc. have attracted theattention of researchers owing to their new interesting fundamental properties and potentialapplications as advanced materials with electronic, magnetic, optical, thermal and catalyticproperties [21e24].

Fig. 14. Batch reactor available at Prof. Tadafumi Adschiri’s laboratory.

53 (2007) 117e166


The intrinsic properties of noble metal nanoparticles strongly depend upon their morphologyand structure. The synthesis and study of these metals have implications for the fundamentalstudy of the crystal growth process and shape control. Majority of the nanostructures of thesemetals alloys and multilayers form under far-from-equilibrium conditions [25]. Among thesemetals, alloys and multilayers, shape anisotropy exhibits interesting properties. Both the hydro-thermal and hydrothermal supercritical water techniques have been extensively used in thepreparation of these nanoparticles.

Zhu et al. have reported the synthesis of silver dendrite nanostructures using anisotropicnickel nanotubes [22] via mild hydrothermal reactions. The nickel nanotubes acted as a reduc-ing agent. The crystal morphologies which changed from dendrite to compact crystals were in-vestigated during the evolution of the reaction system from non-equilibrium to quasi-equilibrium conditions. Here the strong shape anisotropy of the Ni nanotube has influencedthe formation of Ag dendritic nanostructures. When a PVP surfactant was used, the nanostruc-tures were replaced by bulk or compact particles. Figs. 15 and 16 show the characteristic pho-tographs of Ag nanocrystals and Ag compact crystals [22].

Several magnetic nanoparticles have been reported in the literature. Xie et al. and Liu et al.have reported the hydrothermal synthesis of cobalt nanorods and nanobelts with and withoutsurfactants [24,26]. When a micro-emulsion was used, cobalt nanorods with hcp structureshave been obtained at 90 �C, with an average particle size of 10 nm diameter and 260 nm length[26]. Similarly, Co nanobelts via a surfactant assisted hydrothermal reduction process at 160 �Cfor 20 h have been reported by Xie et al. Liu et al. have reported a complex-surfactant-assistedhydrothermal route to ferromagnetic nickel nanobelts at about 110 �C in 24 h [24,27]. TheseNi-nanobelts show remarkably enhanced ferromagnetic properties. Here the key factors inthe preparation of these Ni-nanobelts are the pre-formation of the Ni complex Ni(C4H2O6)2�,the presence of surfactant SDBS and the selective use of the reducing agent NaH2PO2. Such anapproach can be extended to the hydrothermal preparation of nanobelts of several other transi-tional metals and their alloys.

Fig. 15. TEM images of Ag dendrites (photos: courtesy Prof. Y.T. Qian).

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Niu et al. have prepared NieCu alloy nanocrystallites at low temperatures under hydrother-mal conditions [28]. These nanoscale metallic alloys like CuNi, AgPd, AuPt can be applied insmall scale electronic devices. The authors have used a polymeresurfactant to obtain these NieCu alloy nanoparticles at about 80 �C. The average diameter of the particles is about 12 nm.The most vital factor in the preparation of these nanoparticles is the simultaneous reductionof nickel and copper metals, which enables the ready inter-diffusion of the different atoms.

In recent years supercritical conditions have provided reactions for synthesizing nanoparticlesof Ag, Au, Pd, In, Pt, Si, Ge, Cu, etc., and are becoming very popular as a consequence of fastkinetics and rapid particle production with the shortest residence time. There are several reportson the preparation of nanoparticles under SCW conditions. The reader can refer to refs. [29e32].

Similarly, the coating of nanocrystalline films of Cu, Ni, Ag, Au, Pt, Pd, Rh, etc., on siliconwafers for microelectronics, data storage, etc., has been reported [33]. Such an approach hasbeen extended to several other materials like the coating of nanocrystalline carbon on Si wafers,etc.

Thus the hydrothermalesolvothermal and hydrothermaleSCW offer unique advantages overthe preparation of these metal nanoparticles over other conventional methods.

5.2. Hydrothermal processing of advanced metal oxide nanomaterials

Today the processing of metal oxides under hydrothermal conditions constitutes an impor-tant aspect of hydrothermal processing of materials because of its advantages in the preparationof highly monodispersed nanoparticles with a control over size and morphology. There are

Fig. 16. TEM image of Ag compact crystals with the addition of PVP in the reaction system (photo: courtesy Prof. Y.T.

Qian).


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thousands of reports in the literature, which also include a vast number of publications on SCWtechnology for the preparation of metal oxides. The most popular among these metal oxides areTiO2, ZnO, CeO2, ZrO2, CuO, Al2O3, Dy2O3, In2O3, Co3O4, NiO, etc. Metal oxide nanopar-ticles are of practical interest in a variety of applications including high-density informationstorage, magnetic resonance imaging, targeted drug delivery, bio-imaging, cancer therapy, hy-perthermia, neutron capture therapy, photocatalytic, luminescent, electronic, catalytic, optical,etc. Majority of these applications require particles of pre-determined size and narrow size dis-tribution with a high dispersibility. Hence, a great variety of modifications are used in the hy-drothermal technique. However, for the sake of convenience, the synthesis of the most popularmetal oxides such as TiO2 and ZnO will be discussed separately.

Perrotta and Al’myasheva et al. have reviewed the hydrothermal synthesis of corundumnanoparticles under hydrothermal conditions [13,34]. A high specific surface area corundumhas been synthesized through the conversion of diaspore to corundum under hydrothermal con-ditions. This nanosized alumina has great application potential. The authors were able to de-velop a new transitional alumina reaction sequence that gave rise to an alpha intermediatestructure, a0-Al2O3 with a very high surface area. Also they have investigated the thermody-namic basis and equilibrium relationships for the nanocrystalline phases.

Jiao et al. have reported the hydrothermal preparation of ZrO2 nanocrystallites using organicadditives [35]. Phase-pure tetragonal and monoclinic zirconia nanocrystallites of various parti-cle sizes and morphologies were prepared in the presence of polyhydric alcohols such as glyc-erols and di- and tri-ethanolamine, which gave a tetragonal phase, while alkyl halides favouredthe formation of monoclinic ZrO2. The as-prepared tetragonal zirconia particles were sphericalor elliptical in shape and w8e30 nm in size, whereas the monoclinic zirconia particles werespindle-like and w20e40 nm in size.

Sun et al. have reported the solvothermal preparation of CeO2 nanorods 40e50 nm in diam-eter and 0.3e2.2 mm in length by adding ethylenediamine [36]. The morphology was controlledby adjusting solvent composition, surfactant, cerium source, reaction temperature and duration.The UVevis absorption and photoluminescence spectra of CeO2 nanorods show unusual red-shift and enhanced light emission, respectively, compared with that of bulk CeO2. This mightbe due to the abundant defects in CeO2 nanorods and the shape-dependent effect.

Wang et al. have reported the synthesis of Dy2O3 nanorods under hydrothermal conditions at180 �C in about 24 h [37]. Dy2O3 was dissolved in concentrated HNO3 and the pH was adjustedto 7e8 using 10% KOH solution. Then the precipitate was transferred to an autoclave for hy-drothermal treatment. The thermal decomposition of Dy(OH)3 gave rise to Dy2O3 nanorods.

Sorescu et al. have synthesized nanocrystalline rhombohedral In2O3 under hydrothermalconditions at about 200 �C in 4 h [38]. This In2O3 has a corundum structure and is a high pres-sure phase crystallizing with a rhombohedral structure. The hydrothermally treated product waspost-annealed at 500 �C.

Several workers have prepared the a-Fe2O3 (hematite) phase as nanoparticles under hydro-thermal conditions (using both aqueous and non-aqueous solvents) with or without surfactants[39e41]. These hematite particles find extensive applications such as catalysts, pigments, re-cording medium, sensors, etc. Hydrothermal method shows advantages over conventionalmethods like solegel and hydrolysis of iron salts [42]. Surfactants like sodium dodecylsulfo-nate (SDS), sodium dodecylbenzene sulphonate (DBS), cetyltrimethyl ammonium bromide(CTAB) and hexadecylpyridinium chloride (HPC) have been used. Fe(NO3)3$9H2O orFeC2O4 was used as the source of iron. NaOH, or N,N-dimethylformamide (DMF) was usedas a solvent. The experimental temperature ranges from 180 to 250 �C in most of the cases.


The typical size of the products varies from 20 to 200 nm depending upon the starting materialsand the experimental temperature. Iron oxides of spinel and magnetic structures are very impor-tant for their unique magnetic properties, which can be varied systematically through dopantslike Co, Ni, Zn, Mn, etc. Cote et al. have prepared CoFe2O4 nanoparticles through hydrothermalmeans within a temperature range 200e400 �C and pressure 25 MPa [43]. A complete mecha-nism of formation of CoFe2O4 has been discussed in ref. [44]. It was found necessary to controlthe pH and experimental temperature to obtain a desired phase with a size of 100 nm.

Wu et al. have prepared nanowire arrays of Co-doped magnetite under hydrothermal condi-tions at 200 �C using ferrous chloride, cobalt chloride and sodium hydroxide. These nanowiresare believed to possess a single magnetic domain which can be regarded as small-wire likemagnets [45].

Wan et al. have proposed a soft-template-assisted hydrothermal route to prepare single crystalFe3O4 nanorods with an average diameter of 25 nm and length of 200 nm at 120 �C in 20 h [46].The formation of these Fe3O4 nanorods has been ascribed to ethylenediamine, which plays a cru-cial role not only as a base source but also as a soft-template to form single crystal Fe3O4 nanorods.Fig. 17 shows the Fe3O4 nanorods obtained through a soft-template-assisted hydrothermal route.

Kominami et al. have prepared Ta2O5 nanoparticles through solvothermal routes and havestudied their photocatalytic properties [47]. They used tantalum pentabutoxide (TPB) in tolueneat 200e300 �C in the presence of water. Ta2O5 powder of 20e100 nm size showing high sur-face area of >200 m2 g�1 was obtained.

Adschiri and co-workers [48e53] have worked out in detail a continuous synthesis of finemetal oxide particles using supercritical water as the reacting medium. They have shown thatfine metal oxide particles are formed when a variety of metal nitrates are contacted with super-critical water in a flow system. They postulated that the fine particles were produced becausesupercritical water causes the metal hydroxides to rapidly dehydrate before significant growthtakes place. The two overall reactions that lead from metal salts to metal oxides are hydrolysisand dehydration:

MðNO3Þ2þ xH2O/MðOHÞxþ xHNO3

MðOHÞx/MOx=2þ1

2xH2O

Processing in SCW increases the rate of dehydration such that this step occurs while the par-ticle size is small and the reaction rate is less affected by diffusion through the particle. Fur-thermore, the gas-like viscosity and diffusivity of water in the critical region lead toa negligible mass transfer limitation. The net effect is that the overall synthesis rate is verylarge. The high temperature also contributes to the high reaction rate. Several metal oxides in-cluding a-Fe2O3, Fe3O4, Co3O4, NiO, ZrO2, CeO2, LiCoO2, a-NiFe2O4, Ce1�xZrxO2, etc. havebeen prepared through this technique.

Fig. 18aec shows the nanoparticles prepared by Adschiri and co-workers. Reverchon andAdami have reviewed the preparation of these metal oxide nanoparticles under SCF conditions[29].

5.3. Hydrothermal processing of TiO2 and ZnO nanoparticles

The processing of TiO2 and ZnO nanoparticles occupies a unique place in hydrothermal pro-cessing of advanced materials owing to their importance as photocatalysts. There are more than


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1000 articles dealing with the processing of these materials under hydrothermal and SCW con-ditions, and their properties.

The hydrothermal processing of TiO2 has been carried out by a large number of workers[54e61]. It is the most important material being studied extensively in the last few years owingto its unique properties. TiO2 shows maximum light scattering with virtually no absorption. It isnon-toxic and chemically inert. This has been employed extensively in studies of heterogeneousphotocatalysis and has been accepted as one of the best photocatalysts for the degradation ofenvironmental contaminants. The process involves the absorption of a photon by TiO2, leadingto the promotion of an electron from the valence band to the conduction band and thus produc-ing an electron hole. The electron in the conduction band is then removed by reaction with O2

in the outer system; the hole in the valence band can react with OH� or H2O species, which areabsorbed on the surface of the TiO2 to give the hydroxyl radical. This hydroxyl radical initiates

Fig. 17. FESEM and TEM photographs of Fe3O4 nanorods (photos: courtesy Prof. Y.T. Qian).


the photocatalytic oxidation, a pollution control technology or detoxification technology, whichdestroys the organic chemical contaminants in air, water, and soil. It can be used to treat pol-luted water (both surface and ground water, similarly waste and drinking water) and soil. Thetechnique can be used as an industrial pollution management technique for cleaning up gaseousand aqueous waste streams containing organic compounds. The photocatalytic activity of TiO2

depends upon its crystal structure (anatase, or rutile), surface area, size distribution, porosity,and presence of dopants, surface hydroxyl group density, etc. These factors influence directlythe production of electronehole pairs, the surface adsorption and desorption process and theredox process. TiO2 is also used as a photoanode in photoelectrochemical solar cells.

The hydrothermal method has many advantages e a highly homogeneous crystalline productcan be obtained directly at a relatively lower reaction temperature (<150 �C); it favours

Fig. 18. (a) TEM photograph of Fe3O4 particles obtained at 320 �C, 30 MPa, (b) TEM photographs of particles obtained

and (c) TEM photograph of CeO2 particles produced under supercritical conditions (T¼ 400 �C, P¼ 30 MPa, residence

time¼ 0.4 s).


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a decrease in agglomeration between particles, narrow particles size distribution, phase homo-geneity, and controlled particle morphology; it also offers a uniform composition, purity of theproduct, monodispersed particles, control over the shape and size of the particles, and so on.

Several authors have studied in detail the mild hydrothermal synthesis of TiO2 particles andthe influence of various parameters like temperature, experimental duration, pressure (percent-age fill), solvent type, pH, and the starting charge on the resultant product.

The synthesis of TiO2 is usually carried out in small autoclaves of the Morey type, providedwith Teflon liners. The conditions selected for the synthesis of TiO2 particles are: T¼<200 �C,P< 100 bars. Such pressure temperature conditions facilitate the use of autoclaves of simpledesign provided with Teflon liners. The use of Teflon liners helps to obtain pure and homoge-neous TiO2 particles. Though the experimental temperature is low w150 �C, TiO2 particleswith a high degree of crystallinity and desired size and shape could be achieved through a sys-tematic understanding of the hydrothermal chemistry of the media. Here it is appropriate tomention that the size of the titania particles is the most critical factor for the performance ofmaterial with photocatalytic activity, and the monodispersed nanoparticles are the most suitableones. It has been shown that the particle size is a crucial factor in the dynamics of the electronehole recombination process, which offsets the benefits from the ultra high surface area of nano-crystalline TiO2. The dominant e�/hþ recombination pathway may be different for TiO2. Dif-ferent particle size regimes have been established for improving the photocatalytic efficiency ofdifferent systems [54].

Several solvents like NaOH, KOH, HCl, HNO3, HCOOH and H2SO4 were treated as min-eralizers and it was found that HNO3 was a better mineralizer for obtaining monodispersednanoparticles of titania with homogeneous composition under the present experimental condi-tions [54]. Titania has two important polymorphic forms such as rutile and anatase, both show-ing photocatalytic properties. The authors have used different starting charges such as reagentgrade anatase, sintered anatase (at about 800e900 �C for 10 h), TiCl4 and titanium gel. In eachcase the resultant product was TiO2, however, with different ratios of rutile and anatase depend-ing upon the charge, as confirmed from the X-ray powder diffraction studies. Though the rutilephase is more dominant in the resultant product, the presence of a small amount of anatase per-sisted, except when the experimental temperature was approximately 200 �C. When sinteredanatase or titanium gel is used as a charge, it yields better results such as the resultant productcontained more or less uniformly sized or monodispersed particles with a high degree of crys-tallinity, and interestingly, the rutile phase was formed as a prominent phase with a better yield.Better results, in this sense, meant good photocatalytic activity, because the monodispersed par-ticles had a high degree of crystallinity. Similarly, the authors have tried TiCl4 as a charge, andresultant product contained both anatase and rutile. The formation of a single phase required theproper selection of pH of the media as well as the crystallization temperature. The present au-thors have carried out the TiO2 synthesis within a wide range of pH of the media. When the pHof the medium was low (pH¼ 1e2) only rutile phase was formed. When the pH was kept evenlower, i.e. in the negative range, the product contained a small amount of anatase also. As thepH of the medium increased, the product contained essentially anatase with very small amountsof rutile. Thus, with the addition of KOH or NaOH, the formation of anatase phase was fav-oured. With a further increase in the pH, i.e. beyond 12, in the present experimental tempera-ture, only an amorphous material was obtained. An increased temperature results in theformation of alkali titanates. Thus it is necessary to maintain a proper acidity in the systemin order to obtain a homogeneous rutile phase. Similarly, control over the temperature, timeand pH of the medium helps in the preparation of a desired particle size and shape. When


the reaction temperature and time were increased, it resulted in the formation of faceted grainsof bigger size. The following experimental conditions were maintained for the preparation ofultra fine rutile particles of TiO2:

Nutrient: pre-heated anatase phase or Ti gelTemperature: 150 �CDuration: 40 hpH: 2Percentage fill: 60%Mineralizer: 1.5 M HCl

Qian et al. have reported the preparation of ultra fine powders of TiO2 by hydrothermal H2O2

oxidation starting from metallic Ti [56]. This can be done in two steps: (i) oxidation of Ti withan aqueous solution of H2O2 and ammonia to form a gel (TiO2, H2O); (ii) hydrothermal treat-ment of gel under various conditions. It is expressed as follows:

Tiþ 3H2O2þ 2OH� ��!oxidationTiO2�

4 þ 4H2O

2TiO2�4 þ 2ðxþ 1ÞH2O ��!heating

2TiO2$xH2OþO2þ 4OH�

TiO2$xH2O�!hydrothermal

treatmentTiO2 þ xH2O

It is well known that the photocatalytic activity in TiO2 increases with the addition of MoO3,WO3 or other active element. The authors [54] have introduced WO3 into the composition ofTiO2 from 5 and 10 wt.% by adding the required amount of WO3 into the nutrient (starting ma-terials) and have tested all the samples in the photocatalytic degradation of hydrocarbons. It isto be noted that the introduction of WO3 up to 10 wt.% did not change the homogeneity of theresultant product: there was a slight increase in the cell volume. Also, the grain morphology andsize did not alter significantly.

In some experiments, the authors [54] have introduced a very small quantity of tetra butylammonium hydroxide or ethanol or urea. Addition of these organics enhances the crystalliza-tion kinetics greatly and also increases the TiO2 yield. However, the concentration of these or-ganics was maintained at <0.1 wt.%, as it alters the size and shape of the particles. Fig. 19a andb shows the TEM and SEM micrographs of TiO2 powder prepared by hydrothermal treatmentof gel. Chen et al. have prepared TiO2 powders with different morphologies by an oxidationhydrothermal combination method [55]. The authors have discussed the effects of carboxy-methyl cellulose sodium (CMC), HNO3, Al3þ and Kþ (F�) additives on the particle shapeand crystalline structure. They have also studied the crystallization of TiO2 in great detail,like the influence of hydrothermal conditions, pH, reaction temperature, time and mineralizer.

ZnO is a promising material of photonics because of its wide bandgap of 3.37 eV and highexciton binding energy of 60 meV. The wide bandgap makes ZnO a suitable material for shortwavelength photonic applications while the high exciton binding energy allows efficient exci-ton recombination at room temperature. In recent years ZnO nanostructures have attractedmuch attention due to their exceptional properties compared with bulk materials. A varietyof morphologies have been demonstrated for ZnO nanostructures such as nanoparticles [62],nanowires [63,64], nanorods [65,66], tetrapod nanowires [67,66,68], nanobelts/ribbons[64,69], film structures [70], bone-like structures [71], nanosheets [72], nanopropeller arrays


[73], nanorings/helixes [74], etc. In addition to the hydrothermal technique, various other tech-niques have been employed to prepare ZnO nanostructures like metal organic vapour phase ep-itaxy, laser ablation and thermal evaporation. Also, metals like Au, Zn, etc. are used frequentlyto get the desired nanostructures.

A typical hydrothermal experimental run to synthesize ZnO particles is given below. We canuse various Zn sources along with different mineralizers and additives. The experiments can becarried out within a temperature range, 100e250 �C. The Zn source, solvent, pH, experimentaltemperature and the additives control the size and shape of the particles.

A required amount of ZnCl2 was taken in a Teflon liner the mineralizer solution was addedto it and they were then placed inside a reactor. The reactor assembly was then placed inside thefurnace and the temperature of the furnace was set to a desired temperature. After the experi-mental run for a particular duration (5e50 h), the reactor was quenched with an air jet and coldwater and the liner was taken out. The resultant product inside the liner was separated from thesolution and then rinsed with HCl (0.1 M) (when alkaline solvents are used) and NaOH (0.1 M)(when acidic solvents are used) to remove any residual alkalinity/acidity in the product andthoroughly washed with double distilled water. The product was finally dried at 35e40 �C ina dust proof environment. The characteristics of the final product of any hydrothermal synthesisor treatment depend mainly on the experimental parameters like nutrient selection, experimen-tal temperature and pressure, pH of the medium, mineralizer, experimental duration, etc., as thephotodegradation efficiency is proportional to the particle size of the photocatalyst used [62].Hence a mild experimental temperature (150 �C) and a low concentration (1 M NaOH) solvent

Fig. 19. (a) TEM micrographs of TiO2 powder (Photos: Courtesy Prof. Y.T. Qian) and (b) representative SEM photo-

graph of hydrothermally synthesized TiO2 nanoparticulates [62].

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were considered for the hydrothermal synthesis. The hydrothermal experimental durationwas increased from 5 to 50 h and its effect on the formation of pure ZnO phase was studied.It was found that the formation of pure ZnO phase under such low temperature condition re-quires a minimum duration of 40 h. Fig. 20 shows the hydrothermally synthesized ZnOparticulates.

Recently Zhang et al. have obtained ZnO ranging from microcrystals to nanocrystals withdifferent shapes through a capping molecule-assisted hydrothermal process [75]. The flower-like, disk-like, dumbbell-like ZnO microcrystals of hexagonal phase and the capping moleculeslike ammonia, citric acid, polyvinyl alcohol, etc. were used. Similarly, the authors [76,77] havefabricated many interesting ZnO microcrystals such as nanorings, nanobows and long rings tak-ing advantage of the action of electrostatic polar charge by the addition of a few impurities tothe growth process. Some researchers have used hydrothermal synthesis in SCW to producehighly monodispersed nanoparticles of ZnO in the range 39e320 nm [78].

Thus the ZnO particulates synthesis is challenging to materials’ scientists to obtain a desiredmorphology.

5.4. Hydrothermal processing of metal sulphides nanoparticles

Sulphides of various divalent, trivalent and pentavalent metals form an important group ofmaterials for a variety of technological applications. They popularly form IIeVI, IIIeVI,VeVI group of semiconductors which are being studied extensively with respect to their dif-ferent morphologies and particle size, which in turn, greatly influence their properties. Thereare several hundreds of reports on these sulphides such as CdS, PbS, ZnS, CuS, NiS, NiS2,NiS7, Bi2S3, AgIn5S8, MoS, FeS2, InS, Ag2S, and so on, prepared through hydrothermal or sol-vothermal routes with or without capping agents/surfactants/additives to alter their morphol-ogies and sizes as desired.

Similarly, the rapid expansion of supercritical solutions (RESS) has been used by Sunet al. to prepare nanoparticles of CdS and PbS [79,80] and Zhang et al. [81,82]. ZnS nanopar-ticles in reverse micelles processes assisted by SCF have been prepared. Here let us discussbriefly the synthesis of some important sulphides as nanoparticles using the hydrothermaltechnique.

Fig. 20. Hydrothermally synthesized ZnO particles [62].

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Among IIeVI group semiconductor nanomaterials, AX (A¼Cd, Pb, Zn, X¼ S, Se, Te),CdS is an important one. These AX nanocrystals have important applications in solar cells,light emitting diode, nonlinear optical materials, optoelectronic and electronic devices, biolog-ical labeling, thermoelectric coolers, thermoelectronic and optical recording materials, etc. Fur-ther, these compounds can exhibit varying structures such as zincblende, wurtzite, halite, etc.Several papers have been published recently reporting the synthesis of chalcogenides by hydro-thermal method [83e86]. On the whole for sulphides crystallization, non-aqueous solvents arefound to be more favourable and also in bringing down the PT conditions of crystallization.Qian’s group has reported the hydrothermal synthesis (using non-aqueous solvents) of nano-crystalline CdS in some coordinating solvents such as ethylenediamine and pyridine [87e89]. Li et al. have used thioacetamide as the sulphide source, as it easily releases sulphideions, a process which is beneficial in lowering the reaction temperature and shortening the re-action period [90]. The hydrothermal route is more popular than all the other methods reportedin the literature because of the lower temperature, shorter experimental duration and controlover the size and morphology. The experiments are usually carried out in the temperature range150e200 �C.

Yao et al. have obtained nanowires of CdS through hydrothermal reactions using ethylenedi-amine as the reaction medium [91]. The experimental temperature was 140 �C. Chu et al. havereported the shape controlled synthesis of CdS nanocrystals in mixed solvents [92]. They couldobtain CdS nanotetrahedron, pencil-shaped nanorods, tetrapod, prickly spheres, high aspect ra-tio hexagonal nanoprisms, by adjusting the ratio of two solvents, viz. ethylenediamine and eth-ylene glycol under solvothermal conditions, and the experimental time and temperature. Theydid not use surfactants or other templates in the preparation of CdS nanoparticles. The exper-iments were carried out at 180 �C for 5 h. Fig. 21 shows CdS nanoparticles prepared under hy-drothermal conditions. Li et al. pointed out that the morphology of the resultant CdSnanoparticles could be determined by shifting the reactions between thermodynamically con-trolled and kinetically controlled conditions [93].

Nie et al. have prepared ZnxCd1�xS nanorods under hydrothermal conditions using ZnCl2,CdCl2$2.5H2O and (NH4)2S with ethylenediamine aqueous solutions, at 180 �C for 48 h[94]. By changing the molar ratio of ZneCd in the reactants, different compositions ofZnxCd1�xS nanorods have been prepared.

Gorai et al. have synthesized InS through hydrothermal route within a temperature range120e230 �C using indium metal and thioacetamide [95]. This IIIeVI compound semiconduc-tor has important applications in optoelectronic and photovoltaic industries. However, the au-thors failed to obtain the nanocrystals; instead, they have prepared micron to sub-micron sizecrystals of InS.

Chen and Gao have prepared Ag2S nanosheets using waterealcohol homogeneous mediumunder hydrothermal conditions [96]. They have used silver nitrate with water and ammonia so-lution at 160 �C for 10 h.

Li et al. have prepared monodispersed MoS2 particles 70 nm size at 180 �C for 16 h [97].These particles have applications as a hydrodesulfurization catalyst, solid state lubricant, elec-trode in high-energy density batteries and intercalation host to form new materials. The synthe-sis of MoS2 was carried out using (NH4)2Mo3S13 and hydrazine monohydrate (N2H4eH2O) at180 �C for 16 h.

Zhou et al. have reported the synthesis of ZnS nanoplates under mild hydrothermal condi-tions using non-aqueous solvent in a temperature range 160e200 �C for 24 h [98]. They haveused sulfur powder and ZnCl2 in the presence of ethylenediamine.


Wei et al. have synthesized ZnS hollow nanostructures under hydrothermal conditions usinga single surfactant emulsion template at 120 �C for 24 h [99]. A mixture of 0.5 m Zn(NO3)2 and0.5 m thiourea solutions was mixed into the emulsion that was treated hydrothermally to obtainhollow nanostructures.

Zhang et al. have prepared ZnS nanocrystallites through mild hydrothermal decomposition[100]. The use of zinc acetate and sodium diethyldithiocarbamite with an experimentaltemperature of 150e200 �C for 12e72 h produces ZnS nanocrystallites with a differentmorphology.

Zhang et al. have developed the hydrothermal growth of PbS from nanocubes to dendritesusing Pb(NO3)2 and dithizone as reagents and ethylenediamine as solvent at about 140 �Cfor 5 h. They obtained monodispersed PbS cubic phase particles of w70 nm [101].

Gautam and Seshadri have successfully prepared PbS and PbSe nanocrystals under hydro-thermal conditions using non-aqueous solvents [102]. According to the authors [103] the reac-tion of S and Se with a Pb salt in ethylenediamine solvent is reported to give the respectivephase with particles sizes in the range 20e100 nm. Alkaline aqueous synthesis of PbS andPbSe has been reported by Qian and co-workers, where 20e35 nm PbS/PbSe particles are ob-tained by reacting lead acetate with S/Se [104]. Hydrothermal synthesis of particles of PbSesmaller than 50 nm has also been reported [105].

Fig. 21. TEM and SEM images of CdS products obtained at 180 �C for 5 h in mixed solvents with different volume

ratios: (a) 5% of en, TEM image with SEM image as inset; (b) 15%, TEM image with SEM image as inset;

(c) 65%, TEM image and (d) 100%, SEM image with the upper right inset showing a magnified picture of the hexagonal

ends of the long rods, and the lower left inset showing the HRTEM mage of a nanorod. The scale bars in the

TEM and SEM images all represent 100 nm. The scale bar in the HRTEM image is 5 nm (Photos: Courtesy Prof.

Yan Li).


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Biswas et al. have reported the hydrothermal synthesis of a-MnS nanocrystallites withrock salt structure using a non-aqueous solvent [106]. The crystals are well-faceted, pyra-mid-like single crystals of a-MnS. The experimental temperature was varied from 100 to250 �C to get the optimum condition for the formation of a-MnS crystals with a definedmorphology. Manganese acetate and thiourea were used as precursors with benzene as thesolvent.

Liu et al. have reported the synthesis of CdSxSe1�x (0< x< 1) nanorods by hydrothermalmethod using non-aqueous solvents [107]. The nanorods with diameters of 10e20 nm andlength up to 100e150 nm were successfully synthesized at 140 �C in about 10 h. Fig. 22 showsnanorods of CdS0.8Se0.8 synthesized hydrothermally [107].

Several authors have studied the synthesis of bismuth, antimony sulphide nanorods andnanowires under hydrothermal conditions using both aqueous and non-aqueous solvents withand without surfactants [108e110]. An inorganic surfactant (BiCl4

�eCTAþ) with a lamellarstructure was mixed with thioacetamide solution and heated at 140 �C for 48 h under hydrother-mal conditions to obtain Bi2S3 nanowires (Fig. 23) [109]. Zhang et al. have used pure Bi (NO3)3

and water along with thiourea and Na2S to obtain long Bi2S3 nanowires at 200 �C in 16 h [110].Xie et al. have prepared nanorods of Bi2S3 and Sb2S3 using a precursor M(S2CNEt2)3 (M¼ Bi,Sb) in the presence of water at 115e170 �C in 10 h [108].

Fig. 22. TEM images of CdSxSe1�x products with different reaction time: (a) 1 h; (b) 2 h; (c) 3 h and (d) 4 h (photos:

courtesy Prof. Yong Liu).


Hydrothermal synthesis of NiS, NiS2, NiS7, CuS has been carried out by several authorsusing some surfactants, which assisted in controlling their size and shape [111e113].

Thus sulphides occupy a prominent place in both hydrothermal technology and nanotechnol-ogy owing to their unique properties. From the above discussion it is clear that solvothermalroute is a more preferred one than the aqueous solution route for sulphides.

5.5. Hydrothermal synthesis of carbon nanoforms

The synthesis of different carbon polymorphs such as graphite, diamond, amorphous carbonor diamond-like carbon, fullerenes, carbon nanotubes, etc. has attracted considerable interestfor a long time because of their importance in science and technology. There are uncertaintiesabout the phase stabilities of these polymorphs, as some of them do not find a place in the car-bon pressureetemperature (PeT) diagram and are also known for their contrasting physicalproperties. The exact physico-chemical phenomena responsible for their formation are yet tobe understood. Attempts to synthesize these forms with varied conditions and techniques,sometimes even violating the thermodynamic principles, have met with a fair amount of suc-cess. The stabilities of graphite and diamond in nature were mainly controlled by PeTefO2

in the CeOeH system [114e119]. The role of CeOeH fluids [120,121], as well as the hydro-thermal and organic origin [122,123] of these polymorphs, especially with reference to dia-mond genesis, prompted the material scientists to explore the possibility of synthesizingthem at fairly low pressure and temperature conditions. The hydrothermal technique is highlypromising for reactions involving volatiles, as they attain the supercritical fluid state and super-critical fluids are known for their greater ability to dissolve non-volatile solids [124]. Siliconcarbide powder has been used for the synthesis of carbon polymorphs [125e127] and Gogotsiet al. [128] have reported decomposition of silicon carbide in supercritical water and havediscussed the formation of various carbon polymorphs. Basavalingu et al. have explored thepossibilities of producing carbon polymorphs under hydrothermal conditions through decompo-sition of silicon carbide in the presence of organic compounds instead of pure water [129]. Theorganic compounds decompose into various CeOeH fluids; the main components are CO, OH,

Fig. 23. TEM images of the Bi2S3 nanowires by adopting ethanol as solvent at 140 �C for 48 h and by synthesizing by

the direct reaction of bismuth salts with thioacetamide at 140 �C for 48 h in the aqueous solution (Photos: Courtesy

Prof. Y.T. Qian).

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53 (2007) 117e166

CO2 and C1Hx radicals. It is very well known that these fluids play a significant role in creatinga highly reducing environment in the system and also assist in the dissociation of silicon car-bide and precipitation of the carbon phase. The study of solid and gaseous inclusions in dia-mond also indicated the CeOeH fluids as the source for nucleation and growth of diamondsin nature [119].

The experiments were carried out in the pressure temperature range 200e300 MPa and 600e800 �C using externally heated RoyeTuttle test tube type autoclaves (MRA-114 R/S), made upof Rene/Stellite super alloy steel. The starting charge of b-SiC powder having specific area lessthan 8 m2 g�1 was used along with organic compounds. The organic compounds used were re-agent grade formic acid, oxalic acid, malonic acid, maleic acid, glycolic acid, and citric acid. Thestarting materials comprising b-SiC and an organic compound without water were sealed in an-nealed gold capsules (50e60 mm length and 4.5 mm i.d. having a wall thickness of 0.1 mm),thus restricting the amount of water in the system to the water released through the dissociationof organic compounds. We found that the excess of water in the system would decrease the yieldof carbon precipitation and the thermodynamic calculation of Jacobson et al. [130] indicated thatthe formation of free carbon is expected in the low water to carbide ratio. Further, in the highpressure metal-carbon experimental system the free excess water in the system inhibits the for-mation of diamond [131], and the formation of graphite is more favourable. The sealed capsuleswere placed in the autoclaves for hydrothermal treatment. After the experimental run the cap-sules were removed carefully and were cut open. In most of the runs gas with a pungent smellwas evolved and the run products were carefully dried and subjected to characterization.

The authors have reported earlier about the decomposition of silicon carbide in the presenceof an organic compound at a temperature above 700 �C and pressure above 100 MPa [129,132].It was found that the silicon carbide decomposed either to quartz or to cristobalite along withfree carbon particles in the presence of both water and organic compounds, but the yield of car-bon particles had improved when there was no excess water in the system. The carbon particlesformed were discrete or linked spherical shaped particles having pores, the pores were elon-gated, irregular in shape with pore diameter of 20e30 nm. It demonstrates that the CeOeHsupercritical fluids produced through decomposition of organic compounds will have great in-fluence in decomposing the silicon carbide and precipitating the free elemental carbon. How-ever, the hypothesis of an organic origin for graphite and diamond (at least a part of it) andmethane as a more favourable transport medium for the synthesis of diamond in chemical va-pour transport technique [122,123] as well as the varying scenario of natural diamond forma-tion has encouraged us to carry out more rigorous experiments. Hence, by varying the relativeproportion of silicon carbide to organic compound ratio, experiments in the silicon carbideeor-ganic compound systems were carried out. Earlier the diamond growth region has been dis-cussed by comparing the tri-linear plots of CeOeH of organic compounds used with that ofthe similar diagram of Bachmann et al., Rumble III, Hoering, and discussed by DeVries[121,133,134] indicates favourable chemical environment for the diamond formation. Further,Schmidt and Benndorf were of the opinion that the increased concentration of atomic hydrogenand the C1Hx radicals by dissociating the organic compound in a closed system is an ideal en-vironment for the stabilization of carbon phase especially for the sp3 e hybridized carbon underthe sub-natural conditions [135]. The authors [136,137] have noticed not only the improvementin the yield of carbon particles but also the change in the shape of the carbon particles precip-itated to spherical, ovoid and scaly material having metallic luster. Fig. 24 shows the SEM im-ages of (a) spherical particles; (b) enlarged image of pores; (c) ovoid shaped carbon; and (d)spherules with scaly material.


After a careful examination of these carbon particles under high resolution Scanning Elec-tron Microscope, it was found that some of the spherical particles were porous, hollow and thebroken pieces of these carbon particles exhibited the growth of very minute crystallites whichhad adhered to the inner walls’ spheres and these crystals showed well developed octahedralfacets (Fig. 25). The growth of these crystallites resembles the zeolite crystals grown in voidsand cavities of volcanic flows. The authors [136,137] have also noticed different stages in thedevelopment of hollow and porous carbon phase formation, i.e. fluid like material is coming outof the solid spherical or ovoid particles through a vent, resulting in hollow ovoid or spheres(Fig. 26). Quantification of the mechanism leading to the formation of desired shape andtype of carbon formation is difficult since the phenomenon observed as above is not uniqueto any of the particular conditions of formation.

Fig. 24. SEM images showing (a) spherical particles; (b) enlarged image of pores; (c) ovoid shaped carbon and (d)

spherules with scaly material [136].

Fig. 25. SEM images of diamond particles showing well developed octahedral facets adhered to the inner walls of the

broken spherical particles [136].


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Hitherto, the attempts on the hydrothermal synthesis of diamond/sp3 e hybridized carbonsucceeded in the overgrowth of seeded diamond crystal and in some, they even used the metalcatalysts, but in the study of authors [136,137] they did not use either the diamond seed or themetal catalysts. Thus, they demonstrated that, not only the overgrowth but also the nucleationand crystallization of diamond were possible under hydrothermal conditions, particularly undersub-natural conditions.

5.6. Hydrothermal preparation of nanotubes

The discovery of carbon nanotubes (CNTs) in 1991 has opened a new era in materials sci-ence and nanotechnology research to prepare several other types of nanotubes like barium ti-tanate (BTNT) and strontium titanate nanotubes (STNTs), titania nanotube (TNT), siliconnanotube (SNT), antimony nanotubes (SbNTs), gallium nitride nanotube (GNT), etc. [138e144].

CNTs show outstanding electrical and mechanical properties. They are potential buildingblocks for microelectronics and macroenanoelectromechanical devices [145]. Most of theseapplications require CNTs in some kind of alignment in order to use them as field effect tran-sistors, electron-field emitters, etc. In view of the large number of well-defined carbonecarbonsingle and double bonds in most carbon nanotubes, carbon nanotube at its very essence is poly-meric. Having a conjugated all-carbon structure, carbon nanotubes have indeed been demon-strated as possessing some similar optoelectronic characteristics to conjugated polymers. Justas conjugated polymers have widely been regarded as quasi-one-dimensional semiconductors,carbon nanotubes can be considered quantum wires [146,147]. The interesting electronic andphotonic properties, coupled with their unusual molecular symmetries have made carbon nano-tubes very attractive for many potential applications including single molecular transistors,scanning probe microscope tips, gas and electrochemical energy storage, catalyst, protein/DNA supports, molecular-filtration membranes and artificial muscles [148e151]. There aretwo types of carbon nanotubes: single wall carbon nanotubes (SWCNTs) and multiwall carbonnanotubes (MWCNTs). The most popularly used method of preparation of CNTs is the CVDmethod, in the presence of a catalyst metal. However, the pyrolysis of hydrocarbons is alsoused, but it yields poorly ordered crystals and needs the presence of nanosized metal particlesacting as catalyst. The growth mechanism is similar as in the gas phase in a high temperatureregime under vacuum or in an inert atmosphere. Hydrothermal routes may lead to a reproduciblefabrication method of crystalline nanocarbons. In fact natural MWCNTs appear always

Fig. 26. SEM images showing fluid like material coming out of solid spherical and ovoid particles [136].


associated with rocks of hydrothermal origin [121]. Formation of MWCNT in hydrocarbonfluids in supercritical conditions has been previously observed always associated with the pres-ence of substantial amounts of amorphous carbon in the condensed solids. A recent publicationreports the MWCNTs present in coal and carbonaceous rocks.

Gogotsi et al. report the synthesis of hydrothermal CNTs using a mixture of pieces of high-density polyethylene (PE) sheet or ethylene glycol (EG) and water and Ni powder, sealed ingold capsules reacted at high pressure and temperature [152]. These authors also suggestthat virtually any liquid, solid or gaseous carbon source can be used. Fig. 27 shows TEM mi-crographs of carbon nanotubes produced from PE/water/Ni mixtures. These authors have ob-tained four types of particles: (i) tubes with diameter 70e150 nm, (ii) Ni tipped tubes, (iii)tubes terminating with ball-like ends and (iv) isolated pyrocarbon spheres. Tubes obtainedby this method have the lowest degree of graphitization. Nanotubes in the size range 50e100 nm are characterized by a large number of internal closures and torturous profiles whilemicrotubes tend to be straight with fewer internal closures. TEM images of all tubes revealwide internal openings and well ordered graphitic layers. In tubes whose outer diameters arein the range 50e150 nm, graphitic layers are sometimes observed to terminate inside and out-side the tube at low angles. The unique feature of hydrothermal graphite tubes is the entrapmentof apparent liquid inclusions. The authors explain that a high-density fluid phase, from whichgrowth occurs, changes its compositions rapidly as carbon condenses, traversing a range ofcompositions during which different phenomena take place. This explains the wide variancein tube structure.

Yoshimura and group have studied the nanostructural evolution of CNTs under hydrothermalconditions in detail [153,154]. They have used commercial grade SWCNTs from Bucky, USA,in the Tuttle-type autoclaves (provided with gold capsules) filled with double distilled water asa solvent. The experimental temperature was varied from 200 to 800 �C and a pressure of100 MPa over a duration of 30 m to 48 h. Fig. 28 shows the stability diagram of SWCNTs (tem-perature vs. time) in pure water under 100 MPa pressure. This study shows that SWCNTsslowly change over to MWCNTs and polyhedral graphitic nanoparticles under hydrothermalconditions above 550 �C and after 800 �C and 48 h of treatment, SWCNTs completely trans-form into MWCNTs and polyhedral carbon nanoparticles. Motiei et al. [155] have obtained

Fig. 27. TEM micrographs of carbon nanotubes produced from PE/water/Ni mixtures. Hydrothermal nanotubes are

characterized by a wide channel (95 nm) and thin walls (w10 nm) (a). The lattice fringe image (b) shows highly ordered

graphitic wall structure [152].


carbon nanotubes and MgO by reacting carbon dioxide with Mg in a closed cell for 3 h at1000 �C with a calculated pressure of approximately 10 kbar. After removing MgO by treatingthe products with aqueous HCl, the remnant contained CNTs and fullerenes. Lee et al. [156]have carried out the synthesis of CNTs and carbon filaments in supercritical toluene (servingas both the carbon source for CNT formation and the solvent in the chemical reaction) usingferrocene, Fe or FePt nanocrystals as growth catalysts, at 600 �C and w12.4 MPa. The openingand thinning of CNTs will be of interest for applications especially in the drug delivery sys-tems. Such an attempt has been done by Chang et al. [157] for the first time using the super-critical water in the presence and absence of oxygen to study the opening and thinning ofMWCNTs. They have examined the influence of variation of pressure, temperature and timeon the opening and thinning of MWCNTs. The presence of oxygen contributed greatly tothe thinning of the MWCNTs.

In recent years CNTs are encapsulated with a great variety of active complexes, biologicalmolecules, functionalized surfaces based on polymers and carbon nanotubes for biomedical andoptoelectronic applications. Although the hydrothermal technique offers some special advan-tages for processing such advanced materials, there are some disadvantages with respect tothe CNTs. So far there are no reports in the literature on the preparation of SWCNTs andalso the commercial production of CNTs of either type unlike the vapour phase techniques.However, the hydrothermal technique definitely provides some high quality CNTs with a controlover the tube diameter and tube structures.

Recently Zhao et al. have reported the synthesis of thin films of barium titanate and bariumstrontium titanate nanotubes on titanium substrates [139]. Solutions of 0.1 m barium hydroxide(pH¼ 13.4) and mixture of barium hydroxide and strontium hydroxide (pH¼ 13.2) with equalmole ratio in CO2-free deionized water were poured, respectively, into hydrothermal vesselslined with Teflon. Titanium foils that had been anodized under different conditions were putinto the above vessels. The autoclaves were then placed into a bakeout furnace at 200 �C for30e200 m. After that the titanium substrates were removed from the hydrothermal vessel,rinsed in CO2-free deionized water and dried. This work indicates the possibility of fabrication

Fig. 28. Stability diagram of SWCNTs (temperature vs. time) in pure water under 100 MPa pressure [154].

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of nanotube arrays of barium strontium titanate with different ratios of barium to strontium. Themole fraction of barium and strontium in BST films is not the same as that in the originalsolution.

Yan et al. have reported the synthesis of monodispersed barium titanate nanocrystals throughhydrothermal recrytallization of BaTiO3 nanospheres [158]. Hydrothermal reaction has yieldedpure, well controlled size and shape of particles with monodispersity. The change in experimen-tal temperature and experimental duration also changes the size and shape of the particles. Theaverage particle size varies from 30� 5 to 80� 10 nm obtained in a temperature range 180e240 �C in about 3 h.

Zhu et al. have synthesized BaTiO3 nanocrystals under hydrothermal conditions [159]. Bar-ium titanate nanocrystals were synthesized using titanium hydroxide and barium hydroxide asstarting materials. The pH of the precursor was held at 8.5 using 4 N NH4OH. Similarly, in an-other experiment, barium hydroxide and titanium dioxide were used along with KOH solvent at220 �C for 3 days. The particle size is around 200 nm.

Xie et al. have synthesized bismuth titanate using an isopropanol-assisted hydrothermalroute. Nearly spherical Bi2TiO20 nanocrystals were synthesized using Bi(NO3)3$5H2O andTi(SO4)2 as reactants at 140 �C for 1 h. The Bi12TiO20 nanocrystals had a diameter of 10 nmand displayed excellent light absorption properties [160].

In the past few years silicon nanostructures have been extensively studied for their applica-tions in optoelectronic devices [161]. The progress from silicon nanotubes to nanowires hasbeen achieved at 50 �C for 60 m in the presence of HF and AgNO3 solution [162]. The forma-tion of intermediate Si nanostructures (undetached Si nanotubes) is suggested to be responsiblefor the growth of triangular shaped silicon nanowires. The diameter of the nanowires rangesfrom 30 to 200 nm, and interestingly, as in CNTs, here also each Si nanowire has a Ag cappednanolayer on the free end. The authors have given a complete mechanism of the formation of Sinanowires.

Adschiri and group have prepared the BaTiO3 group of compounds as monodispersed nano-particles under sub- and supercritical water conditions [163,164]. They have used anatase phaseTiO2 solution and barium hydroxide dissolved in distilled water. The flow reactor and batchreactor have been used by these authors to obtain well developed barium titanate nanoparticlesat 420 �C. The pressure is maintained usually at 30 MPa. The residence time of the solution inthe reactor was about 20 s. At the exit of the reactor, the fluid was rapidly quenched by using anexperimental water jacket. During the experimental run, the pressure was maintained by usinga back-pressure regulator positioned after a cooling unit. In case of batch reactors, the reactionwas performed in 10 m and the reactors were quenched in a water bath at room temperature.Fig. 29 shows the BaTiO3 nanoparticles at 400 �C and 30 MPa, using a flow reactor and a batchreactor. The size of BaTiO3 nanoparticles from flow type experiment was smaller and the par-ticle size distributed was narrower than that from batch type reactor experiment. The flow typeexperiments provide rapid and homogeneous nucleation of BaTiO3.

Thus there is ample scope for this type of work on the processing of Ba, Pb and Sr titanatesusing flow reactors.

5.7. Hydrothermal processing of hydroxyapatite (HAp)

Hydroxyapatite (HAp) with the chemical formula corresponding to Ca10(PO4)6(OH)2 hasbeen extensively used in medicine for implant fabrication and is one of the most biocompatiblematerials owing to its similarity with mineral constituents found in hard tissue (i.e. teeth and


53 (2007) 117e166

bones) [165,166]. It is commonly the material of choice for fabrication of dense and porous HApbioceramics [167]. Its general uses include biocompatible phase/reinforcement in composites[168], coatings on metal implants, and granular fill for direct incorporation into human tissues[165e167]. The authors have studied extensively the hydrothermal synthesis of HAp by adapt-ing the intelligent engineering approach based on thermodynamic principles [19,169e171].

Experimental conditions were planned based on calculated phase boundaries in the systemCaOeP2O5eNH4NO3eH2O at 25e200 �C. HAp powders were then hydrothermally synthe-sized in stirred autoclaves at 50e200 �C and by the mechanochemicalehydrothermal methodin a multi-ring media mill at room temperature. The synthesized powders were characterizedusing X-ray diffraction, infrared spectroscopy, thermogravimetry, chemical analysis and elec-tron microscopy. Hydrothermally synthesized HAp particle morphologies and sizes were con-trolled through thermodynamic and non-thermodynamic processing variables, e.g. synthesistemperature, additives and stirring speed. Hydrothermal synthesis yielded well crystallized nee-dle-like HAp powders (size range 20e300 nm) with minimal levels of aggregation. Conversely,room temperature mechanochemicalehydrothermal synthesis resulted in agglomerated, nano-sized (w20 nm), mostly equiaxed particles regardless of whether the HAp was stoichiometric,carbonate-substituted, or contained both sodium and carbonate. The thermodynamic modelappears to be applicable for both stoichiometric and non-stoichiometric compositions. The me-chanochemicalehydrothermal technique was particularly well suited for controlling carbonatesubstitution in HAp powders in the range 0.8e1.2 wt.%. The use of organic surfactants, pH ornon-aqueous solvents facilitated the preparation of stable colloidal dispersions of these mecha-nochemicalehydrothermal-derived HAp nanopowders.

The mechanochemicalehydrothermal synthesis utilizes aqueous solution as a reaction me-dium. Mechanochemical activation of slurries can generate local zones of high temperatures(up to 450e700 �C) and high pressures due to friction effects and adiabatic heating of gas bub-bles (if present in the slurry), while the bulk system is close to room temperature [172]. Con-sequently, the thermodynamics of the local reaction environment favour reactions which mayotherwise be kinetically inhibited at the bulk system temperature and pressure. Low-cost rawmaterials can be used for most hydrothermal and mechanochemicalehydrothermal processeswhich, when coupled with the use of conventional autoclaves and mills, can lead toward thedevelopment of low-cost powder synthesis processes.

Fig. 29. TEM images of BaTiO3 nanoparticles at 400 �C and 30 MPa (a) flow type reactor and (b) batch type reactor

[163].


Experimental conditions for hydrothermal synthesis of HAp were based upon calculatedphase boundaries in the system CaOeP2O5eNH4NO3eH2O between 25 and 200 �C. Phase di-agrams were calculated at each experimental temperature using commercial thermochemicalprocess simulation software. Briefly, standard state chemical potentials at the temperature ofinterest are calculated either from temperature-dependent equilibrium constant functions foreach species. The standard state heat capacities e both used in conjunction with solute and sol-vent activity coefficients. The equations used to calculate the latter quantities were documentedby Lencka and Riman [173].

The standard state quantities (DGf0, DHf

0, Sf0) for the solute species were generally taken from

the standard references and data bank [174e176].Changes in solute free energies were calculated as functions of temperature and pressure us-

ing the modified HalgesoneKirkhameFlowers (HKF) model.Computed phase diagrams for hydrothermal and mechanochemicalehydrothermal synthesis

of HAp at selected temperatures, with results from experimental validation studies, are shownin Fig. 6. Experimental phase assemblages from the synthesis agreed well with those predictedby the thermochemical calculations. At the phosphorus concentrations used in this study, wehave found that at 25 �C the HAp stability field exists at equilibrium pH> 4.8, whereas at200 �C the HAp stability field extends to an equilibrium pH as low as 2.9.

FESEM photographs of selected batches of HAp crystals synthesized at 200 �C in 1 wt.%KCl (aq) and 50 vol.% 2-propanol (aq) are shown in Fig. 30. HAp crystals synthesized in 50vol.% 2-propanol (aq) had low aspect ratios ranging between 2 and 3 and diameters between20 and 40 nm (Fig. 30a). Conversely, uniform nanosized needles (dimensions of about20� 100e160 nm, aspect ratio of 5e8) (Fig. 30b) were formed when 1 wt.% KCl additivewas used. HAp crystals prepared under similar conditions but without additives werew20� 50e100 nm in size, yielding aspect ratios between 3 and 5. Here the authors explainthe possible mechanism of the morphology control for HAp [19,171].

Formation of ACP prior to hydrothermal reaction may explain why either equiaxed nanopar-ticles or anisotropic needles over the range of experimental conditions are formed. Since ACPhas a sufficiently low solubility such that the initial precipitate does not completely dissolveunder the experimental conditions, it is likely that the ACP particles acted as templates for

Fig. 30. HAp crystals prepared hydrothermally at 200 �C for 24 h using moderate stirring. Room temperature pH of

precursor slurries was 10. (a) Powders crystallized in 50 vol.% 2-propanol in H2O (aq) and (b) powders crystallized

with 1 wt.% KCl (aq) [19].


HAp crystallization via interface reaction rate control. The formation of anisotropic particlesmay be due to reaction conditions that promote a greater degree of dissolutioneprecipitationthat competes with interface reaction rate control. Greater flexibility in tailoring HAp crystalsize and morphology by the hydrothermal technique could be achieved through precipitationfrom homogeneous solutions containing both Ca and P. Use of chelating agents for Ca, suchas lactic acid or EDTA, prevents formation of ACP upon mixing sources of Ca and P atroom temperature. However, published work with these chelating agents shows that anisotropicfibers are also formed [177,178]. Thus, future work will need to identify additives that inhibitanisotropic particle growth.

Mechanochemicalehydrothermal synthesis was performed at room temperature utilizingsystem compositions so that the model calculations indicated would yield phase-pure HAppowders. These conditions were effective for synthesis of phase-pure undoped HAp, carbon-ate-substituted HAp (CO3HAp), or coupled sodium and carbonate-substituted HAp (NaCO3-

HAp). Fig. 31 shows as-prepared carbonated HAp powders with aggregates of nanosizedHAp crystals.

Fig. 31. Carbonated HAp powders with aggregates of nanosized HAp crystals [170].

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Several authors have contributed greatly on the synthesis of HAp. But what is most challeng-ing to the materials scientist is the control of aspect ratio in HAp. In this respect the hydrother-mal technology has many advantages over other techniques. In recent years there is a growingtendency to prepare carbonate/magnesium/strontium coupled with magnesium and carbonatesubstitutions in calcium phosphate, which imitates the inorganic portion of the natural bonecomposition.

There are hundreds of reports on the hydrothermal preparation of nanocrystalline particlesfor various applications. To mention a few, the preparation of LaPO4, CePO4, AlPO4, ferrites,phosphors like Eu:Y2Sn2O7, Nd:YAlO3, GaN, GaP, Ga3P, vanadates, fluorides, carbonates, gar-nets, C3N4, hydroxides, etc., using both hydrothermal and solvothermal routes [179e186]. Onthe whole, the solvothermal routes or the use of non-aqueous solutions are becoming more pop-ular even for ultra high melting compounds like GaP and diamond. Further, the solvothermalroute can minimize the entry of undesired carbonate or hydroxyl molecules into the final com-pounds. Similarly, the stoichiometry of the starting materials, and in some cases, the change invalency of the metals can be well controlled under both hydrothermal and solvothermal routes.However, the experimentater has to bear in mind that though non-aqueous solvents show veryhigh reactivity, one has to understand the exothermic and endothermic reactions taking placeinside the autoclave and also the pressure surge and release or formation of highly dangerouscomponents with a high volatility. If these things are checked in advance then the method canbe well suited for the advanced nanomaterial synthesis.

6. Hydrothermal processing of composites

For the nature-invented composites, wood (cellulose and lignin) and bone (the polymer col-lagen and the mineral, hydroxyapatite) are specific examples. Man-made composites are alsopopular, especially in the era of nanotechnology. Composites are highly useful in the fabrica-tion of high performance microelectronic devices consisting of multilayer substances: multiplelayers of ceramic (alumina), metal and thin-film organic insulators. Tailoring the properties ofthe interface between the reinforcing component and the matrix is a major application of chem-istry in improving the performance of composites. The surface treatments, now used to modifythe surface properties of reinforcing fibers in composites, are largely empirical. In this regard,the hydrothermal method of processing the materials to obtain composites and multilayers ofceramics and coating of substrates on other materials is very significant. The literature dataavailable on the hydrothermal processing of composites are so vast that it is impossible to dis-cuss each and every material in this review. Hence the authors discuss only some selected com-posites processing like HAp and some other recent materials processed in their laboratories ofthe authors.

The coating of HAp films has been attempted on bioinert materials of high strength/ortoughness, such as polycrystalline alumina (Al2O3), zirconia (ZrO2), Ti metal, TiC, etc. HApcoatings provide stable fixation of the implant to bone and minimizes adverse reaction by pro-vision of a biocompatible phase. Moreover, the HAp coatings decrease the release of metal ionsfrom the implant to the body, and shield the metal surface from environmental attack. The coat-ing under hydrothermal conditions has been carried out effectively by many workers and allthese references have been listed by Suchanek and Yoshimura [187]. Byrappa and Yoshimurahave reviewed the HAp composites in ref. [1]. There are several varieties of HAp-based com-posites like HAp/bioactive glass composites, HAp/polymer composites, HAp/HAp (whisker),etc. Among the commonly used organics for HAp/polymer composites are phosphorylated


cotton fibers, polymeric substrates, polymethyl-methacrylate, poly[bis(sodium carboxylatophenoxy) phosphazene] [188].

The preparation of TiO2 based composites with SiO2, activated carbon (AC), Al2O3, zeolites,etc., have been reported in the literature. Several authors have worked extensively on the prep-aration of TiO2eHAp composites. Recently the authors [189] have prepared anatase nanocrys-tals deposited on hydroxyapatite under hydrothermal conditions. The HAp was preparedinitially using CaCO3, H3PO4 and HNO3. Ammonia solution was used to adjust the pH. Laterto synthesize TiO2-deposited HAp, 0.2 g of the above synthesized HAp or commercial HApparticles were added into a pH adjusted neutral solution of 4 ml titanium amine complexand distilled water. The precursor was treated in an autoclave at 180 or 120 �C for various re-action times. The anatase crystals nucleated by heterogeneous nucleation and grew on the HApsurface. A higher experimental temperature of 180 �C was effective for producing highly crys-tallized anatase with rod like crystals of 100e150 nm in length on HAp crystals.

In recent years several new composites for enhancing photocatalytic properties have been re-ported [190e192]. Wu et al. have reported the synthesis of HNbWO6/Mo nanocomposites forphotocatalytic applications [190]. This nanocomposite was obtained in several steps involvingthe preparation of HNbWO6, and LiNbWO6, then [Fe3(CH3COO)7(OH) (H2O)2]NO3 was addedwith Deggusa P-25 grade unsupported TiO2. M2þ (M¼Mn, Ni and Cu) or M3þ (M¼ Cr and Fe)ions were incorporated into the interlayer of HNbWO6 by hydrothermal reactions of HNbWO6

with 1 M M(NO3)2 or M(NO3)3 aqueous solution in an autoclave at 120 �C for 12 h. After beingfiltered and washed with water, the precipitate was heated at 250 �C for 3 h so as to decomposeany water remaining in the interlayer of HNbWO6. The samples obtained thus were designatedas HNbWO6/Cr2O3, HNbWO6/MnO, HNbWO6/NiO and HNbWO6/CuO.

Byrappa et al. have prepared an activated carbon:titania (AC:TiO2) nanocomposite photoca-talyst under hydrothermal conditions [57,191]. Activated carbon has long been recognized asone of the most versatile adsorbent materials used for the effective removal of low concentra-tions of organic and inorganic species from solution and the industrial wastewater.

There is much scope for the impregnation of a suitable semiconductor onto the activated car-bon surface layers to prepare a carbon/semiconductor photocatalyst. The adsorption capacitiesand the feasible removal rates could be substantially boosted by the impregnation of the acti-vated carbon with suitable semiconductors. Here the authors have used activated carbon as aninert porous carrier material for distributing TiO2 to be accessible to reactants for photocatalyticdegradation. Commercially available activated carbon was used along with coconut shell basedactivated carbon. Activated carbon was crushed into small particles and separated by 50e80mesh. First activated carbon was washed with double distilled water until the black colourof washings disappeared, followed by soaking in 5% HCl solution with constant shaking for24 h. Further it was washed with distilled water till the pH of washings became neutral. Finallythe product was dried at about 80 �C. Then a required amount of activated carbon (2 g) wastaken in a Teflon liner containing a desired amount (10 ml) of different molar concentrationsof HNO3 and NaOH as solvents. The active metal oxides such as TiO2 and ZnO were takenin the form of respective oxides or gels. This mixture was stirred well using a magnetic stirrerfor 2 h. Later the Teflon liner was placed in an autoclave, which was kept inside a furnace pro-vided with a temperature programmer controller. The temperature of the furnace was raisedslowly up to a pre-determined temperature (150e200 �C) for a period of 8e24 h. AC:ZnOshows better catalytic properties than AC:TiO2. However, the greatest disadvantage of thesecomposites is the high cost of activated carbon, but definitely, activated carbon:metal oxidebased composites have been proven to be better photocatalysts than the pure metal oxides.

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Recently the authors [192] have reported the hydrothermal preparation of Nd2O3 coated ti-tania composite particles for photocatalytic applications. They used 1 M NaOH at 250 �C andP w 80 bars with an experimental duration of 5e72 h. A different wt.% of Nd2O3 was used forthe coating in order to reveal the role of Nd2O3. Highly monodispersed nanocomposite particleswere obtained. Fig. 32 shows the schematic representation of the formation of Nd2O3 coatedtitania designer composite particulates.

Fig. 32. The schematic representation of the formation of Nd2O3 coated titania designer composite particulates [191].

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In recent years functionalized surfaces based on CNTs and polymers, DNA-assisted disper-sion and separation of CNTs and some special biomedical and optoelectronic applications havebeen prepared [193,194]. Hydrothermal technique has been effectively used to synthesize suchnanocomposites.

Similarly the encapsulation of active molecules such as ZnO, TiO2, zeolites, etc., into CNTshas also been achieved.

7. Recycling waste treatment and alteration under hydrothermal supercritical conditions

Modern human society has been sustained both by the remarkable development of advancedmaterials and by the huge consumption of energy and resources. As we cannot withdraw our-selves from using them now and even in the near future, the wastes of materials and chemicalsas well as the dissipation of energy and heat will increase markedly and cause increasing en-vironmental problems on earth [195]. The global environment dominates human lives. Globalenvironmental problems will lead to great changes in the social structure in the 21st century.These changes will affect not only human life and industrial activity, but will also force signif-icant reforms on the fundamental concepts for manufacturing goods [196]. Considering thesespecifications we must search for materials that are (1) less hazardous to human life and pref-erably compatible with human beings and other living species; and (2) environmentally friendlyprocessing to fabricate, to manipulate, to treat, to reuse, to recycle, and to dispose these mate-rials. The term ‘materials cycle’ is generally used to designate raw materials synthesis for thefabrication of functional products, and their subsequent disposal. It is well known that all ma-terials are initially extracted from the earth, and then converted into functional products throughvarious processes of materials processing, usually involving very high temperature/energy andcost, which in turn contribute to the global warming. These materials may be disposed of byburial in the earth or recycled [197]. Thus the important subjects for technology in the 21st cen-tury are predicted to be the limitation of environmental corruption by appropriate control ofliving, resources and energy. This has led to the development of a new concept related tothe processing of advanced materials in the 21st century, viz. industrial ecology e the scienceof sustainability [12].

Many believe that implementing industrial ecology will be a principal challenge for businessand society in the 21st century. The race has to become the most innovative, most visionary, andmost effective company for understanding industrial ecology by implementing designs for theenvironment (DFE).

8. Perspectives

Development of simple patterning methods with nanometer resolution, acceleration of thekinetics of synthesis, framing (deducing) the theoretical approaches to the growth of materialsfrom solutions, development of in situ observation techniques, in situ surface modification,achieving high dispersion are some of the examples of the emerging research subjects in hydro-thermal technology. During the 21st century, hydrothermal technology on the whole will not bejust limited to the crystal growth, or leaching of metals, but it is going to take a very broadshape covering several interdisciplinary branches of science. For example, the hydrothermaltechnique is viewed as the most suitable technique to prepare the materials for advanceddrug delivery systems, hyperthermia, neutron capture therapy, bio-imaging, fluorescent label-ing, and so on. These applications require the control of the size and shape of the synthesized


nanocrystals. In situ surface modification under hydrothermal conditions helps significantly inpreparing nanocrystals with a highly controlled size, shape and dispersibility. Such studies canbe carried out with the help of organic molecules or surfactants, or capping agents includingseveral peptides and amines. This leads to the self assembly of nanoparticles totally dispersedwith a definite size and shape. Also hydrothermal technology greatly helps in preparing the hy-brid materials, which is the latest trend in nanoparticles fabrication. The reader can obtain moreinformation on these recent aspects in ref. [198]. Therefore, it has to be viewed from a differentperspective, as it offers several new advantages like homogeneous precipitation using metalchelates, decomposition of hazardous and/or refractory chemical substances, monomerizationof high polymers like polyethylene or tetraphtalate, and other environmental engineeringand chemical engineering issues dealing with recycling of rubbers and plastics instead of burn-ing. Further, the growing interest in enhancing the hydrothermal reaction kinetics using micro-wave, ultrasonic, mechanical, and electrochemical energies will be distinct [2]. Also theduration of experiments is being reduced by at least 2e3 orders of magnitude which in turnmake the technique more economic. This takes hydrothermal technology to a new directioncalled multi-energy processing of the materials, which opens enormous potential which is yetto be explored by the mankind. This not only enhances the reaction kinetics in materialscrystallization, but it evolves a new concept like chemistry at the speed of light. This hasalso led to the other concepts called Instant Hydrothermal System, and Automatic HydrothermalVending Machine to synthesize particles instantly. These aspects have been discussed in detailby Yoshimura and Byrappa [199]. With an ever-increasing demand for composite nanostruc-tures, the hydrothermal technique offers a unique method for coating of various compoundson metals polymers, and ceramics as well as the fabrication of powders or bulk ceramic bodies.

The first quarter of the 21st century belongs to nanotechnology which has a strong bearingon human life and ecology. Hydrothermal technology has a great impact on nanotechnologyowing to the aforesaid advantages. The combination of SCW hydrothermal and nanotechnologyshould be able to answer many of the problems associated with advanced materials processingin the 21st century.

9. Conclusions

Advanced materials processing using hydrothermal technology has lots of advantages owingto the adaptability of the technique, which is also environmentally benign. The use of non-aque-ous and a host of other mixed solvents employed in materials processing have brought down thePT conditions of advanced materials processing. The use of thermodynamic computation helpsto intelligently engineer the materials processing and makes it the most cost effective techniqueeven for high hardness or superhard and ultralow solubility materials. The great advantages ofhydrothermal technology for nanomaterials processing are the production of particles that aremonodispersed with total control over their shape and size in addition to their chemical homo-geneity with the highest dispersibility. A great variety of advanced nanomaterials whethernanoparticles, or nanocomposites covering metals, metal oxides, semiconductors includingIIeVI and IIIeV compounds, silicates, sulphides, hydroxides, tungstates, titanates, carbon, ze-olites, ceramics, composites, etc. have been processed using hydrothermal technology. The useof multi-energy systems like microwaveehydrothermal, or electrochemicalehydrothermal, ormechanochemicalehydrothermal drives this technology to a new and totally unexplored avenuein the 21st century. Also the use of capping agents, surfactants and other organic moleculescontribute greatly to the surface modification of these nanocrystals to obtain the desired


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physico-chemical characteristics. In fact the lowering of PT conditions of nanomaterials pro-cessing has made it the most environmentally friendly and effective technique enabling maxi-mum yields. Hydrothermal technology bears a special mention in advanced materialsprocessing through its ability to significantly accelerate the kinetics of synthesis, to modelthe theoretical approaches from solutions, to develop in situ observation techniques, to evolveSCW and SCF technologies for decomposing and recycling toxic hazardous chemical wastes aswell as the monomerization of high polymer wastes and other environmental engineering andchemical engineering issues like recycling of rubbers and plastics instead of burning, etc. Thecombination of hydrothermal technology and nanotechnology can answer most of the problemsassociated with advanced materials processing in the 21st century.

Acknowledgements

The authors wish to acknowledge Prof. Brian J. Mullin, Editor-in-Chief, Progress in CrystalGrowth and Characterization of Materials, for his constant support and critical commentsin preparing this review. Also the authors acknowledge the referee for critical comments toimprove the quality of this review. Our thanks to Prof. B. Basavalingu, DOS in Geology,University of Mysore, India, for his valuable academic support in preparing this review.

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Prof. K. Byrappa graduated from the University of Mysore, India, with Ranks and Gold Medals.

Then he left India to pursue Ph.D. work at the Moscow State University, Russia during 1981 and

worked as a Post-Doctoral Fellow there for one year. He is specialized in the Crystal Growth and

Material Processing using the Hydrothermal Technique. He has published over 163 research

papers in the International Journals, and also authored and edited several books from Springer,

Elsevier, Noyes Publishers, John Wiley & Sons, and so on. He is the member of several Interna-

tional Bodies like International Commission on Crystal Growth and Characterization of Mate-

rials (part of the International Union of Crystallography); International Commission on

Mineral Synthesis and Interface Processes; Council Member of the Asian Association for Crys-

tallography, Council Member of the Indian National Science Academy Crystallography Associ-

ation, etc. Prof. Byrappa is a recipient of Sir C.V. Raman Award for 1998 in Physical Sciences,

two times recipient of the Golden Jubilee Award of the University of Mysore for the years 1986 and 1992. During 2005, he

was awarded the Materials Research Society of India Medal in recognition of his contribution of Materials Science. He is

serving as Associate Editor and Editorial Board Members of several International Journals. Prof. Byrappa has widely trav-

eled and worked in several international laboratories all over the world. At the University of Mysore, Prof. Byrappa has

a well established Hydrothermal Research Laboratory, involving multi-disciplinary research team.

Prof. T. Adschiri graduated from the University of Tokyo, Tokyo, Japan in the year 1981.

Then he received his Masters and also doctorate (1983) degrees in Chemical Engineering

from the same University. After serving as a Special Research Fellow for sometime, he began

his career as an Assistant Professor in the Department of Chemical Engineering, University of

Tokyo during 1987e1989, then as Assistant Professor in the Department of Chemical Engi-

neering, Tohoku University during 1989e1991. Later, he worked as Associate Professor for

10 years (1991e2001) in the same department. In the year 2001 he was appointed as a Full

Professor in the Institute of Multidisciplinary Research for Advanced Materials, Tohoku Uni-

versity, Japan. He is the member of several National and International commissions, like are

Executive committee of Society of Chemical Engineers, Japan, Committee of Advanced Nano-

technology (NEDO), Committee of Global Environmental Protection (Institute of Advanced

Energy) and Committee of RITE (Research Institute for Innovative Technology for the Earth). Similarly he is also

a member of several Professional Societies like Society of Chemical Engineers Japan, Chemical Society of Japan, En-

ergy Society of Japan, Petroleum Society of Japan, The Japan Society of High Pressure Science and Technology, In-

ternational Society for the Advancement of Supercritical Fluids, American Institute of Chemical Engineering

(AIChE), American Association for the Advancement of Science (AAAS). His dedication and hard work in the field

of scientific research have brought him many awards and honours. He was awarded the Best Research Prize in 2001

53 (2007) 117e166


by the Japan Chemical Society; Advanced Research 2000 Suzuki Prize of General Petroleum in 2000 by the Energy

Society Japan; Best Paper, International Conference of Materials Engineering in 1999. He has authored more than

20 books, over 100 articles and has 30 patents to his credit. He has organized three International Conferences/Symposia.

Also he has delivered more than 19 invited/plenary lectures at International Conferences. Prof. Adschiri has established

a very good laboratory for hydrothermal and solvothermal research at the Institute of Multidisciplinary Research for

Advanced Materials (IMRAM), Tohoku University, Japan.

hydro thermal technology for nanotechnology

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