Journal of Colloid and Interface Science 329 (2009) 73–80

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

www.elsevier.com/locate/jcis

Solvothermal synthesis and properties control of doped ZnO nanoparticles

Samanta Cimitan b,∗, Stefania Albonetti a,∗, Laura Forni a, Francesca Peri b, Dario Lazzari b

a Department of Industrial Chemistry and Materials, University of Bologna, INSTM, Research Unit of Bologna, Viale Risorgimento 4, 40136 Bologna, Italyb Ciba Specialty Chemicals S.p.A., via Pila 6/3, Pontecchio Marconi (BO), Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 May 2008Accepted 23 September 2008Available online 27 September 2008

Keywords:Nanoparticles synthesisIndium doped ZnOGallium doped ZnOOptical properties

Indium and gallium doped ZnO nanoparticles have been prepared by a hydrothermal reaction inethanol and methoxyethanol. A comprehensive study of the preparation process, including a thoroughinvestigation by TG-FTIR and TG-MS of the thermal-purification procedure, is presented. Moreover,the effect of thermal conditions and dopant concentration on the structural and optical properties isdiscussed on the basis of XRD, TEM and UV–vis–NIR results. Reported data indicated that the useof methoxyethanol as a solvent allows an enhanced control of nanoparticle size and favours dopantincorporation into zinc oxide. Near infrared absorption of these materials can be strongly affected byincreasing the doping level and upon treating nanoparticles under reducing atmosphere. Preliminarystudy indicated that this effect is greatly enhanced for gallium-doped zinc oxide.

© 2008 Elsevier Inc. All rights reserved.

1. Introduction

Nanocomposites are advanced materials obtained by combin-ing two phases, one of which having at least one dimensionin a nano-sized scale range. With the exception of clay-polymernanocomposites, which have been widely investigated [1] and to-day are at a commercial stage, the research activity on this fieldis still very limited [2,3]. As an example, semiconductor crystallinenanoparticles could be utilized for the preparation of conductivenano-composites materials that can be exploited for a number ofapplications in different technological fields. In this respect, trans-parent conducting oxides (TCOs) attract particular interest. Thesesemiconductor materials are widely used to produce IR reflect-ing or heatable layers, electromagnetic shielding or dissipatingstatic [4], transparent infra red shields [5,6]. Normally, multilayerfilms or coatings are in use as solar control coatings, such as mightbe used on window glass [7,8]. The most widely used TCO istin-doped indium oxide (ITO) [9] but the development of a inex-pensive material with physical properties similar to those of com-mercial ITO is important because the expanding industrial needsand the cost and scarcity of indium [10]. However, the simultane-ous occurrence of high optical transparency in the visible regionand the high electrical conductivity is not possible in an intrinsicstoichiometric material. The only way to obtain good transpar-ent conductors is to create electron degeneracy in a wide bandgap (greater than 3 eV) oxide by controllably introducing non-stoichiometry and/or appropriate dopants. Selecting dopants ionic

* Corresponding authors.E-mail address: stefania.albonetti@unibo.it (S. Albonetti).

0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2008.09.060

sizes have to approximate those of the oxide host materials, andtherefore should be essentially non-disruptive to the host crys-tal lattice, thereby reducing electron scattering and increasing filmconductivity.

Pure ZnO is an n-type semiconductor with a wide band gap of3.37 eV [11] and it has been extensively studied for a long timebecause its use in a wide range of applications such as synthesisof methanol [12–14] and the stabilization of polyethylene [15]. Itis highly resistant to chemical attack and has a good adherence tomany substrates. High transparency in the visible region and re-fractive index value (it is of the order of 1.7–2) enable it to act asan antireflection coating, conductive electrode and window layerin solar cells [16]. ZnO films are also used as light emitting de-vices [17], gas sensors [18] and acoustic wave filters. Doping of ZnOcan be achieved by replacing Zn2+ atoms with the atoms of ele-ments of higher valency such as Ga, Al and In, inducing dramaticchanges in its electrical and optical properties [10]. The efficiencyof the dopant element depends on its electronegativity and ionicsradius but it is also strongly influenced by the synthesis method.In most case, doped ZnO is produced in the form of thin films ap-plying various methods, such as chemical vapour deposition [19] orvapour-phase oxidation [20]. However, these techniques are highlyrestricted with respect to substrate geometry, substrate structureand cost effectiveness. Moreover, high temperature is often re-quired to achieve fully crystalline films.

Since, there is today a great interest to coat systems which donot withstand high temperature, such as plastic substrates or al-ready preformed glass devices, an answer to these problems couldbe the wet chemical deposition of suspensions containing crys-talline nanoparticles [21,22].

74 S. Cimitan et al. / Journal of Colloid and Interface Science 329 (2009) 73–80

Various methods of synthesis have been developed in order toobtain nanoscale materials with controlled properties. Two typeof synthesis, that gave good results to this respect, are the so-called polyol method developed by Fievet and co-workers [23,24]and Feldmann and co-workers [25,26] and the solvothermal meth-ods [27–29]. In these procedures, whose mechanisms are non-hydrolytic, molecular precursors are decomposed in the reactionvessel and the formation and crystallization of the oxide speciesis promoted by the energy provided by the process temperatureand/or pressure. Moreover, the solvents act as reactant as well ascontrol agent for particle growth, allowing the synthesis of highpurity nanomaterials [30–33].

In the present paper, we describe the effects of indium and gal-lium doping on structural, electrical and optical properties of ZnOnanoparticles prepared by hydrothermal reaction in ethanol andmethoxyethanol. This synthetic approach has the advantage that itis a procedure for the synthesis of oxide nanopowders at moder-ate temperatures, is very easy to perform and leads to quantitativeyields of doped nanoparticles. These points are of particular im-portance given the interest for such nanomaterials in commercialapplication. In particular, we focused on the optimization of dopedzinc oxides preparation, with the final aim of achieving stable solsand nano powders to be used as incorporated additive with se-lective optical properties. The synthesis and characterization ofnanostructures of such binary metal oxides such as In and Ga-doped have been studied, and hereby elucidated into details. Thusa comprehensive study of the preparation process is presented,including a thorough investigation of the thermal-purification pro-cedure and the effect of heat treatment conditions on optical prop-erties.

2. Experimental

All chemicals were from Sigma Aldrich and used as such.Ga(NO3)3·xH2O was dried in a kugelrohr oven set to 150 ◦C for8 h (15 mmHg). The autoclave apparatus was a 300 ml capacityhastelloy C22 water cooled by a thermostatic unit Julabo 5HC. Theexperiments were repeated successfully within a 250 ml capacitysteel autoclave, paired by silicon oil Haake thermostatic unit. Thekugelrohr oven was a rotary Büchi apparatus equipped with a vac-uum pump, functioning at 15 mmHg.

2.1. Preparation of doped ZnO nanoparticles

2.1.1. In:ZnO14.0 g Zn(CH3CO2)2 and the necessary amount of In(CH3CO2)3

to obtain the desired Zn:In ratio, were dissolved in 166 ml ofmethoxyethanol (>99.5%) or ethanol (>99.5%) within the auto-clave apparatus. The temperature was raised till 150 ◦C and thedeveloped pressure was about 2 bar and 10 bar, respectively. Oncethe temperature was set the mixture was left for an hour at such atemperature and then lowered to room temperature. The obtainedsol was bluish and stable. Straight after the end of the reactiona sample was taken for dynamic light scattering (DLS) analysis.The crude was dried by evaporating the solvent and subsequentlyby one or more thermal treatment within a rotary kugelrohr ovenset to 150 ◦C for 19 h under N2. Table 1 reports the In–ZnO pow-ders synthesised in methoxyethanol and ethanol containing differ-ent amount of In. The samples are named according to the usedsolvent (E for ethanol and M for methoxyethanol) and to the Inconcentration: that is, IZO-M6 corresponds to the sample synthe-sized in methoxyethanol with 6 wt% of In doping respect to Zn,and so on.

2.1.2. Ga:ZnO8.42 g Zn(CH3CO2)2 and 1.15 g Ga(NO3)3·xH2O were dissolved

in 100 ml of methoxyethanol (>99.5%) within the autoclave ap-

Table 1In-doped ZnO samples synthesized in different solvents.

Sample Reactionsolventa

In(w/w%)

Particle size,DLS (nm)

PI

IZO-E2 ET 2 63 0.084IZO-E6 ET 6 58 0.090IZO-E12 ET 12 122 0.134IZO-M0 MET 0 60 0.072IZO-M2 MET 2 48 0.055IZO-M6 MET 6 47 0.107IZO-M12 MET 12 59 0.147

a Ethanol (ET), methoxyethanol (MET).

paratus. The temperature was raised till 150 ◦C and the developedpressure was about 2 bar. Once the temperature was set the mix-ture was left for 15 min at such a temperature and then lowered toroom temperature. The obtained sol was milky and stable. Straightafter the end of the reaction a sample was taken for DLS analysis.The crude was dried by evaporating the solvent and subsequentlyby one or more thermal treatments within a rotary kugelrohr ovenset to 150 ◦C for 19 h.

Elemental analysis by ICP was done on the solvent of end-reaction obtained after nanoparticles separation. This analysis con-firmed that this method of synthesis leads to quantitative yields ofdoped nanoparticles and that the stoichiometric amount of In andGa was inserted into ZnO powder during the mixed oxide prepara-tion procedure.

2.1.3. Powder reduction treatmentPowder reduction was carried out in a fixed bed glass reac-

tor at atmospheric pressure. A K-type thermocouple was placedinto the sample to monitor the temperature. Each run utilizedapproximately 5 g of powder in the form of 30–60 mesh (250–595 μm) particles. The total volumetric flow through the catalystbed was held constant at 60 ml/min (measured at atmosphericpressure and room temperature), 50% hydrogen and 50% nitro-gen.

2.2. Structural characterization

2.2.1. Nanoparticle size analysisNanoparticle sols were analysed by dynamic light scattering

(DLS) using a Malvern HPPS 5000 Instrument and a Malvern ZetaNano Sizer. The instrument being equipped with a 3.0 mW He–Nelaser (633 nm) and an avalanche photodiode as detector. Correla-tion functions were recorded at a constant scattering angle of 90◦ .Sols were diluted in methoxyethanol or ethanol and sonicatedprior to the analysis. The hydrodynamic diameter measure givenby the z-average value, the polydispersity index and the volumedistribution were taken as indicative results for the size of the par-ticles.

2.2.2. TGA-IR-MS studyTGA-IR-MS curves were recorded simultaneously on the Mettler

TGA/SDTA 851e-Nicolet Nexus 470 FTIR–Balzers Quadrupole MS.Experiments can be done under air or argon (50 ml/min) in 900 μlalox crucibles between 30 and 800 ◦C with 10 ◦C/min. A samplewas heated up from 30 ◦C to 600 ◦C at 10 ◦C/min analyzed by TGAfollowed by evolved gas analysis in order to measure the temper-ature range in which decomposition occurred and to identify thevolatile compounds produced. This was of interest to understandthe material’s composition and what can make it impure, eventu-ally influencing desired optical properties. The gaseous productswere analyzed using a mass spectrometer (MS) coupled with aFourier transform infrared spectrometer (FTIR).

S. Cimitan et al. / Journal of Colloid and Interface Science 329 (2009) 73–80 75

Fig. 1. TGA-IR curve for sample IZO-M6.

2.2.3. TEM analysisA Zeiss EM 910 with a LaB6 cathode transmission electron mi-

croscope was used in the current study. TEM images were taken atan acceleration voltage of 100 kV.

2.2.4. Powder X-ray diffraction measurementsPowder X-ray diffraction measurements were carried out at

room temperature with a Bragg/Brentano diffractometer system(Philips PW1050/81-PW1710), equipped with a graphite monochro-mator in the diffracted beam, operating at 40 kV and 40 mA. Cuanode (CuKα radiation λ = 0.15418 nm) was used as X-ray source.Data were collected in the range 10–80◦ (2θ ), step size 0.05 and1 s time per step. Scherrer equation was used to determine thecrystallite size. The Warren correction was used for instrumen-tal line broadening, without considering the effect of microstrainand/or disorder effects. Silicon was used as external standard.The lattice constants were determined by least-square refinementswith the “celref” routine [34], from the well-defined positions ofthe most intense peaks. The crystallographic parameters a and cwere calculated for a hexagonal cell on the basis of P63mc spacegroup.

2.2.5. Diffuse reflectance analysisUV–vis spectroscopy was used to characterize the optical ab-

sorption properties. UV–vis spectra were recorded in the diffusereflectance mode (R) on a Perkin-Elmer Lambda 19 spectropho-tometer, equipped with a 60-mm integrator sphere, using BaSO4 asthe reference sample. Spectra were recorded in air at room temper-ature and the data transformed through a Kubelka–Munk functionrelating the reflectance of the sample (R∞) to sample concentra-tion; mathematically F (R) = (1 − R∞)2/2R∞ .

2.2.6. Temperature-programmed reduction tests (TPR)The reduction behavior of doped ZnO species was studied by

means of TPR using a Termoquest TPDRO instrument. The reduc-ing gas was composed of 5 vol% of H2 in Ar. The general procedurecan be divided into a pre-treatment and a measuring step. 0.1 g ofeach sample, shaped in granules of 250–595 μm size, were loadedin a quartz glass reactor, and first heated in flowing dry air at

100 ◦C for 1 h to eliminate adsorbed water. After cooling to roomtemperature, air was replaced by hydrogen as reducing gas andthe temperature program was started: 10 ◦C/min from room tem-perature up to 900 ◦C. The temperature of the catalytic bed wasdetected by a thermocouple. The outlet gas from the reactor wasanalyzed with a TCD detector.

3. Results and discussion

3.1. Synthesis and purification of the nanoparticles

The preparation of doped ZnO by hydrothermal synthesisyielded nanoscale particles suspended in ethanol or methoxyeth-anol. The resulting suspensions were colloidally stable for severalweeks. Laser diffraction investigation (DLS) was applied to examinethe particle hydrodynamic diameter and the particle size distribu-tion in suspension.

These data can strongly differ from that obtained by XRD due tothe different analytical techniques used, which provide informationon the size of the diffracting nanocrystals (XRD) and on the hydro-dynamic diameter of the scattering object, including the eventualamorphous surface layer and the solvatation sphere (DLS).

Table 1 summarizes DLS data on In-doped samples and indi-cated that the average particle diameter of suspensions dependsboth on the used solvent and on In content. In general, as indicatedby the polydispersity index (PI), the size distribution is closelyrelated to the average particle diameter: the larger the particlediameter, the broader the particle size distribution. Nevertheless,all suspensions revealed an uniform dispersion of nanoparticles (PI< 0.2). Moreover, the use of methoxyethanol as a solvent allowsa better control of hydrodynamic particle size—and thus of sus-pension stability—probably due to its higher complexation abilitycompared to ethanol, while increasing the indium content leads toparticles with larger dimensions. Since DLS measurements rely onthe Brownian motion present among the measured suspensions,this observed increase could be attributed to some changes in thedipolar interaction among doped ZnO nanoparticles, that can leadto slight particle aggregation.

76 S. Cimitan et al. / Journal of Colloid and Interface Science 329 (2009) 73–80

Scheme 1. Proposed mechanism for doped ZnO nanoparticles.

TGA followed by evolved gas analysis was utilized in order toidentify the residual volatile compounds evolved during the ther-mal treatment of nanoparticles. Fig. 1 reports the TGA-FTIR anal-ysis on IZO-M6 sample as an example. Up until 600 ◦C, two massloss steps can be identified in the TGA curve at 140 and 280 ◦C,respectively. By analyzing the TGA-FTIR spectra which output is de-picted in Fig. 1, we can easily see that these three steps are linkedto methoxyethanol, acetic acid and CO2 development. From thesedata it can be supposed that between 140 ◦C and 230 ◦C somepart of the organic residuals is progressively desorbed from thesurface while at higher temperature the carbonaceous residual ismainly oxidized to CO2. The acetic acid IR pattern is very muchin line with the databank reference spectrum, this confirming thatsome of the acetate coming from the zinc precursor is transformedinto acetic acid and electrostatically coordinated within the mate-rial structure. A small amount of acetate is also detectable via MS.TGA-MS confirm the residuals identification. Four m/z peaks canbe identified: the m/z 59 and 60 indicated the presence of acetateand acetic acid (about 1.46% in terms of mass loss from the TGAcurve). The presence of traces of methoxyethanol coming from thereaction solvent is not detectable via MS but can be seen fromthe TGA-IR study. The m/z 44 is for CO2 and it refers to the ma-jor mass loss of ca. 10%, and starts to be detected at about 230 ◦C.A peak m/z 81 likely referring to Zn(OH)∗ moiety is also presentafter 200 ◦C, suggesting that a form of instable zinc hydroxide isprobably located within the crystalline structure of the material.The TGA-FTIR analysis is in line with the TGA-MS analysis: tracesof methoxyethanol and small amounts of water are also identifi-able.

3.2. Mechanism of particle formation

Even though the mechanism leading to the oxide formation willneed further detailed investigation, a possible reaction mechanismcan be proposed on the basis of our experimental results. In fact,the formation of doped ZnO nanoparticles could be based on a hy-drolytic process initiated by the OH ions that can be generated as aresult of an esterification reaction of acetate ions with alcohol, asalready reported [35,36]. Since the generation of methoxyethanolacetate, in our conditions, was confirmed by gas chromatography–mass spectrometry (GC–MS) analysis of the surnatant solution af-ter the inorganic nanoparticles had been removed, we can assumethat the hydrolysis of zinc acetate is the first step of the overall re-action. Alcohols, in fact, provide an appropriate medium for the hy-drolysis of carboxylates [37] that yield ZnO through the condensa-tion route. Thus, the esterification of acetate with methoxyethanolcould lead to the release of hydroxyl groups binded to the zincspecies whose condensation lead to ZnO and water, as indicated inScheme 1.

ZnO crystals can be formed either directly by homogeneousprecipitation, or through redissolution of metastable hydroxo-intermediates, such as zinc hydroxide or zinc hydroxyacetate thatchange into the more stable ZnO. Layered zinc hydroxyacetatehave been obtained at near room temperature processes fromZn(CH3CO2)2 in alcoholic media [38,39] but these species shouldbe unstable in our reaction conditions.

Fig. 2. XRD patterns of IZO synthesised in ethanol with different indium concentra-tions: (a) IZO-E2 sample; (b) IZO-E6 sample; (c) IZO-E12 sample.

Fig. 3. XRD patterns of IZO synthesised in methoxyethanol with different indiumconcentrations: (a) IZO-M0 sample; (b) IZO-M2 sample; (c) IZO-M6 sample, (d) IZO-M12 sample.

3.3. Structural and chemical properties of obtained powders

3.3.1. X-ray diffraction analysisThe XRD patterns of the ZnO based samples synthesised in

methoxyethanol and ethanol at increasing In loadings are shownin Figs. 2 and 3. Powders as obtained are crystalline and show astable wurtzite structure. All reflections were assigned to hexag-onal P63mc structure of ZnO and are indexed on the basis ofJCPDS card No. 36-1451. No phase corresponding to indium oxideor other compounds was detected. After In doping, the diffractionpeaks shift slightly towards smaller diffraction angles compared tothe ZnO crystals, suggesting an increase of lattice constant, as al-ready observed by others authors [40]. Moreover, the peak widthof doped nanoparticles is much broader than that of undopedmaterials, indicating a lattice distortion due to the indium intro-duction. It is noticed that the most significant broadening takesplace with the higher dopant content regardless of the solventused. This result clearly indicates the influence of the In atoms onthe formation of ZnO crystallites. A similar effect was reported inthe literature [41].

However, some considerations have to be addressed to the useof different solvents. First of all, when solvent is ethanol, some

S. Cimitan et al. / Journal of Colloid and Interface Science 329 (2009) 73–80 77

Table 2Powder grain size for In-containing samples calculated respect to different latticeplanes.

Sample Grain size (nm) vs plane

100 002 101 102 110 103

IZO-E2 20 17 19 18 18 17IZO-E6 16 8 13 10 15 8IZO-E12 16 2 16 10 14 10IZO-M0 23 22 22 20 21 19IZO-M2 18 15 17 9 17 15IZO-M6 18 6 12 9 15 7IZO-M12 16 3 11 7 13 5

spectrum (IZO-E2) reveals the characteristic peaks for crystallineZnO along with several other peaks, most of which correspondto Zn(CH3CO2)2 formed during drying. The presence of unreactedZn(CH3CO2)2 is related to the degree of completion of nucleationand growth; thus the disappearance of the Zn(CH3CO2)2 peaks uti-lizing methoxyethanol indicated that the nucleation and growth inethanol is very slow probably due to the low concentration of oxy-gen in the molecule. In addition, it seems that increasing the Incontent improves ZnO purity.

The grain size of the powders were calculated using the Scher-rer formula D = (0.9λ)/[(β ′) cos θ] where D represents the diame-ter of the crystallites, λ is the wavelength of CuKα line. β ′ is theFWHM of the reflection at a given 2θ Bragg angle cleaned for theinstrumental broadening [(β2 − β2

st)−1/2] where β represents the

FWHM of the measured reflection in the sample and βst is theFWHM of a suitable reflection of a strain free sample with verylarge crystallites, in the same 2θ region. Standard deviation for allcalculation was lower than 0.5 nm. Independently of the solventused, the grain size was found to decrease with increasing In load-ing, as reported in Table 2. In particular, such grains decrease ismore consistent along some lattice plane (i.e. 002, 102 and 103).Taking into account the grain dimensions a and c respectively fromthe (100) and (002) diffraction peaks, these data seems to indi-cate that the products formed in absence of dopant were sphericalor ellipsoidal grains, with average diameters of 15–20 nm. On thecontrary, the presence of indium, favour the formation of rods,grown preferentially along the a axis. In fact, the estimation of thecrystallite size at increasing In content was constant along that di-rection, while in the c direction decreases from 20 nm to 3 nm.This means that the crystal growth of wurtzite ZnO in the c direc-tion was highly inhibited by indium introduction. Recently, it wasreported that specific anion, such as citrate, can be selectively ad-sorbed on the positively charged Zn2+ face on the (001) plane ofthe ZnO structure leading to a suppression of the growth along thec direction by the absorption [42]. Thus, this significant change inthe ZnO growing behaviour in presence of In seems to indicate aenhanced absorption of anion, such as acetate or others organics,on the (001) plane due to the doping.

On the basis of cell parameters refinements, it is possible to ob-serve a modification of the primitive cell volume as a function of Inloading, as reported in Fig. 4. The increasing trend of the primitivecell volume with In loading suggests that In atoms are occludedin the ZnO lattice and it is likely that the slightly larger In3+ ionsmight replace the smaller Zn2+ ions in the ZnO crystals. Such trendis in agreement with the linear Vegard’s Law as already reportedfrom others authors [43] due to the slight difference between Znand In ionic radii [44]. Nevertheless, this effect could reasonablyalso be affected by the larger repulsive forces arising from theadditional positive charge of the In cations. To balance this extracharge, free electrons can be released into the conduction band, in-creasing carrier concentration and consequently resulting in higherconductivity consistently with what has been shown previously inthe literature with gallium doped ZnO [45,46]. Thus, as their con-

Fig. 4. Unit cell volumes derived from X-ray refinements as a function of the amountof In-dopant in the zinc oxide.

centration increases with increasing level of indium doping, theunit cell should increase as well.

In addition, from this data it is possible to observe a split tohigher cell volumes for the powders synthesised in methoxyetha-nol in comparison to the ethanol one. This result seems to suggestthat this solvent might emphasize the insertion of In ions in theZnO lattice increasing charge repulsions among the atoms of thecrystals yielding an increase of lattice constrains.

3.3.2. TEM analysisFigs. 5a and 5b provide representative TEM overview images of

the ZnO and In/ZnO nanoparticles synthesized in methoxyethanol.In the case of ZnO (Fig. 5a), although the particles agglomerated,due to the lack of any stabilizing surfactants, they are clearly dis-tinguishable from each other. They exhibit nearly spherical mor-phology with an average diameter of about 20 nm, agreeing wellwith the XRD data. The presence of In profoundly influences theformation of ZnO and the shape of particles varies from roundedto rod-like. The determination of the exact primary particles size inthis images become difficult, but it roughly indicated that the in-troduction of the In ions in ZnO, beside changing the nanoparticlesmorphology, also decreases the primary particle size, as alreadyshowed by XRD measurements.

3.3.3. UV–vis–NIR spectroscopy measurementsA typical example of the optical properties of doped ZnO

nanopowders synthesized in methoxyethanol is shown in Fig. 6reporting DR–UV–vis–NIR spectra for materials at different In con-tent. Kubelka–Munk treated data of diffuse reflectometry are anal-ogous to absorption results, and allow relative comparisons of ab-sorption for bulk samples. The typical UV-absorption bands of ZnOnanoparticles, at about 360–380 nm, can be clearly seen for allsamples. Moreover, reported data indicated a high transmissionin the visible range for all powders. Indium introduction causesthe appearance of absorbance in the near IR range; the influenceof n-doping due to indium presence is, in fact, clearly seen bythe strong absorption occurring in the range from 900 nm < λ <

2000 nm, due to the presence of free carrier in the materials [47].This absorption band increases as the In concentration increases,reaching a maximum value at In = 6 w/w%. Further increase inthe [In]/[Zn] value leads to a decrease of absorption in this range.

The effect of In on the near IR range absorption was alsoconfirmed for samples synthesized in ethanol (see Supporting in-formation). Nevertheless, for these materials the absorption be-tween 900 and 2000 nm was less significant and reach a max-imum for 2 w/w% In, confirming the lower ability of ethanol infavouring the doping of ZnO, as also indicated by XRD analysis.

78 S. Cimitan et al. / Journal of Colloid and Interface Science 329 (2009) 73–80

Fig. 5. TEM overview image of IZO-M0 (a) and IZO-M6 (b).

Fig. 6. DR–UV–vis–NIR spectra of methoxyethanol synthesized ZnO containing dif-ferent content of indium.

3.4. Characterization of the thermally treated nanoparticles

In a preparation of nanoparticles containing composites, vari-ous processes such as particle synthesis, thermal treatment anddispersion are involved. As we have already reported, the mostimportant process among them is to be particle synthesis. In fact,particle to be dispersed must be synthesized in order to maximizethe needed properties. Nevertheless, there is little study for therelationship between particles properties and manufacturing vari-ables, such as thermal treatment environment and temperature. Inparticular, thermal treatment studies in different atmospheres andtemperatures may be useful since these conditions may change op-tical properties without altering the material morphology.

Fig. 7. DR–UV–vis–NIR spectra of indium doped ZnO (IZO-M6) as is (—) and thermaltreated in air at 350 ◦C (!).

In our case, when prepared under N2, the In-doped zinc ox-ide powders showed a light blue colour for low doping levels anda blue-black colour for heavier doping. However, even the mosthighly coloured products have the optical absorption so low in thevisible region of the spectrum that, also in this case, they can beconsidered transparent conductors. Nevertheless, this blue colourrapidly turned to light green upon heating the powders in air to350 ◦C or above, while the absorption band in the NIR region dis-appeared (Fig. 7). These data indicated that in the preparation ofhighly conductive and transparent doped ZnO nanoparticles, it hasto be taken in account that the control of the oxidation of Zn ismuch more difficult than that of other binary compounds, such asSnO2 and In2O3, because Zn is more chemically active in an oxidiz-ing atmosphere than either Sn or In. This effect may be attributedto the binding energy between Zn and O. Thus, the activity andamount of oxygen during thermal treatment of nanoparticles mustbe carefully verified.

In addition, the possibility of partially reducing doped ZnO wasinvestigated by temperature-programmed reduction tests with H2(H2-TPR). The reduction is measured by monitoring hydrogen con-sumption from diluted hydrogen in inert gas mixture while in-creasing the temperature of the sample at constant rate. In thisway, if one or more reduction events occur at different tempera-tures, the reduction profile can be easily obtained. The temperatureof the peak maxima are characteristic of the reduction process andmay be used for “finger-print” identification for different reducingphases. In our case, the TPR tests were carried out in order to de-termine the temperature range at which it was possible to partiallyreduce doped ZnO. The TPR-profile of the IZO-M6 and GaZO sam-ples is reported as an example in Fig. 8. Samples show broadenedpeaks because of the heterogeneity of the sites but in both cases,three main zone of reduction were present, with a maximum at350–400 ◦C, at 550–600 ◦C and at about 800 ◦C. On the basis ofthese information, In and Ga doped samples were treated underreducing conditions at 500 ◦C and 650 ◦C. While with the lowertemperature treatment no formation of metallic Zn was observed,the reduction of doped samples at 650 ◦C yielded Zn, indicating apartial degradation of the oxide structure.

On the contrary, treating doped zinc oxide under reducing con-ditions at 500 ◦C causes a deeper dark blue colour to develop,which turns to light green if the sample is heated in air atabout 350 ◦C for few hours. In addition, the NIR absorption of re-duced sample significantly increased (Fig. 9).

From these results it is suggested that, at 500 ◦C, with the aid ofreducing atmosphere of H2, the interstitial O-atoms were removed

S. Cimitan et al. / Journal of Colloid and Interface Science 329 (2009) 73–80 79

Fig. 8. TPR analysis of ZnO doped with In (—) and with Ga ( ).

Fig. 9. DR–UV–vis–NIR spectra of indium doped ZnO (IZO) and gallium doped ZnO(GaZO) as is (—, ) and reduced at 500 ◦C (1, ).

as possible as it could and hence a plentiful corresponding freecarries were released. Preliminary studies indicated that this effectis greatly enhanced thermal treating in H2 the zinc oxide dopedwith gallium ions (Fig. 9, GaZO sample).

4. Summary

Crystalline In- and Ga-doped ZnO nanoparticles have been pre-pared using a hydrothermal synthesis in ethanol and methoxyeth-anol. The synthesis of doped ZnO materials in alcohols results instable colloids of nanometer-sized particles. This method is easy tocarry out and leads to quantitative yields of doped ZnO nanoparti-cles.

The use of methoxyethanol as a solvent allows an enhancedcontrol of nanoparticle size and purity; moreover, it permits a su-perior incorporation of the dopant into zinc oxide.

Independently of the used solvent, the presence of indiumfavours the formation of rods, grown preferentially along the aaxis. In fact, the estimation of the ZnO crystallite size, at increasingIn content, exhibited a significant decrease along the c axis, indi-cating that the crystal growth in this direction was highly inhibitedby indium introduction. These XRD-derived results were confirmedby TEM observations.

The possibility to produce transparent materials with highabsorption properties in the NIR range has been demonstratedthrough the occlusion of In-dopant into the zinc oxide lattice andthe use of short reducing treatments. Preliminary studies indi-

cated that this effect is greatly enhanced using gallium as a dopingagent.

Acknowledgments

The authors would like to thank Dr. M. Gazzano (CNR-ISOF) formany useful discussions and help in XRD characterization.

Dr. Pierre Fux and Dr. Peter Muehlschlegel from CIBA Basel arehighly acknowledged for TGA-IR-MS and TEM investigations, re-spectively.

Supporting information

The online version of this article contains additional supportingmaterial.

Please visit DOI: 10.1016/j.jcis.2008.09.060.

References

[1] M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1.[2] H. Althues, J. Henle, S. Kaskel, Chem. Soc. Rev. 36 (2007) 1454.[3] G. Carotenuto, M. Valente, G. Sciumé, T. Valente, G. Pepe, A. Ruotolo, L. Nicolais,

J. Mater. Sci. 41 (2006) 5587.[4] D.S. Ginley, C. Bright, MRS Bull. 25 (2000) 15.[5] M. Dobler, W. Hoheisel, H. Schmidt, R. Nonninger, M. Schichtel, M. Jost, US

2003/0122144 A1 (2003), assigned to Bayer Polymer LLC (US).[6] M. Dobler, W. Hoheisel, H. Schmidt, R. Nonninger, M. Schichtel, M. Jost, US

7,074,351 B2 (2006), assigned to Leibniz-Institut fur Neue Materialien gem.GmbH (DE).

[7] S. Schelm, G.B. Smith, P.D. Garrett, W.K. Fisher, J. Appl. Phys. 97 (2005) 124314-1.

[8] G.B. Smith, C.A. Deller, P.D. Swift, A. Gentle, P.D. Garrett, W.K. Fisher,J. Nanopart. Res. 4 (2002) 157.

[9] C.G. Granqvist, A. Hultaåker, Thin Solid Films 411 (2002) 1.[10] T. Minami, Semicond. Sci. Technol. 20 (2005) S35.[11] D.P. Norton, Y.W. Heo, M.P. Ivill, K. Ip, S.J. Pearton, M.F. Chisholm, T. Steiner,

Mater. Today 7 (2004) 34.[12] M. Kurtz, N. Bauer, H. Wilmer, O. Hinrichsen, M. Muhler, Chem. Ing. Tech. 76

(2004) 42.[13] B.A. Peppley, J.C. Amphlett, L.M. Kearns, R.F. Mann, Appl. Catal. A 179 (1999)

21.[14] S.A. French, A.A. Sokol, S.T. Bromley, C.R.A. Catlow, P. Scherwood, Top. Catal. 24

(2003) 24, 161.[15] H. Zhao, R.K.Y. Li, Polymer 47 (2006) 3207.[16] T. Yamamoto, T. Yamada, T. Sakemi, S. Shirakata, M. Osada, S. Kishimoto, K.

Awai, H. Makino, T. Mitsunaga, Ceram. Trans. 196 (2006) 475.[17] A.B. Djurisc, Y.H. Leung, Small 2 (2006) 944.[18] X.L. Cheng, H. Zhao, L.H. Huo, S. Gao, J.G. Zhao, Sens. Actuators B 102 (2004)

248.[19] J.-J. Wu, S.-C. Liu, J. Phys. Chem. B 106 (2002) 9546.[20] J.Q. Hu, Q. Li, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Mater. 14 (2002) 1216.[21] C. Goebbert, R. Nonninger, M.A. Aegerter, H. Schmidt, Thin Solid Films 351

(1999) 79.[22] J. Ederth, P. Johnsson, G.A. Niklasson, A. Hoel, A. Hultaåker, P. Heszler, C.G.

Granqvist, A.R. Van Doorn, M.J. Jongerius, D. Burgard, Phys. Rev. B 68 (2003)155410-1.

[23] G. Viau, P. Toneguzzo, A. Perrard, O. Acher, F. Fièvet-Vincent, F. Fievet, ScriptaMater. 44 (2001) 2263.

[24] L. Poul, S. Ammar, N. Jouini, F. Fievet, F. Villain, J. Sol-Gel Sci. Technol. 26 (2003)261.

[25] C. Feldmann, H.O. Jungk, Angew. Chem. Int. Ed. 40 (2001) 359.[26] C. Feldmann, Adv. Funct. Mater. 13 (2003) 101.[27] M. Niederberger, G. Garnweitner, Chem. Eur. J. 12 (2006) 7282.[28] M. Niederberger, G. Garnweitner, N. Pinna, G. Neri, Prog. Solid State Chem. 33

(2005) 59.[29] G. Garnweitner, M. Niederberger, J. Am. Ceram. Soc. 89 (2006) 1801.[30] M.L. Kahn, M. Monge, V. Colliere, F. Senocq, A. Maisonnat, B. Chaudret, Adv.

Funct. Mater. 15 (2005) 458.[31] M. Niederberger, G. Garnweitner, J. Buha, J. Polleux, J. Ba, N. Pinna, J. Sol-Gel

Sci. Technol. 40 (2006) 259.[32] M. Epifani, R. Diaz, J. Arbiol, E. Comini, N. Sergent, T. Pagnier, P. Siciliano, G.

Faglia, J. Morante, Adv. Funct. Mater. 16 (2006) 1488.[33] J. Buha, I. Djerdj, M. Niederberger, Cryst. Growth Des. 7 (2007) 113.[34] U.D. Altermatt, I.D. Brown, Acta Crystallogr. A 43 (1987) 125.[35] G. Clavel, M.G. Willinger, D. Zitoun, N. Pinna, Adv. Funct. Mater. 17 (2007) 3159.[36] X. Zhong, Y. Feng, Y. Zhang, I. Lieberwirth, W. Knoll, Small 3 (2007) 1194.

80 S. Cimitan et al. / Journal of Colloid and Interface Science 329 (2009) 73–80

[37] G. Rodriguez-Gattorno, P. Santiago-Jacinto, I. Rendon-Vazquez, J. Nemeth,I. Dekany, D. Diaz, J. Phys. Chem. B 107 (2003) 12597.

[38] L. Poul, N. Jouini, F. Fievet, Chem. Mater. 12 (2000) 3123.[39] E. Hosono, S. Fujihara, T. Kimura, H. Imai, J. Colloid Interface Sci. 272 (2004)

391.[40] Y.W. Chen, Y.C. Liu, S.C. Lu, C.S. Xu, C.L. Shao, C. Wang, J.Y. Zhang, Y.M. Lu, D.Z.

Shen, X.W. Fan, J. Chem. Phys. 123 (2005) 134701.[41] Y. Cao, L. Miao, S. Tanemura, Y. Hayashi, M. Tanemura, Adv. Mater. Res. 11–12

(2006) 159.

[42] Z.R. Tian, J.A. Voight, B. McKeinzie, M.J. McDermott, M.A. Rodriguez, H. Konishi,H. Xu, Nat. Mater. 2 (2003) 821.

[43] J.U. Brehm, M. Winterer, H. Hahn, J. Appl. Phys. 100 (2006) 064311.[44] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.[45] R. Wang, A.W. Sleight, D. Cleary, Chem. Mater. 8 (1996) 433.[46] A.J. Freeman, K.R. Poeppelmeier, T.O. Mason, R.P.H. Chang, T.J. Marks, MRS

Bull. 25 (2000) 45.[47] R. Wang, A.W. Sleight, R. Platzer, J.A. Gardner, J. Solid State Chem. 122 (1996)

166.