Chapter-4

Nanoporous materials

1 Introduction

Porous materials are like music: the gaps are as important as the filled-in bits. The presence

of pores (holes) in a material can render itself all sorts of useful properties that the

corresponding bulk material would not have. The definition of pore size according to the

International Union of Pure and Applied Chemistry (IUPAC) is that micropores are smaller

than 2 nm in diameter, mesopores 2 to 50 nm and macropores larger than 50 nm. However

this definition is somewhat in conflict with the definition of nanoscale objects. Nanoporous

materials are a subset of porous materials, typically having large porosities (greater than 0.4),

and pore diameters between 1- 100 nm. In the field of chemical functional porous materials,

it is better to use the term "nanoporous" consistently to refer to this class of porous materials

having diameters between 1 and 100 nm. For most functional applications, pore sizes

normally do not exceed 100 nm anyway.

Introducing porosity in to a nanostructure material can alter some of its properties

significantly. In general, a porous material has a porosity volume fraction between 20 to 95%.

Depending on the connectivity of the pores, these materials can further subdivided into open

pore and closed pore materials. Open pore- Pores are interlinked with each other spreading

from the interior of the material to its surface. Closed pore- Pores are isolated from each

other. Open porous materials have their potential applications in various fields like

adsorption, catalysis etc. Closed porous materials are used for structural and thermal

insulating materials.

Nanoporous materials contain pores having a volume fraction more than 40% and the pore

size varies from 1 to 100 nm. Because of high volume fraction of pores, manoporous

materials exhibit low density compared to that of the parent material, hence they are also

referred as Nanofoams. Metallic nanofoams inherit the metallic character of their parent

metal such as high thermal and electrical conductivity, ductility, strength etc. along with

display properties due to high surface area to volume ratio and strength to density ratio.

2 Properties of nanoporous materials

Nanoporous materials possess a unique set of properties that the bulk correspondent materials

do not have such as high specific area, fluid permeability and molecular sieving and shape-

selective effects. Different nanoporous materials with varying pore size, porosity, pore size

distribution and composition have different pore and surface properties that will eventually

determine their potential applications. For different applications there are different sets of

performance criteria that would require different properties.

1. High adsorption capacity: Fundamental properties that affect this parameter are

specific surface area, surface chemical nature, and pore size. These parameters

determine how much adsorbates can be accumulated by per unit mass of adsorbents.

2. High selectivity: For multicomponent mixture, selectivity is highly desired for

separation. The selectivity of an adsorbent will depend on the pore size, shape and

pore size distribution as well as the nature of the adsorbate components.

3. Favorable adsorption kinetics: Adsorption kinetics is determined by the particle

(crystallite) size, the macro-, meso and microporosity of the adsorbent. Sometimes,

binder type and amount would also affect the interparticle transport thus the global

adsorption process kinetics. A favourable kinetics means that the adsorption rate is

fast or controllable depending on the requirement of a particular application.

4. Excellent mechanical properties: Obviously, adsorbents need to be mechanically

strong and robust enough to stand attrition, erosion and crushing in adsorption

columns or vessels. High bulk density and crushing strength, and attrition resistance

are desirable.

5. Good stability and durability in use. Adsorbents are often subject to harsh chemical,

pressure and thermal environments. Good stability in those environments is essential

in ensuring long life or durable utilization. As synthesized nanoporous materials may

or may not have all these desirable properties depending on the synthesis systems,

methods and processing conditions. Obviously the practical challenges in making

good adsorbent materials will be to obtain high-adsorption-capacity adsorbents in a

simple and cost effective manner, to make sure the above requirements/criteria are

met as much as possible.

In many cases, postsynthesis modification is required to impart certain functionality or

improve certain property due to the inability of the synthesis route to achieve them during

the process of synthesis. There are many research efforts devoted to this area. If used as

catalyst support or catalysts, nanoporous materials involved are required to have not only

the above properties but also suitable surface chemistry characteristics such as acidity or

basicity, and shape selectivity is often important.

3 Applications of nanoporous materials

3.1 Environmental separations:

As the regulatory limits on environmental emissions become more and more stringent,

industries have become more active in developing separation technologies that could

remove contaminants and pollutants from waste gas and water streams. Adsorption

processes and membrane separations are two dominating technologies that have attracted

continuous investment in R&D. Adsorbent materials and membranes (typically

nanoporous) are increasingly being applied and new adsorbents and membranes are

constantly being invented and modified for various environmental applications such as

the removal of SO2, NOx, and SPMs emissions. Adsorbents of the traditional types such

as commercially available activated carbons, zeolites, silica gels, and activated alumina

have estimated worldwide market exceeding US$1.5 billion per year. New adsorbent

materials with well-defined pore sizes and high surface areas are being developed and

tested for potential use in energy storage and environmental separation technologies.

3.2 Clean energy production and storage

• Future energy supply is dependent on hydrogen as a clean energy carrier. Hydrogen

can be produced from fossil fuels, water electrolysis and biomass. However, the

current debates on the hydrogen economy are intimately linked to the clean

production of hydrogen from fossil fuels such as natural gas and coal. Due to the low

cost and wide availability of coal, coal gasification to syngas and then to hydrogen

through the water gas shift reaction is a promising route to cheap hydrogen. The

success of such a hydrogen production route will be only possible provided that

carbon dioxide is sequestered safely and economically. Key to the cost effective

conversion of coal to hydrogen and carbon capture is nanomaterials development such

as catalyst for the water gas shift (WGS) reaction and inorganic membranes for

hydrogen/CO2 separation. In the future hydrogen economy, hydrogen will be the

dominant fuel, and converted into electricity in fuel cells, leaving only water a

product. Fuel cell development has been very rapid in recent year. However, there are

many technological challenges before fuel cells become commercially viable and

widely adopted. Many of the problems are associated with materials notably related to

electrocatalyst, ion-conducting membranes and porous supports for the catalyst.

Certain nanoporous materials such as carbon nanotubes and zirconium phosphates

have already shown promise for application in fuel cells.

• Hydrogen storage will be also essential in hydrogen economy infrastructure.

Currently there are no optimal systems for hydrogen storage. Hydrogen can be stored

in gaseous, liquid or more recently in solid forms. Nanostructured materials such as

carbon nanotubes again show promise as an adsorbent. Despite many controversial

reports in the literature, hydrogen storage in carbon nanotubes may one day become

competitive and useful.

• Another type of nanostructured carbons is templated by using 3-D ordered

mesoporous silicates. It has been shown that this type of carbons exhibit interesting

and superior performance as supercapacitor and electrode materials for Li-ion battery

applications.

3.3 Sensors and actuators

Nanoparticles and nanoporous materials possess large specific surface areas, and high

sensitivity to slight changes in environments (temperature, atmosphere, humidity, and light).

Therefore such materials are widely used as sensor and actuator materials. Gas sensors reply

on the detection of electric resistivity change upon change in gas concentration and their

sensitivity is normally dependent on the surface area. Gas sensors based on nanoporous metal

oxides such as SnO2, TiO2, ZrO2, and ZnO are being developed and applied in detectors of

combustible gases, humidity, ethanol, and hydrocarbons.

4 Nanoporous metal by dealloying

The dealloying process is a method that uses corrosion to selectively remove the least noble

element(s) within an alloy, resulting in a nanoporous material of the nobler element. This

process results in the formation of a nanoporous sponge composed almost entirely of the

nobler alloy constituents. For example, immersion of a Ag70Au30 bulk alloy in nitric acid

selectively removes the silver atoms leaving behind a nanoporous gold network.

When an Au–Ag alloy is used as the anode in an electrochemical cell, anodic oxidation

occurs if a potential difference is maintained at the electrodes. Silver oxidation represents one

of the principal reactions that takes place in the electrochemical cell during the dealloying

process, but also other processes may occur; each silver atom from the anode loses one

electron and forms a silver ion as described by Ag(s)→Ag++e

-. The positive silver ion moves

through the electrolyte to the cathode, where it is electrodeposited, provided the dealloying

potential is higher than the reduction potential for silver formation. For an Au–Ag alloy,

where only silver atoms dissolve in the electrolyte, the current probed by the ammeter

represents the silver dissolution current. At the end of each process the dealloying current

intensity drops down.

SEM image showing nanoprous Gold after dealloying Ag from Au26%Ag74% (at. %)

The ligament diameter and pore size decreases with its corresponding applied dealloying

potential.

5 Ordered nanoporous material

Using all the above mentioned techniques and many more available processes in literature a

highly nano-range sized porous structure can be synthesized, but typically these materials are

having a disordered pore morphology which hinders the effective transmission of stress

between ligaments resulting in nonuniform and poor overall mechanical properties of the

material. Hence the idea of preparing material with well-ordered nanopores of uniform

morphology arrived, but it a very challenging task to prepare material with such well-ordered

pores in nanoscale.

A mixture of polymers with different chemical composition phase separate macroscopically,

but in case of a block coplolymer the covalent linkage between the homopolymers prevents

macrophase separation. Again because of unfavorable interaction parameter, phase separation

still occurs but at microscopic level known as microphase separation in molten and solid

state. To minimize the enthalpy of interaction there is a stretching in the polymer chains and

to maximize the conformational entropy there is a resistance to the chain stretching, as a

consequence of it there exists a balance between enthalpy and entropy which ultimately

decides the microphase separation behavior in block copolymers.

The block copolymer microphase separation can be controlled by composition, segment-

segment interaction parameter and degree of polymerization. To get an idea for the

combination of these parameters on the actual phase morphology of the block copolymer

system theoretical mean field phase diagram has been constructed. The Flory-Huggins

interaction parameter (χAB represents the segment-segment interaction parameter which

combines the temperature (T), no. of nearest neighbor monomers (Z) and the interaction

energy between A-A (εAA), B-B (εBB) and A-B (εAB) monomers as shown in the Equation

below.

(

)

( )

Mean-field phase diagram for conformationally symmetric diblock melts. Phase are

Lamellar (L), Gyroid (G and G’), Cylindrical (C and C’) and Spherical (S and S’).

Dashed lines denote extrapolated phase boundaries, and the dot denotes the mean-field

critical point. Red color represents A monomer and Black represents B monomer

The well-known thermodynamically stable ordered structures of block copolymers are body

centered cubic spheres, hexagonal cylinders and bicontinuous or double Gyroid structure. In

contrast to all other phases gyroid structure has some superior properties as listed below:

(i) both the polymer phases are continuous throughout the entire domain, and

(ii) extremely high specific surface area can be obtained.

Obtaining a gyroid structure from a system of block copolymer comprising of gyroid phase

consists of three sequential processes. This process is known as template directed synthesis.

5.1 Selective removal of the polymeric gyroid phase

This procedure is aimed at selective removal of the Gyroid phase without affecting

the polymer which comprises the matrix phase.

The process to be employed for getting a successful removal of the gyroid phase

depends on the chemical nature of both the polymers which comprise the block

copolymer.

Ozonolysis, UV degradation, selective leaching using suitable chemical solution are

some of the well-known methods described in literature.

After successful removal of the gyroid network the structure is known as gyroid

template or Inverse gyroid, as the gyroid site is now occupied by pores.

5.2 Backfilling of the template with the desired metal

For metal gyroid there are two most commonly used technologies: electrochemical

and electroless plating.

In case of electrochemical plating an external current is applied across the two

opposite free surfaces.

In case of electroless plating, the template is immersed in an electroless plating bath.

Metal deposition in such process is carried out by autocatalytic method. This process

certainly does not require any external current source.

5.3 Removal of the polymer template

After successful replication of the metal gyroid the polymer template is removed by

any of the various processes available, such as dissolution, UV degradation, pyrolysis

etc.

6 Synthesis of Nickel double gyroid

The adopted synthesis technology is Copolymer template directed synthesis. The

block copolymer used for this purpose is Polystyrene – b – poly (D,L – Lactide)

containing 39.8 wt% PLA. Initially the Styrene block was prepared by radical

polymerization, then copolymerization was performed by addition of PLA through

ring opening polymerization. The robust control over both these polymerization

processes enables precise compositional control, aiming towards the composition for

narrow ranged Gyroid phase in Phase diagram.

Copolymer film of about 2 µm thickness is prepared on a fluorine-doped Tin Oxide

(FTO) coated glass. Conversion of this copolymer into self-assembled Double Gyroid

(DG) morphology is achieved by annealing this film at 173 °C for 20 minutes. The

final step for preparing the template is then selective removal of the DG PLA phase,

which is achieved by dissolving the film in a mild alkaline solution for some time.

A continuous porous structure throughout the template is an essential criterion for

further successful electroplating. For achieving this goal, preferential wetting of the

FTO glass substrate by any of the polymer is to be avoided. Thus prior to

copolymerization the FTO glass substrate was cleaned using Piranha and

subsequently a monolayer of Octyltrichlorosilane (OTS) is deposited on it.

Modification on the FTO substrate was further done by photolithography to obtain

isolated SU-8 pattern, which inhibits electroplating locally.

Electroplating of Ni into the porous template was carried out by maintaining a

constant voltage of -1.05 V against a standard Ag/AgCl electrode at 50 °C using

commercial nickel plating solution. The rate of nickel deposition was 0.288 mgC-1

.

Finally a uniform Ni deposited film of thickness 2 µm thickness with plated area 1

mm2 was obtained.

The next step lies in removal of the polymer PS template, which is achieved by

dissolving the sample in toluene for some time.

Now the sample obtained is a freely standing Ni DG.

Schematic representation of Steps involved in Ni DG synthesis; (a) Preparation of substarte,

(b) Microphase separation into DG morphology after copolymerization and annealing in PS-

b-PLA block copolymer (Red phase shows the styrene matrix and blue phase shows the DG

lactide), the front face shows the characteristic [211] plane of double wave pattern, (c)

Selective removal of PLA to obtain porous PS template, (d) Electroplating of Ni into the

pores of the template, (e) Dissolution of the polymeric matrix yielding a free standing Ni DG

morphology