Template assisted synthesis and assembly of nanoparticles 1

On the other hand, Park and Hyeon et al15 also reviewed chemical synthesis methods of nanoparticles including the reduction method, thermal decomposition method and nonhydrolytic sol-gel

Chapter 1 Introduction

1.1 General review

During the past decade, scientists have developed techniques for synthesizing and characterizing many new materials with at least one of the dimensions in the nanometer scale Nanoscale materials can also be classified as metal and semiconductor nanoparticles, nanowires, nanocubes, and various types of artificial supramolecular composites Properties of materials of nanometric dimensions are significantly different from the compositional atoms as well as the corresponding bulk materials Suitable control of the nanometer scale structures can lead to exciting new electrical and optical properties of nanoscale materials, associated with size and spatial organization All these findings including positive results in devices made of nanoscale components have attracted growing interest in nanoscience and nanotechnology

Nanoparticles refer to those particles whose size is below 100 nm Due to quantum confinement effect, nanoparticles have displayed outstanding electrical or optical properties, which lead to numerous potentials and applications in advanced technology On the other hand, nano-sized particles have been practically utilized to improve the functional and performance quality of bulk material systems in which they are incorporated, e.g polymer nanocomposites, coatings, ceramic composite

materials, data storage and processing systems, dispersions, sensors and catalytic processes Detailed study of nanoparticles in various areas has and will continue to become the main research topic in coming years

In general, there are two approaches to nanoparticles production that are commonly referred to as ‘top-down’ and ‘bottom-up’ methods In ‘top-down’ methods, nanoparticles are generated from the size reduction of bulk materials They are generally made through physical, or a combination of physical and chemical processing methods Such methods include high-energy milling, mechano-chemical processing, laser ablation, inert-gas evaporation technique, sputtering and vapor condensation etc ‘Bottom-up’ approaches generate nanoparticles from atomic or molecular level and thus are predominantly chemical processes Commonly used techniques are conventional salt crystallization/precipitation method1, precursor decomposition2-4, interfacial polycondensation5, sol-gel method6, 7, chemical vapor deposition (CVD)8, 9, template-assisted method10, electrochemical method, combination of physical deposition and reaction11, hydrothermal12 etc

1.2 Chemical synthesis methods of nanoparticles

Chemical synthesis of nanoparticles has been reviewed by several authors Trindade and O’Brien13 have reviewed the arrested precipitation method, synthesis in structured medium, and molecular precursor method O’Brien14 has further divided

chemical synthesis methods into four categories, i.e colloidal route, organometallic route, growth-in-confined structure matrices, and gas-phase synthesis of nanoparticles

On the other hand, Park and Hyeon et al15 also reviewed chemical synthesis methods

of nanoparticles including the reduction method, thermal decomposition method and nonhydrolytic sol-gel method In addition, Cushing and O’Connor16 reviewed the preparation of inorganic nanoparticles by coprecipitation, sol-gel process, microemulsions, hydrothermal/solvolthermal process, templated synthesis, biomimetric and surface derivatized synthesis

Although there are numerous methodologies employed in the synthesis of nanoparticles, many improvements and better methods are also being reported continually in the last few years For example, in order to produce nanoparticles in large scale, microcapillary and integrated microchannel reactors were used Kilogram scale production of organic compounds has been demonstrated with benchtop microreactors17 Another example is the introduction of supercritical fluids (H2O,

CO2), whose density, viscosity and solvation properties are intermediate between those of the vapor and liquid phase These supercritical fluids are nonflammable, nontoxic, easily accessed materials and have recently gained attention as benign solvents for the synthesis of inorganic nanoparticles18, 19 Others such as ionic liquid, sonochemical synthesis, laser ablation and microwave assisted synthesis also provide alternative routes to synthesize nanomaterials20

In the following sub-sections, we will briefly review the precursor decomposition

method and template-assisted synthesis method as they are employed in this thesis

1.2.1 Precursor decomposition method

Precursor decomposition method is one of the main routes of chemical synthesis for nanoparticles This approach became increasingly popular after Murray et al21reported the synthesis of monodispersed CdE nanoparticles (E = S, Se, Te) from (CH3)2Cd and E(Si(CH3)3)3 (where E = TOPSe or TOPTe; TOP is tri-n-octylphosphine) One of the limitations of the TOPO method is the use of hazardous compounds such as dimethylcadmium ((CH3)2Cd) at high temperatures One of the approaches for overcoming this problem is the use of single molecular precursors, i.e

a single compound containing all elements required within the nanocrystallite, such as alkyldiseleno- or alkyldithiocarbamato complexes They were initially used by O’Brien’s group to produce thin films by metal organic chemical vapor deposition (MOCVD)22, 23 TOPO capped CdS/CdSe materials were later reported by Trindade and O’Brien as the first semiconductor nanoparticles prepared through single molecular precursor decomposition method24, 25

General procedure of nanoparticles synthesis through molecular precursor involves the dispersion of the precursor in TOP, followed by its injection into hot TOPO (250°C) The decomposition of the precursor drives the formation of the nanoparticles with termination of growth occurring when the precursor supply is depleted After the initial injection there is a rapid burst of nucleation, which is followed by controlled

growth of the nuclei by Ostwald ripening The resultant nanoparticles are passivated

by TOPO, which prevents agglomeration The nanoparticles are isolated by a process using a non-solvent (e.g methanol), which is added to the reaction mixture to increase the average polarity of the solution and consequently decrease the energy barriers to flocculation The flocculent precipitate obtained is separated by centrifugation and redispersed in toluene to give an optically clear solution of TOPO-capped nanoparticles

O’Brien’s group explored the decomposition of various other single molecular precursors26-31 and the effect of different organic ligands such as TOPO, octylamine32and hexadecylamine33 in the synthesis of CdS, CdSe, PbS34, PbSe35, Cu2S36, InS30, InSe30, CdP37, ZnO32, InAs38, GaAs33, ZnS39, ZnSe40 nanoparticles Further investigations of single source precursors by O’Brien’s group have shown that nanoparticles with different shapes and structures (InS nanorod9, CdS nanorod41,

Sb2Te3 nanoplate42, CdSe/CdS and CdSe/ZnSe core/shell, and CdSe/CdS alloy43 etc) can be successfully prepared by this methods

Inspired by their work, various metal chalcogenide/oxide/phosphide/ nanomaterials such as Ag nanoparticles44, Ag2S nanopartilcles45, PdS nanoparticles46, FePt nanoparticles47, MnO, Mn3O4, Mn2O3 nanoparticles48, AlN, GaN, InN nanowires49, CuS nanowires50, B4C nanofiber51, FeP nanorod/nanowires52, Fe2P nanorod53 CuInS2 and CuInSe2 nanoparticles54, AgInSe nanorod55 etc have been synthesized using single molecular precursor method

The utilization of such precursors offers various advantages over the usual metathesis synthetic routes Essentially these precursors are less toxic and hygroscopic, if compared to the earlier used hydrogen chalcogenides and pyrophoric alkylmetals In addition, the stoichiometric control of the final product becomes easier to manage3

Several conventional and nonconventional routes including solution phase thermal decomposition56, 57, chemical vapor deposition, reverse micelle method48, microwave irradiation58, hydrothermal method50, template-assisted method59 and deposition at the interface of water-toluene31 can be used with single molecular precursors to prepare nanoparticles This led to the development of wide variety of materials ranging from metals to semiconductors and insulators

1.2.2 Templates-assisted method to prepare nanomaterials

The trend in nanomaterial synthesis has been the development of smaller, highly uniform, lower dimensionality materials (quantum dots) where the quantum confinement effects are large As interest in these materials shifts from fundamental studies to the use of these materials in device applications, the focus has shifted to the development of a reliable and reproducible method for large scale synthesis (e.g greater than 1 g) of nanomaterials exhibiting mono-dispersity in shape and size As templates such as polymer, silica etc play an important role in controlling the size and dispersity of nanoparticles, the study of template-assisted method attract more and

more attention

Template directed synthesis is a straightforward method where the template simply

serves as a scaffold within (or around) which a different material is generated in situ

and shaped into a nanostructure with its morphology complementary to that of a template Composite materials synthesized by template-assisted method have synergetic or complementary behaviors between the nanoparticles and templates It is much easier to explore the application of these materials because of the improved properties Additional advantages are the possibility of controlling the shape, morphology and topology of domains or pore structure, from the macroscopic to the nanometer-size Unique properties of composites become more pronounced when one

of the fractions occurs at the nanometric scale

In brief, this approach offers great modularity and versatility in the design, synthesis, and characterization of the respective components, while providing a fundamental insight into the self-assembly process

1.3 Templates used in the preparation of nanoparticles

The idea to use templates is an old idea of mankind to produce reliably and reproducibly household goods and artworks needed (e.g pottery) in everyday life The general procedure to prepare nanoparticles is summarized as following: First, a template is filled or covered with a soft precursor material to bring the material into

the desired form Then, through a chemical reaction or a physical process the products are formed within the template Later, the template can be removed to obtain the desired product The template method has the advantage that its size and shape can be reproducibly controlled in larger scale

Templates can be as small as single molecules, e.g DNA, or large such as anodized alumina oxide or mesoporous silica with micron-size pores They can be classified into two groups: hard templates and soft templates, and these will be discussed in the following paragraphs

1.3.1 Hard templates

Common examples of hard templates include gold disk, porous anodic alumina membrane (AAO)60-63, zeolite64, mesoporous silica65, 66, carbon nanotubes67-69, facetted alumina template70, indium-tin oxide glass71 etc These templates are rigid on the surface and in principle the placement and dimensions of different components can be controlled during synthesis In most cases, these materials must be modified

by making use of specific chemical and physical interactions in order to increase the affinity to drive the precursor materials into the desired place72, 73

Hard template can be divided into two main groups One group is planar substrates such as gold plate, indium-tin oxide glass, block copolymer films, and silicon wafer, which are usually employed as supporting platform These substrates are modified by organic ligands or deposition74 The other group is micro-nanoporous or tubular

templates such as anodic aluminum oxide (AAO), porous metal oxide thin film75, zeolite, mesoporous silica, and carbon nanotubes which are commonly being used as templates for the fabrication of nanowires and nanorods

Among these hard templates, AAO with satisfying physical stability and chemical inertness is one of the most popularly used porous templates It has cylindrical pores

of 30-200 nm diameter depending on the anodizing parameters and almost parallel porous structures.76 Thus it offers efficient method for the synthesis of well-distributed tubular and fibrillar nanostructures within the pores of an alumina membrane.77 Hurst et al reviewed the multi-segmented one-dimensional nanorods prepared by hard template synthetic method78 They have summarized the synthesis and applications of multi-segmented one-dimensional nanorods and nanowires with metal, semiconductor, and polymer using ion-track-etched or AAO templates

Carbon nanotubes (CNTs) have also been intensively studied as template because

of their outstanding properties Detailed review on the preparation of CNT nanocomposites will be separately given in Section 1.6

1.3.2 Soft templates

Soft templates include polymer beads79, 80, conducting polymer81, polymer micelles82, 83, polymer gel84, 85, vesicles86, Langmuir-Blodget films87-89, Langmuir monolayer90, 91, self-assembly monolayer92, 93, surfactant94, clay95, dendrimer96-98, DNA99, 100, virus101 etc Polymeric templates are the major component of soft

templates One of the functions of soft template is to constrict the size and distribution of nanoparticles The properties of the designed nanocomposites will depend both on the characteristics of the nanoparticles and on the nature of the soft templates

There has been considerate progress in the use of polymer as template for the controlled synthesis of nanoparticles Versatile synthetic routes have been developed

to produce nanoparticles in polymer microdomains Many reviews and feature articles covering different aspects of this field have already appeared For example, Liu et

al102 reviewed the polymer-assisted fabrication of nanomaterials with emphasis on ordered polymeric nanostructures Examples have been demonstrated including self-assembled amphiphilic block co-polymers/surfactants, cross-linkable polymers, dendrimers, microemulsions, latex particles, biomacromolecules, electric- or shear-induced structures as templates to fabricate inorganic, organic/inorganic composites and polymeric materials with nanoscale modifications Föster103 also presented developments in using block copolymers as templates for the synthesis of metal, semiconductor and ceramic nanoparticles while Zhang85 reported approaches employing polymer microgels as templates

Another commonly used polymeric soft template is dendrimer This subject has been reviewed by many authors.104-107 Esumi108 gave a detailed description on the synthesis and characterization of dendrimer/metal (Au, Pt, Ag) as well as dendrimer/semiconductor (CdS, CuS) nanoparticles Goodson et al96 also reviewed

the synthesis, optical properties and potential applications of dendrimer-metal nanocomposites The desirable characteristics of dendrimer-encapsulated nanoparticles include good solubility in nearly any solvent107, catalytically active 105, 109

, functionalized nanoparticles surface106 and highly luminescent110 Because of the excellent tractability of those polymeric materials, much research efforts have been directed towards methodologies and processes for the incorporation of nanoparticles into polymeric matrixes

All in all, both hard and soft templates have been used to prepare nanoparticles with regular and uniform shape and size However, the varieties of soft templates provide higher chance to obtain nanocomposites with variable properties or behaviors

By adjusting the properties of soft templates, it is easy to obtain materials with predetermined properties, whereas hard templates can only be used as the substrate All this advantages of soft templates make them widely used in nanocomposites preparation, especially the semiconductor nanoparticles nanocomposites In the following sections, we will give brief introduction to several templates & nanocomposites that are subjects of this thesis

1.4 Polyaniline involved nanocomposites

Nanocomposites are materials that comprise at least two compositions, while at least one of the compositions is in nanoscale regime dispersing in the other matrix

composition The matrix composition may be single (e.g polymer) or components (e.g polysilane/poly(methacrylic acid) core-shell micelle83) Combining two or more different nanoscale functionalities, nanocomposites become attractive candidates as advanced nanomaterials Research carried out over the past decade has revealed that nanocomposites exhibit markedly improved mechanical, thermal, optical and physical-chemical properties compared to conventional composite materials

multi-Polyaniline (PANi), as one of the most important conducting polymers, has been investigated intensively for many years This is because of its many chemical, electrical, and optical properties, as well as its unique redox tunability.111, 112 Its electronic properties can be reversibly controlled both by protonation and by changing the oxidation state of the polymer Such attractive features make the polyaniline/nanoparticles nanocomposites promising materials to be used in the nanotechnology industries

Generally, there are several methods to synthesize nanocomposites containing PANi The first method is to directly coat the polyaniline film with nanoparticles by evaporation or deposition113, 114 Gaponik115, Tian116 et al have prepared CdTe and Au nanoparticles doped PANi films as light-emitting devices by dipping PANi film into the respective colloid solution Kang and coworkers117, 118 have investigated the interactions between evaporated metal atoms with PANi films However, the nanocomposites prepared by this method are mainly located on the surface of the

PANi films Hence only the surface property was modified and these films have been used as electrodes because of their electronic conductivity

The second method involves polymerization of aniline in nanoparticles solution Wan et al119 prepared and investigated the electric and ferromagnetic behavior of PANi/Fe3O4 nanocomposites while Pethkar et al120 prepared and characterized PANi/CdS composite films using electrochemical induced in-situ polymerization Most recently, Dong et al121 reported Au/PANi nanoparticles can be achieved by in-situ polymerization of aniline in polymer/Au microgel Pillalamarri et al122 reported the synthesis of Au/PANi by surface initiated polymerization of aniline from gold nanoparticles surface This method is chosen by many researchers because of the easy incorporation of nanoparticles in PANi while the size and shape of nanoparticles can

be controlled separately The key step of this method is the pre-synthesis of nanoparticles

The third method involves one-pot preparation of PANi nanocomposites, which means aniline is polymerized while nanoparticles are synthesized at the same time For example: Li et al123 have synthesized NiZn ferrite/polyaniline nanocomposites with core shell structure and Zarbin ’s group124 synthesized (Ti, Sn)O2/PANi nanocomposites For Khanna et al125, they synthesized Ag/PANi nanocomposites via

an in-situ photo-redox mechanism Sui and Chu et al126 prepared PANi/AgCl, PANi/BaSO4, PANi/TiO2 by mixing aniline and the corresponding salt into reverse micelle solution Kinyanjui 127 and Mallick 128 also described the formation of

Au/PANi composites through one-pot reaction The advantage of this method is the simple synthesis of the nanocomposites by mixing all the reagents in one-pot However, since various reagents are mixed together, there is high chance to have side reaction or compatibility problem The morphology and size distribution of nanocomposites prepared by this method are not good as well125, 127, 128

The fourth and recent method synthesizes nanoparticles in polyaniline solution Khanna et al129 synthesized PANi/CdS nanocomposites in which the CdS was generated from organometallic cadmium precursor in PANi solution γ-Fe2O3nanoparticles were synthesized in PANi/DBSA (surfactant) solution by Yang et al130 Hatchett reported the preparation of Au cluster in PANi film through electrochemical route.131 Recently, Gallon prepared Palladium nanoparticles on PANi nanofiber and investigated the activity of this materials as semi-heterogeneous catalyst.132 In this method, polyaniline not only can serve as the template or matrix but also as a reducing agent132, 133 However, the reducing function of PANi will affect its conductivity and other electrical property In addition, the morphology and size distribution of the nanoparticles are not well controlled

In summary, metal/semiconductor nanoparticles have been incorporated into the PANi template through different pathways and the optical properties;115, 129 magnetic properties;123, 134 electric/electrochemical properties135, catalytic activity132 of final PANi/nanoparticles composites have been investigated Results showed that these PANi nanocomposites could be potential candidates for application in catalysis,

biosensor, memory device and others

1.5 Clay related nanocomposites

Clays are classified on the basis of their crystal structure and the amount and locations of charge per basic cell Montmorillonites (from Montmorillon, a town in the Poitou area, France) are the most abundant minerals within the smectite group of 2:1 clay minerals Montmorillonite has been applied extensively as filler in the field

of polymeric composite materials since the particle size is smaller than 2 nm and polymer diffusion into the particles is possible The basic building blocks consist of

the tetrahedral layers and the octahedral layers Figure 1.1 showed a schematic

structural diagram of Montmorillonite

Figure 1.1 Schematic structural diagram of montmorillonite (1) oxygen, (2)

hydroxyl, (3) aluminium and magnesium, (4) silicon.136

Clays have many applications such as being catalysts for the petroleum industry while clay supported reagents can be used to control the stereochemical outcome of organic reactions137 Hence, the study on clay involved nanocomposites is always one

of the major topics in recent years Most clay-containing nanocomposites are polymeric nanocomposites since nanometer-sized clay was added Owing to the nanometer size, mechanical, thermal, optical and physicochemical properties were markedly improved when compared with neat polymers The improvements include moduli, strength, heat resistance, barrier properties, flammability etc For example Gao138 has reviewed the progress, advantages, limitations, and current problems of clay/polymer nanocomposites

For nanoparticles/clay composites, only a few preparation methods were reported

One method is to prepare nanoparticles in situ in clay suspension Németh139 and Mogyorósi140 reported the synthesis and characterization of ZnO/clay and TiO2/clay nanocomposites using this method Han et al141 prepared CdS/clay using in-situ hydrothermal decomposition method Silimar hydrothermal method was employed by Hui142 to prepare ZnO/clay nanohybrid materials The advantage of this method is nanocomposites can be prepared by one-step reaction However, it is not easy to control the morphology and size distribution of the nanoparticles

Another commonly used method is to combine the ion exchange process/heterocoagulation with hydrothermal decomposition or calcination Han143

have synthesized CdS/clay by ion exchange process followed by hydrothermal method while K rösi144 prepared SnO2/clay nanocomposites by heterocoagulation followed by calcination Using this method, the preparation of Fe2O3/clay145, 146, MgO/clay147 nanocomposites have been reported recently TiO2/clay nanocomposites have been synthesized by Mogyorósi only through heterocoagulation140 The limitations of this method are: i) the precursor must be water-soluble and ii) aggregation during the thermal decomposition process is difficult to avoid Other methods such as layer-by-layer assembly (LBL) method148, coprecipitation method149, electrodeposition method150 have also been explored

Since most of the reported preparations of metal/semiconductor nanoparticles occur in organic solvents, these methods are not compatible with clays that are hydrophilic in nature In this case, pretreatment of either the clays or the nanoparticles

is necessary The most popular methods for clay modification is through ion exchange using amino acids151, organic ammonium salts152, or tetra organic phosphonium153 These species convert the clay surface from hydrophilic to organophilic Clays modified in this way are known as ‘organo-clay’ Investigations on organo-clay employed in the polymer/clay nanocomposites have been carried for several years The formation of semiconductor/metal nanoparticles in organo-clay matrix (hexadecylpyridinium or hexadecylammonium montmorillonite) has only been reported by Dékány’s group using the clay/liquid interface reaction method CdS, ZnS, TiO2 and Pd nanoparticles synthesized in the organo-clay interface, which is a 0.5-5

nm alcohol-rich absorption layer This layer was formed by dispersing organo-clay in ethanol-cyclohexane or methanol-cyclohexane binary mixture.154-156 Their studies

have shown that the sizes of semiconductor/metal nanoparticles, grown in situ in the

interlayer, are determined by the volume of the adsorption layer and the concentration

of the precursors introduced However, there are a few limitations to this method, such as i) one of the solvent in the binary mixture should be preferentially adsorbed at organo-clay interface, and ii) the semiconductor/metal precursors should be highly soluble in one solvent but not in the other solvent

1.6 Multi-walled carbon nanotube used as hard template

Following the discovery of fullerene molecules C60 in 1985,157 Mintmire et al in

1991 proposed that one-dimensional elongation or rolling a sheet of graphite into a small tube is possible In the same year, Iijima observed for the first time the existence of carbon nanotubes together with the fullerene molecules and found that these carbon nanotubes (CNTs) are comprised of several concentric graphitic cylinders that are capped with carbon pentagons at each end, similar to the fullerene molecules.158

According to the number of the graphitic sheets present in their wall structure, CNTs are further classified into two basic types: single-walled nanotubes SWCNT (a single graphite sheet seamlessly wrapped) and multi-walled nanotubes MWCNT (an

array of cylindrical tubes that are concentrically nested) The electronic properties of perfect MWCNTs are rather similar to those of perfect SWCNTs, because the coupling between the cylinders is weak in MWCNTs.159 Baughman et al have introduced the potential/realized applications for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects.159 CNTs have been used as a support material for the dispersion and stabilization of metal/semiconductor nanoparticles After the first report on decorating CNTs with Ru nanoparticles160, more metal nanoparticles such as Pd161-167, La161, Pt164, 166-187, Rh183, 188

, Ag164, 166, 167, 186, Au163, 166, 167, 172, 181-183, 186, 189-197, Ni198-200, Cu167, 172, 201, Fe202, including alloy nanoparticles such as FeNi203, CoNiFe204, PtRu205, 206, RuSn205, PtRuIr207 have been deposited on CNT through various different methods These metals are commonly applied in heterogeneous catalytic reactions and their properties can be enhanced when CNTs are employed as support materials Applications of these hybrid materials as functional components in supercapacitors204, gas sensors165, biosensors208, and hydrogen storage199 have been reported

On the other hand, metal sulfide/selenide (CdS/SWCNT209-211, CdS/MWCNT168,

212, 213

, CdSe/SWCNT211, 214-216, CdSe/MWCNT217, 218, CdTe/SWCNT219,

Ag2S/MWCNT4, HgS/MWCNT4, ZnS/MWCNT217, 220, WS2/MWCNT221) as well as metal oxide (such as Cu2O/MWCNT222, 223, Co3O4/MWCNT224, SnO2/MWCNT225,

SnO/MWCNT226, SnO2/SWCNT227, ZrO2/MWCNT228 ZnO/MWCNT218, 229, MgO/MWCNT229, RuO2/MWCNT230, SiO2/MWCNT168, 218, TiO2/SWCNT211, have been deposited onto CNTs These nanoscale hybrid systems derived from inorganic solids and CNTs have unique optical, electrical, and mechanical properties and are promising materials for nano-electronics and other applications

Many preparation methods have been reported in the literatures to immobilize nanoparticles on CNTs and they can be divided into two main pathways: i) the

formation of metal/semiconductor nanoparticles directly on the CNT surface (in situ

preparation method); or ii) connecting metal/semiconductor nanoparticles on the CNT surface using covalent linkages through organic ligand (indirect method)

For in situ preparation method, two pathways including direct deposition and

precursor decomposition/reduction have been applied in the preparation of nanoparticles For example, metal nanoparticles were created on the surface of CNT directly by electrodeless deposition167, 178, 182, 198, 231 electrodeposition164, 179, sol-gel process, sonochemical method, microemulsion method163, 229 supercritical liquid174, 206, 231

method and intermittent microwave irradiation184 Semiconductor nanoparticles such as CdS209, 212, ZnS220 were directly formed on the CNT surface through direct precipitation For precursor decomposition/reduction way, heat166, 232, 202, light, current162 and reducing chemicals (e.g H2166, 176, 199, 201, 203, NaBH4170, sodium citrate189, ethylene glycol173, 177, 185, 186, formaldehyde177, polyethylene glycol183) have been used to produce metal nanoparticles on the surface of CNTs These nanoparticles

can be deposited onto the CNT walls mostly through van der Waals interactions, which in some cases seem to be sufficiently strong to guarantee adhesion

The indirect method dominates the preparation of semiconductor/CNT nanomaterials Amide bond has been widely used to immobilize semiconductor particles on CNT surface211, 214, 217, 221 In addition, polymer (e.g polyallylamine hydrochloride218), and surfactant (e.g sodium dodecyl sulfate210) etc can also be used

as alternative linker molecules Although the in situ synthesis method is simple and

effective, this method usually leads to the formation of inhomogeneous crystalline structures with large polydispersity in shape and size

In the more complicated indirect method, nanoparticles were formed before attaching onto CNTs so that their sizes or shapes are controllable One particular advantage of indirect method is that the connection of nanoparticles to CNT is under certain circumstances reversible In most cases, however, the CNT surfaces must be functionalized by harsh oxidative processes such as refluxing in HNO3/H2SO4, followed by the use of carboxylic acid or direct sidewall reactions These functionalization approaches tend to considerably degrade the mechanical and electronic performance of CNTs as a large number of defects are introduced

Since there are many advantages using CNT as supporting materials, exploration

on the application of CNT functionalized metal/semiconductor nanoparticles and other nanoparticles will continue to expand in the future Future development will focus on how to find synthetic methodologies to produce industrially useful materials

with reproducible properties and performances

1.7 Template-assisted assembly of nanoparticles

The assembly of nanoparticles is critical to further enhance their applications in sensors, catalysis, medical diagnostics, and information storage etc Intensive studies have been devoted to assembling nanoparticles into ordered two- or three-dimensional superstructures The properties of the assembled nanoparticles may be different from those of the individual nanoparticles and their bulk phases.233

Assembly is usually based on rational control of non-covalent interactions such as hydrogen bonds234, π-π stackings, electrostatic or van der Waals interactions Van del Waals force is one of the most common intermolecular or interparticle forces in the formation of ordered structure However, particles held together by such a nonspecific force are not fully controllable and also not chemically or mechanically stable on macroscopic materials scale

In contrast, assembly through chemically specific binding (e.g., covalent binding

or acid-base interaction235) could overcome the weakness, though observations of large domains of highly ordered organization are very limited For example, the thiol-

236, 237

, dithiol or amine-based238 chemistry is often exploited in approaches toward nanoparticles assembly Polymers functionalized with molecular recognition groups239-241, carboxylic acid242, thiolate ligands243, 244, pyridine groups245,

tetradentate thioether ligands246, tetraalkylammonium247, citrate248 and surfactants249have been intensively studied as mediator during the formation of spherical or related assemblies of gold nanoparticles

Self-assembly of nanomaterials not only depends on interparticle interactions, but also depends on the particle size distribution and particle shape Spontaneous self-assembly of spherical nanoparticles occurs only with particles of a narrow size distribution, i.e those with a polydispersity < 5%.250, 251 In contrast, nonspherical nanoparticles show different types of self-assembly Several studies addressed the organization of superstructure of BaCrO4252, 253, CdSe254, CdS255, FePt256, Au249 and

Sb2O3257 etc nanorods Recently, Jana studied the assembly of gold and silver nanoparticles with various aspect ratio and showed that self-assembly of nanoparticles is strongly induced by the shape of the particles258 Both the chemical structures of the component molecules and environmental factors significantly affect these assembled supramolecular architectures

Methods used to control the nanoparticles assembly include electric force fields255, magnetic force259, UV photolithography260, temperature261, colloidal deposition over pre-patterned substrates262, 263, copolymer microphases264-266, electrophoresis of metallic particles in solution267, 268, fluid-assisted dewetting269, 270, microcontact printing and dewetting271, 272, and the formation of nanopatterns at the air-water interface using the Langmuir-Blodgett technique253, 273

Template-assisted colloidal assembly has two major advantages over the

non-templated assembly Firstly, colloidal crystals grown on templates are less defective and possess a longer range order in their structure Secondly, by using templates, new crystal structures can be engineered, which will otherwise be hard to achieve There are two important forces controlling this process The first is the mediation force as a result of the coordination ability, which can be manipulated by the capping groups on the nanoparticles The second is the templating effect exerted by the surfactant reactivity

CNTs216, 274, 275, polymeric molecules265, DNA276-278, and other biomolecules279have been used as templates Various metal oxides (SiO2280, Al2O3281, 282, TiO2268, ZrO2283 etc.) with different morphologies such as spheres284, rods268, mesoporous channels285 and cylindrical nanotubes283 have also been exploited as substrates Several approaches such as flexible aliphatic molecules with all kinds of functional groups and electron beam lithography286, 287, reactive ion etching techniques288 or scanning probe techniques289, 290 have been reported to modify substrates to produce the desired templates Such topographically patterned or chemically patterned surface

is normally used as a template for forming particle structures that are either commensurate or incommensurate with the template geometry

Most recently, capillary force and convective flow have been reviewed as driving force during the assembly of nanoparticles by Malaquin and coworkers.291 They found that the hydrodynamic drag exerted on the particles in the suspensions plays a key role in the assembly process The velocity and direction of particles in the

suspension can be controlled by varying the temperature of colloidal suspension Other parameters, such as substrate velocity, wetting properties and pattern geometry, have also been investigated Patterns produced through dewetting of thin films of charged polymer solutions used as liquid template to obtain large arrays of organized nanoparticles have also been investigated by Rezende and colleagues.292

Although various studies on the applications of functional nanostructures have been carried out293-295, considerable challenges still lie ahead, especially in terms of the design and preparation of three-dimensional structures with precisely controllable geometry Another future mission for research would be the investigation of multi-component systems that are made up of several different nano-sized building blocks towards the development of novel materials

1.8 Scope of thesis

In spite of the numerous studies on nanostructure materials, tedious synthetic procedure and hazardous chemicals are often involved to control the size and shape of nanoparticles Therefore exploration on simple and convenient synthesis method and effective assembly pathway of nanomaterials remains a key issue in nanotechnology The aim of this project was to develop new synthesis and assembly methods – template combining single-precursor method to prepare size-controllable metal sulfide nanoparticles in/on different templates at room temperature Compared to

conventional synthesis method of nanoparticles, this simple and convenient method can prepare multi-functional nanocomposites which possess the properties of metal sulfide nanoparticles and templates For the first part of this thesis (Chapter 3-5), metal sulfide (Ag2S, PbS) nanoparticles were prepared from molecular precursors under mild condition in solution in/on three different templates, namely PANi, clay, and CNTs

A simple and versatile decomposition method has recently been established in our research group to synthesize metal sulfide (Ag2S, CdS, In2S3, PbS, CuxS and NiS) nanoparticles by decomposing the corresponding metal thiobenzoate precursor with amine There are two pathways to decompose the precursor One is room temperature decomposition by amine296 and another is hot-injection decomposition method, which has been used to synthesize metal sulfide nanocrystals with controllable shape and size297, 298 In this thesis, we employed the room temperature decomposition method

to synthesize semiconductor Ag2S or PbS in the presence of template

In Chapter 3, we describe a generic method for producing high-quality silver sulfide nanoparticles in the nanometric range in PANi matrix Furthermore, it is carried out under mild conditions - room temperature and ambient pressure The application of these PANi/Ag2S nanocomposites as silver ion selective electrode was also investigated

In Chapter 4, we discuss the formation of organo-clay/PbS nanocomposites The unique layered structure of organo-clay was found to provide a confined medium for

the formation of nanoparticles The resultant organo-clay/PbS nanocomposites exhibited near-infrared photoluminescence and the infrared luminescence can be tuned by varying the composition

In Chapter 5, we discuss the synthesis of PbS nanoparticles on the wall of MWCNTs or on top of MWCNTs arrays through one-step decomposition reaction Due to the special conductivity of CNTs arrays, field emission and photocurrent of PbS/CNTs on silicon wafer have been measured, which provided useful information

on its potential application in solar cell

In Chapter 6, the assembly of Ag2S and CuxS nanoparticles in different ordered matrix was presented Nanoparticles with cubic, rod and disk shape have been deposited on polystyrene (PS) templates which are formed through the assembly

well-of PS micro-beads or through lithography The influence well-of the volume fraction well-of the nanoparticles suspending solution and the withdrawal speed of the template on the formation of array structures was investigated