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Part of our project concerns the synthesis of extended metal oxide structures using building blocks on functionalised Si(111) surfaces. Metal oxide (M-O-M) linkage formation was expected by anchoring the building blocks and subsequent hydrolysis on surfaces. We are expecting that metal alkoxides would react more slowly compared to solution phase reactions.

Surface sol-gel process

1.7.1. Metal alkoxides

Controlled synthesis of metal oxide thin films on functionalised surfaces is still very challenging and it has received huge attention from the scientific community recently because of its potential applications in the field of electronics. Many thin films are prepared by sol-gel processing conducted under mild experimental conditions. Controlled growth of metal oxide thin films with atomic precision has been accomplished by sequential surface chemical reactions using the Chemical Vapour Deposition (CVD) technique.61 Atomic Layer Deposition (ALD) is also one of the processes used for the formation of oxide thin films on surfaces. The precursor molecule mostly metal alkoxides and metal halides, chemisorbs or reacts with surface groups saturatively and further exposure to a second precursor leads to controlled growth of metal oxide thin films.62,63 Recently T. Kunitake et.al 64,65 reported a novel method to fabricate metal oxide thin films by means of stepwise adsorption of metal alkoxides from solution., monitoring the reactions by quartz crystal microbalance (QCM). This has been termed as surface sol-gel process and contains four major steps; chemisorptions of alkoxide, rinsing, hydrolysis of chemisorbed alkoxide and drying. In addition they found that polyhydroxyl compounds, polymers and small molecules strongly adsorb onto metal oxide layers formed by surface sol-gel process.

In our efforts, we are trying to synthesise metal oxide thin films by anchoring the possible precursor i.e. metal alkoxides or metal halides to ~30% hydroxyl functionalised Si (111) surfaces.66 The preliminary studies carried out with metal alkoxides on functionalised Si(111) surfaces and the details of reactivity studies are discussed in Chapter 5.

1.7.2. Polyoxometalates

Our approach with regards to synthesising extended metal structures on the functionalised surfaces is to attach the diheterometallic (especially trans) Lindqvist type POMs initially, followed by subsequent reactions with another POM leading extended metal oxide structures. Errington et.al attempted to synthesise the diheteroemetallic Lindqvist POMs and as discussed in section1.6.2.

Multilayer films of polyoxometalate ions have been assembled by using electrostatic alternate adsorption techniques. Klemperer et.al started the surface chemistry technique to construct monolayers of POMs on various surfaces e.g. Ag(111) and Au(111). 67 Keggin POM monolayers were prepared by immersion of Ag(111) and Au(111) in to acidic solutions of silicotungstate [SiW12O40]4-. This was followed by

Errington and his co-workers, who have successfully attached alkoxido derivatised heterometallic Lindqvist type POMs [(RO)M’M5O18]3-(M’=Ti, Zr, Nb, Ta, Hf and Sn)

to functionalised Si(111) surfaces. These surface alkoxido groups are readily accessible to attach to functionalised silica surfaces and they have successfully attached the novel TiW5 polyoxometalate to undecanol derivatised silicon (Si)

surfaces. This was the first demonstration of the covalent surface immobilisation of polyoxometalates.68The basic principle being that a reacting group is erected on the surface, which binds to the POMs and then these surfaces are then studied by AFM, XPS or by FTIR techniques.69

Several techniques such as Langmuir-Blodgett, sol-gel processing and layer-by-layer self assembly have been employed to produce hybrid inorganic-organic materials. Decher et. al 70,71,72,73 originally developed the layer-by-layer assembly technique to prepare multilayer assemblies of organic polymers and this technique is simply based on adsorption of oppositely charged species from dilute solutions. This method allows the use of various inorganic building blocks. Hu et.al 74 reported the first organic- inorganic composite films of a rare earth containing POM, Na9{EuW10O36] and

poly(allylamine hydrochloride) by the layer- by- layer self assembly method. Cabuil et.al75,76 have successfully demonstrated the covalent attachment of thiol derivatised

[γ-SiW10O36(RSi2)O]4- (R=HSC3H6) POM on gold nanoparticles. The organic part R

plays a critical role which easily makes covalent link to gold particles through the thiol group and covalent links with polyanion through the trimethoxy silane group.

Currently, we are aiming to synthesise diheterometallic alkoxido derivatised Lindqvist type POMs and reactivity studies with ~30% -OH functionalised Si (111) surfaces will be developed in future. The reactivity of tin Lindqvist type POM anions [(MeO)SnW5O18]3-with 30% functionalised Si(111) surfaces is discussed in Chapter 5. 1.8. Applications of sol-gel process

1.8.1. Reversible cathodes for Li Batteries

Transition metal chalcogenides have been extensively used as reversible cathodes for Li ion batteries especially V2O5, which exhibits a three dimensional layer structure

rather than being van der Waals host. It is hoped that better reversibility can be achieved with amorphous oxides for which structural changes should be limited and thus vanadate glasses have been suggested as reversible cathodes for lithium batteries.77,78

1.8.2. Catalysts

The sol-gel process offers many advantages for making catalysts. Since the homogenous mixing can be achieved at the molecular scale, the chemical reactivity of the oxide surface can be greatly increased and usually provides powders with specific pore dimensions and large surface area. The average size of the particles can be varied in the range 30-120 Å by diluting the alkoxide precursor during the synthesis. If drying is performed in hypercritical conditions, a highly porous material called an aerogel is obtained. Aerogels exhibit better catalytic properties in terms of selectivity, activity. etc than xerogels.79,80

1.8.3. Materials

The Sol-gel process is widely used to make multi-component ceramics or glasses. Metal alkoxides are usually used for initial precursors and various ceramic synthetic routes and their applications are listed in the following table.

Ceramics Initial precursors Uses BaTiO3 Ba(OEt)2or Ba(OPrn)281,82

Ti(OEt)4or Ti(OPri)4

Ferroceramics

PbTiO3 Pb(OAc)2and Ti(OPri)483,84,85 High dielectric constant

capacitors. NiFe2O4 Fe (II), Ni(II) hydroxides86 Ferromagnetic films.

LaYO3 La(OEt)3and Y(OEt)387 Thermomechanical

ceramics. Y3Al5O12 Y(NO3)3and Al(OPri)388 Translucent gel.

Table 1: Applications of various meral oxides in ceramics field.

1.8.4. Bio-Applications

The sol-gel method was first used for the powderless processing of glasses and ceramics. The encapsulation of fragile bio molecules within sol-gel glasses has been recently addressed and cells, antibodies and enzymes are safely trapped by the sol-gel materials where they retain the bioactivity or sometimes enhance their activity. The increased activities of enzymes in sol-gel materials may be explained in terms of structural changes which are observed in enzymes and the sol-gel matrix cage upon heating. Therefore a real burst of interest in biological applications of sol-gel materials has occurred during recent years and now scientists around the world have been working on bio-sensors or bio-catalysts.89