Films, formation from sols

Recently, we reported that thin films with nanocrystals such as boehmite (AlOOH) [7-10], Al-Zn layered double hydroxide (LDH) [11], and anatase [12] were prepared through hot water treatment of the gel films at temperatures less than 100 C. In this process, precursor gel films prepared using the sol-gel process were immersed only in hot distilled water under ambient atmosphere at temperatures lower than 100 C. Although studies of oxide film formation from solution are usually based on the precipitation of crystals from a saturated solution of the precursor [1-6], in the present process, precursor amorphous gel films prepared using the sol-gel process are immersed in hot distilled water to form nanocrystals on the thin film surface. 

Kozuka H. On ceramic thin film formation from gels Evolution of stress, cracks and radiative striations. J. Ceram. Soc. Jpn 2003c 111 624-632 Kozuka H., Komeda M. Effect of the amount of water for hydrolysis on cracking and stress evolution in alkoxide-derived sol-gel silica coating films. J. Ceram. Soc. Jpn. 2004a 112 S223-S227... 

Al-Obaidi A H R, Rigby S J, Hegarty J N M, Bell S E J and McGarvey J J 1996 Direct formation of silver and gold metal liquid-like films (MELLFS) from thiols and sols without organic solvents SERS and AFM studies ICORS 96 XVth Int. Conf on Raman Spectroscopy ed S A Asher and P B Stein (New York Wiley) pp 590-1... 

Kinetic stability of lithium and the lithiated carbons results from film formation which yields protective layers on lithium or on the surfaces of carbonaceous materials, able to conduct lithium ions and to prevent the electrolyte from continuously being reduced film formation at the Li/PC interphase by the reductive decomposition of PC or EC/DMC yielding alkyl-carbonates passivates lithium, in contrast to the situation with DEC where lithium is dissolved to form lithium ethylcarbonate [149]. EMC is superior to DMC as a single solvent, due to better surface film properties at the carbon electrode [151]. However, the quality of films can be increased further by using the mixed solvent EMC/EC, in contrast to the recently proposed solvent methyl propyl carbonate (MPC) which may be used as a single sol-... 

Another important aspect is the proximity of particle layer and thin or thick films, whereby the border between thin and thick films remains ambiguous. Conventional processing of particle assemblies comprises molding of the particle mass, like dry compression or wet casting. In the case of nanoparticles, to form a film is much more rational than conventional powder processing. In a dry procedure, production of nanoparticles and film formation in most cases take place simultaneously. An assembly of nanoparticles can also be formed in a wet procedure by precipitation on the templates or even by simple dip or spin coating from a sol containing nanoparticles. 

All films prepared for contact angle studies were cast from an excess of dry polymer swollen and fluidized by a small quantity of added liquid (usually triple-distilled water, the last two distillations being in an allquartz apparatus). The pH of these weak gels at the initiation of film formation was always 5. All our surface films were gel-dried rather than sol-dried. Although swelling may not have reached equilibrium prior to film drying, the films were only about 1 mm thick and their surface properties, as determined by contact angle measurements, were always reproducible within 5°. 

Brinker and coworkers [49] reported the synthesis of microporous silica membranes on commercial (membralox) y-alumina supports with pore diameters of 4.0 nm. Ageing of the silica sols was shown to be effective to form discrete membrane layers with an estimated thickness of 35 nm on top of the support and to inhibit pore penetration of the silica. Sols with gyration radii Rg < (radius of support pores) penetrate the support to a depth of about 3 im, which is the thickness of the y-alumina support layer. Minimization of the condensation rate during film formation was considered to decrease the width of the pore size distribution without changing the average pore radius, which was estimated to be 0.35 < Tp < 0.5 nm. The porosity of films deposited on dense supports was about 10% as calculated from refractive index measurements. 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

Silica in the form of thin films as well as oxide monoliths, fibers, and powders can be prepared from sol-gel method. In contrast with the fabrication of conventional inorganic glasses at much higher melting temperature, sol-gel processing is performed at low temperatures to produce oxide materials with desirable hardness, optical transparency, chemical durability, tailored porosity, and thermal resistance. The sol-gel method involves formation of a colloidal suspension (sol) and gelation to form a network in a continuous liquid phase (gel). One starts with an aqueous solution containing oxides or alkoxides, mutual solvent, and catalyst. Usually an external catalyst is added like mineral acids and ammonia as well as acetic acid, KOH, amines, KF, and HF for rapid and... 

The term controlled index means that the refractive index can be made smaller than that of the bulk precursor by controlling the microstructure via the porosity. When silica is deposited, for example, the film index n can be varied over a wide range (7) from n = 1.1 to 1.5. This process control makes sol-gel coatings interesting for many optical, electronic, and sensor applications, but the evolution of the microstructure during film formation is not well understood, in spite of efforts to survey the variables (8, 9). This chapter reviews the important factors determining the microstructure of dip-coated films and explores at length two of them, evaporation and surface tension. 

Crystallization and reduction of sol-gel prepared zinc oxide films derived from zinc acetate by irradiation with an UV lamp (185 and 254 nm) was studied (Asakuma et al. 2003). UV irradiation induced the formation of hexagonal ZnO crystals from amorphous ZnO films preheated at 100 C, while irradiation of porous ZnO films preheated at 60 C led also to formation of metallic zinc. Composite ZnO/Cu and ZnO/Ag/Cu nanostructures were prepared via the photocatalytic reduction (wavelength 310-390 nm) of cuprous chloride and silver nitrate over the chemically prepared ZnO nanoparticles in aqueous solution (Shvalagin et al. 2004). Amorphous ZnO thin films were prepared... 

We found that the surface morphology of thin films derived from mixture of TEOS and MTES sols can be controlled by changing molecular weights of siloxane polymers in TEOS and MTES (Takahashi, 1997). We describe the method to produce low refractive index film and discuss the mechanism of formation of different surface morphologies. A new layer to reduce the reflection of the automotive windshield for drivers has been developed by sol-gel method. 

---