R. Subasri

Synthesis of a hydrogen producing nanocrystalline ZnFe2O4 visible light photocatalyst using a rapid microwave irradiation method

A rapid microwave solid-state synthesis method is systematically investigated to achieve a H2 producing visible light active spinel photocatalyst. ZnFe2O4 nanocrystallites were obtained by microwave irradiation of precursor compacts under optimized conditions. This investigation led to a uniform sized nanocrystalline photocatalyst that yielded a quantum-yield of H2 evolution ∼3.8 times higher than that of conventionally synthesized ZnFe2O4. The synthesis parameters – microwave power, synthesis temperature, and time, were found to control the physico-chemical properties viz phase formation kinetics, phase purity, crystallinity, specific surface area and photochemical efficiency, of the synthesized photocatalyst. The study reveals that the threshold microwave power of ≥3 kW was necessary to obtain a spinel phase structure, while lower power (<3 kW) could not induce the crystallization even after prolonged low-power irradiation of 180 min. At the threshold power, a minimum of 10 min. synthesis time was enough to obtain uniform sized nanocrystallites, indicating that the synthesis method is ∼24 times faster than the solid state reaction method, which needs nearly 4 h. The particle morphology evolution with irradiation time from 10–150 min. exhibited de-crystallization phenomena. Longer irradiation displayed a morphological crystallization probably induced due to the simultaneous area and volumetric heating effect. The possible “formation mechanism” of these uniform nanocrystallites has been presented here for qualitative understanding. Thus synthesized photocatalysts generated hydrogen from a water–methanol mixture even without the co-catalyst loading. The ferrite photocatalyst was found to decolorize methylene blue dye with a maximum decay constant of 0.232 h−1, thereby demonstrating its capability in the pollutant decomposition applications, all under visible light photons.

Biofouling studies on nanoparticle-based metal oxide coatings on glass coupons exposed to marine environment

Titania, niobia and silica coatings, derived from their respective nanoparticle dispersions or sols and fabricated on soda lime glass substrates were subjected to field testing in marine environment for antimacrofouling applications for marine optical instruments. Settlement and enumeration of macrofouling organisms like barnacles, hydroides and oysters on these nanoparticle-based metal oxide coatings subjected to different heat treatments up to 400 degrees C were periodically monitored for a period of 15 days. The differences observed in the antifouling behaviour between the coated and uncoated substrates are discussed based on the solar ultraviolet light induced photocatalytic activities as well as hydrophilicities of the coatings in case of titania and niobia coatings and the inherent hydrophilicity in the case of silica coating. The effect of heat treatment on the photocatalytic activity of the coatings is also discussed.

Sol-gel nanocomposite hard coatings

Sol-gel processing is a versatile technology for obtaining multifunctional (hydrophobic, scratch, abrasion and corrosion resistant, anti-bacterial, anti-reflective, etc.) coatings on different substrates, and a judicious selection of precursors is required to form a sol for obtaining coatings with the required properties. Organic–inorganic hybrid nanocomposite coatings blend the properties of organic polymeric materials with those of ceramics: the inorganic components improve properties like scratch resistance, durability, impact strength, and the gloss of the coating, while the organic moieties increase the flexibility, critical thickness and impart low temperature coating curability. For several industrial applications, hard coatings need to be deposited on various substrates (with varying temperature sensitivities) to improve their mechanical properties. These coatings are usually characterized by their scratch hardness, abrasion resistance, nanoindentation hardness and adhesion. This chapter presents a comprehensive review of the sol-gel derived nanocomposite hard coatings investigated on different substrates, along with typical examples of industrial applications, as appropriate.

Effect of plasma pretreatment on adhesion and mechanical properties of sol-gel nanocomposite coatings on polycarbonate

Adhesion of the coatings to the substrate plays a vital role in improving mechanical properties of sol-gel coatings especially when deposited on plastics. In the present study, an attempt was made to study the effect of atmospheric air plasma surface activation on the adhesion of sol-gel nanocomposite coatings on polycarbonate substrate. The sol was synthesized by the hydrolysis and condensation of an epoxy silane (3-Glycidoxypropyl trimethoxy silane) along with titanium tetraisopropoxide. Coatings were deposited on 100 × 100 mm substrates by spray coating and subsequently subjected to UV curing followed by thermal curing at 130 °C for 1 h. One set of the substrates was subjected to atmospheric air plasma surface pretreatment prior to coating deposition. Coating thickness, adhesion as well as mechanical properties like pencil hardness, scratch resistance and taber abrasion resistance were evaluated for coatings deposited over plasma-treated and -untreated surfaces. It was found that plasma surface pretreatment has improved the pencil hardness from H to 2H and adhesion of coatings from 2B to 4B. Results obtained from the above experiments were discussed on the basis of surface modification using plasma pretreatment.

Multifunctional sol-gel coatings for protection of wood

Abstract Organic–inorganic hybrid sol-gel coatings derived from sols synthesized using two organically modified precursors were generated on wood specimens using dip and spray coating techniques. One of the sols was synthesized using methacryloxypropyltrimethoxysilane (MPTMS) and nanoscaled boehmite particles and the other was synthesized using dimethyldiethoxysilane (DMDEOS) and tetraethoxysilane (TEOS). The coatings were generated on the wood specimens by dipping into the sols for specific time periods and by using spray coating method. The coatings were cured at different temperatures for varying soaking times. The dipping times, curing temperature, and soaking times were varied between 24–96 h, 130–200°C and 1–5 h, respectively. The coated samples were characterized for their water contact angles, microstructure, and resistance to water uptake. The coatings derived from MPTMS+boehmite sol were seen to exhibit better performance than DMDEOS+TEOS sol by forming a good barrier on the wood surface, thereby providing superior resistance to water and weather. Dipping of wood into the sol was seen to provide better protection when compared to spray coating. Dipping into the MPTMS+boehmite sol for 24 h and curing at 130°C for 2 h were found to be the optimal processing parameters yielding the best properties.

One-Step Anodization/Sol-Gel Deposition of -Doped Silica-Zirconia Self-Healing Coating on Aluminum

A novel process was used for the preparation of dense, thick, and stable silica-zirconia coatings on aluminum by an in situ anodization along with sol-gel deposition. Anodic electrophoretic deposition was carried on aluminum using a SiO2-ZrO2 sol that was synthesized from an epoxy modified silane and zirconium n-propoxide along with a cerium salt (Ce(NO3)3·6H2O). Current density and time were varied during the deposition. The optimal parameters that yielded uniform coatings were determined. Coatings were characterized for their crystallinity, scratch hardness, and microstructure. The barrier properties of the coatings were tested using potentiodynamic polarization studies, electrochemical impedance spectroscopy, and neutral salt spray tests. Grazing angle incidence X-ray diffraction studies revealed that the coating comprised crystalline Al2SiO5 along with an amorphous phase. The novelty of the process was that the crystalline aluminosilicate phase was formed even at room temperature and could be deposited on aluminum by a simultaneous anodization of aluminum and sol-gel deposition. The coated substrates withstood more than 400 hours of salt spray tests. Polarization measurements reveal that the composite layer of aluminosilicate along with the Ce3