Venu Sreekala Smitha

UV curable hydrophobic inorganic–organic hybrid coating on solar cell covers for photocatalytic self cleaning application

A process for obtaining a low temperature, UV curable inorganic–organic hybrid coating having excellent photoactive properties which is suitable for glass surfaces such as solar cell covers, automobile rear view mirrors and several electronic devices is presented. Nano titania sol having an average particle size of 19 nm has been synthesized using the aqueous sol–gel route starting from titanyl sulphate and was made highly photoactive by crystallizing through a simple microwave treatment which is of considerable importance in the synthesis of hybrid coatings where conventional heat treatment is not possible. The photoactive titania particles were then partially covered by silica particles by optimizing the amount of the silane moiety. A multifunctional monomer composition was further prepared and coated on a solar cell cover glass followed by a second layer coating using photoactive titania particles by dip coating and was cured under UV for six hours. The coated substrate shows photocatalytic efficiency as well as transparency >90% and further, a water contact angle >90°. Titania particles were uniformly distributed on the surface of the top coating as revealed by AFM and the coating had a total thickness of approximately 2.5 μm. The appropriate ratio of organic silanes/acrylates imparts strong adhesion to the substrate, while the inorganic components composed of a hybrid mixture of silica/titania enhanced the self cleaning efficiency. Such multifunctional coatings on solar cell covers have considerable advantage in preventing surface contamination from the exposed atmosphere which decreases the efficiency of such panels in the 15–25% range.

Novel multifunctional titania–silica–lanthanum phosphate nanocomposite coatings through an all aqueous sol–gel process

A novel nanocomposite coating containing titania, silica and lanthanum phosphate prepared through an all aqueous sol-gel route exhibits excellent self-cleaning ability arising from the synergistic effect of the constituents in the nanocomposite. A highly stable titania-silica-lanthanum phosphate nanocomposite sol having particle size in the range of 30-50 nm has been synthesized starting from a titanyl sulphate precursor, which was further used for the development of photocatalytically active composite coatings on glass. The coatings prepared by the dip coating technique as well as the nanocomposite powders are heat treated and characterized further for their morphology and multifunctionality. The nanocomposite containing 1.5 wt% LaPO4 has shown a surface area as high as 138 m(2) g(-1) and a methylene blue degradation efficiency of 94% in two hours of UV exposure. The composite coating has shown very good homogeneity evidenced by transparency as high as 99.5% and low wetting behaviour. The present novel approach for energy conserving, aqueous derived, self-cleaning coatings may be suitable for large scale industrial applications.

Effect of precursor particle size distribution on the morphology and low wetting behavior of photocatalytic nanocoatings on glass surfaces

The effect of particle size distribution of coating precursor on the morphology and low wetting character of photocatalytic nanocoatings is investigated in the present work. TiO2–SiO2–Al2O3 nanocomposite coatings containing Al2O3 particles of size in the range 20–200 nm have been prepared on glass substrates by an aqueous sol–gel process. A composite sol of titanium dioxide containing 30 mol% silica comprises the matrix sol to which alumina particles (1–10 mol%) having sizes in the range 20 to 200 nm are introduced as stabilized dispersions and further coated on glass substrates by the dip-coating method followed by annealing of the coatings at 400 °C. A composite coating containing 2 mol% boehmite derived alumina (TS-B-2) has been found to be more photoactive under visible light and was low wetting in nature. The higher photocatalytic activity of the TS-B-2 nanocomposite is attributed to the presence of phase pure anatase with crystallite size of 3.7 nm and high surface area of 315 m2 g−1, while the low wetting character is attributed to the hierarchical morphology resulting in uniform surface roughness. The present study significantly highlights the possibility of designing composite precursors containing desired constituent particle sizes to produce nanocoatings differing in grain sizes, surface roughness and morphology, resulting in increased self-cleaning and low wetting properties.

Photoluminescent, self-cleaning titanium oxide nanocomposites with multifunctional properties

Photoluminescent and self-cleaning properties of an Eu3+ doped titania–silica–lanthanum phosphate nanocomposite (Eu-TSL) prepared by an aqueous sol–gel process and films fabricated on glass substrates by dip coating are investigated in the present work. These nanocomposites containing Eu3+ as dopants exhibit red luminescence upon visible light excitation, after heat treatment at 400 °C. For an excitation wavelength of 465 nm, the PL spectra of Eu-TSL show intense peaks at 613 nm (5D0–7F2), characteristic of Eu3+ ions. This red emission has a life time of ∼0.5 ms. 1 mol% Eu added TSL shows ∼96% methylene blue dye degradation after two hours UV light exposure which is attributed to the presence of anatase phase, optimum crystallite size (5.7 nm) and enhanced specific surface area (147 m2 g−1). Eu-TSL was found to be five times more efficient in decolourization of methylene blue than Eu-TiO2 from the reported literature. Eu-TSL retains all the major properties including photoactivity, transparency (∼97%) and low wettability (∼80°). In addition, red emission could be integrated at very low europium doping levels (1 mol%). Even though europium doping on titania is reported widely, a multifunctional luminescent and self-cleaning composition based on Eu-TSL synthesized by an aqueous sol–gel route is reported for the first time.

Temperature assisted acid catalyzed peptization of TiO2; facile sol–gel approach for thermally stable anatase phase

High temperature stable, phase pure anatase has been successfully synthesized by temperature assisted acid catalyzed peptization of hydrous titania through an aqueous sol–gel method. This facile method does not contain any metal or non metal dopants and is very effective in extending the anatase to rutile phase transformation by 200 °C. The temperature maintained during the peptization process has a significant effect on the particle size and morphology of titanium dioxide. The sol synthesized by peptization at 60 °C has the lowest particle size of 65.2 nm and after calcination at 800 °C, has a crystallite size of ∼53 nm. The relatively higher particle size of other samples is attributed to the aggregation of particles at respective peptization temperatures. An optimum temperature of 60 °C is effective in minimising the aggregation behaviour of titania sol which further increases the photocatalytic activity. Titanium dioxide peptized at 60 °C has higher photocatalytic activity (∼86%) than the one peptized at 30 °C (∼74%) under UV-A exposure for 100 minutes with apparent rate constants 0.02 and 0.01 min−1 respectively. An increased anatase to rutile transformation temperature of 800 °C will be very useful in increasing the annealing especially in ceramic substrates. Thus highly photoactive nanocrystalline titania containing thermally stable anatase that can be used for high temperature photocatalytic applications has been synthesized.

Energy revamping of solar panels through titania nanocomposite coatings; influence of aqueous silica precursor

An all-inorganic nanocomposite coating of titania has been found to revamp the reduction in efficiency of solar panels arising due to atmospheric pollution. A multifunctional, photoactive and hydrophobic titania–silica–lanthanum phosphate (TSL) nanocomposite having excellent anatase thermal stability is reported for the first time as alternative coatings on solar panel covers for maintaining the energy efficiency of panels over a period of time under outdoor conditions. The precursor source of silica has been found to significantly influence the photoactivity of titania in the nanocomposite under investigation. Polymeric silica is derived from the tetraethylorthosilicate precursor while particulate silica is aqueous colloidal silica. The enhanced photoactivity of the particulate titania–silica composition is attributed to the presence of phase pure anatase having an optimum crystallite size and surface area, achieved in the presence of silica. Lanthanum phosphate which has low water wettability is further influenced by particulate silica in providing better non-wetting behavior to the TSL nanocomposite. The presence of particulate silica in the TSL composite is superior to that containing polymeric silica in terms of crystallinity, photoactivity and low wettability. Thus, functional titania coatings possessing synergistic properties of the constituents can be designed from a less expensive and effective silica precursor. The all-inorganic system investigated in the present work is environmentally friendly and is stable over a wide range of temperatures. The feasibility of large-scale preparation and application of the particulate TSL nanocomposite coatings on solar panels under outdoor exposure has been demonstrated. The present novel approach for the development of aqueous-derived, self-cleaning, energy conserving coatings will lead to the energy revamping of solar panels and may have large-scale industrial applications.

ORMOSIL–ZrO2 hybrid nanocomposites and coatings on aluminium alloys for corrosion resistance; a sol–gel approach

Corrosion resistant coatings are prepared from a hybrid nanocomposite aerogel derived from tri-functional silanes, methyltrimethoxysilane (MTMS) and glycidyloxypropyl trimethoxysilane (GPTMS) and from a zirconium isopropoxide (ZIP) precursor which acts as an inorganic nano-dispersion in an organically modified silane (ORMOSIL) matrix. A series of hybrid compositions of MTMS and GPTMS are prepared in which the amount of ZIP is varied. The variations in the pH, viscosity and gelation time of the prepared compositions are monitored. The wet alcogels thus obtained are homogenized in a solvent using an ultrasonicator followed by coating the suspension on aluminium alloys and glass substrates using a dip coating unit. The prepared coatings are further dried and annealed at 400 °C for 1 h. The wet alcogels are also dried under ambient conditions for seven days resulting in hybrid nanocomposite aerogel monoliths and are calcined at 400 °C. The hybrid nanocomposites and coatings are further characterized using X-ray diffraction analysis, Fourier transform infrared spectroscopy, BET surface area analysis, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible spectroscopy, potentiodynamic polarization and EIS measurements. The hybrid nanocomposite coated aluminium alloy shows enhanced corrosion protection when compared to the uncoated aluminium alloy. The anticorrosive feature of the ORMOSIL–ZrO2 hybrid nanocomposite coatings makes them an important candidate in the field of protective environment resistant coatings.

Sol-gel derived functional coatings of titania-silica-lanthanum phosphate nanocomposite

A composite coating containing silica and lanthanum phosphate in nanotitania is expected to introduce multi functionality. Nano titania-silica-lanthanum phosphate sol with an average particle size of 59 nm has been synthesized using an all aqueous sol-gel route starting from titanyl sulphate which was further used for the development of photocatalytically active nanotitania composite coatings on ceramic glazed substrate by spray coating technique. The coated substrates were annealed at 700 °C. The photoactivity of the prepared nanocomposite coating was found to increase by the incorporation of silica in the titania matrix. The enhanced photoactivity and non-wettability was due to the synergistic effect of mixed oxide additives. Thus, by combining the inherent properties of the components, a novel nanocomposite material exhibiting self-cleaning ability and high photocatalytic efficiency was synthesized. The prepared titania-silica-lanthanum phosphate nanocomposite calcined at 700 °C has given a BET surface area of 244 m2/g and an average grain size of 5.6 nm was observed from TEM. SEM images of the coated surface showed clear evidence of the nanocoating. The present approach for the synthesis of novel and functional composite coating over tile substrate can be extended further to other substrates, leading to a variety of applications useful to society.