Dibyendu Ganguli

Sol–gel electrochromic coatings and devices: A review

A brief updated review is made on sol-gel-derived electrochromic films (some of which used as ion storage films) of different chemical systems. Performances of selected films measured in electrochemical cells or in devices are discussed and the degradation problems experienced by different authors enumerated.

Dense silica microspheres from organic and inorganic acid hydrolysis of TEOS

Abstract It has been observed that any acid–water mixture (irrespective of the type of acid organic and inorganic) in the pH range 1.35–2.25 can produce silica microspheres from tetraethyl orthosilicate (TEOS) with only a restriction of the water/TEOS molar ratio which varies in the ranges 1.0–1.5 for strong acids and 1.5–4.0 for weak acids. Microspheres have been found to form in situ from an intermediate heavier liquid phase of polysilicic acid which has been segregated and separated from the sol during the progress of the reaction. FTIR spectral analysis reveals that this intermediate liquid predominantly consists of four-membered siloxane rings (Si 4 O 12 ) as identified by their absorption frequencies in the ranges 550–600 and 670–710 cm −1 . The segregated liquid phase has been transformed into glass-like non-porous dense microspheres during stirring. FTIR analyses of these silica spheres are similar to that of fused silica. Density, surface area and size of the dried (150°C) microspheres vary in the ranges 2.10–2.16 g cm −3 , 0.5–0.9 m 2 /g and 10–60 μm, respectively. Their high densities and FTIR spectral data indicate a similar structure to that of silica glass.

Indium tin oxide nano-particles through an emulsion technique

Abstract Spheroidal indium tin oxide (ITO) nano-particles with In/Sn=85:15 (at.%) were prepared by an emulsion technique under mechanical agitation without the aid of any surface-active agent. X-ray powder diffraction (XRD) indicated the stabilization of cubic ITO as the only phase at 250 °C. Particle size analysis showed a narrow distribution of 20–58 nm range particles with a mean size of 38 nm, thus, confirming their nano-structured nature. The pellets sintered at 1350 °C for 6 h were found to have 95–97% of theoretical density. Thus, well sinterable nano-particles of ITO powders were prepared at a significantly low temperature compared to that in the reported methods.

Tailoring of room temperature excitonic luminescence in sol-gel zinc oxide-silica nanocomposite films

Abstract Zinc oxide–silica (ZnO/SiO 2 =20:80, molar ratio) nanocomposites consisting of ZnO nanoparticles embedded in a dielectric matrix were prepared by a sol–gel technique (spin coating). Optical transmittance, Raman effect and photoluminescence measurements of the composites indicated effective capping of the ZnO nanoparticles (radii 1.4–1.5 nm) in the host showing practically no variation of particle size with the post-deposition-annealing treatments. The blue shift of the band gap (4.23–4.29 eV) from that of bulk ZnO (3.3 eV) indicated strong carrier confinement for samples annealed at T ≤773 K. Highly intense UV emission (approx. 4.2 eV) at room temperature could be obtained by annealing the composites in static oxygen atmosphere, while the visible defect-related luminescence (approx. 2.62 and 2.43 eV) could be reduced, resulting in a high intensity ratio (approx. 27) of the two.

Optical and electrochromic properties of sol-gel WO3 films on conducting glass

Abstract The precursor containing peroxotungstic acid (PTA) was prepared by the reaction of tungstic acid with H 2 O 2 (30%) solution in water–ethanol mixture. This was utilized for the deposition of WO 3 · n H 2 O film on conducting (F-doped SnO 2 coated) flat glass substrate by the dipping technique. The films were cured in the temperature range of 200–300 °C in air. The optimum thickness of the film (300–500 nm) was obtained by successive operations (3–5 numbers). Films of around 500 nm thickness exhibited 60–70% transmission in the visible region. Electrochromic properties (colouration↔bleaching) of the films were studied by cyclic voltammetry (CV) using a classical three-electrode potentiostatic cell system. The cell system consists of a WO 3 -coated sample as working electrode, a platinum rod as counter electrode, Ag/AgCl as reference electrode and 0.5 M LiClO 4 in propylene carbonate as an electrolyte. Several voltammograms were recorded within the voltage range of +1.5 to −1.8 V with a scan rate of 50 mV/s. A continuous curve was observed for the films at a certain voltage sweep where a random insertion of Li + occurred reversibly in a definite crystallographic site [WO 3 (colourless)+ n Li + + n e − ↔ Li n WO 3 (blue)]. A dark blue colouration (% T =20–30% in the visible region) was observed under a constant voltage of −1.8 V whereas bleaching occurred at +1.0 V (% T =60–70%), which was studied simultaneously along with the voltage sweep of CV. The colouration time ( T col ) and the bleaching time ( T bl ) were almost equal as revealed by the simultaneous study of the second signal (photomultiplier output coupled with the electrochemistry system) during colouration↔bleaching. Coatings of about 500 nm thickness exhibited more that 500 cycles (colouration↔bleaching). The reversibility of the cycle remained good but the intensity of the colouration decreased with the number of cycles. This was possibly due to the structural deformation of WO 3 films for its long time exposure to the electrolytic solution. The cathodic current ( I c ) and anodic current ( I a ) increased with increasing thickness.

FTIR studies of gel to glass conversion in TEOS-fumed silica-derived gels

Abstract Hybrid gels, prepared from mixtures of tetraethyl orthosilicate (TEOS) solution and fumed silica (alkoxide: fumed silica = 1 : 1.25) were examined by FTIR at different stages of drying/densification up to glass formation. Transparent, bubble-free glasses containing low hydroxyl were obtained at up to 1525°C under vacuum, oxygen, carbon tetrachloride and helium atmospheres. Spectroscopic investigations revealed a free, isolated silanol stretching (v1) band at around 3740 cm−1 split into two to three overlapping peaks after heat treatment of the gels at 600–950°C. This phenomenon is suggested to be due to either the presence of two types of isolated silanols (single and geminal) and/or an effect of the hybrid microstructure of the gels with widely varying spatial relationships among SiOH groups, leading to closely related, weak interactions.

Spectroscopic properties of cerium in glasses and their comparison with crystals

Abstract Glasses containing Ce were prepared by the sol–gel method at elevated temperatures. Two parallel series one in air and another in nitrogen atmosphere, were obtained. The main constituent of the glasses was silica SiO 2 obtained by hydrolysis of ethoxysilane (TEOS). Parallel series of glasses were prepared with addition of boron and aluminum. Spectroscopic properties of the glasses were mainly studied by absorption spectroscopy, luminescence spectroscopy and life-time measurements. The comparison of the spectral properties of the glasses was made with existing information on spectroscopy of cerium in crystals and conventional glass.

Optical properties of Cd1−xZnxS nanocrystallites in sol-gel silica matrix

Cd1?xZnxS nanoparticles with different compositions (0?x?1) embedded in silica matrix were prepared in thin film form by the sol?gel technique. The film texture and structural transformation with increasing x value were studied from transmission electron micrographs and electron diffraction patterns. Optical study indicated increase in optical bandgap and decrease in refractive index with increasing zinc content (x value). Photoluminescence studies showed emission from surface states with emission energy blue shifted with increasing zinc content in the films. Annealing behaviour of the films with increasing annealing temperature and time was studied.

Monolithic zirconia gels from metal-organic solutions

It has recently been shown by Ganguli and Kundu [1] that amorphous and transparent coatings of pure ZrO 2 could be prepared by controlled hydrolysis of zirconium propoxide and subsequent polymerization by using a non-polar solvent like cyclohexane instead of the usual alcoholic solvents employed for preparing coatings and monoliths [2-7]. The non-polar, hydrophobic solvent was used to avoid precipitation caused by the high rate of hydrolysis of Zr(OC3H7)4. The present work is a continuation of the above study, leading to the preparation of pure, transparent and amorphous zirconia gels (including monoliths), using zirconium propoxide and cyclohexane. It was observed during this work that the rate of evolution of the volatiles from the gel-forming compositions could play a vital role in determining the nature of the product. This has also been systematically examined. Technical grade zirconium n-propoxide (Fluka) and Analar grade cyclohexane (BDH) were used for preparing the zirconia gels. The cyclohexane was thoroughly dried before use by repeated contact under stirring with activated silica gel and molecular sieve 4A. Solutions of the propoxide in cyclohexane (in the range 4 to 10 equivalent wt % ZrO2) were prepared in

Hollow alumina microspheres from boehmite sols

Hollow alumina microspheres were obtained from emulsified boehmite sols by an ion extraction method. The viscosity and the equivalent alumina content of the sols were found to affect the characteristics of the derived microspheres. High-viscosity sols produced broken microspheres. A temperature of about 400–500 °C for boehmite to γ-Al2O3 transformation in the gel microspheres was observed by differential thermal analysis and X-ray diffraction. Complete crystallization of the gel microspheres to α-Al2O3 occurred at 1200 °C. A tentative mechanism for the formation of the hollow microspheres is presented.