Sol-gel SiO2/TiO2 bilayer films with self-cleaning and antireflection properties

We report here that a facile sol-gel dip-coating technique can be used to fabricate a SiO₂/TiO₂ bilayer film with self-cleaning and antireflection properties. The bottom SiO₂ layer acts as an antireflection coating due to its lower refractive index; the top TiO₂ layer acts as a self-cleaning coating generated from its photocatalysis and photo-induced superhydrophilicity. The maximal transmittance of SiO₂/TiO₂ bilayer film at normally incident light can be reached 96.7%, independent of the high refractive index and coverage of TiO₂ nanoparticles. However, the photocatalytic activity of the bilayer film shows a close dependence on coverage of TiO₂ nanoparticles. After illuminated by ultraviolet light, the SiO₂/TiO₂ bilayer films are superhydrophilic with water contact angle less than 2°, which favors greatly the self-cleaning function of the films.     

Electrodeposition of TiO2/SiO2 nanocomposite for dye-sensitized solar cell

For the working electrode of dye-sensitized solar cell (DSC), TiO₂/SiO₂ nanocomposite materials were electrodeposited on transparent fluorine doped tin oxide-coated glass by cathodic electrodeposition at room temperature. The electrode and DSC fabricated with TiO₂/SiO₂ nanocomposite were characterized with photocurrent density, X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) and a photovoltaic performance test. On the electrodeposition, the addition of an appropriate amount of SiO₂ in the bath containing TiO₂ slurry was essential to achieve the superior crystallinity, photocurrent density and photovoltaic performance of the resulting TiO₂/SiO₂ electrode, which was significantly superior to a bare TiO₂ electrode. This enhanced performance of optimized TiO₂/SiO₂ electrode was ascribed to the role of SiO₂ as an energy barrier, increasing the physical separation of injected electrons and oxidized dyes/redox couple, and thereby retarding the recombination reactions in the resulting DSC.     

Study of sensitivity and response kinetics changes for SnO2 thin-film hydrogen sensors

We present results of investigations devoted to searching of ways for sol–gel derived SnO₂ thin-film hydrogen sensors performance improvement. To that end we studied the as-prepared sensors parameters changes in real time during short- and long-term operation in gas–air mixture and known operating temperature conditions. It was established that in comparison with as-prepared sensor parameters such initial operation mode leads to the rise of the sensors sensitivity on more than two orders of magnitude, improvement of the sensor parameters stability in time. Obtained experimental data are in a good accordance with known theoretical results.     

Tri-layer antireflection coatings (SiO2/SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique

Antireflection coatings (ARCs) have become one of the key issues for mass production of Si solar cells. They are generally performed by vacuum processes such as thermal evaporation, reactive sputtering, and plasma-enhanced chemical vapor deposition. In this work, a sol–gel method has been demonstrated to prepare the ARCs for the non-textured monocrystalline Si solar cells. The spin-coated TiO₂ single-layer, SiO₂/TiO₂ double-layer and SiO₂/SiO₂–TiO₂/TiO₂ triple-layer ARCs were deposited on the Si solar cells and they showed good uniformity in thickness. The measured average optical reflectance (400–1000 nm) was about 9.3, 6.2 and 3.2% for the single-layer, double-layer and triple-layer ARCs, respectively. Good correlation between theoretical and experimental data was obtained. Under a triple-layer ARC condition, a 39% improvement in the efficiency of the monocrystalline Si solar cell was achieved. These indicate that the sol–gel ARC process has high potential for low-cost solar cell fabrication.     

Analysis of the effect of hydrogen-radical annealing for SiO2 passivation

The effect of hydrogen-radical annealing for SiO₂ passivation was examined. The annealing effect was analyzed by measuring effective lifetime and C-V characteristics and was compared with the effects of forming gas annealing (FGA) and hydrogen RF plasma annealing. The effect of hydrogen-radical annealing is much higher than those of FGA and hydrogen RF plasma annealing. It was also confirmed that both changes of surface recombination velocity and interface state density showed the same tendency. Furthermore, the investigations of hydrogen-radical density showed that by using microwave afterglow method, hydrogen-radicals could be generated much more than that by RF plasma. Accordingly, more interface trap density could be terminated and surface recombination velocity was effectively decreased by using microwave afterglow method.     

Synthesizing hydrogen from ammonia over Ru incorporated SiO2 type nanocomposite catalysts

In this study, Ruthenium incorporated SiO₂ type nanocomposite catalysts were prepared for CO_x free hydrogen production by using one-pot hydrothermal synthesis procedure. Experiments which were carried out under the flow of pure ammonia (300 ml/min) presented that 86% ammonia conversion was obtained at 500 °C over the catalyst having Ru/Si molar ratio of 0.060. Using promoter in the preparation of catalyst enhanced the catalytic activity especially for the ones prepared at low ruthenium loadings. While the catalyst that was prepared at a Ru/Si molar ratio of 0.010 without promoter gave negligible activity at 500 °C, the promoted one gave 33% conversion at 500 °C and 73% at 600 °C. Experiments were also repeated with lower feed flow rate values of ammonia such as 60 ml/min and 5 ml/min. It was seen that catalyst prepared at a Ru/Si molar ratio of 0.010 with promoter gave conversion value over 80% at 400 °C under the feed flow rate of 5 ml/min.     

Carrier transport in high-efficiency ZnO/SiO2/Si solar cells

Carrier transport in ZnO/SiO₂/n-Si solar cell has been theoretically analyzed with a consideration that the photo-carrier transport from silicon to ZnO layer through the barrier is dominated by quantum mechanical tunneling process of minority carrier. It was found that the highest efficiency of the cell could be achieved at SiO₂ layer thickness of around 20 Å. The efficiency of the cells decreases as the surface states density Q_ss becomes higher. Moreover, the efficiency increases as the electron concentration of ZnO layer is increased due to the decrease of work function of ZnO. It was also found that the lower transmittance of the high carrier concentration ZnO due to the free-carrier absorption at infrared wavelength region does not give any significant effect to the cell performance. The efficiency of higher than 25% is achievable by optimizing the involved device parameters.     

Photocatalytic hydrogen evolution over CuCrO2

We have been studying the technical feasibility of a photochemical H₂ evolution based on a dispersion of CuCrO₂ powder in aqueous electrolytes containing various reducing agents (S²⁻, and ). The title oxide combines a fair resistance to corrosion with an optimal band gap E_g of 1.32 eV. The intercalation of a small amount of oxygen should be accompanied by a partial oxidation of Cu⁺ into Cu²⁺ implying a p-type semiconductivity. The S²⁻ oxidation inhibits the photocorrosion and the H₂ evolution increases parallel to polysulfides formation. Most of H₂ is produced when p-CuCrO₂ is connected to n-Cu₂O formed in situ. H₂ liberation proceeds mostly on CuCrO₂ while the oxidation of S²⁻ takes place over Cu₂O surface and the hetero system Cu₂O/CuCrO₂ is optimized with respect to some physical parameters. The photoactivity is dependent on preparation conditions and lowering the synthesis temperature through nitrate route leads to an increase in specific surface area S_sp. The photoelectrochemical H₂ production is a multistep process where the rate determining step is the arrival of electrons at the interface because of their low mobility. Prolonged irradiation (>80 min) leads to a pronounced decrease of the photoactivity; the tendency toward saturation is due to the undesired back reduction of polysulfides in a closed system and to their strong absorption in the visible region (λ_max = 520 nm).     

TiO2 and TiO2–SiO2 thin films and powders by one-step soft-solution method: Synthesis and characterizations

Simple soft-solution method has been developed to synthesize films and powders of TiO₂ and mixed TiO₂–SiO₂ at relatively low temperatures. This method is simple and inexpensive. Furthermore, reactor can be designed for large-scale applications as well as to produce large quantities of composite powders in a single step. For the preparation of TiO₂, we used aqueous acidic medium containing TiOSO₄ and H₂O₂, which results in a peroxo-titanium precursor while colloidal SiO₂ has been added to the precursor for the formation of TiO₂–SiO₂. Post annealing at 500 °C is necessary to have anatase structure. Resulting films and powders were characterized by different techniques. TiO₂ (anatase) phase with (1 0 1) preferred orientation has been obtained. Also in TiO₂–SiO₂ mixed films and powders, TiO₂ (anatase) phase was found. Fourier transform infrared spectroscopy (FTIR) results for TiO₂ and mixed TiO₂–SiO₂ films have been presented and discussed. The method developed in this paper allowed obtaining compact and homogeneous TiO₂ films. These compact films are highly photoactive when TiO₂ is used as photo anode in an photoelectrochemical cell. Nanoporous morphology is obtained when SiO₂ colloids are added into the solution.     

Comparative study on the promotion effect of Mn and Zr on the stability of Ni/SiO2 catalyst for CO2 reforming of methane

The stability of Mn-promoted Ni/SiO₂ catalyst for methane CO₂ reforming was investigated comparatively to that of Zr-promoted Ni/SiO₂. The catalysts were prepared by the same impregnation method with the same controlled promoter contents and characterized by TPR, XRD, TG, SEM, XPS and Raman techniques. The addition of Mn to Ni/SiO₂ catalyst promoted the dispersion of Ni species, leading to smaller particle size of NiO on the fresh Ni–Mn/SiO₂ catalyst and the formation of NiMn₂O₄, which enhanced the interaction of the modified support with Ni species. Thus, the Ni–Mn/SiO₂ catalyst showed higher activity and better ability of restraining carbon deposition than Ni/SiO₂ catalyst. Besides, it exhibited stable activity at reaction temperatures over the range from 600 °C to 800 °C. However, the introduction of Zr increased the reducibility of Ni–Zr/SiO₂, and the catalyst deactivated much more dramatically when the reaction temperature decreased due to its poor ability of restraining carbon deposition, and its activity decreased monotonically with time on stream at 800 °C.     

Low-temperature preparation of AgIn5S8/TiO2 heterojunction nanocomposite with efficient visible-light-driven hydrogen production

AgIn₅S₈ and AgIn₅S₈/TiO₂ heterojunction nanocomposite with efficient photoactivity for H₂ production were prepared by a low-temperature water bath deposition process. The resultant AgIn₅S₈ shows an absorption edge at ∼720 nm, corresponding to a bandgap of ∼1.72 eV, and its visible-light-driven photoactivity (100.1 μmol h⁻¹) for H₂ evolution is 9 times higher than that (11 μmol h⁻¹) of the product derived from a hydrothermal process, while the obtained AgIn₅S₈/TiO₂ heterojunction nanocomposites prepared by using commercially available TiO₂ nanoparticles (P25) as TiO₂ source exhibit remarkably improved photoactivity as compared to the pristine AgIn₅S₈, and the AgIn₅S₈/TiO₂ nanocomposite with molar ratio of 1:10 shows a maximum photocatalytic H₂ evolution rate (371.1 μmol h⁻¹), which is 4.3 times higher than that (85 μmol h⁻¹) of the corresponding AgIn₅S₈/TiO₂ nanocomposite derived from a hydrothermal method. This significant enhancement in the photocatativity of the present AgIn₅S₈/TiO₂ nanocomposite can be ascribed to the better dispersion of the AgIn₅S₈ formed on TiO₂ nanoparticle surfaces and the more intimate AgIn₅S₈/TiO₂ heterojunction structure during the water bath deposition process under continuously stirring as compared to the corresponding nanocomposite derived from a hydrothermal method. This configuration of nanocomposite results in fast diffusion of the photogenerated carriers in AgIn₅S₈ towards TiO₂, which is beneficial for separating spatially the photogenerated carriers and improving the photoactivity. The present findings shed light on the tuning strategy of spectral responsive region and photoactivity of photocatalysts for efficient light-to-energy conversion.     

Catalytic behavior of Ni/ZrxTi1−xO2 and the effect of SiO2 doping in oxidative steam reforming of n-butane

Catalytic behaviors of TiO₂-, Zr_0.5Ti_0.5O₂-, and ZrO₂-supported Ni catalysts were investigated for oxidative steam reforming of n-C₄H₁₀ at 723 K. The composite oxide support, Zr_0.5Ti_0.5O₂, shows high specific surface area (136 m²/g), leading to fine Ni particles. Thus, the Ni/Zr_0.5Ti_0.5O₂ catalyst exhibits higher and more stable activity than that exhibited by other catalysts. However, relatively large amounts of coke are deposited on the catalyst during reaction. Thus, to retard carbon deposition, the influence of SiO₂ additive was studied. Large amounts of SiO₂ additive (5 or 10 mol%) decrease initial activity; at 10 mol%, degradation is also induced by oxidation of Ni⁰. However, small amounts of SiO₂ additive (1.5 mol%) effectively retard coking without lowering initial activity. The resultant Ni/Zr_0.5Ti_0.5O₂–SiO₂ (1.5 mol%) catalyst exhibits high and stable activity without coking.     

The structural characterization of (NH4)2B10H10 and thermal decomposition studies of (NH4)2B10H10 and (NH4)2B12H12

The structure of (NH₄)₂B₁₀H₁₀ (1) was determined through powder XRD analysis. The thermal decomposition of 1 and (NH₄)₂B₁₂H₁₂ (2) was examined between 20 and 1000 °C using STMBMS methods. Between 200 and 400 °C a mixture of NH₃ and H₂ evolves from both compounds; above 400 °C only H₂ evolves. The dihydrogen bonding interaction in 1 is much stronger than that in 2. The stronger dihydrogen bond in 1 resulted in a significant reduction by up to 60 °C, but with a corresponding 25% decrease in the yield of H₂ in the lower temperature region and a doubling of the yield of NH₃. The decomposition of 1 follows a lower temperature exothermic reaction pathway that yields substantially more NH₃ than the higher temperature endothermic pathway of 2. Heating of 1 at 250 °C resulted in partial conversion of B₁₀H₁₀²⁻ to B₁₂H₁₂²⁻. Both 1 and 2 form an insoluble polymeric material after decomposition. The elements of the reaction network that control the release of H₂ from the B₁₀H₁₀²⁻ can be altered by conducting the experiment under conditions in which pressures of NH₃ and H₂ are either near, or away from, their equilibrium values.     

An experimental investigation of H2 emissions of a 2004 heavy-duty diesel engine supplemented with H2

Hydrogen (H₂) emissions characteristics of H₂-diesel dual fuel engine were measured using a 2004 turbocharged heavy-duty diesel engine with H₂ supplemented into the intake air. The emissions of H₂ were measured using an Electron Pulse Ionization (EPI) Mass Spectrometer (MS). The effect of the amount of H₂ added, the engine load, and diesel fuel flow rates on the emissions of H₂ and its combustion efficiency in the engine were investigated.     

Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor

Highly-ordered, vertically oriented TiO₂ nanotubes are synthesized, and their hydrogen sensing properties are investigated. Self-organized TiO₂ nanotube arrays are grown by anodic oxidation of a titanium foil in an aqueous solution that contains 1 wt% hydrofluoric acid at 20 °C. We use a potential ramp at a rate of 100 mV s⁻¹, increasing from the initial open-circuit potential (OCP) to 20 V, and this final potential of 20 V is then held constant during the anodization process. The fabricated TiO₂ nanotubes are approximately 1 μm in length and 90 nm in diameter. For the sensor measurements, two platinum pads are used as electrodes on the TiO₂ nanotube arrays. The hydrogen sensing characteristics of the sensor are analyzed by measuring the sensor responses ((I − I₀)/I₀) in the temperature interval of 20–150 °C. We find that the sensitivity of the sensor is approximately 20 for 1000 ppm H₂ exposure at room temperature, and increases with increasing temperature. The sensing mechanism of the TiO₂ nanotube sensor could be explained with chemisorption of H₂ on the highly active nanotube surface.     

Effect of SiO2 additives on the PEM fuel cell electrode performance

The Nafion membrane used as an electrolyte in the Polymer Electrolyte Membrane Fuel Cell (PEMFC) needs hydration to retain the proton conductivity. In PEMFC operation the reactant gas needs to be humidified either externally or internally. To reduce the cost, weight and complexities of the PEMFC system, it is beneficial to operate the PMEFC without humidification of the reactant gases because it eliminates the need for a complex gas-humidification sub-system. In recent years, worldwide R&D efforts have been made to remove the external humidifying unit from the PEMFC system by endowing the membrane electrode assembly (MEA) with self-humidifying ability. Efforts have been made to minimize humidification of the ionic polymer by introducing SiO₂ either in the catalyst layer or on the gas diffusion layer or on the membrane directly. In-house has made two silica powders, 1. Aerogel silica, surface area is 582 m²/g and 2. Silica powder with surface area of 45 m²/g is incorporated in the fuel cell electrode. This improves the hydrophilic and protonation properties of the SiO₂ powders when treated with H₂SO₄. Initial experiments under humidified conditions showed that the Silica powder, which was not treated with H₂SO_4, gave marginally lower performance compared to the H₂SO₄ treated sample. The polarization behaviors of the electrode with and without SiO₂ in the catalyst layer were studied. The PEMFC was also studied under different humidity conditions. The electrodes and the PEMFC were characterized by different electrochemical techniques like cyclic voltammetry and Electrochemical Impedance Spectroscopy (EIS). The results are presented in this paper.     

Hydrogen generation by hydrolysis of sodium borohydride on CoB/SiO2 catalyst

Generation of hydrogen by hydrolysis of alkali metal hydrides has attracted attention. Unsupported CoB catalyst demonstrated high activity for the catalytic hydrolysis of NaBH₄ solution. However, unsupported CoB nanoparticles were easy to aggregate and difficult to reuse. To overcome these drawbacks, CoB/SiO₂ was prepared and tested for this reaction. Cobalt (II) acetate precursor was loaded onto the SiO₂ support by incipient-wetness impregnation method. After drying at 100 °C, Co cations were deposited on the support. The dried sample was then dispersed in methanol/water solution and then fully reduced by NaBH₄ at room temperature. The catalyst was characterized by N₂ sorption, XRD and XPS. The results indicated that the CoB on SiO₂ possessed amorphous structure. B and Co existed both in elemental and oxidized states. SiO₂ not only affected the surface compositions of CoB, but also affected the electronic states of Co and B. B⁰ could donate partial electron to Co⁰. The structure effect caused by the SiO₂ support helped to prevent CoB nanocluster from aggregation and therefore the activity increased significantly on hydrolysis of alkaline NaBH₄ solution. The CoB/SiO₂ catalyst showed much higher activity than the unsupported CoB catalyst. At 298 K, the hydrogen generation rate on CoB/SiO₂ catalyst was 4 times more than that on the unsupported CoB catalyst. The hydrogen generation rate was as high as 10,586 mL min⁻¹ g⁻¹ catalyst at 298 K. CoB/SiO₂ is a very promising catalyst for this reaction.     

Stabilized zirconia-based sensor utilizing SnO2-based sensing electrode with an integrated Cr2O3 catalyst layer for sensitive and selective detection of hydrogen

A highly selective hydrogen (H₂) sensor has been successfully developed by using an yttria-stabilized zirconia (YSZ)-based mixed-potential-type sensor utilizing SnO₂ (+30 wt.% YSZ) sensing electrode (SE) with an intermediate Al₂O₃ barrier layer which was coated with a catalyst layer of Cr₂O₃. The sensor utilizing SnO₂ (+30 wt.% YSZ)-SE was found to be capable of detecting H₂ and propene (C₃H₆) sensitively at 550 °C. In order to enhance the selectivity towards H₂, a selective C₃H₆ oxidation catalyst was employed to minimize unwanted responses caused by interfering gases. Among the examined metal oxides, Cr₂O₃ facilitated the selective oxidation of C₃H₆. However, the addition or lamination of Cr₂O₃ to SnO₂ (+30 wt.% YSZ)-SE was found to diminish the sensing responses to all examined gases. Therefore, an intermediate layer of Al₂O₃ was sandwiched between the SE layer and the catalyst layer to prevent the penetration of Cr₂O₃ particles into the SE layer. The sensor using SnO₂ (+30 wt.% YSZ)-SE coated with a catalyst layer of Cr₂O₃ as well as an intermediate layer of Al₂O₃ exhibited a sensitive response toward H₂, with only minor responses toward other examined gases at 550 °C under humid conditions (21 vol.% O₂ and 1.35 vol.% H₂O in N₂ balance). A linear relationship was observed between sensitivity and H₂ concentration in the range of 20–800 ppm on a logarithmic scale. The results of sensing performance evaluation and polarization curve measurements indicate that the sensing mechanism is based on the mixed-potential model.     

Nafion/SiO2 hybrid membrane for vanadium redox flow battery

Sol–gel derived Nafion/SiO₂ hybrid membrane is prepared and employed as the separator for vanadium redox flow battery (VRB) to evaluate the vanadium ions permeability and cell performance. Nafion/SiO₂ hybrid membrane shows nearly the same ion exchange capacity (IEC) and proton conductivity as pristine Nafion 117 membrane. ICP-AES analysis reveals that Nafion/SiO₂ hybrid membrane exhibits dramatically lower vanadium ions permeability compared with Nafion membrane. The VRB with Nafion/SiO₂ hybrid membrane presents a higher coulombic and energy efficiencies over the entire range of current densities (10–80 mA cm⁻²), especially at relative lower current densities (<30 mA cm⁻²), and a lower self-discharge rate compared with the Nafion system. The performance of VRB with Nafion/SiO₂ hybrid membrane can be maintained after more than 100 cycles at a charge–discharge current density of 60 mA cm⁻². The experimental results suggest that the Nafion/SiO₂ hybrid membrane approach is a promising strategy to overcome the vanadium ions crossover in VRB.     

An improved H2/O2 mechanism based on recent shock tube/laser absorption measurements

An updated H₂/O₂ reaction mechanism is presented that incorporates recent reaction rate determinations in shock tubes from our laboratory. These experiments used UV and IR laser absorption to monitor species time-histories and have resulted in improved high-temperature rate constants for the following reactions: H+O₂=OH+OH₂O₂(+M)=2OH(+M)OH+H₂O₂=HO₂+H₂OO₂+H₂O=OH+HO₂ The updated mechanism also takes advantage of the results of other recent rate coefficient studies, and incorporates the most current thermochemical data for OH and HO₂. The mechanism is tested (and its performance compared to that of other H₂/O₂ mechanisms) against recently reported OH and H₂O concentration time-histories in various H₂/O₂ systems, such as H₂ oxidation, H₂O₂ decomposition, and shock-heated H₂O/O₂ mixtures. In addition, the mechanism is validated against a wide range of standard H₂/O₂ kinetic targets, including ignition delay times, flow reactor species time-histories, laminar flame speeds, and burner-stabilized flame structures. This validation indicates that the updated mechanism should perform reliably over a range of reactant concentrations, stoichiometries, pressures, and temperatures from 950 to greater than 3000 K.