Decolourization of reactive dyes by thin film immobilized surface photoreactor using solar irradiation

The photocatalytic degradation of four reactive dyes using TiO₂ was investigated in suspended and immobilized systems under solar irradiation. Batch degradation experiments were carried out at initial concentrations ranging from 25 to 100 mg l⁻¹ and at a catalyst loading of 0.5–1 g l⁻¹. The studies on batch photocatalytic degradation of four dyes, showed about 30–70% colour removal depending on the initial dye concentration, dye structure (functional group and reactivity of dyes) and the amount of catalyst. The thin film immobilized surface photoreactor was able to give nearly 90–98% colour removal depending on the initial concentration and exposure time. Flow rate has noticeable effect on colour removal particularly at higher concentration (100 mg l⁻¹). High colour removals obtained with solar radiation indicated effectiveness of this process and its potential for practical application.     

Elimination of pesticides and their formulation products from drinking water using thin film continuous photoreactor under solar radiation

There is an urgent need for the development of inexpensive, but reliable and efficient photocatalyst which can work under solar radiation for drinking water application. Hence the treatment options to be tried out for drinking water contaminated with pesticides and their formulation products should be cost effective and affordable. In this study, we developed a cheap and efficient photocatalyst and continuous photoreactor for the removal of pesticides from drinking water under solar radiation. Continuous photodegradation experiments were carried out with synthetically prepared commercial grade methyl parathion (Folidon 50% E.C.), dichlorvos (DDVP 70% E.C.), and analytical grade lindane. Photodegradation of mixed pesticide was carried out using both Degussa P-25 TiO₂ and N-doped TiO₂ with identical mass concentrations (50 μg/L) of all the three pesticides under UV, visible and solar radiation. Continuous reactor was operated for more than 24 h (6 h each on 4 days) for mixed pesticide degradation. N-doped TiO₂ showed 100% degradation for all the three pesticide under solar radiation. Photodegradation of mixed pesticide showed methyl parathion, dichlorvos and lindane were degrading simultaneously. However, the rate of reaction was completely different from single pesticide degradation. N-doped TiO₂ showed higher photocatalytic activity under solar radiation compared to UV and visible light. GC–MS analysis of mixed pesticide degradation showed more than 16 peaks in the middle of the reaction. Among these peaks, three intermediates such as hexachloro-benzene and para-nitrophenol and dichlorovinyl-O-methyl phosphate were identified in the middle of the reaction. However, at the end of the reaction (reactor outlet) none of the intermediates were observed.     

Visible-light-assisted photocatalytic degradation of gaseous formaldehyde by parallel-plate reactor coated with Cr ion-implanted TiO2 thin film

Sol–gel nano titanium dioxide (TiO₂) thin film can be activated by the ultraviolet (UV) radiation available in sunlight to perform solar photocatalysis. The useful spectral range can be extended from UV to visible light by implantation of metal ion into the TiO₂ lattice. As a result, the solar visible light can be utilized more efficiently to enhance the solar photocatalysis. In this study, visible-light-assisted photocatalytic glass reactors were built by parallel borosilicate glass plates coated on the upper surfaces with sol–gel TiO₂ thin films implanted with chromium (Cr) ion. The properties of the Cr/TiO₂ thin films were fully characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravity (TG) analysis, scanning-electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. In the performance tests, a metal halide lamp was used as an external light source to resemble the solar visible spectral radiation. The performance of a Cr/TiO₂ photoreactor was measured in terms of its photocatalytic degradation of gaseous formaldehyde in a single pass of contaminated air flowing through the photoreactor. The experimental results demonstrated the promise of using light-transmitting glass substrate to allow transmission and distribution of light from an external source to achieve solar photocatalysis. In the design of a parallel-plate photoreactor, it is important to properly control the Cr ion loading so that each Cr/TiO₂-coated glass plate absorbs a portion of the incident light for its photocatalytic activation and allows light transmission available for the remaining coated plates.     

Solar photocatalytic degradation of a reactive azo dye in TiO2-suspension

The photocatalytic decolourisation and degradation of an azo dye reactive orange 4 (RO4) in aqueous solution with TiO₂-P25 (Degussa) as photocatalyst in slurry form have been investigated using solarlight. There is a significant difference in adsorption of dye on TiO₂ surface with the change in solution pH. The effect of various photocatalysts such as TiO₂-P25, TiO₂ (anatase), ZnO, CdS, Fe₂O₃, SnO₂ on the decolourisation and degradation has been studied. The order of reactivity of photocatalysts is TiO₂-P25>ZnO>TiO₂ (anatase). CdS, Fe₂O₃ and SnO₂ have negligible activity on RO4 decolourisation and degradation. The effects of various parameters such as catalyst loading, pH and initial concentration of the dye on decolourisation and degradation have been determined. The degradation was strongly enhanced in the presence of electron acceptors such as H₂O₂, (NH₄)₂S₂O₈ and KBrO₃. The effects of dye-assisting chemicals such as Na₂CO₃, NaCl have been carried out. Addition of these chemicals inhibits the removal rate. The photodecolourisation and degradation kinetics are discussed in terms of Langmuir–Hinshelwood kinetic model.     

Solar photocatalytic degradation of gaseous formaldehyde by sol–gel TiO2 thin film for enhancement of indoor air quality

Solar photocatalytic degradation of formaldehyde in the gaseous phase has been investigated. The tested photoreactor is made of a borosilicate glass tube with the inner surface coated with a sol–gel TiO₂ thin film. In a pseudo-first-order Langmuir–Hinshelwood (L–H) model, the maximum reaction rate constant obtained is 0.148 min⁻¹ under an exposure to sunlight with solar UVA irradiance of 1.56 mW/cm². The solar photolysis effect is found to be negligible. It is also found that the sol–gel TiO₂ thin film has a lower apparent photonic efficiency of solar photocatalysis than a Degussa P25 TiO₂ coating. However, for the photonic efficiency taking into account the absorbed and scattered photons only and, in other words, excluding the transmitted photons, the thin film has a higher value. Based on a total of 28 measured data, an empirical-correlation equation has been developed to express the reactant residue with respect to the solar UVA irradiance and exposure time. A reasonable agreement between the correlation and experimental data is obtained. The findings of this investigation can be applied to design optimization of a honeycomb photoreactor made up of TiO₂-coated glass tubes or polygonal cells.     

基于平行流铝扁管吸附床传热性能的模拟研究

吸附床是吸附式制冷系统的关键部件。吸附床的换热能力对吸附式制冷系统的各项性能有显著影响。文章针对应用于吸附床的传统换热器和扁管换热器的不足之处,设计出一种新型平行流铝扁管吸附床,并建立了该吸附床的二维传热模型,以温度随时间的变化情况为分析指标,分析翅片的间距、高度、厚度,以及吸附剂体积分数等因素对吸附床传热性能的影响,从而优化调整吸附床的结构,提高其换热性能。分析结果表明:当翅片高度约为70 mm时,吸附床的换热能力达到峰值;当翅片厚度大于1.5 mm时,翅片厚度的增加对吸附床传热性能的影响比较微弱;当吸附剂体积分数由0.25逐渐增大至0.45时,吸附剂的等效传热系数约增加了50%。     

Fabrication of thin film nanocrystalline silicon solar cell with low light-induced degradation

Nanocrystalline silicon thin films have been deposited at different total gas flow rates and plasma excitation frequencies and samples with similar crystalline volume fraction have been compared. In hydrogenated nanocrystalline silicon solar cells, amorphous component is not necessarily the only determining factor for light-induced degradation. Smaller grain size less than 3 nm diameter and intermediate range order provide a better stability in the i-layer near the p/i interface, thus improving the overall stability of the solar cell. Light-induced degradation (LID) of efficiency of the cell mainly depends on the light-induced degradation of short-circuit current density and light-induced degradation of fill factor (FF). Degradation of open-circuit voltage is less than 1%. Minimum degradation of efficiency obtained in this work is 2%.     

Hydrogen generation from photocatalytic water splitting over TiO2 thin film prepared by electron beam-induced deposition

Photocatalytic TiO₂ thin films were prepared via an electron beam-induced deposition (EBID) method. The effects of post-calcination treatment on the properties of the prepared TiO₂ thin films were studied. X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometry (SEM-EDS), and UV–V is absorption spectrometry were performed to reveal the crystallinity, surface morphology, chemical composition, and light absorbance of the prepared TiO₂ thin films. The photoelectrochemical characteristics of the TiO₂ thin films were investigated with a potentiostat. Under UV irradiation, a photocurrent of ˜2.1 mA was observed for the TiO₂ thin film with post-calcination at 500 °C. A water-splitting reaction was conducted over the TiO₂ thin film with the best photoelectrochemical performance. The yields of hydrogen and oxygen were 59.8 and 30.6 μmole, respectively, after 8 h of reaction under UV irradiation.     

Dynamic behavior of surface film on LiCoO2 thin film electrode

Electrochemical oxidation behavior of non-aqueous electrolytes on LiCoO₂ thin film electrodes were investigated by in situ polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy (XPS). LiCoO₂ thin film electrode on gold substrate was prepared by rf-sputtering method. In situ PM-FTIR spectra were obtained at various electrode potentials during cyclic voltammetry measurement between 3.5 V vs. Li/Li⁺ and 4.2 V vs. Li/Li⁺. During anodic polarization, oxidation of non-aqueous electrolyte was observed, and oxidized products remained on the electrode at the potential higher than 3.75 V vs. Li/Li⁺ as a surface film. During cathodic polarization, the stripping of the surface film was observed at the potential lower than 3.9 V vs. Li/Li⁺. Depth profile of XPS also showed that more organic surface film remained on charged LiCoO₂ than that on discharged one. AFM images of charged and discharged electrodes showed that some decomposed products deposited on charged electrode and disappeared from the surface of discharged one. These results indicate that the surface film on LiCoO₂ is not so stable.     

Hydrogen production by photocatalytic water-splitting using Cr- or Fe-doped TiO2 composite thin films photocatalyst

Cr- or Fe-ion-doped TiO₂ thin films have been synthesized by radio-frequency magnetron sputtering and a sol–gel method to study hydrogen generation by photocatalytic water-splitting under visible light irradiation. The doping method, dopant concentration, charge transfer from metal dopants to TiO₂, and type of dopants used for modification of TiO₂ were investigated for their ability to enhance photocatalytic activity. UV–Visible spectra show that the metal-doped-TiO₂ obtained by sputtering is much more efficient than that obtained by the sol–gel technique at inducing a red shift of the absorption edge in the visible light range. Low concentration metal ion doping must be done near the conducting indium tin oxide (ITO) – TiO₂ interface to avoid the formation of recombination centers for photo-generated electron–hole pairs. H₂ production rate (μmol/h) is higher for Fe-doped TiO₂ (15.5 μmol/h) than for Cr-doped TiO₂ (5.3 μmol/h) due to the ability of Fe ions to trap both electrons and holes, thus avoiding recombination, while Cr can only trap one type of charge carrier. A constant H₂ generation rate is obtained for long periods of time by all the investigated TiO₂ films because of the separate evolution of H₂ and O₂ gases, thus eliminating the back-reaction effect.     

A comparative study of tartrazine degradation using UV and solar fixed bed reactors

The fixed bed reactor was combined with a solar photoreactor and UV lamp reactor. This hybrid photoreactor used a heterogeneous photocatalysis process (TiO₂/UV) as a tertiary treatment for the degradation of tartrazine dye in water. The solar reactor removed almost all organic compounds from the wastewater. Photocatalysis was optimized using a parametric study to improve the influence of different parameters on the degradation efficiency. Color removals when using solar, UV lamp and hybrid reactors present 99%, 30%, and 99%, respectively. Tartrazine removal kinetics followed a pseudo-first order model. The hybrid solar and UV lamp system combination was a feasibility choice for removing both dyes from wastewater.     

Multi-electron storage of photoenergy using Cu2O–TiO2 thin film photocatalyst

UV–vis irradiation of thin films of TiO₂ (ITO/TiO₂) and Cu₂O/TiO₂ (ITO/Cu₂O/TiO₂) coated on conducting glasses generate H₂ from H₂O and once the illumination is ceased, the H₂ production was still noticeable under dark for ITO/Cu₂O/TiO₂ at a lesser production rate for up to 2 h. No such dark reactions were observed for ITO/TiO₂ or TiO₂-coated copper metal foil (Cu/TiO₂). It was noticed that the irradiation of ITO/Cu₂O/TiO₂ leads to formation of trapped electrons and this stored energy leads to generate H₂ from H₂O in the dark.     

Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2

The photocatalytic activity of commercial ZnO powder has been investigated and compared with that of Degussa P25 TiO₂. Laboratory experiments with acid brown 14 as the model pollutant have been carried out to evaluate the performance of both ZnO and TiO₂ catalysts. Solar light was used as the energy source for the photocatalytic experiments. These catalysts were examined for surface area, particle size and crystallinity. The effect of initial dye concentration, catalyst loading, irradiation time, pH, adsorption of acid brown 14 on ZnO and TiO₂, intensity of light and comparison of photocatalytic activity with different commercial catalysts were studied. The progress of photocatalytic degradation of the acid brown 14 has been observed by monitoring the change in substrate concentration of the model compound employing HPLC and measuring the absorbance in UV–Visible spectrophotometer for decolourisation. The photodegradation rate was determined for each experiment and the highest values were observed for ZnO suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta than TiO₂. The complete mineralisation was confirmed by total organic carbon (TOC) analysis, COD measurement and estimation of the formation of inorganic ions such as NH₄⁺, NO₃⁻, Cl⁻ and SO₄²⁻.     

Development of high efficiency large area silicon thin film modules using VHF-PECVD

This paper reviews recent work on the development of thin film silicon solar modules and cost-effective production technology. Noting the potential of VHF-PECVD for high rate and high quality deposition, we initiated development of a-Si solar modules. In the first stage, we succeeded in up-scaling a-Si high quality uniform deposition at a high rate of over 1.0 nm/s to a substrate area of 1.1 × 1.4 m² to achieve high productivity. Next, the large area a-Si solar modules with stable aperture efficiency of 8% were developed, and the commercial production of a-Si solar modules commenced in October 2002. In the second stage, aiming at stable efficiency of 12%, which could make the PV power generating cost below residential electricity prices in combination with cost-effective production technology, we have been developing a-Si/μc-Si tandem solar modules. Recently, tandem modules of 40 × 50 cm² in size with a μc-Si i-layer prepared at a deposition rate of 2.1 nm/s yielded initial conversion efficiencies of 11.1%. As for small sized μc-Si single cells, technologies with a high deposition rate of 2.5 nm/s and efficiency of 8.8% have already been developed. In addition, by improving the up-scaling and light-trapping techniques, we will achieve our current goal of 12% stable efficiency for a-Si/μc-Si tandem modules at a deposition rate of over 2.0 nm/s, leading to cost-effective mass production.     

Deposition of thin film silicon using the pulsed PECVD and HWCVD techniques

We report on the use of pulsed plasma-enhanced chemical vapor deposition (P-PECVD) technique and show that “state-of-the-art” amorphous silicon (a-Si:H) materials and solar cells can be produced at a deposition rate of up to 15 Å/s using a modulation frequency in the range 1–100 kHz. The approach has also been developed to deposit materials and devices onto large area, 30 cm×40 cm, substrates with thickness uniformity (<5%), and gas utilization rate (>25%). We have developed a new “hot wire” chemical vapor deposition (HWCVD) method and report that our new filament material, graphite, has so far shown no appreciable degradation even after deposition of 500 μm of amorphous silicon. We report that this technique can produce “state-of-the-art” a-Si:H and that a solar cell of p/i/n configuration exhibited an initial efficiency approaching 9%. The use of microcrystalline silicon (μc-Si) materials to produce low-cost stable solar cells is gaining considerable attention. We show that both of these techniques can produce thin film μc-Si, dependent on process conditions, with 1 1 1 and/or 2 2 0 orientations and with a grain size of approx. 500 A. Inclusion of these types of materials into a solar cell configuration will be discussed.     

Superstrate-type CuInSe2-based thin film solar cells by a low-temperature process using sodium compounds

Superstrate-type solar cells with a Au/CuInSe₂(CIS)/In_xSe_y,/ZnO : Al/glass structure were investigated. The CIS films were deposited by coevaporation method with intentionally incorporated Na₂S at a substrate temperature of 350°C. Even at relatively low substrate temperatures, sodium compounds enhanced the (1 1 2) preferred orientation of the chalcopyrite structure, and also improved the cell performance. The In_xSe_y buffer layers disappeared after CIS deposition by interdiffusion. Preliminary cells yielded an efficiency of 7.5% with V_oc, = 430 mV, J_sc = 29.4 mA/cm² and FF = 0.60. The light soaking and forward bias effects were observed for these cells.     

Numerical prediction of film cooling effectiveness over flat plate using variable turbulent prandtl number closures

Numerical simulations were performed to predict the film cooling effectiveness on the fiat plate with a three- dimensional discrete-hole film cooling arrangement.The effects of basic geometrical characteristics of the holes,i.e.diameter D,length L and pitch S/D were studied.Different turbulent heat transfer models based on constant and variable turbulent Prandtl number approaches were considered.The variabiUty of the turbulent Prandtl number Pr_t in the energy equation was assumed using an algebraic relation proposed by Kays and Crawford,or employing the Abe,Kondoh and Nagano eddy heat diffusivity closure with two differential transport equations for the temperature variance kg and its destruction rate ε_θ.The obtained numerical results were directly compared with the data that came from an experiment based on Transient Liquid Crystal methodology.All implemented models for turbulent heat transfer performed sufficiently well for the considered case.It was confirmed,however,that the two- equation closure can give a detailed look into film cooling problems without using any time-consuming and inherently unsteady models.     

Development of thin film solar cell based on Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (hereafter CZTS) thin films were successfully formed by vapor-phase sulfurization of precursors on a soda lime glass substrate (hereafter SLG) and a Mo-coated one (hereafter Mo-SLG). From the optical properties, we estimate the band-gap energy of this thin film as 1.45–1.6 eV which is quite close to the optimum value for a solar cell. By using this thin film as an absorber layer, we could fabricate a new type of thin film solar cell, which was composed of Al/ZnO:Al/CdS/CZTS/Mo-SLG. The best conversion efficiency achieved in our study was 2.62% and the highest open-circuit voltage was 735 mV. These device results are the best reported so far for CZTS.     

Effect of gas flow rates on PECVD-deposited nanocrystalline silicon thin film and solar cell properties

Nanocrystalline silicon films have been deposited at a plasma excitation frequency of 54.24 MHz by varying the flow rates of SiH₄+H₂ mixture in the reaction chamber. It has been found that with increase in gas flow rate from 100 to 300 sccm the defect density, microstructural defect fraction and the crystalline volume fraction in the film decrease. Films deposited at optimum total gas flow rate of 200 sccm with comparable crystalline volume fraction have shown better structural and optoelectronic properties compared to the films deposited at 100 sccm total gas flow rate for application in solar cell. Solar cells have been fabricated using these layers as absorber layers and the maximum cell efficiency obtained is 6.2% (AM1.5, 28 °C) at 200 sccm total gas flow rate. It has been found that material prepared using higher total gas flow rate of 200 sccm together with higher hydrogen dilution is better suited for solar cell application.     

CuInSe2 thin film preparation through a new selenisation process using chemical bath deposited selenium

Copper indium selenide thin films were prepared through a novel and an eco-friendly selenisation process. In this method, selenium film required for selenisation was prepared using chemical bath deposition technique, at room temperature. Thus, totally avoided usage of highly toxic H₂Se or selenium vapour. Here, the process involved annealing the Stacked layer, Se/In/Cu in which Cu and In were deposited using vacuum evaporation technique. Investigations on the solid-state reaction between the layers were done by analysing structural and optical properties of films formed at different annealing temperatures. Optimum annealing condition for the formation of copper indium selenide thin film was found to be 673 K for 1 h in high vacuum. Compositional dependence of the growth process was also studied using various Cu/In ratios. Optical band gap was decreased with increase in Cu/In ratio. Carrier concentration and hence conductivity were found to be increased with increase in Cu/In ratio. The films obtained were p-type and highly Cu-rich films were degenerate.