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⁻.     

An effective use of nanocrystalline CdO thin films in dye-sensitized solar cells

Thin films of cadmium oxide (CdO) were synthesized by layer-by-layer deposition method on indium doped tin oxide (ITO) substrates. Post-deposition annealing at 250 °C for 24 h produced pure phase CdO films by removal of trace amount of cadmium hydroxide, as confirmed from X-ray diffractogram. First time employment of CdO in place of TiO₂ in dye-sensitized solar cells is reported to check feasibility and cell performance. A dye-sensitized nanocrystalline CdO photo-electrode was obtained by adsorbing cis-dithiocyanato (4,4′-dicarboxylic acid-2,2′-bipyridide) ruthenium (II) (N3) dye by keeping at 45 °C for 20 h. The efficiency of dye-sensitized nanocrystalline CdO thin film solar cell was increased from 0.24% to 2.95% due to dye adsorption. This must be highest reported conversion efficiency for other metal oxides than TiO₂based dye-sensitized solar cells.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells

Dye-sensitized solar cells based on nanoporous oxide semiconductor thin films such as TiO₂, Nb₂O₅, ZnO, SnO₂, and In₂O₃ with mercurochrome as the sensitizer were investigated. Photovoltaic performance of the solar cell depended remarkably on the semiconductor materials. Mercurochrome can convert visible light in the range of 400–600 nm to electrons. A high incident photon-to-current efficiency (IPCE), 69%, was obtained at 510 nm for a mercurochrome-sensitized ZnO solar cell with an I⁻/I₃⁻ redox electrolyte. The solar energy conversion efficiency under AM1.5 (99 mW cm⁻²) reached 2.5% with a short-circuit photocurrent density (J_sc) of 7.44 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.52 V, and a fill factor (ff) of 0.64. The J_sc for the cell increased with increasing thickness of semiconductor thin films due to increasing amount of dye, while the V_oc decreased due to increasing of loss of injected electrons due to recombination and the rate constant for reverse reaction. Dependence of photovoltaic performance of mercurochrome-sensitized solar cells on semiconductor particles, light intensity, and irradiation time were also investigated. High performance of mercurochrome-sensitized ZnO solar cells indicate that the combination of dye and semiconductor is very important for highly efficient dye-sensitized solar cells and mercurochrome is one of the best sensitizers for nanoporous ZnO photoelectrode. In addition, a possibility of organic dye-sensitized oxide semiconductor solar cells has been proposed as well as one using metal complexes.     

Generation of inhomogeneous photocurrent in solid-state TiO2dyeCuI cells and effect of ligands attached to surfactant on morphology of CuI films

Microwave conductivity is observed in TiO₂ and CuI films at room temperature. Organic dye pyrogallor red is found to sensitize TiO₂ and CuI in solid-state TiO₂dyeCuI cells. Effect of ligands attached to crystal growth inhibitors on morphology of CuI films is studied. A weaker crystal growth inhibition was observed when size of ligands attached to surfactant is increased. Reasonable explanations are given for observed inhomogeneous photocurrent and its degradation under illumination.     

Improved performance of a dye-sensitized solar cell using a TiO2/ZnO/Eosin Y electrode

TiO₂/ZnO/Eosin Y structure films were prepared by a one-step cathodic electrodeposition method and used as a photoanode in a dye-sensitized solar cell (DSSC). Using this TiO₂/ZnO/Eosin Y electrode in DSSC, the degradation of the cell with time was reduced and I_SC, V_OC and fill factor values were increased. The use of a thin ZnO layer, permitted the formation of an energy barrier at the electrode/electrolyte interface, thus reducing recombination rate and improving cell performance. In addition, the adsorbed dye molecules prepared by one-step cathodic electrodeposition with ZnO were very stable compared with that prepared by conventional immersing method, as evidenced by UV/vis absorption spectroscopy measurements.     

Photocatalyzed N-demethylation and degradation of methylene blue in titania dispersions exposed to concentrated sunlight

S imulated wastewaters that con tain methylene blue (MB) were bleached in a photocatalytic aqueous TiO₂ dispersion illuminated by concentrated sunlight using a parabolic round concentrator reactor (PRCR). The kinetic analysis was carried out well when the temporal concentration variation was a function of the concentrated light energy irradiated. The p hotocatalyzed N-demethylation of MB takes place concurrently with photocatalytic decomposition of MB by pseudo- first-order kinetics. The dependence of the photo- decomposition kinetics on the initial concentration of MB is consistent with the Langmuir– Hinshelwood model. Elimination of TOC (total organic carbon) also occurs by pseudo- first-order kinetics prior to full bleaching of the aqueous TiO₂ dispersion, after which the TOC level decreases only slightly. Also, compared with the open to air, the photodegradation of MB is not influenced by molecular oxygen bubbl ing continuously through the reactant suspensions during illumination . TiO₂ loadings and flow rates markedly a ffect the degradation o f MB. Under concentrated sunlight, t he relative photonic efficiency of MB photodegradation i s ζ_rel=0.49 (relative to phenol) . T he efficiency ζ_rel for the degradation of MB is independent of photoreactor geometry (cylindrical bottle reactor versus round-bot tomed flask), of light sources (solar light concentrator versus a Hg lamp) and of the operating mode used (flow versus batch operation).     

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.     

Investigations on ZnxCd1−xO thin films obtained by spray pyrolysis

Zn_xCd_1−xO thin films were prepared on glass substrates by spray pyrolysis technique. The precursor solutions were obtained by varying the concentration of Zn(NO₃)₂·6H₂O and Cd(NO₃)₂·4H₂O in bi-distilled water. The structural properties have been studied using X-ray diffraction spectra. All the structures include the basic compounds, i.e. ZnO and CdO. The orientation and the crystalline phases of the deposited films were specified. With the addition of Zn to the precursor solution, we can observe the preferential orientation of the CdO in the [2 0 0] direction. The electrical measurements were performed using method of four contacts. Thin films transmittances, in the 1.5–4.3 eV range, for different compositions have been measured and the optical gaps have been determined. The variations are explained considering the gaps of the two pure films. The influence of increased Cd concentration in the films on the structural, electrical and optical properties is investigated in this study.     

Assessment and influence of operational parameters on the TiO2 photocatalytic degradation of sodium benzene sulfonate under highly concentrated solar light illumination

Sodium benzene sulfonate (BS) was decomposed in aqueous TiO₂ dispersions under highly concentrated solar light illumination to examine the photocatalytic characteristics of a parabolic round concentrator (PRC) reactor to degrade the pollutant without visible light absorption. The effects of such operational parameters as initial concentration, volume of the aqueous BS solution, oxygen purging, and TiO₂ loading on the kinetics of decomposition of BS were investigated. An effective photodegradation necessitates a suitable combination of initial volume and concentration of BS solution. Relative to atmospheric air, oxygen purging significantly accelerates the degradation process at high initial concentrations of BS (0.40 mM or 1.0 mM). Optimal TiO₂ loading was 9 g l⁻¹, greater than previously reported. Elimination of TOC (total organic carbon) followed pseudo first-order kinetics in the initial stages of the photodegradation process. The relative photonic efficiency for the photodegradation of BS is ζ_rel=1.0.     

Photocatalytic and electronic properties of TiO2 powders elaborated by sol–gel route and supercritical drying

Nanosized TiO₂ powders for photocatalytic degradation of organic pollutants are prepared by sol–gel method. The –Ti–O–Ti– network is synthesized by hydrolysis and controlled condensation of titanium isopropoxide Ti(O–iC₃H₇)₄. The resulting alcogels are dried under air to form xerogels or under supercritical CO₂ to form aerogels. In both cases, drying is followed by thermal treatment under air and gives crystallized white and dry powders.These powders are characterized by XRD, SEM, and specific surface area measurements. Their electronic properties are determined through Time Resolved Microwave Conductivity. Their photocatalytic activities are tested in the photodegradation of phenol in water.Results establish a strong correlation between synthesis, structure, charge-carrier lifetimes and photocatalytic activity. The influence of the supercritical drying on the final properties of materials is also evidenced.     

A comparative study on photoelectrochemical activity of ZnO/TiO2 and TiO2/ZnO nanolayer systems under visible irradiation

TiO₂/ZnO and ZnO/TiO₂ nanolayer thin films were synthesized using sol-gel method. Optical analysis revealed high transmittance of the films in the visible range with almost the same bandgap energy for the both systems. XPS technique shows stoichiometric formation of TiO₂ and ZnO on the surface of TiO₂/ZnO and ZnO/TiO₂ layers, respectively. According to AFM observations and its data analysis, the TiO₂/ZnO films exhibited a higher surface roughness and more effective interfaces with electrolyte during redox reactions. Based on photoelectrochemical measurements, TiO₂/ZnO nanolayer photoanode possesses a lower charge transfer resistance and higher transient time for charge carriers (e⁻ and h⁺) and hence a higher photocurrent density under visible light irradiation as compared with the ZnO/TiO₂ nanolayer system.     

Characterization by resonance Raman spectroscopy of sol–gel TiO2 films sensitized by the Ru(PPh3)2(dcbipy)Cl2 complex for solar cells application

The development of TiO₂ photoelectrochemical cells is largely dependent on the elaboration of efficient dyes molecules to sensitize the nanocrystalline titania surfaces. In the present work, a new system functioning on this basis is proposed: an Ru(II) complex bearing two triphenylphosphine (PPh₃) and one 2,2′-bipyridyl-4,4′-dicarboxylate (dcbipy) chromophoric ligands, deposited on TiO₂ prepared through a simple sol–gel process is tested in a wet regenerative photoelectrochemical cell. The grafting of the complex on TiO₂ surface is characterized using Raman spectroscopy. The measurement of the full-width at half-maximum (FWHM) of the TiO₂ lower frequency E_g Raman line allowed to compare the crystallite sizes of the sol–gel films (13–14 nm) with those of nanocrystalline anatase films (7–8 nm). Very intense Raman bands were observed (ex situ and in situ) for the Ru(PPh₃)₂(dcbipy)Cl₂ complex chemisorbed on TiO₂. The most important vibrations were unambiguously assigned to the PPh₃ or to dcbipy ligands. Altering the potential applied to the electrode for a given laser excitation energy can selectively enhance the vibrational modes of PPh₃. A reversible shift of the dcbipy Raman lines was also observed. The electronic absorption spectrum and the electrochemical data are taken into account in order to explain the obtained results for the chemisorbed dye on both nanocrystalline and sol–gel films.     

Patterns of efficiency and degradation in dye sensitization solar cells measured with imaging techniques

A larger number of dye sensitization solar cells based on cis-Ru^II(LH₂)₂(NCS)₂ with LH₂=2,2′-bipyridyl-4,4′-dicarboxylic acid with an electrolyte consisting of 0.5 M LiI, 50 mM I₂, 0.2 M tert.-butyl pyridine in acetonitrile have been studied, using spatially resolved photocurrent imaging techniques. Measurements have been made after preparation and periodically during a longer period of simulated solar light illumination. The observed phenomena have been grouped into five categories. The first one concerns significant inhomogeneities reflecting the TiO₂-layer preparation technique used. The second category concerns an inhomogeneous deterioration of the dye sensitization cell during illumination. The third phenomenon involves photodegradation itself, which can be visualized by selectively illuminating the dye sensitization solar cell. Changes observed in the composition of the electrolyte, typically indicated by a bleaching of the iodide/iodine solution were also observed. Finally, the fifth category to be considered deals with a loss of electrolyte and the parallel appearance of gas bubbles in the solar cell. All these phenomena may coexist, being responsible for the overall process of degradation. The different mechanisms are discussed and analyzed in an effort to determine parameters critical for increasing efficiency and stability of dye sensitization solar cells.     

Effect of surface modification of TiO2 on the photovoltaic performance of the quasi solid state dye sensitized solar cells using a benzothiadiazole-based dye

We report the preparation of nanoporous TiO₂ electrode modified with an insulating material—BaCO₃ and used as electrode for quasi solid state dye sensitized solar cells (DSSCs), with a benzothiadiazole-based dye (BTDR2) as sensitizer and PEDOT:PSS coated FTO as counter electrode. We found that the BaCO₃ modification significantly increases the dye adsorption, resulting from the fact that the surface of BaCO₃ is more basic than TiO₂. The performance of the DSSCs with and without BaCO₃ modified TiO₂ electrodes were analyzed by cyclic voltammograms, optical absorption spectra, current-voltage characteristics in dark and under illumination and electrochemical impedance spectra. The photovoltaic performance has been significantly improved for the BaCO₃ modified electrode as compared to bare TiO₂ electrode having the same other components in the DSSCs. The value of the overall power conversion efficiency (η) improves from 2.42% to 4.38% under illumination intensity, when BaCO₃ modified electrode is used instead of bare TiO₂ electrode. The improvement in η has been attributed to the increased dye adsorption, reduction in recombination rate and enhancement in the electron lifetime when the modified TiO₂ electrode is used.     

Anchorage of N3 dye-linked polyacrylic acid to TiO2/electrolyte interface for improvement in the performance of a dye-sensitized solar cell

A synthetic route was developed to link N3 dye to polyacrylic acid (PAA) using ethylenediamine (en) as the linker. The resulting complex, PAA–en–N3, was then coated onto a TiO₂ film. The modified TiO₂ film electrode (hereafter PAA–en–N3/TiO₂), when used as the photoanode in a dye-sensitized solar cell (DSSC), exhibited enhanced solar energy conversion efficiency compared with that of the usual DSSC with the N3/TiO₂ film electrode. The increase in efficiency was attributed to the increased open-circuit voltage (V_oc) and short-circuit photocurrent (J_sc). The increase in V_oc was attributed to the formation of a hydrophobic PAA–en–N3 layer on the TiO₂/electrolyte interface, while the increase in J_sc was attributed to the additional dye acquired by the TiO₂ film from the PAA–en–N3 complex.     

Effect of loaded silver nanoparticles on TiO2 for photocatalytic degradation of Acid Red 88

Nanometer sized Ag–TiO₂ nanoparticles were prepared (by photoreduction of Ag⁺ ions) in order to assess its photocatalytic degradation ability of target pollutant (textile dye; Acid red 88) upon visible light irradiation. Furthermore, oxidative reagents such as peroxomonosulfate (PMS), peroxodisulfate (PDS) and hydrogen peroxide (H₂O₂) were added to the photocatalytic system, which may act as an alternative electron acceptor and result in a notably enhanced degradation rate (seven-fold increase with PMS as oxidant) of pollutant destruction. Mineralization of target pollutant was also performed by total organic carbon (TOC) analysis and from the results, it was confirmed that 65% mineralization was achieved in 7 h using PMS as electron acceptor. Overall, this system is relatively inexpensive, reproducible, extremely stable and efficient in complete degradation of dye in aqueous solution. In order to obtain maximum information about the performance of Ag–TiO₂ photocatalyst, we did experiments under different operating conditions, i.e., variation of amount of catalyst, concentration of dye and electron acceptors. In addition to the above, a comparative study on the photocatalytic activities of TiO₂ was also made.     

Al2O3-coated nanoporous TiO2 electrode for solid-state dye-sensitized solar cell

This paper reports the preparation of a core-shell nanoporous electrode consisting of an inner TiO₂ porous matrix and a thin overlayer of Al₂O₃, and its application for solid-state dye-sensitized solar cell using p-CuI as hole conductor. Al₂O₃ overlayer was coated onto TiO₂ porous film by the surface sol–gel process. The role of Al₂O₃ layer thickness on the cell performance was investigated. The solar cells fabricated from Al₂O₃-coated electrodes showed superior performance to the bare TiO₂ electrode. Under illumination of AM 1.5 simulated sunlight (89 mW/cm²), a ca. 0.19 nm Al₂O₃ overlayer increased the photo-to-electric conversion efficiency from 1.94% to 2.59%.     

Combination effect of ZnO and activated carbon for solar assisted photocatalytic degradation of Direct Blue 53

The degradation efficiency of AC–ZnO composite photocatalyst was evaluated with solar light using an azo dye Direct blue 53 (DB53) at room temperature. Activity measurements performed under solar radiation have shown good results for the photodegradation of DB53. The synergistic effect observed was ascribed to an extended adsorption of DB53 on activated carbon followed by its transfer to ZnO where it was photocatalytically degraded. The enhanced photocatalytic activity of AC–ZnO when compared to ZnO is found to be due to this synergistic effect. A study on the effects of various parameters like concentration of dye, amount of catalyst and initial pH on the photodegradation of DB53 has been carried out to find optimum conditions.     

Improvement of a-Si solar cell properties by using SnO2:F TCO films coated with an ultra-thin TiO2 layer prepared by APCVD

TiO₂-overcoated SnO₂:F transparent conductive oxide films were prepared by atmospheric pressure chemical vapor deposition (APCVD) and an effect of TiO₂ layer thickness on a-Si solar cell properties was investigated. The optical properties and the structure of the TiO₂ films were evaluated by spectroscopic ellipsometry and X-ray difractometry. a-Si thin film solar cells were fabricated on the SnO₂:F films over-coated with TiO₂ films of various thicknesses (1.0, 1.5 and 2.0 nm) and I–V characteristics of these cells were measured under 1 sun (100 mW/cm² AM-1.5) illumination. It was found that the TiO₂ film deposited by APCVD has a refractive index of 2.4 at 550 nm and anatase crystal structure. The conversion efficiency of the a-Si solar cell fabricated on the 2.0 nm TiO₂-overcoated SnO₂:F film increased by 3%, which is mainly attributed to an increase in open circuit voltage (V_oc) of 30 mV.