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.     

Carbon nitride quantum dots (CNQDs)/TiO2 nanoparticle heterojunction photocatalysts for enhanced ultraviolet-visible-light-driven bisphenol a degradation and H2 production

The extension of the absorption band of solar energy is an efficient strategy to dramatically enhance the application value of TiO₂. Based on this, we have prepared carbon nitride quantum dots (CNQDs)/TiO₂ nanoparticle heterojunctions by mixing TiO₂ and the as-prepared CNQDs by the simple mechanical stirring method. The synthesized CNQDs-x/TiO₂ composites were systematically characterized in term of their physicochemical properties, the performance of photocatalytic degradation of Bisphenol A, and their photocatalytic hydrogen evolution performance under stimulated sunlight. The CNQDs/TiO₂ nanoparticle exhibited a lattice spacing of 0.352 nm, assigning to the (101) crystal plane of anatase phase TiO_2. Intriguingly, the modification of TiO₂ nanoparticle with CNQDs can indeed get a narrower optical band gap of 3.02 eV, with a wider absorption range extending to visible light region and could enhance their overall photocatalytic performance over the commercially TiO₂ nanoparticles. In Addition, it was demonstrated that the ratios of CNQDs to TiO₂ exhibited obvious influence on the photocatalytic performance of the obtained composite catalysts.By contrast to the pure TiO₂, all the CNQDs-x/TiO₂ composites displayed higher photocatalytic activities, and the CNQDs-2/TiO₂ possessed the highest photocatalytic degradation capacity towards bisphenol A with a reaction rate constant 0.30 (0.17 for pure TiO₂). Meanwhile, the H₂ production rate of CNQDs-2/TiO₂ sample is about 30 μmol g⁻¹ h⁻¹ higher than that of the pure TiO₂ nanoparticles. Moreover, the photocurrent intensity of CNQDs-2/TiO₂ was about 25 times higher compared to that of pure TiO₂ nanoparticles. Therefore, our research results can provide valuable guidance for exploring high-performance photocatalytic materials.     

Hydrogen generation under sunlight by self ordered TiO2 nanotube arrays

Highly ordered TiO₂ nanotube arrays generate a considerable interest for hydrogen generation by an electrochemical photocell, since ordered architecture of nanotube arrays provides a unidirectional electric channel for electron's transport. Here, we report the hydrogen generation by highly ordered TiO₂ nanotube arrays under actual sunlight in KOH electrolyte. The two-electrode electrochemical cell included an adjustable anode compartment capable of tracing the trajectory of the sun and a set of alkaline batteries connected with a rheostat for application of external bias. The results showed that the photocurrent responses of nanotube arrays match well with the intensity of solar irradiance on a clear summer day. Addition of ethylene glycol into KOH electrolyte as a hole scavenger enhanced the rate of hydrogen generation. A maximum photocurrent density of 31 mA/cm² was observed at 13:30 h, by focusing the sunlight with an intensity of 113 mW/cm² on the surface of the TiO₂ nanotube arrays in 1 M KOH electrolyte with 10 vol% ethylene glycol under an applied bias of 0.5 V. The observed hydrogen generation rate was 4.4 mL/h cm² under the focalized solar irradiance with an intensity between 104 mW/cm² and 115 mW/cm² from 10:00 to 14:20 h.     

Properties of spray deposited titanium dioxide thin films and their application in photoelectrocatalysis

Thin films of titanium dioxide were deposited onto optically transparent, electrically conducting substrates (fluorine doped tin oxide on glass). The two oxide layers, SnO₂ and TiO₂, were deposited sequentially by spray pyrolysis. TiO₂ films of up to 800 nm thickness were prepared by varying the quantity of sprayed solution (titanyl acetylacetonate in methanol), at a growth rate of 0.15 nm/s.The effect of film thickness on the structural, optical and photoelectrochemical properties of TiO₂ films was studied. Scanning electron microscopy showed that the polycrystalline anatase films were compact. The grain size increased up to 1100 nm with increase in film thickness, whereas the crystallite size remained constant (40 nm) as shown by X-ray diffraction. The films had a transmittance of more than 70% in the visible region.Junctions of the semiconducting films with aqueous electrolytes were rectifying and photoactive. Films of 330 or 600 nm were thick enough to exhibit maximum photoelectrochemical response for light of a wavelength of 313 or 365 nm, respectively. Under depletion conditions, an IPCE (incident photon to current conversion efficiency) of 0.8 for a 330 nm thick film at 313 nm was obtained.Oxalic acid degradation under UVA light and under sunlight, applying electrical bias, was demonstrated using these electrodes.     

Visible light-sensitized photocatalyst immobilized on beads by CVD in a fluidizing bed

In this work, chemical vapor deposition in a fluidized bed was employed for the immobilization of a photocatalyst on beads. By introducing carbon (TiOC), nitrogen (TiON) and iron (Fe/TiO₂) into TiO₂ matrix, samples were prepared, respectively. The X-ray diffractometer pattern of TiOC revealed a mixture of the anatase and rutile crystalline forms (65% and 34%, respectively) at the reaction temperature of 600 °C. TiOC absorbed in the visible region and showed photocatalytic activity in terms of isopropyl alcohol (IPA) degradation under a fluorescent lamp and blue light-emitting diode. TiON exhibited a shoulder in the absorption near 520 nm and was photocatalytically active under visible light at 460 nm. In the case of the Fe/TiO₂ species, lowering the bubbler temperature, i.e. reducing the amount of doped-iron, caused the rate of CO₂ evolution to increase during IPA degradation. Among studied species, Fe/TiO₂ under the condition of the bubbler temperature at 109 °C with a flow rate of 10 cm³/min showed highest photocatalytic activity in IPA degradation.     

A novel photoelectrochemical cell with self-organized TiO2 nanotubes as photoanodes for hydrogen generation

A photoelectrochemical (PEC) cell with an innovative design for hydrogen generation via photoelectrocatalytic water splitting is proposed and investigated. It consisted of a TiO₂ nanotube photoanode, a Pt/C cathode and a commercial asbestos diaphragm. The PEC could generate hydrogen under ultraviolet (UV) light-excitation with applied bias in KOH solution. The Ti mesh was used as the substrate to synthesize the self-organized TiO₂ nanotubular array layers. The effect of the morphology of the nanotubular array layers on the photovoltaic performances was investigated. When TiO₂ photocatalyst was irradiated with UV-excitation, it prompted the water splitting under applied bias (0.6 V vs. Normal Hydrogen Electrode, NHE.). Photocurrent generation of 0.58 mA/cm² under UV-light irradiation showed good performance on hydrogen production.     

Effect of Ag/N codoping on photoactivity of TiO2 thin film

Abstract

Nano Ag/N–TiO₂ films with porous hadites as carriers have been successfully prepared by a modified sol–gel process. The performance of the films was characterised by X-ray diffraction, UV-vis and energy dispersive X-ray spectroscopy. The photocatalytic activities and sterilisations of the films were estimated with the waste water contained oil and the fresh cocci as polluted models. The experiments showed that the codoping of Ag and N modified observably the photocatalytic activity of TiO₂. The degradation rate of Ag/N–TiO₂ films on oil in water and bacteria was three times more than pure TiO₂ films on porous hadites. The porous hadites loaded Ag/N–TiO₂ films may be used for waste water processing, sterilisation and filtering.     

Enhanced hydrogen fuel production using synergistic combination of solar radiation and TiO2 photocatalyst coupled with Burkholderia cepacia lipase

Promising and sustainable alternatives for hydrogen production have been investigated. Among these, the use of enzymes may represent an efficient alternative. In this work, an increase in hydrogen production under simulated sunlight by combining TiO₂ and TiO₂/MgCl₂ with Burkholderia cepacia lipase (BCL) was reported. The samples (TiO₂, TiO₂/MgCl₂, and TiO₂/MgCl₂/BCL) were characterized by X-ray diffraction (XRD), thermo-gravimetric (TGA), N₂ adsorption-desorption isotherms (BET), scanning electron microscopy (SEM), and UV–Visible absorption spectra. Hydrogen production tests were performed in aqueous methanol solutions under simulated sunlight. The results demonstrated that 130-times greater increase in hydrogen production rate was observed by the addition of BCL than individual TiO₂. This better hydrogen production rate can be attributed to the increase in the amount of reduction equivalent of the system due to the oxidation of protein peptides caused by TiO₂. In conclusion, the results indicate that hydrogen production efficiency can be significantly improved when integrating TiO₂/MgCl₂ with BCL.     

Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting

TiO₂ thin films have been synthesized by radio-frequency magnetron sputtering and sol–gel method to study the hydrogen generation by photocatalytic water splitting under visible light irradiation. Photoelectrochemical cell with chemical bias, involving photo-anode in form of TiO₂ film deposited on conducting indium tin oxide (ITO) film and Pt as cathode, is developed. The effect of conducting ITO layer on photo-voltage is studied by varying the thickness of ITO films. Constant H₂ generation rate is obtained for long period of time by both the TiO₂ films because of the separated evolution of H₂ and O₂ gas, thus eliminating the back-reaction effect. Sputter-deposited film as compared to sol–gel-synthesized film showed better H₂ generation rate, mainly explained in terms of the higher visible light absorption achieved by oxygen vacancies created in the TiO₂ film by the energetic target ions during deposition in pure Ar gas pressure.     

Enhanced photocatalytic performance of hollow spherical CdS QDs@ZrO2–TiO2 composites with double Z-scheme heterostructures

In this paper, CdS QDs@ZrO₂–TiO₂ hollow spherical composites with double Z-scheme heterostructure modified by CdS QDs were prepared by the sol–gel method and vacuum impregnation method using polystyrene (PS) microspheres synthesized by the self-assembly technique as templates. A series of characterization results show that CdS QDs@ZrO₂–TiO₂ composites treated by PS microspheres have the structure of hollow spheres with uniform size and orderly arrangement. Moreover, the double Z-scheme heterojunction formed between CdS QDs, ZrO₂, and TiO₂ in the composite can optimize the charge transfer path and improve the charge separation efficiency. Results of the photodegradation and the photo-splitting water of CdS QDs@ZrO₂–TiO₂ composites show that, compared with other systems, CdS QDs@ZrO₂–TiO₂ composites have the highest photocatalytic degradation rate of crystal violet, and meanwhile, the photocatalytic hydrogen production rate of the composite under simulated sunlight is more than 100 times that of TiO₂. The good absorption of visible light, the successful construction of double Z-scheme heterojunction, and the unique hollow sphere structure of CdS QDs@ZrO₂–TiO₂ composites are the key factors for enhanced photocatalytic performance.     

Enhanced remediation of simulated wastewaters contaminated with 2-chlorophenol and other aquatic pollutants by TiO2-photoassisted ozonation in a sunlight-driven pilot-plant scale photoreactor

Solar photoassisted remediation of a simulated wastewater contaminated with 2-chlorophenol (2-CP) was carried out by various advanced oxidation processes in a pilot-plant-scale Pyrex glass tubular-type photoreactor using solar cell derived electricity to drive the whole setup. The UV-assisted degradation and mineralization (loss of total organic carbon, TOC) of 2-CP in the co-presence of TiO₂ and ozone (UV/TiO₂/O₃) was enhanced significantly compared with ozonation alone (UV/O₃) and conventional TiO₂ photocatalysis in oxygenated dispersions (UV/TiO₂/O₂). The mineralization process was monitored by TOC assays and chloride ion analyzes. The utilization of immobilized TiO₂ was also examined with a TiO₂-coated glass photoreactor and compared with dispersed TiO₂ in aqueous media. In addition, simulated wastewaters contaminated with the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), the endocrine disruptor bisphenol-A (BPA), the anionic surfactants sodium dodecylbenzenesulfonate (DBS) and sodium butylnaphthalenesulfonate (BNS), together with the cationic benzyldodecyldimethyl- ammonium bromide (BDDAB) surfactant were also efficiently remediated under otherwise identical conditions. The TiO₂-photoassisted ozonation of organic wastewater contaminants is a promising synergistic methodology to accelerate the remediation of such wastewaters, even when the TiO₂ is immobilized which can lead to reduced costs of operation.     

Photocatalytic degradation of TCE in water using TiO2 catalyst

Wastewater is generally released untreated into the rivers and streams in developing countries. Industrial wastewater usually contains highly toxic pollutants, cyanides, chlorinated compounds such as trichloroethylene (TCE). Ultraviolet (UV) radiation from sunlight also decomposes organic compounds by oxidation process. However, the process is less effective due to large amount of toxic effluent entering the main stream water. The solar radiation can effectively be applied to accelerate the process by using suitable catalyst for economically cleaning the major fresh water sources. This paper describes photocatalytic degradation of trichloroethylene in aqueous solution using TiO₂. Variable parameters such as initial concentration of TCE, type and concentration of TiO₂ and reaction time are investigated. The powder TiO₂ is found more effective than the sand TiO₂ for decomposing TCE. The effect of sand TiO₂ as photocatalyst is investigated at various water depths. It is observed that up to 45 mm water depth, sand TiO₂ shows photo-degradation of TCE. The degradation rate increases as the concentration of TCE is increased up to 45 μl of TCE per litre of water. Similarly the photocatalytic degradation increases with TiO₂ concentration up to 0.7 g L⁻¹ of solution but then starts decreasing. The optimum values of TiO₂ and TCE concentration obtained are 0.7 g and 35 μl L⁻¹ of the solution, respectively.     

Visible light-induced degradation of blue textile azo dye on TiO2/CdO–ZnO coupled nanoporous films

The photodegradation of a typical textile blue azo dye, followed by UV–VIS spectra analysis, has been carried out successfully under white light illumination on TiO₂/CdO–ZnO nanoporous coupled thin films. A relatively fast degradation occurs in dye solutions with concentrations of 100 mg/l (pH=3), at temperatures of 85°C, and with the aid of 400 mg/l hydrogen peroxide. Photodegradation also occurs on nanoporous TiO₂ films but with significant lower efficiency than on TiO₂/CdO–ZnO coupled nanoporous films. Dye photodegradation does not occur on TiO₂/CdO or TiO₂/ZnO nanoporous films, suggesting that both CdO and ZnO components are required on the sensitization of TiO₂ nanoporous films. A combined effect of new sensitizing interband states (response to white illumination) and/or rectification phenomena (improved charge separation) may be responsible of the higher photocatalytic activity of the TiO₂/CdO–ZnO nanoporous films. Similarly, the alternative route for visible degradation, the photosensitized degradation mechanism, could also benefit from the coupled nanoporous films due to a higher driving force for electron injection (dye oxidation).     

Hydrogen from hydrogen sulfide cleavage. Improved efficiencies via modification of semiconductor particulates

Visible light irradiation of CdS dispersions containing hydrogen sulfide in alkaline aqueous media leads to efficient hydrogen evolution. An improvement of the process is observed when CdS particles are loaded with RuO₂. A combination of two semiconductor particulates, CdS and TiO₂, shows a remarkable behavior: when RuO₂ is deposited onto TiO₂, the efficiency of the process is increased. The dependence of the hydrogen evolution rate on the preparation of CdS and TiO₂ particles is shown to be an important factor. For the best catalyst combination and under direct sunlight irradiation, the overall conversion efficiency of solar energy is 2% (CdS (M 9)/RuO₂, 0.2% + TiO₂(M4bis)/RuO₂, 0.2%).     

A highly active Pt nanocatalysts supported on RuO2 modified TiO2-NTs for methanol electrooxidation with excellent CO tolerance

Poisoning devitalization of Pt catalyst caused by the absorption of carbon monoxide is an important issue in direct Methanol Fuel Cell (DMFC). To solve this problem, this work introduced a novel nano-structured Pt catalytic electrode, in which RuO₂ modified TiO₂ nanotube arrays (TiO₂-NTs) was used as a carrier for the load of Pt nanocatalysts. Specifically, RuCl₃ sol was filled into the voids of TiO₂-NTs under vacuum condition, followed by thermal decomposition to form RuO₂/TiO₂-NTs support, and then Pt particles were loaded on the RuO₂/TiO₂-NTs support by pulse potential electrodeposition from H₂PtCl₆ aqueous solution. The electrochemical results show that the methanol oxidation current on Pt/RuO₂/TiO₂-NTs is much higher than that on Pt/TiO₂-NTs. In addition, the current attenuation on Pt/RuO₂/TiO₂-NTs with the increased scan cycle is also decreased. The Pt/RuO₂/TiO₂-NTs electrode with 8 g m⁻² RuO₂ exhibits the most stable performance, indicating a strong effects of anti CO poisoning endowed by RuO₂. In Nyquist diagrams, one capacitance arc representing the action of deprivation of H atom appears in the first quadrant and one inductance arc representing the action of deprivation of CO appears in the fourth quadrant. From the fitting results, both the reaction resistance R_ct and the inductance L decrease with the argument of RuO₂ content under bias potential of 600 mV, and in this case CO oxidation is the rate controlling step.     

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.     

Experimental investigation and analysis of a new photoelectrochemical reactor for hydrogen production

In this research paper, an experimental investigation of photoactive material titanium dioxide (TiO₂) coated on 180 cm² 316 stainless steel anode is undertaken to study the photoresponse on photoelectrochemical (PEC) hydrogen production. The TiO₂ nanoparticles are first prepared via sol-gel method. A large surface 316 stainless steel anode is coated with TiO₂ nanoparticles by a dip coating apparatus at a withdraw rate of 2.5 mm/s. The nanoparticles are carried on the stainless steel substrate by two-step annealing procedure. The potentiostatic studies confirm the photoactivity of TiO₂ nanoparticles in a photoelectrochemical reactor when exposed to solar ultraviolet (UV) light. The photon to current efficiency measurements carried out on the PEC reactor with TiO₂ coated large surface stainless steel as photoanode demonstrate a significant increase of photoresponse in UV light compared to the uncoated stainless steel prepared under similar conditions. Upon illumination at a power density of 600 W/m², the hydrogen production is observed in TiO₂ coated stainless steel substrate at a measured rate of 51 ml/h while no illumination conditions show a production rate of 42 ml/h. In comparative assessments, the TiO₂ coated substrate shows an increase in photocurrent of 10 mA with an energy efficiency of 1.32% and exergy efficiency of 3.42% at an applied potential of 1.6 V. The present results show a great potential for titanium nanoparticles semiconductor metal oxide in photoelectrochemical hydrogen production application.     

Enhanced heterogeneous ferrioxalate photo-fenton degradation of reactive orange 4 by solar light

A combined homogeneous and heterogeneous photocatalytic decolourisation and degradation of a chlorotriazine Reactive azo dye Reactive Orange 4 (RO4) have been carried out using ferrous sulphate/ ferrioxalate with H₂O₂ and TiO₂-P25 particles. Solar/ferrous/H₂O₂/TiO₂-P25 and solar/ferrioxalate/H₂O₂/TiO₂-P25 processes are found to be more efficient than the individual photo-Fenton and solar/TiO₂-P25 processes. A comparison of these two processes with UV/ferrous/H₂O₂/TiO₂-P25 and UV/ferrioxalate/H₂O₂/TiO₂-P25 reveals that ferrioxalate is more efficient in solar light whereas ferrous ion is more efficient in UV light. The experimental parameters such as pH, initial H₂O₂, Fe²⁺, ferrioxalate and TiO₂-P25 concentration strongly influenced the dye removal rate in solar processes. The optimum operating conditions of these two combined processes are reported.     

Solar photocatalysis, photodegradation of a commercial detergent in aqueous TiO2 dispersions under sunlight irradiation

A commercial detergent whose major components are an anionic surfactant and a fluorescent whitening agent can be photodegraded in aqueous TiO₂ dispersions under irradiation with concentrated sunlight in the presence of air. The degradation process followed apparent first-order kinetics in terms of the total sunlight energy impinging on the photoreactive system. The effects of (a) TiO₂ loading, (b) circulation flow rate, and (c) pH of the reactant solution on the kinetics of decomposition of the detergent were examined. Under the prevailing conditions, the optimal operational parameters for this detergent were, respectively: TiO₂ loading, 6 g l⁻¹; circulation flow rate, 4.9 l min⁻¹; and pH, 4.9. The rate of increase of the surface tension was greater than the rate of decrease of the concentration of the detergent. This study adds to our knowledge base in the effective use of sunlight irradiation to detoxify wastewaters containing undesirable detergents.     

N-Cdots-decorated TiO2(B)/anatase microspheres with high photocatalytic performance in visible light

Nitrogen-doped carbon dots (N-Cdots) with enhanced absorption in visible light region were combined with TiO₂(B)/anatase (TiO₂(A + B)) microspheres consist of nanosheets via a one-step synthesis. The successful connection of N-Cdots and TiO₂ substrate was confirmed by the appearance of new Ti–O–C bond. The degradation rate of Rhodamine B (RhB) for N-Cdots@TiO₂(B)/anatase was improved to seven times higher than that of initial TiO₂(B) microspheres under full spectrum illumination. N-Cdots@TiO₂(B)/anatase photocatalyst also exhibited high photocatalytic ability under visible light irradiation. The possible mechanism was proposed in two irradiation conditions. N-Cdots act as a semiconductor to generate electrons and holes. The electrons were transferred to the conduction band of TiO₂(B) and anatase, which improved the visible light utilization efficiency. Meanwhile, the heterophase structure of TiO₂(A + B) substrate restrained the recombination of electron-hole pairs. It is the synergistic effect of heterophase and N-Cdots that enhanced the photocatalytic performance.