Photocatalytic activity of Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions

Cu₂O/TiO₂, Bi₂O₃/TiO₂ and ZnMn₂O₄/TiO₂ heterojunctions were studied for potential applications in water decontamination technology and their capacity to induce an oxidation process under VIS light. UV–vis spectroscopy analysis showed that the junctions-based Cu₂O, Bi₂O₃ and ZnMn₂O₄ are able to absorb a large part of visible light (respectively, up to 650, 460 and 1000 nm). This fact was confirmed in the case of Cu₂O/TiO₂ and Bi₂O₃/TiO₂ by photocatalytic experiments performed under visible light. A part of the charge recombination that can take place when both semiconductors are excited was observed when a photocatalytic experiment was performed under UV–vis illumination. Orange II, 4-hydroxybenzoic and benzamide were used as pollutants in the experiment. Photoactivity of the junctions was found to be strongly dependent on the substrate. The different phenomena that were observed in each case are discussed.     

Electrical properties of TiO2/SEO nanocomposites: From macro to nano

Tunneling atomic force microscopy (TUNA) was successfully used to confirm that electro-devices based on TiO₂-SEO nanocomposites can find possible application in solar power conversion field. Investigated electro-devices show different current flow depending on the layer combination. The highest capacity was shown by the electro-devices with a PEDOT-PSS layer on the top, being the average current values ∼200 pA at 10 V applied voltage. The conductivity value measured by Keithley indicated that the system ITO/1:PEDOT-PSS/1:TiO₂-SEO electro-device shows the highest conductivity level, 0.16 S/cm. Thus, these systems have high potential to find application as solar cell devices.     

TiO2 nanotubes and CNT–TiO2 hybrid materials for the photocatalytic oxidation of propene at low concentration

In this work, we investigated titanium dioxide (TiO₂) nanotubes and CNT–TiO₂ hybrid materials for the photocatalytic oxidation (PCO) of propene at low concentration (100 ppmv) in gaseous phase. The materials were prepared via sol–gel method using sacrificial multi-walled carbon nanotubes (CNT) as templates and subsequent heat treatments to obtain the desired crystalline phase (anatase, rutile or a mixture of both) and eventually to remove the carbon template. We also studied rutile nanotubes for the first time and demonstrate that the activity strongly depends on the crystalline composition, following rutile < anatase < anatase/rutile mixture. The enhanced activity of the anatase–rutile mixture is attributed to the decrease in the electron–hole pair recombination due to the multiphasic nature of the particles. The key result of this work is the exceptional performance of the CNT–TiO₂ hybrid, which yielded the highest observed photocatalytic activity. The improved performance is attributed to synergistic effects due to the hybrid nature of the material, resulting in small anatase crystalline sizes (CNT act as heat sinks) and a reduced electron–hole pair recombination rate (CNTs act as electron traps). These results demonstrate the great potential of hybrid materials and stimulate further research on CNT-inorganic hybrid materials in photocatalysis and related areas.     

Fabrication of TiO2 nanotubes by using electrodeposited ZnO nanorod template and their application to hybrid solar cells

Vertically aligned TiO₂ nanotubes have been fabricated on the indium-doped tin oxide (ITO) by a simple and versatile technique using the electrochemically deposited ZnO nanorods, oriented along the c-axis, as a template in the spin-on based sol-gel reaction of a Ti precursor. The diameter, length, and shape of TiO₂ nanotubes were controlled by changing the initial ZnO nanorod template and the spin conditions during sol-gel process of a Ti precursor. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were used to confirm the successful formation of TiO₂ nanotubes and characterize their structure and morphology. Furthermore, as an application of the TiO₂ nanotubes, hybrid solar cells based on TiO₂ and poly[2-methoxy,5-(2′-ethyl-hexyloxy)1,4-phenylenevinylene] (MEH-PPV) were successfully fabricated.     

Immobilized TiO2 photocatalyst during long-term use: decrease of its activity

When TiO₂ is immobilized on organic fibres, pumice stone or polymer film, the photocatalytic efficiency decreases slowly during long-term use. The efficiency of immobilized photocatalysts were tested on 500 ml of a 5×10⁻⁵ M solution of acid orange-7 (a classical azo dye) before and after treatment of 10⁻³ M solution of acid orange-7 during 4 weeks. It was observed that the efficiency was reduced approximately four, five and 10 times with polymer film containing TiO₂, TiO₂ on organic fibres and TiO₂ on pumice stone, respectively, after 4 weeks of use. Volumes treated were 40, 45 and 60 l, respectively. Nevertheless, the decomposition rate stays a little higher with TiO₂ on pumice stone than with the two other catalysts tested. The photocatalytic activity of immobilized TiO₂ was significantly reduced also during treated with wastewaters. For immobilized photocatalysts used, the decrease of activity is considered to be caused by the elimination of some particles from the catalyst surface during use and also by fouling of catalyst surface by the formation of by-products during the course of degradation process.     

Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO2

A visible-light-active TiO₂ photocatalyst was prepared through carbon doping by using glucose as carbon source. Different from the previous carbon-doped TiO₂ prepared at high temperature, our preparation was performed by a hydrothermal method at temperature as low as 160 °C. The resulting photocatalyst was characterized by XRD, XPS, TEM, nitrogen adsorption, and UV–vis diffuse reflectance spectroscopy. The characterizations found that the photocatalyst possessed a homogeneous pore diameter about 8 nm and a high surface area of 126 m²/g. Comparing to undoped TiO₂, the carbon-doped TiO₂ showed obvious absorption in the 400–450 nm range with a red shift in the band gap transition. It was found that the resulting carbon-doped TiO₂ exhibits significantly higher photocatalytic activity than the undoped counterpart and Degussa P25 on the degradation of rhodamine B (RhB) in water under visible light irradiation (λ > 420 nm). This method can be easily scaled up for industrial production of visible-light driven photocatalyst for pollutants removal because of its convenience and energy-saving.     

Carbon nanotubes/titanium dioxide (CNTs/TiO2) nanocomposites prepared by conventional and novel surfactant wrapping sol–gel methods exhibiting enhanced photocatalytic activity

In this work, a conventional sol–gel method was used to prepare CNTs/TiO₂ nanocomposites with different carbon loading in the range up to 20% CNTs/TiO₂ by weight. The bare CNTs (multi-walled), and the composites were characterized by a range of analytical techniques including TEM, XRD, BET and TGA–DSC. The results show the successful covering of the CNTs with discrete clusters of TiO₂ and bare CNTs surfaces which after annealing at 500 °C led to mesoporous crystalline TiO₂ (anatase) clusters. The photocatalytic activities of the nanocomposites were monitored from the results of the photodegradation of methylene blue (MB). The optimum CNTs/TiO₂ ratio in the composites prepared by conventional sol–gel method was found to be in the range from 1.5% to 5% by weight under the experimental conditions investigated. The maximum increase in activity was found to be 12.8% compared to the pure TiO₂ sample.In contrast, the synthesis of CNTs/TiO₂ nanocomposites by a novel surfactant wrapping sol–gel method [B. Gao, C. Peng, G.Z. Chen, G. Li Puma, Appl. Catal. B: Environ. 85 (2008) 17.] led to a uniform and well-defined nanometer-scale titania layer on individual CNTs. The nanocomposites were found to enhance the initial oxidation rate of methylene blue by onefold compared to the pure TiO₂ sample. This larger degree of rate enhancement is attributed to the supporting role of the CNTs and surface properties prepared by this novel modified sol–gel method.     

The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO2 supported on glass bead

In this investigation, a packed bed filled with coated titanium dioxide glass beads to study the kinetics of photocatalytic degradation of trichloroethylene under irradiation of 365 nm UV light. In the range of 100–500 ml/min of flowrate, the reaction rate for 6 μM TCE increased with an increasing flowrate upto 300 ml/min, while was not affected by the flowrate at the values higher than 300 ml/min. For moisture in the range of 9.4–1222.2 μM, the reaction rate of TCE was decreased with an increasing humidity. The adsorbed water on the catalyst surface could compete with the adsorption of TCE on the sites. The reaction rate of 6 μM TCE increased as the light intensity increased, and was proportional to the 0.61 order of the light intensity. Among the three L–H bimolecular form models, the experimental data had the best fit for one of models:

     

Photocatalytic degradation of Chromotrope 2R using nanocrystalline TiO2/activated-carbon composite catalysts

Composite catalysts made of nanocrystalline TiO₂ and carbon were prepared by a modified sol–gel method over activated carbon (AC). The composite catalysts were characterized by N₂ adsorption–desorption isotherm, TG, diffuse reflectance UV–vis spectroscopy, XRD and SEM. The photocatalytic activity was tested on the degradation of Chromotrope 2R (C2R) in aqueous medium under UV radiation. The composite catalysts exhibited higher activities than commercial Degussa P25 alone and the photocatalytic process was more efficient than the pure photolytic degradation. A modified Langmuir–Hinshelwood approach was used to study the kinetics and to determine the adsorption equilibrium constant and the reaction rate constant. Two different mechanisms are proposed and discussed in order to explain the observed synergy.     

Tubular V-free and V-doped TiO2 derived from H2Ti12O25 hollow spheres: Vapour-thermal synthesis,morphology evolution,and photocatalytic performance

Vanadium (V)-free and V-doped H₂Ti₁₂O₂₅ hollow spheres (HTOHSs) were first synthesised via vapour-thermal method at 290?°C using common chemicals and solvents, over super-critical temperature (243?°C) of ethanol (vapour source). Then, they were annealed at 600?°C under different processing conditions to obtain titanium dioxide (TiO₂) photocatalysts. All catalysts were characterised by means of XRD, SEM, TEM, XPS, UV–vis DRS, FT-IR, N₂ adsorbtion-desorbtion and fluorescence lifetime. Results indicated that after the treatment in air, V-free HTOHSs were transformed into open-ended hollow tubes with uniform length of ~5?µm and diameter of ~1?µm, and walls of hollow tubes consisted of aggregated nanosheets. Furthermore, HTOHS crystallised into anatase TiO₂ (white) phase. The treatment in N₂ atmosphere led to the breaking of longer tubes into shorter ones. In contrast, after the treatment in N₂ atmosphere or by reduction using NaBH₄ as a reductant, V-doped HTOHS resulted in the formation of anatase TiO₂ (black) samples and consisted of fewer tubes and more deformed spheres. In this study, fluorescence lifetime (τ) of photo-generated carriers corresponded well with the ratio of oxygen vacancy, indicating that oxygen vacancy dominates the lifetime even though it is very sensitive to many factors. The evolution of structure and morphology and photocatalytic mechanism were analysed and discussed.     

Preparation of carbon-coated TiO2 nanostructures for lithium-ion batteries

Carbon-coated TiO₂ one-dimensional nanostructures are synthesized by hydrothermal reaction followed by post-calcination at various temperatures. Post-calcination induces crystallization of TiO₂ and the complete crystallization of anatase phase is observed at 600 °C of the calcination temperature. Carbon-coated TiO₂ nanostructures show relatively poor crystallinity as compared with the pristine counterparts, but their lithiation capacity and high rate capability are improved throughout all calcination temperatures. The coated carbon suppresses severe agglomeration of TiO₂ nanotubes which allows easy access of Li-ions and electrons to the whole surface of primary nanotubes, leading to the better lithiation performance. Higher calcination temperatures cause excessive growth of nanotube walls, leading to the collapse of tubular morphology and deterioration of lithiation performance. At 700 °C of the calcination temperature, the enhanced electronic conductivity from the graphitization of the coated carbon seems to be the main reason for the improved capacity of TiO₂ nanowires.     

Preparation and characterizations of a new PS/TiO2 hybrid membranes by sol-gel process

New organic-inorganic hybrids based on PS/TiO₂ hybrid membranes were prepared by sol-gel and phase inversion process. The membranes were characterized in terms of morphology, structure, hydrophlicity, UF performance and thermal stability. The results showed that macrovoids were nearly suppressed with formation of a sponge like membrane structure. The TiO₂ particles were uniformly dispersed in membrane. The nanodispersed inorganic network formed after sol-gel process and the strong interaction between inorganic network and polymeric chains led to the improvement of porosity and thermal stability. In particular hydrophilicity and permeability increased drastically with the increasing of TiO₂ content in the range of 0-9.3 wt%, without changing retention properties of membrane. However, high-TiO₂ concentration induces nanoparticles aggregate, resulting in the decline of hydrophilicity and permeability. Thus, PS/TiO₂ hybrid membranes with proper TiO₂ content are desirable to meet some specific requirements in industrial separation.     

A quantitative evalution of the photocatalytic performance of TiO2 slurries

This paper reports the results of a comparison between two TiO₂ photocatalysts that differ for particle size and absorption/scattering optical properties. The catalyst with larger particles and lower surface area performed better in the degradation of phenol than the specimen with smaller particles and larger surface area. Following carefully designed experiments, it is possible to assess the relative role of light absorption/scattering properties and catalyst-related efficiency by means of a basic kinetic model for the rate of photocatalytic reactions. Explicit relationships are derived in the framework of the steady-state approximation for the quantum yield as a function of one a-dimensional number collecting surface kinetic constants for charge carrier reactions at the interface, absorbed light and surface substrate concentrations. The dimensionality change to volume-defined quantities allows derivation of the explicit dependence of the quantum yield on substrate concentration and partition constants, catalyst concentration, and the rate of volumetric light absorption. Following this approach, the rate expression for slurry systems, valid in the absence of back reactions, is directly derived. Some further simplification of the rate equation for the case of low quantum yield regime leads to analytical relationships able to account for the dependence of the rate on catalyst concentration and absorbed light in the case of stirred and unstirred conditions. The reported properly designed experiments allow the estimation of catalyst-specific micro-kinetic constants.     

Highly porous TiO2 films from anodically deposited titanate hybrids and their photoelectrochemical and photocatalytic activity

Hybrid films of TiO₂ and benzoquinone, its derivative 2-methyl-benzoquinone or the dye 2,9,16,23-tetrasulfophthalocyaninatonickel(II) were prepared by anodic electrodeposition from titanium alkoxide solutions. Calcination of the films at 450 °C led to removal of the organics and the formation of crystalline and highly porous TiO₂ films as seen in XRD and Kr adsorption measurements, respectively. In dye-sensitized solar cells the films achieved an overall light-to-electricity conversion efficiency of 0.8% despite a low film thickness of 0.55 μm. In the photocatalytic decomposition of methylene blue the films showed photonic efficiencies of up to 0.09% for film thicknesses around 0.5 μm, which is much higher than those of comparable TiO₂ films prepared by sol–gel method.     

Photocatalytic treatment of water-soluble pesticides by photo-Fenton and TiO2 using solar energy

The technical feasibility and performance of photocatalytic degradation of four water-soluble pesticides (diuron, imidacloprid, formetanate and methomyl) have been studied at pilot scale in two well-defined systems of special interest because natural-solar UV light can be used: heterogeneous photocatalysis with titanium dioxide and homogeneous photocatalysis by photo-Fenton. The pilot plant is made up of compound parabolic collectors (CPCs) specially designed for solar photocatalytic applications. Experimental conditions allowed disappearance of pesticide and degree of mineralisation achieved in the two photocatalytic systems to be compared. In order to assure that the photocatalytic results are consistent, hydrolysis and photolysis tests have been performed with the four pesticides. The initial concentration tested with imidacloprid, formetanate and methomyl was 50 and 30 mg/l with diuron, and the catalyst concentrations were 200 mg/l and 0.05 mM with TiO₂ and iron, respectively. Total disappearance of the parent compounds and 90% mineralisation have been attained with all pesticides tested, methomyl being the most difficult to be degraded with both treatments. First-order rate constants, initial rate, time necessary for mineralising 90% of the initial TOC and hydrogen peroxide consumption were calculated in all cases, enabling comparison both of treatments and of the selected pesticide reactivity.     

Photocatalytic degradation of imidazolinone fungicide in TiO2-coated optical fiber reactor

The photocatalytic degradation of the fungicide fenamidone is studied in a TiO₂-coated optical fiber photoreactor. Fenamidone is slowly transformed with a kinetic order of 1 and a degradation rate of 0.02 h⁻¹. Intermediate products were isolated and identified by means of solid phase extraction (SPE) coupled to liquid chromatography-mass spectrometry techniques (LC-MS). A proposed degradation pathway of fenamidone is presented, involving mainly hydroxylation and oxidation reactions. Carboxylic acids and sulfate ions resulting from the same reaction in a powder reactor were also identified.     

Modified titanium electrodes: Application to Ti/TiO2/PbO2 dimensionally stable anodes

Titanium is a valve metal able to withstand corrosion, due to the presence of a passivating layer of titanium oxide on its surface. But, due to that more or less insulating layer, titanium cannot be used directly as an anodic material. However, modification of the surface of a Ti/TiO₂ substrate may lead to the formation of new structures: Ti/TiO₂/M or Ti/TiO₂/OX, in which M is a metal such as platinum and OX a conducting oxide exhibiting electrocatalytic properties. These structures have interesting electrochemical properties and may be used as efficient electrode materials.In this paper, after a review of the electrochemical behaviour of these structures, we give new results concerning the selective electrodeposition of lead dioxide on Ti/TiO₂ substrates and we propose an interpretation of the results taking into account the dielectric properties of the underlying TiO₂. It is shown that there is a dramatic decrease of the resistance of the electrode when a PbO₂ layer is electrodeposited onto a Ti/TiO₂ structure. That effect allows the preparation of electrodes (low-cost DSAs) that may be used as anodes in spite of the presence of the underlying TiO₂ layer, that layer being useful to avoid corrosion of the titanium substrate. At last, the effect of stabilization of the underlying TiO₂ layer is discussed.