Showing papers in "Journal of Photochemistry and Photobiology A-chemistry in 2010"

What is Degussa (Evonik) P25? Crystalline composition analysis, reconstruction from isolated pure particles and photocatalytic activity test

Abstract: Anatase and rutile crystallites were isolated from Degussa (Evonik) P25 by selective dissolution with a hydrogen peroxide–ammonia mixture and diluted hydrofluoric acid, respectively, and used as standard samples for calibration curves of X-ray diffraction analyses. The results showed that P25 contains more than 70% anatase with a minor amount of rutile and a small amount of amorphous phase. The composition anatase/rutile/amorphous could be determined by analysis of P25 mixed with an internal standard, nickel(II) oxide. However, it was also found that the composition of P25 used in this study was inhomogeneous and changed depending on the position of sampling from the same package. Comparison of activities of original P25 and reconstructed P25 with those of isolated anatase and rutile particles suggested a less-probable synergetic effect of the co-presence of anatase and rutile.

Well-aligned arrays of vertically oriented ZnO nanowires electrodeposited on ITO-coated glass and their integration in dye sensitized solar cells

Abstract: We report on the effects of post-growth hydrothermal treatment and thermal annealing on properties of vertically aligned ZnO nanowires arrays (NWs). The samples were electrochemically deposited (ECD) on indium–tin oxide (ITO)-coated glass substrates and subjected to post-growth hydrothermal treatment (HT) at 150 °C and, for the purpose of comparison, to conventional thermal annealing (CTA) in a furnace at 150, 400, and 600 °C in air. Sample characterization was realized using X-ray diffraction (XRD), SEM, TEM, selected-area electron diffraction (SAED) and photoluminescence (PL). Thermal annealing does not induce significant changes of morphology, but influences the structural and optical properties. At the same time we found that the HT induces more significant improvement of properties of ZnO nanowires arrays (ZnO NWs) on ITO. The results show that the ECD ZnO NWs are single-crystalline with hexagonal structure and c-axis perpendicular to ITO substrate. Only one peak at about 379 nm was observed in the photoluminescence spectra at room temperature which showed an intensity increase after hydrothermal treatment. This corresponds to the increase of the optical quality of ZnO NWs. The best optical quality for ZnO NWs was found after the hydrothermal treatments at 150 °C in our experiment. The high-quality electrodeposited NW layers have been used, after sensitization with the highly absorbing D149 dye, as a photoanode in dye sensitized solar cells (DSCs) and the impact of post-growth treatment of the nanowires on DSCs performances has been evaluated. The photocurrent of the solar cells increased significantly after HT or CTA at 150 °C leading to a maximum overall photovoltaic conversion efficiency (PCE) of 0.66% at 100 mW/cm2, based on short-circuit photocurrent density, open-circuit voltage and fill factor of 3.283 mA/cm2, 0.606 V and 33.3%, respectively. The obtained results are interesting in view of the low layer roughness and pave the way for implementation of high-quality electrodeposited ZnO NW arrays in DSCs fabrication.

Surface modification of ZnO with Ag improves its photocatalytic efficiency and photostability

Abstract: Ag/ZnO photocatalysts with different Ag loadings were prepared by photocatalytic reduction of Ag+ on ZnO with ethanol as hole scavenger. It was found that loading an appropriate amount of Ag on ZnO not only enhances its photocatalytic activity, but also improves its photostability. The Ag/ZnO photocatalysts were characterized with XRD, BET, DRUV–vis, Raman, PL, and photoelectrochemical measurement. No matter what the Ag loading is higher or low, silver exists in the form of metallic species in the Ag/ZnO photocatalysts. The enhancement of photocatalytic activity is due to the fact that the modification of ZnO with an appropriate amount of Ag can increase the separation efficiency of photogenerated electrons and holes in ZnO, and the improvement of photostability of ZnO is attributed to a considerable decrease of the surface defect sites of ZnO after the Ag loading. The chemisorption of molecular oxygen and the chemisorption of atomic oxygen on Ag in the Ag/ZnO photocatalysts were observed. It was found that the metallic Ag in the Ag/ZnO photocatalysts does play a new role of O2 chemisorption sites except for electron acceptor, by which chemisorbed molecular oxygen reacts with photogenerated electrons to form active oxygen species, and thus facilitates the trapping of photogenerated electrons and further improves the photocatalytic activity of the Ag/ZnO photocatalysts.

Fundamentals and misconceptions in photocatalysis

Abstract: Photocatalysis has presently become a major discipline owing to two factors: (i) the intuition of the pioneers of last 20th century and (ii) the mutual enrichment of scientists arising from different fields: photochemistry, electrochemistry, analytical chemistry, radiochemistry, material chemistry, surface science, electronics, and hopefully catalysis. However, heterogeneous photocatalysis belongs to catalysis, which means that all the bases of this discipline must be respected and consequently, it has become imperative to refocus the frame of photocatalysis to avoid misfits and misconceptions: (i) proportionality of the reaction rate to the mass of catalyst (below the plateau due to a full absorption of photons); (ii) implication of the Langmuir–Hinshelwood mechanism of kinetics with the initial rate being proportional to the coverages θ in reactants; (iii) obtention of conversions beyond the stoichiometric threshold defined as the number of potential active sites initially present at the surface. Photonics should be respected with the photocatalytic activity being (i) parallel to the absorbance of the photocatalyst and (ii) proportional to the radiant flux Φ, enabling one to determine the quantum yield defined as the ratio of the reaction rate r (in molecules converted per second) to the efficient photonic flux (in photons per second) received by the solid. True photocatalytic normalized tests should be established to prove the real catalytic activity of irradiated solids, independent of non-catalytic side-reactions. In particular, dye decolorization is a misleading test, which only provides a “visible” and apparent “disappearance” of the dye, purely photochemical but not photocatalytic. Thermodynamics have also to be respected. The decrease of photon energy to the visible may be thermodynamically detrimental for the generation of highly active species such as OH°. Concerning solid state chemistry, it is now eventually admitted that cationic doping is detrimental for photocatalysis. Anionic doping must be rapidly clarified or otherwise abandoned. In conclusion, all these recommendations have to be addressed and experiments have to be operated in suitable conditions before claiming that one deals with a true photocatalytic reaction, whose veracity can be proved by following a protocol suggested at the end.

The hydrothermal synthesis of BiOBr flakes for visible-light-responsive photocatalytic degradation of methyl orange

Abstract: Flake-like BiOBr semiconductors have been prepared using hydrothermal synthesis with meticulous control of synthesis parameters and used for photocatalytic degradation of methyl orange. XRD, SEM and UV-vis characterizations have been performed to study the obtained BiOBr materials. The results indicate that the morphology and crystallite size of BiOBr depend significantly on the temperature and duration of the hydrothermal syntheses. Diffuse UV-vis spectra show the BiOBr materials to be indirect semiconductors with an optical bandgap of approximately 2.92 eV, which is essentially unaffected by the synthesis parameters. The hydrothermal-synthesized BiOBr flakes exhibit noticeable activity for photodegradation of methyl orange under visible light (>400 nm) illumination, with the BiOBr synthesized by hydrothermal treatment at 120 degrees C for 6 h exhibiting superior photocatalytic performance in these flakes. The excellent activity and photo-stability reveal that BiOBr is a promising visible-light-responsive photocatalyst. (C) 2010 Elsevier B.V. All rights reserved.

Photoelectrochemical water splitting at nanostructured ZnFe2O4 electrodes

Abstract: Semiconducting nanocrystalline ZnFe 2 O 4 thin films were deposited by aerosol-assisted chemical vapour deposition (AACVD) for photoelectrochemical (PEC) water splitting. The effect of deposition parameters such as solvent type, temperature and deposition time on PEC properties has been investigated. The SEM analysis illustrated that the morphology of the films changes significantly with the change of solvent. The films deposited from ethanolic precursor solution have a morphology consisting of interconnected cactus-like ZnFe 2 O 4 structure growing vertically from the FTO substrate. The current–voltage characterization proved that the nanocrystalline ZnFe 2 O 4 electrodes exhibit n -type semiconducting behaviour and the photocurrent was found strongly dependent on the deposition solvent, deposition temperature and deposition time. The maximum photocurrent density of 350 μA/cm 2 at 0.23 V vs. Ag/AgCl/3 M KCl (∼1.23 V vs. RHE) was obtained for the ZnFe 2 O 4 electrode synthesized using the optimum deposition temperature of 450 °C, the deposition time of 35 min, and 0.1 M solution of ( 1 ) in ethanol. The electrode gave an incident photon to electron conversion efficiency of 13.5% at an applied potential of 0.23 V vs. Ag/AgCl/3 M KCl at 350 nm. The donor density of the ZnFe 2 O 4 was 3.24 × 10 24 m −3 and the flatband potential is approximately −0.17 V, which remarkably agrees with the photocurrent onset potential of −0.18 V vs. Ag/AgCl/3 M KCl.

2-Nitrobenzaldehyde as a chemical actinometer for solution and ice photochemistry

Abstract: 2-Nitrobenzaldehyde (2NB) is a convenient, photochemically sensitive, and thermally robust actinometer. Although 2NB has been used in a number of solution and ice experiments in the laboratory, the quantum efficiencies and molar absorptivities of 2NB have not been critically evaluated, especially on ice. Using a series of laboratory and field measurements we have measured the light absorbance and photochemical properties of 2-nitrobenzaldehyde in solution (water and/or acetonitrile) and in/on water ice. Our results show that the molar absorptivities of 2NB are only weakly dependent upon temperature and that the quantum yield is independent of temperature in water; the quantum yield is also independent of wavelength, as shown by past reports. Furthermore, we find that the photochemistry of 2NB in/on water ice is the same as in liquid water. While most studies employing 2NB cite and use a quantum yield of 0.50, based on a review of the literature, and on our new experimental data, we recommend a quantum yield of 0.41 for 2NB photolysis for both solution and water ice.

Photocatalytic study of BiOCl for degradation of organic pollutants under UV irradiation

Abstract: BiOCl exhibited high photocatalytic activities for the degradation of rhodamine B, methyl orange and phenol. Surface chloride ions were adverse to the BiOCl photocatalysis and dissociated from BiOCl via reaction with photogenerated holes and electrons under UV irradiation. Conduction band electrons of BiOCl directly reduced either chlorine radical or the azo-bond of MO during the photocatalytic process. Hydroxyl radical was the main oxidative species in the BiOCl photocatalysis, whose generation can be accelerated via enhancing the conduction band electron consumption by MO. After the photocatalytic reaction, the dissolved chloride ion would spontaneously recombine back to the BiOCl photocatalyst, hence qualifies BiOCl as a practical high-activity photocatalyst with long lifetime.

Sustainable H2 gas production by photocatalysis

Abstract: The photocatalytic, ambient temperature, reforming of oxy-hydrocarbons, used to generate hydrogen gas, has been studied in some detail over Pd-titania systems and a general mechanism is developed. In the case of water–methanol mixtures, methanol adsorption on the metal surface results in hydrogen gas evolution and the formation of adsorbed CO. The latter is subsequently oxidised by an oxygen species (O−) generated in the semiconductor by UV light. Desorption of the resulting CO2 enables another methanol molecule to adsorb, restarting the cycle. Water is reduced at the titania hole replacing the lost anion and generating further hydrogen. We will discuss the evidence for this mechanism and extend it to consider the photocatalysis of higher alcohols including ethanol, butanol and glycerol.

Inactivation of clinically relevant pathogens by photocatalytic coatings

Abstract: Novel disinfection methods are being sought to provide additional means of protection in a number of areas where disease outbreaks could lead to illness or fatalities For example, the risk of contamination arising from contact with surfaces and medical devices has received much attention due to the rise in incidence of healthcare acquired infections It is possible that reducing bio-burden on these sites may supplement the disinfection protocols currently in place and help reduce risk of infection Photocatalytic surfaces offer promise as innovative and cost-effective biocidal engineering solutions which address these specific problems whilst maintaining stringent health and safety controls A method was developed to assess the disinfection efficiency of photocatalytic surfaces allowing (a) determination of pathogen viability as a function of treatment time; (b) assessment of the surface for viable surface bound organisms following disinfection; (c) measurement of the re-growth potential of inactivated organisms This method was used to demonstrate the inactivation of extended-spectrum beta-lactamase Escherichia coli , methicillin resistant Staphylococcus aureus, Pseudomonas aeruginosa and Clostridium difficile spores using immobilised films of commercial titania nanoparticles 999% reduction in viability (a 3-log kill) was observed for all bacterial cells within 80 min photocatalytic treatment Complete surface inactivation was demonstrated and bacterial re-growth following photocatalytic treatment was not observed Greater than 99% inactivation (26-log reduction) was observed when the photocatalytic surfaces were challenged with C difficile spores The efficacy of photocatalytic disinfection to inactivate Staphyloccocus epidermidis cells within a biofilm was also demonstrated, with 3 h treatment rendering 965% ± 6 of the biofilm cells on the TiO 2 coated substrate non-viable Disinfection of cells throughout the 3–4 μm thick biofilm was observed

Antimicrobial activity of titania/silver and titania/copper films prepared by CVD

Abstract: We have previously reported the production of photocatalytically active films of TiO2/Ag and TiO2/CuO grown by atmospheric pressure thermal CVD that had high antimicrobial activity. The present study compares the activity of dual layers and co-deposited TiO2-CuO with single layers. We also compared the BS ISO 27447:2009 method with our previously reported method for determining photocatalytic antimicrobial activity and showed that although the activity was reduced in the BS method, probably due to the lower UV irradiation used, there was still a good antimicrobial activity. The results showed that Ag-TiO2 surfaces retained photocatalytic self-cleaning activity measured by stearic acid oxidation whereas Cu-TiO 2, both layered and co-deposited had very low activity. However, both were antimicrobial against Escherichia coli with activity of the Cu-TiO 2 films greatly enhanced by irradiation possibly via a photo-Fenton type reaction. The activity of the Ag-TiO2 films against Pseudomonas aeruginosa and MRSA (methicillin resistant Staphylococcus aureus) showed reduced killing activity with an environmental isolate of P. aeruginosa and the MRSA showing only 3 log and 1.5 log reductions respectively. The implications for their use for reduction of surface contamination by microorganisms as part of control measures for healthcare associated infections are discussed.

Charge transport in nanostructured materials for solar energy conversion studied by time-resolved terahertz spectroscopy

Abstract: Spectra of far-infrared conductivity contain useful information on charge transport at nanoscopic length scales. However, decrypting the mechanisms and parameters of charge transport from the measured spectra is a complex task in nanostructured systems: in particular, the conductivity is strongly influenced by charge carrier interaction with surfaces or interfaces between constituents of the composite material as well as by local field effects. Here we review our work on transient far-infrared conductivity in polymer:fullerene bulk heterojunctions and in bare and dye-sensitized semiconductor nanoparticles. Measurements performed by time-resolved terahertz spectroscopy are complemented by Monte-Carlo calculations which clearly link the charge transport properties and the terahertz conductivity spectra.

Visible light superoxide radical anion generation by tetra(4-carboxyphenyl)porphyrin/TiO2: EPR characterization

Abstract: The generation of superoxide radical anion O2− from tetra(4-carboxyphenyl)porphyrin (TCPP) adsorbed on TiO2 in DMSO and irradiated by visible light was studied using EPR spectroscopy and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as spin trap. A chemical filter was used to remove light with wave lengths <500 nm. A multiline EPR spectrum was observed, characteristic of a mixture of two adducts, the first corresponding to DMPO–O2− and the second to the so-called nitroxide-like radical. Hyperfine coupling constants determined for the DMPO–O2− adduct are: aN = 14.1 G, aHβ = 10.8 G and aHγ = 1.4 G, and for the nitroxide-like radical adduct aN = 14 G. An increased intensity of the EPR lines corresponding to the nitroxide-like radical adduct was observed under irradiation without chemical filter, which suggests a possible DMPO–O2− decomposition. No singlet oxygen could be detected by EPR spectroscopy using 2,2,6,6-tetramethyl-4-piperidone (TEMP) as spin trap and by chemical trapping using anthracene as the trap.

Fluorescence quenching phenomena facilitated by excited-state hydrogen bond strengthening for fluorenone derivatives in alcohols

Abstract: Spectroscopic studies on benzo[b]fluorenone (BF) solvatochromism in several aprotic and alcoholic solvents have been performed to investigate the fluorescence quenching by hydrogen bonding and proposed a weaker ability to form intermolecular hydrogen bond of BF than fluorenone (FN). In this work, the time-dependent density functional theory (TD-DFT) method was used to study the excited-state hydrogen bonding of both FN and BF in ethanol (EtOH) solvent. As a result, it is demonstrated by our theoretical calculations that the hydrogen bond of BF-EtOH complex is almost identical with that of FN-EtOH. Moreover, the fluorescence quantum yields of FN and BF in the alcoholic solvent is efficiently dependent on the energy gap between the lowest excited singlet state (fluorescent state) and ground state, which can be used to explain the fluorescence quenching by the excited-state hydrogen bond strengthening. (C) 2009 Elsevier B.V. All rights reserved.

Photochemical efficiency of Fe(III)-EDDS complex: *OH radical production and 17β-estradiol degradation

Abstract: In this work, the photochemical impact of Fe(III)-EDDS complex on the quantum yield of *OH formation and on the degradation of 17 -estradiol (E2) was investigated. The quantum yield of *OH formation by photolysis of Fe(III)-EDDS increased with increasing pH in the range of 3.0-9.0. The effect of Fe(III)-EDDS concentration, irradiation wavelength and oxygen on the quantum yield of *OH formation was also investigated. The degradation of E2 was influenced by the concentration of Fe(III)-EDDS, the solution pH, oxygen and Fe(III) concentrations. This work demonstrates that Fe(III)-EDDS is a photoactive iron species especially at higher pHs and could play an important role in the transformation of organic compounds in the natural environment.

Photo-induced treatment of breast epithelial cancer cells using nanostructured titanium dioxide solution

Abstract: The photo-induced bioactivity of titanium dioxide was investigated in terms of determining the conditions for photocatalytic treatment of cancer cells and also exploring the molecular mechanisms involved in this process. Cultured MCF-7 and MDA-MB-468 breast cancer epithelial cells were irradiated, using UV-A light (wavelength 350 nm) for 20 min, in the presence of nanostructured titania aqueous dispersions prepared using the sol–gel technique. Detailed characterization of the titania sols confirmed the presence of the photocatalyst in the form of anatase nanoparticles. Two different techniques were employed to examine the effects on cell cycle and the viability of the treated culture: propidium iodide (PI) flow cytometric (FACScan) assays permitted the identification of treatment effects on the cell cycle and cell viability analysis (MTT assays) allowed the definition of the precise percentage of cells that are still alive and functionable, after the treatment. A selective action of both TiO2 nanoparticles and photocatalytically activated titania was observed on the highly malignant MDA-MB-468 cells. Upon irradiation, these cells were induced to undergo apoptotic cell death, compared to the MCF-7 cells which were still unimpaired. This was profoundly revealed via Western blot analysis. The molecular mechanism of apoptosis is associated at least in part with increase of caspase-3-mediated poly(ADP-ribose) polymerase (PARP) cleavage.

TiO2 photocatalytic degradation of phenylarsonic acid

Abstract: Phenyl substituted arsenic compounds are widely used as feed additives in the poultry industry and have become a serious environmental concern. We have demonstrated that phenylarsonic acid (PA) is readily degraded by TiO 2 photocatalysis. Application of the Langmuir–Hinshelwood kinetic model for the initial stages of the TiO 2 photocatalysis of PA yields an apparent rate constant ( k r ) of 2.8 μmol/L min and the pseudo-equilibrium constant ( K ) for PA is 34 L/mmol. The pH of the solution influences the adsorption and photocatalytic degradation of PA due to the surface charge of TiO 2 photocatalyst and speciation of PA. Phenol, catechol and hydroquinone are observed as the predominant products during the degradation. The roles of reactive oxygen species, OH, 1 O 2 , O 2 − and h VB + were probed by adding appropriate scavengers to the reaction medium and the results suggest that OH plays a major role in the degradation of PA. By-product studies indicate the surface of the catalyst plays a key role in the formation of the primary products and the subsequent oxidation pathways leading to the mineralization to inorganic arsenic. TiO 2 photocatalysis results in the rapid destruction of PA and may be attractive for the remediation of a variety of organoarsenic compounds.

Transparent visible light activated C-N-F-codoped TiO2 films for self-cleaning applications

Abstract: The current work reports a potential technology for fabricating visible light activated doped TiO2 coatings for self-cleaning applications. Transparent C–N–F-codoped TiO2 films with enhanced visible light photocatalytic activity and non-light activated superwettability were successfully prepared by a simple layer-by-layer dip-coating method using TiO2 sol and NH4F methanol solution as precursors. The current coating method prevents the reactions of F− ions with the glass substrate and hence resulting in a uniform and transparent coating. It also creates TiO2 coating with high surface roughness without an additional pore-inducing agent and generates non-irradiated superhydrophilic surface. Contact angles of the C–N–F-codoped TiO2 films were 2.3–3.1° in the absence of any illumination and they rose slowly in the dark (<1.8° in 30 days). The C–N–F-codoped TiO2 films showed strong visible-light absorption and enhanced photocatalytic activity for stearic acid decomposition under visible light irradiation, which was 5 times higher than that of C-doped TiO2 film. Our DFT calculations also showed that increasing N:F doping ratio leads to band gap narrowing of TiO2.

Photoreaction of ethanol on Au/TiO2 anatase: Comparing the micro to nanoparticle size activities of the support for hydrogen production

Abstract: The work presents the dark and photocatalytic reactions of ethanol over Au particles deposited on TiO2 anatase nano (≤10 nm) and micro (ca. 0.15 μm) particle catalysts. The Au particles are of uniform and similar dimension (mean particle size = ca. 5 and 7 nm on the micro– and nano-sized TiO2, respectively). XPS Au4f indicated that in both cases Au particles are present in their metallic state with no evidence of charge transfer to (or from) the semiconductor. Under dark conditions, ethanol adsorption leads to stable ethoxide species (from in situ Infrared analysis) up to ca. 550 K at which point conversion to acetaldehyde by dehydrogenation and ethylene by dehydration occurs (from temperature programmed desorption (TPD) analysis). Liquid slurry photoreaction indicated the production of hydrogen with a rate ≈2 L/kgCatal min on 2 wt.% Au/TiO2 anatase nanoparticles under UV photo irradiation of comparable intensity to solar radiation. While the reaction rate per unit mass was lower on the micro-sized Au/TiO2, it simply scaled up to an equivalent rate for the nano-sized Au/TiO2 catalyst when normalised by unit area, indicating the absence of a particle size effect of the semiconductor on the electron transfer reaction within the range 10–150 nm).

Novel (and better?) titania-based photocatalysts: Brookite nanorods and mesoporous structures

Abstract: Herein, recent results concerning the synthesis of tailored anatase/brookite mixtures and of pure brookite TiO2 nanorods will be reported employing a simple hydrothermal method, i.e., the reaction of aqueous solutions of the titanium(IV) bis(ammoniumlactate) dihydroxide complex with urea. Highly ordered hexagonal P6m mesoporous Pd or Au doped TiO2 nanocomposites have also been synthesized using the F127 triblock copolymer as a template. Utilizing these nanomaterials, the effect of the phase composition, the role of the surface area, and of the ordered mesoporous structure on the photocatalytic activity of titania-based photocatalysts have been investigated. The results revealed that anatase/brookite mixtures and brookite nanorods exhibit higher photocatalytic activity than anatase nanoparticles and even higher than Aeroxide (Evonik) TiO2 P25 for the photocatalytic H2 evolution from aqueous methanol solution, despite the fact that the former have lower surface areas. This behavior is explained by the fact that the flatband potential of brookite nanorods is shifted by 140 mV more cathodically than the flatband potential of anatase nanoparticles and/or by the better charge carrier separation in the case of anatase/brookite mixtures. Hexagonal P6m mesoporous Au and Pd/TiO2 nanoarchitectures showed ∼3–4 times higher activity for the photooxidation of CH3OH than Pd photodeposited on commercial Sachtleben Hombikat UV-100. The increased HCHO formation rate revealed that the photocatalytic oxidation efficiencies within the mesoporous Pd/TiO2 system are (in spite of its lower surface area) superior to that of Pd/UV-100. The key to this success is the preparation of Pd/TiO2 networks with an ordered mesoporous structure which at the same time render the methanol diffusion into the bulk of the photocatalysts facile and hence provide fast transport channels for the methanol molecules.

Photocatalytic nitrate reduction over Pt–Cu/TiO2 catalysts with benzene as hole scavenger

Abstract: Nitrate and benzene are commonly identified contaminants in groundwater. In this study, a series of TiO 2 supported Pt–Cu bimetallic catalysts were prepared and photocatalytic nitrate reduction in the presence of benzene was investigated. The catalysts were characterized by XRD, N 2 adsorption, TEM, X-ray photoelectron spectroscopy and IR spectroscopy of CO adsorption. The results showed that Pt–Cu alloy was formed in TiO 2 supported bimetallic catalysts except for the bimetallic catalyst with TiO 2 calcined at 700 °C as the support. In addition, higher alloy dispersion and smaller metal particle sizes could be obtained on TiO 2 calcined at 300 °C compared to those calcined at 500 and 700 °C. For photocatalytic nitrate reduction in the presence of benzene, nitrate was mainly converted to ammonia or nitrite over Pt/TiO 2 or Cu/TiO 2 , respectively, whereas TiO 2 supported Pt–Cu bimetallic catalysts exhibited a considerable N 2 selectivity for photocatalytic nitrate reduction. The catalytic activity and N 2 selectivity of the supported bimetallic catalyst was strongly dependent on TiO 2 calcination temperature, Pt/Cu ratio and metal loading amount. The bimetallic catalyst with TiO 2 calcined at 300 °C as the support, Pt loading amount of 5 wt.% and Pt/Cu ratio of 4/1 displayed higher N 2 selectivity compared with other bimetallic catalysts. The present results demonstrate the selective nitrate reduction over Pt–Cu/TiO 2 catalysts with benzene as the hole scavenger, highlighting the validity of simultaneous removal of aqueous nitrate and benzene by photocatalysis.

Low temperature chemically sintered nano-crystalline TiO2 electrodes for flexible dye-sensitized solar cells

Abstract: Chemically sintered, mesoporous TiO 2 electrodes with improved interparticle contact and bond strength were prepared on indium tin oxide (ITO) coated plastic substrates in the absence of any organic binders, using hydrochloric acid as the sole reagent to encourage interparticle connectivity. The degree of chemical sintering as a function of the HCl concentration was evaluated using a newly developed nanoscratch technique. The rheological properties of the titania paste was influenced by the HCl, as was the resultant bonding of the TiO 2 grains both to each other and to the substrate. The mechanism for the low temperature chemical sintering could be due to the surface modification of titania particles during the HCl treatment and subsequent improved chemical bonding at particle contact points. The improved strength and chemical bonding of the particle network led to improved photovoltaic properties of the resulting solar cells. The highest value of light to electrical energy conversion efficiency obtained was 5% for the plastic-based dye-sensitized solar cells, under 1 sun (100 mW cm −2 ). This thus represents a novel method for ensuring the appropriate titania nanomorphology using room temperature treatment on a flexible substrate, as compared to the required thermal treatments in excess of 400 °C normally required in conventional ITO glass-based dye-sensitized solar cell devices.

Photocatalytic bleaching of p-nitrosodimethylaniline and a comparison to the performance of other AOP technologies

Abstract: In this paper, the performance of a developed photo reactor has been studied using the process of bleaching of p-nitrosodimethylaniline (RNO). The effect of available surface area in the reactor has been tested with the TiO2 suspension system as reference, as well as the influence of operating parameters as pH and aerobic/anaerobic conditions. The bleaching of RNO by UV light alone was found to be significantly slower compared to the bleaching observed when applying photocatalysis, but the initial bleaching rate when applying TiO2 in suspension was still 7 times faster compared to the most efficient immobilized TiO2 treatment set up. An approximate linear correlation between the surface area of the TiO2 available for reaction and the initial rate of bleaching was found. In addition, the kinetics of the photocatalytic bleaching was found to obey L–H kinetics. The pH of the solution was found to influence the rate of bleaching presumably due to the change in the surface charge of TiO2. Also aerobic/anaerobic conditions and the presence of hydroxyl radical scavenger had a profound influence on the bleaching rate, where the reductive bleaching process was found to be much faster than the oxidative bleaching paths. When comparing the obtained rates of bleaching of RNO in the photocatalytic reactors to two competitive AOPs, UV/S2O8 2− and conductive-diamond electrochemical oxidation (CDEO), the CDEO process was the most energy efficient for bleaching of RNO.

Controlled mesoporous self-assembly of ZnS microsphere for photocatalytic degradation of Methyl Orange dye

Abstract: In this article, we have reported self-assembled synthesis of pore tunable, mesoporous ZnS microsphere and its photocatalytic activity. The photocatalytic activity was evaluated using Methyl Orange (MO) as model pollutant under UV light irradiation. The mesoporous photocatalysts was synthesized using hydrothermal method without using any templates or catalysts in large scale. The products were characterized by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectrometry (EDX). The nitrogen adsorption analysis confirmed the presence of mesoporous structure. The pore size of the photocatalysts could be enlarged from 2.6 to 7.2 nm by changing the concentration of the zinc precursor. The microsphere formation has been facilitated by self-assembly followed by Ostwald ripening process. In conclusion, this simple synthetic methodology open new vistas to prepare mesoporous photocatalysts in large scale.

Elimination of estrogen and its estrogenicity by heterogeneous photo-Fenton catalyst β-FeOOH/resin

Abstract: β-FeOOH loaded resin (β-FeOOH/resin) was synthesized through in situ hydrolysis of Fe(III)-exchanged resin and its physiochemical properties were characterized. The typical environmental endocrine disruptor, natural hormone 17β-estradiol (E2) was removed by heterogeneous photo-Fenton reaction in presence of β-FeOOH/resin and H 2 O 2 under weak UV irradiation. E2 degradation was effectively achieved by hydroxyl radicals that were generated in the heterogeneous photo-Fenton process. pH was an important factor that affected efficiency of E2 degradation and catalyst's surface activity which were determined by X-ray photoelectron spectrum (XPS). The mechanical stability and photo activity of β-FeOOH/resin were tested by several cycles of photo catalytic degradation and FTIR. The reduction of estrogenicity of E2 would be very important to the safety of treated water so as to avoid secondary pollution. It seems that the heterogeneous Fenton oxidation would be a promising method to eliminate the steroid estrogenic compounds.

All-solid-state electrolytes consisting of ionic liquid and carbon black for efficient dye-sensitized solar cells

Abstract: All-solid-state electrolytes-based dye-sensitized solar cells (DSSCs) are constructed using a mixture of carbon black and 1-methyl-3-propylimidazolium iodide (PMII) ionic liquid without the addition of iodine, TiO 2 /FTO glass, N719, and FTO glass as electrolyte, working electrode, light harvesting material and counter electrode, respectively. The influences of the electrolyte composition (weight ratio of carbon black and PMII), TiO 2 film thickness and the compact layer on the photovoltaic parameters of DSSCs have been investigated in detail. Electrochemical impedance spectroscopy (EIS) measurement is used to analyze the influence of electrolyte composition on the photovoltaic performance. The DSSC based on a 16.2 μm TiO 2 nanocrystalline film and an all-solid-state electrolytes containing 60 mg carbon black and 100 mg PMII exhibits a power conversion efficiency of 6.37%, short-circuit current density of 15.33 mA cm −2 , open-circuit voltage of 644 mV and fill factor of 64.5%, measured at AM 1.5 G one sun (100 mW cm −2 ) illumination.

Location and catalytic role of iron species in TiO2:Fe photocatalysts: An EPR study

Abstract: A usual approach to improve the photocatalytic activity of wide band gap oxide semiconductors is through doping with transition metals. Dopants are primarily selected according to their capacity to introduce energy levels in the oxide band gap, promoting light absorbance. In the present work, we carefully characterize iron-doped titania powders obtained by a sol–gel route. Iron cations are introduced in the initial solution, before gelification, what promotes their lattice localization. X-ray diffraction and electron spin resonance spectroscopy are used to determine the temperature evolution of the crystalline structure and the dopant local environment. These results allow us to correlate the structural, electronic and chemical properties of the iron-doped titania powders with their activity towards the photocatalytic degradation of methylene blue (MB). The obtained results showed that Fe 3+ cations far from improve the photocatalytic properties of bare TiO 2 , decreased the activity towards methylene blue degradation. We associated such decrease with the role played by the iron cations depending on their position inside the lattice of TiO 2 . And second, iron cations located near or at the surface tend to form iron-based structures, such as iron titanate or pseudobrookite which are highly inactive as photocatalyst.

Influence of nitrogen chemical states on photocatalytic activities of nitrogen-doped TiO2 nanoparticles under visible light

Abstract: Nitrogen-doped TiO 2 (N-TiO 2 ) nanoparticles with a homogeneous anatase structure were synthesized using three different chemical methods. X-ray photoelectron spectra (XPS) analysis shows that nitrogen was successfully doped into TiO 2 nanoparticles and the nitrogen atoms are present in both substitutional and interstitial sites. The electron binding energy (BE) of N 1s core level is found to depend on the synthesis methods. Changes in Ti–O bond lengths of the substitutional and interstitial N doped-TiO 2 were calculated by computational geometry optimization, and confirmed by Raman shift analysis. Differences in UV–vis light absorption and visible-light-induced photocatalytic activity of three N-TiO 2 samples were attributed to their different nitrogen states within TiO 2 lattice, which would create different defect levels. The defect levels were confirmed by photoluminescence (PL) analysis and density of states (DOSs) calculation. From one to one correspondence between XP spectrum and photocatalytic activities, it is concluded that nitrogen atoms in substitutional sites enhances the photocatalysis of TiO 2 under visible light more effectively than nitrogen atoms in interstitial sites.

Singlet oxygen generation in the presence of titanium dioxide materials used as sunscreens in suntan lotions

Abstract: The goal of this study was to test photoreactivity of commercially available suntan lotions and their components (mainly TiO2). Isolated water insoluble components containing titanium dioxide appeared almost non-active in the tests of 4-chlorophenol degradation, however a fast UV-light induced degradation of azur B and oxidation of α-terpinene to ascaridol in the presence of these materials was observed. The photoreactivity of suntan lotions and their components was compared to the photoactivity of phenalenone (an efficient 1O2 photosensitizer). The results have proven a relatively low efficiency of hydroxyl radicals formation, however significant rates of reactions involving singlet oxygen were observed in the presence of either the components of the cosmetics or the suntan lotions used as received. Moreover, an efficient photocurrent generation by photoelectrodes made of isolated TiO2 materials reflects their photoredox properties. Although singlet oxygen scavengers used as additives in suntan lotions might decrease the risk related to generation of this reactive oxygen species, producers of cosmetics containing titanium dioxide should consider testing TiO2 photoactivity in reference to 1O2 generation.

Flux synthesis of AgNbO3: Effect of particle surfaces and sizes on photocatalytic activity

Abstract: The molten-salt flux synthesis of AgNbO 3 particles was performed in a Na 2 SO 4 flux using 1:1, 2:1 and 3:1 flux-to-reactant molar ratios and heating to 900 °C for reaction times of 1–10 h. Rectangular-shaped particles are obtained in high purity and with homogeneous microstructures that range in size from ∼100 to 5000 nm and with total surface areas from 0.16 to 0.65 m 2 g −1 . The smallest particle-size distributions and highest surface areas were obtained for the largest amounts of flux (3:1 ratio) and the shortest reaction time (1 h). Measured optical bandgap sizes of the AgNbO 3 products were in the range of ∼2.8 eV. The photocatalytic activities of the AgNbO 3 particles for H 2 formation were measured in visible light ( λ > 420 nm) in an aqueous methanol solution and varied from ∼1.7 to 5.9 μmol H 2 g −1 h −1 . The surface microstructures of the particles were evaluated using field-emission SEM, and the highest photocatalytic rates of the AgNbO 3 particles were correlated with the formation of high densities of ∼20–50 nm terraced surfaces. By comparison, the solid-state sample showed no well-defined morphology or microstructure. Thus, the results presented herein demonstrate the utility of flux-synthetic methods in targeting new particles sizes and surface microstructures for the enhancement and understanding of photocatalytic reactivity over metal-oxide particles.