Showing papers in "Journal of Photochemistry and Photobiology A-chemistry in 2009"

Effect of Fe3+ ion doping to TiO2 on the photocatalytic degradation of Malachite Green dye under UV and vis-irradiation

Abstract: Fe 3+ ion-doped TiO 2 particles were synthesized by the hydrothermal process at 225 °C. Titanium isopropoxide (Ti(OPr i ) 4 ) was used as precursor material. Both undoped and doped TiO 2 particles were used to coat glass surface. The coated surface was examined with respect to their photocatalytic performance for degradation of Malachite Green (MG) dye in aqueous solution under UV and vis-light irradiation. The particles and the films were characterized by XRD, SEM and UV/vis/NIR techniques. The results showed that crystallite size of the hydrothermally synthesized TiO 2 particles are in nanoscale. Anatase was only the crystalline phase. Doping of Fe 3+ ion improved the photodegradation performance of TiO 2 coated surfaces. Degradation performance of Fe 3+ doped TiO 2 coated surfaces determined under UV- and vis-irradiation conditions was higher than the undoped TiO 2 coated surface. It was concluded that the photodegradation of 2.5 mg L −1 MG under UV-light irradiation with the catalysis of Fe 3+ -doped TiO 2 follows the pseudo-first-order reaction kinetics with the rate constant of 0.0202 min −1 .

Treatment of azo dye production wastewaters using Photo-Fenton-like advanced oxidation processes: Optimization by response surface methodology

Abstract: Treatability of synthetic azo dye production wastewaters from Acid Blue 193 and Reactive Black 39 production and real Reactive Black 39 production effluent via Photo-Fenton-like process was investigated. Response surface methodology was employed to assess individual and interactive effects of critical process parameters (Fe 3+ , H 2 O 2 concentrations; initial chemical oxygen demand (COD) and reaction time) on treatment performance in terms of color, COD and total organic carbon (TOC) removal efficiencies. Optimized reaction conditions for synthetic AB 193 production wastewater were established as Fe 3+ = 1.5 mM; H 2 O 2 = 35 mM for CODs ≤ 200 mg/L and a reaction time of 45 min. Under these conditions, 98% color, 78% COD and 59% TOC removals were experimentally obtained and fitted the model predictions well. The same model also described the treatment of synthetic Reactive Black 39 production wastewater satisfactorily. Experimentally achieved removals were considerably lower than model predictions for real Reactive Black 39 production effluent due to its high chloride content.

Inactivation of Escherichia coli on immobilized TiO2 using fluorescent light

Abstract: There are many circumstances where it is necessary or desirable to remove or to kill microorganisms found on surfaces. In this paper, we present evidence of the photocatalytic inactivation of Escherichia coli (ATCC8739) cells deposited on TiO 2 loaded membrane filters during irradiation with fluorescent light. The TiO 2 selected was the photocatalyst PC105, with loadings ranging from 520 to 15,590 mg m −2 . Irradiation was produced by eight 8 W lamps with visible light, and UV (290–400 nm) at 0.05–0.12 W m −2 intensity. E. coli inactivation as a function of time was monitored for up to 120 min. In the presence of fluorescent light, the inactivation rate of E. coli increased with a decrease in the TiO 2 loading, giving the best results at 520 mg m −2 loading, with complete inactivation achieved after 2 h of exposure. TiO 2 loading higher than 6236 mg m −2 resulted in decreased inactivation. SEM images of photocatalyst and bacteria show that increasing the particle contact with the bacteria enhanced the disinfection process. Thus excess TiO 2 did not enhance the antibacterial effect, once maximum cell–photocatalyst contact had been achieved.

Architecture of supramolecular metal complexes for photocatalytic CO2 reduction: III: Effects of length of alkyl chain connecting photosensitizer to catalyst

Abstract: New Ru(II)–Re(I) binuclear complexes [{Ru(dmb)2}-LL-{Re(CO)3Cl}]2+(dmb = 4,4′-dimethyl-2,2′-bipyridine) with 1,2-bis(4′-methyl-[2,2′]bipyridinyl-4-yl)-ethane (MebpyC2H4Mebpy), 1,4-bis(4′-methyl-[2,2′]bipyridinyl-4-yl)-butane (MebpyC4H8Mebpy), and 1,6-bis(4′-methyl-[2,2′]bipyridinyl-4-yl)-hexane (MebpyC6H12Mebpy) as bridge ligands (LL) have been synthesized, and their photocatalytic activity for CO2 reduction has been investigated. The most efficient photocatalyst had MebpyC2H4Mebpy as the bridge ligand, but no difference in photocatalysis was observed between the diads with MebpyC4H8Mebpy and MebpyC6H12Mebpy. Weak interaction between the Ru and Re sites was observed only through the MebpyC2H4Mebpy ligand but not through the other bridge ligands. This interaction induces a higher reductive quenching efficiency of the 3MLCT excited state of the diad with MebpyC2H4Mebpy by the reductant, BNAH, and consequently the quantum yield of CO2 reduction is higher.

New NIR-emitting complexes of platinum(II) and palladium(II) with fluorinated benzoporphyrins

Abstract: New platinum(II) and palladium(II) complexes with fluorinated benzoporphyrins are prepared and characterized. The photo-physical and electrochemical properties, as well as quenching by oxygen are investigated. The complexes possess highly efficient room-temperature NIR phosphorescence and are excitable with blue- and red-light. The fluorinated derivatives show improved photo-physical properties and photo-stability. The new dyes are particularly suitable as indicators for the use in optical oxygen sensors.

Titanium dioxide and composite metal/metal oxide titania thin films on glass: A comparative study of photocatalytic activity

Abstract: Titania and composite (Ag, Au, W) titania coatings were prepared on glass microscope slides via a sol-gel dip-coating method. The resulting coatings were characterised by X-ray diffraction, Raman, scanning electron microscopy (SEM), wavelength dispersive X-ray (WDX), Brunauer-Emmett-Teller (BET) surface area analysis, atomic force microscopy (AFM) and UV-visible absorption techniques. Photocatalytic activity of the coatings was determined by photomineralisation of stearic acid under 254, 365 nm and white light sources monitored by FT-IR spectroscopy. These activities at 254 and 365 nm light were represented as formal quantum efficiencies (FQEs) after determination of photon flux. Water contact angle measurements were made before and after irradiation with monochromatic 254 or 365 nm light; all films demonstrated photo-assisted super hydrophilicity (PSH). A Ag:Au titania composite coating was found to be the most significant photoactive film. The mode of improved photocatalytic activity was postulated in terms of a charge separation model. The Ag:Au TiO2 coating showed potential as a useful coating for hard self-cleaning surfaces due to its robustness, stability to cleaning and reuse and its photoactive response to indoor lighting conditions. (C) 2009 Elsevier B.V. All rights reserved

Photosensitization of TiO2 by Ag2S and its catalytic activity on phenol photodegradation

Abstract: A process of TiO 2 photosensitization by coupling it with a narrow band gap semiconductor has been investigated here. Distinct TiO 2 /Ag 2 S nanocomposites were prepared by a single-source decomposition method. After sensitization, the TiO 2 materials were evaluated as photocatalysts on the degradation of aqueous phenol solutions. The experimental results show that the nanocomposites photocatalytic activity is related with the existence of Ag 2 S over the TiO 2 surface. The best catalytic results, for phenol photodegradation, were obtained using a TiO 2 /Ag 2 S material with a surface Ti/Ag atomic ratio equal to 2.40. With this material the complete photodegradation of a 0.20 mM phenol solution was achieved within 90 min, which is considerably faster when compared with the use of TiO 2 . The occurrence of main degradation byproducts was also investigated.

Enhanced photocatalytic activity under visible light in N-doped TiO2 thin films produced by APCVD preparations using t-butylamine as a nitrogen source and their potential for antibacterial films

Abstract: Atmospheric pressure chemical vapour deposition (APCVD) of N-doped titania thin films has been achieved from titanium (IV) chloride, ethyl acetate and t-butylamine at a deposition temperature of 500 °C and the films characterised by XRD, Raman spectroscopy, XPS, SEM, UV–visible–NIR spectroscopy, contact angle measurements and stearic acid degradation. The films were compared to two industrial self-cleaning products: Activ™ and BIOCLEAN™ and shown to be significantly better in both photocatalysis and superhydrophilicity, two preferential properties of effective self-cleaning coatings. X-ray diffraction showed the films have the anatase TiO2 structure. High resolution X-ray photoelectron spectroscopy was consistent with small quantities of nitrogen (0.15–0.7 at.%) occupying an interstitial site (N 1s ionisation at ∼400 eV). This work sheds light on the current confusion within the literature as to the role of nitrogen in the enhancement of the photocatalytic properties of thin films with direct evidence that selective doping at the interstitial site (ionisation ∼400 eV by XPS) has a pronounced effect on enhancing photocatalysis. Surprisingly in the majority of films no XPS peak for N for O substitution was observed (ionisation ∼396 eV by XPS). This is to our knowledge the first example of an N-doped titania film with only interstitial doping. These films showed significant photocatalysis with visible light. The best films were tested for their antimicrobial properties and found to be an effective agent for the destruction of Escherichia coli using lighting conditions commonly found in UK hospitals.

N, S co-doped and N-doped Degussa P-25 powders with visible light response prepared by mechanical mixing of thiourea and urea. Reactivity towards E. coli inactivation and phenol oxidation

Abstract: Degussa P-25 powder was mixed mechanically with thiourea and urea and then annealed during 1 h at 400 °C. Diffuse reflectance spectroscopy (DRS) revealed that treated powders absorb visible light (between 400 and 550 nm) and by Kubelka–Munk relations it was possible to estimate their band-gap energies, being 2.85 and 2.73 eV to thiourea and urea treated powders, respectively. X-ray photoelectron spectroscopy (XPS) showed in the case of thiourea treated P-25, hints of interstitial N-doping and anionic S-doping. On the other hand, P-25 treated with urea showed only the presence of interstitial N-doping. Atomic concentrations measured by XPS showed highest content of N species on TiO2 surfaces in urea treated P-25 (2.7 at%) while thiourea treated powders presented low concentrations of N and S (0.61 and 0.68 at%, respectively). Specific surface area (SSA) was measured by BET method, obtaining values of 52 and 46 m2 g−1 for undoped P-25 and undoped P-25 annealed at 400 °C during 1 h. N, S co-doped and N-doped P-25 showed SSA of 42 and 40 m2 g−1, respectively. We suggest that probably OH radical is not directly involved in oxidative processes taking place when N, S co-doped Degussa P-25 was exposed to visible light.

Effects of hydroxyl radicals and oxygen species on the 4-chlorophenol degradation by photoelectrocatalytic reactions with TiO2-film electrodes

Abstract: The supported nano-TiO 2 electrode was prepared by sol–gel and hydrothermal method, and the photoelectrocatalytic degradation of 4-chlorophenol (4-CP) under UV irradiation has been investigated to reveal the roles of hydroxyl radicals and dissolved oxygen species for TiO 2 -assisted photocatalytic reactions. The degradation kinetics, the formation and decay of intermediates, the isotopic tracer experiments with H 2 O 18 , the removal yield of total organic carbon and the formation of active radical species in the presence of oxygen or not were examined by HPLC, GC–MS, TOC and spin-trap ESR spectrometry. It was found that most of OH radicals in the primary hydroxylated intermediates derived from the oxidation of adsorbed H 2 O or HO − by photo-holes in the electrochemically assisted TiO 2 photocatalytic system. It also indicates that the enhancement in the separation efficiency of photogenerated charges by applying a positive bias (+0.5 V vs SCE) has little role in the following decomposition and mineralization of these hydroxylated intermediates in the absence of oxygen. According to above experimental results, the pathway of 4-CP photocatalytic degradation was deduced initially. Due to the combined effect of OH radicals and dissolved oxygen species, the hydroxylated 4-chlorphenol, via cis, cis -3-chloromuconic acid, was decomposed into low molecular weight acid and CO 2 .

TiO2 nanotubes as recyclable catalyst for efficient photocatalytic degradation of indigo carmine dye

Abstract: TiO 2 nanotubes have been synthesized in a hydrothermal system. The nanotubes were characterized by scanning electronic microscopy (SEM), FT-Raman spectroscopy and surface charge density by surface area analyzer. These nanocatalysts were applied to photocatalyse indigo carmine dye degradation. Photodegradation ability of TiO 2 nanotubes was compared to TiO 2 anatase photoactivity. Indigo carmine dye was completely degraded at 60 and 110 min of reaction catalysed by TiO 2 nanotubes and TiO 2 anatase, respectively. TiO 2 nanotubes presented high photodegradation activity at pH 2 and TiO 2 anatase at pH 4. TiO 2 nanotubes were easily recycled whereas the reuse of TiO 2 anatase was not effective. Nanotubes maintained 90% of activity after 10 catalytic cycles and TiO 2 anatase presented only 10% of its activity after 10 cycles.

Solvent effect on two-photon absorption and fluorescence of rhodamine dyes.

Abstract: For a series of rhodamine dyes, two-photon absorption (TPA) and two-photon fluorescence (TPF) have been performed in different solvents. Solvent-dependent TPA spectra of these dyes were measured with open aperture z-scan method and compared to their respective single-photon spectra at equivalent energies. In the TPA spectra, relative peak intensities and positions are highly solvent dependent, which could be a result of vibronic couplings that depend on solvent environment. Measured TPA cross-sections of rhodamine dyes are consistently higher in nonpolar solvents. Certain complementary and similarity between TPA and TPF are also elucidated. Finally, a two-photon figure-of-merit is presented for these dyes in different solvents as a function of wavelength.

Spectroscopic properties of lanthanoid benzene carboxylates in the solid state: Part 1

Abstract: Europium and terbium complexes of benzoic acid (HBA), 9-anthracene carboxylic acid (9-HACA), 1-naphthoic acid (1-HNA), 2-naphthoic acid (2-HNA), cinnamic acid (HCA), o-phenylbenzoic acid (o-HPhBA), p-phenylbenzoic acid (p-HPhBA), o-methylbenzoic acid (o-HMeBA), m-methylbenzoic acid (m-HMeBA), p-methylbenzoic acid (p-HMeBA), p-tert-butylbenzoic acid (p-tertHBuBA), phthalic acid (H2Phth), isophthalic acid (H2-IsoPhth) and terephthalic acid (H2-TerePhth) were synthesised by aqueous metathesis reactions The compositions were determined by microanalysis, DTA, and EDTA volumetric methods Structural conclusions were drawn from powder XRD patterns and IR spectra The optoelectronic properties were determined in the solid state, including reflectance, excitation, emission, and excitation efficiency, complemented by considering triplet state energies, which were derived from phosphorescence spectra of the corresponding gadolinium complexes Besides the emission efficiencies, the splitting pattern of the 5D0 → 7FJ transitions of Eu3+ were used to derive structural information and the intensities relative to the 5D0 → 7FJ were used to gain information about the mechanism, adding electronic allowed dipole character to the forbidden emissions

Peripheral and non-peripheral tetrasubstituted aluminium, gallium and indium phthalocyanines: Synthesis, photophysics and photochemistry

Abstract: The synthesis, photophysical and photochemical properties of soluble triethyleneoxythia substituted aluminium, gallium and indium phthalocyanines ( 6a – c and 7a – c ) are reported for the first time. The new compounds have been characterized by elemental analysis, FT-IR, 1 H NMR spectroscopy, electronic spectroscopy and mass spectra. General trends are described for quantum yields of photodegradation, fluorescence and fluorescence lifetimes as well as singlet oxygen quantum yields of these compounds. The effects of the position of the substituents and the nature of the metal ion on the photophysical and photochemical parameters of the Al(III), Ga(III) and In(III) phthalocyanines ( 6a – c and 7a – c ) are also reported. Photophysical and photochemical properties of phthalocyanine complexes are very useful for photodynamic therapy (PDT) applications. The singlet oxygen quantum yields ( Φ Δ ), which give an indication of the potential of the complexes as photosensitizers in applications where singlet oxygen is required (Type II mechanism) ranged from 0.27 to 0.90. The substituted Al(III), Ga(III) and In(III) phthalocyanines showed high singlet oxygen quantum yields. High singlet oxygen quantum yields are very important for Type II mechanism. Thus, these complexes show potential as Type II photosensitizers.

Layer-by-layer TiO2 films as efficient blocking layers in dye-sensitized solar cells

Abstract: Charge recombination at the conductor substrate/electrolyte interface has been prevented by using efficient blocking layers of TiO2 compact films in dye-sensitized solar cell photoanodes Compact blocking layers have been deposited before the mesoporous TiO2 film by the layer-by-layer technique using titania nanoparticles as cations and sodium sulfonated polystyrene, PSS, as a polyanion The TiO2/PSS blocking layer in a DSC prevents the physical contact of FTO and the electrolyte and leads to a 28% increase in the cell's overall conversion efficiency, from 57% to 73%

Fluorescence modulation and photodegradation characteristics of safranin O dye in the presence of ZnS nanoparticles

Abstract: ZnS nanoparticles were synthesized using a chemical precipitation method and were characterized with FTIR, transmission electron microscope (TEM), X-ray diffraction analysis (XRD) and UV–vis absorption. XRD analysis shows that the diameter of the particles is 1.6 nm. The interaction between ZnS nanoparticles and safranin O (SO) dye was studied with UV–vis absorption as well as fluorescence emission and excitation spectra. The results show fluorescence enhancement from dye molecules with nanoparticles upon excitation at 325 nm. In contrast, the fluorescence of the dye monitored at 520 nm is quenched by ZnS nanoparticles. ZnS nanoparticles were used as a photocatalyst in order to degrade SO dye. A maximum degradation efficiency of 51% of the dye has been achieved in the presence of ZnS as a nanophotocatalyst at pH 7. Photodegradation of the dye follows second-order kinetics.

Reconsideration to the deactivation of TiO2 catalyst during simultaneous photocatalytic reduction of Cr(VI) and oxidation of salicylic acid

Abstract: Photocatalytic reduction/oxidation and deactivation of TiO 2 photocatalyst was investigated in the systems composed of Cr(VI) and salicylic acid. The selection of analysis method of Cr(IV) was very important to the monitoring of the photocatalytic process. It was found that as previously reported, serious deactivation of TiO 2 catalyst in the simultaneous photo-reduction of Cr(VI) and oxidation of salicylic acid was incorrectly observed if the Cr(VI) level was analyzed by directly monitoring the absorbance at characteristic 348 nm band of Cr(VI), because it seriously suffers from the interferences of the intermediates generated from the degradation of salicylic acid. By using an appropriate method to determine the Cr(VI) concentration, it was observed that all the added Cr(VI) could be reduced, not showing marked deactivation of the photocatalyst. A long time photocatalytic reduction of Cr(VI) under UV illumination induced the deposition of Cr(III) species on the surface of TiO 2 particles, which could cause a mild deactivation of the photocatalyst. However, the accompanied oxidation of salicylic acid was demonstrated to depress the deactivation effect of the deposited Cr(III) species on the photocatalytic activity of the TiO 2 photocatalyst.

Photocatalytic oxidation of nitrogen monoxide using TiO2 thin films under continuous UV light illumination

Abstract: A deactivating behavior of TiO2 photocatalysts in NO2 oxidation to HNO3 was studied with use of TiO2 nanoparticulate thin films (0.5−1.5 μm thick) under UV light illumination over 10 h. The photocatalytic activity was decreased with accumulation of HNO3 on the TiO2 surface. For thicker TiO2 films, the deactivation rate was found to be slower. The amount of HNO3 trapped on the TiO2 surface was increased and finally saturated, at which the largest amount of HNO3 was proportional to the thickness of the film. On the basis of the results, we concluded that the produced HNO3, inhibiting the reaction as a physical barrier, must be able to diffuse on the TiO2 surface at a rate of at least more than 1.5 μm h−1, and finally distributes homogeneously on the whole film. The maximum density of HNO3 accumulated on the TiO2 surface was estimated to be ∼2 molecules nm−2 under standard conditions. Finally, when the steady state is reached, the photocatalytic activity remained ∼8% of the initial one.

Spiropyran-based reversible, light-modulated sensing with reduced photofatigue

Abstract: Switchable materials have tremendous potential for application in sensor development that could be applied to many fields. We are focusing on emerging area of wireless sensor networks due to the potential impact of this concept in society. Spiropyran-based sensors are probably the most studied type of photoswitchable sensing devices. They suffer from many issues but photofatigue, insufficient selectivity and lack of sensitivity are probably the most important characteristics that hinder their wider application. Here, we are address these issues and demonstrate that covalent attachment of modified spiropyran into a polymeric film significantly reduces photodegradation. The observed signal loss after 12th cycle of switching between the spiropyran and merocyanine forms is only about 27% compared to the loss of 57% of the initial signal in an equivalent experiment based on non-immobilized spiropyran. This has enabled us to demonstrate at least five reversible cycles of detection of an ion of interest (in our case H+) with minimal signal loss. Furthermore, we demonstrate that the sensitivity can be increased by incorporation of additional binding groups in the parent spiropyran molecule. Using molecular modelling to calculate the relevant bond lengths as a measure of interaction between MC and H+, the calculated increase of H-bond strength is approximately an order of magnitude for a derivative containing a methoxy group incorporated in the o-position of the parent spiropyran in comparison to the equivalent unsubstituted phenol. This theoretical result was found to correspond very well with experimental observation. As a result, we have increased the sensitivity to H+ by approximately one order of magnitude.

Effect of substituents on the photoluminescence performance of Ir(III) complexes: Synthesis, electrochemistry and photophysical properties

Abstract: We have prepared and characterized a series of substituted imidazole ligands namely dmmppi, dmmpfpi, dmdmppi and dmdmpfpi. These compounds will readily undergo cyclometalation with iridium trichloride and form di-irrido and the six coordinated iridium(III) dopants of the substituted imidazole ligands. They emit green colour both in solid and in solution phase. The peak emission wavelength of the dopants ( λ max = 428–497 nm) can be finely tuned depending upon the electronic properties of the phenyl, fluorophyenyl, methoxy phenyl and dimethoxyphenyl substituents as well as their positions in the imidazole ring. These iridium complexes namely Ir(dmmppi) 2 (pic) 1a , Ir(dmmpfpi) 2 (pic) 1b , Ir(dmdmppi) 2 (pic) 1c and Ir(dmdmpfpi) 2 (pic) 1d were characterized by 1 H NMR, MS and elemental analysis. All these iridium complexes 1a – 1d show unusual high HOMO levels ( E HOMO = 5.21–5.41 eV) and high phosphorescence. These complexes emit green light with exceedingly high efficiency.

Near Ultraviolet and Visible light photoelectrochemical degradation of organic substances producing electricity and hydrogen

Abstract: A two-compartment, chemically biased photoelectrochemical cell was used to photodegrade several organic substances and produce electricity and hydrogen. The photoanode of the cell was based on commercial nanocrystalline titania, which may be functionalized with CdS to absorb Visible light. The dark cathode was made also of commercial nanocrystalline titania with cast Pt nanoparticles. The cell was run under Near Ultraviolet or Visible radiation and the efficiencies in the two cases have been compared. Short chain-length alcohols and glycerol produced the highest energy yield but water-soluble waste materials are also interesting for the same purpose. The overall efficiency of cells made by CdS-functionalized titania was higher than those made of pure titania. However, larger currents were obtained with pure TiO2 under Near Ultraviolet excitation. Under anaerobic conditions, molecular hydrogen can be produced at the dark cathode by the reduction of hydrogen ions generated at the photoanode.

Photocatalytic and photoelectrochemical studies on N-doped TiO2 photocatalyst

Abstract: One of the commercial N-doped TiO2 powders (Sumitomo Chemicals, TPS 201) was studied as a model material by photocatalytic and photoelectrochemical methods in order to evaluate the photoactivity of N-doped TiO2 materials and the possibility of their applications in solar photocatalysis. The N-doped TiO2 powder (TPS) was able to degrade and mineralize phenol under solar or visible light (VL) irradiation, and the degradation rate was strongly dependent on the suspension concentration. Photoelectrochemical studies showed that the VL-irradiated TPS electrode was able to oxidize water, phenol, as well as maleic acid, an open-ring oxidized product of phenol, consistent with the results of photocatalytic studies. Calcining the TPS powder at/over 773 K was found to lower the absorption in the VL region and the photocatalytic activity under VL irradiation, but improve the photocatalytic activity under solar irradiation, suggesting that the mid-gap states introduced by nitrogen doping also worked as recombination centers. Deposition of Pt (0.2 wt%) on the TPS photocatalyst thus greatly increased the degradation rate of phenol under either solar or VL irradiation due to the suppression of charge recombination, and the degradation rate was found to be higher than a pristine TiO2 photocatalyst (ST-01, anatase) either with or without loading of Pt. The potential application of the N-doped TiO2 in solar photocatalysis was discussed on the basis of above-mentioned studies.

Nitrogen-doped and nitrogen-fluorine-codoped titanium dioxide. Nature and concentration of the photoactive species and their role in determining the photocatalytic activity under visible light”

Abstract: Two series of N-doped and N, F-codoped titanium dioxide samples having different loading of impurity were prepared using sol–gel synthesis and successive calcinations in air. Ammonium chloride and ammonium fluoride were used as source of dopants in the two cases. In all cases the insertion of nitrogen is observed which determines the optical absorption in the visible of the materials and their photocatalytic activity under solar light. This was investigated following the decomposition of methylene blue in aqueous phase. Two photoactive species (describable in terms of NO2− and NO3− bulk species) determine the optical absorption in the visible range. The former one (paramagnetic) is detectable by Electron Paramagnetic Resonance (EPR) and its intensity in all samples is roughly proportional to the optical absorption at 440 nm. The photocatalytic activity under solar light directly depends on the concentration of the nitrogen photoactive species in the solid till a saturation limit, which in turn, depends on the concentration of ammonium ions in the starting solution and on the heating rate during calcinations. The use of ammonium fluoride in the synthesis promotes N insertion and increments the photocatalytic activity due to the fact that, differently from Cl− ions, F− ions substitutes oxygen in the solid lattice giving rise to electronic effects which favour N photoactive species incorporation in the solid.

Substituent and solvent effects on excited state intramolecular proton transfer in novel 2-(2′-hydroxyphenyl)benzothiazole derivatives

Abstract: A group of novel 2-(2′-hydroxyphenyl)benzothiazole derivatives 1–5 were synthesized with electron-donating or -withdrawing substituent introduced in para position of N atom in benzothiazolyl ring. The excited state intramolecular proton transfer (ESIPT) in 1–5 along with non-substituted 2-(2′-hydroxyphenyl)benzothiazole 6 was studied by means of UV–vis absorption and steady-state fluorescence in solutions. Compounds 1–6 exhibit dual fluorescences including purple normal emission and green tautomer emission. Systematical comparison of the fluorescence of any analogue in a series of solvents ranging from protic ethanol to non-polar hexane demonstrated that polar solvents favor the normal emission while non-polar solvents facilitate ESIPT process and tautomer formation and emission. In either protic or non-polar solvent the tautomer emission intensity of 1–6 decreases consecutively in the order of decreasing electron-donating ability or increasing electron-withdrawing ability of the substituents, on the premise of identical normal emission intensity. This indicates that electron-donating substituents in these derivatives favor ESIPT process and tautomer emission. Competition of intra- and intermolecular hydrogen bonding was studied in dioxane-water binary solvent. It is demonstrated that intermolecular hydrogen bonding with protic solvent impedes ESIPT and tautomer emission. The fluorescent behaviors of 1–6 were interpreted in terms of the population of ground-state rotamers responsible for normal and tautomer emission respectively.

Sol-gel modified TiO2 powder films for high performance dye-sensitized solar cells

Abstract: Techniques in TiO2 film fabrication process are a very important aspect in construction of efficient dye-sensitized solar cells (DSSCs). Among fundamental factors in high efficiency solar cells is the development of a highly porous nanocrystalline film in a relatively thick and mechanical stable TiO2 network. In present work, two types of commercial titania powders with high loading (74% (wt.%) in the final films), Degussa P25 powders and Ishihara ST-01 powders, have been successfully incorporated into a mesoporous structured TiO2 crystalline network. The good mechanical stability of both types of films involves the presence of a titanium alkoxide solution mixed with non-ionic surfactant Tween 20 as a suitable binder precursor to immobilize commercial titania nanoparticles on a SnO2:F conductive glass via a dip coating sol–gel process. The results of photoelectrochemical characterization of the films in a dye-sensitized solar cell showed that both films presented high conversion efficiency (about 6%) with only a slight difference on their photovoltaic behavior, when a quasi-solid state electrolyte was employed. The crystalline structure of the foreign titania particles, the particle size and the existence of any agglomeration, as well as the surface area and the pore structure of these films made of different titania powder sources were examined in detail and are discussed in relation to the relatively high solar to electrical energy conversion efficiency obtained in such quasi-solid state DSSCs.

Evaluation of two types of TiO2-based catalysts by photodegradation of DMSO in aqueous suspension

Abstract: Among the different methods to evaluate the photocatalytic activity of TiO2-based photocatalysts, the most employed has been the methylene blue method. However, several questions have emerged during the lasts years about its reliability, because this compound can absorb light in the visible range. In this work a new compound is used as photocatalytic test-substance, the dimethylsulfoxide (DMSO), which will be used in the ISO standard (ISO/CD 10676) for the determination of the photocatalytic efficiency of systems used for water treatment. This solvent is a hydroxyl scavenging agent, which can interact with OH. The study is focused on the degradation of this molecule by two different catalysts: Degussa P25 and Hombikat UV100. The reaction is studied by the monitoring of two of the intermediates formed during the reaction: the methansulfonate (CH3SO3−) and the sulfate (SO42−), by means of the ion chromatography technique, and by measuring the TOC. In general, TiO2 Degussa P25 shows better results in terms of generation of two intermediates (methansulfonate and sulfate), TOC removal, and photonic efficiencies. Although Hombikat UV100 has six times larger specific surface than the other photocatalyst, the reason for the higher photoactivity of Degussa P25 can lie in the slower recombination rate of the electron/hole pairs, and also in the kind of aggregates formed in aqueous solution.

Revisiting the electronic excited-state hydrogen bonding dynamics of coumarin chromophore in alcohols: Undoubtedly strengthened not cleaved

Abstract: In the present work, the electronic excited-state hydrogen bonding dynamics of coumarin chromophore in alcohols is revisited. The time-dependent density functional theory (TDDFT) method has been performed to investigate the intermolecular hydrogen bonding between Coumarin 151 (C151) and methanol (MeOH) solvent in the electronic excited state. Three types of intermolecular hydrogen bonds can be formed in the hydrogen-bonded C151–(MeOH) 3 complex. We have demonstrated again that intermolecular hydrogen bonds between C151 and methanol molecules can be significantly strengthened upon photoexcitation to the electronically excited state of C151 chromophore. Our results are consistent with the intermolecular hydrogen bond strengthening in the electronically excited state of Coumarin 102 in alcoholic solvents, which has been demonstrated for the first time by Zhao et al. At the same time, the electronic excited-state hydrogen bond cleavage mechanism of photoexcited coumarin chromophores in alcohols proposed in some other studies about the hydrogen bonding dynamics is undoubtedly excluded. Hence, we believe that the two contrary dynamic mechanisms for intermolecular hydrogen bonding in electronically excited states of coumarin chromophores in alcohols are clarified here.

Correlation of oxidative and reductive dye bleaching on TiO2 photocatalyst films

Abstract: A series of titania films of different thicknesses (60–420 nm), and therefore activities, are produced using a sol–gel method and assessed for photocatalytic activity using three test methods. The first two involve the photo-oxidative bleaching of common dyes, namely methylene blue (MB), a cationic thiazine dye, and Acid Orange 7 (AO7), an anionic, azo dye, in aerated aqueous solution. The third test uses an ink comprising, amongst other things, a redox dye, Resazurin (Rz), and an easily oxidized species, glycerol. When deposited on a titania film and irradiated with UV light the colour of this Rz ink (initially blue) changes to pink due to the titania-sensitized reduction of Rz. The initial rates of destruction of MB and AO7 and the initial rate of reduction of Rz depend directly upon the thickness of the titania films, indicating a strong correlation between the different tests. The significance of these results is discussed in the context that the MB test is currently being considered by International Organization for Standardization (ISO) as a standard method for assessing the photocatalytic activities of the surfaces. Compared with MB, Rz ink test is much faster and easier to implement, which suggests that there would be significant advantages to employing the Rz test, instead of another dye test, for assessing the photocatalytic thin films activities.

Photoelectrocatalytic degradation of ethylene by a combination of TiO2 and activated carbon felts

Abstract: Postharvest loss of quality is an important problem in the food and horticultural product industry. One of the major factors contributing to loss of quality is the uncontrolled exposure of the products to small amounts of ethylene gas during storage. In this study we investigated the photoelectrocatalytic (PEC) degradation of ethylene gas at a temperature of 3 ± 1 °C and relative humidity of 90 ± 3% on an activated carbon felts (ACF)-supported photocatalyst titanium dioxide photoelectrode [TiO 2 /ACF] or on a photoelectrode which had been modified by coating the ACF support with platinum [TiO 2 /ACF-Pt]. The apparent pseudo-first-order kinetic model was used to describe the PEC degradation of ethylene. The key designing parameters for a PEC reactor affecting the degradation efficiency in terms of the rate constant of this model were studied, including the bias voltage and the light intensity. Degradation of ethylene by applying a bias voltage to the [TiO 2 /ACF] |Nafion|[TiO 2 /ACF] electrode-membrane assembly or to the [TiO 2 /ACF-Pt] |Nafion|[TiO 2 /ACF-Pt] electrode-membrane assembly enhanced the efficiency of photocatalytic (PC) degradation. The combination of the ACF support modified with platinum and the applied bias voltage were found to have an additive enhancement effect on the rate constant compared to PEC degradation carried out using the unmodified ACF support. With respect to the [TiO 2 /ACF-Pt] |Nafion|[TiO 2 /ACF-Pt] electrode-membrane assembly, a kinetic model was established using response surface methodology to describe the relationship between the rate constant and the affecting parameters. Optimized parameters were found to be a light intensity of 3.1 mW cm −2 with a bias voltage of 47.5 V.

Effect of thickness of ALD grown TiO2 films on photoelectrocatalysis

Abstract: Atomic layer deposited TiO2 films were used for photoelectrocatalysis for the first time. Films with different thicknesses were grown and comprehensively characterized by structural and photoelectrochemical techniques. Methylene blue was used as a model substance in photocatalytic and photoelectrocatalytic degradation studies. In both cases, degradation rate was observed to saturate above certain film thickness. The reason for saturation could not be explained by the electrical parameters alone, and it was suggested that film morphology and crystalline structure are of major importance in this case.