Photoreduction Reaction Research Articles

Robust Hollow Spheres Consisting of Alternating Titania Nanosheets and Graphene Nanosheets with High Photocatalytic Activity for CO2 Conversion into Renewable Fuels

AbstractRobust hollow spheres consisting of molecular‐scale alternating titania (Ti0.91O2) nanosheets and graphene (G) nanosheets are successfully fabricated by a layer‐by‐layer assembly technique with polymer beads as sacrificial templates using a microwave irradiation technique to simultaneously remove the template and reduce graphene oxide into graphene. The molecular scale, 2D contact of Ti0.91O2 nanosheets and G nanosheets in the hollow spheres is distinctly different from the prevenient G‐based TiO2 nanocomposites prepared by simple integration of TiO2 and G nanosheets. The nine times increase of the photocatalytic activity of G‐Ti0.91O2 hollow spheres relative to commercial P25 TiO2 is confirmed with photoreduction of CO2 into renewable fuels (CO and CH4). The large enhancement in the photocatalytic activity benefits from: 1) the ultrathin nature of Ti0.91O2 nanosheets allowing charge carriers to move rapidly onto the surface to participate in the photoreduction reaction; 2) the sufficiently compact stacking of ultrathin Ti0.91O2 nanosheets with G nanosheets allowing the photogenerated electron to transfer fast from the Ti0.91O2 nanosheets to G to enhance lifetime of the charge carriers; and 3) the hollow structure potentially acting as a photon trap‐well to allow the multiscattering of incident light for the enhancement of light absorption.

Improving the Performance of a Zinc Oxide Nanowire Ultraviolet Photodetector by Adding Silver Nanoparticles

A significant enhancement in the performance of a ZnO nanowire-based UV detector has been achieved through fabricating a Ag/ZnO heterostructure by a photoreduction reaction. This enhancement is due to a Schottky barrier at the interface of Ag and ZnO because of the formation of a thin AgOx layer. An efficient and low-working-temperature method was proposed for eliminating the familiar persistent photoconductivity in ZnO nanostructure-based photodetectors, and the approach can be widely applied to other photodetectors employing the Schottky barrier phenomenon.

Layer-by-layer inkjet printing of fabricating reduced graphene-polyoxometalate composite film for chemical sensors

Graphene oxide (GO) nanosheets and polyoxometalate such as H(3)PW(12)O(40) (PTA) are prepared into a multilayer film via a layer-by-layer inkjet printing method. The GO/PTA composite thin film shows linear, uniform and regular layer-by-layer growth. Under UV-irradiation, a photoreduction reaction takes place in the film which converts GO to reduced GO (rGO) due to the photoreduction activity of polyoxometalate clusters. According to the cyclic voltammograms measurement, the rGO/PTA composite film displays good electrocatalytic activity for the oxidation of dopamine (DA). The oxidation peak current (I(pa)) increases gradually with increasing the dopamine concentration, which may be used in electrochemical biosensors.

Hierarchical TiO2 Microspheres: Synergetic Effect of {001} and {101} Facets for Enhanced Photocatalytic Activity

Well-faceted nanocrystals of anatase TiO(2) with specific reactive facets have attracted extraordinary research interest due to their many intrinsic shape-dependent properties. In this work, hierarchical TiO(2) microspheres consisting of anatase nanosheets or decahedrons were synthesized by means of a facile hydrothermal technique; meanwhile, the percentage of {001} facets can be tuned from 82 to 45%. Importantly, by investigating the photo-oxidation reactions for ˙OH radical generation and photoreduction reactions for hydrogen evolution, the TiO(2) microspheres consisting of nano-decahedrons with 45% {001} facets show superior photoreactivity (more than 4.8-times) compared to the nanosheets with 82% {001} facets. By analyzing the results of scanning electron microscopy (SEM), photoluminescence (PL) and first-principles density functional theory (DFT) calculations, a model of charge separation between the well-formed {001} and {101} facets is proposed, and the enhanced photocatalytic efficiency is largely attributed to the efficient separation of photogenerated charges among the crystal facets co-exposed.

Influence of the chloro substituent position on the triplet reactivity of benzophenone derivatives: a time‐resolved resonance Raman and density functional theory study

A nanosecond time‐resolved resonance Raman (ns‐TR3) spectroscopic investigation of the photoreduction reactions and ability of several chloro‐substituted benzophenone (Cl‐BP) triplets is described. The TR3 results show that the 3‐chlorobenzophenone (3‐Cl‐BP), 4‐chlorobenzophenone (4‐Cl‐BP) and 4,4′‐dichlorobenzophenone (4,4′‐dichloro‐BP) triplets exhibit similar hydrogen abstraction ability with the parent BP triplet. In 2‐propanol, the 3‐Cl‐, 4‐Cl‐ and 4,4′‐dichloro‐diphenylketyl (DPK) radicals were observed and they appear to react with dimethylketyl radicals at the para‐position to form a light absorption transient species. These transient species were characterized with TR3 spectra, and identified with the help of results from density functional theory calculations. In an acetontitrile/water (MeCN:H2O) 1:1 mixed solvent, these DPK radicals were also observed but with slower formation rates. However, the 2‐Cl‐DPK radical was observed to form with a lower yield and a significantly slower formation rate than the other chloro‐substituted benzophenones examined here in 2‐propanol under the same experimental conditions. These results reveal that the 2‐chloro substituent reduces the hydrogen abstraction ability of the substituted BP triplet, which was not as expected based on the assumption that the electron‐withdrawing group could increase its photoreduction ability. This unusual ortho effect of the chlorine substitution is briefly discussed. Copyright © 2011 John Wiley & Sons, Ltd.

High-Yield Synthesis of Ultrathin and Uniform Bi2WO6 Square Nanoplates Benefitting from Photocatalytic Reduction of CO2 into Renewable Hydrocarbon Fuel under Visible Light

Ultrathin and uniform Bi(2)WO(6) square nanoplates of ∼9.5 nm thickness corresponding to six repeating cell units were prepared in the presence of oleylamine using a hydrothermal route. The Bi(2)WO(6) nanoplates show great potential in the utilization of visible light energy to the highly efficient reduction of CO(2) into a renewable hydrocarbon fuel. On the one hand, the ultrathin geometry of the nanoplates promotes charge carriers to move rapidly from the interior to the surface to participate in the photoreduction reaction. This should also favor the improved separation of photogenerated electron and hole and a lower electron-hole recombination rate; on the other hand, the Bi(2)WO(6) square nanoplate is proven to provide the well-defined {001} facet for two dominantly exposed surfaces, which is a prerequisite for the high level of photocatalytic activity of CO(2) fixation.

Layer-by-Layer Assembly and UV Photoreduction of Graphene–Polyoxometalate Composite Films for Electronics

Graphene oxide (GO) nanosheets and polyoxometalate clusters, H(3)PW(12)O(40) (PW), were co-assembled into multilayer films via electrostatic layer-by-layer assembly. Under UV irradiation, a photoreduction reaction took place in the films which converted GO to reduced GO (rGO) due to the photocatalytic activity of PW clusters. By this means, uniform and large-area composite films based on rGO were fabricated with precisely controlled thickness on various substrates such as quartz, silicon, and plastic supports. We further fabricated field effect transistors based on the composite films, which exhibited typical ambipolar features and good transport properties for both holes and electrons. The on/off ratios and the charge carrier mobilities of the transistors depend on the number of deposited layers and can be controlled easily. Furthermore, we used photomasks to produce conductive patterns of rGO domains on the films, which served as efficient microelectrodes for photodetector devices.

Hybrid Coatings Containing Silver Nanoparticles Generated In situ in a Thiol‐Ene Photocurable System

AbstractNovel silver/polymer composites based on thiol‐ene chemistry are prepared by an in situ bottom‐up approach. The in situ synthesis of silver particles inside the polymer matrix is achieved in one pot by photoreduction reaction in presence of a silver precursor and the concurrent crosslinking reaction. XPS analysis confirms the formation of silver particles; TEM morphological investigation shows a very good dispersion and distribution of the nanometric silver particles within the thiol‐ene network. Antimicrobial properties of the photocured hybrids are also evaluated. magnified image

Reaction Mechanisms and Structural Characterization of the Reactive Intermediates Observed after the Photolysis of 3-(Hydroxymethyl)benzophenone in Acetonitrile, 2-Propanol, and Neutral and Acidic Aqueous Solutions

Nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy was employed to investigate the photoinduced reactions of 3-(hydroxymethyl)benzophenone (1) in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions. Density functional theory calculations were utilized to help the interpretation of the experimental spectra. In acetonitrile, the neutral triplet state 1 [denoted here as (m-BPOH)(3)] was observed on the nanosecond to microsecond time scale. In 2-propanol this triplet state appeared to abstract a hydrogen atom from the solvent molecules to produce the aryphenyl ketyl radical of 1 (denoted here as ArPK of 1), and then this species underwent a cross-coupling reaction with the dimethylketyl radical (also formed from the hydrogen abstraction reaction) to form a long-lived light absorbing transient species that was tentatively identified to be mainly 2-(4-(hydroxy(3-(hydroxymethyl)phenyl)methylene)cyclohexa-2,5-dienyl)propan-2-ol. In 1:1 H(2)O:CH(3)CN aqueous solution at neutral pH, (m-BPOH)(3) reacted with water to produce the ArPK of 1 and then underwent further reaction to produce a long-lived light absorbing transient species. Three photochemical reactions appeared to take place after 266 nm photolysis of 1 in acidic aqueous solutions, a photoreduction reaction, an overall photohydration reaction, and a novel photoredox reaction. TR(3) experiments in 1:1 H(2)O:CH(3)CN aqueous solution at pH 2 detected a new triplet biradical species, which is associated with an unusual photoredox reaction. This reaction is observed to be the predominant reaction at pH 2 and seems to face competition from the overall photohydration reaction at pH 0.

Photodeposition of Co-Based Oxygen Evolution Catalysts on α-Fe2O3 Photoanodes

Cobalt-based oxygen evolution catalysts contain-ing phosphates (Co-Pi OEC) were photochemically deposited onto the surface of n-type α-Fe2O3 electrodes to enhance solar O2 production. α-Fe2O3 films used in this study were prepared by electrodepositing Fe films followed by thermal oxidation at 500 °C. The use of a nonaqueous plating solution made it possible to deposit adherent and uniform Fe films, which is difficult to achieve in an aqueous medium. Photodeposition of Co-Pi OEC was carried out by using photogenerated holes in the valence band of α-Fe2O3 to oxidize Co2+ ions to Co3+ ions in a phosphate buffer solution, which resulted in the precipitation of Co-Pi OEC on the α-Fe2O3 surface. Two different deposition conditions, open circuit (OC) and short circuit (SC) conditions, were studied comparatively to understand their effect on the growth and composition of Co-Pi OEC deposits. The results showed that the SC condition where the photoreduction reaction is physically separated from the photo-oxidation ...

Proton-controlled photoisomerization of 1-(2-pyridyl)-2-(2-quinolyl)ethylene

Quantum-chemical calculations (B3LYP/6-31G*) predict the formation of intramolecular hydrogen bond (IMHB) in the monoprotonated Z-isomer of 1-(2-pyridyl)-2-(2-quinolyl)ethylene (2P2Q), with this bond stabilizing the isomer relative to its E-counterpart. An experimentally observed increase in the quantum yield of trans-cis photoisomerization (φtc) by more than an order of magnitude (from 0.033 to 0.42 in acetonitrile) on passing from the neutral to the monoprotonated form of 2P2Q can be associated with IMHB, which manifested itself in the spectral properties of the Z-isomer. The IMHB breaks in the diprotonated form, and the value of φtc decreases back to the initial value. In addition to the photoisomerization, the photoreduction and photoaddition reactions of solvent molecules have been observed in an ethanol solution of 2P2Q.

Photocatalytic decolorization of soluble dyes by a bis-ions coexistence system of NH4+ and NO3− with high photoreduction ability

In this paper, we found that the acidic and basic dyes were easily decolorized by a bis-ions coexistence system of NH4+ and NO3− under UV light irradiation. The coexistence of NH4+ and NO3− is a necessary condition for the photocatalytic decolorization of soluble dyes. The photocatalytic decolorization of methyl orange (MO) and methylene blue (MB) follows the first order rate kinetics. The location of an absorption peak in the visible region is blue-shifted with the increase in the illumination time. It is proposed that the photocatalytic decolorization of soluble dyes in the bis-ions coexistence system of NH4+ and NO3− is a photoreduction reaction, in which the ammonium nitrate acts as a photocatalyst. The chromophore of acidic and basic dyes reacts with hydrogen and then results in their rapid decolorization.

ChemInform Abstract: Photochemistry of Tetrabenzo(c,e,i,k)(1,2,7,8)tetraazacyclododecine in 2-Propanol.

The photochemical properties of the tetraazacyclododecine 1 were investigated. The trans,trans isomer 1tt can be photoisomerized yielding the stable cis,cis isomer 1ee. The isomerization requires two photons and proceeds via the unstable trans,cis isomer 1tc. In nitrogen-purged 2-propanol as solvent 1 yields five stable photoproducts as a result of a number of consecutive photoreduction reactions. Our attempts to convert 1 by light into the tetraazetidine 4 were not successful.

Resonance Raman spectroscopic and density functional theoretical study of dynamical structures of Cu(II)-bisacetylacetone

The A- and B-band resonance Raman spectra of Cu(II)-bisacetylacetone in methanol solution were obtained and their intensity patterns were found to be significantly different. It appears that the A-band structural dynamics is mostly along the photophysically and photochemically important C γC β/C βC α and Cu–O stretch reaction coordinates, while the B-band structural dynamics is mainly along photochemically important Cu–O stretch reaction coordinate. The significantly different structural dynamics provide an evident that the primary photoreduction reactions of Cu(II)-bisacetylacetone are wavelength dependence.

Properties of photoreduction reaction of cytochrome b559 in photosystem II membrane fragments

Properties of photoreduction reaction of cytochrome b559 in photosystem II membrane fragments

Products and mechanisms of photochemical transformations of o-quinones

The photochemical transformations of quinones by the action of light at λ > 500 nm, namely, the photodecarbonylation and photoreduction reactions were studied with the use of a series of o-benzoquinones and 9,10-phenanthrenequinone as examples. The two-stage mechanism of the decarbonylation reaction of o-benzoquinones was established. At the first stage, rearrangement of a photoexcited quinone molecule into a bicyclic compound that spontaneously decomposes in the dark reaction into cyclopentadienone and CO takes place. It has been found that the formation of the photoreduction products of both o-benzoquinones and 9,10-phenanthrenequinone in the presence of various H donors (N,N-dimethylanilines and polymethylbenzenes) follows the same mechanism. In the first step, a phenol ether is produced, which subsequently undergoes quantitative transformation into pyrocatechol or ketol via the heterolytic mechanism. The stability of phenol ethers is determined by the structure and redox properties of the reactants.

Surface photovoltage studies of nonstoichiometric rutile titanium dioxide

The aim of the present investigation was to assess how processing at elevated temperatures impacts upon the room temperature electronic structure of rutile TiO2. Consequently, strongly reduced and oxidized rutile TiO2 pellets were processed under controlled conditions and studied using surface photovoltage spectroscopy. Under illumination with photon energies above the band gap energy, reduced and oxidized rutile, respectively, displayed positive and negative surface photovoltages. This result indicates that undoped TiO2 may be tailored to promote either photoreduction or photo-oxidation reactions. Several transitions involving surface and bulk electronic states within the band gap were observed in oxidized rutile, but in contrast, similar states could not be readily distinguished in reduced rutile. A band diagram of oxidized rutile is proposed.

Effect of divalent anions on photodegradation kinetics and pathways of riboflavin in aqueous solution

The present investigation is based on a study of the effect of buffer and non-buffer divalent anions (phosphate, sulphate, tartrate, succinate, malonate) on the kinetics, product distribution and photodegradation pathways of riboflavin (RF) at pH 6.0–8.0. RF solutions (5 × 10 −5 M) were photodegraded in the presence of divalent anions (0.2–1.0 M) using a visible light source and the photoproducts, cyclodehydroriboflavin (CDRF), formylmethylflavin (FMF), lumichrome (LC) and lumiflavin (LF) were assayed by a specific multicomponent spectrophotometric method. RF degradation in the presence of divalent anions follows parallel first-order kinetics to give CDRF and LC as the final products through photoaddition and photoreduction reactions, respectively. The divalent anion-catalysed CDRF formation is affected in the order: phosphate > sulphate > tartrate > succinate > malonate, showing maximum activity of the anions around pH 7. The divalent anions cause deviation of the photoreduction pathway in favour of the photoaddition pathway to form CDRF. The first- and second-order rate constants for the reactions involved in the photodegradation of RF have been determined and the rate–pH profiles and pathway relationships discussed. The catalytic activity of the divalent anions appears to be a function of the relative strength and chemical reactivity of the RF–divalent anion complex acting as a mediator in the photoaddition reaction.

A comprehensive aerosol spray method for the rapid photocatalytic grid area analysis of semiconductor photocatalyst thin films

Indicator inks, previously shown to be capable of rapidly assessing photocatalytic activity via a novel photo-reductive mechanism, were simply applied via an aerosol spray onto commercially available pieces of Activ™ self-cleaning glass. Ink layers could be applied with high evenness of spread, with as little deviation as 5% upon UV–visible spectroscopic assessment of 25 equally distributed positions over a 10 cm × 10 cm glass cut. The inks were comprised of either a resazurin (Rz) or dichloroindophenol (DCIP) redox dye with a glycerol sacrificial electron donor in an aqueous hydroxyethyl cellulose (HEC) polymer media. The photo-reduction reaction under UVA light of a single spot was monitored by UV–vis spectroscopy and digital images attained from a flat-bed scanner in tandem for both inks. The photo-reduction of Rz ink underwent a two-step kinetic process, whereby the blue redox dye was initially reduced to a pink intermediate resorufin (Rf) and subsequently reduced to a bleached form of the dye. In contrast, a simple one-step kinetic process was observed for the reduction of the light blue redox dye DCIP to its bleached intermediates. Changes in red–green–blue colour extracted from digital images of the inks were inversely proportional to the changes seen at corresponding wavelengths via UV–visible absorption spectroscopy and wholly indicative of the reaction kinetics. The photocatalytic activity areas of cuts of Activ™ glass, 10 cm × 10 cm in size, were assessed using both Rz and DCIP indicator inks evenly sprayed over the films; firstly using UVA lamp light to activate the underlying Activ™ film (1.75 mW cm −2) and secondly under solar conditions (2.06 ± 0.14 mW cm −2). The photo-reduction reactions were monitored solely by flat-bed digital scanning. Red–green–blue values of a generated 14 × 14 grid (196 positions) that covered the entire area of each film image were extracted using a custom-built program entitled RGB Extractor(C). A homogenous degradation over the 196 positions analysed for both Rz (Red colour deviation = 19% UVA, 8% Solar; Green colour deviation = 17% UVA, 12% Solar) and DCIP (Red colour deviation = 22% UVA, 16% Solar) inks was seen in both UVA and solar experiments, demonstrating the consistency of the self-cleaning titania layer on Activ™. The method presented provides a good solution for the high-throughput photocatalytic screening of a number of homogenous photocatalytically active materials simultaneously or numerous positions on a single film; both useful in assessing the homogeneity of a film or determining the best combination of reaction components to produce the optimum performance photocatalytic film.

Prebiotic Metabolism: Production by Mineral Photoelectrochemistry of α-Ketocarboxylic Acids in the Reductive Tricarboxylic Acid Cycle

A reductive tricarboxylic acid (rTCA) cycle could have fixed carbon dioxide as biochemically useful energy-storage molecules on early Earth. Nonenzymatic chemical pathways for some steps of the rTCA cycle, however, such as the production of the alpha-ketocarboxylic acids pyruvate and alpha-ketoglutarate, remain a challenging problem for the viability of the proposed prebiotic cycle. As a class of compounds, alpha-ketocarboxylic acids have high free energies of formation that disfavor their production. We report herein the production of pyruvate from lactate and of alpha-ketoglutarate from pyruvate in the millimolar concentration range as promoted by ZnS mineral photoelectrochemistry. Pyruvate is produced from the photooxidation of lactate with 70% yield and a quantum efficiency of 0.009 at 15 degrees C across the wavelength range of 200-400 nm. The produced pyruvate undergoes photoreductive back reaction to lactate at a 30% yield and with a quantum efficiency of 0.0024. Pyruvate alternatively continues in photooxidative forward reaction to alpha-ketoglutarate with a 50% yield and a quantum efficiency of 0.0036. The remaining 20% of the carbon follows side reactions that produce isocitrate, glutarate, and succinate. Small amounts of acetate are also produced. The results of this study suggest that alpha-ketocarboxylic acids produced by mineral photoelectrochemistry could have participated in a viable enzyme-free cycle for carbon fixation in an environment where light, sulfide minerals, carbon dioxide, and other organic compounds interacted on prebiotic Earth.