Photooxidation Of Water Research Articles

Photocatalytic H2O2 production driven by cyclodextrin-pyrimidine polymer in a wide pH range without electron donor or oxygen aeration

Photocatalytic H2O2 generation has drawn growing attention for on-site H2O2 production. However, current photocatalytic systems still need organic electron donor, oxygen aeration, and acidic pH to elevate the production efficiency. We report a crosslinking polymer catalyst constructed by β-cyclodextrin aldehyde and 4-amino-6-hydroxy-2-mercaptopyrimidine to address the above challenges. This novel catalyst achieves an outstanding yield rate of 557.2 μM g−1 h−1 for photocatalytic H2O2 production in pure water and can maintain its performance over a wide pH range (1–11) without electron donor or oxygen aeration. The mechanism investigation shows that the high generation performance is attributed to the mutual promotion of water photooxidation and oxygen photoreduction. Moreover, this catalyst is demonstrated for in situ sterilization in various water bodies under visible light irradiation. The reported polymer photocatalyst addresses some of the obstacles in photocatalytic H2O2 generation, and will promote practical applications of H2O2 generation from solar energy.

Synthesis of Metal-Free Chiral Covalent Organic Framework for Visible-Light-Mediated Enantioselective Photooxidation in Water.

Although chiral covalent organic frameworks (CCOFs) presence grows in thermal asymmetric catalysis, their application in equally important asymmetric photocatalysis has yet to begin. Herein, we first report a propargylamine-linked and quaternary ammonium bromide decorated porphyrin-CCOF which can highly promote visible-light-driven enantioselective photooxidation of sulfides to sulfoxides in water and in air. This methodology has also been applied to the synthesis of (R)-modafinil, a wakefulness-promoting medication used for the treatment of excessive sleepiness. This research might open a new way for the application of CCOFs in asymmetric photocatalysis.

Constructing crystalline redox catalyst to achieve efficient CO2 photoreduction reaction in water vapor

Artificial photosynthesis, coupling CO2 photoreduction and water photooxidation reactions, is an important and green means to convert CO2. Compared with traditional multiphasic heterojunctions, monophasic photocatalysts for artificial photosynthesis are still very rare. Here we construct a crystalline overall reaction catalyst, RO-4, by connecting reductive {CuI8} and oxidative {PMo8V6O42} clusters with bridging oxygen atoms, which allow the photogenerated electrons to transfer easily between the redox clusters. Under light irradiation, RO-4 can achieve efficient artificial photosynthesis with a high CO production activity of 20.06 μmol g-1h−1 (>99.5% selectivity) along with O2 release. Furthermore, in situ DRIFTS and DFT calculations uncover the related reaction intermediates and CO2 photoreduction mechanism. This work validates a feasible strategy to engineer monophasic crystalline RO photocatalysts for efficient artificial photosynthetic overall reaction.

Water photo-oxidation on self-assembled organic/Co3O4 metal junctions in biphasic systems

A microwater|butyronitrile (W|BCN) interface was used to study the effect of the interfacial chemical polarization on the efficiency of the visible-light induced water oxidation reaction (WOR) by the electron acceptor/photosensitizer TTF/TTF•+self-assembled on the surface of Co3O4 microparticles, acting as redox electrocatalysts. The polarized W|BCN interface acts as a selective platform for the separation of the products of the photoreaction, TTF and protons, which has a strong positive effect on the efficiency of the WOR. The resulting cobalt oxide-organic microwire junction is as efficient as its Pt analog.

Parallel water photo-oxidation reaction pathways in hematite photoanodes: implications for solar fuel production

Potentiodynamic discharge measurements of hematite photoanodes, supported by micro-kinetic modeling, indicate that parallel reaction pathways prevail in water photo-oxidation. Their co-existence may lead to collective phenomena with complex dynamics.

Development of Yttrium and Iron Oxide Thin Films via AACVD Method for Photooxidation of Water

Development of Yttrium and Iron Oxide Thin Films via AACVD Method for Photooxidation of Water

Tungsten Trioxide Photoanode-Palladium Cathode Cell for Solar Seawater Splitting

N-type semiconductor-tungsten trioxide (WO3) finds application in a large variety of fields including photocatalysis, gas sensors, smart windows or as a substrate for heterogeneous catalysts. This presentation will focus on recent improvements in optoelectronic and photoelectrochemical (PEC) properties of WO3 thin films achieved in our laboratory. The semitransparent WO3 films are formed on F-doped tin oxide (FTO) conductive glass substrates, using a sol-gel method, by depositing the precursor layer-by-layer and annealing sequentially in the flow of oxygen. Use of appropriate mixtures of tungstic acid and organic structure-directing agents that form the WO3 film precursor, combined with a two-step annealing, allows the formation of highly crystalline nanostructured (NS) electrodes with controlled, large porosity. This renders possible the permeation of the whole mesoporous film by the electrolyte. Annealing of the as-deposited samples in oxygen above 500°C ensures structural ordering with formation of monoclinic WO3. Since none among photostable (metallic oxide) semiconductors exhibits band-edge energy levels that match those of hydrogen and oxygen evolution reactions to allow unassisted water splitting, the present efforts focus on minimizing the bias voltage required to perform visible light-driven photooxidation of water. In fact, a bias voltage of the order of 1 V may be provided by several single-junction PV cells likely to operate in a tandem device with the PEC cell. Despite visible-light absorption range of the WO3 films restricted to 500 nm, under standard conditions, i.e. at 1.23 V vs RHE and under simulated AM 1.5G solar light (100 mW/cm2), stable anodic water splitting photocurrents exceeding 4.5 mA/cm2 are regularly attained.This presentation will principally focus on photoelectrolysis experiments employing synthetic seawater electrolyte, in a two compartment (separated by a sintered glass diaphragm) cell, that showed dominant (with 60-70% Faradaic efficiency) formation of chlorine at the WO3 photoanode with remarkably stable photocurrents reaching 4.7 mA/cm2. In fact, as demonstrated in an earlier work from our laboratory1, the WO3 photoelectrodes exhibit preferentially oxidation of anions (including Cl-) of the acidic aqueous electrolytes. However, in contrast with the Cl- ions, the photooxidation of oxy-anions of acidic electrolytes leads generally to the "passivation" of the WO3 electrodes due to the formation of the surface layers of peroxo species.1 The only identified exception is the methane-sulfonic acid that allows oxygen generation at the WO3 photoanode with large stable photocurrents.2 In the case of seawater photoelectrolysis, the operating potential point at which the photocurrent approaches saturation occurs at ca 0.4 V below E(Cl-/Cl2)=1.36 V vs SHE. Consistently, the incident photon conversion efficiencies (IPCEs) reach a maximum of 85% over 390-410 nm range of wavelengths.3 Good mechanical and chemical stability of the WO3 electrodes in the presence of chlorine evolved from seawater was confirmed by monitoring the photocurrents over a 20 hours-long continuous photoelectrolysis runs.A schematic diagram of the seawater splitting PEC cell, including a NS WO3 film photoanode and a Pd sheet cathode, used in our experiments is shown below.Fig. 1 Schematic diagram of the seawater splitting PEC cell.The current of CO2 gas passing through the seawater represents a possible option for maintaining slightly acidic pH in the cathodic cell compartment. The important advantage of the adopted PEC cell configuration is to circumvent the problem of gas separation, i.e. chlorine/oxygen formed at the photoanode and hydrogen generated at the Pd cathode that is stored in the form of a Pd-H hydride. The storage of hydrogen generated in a PEC water splitting device in the form of a hydride, opens the possibility of its subsequent use as an anode in a hydride-oxygen battery.Such sunlight-driven PEC seawater splitting device can be implemented in remote places to provide on-site relatively small amounts of chlorine for disinfecting purposes including purification of drinking water.More generally, the prospect of using seawater electrolyte for a larger scale PEC hydrogen generation would release the strain on vital fresh water resources. Considering relatively low concentration of Cl- ions in seawater, roughly equivalent to 0.5 M NaCl, the photoelectrolysis delivering photocurrents in the range of mA/cm2 appears well suited for such applications.

Effect of morphology and non-metal doping (P and S) on the activity of graphitic carbon nitride toward photoelectrochemical water oxidation

The production of molecular hydrogen by photoelectrochemical dissociation (PEC) of water is a promising technique, which allows the direct transformation of solar energy into hydrogen, an energy vector acclaimed by the scientific community and policymakers. Hydrogen stores solar energy and will help overcome the energy crisis and associated environmental problems. Currently, the design and development of innovative photocatalysts with strong photoelectrochemical activity remain a major challenge, and the subject of intense research activity within the international scientific community. Here we describe the synthesis and photoelectrochemical properties of one-dimensional nanostructures of graphitic carbon nitride (1D-gC3N4) doped with phosphorus or sulfur (1D-P-gC3N4 &1D-S-gC3N4, respectively). A new synthesis method using supramolecular melamine, ammonium dihydrogen phosphate, and tri-thiocyanuric acid as precursors has been developed. The samples were characterized by powder-X Ray diffraction (p-XRD), X-Ray spectroscopy (EDS), transmission electron microscopy (TEM), Ultraviolet–visible (UV–Vis) spectroscopy, Fourier transform infrared spectra (FT-IR) and photoluminescence (PL) analysis. The activity towards the photo-oxidation of water was studied by linear scanning voltammetry (LSV). Compared to 3D material, the activity was found to be significantly improved, thanks in particular to the 1D morphology of gC3N4. It was further strengthened by doping with phosphorus and sulfur. The photo-oxidation mechanism of water was analyzed by photoelectrochemical impedance spectroscopy (PEIS). The measurements show that the resistance to charge transfer at the electrode/electrolyte interface can be greatly reduced by controlling the morphology of gC3N4, and that doping with phosphorus and sulfur also plays a positive role. The PEIS analysis makes it possible to demonstrate that the lifetime of the photo-generated electrons in 1D-gC3N4 is increased compared to 3D-gC3N4, and that doping with phosphorus or sulfur further improves it. The width of the forbidden bands and the position of the valence and conduction bands of the different materials were determined by Mott - Schottky type measurements.

Visible light responsive photocatalytic hydrogen production over composites of anodized TiO2 nanotube array and graphitic carbon nitride measured with a gas circulating reactor

Gas phase photocatalysis over composites of electrochemically anodized titania nanotube (TNT) arrays and graphitic carbon nitride (g-C3N4) nanoparticles were examined with a home-made, gas-circulating reactor system. g-C3N4 nanoparticles were deposited onto anodized TNT surfaces by facile thermal polymerization using melamine and urea as precursors. The existence of g-C3N4 on TNT arrays clearly improved hydrogen production from gas phase photocatalytic decomposition of water/methanol mixture under visible light (VIS) irradiation (370–800 nm). The enhanced production of hydroxyl radicals via water photooxidation for TNT/g-C3N4 composites was also ascertained by photoluminescence measurement using terephthalic acid as a radical trap. These results deriving from synergetic effects of TNT and g-C3N4 can be attributed to the photogenerated hole–electron separation enhanced at TNT/g-C3N4 interface, suggesting that g-C3N4 is a metal-free sensitizer for enabling VIS responsive photocatalysis in anodized TNTs even for gas phase reactions.

2D ternary nitrides XNY (X=Ti, Zr, Hf; Y[dbnd]F, Cl, Br) with applications as photoelectric and photocatalytic materials featuring mechanical and optical anisotropy: A DFT study

Using density functional theory, we systematically studied the stability, electronic structure and optical properties of two-dimensional ternary nitrides XNY (X ​= ​Ti, Zr, Hf; YF, Cl, Br). Through phonon spectrum calculations and ab initio molecular dynamics simulations, we found that XNY is dynamic and thermodynamically stable. The calculated results of cohesive energy indicate the possibility of synthesis experimentally. XNY is a direct band gap semiconductor, and the band gap is larger than the 1.23 ​eV required for water splitting. In addition, XNY has anisotropic mechanical and optical properties. In addition, XNY has absorption of visible light, and the band-edge arrangement of ZrNF and HfNCl is feasible for water photo oxidation and photo reduction. These comprehensive properties make 2D ternary nitride XNY a candidate material for direction-dependent optoelectronic devices, and it is expected to be applied in the field of photo catalysis.

CuCoOx/BiVO4 multifunctional catalyst for organics degradation, water oxidation, and O2 reduction under visible light

Monoclinic BiVO4 (BiV) is an excellent photoanode for water oxidation, but it is a poor photocatalyst for organic oxidation due to slow O2 reduction. Herein we report a 94-fold increased photocatalytic activity of BiV through a surface deposited CuCoOx. The model reaction was phenol degradation in aqueous solution under visible light. CuCoOx, CuOx, and CoOx were prepared in butanol, separately, while CuCo2O4 was prepared in aqueous solution. Solid characterization showed that CuCo2O4 was Cu0.92Co2.08O4, but CuCoOx was a mixture of Cu0.92Co2.08O4, CuOx, and CoOx. Notably, the rate of phenol oxidation on CuCoOx/BiV was not only larger than those on Pt/BiV, CuCo2O4/BiV, CuOx/BiV, and CoOx/BiV, but also larger than the sum of the rates obtained for the latter three. Photoluminescence study revealed that all co-catalysts improved the efficiency of charge separation of BiV, with the trend similar to that for phenol photo-oxidation. Electrochemical study with a BiV film electrode showed that among four co-catalysts, CuOx was the most active for O2 reduction, CoOx for water oxidation, and CuCo2O4 for water photo-oxidation. According to the measured band edge potentials for semiconductors, a possible charge transfer from BiV to co-catalyst is proposed, including the electron transfer for CuOx/BiV, the hole transfer for CoOx/BiV and CuCo2O4/BiV, and the sequential electron transfer for CuCoOx/BiV.

M-Dependent activity of MCo2O4 spinels for water splitting and H2 production on Zn0.5Cd0.5S under visible light

Production of H2 from water splitting is challenging. Herein, we report an increased production of H2 on Zn0.5Cd0.5S (ZnCdS) in aqueous solution under a 420 nm LED lamp, via 4 wt % MCo2O4 (M = Mg, Cr, Mn, Fe, Co, Ni, Cu, and Zn). Without addition of sacrifices, the rate of H2 evolution (RH, in a unit of mmol g―1 h―1) was 0.0154 for ZnCdS, and 0.057―0.551 for MCo2O4/ZnCdS. On addition of Na2S/Na2SO3, the corresponding RH increased to 18.5 and 31.5―78.2, respectively. In both cases, no H2 was observed for MCo2O4. However, the RH obtained in presence of Na2S/Na2SO3 had a negative correlation with Ms and R2, respectively, where Ms was the magnetization degree of MCo2O4, and R2 was the charge transfer resistance for MCo2O4/ZnCdS at solid-liquid interface. Electrochemical measurement showed that MCo2O4/ZnCdS was more active than ZnCdS for proton reduction, water oxidation, and water(sulfide/sulfite) photo-oxidation, respectively. According to the solid photoluminescence, band parameters, and open circuit potentials, formation of a p-n junction is proposed, which facilitates the photoelectron transfer to ZnCdS for proton reduction, and the photohole transfer to MCo2O4 for water (sulfide/sulfite) oxidation. It is also proposed that the magnetic field of MCo2O4 inhibits the charge transfer between MCo2O4 and ZnCdS. This work suggests that MCo2O4 spinels are good co-catalysts, but their magnetization need to be minimized.

Two-Dimensional Functionalized Germananes as Photoelectrocatalysts.

Succeeding graphene, monoelemental two-dimensional (2D) materials such as germanene and silicene, coined as "Xenes", have attracted vast scientific and technological interests. Adding covalently bonded hydrogen on both sides of germanene leads to germanane (i.e., hydrogen-terminated germanene, GeH). Further, the covalent functionalization of germanane allows the tuning of its physical and chemical properties. Diverse variants of germananes have been synthesized, but current research is primarily focused on their fundamental properties. As a case in point, their applications as photo- and electrocatalysts in the field of modern energy conversion have not been explored. Here, we prepare 2D germanene-based materials, specifically germanane and germananes functionalized by various alkyl chains with different terminal groups-germanane with methyl, propyl, hydroxypropyl, and 2-(methoxycarbonyl)ethyl-and investigate their structural, morphological, optical, electronic, and electrochemical properties. The bond geometries of the functionalized structures, their formation energies, and band gap values are investigated by density functional theory calculations. The functionalized germananes are tested as photoelectrocatalysts in the hydrogen evolution reaction (HER) and photo-oxidation of water. The performance of the germananes is influenced by the functionalized groups, where the germanane with -CH2CH2CH2OH termination records the lowest HER overpotentials and with -H termination reaches the highest photocurrent densities for water oxidation over the entire visible spectral region. These positive findings serve as an overview of organic functionalization of 2D germananes that can be expanded to other "Xanes" for targeted tuning of the optical and electronic properties for photo- and electrochemical energy conversion applications.

(Invited) Characterization of Charge Transfer and Recombination Processes at Metal Oxide Semiconductors for Solar Water Splitting

Metal oxide (MO) semiconductors based on abundant, low-cost materials are attractive for application in photoelectrochemical water splitting systems related to their inherent stability, specifically against oxidation. In addition, these materials provide a wide-ranging versatility in terms of composition, physical and chemical properties, and configuration of multi-component architectures. The semiconducting MO must at least comply with three requirements: (i) absorb sunlight efficiently; (ii) the valence and conduction band edges must be situated at the appropriate energies; and (iii) the electrochemical reaction must have favorable kinetics compared with charge separation, transport and recombination kinetics. Many systems have been evaluated for photo-oxidation of water in the past 10 years, with some of the more popular materials being Fe2O3, WO3, BiVO4, TiO2, etc.In order to improve the efficiency of a solar water splitting system, the specific loss mechanisms need to be elucidated; based on these insights strategies may then be developed to improve performance. However, it is not straightforward to determine the precise loss mechanisms and generally non-steady state methods need to be applied in combination with steady-state techniques. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful technique to study the carrier dynamics in a photoelectrochemical cell: the photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge separation, charge transfer and surface recombination may be deduced. The kinetic model applicable to the specific system determines the interpretation of the results. On the other hand, theoretical modeling is a very powerful method to validate the interpretation of the results, and to complement the experimental studies in order to obtain a clear picture of the dominant processes and effects.In this presentation, the results from IMPS studies on a variety of systems will be discussed and strategies to improve performance are presented and tested, both experimentally and theoretically: (i) planar, compact WO3 / BiVO4 heterojunction systems are compared with nanorod array WO3 / BiVO4 heterojunctions, with and without a hole scavenger; (ii) CuBi2O4 with and without catalysts and an alternative electron acceptor; and (iii) specific examples of other materials to highlight the proposed interpretations. The results are compared with calculations of the IMPS response using a classic kinetic model and a continuity equation approach in order to incorporate the effects of charge transport and electric field within the electrode.The authors gratefully acknowledge funding from CONACYT, SENER, and CICY through the Renewable Energy Laboratory of South East Mexico (LENERSE; Project 254667; SP-4), and CONACYT under the Basic Sciences (CB) project A1-S-28734. We also thank the Ministerio de Ciencia e Innovación, the Agencia Estatal de Investigación (AEI), and the European Regional Development Fund of the European Union through Project PID2019-110430GB-C22. AR thanks the Spanish Ministry of Education, Culture and Sports via a PhD grant (FPU2017-03684). Finally, we would like thank Dr. Antonio Zapien and the NANOMXCN initiative to foment scientific collaboration between Mexico and China for support.

Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin-film metal oxide photoanodes for solar water splitting.

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, ξ(λ), represents the wavelength-dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract ξ(λ) through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo-oxidation, and observed that it is self-consistent for different illumination conditions and applied potentials. We found agreement between the extracted ξ(λ) spectrum and the photoconductivity spectrum measured by time-resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite's absorption spectrum. Low-energy non-contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO2 and BiVO4 photoanodes, demonstrating the broader utility of the method for determining ξ(λ).

Effect of Chitosan on the Activity of Water-Soluble and Hydrophobic Porphyrin Photosensitizers Solubilized by Amphiphilic Polymers.

In this work, we studied the photocatalytic activity of photosensitizers (PSs) of various natures solubilized with polyvinylpyrrolidone (PVP) and ternary block copolymer ethylene and propylene oxide Pluronic F127 (F127) in a model reaction of tryptophan photo-oxidation in water in the presence of chitosan (CT). Water-soluble compounds (dimegin and trisodium salt of chlorin e6 (Ce6)) and hydrophobic porphyrins (tetraphenylporphyrin (TPP) and its fluorine derivative (TPPF20)) were used as PSs. It was shown that the use of chitosan (Mw ~100 kDa) makes it possible to obtain a system whose activity is comparable to that of the photosensitizer-amphiphilic polymer systems. Thus, the previously observed drop in the photosensitizing activity of PS in the presence of a polysaccharide and amphiphilic polymers (AP) was absent in this case. At the same time, chitosan had practically no inhibitory effect on hydrophobic porphyrins solubilized by Pluronic F127.

Water oxidation by P25 TiO2 photoanodes in acidic solution

P25 TiO2 photoanodes are used to photo-oxidise water in two different acids, 0.5 M H2SO4 and 1 M HClO4. In the former acid, the linear sweep voltammogram, LSV, appears to exhibit two photocurrent waves, whilst only one in the latter. In 0.5 M H2SO4, the recorded LSV coupled with a low faradaic efficiency (0.58) for the photooxidation of water to O2, fO2, and a significant level of persulfate, fS2O8 = 0.12, shows that the electrochemical kinetics are not simply those for water oxidation. In 1 M HClO4, the LSV coupled with a high fO2 value (0.91) suggest that the photocurrent is due to water oxidation. Photo-induced absorption spectroscopy, PIAS, measurements made using the P25 TiO2 photoanode reveal a steady state absorbance change, ΔAbsss, associated with the steady-state concentration of surface accumulated holes, [h+]ss, which varies with: (i) monitoring wavelength, with a peak at ca. 500 nm, and (ii) applied potential, flattening off at ca. 0.7 V vs Ag/AgCl. PIAS measurements, coupled with concomitant transient photocurrent (TC) measurements, on the P25 TiO2 photoanode polarised at 1.3 v vs Ag/AgCl, in 1 M HClO4, show that the oxidation of water is second order with respect the concentration of the surface-accumulated, photogenerated holes, [h+]ss, which have a calculated turnover frequency of 19 s−1, under 1 sun irradiation. This is the first reported example of the use of PIAS/TC to probe the photoelectrochemical kinetics exhibited by a mesoporous semiconductor photoanode derived from a powder, for water oxidation and the significance of such is discussed briefly.

Water Photo-Electrooxidation Using Mats of TiO2 Nanorods, Surface Sensitized by a Metal–Organic Framework of Nickel and 1,2-Benzene Dicarboxylic Acid

Photoanodes comprising a transparent glass substrate coated with a thin conductive film of fluorine-doped tin oxide (FTO) and a thin layer of a photoactive phase have been fabricated and tested with regard to the photo-electro-oxidation of water into molecular oxygen. The photoactive layer was made of a mat of TiO2 nanorods (TDNRs) of micrometric thickness. Individual nanorods were successfully photosensitized with nanoparticles of a metal–organic framework (MOF) of nickel and 1,2-benzene dicarboxylic acid (BDCA). Detailed microstructural information was obtained from SEM and TEM analysis. The chemical composition of the active layer was determined by XRD, XPS and FTIR analysis. Optical properties were determined by UV–Vis spectroscopy. The water photooxidation activity was evaluated by linear sweep voltammetry and the robustness was assessed by chrono-amperometry. The OER (oxygen evolution reaction) photo-activity of these photoelectrodes was found to be directly related to the amount of MOF deposited on the TiO2 nanorods, and was therefore maximized by adjusting the MOF content. The microscopic reaction mechanism which controls the photoactivity of these photoelectrodes was analyzed by photo-electrochemical impedance spectroscopy. Microscopic rate parameters are reported. These results contribute to the development and characterization of MOF-sensitized OER photoanodes.

A kinetic study of the photooxidation of water by aqueous cerium(IV) in sulfuric acid using a diode array spectrophotometer

The photooxidation of water by aqueous cerium(IV) has been investigated in dilute sulfuric acid quantitatively using a diode array spectrophotometer. An overview of experimental data on the speciation of cerium(IV) showed that several different absorbing species are present under almost all conditions, which makes straightforward interpretation of photochemical data very difficult. At submillimolar levels of cerium(IV) in 0.10 mol dm–3 sulfuric acid, a consistent quantum yield of 0.12 for the loss of cerium(IV) could be determined, but even under these conditions, clear evidence of a parallel non-photochemical pathway was obtained. At higher concentrations of the metal ion, significant deviations from the expected direct proportionality between the illumination ratio and the initial rate were confirmed, which can only be interpreted by a complicated system of parallel, coupled photochemical and non-photochemical pathways. The inhibitory effect of aqueous cerium(III) in the process was also demonstrated. Based on indirect evidence, it seems likely that sulfate ion radicals play a significant role in the mechanism.

Photooxidation of Water with Thin Film Tungsten Oxides

Tungsten oxide semiconductors are used to photooxidize water. Students prepare WO3, CuWO4, and mixed composition thin film photoanodes. The syntheses are simple, allowing for the quick production of a range of materials. Photoanodes are tested in a photoelectrochemical cell assembled from common equipment, allowing for the direct comparison of photoanode performance. Composite photoanodes perform best, leading to a discussion of what limits overall performance in these materials. Additional instrumental methods such as SEM, EDS, XRD, and diffuse reflectance UV–vis spectrophotometry, if available, can be incorporated readily into the experiment, providing additional insights into material properties. Students are introduced to solid-state chemistry, semiconductors, and the photooxidation of water in the context of the hydrogen economy.