Water Photolysis Research Articles

Durable and efficient photoelectrochemical water splitting using TiO2 and 3C–SiC single crystals in a tandem structure

Hydrogen can be generated by the photolysis of water with semiconductors as photocatalysts. The most important characteristics of water splitting are its durability and efficiency. Furthermore, TiO 2 and 3C–SiC are durable photocatalysts. Because of their different band gaps and conductivity types, a tandem structure combined with these materials functions as an efficient and durable solar water splitting system. It was observed that the photocurrent from the tandem structure was higher at low pH, as noted by the pH dependence of the photocurrent at the TiO 2 photoanode. The photocurrent from the tandem structure had a light intensity dependence similar to the photocurrent from the TiO 2 photoanode. Therefore, the performance of the tandem structure appears to be limited by the photocurrents from the TiO 2 photoanode. We obtained a maximum applied bias photon-to-current conversion efficiency of approximately 0.74% from the tandem structure with no additional bias applied. We also confirmed the presence of a stable photocurrent from the tandem structure for over ~100 days. •The pH dependence and the irradiation light intensity dependence of TiO 2 , 3C–SiC single crystals and the tandem structure combined were characterized. •The lower the pH, the higher the irradiation light intensity, the better the performance of TiO 2 , 3C–SiC single crystals and the tandem structure. •The measured value of the hydrogen generation is 85% of the calculated value which means that hydrogen is almost completely generated in the actual hydrogen and oxygen generation experiment. •The tandem structure shows a 100 days' long-term durability which is pretty longer than most of the photoelectrodes and photocatalysts.

Layered double hydroxide photocatalysts for solar fuel production

Splitting water or reducing CO2via semiconductor photocatalysis to produce H2 or hydrocarbon fuels through the direct utilization of solar energy is a promising approach to mitigating the current fossil fuel energy crisis and environmental challenges. It enables not only the realization of clean, renewable, and high-heating-value solar fuels, but also the reduction of CO2 emissions. Layered double hydroxides (LDHs) are a type of two-dimensional anionic clay with a brucite-like structure, and are characterized by a unique, delaminable, multidimensional, layered structure; tunable intralayer metal cations; and exchangeable interlayer guest anions. Therefore, it has been widely investigated in the fields of CO2 reduction, photoelectrocatalytic water oxidation, and water photolysis to produce H2. However, the low carrier mobility and poor quantum efficiency of pure LDH limit its application. An increasing number of scholars are exploring methods to obtain LDH-based photocatalysts with high energy conversion efficiency, such as assembling photoactive components into LDH laminates, designing multidimensional structures, or coupling different types of semiconductors to construct heterojunctions. This review first summarizes the main characteristics of LDH, i.e., metal-cation tunability, intercalated guest-anion substitutability, thermal decomposability, memory effect, multidimensionality, and delaminability. Second, LDHs, LDH-based composites (metal sulfide-LDH composites, metal oxide-LDH composites, graphite phase carbon nitride-LDH composites), ternary LDH-based composites, and mixed-metal oxides for splitting water to produce H2 are reviewed. Third, graphite phase carbon nitride-LDH composites, MgAl-LDH composites, CuZn-LDH composites, and other semiconductor-LDH composites for CO2 reduction are introduced. Although the field of LDH-based photocatalysts has advanced considerably, the photocatalytic mechanism of LDHs has not been thoroughly elucidated; moreover, the photocatalytic active sites, the synergy between different components, and the interfacial reaction mechanism of LDH-based photocatalysts require further investigation. Therefore, LDH composite materials for photocatalysis could be developed through structural regulation and function-oriented design to investigate the effects of different components and interface reactions, the influence of photogenerated carriers, and the impact of material composition on the physical and chemical properties of the LDH-based photocatalyst.

Synergistic photo/electrocatalysis for energy conversion and storage

Synergistic photo/electrocatalysis for energy conversion and storage

Research on the Hydrogen Production Technology

Hydrogen energy has attracted widespread attention because of its renewable and zero-emission characteristics. It is also a secondary energy source with broad development prospects, so the hydrogen energy industry is an emerging industry of strategic significance. In order to effectively promote the development of hydrogen energy industry, major countries in the world have attached great importance to the research, development and application of hydrogen energy technology, and its development and utilization technology has become an important direction of the new round of world energy technology reform. This paper mainly compares the three widely used methods of hydrogen production, namely, thermochemical recombination, electrolysis of water and photolysis of water, and analyzes the advantages and defects of each method, and lists the solutions to the different defects.

H2S Removal from Groundwater by Chemical Free Advanced Oxidation Process Using UV-C/VUV Radiation.

Sulfide species may be present in groundwater due to natural processes or due to anthropogenic activity. H2S contamination poses odor nuisance and may also lead to adverse health effects. Advanced oxidation processes (AOPs) are considered promising treatments for hydrogen-sulfide removal from water, but conventional AOPs usually require continuous chemical dosing, as well as post-treatment, when solid catalysts are applied. Vacuum-UV (VUV) radiation can generate ·OH in situ via water photolysis, initiating chemical-free AOP. The present study investigated the applicability of VUV-based AOP for removal of H2S both in synthetic solutions and in real groundwater, comparing combined UV-C/VUV and UV-C only radiation in a continuous-flow reactor. In deionized water, H2S degradation was much faster under the combined radiation, dominated by indirect photolysis, and indicated the formation of sulfite intermediates that convert to sulfate at high radiation doses. Sulfide was efficiently removed from natural groundwater by the two examined lamps, with no clear preference between them. However, in anoxic conditions, common in sulfide-containing groundwater, a small advantage for the combined lamp was observed. These results demonstrate the potential of utilizing VUV-based AOP for treating H2S contamination in groundwater as a chemical-free treatment, which can be especially attractive to remote small treatment facilities.

Isotopic fractionation of water and its photolytic products in the atmosphere of Mars

The current Martian atmosphere is about five times more enriched in deuterium than Earth’s, providing direct testimony that Mars hosted vastly more water in its early youth than nowadays. Estimates of the total amount of water lost to space from the current mean D/H value depend on a rigorous appraisal of the relative escape between deuterated and non-deuterated water. Isotopic fractionation of D/H between the lower and the upper atmospheres of Mars has been assumed to be controlled by water condensation and photolysis, although their respective roles in influencing the proportions of atomic D and H populations have remained speculative. Here we report HDO and H2O profiles observed by the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) in orbit around Mars that, once combined with expected photolysis rates, reveal the prevalence of the perihelion season for the formation of atomic H and D at altitudes relevant for escape. In addition, while condensation-induced fractionation is the main driver of variations of D/H in water vapour, the differential photolysis of HDO and H2O is a more important factor in determining the isotopic composition of the dissociation products. Three years of observations with the ExoMars TGO Atmospheric Chemistry Suite have clarified the mechanism of atmospheric water loss from Mars, and particularly the fraction of deuterium compared with hydrogen. Analysis of several isotopologues of water in the mid-atmosphere of Mars shows that atomic H and D are produced at perihelion in relative amounts controlled by photolysis. These atoms are able to escape from the atmosphere.

Influence of sulfate and the interactions of major organic and inorganic solutes on the formation of nitrite during VUV photolysis of nitrate-rich water

Vacuum ultraviolet (VUV) irradiation, is an effective process for degrading micropollutants in water. However, its wide scale applications are affected by the potential formation of nitrite during irradiation of nitrate-containing water. Through detailed experimental and mechanistic studies, this paper investigated the effects of sulfate along with other major organic and inorganic solutes such as natural organic matter (NOM) and chloride on nitrite formation. Sulfate, at environmentally relevant concentrations, showed little impact. However, when sulfate present at higher concentrations (e.g., 300 mg/L of SO42-–S), it led to reduced nitrite formation. Using UV254/persulfate process as control, it was established that sulfate radical (SO4•-) which is formed at initial sulfate reacts with nitrite, to reform nitrate. Besides sulfate, NOM and chloride have each affected the formation of nitrite in the solution. NOM at 6 mg/L resulted in 200% increase in nitrite formation, whereas 30 mg/L chloride led to 50% lower nitrite formation. Of all the solutes tested in this study, NOM showed to be the most dominant solute to increase nitrite formation, indeed it rendered the roles of other solutes insignificant. When degrading carbamazepine (CBZ), as model micropollutant, sulfate radicals formed during the VUV process showed to improve the degradation efficiency without increasing nitrite formation.

Tandem photoelectrochemical and photoredox catalysis for efficient and selective aryl halides functionalization by solar energy

Tandem photoelectrochemical and photoredox catalysis for efficient and selective aryl halides functionalization by solar energy

A Tunable Multivariate Metal-Organic Framework as a Platform for Designing Photocatalysts.

Catalysts for photochemical reactions underlie many foundations in our lives, from natural light harvesting to modern energy storage and conversion, including processes such as water photolysis by TiO2. Recently, metal–organic frameworks (MOFs) have attracted large interest within the chemical research community, as their structural variety and tunability yield advantages in designing photocatalysts to address energy and environmental challenges. Here, we report a series of novel multivariate metal–organic frameworks (MTV-MOFs), denoted as MTV-MIL-100. They are constructed by linking aromatic carboxylates and AB2OX3 bimetallic clusters, which have ordered atomic arrangements. Synthesized through a solvent-assisted approach, these ordered and multivariate metal clusters offer an opportunity to enhance and fine-tune the electronic structures of the crystalline materials. Moreover, mass transport is improved by taking advantage of the high porosity of the MOF structure. Combining these key advantages, MTV-MIL-100(Ti,Co) exhibits a high photoactivity with a turnover frequency of 113.7 molH2 gcat.–1 min–1, a quantum efficiency of 4.25%, and a space time yield of 4.96 × 10–5 in the photocatalytic hydrolysis of ammonia borane. Bridging the fields of perovskites and MOFs, this work provides a novel platform for the design of highly active photocatalysts.

Microalgal photosynthesis induces alkalization of aquatic environment as a result of H+ uptake independently from CO2 concentration – New perspectives for environmental applications

The photosynthetic process in microalgae and the extracellular proton environment interact with each other. The photosynthetic process in microalgae induces a pH increase in the aquatic environment as a result of cellular protons uptake rather than as an effect of CO2 consumption. The photosynthetic water photolysis and the reduction/oxidation cycle of the plastoquinone pool provide lumen with protons. Weak bases act as “permeant buffers” in lumen during the photosynthetic procedure, converting the ΔpH to Δψ. This is possibly the main reason for continuous light-driven proton uptake from the aquatic environment through cytosol and stroma, into the lumen. The proton uptake rate and, therefore, the microalgal growth is proportional to the light intensity, cell concentration, and extracellular proton concentration. The low pH in microalgae cultures, without limitation factors related to light and nutrients, strongly induces photosynthesis (and proton uptake) and, consequently, growth. In contrast, the mitochondrial respiratory process, in the absence of photosynthetic activity, does not substantially alter the culture pH. Only after intensification of the respiratory process, using exogenous glucose supply leads to significantly reduced pH values in the culture medium, almost exclusively through proton output. Enhanced dissolution of atmospheric CO2 in water causes the phenomenon of ocean acidification, which prevents the process of calcification, a significant process for numerous phytoplankton and zooplankton organisms, as well for corals. The proposed interaction between microalgal photosynthetic activity and proton concentration in the aquatic environment, independently from the CO2 concentration, paves the way for new innovative management strategies for reversing the ocean acidification.

Synthesis of LiNbO3 nanocrystals by microwave-assisted hydrothermal method: formation mechanism and application to hydrogen evolution reaction

Lithium niobate (LiNbO3) is an important ferroelectric material with a wide range of applications. In this work, the use of microwave radiation to improve the conditions of hydrothermal synthesis of pure nanocrystalline LiNbO3 was proposed. The structural characterization showed that nanoparticles of a pure LiNbO3 phase were synthesized. The scanning electron microscopy and transmission electron microscopy analysis revealed a high-quality nanoparticle with an average diameter of ca. 42 and 65 nm for 2 and 3 h of synthesis time, respectively. These results represent a time-saving using a microwave-assisted hydrothermal method for LiNbO3 preparation, with excellent quality at the nanometer scale. The nanoparticles were employed as photocatalysts for hydrogen evolution reaction through water splitting, reaching the rate of 10.5 µmol h−1 g−1. It had shown to be a potential candidate for water photolysis due to the small particle size and high crystallinity. Moreover, it also allows the direct and fast synthesis of LiNbO3 on carbon fiber, multi-walled carbon nanotubes and nanocellulose, which were used as supporting materials for the fabrication of novel nanocomposites.

Metallic two-dimensional metal-organic framework arrays for ultrafast water splitting

This work reports a universal dissolution-recrystallization method to synthesize two-dimensional (2D) conductive metal-organic framework (MOF) arrays. The MOF arrays have exhibited excellent structural characteristics for electrochemical reactions, including ultrathin architecture, metallic conductivity, and hierarchical macro-/micro-porosity. Particularly, 2D nickel, iron-MOF arrays act as bifunctional electrocatalysts for water splitting, which deliver a current density of 500 mA cm−2 at an overpotential of 305 mV for anodic oxygen evolution reaction (OER), 359 mV for hydrogen evolution reaction (HER), and ~640 mV for overall reactions. The electrode also shows prolonged stability against bulk water splitting (up to 20 h at 500 mA cm−2). In addition, the water splitting system has been further coupled with commercial solar cells to simulate natural water photolysis, demonstrating a high solar-to-hydrogen efficiency of 17.69%. Mechanism study thorough density function theory (DFT) computations shows strong synergistic effect between Ni and Fe active sites that can reduce the Gibbs energy barrier of hydroxyl dissociation step (OH* → O*) during OER and H* desorption step during HER simultaneously. These promising results open up enormous opportunities of designing versatile low-dimensional conductive MOF architectures for renewable energy applications.

Dimethoate degradation by VUV/UV process: Kinetics, mechanism and economic feasibility

Vacuum ultraviolet/ultraviolet (VUV/UV) process has been applied to water treatment recently, but little is known about its efficacy and mechanism for pesticide degradation. This study investigated the degradation kinetics and mechanism of a typical organophosphorus pesticide, dimethoate (DMT) by VUV/UV, and then the economic feasibility was assessed. DMT degradation followed well the pseudo-first-order reaction kinetics at an initial concentration of ≤5.0 mg L−1. DMT was degraded by 97.8% after 10 min of VUV/UV exposure (VUV fluence = 12 mJ cm−2), whereas by only 5.2% after 10 min of UV exposure (UV fluence = 156 mJ cm−2). The apparent quantum yield of DMT degradation by VUV/UV was determined to be 0.19, and at most 50.7% of hydroxyl radicals (HO•) generated from VUV photolysis of water could be utilized for DMT degradation. As the pH increased from 5.0 to 9.0, the DMT degradation rate decreased from 0.43 to 0.23 min−1. DMT degradation pathways in the VUV/UV process were proposed based on identified organic intermediates and inorganic ions. SO42− was first released due to HO• attack on the SP bond of DMT, which governed the DMT degradation efficiency; while the release of PO43− was pertinent to the DMT mineralization efficiency. DMT solution toxicity was significantly reduced after VUV/UV treatment. An electrical energy-per-order (EEO) value of 0.57 kWh m−3 Order−1 demonstrated the economic feasibility of the VUV/UV process for DMT removal in small-scale drinking water treatment.

Probing nanostructure MoS2 as catalyst in light activated and electro activated hydrogen evolution reaction

Molybdenum disulfide (MoS2) is a smart compound among Transition Metal DiChalcogenides (TMDC). Properties like tuneable bandwidth, high electrical conductivity, large number of active edge sites, low corrosion etc., help MoS2 nanostructures to participate in different energy applications. Catalytic activity in water splitting to produce hydrogen molecules is such a prominent and confirmed application. To enhance the kinetics of hydrogen evolution reaction (HER) different mechanisms and different types of catalysts can be used depends upon the chosen reaction environment. Each type of catalyst plays different role in hydrogen evolution. MoS2 nanostructure is a promising efficient compound with distinguished catalytic behavior as electrocatalyst, co-catalyst of photocatalyst and photoelectrocatalyst. Electrocatalyst participate in electrolysis of water and other type of catalysts participate in artificial photolysis of water. Different phases, shapes and morphologies of MoS2 nanostructure behaves differently in catalysis. Here a review on the role of MoS2 as each type of catalyst is done. Their depending parameters like phase, morphology, shape, defects, doping, heterostructures etc. and HER characteristics like overpotential, Tafel slope, charge transfer resistance, double layer capacitance, durability, hydrogen evolution rate and quantum yield are reviewed and summarized here.

Enhanced stability and activity of Cu–BTC by trace Ru3+ substitution in water photolysis for hydrogen evolution

Constructing stable, efficient and cost-effective cocatalysts is of great significance for photocatalytic H2 evolution in a dye-sensitization system.

Photoelectrochemical performance of TiO2 nanotube arrays modified with Ni2P Co-catalyst

Solar-driven water splitting to produce hydrogen is an important solution to the problem of energy shortage and environmental pollution. The photolysis of water to produce hydrogen requires highly efficient and stable photocatalysts, and the anode used as catalyst for oxygen evolution is a bottleneck in this process. In this paper, the a-TNTAs/Ni2P composite photo-anode was constructed by electrodeposition to anchor the Ni2P co-catalyst for oxygen evolution at the active site of TiO2 nanotube arrays (TNTAs). The a-TNTAs/Ni2P delivered excellent oxygen evolution at a photocurrent density of 1.058 mA cm-2, an improvement of 2.78 times, 13.2 times, and 15.8 times over a-TNTAs, TNTAs/Ni2P, and TNTAs photo-anodes, respectively. The Mott-Schottky curve showed that Ni2P as co-catalyst for oxygen evolution accelerated the rates of separation and transfer of the photogenerated electrons. This research provides a simple and efficient method to promote the OER performance of optical semiconductors.

The impact of chloride and chlorine radical on nitrite formation during vacuum UV photolysis of water

The impact of chloride ion and chlorine radical on the formation of nitrite was investigated under Vacuum-UV (VUV) photolysis of nitrate contaminating water. An increase in chloride concentration reduced nitrite formation in part due to the relatively high VUV absorption of chloride. The use of various radical scavengers, such as acetate and acetone, helped delineate the specific roles of hydroxyl radical (HO) and chlorine radical (Cl) in oxidation and VUV photolysis of nitrate, and the subsequent formation of nitrite. HO reduced nitrite formation due to its high reaction rate constant with nitrite. Nitrite formation in both chloride and dissolved organic carbon (DOC) containing solution depended primarily on their relative concentrations. Carbamazepine (CBZ) was also used to analyze the effect of Cl on both the degradation of CBZ and the formation of nitrite. Cl showed to significantly increase the degradation of CBZ, but it had little impact on the formation of nitrite. This paper, utilizing detailed experimental data combined with kinetic modeling and mechanistic analysis of VUV photolysis in the presence of chloride and nitrate, provides the necessary scientific guidance towards more effective and optimized applications of VUV technology for drinking water treatment.

Water Photolysis and Its Contributions to the Hydroxyl Dayglow Emissions in the Atmospheres of Earth and Mars.

Airglow is a well-known phenomenon in the Earth's upper atmosphere, which arises from the emissions of energetic atoms and molecules. The Meinel band emission from high vibrationally excited OH(X) radicals is one of the more important contributors to the airglow from the mesosphere/lower thermosphere. The H + O3 reaction has long been regarded as the dominant source of these OH(X, high v) radicals. Here we demonstrate that vacuum ultraviolet (VUV) photolysis of water vapor at λ ∼ 112.8 nm represents another source of exceptionally highly vibrationally excited OH(X) radicals, with a nascent vibrational state population distribution that maximizes at v = 9 and extends to at least the v = 15 level. Atmospheric chemistry modeling indicates that OH(X, high v) radicals from H2O photolysis might be detectable in the OH Meinel band dayglow in the upper atmosphere of Earth and should dominate the corresponding emission from the Martian atmosphere. VUV photolysis of H2O also produces electronically excited OH(A) radicals, and simultaneous detection of emissions from OH(X, high v) and OH(A) is shown to offer a route to identifying high-oxygen exoplanetary atmospheres.

Functions of MnOx in NaCl Aqueous Solution for Artificial Photosynthesis.

SummaryPhotoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl− ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O2 and HClO production in aqueous solution containing Cl− on photoanodes modified with various metal oxides was investigated. The modification of MnOx resulted in the promotion of the O2 evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.

Noble‐metal‐free MOF Derived ZnS/CeO2 Decorated with CuS Cocatalyst Photocatalyst with Efficient Photocatalytic Hydrogen Production Character

AbstractInspired by the rich morphology of ZnO and cation‐exchange technology, regular dodecahedron‐like CeO2/ZnO was obtained by the calcination of Ce doped ZIF‐8 precursor in this study, and CeO2/ZnS was obtained by the in‐situ vulcanization of CeO2/ZnO. This design not only maintained the morphology of ZIF‐8, but also enhanced the oxidation‐reduction capacity of the catalyst. When the Ce content was 10 wt %, 10‐CeO2/ZnS sample showed the best photocatalytic hydrogen production performance in all X−CeO2/ZnS samples. Loading of co‐catalysts can effectively enhance the surface hydrogen reduction in photocatalytic water splitting by introducing a positive Schottky barrier. CuS was regarded as a promising cocatalyst to replace the noble metals due to its low cost and equivalent or even better performance. The in‐situ cation exchange method was used to load co‐catalyst CuS on the 10‐CeO2/ZnS composites to obtain the 10‐CeO2/ZnS‐CuS composites which had the large specific surface area, regular dodecahedral structure and much excellent photolysis of water to produce hydrogen characters. This work provided a new strategy for the advantages combining of co‐catalyst, p‐n junction as well as the porous structure to enhance the photocatalytic characters.