Hole Scavenger Research Articles

Photocatalytic reduction of nitrate pollutants by novel Z-scheme ZnSe/BiVO4 heterostructures with high N2 selectivity

Eliminating nitrate pollutants in water efficiently and environmentally friendly has attracted widespread attention. As a promising technology to solve problems of environmental pollution and energy shortage, photocatalysis also displayed great potential in nitrate removal. In this work, a novel Z-scheme ZnSe/BiVO4 heterostructures were synthesized by in-situ growth of ZnSe nanospheres on BiVO4, and employed for photocatalytic reduction of nitrate in water. Experiments demonstrated that ZnSe/BiVO4 composites obtained improved photocatalytic activity compared with pristine ZnSe and BiVO4, and an optimal performance with nitrate removal rate as 89.84% and N2 selectivity as 91.03% in 50 min was achieved when initial nitrate was 100 mgN/L. The enhancements may be due to the boosted light harvesting capability and stronger redox ability of photogenerated carriers provided by Z-scheme ZnSe/BiVO4 heterostructures. Moreover, HCOOH played a better role as hole scavenger than CH3COOH and C2H5OH. The HCOOH dosage and initial solution pH greatly affected the photocatalytic removal of nitrate. Effects of impurity anions in water such as Cl−, SO42− and CO32− on nitrate removal were also investigated. The charge transfer pathway as well as photocatalytic mechanism for nitrate removal over ZnSe/BiVO4 heterostructures were proposed at the end.

High spatial resolution imaging of the charge injection yield at hematite using scanning electrochemical cell microscopy

In-depth studies of charge injection and separation processes are important in characterizing the photoelectrochemical (PEC) properties of semiconductor materials, but high spatial resolution observations of the charge injection yield of such materials have not yet been reported. Here, scanning electrochemical cell microscopy (SECCM) is used for spatially resolved electrochemical imaging of the charge injection yield for a model material (hematite). Hydrogen peroxide is introduced as a hole scavenger to reduce the recombination of electrons and holes in the material, thereby achieving a division between charge injection and separation processes during PEC oxidation. Then, by calculating the ratio between the photocurrents before and after the addition of hydrogen peroxide, the distribution of the charge injection yield in the hematite film can be obtained at the nanoscale. The co-imaging of the morphology and the charge injection yield of hematite using SECCM makes it possible to observe the inverse dependence of the charge injection yield on the film thickness, which indicates that the main influence on the photocurrent is the charge injection process. The successful development of this nanoscale imaging method could provide more information to help elucidate the mechanism of the PEC process and, eventually, to aid in the design of more effective PEC materials.

Activated carbon-supported AgMoOS bimetallic oxysulfide as a catalyst for the photocatalytic hydrogen evolution and pollutants reduction

Activated carbon (AC) with high porosity and surface area as a catalyst carrier can enhance the catalytic activity and stability of metal nanoparticles with well dispersion to avoid aggregation. Herein, silver molybdenum oxysulfide (AgMoOS) nanoparticles supported by activated carbon with different porous sizes were synthesized through a facile method for photocatalytic H2 evolution reaction and pollutants reduction. The highest H2 evolution of 338.9 µmol/h was obtained by microporous AC-supported AgMoOS ([email protected]) under visible-light irradiation using Na2S/Na2SO3 as a hole scavenger reagent. Likewise, 76.9% of the 4-NP reduction is achieved by in-situ generated proton within 160 min. AC could lower the recombination rate of photo carriers due to its pore channels. Under the dark condition, 10 mg [email protected] completely reduced MB, MO, RhB, 4-NP, and Cr6+ within 4, 4, 9, 9, and 12 min, respectively. This excellent catalytic activity is due to a good dispersion of AgMoOS nanoparticles on AC-1, electron hopping transport of Mo4+→ Mo6+ ions, and surface oxygen vacancy. [email protected] catalyst also exhibits good stability and efficiency that provides a practical catalytic application to reduce chemical pollutants and environmental remediation.

Photocatalytic removal of nitrate from water using activated carbon-loaded with bimetallic Pd-Ag nanoparticles under natural solar radiation

The increase of nitrate contamination of surface and groundwater has raised a concern, because of its impact on both environment and human health. In this study, photocatalysis for nitrate treatment in water was conducted using activated carbon prepared from date palm stone decorated with single and bimetals nanoparticles using solar radiation. The prepared catalyst was characterized by XPS, SEM, EDX, TGA, and surface area analyzer instruments. The photocatalytic effect of the prepared catalyst was studied using nitrate solution and acid scavengers with different concentrations. Significant photocatalytic activity was observed for activated carbon when modified with Pd–Ag. The obtained results showed that the conversion of nitrate takes place mainly through nitrogen gas (N2) rather than nitrite (NO2–) or ammonium (NH4+). Formic acid as a hole scavenger with 0.05 M shows the maximum conversion for nitrate. The prepared photocatalyst shows stability for five cycles of nitrate ion reduction. Moreover, the results showed that the prepared catalyst could be applied for the removal of nitrate from groundwater and river water samples.

Progress in the Photoreforming of Carboxylic Acids for Hydrogen Production

Photoreforming is a process that connects the redox capability of photocatalysts upon light illumination to simultaneously drive the reduction of protons into hydrogen and the oxidation of organic substrates. Over the past few decades, researchers have devoted substantial efforts to enhancing the photocatalytic activity of the catalyst in hydrogen production. Currently, the realization of the potential of photocatalysts for simultaneous hydrogen production with value-added organics has motivated the research field to use the photo-oxidation path. As a distinct benefit, the less energetically demanding organic reforming is highly favorable compared to the slow kinetics of oxygen evolution, negating the need for expensive and/or harmful hole scavengers. Photocatalyst modifications, such as secondary component deposition, doping, defect, phase and morphology engineering, have been the main strategies adopted to tune the photo-oxidation pathways and oxidation products. The effect of the reaction parameters, including temperature, pH, reactant concentration and promising reactor strategies, can further enhance selectivity toward desired outcomes. This review provides a critical overview of photocatalysts in hydrogen production, including chemical reactions occurring with semiconductors and co-catalysts. The use of various oxygenates as sacrificial agents for hydrogen production is outlined in view of the transition of fossil fuels to clean energy. This review mainly focuses on recent development in the photoreforming of carboxylic acids, produced from the primary source, lignocellulose, through pyrolysis. The photo-oxidation of different carboxylic acids, e.g., formic acid, acetic acid and lactic acid, over different photocatalysts for hydrogen production is reviewed.

Molecular Metallocorrole-Nanorod Photocatalytic System for Sustainable Hydrogen Production.

Solar‐driven photocatalytic generation of hydrogen from water is a potential source of clean and renewable fuel. Yet systems that are sufficiently stable and efficient for practical use have not been realized. Here, nanorod photocatalysts that have proven record activity for the water reduction half reaction were successfully combined with molecular metallocorroles suitable for catalyzing the accompanying oxidation reactions. Utilization of OH−/⋅OH redox species as charge transfer shuttle between freely mixed metallocorroles and rods resulted in quantum efficiency that peaked as high as 17 % for hydrogen production from water in the absence of sacrificial hole scavengers. While typically each sacrificial scavenger is able to extract but a single hole, here the molecular metallocorrole catalysts were found to successfully handle nearly 300,000 holes during their lifespan. The implications of the new system on the prospects of realizing practical overall water splitting and direct solar‐to‐fuel energy conversion were discussed.

(Invited, Digital Presentation) Controlling the Energy Structure of Ag(In,Ga)S Quantum Dots for Photocatalytic H2 Evolution

Quantum dots (QDs) composed of group I-III-VI semiconductors, such as CuInS2 and AgInS2, exhibit unique size- and composition-dependent physicochemical properties different from bulk materials due to the quantum size effect. Furthermore, these QDs have attracted intense attention for developing highly efficient solar energy conversion systems, because of the less toxicity and the large absorption coefficients in visible and near-IR regions. Recently we have successfully prepared alloy QDs composed of ZnAgInS or ZnAgInSe semiconductors, the Eg of which was tunable in the visible or near-IR wavelength regions, respectively [1, 2]. Their photocatalytic H2 evolution activity was controlled by the chemical composition and size of QDs. On the other hand, the Eg of bulk I-III-VI semiconductors can be controlled by alloying different metal cations of group III elements. Thus, in this study, we prepare spherical QDs composed of less-toxic AgIn(1-x)GaxS2 (AIGS(x)) alloy semiconductor and report their composition-dependent photocatalytic activity for H2 evolution under visible light irradiation.AIGS(x) QDs with different x values were synthesized by thermal decomposition of Ag(OAc), In(acac)3, Ga(acac)3, and S powders in a mixture solution of oleylamine and dodecanethiol. The chemical composition of resulting QDs, that is, x value, could be controlled by varying the Ga/(In+Ga) ratio in preparation. Thus-obtained AIGS QDs were surface-coated with ZnS shell of various thickness, resulting in the formation of core-shell-structured QDs, AIGS(x)@ZnS. Finally, the ligands modified on the nanocrystal surface were changed from dodecanethiol to mercaptopropionic acid to improve the dispersibility of QDs in an aqueous solution.The obtained AIGS QDs had a spherical shape with size of ca. 4 nm. The ZnS thickness of AIGS(x)@ZnS was varied from 0 to ca. 2 nm. The photocatalytic activity of AIGS(x)@ZnS QDs was investigated for H2 evolution as a model reaction. The irradiation to AIGS(x)@ZnS QDs suspended in an aqueous solution containing Na2S as a hole scavenger was carried out using Xe lamp light (λ> 350 nm). With the elapse of light irradiation, the amount of H2 evolved linearly increased, regardless of the composition of AIGS core or the ZnS shell thickness. The H2 evolution rate was remarkably dependent on the composition of AIGS core: A volcano-type dependence was observed between the H2 evolution rate and the Ga fraction. The highest photocatalytic activity was obtained for AIGS(x)@ZnS QDs with x=0.6 and the ZnS shell thickness of ca. 2 nm. This behavior can be explained by the changes in the stability of AIGS core in aqueous solutions, the conduction band minimum level of QDs, and the amount of photon absorption. References Kameyama, T. Takahashi, T. Machida, Y. Kamiya, T. Yamamoto, S. Kuwabata, and T. Torimoto, J. Phys. Chem. C, 2015, 119, 24740-24749.P.-Y. Hsieh, T. Kameyama, T. Takiyama, T. Yamamoto, Y.-J. Hsu and T. Torimoto, J. Mater. Chem. A 2020 , 8, 13142-13149.

(Invited) Electrified Membranes for Transformation of Nitrate in Wastewaters

The release of nitrate to the environment from wastewater effluent and agricultural runoff contributes to groundwater contamination, harmful algal blooms, and disruption of biogeochemical nitrogen flows. Typical treatment methods are based on nitrate separation, which produces waste streams that are often discharged to the environment. Alternatively, nitrate conversion via electrochemical reduction eliminates the production of concentrated waste streams while avoiding the addition of reductant or hole scavenger chemicals to accomplish the reaction. However, major challenges for nitrate removal from water via electrochemical conversion involve reducing the use of expensive precious metal electrocatalysts while also improving the reaction activity and selectivity, catalyst stability, and mass transport of nitrate to electrocatalyst active sites. The use of electrochemical membranes as multifunctional porous flow-through electrodes could potentially address these challenges based on improved mass transport and altered kinetics under flow conditions within membrane pores.Conductive membranes were fabricated using polymers combined with carbonaceous materials such as reduced graphene oxide (rGO) and carbon nanotubes. The rGO was functionalized with non-precious transition metal oxynitride electrocatalysts, where these catalysts showed higher nitrate conversion activity compared to the unsupported transition metal nitrides. The influence of catalyst materials, membrane fabrication process, and filtration conditions on nitrate reduction activity and selectivity were evaluated. In addition to the environmental impacts of closing the nitrogen loop by converting nitrate into innocuous N2, selective nitrate reduction to ammonia provides opportunities for recovery as fertilizer or carbon-free renewable energy storage. The prospects for reactive nitrogen recovery based on nitrate electrochemical conversion to ammonia were analyzed for various potential source waters.

A Spectroscopic Investigation of Photochemical Nitrogen Fixation

Photochemical nitrogen transformations were initially pioneered by a prominent Indian soil scientist, N.R. Dhar, in the 1930-40s[1]. Others continued to evaluate this process in the presence of liquid water, water vapor, and oxygen. These results remained unverified until 1977, when Schrauzer and Guth also began to investigate this process on natural minerals[2]. Since the 1970s photochemical nitrogen fixation has been widely studied on a number of photocatalyst[3]. Most conclude that nitrogen is reduce to ammonia through a direct electron-transfer based process. However, the reaction mechanism has not been clearly mapped using spectroscopic techniques. Here, we examine the reaction pathway for nitrogen fixation to ammonia on titania based photocatalyst using multiple spectroscopic techniques. We also aim to focus our talk on explaining the role hole scavengers play in enabling this reaction.[1]Dhar, N. R., and N. N. Pant. "Nitrogen loss from soils and oxide surfaces." Nature 153.3873 (1944): 115-116.[2] GN Schrauzer and TD Guth. Photocatalytic reactions. 1. Photolysis of water and photoreduction of nitrogen on titanium dioxide. Journal of the American Chemical Society, 99(22):7189–7193,1977.[3]Medford, Andrew J., and Marta C. Hatzell. "Photon-driven nitrogen fixation: current progress, thermodynamic considerations, and future outlook." Acs Catalysis 7.4 (2017): 2624-2643.

Enhanced and stable photoelectrochemical H2 production using a engineered nano multijunction with Cu2O photocathode

Cuprous oxide (Cu2O) is one of the ideal photocathodes explored for solar water splitting applications due to its suitable optical properties and band edge positions. However, state-of-the-art Cu2O employs Au back contact for hole extraction and Pt or Ru catalyst for water reduction reactions. Moreover, photo-corrosion of Cu2O during the AM 1.5 G illumination is one of the serious challenges that limit the efficiency of water splitting reactions. In this work, a multijunction strategy in which the Cu2O is sandwiched between the stoichiometrically engineered hole extraction layer and an efficient, non-toxic MoOx catalyst layer is proposed for the enhanced charge separation and stable H2 production activity. The optimized multijunction system exhibits the highest photocurrent of 6.1 mA cm−2 at 0 V vs RHE reported for noble metal-free Cu2O photocathodes. Furthermore, a significant anodic shift in onset potential was noticed. In the multijunction, the tuned layers of NiOx, aluminum-doped zinc oxide, and MoOx act as hole scavenger, electron tunneler, and H2 catalyst, respectively. Importantly, the proposed nanolayers multijunction system demonstrates the effective utilization of noble metal and sulfide-free components for stable and enhanced H2 productions employing Cu2O photocathodes.

Sustainable and stable hydrogen production over petal-shaped CdS/FeS2 S-scheme heterojunction by photocatalytic water splitting

One-dimensional CdS nanorods have garnered interest because of their highly visible light response, narrow bandgap, and negative potential at the conduction band edge, which are suitable for proton reduction. However, their poor charge separation and surface photocorrosion remain unresolved. In this study, CdS was synthesized with a 3D dendrite-like morphology to reduce its surface instability and junctioned with inexpensive FeS2 particles to extend the absorption region toward visible light and improve its photoactivity. The photocurrent density was increased 8.3 times, and the photoluminescence reduced by half in petal-shaped CdS/15% FeS2 compared with pure CdS. The petal-shaped CdS/15% FeS2 heterojunction catalyst exhibited significantly enhanced photostability and photocatalytic activity; when 10% lactic acid was used as a hole scavenger, the hydrogen generation rate was 22.91 mL g−1 for 10 h in pure CdS particles and 107.56 mL g−1 in the petal-shaped CdS/15% FeS2 particles. Moreover, the amount of hydrogen generated was maintained until 8th recycling experiments. The Cd and S ions eluted via photocorrosion were not detected after the reaction was complete. This was attributed to the petal-shaped CdS/FeS2 heterojunction system which protected the unstable CdS surface owing to its controlled morphology. The FeS2 junction improved visible light absorption facilitating the separation of photogenerated charges.

Photocatalytic CO2 Reduction Coupled with Alcohol Oxidation over Porous Carbon Nitride

The photocatalytic transformation of CO2 to valuable man-made feedstocks is a promising method for balancing the carbon cycle; however, it is often hampered by the consumption of extra hole scavengers. Here, a synergistic redox system using photogenerated electron-hole pairs was constructed by employing a porous carbon nitride with many cyanide groups as a metal-free photocatalyst. Selective CO2 reduction to CO using photogenerated electrons was achieved under mild conditions; simultaneously, various alcohols were effectively oxidized to value-added aldehydes using holes. The results showed that thermal calcination process using ammonium sulfate as porogen contributes to the construction of a porous structure. As-obtained cyanide groups can facilitate charge carrier separation and promote moderate CO2 adsorption. Electron-donating groups in alcohols could enhance the activity via a faster hydrogen-donating process. This concerted photocatalytic system that synergistically utilizes electron-hole pairs upon light excitation contributes to the construction of cost-effective and multifunctional photocatalytic systems for selective CO2 reduction and artificial photosynthesis.

Ultrafast charge transfer coupled to quantum proton motion at molecule/metal oxide interface.

Understanding how the nuclear quantum effects (NQEs) in the hydrogen bond (H-bond) network influence the photoexcited charge transfer at semiconductor/molecule interface is a challenging problem. By combining two kinds of emerging molecular dynamics methods at the ab initio level, the path integral-based molecular dynamics and time-dependent nonadiabatic molecular dynamics, and choosing CH3OH/TiO2 as a prototypical system to study, we find that the quantum proton motion in the H-bond network is strongly coupled with the ultrafast photoexcited charge dynamics at the interface. The hole trapping ability of the adsorbed methanol molecule is notably enhanced by the NQEs, and thus, it behaves as a hole scavenger on titanium dioxide. The critical role of the H-bond network is confirmed by in situ scanning tunneling microscope measurements with ultraviolet light illumination. It is concluded the quantum proton motion in the H-bond network plays a critical role in influencing the energy conversion efficiency based on photoexcitation.

Light-assisted synthesis of Au/TiO2 nanoparticles for H2 production by photocatalytic water splitting

A series of Au/TiO2 photocatalysts was synthesized via the light assistance through the photo-deposition for H2 production by photocatalytic water splitting using ethanol as the hole scavenger. Effect of solution pH in the range of 3.2–10.0 on the morphology and photocatalytic activity for H2 production of the obtained Au/TiO2 photocatalysts was explored. It was found that all Au/TiO2 photocatalysts prepared in different solution pH exhibited comparable anatase fraction (∼0.84–0.85) and crystallite size of TiO2 (21–22 nm), but showed different quantity of deposited Au nanoparticles (NPs) and other properties, particularly the particle size of the Au NPs. Among all prepared Au/TiO2 photocatalysts, the Au/TiO2 (10.0) photocatalyst exhibited the highest photocatalytic activity for H2 production, owning to its high metallic state and small size of Au NPs. Via this photocatalyst, the maximum H2 production of 296 μmol (∼360 μmol/g⋅h) was gained at 240 min using the 30 vol% ethanol as the hole scavenger at the photocatalyst loading of 1.33 g/L under the UV light intensity of 0.24 mW/cm2 with the quantum efficiency of 61.2% at 254 nm. The loss of the photocatalytic activity of around 20% was observed after the 5th use.

Enhanced photocurrent density for photoelectrochemical catalyzing water oxidation using novel W-doped BiVO4 and metal organic framework composites

Doping heteroatoms and decorating co-catalyst are intensively applied to improve photocatalytic ability of BiVO4. In this study, it is the first time to design W-doped BiVO4 coupling MIL-101(Fe) as photocatalyst for water oxidation using electrodeposition and hydrothermal processes. Similar system with Mo as dopant has been reported, but the dopant plays important roles on electrochemical performance. It is worthy to study the efficient system with different dopant. Doping amount of W is optimized to achieve high carrier density without creating serious recombination sites. MIL-101(Fe) is decorated on W-doped BiVO4 to suppress surface recombination, create accessible active sites and improve water oxidation kinetics. Optimized W-doped BiVO4/MIL-101(Fe) electrode shows a high photocurrent density of 4.00 mA/cm2 at 1.23 V versus reversible hydrogen electrode (VRHE) under air mass 1.5-global simulated light illumination without hole scavenger in electrolyte, due to large electrochemical surface area, high carrier density and small charge-transfer resistance. The W-doped BiVO4 and BiVO4 electrodes merely show photocurrent densities of 2.96 and 1.72 mA/cm2 at 1.23 VRHE, respectively. Photocurrent retention higher than 95.5% is obtained for W-doped BiVO4/MIL-101 (Fe) electrode under continuous illumination for 6300 s, suggesting lasting photocatalytic ability of this novel W-doped BiVO4/MIL-101(Fe) electrode.

Morphological studies and photocatalytic degradation of methylene blue by zinc sulfide nanoparticles

"We report the preparation, morphological and photocatalytic studies of zinc sulfide nanoparticles from zinc(II) dithiocarbamate precursors. Powder X-ray diffraction patterns were index to the wurtzite crystalline phase of zinc sulfide while transmission electron microscopy micrographs revealed spherically shaped particles with sizes in the range 2.8 to 5.2 nm. The ZnS nanoparticles were used as photocatalysts for the degradation of methylene blue with degradation efficiency of 71.41% for ZnS1 and 85.95% for ZnS2 nanoparticles at pH 9.43. The effect of catalyst concentration, pH, radical scavengers, and hole (h+ ) scavengers on the photodegradation of methylene blue dye were evaluated."

Environmental application of chlorine-doped graphitic carbon nitride: Continuous solar-driven photocatalytic production of hydrogen peroxide

Solar-driven photocatalytic generation of H2O2 over metal-free catalysts is a sustainable approach for value-added chemical production. Here, we synthesized chlorine-doped graphitic carbon nitride (Cl-doped g-C3N4) through a solvothermal method to effectively produce H2O2 with a rate of 1.19 ± 0.06 µM min−1 under visible light irradiation, which was improved by 104 times compared to pristine g-C3N4. Continuous net production of H2O2 was realized at a rate of 2.78 ± 0.10 µM min−1 up to 54 h with isopropanol as the hole scavenger, whereas H2O2 production was only sustained for ~ 6 h without scavengers. Both molecular simulations and advanced spectroscopic characterizations elucidated that the Cl dopant increased the charge transfer rate, decreased the bandgap, and reduced the activation energy of the rate-limiting step of O2 reduction, all of which favored H2O2 production. This work implemented a novel metal-free photocatalyst for sustainable H2O2 production and elucidated the mechanism for promoting H2O2 production that can guide future photoreactive nanomaterial design.

Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO2 Photoelectrodes.

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO2 as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO2 surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO2. One of the complexities that have recently been discovered is the size-dependent hole-transfer mechanism. In this work, we reveal another anomalous behavior in the hole-transfer process that the hole scavenging ability of the electrolyte also plays a role in determining the hole-transfer mechanism in the NC-TiO2 system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na2SO3), the hole transfer in Au18-TiO2 occurs directly through the highest occupied molecular orbital (HOMO) of Au18 NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au18-TiO2 does not occur through the HOMO and shifts to surface state-assisted hole transfer. Due to surface state charging, this surface state-assisted hole-transfer mechanism results in delayed photocurrent response in Au18-TiO2. Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed.

Laddered type-1 heterojunction: Harvesting full-solar-spectrum in scavenger free photocatalysis

Preparation of scavenger free, solar photocatalyst is challenging for large-scale dye-degradation. We report for the first time, a novel solar photocatalyst with laddered type-1 heterojunction. Photocatalyst was realised by selective deposition of a fine ZnO shell onto nanospherical, mesoporous zinc ferrite core using a special two-step microwave-assisted-solvothermal-technique. Synthesis and processing conditions were tuned to harvest entire solar spectrum owing to oxygen vacancies. Under direct sunlight our photocatalyst completely degraded Rhodamine-B without the need for hole scavenger. Scavenging and probing-tests implied the participation of hydroxyl, and superoxide-radicals for Rhodamine-B degradation. NIR light absorption played a vital role in the photodegradation along with the laddered transitions enabled by well positioned bands that avoid recombination. The catalyst was tested for the reproducible activity upto 5 reuse cycles without loss of significant activity. Our approach utilizes (i) direct sunlight (ii) relatively inexpensive and (iii) reusable materials for scavenger free dye degradation and encourages a viable route for ideal solar-photocatalyst.

Surface Anchoring and Active Sites of [Mo3S13]2- Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution.

Achieving light-driven splitting of water with high efficiency remains a challenging task on the way to solar fuel exploration. In this work, to combine the advantages of heterogeneous and homogeneous photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate [Mo3S13]2– clusters onto photoactive metal oxide supports to act as molecular co-catalysts for photocatalytic water splitting. We demonstrate that strong and surface-limited binding of the [Mo3S13]2– to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S22– groups, upon which Mo–O–Ti bonds with the hydroxylated TiO2 surface are established. The heterogenized [Mo3S13]2– clusters are active and stable co-catalysts for the light-driven hydrogen evolution reaction (HER) with performance close to the level of the benchmark Pt. Optimal HER rates are achieved for 2 wt % cluster loadings, which we relate to the accessibility of the TiO2 surface required for efficient hole scavenging. We further elucidate the active HER sites by applying thermal post-treatments in air and N2. Our data demonstrate the importance of the trinuclear core of the [Mo3S13]2– cluster and suggest bridging S22– and vacant coordination sites at the Mo centers as likely HER active sites. This work provides a prime example for the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and gives the incentive to explore other thio(oxo)metalates.