Water Photosplitting Research Articles

PdS-ZnS-Doped Electrospun Polymer Nanofibers as Effective Photocatalyst for Hydrogen Evolution

Poly(vinyl acetate) nanofibers doped with PdS-ZnS nanoparticles (PdS-ZnS/PVAc nanofibers) were fabricated via an electrospinning technique. PdS-ZnS nanoparticles were in situ synthesized by adding (NH4)2S solution to poly(vinyl acetate)/zinc acetate/palladium acetate solution. Electrospinning of the formed colloidal solution led to the formation of poly(vinyl acetate) nanofibers containing uniformly distributed PdS-ZnS nanoparticles. The prepared samples were characterized by field emission scanning electron microscopy, X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy. In photocatalytic activity investigation, the PdS-ZnS/PVAc nanofibers showed remarkably enhanced performance towards water photosplitting under solar irradiation compared to the ZnS/PVAc nanofibers. This enhanced performance is attributed to the synergistic effects of heterostructured PdS-ZnS nanoparticles, which can improve photogenerated charge migration and solar light absorption.

Preferential heteroatom substitution into ZnIn2S4 nanosheets boosts photocatalytic hydrogen production

Interior modification through preferential substitution of heteroatoms in 2D materials can rescue surface reactive sites for enhanced charge kinetics and photo-activity. Here, Ru and Ni heteroatoms were successfully incorporated in ZnIn2S4 (ZIS) nanosheets via a simple hydrothermal technique. Ru-ZIS photocatalysts exhibited improved photo–water splitting activity (26,400 µmol. h−1g−1) and good stability than that of Ni-ZIS and pristine ZIS under simulated sunlight, which may be related to the increase in reactive sites with Ru substitution, optimized adsorption-free energy of the reaction intermediates and charge kinetics. Ru substitution influenced Zn site availability and thus coordinated bonding with surrounding S ions. Ru-ZIS showed excellent hydrogen evolution performance and minimal change in Gibbs free energy (ΔGH, ∼0 eV). By analyzing the density of states, both Ru and S sites had greater H* intermediate absorption abilities. This study elaborates on the effect of preferential heteroatom substitution on photocatalytic performance and has widespread applications.

Insights into interface engineering of g-C3N4/NaNbO3 heterojunction for photoelectrochemical water splitting

Green hydrogen is a very attractive energy source, a widely studied strategy among scientists to produce it is using metal oxide semiconductors as photoanodes for photoelectrochemical water splitting. The main challenge is to produce a photocatalyst that is stable and efficient enough to be commercially viable. In this sense, several different combinations of materials have been studied. Here, we report the preparation of g-C3N4/NaNbO3 coupled to Pt-based species (Pt-g-C3N4/NaNbO3) and its potential application as a photoanode for water photosplitting. NaNbO3 was chosen for its suitable properties, g-C3N4 was used to enhance the visible light activity, and Pt species were added to perform as a co-catalyst avoiding recombination of photogenerated charges. The main innovations are the reduced time and energy-consuming procedure to prepare NaNbO3 and the direct polymerization of the g-C3N4 on the NaNbO3. Results demonstrate the successful synthesis of the Pt-g-C3N4/NaNbO3 heterojunction; however, the resultant system showed low efficiency, probably for forming a layer of g-C3N4 on the top of NaNbO3 and electrically active surface states at the semiconductor/solution interfaces. Besides that, this study contributes valuable insights into the design of photoelectrodes for sustainable hydrogen production, especially due to the detailed research on the formation of the heterojunction.

Synergistic doping with Ag, CdO, and ZnO to overcome electron-hole recombination in TiO2 photocatalysis for effective water photo splitting reaction.

This manuscript is dedicated to a comprehensive exploration of the multifaceted challenge of fast electron-hole recombination in titanium dioxide photocatalysis, with a primary focus on its critical role in advancing the field of water photo splitting. To address this challenge, three prominent approaches-Schottky barriers, Z-scheme systems, and type II heterojunctions-were rigorously investigated for their potential to ameliorate TiO2's photocatalytic performance toward water photo splitting. Three distinct dopants-silver, cadmium oxide, and zinc oxide-were strategically employed. This research also delved into the dynamic interplay between these dopants, analyzing the synergetic effects that arise from binary and tertiary doping configurations. The results concluded that incorporation of Ag, CdO, and ZnO dopants effectively countered the fast electron-hole recombination problem in TiO2 NPs. Ag emerged as a critical contributor at higher temperatures, significantly enhancing photocatalytic performance. The photocatalytic system exhibited a departure from Arrhenius behavior, with an optimal temperature of 40°C. Binary doping systems, particularly those combining CdO and ZnO, demonstrated exceptional photocatalytic activity at lower temperatures. However, the ternary doping configuration involving Ag, CdO, and ZnO proved to be the most promising, surpassing many functional materials. In sum, this study offers valuable insights into how Schottky barriers, Z-scheme systems, and type II heterojunctions, in conjunction with specific dopants, can overcome the electron-hole recombination challenge in TiO2-based photocatalysis. The results underscore the potential of the proposed ternary doping system to revolutionize photocatalytic water splitting for efficient green hydrogen production, significantly advancing the field's understanding and potential for sustainable energy applications.

An epitaxial La2CuO4 thin film photocathode for water splitting under visible light

A semiconductive oxide, La2CuO4 (LCO), was investigated as a potential material to compose photocathode for sunlight-driven hydrogen evolution by splitting water. LCO, despite involving partially filled Cu 3d orbitals, behaves as a semiconductor and absorbs visible light on the bandgap formed by significant Coulomb repulsion between the electronic orbitals. An epitaxial LCO film was grown on a SrRuO3/SrTiO3 (SRO/STO) single-crystal substrate by pulsed laser deposition to obtain a photocathodic specimen for water photo-splitting. An LCO photocathode dressed with a Pt cocatalyst for hydrogen evolution (Pt/LCO/SRO/STO) exhibited a cathodic photocurrent with a density of 0.4 mA cm−2 at 0 VRHE under simulated AM1.5 G sunlight. This photocathode responded to incident light up to 800 nm, which is one of the longest wavelengths so far reported for an oxide photoelectrode. Together with a counter-electrode for oxygen evolution, the Pt/LCO/SRO/STO photocathode generated hydrogen with the expected H2 : O2 = 2 : 1 stoichiometric ratio with a Faradaic efficiency of approximately 80%.

S-scheme heterojunction of crystalline carbon nitride nanosheets and ultrafine WO3 nanoparticles for photocatalytic CO2 reduction

Highly crystalline carbon nitride polymers have shown great opportunities in overall water photosplitting; however, their mission in light-driven CO2 conversion remains to be explored. In this work, crystalline carbon nitride (CCN) nanosheets of poly triazine imide (PTI) embedded with melon domains are fabricated by KCl/LiCl-mediated polycondensation of dicyandiamide, the surface of which is subsequently deposited with ultrafine WO3 nanoparticles to construct the CCN/WO3 heterostructure with a S-scheme interface. Systematic characterizations have been conducted to reveal the compositions and structures of the S-scheme CCN/WO3 hybrid, featuring strengthened optical capture, enhanced CO2 adsorption and activation, attractive textural properties, as well as spatial separation and directed movement of light-triggered charge carriers. Under mild conditions, the CCN/WO3 catalyst with optimized composition displays a high photocatalytic activity for reducing CO2 to CO in a rate of 23.0 µmol/hr (i.e., 2300 µmol/(hr·g)), which is about 7-fold that of pristine CCN, along with a high CO selectivity of 90.6% against H2 formation. Moreover, it also manifests high stability and fine reusability for the CO2 conversion reaction. The CO2 adsorption and conversion processes on the catalyst are monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), identifying the crucial intermediates of CO2*−, COOH* and CO*, which integrated with the results of performance evaluation proposes the possible CO2 reduction mechanism.

Bifunctional carbon dots as cocatalyst and reactor decorating an organic photocatalyst for H2 production from water-splitting in an emulsion

CDs were used to modify an organic semiconductor of p(DB) and the optimal sample exhibits efficient H2 evolution under visible light in an o-xylene-in-water emulsion by photo-splitting of water. The CDs play both the roles of cocatalyst and reactor.

Multication and Structure Regulation: Utilizing X‐Doped (X = Co, Mn, Cu) ZnS/CoO Hollow Composites to Spatially Propel the Charge for Superior Solar Hydrogen Evolution

Developing polyhedral hollow structures with a concise and flexible method is intensively pursued to construct the high‐efficiency photocatalytic system. Herein, a general ion exchange‐induced strategy is developed to synthesize various hollow X‐doped (X = Co, Mn, Cu) ZnS with tunable interior components. To make the reactivity superior, abundant CoO nanoparticles are integrated into doped ZnS hollow microstar as visible‐light‐responsive photocatalysts for efficient H2 evolution. This distinct heterostructure gives full play to the following structural and compositional virtues: (i) multifacet light reflections in cavity offer massive photoexcited active centers for water photosplitting; (ii) the built‐in electric field (formed at the p‐n junction) accelerates interfacial charge separation and enhance the reducibility of photogenerated electrons, which endows the doped ZnS/CoO composite superior H2 evolution rate (1763.17 μmol h−1 g−1) and stable cyclability without needing any cocatalyst. More importantly, the formation of the potential barrier at the p‐n nanointerface and essential hydrogen evolution reaction steps are investigated to gain insight into the catalytic mechanism of the doped ZnS/CoO system.

A novel method for fabrication of electrospun cadmium sulfide nanoparticles- decorated zinc oxide nanofibers as effective photocatalyst for water photosplitting

This research investigates using CdS nanoparticles (NPs)-decorated ZnO nanofibers (NFs) as a novel photocatalytic for water splitting. For electrospinning, a colloidal solution, was used to prepare NFs, using a solution of zinc acetate dehydrate, cadmium acetate dehydrates, poly (ε-caprolactone), and ammonium sulfide. The prepared NFs were calcinated under a low-temperature condition of 200 °C, a short time of 30 min, and an argon/hydrogen (Ar/H2) atmosphere. The use of the standard techniques led to the formation of crystalline CdS NPs-decorated ZnO NFs. Compared to native ZnO NFs and CdS NPs produced using similar conditions, the photocatalytic performance of the produced NFs was favorable regarding hydrogen production. For the composite NFs, native ZnO NFs, and CdS NPs, hydrogen evolution equaled approximately 820 µmolh-1 g-1, 115 µmolh-1 g-1, and 180 µmolh-1 g-1, respectively. The excellent photostability of the NFs may be related to the nanofibrous morphological sability to facilitate the transport of photocarriers and enhance the number of active sites on the surface of photocatalytic materials. The reported approach raises the potential for novel strategies to synthesize other NFs with innovative processes and the electrospinning technique.

CuS nanosheet-induced local hot spots on g-C3N4 boost photocatalytic hydrogen evolution

An ideal model composite made of CuS nanosheet (photothermal material) and g-C3N4 was constructed through in-situ assembly procedure, along with integration of dual photochemical effect and photothermal effect. Consequently, optimized CN/CuS composite approaches remarkable photocatalytic performance improved by 44.5 times with regarding to that of pristine CN. This superiority can be assignable to inherent characteristic of CuS as photochemical component, with improved charge separation and enriched surface active-sites. Additionally, the critical contribution of photothermal effect in boosting water photosplitting was also experimentally validated. CuS nanosheet as hot spots enables rapid temperature increment around photocatalysts under visible/near-infrared (NIR) light irradiation. The heat converted from solar is conducive to increase carrier density, accelerate carrier mobility, alleviate onset potential and facilitate surface redox kinetics, so as to promote photocatalytic activity. It is believed that synergetic incorporation of photothermal and photochemical conversion could be expanded to other photocatalytic systems towards effective solar energy conversion.

Nitrogen-Doped Zinc Oxide for Photo-Driven Molecular Hydrogen Production.

Due to its thermal stability, conductivity, high exciton binding energy and high electron mobility, zinc oxide is one of the most studied semiconductors in the field of photocatalysis. However, the wide bandgap requires the use of UV photons to harness its potential. A convenient way to appease such a limitation is the doping of the lattice with foreign atoms which, in turn, introduce localized states (defects) within the bandgap. Such localized states make the material optically active in the visible range and reduce the energy required to initiate photo-driven charge separation events. In this work, we employed a green synthetic procedure to achieve a high level of doping and have demonstrated how the thermal treatment during synthesis is crucial to select specific the microscopic (molecular) nature of the defect and, ultimately, the type of chemistry (reduction versus oxidation) that the material is able to perform. We found that low-temperature treatments produce material with higher efficiency in the water photosplitting reaction. This constitutes a further step in the establishment of N-doped ZnO as a photocatalyst for artificial photosynthesis.

Photo-splitting of water toward hydrogen production and active oxygen species for methane activation to methanol on Co-SrTiO3

Direct activation of the C-H bond of CH4 under moderate conditions is highly desired but a great challenge. This work proposes a strategy for activation of the C-H bond in CH4 via the active O species generated from photo-splitting of H2O, simultaneously generating value-added CH3OH and H2. This has been achieved on SrTiO3-supported Co particles with abundant Co-SrTiO3 interfacial sites in which H2O is photo-oxidized on SrTiO3 to H2O2, which then decomposes to the active OH radical to attack the activated CH4 on the Co particle. As a result, 1.84 mmol h−1 on per gram of Co (mmol gCo−1 h−1) of CH3OH with an extraordinary selectivity of 98.7% was obtained, and the efficiency of H2 production improved to 2.4 times higher than that from overall photocatalytic H2O splitting.

Recent advances in hydrogen production using MXenes-based metal sulfide photocatalysts

At present, the composition and crystalline structure of transition metal nitrides or carbides (MXenes) and their derivatives are continuously expanding due to their unique physicochemical properties, especially in the photocatalytic field. Advances over the past four years have led to improved preparation of new MAX phases, resulting in new MXenes with excellent photo-thermal effect, considerable specific surface area, long-term stability and optimum activity. Since MXenes have good electrical conductivity and their bandgap is adjustable under the visible light range, this group is one of the best promising candidates for hydrogen production from photo-splitting of water as an environment-friendly method of converting sunlight to chemical energy. Progress in noble metal-free photocatalyst associated with more understanding of the fundamental mechanism of photocatalysis has enabled a proper choice of cocatalyst with better efficiency. In this study, the photocatalytic production of hydrogen through MXens as a support and co-catalyst on metal sulfide is summarized and discussed. Recent advances in the design and synthesis of MXenes-based metal sulfide nanocomposites to increase the efficiency of photocatalytic hydrogen production are then highlighted. Finally, the challenges and future prospects for the development of MXenes-based metal sulfide composites are outlined.

Photocatalytic reductive and oxidative ability study of pristine ZnO and CeO2-ZnO heterojunction impregnated with Cu2O

In the present work the Cu2O-ZnO heterosystem and the novel Cu2O-CeO2-ZnO triphasic heterojunction were synthesized impregnating the ZnO and the CeO2-ZnO system surfaces with 0.5% in weight of Cu2O, respectively. While X-ray powder diffraction (XRPD) evidenced any alteration in the matrix phases, the UV–vis and electron paramagnetic resonance (EPR) spectroscopy highlighted a drastic change in the optical and electronic behaviour respect to the non-impregnated samples. Deeply investigations allowed us identifying the photo-stability of the surface decorative Cu2O phase and the presence of additional copper species, namely Cu2+ and Cu(0), coming as “waste” from the employed impregnation route. The presented outcomes brought us to suggest a beneficial impact of these species promoting the overall photocatalytic process. Finally, both the oxidative and reductive photocatalytic activity of the produced materials has been evaluated by means the H2 generation from the water photosplitting process and through the photodegradation of the tolytriazol molecule, respectively, where the novel Cu2O-CeO2-ZnO heterojunction exhibited the best performance upon UV–vis and purely visible irradiation.

Sulfide-Based Photocatalysts Using Visible Light, with Special Focus on In2S3, SnS2 and ZnIn2S4

Sulfides are frequently used as photocatalysts, since they absorb visible light better than many oxides. They have the disadvantage of being more easily photocorroded. This occurs mostly in oxidizing conditions; therefore, they are commonly used instead in reduction processes, such as CO2 reduction to fuels or H2 production. Here a summary will be presented of a number of sulfides used in several photocatalytic processes; where appropriate, some recent reviews will be presented of their behaviour. Results obtained in recent years by our group using some octahedral sulfides will be shown, showing how to determine their wavelength-dependent photocatalytic activities, checking their mechanisms in some cases, and verifying how they can be modified to extend their wavelength range of activity. It will be shown here as well how using photocatalytic or photoelectrochemical setups, by combining some enzymes with these sulfides, allows achieving the photo-splitting of water into H2 and O2, thus constituting a scheme of artificial photosynthesis.

Organic Semiconductor/Carbon Dot Composites for Highly Efficient Hydrogen and Hydrogen Peroxide Coproduction from Water Photosplitting.

Coproduction of hydrogen (H2) and hydrogen peroxide (H2O2) from water splitting is one of the most promising ways to alleviate the energy crisis and environmental pollution. Here, we first show the synthesis and photocatalytic property of an organic semiconductor (DAnTMS compound) from 9,10-dibromoanthracene and trimethylsilylacetylene. Then, a metal-free photocatalyst of a DAnTMS/carbon dot (DAnTMS/CD) composite was designed and fabricated, which achieved the efficient photocatalytic production of H2 and H2O2 without usage of any organic solvents and sacrificial agents. Under visible light, the DAnTMS/CD composite could produce H2O2 with a maximum rate of 396.7 μmol g-1 h-1 and H2 with a maximum rate of 265.0 μmol g-1 h-1 in pure water. Transient photovoltage tests showed that CDs changed the interfacial electron transfer kinetics and served as the active site for highly efficient H2 evolution. This work provided a deep insight into the function of CDs in regulating the catalytic property of organic photocatalysts.

The Subsurface Diffusion of Hydrogen on Rutile TiO2 Surfaces: A Periodic DFT Study

Titanium oxide (TiO2) is an oxide with a wide range of technological and industrial applications. The interaction of hydrogen with TiO2 surface is an important step in many catalytic processes, such as spillover or water photosplitting. In the last years, the role of the subsurface hydrogen has been invoked many times to explain different phenomena, from black titania to reversible storage. In this work, we systematically investigate the paths for hydrogen diffusion from surface into subsurface focusing on the surface topology, the thermal and isotopic effects, and the degree of reduction of the substrate, by means of state of the art periodic DFT calculations. We find differences in the behavior of the rutile TiO2 surfaces (001), (100) and (110). Reaction energies are lightly exothermic, from − 0.02 to − 0.15 eV, and activation energies range from 0.39 to 1.00 eV. Deuterium diffusion from rutile (110) surface to the bulk, as well as thermal effects, were found to affect less than 0.04 eV the energy profile. On the contrary, the degree of reduction of the rutile (110) was found to noticeably decrease the activation barrier down to 0.8 eV for the (110) slab. We analyze structural, electronic and reactivity parameters that affect the kinetic barriers for the surface to subsurface H diffusion, and conclude that topology and reduction degree are valid strategies to tune the surface-to-bulk migration process.

Controlled Periodic Illumination Enhances Hydrogen Production by over 50% on Pt/TiO2.

Efficient solar water photosplitting is plagued by large overpotentials of the HER and OER. Even with a noble metal catalyst, the hydrogen evolution reaction can be limited by the strong M–H bonding over some metals, such as Pt, Pd, and Rh, inhibiting hydrogen desorption. H absorption is regulated by the potential at the metal nanoparticles. Through controlled periodic illumination of a Pt/TiO2 suspension, we hypothesized a fast variation of the photopotential that induced catalytic surface resonance on the metal, resulting in more than a 50% increase of the efficiency at frequencies higher than 80 Hz.

Study of Rh/TiO2\u2013SiO2 system in photolytic water splitting

Titania silicates (TiO2–SiO2) of various compositions were prepared by a sol–gel method. RhCl3 was used in 0.2 wt% of Rh quantity on the gel surfaces, which were subsequently exposed to UV irradiation in water under a 355-W UV lamp. λ > 370 nm. Both Rh [I] and Rh [III] surface complexes were formed, depending on the gels composition. They also exhibited various efficiency in photo water splitting reaction, the photocatalysts possessing as supports titania-silica gels containing 40 and 50 mol % of TiO2, were the most effective. According to our proposal, the rhodium complexes took part in the oxidative addition and reductive elimination cycle, where two water molecules in photo water splitting reaction, yielding hydrogen – H2 and two hydroxyl radicals. The two hydroxyls radicals, catalytically converted to oxygen atom and water, in the second cycle of the investigated water splitting reaction. Titanium octahedra and silicon tetrahedra linked by Ti–O–Si bridges were thought to be the grouping responsible for activity of the investigated catalysts. Their largest amounts are on the gels containing 40–58 mol % TiO2.

Recent Advances in Photocatalytic Oxidation of 5‐Hydroxymethylfurfural

AbstractConversion of biomass‐derived platform compounds into value‐added chemicals will reduced the current dependence on carbon resources from fossil reserves. In recent years, catalytic oxidation of 5‐hydroxymethylfurfural (HMF) has received extensive attention due to the widespread application of 2,5‐diformylfuran (DFF) and 2,5‐furandicarboxylic acid (FDCA). A photocatalyst system is a cost‐effective way of O2 utilization as a green chemical oxidant for HMF oxidation. Moreover, replacement of the kinetically sluggish water oxidation with HMF oxidation can promote hydrogen production during photo water splitting. This review provides a concise and up‐to‐date overview of photocatalyst systems for HMF oxidation, giving special attention to catalyst material design, product selectivity, mechanistic insight, and catalyst stability. The chemistry discussed in this review is an attempt to provide inspiration for researchers to develop efficient photocatalyst systems for solar biorefinery.