Photocatalyst For Water Oxidation Research Articles

Dual Cocatalysts on Covalent Organic Framework as Charge Transfer Mediator Facilitating Photocatalytic Water Oxidation

AbstractCovalent organic frameworks (COFs) have been regarded as promising photocatalytic materials. However, the sluggish diffusion of photoinduced charges in a single‐component COF remains the obstacles that have to be overcome. Here, we demonstrate a synthesis of a COF bearing dual cocatalysts Co3O4 and Re complex as photocatalyst for water oxidation. The dual cocatalysts are coordinated on the COF, and enable the spatial separation of photoinduced charges. The generated electrons were attracted to Re moiety, while the holes were localized on Co3O4 cocatalyst for water oxidation to produce O2 product. The synergy of dual cocatalysts of COF promote the charge separation and redox reaction. Moreover, hydroxyl radicals generated from the hydration of COF contribute to O−O bond formation, which is the key step of O2 formation. Accordingly, an excellent O2 production rate of 400 μmol g−1 h−1 under visible light irradiation was obtained. This work may pave the way to the development of highly efficient COF photocatalysts for solar energy conversion.

Voltage-Driven Molecular Photoelectrocatalysis of Water Oxidation.

Molecular photocatalysis and photoelectrocatalysis have been widely used to conduct oxidation-reduction processes ranging from fuel generation to electroorganic synthesis. We recently showed that an electrostatic potential drop across the double layer contributes to the driving force for electron transfer (ET) between a dissolved reactant and a molecular catalyst immobilized directly on the electrode surface. In this article, we report voltage-driven molecular photoelectrocatalysis with a prevalent homogeneous water oxidation catalyst, (bpy)Cu (II), which was covalently attached to the carbon surface and exhibited photocatalytic activity. The strong potential dependence of the photooxidation current suggests that the electrostatic potential drop across the double layer contributes to the driving force for ET between a water molecule and the excited state of surface-bound (bpy)Cu (II). Scanning electrochemical microscopy (SECM) was used to analyze the products and determine the faradaic efficiencies for the generation of oxygen and hydrogen peroxide. Unlike electrocatalytic water oxidation by (bpy)Cu (II) in the dark, which produces only O2, the voltage-driven photooxidation includes an additional 2e- pathway generating H2O2. DFT calculations show that the applied voltage and the presence of light can alter the activation energy for the rate-determining water nucleophilic attack steps, thereby increasing the reaction rate of photo-oxidation of water and opening the 2e- pathway. These results suggest a new route for designing next-generation hybrid molecular photo(electro)catalysts for water oxidation and other processes.

Strategic integration of nickel tellurium oxide and cobalt iron prussian blue analogue into bismuth vanadate for enhanced photoelectrochemical water oxidation

Bismuth vanadate (BVO) with a small band gap and suitable band edges is regarded as one of the promising photocatalysts for water oxidation. However, the short charge-transfer path limits its photocatalytic performance. Establishing a heterojunction and incorporating a co-catalyst are feasible methods to improve the photocatalytic ability of BVO by enhancing carrier transfer rates and reducing in-electrode resistances. In this study, nickel tellurium oxide (NTO) and cobalt iron Prussian blue analogues (CoFePBA) are incorporated into the BVO electrode to respectively develop a heterojunction and decorate co-catalyst for efficiently catalyzing the water oxidation reaction for the first time. Different amounts of CoFePBA are deposited on the NTO/BVO electrode by varying the electrodeposition durations to enhance exited charge generations and maintain high absorbance of incident light. The largest photocurrent density of 6.55 mA/cm2 at 1.23 V versus reversible hydrogen electrode is attained for the optimal CoFePBA/NTO/BVO electrode prepared using an electrodeposition duration of 2 min. Excellent catalytic stability is also achieved, with the photocurrent retention of 91.9% after illuminating the electrode for 5000 s. This study provides blueprints for incorporating novel electrochemically active materials in the BVO system to realize heterojunction and co-catalyst strategies, thereby attaining excellent photocatalytic ability toward water oxidation.

Facet-Engineered BiVO4 Photocatalysts for Water Oxidation: Lifetime Gain Versus Energetic Loss.

A limiting factor to the efficiency of water Oxygen Evolution Reaction (OER) in metal oxide nanoparticle photocatalysts is the rapid recombination of holes and electrons. Facet-engineering can effectively improve charge separation and, consequently, OER efficiency. However, the kinetics behind this improvement remain poorly understood. This study utilizes photoinduced absorption spectroscopy to investigate the charge yield and kinetics in facet-engineered BiVO4 (F-BiVO4) compared to a non-faceted sample (NF-BiVO4) under operando conditions. A significant influence of preillumination on hole accumulation is observed, linked to the saturation and, thus, passivation of deep and inactive hole traps on the BiVO4 surface. In DI-water, F-BiVO4 shows a 10-fold increase in charge accumulation (∼5 mΔOD) compared to NF-BiVO4 (∼0.5 mΔOD), indicating improved charge separation and stabilization. With the addition of Fe(NO3)3, an efficient electron acceptor, F-BiVO4 demonstrates a 30-fold increase in the accumulation of long-lived holes (∼45 mΔOD), compared to NF-BiVO4 (∼1.5 mΔOD) and an increased half-time, from 2 to 10 s. Based on a simple kinetic model, this increase in hole accumulation suggests that facet-engineering causes at least a 50-100 meV increase in band bending in BiVO4 particles, thereby stabilizing surface holes. This energetic stabilization/loss results in a retardation of OER relative to NF-BiVO4. This slower catalysis is, however, offset by the observed increase in density and lifetime of photoaccumulated holes. Overall, this work quantifies how surface faceting can impact the kinetics of long-lived charge accumulation on metal oxide photocatalysts, highlighting the trade-off between lifetime gain and energetic loss critical to optimizing photocatalytic efficiency.

Photocatalytic water oxidation by iron and copper phthalocyanine complexes immobilized on Fe3O4@SiO2@TiO2 under visible light irradiation

In this research, the heterogeneous catalysts of Fe3O4@SiO@TiO‐FePc and Fe3O4@SiO@TiO‐ CuPcN (FePc = iron (II) phthalocyanine and CuPcN = copper (II) tetraazaphthalocyanine), termed respectively as nanocomposites 1 and 2, were prepared by immobilizing the FePc and CuPcN complexes on the Fe3O4@SiO@TiO surface and used for the photocatalytic water oxidation reaction. Since phthalocyanine complexes have received very little attention as photocatalysts for water oxidation, the advantage of the synthesized photocatalyst owes to the ability of the phthalocyanine complex grafted on the titanium substrate to oxidize water in the photocatalytic reaction. The prepared nanocomposites were then studied by diffraction (XRD), scanning electron microscopy (SEM), energy‐dispersive X‐ray (EDX), Fourier transform infrared (FT‐IR) spectroscopies, thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), inductively coupled plasma‐optical emission spectroscopy (ICP‐OES), Brunauer–Emmett–Teller (BET) analysis, ultraviolet diffuse reflectance (DRS), and photoluminescence (PL) spectroscopies. The photocatalytic response of water oxidation by nanocomposites 1 and 2 was investigated in the presence of silver nitrate as an electron recipient at ambient temperature. The progress of the water oxidation reaction was monitored with an oxygen meter. The amount of oxygen generated in the presence of nanocomposites 1 and 2 under visible light irradiation was found to be 2.6 and 2.1 mg L−1, respectively. Owing to the existence of magnetic material, these nanocomposites can be effortlessly removed from the response medium with an external magnet. The involved nanocomposites can be used up to three times in catalytic water oxidation reactions with no significant change in their photocatalytic activity.

One-Step Decoration of Subnanometer MoOx Clusters on Bi11VO19 Nanotubes for Visible-Light-Driven Water Oxidation.

For the sluggish reaction kinetics due to a four-electron transfer process, water oxidation is always a major obstacle to solar splitting of water to hydrogen. It remains a tough challenge to develop efficient nonnoble-metal photocatalysts for water oxidation. Herein, we decorate the host photocatalyst of Bi11VO19 nanotubes with the coatalyst of subnanometer MoOx clusters (denoted as Bi11VO19/MoOx hetero-nanotubes) via a one-step cation-exchange solvothermal reaction using Na2V6O16 nanowires as the hard template. It is observed that the morphology and microstructure of Bi11VO19/MoOx hetero-nanotubes vary with the dosage of Mo source and polyvinylpyrrolidone, as well as with the solvent composition. The optimized Bi11VO19/MoOx hetero-nanotubes significantly enhance the photooxidation of water to oxygen with visible light, delivering an oxygen production rate of 790 μmol g-1 h-1, which is 12 times that of bare Bi11VO19 nanotubes. In situ X-ray photoelectron spectroscopy and (photo)electrochemical characterization suggest that the enhanced photoactivity may be caused by the decorated cocatalyst of MoOx clusters, which extracts electrons from Bi11VO19 nanotubes, leaving an abundance of holes for water photooxidation. This work demonstrates a potential strategy to develop photocatalysts for energy conversion by constructing Bi11VO19-based nanostructures.

BiVO4/CuIn5S8 heterojunction with rich sulphur vacancies for boosting visible light-driven photocatalytic water oxidation

The efficient conversion of water into clean energy using solar energy presents a sustainable solution to environmental challenges, albeit constrained by the slow kinetics of the oxygen evolution reaction. This study introduces a series of novel BiVO4/CuIn5S8 heterojunctions, developed and employed as photocatalysts for water oxidation. The photocatalytic activity of these heterojunctions was systematically enhanced by optimizing the BiVO4/CuIn5S8 ratio and hydrothermal reaction temperature. Notably, the heterojunction comprising BiVO4 coupled with 5 % CuIn5S8, synthesized at 180 °C (BiVO4/CuIn5S8/180–5), exhibited superior photocatalytic performance compared to its individual components. This enhancement was further augmented by the introduction of sulfur vacancies (VS) in VS-BiVO4/CuIn5S8/180–5, achieving approximately 3 and 1.4 times the photocatalytic activity of BiVO4 and BiVO4/CuIn5S8/180–5, respectively. Our findings demonstrate that CuIn5S8 effectively grows on decahedral BiVO4, forming n-n heterojunctions that not only improve visible light absorption but also facilitate the separation of photogenerated carriers. The incorporation of VS significantly lowers the charge carrier recombination rate and promotes the formation of additional active sites for H2O adsorption, thereby further elevating the efficiency of photocatalytic water oxidation.

Recyclable and dual-functional Ag3PO4/3D graphene aerogel photocatalysts for simultaneous water oxidation and pollutant degradation

Exploiting recyclable and reusable photocatalysts in pollutants elimination and water splitting has drawn extensive research attention for solving the increasing energy and environmental issues. Herein, we designed a high-performance dual functional Ag3PO4/three-dimensional (3D) graphene aerogel (APGA) photocatalyst, in which the 3D GA with extremely lightweight and high mechanical stability served as the support for Ag3PO4 particles, preventing the serious weight loss during the recovery process. Moreover, different from the traditional powder photocatalysts, the application of 3D GA support avoided the secondary contamination caused by the catalyst residue during the reaction. The abundant hydroxyl groups on the surface of porous 3D GA prevented the aggregation of Ag3PO4 particles, forming a close interfacial contact. Profiting from the large number of active sites and high conductivity of 3D GA, the optimized APGA-12 sample showed excellent O2 generation (814.1 μmol·g−1) and synchronous Cr (VI) reduction (87.5 %) efficiency. In addition, the APGA-12 sample also exhibited excellent carbamazepine (CBZ) degradation activity (99 % within 30 min) in the presence of peroxymonosulfate (PMS). Cycling experiments combined with the scaled-up reactions verified the good mechanical and chemical stability of APGA samples. This work provided a promising strategy for solving the problems of catalyst recycling in the field of pollutant degradation and water oxidation.

First-principles study of direct Z-scheme GaS/WTe2 van der Waals heterostructure as photocatalyst for water splitting

The effective separation of charge carriers and the redox capacity for water splitting are two crucial factors that influence photocatalysis. Here, we studied the photocatalytic properties of GaS/WTe2 van der Waals heterostructure (vdWH) using first-principle calculations. The results show that this heterostructure acts as a direct Z-scheme with GaS as the water oxidation photocatalyst and WTe2 as the water reduction photocatalyst. Photogenerated charge carriers are efficiently separated, with electrons in GaS combining with holes in WTe2, while other charge carriers with high redox ability remain in the respective materials for water splitting. More importantly, the heterostructure exhibits excellent performance in all acid and alkali solutions (pH 0–14) and has strong light absorption in the ultraviolet (UV) and visible region. The power conversion efficiency(PCE) reaches 19.73%. This work reveals that GaS/WTe2 heterostructure is a highly promising material for photocatalytic water splitting.

Ultrathin covalent organic framework nanosheets for enhanced photocatalytic water oxidation.

Photocatalytic water oxidation is a key half-reaction for various solar-to-fuel conversion systems but requires simultaneous water affinity and hole accumulation at the photocatalytic site. Here, we present the rational design and synthesis of an ionic-type covalent organic framework (COF) named tetraphenylporphyrin cobalt and cobalt bipyridine complex (CoTPP-CoBpy3) COF, combining cobalt porphyrin and cobalt bipyridine building blocks as a photocatalyst for water oxidation. The good dispersibility of porous large-size (>2 micrometers) COF nanosheets (≈1.45 nanometers) facilitates local water collection; the ultrafast triplet-state charge transfer (1.8 picoseconds) and prolonged charge separation (1.2 nanoseconds) further contribute to the efficient accumulation of holes in the CoTPP moiety, leading to a photocatalytic dioxygen production rate of 7323 micromoles per gram per hour. Moreover, we have identified an end-on superoxide radical (O2·) intermediate at the active site of the CoTPP moiety and proposed an electron-intermediate cascade mechanism that elucidates the synergistic coupling of electron relay (S1-T1-T1') and intermediate evolution during the photocatalytic process.

Modifying BiVO4 as a photocatalyst for water oxidation using constant-duration alkaline-etched post treatments

Alkaline etching refines surface properties of BiVO4. Three alkaline etching methods including hydrothermal, soaking, and electrodeposition processes are applied to tailor surface properties of BiVO4 as photocatalyst for OER.

Bifunctional conjugated polymer photocatalysts for visible light water oxidation and CO2 reduction: function- and site-selective hybridisation of Ru(ii) complex catalysts

Function-/site-selective hybridisation of two specific Ru(ii) complex catalysts on donor–acceptor conjugated polymers enables bifunctional visible-light water oxidation and CO2 reduction.

A novel method for preparing BiOI nanoplates and their use as precursors to synthesize porous BiVO4 water oxidation photocatalysts.

In this work, we report an innovative method for synthesizing BiOI nanoplate powder by a slow basification of an aqueous solution constituted of Bi(NO3)3 and KI. The basification was done with NH3 vapor which was naturally generated on top of an NH4OH solution kept in a closed space. The impact of the basification rate on the morphology and crystallinity of the BiOI product was investigated. Herein, we also report on the use of newly produced BiOI nanoplate powder together with the VO(acac)2 precursor for fabricating BiVO4 photoanodes for solar driven water splitting applications. We also discuss how the morphology of BiOI nanoplates and their orientation on a fluorine doped tin oxide substrate will affect the morphology, topology and photocatalytic performance of the electrode. The BiVO4 photoanode showed a photocatalytic current density of 0.55 mA cm-2 at 1.23 V vs. the Reversible Hydrogen Electrode (RHE) when assayed in a pH 7 phosphate buffer electrolyte and under 1 sun illumination.

Efficiently catalyzing photo-oxidation of water by surface engineering of bismuth vanadate with nickel molybdenum oxide coverages

Bismuth vanadate with suitable band edges is one of the efficient photocatalysts for water oxidation. Establishing heterojunction can improve electron diffusion lengths and photocatalytic ability of BiVO4. In this work, nickel molybdenum oxide (Ni–Mo–O) and BiVO4 heterojunction is established by the hydrothermal process with different nickel to molybdenum precursor ratios. The Ni–Mo–O/BiVO4 electrode is applied as photoanodes for water oxidation. The growth mechanism of Ni–Mo–O on BiVO4 surface is proposed. The Ni–Mo–O deposition amount is optimized regarding to light absorbance and charger transportation. The largest photocurrent density of 1.72 mA/cm2 at 1.23 VRHE is obtained for the optimal NiMoO4/BiVO4 electrode (NM12) in the electrolyte without hole scavenger, while the pure BiVO4 electrode only shows a photocurrent density of 0.90 mA/cm2. The NM12 electrode even presented an impressive photocurrent density of 5.39 mA/cm2 at 1.23 VRHE in the electrolyte with the hole scavenger, owing to the abundant active sites and higher light absorbance as well as the favorable synergistic effects from its suitable Ni to Mo ratio. The NM12 electrode also shows excellent long-term stability with the photocurrent retention of 83% after illumination for 6500 s. This work opens a blueprint for establishing a novel heterojunction with the adjustable metal ratio and coverage on BiVO4.

Exploring modern developments in diverse 2D photocatalysts for water oxidation

Exploring modern developments in diverse 2D photocatalysts for water oxidation

Bismuth Tungstate Nanoplates-Vis Responsive Photocatalyst for Water Oxidation.

The development of visible-light-responsive (VLR) semiconductor materials for effective water oxidation is significant for a sustainable and better future. Among various candidates, bismuth tungstate (Bi2WO6; BWO) has attracted extensive attention because of many advantages, including efficient light-absorption ability, appropriate redox properties (for O2 generation), adjustable morphology, low cost, and profitable chemical and optical characteristics. Accordingly, a facile solvothermal method has been proposed in this study to synthesize two-dimensional (2D) BWO nanoplates after considering the optimal preparation conditions (solvothermal reaction time: 10-40 h). To find the key factors of photocatalytic performance, various methods and techniques were used for samples' characterization, including XRD, FE-SEM, STEM, TEM, HRTEM, BET-specific surface area measurements, UV/vis DRS, and PL spectroscopy, and photocatalytic activity was examined for water oxidation under UV and/or visible-light (vis) irradiation. Famous commercial photocatalyst-P25 was used as a reference sample. It was found that BWO crystals grew anisotropically along the {001} basal plane to form nanoplates, and all properties were controlled simultaneously by tuning the synthesis time. Interestingly, the most active sample (under both UV and vis), prepared during the 30 h solvothermal reaction at 433 K (BWO-30), was characterized by the smallest specific surface area and the largest crystals. Accordingly, it is proposed that improved crystallinity (which hindered charge carriers' recombination, as confirmed by PL), efficient photoabsorption (using the smallest bandgap), and 2D mesoporous structure are responsible for the best photocatalytic performance of the BWO-30 sample. This report shows for the first time that 2D mesoporous BWO nanoplates might be successfully prepared through a facile template-free solvothermal approach. All the above-mentioned advantages suggest that nanostructured BWO is a prospective candidate for photocatalytic applications under natural solar irradiation.

Distinguished heterojunction and co-catalyst behaviors on tungsten doped BiVO4 for photoelectrochemical catalytic water splitting reaction

Bismuth vanadate (BiVO4) is one of the most promising photocatalysts for water oxidation. The short charge transfer path of BiVO4 limits its electrochemical performance. Incorporating foreign materials into BiVO4 is widely usedto resolve this problem. The heterojunction or co-catalyst/photocatalyst system can possibly be established in the foreign material/BiVO4 system. However, the definition of these two systems is sometimes misleading. It is necessary to distinguish the behavior of the foreign material and understand its effects on water oxidation catalytic ability. In this study, three metal organic frameworks or their derivatives, MIL(Fe), ZIF67, and oxidized ZIF67, are coupled with BiVO4 as the catalysts for water oxidation. By observing optical properties and linear sweep voltammetry curves, the behavior of foreign materials can be simply defined. The in-situ synthesized MIL(Fe)/W doped BiVO4 (MIL/W:BVO) shows a heterojunction behavior, while the ex-situ synthesized ZIF67-decorated and oxidized ZIF67-decorated W:BVO present co-catalyst behaviors. The highest photocurrent density of 4.14 mA/cm2 at 1.23 VRHE and the smallest charge-transfer resistance of 67.0 Ω are obtained for the MIL/W:BVO electrode, implying a better modification of heterojunction with effective charge transfer cascades. Agood long-term stability with a current retention higher than 90% is also obtained for MIL/W:BVO after 5000 s under illumination.

Boosted Z-scheme photocatalytic overall water splitting with faceted Bi4TaO8Cl crystals as water oxidation photocatalyst

We have identified a facet-assisted mechanism for photocarrier separation in flux-treated Bi4TaO8Cl crystals that expose mainly {001} and {110} facets. These two crystal facets have different energy states that provide an internal electric field to split photocarriers towards different crystal facets. We have further leveraged the phenomenon by facet-selective deposition of CoOx cocatalyst at {001} crystal facets where holes are accumulated to expedite interfacial charge transfer reactions. The coupling of these two facet-engineering techniques brings exquisite control over the flow of photocarriers which cascade continuously from bulk to the surface for catalytic reactions. Under optimal conditions, the CoOx-deposited Bi4TaO8Cl achieves an unprecedentedly high apparent quantum efficiency of 27% at 420 ± 20 nm for water oxidation into O2. Stable overall water splitting into H2 and O2 (molar ratio 2:1) under visible light irradiation has also been realized in a Z-scheme system employing the CoOx-deposited Bi4TaO8Cl as the O2-evolution moiety.

Surface Termination Tuning Photoexcited-Carrier Dynamics and Carrier Transfer Pathway on Silicon for High-Performance Photocatalytic H2 Evolution from Pure Water

Tuning surface termination can improve the carrier dynamics and photoredox performance of photocatalysts by modifying the surface physicochemical properties. Herein, we prepared a Si photocatalyst by magnesium-thermal reduction and then treated it with trifluoroacetic acid (TFA). The as-obtained TFA-terminated Si photocatalyst (TFA-Si) improves double-layer capacitance and electrochemical/Brunauer–Emmet–Teller surface area, rendering more active sites. Moreover, the TFA modification causes an increase in wettability, charge carrier density, and photocurrent and a reduction in electrochemical impedance and overpotential, which enhances the adsorption/reduction of water molecules and the photoexcited-carrier dynamics for photocatalysis. TFA-Si shows high photocatalytic H2 evolution activity (1827 μmol·g–1·h–1) in pure water under visible light. The AQY of TFA-Si reached 6.1% at 400 nm. Interestingly, TFA molecules promote the oxidation capacity of Si photocatalysts for water oxidation through a hole transfer pathway, boosting its activity and stability. This work broadens the applications of Si-based materials for use in photocatalysis.

All-Solid-State C3N4/NixP/Red Phosphorus Z-Scheme Heterostructure for Wide-Spectrum Photocatalytic Pure Water Splitting

Z-scheme photocatalysts encouraged by natural photosynthesis have received increasing attention for pure water splitting. However, there have been only a few instances of effective Z-scheme nanosystems utilizing nonmetal photocatalysts for both water reduction and oxidation. In this study, we used carbon nitride (CN), metallic NixP, and crystalline red phosphorus (RP) to build a solid-state Z-scheme photocatalytic system, which worked as reduction sites, an electron mediator, and oxidation sites, respectively. The light absorption capability up to ∼600 nm enabled the photocatalyst to realize water splitting under broad-spectrum illumination. Detailed analysis suggested that the photocatalytic hydrogen production rate was apparently enhanced on account of effective spatial separation of light-induced charges owing to the intimate contact between the NixP mediator and photocatalyst components as well as the suitable energy band alignment. Meanwhile, hydrogen peroxide instead of oxygen was generated from water oxidation, which can solve the separation and safety issues of the synchronized production of hydrogen and oxygen and thus facilitated the feasible application of photocatalytic hydrogen production.