Photocatalytic Water Splitting Research Articles

A Protocol for Unveiling the Nature of Photocatalytic Hydrogen Evolution Reactions: True Water Splitting or Sacrificial Reagent Acceptorless Dehydrogenation?

Photocatalytic water splitting for hydrogen evolution is a highly topical subject in academic research and a promising approach for sustainable fuel production from solar energy. Due to the mismatched energy diagram of the photosensitizer (especially semiconductor-based materials where band-edge engineering is not trivial) and the redox potential of the half-reactions of water splitting, photocatalytic H2 generation from water splitting is usually accelerated by the addition of hole scavengers, i.e. sacrificial reagents such as alcohols, amines, and thiols. However, the source of the protons of the evolved H2 is often neglected, and it is questionable whether such systems are really water splitting. Here, we discuss recent reports on sacrificial reagent-assisted photocatalytic water splitting and present our recent findings, which showcase that the sacrificial reagent in the investigated photocatalytic water splitting systems inherently undergoes acceptorless dehydrogenation, with H2O serving as the proton shuttle, the amount of which doesn't change during the course of the reaction.

A Protocol for Unveiling the Nature of Photocatalytic Hydrogen Evolution Reactions: True Water Splitting or Sacrificial Reagent Acceptorless Dehydrogenation?

AbstractPhotocatalytic water splitting for hydrogen evolution is a highly topical subject in academic research and a promising approach for sustainable fuel production from solar energy. Due to the mismatched energy diagram of the photosensitizer (especially semiconductor‐based materials where band‐edge engineering is not trivial) and the redox potential of the half‐reactions of water splitting, photocatalytic H2 generation from water splitting is usually accelerated by the addition of hole scavengers, i.e. sacrificial reagents such as alcohols, amines, and thiols. However, the source of the protons of the evolved H2 is often neglected, and it is questionable whether such systems are really water splitting. Here, we discuss recent reports on sacrificial reagent‐assisted photocatalytic water splitting and present our recent findings, which showcase that the sacrificial reagent in the investigated photocatalytic water splitting systems inherently undergoes acceptorless dehydrogenation, with H2O serving as the proton shuttle, the amount of which doesn't change during the course of the reaction.

First‐Principles Study on Electronic and Optical Properties of Novel Potential Photocatalytic Water‐Splitting Material: Blue‐P/Hf2CO2 vdW Heterostructure

ABSTRACTIn this work, we set a series of blue‐P/Hf2CO2 vdW heterostructures by stacking blue‐P and Hf2CO2 monolayers together. Then the most stable structure, the AD‐stacking blue‐P/Hf2CO2 vdW structure (the HS), is selected out for further investigation. The electronic and optical properties of the HS are studied for exploring its potential applications. Result of its electronic structure investigation indicates that the HS is a type‐I band arrangement. By applying different biaxial strains parallel to the stacking direction to the HS, we regulate its band gap and band edge positions. Results show a suitable strain not only can adjust the band alignment type of the HS change from type‐I to type‐II but also regulate the band structure to a suitable band edge positions for photocatalytic water splitting. The band edge position of HS (2%) across the oxidation and reduction potential indicates that it is a potential photocatalyst of water splitting at pH = 7. By calculating the absorption coefficient, we found the HS (2%) has a good light‐harvesting ability in visible light and UV region, which further proves it has the potential as the sunlight‐driven photocatalyst for water splitting.

ZrS2/Ga2SSe heterojunction: A direct Z-scheme heterojunction with excellent photocatalytic performance across the entire pH range and high solar-to-hydrogen efficiency

Developing photocatalytic water splitting materials for hydrogen production is a crucial strategy for achieving sustainable energy utilization. This study proposes a ZrS2/Ga2SSe van der Waals heterojunction and conducts an in-depth analysis through first-principle calculations. The results show that the heterojunction possesses an indirect bandgap of 1.33 eV, which contributes to its superior optical performance. Its band structure and built-in electric field promote the Z-scheme electron transfer mechanism. High carrier mobility in the heterojunction improves photocatalytic efficiency. Strain engineering further enhances its electronic and optical properties, allowing water splitting across all pH levels within a biaxial strain range of −4% to +2%. Under a −6% compressive strain, there is a 63.06% enhancement in optical performance within the visible light spectrum, and the solar-to-hydrogen efficiency reached 10.93%. Gibbs free energy analysis indicates that the hydrogen evolution reaction, powered by photogenerated electrons, requires only 0.19 eV of energy. Therefore, the ZrS2/Ga2SSe heterojunction is identified as a promising visible-light-driven catalyst for water splitting.

Two-dimensional pnictogen carbides and their Janus structures: Potential on visible-light photocatalytic water splitting for hydrogen production

This study explored eight novel and highly stable monolayer configurations: C3X2 (X = N, P, As, Sb, and Bi) and C3XY (X = P and As, YAs and Sb) by analyzing their electronic, optical, and photocatalytic properties based on first-principles density functional theory. The findings indicated that the C3X2 monolayers, except for C3Bi2, exhibit indirect band gap semiconductor properties, and their band gaps are 1.497, 1.862, 2.272, 1.448, and 1.561 eV, respectively. Notably, only C3P2 and C3As2 fulfilled the prerequisites for photocatalytic water splitting, boast high electron mobility (up to ∼103 cm2/Vs), strong absorption in the visible light range, and high solar-to-hydrogen (STH) efficiency (30.16% for C3P2 and 23.05% for C3As2). Moreover, the Janus C3PAs, C3PSb, and C3AsSb monolayers exhibited indirect band gaps ranging from 1.616 to 2.070 eV and satisfied the band structure criteria for photocatalytic water splitting. These three Janus monolayers outperformed C3X2 in terms of optical absorption, carrier mobility, and STH efficiency owing to their inherent electric fields. The STH efficiency of C3PSb and C3AsSb reach 31.26% and 29.17%, respectively, with electron mobility reaching up to 4642.01 and 11902.64 cm2/Vs, respectively, indicating their exceptional potential for photocatalytic water splitting.

Computational study of hydrostatic pressure effect on MgSiO3 perovskite oxide for photocatalytic water splitting application

Despite the challenges of developing efficient photocatalysts for water splitting, we demonstrate that MgSiO3 perovskite oxide, under applied external stress, significantly enhances photocatalytic activity. The use of earth-abundant and low-cost photocatalysts for high-efficiency photocatalytic water splitting is extremely important. Using density functional (DFT) calculations, we present increased photocatalytic water splitting activity of MgSiO3 perovskite oxide by applying external stress. Our findings indicate that raising the concentrations of pressure can lower the lattice constants and volume of MgSiO3. An anisotropic elastic material with high stiffness that changes from brittle to ductile at high pressure (10–30 GPa). It is stable under various circumstances owing to its high Debye temperature, thermal conductivity, and melting temperature, all of which are thermodynamic properties. MgSiO₃ is an indirect bandgap (Γ →R) p-type semiconductor with a band gap energy is 2.627 eV at 0 GPa. However, the band gap energies increase as enhanced stress is confirmed by its electronic characteristics, making it appropriate for photocatalytic water-splitting applications. Strong anisotropy, effective light absorption, and reflective properties were revealed by optical investigations, which bode well for photocatalytic applications. Moreover, the optical conductivity suggests its potential for light amplification by showing a gain behavior in the visible light region. Crucially, because of its advantageous band-edge placements, MgSiO₃ shows great promise for photocatalytic water-splitting applications. This work paves the path for more investigation into effective water-splitting technologies by demonstrating the multidimensional properties and novel and inventive nature of MgSiO3 under high pressure, as well as by demonstrating its feasibility for a variety of photocatalytic applications.

Photocatalyzed oxidation of water on oxygen pretreated rutile TiO2(110)

The oxygen evolution reaction (OER) is the bottleneck in the overall photocatalytic splitting of water. The active sites (terminal titanium or bridging oxygen) and active species (molecular or dissociative water) of the initial step of the photocatalyzed OER on the prototypical photocatalyst TiO2, remain debatable. Herein, the photocatalytic chemistry of monolayer water on oxygen-pretreated TiO2(110) (o-TiO2(110)) and reduced TiO2(110) (r-TiO2(110)) surfaces initiated by 400 nm light illumination was investigated by time-dependent two-photon photoemission spectroscopy (TD-2PPE). The photoinduced reduction of the H2O/o-TiO2(110) interface rather than the H2O/r-TiO2(110) interface was detected by TD-2PPE. The difference in 2PPE originated from the presence of the terminal hydroxyl anions (OHt¯) on H2O/o-TiO2(110), as identified by X-ray photoelectron spectroscopy and temperature-programmed desorption. Therefore, the evolution of the electronic structure of H2O/o-TiO2(110) was attributed to the photocatalyzed oxidation of the terminal hydroxyl anions, which most likely formed gaseous •OH radicals, reducing the interface. This work suggested that the oxidation of hydroxyl anions on top of the terminal titanium ions on TiO2, which were excluded previously in solution, need to be considered in the mechanistic studies of the photocatalyzed OER.

Novel Two-dimensional Tellurophosphates: Visible Light Photocatalysts for Water Splitting and Carbon Dioxide Reduction

Exploring efficient two-dimensional (2D) photocatalysts for solar-driven water splitting into hydrogen and oxygen is crucial for sustainable hydrogen energy production. In this study, we investigated the suitability of novel 2D tellurophosphates M2P2Te6 (M=Mg, Ca, Sr, Ba, Ra) for photocatalytic overall water splitting using density functional theory (DFT) approach. The stability of tellurophosphates was assessed based on formation energies, phonon dispersions, ab initio molecular dynamics, and elastic modulus results. These monolayers have desirable direct bandgaps (∼1.6 – 2.2 eV) for efficient light absorption, and their valence and conduction bands ideally straddle both the oxidation and reduction potential of water along with the ability to reduce CO2 to hydrocarbons or carbonaceous products. Moreover, Gibbs free energies of Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) of tellurophosphate monolayers are also computed and display that these monolayers are viable photocatalysts under the influence of applied external potential. The tellurophosphates monolayers show strong optical absorption, with an absorption coefficient reaching up to 105 cm-1, across both the visible and UV regions of the solar spectrum, accompanied by high carrier mobility (order of 103 cm2 V-1s-1). The predicted solar-to-hydrogen efficiency of all monolayers exceeds 12%, fulfilling the criteria for commercial and economic viability in solar hydrogen production. The theoretical findings of current study imply that novel 2D tellurophosphate monolayers are promising candidates for optoelectronics, especially as photocatalysts for overall water splitting and CO2 reduction.

Enhanced photocatalytic hydrogen and oxygen evolution activity by two-dimensional van der Waals AlSb/ZnO heterostructure: A first-principles study

Photocatalytic water splitting facilitates the generation of green hydrogen energy by transforming renewable solar energy into chemical energy. This paper investigated the electronic, optical, and photocatalytic properties of AlSb/ZnO van der Waals (vDW) heterostructure using density functional theory. AlSb/ZnO, a semiconductor heterostructure with an indirect bandgap, was energetically stable with a formation energy of −0.4 eV as well as dynamically and thermally stable. The band edges of the 2D heterostructure maintained the essential redox energy levels for complete water splitting, allowing hydrogen and oxygen generation. Moreover, the heterostructure exhibited a high absorption coefficient in the visible region of the solar spectrum compared to the individual AlSb and ZnO monolayers. We revealed low barrier potential at neutral pH conditions for oxygen and hydrogen evolution reactions from the Gibbs free energy calculation without any external potential. Our study additionally showed the solar-to-hydrogen efficiency of 27.93% and a carrier utilization efficiency of 42.83%, indicating the AlSb/ZnO heterostructure as an excellent water splitting photocatalyst with remarkable potential for fuel cells and energy storage applications.

Nanoscale engineering of semiconductor photocatalysts boosting charge separation for solar-driven H2 production: Recent advances and future perspective

Abstract Photocatalytic hydrogen (H₂) production is regarded as an efficient method for generating renewable energy. Despite recent advancements in photocatalytic water splitting, the solar-to-hydrogen conversion efficiency of photocatalysts remains well below the 10% target needed for commercial viability due to ongoing scientific challenges. This review comprehensively analyzes recent advancements in nanoscale engineering of photocatalytic materials, emphasizing techniques to enhance photogenerated charge separation for efficient solar hydrogen production. Here we highlight the nanoscale engineering strategies for effective charge separation including crystal engineering, junction engineering, doping-induced charge separation, tailoring optoelectronic properties, hierarchical architecture, defects engineering, various types of heterojunctions, and polarity-induced charge separation, and discuss their unique properties including ferroelectric on spatial charge separation along with the fundamental principles of light-induced charge separation/transfer mechanisms, and the techniques for investigation. This study, critically assesses strategies for effective photogenerated charge separation to enhance photocatalytic hydrogen production and offers guidance for future research to design efficient energy materials for solar energy conversion.

Unveiling the Key Obstacle in Photocatalytic Overall Water Splitting Reaction on Poly (heptazine imide) Semiconductors.

Poly (heptazine imide) (PHI), a classic 2D polymeric photocatalyst, represents a promising organic semiconductor for photocatalytic overall water splitting (POWS). However, since the key bottleneck in POWS of PHI remains unclear, its quantum efficiency of POWS is extremely restrained. To identify the key obstacle in POWS on the PHI, a series of PHI with different stacking modes is synthesized by tuning interlayer cations. The structural characterizations revealed that tuning the interlayer cations of PHI can induce rearrangements in interlayer stacking modes. Additionally, charge carriers dynamics uncover that optimizing the interlayer stacking modes of PHI can promote exciton diffusion and prolong the photoexcited electron lifetimes, thus improving the concentration of surface-reaching charge. More importantly, this confirms that the POWS activity of PHI is closely correlated with the interlayer stacking modes. This work offers new insight into structural regulation for governing charge-transport dynamics and the activity of 2D polymeric photocatalysts.

Investigating the role of internal electric fields on interfacial charge transfer dynamics in 2D MoSTe/WSeTe heterojunction for photocatalytic water splitting

Introducing the degrees of freedom brought by intrinsic dipole moments into heterojunctions allows for effective manipulation through the combination of different interfaces, thus offering more possibilities for designing high-performance photocatalysts. The influence of internal electric fields (EIN) in Janus materials on heterojunction interface charges is a crucial aspect of understanding the intrinsic mechanisms of charge transfer at interfaces. To address this, we construct four contact modes of MoSTe/WSeTe heterojunctions, considering the work function difference as a mediator to quantify two different electric fields. It is estimated that the influence of the interfacial electric field (EIF) on interfacial charge transfer is approximately 1.6 times that of the EIN (pointing outwards the interface). Moreover, the direction of the EIN varies, resulting in different impacts on interface charges. Subsequently, we also discuss the conditions under which different charge transfer modes are formed at different contact surfaces. On the other hand, the heterojunction of the Te-S mode can serve as a direct Z-Scheme photocatalyst for overall water splitting, and the presence of vacancies is necessary to address the issue of surface passivation. Our study not only refines the intrinsic mechanism of interfacial charge transfer in Janus-based heterojunctions but also predicts a direct Z-Scheme photocatalyst capable of achieving overall water splitting.

Photocatalytic Overall Water Splitting with a Solar-to-Hydrogen Conversion Efficiency Exceeding 2 % through Halide Perovskite.

Photocatalytic water splitting using semiconductors is a promising approach for converting solar energy to clean energy. However, challenges such as sluggish water oxidation kinetics and limited light absorption of photocatalyst cause low solar-to-hydrogen conversion efficiency (STH). Herein, we develop a photocatalytic overall water splitting system using I3 -/I- as the shuttle redox couple to bridge the H2-producing half-reaction with the O2-producing half-reaction. The system uses the halide perovskite of benzylammonium lead iodide (PMA2PbI4, PMA=C6H5CH2NH2) loaded with MoS2 (PMA2PbI4/MoS2) as the H2 evolution photocatalyst, and the RuOx-loaded WO3 (WO3/RuOx) as the O2 evolution photocatalyst, achieving a H2/O2 production in stoichiometric ratio with an excellent STH of 2.07 %. This work provides a detour route for photocatalytic water splitting with the help of I3 -/I- shuttle redox couple in the halide perovskite HI splitting system and enlightens one to integrate and utilize multi catalytic strategies for solar-driven water splitting.

Photocatalytic Overall Water Splitting with a Solar‐to‐Hydrogen Conversion Efficiency Exceeding 2 % through Halide Perovskite

AbstractPhotocatalytic water splitting using semiconductors is a promising approach for converting solar energy to clean energy. However, challenges such as sluggish water oxidation kinetics and limited light absorption of photocatalyst cause low solar‐to‐hydrogen conversion efficiency (STH). Herein, we develop a photocatalytic overall water splitting system using I3−/I− as the shuttle redox couple to bridge the H2‐producing half‐reaction with the O2‐producing half‐reaction. The system uses the halide perovskite of benzylammonium lead iodide (PMA2PbI4, PMA=C6H5CH2NH2) loaded with MoS2 (PMA2PbI4/MoS2) as the H2 evolution photocatalyst, and the RuOx‐loaded WO3 (WO3/RuOx) as the O2 evolution photocatalyst, achieving a H2/O2 production in stoichiometric ratio with an excellent STH of 2.07 %. This work provides a detour route for photocatalytic water splitting with the help of I3−/I− shuttle redox couple in the halide perovskite HI splitting system and enlightens one to integrate and utilize multi catalytic strategies for solar‐driven water splitting.

Modified Titanium Dioxide Nanoparticles for Photocatalytic Splitting of Water and Its Application in Environmental Remediation as a Potential Alternative

Herein we reviewed Titanium dioxide (TiO2) nanoparticles and how they are applied in the photocatalytic splitting of water to generate hydrogen but also to remove both organic and inorganic pollutants. Hydrogen generation through the splitting of water by TiO2 which is a cheap, efficient, and non-toxic photocatalyst has found wide application in clean energy production. The efficiency of the photocatalyst can be improved by using water or sacrificial agents as electron donors, thereby enhancing hydrogen production. In addition, this review explores the fundamental principles, mechanisms, and applications of TiO2-based photocatalysis. It further, highlights the structural properties, bandgap engineering, and surface modifications that influence the catalytic activity of TiO2. Additionally, we discuss the doping of TiO2 with both metals and non-metals to narrow the energy band gap which lowers the recombination effects. Finally, it introduces the potential applications of TiO2-based photocatalysis in the photodegradation of organic and inorganic pollutants.

Photocatalytic hydrogen production and sulfamerazine degradation via a novel dual S-scheme photocatalyst: Nanocomposite synthesis, characterization and mechanism insights

Creating highly effective photocatalysts is crucial for harnessing solar energy to degrade pollutants and produce hydrogen (H₂). In this study, we successfully synthesized a novel dual S-scheme iron oxide (Fe₂O₃)/bismuth oxide (Bi₂O₃)/titanium dioxide (TiO₂) ternary photocatalyst using a straightforward method. This photocatalyst was employed for efficient photocatalytic water splitting and the degradation of the antibiotic sulfamerazine (SMZ) under visible light. Various characterization and photoelectrochemical techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area analysis (BET), photocurrent measurements, Mott-Schottky analysis, photoluminescence (PL), electrochemical impedance spectroscopy (EIS), and electron spin resonance (ESR), were utilized to analyze the synthesized materials. Among the synthesized nanocomposites, the 15 wt% Fe₂O₃/Bi₂O₃/TiO₂ (15FeBi/TiO₂) composite demonstrated exceptional photocatalytic efficiency, achieving 98 % SMZ degradation and a hydrogen production rate of 590.36 μmol/g·h. Experimental results, including scavenging tests and ESR findings, highlighted the crucial role of hydroxyl radicals (•OH) and superoxide radicals (•O₂−) in the photocatalytic process. Moreover, liquid chromatography-mass spectrometry (LC-MS) results proposed three degradation pathways, and quantitative structure-activity relationship (QSAR) analysis showed that the toxicity of intermediates was effectively reduced. The 15FeBi/TiO₂ photocatalyst also exhibited excellent reusability, retaining about 85 % of its initial activity after five cycles, and proved effective against various pollutants and in real water matrices. This research contributes to the design and development of high-activity heterojunction photocatalysts for superior clean energy generation and pollutant degradation under visible light.

Aliovalent metal cation doping of La5Ti2AgO7S5 particles for improved photocatalytic and photoelectrochemical water splitting.

The doping of photocatalytic La5Ti2AgO7S5 particles with aliovalent metal cations was investigated. The incorporation of lower-valence Al3+ cations at Ti4+ sites improved the photocatalytic hydrogen evolution activity. In addition, the anodic photocurrent during oxygen evolution was increased upon adding higher-valence Ta5+ ions. The effect of doping on the carrier density in the photocatalytic particles was also examined in this work.

Efficient Photocatalytic Hydrogen Evolution via Cocatalyst Loaded Al-doped SrTiO3

The growing reliance on fossil fuels is causing significant environmental issues, prompting the search for renewable energy sources. Hydrogen energy, which produces only water vapor, is a promising solution. This study focuses on developing an aluminum-doped SrTiO3 photocatalyst with dual cocatalysts (Rh/Cr2O3 and CoOOH) for efficient photocatalytic water splitting. Using a simple chemical deposition method, high-purity and crystalline SrTiO3 was synthesized and thoroughly characterized. The results show that the modified SrTiO3 achieved significantly higher photocatalytic activity, with Rh/Cr2O3/SrTiO3@Al/CoOOH producing 11.04 mmol g–1 h–1 of H2 and 4.69 mmol g–1 h–1 of O2. This work demonstrates the effectiveness of dual cocatalyst deposition and aluminum doping in enhancing photocatalytic performance by improving charge separation and reducing recombination.

A density functional study of type I to type II crossover in g-C3N4/CoN4 heterostructure in presence of external perturbation

Herein, we report the impact of external perturbation on the photocatalytic performance of g-C3N4/CoN4 heterojunction and concomitant transition from type-I to type-II. The state-of-the-art theoretical modeling presented here is the first study on dynamic role of strain and electric field on g-C3N4/CoN4 heterojunction and its potential application as a solar-driven water splitting reaction. Utilizing the density functional theory (DFT) calculation, V. N. et al. recently reported the photocatalytic activity of g-C3N4/MN4 (M = Mn, Fe and Co) heterojunction [J. Comput. Chem. 2024, 45, 2518-2529, https://doi.org/10.1002/jcc.2746412] and subsequently established g-C3N4/CoN4 as a type I heterojunction for photocatalytic water splitting reaction [Phys. Chem. Chem. Phys. 2024, 26, 21117–21133]. In the present study, we applied external perturbation in the form of mechanical strain, electric field and evaluated the electronic structure properties of the system along with the detailed examination of concomitant optical and magnetic properties of g-C3N4/CoN4 heterojunction. The switching of photocatalytic feature from type I to type II originates due to the crossover between valence band maxima (VBM) of g-C3N4 and CoN4 in presence of external perturbation. Notably, the resulting system acts as a type II photocatalyst for water splitting reaction while applying compressive (negative) strain of −2 to −6% and an electric field of −0.5 and +0.5 V/Å, respectively. Accumulation of photogenerated electrons and holes separately in g-C3N4 and CoN4 units confirms the perceptible separation of charge carriers and thereby reduces the recombination compared to un-perturbed system. A red-shifted absorption maximum (689 nm), superior charge transfer (0.9e), and clear separation of photogenerated electrons and holes are the origin of enhanced photocatalytic efficiency of g-C3N4/CoN4 heterojunction in the presence of external perturbation. We believe that our work will provide enough evidence to the experimentalists to achieve such a system in practice leading to human-friendly applications.

Dynamic structural twist in metal-organic framework twists the clockwork spring of photocatalytic overall water splitting

Dynamic structural twist in metal-organic framework twists the clockwork spring of photocatalytic overall water splitting