Constructing a new 2D Janus black phosphorus/SMoSe heterostructure for spontaneous wide-spectral-responsive photocatalytic overall water splitting

Janus MoSSe monolayer with built-in electric dipole, as another emerging two-dimensional (2D) material after MoS₂, is predicted to be an ideal photocatalyst for overall water splitting. However, in spite of the excellent hydrogen evolution reaction (HER) activity of Se-surface, the extremely poor oxygen evolution reaction (OER) activity of S-surface hinders the achievement of photocatalytic overall water splitting. Herein, we construct a new 2D van der Waals heterostructure consisting of high-OER-active black phosphorus (BP) and Janus MoSSe monolayer, and demonstrate a new strategy of Janus BP/SMoSe heterostructure to achieve wide-spectral-responsive photocatalytic overall water splitting. The electronic structures and optical properties of two different heterostructures, BP/SMoSe and BP/SeMoS, are systematically investigated via first principles density, exhibiting a type-II band arrangement. Unlike BP/SeMoS, the BP/SMoSe heterostructure shows excellent optical properties, such as a large dielectric constant of 8.14 and a small optical absorption boundary of 0.10 eV. Furthermore, BP/SMoSe heterostructure possesses greater light absorption intensity and a broader light absorption range. It is found that the BP/SMoSe heterostructure exhibits proper band alignment and enhanced intrinsic dipole, which is favorable to obtain high electron-hole separation efficiency. This work provides a feasible strategy of 2D Janus BP/SMoSe heterostructure for approaching almost perfect overall water-splitting photocatalysis.     

Two dimensional Janus SGaInSe(SeGaInS)/PtSe2 van der Waals heterostructures for optoelectronic and photocatalytic water splitting applications

Hydrogen generation by photocatalytic water splitting is considered as a viable and clean energy source for dealing with energy shortages and environmental pollution challenges. By means of first-principles calculations, the SGaInSe(SeGaInS)/PtSe₂ van der Waals (vdWs) heterostructures are confirmed to be energetically, dynamically, and thermally stable, indicating that they have a lot of potential for experimental implementation. The Model-I of SGaInSe/PtSe₂ heterostructures possesses type-II indirect band alignment, while the other three heterostructures retain type-I band alignment, which is further tuned to type-II with the application of strain. The charge transfer to SGaInSe/SeGaInS layer from PtSe₂ layer generates built-in electric field that effectively resists the recombination of photo-generated electron-hole pairs. At pH = 0, the band edge positions of both heterostructures completely straddle the redox potentials. The Model-I of SeGaInS/PtSe₂ heterostructures with biaxial ?2% compressive strain makes the band edges to do complete water splitting in natural environment (pH = 7). In the visible range of the irradiating spectrum, our designed heterostructures have enhanced imaginary part of the dielectric function and absorption coefficient up to 10⁵ cm^?1. Moreover, with the biaxial compressive (tensile) strains, the blue-shift (red-shift) in absorption spectra is examined. Our study extends the applications of Janus monochalcogenides/PtSe₂ vdWs heterostructures and supports to design of more heterostructures-based photocatalysts and optoelectronic devices.     

A first-principles study of electronic structure and photocatalytic performance of two-dimensional van der Waals MTe2–As (M = Mo,W) heterostructures

To efficiently produce green energy and to overcome energy crises and environmental issues, photocatalytic water splitting has become the core heart of recent research. Fabricating heterostructures with type-II band alignment can enhance the photocatalytic activity. By first-principles computations, we study Mo(W)Te₂–As van der Waals (vdW) heterostructures as promising photocatalysts for overall water splitting. The bandgap, band edge position and optical properties can be modified by biaxial strain. With appropriate compressive strain of 2% and 3%, the WTe₂–As heterostructures show transition from type-I to type-II band alignment, which could slow down electron-hole pair recombination. Compared with Mo(W)Te₂ and As monolayers, the band edge of Mo(W)Te₂–As heterostructures is on favorable positions for straddling the water redox potentials. Moreover, Mo(W)Te₂–As heterostructures fascinatingly show strong absorption peaks in both visible and near ultra-violet region, making them promising candidates for overall water splitting photocatalysts.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

New gallium chalcogenides/arsenene van der Waals heterostructures promising for photocatalytic water splitting

Single two-dimensional (2D) GaS and GaSe were studied as photocatalysts, yet the overall performance is limited by the low optical absorption and inefficient separation of photogenerated electron-hole pairs. Constructing van der Waals (vdW) heterostructures is an ideal way to overcome the deficiency of single 2D gallium chalcogenides. This work unravels that gallium chalcogenides/arsenene (GaX/As, X = S, Se) are the promising vdW heterostructures that show significantly improved photocatalytic performance by means of first-principles calculations. The GaX/As heterostructures possess suitable band alignment and bandgap satisfying the requirements for photocatalysts. Contrary to the pristine monolayers, the Se_0.5GaS_0.5/As and S_0.5GaSe_0.5/As heterostructures undergo indirect-direct bandgap transition by varying the interlayer distances; moreover, they exhibit high carrier mobility (～2000 cm² V^?1 s^?1 for electrons) and transport anisotropy, efficiently facilitating the migration and separation of photogenerated electron-hole pairs. Finally, all GaX/As heterostructures show significantly enhanced optical absorption beyond the isolated GaX monolayers under visible-light irradiation. These extraordinary properties render GaX/As heterostructures as competitive photocatalysts for water splitting to produce hydrogen.     

Review of synergistic photo-thermo-catalysis: Mechanisms,materials and applications

Solar energy is potentially the most promising type of renewable energy for large-scale utilization in the future, thus maximizing the use of solar energy has long been long pursued. Photo-thermo-catalysis (PTC) has presented a novel strategy that could utilize the full-spectrum sunlight to stimulate the synergy between photocatalysis (PC) and thermocatalysis (TC), which not only achieves high utilization efficiency of solar energy but also minimizes the energy consumption compared to sole PC and TC. This review strives to give a comprehensive overview of major advances of PTC. It starts with the fundamental mechanisms of PTC categorized by either heating mode (local and global) or photo-thermal synergic mode (thermal-assisted photocatalysis, photo-assisted thermocatalysis, photo-driven thermocatalysis and photo-thermal co-catalysis). Then, various photo-thermal materials are illustrated, including metals, semiconductors, carbon materials, etc. After that, we focus on the diverse applications of PTC, specifically in the fields of energy (CO₂ reduction and H₂ evolution), environment (VOCs and 4-NP degradations) and organic synthesis (Suzuki coupling and cyclocondensation reactions). Special emphasis is placed on the synergism of photo and thermal effect that leads to enhanced catalytic performances in PTC. Finally, the challenges and perspectives of PTC are discussed. We hope this review could shed some light on the fundamental mechanisms of PTC reactions and serve as a clearer guidance for synergistically high-efficient solar energy utilization systems in the future.     

Type-II van der Waals heterostructures of GeC,ZnO and Al2SO monolayers for promising optoelectronic and photocatalytic applications

Two-dimensional materials stacked via van der Waals (vdW) forces provide a revolutionary route toward high-performance optoelectronic and renewable energy devices. Here, we report vdW heterostructures (vdWHs) consisting of GeC, ZnO and Al₂SO monolayers on first-principles computations. GeC (ZnO)–Al₂SO vdWHs are both stable type-II semiconductors with indirect (direct) band gaps. This significantly suppresses the recombination of photogenerated charge carriers across the interface, making them promising for light detection and harvesting applications. Charge transfer from GeC (Al₂SO) layer to Al₂SO (ZnO) layer leads to p-doping in GeC (Al₂SO) and n-doping in Al₂SO (ZnO) of GeC (ZnO)–Al₂SO vdWHs. In contrast to pristine monolayers, higher carrier mobility promotes charge transfer to the surface and reduces carrier recombination in GeC (ZnO)–Al₂SO vdWHs. Further, the absorption spectra indicate redshift (blueshift) and reveal more solar light is absorbed by GeC (ZnO)–AlS₂O vdWHs in the visible (ultraviolet) region. The band edge positions suggest that GeC–Al₂SO vdWHs can reduce water into H₂ but fails to perform an oxidation reaction at pH = 0. More interestingly, ZnO–Al₂SO vdWHs can perform redox reactions, making them prominent for overall water-splitting reactions. Our computational findings provide a path for the design of vdWHs for future optoelectronic and photovoltaic devices.     

A current perspective for photocatalysis towards the hydrogen production from biomass-derived organic substances and water

Recently, an increasing interest has been devoted to produce chemical energy – hydrogen (H₂) by converting sustainable sunlight energy via water splitting and reforming of renewable biomass-derived organic substances. These photocatalytic processes are very promising, sustainable, economic, and environment-friendly. Herein, this article gives a concise overview of photocatalysis to produce H₂ as solar fuel via two approaches: water splitting and reforming of biomass-derived organic substances. For the first approach – photocatalytic water splitting, there are two reaction types have been used, including photoelectrochemical (PEC) and photochemical (PC) cell reactions. For the second approach, biomass-derived oxygenated substrates could undergo selective photocatalytic reforming under renewable solar irradiation. Significant efforts to date have been made for photocatalysts design at the molecular level that can efficiently utilize solar energy and optimize the reaction conditions, including light irradiation, type of sacrificial reagents. Critical challenges, prospects, and the requirement to give more attention to photocatalysis for producing H₂ are also highlighted.     

Inducing a deep impurity level in Li2SnO3 by ionic Bi–O bonding to enhance light absorption in photocatalysis

Orbital engineering is an important strategy for modulating light absorption in photocatalysis. Here, Bi doping of the oxide photocatalyst Li₂SnO₃ to enhance light absorption was rationally designed by orbital engineering. Based on density functional theory, owing to the lower Bi 6s energy level compared with that of Sn 5s, a deep impurity energy level induced by ionic Bi–O bonds is generated in the middle band gap of Li₂SnO₃. The impurity energy level can facilitate the utilization efficiency of light absorption, leading to remarkably enhanced photocatalytic performance. For Li₂Sn_0.9Bi_0.1O₃, the photodegradation rate of tetracycline solution reached 76% within 30 min, which was approximately 2.6 times higher than that for Li₂SnO₃. A radical trapping experiment revealed that the holes (h⁺) play a dominant role in the elimination reaction. Finally, liquid chromatography–mass spectrometry was performed to monitor the photocatalytic process. This study lays a foundation for rational design of photocatalysts with excellent light absorption for photocatalysis.     

Recent progress in metal-doped TiO2, non-metal doped/codoped TiO2 and TiO2 nanostructured hybrids for enhanced photocatalysis

Titanium dioxide remains a benchmark photocatalyst with high stability, low cost, and less toxicity, but it is active only under UV light; thus, in practical applications using visible light, its catalytic reactions are stalled. To enhance its catalytic activity under visible light, non-metal/codoped TiO₂ structures are being studied. These structures improve the photocatalytic activity of TiO₂ in visible light by reducing its energy bandgap. This might be useful in wastewater treatment for the photocatalytic degradation of organic contaminants under visible and UV light irradiation. In this intensive review, we describe recent developments in TiO₂ nanostructured materials for visible-light driven photocatalysis, such as (i) mechanistic studies on photo-induced charge separation to understand the photocatalytic activity and (ii) synthesis of non-metal doped/codoped TiO₂ and TiO₂ nanostructured hybrid photocatalysts. Furthermore, the effects of various parameters on their photocatalytic efficiency, photodegradation of various organic contaminants present in wastewater, and photocatalytic disinfection are delineated.     

MoS2-mesoporous LaFeO3 hybrid photocatalyst: Highly efficient visible-light driven photocatalyst

Perovskite type materials have high potential photocatalytic application towards both hydrogen energy generation and organic dye degradation due to their high stability and good reusability. Here, it is the first analysis of photocatalytic degradation of RhB and hydrogen energy evolution under visible light over MoS₂/LaFeO₃ nanocomposite. The physicochemical properties of the materials were characterized using a range of techniques such as XRD, TEM, XPS, FTIR, PL, photocurrent, etc. The optical properties of the nanocomposite show good absorption in UV-Vis spectra as compared to the bare LaFeO₃. In this study, MoS₂/LaFeO₃ nanocomposite was synthesized through single step in situ hydrothermal processes with a narrow bandgap, enhanced photocatalytic application under visible light. This novel MoS₂/LaFeO₃ nanocomposite is an efficient and promising photocatalyst for both hydrogen energy evolution and organic dye degradation.     

Graphitic carbon nitride/few-layer graphene heterostructures for enhanced visible-LED photocatalytic hydrogen generation

Few-layer graphene (FLG, 2–7 nm thickness) prepared by catalytic chemical vapour deposition (c-CVD), and bulk graphitic carbon nitride (g-C₃N₄; GCN) were assembled to develop novel 2D/2D xFLG_y/GCN heterostructures. The impact of FLG loading and morphology on the activity of GCN has been evaluated towards H₂ generation from water splitting under visible-LED irradiation. The heterostructures, characterised by UV–vis DRS, photoluminescence, EPR, Raman, AFM, XRD, XPS, SEM/TEM/STEM and photocurrent, present strong interfacial interaction and show higher photocatalytic activity than pure GCN. The best performing material, 2FLG₁₀/GCN, generated 1274 g^?1 h^?1 of H₂, i.e., 4-times higher than pure GCN. The improved photoactivity was ascribed to a synergistic effect between GCN and FLG, owing to: i) efficient charge separation of photoinduced electron-hole pairs through electron transfer from GCN to FLG, ii) increased surface area, and iii) enhanced visible light absorption. Moreover, the best performing composite presents high stability after four successive cycles with no significant change in its activity.     

A review on selected heterogeneous photocatalysts for hydrogen production

In this study, visible light‐driven heterogeneous photocatalysts for hydrogen production are comparatively assessed based on technical, environmental, and cost criteria. The photocatalysis systems are compared with respect to their (i) rate of hydrogen generation per gram; (ii) rate of hydrogen generation per m² of the specific surface area; and (iii) the band gap energy. The photocatalysis systems are also compared and discussed in terms of flammability, reactivity, and their impact on living systems' health. Furthermore, the costs of the required components of the photocatalysis systems are ranked. In addition to individual photocatalyst comparison, seven photocatalyst groups are ranked and compared. The results show that TiO₂‐C‐362 and Ag_0.03Mn_0.40Cd_0.60S show the highest in terms of µmol/h‐g_cat and µmol/h‐m²_cat, respectively, and TiO₂‐C‐362 has the highest overall rankings. The Zn/In/S‐based photocatalyst groups show the highest hydrogen production rate in terms of µmol/h‐gcat and µmol/h‐m²_cat. Overall, Cd/S/Zn has the highest rankings when cost and health and environmental impact criteria are taken into account. Copyright © 2014 John Wiley & Sons, Ltd.     

Design and fabrication of hollow structured Cu2MoS4/ZnIn2S4 nanocubes with significant enhanced photocatalytic hydrogen evolution performance

The unique architecture is very significant for photocatalysts to achieve high photocatalytic efficiency. Herein, hollow Cu₂MoS₄/ZnIn₂S₄ heterostructural nanocubes with intimate-contact interface have been prepared for the first time via a self-template way, which can promote the photocatalysis hydrogen evolution. First, novel hollow structured Cu₂MoS₄ nanocubes were successfully synthesized using Cu₂O as a precursor, then the ZnIn₂S₄ nanosheets were in-situ grew on the surface of hollow Cu₂MoS₄ nanocubes. The unique hollow heterostructures have markedly enhanced photocatalytic efficiency, and 15 wt% Cu₂MoS₄/ZnIn₂S₄ sample exhibits the highest hydrogen production rate of 8103 μmol·h⁻¹·g⁻¹, which is approximately four times higher than pure ZnIn₂S₄. The improved photocatalytic performance is mainly attributed to the following two points: (1) the hollow nanocube structure can provide rich active sites and increase light absorption; (2) forming a built-in electric field is conducive to transfer the holes generated by ZnIn₂S₄ to Cu₂MoS₄, which can effectively promote charge separation. This work may provide insights for the design of hollow architecture cage materials for high photocatalytic performance.     

Visible-light-assisted photocatalytic degradation of gaseous formaldehyde by parallel-plate reactor coated with Cr ion-implanted TiO2 thin film

Sol–gel nano titanium dioxide (TiO₂) thin film can be activated by the ultraviolet (UV) radiation available in sunlight to perform solar photocatalysis. The useful spectral range can be extended from UV to visible light by implantation of metal ion into the TiO₂ lattice. As a result, the solar visible light can be utilized more efficiently to enhance the solar photocatalysis. In this study, visible-light-assisted photocatalytic glass reactors were built by parallel borosilicate glass plates coated on the upper surfaces with sol–gel TiO₂ thin films implanted with chromium (Cr) ion. The properties of the Cr/TiO₂ thin films were fully characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravity (TG) analysis, scanning-electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. In the performance tests, a metal halide lamp was used as an external light source to resemble the solar visible spectral radiation. The performance of a Cr/TiO₂ photoreactor was measured in terms of its photocatalytic degradation of gaseous formaldehyde in a single pass of contaminated air flowing through the photoreactor. The experimental results demonstrated the promise of using light-transmitting glass substrate to allow transmission and distribution of light from an external source to achieve solar photocatalysis. In the design of a parallel-plate photoreactor, it is important to properly control the Cr ion loading so that each Cr/TiO₂-coated glass plate absorbs a portion of the incident light for its photocatalytic activation and allows light transmission available for the remaining coated plates.     

A critical review in strategies to improve photocatalytic water splitting towards hydrogen production

Water splitting for hydrogen production under light irradiation is an ideal system to provide renewable energy sources and to reduce global warming effects. Even though significant efforts have been devoted to fabricate advanced nanocomposite materials, the main challenge persists, which is lower efficiency and selectivity towards H₂ evolution under solar energy. In this review, recent developments in photo-catalysts, fabrication of novel heterojunction constructions and factors influencing the photocatalytic process for dynamic H₂ production have been discussed. In the mainstream, recent developments in TiO₂ and g-C₃N₄ based photo-catalysts and their potential for H₂ production are extensively studied. The improvements have been classified as strategies to improve different factors of photocatalytic water splitting such as Z-scheme systems and influence of operating parameters such as band gap, morphology, temperature, light intensity, oxygen vacancies, pH, and sacrificial reagents. Moreover, thermodynamics for selective photocatalytic H₂ production are critically discussed. The advances in photo-reactors and their role to provide more light distribution and surface area contact between catalyst and light were systematically described. By applying the optimum operating parameters and new engineering approach on photoreactor, the efficiency of semiconductor photocatalysts for H₂ production can be enhanced. The future research and perspectives for photocatalytic water splitting were also suggested.     

Sulfur-doped 2D/3D carbon nitride-based van der Waals homojunction with superior photocatalytic hydrogen evolution and wastewater purification

The weaker van der Waals force between layers inhibits the interlayer electron migration, which greatly limiting the enhancement of the photocatalytic activity over graphitic carbon nitride (g-C₃N₄). Herein, the metal-free sulfur-doped 2D/3D van der Waals (vdW) homojunction (2D/3D CNSCN) that containing 2D sulfur-doped g-C₃N₄ nanosheets (CNS) and 3D g-C₃N₄ flower-like hierarchical structure (CN) was fabricated. Thanks to the cooperative effect of 2D-3D structural and good compatibility between CNS and CN, an in situ formed 2D/3D vdW homojunction served as the driving force for promoting charge carrier separation and transfer. The optimal photocatalytic H₂ evolution of 2D/3D CNSCN reached up to 2196 μmol h^?1g^?1, which was 4.1 and 3.2 times higher than that of CN and CNS, respectively. 10 mg of the 2D/3D CNSCN photocatalyst was able to totally remove of rhodamine B (RhB) solution in less than 80 min under the visible light. This study provides new opportunities to construct novel 2D/3D mixed-dimensional vdW homojunction, and broad interest in vdW homojunction research for the applications in energy conversion and environmental protection field.     

Deposition of CdS and Au nanoparticles on TiO2(B) spheres towards superior photocatalytic performance

TiO₂(B)/CdS/Au and TiO₂(B)/Au/CdS heterostructures were synthesized to investigate the effect of the selected deposition of CdS and Au nanoparticles (NPs) on H₂ generation. TiO₂(B) spheres (phase B) consisted of nanosheets were synthesized via a hydrothermal reaction. The deposition of CdS and Au NPs were carried out using wet-chemical method and a reduction reaction, respectively. The size and amount of Au and CdS NPs were adjusted to optimize the resulting properties and discuss the change of band gap. Two kinds of heterogeneous revealed different photocatalytic hydrogen generation which indicated the position of Au NPs affect the transfer of photogenerated carriers. The hydrogen production rate of TiO₂(B)/CdS/Au heterostructures reached up to 12100 μmol g⁻¹ h⁻¹, which is about 3.8 times of that of pure TiO₂(B) spheres. This is ascribed to the structure of heterostructures. CdS NPs increase the separation of photogenerated electrons and Au NPs accelerated the transfer of the electrons. The result provided a utilizable strategy for efficient photocatalysis H₂ generation.     

Synthesis of carbon-doped SnO2 nanostructures for visible-light-driven photocatalytic hydrogen production from water splitting

Environmental issues: global warming, organic pollution, CO₂ emission, energy shortage, and fossil fuel depletion have become severe threats to the future development of humans. In this context, hydrogen production from water using solar light by photocatalytic/photoelectrochemical technologies, which results in zero CO₂ emission, has received considerable attention due to the abundance of solar radiation and water. Herein, a single-step thermal decomposition procedure to produce carbon-doped SnO₂ nanostructures (C–SnO₂) for photocatalytic applications is proposed. The visible-light-driven photocatalytic performance of the as-prepared materials is evaluated by photocatalytic hydrogen generation experiments. The bandgaps of the photocatalysts are determined by ultraviolet–visible diffused reflectance spectroscopy. The crystallinity, morphological features (size and shape), and chemical composition and elemental oxidation states of the samples are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The proposed simple thermal decomposition method has significant potential for producing nanostructures for metal-free photocatalysis.     

Constructing Janus SnSSe and graphene heterostructures as promising anode materials for Li-ion batteries

Developing efficient anode materials for Li-ion batteries is becoming increasingly important but is still challenging to collect relevant information about their adsorption and diffusion. Herein, by means of density functional theory (DFT) computations, the Janus SnSSe, and graphene van der Waals heterostructures (ie, SSnSe/G and SeSnS/G) are systematically investigated by first principles calculations, aiming at constructing promising anode materials for Li-ion batteries (LIBs). The results have demonstrated that the SnSSe/G heterostructures exhibits a semimetal-to-metal transition after incorporating Li, indicating enhanced conductivity compared to monolayer Janus SnSSe or graphene. Moreover, the SnSSe/G heterostructures can maintain favorable structural stability and ultrahigh stiffness well after applying the strain or adsorption of lithium atoms, thereby ensuring the pulverization resistance. In addition, the energy barriers of Li atoms diffusion are very low, which are expected to achieve a fast charge/discharge rate. Meanwhile, the estimated storage capacity of Li on SnSSe/G heterostructures could achieve 472.66 mA h g^?1, which greatly improves the storage capacity. These interesting results show that Janus SnSSe/G heterostructures could be used as excellent anode materials for LIBs.