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.     

PtS2/g-C3N4 van der Waals heterostructure: A direct Z-scheme photocatalyst with high optical absorption,solar-to-hydrogen efficiency and catalytic activity

Using two-dimensional semiconductors to build heterojunction as photocatalyst for water splitting is an important green and clean energy technology and has wide development prospects. Here, the monolayered PtS₂ and g-C₃N₄ are used to build the direct Z-scheme van der Waals (vdW) heterostructure, and the structure, electrical, Bader charge, optical properties and solar-to-hydrogen efficiency are calculated in detail through first-principle calculations. The direct Z-scheme PtS₂/g-C₃N₄ vdW heterostructure has an inherent type-II band alignment that enables it to reduce the photogenerated carriers aggregation, and it also possesses a decent band edge position to fully induce the redox reactions of decomposed water. The charge density shows that PtS₂ monolayer is negatively charged while g-C₃N₄ monolayer is positively charged, and the interface potential drop of PtS₂/g-C₃N₄ vdW heterostructure forms a built-in electric field with the direction from g-C₃N₄ to PtS₂. The PtS₂/g-C₃N₄ vdW heterostructure has suitable optical property, outstanding solar-to-hydrogen efficiency, high catalytic activity and thus a promising application prospect for photocatalytic water splitting.     

2D Triphosphides: SbP3 and GaP3 monolayer as promising photocatalysts for water splitting

Two-dimensional (2D) triphosphides, as an important family of 2D materials, have attracted more attention. Here, by using density functional theory (DFT) computations, we explored the remaining XP₃ monolayers (X = the metal of group IIIA-VA), and then proposed two novel 2D triphosphides, antimony triphosphide (SbP₃) and gallium triphosphide (GaP₃) monolayer. SbP₃ and GaP₃ monolayer are structurally stable semiconductors with indirect bandgaps of 2.36 eV and 1.45 eV, respectively. They not only show strong light absorption coefficients (up to 10⁵ cm⁻¹) in the visible and ultraviolet regions, but also their band edge positions straddle the redox potentials of water, making them promising catalysts for water splitting. Under strain, SbP₃ monolayer can effectively reduce the bandgap and GaP₃ monolayer can achieve the transition from indirect to direct bandgap, which can improve the photocatalytic efficiency accordingly. Our work would stimulate the fabrication of SbP₃ and GaP₃ monolayer and explore their potential applications in electronic and optoelectronic devices.     

SiI2 monolayer as a promising photocatalyst for water splitting hydrogen production under the irradiation of solar light

The feasibility of SiI₂ monolayer as the candidate for photocatalytic water splitting for hydrogen generation under the irradiation of the solar light is explored. The geometrical structure, the electronic and optical properties, the mobility of carrier and strain engineering of the monolayer are investigated based on the first-principles calculations. The results demonstrate SiI₂ monolayer possesses an indirect gap of 2.33 eV (HSE06), and both the band edge and the bandgap match the redox potential conditions of the water splitting for hydrogen generation. There is an obvious optical absorption in the visible light and near-ultraviolet region and can be enhanced by the compressive strain. Moreover, the mobility of the electron is significantly different from that of the hole, implicating that the effective spatial charge separation is expectable and the ratio of the recombination of the photogenerated charge pairs is low. The primary adsorption site of the water molecule is identified. The Gibbs free energy and the adsorption energies are calculated to demonstrate the H₂ generation from the water molecule splitting on the monolayer. All the considered properties support that SiI₂ monolayer can be achieved as a promising candidate for the photocatalytic water splitting for hydrogen production under the irradiation of the solar light.     

An efficient Z-scheme (Cr,B) codoped g-C3N4/BiVO4 photocatalyst for water splitting: A hybrid DFT study

A systematic theoretical research on the geometrical, electronic, optical, charge transfer, and photocatalytic mechanisms of pure, Cr-doped, B-doped, and (Cr, B) codoped g-C₃N₄/BiVO₄ heterostructures using a hybrid density functional approach has been carried out. The face-to-face g-C₃N₄/BiVO₄ composed of two-dimensional materials of g-C₃N₄ and BiVO₄ (010) surface, can introduce a built-in electric field, which promotes interface charge transfer and prevents the electron-hole pair recombination, and causing g-C₃N₄ monolayer with negative charge and BiVO₄ (010) surface with positive charge. Under visible light irradiation, electrons are excited to the conduction band minimum (CBM) of the BiVO₄ (010) surface undergoing the hydrogen evolution reaction (HER), while the holes remain in the valence band maximum (VBM) of g-C₃N₄ monolayer aiding the oxygen evolution reaction (OER). The band edge potentials of BiVO₄ (010) surface is higher than that of g-C₃N₄ monolayer, which ensures a stronger redox reaction potential and therefore belongs to a typical Z-scheme heterostructure. In addition, the Cr or/and B (co)doping introduces the Cr-3d or/and B-2p states to reduce the bandgap and generate impurity levels, thus enhancing solar energy utilization rate and expanding the optical absorption in the visible-light range. The optical absorption intensity of the (Cr, B) codoped g-C₃N₄/BiVO₄ is superior to pure and Cr or B doped g-C₃N₄/BiVO₄, confiriming the synergistic effect of Cr-3d and B-2p states. Thus, this research is helpful to design a novel and potential Z-scheme photocatalyst useful for the photocatalytic water splitting.     

An emerging bidirectional auxetic post-phosphorene ε-SnO monolayer: A promising Janus semiconductor with photocatalytic activity for solar-driven water splitting reaction

The design of two-dimensional (2D) auxetic semiconductors satisfying the rigorous requirements of photocatalytic water-splitting remains challenging. Anisotropic Janus monolayers display excellent potential for water splitting owing to their high photocatalytic activity, while their scarcity proves to be a disadvantage for wide application. Herein, we propose an anisotropic auxetic Janus 2D photocatalyst, a structurally stable ε-SnO monolayer, using first-principles calculations. Monolayer ε-SnO exhibits extraordinary flexibility due to its ultralow Young's moduli and high critical crack strain. Particularly, the large negative in-plane Poisson's ratios are predicted to be ?0.14/-0.17 along the x-/y-direction, which are larger than most previously reported 2D materials. It has a wide indirect bandgap, straddling the redox potentials of water, which varies with the in-plane strain. Radiation-induced carriers can drive the simultaneous occurrence of both hydrogen (HER) and oxygen (OER) evolution half reactions, even when they are under strain. The anisotropic carrier mobility can reach 625.86 cm²·V^?1·s^?1 and the absorption coefficients are predicted to reach up to the order of 10⁵ cm^?1, which is favorable for the photoexcited carrier to migrate to the active sites for water splitting. Interestingly, after transition metal atoms (from Sc to Zn) decoration, Mn/ε-SnO and Cr/ε-SnO as high-efficient single-atom HER photocatalysts are capable of driving HER with ultralow overpotentials of ?0.002 V and ?0.033 V, respectively, outperforming commercial Pt (?0.09 V). Meanwhile, the Cu/ε-SnO with a low-overpotential (0.385 V) is significantly better for neutral OER than IrO₂ (0.55 V) that is widely accepted in industrial applications.     

Novel hydrogen-doped SrSnO3 perovskite with excellent optoelectronic properties as a potential photocatalyst for water splitting

Developing and designing novel electrodes for photocatalytic water splitting using computational analysis has become a crucial interest recently through bulk and surface calculations of the investigated materials. Doping wide band gap metal oxides has proven to be an efficient method for optical properties enhancement and band gap engineering. Herein, first-principles calculations were employed to investigate the possibility to engineer the optical and structural properties of SrSnO₃ perovskite as a potential catalyst for photo-driven hydrogen production. Specifically, the synergistic effect of hydrogen doping and oxygen vacancies (O_V) on the optoelectronic properties of SrSnO₃ was for the first time investigated and discussed in detail. The interstitial hydrogen defects (H_i) are energetically favorable compared with the substitutional hydrogen defects. Mono- and co-hydrogen occupied oxygen vacancies sites were further examined. Interstitial hydrogen doping was found to introduce a shallow defect state below the conduction band minimum (CBM) forming a band tailing and increasing the dielectric constant. Thus, it could be used in gate dielectric applications. The created defect states upon doping were found to depend directly on the defect site and the defect concentration. At high concentration of oxygen vacancies defect, the H_OV-O_V structural configuration showed localized and shallow defect states with a band gap of 1.3 eV below the CBM. It also considerably increased the dielectric constant with optical absorption enhancement, compared to the pristine SrSnO₃ counterpart. With optimum Gibbs free energy of hydrogen evolution reaction (HER) and theoretical band gap straddling of the oxygen and hydrogen evolution potentials, low exciton binding energy, and high permittivity, the H_OV-O_V structure is an ideal novel candidate catalyst for photocatalytic water splitting.     

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.     

Multifunctional MoS2 ultrathin nanoflakes loaded by Cd0.5Zn0.5S QDs for enhanced photocatalytic H2 production

Two‐dimensional MoS₂ has been widely used as hydrogen evolution reaction (HER) cocatalyst to load onto nanostructured semiconductors for visible light‐response photocatalytic hydrogen production. However, its another important role as light harvester because of the band‐gap tunable property and beneficial band position has been rarely exploited. Herein, few layer‐thick MoS₂ nanoflakes with extended light absorption over the range of 400 to 680 nm and a photocatalytic HER rate of 0.98 mmol/h/g have been obtained. Then 7‐nm‐sized Cd_0.5Zn_0.5S quantum dots (QDs) are selectively grown upon ultrathin MoS₂ nanoflakes for enhanced photocatalytic H₂ generation. Upon the photocatalytic, light absorption, and charge transfer properties of the MoS₂‐Cd_0.5Zn_0.5S composites evolved with the amount of MoS₂ from 0 to 3 wt%, the multiple roles of MoS₂ as long‐wavelength light absorber, in‐plane carrier mediator, and edge site‐active HER catalyst have been revealed. An optimum H₂ generation rate of 8863 μmol/h/g and a solar to hydrogen (STH) efficiency of 2.15% have been achieved for 2 wt% MoS₂‐Cd_0.5Zn_0.5S flakes. Such a strategy can be applied to other cocatalysts with both the light response and HER activity for efficient photocatalytic property.     

Z-scheme MoO3-2D SnS nanosheets heterojunction assisted g-C3N4 composite for enhanced photocatalytic hydrogen evolutions

In this work, the 2D SnS/g-C₃N₄ nanosheets have been successfully prepared by a facile ultrasonic and microwave heating approach, which formed intimate interfacial contact and suitable energy band structure. The optimized sample displayed enhanced photocatalytic hydrogen evolution from water assisted with Pt co-catalyst, which is much higher than that of pure g-C₃N₄. After loaded with MoO₃ particles, the stability of photocatalysts displayed significate improvement due to the formed Z-scheme heterojunction. With the characterization, the enhanced hydrogen evolution reaction (HER) performance might be ascribed to the improved light-harvesting capability of the composite, lowered charge-transfer resistance, increased electrical conductivity and the co-catalyst effect of SnS. This study provides insights about SnS assisted HER photocatalysts and a new strategy to improve the stability of metal sulfides photocatalysts.     

Photophysical and photocatalytic water splitting performance of stibiotantalite type-structure compounds,SbMO4 (M = Nb,Ta)

Metal oxides with ferroelectric properties are considered to be a new family of efficient photocatalysts. Here, we investigate stibiotantalite type-structure compounds, SbMO₄ (M = Nb, Ta), with layered crystal structures, and ferroelectric properties as photocatalysts for hydrogen generation from the splitting of pure water. Both compounds were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic water splitting performance were characterized and evaluated. Diffuse reflectance analysis showed that both compounds have moderate band gaps of 3.7 eV for SbTaO₄ and 3.1 eV for SbNbO₄ (cf. 3.0 eV for TiO₂). Mott–Schottky analysis reveals that their conduction-band edge potentials are higher than the water reduction (hydrogen evolution) potential (0 V vs. RHE), indicating both compounds can generate hydrogen from water splitting. The photocatalytic water splitting performance was conducted by using pure water and UV-light irradiation, and photocatalytic H₂ production was confirmed for both compounds. After loading RuO₂ cocatalyst, the rates of hydrogen evolution of SbNbO₄ and SbTaO₄ were 24 μmol/g h and 58 μmol/g h, respectively. It was concluded that both compounds can be used as photocatalysts for water splitting under UV irradiation. The photocatalytic activity difference in both compounds was discussed with regard to electronic band structure and dipole moment difference, resulting from their crystal structures.     

Bandgap engineering in MnPS3 and ZnPS3 for photocatalytic water splitting: A first-principles study

By virtue of their wide band gaps, the photocatalytic ability of MnPS₃ and ZnPS₃ for water splitting is subject to narrow visible light adsorption. In this work, the first-principle calculations are adopted to study the impact of biaxial strain and electric field on photocatalytic performance of MnPS₃ and ZnPS₃. Our theoretical calculations demonstrate that MnPS₃ under ?10% biaxial strain, ?5% biaxial strain and a 0.05 V/Å electric field possess appropriate band gaps and band edge positions required for photocatalytic water splitting as well as pronounced absorbance in the visible and ultraviolet spectrum. Whereas, the band structures for ZnPS₃ can not be altered significantly either by applying biaxial strain or electric field. Through the free energy change analysis, it is found that water splitting reactions on MnPS₃ under ?10% biaxial strain as well as ?5% biaxial strain can be thermodynamically spontaneous at pH 7–10 using natural sunlight.     

Influence of exposed facets,morphology and hetero-interfaces of BiVO4 on photocatalytic water oxidation: A review

The photocatalytic water splitting has a stupendous role to generate renewable hydrogen. However, the overall water splitting reaction is limited by the sluggish oxidation step. Bismuth vanadate (BiVO₄) has been identified as one of the most promising n-type semiconductors for photocatalytic water oxidation due to its small energy gap and favorable band alignment. Thus, it is necessary to summarize the recent progress done on BiVO₄ for the guidance of future research. In this review, we have discussed recent strategies that have been adopted to boost its photoconversion efficiency at three different levels: (1) facet control, (2) morphological control and, (3) interfacial control. The roles of high indexed facets, anisotropic morphologies, and mediator-based Z schemes are comprehensively discussed and the emphasis is given to find the suitable structural, morphological, and interfacial combination of BiVO₄ for efficient water oxidation.     

Indirect Z-scheme hydrogen production photocatalyst based on two-dimensional GeC/MoSi2N4 van der Waals heterostructures

The stacked two-dimensional materials with suitable band gap are crucial for photocatalytic hydrogen production. Here, using first-principles calculations, the GeC/MoSi₂N₄ heterojunction with a band gap of 1.80 eV is calculated thoroughly. The indirect band alignment of Z-scheme and high carrier mobility boost the separation of electron-hole pairs, allowing more electrons and holes participating in the reactions. Additionally, the band-edge potential perfectly satisfies the requirements for redox potential of water splitting. Furthermore, the Gibbs free energy (−0.552 eV) close to zero indicates the heterojunction can conduct HER exceedingly well, providing a guarantee for photocatalytic hydrogen production. Remarkably, the light absorption coefficient peak is about 1.39 × 10⁵ cm⁻¹ within the visible light range enables the heterojunction to absorb more visible light from the spectrum. In short, results demonstrate the GeC/MoSi₂N₄ heterojunction is a promising photocatalyst for visible light water splitting, which will pave the way for the development of water splitting hydrogen production.     

Two-dimensional g-C6N6/SiP-GaS van der Waals heterojunction for overall water splitting under visible light

The energy crisis caused by the decrease of fossil fuels and the environmental pollution problems related to combustion can be resolved through the green technology of photocatalytic water splitting to produce hydrogen. A novel g-C₆N₆/SiP-GaS van der Waals heterojunction is designed and its structural, optoelectronic and photocatalytic properties are systematically investigated by using the first-principles method. The results indicate that the g-C₆N₆/SiP-GaS heterojunction is a type-Ⅱ heterojunction and the band edge straddles the redox potential of water splitting. The g-C₆N₆/SiP-GaS heterojunction exhibits good optical absorption in the visible-light range. It is worth noting that the optoelectronic properties and thermodynamic feasibility of the heterojunction can be modulated by the biaxial strain. Interestingly, all the optical absorption, OER and HER can be effectively improved under the strain of ?4%. The work supplies a strategy for the design and making of novel heterojunction for a highly effective photocatalyst in water splitting.     

Structural,elastic, phononic,optical and electronic properties investigation of two-dimensional XIS (X=Al,Ga, In) for photocatalytic water splitting

Two-dimensional (2D) materials have been widely developed due to their attractive properties. Here, by using density functional theory (DFT) calculations, for the first time, we explore potential applications of the novel XIS (X = Al, Ga, In) monolayer 2D materials on photocatalytic water splitting. A series of simulations were carried out to predict and study the structural, elastic, phononic, optical and electronic properties of 2D XIS materials. The results show that GaIS and InIS demonstrate low thermal conductivity. For optical properties, AlIS shows strong light absorption coefficients and refractive index only under ultraviolet (UV) light, while GaIS and InIS show stronger performance under both visible light and UV light with the band edge positions spanned the redox potential of water. The reasonable band positions and bandgaps make them promising photocatalysts for water splitting. This work reveals the potential applications of monolayer 2D XIS in thermal, electronic, and photocatalytic water splitting.     

Pentagonal transition-metal (group X) chalcogenide monolayers: Intrinsic semiconductors for photocatalysis

Two-dimensional (2D) materials attract enormous attention and show promising applications in many fields of science and technologies (nanodevices, energy storage/harvest and catalytic processes, etc.). Pentagonal compounds emerge as a new family in 2D materials along with classic trigonal transition metal dichalcogenides and MXenes, which have been intensively investigated to date. Encouraged by the successful synthesis of pentagonal PdSe₂ using CVD method, we explore nine pentagonal monolayers, MX₂ (M = Ni, Pd & Pt, and X = S, Se & Te), based on the first-principles calculations. We find that all MX₂ are dynamically and thermodynamically stable, and intrinsic semiconductors. Our results show that PdTe₂ exhibits excellent potential application in solar water splitting due to optimal band gap and suitable band-edge positions matching with both the water reduction and oxidation potentials (0 and 1.23 V vs. NHE). We further find that the majority of MX₂ monolayers (except NiTe₂) are applicable in photocatalytic oxygen production. Our findings are expected to shed light on the possible synthesis of pentagonal MX₂ and their application in photocatalytic water splitting.     

Fabrication of 0D/1D/2D ZnS–CuS nanodots/GNRs/g-C3N4 heterojunction photocatalyst for efficient photocatalytic overall water splitting

Photocatalytic water splitting has become a significant challenge in modern chemistry. In this process, the rate-determining step is the hydrogen evolution reaction (HER). In the present work, a surface modification approach for graphitic carbon nitride (g-C₃N₄) was applied to improve its photocatalytic HER. 0D ZnS–CuS nanodots were synthesized with the hydrothermal method as a co-catalyst to enhance the capability and stability of water splitting in the presence of visible light irradiation. Also, graphene nanoribbons were synthesized from CNTs unzipping to reduce the energy barrier of HER, improve the HE kinetic, and enhance the catalytic performance of the g-C₃N₄. By using ZnS–CuS/GNRs₍₂₎/g-C₃N₄ photocatalyst, a low onset potential of 130 mV, slight Tafel slope of 41 mV dec^?1, as well as excellent stability of 2000 s was obtained in acidic media. This efficient performance is attributed to the increased visible light absorption level in the proposed photocatalyst and the high stability in electron-hole pairs.     

Essential effect of proton coupled electron sequent transfer on photocatalytic water complete dissociation: A DFT study

Photocatalytic water splitting for hydrogen energy is one of the most promising ways to solve the energy crises. The mechanism is unclear on the sequence of proton coupled electron transfer (PCET) in photocatalytic water dissociation catalyzed by organic material. Here, the water splitting catalyzed by zinc porphyrin with boron dipyrrin (ZnPP-BDP) is systematically investigated by density functional theory (DFT) calculations. The H₂O/ZnPP in valence state based on the electron transfer pulls the trigger on the water splitting process. For the oxygen evolution reaction (OER) step on ZnPP, the processes for the two H ions dissociate from water are exothermic with the −1.06 and −0.95 eV energies respectively, and the O/ZnPP system is formed. The second H₂O molecule on O/ZnPP system can provide the extra oxygen atom to produce the free O₂ with 1.22 eV energy barrier. For the hydrogen evolution reaction (HER) step on BDP, the H ions dissociated from OER process can be captured by BDP to form free H₂ with 2.24 eV/molecule energy released. In this attractive clean cyclic process, the ZnPP-BDP can continuously catalyze the H₂O dissociated into free H₂ and O₂ by the shifting potential barrier. Besides, the proton coupled electron sequent transfer possess an essential role in photocatalytic water dissociation. It is very instructive to design sustainable photodecomposition catalysts for both OER and HER, and to design extraordinary biomimetic photosynthesizers for clean energy.     

Janus Ga2SeTe and In2SeTe nanosheets: Excellent photocatalysts for hydrogen production under neutral pH

In the past few years, Janus nanosheets have attracted much interest according to their specific structure and considerable potential to address the energy and environmental issues. Herein, the electronic, optical and photocatalytic properties of two-dimensional Janus Ga₂SeTe and In₂SeTe have been studied using ab-initio computations based on the density functional theory. The obtained results show that these nanomaterials exhibit a semiconductor behavior with direct and moderate bandgaps using hybrid HSE06 functional. Subsequently, the understudied compounds present suitable optical conductivity, absorption, transmission and reflectivity for water splitting under the ultraviolet–visible light irradiation. Interestingly, the band edge positions of Janus Ga₂SeTe and In₂SeTe excellently straddle the redox potentials of water under neutral pH. Additionally, the free energy values for the formation of H₂ from H adsorbed on the Ga₂SeTe and In₂SeTe compounds are respectively 1.304eV and 0.976eV at pH = 7. More excitingly, the present study proposes strain engineering approach to improve the photocatalytic performance of the Janus Ga₂SeTe and In₂SeTe monolayers. Specifically, the investigated semiconductors show more appropriate band edge alignment and better hydrogen evolution reaction activity under biaxial tensile strain, which fulfil the water splitting requirements at neutral pH conditions. Our findings conclude that the Janus Ga₂SeTe and In₂SeTe nanosheets are promising candidates for photocatalytic hydrogen production.