Photocatalyst For Overall Water Splitting Research Articles

The band structure engineering of fluorine-passivated graphdiyne nanoribbons via doping with BN pairs for overall photocatalytic water splitting

In this work, we systematically study the electronic band structures of fluorine-passivated graphdiyne nanoribbons (F_GDYNRs) doped with BN pairs using first-principles density functional theory calculations. The calculation results show that that fluorine passivation and heteroatom doping play different roles in modifying the electronic structures of F_GDYNRs. The former helps lower the position of the valence band of the graphdiyne nanoribbons (GDYNRs) while the latter significantly opens the band gap of GDYNRs. The doped F_GDYNRs have direct band gaps of 1.8-2.9 eV, and their valence and conduction bands perfectly straddle both the oxidation and reduction potential of water. This work demonstrates that F_GDYNRs, via doping with BN pairs, possess high catalytic activity for water splitting, which will shed light on the design of metal-free low-dimensional photocatalysts.

Computational studies on triphenyldiyne as a two-dimensional visible-light-driven photocatalyst for overall water splitting.

The high carrier mobility, porous configurations and tunable electronic structures of two-dimensional (2D) carbon materials hold great promise in energy conversion and storage. However, few of them are capable of photocatalytic overall water splitting. Here, by means of first-principles calculations within the quasi-particle approximation and the Bethe-Salpeter equation, we demonstrated a unique framework of triphenylenes (sp2) and acetylenic linkages (sp), namely triphenyldiyne (TDY) that has the electronic band structure suitable for photocatalytic overall water splitting along with pronounced optical absorbance in visible light. The redox ability of its photogenerated electrons is high enough to drive the hydrogen evolution reaction (HER). Through Ni doping with TDY, its overpotential for the oxygen evolution reaction (OER) can be reduced to match the redox ability of its photogenerated holes, enabling the photocatalytic overall water splitting in sunlight without the need of sacrificial reagents. This work offers not only a low-cost, earth-abundant and environmental-friendly photocatalyst, but also a promising strategy for designing highly efficient photocatalysts for overall water splitting.

Metal-free highly efficient photocatalysts for overall water splitting: C3N5 multilayers.

As a promising means of renewable energy storage, the production of molecular hydrogen and oxygen from photocatalytic water splitting has gained increasing interest. The optimal photocatalyst for water splitting should have high solar energy conversion efficiency and strong photocatalytic redox ability to drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, few photocatalysts have been reported to fulfil these two contradictive requirements. Here, we demonstrated from first-principles calculations that the recently synthesized two-dimensional carbon nitride (C3N5) multilayers can serve as promising candidates to reach this goal. The intrinsic electric field which is more pronounced in the multilayers alters the band alignment of the photocatalysts, making the HER and OER be driven solely by the photogenerated carriers. The thickness-dependent electronic band gap (2.95-2.16 eV) along with the high carrier mobility broadens the energy range of light absorption and promotes carrier separation and transfer, leading to high solar energy conversion efficiency. Our computational results offer not only low-cost, Earth-abundant and environmentally friendly photocatalysts but also a promising strategy for the design of photocatalysts for highly efficient overall water splitting without using sacrificial reagents.

Two-dimensional penta-SiAs2: a potential metal-free photocatalyst for overall water splitting

We thoroughly investigated the photocatalytic performance of novel 2D penta-SiAs2 as an efficient photocatalyst based on first-principles calculations.

2D van der Waals heterostructures of graphitic BCN as direct Z-scheme photocatalysts for overall water splitting: the role of polar π-conjugated moieties.

The application of two-dimensional (2D) graphitic carbon-based materials in photocatalysis has been limited to date, because the nature and role of π-conjugated moieties in them remain unclear. Herein we propose and study bilayer BC3/C3N and BC3/BC6N van der Waals heterostructures as direct Z-scheme photocatalysts for overall water splitting using density functional theory calculations. The roles of polar π-conjugated moieties in the formation, stacking configuration, and electronic and optical properties of bilayer van der Waals heterostructures are discussed. It is shown that polar π-conjugated moieties of graphitic BCN monolayers lead to a favorable π-π interaction, determining the most stable stacking configuration, and a long-range charge transfer between components. The former makes the electronic band structure of heterostructures favor photocatalytic water splitting in efficiency and energetics. The latter generates a built-in electric field for the interface recombination of photogenerated electron-hole pairs, indicating a Z-scheme mechanism. The delocalized nature of π-conjugated electrons in monolayer components allows for high carrier mobility of bilayer heterostructures, promoting the photocatalytic reactions on graphitic BCN monolayers. These findings show that 2D π-conjugated materials, including graphitic carbon-based materials and biological systems, have great potential in the design and development of 2D metal-free direct Z-scheme photocatalysts for environmental purification and energy conversion.

A two-dimensional h-BN/C2N heterostructure as a promising metal-free photocatalyst for overall water-splitting.

The construction of a heterostructure (HS) is an effective strategy to modulate the desired properties of two-dimensional (2D) materials and to extend their applications. In this paper, based on the density functional theory, we predict a metal-free type-II HS formed by h-BN and C2N single layers. The h-BN/C2N HS possesses a smaller bandgap than individual h-BN and C2N single layers, and it exhibits excellent visible light absorption. Importantly, its band edge positions satisfy the requirements for spontaneous water-splitting. With the assistance of the built-in electric field across the HS and the band offset, the photoinduced carriers can be readily spatially separated. Free energy calculations indicate the high catalytic activity for water oxidation and reduction reactions. The performance can be further enhanced by strain, which modulates the bandgap and the band edge positions of the HS. The band alignment may undergo a transition from type-I to type-II under strain, offering an effective switch for the reaction. The appropriate bandgap, suitable band edge positions, and effective carrier separation make the h-BN/C2N HS a promising candidate for use as a photocatalyst in water-splitting.

Direct Z-scheme photocatalytic overall water splitting on two dimensional MoSe2/SnS2 heterojunction

The direct Z-scheme photocatalysts for overall water decomposition have aroused much concern on account of their strong redox ability and efficient separation of photogenerated electron-hole pairs. In the present work, we have constructed the two dimensional (2D) van der Waals (vdW) MoSe2/SnS2 heterojunction and investigated its electronical and optical properties by applying hybrid functional calculations. The calculated band structures have implied that the MoSe2/SnS2 heterostructure as a direct Z-scheme photocatalyst can make the best of visible light. The induced built-in electric field can effectively improve the separation efficiency of the photoinduced carriers. Moreover, compared with the MoSe2 and SnS2 monolayers, the absorption intensity of MoSe2/SnS2 heterojunction is reinforced in the visible light range. Therefore, MoSe2/SnS2 nanocomposite shows a bright application prospect as a direct Z-scheme visible-light-driven photocatalyst for overall water splitting.

Highly-efficient overall water splitting in 2D Janus group-III chalcogenide multilayers: the roles of intrinsic electric filed and vacancy defects

Two-dimensional (2D) van der Waals materials have been widely adopted as photocatalysts for water splitting, but the energy conversion efficiency remains low. On the basis of first-principles calculations, we demonstrate that the 2D Janus group-III chalcogenide multilayers: InGaXY, M2XY and InGaX2 (M = In/Ga; X, Y = S/Se/Te), are promising photocatalysts for highly-efficient overall water splitting. The intrinsic electric field enhances the spatial separations of photogenerated carriers and alters the band alignment, which is more pronounced compared with the Janus monolayers. High solar-to-hydrogen (STH) efficiency with the upper limit of 38.5% was predicted in the Janus multilayers. More excitingly, the Ga vacancy of InGaSSe bilayer effectively lowers the overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to the levels provided solely by the photogenerated carriers. Our theoretical results suggest that the 2D Janus group-III chalcogenide multilayers could be utilized as highly efficient photocatalysts for overall water splitting without the needs of sacrificial reagents.

First-principles investigation of β-Ge3N4 loaded with RuO2 cocatalyst for photocatalytic overall water splitting

Abstract β-Ge3N4 loaded with nanoparticulate RuO2 as a cocatalyst is the first successful non-oxide photocatalyst for overall water splitting. To get an insight into the working mechanism of this particular photocatalytic system, we have calculated geometrical structures of low-index surfaces for β-Ge3N4. Analysis of surface energies indicates that the most preferentially exposed surface is (100). The band gap of surface is narrower than that of bulk due to the dangling bonds. Dissociative water adsorption on (100) surface is thermodynamically favorable. The adsorption behavior of (RuO2)n (n = 2, 3, and 4) clusters on the β-Ge3N4(100) surface has been explored. It is found that all the clusters bind to (100) surface strongly by forming interfacial bonds so that the adsorptions are exothermic processes. The calculation on density of states for β-Ge3N4(100) surface loaded with (RuO2)n clusters reveals that photo-induced electrons tend to accumulate on (RuO2)n clusters and holes tend to stay in β-Ge3N4. Based on the theoretical indication of Type-II staggered band alignment, we have proposed that in photocatalytic water splitting reaction, oxygen evolution reaction is inclined to occur on the surface of β-Ge3N4 while hydrogen evolution reaction is apt to occur on (RuO2)n clusters. In a word, loading RuO2 nanoparticles as a reduction cocatalyst benefits the charge separation in β-Ge3N4. Furthermore, attaching (RuO2)n clusters onto β-Ge3N4(100) surface results in the redshift of absorption edge and the increase of absorption intensity. Our calculations have reasonably explained the experimental observation on the decomposition of water into H2 and O2 after loading RuO2 cocatalyst in β-Ge3N4 photocatalyst.

Surface Photovoltage Spectroscopy Observes Sub-Band-Gap Defects in Hydrothermally Synthesized SrTiO3 Nanocrystals

SrTiO3 has been of interest as a photocatalyst for overall water splitting, but the energy conversion efficiency of this material is limited by recombination at surface and lattice defects. Here, w...

Controllable synthesis of high-efficient TaOxNy and Ta3N5 photocatalysts through vacuum nitriding using melamine as a nitrogen source

Tantalum (oxy) nitrides have been considered as very promising visible-light-driven photocatalysts for overall water splitting. The traditional tantalum (oxy) nitrides preparation using NH3 gas at high temperature (850-1150 °C) is not favored for the dangerous process and complexity. Furthermore, the undesirable defects (O impurities and N vacancies) in tantalum (oxy) nitrides reduce the photocatalytic activity. Herein, we developed a safer and simpler approach for controllable preparing high-efficient TaOxNy and Ta3N5 through vacuum nitriding using melamine as a nitrogen source. The effect of vacuum nitriding conditions on the microstructure, composition, photo-adsorption and photocatalytic H2 evolution of the samples were studied. TaOxNy and Ta3N5 can be obtained with the pressure below 2 × 104 Pa, temperature of 500–700 °C and holding time for 6 h. Compared to the samples synthesized by traditional NH3 nitriding, the as-prepared TaOxNy and Ta3N5 by vacuum nitriding exhibited enhanced photocatalytic H2 evolution owing to the lower defects, higher charger separation rate and higher specific surface area. Furthermore, according to the nitrogen potential theory, vacuum condition increased the nitrogen potential and promoted the collision of active nitrogen with the sample, which achieved more effective nitrogen replacement. This study contributes to the simple synthesis of high-efficient tantalum (oxy) nitrides with lower defects.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies.

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Hydrogen Production Improvement on Water Decomposition Through Internal Interfacial Charge Transfer in M3(PO4)2-M2P2O7 Mixed-Phase Catalyst (M = Co, Ni, and Cu)

In this study, three types of Nasicon-type materials, Co3(PO4)2-CO2P2O7, Ni3(PO4)2-Ni2P2O7, and Cu3(PO4)2-Cu2P2O7, were synthesized as mixed-phase catalysts (MPCs) for evaluating their potential as new photocatalytic candidates (called Co3(PO4)2-CO2P2O7mpc, Ni3(PO4)2-Ni2P2O7mpc, and Cu3(PO4)2-Cu2P2O7mpc herein). Based on various physical properties, it was confirmed that there are two phases, M3(PO4)2 and M2P2O7, in which a similar phase equilibrium energy coexists. These colored powders showed UV and visible light responses suitable to our aim of developing 365-nm light-response photocatalysts for overall water-splitting. The photocatalytic performance of Ni2(PO4)3-Ni2P2O7 MPC showed negligible or no activity toward H2 evolution. However, Co2(PO4)3-Co2P2O7 MPC and Cu3(PO4)2-Cu2P2O7 MPC were determined as interesting materials because of their ability to absorb visible light within a suitable band. Moreover, an internal interface charge transfer was suggested to occur that would lower the recombination rate of electrons and holes. For Cu3(PO4)2-Cu2P2O7 MPC, the charge separation between the electron and hole was advantageously achieved, a water-splitting reaction was promoted, and hydrogen generation was considerably increased. The performance of a catalyst depended on the nature of the active metal added. In addition, the performance of the catalyst was improved when electrons migrated between the inter-phases despite the lack of a heterojunction with other crystals.

Oxysulfide photocatalyst for visible-light-driven overall water splitting.

Oxysulfide semiconductors have narrow bandgaps suitable for water splitting under visible-light irradiation, because the electronegative sulfide ions negatively shift the valence band edges of the corresponding oxides1,2. However, the instability of sulfide ions during the water oxidation is a critical obstacle to simultaneous evolution of hydrogen and oxygen3. Here, we demonstrate the activation and stabilization of Y2Ti2O5S2, with a bandgap of 1.9 eV, as a photocatalyst for overall water splitting. On loading of IrO2 and Rh/Cr2O3 as oxygen and hydrogen evolution co-catalysts, respectively, and fine-tuning of the reaction conditions, simultaneous production of stoichiometric amounts of hydrogen and oxygen was achieved on Y2Ti2O5S2 during a 20 h reaction. The discovery of the overall water splitting capabilities of Y2Ti2O5S2 extends the range of promising materials for solar hydrogen production.

Hexagonal CuCl Monolayer for Water Splitting: A DFT Study

In the past few years, enormous efforts have been put to develop earth-abundant semiconductor based photocatalysts for overall water splitting. In the present work, we predict a hexagonal CuCl mono...

Understanding Charge-Transfer and Kinetic Processes in Particulate Photoelectrode Morphologies for Solar Water Splitting

Z-scheme semiconductor particles, either as suspended solids in an aqueous solution1–3 with soluble redox shuttles, or as particulate sheets4,5 with solid-state electronic contacts, is a promising two-step approach to use solar energy to split water to produce hydrogen and oxygen. For Z-scheme particles to operate efficiently, it is important to realize efficient electron-transfer from the oxygen evolution particle to the hydrogen evolution particle, and to selectively drive the reactions of interest at the semiconductor-electrolyte interface. I will present on the development of a numerical modeling framework to evaluate the transport and kinetics of photogenerated charge-carriers inside a spherical semiconductor particle and across the semiconductor–electrolyte interface for Z-scheme systems with and without soluble redox shuttles. The boundary conditions applied at the electron-transfer interfaces are formulated to take into account all plausible particle surface-mediated redox reactions. The model is used to elucidate the impacts of particle size, recombination pathways, kinetic parameters and the electrochemical potential of the redox species on the performance of these particles. Results are further interpreted to identify strategies to boost the energy conversion efficiency and quantify the influence of the electrode morphologies on the overall rates of hydrogen production in Z-scheme solar water splitting systems. References Fabian, D. M. et al. Particle suspension reactors and materials for solar-driven water splitting. Energy Environ. Sci. 8, 2825–2850 (2015).Bala Chandran, R., Breen, S., Shao, Y., Ardo, S. & Weber, A. Z. Evaluating particle-suspension reactor designs for Z-scheme solar water splitting via transport and kinetic modeling. Energy Environ. Sci. 11, 115–135 (2018).Keene, S., Bala Chandran, R. & Ardo, S. Calculations of theoretical efficiencies for electrochemically-mediated tandem solar water splitting as a function of bandgap energies and redox shuttle potential. Energy Environ. Sci. (2019). doi:10.1039/C8EE01828FChen, S., Takata, T. & Domen, K. Particulate photocatalysts for overall water splitting. Nat. Rev. Mater. 2, 17050 (2017).Goto, Y. et al. A Particulate Photocatalyst Water-Splitting Panel for Large-Scale Solar Hydrogen Generation. Joule 0, (2018). Figure 1

N-doped defective graphene decorated by strontium titanate as efficient photocatalyst for overall water splitting

This manuscript reports the synergic effect for overall water splitting of N-doped defective graphenes obtained from the pyrolysis of biomass wastes, upon deposition at 1 wt% of Al-doped SrTiO3 containing RhCrOx nanoparticles (RhCrOx/STO:Al) as co-catalyst, exhibiting a photocatalytic activity of 153 mmol of H2 per g of total photocatalyst and 74 mmol/g of O2 at 24 h irradiation with UV–vis light reaching an incident photon to hydrogen efficiency of 0.67% at 360 nm. This hybrid N-doped defective graphene-RhCrOx/STO:Al photocatalyst was stable in 4 consecutive photocatalytic runs. The obtained results indicate that just a small amount of RhCrOx/STO:Al acting as co-catalysts supported on N-doped defective graphene is sufficient to enhance significantly its photocatalytic activity, making N-doped graphene suitable for overall splitting.

Two-dimensional amorphous CoO photocatalyst for efficient overall water splitting with high stability

CoO nanocrystal is a well-known photocatalyst for overall water splitting; however it suffers from a very short lifetime of only 1 h. The poor stability is derived from carrier recombination-induced thermal oxidation. Amorphous materials are generally omitted from photocatalysts due to the tailing effect. Here, we report that efficient solar overall water splitting into H2 and O2 with an expected 2:1 stoichiometry is achieved by two-dimensional amorphous CoO nanoflakes without any cocatalysts or scavengers, and no deactivation is detected even after working for 120 h. We also establish that the mediation of mid-gap states resulting from the amorphization is responsible for the carrier separation and consequent high stability. Further, graphene oxide is grafted to construct a bionic Z-scheme structure, promoting the gases evolution rates by a factor of 13. Overall, these findings reveal the potential of CoO for practical applications in the future hydrogen economy, and shed new light on the design of novel amorphous photocatalysts.

Optimized band gap and fast interlayer charge transfer in two-dimensional perovskite oxynitride Ba2NbO3N and Sr2NbO3/Ba2NbO3N bonded heterostructure visible-light photocatalysts for overall water splitting

Searching for promising visible-light photocatalysts for overall water splitting into hydrogen and oxygen is a very challenging task to solve the energy crisis and environment pollution. The widely-used tantalate and niobate perovskite photocatalysts have two drawbacks, i.e., the large energy band gap (∼3.2–4.6 eV) and low electron (hole) mobility 102 (101) cm2 V−1 s−1, which greatly limit their photocatalytic performance. Here, based on the powerful first-principles and accurate GW calculations, we design several novel two-dimensional (2D) Ruddlesden-Popper (RP) type (n = 1) perovskite oxynitrides A2BO3N (A = Ca, Sr, Ba and B = Ta, Nb) and their bonded heterostructures and comprehensively investigate their interlayer coupling, electronic structures, transport and photocatalytic characteristics. We find that 2D A2BO3N oxynitrides have a reduced direct band gap at Γ-point, especially for three-layer (3L) Ba2NbO3N and 1L-Sr2NbO3N/1L-Ba2NbO3N bonded heterostructure with the optimized band gap ∼2.0 eV. Compared with tantalate and niobate perovskite oxides, the electron (hole) mobility increases 1–2 orders of magnitude up to 103–104 (102–103) cm2 V−1 s−1. A fast electron-hole vertical transport across the heterointerface and remarkable electron-hole separation can be realized in 1L-Sr2NbO3N/1L-Ba2NbO3N bonded heterostructure due to its strong interface Ba-O and Sr-O bonds and type-II band offset. Compared with the well-known photocatalysts, such as BiVO4 and MoS2/g-C3N4, an improved optical absorption (8 × 104 cm−1) in A2BO3N is obtained in the visible region. The 2D RP-type perovskite oxynitrides 3L-Ba2NbO3N and 1L-Sr2NbO3N/1L-Ba2NbO3N are powerful visible-light photocatalysts for overall water splitting.

Particulate Photocatalysts for Water Splitting: Recent Advances and Future Prospects

Water splitting via photocatalysis has the potential to produce clean, renewable hydrogen. Although various new photocatalysts have been reported, developing semiconductor materials that efficiently convert the energy in sunlight remains the primary challenge. This Perspective focuses on representative examples of particulate photocatalysts for overall water splitting. The design of both photocatalytic materials and overall systems intended for large-scale operation is discussed.