Materials For Photocatalytic Water Splitting Research Articles

Strain engineering for C2N/Janus monochalcogenides van der Waals heterostructures: Potential applications for photocatalytic water splitting

A type-II van der Waals (vdW) heterostructure that can inhibit carrier recombination and improve the photocatalytic efficiency of water splitting has received much attention for photocatalytic water splitting. In this work, two-dimensional Janus monochalcogenides, In2SSe(In2STe, In2SeTe) and C2N, are designed to type-II vdW heterostructures for photocatalytic water splitting. First-principles calculation results show that C2N/In2SSe and C2N/In2STe heterostructures have a type-I band alignment, and that C2N/In2SeTe have a unique type-II band alignment. Under biaxial strain, both C2N/In2SSe and C2N/In2STe heterostructures can have a type-I to type-II band alignment and an indirect-direct band-gap transition. Systematically electronic structure and band alignment results show that C2N/In2SSe and C2N/In2SeTe vdW heterostructures are more suitable for photocatalytic water splitting under strain than C2N/In2STe heterostructure. In addition, the optical absorption of C2N/In2SSe and C2N/In2SeTe vdW heterostructures can be significantly improved under biaxial strain. Detailed strain engineering results show that C2N is more sensitive to strain than Janus monochalcogenides due to the planar properties of sp2 hybrization of C2N, which dominate the band gap and band transition in heterostructures under strain. These results indicate that C2N/In2SSe and C2N/In2SeTe vdW heterostructures can be used as photocatalytic water splitting materials, providing a motivation for further research on Janus heterostructures.

Crystal phase dependent solar driven hydrogen evolution catalysis over cobalt diselenide

The design of robust metallic transition metal dichalcogenides (TMDs)-based materials for photocatalytic water splitting is desirable but challenging. Here we report the exploration and crystal phase engineering of metallic CoSe2 as cocatalysts for photocatalytic hydrogen evolution. The potential of orthorhombic and cubic CoSe2 (i.e. o-CoSe2, c-CoSe2) as cocatalysts are evaluated by theoretical simulation. Experimentally, o-CoSe2 and c-CoSe2 are rationally anchored to ultrathin g-C3N4 nanosheets, which provides a platform to evaluate the practical photocatalytic activity. The experimental results reveal the crystal phase dependent hydrogen evolution catalysis, i.e., the hydrogen evolution can be proceeded more efficiently on o-CoSe2 surface compared to c-CoSe2. We demonstrate that o-CoSe2 possesses more favorable hydrogen adsorption capacity and can thus tune the rate-determining step, i.e., the surface conversion from H+ to H2, leading to enhanced performance. The presented results provide insights into the role of phase engineering and a strategy for enhanced catalysis.

Recent Achievements in Development of TiO2-Based Composite Photocatalytic Materials for Solar Driven Water Purification and Water Splitting.

Clean water and the increased use of renewable energy are considered to be two of the main goals in the effort to achieve a sustainable living environment. The fulfillment of these goals may include the use of solar-driven photocatalytic processes that are found to be quite effective in water purification, as well as hydrogen generation. H2 production by water splitting and photocatalytic degradation of organic pollutants in water both rely on the formation of electron/hole (e−/h+) pairs at a semiconducting material upon its excitation by light with sufficient photon energy. Most of the photocatalytic studies involve the use of TiO2 and well-suited model compounds, either as sacrificial agents or pollutants. However, the wider application of this technology requires the harvesting of a broader spectrum of solar irradiation and the suppression of the recombination of photogenerated charge carriers. These limitations can be overcome by the use of different strategies, among which the focus is put on the creation of heterojunctions with another narrow bandgap semiconductor, which can provide high response in the visible light region. In this review paper, we report the most recent advances in the application of TiO2 based heterojunction (semiconductor-semiconductor) composites for photocatalytic water treatment and water splitting. This review article is subdivided into two major parts, namely Photocatalytic water treatment and Photocatalytic water splitting, to give a thorough examination of all achieved progress. The first part provides an overview on photocatalytic degradation mechanism principles, followed by the most recent applications for photocatalytic degradation and mineralization of contaminants of emerging concern (CEC), such as pharmaceuticals and pesticides with a critical insight into removal mechanism, while the second part focuses on fabrication of TiO2-based heterojunctions with carbon-based materials, transition metal oxides, transition metal chalcogenides, and multiple composites that were made of three or more semiconductor materials for photocatalytic water splitting.

Photoinduced Carrier Dynamics at the Interface of Black Phosphorus and Bismuth Vanadate.

Two-dimensional (2D) heterostructures of black phosphorus (BP)/bismuth vanadate (BVO) have attracted much attention due to their potential uses in photocatalytic water splitting. However, the interfacial photoinduced electron- and hole-transfer dynamics are not explored computationally. Herein, we have used density functional theory (DFT) calculations and DFT-based fewest-switches surface-hopping dynamics simulations to investigate the light-driven electron and hole dynamics taking place at the interface of BP and the BVO(010) surface. Our results show that the BP monolayer is adsorbed on BVO(010) via van der Waals interaction. Upon irradiation, the electron transfer takes place from BP to BVO(010) within 500 fs but with two distinct processes. In the first phase, the electron transfer proceeds adiabatically and is mainly driven by atomic motions. In the second phase, the electron transfer decays very slowly. The hole-transfer dynamics from BVO(010) to BP exhibits a similar ultrafast decay in the first stage followed by a slow decay; however, there is a comparable amount of hole trapped in a BP state due to a large energy gap from its higher state. These insights may be useful for the design of novel photocatalytic water-splitting materials.

Asymmetric embedded benzene ring enhances charge transfer of carbon nitride for photocatalytic hydrogen generation

Preventing the high carrier recombination rate of graphitized C3N4 (GCN) is an urgent problem to be solved for its application as a photocatalyst for hydrogen production. Here, we first rationally embed the benzene ring in GCN to modify the local symmetry without changing its long-order structure. Theoretical calculation predicts that this design can change the electronic structure and promote the effective charge transfer in GCN. The benzene ring embedded GCN is successfully prepared by copolymerization using dicyandiamide and terephthalonitrile as precursors. Photoluminescence (PL) and time-resolved transient PL (TRPL) spectra confirm that the electron transfer efficiency of the benzene ring embed GCN is greatly improved. This nanomaterial displays 10.8 times higher photocatalytic hydrogen production rate than that of pristine GCN with the apparent quantum yield of 11.3% at 400 nm and 9% at 420 nm. This work provides a novel strategy for designing high-efficiency two-dimensional (2D) photocatalytic materials for water splitting.

Fabrication novel WO3@Cu core-shell nanoparticles for high-efficiency hydrogen generation under visible light by photocatalytic water splitting

In this study, WO3@Cu core-shell nanoparticles have been successfully synthesized by sol–gel route using sodium dodecyl benzene sulfonate(SDBS) as dispersing agent. The fabricated WO3@Cu has been characterized by X-ray powder diffraction, energy dispersive X-ray spectrometer, transmission electron microscope, and scanning electron microscope. The experimental results showed that the hydrogen production rate of WO3@Cu is up to 37.78 μmol/(h·g) under visible light irradiation. There was no considerable decline of the hydrogen production rate after continuous reaction for 120 h, indicating that the obtained catalysts exhibited excellent stability. Furthermore, the possible mechanism of photocatalytic water splitting on WO3@Cu has been proposed. Thus, WO3@Cu can be considered as an ideal material for photocatalytic water splitting in the future.

First-principles investigation on hydrogen evolution reaction in KNbO3 (100)/g-C3N4 heterojunction

Constructing the two-dimensional (2D) heterojunction photocatalysts is a highly efficient strategy for utilizing solar energy and speeding up the separation rate of photogenerated charge carriers. In this contribution, we systematically investigate the electronic and photocatalytic performances of the 2D KNbO3/g-C3N4 heterojunction using first-principles calculations, along with the origin of the photocatalytic performance. Our results indicate that the KNbO3/g-C3N4 heterojunction is a very promising photocatalytic material for water splitting, particularly for the hydrogen evolution reaction, well consistent with the reported experimental results. The KNbO3/g-C3N4 heterojunction owns a type-Ⅱ band alignment. Moreover, the density charge difference (DCD) and Mulliken population analysis show that the g-C3N4 monolayer can absorb visible light and effectively improve the optical properties of KNbO3. After g-C3N4 combined with KNbO3 (100) surface, the band gap has been significantly reduced and a red shift occurs in the visible light region. Additionally, the application of an external electric field and biaxial strain have been evident as efficient methods to obtain modulated band gaps for the heterojunctions. Interestingly, a linear evolution trend of the band gap is presented for the heterojunctions under the compression and tensile strain. Heterojunctions have a tendency to convert from semiconductors to conductors. Overall, these findings provide theoretical support for the research about KNbO3/g-C3N4 heterojunctions and pave a way of enhancing the photocatalytic performance under visible light irradiation.

(Keynote) Stable Mixed-Anion Photocatalysts for Visible-Light-Induced Water Splitting

Photo-induced water splitting using semiconductor photocatalysts has attracted considerable attention for producing H2 as a clean energy carrier, while the effective utilization of visible light is imperative to achieve the desired efficiency for practical applications.[1, 2] Recently, mixed-anion compounds such as oxynitrides have been intensively studied as promising candidates since one can expect that higher energy p orbitals of non-oxide anions (e.g., N-2p) elevate their valence band maximum (VBM) values. Unfortunately, most of them are subject to facile self-oxidation by photogenerated holes, while highly dispersed cocatalyst particles certainly improve the stability of some oxynitrides.[3] We have recently demonstrated that Sillén–Aurivillius type perovskite oxyhalides such as Bi4NbO8Cl can stably and efficiently oxidize water to O2 under visible light without any surface modifications, and also exhibits a stable Z-scheme water splitting when coupled with a H2-evolving photocatalyst.[4] It was revealed that the VBMs of these materials consist mainly of O-2p orbitals, instead of Cl-3p (or Br-4p), but their positions are much more negative than those of conventional oxides. [5, 6] Thus, they possess narrow bandgaps for visible light absorption as well as sufficiently negative CBMs for water reduction. DFT calculation visualized a fairly strong hybridization between the Bi-6s and O-2p orbitals, which can explain why the O-2p orbitals are elevated in energy, combined with the result on Madelung site potential analysis that can rationalize the origin of high energy of O-2p orbital in these materials. Since O– anions are known to be relatively stable, photogenerated holes populated at the O-2p orbitals will not lead to self-decomposition but to oxidize water. These results could provide new strategies for developing durable photocatalytic materials for water splitting under visible light, by manipulating the interaction between post-transition metal s orbitals and O-2p orbitals. [1] R. Abe, J. Photochem. Photobiol. C: Photochemistry Reviews, 11 (2011) 179. [2] R. Abe, J. Tang et al., Chem. Rev. 118 (2018) 5201. [3] R. Abe, M. Higashi, K. Domen, J. Am. Chem. Soc., 132, (2010) 11828. [4] H. Fujito, H. Kunioku, H. Kageyama, R. Abe et al., J. Am. Chem. Soc., 38 (2016) 2082. [5] D. Kato, H. Kunioku, R. Abe, H. Kageyama et al. J. Am. Chem. Soc., 139 (2017) 18725. [6] H. Kunioku, H. Kageyama, R. Abe et al., J. Mater. Chem. A, 6 (2018) 3100. Key Words: Photocatalyst, Semiconductor, Water splitting, Solar energy

Polymeric Donor-Acceptor Heterostructures for Enhanced Photocatalytic H2 Evolution without Using Pt Cocatalysts.

Polymeric carbon nitride (CN) is a promising material for photocatalytic water splitting. However, CN in its pristine form tends to show moderate activity due to fast recombination of the charge carriers. The design of efficient photocatalytic system is therefore highly desired, but it still remains a great challenge in chemistry. In this work, a pyrene-based polymer able to serve as an electron donor to accelerate the interface charge carrier transfer of CN is presented. The construction of donor-acceptor (D-A) heterojunction was confirmed to significantly restrain the charge recombination and, thus, improve the proton reduction process. This study provides a promising strategy to achieve solar H2 production in an efficient and low-cost manner.

Band-gap engineering in AB(OxS1−x)3 perovskite oxysulfides: a route to strongly polar materials for photocatalytic water splitting

DFT calculations predict BaZrxTi1−xO2S to combine ferroelectricity and small band gaps making them promising materials for photocatalytic water splitting.

Synthesis of a semi-conductor-like MOF with black phosphorous as a composite for visible light-driven photocatalysis.

The fabrication of nanocomposites of a metal–organic framework with two-dimensional materials for photocatalytic water splitting and environmental remediation has been the focus of the scientific community for many years now. In the work reported here, MIL-125 (Ti) was functionalized using few-layer black phosphorus (FLBP) via a two-step hydrothermal and sonication approach. The as-synthesized composite materials were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) and photoluminescence (PL) spectroscopy. The techniques revealed that the nanocomposites exhibited circular blocks and a sheet-like morphology; they also showed enhanced visible light absorption and thus could be applied in visible-light-driven photocatalytic water treatment or water splitting. Electrochemical impedance spectroscopy (EIS) showed that the composite referred to as 4%BpMIL displayed the lowest recombination of all the composites (MIL-125(Ti) > 12%BpMIL> 6%BpMIL > 4%BpMIL > FLBP) with high effectual charge transfer between FLBP and MIL-125(Ti) via a p–n junction.

Nanorod Array of SnO2 Quantum Dot Interspersed Multiphase TiO2 Heterojunctions with Highly Photocatalytic Water Splitting and Self-Rechargeable Battery-Like Applications.

The ever-growing demand for sustainable and renewable power sources has led to the development of novel materials for photocatalytic water splitting, but enhancing the photocatalytic efficiency remains a core problem. Herein, we report a conceptual effective and experimental confirmed strategy for SnO2 quantum dot (QD) interspersed multiphase (rutile, anatase) TiO2 nanorod arrays (SnO2/RA@TiO2 NRs) to immensely enhance the carrier separation for highly efficient water splitting by merging simultaneously the QD, multiphase, and heterojunction approaches. Under this synergistic effect, a doping ratio of 25% SnO2 QD interspersed into multiphase TiO2 NRs exhibited a superior optical adsorption and excellent photocurrent density (2.45 mA/cm2 at 1.0 V), giving rise to a largely enhanced incident light to current efficiency in the UV region (45-50%). More importantly, this material-based device can act as power supply with a voltage of ∼2.8 V after illumination, which can automatically self-recharge by reacting with oxygen vacancy and water molecule to realize reuse. The current study provides a new paradigm about heightening the carrier separation extent of QD interspersed multiphase heterojunctions, fabricating a new solar-energy-converting material/device, and achieving a highly photocatalytic water splitting/self-charging battery-like application.

CuO photoelectrodes synthesized by the sol–gel method for water splitting

CuO is an attractive photocatalytic material for water splitting due to its high earth abundance and low cost. In this paper, we report the deposition of CuO thin films by sol–gel dip-coating process. Sol deposition has attractive advantages including low-cost solution processing and uniform film formation over large areas with a fairly good control of the film stoichiometry and thickness. Pure CuO phase was obtained for calcination temperatures higher than 360 °C in air. The CuO photocurrents for hydrogen evolution depend on the crystallinity and the microstructure of the film. Values of −0.94 mA cm−2 at pH = 8 and 0 V vs. RHE were achieved for CuO photoelectrodes annealed at 400 °C under air. More interestingly, the stability of the photoelectrode was enhanced upon the sol–gel deposition of a TiO2 protective layer. In this all sol–gel CuO/TiO2 photocathode, a photocurrent of −0.5 mA cm−2 is achieved at pH = 7 and 0 V vs. RHE with a stability of ~100% over 600 s.

Recent Progress on Black Phosphorus‐Based Materials for Photocatalytic Water Splitting

AbstractEmploying photocatalytic technology for efficient water splitting to hydrogen evolution is a sustainable and renewable solution for energy and environmental issues. Large‐scale utilization of solar hydrogen requires the appropriate usage of photocatalysts with high efficiency and their easy integration into solar devices. Recently, 2D materials have been widely studied because of their potential utility in device fabrication. Black phosphorus (BP) is a newly developed 2D direct bandgap semiconductor with an impressive tunable bandgap and fast carrier migration characteristics. In this review, the optical and electronic characteristics of 2D phosphorene‐based materials are mainly discussed in terms of water splitting reaction. Density functional theory calculations are first summarized, followed by brief discussions on the direct application of BP in photocatalytic water splitting based on recent reports. This review might assist researchers to design new materials for photocatalytic water splitting.

Surface properties of lead-free halide double perovskites: Possible visible-light photo-catalysts for water splitting

Halide double perovskites based on combinations of monovalent and trivalent cations have been proposed as promising lead-free alternatives to lead halide perovskites. Among the newly synthesized compounds Cs2BiAgCl6, Cs2BiAgBr6, Cs2SbAgCl6, and Cs2InAgCl6, some exhibit bandgaps in the visible range and all have low carrier effective masses; therefore, these materials constitute potential candidates for various opto-electronic applications. Here, we use first-principles calculations to investigate the electronic properties of the surfaces of these four compounds and determine, for the first time, their ionization potential and electron affinity. We find that the double perovskites Cs2BiAgCl6 and Cs2BiAgBr6 are potentially promising materials for photo-catalytic water splitting, while Cs2InAgCl6 and Cs2SbAgCl6 would require controlling their surface termination to obtain energy levels appropriate for water splitting. The energy of the halogen p orbitals is found to control the conduction band level; therefore, we propose that mixed halides could be used to fine-tune the electronic affinity.

Anion Order and Spontaneous Polarization in LaTiO_{2}N Oxynitride Thin Films.

The perovskite oxynitride LaTiO_{2}N is a promising material for photocatalytic water splitting under visible light. One of the obstacles towards higher efficiencies of this and similar materials stems from charge-carrier recombination, which could be suppressed by the surface charges resulting from the dipolar field in polar materials. In this study, we investigate the spontaneous polarization in epitaxially strained LaTiO_{2}N thin films via density functional theory calculations. The effect of epitaxial strain on the anion order, resulting out-of-plane polarization, energy barriers for polarization reversal, and corresponding coercive fields are studied. We find that for compressive strains larger than 4% the thermodynamically stable anion order is polar along the out-of-plane direction and has a coercive field comparable to other switchable ferroelectrics. Our results show that strained LaTiO_{2}N could indeed suppress carrier recombination and lead to enhanced photocatalytic activities.

Bombyx mori silk/titania/gold hybrid materials for photocatalytic water splitting: combining renewable raw materials with clean fuels.

The synthesis, structure, and photocatalytic water splitting performance of two new titania (TiO2)/gold(Au)/Bombyx mori silk hybrid materials are reported. All materials are monoliths with diameters of up to ca. 4.5 cm. The materials are macroscopically homogeneous and porous with surface areas between 170 and 210 m2/g. The diameter of the TiO2 nanoparticles (NPs) – mainly anatase with a minor fraction of brookite – and the Au NPs are on the order of 5 and 7–18 nm, respectively. Addition of poly(ethylene oxide) to the reaction mixture enables pore size tuning, thus providing access to different materials with different photocatalytic activities. Water splitting experiments using a sunlight simulator and a Xe lamp show that the new hybrid materials are effective water splitting catalysts and produce up to 30 mmol of hydrogen per 24 h. Overall the article demonstrates that the combination of a renewable and robust scaffold such as B. mori silk with a photoactive material provides a promising approach to new monolithic photocatalysts that can easily be recycled and show great potential for application in lightweight devices for green fuel production.

Novel SrZrO3-Sb2O3 heterostructure with enhanced photocatalytic activity: Band engineering and charge transference mechanism

The development of efficient and stable photocatalytic materials for water splitting and degradation of organic compounds is one of the most important goals of the scientists in the area of research for energy and environmental applications. Herein, we designed through band engineering a heterostructure consisting of SrZrO3 particles dispersed on the surface of Sb2O3, as an efficient and stable catalyst for photocatalytic water splitting and the degradation of the pharmaceutical compound tetracycline under UV and UV–vis light. SrZrO3–Sb2O3 heterostructure exhibited an enhanced electron-hole separation, transference and utilization, resulting in an improved photocatalytic activity. Recyclability tests and evaluation of the effect of the irradiation source are presented. The heterostructure showed an H2 evolution rate of 330 μmol g−1 h−1 under UV light, which is more than 8 times the activity of pure SrZrO3. Under same irradiation conditions (same lamp), the heterostructure reached a 70% of degradation of tetracycline, increasing 30% the activity of SrZrO3. These results indicate that SrZrO3 is efficiently dispersed on the surface of Sb2O3 and confirm the appropriate band alignment between the semiconductors in the heterostructure, allowing an efficient transference of charges. According to this, the increased photocatalytic activity of the heterostructure SrZrO3–Sb2O3 was adjudicated to the effective charge separation and transference in the interface of the heterostructure. A mechanism of the process is proposed and discussed by photoluminescence and electrochemical measurements.

Vacancy-doped homojunction structural TiO2 nanorod photoelectrodes with greatly enhanced photoelectrochemical activity

In this study, we report a simple, effective and universal approach to fabricate homojunction structural TiO2 nanorod array photoelectrodes through doping of gradient distributed oxygen vacancies introduced by annealing in high vacuum with controlled time. The homogeneous junction structure promotes the separation and transport of photoexcited charge carriers, and the photocurrent density of TiO2 nanorod arrays annealed in high vacuum for 2 h was 20 times higher than that of the pristine TiO2 nanorod arrays. The results indicate that annealing in high vacuum with controlled time could form a homojunction structure, which is an effective approach for further enhancing the performance of UV–visible light driven photocatalytic materials for water splitting.

C60/graphene/g-C3N4 composite photocatalyst and mutually- reinforcing synergy to improve hydrogen production in splitting water under visible light radiation

g-C3N4 as a new metal-free photocatalytic material for water splitting has attracted much attention in recent years, but its photocatalytic efficiency needs further improvement. Here we synthesized novel C60/graphene/g-C3N4 composite photocatalytic materials with high hydrogen generation ability for water splitting under visible light radiation (λ>420nm). These materials take full advantage of the electron conduction expressing of graphene and the superior-strong electron-attracting ability of C60. The mutually-reinforcing synergy between graphene and C60 improves the migration and utilization efficiency of photo-generated electrons and accelerates the separation of photo-generated charges, thus significantly enhancing the hydrogen generation capacity of g-C3N4. The hydrogen production amount and rate of C60/graphene/g-C3N4 (10mg/L C60 and graphene) after 10h are 5449.5µmol/g and 545µmol/g/h, which is 539.6 times of pure g-C3N4 under the same condition. The values are 50.8 and 4.24 times of graphene/g-C3N4 (10mg/L graphene) and C60/g-C3N4 (10mg/L C60), respectively. The apparent quantum yield of C60/graphene/g-C3N4 (10mg/L C60 and graphene) in 97h is about 7.2%. The improvement of hydrogen generation activity in 97h suggests the high long-time stability of C60/graphene/g-C3N4 in photocatalytic water spitting. The photocatalytic ability of C60/graphene/g-C3N4 can be controlled by regulating the addition of graphene and C60. The mutually-reinforcing synergy between graphene and C60 was proved by X-ray photoelectron spectroscopy, photoluminescence spectrum and organic electron acceptors of MV2+. Thus, the joint action of C60 and graphene promotes the migration, separation and utilization of photo-generated electrons, which is responsible for the significant enhancement of photocatalytic performance.