Visible Light Driven Water Splitting Research Articles

Band-structure tunability via modulation of planar buckling in ZnO monolayer: Manifestation in optoelectronic and photocatalytic properties

Two-dimensional ZnO materials, with their exceptional characteristics, hold great potential for novel nanoelectronic devices and catalysts. However, the strategy of modulating the electronic properties is still ongoing. To address this challenge, we conducted first-principles calculations to systematically investigate the structural stability, electronic properties and tunability of photocatalytic performance in ZnO monolayers with different planar buckling. The results indicate that ZnO monolayers with different buckling exhibit dynamical and thermal stability. The band edge positions of ZnO monolayers could be effectively controlled by planar buckling, and ZnO monolayers with suitable buckling can serve as potential candidates for visible-light driven water splitting for hydrogen generation. Furthermore, the narrowed band gaps in ZnO monolayers with increasing of planar buckling suggest their potential for use in the design of optoelectronic devices in nanoscale systems. In addition, the stability of buckled ZnO monolayers on Ag/Pt substrates and planar ZnO monolayer on semiconducting blue phosphorus (BlueP) are explored. Interestingly, Ag and Pt metal substrates have been found to be suitable for the growth of buckled ZnO monolayers. While BlueP monolayer is believed to serve as an excellent substrate for the growth of planar ZnO monolayers. These findings offer new strategy for designing planar or buckled ZnO monolayers with potential applications in futuristic nano-optoelectric and photocatalysis systems.

NH4Cl-assisted synthesis of TaON nanoparticle applied to photocatalytic hydrogen and oxygen evolution from water

Oxynitride semiconductors are promising photocatalyst materials for visible light-driven water splitting, while the synthesis of well crystalized oxynitride still remains challenge. In present work, narrow-bandgap TaON nanoparticles are synthesized via heating a vacuum-sealed mixture of KTaO3, Ta and NH4Cl. This method possesses multiple advantages in terms of lower calcination parameter, higher N conversion efficiency and superior photocatalytic activity in comparison with the traditional thermal ammonolysis using NH3 gas as a nitrogen source. Through the analysis of intermediates produced upon the elevation of heating temperature, a gas–solid-phase reaction between TaCl5 and Ta2O5 is demonstrated as the final step, which is conducive to decreasing thermal energy barrier and accelerating nitridation process. Precise control of preparation conditions, including calcination temperature and duration, allows for the regulation of surface O/N ratio of TaON particles to unity, resulting in optimized photocatalytic activity. Photoelectrochemical assessment and intensity modulated photocurrent spectroscopy provide convincing evidence for improved charge transfer efficiency of photoexcited holes at TaON surface. A Z-scheme overall water splitting is accomplished by employing the TaON as an effective oxygen evolution photocatalyst, SrTiO3:Rh as a hydrogen evolution photocatalyst, and reduced graphene oxide (rGO) as a solid-state electron mediator. This work presents a promising strategy for the synthesis of high-quality oxynitride materials in application to photocatalytic water splitting.

Photocatalytic Oxygen Evolution under Visible Light Mediated by Molecular Heterostructures.

Due to their structural and property tunability, semiconductive conjugated polymers (CPs) have emerged as promising candidates for photocatalytic water splitting. Compared with inorganic materials, the photocatalytic performance of mono-component polymers was limited by the fast recombination of photoexcited charge carriers, and they always needed to catch up to expectations. To this end, researchers established molecular donor-acceptor heterostructures, which could notably promote oxygen production efficiency due to their more effective charge carrier separation. In this work, easy Schiff base reactions between side-chain -CHO groups and terminal -NH2 groups were used to introduce benzene and perylene diimide (PDI) into the molecular heterostructure to serve as electron donors (D) and electron acceptors (A). In particular, for the first time, we employed the molecular heterostructures of CPs to promote photocatalytic O2 production. One prepared molecular heterostructure was demonstrated to improve oxygen generation rate (up to 0.53 mmol g-1 h-1) through visible light-driven water splitting. Interestingly, based on the photoelectric properties, a stepwise two-electron/two-electron pathway constituted the photocatalytic mechanism for oxygen production with the molecular heterostructure. These results provide insights into designing and fabricating high-performance molecular heterostructures for photocatalytic oxygen production.

Momentum-resolved electronic structure of LaTiO2N photocatalysts by resonant Soft-X-ray ARPES

Oxynitrides are promising materials for visible light-driven water splitting. However, limited information regarding their electron-momentum resolved electronic structure exists. Here, with the advantage of the enhanced probing depth and chemical state specificity of soft-X-ray ARPES, we determine the electronic structure of the photocatalyst oxynitride LaTiO2N and monitor its evolution as a consequence of the oxygen evolution reaction. After the photoelectrochemical reactions, we observe a partial loss of Ti- and La-N 2p states, distortions surrounding the local environment of titanium atoms and, unexpectedly, an indication of an electron accumulation layer at or near the surface, which may be connected with either a large density of metallic surface states or downward band bending. The distortions and defects associated with the titanium 3d states lead to the trapping of electrons and charge recombination, which is a major limitation for the oxynitride LaTiO2N. The presence of an accumulation layer and its evolution suggests complex mechanisms of the photoelectrochemical reaction, especially in cases where co-catalysts or passivation layers are used.

Interface Cd-N bond bridge accelerating charge separation to the enhanced visible light driven hydrogen production from water splitting on polyaniline@Cd-Zn-S photocatalyst

The key to the industrialization of eco-friendly hydrogen production from visible light driven water splitting is the development of photocatalyst with high performance and superior stability. Herein, a novel core-shell structure polyaniline@Cd-Zn-S (Cd/Zn = 0.8/0.2) nanorod photocatalyst with the shell thickness at ∼3 nm was constructed successfully. With polyaniline served as core, the construction of polyaniline@Cd-Zn-S reduced the charge transfer resistance and released reactive sites maximumly which could benefit the hydrogen production. The constructed polyaniline@Cd-Zn-S exhibited excellent photocatalytic hydrogen production efficiency and stability with the assistance of interface Cd-N bonds. The introduction of Cd-N bonds at the interface was inferred by both experimental and DFT simulated results. It served as the bridge between shell Cd-Zn-S and core polyaniline and promoted the directional transmission and efficient separation of photo generated charge carriers. Meanwhile, the improved directional transmission of photogenerated holes restricted photo corrosion effect of the Cd-Zn-S effectively and contributed to the photocatalytic stability. The hydrogen production without any cocatalyst increased more than 6 times to 721 μmol·h−1 after the construction of core-shell structure, and remain stable for 12 consecutive hours without photocatalytic performance reduction. This work proposed a new inspiring structural model for hetero junction construction of photocatalyst, and brought an interesting enlightenment to the development of eco-friendly hydrogen production technology.

Interfacial charge separation of nickel phosphide anchored on anatase-hematite heterojunction for stimulating visible light driven hydrogen generation

Anchoring of transition metal phosphides as cocatalysts onto the electron outlet of photocatalytically active systems demonstrates the unprecedented potential for increasing the rate of photocatalytic hydrogen generation. Herein, the role of NiP nanoparticles as cocatalysts for enhancing the charge transfer mechanism towards photocatalytic hydrogen generation is explored. A general chemical deposition strategy is adopted for the controlled surface anchoring of NiP cocatalyst onto the photocatalytic system, and the obtained NiP anchored photocatalyst is found to exhibit superior photocatalytic performance towards visible light driven water splitting. The enhanced photocatalytic activity is attributed to the synergistic interaction of NiP cocatalysts to facilitate strong electronic interactions at the heterojunctions, which are beneficial for the rapid transfer of photogenerated electrons to protons. The present approach may widen the horizon for design of many other noble-metal-free metal phosphides as highly efficient cocatalysts for photocatalytic hydrogen generation.

Two-dimensional ZnO/BlueP van der Waals heterostructure used for visible-light driven water splitting: A first-principles study

Solar driven water splitting for hydrogen generation has been considered as an important method for collecting clean energy. Herein, based on first-principles calculations, we propose that ZnO/BlueP van der Waals heterostructure can realize overall water splitting reaction for hydrogen generation. Strikingly, the band-gap of 1.83 eV is appropriate, and band alignments straddle the water redox potentials, ensuring the occurrence of hydrogenevolutionreaction and oxygen evolution reaction. Charge density distribution and carrier mobility exhibit significant charge separation and transfer. Visible-light response is improved compared with those of the isolated monolayers. Moreover, hydrogenevolutionreaction is actually realized on the ZnO layer, while oxygen evolution reaction is implemented on the BlueP layer. Through the investigation of the adsorption and dissociation reactions of H2O, we observe that two neighboring H*s prefer to combine to form H2 by overcoming a lowered energy barrier of 0.75 eV. Strain effect indicates that the lateral compressive strain of −4% to 0% and the vertical tensile strain of 0% to +6% can effectively tune band-gap and band alignments. The results indicate that ZnO/BlueP vdW heterostructure is probable highly efficient photoelectric material used for visible-light driven water splitting for hydrogen generation.

Size effect of bifunctional gold in hierarchical titanium oxide-gold-cadmium sulfide with slow photon effect for unprecedented visible-light hydrogen production

Gold nanoparticles (Au NPs) with surface plasmonic resonance (SPR) effect and excellent internal electron transfer ability have widely been combined with semiconductors for photocatalysis. However, the in-depth effects of Au NPs in multicomponent photocatalysts have not been completely understood. Herein, ternary titanium oxide-gold-cadmium sulfide (TiO2-Au-CdS, TAC) photocatalysts, based on hierarchical TiO2 inverse opal photonic crystal structure with different Au NPs sizes have been designed to reveal the SPR effect and internal electron transfer of Au NPs in the presence of slow photon effect. It appears that the SPR effect and internal electron transfer ability of Au NPs, depending on their sizes, play a synergistic effect on the photocatalytic enhancement. The ternary TAC-10 photocatalyst with ~ 10 nm Au NPs demonstrates an unprecedented hydrogen evolution rate of 47.6 mmolh-1g−1 under visible-light, demonstrating ~ 48% enhancement comparing to the sample without slow photon effect. In particular, a 9.83% apparent quantum yield under 450 nm monochromatic light is achieved for TAC-10. A model is proposed and finite-difference time-domain (FDTD) simulations reveal the size influence of Au NPs in ternary TAC photocatalysts. This work suggests that the rational design of bifunctional Au NPs coupling with slow photon effect could largely promote hydrogen production from visible-light driven water splitting.

Advances in engineering perovskite oxides for photochemical and photoelectrochemical water splitting

Solar-driven water splitting is an efficient process for converting solar energy into chemical energy. In this process, semiconductor materials are excited by solar energy to generate free electrons to participate in the water-splitting reaction. Among these semiconductor materials, inorganic perovskite oxides have a spatial structure that is easy to control and thereby lead to different energy band structures and photocatalytic properties. More importantly, perovskite oxides can be compounded with other organic/inorganic materials to promote charge separation and improve apparent quantum yield. However, the low solar-to-hydrogen conversion efficiency has not yet reached the requirements of practical applications. In this review, the fundamental principles of solar-driven water splitting based on perovskite materials are introduced according to the most recently published results. In addition, the innovative modification techniques for water splitting based on perovskite oxides have been summarized, focusing on the following methods: element doping, homo/heterojunction formation, Z-scheme, plasmon effect, dye sensitization, carbon enhancement, and surface modifications. Note that the applications in the visible light wavelength range have been described, with emphasis among all these modification materials. Furthermore, the recent water-splitting reaction systems for practical applications are briefly discussed. As a summary, we outline the challenges and potential utilization associated with visible light–driven water splitting based on perovskite oxides for future commercial applications. This review describes various modification methods to improve photochemical performance of perovskite oxides as well as illustrates the potential to employ perovskite oxides as a key material for the practical application of water splitting.

Constructing CdSe QDs modified porous g-C3N4 heterostructures for visible light photocatalytic hydrogen production

Constructing heterostructures with narrow-band-gap semiconductors is a promising strategy to extend light absorption range of graphitic carbon nitride (g-C3N4) and simultaneously promote charge separation for its photocatalytic activity improvement. However, its highly localized electronic states of g-C3N4 hinder photo-carrier migration through bulk towards heterostructure interfaces, resulting in low charge carrier separation efficiency of solid bulk g-C3N4-based heterostructures. Herein, porous g-C3N4 (PCN) material with greatly shortened migration distance of photo-carriers from bulk to surface was used as an effective substrate to host CdSe quantum dots to construct type II heterostructure of CdSe/PCN for photocatalytic hydrogen production. The homogeneous modification of the CdSe quantum dots throughout the whole bulk of PCN together with proper band alignments between CdSe and PCN enables the effective separation of photo-generated charge carriers in the heterostructure. Consequently, the CdSe/PCN heterostructure photocatalyst gives the greatly enhanced photocatalytic hydrogen production activity of 192.3 μmol h−1, which is 4.4 and 8.1 times that of CdSe and PCN, respectively. This work provides a feasible strategy to construct carbon nitride-based heterostructure photocatalysts for boosting visible light driven water splitting performance.

Tunable cobalt doping titanium nitride (TixCoyN) interlaced nanotubes enable an enhanced electronic synergy on visible-light driven hydrogen evolution

Overcoming the limited light harvest and sluggish charge transfer efficiency has remained a long-standing challenge for the practical application of visible-light driven hydrogen evolution. In this work, a novel cobalt doped titanium nitride interlaced nanotube was synthesized through a facile hydrothermal approach and subsequent ammonia gas nitrification route. The optimized Ti0·75Co0·25N (molar ratio of Ti:Co:N = 0.75:0.25:1) catalyst delivered the highest hydrogen yield in visible-light-induced water splitting, which was almost 2 times higher than the pristine TiN did. Further structure characterizations have unraveled that the cobalt atoms were partially substituted the titanium atoms in crystallite lattice, generating a significant synergistic regulation on band structure and electronic state. As a result, the narrowed energy bandgap, improved charge carrier mobility and lowered over-potential contributed to the high electron-hole separation efficiency and smoothed interface charge transfer, yielding an enhanced photocatalytic activity. This work is a significant contribution to the design of efficient and stable visible light driven water splitting catalysts.

Completely Solar-Driven Photoelectrochemical Water Splitting Using a Neat Polythiophene Film

Summary π-Conjugated polymers are emerging as appealing photoelectrode materials for the photoelectrochemical hydrogen evolution reaction via water splitting, which has otherwise been extensively explored using inorganic semiconductors. Here, we report the performance of a pure organic semiconductor film as a catalyst for hydrogen production via visible light-driven water splitting. The neat and unsubstituted polythiophene film, characterized with a well-filled grain morphology of the crystalline polymer, is prepared by a facile polymerization method. The photovoltage of 1.38 V versus RHE at pH 12 enables solar-driven one-electron-per-photon water splitting in combination with a traditional water-oxidation catalyst to produce hydrogen and oxygen separately. The high photoelectrocatalytic hydrogen evolution rate of 1.02 mol(H2) h−1 g−1 or 0.75 mA/cm2 at 0 V versus RHE is also achieved with high durability. This study may open a new window for π-conjugated polymers for ultimately sustainable hydrogen production.

Enhanced visible light absorption of TiO2 nanorod photoanode by NiTiO3 decoration for high-performance photoelectrochemical cells

TiO2 nanorod (NR) and NiTiO3 (NTO) enhanced TiO2 NR photoanodes were studied for visible light driven water splitting in photoelectrochemical cells (PECs). The TiO2 and NTO/TiO2 growth on the FTO substrates were synthesised via the hydrothermal method. The Ni2+ content was the control factor to fabricate various different NTO/TiO2 heterostructures. The structure and surface morphology of NTO formation on TiO2 surfaces were investigated via SEM and TEM. Chemical absorption species and chemical bonding at interfaces were investigated by EDS and XPS spectroscopy. The optical absorption of the materials was studied by UV–visible and PL spectroscopy. Details on the influencing parameters on the electrochemical and photoelectrochemical properties were discussed. In this study, the 0.25NTO/TiO2 photoanode exhibited the best performance. This was because the electron lifetime, charge transfer resistance, donor density, and band gap improved.

Recent Advances of Epitaxial BiVO4 Thin Film: Preparation and Physical and Photoelectrochemical Properties

Semiconductor photocatalysis has two important applications: environmental remediation and H2 production. Bismuth vanadate has attracted great interests as an ideal candidate for use as high-performance photocatalysts for visible light-driven water splitting and photoanode in photoelectrochemical cells. Single crystal film is invaluable to deepen the fundamental understanding of materials. The object of this article was to provide a brief review on the advances in the preparation of epitaxial BiVO4 thin film, the physical and photoelectrochemical properties. The epitaxial BiVO4 thin film could be fabricated on (001) yttria-stabilized cubic zirconia or SrTiO3(001) by molecular beam epitaxy, chemical vapor deposition, and, pulsed laser deposition. Three crystal structures, namely, monoclinic scheelite, monoclinic clinobisvanite, and orthorhombic, were mentioned in the literatures. The optical band gap was reported to be 2.5~2.7 eV for a direct transition. The highest photocurrent of 4.83 mA cm−2at 1.23 V vs. the RHE was obtained at the sample with Lu2O3 as a hole blocking layer.

Predicting the Most Suitable Surface Candidates of Ta3N5 Photocatalysts for Water-Splitting Reactions Using Screened Coulomb Hybrid DFT Computations

Ta3N5 is one of the mostly used photocatalysts for visible light-driven water splitting. Using accurate first-principles calculations based on the density functional theory (DFT) with the screened ...

Hydrothermal synthesis of Co2P-modified MoS2: a highly efficient non-precious metal catalyst of light

For visible light-driven water splitting, an effective non-noble metal semiconductor photocatalyst is highly desirable. Co2P was synthesized from red phosphorus and cobalt nitrate to increase the photocatalytic activity of MoS2. Co2P was loaded onto the surface of MoS2, and the photo-catalytic activity (9.15 mmol g−1 h−1) was three times that of MoS2 (3.14 mmol g−1 h−1), which was twice times that of Co2P (4.60 mmol g−1 h−1). Characterization by fluorescence and electrochemistry indicates that the composite photocatalyst improves the efficiency of photo-generated charge separation. Composite catalyst described by the BET specific surface area greater can provide more active sites. Therefore, after loading Co2P, the photocatalytic hydrogen production efficiency of MoS2 can be effectively improved.

Cover Feature: SnS2 Nanosheets/H‐TiO2 Nanotube Arrays as a Type II Heterojunctioned Photoanode for Photoelectrochemical Water Splitting (ChemSusChem 5/2019)

The Cover Feature shows a SnS2 nanosheet/hydrogenated-TiO2-Nanotube Array Photoanode for visible light-driven water splitting. Through hydrogen treatment, defects are created in TiO2 nanotubes, which converts a type I junction to type II in the presence of SnS2. More information can be found in the Communication by Lin, Liu et al. on page 961 in Issue 5, 2019 (DOI: 10.1002/cssc.201802691).

Band Gap Modulation of Tantalum(V) Perovskite Semiconductors by Anion Control

Band gap magnitudes and valence band energies of Ta5+ containing simple perovskites (BaTaO2N, SrTaO2N, CaTaO2N, KTaO3, NaTaO3, and TaO2F) were studied by diffuse reflection absorbance measurements, density-functional theoretical calculations, and X-ray photoelectron spectroscopy. As a universal trend, the oxynitrides have wider valence bands and narrower band gaps than isostructural oxides, owing to the N 2p contribution to the electronic structure. Visible light-driven water splitting was achieved by using Pt-loaded CaTaO2N, together with a sacrificial agent CH3OH.

Synthesis of novel Mn-doped Fe2O3 nanocube supported g-C3N4 photocatalyst for overall visible-light driven water splitting

A novel Mn-doped Fe2O3 modified g-C3N4 nanosheets composite has been prepared for overall water splitting. Structure characterizations reveal that the Mn are successfully doped in the Fe2O3 nanoparticle and a close interface between g-C3N4 and Mn-doped Fe2O3is obtained. The optimized FMC-10 photocatalyst has a H2 evolution of 51 μmol h−1. The improved photocatalytic performance can be ascribed to the synergy effect of both Mn-doped Fe2O3and g-C3N4. Doping Fe2O3 with Mn promotes photo-induced charges and increases the charge transfer for the improved conductivity of the bulk Fe2O3, while favorable contacting with g-C3N4 enhances the charge separation ability.

Vapor growth of binary and ternary phosphorus-based semiconductors into TiO2 nanotube arrays and application in visible light driven water splitting.

We report successful synthesis of low band gap inorganic polyphosphide and TiO2 heterostructures with the aid of short-way transport reactions. Binary and ternary polyphosphides (NaP7, SnIP, and (CuI)3P12) were successfully reacted and deposited into electrochemically fabricated TiO2 nanotubes. Employing vapor phase reaction deposition, the cavities of 100 μm long TiO2 nanotubes were infiltrated; approximately 50% of the nanotube arrays were estimated to be infiltrated in the case of NaP7. Intensive characterization of the hybrid materials with techniques including SEM, FIB, HR-TEM, Raman spectroscopy, XRD, and XPS proved the successful vapor phase deposition and synthesis of the substances on and inside the nanotubes. The polyphosphide@TiO2 hybrids exhibited superior water splitting performance compared to pristine materials and were found to be more active at higher wavelengths. SnIP@TiO2 emerged to be the most active among the polyphosphide@TiO2 materials. The improved photocatalytic performance might be due to Fermi level re-alignment and a lower charge transfer resistance which facilitated better charge separation from inorganic phosphides to TiO2.