Donor‐Acceptor Fulvalene‐Analogous Small Molecules for Efficient Photocatalytic Hydrogen Evolution

Evolutionary mechanism of Ni-ZIF/CdS calcination for efficient photocatalytic hydrogen evolution

Construction of Z-scheme heterostructure is of great significance for realizing efficient photocatalytic water splitting. However, the conscious modulation of Z-scheme charge transfer is still a great challenge. Herein, interfacial Mo-S bond and internal electric field modulated Z-scheme heterostructure composed by sulfur vacancies-rich ZnIn2S4 and MoSe2 was rationally fabricated for efficient photocatalytic hydrogen evolution. Systematic investigations reveal that Mo-S bond and internal electric field induce the Z-scheme charge transfer mechanism as confirmed by the surface photovoltage spectra, DMPO spin-trapping electron paramagnetic resonance spectra and density functional theory calculations. Under the intense synergy among the Mo-S bond, internal electric field and S-vacancies, the optimized photocatalyst exhibits high hydrogen evolution rate of 63.21 mmol∙g−1·h−1 with an apparent quantum yield of 76.48% at 420 nm monochromatic light, which is about 18.8-fold of the pristine ZIS. This work affords a useful inspiration on consciously modulating Z-scheme charge transfer by atomic-level interface control and internal electric field to signally promote the photocatalytic performance.

A binary nanocomposite composed of two-dimensional (2D) ultrathin ZnIn2S4 nanosheets and one-dimension (1D) TiO2 nanobelts was prepared and applied as a noble-metal-free photocatalyst for hydrogen evolution under solar-light irradiation. The TiO2 nanobelt/ZnIn2S4 nanosheet heterojunction nanocomposites show higher light absorption capacity, larger surface area and higher separation of charge carriers in comparison to pristine TiO2 and ZnIn2S4. As a result, the hydrogen production over the TiO2/ZnIn2S4 nanocomposite with 15 wt% TiO2 can reach up to 348.21 μmol · g−1 · h−1, even without noble metals, which is about 26 and 2.3 times higher than the pristine TiO2 and ZnIn2S4, respectively. Meanwhile, a possible photocatalytic mechanism of TiO2/ZnIn2S4 heterojunction nanocomposites was proposed and corroborated by photoluminescence (PL) spectroscopy and photoelectrochemical (PEC) results. This work paves a way for developing low-cost and high-efficiency noble-metal-free photocatalytic systems for solar-to-hydrogen evolution.

S-scheme heterojunction of polyfluorene derivatives coupled with ZnxCd1-xS nanoparticles for efficient and stable photocatalytic hydrogen evolution

The current bottleneck in the development of efficient photocatalysts for hydrogen evolution is the limited availability of high‐performance acceptor units. Over the past nine years, dibenzo[b,d]thiophene sulfone (DBS) has been the preferred choice for the acceptor unit. Despite extensive exploration of alternative structures as potential replacements for DBS, a superior substitute remains elusive. In this study, a symmetry‐breaking strategy was employed on DBS to develop a novel acceptor unit, BBTT‐1SO. The asymmetric structure of BBTT‐1SO proved beneficial for increasing multiple moment and polarizability. BBTT‐1SO‐containing polymers showed higher efficiencies for hydrogen evolution than their DBS‐containing counterparts by up to 166 %. PBBTT‐1SO exhibited an excellent hydrogen evolution rate (HER) of 222.03 mmol g−1 h−1 and an apparent quantum yield of 27.5 % at 500 nm. Transient spectroscopic studies indicated that the BBTT‐1SO‐based polymers facilitated electron polaron formation, which explains their superior HERs. PBBTT‐1SO also showed 14 % higher HER in natural seawater splitting than that in deionized water splitting. Molecular dynamics simulations highlighted the enhanced water‐PBBTT‐1SO polymer interactions in salt‐containing solutions. This study presents a pioneering example of a substitute acceptor unit for DBS in the construction of high‐performance photocatalysts for hydrogen evolution.

Hollow tubular Co9S8 grown on In2O3 to form S-scheme heterojunction for efficient and stable hydrogen evolution

S-scheme heterojunction of hollow corncob-like ZnIn2S4/LaFeO3 for water splitting and tetracycline degradation

Water splitting driven by solar energy has attracted much attention as a means of renewable solar hydrogen production. Photocatalytic water splitting is a representative approach utilizing particulate semiconductors.1,2 When a particulate semiconductor photocatalyst absorbs a photon, an electron is excited from the valence band to the conduction band, and a positive hole is left in the valence band. These excited electrons and holes can drive reduction and oxidation reactions on each photocatalyst particle, respectively. A photocatalyst can split water into hydrogen and oxygen thermodynamically when the band gap straddles the potentials of the hydrogen evolution reaction (0 V vs. RHE) and the oxygen evolution reaction (+1.23 V vs. RHE); however, to drive the reaction efficiently, it is generally necessary to load nanoparticulate metals and metal oxides, denoted as cocatalysts, on the photocatalyst as active sites for the hydrogen evolution reaction and the oxygen evolution reaction. Note that it is also possible to split water using two kinds of photocatalysts for hydrogen evolution and oxygen evolution, respectively. Such a reaction scheme based on two-step excitation is called Z-scheme. A solar-to-hydrogen energy conversion efficiency (STH) of 5% or higher is considered to be necessary for practical operation of photocatalytic solar hydrogen production. To attain such a high STH at reasonable quantum efficiencies, it is necessary to activate and stabilize narrow band gap photocatalysts.2 Scalability of the system is also a critical issue.3 Water splitting systems based on particulate photocatalysts are promising in terms of scalability because they do not need any secure electric circuits and thus can be spread over a wide area easily. Conventionally, photocatalytic water splitting has been studied using suspension of particulate photocatalysts. However, this approach is not feasible in practice because it is challenging to design a cheap photocatalytic reactor that can maintain a large amount of water over a wide area. It is probably a more feasible to use photocatalysts processed into panels and construct modules similar to photovoltaic applications.4 This approach, fixing particulate photocatalysts onto substrate, is also expected to be advantageous over the conventional powder suspension approach in operation and maintenance, because power input to stir a large amount of photocatalyst suspension is unneeded and used photocatalysts can be readily replaced with fresh ones. Recently, the authors’ group developed photocatalyst sheets based on particulate hydrogen evolution photocatalyst (HEP) and oxygen evolution photocatalyst (OEP) embedded into conductive layers by particle transfer for efficient and scalable water splitting (Figure).5–8 The STH of water splitting using photocatalyst sheets consisting of La- and Rh-codoped SrTiO3 (SrTiO3:La,Rh) as a HEP and BiVO4 as an OEP was 0.2%, superior to those for the corresponding powder suspension systems.6 The photocatalyst sheet maintained the high water splitting activity over a wide range of pH values and even in pure water. Moreover, the STH was improved to 1.1% through improvements in the preparation process of the photocatalyst sheet and the reaction conditions.7 The high activity of the photocatalyst sheet is due to the efficient electron transfer between HEP and OEP particles via the underlying conductive layer. In addition, evolution of hydrogen and oxygen in close proximity allows to prevent generation of pH gradient during the water splitting reaction.8 Therefore, the photocatalyst sheet is scalable directly without sacrificing the high activity. However, the absorption edge wavelengths of SrTiO3:La,Rh and BiVO4 are 540 nm at most. It is still important to develop photocatalysts with longer absorption edge wavelengths. In this talk, recent progress and future challenges in photocatalytic water splitting and system development will be presented. References Hisatomi et al., Chem. Soc. Rev. 2014, 43, 7520. Hisatomi et al., Catal. Lett. 2015, 145, 95. Maeda et al., J. Phys. Chem. Lett. 2010, 1, 2655. Xiong et al., Catal. Sci. Technol. 2014, 4, 325. Minegishi et al., Chem. Sci. 2013, 4, 1120. Wang et al., J. Catal. 2015, 328, 308. Wang et al., Nat. Mater. 2016, 15, 611. Wang et al., Faraday Discuss., in press. Figure 1

Multi-branched AgAuPt nanoparticles for efficient electrocatalytic hydrogen evolution: Synergism of tip-enhanced electric field effect and local electric field effect

Photocatalytic hydrogen evolution from pure water and bifunctional photocatalytic reaction are realized using a CoP/CdS/WS2 photocatalyst, which has a p–n–n tandem heterostructure.

Cadmium sulfide (CdS) as one of the most common visible-light-responsive photocatalysts has been widely investigated for hydrogen generation. However, its low solar–hydrogen conversion efficiency caused by fast carrier recombination and poor catalytic activity hinders its practical applications. To address this issue, we develop a novel and highly efficient nickel–cobalt phosphide and phosphate cocatalyst-modified CdS (NiCoP/CdS/NiCoPi) photocatalyst for hydrogen evolution. The dual-cocatalysts were simultaneously deposited on CdS during one phosphating step by using sodium hypophosphate as the phosphorus source. After the loading of the dual-cocatalysts, the photocurrent of CdS significantly increased, while its electrical impedance and photoluminescence emission dramatically decreased, which indicates the enhancement of charge carrier separation. It was proposed that the NiCoP cocatalyst accepts electrons and promotes hydrogen evolution, while the NiCoPi cocatalyst donates electrons and accelerates the oxidation of sacrificial agents (e.g., lactic acid). Consequently, the visible-light-driven hydrogen evolution of this composite photocatalyst greatly improved. The dual-cocatalyst-modified CdS with a loading content of 5 mol % showed a high hydrogen evolution rate of 80.8 mmol·g–1·h–1, which was 202 times higher than that of bare CdS (0.4 mmol·g–1·h–1). This is the highest enhancement factor for metal phosphide-modified CdS photocatalysts. It also exhibited remarkable stability in a continuous photocatalytic test with a total reaction time of 24 h.

Fabrication of S-scheme Zn1-xCdxS/NiO heterojunction for efficient photocatalytic hydrogen evolution

Herein, transition metal chalcogenides of pristine cobalt sulfides are rationally designed to act as robust bifunctional photocatalysts for visible‐light‐driven water splitting for the first time. Through moderate solvothermal route, cobalt sulfides are synthesized in situ growth and observed by scanning electron microscope image analysis. Noteworthily, 3D hierarchical cobalt sulfides acting as bifunctional photocatalysts are implemented to catalyze the visible‐light‐driven oxygen evolution reaction and hydrogen evolution reaction. This efficient, earth‐abundant, and nonnoble water splitting catalyst for artificial photosynthesis is thoroughly analyzed by various spectroscopic techniques with the aim of investigating its photocatalytic mechanism under visible‐light illumination. The main catalyst of CoS‐2 exhibits considerable H2 evolution rate of 1196 µmol h−1 g−1 and O2 yield of 63.5%. The efficient activity is attributed to the effective electron transfer between the photosensitizer and catalyst, which is verified by transient absorption experiments. The effective electron transfer between the photosensitizer and catalyst during water oxidation is verified by the dramatic decline of [Ru(bpy)3]3+ concentration in the presence of the catalyst CoS‐2. At the same time, transient absorption experiments support a rapid electron transfers from 3EY* (excited photosensitizer eosin‐Y) to the catalyst CoS‐2 for efficient hydrogen evolution.

A highly efficient photocatalytic hydrogen evolution system without an electron mediator such as methyl viologen (MV(2+)) has been constructed using 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt-PVP) and NADH (beta-nicotinamide adenine dinucleotide, reduced form) as the photocatalyst, hydrogen evolution catalyst and electron donor, respectively. The photocatalyst (Acr(+)-Mes) undergoes photoinduced electron transfer (ET) from the Mes moiety to the singlet excited state of the Acr(+) moiety to produce an extremely long-lived ET state, which is capable of oxidizing NADH and reducing Pt-PVP, leading to efficient hydrogen evolution. The hydrogen evolution efficiency is 300 times higher than that in the presence of MV(2+) because of the much faster reduction rate of Pt-PVP by Acr(*)-Mes compared with that by MV(*+). When the electron donor (NADH) is replaced by ethanol in the presence of an alcohol dehydrogenase (ADH), NADH is regenerated during the photocatalytic hydrogen evolution.

g-C3N4/ITO/Co-BiVO4 Z-scheme composite for solar overall water splitting