Boron-doped nitrogen-deficient carbon nitride-based Z-scheme heterostructures for photocatalytic overall water splitting

Photocatalytic overall water splitting can be achieved using Z-scheme systems that mimic natural photosynthesis by combining dissimilar semiconductors in series. However, coupling well-suited H2- and O2-evolving components remains challenging. Here, we fabricate a Z-scheme system for photocatalytic overall water splitting based on boron-doped, nitrogen-deficient carbon nitride two-dimensional (2D) nanosheets. We prepare ultrathin carbon nitride nanosheets with varying levels of boron dopants and nitrogen defects, which leads to nanosheets that can act as either H2- or O2-evolving photocatalysts. Using an electrostatic self-assembly strategy, the nanosheets are coupled to obtain a 2D/2D polymeric heterostructure. Owing to their ultrathin nanostructures, strong interfacial interaction and staggered band alignment, a Z-scheme route for efficient charge-carrier separation and transfer is realized. The obtained heterostructure achieves stoichiometric H2 and O2 evolution in the presence of Pt and Co(OH)2 co-catalysts, and the solar-to-hydrogen efficiency reaches 1.16% under one-sun illumination.

Ferroelectric polarization enabled spatially selective adsorption of redox mediators to promote Z-scheme photocatalytic overall water splitting

Photocatalytic water splitting using semiconductors is a promising approach for converting solar energy to clean energy. However, challenges such as sluggish water oxidation kinetics and limited light absorption of photocatalyst cause low solar-to-hydrogen conversion efficiency (STH). Herein, we develop a photocatalytic overall water splitting system using I3 -/I- as the shuttle redox couple to bridge the H2-producing half-reaction with the O2-producing half-reaction. The system uses the halide perovskite of benzylammonium lead iodide (PMA2PbI4, PMA=C6H5CH2NH2) loaded with MoS2 (PMA2PbI4/MoS2) as the H2 evolution photocatalyst, and the RuOx-loaded WO3 (WO3/RuOx) as the O2 evolution photocatalyst, achieving a H2/O2 production in stoichiometric ratio with an excellent STH of 2.07 %. This work provides a detour route for photocatalytic water splitting with the help of I3 -/I- shuttle redox couple in the halide perovskite HI splitting system and enlightens one to integrate and utilize multi catalytic strategies for solar-driven water splitting.

Surface modification of carbon nitride with single Co sites via a solvent-driven strategy promoting high‐efficiency photocatalytic overall water splitting

Synthesis of 3D mesoporous g-C3N4 for efficient overall water splitting under a Z-scheme photocatalytic system

The two-dimensional (2D) Z-scheme system is an effective approach for hydrogen production via photocatalytic water splitting (PWS). This study established a 2D van der Waals (vdW) SnC/Sc2CCl2 heterojunction for PWS. The electronic and optical properties of the designed heterojunction were determined using first-principles methods, showing that the heterojunction, acting as a Z-scheme photocatalyst (ZSP), formed an induced internal electric field to achieve effective electron-hole separation. The strong redox ability (∼1.5 eV) and moderate energy barrier of the SnC/Sc2CCl2 heterojunction further enabled efficient PWS. Moreover, the PWS process benefited from the heterojunction's favorable absorption coefficient (105 cm-1) and solar-to-hydrogen conversion efficiency (21.36%) under visible light. The proposed Z-scheme SnC/Sc2CCl2 heterojunction is a promising candidate for photocatalytic overall water splitting (POWS) across a pH range of 0-14.

Solar light utilization for photocatalytic overall water splitting (POWS) is a promising alternative to electrolysis to produce hydrogen, since photocatalytic water splitting is simple, and can be operated at low-cost. Transformation and storage of solar energy in the form hydrogen can significantly reduce the rate of the greenhouse gas emissions. Given the low-cost and simplicity of photocatalytic hydrogen production, this thesis has focused on designing an efficient photocatalytic water splitting system. SrTiO3 has been shown to be capable of driving photocatalytic overall water splitting under UV light illumination. In this thesis, SrTiO3 is used as the photocatalyst and the performance of SrTiO3 is discussed in the POWS reaction. Several strategies have been applied to understand particular functions of the SrTiO3-based photocatalyst in photocatalytic overall water splitting with following aspects: i) the photocatalytic transients are collected to understand how Ni/NiO co-catalyst is changed during illumination; ii) Mg is doped into the structure of SrTiO3 to improve the photocatalytic activity; iii) Cr2O3 is introduced on Mg:SrTiO3-NiOx composite to improve the stability of photocatalytic gas evolution; iv) state-of-the art semiconductors, Al:SrTiO3 and Mg:SrTiO3, are compared in the same conditions. Generally, the effect of the various modifications on the photocatalytic gas evolution rates has been revealed by reliable on-line GC measurements. Due to the fast detection mode of the GC, the applied setup allows to determine transients in gas-evolution to reveal activity and stability of the tested photocatalysts.

The self-assemble 0.5Pt-ZnIn2S4/rGO/Co3O4-BiVO4 (110) Z-scheme system photocatalysts were synthesized successfully, in which 0.5Pt-ZnIn2S4, rGO and Co3O4-BiVO4 (110) were as H2-photocatalyst, electron mediator and O2-photocatalyst, respectively. Herein, the preferred exposed (110) crystal facets of BiVO4 have great contribution to optimizing its photocatalytic oxidation performance. The Z-scheme system samples were used for photocatalytic water splitting to H2 and O2 in stoichiometric ratio without any sacrificial agents under visible light irradiation. SEM results clearly revealed the morphology and structure of Pt-ZnIn2S4, rGO and Co3O4-BiVO4 (110), suggesting that the three samples closely contacted with each other. Thus the electrons in the Z-scheme photocatalyst system could flow continuously in the photocatalytic water splitting process, which would induce the highly separation rate of photoinduced charge carriers and then accelerated the photocatalytic performance. Furthermore, rGO and Co3O4 were shown to be the critical factors for the improvment of photoinduced electron–hole pairs, thereby influencing the photocatalytic performance. In 0.5Pt-ZnIn2S4/2rGO/5Co3O4-BiVO4 (110) photocatalyst, the amount of the H2 and O2 could reach 294.3 μmol/g and 143.4 μmol/g, respectively (in 12 h). The mechanism for the photocatalytic overall water splitting was also discussed in detail. We confirmed that Z-scheme 0.5Pt-ZnIn2S4/rGO/Co3O4-BiVO4 (110) exhibit photocatalytic overall water splitting into H2 and O2 (294.3 and 143.4 μmol/g in 12 h). Here, 0.5Pt-ZnIn2S4, rGO and Co3O4-BiVO4 (110) act as H2-photocatalyst, electron-mediator and O2-photocatalyst, respectively. And the contact interface is favourable and then helpful for the electrons flow continuously during the photocatalytic reaction process. The electron mediator rGO and cocatalyst Co3O4 are responsible for the photoactivity.

Water oxidation reactions over a surface treated PtOx/WO3 photocatalyst in the presence of IO3− or I3− ions were investigated. When the IO3− ion was used as an electron acceptor, PtOx/WO3 treated by thermal treatment using Cs2CO3 aqueous solution followed by H+-exchange-treatment (PtOx/H-Cs-WO3) showed ca. 4 times higher activity compared with PtOx/WO3 without surface treatment. The apparent quantum yield at 420 nm was 20%. Moreover, when the I3− ion was used as the electron acceptor, the activity of PtOx/WO3 was negligible, while PtOx/H-Cs-WO3 showed a high activity for water oxidation. The cesium surface treatment also had an excellent effect for the improvement of photocatalytic performance of TiO2. The activity of a water splitting reaction under visible light by the Z-scheme system that combined Pt/SrTiO3:Cr/Ta with PtOx/H-Cs-WO3 was 3 times higher than that using PtOx/WO3 as an O2 evolution photocatalyst. Moreover, owing to the contribution from both IO3− and I3− ions as redox mediators, the water splitting reaction stoichiometrically proceeded in a wider pH region (pH 2–9).

Energy and environment are the key global challenges in the 21st century. Solar energy is considered the most promising clean and sustainable energy resource due to its university, inexhaustible and environmental friendliness. One of the most viable means of solar energy conversion and utilization is artificially converting solar energy into chemical energy as natural photosynthesis does. H2 produced from water splitting and CO2 reduction, are the major research topics of artificial photosynthesis. However, the effective conversion of solar energy into chemical energy by cost-effective artificial means on a large scale remains elusive. Metal oxide-based photocatalysts are the most studied materials for photocatalytic water splitting and CO2 reduction. Particularly, perovskite oxides with the chemical formula of ABO3 have been intensively studied as semiconductor photocatalysts. However, overwhelmingly of the perovskite oxides are only active under UV-light-irradiation, which limited their potential in solar energy application. Therefore, breakthrough technology and step-change materials, particularly, visible-light-responsive photocatalysts are highly desirable for the development of photocatalytic systems. In recent years, copious progress has been made in designing materials that function under visible-light-irradiation. So far, the most successful strategy is anion doping of oxide semiconductors, such as nitrogen or sulfur dope to form oxynitrides and oxysulfides, respectively. For example, nitrogen-doped oxynitrides such as (Ga1-xZnx)(N1-xOx) and ANbO2N (A=Sr, Ba, and La)1, and sulfur-doped oxysulfides such as Sm2Ti2S2O5 2 showed efficient photocatalytic overall water splitting activities under visible-light-irradiation. Particularly, oxynitrides (Ga1-xZnx)(N1-xOx) based photocatalyst sheet, showed remarkable photocatalytic overall water splitting activity with AQE of more than 30% at l»420 nm3. However, these oxynitrides and oxysulfides suffer stability problems due to photocorrosion, hindering their potential practical application in photocatalytic applications. Recently, it has been theorized that double perovskite oxides (DPOs) with the chemical formula of A2BʹB"O6 can function as efficient and stable visible-light-responsive photocatalysts for photocatalytic water splitting and CO2 reduction. However, DPOs have been rarely studied for photocatalytic water splitting and CO2 reduction due to the difficulty of obtaining pure phase materials and the paucity of exposed active sites. Thus, developing efficient and stable visible-light-responsive DPOs photocatalysts for photocatalytic water splitting and CO2 reduction becomes important. In this regard, recently, we have demonstrated a series of efficient and stable visible-light-responsive DPOs photocatalysts for photocatalytic water splitting. For instance, Sr2CoWO6 and Sr2CoTaO6 can serve as an efficient and stable bifunctional photocatalyst for both photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction(HER)4, 5, and Sr2NiWO6 can efficiently drive the photocatalytic OER6. Even though these DPOs have been demonstrated as visible-light-responsive photocatalysts with suitable CB and VB positions that straddle the theoretical potentials for overall water splitting, however, one-step overall water splitting has not been achieved so far, and their potential for photocatalytic CO2 reduction has not been studied yet. Overall water splitting under visible-light-irradiation based on particulate photocatalysts is a challenging reaction, which is regarded as one of the “Holy Grail” of sciences. And oxide semiconductors showing both photocatalytic OER and HER activities are rare. Nevertheless, bifunctional photocatalytic OER and HER were successfully demonstrated based on Sr2CoWO6 and Sr2CoTaO6. Further improvement of the material designs and developing appropriate cocatalysts may play a key role in achieving one-step overall water splitting under visible-light-irradiation based on DPOs photocatalysts. This work is mainly to explore the possibility of utilizing DPOs materials as visible-light-responsive photocatalysts for photocatalytic water splitting and CO2 reduction reaction, which is challenging work but has great potential value in advancing science and technology in photocatalysis. We anticipated that this work will provide a novel and rational strategy for improving the light absorption, charge separation and charge utilization in DPOs photocatalysts. References Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. The Journal of Physical Chemistry B 2006, 110 (26), 13107-13112.Ma, G.; Kuang, Y.; Murthy, D. H.; Hisatomi, T.; Seo, J.; Chen, S.; Matsuzaki, H.; Suzuki, Y.; Katayama, M.; Minegishi, T. The Journal of Physical Chemistry C 2018, 122 (25), 13492-13499.Kato, H.; Asakura, K.; Kudo, A. Journal of the American Chemical Society 2003, 125 (10), 3082-3089.Idris, A. M.; Liu, T.; Shah, J. H.; Zhang, X.; Ma, C.; Malik, A. S.; Jin, A. Solar RRL 2020, 4 (3), 1900456.Idris, A. M.; Liu, T.; Hussain Shah, J.; Han, H.; Li, C. ACS Sustainable Chemistry&Engineering 2020, 8 (37), 14190-14197.Idris, A. M.; Liu, T.; Hussain Shah, J.; Malik, A. S.; Zhao, D.; Han, H.; Li, C. ACS Applied Materials&Interfaces 2020, 12 (23), 25938-25948.

Photocatalytic water splitting using semiconductors is a promising approach for converting solar energy to clean energy. However, challenges such as sluggish water oxidation kinetics and limited light absorption of photocatalyst cause low solar‐to‐hydrogen conversion efficiency (STH). Herein, we develop a photocatalytic overall water splitting system using I 3 − /I − as the shuttle redox couple to bridge the H 2 ‐producing half‐reaction with the O 2 ‐producing half‐reaction. The system uses the halide perovskite of benzylammonium lead iodide (PMA 2 PbI 4 , PMA=C 6 H 5 CH 2 NH 2 ) loaded with MoS 2 (PMA 2 PbI 4 /MoS 2 ) as the H 2 evolution photocatalyst, and the RuO x ‐loaded WO 3 (WO 3 /RuO x ) as the O 2 evolution photocatalyst, achieving a H 2 /O 2 production in stoichiometric ratio with an excellent STH of 2.07 %. This work provides a detour route for photocatalytic water splitting with the help of I 3 − /I − shuttle redox couple in the halide perovskite HI splitting system and enlightens one to integrate and utilize multi catalytic strategies for solar‐driven water splitting.

Green hydrogen (H2) has been identified as a promising alternative to fossil fuel. Compared with traditional methods, such as steam methane reforming and coal gasification, electro-(photo)catalysis of water splitting provides a clean and sustainable way to produce green H2. However, electro-(photo)catalytic water splitting still suffers from sluggish kinetics and high-power consuming. Chemical-assisted electro-(photo)catalytic water splitting, with concurrent evolution of H2 and high value-added chemicals (HVACs), has recently drawn great attention. In such system, oxygen evolution process has been replaced by small organics or other chemicals with low oxidation reaction potential to reduce the energy gap. In this review, we will review recent important advances on how to design the electro-(photo)catalytic systems for concurrent evolution of H2 and HVACs. We first introduce the design principles and fundamentals of chemical-assisted electro-/photocatalytic water splitting. Then we focus on the different reaction types at anode for electro-(photo)catalysis, in which specific chemicals, especially small molecule, can be produced from biomass, alkyl alcohols and so on, with high efficiency and selectivity, coupled with promoted H2 generation. Finally, major challenges and perspectives relevant to the catalyst design, catalytic mechanisms and application of electro-(photo)catalytic concurrent evolution of H2 and HVACs will be provided.

Molten salt-assisted synthesis of nitrogen-vacancy crystalline graphitic carbon nitride with tunable band structures for efficient photocatalytic overall water splitting

Plasmonic-assisted solar-driven photocatalytic water splitting for hydrogen production represents a sustainable strategy for green energy generation. However, conventional plasmonic enhancement via photo-induced electron injection into water molecules faces a key limitation: weak interfacial H2O adsorption characterized by restriction solely to H-atom-mediated interactions, which severely constrains reaction kinetics. Simultaneously, the novel quantum-corrected plasmonic effect could improve the electron models in optoelectronic device; however, within the domain of photocatalytic water splitting, experimental validations of this effect remain relatively scarce. Here, we harness the quantum-corrected plasmonic effect via Au25 nanocluster incorporation to realize and enhance photocatalytic overall water splitting performance, facilitated by modulated surface adsorption behavior through electron-deficient Auδ+ active sites originating from the size effect and interband transitions. In this case, the Auδ+ active sites enhance the antibonding-orbital occupancy of adsorbed Au-O species, accelerating both multipath electron injection and the activation process of water molecules, ultimately facilitating the cleavage of H─O bonds. Consequently, it achieves H2 and O2 evolution rates of 1.07 and 0.54mmol h-1 under light irradiation with catalyst ZnIn2S4-Au25, resolving the long-standing challenge of incomplete overall water splitting for sole Au nanoparticle-decorated photocatalysts, providing a promising strategy for photocatalytic overall water splitting and new insights into designing nanocluster-based photocatalysts.

Strong coupling effect at the interface of cobalt phosphate-carbon dots boost photocatalytic water splitting