Recent Progress in Energy-Driven Water Splitting

Nanostructured Ni Based Anode and Cathode for Alkaline Water Electrolyzers

Owing to the progressive abandoning of the fossil fuels and the increase of atmospheric CO2 concentration, the use of renewable energies is strongly encouraged. The hydrogen economy provides a very interesting scenario. In fact, hydrogen is a valuable energy carrier and can act as a storage medium as well to balance the discontinuity of the renewable sources. In order to exploit the potential of hydrogen it must be made available in adequate quantities and at an affordable price. Both goals can be potentially achieved through the electrochemical water splitting, which is an environmentally friendly process as well as the electrons and water are the only reagents. However, these devices still require a lot of research to reduce costs and increase efficiency. An approach to improve their performance is based on nanostructured electrodes characterized by high electrocatalytic activity. In this work, we show that by using template electrosynthesis it is possible to fabricate Ni nanowires featuring a very high surface area. In particular, we found that water-alkaline electrolyzers with Ni nanowires electrodes covered by different electrocatalyst have good and stable performance at room temperature as well. Besides, the results concern nickel-cobalt nanowires electrodes for both hydrogen and oxygen evolution reaction will be presented and discussed. Finally, preliminary tests concerning the use of Ni foam differently functionalized will be shown. For each electrode, electrochemical and electrocatalytic tests aimed to establishing the performance of the electrolyzers were carried out. Long term amperostatic test carried out in aqueous solution of KOH will be reported as well.

MoS2 nanotubes loaded with TiO2 nanoparticles for enhanced electrocatalytic hydrogen evolution

Efficient and stable non-precious metal catalysts composed of earth-abundant elements are crucial to the hydrogen evolution reaction (HER) in high-energy conversion efficiency. Herein, TiO₂/MoS₂-NTs catalyst, in which the MoS₂ nanotubes were loaded with TiO₂ nanoparticles, have been synthesized via a facile solvothermal and hydrothermal method. The as-prepared TiO₂/MoS₂-NTs electrocatalyst demonstrated enhanced electrocatalytic hydrogen evolution performance compared with MoS₂-NTs. Electrochemical measurements reveal the overpotential and Tafel slope of as-prepared TiO₂/MoS₂-NTs are −0.21 V and 42 mV dec⁻¹. The HER improvement is proposed to be attributed to the increased edge sites results from the interfaces and synergic effect between TiO₂ nanoparticles and MoS₂ nanotubes.

Polarization curves of TiO₂, MoS₂-NTs and TiO₂/MoS₂-NTs in 0.5 M H₂SO₄ solution.

FexNi9-xS8 (x = 3-6) as potential photocatalysts for solar-driven hydrogen production?

The efficient reduction of protons by non-noble metals under mild conditions is a challenge for our modern society. Nature utilises hydrogenases, enzymatic machineries that comprise iron- and nickel- containing active sites, to perform the conversion of protons to hydrogen. We herein report a straightforward synthetic pathway towards well-defined particles of the bio-inspired material FexNi9-xS8, a structural and functional analogue of hydrogenase metal sulfur clusters. Moreover, the potential of pentlandites to serve as photocatalysts for solar-driven H2-production is assessed for the first time. The FexNi9-xS8 materials are visible light responsive (band gaps between 2.02 and 2.49 eV, depending on the pentlandite's Fe : Ni content) and display a conduction band energy close to the thermodynamic potential for proton reduction. Despite the limited driving force, a modest activity for photocatalytic H2 has been observed. Our observations show the potential for the future development of pentlandites as photocatalysts. This work provides a basis to explore powerful synergies between biomimetic chemistry and material design to unlock novel applications in solar energy conversion.

One-Step Synthesis of NiFe Layered Double Hydroxide Nanosheet Array/N-Doped Graphite Foam Electrodes for Oxygen Evolution Reactions

Developing cost-effective and highly efficient oxygen evolution reaction (OER) electrocatalysts is vital for the production of clean hydrogen by electrocatalytic water splitting. Here, three dimensional nickel-iron layered double hydroxide (NiFe LDH) nanosheet arrays are in-situ fabricated on self-supporting nitrogen doped graphited foam (NGF) via a one-step hydrothermal process under an optimized amount of urea. The as prepared NiFe LDH/NGF electrode exhibits a remarkable activity toward OER with a low onset overpotential of 233 mV and a Tafel slope of 59.4 mV dec-1 as well as a long-term durability. Such good performance is attributed to the synergy among the doping effect, the binder-free characteristic, and the architecture of the nanosheet array.

An anion exchange reaction: an effective approach to prepare alloyed Co-Fe bimetallic disulfide for improving the electrocatalytic activity

A mild anion exchange approach is proposed for the synthesis of alloyed Co-Fe bimetallic disulfide. Abundant structural disorders and tunable compositions are effectively induced by the anion exchange process. The alloyed Co-Fe bimetallic disulfide exhibits a low overpotential of 205 mV to reach a current density of 100 mA cm-2 in an acidic electrolyte, which is significantly improved compared to the performance of individual disulfide. It is believed that this work paves a new strategy for the synthesis of bimetallic compounds and highlights the importance of tuning the catalyst composition for achieving high catalytic performance.

MOF-templated cobalt nanoparticles embedded in nitrogen-doped porous carbon: a bifunctional electrocatalyst for overall water splitting

Development of cost-effective and efficient non noble metal electrocatalysts has immense importance towards sustainable energy technologies. Herein, a newly constructed porous Co(ii)-metal organic framework (MOF) has been utilized for the synthesis of cobalt nanoparticles embedded in N-doped porous carbon, (Co@NPC), via a facile MOF-annealing strategy, at an optimum temperature of 800 °C under an argon atmosphere. DMF molecules present in the form of solvated guests and cations within the 3D-framework serve as a source for N-doping during the formation of the porous graphitic carbon upon carbonization. The nanocomposite was found to encapsulate homogeneously dispersed cobalt nanoparticles within the N-doped porous carbonaceous matrix. The synergistic effect of cobalt nanoparticles and the heteroatom-doped carbon framework makes Co@NPC electrochemically active towards both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline conditions. Furthermore, Co@NPC exhibits outstanding performance as a bifunctional electrocatalyst towards electrochemical water splitting with remarkable stability and durability. It achieves a current density of 10 mA cm^-2 at a low cell voltage of 1.66 V in 1 M NaOH solution which is comparable with that of most of the self-templated ZIF-derived non-noble metal electrocatalysts.

Thermodynamics, Electronic Structure, and Vibrational Properties of Sn n (S1-x Se x ) m Solid Solutions for Energy Applications

The tin sulfides and selenides have a range of applications spanning photovoltaics and thermoelectrics to photocatalysts and photodetectors. However, significant challenges remain to widespread use, including electrical and chemical incompatibilities between SnS and device contact materials and the environmental toxicity of selenium. Solid solutions of isostructural sulfide and selenide phases could provide scope for optimizing physical properties against sustainability requirements, but this has not been comprehensively explored. This work presents a detailed modeling study of the Pnma and rocksalt Sn(S_1-x Se _x ), Sn(S_1-x Se _x )₂, and Sn₂(S_1-x Se _x )₃ solid solutions. All four show an energetically favorable and homogenous mixing at all compositions, but rocksalt Sn(S_1-x Se _x ) and Sn₂(S_1-x Se _x )₃ are predicted to be metastable and accessible only under certain synthesis conditions. Alloying leads to a predictable variation of the bandgap, density of states, and optical properties with composition, allowing SnS₂ to be "tuned down" to the ideal Shockley-Queisser bandgap of 1.34 eV. The impact of forming the solid solutions on the lattice dynamics is also investigated, providing insight into the enhanced performance of Sn(S_1-x Se _x ) solid solutions for thermoelectric applications. These results demonstrate that alloying affords facile and precise control over the electronic, optical, and vibrational properties, allowing material performance for optoelectronic applications to be optimized alongside a variety of practical considerations.

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising carbon-free approach to produce hydrogen from water. A photoelectrochemical cell consists of a semiconductor photoelectrode in contact with an aqueous electrolyte. Its performance is sensitive to properties of the photoelectrode/electrolyte interface, which may be tuned through functionalization of the photoelectrode surface with organic molecules. This can lead to improvements in the photoelectrode's properties. This Minireview summarizes key computational investigations on using molecular functionalization to modify photoelectrode stability, barrier height, and catalytic activity. It is discussed how first-principles density functional theory, first-principles molecular dynamics, and device modeling simulations can provide predictive insights and complement experimental investigations of functionalized photoelectrodes. Challenges and future directions in the computational modeling of functionalized photoelectrode/electrolyte interfaces within the context of experimental studies are also highlighted.

Scaling Up Electrodes for Photoelectrochemical Water Splitting: Fabrication Process and Performance of 40 cm2 LaTiO2 N Photoanodes

A scalable process for fabrication of particle-based photoanodes is developed. The electrodes are versatilely made of photocatalytically active semiconductor particles, in this case LaTiO₂ N, and optionally coated with cocatalysts and protecting components, all immobilized on a conducting substrate. The involved fabrication steps are restricted to scalable processes such as electrophoretic deposition, annealing in air, and dip coating. Special care is taken to ensure efficient charge transport in-between particles and to the substrate by incorporating conducting connectors. By adapting the fabrication steps, the electrode geometrical dimension is increased from the size of a typical lab electrode of 1 to 40 cm² . The quality of the scale-up process is characterized by comparing the photoanodes in terms of thickness, light-absorption properties, and morphology. For several compositions, the electrochemical performance of both electrode sizes is assessed by measuring the photocurrents and faradaic efficiencies. The comparison revealed a complex upscaling behavior and showed that the photoelectrode size affects performance already on the 0.1 m scale.

Photoelectrochemical Water Splitting with p-Type Metal Oxide Semiconductor Photocathodes

Photoelectrochemical (PEC) water splitting is a promising way to produce clean and sustainable hydrogen fuel. Solar hydrogen production by using p-type metal oxide semiconductor photocathodes has not been studied as extensively as that with n-type metal oxide semiconductor photoanodes and p-type photovoltaic-grade non-oxide semiconductor photocathodes. Copper-based oxide photocathodes show relatively good conductivity, but suffer from instability in aqueous solution under illumination, whereas iron-based metal oxide photocathodes demonstrate more stable PEC performance but have problems in charge separation and transport. Herein, an overview of recent progress in p-type metal oxide-based photocathodes for PEC water reduction is provided. Although these materials have not been fully developed to reach their potential performance, the challenges involved have been identified and strategies to overcome these limitations have been proposed. Future research in this field should address these issues and challenges in addition to the discovery of new materials.

Enhanced photoelectrochemical hydrogen production efficiency of MoS2-Si heterojunction

Photoelectrochemical water splitting is one of the viable approaches to produce clean hydrogen energy from water. Herein, we report MoS₂/Si-heterojunction (HJ) photocathode for PEC H₂ production. The MoS₂/Si-HJ photocathode exhibits exceptional PEC H₂ production performance with a maximum photocurrent density of 36.33 mA/cm², open circuit potential of 0.5 V vs. RHE and achieves improved long-term stability up to 10 h of reaction time. The photocurrent density achieved by MoS₂/Si-HJ photocathode is significantly higher than most of the MoS₂ coupled Si-based photocathodes reported elsewhere, indicating excellent PEC H₂ production performance.

Toward practical solar hydrogen production - an artificial photosynthetic leaf-to-farm challenge

Solar water splitting is a promising approach to transform sunlight into renewable, sustainable and green hydrogen energy. There are three representative ways of transforming solar radiation into molecular hydrogen, which are the photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic-electrolysis (PV-EC) routes. Having the future perspective of green hydrogen economy in mind, this review article discusses devices and systems for solar-to-hydrogen production including comparison of the above solar water splitting systems. The focus is placed on a critical assessment of the key components needed to scale up PEC water splitting systems such as materials efficiency, cost, elemental abundancy, stability, fuel separation, device operability, cell architecture, and techno-economic aspects of the systems. The review follows a stepwise approach and provides (i) a summary of the basic principles and photocatalytic materials employed for PEC water splitting, (ii) an extensive discussion of technologies, procedures, and system designs, and (iii) an introduction to international demonstration projects, and the development of benchmarked devices and large-scale prototype systems. The task of scaling up of laboratory overall water splitting devices to practical systems may be called "an artificial photosynthetic leaf-to-farm challenge".

Mesoporous Co x Sn(1-x)O2 as an efficient oxygen evolution catalyst support for SPE water electrolyzer

SPE water electrolysis is a promising method of hydrogen production owing to its multiple strengths, including its high efficiency, high product purity and excellent adaptability. However, the overpotential of the oxygen evolution reaction process and consumption of Ir during charging in SPE water electrolysis will inevitably result in large energy loss and then high cost. Under these circumstances, we propose a novel 40IrO₂/Co _x Sn_(1-x)O₂ (x = 0.1, 0.2, 0.3) anode catalyst, where the Co _x Sn_(1-x)O₂ support is synthesized by a hydrothermal method and IrO₂ is synthesized by a modified Adams fusion method. After modifying the component of Co _x Sn_(1-x)O₂, the 40IrO₂/Co _x Sn_(1-x)O₂ exhibits an increased specific surface area, electrical conductivity and surface active sites. Moreover, a single cell is fabricated by Pt/C as cathode catalyst, 40IrO₂/Co _x Sn_(1-x)O₂ as anode catalyst and Nafion 117 membrane as electrolyte. The 40IrO₂/Co_0.2Sn_0.8O₂ exhibits the lowest overpotential (1.748 V at 1000 mA cm^-2), and only 0.18 mV h^-1 of voltage increased for 100 h durability test at 1000 mA cm^-2. Consequently, Co _x Sn_(1-x)O₂ is a promising anode electrocatalyst support for an SPE water electrolyzer.

Carbon-Based Photocathode Materials for Solar Hydrogen Production

Hydrogen is considered a promising environmentally friendly energy carrier for replacing traditional fossil fuels. In this context, photoelectrochemical cells effectively convert solar energy directly to H₂ fuel by water photoelectrolysis, thereby monolitically combining the functions of both light harvesting and electrolysis. In such devices, photocathodes and photoanodes carry out the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. Here, the focus is on photocathodes for HER, traditionally based on metal oxides, III-V group and II-VI group semiconductors, silicon, and copper-based chalcogenides as photoactive material. Recently, carbon-based materials have emerged as reliable alternatives to the aforementioned materials. A perspective on carbon-based photocathodes is provided here, critically analyzing recent research progress and outlining the major guidelines for the development of efficient and stable photocathode architectures. In particular, the functional role of charge-selective and protective layers, which enhance both the efficiency and the durability of the photocathodes, is discussed. An in-depth evaluation of the state-of-the-art fabrication of photocathodes through scalable, high-troughput, cost-effective methods is presented. The major aspects on the development of light-trapping nanostructured architectures are also addressed. Finally, the key challenges on future research directions in terms of potential performance and manufacturability of photocathodes are analyzed.

Highly Crystalline K-Intercalated Polymeric Carbon Nitride for Visible-Light Photocatalytic Alkenes and Alkynes Deuterations

In addition to the significance of photocatalytic hydrogen evolution, the utilization of the in situ generated H/D (deuterium) active species from water splitting for artificial photosynthesis of high value-added chemicals is very attractive and promising. Herein, photocatalytic water splitting technology is utilized to generate D-active species (i.e., Dad) that can be stabilized on anchored 2nd metal catalyst and are readily for tandem controllable deuterations of carbon-carbon multibonds to produce high value-added D-labeled chemicals/pharmaceuticals. A highly crystalline K cations intercalated polymeric carbon nitride (KPCN), rationally designed, and fabricated by a solid-template induced growth, is served as an ultraefficient photocatalyst, which shows a greater than 18-fold enhancement in the photocatalytic hydrogen evolution over the bulk PCN. The photocatalytic in situ generated D-species by superior KPCN are utilized for selective deuteration of a variety of alkenes and alkynes by anchored 2nd catalyst, Pd nanoparticles, to produce the corresponding D-labeled chemicals and pharmaceuticals with high yields and D-incorporation. This work highlights the great potential of developing photocatalytic water splitting technology for artificial photosynthesis of value-added chemicals instead of H2 evolution.

A porous Ni-O/Ni/Si photoanode for stable and efficient photoelectrochemical water splitting

In this study a porous Ni-O/Ni/Si photoanode with superior photoelectrochemical activity and stability was obtained.

Controlled growth of ZnS/ZnO heterojunctions on porous biomass carbons via one-step carbothermal reduction enables visible-light-driven photocatalytic H2 production

Controlled growth of ZnS/ZnO heterojunctions on porous biomass carbons has been achieved via a one-step carbothermal reduction strategy.

Photoelectrochemical study of carbon-modified p-type Cu2O nanoneedles and n-type TiO2−x nanorods for Z-scheme solar water splitting in a tandem cell configuration

Nanostructured photoelectrodes with high surface-area and tunable optical-electrical properties can potentially benefit a Z-scheme photoelectrochemical water splitting systems to generate solar fuels at no external bias.

Liquid phase hydrogenation of CO2 to formate using palladium and ruthenium nanoparticles supported on molybdenum carbide

We report the development of palladium nanoparticles supported on Mo₂C as an active catalyst for the liquid-phase hydrogenation of CO₂ to formate under mild reaction conditions (100 °C and 2.0 MPa of a 1 : 1 CO₂ : H₂ mixture).

Round-the-Clock Photocatalytic Hydrogen Production with High Efficiency by a Long-Afterglow Material

Long afterglow materials can store and release light energy after illumination. A brick-like, micrometer-sized Sr₂ MgSi₂ O₇ :Eu²⁺ ,Dy³⁺ long-afterglow material is used for hydrogen production by the photocatalytic reforming of methanol under round-the-clock conditions for the first time, achieving a solar-to-hydrogen (STH) conversion efficiency of 5.18 %. This material is one of the most efficient photocatalysts and provides the possibility of practical use on a large scale. Its remarkable photocatalytic activity is attributed to its unique carrier migration path and large number of lattice defects. These findings expand the application scope of long afterglow materials and provide a new strategy to design efficient photocatalysts by constructing trap levels that can prolong carrier lifetimes.

Quasi-Amorphous Metallic Nickel Nanopowder as an Efficient and Durable Electrocatalyst for Alkaline Hydrogen Evolution

Nickel is regarded as the best alternative metal electrocatalyst to platinum for hydrogen evolution reaction (HER). Success in developing a quasi-amorphous metallic nickel (QAMN) nanopowder catalyst using a two-step chemical route for efficient and durable HER in alkaline solution is reported. It is found that the QAMN electrocatalyst exhibits essentially zero overpotential at the cathodic onset while delivering 10 mA cm-2 at an overpotential of only 240 mV; both performances are far better than what was reported previously using prior metallic nickel catalysts. Taking into account increased surface area, further enhanced activity is attributed to the superior intrinsic activity. Meanwhile, the QAMN catalyst shows excellent stability in accelerated and interrupted polarization in alkaline solution for tens of hours. This study provides a new chemical means to prepare amorphous metallic materials for more efficient catalysis.

First-Principles Determination of Active Sites of Ni Metal-Based Electrocatalysts for Hydrogen Evolution Reaction

The determination of active sites of materials is essential for the molecular design of high-performance catalysts. In this study, the first-principles method is applied to investigate the active sites of low-cost Ni metal-based electrocatalysts for hydrogen evolution reactions (HER), which is a promising alternative to expensive Pt metal-based catalysts. The adsorption of hydrogen on different sites of pristine and partially oxidized Ni(111) surface is investigated. All of the possible configurations have been systematically investigated here with the consideration of their Boltzmann distribution. Using the Gibbs free energy of intermediate H atoms (Δ G_H*) as a descriptor, it is found that the Δ G_H* increases with the increase of the coverage of oxygen atoms. The slightly oxidized surface Ni atoms are theoretically identified to be the best catalytic centers for the electrocatalytic HERs when the coverage of oxygen is considerably low. On the basis of the analyses of Bader charge distribution and density of states, our results reveal that the superior performance of the slightly oxidized surface Ni atoms can be ascribed to the optimal electronic properties.

Reactive template-induced core-shell FeCo@C microspheres as multifunctional electrocatalysts for rechargeable zinc-air batteries

Sluggish kinetics and thermodynamic unfavorability restrict electrocatalysis for energy storage and conversion reactions such as oxygen reduction/evolution and hydrogen evolution reactions. Herein, we report the synthesis and electrochemical performance of novel core-shell nanoparticles@porous carbon microspheres. A unique core-shell architecture of dual-phase FeCo-based nanoparticles@heteroatom-doped carbon microspheres (FeCo@C MS) has been prepared via a two-step carbonization process from a reactive multifunctional core-double shell template. With the advantages of heterogeneous composition and architectural structure, the obtained FeCo@C MS exhibits excellent performances for the electrochemical oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), which are comparable to those of commercial Pt/C catalyst. As an excellent cathode catalyst of the Zn-air battery (ZAB), FeCo@C MS exhibits high discharge voltage of 1.27 V, high specific capacity of 503 mA h gZn-1, an energy density of 639 W h kgZn-1, and better cycling durability than the battery having a mixture of 20 wt% Pt/C and RuO2. This approach provides a new way to design structures with controlled morphology and excellent multifunctional electrocatalytic activity.

Metal Organic Framework Derived Materials: Progress and Prospects for the Energy Conversion and Storage

Exploring new materials with high efficiency and durability is the major requirement in the field of sustainable energy conversion and storage systems. Numerous techniques have been developed in last three decades to enhance the efficiency of the catalyst systems, control over the composition, structure, surface area, pore size, and moreover morphology of the particles. In this respect, metal organic framework (MOF) derived catalysts are emerged as the finest materials with tunable properties and activities for the energy conversion and storage. Recently, several nano- or microstructures of metal oxides, chalcogenides, phosphides, nitrides, carbides, alloys, carbon materials, or their hybrids are explored for the electrochemical energy conversion like oxygen evolution, hydrogen evolution, oxygen reduction, or battery materials. Interest on the efficient energy storage system is also growing looking at the practical applications. Though, several reviews are available on the synthesis and application of MOF and MOF derived materials, their applications for the electrochemical energy conversion and storage is totally a new field of research and developed recently. This review focuses on the systematic design of the materials from MOF and control over their inherent properties to enhance the electrochemical performances.

Effect of Intrinsic Properties of Anions on the Electrocatalytic Activity of NiCo2O4 and NiCo2O x S4-x Grown by Chemical Bath Deposition

Electrochemical water (H₂O) splitting is one of the most promising technologies for energy storage by hydrogen (H₂) generation but suffers from the requirement of high overpotential in the anodic half-reaction (oxygen evolution), which is a four-electron process. Though transition-metal oxides and oxysulfides are increasingly researched and used as oxygen evolution electrocatalysts, the bases of their differential activities are not properly understood. In this article, we have synthesized NiCo₂O₄ and NiCo₂O _x S_4-x by a chemical bath deposition technique, and the latter has shown better oxygen evolution performance, both in terms of stability and activity, under alkaline conditions. Comprehensive analysis through time-dependent cyclic voltammetry, microscopy, and elemental analysis reveal that the higher activity of NiCo₂O _x S_4-x may be attributed to the lower metal-sulfur bond energy that facilitates the activation process to form the active metal hydroxide/oxyhydroxide species, higher electrochemically active surface area, higher pore diameter and rugged morphology that prevents corrosion. This work provides significant insights on the advantages of sulfur-containing materials as electrochemical precatalysts over their oxide counterparts for oxygen evolution reaction.

Solar Photochemical Reduction and Oxidation of Water and Related Aspects

Conversion of solar energy to useful chemicals has become necessary for finding solutions to energy and environmental issues. One of the means is to use of solar energy for the reduction of water to generate hydrogen or for the reduction of CO[Formula: see text] to useful chemicals. In spite of substantial effort, the discovery of stable and efficient photocatalysts remains a challenge, although some encouraging results have been reported. In this article, we provide a brief perspective of the current status of solar water splitting and reduction of CO[Formula: see text].

Dibenzothiophene-S,S-Dioxide-Based Conjugated Polymers: Highly Efficient Photocatalyts for Hydrogen Production from Water under Visible Light

Three dibenzothiophene-S,S-dioxide-based alternating copolymers were synthesized by facile Suzuki polymerization for visible light-responsive hydrogen production from water (> 420 nm). Without addition of any cocatalyst, FluPh2-SO showed a photocatalytic efficiency of 3.48 mmol h^-1 g-¹ , while a larger hydrogen evolution rate (HER) of 4.74 mmol h^-1 g^-1 was achieved for Py-SO, which was ascribed to the improved coplanarity of the polymer that facilitated both intermolecular packing and charge transport. To minimize the possible steric hindrance of FluPh2-SO by replacing 9,9'-diphenylfluorene with fluorene, Flu-SO exhibited a more red-shifted absorption than FluPh2-SO and yielded the highest HER of 5.04 mmol h^-1 g^-1 . This work highlights the potential of dibenzothiophene-S,S-dioxide as a versatile building block and the rational design strategy for achieving high photocatalytic efficiency.

Visible-Light-Driven Photocatalytic Activity of SnO2-ZnO Quantum Dots Anchored on g-C3N4 Nanosheets for Photocatalytic Pollutant Degradation and H2 Production

A zero-dimensional/two-dimensional heterostructure consists of binary SnO₂-ZnO quantum dots (QDs) deposited on the surface of graphitic carbon nitride (g-C₃N₄) nanosheets. The so-called SnO₂-ZnO QDs/g-C₃N₄ hybrid was successfully synthesized via an in situ co-pyrolysis approach to achieve efficient photoactivity for the degradation of pollutants and production of hydrogen (H₂) under visible-light irradiation. High-resolution transmission electron microscopy images show the close contacts between SnO₂-ZnO QDs with the g-C₃N₄ in the ternary SnO₂-ZnO QDs/g-C₃N₄ hybrid. The optimized hybrid shows excellent photocatalytic efficiency, achieving 99% rhodamine B dye degradation in 60 min under visible-light irradiation. The enriched charge-carrier separation and transportation in the SnO₂-ZnO QDs/g-C₃N₄ hybrid was determined based on electrochemical impedance and photocurrent analyses. This remarkable photoactivity is ascribed to the "smart" heterostructure, which yields numerous benefits, such as visible-light-driven fast electron and hole transfer, due to the strong interaction between the SnO₂-ZnO QDs with the g-C₃N₄ matrix. In addition, the SnO₂-ZnO QDs/g-C₃N₄ hybrid demonstrated a high rate of hydrogen production (13 673.61 μmol g^-1), which is 1.06 and 2.27 times higher than that of the binary ZnO/g-C₃N₄ hybrid (12 785.54 μmol g^-1) and pristine g-C₃N₄ photocatalyst (6017.72 μmol g^-1). The synergistic effect of increased visible absorption and diminished recombination results in enhanced performance of the as-synthesized tin oxide- and zinc oxide-modified g-C₃N₄. We conclude that the present ternary SnO₂-ZnO QDs/g-C₃N₄ hybrid is a promising electrode material for H₂ production and photoelectrochemical cells.