There is a growing interest in the conversion of water and solar energy into clean and renewable H2 fuels using earth-abundant materials due to the depletion of fossil fuel and its serious environmental impact. This critical review highlights some key factors influencing the efficiency of heterogeneous semiconductors for solar water splitting (i.e. improved charge separation and transfer, promoted optical absorption, optimized band gap position, lowered cost and toxicity, and enhanced stability and water splitting kinetics). Moreover, different engineering strategies, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering of heterogeneous semiconductors are summarized and discussed thoroughly. The synergistic effects of the different engineering strategies, especially for the combination of co-catalyst loading and other strategies seem to be more promising for the development of highly efficient photocatalysts. A thorough understanding of electron and hole transfer thermodynamics and kinetics at the fundamental level is also important for elucidating the key efficiency-limiting step and designing highly efficient solar-to-fuel conversion systems. In this review, we provide not only a summary of the recent progress in the different engineering strategies of heterogeneous semiconductors for solar water splitting, but also some potential opportunities for designing and optimizing solar cells, photocatalysts for the reduction of CO2 and pollutant degradation, and electrocatalysts for water splitting.

Engineering heterogeneous semiconductors for solar water splitting

Direct Z-scheme photocatalysts: Principles, synthesis, and applications

In recent years, heterogeneous photocatalysis has received much research interest because of its powerful potential applications in tackling many important energy and environmental challenges at a global level in an economically sustainable manner. Due to their unique optical, electrical, and physicochemical properties, various 2D graphene nanosheets-supported semiconductor composite photocatalysts have been widely constructed and applied in different photocatalytic fields. In this review, fundamental mechanisms of heterogeneous photocatalysis, including thermodynamic and kinetics requirements, are first systematically summarized. Then, the photocatalysis-related properties of graphene and its derivatives, and design rules and synthesis methods of graphene-based composites are highlighted. Importantly, different design strategies, including doping and sensitization of semiconductors by graphene, improving electrical conductivity of graphene, increasing eloectrocatalytic active sites on graphene, strengthening interface coupling between semiconductors and graphene, fabricating micro/nano architectures, constructing multi-junction nanocomposites, enhancing photostability of semiconductors, and utilizing the synergistic effect of various modification strategies, are thoroughly summarized. The important applications including photocatalytic pollutant degradation, H2 production, and CO2 reduction are also addressed. Through reviewing the significant advances on this topic, it may provide new opportunities for designing highly efficient 2D graphene-based photocatalysts for various applications in photocatalysis and other fields, such as solar cells, thermal catalysis, separation, and purification.

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Graphene in Photocatalysis: A Review

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

Heterogeneous photocatalysis using semiconductors and renewable solar energy has been regarded as one of the most promising processes to alleviate, and even solve, both the world crises of energy supply and environmental pollution. In the past few years, many encouraging achievements have been made in the research area of graphene-based semiconductor photocatalysts. Among them, CdS/graphene nanocomposites have attracted extensive attention as an important kind of photocatalyst in chemical and material science, due to its superior photocatalytic activity and photostability under visible-light irradiation. The aim here is to address the enhancement mechanism of the photocatalytic performance of CdS/graphene composite photocatalysts, and systematically summarize recent progress regarding the design and synthesis of CdS/graphene nanocomposites. These nanocomposites are promising for a great diversity of applications in visible-light photocatalytic fields, including artificial photosynthetic systems (photocatalytic hydrogen production and CO2 reduction), environmental remediation, and organic photosynthesis. Special attention is given to the photocatalytic hydrogen production and pollutant photodegradation over CdS/graphene nanocomposite photocatalysts. Furthermore, perspectives on CdS/graphene-based materials are discussed, including the various remaining challenges for large-scale applications, identifying prospective areas for related research in this field.

CdS/Graphene Nanocomposite Photocatalysts

Natural photosynthesis is usually recognized as an efficient mechanism to achieve solar energy conversion. We construct a CdS/WO3 nanojunction achieving a Z-scheme for clean hydrogen fuel evolution by mimicking the natural photosynthesis. Although WO3 alone cannot be used for H2 evolution from water splitting, it can significantly increase the H2 evolution activity of CdS through a Z-scheme mechanism with lactate as electron donor. The CdS/WO3 photocatalyst has a high H2 evolution rate of 369 μmol g–1 h–1 at a CdS concentration of 20 wt %, which is 5 times as high as that of CdS with lactic acid as electron donor. For further improving the hydrogen production rate, we introduce the noble metal Pt to ameliorate the charge transport between CdS and WO3. Good H2 evolution rates up to 2900 μmol g–1 h–1 were obtained with WPC3, which is about 7.9 times the rate of WC20 with visible radiation. The interesting thing is that the photocatalytic mechanism of CdS/WO3 is different from the previously reported mechani...

Highly Efficient CdS/WO3 Photocatalysts: Z-Scheme Photocatalytic Mechanism for Their Enhanced Photocatalytic H2 Evolution under Visible Light

Cobalt hydroxide/cadmium sulfide composite was prepared using an easy coprecipitation strategy. The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) confirmed that Co(OH)2 nanometer particles were modified on CdS. Even without noble-metal cocatalyst, the photocatalytic H2 evolution over CdS after Co(OH)2 loaded was evidently increased. The most excellent Co(OH)2 of 6.8 mol %, resulted in a H2 generation rate of 61 μmol h–1 g–1, which exceeded that of pure CdS by a factor of 41 times. Surface photovoltage (SPV) and surface photocurrent (SPC) investigations revealed that the photogenerated electrons could be captured by the loaded Co(OH)2 nanoparticles. The interface formed between Co(OH)2 and CdS is vital to the enhancement of photocatalytic H2 generation. Electrochemical measurement results indicated that another reason for the enhanced photocatalytic activity of Co(OH)2/CdS catalyst is that Co(OH)2 has outstanding H2 generation activity.

Enhanced Photocatalytic H2 Generation on Cadmium Sulfide Nanorods with Cobalt Hydroxide as Cocatalyst and Insights into Their Photogenerated Charge Transfer Properties

Gold-modified ZnO nanocomposites for photo-Fenton-like catalysis of Escherichia coli disinfection

Pd Nanoparticle-Modified Tio2-X Composites as Efficient Visible-Light Active Photocatalysts for Tetracycline Degradation and Microbial Disinfection

In this study, Pd-ZnO nanocomposite was synthesized via the facile solvothermal method. Then, PdO-ZnO was obtained by continuing calcination of the synthesized Pd-ZnO. X-ray Diffraction (XRD), Transmission Electron Microscope (TEM), and X-ray Photoelectron Spectroscopy (XPS) tests showed that Pd was converted to PdO in the process of calcination. The photocatalytic sterilization experiment showed that in the 3 % Pd-ZnO system, 2 × 107 CFU/mL of E. coli was completely killed within 3 h, while 3 % PdO-ZnO took 1.5 h. In electron spin resonance (ESR) tests on PdO-ZnO, The ·OH and ·O2− signals can be detected under light and within 20 min after the end of light. 1 × 105 CFU/mL of E. coli were killed within 1 h by the optical “memory” effect of PdO-ZnO. PdO-ZnO inactivates E. coli by disrupting the membrane structure of bacteria as demonstrated by scanning electron microscopy (SEM) test. PdO-ZnO is non-toxic and can be demonstrated through cytotoxicity experiments on HTT2. 

Bing K. Liu

Papers

Highly Efficient CdS/WO3 Photocatalysts: Z-Scheme Photocatalytic Mechanism for Their Enhanced Photocatalytic H2 Evolution under Visible Light

Enhanced Photocatalytic H2 Generation on Cadmium Sulfide Nanorods with Cobalt Hydroxide as Cocatalyst and Insights into Their Photogenerated Charge Transfer Properties

Gold-modified ZnO nanocomposites for photo-Fenton-like catalysis of Escherichia coli disinfection

Pd Nanoparticle-Modified Tio2-X Composites as Efficient Visible-Light Active Photocatalysts for Tetracycline Degradation and Microbial Disinfection

“Memory catalysis”: Extending the photocatalytic antibacterial performance of a palladium-modified ZnO nanocomposite