The unique anisotropy and electronic properties of 2D materials have sparked immense interest in their fundamental electrochemistry and wide spectrum of applications. Beginning with the prototype 2D material — graphene — studies into an extensive library of other ultrathin layered structures have gradually emerged. Among these are the transition metal dichalcogenides, layered double hydroxides, metal carbides and nitrides (MXenes) and the black phosphorus family of monoelemental compounds. In this Review, we discuss the similarities of these 2D materials and highlight differences in their electrochemical and electrocatalytic properties. Recent progress on 2D materials for energy-related electrocatalysis in industrially important reactions is presented. Together this shows that dimensionality and surface characteristics are both vital aspects to consider when designing and fabricating compounds to achieve desired properties in specific applications. Two-dimensional materials have been in the spotlight since the discovery of graphene, and over time an extensive library of other ultrathin layered structures have emerged. In this Review, Pumera and Chia gather and discuss the features of this class of materials and review their recent applications in electrocatalysis.

Characteristics and performance of two-dimensional materials for electrocatalysis

High-precision gas sensors operated at room temperature are attractive for various real-time gas monitoring applications, with advantages including low energy consumption, cost effectiveness and device miniaturization/flexibility. Studies on sensing materials, which play a key role in good gas sensing performance, are currently focused extensively on semiconducting metal oxide nanostructures (SMONs) used in the conventional resistance type gas sensors. This topical review highlights the designs and mechanisms of different SMONs with various patterns (e.g. nanoparticles, nanowires, nanosheets, nanorods, nanotubes, nanofilms, etc.) for gas sensors to detect various hazardous gases at room temperature. The key topics include (1) single phase SMONs including both n-type and p-type ones; (2) noble metal nanoparticle and metal ion modified SMONs; (3) composite oxides of SMONs; (4) composites of SMONs with carbon nanomaterials. Enhancement of the sensing performance of SMONs at room temperature can also be realized using a photo-activation effect such as ultraviolet light. SMON based mechanically flexible and wearable room temperature gas sensors are also discussed. Various mechanisms have been discussed for the enhanced sensing performance, which include redox reactions, heterojunction generation, formation of metal sulfides and the spillover effect. Finally, major challenges and prospects for the SMON based room temperature gas sensors are highlighted.

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Advances in designs and mechanisms of semiconducting metal oxide nanostructures for high-precision gas sensors operated at room temperature

The facet-engineered surface and interface design for photocatalytic materials has been proven as a versatile approach to enhance their photocatalytic performance. This review article encompasses some recent advances in the facet engineering that has been performed to control the surface of mono-component semiconductor systems and to design the surface and interface structures of multi-component heterostructures toward photocatalytic applications. The review begins with some key points which should receive attention in the facet engineering on photocatalytic materials. We then discuss the synthetic approaches to achieve the facet control associated with the surface and interface design. In the following section, the facet-engineered surface design on mono-component photocatalytic materials is introduced, which forms a basis for the discussion on more complex systems. Subsequently, we elucidate the facet-engineered surface and interface design of multi-component photocatalytic materials. Finally, the existing challenges and future prospects are discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201600216

Facet-Engineered Surface and Interface Design of Photocatalytic Materials

Utilization of diffusive solar energy through photocatalytic processes for environmental purification and fuel production has long been pursued. However, efficient capture of visible-near-infrared (NIR) photons, especially for those with wavelengths longer than 600 nm, is a demanding quest in photocatalysis owing to their relatively low energy. In recent years, benefiting from the advances in photoactive material design, photocatalytic reaction system optimization, and new emerging mechanisms for long-wavelength photon activation, increasing numbers of studies on the harnessing of visible-NIR light for solar-to-chemical energy conversion have been reported. Here, the aim is to comprehensively summarize the progress in this area. The main strategies of the long-wavelength visible-NIR photon capture and the explicitly engineered material systems, i.e., narrow optical gap, photosensitizers, upconversion, and photothermal materials, are elaborated. In addition, the advances in long-wavelength light-driven photo- and photothermal-catalytic environmental remediation and fuel production are discussed. It is anticipated that this review presents the forefront achievements in visible-NIR photon capture and at the same time promotes the development of novel visible-NIR photon harnessing catalysts toward efficient solar energy utilization.

Visible‐to‐NIR Photon Harvesting: Progressive Engineering of Catalysts for Solar‐Powered Environmental Purification and Fuel Production

Photocatalysis is a promising route to convert solar energy into chemical energy directly, providing an alternative solution to environment and natural resource problems. Theoretically, all photocatalytic reactions are driven by charge carriers whose behavior can be divided into charge generation, separation, migration and surface reactions. Efficiencies of charge utilization in every step determine the overall performance of photocatalysis. Yolk–shell (YS) structures can provide an ideal platform for the efficient utilization of charge carriers. Typically, a YS structure is constructed from a hollow shell and an inner core, which can enhance light scattering in the hollow space and provide a large surface to create sufficient active sites, both of which can significantly improve the efficacy of charge utilization. Additionally, many strategies can be adopted to modify the YS structure for further enhancement of charge behaviors in every step. Existing reviews about YS structures mainly concentrate on the universality of the application of YSs, while the strategies to improve photocatalytic performance based on YSs have not been elaborately illustrated. This review describes the classification, synthesis, formation mechanism of YS structures and the rational regulation of the behaviors of photogenerated charge carriers, aiming at their effective utilization based on YS structures in heterogeneous photocatalytic reactions.

Rational design of yolk–shell nanostructures for photocatalysis

We report a robust and highly active photocatalyst for the C3H6 evolution reaction that is constructed by surfactant-free growth of oxygen vacancy-rich MoO3−x ultrathin nanobelts. Under visible-light irradiation, the new catalyst can selectively (95% selectivity) dehydrate isopropyl alcohol into C3H6 with yields above 98%.

Direct growth of defect-rich MoO3−x ultrathin nanobelts for efficiently catalyzed conversion of isopropyl alcohol to propylene under visible light

Yolk–shell architectures have attracted extensive attention owing to their unique structure and infusive applications. MoS2 is regarded as one of the most promising catalytic materials for hydrogen evolution by the splitting of water. In this work, a simple self-template solvothermal approach is developed for the synthesis of novel MoS2 yolk–shell microspheres with a hierarchical porous structure by reacting MoO2 microspheres with L-cysteine. A dissolutionrecrystallization formation mechanism is proposed for the MoS2 yolk–shell microspheres. Owing to structural superiority, the new material architecture exhibits improved photoelectrochemical properties, including efficient hydrogen evolution reaction catalytic activities, a high photocurrent density, a small overpotential, and a low charge-transfer resistance.

MoS2 yolk–shell microspheres with a hierarchical porous structure for efficient hydrogen evolution

Molybdenum nitride (δ–MoN) is an important functional material due to its impressive catalytic, energy storage, and superconducting properties. However, the synthesis of δ–MoN usually requires extremely harsh conditions; thus, the insight into δ−MoN is far behind that of oxides and sulfides of molybdenum. Herein, we report that ultrathin δ−MoN nanosheets are prepared at 270 °C and 12 atm. WN, VN, and TiN nanosheets are also synthesized by this method. The δ−MoN nanosheets show strong surface plasmon resonance, high conductivity, excellent thermal and chemical stability as well as a high photothermal conversion efficiency of 61.1%. As a promising surface enhanced Raman scattering substrate, the δ−MoN nanosheets exhibit a 8.16 × 106 enhanced factor and a 10−10 level detection limit for polychlorophenol. Molybdenum nitride is promising for catalysis, energy storage and Raman scattering, but it is synthesized under harsh conditions. Here the authors synthesize highly crystalline molybdenum nitride nanosheets using a relatively mild, non-aqueous solvothermal approach that can be extended to other nitrides.

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Low temperature synthesis of plasmonic molybdenum nitride nanosheets for surface enhanced Raman scattering.

Cubic-perovskite (SrTiO3) has a great potential to be utilized in a wide range of applications. However, currently very little is known about the shape and facet effects of SrTiO3 on their physicochemical properties. This is largely owing to the difficulties in controlling the morphology of facets during synthesis. Herein, we describe a facile route for the facet- and size-controlled fabrication of singlecrystalline SrTiO3 triangular prisms with highly exposed {101} side faces and {111} end faces. Theoretical and experimental studies of their photocatalytic performance have shown that triangular prisms exhibit superior photocatalytic activities than nanocubes with exposed (001) faces for the degradation of organic contaminants, which may be primarily attributed to the much higher surface energy of {101} facets (2.97 J/m2) and {111} facets (2.80 J/m2) than of that of {001} facets (0.98 J/m2).

Synthesis and facet-dependent photocatalytic activity of strontium titanate polyhedron nanocrystals

Compared with noble metals, improving the sensitivity of semiconducting surface-enhanced Raman scattering (SERS) substrates is of great significance to their fundamental research and practical appl...

Yahui Li

Papers

Direct growth of defect-rich MoO3−x ultrathin nanobelts for efficiently catalyzed conversion of isopropyl alcohol to propylene under visible light

MoS2 yolk–shell microspheres with a hierarchical porous structure for efficient hydrogen evolution

Low temperature synthesis of plasmonic molybdenum nitride nanosheets for surface enhanced Raman scattering.

Synthesis and facet-dependent photocatalytic activity of strontium titanate polyhedron nanocrystals

Alternative to Noble Metal Substrates: Metallic and Plasmonic Ti3O5 Hierarchical Microspheres for Surface Enhanced Raman Spectroscopy.