Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

Semiconductor photocatalysts have received much attention in recent years due to their great potentials for the development of renewable energy technologies, as well as for environmental protection and remediation. The effective harvesting of solar energy and suppression of charge carrier recombination are two key aspects in photocatalysis. The formation of heterostructured photocatalysts is a promising strategy to improve photocatalytic activity, which is superior to that of their single component photocatalysts. This Feature Article concisely summarizes and highlights the state-of-the-art progress of semiconductor/semiconductor heterostructured photocatalysts with diverse models, including type-I and type-II heterojunctions, Z-scheme system, p–n heterojunctions, and homojunction band alignments, which were explored for effective improvement of photocatalytic activity through increase of the visible-light absorption, promotion of separation, and transportation of the photoinduced charge carries, and enhancement of the photocatalytic stability.

State‐of‐the‐Art Progress in Diverse Heterostructured Photocatalysts toward Promoting Photocatalytic Performance

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

Compound Copper Chalcogenide Nanocrystals

The development of advanced energy storage devices is at the forefront of research geared towards a sustainable future. Nanostructured materials are advantageous in offering huge surface to volume ratios, favorable transport features, and attractive physicochemical properties. They have been extensively explored in various fields of energy storage and conversion. This review is focused largely on the recent progress in nanostructured Mo-based electrode materials including molybdenum oxides (MoOx, 2 ≤ x ≤ 3), dichalconides (MoX2, X = S, Se), and oxysalts for rechargeable lithium/sodium-ion batteries, Mg batteries, and supercapacitors. Mo-based compounds including MoO2, MoO3, MoO3−y (0 < y < 1), MMoxOy (M = Fe, Co, Ni, Ca, Mn, Zn, Mg, or Cd; x = 1, y = 4; x = 3, y = 8), MoS2, MoSe2, (MoO2)2P2O7, LiMoO2, Li2MoO3, etc. possess multiple valence states and exhibit rich chemistry. They are very attractive candidates for efficient electrochemical energy storage systems because of their unique physicochemical properties, such as conductivity, mechanical and thermal stability, and cyclability. In this review, we aim to provide a systematic summary of the synthesis, modification, and electrochemical performance of nanostructured Mo-based compounds, as well as their energy storage applications in lithium/sodium-ion batteries, Mg batteries, and pseudocapacitors. The relationship between nanoarchitectures and electrochemical performances as well as the related charge-storage mechanism is discussed. Moreover, remarks on the challenges and perspectives of Mo-containing compounds for further development in electrochemical energy storage applications are proposed. This review sheds light on the sustainable development of advanced rechargeable batteries and supercapacitors with nanostructured Mo-based electrode materials.

Nanostructured Mo-based electrode materials for electrochemical energy storage

Polyol synthesis of nanoparticles: status and options regarding metals, oxides, chalcogenides, and non-metal elements

Influence of morphologies and pseudocapacitive contributions for charge storage in V2O5 micro/nano-structures

Two kinds of Cu2O–ZnO nano-heteroarchitectures were fabricated through a hydrothermal method. By utilizing polyethyleneimine (PEI) as a binding agent, ZnO nanoparticles were uniformly self-assembled onto Cu2O micro structures. Photoluminescence (PL), field emission and photocatalytic activities of the composite samples were investigated and compared with those of the pure Cu2O samples. The results reveal that ZnO nanoparticles combined on Cu2O microstructures remarkably changed the PL signals, and significantly enhanced the field emission and photocatalytic activities. The novel PL properties, enhanced field emission and photocatalysis activities are attributed to the electron transition and the inhibition of photo-induced electron–hole pairs recombination, which stem from interfacial defect states and energy band differentials at the interface of Cu2O and ZnO. The surface nano-protrusions, large surface area and better dye adsorption induced by ZnO nanoparticles are responsible for the field emission and photocatalysis improvements as well.

Facile synthesis of p-type Cu2O/n-type ZnO nano-heterojunctions with novel photoluminescence properties, enhanced field emission and photocatalytic activities

A novel hierarchical three-dimensional (3D) Cu2S@ZnO heteroarchitecture was successfully synthesized by a facile three-step synthetic process. FESEM and TEM analyses revealed the formation of the hierarchical 3D structure of the as-prepared Cu2S@ZnO. The purity and crystalline phase of the individual component in the 3D Cu2S@ZnO were determined by a powder X-ray diffractogram. The formation of the hierarchical 3D structure of the as-prepared Cu2S@ZnO leads to multiple p–n junctions formed at p-Cu2S@n-ZnO interfaces, which promote the generation and separation of photoinduced electrons and holes and thus improves the photocatalytic efficiency tremendously. According to the data, after irradiation for 40 min, the remaining MB in solution is about 35% for Cu2S nanoflowers and 9% for Cu2S@ZnO binary nanoflowers. Also, the increases in the overall aspect ratio and the ZnO nanoparticles act as extra emission sites, which help to enhance the field emission properties. The turn-on fields and threshold fields for the Cu2S nanoflowers as well as the Cu2S@ZnO nanoflower hybrid emitters are 3.3 and 9.2 V μm−1, 2 and 6.3 V μm−1, respectively. Compared with the Cu2S nanoflowers, the Cu2S@ZnO hetero-nanoflowers show excellent improvements in both photocatalytic and field emission applications.

Cu2S@ZnO hetero-nanostructures: facile synthesis, morphology-evolution and enhanced photocatalysis and field emission properties

A polyol method was first employed to synthesize MoS2@MWNT nano-flower composites. Due to their high conductivity, large aspect ratio and sharp edges, an excellent field emission performance was obtained relative to pure MoS2 with the turn-on field decreasing from 5.3 V μm−1 to 2.7 V μm−1 and the threshold field decreasing from 8.0 V μm−1 to 5.3 V μm−1. When used as anode materials for Li-ion batteries in an aqueous electrolyte, the composites exhibit a highest specific capacity of 1217 mA h g−1 at a current of 100 mA g−1, excellent cycling stability and high-rate capability, and a high specific capacity of 851 mA h g−1, which was still retained at a high current density of 1600 mA g−1.

Shouchuan Li

Papers

Influence of morphologies and pseudocapacitive contributions for charge storage in V2O5 micro/nano-structures

Facile synthesis of p-type Cu2O/n-type ZnO nano-heterojunctions with novel photoluminescence properties, enhanced field emission and photocatalytic activities

Cu2S@ZnO hetero-nanostructures: facile synthesis, morphology-evolution and enhanced photocatalysis and field emission properties

Synthesis of a MoS2@MWNT nanostructure with enhanced field emission and electrochemical properties

Controlled synthesis of Cu2S microrings and their photocatalytic and field emission properties