In this Review, we aim to provide an updated summary of the research related to hollow micro- and nanostructures, covering both their synthesis and their applications. After a brief introduction to the definition and classification of the hollow micro-/nanostructures, we discuss various synthetic strategies that can be grouped into three major categories, including hard templating, soft templating, and self-templating synthesis. For both hard and soft templating strategies, we focus on how different types of templates are generated and then used for creating hollow structures. At the end of each section, the structural and morphological control over the product is discussed. For the self-templating strategy, we survey a number of unconventional synthetic methods, such as surface-protected etching, Ostwald ripening, the Kirkendall effect, and galvanic replacement. We then discuss the unique properties and niche applications of the hollow structures in diverse fields, including micro-/nanocontainers and rea...

Synthesis, Properties, and Applications of Hollow Micro-/Nanostructures

Metal oxide resistive-type nano-scale gas sensors have been investigated for their low cost, high sensitivity, and environmentally friendly fabrication. In these sensors, electrical resistance measurements are used to detect the presence of gas. In n-type metal oxides, resistance is increased by coverage of adsorbed oxygen and lowered by removal of adsorbed oxygen through reactions with reducing gasses. The sensitivity and selectivity of these sensors have been improved by incorporation of heterostructures. Heterostructures may improve sensor performance through facilitating catalytic activity, increasing adsorption, and creating a charge carrier depletion layer that produces a larger modulation in resistance. Synergistic effects in these gas sensors describe the improved sensor signal due to these combined effects which act to amplify the reception and transduction of the sensor signal. Receptive mechanisms may be improved by increasing adsorption and reactivity. Transduction mechanisms may be improved by restriction of the major charge conduction channels which helps to maximize resistance modulation. In this review, the synergistic effect achieved by combining these two mechanisms are examined. Fundamental properties of the metal oxide surface are used to provide insight for the large body of experimental evidence available for metal oxide resistive-type gas sensors. This review aims to connect experimental evidence to conceptual mechanistic descriptions by examining adsorption processes, charge transfer, reaction mechanisms, morphology, and ambient gas interactions.

Synergistic effects in gas sensing semiconducting oxide nano-heterostructures: A review

Investigations into the mechanisms governing the behavior of metal oxide gas sensors continue to be of great interest. Oxygen vacancies are a ubiquitous defect in this class of materials and their characteristics can be affected by synthesis, processing and operating parameters. The primary role of oxygen vacancies in modifying sensing performance cited in the gas sensing literature is based on a modulation of the amount of surface adsorbed oxygen or alternatively, the baseline resistance. Unfortunately, this generalized description does not provide a complete representation of the role of oxygen vacancies that would aid in more a fundamental understanding of their role in gas sensing. To this end, an attempt is made to distinguish between the role of surface and bulk oxygen vacancies where emphasis on proper characterization is first highlighted. The influence of surface oxygen vacancies on factors affecting adsorption, such as surface structure, are examined to gain understanding on improved sensing performance. The effect of bulk oxygen vacancy concentration and distribution on sensing are also discussed. Finally, the importance of these concepts within the context of doped and heterostructured gas sensors are then briefly discussed.

Role of Oxygen Vacancies in Nanostructured Metal-Oxide Gas Sensors: A Review

Resistance-based H2S gas sensors using metal oxide nanostructures: A review of recent advances.

The alarming rise of indoor pollution and the need to combat the associated negative effects have promoted increasing attention in modernizing the chemical sensing technologies by newly designed materials with rich and tunable functionalities at atomic or molecular levels. With the appealing physical, chemical, optical, and electronic properties for various potential applications, the state-of-art gas-sensing nanomaterials and their future perspectives are well-documented and summarized in this paper. Specifically, the key performance attributes are addressed in detail such as the sensitivity, selectivity, reversibility, operating temperature, response time, and detection limit. As such, this review provides both critical insights in exploring and understanding various gas sensing nanomaterials and points out limitations and opportunities for further developments, such as morphology control, doping and surface alteration, atomic-scale characterization, and applications in different fields. Finally, the challenges and outlooks are discussed on the basis of the current developments.

Functional gas sensing nanomaterials: A panoramic view

Designing composite nanomaterials is one of the most important key techniques for improving gas sensing performance. In this paper, a design of the ethanol sensor based on ZnO–SnO 2 heterostructure is reported. The hollow SnO 2 nanofibers are first synthesized by using the electrospinning method, and then the ZnO shell is subsequently grown on the fibers via the hydrothermal method. The ZnO–SnO 2 core–shell structure is confirmed by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), scanning electron microcopy (SEM), transmission electron microscopy (TEM) and elemental mapping analysis. The gas sensing behaviors of the fabricated sensors are systematically investigated. Under optimum operating temperature (200 °C) at 100 ppm ethanol, the response of ZnO–SnO 2 sensor is 392.29, which is 11 times larger than that of SnO 2 sensor (about 35.02). The response and recovery time of ZnO–SnO 2 sensor are 75 s and 12 s, while that of SnO 2 sensor are 86 s and 14 s, respectively. The results reveal that ZnO–SnO 2 core–shell structure enhances the sensing performance and shortens the response/recovery time, which is attributed to unique hollow structure, oxygen vacancies and n–n heterojunction. In addition, the energy band structure of ZnO–SnO 2 heterojunction and the ethanol sensing mechanism are analyzed.

Enhanced ethanol sensing performance of hollow ZnO–SnO2 core–shell nanofibers

SnO 2 –ZnO hetero-nanofibers were fabricated by a facile electrospinning method and calcination in this study. The SnO 2 –ZnO nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) and X-ray diffraction (XRD). The gas sensor prepared by the SnO 2 –ZnO hetero-nanofibers revealed better ethanol sensing performance than pure ZnO and pure SnO 2 nanofibers, good stability and excellent selectivity at the optimum temperature of 300 °C. The response and recovery time to 100 ppm ethanol were about 25 s and 9 s, respectively. The growth mechanism of the hetero-nanofibers was discussed, as well as the ethanol adsorption–desorption mechanism.

S.H. Yan

Papers

Enhanced ethanol sensing performance of hollow ZnO–SnO2 core–shell nanofibers

Synthesis of SnO2–ZnO heterostructured nanofibers for enhanced ethanol gas-sensing performance

Synthesis of hollow SnO2 nanobelts and their application in acetone sensor

Hierarchical heterostructure of porous NiO nanosheets on flower-like ZnO assembled by hexagonal nanorods for high-performance gas sensor

Improvement of acetone gas sensing performance of ZnO nanoparticles