Metal Oxide Heterostructures for Improving Gas Sensing Properties: A Review

Abstract

Metal oxide semiconductor gas sensors are widely used to detect toxic and inflammable gases in industrial production and daily life. The main research hotspot in this field is the synthesis of gas sensing materials. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can exhibit superior gas sensing performance in response and selectivity compared with single phase. This review focuses on mainly the synthesis methods and gas sensing mechanisms of metal oxide heterostructures. A significant number of heterostructures with different morphologies and shapes have been fabricated, which exhibit specific sensing performance toward a specific target gas. Among these synthesis methods, the hydrothermal method is noteworthy due to the fabrication of diverse structures, such as nanorod-like, nanoflower-like, and hollow sphere structures with enhanced sensing properties. In addition, it should be noted that the combination of different synthesis methods is also an efficient way to obtain metal oxide heterostructures with novel morphologies. Despite advanced methods in the metal oxide semiconductors and nanotechnology field, there are still some new issues which deserve further investigation, such as long-term chemical stability of sensing materials, reproducibility of the fabrication process, and selectivity toward homogeneous gases. Moreover, the gas sensing mechanism of metal oxide heterostructures is controversial. It should be clarified so as to further integrate laboratory theory research with practical exploitation.

Keywords: gas sensing mechanism; heterostructures; metal oxide semiconductor; sensing materials; synthesis methods.

Figure 1

Records of the number of metal-oxide-heterojunctions-related published papers (a) or metal-oxide-heterostructures-related published papers (b) and the number of their respective times cited (c,d) since the year 2000. The search string: TITLE-ABS-KEY (metal and oxide and heterojunctions/heterostructures and gas and sensors) (internet search of Web of Science on 28 June 2019).

Figure 2

Formation of core–shell structures of charge carriers in (a) n-type and (b) p-type oxide semiconductors. Reproduced with permission from Ref. [25].

Figure 3

Microstructure of ZnO@ZIF-8 core–shell nanorod film: (a) SEM image; (b,c) TEM images; (d) HAADF-STEM image; (e,f) EDS element scanning of area-1 and area-2 in image (d); (g) cross-section image and EDXS mappings (reprinted with permission from Ref. [63]).

Figure 4

FESEM and TEM images of (a–d) pristine SnO₂, (e–h) 20 mol%In₂O₃-SnO₂ nanocomposite and (i–l) 20 mol%In₂O₃-Sn_0.92In_0.08O₂ nanocomposite (reprinted with permission from Ref. [66]).

Figure 5

(a) FESEM image of pure ZnO nanoflowers; (b) FESEM image of NiO-decorated ZnO nanostructures (reprinted with permission from Ref. [68]).

Figure 6

Field emission scanning electron microscopy (FESEM) images of (a) TiO₂-SnO₂ core–shell nanofibers (NFs), (b) SnO₂ NFs, and (c) TiO₂ core–shell nanofibers, transmission electron microscopy (TEM) images of (d) TiO₂-SnO₂ core–shell NFs, (e) SnO₂ NFs, and (f) TiO₂ core–shell NFs (reprinted with permission from Ref. [76]).

Figure 7

(a,b) TEM image and corresponding HRTEM image of a single pure ZnO microflower; (c,d) TEM image and corresponding HRTEM image of NiO-decorated ZnO to form NiO-ZnO composite microflower; and (e–h) The corresponding EDS elemental mapping images. (Reprinted with permission from Ref. [89]).

Figure 8

(a,b) SEM micrographs of CuO particle-decorated SnO₂ nanowires deposited on Al₂O₃ substrate; (c) STEM image of SnO₂@CuO heterostructures (left) and EDS elemental maps (right) (reprinted with permission from Ref. [91]).

Figure 9

(a) A significant improvement in sensor response to H₂S is observed when forming p–n junctions. (b) Sensing mechanism of CuO sensitivity to H₂S is explained. At the interface between SnO₂ and CuO, a depletion zone forms in air. After H₂S joins in, p-CuO particles react with it and transforms to CuS, resulting in decreasing the depletion region. (Reprinted with permission from Ref. [92]).

Figure 10

The energy band diagram of the gas sensing mechanism for (a) W₁₈O₄₉ and SnO₂ materials and (b) the SnO₂/W₁₈O₄₉ nanostructures. Reprinted with permission from Ref. [95].

Figure 11

(a) TEM image of mesoporous NiO@CuO nanosheets; (b,c) heterojunction between NiO nanosheets and CuO nanoparticles at the interface; (d) sensing mechanism of NiO@CuO gas sensors exposed to air and NO₂. Reprinted with permission from Ref. [98].

Figure 12

(a–d) Gas sensing response of SnO₂-Cr₂O₃ heterostructure nanocomposite towards CO and H₂ via changing Cr₂O₃ content. Reprinted with permission from Ref. [114].