SnO2 core-shell hollow microspheres co-modification with Au and NiO nanoparticles for acetone gas sensing
Graphical abstract
Introduction
Detection of gas types and concentrations is of great significance in environment, fire protection, medical and other field. Gas sensors are devices used to detect the target gas in environment. According to the working principle, the gas sensors are divided into electrochemical sensors [1], quartz microbalance sensors [2], metal oxide sensors [3], and so on. Gas sensors based on metal oxides have been researched and widely used for the detection of volatile organic compounds (VOCs), flammable and explosive gases, and hazardous gases because of their advantages such as high response, easy manufacturing, and low cost [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. As a typical wide band semiconductor, tin oxide (SnO2) has been considered as one of the primary sensing materials to detect ethanol, acetone, n-butanol, and formaldehyde [[20], [21], [22], [23]]. However, the current drawbacks of pure SnO2 gas sensors are low response, slow response-recovery speed, and poor selectivity [[24], [25], [26]].
In order to improve the gas sensing properties of pure SnO2, several microstructures have been investigated including nanowires, nanosheets, and nanofibers, [[27], [28], [29], [30], [31]]. Recently, there has been great interest in metal oxides with a core-shell microstructure and they have been widely studied in various areas such as PANI @ TiO2 microspheres, CuO @ carbon nanofibers, Zn2SiO4@SiO2 nanoparticles, Ag@Co@TiO2 nanoparticles, and CdS@Cr2O3 nanorod [[32], [33], [34], [35], [36]]. It is well known that hollow structures provide a larger specific surface area than solid spheres [37,38], and thus, they are accompanied by more active sites for advanced gas sensing. Hence, the gas sensing of SnO2 based materials may be improved by the core-shell hollow microstructure design. However, the gas sensing of SnO2 core-shell hollow structures has rarely been reported. In addition to the novel structure of the design, establishment of the heterostructure using two different types of metal oxides is also considered as an effective way to enhance the gas sensing properties. For example, Kwon and co-workers prepared NiO-SnO2 heterojunctions for NO2 and H2 gas sensors [39]. Lee's group reported Cr2O3-ZnCr2O4 heterostructures through a galvanic replacement method for xylene gas sensing [40]. Yao et al. synthesized a metal-organic framework (MOF)-on-MOF heterostructure for enhanced gas sensing [41]. In a previous work, we also fabricated In2O3/ZnO and Zn2SnO4/ZnO heterostructures for n-butanol and triethylamine gas sensors. The heterointerface between different materials provide channels for electron transformation, which is helpful for the oxygen adsorption and sensing reaction. Furthermore, surface modification using precious metal (such as Pt, Au, Pd, and Ag) is another effective method that is used to improve the gas sensing properties of metal oxide gas sensors. Tofighi and co-workers prepared Au, Pd, and AuPd modified SnO2 nanoparticles for advanced gas sensing [15]. Somacescu et al. reported Pd modified Zn doped SnO2 nanoparticles for NO2 detection [42]. Hung's group fabricated SnO2 nanowires modified with Pd nanoparticles for a H2 gas sensor [43]. Au decorated porous ZnO microspheres and SnO2 hierarchical structures were also reported for ethanol and n-butanol sensors. The mechanism of precious metal modification has been ascribed to Fermi-level control or spill-over. All the previous examples illustrate that gas-sensing performance could be improved by designing particular structures, constructing heterostructures, and modifying surfaces. Unique core-shell structures provided much active site for metal oxides (MOS) to improve their gas sensing properties [37,38]. The p-n junctions between p and n types of semiconductors promoted the electron transfer for advanced gas sensing performance [39]. The catalysis of precious metal nanoparticles were considered of beneficial to enhance the gas sensing properties of MOS sensors [15,43]. However, the synergistic effect of microstructure, heterojunction, and precious metal modification has rarely been reported. Therefore, it is meaningful to develop a p-n heterostructures modified with precious metal nanoparticles for excellent gas sensing properties.
In this work, SnO2 core-shell hollow microspheres co-modified with Au and NiO nanoparticles (Au-NiO@SnO2) were prepared by a facile chemical precipitation method combined with a chemical reduction approach. The as-prepared Au-NiO@SnO2 core-shell hollow structures displayed improved gas sensing performance toward acetone in comparison with pure SnO2, NiO@SnO2, and Au@SnO2 samples. The mechanism for the enhanced gas sensing was proposed based on the analysis of the unique microstructure, the heterojunction between NiO and SnO2, and the catalytic action of Au nanoparticles.
Section snippets
Materials
SnCl4·5H2O, HAuCl4·4H2O, NaHB4, and L-lysine (C6H14N2O2) were purchased from Sinopharm Chemical Reagent Co., Ltd. NiC4H6O4·4H2O and C4H6N2 were supplied by Shanghai Macklin Biochemical Co., Ltd. Anhydrous methanol (CH3OH) was supplied by Chengdu Kelong Chemical Co., Ltd. All reagents were analytical grade and unaltered.
Synthesis of SnO2 core-shell microspheres
SnCl4 · 5H2O (12.4 mmol) and C4H6N2 (43.8 mmol) were dissolved in 200 mL CH3OH under magnetic stirring. After dissolution, the mixture was transferred into a water bath equipped
Characterization of Au-NiO-SnO2 core-shell microspheres
The crystal structure of as-prepared Au-NiO@SnO2 core-shell microspheres was investigated by XRD. As shown in Fig. 1, most of the diffraction peaks of the product were indexed well with tetragonal rutile SnO2 (JCPDS 41–1445). The diffraction peaks at 2 theta of 26.6°, 33.9°, 37.9°, 51.8°, 54.7°, 57.8°, 61.9°, 64.7°, 65.9°, 71.3°, 78.7°, 81.1°, and 83.7° were ascribed to the (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), (321), (400), and (222) crystal planes of SnO2,
Conclusion
Au-NiO@SnO2 core-shell hollow microspheres were successfully synthesized using a facile chemical precipitation method combined with a chemical reduction approach. The core-shell microstructures were verified through SEM and TEM. The formation of p-n heterojunctions and AuNPs enabled the Au-NiO-SnO2 core-shell microspheres to exhibit a remarkable sensing performance for detecting acetone. The sensors based on Au-NiO-SnO2 introduced fast response-recovery time (4 s and 5 s, respectively),
Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgements
This work was supported by Distinguished Taishan Scholars in Climbing Plan (No. tspd20161006), Shandong Provincial Natural Science Foundation (No. ZR2019MEM049), the Natural Science Foundation of China (No.51772176, 51971121), and Shandong Province Key Laboratory of Mine Mechanical Engineering (2019KLMM101).
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