Construction and enhanced gas sensing performances of CuO-modified α-Fe2O3 hybrid hollow spheres
Introduction
In recent years, three-dimensional (3D) advanced functional nanostructures controlled assembled by 0D nanoparticles [1], [2], 1D nanorods/nanowires [3], [4] or 2D nanoplates/sheets [5], [6] have received great research interest, due to their advanced geometric structure which can provide more chances for exploring novel properties and superior devices. For the purpose of gas sensors, the semiconductor metal oxide (SMO) hollow sphere (HS) nanostructures are drawing increasing research interest associated with encapsulation of large quantities of guest molecules or large-sized guests within the empty core domain [7], [8].
α-Fe2O3, a typical n-type transition SMO with a band gap of 2.1 eV, has been widely applied in water splitting, lithium-ion battery, gas sensor, catalyst, and solar energy conversion [9], [10], [11], [12]. Some efforts have made in enhancing response of α-Fe2O3 gas sensor. In particular, chemical surface modification is a valuable means to produce specific active sites. For instance, the sensing performance of α-Fe2O3 can be advantageously tailored by modifying with noble metal nanoparticles (e.g., Ag, Pt, Au), resulting in beneficial effects on sensitivity, response time, and operating temperature [13], [14], [15]. Compared with the noble metals, the modifiers of non noble metals present potential advantages in cost and preparation produce. Modifiers with opposite conductivity types can provide an impressing effect in improving the sensor parameters, which can form p–n heterojunctions on the interface between two types of SMOs [16]. It has indicated that CuO, a p-type SMO with a narrow band gap of 1.21 eV [17], is an effective modifier in SnO2 and ZnO sensors for CO [18], NO [19] and H2S [20], [21] detection.
Among various preparation methods for HSs, template methods can provide a controlled synthesis manner in morphology. Generally, polymer, silica, metal, metal oxide or carbon spheres are used as templates to synthesize various HS nanostructures. Some recent researches have demonstrated that using carbon spheres (CSs) as novel green templates to synthesize SMO HSs, including WO3, ZnO, SnO2, and CeO2, is a simple, friendly and efficient approach [8], [22], [23], [24]. Although single SMO HSs can be easily prepared by the chemical synthesis, the construction of hybrid HSs with two or more components as the shells remains a major challenge. Several 3D SMO hybrid HSs have been prepared for different applications. For example, Li and Zeng synthesized Sn-doped TiO2 HSs via an Ostwald ripening process [25]. Wang et al. prepared ZnO–SnO2 HSs for photocatalytic degradation of MO using a hydrothermal method [26]. Chen et al. have fabricated SnO2@TiO2 HSs via a SiO2 template approach for enhanced lithium storage properties [27]. Xuan et al. prepared Fe3O4/TiO2 hybrid HS photocatalysts through a PSA template method [28]. To the best of our knowledge, the well-defined CuO-modified α-Fe2O3 hybrid HSs for gas sensor have seldom been investigated. Herein, we develop the facile CS template strategy to one-pot construct CuO-modified α-Fe2O3 hybrid HS nanostructures. The gas sensing performance measurement shows that, compared with the single α-Fe2O3 HS sensor, the CuO-modified α-Fe2O3 HS sensor has a remarkably enhanced response to several volatile organic compounds (VOCs) including methanol, ethanol and acetone.
Section snippets
Sample preparation
Iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O), copper nitrate thrihydrate (Cu(NO3)2·3H2O), N,N-dimethylformamide (DMF), absolute ethanol and glucose were all of analytical grade. CS templates were first prepared in an improved procedure [8]. 80 mL of 0.5 mol/L glucose aqueous solution was sealed in a 100 mL capacity Teflon-lined stainless steel autoclave at 160 °C for 6 h. After reaction, the obtained precipitates were centrifuge-rinsed with distilled water and absolute ethanol for several times
Composition and morphology
The morphology and structure of the CuO-modified α-Fe2O3 hybrid are investigated by SEM micrograph. Fig. 2a indicates that the original morphology of the prepared CS templates is spherical with the diameter ranging from 0.3 μm to 0.5 μm. As shown in Fig. 2b, the as-obtained CuO-modified α-Fe2O3 hybrid consists of spherical microstructures with the diameter ranging from 88 nm to 176 nm, which is smaller than that of CSs. This implies that a large shrinkage has happened in the annealing process for
Conclusions
In summary, the well-defined 3D CuO-modified α-Fe2O3 hybrid HS nanostructures with thin shell have successfully been constructed via the one-pot CS template strategy. The α-Fe2O3 HSs can be also modified by other p-type SMOs such as Co3O4 and Mn3O4 using the facile CS template method. Several VOCs including methanol, ethanol and acetone are used as probe gases to investigate the gas sensing performance of the prepared products. The 3D hybrid HS sensors exhibit not only high response to ppm
Acknowledgments
This work was supported by National Natural Science Foundation of China (No. 21177066) and 111 Project (B12015).
Yanfei Kang is currently studying for her MS degree in chemistry in Nankai University with a research area of gas sensing materials.
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Yanfei Kang is currently studying for her MS degree in chemistry in Nankai University with a research area of gas sensing materials.
Liwei Wang is currently a candidate for PhD degree of chemistry at Nankai University. His research is focused on the development of gas sensing materials.
Yanshuang Wang is currently studying for her MS degree in chemistry in Nankai University with a research area of gas sensing materials.
Hongxin Zhang is currently studying for her MS degree in chemistry in Nankai University with a research area of gas sensing materials.
Yao Wang is currently studying for her bachelor degree in chemistry in Nankai University with a research area of materials chemistry.
Danting Hong is currently studying for her bachelor degree in chemistry in Nankai University with a research area of materials chemistry.
Yaqing Qv is currently studying for her bachelor degree in chemistry in Nankai University with a research area of materials chemistry.
Shurong Wang received her PhD in chemistry in 2007 and is currently associate professor in Department of Chemistry, Nankai University, focusing on catalysis and gas sensors.