Full Length ArticleMetal organic frameworks-derived sensing material of SnO2/NiO composites for detection of triethylamine
Graphical abstract
SnO2/NiO composite was prepared by hydrothermal followed by calcination using metal-organic framework (MOF) as sacrificial templates. The composite still maintains the morphology of precursor. The SnO2/NiO composite with molar ratio of 9:1 reaches the best response to 10 ppm triethylamine at 70 °C and shows good selectively. Such outstanding performance is attributed to porous structure and formation of p-n heterojunctions at interface of composite.
Introduction
Recently, metal-organic frameworks (MOFs) consisting of networks formed by metal ions connected by multidentate organic groups acting as ligands [[1], [2]], have been widespread attention due to their regular structure, controllable aperture size, the multivariate material centers and diversity ligands [[3], [4]]. Hence MOFs show wide applications in storage and separation of fuel gases [[5], [6]], CO2 sequestration [7], catalysis [8] and light harvesting [9]. Most of MOFs are focused on transition metal elements as the metal center, such as Zn, Co, Ti and Ni [[10], [11], [12], [13]]. Particully, the MOFs prepared with Sn as the metal center was rarely reported. For example, Zhou et al. prepared SnO2-based composites using MOFs as precursors and showed good electrochemical properties [14]. Moreover, MOFs have been demonstrated as a promising precursor to synthesize porous complex metal oxide nanostructures [15], and the resulting metal oxide have some advantages than that from other methods, such as large surface area and high porosity because organic ligand can afford abundant pores to the sequent nanostructures after calcination [16]. Another advantage of MOFs is that they can simultaneously contain different species of metal irons with different ratios [17]. The metal oxides can be prepared by pyrolyzation of MOFs precursors with an appropriate calcination temperature in order to make the composites to remain the morphology of MOFs, because the metal ions can be highly dispersed and separated by organic ligand in the MOFs and inhibit the aggregation of metal oxide nanoparticles during the MOF decomposition process. Therefore, MOF-derived metal oxides show the potential sensing properties as sensing materials. Though a few kinds of MOF-derived composites of metal oxide have been synthesized [[14], [18]], to the best of our knowledge, there is little effort developing composites of the MOF as the gas sensing materials for detection of toxic gases.
Among many gas sensing materials, although SnO2 is an attractive candidate to detect toxic gases [19], developing the high-quality SnO2 sensors are hindered by the poor selectivity and high operating temperature that leads to high power consumption and low stability. Nowadays, vast quantities of efforts have been devoted to enhance the sensing performance of material, such as surface modification, element doping and incorporating another oxide to form composite [[20], [21]]. Especially, the p-n heterojunctions has been proved to be an effective way to improve gas sensing properties [22]. Compared with traditional n-type metal oxides, p-type metal oxide NiO has attracted growing interest in recent years because for the p-type oxides, amount of oxygen adsorption is known to be significantly higher than that of n-type oxide semiconductors. Zhang et al. [23] prepared NiO/SnO2 composites by hydrothermal method and reached response of 7–45 ppm NO2 at room temperature. Lv et al. [24] fabricated SnO2/NiO nanometer polycrystalline composite by a chemical co-precipitation method and obtained response of 2.6–0.06 ppm formaldehyde at 300° C. Ju et al. [25] reported NiO/SnO2 hollow sphere and the material showed a good response to 10 ppm TEA at 220 °C. Triethylamine (TEA), as a volatile organic compound (VOC), is released from wastewater, dead fish and marine products and TEA also causes health hazards, especially on respiratory, causing pulmonary edema and even death. However, the TEA sensors reported in literatures required a higher operating temperature [[25], [26], [27], [28], [29], [30], [31]]. Detection of TEA at low temperature are rarely reported [32]. For comparison, the sensing performances of SnO2 and SnO2/NiO composites synthesized in this work and other materials reported in literatures to TEA are listed in Table 1.
In this study, we selected the Sn/Ni-based metal-organic framework as the MOF precursor for preparing SnO2/NiO nanostructures. The layered structure Sn/Ni-based MOFs with the ligand of p-benzene-dicarboxylic acid (PTA) were synthesized by hydrothermal method, followed by a calcination procedure to get SnO2/NiO composites. The effect of the calcination temperature and composition of precursors on morphologies, structures and sensing properties of SnO2/NiO composites has been investigated. And the mechanism of hetrojunction for improving SnO2 gas sensing has also been discussed in details.
Section snippets
Synthesis of Sn/Ni-based MOF
All chemicals are of analytical grade, and were used without any further purification. Sn-based MOF and Sn/Ni-based MOFs were synthesized via a typical hydrothermal method: 0.8123 g SnCl2·6H2O (3.6 mmol) and 0.1163 g Ni(NO3)2·6H2O (0.4 mmol) and p-benzene-dicarboxylic acid (PTA)(4 mmol) were added into the mixed solvent of 60 ml distilled water and N,N-dimethylformamide (DMF) (the ratio of distilled water and DMF is 2:1) with vigorous stirring at room temperature for 30 min. The mixture was
Morphology and structure of SnO2/NiO composite
The as-synthesized product was examined by FT-IR spectrum to analyze the coordination mode of PTA to Sn2+ and Ni2+ for the MOFs. We recorded the FT-IR spectrum of PTA and Sn/Ni-based MOF for comparison, the results were shown in Fig. 2. The FT-IR spectrum of aromatic carboxylic acid has been studied extensively and every absorption peak was assigned to corresponding vibration. The FT-IR spectrum of PTA ligand was shown in Fig. 2(a), the peaks at 1690 and 1423 cm−1 were attributed to the υ(CO)
Conclusions
The SnO2/NiO composites were synthesized using the ligand of p-benzene-dicarboxylic acid and the metal center ions of Sn and Ni as sacrificial templates. An appropriate calcination temperature of 550° C is determined for synthesized SnO2/NiO composite that not only has regular structure and morphology, but also exhibits high response to TEA at low operating temperature. Particularly, the composite (9Sn1Ni) exhibits high response of 14.03, excellent selectivity and good stability at lower
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 51772015), Beijing Engineering Center for Hierarchical Catalysts and Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China.
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