Elsevier

Applied Surface Science

Volume 578, 15 March 2022, 151983
Applied Surface Science

Full Length Article
Optimal Pt mesoporous layer modified nanocomposite film with highly sensitive detection of ethanol at low temperature

https://doi.org/10.1016/j.apsusc.2021.151983Get rights and content

Highlights

  • A novel strategy to construct Pt mesoporous layer by complete MEMS process was proposed.

  • The formation and optimization of Pt mesoporous layer was controlled by the sputtering time of Pt and the annealing temperature, respectively.

  • The Pt mesoporous modified multi-layer TiO2/SnO2 nanocomposite thin film had the response of 3.32 to 0.1 ppm of ethanol at the low working temperature of 130 ℃.

Abstract

Developing highly sensitive and selective gas sensing MOS materials with low working temperature is urgent to fabricate high-performance ethanol gas sensors. Herein, we report a Pt mesoporous layer modified multi-layer TiO2/SnO2 nanocomposite thin film for improving sensitivity and selectivity of detecting ethanol at a low temperature. The Pt mesoporous layer is prepared by simple sputtering techniques, which are completely compatible with the MEMS process. The characterization results show the Pt sputtering time of 30 s and the annealing temperature of 500℃ are the opening condition and forming condition of Pt mesoporous layer, respectively, which has the most mesoporous of 5–50 nm. Because the Pt mesoporous layer has copious mesoporous and there is an abundant Schottky barrier between Pt and TiO2, the thin film (optimal parameters: 30 s, 500℃) based sensor has the highest sensitivity to ethanol at 130 ℃ and its response is as high as 3.32 to 0.1 ppm of ethanol, so the thin film is regarded as the optimal Pt mesoporous layer. Thus, the Pt mesoporous layer on the multi-layer TiO2/SnO2 nanocomposite thin film prepared by sputtering techniques is a simple but effective method to largely improve the gas sensing properties of the nanocomposite film.

Introduction

Volatile organic compounds (VOCs), such as ethanol, acetone, formaldehyde, and benzene, etc, are inimical to human health and the environment due to their mutagenicity, teratogenicity, and poisonousness [1]. Ethanol, as one kind of the common and extreme VOCs in life, is widely used in manufacturing [2], biomedical [3], and food industries [4]. However, some serious symptoms like narcosis and impaired perception will happen while a person gets in amounts of ethanol [5]. Thus, the detection of ethanol is of great significance for protecting human health. There was an abundant investigation about the ethanol gas sensors based on the metal oxide semiconductors (MOS) which are extensively used as the gas sensing materials for their stable, remarkable, and reversible surface resistance changes in the gas ambiance [6], [7], [8].

In recent three decades, research on gas sensing properties of various MOS (i.e., SnO2[9], [10], [11], TiO2[12], [13], [14], ZnO [15], Fe2O3[16], In2O3[17], [18], WO3[19], Co3O4[20], [21], V2O5[22] have been investigated and widely used to detect ethanol. Among them, SnO2 as the n-type MOS material with a wide bandgap of 3.6 eV, attracted much attention for its excellent chemical and electrical properties [23], [24], [25], [26]. To date, many studies have demonstrated good properties of SnO2 with various microstructures (i.e., flower-like, nanofibers, hollow nanoparticles) which can provide high surface areas. For example, Zeng et al. prepared the flower-like SnO2 nanostructures showing the response of about 17 to 400 ppm of ethanol at 350℃ [27]. Zhang et al. revealed that SnO2 nanofibers had a response of about 19 to 200 ppm of ethanol at 330℃ [28]. Zito et al. found that the hollow SnO2 particles can detect 100 ppm of ethanol with the response of 63.4 at 300℃ [29]. Although the response to ethanol can be improved by different microstructures of SnO2, there were still some problems, such as relatively low sensitivity and high working temperature.

Previous research has proven that some measures (i.e., using heterojunctions [30], metal surface modification [31], [32]) can effectively enhance the MOS gas sensing sensitivity and decrease the working temperature. The heterojunctions may improve gas sensing properties due to their synergistic effects in the composites [33], [34]. For example, Su et al. found the response of TiO2@SnO2 nanosphere to 10 ppm of formaldehyde reached 49.3, which is better than that of TiO2 nanosphere under the same testing condition [35]. Feng et al. revealed that the response of TiO2//SnO2 nanofibers to 10 ppm of ethanol exhibited about 7.5 at 368℃ [36]. Wang et al. prepared the TiO2-SnO2-TiO2 composite nanofibers showing the response of 13.3 to 1 ppm of acetone at 280℃ [37]. Therefore, the gas sensing properties of SnO2 can be enhanced by heterojunctions formed with TiO2 and greatly improved by multilayer heterojunctions of multi-layer TiO2/SnO2. However, the high working temperature is still a problem on many occasions. As another effective method, the metal surface modification can enormously enhance the sensing properties and decrease the working temperature because of their catalysis activity to gas and Schottky Barrier formed at the interface between metal and MOS. Huang et al. revealed that the optimal working temperature of the Pt modified SnO2 nanorod is lower than the pure SnO2 nanorods [38]. Mohammad et al. confirmed that Ag modified SnO2 based sensor attains the high response as well as the low working temperature of 180℃ compared to the pure SnO2 based sensor [32]. Therefore, the metal-modified heterojunctions are possible to enhance the gas sensing sensitivity at the low working temperature.

In this paper, we proposed a method to fabricate the high-performance MEMS gas sensor for detecting ethanol. To enhance the gas sensing sensitivity of MOS material and decreasing the working temperature by the above-mentioned measures (microstructures, heterojunctions, and metal modification), a Pt mesoporous layer modified multi-layer TiO2/SnO2 (Pt/TiO2/SnO2) nanocomposite thin film was prepared by magnetron sputtering technique. This Pt mesoporous layer activated multi-layer MOS materials are compatible with the MEMS process, and as far as this method has not been reported yet. The contribution of different Pt sputtering time and annealing temperature to the formation of Pt mesoporous layer was investigated, showing the Pt sputtering time of 30 s and the annealing temperature of 500℃ are the opening condition and forming condition of Pt mesoporous layer, respectively, which has the most mesoporous of 5–50 nm. The MEMS ethanol gas sensors with the Pt/TiO2/SnO2 nanocomposite thin film has the high sensitivity to ethanol at 130℃ and shows the response of 3.32 to 0.1 ppm of ethanol, which can be attributed to the plenty of Pt mesoporous and Schottky Barrier formed at the interface between Pt and TiO2. The Pt mesoporous layer and multilayer heterojunctions created entirely by sputtering techniques verify the effectiveness of this method for preparing enhanced MOS gas sensing materials for the gas sensors.

Section snippets

Preparation of Pt/TiO2/SnO2 nanocomposite thin film

Pt mesoporous layer modified TiO2/SnO2 nanocomposite thin film was completely prepared by a sputtering technique. The pure Pt (three-inch, 99.99%, Zhongnuo New Material Technology Co., Ltd.), SnO2(three-inch, 99.99%, Zhongnuo New Material Technology Co., Ltd.), and TiO2(three-inch, 99.99%, Zhongnuo New Material Technology Co., Ltd.) target were used to deposit Pt/TiO2/SnO2 nanocomposite thin film by magnetron sputtering in the order from Table 1. Five sets of parameters for the sputtering time

Structural and morphological characteristics

The microstructure and morphologies of the as-prepared samples S1, S2, S3, S4, S5 were investigated by FESEM observation. Fig. 3 shows the Pt layer with the different sputtering times of Pt at the same annealed temperature of 400℃ in N2. The FESEM images in (Fig. 3a-b) clearly show the smooth plane of sample S1 completely prepared by sputtering. The Pt particles turned into larger from sample S2 to sample S3. The Pt mesoporous layer is formed at sample S4 and disappears in sample S5, as shown

Conclusion

In summary, a complete MEMS process was developed to prepare the Pt mesoporous layer modified the multi-layer TiO2/SnO2 nanocomposite thin film as the gas sensing material for ethanol sensing, which can be integrated with the microheater and IDE. The morphology and microstructure of the Pt/TiO2/SnO2 nanocomposite thin film were characterized by various instruments, which demonstrates that the optimal preparing process of Pt mesoporous is 30 s sputtering time of Pt and annealing temperature of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work is supported by Science and Technology Planning Project of Shenzhen (Grant No. JSGG20191129102808151), NSAF (Grant No. U1930205), Key Research and Development Projects of Shaanxi Province (Grant No. 2019GY-121, 2020GY-137), Fundamental Research Funds for the Central Universities (Grant No. xzd012019020). We appreciate the support from International Joint Laboratory for Micro/Nano Manufacturing and Measurement Technologies

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