A rational design of hollow nanocages Ag@CuO-TiO2 for enhanced acetone sensing performance
Graphical abstract
Rational designed Ag@CuO-TiO2 multi-components hollow nanocages exhibited improved sensor performances for acetone.
Introduction
With the development of the economy, issues of environment and healthcare have attracted considerable attention. Volatile organic compounds (VOCs), one of the primary environmental pollutants, are harmful to the human health. Besides, there are few thousands kind of VOCs contained in exhaled breath, some of which can be used as important indicators for diagnosis of diseases [1,2]. For example, acetone is a volatile and potentially hazardous chemical reagent, extensively used in laboratory and chemical industry [3]. As reported, the exposure to some concentration of acetone atmosphere can anesthesia the central nervous system and cause a series of damages to human body. Likewise, inhaling some amount of acetone can cause respiratory tract infection, dizziness, loss of strength and some physical discomfort [2]. Furthermore, the flash point (−20 °C) and high effumability increase the probability of explosion and flash fire. Therefore, it is significantly important to detect the level of acetone in workplace to ensure the human safety and health. As we know, some diseases are connected with specific VOCs in exhaled breath. The detection of acetone can be used as a suitable diagnose means of the diabetes. Hence, making a reliable acetone gas sensor with high sensitivity is of great importance.
Semiconductor oxides sensors have attracted much attention for their low price and simple fabrication, which are considered as a convenient tool to detect the combustible and toxic gases [4]. Among the available oxide semiconductor sensors, TiO2 has attracted much research interest in different fields, such as lithium storage, catalysts and gas sensors [[9], [10], [11], [12]]. However, as an n-type semiconductor, TiO2 exhibits a large band gap energy, high working temperature and a relatively high resistance. The inherent limitation of TiO2 might lead to the potential practice in gas sensing.
A single nanomaterial can sometimes limit the various properties and affect the performance in application fields. Sensing performances can be enhanced by forming hierarchical structures, modification with noble metals and multicomponent heterostructures, demonstrated by numerous previous reports [[3], [4], [5], [6], [7], [8]]. For example, the CuO–ZnO hetero-junctions combined with reduced graphene oxide were designed, which exhibited a high response to acetone [3]. Multicomponent nanomaterials with different physical or chemical characteristics have tremendous potential applications due to the synergistic effect of various novel specialties.
TiO2 can be synthesized by introducing other metal oxide materials, such as, Co3O4 and ZnO, to improve the sensing performance [4,13]. CuO, a p-type metal oxide with narrow band gap of 1.7 eV, is believed to be a promising candidate sensor. Some literatures have reported the gas sensing characteristics of CuO [1].
In addition, hollow nanostructured semiconductor oxides with well-defined morphologies, size, and compositions have attracted much attention and extensive research because of the high specific surface area, interior voids, low density and good surface permeability. It can affect the adsorption, diffusion and interfacial reaction of gases on surface, which possesses a great impact on the gas sensing performance [[14], [15], [16]]. Moreover, the nanostructured gas sensors usually offer a high specific area and numerous reaction sites for the target gases [12].
Therefore, hollow multicomponent heterostructures have caused great interest for their unique and superior properties owing to the synergetic effects of both structures and composition nanomaterials. Presently, Au@SnO2/Fe2O3 core-shell nanoneedles were manifested a superior sensing material toward trimethylamine gas, which is ascribed to the Schottky contact and N-N heterojunction [14]. Zhou et al. reported the porous Cu2O/CuO cubes with enhanced gas sensing by calcination the precursor at different temperatures [17]. However, the attempt for preparing hollow structured multicomponent materials still remains a big challenge in terms of cost, efficiency, environmental and technical issue.
Inspired by these, we designed and fabricated hollow nanostructured TiO2 functionalized with Ag@CuO to improve the acetone sensing performances. The approach we proposed is a very simple, cost-effective hydrothermal route without tedious and multiple-synthesis processes. The preparation of Ag@Cu2O was accomplished with addition of Ag+ in the precursor of Cu2O, without involving the consecutive deposition of Ag on the surfaces of presynthesized Cu2O. Cu2O is the self-template that forms the hollow structures, as well as a scaffold and frame that maintains the original octahedral shape and prevents the final structure from collapsing. The subsequently deposition of TiO2 layer tightly anchored to Ag@Cu2O avoids the aggregation of p-type or n-type nanoparticles and ultimately forms p–p junctions or n–n junctions. The as-prepared hollow Ag@CuO-TiO2 nanocages were used as gas sensors to test acetone, ethanol, methanol and other gases, which showed a superior performance towards acetone at 200 °C. The remarkably enhanced gas responses can be mainly ascribed to the synergistic effects of functional factors, such as abundant p − n heterojunctions, specific hollow structure and catalytic effect of Ag metal.
Section snippets
Reagents
Copper sulfate pentahydrate (CuSO4·5H2O), potassium sodium tartrate, potassium hydroxide (KOH), silver nitrate (AgNO3) and glucose (C6H12O6) were purchased from Guoyao Chemical Reagent Company (Shanghai, China). Titanium fluoride (TiF4) was commercially available. All chemicals were used without further purification.
Preparation of hollow Ag@CuO-TiO2 nanocage
Preparation of Ag@CuO-TiO2 was modified according to the previous work [18]. 10 mg Ag@Cu2O octahedra were ultrasound dispersed into 25 ml H2O. Then, 0.4 ml of TiF4 aqueous solution
Results and discussion
Scheme 1 illustrates the synthesis process of Ag@CuO-TiO2 hollow nanocages. The layer of TiO2 was formed on the scaffold of Ag@Cu2O by the interfacial reaction between Cu2O and the precursor of TiF4 under the hydrothermal treatment. Solid Cu2O crystals were gradually etched by HF released from the hydrolysis of TiF4. After annealing process, uniform Ag@CuO-TiO2 hollow nanocages were obtained without the involving of additional templates.
Gas sensing properties
Working temperature is a crucial factor for semiconductor sensors. To determine the optimum operating temperature of sensors, we investigated the responses of Ag@CuO-TiO2 to 100 ppm acetone at the temperature from 175 °C to 300 °C. Seen in Fig. 5a, as the increase of working temperature, the response of Ag@CuO-TiO2 was raised and reached the highest response at 200 °C. When further increasing the temperature, the response decreased sharply. This phenomenon could be explained as follows: the
Conclusion
In summary, Ag@CuO-TiO2 hollow nanocages were fabricated using a facile hydrothermal method. The hollow nanocaged Ag@CuO-TiO2 composites consist of uniform mixed n-type TiO2 and p-type CuO coated with metal Ag nanoparticles. The well-designed sensor materials was used to test acetone and displayed an enhanced sensing performance compared to the previous reports and unmodified TiO2, indicating that the as-prepared sensor has the potential and ability to be a promising gas sensors. More
Author contributions
Guangxia Wang, Yongming Sui, and Peng Sun proposed the research direction and guided the project. Guangxia Wang, Ziyu Fu carried out the experiments and analyzed the experimental data. Tianshuang Wang and Weiwei Lei helped the manuscript language. Bo Zou provided the proof of the article. Guangxia Wang, Yongming Sui wrote the paper.
Notes
The authors declare no competing financial interests.
Acknowledgements
This work is supported by the National Science Foundation of China (Nos. 11774124 and 21725304); the Chang Jiang Scholars Program of China (No. T2016051); Technology Development Program of Jilin Province (20180101285JC); JLU Science and Technology Innovative Research Team (2017TD-01). This work is also supported by Graduate Innovation Fund of Jilin University.
Guangxia Wang She is currently working toward the PhD degree in the State Key Laboratory of Superhard Materials, College of Physics, Jilin University, China. Presently, she is interested in the synthesis and characterization of the semiconducting oxides materials with specific structure and relevant application.
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Guangxia Wang She is currently working toward the PhD degree in the State Key Laboratory of Superhard Materials, College of Physics, Jilin University, China. Presently, she is interested in the synthesis and characterization of the semiconducting oxides materials with specific structure and relevant application.
Ziyu Fu She is currently working toward the MS degree in the State Key Laboratory of Superhard Materials, College of Physics, Jilin University, China. Her research interests include the synthesis of functional materials and their applications in gas sensors.
Tianshuang Wang received his BS degree from the Electronics Science and Engineering department, Jilin University, China in 2015. Presently, he is a graduate student and interested in the synthesis and characterization of the semiconducting functional materials and gas sensors.
Weiwei Lei received his PhD degree in Condensed Matter Physics from Jilin University in 2010. From 2010 to 2011, he worked as a research fellow at Max Planck Institute of Colloids and Interfaces in Germany. Afterwards, he was awarded an Alfred Deakin Postdoctoral Research Fellowship and ARC Discovery Early Career award at the Institute for Frontier Materials (IFM), Deakin University, Australia. Now, he is a Senior Research Fellow leading Plasma technology group in IFM. His research includes the synthesis and functionalisation of two- and three-dimensional nanomaterials and their applications in energy conversion and storage, and water purification.
Peng Sun received his PhD degree from the Electronics Science and Engineering department, Jilin University, China in 2014. Now, he is engaged in the synthesis and characterization of the semiconducting functional materials and gas sensors.
Yongming Sui majored in condensed matter physics, and received his PhD degree from State Key Laboratory of Superhard Materials, Jilin University, China in 2010. He was appointed the lecturer in State Key Laboratory of Superhard Materials, Jilin University in July 2010. Now, he is engaged in the synthesis, characterization, and the potential application of the semiconducting functional materials, and nanocomposites.
Bo Zou is a Professor of State Key Laboratory of Superhard Materials, Jilin University, China. He received his PhD degree (2002) in the field of polymer chemistry and physics from Jilin University. Presently, His scientific interests focus on high-pressure chemistry and functional semiconductor nanocrystals.