Gas-sensing behaviors of TiO2-layer-modified SnO2 quantum dots in self-heating mode and effects of the TiO2 layer
Introduction
Gas sensors are vital for the human safety. Currently, resistive sensors are highly popular since they are sensitive, have low price and a simple design and operation [[1], [2], [3], [4], [5]]. Nevertheless, they suffer from poor selectivity and high power consumption. To overcome such drawbacks, various strategies have been proposed [6,7]. One good solution is to employ quantum dots (QDs) as a sensing material [8]. A QD is a semiconductor with the size of the crystal being on the same order as the size of the exciton bohr radius. QDs are nanomaterials which exhibit peculiar electrical characteristics because of the ultra-fine grain sizes [9]. Another approach is morphology engineering, such as the use of core-shell (C–S) nanomaterials [10]. In C–S structures, a core is covered by a thin shell of another material, which maximizes the interfaces between the two materials and can elicit the appearance of new properties in the resultant structure [11].
SnO2 is highly popular for sensing studies because of its high carrier mobility, high physicochemical stability, low price, and intrinsically high response to a variety of gases [12]. Accordingly, it is widely used as a gas sensor in pristine [13] or C–S forms [14]. However, in its pristine form, the sensing temperature is often high, even for SnO2 QDs. For instance, Sedghi et al. [9] reported CO gas-sensing response for SnO2 QDs at 225 °C, which is high for real applications and consumes a considerable amount of energy. SnO2-based C–S nanostructures can perform at 25–80 °C [15]. However, at room temperature, water vapor may limit the sensitivity of gas sensors. A useful strategy to overcome this problem is to use gas sensors in self-heating mode [16], wherein a voltage is applied to the sensor and drifting electrons along the pathway make contact with other electrons and ions in the lattice structure, losing a portion of their kinetic energy to heat. This heat generated as a result of applied voltage can be used to heat gas sensors. This self-heating strategy not only minimizes the effects of water vapor in the environment but also significantly reduces power consumption relative to high-temperature gas sensors.
Among various metal oxide semiconductors, TiO2 is highly popular for gas sensing studies [17]. In particular, TiO2 is a low cost n-type semiconductor, possessing the advantages of being nontoxic, insoluble, biocompatible, and highly stable in different environments [18]. Furthermore, TiO2 has mainly three kinds of solid phases and thus phase transformation will induce significant modification of sensing properties, contributing to the development of high-performance sensors [17]. In regard to TiO2/SnO2 heterointerfaces, in the present work, TiO2 layer has been deposited on the clusters of SnO2 QDs and therefore the exposed TiO2 plays a major role of sensing behaviors. Owing to the work-function differences, TiO2 will be electron-depleted upon the contact with SnO2. The smaller electron concentration/conduction volume is regarded to be favorable for better sensing. In addition, various heterostructures consisting of TiO2 have been examined for their gas sensing properties [[19], [20], [21], [22]]. Up to the present, TiO2/SnO2 heterointerfaces have been employed for gas sensing, in a variety of structures, including SnO2-TiO2 composites [[23], [24], [25], [26], [27]], TiO2-SnO2 core-shell nanofibers [28], SnO2-coated TiO2 nanobelts [29], TiO2-doped SnO2 thick films [30], SnO2-on-TiO2 layered thin films [31], and SnO2 nanopaticles-decorated TiO2 nanofibers [18]. Thus, in this study, we prepared SnO2 QDs and then deposited a thin layer of TiO2. The atomic layer deposition (ALD) technique was utilized to precisely control the thickness of TiO2 by the number of ALD cycles. Similar to SnO2, TiO2 is an n-type (Eg =3.18 eV) semiconductor [32], with a charge mobility of 0.4 cm2/V s [33], that has been used as a sensing material [34,35]. TiO2 layers with thicknesses of 10, 30, or 60 nm were coated on the surface of SnO2 QDs for studies on gas sensors in self-heating mode. The response of gas sensors fabricated in the presence of either NO2 or CO gas considerably depends on the TiO2 thickness. Up to the present, TiO2 or TiO2-comprising heterostructures have been used for sensing a couple of gases, such as ozone [19], O2 [31], CO [20], C7H8 [21], ethanol [26,29,30], and H2 [25,36]. This paper not only provides the first report on NO2 sensing, but also compares the sensing behaviors of NO2 and CO gases. To the best of our knowledge, this is the first article that not only reports the gas sensing capabilities of TiO2-layer-modified SnO2 QDs but also examines the effects of TiO2 layer thickness on the QD sensors; we hope that this study leads to advances in similar systems in this field.
Section snippets
Synthesis of SnO2 QDs
Fig. 1 presents a schematic of the procedure for the preparation of TiO2-layer-modified SnO2 QDs. SnO2 nanocrystals were prepared by precipitating α-stannic acid gel using an aqueous ammonia solution and H2SnCl6 solution (SnCl4·5H2O, Sigma-Aldrich) as tin precursors. The ammonia solution was added to the H2SnCl6 solution drop wisely until reaching a pH of 6.5-7.0. Using a centrifuge, the as-obtained gel-like precipitate of α-stannic acid was separated and then washed several times with
Morphological and compositional characterization
Fig. 2a exhibits an FE-SEM micrograph of the electrodes, and Fig. 2b indicates an FE-SEM micrograph of synthesized SnO2 QDs on the electrodes. An FE-SEM micrograph of SnO2 QDs is presented in Fig. 2c, and SnO2-TiO2 C–S QDs, with different layer thicknesses, are displayed in Fig. 2d–f. Fig. 3a–c displays FE-SEM side-view images of the TiO2 films deposited by ALD. In fact, the thickness of TiO2 was controlled by the number of ALD cycles. With 500, 1000, and 2000 ALD cycles, the thickness of TiO2
Conclusions
In summary, we synthesized SnO2 QDs with subsequent depositions of TiO2 layers with different thicknesses using ALD for gas-sensing studies. Before sensing tests, the products were fully investigated by SEM, TEM, and EDAX, demonstrating that the desired composition and morphology were achieved. The gas-sensing tests were conducted under externally heated conditions as well as self-heating mode without an external heat source. Using applied voltages ranging from 1 to 20 V, gas-sensing tests were
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
CRediT authorship contribution statement
Jae-Hyoung Lee: Data curation, Investigation, Validation, Formal analysis, Writing - original draft. Ali Mirzaei: Methodology, Visualization, Writing - original draft, Writing - review & editing. Jae-Hun Kim: Investigation, Data curation, Validation. Jin-Young Kim: Investigation, Validation. Abulkosim F. Nasriddinov: Data curation, Investigation. Marina N. Rumyantseva: Writing - review & editing, Funding acquisition, Resources. Hyoun Woo Kim: Supervision, Project administration, Writing -
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03013422). This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2019R1A2C1006193).
Jae-Hyoung Lee received his BS degree from Inha University, Republic of Korea in 2014. In February 2016, he received MS degree from Inha University, Republic of Korea. He is now working as a Ph. D. candidate at Inha University, Republic of Korea. He has been working on oxide nanofiber gas sensors.
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Jae-Hyoung Lee received his BS degree from Inha University, Republic of Korea in 2014. In February 2016, he received MS degree from Inha University, Republic of Korea. He is now working as a Ph. D. candidate at Inha University, Republic of Korea. He has been working on oxide nanofiber gas sensors.
Ali Mirzaei received his Ph.D. degree in Materials Science and Engineering from Shiraz University in 2016. He was visiting student at Messina University, Italy in 2015. Since 2016 he is postdoctoral fellow at Hanyang University in Seoul. He is interested in the synthesis and characterization of nanocomposites for gas sensing applications.
Jae-Hun Kim received his B.S. degree from Gyeongsang National University, Republic of Korea in 2013. In February 2015, he received M.S. degree from Inha University, Republic of Korea. He is now working as a Ph.D. candidate at Inha University, Republic of Korea. He has been working on oxide nanowire gas sensors.
Jin-Young Kim received his B.S. degree from Inha University, Republic of Korea in 2017. He is now working as a M. S. degree at Inha University, Republic of Korea. He has been working on oxide nanowire gas sensors.
Abulkosim F. Nasriddinov is pursuing his Ph.D course under the guidance of Prof. Marina. N. Rumyantseva in Moscow State University. He is interested in the synthesis of nanostructural materials and modification for gas sensing applications.
Marina. N. Rumyantseva received her Ph.D. in chemistry in 1996 from Moscow State University (MSU) and Grenoble Polytechnic Institute, her doctoral degree in chemistry in 2009 from the Chemistry Department of MSU. Since 2013 she is full professor of MSU. She is heading the sensor group of the Laboratory of Semiconductor and Sensor Materials, Moscow State University. Her research activities are dealt with nanostructural materials synthesis and modification for gas sensing applications.
Hyoun Woo Kim joined the Division of Materials Science and Engineering at Hanyang University as a full professor in 2011. He received his B.S. and M. S. degreesfrom Seoul National University and his Ph.D. degree from Massachusetts Institute of Technology (MIT) in electronic materials in 1986, 1988, and 1994, respectively. He was a senior researcher in the Samsung Electronics Co., Ltd. from 1994 to 2000.He has been a professor of materials science and engineering at Inha Universityfrom 2000 to 2010. He was a visiting professor at the Department of Chemistryof the Michigan State University, in 2009. His research interests include the one-dimensional nanostructures, nanosheets, and gas sensors.
Sang Sub Kim joined the Department of Materials Science and Engineering, Inha University, in 2007 as a full professor. He received his B.S. degree from Seoul National University and his M.S and Ph.D. degrees from Pohang University of Science and Technology (POSTECH) in Material Science and Engineering in 1987, 1990, and 1994, respectively. He was a visiting researcher at the National Research in Inorganic Materials (currently NIMS), Japan for 2 years each in 1995 and in 2000. In 2006, he was a visiting professor at Department of Chemistry, University of Alberta, Canada. In 2010, he also served as a cooperative professor at Nagaoka University of Technology, Japan. His research interests include the synthesis and applications of nanomaterials such as nanowires and nanofibers, functional thin films, and surface and interfacial characterizations.
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Jae-Hyoung Lee and Ali Mirzaei had equal contributions as co-first authors.