One-step fabrication of b-Ga 2 O 3 –amorphous-SnO 2 core–shell microribbons and their thermally switchable humidity sensing properties† Kewei Liu, * Makoto Sakurai * and Masakazu Aono * Received 27th February 2012, Accepted 24th April 2012 DOI: 10.1039/c2jm32230g We reported the fabrication of a highly sensitive, fast, and thermally switchable humidity sensor based on a b-Ga 2 O 3 –amorphous-SnO 2 core–shell microribbon, which was synthesized via a simple one-step chemical vapour deposition. The as-grown microribbons were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) and the results indicated that the microribbon has a well-defined core–shell structure with b-Ga 2 O 3 in the core and amorphous SnO 2 in the shell. At 25  C, the conductivity of the humidity sensor at 75% relative humidity (RH) was three orders of magnitude larger than that in dry air (5% RH). The response time and recovery time were 28 and 7 s, respectively, when RH was switched between 5 and 75%. Interestingly, by changing the temperature between 12 and 40  C at 75% RH, the sensitivity can be tuned between 10 5 (12  C) and 10 2 (40  C). Typical thermally switchable properties of b-Ga 2 O 3 –amorphous-SnO 2 core–shell microribbons at 75% RH were demonstrated using a heating–cooling cycle between 20 and 30  C. The possible mechanisms have been proposed based on the novel core–shell structures and water adsorption–desorption processes. Our findings pave the way for new types of humidity sensors and thermal switches. 1. Introduction Because of their special structural, physical and chemical char- acteristics, core–shell coaxial structured materials have been widely used in various fields. 1–4 In particular, these core–shell structures exhibit enhanced gas sensing properties with high sensitivity, selectivity and dynamic repeatability. 5–9 These performance enhancements are usually attributed to the heter- ojunction at the core–shell interface. 6–9 Recently, sensors that can respond to humidity and temperature have aroused great interest owing to their potential application to industrial processes, as well as in the fields of biomedicine and human comfort. 10–14 Different oxide materials have been investigated, such as ZnO, SnO 2 , In 2 O 3 , and their compounds with a variety of shapes including wires, tubes, belts, and films. However, very little information on semiconductor humidity sensors based on core– shell structures can be found. 15 In this work, we fabricated b-Ga 2 O 3 –amorphous-SnO 2 core– shell microribbons using a simple one-step chemical vapour deposition (CVD). 16 SnO 2 is the most commonly used material for gas sensors and has a good sensitivity to relative humidity (RH) in air at low temperatures. 14,17–20 On the other hand, b- Ga 2 O 3 , owing to its good physical, chemical and thermally stable properties, has a wide range of applications, such as in photo- detectors and high-temperature gas sensors. 21–24 Therefore, their combination as a core–shell structure is expected to exhibit novel properties in the gas sensor field, such as tunable sensitivity and selectivity for different gases, and a wide operating temperature range. Moreover, the difference in work function between Ga 2 O 3 and SnO 2 can induce the depletion region in SnO 2 , resulting in a decrease in background signal intensity and the increase in response speed, which play an important role in detecting reducing gas. b-Ga 2 O 3 –SnO 2 core–shell microribbons fabricated by one-step CVD have the following advantages over those fabricated by the two-step method, in which the core and shell grow independently. 25–27 The one-step method saves time and cost in the fabrication process. More interestingly, the SnO 2 shell in the present system has an amorphous structure and a large protrusion. Because these properties produce large active chan- nels and high surface areas, which are required for gas sensors, 28 the humidity sensor based on the Ga 2 O 3 –SnO 2 core–shell structure showed good humidity-sensing performance at room temperature. The mechanism of the RH- and temperature- dependent sensitivity is discussed in detail. 2. Experimental Synthesis of b-Ga 2 O 3 –amorphous-SnO 2 core–shell microribbons The b-Ga 2 O 3 –amorphous-SnO 2 core–shell microribbons used in this study were synthesized inside a ceramic boat in a horizontal International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan. E-mail: Liukewei2007@yahoo.com.cn; Liu.Kewei@nims.go.jp; Sakurai.Makoto@nims.go.jp; Aono.Masakazu@nims.go.jp † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2jm32230g 12882 | J. Mater. Chem., 2012, 22, 12882–12887 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 12882 www.rsc.org/materials PAPER