Met. Mater. Int., Vol. 22, No. 4 (2016), pp. 730~736 doi: 10.1007/s12540-016-6028-3 Room-Temperature Hydrogen Gas Sensing Properties of the Networked Cr 2 O 3 -Functionalized Nb 2 O 5 Nanostructured Sensor Sunghoon Park 1 , Hyejoon Kheel 1 , Gun-Joo Sun 1 , Hyoun Woo Kim 2 , Taegyung Ko 1 , and Chongmu Lee 1, * 1 Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea 2 Department of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea (received date: 8 January 2016 / accepted date: 29 February 2016) Cr2O3-functionalized Nb2O5 nanoparticles were synthesized via a facile hydrothermal route. The multiple-net- worked Cr2O3-functionalized Nb2O5 nanostructured sensor showed enhanced H2 gas sensing performance compared to its pristine Nb2O5 nanostructure counterpart. The Cr2O3-functionalized Nb2O5 nanostructure sensor showed responses of 5.24 to 2 ppm of H2 at room temperature, whereas the pristine Nb2O5 nanoparticle sensors showed responses of 2.29. The former also exhibited a faster response to H2. The multiple-networked pristine and Cr2O3-functionalized Nb2O5 nanostructured sensors were stronger and much shorter, respectively, than other nanomaterial-based Schottky diode-type sensors and Nb2O5-based Schottky diode-type sensors. The underly- ing mechanism for the enhanced sensing performance of the Cr2O3-functionalized Nb2O5 nanostructured sensor towards H2 gas is discussed in detail. Particular emphasis is placed on the role of the Cr2O3-Nb2O5 p-n junction in the Cr2O3-functionalized Nb2O5 nanostructure sensor. Keywords: sensors, nanostructured materials, sol-gel, electrical properties, X-ray diffraction 1. INTRODUCTION Over the past several decades, oxide semiconductor-based gas sensors have been studied extensively because of their high sensitivity, but the high operating temperature is one of the limitations of oxide semiconductor-based gas sensors. Several techniques, including metal catalyst doping [1-3], heterostructure formation [4-7] and UV light irradiation [8,9], have been stud- ied to enhance the sensing performance of nanostructured oxide semiconductor-based gas sensors at low temperatures. Of the above techniques, heterostructure formation techniques to enhance the sensing performance of MOS sensors have been studied intensively [10-12]. Heterostructures can be formed by the surface functionalization of metal oxide semiconductor nanostructures with oxide semiconductor nanoparticles [4,5] or metal oxide semiconductor core-shell nanostructures [6,7]. These heterostructures are commonly synthesized by a com- bination of complicated dry and wet techniques. Niobium pentoxide (Nb 2 O 5 ) is an oxide semiconductor with many attractive properties such as good chemical stability, high refractive index, and low film stress [13,14]. Owing to these properties, Nb 2 O 5 has many applications, such as solar cells, batteries, electrochromic coatings, biocompatible materi- als, catalysts, and gas sensors [15]. Nb 2 O 5 -based gas sensors, however, have attracted less attention than sensors based on other metal oxide semiconductors such as SnO 2 , ZnO 2 , In 2 O 3 , TiO 2 , and WO 3 . The successful development of a Schottky diode-type sensor based on Nb 2 O 5 [15], and Nb 2 O 5 -based gas sensors have been reported [16-19]. In 2008, Cvelbar et al. reported Nb 2 O 5 nanowire array sensors for the detection of oxygen gas [16]. In recent years, the strong responses of Nb 2 O 5 - based gas sensors, particularly those sensitive to hydrogen (H 2 ), have been reported. Regarding the detection of hydrogen gas, several groups fabricated Nb 2 O 5 –based Schottky diode sen- sors and examined their H 2 gas sensing performance, as listed in Table 1 [15,17,19-21]. Wang et al. obtained the response and response times of ~100% and 1.67 min, respectively, to 2,000 ppm H 2 gas at room temperature using Pt/Nb 2 O 5 Schottky diode sensors [17]. Rania et al. detected hydrogen gas using Pt/Nb/Nb 2 O 5 /Pt metal–semiconductor–metal structure sensors [18]. More recently, Park et al. reported the H 2 gas sensing performance of a multiple-networked Nb 2 O 5 /ZnO core-shell nanowire sensor at 300 °C [19]. Nevertheless, Nb 2 O 5 -based gas sensors that can be operated at room-temperature require further improvement in sensitivity for practical use. In this study, networked Cr 2 O 3 -functionalized Nb 2 O 5 nano- structures were fabricated using a facile and cost-effective wet *Corresponding author: cmlee@inha.ac.kr KIM and Springer