Extraordinary Improvement of Gas-Sensing Performances in SnO 2 Nanofibers Due to Creation of Local p-n Heterojunctions by Loading Reduced Graphene Oxide Nanosheets Jae-Hyoung Lee, † Akash Katoch, † Sun-Woo Choi, † Jae-Hun Kim, † Hyoun Woo Kim,* ,‡ and Sang Sub Kim* ,† † Department of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea ‡ Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea * S Supporting Information ABSTRACT: We propose a novel approach to improve the gas-sensing properties of n-type nanofibers (NFs) that involves creation of local p-n heterojunctions with p-type reduced graphene oxide (RGO) nanosheets (NSs). This work investigates the sensing behaviors of n-SnO 2 NFs loaded with p-RGO NSs as a model system. n-SnO 2 NFs demonstrated greatly improved gas-sensing performances when loaded with an optimized amount of p-RGO NSs. Loading an optimized amount of RGOs resulted in a 20-fold higher sensor response than that of pristine SnO 2 NFs. The sensing mechanism of monolithic SnO 2 NFs is based on the joint effects of modulation of the potential barrier at nanograin boundaries and radial modulation of the electron-depletion layer. In addition to the sensing mechanisms described above, enhanced sensing was obtained for p-RGO NS-loaded SnO 2 NFs due to creation of local p-n heterojunctions, which not only provided a potential barrier, but also functioned as a local electron absorption reservoir. These mechanisms markedly increased the resistance of SnO 2 NFs, and were the origin of intensified resistance modulation during interaction of analyte gases with preadsorbed oxygen species or with the surfaces and grain boundaries of NFs. The approach used in this work can be used to fabricate sensitive gas sensors based on n-type NFs. KEYWORDS: sensing mechanism, SnO 2 nanofibers, reduced graphene oxide, electronic sensitization 1. INTRODUCTION One-dimensional (1D) semiconductor oxide structures have been intensively studied for gas sensing applications for several reasons. The 1D morphology of these structures is suitable for directional carrier transport. Second, the structures have size confinement along two coordinates and hence the smallest dimension, which is most efficient for translating gas recognition into an electrical signal. 1,2 Third, because of their high surface-to-volume ratio, nearly the entire bulk of these materials is readily affected by gas molecules. Fourth, these structures are relatively easy to integrate at low cost. Accordingly, tremendous efforts have been dedicated to synthesizing 1D structures and characterizing their gas sensing performances. Among a variety of 1D nanostructures, electrospun nano- fibers (NFs) have extraordinary sensing characteristics, which have been ascribed to efficient diffusion of gas molecules through a web-like structure of NFs. 3 In addition, the polycrystalline nature of electrospun NFs, which consist of many grains, provides potential barriers between grains within NFs in addition to those between individual NFs. 4 In particular, electrospun NFs are easily produced on a large scale due to the facile preparation process. 5 Tin dioxide (SnO 2 ) is a well-known n-type wide band gap semiconductor (E g = 3.6 eV, at 300 K). Due to its attractive characteristics such as nontoxicity, low-cost preparation, and simple fabrication, SnO 2 is regarded as one of the most promising sensing materials to detect a wide variety of pollutant gases. 6,7 Accordingly, SnO 2 electrospun NFs are promising candidates for gas sensors. To improve the sensing character- istics of SnO 2 NFs, functionalization techniques such as Al doping, 8 Pt doping, 9 CuO-SnO 2 composites, 10 In 2 O 3 -SnO 2 composites, 11 and loading of La 0.7 Sr 0.3 FeO 3 nanoparticles 12 have been investigated. Heterostructures based on semiconductors have been fabricated for use in advanced gas-sensor applications. Basu et al. reported that a Pd/ZnO interface provided a site for the adsorption of H 2 gas and the subsequent chemical reaction, which increased the current flow and thus the sensitivity of the sensor relative a sensor without a Pd/ZnO interface. 13 In a CuO/ZnO thin-film heterojunction, the generated interface states have been shown to affect current flow. 14 Ling et al. Received: October 17, 2014 Accepted: January 20, 2015 Published: January 20, 2015 Research Article www.acsami.org © 2015 American Chemical Society 3101 DOI: 10.1021/am5071656 ACS Appl. Mater. Interfaces 2015, 7, 3101-3109