Half-Pipe Palladium Nanotube-Based Hydrogen Sensor Using a Suspended Nanofiber Scaffold Minkyu Cho, Jianxiong Zhu, Hyeonggyun Kim, Kyungnam Kang, and Inkyu Park* Mechanical Engineering and KI for NanoCentry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea * S Supporting Information ABSTRACT: A half-pipe palladium nanotube network (H-PdNTN) structure was developed for high-performance hydrogen (H 2 ) sensor applications. To fabricate the sensor, suspended poly(vinyl alcohol) (PVA) nanofiber bundles were electrospun on a conductive substrate, followed by a palladium (Pd) deposition on top of the PVA nanofiber bundles. Then, Pd- deposited PVA nanofibers were transferred to a host substrate, and the PVA nanofiber templates were selectively removed. Various material analyses confirmed that the PVA nanofibers were successfully dissolved leaving a half-pipe-shaped Pd nanotube network. The fabricated Pd nanotube-based sensors were tested for H 2 responses with different gas concentrations. The 4 nm thick sensor showed the highest response (ΔR/R 0 ) to H 2 gas. Platinum (Pt) decoration of the sensor showed an improved response speed compared to that of the pristine sensor via the catalytic function of Pt. Additionally, the sensor exhibited good H 2 selectivity against other interfering gases. The H-PdNTN H 2 sensor provides a facile and cost- effective way to fabricate high-performance H 2 sensors. KEYWORDS: electrospinning, electrospun nanofiber, metal nanostructure, palladium nanotube, hydrogen sensor ■ INTRODUCTION The hydrogen (H 2 ) economy is rising as a new energy ecosystem for the future. To maintain a stable hydrogen ecosystem, every sector of the system, namely, the source, infrastructure, and fuel storage system, must be secure. The H 2 sensor plays a crucial role in the H 2 economy for early detection of H 2 leakage. There are many types of H 2 sensors: optical fiber type, 1-7 catalytic combustion type, 8-11 chemo- resistive type, 12-23 etc. Among various types of chemoresistive- type sensors, Pd-based H 2 sensors have been studied by many researchers because of their advantages, such as simplicity, low power consumption, and fast response. The principle of the Pd-based H 2 sensor is the resistance change by palladium hydride (PdH x ) formation, which is generated from the absorption, dissociation, and diffusion of H 2 into the Pd bulk. 24-26 To enhance the performance of Pd-based H 2 sensors, one of the strategies is to reduce the physical dimension of the sensor such as Pd nanowire for faster response. Various methods to fabricate Pd nanostructure-based H 2 sensors have been reported. A Pd nanowire-based H 2 sensor was demonstrated via an electrophoresis method in which a Pd electrolyte solution was drop-cast onto a poly(methyl methacrylate) channel, followed by applying a current across the channel to grow a Pd nanowire. 27 However, this method suffers from complicated steps for fabricating two electrodes and a 100 nm width channel. Yang suggested a method for growing Pd nanowires laterally using an electro- deposition method. 25,28 In this approach, the width of the nanowire is controlled by the lateral etching of the nickel film underneath the patterned photoresist. However, the wet etching requires stringent control of the etching time and the solution concentrations. To increase the surface area-to- volume ratio of the Pd nanostructure sensors even further, some researchers adopted template methods to realize three- dimensional (3D) Pd nanostructure-based H 2 sensors. Zeng et al. and Cherevko et al. demonstrated Pd nanowire sensors by depositing Pd on an anodized aluminum oxide that showed a good sensing performance. 29,30 Lim et al. demonstrated a method of forming tubular Pd nanostructures by liquid-phased reduction of Pd ions along a ZnO nanowire template with in situ dissolution of the ZnO nanowires. 31 While various methods to realize nanostructure-based H 2 sensors have been demonstrated, a facile method of fabricating a one-dimensional metal nanostructure has been introduced using electrospinning: Wu et al. demonstrated a method of fabricating transparent gold (Au) electrodes using electrospun nanofibers as templates. 32 Au nanotrough electrodes showed higher transparency and conductivity compared to other existing transparent electrodes. In this work, by adopting that approach, Pd was deposited onto poly(vinyl alcohol) (PVA) nanofibers, followed by a selective removal of the fibers to make half-pipe-shaped Pd nanotubes as a high-performance H 2 sensor. The sensor showed rapid responses to H 2 gas with a response (ΔR/R 0 ) of 2.1% and a response time (τ 0-80% ) of 11 s to 1.8% H 2 concentrations. The effects of thickness and temperature on the sensing performance were also investigated Received: November 15, 2018 Accepted: March 15, 2019 Published: March 15, 2019 Research Article www.acsami.org Cite This: ACS Appl. Mater. Interfaces 2019, 11, 13343-13349 © 2019 American Chemical Society 13343 DOI: 10.1021/acsami.8b19848 ACS Appl. Mater. Interfaces 2019, 11, 13343-13349 Downloaded by KOREA ADVANCED INST SCI AND TECHLGY at 01:13:31:797 on June 27, 2019 from https://pubs.acs.org/doi/10.1021/acsami.8b19848.