10×-Enhanced Heterogeneous Nanocatalysis on a Nanoporous Gold Disk Array with High-Density Hot Spots Md Masud Parvez Arnob, † Camille Artur, † Ibrahim Misbah, † Syed Mubeen, ‡ and Wei-Chuan Shih* ,†,§,∥,⊥ † Department of Electrical and Computer Engineering, § Department of Biomedical Engineering, ∥ Program of Materials Science and Engineering, and ⊥ Department of Chemistry, University of Houston, Houston, Texas 77204, United States ‡ Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, Iowa 52242, United States * S Supporting Information ABSTRACT: Certain noble metal nanostructures as hetero- geneous photocatalysts have drawn significant attention in the recent past because of their unique optical properties which lead to the excitation of localized surface plasmon resonance (LSPR). The LSPR concentrates electromagnetic fields to the surfaces and its relaxation processes can convert photon energy to energetic charge carriers or heat, which can be subsequently harvested to enhance surface catalysis. Here, we report the catalytic performance of a novel plasmonic nanostructure, disk-shaped nanoporous gold (NPG) nano- particles or simply NPG disks, using a well-tested reduction pathway of resazurin to resorufin. We show that the catalytic reaction rate of NPG disks is enhanced by 10-fold upon external light illumination because of the excitation of LSPR. The plasmon-enhanced catalytic reaction follows a linear-to-superlinear transition in the rate dependence on the input light power. In addition, the light input results in a room temperature reaction rate equivalent to that of an ambient temperature of 70 °C. Together, the results support that hot charge carriers play the dominant role in the enhancement. KEYWORDS: nanoporous gold in photocatalysis, LSPR, plasmon enhanced photocatalysis, hot electrons 1. INTRODUCTION Heterogeneous catalysis, where the catalyst and reactants are in different phases, plays a significant role in chemical conversion, energy production, and pollution mitigation. 1 Nowadays, many of the commercially important catalytic processes (oxidation of hydrocarbons, oxidation of CO, reduction of NO, etc.) are based on heterogeneous catalysis. 2,3 Traditional heterogeneous catalysts include the late transition metals, such as palladium, platinum, ruthenium, rhodium, and iridium, which are typically supported by a high surface area material (zeolites, alumina, carbon, etc.) to enhance efficiency (the support participates in the reaction) and/or reduce cost. 4−6 However, most of these schemes require high temperature and suffer from thermal instability and susceptibility to various “poisoning” issues. 6−8 Gold nanostructures of size less than 10 nm, when properly supported on a suitable metal oxide, have exhibited surprisingly high catalytic activity for CO oxidation at low temperatures. 9 Its catalytic property depends primarily on the size of the particles and the nature of the support. 10,11 The use of some supports, including α-Fe 2 O 3 and TiO 2 as well as CeO 2 , results in highly active catalysts, whereas others, such as Al 2 O 3 and SiO 2 , showed usually low or no activity. 7,12 To decouple from the dependence on the support, support- free Au catalysts have received growing attention. 13,14 An effective means to prepare support-free Au catalysts is by selective removal of Ag from an Au−Ag alloy with the product known as nanoporous gold (NPG) material. 15,16 NPG features a sponge-like porous morphology with the characteristic bicontinuous pore/ligament size in the nm length scale (typically on the order of 10 nm). As shown by several groups, NPG can be used for oxidation of CO, alcohols, and sugars with surprisingly high catalytic activity at low temper- atures. 6,16−18 The origin of the observed catalytic activity of NPG has been attributed to low-coordinated atoms residing in steps and kinks as active catalytic sites and the role played by residual Ag atoms. 16,19−21 However, the catalytic property of NPG under light illumination condition has not been elucidated. Recently, noble metal nanostructures as catalysts have attracted further attention because of localized surface plasmon resonances (LSPRs), where the alternating electromagnetic field of the incident light causes the free electron gas to oscillate. When the incident light frequency matches the LSPR frequency, optical extinction (absorption + scattering) reaches 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, 13499-13506 © 2019 American Chemical Society 13499 DOI: 10.1021/acsami.8b19914 ACS Appl. Mater. Interfaces 2019, 11, 13499−13506 Downloaded by UNIV OF HOUSTON MAIN at 12:07:31:740 on May 28, 2019 from https://pubs.acs.org/doi/10.1021/acsami.8b19914.