CALLEJAS ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Electrocatalytic and Photocatalytic Hydrogen Production from Acidic and Neutral-pH Aqueous Solutions Using Iron Phosphide Nanoparticles Juan F. Callejas, †,§ Joshua M. McEnaney, †,§ Carlos G. Read, †,§ J. Chance Crompton, ‡ Adam J. Biacchi, † Eric J. Popczun, † Thomas R. Gordon, † Nathan S. Lewis, * ,‡ and Raymond E. Schaak * ,† † Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States and ‡ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States. § These authors contributed equally. P latinum is the most widely used ma- terial for the electrocatalytic and photocatalytic production of molecu- lar hydrogen (H 2 ) from water through the hydrogen-evolution reaction (HER). Al- though Pt is highly active and stable under the often harsh operational conditions used in electrolyzers and photoelectrochemical cells, 1 Pt is expensive and scarce. 2 Hence, several new Earth-abundant HER catalysts have emerged, including MoS 2 , 3,4 NiÀMo, 5 CoSe 2 , 6 CoS 2 , 7 Ni 2 P, 8,9 CoP, 10,11 MoP, 12,13 and WP, 14 as well as other related materials. 15À19 Iron-based alternatives are especially attractive because Fe is the most abundant transition metal, comprising ∼5% of the Earth's crust. 20 Accordingly, the price of iron is typically at least 2 orders of magnitude less than that of other highly abundant and catalytically relevant metals for the HER, including Ni and Co. 20 Iron- based clusters also have been found to be the catalytically active sites in [FeFe] and [Fe]-only hydrogenases, which are highly active and efficient biological HER catalysts. 21 A few moderately active iron-based hetero- geneous HER catalysts have been repor- ted, including porous FeP nanosheets, 22 pyrrhotite-type FeS nanoparticles, 23 and polycrystalline films of pyrite-type FeS 2 . 16 However, highly active HER catalysts com- posed of high-quality iron-based nano- particulate materials, which are among the most desired because of their cost, abun- dance, and processability, have not yet been identified. We report herein that iron phosphide (FeP) nanoparticles are exceptionally active as both electrocatalysts and photocatalysts for sustained hydrogen production in either * Address correspondence to schaak@chem.psu.edu, nslewis@caltech.edu. Received for review August 28, 2014 and accepted September 24, 2014. Published online 10.1021/nn5048553 ABSTRACT Nanostructured transition-metal phosphides have recently emerged as Earth-abundant alternatives to platinum for catalyzing the hydrogen-evolution reaction (HER), which is central to several clean energy technologies because it produces molecular hydrogen through the electrochemical reduction of water. Iron- based catalysts are very attractive targets because iron is the most abundant and least expensive transition metal. We report herein that iron phosphide (FeP), synthesized as nanoparticles having a uniform, hollow morphology, exhibits among the highest HER activities reported to date in both acidic and neutral-pH aqueous solutions. As an electrocatalyst operating at a current density of À10 mA cm À2 , FeP nanoparticles deposited at a mass loading of ∼1 mg cm À2 on Ti substrates exhibited overpotentials of À50 mV in 0.50 M H 2 SO 4 and À102 mV in 1.0 M phosphate buffered saline. The FeP nanoparticles supported sustained hydrogen production with essentially quantitative faradaic yields for extended time periods under galvanostatic control. Under UV illumination in both acidic and neutral-pH solutions, FeP nanoparticles deposited on TiO 2 produced H 2 at rates and amounts that begin to approach those of Pt/TiO 2 . FeP therefore is a highly Earth-abundant material for efficiently facilitating the HER both electrocatalytically and photocatalytically. KEYWORDS: hydrogen evolution reaction . metal phosphides . nanoparticles . electrocatalysis . photocatalysis ARTICLE