Sulfur and Nitrogen Dual-Doped Molybdenum Phosphide Nanocrystallites as an Active and Stable Hydrogen Evolution Reaction Electrocatalyst in Acidic and Alkaline Media Mohsin Ali Raza Anjum and Jae Sung Lee* School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, South Korea * S Supporting Information ABSTRACT: Sulfur and nitrogen dual-doped molybdenum phosphides (MoP/SN) are synthesized via a (thio)urea- phosphate-assisted strategy in which the reductant (thio)urea acts as S and N source and phosphoric acid provides the P atom. The MoP/SN nanoparticles are generated by in situ phosphidation of indigenously synthesized ammonium phos- phate-coated P-doped MoS x nanoparticles in a hydrogen atmosphere. Then, MoP/SN is anchored on graphene to obtain a hybrid electrocatalyst (MoP/SNG) that exhibits high activity and stability for electrochemical hydrogen evolution from water in both acidic and basic electrolytes, outperforming most MoP-based electrocatalysts reported in the literature. The dual doping and hybridization with graphene enhance electron conductivity of MoP and stabilize small MoP nanoparticles to increase activity and stability, especially in acid electrolytes. KEYWORDS: molybdenum phosphide, S and N dual doping, hydrogen evolution reaction, electrocatalysts, urea-phosphate route T he electrochemical hydrogen evolution reaction (HER) from water splitting has attracted significant attention recently for producing sustainable hydrogen with electricity generated from renewable energy sources. The biggest challenge in HER research is to replace the most common and best incumbent Pt catalysts with inexpensive nonprecious metals. 1 Transition metal carbides, sulfides, borides, nitrides, and phosphides have been extensively studied as candidates for non-Pt electrocatalysts. 1-5 Transition metal phosphides (TMPs) have attracted significant attention over the past few years due to their superior electrical conductivity, mechanical strength, and chemical stability relative to those of other transition metal compounds. 4-6 They are proven to be high- performance electrocatalysts with excellent activity, stability, and nearly 100% Faradaic efficiency in acidic, alkaline, and neutral media for HER. 6-11 Experimental and theoretical investigations have revealed that the atomic percentage of P atoms in the lattice of transition metals (Fe, Co, Ni, Cu, Mo, and W) plays a crucial role with the more electronegative P atom acting as a basic trap of positively charged protons during the reaction. 6,12-14 Furthermore, an appropriate atomic ratio of metals and P, especially in metal-rich phosphides, offers excellent conductivity and more noble metal-like properties relative to those of the parent transition metals. 15 Despite the success of TMPs as good electrocatalysts for HER, there are still many remaining challenges to improve their performance and stability further by tuning their electronic structures, maximizing electrical conductivity, doping with other elements, and protecting P 3- in TMP, the least stable oxidation state of P, from oxidation. Introduction of more electronegative P atoms into metals may greatly restrict the electron delocalization in the metal, resulting in lower conductivity. 16 However, with an appropriate atomic ratio of metal and P or doping of other heteroatoms such as S or N, TMPs can exhibit a metallic character or even superconductivity, especially for the metal-rich phosphides. 17 Recently, similar strategies have been applied to enhance the HER activity of MoP: (i) doping with S to form molybdenum phosphosulfide (MoP|S) on the surface of MoP by a postsulfidation 8 or postphosphidation of MoS 2 to form MoS 2(1-x) P x solid solution, 18 (ii) compounding with carbon materials like graphene 19 or porous carbons, 20 and (iii) promotion with a TM (Co, Fe, or W). 21-24 The TMP nanostructures are generally synthesized via three common ways: (i) Solution-phase synthesis using organic phosphine like tri-n-octylphosphine (TOP), triphenylphosphine (TPP) or tri- n-octylphosphine oxide (TOPO) as a P source in high-boiling- point solvents (e.g., oleylamine) in an inert atmosphere. 17,25 (ii) Gas-solid reaction, in which extremely toxic and lethal PH 3 gas is used as P source directly or produced in situ from hypophosphite; 26 in this method, post-treatment is mandatory with inert gas to remove residual PH 3 . (iii) High-temperature reduction of metal phosphates to form bulk TMP. 17 Received: February 19, 2017 Published: March 16, 2017 Research Article pubs.acs.org/acscatalysis © 2017 American Chemical Society 3030 DOI: 10.1021/acscatal.7b00555 ACS Catal. 2017, 7, 3030-3038 Downloaded via ULSAN NATL INST SCIENCE AND TECHLGY on August 16, 2018 at 06:57:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.