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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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00555 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017
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ACS Catalysis
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. KEYWORDS: Molybdenum phosphide; S and N dual doping; hydrogen evolution reaction; electrocatalysts; urea-phosphate route.
ABSTRACT: S and N 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 while phosphoric acid provides P atom. The MoP/SN nanoparticles are generated by in-situ phosphidation of indigenously-synthesized ammonium phosphate-coated P-doped MoSx nanoparticles in 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 of MoP-
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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.
The electrochemical hydrogen evolution reaction (HER) from water splitting has attracted a great attention recently to produce sustainable hydrogen with electricity generated from renewable energy sources. The biggest challenge in HER research is to replace the most common and the best incumbent Pt catalysts with inexpensive non-precious 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 massive attention last few years due to their superior electrical conductivity, mechanical strength, and chemical stability relative to other transition metal compounds.4-6 They are proven to be highperformance electrocatalysts with excellent activity, stability, and nearly ≈100% Faradic efficiency in acidic, alkaline and neutral media for HER.6-11 Experimental and theoretical investigations have revealed that atomic percentage of P atoms into 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 Also, an appropriate atomic ratio of metals and P, especially in metal-rich phosphides, offers excellent conductivity and more noble metal-like properties relative to parent transition metals.15 In spite of 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
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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 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 post-sulfidation,8 or postphosphidation of MoS2 to form MoS2(1-x)Px 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 by using organic phosphine like tri-n-octylphosphine (TOP), tri-phenylphosphine (TPP) or tri-n-octylphosphine oxide (TOPO) as a P source in high-boiling-point solvents (e.g. oleylamine) in inert atmosphere.17,25 ii) Gas-solid reaction, in which extremely toxic and lethal PH3 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 PH3. iii) High temperature reduction of metal phosphates to form bulk TMP.17 Herein, a simple, economical and eco-friendly (thio)urea-phosphate-assisted strategy is demonstrated to synthesize S and N co-doped MoP (MoP/SN) nanocrystallites, which display excellent electrocatalytic activity and stability for HER both in acidic and alkaline media. The MoP nanocrystallites show their best performance when they are grown on S and N dual-doped graphene (MoP/SNG) displaying one of the best HER activities among reported non-noble metal electrocatalysts, and still maintaining excellent stability in aqueous acidic and alkaline solutions.
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In particular, the stability in acids makes them a promising practical electrocatalyst because there are only a few acid-stable non-precious metal electrocatalysts known so far. Distinct from earlier studies, our synthesis relies on in-situ sulfidation reaction between ployoxometalates (POMs) and (thio)urea-phosphates to form ammonium phosphate-coated, reduced P-doped MoSx, followed by the reductive phosphidation in H2 atmosphere. To the best of our knowledge, this is the first systematic report on fabrication of MoP electrocatalysts by using phosphoric acid as a P source and reductant (thio)urea as S and N sources while controlling the phase and size of the nanoparticles. In addition, this method induces self-doping of N and S atoms into the skeleton of phosphides as well as graphene support that plays crucial roles to stabilize the MoP nanoparticles (NPs) and P-3 state by making Mo/P-N and Mo/P-S bonds, to increase the electrical conductivity, and to provide ample S and N sites for proton adsorption. Avoiding use of expensive, toxic and corrosive chemicals like PH3, hypophosphites, organic phosphine and high-boiling organic solvents is another advantage of this synthesis method of MoP nanoparticles. The present synthetic strategy could also be applied to other transition metals (e.g. Co, Fe, Ni and W) phosphides for large scale production due to its simplicity, and use of cheap and environmentally-benign precursors.
RESULTS AND DISCUSSION Urea-phosphate-assisted synthesis for molybdenum phosphide Polyoxometalates (POMs) are early transition-metals (Mo, W) polyatomic anionic clusters composed of one metal oxide and a main group oxyanion (phosphate or silicates), and are good oxidants and versatile inorganic building blocks for the construction of various hybrid materials.27-29 Thiourea is a reducing agent which liberates H2S gas and urea especially in the
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presence of acids (H3PO4 and POM).29 These properties led us to design a procedure to synthesize S, N co-doped MoP nanocrystallites (MoP/SN) by simply using phosphomolybdic acid (H3PMo12O40·nH2O) to produce ammonium phosphate-coated, P-doped MoSx (precursor) via hydrothermal reduction with thiourea followed by hydrogen reduction of the precursor as illustrated in Scheme 1, and described in detail in Experimental Section. A Keggin-type H3PMo12O40 is reduced with thiourea (Eqn. 1) to form a bluish mixed-valence reduced species [PMo12O40]6- without the loss of the Keggin structure.30 During continuous boiling under autogenous pressure, [PMo12O40]6- turns to P-doped MoSx nanoparticles (NPs) by reduction with H2S gas liberated from thiourea in the acidic solution (Eqn. 2) with urea-phosphate as a side product. Ammonium ortho-polyphosphates and biuret encapsulate the P-doped MoSx NPs upon pyrolysis of urea-phosphate (Eqn. 3).31 Then, ammonium phosphate-encaged P-doped MoSx NPs are converted to MoP/SN upon reduction by H2. Exfoliated graphene oxide (GO) suspension is introduced directly into the hydrothermal step to obtain MoP/SN NPs supported on graphene (MoP/SNG).
[ ] + ( ) [ ] + + ( ) (1) ∆
[ ] + − + (2) ∆
( ) + ( ) . (#$$. %ℎ'%ℎ()*) + +('*' (3) These phase changes during the synthesis steps were confirmed by powder X-ray diffraction (XRD) patterns of MoP/SN electrocatalysts at different reduction temperatures under H2 as shown in Figure 1a. Pure MoP of hexagonal WC-type structure (space group P6m2) is obtained above 600 °C and small peaks of molybdenum phosphate are observed in the samples reduced below 600 °C. Doping of N and S does not significantly alter the bulk crystal structure of MoP. The crystallite size of MoP was obtained by applying the Scherrer equation to X-ray line
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broadening, which increases along with the reduction temperature as shown in Figure 1b. The XRD pattern of P-doped MoSx shows P-Mo3S4 as the main phase (Figure S1a) while MoP/N-650 and graphene-supported catalysts show the same phase behavior as the unsupported MoP/SN catalysts as shown in Figure S1b-c of Supporting Information (SI). Scanning electron microscopy (SEM) images of the as-synthesized MoP/SN-650 °C and MoP/SNG-20 (with 20 wt% graphene) catalysts show that particles form a closely interconnected porous network (Figure S2). No difference in morphology is observed after loading the SN-doped MoP nanoparticles on graphene (MoP-SN/G-20, Figure S4a-b). The surface morphologies of MoP/N-650 (N-doping only) and a physical mixture of MoP-SN/G-20 are also shown in Figure S3 and Figure S4a-b. Elemental mappings by energy-dispersive-X-rayspectroscopy (EDX)-SEM images of MoP/SN-650 (Figure S5) and MoP/SNG-20 (Figure S6) clearly indicate uniform elemental distributions throughout the particles. (EDS)-STEM mappings of MoP/SNG-20 and MoP/SN-650 in Figure S7a and S8 of SI also demonstrate similar uniform elemental distributions. Thus, MoP NPs are homogeneously distributed over graphene and uniformly doped with S, N and C. EDS-STEM of MoP/SNG-20 (Figure S7b) clearly indicates that there is no significant change in the elemental composition after HER durability test. The nature of chemical bonding in as-prepared MoP/SN and MoP/SNG-20 was investigated by X-ray photoelectron spectroscopy (XPS) in Figure 2. The Mo3d region (Figure 2a) for both MoP/SN and MoP/SNG indicate a prominent doublet at 228.2 eV and 231.3 eV of Moδ+ (0
