Surface oxidation of transition metal sulfide and phosphide nanomaterials
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Surface oxidation of transition metal sulfide and phosphide nanomaterials Zishan Wu1,2, Ling Huang2,†, Huan Liu1,2,‡, Min Li3, and Hailiang Wang1,2 () 1
Department of Chemistry, Yale University, New Haven, Connecticut 06511, USA Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, USA 3 Materials Characterization Core, Yale West Campus, West Haven, Connecticut 06516, USA † Present address: Engineering Laboratory for Next Generation Power and Energy Storage Batteries, Tsinghua Shenzhen International Graduate School, Shenzhen 518055, China ‡ Present address: College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, China 2
© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 21 September 2020 / Revised: 22 October 2020 / Accepted: 29 October 2020
ABSTRACT Many transition metal sulfides and phosphides are susceptible to surface oxidation under ambient conditions. The formed surface oxidation layer, which is likely to further restructure under reaction conditions, alters the chemical properties of the pristine material but has not been well studied. In this work, we for the first time use X-ray photoelectron spectroscopy to quantify the natural surface oxidation of transition metal phosphide and sulfide nanoparticles and employ a simplified Deal-Grove model to analyze the kinetics. We show that CoS2 oxidizes faster than CoS whereas CoP2 is more difficult to oxidize compared to CoP, and there exists an inverse correlation between the surface oxidation rate and the Co-S/P distance in the pristine structure. More inclusive investigation unveils different types of surface oxidation behavior: CoS, NiS and FeS are limited by their reactivity with oxygen; CoS2 is the most reactive and its oxidation is governed by oxygen diffusion; CoP2 is influenced by both reactivity and diffusion; CoP, Ni2P, Cu3P and MoP exhibit high initial oxidation degrees and the kinetics are not well-defined; MoS2 is largely stable against oxidation.
KEYWORDS sulfides, phosphides, surface oxidation, kinetics, X-ray photoelectron spectroscopy (XPS)
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Introduction
Transition metal phosphides and sulfides are emerging electrocatalyst materials for energy storage and conversion applications [1–8]. For example, CoP is a relatively low cost material with high catalytic activity and stability for the electrochemical hydrogen evolution reaction which is useful for clean fuel production from water [9–12]. Many phosphides and sulfides can also catalyze the oxygen evolution reaction, i.e., the anodic half reaction of water electrolysis, although the active phase is usually (oxy)hydroxides formed under the electrochemical conditions [13–16]. Transition metal phosphides and sulfides are often oxidized on the surface under ambient conditions [17–20]. Our prior work has shown that the surface oxidation layer undergoes element-specific restructuring under working conditions depending on the electrolyte and electrode potential, which greatly influences and in so
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