Low-Pressure Plasma Synthesis of Ni/C Nanocatalysts from Solid Precursors: Influence of the Plasma Chemistry on the Morphology and Chemical State

Abstract

Nanocatalyst materials based on metal nanoparticles (NPs) deposited on mesoporous carbon substrates are widely used in catalysis and energy storage; however, conventional wet-chemical deposition methods based on the reduction of metal salts are not always the best choice when looking for a process ensuring easy scalability and low environmental impact. Moreover, additional surface functionalization steps, such as the addition of nitrogen- or oxygen-containing groups, are more and more explored to increase the activity or the chemical stability of catalysts. In this work, we investigate a new methodology for the fabrication of nickel/carbon nanocatalysts relying on a low-pressure radio frequency plasma treatment of solid (powder) precursors. A mesoporous carbon xerogel is used as support for nickel NPs synthesized through the decomposition of an organometallic nickel precursor in a plasma discharge. Different plasma treatment conditions and chemical environments are applied by varying the plasma power and the gas mixture injected into the plasma chamber (Ar, N2, NH3, and O2). The nucleation kinetics of nickel NPs, their morphology evolution, and chemical state were fully characterized by combining analytical techniques such as in situ optical emission spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Results indicate that the plasma chemistry and conditions strongly influence the organometallic compound decomposition as well as the size and the oxidation state of the homogeneously dispersed nickel NPs. We compare the organometallic precursor degradation efficiency for each plasma by defining a rational "activation power" associated with each plasma chemistry. Moreover, simultaneous carbon substrate functionalization is obtained through plasma treatment, which demonstrates the high versatility of the plasma fabrication for developing green and efficient catalysts and energy materials.