Room-Temperature Wet Chemical Synthesis of Au NPs/TiH<sub>2</sub>/Nanocarved Ti Self-Supported Electrocatalysts for Highly Efficient H<sub>2</sub> Generation
2017-08-03T00:00:00Z (GMT) by
Self-supported electrocatalysts are a new class of materials exhibiting high catalytic performance for various electrochemical processes and can be directly equipped in energy conversion devices. We present here, for the first time, sparse Au NPs self-supported on etched Ti (nanocarved Ti substrate self-supported with TiH<sub>2</sub>) as promising catalysts for the electrochemical generation of hydrogen (H<sub>2</sub>) in KOH solutions. Cleaned, as-polished Ti substrates were etched in highly concentrated sulfuric acid solutions without and with 0.1 M NH<sub>4</sub>F at room temperature for 15 min. These two etching processes yielded a thin layer of TiH<sub>2</sub> (the corrosion product of the etching process) self-supported on nanocarved Ti substrates with different morphologies. While F<sup>–</sup>-free etching process led to formation of parallel channels (average width: 200 nm), where each channel consists of an array of rounded cavities (average width: 150 nm), etching in the presence of F<sup>–</sup> yielded Ti surface carved with nanogrooves (average width: 100 nm) in parallel orientation. Au NPs were then grown <i>in situ</i> (self-supported) on such etched surfaces via immersion in a standard gold solution at room temperature without using stabilizers or reducing agents, producing Au NPs/TiH<sub>2</sub>/nanostructured Ti catalysts. These materials were characterized by scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), grazing incidence X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS). GIXRD confirmed the formation of Au<sub>2</sub>Ti phase, thus referring to strong chemical interaction between the supported Au NPs and the substrate surface (also evidenced from XPS) as well as a titanium hydride phase of chemical composition TiH<sub>2</sub>. Electrochemical measurements in 0.1 M KOH solution revealed outstanding hydrogen evolution reaction (HER) electrocatalytic activity for our synthesized catalysts, with Au NPs/TiH<sub>2</sub>/nanogrooved Ti catalyst being the best one among them. It exhibited fast kinetics for the HER with onset potentials as low as −22 mV vs. RHE, high exchange current density of 0.7 mA cm<sup>–2</sup>, and a Tafel slope of 113 mV dec<sup>–1</sup>. These HER electrochemical kinetic parameters are very close to those measured here for a commercial Pt/C catalyst (onset potential: −20 mV, Tafel slope: 110 mV dec<sup>–1</sup>, and exchange current density: 0.75 mA cm<sup>–2</sup>). The high catalytic activity of these materials was attributed to the catalytic impacts of both TiH<sub>2</sub> phase and self-supported Au NPs (active sites for the catalytic reduction of water to H<sub>2</sub>), in addition to their nanostructured features which provide a large-surface area for the HER.