Green synthesis of carbon-supported nanoparticle catalysts by physical vapor deposition on soluble powder substrates

Metal and metal oxide nanoparticles (NPs) supported on high surface area carbon (NP/Cs) were prepared by the physical vapor deposition of bulk materials on an α-D-glucose (Glu) substrate, followed by the deposition of the NPs on carbon supports. Using Glu as a carrier for the transport of NPs from the bulk materials to the carbon support surfaces, ultrafine NPs were obtained, exhibiting a stabilizing effect through OH moieties on the Glu surfaces. This stabilizing effect was strong enough to stabilize the NPs, but weak enough to not significantly block the metal surfaces. As only the target materials and Glu are required in our procedure, it can be considered environmentally friendly, with the NPs being devoid of hazardous chemicals. Furthermore, the resulting NP/Cs exhibited an improvement in activity for various electrochemical reactions, mainly attributed to their high surface area.


Materials
Commercial electrocatalysts (Pt/C, Ir/C, Pd/C) were purchased from Premetek Inc. and utilized without further heat treatment for the electrochemical analysis. 4-inch metal and metal oxide targets (Pt, Ir, Pd, Au, Ni, Y, NiO, CoO and FeO) were purchased from United Vacuum & Materials (Purity 99.99%). High surface carbon was purchased from Cabot co.
(Vulcan XC-72R, BET: 237 m 2 /g). TiO2 nanopowder was purchased from Sigma Aldrich (particle size < 100 nm) and CNT was purchased from Carbon Nano-material Technology Co. The GO was supplied by Carbon convergence material research center in KIST Jeonbuk branch composite material technology institute and prepared using Hummers' method (J. Am. Chem. Soc., 80 (1958) 1339. The solvent of Nafion solution was a mixture of alphatic alcohol (85~80%) and water (15~20%).

Electrochemical analysis
Preparation of catalyst coated glassy carbon electrode. The catalyst layer was obtained as follows: (i) first, a slurry was prepared by ultrasonic agitation of a mixture of 800 μL of DI water, 10 mg of catalyst, and 60 μL of Nafion solution (Aldrich: 5 wt% Nafion) for 10 min; (ii) second, 5 μL of the slurry was pipetted and spread on the carbon disc; (iii) third, the electrode was dried at 60 C for 10 min.

Oxygen reduction reaction (ORR) polarization
Prior to measuring ORR polarization curves, multiple cyclic voltammetry (CV) was carried out in deaerated 0.1 M HClO4 solution at the can rate of 50 mV s -1 in a potential window of 0.05 -1.0 V vs RHE, at room temperature. The ORR activity was evaluated by the rotating disk electrode (RDE) technique in O2-saturated 0.1 M HClO4 solution with a sweep rate of 5 mV s -1 at 1600 rpm, at room temperature.

Oxygen evolution reaction (OER) polarization
OER polarization curve was recorded using cyclic voltammetry (CV) at a scan rate of 5 mV s -1 and 2500 rpm in N2 saturated 0.1 M KOH solution. The OER activity polarization curve was stabilized after around 10 th cycles of CV analysis and, thus, the OER activity was determined using the stabilized polarization curve. Following each measurement, 0 V vs. RHE was established by performing the hydrogen oxidation and hydrogen evolution reaction in the same electrolyte. An impedance spectrum was recorded with a peak-to-peak amplitude of 10 mV, at frequencies from 1 kHz to 500 kHz for evaluating the Ohmic resistance, which is obtained from the high-frequency intercept (or minimum) on the horizontal (real) axis of the Nyquist plot; typically, it was 31 Ω.
The OER activity of commercial CoO was measured using CoO particles deposited on a thin film of carbon support layers (CoO-C).To prepare a thin layer of carbon support (0.237 g cmgeo -2 ), a suspension of Vulcan XC-72 (0.08 g) in 1,2-propanon (8.6 mL) was prepared.
Then, 5 μL of the suspension was dropped onto the RDE electrode and dried in a gentle Ar stream. After drying, a 5 μL suspension of commercial CoO (0.02 g) in 1,2-propanol (8.6 mL) was pitted onto the carbon layer and dried. The amount of CoO was 0.059 g cmgeo -2 . Finally, a 5 μL diluted Nafion solution (10 vol.% in 1,2-propanol) was dropped and dried.

Glucose oxidation reaction
The activity of the PtxAu1-x/C alloys toward electrochemical oxidation of glucose was determined using cyclic voltammetry (CV) and chronoamperometry (CA) technique. CV was 25 conducted in the potential range from −1.0 V to 1.0 V vs. RHE with a scan rate of 50 mV/s in an electrolyte composed of 0.5 M phosphate buffer solution (PBS, pH 7) and 0.05M glucose. CA was performed at 0.6V vs. RHE for 3600s. All of the electrolytes were purged with N2 gas for 30 min to remove any dissolved gas prior to the electrochemical tests.

Formic acid oxidation reaction
The electrochemical activity of formic acid oxidation reaction (FOR) on Pd/C and commercial Pd/C were measured using cyclic voltammetry (CV) in a N2-saturated 0.1 M HClO4 + 0.1 M HCOOH solution at 50 mV s -1 .