allows for the creation of high surface area catalysts that expose crystal faces not present in the sputtered thin films. Lastly, inspiration was gathered from the CO hydrogenation literature which uses Cu/ZnO catalysts industrially. Unfortunately, because these catalysts are on oxide supports they are ill-suited as reductive electrocatalysts;
fortunately, several bimetallic catalysts such as nickel-gallium do not require an oxide support and were tested here as electrocatalysts.
nanoparticle drop casting was performed on a polished carbon electrode. Due to the highly negative potentials needed for CO2R, carbon is one of the few conductive substrates that will not substantially catalyze HER at these potentials.
Intermetallic Thin Film Synthesized by Solvothermal Methods
Solvothermal methods were also used as a means of easy catalyst synthesis.
Generally, aqueous solutions of the target metal salts were generated by dissolving water soluble salts that contained easily decomposed anions. Mostly metal nitrates were used except where solubility or availability limited their application. In these rare cases other salts such as sulfates were used. Solutions were mixed to appropriate ratios at ~ 0.1 M. A cartoon representation of this is shown in Figure 2-1. While the rough morphology of films produced using this method was less than desirable, the ease and adaptability of this synthetic process made it an attractive option. Films made by this technique were also characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), to confirm the desired phase was obtained. Occasionally, phase pure materials were not obtained and changes to the annealing conditions or ratios of metal salts were tuned.
Generally speaking, higher annealing temperatures gave more phase pure materials.
Intermetallic Nanoparticle Synthesized by Solvothermal Methods
Co-reduction of Metallic Precursors to form RhBi2: 100 mg of RhCl3, 200 mg Bi(acac)3, PVP, and 15 mL tetraethylene glycol were added to a 50 mL three-necked, round-bottom flask equipped with a reflux condenser attached to a Schlenk line, a thermometer adapter for a thermocouple, and a borosilicate stir bar. The solution was heated to 120 °C under vacuum to remove low boiling point solvents. The solution was then placed under N2 and refluxed at 314 °C for 1 h. The solution was then cooled rapidly
Figure 2-1. Cartoon representation of the solvothermal method used here.
to room temperature by removing the heating mantle. The solution was then transferred to a centrifuge tube and 5 mL of ethanol and 15 mL of hexanes added. The tube was then centrifuged at 12,000 rpm. The supernatant was poured off and the nanoparticles washed twice more.
Solution Based Conversion of Rh Nanoparticles to RhBi2: 100 mg of RhCl3, PVP, and 15 mL of tetraethylene glycol was added to a 50 mL three-necked, round-bottom flask equipped with a reflux condenser attached to a Schlenk line, a thermometer adapter for a thermocouple, and a borosilicate stir bar. The solution was heated to 120 °C under
vacuum to remove low boiling point solvents. The solution was then placed under N2 and refluxed at 314 °C for 30 min. The solution rapidly turned black as the Rh nanoparticles formed. 200 mg of Bi(acac)2 was then dissolved in 5 mL of TEG and injected through the rubber septum dropwise. The reaction was allowed to reflux again for another thirty minutes. The solution was then cooled rapidly to room temperature by removing the heating mantle. The solution was then transferred to a centrifuge tube and I5 mL of ethanol and 15 mL of hexanes added. The tube was then centrifuged at 12,000 rpm. The supernatant was poured off and the nanoparticles washed twice more.
Synthesis of IrPb Nanoparticles: 250 mg of Pb(acac)2 and 25 mL of TEG was added to a 50 mL three-necked, round-bottom flask equipped with a reflux condenser attached to a Schlenk line, a thermometer adapter for a thermocouple, and a borosilicate stir bar. The solution was heated to 120 °C under vacuum to remove low boiling point solvents. The solution was then heated to refluxing for thirty minutes. The solution was then removed from heat by hand using an insulated glove. The solution was then poured into 250 mL of chilled ethanol. Black Pb nanoparticles settled on the bottom almost immediately. The supernatant was poured off and the nanoparticles scraped into a 50 mL three neck flask.
250 mg of IrCl3 and 10 mL TEG was then added and the flask placed under vacuum at 120 °C. The solution was then put under N2 and heated to reflux for 1 hr. The resulting nanoparticles were collected through centrifugation.
Nitride, Carbide, and Sulfide Synthesis
Nitrides were synthesized via reactive sputter deposition under a nitrogen atmosphere. The target was a metal, or in some instances 2 targets were used for co- sputter deposition, and the power of the ion source was tuned to the appropriate setting
depending on the material. For carbides, co-sputter deposition was performed with the desired metal target and a carbon target. The carbon target was kept at a very low power
<30 W to prevent the deposition of mostly carbon. Sulfides were synthesized using the solution-phase deposition outlined later.
Materials Characterization
Electrodes were characterized by several techniques both before and after catalysis including SEM, energy dispersive x-ray spectroscopy (EDS), XRD, and X-ray
photoelectron spectroscopy (XPS). Films which showed inhomogeneous structure after sputter deposition were then annealed under forming gas (5% H2 / 95% N2) to generate a homogeneous polycrystalline film. In example of this is shown in Figure 2-2. Films of Ag3Sn appeared to deposit as Sn islands on an Ag film. After annealing the films become homogeneous and are of the desired stoichiometry. SEM was performed on a FEI Nova NanoSEM at an accelerating voltage of 15 kV. X-ray diffraction (XRD) was performed on a Bruker-AXS D8 Advance diffractometer with Cu Kα radiation and a LynxEye 1-D detector in order to verify a crystalline material of the proper phase was produced. An example XRD is shown in Figure 2-3. The XRD shows a peak corresponding to excess Sn at 60 ° 2ϴ that disappears after annealing, as well as several new peaks at 38 ° 2ϴ and 80 ° 2ϴ. A total of 8 samples were made by sputter deposition and include: Ag3Sn, NiPdx
(where 1 < x < 3), NiSn, TiNC, TiN, WN, WNC, and TiWN. XPS was performed on an Axis Ultra X-ray photoelectron spectrometer (Kratos, Manchester, U.K) with a
monochromatic Al Kα source (1486.7 eV) and a concentric hemispherical analyzer with a pass energy of 20 eV, with the photoelectrons captured normal to the surface. Binding energies were calibrated against the “adventitious” C 1s peak (taken to be 284.6 eV).
Figure 2-2. SEM of Ag3Sn (top) before and (bottom) after annealing at 400 °C.
Figure 2-3. XRD of Ag3Sn films before vs. after annealing at 400 °C.