To untangle these effects, in this study we used DC magnetron sputtering to prepare low roughness Cu2O thin films (TFs) with a controlled oxide thickness of 100 nm, and used these as electrocatalysts for the reduction of CO2. Interestingly, these model electrodes did not display the enhanced selectivity towards ethanol and ethylene usually reported for CuOx, but instead led to a product distribution similar to the one attained with polycrystalline copper. Consistently, grazing angle X-ray diffraction measurements of the TFs prior to and after chronoamperometric (CA) CO2-reduction at ‑ 1.0 V vs. the reversible hydrogen electrode (RHE) suggested the co-reduction of the copper oxide in the course of the reaction. To verify this and additionally study the TFs’ surface oxidation state, complementary X-ray photoelectron spectroscopy (XPS) and Ar-sputtering measurements were performed; chiefly, sample re-oxidation upon disassembly of the electrochemical cell and/or transfer to the XPS were avoided by performing the electrochemical measurements in a N2-filled glovebox and transferring the sample in an air-tight chamber, respectively. As illustrated in Figure 1, this resulted in clear changes in the Cu-2p and -LMM spectra – specifically, whereas in the former no significant differences were observed when the post-CA samples were transferred in air or N2 (likely due to the identical Cu-2p spectra exhibited by Cu0 and Cu2O),6 the Auger spectra were much more sensitive to the transfer means, and the excellent agreement6 between the spectrum of the N2-transferred sample and reports for Cu0 confirmed the complete reduction of the TF upon CO2-electroreduction.
Ultimately, these results indirectly point at the surface roughness as the factor determining the enhanced alcohol selectivity of CO2-reduction electrocatalysts derived from (reduced) copper oxides, while highlighting the importance of air-free sample transfer for the post-mortem assignment of surface oxidation states.
References
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Figure 1. Cu-2p and -LMM spectra (left vs. right) of 100 nm thick Cu2O thin film electrodes before and after 30 minutes of chronoamperometry (CA) at -1.0 V vs. RHE in CO2-saturated 0.05 M Cs2CO3. Note that blue (i.e., middle) lines correspond to the spectra recorded when the electrode was transferred to the XPS in air (“trans. air”), whereas green (bottom) ones were recorded by disassembling the cell in a N2-filled glovebox and transferring the electrode to the XPS inside an air-tight chamber (“trans. N2”).