In situ electrochemical APXPS analysis of ALD grown Cu catalyst for CO₂ reduction

The grand challenge in artificial photosynthesis is to produce liquid solar fuels from water and carbon dioxide. Unfortunately, current materials solutions for a solar fuel reactor lack efficiency, selectivity towards liquid fuel products, and long-term stability. Cu based catalysts are so far the most promising materials for the carbon dioxide reduction reaction (CO₂RR), whereas the selectivity of Cu catalyst towards liquid products is strongly affected by the exact chemical composition and structure. Recently, Eilert et al. suggested, based on in situ electrochemical ambient-pressure X-ray photoelectron spectroscopy (APXPS) measurements on Cu foil, that the high activity of oxide-derived copper towards CO₂RR stems from subsurface oxygen that was proposed to increase the CO binding energy and thus enhance the production of methanol and multicarbon products [1]. This contradicts the alternative hypothesis that assigns the activity to Cu₂O on the surface, albeit no copper oxide should be stable under reductive CO₂RR conditions.

In this work, ALD grown Cu oxide thin film was analyzed by electrochemical APXPS at the HIPPIE soft X-ray beamline, MAX IV Laboratory. The sample was probed after heat treatment at 300 °C and after operation under cathodic gas evolution conditions in 0.1 M KHCO₃. The chemical state of Cu was analysed from Cu 2p and Cu LMM transitions that were measured in the presence of adsorbed electrolyte film. Cathodic electrochemical treatment induced complete reduction of Cu oxide (CuO) to metallic Cu. Neither Cu₂O nor sub-surface oxygen was detected on the surface which provides support to the metallic state of Cu in oxide-derived Cu electrocatalysts. However, we observed X-ray beam induced transformation of Cu to Cu₂O, which may have contributed to the existing discrepancy in the field. In the experiment, beam induced effects were prevented by continuous rastering of the sample during the data acquisition.
In summary, under CO₂RR conditions our oxide-derived Cu catalyst showed only metallic Cu without any evidence of Cu oxides or incorporated oxygen species. Although, the debate on subsurface oxygen is still open, the research serves as an advancement towards understanding and fabrication of ALD grown Cu oxide catalysts combined with ALD TiO₂ [2, 3] coatings for photocathodes in solar fuel cells to produce methanol and multicarbon products from carbon dioxide, water, and sunlight.

[1] A. Eilert, F. Cavalca, F.S. Roberts, J. Osterwalder, C. Liu, M. Favaro, E.J. Crumlin, H. Ogasawara, D. Friebel, L.G.M. Pettersson, and A. Nilsson, J. Phys. Chem. Lett. 8, 285–290 (2017).
[2] M. Hannula, H. Ali-Löytty, K. Lahtonen, E. Sarlin, J. Saari, and M. Valden, Chem. Mater. 30, 1199–1208 (2018).
[3] H. Ali-Löytty, M. Hannula, J. Saari, L. Palmolahti, B.D. Bhuskute, R. Ulkuniemi, T. Nyyssönen, K. Lahtonen, and M. Valden, ACS Appl. Mater. Interfaces 11, 2758–2762 (2019).