Wavelength-Tailoring Copper Oxidation States for Tunable Photoelectrochemical Syngas Generation

The photoelectrochemical CO₂ reduction reaction (PEC CO₂RR) to syngas is of great significance for meeting the needs of the green chemical industry, and controlling the CO/H₂ ratio is an important issue. However, the reliance on thin-film semiconductor photocathodes significantly limits the available fabrication methods, and some of the proposed schemes have not been able to precisely tune the CO/H₂ ratio by indirectly regulating the electronic structure of active sites. In order to overcome the limitations of traditional fabrication methods, this work proposes a simple photodeposition method for loading Cu_2<i>-x</i>Te onto 1% S-doped ZnTe/ZnO to regulate the oxidation state of Cu between +1 and +2 by precisely controlling the deposition light wavelength from violet to red. With shorter deposition light wavelengths, the photon energy increases, leading to a reduced valence state of Cu. As the Cu oxidation state decreases, the band structure of Cu_2-<i>x</i>Te-ZnTe can be modulated, with the overall <i>d</i>-band center shifting toward the Fermi level. Besides, the electron density around the Cu active sites increases due to the shorter Cu-Cu bond, resulting in stabilized reaction intermediates and a faster charge transfer process, leading to higher CO selectivity with suppressed hydrogen evolution reaction. As a result, Cu@S-ZnTe/ZnO shows a tunable CO/H₂ molar ratio ranging from 0.45 to 1.70 by adjusting the oxidation state of Cu, which can be precisely controlled by simply varying the deposition light wavelength with a specific filter. This demonstrates the great potential of the proposed photodeposition method and the resulting photoelectrocatalyst for practical PEC CO₂RR applications.