Effect of Wavelength-Scale Cu₂O Particles on the Performance of Photocathodes for Solar Water Splitting

Grain particle size-controlled Cu₂O structures were fabricated by oxidizing copper microfilms by soaking them in a 4 M NaOH solution. A microfilm of Cu₂O was obtained from one-step heating (80 °C for 1 h) of a multilayered film of Cu/Au/Ti (1000/50/10 nm) on a glass substrate followed by naturally cooling under ambient conditions. On the other hand, molding by a micro contact stamp during fabrication of the Cu₂O structure was carried out to obtain Cu₂O structures with a relatively smaller particle size than that of the Cu₂O microfilm, which was obtained by confinement of the Cu₂O crystal growth within the molded space of the micro contact stamp. The Cu₂O phases in the microfilm and the molded Cu₂O samples were confirmed with X-ray diffraction, micro-Raman spectroscopy, and diffuse reflectance spectroscopy. The light absorption capabilities of the Cu₂O samples were characterized by finite-difference time-domain simulations; the wavelength-scale particle size resulted in absorbance enhancement near the Cu₂O band gap, and the mirror effect of the Au film underneath the Cu₂O samples strengthened the absorbance over the entire monitored wavelength region. Moreover, photoelectrochemical characterization proved that the photocurrent of the wavelength-scale Cu₂O particles (obtained with the molding procedure) increased by approximately 2-5 times in the cathodic process, and the onset potential decreased by approximately 0.3 eV compared to that of the Cu₂O microfilm. The enhanced light absorption and relatively large surface area, due to the wavelength-scale particle size of Cu₂O in the molded sample, would result in an enhancement of the photocatalytic activity of Cu₂O materials.