Surface sensitization of TiO2 nanorod mats by electrodeposition of ZIF-67 for water photo-oxidation

Results reported in this research paper are related to the photo-oxidation of water into molecular oxygen, using n-type photoelectrodes operated with the support of an external electric bias. These photoanodes are made of 1 cm2 rectangular glass substrates, surface-covered by thin films of conductive fluorine-doped tin oxide (FTO). Thin mats of titanium dioxide nanorods (TDNRs) have been deposited over the FTO surfaces. The TDNRs have then been sensitized by surface deposition of cobalt dodecahedral Zeolitic Imidazolate Framework (ZIF-67) metal organic framwork (MOF). ZIF-67 has been deposited using a two-step process: (i) deposition of cobalt hydroxide by chronoamperometry; (ii) in-situ transformation of cobalt hydroxide into ZIF-67 by chemical reaction with a solution of 2-methylimidazol. The amount of ZIF-67 has been adjusted by monitoring the duration of the electro-reduction step. The photoanodes have been characterized by scanning electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and UV–Vis absorption spectroscopy. The photoelectrochemical responses of the different photoanodes have been measured under chopped illumination conditions. Dark currents as high as 100–150 μA/cm2 and photocurrents of only 10–15 μA/cm2 have been measured on fresh samples with large MOF contents, at an operating potential of +1.0 V vs. saturated calomel electrode, 25 °C, under UV–Vis irradiation. No dark current and photocurrents close to 40 μA/cm2 have been measured on samples with low MOF contents. The dynamics of charge transfer leading to the formation of molecular oxygen has been analyzed by photoelectrochemical impedance spectroscopy to understand the interplay between the OER electroactivity and photoelectroactivity. It is shown that large MOF contents induce two contradictory effects: the first one (faster charge transfer kinetics) is desirable while the second one (faster charge recombination kinetics) is not desirable. Only limited MOF contents are beneficial to maximize the photocurrent density.