Novel nano-architectured water splitting photoanodes based on TiO_2-nanorod mats surface sensitized by ZIF-67 coatings

摘要

We report on the synthesis, characterization and application to water photo-dissociation of micrometer-long self-ordered TiO2 nanorod (TDNR) mats, surface sensitized by deposition of cobalt dodecahedral Zeolitic Imidazolate Framework (ZIF-67) coatings. TDNR mats have been grown over glass/FTO substrates from titanium (IV) butoxide, by using a solvothermal procedure at 150 degrees C. Homogeneous micrometer-thick mats of similar to 120 nm wide TDNRs (surface density similar to 15-20 nanorods per mu m(2)) have been obtained. The TDNR mats have been impregnated by crystalline coatings of ZIF-67 MOF. Four different photoelectrodes, containing different amounts of ZIF-67, have been prepared and characterized. X-ray photoelectron spectroscopy analysis revealed that the oxidation state of Co is Co-II. The presence of MOF at the surface of TDNRs has been found to greatly impact the optical properties of TiO2: light absorption is shifted to the Vis region. The apparent band-gaps of the composites were determined from Tauc's plots. The photoelectrodes have been used as photoanodes for water photooxidation in Na2SO4 aqueous solutions. I-V curves have been measured under chopped illumination conditions by linear sweep voltammetry (LSV), under Vis and UV-Vis irradiation. The dynamics of charge transfer at the interface has been investigated by photoelectrochemical impedance spectroscopy (PEIS). In the dark, all photoelectrodes present a capacitive behavior with a high impedance. Under Vis irradiation, only one time constant is observed, and all photoelectrodes impedances are found in the 10(5)-10(6) Omega cm(2) range. Such values are consistent with the observed photocurrents. Under UV-Vis light, PEIS spectra appeared flattened and therefore the data have been fitted with two time constants, with a model considering the presence of surface states the interface. The photoanode presenting the highest photocurrent (16 h of MOF growth) also displays the lowest value for the charge transfer resistance from trap states, clearly evidencing the role of the MOF as a surface co-catalyst with a beneficial effect for water splitting. The presence of MOF at the surface has been also found to increase the rate of charge recombination. The observed photocurrent has been found to be a result of the dynamic interplay between charge transfer and recombination microscopic processes. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.