PEC water splitting using mats of calcined TiO2 rutile nanorods photosensitized by a thin layer of Ni-benzene dicarboxylic acid MOF

Abstract

Photoelectrodes made of calcined TiO2 nanorods (c-TDNRs), surface photosensitized by the deposition of Ni-benzene dicarboxylic acid (BDC) MOF nanocoatings, have been fabricated utilizing a four-step method. First, mats of titanium dioxide nanorods have been synthesized at the surface of FTO/glass substrates from a solution of titanium (IV) n-butoxide. Then the mats have been calcined at 350 °C to remove structural defects which act as recombining sites during photoelectrochemical (PEC) experiments. The average diameter of individual TiO2 nanorods measured on c-TDNR is 190 nm. Then, nickel hydroxide has been electrodeposited by cyclic voltammetry from a pH-neutral aqueous solution of nickel nitrate. Then the hydroxide has been chemically transformed into a MOF of nickel and BDC, using a solvothermal treatment at 450 °C. A 50 nm thick layer of Ni-BDC MOF was thus deposited at the surface of c-TiO2 nanorods. The resulting photoelectrodes have been characterized by SEM, TEM, XRD, XPS, FTIR and UV-Vis. absorption spectroscopy. They have been used as photoanodes to perform PEC water splitting experiments. The Ni-MOF was found to play the double role of surface photosensitizer (with a photoactivity mainly in the UV + Vis. range) and electrocatalyst. An overall current density of 4.48 mA cm−2 was measured at room temperature under UV + Vis irradiation at 100 mW cm−2, and a current density of 3.08 mA cm−2 was measured under dark condition. The calcination of TDNRs and the addition of the nickel MOF were both found to be significantly beneficial. The combination of photoactivity and electroactivity is interesting in view of photo-assisted water electrolysis applications.