Effect of Metal Substrate on Photo(electro)catalytic Activity of B-Doped Graphene Modified TiO₂ Thin Films: Role of Iron Oxide Nanoparticles at Grain Boundaries of TiO₂

TiO₂ thin films deposited on stainless steel (SS) substrates exhibit low photocatalytic (PC) activity when calcined above 350 °C. The cause is the presence of Fe³⁺ into the film due to its diffusion from SS to the surface during the calcination process. Over the past two decades, most researchers accepted the idea that Fe³⁺ acts as recombination center of photogenerated electrons and holes, although the role of Fe³⁺ has not been studied. To understand the effect of Fe³⁺ on the PC and photoelectrocatalytic (PEC) activity of TiO₂ films, boron-doped graphene-modified TiO₂ (BTG) films supported on SS and Ti were prepared by a sol-gel method. The surface of BTG films was characterized by GIXRD, FESEM, XPS, voltammetry, and Mott-Schottky analysis. Photo(electro)chemical properties of BTG films were investigated by open-circuit potential measurements, photovoltammetry, and photocurrent transients, while their PC and PEC activities were evaluated using phenol degradation under ultraviolet irradiation. In addition to previous findings that show Fe³⁺ ions exist as α-Fe₂O₃, surface-chemical composition of the calcined BTG films revealed the presence of α-FeOOH. Iron oxides facilitate carrier recombination by increasing the amount of grain boundaries in the BTG film, which hindered electron mobility. Our findings invalidate the recombination center hypothesis, which remained for two decades.