Activation/ Deactivation of Surface States of Heterostructure (Anatese-TiO2/Au) Via Addition of Materials with High Band Gap, Rutile TiO2 (R-TiO2) or Low Band Gap Silicon (Si), and Fabrication Strategy

Abstract

The addition of high band gap or low band gap material not only modulates the optical absorption cross-section but also affects the surface state reactivity of heterostructure towards the photoelectrochemical (PEC) water splitting. Further, the deposition strategy to assemble the heterostructure also affects the effectiveness of the heterostructure. To explore such issues, we fabricated the two sets of heterostructures containing (A-TiO2, R-TiO2) and (Si, A-TiO2) via the click chemistry approach through two different strategies, i.e., layer-by-layer heterostructure (LLH), and randomly deposited heterostructure. In LLH strategy, only the addition of high band gap material R-TiO2 enhances the reactivity of the surface states. On the other hand, the addition of the low band gap material Silicon (Si) hampers the reactivity of the surface states of the heterostructure deposited via the LLH. Further, by changing the deposition strategy to RDH, the reactivity of surface states diminishes by adding either the high band gap or the low band gap material. These observations were rationalized from the framework of band structure/band-alignment in the heterostructure and also via electrochemical impedance spectroscopy (EIS).In LLH, the addition of the A-TiO2 quenches electrons from the conduction band (CB) of R-TiO2, and the holes of A-TiO2 are transferred to R-TiO2. The holes from R-TiO2 participate in activating the surface states present at the surface. On the other hand, the addition of silicon to LLH, having CB above the CB of A-TiO2, does not allow the electrons to transfer from A-TiO2 to Si; hence, the CB electrons from A-TiO2 transfer to surface states, which enhances the energy level of surface states, making it more negative than the VB of Si. The negative shift of surface states of A-TiO2 makes it the recombination center. Interestingly, changing the fabrication strategy from LLH to RDH makes the nature of surface states from reactive to recombination states due to the interfacing of all materials with the underlying substrates.EIS was utilized to measure and analyze the low-frequency capacitance due to the reactive surface state. The low-frequency capacitance of LLH with A-TiO2 shows the maxima at onset potential, confirming the role of reactive surface states. On the contrary, low-frequency capacitance increases monotonically for LLH with Si addition, showing the effect of recombination surface states. Overall, the study provides pointers on the interplay between band positions and electron-hole separation in heterostructures and how they can be effectively used to enhance surface reactivity in PEC water splitting.