(233c) Spatial Tailoring of Dopant Position in Solids for Enhanced Visible Light Photocatalytic Performance

Transition metal (TM) doped semiconductor materials are extensively employed for light harvesting and photocatalytic applications to increase the charge mobility for better performance. The position and site occupancy of the dopant dictate the physical properties of these materials such as light absorption, separation of charge carriers, and the surface reaction kinetics. Commonly used bulk characterization techniques are often indifferent to surface vs bulk dopant occupancy. Therefore, this work addresses the phenomenon of spatially controlled dopant incorporation for controlled optical and photocatalytic properties.

In this work, highly doped TiO₂:Ni²⁺ (15 mol%) nanoparticles are synthesized via sol-gel chemistry. The drying and annealing of the aged sol are shown to affect the segregation of NiO onto the surface of TiO₂. Specifically, it is possible to control the dopant position by varying the moisture exposure time, indicating that the ambient water layer on the surface of TiO₂ plays an important role on surface energetics. The effect of moisture and annealing rate on the dopant position was systematically studied using steady state and time resolved methods (XRD, UV-Vis, Raman, TGA/DSC, and FTIR) to extract crystallization temperatures, optical absorption, bond formation, and surface hydroxyl concentration. XRD results show that the vacuum dried TiO₂:Ni powders formed a doped anatase phase while the air dried powders formed segregated anatase and NiO phases upon annealing. Furthermore, rapid annealing of the air dried TiO₂:Ni powders formed a metastable anatase doped phase while slow annealing resulted in the segregation of NiO phase, which can also be observed in TGA/DSC. The annealing processes result in a shift from discrete peaks to a broad absorption based on the dopant position. The evolving local environment is probed via x-ray absorption spectroscopy (XAS) and High-Resolution TEM to elucidate the difference in the dopant local environment with photocatalytic performance using photoluminescence (PL) spectrometer. Moreover, the charge carrier recombination kinetics in these NPs can investigated using time-resolved PL measurements to understand the reaction pathways. This ability to incorporate higher dopant concentrations while engineering the dopant position in nanostructures will assist in the development of improved solar cells and photocatalytic devices. Finally, a similar trend of dopant segregation was observed with other first row TM doped TiO₂ powders upon slow annealing, suggesting that the segregation of TM dopants in TiO₂ is a function of the dopant size.