(660e) Structure-Photocatalytic Relationships of Well-Defined TiO2 Nanodomains

Authors: 
Roberts, C. A., Lehigh University
Wachs, I. E., Lehigh University
Puretzky, A. A., Oak Ridge National Laboratory
Phivilay, S. P., Lehigh University


The goal of this study was to examine the photocatalysis of well-defined TiO2 nanodomains supported on SiO2 and to determine their structure-photocatalytic relationships. Understanding how catalytic structure relates to photocatalytic properties (photoluminescence, electron excitation, intermediate and product formation) in these catalysts of known structure will lead to more rapid development in the discovery of improved photocatalysts for specific reactions. Thus, 1-60% TiO2/SiO2 catalysts were synthesized by incipient wetness impregnation of Ti-isopropoxide into the SiO2 support (Cab-O-Sil), with drying followed by calcination at 500 oC. The molecular and electronic structures of the TiO2 nanodomains were determined with in situ Raman and UV-vis spectroscopy. The nature of the TiO2 nandomain was found to change in the following manner as a function of the titania loading: isolated site (1% TiO2/SiO2) 2/SiO2) < 2D sheets (20-40% TiO2/SiO2) < 3D nanoclusters (60% TiO2/SiO2). Studies were conducted using in situ photoluminescence (PL) spectroscopy to determine if the type of TiO2 nanodomain present in the sample affects the emission spectrum. Samples were dehydrated under flowing 10% O2 at 400 oC in order to avoid the quenching effect of water on PL emission. Spectra were collected on a Jobin-Yvon Fluorolog system which also allowed collection of PL maps. Studies were also conducted in situ using a 76 MHz pulsed tunable laser, tuned to 400 nm excitation, and a gated Picostar detector with time resolution in picoseconds. The same dehydration procedure was used and the lifetime of excited states of the various nano-domain containing samples was determined by changing the delay time of the detector. Production of H2 was monitored by gas chromatography for the water splitting reaction in a UV irradiated batch reactor at room temperature.

The results show several main trends. The PL spectra show that as the percent loading of titania increases, the peak emission occurs at higher wavelength excitation, meaning those samples are more easily excited with lower energy irradiation. However, the excitation lifetime measurements indicate the the lower percent loading samples (those containing isolated TiO4 sites) have slower decay rates, meaning there is a greater opportunity for reactions to occur. This finding offers an explanation for the observed higher production of H2 during water splitting by the lower titania loading catalysts when normalized by exposed Ti site.