Evidence for Crystal-Face-Dependent TiO₂ Photocatalysis from Single-Molecule Imaging and Kinetic Analysis

Abstract

According to the concept of active sites, the activity of heterogeneous catalysts correlates with the number of available catalytic sites and the binding affinity of the substrates. Herein, we report a single-molecule, single-particle fluorescence approach to elucidate the inherent photocatalytic activity of exposed surfaces of anatase TiO(2), a promising photocatalyst, using redox-responsive fluorogenic dyes. A single-molecule imaging and kinetic analysis of the fluorescence from the products shows that reaction sites for the effective reduction of the probe molecules are preferentially located on the {101} facets of the crystal rather than the {001} facets with a higher surface energy. This surprising discrepancy can be explained in terms of face-specific electron-trapping probability. In situ observation of the catalytic events occurring at the solid/solution interfaces reveals the hidden role of the crystal facets in chemical reactions and their impact on the efficiency and selectivity of heterogeneous (photo)catalysts.

Superresolution fluorescence mapping of single-nanoparticle catalysts reveals spatiotemporal variations in surface reactivity

“…Over long times, the two ends of the nanorod are more productive than the middle, although the two ends are not equally active. This heterogeneous distribution of reactivity or active sites on single nanocatalysts has been extensively observed in other single-nanocatalyst systems (2,4,9,11) and is in agreement with a recent report on 1D SiO 2 -coated Au nanorods (2).…”

“…Furthermore, the wide-field imaging format of our approach offers the capability to study multiple nanorods in the same frame, thus providing higher data collection efficiency. Similar imaging techniques have also been used in long-time-averaged analyses to differentiate catalysis on different facets of metal hydroxide and oxide microcrystals (6,9,12), resolve catalytic domains (13), and map static spatial patterns of reactivity (2,4).…”

“…For example, defects, corners, and edges on nanoparticle surfaces have been shown by singlemolecule, superresolution imaging of single nanocatalysts to be more active than other sites. The relative abundance of these sites on nanoparticle catalysts is important for explaining their observed reactivity (2,4,(6)(7)(8)(9)(10)(11)(12)(13).However, prior reports are based on longtime integrations of catalytic activity and thus do not provide information about any dynamic changes in catalytic activity, such as the time-dependent evolution of different active sites. Studies of time-varying catalytic activity are thus arguably more informative than time-averaged observations.…”

Effects of surface stability on the morphological transformation of metals and metal oxides as investigated by first-principles calculations

“…In particular, Liu et al reported that the photoreactivity of (010) facets of TiO 2 is higher than that of the (101) facets, while the (001) facets showed the lowest photoreactivity [116]. However, more recent research has suggested that different facets may play different roles in photoreaction [117,118].…”

Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4

“…Thus, certain facets of a semiconductor prefer reduction while others favour oxidation [18][19][20] . However, reports on the reduction and oxidation reaction facets on the same semiconductor crystal (such as TiO 2 ) were often contradictory in the literature [21][22][23][24][25][26] . For example, it was reported that rutile {011} and anatase {001} faces provided the sites for oxidation, while the rutile {110} and anatase {101} faces offered the sites for reduction 24 .…”