Photogenerated Defects in Shape-Controlled TiO₂ Anatase Nanocrystals: A Probe To Evaluate the Role of Crystal Facets in Photocatalytic Processes

Abstract

The promising properties of anatase TiO2 nanocrystals exposing specific surfaces have been investigated in depth both theoretically and experimentally. However, a clear assessment of the role of the crystal faces in photocatalytic processes is still under debate. In order to clarify this issue, we have comprehensively explored the properties of the photogenerated defects and in particular their dependence on the exposed crystal faces in shape-controlled anatase. Nanocrystals were synthesized by solvothermal reaction of titanium butoxide in the presence of oleic acid and oleylamine as morphology-directing agents, and their photocatalytic performances were evaluated in the phenol mineralization in aqueous media, using O2 as the oxidizing agent. The charge-trapping centers, Ti3+, O–, and O2 –, formed by UV irradiation of the catalyst were detected by electron spin resonance, and their abundance and reactivity were related to the exposed crystal faces and to the photoefficiency of the nanocrystals. In vacuum conditions, the concentration of trapped holes (O– centers) increases with increasing {001} surface area and photoactivity, while the amount of Ti3+ centers increases with the specific surface area of {101} facets, and the highest value occurs for the sample with the worst photooxidative efficacy. These results suggest that {001} surfaces can be considered essentially as oxidation sites with a key role in the photoxidation, while {101} surfaces provide reductive sites which do not directly assist the oxidative processes. Photoexcitation experiments in O2 atmosphere led to the formation of Ti4+–O2 – oxidant species mainly located on {101} faces, confirming the indirect contribution of these surfaces to the photooxidative processes. Although this work focuses on the properties of TiO2, we expect that the presented quantitative investigation may provide a new methodological tool for a more effective evaluation of the role of metal oxide crystal faces in photocatalytic processes.

Surface features of TiO2 nanoparticles: combination modes of adsorbed CO probe the stepping of (101) facets

“…4,5 Among TiO 2 polymorphs, anatase is widely employed because of its higher activity 6,7 and, consequently, a number of preparation methods of TiO 2 anatase nanoparticles (NP) have been developed, ranging from commercially exploited sol-gel and pyrolysis (as for the well-known TiO 2 P25 by Evonik) procedures, to hydrothermal synthesis in the presence of shape-controllers. 8,9 In many cases, the most frequently observed surfaces for such TiO 2 NP are of the (101) type, because of its thermodynamic stability. 10,11 However, stepping of (101) surfaces can occur, resulting in local structures with a different reactivity.…”

Layered sphere-shaped TiO 2 capped with gold nanoparticles on structural defects and their catalysis of formaldehyde oxidation

“…Au/TiO 2 -300 yielded an ESR signal at g = 1.991, which was attributed to Ti 3+ in the anatase lattice. Other observed g-values, 1.967 and 1.985, have been assigned to paramagnetic Ti 3+ (Dimitrijeric et al, 2011;D'Arienzo et al, 2011). Surface Ti 3+ would tend to adsorb atmospheric oxygen, which would then be reduced to O 2 − , producing an ESR signal at g = 2.02 (Anpo et al, 1999;Zuo et al, 2010).…”

CdMoO4 Micro-ellipsoids: Controllable Synthesis, Growth Mechanism, and Photocatalytic Activity

“…However, the changes in the photocatalytic activities of CdMoO 4 products prepared under different pH are not significant. It is well-known that the photocatalytic activity of catalyst is related to its phase compositions, structures, and morphologies, which are greatly influenced by the synthesis conditions [30][31][32][33] . It can be noticed that the photocatalytic activities of CdMoO 4 products are highly related to its crystallinity (Figure 1b-1d) except for the CdMoO 4 products prepared under different pH.…”