Black titania by sonochemistry: A critical evaluation of existing methods

Abstract

In the field of photocatalysis, the fabrication of black titania is a booming topic, as it offers a system with improved solar light harvesting properties and increased overall efficiency. The darkening of white TiO₂ powders can be ascribed to surface hydroxylation, oxygen vacancies, Ti³⁺ centres, or a combination thereof. A handful of studies suggests these defects can be conveniently introduced by acoustic cavitation, generated during sonochemical treatment of pristine TiO₂ powders. In reproducing these studies, P25 TiO₂ samples were ultrasonicated for various hours with a power density of 8000 W/L, resulting in powders that indeed became gradually darker with increasing sonication time. However, HAADF-STEM revealed that extensive erosion of the sonotrode tip took place and contaminated the samples, which appeared to be the primary reason for the observed colour change. This was confirmed by UV-Vis DRS and DRIFTS, that showed no significant alteration of the catalyst surface after sonication. EPR measurements showed that only an insignificant fraction of Ti³⁺ centres were produced, far less than in a TiO₂ sample that was chemically reduced with NaBH₄. No evidence of the presence oxygen vacancies could be found. The enhanced photocatalytic activities of ultrasonicated materials reported in literature can therefore not be ascribed to the synthesis of actual black (defected) TiO₂, but rather to specific changes in morphology as a result of acoustic cavitation. Also, this study underlines the importance of considering probe erosion in sonochemical catalyst synthesis, which is an unavoidable side effect that can have an important impact on the catalyst appearance, properties and performance.

Keywords: Black titania; Lattice defects; Oxygen vacancies; Photocatalysis; Sonochemistry; Ti(3+) centres; Titanium dioxide (TiO(2)); Ultrasonic synthesis.

Graphical abstract

Fig. 1

Calibration curves with linear fit during the first 30 s of sonication. A Hielscher UP200Ht probe fitted with a S26d2 tip was run at 100% amplitude in continuous mode with a polished probe tip (black ▪) or an unpolished probe tip after 7 h of preceding operation (red •). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Colour comparison of pristine P25 and P25 ultrasonicated for different hours at 100% amplitude in continuous mode.

Fig. 3

Suspension of P25 in water (a) before sonication, (b) immediately after 4 h of sonication at 100% amplitude in continuous mode, and (c) after sonication and settling overnight.

Fig. 4

(a) HAADF-STEM image of P25 TiO₂ with eroded probe tip particles, and EDX mapping of (b) Ti, (c) O, (d) Al, and (e) V.

Fig. 5

XRD patterns of pristine P25, and P25 ultrasonically treated for 1, 4, and 7 h at 100% amplitude in continuous mode.

Fig. 6

UV–Vis DR spectra of pristine P25, and P25 ultrasonically treated for 1, 4, and 7 h at 100% amplitude in continuous mode. Vertical dashed line indicates the onset of the absorption edge.

Fig. 7

DRIFT spectra of pristine P25, and P25 ultrasonically treated for 1, 4, and 7 h at 100% amplitude in continuous mode.

Fig. 8

Comparison of X-band CW-EPR spectra of P25, P25 ultrasonically treated for 4 h at 100% amplitude in continuous mode, and chemically reduced P25. The inset zooms in on the EPR peaks of P25 prior and after ultrasonic treatment. All spectra were measured at 10 K in the presence of air, and are shown normalized to the mass. The spectra are also corrected for the differences in the microwave frequency.

Fig. 9

Photocatalytic activity during an acetaldehyde degradation test. The error bars represent the standard deviation on the measurements.