Nanostructured TiO2 cavitation agents for dual-modal sonophotocatalysis with pulsed ultrasound

Abstract

Current sonochemical methods rely on spatially uncontrolled cavitation for radical species generation to promote chemical reactions. To improve radical generation, sonosensitizers have been demonstrated to be activated by cavitation-based light emission (sonoluminescence). Unfortunately, this process remains relatively inefficient compared to direct photocatalysis, due to the physical separation between cavitation event and sonosensitizing agent. In this study, we have synthesized nanostructured titanium dioxide particles to couple the source for cavitation within a photocatalytic site to create a sonophotocatalyst. In doing so, we demonstrate that site-controlled cavitation from the nanoparticles using pulsed ultrasound at reduced acoustic powers resulted in the sonochemical degradation methylene blue at rates nearly three orders of magnitude faster than other titanium dioxide-based nanoparticles by conventional methods. Sonochemical degradation was directly proportional to the measured cavitation produced by these sonophotocatalysts. Our work suggests that simple nanostructuring of current sonosensitizers to enable on-site cavitation greatly enhances sonochemical reaction rates.

Keywords: Cavitation nuclei; Pulsed ultrasound; Sonophotocatalysis; Titanium dioxide nanoparticles.

Graphical abstract

Fig. 1

Schematic diagram for TFNs mechanism of reaction. (a) Schematic illustration of gas entrapment by TFNs and generation of cavitation event by ultrasonic exposure. Upon bubble collapse, thermal effects, radical formation, and sonoluminescence are induced. (b) Pyrolysis of water caused by cavitation results in generation of H⁺ and hydroxyl radicals. Sonoluminescence will result in photoactivation of the TiO₂, further generating singlet oxygen and hydroxyl radicals. Chemicals in the fluid media, such as methylene blue, can interact with these newly generated radicals to accelerate chemical processing to degradation products (DP). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Morphological and crystal characterization of TFNs particles. (a) Representative XRD spectra from TFNs. Peak locations matched well with anatase structure and planes are given for each peak. (b) SEM micrograph exhibiting TFNs surface structure to have a rough and “porous” morphology. Additionally, the particles assumed a mixture of fully formed, porous, and broken shell morphology. (c) By TEM imaging, the hollow TFNs structure is visualized by the contrast difference from the edge to particle centre. (d) Higher magnification TEM presented the fractured shell morphology of the TFNs (red outline) and gaps in the nanoshell can be can further be observed as lighter pixel intensity spots (yellow arrows). Scale bars are 100 nm for all images. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Cavitation potential of TFNs at various acoustic pressures. Representative cavitation response for TFNs, water, and degassed TFNs is illustrated following irradiation (a) with a 1.1 MHz HIFU transducer and (b) a 0.5 MHz transducer for a range of pressures. The cavitation threshold was defined as the pressure at which the cavitation probability was 50%. The table inset in (a) and (b) describes the mean cavitation threshold and standard deviation from a triplicate for each sample. (c) The power spectral density was further analyzed to validate quality of noise (Below the cavitation threshold, all samples presented As the particles were excited above the cavitation threshold, the power spectral density curve expressed greater evidence of broadband cavitation at both frequencies. Comparatively, as cavitation was not as prominent for both the degassed TFNs and water only samples, the spectral curves did not present as strong broadband signals. (for a and b tables, n = 3, μ ± σ).

Fig. 4

Rate kinetics for MB photo/sonophotodegradation of TFNs particles. (a) Photocatalytic degradation of methylene blue probe was assessed over 30 min with or without TFNs in solution from which the first order rate kinetics were assessed for the total light irradiation time. Comparatively, the rate kinetics for sonocatalytic degradation of MB was assessed over the first 3 min of irradaition at (b) 1.1 MHz and (c) 0.5 MHz. Given the use of pulsed ultrasound, the first order rate kinetics was calculated as a function of ultrasound exposure time (33% total irradiation time). A tabulated summary of MB degradation rates at different stimuli and ultrasound frequencies is given in below the rate kinetics plots. (n = 3, μ ± σ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

TFNs cavitation dynamics and correlation to MB degradation. For the first 3 min of ultrasound irradiation, the PCD signal energy received (arbitrary units), which was proportional to the cavitation response, was quantified and correlated to the corresponding moles of MB remaining in solution. A linear regression was used to quantify the relationship between cavitation and probe degradation. (a) At irradiation with 1.1 MHz and (b) 0.5 MHz, it was observed that MB degraded proportionally to cavitation generated and this was accelerated by irradiation of the TFNs particles. The acoustic response of the particles was visualized by assessing the acoustic cavitation intensity vs time. When irradiated with either (c) 1.1 MHz or (d) 0.5 MHz, cavitation is more clearly sustained over longer exposure times with the TFNs compared to the degassed and no TFNs groups, leading to greater decolorization of the probe.

Fig. 6

Comparative overview TiO₂ nanoparticles for sonodegradation of MB. Rate kinetics comparing TiO₂-based sonodegradation of MB between existing literature and the current work. Data is compiled from Supplementary Table 2, with each data point associated with its corresponding reference. Given variations in reaction volumes and particle/probe concentrations used, the rate kinetics was calculated proportionally to the particle mass used in the reaction container.