| SONOHYDROTHERMAL SYNTHESIS OF NANOMATERIALS Sergey Nikitenko1; 1INSTITUT DE CHIMIE SéPARATIVE DE MARCOULE, Bagnols-sur-Cèze, France; PAPER: 24/AdvancedMaterials/Regular (Oral) SCHEDULED: 12:45/Wed. 30 Nov. 2022/Saitong ABSTRACT: Hydrothermal treatment (10≤P≤200 bar and 150≤T≤325°C) has been proven to be efficient for the synthesis of materials with advanced properties [1]. Power ultrasound is also applied for similar purposes [2]. We have developed an innovative reactor providing simultaneous ultrasonic and hydrothermal treatment, called sonohydrothermal (SHT) reactor, which allows to benefit from the advantages of both techniques. Physical and chemical effects of power ultrasound derive from acoustic cavitation, that is, formation, growth, and implosive collapse of gas-filled microbubbles in a liquid subjected to ultrasonic waves (f>16 kHz). Acoustic noise spectra (ANS) revealed that the effects of 20 kHz ultrasound in hydrothermal water are mostly driven by stable cavitation. In the entire range of studied conditions the ANS exhibit several harmonics (nf<sub>0</sub>, f<sub>0</sub>=20 kHz) indicating nonlinear bubble oscillations synchronized with the fundamental frequency f<sub>0</sub>. However, the spectra at 200°C and pressure of 14 bar are more specific and characterized by strong subharmonic (f<sub>0</sub>/2) and multiple ultraharmonic (nf<sub>0</sub>/2) bands. In addition, these spectra exhibit numerous stochastic oscillations in the vicinity of principal lines indicating strong contribution of chaotic bubble behavior. Addition of TiO<sub>2</sub> nanopowder to SHT reactor heated at 200°C causes the disappearance of subharmonics, ultraharmonics and stochastic oscillations, which can be explained by the stabilization of oscillating bubbles due to the Pickering-like effect. SHT treatment (T=150-200°C, P=6-14 bar) of titanium metal nanoparticles in pure water provides a facile synthetic route to prepare core-shell Ti@TiO<sub>2</sub> nanoparticles composed of quasi-spherical metallic Ti core (20-80 nm) coated by 5-15 nm crystals of defect-free anatase with small amounts of rutile [3]. Ti@TiO<sub>2</sub> NPs exhibit strong photothermal effect in H<sub>2</sub> production from aqueous solutions of glycerol and pure water as well [4]. The apparent activation energy (E<sub>a</sub>=32±2 kJ·mol<sup>-1</sup>) assumes that photothermal effect arises from diffusion of intermediates or from water dynamics at the surface of catalyst. References: [1] A. Rabenau, Angew. Chem. Int. Edit. 24 (1985) 1026-1040. [2] J.H. Bang, K.S. Suslick, Adv. Mater. 22 (2010) 1039-1059. [3] S.I. Nikitenko, T. Chave, C. Cau, H.-P. Brau, V. Flaud, ACS Catal. 5 (2015) 4790-4795. [4] S.I. Nikitenko, T. Chave, X. Le Goff, Part. Part. Syst. Char. 2018, 35, 1800265. |
