Single-bubble sonoluminescence of aqueous suspensions of ZnS or Tb(acac)3·H2O nanoparticles

Abstract

Aqueous suspensions of nanoparticles (below 50 nm in size) of ZnS or Tb(acac)3·H2O crystals were prepared by dispersion under multibubble sonolysis. Single-bubble sonoluminescence (SL) of these colloidal suspensions in an ultrasonic standing wave was found. For a ZnS suspension, the SL spectrum of a bubble moving at acoustic pressure pa = 1.29 bar, recorded with 12 nm resolution, exhibited a ZnS band with a maximum at 470 nm against the background of a wide continuum of bubble luminescence (200–700 nm). At a resolution of 1.2 nm, numerous lines of atomic (Zn, S) and ionic (S+, S2+, S3+) emitters were present in the 270–670 nm range. For a Tb(acac)3 suspension at 12 nm resolution, Tb3+ quasilines were detected at 488, 544, 585, and 618 nm (5D4 – 7Fj transitions, j = 6,5,4,3). At 1.2 nm resolution, no terbium atomic lines were found. The ZnS band and Tb3+ quasilines were present, although with a lower intensity (at a constant continuum intensity), in the SL spectra of suspensions for a stationary (pa = 1.07 bar) bubble. The atomic emitter lines for a stationary bubble were absent in the spectra of both suspensions. The ZnS band and Tb3+ quasilines in the SL spectra were similar to those in the photoluminescence (PL) spectra. However, the spectrum of a moving bubble in a Tb(acac)3 suspension differed from the PL spectrum in the relative intensities of the quasilines for some transitions. It was concluded that a non-equilibrium plasma was ignited in a stationary or moving bubble in suspensions under periodic compression of its gas content. This was the main source of luminescence during sonolysis, which was observed as thermal emission bursts in a plasma with a spectral continuum. In addition, sonophotoluminescence was generated as re-emission of the continuum luminescence partially absorbed by nanoparticles in suspensions; this occurred as flashes of ZnS or Tb3+ characteristic luminescence. Additionally, for a moving bubble, there was luminescence of these emitters associated with the entry of nanoparticles into the bubble and the collisional excitation of emitters in the plasma. In the case of a ZnS suspension, there was luminescence of atomic emitters (Zn, S) that formed upon decomposition and excitation of fragments of the initial emitter (ZnS) in a bubble plasma. The absence of a similar component in the SL spectrum of the Tb(acac)3 suspension may be attributable to low efficiency of the collisional excitation of terbium atoms, which were undoubtedly formed upon decomposition of nanoparticles in a bubble plasma. The emitters and mechanisms of the studied single-bubble sonoluminescence in colloidal suspensions were compared with those for the previously considered (J. Luminescence. 252 (2022) 119389) multibubble sonotriboluminescence in non-colloidal suspensions of ZnS or Tb(acac)3·H2O crystals.