Activating Molecules, Ions, and Solid Particles with Acoustic Cavitation

Uranyl ion sonoluminescence is extremely weak in HClO₄ solutions: though typical light absorption by UO₂²⁺ ions is observed below 500 nm, emission lines from excited (UO₂²⁺)* (centered at 512 nm and 537 nm) are hardly seen (Figure 1). The SL of UO₂²⁺ is quenched. This quenching can be attributed to reduction of the excited uranyl ion by a coordinated water molecule ^11-13:

(UO₂²⁺)* + H₂O → UO₂⁺ + H⁺ + OH^•

Figure 1: Sonoluminescence spectra of HClO₄ (0.2 M; 65 W) and UO₂²⁺ (0.1 M) in HClO₄ (0.2 M; 81 W) at 203 kHz, 10 °C, Ar. Black curve shows the emission spectrum of 0.2 M perchloric solution without uranium in the presence of argon. This spectrum consists of OH radical emission centered at 310 nm and a broad continuum spanning from the UV to the near IR spectral range. Blue curve demonstrates the emission spectrum of the same solution but in the presence of uranyl ions (0.1 M). The photons emitted by collapsing bubbles in the range of 250-450 nm are absorbed by UO₂²⁺ species almost completely. However, emission lines from excited (UO₂²⁺)* at 512 nm and 537 nm are very weak. Moreover, these emission lines can be observed only at 203 kHz which also exhibits the most intense sonoluminescence from sonicated water in the studied range of frequencies. Reproduced with permission from ⁶.

Besides, the mass spectrometer measures a decrease by 40% in H₂ formation rate during the sonolysis of HClO₄ solutions when UO₂²⁺ concentration is increased from 50 to 100 mM. This decrease indicates that the excited uranyl ions can also be quenched by products of the sonochemical water split (H₂ and H₂O₂):

(UO₂²⁺)* + H₂ → UO₂⁺ + H⁺ + H

(UO₂²⁺)* + H → UO₂⁺ + H⁺

(UO₂²⁺)* + H₂O₂ → UO₂⁺ + H⁺ + HO₂^•

This quenching pattern resembles quenching of the radioluminescence of uranyl ions by radiolytical products of water ¹⁴.

By contrast, in H₃PO₄ solutions the SL of UO₂²⁺ is intense and, in the right conditions of US frequency and UO₂²⁺ concentration, even visible to the naked eye (Figure 2). The light emission zone shows a layered structure arising from the standing acoustic wave field inside the sonochemical reactor: brighter zones correspond to the antinodes of acoustic pressure, where the concentration of cavitation bubbles is highest.

Figure 2: Photograph of UO₂²⁺ sonoluminescence (3.1 x 10^-2 M) in 0.5 M H₃PO₄ at 203 kHz, 63 W, 10 °C, Ar, exposure time 30 sec. The picture is taken through the quartz window of the sonoreactor. The halo around the major light emission zone originates from light reflection in the reactor’s cooling jacket. Scale bar 2.5 cm. The light emission zone is layered indicating that there is a standing acoustic wave field inside the reactor. Reproduced with permission from ⁶.

The intense SL of UO₂²⁺ in H₃PO₄ (Figure 3) is enabled by strong complexation of UO₂²⁺ by phosphate ions ¹⁵, which protects uranyl from quenching. Through the formation of uranyl phosphate complexes (UO₂²⁺)* lifetime gets almost 200 times longer¹⁶ and UO₂²⁺ molar extinction coefficient twice as much. These two effects lead to an enhancement of UO₂²⁺ sonoluminescence, while the pure acids show similar SL intensities.

Figure 3: Sonoluminescence spectra of H₃PO₄ (0.5 M) and UO₂²⁺ (5 x 10^-2 M) in H₃PO₄ (0.5 M) at 203 kHz, 61 W, 10 °C, Ar. The strong emission lines at 496 nm, 517 nm and 540 nm (blue curve) are attributed to (UO₂²⁺)* emission. Reproduced with permission from ⁶.

Excitation of uranyl ions may proceed by two mechanisms, both of which can be provided by ultrasound. In the first excitation pathway, excitation proceeds by absorption of part of the light emitted by cavitation bubbles at collapse. In the second pathway excitation is induced by collisions at the bubble interface with the "hot" particles from the sonochemical plasma, whereby kinetic and excitation energy from the “hot” particles may be transferred to uranyl ions. To determine which excitation mechanism prevails, the intensity of uranyl SL is measured for different uranyl concentrations in H₃PO₄ at 204 kHz. As can be seen in Figure 4, for very dilute solutions ([UO₂²⁺] < 1 x 10^-4 M) the intensity of uranyl SL increases linearly with UO₂²⁺ concentration. For higher concentrations up to 2 x 10^-2 M the intensity of uranyl SL is almost independent from UO₂²⁺ concentration. These two domains correspond to a photoexcitation mechanism, where the emission is proportional to the number of absorbed photons. It is therefore proportional to UO₂²⁺ concentration until absorption of the UV part of the SL spectrum becomes too important. It is then approximately constant.

Figure 4: Variation of (UO₂²⁺)* relative emission peak heights at 516 nm and 539 nm as a function of UO₂²⁺ concentration in H₃PO₄ (0.5 M) for diluted solutions; at 203 kHz, 82 W, 10 °C, Ar. The seeming maximal intensity of uranium sonoluminescence observed at [UO₂²⁺]~0.001 M is related to uranium self-absorption that decreases the intensity at higher concentrations (see Ref. ³).

In more concentrated solutions ([UO₂²⁺] > 3 x 10^-2 M), when the amount of uranyl ions at the interface of collapsing bubbles becomes high enough, uranyl SL starts to increase again with uranyl concentration. This additional luminescence is due to a second excitation mechanism, namely by collisions with “hot” particles formed in the plasma of cavitation bubbles at collapse.

Figure 5: Variation of (UO₂²⁺)* relative emission peak heights at 516 nm and 539 nm as a function of UO₂²⁺ concentration in H₃PO₄ (0.5 M) for more concentrated solutions; at 203 kHz, 82 W, 10 °C, Ar. Reproduced with permission from ⁶.

By coupling gas mass spectrometer, UV/Vis spectroscopy and ICP-OES measurement, it’s possible to determine the reduction kinetics of platinum ions within our conditions (Figure 7). The results indicate that the Pt(IV) reduction is a two-step mechanism with the subsequent reduction of Pt(IV) ions into Pt(II) and finally in Pt(0) as reported in the literature ¹⁷. The sharp increases of CO₂ signal in the gas phase (Figure 7) is evidence that the main reducing agent in the system is CO following the equations 1 and 2:

PtCl₆^2- + CO + H₂O → PtCl₄^2- + CO₂ + 2H⁺ + 2Cl^– (1)

PtCl₄^2- + CO + H₂O → Pt⁰ + CO₂ + 2H⁺ + 4Cl^– (2)

Figure 7: Evolution of the concentrations of platinum(IV) and (II) in solution and estimated percentage of metallic Pt (dash lines). Hydrogen and CO₂ emission curves obtained during ultrasonic irradiation in pure water under Ar/CO atmosphere (solid lines). T = 20±1 °C and P_ac = 0.35 W/ml. Ultrasonic irradiation was triggered at 10 min on the time axis and platinum solution was added 30 min later. This figure has been modified from ⁷.

Experiments in pure water under argon atmosphere confirm that the reduction of platinum ions can occur by in situ formed hydrogen. Indeed, acoustic cavitation can induce the homolytic split of water molecules inside cavitation bubbles into H and OH° radicals. Recombination of H atoms and OH° radicals leads respectively to the formation of H₂ and hydrogen peroxide into solution ¹⁸. The presence of oxidative species namely OH° radicals and/or hydrogen peroxide in the system can be a hindrance for the reduction process. Recently it was reported that the addition of a small amount of CO within the gas phase, can totally scavenge the hydroxyl radicals formed under ultrasonic irradiation (equation 3) ¹⁹. Note that this reaction leads also to an increase of H₂ formation rate, compared to the sonolysis of pure water under Ar, in agreement with equations 3 and 4 ⁷.

CO + OH^• → CO₂ + H (3)

2H → H₂ (4)

Thus, CO is not only a reducing agent but also plays the role of OH radical scavenger. However bubbling CO into a Pt(IV) solution without ultrasonic irradiation at room temperature doesn’t lead to the formation of Pt nanoparticles but to a pink colloidal suspension of platinum carbonyl species ²⁰. Thus, the carbon monoxide ability to reduce platinum ions at room temperature is enhanced due to the local heating in the liquid shell surrounding the bubble during its collapse ⁷. In these conditions the sonochemical reduction at room temperature could be observed within few hours and lead to the formation of metallic Pt particles within the range of few nanometers as observed in Figure 8. This procedure can be considered for a large array of experimental conditions since only the gas phase has to be changed and no further reagent need to be added within the liquid medium. Noteworthy, the design of a reduction process carried out at low ultrasonic frequency will be of utmost interest for the controllable Pt NP deposition on various specific supports even on thermosensitive materials like polymers ²¹.

Figure 8: High resolution TEM image of Pt nanoparticles obtained under ultrasonic irradiation in pure water with Ar/CO atmosphere. Picture of obtained brown colloid is given as an inset with an overview of Pt nanoparticles at low magnification. This figure has been modified from ⁷.

The 5 hr sonolysis of a PuO₂ suspension in pure water under Ar bubbling leads to the formation of a green colloid solution which remains stable for months after centrifugation (Figure 9). The application of ultrasound in heterogeneous systems is known to generally enhance erosion, fracture, and dispersion of solid surfaces ^22-24. However, the same experiment performed with CeO₂ or ThO₂ as surrogates of PuO₂ do not lead to the formation of colloids. A possible reaction between PuO₂ and sonochemically generated chemical species could explain this phenomenon. When increasing the duration of ultrasound irradiation from 5 hr to 11 hr, the concentration of colloid is found to be increased. UV-Vis spectra obtained with these conditions are different than the one observed for Pu colloids obtained by conventional techniques (Figure 10), for example by dilution of concentrated Pu(IV) solution ²⁵.

Figure 9: Scheme of the experiment leading to the sonochemical formation of Pu colloids.

Figure 10: UV-Vis spectra characterizing the Pu colloids obtained from PuO₂ with ultrasound under Ar in pure water (brown curve) and by the dilution of Pu(IV) in pure water (blue curve).

Chemical species generated under ultrasound irradiation are attributed to the acoustic cavitation phenomenon ⁷. Under Ar bubbling, the species detected during the sonication of pure water are H₂ and H₂O₂ in agreement with the following equations:

H₂O → H + OH°

H + H → H₂

HO° + °OH → H₂O₂

In order to clarify the chemical mechanism involved in the formation of Pu colloids, the presence of H₂O₂ can be measured by the TiOSO₄ colorimetric method during sonolysis, by determining the generation of Ti(IV)·H₂O₂ complexes at ~410 nm. Using our experimental settings, the H₂O₂ formation rate is measured to be ~0.7 µM/min and follows a zero order kinetic law in agreement with the literature for similar experiments performed with reactors having the same geometry. In the presence of suspended PuO₂, analyses reveal that the sonolysis of such a solution do not lead to the accumulation of H₂O₂ in solution suggesting a chemical mechanism occurring between PuO₂ and H₂O₂. In order to check this hypothesis, the same experiment can be performed in the presence of a 5 x 10^-2 M H₂O₂ in solution. However, these settings do not lead to the formation of Pu colloids according to the UV-Vis absorption spectra.

The sonochemical formation of Pu colloids can therefore be investigated in pure water under experimental conditions that avoid the formation of H₂O₂. The replacement of Ar atmosphere by Ar/CO (10%) atmosphere allows quenching OH° radicals formed during sonolysis ⁷. In agreement with the literature, Ar/CO bubbling prevents the accumulation of H₂O₂ in solution. However, these experimental conditions lead to the formation of Pu colloids having a UV-Vis absorption spectrum that agrees with what observed under Ar atmosphere (Figure 10). Consequently, H₂O₂ appears to not be the limiting species for the generation of Pu colloids under ultrasound irradiation. Furthermore, Pu colloids can be formed in the absence of H₂O₂ as well. Note also that the sonolysis of PuO₂ suspended in a nitric acid solution do not lead to the formation of Pu colloids. One can conclude that plutonium colloids are formed via Pu(IV) reduction by hydrogen (H₂) originated from water sonolysis followed by the re-oxidation of Pu(III) in solution and formation of hydrolyzed Pu(IV) ultrasmall particles.