A novel sonochemical approach to synthesize platinum (Pt) nanoparticles has been developed to produce smaller, and more monodisperse particles than traditional chemical reduction methods such as the sodium borohydride (NaBH₄) reduction method. It was also found that careful experimental manipulation of the ultrasonic conditions such as the ultrasonic frequency (f), the acoustic power (P_acoustic) and the solution volume (V) may be employed to tailor the sonochemical radical yield to the same value for two different ultrasonic systems (20 kHz provided by an ultrasonic probe or sonifier, and 408 kHz by an ultrasonic plate transducer) leading to a similar reduction rate, and as a consequence, to Pt-nanoparticles with similar sizes. The reduction of Pt(IV) was found to be a first order reaction with a rate constant (k) of 0.062 min⁻¹ for both ultrasonic systems, and the particle sizes from TEM measurements were found to be (2.2 ± 0.5) nm and (2.3 ± 0.4) nm for the 20 kHz and the 408 kHz samples, respectively. In comparison, the chemical reduction method resulted in particle sizes of (3.0 ± 0.5) nm. It was also found that the sonochemical synthesis at 408 kHz was more efficient than at 20 kHz with an initial sonochemical efficiency of (0.20 ± 0.04) μmol kJ⁻¹ at 408 kHz and (0.060 ± 0.004) μmol kJ⁻¹ at 20 kHz. The sonochemical synthesis at 408 kHz also yielded nanoparticles of higher purity. Interestingly, direct sonication into the reaction vessel using the 20 kHz sonication probe produced impurities of Ti, V and Al in the solution, indicating that direct sonication at lower frequencies led to (i) probe material (made of Ti alloy – Ti-6Al-4V) erosion induced by cavitation and (ii) a continuous supply of micro meter sized impurities. It was also observed that the introduction of impurities increased the reduction rate constant of Pt(IV) through heterogeneous nucleation (k = 0.12 min⁻¹) as was demonstrated for the 408 kHz system under controlled addition of Ti/Al/V seed microparticles. Direct sonication using a sonifier should therefore be avoided when high purity catalytic materials are required.

Optimizing the surface area of nanoparticles is key to achieving high catalytic activities for electrochemical energy conversion devices. In this work, the frequency range (200 kHz–500 kHz) for maximum sonochemical radical formation was investigated for the sonochemical synthesis of Pt-nanoparticles to assess whether an optimum frequency exists or if the entire range provides reproducible particle properties. Through physical and electrochemical characterization, it was found that the frequency dependent mechanical effects of ultrasound resulted in smaller, more open agglomerates at lower frequencies with agglomerate sizes of (238 $\pm$ 4) nm at 210 kHz compared to (274 $\pm$ 2) nm at 326 kHz, and electrochemical surface areas of (12.4 $\pm$ 0.9) m²g⁻¹ at 210 kHz compared to (3.4 $\pm$ 0.5) m²g⁻¹ at 326 kHz. However, the primary particle size (2.1 nm) and the catalytic activity towards hydrogen evolution, (19 $\pm$ 2) mV at 10mA cm²,remained unchanged over the entire frequency range. Highly reproducible Pt-nanoparticles are therefore easily attainable within a broad range of ultrasonic frequencies for the sonochemical synthesis route.

Maintaining nanoparticle properties when scaling up a chemical synthesis is challenging due to the complex interplay between reducing agents and precursors. A sonochemical synthesis route does not require the addition of reducing agents as they are instead being continuously generated in-situ by ultrasonic cavitation throughout the reactor volume. To optimize the sonochemical synthesis of nanoparticles, understanding the role of radical scavengers is paramount. In this work we demonstrate that optimum scavenger concentrations exist at which the rate of Ag-nanoparticle formation is maximized. Titanyl dosimetry experiments were used in conjunction with Ag-nanoparticle formation rates to determine these optimum scavenger concentrations. It was found that more hydrophobic scavengers require lower optimum concentrations with 1-butanol < 2-propanol < ethanol < methanol < ethylene glycol. However, the optimum concentration is shifted by an order of magnitude towards higher concentrations when pyrolytic decomposition products contribute to the reduction. The reduction rate is also enhanced considerably.

Reducing the amount of noble metals in catalysts for electrochemical conversion devices is paramount if these devices are to be commercialized. Taking advantage of the high degree of particle property control displayed by the sonochemical method, we set out to synthesize Cu@Pt bimetallic nanocatalysts in an effort to improve the mass activity towards the hydrogen evolution reaction. At least 17 times higher mass activity was found for the carbon supported Cu@Pt bimetallic nanocatalyst (737 mA mg⁻¹, E = $-20$$\mathrm{m}$$\mathrm{V}$) compared to carbon supported Pt nanocatalysts prepared with the same ultrasound conditions (44 mA mg⁻¹, E = $-20$$\mathrm{m}$$\mathrm{V}$). The synthesis was found to proceed with the sonochemical formation of Cu and Cu₂O nanoparticles with the addition of PtCl₄ leading to galvanic displacement of the Cu-nanoparticles and the formation of a Pt-shell around the Cu-core.