Size-tunable synthesis of Ag@Pt core–shell nanoparticles as highly active electrocatalysts using ultrafine bubbles and ultrasound

Abstract

In this study, core–shell nanoparticles (NPs) composed of an Ag core and a Pt shell (Ag@Pt NPs) were synthesized. The Ag core was first synthesized using a combination of ultrasound and chemical reduction, followed by the deposition of the Pt shell by chemical reduction in the presence of a non-ionic surfactant and ultrafine bubbles (UFBs). The Pt shell thickness increased in the presence of UFBs. During the shell formation, Pt NPs adsorbed onto the surface of UFBs through electrostatic and hydrophobic interactions. Subsequently, UFBs approached the surface of the Ag core and provided additional Pt NPs onto the core surface, thereby increasing the Pt shell thickness. This demonstrates that the Pt shell thickness can be controlled via UFBs, which enables the efficient utilization of Pt NPs as shell materials. The catalytic activity of Ag@Pt NPs for the oxidation of methanol was evaluated. At Pt/Ag molar ratios of 1.0 and 2.0, the catalytic activity was improved as the Pt shell thickness decreased. For Pt/Ag molar ratios of 0.50 and 0.66, however, Ag@Pt NPs synthesized with UFBs exhibited better catalytic performance than those without UFBs. Morphological analysis suggests that at a Pt/Ag molar ratio of 0.66, UFB-assisted delivery of Pt NPs onto the Ag core contributed to the formation of a thin Pt shell with enhanced Pt coverage on the Ag core, thereby maximizing the catalytic performance. This work establishes a basis for the efficient and environmentally friendly synthesis of highly active metallic NP electrocatalysts.

1. Introduction

Global warning has become an urgent challenge, motivating extensive research into clean and efficient energy conversion technologies. Among these, direct methanol fuel cells (DMFCs) have attracted considerable attention due to their potential as compact and energy-efficient power sources [1], [2]. Methanol is an abundant, inexpensive, and easily handled liquid fuel, which can be stored and transported more conveniently than hydrogen, making it highly suitable for portable and small-scale energy applications. In DMFCs, the methanol oxidation reaction (MOR) at the anode is a critical process, where methanol is electrochemically oxidized to generate electricity. However, MOR proceeds through a complex 2-step pathway, often accompanied by the formation of a CO intermediate. The CO species strongly adsorbs onto catalyst surfaces, poisoning the active sites of catalysts and significantly reducing catalytic performance [1], [2]. Hence, developing catalysts that are both highly active and resistant to CO poisoning remains a central challenge in the field.

Pt-based catalysts are widely recognized as the most effective materials for MOR due to their high intrinsic catalytic activity and low overpotential.^1-3) Nevertheless, the scarcity and high cost of Pt severely limit their practical and large-scale applications. This has driven extensive research into strategies aimed at reducing Pt usage while maintaining or even enhancing catalytic performance. Typical approaches include combining Pt with less expensive metals to form composite materials, controlling shape and size of Pt nanoparticles (Pt NPs), and designing advanced nanostructures to maximize Pt utilization. Among these, core–shell NPs have emerged as particularly promising [3], [4], [5], [6], [7]. In core–shell architectures, a cheaper metal core, such as Au, Fe, or Cu, is coated with a Pt shell. This design not only reduces the amount of Pt required but also introduces synergistic effects between the core and shell, which can enhance catalytic activity and long-term stability. For instance, Au@Pt NPs have been extensively studied as a promising candidate for MOR catalysts [5]. Au cores are chemically stable, easily synthesized due to its favorable reduction potentials, and can therefore serve as effective templates for Pt shell growth. Previous studies [5] have shown that Au@Pt NPs with precisely controlled core diameters and shell thicknesses exhibit enhanced MOR activity and stability compared to conventional Pt catalysts. The geometric features of the Pt shell, including surface area, morphology, and thickness, play a crucial role in determining catalytic performance, highlighting the importance of precise structural control in catalyst design.

Nonetheless, the scarcity and cost of Au have also been increasingly becoming higher. As an alternative core material, Ag has attracted attention due to its lower cost, high conductivity, and chemical stability [3], [4]. Ag can also modulate the electronic structure of the Pt shell, potentially enhancing catalytic performance. However, conventional Ag@Pt NPs often face challenges such as inefficient Pt utilization and difficulties in controlling shell morphology [3], [4]. Therefore, developing advanced synthesis methods that provide precise control over particle size and shell thickness is critical for realizing high-performance Ag@Pt NP catalysts.

Fine bubbles are defined as bubbles with a diameter smaller than 100 µm and are further classified into microbubbles, bubbles from 1 to 100 µm in diameter, and ultrafine bubbles (UFBs), bubbles with a diameter smaller than 1 µm [8], [9]. UFBs have recently emerged as versatile tools in the green synthesis of NPs due to their small size, long-term stability in aqueous solutions, and unique interfacial properties [10], [11], [12]. The UFB-assisted synthesis of NPs is expected to yield the impurity-free surface of NPs. In our previous study [7], we clarified that the mean diameter of Au NPs sonochemically synthesized from HAuCl₄ in the presence of UFBs decreased with number concentration of UFBs. Furthermore, we suggested that the presence of UFBs impacted the Pt shell thickness in the synthesis of Au@Pt NPs [6]. For characterization, the size distribution, mean diameter, and number concentration of UFBs are measured using dynamic light scattering, NP tracking analysis [13], and other relevant techniques. Prior to measurements, the UFB number concentration is adjusted to fall within the detectable range of the respective instruments [14].

Sonochemical reduction is another green synthetic method that leverages ultrasound irradiation to generate acoustic cavitation [15], [16], [17], [18], [19], [20], [21], [22]. The collapse of cavitation bubbles produces localized high-temperature and high-pressure conditions, generating reactive species, including H radicals that reduce metal ions to atoms. This approach enables the synthesis of small, uniform NPs without additional reducing agents or surfactants [15], [16], [17], [18], [19]. Furthermore, the interplay between ultrasonic irradiation and UFBs can be exploited to finely control nucleation of Au NPs, as UFBs serve as additional cavitation nuclei in sonochemical reduction of Au ions [7].

Building on our previous work [6] on Au@Pt NPs, the present study extends the combined UFB and sonochemical strategy to Ag@Pt NPs. By leveraging the unique interfacial properties of UFBs and the environmentally friendly approach of ultrasound, we aim to improve structural control and Pt utilization in the synthesis of Ag@Pt NPs. Furthermore, the catalytic activity of Ag@Pt NPs for MOR was evaluated by cyclic voltammetry (CV) to identify the optimal particle structure for MOR.

2. Experimental methods

2.1. Ultrasonic apparatus

A sample in a reaction vessel made of glass was indirectly irradiated with ultrasound. The sample volume was 4.0 mL. A plate-type transducer (PZT, Honda Electronics) was driven by a power amplifier (1040 L, ENI), which amplified a continuous sine wave generated by a signal generator (WF1943, NF). The effective electric power applied to the transducer was calculated from the transducer voltage and the current measured by an oscilloscope (TDS3014B, Tektronix) and a current probe (TCP202, Tektronix), respectively [20]. The ultrasonic frequency was set at 488 kHz. The outline of the indirect-irradiation ultrasonic reactor is shown in Fig. 1. The sample temperature was 283 K. With the bottom of the reaction vessel approximately 6 mm away from the transducer, the vessel position was adjusted by Z-Stage to maximize the impedance of the transducer [22]. The ultrasonic power measured by calorimetry was 25 W.

Fig. 1.

Indirect-irradiation ultrasonic reactor used in the experiment.

2.2. Materials

AgNO₃, K₂PtCl₄, ascorbic acid, and acetone were purchased from Fujifilm Wako Pure Chemicals. Pluronic F127 (PF127), a non-ionic surfactant, was purchased from Merck. PF127 (M.W. ca. 12,500) is a non-ionic triblock copolymer composed of polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO). Ultrapure water was produced using a system equipped with both Elix-UV20 and Milli-Q Advantage for laboratory use (Merck). UFB water was produced from ultrapure water and air by the pressurized dissolution method (ultrafineGalf FZ1N, IDEC) [8]. Then UFB water was concentrated accordingly using a rotary evaporator. In the rotary evaporator, the temperatures of water in the heating bath and coolant of the concentrator were 323 and 253 K, respectively. The concentration operation was performed at a rotation speed of 30 rpm and a decompression pressure of 7.0 kPa [14]. For evaluation of the electrocatalytic activity of Ag@Pt NPs, CH₃OH, H₂SO₄, and Nafion^T.M. were purchased from Fujifilm Wako Pure Chemicals. Nafion^T.M. is a perfluorosulfonic acid ionomer consisting of a hydrophobic fluorocarbon backbone and hydrophilic sulfonic acid side groups, which enable high proton conductivity.

2.3. Synthesis of Ag@Pt NPs

First, 1.0 mL of a 20 mM AgNO₃ aqueous solution, 1.0 mL of a 0.6 M ascorbic acid aqueous solution, 2.0 mL of ultrapure water, and 0.43 g of PF127 were mixed. The solution was then irradiated with ultrasound under an air atmosphere for 20 min. Subsequently, 0.50 to 2.0 mL of a 20 mM K₂PtCl₄ aqueous solution was added to the system. In other words, the Pt/Ag molar ratio was varied in the range of 0.50 to 2.00. After this operation, the sample solution was mixed with 6.0 mL of either UFB water or ultrapure water. Then the sample was kept undisturbed for 24 h. During this process, the progress of K₂PtCl₄ reduction was easily monitored via the observation of the solution color. While the initial solution exhibited a yellowish color due to the presence of naked Ag NPs, the solution gradually turned black in the next 24 h, indicating the deposition of Pt NPs in the sample solution. No further color change was observed after 24 h, suggesting the completion of the [PtCl₄]^2- reduction. Then the product was collected by performing centrifugation at 10,000 rpm for 20 min (11,000 g, Eppendorf 5417R). The surfactants and residual ascorbic acid were removed through three washing and centrifugating cycles using acetone and ultrapure water. Finally, the product was redispersed in water, and the resulting dispersion of NPs was used as the sample.

2.4. Characterization

Synthesized NPs were observed using transmission electron microscopy (TEM, JEM-2100 Plus, JEOL), energy dispersive X-ray spectroscopy (EDS), and high-angle annular dark-field scanning TEM (HAADF–STEM, JEM-ARM 200F Cold, JEOL). TEM was operated at 200 kV. The TEM samples were prepared by dropping the solution of NPs onto a carbon-coated copper grid (200 mesh) and drying the grid at a room temperature. The chemical states and elemental compositions were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, VG Scientific). The number concentration of UFBs with a diameter ranging from 30 to 1,000 nm was measured by NP tracking analysis (NanoSight LM10, Malvern) [13]. The number concentration of UFBs was defined as the total UFB number detected in 1 mL of ultrapure water. The zeta potential of NPs in an aqueous solution was measured by electrophoretic light scattering (Zetasizer Nano ZS, Malvern).

2.5. Evaluation of electrocatalytic activity

Electrochemical measurements were performed using a CHI800 (CH Instruments). All electrodes used in this study were purchased from BAS Inc. To evaluate the electrocatalytic activity of Ag@Pt NPs for MOR, CV measurements were performed in 3 mL of an aqueous solution containing 0.5 M H₂SO₄ and 0.5 M CH₃OH (scan rate: 0.1 V/s). The electrodes used in CV measurements consisted of a working electrode (glassy carbon modified with metallic NPs using Nafion^T.M. as a binder), a reference electrode (Ag/AgCl), and a counter electrode (Pt wire). Assuming complete reduction of the Ag and Pt precursors, the mass ratio of Nafion^T.M. to Ag and Pt atoms was set to 3.00. After 5 μ L of the colloidal solution was dropped onto the working electrode surface, the electrode was dried at a room temperature for 24 h in an upright position prior to CV measurements.

3. Results and discussion

3.1. Effect of ultrasound on Ag NPs and confirmation of core–shell structure

Fig. 2 shows the effect of ultrasonic irradiation on the size distribution of Ag NPs synthesized by chemical reduction, based on TEM analysis. In Fig. 2, except for the end points, the x-axis tick values represent the midpoints of the Ag NP diameter intervals. For example, a tick value of 5 corresponds to the fraction of Ag NPs with diameters ranging from 2.5 nm to less than 7.5 nm. For the end points, the tick values of 0 and 50 correspond to the fractions of Ag NPs with diameters of < 2.5 nm and ≥ 47.5 nm, respectively. The combination of chemical reduction and ultrasonic irradiation resulted in fewer large NPs and a narrower size distribution, compared with the synthesis without the aid of ultrasound. This is because the chemical and mechanical effects of shock waves generated by a series of cavitation facilitated particle nucleation and suppressed particle aggregation, thereby yielding homogeneous Ag NPs. In addition to chemical reduction by ascorbic acid, ultrasonic irradiation facilitated the reduction of Ag⁺ by H radicals generated in the pyrolysis of water molecules. Furthermore, cavitation collapse produced high-velocity microflows and shock waves, resulting in mechanical effects such as particle fragmentation. This underscores the effectiveness of ultrasound irradiation in chemical reduction syntheses of NPs for catalytic applications, as smaller particles yield larger total surface areas. Regarding the size of Ag NPs synthesized in this method, the mean diameter was 17.2 nm. All subsequent results were obtained using Ag NPs synthesized by chemical reduction under ultrasonic irradiation.

Fig. 2.

Size distributions of Ag NPs synthesized by chemical reduction with and without ultrasonic irradiation.

To confirm the core–shell structure of NPs synthesized in the experimental method described in Section 2.3, EDS measurements were conducted. As an example, the EDS results of a 1.00Pt/Ag molar ratio are shown in Fig. 3. Fig. 3 (a) shows a HAADF-STEM micrograph of NPs synthesized at a Pt/Ag molar ratio of 1.00 without UFBs, and Fig. 3 (b), (c), and (d) are elemental mappings of NPs corresponding to (a). Yellow-colored dots indicate Ag, whereas green-colored dots indicate Pt. Fig. 3 (e) shows the elemental line scan over the entire NP region along the red line in Fig. 3 (a). The results demonstrate that the center, composed of Ag, was surrounded by Pt, indicating that Ag and Pt were distributed according to the structure of Ag@Pt NPs. Thus, Ag@Pt NPs were successfully synthesized by combining ultrasonic irradiation with chemical reduction in the presence of PF127. Note that K₂PtCl₄ was selected as the Pt precursor. Since the standard reduction potential of K₂PtCl₄ [PtCl₄^2-/Pt] (0.76 V vs. SCE) is lower than that of AgNO₃ [Ag⁺/Ag] (0.80 V vs. SCE), the use of K₂PtCl₄ is expected to suppress galvanic oxidation of Ag by [PtCl₄]²⁻ during Pt shell growth.³⁾ Fig. S1 shows the size distributions of Ag NPs (Ag cores) before and after Pt shell growth. The results obtained at all investigated Pt/Ag molar ratios are shown in (a), while comparisons at Pt/Ag molar ratios of 0.50, 0.66, 1.00, and 2.00 are shown in (b), (c), (d), and (e), respectively. No significant change in the size distribution of Ag NPs was observed after Pt shell growth. The mean diameter of Ag NPs before Pt shell growth was 17.2 nm, while the mean Ag core diameter after Pt shell growth was 17.8 nm across the four Pt/Ag molar ratios shown in (b)–(e). These results indicate that the Ag core diameter was largely maintained during Pt shell growth. Furthermore, if substantial Ag dissolution had occurred through galvanic replacement, the released Ag species would be expected to react with Cl⁻ to form AgCl under the present experimental conditions. However, no AgCl particles were observed in the TEM micrographs or detected by EDS analysis, which provides additional supplementary evidence against substantial Ag dissolution. Nevertheless, it remains unclear whether any minor Ag dissolution occurred under the present experimental conditions, and the present results should not be used as a basis for definitively excluding galvanic replacement. Therefore, based on these results, we conclude that the Ag core was largely preserved during Pt shell formation, while acknowledging that the possibility of some degree of galvanic replacement cannot be excluded. To definitively determine whether galvanic replacement occurred, Ag in the recovered Ag@Pt NPs and supernatant/filtrate should be quantitatively analyzed by ICP-OES or ICP-MS following appropriate separation, such as ultrafiltration.

Fig. 3.

(a) HAADF-STEM micrograph of NPs synthesized at a Pt/Ag molar ratio of 1.00 without UFBs. (b) Elemental mapping of NPs corresponding to (a), (c) Ag dots, (d) Pt dots. Red-colored dots indicate Ag, whereas green-colored dots indicate Pt. (e) Elemental line scanning over the entire NP region along the red line in (a).

The role of PF127 in Ag@Pt core–shell formation is shown in Fig. S2. PF127 was introduced primarily as a structure-directing agent rather than as a reducing agent [3], [5], [23]. After the synthesis of Ag NPs, the PPO blocks of PF127 adsorbed onto the hydrophobic surface of Ag NPs. Meanwhile, the PEO blocks were hydrated in the aqueous medium [23], promoting the accumulation of [PtCl₄]^2- near the Ag NP surface through electrostatic and hydrophilic interactions. Upon chemical reduction of [PtCl₄]^2- to Pt NPs, the newly formed Pt NPs were immobilized on the Ag NP surface through hydrophobic interactions, thereby suppressing the aggregation of Pt NPs. Although Pluronic surfactants have been reported to exhibit reducing ability toward certain metal precursors, such as [AuCl₄]^- [24], [25], their reducing ability toward the metal precursors used in this study was expected to be weaker. Therefore, while a minor contribution of PF127 to the reduction process cannot be completely excluded, its primary role in this study was to facilitate the localization of the Pt precursor around the Ag NPs and to stabilize the resulting Ag@Pt core–shell structure.

Furthermore, XPS measurements were performed to investigate the surface chemical composition of Ag@Pt NPs. XPS provides information about elemental compositions and chemical states of atoms on a material surface. Due to its shallow analysis depth (1–10 nm), XPS is widely used to characterize surface compositions, oxidation states, and chemical bonding [26], [27]. Fig. 4 shows the XPS results of a 1.00Pt/Ag molar ratio without UFBs. As shown in Fig. 4 (a), two strong peaks at binding energies of approximately 73 and 76 eV are consistent with Pt 4f_7/2 and Pt 4f_5/2 of Pt⁰. Meanwhile, as observed in Fig. 4 (b), the absence of the Ag 3d_5/2 and Ag 3d_3/2 peaks corresponding to Ag⁰ in the binding energy range of 360 to 380 eV further supports the successful formation of the Ag@Pt core–shell structure, as Ag-derived photoelectrons were attenuated by the Pt shell during the XPS measurement. In general, the detectable depth in XPS is limited to several nanometers because the photoelectron intensity decreases exponentially with shell thickness according to the inelastic mean free path. For Ag 3d photoelectrons excited by Al Kα radiation, the inelastic mean free path in Pt is typically on the order of ∼ 1–2 nm. Consequently, when the Pt shell thickness exceeds approximately 3–5 nm (about 2–3 times the inelastic mean free path), the Ag core signal becomes strongly attenuated and falls below the detection limit [27].

Fig. 4.

XPS spectra of NPs synthesized at a Pt/Ag molar ratio of 1.00 without UFBs: (a) Pt 4f and (b) Ag 3d regions.

3.2. Effect of UFBs on Pt shell formation

Fig. 5 (a1) and (a2) show TEM micrographs of Ag@Pt NPs (Pt/Ag molar ratio: 1.00, UFB concentration: (a1) 0 mL⁻¹, (a2) 9.37 × 10⁸ mL⁻¹). In the absence of UFBs, the shell thickness was small overall, whereas the presence of UFBs led to an apparent increase in the Pt shell thickness. In Fig. 5 (b1) and (b2), TEM micrographs of Ag@Pt NPs at a Pt/Ag molar ratio of 2.00 are presented (UFB concentration: (b1) 0 mL⁻¹, (b2) 9.37 × 10⁸ mL⁻¹). A similar increasing trend was also observed at this molar ratio. Fig. 5 (c) quantifies these findings above. The Pt shell thickness was calculated by averaging the shell thickness at three points 120 degrees spaced apart on the shell circumference for each of Ag@Pt NPs. At both Pt/Ag molar ratios, the Pt shell thickness increased in the presence of UFBs. These results highlight precise control of the Pt shell thickness via UFBs, thereby enabling efficient use of Pt NPs as shell materials. In contrast, in the absence of UFBs, a large amount of Pt precipitate was observed at the bottom of the glass vial. This indicates that most Pt NPs aggregated and precipitated, and that only a limited amount was deposited on the surface of Ag@Pt NPs. The Pt shell thickness distributions of Ag@Pt NPs synthesized with/without UFBs at Pt/Ag molar ratios of 1.00 and 2.00 are shown in Fig. S3 (a) and (b), respectively. The analysis revealed that, in the presence of UFBs, the overall shell thickness distribution exhibited a tendency to shift toward greater thickness.

Fig. 5.

TEM micrographs of Ag@Pt NPs synthesized at different UFB concentrations at a Pt/Ag molar ratio of (a) 1.00 and (b) 2.00: (a1), (b1) 0 mL⁻¹, (a2), (b2) 9.37 × 10⁸ mL⁻¹. (c) Effect of UFBs on the mean thickness of the Pt shell for Pt/Ag molar ratios of 1.00 and 2.00.

Fig. 6 proposes the mechanism of the Pt shell formation with and without UFBs. During the formation of the Pt shell, Pt NPs deposited in the bulk solution are considered to play a crucial role in increasing the Pt shell thickness. The pH of the sample solution at which it was kept undisturbed for 24 h was 3.27. Under the experimental conditions, the measured zeta potentials of Pt NPs and Ag@Pt NPs (Pt/Ag: 1.00, synthesized without UFBs) were −13.6 and −20.6 mV, respectively. The zeta potential of UFBs was estimated to be approximately + 14.2 mV,¹²⁾ as the influence of the nonionic surfactant PF127 on the zeta potential of UFBs was negligible. As a result, negatively charged Pt NPs were attracted to the positively charged UFB surface through electrostatic and hydrophobic interactions. In addition to Pt NPs, PF127 also adsorbed onto the surface of UFBs, suppressing the aggregation of Pt NPs on the UFB surface. Since the absolute value of the zeta potential of UFBs was greater than that of Pt NPs, Pt NP–covered UFBs were regarded as positively charged. Consequently, they approached negatively charged Ag@Pt NPs via electrostatic and hydrophobic interactions, which was followed by Pt NPs on the UFB surface migrating to the Ag@Pt NP surface by adhering to pre-existing Pt NPs constituting the shell of Ag@Pt NPs, owing to the intrinsic tendency of Pt NPs to aggregate. This transfer of Pt NPs from UFBs to Ag@Pt NPs caused an increase in the Pt shell thickness. It should be noted that, as mentioned in Section 3.1, during Pt shell growth, the possibility of some degree of Ag dissolution due to galvanic replacement cannot be excluded. Therefore, the mechanism proposed in Fig. 6 should be regarded as a plausible interpretation of the observed experimental data rather than as a definitive mechanistic conclusion.

Fig. 6.

Illustration of the mechanism of the Pt shell formation in the presence of UFBs.

3.3. Evaluation of electrocatalytic activity

The catalytic activity of Ag@Pt NPs for MOR was investigated by CV. A series of relevant chemical reactions is described as follows [1], [2], [3]

$${\text{CH}}_{\text{3}}{\text{OH}}_{\text{ad}}\to {\text{CO}}_{\text{ad}}+{\text{4H}}^{+}+{\text{4e}}^{-}$$ (1)

$${\text{CO}}_{\text{ad}}+{\text{H}}_2{\text{O}}_{\text{ad}}\to {\text{CO}}_2+{\text{2H}}^{+}+{\text{2e}}^{-}$$ (2)

In Reaction (1), CH₃OH_ad, adsorbed on the Pt shell, is oxidized to CO_ad. Subsequently, CO_ad is oxidized to CO₂ in Reaction (2), thereby regenerating a previously occupied active site on the Pt shell.

Fig. 7 shows Pt-mass-normalized cyclic voltammograms of MOR catalyzed by Ag@Pt NPs at different Pt/Ag molar ratios, measured over a potential range of −0.2 to + 1.2 V (vs. Ag/AgCl). The current density peak I₁ is defined as the anodic peak current density measured for Reaction (1), whereas I₂ corresponds to the anodic peak current density measured for Reaction (2). The CO_ad poisoning tolerance can thus be quantified as a ratio of these two current densities, I₁/I₂.³⁾ In Fig. 7, the peak I₁ was observed at approximately 0.6 V (vs. Ag/AgCl), whereas I₂ was observed at approximately 0.27 V (vs. Ag/AgCl). Although not shown in Fig. 7, no catalytic activity was observed for Ag NPs without the Pt shell. At a Pt/Ag molar ratio of 1.00, Ag@Pt NPs synthesized without UFBs exhibited a slightly higher catalytic activity than those prepared with UFBs, as evidenced by the comparison of I₁ at approximately 0.6 V (vs. Ag/AgCl). This is because as observed in Fig. 5, the Pt shell thickness increased in the presence of UFBs, and the thicker Pt shell was less favorable for CO_ad removal due to the increased adsorption energy of CO_ad [1], [2], [3], [28], [29], further inhibiting Reaction (1), which can be attributed to the diminished synergistic effects from the Ag core [1], [2], [3], [28], [29]. As the Pt shell became thicker, Pt atoms on the shell surface increasingly resembled bulk Pt, leading to stronger CO adsorption and consequently hindering its oxidative removal. This is further supported by the lower CO_ad poisoning tolerance I₁/I₂ observed for Ag@Pt NPs prepared with UFBs compared to those without UFBs at this Pt/Ag molar ratio. However, at a Pt/Ag molar ratio of 0.66, Ag@Pt NPs synthesized with UFBs exhibited a higher I₁ than those without UFBs. A detailed discussion of this matter is provided in the following paragraphs.

Fig. 7.

Pt-mass-normalized cyclic voltammograms of MOR catalyzed by Ag@Pt NPs at different Pt/Ag molar ratios. The UFB concentration was 3.72 × 10⁸ mL⁻¹.

Fig. 8 (a) shows the relationship between the Pt/Ag molar ratio and the mean thickness of the Pt shell, whereas Fig. 8 (b) shows the relationship between the Pt/Ag molar ratio and the current density obtained from CV measurements. The mean thickness of the Pt shell increased with Pt/Ag molar ratio. Fig. 8 (c) shows the relationship between the mean thickness of the Pt shell and the current density. At Pt/Ag molar ratios of 1.00 and 2.00, Ag@Pt NPs synthesized without UFBs slightly outperformed those with UFBs. However, this magnitude order was reversed at a Pt/Ag molar ratio of 0.66, at which with the aid of UFBs, the highest catalytic activity was achieved among all the tested samples. Thus, in this study, the catalytic activity was maximized for Ag@Pt NPs with an Ag core diameter of 17.2 nm and a Pt shell thickness of 4.86 nm. Ag@Pt NPs exhibited enhanced catalytic activity due to both electronic and lattice strain effects induced by the Ag core. The synergistic effects modulated the d-band center of Pt atoms on the shell surface, optimizing the adsorption energies for the reactants [1], [2], [28], [29]. However, when the Pt shell was too thin (lower than 4.75 nm), the adsorption of H₂O_ad on the Pt shell surface became excessively strong in Reaction (2), thereby inhibiting the progression of Reaction (2) and reducing overall catalytic activity [1], [2], [28], [29].

Fig. 8.

(a) Relationship between the Pt/Ag molar ratio and the mean thickness of the Pt shell. (b) Relationship between the Pt/Ag molar ratio and the current density measured by CV. (c) Relationship between the mean thickness of the Pt shell and the current density.

Fig. 9 (a) and (b) show HAADF-STEM micrographs of Ag@Pt NPs synthesized with and without UFBs at a Pt/Ag of 0.66, respectively. In HAADF-STEM micrographs, the image intensity follows Z-contrast, where it scales approximately with a power of the atomic number (Z). In the case of Ag@Pt NPs, Pt (Z = 78) possesses a much higher atomic number than Ag (Z = 47), resulting in stronger electron scattering. Consequently, the Pt shell appears brighter, while the Ag core appears relatively darker, enabling clear visualization of the core–shell structure. In the presence of UFBs, the thin and predominantly uniform Pt shell was present over the Ag core, and the measured lattice fringe spacing of 0.23 nm in the Pt shell region corresponds to the (1 1 1) plane of face-centered cubic (fcc) Pt. The UFB-induced improvement in shell thickness uniformity was also observed for Au@Pt NPs [6]. This shell morphology was thought to be the most favorable for catalyzing MOR. In the absence of UFBs, however, exposure of the Ag core surface was more frequently observed.

Fig. 9.

HAADF-STEM micrographs of Ag@Pt NPs synthesized (a) with and (b) without UFBs at a Pt/Ag molar ratio of 0.66.

The Pt shell thickness distributions of Ag@Pt NPs synthesized with/without UFBs at the Pt/Ag molar ratio are shown in Fig. 10. In Fig. 10, except for the end points, the x-axis tick values represent the midpoints of the Pt shell thickness intervals. For example, a tick value of 2 corresponds to the fraction of Pt shells with thicknesses ranging from 1 nm to less than 3 nm. For the end points, the tick values of 0 and 16 correspond to the fractions of Pt shells with thicknesses of < 1 nm and ≥ 15 nm, respectively. The analysis revealed that those synthesized without UFBs exhibited a higher fraction of Pt shells with thicknesses of < 1 nm. This range roughly corresponds to the proportion of the Pt shell with a thickness of 0 nm, which is, in other words, the exposed surface of the Ag core. Thus, in the absence of UFBs, a lower Pt coverage on the Ag core hindered an enhancement in the activity, since Ag NPs alone possessed no catalytic activity. On the other hand, the fraction of Pt shells with thicknesses of 1–5 nm was higher in the presence of UFBs than in their absence. These structural changes are thought to be responsible for the enhanced catalytic activity toward MOR. These findings suggest that the UFB-assisted delivery of Pt NPs onto the Ag core led to the formation of a thin Pt shell with a higher Pt coverage on the Ag core, thereby maximizing the catalytic performance of Ag@Pt NPs for MOR. It should be noted that, due to the resolution limit of the HAADF-STEM observations, nanoscale pinholes cannot be reliably identified from the Pt shell thickness distribution obtained from the HAADF-STEM micrographs, and such defects may occur locally even in regions where the mean Pt shell thickness is relatively large. Although the presence of such nanoscale pinholes cannot be completely excluded even for Ag@Pt NPs synthesized with UFBs, the substantial and predominantly continuous Pt coverage around the Ag core observed in the HAADF-STEM micrographs suggests that such defects, if present, are unlikely to have been a major factor in the difference in catalytic activity between the samples synthesized with and without UFBs.

Fig. 10.

Pt shell thickness distributions of Ag@Pt NPs synthesized with/without UFBs (Pt/Ag = 0.66).

Furthermore, as presented in Table 1, Pt/Ag molar ratios of 0.50 and 0.66 demonstrated higher CO_ad poisoning tolerance I₁/I₂ in the presence of UFBs than those without UFBs. This result is likely attributable to the more uniform deposition of Pt NPs over the Ag core surface, as observed in Fig. 9 (a). In Fig. 8 (c), the current density was maximized at a Pt shell thickness of 4.86 nm (Pt/Ag: 0.66, synthesized with UFBs). Thus, the combination of UFBs and a low Pt/Ag molar ratio yielded the optimal Ag@Pt core–shell structure, where prompt CO_ad removal and moderate H₂O adsorption were balanced in Reaction (2), achieving the highest catalytic activity. This combination enabled the efficient utilization of a small amount of Pt NPs as superior electrocatalysts for MOR.

Table 1.

Relationship between the experimental conditions and the CO poisoning tolerance of Ag@Pt NPs.

Pt/Ag molar ratio	0.50 (UFBs)	0.50 (w/o UFBs)	0.66 (UFBs)	0.66 (w/o UFBs)
I1/I2	3.33	1.74	1.98	1.32

4. Conclusions

Ag@Pt NPs were successfully synthesized through a combination of ultrasound and chemical reduction in the presence of PF127. Ultrasonic irradiation during the reduction of Ag⁺ by ascorbic acid enabled the synthesis of homogeneous Ag NPs. The presence of UFBs during Pt shell growth led to an increase in the thickness of the shell. During this process, Pt NPs adsorbed onto the UFB surface through electrostatic and hydrophobic interactions. UFBs subsequently approached the surface of the Ag core, facilitating the delivery of additional Pt NPs and thereby promoting further Pt shell deposition. This proposed mechanism provides a plausible interpretation of the observed increase in Pt shell thickness. Ag@Pt NPs (Pt/Ag ≧ 1.0) synthesized without UFBs outperformed those with UFBs. This trend was reversed at a Pt/Ag molar ratio of 0.66, where the presence of UFBs resulted in the formation of Ag@Pt NPs with reduced exposure of the Ag core. In summary, this composition, combined with UFBs, achieved the highest catalytic activity of all the samples. Our findings in this study pave the way for the efficient and environmentally friendly synthesis of highly active metallic NP electrocatalysts, addressing the global challenge of resource constraints for achieving a sustainable green society.

CRediT authorship contribution statement

Yuki Mizuno: Writing – original draft, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Jiye Jin: Resources. Tsuyoshi Yamaguchi: Writing – review & editing, Project administration. Keiji Yasuda: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article: