Synthesis of core/shell structured Pd₃Au@Pt/C with enhanced electrocatalytic activity by regioselective atomic layer deposition combined with a wet chemical method

Abstract

We report the synthesis of Pd₃Au@Pt/C by atomic layer deposition (ALD) combined with a wet chemical method. Initially, nearly uniform Pd₃Au nanoparticles (NPs) (∼4.7 nm) supported on carbon with a loading of ∼20 wt% were prepared by reducing metal complexes confined in reverse micelles adsorbed on carbon. Next, a Pt thin layer (less than 1 nm) was selectively deposited on purified Pd₃Au particles instead of on carbon by regioselective ALD that likely arises from the coverage of nucleation sites on carbon by surfactant molecules in conjunction with catalytic decomposition of the Pt precursor on Pd₃Au. The resulting core–shell structured material was subject to TEM, HAADF-STEM/EDX, XRD, XPS and electrochemical measurements. Interestingly, Pd₃Au@Pt/C demonstrates a significantly improved electrocatalytic activity toward both formic acid oxidation reaction (FAOR) and oxygen reduction reaction (ORR) compared with Pd₃Au/C and state-of-the art Pt/C. This study opens up a unique avenue for the synthesis core/shell structured materials with potential applications in catalysis, electrocatalysis, and so on.

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Introduction

Pt-based materials have been applied in sensing,¹ hydrogen storage,² fuel cells^3–7 and so on. However, the limited supply and prohibitive cost of Pt remains a challenge for wide-spread application. The optimization of the performance of Pt-based materials can contribute to gaining Pt usage efficiency and the necessary cost reduction. This can be realized by manipulating the size, morphology, and composition of Pt. In this regard, core/Pt-shell NPs have attracted great attention because of the high utilization efficiency of Pt and the improved activity^8–13 that has been assigned to the electronic effect and geometric effect induced by the interaction between the core and Pt-shell.^14,15 It is worth pointing out that Pd₃Au@Pt NPs have been predicted to possess a high electrocatalytic activity for both ORR and FAOR.^9,16 So, it is necessary to synthesize Pd₃Au@Pt NPs, which have been rarely reported in the previous studies.

Typical wet chemical synthesis of core/shell structured NPs includes seeding method,^17–21 Cu underpotential deposition (Cu-UPD),^8,10–12 and dealloying.^15,22,23 As a gas phase technique, ALD is capable of conformably coating a substrate at the atomic level relying on self-limiting surface reactions.^24,25 Recently, ALD has been applied to the synthesis of core/shell structured metallic NPs supported on oxides.^26–30 However, the core/shell structured NPs supported on carbon by ALD remains a challenge possibly because of the lack of active nucleation sites on carbon, leading to insufficient chemisorption of metal precursors and thus severe particle aggregation.^31,32 Accordingly, pre-formation of uniform Pd₃Au cores on carbon becomes critical for the synthesis of Pd₃Au@Pt/C NPs.

In this study, we present a new synthetic approach for the creation of core/shell structured Pd₃Au@Pt/C by ALD combined with a wet chemical method. The nearly uniform Pd₃Au core on carbon was firstly prepared by reducing metallic complexes confined in reversed micelles and then Pt-shell was selectively deposited on Pd₃Au by ALD likely due to the coverage of nucleation sites on carbon by surfactant molecules in conjunction with catalytic decomposition of Pt precursor on Pd₃Au. The resulting core–shell structured electrocatalyst demonstrates significantly improved activity toward FAOR and ORR compared with Pd₃Au/C and state-of-the art Pt/C. This study provides a unique route to prepare core/shell structured NPs with potential applications in electrocatalysis, catalysis, and so on.

Experimental

Chemicals and materials

Potassium chloropalladate (K₂PdCl₄) and sodium tetrachloroaurate (NaAuCl₄) were purchased from Fengchuan Chemical Reagent Co. (Tianjin, China). Trimethyl-(methylcyclopentadienyl) platinum (Pt(MeCp)Me₃, Strem Chemicals, 99%), O₂ (Air–gas, 99.999%), cetyltrimethylammonium bromide (CTAB) (Sigma-Aldrich), chloroform (Sinopharm Chemical Reagent), and sodium borohydride (NaBH₄, Acros organics) were used as received. All aqueous solutions were prepared with ultrapure water (18.2 MΩ cm at 25 °C) produced from a Millipore water system (Synergy® UV, France).

Synthesis of Pd₃Au/C

10 mL of 20 mM K₂PdCl₄ aqueous solution is mixed with 10 mL of chloroform solution containing 40 mM CTAB. The mixture was left under mild stir at 25 °C for 1 h to guarantee the complete transfer of the palladium complexes to the interior of CTAB reversed micelles formed in the chloroform phase, obtaining Pd(II) ion-containing reversed micelles. Similarly, Au(III) ion-containing reversed micelles in the chloroform phase were prepared with equal volume of 20 mM NaAuCl₄ aq. and 20 mM CTAB chloroform solution. VXC-72 carbon black (about 60.0 mg) was dispersed in 10 mL of the mixture (7.5 mL Pd ion-containing chloroform solution and 2.5 mL Au ion-containing chloroform solution) under sonication for at least 5 min, forming metallic ion-containing reversed micelles absorbed on carbon in chloroform. Next, 90 mL of deionized water was added in and then 10 mL of 300 mM NaBH₄ was added under stirring at 1600 rpm. The mixture turned black immediately, accompanied by bubbling. Finally, we purified the electrocatalyst simply by washing with copious amount of hot water.

Carbon was also treated by above procedure without the addition of metal complexes and labelled as C-PT. VXC-72 carbon black (about 60.0 mg) was dispersed in 10 mL of chloroform containing 40 mM CTAB under sonication for at least 5 min, forming reversed micelles absorbed on carbon. Next, 90 mL of deionized water was added in and then 10 mL of 300 mM NaBH₄ was added under stirring at 1600 rpm. Finally, we purified the carbon simply by washing with copious amount of hot water.

ALD synthesis of Pd₃Au@Pt/C

Platinum deposition was conducted on an ALD instrument (A-DEP, Albuquerque, New Mexico). The purified and dried Pd₃Au/C powder was placed inside of a rotational reactor wrapped by hot jacket. The deposition temperature was 150 °C, while the container of the Pt precursor was kept at 65 °C (800 mTorr) to provide a steady state flux of MeCpPtMe₃ to the reactor. Gas line temperature was held at around 70 °C to avoid precursor condensation. Highly pure O₂ (99.999%) was used as the counter reactant, and high-purity Ar (99.9995%) was used as purging gas and carrier gas. Each ALD cycle includes 30 s of MeCpPtMe₃ pulse, 120 s of adsorption time, 46 s Ar purge, 75 s of O₂ pulse, 120 s of adsorption time and 46 s of Ar purge. In the presence of Pd₃Au/C powder, 5–25 ALD cycles were adopted to synthesize Pd₃Au@Pt/C.

For comparison, platinum was also deposited on bare VXC-72 carbon black with and without phase transfer processing by using the same Pt ALD process as that employed for Pd₃Au@Pt/C. These samples were labeled as Pt/C (ALD) and Pt/C-PT (ALD), respectively. Additionally, in the absence of Pt precursor Pd₃Au/C was also treated by the same ALD process to examine the heating effect of ALD process. This sample was indexed as Pd₃Au/C (heat).

TEM and XRD

Transmission electron microscope (TEM) (JEM-2100, 200 KeV) was used to image obtained Pd₃Au/C, Pd₃Au@Pt/C and Pd₃Au/C (heat). The samples for TEM analysis were prepared by adding drops of colloidal suspension onto a standard copper grid and wicking away the excess liquid with a filter paper. The grids were air-dried for at least 2 h before imaging. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) analyses (JEM ARM200F, 200 keV) were performed to analyze the core/shell structure. The X-ray diffraction (XRD) pattern of Pd₃Au/C and Pd₃Au@Pt/C was collected on a Rigaku D/Max2500V/PC powder diffractometer with a Cu K_α radiation source (40 kV, 100 mA, λ = 0.15432 nm) in the 2θ range of 5–80° at a scanning rate of 5°·min⁻¹. X-ray photoelectron spectroscopic (XPS) measurements were carried out using an ESCALAB 250Xi spectrometer with an Al Kα radiation source.

Thermal analysis, inductively coupled plasma-optical emission spectroscopy and UV-visible spectroscopy

To measure the loading of Pd₃Au/C, thermogravimetry analysis (TGA) for Pd₃Au/C was performed on STD Q600 with dried air atmosphere at a heating rate of 10 °C min⁻¹. Pd₃Au@Pt/C were pyrolyzed under air atmosphere, and then the residual was dissolved in hot aqua regia solution (HCl : HNO₃ = 3 : 1). Next, the solution was diluted for further analysis. Finally, the concentrations of Pt in acidic solution were measured using inductively coupled plasma-optical emission spectroscopy (ICP-OES, PerkinElmer ICP-OES 7300DV).

UV-visible spectra were obtained with a spectrophotometer (Analytic Jena, Specord S600) and a 0.2 cm path length quartz cell.

Electrochemical measurements

All the electrochemical measurements were performed using an Autolab potentiostat/galvanostat (Echo Chemie BV Model PGSTAT-302N, Netherlands). A conventional three-electrode electrochemical cell was used, including a saturated calomel electrode (SCE) as reference electrode that is connected to the cell by a salt bridge (agar gel containing saturated KNO₃) for collecting cyclic voltammetry (CV) and ORR or mercurous sulfate electrode as reference electrode for linear sweep voltammetry (LSV) and chronoamperometry (CA) of FAOR, a platinum mesh (1 cm × 1 cm) as counter electrode, and a working electrode. All potentials in this work were referred to reversible hydrogen electrode (RHE). All electrochemical measurements were performed at 25 ± 0.5 °C.

Certain amount of water, ethanol, Nafion solution (5 wt% Dupont) (v : v : v = 1 : 9 : 0.06) and electrocatalyst were mixed and sonicated in a water bath for 2 min, obtaining a catalyst ink (1 mg mL⁻¹). The working electrodes were glassy carbon rotating disk electrodes (RDE), 5 mm in diameter (0.19625 cm²). Glassy carbon was polished with 0.05 μm Bio-Analytical Systems (BAS) alumina paste. 10 μL of catalyst ink was transferred onto RDE and then evaporated the solvent in the air.

CV curves were recorded at 25 ± 0.5 °C in N₂-saturated 0.1 M HClO₄ aq. with a sweep rate of 50 mV s⁻¹. CV measurements were performed for multiple cycles until obtaining a stable curve. ORR polarization curve of the catalysts was recorded at 25 ± 0.5 °C in O₂-saturated 0.1 M HClO₄ aq. with a positive sweep rate of 10 mV s⁻¹ at 1600 rpm. Current densities in the ORR polarization curves were normalized to the geometric area of the glassy carbon RDE. The kinetic current was calculated based on the Koutecky–Levich equation:

where i is the experimentally measured current density at 0.9 V (vs. RHE), i_d is the diffusion-limited current density, and i_k is the kinetic current density. The kinetic current density was normalized to Pt loading in order to obtain mass activity (MA).

The electrocatalytic activity towards FAOR was evaluated using LSV in a N₂-saturated aq. containing CHOOH (0.5 M) and H₂SO₄ (0.5 M) at 25 ± 0.5 °C. The potential was swept between 0 and 1.1 V (vs. RHE) with a scan rate of 10 mV s⁻¹ at 900 rpm. The CA curves for Pd₃Au@Pt/C and commercial Pt/C (JM) were also recorded in N₂-saturated aqueous solution containing 0.5 M formic acid (FA) and 0.5 M H₂SO₄ with a rotation rate of 900 rpm at 0.5 V (vs. RHE) for 1 h at 25 ± 0.5 °C.

Results and discussion

To well control over the size, Pd₃Au/C was synthesized by a phase-transfer method according to our previous study.^33,34 In a typical synthesis, 10 mL of a 20 mM K₂PdCl₄ aq. was mixed with 10 mL of chloroform solution containing 40 mM CTAB under stirring. The electrostatic interaction between positively charged CTAB and negatively charged metallic complexes PdCl₄²⁻ drives metallic complexes to automatically transfer from aqueous phase to the interior of reversed micelles in chloroform, forming the Pd ion-containing reversed micelles in chloroform. Similarly, the Au ion-containing reversed micelles in chloroform was also obtained by using 10 mL of 20 mM CTAB and 10 mL of 20 mM NaAuCl₄ aq. The UV-visible spectrum and photograph show that the complete transfer of metallic ions from aqueous phase to chloroform in Fig. S1.† Next, carbon powder (XVC-72) was added into the mixture of Pd- and Au-containing reverse micelles at a molar ratio of 3 : 1 under sonication, obtaining metallic ion-containing reversed micelles absorbed on carbon. Then, Pd(II) and Au(III) confined in micellar interior were reduced by aqueous NaBH₄. Finally, Pd₃Au/C was purified with copious amount of hot water that improves the solubility of CTAB absorbed on the surface of Pd₃Au/C.

TEM images (Fig. 1a) and particle size distribution histogram (Fig. S2a†) reveals that the average size of as-prepared Pd₃Au is about 4.7 nm with a size distribution of 34.0% as determined by manually measuring no less than 200 randomly selected individual NPs. Pd₃Au NPs are uniformly dispersed on the surface of carbon without evident agglomeration. Normally, Au- and/or Pd-based nanoparticles tend to grow into chunks without size and size distribution control because of the fast reduction kinetics of Au(III) and Pd(II) by NaBH₄. In our case, the growth of Pd₃Au was confined in the reversed micelles, thus achieving the desired size and size distribution control. The metal loading of Pd₃Au/C was determined to be 20.4 wt% by TGA (Fig. S3†). To confirm the formation of Pd₃Au alloy, Pd₃Au/C was subjected to XRD measurement. Except the diffraction peak of carbon support at 24.7°, the three characteristic peaks at 39.0°, 44.7° and 66.3° are coming from (111), (200) and (220) of face center cubic Pd₃Au alloy (Fig. 1b). The diffraction peaks of Pd₃Au negatively or positively shift with respect to those of pure Pd or Au and no diffraction peak of pure Pd or Au is observed. This verifies that we have successfully prepared Pd₃Au alloy NPs supported on carbon.

Fig. 1 (a) TEM image of Pd₃Au/C prepared by reducing metal complexes confined in reverse micelles adsorbed on carbon; (b) XRD pattern of Pd₃Au/C. Red and blue vertical lines represent the fcc Pd (JCPDS file 46-1043) and the fcc Au (JCPDS file 04-0784), respectively.

We chose MeCpPtMe₃ and O₂ gas for Pt ALD process. In order to prepare the desired core–shell nanostructure, a series of ALD parameters were examined and optimized on our ALD instrument (see details in the Experimental part). In particular, experiments were conducted to study the influence of Pt ALD cycles on ORR performance (Fig. 2). It appears that the ORR performance improves with the increase of ALD cycling number from 5 to 10 to 15, which has reached the best performance at 15 cycles. Further increase of ALD cycling number to 25 did not apparently enhance ORR performance. Eventually, 15 cycles of Pt ALD process were applied to cover Pd₃Au particles at 150 °C. To quantify the deposited amount of Pt, the electrocatalyst was pyrolyzed under dried air atmosphere, and then the residual was dissolved in hot aqua regia solution. In the following, the concentration of Pt in solution was measured using ICP-OES. The loading of platinum on Pd₃Au/C deposited by ALD was determined to be 3.8 wt%.

Fig. 2 (a) CV curve of Pd₃Au@Pt/C with different ALD cycles collected in N₂-saturated 0.1 M HClO₄ aq. with a sweep rate of 50 mV s⁻¹ at 25 ± 0.5 °C between 0 and 1.2 V (vs. RHE); (b) ORR polarization curve of Pd₃Au@Pt/C with different ALD cycles collected at 25 ± 0.5 °C in O₂-saturated 0.1 M HClO₄ aq. with a positive scan rate of 10 mV s⁻¹ at 1600 rpm.

Consistent with the successful ALD process of Pt, the average size of Pd₃Au NPs slightly increases from 4.7 to 5.5 nm (Fig. 3a), indicating a plausible well dispersion of ALD deposited Pt. Additionally, there is no evident agglomeration observed, again suggestive of the desired deposition of Pt on Pd₃Au NPs. However, the contrast between Pt and Pd₃Au is relatively low likely due to the close atomic number between Pt and Au as well as the possible thin nature of the shell. Consequently, HRTEM image cannot allow us to clearly visualize the presence of Pt shell as shown in Fig. 3b. To investigate the heating effect of ALD process, Pd₃Au/C was treated by the same ALD process in the absence of Pt precursor. The average size of resultant Pd₃Au/C (heat) is about 5.8 nm (Fig. 3c), larger than Pd₃Au particles after Pt ALD process. We tentatively propose that the formation of Pt shell restrains the size increase of particles. It is worth conducting further studies to elucidate the origin of size increase after Pt deposition by ALD. Consequently, the XRD pattern of Pd₃Au/C before and after Pt ALD process was studied carefully (Fig. 3d). To our surprise, there is almost no change occurred for the main diffraction peaks of Pd₃Ad alloy after the Pt ALD process, and no diffraction peaks corresponding to Pt NPs appear for the sample after Pt ALD process. The absence of the characteristic diffraction peaks of Pt can be attributed to the formation of thin Pt shell that lacks diffraction matter as observed by others.^12,35,36 Since we did not observe the formation of Pt nanoparticles attached to Pd₃Au in TEM and HRTEM image, neither in XRD pattern, the deposited Pt should be fairly evenly distributed on Pd₃Au and the thickness of Pt sell should be less than 1 nm. For additional comparison, bare VXC-72 carbon black without and with phase transfer processing was treated by the same Pt ALD process to obtain Pt/C (ALD) and Pt/C-PT (ALD). For carbon without phase transfer processing, the hydrogen desorption peaks of Pt/C (ALD) becomes larger at the potential range from 0.1 to 0.4 V (vs. RHE), corresponding to characteristic hydrogen desorption of Pt (Fig. 4a). And the ORR performance significantly improves (Fig. 4b). This implies that Pt deposits on carbon without phase transfer processing. In contrast, Pt cannot be deposited on carbon with phase transfer processing during Pt ALD process. There is no hydrogen adsorption/desorption peaks observed between 0.1 and 0.4 V (vs. RHE) and the corresponding ORR performance is extremely low as compared with that of commercial Pt/C (Fig. 4). This may result from that the nucleation sites on carbon after phase transfer processing are effectively blocked by surfactants even after being washed with hot water, which is consistent with the phenomena reported by Chen et al.²⁹ This experiment indicates that Pt has been selectively deposited on Pd₃Au nanoparticles, which is good agreement with the above results.

Fig. 3 (a) TEM image of Pd₃Au@Pt/C obtained by regioselective Pt ALD process; (b) HRTEM image of Pd₃Au@Pt/C; (c) Pd₃Au/C treated by the same ALD process in the absence of Pt precursor; (d) XRD pattern of Pd₃Au/C and Pd₃Au@Pt/C. Green, red and blue vertical lines represent fcc Pt (JCPDS file 04-0802), fcc Pd (JCPDS file 46-1043), and fcc Au (JCPDS file 04-0784), respectively.

Fig. 4 (a) CV curve of Pt/C (jm), Pt/C (ALD) and Pt/C-PT (ALD) collected in N₂-saturated 0.1 M HClO₄ aq. with a sweep rate of 50 mV s⁻¹ at 25 ± 0.5 °C between 0 and 1.2 V (vs. RHE); (b) ORR polarization curve of Pt/C (JM), Pt/C (ALD) and Pt/C-PT (ALD) collected at 25 ± 0.5 °C in O₂-saturated 0.1 M HClO₄ aq. with a positive scan rate of 10 mV s⁻¹ at 1600 rpm.

The resulting core/shell structured electrocatalyst was characterized by HAADF-STEM in conjunction with EDX analysis. The element mapping of Pt, Pd and Au of Pd₃Au@Pt/C shows that the three element overlap with one another and element Pt surrounds Pd and Au, indicating the formation of Pt shell (Fig. 5a–e). EDX line profile taken across a typical Pd₃Au@Pt nanoparticle confirms the presence of a Pt thin layer (less than 1 nm) (Fig. 5f and g). Taken together, the Pt, Pd and Au elemental mapping and the line profile prove the formation of a core/shell structure after Pt ALD process.

Fig. 5 (a) HAADF-STEM image of Pd₃Au@Pt/C; elemental mapping of (b) Pd, (c) Au, (d) Pt and (e) Pd₃Au@Pt for the area in (a); (f) HAADF-STEM image of a typical Pd₃Au@Pt nanoparticle on carbon black. The red line indicates the electron probe's scanning direction; (g) line-scan profiles of Pt, Pd, and Au showing a core–shell structure.

It seems that Pt ALD process has selectively occurred on the surface of Pd₃Au NPs instead of on carbon. One possible reason is that the active sites for the nucleation of Pt on carbon have been blocked by surfactant molecules. The other reason is that Pd₃Au may catalyze the decomposition of Pt precursor on Pd₃Au,^30,37,38 facilitating the nucleation and growth of Pt on Pd₃Au. It has been shown that Pt can be selectively deposited on the surface of Pt group metallic NPs instead of on the oxide support like Al₂O₃.^{27,29,30,37,38} A catalytic ALD process has been proposed in these cases, analogous to our study. We proposed a possible ALD process, mainly including chemisorption of platinum precursor and subsequent decomposition by oxygen as shown in Fig. 6. Two types of Pt nanostructures might be achieved after ALD process, namely incomplete and complete platinum thin layer deposited on the Pd₃Au core. Therefore, the Pt ALD process in combination with wet-chemical method is an alternative route for the creation of the core–shell structured Pd₃Au@Pt/C as expected.

Fig. 6 Schematic illustration of feasible ALD process for the deposition of Pt on Pd₃Au/C. (1) During the MeCpPtMe₃ exposure, some of the precursors reacts with oxygen-containing functional groups on the surface of Pd₃Au NPs, and the limited surface oxygen provides the self-limiting growth necessary for ALD; (2) during the subsequent oxygen exposure, oxygen reacts with the chemisorbed precursor and forms oxygen species on the Pt surface; step (1) and (2) constitute a complete ALD cycle. O* represents an active surface species. Two Pt nanostructures might be achieved after 15 ALD cycles: incomplete and complete platinum thin layer.

To investigate the electrocatalytic properties, CV, ORR and FAOR of Pd₃Au@Pt/C, Pd₃Au/C, Pd₃Au/C (heat) and commercial 20 wt% Pt/C (JM) were evaluated. As shown in Fig. S4,† the hydrogen desorption peaks of Pd₃Au particles get enlarged after being treated by ALD process. This may be due to the surface atoms reconstruction driven by the heating effect of ALD process. In other words, the enrichment of Pd atoms on surface may have happened because of ALD heating. The hydrogen desorption peaks of Pd is relatively large owning to the easy diffusion of hydrogen atoms in or out the lattice of Pd or forming PdH.^19,39 Compared with that of Pd₃Au/C (heat), the hydrogen desorption peaks of Pd₃Au@Pt/C get diminished in the potential range from 0 to 0.1 V (vs. RHE), and becomes larger at the potential range from 0.1 to 0.4 V (vs. RHE), corresponding to characteristic hydrogen desorption of Pt. This implies that the coverage of the surface of Pd₃Au core by ALD deposited Pt and the formation of Pt shell. Since the hydrogen desorption peak of Pd₃Au@Pt/C at 0–0.1 V (vs. RHE) merely decreases about 50% relative to that of Pd₃Au/C (heat), our Pd₃Au@Pt/C more than likely possesses an incomplete Pt thin shell. Otherwise, the hydrogen desorption peak of Pd₃Au@Pt/C at 0–0.1 V (vs. RHE) should entirely disappear.

The ORR activities of these samples were evaluated in O₂-saturated 0.1 M HClO₄ aq. with a rotation speed of 1600 rpm at 25 ± 0.5 °C (Fig. 7a). In terms of half-wave potential, Pd₃Au/C exhibits the lowest ORR activity. After heat-treatment by ALD, the ORR activity of Pd₃Au/C (heat) improves a little bit. However, the ORR activity of Pd₃Au@Pt/C is close to that of commercial 20 wt% Pt/C. After the deposition of Pt on Pd₃Au, the half-wave potential of ORR positively shifts about 128 mV compared with that of Pd₃Au/C (heat). Moreover, the mass activity of Pd₃Au@Pt/C is 558 mA mg_Pt⁻¹, 2.4 times commercial 20 wt% Pt/C (JM) (233 mA mg_Pt⁻¹) (Fig. 7b). This suggests that a high utilization efficiency and high dispersion of Pt have been achieved, corroborating the formation of thin Pt shell.

Fig. 7 (a) ORR polarization curve of Pt/C (JM), Pd₃Au@Pt/C, Pd₃Au/C (heat) and Pd₃Au/C collected at 25 ± 0.5 °C in O₂-saturated 0.1 M HClO₄ aq. with a positive scan rate of 10 mV s⁻¹ at 1600 rpm; (b) mass activity given as kinetic current densities (j_k) normalized in reference to Pt mass at 0.9 V (vs. RHE) for commercial Pt/C (JM) and Pd₃Au@Pt/C; (c) and (d) linear sweep voltammogram curve of FA for commercial Pt/C (JM) and Pd₃Au@Pt/C at 25 ± 0.5 °C in N₂-saturated 0.5 M H₂SO₄ aq. containing 0.5 M formic acid with a positive scan rate of 10 mV s⁻¹ at 900 rpm.

Fig. 7c and d show the FAOR activity of Pd₃Au@Pt/C and commercial 20 wt% Pt/C (JM) that was evaluated in N₂-saturated 0.5 M H₂SO₄ aq. containing 0.5 M FA at a scanning rate of 10 mV s⁻¹. Pd₃Au@Pt/C exhibits two oxidation peaks at ca. 0.50 V and 0.84 V, respectively, which represents typical dual pathway FAOR mechanism.⁴⁰ FAOR frequently involves both direct and indirect pathway. In the direct pathway, FA is oxidized into CO₂ at ca. 0.5 V via active intermediate (e.g. formate). In the indirect pathway, FA is oxidized into CO₂ at ca. 0.84 V via adsorbed CO (CO_ads) intermediate. The indirect pathway always exists in the FA oxidation for the case of Pt.^40,41 However, the FA oxidation on Pd-based electrocatalysts happens only through direct pathway.⁴⁰ For the case of Pd₃Au@Pt/C, both direct and indirect pathways play a role in FAOR (Fig. 7c). As shown in Fig. 7d, the FAOR activity of Pd₃Au@Pt/C was determined to be 2.15 mA μg_Pt⁻¹ at ca. 0.5 V and 2.51 mA μg_Pt⁻¹ at ca. 0.84 V, respectively, that is much better than Pt/C (JM) (0.135 mA μg_Pt⁻¹, 0.877 mA μg_Pt⁻¹). Once more, the co-existence of indirect and direct FA oxidation peak suggests the core/shell structured Pd₃Au@Pt/C should have an incomplete Pt shell. Otherwise, if the Pt shell was fully formed, the FA direct oxidation peak should be small.

Besides well dispersion of platinum in thin shell, the significant improvement of electrocatalytic activity of Pd₃Au@Pt/C can be explained by electronic effect and/or geometric effect induced by the interaction between Pd₃Au core and Pt-shell. As shown in Fig. 3d, all the diffraction peaks of Pd₃Au@Pt/C overlap with those of Pd₃Au/C. The calculated lattice constant of Pd₃Au@Pt/C is 3.9725 Å, slightly extended with respect to that of Pt/C (3.9231 Å) based on Bragg's law using (220) diffraction peak (at ca. 66.3°). Possibly, there is a tensile strain effect occurred for the Pt shell coated on Pd₃Au core. Consequently, a lateral tensile strain should exist in the platinum shell of Pd₃Au@Pt/C compared with Pt/C.¹⁶ In addition, the electronic effect was investigated by XPS, as shown in Fig. 8. Compared with Pt/C (JM), there is an evident negative shift (0.27 eV) in the binding energy for Pd₃Au@Pt/C, implying that electrons have transferred from Pd₃Au core to Pt shell. This can lead to downshift of the d-band center of Pt so as to lower the surface oxygen affinity, which may also facilitate the enhancement of electrocatalytic activity.^40,42–44

Fig. 8 (a) XPS full spectrum of Pt/C (JM), Pd₃Au/C and Pd₃Au@Pt/C; (b) high-resolution Pt 4f spectra of Pt/C (JM) and Pd₃Au@Pt/C.

The stability of Pd₃Au@Pt/C and commercial Pt/C (JM) for FAOR was examined by collecting CA curve at 0.50 V, which is vital for the practical application of the electrocatalysts in direct formic acid fuel cells. As shown in Fig. S5,† in comparison with Pt/C (JM), Pd₃Au@Pt/C always gives a higher current density during the whole process. The activity of Pd₃Au@Pt/C is about 1.8 times of Pt/C (JM) based on the current density even after 3600 s. The better stability of Pd₃Au@Pt/C for FAOR may result from the less amount of CO_ads formation than that of Pt/C (JM).

Conclusions

In conclusion, we have developed a new approach for the synthesis of core/shell structured electrocatalysts by regioselective atomic layer deposition combined with a wet-chemical method. Nearly uniform Pd₃Au NPs supported on carbon were firstly prepared by reducing metal complexes confined in reverse micelles adsorbed on carbon. Next, Pt thin shell was selectively deposited on Pd₃Au particles instead of on carbon by ALD, likely originating from the coverage of nucleation sites on carbon by surfactant molecules in combination with catalytic decomposition of Pt precursor on Pd₃Au. Pd₃Au@Pt/C demonstrates a significantly improved electrocatalytic activity toward both FAOR and ORR compared with Pd₃Au/C and the state-of-the art commercial Pt/C. We realized that the regioselective ALD combined with wet chemical synthesis needs to be further improved to produce better electrocatalysts in the future. This study opens up a unique avenue to prepare core/shell structured electrocatalysts with potential applications in catalysis, electrocatalysis, sensors, and so on.

Acknowledgements

This work was partially supported by National Key Basic Research Program of China (973 project, no. 2012CB215500), National Natural Science Foundation of China (project no. 21003114, 21103163, 21306188, 21373211), Liaoning BaiQianWan Talents Program (project No. 201519), Program for Liaoning Excellent Talents in University (project No. LR2015014), Dalian Excellent Young Scientific and Technological Talents (project No. 2015R006), and the Fundamental Research Funds for the Central Universities (project No. DUT15RC(3)001, DUT15ZD225).

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Footnote

† Electronic supplementary information (ESI) available: UV-visible spectrum, TGA curve, TEM image, and electrochemistry measurements (Fig. S1–S5). See DOI: 10.1039/c6ra04990g

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