Surface-Pt-rich AgPtAu trimetallic nanotrough arrays for boosting alcohol electrooxidation†

Jing-Jing Li ^a, Wen-Chao Geng ^b, Ling Jiang ^a and Yong-Jun Li *^a
aState Key Lab of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China. E-mail: liyje@hnu.edu.cn; Tel: +86-731-88821603
^bSchool of Chemical and Printing-Dyeing Engineering, Henan University of Engineering, Zhengzhou 450000, China

First published on 24th March 2023

---

---

Introduction

“Greenhouse gases” associated with the continuous consumption of fossil energy resources have shown a serious threat to the environment; developing green sustainable energy is the only alternative.^1–3 To date, wind energy, solar power and chemical energy have been widely investigated^4–6 and are used in daily life and industrial production. The direct alcohol fuel cell (DAFC), as one of the chemical power sources, has been forecasted to play an important role in new energies owing to its high energy density, easy portability and low pollutant emissions.^7–9 However, the DAFC suffers from shortages of Pt catalyst and its weak durability during operations^10–12 and thus has not been commercialized. Therefore, it is imperative to increase the usage of Pt atoms and to improve its tolerance towards CO-like carbonaceous intermediates.

To date, many efforts have been made to improve the electrocatalytic performance of Pt-based catalysts by tuning their elemental compositions and morphologies,^13–15 where Pt-based bimetallic electrocatalysts, such as PtAu,^16,17 PtPd,^18,19 PtNi,^20,21 PtAg^22,23 and PtCu,^24,25 have been widely investigated, providing fresh impetus to DAFC commercialization. Ag as an oxytropic element, if being alloyed with Pt, can effectively provide OH_ad²⁶ for oxidizing CO-like carbonaceous intermediates adsorbed strongly on Pt atoms and thus enhance the catalytic activity of Pt-based catalysts.²⁷ Nevertheless, Ag is well known to be more chemically active than Pt, and apt to being oxidized into silver oxide during the potential window of alcohol electrooxidation. Generally, AgPt electrocatalysts containing ∼10 at% or more less Ag exhibit relatively better catalytic performance,²⁸ which indicates that the introduction of small amounts of Ag is indeed beneficial for catalysis, but the utilization efficiency of Pt is not greatly improved. Au is akin to Ag but more stable thermodynamically than Ag, and thus, PtAu alloy catalysts can inherit the advantage of PtAg but bypass its disadvantages. As demonstrated in many previous reports,^29,30 PtAu nanostructured catalysts are indeed capable of improving the catalytic activity and increasing the utilization of Pt atoms. For example, a Pt₇Au₉₃ nanowire maintained 27% of its initial activity even after 900 cycles of ethanol electrooxidation,³¹ showing much better durability than commercial Pt/C.

Regardless of its long-term durability, the high activity of a PtAg or PtAu catalyst mainly accounts for the weakening of CO_ad on Pt atoms and the formation of abundant OH_ad on Ag or Au.^28,29 The CO adsorption energy is governed by the electron density around Pt, which plays a crucial role in the oxidation potential of CO_ad.^30,32 For example, Pt catalysts supported on the CeO₂ substrate^30,33,34 exhibit weak adsorption towards CO owing to the transfer of CeO₂ 4f electrons to Pt, capable of weakening the poisoning of CO. Although PtAu shows better catalytic performance towards alcohol electrooxidation, Au is similar in electronegativity to Pt and cannot donate electrons to Pt. Fortunately, Ag is less electronegative, and if less Ag is doped into the PtAu catalyst, the AgPtAu trimetallic catalyst may not only exhibit the robust durability similar to PtAu, but also increase PtAu catalytic activity and the utilization of Pt atoms.

To the best of our knowledge, although there are a few studies of AgPtAu trimetallic nanostructures,^35,36 designing surface-Pt-rich AgPtAu nanostructures has been rarely reported, in particular for the DAFC. Herein, using an interfacial Ag nanowire array as a sacrificial template, we successfully fabricated surface-Pt-rich composition-varied AgPtAu trimetallic nanotrough arrays at two-phase interfaces. Optimized Ag₁₁Pt₅Au₈₄ nanotrough arrays integrate the positive catalytic effects of morphology and composition on methanol or ethanol electrooxidation, exhibiting improved long-term durability and more than ∼3 times the specific activity of commercial Pt/C.

Materials and methods

Chemicals and materials

Ethylene glycol (EG, 99%), ethanol (C₂H₅OH, 99.7%), silver nitrate (AgNO₃, 99.8%), sodium borohydride (NaBH₄, 96%) and hexachloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 37 wt%) were supplied by Sinopharm Chemical Reagent Co. Ltd (China). Polyvinylpyrrolidone (PVP, MW ≈ 29000) and gold(III) chloride trihydrate (HAuCl₄·3H₂O, ≥49.0% Au basis) were purchased from Sigma-Aldrich. All chemical reagents were used as received except that EG was refluxed at 196 °C for 5 h to remove trace water. All solutions were prepared with Milli-Q water (18.2 MΩ cm).

Synthesis of Ag nanowires

Monodisperse silver nanowires were prepared according to a previously reported polyol method with minor modifications.³⁷ Typically, 10 mL of refluxed EG was heated at 160 °C for 1 h in a three-necked flask, to which were added two EG solutions at the same rate (0.2 mL min⁻¹) under vigorous stirring: AgNO₃ (5 mL, 0.2 mol L⁻¹) and PVP (5 mL, 0.3 mol L⁻¹). After 45 min, the solution turned to opaque gray, indicative of the formation of Ag nanowires. The resulting product was collected and washed with water (1 × 10 mL) by centrifugation. The purified Ag nanowires were redispersed in 9 mL of ethanol for further use.

Fabrication of surface-Pt-rich AgPtAu nanotrough arrays

Ag nanowires were firstly assembled into a monolayer array at a toluene/water interface according to our previous reports.^38,39 Briefly, Ag nanowire/ethanol solution (1 mL) and toluene (2 mL) were mixed in a 10 mL beaker, into which 8 mL of H₂O was rapidly poured. Ag nanowire monolayer arrays were observed to appear at the toluene/water interface. H₂PtCl₆ solution (35 μL, 20 mmol L⁻¹) was injected into the water phase and gently stirred for 30 s (tip: do not destroy the as-formed interfacial monolayer), and subsequently, the reaction system was kept closed for the galvanic replacement between the interfacial Ag nanowires and PtCl₆²⁻. After 12 h, the reaction system was opened. After the upper toluene phase was evaporated, Ag₇₇Pt₂₃ nanotrough arrays were obtained at the water/air interface (the subscript numbers of Ag₇₇Pt₂₃ represent the atom number of the corresponding atom per 100 atoms, which was determined by inductively coupled plasma-optical emission spectroscopy, ICP-OES, Table S1†).

To fabricate AgPtAu nanotrough arrays, a certain volume (100 μL, 50 μL, or 20 μL) of 25.40 mmol L⁻¹ HAuCl₄ aqueous solution was injected into the water phase. After 12 h, AgPtAu nanotrough arrays were formed and can be transferred onto any solid substrate (e.g., Si wafer, Cu grid, or glassy carbon) by touching the side immersed in the water phase (Scheme 1f) or the side open to the air (Scheme 1g) for characterization and uses.

Electrochemical measurements

All electrochemical measurements were performed in a standard three-electrode cell. A platinum gauze and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. All given potentials were referred to SCE unless otherwise specified.

The working electrode is a glassy carbon electrode (GCE, 3 mm in diameter) modified with a specific catalyst. Ag₇₇Pt₂₃ and AgPtAu nanostructures were fabricated directly by touching interfacial nanostructures with the GCE. Prior to use, the GCE was polished with Al₂O₃ powder with diameters of 1 and 0.5 μm in turn, and flushed with copious amounts of water. For comparison, a commercial Pt/C electrode was also prepared: 6 μL of Pt/C ethanol suspension (1.0 mg mL⁻¹) was applied over a GCE surface and dried under ambient conditions. All catalyst layers on the GCEs were stabilized with 3 μL of Nafion ethanol solution (0.5 wt%) by a spinning drop method.

Before electrocatalysis, all working electrodes were cleaned in N₂-saturated 0.5 mol L⁻¹ aqueous H₂SO₄ solution by scanning between −0.2 and 1.2 V until stable cyclic voltammograms were obtained. The electrocatalytic performance of each catalyst was evaluated in N₂-saturated 1.0 mol L⁻¹ KOH solution containing 1.0 mol L⁻¹ CH₃OH or 1.0 mol L⁻¹ CH₃CH₂OH by cyclic voltammetric and amperometric i–t techniques.

The CO stripping test was conducted as follows: the potential was fixed to −0.8 V, and the flow of CO was sustained in 1.0 mol L⁻¹ KOH solution for 20 min, and then followed by a sustained flow of N₂ for 20 min to remove the residual CO in the solution. At a sweep rate of 50 mV s⁻¹ in 1.0 mol L⁻¹ KOH solution, cyclic voltammetry was performed in the range of −0.8–0.4 V.

Characterization and instruments

All electrochemical experiments were performed on a CHI660D workstation (Chenhua, Shanghai). Transmission electron microscopic (TEM) images and high-resolution TEM (HRTEM) images were obtained on a JEM-3010 microscope (JEOL, Japan) with an Oxford INCA detector, operating at 300 kV. Scanning electron microscopic (SEM) images were obtained on an S-4800 microscope (JEOL, Japan). Energy dispersive spectroscopic (EDS) elemental mapping images were obtained in a high-angle annular dark-field (HAADF) mode on a Themis Z (3.2) condenser spherical aberration-corrected transmission electron microscope. X-ray diffraction (XRD) measurements were performed on a Shimadzu XRD-6100 θ–2θ X-ray diffractometer. UV-vis spectra were obtained on a Shimadzu UV-1800 spectrophotometer (Japan). The mass and the Ag:Pt:Au atomic ratio of catalyst were determined by ICP-OES on an IRIS Intrepid II XSP instrument (ThermoFisher). X-ray photoelectron spectroscopy (XPS) was performed on a K-Alpha XPS spectrometer (ThermoFisher, ESCALAB Xi+, UK) with Al Kα X-ray radiation (1486.6 eV) for excitation.

Results and discussion

From our previously reported water-triggered interfacial self-assembly of nanoparticles,⁴⁰ Ag nanowires with an average diameter of ∼100 nm (Fig. S1†) were assembled at a toluene/water interface from Ag nanowire solution. These interfacial Ag nanowires show a monolayer array with a side-by-side arrangement (Fig. 1a). After reacting successively with H₂PtCl₆ and HAuCl₄ in the water phase, the interfacial Ag nanowire array was transformed into a 2D AgPtAu trimetallic nanotrough array (Scheme 1a–e).

When 100 μL of HAuCl₄ was used, the as-prepared AgPtAu nanostructure has the Ag:Pt:Au atomic ratio of ∼11:5:84 (ICP-OES results) and thereafter was named as Ag₁₁Pt₅Au₈₄ (Fig. 1b–d). Interfacial etching is an asymmetrical chemical reaction, which may result in an asymmetrical morphology. When interfacial Ag₁₁Pt₅Au₈₄ was transferred from the water/air interface by touching its bottom surface from the water phase (Scheme 1f), what we observed is that the top side is composed of many close-packed nanotroughs (Fig. 1b). This trough-like nanostructure can be further corroborated by the TEM result (Fig. 1c): the center area is more transparent than the two sides. The external surface of Ag₁₁Pt₅Au₈₄ is extremely rough (Fig. 1c) because the as-formed nanoparticles grow outwards, as observed from the nanotrough bottom (Fig. 1d). The TEM image (Fig. 1c) shows that the external and internal diameters of Ag₁₁Pt₅Au₈₄ are ∼200 and 60 nm, respectively, by measuring 300 locations (Fig. S2†). The external average diameter of Ag₁₁Pt₅Au₈₄ is much larger than that of the original Ag nanowire (∼100 nm) because of the outward expansion of the nanotrough wall and the outward growth of nanoparticles. When interfacial Ag₁₁Pt₅Au₈₄ was transferred by touching its top surface from the air (Scheme 1g), what we observed is the bottom surface consisting of intertwined pillar-like nanoparticles (Fig. 1d) rather than the nanotroughs. These pillar-like nanoparticles connect with each other, integrating individual Ag₁₁Pt₅Au₈₄ nanotroughs into an array. The imaging characterization above clearly builds up a picture that Ag₁₁Pt₅Au₈₄ is a morphology-asymmetrical nanostructure: the top side appears to be a trough-like morphology, and the bottom side is a pillar-like nanoparticle assembly (Fig. 1e).

The HRTEM images (Fig. 2a–c) show that, for the center area (Fig. 2a and b), the interspacing between neighboring lattice fringes measures ∼0.232 nm, larger than that of Pt(111) (0.228 nm) but smaller than that of Ag or Au(111) (0.235 nm), which can be ascribed to AuPt(111) or AgPt(111).⁴¹ This result suggests that some PtAu or PtAg alloys may be formed in Ag₁₁Pt₅Au₈₄ (we cannot determine AuPt or AgPt from the HRTEM images because of the similarity of Ag and Au). However, at the edge of pillar-like particles, Pt(111) is also detectable (Fig. 2a and c), suggesting that Ag₁₁Pt₅Au₈₄ is a surface-Pt-rich alloy nanostructure, which is consistent with line scan results (Fig. S3†). The overlays of Ag + Pt, Ag + Au, Pt + Au and Ag + Pt + Au elemental mapping images (Fig. 2d) further show that the distribution of Ag, Pt and Au elements is not completely uniform from the internal to the external surface along the cross-section of the Ag₁₁Pt₅Au₈₄ wall, namely PtAg|AgAu|Au|AuPt. The existence of pure Au species was further confirmed by XRD characterization. Au(111), −(200), −(220) and −(311) diffraction peaks of face-centered cubic Au (JCPDS no. 4-784)⁴² appear clearly at ∼38.44°, 44.62°, 64.74° and 77.73°, respectively, but separated diffraction peaks belonging to pure Pt have not been observed (Fig. S4†). XRD analyses together with the elemental mapping results indicate that Ag₁₁Pt₅Au₈₄ is a surface-Pt-rich alloy-like trimetallic nanotrough array.

The characteristic peaks of Ag, Pt and Au can be clearly observed in the XPS full spectrum (Fig. 3a). The XPS spectra (Fig. 3b and c) show that each energy band of Ag and Au of Ag₁₁Pt₅Au₈₄ is perfectly symmetrical, consistent with the standard patterns of Ag⁰ 3d and Au⁰ 4f,⁴³ indicating that Ag and Au species are in the metallic state. Nevertheless, compared with Ag nanowires (Ag 3d_5/2, 367.4 eV; 3d_3/2, 373.4 eV),⁴⁴ Ag 3d_5/2 and 3d_3/2 of Ag₁₁Pt₅Au₈₄ are located at higher energies of 367.72 and 373.71 eV, respectively (Fig. 3b); compared with pure Au (4f_7/2, 84.00 eV; 4f_5/2, 87.67 eV), Au 4f_7/2 and 4f_5/2 of Ag₁₁Pt₅Au₈₄ are located at lower energies of 83.97 and 87.64 eV, respectively (Fig. 3c); Pt⁰ 4f_7/2 and 4f_5/2 binding energy bands are centered at 70.90 and 73.99 eV, respectively (Fig. 3d), with an ∼0.3 eV shift towards the lower energy direction when compared with pure Pt (4f_7/2, 71.20 eV; 4f_5/2, 74.53 eV). These results indicate that, due to the small electronegativity of Ag (1.93),⁴⁵ more electrons accumulate around Au and Pt.⁴⁶ The localized charge transfer may modify the electronic structure of Ag₁₁Pt₅Au₈₄, helpful for improving the tolerance of Pt towards the CO-like species.^47,48 Additionally, Pt 4f binding energy shows shoulder bands owing to the presence of Pt²⁺ and Pt⁴⁺, which is a common phenomenon for a Pt-based hybrid nanostructure.⁴⁹

To understand the growth mechanism of Ag₁₁Pt₅Au₈₄, we tracked the morphological evolution of interfacial Ag nanowires experiencing the successive etching of PtCl₆²⁻ and AuCl₄⁻ ions in the water phase. After Ag nanowires were assembled at a water/toluene interface, PtCl₆²⁻ was immediately added into the water phase to react with interfacial Ag nanowires in a sealed state. After 12 h, the upper toluene phase was removed by evaporation, and interfacial Ag₇₇Pt₂₃ was collected. When we touched the bottom surface of Ag₇₇Pt₂₃ from the water phase, as illustrated in Scheme 1f, we observed that Ag₇₇Pt₂₃ is composed of nanotroughs (Fig. 4a). When interfacial Ag₇₇Pt₂₃ was transferred by touching the top surface from the air (Scheme 1g), Ag₇₇Pt₂₃ is still similar in morphology to the original Ag nanowires but much rougher (Fig. 4b): many tiny decorated particles can be observed. The TEM image (Fig. 4c) shows that the two sides of each Ag₇₇Pt₂₃ are much darker in contrast than the interior area, further confirming that each Ag₇₇Pt₂₃ unit is a nanotrough. By measuring 300 locations of different Ag₇₇Pt₂₃ nanotroughs, the average external diameter is ∼150 nm, larger than that of the original Ag nanowires. This result suggests that Pt⁰ atoms produced by the galvanic replacement adhere at the Ag nanowire surface, gradually grow towards the water phase, and thus result in the expansion of the diameter of the original Ag nanowires. At the interior area of nanotroughs (Fig. 4d), the interspacing between neighboring lattices measures 0.232 nm, corresponding to AgPt(111),⁵⁰ indicative of the formation of the AgPt alloy; in contrast, at the edge, Pt(111) is detected with a lattice spacing of 0.228 nm (ref. 51) (Fig. 4e). The HAADF-EDS elemental mapping results (Fig. 4f) further reveal that Ag and Pt species are not distributed uniformly over the wall of an Ag₇₇Pt₂₃ nanotrough: the external surface contains more Pt than the internal surface, consistent with the line scan results (Fig. S5†). The XRD patterns do not show separated Pt diffraction peaks (Fig. S4†), and noticeably, Ag(111) and −(200) peaks shift towards the high-angle direction. These combined results indicate that Ag₇₇Pt₂₃ is a surface-Pt-rich alloy nanotrough. Similar to Ag₁₁Pt₅Au₈₄, most Ag atoms of Ag₇₇Pt₂₃ exist in the metallic state (Fig. S6†); nevertheless, the Pt binding energy center shows a slightly negative shift due to the electron transfer from Ag to Pt.

After Ag₇₇Pt₂₃ nanotroughs were formed, the upper toluene phase was removed, and 100 μL of HAuCl₄ aqueous solution was added into the water phase to transform Ag₇₇Pt₂₃ into Ag₁₁Pt₅Au₈₄ (Fig. 1). Obviously, Ag₁₁Pt₅Au₈₄ inherits the morphology of Ag₇₇Pt₂₃. Compared with Ag₇₇Pt₂₃, the wall of the Ag₁₁Pt₅Au₈₄ nanotrough becomes much thicker and rougher. Moreover, owing to the mutual diffusion of different atoms during the etching process, final Ag₁₁Pt₅Au₈₄ tends to be an alloy. By comparing the compositions of Ag₇₇Pt₂₃ and Ag₁₁Pt₅Au₈₄, we found that most Ag atoms of Ag₇₇Pt₂₃ are replaced by Au, and the Pt atom number of Ag₇₇Pt₂₃ is also reduced, indicative of the reaction occurring between Au³⁺ and Pt⁰. Thermodynamically, Au³⁺ can oxidize Pt⁰ into Pt²⁺, but the reaction rate is extremely sluggish.⁵² Our results suggest that two-phase interfaces can accelerate the reaction rate between Au³⁺ and Pt⁰.

For Ag₁₁Pt₅Au₈₄, the formation of pillar-like nanoparticles on the bottom may be related to the directional electron transfer during interface-confined Ag oxidation⁵³ and the morphology-inducing effect of surfactant molecules (PVP).⁵⁴ When the as-prepared Ag nanowires (PVP as a stabilizer) were washed with ethanol several times, highly purified Ag nanowires cannot be transformed into morphology-well-defined AgPtAu nanostructures (Fig. S7†) under the same etching conditions as the formation of Ag₁₁Pt₅Au₈₄: many differently sized larger particles are distributed disorderedly over the bottom of each unit of interface products (Fig. S7a†), but the top side still shows a trench (Fig. S7b†).

To adjust the Ag:Pt:Au atomic ratio of the AgPtAu nanotrough, we reduced the feeding amount of HAuCl₄ (<100 μL) in the water phase. When 20 and 50 μL of HAuCl₄ were added, Ag₄₅Pt₁₈Au₃₇ (Fig. 5a–c) and Ag₂₁Pt₁₀Au₆₉ (Fig. 5d–f) were fabricated, respectively (ICP-OES results, Table S1†). The SEM (Fig. 5a, b, d and e) and TEM (Fig. 5c and f) images together demonstrate that both Ag₄₅Pt₁₈Au₃₇ and Ag₂₁Pt₁₀Au₆₉ are nanotroughs: the top side is evacuated (Fig. 5a and d), and the bottom side (Fig. 5b and e) looks just like that of Ag nanowires. Compared with Ag₁₁Pt₅Au₈₄ (Fig. 1c), Ag₄₅Pt₁₈Au₃₇ (Fig. 5c) and Ag₂₁Pt₁₀Au₆₉ (Fig. 5f) have much smoother bottom surfaces although Ag₂₁Pt₁₀Au₆₉ is much rougher than Ag₄₅Pt₁₈Au₃₇, and no larger pillar-like particles are observed. Similarly, Pt(111) facets are detectable at the edge of both Ag₄₅Pt₁₈Au₃₇ and Ag₂₁Pt₁₀Au₆₉ (Fig. S8†). All these results indicate that the more HAuCl₄ aqueous solution is added, the more Au, and the less Ag and Pt are contained in the final AgPtAu nanotrough. Additionally, UV-vis spectra based on the surface plasma resonance of the nanoscale Ag and Au (Fig. S9†) further confirm the transformation of Ag nanowires into composition-varied AgPtAu nanotroughs.

The formation of a composition-varied AgPtAu nanotrough can be well explained by our previously reported interface-confined hydration-layer-assisted etching mechanism.^39,54 H₂PtCl₆ in the water phase preferentially reacts with the bottom side of Ag nanowires (Scheme 2a). PtCl₆²⁻ is reduced to Pt⁰, forming a dense Pt layer (Scheme 2b), which, as a nanomask, blocks the further oxidation of the side of the Ag nanowire located in the water phase. However, the upper part of the Ag nanowire covered with a hydration layer cannot contact plentiful PtCl₆²⁻ species, and is still oxidizable. In this situation, Ag⁰ at the top side is oxidized into Ag⁺. As-formed Ag⁺ ions migrate to the interface or into the solution via the hydration layer (Scheme 2c), and simultaneously, the electrons are transferred to PtCl₆²⁻ in the water phase via the residual Ag and the Pt layer of the bottom side (Scheme 2c). The continuous oxidation of the top side thickens the Pt layer of the bottom side and creates a trench at the top surface. Finally, a PtAg nanotrough is formed (Scheme 2d). Following the same mechanism, an interfacial PtAg nanotrough is transformed into an AgPtAu nanotrough by reacting with AuCl₄⁻ in the solution.

Generally, an Ag nanowire in solution reacting with H₂PtCl₆ results in a hollow AgPt⁵⁵ or Pt nanotube.⁵⁶ However, in the electrooxidation of formic acid,⁴⁷ the AgPt nanotube lacks long-term durability owing to the continuous dissolution of Ag; the Pt nanotube is still apt to be poisoned. In our case, the Ag nanowire was transformed into a morphology-unique AgPt nanotrough, and Au is further introduced, which may be helpful for weakening the poisoning of CO^57–60 and enhancing the catalytic performance towards small organic molecules.

To verify our assumption, we employed methanol and ethanol as model molecules to examine the electrocatalytic performance of AgPtAu nanotrough arrays. Ag₄₅Pt₁₈Au₃₇, Ag₂₁Pt₁₀Au₆₉ and Ag₁₁Pt₅Au₈₄ nanotroughs in H₂SO₄ solution show the characteristic reduction peaks of Pt and Au species in electrochemical cyclic voltammograms (Fig. 6a): ∼0.13 and 0.82 V for the Pt and Au species, respectively, and with the increase of Au content, the reduction peak current at ∼0.82 V increases correspondingly. In contrast, Ag₇₇Pt₂₃ shows only the reduction peaks of the Pt species. These results suggest that both Au and Pt are exposed to the solution. The electrochemically active surface areas (ECSAs) of Pt components are determined by integrating the peak areas of hydrogen absorption and desorption (Fig. S10a†).⁶¹ Ag₁₁Pt₅Au₈₄ shows the largest ECSA (∼55 m² g_Pt⁻¹) among all samples, approximately two times larger than that of commercial Pt/C catalysts (Table S2†), owing to the integration of the porous trough-like structure and fine pillar-like nanoparticles. Either from ECSA- (Fig. S10b†) or mass-normalized (Fig. S10c†) current density, Ag₇₇Pt₂₃ and all AgPtAu nanotrough arrays show better catalytic activity than commercial Pt/C for the methanol oxidation reaction (MOR); moreover, AgPtAu arrays are better than Ag₇₇Pt₂₃, indicating that the introduction of Au can efficiently improve the activity of AgPt. To show the difference of catalytic activity, the forward oxidation current densities of all samples are summarized in Fig. 6b. As for the three AgPtAu nanotrough arrays, Ag₁₁Pt₅Au₈₄ shows the best specific catalytic activity: the ECSA- and mass-normalized activities are up to ∼44 mA cm⁻¹ and 2270 mA mg_Au+Pt⁻¹, respectively, ∼15 and 3 times larger than the corresponding activities of commercial Pt/C (∼2.8 mA cm⁻² and 796 mA mg_Pt⁻¹). The catalytic activity of Ag₂₁Pt₁₀Au₆₉ or Ag₄₅Pt₁₈Au₃₇, both inferior to Ag₁₁Pt₅Au₈₄, is still ∼2 times the mass-normalized activity of Pt/C. Besides, the reverse oxidation in the MOR indicates the removal of carbonaceous species, which are not fully oxidized in the forward scan.⁶² The ratio of the forward to the backward oxidation peak current densities, j_f/j_b (Fig. 6c), was used to roughly assess the tolerance of catalyst to CO-like intermediates. The larger the value of j_f/j_b, the stronger the tolerance is of a catalyst.⁶³ Ag₁₁Pt₅Au₈₄ shows the best tolerance to CO-like intermediates, but the tolerance of Ag₇₇Pt₂₃, Ag₂₁Pt₁₀Au₆₉ and Ag₄₅Pt₁₈Au₃₇ is still better than that of Pt/C.

To corroborate the j_f/j_b results, we further employed an amperometric i–t technique. Ag₁₁Pt₅Au₈₄, Ag₄₅Pt₁₈Au₃₇ and Ag₂₁Pt₁₀Au₆₉ show better durability than Ag₇₇Pt₂₃ and Pt/C (Fig. 6d). After 4000 s continuous electrooxidation of methanol, Ag₁₁Pt₅Au₈₄ exhibits the best durability and shows a mass-normalized activity of ∼698 mA mg_Au+Pt⁻¹, ∼2 times better than that of Ag₂₁Pt₁₀Au₆₉ and ∼9 times better than that of any of Ag₄₅Pt₁₈Au₃₇, Ag₇₇Pt₂₃ and Pt/C. A combination of amperometric and cyclic voltammetric techniques further confirms the robust durability of Ag₁₁Pt₅Au₈₄ (Fig. S10d†). After ten cycles, the retention of the forward oxidation current density on Ag₁₁Pt₅Au₈₄ is still 35% of the initial value, much better than Pt/C.

Moreover, AgPtAu nanotrough arrays show excellent electrocatalytic performance towards the ethanol oxidation reaction (EOR) (Fig. S11a–c†). Ag₁₁Pt₅Au₈₄ still shows the best catalytic performance among Ag₇₇Pt₂₃, AgPtAu nanotrough arrays and Pt/C: ECSA- and mass-normalized activity are ∼73 mA cm⁻¹ and 2290 mA mg_Au+Pt⁻¹, respectively, ∼24 and 3 times larger than the corresponding values of commercial Pt/C. Ethanol can be electrooxidized into more CO-like intermediates than methanol, which may severely poison the catalyst. Nevertheless, in our case, AgPtAu nanotrough arrays still have higher j_f/j_b values than those of Pt/C (Fig. S11d†).

In either ECSA- or mass-normalized activity, an Ag₁₁Pt₅Au₈₄ nanotrough array is much better than most previously reported AgPtAu catalysts and Pt/C for the MOR or the EOR (Tables S3 and S4†), which could be attributed to the dual effects of composition and structure. Alcohol molecules are electrochemically dehydrogenated by dissociative adsorption, forming carbonaceous intermediates including CO, where the oxidation of CO_ad is considered to be the determining step of electrocatalytic oxidation of methanol under alkaline conditions: Pt–CO_ad + Pt–OH + OH⁻ → 2Pt + CO₂ + H₂O + e⁻. The presence of the oxophilic metals, Au and Ag, is favorable for the adsorption of OH⁻ on AgPtAu surfaces, promoting the oxidation of CO_ad on the Pt active site.⁶⁴ On the other hand, according to the d-band center theory,^45,65 the d-band center of Pt of AgPtAu is shifted upward compared with pure Pt because of the large lattice constants of Au and Ag, which can also increase the adsorption capacity of OH_ad on Pt. Furthermore, the Ag component of AgPtAu can regulate the electronic structure of Pt by donating electrons, reduce the binding energy of CO_ad on Pt, and thus make CO oxidation much easier. The excellent anti-poisoning ability of Ag₁₁Pt₅Au₈₄ is further confirmed by CO stripping experiments (Fig. S12†). Furthermore, the more Au is introduced into Ag₇₇Pt₂₃, the more the peak of CO shifts towards a negative direction (Fig. S13†), evidently signifying the role of Au in the improvement of CO tolerance. Additionally, the Ag₁₁Pt₅Au₈₄ nanotrough has a curved 2D surface-Pt-rich nanostructure, capable of effectively improving the utilization of Pt atoms and favorable for accelerating the mass transfer.

Conclusions

Composition-controllable surface-Pt-rich AgPtAu nanotrough arrays were fabricated at a two-phase interface by successively etching interfacial Ag nanowire arrays with PtCl₆²⁻ and AuCl₄⁻ in the water phase. The formation of AgPtAu nanotrough arrays can be attributed to the interface-confined hydration-layer-assisted etching mechanism. Optimized Ag₁₁Pt₅Au₈₄ exhibited an outstanding catalytic performance towards alcohol electrooxidation in alkaline solution owing to the collective effects of composition and structure. This study not only develops the knowledge of the interface reaction for fabricating complex multi-metallic nanostructures, but also provides new insight into the way of producing high-performance electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 21872048).

References

1. J. Li, C. Wang, H. Shang, Y. Wang, H. You, H. Xu and Y. Du, Chem. Eng. J., 2021, 424, 130319 CrossRef CAS .
2. Y. Choi, M. Gu, J. Park, H. K. Song and B. S. Kim, Adv. Energy Mater., 2012, 2, 1510–1518 CrossRef CAS .
3. S. Xiao, L. Zhu, S. M. Osman, Y. Feng, S. Zhang, G. Li, S. Zeng, R. Luque and B. H. Chen, Green Chem., 2022, 24, 5813–5821 RSC .
4. G. Nagaraju, S. C. Sekhar, B. Ramulu and J. S. Yu, Small, 2019, 15, 1805418 CrossRef PubMed .
5. W. Chen, G. Li, A. Pei, Y. Li and Y. Cui, Nat. Energy, 2018, 3, 428–435 CrossRef CAS .
6. B. Luo, D. Ye and L. Wang, Adv. Sci., 2017, 4, 1700104 CrossRef PubMed .
7. A. A. Dubale, Y. Zheng, H. Wang, R. Hübner, Y. Li, J. Yang, J. Zhang, N. K. Sethi, L. He and Z. Zheng, Angew. Chem., 2020, 132, 13995–14003 CrossRef .
8. L. Huang, M. Wei, N. Hu, P. Tsiakaras and P. K. Shen, Appl. Catal., B, 2019, 258, 117974 CrossRef CAS .
9. T. Li, Y. Shi, Y. Huang, R. Chen, Y. Zhang, J. Huo, Y. Zou, G. Yu, J. Luo, C. Dong and S. Wang, Nano Energy, 2018, 53, 604–612 CrossRef .
10. N. Sui, R. Yue, Y. Wang, Q. Bai, R. An, H. Xiao, L. Wang, M. Liu and W. W. Yu, J. Alloys Compd., 2019, 790, 792–798 CrossRef CAS .
11. L. Xiao, G. Li, Z. Yang, K. Chen, R. Zhou, H. Liao, Q. Xu and J. Xu, Adv. Funct. Mater., 2021, 31, 2100982 CrossRef CAS .
12. W. Ren, W. Zang, H. Zhang, J. Bian and C. Cheng, Carbon, 2018, 142, 206–216 CrossRef .
13. G. Xu, S. Rui, J. Liu, L. Zhang, G. Xia, G. Rui, B. Liu and J. Zhang, J. Mater. Chem. A, 2018, 6, 12759–12767 RSC .
14. A. Glüsen, F. Dionigi, P. Paciok, M. Heggen, M. Müller, L. Gan, P. Strasser, R. E. Dunin-Borkowski and D. Stolten, ACS Catal., 2019, 9, 3764–3772 CrossRef .
15. W. Hong, J. Wang and E. Wang, Small, 2014, 10, 3262–3265 CrossRef CAS PubMed .
16. M. L. Wu, D. H. Chen and T. C. Huang, Chem. Mater., 2001, 13, 599–606 CrossRef CAS .
17. F. Li, Y. Ding, X. Xiao, S. Yin, M. Hu, S. Li and Y. Chen, J. Mater. Chem. A, 2018, 6, 17164–17170 RSC .
18. Z.-H. Lin, Z.-Y. Shih, H.-Y. Tsai and H.-T. Chang, Green Chem., 2011, 13, 1029–1035 RSC .
19. F. Li, X. Gao, S. Li, Y. Chen and J. Lee, NPG Asia Mater., 2015, 7, e219–e219 CrossRef CAS .
20. F. Gao, Y. Zhang, P. Song, J. Wang, B. Yan, Q. Sun, L. Li, X. Xing and Y. Du, Nanoscale, 2019, 11, 4831–4836 RSC .
21. Y. Zhang, F. Gao, P. Song, J. Wang, T. Song, C. Wang, C. Chen, L. Jin, L. Li and X. Zhu, J. Power Sources, 2019, 425, 179–185 CrossRef CAS .
22. W. Yao, X. Jiang, M. Li, Y. Li and Y. Tang, Appl. Catal., B, 2020, 282, 119595 CrossRef .
23. W. Zhang, J. Yang and X. Lu, ACS Nano, 2012, 6, 7397–7405 CrossRef CAS PubMed .
24. I. Mintsouli, J. Georgieva, S. Armyanov, E. Valova, G. Avdeev, A. Hubin, O. Steenhaut, J. Dille, D. Tsiplakides and S. Balomenou, Appl. Catal., B, 2013, 136, 160–167 CrossRef .
25. F. Xu, S. Cai, B. Lin, L. Yang, H. Le and S. Mu, Small, 2022, 18, 2107387 CrossRef CAS PubMed .
26. Q. Shi, P. Zhang, Y. Li, H. Xia, D. Wang and X. Tao, Chem. Sci., 2015, 6, 4350–4357 RSC .
27. S. Chen, S. Thota, X. Wang and J. Zhao, J. Mater. Chem. A, 2016, 4, 9038–9043 RSC .
28. S. T. Nguyen, H. M. Law, H. T. Nguyen, N. Kristian, S. Wang, S. H. Chan and X. Wang, Appl. Catal., B, 2009, 91, 507–515 CrossRef CAS .
29. X. Ge, R. Wang, P. Liu and Y. Ding, Chem. Mater., 2007, 19, 5827–5829 CrossRef CAS .
30. D. Van Dao, T. D. Le, G. Adilbish, I.-H. Lee and Y.-T. Yu, J. Mater. Chem. A, 2019, 7, 26996–27006 RSC .
31. Q. Gan, L. Tao, L. Zhou, X. Zhang, S. Wang and Y. Li, Chem. Commun., 2016, 52, 5164–5166 RSC .
32. S. Kwon, D. J. Ham, T. Kim, Y. Kwon, S. G. Lee and M. Cho, ACS Appl. Mater. Interfaces, 2018, 10, 39581–39589 CrossRef CAS PubMed .
33. V. T. T. Ho, C.-J. Pan, J. Rick, W.-N. Su and B.-J. Hwang, J. Am. Chem. Soc., 2011, 133, 11716–11724 CrossRef CAS PubMed .
34. M. A. Scibioh, S.-K. Kim, E. A. Cho, T.-H. Lim, S.-A. Hong and H. Y. Ha, Appl. Catal., B, 2008, 84, 773–782 CrossRef CAS .
35. A. Londono-Calderon, C. Campos-Roldan, R. González-Huerta, M. Hernandez-Pichardo, P. del Angel and M. Yacaman, Int. J. Hydrogen Energy, 2017, 42, 30208–30215 CrossRef CAS .
36. Z. Ye, Q. Wang, J. Qiao, B. Ye and G. Li, J. Electroanal. Chem., 2019, 854, 113541 CrossRef CAS .
37. H. Shi, B. Hu, X. Yu, R. Zhao, X. Ren, S. Liu, J. Liu, M. Feng, A. Xu and S. Yu, Adv. Funct. Mater., 2010, 20, 958–964 CrossRef CAS .
38. Y. Li, W. Huang and S. Sun, Angew. Chem., Int. Ed., 2010, 45, 2537–2539 CrossRef PubMed .
39. H. Xue, W. Shen, W. Geng, X. Yin, Y. Yang, S. Guo and Y. Li, Chem. Commun., 2018, 54, 7227–7230 RSC .
40. C. Liu, Y. Li, M. Wang, Y. He and E. S. Yeung, Nanotechnology, 2009, 20, 065604 CrossRef PubMed .
41. J. Fang, L. Zhang, J. Li, L. Lu, C. Ma, S. Cheng, Z. Li, Q. Xiong and H. You, Nat. Commun., 2018, 9, 521 CrossRef PubMed .
42. A. Mashentseva, D. Borgekov, D. Niyazova and M. Zdorovets, Pet. Chem., 2015, 55, 810–815 CrossRef CAS .
43. J. Moulder, J. Chastain and R. King, Chem. Phys. Lett., 1992, 220, 7–10 Search PubMed .
44. H. Mao, J. Feng, X. Ma, C. Wu and X. Zhao, J. Nanopart. Res., 2012, 14, 1–15 CrossRef .
45. X. Fu, C. Wan, A. Zhang, Z. Zhao, H. Huyan, X. Pan, S. Du, X. Duan and Y. Huang, Nano Res., 2020, 13, 1472–1478 CrossRef CAS .
46. J. Yang and J. Y. Ying, Angew. Chem., Int. Ed., 2011, 50, 4637–4643 CrossRef CAS PubMed .
47. J. H. Kim, S. M. Choi, S. H. Nam, M. H. Seo, S. H. Choi and W. B. Kim, Appl. Catal., B, 2008, 82, 89–102 CrossRef CAS .
48. H. H. Li, S. Zhao, M. Gong, C. H. Cui, D. He, H. W. Liang, L. Wu and S. H. Yu, Angew. Chem., Int. Ed., 2013, 52, 7472–7476 CrossRef CAS PubMed .
49. J. Sun, T. Li, X. Li, J. Pan, X. Hao and T. Zhu, J. Alloys Compd., 2020, 831, 154871 CrossRef CAS .
50. F. Li, Y. Kang, H. Liu, Y. Zhai, M. Hu and Y. Chen, J. Colloid Interface Sci., 2018, 514, 299–305 CrossRef CAS PubMed .
51. Y. Ouyang, H. Cao, H. Wu, D. Wu, F. Wang, X. Fan, W. Yuan, M. He, L. Y. Zhang and C. M. Li, Appl. Catal., B, 2020, 265, 118606 CrossRef CAS .
52. N. Gowthaman, B. Sinduja, S. Shankar and S. A. John, Sustainable Energy Fuels, 2018, 2, 1588–1599 RSC .
53. Y. Bi, H. Hu and G. Lu, Chem. Commun., 2010, 46, 598–600 RSC .
54. W. C. Geng, Y. J. Zhang, L. Yu, J. J. Li, J. L. Sang and Y. J. Li, Small, 2021, 17, 2101499 CrossRef CAS PubMed .
55. X. Gu, C. Wang, L. Yang and C. Yang, J. Nanosci. Nanotechnol., 2017, 17, 2843–2847 CrossRef CAS PubMed .
56. Q. Shi, Y. Song, C. Zhu, H. Yang, D. Du and Y. Lin, ACS Appl. Mater. Interfaces, 2015, 7, 24288–24295 CrossRef CAS PubMed .
57. Z. Peng and Y. Hong, Nano Res., 2009, 2, 406–415 CrossRef CAS .
58. M. Valden, X. Lai and D. W. Goodman, Science, 1998, 281, 1647–1650 CrossRef CAS PubMed .
59. X. Ge, R. Wang, P. Liu and D. Yi, Chem. Mater., 2007, 19, 5827–5829 CrossRef CAS .
60. J. Zeng, J. Yang, J. Y. Lee and W. Zhou, J. Phys. Chem. B, 2006, 110, 24606–24611 CrossRef CAS PubMed .
61. W. Kuang, Z. Jiang, H. Li, J. Zhang, L. Zhou and Y. Li, ChemElectroChem, 2018, 5, 3901–3905 CrossRef CAS .
62. L. Chen, N. Chen, Y. Hou, Z. Wang, S. Lv, T. Fujita, J. Jiang, A. Hirata and M. Chen, ACS Catal., 2013, 3, 1220–1230 CrossRef CAS .
63. Z. Liu, X. Ling, X. Su and J. Y. Lee, J. Phys. Chem. B, 2004, 108, 8234–8240 CrossRef CAS .
64. G. Chen, D. Xia, Z. Nie, Z. Wang, L. Wang, L. Zhang and J. Zhang, Chem. Mater., 2007, 19, 1840–1844 CrossRef CAS .
65. V. Stamenkovic, B. S. Mun, K. J. J. Mayrhofer, P. N. Ross, N. M. Markovic, J. Rossmeisl, J. Greeley and J. K. Nørskov, Angew. Chem., Int. Ed., 2006, 45, 2897–2901 CrossRef CAS PubMed .

---

Footnote

† Electronic supplementary information (ESI) available: Size distribution, XRD patterns, additional material characterization, XPS spectra, ICP-OES results and electrochemical data. See DOI: https://doi.org/10.1039/d3gc00596h

This journal is © The Royal Society of Chemistry 2023