Robust synthesis of ultrathin Au–Ag nanowires as a high-surface-area, synergistic substrate for constructing efficient Pt-based catalysts

Shumeng Zhang , Lei Zhang , Zhaojun Liu , Moxuan Liu , Qikui Fan , Kai Liu and Chuanbo Gao *
Frontier Institute of Science and Technology, State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710054, China. E-mail: gaochuanbo@mail.xjtu.edu.cn

Received 14th June 2018 , Accepted 10th August 2018

First published on 10th August 2018


Ultrathin Au (or Ag) nanowires represent an excellent substrate for atomic layer deposition of Pt to afford highly active and cost-effective catalysts due to the large surface area and possible synergistic effect. An ideal synthesis of such nanowires should avoid using strong capping agents for convenient post-synthesis treatments and should be easily scaled up and reproduced in a high yield, which remains a challenge. Here, we report a novel strategy to synthesize sub-2 nm Au–Ag alloy nanowires with high quality in N,N-dimethyl formamide (DMF), which relies on Ag modification of the nanocrystal surface and Ag–halide interactions for regulating the one-dimensional growth of the nanowires, without involving strong capping agents that are usually required in conventional syntheses. Sub-monolayer Pt atoms were successfully deposited on these ultrathin Au–Ag alloy nanowires without forming ensembles despite a high loading amount (up to 20% in terms of Pt/(Au + Ag)) due to the large surface area. The resulting Au–Ag@Pt core/shell nanowires demonstrate superior activities in the formic acid oxidation reaction (FAOR) due to the synergistic ligand effect and the absence of Pt ensembles. We believe that the novel synthesis and the demonstration of these ultrathin Au–Ag alloy nanowires as a general platform for constructing cost-effective noble metal catalysts open new opportunities in designing catalysts for a broad range of reactions.


Introduction

Pt and Pt-alloy nanocrystals are a family of superior catalysts for a broad range of reactions. The low abundance of Pt in the earth's crust leads to a high cost of catalysts, which has stimulated great efforts from researchers around the world to minimize the use of Pt in catalytic applications. Typically, Pt nanocrystals have been synthesized with an ultrasmall size or thickness to maximize the proportion of the Pt atoms exposed on the surface.1–7 Compared with ultrasmall Pt nanoparticles which usually suffer from sintering or detachment from the support in catalysis, the deposition of atomic layers of Pt on secondary metal nanocrystals represents a more promising strategy, which not only enables long-term stability and excellent control over the surface structure and thickness of the Pt thin layer, but also introduces synergistic ligand or strain effects by the secondary metals to dramatically enhance the activity of Pt catalysts.8–13 However, secondary metal nanocrystals for the deposition of Pt are typically limited to Pd nanocrystals in recent research,8–13 which are within the scope of precious metals and are usually low in surface area for the Pt deposition due to the large crystal size, leaving a lot of room for further reducing the overall cost of catalysts for practical applications.

Among different metal nanocrystals as the substrate for the atomic layer deposition of Pt in the design of catalysts, one-dimensional Au and Ag nanowires with an ultrasmall diameter represent a promising candidate due to their large surface area, the relatively high abundance of the elements in the earth's crust, their synergistic effects with Pt for enhanced catalytic activities, their high stability due to the suppressed ripening, and their improved electronic transport characteristics.14–16 These nanowires have drawn substantial attention in the past few years due to their potential for application in catalysis,17–20 nanoelectronics,21,22 and molecular sensing.23,24 Compared with the synthesis of the Au nanowires on a substrate,25–28 a wet-chemical synthesis is more advantageous in terms of the convenient scale-up of the synthesis for mass production. However, many attempts to synthesize Au nanowires in aqueous wet-chemical systems rely on hexadecyltrimethylammonium bromide (CTAB) as the capping agent, which usually leads to Au nanowires with unfavorably large diameters (10–50 nm) or compromised yields.29–32 Although porous nanowire networks of Au could be obtained by quick reduction of a Au salt with a borohydride in an aqueous solution, the nanowires are often highly polycrystalline with less control over their sizes.18,19,33 Therefore, to date, the most successful synthesis of ultrathin Au nanowires has been achieved by the template-directed growth or oriented attachment in an oil phase by employing oleylamine, oleic acid, or amidoamine as capping agents.21,23,34–45 The resulting Au nanowires possess an ultrasmall diameter of ∼2 nm, uniform morphology, and single crystallinity. Despite this great success, the presence of a substantial amount of strong capping agents on the surface of these nanowires represents a major obstacle for their quick expansion in a broad range of applications, due to the difficulty in the effective removal of the strong capping agents without losing the structural integrity of the nanowires.27,46 An ideal synthesis should produce ultrathin Au (or Ag) nanowires in a high yield with excellent controllability of the ultrathin thickness (and thus high surface area), reproducibility, and scalability, and use readily exchangeable capping agents for convenient post-synthesis processes, which, however, remains highly desirable, especially for constructing efficient Pt-based catalysts as discussed in this work.

Herein, we report a novel robust strategy to achieve a scalable synthesis of single-crystalline Au–Ag alloy nanowires with an ultrasmall diameter of ∼1.8 nm and a length of few micrometers in a polar solvent (N,N-dimethyl formamide, DMF). Different from conventional methods, our synthesis of these ultrathin Au–Ag alloy nanowires does not rely on strong organic capping agents, although polyvinylpyrrolidone (PVP) is present as a readily exchangeable dispersant for the enhanced colloidal properties of the nanowires. Instead, we introduce a new capping effect by involving a Ag salt and a halide (Cl and Br) in this synthesis. By the co-reduction of the Ag salt, the surface of the Au seeds is modified with Ag, which readily adsorbs halides by the Ag–halide interactions. The capping effect of the halides regulates the one-dimension growth of the nanocrystals into nanowires and hinders their growth in lateral directions to afford ultrasmall diameters. These ultrathin Au–Ag alloy nanowires prove to be an excellent substrate for the deposition of Pt with controlled thickness. They provide not only a high surface area, which enables loading of a high amount of Pt on these nanowires despite a sub-monolayer thickness (thus the absence of Pt ensembles), but also a strong synergistic ligand effect to the Pt atoms, leading to excellent activity in the formic acid oxidation reaction (FAOR). We believe that this work provides a novel route for the scalable synthesis of ultrathin Au–Ag alloy nanowires with a conveniently processable surface, and the demonstration of these nanowires as an excellent platform for constructing highly efficient noble metal catalysts opens new opportunities in the catalyst design of fuel cells and a broader range of applications.

Experimental section

Materials

DMF, PVP (Mw 40[thin space (1/6-em)]000), silver nitrate (AgNO3, 99%), chloroplatinic acid hexahydrate (H2PtCl6·6H2O), chloroauric acid (HAuCl4), L-ascorbic acid (AA), acetonitrile (CH3CN), sodium hydroxide (NaOH), sodium bromide (NaBr), potassium iodide (KI), and sodium chloride (NaCl) were purchased from Sigma-Aldrich. N,N-diethylhydroxylamine (DEHA) was purchased from Adamas-beta. All chemicals were used as purchased without further purification.

Synthesis of ultrathin Au–Ag alloy nanowires

In a typical synthesis, 8 mL of PVP (5 wt% in DMF), 3.2 mL of DEHA, 1.2 mL of AgNO3 (0.1 M), and 0.6 mL of HAuCl4 (0.25 M) were added to 168 mL of DMF in sequence in a flask in an ice-water bath (0 °C) and stirred for 5 min. The reaction solution was then transferred into a 50 °C water bath and stirred for another 30 min. The resulting ultrathin Au–Ag alloy nanowires were collected by centrifugation, washed with H2O, and redispersed in 50 mL of H2O.

Synthesis of ultrathin Au–Ag@Pt core/shell nanowires

The epitaxial growth of Pt on the ultrathin Au–Ag alloy nanowires was achieved by our previously reported method with modifications.47 In a typical synthesis, 13 mL of CH3CN, 2 mL of H2O, 4 mL of PVP (5 wt%), 20 μL of H2PtCl6 (0.1 M), 200 μL of NaNO2 (0.2 M), 800 μL of NaOH (1 M) and 800 μL of AA (0.5 M) were added in sequence to 12 mL of the aqueous sol of the Au–Ag alloy nanowires under vigorous stirring at 30 °C. After stirring for 18 h, Au–Ag@Pt core/shell nanowires were obtained, which were collected by centrifugation and washed with water. The Pt/(Au + Ag) ratio was measured to be ∼0.05 by the inductively coupled plasma mass spectrometry (ICP-MS) analysis.

The Pt/(Au + Ag) ratio can be tuned by adjusting the amount of H2PtCl6 in the typical synthesis. These Au–Ag@Pt core/shell nanowires are denoted as Pt-x, where x is the Pt/(Au + Ag) ratio determined by ICP-MS. The Pt-0.01, Pt-0.15, Pt-0.20, Pt-0.31, and Pt-0.54 nanowires were obtained with 6, 50, 68, 100, and 200 μL of H2PtCl6 (0.1 M), respectively. The Pt-1.47 nanowires were synthesized with 600 μL of H2PtCl6 (0.1 M) in a flask filled with H2 at 1 atm and 50 °C, while all the other parameters were kept the same.

Electrochemical measurements

All electrochemical measurements were carried out on a CHI 760E electrochemical workstation (Shanghai Chenhua) with a three-electrode configuration at 25 °C. Pt foil and a Ag/AgCl electrode were used as the counter and reference electrodes, respectively. Commercial Pt/C (JM, 20%Pt/XC72R) and Pd/C (Pd 10%, Sigma-Aldrich) were used as reference catalysts. All potentials are reported with respect to the reversible hydrogen electrode (RHE). The catalyst was supported on carbon black (Vulcan XC-72) and dispersed in a mixture of isopropanol and 5% Nafion (volume ratio, 5[thin space (1/6-em)]:[thin space (1/6-em)]0.02). After ultrasonication for 1 h, a homogeneous ink was obtained. The working electrode was prepared by depositing a known volume of the ink with the same amount of Pt (1 μg) on the polished surface of a rotating disk electrode (RDE, 0.196 cm−2). Before recording the electrochemical data, the working electrode was activated with cyclic voltammetry (CVs) from 0.05 to 1.10 V in N2-saturated 0.5 M H2SO4 (100 mV s−1) until a steady curve was obtained. The FAOR was studied by recording CV curves at a scan rate of 50 mV s−1 from 0.014 to 1.214 V in 0.5 M H2SO4 and 0.25 M HCOOH. The durability of the catalysts was examined by chronoamperometry at a practical operating voltage of 0.3 V at 25 °C.

CO stripping of the catalysts was conducted to determine the electrochemically active surface area (ECSA) of Pt. CO was adsorbed by holding the activated working electrode at 0.1 V in CO-saturated 0.5 M H2SO4 for 30 min. Then, CO was removed from the electrolyte by purging with N2 for 30 min. The CO-stripping voltammetry data were collected from 0.05 to 1.4 V at a scan rate of 50 mV s−1. The background current associated with the double-layer charging and surface oxidation was subtracted from these curves. The ECSAs of the Pt and Au were calculated based on the charge associated with the CO stripping at a reference value of 420 μC cm−2 and the reduction of the Au oxide with a reference value of 400 μC cm−2, respectively.48–50

Characterization

Transmission electron microscopy (TEM) was performed on a Hitachi HT-7700 at 100 kV. High-resolution TEM (HRTEM) was performed on a JEM-F200-TEM at 200 kV. X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab powder X-ray diffractometer equipped with Cu Kα radiation. UV-vis-near-infrared (NIR) spectra were obtained on an Ocean Optics HR2000 + ES UV-vis-NIR spectrophotometer with a DH-2000-Bal light source. ICP-MS was performed on a PerkinElmer NexION 350D. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB Xi + equipped with monochromatic Al Kα radiation. Energy-dispersive X-ray spectroscopy (EDS) was carried out on a Hitachi SU3500 scanning electron microscope at 15 kV.

Results and discussion

Synthesis of ultrathin Au–Ag alloy nanowires

The ultrathin Au–Ag alloy nanowires were synthesized in a polar solvent of DMF by employing HAuCl4 as a source of Au and Cl, AgNO3 as a source of Ag, DEHA as a weak reducing agent, and PVP as a dispersant to ensure the colloidal properties of the nanowires. Additional Cl (NaCl) may help maintain a high yield of the ultrathin Au–Ag alloy nanowires. A sol of the Au–Ag nanowires was obtained at a high concentration appearing in a deep color (Fig. 1a, inset). A typical lab-scale synthesis produces 40 mg of the nanowires from a reaction volume of ∼180 mL and could be readily scaled up for large-scale applications. After dilution, the sol of the ultrathin Au–Ag alloy nanowires appears in a yellowish color, which is typical of Au–Ag nanocrystals with high aspect ratios (Fig. 1a, inset). No reddish color could be observed, indicating the absence of spherical impurities and thus the high yield of the product. In addition, this synthesis is convenient and time-efficient with a typical reaction time of ∼35 min, which is much speedier than most conventional syntheses of Au nanowires in an oil phase.
image file: c8ta05663c-f1.tif
Fig. 1 Synthesis of ultrathin Au–Ag alloy nanowires: (a) low-magnification TEM image of the nanowires. Inset: photograph of a sol of the nanowires obtained from a large-scale synthesis before and after dilution. (b) HRTEM images of the nanowires, showing the growth along the [110] direction. Inset: the corresponding Fourier diffractograms. (c) EDS of the nanowires. (d) EDS elemental mappings of the nanowires. (e) Evolution of the UV-vis-NIR spectra during the synthesis of the Au–Ag alloy nanowires. (f) Growth intermediates of the nanowires obtained after different lengths of the reaction time.

The morphology and structure of the ultrathin Au–Ag alloy nanowires were examined by TEM imaging (Fig. 1a and b). It is clear that uniform nanowires have been synthesized in a high yield with an average diameter of ∼1.8 nm (for histogram see Fig. S1, ESI) and a length of few micrometers. The HRTEM images reveal the high single crystallinity of the nanowires and their crystal growth along the [110] direction (Fig. 1b). It is worth noting that the ultrathin Au nanowires from a conventional oil-phase synthesis were usually grown in the [111] direction.21,23,34–45 The different growth directions of the ultrathin nanowires confirm dissimilar capping effects involved in the syntheses. The EDS elemental analyses demonstrate uniform distributions of Au and Ag in the nanowires (thus high stability, Fig. S2, ESI), and Cl is present on the surface of the nanowires even after repetitive washing with ammonia, confirming the strong adsorption of the chloride on the nanowire surface (Fig. 1c and d, and S3, ESI, the capping effects discussed later in detail). The stepwise growth of the ultrathin Au–Ag alloy nanowires can be monitored by UV-vis-near infrared (NIR) spectroscopy and TEM imaging (Fig. 1e and f). Ultrasmall Au nanospheres at a low concentration were produced at an early stage, which gave rise to a light pink sol without pronounced localized surface plasmon resonance (LSPR) properties. Subsequently, the nanospheres were elongated into one-dimensional nanorods and then nanowires. Two LSPR bands appeared at ∼460 nm and in the NIR range (>1050 nm), corresponding to the out-of-plane and in-plane dipole-mode LSPR of the nanowires, respectively. The LSPR intensity increased continuously with time, accompanying the growth of the nanowires. A dark yellowish sol of the nanowires was eventually obtained as the final product after 30 min.

Formation mechanism of the ultrathin Au–Ag alloy nanowires

Our approach to synthesize ultrathin Au–Ag alloy nanowires relies on the capping effect of a halide on the Ag-modified nanocrystal surface by the Ag–halide interaction. Therefore, both the Ag modification and the halide play a pivotal role in this synthesis, which has been confirmed by a series of control experiments.

To verify the role of Ag, we first conducted a synthesis in the absence of AgNO3, which produced bulky Au nanocrystals (∼100 nm) with sharp nanotips instead of the nanowires (Fig. 2a). Two LSPR bands were observed at ∼600 and 750 nm in the UV-vis-NIR spectrum, which could be attributed to the anisotropic morphology of the nanocrystals (Fig. 2d). When a low concentration of AgNO3 (HAuCl4/AgNO3 = 3.75) was introduced, the size of the resulting Au–Ag nanocrystals became significantly reduced due to the strong adsorption of the chloride on the Ag-modified crystal surface (Fig. 2b). However, only sparse nanowires were found in the product, which could be attributed to the insufficient capping effect for the anisotropic crystal growth, giving rise to a red sol with an LSPR extinction at ∼530 nm (Fig. 2d). With the increasing concentration of AgNO3 (HAuCl4/AgNO3 = 2.5, 1.25, 0.50, 0.25 and 0.10) in a typical synthesis, ultrathin nanowires of high quality were eventually obtained as a deep yellowish sol (Fig. 2c and S4, ESI), with the in-plane dipole mode LSPR appearing in the NIR range of the spectrum (Fig. 2d). The strong dependence of the formation of the ultrathin nanowires on the concentration of AgNO3 in the precursors well elucidates the vital role of Ag in establishing an appropriate capping effect and regulating the one-dimensional growth of the nanowires. These results also suggest the high versatility of this synthesis in producing ultrathin Au/Ag alloy nanowires with varying compositions (Au/Ag = 0.17–2.2 by EDS) and properties (Fig. S4 and S5, ESI).


image file: c8ta05663c-f2.tif
Fig. 2 Role of Ag in the formation of the Au–Ag alloy nanowires: (a) TEM image of the product synthesized in the absence of AgNO3. (b and c) TEM images of the products synthesized with HAuCl4/AgNO3 ratios of 3.75 and 1.25, respectively, in the precursors. Insets: photographs of the respective sols. (d) UV-vis-NIR spectra of the products corresponding to (a–c).

To examine the role of the halide, the effect of the concentration of Cl was investigated on the formation of the ultrathin Au–Ag alloy nanowires. First, Au–Ag nano-tadpoles were synthesized by modifying the typical synthesis of the nanowires with an unfavorable HAuCl4/AgNO3 ratio of 0.5 (Fig. 3a). After the introduction of additional NaCl (NaCl/AgNO3 = 4), these nano-tadpoles were successfully transformed into ultrathin nanowires of high quality (Fig. 3b). These nanowires could be obtained from a synthesis with a wide range of Cl/Ag+ ratios (Fig. S6, ESI). It is reasonable that at a low HAuCl4/AgNO3 ratio of 0.5, the concentration of the Cl (from HAuCl4) was lower than that required for establishing a sufficient capping effect for the growth of the nanowires, while this strong interaction was fully achieved in the presence of additional NaCl, which clearly confirms the critical role of the halide in this synthesis. In addition, we found that Br could be used as the halide without causing significant changes in the morphology and overall quality of the ultrathin Au–Ag alloy nanowires (Fig. 3c). However, when the halide was changed to I, instead of nanowires, only nanospheres of ∼5 nm could be obtained (Fig. 3d). A plausible explanation is that the I binds on different facets of the metal nanocrystals compared with Cl and Br.51 In addition, the binding energy of I on the metal surface is much stronger, which may cause problems in the reaction kinetics to achieve controlled one-dimensional growth of the nanowires.52 Therefore, all evidence confirms that an appropriate capping effect of a halide is required for the successful synthesis of the ultrathin Au–Ag alloy nanowires. It is worth noting that PVP was also present in the synthesis system to ensure the high dispersity of the nanowires in the DMF. However, a control experiment indicates that the ultrathin Au–Ag alloy nanowires could be obtained in the absence of PVP, which unambiguously rules out the effect of the PVP on regulating the anisotropic growth of the nanowires (Fig. S7, ESI).


image file: c8ta05663c-f3.tif
Fig. 3 Role of the halide in the formation of the ultrathin Au–Ag alloy nanowires: (a) Au–Ag nano-tadpoles obtained from a typical synthesis with a HAuCl4/AgNO3 ratio of 0.5. (b) TEM image of the Au–Ag alloy nanowires obtained by adding NaCl to the synthesis system of (a). (c and d) TEM images of the Au–Ag alloy nanowires/nanospheres obtained in the presence of NaBr and KI (NaBr(or KI)/AgNO3 = 0.7) in a typical synthesis, respectively.

It is worth noting that the synthesis of the ultrathin Au–Ag alloy nanowires was conducted in DMF, which proves to be a good polar solvent due to its excellent coordinating capability with the metal salts and consequently favorable reduction kinetics (Fig. S8, ESI).53–55 For comparison, when H2O was used as the solvent in place of DMF, only large nanoparticles could be obtained with a rough surface due to the quick reduction of the metal salts (Fig. S8, ESI). In addition, the use of DEHA as the reducing agent also contributed to the controllable reaction kinetics for the growth of the ultrathin nanowires.53,54 Our results confirm that the use of ascorbic acid in place of DEHA can also produce ultrathin Au–Ag alloy nanowires at a slow reaction rate. When the reaction was accelerated by deprotonating the ascorbic acid, only irregular nanoparticles were obtained as the final product (Fig. S9, ESI). Therefore, a combination of DMF and DEHA represents the preferred condition for the robust synthesis of the ultrathin Au–Ag alloy nanowires with high reproducibility.

Epitaxial growth of Pt on the ultrathin Au–Ag alloy nanowires

The robust synthesis of the ultrathin Au–Ag alloy nanowires paves a way to the deposition of Pt with controlled atomic thicknesses as a cost-effective and efficient catalyst for a broad range of reactions. Here, the epitaxial growth of Pt on the Au–Ag alloy nanowires was achieved by following our previously reported method with modifications.47 In essence, the presence of the nitrite and acetonitrile (ligands) greatly hinders the formation of Pt islands on the Au–Ag substrates. We hypothesize that the adsorption of the ligands on the Au (or Ag) surface is much weaker than that on the Pt surface, which favors the preferential formation of Pt–Au (or Pt–Ag) bonds rather than Pt–Pt bonds and thus layer-by-layer growth of Pt on the Au–Ag nanowires with high controllability. As a result, Au–Ag@Pt core/shell nanowires with different thicknesses of Pt (Pt-x: x represents the Pt/(Au + Ag) ratio) were successfully prepared (Fig. 4). Elemental mapping analysis of typical Au–Ag@Pt core/shell nanowires (Pt-0.15) confirms that Pt has been successfully grown on the Au–Ag alloy nanowires (Fig. 4a). The HRTEM images, the corresponding Fourier diffractograms and the XRD patterns of the Pt-0.15 and Pt-0.54 nanowires confirm a single lattice with an almost identical size to that of the Au–Ag alloy nanowires, which suggests that the Pt atoms were deposited on the surface of the Au–Ag nanowires by epitaxial growth (Fig. 4b and c and S10, ESI). The growth of Pt was further proved by TEM imaging by investigating the change in the average diameter of the Au–Ag@Pt core/shell nanowires with varying Pt/(Au + Ag) ratios (Fig. 4d–i). The diameters of the Pt-0.05, -0.15, -0.20 and -0.31 nanowires are almost the same (∼2.0–2.2 nm), while those of the Pt-0.54 and -1.47 nanowires become significantly larger (∼2.6 and 3.4 nm). This hints that the Pt grown on the Au–Ag alloy nanowires was in a sub-monolayer thickness when the Pt/(Au + Ag) ratio is ≤ 0.31, and the growth of Pt multilayers occurred when the Pt/(Au + Ag) ratio becomes larger than 0.54.
image file: c8ta05663c-f4.tif
Fig. 4 Synthesis of Au–Ag@Pt core/shell nanowires (Pt-x) with a tunable atomic thickness of Pt: (a) EDS elemental mappings of the Au–Ag@Pt core/shell nanowires (Pt-0.15). (b and c) HRTEM images of the Au–Ag@Pt nanowires, Pt-0.15 and Pt-0.54, respectively. Inset: the corresponding Fourier diffractograms. (d–i) TEM images of the Pt-0.05, -0.15, -0.20, -0.31, -0.54, and -1.47, respectively. Inset: histograms of the diameter distribution of the nanowires. The diameters are expressed in terms of “mean ± standard deviation”.

The gradual coverage of the Pt atomic layers on the surface of the ultrathin Au–Ag alloy nanowires was verified by the electrochemical analysis (Fig. 5). The electrochemically active surface area (ECSA) of the Pt on the Au–Ag nanowires can be determined by the CO stripping (Fig. 5a and b). When the Pt/(Au + Ag) ratio is smaller than 0.31, all the Au–Ag@Pt core/shell nanowires possess a very close ECSA value, which is as high as ∼165 m2 gPt−1. Assuming that the Pt atoms form a monolayer or a sub-monolayer, the surface area could be theoretically estimated to be πr2NA/m = 185 m2 g−1, where r is the radius of a Pt atom (138 pm), NA is the Avogadro constant (6.02 × 1023 mol−1), and m is the molar mass of Pt (195 g mol−1). Therefore, the ECSAs of the Au–Ag@Pt core/shell nanowires (Pt-x, x ≤ 0.31) are very close to the theoretical surface area of monolayer/sub-monolayer Pt, which is a clear sign of the growth of sub-monolayer Pt on the Au–Ag alloy nanowires in this synthesis. When the Pt/(Au + Ag) ratio is larger than 0.54, a significant decrease in the ECSA of the Au–Ag@Pt core/shell nanowires can be observed, suggesting that the Pt grows into multilayers on the Au–Ag nanowires. The exposure of Pt and Au can be further evidenced by their oxidation and reduction in the cyclic voltammetry (CV) measurements in N2-saturated H2SO4 (Fig. 5c). In cathodic scans, two peaks can be observed at ∼1.1 and 0.57 V vs. RHE, which corresponds to the reduction of the Au oxide and Pt oxide, respectively (Fig. S11, ESI). It is clear that with increasing Pt/(Au + Ag) ratios, the peak at 1.1 V (reduction of the Au oxide) becomes continuously weaker. This peak becomes almost indiscernible when the Pt/(Au + Ag) ratio is 0.31. Quantitative analysis indicates that the Au surface is completely covered by the Pt atoms when the Pt/(Au + Ag) ratio reaches 0.54 and higher values (Fig. 5d).


image file: c8ta05663c-f5.tif
Fig. 5 (a) CO stripping of the Au–Ag@Pt core/shell nanowires (Pt-x). (b) ECSAs of the Au–Ag@Pt core/shell nanowires calculated from the CO stripping curves. (c) CV curves of the Au–Ag@Pt core/shell nanowires in N2-saturated 0.5 M H2SO4, showing the diminishing Au surface with increasing Pt/(Au + Ag) ratios. (d) Change of the surface area of Au normalized to the mass of Au with increasing Pt/(Au + Ag) ratios.

Therefore, all evidence confirms that the growth of Pt on the ultrathin Au–Ag alloy nanowires proceeds in an atomic layer-by-layer mode, and the complete formation of a Pt monolayer is achieved when the Pt/(Au + Ag) ratio is in the range of 0.31–0.54. This value is also consistent with the theoretical value obtained by geometrical calculation (∼0.53).

Electronic properties of the Au–Ag@Pt core/shell nanowires

The ultrathin Au–Ag alloy nanowires provide not only a high surface area for atomic layer deposition of Pt to achieve a high proportion of surface atoms, but also a synergistic effect on the surface Pt atoms to significantly enhance their catalytic activities.56,57 This synergistic effect can be readily observed in the electrocatalytic CO stripping (Fig. 5a). With decreasing Pt/(Au + Ag) ratios from 1.47 to 0.05, the CO adsorbed on the Au–Ag@Pt nanowires could be oxidized at continuously elevated potentials from 0.84 to 1.28 V vs. RHE. It is recognized that the electron back-donation from the Pt d orbitals to the CO 2π* orbital accounts for the increased binding energy of CO on the Pt surface.58,59 With CO as a probe, it can be inferred that the Pt atoms are more electron-rich at low Pt/(Au + Ag) ratios.

The electron transfer from Au to Pt can be further confirmed by XPS (Fig. 6). All the core-level Au 4f XPS spectra of the Au–Ag and the Au–Ag@Pt core/shell nanowires can be fitted to one set of the 4f5/2 and 4f7/2 doublet components, with the 4f7/2 component appearing at ∼83.45 and ∼83.8 eV for the Au–Ag alloy and Au–Ag@Pt core/shell nanowires, respectively, corresponding to the Au(0) species (Fig. 6a). The shift of the binding energy by ∼.35 eV indicates that the Au sites are more electron-deficient after deposition of Pt. The Pt 4f XPS spectra of the Au–Ag@Pt core/shell nanowires can be fitted to two sets of the 4f7/2 and 4f5/2 doublet components, corresponding to the Pt(0) and Pt(II) species (Fig. 6b). The proportion of the Pt(II) species increases with decreasing Pt/(Au + Ag) ratios, which can be ascribed to the thickness-dependent tendency of the surface Pt toward oxidation by ambient air. It is interesting that when the Pt/(Au + Ag) ratio decreases from 1.47 to 0.05, a continuous shift in the Pt 4f7/2 binding energy can be observed from 71.05 to 70.38 eV, indicating that the Pt sites become increasingly electron-rich. Therefore, a conclusion could be drawn that electron transfer occurs from Au to Pt and the Pt is more electron-rich at low Pt/(Au + Ag) ratios, which is highly consistent with the CO stripping results. The efficient and thickness-dependent electron transfer from Au (and Ag) to Pt enables a strong synergistic ligand effect that can impose a pronounced influence on the catalytic activities of the Au–Ag@Pt core/shell nanowires.56,57


image file: c8ta05663c-f6.tif
Fig. 6 (a) Au 4f XPS of the Au–Ag alloy nanowires and the Au–Ag@Pt core/shell nanowires (Pt-x). (b) Pt 4f XPS of the Au–Ag@Pt core/shell nanowires. Arrows indicate the shift of the Au 4f7/2 or Pt 4f7/2 peak positions.

Electrocatalytic performance of the Au–Ag@Pt core/shell nanowires

To evaluate the catalytic properties of the Au–Ag@Pt core/shell nanowires with the tunable atomic thickness of Pt, we systematically examined their performance in the electrocatalytic formic acid oxidation reaction (FAOR) (Fig. 7). The FAOR is known to proceed in dual pathways: the direct pathway by dehydrogenation and the indirect pathway by dehydration involving CO as an intermediate.60–62 The indirect pathway causes poisoning of the Pt catalyst, which should be avoided for enhanced catalytic activity and durability. It is recognized that Pt ensembles are required to trigger the indirect pathway of the formic acid oxidation.60–62 It is therefore inferred that our Au–Ag@Pt core/shell nanowires with controlled sub-monolayer Pt may serve as an excellent catalyst for this reaction due to the strong synergistic effect, as discussed above, and the absence of the Pt ensembles.
image file: c8ta05663c-f7.tif
Fig. 7 Electrocatalytic performance of the Au–Ag@Pt core/shell nanowires (Pt-x) with different Pt/(Au + Ag) ratios in the FAOR, in comparison with the commercial Pt/C and Pd/C catalysts: (a) CV curves of the Pt-x, Pt/C and Pd/C catalysts in N2-saturated HCOOH(0.25 M)/H2SO4(0.5 M) at a scan rate of 50 mV s−1. (b and c) Specific (js) and mass (jm) activities of the catalysts in the FAOR. The values of the Pt-x and the Pt/C catalysts were obtained at 0.58 V vs. RHE, and those of the Pd/C were obtained at 0.37 V vs. RHE. (d) Chronoamperometric curves of the Pt-0.05 and the Pt/C in N2-saturated HCOOH(0.25 M)/H2SO4(0.5 M) at 0.3 V vs. RHE.

Fig. 7a shows the CVs of the Au–Ag@Pt core/shell nanowires (Pt-x) in N2-saturated HCOOH(0.25 M)/H2SO4(0.5 M), with the commercial Pt/C and Pd/C catalysts as references (Fig. S12, ESI). The FAOR with the Pt-1.47 catalyst is mainly through an indirect pathway, resembling the commercial Pt/C, which can be evidenced by the predominant CO oxidation peak at ∼0.9 V in the anodic scan. The FAOR with the Pt-0.54 and -0.31 catalysts showed both oxidation peaks at ∼0.58 and ∼0.9 V, implying that the reaction proceeded in dual direct and indirect pathways. Only the direct pathway was involved in the FAOR with the Pt-0.20, -0.15 and -0.05 catalysts, which showed a single oxidation peak at ∼0.58 V. It is clear that with the decreasing thickness of Pt (quantified using the Pt/(Au + Ag) ratio), the formic acid oxidation mechanism shifted from the indirect pathway to a dual indirect and direct pathway, and eventually to a sole direct pathway. The sole direct pathway was achieved only when the Pt atoms were far from the complete coverage of the Au–Ag alloy nanowires. It is also observed that with the decreasing Pt/(Au + Ag) ratio, the catalytic activities of the Au–Ag@Pt core/shell nanowires increased monotonically (Fig. 7b and c). The Pt-0.05 catalyst displayed a specific activity of 4.1 mA cm−2 and a mass activity of 6.8 mA μgPt−1 at 0.58 V, which were ∼45 and 68 times those of the commercial Pt/C, and ∼5.2 and 17 times those of the commercial Pd/C (peak current densities, at 0.37 V). It is worth noting that a further decrease in the Pt/(Au + Ag) ratio leads to even enhanced activities. For example, the Pt-0.01 catalyst shows a specific activity of 4.5 mA cm−2 and a mass activity of 7.1 mA μgPt−1 at 0.58 V (Fig. S13 and S14). This trend of the catalytic activity can be attributed to the synergistic ligand effect, in addition to the absence of the Pt ensembles. As discussed above, dramatic electron transfer from Au and Ag to Pt occurred in the Au–Ag@Pt core/shell nanowires, which may significantly strengthen the interaction between the catalyst surface and the formic acid and hence enhance the rate of its oxidation. Therefore, it can be inferred that the ultrathin Au–Ag alloy nanowires conveniently synthesized in this work in a large scale provide a high surface area and the synergistic effects for the deposition of Pt far from a monolayer despite a high loading amount (the ratio of Pt/(Au + Ag), 1–20%), which paves a way to the excellent activity in the FAOR catalysis. It is worth noting that the controlled atomic deposition of Pt on the Au–Ag alloy nanowires in this work ensures the absence of Pt ensembles at low Pt/(Au + Ag) ratios, which is also critically important for the optimization of the catalyst, superior to some previous reports with unavoidable formation of Pt ensembles on the Au substrates.20

Moreover, the chronoamperometric measurements of the FAOR were carried out to evaluate the catalytic durability of the Pt-0.05 catalyst compared with the commercial Pt/C (Fig. 7d and S15). The current densities were recorded at a practical operating voltage of 0.3 V for 50 min.63 The steady-state Pt mass activity of the Pt-0.05 catalyst remained two orders of magnitude higher than that of the commercial Pt/C, which demonstrates the high durability of the Au–Ag@Pt core/shell nanowires in the FAOR catalysis.

Conclusions

In summary, we have developed a novel strategy to synthesize ultrathin Au–Ag alloy nanowires (diameter, ∼1.8 nm) in a high yield from a polar solvent without involving strong capping ligands that are usually used in conventional oil-phase syntheses. The key to the synthesis is the surface modification of nanocrystals by Ag and the Ag–halide interactions for regulating the one-dimensional growth of the nanocrystals into nanowires. This synthesis is reproducible and conveniently scalable, thanks to the robust formation mechanism of the ultrathin nanowires. We further demonstrated the application of these nanowires as a high-surface-area substrate for the atomic deposition of Pt for constructing highly active and cost-effective electrocatalysts. Due to the high surface area, Pt atoms have been deposited on the Au–Ag alloy nanowires without forming ensembles despite a relatively high loading amount (Pt/(Au + Ag), up to 20%). Thanks to the absence of the Pt ensembles and the synergistic ligand effect, the resulting Au–Ag@Pt core/shell nanowires showed excellent activity in the FAOR electrocatalysis. A typical catalyst (Pt-0.05) demonstrated specific and mass activities, which were ∼45 and 68 times those of the commercial Pt/C, respectively. We believe this novel synthesis of the ultrathin Au–Ag alloy nanowires and the demonstration of these nanowires as a high-surface-area substrate for constructing noble metal catalysts with atomic thickness could inspire new endeavors in the design of catalysts for superior activities in a series of catalytic applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

C. G. acknowledges the support from the National Natural Science Foundation of China (21671156 and 21301138) and the Tang Scholar Program from Cyrus Tang Foundation. The authors thank the Instrument Analysis Center of Xi'an Jiaotong University for access to HRTEM, ICP-MS, and XPS.

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Footnote

Electronic supplementary information (ESI) available: Additional XRD, UV-vis-NIR, TEM, EDS, and electrochemical results. See DOI: 10.1039/c8ta05663c

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