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Organics- and Surfactant-free, Molten Salt Medium Controlled Synthesis of Pt-M (M = Cu, Pd) Bi- and Tri-metallic Nanocubes and Nanosheets Pengtao Qiu, Jinglei Bi, Xiaojing Zhang, and Shengchun Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00193 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017

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ACS Sustainable Chemistry & Engineering

Organics- and Surfactant-free, Molten Salt Medium Controlled Synthesis of Pt-M (M = Cu, Pd) Bi- and Tri-metallic Nanocubes and Nanosheets PengtaoQiu, †Jinglei Bi, †Xiaojing Zhang, † and Shengchun Yang*,†, ‡ †

School of Science, Key Laboratory of Shaanxi for Advanced Materials and

Mesoscopic Physics, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China. ‡

Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou

Academy of Xi’an Jiaotong University, 215000, Suzhou, People’s Republic of China

KEYWORDS: inorganic molten salt, Pt-based alloys, shape-controlled, graphene oxide, stabilizer

*

Address correspondence to [email protected].

1 ACS Paragon Plus Environment


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ABSTRACT

Herein we present a novel synthetic strategy for the shape-controlled synthesis of Pt-based alloy nanoparticles (NPs) in inorganic molten salt without using any organic surfactants or capping agents. Graphene oxide (GO) was chosen as the stabilizer in inorganic molten salt synthetic strategy, due to the existence of various oxygen-containing functional groups on its surface, which could adsorb, anchor and stabilize the metal ions or NPs. After GO was added in molten salt, the PtPd nanosheets could be formed on its surface in H2 atmosphere. In addition, when KI was chosen as the shape-inducing agent to selectively adsorb on and fully protect the (100) facets of alloys, PtPd nanocubes with core-shell structure, PtCu nanohemicubes and PtPdCu nanocubes were prepared successfully on the surface of GO in molten salt. To introduce GO as the stabilizer in molten salt proves a new approach in synthesis of Pt-based nanocrystals with controlled morphologies.


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INTRODUCTION Pt-based nanocrystals with well-defined morphology have been extensively investigated and used as catalysts for various electrocatalytic reactions.1-5 The morphology, which modifies the exposed surface electronic and crystalline structure, has much influence on the catalytic performance.6-7 Generally, for controlling the morphology of the nanocrystals, organic solvents or surfactants (such as PVP, CTAB/CTAC, oleylamine/oleic acid, etc.) are essentially required.8-13 However, these organic reagents, which always strongly adsorbed on the surface of nanocatalysts, are very difficult to be completely removed.14-19 The adsorption of reactants onto the surface active sites would be limited by the residual organic reagents during the catalytic

reactions,

thus

severely

limiting

the

improvement

in

catalytic

performance.20-21 In addition, these organic-based synthetic methods also suffer the disadvantages of the complex synthetic process, high toxicity of the solvent, high material cost, etc. Therefore, the design of surfactant-free (even organics-free) synthetic strategies for effective catalysts is imperative.

Inorganic molten salt has been recently used to synthesize noble metal NPs, porous carbon nanostructures and metal oxide nanocrystals.22-25 In such syntheses, the inorganic salts melt above the eutectic melting point, being used as the reaction medium. Molten salt solvent shares many unique characteristics, such as the good solubility at high temperature, easy isolation of the product, inappreciable vapor pressure, wide operating temperature range, low material cost, recyclability, etc.26-29



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However, comparing with organic-based syntheses, morphosynthesis of nanocrystals in molten salt has posed a serious challenge: typically, (1) due to the large polarity difference, the generally used organic capping reagents cannot be dissolved in the molten salt, thus making it impossible to use the organics to control the morphologies of the noble metal NPs; (2) until now, the kinetics and thermodynamics in the nucleation and growth of metal clusters in molten salt are still unclear compared with that in the organic or aqueous solutions, which causing a difficulty to induce the anisotropic growth of metallic nucleus; (3) NPs formed in molten salt have very high surface energy and tend to minimize the energy by aggregation, owing to the absence of stabilizer or capping reagents.

For controlling the morphology of nanocrystals in molten salt, a novel inorganic surfactant or stabilizer is quite necessary. Recently, GO has been used as a stabilizer or structure-directing agent in some hydrothermal synthesis. For example, Li and co-workers demonstrated that GO could be used as the stabilizer, carrier and reducing agent in the preparation of Pt/GO nanocomposites via the direct reaction between Na2PtCl4 and GO in aqueous solution.30 Lu and co-workers presented that GO could act as a structure-directing agent in the controlled synthesis of PtPd alloy concave nanocubes (NCs) enclosed by high index facets in DMF solution.31 But these reported synthetic strategies either lacked effective control over the morphology or used the toxic organics. It is noteworthy that oxygen-containing functional groups of GO (e.g. hydroxyl, carbonyl, carboxyl and epoxide groups) could be used as binding sites for anchoring and stabilizing metal ions or NPs not only in aqueous or organic solution 4 ACS Paragon Plus Environment

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but also in molten salt. For example, in our previous work, we found that small gas molecules such as H2 could be used to induce the anisotropic growth of Pt NPs to form single crystal Pt nanosheets (NSs) via an oriented attachment (OA) process on the surface of GO in molten salt without adding any organics.32 Therefore, using GO to stabilize metal ions or NPs, the morphosynthesis of nanomaterials in molten salt would become possible.

In current research, we successfully synthesized the reduced GO (RGO) supported single crystal PtPd nanocrystals with two different morphologies (NSs and NCs), PtCu nanohemicubes (NHCs) and trimetallic PtPdCu NCs in the inorganic KNO3–LiNO3 eutectic mixture molten salt without using any organics. In this novel synthetic strategy, inorganic molten salt was used as the solvent; H2, KI and GO were used as the reducing, shape-inducing and stabilizing agent, respectively. Compared to the traditional organic-based methods, the syntheses of nanomaterials in molten salt present

many

obvious

advantages,

such

as

recyclable,

low-cost

and

environmental-friendly.

EXPERIMENTAL SECTION

Chemicals. Lithium nitrate (LiNO3) and sodium tetrachloropalladate(II) (Na2PdCl4) were provided by Shanghai Aladdin Reagent. Potassium nitrate (KNO3), copper nitrate trihydrate (Cu(NO3)2·3H2O) and potassium iodide (KI) were obtained from Tianjin Fengchuan Chemical Reagent. Graphene oxide (GO) was supplied by Carmery

Materials

Technology

Co.,

Ltd.

(Taiyuan

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Shanxi).

Potassium 5


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Tetrachloroplatinate(II) (K2PtCl4) was supplied by Sino-platinum Metals CO., Ltd. All the chemicals were used as received.

Preparation of PtPd NSs/RGO. In the typical synthesis of PtPd NSs/RGO, K2PtCl4 (10.38 mg) and Na2PdCl4 (7.35 mg) were dissolved into the mixture of 5.5 mL GO aqueous solution (2 mg/mL) and 34.5 mL water, and sonicated for 1 h. Then added a mixture of KNO3 (13.2 g) and LiNO3 (6.8 g) into the obtained solution. This resulted mixture was slowly heated to 200 ºC (allowing the water being evaporated) and kept for 1 h in hydrogen (H2) atmosphere with magnetic stirring. Finally, the product was collected and washed by centrifuging at 12000 rpm for 3 min with distilled water.

Preparation of PtPd NCs/RGO. The PtPd NCs/RGO was prepared using the standard procedure for synthesizing the PtPd NSs/RGO, but KI (50 mg) was added. Typically, K2PtCl4 (10.38 mg), Na2PdCl4 (7.35 mg) and KI (50 mg) were dissolved into the mixture of 5.5 mL GO aqueous solution (2 mg/mL) and 34.5 mL water, and sonicated for 1 h. Then added the mixture of KNO3 (13.2 g) and LiNO3 (6.8 g). This resulted mixture was slowly heated to 200 ºC and kept for 1 h in H2 atmosphere with magnetic stirring. Finally, the samples were collected by centrifuging and washing with distilled water several times.

Preparation of PtCu NHCs/RGO. In the typical synthesis of PtCu NHCs/RGO, K2PtCl4 (10.38 mg), Cu(NO3)2·3H2O (6.04 mg) and KI (33.2 mg) were dissolved into the mixture of 2.5 mL GO aqueous solution (2 mg/mL) and 37.5 mL water, and 6 ACS Paragon Plus Environment

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sonicated for 1 h. Then added the mixture of KNO3 (13.2 g) and LiNO3 (6.8 g). This resulted mixture was slowly heated to 200 ºC and kept for 0.5 h in H2 atmosphere with magnetic stirring. Finally, the samples were collected by centrifuging and washing with distilled water several times. Especially, PtCu NHCs/RGO was also washed with ammonium hydroxide several times to remove the residual CuI.

Preparation of PtPdCu NCs/RGO. In the typical synthesis of PtPdCu NCs/RGO, K2PtCl4 (10.38 mg), Na2PdCl4 (7.35 mg), Cu(NO3)2·3H2O (6.04 mg) and KI (46.4 mg) were dissolved into the mixture of 5 mL GO aqueous solution (2 mg/mL) and 35 mL water, and sonicated for 1 h. Then added the mixture of KNO3 (13.2 g) and LiNO3 (6.8 g). This resulted mixture was slowly heated to 200 ºC and kept for 1 h in H2 atmosphere with magnetic stirring. Finally, the samples were collected by centrifuging and washing with distilled water and ammonium hydroxide several times.

Characterizations. A JEOL JEM-2100 instrument at an accelerating voltage of 200 kV was used to take Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images. FEI Tecnai G2 F20 or Talos F200X microscope equipped with an energy dispersive X-ray analysis (EDXA) X-ray detector

was

utilized

for

High-angle

annular

dark-field

scanning

TEM

(HAADF-STEM) and X-ray energy dispersive spectra (EDS) elemental mapping measurements. A confocal microprobe Raman spectrometer (Horiba Jobin Yvon LabRAM HR800) with the 633 nm He–Ne laser line was used to measure Raman



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spectra. An X-ray diffractometer (Bruker D8 Advance) with a Cu KR X ray source (λ =1.5405 Å) recorded the X-ray diffraction (XRD) patterns. An Axis Ultrabld Kratos with monochromatic AlKa radiation (150 W, 5 kV at 1486.6 eV) and the chamber pressure for the spectrometer kept at 10-9 Torr was used to study X-ray photoelectron spectroscopy (XPS). An ICP-AES 5300 was used to determine the amount of metal loaded on working electrode.

Electrochemical measurements. A Pine AFCBP1 Instrument was used to perform all electrochemical measurements by a standard three-electrode electrochemical cell in which a glassy carbon electrode (GCE) as working electrode, a Pt mesh as the counter electrode and an Ag/AgCl (3 M KCl) electrode as the reference. All the measured potentials (vs. Ag/AgCl) have been converted to the reversible hydrogen electrode (RHE) according to Nernst equation (ERHE = EAg/AgCl + 0.059pH + 0.197). The as-synthesized catalysts were dispersed in distilled water and sonicated for 1h to obtain a uniform suspension. Then 10 µL of the obtained suspension was added onto a GCE (5.0 mm in diameter). The electrode was treated with UV/Ozone (wavelength 185 and 254 nm) for 1 h to oxidize and remove the residual iodide ions. After that, 15 µL of 0.05 wt% Nafion solution was used to cover the catalyst onto the GCE. Based on ICP-MS measurement, the Pt mass on GCE of the PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO and PtPdCu NCs/RGO catalysts were 2.40 µg, 1.56 µg, 2.08 µg and 3.18 µg, respectively. Cyclic voltammetry (CV) measurements were performed in deoxygenated 0.5 M H2SO4 solution with voltage range from 0.04 to 1.24 V (vs. RHE) at a scan rate of 50 mV s-1. And the methanol catalytic oxidation was performed in the 8 ACS Paragon Plus Environment

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deoxygenated 0.5 M H2SO4 solution containing 0.5 M methanol in the same experimental settings. For comparison, similar measurements were performed with commercial Pt/C (JM, 20 wt%).

RESULTS AND DISCUSSION

The RGO-supported PtPd nanocrystals with two different morphologies (NSs and NCs) were prepared in KNO3-LiNO3 eutectic mixture molten salt without using any organics. The morphology of PtPd would change from sheets to cubes by simply adding KI in the synthesis process. After that, PtCu with special hemicubic shape and trimetallic PtPdCu with cubic shape are also successful synthesized with the assistance of GO and KI in the molten salt synthetic strategy, as illustrated in Scheme 1. It should be noted that in the whole synthesis process, no organics were used.

Scheme 1. Illustration of the synthesis of PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO and PtPdCu NCs/RGO in inorganic molten salt with the assistance of GO.



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The morphology and structure analysis of the as-prepared samples were illustrated by TEM (Figure 1). As shown in Figure 1a1 and a2, the as-prepared PtPd sample definitely presents a sheet-like structure with irregular polygonal morphology and well disperses on the surface of RGO with measured diameter ranging from 20.34 to 75.34 nm (Figure S1a). The sheet-like morphology was further elucidated by a set of TEM images obtained at different tilting angles with X as the axis of rotation, as shown in Figure S2a. When controlling the tilting angles from −35° to 30°, the morphology of the PtPd NS/RGO has changed from sheet to rod. Furthermore, we can clearly see that the TEM image obtained at tilting angles of 30 degrees almost showed the cross section of the sheet. Thus the thickness of this sheet could be measured to about 10 nm. After that, some vertical PtPd NSs on the wrinkled area of the RGO were also found, as shown in Figure S2b. Based on this TEM image, the thickness of these vertical sheets could be measured to about 13, 10 and 15 nm, respectively. The SEAD pattern (Figure 1a3) of an individual PtPd NS shows the hexagonal symmetry spots, which could be indexed to the face-centered-cubic (FCC) structure, indicating that the PtPd NS is single crystal. The HRTEM image (Figure 1a4) viewed along the zone axis (confirmed by the insert of Figure 1a4) displays the well-defined crystal lattices fringes and no obvious grain boundaries between Pd and Pt due to the same crystal structure and the extremely high lattice match ratio.33-34 Moreover, the fringe spacing of 0.22 nm can be assigned to the interplanar (111) distance of the FCC PtPd alloy.35 For the as-prepared PtPd NCs/RGO (Figure 1b1 and b2), it can be seen that PtPd nanocrystals endowed with a cubic shape and uniformly deposited on the 10 ACS Paragon Plus Environment

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RGO sheets with an average side length of 22.2 nm (Figure S1b). The single crystal nature of the PtPd NCs was also elucidated by the SEAD pattern (Figure 1b3). And the HRTEM image (Figure 1b4) viewed along the zone axis displays the lattices fringes with lattice spacing of ca. 0.19 nm, which can be indexed to the {200} planes of the FCC PtPd alloy.36-37

Figure 1. TEM images of the as-prepared PtPd NSs/RGO (a1 and a2), PtPd NCs/RGO (b1 and b2), PtCu NHCs/RGO (c1 and c2) and PtPdCu NCs/RGO (d1 and d2). The SAED patterns of the single PtPd NS/RGO (a3), PtPd NC/RGO (b3), PtCu NHC/RGO (c3) and PtPdCu NC/RGO (d3). And HRTEM images of PtPd NSs/RGO (a4), PtPd NCs/RGO (b4), PtCu NHCs/RGO (c4) and PtPdCu NCs/RGO (d4), the inserts in HRTEM images are the corresponding FFT images, respectively.



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Interestingly, the as-prepared PtCu NPs/RGO endowed with a special hemicube shape, as shown in Figure 1c1 and c2. The special hemicube morphology would be further analysed later (Figure 2). We had statistically measured the edge length from the foursquare surface of the PtCu NHCs/RGO. And the average side length was 26.54 nm (Figure S1c). The SEAD pattern (Figure 1c3) of an individual PtCu NHC also reveals that the as-prepared PtCu NHCs are single crystal. The HRTEM image (Figure 1c4) of PtCu NHCs along the zone axis shows the fringes with lattice spacing of ca. 0.19 nm, which can be indexed to the {200} planes of a FCC lattice.38 Likewise, Figure 1d1 and d2 show the cubic PtPdCu on RGO sheets with an average side length of 40.86 nm (Figure S1d). The single crystal nature of an individual PtPdCu NC is indicated by a SEAD pattern (Figure 1d3). And HRTEM image (Figure 1d4) of PtPdCu NCs along the zone axis shows the fringes with lattice spacing of ca. 0.19 nm, which can be indexed to the{200} planes of a FCC PtPdCu alloy.11 It is notable that, when KI was not added in the synthesis process, sheet-like PtPd bimetallic nanocrystals could be prepared, as similar as sheet-like Pt nanocrystals in our previous work,32 maybe due to the good miscibility and small lattice mismatch in structure between FCC Pt and Pd.39 However, if Cu was introduced into Pt or PtPd morphosysthesis and without using KI, only the uneven PtCu or PtPdCu NPs and irregular aggregates can be observed in the products, as shown in Figure S3, indicating that the addition of Cu destroyed the shape inheritance from Pt or PtPd NSs. 12 ACS Paragon Plus Environment

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The morphology of the special PtCu NHCs/RGO was further elucidated by using a FEI Talos F200X microscope, as shown in Figure 2. We can clearly see that a pair of the parallel surfaces of the PtCu NHC marked in the white circle region has been quite vertical when the tilt at an angle of −20 degrees. Thus the distance of the pairs, which is about 23 nm, could be indexed to one of the edge lengths of the PtCu NHC. All of the TEM images obtained at tilting angles of −30, −20 and −10 degrees show a foursquare nature of the PtCu NHC upper surface, indicating another edge length is also about 23 nm. But the last edge length, which could be measured in the TEM image obtained at a tilting angle of 50 degrees, is about 14 nm, almost half of the other edge lengths. Based on these analyses, the corresponding models of the marked PtCu NHC can be illustrated as shown in the bottom left insets of TEM images.

Figure 2. A set of TEM images of PtCu NHCs/RGO obtained at different tilting angles with X as the axis of rotation from −30° to 60°, the scale bar applies to all images and the insets show the corresponding models of the PtCu NHC in the white circle region.



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The internal crystalline structures of the as-prepared samples were further characterized by X-ray diffraction (XRD), as shown in Figure 3. All of the as-prepared samples displayed three characteristic diffraction peaks that could be well assigned to the single-phase peaks with a typical FCC lattice structure in the XRD patterns. It should be noted that both of the as-prepared PtPd NSs/RGO and PtPd NCs/RGO samples are coincidentally located between pure Pt (JCPDS No.04-0802) and Pd (JCPDS No.46-1043) patterns, and the PtCu NHCs/RGO sample is located between pure bulk Pt (JCPDS No. 04-0802) and Cu (JCPDS No. 89-2838). These results further confirm their bimetallic alloyed nanostructure.9,

31, 40

Likewise, the

as-prepared PtPdCu NCs/RGO are coincidentally located between the as-prepared PtPd NCs/RGO and PtCu NHCs/RGO, indicating the existence of trimetallic PtPdCu phase in the nanocrystals.11 Furthermore, the EDS spectra of the as-prepared samples were shown in Figure S4. And based on the EDS spectra, the Pt atomic ratio of the PtPd NSs/RGO, PtPd NCs/RGO and PtCu NHCs/RGO is 48.03%, 45.07% and 77.72%, respectively. Additionally, the ICP-AES measurements reveal the Pt atomic ratio of these bimetallic alloys to be 53.33%, 53.12% and 77.52%, respectively. Thus, the contents of Pt in the PtPd NSs/RGO and PtPd NCs/RGO are almost same as that in the precursors, indicating these successful reductions. But the content of Cu in the PtCu NHCs/RGO is much lower than that in the precursors, maybe due to the formation of insoluble CuI during the synthesis process. For PtPdCu NCs/RGO, the atomic content of Pt, Pd and Cu from the EDS results is 41.79%, 44.21% and 14.00%, 14 ACS Paragon Plus Environment

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respectively. And the low atomic content of Cu in PtPdCu NCs/RGO may be due to the formation of insoluble CuI during the synthesis process as well.

Figure 3. XRD patterns of the as-prepared PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO and PtPdCu NCs/RGO catalysts, respectively. The standard patterns of Pt (JCPDS No.04-0802), Pd (JCPDS No.46-1043) and Cu (JCPDS No.04-0836) are included for comparison.

The elemental distributions of the as-prepared samples were identified by the HAADF-STEM-EDS mapping images and line scanning profiles, as shown in Figure 4. A typical image of PtPd NSs/RGO is shown in Figure 4a. It can be seen that the distributions of both Pt and Pd are across the whole sheets, illustrating the formation of the PtPd alloyed phase. But for the PtPd NCs/RGO (Figure 4b), element Pd is concentrated in the center region, while Pt is distributed on the shell surface, indicating that PtPd NCs have core–shell structure. Moreover, the line scanning profile across a single PtPd NC (Figure 4e) further demonstrates the core–shell structure of PtPd NC with Pd rich core and Pt rich shell, which also illustrates the 15 ACS Paragon Plus Environment


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formation of alloyed PtPd in our synthetic process. A typical elemental mapping pattern of PtCu NHCs (Figure 4c) demonstrate the homogeneous distribution of Pt and Cu across the whole hemicubes, also illustrating the formation of the alloyed PtCu. Moreover, the homogeneous distribution of Pt, Pd and Cu is also found on the whole PtPdCu NC, as shown in Figure 4d.

Figure 4. HAADF-STEM-EDS mapping images of the as-prepared PtPd NSs/RGO (a), PtPd NCs/RGO (b), PtCu NHCs/RGO (c) and PtPdCu NCs/RGO (d). And HAADF-STEM images with cross-sectional compositional line profiles of PtPd NCs/RGO (e). Pt 4f XPS spectra of PtPd NSs/RGO (f1), PtPd NCs/RGO (f2), PtCu NHCs/RGO (f3) and PtPdCu NCs/RGO (f4), respectively; Pd 3d XPS spectra of PtPd NSs/RGO (g1), PtPd NCs/RGO (g2) and PtPdCu NCs/RGO (g3), respectively; Cu 2p XPs spectra of PtCu NHCs/RGO (h1) and PtPdCu NCs/RGO (h2), respectively.


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Furthermore, the near surface valence state of the as-prepared samples were investigated with XPS, as shown in Figure 4f-h. All of the surface energies were calibrated by referencing the spectra to the C1s peak for the C-C bond at a binding energy of 284.82eV. For both PtPd NSs/RGO and PtPd NCs/RGO, the Pt 4f deconvoluted XPS spectra (Figure 4f1and f2) show two pairs of peaks. The stronger peaks at 71.09 (Pt 4f7/2) and 74.36eV (Pt 4f5/2) could be assigned to metallic Pt0. And the other pairs at 72.45 (Pt 4f7/2) and 75.66eV (Pt 4f5/2) could be attributed to Pt2+ in PtO or Pt(OH)2.41 Notably, the metallic Pt0 is the predominant species in these samples. The Pd 3d deconvoluted XPS spectra of both PtPd NSs/RGO (Figure 4g1) and PtPd NCs/RGO (Figure 4g2) have showed two pairs of peaks as well. The pair of stronger peaks could be assigned to the binding energies of Pd3d5/2 and Pd3d3/2 of metallic Pd0, respectively. And the other pairs could be ascribed to the binding energies of Pd3d5/2 and Pd3d3/2 of Pd2+ species, respectively. Likewise, the metallic Pd0 is the predominant species in both PtPd NSs/RGO and PtPd NCs/RGO. Similarly, in the Pt 4f deconvoluted XPS spectrum of PtCu NHCs/RGO (Figure 4f3), the stronger peaks at 70.86 (Pt 4f7/2) and 74.20eV (Pt 4f5/2) could be attributed to metallic Pt0, which is the predominant species in the sample. And the other pair at 72.47 (Pt 4f7/2) and 75.55eV (Pt 4f5/2) could be ascribed to Pt2+ species. It should be also noted that the binding energy of Pt 4f observed for the PtCu NHCs/RGO has a negative shift compared to that of pure Pt/RGO,32 suggesting that electrons have been transferred from Cu to Pt.20 The Cu 2p divided XPS spectrum of PtCu NHCs/RGO (Figure 4h1) also showed that most of the Cu was in form of metallic Cu0.42 For the sample of 17 ACS Paragon Plus Environment


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PtPdCu NCs/RGO nanocomposite, the peak intensity of metallic Pt0 (at 70.77 and 74.12eV) is also much higher than that of Pt2+ (at 72.25 and 75.34eV), revealing that the metallic Pt is the predominant species in the sample, as shown in the Pt 4f deconvoluted XPS spectrum (Figure 4f4). And both the Pd 3d and Cu 2p divided XPS spectra (Figure 4g3 and h2) also show that most of the Pd and Cu are in the form of metallic Pd0 and Cu0, respectively. Therefore, the XPS analysis further indicates the efficient reduction of the metal precursors in all of the as-prepared samples.

The Raman spectra of the as-prepared PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO, PtPdCu NCs/RGO and the original GO were shown in Figure 5a. All of the Raman spectra display two distinguishing characteristic peaks at around 1325 and 1594 cm−1, correlating with the D and the G band, which can be indexed to disruption of the sp2 hybridized carbon atoms and the first-order scattering of the E2g phonons for the sp2 carbon lattice, respectively.21, 43 Usually, the intensity ratio of the D to G bands (ID/IG) could evaluate the reduction degree of GO, in which the ID/IG ratio increases when GO was reduced.41, 44 As shown in Fig. 5a, the ID/IG ratio of the as-prepared PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO and PtPdCu NCs/RGO is 1.294, 1.591, 1.269 and 1.302, respectively. All of the samples have a higher ID/IG ratio than the original GO (1.102), indicating a successful reduction of GO in the molten salt synthetic strategy. It should be noted that, the as-prepared PtPd NCs/RGO shows a higher ID/IG value than that of the PtPd NSs/RGO, signifying that the iodide ions can facilitate the GO reduction in the molten salt synthesis. But both PtCu NHCs/RGO and PtPdCu NCs/RGO have a lower ID/IG value than PtPd 18 ACS Paragon Plus Environment

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NCs/RGO, maybe due to the decreased content of iodide ions in such syntheses, which caused by the reaction between iodides and copper ions. The more complete reduction of GO leads to higher conductivity, which will facilitate the charge transport through the catalyst.44 The reduction degree of GO in the as-prepared samples was also studied by XPS, as shown in Figure 5b. The C 1s XPS spectra could be deconvoluted into three main components which centered at 284.82, 286.83 and 288.78eV, corresponding to the C-C/C=C, C-O and C=O groups, respectively.32, 44 In the XPS spectra, the proportion of C-C/C=C in the as-prepared PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO and PtPdCu NCs/RGO is 84.37, 87.81, 75.14 and 85.00 %, respectively. Notably, all of the as-prepared samples have a higher proportion of C-C/C=C than the original GO (68.09 %), further indicating the successful reduction of GO in all of the as-prepared samples. And the GO reduction for the sample of PtPd NCs/RGO is also more complete than that of PtPd NSs/RGO, PtCu NHCs/RGO and PtPdCu NCs/RGO, consisting with the result of Raman spectra.

Figure 5. Raman spectra (a) of the PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO, PtPdCu NCs/RGO and GO. C1s XPS spectra for the original GO (b1), PtPd NSs/RGO (b2), PtPd NCs/RGO (b3), PtCu NHCs/RGO (b4) and PtPdCu NCs/RGO (b5), respectively.



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In the synthesis, GO is very critical in the formation of the Pt-based nanocrystals. The morphology of the original GO was illustrated by SEM. As shown in Figure S5a, the GO shows a paper-like structure with corrugated morphology. And the existence of the oxygen-containing functional groups on its surface was also surveyed by FTIR (Figure S5b). The peaks located at 3360, 1721, 1222 and 1061 cm-1 correspond to the stretching vibrations of O-H, C=O, C-OH and C-O groups, respectively.32 Our additional experiments showed that the PtPd prepared in the absence of GO only yielded uneven aggregated PtPd NPs with whether adding KI or not, as shown in Figure S6. In molten salt, NPs have very clean surface but high surface energy owing to the absence of organics, thus tending to minimize their energy by aggregation. But when GO was dispersed into the molten salt, the oxygen-containing functional groups on the surface of GO acted as binding sites for anchoring the metal ions or NPs to restrain the aggregation, so as to inducing the anisotropic growth of NPs.31-32 In our current work, when introducing H2 atmosphere, PtPd NPs assembled sheet-like aggregates, and then fused to form the single crystal PtPd NSs on the surface of GO in molten salt, as discussed in our former research.25 Further adding KI, which usually used as a morphology controller for noble metal nanocrystals,45-46 the as-prepared PtPd NPs changed from the sheet-like shape to cubic shape with core-shell structure. The standard reduction potential of PdCl42–/Pd is typically more negative than that of PtCl42–/Pt. But when adding KI, the Cl– ligand of PtCl42– and PdCl42– can be readily replaced by I– through a ligand exchange reaction, causing the reduction potential of 20 ACS Paragon Plus Environment

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Pt2+/Pt to be negative than that of Pd2+/Pd.13, 45 Thus the existence of KI resulted in the facile reduction of Pd precursors relative to Pt precursors and induced the formation of core-shell structure.46-47 Furthermore, KI could selectively adsorb on and fully protect the (100) facets of alloys, thus improving the formation of cubic PtPd NPs. 9, 31 This synthetic strategy is also appropriate for preparing PtCu NHCs and trimetallic PtPdCu NCs with using KI. Similarly, I- ions also facilitate the production of Cu(Ⅰ, Ⅱ)-iodide complexes, and selectively adsorb on the (100) facets as well, thus maybe leading to the formation of PtCu NHCs and trimetallic PtPdCu NCs. Further study of the PtCu NHCs formation would be discussed in our future research.

The as-prepared PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO and PtPdCu NCs/RGO used as catalysts were investigated in electrocatalytic reaction, as shown in Figure 6. The CV curves of the as-prepared catalysts and commercial Pt/C were performed in deoxygenated 0.5 M H2SO4 solution, as shown in Figure 6a. The electrochemical active surface area (ECSA) can be obtained from integrating the electric charge of hydrogen adsorption/desorption after double-layer correction.20, 48 The measured ECSAs are listed in Table S1. The lower ECSA for the as-prepared catalysts than commercial Pt/C is most likely due to the large nanoparticle size or the surface occupied by Pd or Cu species which has little contribution to the ECSA.12 But among the as-prepared electrocatalysts, PtPd NCs/RGO (35.3 m2g-1) shows the largest ECSA, maybe due to the smallest size and the core-shell structure.



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Figure 6. CV curves of the as-prepared PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO, PtPdCu NCs/RGO and commercial Pt/C catalysts measured in (a) N2-saturated 0.5 M H2SO4 and (b) 0.5 M CH3OH/0.5 M H2SO4 solution with a scanning rate of 50 mV s-1. (c) Mass activity and specific activity for the as-prepared catalysts and commercial Pt/C catalyst. (d) CA curves of the as-prepared catalysts and commercial Pt/C catalyst at 0.87 V in a 0.5 M CH3OH/0.5 M H2SO4 solution.

The catalytic performance of these catalysts and commercial Pt/C toward methanol electrooxidation was carried out in a 0.5 M H2SO4 solution containing 0.5 M methanol, as shown in Figure 6b. All current density values have been normalized to the ECSAs of Pt. For all the catalysts, the peak at about 0.88 V (vs. RHE) during the forward scan is associated with the electrooxidation of methanol, forming CO2 and Pt adsorbed intermediates (mainly as CO). And the anodic peak between 0.62 and 0.77 V (vs. RHE) during the backward scan is attributed to the reactivation of 22 ACS Paragon Plus Environment

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oxidized Pt with the additional oxidation of the adsorbed carbonaceous species to CO2.42, 49 As shown in Table S1, the specific activity (also show in Figure 6c) of all the as-prepared catalysts is much higher than that of the commercial Pt/C catalyst. The possible reason for the enhanced specific activity could be the clean surface of the products obtained in molten salt and the synergetic effects between Pt and second or third metal. Noticeably, the as-prepared PtPd NCs/RGO shows the peak current density about 3.3 times larger than commercial Pt/C catalyst, which is also higher than the as-prepared PtPd NSs/RGO (about 2.6 times larger than commercial Pt/C catalyst), maybe due to the (100)-facet-enclosed PtPd alloy has the higher catalytic activity for methanol oxidation than the (111)-facet-enclosed PtPd alloy.50-51 Normalized by the loading mass of Pt, as show in Figure 6c and Table S1, the mass activity of the PtPd NSs/RGO, PtPd NCs/RGO, PtCu NHCs/RGO and PtPdCu NCs/RGO is 189.4, 404.5, 65.0 and 195.9 mA mgPt−1, respectively. Among the as-prepared catalysts, PtPd NCs/RGO also shows a highest mass activity of nearly 2.1 times larger than commercial Pt/C catalyst (194.7mA mgPt−1). The reason for the enhanced mass activities of the PtPd NCs/RGO could be attributed to the core-shell structure, the smallest size and the synergetic effects between Pt and Pd. Furthermore, comparing with recently reported PtPd alloys,9, 31, 52-56 the PtPd NCs/RGO also shows the comparable electrocatalytic performance for methanol electrooxidation, as shown in Table S2.

In addition, the ratio of forward to backward current (If/Ib) is often used to measure the oxidation ability for the intermediate species (CO) accumulated on 23 ACS Paragon Plus Environment


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electrode surface.49, 57 A higher If/Ib ratio represents a relatively complete oxidation of methanol with producing less poisoning species. The If/Ib ratio results are also listed in Table S1. Among the catalysts, the as-prepared PtCu NHCs/RGO exhibits the highest If/Ib ratio, indicating that the incorporation of Cu into catalysts improves the ability for tolerance toward CO poisoning. All the as-prepared catalysts exhibit higher If/Ib ratio than commercial Pt/C catalyst, suggesting the ability for much more tolerance toward CO poisoning during electrocatalysis, which due to the presence of the second or third metal changes the electronic properties of Pt for CO adsorption.12 The catalytic stability of the as-prepared catalysts and commercial Pt/C were evaluated by chronoamperometric (CA) measurements performed at 0.87 V (vs. RHE) for 1000 s, as shown in Figure 6d. It has been observed that all the catalysts experienced the degradation of oxidation current density, due to accumulation of poisoning intermediates (CO) on the surface of Pt during the methanol oxidation reaction process. It should be noted that the as-prepared core-shell PtPd NCs/RGO catalyst maintained the highest current for the entire time course, which indicating that the PtPd NCs/RGO catalyst had a better catalytic stability for methanol electro-oxidation. Furthermore, the PtPd NCs/RGO catalyst shows no morphology or size change after the MOR stability tests, as shown in Figure S7a and b. But the commercial Pt/C catalyst (Figure S7c and d) shows distinct aggregation and increased size. The better catalytic stability of the PtPd NCs/RGO catalyst may be due to the introduction of Pd (improved the removal of CO adsorption)

47

and the core-shell

structure (labile Pd core protected by the compact Pt shell) 58. 24 ACS Paragon Plus Environment

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CONCLUSIONS

In summary, a novel and eco-friendly synthetic strategy has been developed for the shape-controlled synthesis of Pt-based alloy NPs in inorganic molten salt without using any organics. GO have the capability of stabilizing the metal ions or NPs, and act like surfactants or capping agents to restrain particles aggregation in molten salt. With the assistance of GO, sheet-like PtPd alloys had been formed successfully in inorganic molten salt. Moreover, by introducing KI as the shape-inducing agent to selectively adsorb on and fully protect the (100) facets of alloys, cubic PtPd, PtCu and PtPdCu alloys have been prepared successfully in molten salt as well. Especially, the existence of KI also resulted in the facile reduction of Pd precursors relative to Pt precursors, leading to form the core-shell structure in the synthesis of cubic PtPd alloys in molten salt. This novel molten salt synthetic strategy has the capability to prepare different kinds of Pt-based alloys with controlled shapes. Therefore, this work provides an alternative strategy for the synthesis of some shaped metallic NPs through a totally inorganic method.

ASSOCIATED CONTENT

Supporting Information Available: Figure S1-7. Table S1-2. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding size distribution histograms, TEM images, EDS spectra, SEM image, FTIR spectrum, Tables of the catalytic activity.



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AUTHOR INFORMATION

Corresponding Author *Address correspondence to [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We thank Liqun Wang from the School of Science, Xi’an Jiaotong University, for her support of TEM and SEM characterizations. This work is supported by National Natural Science Foundation of China (No. 51271135), the Fundamental Research Funds for the Central Universities, and the Natural Science Foundation of Shanxi Province (No. 2015JM5166). REFERENCES 1.

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For Table of Contents Use Only

Title: Organics- and Surfactant-free, Molten Salt Medium Controlled Synthesis of Pt-M (M = Cu, Pd) Bi- and Tri-metallic Nanocubes and Nanosheets

Authors: PengtaoQiu, †Jinglei Bi, †Xiaojing Zhang, † and Shengchun Yang*,†, ‡

Synopsis: Organics-free synthesis of shaped Pt-based alloys in molten salt with using graphene oxide as stabilizer was presented.


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