Strain Engineering to Enhance the Electrooxidation Performance of

Feb 12, 2018 - For MOR, this tensile-strained AL-Pt electrocatalyst showed obviously higher specific activity (7.195 mA cm–2) and mass activity (1.0...
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Strain engineering to enhance the electrooxidation performance of atomic-layer Pt on intermetallic Pt3Ga Quanchen Feng, Shu Zhao, Dongsheng He, Shubo Tian, Lin Gu, Xiaodong Wen, Chen Chen, Qing Peng, Dingsheng Wang, and Yadong Li J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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Strain engineering to enhance the electrooxidation performance of atomic-layer Pt on intermetallic Pt3Ga Quanchen Feng,†, ∆ Shu Zhao,‡, ∆ Dongsheng He,§ Shubo Tian,† Lin Gu,ǁ Xiaodong Wen,┴ Chen Chen,† Qing Peng,† Dingsheng Wang,*,† Yadong Li† †

Department of Chemistry, Tsinghua University, Beijing 100084, China. Beijing Guyue New Materials Research Institute, Beijing University of Technology, Beijing 100124, China. § Materials Characterization and Preparation Center, South University of Science and Technology of China, Shenzhen 518055, China. ǁ Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. ┴ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China. ‡

Supporting Information Placeholder ABSTRACT: Strain engineering has been a powerful strategy to finely tune the catalytic property of materials. We report a tensilestrained 2–3-atomic-layer Pt on intermetallic Pt3Ga (AL-Pt/Pt3Ga) as an active electrocatalysts for methanol oxidation reaction (MOR). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy characterization showed that the AL-Pt possessed a 3.2% tensile strain along the [001] direction while had a negligible strain along the [100]/[010] direction. For MOR, this tensile strained AL-Pt electrocatalyst showed obviously higher specific activity (7.195 mA cm−2) and mass activity (1.094 mA/µgPt) than those of its unstrained counterpart and commercial Pt/C catalysts. Density functional theory calculations demonstrated that the tensile-strained surface was more energetically favorable for MOR than the unstrained one and the stronger binding of OH* on stretched AL-Pt enabled the easier removal of CO*.

To reduce our dependence on non-renewable and environmentally unfriendly fossil fuels, fuel cell is a potential candidate, which could supply us with sustainable and clean energy conversion.1–7 Crucial to achieve this future goal is the development of electrocatalysts with both improved activity and stability.8–11 To date, Pt-based nanomaterials have been recognized as the best electrocatalysts for both anodic (oxidation of methanol, ethanol and hydrogen gas) and cathodic (reduction of oxygen gas) reactions.12–15 Nowadays, how to further improve the electrocatalytic performance of Pt-based nanomaterials has been studied intensively. Strain engineering on catalyst surface has been used to precisely control and optimize their electrocatalytic activity.16–23 The resultant surface strain alters the adsorption/desorption properties of the relevant reaction species, and thus enhancing the electrocatalytic performance.16,21 Although some improvements in electrocatalytic performance have been achieved in recent years, they are commonly considered to be contributed from the compressive strain effect.23–28 Because tensile strain effect has been reported to

be detrimental to activity enhancement, it is believed to be undesirable and has been previously ignored.26–30 Here, we demonstrate that a tensile-strained 2–3-atomic-layer Pt on intermetallic Pt3Ga (AL-Pt/Pt3Ga), can be prepared by a simple method. Due to the lattice mismatch between the inner Pt3Ga and outer AL-Pt, surface strain is generated. As a result, the AL-Pt has a 3.2% tensile strain along the [001] direction and a negligible strain along the [100]/[010] direction. Interestingly, this tensile-strained AL-Pt/Pt3Ga catalyst exhibited obviously higher specific activity and mass activity than commercial Pt/C and unstrained Pt nanocrystals (NCs) catalysts. First, we synthesized Pt3Ga intermetallic nanocrystals (IMNCs) by co-reducing platinum (II) acetylacetonate [Pt(acac)2] and gallium (III) chloride (GaCl3) in 1-octadecene (ODE)/oleylamine (OAm) mixed solvent at 300 °C for 30 min. Then we supported the product onto Vulcan XC-72 conductive carbon black. After washing with acetic acid at 70 °C for 12 h, we obtained the ALPt/Pt3Ga catalyst (Figure S1). During this acid-washing process, not only could amorphous GaOx and surfactant (OAm) be easily removed, but also could the Ga atoms on the first few subsurface be etched away. This allows the diffusion and rearrangement of Pt atoms to take place, and therefore renders the formation of outer AL-Pt on inner Pt3Ga. High-resolution transmission electron microscopy (HRTEM) image revealed the single-crystallinity of individual Pt3Ga nanoparticle (Figure S2). Figure S3 showed that the Pt/Ga composition, measured by energy-dispersive X-ray spectroscopy (EDX), was 76.7/23.3 (3.3/1), which was consistent with the inductively coupled plasma atomic emission spectroscopy (ICP-AES) result (3.6/1). This higher Pt/Ga composition than theoretical value (3/1) may originate from the formation of AL-Pt on Pt3Ga. Figure 1a shows typical X-ray diffraction (XRD) patterns of supported AL-Pt/Pt3Ga and Pt NCs. The broad peaks in both samples at around 25° are assigned to the carbon support. The five diffraction peaks corresponding to (111), (200), (220), (311) and (222) planes of face-centered cubic (fcc) structure are evident in both samples. Besides, AL-Pt/Pt3Ga has six additional reflections of 100, 110, 210, 211, 300, and 310. All peaks of AL-Pt/Pt3Ga match well with the Pt3Ga phase [space group Pm-3m, Joint

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Committee on Powder Diffraction Standards (JCPDS) File Card No. 65-8000, a = 3.892 Å].31 Compared with Pt NCs, peak positions of AL-Pt/Pt3Ga are right-shifted to higher degrees (Figure 1b and Figure S4), indicating a contraction of the unit cell of pure metallic Pt originating from the relatively smaller atomic radius of Ga (122 pm) than Pt (139 pm). However, due to its few layer nature and relatively low content, AL-Pt is unable to generate observable diffraction peaks and could not be identified in XRD characterization.

Figure 1. (a) XRD diffraction patterns of supported AL-Pt/Pt3Ga and Pt NCs samples. (b) Enlarged region of the (220) diffraction peaks of both samples. To confirm the presence of AL-Pt on Pt3Ga, we employed atomic-resolution high-angle annular dark-field scanning TEM (HAADF-STEM) to determine the atomic structure of ALPt/Pt3Ga. In Figure 2a, an individual 10-nm AL-Pt/Pt3Ga is viewed along the [010] zone axis. In HAADF-STEM images, Ga and Pt atoms are represented in two columns with different contrast. Along this [010] zone axis, the inner Pt3Ga phase can be viewed as a Pt-Ga mixed (200) layer next to a pure Pt (200) layer. The corresponding fast Fourier transform (FFT) pattern shows the presence of both (100) and (110) superlattice spots, characteristic of its intermetallic structure (Figure 2b). In the unit cell, Ga atoms are at the eight vertices of the cube while Pt atoms are at the centers of six facets (Figure 2c). Figure 2, d to f, display the enlarged images from the selected areas marked by the yellow rectangles in Figure 2a. Image simulation and atomic model are overlapped on the experimental images. The Ga atoms near the surface are completely replaced by Pt atoms, giving birth to a 2–3-atomic-layer pure Pt (Figure S5). As shown in Figure 2, g and h, the integrated pixel intensity profiles are taken from the inner Pt3Ga (marked by yellow arrows in Figure 2a) and outer AL-Pt (marked by yellow rectangle in Figure 2d), respectively. The averaged (100) spacing of inner Pt3Ga is calculated to be 3.83 Å, consistent with the theoretical value of Pt3Ga (3.89 Å). The averaged (100) and (001) spacings of outer AL-Pt are measured to be 3.86 Å and 3.98 Å, respectively. These results reveal that the outer AL-Pt is in a 3.2% tensile strain along the [001] direction and a negligible strain along the [100]/[010] direction (Table S1).

Figure 2. (a) HAADF-STEM image of an individual 10-nm ALPt/Pt3Ga viewed along the [010] zone axis. (b) FFT pattern of the AL-Pt/Pt3Ga shown in (a). (c) Unit cell of the intermetallic Pt3Ga phase. Yellow and blue spheres represent Pt and Ga atoms, respectively. (d to f) Enlarged high-resolution HAADF images taken from the selected areas indicated by yellow rectangles in (a). Simulated HAADF images and atomic models are overlapped on the experimental images. (g and h) The line intensity profiles taken along the atomic layers marked by the yellow arrows in (a) and yellow rectangle in (d), respectively. Scale bar: (a) 2 nm, (d to f) 0.5 nm.

Figure 3. (a) HAADF-STEM image of one single Pt NC viewed along the [010] zone axis. (b) FFT pattern of the Pt NC shown in (a). (c) Unit cell of the monometallic Pt. Yellow spheres represent Pt atoms. (d) An enlarged high-resolution HAADF image taken

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Journal of the American Chemical Society from the selected area indicated by yellow rectangle in (a). Simulated HAADF image and atomic model are overlapped on the experimental image. (e and f) The line intensity profiles taken along the atomic layers marked by the yellow arrows in (a) and yellow rectangle in (d), respectively. Scale bar: (a) 2 nm, (d) 0.5 nm. To compare with the tensile-strained AL-Pt/Pt3Ga structure, we prepared the unstrained Pt NCs as a reference sample (Figure S6). Figure 3a shows a single Pt NC viewed along the [010] zone axis. All columns of Pt have equal intensity. FFT pattern reveals the absence of the (001) super period (Figure 3b). In the unit cell of Pt, the Pt atoms occupy both the eight vertices of the cube and the centers of six facets (Figure 3c). Figure 3d exhibits the enlarged image from the selected area marked by the yellow rectangle in Figure 3a. Image simulation and atomic model are overlapped on the experimental image, confirming the same atomic structure of the outer region with that of the inner region. Figure 3e and f are the intensity profiles taken from the inner region (marked by yellow arrows in Figure 3a) and outer region (marked by yellow rectangle in Figure 3d), respectively. The averaged Pt (100) spacing is determined to be 3.86 Å, consistent with the lattice constant of Pt (3.92 Å). The averaged Pt (100) and (001) spacings in the outer region are measured to be 3.86 Å and 3.87 Å, respectively. These three values are almost identical, indicating the unstrained nature of Pt NC.

mercial Pt/C. Chronoamperometry (CA) curves (Figure 4c) were obtained by keeping the potential at 0.65 V vs Ag/AgCl. The current densities of Pt NCs/C and commercial Pt/C decayed rapidly while that of AL-Pt/Pt3Ga remained the largest. In Figure 4d, ALPt/Pt3Ga showed the highest specific activity (7.195 mA cm−2), which was 2.5- and 8.5-fold higher than that of Pt NCs (2.925 mA cm−2) and commercial Pt/C (0.851 mA cm−2), respectively. Besides, in Figure S9, the mass activity of AL-Pt/Pt3Ga (1.094 mA/µgPt) was also the largest among the three catalysts, which was 3.7- and 1.9-fold higher than that of Pt NCs (0.299 mA/µgPt) and commercial Pt/C (0.584 mA/µgPt), respectively. The stability of AL-Pt/Pt3Ga was also studied by potential cycling. The CV curves before and after 1000 potential cycles (Figure S10) exhibited that AL-Pt/Pt3Ga catalyst had negligible loss in peak current density. Atomic-resolution ADF-STEM image of AL-Pt/Pt3Ga (Figure S11) after potential cycles showed that the AL-Pt/Pt3Ga structure was preserved, demonstrating its excellent stability. This result can be attributed to the structural stability of the inner Pt3Ga intermetallic phase.10,21

Figure 5. Calculated reaction pathways of MOR on unstrained Pt (100) and stretched Pt (100) surfaces at pH = 0.25, U = 0.88 V with respect to the RHE. All the elementary steps involve the dissociation of a (H+ + e-) pair. The insets are the optimized structures of intermediates (Pt: yellow, C: black, O: red, H: white). Figure 4. CV curves of the three catalysts in (a) 0.5 M H2SO4 solution and (b) 0.5 M H2SO4 + 1 M methanol solution at a sweep rate of 50 mV/s. (c) CA curves of the three catalysts at 0.65 V vs Ag/AgCl. (d) Specific activity and mass activity of the three catalysts at 0.668 V vs Ag/AgCl. Then we studied the electrochemical properties of the ALPt/Pt3Ga as well as the Pt NCs and commercial Pt/C catalysts (Figure S7) for methanol oxidation reaction (MOR). Cyclic voltammograms (CVs) of these three catalysts were recorded in N2saturated 0.5 M H2SO4 solution at a sweep rate of 50 mV/s (Figure 4a). Then the methanol polarization curves were measured in N2-saturated 0.5 M H2SO4 and 1 M methanol solution at a sweep rate of 50 mV/s (Figure S8). The specific activity curves were obtained by normalizing the currents with the surface area (Figure 4b). The AL-Pt/Pt3Ga catalyst exhibited the highest current density among the three samples, indicating the high activity of tensilestrained AL-Pt. The current density ratio of the forward scan (Jf) to backward scan (Jb) is often used to evaluate the anti-poisoning ability to carbonaceous species.1,29,32 The Jf/Jb ratio of ALPt/Pt3Ga (1.04) was higher than that of commercial Pt/C (0.79), demonstrating its better resistance against poisoning than com-

We carried out density functional theory (DFT) calculations to investigate the reaction mechanisms of MOR on both tensilestrained and unstrained Pt (100) surfaces. Figure S12 shows the reaction network. We calculated the reaction free energy using the computational hydrogen electrode (CHE) method.33,34 On both Pt (100) surfaces, the dehydrogenation of methanol is much easier to break C-H bond compared to O-H bond (Table S2 and S3). MOR on Pt (100) surface undergoes an indirect pathway, i.e. CH3OH → CH2OH*→ CHOH*→ COH*→ CO*, followed by CO interaction with surface hydroxyl (OH*) to form CO2, agreeing with previous study.35 Figure S13 displays the optimized structures. All intermediates have the same structures on both surfaces. Figure 5 shows that the stretched surface is more energetically favorable than the unstrained one. All electron-transfer elementary steps are exothermic, indicating the high MOR reactivity on the stretched catalyst. This results from the tensile strain that makes all the intermediates bind more strongly, especially CO* + OH*. The stronger binding of OH* indicates that on stretched Pt (100), water is more easily activated, allowing for the removal of CO*. Moreover, the higher reactivity for the stretched catalyst fits with the upshift of the d-band center of the stretched surface (-2.54 eV) as compared to the unstrained surface (-2.77 eV) and, therefore, is of electronic origin.35,36

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In conclusion, we report a tensile-strained 2–3-atomic-layer Pt on intermetallic Pt3Ga as an active electrocatalyst for MOR. This tensile-strained AL-Pt catalyst showed obviously better specific activity and mass activity than its unstained counterpart and commercial Pt/C catalysts and excellent durability in MOR. This work highlights the beneficial effect of tensile strain on electrochemical performance of Pt-based electrocatalysts.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental procedures, characterization methods, extended figures and tables (PDF)

AUTHOR INFORMATION Corresponding Authors *[email protected]

Author Contributions ∆

Q.F. and S.Z. contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by China Ministry of Science and Technology under Contract of 2016YFA (0202801), and the National Natural Science Foundation of China (21521091, 21390393, U1463202, 21471089, 21671117).

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