Solid Synthesis of Ultrathin Palladium and Its Alloy Nanosheets on

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Solid Synthesis of Ultrathin Palladium and Its Alloy Nanosheets on RGO with High Catalytic Activity for Oxygen Reduction Reaction Chunyong He, Juzhou Tao, and Pei Kang Shen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03190 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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ACS Catalysis

Solid Synthesis of Ultrathin Palladium and Its Alloy Nanosheets on RGO with High Catalytic Activity for Oxygen Reduction Reaction Chunyong He1, 2*, Juzhou Tao1, 2* Pei Kang Shen3* 1

2

Dongguan Neutron Science Center, Dongguan 523803, China Institute of High Energy Physics, Chinese Academy of Sciences (CAS)，Beijing 100049,

China 3

Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University,

Nanning, Guangxi, 530004, PR China * e-mail: [email protected]; [email protected]；[email protected] ABSTRACT The special surface structure and distinctive quantum confinement of electrons provide two-dimensional (2D) materials with extraordinary properties. Ultrathin 2D noble metal nanosheets consisting of single or few atomic layers in particular, demonstrate high catalytic activities. Here, we demonstrate solid-phase synthesized ultrathin palladium (Pd) and its alloy nanosheets (PdFe, PdCo and PdNi) on RGO using carbon monoxide as reducing and surfaceconfining agent. No templates and surfactants were needed in the process, making it one of the most promising routes towards preparing such nanomaterials in large quantity and at low price.

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The ultrathin Pd and its alloy nanosheets on RGO hybrids show high catalytic activities and durability.

KEYWORDS: two-dimensional • palladium • reduced graphene oxide • solid synthesis • oxygen reduction reaction

1. INTRODUCTION Since the identification of graphene in 2004,1 single- or few-layer 2D nanomaterials become intriguingly interesting because of their remarkable properties including high mechanical integrity, ultrahigh specific surface area, excellent electronic properties, unique electrochemical properties and so on, revealing great potentials in such a wide range of applications as field effect transistors,1-3 optoelectronic devices,4 topological insulators,5 energy storage and conversion,6-9 catalysis,10-15 sensing16,

17

and biomedicine18. Currently two kinds of 2D

nanomaterials are being intensively developed, one is the graphene-like layered compounds such as transition metal dichalcogenides (MoS2,16 WS2,18 MoSe219 and WSe219), layered transition metal oxides (Ti0.87O2,20 TaO3,21 and ZnO22), MXenes,7 g-C3N4,10 boron nitride23 and layered double hydroxides24; the other is the non-layered structured 2D nanomaterials, including noble metals (Au,25 Pd12 and Rh26), base metal (Fe27), non-layered structured transition metal oxides (WO3,28 CeO2,29 and SnO230) and non-layered structured metal chalcogenides (CuS,31 SnSe32 and CdSe33). The layered compounds, stacked by Van der Waals interaction of interlayer bonding, can be converted into ultrathin 2D nanomaterials by top-down fabrication using mechanical


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exfoliation1 or liquid exfoliation34, 35. The non-layered materials so far are only prepared through bottom-up route. The unique properties and versatility of palladium (Pd) has contributed to its applications in many areas such as catalysis,36 electronics,37 hydrogen storage,38 sensors,39 and jewelry. By tuning their electronic state, atomic arrangement, exposed catalytic surface and coordination environment through tailoring of the underlying morphology and structure, properties of the Pdbased heterogeneous catalysts can be optimized and customized40-42 A prominent examples is the shape controlled synthesis of Pd metal nanocrystals via wet-chemical method, which involves organic compounds with different functional groups, such as surfactants, biomaterials, polymers and fatty ligands as capping and dispersing agents, their interactions with Pd surfaces both mediate the growth orientation and stabilize the nanocrystals against agglomeration. These organic agents however are difficult to remove from the catalyst surface, leading to inhibited activities of the as-prepared nanomaterials12, 43 Some small molecules or ions (e.g., CO, amines, Fe3+, Br−) have been reported as surface confining agents to synthesize well-defined Pd nanoarchitectures with specific explored facets, which can not achieved by other synthetic strategies.43 In this method, small molecules or ions were preferentially and selectively adsorbed onto a specific facet of the Pd, thus controlling the facet growth orientation and playing the role of a surface confining agent. Palladium and its alloy nanosheets less than 10 atomic layer thick have been successfully synthesized in this way when CO was used as surface confining agent, since the CO molecules would adsorbed on basal (111) facet of palladium nanosheets strongly, the growth along the [111] direction would be prohibited, which would result in sheet-like structure.12, 44 However, this prepare method of palladium nanosheets is still based on a wetchemical process using CTAB, PVP or other polymers as the capping and dispersing agents. The


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complete removal of the capping and dispersing agents would give rise to the reconstruction of the as-prepared Pd nanosheets surface and lead to the sheet restacking back to three-dimensional configuration, which is deleterious to the study of catalytic activity.45, 46 So far, the CO-confined growth of Pd nanosheets has been restricted to wet-chemical reactions, because without the assistance of organic capping agents in liquid phase, the CO can not play the surface confining role solely. Compared with wet-chemical synthesis, the solid synthesis of Pd nanosheets has two potentially very attractive advantages: one is no need of organic capping agent resulting in clean Pd nanosheet surface, the other is that materials can be produced in larger quantity and at lower cost. On that account, we report here a new solid synthesis method to prepare ultrathin Pd and its alloy nanosheets on reduced graphene oxide (RGO) support, which is both template- and organic-free. A novel sandwich-like confined growth mechanism is proposed to describe the confined growth of Pd nanosheets on RGO based on the strong adsorption of CO on Pd (111) as well as the strong interaction between Pd (111) and RGO surface.

2. RESULTS AND DISCUSSION Material characterization. Transmission electron microscopy (TEM) was used to investigate the morphology and structure of the as-synthesized Pd nanosheets on RGO (Pd NSs/RGO) (Fig. 1A and Supplementary Information Fig. S1), which show that ultrathin Pd nanosheets lay on RGO substrate flatly and homogeneously with the average size of ～20 nm, except for a small fraction overlapping with each other. The XRD pattern of Pd NSs/RGO reveals the face-centered cubic (fcc) structure of the Pd nanosheets (Fig. S2). The high-resolution TEM (HRTEM) of the Pd nanosheet shows jagged edge, which is conducive to the catalytic activities of the catalysts


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(Fig. 1B).42 Lattice fringes with interplanar spacing of 0.228 nm and 0.138 nm correspond to the Pd (111) and Pd (220) facets, respectively (Fig. 1C). The corresponding fast Fourier transformation (FFT) pattern (the inset in Fig. 1C) suggests that the Pd nanosheets have the dominant (111) basal plane, and the helical axis is along the [-2 2 0] direction. TEM and HRTEM analyses of more individual Pd nanosheets are presented in Fig. S3. A portion of the Pd nanosheets displays stacking faults parallel to the basal (111) planes (Supplementary Fig. S4), which probably associated with the Pd nanosheets.12 The high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images (Fig. 1D and 1E) show uniform contrast of the Pd nanosheets except for those overlapping areas, which reveal sheet-like morphology of the Pd nanocrystals. High-resolution HAADF-STEM image of the Pd nanosheet also demonstrates its fcc structure (Fig. 1F). Since the high-resolution HAADF-STEM signal originates from electron scattering by atomic nuclei and is proportional to the specimen thickness and the atomic mass, its intense profiles can be used to reconstruct the real three-dimensional structure of nanomaterials.47, 48 In our case, the high-resolution HAADF-STEM intensity profiles are parallel to the background (Fig. S5), indicating the sheet-like structure of Pd nanosheet. The energy-dispersive x-ray spectroscopy (EDS) pattern of Pd NSs/RGO indicates co-existence of C, O and Pd (Fig. S6). The atomic force mircoscope (AFM) analyses show that the ultrathin Pd nanosheets have a typical overall thickness ~1.2 nm (Fig. 1G and 1H), suggesting they are about six atomic layers and consistent with the HAADF-STEM intensity profiles above.


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Figure 1. (A) representative bright field HEM image of Pd NSs/RGO. (B) HRTEM image of the Pd nanosheet edge, red dashed line highlighting the rough edge. (C) HRTEM image of Pd NSs/RGO, the inset showing the corresponding fast Fourier transformation (FFT) pattern; the HRTEM analyses and the corresponding FFT pattern indicating that the Pd nanosheets have the dominant (111) basal planes, and the helical axis along the [-2 2 0] direction. (D & E) HAADFSTEM images of the Pd NSs/RGO. (F) High-resolution HAADF-STEM image of the Pd NSs/RGO, the inset its corresponding FFT pattern. (G) AFM image of the Pd NSs/RGO. (H) The corresponding AFM height profile along the black line in (G), thickness of the Pd nanosheets is estimated 1.2 ± 0.2 nm.


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Growth mechanism. The intrinsically isotropic growth of the noble metals poses a great challenge to their synthesis into 2D nanosheets.49 We investigated two key factors that are involved in the confined growth of Pd nanosheets on RGO substrate. First is the role played by adsorbed CO on Pd nanosheet surface in its growth. The fourier transform–infrared (FT-IR) spectra of the freshly prepared samples verified they are predominantly in a bridge configuration (Fig. S7). And the CO stripping voltammetry curve of the Pd NSs/RGO exhibits the electrooxidation of irreversibly adsorbed CO, as indicated by a dominant peak at 1.01 V (vs. reversible hydrogen electrode (RHE)) (Fig. S8), which corresponds to the CO stripping from Pd(111).50 Density functional theory (DFT) calculations were also performed to examine the interaction between CO and Pd surface by placing CO molecules at high-symmetry adsorption sites on Pd(100), Pd (110) and Pd(111), as illustrated in Fig. S9. The calculated average adsorption energy of CO on the Pd(111) (2.93 ev) is higher than that on the Pd (100) (2.67 ev) and the Pd (110) (2.18 ev), consequently the strongest adsorption of CO on Pt (111) prevents growth along the [111] direction and leads to the sheet-like structure. Clearly CO is critical to the formation of ultrathin Pd nanosheets, without which only ordinary granules appear on RGO (Fig. S10). Secondly, the 2D RGO substrate is also essential to the anisotropic growth of the Pd nanosheets. In the absence of RGO, there is no specific morphology of Pd nanocrystal obtained even when CO was applied as reducing agent in the solid synthesis of Pd nanocrystal on carbon black (Fig. S11) and carbon nanotubes (Fig. S12). This implies the 2D RGO functions not only as the support, but also an effective surface confining agent. On this account, the interactions between Pd (100) / (110) / (111) and RGO were also examined through DFT calculations, with the theoretical models of this Pd/RGO system shown in Fig. S13. The calculated average


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adsorption energies between Pd (100) / (110) / (111) and RGO are 1.17 J m-2, 0.45J m-2 and 2.54 J m-2, respectively, this large interaction between Pd and RGO layer can anchor the Pd nanosheets and stabilize them against agglomeration. The strongest interaction between Pd (111) and RGO is related to the finer lattice matching between the two, which also contributes to the better epitaxially coordinated placement of Pd atoms to the RGO. As a result, epitaxial growth of the Pd (111) on RGO surface is energetically disfavored, allowing CO the priority to be absorbed onto the Pd (111), This sandwich-like confinement mechanism prevents growth along the [111] direction, leading to the formation of single or few-layered ultrathin Pd nanosheets. Synthesis of PdM (M=Fe, Co and Ni) alloys nanosheets. An effective strategy to improve the catalytic activity of Pd is through fabrication of bimetallic Pd-based heterogeneous catalysts, as the synergetic effects introduced by the other metal open possibilities of tuning the electronic state, strain and Pd coordination environment.44 We also prepared bimetallic nanosheets of PdFe, PdCo and PdNi on RGO hybrids (PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO, henceforth PdM NSs/RGO) via a similar solid synthesis method (see synthesis details in METHODS section). The incorporation of Fe, Co and Ni atoms into the Pd fcc structure would result in the formation of ordered intermetallic phases with a concomitant lattice contraction, which was confirmed by the shift of the diffraction peaks to higher angle for the bimetallic nanosheets compared with that of the Pd NSs/RGO (Fig. S14). The incorporation of Fe, Co and Ni atoms in the ultrathin Pd nanosheets would also adjust the compressive strain of the Pd surface, resulting in a change in the Gibbs free-energy of the intermediate state and providing an effective approach to improve catalytic activities of the Pd.49, 51 The bimetallic Pd nanosheets on RGO have the similar average length of ～20 nm, and the same crystalline structure (Fig. S15), suggesting a possibly identical growth mechanism. The HAADF-STEM images of PdM


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ACS Catalysis

NSs/RGO display sheet-like structure of respective nanocrystals (Figs. 2A, 2C and 2E). Elemental mappings based on the EDS confirmed the bimetallic composition of PdFe, PdCo and PdNi, (Figs. 2B, 2D and 2F), in which Fe, Co Ni and Pd also exhibited a relatively homogeneous distribution. The chemical compositions of these bimetallic nanosheets/RGO hybrids were determined from the line-profile analysis (Fig. S16), the obtained atomic ratios of Pd to metal at 54:46, 53:47 and 55:45, respectively (Fig. S17), matching very well to separate results of the inductively coupled plasma mass spectrometry (ICP-MS) measurement, which gave the respective atomic ratios at 53:47, 51:49 and 54:46. The AFM height profiles show that the thickness of the PdFe, PdCo and PdNi bimetallic nanosheets are ~ 1.37 nm, ~ 0.96 nm and ~ 1.30 nm, respectively (Fig. S18), which are thinner than most of the reported Pt/Pd alloy nanosheets.52-54 The high-resolution HAADF-STEM intensity profiles of the bimetallic nanosheets are also parallel to the background (Fig. S19), indicating their sheet-like structures and consistent with the AFM analyses. Raman spectra of the samples were carried out to investigate the reduction reversion of the GO. The intensity ratio of G band (~1577 cm−1, sp2 carbon) to D band (~1350cm−1, disordered carbon) (IG/ID) can be used to characterize the graphitization degree of carbon material.55 The IG/ID of the GO, bare RGO, Pd NSs/RGO, PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO are 0.78, 1.47, 1.52, 1.61, 1.58 and 1.66, respectively (Fig. S20). Compared with GO, the IG/ID ratios of bare RGO, Pd NSs/RGO, PdM NSs/RGO increase considerably, indicating the reversion from GO to the pristine graphitic structure for RGO support. Such increased graphitization would result in higher graphitization degree of RGO, which benefits the durability of the catalyst.56


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ACS Catalysis

Pd3d2/3 Pd(0) Pd( )

Pd(0) Pd( )

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Figure 2. HAADF-STEM images of PdFe NSs/RGO (A), PdCo NSs/RGO (C) and PdNi NSs/RGO (E), their corresponding STEM-EDS elemental mapping images (B), (D) and (F), the respective high-resolution Pd 3d XPS spectra (G), (H) and (I).

XAFS of Pd and its alloys nanosheets. The structural and electronic properties of Pd NSs/RGO and PdM NSs/RGO were investigated by X-ray absorption fine structure (XAFS) (Fig. S21). The main absorption peaks in the Pd K-edge X-ray absorption near-edge structure (XANES) spectra of Pd/C, Pd NSs/RGO and PdM NSs/RGO have resembling features to Pd foil, indicating similar metallic character with few oxide species or other attached contaminants (Fig. 3A). Since all of the heterogeneous electrocatalytic reactions occur on the interface, in which the oxide layer or organic capping agent coating could block active sites of electrocatalysts, the


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absence of any organic capping and dispersing agents on such clean surfaces of Pd, PdFe, PdCo and PdNi nanosheets give us hopes for significant boost in electrochemical activity. The electronic properties and surface states of Pd NSs/RGO and PdM NSs/RGO were investigated by X-ray photoelectron spectroscopy (XPS), shown as in Fig. S22A. The fitting curves of the highresolution Pd 3d XPS spectra of Pd/C show a fraction of Pd(II) at higher binding energies of 336.55 eV and 341.85 eV, which likely originates from the oxidation of Pd nanoparticle surface when explored to air (Fig. S22B).57 Whereas the high-resolution Pd 3d XPS spectra of Pd NSs/RGO (Fig. S22C), PdFe NSs/RGO (Fig. 2G), PdCo NSs/RGO (Fig. 2H), and PdNi NSs/RGO (Fig. 2I) show the dominant metallic Pd(0). Compared with Pd/C, the Pd(II) peak in Pd NSs/RGO and PdM NSs/RGO are much smaller, indicating clean exposed Pd or bimetallic Pd surface. The binding energies of Pd(II) in PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO are 337.70 eV and 343.0 eV, which are different from those of Pd/C and Pd NSs/RGO. Clearly, parts of Pd atoms in the bimetallic nanosheets are in different chemical situations, which likely results from the incorporation of Fe, Co and Ni atoms into the Pd fcc lattice.58 The highresolution C 1s XPS spectra of Pd NSs/RGO and PdM NSs/RGO show the dominating peak at 284.6 eV (Figs. S22G, S22H, S22I and S22J), which is assigned to the graphite-like sp2 bonded carbon, indicating most of the carbon atoms in Pd NSs/RGO and PdM NSs/RGO are sp2 carbon.59


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Figure 3. (A) Pd K-edge XANES spectra of Pd foil, Pd/C, Pd NSs/RGO and PdM NSs/RGO. (B) Fourier transform magnitudes of the k3-weighted Pd K-edge EXAFS obtained from the Pd foil, Pd/C, Pd NSs/RGO and PdM NSs/RGO (solid lines), and respective first-shell fit data (dashed lines).

Extended X-ray absorption fine structure (EXAFS) provided detailed information on the intraparticle composition and degree of alloying of Pd and Fe/Co/Ni. The Fourier transformation data of the k-weighted EXAFS were fitted using the Artemis program in the IFEFFIT software package. Fig. 3B shows the radial distribution function over the range of 3.1–13.7 Å−1, and the corresponding first shell fit to the experimental data. Compared with the single peaks of Pd foil, Pd/C and Pd NSs/RGO, those of bimetallic nanosheets both shifted to smaller bonding length and broadened into overlapping peaks, suggesting formation of Pd-Fe, Pd-Co and Pd-Ni bonds. The fitted parameters of the Pd foil, Pd/C, Pd NSs/RGO and the bimetallic Pd nanosheets are summarized in Table S1. The Pd/C gives the coordination number (CN) of 6.9 ± 0.29 with the Pd–Pd length of 2.75 Å from the first shell analysis, which is smaller than that of the Pd foil (CN = 11.3± 0.4) because the nanoscale of the Pd nanocrystals would significantly increase the


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proportion of surface Pd sites. The CN of the Pd NSs/RGO is 9.6 ± 1.2 with the Pd–Pd distance (RPd–Pd) of 2.75 Å. The CNs of metallic Pd–Pd bonds in PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO are 4.1 ± 1.3, 4.2 ± 1.2 and 4.4 ± 1.3, respectively. We note that the respective RPd–Pd obtained from PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO are 2.74 Å, 2.70 Å and 2.70 Å, a litter shorter than those of Pd/C and Pd NSs/RGO due to the lattice contraction after the Fe/Co/Ni incorporation and in accord with XRD results. In addition, the CNs of the fitted Pd-Fe, Pd-Co and Pd-Ni nanosheets are 3.7 ± 1.4, 4.1 ± 1.3 and 4.1 ± 1.6 with the RPd–Fe, RPd–Co and RPd–Ni of 2.62 Å, 2.61 Å and 2.59 Å, respectively. The calculated average adsorption energies of CO on the PdM (100) / (110) / (111), and the calculated average adsorption energies between PdM (100) / (110) / (111) and RGO of PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO are displayed in Table S2, which show that the PdM (111) still the favorable surface for CO adsorption, and the interaction between PdM (111) and RGO is still the strongest. Overall, the growth of PdM NSs are also dominated by the sandwich-like confinement mechanism. ORR performance of the Pd and its alloys nanosheets. For the oxygen reduction reaction (ORR) in fuel cells (and metal-air batteries), platinum (Pt) is the highly efficient reference electrocatalyst. However, the broad deployment of Pt-based electrocatalysts in these applications suffers from the high cost and scarcity of Pt.54, 60, 61 Although remarkable progress has been made on developing high-performance Pt-based electrocatalysts with minimized Pt content to lower the cost, one notable example being Pt3Ni nanoframes with extraordinary mass activity of 5.7 A mg−1Pt toward the ORR,61 this approach still faces challenges with intermediate (such as COabs) poisoning and selectivity. Pd is one of the idea alternatives to replace Pt with its higher natural abundance, higher selectivity in the acid electrolyte, and being less expensive36, 57. The ORR activities of the Pd NSs/RGO and PdM NSs/RGO were evaluated in 0.1 M HClO4 solution, with


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the state-of-the-art commercial Pd/C and Pt/C electrocatalysts as reference. Fig. 4A shows the ORR polarization curves of the Pt/C, Pd NSs/RGO and PdM NSs/RGO catalysts preformed as thin films on a rotating ring-disk electrode (RRDE) in an O2-saturated 0.1 M HClO4 solution under 1,600 rpm. The half-wave potential (E1/2) of the Pd/C is about 0.830 V (vs. RHE), which is lower than Pt/C (0.889 V), suggesting the sluggish intrinsic catalytic activity of Pd. The E1/2 of Pd NSs/RGO is 0.876 V, which is higher than Pd/C, indicating that the specific 2D structure of the Pd nanosheets benefits the catalystic ORR. However, it is still lower than that of Pt/C. The E1/2 of PdFe NSs/RGO (E1/2=0.897 V), PdCo NSs/RGO (E1/2=0.908 V) and PdNi NSs/RGO (E1/2=0.918 V) are higher than those of Pt/C and Pd NSs/RGO, indicating that alloying with another metal would further improve the Pd ORR activity. The currents detected on the ring electrode (ir) are negligible on Pt/C, Pd NSs/RGO and PdM NSs/RGO whereas the ir on Pd/C is much bigger than the others. On the other hand, the high yield of the peroxide species (HO2-1) in the ORR mechanism may damage the catalyst and/or the membrane in the Membrane Electrode Assembly (MEA).60 The transferred electron number (n) per oxygen molecule involved in the ORR and the HO2-1 yield of the catalysts are shown in Fig. 4B. The Pt/C, Pd NSs/RGO and PdM NSs/RGO catalyze the ORR through a four-electron process over 0.9 V (vs. RHE), and the HO2-1 yields are negligible. On the Pd/C electrode, the HO2-1 yield is much higher and part of the O2 was reduced through a two-electron process. Fig. 4D compares the mass activities (im) of various catalysts at 0.9 V (vs. RHE). The im of Pd NSs/RGO is 73 mA mg-1Pd, which is much higher than Pd/C (27 mA mg-1Pd) and a little lower than Pt/C (96 mA mg-1Pt). And the im of PdNi NSs/RGO (318 mA mg-1Pd) is the highest among these catalysts, which is about 11.8 and 3.3 times of the Pd/C and Pt/C, respectively, suggesting a significent improvement in ORR activity.


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specific mass a activity@ ctivity@ at 0.9V at 0.9V

0.4

0.0

350

Pt/Pd

140

0.6

0.2

50


60

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PdNi NSs/RGO PdCo NSs/RGO PdFe NSs/RGO Pd NSs/RGO Pt/C Pd/C

Pt/Pd

1.0

-2

0.6

D

C j / mA cm

0.4


Pt/Pd

-4

Specific activities / mA cm -2

-2

3.8

H 2O 2 / %

j / mA cm

-2

0

Normalized im %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PdNi NSs/RGO PdCo NSs/RGO PdFe NSs/RGO Pd NSs/RGO Pt/C Pd/C

300 250 200 150 100 50 0

before

durabil it

after du rability y test@ test@at at 0.9V 0.9V

Figure 4. (A) RRDE polarization curves of Pd/C, Pt/C, Pd NSs/RGO, PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO catalysts in oxygen‐saturated 0.1 M HClO4 solution at 25 oC


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with rotation rate of 1600 rotations per min (rpm), the sweep rate is 5 mV s-1. (B) The electron transfer number (n) and the percentage of the H2O2 yield of the catalysts. (C) Cyclic voltammograms (CVs) of the catalysts in N2‐purged 0.1 M HClO4 solution with a sweep rate of 20 mV s-1. (D) Specific and mass activities of the catalysts at 0.9 V. (E) The normalized mass activity of the Pd/C, Pt/C, Pd NSs/RGO, PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO at the intial, 100th, 500th, 1,000th, 2,000th, and 5,000th CV cycle. (F) Mass activity of the catalysts before and after 5,000 CV cycles.

Since Pd does not only adsorbs but also absorbs hydrogen in the range between 0.05 to 0.2 V (vs. RHE), the electrochemically active surface area (ECSA) of Pd-based catalysts cannot be properly calculated from this region alone. Instead, the ECSA of Pd-based catalysts should be calculated from the regions for formation and reduction of Pd-O monolayer at the Pd surface (Fig. S23).56, 62 The ECSA of the Pd/C, Pt/C, Pd NSs/RGO, PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO are 46.2, 57.5, 47.7, 64.4, 69.6 and 70.2 m2 g-1 (Fig. 4C, Table S3), respectively. As shown in Fig.4D, the PdNi NSs/RGO exhibited the highest specific activity (jk, calculated by normalizing the electrode current to the ECSA), with a value of 0.61 mA cm−2 at 0.9 V (vs. RHE), which was 4.3 and 3.4 times greater than those of Pd/C (0.14 mA cm−2) and Pd NSs/RGO (0.18 mA cm−2). The electrochemical durability of the catalysts was evaluated by continuous potentiodynamic sweep between 0.6 and 1.2 V (vs. RHE) in 0.1 M HClO4 solution. The Pd NSs/RGO and PdM NSs/RGO show no shift in ORR polarization curves after 5,000 sweeping cycles (Fig. S24C, S24D, S24E and S24F). Fig. 4E shows the normalized im of the catalysts at the intial, 100th,


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500th, 1,000th, 2,000th, and 5,000th potential cycles. For the PdM NSs/RGO, the im increased after the first few cycles, which is attributed to surface roughening and contaminant removal from the sample surface.51 After 5,000 sweeping cycles only 6.8% loss of mass activity was found for the PdNi NSs/RGO, and less than 10% loss for the Pd NSs/RGO, PdFe NSs/RGO and PdCo NSs/RGO (Fig. 4F). Under the same condition, the commercial Pd/C and Pt/C showed a large negative shift in ORR polarization curves (Fig. S24A and S24B), and a significant loss of mass activity at 51.8% and 43.2%, respectively(Fig. 4F). The structures of the Pd NSs/RGO and PdM NSs/RGO before and after the durability tests were characterized by TEM, HRTEM and STEM-EDS elemental mapping (Figs. S25, S26, S27 and S28), which show only negligible changes of morphology and composition for the Pd NSs/RGO and PdM NSs/RGO. By contrast, the commercial Pd/C and Pt/C catalysts exhibited an obvious increase in nanoparticle size after 5,000 CV cycles, as the Pd/Pt particles coalesced and aggregated (Fig. S29 and S30) to much larger grain size, which is ascribed to Ostwald ripening63 and in accord with the observed decrease of im for the Pd/C and Pt/C catalysts. The high electrochemcial durability of the Pd NSs/RGO and PdM NSs/RGO mainly originates from their special structure, in which the welldefined 2D nanosheets can hinder the change of surface morphology, and the strong interaction between the nanosheets and RGO support attach one to the other, preventing coalescence of the former. Since the crossing of organic fuels such as methanol over the polymer membrane to the cathode severely degrades the cell performance,64 future breakthrough of the current polymer electrolyte membrane fuel cells depends on developing highly active and selective catalysts on both electrodes. The RRDE polarization curve of the PdNi NSs/RGO catalyst recorded at room temperature in O2‐saturated 0.1 M HClO4 + 1.0 M CH3OH solution, showed no obvious shift


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(Fig. 5B) in comparison with that in the O2 ‐ saturated 0.1 M HClO4 solution, indicating excellent selective catalytic property of the PdNi NSs/RGO catalysts. Fig. 5B compares the RRDE polarization curves of the Pt/C catalyst recorded in O2‐saturated 0.1 M HClO4 solution with and without 1.0 M CH3OH solution. A methanol oxidation peak clearly appeared, which offsets the ORR current and indicates the ORR was seriously inhibited.

60 40 20 0 10 8 6 4 2 0 -2 -4 -6

B

0.0

i / µA

10 5 0

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-2

0.1 M HClO4 0.1 M HClO4+ 1.0 M CH3OH

j / mA cm

-2

i / µA

A

j / mA cm

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Pt/C

-2

0.1 M HClO4+ 1.0 M CH3OH

PdNi NSs/RGO

-4

-6

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0.4

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0.8


1.0

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Figure 5. Rotating ring disk electrode (RRDE) polarization curves of Pt/C, (A) and PdNi NSs/RGO (B) catalysts in O2‐saturated 0.1 M HClO4 solution with and without 1.0 M CH3OH solution at a sweep rate of 5 mV s-1 and a rotation rate of 1600 rpm. 3. CONCLUSIONS In summary, we report a novel solid-phase method to synthesize ultrathin Pd nanosheets on RGO surface using carbon monoxide as the reducing and surface confining agent. The experimental results and theoretical calculations suggest a sandwich-like confined growth mechanism to the successful synthesis, which is based on the fortuitous combination of Pd (111) interacting strongly and preferably to both CO and RGO support. The Pd NSs/RGO, PdFe NSs/RGO, PdCo NSs/RGO and PdNi NSs/RGO exhibit outstanding ORR activity and extremely


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high electrochemical durability. The mass activity of PdNi NSs/RGO particularly is about 4.4 times and 2.2 times greater than those of the commercial state-of-art Pd/C and Pt/C catalysts. This solid synthesis method can be applied to prepare other Pd based alloy nanosheets on RGO, and it is also possible to be branched out into the exploration of unknown noble metal lowdimensional nano-architectures. 4. METHODS Synthesis of graphite oxide. A modified Hummers method was used to prepare graphite oxide. Typically, 2.0 g graphite powder (325 mesh, XFNANO Material Technologic Co.Ltd., Nanjing, China) was added to concentrated H2SO4 (46 mL) under stirring in an ice bath and the mixture was stirred for 15 minutes. Under vigorous agitation, KMnO4 (10.0 g) and NaNO3 (5.0 g) were added slowly to keep the temperature of the suspension lower than 20 oC. The product was then transferred into an oil bath at 40 oC and vigorously stirred for about 35 minutes. Afterwards, 50 ml of deionized water was added to the mixture slowly, and the temperature of the suspension was kept at 95 oC for 30 minutes. Deionized water (200 mL) and 30% H2O2 (10 ml), in turn, were added to the mixture. Finally, to remove residual metal ions, the mixture was filtered and washed by 1M HCl solution (250 mL) and plenty of deionized water. Synthesis of ultrathin palladium nanosheets on RGO surface (Pd NSs/RGO). 0.15 g GO was dissolved in 150 mL deionized water, followed by 2 h of ultrasonic agitation to form chemically exfoliated graphene oxide suspension. A Na2PdCl4 solution of 0.15 g sodium chloropalladite dissolved in 50 mL deionized water was added into the suspension under ultrasonic agitation. After 0.5 h of further ultrasonic agitation, the mixed suspension was placed inside a water bath, increased to 90 oC temperature, stirred continuously until most of the moisture has evaporated and it turned into hydrogel-like mixture, which was then freeze dried.


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The freeze-dried mixture was placed in the center region of a quartz tube inside a quartz tube furnace at ambient pressure. High purity argon was pumped into the furnace at a rate of 300 sccm for 30 min to expel oxygen in the furnace, the temperature then increased to 500 °C at a rate of 8 °C/min under 60 sccm Ar and 30 sccm CO, and maintained at 500 °C for 1 h. Finally, CO was turned off and sample cooled to room temperature under Ar. Synthesis of ultrathin palladium alloy nanosheets on RGO surface. The preparation steps are mostly identical to those for the Pd NSs/RGO. In addition, 0.215 g potassium ferrocyanide, or 0.21 g sodium cobaltinitrite, or 0.127 g nickel acetate, was dissolved in 50 mL deionized water and added into the suspension mixture of GO and sodium chloropalladite solution, and kept under ultrasonic agitation for additional 0.5 h. Structural characterization. The X-ray diffraction (XRD) patterns of the samples were carried out on a a Rigaku D/Max-III using Cu Kα radiation at the 2θ between 10° and 80°, the scan rate is 6° min−1. An XPS apparatus (ESCALAB 250, Thermo-VG Scientific Ltd.) was used to perform the X-ray photoelectron spectroscopy (XPS) of the samples. To fit the high-resolution XPS spectra of Pd 3d, Fe 2p, Co 2p and Ni 2p, a Shirley background was substracted, and asymmetrical functions were applied. Raman spectra were collected on a Raman spectrometer (Renishaw Corp., UK) using a He/Ne laser of 514.5 nm wavelength. A scanning probe microscope (SPM, Dimension Fastscan, Bruker) was applied to measure the atomic force microscopy (AFM) images of the samples at tapping mode. Transmission electron microscopy (TEM), high-angle annular dark-field scanning TEM (HAADF-STEM) and STEM-energydispersive x-ray spectroscopy (STEM-EDS) elemental mapping were performed on a field emission transmission electron microscope (FETEM, FEI Tecnai G2 F30) operating at 300 kV.


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The metal (Pd, Fe,Co and Ni) loadings of the catalysts were determined by the inductively coupled plasma atomic emission spectroscopy (iCAP 6500 Duo, ThermoFisher, ICP-AES). XAFS experiment and data analyses. The XAFS of the samples at Pd–K edge was carried out at beamline BL14W1 in Shanghai Synchrotron Radiation Facility (SSRF), China. The source of beamline BL14W1 is a 38-pole wiggler device with maximum magnetic field of 1.2 T and magnet period 80 mm. The XAFS measurements were performed at unfocused mode with a liquid nitrogen cooled double crystal monochromator (DCM) with Si (311), the data were collected under transmission-mode by using a 32-element high pure Ge solid detector. The photon flux at the sample position was 7.2 × 1011 photons per second. IFEFFIT software package was used to analyze the XAFS data. Firstly, the raw spectra were calibrated, averaged, pre-edge background subtracted and post-edge normalized using the Athena program in the IFEFFIT software package.65 Then, a radial distribution function was obtained from Fourier transformation of the k-weighted EXAFS (k·χ(k)) performed over a range of 3.5– 13.5 Å−1. Finally, the data were fitted using the Artemis program in the IFEFFIT. Density functional calculations. Cambridge Sequential Total Energy Package (CASTEP) was used to do density functional theory (DFT) calculations.

66

We used the Perdew-Burke-

Ernzerhof (PBE) as the default exchange-correlation functional, and generalized gradient approximations (GGA) was also used.

67-68

The cutoff energy for the plane-wave expansion was

set to 400 eV, which gives well-coveraged total energies and structures. The electronic wave functions at each k-point were expanded in terms of a plane-wave basis set. A single k-point at the center of the Brillouin zone was used. Ultrasoft pseudopotential (USP) was applied,69 and the k-points sampling were generated following the Monkhorst-Pack procedure with a 5×5×1


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mesh.70 The Pd (100), Pd (110) and Pd (111) surfaces were modeled by a 2×2 supercell with a five-layer slab and a vacuum region of more than 15 Å. The unit cell was first relaxed by geometry optimization until energy values change less than 2.0×10−6 eV/atom and average force on the atoms was less than 0.05 eV Å−1. The adsorption energy (Eads) was defined as: ∆Echem ＝ E(2x2)CO on Pd – EPd facet – ECO

(1)

in which E(2x2)CO on Pd is energy of the slab with adsorbates; EPd facet energy of the slab, and ECO energy of the CO. The calculated average adsorption energy between Pd (100) / (110) / (111) and RGO were also obtained using the same method. The distance between Pd (100) / (110) / (111) and RGO was set as 0.274 nm. The adsorption energy reflects the bonding strength of an interface and is defined as: ∆Echem ＝ ERGO on Pd – EPd – ERGO

(2)

in which ERGO on Pd is energy of the slab with adsorbates; EPd energy of the slab, and ERGO energy of the RGO. Electrochemical measurements. The ORR performances of the catalysts were performed by using rotating ring/disk electrode (RRDE) on a bipotentiostat (Pine Instrument Company, USA). To keep temperature at 25 oC, the electrochemical cell was put in water bath. The reference electrode is a reversible hydrogen electrode (RHE). Since the counter electrode of Pt may affect the ORR performance (especially the durability performance) of the Pd and its alloy nanosheets, a graphite foil (1.0×10 cm-2) was used as counter electrode. The working electrode is a RRDE


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(the glassy carbon disk (GCD) is 5.6 mm in diameter). Before electrochemical measurements, the samples were deposited on GCD. The prepare processes can be descripted as follows: firstly, 1.0 mL 0.05 wt% Nafion solution (DuPont, USA) was mixed with 1.0 mL absolute ethyl alcohol, and 5.0 mg catalyst was put in the mixture, ultrasonic treatment for an hour to form a welldispersed ink; then a certain amount of the ink was deposited on the GCE; finally, the electrode was dried under infrared lamp for 5 min to obtain a catalyst thin film. The noble metal (Pt/Pd) loading was keep at 0.008 mg cm-2 for all the catalysts. Since CO molecules were adsorbed on the fresh palladium and its alloy nanosheets, the samples were activated before any electrochemical active measurements through CVs for several cycles in the electrochemical window of 0.05 to 1.2 V (vs. RHE) until the adsorbed CO molecules were removed completely. The catalytic activities of the palladium and its alloy nanosheets/RGO hybrids for oxygen reduction reaction (ORR) were carried out in O2-saturated 0.1 M HClO4 solution via CV. The electrochemical window was kept at 0.05 to 1.1 V (vs. RHE) and the scan rate is 2 mV s−1. The electrochemical active surface area (ECSA) of the catalyst was evaluated by CVs in a nitrogen-saturated 0.1 M HClO4 solution. The ECSA of the Pt/C was calculated by using the adsorption/desorption region, where occur the adsorption of a hydrogen monolayer on the Pt surface (210 µC cm2).

60

For Pd-based catalysts, the columbic charge for the reduction of Pd-O

monolayer, formed on Pd catalysts at the forward scan, was applied to evaluate the ECSA of the samples. The ECSA calculations were based on the equation: ECSA=Q/SL, where L is the Pd loading, Q is the collected charge that calculated from the Pd-O stripping, S is a constant of 424 µC cm−2 that assuming a monolayer of Pd-O on the surface.71


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The sample durability was studied by continuous potentiodynamic sweep between 0.05 and 1.1 V (vs. RHE) in O2-saturated 0.1 M HClO4 solution at 30 oC with a scan rate of 100 mV s−1, and the catalytic activities of the samples for ORR were recorded at 100, 200, 500, 1,000, 2,000 and 5,000 cycles. To investigate the selectivity of the Pd and its alloy nanosheets, the ORR polarization curves of the Pt/C and PdNi NSs/RGO catalysts were measured using thin films on a rotating ring-disk electrode (RRDE) in O2-saturated 0.1 M HClO4 +1.0 M CH3OH solution under 1,600 rpm. For comparison, commercial 20 wt% Pd/C and Pt/C (TKK, Japan) were measured under the same conditions.

ASSOCIATED CONTENT Conflict of Interest. The authors declare no competing financial interest. Acknowledgments. This work was supported by the Natural Science Foundation of Guangdong Province, China (2015A030310005), the 100 Talents Project of Chinese Academy of Sciences, China (H9291440S3) and Natural Science Foundation of Guangdong Province, China (2016B090918006). The authors thank the beam time from beamline BL14W1 at Shanghai Synchrotron Radiation Facility (SSRF).

Supporting Information. XRD, TEM, STEM, STEM-EDS elemental mapping, FTIR, AFM, Raman, XPS, computational model and electrochemical measurements of the catalysts are presented in the Supporting Information.


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Corresponding Author. * Address correspondence to [email protected]; [email protected] ；[email protected]

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Table of Contents graphic


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Solid Synthesis of Ultrathin Palladium and Its Alloy Nanosheets on

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