Surface Charge Polarization at the Interface: Enhancing the Oxygen

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Surface Charge Polarization at the Interface: Enhancing the Oxygen Reduction via Precise Synthesis of Heterogeneous Ultrathin Pt/PtTe Nanowire Hui-Hui Li, Mao-Lin Xie, Chun-Hua Cui, Da He, Ming Gong, Jun Jiang, Ya-Rong Zheng, Gang Chen, Yong Lei, and Shu-Hong Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02769 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016

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Chemistry of Materials

Surface Charge Polarization at the Interface: Enhancing the Oxygen Reduction via Precise Synthesis of Heterogeneous Ultrathin Pt/PtTe Nanowire ▽

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Hui-Hui Li,†, Mao-Lin Xie,‡ , Chun-Hua Cui,† , Da He, † Ming Gong,§ Jun Jiang, ‡* Ya-Rong Zheng, ‖ † Gang Chen, Yong Lei# and Shu-Hong Yu†* †

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ Collaborative Innovation Center of Chemistry for Energy Materials, Hefei Science Center CAS, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. § Engineering and Materials Science Experiment Center, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. ‖

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China.

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Ilmenau University of Technology, Institute of Physics and IMN MacroNano®(ZIK), Prof. Schmidt Str. 26, 98693 Ilmenau, Germany ABSTRACT: Hetero-interface tailoring is of importance for the catalytic activity enhancement of nanomaterials in various reactions owing to the charge polarization at the interface. We report an accurate manipulation of a replacement reaction for Pt/PtTe hetero-nanowire (NW) and PtTe NW fabrication. Their geometric structures are controlled by using the aged K2PtCl4 in ethylene glycol (EG) as Pt precursors with self-tuned coordination state and structure. The aging process enables the slow conversion of PtCl42- to linear structured PtmCln leading to their tunable oxidation ability. The Pt-Pt bond formation during aging has been revealed by time-resolved in-situ UV-vis and X-ray absorption fine structure (XAFS) spectroscopies. The coexistence of highly active Te template, PtCl42- ions and linear structured PtmCln complexes allow Pt NPs in situ growth on the PtTe NWs, forming controllable interface structure served as the catalytic active sites and charge transfer location. In combination with theoretical calculations, we highlight that enhanced activity toward oxygen reduction resulted from the surface charge polarization at the Pt/PtTe hetero-interface and the long-term stability benefited from the structural stability of the interface.

■ INTRODUCTION The heterogeneous catalysts show increasing importance to catalytic optimization for their composition segregation and active interface structure in favor of binding, transformation and transfer of surface species such as electrons, adsorbents or intermediates.1-3 Currently, constructing multi-metallic materials, applying supports and modifying ligands on the surface are mainly strategies to tailor the interface structure and improve the electronic structure by controlling material components with different electronic properties, creating strong metal-support interactions and donating electrons from ligands to enhance performance.4-8 For example, the ethylenediamine makes the surface of Pt NWs highly electron rich. Such an interfacial effect makes the Pt NWs favor the adsorption of electro-deficient reactants, enhancing the catalytic selectivity.4 Interface confine effect between FeOx and Pt substrates were used to stabilize the coordinatively unsaturated ferrous sites for the promotion of CO oxidation.9 Fine control of multimetallic catalysts with alloyed structure, core-shell structure

and heterojunction structure, also introducing the change of electronic structure and enhanced catalytic performance, such as oxygen reduction,10, 11 CO oxidation,5, 12 hydrogen evolution reaction,13 reduction of CO2,14 and so on.15 Given the importance of interface effect, the rational design and precise control over the well-defined interface structure has become an important task in the synthesis of catalysts, due to the illdefined structure, wide size distribution and different components make the researcher difficult to isolate the catalytic effect of each individual parameter (shape, size, component, etc.) and certain the precise catalytic mechanism. In this sense, engineering the heterogeneous materials with accurate composition segregation, uniform size distribution and controllable interface structure by a facile strategy to precisely recognize the interface effect on catalysis is highly desirable. Here, we present a facile “precursor solution-aging” approach to synthesize composition and interface controlled heterogeneous Pt/PtTe ultrathin NWs, which offer a platform for investigating the interface effect on oxygen dissociation and -(OH) desorption in oxygen reduction reaction (ORR).

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The coexistence of PtCl42- ions and PtmCln complexes is the key for the successful synthesis of heterogeneous Pt/PtTe ultrathin NWs. Highly active Te template, PtCl42- ions and linear structured PtmCln complexes allow Pt NPs in situ growth on the PtTe NWs, forming controllable interface structure served as the catalytic active sites and charge transfer location. To investigate the interface effect, we prepared homogeneous PtTe NWs with the same one-dimensional (1D) structure, ultrathin size and composition with Pt/Pt51Te49 NWs for comparison. Such similarities prevent definitive correlation between 1D structure/size/composition-activity. The improved activity for electrocatalytic applications was benefited from the strong surface charge polarization induced by a few free electrons in PtTe flow into Pt.

■ EXPERIMENTAL SECTION Precursor Processing. We used two kinds of the Pt precursors. The first kind is K2PtCl4 crystal dispersed in the EG solvent without aging and mainly contains PtCl42- ions. We labeled the first kind Pt precursor as 0h-aged K2PtCl4. The second kind is K2PtCl4 crystal dispersed in the EG solvent aging for 7h and mainly contains PtmCln complexes. We labeled such Pt precursor as 7h-aged K2PtCl4. Synthesis of ultrathin Te NWs. Te NWs were prepared according to a procedure developed by our group recently. Briefly, 10.0 g PVP and 0.89 g Na2TeO3 (4.0 mmol) were dissolved in 350 mL of DIW under vigorous magnetic stirring at room temperature before 17 mL of hydrazine hydrate (85%, w/w %) and 33 mL of aqueous ammonia solution were added. The mixture was heated to 180 oC and kept for 3 h in a Teflon vessel held in a stainless steel autoclave (500 mL in total volume) and then was allowed to cool to room temperature. Synthesis of heterogeneous Pt/PtTe and homogeneous PtTe NWs. Pt/PtTe and PtTe NWs were obtained through a galvanic replacement at the expense of Te NWs. Firstly, acetone was added into as-prepared Te NWs solution above, which was collected by strong shaking to separate Te NWs from the solution and then dispersed into EG solution with vigorous magnetic stirring. Then, 0.48 ml 0h-aged K2PtCl4 (25 mM) EG solution was immediately added dropwise to Te NWs (0.05 mmol) EG solution under magnetic stirring at room temperature. The mixed solution was shaken at a rotation rate of 260 rpm using an Innova 40 Benchtop Incubator Shaker for 5 h at 50oC. Then acetone was added into the resulting products to precipitate Pt/Pt37Te63 catalysts. The products were collected through centrifugation (2000 rpm, 2 min) and then washed with absolute ethanol for several times. To obtain different compositions of Pt-based catalysts, we varied the volumes of Pt precursors during the process. And, 0.96 ml and 1.92 ml 0haged K2PtCl4 (25 mM) EG solution were added to obtain Pt/Pt51Te49 and P/Pt60Te40 catalysts, respectively. To obtain PtTe catalysts, we added 0.96 ml 7h-aged K2PtCl4 (25 mM) EG solution into 30 ml Te NWs EG solution. The mixed solution was shaken at a rotation rate of 260 rpm using an Innova 40 Benchtop Incubator Shaker for 7 h at 50oC. Processing the remaining is the same with Pt/PtTe catalysts. This kinetic refined synthesis method was robust and readily extended to more complex hetero-nanostructure catalysts system by adding Pd or Au atoms. Synthesis of heterogeneous and homogeneous PtPdTe

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and PtAuTe ultrathin NWs. Heterogeneous and homogeneous PtPdTe NWs were obtained using 0.60 ml 0h-aged K2PtCl4 and 7h-aged K2PtCl4 EG solution mixed with K2PdCl4 (0.36 ml) solution, respectively. Heterogeneous and homogeneous PtAuTe NWs were also obtained using 0.86 ml 0h-aged K2PtCl4 and 7h-aged K2PtCl4 EG solution mixed with HAuCl4 (0.10 ml), respectively. The synthesis process is similar with that of heterogeneous PtTe NWs. UV-vis and XAFS Meassurment. The time-resolved insitu UV-vis spectra of K2PtCl4 dispersed in EG were measured at room temperature using a DUV—3700 UV-vis spectrometer. X-ray adsorption measurements (XAFS) were conducted at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF). The storage ring of SSRF was operated at energy of 3.5 GeV and the current in the range of 150–210 mA. The synchrotron radiation was monochromatized with a Si(111) monochromator. In-situ XAFS spectroscopy at the Pt L3-edge was performed at room temperature in transmission mode. The radiation intensity (incident intensity and transmitted beam intensity) were monitored using ionization chambers filled with pure N2. The 0h-aged K2PtCl4 and 7h-aged K2PtCl4 solution were injected into Teflon cell sealed with Kapton windows to achieve detection. XAFS Data Analysis. The Data processing was performed using the program ATHENA with the use of the IFEFFIT software package. The k3-weighted EXAFS spectra were calibrated, averaged, subtracted the post-edge background from overall absorption using standard procedures. Subsequently, the data in the k-space ranging from 2.2-12.7 Å−1 were Fourier transformed into R-space to separate the EXAFS contributions from different coordination shells. The FEFF8.4 code was used to calculate the backscattering amplitude and phase shift during the curve fitting of Pt L3-edge EXAFS to determine the structural parameters. The theoretical Pt–Pt and Pt-Cl photoelectron amplitudes and phases were calculated for the bulk Pt fcc structure and K2PtCl4 structure. Electrochemical measurements. Electrochemical measurements were carried out with a three electrode system on an electrochemical workstation (Autolab, Swiss). A platinum foil, Ag/AgCl (3.5 M) and glassy carbon rotating disk electrode (RDE) (PINE, 5 mm diameter, 0.196 cm2) were used as the counter, reference and working electrodes, respectively. To prepare the working electrode, isopropyl alcohol suspensions of the PtTe and PtPdTe based catalysts were droped on the surface of RDE and followed adding 3 µL 0.02 wt% Nafion (diluted from 5 wt% Nafion, Sigma-Aldrich). For comparison, the Pt/C catalysts (20 wt%, Johnson Matthey) dispersion was prepared by mixing 5 mg of catalyst in 5 mL of solution containing 5mL of isopropyl alcohol and 25 µL of 5 wt% Nafion solution followed by 30 min of ultrasonication. The Pt loading of Pt/PtTe NWs, PtTe NWs and Pt/C catalysts are 10 µg. The PtPd loading of the homogeneous and heterogeneous PtPdTe NWs are 2.5 µg. The cyclic voltammetry (CV) measurements were conducted at room temperature in Ar-saturated 0.1 M HClO4 solution at 50 mV s-1. The ORR measurements were conducted in O2-saturated 0.1 M HClO4 solution at 5 mV s-1 and at the rotation rate of 1600 rpm. The durability testing was performed under half-cell conditions in HClO4 solution. The electrode was cycled from 0.6 V to 1.1 V in 0.1M HClO4 solu

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Scheme 1 Heterogeneous Pt/PtTe NW catalysts (a) and homogeneous PtTe NW catalysts (b) were prepared by a dynamically controlled galvanic replacement reaction using the precursor 0h-aged K2PtCl4 EG solution and 7 h-aged K2PtCl4 EG solution, respectively.

tion for 30k cycles and left open to the atmosphere to allow for the replenishment of the dissolved oxygen in the electrolyte. The ECSA and electrochemical activities were measured after every 5,000 cycles. For comparison, Pt/C catalysts were measured under the same procedure as described above. First-principles simulations. The Vienna ab initio Simulation package (VASP) was employed to simulate the geometric, electronic, and catalytic properties of model systems of Pt, PtTe, and Pt/PtTe, at the spin-polarized density functional theory (DFT) level. The computations are performed with the frozen-core all-electron projector augmented wave (PAW) model and the generalized gradient approximation (GGA) and Perdew–Burke–Ernzerhof (PBE) functions. By simulating the electronic structures and molecular interactions in several atomic model systems (see the following information for more details), we predicted the electrochemical performance of the designed NW films.

■ RESULTS AND DISCUSSION To explore the precise synthesis mechanism of heterogeneous NW, 0h-aged K2PtCl4 and 7h-aged K2PtCl4 EG solution (see the experimental section) were prepared for the galvanic replacement reaction. High-quality ultrathin Te NWs were selected as active sacrificial templates and to confine the ultrathin size of target products.16, 17 As illustrated in Scheme 1, depending on the coordination state and structure of the Pt precursors, the reaction kinetics can be tuned in a controllable manner and thus the final shape. When 0h-aged K2PtCl4 involving mainly PtCl42- ions were used as the Pt precursor, ultrathin PtTe NWs with surface decorated Pt NPs would be obtained (marked as Pt/PtTe catalysts). During the reaction, the initial precursors of PtCl42- ions would gradually polymerized to Cl32-Pt-PtCl32- dimer, then may polymerized to PtmCln complexes refining the whole reaction kinetics. The dosage of the Pt precursors (PtCl42- ions) affected the ratio of PtCl42- ions and formed PtmCln complexes, as well as Pt content and hetero-interface structure of the target NWs. While 7h-aged K2PtCl4 involving already converted PtmCln complexes were

used as the Pt precursor, homogeneous PtTe ultrathin NWs without interface would be obtained (marked as PtTe catalysts), due to the initial nucleation and growth process were influence by the highly directionality and lower kinetics of the linear structured PtmCln complexes attaching on the Te NWs. To prove the generality of this method, we added a third element (Pd or Au) to the system and successfully prepared heterogeneous and homogeneous PtPdTe NWs and PtAuTe NWs (Figure S1). To confirm the variation of the coordination state and structure of Pt precursors dynamically in EG solvent, the process has been proved by the time-resolved in-situ UV-vis and XAFS spectroscopic studies. When K2PtCl4 was dispersed in EG solution at room temperature, the color of the solvent changed from the initially transparent orange-red to yellow after 7h (inset in Figure 1a).18-20 As shown in Figure 1a, the UV-vis spectra evidenced the formation of PtmCln complexes and local structural change. Obviously, the characteristic absorption peak of PtCl42- ions at ca. 228 nm declined with increasing of the aging time, while the peak at ca. 248 nm newly appeared and increased, indicating the formation of the PtmCln complexes.19, 21-23 To further distinguish the intermediate species and verify the formation of Pt-Pt bond, we performed insitu Pt L3-edge XAFS measurements on 0h-aged K2PtCl4 and 7h-aged K2PtCl4 EG solution, respectively. As shown in Figure 1b, the Fourier transforms (FTs) of EXAFS data demonstrate that the enhancement of the intensity of the peak at 2.48 Å for 7h-aged K2PtCl4 assigned to the Pt-Pt bond, while depressed the peak at 1.93 Å assigned to the Pt-Cl bond. The obtained coordination number of the Pt-Pt shell as shown in Table S1 for the 7h-aged K2PtCl4 is 1.3, implying the formation of Pt-Pt bond. Further, the coordination number of the Pt-Cl shell is 3.1, indicating the Cl32-Pt-PtCl32- dimer may as the main species. We also perform the X-ray absorption nearedge structure (XANES) simulations for Cl32-Pt-PtCl32- dimer (Figure 1c). The spectrum of 7h-aged K2PtCl4 EG solution shows similar features (the shape and the position peak at ~

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Figure 1. (a) Temporal evolution of UV-vis absorption spectra of K2PtCl4 in EG solution. The PtCl42- ions show the maximum at around 228 nm. The appearance of the peak at 248 nm and the disappearance of the peak at 228 nm indicated the formation of PtmCln complexes. The inset is the photos of 0h-aged K2PtCl4 EG solution and 7h-aged K2PtCl4 EG solution, respectively. Pt L3-edge XANES calculations for experimental spectra of K2PtCl4 in EG solution aging for 0h and 7h, respectively. To investigate the formation of Pt-Pt bond, the XANES (b) and the corresponding FTs (c) spectra were compared.

11579 eV) to those of the Cl32-Pt-PtCl32- dimer, but different with that of 0h-aged K2PtCl4 EG solution.18-20 Based on the analysis above, it suggests that at the early stage, only few PtCl42- ions were reduced. When aging time proceeded in the EG solution, more and more PtCl42- ions were reduced and may be reduced to Pt(I) forming Cl32-Pt-PtCl32- dimer via Pt-Pt bonding. By further increasing the aging time, these dimers may polymerized into a tetramer and subsequent longer linear PtmCln complexes.19 Interestingly, the linear structured PtmCln complexes are stable for several weeks at 4-7oC and then could be used as oxidants, providing precise synthesis condition. On the basis of the energy-dispersive X-ray spectroscopy (EDS) analysis on a random selection of the catalysts (see Figure S2), it is proved to be a composition-tunable preparation method. By adding different dosage of 0h-aged K2PtCl4, Pt/Pt37Te63, Pt/Pt51Te49 and Pt/Pt60Te40 catalysts (the subscript numbers shows the real atomic ratio of total Pt to Te in the catalysts) with tunable atomic ratio were successfully prepared. By adding the same dosage of 7h-aged K2PtCl4 as Pt/Pt51Te49 catalysts, homogeneous Pt48Te52 catalysts could be obtained. As shown in Figure 2 and S3, the transmission electron microscopy (TEM) images of the Pt/Pt51Te49 catalysts reveal a narrow diameter distribution of the NPs ranging from 2 to 4 nm decorated on the NW skeleton, inducing a special and heterogeneous composition interface between the NP and NW. The Pt/Pt37Te63 and Pt/Pt60Te40 catalysts distributed smaller (1-2 nm) but less and more but larger size (4-8 nm) of Pt NPs on the surface of the NW skeleton, respectively. This result shows that the concentration of PtCl42- ions is the decisive factor of the formed interface structure. The X-ray diffraction (XRD) pattern in Figure 2f proves the existence of pure Pt phase in Pt/Pt51Te49 catalysts, but none in Pt48Te52 catalysts. To accurately determine the location of the distribution of pure Pt, the high resolution TEM (HRTEM) (Figure 2e) and the high-angle annular dark-field scanning TEM (HAADF-STEM) and STEM-EDS elemental mapping were used to gather more information (Figure 2g). As shown in the HRTEM images, the lattice distance of the NPs are 0.230 nm matching Pt (111) and is consistent with the result of

the XRD analysis. The intensity profile in a line scan across the NP on NW indicates that the NP is dominated by Pt. It is clear from the elemental mapping that Pt and Te atoms were evenly distributed on the NW backbone, but almost Pt dispersed on the NPs, which agree with the HRTEM observation. To ex-situ monitor the crystal phase of the products during the galvanic replacement process, the XRD pattern of the products terminated by different replacement reaction time was performed (Figure S4). Using 0h-aged K2PtCl4 as precursors, the peak of pure Pt is firstly and quickly appeared while the replacement reaction was only carried out for 5 min at room temperature. The pure metallic Pt and Te phases were confirmed by XRD, indicating that PtCl42- ions provide a high reaction rate. The high reaction rate induced that PtCl42- ions may only oxidized the Te atoms on the Te NWs surface quickly and were reduced into zero valent Pt (0) atoms which then aggregated to be a NP. When the replacement reaction proceeded at 50oC, unreacted PtCl42- ions could be partially reduced and polymerized to PtmCln complexes simultaneously and quicker than that at room temperature. To prove this, we performed in-situ UV-vis spectroscopic study for the K2PtCl4 EG solution aging at 50oC and 80oC. As shown in Figure S5, the peak at ca. 248 nm presenting the initial formation of the PtmCln complexes was appeared and increased after the sample aging for 30min and 10min, respectively. Obviously, only 70 min was enough for K2PtCl4 polymerizing to PtmCln complexes at 50oC. During the reaction, the precursors changed from PtCl42- ions to the mixture of PtCl42- ions and Cl32-Pt-PtCl32dimer or longer PtmCln complexes. The highly directionality of PtmCln complexes may enable them attaching on the Te NWs and oxidized Te atoms. Meanwhile, the reaction kinetics may be lowered by the reversible reactions between PtmCln complexes and the shorter complexes forming PtTe NW. When the replacement reaction time prolonged to 1h, the PtTe peak show up, but the intensity of the peak is weaker than that of pure Pt. The time of PtTe formation is consistent with the results of UV-vis spectra (Figure S5). With the replacement reaction time ex tended to 5h, the intensity of PtTe is higher than that at 1h. This experiment conclusively shows that the first formation of Pt NP on the NWs and followed the

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Figure 2. (a-d) TEM images of the Pt/Pt37Te63, Pt/Pt51Te49, Pt/Pt60Te40 and Pt48Te52 NWs. (e) HRTEM images of the nanoparticles on the Pt/Pt51Te49 catalysts. (f) XRD pattern of Pt/Pt51Te49 catalysts and Pt48Te52 catalysts. (g) HAADF image of Pt/Pt51Te49 NWs and the distribution of a Pt nanoparticle formed on the PtTe NWs, obtained by line analysis of EDS. HAADF-STEM characterization of Pt/Pt51Te49 NWs, indicating the main component of the NPs is Pt.

formation of PtTe, confirming the synthesis mechanism. Basing on the same Pt mass, such unique hetetogeneous Pt/PtTe ultrathin NWs with Pt NPs on the surface with heterointerface expose more active sites. Meanwhile, the migration, aggregation and ripening of Pt NPs can be prevented as the Pt NPs in situ growth on the PtTe NW surface. Moreover, this ultrathin PtTe and Pt/PtTe 1D nanostructure can be considered as self-supporting structured catalysts to avoid extra carboncorrosion induced degradations. Such NWs with a length up to hundreds of nanometers are also flexible and intertwisted with each other to form NW network. We found that these NWs can form a film easily through casting process, which are advantageous for electron and mass transport (Figure S6).17, 24 So, Pt/PtTe and PtTe catalysts have a lot of similarities (above described intrinsic structural features) besides the interface structure. Such precise synthesis and control of the catalysts allow us to isolate the catalytic effect of each individual parameter advantageous for catalysis and help us to organize the interface effect of heterogeneous catalysts. The obtained geometric information of Pt/PtTe NWs thus enabled us to do theoretical investigations on model structures

so as to examine electrochemical properties.25 To determine whether the important variable that is altered by the Pt surface or the interface between Pt and PtTe, we built three atomic models. Three atomic models were built to simulate the PtTe surface, fully-covered Pt/PtTe surface (representing the PtTe NWs fully covered by Pt) and half-covered PtTe-Pt/PtTe surface (representing the interfacing part between the exposed PtTe surface and Pt NP of the Pt/PtTe NW) (Figure S7). Our simulations of electronic structures found that the work function of Pt (5.6 eV) is higher than that of the PtTe alloy (4.8 eV), as shown in Figure 3a. At the PtTe and Pt interface, the Fermi level/work function difference drives a few free electrons in PtTe to flow into Pt, causing surface charge polarization as illustrated in Figure 3b. Although our calculations found more charges being donated to Pt in the fully-covered Pt/PtTe than in PtTe-Pt/PtTe (Table 1), the polarization effect is much stronger in PtTe-Pt/PtTe when considering its smaller size of covered Pt. This explains that the PtTe-Pt/PtTe interface helps to improve O2 adsorption, which triggers ORR reactions.

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Figure 3. (a) The computed potential energy surface along the interface layer of the PtTe and Pt system with (110) facets, in which their Fermi levels are set to zero. (b)The computed difference charge distribution in Pt/PtTe and PtTe-Pt/PtTe system, in which free electrons move from PtTe into Pt part.

The optimized geometries for the three model systems to adsorb O2 are displayed in Figure S8, giving adsorption energies in Table 1 with the order of PtTe-Pt/PtTe > Pt/PtTe > PtTe. Meanwhile, the O-O bond lengths are prolonged from 1.42 Angstrom to 1.46 Angstrom at all these three surfaces (Figure S8 and Table 1), implying high O2 dissociation ability.26 The dissociation of O2 produces -(OH) which will soon be adsorbed to the Pt sites on surface (Figure S9). The adsorption energy of -(OH) is smaller on the PtTe-Pt/PtTe interface than on PtTe or fully covered Pt/PtTe surfaces (Table 1). Consequently, further simulations revealed that the activation energy barrier for breaking the Pt-(OH) bond in PtTe-Pt/PtTe is the lowest (Figure S10 and Table 1), suggesting the interface helping it as the easiest one to reduce O2 to H2O. Therefore, one can conclude that the surface charge polarization effect at the PtTe-Pt/PtTe interface could synergize the adsorption and dissociation of O2 with the activation of Pt-(OH) at the same site, so that lead to high electrochemical performance in ORR applications. The synergizing effect would balance the multistep reaction process so that might improve the stability of materials during ORR.27-30 Table 1. Simulated surface polarization charge flowing from PtTe to Pt in a unit cell, O2 adsorption energy, O-O bond lengths, (OH) adsorption energy, and Pt-(OH) bonding breaking (activation) energy. The adsorption energy is calculated with Eads = Emolecule + Esubstrate – Emolecule/substrate.

The different Pt particle size and numbers of Pt/PtTe NWs differ in terms of area of the interface, so we could directly analyze the impact of surface charge polarization for ORR. The electrochemical properties of the catalysts are studied by cyclic voltammetry.31-34 In order to evaluate the role of interface structure in the oxygen reduction, we have prepared a reference simple (Pt48Te52 NWs) with the same structural features and similar composition with Pt/Pt51Te49 NWs besides the interface structure. Figure 4a plots the CV of the PtTe, Pt/PtTe and Pt/C catalysts. Obviously, the Pt/PtTe and Pt48Te52 NWs have a clearly positive shift (30 mV) in the formation of

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adsorbed hydroxyl species (OHad) in comparison with Pt/C catalysts, indicating faster desorption of OHad from the Pt/PtTe and Pt48Te52 NWs. This result is consist with our DFT calculations that the desorption of OHad on PtTe surface become easier than that on pure Pt surface, due to its smaller adsorption energy of OHad. The ECSA of the electrocatalysts can be estimated by integrating the underpotentially deposited hydrogen adsorption/desorption (Hupd) charge from the electrode surface after the double-layer correction. Based on the same Pt loading, the Pt/C catalysts have the highest ECSA, due to more Pt exposure and higher surface area of the nanoparticles (2-5 nm). According to the magnified CV in the inserts, all Pt/PtTe catalysts have the higher ECSA comparing with Pt48Te52 catalysts owing to the formation of Pt NPs and thus active interfacial sites. Comparing with Pt/Pt51Te49 NWs, the lower ECSA of Pt/Pt37Te63 and Pt/Pt60Te40 catalysts are most likely due to the smaller size but less amount and the similar amount but larger size of Pt NPs on the PtTe NWs, respectively, correspondingly inducing lower interface number or interfacial area.

Figure 4. Electrochemical activities of Pt/PtTe catalysts with different composition versus Pt48Te52 and Pt/C catalysts. a) CV profiles of the catalysts recorded in Ar-saturated 0.1 M HClO4 solution at a sweep rate of 50 mV s−1. b) ORR polarization curves for the catalysts in O2-saturated 0.1 M HClO4 solution with a sweep rate of 5 mV s−1 and a rotation rate of 1600 rpm. The colour scheme in (b) applies to (a). Specific activity (c) and mass activity (d) are depicted as kinetic-current densities (jk) normalized to the ECSA and the loading mass of the metal, respectively.

Catalytic activities for the ORR of Pt/Pt37Te63, Pt/Pt51Te49, Pt/Pt60Te40, Pt48Te52 and Pt/C catalysts are evaluated in Figure 4 in O2-saturated 0.1 M HClO4 solution at a sweep rate of 5 mVs−1 at 1600 rpm. The polarization curves of all Pt/PtTe catalysts, especially the Pt/Pt51Te49 catalysts, for the ORR indicate a positive shift in the half-wave potential (E1/2) than that for the Pt48Te52 catalysts, indicating a higher catalytic activity for the Pt/PtTe catalysts. As shown in Figure 4c and 4d, the heterogeneous Pt/Pt51Te49 catalysts exhibited the highest mass and specific activity comparing with the homogeneous Pt48Te52 and Pt/C catalysts at 0.85V (vs RHE). The specific activity and mass activity are depicted as kinetic-current densities (jk) normalized to the ECSA and the loading mass of the metal, respectively. Comparing with Pt/C catalysts, Pt48Te52 NWs exhibited lower mass activity but higher specific activity

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despite of their much lower ECSA. Although the pure Pt surface of Pt/C catalysts are advantageous for oxygen adsorption and dissociation, the coverage of OHad species block new oxygen adsorption, controlling the reaction kinetics. Comparing with Pt48Te52 catalysts, the Pt/Pt51Te49 catalysts (similar composition, ultrathin size and 1D structure with Pt48Te52 NWs) exhibited both higher specific and mass activity, owing to the synergizing effect of Pt NP and PtTe NW with enhanced O2 dissociation and weaker -(OH) adsorption for the charge polarization at the interface (active sites).35 This comparison displayed a strong interface-dependent activity. To prove the interface-dependent activity, we further examined the interfacial effect for PtPdTe NWs. The homogeneous PtPdTe NWs and heterogeneous Pt/PtPdTe NWs (atomic ratio Pt:Pd=6.5:1 and 3.6:1) were synthesized in a similar fashion with PtTe and Pt/PtTe NWs. The catalytic activity of PtPdTe based NWs (PtPd loading 2.5µg) toward the ORR were tested under the same condition. As shown in Figure S11, by adding the third Pd element, the Pt/PtPdTe NWs exhibited higher mass activity and specific activity than that of Pt/PtTe NWs. The homogeneous PtPdTe NWs show a slight enhancement in catalytic activity than that of homogeneous PtTe NWs, but poorer than that of heterogeneous Pt/PtTe NWs, implying the interfacial effect is obvious for enhancing ORR activity. However, the specific activity of heterogeneous Pt/PtPdTe NWs exhibit 2.1 and 2.2 time enhancement than that of Pt/PtTe NWs and PtPdTe NWs, respectively. These experimental results for catalytic activity comparison indicated that the heterogeneous NWs show better activity for their interfacial effect.

Pt/Pt51Te49 catalysts were tested following 10k, 20k and 30k accelerated stability test cycles, as well as commercial Pt/C catalysts (0.6 and 1.1V vs RHE) (Figure 5 and 6) 11, 36. The CV measurements show a loss of 21.2% in the ECSA for the Pt/Pt51Te49 catalysts, but 65.6% for the Pt/C catalysts (Figure 5). Interestingly, the Pt/Pt51Te49 catalysts had a positive shift of 10 mV in the half-wave potential in the polarization curve after 10k cycles’ stability testing (Figure 6a). The improved activity of the Pt/Pt51Te49 catalysts may be induced by the rearrangement of the Pt atoms during the 10k potential cycling (electrochemical annealing). The half-wave potential of the Pt/Pt51Te49 catalysts had no substantial change after 30k potential cycling, while the Pt/C catalysts had a negative shift of 20 mV in half-wave potential (Figure 6b). As shown in Figure 6c and d, the specific activity of Pt/C catalysts was increased due to the sharply decreased ECSA and more active surface after potential cycling (electrochemical annealing). The half-wave potential and mass activity decreased quickly with the potential cycling increases. So the increased specific activity can’t represent the intrinsic activity completely. However, for Pt/Pt51Te49 NWs, both of specific activity and mass activity, as well as the half-wave potential of Pt/Pt51Te49 NWs increased after the stability tests.

Figure 6. (a and b) Initial and post potential cycling ORR polarization curves for Pt-Pt51Te49 catalysts and Pt/C catalysts, respectively. (c and d) The specific activity and mass activity are shown as a function of durability cycling for the Pt-Pt51Te49 catalysts and Pt/C catalysts at 0.85 V (versus RHE).

Figure 5. Comparison of the stability of Pt-Pt51Te49 cata-lysts and Pt/C catalysts. (a and b) Initial and post potential cycling CV curves for Pt-Pt51Te49 catalysts and Pt/C catalysts, respectively, at room temperature in Ar-saturated 0.1 M HClO4 solution at the sweep rate of 50 mV s-1. (c and d) Relative Pt area and ECSA as measured by HUPD as a function of accelerated stability test cycles, respectively.

The degradation of the electrocatalysts, except for the cost, remains another important issue for the commercialization of the fuel cells. To evaluate the structure and ORR stability, we further examined the stability of the Pt/Pt51Te49 and commercial Pt/C catalysts. Stability testing was performed by cycling the potential between 0.6 and 1.1V vs RHE 30k times at 50 mV s-1 in an air-saturated 0.1M HClO4 solution at room temperature. The specific ECSA and ORR activity of the

To further explore the reason of the enhanced stability, the morphology of Pt NP size of Pt/Pt51Te49 the Pt/C catalysts were characterized after stability tests. The NP of Pt/C catalysts increased from 2-5 nm to 5-25 nm, suggesting considerable NP ripening and aggregation. However, the surface at the Pt/Pt51Te49 interface become more smooth, may due to the Pt atomic restructuring via electrochemical annealing (30k stability test cycles). The Pt NP on the Pt/Pt51Te49 NWs without aggregation and migration almost has no change in size, implying the structure stability and no destruction of interface structure (Figure S12). The TEM images allow us to conclude that the sharply decreased ECSA of Pt/C catalysts was ascribed to the Pt NP ripening and aggregation. However, the Pt/Pt51Te49 NWs exhibited much better stability due to the existed interface structure on the NWs (interfacial stabilization), further proved that the active sites of heterogeneous Pt/PtTe NWs at the interface.

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■ CONCLUSION

■ REFERENCES

In summary, we introduce a dynamically self-tuned Pt precursors formed through aging PtCl42- in EG and reveal their variation of coordination state and formation of linear structure through Pt-Pt bond formation during the aging process. The Pt precursors with aging time-dependent oxidation ability allow to accurately controlled synthesis hetero-structured Pt/PtTe and homo-structured PtTe NW via galvanic replacement reaction. This precise synthesis and controllable interface structure are achieved with the aim of investigating the interfacial effect on catalytic performance while ruling out the complex structure factors (shape, size, component, etc.). The coexistence and ratio between PtCl42- ions and linear structured PtmCln complexes are largely responsible for the formation and interface area of Pt/PtTe NWs. As electrocatalysts for the ORR, the strong surface charge polarization effect in Pt/Pt51Te49 catalysts caused better performance relative to homogeneous PtTe NWs and Pt/C. Theoretical calculations suggest that interface between Pt NP and PtTe NW improve O2 dissociation and – (OH) desorption, thus it enhances the performance of the Pt/PtTe catalysts. This clearly defined structure-processingperformance correlation has been ascribed to the amount of available interface sites present on the self-supporting NWs. More generally, it could be successfully extended to synthesize other heterogeneous Pt-based (e.g., PtPdTe and PtAuTe) catalysts and illustrates a promising and efficient way to design active interface for catalysis application.

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■ASSOCIATED CONTENT Supporting Information. chemicals, instruments, detailed structure description, computational details and other experimental results. This information is available free of charge via the internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected]

Notes The authors declare no competing financial interest.

Author Contributions ▽

H. H. L., M. L. X. and C. H. C. contributed equally to this work.

■ ACKNOWLEDGMENT S.H.Y. acknowledges the funding support from the National Natural Science Foundation of China (Grants 21431006, 21407140, 91227103), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), the National Basic Research Program of China (Grants 2014CB931800, 2013CB933900), and Scientific Research Grant of Hefei Science Center of Chinese Academy of Sciences (Grants 2015HSC-UE007, 2015SRG-HSC038). J.J. acknowledges the funding support from National Basic Research Program of China (Grants 2014CB848900), the National Natural Science Foundation of China (Grants 21473166). H.H.L. is grateful for the China Postdoctoral Science Foundation (Grant 2014M560519).

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Graphic Figure

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Surface Charge Polarization at the Interface: Enhancing the Oxygen

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