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Influence of sp-sp Carbon Nano-Domains on Metal/Support Interaction, Catalyst Durability and Catalytic Activity for the Oxygen Reduction Reaction Carlos A. Campos-Roldán, Guadalupe Ramos-Sanchez, Rosa de Guadalupe GonzalezHuerta, Jorge Roberto Vargas-Garcia, Perla B. Balbuena, and Nicolas Alonso-Vante ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06886 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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ACS Applied Materials & Interfaces

Influence of sp3-sp2 Carbon Nano-Domains on Metal/Support Interaction, Catalyst Durability and Catalytic Activity for the Oxygen Reduction Reaction Carlos A. Campos-Roldán1,2,3, Guadalupe Ramos-Sánchez4, Rosa G. Gonzalez-Huerta2, Jorge R. Vargas García3, Perla B. Balbuena5, Nicolas Alonso-Vante1* 1

IC2MP, UMR-CNRS 7285, University of Poitiers, 4 rue Michel Brunet, 86022 Poitiers, France Instituto Politécnico Nacional-ESIQIE, Laboratorio de Electroquímica y Corrosión, UPALM, 07738, CDMX, México 3 Instituto Politécnico Nacional-ESIQIE, Departamento de Metalurgia y Materiales, UPALM, 07738, CDMX, México 4 CONACYT – Universidad Autónoma Metropolitana-Iztapalapa, San Rafael Atlixco 186, Iztapalapa, Vicentina, 09340 CDMX, México. 5 Artie Mc. Ferrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, USA 2

ABSTRACT In this work, platinum nanoparticles were impregnated by two different techniques, namely the carbonyl chemical route and photo-deposition, onto systematically surface-modified multi-walled carbon nanotubes. The different interactions between platinum nanoparticles with sp2-sp3 carbon nano-domains were investigated. The oxidation of an adsorbed monolayer of carbon monoxide, used to probe electronic catalytic modification suggests a selective nucleation of platinum nanoparticles onto sp2 carbon nano-domains when photodeposition synthesis is carried out. XPS attests the catalytic center electronic modification obtained by photo-deposition. DFT calculations were used to determine the interaction energy of a Pt cluster with sp2 and sp3 carbon surfaces as well as with oxidized ones. The interaction energy and electronic structure of the platinum cluster presents dramatic changes as function of the support surface chemistry, which also modifies its catalytic properties evaluated by the interaction with CO. The interaction energy was calculated to be 8 fold higher on sp3 and oxidized surfaces in comparison to sp2 domains. Accelerated Stability Test (AST) was applied only on the electronic modified materials to evaluate the active phase degradation and their activity towards oxygen reduction reaction (ORR). The stability of photo-deposited materials is correlated with the surface chemical nature of supports indicating that platinum nanoparticles supported onto multi-walled carbon nanotubes with the highest sp2 character show the higher stability and activity towards ORR.

Keywords: Electrocatalyst, sp3-sp2 domains, durability, DFT, ORR.

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1. INTRODUCTION Climate change and global warming have raised concerns about the use of finite nonrenewable resources thus speeding up the search of means to harvest renewable energy and using more efficient means to transform it, consequently leading to a more sustainable world.1 Proton Exchange Membrane Fuel Cell (PEMFC) is considered an ideal power source for mobile and stationary applications. When using hydrogen as fuel and air as oxidant the only sub-product is water; therefore no contaminants are released at an energy transformation efficiency higher than heat machines.2 To date PEMFC operates in acidic electrolytes in which the Oxygen Reduction Reaction (ORR) is the limiting step diminishing efficiency and leading to the use of huge platinum loads or else carbon supported Pt-based nanoparticles. Nanoparticulated Pt approaches have resulted in the increase of the utilization of Pt, higher mass activity and thereof reduced Pt loads. However, the use of carbon support provides new challenges on the electrode durability; because of, either, the agglomeration of particles or the oxidation of the support itself.3-5 Nanoparticle agglomeration can be related to the strength of the support-nanoparticle interaction: weaker interaction provides high nanoparticle mobility while strong interaction might be detrimental enhancing the oxidation rates of the support or the nanoparticle itself. On the other hand, the oxidation of the support might be influenced by several factors such as operation voltage, surface chemistry and carbon structure. Until today, several carbon structures have been reported as having an effect on the reactions occurring on the fuel cells.3 Carbon blacks, owed to their low cost and high availability are the immediate choice as support for low temperature fuel cells, although the presence of volatile contaminants,6 mostly amorphous quasi-graphitic structure and heterogeneous surface lead to dissimilar results and poor stability. Several modifications to black carbons have been proposed. Among them, the most effective ones are the acid7 and heat5 treatments intended to form specific surface chemistry (mostly oxygen functional groups) and to reconstruct the ordered graphitic structure respectively. The effect of such treatments has conducted to contradictory conclusions, which may simply arise from the inhomogeneity of the starting material. The development of new carbonaceous materials opens up the route for controlled structure and microstructure. Among them, carbon materials with high graphitization degree, as support materials, such as multi-walled carbon nanotubes (MWCNT) proved to be effective to improve the catalyst durability.8-9 The origin of the enhanced stability is still debatable. Moreover, the effect of the support/nanoparticle interaction has been attributed to the modification of the nanoparticle electronic structure which in turn modifies its electrocatalytic properties for instance for the oxidation of carbon monoxide.10 The modification of the electronic structure of the nanoparticle as results of the interaction with the support has been observed by X-Ray Photoelectron Spectroscopy (XPS) experiments,1113 X-Ray Absorption Spectroscopy (XAS),14 electrochemical methods, coupled experimental and Density Functional Theory (DFT) experiments.10, 15-18 In particular DFT simulations have been useful to confirm that the support is capable to modify the nanoparticle electronic structure thus changing the catalytic properties.19-21 Strong interactions between platinum clusters and sp2 carbon material domains, such as highly oriented pyrolytic graphite (HOPG),15 multi-walled carbon nanotubes,10 reduced graphene oxide and nitrogen-doped reduced graphene oxide18 have been reported. However, the effects of the support structure and surface chemistry remain elusive. In this work, the


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effects of sp2 and sp3 nano-domains of systematically modified multi-walled carbon nanotubes on the nucleation of catalytic centers deposited by two different techniques, namely, carbonyl chemical route (CCR) and photo-deposition (PD) are discussed by a theoretical/experimental approach. The ORR activity as well as the stability, and dynamics of the Pt NPs onto the substrate are investigated and the results reported on the basis of the effect of the support structure.

2. EXPERIMENTAL 2.1. Chemicals The following precursors were purchased and used without further purification: Na2PtCl6⋅6H2O and H2PtCl6⋅6H2O (Alfa-Aesar), CH3COONa from Sigma-Aldrich, methanol and isopropanol from Sigma-Aldrich, black carbon Vulcan XC-72 (C) from Cabot, and home-made pristine multi-walled carbon nanotubes (CNT) prepared as previously reported. 22 2.2. Chemically treated multi-walled carbon nanotubes Purification (f-CNT). 100 mg of home-made CNT were treated in 7.6% vol. HNO3 solution and sonicated at 42 kHz at room temperature for 20 min. The suspension was filtered and washed with deionized water, thereafter dried under vacuum. Exfoliation (e-CNT). 250 mg of home-made CNT was dispersed into 15 mL of concentrated H2SO4 and stirred for 1 h, followed by the addition of 1.5 g of KMnO4 at 50 °C. After complete addition of KMnO4, 40 mL of deionized water and 3 mL 30 % H2O2 were added in order to quench the reaction.23 The powder was collected by centrifugation, washed with deionized water and dried at 60 ºC overnight. 2.3 Catalyst synthesis

2.3.1 Carbonyl Chemical Route (CCR) Pt-carbonyl complexes were synthesized through the reaction of Pt-salt (Na2PtCl6⋅6H2O) with CO using methanol as a solvent at 55 ºC for 24 h with constant stirring. Afterwards, the support material (e.g., C, CNT, f-CNT or e-CNT) was added to the carbonyl complex solution under N2 atmosphere for 12 h in order to obtain a catalyst loading of 20 wt. % onto the support. The solvent was evaporated, the resulting powder was thoroughly washed with Milli-Q water and vacuum filtered. The recovered powder was dried overnight (T=60 °C). Finally, the resulting catalyst was heat-treated at 200 ºC for 1 h under H2 (99.999%). 2.3.2 Photo-deposition (PD) The support material (e.g., C, CNT, f-CNT or e-CNT) was mixed in N2-saturated Milli-Q water in a photo-reactor equipped with an optical quartz window. An isopropanol solution containing H2PtCl6⋅6H2O was added into the photo-reactor and stirred for 3 h under UV light (Xe lamp, 159 W). Visible and infrared light was filtered by a hot mirror UV, while a water filter (long-pass filter GG40) was used to prevent the sample heating. After the irradiation time, the resulting powder was thoroughly washed with Milli-Q water, vacuum filtered and dried overnight.



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2.4 Physical-Chemical Characterization Raman Spectroscopy. Raman spectra were obtained in a Horiba Jobin Yvon Spectrometer HR 800 with a 532 nm laser in the range of 100-3100 cm-1. Deconvolution analyses were performed using the Fityk free-software. Transmission Electron Microscopy. A High Resolution Transmission Electron Microscope (HRTEM) JEM 220FS was used to image the morphology of supports. Samples were prepared by suspending CNT into isopropanol, by using ultrasound during 15 min. Then, an aliquot was impregnated on a copper grid. Finally, the samples were dried at room temperature. XPS Measurements. A Thermo Scientific Kα X-ray Photoelectron Spectrometer was used, with an Al-Kα X-Ray (1487 eV) monochromator to obtain C1s and Pt 4f photoemission lines for studying the chemical surface structure of materials, at room temperature at a base pressure of 10-9 Pa. The C 1s and Pt 4f lines were deconvoluted, after Shirley background removal, to identify the predominant functional groups and oxidation peaks, respectively, using Voigt peaks. Electrochemical Measurements. Catalytic inks containing the active materials were prepared as follows: a mixture containing 5 mg of catalyst and 3 mL of stock solution (20% isopropanol, 0.4 % Nafion® Sigma-Aldrich and 79.6% Milli-Q water) was dispersed in an ultrasonic bath for 30 min. Then, 3 µL of the mixture was dropped onto a glassy carbon electrode (3 mm diameter) and dried at room temperature. The electrochemical measurements were carried out at 25 ºC using a potentiostat/galvanostat (Autolab PGSTAT 30) in a typical one-compartment three-electrode cell with 0.1 M HClO4 as electrolyte. A Pt mesh and a reversible hydrogen electrode (RHE) were respectively used as counter and reference electrode. We used the adsorption of carbon monoxide molecule to probe the electronic state of Pt NPs onto the various supports. Thus, CO stripping measurements were carried out by bubbling CO for 3 min and N2 for 15 min. Then the potential sweep was done at 5 mV s-1 in anodic direction. For the materials stability test, a gold electrode (0.0707 cm2 geometric area) was used instead of the glassy carbon in order to avoid carbon corrosion currents. Cyclic Voltammograms (CVs) were recorded at 50 mVs-1 between 0.6 and 1.0 V for 5000 cycles. Every 500 cycles the Electrochemical Surface Area (ECSA) was measured by integrating the Hupd region (anodic contribution), as well as the ORR activity. ORR activity was measured every 500 cycles by the Rotating Disk Electrode technique in O2 saturated 0.1 M HClO4, at 900 rpm, and 5 mV s-1. 2.6 Computational details Periodic DFT calculations were used to elucidate the role of the support on the interaction with platinum. The Vienna Ab-Initio Software Package (VASP)24-27 was used for all calculations, spin polarized calculations were performed using the projected augmented wave (PAW) pseudopotentials and the PBE exchange correlation functional.28-29 The sp3 and sp2 domains were simulated as surfaces of graphene and diamond, the former as a single 5x5 graphene layer separated by 12 Å vacuum space while the later as (111) diamond surfaces. Pt species were allowed to interact with the surfaces and also with a sp3


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oxygen terminated surface. The surfaces are intended to determine their effects on the limits of the sp3 and sp2 domains, intermediate behaviors are expected depending on the support synthesis and treatments. 3. RESULTS AND DISCUSSION 3.1. Effect of exfoliation and functionalization of MWCNT nano-domains Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and XPS techniques were carried out in order to identify the chemical modifications produced at the CNT surface by the purification and exfoliation processes. Figure 1 contrasts the Raman, and XPS spectra of C, CNT, f-CNT and e-CNT, whereas Figure S1 (in Supplementary Information) depicts the FTIR spectra of C, CNT, f-CNT and e-CNT. The energy interval of 800 and 2000 cm-1 for C, CNT, f-CNT and e-CNT are shown in Figure 1(a), the insets are the corresponding HRTEM images of the material. Typical features of carbonaceous materials are observed. However, in this spectral region, there are overlapping bands corresponding to the different functional groups. The D and G signals of MWCNT are observed at ca. 1350 cm-1, and ca. 1580 cm-1, respectively.30-33 Carbon black (Vulcan) shows a very intense and wide D band, in correspondence to a high degree of interstitial disorder (amorphous) along the c-axis between the crystallite planes.10 On the other hand, CNT presents an amorphous carbon layer (see HRTEM micrograph) on the surface of MWCNTs. This amorphous carbon, added to disorder between crystalline planes, endows a considerable contribution of its D band intensity and width.

Figure 1. a) Raman Spectra from 800-2000 cm-1 using λlaser=532nm, (insets) HRTEM images; and b) C 1s XPS spectra of C, CNT, f-CNT and e-CNT. It is evident that the crystalline domains of carbon samples after purification (f-CNT) become higher, as shown by the low intensity and width of their D band. Interestingly, G



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band becomes narrower and more intense after this purification process, suggesting an increment of the sp2 character of f-CNT. HRTEM of f-CNT reveals an ordered surface, less amorphous carbon. Finally, e-CNT favors a higher sp3 character as compared to CNT. HRTEM revealed a surface with higher defect density, which is the reason of the increased intensity and width on its D band. Deconvolution analysis was carried out by using Lorentzian line shapes for D and G bands, whereas Gaussian ones were used to fit the D’ band as well as the band centered at ca. 1200 cm-1.17 The D’ band (ca. 1450 cm-1) is well-defined from D band. Both signals are related to defects on MWCNTs’ surface. For carbonaceous materials, it is possible to quantify the degree of graphitization by evaluating the D and G integrated intensity ratio (ID/G). This ratio is commonly used to quantify the amount of defects in the material as well as the inplane crystallite size ( ) , which is a measure of the interdefect distance, using the equation (1).17 = 2.4 x 10 (1) Where is the laser wavelength and is the integrated intensity of the corresponding band. Table 1 summarizes the data for D and G bands from the deconvolution analysis. The results indicate that purification increases the crystalline planes of MWCNTs, in other words, extended sp2 domains are built, whereas exfoliation increases the defect density over the tubular structure, viz., extended sp3 domains. C 1s XPS spectra of C, CNT, f-CNT and e-CNT are reported in Figure 1(b), the wellknown C 1s signal of graphitic surfaces at 284.4 - 284.5 eV is clearly observed.34 The wellpronounced asymmetry of the C 1s peak recorded on graphitic-like materials reflects the high density of states (DOS) near the Fermi energy level (Ef).35 Deconvolution analysis reveals that C shows a weak intensity signal at 284.4 eV, and the main contribution of the spectrum is related with the sp3 character (285.1 - 285.4 eV).34-36 Moreover, at higher binding energy, it is possible to identify width signals, which are related to hydroxyl, carbonyl and carboxyl (or ester) groups,36-38 which is congruent with FTIR spectra of C (see Figure S1 in Supplementary Information). For CNT is possible to observe a higher contribution of sp2 carbon at 284.5 eV, however, after purification treatment (f-CNT) the graphitic contribution of the whole spectra becomes higher and narrower, suggesting the increment of sp2 character on the surface after purification treatment. Whereas, after exfoliation treatment (e-CNT), the main contribution corresponds to sp3 character. Moreover, at higher binding energy, the spectra reveal the hydroxyl, carbonyl and carboxyl (or ester) signals (also revealed by FTIR spectra, see Figure S1 in Supplementary Information), suggesting that these chemical signatures were developed at the materials’ surface by the exfoliation chemical treatment. Table 1 summarizes the data from XPS spectra deconvolution analysis. Table 1. Raman Spectra and XPS analyses of C, CNT, f-CNT and e-CNT La C=C sp2 C-C sp3 C=C sp2 C-C sp3 ID/G (nm) (eV) (eV) (at. %) (at. %) C CNT

1.8027 0.8923

10.66 21.54

284.5 284.5

285.1 285.4


10.70 57.54

68.06 27.91

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f-CNT e-CNT

0.3663 1.5051

52.47 12.77

284.5 284.6

285.4 285.1

72.47 25.57

12.18 56.64

From Table 1, f-CNT presents the highest value of and sp2 character, thus, the largest crystallite size, whereas, the graphitic structures of C, e-CNT and CNT, respectively, possess a high degree of interstitial disorder, meaning, that those materials do not possess extended graphitic domains and the main character is essentially sp3. The sp2 character in the various supports decreases as: f-CNT > CNT > e-CNT > C. By the other hand, the quantity of oxygenated functional groups decreases as: C > e-CNT > CNT > f-CNT. These results suggest that functional groups are formed over the defects onto the carbon surface. 3.2. Nucleation of Pt nanoparticles onto sp3-sp2 MWCNT nano-domains Figure 2a shows the CO-stripping voltammograms of photo-deposited, and chemicalimpregnated Pt onto C, CNT, f-CNT and e-CNT, the CO oxidation at the potential region 0.63 V < E < 0.80 V shows a multi-peak profile. The origin of such CO stripping peak multiplicity is still an open question. A survey of the literature data revealed at least five different explanations for the occurrence of multiple peaks, as follows: (i) preferred orientation of surface planes,39 (ii) the occurrence of the reaction at edges and terrace sites which are particle size dependent,40 (iii) particle-size effects related to the distribution of sizes of the particles,41-46 (iv) particle agglomeration and the presence of defect sites,41, 46-47 and (v) the role of CO surface diffusion between the facets of a nanoparticle.48 For all catalyst synthesized by photo-deposition (PD) three different signals appear: a prepeak at ca. 0.40 V, and two more centered at ca. 0.66 V and 0.75 V, respectively. The prepeak suggest that the oxidation of adsorbed CO takes place on agglomerated sites, constituted by different facets and defects, 49-52 whereas the peak centered at ca. 0.66 V and ca. 0.75 V can be assigned to two different carbon sites interacting with Pt.17 Interestingly, for the PD method the Electrochemical Active Surface Area (ECSA) of the peak centered at ca. 0.66 V is higher than the one centered at ca. 0.75 V (cf. Figure 2b), suggesting a selective nucleation, induced by the electron-hole pair generation, of Pt nanoparticles onto a specific domain of carbon support independently of the carbon nature,10, 17 while by using the CCR method the CO peak prevalence depends on the type of carbon structure.



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Figure 2. a) CO stripping on Pt/C, Pt/CNT, Pt/f-CNT and Pt/e-CNT, and b) ECSA at 0.66 V and 0.75 V as a function of the in-plane crystalline size of supports. Catalyst were prepared by carbonyl chemical route (red line) and photo-deposition (blue line). Measurements done in 0.1M HClO4, at 25 ª C, 5 mV/s. As assessed in previous publications10, 15, 18, 33 Pt prepared by Carbonyl Chemical Route (CCR) onto carbon Vulcan C (Pt/C) reveals a unique CO oxidation peak centered at ca. 0.75 V,53 whereas this catalyst supported onto MWCNT by the same chemical procedure presents two successive CO oxidation peaks, placed at ca. 0.66 V and ca. 0.75 V, respectively. Interestingly, the ECSA of both peaks depends on the crystalline degree of the carbon surface, determined by the in-plane crystallite size ( , see Figure 2b). It turns out that the higher sp3 character dominates the ECSA at 0.75 V; and the higher sp2 character, the higher ECSA at 0.66V. In other words, the PD synthesis method leads to a selective heterogeneous nucleation of platinum nanoparticles onto sp2, whereas CCR leads to a random nucleation onto carbon domains (no specificity). Table 2 summarizes the ratio of ECSA at 0.66V and the total ECSA as a function of . From the Table 2, it is evident that the ECSA at 0.66 V increases with L . Therefore, the CO oxidation potential down-shift observed on Pt NPs supported onto MWCNT, induced by PD can be related to the interaction of Pt nanoparticles with well-ordered graphitic domains (sp2), unlike the CCR method that allows a random nucleation on sp2 and sp3 domains. Thus UV photons generating electron-hole pair formation is favored on the graphitic domains where Pt NPs are photo-deposited.


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Table 2. Comparison between %ECSA (at 0.66V and 0.75V) with the in-Plane crystallite size (La) for the catalyst prepared by carbonyl chemical route (CCR) and photo-deposition (PD). La %ECSA0.66V %ECSA0.75V %ECSA0.66V %ECSA0.75V (nm) (CCR) (CCR) (PD) (PD) Pt/C Pt/CNT Pt/f-CNT Pt/e-CNT

10.66 21.54 52.47 12.77

0 32.60 51.33 7.74

100 64.79 48.66 90.39

61.50 74.04 90.86 72.05

30.20 20.95 5.29 18.95

To have insights of such phenomenon, DFT simulations were performed in which the effect of the support on the adsorption of a CO molecule was analyzed. The CO was allowed to interact with the cluster in the same site in the unsupported Pt6 cluster and interacting with sp2 and sp3 domains. The interaction energy was calculated as the difference of isolated systems (taking into account the changes in geometry) and the interacting one. Bader charge analysis54 was performed in order to determine the trends in electron transfer to the CO molecule. Table 3 shows the interaction energy of CO with the cluster and the changes in charge transfer to the CO molecule. Both quantities are modulated by the presence and the nature of the support. In comparison to the interaction on the unsupported cluster, the interaction energy is higher on the oxygen terminated surfaces, and lower on sp2 and sp3 domains. However, the charge transfer follows an interesting trend since the CO receives a higher electron density on the cluster supported on the sp2 domains than on the oxygen terminated and sp3 surfaces, having similar values than the unsupported cluster. A graphical representation of the charge accumulation depletion caused by the interaction with the support is depicted in Figure 3. Blue areas represent electron density accumulation while those in red represents depletion. Interesting features are observed in Figure 3, the formation of bonds between the support and the nanoparticle is evident as blue areas appear directly between carbon and platinum atoms. This interaction occurs at expenses of charge depletion of platinum atoms at the interface, comparing sp3 and sp2 domains the higher blue regions at the interface is a direct consequence of an enhanced interaction energy. The interaction with the oxygenated surface is fundamentally different, owed to a strong charge transfer process at the interface characterized by a high electron density accumulation in the oxygen and depletion in the platinum atoms, differently to the interaction with sp3 and sp2 domains, the charge transfer is so extreme than even the platinum atoms located on top of the cluster presents charge depletion areas. Therefore, the electronic properties of the cluster are very different which is expected to lead to changes in the interaction with reactive molecules, i.e., CO and O2. The values on Table 3 confirms that when the cluster interacts with the oxygen terminated and sp3 surfaces most of the charge is kept at the interface thus the charge being transferred to the CO molecule is higher only when the Pt cluster interacts with sp2 domains, on the other cases, the amount of charge transferred is similar to systems where the support is not included.



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Table 3. Properties of CO interaction with the supported clusters as function of the surface chemistry Interaction CO charge / energy / eV excess electrons Pt Pt/sp2 Pt/sp3 Pt/Oterminated sp3

-3.01 -2.95 -2.92 -3.18

-0.09 -0.15 -0.09 -0.10

Figure 3. Electron density accumulation (blue) and depletion (red) iso-surfaces (0.01 e-/Å3) as result of the interaction with the support, a) sp2 domains, b) sp3 domains and c) oxygen terminated diamond (111) surfaces. Gray, brown and red spheres represent platinum, carbon and oxygen atoms respectively. 3.3. Metal/Support Interaction on sp3-sp2 MWCNT nano-domains Figure 4 shows the 4f Pt XPS region spectra of Pt/C prepared by CCR, and Pt/CNT, Pt/fCNT and Pt/e-CNT prepared by PD. Typical doublet centered at 74.86 and 71.55 eV are found on Pt/CCCR. However, the Pt 4f emission peaks of materials prepared by PD present a binding energy down-shift attesting that photo-deposited materials are electronically modified as compared to Pt/CCCR,10 which is most likely related to the preferential Pt interaction with sp2 carbon domains. The binding energy down-shift for Pt/CPD, Pt/CNTPD, Pt/f-CNTPD and Pt/e-CNTPD are, respectively, 122, 203, 409 and 162 meV. Therefore, the highest electron density among all samples was found in Pt/f-CNTPD.


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Figure 4. (a) Pt 4f XPS spectra for Pt/C, prepared by carbonyl chemical route (CCR), and Pt/C, Pt/CNT, Pt/f-CNT and Pt/e-CNT, prepared by photo-deposition (PD); and (b) HRTEM micrographs for (1) Pt/e-CNTPD, (2) Pt/f-CNTPD, (3) Pt/CNTPD, (4) Pt/CPD and (5) Pt/CCCR Periodic Density Functional Theory (DFT) calculations were used to elucidate the role of the support on the interaction with platinum. First, a single Platinum atom was allowed to interact with the graphite and diamond surfaces, the goal of this preliminary study is to demonstrate the basic interaction strength and charge delocalization caused by the platinum-support interaction. The modelled surfaces are depicted in Figure 5. In them, the possible interaction sites are identified. In graphene there are three possible sites, bridge (A), top (B), and hollow-type at the center of the hexagon (C), while on the diamond (111) surface we initially simulated three sites corresponding to top (A) in which the Pt lies above a three-fold coordinated C atom, top2 (B) in which the site is above a 4-coordinated carbon atoms and in the space between three A-sites, the C site is a bridge one between two adjacent carbon atoms. The platinum atom was placed directly above the surface at a distance of ~ 2 Å and then relaxed until the force criteria were reached. On one hand, in the sp2 domains the only stable interaction site corresponds to the bridge one, all other sites irremediably shift to the bridge position, the interaction energy calculated as the energy difference between isolated and interacting systems. The interaction energy (adsorption energy) between Pt and sp2 domains is similar to previous reports.55-56 On the other hand, for sp3 domains, the relaxation of the Pt on the A and B sites lead to stable configurations; however, on the bridge site the platinum atom was relaxed into a hollow site interacting with three carbon atoms having no carbon atom in the second sublayer. For diamond (111) oxygen terminated surfaces, the interaction directly with on top of an oxygen atom did not result in a stable configuration; however, those interacting in the space formed by three oxygen atoms resulted in a very stable configuration.



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Figure 5. Models used for sp3 and sp2 domains analyses. A) Periodic graphite totally sp2 carbon, B) diamond 111 surface having sp3 carbon domains. The letters indicate the site for adsorption, on diamond 111 the first layer is depicted in brown and subsurface layers in gray. The interaction energy is an important indicator of the stability of the nanoparticle on the support surface. For instance, if the interaction energy is too weak, the mobility of the nanoparticle is very high leading to migration and possible sintering into bigger nanoparticles. The interaction energy of the single atom is reported in Table 4, the interaction energy of Pt with the sp3 domains is around three times higher than in the sp2 domains on the C site (Figure 5) which is not surprising since the interaction is done directly with three three-fold sp3 coordinated atoms. It is very unlikely that the (111) surface (or any other) is composed of uncoordinated C atoms, according to the experimental procedure, a high number of oxygenated species is expected; therefore, the diamond (111) surface was covered with =O terminal groups. The interaction of the Pt atom with the =O groups is the highest of all the possible sites being slightly higher than the interaction with the pure (111) diamond surface. This indicates that all non-graphitic sites (sp3 domains) which are formed during the synthesis are most likely to serve as anchoring sites for the interaction with positive ions and then as stable sites for the interaction with Pt clusters. Another important characteristic of the nanoparticle-support interaction is the charge transfer features. In pure covalent systems it is expected the charge being distributed equally between the interacting atoms; however, in mixed covalent-ionic systems there are atoms that donate/receive electron density. Table 4 illustrates that in the sp2 systems the charge being transferred is almost null, while in the sp3 systems the charge transfer is two orders of magnitude higher depending on the site (and therefore on the atom coordination). However, compared to the interaction with oxygenated sites, the previous charge transfer processes are eclipsed by the positive formal charge found in Pt interacting with oxygen terminated surfaces, becoming a true positive ion. The charge transfer is of primordial importance since it modifies the electronic structure of the nanoparticle and in consequence changes its properties, especially those related to the electrocatalytic ones.


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Table 4. Interaction energy of the Pt atom and Pt6 with the support and charge transfer properties. Site Interaction energy / eV Platinum charge / excess electrons/atom Graphene Diamond Graphene Diamond O O 111 terminated 111 terminated sp3 sp3 A -1.69 -4.00 -0.002 -0.06 B Shift to A -4.87 -5.28 +0.13 +1.14 C Shift to A -5.11 -5.17 +0.18 +1.15 Pt6 -1.15 -1.02 -8.65 +0.13 +0.27 +2.33

The last row of Table 4 shows the results of the interaction energy per Pt atom for a 6-atom cluster interacting with the surfaces. The cluster was relaxed in vacuum and then was allowed to interact with the surface in the most energetically favorable sites determined for the single atom interactions (A for graphene and C for diamond and O-terminated diamond surfaces). The interaction energy was calculated in a similar way than in previous sections. Interestingly, the trend in interaction energy is reverted, and the higher interaction energy is observed in the sp2 graphite domains while in diamond (111) surface the interaction energy is slightly weaker. Analyzing the interatomic distances in the nanoparticle/support interphase it is found that the Pt-Pt distance was enlarged from 2.56 Å for the cluster in vacuum to 2.99 Å in sp3 domains while the interaction with graphene sp2 domains the distance is only increased to 2.74 Å. The changes in interaction energy thus, can be ascribed to the modification of the cohesive energy in the clusters by the different interaction with the sp3 domains. Thereafter, the interaction energy calculated, taking into account the geometry changes in the Pt cluster, confirms a higher (5.1 eV) interaction energy with sp3 domains in comparison to the sp2 domains. Similarly, to the interaction of a single atom with =O sites, the interaction of the cluster with the (111) diamond surface oxygen terminated led to the highest interaction energy in comparison to all other surfaces. Considering the changes in cluster geometry the interaction energy in oxygen terminated surfaces is -10.1 eV which, compared to the interaction with graphene is one order of magnitude higher and around 1.5 times higher than in the (111) diamond surface. Thus the interaction energy is higher on the sp3 domains which indicates lower nanoparticle mobility. However, this statement does not indicate that the interaction of the cluster with sp3 sites should lead to more stable catalyst in comparison to the interactions with the sp2 domains, since other factors such as support corrosion or cluster oxidation might take place. The charge transfer properties per atom are in line with the interaction of the single atom. The charge transferred to sp3 domains is twice that found in graphene, while the charge transferred in oxygenated surfaces is very much like a real ionic interaction. These changes in charge transfer properties are the responsible for the changes in the nanoparticle electronic structure and therefore for the modification of the catalytic activity towards CO and O2 reactions. Theoretical analysis is consistent with experimentally observed in Pt 4f XPS spectra, where Pt 4f peaks of materials prepared by PD present a binding energy



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down-shift, leaving the photo-deposited materials supported on f-CNT (substrate with > sp2 character) with higher electron density on Pt. 3.4. Platinum nanoparticles stability on sp3-sp2 MWCNT nano-domains Accelerated stability test (AST) was performed on photo-deposited Pt NPs. After 2500 CVs between 0.6 – 1.0 V/RHE. Figure 6 depicts the variation of the remaining ECSA, deduced from the hydrogen underpotential (Hupd) region (see insets, Figure 6a), and the kinetic current, jk, for the ORR measured with the RDE technique and determined at 0.9V/RHE (see insets, Figure 6b) of the same catalytic center supported on various carbons. The stability of both parameters decreases as follows: Pt/f-CNTPD > Pt/CNTPD > Pt/CPD. The ECSA loss of this latter is of 54.8%,15 whereas Pt/CNTPD and Pt/f-CNTPD are 29.23% and 19.77 % of ECSA, respectively. This behavior is reflected in the remained kinetic current (cf. Figure S2 in Supplementary Information). However, a peculiar response was obtained in the first 1000 CVs on Pt/f-CNTPD. Summing up, the stability towards the ORR is as follow Pt/f-CNTPD >Pt/CNTPD >Pt/CPD, and such ORR stability can be related to Pt-sp2 carbon domains interaction.

Figure 6. a) Remaining Electrochemical Active Surface and b) remaining kinetic current for Pt/C, Pt/CNT and Pt/f-CNT, prepared by photo-deposition, during the accelerated durability test. The insets show, respectively, the hydrogen underpotential (Hupd) region in (a) and the ORR variation, in 0.1M HClO4, after initial (0) and final (2500) CV cycles for Pt/CPD and Pt/f-CNTPD.

To visualize up to which extent the interaction (metal-carbon support hybridization) can stabilize the catalytic center, we performed AST and CO stripping combined with TEM measurements, in such a way to observe the dynamic behavior of photo-deposited Pt induced by the oxidation reduction process of the catalytic center. Figure 7 contrasts such dynamics of Platinum, by probing the electronic modification of Pt NPs via the COstripping before and after AST on Pt/CPD, Pt/CNTPD and Pt/f-CNTPD. Likewise, TEM images of the samples before and after 2500 cycles are depicted. Before AST the main oxidation peak on all samples is centered at 0.66 V and a small one at 0.75 V, which, as discussed above, these oxidation peaks are related to the Pt NPs interacting on sp2 and sp3


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domains, respectively. It is apparent that after the AST, the CO oxidation indeed probes, via the oxidation peak electrode potential, that those sites with smaller in-plane are prone to facilitate the re-deposition and/or agglomeration of Pt NPs onto sp3 sites via either a 3D or 2D Ostwald ripening process, i.e., Pt dissolved, traveling along and re-deposited onto the carbon surface (sp2 or sp3). Re-deposition of Pt takes place preferentially onto sp3 sites as the interaction calculated by DFT is preferential on the sp3 domains, as long as the sp2 domains are smaller. This was not the case for Pt/f-CNTPD, since the peak centered at ca. 0.66 V does not move after 2500 cycles but it became broader, cf. TEM picture, that assess that Pt nanoparticles of ca. 5 nm are better dispersed in comparison to Pt/CNT.

Figure 7. (a) CO stripping in 0.1M HClO4 at 25 ªC, with 5 mV/s on catalysts for Pt/C, Pt/CNT and Pt/f-CNT, prepared by photo-deposition, before (blue line) and after (pink line) the stability test; and (b)TEM images before (0 cycles) and after (2500 cycles) the stability test

As demonstrated by XPS Pt 4f deconvoluted spectra and DFT calculations, the higher electron density on Pt occurs when Pt is deposited on sp2 domains, i.e. on Pt/f-CNTPD. This interaction could be responsible for the limited mobility and agglomeration of platinum nanoparticles through sp2 domains, saying nothing about carbon nanotubes corrosion resistance. The effect on the stability of Pt/f-CNTPD could be related to extended π systems, which can anchor particles, strongly modifying platinum d-band properties,18, 35 therefore, even though the Pt-sp2 interaction is much weaker than the Pt-sp3, it is possible to appreciate a coordinating bonding between platinum d-orbital and the π−π carbon network, namely sp2-d hybridization.10 Previous reports,10, 17-18 have demonstrated that the d-band density of states (DOS) of atoms with higher electron density, resulting from the interaction



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between platinum clusters and graphite, has more states near the Ef in comparison to those of the interface Pt atoms having a deficit of electrons.

4. CONCLUSIONS In this work theoretical and experimental evidences demonstrate the consequences of the interaction strength of Pt nanoparticles with sp2 and sp3 domains toward the oxygen reduction reaction (ORR) in acid medium. Combined Raman spectroscopy, DFT, and CO stripping measurements reveal two different interactions between platinum nanoparticles and carbon sp2 or sp3 nano-domains. Additionally, photo-deposition allowed a selective heterogeneous nucleation of platinum clusters onto sp2 nano-domains, whereas the carbonyl chemical route leads to a random deposition of platinum clusters onto carbon nanodomains. The limited mobility of platinum nanoparticles supported onto in-plane extended sp2 nano-domains could be a consequence of the observed electronic density modification of platinum nanoparticles. The presence on the support of mostly sp2 domains augments the durability since re-deposition occurs on sp2 domains. In contrast, the presence of sp3 domains causes the re-deposition of Pt to occur on sp3 sites causing agglomeration. These results provide new avenues for enhancing the stability of platinum nanoparticles supported onto carbon-based materials via engineering their surface chemistry. ASSOCIATED CONTENT Supporting Information FTIR of carbon supports. ORR electrochemistry. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. 5. ACKOWLEDGEMENTS The authors thank the financial support from University of Poitiers. C.A. C.-R. acknowledges financial support from CONACYT-Mexico Nr.: MX-561206. G.R.S acknowledges partial financial support from Red Temática del Hidrógeno (México).

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Table of Contents

sp2 vs sp3 La = 52.47 nm

La = 21.54 nm

La=21.54 nm


Support

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