Article pubs.acs.org/Langmuir
Analysis of the Surface Oxidation Process on Pt Nanoparticles on a Glassy Carbon Electrode by Angle-Resolved, Grazing-Incidence X‑ray Photoelectron Spectroscopy Shota Miyashita,† Mitsuru Wakisaka,‡,§ Akihiro Iiyama,‡ and Hiroyuki Uchida*,‡,⊥ †
Special Doctoral Program for Green Energy Conversion Science and Technology, Interdisciplinary Graduate School of Medicine, Engineering and Agricultural Science, and ‡Fuel Cell Nanomaterials Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan ⊥ Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan S Supporting Information *
ABSTRACT: We have analyzed the surface oxidation process of Pt nanoparticles that were uniformly dispersed on a glassy carbon electrode (Pt/GC), which was adopted as a model of a practical Pt/C catalyst for fuel cells, in N2-purged 0.1 M HF solution by using angle-resolved, grazing-incidence X-ray photoelectron spectroscopy combined with an electrochemical cell (EC-ARGIXPS). Positive shifts in the binding energies of Pt 4f spectra were clearly observed for the surface oxidation of Pt nanoparticles at potentials E > 0.7 V vs RHE, followed by a bulk oxidation of Pt to form Pt(II) at E > 1.1 V. Three types of oxygen species (H2Oad, OHad, and Oad) were identified in the O 1s spectra. It was found for the first time that the surface oxidation process of the Pt/GC electrode at E < ca. 0.8 V (OHad formation) is similar to that of a Pt(111) single-crystal electrode, whereas that in the high potential region (Oad formation) resembles that of a Pt(110) surface or polycrystalline Pt film.
1. INTRODUCTION By virtue of an intensive research and development effort, polymer electrolyte fuel cells (PEFCs) have been commercialized in residential cogeneration systems and fuel cell vehicles in Japan. However, for larger scale commercialization in the global market, the reduction of the system cost, while maintaining the performance and durability, is essential. The development of highly active, durable cathode catalysts for the oxygen reduction reaction (ORR), is one of the most important subjects. Specifically, it is essential to investigate the oxygen species adsorbed on Pt-based electrodes during both the surface oxidation reaction (in inert atmosphere) and the ORR (in the presence of dissolved O2) as a function of electrode potential. Recently, we have for the first time clarified and quantified the oxygen species adsorbed on polycrystalline Pt and Pt skin/Pt-M (M = Co and Fe) alloy surfaces in N2-purged and O2-saturated 0.1 M HF solutions by using X-ray photoelectron spectroscopy (XPS) combined with an electrochemical cell (EC-XPS).1,2 During the ORR, the coverage of atomic oxygen (Oad) was found to increase greatly by alloying of Pt with Co or Fe at Pt skin/Pt-M alloys, and the incremental increase was correlated with the enhancement of the ORR activity. The surface oxidation states of Pt(111), Pt(100), and Pt(110) singlecrystals in N2-purged 0.1 M HF solution were also analyzed by use of EC-XPS, and the structural effects on the surface oxidation processes were clarified on the basis of the coverage © XXXX American Chemical Society
of oxygen species (Oad, OHad, and H2Oad) as a function of the electrode potential.3,4 In PEFCs, Pt or Pt-alloy nanoparticles highly dispersed on carbon black supports (Pt/C or Pt-alloy/C) have been commonly used as the cathode catalysts. The electrooxidation processes of Pt-based nanoparticles have been examined by electrochemical methods,5 X-ray absorption fine structure (XAFS),6−13 X-ray diffraction (XRD),14 theoretical calculations15 and XPS.16−21 For example, in situ or operando XAFS is very useful to analyze the coordination numbers and distances of Pt−Pt and Pt−O bonds of Pt-based nanoparticles, specifically in the high potential region (>0.8 V vs RHE).6−13 However, it is usually difficult for XAFS to distinguish and quantify the oxygen species (Oad and OHad) in a practically important potential range for the ORR (0.5 to 1.0 V vs RHE) due to insufficient energy resolution. Conventional XPS, in spite of its high characteristic energy resolution, cannot be applied for the analysis of the electronic states and oxygen species (Oad, OHad, and H2Oad) adsorbed on, for example, Pt/ C practical catalysts, because the photoelectron signals from Pt nanoparticles dispersed on carbon black supports are too severely influenced by inelastic scattered to be properly Received: April 27, 2017 Revised: August 3, 2017
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DOI: 10.1021/acs.langmuir.7b01446 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. SEM image (left panel) and histogram of particle size distribution (right panel) for Pt/GC model electrode.
deconvoluted.22 In the present research, we have adopted an angle-resolved, grazing incidence XPS (ARGIXPS) technique combined with an electrochemical cell. The ARGIXPS technique, in which the incident angle of X-rays and detection angle of photoelectrons can be adjusted, provides more surfacesensitive information, with smaller signals from background (substrate), than conventional XPS. We have, for the first time, demonstrated the electronic structure of Pt and adsorbed oxygen species during the oxidation process of Pt nanoparticles highly dispersed on a glassy carbon electrode (Pt/GC), which was adopted as a model for practical Pt/C catalysts.
and IO are the normalized intensity of O 1s and measured where Inorm O intensity, respectively. The details of the normalization of the intensity are explained in the Supporting Information.
3. RESULTS AND DISCUSSION 3.1. Characterization of Pt/GC Model Electrode. Figure 1 shows an SEM image of a Pt/GC model electrode, together with the size distribution histogram of the Pt particles. The surface of the GC substrate was polished to a mirror finish but exhibited roughness on the order of tens of nanometers, so that grain boundaries were clearly observed (see SEM of bare GC surface in Figure S1). Pt nanoparticles ranging from ca. 2 to 7 nm were uniformly dispersed on the GC surface, without specific deposition on the grain boundaries. The mean particle size of the Pt particles was 3.0 nm, with a standard deviation of 0.9 nm. The Pt/GC model electrode was electrochemically stabilized by repeating potential cycles between 0.05 and 1.3 V in N2purged 0.1 M HF solution. Figure 2 shows a cyclic voltammogram (CV) of the Pt/GC model electrode after the stabilization.
2. EXPERIMENTAL SECTION Pt nanoparticles prepared by a colloidal method23 were dispersed on a GC substrate (diameter 10 mm, BAS Inc.). After loading the Pt nanoparticles, the Pt/GC electrode was heat-treated at 100 °C in an H2 atmosphere for 15 min and then cooled to room temperature under an Ar atmosphere. A bare GC electrode (Pt-free) was also treated in the same manner as that for the Pt/GC electrode. The morphology of the Pt nanoparticles was observed by scanning electron microscopy (SEM, S-5200, Hitachi High Technologies Co.). All of the EC-ARGIXPS measurements were performed in an ultrahigh vacuum (UHV, base pressure = 2 × 10−8 Pa) chamber combined with an electrochemical (EC) chamber. A hemispherical electron spectrometer (Model 10−360, ULVAC-PHI), an X-ray source (Model 10−360, ULVAC-PHI), and a toroidal monochromator (Model 10−360, ULVAC-PHI) were attached to the UHV chamber. All electrode potentials are referenced to a reversible hydrogen electrode (RHE), which was placed outside the chamber and connected to the electrochemical cell with a Teflon tube. A platinum mesh was used as the counter electrode (CE). The EC-ARGIXPS measurements were performed as follows. First, the working electrode (Pt/GC or GC) was polarized at a given potential between 0.1 and 1.3 V for 5 min in N2-purged 0.1 M HF solution. Then, the working electrode was emersed from the electrolyte solution under potential control, and the EC chamber was rapidly evacuated to ca. 2 × 10−5 Pa. The electrode was quickly transferred to the UHV chamber. The XP spectrum of the electrode was measured within 5 min after the emersion. All of the XP spectra were obtained with monochromatic AlKα radiation (hν = 1486.58 eV) at 350 W. The incident angle of the X-rays was between 1 and 2°, while the detection angles of the photoelectrons were adjusted to 75° and 0°. Binding energies were calibrated by the use of Ag 3d5/2 (368.26 eV), Pt 4f7/2 (71.2 eV) and the Fermi energy EF of Pt (0 eV). The pass energy and resolution of the electron spectrometer were 23.50 and 0.66 eV (fwhm of Ag 3d5/2), respectively. To evaluate changes in the amount of oxygen species adsorbed on Pt surface, the intensities of the O 1s spectra were normalized with respect to the integrated intensity of the Pt 4f peaks, IPt:
IOnorm =
IO IPt
Figure 2. Cyclic voltammogram of Pt/GC model electrode in N2purged 0.1 M HF solution at 50 mV s−1.
The CV shows the features of a typical clean surface of polycrystalline Pt. Then, the amount of Pt loaded MPt on the GC was calculated from the following equation: MPt =
ΔQ H ΔQ H°ECAS
(2)
where ΔQH, ECA, and S are the hydrogen adsorption charge in the potential region from 0.05 to 0.40 V, the electrochemically active area per unit mass of Pt, and the geometric area of the GC substrate, respectively. The value of ECA was calculated from the mean particle size of the spherical Pt particles (3 nm). Assuming ΔQH° = 210 μC cm−2 for a smooth Pt polycrystal,24 the value of MPt calculated was 546 ng cm−2. We also estimated the value of MPt separately as follows. Based on the number of
(1) B
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with 0° (conventional angle). Thus, we were able to obtain more surface-sensitive signals with the detection angle of the photoelectrons at 75°, with the X-ray incident angle between 1 and 2°. Very recently, by use of the ambient-pressure XPS (APXPS) technique with hard X-rays (AP-HAXPES), Takagi et al. have examined the oxidation reactions of Pt/C in a membraneelectrode assembly (MEA).21 They observed peaks at ca. 74.2 (shoulder) and 77.55 eV at a high potential (1.4 V) and assigned them to 4f7/2 and 4f5/2, respectively, corresponding to Pt(II), although they mainly discussed the surface oxidation process in terms of the Pt 3d region, because of the weak intensities of these 4f peaks. 3.3. Changes in O 1s Spectra. Figure 4 shows O 1s spectra for the Pt/GC model electrode after holding E at 0.4 to
Pt particles per unit area (NPt) in the SEM image (Figure 1), MPt, SEM was calculated as the product of the density (21.37 g cm−3) and the total volume of Pt particles (NPt × VPt, 3 nm), assuming a spherical shape for the particles with a mean diameter of 3 nm. The value of MPt, SEM thus calculated (543 ng cm−2) was nearly identical with that determined based on ΔQH. It was very difficult to identify the crystal phase of the Pt nanoparticles on the GC substrate with such a small amount of Pt. However, by use of high-resolution TEM, we have characterized samples that were similarly prepared but supported on high-area materials. In those cases, we observed clear lattice fringes corresponding to the distance between Pt(111) planes of the face-centered cubic (fcc) structure for Pt nanoparticles, which were prepared by the same colloidal method as that employed in the present work and supported on Sn0.96Sb0.04O2−δ,25 TiN,26 and TiC.27 Hence, the crystal phase of Pt nanoparticles dispersed on the GC substrate in the present work is highly likely to also be the fcc structure. 3.2. Change in Pt 4f Spectra for Pt/GC Model Electrode. Figure 3 shows typical Pt 4f spectra for the Pt/
Figure 4. Area-normalized O 1s spectra for Pt/GC model electrode after emersion from N2-purged 0.1 M HF solution. The detection angle of photoelectrons was 75°, with the X-ray incident angle between 1 and 2°. The values of binding energy for H2Oad, OHad, and Oad, cited from ref 1, are shown.
Figure 3. Area-normalized Pt 4f spectra for Pt/GC model electrode after emersion from N2-purged 0.1 M HF solution. The detection angle of photoelectrons was 75°, with the X-ray incident angle between 1 and 2°.
1.0 V in N2-purged 0.1 M HF solution. Each O 1s spectrum was normalized with respect to the integrated area of Pt 4f, as described in the Experimental Section. The intensity of the spectrum in the region from 531 to 529 eV increased with increasing electrode potential, suggesting the increase in oxygen species adsorbed on the Pt surface due to the surface oxidation, because similar O 1s components were observed for bulk polycrystalline Pt and Pt(111) single crystal.1,2 In contrast, a broad peak from 532 to 534 eV was observed at all potentials and shifted to lower binding energy with increasing potential. To clarify the assignment of such O 1s spectrum, we examined the bare GC electrode without any Pt nanoparticles by EC-ARGIXPS. All O 1s spectra were normalized in area with respect to the integrated area of C 1s. Comparing the potential dependence of the O 1s spectrum for the Pt-free GC electrode (Figure 5) with that of the Pt/GC electrode (Figure 4), the broad peak from 532 to 534 eV can be mainly assigned to functional groups on the carbon, such as quinone, carboxyl, and carbonyl.29−31 For the detection of the O 1s spectrum from the Pt particles, the use of the detection angle of 75° was quite beneficial. As shown in Figure S3, the intensity of the spectrum in the region from 531 to 529 eV (assigned to oxygen species adsorbed on Pt) increased with the detection angle of 75° compared with that at 0°. To separate the O 1s signals originating from oxygen species adsorbed on the Pt nanoparticles, a difference spectrum was obtained by subtracting the O 1s spectrum of the GC electrode (without any Pt) from that of the Pt/GC model electrode at each electrode potential. A typical procedure is shown in Figure 6, in which the subtraction was conducted so that the signal
GC model electrode after holding at various potentials E in N2purged 0.1 M HF solution. All spectra were normalized with respect to the integrated area. All of the spectra obtained in the range E = 0.1 to 1.3 V are shown in Figure S2. We deconvoluted the spectrum obtained at E = 0.4 V into two peaks, i.e., the metallic Pt 4f7/2 (71.1 eV) and Pt 4f5/2 (74.6 eV), and calculated the ratio of the integrated area of the former to the that of the latter. The calculated value of 1.14 was somewhat smaller than the theoretical value (1.33);22 this discrepancy might be ascribed to an error of the deconvolution (including the baseline subtraction). The Pt 4f spectra gradually shifted to higher binding energy with increasing electrode potential. These apparent shifts can be explained by changes in the surface core level of Pt 4f due to adsorption of oxygen species formed by the surface oxidation. To our knowledge, this is the first report of the detection of distinct changes in the electronic structure of Pt nanoparticles on a carbon support induced by surface oxidation. At E > 1.1 V, new peaks appeared at 73.6 and 76.9 eV, and the intensity increased with increasing potential. These peaks were assigned to Pt(II), suggesting a bulk oxidation of Pt. This is consistent with a report that O atoms adsorbed on Pt surface undergo an interfacial place exchange process with Pt surface atoms in the potential range 1.1 to 1.2 V.28 It is worth noting that, in Figure S3, the peaks at 73.6 and 76.9 eV (assigned to Pt(II) described above) observed after holding E = 1.3 V, for example, were detected more clearly with the detection angle of 75° than that C
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Figure 7. Deconvolution of difference spectrum for Pt/GC model electrode at 0.9 V. Blue, yellow, and red lines indicate H2Oad, OHad, and Oad, respectively.
Figure 5. Area-normalized O 1s spectra for GC electrode after emersion from N2-purged 0.1 M HF solution. All O 1s spectra were normalized in area with respect to the integrated area of C 1s. The detection angle of photoelectrons was 75°, with the X-ray incident angle between 1 and 2°. The values of binding energy for CO, COOH, and C−OH, cited from ref 29, are shown.
and Oad, at 532.4, 531.1, 530.5, and 529.6 eV, respectively), in which H2Oad, 1 (531.1 eV) was assigned to the first-layer water molecule adsorbed directly on the Pt surface through an oxygen lone pair, and H2Oad, 2 was assigned to second-layer water. However, in the present work, the H2Oad peak at 532.4 eV was overlapped with those of functional groups of the carbon substrate, and thus we were not able to distinguish it. Hereinafter, the adsorbed water observed at 531.1 eV is simply referred to as H2Oad, which can be assigned to the first-layer water molecule. Figure 8d shows the integrated intensity of each oxygen species as a function of E for the Pt/GC model electrode. For comparison, the potential dependences of the coverages of each oxygen species for polycrystalline Pt film and Pt(111) singlecrystal electrodes, measured in our previous works,1,3 are shown in Figure 8e, f. On the Pt/GC model electrode, H2Oad was observed as the dominant species at E < 0.7 V and decreased with increasing E. The formation of OHad commenced at relatively low E (from ca. 0.4 V), and the intensity increased with E, accompanied by a decrease of H2Oad. Specifically, OHad increased steeply together with a sharp decrease in H2Oad from 0.75 V, at which point the anodic current increased appreciably in the linear sweep voltammogram, which is in good agreement with the results of Murthi et al. obtained with XAFS.6 Thus, the anodic current in this potential region is clearly ascribed to OHad formation via reaction 3:
Figure 6. (a) Area-normalized O 1s spectra for Pt/GC (black line) and GC (red line) electrodes after emersion from N2-purged 0.1 M HF solution and (b) obtained difference spectrum at 0.9 V.
intensity at 534 to 532 eV was as low as possible. The resulting difference spectrum shown in Figure 6b resembles that observed for Pt(111) emersed from N2-purged 0.1 M HF solution at 0.9 V.2 Nevertheless, the potential dependence of the O 1s spectrum for the GC electrode may not always be identical with that for the Pt/GC electrode, because it has been recognized that carbon oxidation is catalyzed by Pt particles.32,33 Hence, we should consider a possible error in the subsequent deconvolution process of the O 1s difference spectrum, specifically, the high binding energy region of 531 to 532 eV, which is assigned predominantly to water molecules adsorbed on Pt.1,2 We then deconvoluted the O 1s difference spectra into several asymmetric Lorentzian−Gaussian peaks with linear background by means of XPSPEAK version 4.1.1 Figure 7 shows a typical result of the deconvolution of the difference spectrum at 0.9. V The spectrum was deconvoluted into three oxygen species. The peaks at 531.1, 530.5, and 529.6 eV were assigned to adsorbed water (H2Oad), hydroxyl (OHad), and atomic oxygen (Oad), respectively on the Pt surface. In our previous work on bulk-Pt and Pt(111),1−4 we deconvoluted the O 1s spectrum into four components (H2Oad, 2, H2Oad, 1, OHad,
H 2Oad → OHad + H+ + e−
(3)
At E ≥ 0.7 V, Oad commenced to increase, suggesting the following surface oxidation reaction: OHad → Oad + H+ + e−
(4)
The onset potential of Oad formation (ca. 0.7 V) was more positive than that of OHad, and the intensity of OHad still increased even while Oad formation commenced. In contrast, in the case of the Pt(111) single-crystal electrode (Figure 8f), OHad decreased with the formation of Oad. Thus, it is clarified that reactions 3 and 4 occur in parallel on the Pt/GC electrode, whereas reaction 4 occurs in series with reaction 3 for the case of Pt(111). It was also found for the surface oxidation of Pt(110)4 (data not shown) that the formation of OHad commenced at 0.65 V, followed by Oad formation at 0.70 V. The coverage of OHad did not decrease and was larger than that of Oad up to 1.0 V. In the case of the polycrystalline Pt film, however, OHad and Oad were formed at nearly identical E, and the coverage of both species increased at similar rates with E (Figure 8e). Hence, the surface oxidation process of the Pt/GC electrode at E < ca. 0.8 V (OHad formation) is similar to that of D
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Figure 8. Comparison of Pt/GC, poly-Pt, and Pt(111) single-crystal electrodes in N2-purged 0.1 M HF solution: voltammograms (50 mV s−1) at (a) Pt/GC, (b) poly-Pt, and (c) Pt(111); integrated intensities of each oxygen species as a function of electrode potential at (d) Pt/GC. The potential dependence of the fractional coverage θ of each oxygen species (atomic ratio of oxygen in the species to Pt) at (e) poly-Pt1 and (f) Pt(111).3 Symbols: (○) Oad, (▲) OHad, and (□) H2Oad. O 1s spectra for these electrodes are shown in Figure S5.
the Pt(111) single-crystal electrode, whereas that in the high E region (Oad formation) shows nearly the same tendency as that of Pt(110) or polycrystalline Pt film. Recently, by use of APXPS, the electronic states of Pt and oxygen species adsorbed on Pt surfaces were analyzed only under open circuit conditions.17,19 Takagi et al. has reported the changes in the Pt 3d5/2 level resulting from the oxidation reaction of Pt/C in an MEA; they observed the formation of PtO and PtOH at a relatively high E (1.3 V).18 Hence, the present work is the first to clarify the surface oxidation process of Pt nanoparticles in the practical potential range for PEFCs.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hiroyuki Uchida: 0000-0001-6718-5431 Present Address §
M.W. is currently at Graduate School of Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama 9390398, Japan
4. CONCLUSIONS We have successfully analyzed the electronic states of Pt and the oxygen species adsorbed on Pt nanoparticles dispersed on a GC electrode in N2-purged 0.1 M HF solution as a function of potential by using EC-ARGIXPS. It was demonstrated that the use of grazing incidence X-ray excitation and a detection angle of resulting photoelectrons at 75° provided surface-sensitive information on both Pt 4f and O 1s from Pt nanoparticles more clearly than that for the case of conventional XPS. To separate the O 1s signals originating from oxygen species adsorbed on Pt nanoparticles, we obtained difference spectra by subtracting the O 1s spectrum of the GC electrode from that of the Pt/GC model electrode at each electrode potential. H2Oad, OHad, and Oad were identified in the difference spectra during the surface oxidation of the Pt nanoparticles. Based on the comparison of the potential dependence of the oxygen species for the Pt/GC electrode with those for Pt(111) and polycrystalline Pt film, we have for the first time clarified that the surface oxidation process for Pt/GC at E < ca. 0.8 V (OHad formation) is similar to that of the Pt(111) single-crystal electrode, whereas that at high E (Oad formation) resembles that of Pt(110) or polycrystalline Pt film.
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spectra for Pt/GC, comparison and normalization of O 1s spectra (PDF)
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the HiPer-FC and SPer-FC projects from the New Energy and Industrial Development Organization (NEDO) of Japan. The authors thank Prof. Donald A. Tryk (Fuel Cell Nanomaterials Center, University of Yamanashi) for his kind advice.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01446. SEM image of bare GC substrate, all of the areanormalized Pt 4f spectra for the Pt/GC electrode, O 1s E
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DOI: 10.1021/acs.langmuir.7b01446 Langmuir XXXX, XXX, XXX−XXX
