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Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis
Accepted Manuscript Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis Yong-Tae Kim, Md. Abdul Matin, Young-Uk Kwon PII: DOI: Reference:
S0008-6223(13)00921-4 http://dx.doi.org/10.1016/j.carbon.2013.09.067 CARBON 8410
To appear in:
Carbon
Received Date: Accepted Date:
30 July 2013 20 September 2013
Please cite this article as: Kim, Y-T., Abdul Matin, Md., Kwon, Y-U., Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis, Carbon (2013), doi: http:// dx.doi.org/10.1016/j.carbon.2013.09.067
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Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis Yong-Tae Kim a, Md. Abdul Matin b, Young-Uk Kwon a,b,* a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon
440-746, Korea. b
Department of Chemistry, BK-21 School of Chemical Materials Sciences, Sungkyunkwan
University, Suwon 440-746, Korea. *Corresponding author. Tel: +82-31-290-7070. E-mail:, [email protected]
We report a novel, nanostructured Pt-graphene (Pt-G) hybrid, which is composed of Pt nanowires (NWs) grown directly on intrinsic graphene by using a mesoporous silica thin film as a nanotemplate. The direct junction between the Pt NWs and graphene, as well as the defect- free nature of the graphene grown via chemical vapor deposition, enables the charge transfer from graphene to Pt in Pt-G. The Raman, ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS) data on Pt-G show clear evidence for the charge transfer from graphene to Pt. Through the interaction, the Fermi level of Pt is raised by 0.3 eV. With the electronic structure altered, Pt-G shows an increased tolerance to CO1
poisoning and, hence, enhanced methanol oxidation reaction (MOR) performance with increased If/I b ratio and cycle stability. The present data demonstrate a novel way to exploit the unusual characteristics of graphene for useful applications.
1. Introduction Applications of graphene have been at the center of recent materials research. Among the many practical applications of graphene, hybrids containing graphene have emerged as a promising class of materials [1-7]. The unique geometrical, chemical, electronic, and electrical properties of graphene have been utilized to endow novel and desirable properties to these hybrids for specific applications [8-10]. Recently, hybrids of graphene and noble metals or noble metal alloys have been investigated as electrocatalysts for the oxygen reduction reaction (ORR) or MOR of low-temperature fuel cells [11-17]. Such hybrids showed enhanced electrocatalytic properties compared to those of noble metals alone. The effects of graphene have been attributed to its high electrical conductivity and large surface area, allowing for increased dispersion of noble metal nanoparticles (NPs) [11]. Electronic interaction is one of the more promising means by which graphene can enhance the electrocatalytic properties of noble metals. The prevailing Pt NP catalysts suffer from CO-poisoning in the MOR, as well as sluggish kinetics and low stability in the ORR, which are major bottlenecks in the widespread use of low-temperature fuel cells [18-20]. The incorporation of a transition metal element (M) to Pt is among several approaches attempted to solve these problems. Through the electronic interaction with M, the Fermi level of Pt is raised and, consequently, the binding energy of Pt with CO or an oxygen species can be reduced [21-23]. Since the work function of graphene (~4.5 eV vs. vacuum level) is smaller than that of Pt (~5.2 eV), their electronic interaction can produce a similar effect [24].
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We note, however, that in all of the Pt-G hybrids reported for electrocatalysis, such an electronic interaction appears difficult. The presence of a surface-capping agent to stabilize Pt NPs, which is typically used in the fabrication of Pt-G hybrids in the literature [25-28], prevents direct electronic interaction between graphene and Pt. More seriously, the graphene used in the literature is, in fact, either graphene oxide (GO) or reduced graphene oxide (rGO), which differ from intrinsic graphene in many aspects, particularly in electronic structures [2931]. While GO and rGO have their own beneficial characteristics, their large numbers of functional groups and defects make them unsuitable to induce direct electronic interaction with Pt. Furthermore, the reported work function of rGO is 5.0 eV, very closer to that of Pt [32]. Clearly, the use of intrinsic graphene in the formation of Pt-G hybrid can reveal some many new phenomena that are not possible with rGO. In order to induce electronic interaction and achieve high-performance electrocatalysts from Pt-G hybrids, a method to directly deposit Pt on intrinsic graphene is needed. This, unfortunately, has been an insurmountable challenge, because the graphene basal plane is chemically inert. In this regard, we have previously demonstrated that the use of a nanoporous template formed on the graphene surface makes it possible to grow CdSe nanomaterials directly on the basal plane of graphene without any capping agent involved [33]. This technique can be applied to graphene grown via the chemical vapor deposition (CVD) method, which is free from defects or functional groups. In the present paper, we report the fabrication of a nanostructured Pt thin film grown directly on CVD-graphene. Our analysis data show that the electronic structure of Pt is changed through the interaction with graphene. As a result, the Pt-G hybrid shows significantly enhanced electrocatalytic properties for MOR, with increased tolerance to COpoisoning and remarkably enhanced stability.
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2. Experime ntal 2.1. Preparation of Graphene The graphene films were synthesized via the CVD method on Cu foils, according to the method reported in Ref. 33. A Cu foil of 25 m thickness was placed in a quartz tube under an Ar atmosphere. The temperature was raised to 1000C with a H2 flow (10 sccm), and a reaction gas mixture of CH4 (15 sccm) and H2 (10 sccm) was flown. After the growth, the temperature was lowered at the rate of ca. 10 C sec-1 . The graphene on the back side was removed using O2 plasma, after protecting the front side with a polymethylmethacrylate (PMMA) coating. Graphene film was transferred on an indium tin oxide (ITO) substrate after the Cu foil was etched in an ammonium persulfate solution. 2.2. Synthesis of mesoporous silica thin film The precursor solution of a mesoporous silica thin film (MSTF, Ref. 33), was prepared by dissolving tetraethoxyorthosilicate (TEOS, 99.999%) and a Pluronic triblock copolymer (F127) in a mixed solution of diluted aqueous HCl and absolute ethanol to make the composition TEOS : F-127 : HCl : H2 O : EtOH = 1 : 6.60 x 10-3 : 6.66 x 10-3 : 4.62 : 22.6 (molar ratio). The solution was stirred at 20C for 18 h under a controlled relative humidity of below 20% before use. The solution was spun-coat on graphene/ITO substrates, aged at 80C, and calcined at 400C to form a MSTF template. This template consists of regularlyordered pores in hexagonal symmetry, from the top view. The pore size is 8 nm, and the wall thickness is 4 nm. 2.3. Electrochemical synthesis of Pt NWs on graphene A corner of the MSTF film on the graphene/ITO was etched out to reveal the graphene surface, allowing electrical contact with a wire for the following electrochemical deposition. Using MSTF-coated graphene as the working electrode and an aqueous electrolyte solution 4
of a Pt compound (10 mM H2 PtCl6 ), Pt was deposited into the pores of MSTF. A strongly reducing pulse of -1.5 V was applied for the first 1 sec; the potential was then reduced to 0.1 V for 20 sec at room temperature (RT). After the deposition, the MSTF template was removed with a diluted HF solution to reveal the nanostructure of Pt on the graphene surface. 2.4. Electrochemical measurements The electrocatalytic activity was evaluated using a potentiostat (Ivium compactstat, Ivium technology) and a standard three-electrode electrochemical cell, which was equipped with a working electrode, a Ag/AgCl (3M KCl) reference electrode, and a Pt- net counter electrode. Cyclic voltammograms (CVs) were measured in 0.1 M HClO 4 at a scan rate of 50 mV s-1 under a N 2 -envrionment. The electrocatalytic activity for MOR was investigated using CVs in an electrolyte of 0.1 M HClO 4, containing 0.5 M methanol, at a scan rate of 50 mV s-1 . In order to determine the stability of the sample in the electrochemical environment, the CV was run for 100 potential cycles under a N 2 -environment for 3 times in the same condition as the MOR measurements. The CVs of CO-stripping were performed to electro-oxidize the adsorbed CO at a scan rate of 50 mV s-1 after CO gas (99.5%) was bubbled into the electrolyte for 25 min. All electrochemical measurements (CVs, stability test, and CO-stripping) were performed at RT, and a fresh electrolyte was used for every electrochemical measurement. The reported potentials refer to the reversible hydrogen electrode (RHE), and the current densities are normalized by the electrochemical surface areas (ECSAs) calculated from the CVs. 2.5. Characterization Field emission scanning electron microscopy (FESEM), energy dispersive spectrometer (EDS, JEOL JSM-7100F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-3010) were used to characterize the samples. Raman spectra were recorded with 5
a Renishaw RM-1000 Invia, using a 514 nm excitation and a notch filter of 50 cm-1 . The spectral resolution was 0.2 cm-1 . XPS data were recorded on a Concentric Hemispherical Analyzer, and the binding energies are, with respect to C, 1s at 284.6 eV. The work function data of graphene and Pt-G were obtained from a UPS (AXIS NOVA Kratos Inc.). The UPS measurement utilized a photon source of He I (21.2 eV).
3. Results and Discussion The synthesis procedure of the Pt-G hybrid is illustrated in Fig. 1. We used graphene synthesized via CVD on a Cu foil, which is known to produce primarily monolayer (> 98 %) graphene [34]. A 2×2 cm2 graphene sheet was transferred to an ITO substrate (2.5×2 cm2 ) following the reported method. An MSTF with uniform pores of 8 nm in diameter is formed on the graphene by spin-casting a precursor solution, followed by aging and calcination [31]. Pt was deposited into the pores of the MSTF template via electrochemical deposition in a H2 PtCl6 electrolyte solution. After the deposition, the template was removed with a dilute HF solution to reveal the nanostructure of Pt. In fact, the pore morphology of the MSTF template is more complicated than shown in Fig. 1b. Detailed descriptions of the pore morphology of the MSTF template and the morphology of the nanostructured Pt are given in Supporting Information. Briefly, the MSTF used in the present study is constituted of two different types of pores, one with the vertical channels as shown in the Fig. 1b and t he other with less ordered channels. The vertically standing Pt NWs in Fig. 1c are grown in the region of the vertical channels. The less ordered channels of the MSTF template produce nanostructured Pt with a different morphology (see below). The Pt-G hybrid so synthesized will hereafter be denoted as Pt-G. When Pt-G is used as an electrode, it will be denoted as Pt-G/ITO, since the contact with ITO may influence the
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electrode properties. For comparison, we also prepared a nanostructure Pt film directly grown on ITO (Pt/ITO), using the same template method. Fig. 2a shows a FESEM image of Pt-G. Bundles of Pt NWs of 100-250 nm in length are seen. The inset in the left of Fig. 2a shows a magnified image of a bundle, in which one can see that this bundle is composed of Pt NWs. In the inset in the right of Fig. 2a, we show a magnified image of a region between the bundles. This image also shows Pt NWs, but with a different morphology from that of the bundles. The MSTF template we used in the present synthesis is composed of two types pore structures occupying different regions. Apparently, the bundles are grown in the regions with vertical channels and the other type of Pt NW is grown in the regions with less ordered channels. Because of the different channel orientations, deposition of Pt in the vertical channel is faster than in the other type of channel, resulting in the different heights. These images clearly show that the graphene in the Pt-G is fully covered with nanostructured Pt. The bundles are likely to have formed due to the inhomogeneous nature of the nucleation and growth process of Pt NWs in pulse electrodeposition. Because the MSTF film is entirely free of cracks (Fig. S5), the morphology shown in Fig. 2a is homogeneously formed throughout the 2×2 cm2 graphene surface. Elemental distribution maps of C (of graphene) and Pt show even distributions of both elements (Fig. S6). In our previous work on the growth of Pt NPs using the MSTF template on a Pt-coated Si substrate, we demonstrated that Pt NPs were individually- grown, single crystals [35]. The small, 8 nm-sized pore of the template allowed a single nucleus per pore. With just one Pt nucleus, the Pt NP grown on such a nucleus would be single crystalline. Since the underlying principles are unchanged in the present system, we believe the same is true for the nanostructured Pt in Pt-G. Because the very small amount of Pt in the Pt-G hybrid prevents phase identification via conventional X-ray diffraction, we used HRTEM to investigate the crystallinity and 7
morphology of Pt in Pt-G. The HRTEM image in Fig. 2c is a top view of a Pt-G hybrid film before the removal of the MSTF template. Most of the pores on the template are filled with Pt; therefore, the diameter of the Pt NWs in Fig. 2a and 2b is predetermined to be 8 nm, the pore size of the template. The selected-area electron diffraction (SAED) pattern shows a ring pattern with d = 0.23, 0.19, and 0.14 nm which are indexed as the (111), (200), and (220) planes of the fcc structure of Pt (inset of Fig. 2c), respectively. The magnified HRTEM image on a dot in Fig. 2c is shown in Fig. 2d, displaying a Pt NW along its longitudinal direction. The lattice fringe pattern with a spacing of 0.23 nm can be identified as the (111) planes of Pt. Raman spectroscopy was used to investigate the state of graphene in Pt-G. For comparison, graphene without deposited Pt was also measured, as shown in Fig. 3. The spectra were taken of samples on ITO substrates. The D band, which normally appears at ~1320 cm-1 , is entirely absent in the spectra of graphene and Pt-G (Fig. 3a). This is an indication that our graphene is defect- free and also that the procedure to form Pt-G does not induce any defect formation of graphene [34]. The 2D band intensity of Pt-G hybrid is significantly reduced from that of graphene. According Casiraghi et al., the intensity ratio between the G and 2D band (I(G)/I(2D)) is related to the Fermi level position of graphene, as in the following equation: {I(G)/I(2D)} = C′(rep + 0.07EF), where C′ is a constant, rep is the emission of phonons, and EF is the Fermi level of graphene [36, 37]. Based on this relation, the Fermi level shifts from the Dirac point were calculated to be 0.17 eV for graphene and 0.42 eV for Pt-G. Upon deposition of Pt, the Fermi level of graphene is lowered by 0.25 eV, indicating a charge transfer from graphene to Pt in Pt-G. The positions and widths of the G and 2D bands of graphene are known to be sensitive indicators for the electron or hole-doping level of graphene (Fig. 3b and 3c). The G and 2D bands of our graphene occur at 1579 and 2682 cm-1 , with a full- width-at-half- maximum 8
(FWHM) of 19 and 33 cm-1 , respectively. These values agree well with those reported on high-quality monolayer graphene in the literature [34]. In the spectrum of Pt-G, the band positions are blue shifted to 1583 and 2684 cm-1 . The FWHM of the G band is slightly decreased, but the FWHM of the 2D band is greatly increased to 48 cm-1 , which is also a signature of graphene hole creation. Additional evidence of the charge transfer from graphene to Pt in Pt-G is obtained from the UPS, taken with He I (21.2 eV) as the excitation source (Fig. 4a). The work functions of graphene and Pt-G measured 4.5 and 4.8 eV, respectively. The difference of 0.3 eV agrees with the Fermi level shift of 0.25 eV estimated from the I(G)/I(2D) ratios of the Raman spectra. These shifts also agree with the reported Fermi level shifts (0.3 eV) of graphene in contact with Pt in the literature, experimentally on graphene grown on a Pt(111) substrate [38], and theoretically on graphene in contact with the Pt(111) surface [39]. In these studies, the reported Fermi level shifts are obtained from graphene in full contact with Pt. The same magnitude of Fermi level shift in our sample indicates that the graphene in our Pt-G is fully covered with Pt NWs. In Ref. 39, the authors report that the magnitude of Fermi level shift is a function of the distance between graphene and the Pt atoms. The shift of 0.3 eV corresponds to the distance of dC-Pt = ~0.33 nm, which implies a weak chemical interaction between graphene and the Pt NWs in our Pt-G. In order to probe the effect of the electronic interaction between the Pt NWs and graphene from the Pt-side, we studied the XPS on the core-level of Pt. The XPS spectra of Pt/ITO and Pt-G/ITO are similar to each other, showing the 4f7/2 and 4f5/2 doublets of Pt(0), Pt(II) and Pt(IV) (Fig. 4b and 4c, respectively), typical for nanostructured Pt [16, 20, 40]. The O 1s XPS on Pt-G/ITO shows that the majority of oxygen atoms in this sample are in the forms of PtO and Pt(OH)4 (Fig. S7). Since the underlying graphene almost completely covered by Pt NWs a few hundred nanometers long, the O 1s XPS signals are primarily from the NW surfaces, 9
explaining the origins of these Pt(II) and Pt(IV) peaks. In the Pt XPS, the peaks of Pt-G/ITO are about 0.2 eV lower in binding energy than the corresponding peaks of Pt/ITO (Table S1), indicating that the Pt in Pt-G/ITO has more electrons than Pt in Pt/ITO, which is in agreement with the above discussed charge transfer from graphene to Pt in Pt-G [40]. Pt-G/ITO and Pt/ITO were compared for their electrochemical and electrocatalytic properties. Fig. 5a shows the CVs of Pt-G/ITO and Pt/ITO measured in a N 2 -saturation solution of 0.1 M HClO 4 , with a scan rate of 50 mV s-1 , at room temperature. Both electrodes show similar CVs, with hydrogen adsorption/desorption peaks at 0.00-0.32 V and oxide formation/decomposition at 0.55-1.5 V: characteristic features of Pt. The graphene in PtG/ITO appears to be electrochemically inactive, given that there is no other peak. The ECSAs were calculated to be 2.65 cm2 for Pt-G/ITO, and 2.41 cm2 for Pt/ITO from the integrated charges of the hydrogen desorption waves, with a reference value of 0.210 mC cm-2 [41]. On the contrary, the two electrodes show different CO-stripping curves (Fig. 5b). The COstripping data were measured in an electrolyte of 0.1 M HClO 4 , saturated with CO (99.5%) at a scan rate of 50 mV s-1 , at RT. The curves show a sharp rise and a precipitous fall of current in the cathodic (backward) scan, characteristic of bulk- like Pt because the electrodes are composed of semi-continuous, nanostructured Pt films. The ECSAs of Pt/ITO and Pt-G/ITO are calculated at 2.82 and 2.71 cm2 , respectively, with a reference value of 0.420 mC cm-2 [42], in good agreement with the ECSAs from the hydrogen desorption waves of the CVs. Compared with Pt/ITO, Pt-G/ITO shows a peak position that is lower by 50 mV. This indicates that the adsorption of CO on Pt-G/ITO is energetically less favorable than on Pt/ITO. The decreased binding energy of CO on the Pt-G/ITO electrode is due to the comparatively higher Fermi level of Pt than that of Pt/ITO. Through the electronic interaction with graphene, Pt is enriched with electrons, weakening its propensity to form bonds to CO and, hence, reducing the effect of CO-poisoning [17, 43]. 10
The MOR activity of Pt-G/ITO and Pt/ITO were measured in an electrolyte of 0.1 M HClO 4 , containing 0.5 M CH3 OH, at scan rate of 50 mV s-1 , at RT (Fig. 5c). The vertical axis in Fig. 5c is the current density, which is calculated by normalizing the current values of the ECSAs from the CO stripping data. The inset shows the CV of a graphene-on-ITO electrode under the same condition. The absence of any faradaic signal confirms the electrochemical inactivity of graphene alone. The onset potential of MOR on Pt-G/ITO commences at 0.295 V, 0.23 V lower than that on Pt/ITO. Common Pt NW electrocatalysts show problematic COpoisoning, the signature of which appears as a large peak in the range of ~0.8-0.85 V (vs. RHE) in the cathodic sweep, due to the oxidation of adsorbed CO [40, 44]. The poison tolerance factors, defined as If/Ib, where If and Ib are the forward and backward currents, are <1, 1.01, and 1.10 for typical Pt catalysts of 20 wt% Pt/C (E-TEK) [45], Pt NPs on Vulcan [19], and 40 wt% Pt/C (E-TEK) [46], respectively. The tolerance factor of Pt/ITO is 1.25. PtG/ITO shows an even larger value of 1.74. The oxidation of adsorbed CO on Pt-G/ITO at a peak potential of 0.73 V is relatively lower than that of Pt/ITO, at 0.78 V, indicating that the adsorbed CO on the surface of Pt-G/ITO is oxidized at relatively lower potentials than that of Pt/ITO. The bond between CO and Pt-G/ITO is weaker than that of Pt/ITO due to the charge transfer from graphene to Pt. All presented data clearly indicate that Pt-G/ITO has noticeably higher performance than Pt/ITO for MOR. This high performance can be ascribed to the change in electronic structure of Pt through the electronic interaction with graphene. The stability test on the Pt-G/ITO electrode was carried out by running potential cycles 100 times under the same condition used for the MOR curves. The peak-current density values are plotted against the cycle number in Fig. 5d. The current density increases during the first ~50 cycles and then stays almost constant. The initial rise in MOR current appears to be related to the diffusion of methanol into the space between the Pt NWs. In the idealized morphology of the nanostructured Pt films of Pt-G/ITO and Pt/ITO, the gap between the 11
NWs would be 4 nm, the wall thickness of the MSTF template. The diffusion of methanol to the Pt surfaces is expected to be a slow process. The number of cycles required to reach the maximum current density is reduced when the electrode is soaked in the electrolyte solution (Fig. S8). This observation corroborates with the slow diffusion into the gaps between the Pt NWs. After reaching the peak at 45-50 cycles, the MOR current decreases very slowly. Contributing to the decay is the depletion of methanol as the cycle number increases. We ran additional stability test cycles by transferring the electrode into a fresh electrolyte solution after every 100 cycles (Fig. 5d). The first set of data during the first 100 cycles is repeated without any detectable decay. In fact, the maximum current density of each 100 cycles was exactly the same, at 13.8 mA cm-2 . These observations indicate that Pt-G/ITO does not lose its electrocatalytic activity upon repeated MOR up to 300 cycles, and presumably, beyond. The morphology of the Pt film on Pt-G/ITO is unchanged following the stability test (Fig. S9), and no damage is detected on the graphene sheet. With these results, we can conclude that Pt-G/ITO is a stable MOR electrocatalyst [21].
4. Conclusions We have synthesized a novel Pt-G hybrid via an electrochemical method through the use of MSTF as a nanoporous template. Our Pt-G is characterized by (1) the use of intrinsic graphene, free from defects or functional groups and (2) the direct fabrication of a nanostructured Pt film on the graphene without any intervening capping agents. As a result, our Pt-G allows electronic interaction between the graphene and Pt. The Raman, UPS, and XPS data on Pt-G show clear evidences for the charge transfer from graphene to Pt. Through the interaction, the Fermi level of Pt is raised by 0.3 eV. With the electronic structure altered, Pt-G shows an increased tolerance to CO-poisoning and, hence, enhanced MOR performance, with increased If/I b ratio and cycle stability. Our data clearly show that graphene can enhance 12
the electrocatalytic properties of Pt through the electronic interaction. The present data demonstrate a novel way to exploit the unusual characteristics of graphene for useful applications.
Figure captions. Fig. 1 Scheme of synthesis and electrodeposition processes for the Pt-G: (a) Graphene on ITO, formed by transferring a CVD graphene sheet onto an ITO substrate using a PMMA layer as a sample handle, (b) MSTF on the graphene on ITO, (c) nanostructured Pt on graphene (Pt-G) on ITO. The formation of MSTF in step I includes the spin-coating of a silica precursor solution, aging, and calcination at 400C. The formation of the nanostructure Pt film in step II includes the electrochemical deposition of Pt into the pores of the MSTF, and the removal of MSTF via a diluted HF solution. Note that this scheme is oversimplification for the pore structure of the MSTF template and the morphology of Pt. More details are in Supporting Information. Fig. 2 (a) A top-view SEM image of Pt-G with magnified views of the marked areas in the insets. (b) A top-view TEM image of Pt-G with MSTF template and (c) a magnified view on a Pt NW in (c). The inset in (c) shows a SAED pattern. Fig. 3 (a) Raman spectra of Pt-G and graphene, and enlarged views of the (b) G band and (c) 2D band regions of graphene. The spectra are on the corresponding samples on ITO substrates. Fig. 4 (a) UV-photoelectron spectroscopy data of graphene and Pt-G. XPS spectra of the Pt 4f core regions: (b) Pt/ ITO and (c) Pt-G/ITO. 13
Fig. 5 (a) CVs (45th cycle) of Pt-G/ITO and Pt/ITO, measured in an electrolyte of 0.1 M HClO 4 at a scan rate of 50 mV/s, (b) CVs of CO-stripping on Pt-G/ITO and Pt/ITO, measured in an electrolyte of 0.1 M HClO 4, saturated with CO for 25 min, and at a scan rate of 50 mV s-1 , (c) CVs of Pt-G/ITO measured in an electrolyte of 0.1 M HClO 4, containing 0.5 M methanol, and at a scan rate of 50 mV/s (Inset is the CV of G/ITO under the same condition), and (d) Results of the stability test performed by cycling potential in the range of 0.0 to 1.15 V (vs. RHE), with a scan rate of 50 mV s-1 , in an electrolyte of 0.1 M HClO 4 , containing 0.5 M CH3 OH, at room temperature. After every 100 cycles, the electrolyte was replaced with a fresh one.
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Figure
Fig. 1 Scheme of synthesis and electrodeposition processes for the Pt-G: (a) Graphene on ITO, formed by transferring a CVD graphene sheet onto an ITO substrate using a PMMA layer as a sample handle, (b) MSTF on the graphene on ITO, (c) nanostructured Pt on graphene (Pt-G) on ITO. The formation of MSTF in step I includes the spin-coating of a silica precursor solution, aging, and calcination at 400C. The formation of the nanostructure Pt film in step II includes the electrochemical deposition of Pt into the pores of the MSTF, and the removal of MSTF via a diluted HF solution. Note that this scheme is oversimplification for the pore structure of the MSTF template and the morphology of Pt. More details are in Supporting Information.
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Fig. 2 (a) A top-view SEM image of Pt-G with magnified views of the marked areas in the insets. (b) A top-view TEM image of Pt-G with MSTF template and (c) a magnified view on a Pt NW in (c). The inset in (c) shows a SAED pattern.
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Fig. 3 (a) Raman spectra of Pt-G and graphene, and enlarged views of the (b) G band and (c) 2D band regions of graphene. The spectra are on the corresponding samples on ITO substrates.
Fig. 4 (a) UV-photoelectron spectroscopy data of graphene and Pt-G. XPS spectra of the Pt 4f core regions: (b) Pt/ ITO and (c) Pt-G/ITO.
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Fig. 5 (a) CVs (45th cycle) of Pt-G/ITO and Pt/ITO, measured in an electrolyte of 0.1 M HClO 4 at a scan rate of 50 mV/s, (b) CVs of CO-stripping on Pt-G/ITO and Pt/ITO, measured in an electrolyte of 0.1 M HClO 4, saturated with CO for 25 min, and at a scan rate of 50 mV s-1 , (c) CVs of Pt-G/ITO measured in an electrolyte of 0.1 M HClO 4, containing 0.5 M methanol, and at a scan rate of 50 mV/s (Inset is the CV of G/ITO under the same condition), and (d) Results of the stability test performed by cycling potential in the range of 0.0 to 1.15 V (vs. RHE), with a scan rate of 50 mV s-1 , in an electrolyte of 0.1 M HClO 4 , containing 0.5
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M CH3 OH, at room temperature. After every 100 cycles, the electrolyte was replaced with a fresh one. ASSOCIATED CONTENT Supporting Information. Photograph of MSTF-G/ITO sample, SEM and EDS images of the Pt-G, O 1s XPS spectrum of Pt-G/ITO, the effect of the soaking time of the Pt-G/ITO electrode in the electrolyte solution (0.1 M HClO 4 containing 0.5 M methanol), SEM images of the Pt-G/ITO (a) before and (b) after stability test and summary of Pt 4f XPS data. AUTHOR INFORMATION Corresponding Author *Department of Chemistry, HRD Center for Creative Convergence Chemical Sciences, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Korea. Fax: +82 31 299 4179; Tel: +82 31 290 7070; E- mail: [email protected] Funding Sources This work was supported by grants NRF-20090081018 (Basic Science Research Program), NRF-2011-0031392 (Priority Research Center Program), NRF-2011-0006268 (Basic Science Research Program). Acknowledge ments We thank CCRF for the TEM, SEM, and EDS data, CNNC for the Raman data, RIC for the XPS data and PAL for the UPS data.
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Title: Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis Yong-Tae Kim, Md. Abdul Matin, and Young-Uk Kwon* ToC figure
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S0008-6223(13)00921-4 http://dx.doi.org/10.1016/j.carbon.2013.09.067 CARBON 8410
To appear in:
Carbon
Received Date: Accepted Date:
30 July 2013 20 September 2013
Please cite this article as: Kim, Y-T., Abdul Matin, Md., Kwon, Y-U., Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis, Carbon (2013), doi: http:// dx.doi.org/10.1016/j.carbon.2013.09.067
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Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis Yong-Tae Kim a, Md. Abdul Matin b, Young-Uk Kwon a,b,* a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon
440-746, Korea. b
Department of Chemistry, BK-21 School of Chemical Materials Sciences, Sungkyunkwan
University, Suwon 440-746, Korea. *Corresponding author. Tel: +82-31-290-7070. E-mail:, [email protected]
We report a novel, nanostructured Pt-graphene (Pt-G) hybrid, which is composed of Pt nanowires (NWs) grown directly on intrinsic graphene by using a mesoporous silica thin film as a nanotemplate. The direct junction between the Pt NWs and graphene, as well as the defect- free nature of the graphene grown via chemical vapor deposition, enables the charge transfer from graphene to Pt in Pt-G. The Raman, ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS) data on Pt-G show clear evidence for the charge transfer from graphene to Pt. Through the interaction, the Fermi level of Pt is raised by 0.3 eV. With the electronic structure altered, Pt-G shows an increased tolerance to CO1
poisoning and, hence, enhanced methanol oxidation reaction (MOR) performance with increased If/I b ratio and cycle stability. The present data demonstrate a novel way to exploit the unusual characteristics of graphene for useful applications.
1. Introduction Applications of graphene have been at the center of recent materials research. Among the many practical applications of graphene, hybrids containing graphene have emerged as a promising class of materials [1-7]. The unique geometrical, chemical, electronic, and electrical properties of graphene have been utilized to endow novel and desirable properties to these hybrids for specific applications [8-10]. Recently, hybrids of graphene and noble metals or noble metal alloys have been investigated as electrocatalysts for the oxygen reduction reaction (ORR) or MOR of low-temperature fuel cells [11-17]. Such hybrids showed enhanced electrocatalytic properties compared to those of noble metals alone. The effects of graphene have been attributed to its high electrical conductivity and large surface area, allowing for increased dispersion of noble metal nanoparticles (NPs) [11]. Electronic interaction is one of the more promising means by which graphene can enhance the electrocatalytic properties of noble metals. The prevailing Pt NP catalysts suffer from CO-poisoning in the MOR, as well as sluggish kinetics and low stability in the ORR, which are major bottlenecks in the widespread use of low-temperature fuel cells [18-20]. The incorporation of a transition metal element (M) to Pt is among several approaches attempted to solve these problems. Through the electronic interaction with M, the Fermi level of Pt is raised and, consequently, the binding energy of Pt with CO or an oxygen species can be reduced [21-23]. Since the work function of graphene (~4.5 eV vs. vacuum level) is smaller than that of Pt (~5.2 eV), their electronic interaction can produce a similar effect [24].
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We note, however, that in all of the Pt-G hybrids reported for electrocatalysis, such an electronic interaction appears difficult. The presence of a surface-capping agent to stabilize Pt NPs, which is typically used in the fabrication of Pt-G hybrids in the literature [25-28], prevents direct electronic interaction between graphene and Pt. More seriously, the graphene used in the literature is, in fact, either graphene oxide (GO) or reduced graphene oxide (rGO), which differ from intrinsic graphene in many aspects, particularly in electronic structures [2931]. While GO and rGO have their own beneficial characteristics, their large numbers of functional groups and defects make them unsuitable to induce direct electronic interaction with Pt. Furthermore, the reported work function of rGO is 5.0 eV, very closer to that of Pt [32]. Clearly, the use of intrinsic graphene in the formation of Pt-G hybrid can reveal some many new phenomena that are not possible with rGO. In order to induce electronic interaction and achieve high-performance electrocatalysts from Pt-G hybrids, a method to directly deposit Pt on intrinsic graphene is needed. This, unfortunately, has been an insurmountable challenge, because the graphene basal plane is chemically inert. In this regard, we have previously demonstrated that the use of a nanoporous template formed on the graphene surface makes it possible to grow CdSe nanomaterials directly on the basal plane of graphene without any capping agent involved [33]. This technique can be applied to graphene grown via the chemical vapor deposition (CVD) method, which is free from defects or functional groups. In the present paper, we report the fabrication of a nanostructured Pt thin film grown directly on CVD-graphene. Our analysis data show that the electronic structure of Pt is changed through the interaction with graphene. As a result, the Pt-G hybrid shows significantly enhanced electrocatalytic properties for MOR, with increased tolerance to COpoisoning and remarkably enhanced stability.
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2. Experime ntal 2.1. Preparation of Graphene The graphene films were synthesized via the CVD method on Cu foils, according to the method reported in Ref. 33. A Cu foil of 25 m thickness was placed in a quartz tube under an Ar atmosphere. The temperature was raised to 1000C with a H2 flow (10 sccm), and a reaction gas mixture of CH4 (15 sccm) and H2 (10 sccm) was flown. After the growth, the temperature was lowered at the rate of ca. 10 C sec-1 . The graphene on the back side was removed using O2 plasma, after protecting the front side with a polymethylmethacrylate (PMMA) coating. Graphene film was transferred on an indium tin oxide (ITO) substrate after the Cu foil was etched in an ammonium persulfate solution. 2.2. Synthesis of mesoporous silica thin film The precursor solution of a mesoporous silica thin film (MSTF, Ref. 33), was prepared by dissolving tetraethoxyorthosilicate (TEOS, 99.999%) and a Pluronic triblock copolymer (F127) in a mixed solution of diluted aqueous HCl and absolute ethanol to make the composition TEOS : F-127 : HCl : H2 O : EtOH = 1 : 6.60 x 10-3 : 6.66 x 10-3 : 4.62 : 22.6 (molar ratio). The solution was stirred at 20C for 18 h under a controlled relative humidity of below 20% before use. The solution was spun-coat on graphene/ITO substrates, aged at 80C, and calcined at 400C to form a MSTF template. This template consists of regularlyordered pores in hexagonal symmetry, from the top view. The pore size is 8 nm, and the wall thickness is 4 nm. 2.3. Electrochemical synthesis of Pt NWs on graphene A corner of the MSTF film on the graphene/ITO was etched out to reveal the graphene surface, allowing electrical contact with a wire for the following electrochemical deposition. Using MSTF-coated graphene as the working electrode and an aqueous electrolyte solution 4
of a Pt compound (10 mM H2 PtCl6 ), Pt was deposited into the pores of MSTF. A strongly reducing pulse of -1.5 V was applied for the first 1 sec; the potential was then reduced to 0.1 V for 20 sec at room temperature (RT). After the deposition, the MSTF template was removed with a diluted HF solution to reveal the nanostructure of Pt on the graphene surface. 2.4. Electrochemical measurements The electrocatalytic activity was evaluated using a potentiostat (Ivium compactstat, Ivium technology) and a standard three-electrode electrochemical cell, which was equipped with a working electrode, a Ag/AgCl (3M KCl) reference electrode, and a Pt- net counter electrode. Cyclic voltammograms (CVs) were measured in 0.1 M HClO 4 at a scan rate of 50 mV s-1 under a N 2 -envrionment. The electrocatalytic activity for MOR was investigated using CVs in an electrolyte of 0.1 M HClO 4, containing 0.5 M methanol, at a scan rate of 50 mV s-1 . In order to determine the stability of the sample in the electrochemical environment, the CV was run for 100 potential cycles under a N 2 -environment for 3 times in the same condition as the MOR measurements. The CVs of CO-stripping were performed to electro-oxidize the adsorbed CO at a scan rate of 50 mV s-1 after CO gas (99.5%) was bubbled into the electrolyte for 25 min. All electrochemical measurements (CVs, stability test, and CO-stripping) were performed at RT, and a fresh electrolyte was used for every electrochemical measurement. The reported potentials refer to the reversible hydrogen electrode (RHE), and the current densities are normalized by the electrochemical surface areas (ECSAs) calculated from the CVs. 2.5. Characterization Field emission scanning electron microscopy (FESEM), energy dispersive spectrometer (EDS, JEOL JSM-7100F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-3010) were used to characterize the samples. Raman spectra were recorded with 5
a Renishaw RM-1000 Invia, using a 514 nm excitation and a notch filter of 50 cm-1 . The spectral resolution was 0.2 cm-1 . XPS data were recorded on a Concentric Hemispherical Analyzer, and the binding energies are, with respect to C, 1s at 284.6 eV. The work function data of graphene and Pt-G were obtained from a UPS (AXIS NOVA Kratos Inc.). The UPS measurement utilized a photon source of He I (21.2 eV).
3. Results and Discussion The synthesis procedure of the Pt-G hybrid is illustrated in Fig. 1. We used graphene synthesized via CVD on a Cu foil, which is known to produce primarily monolayer (> 98 %) graphene [34]. A 2×2 cm2 graphene sheet was transferred to an ITO substrate (2.5×2 cm2 ) following the reported method. An MSTF with uniform pores of 8 nm in diameter is formed on the graphene by spin-casting a precursor solution, followed by aging and calcination [31]. Pt was deposited into the pores of the MSTF template via electrochemical deposition in a H2 PtCl6 electrolyte solution. After the deposition, the template was removed with a dilute HF solution to reveal the nanostructure of Pt. In fact, the pore morphology of the MSTF template is more complicated than shown in Fig. 1b. Detailed descriptions of the pore morphology of the MSTF template and the morphology of the nanostructured Pt are given in Supporting Information. Briefly, the MSTF used in the present study is constituted of two different types of pores, one with the vertical channels as shown in the Fig. 1b and t he other with less ordered channels. The vertically standing Pt NWs in Fig. 1c are grown in the region of the vertical channels. The less ordered channels of the MSTF template produce nanostructured Pt with a different morphology (see below). The Pt-G hybrid so synthesized will hereafter be denoted as Pt-G. When Pt-G is used as an electrode, it will be denoted as Pt-G/ITO, since the contact with ITO may influence the
6
electrode properties. For comparison, we also prepared a nanostructure Pt film directly grown on ITO (Pt/ITO), using the same template method. Fig. 2a shows a FESEM image of Pt-G. Bundles of Pt NWs of 100-250 nm in length are seen. The inset in the left of Fig. 2a shows a magnified image of a bundle, in which one can see that this bundle is composed of Pt NWs. In the inset in the right of Fig. 2a, we show a magnified image of a region between the bundles. This image also shows Pt NWs, but with a different morphology from that of the bundles. The MSTF template we used in the present synthesis is composed of two types pore structures occupying different regions. Apparently, the bundles are grown in the regions with vertical channels and the other type of Pt NW is grown in the regions with less ordered channels. Because of the different channel orientations, deposition of Pt in the vertical channel is faster than in the other type of channel, resulting in the different heights. These images clearly show that the graphene in the Pt-G is fully covered with nanostructured Pt. The bundles are likely to have formed due to the inhomogeneous nature of the nucleation and growth process of Pt NWs in pulse electrodeposition. Because the MSTF film is entirely free of cracks (Fig. S5), the morphology shown in Fig. 2a is homogeneously formed throughout the 2×2 cm2 graphene surface. Elemental distribution maps of C (of graphene) and Pt show even distributions of both elements (Fig. S6). In our previous work on the growth of Pt NPs using the MSTF template on a Pt-coated Si substrate, we demonstrated that Pt NPs were individually- grown, single crystals [35]. The small, 8 nm-sized pore of the template allowed a single nucleus per pore. With just one Pt nucleus, the Pt NP grown on such a nucleus would be single crystalline. Since the underlying principles are unchanged in the present system, we believe the same is true for the nanostructured Pt in Pt-G. Because the very small amount of Pt in the Pt-G hybrid prevents phase identification via conventional X-ray diffraction, we used HRTEM to investigate the crystallinity and 7
morphology of Pt in Pt-G. The HRTEM image in Fig. 2c is a top view of a Pt-G hybrid film before the removal of the MSTF template. Most of the pores on the template are filled with Pt; therefore, the diameter of the Pt NWs in Fig. 2a and 2b is predetermined to be 8 nm, the pore size of the template. The selected-area electron diffraction (SAED) pattern shows a ring pattern with d = 0.23, 0.19, and 0.14 nm which are indexed as the (111), (200), and (220) planes of the fcc structure of Pt (inset of Fig. 2c), respectively. The magnified HRTEM image on a dot in Fig. 2c is shown in Fig. 2d, displaying a Pt NW along its longitudinal direction. The lattice fringe pattern with a spacing of 0.23 nm can be identified as the (111) planes of Pt. Raman spectroscopy was used to investigate the state of graphene in Pt-G. For comparison, graphene without deposited Pt was also measured, as shown in Fig. 3. The spectra were taken of samples on ITO substrates. The D band, which normally appears at ~1320 cm-1 , is entirely absent in the spectra of graphene and Pt-G (Fig. 3a). This is an indication that our graphene is defect- free and also that the procedure to form Pt-G does not induce any defect formation of graphene [34]. The 2D band intensity of Pt-G hybrid is significantly reduced from that of graphene. According Casiraghi et al., the intensity ratio between the G and 2D band (I(G)/I(2D)) is related to the Fermi level position of graphene, as in the following equation: {I(G)/I(2D)} = C′(rep + 0.07EF), where C′ is a constant, rep is the emission of phonons, and EF is the Fermi level of graphene [36, 37]. Based on this relation, the Fermi level shifts from the Dirac point were calculated to be 0.17 eV for graphene and 0.42 eV for Pt-G. Upon deposition of Pt, the Fermi level of graphene is lowered by 0.25 eV, indicating a charge transfer from graphene to Pt in Pt-G. The positions and widths of the G and 2D bands of graphene are known to be sensitive indicators for the electron or hole-doping level of graphene (Fig. 3b and 3c). The G and 2D bands of our graphene occur at 1579 and 2682 cm-1 , with a full- width-at-half- maximum 8
(FWHM) of 19 and 33 cm-1 , respectively. These values agree well with those reported on high-quality monolayer graphene in the literature [34]. In the spectrum of Pt-G, the band positions are blue shifted to 1583 and 2684 cm-1 . The FWHM of the G band is slightly decreased, but the FWHM of the 2D band is greatly increased to 48 cm-1 , which is also a signature of graphene hole creation. Additional evidence of the charge transfer from graphene to Pt in Pt-G is obtained from the UPS, taken with He I (21.2 eV) as the excitation source (Fig. 4a). The work functions of graphene and Pt-G measured 4.5 and 4.8 eV, respectively. The difference of 0.3 eV agrees with the Fermi level shift of 0.25 eV estimated from the I(G)/I(2D) ratios of the Raman spectra. These shifts also agree with the reported Fermi level shifts (0.3 eV) of graphene in contact with Pt in the literature, experimentally on graphene grown on a Pt(111) substrate [38], and theoretically on graphene in contact with the Pt(111) surface [39]. In these studies, the reported Fermi level shifts are obtained from graphene in full contact with Pt. The same magnitude of Fermi level shift in our sample indicates that the graphene in our Pt-G is fully covered with Pt NWs. In Ref. 39, the authors report that the magnitude of Fermi level shift is a function of the distance between graphene and the Pt atoms. The shift of 0.3 eV corresponds to the distance of dC-Pt = ~0.33 nm, which implies a weak chemical interaction between graphene and the Pt NWs in our Pt-G. In order to probe the effect of the electronic interaction between the Pt NWs and graphene from the Pt-side, we studied the XPS on the core-level of Pt. The XPS spectra of Pt/ITO and Pt-G/ITO are similar to each other, showing the 4f7/2 and 4f5/2 doublets of Pt(0), Pt(II) and Pt(IV) (Fig. 4b and 4c, respectively), typical for nanostructured Pt [16, 20, 40]. The O 1s XPS on Pt-G/ITO shows that the majority of oxygen atoms in this sample are in the forms of PtO and Pt(OH)4 (Fig. S7). Since the underlying graphene almost completely covered by Pt NWs a few hundred nanometers long, the O 1s XPS signals are primarily from the NW surfaces, 9
explaining the origins of these Pt(II) and Pt(IV) peaks. In the Pt XPS, the peaks of Pt-G/ITO are about 0.2 eV lower in binding energy than the corresponding peaks of Pt/ITO (Table S1), indicating that the Pt in Pt-G/ITO has more electrons than Pt in Pt/ITO, which is in agreement with the above discussed charge transfer from graphene to Pt in Pt-G [40]. Pt-G/ITO and Pt/ITO were compared for their electrochemical and electrocatalytic properties. Fig. 5a shows the CVs of Pt-G/ITO and Pt/ITO measured in a N 2 -saturation solution of 0.1 M HClO 4 , with a scan rate of 50 mV s-1 , at room temperature. Both electrodes show similar CVs, with hydrogen adsorption/desorption peaks at 0.00-0.32 V and oxide formation/decomposition at 0.55-1.5 V: characteristic features of Pt. The graphene in PtG/ITO appears to be electrochemically inactive, given that there is no other peak. The ECSAs were calculated to be 2.65 cm2 for Pt-G/ITO, and 2.41 cm2 for Pt/ITO from the integrated charges of the hydrogen desorption waves, with a reference value of 0.210 mC cm-2 [41]. On the contrary, the two electrodes show different CO-stripping curves (Fig. 5b). The COstripping data were measured in an electrolyte of 0.1 M HClO 4 , saturated with CO (99.5%) at a scan rate of 50 mV s-1 , at RT. The curves show a sharp rise and a precipitous fall of current in the cathodic (backward) scan, characteristic of bulk- like Pt because the electrodes are composed of semi-continuous, nanostructured Pt films. The ECSAs of Pt/ITO and Pt-G/ITO are calculated at 2.82 and 2.71 cm2 , respectively, with a reference value of 0.420 mC cm-2 [42], in good agreement with the ECSAs from the hydrogen desorption waves of the CVs. Compared with Pt/ITO, Pt-G/ITO shows a peak position that is lower by 50 mV. This indicates that the adsorption of CO on Pt-G/ITO is energetically less favorable than on Pt/ITO. The decreased binding energy of CO on the Pt-G/ITO electrode is due to the comparatively higher Fermi level of Pt than that of Pt/ITO. Through the electronic interaction with graphene, Pt is enriched with electrons, weakening its propensity to form bonds to CO and, hence, reducing the effect of CO-poisoning [17, 43]. 10
The MOR activity of Pt-G/ITO and Pt/ITO were measured in an electrolyte of 0.1 M HClO 4 , containing 0.5 M CH3 OH, at scan rate of 50 mV s-1 , at RT (Fig. 5c). The vertical axis in Fig. 5c is the current density, which is calculated by normalizing the current values of the ECSAs from the CO stripping data. The inset shows the CV of a graphene-on-ITO electrode under the same condition. The absence of any faradaic signal confirms the electrochemical inactivity of graphene alone. The onset potential of MOR on Pt-G/ITO commences at 0.295 V, 0.23 V lower than that on Pt/ITO. Common Pt NW electrocatalysts show problematic COpoisoning, the signature of which appears as a large peak in the range of ~0.8-0.85 V (vs. RHE) in the cathodic sweep, due to the oxidation of adsorbed CO [40, 44]. The poison tolerance factors, defined as If/Ib, where If and Ib are the forward and backward currents, are <1, 1.01, and 1.10 for typical Pt catalysts of 20 wt% Pt/C (E-TEK) [45], Pt NPs on Vulcan [19], and 40 wt% Pt/C (E-TEK) [46], respectively. The tolerance factor of Pt/ITO is 1.25. PtG/ITO shows an even larger value of 1.74. The oxidation of adsorbed CO on Pt-G/ITO at a peak potential of 0.73 V is relatively lower than that of Pt/ITO, at 0.78 V, indicating that the adsorbed CO on the surface of Pt-G/ITO is oxidized at relatively lower potentials than that of Pt/ITO. The bond between CO and Pt-G/ITO is weaker than that of Pt/ITO due to the charge transfer from graphene to Pt. All presented data clearly indicate that Pt-G/ITO has noticeably higher performance than Pt/ITO for MOR. This high performance can be ascribed to the change in electronic structure of Pt through the electronic interaction with graphene. The stability test on the Pt-G/ITO electrode was carried out by running potential cycles 100 times under the same condition used for the MOR curves. The peak-current density values are plotted against the cycle number in Fig. 5d. The current density increases during the first ~50 cycles and then stays almost constant. The initial rise in MOR current appears to be related to the diffusion of methanol into the space between the Pt NWs. In the idealized morphology of the nanostructured Pt films of Pt-G/ITO and Pt/ITO, the gap between the 11
NWs would be 4 nm, the wall thickness of the MSTF template. The diffusion of methanol to the Pt surfaces is expected to be a slow process. The number of cycles required to reach the maximum current density is reduced when the electrode is soaked in the electrolyte solution (Fig. S8). This observation corroborates with the slow diffusion into the gaps between the Pt NWs. After reaching the peak at 45-50 cycles, the MOR current decreases very slowly. Contributing to the decay is the depletion of methanol as the cycle number increases. We ran additional stability test cycles by transferring the electrode into a fresh electrolyte solution after every 100 cycles (Fig. 5d). The first set of data during the first 100 cycles is repeated without any detectable decay. In fact, the maximum current density of each 100 cycles was exactly the same, at 13.8 mA cm-2 . These observations indicate that Pt-G/ITO does not lose its electrocatalytic activity upon repeated MOR up to 300 cycles, and presumably, beyond. The morphology of the Pt film on Pt-G/ITO is unchanged following the stability test (Fig. S9), and no damage is detected on the graphene sheet. With these results, we can conclude that Pt-G/ITO is a stable MOR electrocatalyst [21].
4. Conclusions We have synthesized a novel Pt-G hybrid via an electrochemical method through the use of MSTF as a nanoporous template. Our Pt-G is characterized by (1) the use of intrinsic graphene, free from defects or functional groups and (2) the direct fabrication of a nanostructured Pt film on the graphene without any intervening capping agents. As a result, our Pt-G allows electronic interaction between the graphene and Pt. The Raman, UPS, and XPS data on Pt-G show clear evidences for the charge transfer from graphene to Pt. Through the interaction, the Fermi level of Pt is raised by 0.3 eV. With the electronic structure altered, Pt-G shows an increased tolerance to CO-poisoning and, hence, enhanced MOR performance, with increased If/I b ratio and cycle stability. Our data clearly show that graphene can enhance 12
the electrocatalytic properties of Pt through the electronic interaction. The present data demonstrate a novel way to exploit the unusual characteristics of graphene for useful applications.
Figure captions. Fig. 1 Scheme of synthesis and electrodeposition processes for the Pt-G: (a) Graphene on ITO, formed by transferring a CVD graphene sheet onto an ITO substrate using a PMMA layer as a sample handle, (b) MSTF on the graphene on ITO, (c) nanostructured Pt on graphene (Pt-G) on ITO. The formation of MSTF in step I includes the spin-coating of a silica precursor solution, aging, and calcination at 400C. The formation of the nanostructure Pt film in step II includes the electrochemical deposition of Pt into the pores of the MSTF, and the removal of MSTF via a diluted HF solution. Note that this scheme is oversimplification for the pore structure of the MSTF template and the morphology of Pt. More details are in Supporting Information. Fig. 2 (a) A top-view SEM image of Pt-G with magnified views of the marked areas in the insets. (b) A top-view TEM image of Pt-G with MSTF template and (c) a magnified view on a Pt NW in (c). The inset in (c) shows a SAED pattern. Fig. 3 (a) Raman spectra of Pt-G and graphene, and enlarged views of the (b) G band and (c) 2D band regions of graphene. The spectra are on the corresponding samples on ITO substrates. Fig. 4 (a) UV-photoelectron spectroscopy data of graphene and Pt-G. XPS spectra of the Pt 4f core regions: (b) Pt/ ITO and (c) Pt-G/ITO. 13
Fig. 5 (a) CVs (45th cycle) of Pt-G/ITO and Pt/ITO, measured in an electrolyte of 0.1 M HClO 4 at a scan rate of 50 mV/s, (b) CVs of CO-stripping on Pt-G/ITO and Pt/ITO, measured in an electrolyte of 0.1 M HClO 4, saturated with CO for 25 min, and at a scan rate of 50 mV s-1 , (c) CVs of Pt-G/ITO measured in an electrolyte of 0.1 M HClO 4, containing 0.5 M methanol, and at a scan rate of 50 mV/s (Inset is the CV of G/ITO under the same condition), and (d) Results of the stability test performed by cycling potential in the range of 0.0 to 1.15 V (vs. RHE), with a scan rate of 50 mV s-1 , in an electrolyte of 0.1 M HClO 4 , containing 0.5 M CH3 OH, at room temperature. After every 100 cycles, the electrolyte was replaced with a fresh one.
14
Figure
Fig. 1 Scheme of synthesis and electrodeposition processes for the Pt-G: (a) Graphene on ITO, formed by transferring a CVD graphene sheet onto an ITO substrate using a PMMA layer as a sample handle, (b) MSTF on the graphene on ITO, (c) nanostructured Pt on graphene (Pt-G) on ITO. The formation of MSTF in step I includes the spin-coating of a silica precursor solution, aging, and calcination at 400C. The formation of the nanostructure Pt film in step II includes the electrochemical deposition of Pt into the pores of the MSTF, and the removal of MSTF via a diluted HF solution. Note that this scheme is oversimplification for the pore structure of the MSTF template and the morphology of Pt. More details are in Supporting Information.
15
Fig. 2 (a) A top-view SEM image of Pt-G with magnified views of the marked areas in the insets. (b) A top-view TEM image of Pt-G with MSTF template and (c) a magnified view on a Pt NW in (c). The inset in (c) shows a SAED pattern.
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Fig. 3 (a) Raman spectra of Pt-G and graphene, and enlarged views of the (b) G band and (c) 2D band regions of graphene. The spectra are on the corresponding samples on ITO substrates.
Fig. 4 (a) UV-photoelectron spectroscopy data of graphene and Pt-G. XPS spectra of the Pt 4f core regions: (b) Pt/ ITO and (c) Pt-G/ITO.
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Fig. 5 (a) CVs (45th cycle) of Pt-G/ITO and Pt/ITO, measured in an electrolyte of 0.1 M HClO 4 at a scan rate of 50 mV/s, (b) CVs of CO-stripping on Pt-G/ITO and Pt/ITO, measured in an electrolyte of 0.1 M HClO 4, saturated with CO for 25 min, and at a scan rate of 50 mV s-1 , (c) CVs of Pt-G/ITO measured in an electrolyte of 0.1 M HClO 4, containing 0.5 M methanol, and at a scan rate of 50 mV/s (Inset is the CV of G/ITO under the same condition), and (d) Results of the stability test performed by cycling potential in the range of 0.0 to 1.15 V (vs. RHE), with a scan rate of 50 mV s-1 , in an electrolyte of 0.1 M HClO 4 , containing 0.5
18
M CH3 OH, at room temperature. After every 100 cycles, the electrolyte was replaced with a fresh one. ASSOCIATED CONTENT Supporting Information. Photograph of MSTF-G/ITO sample, SEM and EDS images of the Pt-G, O 1s XPS spectrum of Pt-G/ITO, the effect of the soaking time of the Pt-G/ITO electrode in the electrolyte solution (0.1 M HClO 4 containing 0.5 M methanol), SEM images of the Pt-G/ITO (a) before and (b) after stability test and summary of Pt 4f XPS data. AUTHOR INFORMATION Corresponding Author *Department of Chemistry, HRD Center for Creative Convergence Chemical Sciences, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Korea. Fax: +82 31 299 4179; Tel: +82 31 290 7070; E- mail: [email protected] Funding Sources This work was supported by grants NRF-20090081018 (Basic Science Research Program), NRF-2011-0031392 (Priority Research Center Program), NRF-2011-0006268 (Basic Science Research Program). Acknowledge ments We thank CCRF for the TEM, SEM, and EDS data, CNNC for the Raman data, RIC for the XPS data and PAL for the UPS data.
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Title: Graphene as electronic structure modifier of nanostructured Pt film for enhanced methanol oxidation reaction electrocatalysis Yong-Tae Kim, Md. Abdul Matin, and Young-Uk Kwon* ToC figure
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