Materials 2015, 8, 6484-6497; doi:10.3390/ma8095318
OPEN ACCESS
materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Characterization of Platinum Nanoparticles Deposited on Functionalized Graphene Sheets Yu-Chun Chiang *, Chia-Chun Liang and Chun-Ping Chung Department of Mechanical Engineering, Yuan Ze University, 135 Yuan-Tung Rd., Chung-Li, Taoyuan 32003, Taiwan; E-Mails:
[email protected] (C.-C.L.);
[email protected] (C.-P.C.) * Author to whom correspondence should be addressed; E-Mail:
[email protected]; Tel.: +886-3463-8800 (ext. 2476); Fax: +886-3455-8013. Academic Editor: Wen-Hsiang Hsieh Received: 18 August 2015 / Accepted: 17 September 2015 / Published: 21 September 2015
Abstract: Due to its special electronic and ballistic transport properties, graphene has attracted much interest from researchers. In this study, platinum (Pt) nanoparticles were deposited on oxidized graphene sheets (cG). The graphene sheets were applied to overcome the corrosion problems of carbon black at operating conditions of proton exchange membrane fuel cells. To enhance the interfacial interactions between the graphene sheets and the Pt nanoparticles, the oxygen-containing functional groups were introduced onto the surface of graphene sheets. The results showed the Pt nanoparticles were uniformly dispersed on the surface of graphene sheets with a mean Pt particle size of 2.08 nm. The Pt nanoparticles deposited on graphene sheets exhibited better crystallinity and higher oxygen resistance. The metal Pt was the predominant Pt chemical state on Pt/cG (60.4%). The results from the cyclic voltammetry analysis showed the value of the electrochemical surface area (ECSA) was 88 m2 /g (Pt/cG), much higher than that of Pt/C (46 m2 /g). The long-term test illustrated the degradation in ECSA exhibited the order of Pt/C (33%) > Pt/cG (7%). The values of the utilization efficiency were calculated to be 64% for Pt/cG and 32% for Pt/C. Keywords: platinum; graphene; functionalization; characterization; electrochemical activity
1. Introduction Carbon materials are widely used as the supports for deposition of precious metal nanoparticles in heterogeneous and electrochemical catalysis [1,2]. The performance and cost efficiency of these catalyst
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materials are highly dependent on the dispersion and stability of the catalytic nanoparticles. Platinum (Pt) nanoparticles supported on carbon black (Pt/C) are currently considered the most promising electrocatalyst on proton exchange membrane fuel cells (PEMFCs). Respecting the fact that carbon blacks suffer serious corrosion problems under normal operating conditions, several nanostructured carbon materials have been investigated as Pt catalyst supports. Herein, graphene, a monolayer of carbon atoms in a crystal lattices and the basal plane of graphite composed of a honeycomb arrangement of carbon atoms, receives special interest as a catalyst support in PEMFCs due to its distinct electronic, mechanical and structural properties, such as high electrical and heat conductivity, high chemical stability and large surface-to-volume ratio (theoretical specific surface area of 2620 m2 /g) [3,4]. However, the weak interactions between metal nanoparticles and carbon supports generally result in severe agglomeration, dissolution, reprecipitation or migration of metal particles or the diffusion of metal ionic species, leading to the degradation of electrochemical activity under long-term operations. To enhance the interfacial interactions between the carbon supports and the Pt nanoparticles, the modifications of carbon supports are essential to develop the performance of Pt nanoparticles. Introduction of oxygen-containing functional groups onto the surface of carbon supports is a commonly used method to enhance the attachment and eliminate the agglomeration of Pt nanoparticles. Li et al. [4] prepared composite graphene nanosheets decorated with Pt nanoparticles. Pt nano clusters, consisted of small Pt nanoparticles with mean diameters of about 5–6 nm, were deposited on the basal planes and the edges of the graphene. The value of the electrochemical surface area (ECSA) for the Pt/graphene was 44.6 m2 /g, higher than that of commercial Pt/C (30.1 m2 /g). Kou et al. [5] studied the electrochemical activity of Pt nanoparticles supported on functionalized graphene sheets (FGS) by impregnation methods. The improved performance of Pt/FGS compared to Pt/C (E-TEK) can be attributed to the smaller particle size (average Pt size ~2 nm) and less aggregation of the Pt nanoparticles on FGS. It was also observed Pt-Ru/graphene nanosheets exhibited higher ECSA (47.9 m2 /g, 1.7 times that of Pt-Ru/Vulcan) and methanol oxidation activity (0.113 mA g/m2 , 1.4 times that of Pt-Ru/Vulcan) [6]. Seo et al. [7] found the values of ECSA for Pd/graphene were 54.9 m2 /g, higher than that of Pt/graphene (42.0 m2 /g). In addition, some studies also investigated the effects of the spacers between metal nanoparticles and the graphene sheets, such as carbon nanotubes [8], carbon black [9], or indium tin oxide (ITO) [10]. It was observed that very small and highly dispersed PtRu nanoparticles decorated on a graphene support using simple surfactant-free synthesis processes possessed excellent activity and stability compared with conventional Pt/C (E-TEK) for the electrooxidation of biomass-derived glycerol [11]. The sulfhydryl groups uniformly attached on the graphene nanosheets were used to assemble gold nanoparticles. It was found that the gold particles (in the range of 3–5 nm) were highly scattered on the surface of the functionalized graphene sheets and this hybrid showed high electrochemical activity toward the oxygen reduction reaction and high stability in alkaline media [12]. Cobalt phosphate has also been prepared on porous graphene film by a convenient charge-controlled electrodeposition method, which served as an efficient oxygen evolving catalyst [13]. This graphene-supported catalyst was of high catalytic activity and stability towards water oxidation, superseding those of unsupported ones.
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In this study, Pt nanoparticles were deposited on citric acid-treated graphene sheets (denoted as cG) using a reverse micelle method. Pt nanoparticles supported on carbon black (Pt/C) is currently considered the most promising electrocatalyst on PEMFCs. However, the corrosion of carbon black under operating conditions restricted the applications. Therefore, the graphene was applied to overcome this problem. To enhance the interfacial interactions between the graphene sheets and the Pt nanoparticles, the surface oxides were introduced onto the surface of graphene sheets. The products were characterized by high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The cyclic voltammetry (CV) technique was used to measure the ECSA, stability and utilization efficiency of the catalysts. One commercial Pt/C (Johnson Matthey) was used for comparison. 2. Experimental Section 2.1. Modification of Graphene Sheets The commercial graphene sheets (denoted as GS) used in this study were provided by Legend Star International Co. (New Taipei City, Taiwan). The oxygen-containing functional groups were introduced onto the surface of the graphene sheets by treating them with citric acid (C6 H8 O7 ), as follows: 200 mg of graphene sheets were added to the citric acid aqueous solution (1.6 mM) and subjected to 15 min ultrasonic treatment, followed by vigorous stirring for 1 h using a magnetic stirrer. The ultrasonic treatment was conducted using an ultrasonic cleaner (LEO-1502, LEO Ultrasonic Co., New Taipei City, Taiwan) at a frequency of 46 kHz and a power of 150 W. The solvent was removed by filtering and the samples were heat-treated in a muffle furnace at 300 ˝ C for 30 min. This product was denoted as cG. 2.2. Synthesis of Pt/cG The deposition of Pt particles on the oxidized graphene sheets was performed using the colloidal method [14], involving a two-phase transfer of PtCl6 2´ , followed by reduction in the presence of 1-dodecanethiol (DDT, C12 H25 SH, 98%) (Sigma-Aldrich, Steinheim, Germany). Hexachloroplatinic acid (H2 PtCl6 ¨ 6H2 O) solution (3 mL of 0.086 M in deionized water) (Alfa Aesar, Heysham, England) was mixed with tetraoctylammonium bromide (ToAB, N(C8 H17 )4 Br) (Sigma-Aldrich, Steinheim, Germany) solution (4 mL of 0.18 M in toluene) and vigorously stirred for about 30 min at room temperature. During this process, PtCl6 2´ ions are transferred from the aqueous solution to the organic layer (toluene), using ToAB as the phase-transfer catalyst. The orange-colored organic layer was extracted and 450 mg of DDT (capping agent) was added and stirred for 30 min. 200 mg cG was then added with constant stirring for 1 h. 10 mL of 0.25 M sodium formate (HCOONa) aqueous solution was added drop-wise at 60 ˝ C, to reduce the Pt ion. The solid product was filtered and rinsed with ethanol to remove excess DDT and copious amount of warm deionized water was used to remove the remaining sodium formate. The resulted Pt/cG products were dried at 100 ˝ C for 3 h and further heat-treated at 800 ˝ C for 2 h in a tubular furnace in an argon atmosphere. Figure 1 illustrated the schematic representation of surface modification of graphene sheets and the deposition of Pt nanoparticles. For comparison, the commercial Pt/C with a Pt content of 20 wt % (Johnson Matthey, denoted as JM) was purchased.
800 °C for 2 h in a tubular furnace in an argon atmosphere. Figure 1 illustrated the schematic representation of surface modification of graphene sheets and the deposition of Pt nanoparticles. For comparison, the commercial Pt/C with a Pt content of 20 wt% (Johnson Matthey, denoted as JM) Materials 2015, 8 6487 was purchased.
Figure 1. Schematic representation of surface modification of graphene sheets and the
Figure 1. Schematic representation of surface modification of graphene sheets and the deposition of Pt nanoparticles. deposition of Pt nanoparticles. 2.3. Material Characterizations
2.3. Material Characterizations
HRTEM images were obtained using a transmission electron microscope (Tecnai G2, 200 kV) (Philips/FEI, the Netherlands), to determine the interior microstructure of the graphene HRTEM images Eindhoven, were obtained using a transmission electron microscope (Tecnai G2, 200 kV) sheets and the morphology and size distribution of the Pt particles deposited on the graphene sheets. (Philips/FEI, Eindhoven, the Netherlands), to determine the interior microstructure of the graphene The oxidation resistance of the samples was determined in flowing air (60 cm3 /min) with a heating sheets and the morphology and size distribution of the Pt particles deposited on the graphene sheets. rate of 10 ˝ C/min, using a thermogravimetric analyzer (Dynamic TGA Q500 in TA Instrument 5100) 3 /min) a heating rate The oxidation resistance of the samples was determined in flowing air (60 cm (New Castle, DE, USA) to measure any changes in the weight of the sample as a function of with temperature of 10 °C/min, using analyzer (Dynamic TGAthermogravimetry, Q500 in TA DTG, Instrument (TGA plot) and athethermogravimetric rate of weight loss versus temperature (differential plot). 5100) The residual wastoused to estimate Pt content in deposited on the surface the graphene (New Castle, DE, mass USA) measure anythechanges the weight of theof sample as asheets. function of The XRD takenrate withofanweight X-ray powder diffractometer (Rigaku(differential TTRAX III, Tokyo, Japan) temperature (TGApatterns plot) were and the loss versus temperature thermogravimetry, to give detailed information about the crystallographic structure of the materials. The radiation used was DTG, plot). The residual mass was used to estimate the Pt content deposited on the surface of the Cu Kα with a wavelength of 0.15418 nm, a voltage of 30 kV and a current of 20 mA. The value of 2-theta graphene(2θ) sheets. The XRD patterns were taken with an X-ray powder diffractometer (Rigaku TTRAX III, ranges from 10˝ to 90˝ , where θ is the diffraction angle, with a scanning speed of 4˝ /min. XPS was Tokyo, Japan) to give detailed information about the crystallographic structure the materials. The used to determine the number and type of functional groups present on the surface of theof samples, using radiation aused was Cu Kα with a wavelength of 0.15418 voltage of 30anode kV and a current of 20 mA. spectrophotometer (PerkinElmer PHI 1600, Waltham,nm, MA,aUSA). A twin Mg X-ray source (hνof= 1253.6 wasranges used, at from a voltage 15 90°, kV and a power of 400 For calibration purposes, The value 2-thetaeV)(2θ) 10°of to where θ is the W. diffraction angle, with the a scanning C1s electron binding energy that corresponds to graphitic carbon was set at 284.6 eV. A nonlinear least speed of 4°/min. XPS was used to determine the number and type of functional groups present on the squares curve-fitting program (XPSPEAK software, Version 4.1) was used for deconvolution of the XPS surface of the samples, using a spectrophotometer (PerkinElmer PHI 1600, Waltham, MA, USA). spectra. The FT-IR spectra were recorded using a Fourier transform infrared spectroscope (PerkinElemer A twin anode Mg100, X-ray sourceMA, (hνUSA) = 1253.6 eV) pellets was used, a voltage of region 15 kVtoand a powerthe of 400 W. Spectrum Waltham, with KBr in theat 4000–400 cm´1 characterize For calibration purposes, C1s electron binding energy that corresponds to graphitic carbon was set functional groups onthe the samples.
at 284.6 eV. A nonlinear least squares curve-fitting program (XPSPEAK software, Version 4.1) was used for deconvolution of the XPS spectra. The FT-IR spectra were recorded using a Fourier transform infrared spectroscope (PerkinElemer Spectrum 100, Waltham, MA, USA) with KBr pellets in the 4000–400 cm−1 region to characterize the functional groups on the samples.
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A CHI 613C electrochemical workstation (CH Instruments, Austin, TX, USA) was employed for 2.4. Measurement of Electrochemical Activity Pt samples. A three-electrode electrochemical cell was the electrochemical study of carbon-supported constructed CVelectrochemical measurements,workstation through which the ECSA ofAustin, the Pt TX, nanoparticles determined. A CHI for 613C (CH Instruments, USA) was was employed for r The electrode wasofa carbon-supported thin layer of Nafion -impregnated catalyst sample, cast on cell a vitreous theworking electrochemical study Pt samples. A three-electrode electrochemical was carbon disk of 5 mm in diameter embedded in a Teflon cylinder. A homogeneous ink composed constructed for CV measurements, through which the ECSA of the Pt nanoparticles was determined. of r ® electrocatalyst, Nafionr was ionomer wt %, DuPont), and isopropanol, where the content The working electrode a thin(5layer of Nafion -impregnated catalyst sample, castNafion on a vitreous in carbon the thin-film layer was 25inwt %. A cylinder. Pt wire A andhomogeneous a saturated ink calomel electrode disk of 5electrocatalyst mm in diameter embedded a Teflon composed of ® ® electrocatalyst, ionomerand (5 reference wt%, DuPont), and isopropanol, where Nafion content in the (SCE) were used Nafion as the counter electrodes, respectively. Thethe measurements of CV were thin-film atelectrocatalyst layer was wt%. Pt 4wire andelectrolyte a saturatedsolution calomelatelectrode (SCE) were conducted room temperature using250.5 M HA2 SO as the a scan rate of 20 mV/s used´0.2 as thetocounter andSCE. reference electrodes, respectively. The measurements of CV were conducted at from 1.0 V vs. room temperature using 0.5 M H2SO4 as the electrolyte solution at a scan rate of 20 mV/s from −0.2 to 3. 1.0 Results V vs. and SCE.Discussion 3.The Results and Discussion morphology of the prepared catalyst was observed using HRTEM. Figure 2 shows the HRTEM
images and the distribution of the Pt particle sizes for Pt/cG and Pt/C (JM). The flake-like shapes The morphology of the prepared catalyst was observed using HRTEM. Figure 2 shows the HRTEM of graphene sheets and the wrinkles on the graphene sheets were clearly observed in Figure 2a. Pt images and the distribution of the Pt particle sizes for Pt/cG and Pt/C (JM). The flake-like shapes of nanoparticles were uniformly dispersed on the surface of the graphene sheets (Figure 2a) and carbon graphene sheets and the wrinkles on the graphene sheets were clearly observed in Figure 2a. black (Figure 2b) and the sizes ranged between 0 and 5 nm with some nano clusters. The mean sizes Pt nanoparticles were uniformly dispersed on the surface of the graphene sheets (Figure 2a) and carbon of black the Pt(Figure particles 2.08 ˘ ranged 0.52 nm (Pt/cG), similar thesome study of clusters. Kim et al. and slightly 2b) were and the sizes between 0 and 5 nm to with nano The[11] mean sizes of greater than Pt/C (JM) (1.96 ˘ 0.50 nm). As seen from the data, the Pt nanoparticles deposited on the Pt particles were 2.08 ± 0.52 nm (Pt/cG), similar to the study of Kim et al. [11] and slightly greaterthe graphene sheets particle sizefrom and the distribution to nanoparticles that of Pt/C, deposited and both had a high degree than Pt/C (JM) had (1.96a ±similar 0.50 nm). As seen data, the Pt on the graphene of sheets uniformity. had a similar particle size and distribution to that of Pt/C, and both had a high degree of uniformity.
Figure images (a) and and(b) Pt and particle distribution (c) and (d) (c) of the Figure2.2.HRTEM HRTEM images (a)(b) and Pt size particle size distribution andsamples. (d) of
the samples.
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TheTGA TGAand andDTG DTGprofiles profiles measured measured in in flowing flowing air The air for for all all samples samplesare areshown shownininFigure Figure3. 3. The sampleswere wereheated heated at at aa rate rate of flow of 60 sccm. The temperatures for thefor ˝ The samples of 10 10°C/min C/minininananairair flow of 60 sccm. The temperatures and Pt/C were 715, 581581 andand 414414 °C˝ C themaximum maximumrate rateofofweight weightloss loss(or(oroxidation) oxidation)forforcG,cG,Pt/cG Pt/cG and Pt/C were 715, (Figure 3a,b,d). The maximum rate of weight loss for cG occurred at a temperature higher than that of (Figure 3a,b,d). The maximum rate of weight loss for cG occurred at a temperature higher than that of the carbon nanotubes in the literature. Following deposition with Pt nanoparticles, the maximum rate the carbon nanotubes in the literature. Following deposition with Pt nanoparticles, the maximum rate of of weight loss slightly decreased. Nevertheless, the thermal stability or the resistance to oxidation of weight loss slightly decreased. Nevertheless, the thermal stability or the resistance to oxidation of Pt/cG Pt/cG was still higher than Pt/C (JM). The TGA and DTG profiles of Vulcan XC-72R (the support of was still higher than Pt/C (JM). The TGA and DTG profiles of Vulcan XC-72R (the support of Pt/C Pt/C (JM)) were provided in Figure 3c, whose thermal stability was similar to cG. However, (JM)) provided in nanoparticles Figure 3c, whose thermal stability similar to decreased cG. However, the deposition the were deposition of Pt on the Vulcan carbonwas significantly its resistance to of oxidation. Pt nanoparticles on the Vulcan carbon significantly decreased its resistance to oxidation. Figure Figure 3b depicted two peaks for the DTG profile, which was attributed to the reason that 3b depicted two peaks for DTG profile, which was attributed to the reason sample that different different resistance to the oxidation existed on the Pt/cG samples. The graphene used inresistance this study to oxidation existed on the samples. The graphene used on in this study was sheets multi-layered. The was multi-layered. ThePt/cG Pt nanoparticles were mainlysample distributed the outermost where the Pt thermal nanoparticles were mainly distributed the outermost sheets Pt where the thermal would stability would be inferior to theoninternal sheets without deposition. Thus,stability there were two be inferior sheets without Pt deposition. Thus, there loss wereoftwo peaks on thethe DTG profile. The peaks toonthe theinternal DTG profile. The profile for the TGA weight Pt/cG showed residue was 12.19for wt%. with showed the metal in cG, calculate the profile the This TGAdatum, weightcombined loss of Pt/cG thecatalyst residueresidue was 12.19 wtwas %. used This to datum, combined Pt the content, wasresidue about 10 with metalwhich catalyst inwt%. cG, was used to calculate the Pt content, which was about 10 wt %.
Figure (a) cG; cG; (b) (b) Pt/cG; Pt/cG; (c) (c)Vulcan VulcanXC-72R; XC-72R;(d) (d)Pt/C Pt/C(JM). (JM). Figure3.3.TGA TGAprofiles profilesof ofthe the samples. samples. (a) Figure displayedthe theXRD XRDpatterns patterns for for all all samples. samples. As degree of of Figure 4 4displayed As seen seenfrom fromFigure Figure4a,b, 4a,b,a ahigh high degree ˝ the crystallinityforforcGcGand andPt/cG Pt/cGwas wasobserved. observed. The The diffraction crystallinity diffraction peaks peaksatat2θ 2θangles anglesofof26.4°–83.2° 26.4˝ –83.2in in the XRD pattern of oxidized graphene sheets can be assigned to hexagonal crystalline graphite (JCPDS XRD pattern of oxidized graphene sheets can be assigned to hexagonal crystalline graphite (JCPDS No. 41-1487). The peaks at 2θ angles of 26.4˝ , 42.2˝ , 44.4˝ , 54.5˝ , 77.2˝ and 83.2˝ could be indexed as the reflections from the C(002), C(100), C(101), C(004), C(110) and C(112) planes. Pt/cG exhibited
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No. 41-1487). The peaks at 2θ angles of 26.4°, 42.2°, 44.4°, 54.5°, 77.2° and 83.2° could be indexed as Materials 2015,The 8 peaks 6490 No. 41-1487). at 2θ angles of 26.4°, 42.2°, 44.4°,C(110) 54.5°, 77.2° and 83.2° could be indexed as the reflections from the C(002), C(100), C(101), C(004), and C(112) planes. Pt/cG exhibited the reflections from the C(002), C(100), C(101), C(004), C(110) and C(112) planes.bePt/cG exhibited strong diffraction peaks at 2 θ values of 39.8°, 46.2°, 67.5° and 81.3°, which could indexed as the ˝ ˝ ˝ ˝ strong diffraction peaks at 22 θθPt(200), values of of 39.8°,, and 46.2°, 67.5°facets and 81.3 81.3°, which could be becubic indexed as the the strong diffraction peaks at values 39.8 46.2 , 67.5 and , which could indexed as reflections from the Pt(111), Pt(220) Pt(311) of the face-centered structures reflections from thePt(111), Pt(111), Pt(200), Pt(220) andPt(311) Pt(311) facets the face-centered cubic structures reflections from the Pt(200), Pt(220) and of of the face-centered cubic structures of of Pt crystal (JCPDS No. 4-802). The diffraction peaksfacets for Pt/C (JM) were broader than those for of Pt crystal (JCPDS No. 4-802). diffraction for was Pt/Cgreater (JM) broader were broader thanfor those for Pt crystal (JCPDS No. 4-802). The diffraction peakspeaks forPt/cG Pt/C (JM) were than Pt/cG, Pt/cG, implying the average size ofThe the Pt particles on than that on those Pt/C, consistent Pt/cG, implying the average size the images. Pt particles on Pt/cG wasthan greater than that consistent on consistent implying the average size the Ptofparticles on Pt/cG was greater that on Pt/C, with theis with the conclusions fromofthe HRTEM In summary, the XRD results show thePt/C, Pt precursor with the to conclusions from theusing HRTEM In summary, the XRD results the Ptisprecursor is conclusions from the HRTEM images. Inimages. summary, the XRD results show the Ptshow precursor reduced to reduced the metallic state the colloidal method. reduced to the metallic statecolloidal using the colloidal method. the metallic state using the method.
Figure 4. XRD patterns of the samples. (a) cG; (b) Pt/cG; (c) Pt/C (JM). Figure Figure 4. 4. XRD XRD patterns patterns of of the the samples. samples. (a) (a) cG; cG; (b) (b) Pt/cG; Pt/cG; (c) (c) Pt/C Pt/C (JM). (JM). XPS analysis was used to elucidate the surface compositions and the oxidation states of the XPS analysis analysis was used to elucidate the compositions and the oxidation states of the materials. The XPS survey spectra the surface samples revealed of the most external XPS was used to scan elucidate theof surface compositions andthe thecompositions oxidation states of the materials. materials. Thefrom XPS survey spectra of revealed the samples revealed the compositions ofthe themajor most external surface. XPSscan survey spectra of the GS cG (Figure peaks in The XPS Data survey scanthe spectra of the samples theand compositions of5a) theindicated most external surface. Data surface. Data from the XPS survey spectra of the GS and cG (Figure 5a) indicated the major peaks in the spectra to the C1s O1s(Figure photoelectrons. Figure 5b illustrated elemental from the XPSwere surveydue spectra of the GSand and cG 5a) indicated the major peaks in thethe spectra were the spectra due photoelectrons. toin the C1sratio, andFigure O1s 5b photoelectrons. Figure 5bcompositions illustrateddepth the elemental compositions (measured atomic at.%) onillustrated the surface over the sampling of several due to the C1swere and O1s theorelemental (measured in compositions (measured in atomic ratio, at.%) on the surface or over the sampling depth of several atomic ratio, layersat.%) fromonthe The oxidation of weak acid led to alayers slightfrom increase in the atomic the surface. surface or over the sampling depthcitric of several atomic the surface. atomic layers from the surface. The oxidation of weak citric acid led to a slight increase in the percentage of of oxygen. The oxidation weak citric acid led to a slight increase in the percentage of oxygen. percentage of oxygen.
Figure Figure5.5. Cont. Cont. Figure 5. Cont.
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Figure 5. Chemical atomic Figure 5. Chemical states states of of GS GS and and cG. cG. (a) (a) XPS XPS survey survey scan scan spectra; spectra; (b) (b) surface surface atomic ratios; of high ratios; (c) (c) and and (d) (d) curve curve fitting fitting of high resolution resolution XPS XPS C1s C1s spectra; spectra; (e) (e) and and (f) (f) curve curve fitting fitting of resolution XPS XPS O1s O1s spectra; spectra; (g) (g) FT-IR FT-IR spectra. spectra. of high high resolution The XPS spectra spectra of of the the C1s for GS GS and and cG cG were were illustrated illustrated in in Figure Figure 5c,d. 5c,d. The high-resolution high-resolution XPS C1s region region for Carbon atoms differed in their binding energies depending on whether they are linked to one O atom Carbon atoms differed in their binding energies depending on whether they are linked to one O atom by by a single bond, a double bond, or two oxygen atoms [15]. Deconvolution of the C1s spectra gave a single bond, a double bond, or two oxygen atoms [15]. Deconvolution of the C1s spectra gave seven seven individual component groups. Table 1 summarized the calculated percentages of graphitic and individual component groups. Table 1 summarized the calculated percentages of graphitic and functional functional carbon atoms, where the values were given in at.% of total intensity. Besides graphitic carbon atoms, where the values were given in at.% of total intensity. Besides graphitic carbon, phenolic carbon, phenolic groups were the most predominant functionalities on the surface of the GS. The groups were the most predominant functionalities on the surface of the GS. The percentage of carbon 3 )) obviously increased once the citric acid percentage of carbon atoms in aliphatic structures (C(sp atoms in aliphatic structures (C(sp3 )) obviously increased once the citric acid oxidation was applied.
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Moreover, the content of the –COOH groups was also significantly enhanced and the surface became much more active due to the decrease in the π-π* transitions in aromatic rings. Table 1. Results of the fits of the XPS C1s region, values given in at.% of total intensity. Binding Energy (eV)
Sample
284.6
285.4
286.0
287.6
288.8
290.6
291.6
Carbon Atoms in Polyaromatic Structures (C(sp2 ))
Carbon Atoms in Aliphatic Structures (C(sp3 ))
–OH
C=O
–COOH
Carbonates
π-π*
56.56 51.25
22.88 37.86
7.31 0.25
1.08 0.10
1.54 3.16
2.41 2.17
8.22 5.21
GS cG
To gain a better understanding of the results noted above, high-resolution O1s spectra of GS and cG were illustrated in Figure 5e,f, and four different O functionalities as well as a contribution from chemisorbed water were identified, as reported in the studies by Zielke et al. [16,17]. The calculated percentages of functional O atoms were shown in Table 2. As seen from the data, the citric acid oxidation resulted in the decrease of the –OH groups but the increase of the –COOH groups on graphene sheets, consistent with the result for the high-resolution C1s region. Specifically, except for –OH groups, the percentages of other O functionalities increased after oxidation. The FT-IR spectra in Figure 5g provided another piece of evidence for the above discussion, which was in agreement with the literature [18,19]. The bands centered at ~3470 and ~1400 cm´1 were attributed to deformation of the –OH bond and CO–H groups. The stretching vibrations of the carbonyl or carboxyl groups and the C–O bond were observed at ~1670 and 1100 cm´1 , respectively. Table 2. Results of the fits of the XPS O1s region, values given in at.% of total intensity. Binding Energy (eV) 531.1
532.3
533.3
534.2
536.1
C=O
R-O-C=O, O=C-NH2 , O=C-O-C=O, C-OH, R-O-R
R-O-C=O, O=C-O-C=O
C-OOH
H2 O
GS
23.55
68.53
1.02
1.57
5.33
cG
32.24
52.54
2.20
6.53
6.49
Sample
The major peaks observed in the survey scan spectra of Pt/cG and Pt/C (JM) were due to the C1s, O1s, and Pt4f photoelectrons. Figure 6a showed the atomic ratios of C1s, O1s, and Pt4f for Pt/cG and Pt/C (JM), where the ratios were similar for both samples. The most abundant element on the surface was carbon. The content of oxygen atoms implied the surface was not hydrophilic enough. Moreover, the Pt contents from XPS agreed with those from the TGA profiles. The deconvolution of the XPS spectra over the Pt4f region, shown in Figure 6b,c, showed different states of oxidation for each of the carbon supported Pt samples, consisting of three couples of doublets. The most intense doublet, at 71.0 and 74.35 eV, represents a zero-valence metallic Pt (Pt˝ ), the doublet
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at 72.4 and 75.75 eV is attributed to the presence of an amorphous Pt(II) species, such as PtO or Pt(OH)2 [20], and the broader doublet at 74.9 and 78.25 eV is assigned to the Pt(IV) species. Figure 6d Materials 2015, 8 10 summarized the calculated percentages of the Pt species in different chemical states. The data showed most of the Pt was in the zero-valence metallic state (>50%). The percentages of Pt˝ , Pt(II), and Pt(IV) most of the Pt was in the zero-valence metallic state (>50%). The percentages of Pt°, Pt(II), and Pt(IV) in Pt/cG werewere 60.4,60.4, 20.020.0 andand 19.7 at.% were 53.2%, 53.2%,23.4% 23.4% and 23.4%. Pt/cG in Pt/cG 19.7 at.%and andininPt/C Pt/C (JM) (JM) were and 23.4%. Pt/cG had had ˝ higherhigher Pt , but Pt(II) andand Pt(IV), indicating deposited graphene sheets Pt°, less but less Pt(II) Pt(IV), indicatingthe the Pt Pt nanoparticles nanoparticles deposited on on graphene sheets werewere reduced muchmuch moremore effectively, compared blacks. reduced effectively, comparedtotothose thoseon oncarbon carbon blacks.
Figure 6. Curve fitting high-resolution XPS XPS Pt4f Pt4f spectra (a) (a) Surface Figure 6. Curve fitting of of high-resolution spectrafor forthe thesamples. samples. Surface atomic ratios; Pt/cG; Pt/C(JM); (JM);(d) (d)Chemical Chemical states atomic ratios; (b) (b) Pt/cG; (c)(c) Pt/C statesofofPt. Pt. The cyclic voltammogram for the Pt/cG was displayed in Figure 7 and the cyclic voltammogram of
The for the Pt/cG was displayed in Figure and the cyclic voltammogram thecyclic Pt/C voltammogram (JM) was also presented for comparison. The CV curves7show three characteristic potentialof the Pt/C (JM) wasthe also presented for comparison. The CV(−0.2 curves show regions: hydrogen adsorption/desorption region to 0.1 V), three doublecharacteristic layer plateau potential region (0.1regions: to the hydrogen region V),(0.5 double plateau region (0.1 to 0.5 V) 0.5 V) andadsorption/desorption the formation and reduction of (´0.2 surfaceto Pt 0.1 oxides to 1.0layer V). All voltammograms display a well-defined adsorption/desorption V vs. Compared to that of and the formation hydrogen and reduction of surface Pt region oxidesfrom (0.5−0.2 to to 1.00.1V). AllSCE. voltammograms display a the Pt/C,hydrogen the Pt nanoparticles deposited on the graphene larger loop, especiallyto a that well-defined adsorption/desorption region from sheets ´0.2 possessed to 0.1 V avs. SCE. Compared region, indicating the system’s greater ability to store electricacharges. The ECSA is of thewide Pt/C,double the Ptlayer nanoparticles deposited on the graphene sheets possessed larger loop, especially a a very significant parameter indicating the intrinsic electrocatalytic activity of a Pt catalyst. The values wide double layer region, indicating the system’s greater ability to store electric charges. The ECSA is a of the ECSA for Pt/cG and Pt/C were calculated to be 88 and 46 m2/g Pt, respectively, based on a very significant parameter indicating the intrinsic electrocatalytic activity of a Pt catalyst. The values of monolayer hydrogen adsorption charge of 0.21 mC/cm2 on polycrystalline Pt. A higher ECSA of the 2 the ECSA Pt/cG Pt/C were calculated be 88 and 46 /g Pt, respectively, based on a monolayer Pt/cGfor could be and attributed to the small Pt to nanoparticles andmthe efficient formation of zero-valence 2 hydrogen adsorption charge mC/cm on polycrystalline Pt. on A Pt higher ECSA ofNote the Pt/cG metallic Pt such that the of fuel0.21 gases could attach to the redox sites nanoparticles. that thecould be attributed to themuch smallmore Pt nanoparticles the efficient formation of zero-valence metallic Pt such Pt/cG required activation timeand to develop its electrochemical activity. that the fuel gases could attach to the redox sites on Pt nanoparticles. Note that the Pt/cG required much more activation time to develop its electrochemical activity.
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Figure 7. Cyclic voltammograms and changes in ECSA for all samples. Figure 7. Cyclic voltammograms and changes in ECSA for all samples. To To understand understand the the stability stability of of the the electrochemical electrochemical activity activity of of the the graphene graphene sheets sheets supported supported Pt Pt nanoparticles, the long-term operation of CV analysis up to 600 cycles was conducted under the same nanoparticles, the long-term operation of CV analysis up to 600 cycles was conducted under the same conditions SCE) and conditions (at (at aa scan scan rate rate of of 20 20 mV/s mV/s from from´0.2 −0.2 to to 1.0 1.0 VV vs. vs. SCE) and the the values values of of ECSA ECSA were were evaluated. of the values in ECSA on the number cycles of (Table 3) showed evaluated. The Thedependence dependence of the values in ECSA on the of number cycles (Table degradation 3) showed in ECSA due to dissolution/reprecipitation or migration of Pt particles or the diffusion of Pt ionic degradation in ECSA due to dissolution/reprecipitation or migration of Pt particles or the diffusion of species [21] presented in the order of Pt/C (33%) > Pt/cG (7%). The degradation of electrochemical Pt ionic species [21] presented in the order of Pt/C (33%) > Pt/cG (7%). The degradation of activity of Pt/cG activity was negligible within experimental error It waserror believed the graphene electrochemical of Pt/cG wasthenegligible within thethreshold. experimental threshold. It was corrosion wasgraphene unexpected and the oxidation of the surface was of slowed thedown. loss believed the corrosion was unexpected andPtthe oxidation the Ptdown. surfaceEvidently, was slowed in ECSA uptheto loss 600 in cycles for up Pt/cG was cycles only 21% that was associated with of commercial Pt/C (JM) Evidently, ECSA to 600 for of Pt/cG only 21% that associated with catalysts, implying the electrostatic attractive force between Pt colloids and oxidized graphene sheets commercial Pt/C (JM) catalysts, implying the electrostatic attractive force between Pt colloids and greatly on the Pt/C surpassed (JM) catalysts. addition, the higher electrochemical of oxidizedsurpassed graphenethat sheets greatly that onIn the Pt/C (JM) catalysts. In addition, activity the higher Pt/cG was attributable 2D was nanostructure, area, and good cGgood [8]. electrochemical activitytoofthe Pt/cG attributablelarge to thesurface 2D nanostructure, largeconductivity surface area,ofand The multi-layer of graphene sheets also possesses a higher leadingatohigher a better mass conductivity of structure cG [8]. The multi-layer structure of graphene sheetsporosity also possesses porosity transport reactants. The functional on graphene sheets may leadonto graphene a strong leading toofaelectrolytes better massand transport of electrolytes andgroups reactants. The functional groups metal-support interaction, in resistance of Pt to agglomeration and sintering [5], and therefore sheets may lead to a strongresulting metal-support interaction, resulting in resistance of Pt to agglomeration and enhanced durability. sintering [5], and therefore enhanced durability. 2 Table Table 3. 3. Calculated Calculated ECSA ECSA (m (m2/g) /g) and and activity activity degradation degradation (%) (%) of of the the samples. samples.
Sample Parameter Parameter 30th 30th 100th200th 200th 300th 300th 400th 400th 500th Sample 100th 500th600th 600th 2 /g) 82 88 87 85 84 84 82 ECSA (m Pt/cG ECSA (m2 /g) 82 88 87 85 84 84 82 Degradation (%) 2 3 5 5 7 Pt/cG Degradation 3 33 5 33 335 31 7 ECSA(%) (m2/g) - 46 - 40 2 34 Pt/C (JM)ECSA (m2 /g) 33 Degradation (%)46 - 40 13 34 26 3328 29 2833 33 31 Pt/C (JM) Degradation (%) 13 26 28 29 28 33
In addition, the Pt utilization efficiency (η) is also the essential parameter to describe the catalyst performance, calculated by dividing ECSA the chemical surface area (CSA) [22]. In addition, which the Pt was utilization efficiency (η) is also the by essential parameter to describe the catalyst 3 The CSA is defined 6/ρd, where ρ by is the densityECSA of Pt (=21.09 g/cm ) andsurface d is mean of Pt performance, which as was calculated dividing by the chemical areadiameter (CSA) [22]. nanoparticles (nm). The values of the weredensity calculated be 64% for3 )Pt/cG 32% diameter for Pt/C. The CSA is defined as 6/ρd, where ρ isη the of Pt to(=21.09 g/cm and dand is mean The results implied oxidized graphene sheets were an effective support for Pt depositions.
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of Pt nanoparticles (nm). The values of the η were calculated to be 64% for Pt/cG and 32% for Pt/C. The results implied oxidized graphene sheets were an effective support for Pt depositions. 4. Conclusions The results showed the Pt nanoparticles on Pt/cG were uniformly dispersed on the surface of graphene sheets by the reverse micelle method, where the mean size of the Pt particles was 2.08 ˘ 0.52 nm, and the average size of the Pt particles on Pt/C (JM) was 1.96 ˘ 0.50 nm. The Pt nanoparticles deposited on graphene sheets exhibited better crystallinity and the sample had a higher oxygen resistance. The metal Pt was the predominant Pt chemical state on Pt/cG (60.4%) and Pt/C (53.2%). The results from CV analysis showed the values of ECSA were 88 m2 /g (Pt/cG) and 46 m2 /g (Pt/C), respectively. The long-term test illustrated the degradation in ECSA exhibited the order of Pt/C (33%) > Pt/cG (7%). The values of the η were calculated to be 64% for Pt/cG and 32% for Pt/C. Therefore, Pt nanoparticles deposited on functionalized graphene sheets should produce a promising electrocatalyst. Acknowledgments This work was supported by the Ministry of Science and Technology under NSC 102-2622E-155-013-CC3. Author Contributions Yu-Chun Chiang organized and designed the experimental methods and procedures, analyzed the material property data and wrote the paper. Chia-Chun Liang and Chun-Ping Chung carried out the synthesis of Pt/cG and performed the electrochemical activity measurements. All authors read and approved the final version of the manuscript to be submitted. Conflicts of Interest The authors declare no conflict of interest. References 1. Yin, S.; Mu, S.; Lv, H.; Cheng, N.; Pan, M.; Fu, Z. A highly stable catalyst for PEM fuel cell based on durable titanium diboride support and polymer stabilization. Appl. Catal. B Environ. 2010, 93, 233–240. [CrossRef] 2. He, P.; Liu, M.; Luo, J.L.; Sanger, A.R.; Chuang, K.T. Stabilization of platinum anode catalyst in a H2 S-O2 solid oxide fuel cell with an intermediate TiO2 layer. J. Electrochem. Soc. 2002, 149, A808–A814. [CrossRef] 3. Xu, C.; Wang, X.; Zhu, J. Graphene-metal particle nanocomposites. J. Phys. Chem. C 2008, 112, 19841–19845. [CrossRef] 4. Li, Y.; Tang, L.; Li, J. Preparation and electrochemical performance for methanol oxidation of Pt/graphene nanocomposites. Electrochem. Commun. 2009, 11, 846–849. [CrossRef]
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