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Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol
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Reduced graphene oxide-poly-(2-(dimethylamino) ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol Xiaoguang Jin, Xiaobei Wang, Yue Zhang, Hefang Wang, Yongfang Yang* Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China
article info
abstract
Article history:
The reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag (RGO-
Received 25 January 2018
PDMAEMA-Pt/Ag) nanoscrolls were prepared by rolling up the RGO-PDMAEMA-Pt/Ag sheets
Received in revised form
under freezing conditions. The compositions and structures of RGO-PDMAEMA-Pt/Ag
2 May 2018
sheets and nanoscrolls were characterized by Fourier transform infrared (FTIR) spectros-
Accepted 10 May 2018
copy, thermogravimetric analysis (TGA), scanning electronic microscopy (SEM) and
Available online xxx
transmission electron microscopy (TEM). Because of the bimetallic synergetic effect and unique scrolled structure, RGO-PDMAEMA-Pt/Ag nanoscrolls show excellent catalytic
Keywords:
performance and high electrochemical stability. The ECSA value of RGO-PDMAEMA-Pt/Ag
RGO-PDMAEMA-Pt/Ag
nanoscrolls for Pt is 891 cm2/mg, which is 1.28 times that of the RGO-PDMAEMA-Pt/Ag
Nanoscrolls
sheets (698 cm2/mg). The ratio of the forward oxidation peak current to the reverse peak
Bimetallic nanoparticles
current is about 1.25. The electrochemical results indicate the RGO-PDMAEMA-Pt/Ag
Methanol oxidation
nanoscrolls are promising electrocatalysts for direct methanol fuel cell. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Recently, direct methanol fuel cell (DMFC) has been popular because of its high energy utilization, high energy efficiency and less pollution. For DMFC, platinum (Pt) is one of the most widely used catalysts in methanol oxidation reactions [1,2]. However, the high cost and easy poisoning by carbon monoxide limit the application of Pt in catalytic field. To reduce the usage and the cost of Pt and improve its electrocatalytic performance, many attempts such as alloying with other electrochemically active and more plentiful metals [3,4], preparation of fine nanocatalysts [5,6] and high dispersion of Pt nanoparticles (NPs) on a support with high specific surface area [7], have been
made. Over these years, Pt-based bimetallic catalysts have become one of the important classes of catalysts which can reduce the cost of electrical energy [8e10]. Pt-containing alloys such as PtNi [11], PtCo [12], PtSn [13], and PtCu [14] exhibit superior electrocatalytic activities, compared with monometallic counterparts. Among these metals, silver is regarded as one of the most popular transition alloy metals owing to its low cost and desired synergistic effect [15e18]. The bimetallic Pt-Ag alloy NPs attracted increasing attention owing to its improved activity in many reactions by forming new active sites and inducing synergistic effect. Yang et al. prepared the PtAg2/C-D and PtPd3Ag5/C-D catalysts via a simple and effective chemical reduction to improve the electrocatalytic performance (about 3.35 times as high as that of Pt/C) [19].
* Corresponding author. E-mail address: [email protected] (Y. Yang). https://doi.org/10.1016/j.ijhydene.2018.05.060 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Before commercial viability of DMFC, another important issue to be solved is the long-term durability of electrocatalysts. During the long period of operation, the agglomeration of metal NPs leads to the reduction of catalytic performance. The agglomeration of metal NPs can be reduced by choosing a support which has strong interaction with metal. Therefore, it is necessary for DMFC to find a stable support in electrochemical and strong acid/alkaline environment to run long time. Various types of carbon supports such as carbon Vulcan [20,21], graphene [22] and multiwall carbon nanotubes (MWNTs) [23] are often used to disperse Pt NPs to improve the electrocatalytic activity. As a new carbon nanomaterial, graphene nanoscrolls (GNSs), formed by rolling up the twodimensional graphene sheets, possess the similar properties with the graphene sheets such as high thermal conductivity, excellent electrical conductivity and strong mechanical properties [24e26]. Besides, GNSs, with the open ends, adjustable interlayer galleries and diameter by intercalation or doping, became the optional candidate as the support to load metal NPs. Liu et al. synthesized Pt/reduced graphene oxide nanoscroll (Pt/RGOS) by oxygen implosion in situ rolling up of Pt/RGO sheets and found the Pt/RGOSs possessed significantly higher electrocatalytic activity and stability than Pt/RGO [27]. However, nanoscrolls prepared by rolling up the graphene sheets on which the bimetallic Pt-Ag alloy NPs dispersed have not been reported. In the previous work, we reported a general method to prepare GO-PDA-Au nanoscrolls by rolling up the GO-PDA-Au sheets through lyophilization method [28]. In this study, to reduce the agglomeration of alloy metal NPs, PDMAEMA chains with pyrene terminal groups were introduced to GO sheets via p-p stacking interaction. PDMAEMA chains can act as a stabilizer to disperse metal NPs for its abundant nitrogen atoms which can interact with various metal ions such as Pt, Ag, Pd et al. During the formation of alloy NPs, GO sheets were reduced. The Pt/Ag alloy NPs were uniformly loaded on the surface of RGO-PDMAEMA sheets and were rolled up to form RGO-PDMAEMA-Pt/Ag nanoscrolls by a lyophilization method [29,30]. RGO-PDMAEMA-Pt/Ag nanoscrolls would have the excellent electrocatalytic activity owing to the synergistic effect of Pt/Ag bimetallic alloy and the long stability caused by the introduction of PDMAEMA to GO sheets which effectively avoid the aggregation and falling of Pt/Ag NPs and the unique scrolled structure which reduce the opportunity for metal poisoning. The structure and composition of bimetallic alloy on the GOPDMAEMA sheets were characterized by XRD, XPS and the electrocatalytic activity of methanol oxidation were compared between RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls. The RGO-PDMAEMA-Pt/Ag nanoscrolls will broaden application of graphene-based materials in eletrocatalytic field. The schematic illustration of the procedure for the preparation of RGOPDMAEMA-Pt/Ag nanoscrolls is as follows in Scheme 1.
Experimental section Materials The GO sheets with the average sheets size of 10e20 mm were purchased from Zhejiang carbon Valley Mstar Technology Ltd. 2-
Scheme 1 e The schematic illustration of the procedure for the preparation of RGO-PDMAEMA-Pt/Ag nanoscrolls.
(Dimethylamino) ethyl methacrylate (DMAEMA, 99%) was provided by Acros and was distilled under reduced pressure. Copper bromide (CuBr, 99.5%, from Guo Yao Chemical Company) was washed with glacial acetic acid and dried under vacuum. N,N,N0 ,N0 ,N00 -Pentamethyldiethylenetriamine (PMDETA, 99%), 1pyrenemethanol (98%) and 2-bromo-2-methylpropionyl bromide (98%) were purchased from Aldrich and were used without any further treatment. Triethylamine (AR grade) and THF (AR grade) were bought from Tianjin Chemical Reagent Company and distilled before use. Silver nitrate (AR grade, from Tianjin Yingda Rare Metal Chemical Reagents Company) and chloroplatinic acid (AR grade, from Shanghai Jiuding Chemical Reagents Company) were used directly.
Characterization The structure of pyrene-Br and pyrene-PDMAEMA were characterized by nuclear magnetic resonance spectrometer (NMR 400 MHz). The apparent molecular weight and molecular weight distribution of pyrene-PDMAEMA were determined with gel permeation chromatography (GPC) equipped with a Hitachi L-2130 high-performance liquid chromatography (HPLC) pump. The structure of GO sheets and GO-PDMAEMA sheets were characterized by Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700 FTIR Spectrometric Analyzer) and thermogravimetric analysis (TGA) (PerkineElmer Thermal Analysis SDT/Q600), respectively. The structure of GO sheets, GO-PDMAEMA sheets and RGO-PDMAEMA-Pt/Ag sheets were characterized by X-ray diffraction (XRD) (a D8 Focus diffractometer with Cu Ka radiation), a UV-1800 PC and X-ray photoelectron spectroscopy (XPS) spectra (Kratos Axis Ultra DLD spectrometer employing a monochromated Al Ka X-ray source). The morphologies of the samples were observed by the field-emission transmission electron microscope (FEI-TEM, Tecnai G2 F20) and scanning electronic microscopy (SEM, Nova Nano SEM 450). The thickness of GO sheets and GO-PDMAEMA sheets were characterized by Atomic force microscopy (AFM) (Nanoscope IV atomic force microscope, Digital Instruments). All the products were measured by a Raman spectrometer (Renishaw inVia) with a 532 nm laser excitation source.
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Preparation of pyrene-Br 1-Pyrenemethanol (0.5 g, 2.2 mmol) was dissolved in 5 mL of THF. Then, triethylamine (0.9 mL) was added to the forementioned solution and stirred for 20 min in an ice-salt bath. Next, 2-bromo-2-methylpropionyl bromide (0.5 mL, 4.0 mmol) was dissolved in 5 mL of THF and it was added dropwise into the flask with the ice-salt bath condition. After stirring for 24 h at room temperature, the suspension was filtered and treated with a rotary evaporator in order to remove THF solvent. When CH2Cl2 was added, the solution was washed with 2% HCl solution, 2% Na2CO3 solution and distilled water in turn. Finally, the product was further purified by column chromatography on silica with a mixture of petroleum ether and ethyl acetate (15:1, v/v) and dried under vacuum.
Synthesis of pyrene-PDMAEMA DMAEMA (1.1 mL, 6.5 mmol) and pyrene-Br (0.075 g, 0.2 mmol) were dissolved in 1.5 mL of THF and were degassed by three freeze-pump-thaw cycles. Upon CuBr (0.028 g, 0.199 mmol) and PMDETA (37 ml, 0.199 mmol) were dissolved in 2 mL of THF, the solution was transferred to the Schlenk flask with Atom Transfer Radical Polymerization (ATRP) initiator under a nitrogen atmosphere. After the Schlenk flask was placed in the oil bath and the solution was stirred at 60 C for 24 h, the polymerization was stopped by dipping the Schlenk flask into ice water. The pyrene-PDMAEMA was obtained via precipitation from cold n-hexane after Cu2þ was removed using an Al2O3 column.
Preparation of GO-PDMAEMA sheets Pyrene-PDMAEMA (15 mg, 42.18 mmol), 1 mL of GO solution (10 mg/mL) and 15 mL of deionized water were added into a flask, stirring 24 h at room temperature. Then, the product was washed three times by deionized water and GOPDMAEMA sheets were obtained.
Preparation of RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls GO-PDMAEMA sheets (5 mg) were added into 5 mL of ethylene glycol under ultrasonic dispersion. AgNO3 (1 mmol/L, 0.5 mL) and H2PtCl6 (0.055 mmol/L, 30 mL) were added into the dispersion of GO-PDMAEMA sheets. After stirring for 30 min, NaBH4 (37.83 mg, 1 mmol) was added. Upon stirring at the room temperature for 12 h, the product was washed with ethylene glycol and distilled water three times and dried at vacuum. RGO-PDMAEMA-Pt/Ag sheets (1 mg) was added to 10 mL of deionized water and cooled in liquid nitrogen. Then, it was placed in freeze drier and RGO-PDMAEMA-Pt/Ag nanoscrolls were obtained.
Electrochemical measurements Cyclic voltammetry (CV) studies of methanol electrooxidation were performed using an IM6&ZENNIUM electrochemical
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workstation with a three-electrode system. Before the measurement, the catalysts (1 mg) were dispersed in 0.05 wt% Nafion solution (1 mL) and sonicated several minutes to form a homogeneous dispersion. Next, the dispersion was evenly coated on the surface of the Glassy carbon (GC) disks which were polished with Al2O3 powders and dried in air at room temperature. The electrolytic solutions were deaerated with high purity nitrogen for 15 min and tested with the modified glassy carbon electrode (GCE; 3 mm in diameter) as the working electrode, the saturated calomel electrode (SCE) and the Pt-plate as the reference and counter electrodes. All potentials are given versus the reference electrode of Ag/AgCl saturated KCl.
Results and discussion PDMAEMA chains with terminal pyrene groups were synthesized by ATRP using pyrene-Br as initiator. The 1H NMR spectra of the pyrene-Br ATRP initiator and pyrene-PDMAEMA were shown in Figure S1. As shown in Figure S2, the GPC result demonstrated that the apparent weight-average molecular weight (Mn) of the PDMAEMA was 6.6 K and the molecular weight distribution was 1.28. After the GO sheets were modified by PDMAEMA chains via by p-p stacking, the GOPDMAEMA sheets were obtained. Fig. 1 shows the typical AFM images of GO sheets and GOPDMAEMA sheets. The average thickness of GO sheets is about 1.5 nm. After PDMAEMA chains were attached to the GO sheets, the thickness of GO-PDMAEMA sheets increases to about 5 nm, indicating PDMAEMA chains were successfully anchored on the surface of GO sheets. The FTIR spectra of GO sheets and GO-PDMEMA sheets were shown in Fig. 2A. In FTIR spectrum of GO sheets (curve a), a wide band at 3000-3650 cm1 is attributed to the hydroxyl stretching vibration of the -OH group of the GO sheets. Two absorption peaks at 1709 and 1630 cm1 are associated with the carboxyl stretching and the skeletal vibration of GO sheets [31]. Curve b showed the peaks of PDMAEMA at 2948 cm1 from C-H symmetric and asymmetric stretching of methyl and methylene groups. The characteristic absorptions at 1729 cm1 from the C]O stretching of the ester group of PDMAEMA, 1151 cm1 from C-N stretching of -N(CH3)2 groups, 2823 and 2771 cm1 from C-H stretching of the -N(CH3)2 groups are observed, which demonstrates that PDMAEMA chains were successfully grafted onto the surface of GO sheets [32e34]. Fig. 2B show the TGA curves of GO sheets and GOPDMAEMA sheets. The samples were heated to 800 C at a heating rate of 10 K/min under nitrogen atmosphere. The GO sheets showed a weight loss of 15 wt% below 150 C for adsorption water and a 40 wt% weight loss between 150 C and 600 C owing to the thermal decomposition of -COOH and -OH functional groups on the GO sheets [34]. After GO sheets were modified with PDMAEMA, the TGA results showed the residual content of 18 wt%, which indicates the weight percentage of PDMAEMA is about 27 wt% in GO-PDMAEMA sheets. Figure S3 shows the TGA curve of GO-PDMAEMA-Pt/Ag sheets under oxygen atmosphere. The results showed the residual content of 13.6 wt%, which indicates the weight percentage of Pt and Ag2O was about 13.6 wt% in GO-PDMAEMA-Pt/Ag sheets.
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Fig. 1 e Tapping mode AFM topographic images and height. Profiles of (a) GO sheets and (b) GO-PDMAEMA sheets.
Fig. 2 e FTIR spectra (A) and TGA curves (B) of (a) GO sheets and (b) GO-PDMAEMA sheets.
Once the AgNO3 and H2PtCl6 were added to the GOPDMAEMA sheets aqueous dispersion and reduced via NaBH4, Pt/Ag alloys NPs were loaded on the surface of RGOPDMAEMA sheets, accompanying with the reduction of GO sheets. Fig. 3A shows the XRD spectra of GO sheets, GOPDMAEMA sheets and RGO-PDMAEMA-Pt/Ag sheets. The distinct (002) diffraction peak of GO sheets appears at 11.67 , which corresponded to a d-spacing of 0.76 nm. Upon pyrenePDMAEMA chains were attached to GO sheets, the diffraction (002) peak of GO sheets from 11.67 decreased to 9.59 , corresponding to the increasing interlayer spacing from 0.76 to 0.92 nm. With the formation of Pt/Ag bimetallic NPs under the reduction condition, a weak and broad peak at 26.2 is close to the typical diffraction peak of graphite (d-spacing 3.35 A at 2q ¼ 26.4 ), indicating the reduction of GO sheets [32]. The main diffraction peaks at 38.7 , 44.9 , 65.2 , 81.1 appeared in the XRD spectra of RGO-PDMAEMA-Pt/Ag sheets (curve c). According to the literature, the typical Pt NPs reveal four main diffraction peaks at 39.8 , 46.2 , 67.6 and 81.3 , which are the
corresponding (111), (200), (220) and (311) facets of the face centered cubic (fcc) Pt crystal (JCPDS Card No. 04-0802), while the peaks of the facets of Ag crystal (JCPDS Card No. 04-0783) are located at 38.0 , 44.3 , 64.4 and 77.3 , the corresponding peak positions of RGO-PDMAEMA-Pt/Ag sheets are located between those of Ag and Pt crystals indicated the formation of alloy structure [35]. According to the well-known Scherrer equation, the mean alloy particle size can be calculated to be about 13 nm. The UVevis absorption spectra of GO sheets, GOPDMAEMA sheets and RGO-PDMAEMA-Pt/Ag sheets are shown in Fig. 3B. GO sheets showed a typical peak at 228 nm and a weak peak at about 300 nm due to p-p* transitions of the C-C bonds and p-p* transitions of the C]O bonds, respectively [36]. The absorption peaks of GO-PDMAEMA sheets showed a redshift from 228 to 238 nm for the interaction between GO sheets and pyrene-PDMAEMA [37]. Upon deposition of Pt-Ag alloys on the surface of GO-PDMAEMA sheets, a redshift absorption from 238 to 271 nm, suggests the restoration of the conjugated electronic structure of GO sheets [20,38]. This
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Fig. 3 e XRD (A) and UVevisible absorption spectra (B) of (a) GO sheets, (b) GO-PDMAEMA sheets and (c) RGO-PDMAEMA-Pt/ Ag sheets.
results also proves that GO sheets were reduced during the formation of Pt/Ag alloy NPs. In the UVevis absorption curve of RGO-PDMAEMA-Pt/Ag sheets, a new peak at about 400 nm appears, which is corresponding to the typical surface plasmon resonance of Ag NPs, implying the formation of RGOPDMAEMA-Pt/Ag sheets [39]. The morphology of RGO-PDMAEMA-Pt/Ag sheets was characterized by TEM. The TEM images of RGO-PDMAEMA-Pt/ Ag sheets clearly showed small Pt/Ag NPs were homogeneously distributed on the surface of large GO-PDMAEMA sheets. The mean diameter of Pt/Ag NPs was about 2e4 nm. The size from the Scherrer equation (about 13 nm) is larger than that obtained by TEM, which may be attributed to the fact that large particles were picked in XRD patterns. No aggregation was observed due to the protection of Pt/Ag NPs by the PDMAEMA chains. The high-resolution TEM images showed the oriented and ordered lattice fringes of Pt/Ag NPs (in Fig. 4d), where the d-spacing values of 2.25 A and 2.35 A coincide with that of face centered cubic (fcc) Pt (111) [40,41] and that of face centered cubic (fcc) Ag (111) plain, respectively [42]. After the RGO-PDMAEMA-Pt/Ag sheets were treated in the liquid nitrogen, the RGO-PDMAEMA-Pt/Ag nanoscrolls were formed. Fig. 5 shows the SEM images of GO sheets, GOPDMAEMA sheets, RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls. It was observed obviously that GO sheets exhibited a planar texture (Fig. 5a). When GO sheets were modified by PDMAEMA, a coase plane can be found owing to the anchoring of the PDMAEMA chains (Fig. 5b). Upon Pt/Ag NPs depositing, the SEM images of RGO-PDMAEMA-Pt/Ag sheets showed that small alloy metal NPs were uniformly distributed on the surface of GO sheets (Fig. 5c). When the RGOPDMAEMA-Pt/Ag sheets were treated in liquid nitrogen by freezing drying, a scrolled structure was formed (Fig. 5d). The GO nanoscrolls were also formed by treating GO sheets in liquid nitrogen. The morphology of GO sheets and GO nanoscrolls in aqueous solution and in 0.05 wt% Nafion solution coated on the surface of the glassy carbon were shown in Figure S4. The TEM image shows the diameter of the RGO-PDMAEMAPt/Ag nanoscrolls is about 500 nm (Fig. 6a). It can be clearly observed that RGO-PDMAEMA-Pt/Ag nanoscrolls were rolled
up with several layers and lots of Pt/Ag NPs were deposited evenly on the surface of RGO sheets. The XPS measurement was performed to obtain the surface element states of the samples. The chemical binding energies located at 284.8 and 531.1 eV are attributed to the C1s and O1s of GO sheets, respectively (Fig. 7A). Upon the PDMAEMA chains anchored to the surface of the GO sheets, the weak peak at 398.3 eV corresponding to the N1s binding energy was observed and the percentage of nitrogen was 4.43 wt%, which indicates the percentage of PDMAEMA in the GO-PDMAEMA sheets was about 49.78 wt% [43]. Two characteristic peaks at 76 and 368 eV in RGO-PDMAEAM-Pt/Ag sheets and nanoscrolls were attributed to Pt 4f7/2 and Ag 3d3/2, respectively [37,44], demonstrating Pt and Ag alloys were loaded on the surface of GO-PDMAEMA sheets [45,46]. The molar ratio of Pt and Ag was calculated to be 1.86:1. Two typical peak at 70.9 and 74.1 eV correspond to Pt 4f7/2 and Pt 4f5/2. Compared with pure Pt or monometallic Pt, the higher binding energy (from 70.3 to 70.9 eV) indicates the downshift of Pt f-band center due to alloying and stronger binding of Pt in Pt-Ag alloy [43]. The contents of Pt and Ag NPs were tested by ICP. According to the results of ICP, the contents of Pt and Ag NPs in RGO-PDMAEMA-Pt/Ag sheets were 14.4% and 4.3%, respectively. In addition, the results of ICP showed that the atomic ratio of Pt and Ag NPs is 2.7:1, which is close to the feed ratio (3:1). Compared with the content of Pt and Ag species in the bulk (calculated from ICP OES) and on the surface (estimated from XPS spectra), the lower molar ratio of Pt to Ag on the surface indicates more Ag preferential located on the surface of alloy. The Raman spectra of GO sheets, GO-PDMAEMA sheets, RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls were shown in Fig. 8. Two prominent peaks about 1350 and 1580 cm1 corresponding to D band (associated with the defect-related mode) and G band (related to the graphitic hexagon-pinch mode) of GO sheets were observed for all the samples [47]. The value of ID/IG for GO-PDMAEMA sheets is 0.93, higher than that of GO sheets (0.84), suggesting that some more sp2 carbon atoms in the GO sheets turned into sp3 ones owing to the grafting of PDMAEMA. Upon Pt/Ag NPs were loaded on the surface of GO-PDMAEMA sheets, the value of ID/IG decreased to 0.88, which was caused by more sp2 modes formed by the
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Fig. 4 e TEM images of RGO-PDMAEMA-Pt/Ag sheets.
Fig. 5 e SEM images of (a) GO sheets, (b) GO-PDMAEMA sheets, (c) RGO-PDMAEMA-Pt/Ag sheets and (d) RGO-PDMAEMA-Pt/ Ag nanoscrolls.
deposition of Pt/Ag NPs on the RGO sheets [36]. Compared with RGO-PDMAEMA-Pt/Ag sheets, the ID/IG ratio for the RGOPDMAEMA-Pt/Ag nanoscrolls showed an increase (from 0.88 to 0.96) owing to the increased p-p stacking caused by the scrolled structure [29]. After RGO-PDMAEMA-Pt/Ag sheets were rolled up, a red shift of the G peak from 1592 to 1601 cm1
was observed owing to a slight modification of the intra C-C bond in scrolled structure and a change of the phonon dispersion from the stacked graphene layers [47]. The electrocatalytic performance of RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls for methanol oxidation was investigated in nitrogen-saturated 0.5 M H2SO4 solutions and
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Fig. 6 e TEM images of RGO-PDMAEMA-Pt/Ag nanoscrolls.
Fig. 7 e (A) XPS spectrum of (a) GO sheets, (b) GO-PDMAEMA sheets, (c) RGO-PDMAEMA-Pt/Ag sheets and (d) RGO-PDMAEMAPt/Ag nanoscrolls, (B) Pt 4f7/2 and (C) Ag 3d3/2. 0.5 M H2SO4 solutions containing 0.5 M CH3OH solutions, respectively. The ECSA value provides information regarding the number of available electrochemically active sites which is essential for understanding electrocatalytic activity of the catalysts. It can be obtained via Equation (1): ECSA m2 g1 ¼ Q H mC cm2 210 mC cm2 Pt LPt gPt m2
Fig. 8 e Raman spectra of (a) GO sheets, (b) GO-DMAEMA sheets, (c) RGO-PDMAEMA-Pt/Ag sheets and (d) RGOPDMAEMA-Pt/Ag nanoscrolls.
(1)
QH is the coulombic charge for hydrogen adsorption on Pt sites. LPt is the platinum loading in the working electrode and the value of 210 (mC cm2) represent the charge required to oxidize a monolayer of hydrogen. The ECSA value of the RGO-PDMAEMA-Pt/Ag nanoscrolls for Pt is 891 cm2/mg, which is 1.28 times that of the RGOPDMAEMA-Pt/Ag sheets (698 cm2/mg). The cyclic voltammetry of GO sheets and nanoscrolls for methanol oxidation was also investigated in the same condition (Figure S5). The ECSA value of the GO nanoscrolls (16.65 cm2/mg) is 1.28 times that of the GO sheets (14.75 cm2/mg). The increase proportion of the
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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ECSA value is similar to that of RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls (1.28 times), which indicates that the higher ESCA value for the RGO-PDMAEMA-Pt/Ag nanoscrolls was caused by the unique scrolled structure. Compared with RGO-PDMAEMA-Pt/Ag sheets, the RGOPDMAEMA-Pt/Ag nanoscrolls exhibit higher current at all corresponding potentials and the peak potential shifts by over 19.3 mV in the negative direction (Fig. 9B). The forward peak current for the RGO-PDMAEMA-Pt/Ag nanoscrolls is 46.34 mA/ mg, which is 1.26 times higher than that for the RGOPDMAEMA-Pt/Ag sheets (36.77 mA/mg). The lower onset potential and higher current density of the RGO-PDMAEMA-Pt/ Ag nanoscrolls indicates that RGO-PDMAEMA-Pt/Ag nanoscrolls have more practical significance in the acceptable output voltage range of fuel cells. The different electrocatalytic activity between the RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls was caused by curling of the graphene sheets. The opened tubular structures and different interlayer spaces of the RGO-PDMAEMA-Pt/Ag nanoscrolls promote the mass transfer efficiency for reactant, product and electrolyte in the reaction process, thus increasing the electrocatalytic performance of RGO-PDMAEMA-Pt/Ag nanoscrolls for methanol. Compared with RGO-PDMAEMA-Pt/Ag sheets, the RGOPDMAEMA-Pt/Ag nanoscrolls show higher current towards methanol electrooxidation for the whole testing time, which indicates that the RGO-PDMAEMA-Pt/Ag nanoscrolls have better electrocatalytic activity (Fig. 9C). The ratio I(30 min)/I(0 min)
for the RGO-PDMAEMA-Pt/Ag nanoscrolls of 0.344 is higher than that for the RGO-PDMAEMA-Pt/Ag sheets (0.184), indicating that RGO-PDMAEMA-Pt/Ag nanoscrolls have more desirable stability. Compared with RGO sheets, RGO nanoscrolls decrease the direct contact chance with methanol by providing more pore channels, thus reducing the opportunity for metal poisoning in the electrocatalytic operation process. The introduction of PDMAEMA prevents Pt NPs from dissolving/falling into the reaction solution and improves the stability of alloy NPs. Table 1 listed some data about the performances of methanol oxidation on various catalysts. The comparative results show that the electrocatalytic activity of RGOPDMAEMA-Pt/Ag nanoscrolls is high. The ECSA value of Pt in RGO-PDMAEMA-Pt/Ag nanoscrolls is determined to be 891 cm2/mg, which is much higher than those obtained from LM/Pt (407 cm2/mg), AM/Pt50Co50 (543 cm2/mg) [49], Pt/C (383 cm2/mg) [41] and Pt/graphene (286 cm2/mg) [50]. The ratio of the forward oxidation peak current (If) to the reverse peak current (Ib), (If/Ib) is commonly used to evaluate the tolerance of catalysts toward the poisoning species such as adsorbed CO intermediates formed via decomposition of methanol. The If/ Ib ratio of RGO-PDMAEMA-Pt/Ag nanoscrolls is about 1.25, which is higher than that of the RGO-PDMAEMA-Pt/Ag sheets (1.22), LM/Pt (1.12), AM/Pt50Co50 (1.02) [49], Pt/C (0.83) [42] and Pt/graphene (1.20), Pt (0.71), implying RGO-PDMAEMA-Pt/Ag nanoscrolls have better catalytic tolerance.
Fig. 9 e Typical CV scans for the electrooxidation reactions of methanol on different electrodes (a) RGO-PDMAEMA-Pt/Ag sheets and (b) RGO-PDMAEMA-Pt/Ag nanoscrolls. CV scans between 0 and 1.6 V in nitrogen-saturated 0.5 M H2SO4 solutions (A) and 0.5 M H2SO4 þ 0.5 M CH3OH solutions (B) at 50 mV/s, 298 K. (C) Current-time curves for the electrooxidation reactions of methanol on different electrodes at 0.8 V, 298 K. Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Table 1 e Comparison of performances of methanol oxidation with different catalysts. Electrocatalyst
ECSA (cm2/mg) If/Ib ratio References
Pt/C LM/Pt AM/Pt50Co50 Pt/Graphene RGO-PDMAEMA-Pt/Ag sheets RGO-PDMAEMA-Pt/Ag nanoscrolls
383 407 543 286 698
0.83 1.12 1.02 1.20 1.22
[42] [49] [49] [50] This study
891
1.25
This study
Conclusions Pt/Ag alloy NPs were uniformly loaded on the surface of GOPDMAEMA sheets and RGO-PDMAEMA-Pt/Ag nanoscrolls were formed by curling RGO-PDMAEMA-Pt/Ag sheets through a lyophilization method. The introduction of PDMAEMA to GO sheets can form the small size of alloy NPs, thus increasing the electrochemical performance of the composites. RGOPDMAEMA-Pt/Ag nanoscrolls have the high ECSA value of Pt (891 cm2/mg) and the If/Ib ratio (1.25) and the high ratio I(30 min)/I(0 min) (0.344). The excellent electrochemical performance was caused by the synergistic effect of Pt/Ag bimetallic alloy and the long-term stability was caused by the introduction of PDMAEMA to GO sheets which effectively avoid the aggregation and falling of Pt/Ag NPs and the unique scrolled structure which decrease the contact opportunity with methanol and reduce the opportunity for metal poisoning.
Acknowledgements This work was financially supported by the Natural Science Foundation of Hebei Province (No. E2015202262, No. B2017202226), Hebei Provincial Natural Science Youth Foundation (No. B2016409056) and the National Natural Science Foundation of China (No. 21776058, No. 51603061).
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.05.060.
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Reduced graphene oxide-poly-(2-(dimethylamino) ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol Xiaoguang Jin, Xiaobei Wang, Yue Zhang, Hefang Wang, Yongfang Yang* Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China
article info
abstract
Article history:
The reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag (RGO-
Received 25 January 2018
PDMAEMA-Pt/Ag) nanoscrolls were prepared by rolling up the RGO-PDMAEMA-Pt/Ag sheets
Received in revised form
under freezing conditions. The compositions and structures of RGO-PDMAEMA-Pt/Ag
2 May 2018
sheets and nanoscrolls were characterized by Fourier transform infrared (FTIR) spectros-
Accepted 10 May 2018
copy, thermogravimetric analysis (TGA), scanning electronic microscopy (SEM) and
Available online xxx
transmission electron microscopy (TEM). Because of the bimetallic synergetic effect and unique scrolled structure, RGO-PDMAEMA-Pt/Ag nanoscrolls show excellent catalytic
Keywords:
performance and high electrochemical stability. The ECSA value of RGO-PDMAEMA-Pt/Ag
RGO-PDMAEMA-Pt/Ag
nanoscrolls for Pt is 891 cm2/mg, which is 1.28 times that of the RGO-PDMAEMA-Pt/Ag
Nanoscrolls
sheets (698 cm2/mg). The ratio of the forward oxidation peak current to the reverse peak
Bimetallic nanoparticles
current is about 1.25. The electrochemical results indicate the RGO-PDMAEMA-Pt/Ag
Methanol oxidation
nanoscrolls are promising electrocatalysts for direct methanol fuel cell. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Recently, direct methanol fuel cell (DMFC) has been popular because of its high energy utilization, high energy efficiency and less pollution. For DMFC, platinum (Pt) is one of the most widely used catalysts in methanol oxidation reactions [1,2]. However, the high cost and easy poisoning by carbon monoxide limit the application of Pt in catalytic field. To reduce the usage and the cost of Pt and improve its electrocatalytic performance, many attempts such as alloying with other electrochemically active and more plentiful metals [3,4], preparation of fine nanocatalysts [5,6] and high dispersion of Pt nanoparticles (NPs) on a support with high specific surface area [7], have been
made. Over these years, Pt-based bimetallic catalysts have become one of the important classes of catalysts which can reduce the cost of electrical energy [8e10]. Pt-containing alloys such as PtNi [11], PtCo [12], PtSn [13], and PtCu [14] exhibit superior electrocatalytic activities, compared with monometallic counterparts. Among these metals, silver is regarded as one of the most popular transition alloy metals owing to its low cost and desired synergistic effect [15e18]. The bimetallic Pt-Ag alloy NPs attracted increasing attention owing to its improved activity in many reactions by forming new active sites and inducing synergistic effect. Yang et al. prepared the PtAg2/C-D and PtPd3Ag5/C-D catalysts via a simple and effective chemical reduction to improve the electrocatalytic performance (about 3.35 times as high as that of Pt/C) [19].
* Corresponding author. E-mail address: [email protected] (Y. Yang). https://doi.org/10.1016/j.ijhydene.2018.05.060 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Before commercial viability of DMFC, another important issue to be solved is the long-term durability of electrocatalysts. During the long period of operation, the agglomeration of metal NPs leads to the reduction of catalytic performance. The agglomeration of metal NPs can be reduced by choosing a support which has strong interaction with metal. Therefore, it is necessary for DMFC to find a stable support in electrochemical and strong acid/alkaline environment to run long time. Various types of carbon supports such as carbon Vulcan [20,21], graphene [22] and multiwall carbon nanotubes (MWNTs) [23] are often used to disperse Pt NPs to improve the electrocatalytic activity. As a new carbon nanomaterial, graphene nanoscrolls (GNSs), formed by rolling up the twodimensional graphene sheets, possess the similar properties with the graphene sheets such as high thermal conductivity, excellent electrical conductivity and strong mechanical properties [24e26]. Besides, GNSs, with the open ends, adjustable interlayer galleries and diameter by intercalation or doping, became the optional candidate as the support to load metal NPs. Liu et al. synthesized Pt/reduced graphene oxide nanoscroll (Pt/RGOS) by oxygen implosion in situ rolling up of Pt/RGO sheets and found the Pt/RGOSs possessed significantly higher electrocatalytic activity and stability than Pt/RGO [27]. However, nanoscrolls prepared by rolling up the graphene sheets on which the bimetallic Pt-Ag alloy NPs dispersed have not been reported. In the previous work, we reported a general method to prepare GO-PDA-Au nanoscrolls by rolling up the GO-PDA-Au sheets through lyophilization method [28]. In this study, to reduce the agglomeration of alloy metal NPs, PDMAEMA chains with pyrene terminal groups were introduced to GO sheets via p-p stacking interaction. PDMAEMA chains can act as a stabilizer to disperse metal NPs for its abundant nitrogen atoms which can interact with various metal ions such as Pt, Ag, Pd et al. During the formation of alloy NPs, GO sheets were reduced. The Pt/Ag alloy NPs were uniformly loaded on the surface of RGO-PDMAEMA sheets and were rolled up to form RGO-PDMAEMA-Pt/Ag nanoscrolls by a lyophilization method [29,30]. RGO-PDMAEMA-Pt/Ag nanoscrolls would have the excellent electrocatalytic activity owing to the synergistic effect of Pt/Ag bimetallic alloy and the long stability caused by the introduction of PDMAEMA to GO sheets which effectively avoid the aggregation and falling of Pt/Ag NPs and the unique scrolled structure which reduce the opportunity for metal poisoning. The structure and composition of bimetallic alloy on the GOPDMAEMA sheets were characterized by XRD, XPS and the electrocatalytic activity of methanol oxidation were compared between RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls. The RGO-PDMAEMA-Pt/Ag nanoscrolls will broaden application of graphene-based materials in eletrocatalytic field. The schematic illustration of the procedure for the preparation of RGOPDMAEMA-Pt/Ag nanoscrolls is as follows in Scheme 1.
Experimental section Materials The GO sheets with the average sheets size of 10e20 mm were purchased from Zhejiang carbon Valley Mstar Technology Ltd. 2-
Scheme 1 e The schematic illustration of the procedure for the preparation of RGO-PDMAEMA-Pt/Ag nanoscrolls.
(Dimethylamino) ethyl methacrylate (DMAEMA, 99%) was provided by Acros and was distilled under reduced pressure. Copper bromide (CuBr, 99.5%, from Guo Yao Chemical Company) was washed with glacial acetic acid and dried under vacuum. N,N,N0 ,N0 ,N00 -Pentamethyldiethylenetriamine (PMDETA, 99%), 1pyrenemethanol (98%) and 2-bromo-2-methylpropionyl bromide (98%) were purchased from Aldrich and were used without any further treatment. Triethylamine (AR grade) and THF (AR grade) were bought from Tianjin Chemical Reagent Company and distilled before use. Silver nitrate (AR grade, from Tianjin Yingda Rare Metal Chemical Reagents Company) and chloroplatinic acid (AR grade, from Shanghai Jiuding Chemical Reagents Company) were used directly.
Characterization The structure of pyrene-Br and pyrene-PDMAEMA were characterized by nuclear magnetic resonance spectrometer (NMR 400 MHz). The apparent molecular weight and molecular weight distribution of pyrene-PDMAEMA were determined with gel permeation chromatography (GPC) equipped with a Hitachi L-2130 high-performance liquid chromatography (HPLC) pump. The structure of GO sheets and GO-PDMAEMA sheets were characterized by Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700 FTIR Spectrometric Analyzer) and thermogravimetric analysis (TGA) (PerkineElmer Thermal Analysis SDT/Q600), respectively. The structure of GO sheets, GO-PDMAEMA sheets and RGO-PDMAEMA-Pt/Ag sheets were characterized by X-ray diffraction (XRD) (a D8 Focus diffractometer with Cu Ka radiation), a UV-1800 PC and X-ray photoelectron spectroscopy (XPS) spectra (Kratos Axis Ultra DLD spectrometer employing a monochromated Al Ka X-ray source). The morphologies of the samples were observed by the field-emission transmission electron microscope (FEI-TEM, Tecnai G2 F20) and scanning electronic microscopy (SEM, Nova Nano SEM 450). The thickness of GO sheets and GO-PDMAEMA sheets were characterized by Atomic force microscopy (AFM) (Nanoscope IV atomic force microscope, Digital Instruments). All the products were measured by a Raman spectrometer (Renishaw inVia) with a 532 nm laser excitation source.
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
Preparation of pyrene-Br 1-Pyrenemethanol (0.5 g, 2.2 mmol) was dissolved in 5 mL of THF. Then, triethylamine (0.9 mL) was added to the forementioned solution and stirred for 20 min in an ice-salt bath. Next, 2-bromo-2-methylpropionyl bromide (0.5 mL, 4.0 mmol) was dissolved in 5 mL of THF and it was added dropwise into the flask with the ice-salt bath condition. After stirring for 24 h at room temperature, the suspension was filtered and treated with a rotary evaporator in order to remove THF solvent. When CH2Cl2 was added, the solution was washed with 2% HCl solution, 2% Na2CO3 solution and distilled water in turn. Finally, the product was further purified by column chromatography on silica with a mixture of petroleum ether and ethyl acetate (15:1, v/v) and dried under vacuum.
Synthesis of pyrene-PDMAEMA DMAEMA (1.1 mL, 6.5 mmol) and pyrene-Br (0.075 g, 0.2 mmol) were dissolved in 1.5 mL of THF and were degassed by three freeze-pump-thaw cycles. Upon CuBr (0.028 g, 0.199 mmol) and PMDETA (37 ml, 0.199 mmol) were dissolved in 2 mL of THF, the solution was transferred to the Schlenk flask with Atom Transfer Radical Polymerization (ATRP) initiator under a nitrogen atmosphere. After the Schlenk flask was placed in the oil bath and the solution was stirred at 60 C for 24 h, the polymerization was stopped by dipping the Schlenk flask into ice water. The pyrene-PDMAEMA was obtained via precipitation from cold n-hexane after Cu2þ was removed using an Al2O3 column.
Preparation of GO-PDMAEMA sheets Pyrene-PDMAEMA (15 mg, 42.18 mmol), 1 mL of GO solution (10 mg/mL) and 15 mL of deionized water were added into a flask, stirring 24 h at room temperature. Then, the product was washed three times by deionized water and GOPDMAEMA sheets were obtained.
Preparation of RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls GO-PDMAEMA sheets (5 mg) were added into 5 mL of ethylene glycol under ultrasonic dispersion. AgNO3 (1 mmol/L, 0.5 mL) and H2PtCl6 (0.055 mmol/L, 30 mL) were added into the dispersion of GO-PDMAEMA sheets. After stirring for 30 min, NaBH4 (37.83 mg, 1 mmol) was added. Upon stirring at the room temperature for 12 h, the product was washed with ethylene glycol and distilled water three times and dried at vacuum. RGO-PDMAEMA-Pt/Ag sheets (1 mg) was added to 10 mL of deionized water and cooled in liquid nitrogen. Then, it was placed in freeze drier and RGO-PDMAEMA-Pt/Ag nanoscrolls were obtained.
Electrochemical measurements Cyclic voltammetry (CV) studies of methanol electrooxidation were performed using an IM6&ZENNIUM electrochemical
3
workstation with a three-electrode system. Before the measurement, the catalysts (1 mg) were dispersed in 0.05 wt% Nafion solution (1 mL) and sonicated several minutes to form a homogeneous dispersion. Next, the dispersion was evenly coated on the surface of the Glassy carbon (GC) disks which were polished with Al2O3 powders and dried in air at room temperature. The electrolytic solutions were deaerated with high purity nitrogen for 15 min and tested with the modified glassy carbon electrode (GCE; 3 mm in diameter) as the working electrode, the saturated calomel electrode (SCE) and the Pt-plate as the reference and counter electrodes. All potentials are given versus the reference electrode of Ag/AgCl saturated KCl.
Results and discussion PDMAEMA chains with terminal pyrene groups were synthesized by ATRP using pyrene-Br as initiator. The 1H NMR spectra of the pyrene-Br ATRP initiator and pyrene-PDMAEMA were shown in Figure S1. As shown in Figure S2, the GPC result demonstrated that the apparent weight-average molecular weight (Mn) of the PDMAEMA was 6.6 K and the molecular weight distribution was 1.28. After the GO sheets were modified by PDMAEMA chains via by p-p stacking, the GOPDMAEMA sheets were obtained. Fig. 1 shows the typical AFM images of GO sheets and GOPDMAEMA sheets. The average thickness of GO sheets is about 1.5 nm. After PDMAEMA chains were attached to the GO sheets, the thickness of GO-PDMAEMA sheets increases to about 5 nm, indicating PDMAEMA chains were successfully anchored on the surface of GO sheets. The FTIR spectra of GO sheets and GO-PDMEMA sheets were shown in Fig. 2A. In FTIR spectrum of GO sheets (curve a), a wide band at 3000-3650 cm1 is attributed to the hydroxyl stretching vibration of the -OH group of the GO sheets. Two absorption peaks at 1709 and 1630 cm1 are associated with the carboxyl stretching and the skeletal vibration of GO sheets [31]. Curve b showed the peaks of PDMAEMA at 2948 cm1 from C-H symmetric and asymmetric stretching of methyl and methylene groups. The characteristic absorptions at 1729 cm1 from the C]O stretching of the ester group of PDMAEMA, 1151 cm1 from C-N stretching of -N(CH3)2 groups, 2823 and 2771 cm1 from C-H stretching of the -N(CH3)2 groups are observed, which demonstrates that PDMAEMA chains were successfully grafted onto the surface of GO sheets [32e34]. Fig. 2B show the TGA curves of GO sheets and GOPDMAEMA sheets. The samples were heated to 800 C at a heating rate of 10 K/min under nitrogen atmosphere. The GO sheets showed a weight loss of 15 wt% below 150 C for adsorption water and a 40 wt% weight loss between 150 C and 600 C owing to the thermal decomposition of -COOH and -OH functional groups on the GO sheets [34]. After GO sheets were modified with PDMAEMA, the TGA results showed the residual content of 18 wt%, which indicates the weight percentage of PDMAEMA is about 27 wt% in GO-PDMAEMA sheets. Figure S3 shows the TGA curve of GO-PDMAEMA-Pt/Ag sheets under oxygen atmosphere. The results showed the residual content of 13.6 wt%, which indicates the weight percentage of Pt and Ag2O was about 13.6 wt% in GO-PDMAEMA-Pt/Ag sheets.
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Fig. 1 e Tapping mode AFM topographic images and height. Profiles of (a) GO sheets and (b) GO-PDMAEMA sheets.
Fig. 2 e FTIR spectra (A) and TGA curves (B) of (a) GO sheets and (b) GO-PDMAEMA sheets.
Once the AgNO3 and H2PtCl6 were added to the GOPDMAEMA sheets aqueous dispersion and reduced via NaBH4, Pt/Ag alloys NPs were loaded on the surface of RGOPDMAEMA sheets, accompanying with the reduction of GO sheets. Fig. 3A shows the XRD spectra of GO sheets, GOPDMAEMA sheets and RGO-PDMAEMA-Pt/Ag sheets. The distinct (002) diffraction peak of GO sheets appears at 11.67 , which corresponded to a d-spacing of 0.76 nm. Upon pyrenePDMAEMA chains were attached to GO sheets, the diffraction (002) peak of GO sheets from 11.67 decreased to 9.59 , corresponding to the increasing interlayer spacing from 0.76 to 0.92 nm. With the formation of Pt/Ag bimetallic NPs under the reduction condition, a weak and broad peak at 26.2 is close to the typical diffraction peak of graphite (d-spacing 3.35 A at 2q ¼ 26.4 ), indicating the reduction of GO sheets [32]. The main diffraction peaks at 38.7 , 44.9 , 65.2 , 81.1 appeared in the XRD spectra of RGO-PDMAEMA-Pt/Ag sheets (curve c). According to the literature, the typical Pt NPs reveal four main diffraction peaks at 39.8 , 46.2 , 67.6 and 81.3 , which are the
corresponding (111), (200), (220) and (311) facets of the face centered cubic (fcc) Pt crystal (JCPDS Card No. 04-0802), while the peaks of the facets of Ag crystal (JCPDS Card No. 04-0783) are located at 38.0 , 44.3 , 64.4 and 77.3 , the corresponding peak positions of RGO-PDMAEMA-Pt/Ag sheets are located between those of Ag and Pt crystals indicated the formation of alloy structure [35]. According to the well-known Scherrer equation, the mean alloy particle size can be calculated to be about 13 nm. The UVevis absorption spectra of GO sheets, GOPDMAEMA sheets and RGO-PDMAEMA-Pt/Ag sheets are shown in Fig. 3B. GO sheets showed a typical peak at 228 nm and a weak peak at about 300 nm due to p-p* transitions of the C-C bonds and p-p* transitions of the C]O bonds, respectively [36]. The absorption peaks of GO-PDMAEMA sheets showed a redshift from 228 to 238 nm for the interaction between GO sheets and pyrene-PDMAEMA [37]. Upon deposition of Pt-Ag alloys on the surface of GO-PDMAEMA sheets, a redshift absorption from 238 to 271 nm, suggests the restoration of the conjugated electronic structure of GO sheets [20,38]. This
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Fig. 3 e XRD (A) and UVevisible absorption spectra (B) of (a) GO sheets, (b) GO-PDMAEMA sheets and (c) RGO-PDMAEMA-Pt/ Ag sheets.
results also proves that GO sheets were reduced during the formation of Pt/Ag alloy NPs. In the UVevis absorption curve of RGO-PDMAEMA-Pt/Ag sheets, a new peak at about 400 nm appears, which is corresponding to the typical surface plasmon resonance of Ag NPs, implying the formation of RGOPDMAEMA-Pt/Ag sheets [39]. The morphology of RGO-PDMAEMA-Pt/Ag sheets was characterized by TEM. The TEM images of RGO-PDMAEMA-Pt/ Ag sheets clearly showed small Pt/Ag NPs were homogeneously distributed on the surface of large GO-PDMAEMA sheets. The mean diameter of Pt/Ag NPs was about 2e4 nm. The size from the Scherrer equation (about 13 nm) is larger than that obtained by TEM, which may be attributed to the fact that large particles were picked in XRD patterns. No aggregation was observed due to the protection of Pt/Ag NPs by the PDMAEMA chains. The high-resolution TEM images showed the oriented and ordered lattice fringes of Pt/Ag NPs (in Fig. 4d), where the d-spacing values of 2.25 A and 2.35 A coincide with that of face centered cubic (fcc) Pt (111) [40,41] and that of face centered cubic (fcc) Ag (111) plain, respectively [42]. After the RGO-PDMAEMA-Pt/Ag sheets were treated in the liquid nitrogen, the RGO-PDMAEMA-Pt/Ag nanoscrolls were formed. Fig. 5 shows the SEM images of GO sheets, GOPDMAEMA sheets, RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls. It was observed obviously that GO sheets exhibited a planar texture (Fig. 5a). When GO sheets were modified by PDMAEMA, a coase plane can be found owing to the anchoring of the PDMAEMA chains (Fig. 5b). Upon Pt/Ag NPs depositing, the SEM images of RGO-PDMAEMA-Pt/Ag sheets showed that small alloy metal NPs were uniformly distributed on the surface of GO sheets (Fig. 5c). When the RGOPDMAEMA-Pt/Ag sheets were treated in liquid nitrogen by freezing drying, a scrolled structure was formed (Fig. 5d). The GO nanoscrolls were also formed by treating GO sheets in liquid nitrogen. The morphology of GO sheets and GO nanoscrolls in aqueous solution and in 0.05 wt% Nafion solution coated on the surface of the glassy carbon were shown in Figure S4. The TEM image shows the diameter of the RGO-PDMAEMAPt/Ag nanoscrolls is about 500 nm (Fig. 6a). It can be clearly observed that RGO-PDMAEMA-Pt/Ag nanoscrolls were rolled
up with several layers and lots of Pt/Ag NPs were deposited evenly on the surface of RGO sheets. The XPS measurement was performed to obtain the surface element states of the samples. The chemical binding energies located at 284.8 and 531.1 eV are attributed to the C1s and O1s of GO sheets, respectively (Fig. 7A). Upon the PDMAEMA chains anchored to the surface of the GO sheets, the weak peak at 398.3 eV corresponding to the N1s binding energy was observed and the percentage of nitrogen was 4.43 wt%, which indicates the percentage of PDMAEMA in the GO-PDMAEMA sheets was about 49.78 wt% [43]. Two characteristic peaks at 76 and 368 eV in RGO-PDMAEAM-Pt/Ag sheets and nanoscrolls were attributed to Pt 4f7/2 and Ag 3d3/2, respectively [37,44], demonstrating Pt and Ag alloys were loaded on the surface of GO-PDMAEMA sheets [45,46]. The molar ratio of Pt and Ag was calculated to be 1.86:1. Two typical peak at 70.9 and 74.1 eV correspond to Pt 4f7/2 and Pt 4f5/2. Compared with pure Pt or monometallic Pt, the higher binding energy (from 70.3 to 70.9 eV) indicates the downshift of Pt f-band center due to alloying and stronger binding of Pt in Pt-Ag alloy [43]. The contents of Pt and Ag NPs were tested by ICP. According to the results of ICP, the contents of Pt and Ag NPs in RGO-PDMAEMA-Pt/Ag sheets were 14.4% and 4.3%, respectively. In addition, the results of ICP showed that the atomic ratio of Pt and Ag NPs is 2.7:1, which is close to the feed ratio (3:1). Compared with the content of Pt and Ag species in the bulk (calculated from ICP OES) and on the surface (estimated from XPS spectra), the lower molar ratio of Pt to Ag on the surface indicates more Ag preferential located on the surface of alloy. The Raman spectra of GO sheets, GO-PDMAEMA sheets, RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls were shown in Fig. 8. Two prominent peaks about 1350 and 1580 cm1 corresponding to D band (associated with the defect-related mode) and G band (related to the graphitic hexagon-pinch mode) of GO sheets were observed for all the samples [47]. The value of ID/IG for GO-PDMAEMA sheets is 0.93, higher than that of GO sheets (0.84), suggesting that some more sp2 carbon atoms in the GO sheets turned into sp3 ones owing to the grafting of PDMAEMA. Upon Pt/Ag NPs were loaded on the surface of GO-PDMAEMA sheets, the value of ID/IG decreased to 0.88, which was caused by more sp2 modes formed by the
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Fig. 4 e TEM images of RGO-PDMAEMA-Pt/Ag sheets.
Fig. 5 e SEM images of (a) GO sheets, (b) GO-PDMAEMA sheets, (c) RGO-PDMAEMA-Pt/Ag sheets and (d) RGO-PDMAEMA-Pt/ Ag nanoscrolls.
deposition of Pt/Ag NPs on the RGO sheets [36]. Compared with RGO-PDMAEMA-Pt/Ag sheets, the ID/IG ratio for the RGOPDMAEMA-Pt/Ag nanoscrolls showed an increase (from 0.88 to 0.96) owing to the increased p-p stacking caused by the scrolled structure [29]. After RGO-PDMAEMA-Pt/Ag sheets were rolled up, a red shift of the G peak from 1592 to 1601 cm1
was observed owing to a slight modification of the intra C-C bond in scrolled structure and a change of the phonon dispersion from the stacked graphene layers [47]. The electrocatalytic performance of RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls for methanol oxidation was investigated in nitrogen-saturated 0.5 M H2SO4 solutions and
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Fig. 6 e TEM images of RGO-PDMAEMA-Pt/Ag nanoscrolls.
Fig. 7 e (A) XPS spectrum of (a) GO sheets, (b) GO-PDMAEMA sheets, (c) RGO-PDMAEMA-Pt/Ag sheets and (d) RGO-PDMAEMAPt/Ag nanoscrolls, (B) Pt 4f7/2 and (C) Ag 3d3/2. 0.5 M H2SO4 solutions containing 0.5 M CH3OH solutions, respectively. The ECSA value provides information regarding the number of available electrochemically active sites which is essential for understanding electrocatalytic activity of the catalysts. It can be obtained via Equation (1): ECSA m2 g1 ¼ Q H mC cm2 210 mC cm2 Pt LPt gPt m2
Fig. 8 e Raman spectra of (a) GO sheets, (b) GO-DMAEMA sheets, (c) RGO-PDMAEMA-Pt/Ag sheets and (d) RGOPDMAEMA-Pt/Ag nanoscrolls.
(1)
QH is the coulombic charge for hydrogen adsorption on Pt sites. LPt is the platinum loading in the working electrode and the value of 210 (mC cm2) represent the charge required to oxidize a monolayer of hydrogen. The ECSA value of the RGO-PDMAEMA-Pt/Ag nanoscrolls for Pt is 891 cm2/mg, which is 1.28 times that of the RGOPDMAEMA-Pt/Ag sheets (698 cm2/mg). The cyclic voltammetry of GO sheets and nanoscrolls for methanol oxidation was also investigated in the same condition (Figure S5). The ECSA value of the GO nanoscrolls (16.65 cm2/mg) is 1.28 times that of the GO sheets (14.75 cm2/mg). The increase proportion of the
Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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ECSA value is similar to that of RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls (1.28 times), which indicates that the higher ESCA value for the RGO-PDMAEMA-Pt/Ag nanoscrolls was caused by the unique scrolled structure. Compared with RGO-PDMAEMA-Pt/Ag sheets, the RGOPDMAEMA-Pt/Ag nanoscrolls exhibit higher current at all corresponding potentials and the peak potential shifts by over 19.3 mV in the negative direction (Fig. 9B). The forward peak current for the RGO-PDMAEMA-Pt/Ag nanoscrolls is 46.34 mA/ mg, which is 1.26 times higher than that for the RGOPDMAEMA-Pt/Ag sheets (36.77 mA/mg). The lower onset potential and higher current density of the RGO-PDMAEMA-Pt/ Ag nanoscrolls indicates that RGO-PDMAEMA-Pt/Ag nanoscrolls have more practical significance in the acceptable output voltage range of fuel cells. The different electrocatalytic activity between the RGO-PDMAEMA-Pt/Ag sheets and nanoscrolls was caused by curling of the graphene sheets. The opened tubular structures and different interlayer spaces of the RGO-PDMAEMA-Pt/Ag nanoscrolls promote the mass transfer efficiency for reactant, product and electrolyte in the reaction process, thus increasing the electrocatalytic performance of RGO-PDMAEMA-Pt/Ag nanoscrolls for methanol. Compared with RGO-PDMAEMA-Pt/Ag sheets, the RGOPDMAEMA-Pt/Ag nanoscrolls show higher current towards methanol electrooxidation for the whole testing time, which indicates that the RGO-PDMAEMA-Pt/Ag nanoscrolls have better electrocatalytic activity (Fig. 9C). The ratio I(30 min)/I(0 min)
for the RGO-PDMAEMA-Pt/Ag nanoscrolls of 0.344 is higher than that for the RGO-PDMAEMA-Pt/Ag sheets (0.184), indicating that RGO-PDMAEMA-Pt/Ag nanoscrolls have more desirable stability. Compared with RGO sheets, RGO nanoscrolls decrease the direct contact chance with methanol by providing more pore channels, thus reducing the opportunity for metal poisoning in the electrocatalytic operation process. The introduction of PDMAEMA prevents Pt NPs from dissolving/falling into the reaction solution and improves the stability of alloy NPs. Table 1 listed some data about the performances of methanol oxidation on various catalysts. The comparative results show that the electrocatalytic activity of RGOPDMAEMA-Pt/Ag nanoscrolls is high. The ECSA value of Pt in RGO-PDMAEMA-Pt/Ag nanoscrolls is determined to be 891 cm2/mg, which is much higher than those obtained from LM/Pt (407 cm2/mg), AM/Pt50Co50 (543 cm2/mg) [49], Pt/C (383 cm2/mg) [41] and Pt/graphene (286 cm2/mg) [50]. The ratio of the forward oxidation peak current (If) to the reverse peak current (Ib), (If/Ib) is commonly used to evaluate the tolerance of catalysts toward the poisoning species such as adsorbed CO intermediates formed via decomposition of methanol. The If/ Ib ratio of RGO-PDMAEMA-Pt/Ag nanoscrolls is about 1.25, which is higher than that of the RGO-PDMAEMA-Pt/Ag sheets (1.22), LM/Pt (1.12), AM/Pt50Co50 (1.02) [49], Pt/C (0.83) [42] and Pt/graphene (1.20), Pt (0.71), implying RGO-PDMAEMA-Pt/Ag nanoscrolls have better catalytic tolerance.
Fig. 9 e Typical CV scans for the electrooxidation reactions of methanol on different electrodes (a) RGO-PDMAEMA-Pt/Ag sheets and (b) RGO-PDMAEMA-Pt/Ag nanoscrolls. CV scans between 0 and 1.6 V in nitrogen-saturated 0.5 M H2SO4 solutions (A) and 0.5 M H2SO4 þ 0.5 M CH3OH solutions (B) at 50 mV/s, 298 K. (C) Current-time curves for the electrooxidation reactions of methanol on different electrodes at 0.8 V, 298 K. Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060
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Table 1 e Comparison of performances of methanol oxidation with different catalysts. Electrocatalyst
ECSA (cm2/mg) If/Ib ratio References
Pt/C LM/Pt AM/Pt50Co50 Pt/Graphene RGO-PDMAEMA-Pt/Ag sheets RGO-PDMAEMA-Pt/Ag nanoscrolls
383 407 543 286 698
0.83 1.12 1.02 1.20 1.22
[42] [49] [49] [50] This study
891
1.25
This study
Conclusions Pt/Ag alloy NPs were uniformly loaded on the surface of GOPDMAEMA sheets and RGO-PDMAEMA-Pt/Ag nanoscrolls were formed by curling RGO-PDMAEMA-Pt/Ag sheets through a lyophilization method. The introduction of PDMAEMA to GO sheets can form the small size of alloy NPs, thus increasing the electrochemical performance of the composites. RGOPDMAEMA-Pt/Ag nanoscrolls have the high ECSA value of Pt (891 cm2/mg) and the If/Ib ratio (1.25) and the high ratio I(30 min)/I(0 min) (0.344). The excellent electrochemical performance was caused by the synergistic effect of Pt/Ag bimetallic alloy and the long-term stability was caused by the introduction of PDMAEMA to GO sheets which effectively avoid the aggregation and falling of Pt/Ag NPs and the unique scrolled structure which decrease the contact opportunity with methanol and reduce the opportunity for metal poisoning.
Acknowledgements This work was financially supported by the Natural Science Foundation of Hebei Province (No. E2015202262, No. B2017202226), Hebei Provincial Natural Science Youth Foundation (No. B2016409056) and the National Natural Science Foundation of China (No. 21776058, No. 51603061).
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.05.060.
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Please cite this article in press as: Jin X, et al., Reduced graphene oxide-poly-(2-(dimethylamino)ethyl methacrylate)-Pt/Ag nanoscrolls and its electrocatalytic performance for oxidation of methanol, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.05.060