Lin-Nan Zhoua,
Xiao-Ting Zhanga,
Wen-Jin Shena,
Shi-Gang Sun*b and
Yong-Jun Li*a
aState Key Lab of Chemo/Biosensing and Chemometrics, School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China. E-mail: liyje@hnu.edu.cn
bState Key Lab for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China. E-mail: sgsun@xmu.edu.cn
First published on 15th May 2015
Owing to the high hydrophobicity of the graphene surface and the easy aggregation of Pt nanoparticles, it is very challenging to deposit high loading and a uniform distribution of Pt nanoparticles on graphene nanosheets. Herein, we report a facile approach to produce nanocomposites of Pt nanoparticles and reduced graphene oxide (Pt/RGO) with the intervention of glucose. The number density and arrangement of Pt nanoparticles on RGO nanosheets can be adjusted simply by changing the amount of Pt precursors. With the increase of the amount of Pt precursors, the loading amount of Pt nanoparticles rises and simultaneous Pt nanoparticle arrangement evolves from sparse distribution to a close-packed monolayer, to linear aggregation. HRTEM reveals that Pt nanoparticles are (111)-orientated nanocrystals (NCs) close to 3 nm. Pt24/RGO (Pt, 24 wt%) holds an excellent catalytic performance and stability towards methanol oxidation, ∼3 times in the mass activity better than the commercial Pt/C catalyst (Pt, 20 wt%), due to the close-packed monolayer structure of Pt NCs.
To fabricate Pt nanoparticles with a uniform distribution, graphene nanosheets are usually functionalized by organic linkers, such as polydopamine,5 aminopyrene,6 phosphomoly-bdic acid,7 poly(sodium styrene sulfonate),8 poly(diallyldi-methylammonium chloride),9 p-aminothiophenol-b-cyclodex-trin polymer,10 8-hydroxy-1,3,6-pyrene trisulfonic acid trisodium salt (PyS),11 5,10,15,20-tetrakis(1-methyl-4-pyridinio) porphy-rin tetra(p-toluenesulfonate),12 nickel(II) phthalocy-anine-tetrasulfonic acid tetrasodium salt,13 and ionic liquids.14 In most occasions, the functionalization of graphene surface can effectively avoid the aggregation of Pt nanoparticles, but Pt nanoparticles are sparsely decorated on graphene surface. Few strategies involving functionalized graphene can produce Pt/graphene composites with high-loading Pt nanoparticles9,14b Additionally, although the intervention of organic linkers is very helpful for fabricating uniformly distributed Pt nanoparticles on graphene surface, it also deteriorates the physical properties of graphene due to the disruption of its conjugated structure and decreases the effective contact area between Pt nanoparticles and graphene.9,11 Thus, a post-thermal treatment is sometimes required to remove organic linkers so as to increase the electric conductivity of Pt/graphene composite and expose more active sites of Pt nanoparticles for practical applications,15 which accordingly has a risk of causing the aggregation of Pt nanoparticles.
Thus, organic linker-free methods should be infinitely preferable to producing Pt/graphene composites. The direct growth of Pt nanoparticles on graphene surface can maximize the contact area between graphene and nanoparticles, improving the catalytic activity of Pt/graphene. To meet the hydrophobicity of graphene, supercritical carbon dioxide has been employed as a favorable solvent to successfully prepare Pt/graphene composites by using either dimethyl (1,5-cyclooctadiene) platinum(II) (PtMe2COD) or platinum(II) acetylacetonate (Pt(acac)2) as platinum precursors,16 but the size and distribution of Pt nanoparticles depend on the type of Pt precursor. In spite of the hydrophobicity of graphene powder, reduced graphene oxide (RGO) prepared by chemical methods still has a certain dispersivity in aqueous solution and can be used to prepare Pt/RGO composites as long as it does not undergo a drying process. The typical approach is to reduce Pt precursors in RGO dispersion with NaBH4 under alkaline conditions, but it cannot produce uniformly distributed Pt nanoparticles on RGO surface: Pt nanoparticle aggregates17 and network structures18 are usually observed. Anchoring of Pt nanoparticles on RGO surface is attributed to residue oxygen-containing organic groups of RGO surface.19 Electrochemical deposition can uniformly plant Pt nanoparticles on one side of graphene surface,20 but the loading of Pt nanoparticles is extremely low in order to avoid aggregation. To raise the loading amount, it is imperative to eliminate the aggregation of Pt nanoparticles.21 To address this issue, highly wavy graphene with abundant ripples was used, which not only increases the dispersivity of graphene in aqueous solution, also raises the loading amount of Pt nanoparticles,22 but the size and distribution of Pt nanoparticles on RGO lack the uniformity. To the best of our knowledge, two outstanding studies have fabricated high-loading Pt nanoparticles without severe aggregation on graphene surface by the supercritical carbon dioxide-assisted16b and femto-second-laser-pulse deposition.21 Despite all that, the high loading and uniform distribution of Pt nanoparticles on graphene surface are not easily fabricated due to the easy aggregation of Pt nanoparticles and the hydrophobicity of graphene.
Here, ∼3 nm Pt nanocrystals (NCs) were directly planted on the surface of purified RGO involving glucose as the stabilizer and NaBH4 as the reductant. The participation of glucose not only manipulates the growth of Pt along (111)-orientated direction, but also increases the stability of RGO in aqueous solution. The loading amount and distribution of Pt NCs can be controlled simply by changing the amount of Pt precursors. Pt24/RGO (Pt, 24 wt%) with close-packed Pt NCs exhibits the best electrocatalytic activity and stability towards methanol oxidation among all as-prepared Pt/RGO composites, approximately 3 times in the mass activity better than the commercial Pt/C catalyst (Pt, 20 wt%).
RGO dispersion was directly obtained by adding hydrazine hydrate in GO aqueous dispersion.23 Typically, 80.0 mL of water, 25.2 μL of hydrazine aqueous solution and 628.0 μL of ammonia solution were added into 20.0 mL of as-prepared GO dispersion under vigorous stirring. Subsequently, the mixture was heated to 85 °C, holding for 45 min to produce RGO. Impurities in as-prepared RGO dispersion were removed by centrifugation. The purified RGO precipitate was further redispersed into water to produce RGO dispersion (1.0 mg mL−1) for the following utilization.
By analyzing 300 Pt nanoparticles, the diameter is 2.84 ± 0.03 nm (Inset in Fig. 1F), similar to that of Pt nanoparticles electrodeposited on monolithic porous graphene network,15 which is very suitable to be used as catalyst, but the number density of Pt nanoparticles in our case is much higher. Although similar wavy RGO has been used in the previous report,24 Pt nanoparticles are sparsely decorated on RGO surface and the particle size lacks uniformity, completely different from our results. Such uniform Pt nanoparticles in our study may be attributed to glucose, of which five hydroxyl groups are able to bind Pt nanoparticle surface by the interaction of Pt atom and O atom.25 Moreover, glucose may have a weak non-covalent interaction with RGO due to the existence of CO, assisting the formation of dense and uniform Pt nanoparticles because RGO dispersion can remain stable for several weeks without aggregation if glucose is added, which may be the key point for the construction of the closely-packed Pt nanoparticle monolayer. Of course, the number density of Pt nanoparticles also depends on the amount of Pt precursors, which will be involved later.
HRTEM image (Fig. 2A) shows that each Pt nanoparticle has parallel lattice fringes with a spacing of ∼0.226 nm between two neighboring fringes, indicating that the resulting Pt nanoparticles are nanocrystals (NCs) with abundant (111) facets26 (JCPDF 04-0802), which is attributed to the directing role of glucose additive.25 We also examined the linear aggregates, as shown in Fig. 2B. Lattice fringes extend from one nanocrystal to another, indicating that the linear aggregates are formed by the orientation attachment of Pt NCs rather than the random aggregation. XRD analysis (Fig. 2C) further indicates that Pt NC has a face-centered cubic (fcc) crystal structure with characteristic refraction peaks of Pt(111), (200), (220) and (311) crystalline planes (JCPDF 04-0802).27 The refraction peak of at ∼40° has the strongest intensity, dwarfing other peaks, which is indicative of abundant Pt(111) facets, in accord with the result of HRTEM (Fig. 2A). Apart from the diffraction peaks of Pt facets, there is another diffraction peak at ∼26.35°, which is ascribed to graphite (002) peak of graphene sheets.28
The distribution of Pt NCs on RGO nanosheet has a clear picture: individual RGO nanosheets are covered by close-packed Pt NCs except for the edges of graphene sheets, implying that surface rumples exactly play an important role in accumulating Pt NCs. However, Pt NCs cannot be arranged closely only dependent on RGO rumples.24 In our case, as-prepared Pt NCs not only have a high number density, also show a uniform size-distribution. Such closely-packed Pt NCs on RGO surface are rarely reported,14b which may be attributed to the combination of physical trapping of rumples, chemical bonding of residue oxygen groups of RGO surface and the non-covalent interaction of glucose with RGO.
The existence of residue oxygen groups was demonstrated by C1s and O1s XPS. Asymmetrical C1s binding energy bands were observed in pure RGO (Fig. 3A) and Pt24/RGO (Fig. 3B) and further deconvoluted into four components, respectively: CC, C–O–, C
O and C
OO–, as shown in Fig. 3A and B. As for pure RGO, the corresponding binding energies and the relative atomic amounts are shown in Table S1,† almost consistent with previous reports.24,29 It means that RGO still contains a few oxygen-containing functional groups. Although residual oxygen influences the electrical conductivity of RGO, traces of oxygen in RGO are helpful for the loading of Pt nanoparticles and improving their stability and distribution on RGO surface.30 Moreover, oxygen atoms also increase the hydrophilicity of graphene, making its dispersion more stable. When Pt NCs are loaded on RGO surface, the atomic percentage of C
C groups increases by comparison with pure graphene (Table S1†), which may be ascribed to the repair of the basal plane structure of graphene, implying that the initial C–C groups containing oxygen are partially transformed into C
C groups during the reduction process of Pt precursors. Accordingly, the atomic percentages of C1s of C
O and C
OO– components in Pt24/RGO also decrease (Table S1†), which is attributed to the transformation of C
O and C
OO– components to C
C component and the formation of Pt–O linkages.29 Also, compared with pure RGO, C–O– component has a higher atomic percentage in Pt24/RGO, which may be related to the conversion of C
O to C–O– and the participation of C–O– in C
OOH coming from glucose. This analysis is further confirmed by O1s XPS spectra, as shown in Fig. 3C and D. After deconvolution, O1s XPS spectra of pure RGO (Fig. 3C) and Pt24/RGO (Fig. 3D) contain three oxygen components, respectively: carbonyl oxygen from C
O*O– and C
O*, hydroxyl oxygen from C–O*– and C
OO*–, and oxygen from tiny absorbed H2O (Fig. 3C and D, the corresponding oxygen is marked with the symbol, *). The binding energy and the relative atomic amount of each component are shown in Table S2.† Compared with the spectrum of pure RGO, the atomic percentages of oxygen atoms coming from C
O*O– and C
O* components decrease in Pt24/RGO, in tandem with the increase of the atomic percentages of hydroxyl oxygen atoms in C–O*– and C
OO*– components, which agrees with the results of C1s XPS analysis. Also, the tiny amount of oxygen coming from adsorbed H2O was detected in both Pt/RGO and pure RGO. Additionally, compared with pure RGO, in the XPS pattern of Pt/RGO, the deconvoluted band centers of C1s spectra, apart from C
C component, tend to shift positively while the deconvoluted band centers of O1s spectra, apart from adsorbed H2O, tend to shift negatively, disclosing the difference of the chemical surroundings around C and O atoms. The existence of Pt–O linkage may be related to C–O*– and C
OO*– groups, which is capable of providing heterogeneous nucleation sites for the growth of Pt NCs,30b but not enough for producing uniformly distributed Pt NCs on RGO surface according to the previous reports.17,24 Thus, the introduction of glucose plays a crucial role in the formation of uniform Pt NCs and their close-packed monolayer.
Pt4f XPS spectrum of the Pt/RGO (Fig. 3E) is not symmetrical, implying that some Pt species exist as non-metallic Pt. Pt4f spectrum possesses a pair of peaks. After the deconvolution of Pt4f spectrum, Pt NCs were found to contain Pt(II) and Pt(IV) species. The binding energies centered at 71.61 and 74.81 eV (blue curves) correspond to metallic Pt; the binding energies centered at 72.84 and 75.84 eV (green curves) are attributed to Pt(II) species, such as PtO and Pt(OH)2;31 the broad double peaks at 76.33 and 79.28 eV (purple curves) are assigned to Pt(IV) species, such as PtO2.31 The appearance of Pt(II) and Pt(IV) is caused by the formation of Pt–O linkages, in agreement with the analysis of C1s and O1s XPS spectra. The atomic ratio of Pt(0):
Pt(II)
:
Pt(IV) is about 2.2
:
1.7
:
1 (Table S3†), indicating that almost half of Pt atoms exist as non-metallic Pt, similar to the case of Pt nanoparticles supported on CeO2/graphene.32 Actually, non-metallic Pt also exists in ‘pure’ Pt nanoparticles and the commercial Pt/C. If RGO was not added in the synthesis of Pt24/RGO, as described in Experimental section, ‘pure’ Pt nanoparticles were obtained. XPS analysis (Fig. S1A†) indicates that the atomic ratio of Pt(0)
:
Pt(II)
:
Pt(IV) in Pt nanoparticles is 11.8
:
1
:
1 (Table S3†) and Pt(II/IV) species account for 14.5%. XPS analysis (Fig. S1B†) also indicates that Pt(II/IV) species in Pt/C account for 32.9% with the Pt(0)
:
Pt(II)
:
Pt(IV) ratio of 4.9
:
1.4
:
1. Although non-metallic Pt exists in Pt nanoparticles and Pt/C, the atomic percentage is lower than that of Pt24/RGO, suggesting that the introduction of RGO is responsible for the appearance of partial non-metallic Pt. The decoration of non-crystalline oxygen-contained species (PtO2, PtO or Pt(OH)2) on Pt(0) surface may provide –OH group for promoting the oxidation of organic molecules and produce a certain favorable capability against the corrosion of Pt catalyst during the catalysis process.33
Arrangement of Pt nanoparticles on RGO surface also depends on their number density. To explore the effect of the number density of Pt nanoparticles on the final structure of Pt/RGO, another four samples were synthesized by changing the amount of Pt precursors (the detail of synthesis in ESI and Table S4†). Pt contents in Pt/RGO composites were found to increase with the increase of the molar amount of Pt precursors. Four Pt/RGO samples were determined to contain 17 wt%, 19 wt%, 39 wt% and 52 wt% Pt, respectively, by ICP-OES. The corresponding samples were designated as Pt17/RGO, Pt19/RGO, Pt39/RGO and Pt52/RGO, respectively. TEM characterizations (Fig. S2–S5†) indicate that the number density of Pt nanoparticles on RGO surface gradually increases with the increase of Pt contents. From Pt17/RGO to Pt53/RGO, the arrangement of Pt nanoparticles changes from sparse distribution to close-packed monolayer, to linear aggregation. Pt nanoparticles in Pt17/RGO and Pt19/RGO are sparsely distributed except for the rumple location of RGO (Fig. S2 and S3†). Pt nanoparticles in Pt24/RGO are close-packed (Fig. 1C and D). When Pt content is beyond 24 wt%, Pt nanoparticles in Pt/RGO (namely Pt39/RGO and Pt52/RGO) densely cover RGO surface (Fig. S4 and S5†) and form 2D linear aggregates. Thus, the arrangement of Pt nanoparticles on RGO surface depends not only on the specific synthetic procedure, but also on the amount of Pt precursors. Pt nanoparticles in each sample are (111)-oriented nanocrystals, having the same crystalline structure as those in Pt24/RGO (Fig. 2A). The average size of Pt nanoparticles does not show an obvious change compared with that of Pt24/RGO: the average diameters of Pt NCs in different Pt/RGO nanocomposites fluctuates from 2.6 to 2.9 nm (insets in Fig. S2–S5†).
All Pt/RGO nanocomposites show the same distinguishing characteristics as the commercial Pt/C catalyst in the cyclic voltammetric measurement (Fig. S6†): a pair of adsorption–desorption peaks of hydrogen ranging from −0.2 to 0.05 V, a broad oxidation peak of Pt between 0.5 and 0.9 V, and the reduction peak of platinum oxide at ∼0.50 V. The adsorption–desorption peaks of hydrogen are well known to be closely linked with the electrochemical active surface area (ECSA) of Pt catalyst, which can be calculated by integrating the charge of the hydrogen absorption–desorption peaks.34 ECSAs of all Pt/RGO composites were summarized in Table S4.† Pt39/RGO has the largest ECSA (36.7 m2 g−1), about 2 times larger than that of the commercial Pt/C. With the increase of Pt contents, the ECSA increases from Pt17/RGO to Pt39/RGO. On the contrary, when the mass of Pt reaches up to 52% (i.e. Pt52/RGO), the ECSA decreases due to the severe overlapping of Pt nanoparticles, but all ECSAs of Pt/RGO composites are larger than that of Pt/C, implying that Pt/RGO is able to provide more active sites than Pt/C for chemical reactions occurring on its surface.
To assess the electrocatalytic performance of Pt/RGO towards methanol oxidation, Pt/RGO and the commercial Pt/C catalysts were examined by the cycle voltammetric technique, as shown in Fig. 4A. Cycle voltammograms of Pt/RGO composites and Pt/C have similar features. In the positive-going scan, oxidation peaks appear between 0.5 and 0.8 V due to the direct oxidation of methanol; in the backward scan, another oxidation peaks between 0.2 and 0.6 V are ascribed to the oxidation of intermediate carbonaceous residues caused by the direct oxidation of methanol during the positive-going scan. All the currents were normalized to the mass of Pt. By comparison, all mass activities of Pt/RGO composites and Pt/C, namely the current densities of the positive-going oxidation peaks, are showed in Fig. 4B. With the increase of Pt contents, the mass activity firstly increases, and then decreases. Obviously, Pt content is closely dependent on the electrocatalytic activity of Pt/RGO. Among all Pt/RGO nanocomposites, Pt24/RGO, Pt39/RGO and Pt52/RGO show better catalytic activities than Pt/C; Pt17/RGO and Pt19/RGO have lower activities (∼100 mA mgPt−1), similar to the previous report of Pt/wavy RGO,24 which may be ascribed to the low-loading amount of Pt. Although Pt content increases from Pt24/RGO to Pt52/RGO, the corresponding catalytic activity gradually decreases, which may be attributed to the aggregation of Pt nanoparticles, as shown in Fig. S4 and S5.† Pt24/RGO has the best mass activity (553.4 mA mgPt−1), 3.26 times better than the commercial Pt/C, comparable to Pt nanoparticles loaded on graphene with the help of supercritical carbon dioxide (550.4 mA mgPt−1).16a Also, the mass activity of Pt24/RGO is also far better than previous analogous catalysts (Table 1).14b,17,35 Although dense Pt nanoparticles were produced on graphene surface by reducing PtCl62− with femto-second laser pulses21 and with the assistance of supercritical CO2,16b the mass activities of these two kinds of Pt/graphene towards methanol oxidation (Table 1) are still lower than that of Pt39/RGO (∼359 mA mgPt−1), and far inferior to that of Pt24/RGO in our case.
Catalyst | Mass activity/mA mg−1 | Reference |
---|---|---|
Pt24/RGO | 550.4 | This study |
Pt/ionic liquid functionalized RGO | ∼50 | 14b |
Pt nanoparticle aggregates/RGO | ∼199.6 | 17 |
Pt/graphene obtained through the thermal treatment of GO with Pt(acac)2 | ∼70 | 35 |
Pt/graphene obtained with the assistance of femto-second laser pulses | 295 | 21 |
Pt/graphene obtained with the assistance of supercritical CO2 | 246.7 | 16b |
Pt dendrites/β-cyclodextrin functionalized RGO | 478 | 36 |
Pt/PyS-functionalized graphene sheet | 279.5 | 11 |
Pt/H3PW12O40-functionalized graphene nanosheet (Pt/PW12–graphene) | 353.23 | 37 |
Pt/PANI-HPMo-functionalized RGO | 322 | 7 |
Pt/RGO nanoscroll | ∼450.3 | 38 |
Pt/3D network graphene | 93.31 | 39 |
Pt/Ni/RGO catalyst | ∼413 | 41 |
PtPd NWs/RGO | ∼510 | 42 |
Pt/CeO2/graphene | ∼450 | 32 |
Studies also demonstrated that functionalized RGO with organic linkers is helpful to some extent for improving the electrocatalytic activity of Pt nanoparticles towards methanol oxidation owing to their manipulation to Pt nanoparticle distribution on RGO. But this strategy cannot effectively improve the mass activities to methanol oxidation (Table 1)7,11,36,37 due to the limited loading amount of Pt. Additionally, 3D graphene nanostructure is considered to be a better alternative than flat graphene for loading Pt catalysts because it is able to provide large, accessible, multi-size pores for fast transportation of reactants to the electroactive sites of Pt nanoparticles and has a high electrical conductivity. Design of 3D graphene structure aims to increase the loading amount of Pt by taking advantage of its large surface area. In our case, the number density of Pt nanoparticles on RGO surface can be easily controlled simply by changing the added amount of Pt precursors. The optimized Pt24/RGO with a close-packed Pt NCs is enough to meet the requirement of the catalyst, and shows the better mass activity than Pt/RGO nanoscrolls (Table 1)38 and Pt/3D graphene architecture (Table 1).39
To greatly improve the catalytic performance of Pt nanoparticles, two approaches are usually taken: the first is to introduce another foreign metal to modify the electronic structure of platinum for improving its electrocatalytic activity to methanol oxidation, which is a very effective way to lower the density of states at the Fermi level and to weaken the adsorption of CO-like intermediates on Pt,40 such as, Pt/Ni/RGO,41 PtPd nanowires (NWs)/RGO,42 Pt-on-Pd/RGO,43 Pd/Pt core–shell nanoparticles/RGO,44 Pt/Cu/graphene,45 and Pt/Ru/graphene.46 The second is to rely upon the heterogeneous interaction between Pt and metal oxide that can provide –OH groups for radically enhancing their electrocatalytic activity and durability towards methanol oxidation,47 such as Pt/CeO2/graphene,32 Pt/TiO2/graphene,47 and Pt/SnO2/RGO.48 The enhanced effects of these two strategies are remarkable. Although Pt/RGO prepared in our case without introducing either foreign metallic elements or metal oxide, the optimized Pt24/RGO composite still has a better mass activity than Pt/Ni/RGO,41 PtPd NWs/RGO,42 and Pt/CeO2/graphene32 (Table 1).
After normalization with the ECSAs (Table S4†), the specific activities of all Pt/RGO composites and the commercial Pt/C catalyst were obtained, as shown in Fig. 4B. With the increase of Pt contents, the specific activity of Pt/RGO changes with the same tendency as the mass activity, but only Pt24/RGO and Pt39/RGO show better specific activities than Pt/C. Pt24/RGO still exhibits the best specific activity (∼1.70 mA cm−2), nearly 2 times better than that of the commercial Pt/C, indicating that Pt24/RGO has the great capability of enhancing methanol electrooxidation. The specific activity of Pt24/RGO is far better than branched Pt NWs/RGO,49 Pt/CD-functionalized graphene36 and Pt/PW12–graphene,37 and is comparable to those of Pt/3D graphene monolith (3DGM),20c PtCu dendrites/RGO,45 and Pt hollow nanostructure/RGO.50
Additionally, the current density ratio of the forward oxidation peak to the backward oxidation peak, jf/jb, is usually used to describe the anti-poisoning capability of catalysts. The larger the jf/jb value, the more tolerant the catalyst is towards intermediate carbonaceous species. All jf/jb values of Pt/RGO composites (Table S4†) are larger than Pt/C (0.87), indicating that poisoning intermediates are easily removed from Pt/RGO catalysts. The jf/jb values of Pt17/RGO and Pt24/RGO are 1.24 and 1.23, respectively, ∼1.5 times higher than that of the commercial Pt/C, suggesting that Pt17/RGO and Pt24/RGO have better tolerance towards poisoning intermediates than branched Pt NWs/RGO49 and Pt/3D graphene structure.15 This poisoning tolerance is similar to Pt/exfoliated graphene (jf/jb = 1.20),1 and even comparable to that of bimetallic nanoparticles/graphene, such as PtPd/RGO,42 Pt-on-Pd bimetallic dendrites/graphene nanosheet43 and hollow Pt/Ni/graphene.41 Although the number density of Pt nanoparticles on pristine graphene is similar to our result, the jf/jb value is just 0.89.4
Moreover, the catalytic stability was also further assessed by chronoamperometry, as shown in Fig. 5. At the beginning, all polarization current densities decrease rapidly due to the formation of the double-layer capacitance. Subsequently, the decay of current densities, i.e. lowering of the electrocatalytic activity, may be attributed to the adsorption of poisoning intermediates and the surface reconstruction of catalyst particles. At 2000 s, Pt24/RGO and Pt39/RGO show much better mass activities than Pt/C. The mass activities of Pt17/RGO, Pt19/RGO and Pt52/RGO are close to that of Pt/C. The current density on Pt24/RGO is nearly constant (11.8 mA mgPt−1) since 1700 s, while the current density on Pt39/RGO still tends to decrease, indicating that Pt24/RGO among all as-prepared Pt/RGO possesses satisfactory stability for methanol oxidation, consistent with the cyclic voltammetric result (Fig. 4A).
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Fig. 5 Chronoamperometric curves of Pt/RGO composites and Pt/C at 0.6 V in 0.5 mol L−1 H2SO4 +1.0 mol L−1 CH3OH. The inset show the curves between 1000 s and 2000 s for clarification. |
The electrocatalytic performance of Pt catalyst depends on nanoparticle size, surface facet, surface composition, supporter and loading amount, which collaboratively contributes to the final performance of Pt catalyst. Although all Pt/RGO composites were prepared under the similar conditions, but Pt/RGO composites with different loading amounts exhibit different catalytic activities, which mainly depends on the loading amounts and arrangement of Pt nanoparticles. Compared with Pt/C, Pt24/RGO, Pt39/RGO and Pt52/RGO are better in the case of the mass activity while Pt17/RGO and Pt19/RGO are not. Pt nanoparticles in the commercial Pt/C have an average diameter of 4.2 nm, larger than that of Pt NCs in Pt/RGO composites, which is one possible reason for the worse activity of Pt/C. However, particle size is not a crucial factor because Pt17/RGO and Pt19/RGO with ∼3 nm Pt NCs do not show better activities. Among all Pt/RGO nanocomposites, Pt24/RGO has the best electrocatalytic performance and stability, which may be attributed to the close-packed structure of Pt NCs. we previously demonstrated that close-packed Au/Pt core–shell nanoparticle monolayer, compared with randomly distributed counterparts, has three-fold enhancement in the mass activity towards methanol electrooxidation.51
Footnote |
† Electronic supplementary information (ESI) available: Summary of XPS analysis result and electrochemical characterization of Pt/RGO. See DOI: 10.1039/c5ra03007b |
This journal is © The Royal Society of Chemistry 2015 |