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Journal of Materials Chemistry B
atoms inside of the graphene layers.61 Among the nitrogen doping phases (N1, N2 and N3), nitrogen was predominantly doped in graphene in the form of pyrrolic-N, which prefers to be doped at the edge of graphene in the presence of metal nanoparticles.35 The nitrogen content within the sample has been calculated from the XPS results and it was approximately 9.5 at.%. In addition, from the XPS analysis it was observed that the content of pyridinic and graphitic nitrogen atoms is more within the GN–PtNP nanosheets.
3.3 X-ray diffraction analysis of the N-GN–PtNP nanocomposites Fig. 3 shows the XRD pattern of N-GN–PtNP and GN–PtNP nanocomposites. The reduced graphene (GNs) showed a broad diffraction peak observed at 27.2 in Fig. 3 (inset). This corresponds to the (002) plane of the graphitic carbon, and indicates the presence of reduced graphene in the as prepared nanocomposites. This broadening of the diffraction peak suggests the lack of long-range ordered signatures of the graphene-based nanocomposites. The largely reduced (002) interlayer spacing of 0.35 nm, in comparison with the 0.79 nm interlayer spacing of graphene oxide (GO), revealed that most of the oxygen functional groups that were intercalated into the interlayer space of graphite had been removed during the reduction process. It is known that GO shows a sharp diffraction peak at 2q ¼ 11.2 , suggesting the complete exfoliation of graphite.63 Diminution in the interplanar spacing of reduced GO as compared to GO is due to removal of the intercalated water molecules and the oxide groups that allow graphene nanosheets to be tightly packed.64 The relatively lower intensity peak that occurs at 43 ˚ corresponds to the (001) plane of reduced GO.65 The (2.09 A) number of layers (four) of the reduced GO has been obtained using the Debye–Scherrer equation.65–67 t ¼ 0.9l/b002 cos q002
(1)
n ¼ t/d002
(2)
Fig. 3 XRD patterns of GN–PtNPs (a) and N-GN–PtNP nanocomposites (b). Inset is the XRD pattern of the graphene nanosheets.
This journal is ª The Royal Society of Chemistry 2013
where t is the thickness; b002 is the full width at half maximum (FWHM) corresponding to the (002) plane; n is the number of graphene layers and d002 is the interlayer spacing. Fig. 3a shows that the diffraction peaks at 39.6 , 46.9 , 67.3 , 80.8 and 82.9 are related to the (111), (200), (220), (311) and (222) planes of the face-centered-cubic (fcc) Pt (JCPDS 04-0601), conrming that the Pt precursor has been successfully reduced into Pt nanoparticles during the course of the chemical reduction. The peak corresponding to the (111) plane is more intense than the others, indicating that the (111) plane is the dominating orientation. The average crystallite size for the Pt nanoparticles is calculated from broadening of the (111) diffraction peak using a modied form of the Scherrer equation.65–67 d ¼ 0.9l/b1/2 cos q
(3)
where d is the average particle size (nm), l is the wavelength of ˚ q is the angle at the maximum of the the X-ray used (1.54056 A), peak (rad), and b1/2 is the width of the peak at half height in radians. The calculated average size of the Pt nanoparticles on the graphene is 9 0.5 nm, which is closely matched with that obtained from the FE-SEM and TEM images. Moreover, the broad peak at 2q ¼ 27.5 is due to the (002) plane of the hexagonal structure of the graphene support, indicating that the nature of the GNs doesn't alter aer PtNP functionalization. The XRD diffractogram of N-GN–PtNPs is similar to that of GN–PtNPs (Fig. 3b). For N-GN–PtNPs, the interlayer spacing is ˚ which is little bigger than that of reduced GNs about 3.41 A, ˚ This may be due to defects resulting from nitrogen (3.36 A). doping. However, the N-doping treatment process cannot affect the layers of the GNs. 3.4 Electrochemical oxidation of HCY at a N-GN–PtNP nanocomposite electrode HCY is an important amino acid, which isn't found directly in the diet, but is formed during methionine metabolism.68 It has been documented that the HCY concentration in blood plasma is approximately 5–16 mmol L 1, and higher concentrations of HCY give rise to hyperhomocysteinemia (#100 mmol L 1) or homocystinuria ( 500 mmol L 1).68 It has been shown that hyperhomocysteinemia is associated with folate and cobalamine deciencies, and can lead to early pregnancy loss, mental disorders and tumors.69 Furthermore, a moderate increase in HCY concentration is associated with an increased risk of coronary artery and cerebrovascular diseases,70 including atherosclerosis and thrombosis.71 Ascorbic acid (AA) always coexists with HCY in our body uids, but its concentration is very much higher when compared to HCY concentration. Since these biomolecules coexist in human uids, their simultaneous determination is essential to secure human health from the risk of the above critical diseases. Therefore, highly selective and sensitive determination of HCY is very important from clinical and health viewpoints. We have examined the electrocatalytic activity of N-GN–PtNP GC, GN–PtNP GC, GN GC and unmodied GC electrodes towards the oxidation of HCY. We found that the N-GN–PtNP modied GC electrode showed higher electrocatalytic activity
J. Mater. Chem. B, 2013, 1, 4655–4666 | 4659