Chapter 4: Quantitative Understanding of the Ultra-Sensitive and Selective Detection of
4.3. Results and Discussion
4.3.2. Structural Analysis
4.3.2.1. XRD and Raman Analysis
For the structural analysis, the powder XRD patterns of the as-synthesized GO, WS2 QDs, and their hybrid were recorded, as shown in Fig. 4.2(a). The XRD pattern of GO shows a sharp, intense peak at 2θ ~10.5° corresponding to the characteristic (002) plane of GO.26 Thus, the interplanar spacing ~0.84 nm, calculated from the XRD pattern of GO, is much higher than that of the precursor graphite flakes (~0.34 nm), as presented in the inset. The high degree of oxidation of graphite flakes introduces a vast number of oxygen functional groups in the graphitic planes, which assist in increasing the interplanar spacing in GO.26, 29 The appearance of the new diffraction peaks for (100) and (004) planes is attributed to the defective states or functional groups in graphitic materials.1, 30 In WS2 QDs, the characteristic peak at 2θ ~14.3° for (002) planes confirms their hexagonal crystalline nature.27, 31 As compared to (004) planes of WS2at 2θ ~27.0°, the low intense peak of (002) planes indicates the formation of the ultrathin structure of WS2 QDs.31 Along with the hexagonal planes, the appearance of (006) planes in the XRD pattern signifies the presence of the rhombohedral structure of WS2.31 Moreover, (102) and (106) peaks in the XRD pattern are also the signatures of WS2 content. The XRD pattern of GO/WS2 hybrid confirms the presence of the graphitic component together with WS2 in the hybrid. In the hybrid structure, (002) plane of GO appears at 2θ ~12.2° and (004) plane of WS2 is at 2θ ~25.9°. Interestingly, as compared to the bare GO and WS2 QDs, the change of the peak position of (002) plane of GO and (004) plane of WS2
indicate the close contact of GO and WS2 QDs in GO/WS2 hybrid.
91 | D o p a m i n e S e n s i n g a t p M L e v e l u s i n g G O / W S2Q D s H y b r i d
The comparative Raman spectra of bare GO, WS2 QDs, and GO/WS2 hybrid are presented in Fig.
4.2(b). In the as-synthesized GO sample, the characteristic D and G bands at ~1360 cm-1 and 1594 cm-1, respectively, are observed in the Raman spectrum.1, 32 The intensity ratio of D band to G
Fig. 4.2. (a) XRD pattern and (b) Raman spectra of as-synthesized GO sheet, WS2 QDs, and GO/WS2 hybrid. The inset in (a) shows the XRD pattern of graphite flakes for comparison with GO. The vertical dashed lines indicate the respective peak positions.
band, ID/IG ~1.2 is attributed to the high content of defects in the GO sheet.32 A broad 2D band at
~2743 cm-1 alongside the D+G band at ~2938 cm-1 is also observed in the Raman spectrum of GO.
The broad 2D band indicates the few-layered structure of GO,33 which is in agreement with the AFM analysis. For the case of WS2 QDs, two characteristic Raman modes, E2g (~357.0 cm-1) and
A1g (~421.0 cm-1) correspond to the in-plane and out-of-plane vibration of the W-S bond of the hexagonal WS2, respectively.17, 34 Notably, the frequency differencebetween E2g and A1g modes is
~64.0 cm-1, which corresponds to the bilayer structure of WS2,35 and it is consistent with our AFM analysis. The appearance of the sharp Raman modes confirms the highly crystalline nature of the as-prepared WS2 QDs. For the case of GO/WS2 hybrid, the appearance of the characteristic peaks for both WS2 and GO is clearly observed. In the hybrid structure, the value of ID/IG decreases from
~1.2 to ~1.0, indicating the reduction of defect density/functional groups due to the attachment of WS2 QDs at the defect sites/functional groups of GO, as discussed in Chapter 2, Section 2.3.2 for the interaction of U-GQDs and SWCNTs. In the present case, the existence of various oxygen functional groups and the vacancy/defect states in GO act as the active sites for the efficient interaction of WS2 QDs in GO/WS2 hybrid formation. Noticeably, the 2D Raman band in GO/WS2
hybrid becomes stronger as compared to that of the GO, which indicates the reduction of the functional groups of GO.36
4.3.2.2. XPS Analysis
To acquire the details of various chemical states and the corresponding binding energy, we analyze the high-resolution XPS spectra of bare WS2 QDs, GO, and their hybrid, as presented in Fig. 4.3.
Fig. 4.3(a) shows the W 4f core-level XPS spectrum of WS2 QDs, which is deconvoluted into six
Fig. 4.3. Deconvolution of the high-resolution XPS spectra of (a) W 4f of WS2 QDs and (b) O 1s of GO. Comparison of the high-resolution XPS spectra of (c) C 1s in GO, and GO/WS2 hybrid and (d) S 2p in WS2 QDs, and GO/WS2
hybrid.Each spectrum is fitted with a Shirley baseline.
93 | D o p a m i n e S e n s i n g a t p M L e v e l u s i n g G O / W S2Q D s H y b r i d
Gaussian peakswith a Shirley baseline. The deconvoluted peaks with binding energy ~33.6 eV and 35.7 eV are assigned to W+4 4f7/2 and W+4 4f5/2 states in WS2 QDs, respectively.37, 38 In the W 4f XPS spectrum, the presence of the defect states as W+5 (W+5 4f7/2 ~34.7 eV and W+5 4f5/2 ~37.0 eV ) and W+6 (W+6 4f7/2 ~36.0 eV and W+6 4f5/2 ~37.9 eV) states are observed in WS2 QDs, which are believed to arise from the edge states of WS2 QDs due to the solution exfoliation process.21, 38,
39 For the case of GO, O 1s spectrum consists of three different peaks at ~530.6 eV, 532.2 eV, and 533.4 eV, which are assigned to COOH, C=O, and C–OH/C–O–C functional groups of GO, respectively (see Fig. 4.3(b)).40-42 The comparison of the deconvoluted C 1s spectrum of GO and GO/WS2 hybrid are presented in Fig. 4.3(c). The deconvoluted C 1s spectrum of GO shows the existence of graphitic sp2 hybridized carbon (C=C) at ~284.5 eV together with various oxygen functional groups, such as C–C/C–OH/C–O–C at ~286.0 eV, C=O at ~287.4 eV and COOH at
~288.3 eV.40, 42Interestingly, COOH functional groups of GO is absent in GO/WS2 hybrid, while the contribution C=O increases, as observed from Fig. 4.3(c).40 The deconvoluted S 2p XPS spectra of WS2 QDs and GO/WS2 hybrid are also compared in Fig. 4.3(d). In bare WS2 QDs, the appearance of a peak at ~160.4 eV is due to the sulfur (S) vacancy in WS2 QDs, which is in agreement with the presence of +6 and +5 oxidation states of W.39 Meanwhile, the peaks at ~161.8 eV and 163.0 eV are attributed to the co-existence of the characteristic S 2p3/2, and S 2p1/2 states, respectively. Another prominent peak at ~165.3 eV indicates the presence of SO4-2 states in S 2p XPS spectrum of WS2 QDs.37 Thus, the XPS analysis confirms the abundance of edge related defects in WS2 QDs. Moreover, the deconvoluted S 2p XPS spectrum of WS2 shows a change of the binding energy of the vacancy states after GO/WS2 hybrid formation. Thus, the XPS analysis of C 1s and S 2p spectra reveals that the chemical interaction between GO and WS2 QDs is mainly driven by the interaction between oxygen-rich functional groups GO and S-vacancy states of WS2
QDs. These results clearly indicate the chemical bonding and hybrid formation between the GO and WS2 QDs.