Chapter 4: Quantitative Understanding of the Ultra-Sensitive and Selective Detection of
4.3. Results and Discussion
4.3.3. Optical Analysis
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.
4.3.3.1. UV-vis Absorption Study
The absorption spectrum of WS2 QDs shows an absorption band at ~265 nm in the deep UV region due to the bound exciton transitions from deep level valence band states to the conduction band, as shown in Fig. 4.4(a).43 Additionally, the appearance of the broad absorption tail in the range of 350–700 nm is due to the defect states in WS2 QDs.17 The Tauc plot in the inset of Fig. 4.4(a) shows a direct bandgap of ~4.08 eV for WS2 QDs, which corresponds to the deep valance band to conduction band transition.17 For the case of GO, the maximum absorption is observed at ~260 nm due to the characteristic transition with an indirect bandgap of ~3.75 eV.
Fig. 4.4. (a) Absorption spectra of WS2 QDs and GO sheets. The inset shows the Tauc plot for WS2 QDs, considering the direct bandgap. (b) PLE spectrum of WS2 QDs together with their excitation wavelength-dependent PL spectra.
(c) Comparison of the PL spectra of bare WS2 QDs and GO sheets under 400 nm excitation. (d) The Gaussian deconvolution of the PL spectrum of WS2 QDs at ex =400 nm. (e) The change of the PL intensity of WS2 QDs with the addition of different concentrations of GO (0.05–0.5 mg/mL) and (f) the corresponding non-linear Stern-Volmer plot of I0/I with GO concentration.
4.3.3.2. Photoluminescence Study
Due to the long absorption tail, the excitation wavelength-dependent PL spectral shift is expected in WS2 QDs. The PLE spectrum of WS2 QDs is presented together with the excitation wavelength- dependent PL spectra in Fig. 4.4(b). The PLE spectrum exhibits a strong peak at ~400 nm along with a sideband at ~290 nm. By changing the excitation wavelengths (ex) in the range of 300–420 nm, WS QDs shows a redshift of the emission peaks. Interestingly, the QDs exhibit a relatively
95 | 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
broad emission spectrum for low wavelength excitation (ex), which gradually becomes narrower with the increase of ex.4, 17 Among different excitations, the maximum intensity of the PL peak is observed at ex =400 nm, which is consistent with the PLE spectrum. Note that under 400 nm excitation, GO does not show any significant PL emission as compared to that of WS2 QDs (see Fig. 4.4(c)). The Gaussian deconvolution of the PL spectrum of WS2 QDs at ex =400 nm shows two peaks at ~495 nm, and 557 nm, as shown in Fig. 4.4(d). These peaks are due to the defects in the surface and in-plane defects arising from the S-vacancy (absence of S atoms) in the hexagonal lattice of WS2.17, 44 Note that the S 2p XPS spectrum of WS2 confirmed the presence of S-vacancy, and thus the PL result is consistent with the XPS analysis.
To study the PL behavior of GO/WS2 hybrid, the change of PL intensity of WS2 QDs is monitored at ~496 nm as a function of the concentration of GO (Qg) under ex =400 nm. Fig. 4.4(e) shows the systematic quenching of the PL intensity of WS2 QDs with the increasing concentration of GO in the range of 0.05–0.5 mg/mL. In Fig. 4.4(f), the variation of I0/I with GO concertation is observed to follow a non-linear Stern-Volmer equation, as used in equ. (2.5) of Chapter 2, given by:
𝐼0
𝐼 = 𝑒𝑥𝑝(𝑏𝑄𝑔)𝑚 (4.1)
where I0 and I are the PL emission intensities of WS2 QDs before and after the addition of GO.
‘b’, and ‘m’ are the constants with the values of 3.4 mL/mg and 1.3, respectively. Such a deviation from the linear Stern-Volmer equation indicates the large extent of the quenching in the well- known ‘Sphere of action’ model.45 With ex =400 nm, as the PL contribution is mainly due to the defect states of WS2 QDs (see Fig. 4.4(d)), the PL quenching can be attributed to the attachment of GO at the defect sites of QDs together with the van der Waals interaction.17, 23 The details of the interaction mechanism of GO and WS2 QDs are discussed later. The stability of GO/WS2 is monitored with respect to pH and time, and the results are shown in Fig. 4.5(a-c). With the variation of pH from neutral to basic, the normalized PL spectra of GO/WS2 and the corresponding change of the maximum PL peak position with pH are shown in Fig. 4.5(a, b). The negligible change in the PL spectra, as well as the maximum PL peak position with the variation of pH, reveal the high stability of GO/WS2 in the aqueous medium. The intensity of the PL spectra of GO/WS2
is observed to be constant for 1 h at pH 12, as shown in Fig. 4.5(c).
Fig. 4.5. (a) Normalized PL spectra of GO/WS2 hybrid at different pH, and (b) the corresponding variation of the PL peak position with pH. (c) Stability of the GO/WS2 hybrid with time. Optimization of DA sensing parameters with the variation of (d) GO concentration, (e) pH value, and (f) the reaction time for a fixed concentration of WS2 QDs and DA.