Chemical and Structural Analysis…

Chapter 6: Quantitative Understanding of Charge Transfer Mediated Fe 3+ Sensing and Fast

6.3. Results and Discussion

6.3.2. Chemical and Structural Analysis…

An overview of the chemical composition of N-GQDs and Au@N-GQDs is presented in Fig.

6.2(a) using the XPS survey scan. Three characteristic peaks at ~284 eV, 399 eV, and 531 eV are observed in both N-GQDs and Au@N-GQDs for C 1s, N 1s, and O 1s, respectively,²⁵ while the characteristic peak at ~84 eV for Au 4f appears only in Au@N-GQDs,²⁶ as expected. For the

Fig. 6.2. (a) XPS survey scan spectra of N-GQDs and Au@N-GQDs. (b-d) High-resolution XPS spectra of C1s, N 1s, and O 1s, respectively, of N-GQDs. (e-h) Au 4f, C1s, N 1s, and O 1s XPS spectra of Au@N-GQDs, respectively.

Each spectrum is fitted with a Shirley baseline. The symbols represent the experimental data, and the filled areas correspond to the Gaussian fits of different energy states.

detailed analysis of the chemical states in N-GQDs and Au@N-GQDs, the high-resolution XPS spectra corresponding to C 1s, N 1s, O 1s, and Au 4f are deconvoluted with multiple Gaussian peaks, as shown in Fig. 6.2(b-h). In the deconvoluted C 1s spectrum in N-GQDs (seeFig. 6.2(b)), the peak at ~284.3 eV refers to the graphite-like sp² hybridized state (C=C) and the other peaks at

~285.5 eV, 287.5 eV and 288.9 eV are assigned to C=N, C-O, and C=O/ C-N-C functional groups, respectively.²⁵ For N 1s spectrum of N-GQDs, the binding energy at ~397.9 eV, 399.2 eV, 400.7 eV, and 402.7 eV (see Fig. 6.2(c)) are assigned to pyridinic N, pyrrolic N, graphitic N and oxidized state of N as N-O, respectively.^{27, 28} The pronounced peaks at ~399.2 eV and 400.7 eV confirm the doping of N atom in N-GQDs. In Fig. 6.2(d), three different peaks at ~531.0 eV, 532.1 eV and 533.5 eV in O 1s spectrum endorse the existence of oxygen-rich functional groups C=O, C-OH/C- O-C and COOH, respectively, in N-GQDs.^{25, 27} In the case of Au@N-GQDs, the characteristic Au

4f doublet is deconvoluted with two peaks at ~84.4 eV and 88.1 eV, corresponding to Au 4f7/2, and Au 4f5/2 states, respectively, with a spin-orbit splitting of ~3.7 eV, as shown in Fig. 6.2(e).²⁶ Along with the pure metallic Au, the appearance of a peak at ~85.8 eV is due to the Au⁺ 4f7/2 state endorsing the bonding of Au with N-GQDs.²⁶ In Au@N-GQDs, four different peaks of C 1s appear at ~284.3 eV, 285.7 eV, 287.0 eV, and 288.6 eV, as shown in Fig. 6.2(f). As compared to N-GQDs, the contribution of C=C peak increases from ~57% to 63%, while the contribution of N, O related functional groups decreases. This confirms the reduction of defects from the graphitic array after the attachment of Au NPs. It is noteworthy that the reduction in the binding energy of each state of C 1s in Au@N-GQDs, except C=N state, confirms the increase of the electron density in the Au@N-GQDs hybrid. The upshift in the binding energy of C=N from ~287.5 eV to 287.7 eV is attributed to the electron donation to Au³⁺ ions at the time of reduction reaction. For N 1s spectrum in Au@N-GQDs (see Fig. 6.2(g)), only pyrrolic N (~398.9 eV) and graphitic N (~400.5 eV) peaks appear. The disappearance of the pyridinic N and N-O peak in Au@N-GQDs revels that these functional groups play a vital role in the reduction of Au³⁺. The lone pair electrons of the pyridinic N help in the attachment of the Au NPs with N-GQDs and as a result, the discontinuity in the graphitic network is filled up leading to the reduction of the defect in N-GQDs, while the presence of substantial pyrrolic N (~85%) can increase the n-type conductivity¹⁵ and higher density of states.

In Au@N-GQDs, the modification of the oxidation states of C=O (~530.9 eV), C-OH/C-O-C (~531.9 eV) and COOH (~532.9 eV) functional groups is observed from the deconvoluted O 1s spectrum. The lowering in the binding energy of these functional groups reveals the high electronic charge density in Au@N-GQDs (see Fig. 6.2(h)). The overall increase of the relative atomic % of C 1s and the reduction of N 1s and O 1s contribution in the Au@N-GQDs hybrid, as listed in Table 6.1, confirm the reduction of defects and impurities in the graphene structure. Moreover, the deconvoluted high-resolution spectra confirm the modification of the functional groups.

Table 6.1. Comparison of the atomic concentration (%) of C, N, O, Au in N-GQDs and Au@N- GQDs, as revealed from the XPS analysis.

Characteristic peaks N-GQDs Au@N-GQDs

C 1s 54.0% 66.8%

N 1s 10.0% 5.2%

O 1s 36.0% 27.5%

Au 4f - 0.5%

145 | ^{F e3 +}S e n s i n g a n d F a s t P h o t o r e s p o n s e b y A u @ N - G Q D s

6.3.2.2. XRD Analysis

To study the structure and crystallinity of N-GQDs and Au@N-GQDs, the XRD patterns are presented in Fig. 6.3(a). In the case of N-GQDs, a broad diffraction peak centered at 2θ ~24.6°

corresponds to (002) graphitic plane. The nanoscale size of the particles and the presence of an excess amount of functional groups and defects are responsible for the broadening of the diffraction peak in N-GQDs.^{4, 29} On the other hand, the diffraction corresponding to (002) graphitic planes for Au@N-GQDs is detected at a higher 2θ value (~26.1°) as compared to bare N-GQDs. Due to the higher electron density, the functional groups in the basal plane of N-GQDs mainly participate for the reduction of the Au³⁺ ions, and as a result, Au NPs are mostly attached on the surface of N-GQDs instead of edge sites.³⁰ Thus, due to the attachment of Au NPs, the functional groups are partly eliminated from the basal plane, and as a result, the interplanar spacing may be reduced, giving rise to a peak at a higher 2 value. The reduction of the linewidth of this (002) graphitic peak also indicates the reduction/passivation of the defective sites and functional groups from the graphitic structure. For Au@N-GQDs, two additional diffraction peaks occur at 2θ ~38.3° and 44.6° correspond to (111) and (002) facets, respectively, of fcc Au NPs.

Fig. 6.3. A comparison of the (a) XRD pattern, (b) Raman spectra, and (c) FTIR spectra of N-GQDs and Au@N- GQDs. In (b) the symbol indicates the experimental data and the solid lines represent the fitted curves. The deconvoluted Raman peaks are labeled as D1, D2, D, D3, G, and D4, and the respective peak positions are indicated with vertical dashed lines.

6.3.2.3. Raman Spectral Analysis

For further endorsement of the structural characteristics, Raman spectra of as-synthesized N- GQDs and Au@N-GQDs are presented in Fig. 6.3(b). To acquire insight into the edge states and functional groups, each spectrum is fitted with six Lorentz peaks in the range of ~1100–1700 cm^-

1. The vertical dashed lines on the deconvoluted spectra indicate the positions of D and G bands in N-GQDs at ~1365 cm^-1 and 1590 cm^-1, respectively. The D band arises from the structural defects in the sp² domain of N-GQDs, whereas the G band in N-GQDs is due to the in-plane phonon vibration of C=C carbon components, as mentioned in Chapter 3, Section 3.3.2.2. The appearance of additional Raman bands in N-GQDs and Au@N-GQDs are labelled as D1, D2, D3, and D4, as shown in Fig. 6.3(b). In particular, the Raman peak at ~1300 cm^-1 (D2)is attributed to the O=C-N bond vibration,³¹ and that at ~1422 cm^-1 (D3) corresponds to the vibration of C=O/C-O groups,³² and this confirm the presence of N and O functional groups in N-GQDs. Also, the Raman band at

~1528 cm^-1 in Au@N-GQDs is attributed to the Au-C bond formation due to the strong coupling between N-GQDs and Au NPs in Au@N-GQDs. Its noteworthy that as compared to N-GQDs, a redshift of the G band in Au@N-GQDs from ~1590 cm^-1 to 1585 cm^-1 is due to the charge transfer from Au NPs to N-GQDs³³. The enhancement of electron density in N-GQDs is attributed to the increasing polarizability of the Raman active species. This improvement of the polarizability facilitates the redshift of the Raman bands. Interestingly, after the interaction of N-GQDs with Au NPs, an enhancement in the intensity ratio of ID to IG (ID/IG) from ~0.73 to 1.33 is observed, accompanied by the reduction of the FWHM of D band and the disappearance of D3 band. The enhancement in intensity of D band is primarily due to the preferential attachment of Au NPs at the defect sites of N-GQDs and the local enhancement of field due to the plasmonic absorption by Au NPs.³⁴ In presence of the Au NPs, the intensity of both the G and D bands increase, with a higher enhancement factor for D band particularly due to the attachment of Au NPs at the defect sites.³⁴ Thus, the enhancement in D band intensity here does not necessarily mean the increase in the defect density. Further, the improvement of D4 band (~1615 cm^-1), corresponding to C=N-OH stretching vibration in Au@N-GQDs,³¹ indicates the modification of the functional groups with – OH contributions. Moreover, a drastic reduction in the FWHM of G band from ~60 cm^-1 to~44 cm^-1 is ascribed to the strong vibration of C=C bond in Au@N-GQDs through the enhancement of the local EM field by the SPR absorption of Au NPs, as discussed in Chapter 5, Section 5.3.2.2.

The Raman analysis confirms the modification of the defect states in N-GQDs as well as the

147 | ^{F e3 +}S e n s i n g a n d F a s t P h o t o r e s p o n s e b y A u @ N - G Q D s

increase of the charge density in N-GQDs as a result of metal-carbon composites formation, consistent with the XPS analysis.

Due to the attachment of Au NPs with N-GQDs, the change of the functional groups and modification of different bond vibration are also confirmed from FTIR analysis. The characteristics FTIR spectra of the N-GQDs and Au@N-GQDs are presented in Fig. 6.3(c). The absorption band corresponding to stretching vibration and in-plane bending vibration of C-N and C=N appear at ~1660 cm^-1 and 1522 cm^-1, respectively, which confirm the successful functionalization of GQDs with nitrogen.³¹ An absorption peak at ~1455 cm^-1 arises due to the stretching vibration of phenyl groups in N-GQDs.³¹ Moreover, the absorption band at ~1335 cm^-

1 and 1260 cm^-1 are assigned to the bending vibration of O-H and stretching vibration of C-O bond, respectively, suggesting the presence of abundant oxygen-containing functional groups in N- GQDs.³¹ Additionally, the absorption band at ~976 cm^-1 and 764 cm^-1 are due to the C-H3 rocking vibration and C-O deformation in N-GQDs.³¹ In Au@N-GQDs, strong absorption of C=C stretching vibration at ~1598 cm^-1 is due to the influence of the local EM field enhancement of Au NPs, consistent with the Raman analysis. Noteworthy, the absorption band corresponding to C-N and C=N at ~1660 cm^-1 and 1522 cm^-1, respectively,diminish in Au@N-GQDs as a result of the metal-carbon hybrid formation, again confirming the removal of defect states from N-GQDs.

Interestingly, an absorption band at ~1345 cm^-1 corresponding to O-H vibration becomes sharper as well as stronger in Au@N-GQDs due to the modification of the functional groups, and this is consistent with XPS and Raman analyses.