Figure 1 shows a comparison of the Powder XRD profiles of the pristine and doped GQDs. Doping of GQDs is expected to cause lattice expansion or contraction, depending upon the size of the dopant atom. As we see in figure 1, there are clear shifts in (002) and (110) peaks in all three doped systems, suggesting changes in the lattice parameters[10]. Atomic radii of nitrogen atoms and sulfur atoms are 56 pm and 100 pm respectively, so incorporating them into GQD lattice leads to contraction[19],[20] and expansion respectively[21],[22]. The d(002) spacing values come out to be around 3.4 A0, and the d(110) is around 2.8 A0 for all the GQDs respectively, which are in excellent agreement with other reports as well[19]-[22].
Figure 2 shows the UV-vis spectra of the GQDs where one peak at 236 nm, corresponding to the π to π* transition of C=C bonds, and another at 350 nm, corresponding to the n to π* transition in the C=O bonds [24] in the pristine GQDs[25] are seen. In the doped GQDs, we observe one peak at 235 nm for N-doped GQDs, 240 nm for S-doped GQDs and at 235 nm for the N,S co-doped GQDs, corresponding to π to π* transitions[26]-[28], and another at 350 nm, 340 nm for the pristine and N-doped respectively corresponding to n to π* transition[26],[29]. Interestingly, in the S-doped GQDs, no peak at 340 nm is observed, probably due to the lesser number of C=O bonds, and also because of its symmetry forbidden nature[30]. In the N and N,S co-doped GQDs, the peak at around 340 nm is relatively intense compared to that of the pristine GQDs, perhaps due to a decrease in the oscillator strength of n to π* transition[31].
Figure 3 compares the FTIR spectra of the GQDs. We can observe -OH peaks in all the GQDs, in the neighbourhood of 3400 cm-1 [29]. A doublet corresponding to -C-H stretch is also observed in GQDs at around 2900-2960 cm-1 while -N-H stretch is observed at around 3400 cm-1 [32]. C=C stretch is also observed. In the S-doped and N,S co-doped GQDs, we can see a sulfonyl stretching at around 1438 and 1414 cm-1 respectively. In the range of 1630-1710 cm-1, we can see a peak corresponding to conjugated ketonic C=O stretching, which shows across different GQDs, probably caused by the introduction of dopants into the conjugated system[33]. In earlier work done by our group on synthesizing N,S co-doped GQDs, XPS studies showed the presence of C, O, N and S with atomic concentrations of 68 %, 9.6%, 11.6% 8.2% and 0.1% respectively, which is in agreement with our EDX data. The composition of the GQDs could be understood well in terms of heteroatom doping, and it is well known that different N doped species present in pyridinic and graphitic moiety produce different catalytically active sites. For example, S 2p spectrum has two peaks centred at 164.7 and 169.1 eV respectively indicating the presence of S in two forms while four kinds of N bonding could be seen from 398.16 eV (owing to pyridinic), 399.62 eV ( pyrrolic), 400.73 eV (quartenary) and 401.85 (quarternary valley)REF.
Figure 4 shows the surface morphology as evidenced by the Scanning Electron Micrograph of the N,S co-doped GQDs. The GQDs appear to be granular with more or less uniform particles with no visible signs of agglomeration. The EDX data reveal 55.6% of C, 7.0% of N, 37.3 of O, and 0.1% of S by atomic weight percentage in the sample, showing that the GQDs has been successfully doped with both N and S. However, morphologies of GQDs co-doped with N and S were very similar to that of original GQDs as supported by the , microstructural analysis since structural, topological or edge defects could not be resolved among all these samples.
Fluorescence emission spectra of the GQDs taken using an excitation wavelength of 300 to 410 nm-are shown in Figure 5. In the pristine GQDs, we get a peak at 450 nm in the emission spectrum. Under UV excitation, all the GQDs exhibit blue-green luminescence. We see an excitation-independent luminescence profile in the N-doped GQDs, with the excitation maxima occurring at 450 nm. In the N,S co-doped GQDs, we can observe the excitation maxima to be at around 450 nm, but unlike the case of N-doped GQDs, it is excitation dependent. This can be attributed to the various surface states present on the N,S-GQDs[33]. In the fluorescence spectrum of S-doped GQDs, we can observe dual emission, one peak occurring at around 450 nm, and the other peak at 520 nm respectively. The peak at 450 nm shows excitation independence (inter-band transition), whereas the peak at 520 nm is excitation dependent, perhaps due to the presence of surface groups[33]. These surface groups are also believed to be responsible for the lack of sustained stability especially in dry conditions when the doped GQDs are stored more than few weeks.
Figure 6 depicts superimposed Cyclic Voltammograms of all the GQDs at a constant scan rate of 100 mV/s. In the pristine GQDs, we observe a peak at -0.5 V while for the case of doped GQDs, the peak shifts to -0.45, -0.48 and -0.47 V for N-doped, S-doped and N,S co-doped GQDs respectively. This shows that Oxygen reduction is thermodynamically more favourable in the case of doped GQDs compared to the pristine GQDs. The observed OCV values (210 mV for the pristine, 230 mV, 300 mV and 250 mV for the N-doped, S-doped, and N,S co-doped GQDs) also indicate this order despite more variability ( @ 5 mV) . The current density at a typical voltage like -0.45 V (approximately 650 mV overpotential) is 0.34 mA/cm2 in the pristine GQDs, 0.38 mA/cm2 in the N-doped GQDs, 0.545 mA/cm2 in the S-doped GQDs, and 0.411 mA/cm2 in the case of N,S co-doped GQDs. This is also in excellent agreement with RDE studies on similar GQDs by other groups[31]. Also, from our earlier RDE studies on the ORR performance of N-doped GQDs, we found out that 4 electron pathway is preferred in the case of N-doped GQDs in basic condition[13]. This is further confirmed by the scan rate dependence of the voltammograms indicated in Fig. 7.
It is well known that Oxygen Reduction in alkaline media can undergo via a two-electron pathway, or by a four-electron pathway, based on the electrode materials and pH as shown below.
In N-doped GQDs, the four-electron pathway is preferred for ORR, as confirmed by RDE studies and DFT calculations[10],[13]. Mechanism of ORR in alkaline media on GQDs has been investigated by several groups and a clear picture has emerged suggesting two different modes of oxygen adsorption configuration, namely Yeager and Pauling configurations. DFT calculations clearly show that O2 gets adsorbed in Pauling mode on the surface of N-doped[34] and N,S co-doped GQDs[35]. When we have both N and S as co-dopants, bridged adsorption configurations are more entropically favoured and this is illustrated in the thermodynamically relevant open circuit values in Table.1. However, these benefits are dominated more by kinetic effects as reflected by the higher the exchange current density and Tafel slope values.
The difference in the adsorption configuration in terms of unique structures could contribute in enhancing the performance of N-doped GQDs as compared to that of N, S-doped GQDs. For many of these 2D materials, along with doping, surface states also play a key role depending on this unique adsorption configuration as electronegativity difference can cause localized charge redistribution. However, it is difficult to separate the role of unique structures as the main involvement is through surface states.
Steady-state Galvanostatic polarization measurements were carried out in order to corroborate the results of the Voltammetry to calculate the apparent Tafel slopes and exchange current densities. The parameters derived from the Tafel slopes are given in the table 1, and for comparison purposes, the Tafel slopes of standard bench-mark catalysts are given. Although the values of exchange current density of N-GQDs and N,S-GQDs are very close the Tafel slope change reflects the difference in the conductivity values perhaps suggesting the role of charge redistribution around the dopant hetero-atom.
Catalyst
|
Tafel slope (mV/dec.)
|
Transfer coefficient, α
|
Exchange Current Density (j0, A/cm2)
|
Open Circuit Potential (mV)
|
Pt
|
160 [10]
|
--
|
3 x 10-8
|
--
|
Fe-N-C
|
120 [39]
|
--
|
6.06 x 10-6 [41]
|
--
|
Co-N-C
|
83 [40]
|
--
|
7.07 x 10-6
|
--
|
Pristine GQD
|
80
|
0.7
|
1 x 10-8
|
123
|
N
|
100
|
0.51
|
1.1 x 10-6
|
100
|
N,S
|
120
|
0.49
|
1 x 10-6
|
75
|
S
|
90
|
0.62
|
1 x 10-7
|
230
|
|
|
|
|
|
Table 1: Comparison of the kinetic parameters from Tafel measurements of the GQDs; similar values for bench-marks electrocatalysts are also indicated in O2 saturated 0.1M KOH. Exchange current densities were calculated from the intercepts and apparent Tafel slopes.
Doping leads to an increase in exchange current densities and the order of magnitude increase can be seen in N-GQDs, followed by N,S-GQDs and S-GQDs respectively, showing ORR is kinetically most preferable in the case of N-doped GQDs. There could be several reasons for the higher electrochemical performance of N-doped GQDs as manifested by parameters such as lower onset potential and higher exchange current density compared to those of S-doped and N, S-doped primarily due to lower activation overpotential involved in bond breaking subsequent to the Pauling adsorption configuration. This is also in agreement with an enhancement in conductivity values post doping, as shown by the works of other groups[36]-[38].
From the above fluorescence data, voltammograms, and exchange current densities, we can see that doping causes significant changes in the GQDs and their ORR performance with profound implications for applications such as energy storage. The mechanism of ORR seems to be similar for all the GQDs considered. Perhaps, the excitation-dependent luminescent profile of N,S co-doped GQDs could be leveraged to design “smart electrocatalysts” which can either shift or in quench the luminescence when surface degradation occurs during sustained utilization but further experiments are planned on durability and degradation studies (S-03) to confirm this enticing possibility. These studies clearly indicate the importance of hetero-atom doped metal-free GQDs as a possible replacement of precious metal electrocatalysts since cost reduction and efficiency improvement are possible after establishing their durability and robustness.