Figure 2 (a) shows XRD pattern of synthesized carbon QDs. The XRD pattern of CQDs indicates from a main peak at 2θ = 20 ° and d = 4.5415 A which is related to (002) plane of JCPDS card no. 26-1076 [41]. Raman spectrum of synthesized carbon QDs is shown in Fig. 2 (b). Raman spectrum of CQDs indicates of two sharp peaks at 1311.87 cm− 1 and 1585.82 cm− 1 which are related to D-band and G-band, respectively. The D-band peak (sp3 hybridization) shows the amount of defect, functionalization and edge effect corresponds to the A1g symmetry photons near K-zone boundary and G-band peak (sp2 hybridization) corresponds to the amount of graphitization associated with CQDs. The ratio of G/D, which means the intensity of G-band peak divided on the intensity of D-band of Raman spectrum, obtained about 3.54 that is much higher than other G/D reported ratios [42–45]. The higher G/D ratio confirmed that the synthesized QDs are composed of graphitic crystalline structure. The zeta potential measurement was done to understanding which charge exists on surface of CQDs. Figure 2 (c) shows zeta potential of CQDs, a broad doublet peak in negative side is shown in this figure and the highest peak located in -18 mV. These negative surface charge confirmed a very high stability for CQDs resulting excellent dispersion in aqueous media [46] and can be a good candidate to adsorbing of material with positive charge such as metals ions. It means that synthesized CQDs can an adsorbing metal ions or sensing metal ions due to electrostatic interactions [47].
Zeta potential results confirmed the presence of negative charge moieties on surface of synthesized CQDs, for more understanding about the structure of surface moieties, FT-IR analyze was done. Figure 3 (a) and (b) show FT-IR spectrum of pistachio and CQDs, respectively. The FT-IR spectrum of pure pistachio have a assigned-peak to O-H stretching at 3350 cm− 1 and some peaks corresponding to C-H at 3007 (C-H stretching alkene groups), 2952 and 2854 (CH or CH2 stretching aliphatic groups ), 1461 (CH aliphatic bending groups) [48], 1380 (CH3 in proteins) [49] and 722 cm− 1 (CH ≡ CH). There are another the main peaks at 1746, 1655, 1640, 1546, 1240 and 1162 cm− 1 which can be attributed to C = O, NH3 [50], C = C [51], amid II [52], stretching C-N, C-O-C stretching, C-O ring vibrations in carbohydrates [53], respectively. FT-IR spectrum of reveals a broad and strong peaks at 3225 cm-1 corresponding to OH and NH symmetric and asymmetric stretching, noting that the broadening is due to hydrogen binding [54]. Three peaks at 2963, 2933, 2874, 1453, 1402, 706 cm− 1 attributed CH stretching and bonding vibration [55–56]. Also, there are another main peaks 1672 (C-O-C), 1592 (C = O), 1517 (CON-H), 1325 (C-OH), 1293 (C = S), 1220, 1105 and 1048 (C-O), 1149 and 672 (C-S), 1074 cm− 1 (C-N). As it seen from FTIR spectra due to organics compositions in pistachio, there are some bonding consisting of S and N leading N-S cooped CQDs that has effective impact on the photoluminescence property of CQDs. The persistence COOH is deduced from peaks of C = O ad C-H. The negative charge resulted from zeta potential can be affected by COOH bond leading to good dispersing of CQDs [57].
Figure 4 (a) shows EDS analyze of CQDs. As shown in this figure, carbon and oxygen elements are present in structure. The little amounts of oxygen element in structure is for aqueous media of synthesis process. The amount of nitrogen and sulfur elements was too low and the EDS analyzer could not show them. Figure 2 (b-c) show FESEM and TEM images of CQDs with spherical shape and approximate size of 7 nm.
Figure 5 (a) depicts PL spectrum of CQDs. PL spectrum of CQDs indicates of a doublet broad peak from 300 to 650 nm with two peaks at 360 nm and 485 nm. Figure 5 (b) shows absorbance spectrum of CQDs, in this figure there are two band edge, first a strong band edge centered at 286 nm and another a small band edge at 360 nm which are related to the π-π*transition of C = C and n-π* transition of C = O bands, respectively [42, 45]. During synthesized process the synthesis time was changed and the optical behavior of the synthesized CQDs was investigated. Synthesized times of 1–5 h, the PL spectrum was like Fig. 5 (a) with less intensity but after 6 h from synthesis process the PL spectrum changed and is shown in Fig. 5 (c). As shown in this picture a broad doublet peak changed to a narrow PL peak at 510 nm. It can be explained that with increase synthesis time CQDs grow and further carbonization occur leading to red shift in PL spectrum. Figure 5 (d) depicts the absorbance spectrum of synthesized CQDs after 6 h synthesis process. For investigating effect of solvent on PL intensity of CQDs, 5 mg of synthesized CQDs were dispersed in 5 ml of different solvents and results are shown in Fig. 5 (e). As shown in this picture the highest PL intensity of CQDs was in acetone and lowest of intensity was in dimethylformamide (DMF). These results shows that the PL of synthesized CQDs is dependent to solution due to surface trap or interaction of oxygen, nitrogen and sulfur bonds with environment around of QDS.
Given that the CQDs charge is negative, they may interact to metal ion, that's what we investigated sensitivity of them by different metal ions such as Zn2+, Ni2+, Hg2+, Ag3+, Cd3+, Cu2+, Fe2+, As3+, Pb2+, Mn2+, and MO4+ in water media. The relative PL intensity (I/I0) CQDs after adding of these metals ions with the concentration of 5 mM are shown in Fig. 6 (a). The relative PL intensity for cobalt had higher sensitivity, then it was selected for investigation in lower concentration. Figure 6 (b) shows ln (I0/I) versus Co2+ concentration, which I0 and I are the PL intensity of CQDs in absence and presence of Co2+ ions. The inset picture of Fig. 6 (b) depicts ln (I0/I) versus Co2+ ions concentration in range of 100 µ molar to 280 µ molar. As it can be deduced from this picture, there is a good relationship with linear equation of y = 0.0027 x + 0.193 and R2 = 0.9944 between degree of decreasing in PL intensity and Co2+ concentration. As the obtained results confirmed, only Co2+ ions can decrease PL intensity of CQDs dramatically and these results show that CQDs can be used as Co2+ fluorescence sensor.
As CQDs are nontoxic, application of them for photodegradation of organic dyes from water is important and they can be known as green catalyst. The photocatalyst performance of CQDs were investigated by methylene blue (MB), methylene orange (MO), and rhodamine b (RB) under both UV and visible illuminations. 10 mg of synthesized CQDs was dispersed in 50 ml DI water and after that the prepared solution was stirred for 30 minute in darkness for adsorption-desorption equilibrium. At the end half of the prepared solution was located under UV irradiation and another half were located under a solar simulator system for different time durations. Figure 7 (a) shows the corresponding relative changes versus illumination time. As it is seen in Fig. 7 (a), the photodegradation process under UV illumination was much superior to sun illumination. Also CQDs had better photodegradation with MB than MO and RB.
Figure 7 (b) shows the logarithmic change of relative concentration of samples with different illumination time, and the inset picture of Fig. 7 (b) shows the dye degradation percentage of samples. To better understanding about photodegradation of CQDs with organic dyes, the kinetic law of degradation process of catalyst was calculated by using the following formula [58–60].
In this formula, C is the MB, MO, and RB concentration at a particular time, C0 is the initial MB, MO, and RB concentration, k is the pseudo-linear first-order kinetic constant (min− 1) and t is the reaction time in min.
The obtained results showed that degradation rate of MB by CQDs under UV illumination is bigger than other samples and results are tabulated in table 1. Actually, different reactive species such as electrons (e), holes (h) and hydroxyl (OH) radicals are present during the photodegradation process. The photodegradation process will decrease with the presence of the corresponding scavenger of the reactive species which has a main role in the degradation process. The scavengers that are used in this work are hydrogen peroxide (H2O2) for electrons [61], sodium iodide (NaI) for OH ads and dimethyl sulfoxide (DMSO) for OH bulk [62], and ethylenediaminetetraacetic acid (EDTA) for holes [63]. The obtained results from radical scavengers are show in Fig. 7 (c). The obtained results show, the solution of MB dye in the absence of catalyst under UV irradiation had no degradation. Addition of H2O2 to the photodegradation process decreased the degradation process dramatically while addition of NaI DMSO, and EDTA hadn’t any considerable influence on photodegradation process and it means electrons have a main role in photodegradation of MB by CQDs.