A single-step process presented here describe the facile synthesis of the NP-CD from the pool of imidazole (source of nitrogen (N)), phosphoric acid (source of phosphorus (P)) and polyethylene glycol (source of carbon (C)), via the simplest method of microwave charring for ~4 min. The as-synthesized blue fluorescent NP-CD utilized for the aqueous phase photoreduction of Cr(VI) to Cr(III) under the presence of natural sunlight. Compared to artificial light, natural sunlight shows its significant contribution towards the photoreduction of the Cr(VI) to Cr(III) concerning the time required for the photoreduction is shown in Table 1, which also includes synthesis time for the fabrication of nano-carbons in comparison with the existing reports. Furthermore, the Cr(III) was also precipitated out by the NaOH solution to get treated water from the synthetic contaminated water.

Table 1 Comparative table showing synthesis and application of graphene based carbon nano material in the field of the photoreduction of Cr(VI) Full size table

Spectroscopic characterization

The absorption spectrum, (Fig. 1a) shows peaks at ~210 nm and ~270 nm that correspond to the transitions associated with π–π* (C=C) and n–π* (C = N/P), respectively. The photographic images of the NP-CD in daylight and under the UV light illumination are shown in Fig. 1b. The blue emitting NP-CD show the excitation wavelength dependent flurorescence emission (Fig. 1c), situated mostly between the blue and green region (~360 to ~540 nm) of the visible spectrum, having the highest emission intensity at ~443 nm (excited at 380 nm), and possesing a quantum yield value of ~15%.28 In addition, the photostability of NP-CD was investigated for 5 h at the excitation wavelength of 380 nm and it shows good photostability (Fig. 1d) without any apperent loss in the emission intensity.

Fig. 1 a UV-Vis spectrum; b photographic image in daylight and UV light; c excitation dependent fluorescence emission spectra (excited from 300 nm to 580 nm with the increment of 20 nm towards the higher wavelength) and d photostability at 380 nm excitation for 5 hr of NP-CD Full size image

The possible identification of the surface functionalities29,30 of the NP-CD was carried out by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). XPS spectrum of NP-CD (Fig. 2a) shows the signature peaks associated with four elements; carbon (C 1s ), oxygen (O 1s ), nitrogen (N 1s ), and phosphorus (P 2p and P 2s ) at ~284.4 eV, ~532.1 eV, ~399.8 eV, ~132.6, and ~188 eV, respectively. The inset of Fig. 2a shows a tabular data for the elemental composition from the XPS survey scan. Figure 2b–e shows the high-resolution short scanned XPS spectra of C 1s , N 1s , P 2p , and O 1s , respectively after deconvulation. Short scanned C 1s analysis after deconvulation as displayed in Fig. 2b, showing the presence of six different peaks located at 283.4, 284.2, 284.8, 285.6, 286.2, and 288 eV corresponding to the presence of different binding states of carbon as C=C, C–C, C–N/P, C–O, C=N, and C=O, respectively.31,32 After deconvulation short scan of N 1s show three major peaks at 398.9, 400.6 eV and 402 eV corresponding to the C=N–C (pyridinic N), C–N–H (pyrollic N), and C=N (graphitic N), respectively (Fig. 2c). Similarly, The P 2p short scan after deconvulation shows two peaks at 134.5 eV and 133.5 eV corresponding to the presence of the P–O and P–C, respectively (Fig. 2d).9 The short scan for O 1s region after deconvulation shows the presence of three major peaks at 532.9 eV, 532.3 eV and 531.1 eV corresponding to the presence of C=O/C=N, C–O, and O–N/P, respectively (Fig. 2e).33 FTIR results (Fig. 2f) show a broad band from 3484 to 3200 cm−1 corresponding to the N–H (sharp end) and O–H stretching vibrations.12,16,34 The doublet at ~2925 and ~2873 cm−1 correspond to the C–H stretching vibrations.34 The medium band at ~1745 cm−1 and ~1643 cm−1 correspond to C=O and C=C stretching vibrations. The medium band at ~1456 cm−1 and ~1351 cm−1 correspond to C=N and C–N stretching vibrations, respectively.34,35,36 The medium and sharp peaks ~1068 cm−1 and ~940 cm−1 correspond to the C–O, P–O–C stretching vibrations. The bands between ~600 cm−1 and ~500 cm−1 correspond to the aromatic stretching,12,16,17,35,37 along with presence of the signature peaks of –P–O–C– and =N–C– confirming the doping of N and P in the NP-CD. The presence of different type of functional groups confirmed by the FTIR spectra of NP-CD were also in accordance with binding suggested by the XPS spectra. Additionally, to understand the distribution of the different elements especially, the N and P within the carbonized matrix of NP-CD. We did a XPS analysis concerning the percentage composition of the different elements at three different places in the same sample and presented the data in the form of standard error as C (52.85 ± 0.36)%, O (30.23 ± 0.13)%, N (5.55 ± 0.06)%, P (11.36 ± 0.33)%.

Fig. 2 a XPS survey scan of NP-CD with its corresponding short scan of b C1s; c N1s; d P2p and e O1s; f FTIR spectra Full size image

Microscopic analysis

Transmission electron microscopy (TEM) was used to observe the morphology of NP-CD (Fig. 3). The TEM image of NP-CD (Fig. 3a), shows well dispersed spherical particles with the corresponding size distribution analysis in Fig. 3b. Gaussian fitting of the size distribution curve shows that the average size of the NP-CD is 9.5 nm with the standard deviation of 3.3 nm. The High-resolution TEM (HRTEM) image in Fig. 3c shows the graphitic fringes encircled with the yellow circles. Figure 3d is the HRTEM image of NP-CD, which shows the interplanar layers of the graphitic carbon of 0.22 nm.

Fig. 3 a TEM image of NP-CD; and b its corresponding size distribution; c HRTEM image of NP-CD highlighted with yellow circles showing the arrangements of graphitic packing as interplanar fringes; d HRTEM image showing the interplanar arrangements of NP-CD Full size image

Sunlight Induced photocatalytic reduction of Cr(VI) and plausible mechanism

Using the potassium dichromate as a source of Cr(VI) to prepare the synthetic contaminated water, NP-CD were used here for the aqueous phase photocatalytic reduction of Cr(VI) to Cr(III) under the influence of natural sunlight. The important prospect of the present finding ascribed towards the photoreduction of variable concentrations of Cr(VI) (10 ppm–2000 ppm) to its respective Cr(III) using the same dose of the photocatalyst. The effects of NP-CD and sunlight on photocatalysis were shown in the Fig. 4a. The adsorption–desorption equilibrium was attained via sonicating for 30 min in the daylight (Fig. 4a, b) and afterwards the Cr(VI) was kept under the sunlight, and after every 10 min the photo reduced solutions were collected for the UV–Vis analysis. The residual Cr(VI) concentrations after the photoreduction were investigated by the UV–Vis spectrometer at 540 nm wavelength with the help of di-phenyl carbazide (DPC) assay.6,38,39 In the absence of NP-CD (black line-Fig. 4a) and the sunlight (red line-Fig. 4a), there was almost negligible changes in the initial concentration of Cr(VI). While after the addition of the NP-CD in the presence of sunlight (blue line-Fig. 4a) almost complete photoreduction of the Cr (VI) (400 ppm) had been achieved within the ~110 min. Additionally, the same Fig. 4a shows that NP-CD exhibited only ~10% of adsorption. The quantitative evaluation of Cr(VI) reduction by NP-CD was performed using different concentrations (10, 50, 100, 400, 1000, 2000 ppm) of Cr (VI) with the same catalyst dose (0.07 mg mL−1) just by increasing the sunlight irradiation time. The continuous decrease in the initial concentration of Cr(VI) represented as C/C ο with the time under the sunlight irradiation is shown in Fig. 4b. The photocatalytic reduction of Cr(VI) by NP-CD follows pseudo-first-order kinetics with a significant R2 value (Fig. 4c). The rate constant and it’s corresponding half-life for different Cr(VI) concentration were shown in Fig. 4d. The plausible mechanism for the photocatalytic Cr(VI) reduction by NP-CD, has been displayed in Fig. 4e. Under the influence of sunlight irradiation, holes and electrons are generated in the valence and conduction bands of NP-CD (equation (i)). Further, the photo induced holes react with water molecules to generate highly reactive H+ (as shown in equation (ii)). The photo induced electrons were initially reacting with the Cr(VI) present near the periphery of the NP-CD as shown in equation (iii). These highly reactive protons and electrons cumulatively reduce Cr(VI) to Cr(III) as shown in equation (iv). The overall mechanism is showing schematically to understand the plausible reason for the photocatalytic reduction of toxic Cr(VI) to less toxic Cr(III). Nevertheless to state about the requirement of the acidic pH, as with the increase in pH towards basic side decreases the photoreduction efficiency of NP-CD40. The Cr(III) has been separated from the water by the precipitation in the form of the hydroxides of Cr(III) after the addition of NaOH solution.41

Fig. 4 a Plot of (C/Co) for the photoreduction of the 400 ppm of Cr(VI) by NP-CD under the different condition; b Plot of (C/Co) for Cr(VI) photoreduction by NP-CD under different concentrations of Cr(VI); c pseudo-first-order linear fit data with different concentration of Cr(VI); d rate constant and half-life graph for the different concentration of Cr (VI) and e schematic representation of plausible reaction mechanism of photocatalytic reduction of Cr(VI) to Cr(III) under the sunlight irradiation by NP-CD Full size image

Effect of the contents of N, P on the photoreduction ability of NP-CD towards Cr(VI)

To investigate the effect of N and P content on the photoreduction ability of NP-CD, a control set of experiments was performed using single dopant material such as only N and only P charred with the same amount of polyethylene glycol (control sample) under the similar experimental conditions as being used for the synthesis of NP-CD. Figure 5a shows the prominent effect of combined doping of N-P compared to the un-doped (CD) and singly doped CD as (N-CD and P-CD). As well as, the effect of the amount of the reactant has also been investigated on the photoreduction efficiency by changing the reactant concentration to optimize the reaction conditions and obtain the maximum efficiency of photocatalyst. Figure 5b–d shows the effect of varying concentration of each reactant compared to the control (NP-CD). From the Fig. 5b–d it was concluded that the reactant mixture of 3 g imidazole, 10 mL phosphoric acid and 10 mL polyethylene glycol was the best composition for the fabrication of NP-CD for its potential application in photocatalytic reduction of Cr(VI) to Cr(III). Additionally, under the sunlight irradiation, Fig. 5e shows the extent of photocatalyst loading on the photoreduction efficiency of NP-CD and as expected it was found that on increasing the catalyst dose the rate of the photoreduction becomes faster compared to the lower dose for the same concentrations of Cr(VI) (400 ppm).42 When the dose was 0.14 mg mL−1 the time required for 400 ppm Cr(VI) reduction was 20 min while at 0.07 mg mL−1 the time required for photocatalytic reduction was 110 min. For the presented finding a moderate amount of photocatalyst was being used (0.07 mg mL−1). The NP-CD worked as a stable photocatalyst as observed by its recyclability upto six cycles which shows 98% efficiency for the 400 ppm Cr(VI), is demonstrated in Fig. 5f.

Fig. 5 a Effect of the reactant contents of N and P on the working efficiency of the CD; b effect of imidazole; c effect of phosphoric acid; d and the effect of polyethylene glycol; e effect of catalyst loading; f the photocatalytic performance up to six cycles by the recycling test on the photoreduction efficiency of NP-CD on the 400 ppm Cr(VI) under the sunlight irradiation Full size image

Effect of interfering ions

Further, to explore the possibilities of NP-CD in practical application, the photoreduction ability of NP-CD has been examined in the presence of many other interfering ions. Experimentally separate solutions of the 100 ppm of different interfering43 ions (as chloride (Cl−), sulfate (SO 4 2−), nitrate (NO 3 −), phosphate (PO 4 3−), ferrous (Fe2+), and calcium (Ca2+)) along with a mixture that contains all the different interfering ions,43 were mixed into the solutions of 400 ppm Cr(VI) under the similar experimental conditions as discussed above. All the experiments were carried out at the dose of photocatalyst (0.07 mg mL1) in 120 min of sunlight irradiation. After performing the interference study, a minimal decrease in the reduction efficiency of NP-CD observed as shown in Fig. 6, with different interfering ions and their corresponding mixture compared with the control set containing 400 ppm Cr(VI) solution without any interfering ion. The NP-CD shows an effective and efficient photocatalytic material as the 100 ppm concentration of all the interfering ions and its corresponding mixtures does not affect the photoreduction abilities of NP-CD.

Fig. 6 Photoreduction efficiency of NP-CD under the different presence of the interfering ions Cl–, SO 4 2−, NO 3 −, PO 4 3−, Fe2+ and Ca2+ along with its mixture which contains 100 ppm of each the interfering ionunder the presence of sunlight Full size image

The present finding briefs about the synthesis and exploration of NP-CD as an efficient material for aqueous phase photocatalytic reduction of Cr(VI) to Cr(III) under the influence of natural sunlight. Moreover, the less toxic Cr(III) was removed from the treated water using a simpler process of precipitation. The ease in the recyclability along with no apparent loss in the catalytic efficiency could hold a large, prosperous future of these NP-CD in the field of photocatalytic water remediation. In future, the detailed structural characterization of NP-CD could further provide significant potential as priority materials to execute many hidden applications of doped-CD towards the efficient removal of various toxic inorganic and organic pollutants from the contaminated wastewater. Additionally, a detailed and systematic study to find out the wavelength dependence of the photodegradation efficiency could beimmensely helpful in providing valuable insights into the processes involved.