Synthesis, Purification, and Photocatalytic Evaluation of Nanostructured N 2 dopped Carbogenic Particles

doi:10.21203/rs.3.rs-5260794/v1

Carbon dots (CDs) have brought in significant attention in material science due to their unique characteristics, such as exceptional biocompatibility, physiochemical stability, low toxicity, and photostability. This work aims to synthesize and purify nitrogen-doped carbon dots in a rapid and economical bottom-up (hydrothermal) method. The synthesized material characterization was carried out by utilizing UV spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), Zeta sizer, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The absorption potential of carbon dots, including dye content, pH, irradiation period, and oxidant concentration, was investigated in batch research. The measurement of parameters related to water quality, including total organic carbon (TOC), biological oxygen demand (BOD), and carbon oxygen demand (COD), was evaluated. Response surface methodology (RSM) was used to enhance operational parameters in wastewater treatment. Additionally, the substrate's (black pumice) reusability was examined for up to fourteen cycles.

Carbon Dots

Doping of CDs

fluorescence

Photocatalysis

Characterization

COD

BOD

TOC

Pumice

RSM

reusability

The scarcity of fresh, clean water is one of the biggest problems facing society in the twenty-first century. Because of rising pollution levels and increased water consumption, wastewater must now be treated with water before it can be used again. Additionally, because of the growing population, we must take action to repurpose waste water. Industrial wastewater has been leaking into the rivers which resultantly contaminating the aquifers from decades [1, 2]. Various organic chemicals and metal ions, produced from industrial manufacturing processes like printing, papermaking, dyeing, food sector, etc., are present in the industrial effluent which may comprise poisonous, carcinogenic, teratogenic, and pigmented organic pollutants. These hazardous pollutants are harmful for plants, aquatic life, and humans, if released into the environment untreated. Moreover, it is difficult to naturally eliminate these colored chemicals from wastewater, as generally, there is less than one part per million of dye present in the effluents [3–5]. With the systematic progression, various methodologies have been developed for treating wastewater, among which, adsorption and pollution control method are most widely used techniques. However, photocatalytic degradation have more edge over the other ones as it can convert organic wastewater into CO₂ and H₂O without the need for an intermediary, making it a viable material in environmental technology [6–10]. Carbon nanotubes, graphene, fullerene, and its derivatives are examples of carbogenic nanomaterials that have been acknowledged as sound materials through a variety of applications. 10 nm rang of carbon nanoparticles are the main component of carbogenic materials [11–15]. Many techniques are available for obtaining CNDs, and they can be broadly divided into two categories: top-down and bottom-up techniques [16–19]. In these cases, the size and colloid stability of the CND dispersions can be tuned, although the yield is frequently low and purification steps are required to achieve the desired outcome [20]. Numerous strategies, including as electrochemical processes [21], hydrothermal/solvothermal techniques [22, 23], pyrolysis, microwave assistance, and oxidative cutting [24], have been proposed to accomplish this. Additionally, carbon dots (The surface oxidation by nitric acid) are made using a number of precursors, such as graphite [25], citric acid, glucose, and sucrose [26]. [27, 28] [29]. Over 10,000 distinct types of synthetic pigments and dyes are produced annually worldwide [30]. CNDs are useful nanomaterials for organic pollutant dye degradation [31]. After comparing carbon dots and metal-based quantum dots (QDs), it was found that the former had better qualities than the latter because of their fluorescence features, aqueous solubility, ease of functionalization, chemical inactivity, and less toxicity.

As carbon dots are naturally fluorescent material, their light-emitting factor depends on the excitation wavelength. Surface passivation using a metal-based material also explained how emission affects particle size, with smaller dots having better photoluminescence properties [32–34]. The surface passivation emphasizes the necessity for other synthetic methods to attain more precise control over the dimensions, morphology, and physical characteristics of carbon particles [35–38]. The synthetic process causes the carbon dots to become surface functionalized, most likely as a result of the presence of oxygen-containing groups. Because it increases the particles' dispersibility in a range of solvents, this functionalization is essential [39–41]. The type of surface modification utilized during synthesis determines the dispersibility. These nanoparticles show potential for applications requiring stable colloidal suspensions, such biomedical and materials science research, because they are both monodisperse and dispersible [42–47].

The emission of certain CDs can display uniform or varying photoluminescence (PL) wavelengths owing to the presence of single\multiple band gaps among the valence band and surface positions. This phenomenon is responsible for the excitation of independent and dependent PL wavelength which observed in these CDs [48–50]. Since, with a noticeable energy band gap separating the valence and conduction bands, CDs are photoluminescent semiconductors that have the ability to initiate photocatalytic oxidation and redox reactions. Because of these characteristics, CDs are very well suited for the process of photocatalytic water splitting, which produces oxygen and hydrogen. Moreover, the in vivo utilization of CDs' photocatalytic redox activity has been employed to create antagonistic oxygen species that lead to the demise of tumor hypoxia. Though, as of the rapid backward reactions and electron/hole pair recombination, it has been difficult to create photocatalysts with effective photo to hydrogen conversion, even if solar catalytic water (H2O) splitting is likely for hydrogen generation and photo radiation therapy [51, 52]. With a (2.7 eV) of band gap, graphitic carbon nitride (g-C3N4) is a promising photocatalyst that is positioned for solar light reactions. Accordingly, given that CDs and g-C3N4 have similar polymeric structures and that CDs have different electron sources and acceptors, research should be done on the photocatalytic features of CDs. Recently, CDs have been passivated in heterostructures to increase the activity of different photo stable catalysts and have the capacity to perform a variety of tasks in heterogeneous photocatalysis, including those of photosensitizer and photoelectron mediator acceptor, reducing agent, and enhanced adsorption capacity [53–55].

Moreover, there are a number of benefits that support the use of CDs as photocatalyst. In order to effectively absorb water on their surface, they are first evenly distributed throughout the water, creating a homogeneous phase. Moreover, CDs are non-toxic; they won't damage waterways or have a negative impact on public health. CDs have been used to boost the photocatalytic activity of many metal oxide nanomaterials, including Fe2O3, TiO2, ZnO, SnS2, and BiOBr, but this hasn't always been the case. Less research has been done on the photocatalytic properties of bare CDs, particularly the degradation of photocatalytic dyes. If not, bare CDs' photocatalytic activity is typically negligible [56, 57]. The degradation of dyes is accomplished by a number of methods, including adsorption, flocculation, coagulation, nano-filtration, and biological treatments.

Synthesize N-CNDs was done by hydrothermal method which doesn’t require much time and intricate equipment. Crystallinity and Morphology of the nitrogen-doped material were characterized by using different spectroscopic techniques. The photodegradation activity of nitrogen-doped (N-CDs) on Congo red dye was studied under daylight. The water quality parameters, including COD, BOD, and TOC, were also evaluated by using a UV spectrophotometer before and after the treatment of dye. The central composite design (CCD) was applied to evaluate several parameters such as dye concentration, pH, oxidant concentration, and irradiation time, utilizing response surface methodology (RSM).

Citric acid (C₆H₈O₇), urea (NH₂CONH₂), Hydrogen peroxide (H₂O₂), dis. Water (H₂O), nitric acid (30%), ethanol, surfactant, all were purchased from sigma Aldrich in their pure form. Beakers, weighing balance, hotplate, autoclave, dry oven, micro filter syringes (0.22 μm), and dialysis tubing (14.3 mm) were used for sample filtration.

Synthesis of carbon dots (CDs)

Carbon dots were synthesized done by hydrothermal route which is economically efficient and time saving. Firstly, citric acid (0.05g) and urea (0.3g) were mixed and stirred on hotplate for 50 minutes then shifted the solution into autoclave, placed the autoclave in oven at 180 ֯C for 7hrs. The blue fluorescence was observed under UV lamp [58, 59].

The substrate (pumice) was washed firstly with surfactant then soaked into 30% nitric acid for four hours. After soaking, water was used to wash the substrate then ethanol was sprayed on the pumice and left for the drying. The nitrogen doped citric acid solution was sprayed the pumice.

Fabrication of CD’s:

The 0.1 milimole (mM) solutions of citric acid and urea were prepared in ethanol and methanol (1:3) as solvent for the fabrication of carbogenic material on the pumice surface. The rough surface of pumice act as a good adsorbent. The solution stirred on the hotplate for 50 minutes or until it showed transparency. Then the solution was sprayed on the surface of pumice by keeping length (1feet) of spray nozzle from substrate at temperature 350 °C. when the solution sprayed in hot air on substrate the solvent molecule evaporated and solute particles reacted to the substrate surface as a result a thin coating grew on the surface of pumice.

Characterization of CD’s:

The nitrogen doped carbon dots were characterized done by scanning electron microscopy (SEM) to analysis the surface morphology of CD’s, elemental analysis was done by using EDX, x-ray diffraction (XRD) was used for the crystallographic structure and phase composition analysis, for functional group detection Fourier transformed infrared spectroscopy (FTIR) was used. The size of the carbon dots was determined by using zeta potential.

Statistical analysis after Photo degradation:

The fabricated substrate (pumice) placed into glass beaker containing wastewater and kept under sunlight for degradation. The maximum degradation of wastewater was evaluated by using response surface methodology (RSM) and also optimized the variables i.e. dye concentration (10ppm-70ppm), pH (2-9), time irradiation (1-7hrs) and oxidant concentration (10-70Mm). These parameters were optimized by using software design expert 13. The absorbance of the material was determined before and after treatment and percentage degradation was evaluated by using formula;

While C_i and C_fare before and after treatment absorbance of wastewater.

Table 1: central composite design for the independent variables and their employed levels

Factors	Variables	Unites	Low Act	Higher Act
X1	pH		2	9
X2	Dye Conc.	Ppm	10	70
X3	Oxidant Conc.	mM	10	70
X4	Irradiation Time	Hrs	1	7

Scanning electron microscopy (SEM)

At 180 °C, nucleation speed is faster than growth speed of nanostructures, indicating that the products were composed of numerous separate, small, and uneven particles. The bond length of the carbon atoms in each layer's honeycomb lattice is 0.142 nm, while the spacing between planes is 0.335 nm. The carbogenic material exhibits a high voltage value of N-CDs of 10.0k at 5.00 µm. It also demonstrates this value at 10.0 µm (4.00 k), 30.0 µm (1.50 k), and 500 µm (8.4 k).

Transmission electron microscopy (TEM)

CDs were monodispersing, rang in size from approximately 8 to 15 nm, according to a transmission electron microscope (TEM) image. The TEM image of N-CDs, exhibited fluorophore like morphology. The size of the CDs is approximately smaller; it is most likely that the CDs are difficult to see on the surface material. Carbon dots are simple and dispersed like dots as shown in figure 4.

x-ray diffraction (XRD)

Crystallinity of CDs was investigated by X-ray diffraction (XRD). The average particle size of the crystallite in XRD is 60.12nm. The XRD pattern of CDs revealed a large diffraction peak located at 20.54°. This peak corresponds to the (100) lattice spacing of the carbon-based materials that has, layered, planar structure made up of carbon atoms arranged in a hexagonal way are present in the structure of prepared CDs. Additionally, the XRD pattern is likely responsible for the chaotic structure and amorphous nature of CDs. The average particle size was evaluated by using Debye sheerer formula;

D=kλ/βcosθ eq. (2)

Fourier transformed infrared spectroscopy (FTIR):

Nitrogen metal was successfully doped into the carbon core of carbon dots, as shown by the Fourier-transform infrared (FT-IR) spectroscopy results. The presence of hydroxyl (–OH) functionality is indicated by a broad peak at 3274.5 cm⁻¹. C≡C and C=C stretching vibrations are represented by the peaks at 2215.11 cm⁻¹ and 1623.11 cm⁻¹, respectively. Notably, the peak at 2161.11 cm⁻¹ confirms the existence of C≡O stretching vibrations, further supporting nitrogen metal doping. These observations suggest that nitrogen doped carbon dots have abundant carboxyl and hydroxyl functionalities, potentially allowing for interaction with a wide range of analytes.

Statistical design analysis:

For the optimization of different parameters including dye concentration, time irradiation, pH, and oxidant concentration a central composite design by using response surface methodology (RSM) was applied to check the relationship between variables and percentage degradation of Congo red dye. Design expert 13 pro software was used for the evaluation of these optimized parameters. RSM facilitates the evaluation of various interaction parameters at their optimized degradation efficiency in each thirty runs. The validation of results evaluated by the significance of the model statistically probe value "Prob > F" which was less than 0.005. The model showed excellent correlation between predicted and observed value (R²= 0.4529 adjusted R²= 0.3653). The difference between these R² values is less the 0.2 so the model was reliable for further analysis. By comparing the factor coefficients, the coded equation can be used to determine the relative impact of the components. The final equation in terms of coded each factors in which highest level is coded as positive and lower level coded as negative is given below:

The significance of the model is indicated by its F-value of 5.17. This kind of high F-value has a 1.10% chance of being caused by noise. P-values of less than 0.0500 indicate the significance of the model terms. When the value surpasses 0.1000, the model terms lose their significance. Model reduction can aid your model if it has a lot of unnecessary terms (beyond those needed to preserve hierarchy).

The 0.8912 When compared to pure error, the Lack of Fit F-value suggests that the Lack of Fit is not statistically significant. The probability that a substantial Lack of Fit F-value is due to noise is 13.51%. The model exhibits a "Good Fit" with a non-significant lack of fit.

The statistical parameters that were acquired point to a solid model for forecasting dye degradation in the system under study. Finally, a viable model with good predictive power and sufficient precision is suggested by the examination of these scientific concepts and statistical metrics. This makes it possible to use it to investigate and enhance the dye degradation system's design space.

Table 2: Experimental design which shows level of variables and percentage degradation
Runs	pH	Dye concentration (ppm)	Oxidant concentration (Mm)	Irradiation time (hrs)	Percentage Degradation (%)
1	9	50	10	5	89.86
2	6	5	30	3	75.75
3	3	10	10	5	72.78
4	9	10	10	1	73.33
5	6	30	30	3	78.76
6	3	50	50	1	78.35
7	9	10	10	5	69.01
8	7	30	30	3	70.93
9	12	30	30	3	77.79
10	6	30	30	3	76.74
11	3	10	50	5	89.17
12	6	30	30	3	66.67
13	3	10	10	1	65.55
14	9	50	10	1	81.22
15	6	30	30	3	66.56
16	9	10	50	1	62.76
17	3	50	10	5	67.11
18	3	50	10	1	62.11
19	9	50	50	5	85.05
20	6	30	30	7	70.93
21	6	30	70	3	89.57
22	9	10	50	5	89.17
23	6	30	30	3	81.39
24	6	30	30	3	77.61
25	3	50	50	5	70.27
26	6	30	30	3	78.77
27	9	50	50	1	89.17
28	6	30	10	3	77.77
29	6	70	30	3	77.09
30	3	10	50	1	75.55

Table 3: ANOVA for Linear model (RSM)

source	SS	df	Mean square	F-value	p-value
Model	1292.77	10	129.28	3.23	0.0134	Significant
A-pH	410.52	1	410.52	10.27	0.0047
B-dye conc.	1.33	1	1.33	0.0333	0.8572
C-oxidant conc.	161.89	1	161.89	4.05	0.0586
D-time irradiation	139.25	1	139.25	3.48	0.0775
AB	405.82	1	405.82	10.15	0.0049
AC	51.55	1	51.55	1.29	0.2703
AD	1.27	1	1.27	0.0317	0.8607
BC	19.76	1	19.76	0.4942	0.4906
BD	86.30	1	86.30	2.16	0.1581
CD	15.25	1	15.25	0.3814	0.5442
Residual	759.65	19	39.98
Lack of Fit	547.02	14	39.07	0.9188	0.5911	not significant
Pure Error	212.63	5	42.53
Cor Total	2052.42	29

Table 4: Coefficient of variance (C.V.%) and R-squared confirmation values

Std. Dev.	6.32	R²	0.6299
Mean	76.02	Adjusted R²	0.4351
C.V. %	8.32	Predicted R²	-0.0831
		Adeq Precision	6.5709

Interactions between different parameters evaluated by RSM

Interaction between pH and oxidant concentration

Hydrogen peroxide (H₂O₂) addition is crucial to the photocatalytic breakdown process [60]. It accomplishes two things; Firstly, it increases the production of hydroxyl radicals (OH˙), strong oxidizing agents that cause color deterioration. Second, on the photocatalyst surface, H₂O₂ aids in reducing the recombination of electron-hole pairs (e⁻/h⁺). This recombination process lowers the photocatalyst’s overall efficiency. In figure 7 (a), interaction among oxidant concentration and pH are given in which maximum degradation occurred at 81.39% at pH (6) and oxidant concentration (30mM).

Interaction between dye concentration and pH.

The graphical interface between ph and dye concentration and in figure 7 (b) which depicts the optimum ranges of these two parameters at which Congo red dye degraded maximum. The Congo red dye gave optimum degradation (76.74%) at pH 6 and concentration of dye at 30ppm, beyond this level, there was no further degradation observed [61, 62]. The rate of dye degradation is highly sensitive to pH; dye degraded more rapidly when the pH value went from acidic to basic; on the other hand, if the pH of the wastewater sample was raised, decreased rate of dye degradation because the surface charge of the dye was negative at acidic pH and positive at basic ph. Maximum degradation of congo red dye was achieved in acidic media.

3. Interaction between dye concentration and oxidant concentration

The amount of dye concentration also impacts degradation [63-65]. As illustrated in Figure 7 (c), increasing dye concentration from 10 ppm to 30 ppm increases degradation since there are more dye molecules accessible for the catalyst to attack. However, adding significantly more dye (beyond 30 ppm) reduces efficiency. This is because extra dye molecules can clump together, inhibiting the catalyst's active sites and preventing light from reaching them, which lowers the formation of OH˙ radicals. So maximum degradation (78.79) was achieved at 30ppm dye concentration and 30Mm of oxidant concentration [66-68].

4. Interaction between time irradiation and pH

The relationship between time and and pH gave maximum degradation (78.76) at pH (6) and irradiation time (3hrs) by keeping other two factors constant as shown in figure 7 (b). The most important factor influencing the degradation rate, according to an analysis of the 3D surface and contour diagrams (fig. 7 (d)), was the irradiation period [69, 70]. The findings showed that when the irradiation time was increased from three to five hours, the deterioration efficiency gradually increased. An ideal irradiation time of three hours was suggested at 30ppm dye concentration, as further extending the irradiation time beyond five hours did not result in any further improvements in the degradation rate.

5. Interaction between time irradiation and oxidant concentration

Figure 7 (e) presents a graphical depiction of the change in oxidant concentration over time. The level of H₂O₂ concentration was also determined to be a factor that affects the rate at which degradation occurs. An increase in the concentration of H₂O₂ resulted in a sharp rate of degradation, possibly due to the self-dissociation or reduction of H₂O₂ facilitated by light at the conduction band of the photocatalyst. Nevertheless, beyond the ideal concentration of 30 mM H₂O₂ led to a negative impact on the effectiveness of degradation, irrespective of the duration of irradiation. This decrease can be explained by the consumption of reactive hydroxyl radicals and valence band gaps by an excessive amount of H₂O₂. In addition, the process of radical recombination, which is a competitive reaction, may potentially contribute to impeding the degradation process when larger quantities of H₂O₂ are present.

6. Interaction between time irradiation and dye concentration

The 3hrs of irradiation time with 30ppm concentration of dye investigated by keeping oxidant concentration and pH constant as given in fig 7 (f). The percentage degradation enhances with consumption of time. With the passage of time OH ions gives maximum number of active sites. However, the active sites on the photocatalyst surface were saturated, additional increases led to a drop in the efficiency of degradation. The catalyst absorbs light energy when exposed to light, which causes it to produce reactive oxygen species (ROS), especially hydroxyl radicals (OH˞), on its surface. The dye molecules in the solution are attacked and degraded by these extremely reactive ROS species.

Measurement of untreated and treated textile wastewater treatment:

The measurement carried out by UV spectroscopy in which the sample shows certain absorbance upon which a curve is formed which depicts degradation percentage of the sample. The peak of untreated sample shows that there is no degradation occur at this point. The sample remained unchanged. After treatment of the sample shows a lower curve then untreated sample which means that degradation occurred completely. The sample after treatment gives maximum results of percentage degradation. The maximum degradation occurred at maximum absorbance when we take UV spectra of the sample.

Measurement of water quality parameters (COD, BOD, TOC):

Digestion solution of 1.5mL, 3.5mL of catalytic solution and 2.5mL of sample solution was mixed and then placed the sample into the COD meter for 2hrs at 150 ͦC. Both before treatment and after treatment samples were placed in COD meter. The absorbance was taken from UV spectra of the samples. The mixture of 1.6mL of concentrated sulphuric acid and 2N of 1mL potassium dichromate solution mixture was added into 4mL of sample solution placed into TOC meter for 1.5hrs at 120 ͦC. the absorbance before and after treatment of the samples was evaluated by UV spectra. The biochemical oxygen demand was measured using calibrated digital BOD meter. The special BOD bottles washed with deionized water and 56mL deionized water added into first bottle as blank. Time out of the five bottles filled with 56mL of sample solution and added drops of standard KOH solution before capping them. Then they were placed in their specific position and placed for five days. UV spectra was taken for both before and after treatment of the samples,The decreased in BOD (78%), COD (94%), and TOC (89%) value was observed.

Measurement of photocatalytic activity with time

The sample was placed under sunlight for three hours and investigated its photocatalytic activity with varying time range by using UV spectrometry. The kinetic activity of sample shows the following curve with absorbance on y-axis and time on x-axis.

Measurement of reusability

The reusability of substrate (pumice) was checked. In first seven cycles, the efficiency of substrate remained constant and then decreases gradually up to fourteen cycle. This shows that pumice is efficiently work as substrate for the degradation of wastewater. The reusability factor also depends on sunlight i.e if light intensity poor then it will minimize the results vice versa.

Citric acid and urea were used as precursors in the syntheses of carbogenic material by hydrothermal method. XRD, SEM, TEM, UV-Vis, and zeta potential were applied to evaluation surface passivation on the structural properties of N-CDs. Photocatalytic activity of N-CDs dots was evaluated by UV spectroscopy and also optimization of different parameters including time irradiation, dye concentration, pH, and oxidant concentration with response surface methodology (RSM). The outcomes showed that, when exposed to visible light, C-dots can boost the photocatalytic reaction rate of N-CDs. Water quality parameters like COD, BOD, and TOC were tested by applying ultraviolet spectroscopy, which gave the reduction up to 94%, 78%, and 89%, respectively. The statistical analysis was moreover validated via response surface methodology (central composite design). The fabricated material on substrate (black pumice) was reusable up to fourteen cycles which showed degradation 50%.

Author Contribution

SJ & SP conceptualize the research and perform experimentSP wrote main manuscript MAZ and IAB prepared the figures and help in analysis of instrumental dataSJ & SP perform applications and arranged dataAll authors reviewed the manuscript

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No competing interests reported.

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Synthesis, Purification, and Photocatalytic Evaluation of Nanostructured N 2 dopped Carbogenic Particles

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Abstract

Figures

Introduction

Material and methodology

Results and Discussions

Conclusion

Declarations

Author Contribution

References

Additional Declarations

Supplementary Files

Status:

Version 1