Elsevier

Optik

Volume 242, September 2021, 167311
Optik

Green fabrication of zinc oxide supported carbon dots for visible light-responsive photocatalytic decolourization of Malachite Green dye: Optimization and kinetic studies

https://doi.org/10.1016/j.ijleo.2021.167311Get rights and content

Abstract

A succession of zinc oxide supported carbon dots (CZnO-dots) was fabricated from gum ghatti via a single-pot and straightforward microwave process. The as-fabricated CZnO-dots were employed as the visible-light responsive photocatalyst for the effectual decolourization of Malachite Green dye (MG). The optical band gap was evaluated using a tauc plot in which integration of ZnO to Carbon dots (C-dots) reduces the bandgap compared to bare C-dots and pure ZnO nanoparticles. The increased photocatalytic efficacy was observed in 1 wt% ZnO of CZnO-dots amongst all samples. By the influence of different operating conditions, the CZnO-dots show a degradation efficiency of about 94.8% with an irradiation time of 60 min. Besides capturing the active species in photodegraded products, the possible mechanism behind the photocatalytic degradation of MG dye has been discussed. Notably, the environmental assessment has been measured to develop mung bean plants using treated dye water. The growth of mung bean plants in the treated dye water was almost the same as that found in the control group, while the retarded growth was seen in the MG dye solution. Thus, the as-prepared CZnO-dots can be effectively used for the practical application related to environmental remediation.

Graphical Abstract

The work presented holds the potential of using visible light-responsive photocatalyst CZnO-dots in hands-on applicability related to environmental remediation.

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Introduction

The growth of the social structure and the broad area of industrial production worsens urban water contamination [1]. The indiscriminate dumping of industrial effluents has prompted water contamination in many nations. The carcinogenic essence of the wastes comprising dyes originating from garment mills and food factories induce surface and groundwater pollution that presents possible health threats to livestock, plant life, and humans [2]. Malachite Green dye (MG) is a natural triphenylmethane dye mainly employed in silk, wool, leather, and fish farming industries [3], [4]. However, several studies asserted that MG dye and its metabolite green leucomalachite (LMG) are potentially obstinate owing to their complicated chemical compositions [5]. MG dye inflows more from municipal sewage treatment plants into the environment and triggers genetic damage [6]. It is also prohibited or limited in several nations. However, owing to its high effectiveness and low cost, MG is still illegal in aquaculture [7]. It is crucial to remove MG residue from water systems efficiently. Several conventional MG dye removal methods from wastewater have been implemented in recent decades [8], [9], [10], but such approaches are expensive, laborious, consume high energy, and show poor performance. Likewise, with semiconductor-based photocatalysts, photocatalytic degradation of dyes has induced global interest [11].

Zinc oxide nanoparticles (ZnO NPs) are a potential photocatalyst because they are firmly catalyst, cost-effective, and environmentally friendly [12]. The ZnO NPs nanocatalyst, fast recombination, and wide bandgap (3.37 eV) restrict its applicableness to an extended wavelength region [13]. Visible light active photocatalysts are desirable for the decomposition of harmful chemical pollutants [13], [14]. Sufficient modifications are needed to create new ZnO NPs nanostructures to engulf these issues. Carbon dots (C-dots), a zero-dimensional material, have been spotlighted in recent years due to their low cost, non-toxic, photostability, and photoinduced electron transfer behavior [15], [16], [17], which is used in versatile applications [18], [19], [20]. C-dots turn as an electron pool and improve the separation efficiency of photogenerated charge carriers and enhance photocatalytic performance [21]. ZnO NPs coverage on the C-dots could also become a perfect tactic to enhance the charging isolation and hinder photo corrosion [22].

Sharma et al. [23] used chemical precursor ascorbic acid as the carbon source to design N doped ZnO/C-dots nanocatalyst to degrade MG dye aqueous medium. Likewise, Muthulingam et al. [22] fabricated CQD decorated N-doped ZnO via the chemical method to degrade MG with three different light sources (UV, visible, and daylight) and the enhanced performance was found in daylight source. Bozetine et al. [24] utilized fructose and NaOH as precursors to produce ZnO-carbon dots nanocomposites that degrade rhodamine B under visible light irradiation at room temperature. Uthirakumar and co-workers [25] used a wet chemical process (with a minor modification of Hummer's method) to make CQD decorated N-doped ZnO to degrade MG dye with three distinct light sources. Similarly, Several C-dot-based nanocatalysts (like C-dots/Ag/Ag3PO4, C-dots/Fe2O3, C-dots/CuO) are reported [13], [26], which significantly suppresses the e-/h+ recombination and expands the spectral response to longer wavelengths.

As mentioned above, earlier literature reports focused on the chemical carbon sources to synthesize metal/metal oxide-C-dots as nanocatalyst for various dye degradation. However, it is hard to find info on green synthesized ZnO/C-dots nanocatalyst scheme degrading MG dyes, particularly under visible light irradiation, leading to sustainable chemistry. Herein, we report a simple green fabrication of CZnO-dots using Gum Ghatti, a natural exudate from tree Anogeissus latifolia as a renewable carbon source through microwave pyrolysis approach. As-prepared CZnO-dots, displaying enhanced photocatalytic behavior of MG dye in the aqueous phase under visible light. A detailed analysis of the different operating parameters of CZnO-dots photocatalytic activity is investigated, such as ZnO addition %, catalyst loading, initial dye concentration, and the dye solution's pH. The percentage of ZnO addition improves the active sites on the surface photocatalysts, and the outcomes are compared with those of pure ZnO NPs and bare C-dots. Finally, the possible mechanism of visible-light-responsive CZnO-dots photodegradation is also proposed based on radical scavenging studies.

Section snippets

Materials

Gum ghatti powder was purchased from the State Forest Research Institute, Madhya Pradesh, India. Zinc acetate dihydrate (Zn (CH3COO)2.2H2O), sodium chloride (NaCl), and potassium dichromate (K2Cr2O7) were purchased from Merck, India. Malachite green and ethanol were purchased from LOBA Chemie, India. Double distilled water (DDW) was used for the experiments.

Characterization

Powder X-ray diffraction patterns of CZnO-dots nanocatalyst were procured from 2θ values (20–80°) via BRUKER USA D8 Advance, Davinci, X-ray

Structural properties

Fig. 1a shows the Gum Ghatti derived bare C-dots XRD pattern with the typical broad 2θ peak at 18.1° recognized for the (002) plane, which agrees with graphitic carbon (C-dots). The C-dots d- spacing value was 0.48 nm which was calculated using Bragg law (d = nλ /2 sinθ where, n = 1 positive integer, λ = 1.54 Å wavelength of incident X-ray and θ peak position (002)). The d spacing value is higher on comparing with the "bulk graphite" (0.34–0.35 nm) due to the presence of oxygen-containing

Analysis of CZnO-dots nanocatalyst efficiency towards MG dye decolourization

CZnO-dots are proficiently verified as photocatalyst for decolourization of MG dye under visible light. The outcome of various parameters such as ZnO addition %, CZnO-dots nanocatalyst loading, initial MG dye concentration, and pH range was evaluated using CZnO-dots under visible light.

Conclusion

In a nutshell, the CZnO-dots photocatalysts have been well fabricated and examined to support the occurrence of ZnO and C-dots portions. The MG dye decolourization by CZnO-dots photocatalysts was scrutinized methodically via various parameters like catalyst dosage, dye concentration, and different pH. The CZnO-dots photocatalytic active conditions in the experiment were found to be: 1.5 mg catalyst degraded 94.8% of 0.5 g/L MG dye at pH 8 in 60 min under visible light. The scavenging studies

Funding

No funds, grants, or other support was received.

Descriptions

XRD pattern of CZ-2 sample, FTIR of bare C-dots, FESEM images of C-dots, ZnO NPs, and CZ-2 samples; UV-Vis absorption and PL results; the plot of ln (C0/C) versus time linear fit data; rate constant bar chart; tables of atomic % composition and Degradation rate constants of MG dye.

Declaration of Competing Interest

The authors declare that they have no conflict of interest in the publication.

Acknowledgments

Author Anithadevi acknowledges the Principal Dr P. Wilson and Head of Chemistry Department Dr E. Iyyappan for their encouragement. We honor Madras Christian College, Chennai, Tamil Nadu, for providing the facilities.

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