Green Synthesis of Nano Graphite Materials from Lemon and Orange Peel: A Sustainable Approach for Carbon-Based Materials ^†

Abstract

This research paper presents a study on the preparation and characterization of nano graphite materials (NGMs) from lemon and orange peel powder using the pyrolysis method. The NGMs were analyzed using X-ray Diffractometer (XRD) and Fourier transform infrared spectroscopy (FTIR) for structural and compositional properties. The XRD analysis confirmed the crystalline nature of the NGMs, while the FTIR analysis provided information about the functional groups present in the materials. Our results suggest that NGMs fabricated from lemon and orange peel powder have potential for applications in various fields, including energy storage and catalysis.

1. Introduction

Nano graphite materials (NGM) have received a lot of interest lately because of their distinctive qualities, including chemical stability, high water solubility, affordability, and fluorescent traits (Wang, (2014)). It is possible to roughly divide the synthesis of NGM into “top-down” and “bottom-up” methods [1,2]. By dissolving bigger carbon materials using techniques including arc discharge, laser ablation, and chemical oxidation, NGMs are created in the top-down process [3,4,5]. On the other hand, the bottom-up strategy uses processes including hydrothermal, thermal-decomposition, and microwave approaches to create NGMs from molecular carbon precursors [5,6,7,8,9,10,11]. Many of these synthesis procedures, however, involve numerous steps and are laborious; they frequently need pricey carbon sources and follow-up surface passivation [12,13,14,15].

Numerous green synthesis methods have been studied to address these issues. These methods make use of low-cost and organic carbon sources such chitosan, egg yolk oil, juice from oranges, lemon peel, bee pollen, collagen, humic substances, the hair, nut shells, milk from soybeans, cashew gum, and garlic [6,13,15,16,17,18,19,20,21]. The seasonal variation and foreign contaminants present in natural carbon sources, which might influence the repeatability, shape, and size distribution of NGMs [12], provide difficulties with their utilization. It is still very difficult to achieve uniformity and essential purity in NGMs made from natural carbon sources [6,13,15,16,17,18,19,20]. Additionally, conventional analytical techniques that concentrate on pH-dependent photoluminescence (PL) spectra of NGMs sometimes miss important data from various excitation wavelengths and pH ranges [6,8,9,13,16,17,18,19,20,22].

As a natural carbon source, lemon and orange peels were used in this work to create nano graphite materials (NGMs) by the pyrolysis method. This method makes it possible to create NGMs with a limited size distribution, a feat that has never been accomplished utilizing the pyrolysis of natural carbon sources [7,23]. We investigate how reaction time and temperature affect the pyrolysis procedure, offering information on how to create NGMs with useful features. Notably, the created NGMs show outstanding colloidal solubility, photo-stability, and environmental stability without the need for further surface passivation procedures to improve their fluorescence capabilities [24].

The effective synthesis of NGMs from lemon and orange peels offers up new opportunities for their use in a variety of industries, including sensors, solar cells, supercapacitors, LED technology, printing, and bio-sensing [13,25,26,27,28,29]. The objective of this study is to enhance our understanding of the potential applications of nano graphite materials (NGMs) as humidity sensors by utilizing a sustainable and cost-effective carbon source and applying machine learning analysis techniques. Humidity sensors play a crucial role in various fields, including environmental monitoring, agriculture, and industrial processes. However, the development of high-performance humidity sensors with improved sensitivity, stability, and cost-effectiveness remains a challenge.

In this research, NGMs were synthesized from lemon and orange peel powder using the pyrolysis method. The use of these natural carbon sources provides a sustainable and affordable alternative for the fabrication of NGMs. The synthesis process involved grading, sorting, and washing the peel, followed by crushing/grinding and pyrolysis at 500 °C for 3 h. After synthesis, the NGMs were subjected to sonication, centrifugation, and drying processes to obtain the final material.

To evaluate the suitability of NGMs as humidity sensors, their properties were characterized using various techniques. X-ray diffraction analysis (XRD) was used for phase identification and characterization of crystalline materials based on their diffraction patterns. Fourier transform infrared spectroscopy (FTIR) provided insights into the functional groups present in the NGM experimental methods.

The following steps are for the synthesis of NGMs by orange and lemon peel.

2. Synthesis of NGMs

Nano graphite materials were synthesized by the pyrolysis method. Grading, sorting, and washing lemon and orange peel was carried out to obtain fresh lemon and orange peels, which were then carefully sorted to remove any damaged or discolored parts. The peels were then thoroughly washed with deionized water to remove any surface impurities. The washed lemon and orange peels were dried to remove excess moisture, and then they were crushed or ground into fine particles. This step aimed to increase the surface area of the peels for better pyrolysis efficiency. The crushed or ground peels were subjected to pyrolysis in a controlled environment at a temperature of 500 °C for a duration of 3 h. Pyrolysis involves the decomposition of organic materials in the absence of oxygen, resulting in the formation of carbon-based materials such as NGMs. After pyrolysis, the resulting NGMs were subjected to sonication for 10 min. Sonication involves the use of ultrasonic waves to disperse and break down any aggregated particles, ensuring a more uniform dispersion of NGMs. To separate any remaining impurities or larger particles from the NGMs, centrifugation was performed at a speed of 800 rpm for 10 min. Centrifugation causes the NGMs to settle at the bottom, while impurities and larger particles are separated and collected in the supernatant. The purified NGMs were then dried to remove any residual moisture. This step aimed to obtain dry and stable NGMs suitable for further characterization and application.

2.1. Characterization of NGM

2.1.1. X-ray Diffraction Analysis (XRD)

XRD is a quick analytical method for characterizing crystalline materials based on their diffraction patterns and identifying their phases. Constructive interference is created when X-rays make contact with a crystallite NGM sample, and these diffracted X-rays are then detected, processed, and tallied, exposing the outcomes that are depicted in Figure 1. The XRD spectra for NGMs made from powdered lemon peel are displayed in Figure 1. For NGMs, one sharp peak can be observed at 2θ = 22.8°. With the use of the Scherrer Formula, this peak indicated the presence of a well-ordered layer structure with a 0.365754 nm d-spacing (FWHM = 0.825°) and determined the size of NGM made from lemon peel powder to be 9.818 nm. An as-synthesized NGM sample’s purity may also be determined using XRD. The inadequate carbonization of lemon peel powder resulted in many impurity peaks in the XRD pattern of NGM, as shown in Figure 1 above. Although the XRD is a useful tool for characterizing NGM with a crystallite structure, it cannot be used to determine the essential characteristics of amorphous NGM.

The XRD spectra for carbon nano dots made from the powdered orange peel are displayed in Figure 2. At 2θ = 23°, one distinct peak for NGMs can be seen. This peak demonstrated a well-ordered layer structure with a 0.395199 nm d-spacing (FWHM = 0.689°), and the Scherrer formula was used to determine the size of the NGM made from orange peel powder, which was determined to be 11.760 nm. An as-synthesized NGM sample’s purity may also be determined using XRD. Figure 2 shows that a number of impurity peaks were seen in the XRD pattern of NGMs as a result of the orange peel powder’s inadequate carbonization. The reaction temperature and reaction time are the causes of the as-synthesized NGM’s size fluctuation.

2.1.2. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectra were used to corroborate the surface functional groups and chemical makeup of the as-synthesized NGMs from lemon peel powder. According to Figure 3, the NGMs FTIR spectra include prominent peaks at 3250, 2350, 1620, 1380, and 1110 cm⁻¹, which correspond to O-H, C-H, C=O, C=C, and (C-O or C-H) accordingly. While the O-H, C=O, and C-H bending peaks imply that the NGMs contain O-H, C=O, and C-H surface moieties linked to their surfaces, the existence of the C=C peak suggests that the NGMs are formed of graphitic structure. The surface of the as-synthesized NGMs comprises inherent negatively charged moieties, which are necessary for long-term water solubility, according to FTIR analysis. According to Figure 4, the NGMs FTIR spectra include substantial peaks at 3200, 2380, 1595, 1395, and 1105 cm⁻¹, which correspond to O-H, C-H, C=O, C=C, and (C-O or C-H) accordingly. While the O-H, C=O, and C-H bending peaks imply that the NGMs contain O-H, C=O, and C-H surface moieties linked to their surfaces, the existence of the C=C peak indicates that the NGMs are formed of graphitic structure. The surface of as-synthesized NGMs comprises inherent negatively charged moieties, which are necessary for long-term water solubility, according to FTIR research.

3. Results and Analysis

In this section, we will discuss the results obtained from the X-ray diffraction (XRD) analysis and Fourier-transform infrared spectroscopy (FTIR) experiments. The detailed XRD patterns of the nano-materials (NGMs) can be found in Appendix A, which reveal critical insights into their crystalline nature, crystal structure, and crystallite size. Similarly, the FTIR spectra, along with the identified functional groups, are also presented in Appendix A, offering valuable information about the NGMs’ chemical composition and potential applications.

4. Conclusions

In conclusion, the utilization of waste and by-products in the synthesis of nanomaterials (NGMs) represents a significant advancement, driven by the diverse range of applications associated with these materials. Notably, the composition of the raw materials obtained from waste sources, such as lemon (9.818 nm) and orange (11.760 nm) peel powders, results in varying quantum yields, highlighting the influence of their heterogeneous nature (see

Table 1. Size of NGMs fabricated from fruit peel powder.

Fruit	Size of (NGM) (nm)
Lemon	9.818
Orange	11.760

). Furthermore, the adoption of different reaction conditions and synthesis methods introduces further complexities regarding the chosen route. Even when starting from identical raw materials and employing the same synthesis method, the variation in synthesis parameters leads to distinct effects on the characteristics and properties of the resulting NGMs. This underscores the importance of understanding and optimizing the synthesis process for the desired application of these environmentally friendly nanomaterials.