A Sustainable Method for Achieving High Graphitization from Pomelo Peel

By Megan Craig, M.Sc.Mar 23 2022

Pomelo is one of the world’s most popular fruits., with China contributing to the world's largest supply volume of pomelo production.  As a result, the fruit has a significant economic influence in China.

Image Credit: Spayder pauk_79/Shutterstock.com

Pomelo peels make up roughly 45% of the overall weight of the fruit and are frequently discarded as agricultural trash. This trash has an environmental impact and pollutes the environment; thus, it must be carefully controlled. Pomelo peel is also a potential carbon precursor material since it contains cellulose, hemicellulose, and lignin.

As a result, several procedures and temperature regulations have been explored by researchers to convert pomelo peels into more desirable materials, such as highly ordered and conductive graphitized materials. The team published their research in the MDPI journal crystals.  

The arc discharge technique, laser vaporization, and plasma and thermal chemical vapor deposition are all examples of generic graphitization procedures. However, these approaches cannot be extensively employed due to several constraints leading to a research focus on simplifying the method.  

Different synthesis routes are used in this research to lower the temperature of graphitized carbon materials and transform waste pomelo peels into highly graphitized materials. The synthesis effect of conventional approaches and the new one proposed by the team were compared. Following the team's improvements, a high degree of graphitization effect could be accomplished at a reduced temperature.

Methodology

Two procedures were used to treat discarded pomelo peels (Figure 1). The pomelo peels were washed twice, once with tap water and again with deionized water. They were then dried for 24 hours at 80° C in an oven, and crushed to produce powders. Those less than 147 µm in size were chosen to produce a generally homogenous fine powder.

Figure 1. Schematic flow diagram of the traditional method and the improved method. Image Credit: Zeng, et al., 2022

The materials were analyzed using the Bruker D8 equipment for X-ray powder diffraction (XRD). The surface chemistry and chemical state of sample elements were investigated using X-ray photoelectron spectroscopy (XPS). Thermo Scientific was used to do the XPS.

To examine the degree of order of carbon structure, Raman spectra were collected in a Raman microscope (Via Reflex) with a laser excitation wavelength of 532 nm. The data acquisition duration was 10 seconds, and the scanning range was 500–3500 cm^-1. To confirm the correctness of the data, each sample was examined three times.

Peak deconvolution was used to analyze Raman spectra using Peakfit software. A spectrometer was used to analyze the changes in functional groups during graphitization using Fourier transform infrared (FTIR) spectroscopy.

Lattice fringes were observed using HRTEM. To ensure homogeneous dispersion, all samples for HRTEM analysis were distributed in ethanol and sonicated for 10 minutes. They were then deposited onto carbon-coated grids and dried at ambient temperature.

This was used to detect sample images and lattice fringes. On the FEI Talos F200S, SAED was employed to verify the degree of graphitization.

Results and Discussion

The XRD spectrum was used to determine the degree of graphitization of the samples. Figure 2a shows that the corresponding (002), (100), (101), (004), and (110) diffraction peaks have apparent strong peaks at ca. 26°, 43°, 54°, 78°, and 83°.

Figure 2. (a) XRD patterns of the graphitized pomelo peel; (b) Raman of the graphitized pomelo peel; (c) FWHM of G-band; Deconvolution of Raman spectra for (d) 750-1, (e) 800-1, (f) 900-1, (g) 750-2, (h) 800-2, (i) 900-2. Image Credit: Zeng, et al., 2022

The structural characteristics of graphitic carbon were estimated using the XRD spectrum (Figure 2a).

To compute the diffraction peak of (002), the Bragg equation was used to obtain the plane spacing d(002) as shown in Table 1.

Table 1. Structural parameters were analyzed from the curve of the XRD and Raman spectrum. Source: Zeng, et al., 2022

Figure 2b shows a Raman spectroscopy graph, which is used to investigate the degree of order in samples. The Raman spectra of samples in the range of 1100–1800 cm^-1 were created using a Gauss–Lorentzian curve fitting algorithm, as shown in Figure 2d–i. Figure 2c summarizes the output of the FWHM analysis of the peak-fitted G-band.

The FWHM of the G-band drops steadily as temperature rises, as illustrated in Figure 2c.

A higher degree of order is represented by a higher IG/ID value. The IG/ID values of the 750-1, 800-1, and 900-1 samples are 1.10, 1.17, and 1.19, respectively, as shown in Table 1, suggesting the degree of order of the carbon structure to grow progressively as temperature rises.

The functional groups of the material are reflected in FTIR spectral analysis (Figure 3a).

Figure 3. (a) FTIR of the graphitized peel; (b) N₂ adsorption–desorption isotherms for the graphitized peel; (c) Pore size distribution image of the graphitized peel.  Image Credit: Zeng, et al., 2022

The isotherms of the samples, as shown in Figure 3b, adhere to the typical integral of the IV isotherms. Figure 3c shows the pore size distribution, and the presence of micropores, mesopores, and macropores in the sample agrees with the data in Figure 3b.

750-2, 800-2, and 900-2 looked spheroidized after treatment with procedure two (Figure 4a–c). High-magnification characterization was used to examine the morphology of the pomelo peel after treatment with method two (Figure 4d–f).

The diameter of the nanospheres steadily reduces as the temperature rises, as seen in Figure 4g–i, with the diameters ranging from 750-2, 800-2, and 900-2 being 213.23 nm, 205.32 nm, and 176.8 nm, respectively.

Figure 4. (a,d) SEM images of 750-2; (b,e) SEM images of 800-2; (c,f) SEM images of 900-2; (g–i) Particle size distribution of 750-2, 800-2, 900-2. Image Credit: Zeng, et al., 2022

The sp² to sp³ ratios of samples 750-1, 800-1, and 900-1 are 0.99, 1.06, and 1.09, respectively, as illustrated in Figure 5a–c.

The sp² to sp³ ratios of samples 750-2, 800-2, and 900-2 were 1.17, 1.39, and 1.51, respectively, as shown in Figures 5d and e. It can be seen that the degree of graphitization is closely proportional to temperature, regardless of whether procedure one or method two is utilized.

Figure 5. (a) XPS spectrum of C1s for 750-1; (b) XPS spectrum of C1s for 800-1; (c) C1s spectrum for 900-1; (d) C1s for 750-2; (e) XPS spectrum of C1s for 800-2; (f) XPS spectrum of C1s for 900-2. Image Credit: Zeng, et al., 2022

To learn more about method two, TEM combined with selected area electron diffraction (SAED) used to examine the structure and graphitization of 750-2, 800-2, and 900-2 samples (Figure 6). Figure 6a–c shows the SAED diffraction rings in great detail.

HRTEM images of 750-2, 800-2, and 900-2 are shown in Figure 6d, e. The lattice fringes grow more visible as the temperature rises, as can be seen in the graph.

Figure 6. (a–c) SAED pattern of 750-2, 800-2, 900-2; (d–f) HRTEM images of 750-2, 800-2, 900-2. Image Credit: Zeng, et al., 2022

When the results of XRD, XPS, Raman, TEM, and SAED are combined, it is clear that graphitic carbon is formed by Ni catalysis.

Conclusion

Researchers refined the procedure for producing low-temperature, highly graphitized pomelo peel in this study, turning trash into a valuable material. 

This study compares and contrasts improved and classic graphitization approaches. The degree of graphitization achieved with the new approach is 80.23% at 900 °C, which is somewhat lower than commercial graphite (90.23%) but significantly greater than the sample at 900 °C (48.8%) with the previous method.

At 900 °C, pomelo peels can be turned into highly graphitized nanospheres using the enhanced approach. The temperature plays a crucial role in the catalysis of pomelo peel by Ni, regardless of whether the classic approach or the enhanced method is used.

This research has the potential to transform pomelo peels into useful commodities, minimize pollution, and, most importantly, reduce power consumption and support sustainable development.

Journal Reference:

Zeng, L., Wang, Y., Guo, Y., Dai, X., Chen, L., He, C., Nhung, N. T. H., Wei, Y., Dodbiba, G., Fujita, T. (2022) Improved Method for Preparing Nanospheres from Pomelo Peel to Achieve High Graphitization at a Low Temperature. Crystals, 12(3), p. 403. Available Online: https://www.mdpi.com/2073-4352/12/3/403/htm.

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