3.1. XRD
XRD analysis was carried out to compare the phases of the graphene crystals synthesized from candlenut to all-variation of percentage Ce doped graphene shown in
Figure 1. The presence of nanoscale graphene layers on the interlayer is indicated by the broad and weak peaks detected in the graphene. To calculate the interlayer spacing of graphene, the diffraction peaks must adhere to the Bragg equation, as specified in Equation (1).
The lattice spacing, designated as d, the angle θ created between the inverse ray and the reflecting crystal plane, the wavelength represented by λ, and the reflection series labelled as n all have significant importance. The reduced interlayer distance of the graphene sheets can be ascribed to the heightened disarray and impairment inside the crystal lattice during the process of manufacturing graphene. The XRD diffraction pattern of graphene doped with varying weight percentages of Ce are shown in
Figure 1.
As seen in
Figure 1, the XRD diffractogram graphene results showed peaks at 2θ = 24.7°, due to weak graphene layers which correspond to planes (002) which is close to that of graphite at 2θ = 26° due to the interlayer spacing of 0.334 nm confirms the formation of GNSc. After Ce doped graphene, there is a change in the crystalline phase which is marked by the appearance of another diffractogram on the graph. Although the peaks were found to be shifted left by 1°. JCPDS 00-023-1048 Ce
2O
3.
There are three prominent peaks were identified at 2θ = 25.19, 29.33, and 31.94° (
Figure 1), which corresponds to planes (002), (111) and (200) respectively. This suggests the presence of Ce deposited in graphene. The peak at (111) is relatively more intense as compared to the other peaks and is from the cubic face (FCC) (111) lattice plane of CeO
2 [
21,
22]. Interestingly emergence of new peaks on Ce/graphene indicates the interaction between cerium (Ce) and graphene indeed holds the potential to result in the formation of new crystalline phases.
This interaction implies an intricate many-body phenomenon that has the potential to result in unique crystal structures. Hwang’s research indicates that the phenomenon can be explained by the Kondo effect observed when Ce is intercalated, resulting in localized states near the Fermi energy that hybridize with the graphene π-band [
21,
22]. This hybridization leads to structural changes and the emergence of new peaks.
However, when graphene is doped with Ce, the interaction between Ce and graphene influences several factors, including the appearance of unindexed peaks due to new phases, structural modifications, defects, and impurities. Meanwhile percentage ratio 10%w Ce has a slightly different diffractogram from 20% Ce and 30% C, there is still the influence of graphene due to less Ce concentration so that the weak graphene peak still appears at 10% Ce/Graphene, in this stage 10% Ce doping, there might be a phase transition or the formation of a new crystalline phase at the transition. This could lead to unique diffraction peaks.
3.2. SEM–EDX
Moreover, the SEM EDX data may assist us to make clearer the undoped and doped effect.
The Candlenut graphene has thin and flat surfaces (
Figure 2a) and C element is major element containing of graphene (
Figure 2b–e). The element containing graphene may be seen in
Table 1 (EDX data).
The SEM EDX data proves that graphene is well formed from candlenut as a raw material with remnant product around 19–21%. The morphological structure of graphene is totally different compared to graphite, graphite oxide, graphene oxide and reduced graphene oxide [
23,
24,
25]. Interestingly, the C element containing graphene is more than 90 wt.% and it has flat and thin surfaces respect to large surface area. In addition, TEM was also used to characterize graphene to prove that graphene may be produced from candlenut as a raw material (
Figure 3).
TEM image (
Figure 3) shows the Candlenut graphene has flat, thin, large surface area and d-spacing lattice is 0.33 nm, meaning produced Candlenut graphene is similar to a 2D planar structure of sp
2 hybridized carbon atoms arranged in a honeycomb crystal lattice with an inter-layer spacing [
26].
Subsequently, the property of Candlenut graphene was evaluated with Ce deposited on it. SEM EDX was used to analyze them (
Figure 4).
Figure 4a,c,k clearly show that C and Ce elements exist on 10 wt.% Ce/Graphene, meaning the Ce is well deposited on graphene. The elements composition of 10 wt% Ce/Graphene may be seen in
Table 2.
More depositing performance of graphene as well as a supporting material, evaluating by depositing variation of Ce on graphene (20 and 30 wt.% Ce/Graphene) are shown (
Figure 5I,II).
Figure 5I,II show that Ce particles exist on graphene for both 20 and 30 wt.% Ce/Graphene. The sub-figures show the individual element mapping distribution. These data are very important because (i) graphene base on candlenut shell may be developed as a supporting material; (ii) large scale and sustainable graphene production is possible produced by using simple method and renewable material as a starting material and (iii) there is chemically interaction between Ce and graphene. The elements composition of 20 and 30 wt.% Ce/Graphene can be seen in
Table 3.
Based on SEM–EDX data (
Figure 4 and
Figure 5 and
Table 2 and
Table 3) clearly show the Ce atoms are well deposited on graphene and the preparation method of Ce/Graphene may be recognized. In addition, the GNS role is very important to control distribution and size of Ce particles meanwhile there are other metals such as Ca, K, and Si. These metals are contained in biomass as trace metals however the percentage is very low under 0.05% which may be due to pyrolysis that makes trace metal trapped in compounds.
3.5. XPS
To further study the bonds present in the graphene and 10–30 wt% Ce/Graphene, XPS measurements were conducted. The XPS spectra of graphene and 10–30 wt% Ce/Graphene may be seen in
Figure 8.
Figure 8a shows the XPS graph for C1s, 3 peaks can be observed at 284.8, 286 and 288–290 eV, which corresponds to C-sp
2, C-O and C=O bonds respectively. C=O and sp
2 bonding are typically found in C1s spectrum of Candlenut graphene [
28,
29]. The presence peak at 284.8 eV is solid evidence to prove the formation of graphene. Note, the XPS spectrum of C1s graphite is 284.5 eV. However, graphene still has oxygen functional group, causing its peak shifts to higher binding energy compared to graphite. That is possible due to the formation of graphene using candlenut shell as a starting material. The pyrolyzed candlenut shell normally produces oxygen functional groups embedded on charcoal. It is then reduced by using non-chemical reductor namely activated carbon, the physically interaction between charcoal and activated carbon may be expected both reducing oxygen functional groups and producing carbon containing graphene like Graphene Nano Sheets (GNS) [
30]. Furthermore, the performance of graphene as a supporting material is evaluated with respect to the Ce concentration (10–30 wt% Ce/Graphene) by using XPS. This XPS data is very important to know the chemical interaction between graphene (π-bond) and Ce (f-block metal).
Figure 8b shows there are two broad and relatively weak peaks at XPS graph for Ce 3d
5/2 and Ce 3d
3/2 located at 879 and 901 eV, which coincides with the Ce (III) oxide binding energy of 875 to 920 eV. The separation of spin-orbit components of Ce 3d
3/2 and 3d
5/2 (∆ = 18.6 eV) confirms the presence of Ce dopants within graphene matrix [
31,
32]. The observation from Cd 3d spectrum is consistent with the O 1s spectrum. There are peaks on binding energy 531.5 and 529 eV (
Figure 8c) which correspond to O 1s and metal oxides. Thus, this confirms the presence of Ce oxide with Ce presence in Ce
3+ oxidation state. The reducing state of Ce (IV) to be Ce (III) may possibly occur be assisted by graphene. Interestingly, the presence of graphene and varies of weight amount of Ce on Ce/Graphene may affect the binding energy position, meaning the catalytic activity of Ce metal can be modified by graphene and weight amount of Ce deposited on Candlenut graphene. The role of Candlenut graphene is very pivotal, mainly acting as a supporting material. The large surface area of graphene may be expected to distribute and deposit much more amount of Ce particles on it. The other role of Candlenut graphene is controlling the particle size and catalytic activity of Ce via chemical interaction between graphene and Ce [
33,
34]. These findings are very important to provide and develop the high catalytic activity of Ce catalyst for industry and other applications [
35,
36].
3.6. Raman
Raman spectroscopy was done on commercial and candlenut graphene to analyze the presence of defects and graphitization of carbon-based materials. As seen in
Figure 9.
The peak at 1330 cm
−1 of both materials were matched to the D band which indicates the presence of defects in the crystal lattice which causes the broken symmetry of the hexagonal carbon structure [
37,
38]. The peak at around 1600 cm
−1 is matched to the G band which indicates sp
2 hybrid configurations of carbon atoms [
39]. Peak around 2697 cm
−1 is attributed to the 2D band which is used to distinguish the number of layers for carbon structure. A sharp peak was observed for Commercial Graphene which is expected as it represents a monolayer structure which is typical of graphene. The 2D peak for the Candlenut graphene is broad which indicates the presence of multilayered carbon structure [
40].
The ratio of the intensity of D band over G band (ID/IG) is used to determine the degree of graphitization of the carbon material. The lower the ID/IG value, the higher the degree of graphitization. As can be seen in
Figure 9, the ID/IG (0.98) of the Candlenut graphene is as low as commercial Graphene (0.99) which suggests a very high degree of graphitization and the number of defects. In addition, the Candlenut graphene has lower ID/IG than other biochar that was previously reported [
37,
39]. A material with higher degree of graphitization has been reported to have higher electrical conductivity [
41] and greater electrochemical performance by accelerating ion diffusion [
42].
3.7. Electrochemistry Tests
The catalytic activities of Candlenut graphene and 10–30 wt% Ce/Graphene are necessary to observe. The electrochemistry tests those are CV and LSV may be carried out to answer their catalytic activities.
The CV graphs obtained for Candlenut graphene and Ce/Graphene are shown in
Figure 10. It should be noted the commercial graphene and Pt are used as well as references on both CV and LSV measurements.
Figure 10a obviously shows that the current value of commercial graphene is close to Candlenut graphene, however the Pt current value is the highest among others. Interestingly, the current values of 10 and 20 wt% Ce/Graphene are totally different compared to 30 wt% Ce/Graphene (
Figure 10b). That is possibly caused the Candlenut graphene as a supporting material and the particle sizes Ce effect. These findings are very important due to the small amount of Ce on Candlenut graphene (10 wt% Ce/Graphene) has higher current compared to 30 wt% Ce/Graphene, meaning the small amount Ce attached on Candlenut graphene may be expected to improve the catalytic activity of Ce. Therefore, the usage Ce amount may be reduced, and it tends to economically cost. The other advantage of this finding is the controlling size, or other properties of Ce may be done when it attaches on Candlenut graphene. Note, 0% (
Figure 10a,b) represent to Candlenut graphene.
In addition, the maximum current and area enclosed in the CV graphs may be seen in
Table 4.
Based on
Table 4, the Candlenut graphene has lower maximum current density of 1016 A cm
−1 and smaller enclosed area of 0.1096 as compared to commercial graphene which has a maximum current density of 1224 A cm
−1 and enclosed area of 0.1183, meaning the higher maximum current tends to give higher electrode surface area. However, upon the addition of Ce on Candlenut graphene may decrease its maximum current density and area enclosed numbers. That is caused the Ce state is still Ce oxide. The existence and type of oxygen functional groups on Ce for CeO
2 can affect the current density number of CeO
2 [
43,
44]. For Ce doped Candlenut graphene, 10 wt% Ce shows the best results with a maximum current density of 1081 A cm
−1 and an enclosed area of 1.117, followed by 20 and 30 wt% Ce/Graphene. This shows that the optimal amounts of dopants to add is 10 wt% for Ce/Graphene. It means the 10 wt% Ce/Graphene has a higher concentration of electroactive species thus faster rate higher electron transfer. However, the Pt working electrode produces the highest maximum current density and area enclosed, however, as mentioned in the literature review, Pt can be quite costly, thus the doped graphene samples made from agricultural waste can be considered as a cheaper alternative. Thus, it can be concluded that graphene doped with 10 wt% Ce yielded the best results when looking at the maximum current and enclosed area of the CV curve. The maximum current, or peak current (I
p) can be used to examine the electrochemical properties. The maximum current density is affected by the concentration of electroactive species in the solution, thus higher maximum current density indicates higher concentration of electroactive species. Higher maximum current density also indicates that a faster rate of electron transfer, and the redox reaction is more favorable. The enclosed area in CV curve, or integrated charge, is proportional to the number of electrons transferred during the redox reaction as well as the electroactive species in the solution [
45].
Moreover, analyzing to depict the relationship between overpotential of the anode/cathode and the current density is needed. The LSV analysis and Tafel slope may be expected to address it (
Figure 11 and
Table 5).
Tafel slope is used to describe the link between overpotential of the anode/cathode and the current density. The anodic Tafel slope describes the rate of oxidation reaction, where the electrode becomes more positive after losing electrons, thus it illustrates the rate of electron transfer from electrode to the oxidizing species in electrolyte. The cathodic Tafel slope describes the reduction reaction where the electrode becomes more negative after gaining electrons, thus, it illustrates the rate of electron transfer from reducing species in the electrolyte to the electrode. Therefore, catalysts which have smaller Tafel slope value tend to hasten electrocatalytic reaction and lower overpotential [
46,
47,
48,
49].
The Tafel plot may be seen in
Table 5.
From
Figure 11 and
Table 5, the 10 wt% Ce/Graphene has the best results among 20, 30 wt% Ce/Graphene and Candlenut graphene base on Tafel slope data. That is caused it has lower overpotential and faster electrocatalytic reaction. Briefly, the anodic and cathodic slope numbers of 10, 20, 30 wt% and Candlenut graphene are 0.3091, 0.4167, 0.6111, 0.7564 and −0.2071, −0.2854, −0.3330, −0.3993, respectively. Thus, we can see that 10 wt% Ce/Graphene has the best results. These performances are consistent with the CV results where the 10 wt% Ce/Graphene demonstrates the best electrocatalytic activity compared to 20, 30 wt% Ce/Graphene and Candlenut graphene. Thereby, interaction between Ce and Candlenut graphene may affect and modify their electrocatalytic activity properties. The other important property is corrosion resistance property. The best corrosion resistance results (I
corr and E
corr) are found at 10 wt% Ce/Graphene (−3.3787 μA cm
−2 and 1.8387 V), followed 20, 30 wt% Ce/Graphene and Candlenut graphene (
Table 5). Interestingly, comparing Ce/Graphene and Pt samples based on corrosion resistance, Ce gives better results compared to Pt. A similar result occurs for Candlenut graphene compares Commercial graphene. Therefore, we can conclude that 10 wt% Ce/Graphene has the best corrosion resistance among others thus it indicates more stable electrode potential [
50,
51,
52,
53,
54].
Concerning the electrochemistry tests, Candlenut graphene and 10 wt% Ce catalyst achieve better electrochemical measurement results compared to the other weight percentage doped. The 10 wt% Ce/Graphene produces a higher maximum current density and Ecorr which indicates that it has faster electron transfer and more stable electrode potential. In addition, 10 wt% Ce/Graphene has larger area enclosed within the CV graphs, which indicates higher concentration of electroactive species, gentler anodic and cathodic slope, which indicates lower overpotential and higher electrocatalytic reaction, and higher corrosion resistance, resulting in stronger interactions between Ce and carbon atoms in Candlenut graphene.
3.8. AC Electrical Resistance
To investigate the conductivity of Candlenut graphene and 10–30 wt% Ce/Graphene, we carried out the electrical resistance measurements. The AC electrical resistance measurements are recorded in
Table 6.
At a loading level of 0.24 wt.% (
Table 6), the composites containing Candlenut graphene and Candlenut graphene powder additives exhibit reduced values of AC electrical resistance compared to the Commercial graphene obtained from Sigma Aldrich. This reduction could be attributed to the presence of functional groups on the surface of the graphene structure, which facilitate better compatibility between the conductive additive and the epoxy resin matrix, improving its dispersion. Furthermore, the lack of significant improvement in electrical resistance between loading levels at 0.24 and 2.4 wt.% suggests that electrical percolation had already been achieved at the lower loading level.
The effect of doping the biocarbon (Candlenut graphene) with various cations to improve electrical conductivity is also studied. Nitrogen (N) and transition metal cations dopants such as iron (Fe), zinc (Zn), and nickel (Ni) showed reductions in AC resistance, with nitrogen-doped graphene demonstrating the most significant improvement in electrical conductivity. However, metal cations belonging to the lanthanide series of the periodic table, such as cerium (Ce) do not show significant improvement. This suggests that the electronic structure of the cationic dopant and its effect on the graphene structure play a crucial role in facilitating the transfer of charge across the composite.
Based on all data, we propose the model of converting Candlenut shell to be carbon containing Graphene (
Figure 12).
Figure 12 shows that there are three steps converting Candlenut shells to be carbon containing graphene namely:
Step-1: Candlenut shells contain lignin and cellulose pyrolyzed to form charcoal chips. In this step, the lignin and cellulose should be oxidized and oxygen functional groups containing on it.
Step-2: To reduce oxygen functional groups and reconstruct graphitic carbon structure, pyrolyzed carbon is reduced and attached with activated carbon and further pyrolyzed to generate carbon containing graphene.
Step-3: Finally, in this step, carbon containing graphene is cleaned with distilled water, dried and grinded to separate between graphene and activated carbon, graphene part is collected and characterized.
This route is a very facile route to produce large scale graphene using sustainable raw material.