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Low cost and green approach in the reduction of graphene oxide (GO) using palm oil leaves extract for potential in industrial applications
Journal Pre-proofs Low Cost and Green Approach in The Reduction of Graphene Oxide (GO) Using Palm Oil Leaves Extract for Potential in Industrial Applications M.S Amir Faiz, C.A Che Azurahanim, S.A Raba'ah, M.Z. Ruzniza PII: DOI: Reference:
S2211-3797(19)33093-1 https://doi.org/10.1016/j.rinp.2020.102954 RINP 102954
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Results in Physics
Received Date: Revised Date: Accepted Date:
20 October 2019 2 January 2020 16 January 2020
Please cite this article as: Amir Faiz, M.S, Che Azurahanim, C.A, Raba'ah, S.A, Ruzniza, M.Z., Low Cost and Green Approach in The Reduction of Graphene Oxide (GO) Using Palm Oil Leaves Extract for Potential in Industrial Applications, Results in Physics (2020), doi: https://doi.org/10.1016/j.rinp.2020.102954
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Low Cost and Green Approach in The Reduction of Graphene Oxide (GO) Using Palm Oil Leaves Extract for Potential in Industrial Applications M.S Amir Faiza, C.A Che Azurahanima,b*, S.A Raba’aha,b, and M.Z. Ruznizac aDepartment
of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Characterization and Synthesis Lab, Institute of Advanced Technology, 43400 UPM Serdang, Selangor, Malaysia cDepartment of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia *Corresponding author: [email protected]
bMaterial
Graphene is an exceptional and versatile new material which having worldwide reputation due to its unique properties such as having extremely good electrical conductor, tunable energy band gap, and offers high mechanical strength. The process to derived graphitized carbon into the reduced graphene oxide (rGO) is facing a huge obstacle and the chemical method is proven to be the best choice in order to synthesis rGO in the big scale short amount of time. However, the current chemical method involved the usage of hazardous, toxic and corrosive chemical as reducing agent namely hydrazine to form the sheet layer of graphene. Thus, this will limit the various applications of graphene due to environmental impact and safety issues. As an alternative current research proposed the usage of palm oil leaves extract to replace the hazardous and toxic chemical used as reducing agent. In our project, graphite was used as starting material and was oxidized using modified Hummer’s method. The palm oil leaves extract with the mixture of graphene oxide (GO) solution was refluxed to produce rGO. The as-synthesized green approach rGO material were then characterized using X-ray diffraction (XRD), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), and Raman spectroscopy. The results revealed that the interspace distance between plane increased proportionally as graphite was oxidized, increasing from 0.33 nm to 0.84 nm. The reduction process of GO using palm oil leave extract showed the successful in removing the hydroxyl group and amorphotization of sp2 carbon structures. The reduction process found to be increased in C/O ratios from 1:1 to 3:1. Raman spectroscopy revealed that the G band position was restored comparable to graphite as the reduction process successful achieved. TEM images and selected area electron diffraction (SAED) patterns illustrated the confirmation of the successfully synthesized of the monolayer of graphene sheet. Electrochemical studies carried out for both GO and rGO have positively differentiated and concluded a better voltage-current response of rGO in comparison to GO. The as synthesized rGO in the current project holds various potential for further investigation and industrial applications not limited to just supercapacitor and photocatalyst. Keywords: Biomass, electrochemical, graphene oxide, palm oil, reduced graphene oxide Introduction Graphene refers to a single layer of carbon, usually it is a result of exfoliation of the graphite structure. Due to its unique features for instance high surface area, very excellent electrical, thermal conductivities, and outstanding mechanical strength, graphene has garnered worldwide fame and attention in many field of research for example photocatalyst (Qin et al. 2017) and supercapacitor (Saranya et al. 2016). It is proven that the optimized graphene sheet is highly ordered and possessed numerous extraordinary behaviours such as high surface areas (2630 m2g-1), high Young's modulus (1.0 TPa), high thermal conductivity (~5000 W m-1 K-1) and having strong chemical durability as well as high electron mobility (2.5 x 105 cm2 V-1 s-1) (Aunkor et al. 2016). Graphene can be synthesized using several methods such as chemical vapor deposition (CVD), thermal exfoliation of graphite, epitaxial growth on the electrically insulating surfaces (Sharma et al. 2013; Jin et al. 2010; Sutter et al. 2008).
The major drawbacks with CVD and epitaxial growth on electrically insulating surface consist of their complexity, involved tedious preparation method, small yield and low efficiency (Liao et al. 2011).Therefore, another approach in obtaining graphene with easier preparation and high efficiency is via the reduction of chemically oxidized and exfoliated graphite form into GO. The synthesis of GO was developed by Hummer, Brodie, and Staudenmier (Paulchamy et al. 2015). This method established by the introduction of hydroxyl group between the planar and increased in the distance between the layers of graphite and offers easy to scale up for mass production of GO. Thus, with the use of certain chemical reductants such as hydrazine (Stankovich et al. 2007) looks very promising for a mass scale production of graphene. In addition, the reduction of GO also managed to restore and mimicking the pristine graphene structure. However, the hydrophobic characteristic of the produce graphene and its tendency to easily agglomerate in solvents make it infeasible to fabricate any graphene related materials. Therefore, various research efforts was performed to overcome this problem such as reducing GO in alkaline condition to increase its colloidal stability (Stankovich et al. 2006; Fan et al. 2008). Hydrazine is one of the most widely used solvent but owing to that it is very hazardous, there are various emerging reducing agents were suggested in replacing hydrazine such as sodium hydroxide, alcohols, vitamin C, and sodium borohydride although reported to be less efficient (Liao et al. 2011).In addition, Fan and co-workers also have demonstrated the ability of using aluminium powder to encounter the usage of hazardous reagent. However, the need of using hydrochloric acid in the washing process limited its efficiency (Fan et al. 2010). Another reduction method that currently gaining attention is via the electrochemical method (Rocha et al. 2018). However, previous experiment by Rocha and co-workers, reported that the electrochemically reduced GO contained more defects than the chemically reduced GO and resulted in lower sensing performance. Hence, herein we focused on chemically reduced GO approach without the use of hydrazine. Based on the successful reported researches using biomass such as using clove extract, colocynth leaves and tea polyphenol to replace hydrazine (Suresh et al. 2015; Liao et al. 2011; Wang et al. 2011; Xu et al. 2015; Zhu et al. 2017). This paper will further extend the effort and focus towards the usage of local waste biomass such as plant-based extract as reductant. This project emphases on the easy, abundantly and commercially available palm oil waste biomass. Oil palm plantation is very common in Malaysia, which the oil palm frond disposal recorded to be 12.9 million tonnes (MnT) (Sumathi et al. 2008). The waste biomass usually will be burn up as a method of disposal and some will be recycled to produce green energy green energy such as biogas and this will consequently reduce the environmental and health impact for example air pollution (Abdullah and Sulaim 2013). Continuous research and developments in utilizing palm oil waste in the field of energy was reported whereabout RM 6379 million energy per year was generated. On top of that, recent studies revealed that plant-based biomass waste usually contain both antioxidant activities as well as phenolic compound of butylated hydroxytoluene. Palm oil leaves extract documented to have higher antioxidant properties compared to butylated hydroxytoluene. This findings is important towards the medical application where scientist trying to combat anti-microbial effect and skin disease combat (Ahmad et al. 2018). With the advantages offered by the biomass waste, herein we reported the results based on the utilization of palm oil leaves as new alternative of the reducing agent in order to synthesis rGO. The efficiency will be evaluated thoroughly by its structural, morphology and chemical properties. Experimental Graphite powder was purchased from Sigma Aldrich (99.99 % purity), sodium nitrate (NaNO3) from Bendosen, concentrated sulphuric acid (H2SO4) (95-98%) from Chemiz, potassium permanganate (KMnO4), 30% hydrogen peroxide (H2O2) and diluted hydrochloric acid (HCL) from R&M Chemicals. Palm oil leaves was obtained from Taman Pertanian Universiti (TPU) UPM, Malaysia. Preparation of graphite oxide: The oxidation of graphite was perfomed using method demonstrated by (Paulchamy et al.
2015) with minor adjustment. Basically, 2 g of graphite was added into H2SO4 for 1 hour before 2 g of NaNO3 was added slowly in a controlled temperature equipped with the ice bath. Then 9 g of KMnO4 was added very slowly with interval of 1 minutes for every addition using spatula. The solution then was then diluted using 184 mL of deionized (DI) water in a controlled temperature where the temperature was kept to not exceed 80 ̊C. The solution was then heated at 98 ̊C and then allowed to cool at room temperature before H2O2 solution was further added and the yellowish colour started to appear. The solution was then washed using HCL and DI water using centrifugation method in order to eliminate any KMnO4 residue. Preparation of palm oil leaves extract: Grinded and dried palm oil leave was added into distilled water (1g /10 mL) and heated using magnetic stirrer at 70 ̊C for 30 minutes as well as at room temperature for 3 hours. The solution was then centrifuged to eliminate the sediment left from the extract solution. Synthesis of rGO: GO solution was prepared by sonicated graphite oxide powder in distilled water (2 mg/mL) using probe sonicator for 1 hour in order to exfoliate the graphite oxide and form GO solution. The GO solution was then mixed with the prepared palm oil leaves extract solution at 1:1 ratio. Then, the suspension was refluxed at 100 ̊C for 3 hours. The resulting solution was exposed to centrifugation at 6000 rpm and further dried in the oven for 24 hours. Black powder of reduced GO (rGO) was finally collected and ready for characterization. Characterization X-Ray Diffraction (XRD) analysis To study the changes of structure in samples, XRD was used to calculate the changes of interlayer d spacing value using Bragg’s Law equation. The oxidation and reduction phenomenon can be deducted from the the shifting of the peak. The structural and chemical properties of GO and rGO was characterized using Shimadzu X-Ray diffraction (XRD) with CuKα radiation 1.546 Å. The distance between of plane can be calculated using Bragg’s Law equation: 𝑛 ⋋= 2𝑑 sin 𝜃 Energy-dispersive X-ray (EDX) To determine the oxygen and carbon content for both GO and rGO, EDX characterization was employed before and after reduction process. EDX is an important technique used to quantitatively study the elemental composition in our prepared sample. The samples were drop cast on the aluminium plate with silver as conductive ink. FEI Nova NanoSEM 400 operated at 20.0 kV equipped with EDX was used. Raman spectroscopy Raman spectroscopy is a powerful technique and very important when dealing with graphene related material. The result obtained from Raman can give information regarding the degree of graphitization and defects of sp2 carbon structure in the samples. In this study, GO and rGO was characterized using Raman spectroscopy machine (Raman Witec) equipped with laser at 488 nm wavelength. Transmisson electron microscope (TEM)
To study the morphology of the GO and rGO, TEM was carried out using Thermofisher scientific model L120C. The accelerating voltage used is around 120 kV. Scanning area of electron diffraction (SAED) was also performed in order to observe and compare the atom arrangement of GO and RGO. Electrochemical studies Electrochemical studies of this project were carried out using three electrodes system. Glassy carbon electrode (GCE), platinum wire, and Ag/AgCl electrode were used in this experiment as working, counter, and reference electrodes, respectively. The modification of GCE was carried out by simply polishing it with 0.03 mm alumina powder, rinsed thoroughly with DI water, sonicated with ethanol and again with DI water for 3 minutes. The dried GCE was coated with GO and rGO material by using Nafion solution. Basically, 5 µL of Nafion solution was dropped onto the active materials which are either GO or rGO powder. The GCE was coated using drop cast technique on the electrode with active materials and allow to dry for 10 minutes. The measurement was carried out at room temperature using 6 M KOH electrolyte in the potential range from -0.7 to 0.1 V to give the cyclic voltammetry (CV) curve for each material. Result and discussion XRD is the technique used to identify the element present in the sample and it is very useful to determine the crystallinity of the prepared graphene structure and to determine the layer to layer distance. The changes in the graphene structure was highlighted before and after reduction process using palm oil leaves extract. The previous research applying this green synthesis technique towards reduction of GO can be referred to Table 2. The XRD result for GO presented in Figure 2 shows the appearance of peak at 2θ = 10.5°, indicated the formation of graphite oxide structure and the disappearance of peak at 2θ = 26.6° in Figure 1. The conversion of graphite into graphite oxide resulted in shifting to the left meaning that the oxidation of graphite successfully occurred. The modified Hummer’s method used to prepare GO introduced the hydroxyl group between each graphite layer to form GO. The oxidation of graphite is proportional to the increasing in the distance between each plane which is increased from 0.33 nm to 0.84 nm. The reduction of GO on the other hand, shows the disappearance of the peak (Figure 3) due to the exfoliation of GO. The reduction also gives the outcome of broadening peak appeared at 24.5° which further resulted in the decreasing of distance between each plane from 0.84 nm to 0.36 nm.
150000
Intensity (counts)
a
001 GO
002
100000
50000
0 20
30
40
50
60
70
80
2 (degree)
002
b
Graphite
rGO 100
10
20
30
40
50
2 (degree) Figure 1: XRD results of (a) GO and (b) rGO. Inset figure is XRD result of commercial graphite Table 1: XRD peak and distance between plane of carbon calculated using Bragg's Law equation
Sample
2θ (degree)
d-spacing (nm)
Graphite
26.6°
0.33
Graphite Oxide
10.5°
0.84
rGO
24.5°
0.36
To determine the successful reduction of GO, EDX analysis was performed. Table 2 shows the difference for GO and rGO in respect to the carbon and oxygen content. The reduction process using palm oil leaves extract managed to reduce the oxygen content from 40.34 % to 24.05 % and enhance carbon percentage content where it is increased from 59.66 % to 75.95 %. The C/O ratio of GO found to have improved from 1:1 to 3:1. Table 2: EDX result of GO and rGO, showing difference in carbon and oxygen atomic % content in each sample Sample
Carbon (atomic %)
Oxygen (atomic %)
Total (%)
GO
59.66
40.34
100.00
rGO
75.95
24.05
100.00
Table 3: Comparative study of GO and rGO using different reducing agent Reducing agent
GO (2θ)
d-spacing (nm)
rGO (2θ)
dReduction time spacing (hr) (nm) 0.36 3 hours
References
Palm oil leaves
10.5°
0.84
24.5°
Carrot root
11.2°
0.79
23.96°
NA
48 hours
(Kuila et al. 2012)
Glycine
11.05°
0.80
23.9°
0.372
24 hours
(Bose et al. 2012)
Hydrazine
10.6°
0.83
23.0°
0.38
12 hours
(Park et al. 2011)
Rose water
13.0°
0.78
24.0°
NA
5 hours
(Behzad and Tabrizi 2013)
Clove extract
11.2°
0.79
23.96°
NA
30 min
(Suresh et al. 2015)
C. Colocynthis leaves extract
11.3°
0.77
26.0°
NA
14 hours
(Zhu et al. 2017)
Ascorbic acid
11.9°
0.76
26.5°
0.34
6 hours
(Geeta et al. 2019)
Present study
*NA- not available
TEM is a useful technique to analyse the surface material of GO. The TEM images of the prepared GO (Figure 2 (a-d)) and rGO (Figure 3 (a-c)) show the appearance of wrinkles and thin characteristics of GO sheet. From the image, it is clearly observed that the folding of GO sheet occurred at the edges. The scanning area electron diffraction (SAED) images of nonoverlapped graphene region in Figure 2 (c) indicated the monolayer GO sheets from the SAED pattern obtained showed in Figure 2 (d), produced a clear and single 6-spot image and this findings is in agreement to the previous studies (Wilson et al. 2010; Wilson et al. 2009). The image gave a very clear image of atom arrangement of hexagonal shape which confirmed the typical atom arrangement for graphene. However, for rGO, similar diffraction spots were observed (see Figure 3 (c)), but the main difference is that the later showed less intense spots. This may be due to non-uniformity of the graphene structure as shown in XRD for rGO (see Figure 1 (b)) and increased in ID/IG ratio from Raman analysis. As concluded by Wilson et al. (2010), GO and rGO exhibited similar corrugation properties, similar to the
current observation based on TEM images of both samples. b
a
c
d
Figure 2: TEM images of GO (a,b, and c). Scanning area electron diffraction (SAED) pattern shown in (d)
a
b
c
Figure 3: TEM images of rGO (a and b). Scanning area electron diffraction (SAED) pattern shown in (c)
D
G
2000
Intensity (counts)
a
1500
2D
1000 1000
2000
3000
4000
Wavenumber (cm-1)
b
0
D
1000
G
2000
3000
4000
Wavenumber (cm-1) Figure 4: Raman spectra of (a) GO and (b) rGO. The inset graph in (a) is the Raman spectra of commercial graphite.
Peak position (cm-1)
1700 G peak position
1600 1500
D peak position
1400 1300 1200
Graphite
GO
rGO
Sample Figure 5: Dependency of G and D band towards degree of oxidation
ID/IG ratio
1.5
1.0
0.5
0.0
Graphite
GO
rGO
Sample Figure 6: ID/IG ratio of graphite, graphite oxide, and rGO-N
Since the bonding featured in graphene consist of conjugated and double bond bonding of carbon (Kudin et al. 2008), Raman spectroscopy is a suitable technique to characterize carbon materials in its allotropes such as carbon nanotubes, activated carbon, graphite, and graphene mainly to discern the arrangement whether disordered and ordered carbon in the sample. G band normally assigned to the characteristic of degree of graphitization or sp2 carbon structure. D band normally assigned to the characteristic of defects of structure. Figure 6 and 7 shows the usual spectrum taken by Raman spectroscopy for GO and rGO respectively, showing two prominent peak which are D and G band. The D band of rGO shifted to 1353 cm-1 from 1355 cm-1 in GO. Meanwhile, G band of rGO shifted to 1603 cm-1 from 1605 cm-1 at GO. The G band of graphite at 1575 cm-1 (inset shows in Figure 6) shifted to the right after the oxidation occurred. Then, the reduction of GO resulted in shifting to the left (Figure 7) suggested that the degree of oxidation was successfully decreased (Krishnamoorthy et al. 2013). From these changes illustrated (Figure 8), it can be said the reduction of GO have successfully restored the graphitization as the position of G band as in graphite was achieved again. Another important features of Raman spectroscopy of carbon material are 2D band (2736 cm-1). This band was denoted as an indicator of stacking of
graphite structure at the c-axis and was expected to show a transformation as the oxidation process occurred (Pimenta et al. 2007). The oxidation of graphite led to the broadened peak of 2D band as the stacked structure of graphite was disturbed following the addition of oxygen functional group in between the carbon layer. The distance of each the layer was increased as calculated by the Bragg’s Law equation. Figure 9 shows the variation of ID/IG ratio of the samples. The ratio increases corresponding to the increase of interlayer distance. This is due to defects presence because of hydroxyl group disturbed the arrangement of sp2 carbon hence affect the G band intensity decrease. Following the amorphotization of sp2 after the reduction process, the ID/IG ratio at the maximum.
Current (A)
1 10 -03 5 10 -04 0 100 mV/s 50 mV/s
-5 10 -04
30 mV/s
-1 10 -03 -0.8
5 mV/s
-0.6
-0.4
-0.2
0.0
0.2
Potential applied (V) Figure 7: Cyclic voltammetery of rGO at different scan rates
4 10 -05
Current (A)
2 10 -05 0 100 mV/s
-2 10 -05
50 mV/s
-4 10 -05 -6 10 -05 -0.8
30 mV/s 5 mV/s
-0.6
-0.4
-0.2
0.0
Potential applied (V) Figure 8: Cyclic voltammetry of GO at different scan rates
0.2
Current (A)
1 10 -03 5 10 -04 0 -5 10 -04
GO rGO
-1 10
-03
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Potential applied (V) Figure 9: Cyclic voltammetry of GO and rGO at scan rate of 100 mV/s
Figure 9 and 10 show CV curve of GO and rGO at difference scan rates which are 100, 50, 30, and 5 mV/s. The main differences can be seen from both figures are the shape of the graph plotted from the CV curve for GO and rGO, respectively. For rGO, CV curve resulted in more rectangular shape in comparison to GO, which suggested a better charge propagation at GCE surface defined by electric double layer charging mechanism (Lv et al. 2009). In terms of current response towards both materials (Figure 11), the findings clearly suggested that the as synthesized rGO showing a better voltage-current response contrasted to GO. The presence of hydroxyl group in GO causing to low and poor conductivity as well as reported to be thermally unstable (Paulchamy et al. 2015; Aunkor et al. 2016). Hence, it can be concluded here from only the CV analysis, we can conclude and differentiate the properties of GO and rGO in terms of their electrical conductivity. Conclusion Herein we reported the low cost, green and eco-friendly approach in the reduction of reduction of GO using palm oil leaves extract for various potential in industrial applications. The merit and novelty of the current reported project lies on its low cost and waste to wealth from the unwanted palm oil leaves in Malaysia. Another major successful demonstrated from current finding is in getting the shorter reduction time which is only 3 hours compared to previous reported works. XRD technique and Raman spectroscopy proved the efficiency of using palm oil leaves extract as an alternative reductant agent to replace harmful and highly toxic chemicals in the preparation. The oxidation of graphite resulted in the introduction of hydroxyl group thus increased the spacing of the interlayer distance. The reduction of GO using oil palm leaves extract revealed effective removal of hydroxyl group to produce a monolayer graphene sheets as proven by TEM technique. The C/O ratio have been greatly improved from 1:1 for GO to 3:1 for rGO. Furthermore, the electrochemical studies based on cyclic voltammetry analysis carried out for GO and rGO differentiated their electrical conductivity efficiently. Acknowledgement The authors are grateful to the Ministry of Higher Education of Malaysia for supporting this work under Fundamental Research Grants Scheme (FRGS) (Vot. No. 5524942). We also thankful to Research Management Centre of UPM through the initiative of Dana Tautan (DT9200802) and Ministry of Education Malaysia through FRGS grant (FRGS 5524949) for partially support this project. We also would like to thank NANOTEDD team of Biophysics Lab and Department of Physics, Universiti Putra Malaysia for the assistance and Universiti
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34. Declaration of interests ☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
35. Highlights:
36.
GO was synthesized using modified Hummer’s method from graphite. Morphology and structural analysis by XRD and TEM confirmed the successful formation of graphene. Palm oil leaves extract was used to replace harmful chemical such as hydrazine in the reduction process. The synthesis of rGO was successfully done in short time which is 3 hours. The electrochemical analysis for GO and rGO shows the enhancement of electrical conductivity of rGO after the reduction process.
S2211-3797(19)33093-1 https://doi.org/10.1016/j.rinp.2020.102954 RINP 102954
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Results in Physics
Received Date: Revised Date: Accepted Date:
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Please cite this article as: Amir Faiz, M.S, Che Azurahanim, C.A, Raba'ah, S.A, Ruzniza, M.Z., Low Cost and Green Approach in The Reduction of Graphene Oxide (GO) Using Palm Oil Leaves Extract for Potential in Industrial Applications, Results in Physics (2020), doi: https://doi.org/10.1016/j.rinp.2020.102954
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Low Cost and Green Approach in The Reduction of Graphene Oxide (GO) Using Palm Oil Leaves Extract for Potential in Industrial Applications M.S Amir Faiza, C.A Che Azurahanima,b*, S.A Raba’aha,b, and M.Z. Ruznizac aDepartment
of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Characterization and Synthesis Lab, Institute of Advanced Technology, 43400 UPM Serdang, Selangor, Malaysia cDepartment of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia *Corresponding author: [email protected]
bMaterial
Graphene is an exceptional and versatile new material which having worldwide reputation due to its unique properties such as having extremely good electrical conductor, tunable energy band gap, and offers high mechanical strength. The process to derived graphitized carbon into the reduced graphene oxide (rGO) is facing a huge obstacle and the chemical method is proven to be the best choice in order to synthesis rGO in the big scale short amount of time. However, the current chemical method involved the usage of hazardous, toxic and corrosive chemical as reducing agent namely hydrazine to form the sheet layer of graphene. Thus, this will limit the various applications of graphene due to environmental impact and safety issues. As an alternative current research proposed the usage of palm oil leaves extract to replace the hazardous and toxic chemical used as reducing agent. In our project, graphite was used as starting material and was oxidized using modified Hummer’s method. The palm oil leaves extract with the mixture of graphene oxide (GO) solution was refluxed to produce rGO. The as-synthesized green approach rGO material were then characterized using X-ray diffraction (XRD), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), and Raman spectroscopy. The results revealed that the interspace distance between plane increased proportionally as graphite was oxidized, increasing from 0.33 nm to 0.84 nm. The reduction process of GO using palm oil leave extract showed the successful in removing the hydroxyl group and amorphotization of sp2 carbon structures. The reduction process found to be increased in C/O ratios from 1:1 to 3:1. Raman spectroscopy revealed that the G band position was restored comparable to graphite as the reduction process successful achieved. TEM images and selected area electron diffraction (SAED) patterns illustrated the confirmation of the successfully synthesized of the monolayer of graphene sheet. Electrochemical studies carried out for both GO and rGO have positively differentiated and concluded a better voltage-current response of rGO in comparison to GO. The as synthesized rGO in the current project holds various potential for further investigation and industrial applications not limited to just supercapacitor and photocatalyst. Keywords: Biomass, electrochemical, graphene oxide, palm oil, reduced graphene oxide Introduction Graphene refers to a single layer of carbon, usually it is a result of exfoliation of the graphite structure. Due to its unique features for instance high surface area, very excellent electrical, thermal conductivities, and outstanding mechanical strength, graphene has garnered worldwide fame and attention in many field of research for example photocatalyst (Qin et al. 2017) and supercapacitor (Saranya et al. 2016). It is proven that the optimized graphene sheet is highly ordered and possessed numerous extraordinary behaviours such as high surface areas (2630 m2g-1), high Young's modulus (1.0 TPa), high thermal conductivity (~5000 W m-1 K-1) and having strong chemical durability as well as high electron mobility (2.5 x 105 cm2 V-1 s-1) (Aunkor et al. 2016). Graphene can be synthesized using several methods such as chemical vapor deposition (CVD), thermal exfoliation of graphite, epitaxial growth on the electrically insulating surfaces (Sharma et al. 2013; Jin et al. 2010; Sutter et al. 2008).
The major drawbacks with CVD and epitaxial growth on electrically insulating surface consist of their complexity, involved tedious preparation method, small yield and low efficiency (Liao et al. 2011).Therefore, another approach in obtaining graphene with easier preparation and high efficiency is via the reduction of chemically oxidized and exfoliated graphite form into GO. The synthesis of GO was developed by Hummer, Brodie, and Staudenmier (Paulchamy et al. 2015). This method established by the introduction of hydroxyl group between the planar and increased in the distance between the layers of graphite and offers easy to scale up for mass production of GO. Thus, with the use of certain chemical reductants such as hydrazine (Stankovich et al. 2007) looks very promising for a mass scale production of graphene. In addition, the reduction of GO also managed to restore and mimicking the pristine graphene structure. However, the hydrophobic characteristic of the produce graphene and its tendency to easily agglomerate in solvents make it infeasible to fabricate any graphene related materials. Therefore, various research efforts was performed to overcome this problem such as reducing GO in alkaline condition to increase its colloidal stability (Stankovich et al. 2006; Fan et al. 2008). Hydrazine is one of the most widely used solvent but owing to that it is very hazardous, there are various emerging reducing agents were suggested in replacing hydrazine such as sodium hydroxide, alcohols, vitamin C, and sodium borohydride although reported to be less efficient (Liao et al. 2011).In addition, Fan and co-workers also have demonstrated the ability of using aluminium powder to encounter the usage of hazardous reagent. However, the need of using hydrochloric acid in the washing process limited its efficiency (Fan et al. 2010). Another reduction method that currently gaining attention is via the electrochemical method (Rocha et al. 2018). However, previous experiment by Rocha and co-workers, reported that the electrochemically reduced GO contained more defects than the chemically reduced GO and resulted in lower sensing performance. Hence, herein we focused on chemically reduced GO approach without the use of hydrazine. Based on the successful reported researches using biomass such as using clove extract, colocynth leaves and tea polyphenol to replace hydrazine (Suresh et al. 2015; Liao et al. 2011; Wang et al. 2011; Xu et al. 2015; Zhu et al. 2017). This paper will further extend the effort and focus towards the usage of local waste biomass such as plant-based extract as reductant. This project emphases on the easy, abundantly and commercially available palm oil waste biomass. Oil palm plantation is very common in Malaysia, which the oil palm frond disposal recorded to be 12.9 million tonnes (MnT) (Sumathi et al. 2008). The waste biomass usually will be burn up as a method of disposal and some will be recycled to produce green energy green energy such as biogas and this will consequently reduce the environmental and health impact for example air pollution (Abdullah and Sulaim 2013). Continuous research and developments in utilizing palm oil waste in the field of energy was reported whereabout RM 6379 million energy per year was generated. On top of that, recent studies revealed that plant-based biomass waste usually contain both antioxidant activities as well as phenolic compound of butylated hydroxytoluene. Palm oil leaves extract documented to have higher antioxidant properties compared to butylated hydroxytoluene. This findings is important towards the medical application where scientist trying to combat anti-microbial effect and skin disease combat (Ahmad et al. 2018). With the advantages offered by the biomass waste, herein we reported the results based on the utilization of palm oil leaves as new alternative of the reducing agent in order to synthesis rGO. The efficiency will be evaluated thoroughly by its structural, morphology and chemical properties. Experimental Graphite powder was purchased from Sigma Aldrich (99.99 % purity), sodium nitrate (NaNO3) from Bendosen, concentrated sulphuric acid (H2SO4) (95-98%) from Chemiz, potassium permanganate (KMnO4), 30% hydrogen peroxide (H2O2) and diluted hydrochloric acid (HCL) from R&M Chemicals. Palm oil leaves was obtained from Taman Pertanian Universiti (TPU) UPM, Malaysia. Preparation of graphite oxide: The oxidation of graphite was perfomed using method demonstrated by (Paulchamy et al.
2015) with minor adjustment. Basically, 2 g of graphite was added into H2SO4 for 1 hour before 2 g of NaNO3 was added slowly in a controlled temperature equipped with the ice bath. Then 9 g of KMnO4 was added very slowly with interval of 1 minutes for every addition using spatula. The solution then was then diluted using 184 mL of deionized (DI) water in a controlled temperature where the temperature was kept to not exceed 80 ̊C. The solution was then heated at 98 ̊C and then allowed to cool at room temperature before H2O2 solution was further added and the yellowish colour started to appear. The solution was then washed using HCL and DI water using centrifugation method in order to eliminate any KMnO4 residue. Preparation of palm oil leaves extract: Grinded and dried palm oil leave was added into distilled water (1g /10 mL) and heated using magnetic stirrer at 70 ̊C for 30 minutes as well as at room temperature for 3 hours. The solution was then centrifuged to eliminate the sediment left from the extract solution. Synthesis of rGO: GO solution was prepared by sonicated graphite oxide powder in distilled water (2 mg/mL) using probe sonicator for 1 hour in order to exfoliate the graphite oxide and form GO solution. The GO solution was then mixed with the prepared palm oil leaves extract solution at 1:1 ratio. Then, the suspension was refluxed at 100 ̊C for 3 hours. The resulting solution was exposed to centrifugation at 6000 rpm and further dried in the oven for 24 hours. Black powder of reduced GO (rGO) was finally collected and ready for characterization. Characterization X-Ray Diffraction (XRD) analysis To study the changes of structure in samples, XRD was used to calculate the changes of interlayer d spacing value using Bragg’s Law equation. The oxidation and reduction phenomenon can be deducted from the the shifting of the peak. The structural and chemical properties of GO and rGO was characterized using Shimadzu X-Ray diffraction (XRD) with CuKα radiation 1.546 Å. The distance between of plane can be calculated using Bragg’s Law equation: 𝑛 ⋋= 2𝑑 sin 𝜃 Energy-dispersive X-ray (EDX) To determine the oxygen and carbon content for both GO and rGO, EDX characterization was employed before and after reduction process. EDX is an important technique used to quantitatively study the elemental composition in our prepared sample. The samples were drop cast on the aluminium plate with silver as conductive ink. FEI Nova NanoSEM 400 operated at 20.0 kV equipped with EDX was used. Raman spectroscopy Raman spectroscopy is a powerful technique and very important when dealing with graphene related material. The result obtained from Raman can give information regarding the degree of graphitization and defects of sp2 carbon structure in the samples. In this study, GO and rGO was characterized using Raman spectroscopy machine (Raman Witec) equipped with laser at 488 nm wavelength. Transmisson electron microscope (TEM)
To study the morphology of the GO and rGO, TEM was carried out using Thermofisher scientific model L120C. The accelerating voltage used is around 120 kV. Scanning area of electron diffraction (SAED) was also performed in order to observe and compare the atom arrangement of GO and RGO. Electrochemical studies Electrochemical studies of this project were carried out using three electrodes system. Glassy carbon electrode (GCE), platinum wire, and Ag/AgCl electrode were used in this experiment as working, counter, and reference electrodes, respectively. The modification of GCE was carried out by simply polishing it with 0.03 mm alumina powder, rinsed thoroughly with DI water, sonicated with ethanol and again with DI water for 3 minutes. The dried GCE was coated with GO and rGO material by using Nafion solution. Basically, 5 µL of Nafion solution was dropped onto the active materials which are either GO or rGO powder. The GCE was coated using drop cast technique on the electrode with active materials and allow to dry for 10 minutes. The measurement was carried out at room temperature using 6 M KOH electrolyte in the potential range from -0.7 to 0.1 V to give the cyclic voltammetry (CV) curve for each material. Result and discussion XRD is the technique used to identify the element present in the sample and it is very useful to determine the crystallinity of the prepared graphene structure and to determine the layer to layer distance. The changes in the graphene structure was highlighted before and after reduction process using palm oil leaves extract. The previous research applying this green synthesis technique towards reduction of GO can be referred to Table 2. The XRD result for GO presented in Figure 2 shows the appearance of peak at 2θ = 10.5°, indicated the formation of graphite oxide structure and the disappearance of peak at 2θ = 26.6° in Figure 1. The conversion of graphite into graphite oxide resulted in shifting to the left meaning that the oxidation of graphite successfully occurred. The modified Hummer’s method used to prepare GO introduced the hydroxyl group between each graphite layer to form GO. The oxidation of graphite is proportional to the increasing in the distance between each plane which is increased from 0.33 nm to 0.84 nm. The reduction of GO on the other hand, shows the disappearance of the peak (Figure 3) due to the exfoliation of GO. The reduction also gives the outcome of broadening peak appeared at 24.5° which further resulted in the decreasing of distance between each plane from 0.84 nm to 0.36 nm.
150000
Intensity (counts)
a
001 GO
002
100000
50000
0 20
30
40
50
60
70
80
2 (degree)
002
b
Graphite
rGO 100
10
20
30
40
50
2 (degree) Figure 1: XRD results of (a) GO and (b) rGO. Inset figure is XRD result of commercial graphite Table 1: XRD peak and distance between plane of carbon calculated using Bragg's Law equation
Sample
2θ (degree)
d-spacing (nm)
Graphite
26.6°
0.33
Graphite Oxide
10.5°
0.84
rGO
24.5°
0.36
To determine the successful reduction of GO, EDX analysis was performed. Table 2 shows the difference for GO and rGO in respect to the carbon and oxygen content. The reduction process using palm oil leaves extract managed to reduce the oxygen content from 40.34 % to 24.05 % and enhance carbon percentage content where it is increased from 59.66 % to 75.95 %. The C/O ratio of GO found to have improved from 1:1 to 3:1. Table 2: EDX result of GO and rGO, showing difference in carbon and oxygen atomic % content in each sample Sample
Carbon (atomic %)
Oxygen (atomic %)
Total (%)
GO
59.66
40.34
100.00
rGO
75.95
24.05
100.00
Table 3: Comparative study of GO and rGO using different reducing agent Reducing agent
GO (2θ)
d-spacing (nm)
rGO (2θ)
dReduction time spacing (hr) (nm) 0.36 3 hours
References
Palm oil leaves
10.5°
0.84
24.5°
Carrot root
11.2°
0.79
23.96°
NA
48 hours
(Kuila et al. 2012)
Glycine
11.05°
0.80
23.9°
0.372
24 hours
(Bose et al. 2012)
Hydrazine
10.6°
0.83
23.0°
0.38
12 hours
(Park et al. 2011)
Rose water
13.0°
0.78
24.0°
NA
5 hours
(Behzad and Tabrizi 2013)
Clove extract
11.2°
0.79
23.96°
NA
30 min
(Suresh et al. 2015)
C. Colocynthis leaves extract
11.3°
0.77
26.0°
NA
14 hours
(Zhu et al. 2017)
Ascorbic acid
11.9°
0.76
26.5°
0.34
6 hours
(Geeta et al. 2019)
Present study
*NA- not available
TEM is a useful technique to analyse the surface material of GO. The TEM images of the prepared GO (Figure 2 (a-d)) and rGO (Figure 3 (a-c)) show the appearance of wrinkles and thin characteristics of GO sheet. From the image, it is clearly observed that the folding of GO sheet occurred at the edges. The scanning area electron diffraction (SAED) images of nonoverlapped graphene region in Figure 2 (c) indicated the monolayer GO sheets from the SAED pattern obtained showed in Figure 2 (d), produced a clear and single 6-spot image and this findings is in agreement to the previous studies (Wilson et al. 2010; Wilson et al. 2009). The image gave a very clear image of atom arrangement of hexagonal shape which confirmed the typical atom arrangement for graphene. However, for rGO, similar diffraction spots were observed (see Figure 3 (c)), but the main difference is that the later showed less intense spots. This may be due to non-uniformity of the graphene structure as shown in XRD for rGO (see Figure 1 (b)) and increased in ID/IG ratio from Raman analysis. As concluded by Wilson et al. (2010), GO and rGO exhibited similar corrugation properties, similar to the
current observation based on TEM images of both samples. b
a
c
d
Figure 2: TEM images of GO (a,b, and c). Scanning area electron diffraction (SAED) pattern shown in (d)
a
b
c
Figure 3: TEM images of rGO (a and b). Scanning area electron diffraction (SAED) pattern shown in (c)
D
G
2000
Intensity (counts)
a
1500
2D
1000 1000
2000
3000
4000
Wavenumber (cm-1)
b
0
D
1000
G
2000
3000
4000
Wavenumber (cm-1) Figure 4: Raman spectra of (a) GO and (b) rGO. The inset graph in (a) is the Raman spectra of commercial graphite.
Peak position (cm-1)
1700 G peak position
1600 1500
D peak position
1400 1300 1200
Graphite
GO
rGO
Sample Figure 5: Dependency of G and D band towards degree of oxidation
ID/IG ratio
1.5
1.0
0.5
0.0
Graphite
GO
rGO
Sample Figure 6: ID/IG ratio of graphite, graphite oxide, and rGO-N
Since the bonding featured in graphene consist of conjugated and double bond bonding of carbon (Kudin et al. 2008), Raman spectroscopy is a suitable technique to characterize carbon materials in its allotropes such as carbon nanotubes, activated carbon, graphite, and graphene mainly to discern the arrangement whether disordered and ordered carbon in the sample. G band normally assigned to the characteristic of degree of graphitization or sp2 carbon structure. D band normally assigned to the characteristic of defects of structure. Figure 6 and 7 shows the usual spectrum taken by Raman spectroscopy for GO and rGO respectively, showing two prominent peak which are D and G band. The D band of rGO shifted to 1353 cm-1 from 1355 cm-1 in GO. Meanwhile, G band of rGO shifted to 1603 cm-1 from 1605 cm-1 at GO. The G band of graphite at 1575 cm-1 (inset shows in Figure 6) shifted to the right after the oxidation occurred. Then, the reduction of GO resulted in shifting to the left (Figure 7) suggested that the degree of oxidation was successfully decreased (Krishnamoorthy et al. 2013). From these changes illustrated (Figure 8), it can be said the reduction of GO have successfully restored the graphitization as the position of G band as in graphite was achieved again. Another important features of Raman spectroscopy of carbon material are 2D band (2736 cm-1). This band was denoted as an indicator of stacking of
graphite structure at the c-axis and was expected to show a transformation as the oxidation process occurred (Pimenta et al. 2007). The oxidation of graphite led to the broadened peak of 2D band as the stacked structure of graphite was disturbed following the addition of oxygen functional group in between the carbon layer. The distance of each the layer was increased as calculated by the Bragg’s Law equation. Figure 9 shows the variation of ID/IG ratio of the samples. The ratio increases corresponding to the increase of interlayer distance. This is due to defects presence because of hydroxyl group disturbed the arrangement of sp2 carbon hence affect the G band intensity decrease. Following the amorphotization of sp2 after the reduction process, the ID/IG ratio at the maximum.
Current (A)
1 10 -03 5 10 -04 0 100 mV/s 50 mV/s
-5 10 -04
30 mV/s
-1 10 -03 -0.8
5 mV/s
-0.6
-0.4
-0.2
0.0
0.2
Potential applied (V) Figure 7: Cyclic voltammetery of rGO at different scan rates
4 10 -05
Current (A)
2 10 -05 0 100 mV/s
-2 10 -05
50 mV/s
-4 10 -05 -6 10 -05 -0.8
30 mV/s 5 mV/s
-0.6
-0.4
-0.2
0.0
Potential applied (V) Figure 8: Cyclic voltammetry of GO at different scan rates
0.2
Current (A)
1 10 -03 5 10 -04 0 -5 10 -04
GO rGO
-1 10
-03
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Potential applied (V) Figure 9: Cyclic voltammetry of GO and rGO at scan rate of 100 mV/s
Figure 9 and 10 show CV curve of GO and rGO at difference scan rates which are 100, 50, 30, and 5 mV/s. The main differences can be seen from both figures are the shape of the graph plotted from the CV curve for GO and rGO, respectively. For rGO, CV curve resulted in more rectangular shape in comparison to GO, which suggested a better charge propagation at GCE surface defined by electric double layer charging mechanism (Lv et al. 2009). In terms of current response towards both materials (Figure 11), the findings clearly suggested that the as synthesized rGO showing a better voltage-current response contrasted to GO. The presence of hydroxyl group in GO causing to low and poor conductivity as well as reported to be thermally unstable (Paulchamy et al. 2015; Aunkor et al. 2016). Hence, it can be concluded here from only the CV analysis, we can conclude and differentiate the properties of GO and rGO in terms of their electrical conductivity. Conclusion Herein we reported the low cost, green and eco-friendly approach in the reduction of reduction of GO using palm oil leaves extract for various potential in industrial applications. The merit and novelty of the current reported project lies on its low cost and waste to wealth from the unwanted palm oil leaves in Malaysia. Another major successful demonstrated from current finding is in getting the shorter reduction time which is only 3 hours compared to previous reported works. XRD technique and Raman spectroscopy proved the efficiency of using palm oil leaves extract as an alternative reductant agent to replace harmful and highly toxic chemicals in the preparation. The oxidation of graphite resulted in the introduction of hydroxyl group thus increased the spacing of the interlayer distance. The reduction of GO using oil palm leaves extract revealed effective removal of hydroxyl group to produce a monolayer graphene sheets as proven by TEM technique. The C/O ratio have been greatly improved from 1:1 for GO to 3:1 for rGO. Furthermore, the electrochemical studies based on cyclic voltammetry analysis carried out for GO and rGO differentiated their electrical conductivity efficiently. Acknowledgement The authors are grateful to the Ministry of Higher Education of Malaysia for supporting this work under Fundamental Research Grants Scheme (FRGS) (Vot. No. 5524942). We also thankful to Research Management Centre of UPM through the initiative of Dana Tautan (DT9200802) and Ministry of Education Malaysia through FRGS grant (FRGS 5524949) for partially support this project. We also would like to thank NANOTEDD team of Biophysics Lab and Department of Physics, Universiti Putra Malaysia for the assistance and Universiti
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34. Declaration of interests ☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
35. Highlights:
36.
GO was synthesized using modified Hummer’s method from graphite. Morphology and structural analysis by XRD and TEM confirmed the successful formation of graphene. Palm oil leaves extract was used to replace harmful chemical such as hydrazine in the reduction process. The synthesis of rGO was successfully done in short time which is 3 hours. The electrochemical analysis for GO and rGO shows the enhancement of electrical conductivity of rGO after the reduction process.