Physical, Chemical and Electrical Conductivity Properties of Bismuth Oxide/ Rice Husk-Based Activated Carbon/Graphite Composite Prepared by the Hydrothermal Method

The synthesis of Bismuth Oxide/Rice Husk-based Activated Carbon/Graphite (BO/RH-AC/G) composites using the hydrothermal method with varying concentrations of bismuth nitrate pentahydrate has been conducted. The composites are synthesized from bismuth nitrate pentahydrate, sodium sulfate, and sodium hydroxide precursors added with activated carbon from rice husks and graphite using the hydrothermal method at 110^oC for 5 hours. The characterization results show that the concentration of the added bismuth nitrate pentahydrate precursor affects the physical, chemical, and electrical properties of the resulting composites such as crystallinity, presence of functional groups, pore and surface properties, morphology, element distribution, thermal stability, and electrical conductivity. In the use of smaller bismuth nitrate pentahydrate, composite has the highest crystallinity containing mostly Bi₂O₃ having monoclinic crystal structure, small pore size and high surface area. Each composite showed different pattern of thermal decomposition. Moreover, most products have the same rod-like morphology even though the highest precursor used resulted in lumpy surface shape. The electrical conductivity of composites is higher than that of pure bismuth oxide and rice husk-activated carbon.

Bismuth Oxide (BO); Composite; Graphite (G); Hydrothermal; Rice Husk-activated Carbon (RH-AC)

Metal oxides (Li et al., 2021; Kim et al., 2019) have been studied intensively as somethe most promising candidates for battery anodes due to their high theoretical capacities and low costs. One of the metal oxides that have the potential to be used as an electrode is bismuth oxide (Xu et al., 2022; Nandi and Das, 2020). Expediently, bismuth oxide has high theoretical capacity value (690 mAh.g^-1), volumetric capacity value of 3765 mAh.cm^-1, a potential difference of 2.8 V, is non-toxic, abundant, and relatively cheap (Fang et al., 2017; Li et al., 2013). However, the problem with using bismuth oxide as a battery anode is that it possesses low conductivity value which has a tendency to give a result of the slow charge/ion conductivity of the battery anode. This low conductivity value, however, is able to overcome by adding other materials that have the ability to increase the conductivity value, namely activated carbon and graphite.

Activated carbon is an amorphous carbon consisting of a flat plate with carbon atoms arranged and covalently bonded in a hexagonal lattice (Gilles and Loehr, 1994). Activated carbon has a conductivity value of 6.04 S.m^-1 and a large surface area between 300 m2.g^-1 to 3500 m².g^-1 (Kim et al., 2016). A large surface area is one of the advantageous factors that comprise an electrode material (Ariyanto, Prasetyo, and Rochmadi, 2012), whereby the larger the surface area, the greater the capacitance value (Ciszewski et al., 2015). In addition to the surface area, the volumetric capacity of 1770 mAh.cm^-1 and a potential difference of 0.2 V (Kim et al., 2016) of activated carbon are highly sought-after in an electrode-making material (Bijesh, Selvaraj, and Andal, 2022). Graphite is one of the carbon core materials that act as an electrical conductor and able to be used as an electrode material (Sari, 2015). Graphite has the characteristics of being soft, light, and capable of conducting electricity, though graphite has a low theoretical capacity (372 mAh.g^-1) (Fang et al., 2017).

2.1. Materials

      The materials used in this study are Bi(NO₃)₃.5H₂O (sigma aldrich), H₃PO₄ (Merck), Na₂SO₄ (Merck), NaOH (Merck) bought from Darmstadt-Germany, rice husks, and distilled water.       

2.2. Research Procedure

2.2.1. Rice Husk-Based Activated Carbon Production

The production of rice husk-based activated carbon followed the procedure reported by Arnelli, Santoso, and Astuti (2021). Clean rice husks were charred by pyrolysis at 300^oC for 10 minutes. A total of 5 grams of the carbon produced were activated by the addition of 125 mL 60% H₃PO₄ and processing in the microwave with a power of 400 W for 5 minutes. The activated carbon was then washed with distilled water until the pH was constant. The activated carbon that had been washed was then dried using an oven at 105^oC for 5 minutes. The resulting activated carbon was destructed and sieved to a size of 100 mesh.

2.2.2. Synthesis of Bismuth Oxide/Rice Husk-Activated Carbon/Graphite Composite

       The synthesis of BO/RH-AC/G composites was carried out using the hydrothermal method with variations in Bi(NO₃)₃.5H₂O of 8, 24, and 32 mmol. For the 8 mmol BO/RH-AC/G variation, initially 8 mmol of bismuth nitrate pentahydrate was added with 12 mmol of Na₂SO₄. They were dissolved in 40 mL of distilled water and then stirred using a magnetic stirrer for 45 minutes. Later, 40 mL of 72 mmol of NaOH was added to the solution mixture followed by 0.5 grams of rice husk-activated carbon and 0.1 grams of graphite and the mixture was stirred. The mixture was then poured into a stainless-steel tube and into a hydrothermal reactor, heated at 110^oC for 5 hours. At the final stage, the product was filtered and dried using an oven at a temperature of 110^oC for 1 hour, then sieved to a size of 100 mesh. The same procedure was used to synthesize the 24 mmol and 32 mmol BO/RH-AC/G composites. The three composite products were labeled[BO/RH-AC/G]₁, [BO/RH-AC/G]₂ and [BO/RH-AC/G]₃ for the composites that were synthesized with variations of 8 mmol, 24 mmol, and 32 mmol, respectively.

2.2.3. Characterization of the Bismuth Composite Material

       The characterizations of the three composites were carried out using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Thermo Gravimetric Analyzer-Derivative Thermogravimetri (TGA-DTG), Inductance, Capacitance, and Resistance (LCR), and Gas Sorption Analyzer (GSA) instruments. The characterization of the composite products by FTIR (Shimadzu IRAffinity-1) was carried out in the wave number range of 400 cm^-1 - 4000 cm^-1 to determine the presence of functional groups in the resulting composite products. Analysis of composite products using XRD (Shimadzu 7000) was done with 2theta ()= 0 to 90, to identify the crystal structure of bismuth oxide in the synthesized composites. Furthermore, the Scanning Electron Microscope-Energy Dispersive X-Ray (SEM-EDX) instrument (Jeol JED 6510LA) was used to identify the surface morphology of the sample and the compositions of the composites. A mapping test was also carried out to determine the distribution of the elements that make up the composite materials. The characterization of the composite products using TGA (Mettler Toledo TGA/DSC 3+) was used to determine the thermal stability of the composites. Meanwhile, the electrical conductivities of the composite products were determined using an LCR meter (HIOKI 3532-50). Composite characterization with GSA (Tristar II 3020) was carried out with N₂ gas adsorbate to determine the surface area, pore volume, and pore size of the composites.

3.1 BO/RH-AC/G Composite

       The synthesis of BO/RH-AC/G composites was initiated by reacting the precursor materials consisting of Bi(NO₃)₃.5H₂O, Na₂SO₄, and NaOH according to the procedure reported by Wu et al. (2011). Subsequently, the solution was added with rice husk-activated carbon and graphite, homogenized, and heated in a hydrothermal reactor.

Figure 1 Synthesized (a) [BO/RH-AC/G]₁ (b) [BO/RH-AC/G]₂ and (c) [BO/RH-AC/G]₃ composites

3.2   BO/RH-AC/G Composite Material Characteristics

3.2.1. Composites Functional Groups Structures

Figure 2 FTIR spectra of composite products, graphite, activated carbon, bismuth oxide pentahydrate

Table 1 Functional groups identification from the FTIR analysis

The composite products also contain activated carbon, indicated by the presence of the absorption of the C-O group at the wave numbers of 1118 cm^-1, 1103.10 cm^-1, and 1112.16 cm^-1 for the [BO/RH-AC/G]₁, [BO/RH-AC/G]₂, and [BO/RH-AC/G]₃ products, respectively. The absorption of the C=C group which indicates the presence of activated carbon and graphite was also observed in the three composites at wave numbers of 1631 cm^-1, 1634.94 cm^-1, and 1645.85 cm^-1. The existence of the two absorption areas is consistent with a conducted by (Sastrohamidjojo, 2018), stating that activated carbon is characterized by a sharp absorption at wave number 1626.64 cm^-1 indicating the presence of the C=C group and absorption at wave number 1039.87 cm^-1 that denotes the C-O group. The absorption of the C=C group at the wave number 1630 cm^-1 also indicates the presence of graphite (Huang et al., 2017). The identifications of the functional groups in the composite samples, pure Bi₂O₃, rice husk-activated carbon, and graphite are summarized in Table 1.

3.2.2. Crystal Structures

        The XRD characterization results of the composites are depicted in Figure 3. Composite [BO/RH-AC/G]₃ has sharp peaks at 2 theta () 21.720^o; 28.896^o; and 33.211^o. The peaks have similarities with the peaks in the pure Bi(NO₃)₃.5H₂O diffractogram, namely at 21.943^o; 28.7854^o, and 32.783^o. Meanwhile, the composite [BO/RH-AC/G]₂ possesses no sharp and specific peaks, meaning it is possible that the amounts of -Bi₂O₃ crystals formed in this composite was small making the sample amorphous.

        Figure 3 also displays peaks at 2 theta () 27.418^o; 33.286^o; 46.337^o; 52.403^o, and 54.792^o in the [BO/RH-AC/G]₁ composite product. The peaks of the diffractogram indicate the presence of -Bi₂O₃ crystals with a monoclinic crystal system.

Figure 3 Diffractograms of the [BO/RH-AC/G]₁, [BO/RH-AC/G]₂, and [BO/RH-AC/G]₃ composites, rice husk-activated carbon, graphite, and bismuth nitrate pentahydrate (BNP)

3.2.3. Morphology and Elemental Distribution

       Composite analysis results using SEM presented in Figure 4a and b and shows that the composite products [BO/RH-AC/G]₁ and [BO/RH-AC/G]₂ at magnifications of 1000x and 5000x have almost the same rod-like morphology. The [BO/RH-AC/G]1 composite has rod-like particles with a length of 2.72–20.9  and a width of 2.72–8.28 , while the [BO/RH-AC/G]₂ is 3.63–18.18  long and 0.9–3.36  wide. The shape of the particle is distinct from the shape of the Bi₂O₃ particles as reported by Yakout (2020). Meanwhile, in Figure 4c, the composite [BO/RH-AC/G]₃ has an irregular and a lumpy surface shape. Bismuth oxide synthesized using the hydrothermal method generates rod-like shape particles (Astuti et al., 2022b) meanwhile other methods result in irregular shapes (Astuti et al., 2021; Astuti et al., 2020a; 2020b). This morphology affects the value of the electrical conductivity of bismuth oxide (Astuti et al., 2022b). The particle shapes in one dimension including wires, rods, or tubes are the most magnificent shape for developing the electrochemical behavior due to the large surface areas and the shortened ion diffusion paths (Yousif et al., 2022).

Figure 4 SEM images of (a) [BO/RH-AC/G]₁, (b) [BO/RH-AC/G]₂, (c) [BO/RH-AC/G]₃ at 1000x and 5000x magnifications

Figure 5 Mapping of (a) [BO/RH-AC/G]₁; (b) [BO/RH-AC/G]₂; (c) [BO/RH-AC/G]₃

3.2.4. Composite Thermal Stability

The TGA-DTG curves of the [BO/RH-AC/G]₁ composite (Figure 6a) shows five stages weight loss. The first and second stages occurred at the temperatures between 200–380^oC in which around 0.2% and 13% of mass are lost and are attributed to the removal of NaOH, NaNO₃, and SO42- residues as well as the mass lost from the transformation of the precursors into Bi₂O₃ (Zhang et al., 2011). In stage three, there was a further Bi₂O₃ formation process which occurred at temperatures between 400 – 450^oC with mass loss of around 2% (Ma et al., 2010; Christensen et al., 2003). In stage four, about 0.2% weight reduction was observed 0.2% at 500^oC, denoting the phase transition from -Bi₂O₃ to -Bi₂O₃ (Shen et al., 2012). The fifth stage at 640^oC indicated by a weight loss of about 0.2% is identified as the phase transition from -Bi₂O₃ to -Bi₂O₃ (Klinkova et al., 2007).

There are four stages of mass loss that occurred in [BO/RH-AC/G]² as shown in Figure 6.b. The first stage was able to be observed at temperatures between 200–380^oC where there was a weight loss of about 9% which was able to be attributed to the removal of residues such as NaOH, NaNO₃, and SO₄^2- and the mass loss from the transformation from the precursors to Bi₂O₃ (Zhang et al., 2011). In the second stage, there was a reduction in weight at temperatures between 400–450oC as much as 1.5%, representing the process of further formation of Bi₂O₃ (Ma et al., 2010; Christensen et al., 2003). In the third stage, there was a reduction in weight of 0.5% at the temperatures around 500^oC denoting the phase transition from -Bi₂O₃ to -Bi₂O₃ (Shen et al., 2012). In the last stage, at 640^oC, mass loss of about 2% occurred, which is the phase transition from -Bi₂O₃ to -Bi₂O₃ (Klinkova et al., 2007).

Figure 6 TGA and DTG curves of (a) [BO/RH-AC/G]₁; (b) [BO/RH-AC/G]₂; (c) [BO/RH-AC/G]₃ composites

The [BO/RH-AC/G]₃ composites underwent three stages of mass loss as shown in Figure 6.c. In the first stage, there was a weight loss of 8% at temperatures between 200-275^oC which may be derived from the removal of functional groups (-COO-, -CO-, -OH-, and hydrate) from graphite (Deng et al., 2017). The second stage was accentuated by a mass reduction of 10% at the temperature of about 400^oC which was assigned to the transformation of the precursors to form Bi₂O₃ (Ma et al., 2010). In the third stage, a mass loss of 0.2% occurred at the temperature of about 500^oC which was attributed to the phase transition from  (Shen et al., 2012).

3.2.5 Composite Electrical Conductivity

        The ionic and electrical conductivity of a material is determined by its crystal structure. The electrical conductivity values of the three composite products, pure bismuth oxide (Bi₂O₃), activated carbon and graphite are presented in Table 2.

         Table 2 shows that the electrical conductivity value of pure Bi₂O₃ is very low. So is the case of the rice husk-based activated carbon with its electrical conductivity value. Meanwhile graphite as the dopant has a high conductivity value. The three composites that have been formed have higher electrical conductivity values than pure Bi₂O₃ and activated carbon from rice husks. The rice husk-activated carbon added into the composite functions as a matrix so that the distribution of bismuth oxide particles is more even, which would make it easier for the electrons to flow, thus allowing good electrical conductivity.

        The [BO/RH-AC/G]₃ composite product has the highest electrical conductivity value compared to the [BO/RH-AC/G]₁ and [BO/RH-AC/G]₂ composites. This is because there are more initial precursors contained, in which bismuth nitrate pentahydrate has a high electrical conductivity value, causing the electrical conductivity of the [BO/RH-AC/G]₃ composites to be high.

        The [BO/RH-AC/G]₂ composite has the smallest electrical conductivity value. The value of the electrical conductivity is influenced by the crystallinity of a material. The more crystalline a material is, the higher its electrical conductivity it becomes (Kim, Lee, and Jang, 2014). The XRD data presented that the [BO/RH-AC/G]₃ and [BO/RH-AC/G]₁ are crystalline, while [BO/RH-AC/G]₂ is amorphous. Therefore, the electrical conductivity value [BO/RH-AC/G]₂ is low.

        The [BO/RH-AC/G]₁ has an electrical conductivity value in between the [BO/RH-AC/G]₃ and [BO/RH-AC/G]₂. This is attributed to the successfully formed Bi₂O₃ confirmed through the XRD and FTIR data. The XRD data showed that the composite [BO/RH-AC/G]₁ contained Bi₂O₃ crystals and the FTIR spectra confirmed the presence of the Bi-O group with the highest intensity.

3.3.6 Pore and Surface Properties

Figure 7 Nitrogen adsorption-desorption isotherm graphs of (a) [BO/RH-AC/G]₁; (b) [BO/RH-AC/G]₂; and (c) [BO/RH-AC/G]₃

     Table 3 shows that the more the bismuth nitrate pentahydrate precursor is used, the larger the pore size, the smaller the pore volume, and thus the smaller the surface area. This is because a higher amount of Bi (NO₃)₃.5H₂O in the composite would mean that more activated carbon would be covered resulting in less number of pores, as activated carbon has a large surface area of 18.83 m²/g. Based on the resulting data, [BO/RH-AC/G]₁ has the largest surface area and [BO/RH-AC/G]₃ has the smallest. The data is supported by the XRD data (Figure 3), in which [BO/RH-AC/G]₃ composite formed still contained a lot of bismuth nitrate pentahydrate.

        Larger pore size allows for faster ion transport (Hu et al., 2016). If the pore size is large, the number of pores become less and thus generate lower resistance. This allows more ions to flow more, resulting in a higher electrical conductivity value. This is in accordance with the LCR data (Table 2) in which [BO/RH-AC/G]₃ has the highest electrical conductivity.

    The physical, chemical, and electrical properties of BO/RH-AC/G composite prepared by the hydrothermal method are influenced by the amount of bismuth nitrate pentahydrate precursor added. A real difference is seen in the formation of bismuth oxide (Bi₂O₃) in the resulting composite. In the use of 8 mmol bismuth nitrate pentahydrate, the composite has the highest crystallinity. The high amount of the precursor added also causes the composite to tend to clump. Regarding the thermal stability as well as pore and surface properties, each composite showed successively different patterns of thermal decomposition and surface area and pore size. Meanwhile, the nature of the electrical conductivity indicates that the high amount of precursor added causes the electrical conductivity value to be higher, which may be promoted from the precursor itself.

   The authors fully acknowledged the Institute for Research and Community Services (LPPM) of Diponegoro University for the research funding through the World Class Research Universitas Diponegoro (WCRU) 2021 scheme with the grant No.118-12/UN7.6.1/PP/2021. 

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