Bismuth/bismuth oxide-incorporated reduced graphene oxide nanocomposite: synthesis, characterisation, and photocatalytic activity

This study loaded Bi/Bi2O3 on the surface of reduced graphene oxide (rGO) to perform a two-step facile synthesis of rGO@Bi/Bi2O3 as a bismuth-based nanocomposite. First, Bi/Bi2O3 nanocomposites were synthesised via a solvothermal process using Bi(NO3)3 5H2O as the Bi3+ precursor and dimethyl sulfoxide (DMSO) as the solvent. Second, we exfoliated rGO in water to functionalise Bi/Bi2O3 with a few layers of rGO. Obtained nanocomposites were characterised with scanning electron microscopy and X-ray diffraction. We also measured the nanocomposites’ photocatalytic activity using cationic dyes, specifically methylene blue (MB) and rhodamine B (RhB). Additionally, ultraviolet-visible spectroscopy was used to determine the optical properties of rGO@Bi/Bi2O3. Photodegradation was recorded under differing durations of exposure to visible light. Reaction rates were recorded at 14.6 × 10–4 min−1 and 22.2 × 10–3 min−1 for MB and RhB, respectively, while photodegradation efficiency was logged at 17% and 81%.


Introduction
Water treatment is an increasingly important area of environmental research. In particular, there is considerable interest in applying nanomaterials as a water-treatment method. For example, antifouling nanomembranes based on carbon materials such as reduced graphene oxide (rGO) and cellulose have relevant physical, chemical, and surface properties for water treatment [1][2][3]. However, antifouling membranes can also comprise inorganic materials in the form of polymers, mixed polymers, metals, and ceramics [4,5]. Ceramic membranes are typically comprised of aluminum oxide, silicon carbide, and zirconia, all of which have excellent thermal stability, mechanical properties, biological relevance, low production cost, environmental friendliness, and long lifespan [6]. Various fields make use of a nanosheet with improved thermal and mechanical properties, as well as larger surface area; these nanosheets are derived from hybridising PVA nanoparticles (NPs), metal oxide (MO) NPs, polysaccharide products such as chitosan and cellulose derivatives, generating a modified GO surface [7]. Nanoparticles (notably ZnO and CuO NPs) also have important applications in solar-powered devices, specifically in the form of quantum dots due to their small bandgaps [8]. Water desalination treatments often use GO with titanium dioxide, zinc oxide, and magnetite NPs [9][10][11] that have high water solubility, biological properties, and low cost [12]. In addition, cellulose derivatives such as sodium carboxymethyl cellulose (CMC) have various biological applications [13]. However, CMCs have low solubility and other drawbacks related to unreacted species in the system; to improve solubility, compounds such as glutaraldehyde are typically added, but they are highly toxic [14]. Recent advancements have combined MO nanomaterials and rGO to enhance their suitability for surface functionalization, biomedical implementation, drug systems, and water-soluble applications [15]. Due to their low energy density, these materials can be used as energy storage devices and alternatives to standard batteries [16][17][18].
Bismuth-based NPs (BiNPs) are potential theranostic agents [19] and have been tested in various clinical procedures, such as diagnostic CT imaging [20] and radiotherapy [21][22][23]. For the former, BiNPs can improve device sensitivity and local CT numbers through increasing target absorption and the likelihood of disease detection, while for the latter, the NPs boost radiation effects on target tissue. Recent evidence has suggested that rGO is a radiosensitizing agent [24][25][26][27]. Therefore, Bi-based nanomaterials should improve in performance if combined with rGO. This study fabricated an rGO@Bi/Bi 2 O 3 nanocomposite via loading Bi@Bi 2 O 3 onto the surface of rGO. We then measured the nanocomposite's photocatalytic activity toward dication dyes methylene blue (MB) and rhodamine B (RhB), with the aim of determining its suitability as a radiosensitizer agent.

Fabrication of nanomaterials
The mixture was then heated to 190°C, mechanically agitated for 2 h at that temperature [28,29], and then centrifuged. The resultant precipitate was washed several times with ethanol/acetone (2:1), and then vacuum-dried at 50°C for 12 h, yielding Bi/Bi 2 O 3 as a grey powder.

Synthesis of reduced graphene oxide (rGO) nanostructure
The study generated rGO following methods from Hummers and Offerman [30]. First, 8.0% graphite/H 2 SO 4 was made via combining 8 g ammonium nitrate (NH 4 NO 3 ) and graphite powder with 98% H 2 SO 4 in an ice bath. Potassium permanganate (KMnO 4 ) was then stirred into the mixture for 2 h at 90°C until completely dissolved, forming a GO precipitate. Subsequently, hydrogen peroxide (48 ml) was added to the solution. The resultant mixture was cleaned with hydrochloric acid (10.0%), and then dried at 45°C for 24 h in double-distilled water (DDW). Graphene oxide (1.0 g) was then sonicated in DDW (100 ml) for 15 min until fully mixed. Ammonium hydroxide (5 ml) and hydrazine hydrate (5 ml) were added to the GO solution, and stirred for 30 min This mixture was heated to 90°C in a water bath for 45 min with constant stirring. Darkening of the solution indicated that GO reduction was complete. The final rGO sample was collected and used for further analyses [31].

Synthesis of rGO@Bi/Bi 2 O 3
A 50 mg suspension of rGO in DDW was stirred for 3 h before adding 50 mg of Bi/Bi 2 O 3 dropwise, with vigorous agitation. After stirring overnight, the product was washed with water, then ethanol several times, and finally dried at 70°C. Characterisation was performed using SEM, FT-IR, and X-ray diffraction (XRD).

Photocatalytic activity and electron transfer
Photodegradation of methylene blue (MB) and rhodamine B (RhB) was performed under visible light at differing exposure durations. In a quartz cuvette, 200 μl of stock solution (6.0 mg of rGO@Bi/Bi 2 O 3 in 10 ml DDW) was combined with 100 μl of the appropriate dye (both 1.0×10 -4 M). Double-distilled water was then added until the volume reached 3 ml. The resultant product was exposed to light, and the reaction was followed with UV-vis spectroscopy.  Figure 2 shows the characteristic XRD patterns of rGO, Bi/Bi 2 O 3 , and rGO@Bi/Bi 2 O 3 . The main XRD peak of rGO was 2ϴ=25.02 with a layer spacing of 0.34 nm, indicating that GO was successfully reduced to rGO. The rGO nanosheets were generally exfoliated into a monolayer, resulting in a new lattice structure that is indicated by the broad peak [32]. These findings suggest that differences in intercalated oxide functionalities may cause  pre-exfoliation of vein graphite directed to different interlayer spacings for GO group content. These findings suggest that differences in intercalated oxide functionalities may cause pre-exfoliation of vein graphite directed to different inter-layer spacing for GO group content. The phase patterns of Bi/B i2 O 3 were compared to JCPDS 76-1730, which corresponded to the monoclinic phase of Bi and Bi 2 O 3 NPs (figure 2(a)). As a result, the creation of a pure phase was established [33].

X-ray diffraction (XRD)
The nanocomposite rGO@Bi/Bi 2 O 3 's XRD patterns corresponded to those of Bi/Bi 2 O 3 NPs. This highlighted the presence of Bi/Bi 2 O 3 on the rGO surface. The Scherrer formula was used to determine the crystalline size, and the size of Bi/Bi 2 O 3 was recorded at 48 nm, which agrees well with the results of TEM investigation.

Zeta potential
We detected suspension stabilities (measured using zeta potential) of rGO and rGO@Bi/Bi 2 O 3 in DDW at room temperature, where the stability range was approximately ±30 mV [34]. Nonionic capping reagents and polymers without electrostatic repulsion can be used to determine zeta potential stability. The zeta potentials of rGO and rGO@Bi/Bi 2 O 3 were −31 and −40 mV, respectively (figure 3), emphasising the stability of synthesised nanocomposites.

UV-vis spectroscopy
Distinct from GO, the rGO spectrum (figure 4(a)) shows a strong sharp peak at approximately 258 nm, indicating that oxygen functionality has been almost completely removed and a C=C conjugated graphene structure has been established [35]. Electron transfer and bandgap values of the synthesised nanocomposite determined ROS generation via the Tauc equation (equation (1) where α=absorption coefficient, υ=frequency of light, h=Planck's constant, hυ=photon energy, A=proportionality constant, and E g =bandgap. Bandgap is calculated as n=1/2. To approximate the bandgap using the linear line of the curve, we determined the relationship between (αhυ) 2 and hυ ( figure 4(b)). Bandgaps for Bi/Bi 2 O 3 and rGO@Bi/Bi 2 O 3 were 3.0 and 2.6 eV, respectively.

Photocatalytic activity
We used UV-vis spectroscopy to detect the photocatalytic activity of 6 mg/10 ml rGO/Bi/Bi 2 O 3 under various exposure durations to visible light ( figure 5). We observed a quenching effect of the nanocomposite under visible light, resulting in characteristic peaks at 664 nm for MB and 560 nm for RhB. The rate constants of the reactions were evaluated using equation (

Conclusion
This study fabricated a bismuth-based nanocomposite (rGO@Bi/Bi 2 O 3 ) by loading Bi/Bi 2 O 3 on reduced graphene oxide (rGO), then characterized its structure and photocatalytic activity, in comparison with rGO and Bi/Bi 2 O 3 . We demonstrated that the fabricated nanocomposite can degrade two cationic dyes (methylene blue and rhodamine B) after light exposure. After subjecting the nanocomposite to visible light, we recorded reaction rates of 14.6×10 -4 min −1 and 22.2×10 -3 min −1 for MB and RhB, along with photodegradation efficiencies of 17% and 81%. We believe that our study makes a significant contribution because this is the first reported fabrication of combining rGO and Bi/Bi 2 O 3 to generate a nanomaterial with enhanced properties. The characterization we provided here can act as a starting point for further analysis of this nanocomposite, paving the way for multiple potential applications in water treatments and theranostics.