Enhancing the Photocatalytic Performance of CuBi₂O₄ Through Graphene Oxide Incorporation

Abstract

Copper bismuth oxide (CuBi₂O₄) was produced through a domestic microwave-assisted synthesis using water as an environmentally friendly solvent, whereas graphene oxide (GO) was obtained using a modified Hummers’ method. Structural characterization by XRD confirmed the formation of a pristine tetragonal CuBi₂O₄ phase, with no shift in diffraction peaks after GO incorporation, although a slight reduction in crystallinity was observed. FESEM analysis revealed the characteristic sheet-like morphology of GO and spherical CuBi₂O₄ particles. EDS analysis verified the elemental composition of the prepared composites. Incorporating GO into CuBi₂O₄ narrowed the optical band gap from 2.4 to 2.2 eV and suppressed electron–hole recombination, as evidenced by the lower photoluminescence intensity. Furthermore, BET measurements showed that the specific surface area increased markedly from 3.79 m²/g for pristine CuBi₂O₄ to 16.49 m²/g for the composite. This enhancement may provide additional adsorption sites for malachite green (MG) molecules, promoting their adsorption onto the composite surface before exposure to visible light. The composite achieved 86.5% MG removal within 150 min, with nearly twice the rate constant of pristine CuBi₂O₄. Reactive species trapping experiments indicated that photogenerated holes (h⁺) and superoxide radicals (•O₂⁻) played major roles in the photocatalytic process. Meanwhile, the removal efficiency of pristine CuBi₂O₄ declined by approximately 40% after four consecutive reuse cycles.

1. Introduction

Among the available environmental remediation technologies, photocatalysis has emerged as a promising strategy due to its ability to utilize sunlight as an abundant and sustainable energy source for organic pollutant removal. Through various photoredox reactions, photocatalysis can be applied not only to the degradation of organic pollutants but also to the synthesis of value-added chemicals, depending on the intended application [1]. In this context, bismuth-based photocatalysts have gained increasing research interest in recent years due to their advantageous electronic characteristics and relatively narrow band gaps, which make them promising candidates for the photocatalytic degradation of environmental pollutants. These characteristics enable efficient utilization of visible light, which constitutes more than 40% of the solar spectrum, thereby offering a distinct advantage over ultraviolet (UV)-responsive semiconductors such as the benchmark photocatalyst TiO₂. Consequently, bismuth-based materials, including BiVO₄, Bi₂O₃, BiOBr, BiOCl, and BiOI, have been extensively investigated as visible-light-responsive photocatalysts. Copper bismuth oxide (CuBi₂O₄) is particularly attractive among bismuth-based photocatalysts because of its relatively narrow band gap, tetragonal crystalline structure, and favorable light-absorption characteristics [2,3]. CuBi₂O₄ is a ternary metal oxide that can be formed through the reaction of CuO and Bi₂O₃, resulting in a distinct crystalline phase. The relatively low band gap facilitates efficient absorption of low-energy photons, thereby enhancing visible-light responsiveness and imparting notable photocatalytic activity in heterogeneous systems [4,5,6,7]. Under light irradiation with sufficient photon energy, CuBi₂O₄ absorbs photons and produces electron–hole pairs. These photogenerated charge carriers can participate in surface redox reactions, potentially leading to the formation of reactive oxygen species, including hydroxyl radicals (•OH) and superoxide radicals (O₂^•–). However, the photocatalytic performance of pristine CuBi₂O₄ is often restricted by rapid electron–hole recombination, which reduces the availability of charge carriers for subsequent reactions.

Several strategies, particularly heterojunction construction, have been proposed to overcome the intrinsic limitations of photocatalysts and enhance their photocatalytic performance. For example, Nafees Ahmad et al. [8] reported the synthesis of a novel CuBi₂O₄/polyaniline (PANI) composite via the in situ polymerization of aniline. The composite exhibited a removal efficiency of 96% for ammonia solution, which was significantly higher than those of pristine CuBi₂O₄ (70%) and PANI (78%). The improved photocatalytic activity of CuBi₂O₄/PANI was primarily associated with more efficient separation of photogenerated charge carriers, suppression of electron–hole recombination, and greater surface area. In a related study, Fan Yang et al. [9] reported that integrating spinel CuBi₂O₄ with Bi₂O₃ to form a Z-scheme heterojunction effectively minimized charge-carrier recombination. The resulting CuBi₂O₄/Bi₂O₃ nanofibers, prepared via electrospinning, exhibited enhanced photocatalytic performance toward the degradation of methyl orange (MO).

In another study, Xin Wang et al. [10] further reported the formation of an Ag₂WO₄/CuBi₂O₄ S-type heterojunction, which significantly enhanced the sonocatalytic activity of CuBi₂O₄. Likewise, Tianji Ma et al. [11] developed BiOBr/CuBi₂O₄ composites that suppressed electron–hole recombination through Z-scheme heterojunction formation. Hangmin Guan et al. [12] also successfully prepared In₂O₃/CuBi₂O₄ heterojunctions, achieving improved photoelectric activity due to reduced charge recombination. Thus, these studies consistently demonstrate that heterojunction construction is an effective strategy for enhancing the photocatalytic performance of CuBi₂O₄ by promoting charge separation and suppressing charge carrier recombination.

Based on the literature review, graphene oxide (GO) was selected as a functional component for incorporation into CuBi₂O₄ -based composites in this study. GO is a two-dimensional carbon-based derivative of graphene that preserves the hexagonal lattice of sp²-hybridized carbon atoms, while being decorated with abundant oxygen-containing functional groups, including hydroxyl, epoxide, carbonyl, and carboxyl groups, located on both its basal planes and edges. In contrast to pristine graphene, which exhibits exceptional electrical and thermal conductivity, GO possesses a disrupted π-conjugated structure because of oxidation. This disruption reduces its electrical conductivity but simultaneously enhances its hydrophilicity, surface reactivity, and dispersibility in aqueous and polar solvents. Moreover, the presence of oxygen functionalities confers GO with a large specific surface area, tunable surface chemistry, and considerable mechanical strength. Owing to these unique physicochemical properties, GO has emerged as a promising platform for a wide range of applications, including sensors, energy storage and supercapacitors, batteries, and polymer-based nanocomposites [13,14,15,16,17,18].

When CuBi₂O₄ is coupled with GO, the resulting composite can significantly enhance the separation and migration of photogenerated charge carriers owing to the excellent electron conductivity of GO. In addition, the high adsorption capacity of GO enables the enrichment of organic contaminants from wastewater onto the surface of the composite photocatalyst, thereby increasing the probability of pollutant–catalyst interactions and facilitating more efficient degradation and removal processes.

Numerous studies have reported the successful integration of GO with various metal oxide photocatalysts, including TiO₂, ZnO, BiVO₄, WO₃, CaTiO₃, and BiOBr (Table 1) [19,20,21,22,23,24,25]. In these systems, the overall photocatalytic degradation rate was consistently improved, primarily due to the suppression of electron–hole pair recombination. This suppression enhances the likelihood of redox reactions occurring on the catalyst surface, leading to the generation of reactive radical species and their subsequent interaction with pollutant molecules.

Table 1. Comparison of photocatalytic performance of GO-based composites under different conditions.

Material	Pollutant	Light Source & Conditions	Time (min)	Performance	Ref
TiO2/GO	Acid Orange G	Simulated solar light (500 W/m2, 1500 W Xe lamp)	60	TiO2: 47.6 (%), 0.0112 min−1 TiO2/GO: 99.8%, 0.1092 min−1	[19]
ZnO/GO	MB	Visible halogen lamp (500 W, 9500 lumens)	80	ZnO: 64% ZnO/GO: 91%	[20]
BiVO4/ rGO	Acetaminophen	300 W Xe lamp (λ > 400 nm)	150	BiVO4: 0.008 min−1 BiVO4/rGO:0.0141 min−1	[21]
WO3/GO	MB	UV cabinet (2 × 150 W Xe lamps)	270	WO3:75.79% WO3/GO: 96.30%	[22]
B-CaTiO3/ GO	MB	Visible light (450 nm, Hg lamp, 2.3 mW·cm−2)	180	B/CaTiO3: 31.3%, 0.0019 min−1 CaTiO3/5GO: 87.4%, 0.0106 min−1	[23]
BiOBr/ rGO	MB	Visible light (420 nm, 5 W LED)	75	BiOBr: 67.25%, 0.0090 min−1 BiOBr/RGO:96.41%, 0.0392 min−1	[24]
BiVO4/ rGO	MB	Visible light (halogen lamp, 54 W)	120	BiVO4: 60%, 0.0067 min−1 BiVO4/rGO: 95%, 0.0144 min−1	[25]
CuBi2O4/ GO	MG	30 W visible-light LEDs	150	CuBi2O4: 58.9%, 0.0053 min−1 CuBi2O4/GO: 86.5%, 0.0105 min−1	This work

In this study, GO was synthesized using a modified Hummers method, while CuBi₂O₄ particles and CuBi₂O₄/GO composites were prepared using a household microwave-assisted method. Malachite Green (MG) dye was selected as the target pollutant in this work, serving not only as a standard model for evaluating photocatalytic efficiency but also as a representative environmental hazard. MG has been associated with mutagenic, carcinogenic, and teratogenic effects, raising significant concerns regarding its potential adverse impacts on human health and aquatic ecosystems.

Comprehensive material characterization was carried out to compare pristine CuBi₂O₄, GO, and the CuBi₂O₄/GO composite. The analyses included morphological features, crystal structure and phase composition, surface properties, and optical characteristics, with the objective of explaining how these differences influence the photocatalytic reaction mechanism in the degradation of MG synthetic wastewater. Furthermore, the proposed photocatalytic mechanism was investigated in detail to identify the dominant reactive species. The reusability of the heterogeneous photocatalysts and their potential contribution to the Sustainable Development Goals (SDGs) were also evaluated in this study.

2. Experimental

2.1. Chemicals and Reagents

Graphite powder (Sigma-Aldrich, St. Louis, MO, USA), sulfuric acid (H₂SO₄; QReC, New Zealand), sodium nitrate (NaNO₃; KemAus, Cherrybrook, Australia), potassium permanganate (KMnO₄; CARLO ERBA, Val-de-Reuil, France), 30% hydrogen peroxide (H₂O₂; QReC, New Zealand), and hydrochloric acid (HCl; RCl Labscan, Bangkok, Thailand) were used for the synthesis of graphene oxide (GO). Copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, ≥99.5%; Loba Chemie, Mumbai, India), bismuth(III) nitrate pentahydrate (Bi(NO₃)₃·5H₂O, >98.0%; QReC, New Zealand), and sodium hydroxide (NaOH, >98.0%; QReC, New Zealand) were employed for the synthesis of CuBi₂O₄. All reagents were used as received without further purification.

2.2. Material Characterizations

The crystalline structures of pristine CuBi₂O₄ and its composite were investigated by X-ray diffraction (XRD) using a Philips X’Pert MPD diffractometer (Philips Analytical, Almelo, The Netherlands) equipped with Cu Kα radiation (λ = 0.1546 nm). Surface morphology and elemental composition were analyzed using field-emission scanning electron microscopy (FESEM, AperoS, Thermo Fisher Scientific, Waltham, MA, USA) coupled with energy-dispersive X-ray spectroscopy (EDS). The specific surface areas of the synthesized materials were evaluated using the Brunauer–Emmett–Teller (BET) method based on N₂ adsorption–desorption measurements performed with an Autosorb 1 MP analyzer (Quantachrome, Boynton Beach, FL, USA). Photoluminescence (PL) spectra were recorded using a fluorescence spectrophotometer (FluoroMax-4, Horiba Jobin Yvon, Edison, NJ, USA) with an excitation wavelength of 350 nm. The emission spectra were collected in scanning mode at room temperature using a solid sample holder. Furthermore, the optical absorption properties and band gap energies were investigated using ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) with a Shimadzu UV-3101PC spectrophotometer (Shimadzu Corporation, Kyoto, Japan).

2.3. Synthesis of Graphene Oxide (Go) by Modified Hummer’s Method [26]

Graphite powder (1 g) was dispersed in 20 mL of concentrated H₂SO₄ (98%) under magnetic stirring for 30 min, while the reaction mixture was maintained in an ice bath below 15 °C to control the exothermic reaction. H₂SO₄ served as an acidic medium, facilitating the intercalation of chemical species between graphite layers and promoting subsequent oxidation. Subsequently, 0.5 g of NaNO₃ was added to facilitate graphite intercalation and assist the oxidation process, followed by continuous stirring for an additional 1 h. KMnO₄ (6 g) was then gradually introduced as a strong oxidizing agent to introduce oxygen-containing functional groups into the graphite structure. During this process, the suspension turned dark green, indicating changes in the manganese-containing species during oxidation. The reaction mixture was subsequently diluted with 100 mL of deionized (DI) water and maintained at room temperature for 18 h to promote further oxidation, swelling, and expansion of the graphite interlayer spacing, thereby facilitating subsequent exfoliation into GO sheets. Subsequently, 5 mL of H₂O₂ (30%) was slowly added to terminate the oxidation reaction by reducing residual permanganate ions (MnO₄⁻) to Mn²⁺, resulting in the decolorization of the suspension. The resulting product was purified by centrifugation at 5000 rpm for 5 min, followed by removal of the supernatant and redispersion of the precipitate in fresh 1 M HCl solution. This washing procedure was repeated five times to remove residual manganese species and other metal ion impurities. The precipitate was then repeatedly washed with DI water, with centrifugation and replacement of the supernatant after each cycle, until the supernatant reached approximately neutral pH. The product was redispersed in DI water and ultrasonicated in an ultrasonic bath for 60 min to promote exfoliation into GO sheets. Finally, the GO was dried in an oven at 80 °C for 24 h and stored in a desiccator to minimize moisture absorption.

2.4. Synthesis of CuBi₂O₄ and CuBi₂O₄/GO Composite by Microwave-Assisted Method in Aqueous Medium

The CuBi₂O₄/GO composite was synthesized using a microwave-assisted precipitation method. Initially, aqueous precursor solutions of copper(II) nitrate trihydrate (0.05 mol/dm³) and bismuth(III) nitrate pentahydrate (0.10 mol/dm³) were prepared separately and subsequently combined. In contrast to our earlier work [27], DI water was employed as a more environmentally benign alternative to nitric acid. GO powder was then added to 50 mL of the mixed precursor solution to obtain a CuBi₂O₄/GO composite with a nominal GO content of 20 wt%. This composition was selected for the present study without systematic optimization of different GO loadings. The suspension was placed in a 100 mL Duran glass bottle and homogenized by magnetic stirring for 30 min. Subsequently, 25 mL of 2 mol/dm³ NaOH solution was introduced dropwise, followed by an additional 30 min of stirring. The mixture was then treated under microwave irradiation at a nominal power of 350 W for 15 min using a domestic microwave oven (Samsung MS20A3010AL/ST, Samsung Electronics, Suwon, Republic of Korea; maximum output power of 700 W and chamber capacity of 20 L). The corresponding chemical reactions are presented below:

$$\mathrm{Cu}{\left(\mathrm{N}{\mathrm{O}}_3\right)}_2\cdot 3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\stackrel{{\mathrm{H}}_2\mathrm{O}}{\to }\mathrm{C}{\mathrm{u}}^{2+}\left(\mathrm{aq}\right)+2\mathrm{N}{\mathrm{O}}_3^{-}\left(\mathrm{aq}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

$$\mathrm{Bi}{\left(\mathrm{N}{\mathrm{O}}_3\right)}_3\cdot 5{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\stackrel{{\mathrm{H}}_2\mathrm{O}}{\to }\mathrm{B}{\mathrm{i}}^{3+}\left(\mathrm{aq}\right)+3\mathrm{N}{\mathrm{O}}_3^{-}\left(\mathrm{aq}\right)+5{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$$

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂ (s)

Bi³⁺(aq) + 3OH⁻(aq) → Bi(OH)₃ (s)

Cu(OH)₂(s) + 2Bi(OH)₃(s) → CuBi₂O₄ (s) + 4H₂O(l)

Upon completion of the microwave treatment, the suspension was allowed to reach room temperature through natural cooling. The solid precipitate was recovered by centrifugation, repeatedly washed with DI water, and dried at 100 °C for 2 h. The resulting powder was identified as the CuBi₂O₄/GO composite. Pristine CuBi₂O₄ was also synthesized under identical experimental conditions, except for the omission of GO, to serve as a reference material for comparison.

2.5. Photocatalysis Evaluation

For each photocatalytic experiment, 0.05 g of either pristine CuBi₂O₄ or the CuBi₂O₄/GO composite was dispersed in 50 mL of an aqueous MG solution with an initial concentration (C₀) of 3 ppm. Prior to light irradiation, the mixture was continuously stirred in the absence of light for 30 min to attain adsorption–desorption equilibrium between the photocatalyst surface and MG molecules. Subsequently, the photocatalytic experiments were conducted under visible-light irradiation using two 30 W white LED floodlights positioned approximately 100 cm above the sample surface. The reaction was carried out under continuous irradiation for 120 min in a custom-designed photoreactor equipped with a magnetic stirring system and six 60 mm cooling fans to maintain the reaction temperature at 25 °C. At 30 min intervals (30, 60, 90, 120, and 150 min), aliquots were collected from the reaction suspension and centrifuged to separate the photocatalyst particles. The resulting supernatants were then analyzed to quantify the residual MG concentration (C_t) at each irradiation time. Control experiments were conducted to evaluate MG removal in the absence of a photocatalyst and to compare the photocatalytic performance of pristine CuBi₂O₄ and the CuBi₂O₄/GO composite under the same irradiation conditions. The residual MG concentration was determined by measuring the absorbance of the collected supernatant at 618 nm using a UV–Vis spectrophotometer (UH5300, Hitachi High-Tech Corporation, Tokyo, Japan). The percentage of photocatalytic degradation efficiency (%DE) was subsequently determined according to Equation (1).

$$\%\mathit{DE}=\left({\displaystyle \frac{C_0-C_t}{C_0}}\right)\times 100$$

(1)

Furthermore, the kinetics of MG photodegradation were investigated by applying a pseudo-first-order kinetic model, as described by Equation (2) [28]. This model enables comparison of the rate constants under identical temperature conditions (~35 °C), thereby providing additional confirmation beyond the photocatalytic degradation percentage.

$$\mathit{ln}\left({\displaystyle \frac{C_0}{C_t}}\right)=k_1\times t$$

(2)

where k₁ denotes the pseudo-first-order rate constant (min⁻¹), and C₀ refers to the MG concentration measured after the dark adsorption stage (i.e., post-adsorption equilibrium), which differs from the initial concentration of 3 ppm.

3. Results and Discussion

XRD analysis was performed to investigate the structural transformation of graphite into GO and to characterize the crystalline structure of CuBi₂O₄, as shown in Figure 1. The XRD patterns of graphite and GO (Figure 1a) reveal a significant structural change following oxidation. The characteristic (002) diffraction peak of graphite at 2θ = 26.3° disappeared, while a new diffraction peak emerged at 2θ = 10.5°, corresponding to the (001) plane of GO. These findings confirm the successful oxidation of graphite and the formation of graphite oxide with an expanded interlayer spacing [28].

Figure 1. XRD patterns of (a) graphite and GO and (b) CuBi₂O₄ and CuBi₂O₄/GO composite.

As shown in Figure 1b, the XRD pattern of pristine CuBi₂O₄ exhibits characteristic diffraction peaks at 2θ values of 27.9°, 29.3°, 30.9°, 33.1°, 34.4°, 37.4°, 46.1°, 53.2°, 55.3°, 59.7°, and 65.9°, which correspond to the (211), (220), (002), (310), (112), (202), (411), (213), (332), (521), and (413) lattice planes of the tetragonal CuBi₂O₄ phase (JCPDS No. 42–0334) [29].

Following the incorporation of GO into CuBi₂O₄, the XRD pattern of the CuBi₂O₄/GO composite remained similar to that of pristine CuBi₂O₄, with no significant shifts in the diffraction peak positions. The absence of the characteristic GO diffraction peak may be attributed to its low content and weak diffraction intensity relative to the dominant CuBi₂O₄ phase.

However, the incorporation of GO can inhibit crystal growth during synthesis, leading to smaller crystallite sizes and consequently lower diffraction intensity. This decrease in peak intensity reflects a reduced degree of crystallinity. Similar observations were reported by M.R. Venkatraman et al. [30], who found that the presence of GO sheets lowered the degree of crystallinity in TiO₂ composites compared with pristine TiO₂. To confirm this, the XRD patterns in the 2θ range of 25–30° were analyzed to highlight the (211) peak for the full width at half maximum (FWHM) estimation (Figure 2).

Figure 2. XRD patterns in the 2θ range of 25–30° showing the (211) diffraction peak: (a) CuBi₂O₄ and (b) CuBi₂O₄/GO composite.

The crystallite size was estimated using the Scherrer equation, with FWHM determined using OriginPro8 software. The (211) diffraction peak was selected for analysis due to its high intensity and clear separation from neighboring peaks. The calculated crystallite size decreased from approximately ~43.0 nm for CuBi₂O₄ to ~35.6 nm for CuBi₂O₄/GO, suggesting that GO incorporation influenced the crystallite growth of CuBi₂O₄. This finding is supported by the reduced diffraction peak intensities and slight broadening of the XRD reflections, suggesting a decrease in crystallinity accompanied by a reduction in crystallite size.

Another characteristic feature of GO is its sheet-like morphology, as shown in Figure 3a, where thin, layered structures are clearly observed. Simultaneous EDS analysis confirmed that carbon and oxygen were the dominant elements. The prominent EDS signal at approximately 2.1 keV is attributed to the gold layer deposited on the sample surface during SEM preparation to improve electrical conductivity. As illustrated in Figure 3b, the FESEM micrograph recorded at 5000× magnification reveals that the CuBi₂O₄ particles exhibit a distinct spherical morphology, with particle diameters ranging from approximately 2 to 5 µm. The surfaces of these spheres appear rough and are covered with nanoscale particles, suggesting secondary crystal growth or aggregation on the primary particles. EDS analysis confirmed that both the spherical and flake regions are composed of CuBi₂O₄, containing Bi, Cu, and O as the main constituent elements.

Figure 3. FESEM images of (a) GO, (b) CuBi₂O₄, and (c) composite, with corresponding EDS spectra.

In comparison, the CuBi₂O₄/GO composite (Figure 3c) exhibits larger particles, possibly due to the deposition and aggregation of CuBi₂O₄ particles on GO sheets, which act as a supporting matrix during synthesis. The EDS point analysis confirms the presence of Cu, Bi, O, and C at the selected location, supporting the successful formation of the CuBi₂O₄/GO composite.

As shown in Table 2, the incorporation of GO, which has a high specific surface area of 485.28 m²/g, enhanced the surface area of the CuBi₂O₄/GO composite. The specific surface area increased approximately 4-fold, from 3.79 m²/g for pristine CuBi₂O₄ to 16.49 m²/g for the composite. The BJH desorption cumulative pore volume increased from 0.01 cm³/g for CuBi₂O₄ to 0.05 cm³/g for the composite (Figure 4a). As shown in Figure 4b, all samples exhibited type II adsorption isotherms according to the IUPAC classification. This type indicates that the materials possess large, open surfaces rather than a network of small pores [31]. The higher N₂ adsorption capacity is reflected by the greater amount of N₂ adsorbed, as indicated by the higher values on the y-axis of the isotherms. An increase in specific surface area may improve photocatalytic activity by providing a greater number of accessible surface sites for pollutant adsorption and subsequent photocatalytic reactions. However, photocatalytic activity also depends on other factors, including charge separation efficiency, light absorption, and interfacial charge transfer.

Table 2. BET surface area and pore properties of GO, CuBi₂O₄, and CuBi₂O₄/GO.

Sample	BET Specific Surface Area (m2/g)	BJH Cumulative Pore Volume (cm3/g)	BJH Average Pore Diameter (nm)
GO	485.28	1.79	10.43
CuBi2O4/GO	16.49	0.05	12.24
CuBi2O4	3.79	0.01	13.58

Figure 4. The N₂ adsorption–desorption isotherm of GO, CuBi₂O₄ and the composite.

The optical band gap energies of the synthesized materials were estimated from Tauc plots constructed using ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) data [32,33]. The measured reflectance values were converted into the Kubelka–Munk function, F(R), which was used to construct plots of [F(R)hν]² against photon energy (hν), as presented in Figure 5. By extrapolating the linear portions of the plots to the photon energy axis, the optical band gap energies were determined to be 2.4 and 2.2 eV for pristine CuBi₂O₄ and the CuBi₂O₄/GO composite, respectively. The incorporation of GO resulted in a slight reduction in the apparent optical band gap, suggesting a modification of the optical absorption characteristics of CuBi₂O₄.

Figure 5. Band gap of (a) CuBi₂O₄ and (b) CuBi₂O₄/GO composite.

To further investigate the electronic band structure, the valence band (E_VB) and conduction band (E_CB) edge potentials were estimated using the electronegativity-based relationships given in Equations (3) and (4) [34].

E_VB = χ − E_O + 0.5E_g

(3)

E_CB = E_VB − E_g

(4)

Here, χ denotes the absolute electronegativity of CuBi₂O₄ (4.59 eV) [35], E₀ represents the energy of a free electron on the hydrogen scale (4.5 eV), and E_g is the optical band gap energy obtained from the Tauc analysis. For pristine CuBi₂O₄, an E_g value of 2.4 eV was used to estimate the corresponding band edge positions.

The incorporation of GO into the CuBi₂O₄ composite may introduce carbon-associated localized electronic states near the band edges through interfacial interactions. These states could provide additional pathways for photoexcitation at lower photon energies, potentially contributing to the reduction in the apparent optical band gap from 2.4 to 2.2 eV. The proposed energy states are schematically illustrated in Figure 6a. The recombination behavior of photogenerated charge carriers was further examined by photoluminescence (PL) spectroscopy. PL emission originates from radiative transitions of excited charge carriers to lower-energy states, making it a useful technique for investigating electron–hole recombination processes. Stronger emission generally reflects a greater contribution from radiative recombination, whereas diminished PL intensity may suggest that radiative recombination pathways have been suppressed. Such behavior can be favorable for photocatalysis because more charge carriers may remain available to participate in interfacial redox reactions. As depicted in Figure 6b, pristine CuBi₂O₄ displays a relatively intense PL emission, suggesting pronounced radiative recombination of photogenerated electrons and holes. This process may restrict the availability of charge carriers required for photocatalytic surface reactions, particularly those involved in producing reactive oxygen species (ROS), including superoxide radicals (•O₂⁻) and hydroxyl radicals (•OH). In comparison, the CuBi₂O₄/GO composite exhibits a slightly weaker PL signal, indicating a possible reduction in radiative recombination following GO incorporation. The interaction between CuBi₂O₄ and GO may facilitate interfacial charge transfer and improve charge-carrier utilization during photocatalysis. As a result, the increased participation of photogenerated electrons and holes in surface redox processes could favor ROS production, which subsequently contributes to the oxidative degradation of MG into smaller molecular fragments.

Figure 6. (a) Energy band structure and (b) PL spectra.

As shown in Figure 7a, GO exhibited the highest total MG removal efficiency of 97.8%, primarily due to its strong adsorption capacity. However, its photocatalytic contribution was minimal, as indicated by the nearly constant removal efficiency during the 120 min light irradiation period. In contrast, both pristine CuBi₂O₄ and the CuBi₂O₄/GO composite showed a continuous decrease in MG concentration under light irradiation, indicating photocatalytic degradation. The CuBi₂O₄/GO composite achieved a total removal efficiency of 86.5%, compared to 58.9% for pristine CuBi₂O₄, suggesting that the improved performance resulted from the combined effects of adsorption and photocatalytic degradation.

Figure 7. (a) MG removal efficiency under irradiation (mean ± SD, n = 3) and (b) corresponding pseudo-first-order kinetic plots based on mean values.

To further distinguish photocatalytic degradation from dark adsorption, kinetic analysis was performed using only the data collected during the light irradiation period, as presented in Figure 7b. The initial concentration (C₀) was defined as the MG concentration measured after dark adsorption equilibrium, immediately before light irradiation. The apparent rate constant of the composite (0.0105 min⁻¹) was nearly twice that of pristine CuBi₂O₄ (0.0053 min⁻¹), confirming its superior photocatalytic performance.

The operational stability and reusability of CuBi₂O₄ were assessed through successive photocatalytic degradation experiments using MG as the target pollutant. Each reaction was carried out for 150 min under identical experimental conditions. Following each cycle, the photocatalyst was separated from the reaction suspension by centrifugation, repeatedly rinsed with deionized water, dried in a hot-air oven, and weighed before being introduced into the subsequent experiment. Four consecutive photocatalytic degradation cycles were performed using pristine CuBi₂O₄ as a representative catalyst to evaluate its reusability, as shown in Figure 8a. The initial MG degradation efficiency was 58.9% in the first cycle, consistent with the result presented in Figure 7a, and gradually decreased to 43.4%, 30.9%, and 19.0% in the second, third, and fourth cycles, respectively. Overall, the degradation efficiency decreased by approximately 40 percentage points after four consecutive cycles. To further examine possible structural changes, XRD analysis was performed on the recovered photocatalyst. As illustrated in Figure 8b, the diffraction peaks of the reused CuBi₂O₄ exhibited lower intensities than those of the pristine material. However, the diffraction peak positions remained largely unchanged, suggesting that the main crystalline phase of CuBi₂O₄ was preserved after repeated photocatalytic cycles. Although the photocatalytic degradation efficiency decreased after repeated use, the results suggest that the prepared catalyst retained its structural stability during successive photocatalytic cycles. Therefore, the decline in photocatalytic efficiency may be attributed to the partial blockage of surface-active sites by adsorbed MG molecules and/or residual degradation intermediates, which could reduce the availability of active sites for subsequent photocatalytic reactions.

Figure 8. (a) Reusability over successive 150 min cycles, (b) XRD patterns of the fresh and used catalysts, and (c) radical scavenging experiments.

To elucidate the reactive species governing MG photodegradation, radical and charge-carrier trapping experiments were performed using benzoquinone (BQ), ethylenediaminetetraacetic acid (EDTA), and isopropanol (IPA) as selective scavengers for superoxide radicals (•O₂⁻), photogenerated holes (h⁺), and hydroxyl radicals (•OH), respectively. As illustrated in Figure 8c, the presence of EDTA and BQ markedly suppressed the photocatalytic degradation of MG, with the corresponding removal efficiencies decreasing to 12.2% and 15.5%, respectively. These findings suggest that h⁺ and •O₂⁻ play predominant roles in the photocatalytic process, whereas •OH appears to make a comparatively limited contribution. The relatively weak involvement of •OH may be explained by the estimated valence band potential of CuBi₂O₄ (+1.29 V vs. NHE), which is less positive than the potential generally required for the direct oxidation of H₂O to •OH. Consequently, the formation of hydroxyl radicals through direct hole-mediated water oxidation is thermodynamically unfavorable under the proposed band alignment. This oxidation requires a potential of approximately +2.73 V vs. NHE at pH 0 (or +2.32 V at pH 7) [36]. Therefore, the direct generation of •OH by photogenerated holes (h⁺) is thermodynamically unfavorable, consistent with the relatively weak inhibitory effect observed in the IPA scavenging experiment.

When exposed to light with photon energies sufficient to overcome its band gap, CuBi₂O₄ becomes photoexcited, resulting in the formation of electron–hole pairs, as represented by Equation (5):

CuBi₂O₄ + hv → h⁺ (VB) + e⁻ (CB)

(5)

As photogenerated holes were identified as one of the dominant reactive species, they may directly oxidize MG molecules through interfacial electron transfer, leading to the formation of oxidized intermediates and their subsequent degradation, as described in Equations (6) and (7):

h⁺ + MG → MG⁺•     (VB)

(6)

MG⁺• → Oxidation intermediated (VB)

(7)

The scavenger experiment results agree with those reported by Thammaacheep et al. [37], who identified photogenerated holes as important reactive species in the photocatalytic degradation of tetracycline in aqueous media. Meanwhile, electrons generated in the CB can reduce dissolved molecular oxygen to form superoxide radical anions (•O₂⁻), as expressed in Equation (8).

e⁻ + O₂ → •O₂⁻     (CB)

(8)

MG molecules + •O₂⁻ → CO₂ + H₂O + Other small molecules (CB)

(9)

The generated superoxide radicals can react with MG molecules, leading to the breakdown of their molecular structure through oxidative reactions. Subsequent reactions may produce smaller organic intermediates, which could undergo further oxidation and eventual mineralization into CO₂ and H₂O, as described in Equation (9). Based on the findings from the scavenger experiments, a plausible photocatalytic mechanism for MG degradation was proposed, in which photogenerated holes (h⁺) and superoxide radicals (•O₂⁻) serve as the principal reactive species, as schematically illustrated in Figure 9.

Figure 9. Proposed mechanism of MG photodegradation over the CuBi₂O₄/GO composite.

4. Conclusions

In this study, CuBi₂O₄ was successfully synthesized using a microwave-assisted method with water as an environmentally benign solvent, aligning with green chemistry principles by avoiding the use of organic solvents. Comprehensive material characterization provided evidence of the successful incorporation of GO into CuBi₂O₄. XRD analysis revealed a reduction in diffraction peak intensity and crystallite size following GO addition. SEM images showed the coexistence of two distinct morphologies, corresponding to CuBi₂O₄ particles and GO sheets, while EDS analysis confirmed the presence of carbon alongside the constituent elements of CuBi₂O₄. Furthermore, the incorporation of GO, which possesses a relatively high specific surface area, increased the overall surface area of the composite. Optical characterization also revealed a change in the band gap energy of CuBi₂O₄ after GO incorporation, indicating modified optical absorption properties. The improved photocatalytic performance of the CuBi₂O₄/GO composite is primarily attributed to enhanced charge separation, facilitated interfacial charge transfer, and increased surface area, rather than the slight reduction in band gap energy. The lower PL intensity of the composite compared with pristine CuBi₂O₄ further supports the proposed suppression of radiative electron–hole recombination. Consequently, GO incorporation resulted in an approximately twofold increase in the photocatalytic reaction rate constant under identical temperature conditions.