The synthesized 1% Cu-doped BiVO4 was tested for their antibacterial activity using the disc diffusion method as previously reported30,31. The Gram-positive and Gram-negative bacteria that were employed were Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli), respectively. Both gram-positive and gram-negative bacteria were cultured for a whole night at 37 °C in Lysogeny Broth (LB) medium. Mueller-Hinton agar plates were inoculated with bacterial suspensions at a density of 1.5 × 106 CFU/mL and turbidity of 0.5 McFarland standard. Ten sterile filter discs, each measuring 6 mm in diameter, were then impregnated with 1 mg/mL of 1% Cu-doped BiVO4 suspension. Ethanol served as the negative control, and standard susceptibility discs containing chloramphenicol and streptomycin acted as the positive control. The inhibition zone (mm) of the 1% Cu-doped BiVO4 was measured using a vernier calliper. The experimental results were expressed as mean ± standard deviation (SD) of five replicates.
The crystalline structure and phase composition of the synthesized bare and Cu-doped BiVO were analyzed using X-ray diffraction (XRD). As shown in Fig. 2a, all samples exhibited a monoclinic structure, with prominent peaks indexed to the (121) plane at a 2θ angle of approximately 28.9°, consistent with JCPDS no. 14-0688. The 1 wt% Cu-doped BiVO showed a peak shift to higher 2θ angles, while higher Cu doping levels (3 wt% and 5 wt%) resulted in shifts to lower angles. These shifts are attributed to the substitution of Bi ions (1.03 Å) with smaller Cu ions (0.73 Å), causing lattice contraction. Conversely, the decrease in the diffraction angle at higher Cu doping levels may result from strain induced by the substitution of Cu for V ions, leading to localized lattice distortion and a slight expansion of the unit cell. The disappearance of one of the low-angle peaks (< 20°) in Cu-doped samples compared to bare BiVO can be attributed to lattice distortion and structural rearrangement caused by the substitution of Bi with smaller Cu ions, which suppresses or merges closely spaced crystal planes in this region. Furthermore, the significant increase in peak intensity observed in the Cu-doped samples suggests improved crystallinity and possible preferential orientation induced by Cu incorporation during hydrothermal synthesis. The full-width at half-maximum (FWHM) values and corresponding crystallite sizes were calculated using the Scherrer equation (Table 1). The bare BiVO exhibited the largest crystallite size (27.37 nm), while the 1 wt% Cu-doped BiVO sample showed a reduced size (25.12 nm), indicating that Cu doping promotes defect formation and reduces particle aggregation. Crystallite size slightly increased at higher Cu doping levels (3 wt% and 5 wt%), likely due to the coalescence of smaller particles. The observed trends in peak shifts and crystallite size suggest a trade-off between lattice strain and defect generation with varying Cu doping concentrations.
The changes in lattice parameters confirm the effective incorporation of Cu ions into the BiVO lattice, thereby modifying its electronic and structural properties, which are crucial for optimizing photocatalytic performance.
The functional groups and lattice vibrations of the synthesized bare and Cu-doped BiVO samples were analyzed using Fourier transform infrared (FTIR) spectroscopy, as shown in Fig. 3.
The FTIR spectra revealed characteristic vibrational bands corresponding to BiVO with specific attention to the V-O bond vibrations within the VO tetrahedral structure. Peaks were observed in the range of 750-900 cm, consistent with the stretching modes of V-O bonds. The wavenumber shifts in this region were notable as shown in the inset of Fig. 3, with bare BiVO showing a peak at 728.69 cm, while Cu-doped BiVO samples exhibited peaks at 730.56 cm (1 wt% and 3 wt%) and 724.97 cm (5 wt%). These shifts are attributed to distortions in the BiVO lattice caused by the substitution of Cu ions for Bi or V ions. The ionic radius mismatch between Cu (0.73 Å) and Bi (1.03 Å) induces lattice strain and creates oxygen vacancies, altering the vibrational environment and leading to the observed peak shifts. Additionally, broader and more intense peaks in the Cu-doped samples suggest the formation of oxygen vacancies and changes in bond strength due to the doping process. The presence of oxygen vacancies enhances the local electronic environment, potentially improving charge separation and photocatalytic activity.
The morphology and surface characteristics of the synthesized bare and Cu-doped BiVO were examined using scanning electron microscopy (SEM). As depicted in Fig. 4a, bare BiVO displayed irregularly shaped particles with relatively large sizes and a rough surface morphology, ranging from near-spherical to polyhedral structures. The particles exhibited a loosely packed arrangement with limited porosity. In comparison, the Cu-doped BiVO as shown in Fig. 4b-d showed significant changes in morphology. The particle size decreased progressively with Cu doping, particularly in the 1 wt% Cu-doped BiVO, which exhibited the smallest and most uniformly distributed particles. This reduction in size is attributed to Cu-induced lattice distortion, which inhibits particle aggregation during synthesis. Although BET and BJH pore size distribution analyses are absent in this study, analogous hydrothermally synthesized Cu-doped BiVO systems have been reported to demonstrate increased mesoporosity and surface roughness, attributable to lattice distortion and reduced particle aggregation. These morphological alterations are claimed to enhance the accessibility of reactive sites, thereby correlating with the improved photocatalytic performance observed in this study.
Enhanced porosity likely increases the surface area, which is advantageous for catalytic applications requiring efficient interfacial interactions.
The incorporation of Cu atoms within the BiVO lattice was visually evident, particularly in the 1 wt% Cu-doped sample (Fig. 4b), where the Cu distribution appeared homogeneous across the catalyst surface. Higher Cu doping levels (3 wt% and 5 wt%) resulted in observable clustering, which could be attributed to excess Cu species forming separate phases or aggregates, potentially reducing photocatalytic efficiency as shown in Fig. 4c and d, respectively. Overall, the SEM analysis highlights that Cu doping significantly impacts particle size, morphology, and surface structure. These morphological changes contribute to an increased active surface area and improved photocatalytic performance, particularly for the 1 wt% Cu-doped BiVO, which demonstrates an optimal balance between porosity, particle size, and structural integrity. The incorporation of Cu into the BiVO lattice is supported by XRD peak shifts (Fig. 2) and FTIR band changes (Fig. 3), indicating lattice contraction and modifications in the local bonding environment. The SEM images reflect morphological changes associated with Cu doping but do not provide direct evidence of atomic-level incorporation. The observed highly porous surface morphology in the prepared samples aligns with crystallite size calculations derived from XRD analysis and previous studies on Cu-doped BiVO, which reported suppressed particle growth and enhanced size uniformity. These findings are consistent with prior high-resolution transmission electron microscopy (HRTEM) investigations of Cu-doped BiVO, which confirmed dopant-induced structural modifications.
The band gap energies (E) were estimated via Tauc plots, as shown in Fig. 5. The E for bare BiVO was calculated to be 2.88 eV, consistent with its known optical characteristics. Cu doping significantly influenced the band gap energies. The 1 wt% Cu-doped BiVO showed the lowest band gap of 2.24 eV, representing a substantial narrowing compared to the bare sample. This reduction enhances visible light absorption, facilitating more effective electron excitation and increasing the generation of electron-hole pairs, which are critical for photocatalytic activity. The observed narrowing of the band gap is attributed to the introduction of intermediate energy levels by Cu ions, which modify the electronic structure of BiVO. The smaller band gap facilitates electron excitation, leading to more effective generation of electron-hole pairs for photocatalysis as the ability to absorb light is enhanced due to the broaden region of electromagnetic wave that could excite the electron. Interestingly, as the Cu doping concentration increased to 3 wt% and 5 wt%, the band gap energy increased slightly to 2.55 eV and 2.44 eV, respectively. This trend suggests that excessive doping introduces structural distortions and defect states, which may partially hinder light absorption. Similar trends have been observed in other studies where excessive dopant concentrations lead to the aggregation of dopant species, altering the electronic band structure and reducing photocatalytic performance. The band gap narrowing observed at optimal doping levels (1 wt%) enhances light utilization across a broader spectrum, making the material more efficient under low-intensity UV light.
This anomaly can be explained by the interplay between defect-related and structural effects. At lower Cu concentrations, substitutional Cu ions introduce shallow defect states and oxygen vacancies, which effectively narrow the band gap and improve visible light absorption. However, at higher Cu loadings, excessive doping may lead to local lattice distortions or dopant clustering, creating deep-level defect states that trap carriers without contributing to photocatalytically active transitions. These competing effects result in a non-monotonic variation in band gap with increasing Cu content.
While a more comprehensive evaluation of band gap energies via diffuse reflectance spectroscopy (DRS) utilizing the Kubelka-Munk function was precluded by instrumental constraints, the observed band gap narrowing in our current study demonstrates consistency with prior DRS-based investigations of Cu-doped BiVO, which reported reductions to 2.2-2.4 eV dependent on Cu concentration. Furthermore, despite the absence of direct evidence from photoluminescence (PL), photocurrent response, or electrochemical impedance spectroscopy (EIS) within this study, the observed band gap narrowing, crystallite size reduction, and enhanced photocatalytic activity are consistent with prior reports indicating that Cu doping in BiVO significantly enhances charge separation and transfer characteristics. These correlations suggest that Cu incorporation enhances both the electronic and surface properties of BiVO, thereby contributing to the improved photocatalytic performance observed in this investigation.
The photocatalytic activity of bare and Cu-doped BiVO samples was evaluated using MB degradation , as shown in Fig. 6a. A preliminary adsorption test was conducted by stirring the catalyst-MB solution in the dark for 30 min to achieve adsorption-desorption equilibrium. The results indicate that adsorption performance was relatively low for all samples, with the highest adsorption observed for the 3 wt% Cu-doped BiVO at approximately 8%. This suggests that the degradation efficiency of the catalyst under UV light is primarily due to photocatalytic activity rather than physical adsorption of MB. Control experiments revealed that MB photolysis in the absence of a catalyst was negligible, with no significant degradation observed under UV light over 180 min. This confirms the essential role of the catalyst in facilitating the photodegradation of MB. Among the samples, the 1 wt% Cu-doped BiVO achieved the highest MB degradation efficiency, exceeding 85% after 180 min. The bare BiVO, by comparison, degraded only 8% of MB under identical conditions. The enhanced performance of the 1 wt% Cu-doped BiVO is attributed to its optimized band gap energy (2.24 eV), improved light absorption, and effective charge separation, which facilitate the generation of ROS. As the Cu doping concentration increased to 3 wt% and 5 wt%, the degradation efficiency declined, likely due to excessive dopant-induced defects and clustering, which hinder light absorption and electron-hole pair separation. The degradation kinetics were analyzed using the pseudo-first-order (PFO) model, as shown in Fig. 6b. The calculated rate constant (k) for the 1 wt% Cu-doped BiVO was 0.00306 min, representing a 8.8-fold increase compared to the bare BiVO, which had a rate constant of 0.000346 min. This dramatic improvement highlights the critical role of Cu doping in enhancing photocatalytic performance by improving charge carrier dynamics and promoting the generation of ROS. For higher Cu doping concentrations, the rate constants were 0.000734 min (3 wt%) and 0.00119 min (5 wt%), corresponding to 2.1-fold and 3.44-fold increases, respectively, relative to the bare BiVO. These findings emphasize that while Cu doping is essential for enhancing photocatalytic activity, excessive doping can introduce detrimental effects, such as defect-induced recombination and reduced light absorption. Similar pattern was observed in which the rate constant initially increased and subsequently decreased for the degradation of Rhodamine B using Cu-doped BiVO.
The initial pH of the solution plays a key role in photocatalytic processes, as it influences the degradation of dye molecules by affecting the surface electrical charge properties of the catalyst. The pH value also affects the dissociation ability of compounds, the charge distribution on the catalyst surface, and the oxidation potential of the catalyst's VB. The influence of pH on the photocatalytic degradation of MB using 1 wt% Cu-doped BiVO was investigated. . Fig. 7a shows the degradation efficiencies at three distinct pH levels: acidic (pH 4.7), near-neutral (pH 6.25), and basic (pH 9.7). The results reveal that basic conditions (pH 9.7) yielded the highest degradation efficiency, achieving 62% MB degradation within 180 min. In comparison, the near-neutral condition (pH 6.25) and acidic condition (pH 4.7) resulted in efficiencies of 43% and 13%, respectively. The superior performance under basic conditions is attributed to the favorable surface charge properties of the catalyst and the enhanced generation of ·OH. At pH values above the isoelectric point of BiVO (approximately pH 3.4), the catalyst surface becomes negatively charged, facilitating the adsorption of positively charged MB molecules and promoting ROS generation. The presence of these radicals is enhanced in alkaline environments, leading to more effective breakdown of MB molecules. Conversely, under acidic conditions, the catalyst surface is predominantly positively charged, leading to electrostatic repulsion between the catalyst and MB molecules. Additionally, the excess protons (H) in acidic media can scavenge ·OH, reducing their availability for MB degradation. The degradation kinetics at different pH levels were analyzed using the PFO model, as shown in Fig. 7b. The calculated rate constant (k) was highest under basic conditions (k = 0.00764 min), followed by near-neutral (k = 0.00264 min) and acidic conditions (k = 0.000613 min). The rate constant under basic conditions exhibited a 12.5-fold increase compared to acidic conditions, demonstrating the significant impact of pH on photocatalytic performance.
In this study, three different catalyst loadings of 1 wt% Cu-doped BiVO were investigated: 50 mg, 100 mg, and 200 mg in 100 mL of MB solution. The results presented in Fig. 8a indicate a direct correlation between the amount of catalyst used and the efficiency of the photocatalytic degradation process. Increasing the catalyst loading from 50 to 200 mg led to a significant improvement in degradation efficiency. At 50 mg, MB degradation efficiency was approximately 38%, which increased to 62% and 85% for 100 mg and 200 mg, respectively, after 180 min. The enhanced performance with higher catalyst loading is attributed to the increased availability of active sites, which facilitates greater generation of ROS and accelerates the photocatalytic degradation process. However, further increases in catalyst loading beyond 200 mg may lead to diminishing returns due to excessive turbidity in the reaction mixture. However, beyond a certain threshold, further increases can reduce efficiency due to increased turbidity, which scatters light and diminishes its effectiveness. Although further increases in catalyst loading beyond 200 mg were not tested in this study, previous reports indicate that excessive catalyst concentrations can lead to increased turbidity and particle agglomeration, which scatter light and reduce effective photon penetration, ultimately diminishing photocatalytic efficiency. The degradation kinetics were analyzed using the PFO model, as shown in Fig. 8b. The rate constant (k) increased with catalyst loading, with values of 0.00524 min for 50 mg, 0.00757 min for 100 mg, and 0.00741 min for 200 mg. The results indicate that increasing the catalyst loading from 50 to 200 mg resulted in a 1.4-fold increase in the rate constant. This trend highlights the direct relationship between catalyst loading and the rate of photocatalytic reactions.
The impact of initial MB concentration on photocatalytic degradation efficiency was evaluated using 1 wt% Cu-doped BiVO under low-intensity UV light (13 W). Initial MB concentrations of 5 mg/L, 10 mg/L, and 20 mg/L were tested, with the results presented in Fig. 9a. The degradation efficiency decreased as the initial MB concentration increased, highlighting the inverse relationship between MB concentration and photocatalytic performance. At 5 mg/L, the photocatalytic efficiency reached approximately 85% within 180 min. However, at 10 mg/L and 20 mg/L, the efficiencies dropped to 79% and 17%, respectively. The reduced performance at higher concentration (20 mg/L) is attributed to the increased number of MB molecules competing for the available ROS and active sites on the catalyst surface. Additionally, higher concentrations result in greater light absorption by the MB itself, limiting the photons available to excite the catalyst and reducing the generation of electron-hole pairs. It is noted that the apparent adsorption of MB in the dark for the 10 mg/L solution was slightly higher than the other concentrations, which may reflect minor experimental variability in catalyst dispersion or adsorption kinetics during the equilibrium stage. This does not significantly impact the overall trend, as photocatalytic degradation under UV irradiation remains the dominant mechanism for MB removal across all concentrations tested.
The degradation kinetics for varying initial MB concentrations were analyzed using the PFO model, as shown in Fig. 9b. The calculated rate constants (k) were 0.01194 min for 5 mg/L, 0.00684 min for 10 mg/L, and 0.00104 min for 20 mg/L. These results indicate a 2.7-fold decrease in the rate constant when the MB concentration increased from 5 to 20 mg/L. This trend confirms that higher MB concentrations impede the photocatalytic process by limiting the availability of active sites and reducing the penetration of light into the solution. The observed decline in photocatalytic efficiency and reaction rates with increasing MB concentration is consistent with previous studies.
Radical trapping experiments were conducted to identify the primary ROS responsible for the photocatalytic degradation of MB. Methanol, ascorbic acid (AA), ethylenediaminetetraacetic acid (EDTA), and tert-butanol (Tert-OH) were employed as scavengers for ·OH, ·O, and h, respectively. The results are summarized in Fig. 10. The addition of methanol significantly suppressed MB degradation, confirming that ·OH play a crucial role in the photocatalytic process. Similarly, AA, a scavenger for ·O, caused a substantial reduction in photocatalytic activity, indicating that ·O is also a key reactive species. The introduction of EDTA, which acts as an h scavenger, moderately inhibited the degradation efficiency, suggesting that h contribute to the reaction but are less significant than ·OH and ·O. The moderate suppression observed with Tert-OH further highlights the importance of ·OH in the degradation process. Interestingly, methanol showed stronger inhibition compared to Tert-OH, which can be attributed to its dual role as both a ·OH scavenger and a h scavenger, making it more effective in quenching the reactive species involved in the photocatalytic mechanism. The results of the trapping experiments confirm that ·OH and ·O are the primary ROS responsible for MB degradation, with ·OH being slightly more dominant.
The stability and reusability of the 1 wt% Cu-doped BiVO catalyst were assessed through three consecutive photocatalytic degradation cycles as shown in Fig. 11. Following each cycle, the photocatalysts underwent a washing process with deionized water and ethanol, subsequently being dried at 80 °C prior to subsequent usage. In the first cycle, the MB degradation efficiency reached 85.6%. However, a slight decline was observed in subsequent cycles, with efficiencies of 81.8% and 78.9% recorded for the second and third cycles, respectively. This corresponds to a total reduction of 6.7% in photocatalytic performance over three cycles, indicating excellent stability and reusability of the catalyst. The gradual decline in efficiency can be attributed to the adsorption of degradation byproducts on the catalyst's active sites, a phenomenon known as catalytic poisoning. These byproducts may block the active sites and hinder the catalyst's ability to facilitate photocatalytic reactions. Chemical characterization of the recycled catalyst, as shown in Fig. S2, revealed minor changes in the FTIR spectra, including a slight shift in the V-O vibrational peak from 730.56 cm to 728.69 cm, which can be attributed to the adsorption of organic residues. While XPS analysis before and after photocatalytic use could not be performed in this study due to instrumentation constraints, the excellent reusability performance (> 92% retained activity over three cycles) suggests structural and chemical stability of the catalyst.
A possible photocatalytic degradation mechanism for MB using 1 wt% Cu-doped BiVO under low-intensity UV light irradiation can be proposed, as shown in Fig. 12.
The proposed photocatalytic mechanism involves several interconnected steps: (1) Cu doping reduces the BiVO band gap from 2.88 to 2.24 eV, enhancing light absorption and promoting electron excitation from the VB to the CB; (2) Cu ions act as electron traps, facilitating charge separation and suppressing recombination; (3) photogenerated electrons react with dissolved O to produce ·O radicals, while holes oxidize water or OH to form ·OH radicals; (4) these ROS attack MB molecules, leading to stepwise demethylation, aromatic ring cleavage, and mineralization. The dominance of ·OH and ·O species is corroborated by radical scavenging experiments (Fig. 10), which showed substantial suppression of MB degradation upon the addition of methanol and ascorbic acid.
Similar findings have been reported in other doped ternary oxide systems. For instance, Mg-doped ZnSnO exhibited a substantial band gap reduction (3.82-3.74 eV) and a significant increase in surface area, with oxygen vacancies playing a critical role in enhancing ROS generation and pollutant degradation efficiency. Likewise, Ba-alloyed ZnSnO demonstrated that dopant incorporation induces compressive strain and creates additional defect states, leading to improved charge transfer and higher ·OH radical production. These observations align with our radical scavenger results (Fig. 10), confirming that ·OH radicals are the dominant reactive species in Cu-doped BiVO photocatalysis and highlighting the crucial role of defect engineering in boosting photocatalytic efficiency.
Although LC-MS or HPLC analysis could not be conducted in this study to experimentally confirm intermediates, the proposed pathway aligns with findings from previous photocatalytic studies where MB degradation intermediates, such as azure A/B/C, thionine, and demethylated aromatic fragments, were identified using LC-MS and HPLC analyses. Herein, the proposed degradation pathway of MB likely involves multiple oxidative and reductive processes mediated by ROS such as ·OH and ·O as previously reported. These processes initiate bond cleavage in the MB molecule, leading to the formation of intermediate organic compounds, which subsequently degrade into inorganic end-products such as CO, HO, Cl, SO, and NO. The degradation process begins with the cleavage of the N-CH bond, as it has one of the lowest bond dissociation energies. The methyl groups (-CH) are oxidized to formaldehyde (HCHO) or formic acid (HCOOH). Concurrently, oxidation reactions lead to the conversion of sulfur-containing groups in MB, such as Cl-S groups, into sulfoxide (S=O) and sulfone species, destabilizing the central aromatic structure of the dye molecule. For example, ·OH radicals attack the heteroaromatic ring and sulfur groups, resulting in the opening of the central aromatic structure and the formation of smaller organic compounds. Similar studies have identified key intermediates such as azure A, azure B, azure C, and thionine during MB degradation. These intermediates confirm the stepwise cleavage of bonds in the MB molecule, leading to simpler fragments. The heteroaromatic rings undergo successive oxidations and bond cleavages. The C-S bonds and remaining C-N bonds are progressively broken, resulting in the dissociation of the aromatic rings into simpler structures. Sulfur-containing intermediates are further oxidized to sulfate ions (SO), while nitrogen-containing intermediates oxidize to nitrate ions (NO) and ammonium (NH). As the degradation progresses, the remaining organic intermediates are oxidized into smaller volatile molecules, including CO and HO. Simultaneously, mineral ions such as Cl, SO, and NO are released into the solution. Although no specific experiments were conducted in this study to confirm these intermediates, the proposed pathway aligns with findings from similar photocatalytic studies.
Previous literature on BiVO-based photocatalysts has demonstrated significant advancements in dye degradation under various conditions as shown in Table 2. For example, Fe-loaded BiVO achieved an impressive 81% degradation of MB within 30 min under visible light, while rGO/BiVO BiPO exhibited 94.4% degradation efficiency at pH 9 under similar light conditions. Other studies reported modifications such as Mo-doping, which enabled 88.57% degradation of MB within 120 min under visible light. These findings highlight the effectiveness of doping strategies and structural modifications in enhancing the photocatalytic performance of BiVO. In comparison, the current study achieved 85% MB degradation within 180 min using 1 wt% Cu-doped BiVO under a low-intensity UV lamp (13 W). This result is particularly notable as it was accomplished with significantly lower energy requirements compared to studies utilizing high-energy light sources. Additionally, the hydrothermal synthesis method employed here is both simple and cost-effective, offering scalability without sacrificing performance. The reduced band gap energy (2.24 eV) and improved charge separation achieved through Cu doping further highlight the advancements in photocatalytic efficiency introduced by this study. Beyond photocatalytic performance, this work distinguishes itself by demonstrating the dual functionality of Cu-doped BiVO. While most prior studies focus solely on pollutant degradation, the antibacterial activity of Cu-doped BiVO against S. aureus in dark conditions provides an added advantage. This dual functionality addresses both organic pollutant degradation and microbial contamination, making it a versatile material for environmental remediation. In summary, the findings of this study not only surpass the degradation efficiency of many previously reported BiVO-based photocatalysts under low-energy conditions but also introduce novel functionalities and operational efficiencies. The dual-purpose capability and scalable synthesis approach position Cu-doped BiVO as a promising candidate for practical applications in wastewater treatment and environmental remediation.
The economic viability of synthesizing Cu-doped BiVO photocatalysts for practical applications was evaluated by analyzing the costs associated with raw materials, synthesis methods, and potential scalability. The synthesis of 1 wt% Cu-doped BiVO was evaluated for cost efficiency, yielding approximately 3.5 g of the catalyst at a total production cost of RM 26.48, which corresponds to RM 7.57 per gram. The primary chemicals used in synthesizing Cu-doped BiVO, including Bi(NO)·HO, NHVO, and CuSO·5HO, are relatively accessible and moderately priced. The doping process involves minimal amounts of Cu precursor, typically ranging from 1 to 5% by weight percentage. The synthesis process demonstrated a competitive cost structure compared to other photocatalysts. For instance: BiVO-based composites involving complex heterojunctions or co-catalysts often exceed RM 10.00 per gram due to additional material and processing requirements. Advanced catalysts such as BiVO-rGO hybrids require graphene oxide precursors, significantly increasing material costs to over RM 15 per gram. The hydrothermal method employed for synthesizing Cu-doped BiVO is energy-intensive but offers high yields and superior material properties, such as controlled crystallite size and phase purity. The operational costs associated with maintaining a reaction temperature of 180 °C for 15 h are a significant factor. The operational energy requirement for photocatalytic applications is notably low due to the catalyst's ability to function under low-intensity UV light. Using a low-power (13 W) UV lamp significantly reduces electricity costs, making this catalyst ideal for low-energy, environmentally friendly applications. Additionally, the demonstrated stability and reusability of the Cu-doped BiVO catalyst, with minimal loss of activity over three cycles, further enhance its cost-effectiveness by reducing the need for frequent material replacement. Table 3 depicts the estimated cost associated with the synthesis of the 3% Cu-doped BiVO. To further enhance the cost-efficiency of the synthesis process, several strategies can be explored. Bulk procurement of raw materials, such as Bi(NO)·5HO and NHVO, could significantly reduce material expenses due to economies of scale. Optimizing energy consumption by refining the synthesis process, for instance, by employing lower-temperature calcination or shorter reaction times, may decrease production costs without compromising the quality of the catalyst. Additionally, investigating renewable or waste-derived precursors for raw materials could provide a sustainable and cost-effective alternative, aligning with the growing emphasis on green chemistry. These approaches could further improve the economic viability of Cu-doped BiVO for large-scale environmental remediation applications.
To assess the economic robustness of synthesizing 1 wt% Cu-doped BiVO, a Monte Carlo simulation was performed using MATLAB R2023a. The simulation incorporated ± 10% variability in key cost inputs, including electricity tariff (RM 0.24/kWh), chemical costs, and equipment depreciation, based on market fluctuations. A total of 10,000 iterations were run to evaluate how these uncertainties affect the estimated cost per gram of catalyst. The resulting probability distribution shown in Fig. S3 follows a near-normal shape, indicating that the cost estimation model is statistically stable. The mean cost per gram was RM 7.59, with a standard deviation of RM 0.42. The 95% confidence interval (CI) ranged from RM 6.85 to RM 8.43, suggesting that the cost remains within economically viable margins even when input costs fluctuate moderately.
The antibacterial performance of the 1 wt% Cu-doped BiVO against E. coli and S. aureus was evaluated in dark conditions with results compared to a control (95% ethanol) and positive controls (chloramphenicol and streptomycin) as shown in Fig. 13a and b, respectively. The 1 wt% Cu-doped BiVO exhibited moderate antibacterial activity, with inhibition zones of 7.5 ± 0.79 mm for E. coli and 12.6 ± 0.40 mm for S. aureus, which were lower than those produced by the positive controls (chloramphenicol: 17.8 ± 3.90 mm for E. coli and 30.8 ± 3.30 mm for S. aureus; streptomycin: 20.5 ± 1.20 mm for E. coli and 39.9 ± 1.70 mm for S. aureus) as shown in Table 4.
Notably, the ethanol control showed a slightly larger inhibition zone against E. coli (9.5 ± 1.50 mm) than 1 wt% Cu-doped BiVO, while the photocatalyst had a marginally better effect against S. aureus (12.6 ± 0.40 mm vs. 10.5 ± 0.50 mm). In previous studies of polydopamine-rGO/BiVO, antibacterial test performed under the presence of light will significantly increase its ability to inhibit the growth of bacteria zone. S. aureus, has a thicker peptidoglycan layer but lacks the outer membrane present in Gram-negative bacteria like E. coli. This structural difference makes S. aureus more susceptible to the penetration and action of nanoparticles. Additionally, the mechanisms by which nanoparticles exert their antibacterial effects, such as oxidative stress induction and metal ion release, are more effective against Gram-positive bacteria. Doping also accounts for the effectiveness of the antibacterial property's possess by the catalyst, where previous study revealed that Al-doped BiVO performs better than bare BiVO.
It is important to note that the antibacterial tests in this study were conducted in dark conditions, thereby isolating the effects of Cu doping and associated structural defects without the contribution of photocatalytically generated ROS under light irradiation. The improved antibacterial activity can therefore be attributed to the presence of Cu-induced oxygen vacancies and lattice distortions, which are known to enhance the interaction between bacterial membranes and the catalyst surface. Defect engineering has been shown to modulate the electronic structure of oxide semiconductors by creating localized states that can act as shallow traps, improving surface reactivity. Similar observations were made previously, where defect-rich catalysts exhibited superior antibacterial activity through increased ROS generation and improved surface interactions. Although ROS formation under visible light was not examined here, these findings provide a strong mechanistic context for the enhanced antibacterial performance of Cu-doped BiVO.