Development and Materials Characterization of Hydrothermally Grown Niobium-Doped BiVO₄ for Ciprofloxacin and Methylene Blue Degradation

INTRODUCTION

Semiconductor-based photocatalysis has garnered considerable attention for environmental remediation, particularly in the degradation of persistent organic pollutants in aqueous media 1,2. Among various visible-light-responsive materials, bismuth vanadate (BiVO₄) has emerged as a promising photocatalyst due to its appropriate band gap (~2.4 eV), chemical stability, and non-toxic nature 3,4. However, pristine BiVO₄ suffers from inherent drawbacks, including limited charge carrier mobility, rapid recombination of photoinduced electron-hole pairs, and relatively low surface area, all of which constrain its overall photocatalytic efficiency 5,7,6.

To overcome these limitations, several modification strategies have been investigated, such as heterojunction construction, surface functionalization, and elemental doping 8,9. Among these, doping with transition metals has proven to be an effective method for enhancing the photocatalytic activity of BiVO₄ by altering its electronic structure and facilitating improved charge separation 10. In particular, niobium (Nb) has attracted attention as a dopant due to its similar ionic radius to Bi³⁺ and its capacity to serve as an electron donor when substituted into the BiVO₄ lattice 11,12. Prior studies suggest that Nb incorporation can reduce charge recombination, modulate band edge positions, and introduce beneficial defect states 11,12. Nevertheless, the influence of Nb doping on the morphology and surface chemistry of BiVO₄—key factors governing photocatalytic performance—remains insufficiently explored, especially in the context of pollutant-specific degradation behavior 13.

Furthermore, much of the existing literature emphasizes photocatalytic performance without establishing direct correlations between activity and the underlying physicochemical properties 14,15. A comprehensive understanding of how Nb doping simultaneously affects the crystal structure, surface morphology, optical absorption characteristics, and charge carrier dynamics is essential for the rational design of efficient BiVO₄-based photocatalysts. In addition, the role of surface texturing and its effect on pollutant adsorption, particularly for species with varying chemical functionalities, warrants further investigation.

In this work, Nb-doped BiVO₄ photocatalysts were synthesized via a hydrothermal route with varying Nb concentrations (X = 0, 2, 4, and 6 mol%). The structural, morphological, and optical properties of the samples were systematically characterized. Photocatalytic activities were evaluated under UV irradiation using ciprofloxacin (CIP), a widely used antibiotic, and methylene blue (MB), a model cationic dye. By analyzing degradation kinetics in relation to material characteristics, this study aims to elucidate the dual contributions of electronic modulation and surface texturing induced by Nb doping, and their respective impacts on pollutant-specific photocatalytic efficiency.

MATERIALS AND METHODS

Preparation of Nb-Doped BiVO₄ Nanostructures

Undoped and Nb-doped BiVO₄ samples were synthesized via a hydrothermal method. Bismuth(III) nitrate pentahydrate (2.9 g in 50 mL DI water) and ammonium metavanadate (0.70 g in 30 mL DI water, gently heated to ~60 °C) were initially combined under constant stirring to yield a homogeneous Bi–V precursor solution. Ammonium niobate(V) oxalate hydrate was subsequently introduced into this mixture to achieve nominal Nb doping levels of 2%, 4%, and 6% (i.e., 0.089 g, 0.181 g, and 0.272 g, respectively). The resulting suspension was stirred for an additional 30 minutes to ensure full dissolution and uniform mixing. The prepared reaction mixture was transferred into a Teflon-lined stainless-steel autoclave, sealed, and subjected to hydrothermal treatment at 150 °C for 24 hours. After naturally cooling to room temperature, the solid products were recovered via vacuum filtration, thoroughly rinsed with deionized water, and dried in an oven at 80 °C for 2 hours. The dried powders were then calcined at 500 °C for 2 hours to enhance crystallinity and phase purity, as well as to remove residual organic species from the Nb precursor. The resulting samples are hereafter denoted as NbX-BiVO₄, where X represents the molar percentage of Nb incorporated during synthesis (i.e., Nb0-BiVO₄, Nb2-BiVO₄, Nb4-BiVO₄, and Nb6-BiVO₄).

Table 1. Elemental composition of NbX-BiVO₄ (X = 0, 2, 4, 6) determined by FESEM-EDS, reported as wt% and at% for O, Bi, V, and Nb.

Element	Nb0-BiVO4		Nb2-BiVO4		Nb4-BiVO4		Nb6-BiVO4
	wt%	at%	wt%	at%	wt%	at%	wt%	at%
Oxygen (O)	6.37	37.06	6.01	32.3	6.19	34.47	6.62	35.7
Bismuth (Bi)	78.25	34.84	63.74	26.24	66.18	28.24	62.18	25.68
Vanadium (V)	15.38	28.10	17.61	29.75	13.63	23.85	12.60	21.34
Niobium (Nb)	—	—	12.64	11.71	14.00	13.44	18.60	17.28

Photodegradation Investigation

Photocatalytic activity was assessed in batch experiments (100 mL volume) using a catalyst loading of 50 mg (0.5 g/L). Initial pollutant concentrations were 10 ppm for ciprofloxacin (CIP) and 5 ppm for methylene blue (MB). The suspensions were stirred in the dark for 60 minutes to establish adsorption–desorption equilibrium, followed by UV irradiation using four 10 W Philips TUV lamps (253.7 nm wavelength) under continuous stirring at ambient temperature. At 30-minute intervals, approximately 3 mL aliquots were withdrawn, filtered through syringe filters to remove particulates, and analyzed via UV-Vis spectrophotometry (Shimadzu UV-1200). Pollutant concentrations were determined from calibration curves using characteristic absorption wavelengths of 271 nm for CIP and 663 nm for MB, respectively. Results were expressed as C/C₀ and used to evaluate degradation kinetics, including pseudo-first-order rate constant calculations where applicable.

RESULTS AND DISCUSSION

Figure 1 presents the X-ray diffraction (XRD) patterns of NbX-BiVO₄, illustrating the crystallographic features and phase composition of the synthesized materials. All samples display characteristic diffraction peaks corresponding to monoclinic scheelite BiVO₄, in agreement with the standard reference pattern (PDF #14-0688) 16, confirming this phase as the dominant component (Figure 1a). The most prominent peaks appear at 2θ ≈ 18.7°, 28.9°, 30.5°, 34.5°, and 47.3°, which correspond to the (101), (112), (004), (200), and (204) planes, respectively 17.

Notably, several minor unidentified reflections are evident, particularly in the Nb-doped samples, as emphasized in the magnified region between 2θ = 18° and 36° (Figure 1b). These additional peaks suggest the presence of secondary phases or structural inhomogeneities not associated with the ideal BiVO₄ lattice 18. Their emergence, along with subtle variations in peak intensities and broadening, implies that Nb incorporation induces crystallographic disorder or results in incomplete phase development. These findings indicate that while the employed synthesis conditions successfully produce monoclinic BiVO₄ as the primary phase, they do not fully suppress impurity formation, particularly at elevated doping levels.

Figure 2 shows FESEM micrographs of NbX-BiVO₄ acquired at magnifications of 10,000× and 50,000×, illustrating the morphological evolution as a function of Nb content. At lower magnification (Figure 2a), all samples appear as highly agglomerated clusters composed of submicron primary grains. The undoped sample (Nb0-BiVO₄) exhibits intergrown, plate-like structures with comparatively smooth surfaces 19. In contrast, Nb-doped samples (Nb2-BiVO₄, Nb4-BiVO₄, and Nb6-BiVO₄) display increasingly rough and granular morphologies.

At higher magnification (Figure 2b), surface textures are observed in greater detail. Nb4-BiVO₄ features uniformly distributed hemispherical nodules, while Nb6-BiVO₄ exhibits dense, fine-scale surface asperities. The dimensions of these features range from tens to several hundreds of nanometers, whereas the underlying grain sizes remain approximately 200–500 nm. These findings suggest that Nb incorporation enhances surface roughness and nanoscale texturing, which may contribute to improved interfacial reactivity 20.

Table 1 presents the elemental composition of the synthesized samples as determined by FESEM-EDS, reported in both weight percent (wt%) and atomic percent (at%). As expected, Nb is not detected in the undoped sample (Nb0-BiVO₄), while its concentration increases with the doping level: 11.71 at% in Nb2-BiVO₄, 13.44 at% in Nb4-BiVO₄, and 17.28 at% in Nb6-BiVO₄. These values are consistent with the nominal Nb contents introduced during synthesis. Simultaneously, the atomic percentages of Bi and V decrease with increasing Nb content, suggesting partial substitution within the BiVO₄ lattice or possible surface enrichment of Nb. Oxygen content remains relatively constant (32–36 at%), though slightly below the stoichiometric value for BiVO₄—a common occurrence in EDS due to underestimation of light elements. The Bi:V atomic ratio exhibits modest variation across the samples, reflecting the semi-quantitative limitations of EDS analysis. Overall, these trends confirm successful Nb incorporation while also indicating potential structural or compositional heterogeneities induced during the doping process.

Figure 3 illustrates the optical and photoluminescence (PL) properties of NbX-BiVO₄, highlighting the influence of Nb doping on light absorption behavior and charge carrier dynamics. The UV–Vis absorption spectra (Figure 3a) reveal strong absorption in the UV region for all samples, with visible-light absorption tails extending from the absorption edge. Compared to the undoped sample (Nb0-BiVO₄), Nb2-BiVO₄ and Nb6-BiVO₄ exhibit red-shifted absorption onsets and elevated baseline absorbance, whereas Nb4-BiVO₄ shows a slightly blue-shifted edge. These spectral variations suggest that Nb incorporation modifies the electronic structure, potentially by introducing localized states or perturbing band-edge transitions.

Tauc plots constructed from the absorption data (Figure 3b), assuming direct allowed transitions, yield estimated band gap energies of approximately 3.67, 3.50, 3.72, and 3.38 eV for Nb0-, Nb2-, Nb4-, and Nb6-BiVO₄, respectively 21,22. This non-monotonic trend indicates that low to moderate Nb doping initially narrows the band gap (Nb2), followed by a slight widening (Nb4), and then a pronounced reduction at higher doping levels (Nb6). These results confirm that Nb incorporation influences the optical band structure, enabling partial tuning of the absorption threshold.

The PL spectra (Figure 3c) further support the impact of Nb doping on charge carrier behavior. All samples exhibit broad emission centered near 560 nm, attributed to radiative recombination of photoexcited charge carriers. Notably, PL intensity decreases progressively with increasing Nb content, with Nb6-BiVO₄ showing the most pronounced quenching. This emission reduction implies enhanced charge separation or the presence of additional non-radiative recombination pathways in doped samples, both of which are favorable for photocatalytic activity 23,24. Overall, the optical and PL data demonstrate that Nb doping significantly affects both electronic transitions and recombination dynamics in BiVO₄, offering mechanistic insight into the observed enhancements in photocatalytic performance.

Figure 4 presents time-resolved UV–Vis spectra illustrating the photocatalytic degradation of ciprofloxacin (CIP; panels a–d) and methylene blue (MB; panels e–h) over NbX-BiVO₄ under UV irradiation. For CIP degradation (0–120 min), the characteristic UV absorption bands progressively diminish over time. The spectral attenuation is most significant for Nb2-BiVO₄ and Nb6-BiVO₄, indicating enhanced photocatalytic performance relative to the undoped sample (Nb0-BiVO₄). Nb4-BiVO₄ exhibits an intermediate response. Similarly, MB degradation (0–180 min) is monitored by the decline in intensity of the Q-band centered near 665–670 nm. Substantial reductions in absorbance are observed for Nb2-BiVO₄ and Nb6-BiVO₄, approaching near-complete bleaching, while Nb0-BiVO₄ demonstrates the lowest activity. Again, Nb4-BiVO₄ shows a moderate response. These findings suggest that Nb doping, particularly at 2% and 6%, significantly enhances photocatalytic efficiency. This improvement aligns with previously discussed modifications in electronic structure and surface morphology, underscoring the synergistic contributions of Nb incorporation to photocatalytic activity.

Figure 5 presents the kinetic analysis of photocatalytic degradation of ciprofloxacin (CIP; panels a–c) and methylene blue (MB; panels d–f) using NbX-BiVO₄ photocatalysts. For CIP degradation (Figure 5 versus time (Figure 5b) exhibit linear behavior, confirming that the reaction follows pseudo–first-order kinetics under the employed conditions. The calculated mass-normalized rate constants (Figure 5c) indicate that Nb6-BiVOln(C₀/C) versus time (Figure [FIGREF:5]b) exhibit linear behavior, confirming that the reaction follows pseudo–first-order kinetics under the employed conditions. The calculated mass-normalized rate constants (Figure [FIGREF:5]c) indicate that Nb6-BiVO₄ displays the lowest activity. These trends align with optical data (Figure 3), where Nb6 doping resulted in the most pronounced band gap narrowing and photoluminescence (PL) quenching, indicative of enhanced charge separation.₄ and Nb0-BiVO₄, which show nearly equivalent performance. Nb2-BiVO₄ displays the lowest activity. These trends align with optical data (Figure [FIGREF:3]), where Nb6 doping resulted in the most pronounced band gap narrowing and photoluminescence (PL) quenching, indicative of enhanced charge separation.

For MB degradation (Figure 5d), a substantial concentration decrease occurs during the dark stage, suggesting stronger initial adsorption compared to CIP. The pseudo–first-order model remains applicable (Figure 5e); however, the rate constants (Figure 5f) differ significantly. Nb0-BiVO₄ exhibits the highest degradation rate, followed by Nb4-BiVO₄, while both Nb2-BiVO₄ and Nb6-BiVO₄ show reduced performance. This inverse trend highlights the pollutant-specific nature of photocatalytic behavior, where adsorption affinity and surface interactions may outweigh bulk electronic enhancements.

These findings reveal a clear pollutant-dependent performance pattern. For CIP, Nb6-BiVO₄ exhibits the highest pseudo–first-order rate constant, consistent with enhanced surface morphology and electronic properties. FESEM analysis (Figure 2 features increased nanoscale texture and surface roughness, which enhance the active surface area. Additionally, UV–Vis and PL results (Figure 3) confirm a narrowed band gap and suppressed recombination, promoting improved charge separation. These synergistic effects contribute to superior photocatalytic activity for CIP degradation. In contrast, MB degradation favors the undoped sample (Nb0-BiVO₄ features increased nanoscale texture and surface roughness, which enhance the active surface area. Additionally, UV–Vis and PL results (Figure [FIGREF:3]) confirm a narrowed band gap and suppressed recombination, promoting improved charge separation. These synergistic effects contribute to superior photocatalytic activity for CIP degradation. In contrast, MB degradation favors the undoped sample (Nb0-BiVO₄), which demonstrates the highest rate constant and strongest adsorption during the dark stage. This suggests that surface affinity plays a more dominant role in MB removal. The reduced performance of doped samples may result from altered surface chemistry, such as decreased hydroxyl group density or changes in surface charge, which diminish dye adsorption despite electronic improvements. Overall, while all samples retain the monoclinic BiVO₄ phase, Nb doping introduces significant morphological and electronic modifications that influence photocatalytic outcomes. These results emphasize the importance of aligning photocatalyst design with the chemical nature of the target pollutant to achieve optimal performance.

CONCLUSION

Nb-doped BiVO₄ photocatalysts were successfully synthesized via a hydrothermal route, and their structural, morphological, optical, and photocatalytic properties were systematically investigated. X-ray diffraction (XRD) analysis confirmed that the monoclinic BiVO₄ phase was retained across all samples, although minor unidentified reflections suggested partial crystallographic disorder or secondary phase formation, particularly at higher doping levels. Optical characterizations revealed that Nb doping influenced the electronic structure, as evidenced by shifts in the absorption edge and non-monotonic variations in estimated band gap energies. Photoluminescence (PL) spectra further indicated enhanced charge carrier separation, with emission quenching observed in Nb-doped samples. These findings confirm the dual role of Nb in modifying both band structure and recombination dynamics. Photocatalytic tests demonstrated pollutant-specific activity profiles. For ciprofloxacin (CIP), Nb6-BiVO₄ exhibited the highest degradation rate, attributable to its increased surface area and efficient charge separation. Nb4-BiVO₄ and Nb0-BiVO₄ showed comparable performance, while Nb2-BiVO₄ displayed the lowest activity. In contrast, methylene blue (MB) degradation was more strongly influenced by surface adsorption properties. The undoped BiVO₄ outperformed all Nb-doped variants, indicating that surface affinity rather than electronic enhancement governed MB removal efficiency. These results highlight that Nb doping enhances photocatalytic activity through synergistic morphological and electronic effects, yet the extent of improvement is highly dependent on the physicochemical characteristics of the target pollutant.