Bird’s nest-like Nb₂O₅: preparation, characterization and multifunctional photocatalytic application

ABSTRACT

A bird’s nest-like structure of Nb₂O₅ (Nb₂O₅/PVP) was produced using a hydrothermal method that involved the introduction of PVP. This unique structure provides a micro-reactor for the photocatalytic reaction, which improves the reaction efficiency. The test results demonstrated that the surface properties of Nb₂O₅ were altered by the introduction of PVP. This change caused an enhancement of the water adsorption capacity and ∙OH production yield. Another aspect, the energy band structure was altered by the introduction of PVP. Thus, the photogenerated charge recombination rate was reduced, the photogenerated charge mobility was improved, the reduction of photogenerated electrons was enhanced, and the ∙O₂- production yield was increased. The PVP introduction broadened the light absorption range of Nb₂O₅ from the UV to the visible light region, increasing the light utilization efficiency. The degradation experiments demonstrated that Nb₂O₅/PVP had significantly better catalytic activity than pristine Nb₂O₅. Meanwhile, the photocatalytic degradation efficiency of TC using Nb₂O₅/PVP reached 90.4% in 25 min.

KEYWORDS:

• Nb₂O₅
• photocatalysis
• tetracycline
• hydrothermal method

Introduction

Semiconductor photocatalytic technology is a highly effective method for converting solar energy into chemical energy, making it an ideal choice for individuals and organizations committed to environmental sustainability and energy conservation [1,2]. By utilizing this technology, we can make significant strides towards a more sustainable future. This technology has been widely recognized for its potential applications in wastewater treatment, carbon dioxide conversion, organic compound synthesis, photocatalytic hydrogen production, and bacterial and virus elimination [3–9]. The study of semiconductor photocatalytic technology is of utmost importance.

Photocatalytic reactions involve several key steps, including light absorption, photogenerated charge generation, photogenerated charge complexation, photogenerated charge migration, and active species generation [7,10]. The efficiency of visible light utilization is ensured by a suitable forbidden band width, which results in the production of strong reduction of photogenerated electrons or strong oxidation of photogenerated holes [11]. These processes are well-established and have been extensively studied, leading to a high level of confidence in their effectiveness. Additionally, it is important to acknowledge the potential for further optimization and improvement in this field, and to approach these challenges with a diplomatic and collaborative mindset. The efficiency of the photocatalytic reaction is determined by the activity of the photogenerated charges, the degree of photogenerated charge migration and complexation, and the surface structure of the photocatalytic material [11,12]. These processes are interrelated and critically dependent on the internal (electronic) structure of the photocatalytic semiconductor material, which strongly influences the degree of photogenerated charge migration and complexation, and consequently the yield of active species. The photocatalytic reaction efficiency is determined by the activity of the photogenerated charges, the degree of photogenerated charge migration and complexation, and the surface structure of the photocatalytic material [13,14]. Studying any of these processes in semiconductor photocatalytic materials is highly significant.

It is also very interesting to modify the existing semiconductor photocatalytic materials, especially to prepare photocatalytic materials with different morphologies and unexpectedly find that different morphologies determine different physicochemical properties. Nb₂O₅ is an n-type wide bandgap semiconductor similar to TiO₂ [15,16]. Similarly, Nb₂O₅ is regarded as a potential photocatalytic material because of its benefits such as low toxicity, excellent thermodynamic stability, and relatively high photocatalytic activity [16,17]. However, the photocatalytic activity of pristine Nb₂O₅ has been severely limited by its shortcomings, including a low light absorption intensity, particularly in the visible light regime, a poor crystallinity, and an extremely fast recombination rate of photoinduced carriers. So far, various schemes such as noble metal loading [18,19], metal ion doping [17], and heterojunction assembly [15,20] have been investigated to modify Nb₂O₅ resulting in a significant boost in photocatalytic performance. The study about shape-shifting to optimize the visible light response of Nb₂O₅ and to improve its properties has been little explored. As a result, the systematic study of shape-shifting to expand the absorption range and improve the photocatalytic ability of Nb₂O₅ is an appealing work.

In this work, a bird’s nest-like structure of Nb₂O₅ (Nb₂O₅/PVP) was produced using a hydrothermal method that involved the introduction of PVP into the preparation process. The resultants were characterized using FTIR, XRD, SEM, and TEM to confirm the phase composition, elemental composition, and bird’s nest morphology. This unique structure provides a micro-reactor for the photocatalytic reaction, which improves the reaction efficiency. The FTIR, XPS, and EPR tests demonstrated that the surface properties of Nb₂O₅ were altered by the introduction of PVP. This change caused an enhancement of the water adsorption capacity and the photocatalytic hydroxyl radical production yield. The results of the UV-DRS, PL, and EIS tests indicate that the energy band structure was altered by the PVP introduction. Additionally, the photogenerated charge fusion rate was reduced, the photogenerated charge mobility was improved, the reduction of photogenerated electrons was enhanced, and the photocatalytic yield of superoxide radicals was increased. The PVP introduction o extended the light absorption range of Nb₂O₅ from UV to visible light region, increasing light utilization efficiency. Photocatalytic performance was explored using methyl orange (MO), rhodamine 6B (RhB), and tetracycline (TC) as targets. Degradation experiments demonstrated that Nb₂O₅/PVP had significantly better catalytic activity than pristine Nb₂O₅.

Experimental procedure

Preparation of Nest-like Nb₂O₅

To prepare a homogeneous solution (referred to as solution A), 1 g of niobium oxalate was dissolved in 25 mL of distilled water and stirred at 60°C for 10 min. Then, another homogeneous solution (marked as solution B) was formed by adding 0.9 g of polyvinylpyrrolidone (PVP) to 10 mL of distilled water and stirring for 5 min. The B solution was pipetted drop-by-drop into the A solution while stirring to form a mixed solution. After stirring for 10 min, the mixed solution was confidently poured into a 50 mL hydrothermal reactor. The hydrothermal reactor was transferred to a baking oven and the reaction was conducted at 180°C for 12 h. The solution was confidently filtered after being cooled to room temperature. The final product was gained by drying the filter residue at 80°C for 24 h and grinding it.

Characterization method

The samples’ phase and surface chemical bonds were determined using X-ray diffractometry (TD-3500, Dandong Tongda, China) and Fourier transform infrared spectroscopy (Perkin Elmer L160000A, Massachusetts, U.S.A.). The samples’ morphology was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM were performed with state-of-the-art equipment, including a FEI Gemini300 microscope from ZEISS in Oberkochen, Germany, and a FEI Tecnai G2 F20 microscope in Hillsboro, OR, U.S.A., respectively. The chemical compositions of the samples were measured using an INVIA Raman microprobe (Renishaw Instruments, Wotton-under-Edge, UK) and X-ray photoelectron spectra (XPS, ESCALAB 250Xi, TFS, MA, U.S.A.) with an excitation energy of 1486.6 eV of Al Kα. An ultraviolet-visible diffuse reflectance spectrophotometer (UV-Vis DRS, TU1901, Beijing, China) was utilized to investigate the optical properties of the samples. The samples’ specific surface area and pore structure were analysed using physisorption apparatus (BET, ASAP2460, Micromeritics, U.S.A.) after degassing at 200°C for 12 h. The photoluminescence (PL) spectra were recorded using the Full-functional Steady/Transient Fluorescence Spectrometer (FLS980, Edinburgh, UK). The JES FA200 (JEOL, Japan) spectrometer was used to detect the Electron Spin Resonance (ESR) signals of radical species captured by 5,5-dimethyl-1-pyrroline N-oxide (DMPO). Electrochemical Impedance Spectroscopy (EIS) tests were performed using a three-electrode system and a PP211 electrochemical workstation (Zahner). The working electrode was made of catalyst-modified glassy carbon (3 mm in diameter), the counter electrode was platinum foil, and the reference electrode was Ag/AgCl (saturated KCl). Electrochemical impedance spectroscopy (EIS) measurements were confidently taken at open circuit potential using an AC voltage with an amplitude of 5 mV over a frequency range of 10⁻¹ to 10⁵ Hz. The chosen frequency range was carefully selected to ensure accurate and reliable results.

Photocatalytic application

The photocatalytic activity of the samples was evaluated by degrading macromolecular organic matter (methyl orange (MO), rhodamine 6B (RhB), tetracycline (TC)) under simulated ultraviolet light irradiation (300 W Xenon lamp with visible cut-off filter) or simulated visible-light irradiation (300 W Xenon lamp with ultraviolet cut-off filter). The optical power density was measured at the beginning of the experiment using a light metre (900 mW·cm⁻² for simulated ultraviolet light and 1000 mW·cm⁻² for simulated visible-light). The liquid level to light source distance was maintained at a constant 20 cm throughout the test procedures. Initially, 100 mg of photocatalyst was added to a 100 mL solution containing 10 mg⸳L⁻¹ RhB (MO, TC). The mixture was then fully stirred in the dark for 30 min to achieve adsorption equilibrium. The UV-Vis spectrophotometer (UV-Vis DRS, TU1901, Beijing, China) was used to measure the absorption of a series of reaction solutions (MO 463 nm, RhB 552 nm, and TC 356 nm) at specific irradiation time intervals. The photodegradation rate was calculated as C/C₀, where C₀ is the initial concentration of the reaction solution and C is the concentration of the reaction solution after at different reaction times.

Results and discussion

Phase and structure

As shown in Figure 1a, there are Nb-O bond and Nb-O-Nb bond stretching vibration peaks at 520 cm⁻¹ to 820 cm⁻¹ with coupling [16], while the characteristic absorption peaks between 1500 cm⁻¹ and 2500 cm⁻¹ correspond to the bending vibration of adsorbed H₂O [21,22], and the characteristic absorption peaks between 3400 cm⁻¹ and 3500 cm⁻¹ belong to the hydroxyl group of the sample adsorbed H₂O (−OH) stretching vibration [21,23]. These phenomena were observed in both samples. On the one hand, the presence of Nb₂O₅ is confirmed; on the other hand, the addition of PVP did not change the basic chemical bonding of Nb₂O₅ during the preparation. The peak intensity of adsorbed water is larger for Nb₂O₅/PVP than for pristine Nb₂O₅, indicating that Nb₂O₅/PVP adsorbs water more easily, which is favourable for the photocatalytic reaction.

Figure 1. FTIR spectra (a) and XRD patterns (b) of samples. The high-resolution XPS of Nb and O elements in Nb₂O₅/PVP sample.

The diffraction peaks of Nb₂O₅ and Nb₂O₅/PVP were observed at 2Ɵ = 23.022°, 33.626°, 46.483°, and 55.114°, corresponding to crystal faces (101), (107), (200), and (208) [16,20] (Figure 1b). The peak intensity and sharpness of the characteristic peaks were similar, which confirms that the introduction of PVP did not affect the crystallinity or introduce any new impurities.

In Figure 1(c, d), the high-resolution XPS spectra of Nb and O elements are displayed. These results demonstrate a clear understanding of the chemical composition and properties of the sample. The binding energies of Nb 3d_5/2 and Nb 3d_3/2 were measured at 206.64 eV and 209.41 eV, respectively [15,24]. Additionally, the binding energy of lattice oxygen and adsorbed oxygen were observed at 529.69 eV and 531.18 eV, respectively [25–27].

In summary, it can be confidently stated that the FTIR, XRD, and XPS tests indicate the successful acquisition of high-purity niobium oxide.

The SEM image of Nb₂O₅ in (Figure 2a) displays irregular morphology and severe agglomeration. However, Nb₂O₅/PVP exhibits a regular birdcage-like structure with uniformly distributed internal pores, as shown in (Figure 2b,c). These observations are confirmed by the TEM images in (Figure 2d,e). The birdcage-like structure facilitates an increase in the specific surface area of the sample. The lattice spacing was determined through Fourier and inverse Fourier transforms on the HRTEM. The lattice spacing of 0.3863 nm is confidently attributed to the (101) crystal plane of Nb₂O₅ [15,24], providing evidence that the preparation of Nb₂O₅ was successful. Furthermore, it can be diplomatically stated that the introduction of PVP did not alter its intrinsic crystal structure.

Figure 2. SEM images of Nb₂O₅ (a) and Nb₂O₅/PVP (b, c). TEM (d, e) and HRTEM (f) images of Nb₂O₅/PVP.

The specific surface area of pristine Nb₂O₅ and Nb₂O₅/PVP was 194.1 m²/g and 200.2 m²/g, respectively (as shown in Figure 3a). The average pore size distribution of pristine Nb₂O₅ and Nb₂O₅/PVP was 8.9847 nm and 6.4221 nm, respectively (as shown in Figure 3b). These results confirm our initial hypothesis. Furthermore, both pure Nb₂O₅ and Nb₂O₅/PVP samples can be classified as mesoporous materials due to their average pore size distribution falling between 2 nm and 50 nm [28,29]. The relationship between p/p₀ and the thickness of the adsorption layer is illustrated in Figure 6(c). The thickness of the adsorbed layer increases as p/p₀ increases, indicating multilayer adsorption in numerous mesopores within the samples [27,28]. This finding is consistent with the deduction made from the pore size distribution. The PVP introduction did not alter the surface structure of Nb₂O₅. This is evidenced by the same multilayer adsorption phenomenon and the Type-II isothermal adsorption curve (as shown in Figure 3a).

Figure 3. The N₂ adsorption-desorption isotherms (a), pore properties (b), variation curve of statistical thickness with p/p₀ (c) of pristine Nb₂O₅ and Nb₂O₅/PVP.

Multifunctional photocatalytic application

The photocatalytic activity of the as-prepared samples under visible light was evaluated using simulated dye wastewater solutions of MO and RhB. It is worth noting that neither MO nor RhB underwent self-degradation during the degradation process. As shwon in Figure 4, under visible light irradiation, Nb₂O₅ and Nb₂O₅/PVP demonstrated degradation efficiencies of 33.6% and 75.0% for MO, respectively. The corresponding reaction kinetics were 0.0797 min⁻¹ and 0.05811 min⁻¹. The rate constant for MO degradation by Nb₂O₅/PVP was found to be 5.94 times higher than that of conventional Nb₂O₅, indicating a faster and more efficient degradation process. This could potentially result in cost savings for future applications. These results demonstrate the superior performance of Nb₂O₅/PVP and its potential for practical use in various applications. Additionally, under visible light irradiation, RhB degradation efficiency was observed to be 94.5% for Nb₂O₅ and 100.0% for Nb₂O₅/PVP after only 10 min, with corresponding reaction kinetics of 0.20435 min⁻¹ and 0.30812 min⁻¹. The rate constant for RhB degradation by Nb₂O₅/PVP was 1.51 times higher than that of conventional Nb₂O₅, indicating faster and more efficient reaction. This suggests that the use of Nb₂O₅/PVP could potentially result in cost savings in application.

Figure 4. The photodegradation reaction of MO/RhB using Nb₂O₅ and Nb₂O₅/PVP and the associated reaction kinetics.

The photocatalytic properties of Nb₂O₅/PVP were validated through the photodegradation of TC antibiotic solutions. As shown in Figure 5, the degradation efficiency of TC over Nb₂O₅/PVP was found to be 90.4% after a 25 min reaction, with a rate constant of 0.0824 min⁻¹. These results demonstrate the high effectiveness of Nb₂O₅/PVP as a photocatalyst for the TC degradation. Among the reported photocatalytic materials for the TC degradation under visible light, Nb₂O₅/PVP exhibits the highest degradation efficiency and fastest reaction rate, as demonstrated in Table 1. Meanwhile, the literatures have shown that TiO₂ is time consuming and has low reaction efficiency in the degradation of organic matter [41–43]. This confirms that the catalytic performance of Nb₂O₅/PVP is superior to TiO₂. These findings suggest that Nb₂O₅/PVP holds great potential as a catalytic material for environmental treatment.

Figure 5. The photodegradation reaction of TC using Nb₂O₅/PVP and the associated reaction kinetics.

Table 1. Comparison of visible-light-driven photocatalysis for TC degradation.

Catalyst	Dosage (g/L)	100 mL TC concentration (mg/L)	Efficiency (%)	Reaction rate/time	Reference
Nb2O5/PVP	0.1	10	90.4	0.0824 min^−1 /25 min	This work
barium silicate	0.05	30 mg/L (50 mL)	93.2	2 h	[30]
CuAl−LDH@g− CN	0.18	30	96.0	90 min	[31]
LaCoO3/ZnO	0.1	30 mg/L (40 mL)	70	0.0179 min^−1 /90 min	[32]
SnWO4/BiMoO6	0.06	20	96	0.031min^−1 /90 min	[33]
HC/BiVO4	0.5	50	86.4	1.1552 L mol^−1 min^−1/120 min	[34]
Pd@Fe3O4/MMT with 0.19 g/mL H2O2	0.55	40	84	0.03231 min^−1/60 min	[35]
CN-CoxMe	0.1	10	82	0.01188min^−1 /120 min	[36]
g-C3N4@N-doped Bi2MoO6	0.03	10 (50 mL)	93.46	0.0.34 min^−1 / 120 min	[37]
BOCH-SOF	0.2	25	99	0.033 min^−1/30 min	[38]
BiOI/BiOBr	0.06	10 (160 mL)	93.03	1.4472 min^−1/90 min	[39]
NiCo–BiVO4	0.15	10 (250 mL)	87.2	0.066 min^−1/120 min	[40]

The study evaluated the reuse performance of Nb₂O₅/PVP in degrading RhB solution under visible light irradiation. The results revealed that Nb₂O₅/PVP has high stability and reusability, as the degradation efficiency of RhB remained almost unchanged after three reuses (Fig. 6). Additionally, complete degradation of RhB was achieved. These findings demonstrate the prepared photocatalytic material exhibits high catalytic stability and good reusability.

Figure 6. The reuse performance.

Enhancement mechanism of photocatalysis

The UV-Vis DRS spectra are shown in Figure 7a,b. The absorption range of Nb₂O₅ is restricted to the UV range because of electron transitions from the valence band, primarily composed of O (2s orbits) and Nb (4s and 4p orbits), to the conduct band, primarily composed of O (2p orbits) and Nb (4d and 5s orbits). Moreover, it is worth noting that Nb₂O₅/PVP also exhibits visible light absorption. The statement suggests that the introduction of PVP broadens the absorption range of Nb₂O₅ from the UV light regime to the visible light regime. The band gap energy (Eg) values of Nb₂O₅ and Nb₂O₅/PVP were calculated to be 2.98 eV and 2.62 eV, respectively, which is consistent with previous deductions. Therefore, the addition of PVP decreases the forbidden bandwidth value and enhances the absorption intensity of Nb₂O₅ in the visible light regime.

Figure 7. The UV-vis DRS spectra of Nb₂O₅ (a) and Nb₂O₅/PVP (b), (Inset: The related band gap energy); PL spectra (c) and EIS spectra (d) of Nb₂O₅ and Nb₂O₅/PVP.

It is widely accepted that photoluminescence (PL) signals arise from the recombination of photogenerated carriers in semiconductor upon exposure to light. The intensity of the PL signals is directly proportional to the recombination rate of the carriers [44]. The recombination rate of photogenerated electrons and holes is directly proportional to PL signals. Therefore, higher PL signals indicate a higher recombination rate, while lower PL signals indicate a lower recombination rate [23,45]. The photoluminescence (PL) signals were generated through a photoexcitation process. Electrons from the valence band were excited to the conduction band, thereby forming excited state electrons in the conduction band and holes in the valence band [26,46]. Some of the unstable excitation electrons relax from the conduction band to the valence band, leading to recombination with photoinduced holes, while releasing energy in the form of fluorescence or phosphorescence. This observation suggests that Nb₂O₅/PVP may have a lower concentration of excited electrons and holes, which could be due to the presence of PVP in the composite material. In the visible light region (Figure 7c), it was observed that the PL signal intensity of Nb₂O₅/PVP was lower than that of pristine Nb₂O₅. It can be confidently stated that in the visible light region, the carrier recombination rate of Nb₂O₅/PVP was lower than that of Nb₂O₅. It is important to note that these results were obtained through careful experimentation and analysis. Furthermore, the carrier separation efficiency of Nb₂O₅/PVP was found to be higher than that of Nb₂O₅.

The catalyst’s charge transport ability is determined by analysing the semicircle in the high-frequency section of the Nyquist curve. A smaller semicircle radius indicates lower impedance and better charge transport capacity [44,46]. It can be observed from (Figure 7d) that the semicircular radius of Nb₂O₅/PVP is smaller in the high-frequency regime than that of bare Nb₂O₅. This indicates that the introduction of PVP improves the charge transport ability of Nb₂O₅, thereby facilitating the separation of photogenerated electrons and holes

Based on the experimental results presented above, it is clear that the catalytic activity of Nb₂O₅/PVP is higher than that of Nb₂O₅.

The study confidently employed DMPO spin trapping and ESR technology to examine active species and their yields, while diplomatically investigating the mechanism underlying the enhancement of photocatalytic activity. These findings provide valuable insights into the photocatalytic process. The ESR signal intensity ratio of the 1:1:1:1:1 quartet characteristic was attributed to the DMPO-·O₂⁻ spin adduct [25,27,45], while the intensity ratio of the 1:2:2:1 quartet characteristic ESR signal was attributed to the DMPO-·OH spin adduct [25,44,47]. The generation of ·O₂⁻ and ·OH occurs via one-step reduction and oxidation reactions between photoinduced electrons and absorbed oxygen, and photoinduced holes and absorbed water, respectively. The ESR spectra demonstrate that all samples are capable of generating ·O₂⁻ and ·OH under ultraviolet visible light. Notably, the gsignal intensity of Nb₂O₅/PVP is stronger than that of Nb₂O₅. These results confidently suggest that Nb₂O₅/PVP is a promising candidate for photocatalytic applications, while Nb₂O₅ also exhibits significant potential. The introduction of PVP in Nb₂O₅/PVP significantly increased the amount of ·OH and ·O₂⁻ by 5.32 and 4.15 times, respectively, compared to Nb₂O₅ alone (as shown in Figure 8 and Table 2). This demonstrates the clear enhancement of water adsorption on the niobium oxide surface and the narrowing of the forbidden band width by PVP, resulting in a substantial increase in the reduction of photogenerated electrons.

Figure 8. DMPO spin-trapping ESR spectra in Nb₂O₅ and Nb₂O₅/PVP in dark or under ultraviolet visible light irradiation: (a) and (b) in aqueous dispersion for DMPO-·OH, (c) and (d) in methanol dispersion for DMPO-·O₂⁻.

Table 2. The intensity value of free radical signal and the related square value from ESR test.

samples	·OH signal intensity value	the square of ·OH signal intensity value	·O2^− signal intensity value	the square of ·O2^− signal intensity value
Nb2O5	354.21	125464.72	137.85	19002.62
Nb2O5/PVP	817.50	668306.25	280.83	78865.49

In summary, the reaction mechanism can be described as follows. When Nb₂O₅/PVP is exposed to light radiation, electrons in the valence bands of O (2s orbits) and Nb (4s and 4p orbits) are excited when the energy of the photons is greater than the bandgap energy, resulting in a jump to the conduction bands of O (2p orbits) and Nb (4d and 5s orbits) and the formation of photogenerated electrons with strong reducing ability (Equation 1(1) $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5/\mathrm{PVP}+\mathrm{h}\mathit{\upsilon }\to \mathrm{photoinduced}{\mathrm{h}}^{+}\mathit{\hspace{0.17em}}+\mathit{\hspace{0.17em}}{\mathrm{e}}^{-}$(1) ) [29,44,46]. After the electrons are excited, they jump and form photogenerated holes in the valence band with strong oxidizing ability (Equation 1(1) $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5/\mathrm{PVP}+\mathrm{h}\mathit{\upsilon }\to \mathrm{photoinduced}{\mathrm{h}}^{+}\mathit{\hspace{0.17em}}+\mathit{\hspace{0.17em}}{\mathrm{e}}^{-}$(1) ) [29,44,46]. Photogenerated electrons and dissolved oxygen undergo a one-electron reduction reaction to produce superoxide radicals (Equation 2(2) ${\mathrm{e}}^{-}+\mathit{\hspace{0.17em}}{\mathrm{O}}_2\mathit{\hspace{0.17em}}\to {{\mathrm{O}}_2\mathit{\hspace{0.17em}}}^{\bullet -\mathit{\hspace{0.17em}}}$(2) ) [21,48]. Photogenerated electrons, dissolved oxygen, and adsorbed water undergo a five-electron reaction to form hydroxyl radicals (Equation 3(3) $5{\mathrm{e}}^{-}+\mathit{\hspace{0.17em}}{\mathrm{O}}_2\mathit{\hspace{0.17em}}+2\mathit{\hspace{0.17em}}{\mathrm{H}}_2\mathit{\hspace{0.17em}}\mathrm{O}+\to 4\bullet \mathrm{O}\mathrm{H}$(3) ) [48]. Photogenerated holes and adsorbed water undergo an oxidation reaction via the one-hole pathway to produce hydroxyl radicals (Equation 4(4) ${\mathrm{h}}^{+}+\mathit{\hspace{0.17em}}{\mathrm{H}}_2\mathit{\hspace{0.17em}}\mathrm{O}\to \bullet \mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}\mathit{\hspace{0.17em}}$(4) ) [21,48]. The generated hydroxyl and superoxide radicals, photogenerated electrons and holes combine to attack the fragile chemical bonds of the organic matter, ultimately breaking down the large organic molecules into small, non-toxic inorganic molecules (Equation 5(5) ${{\mathrm{O}}_2\mathit{\hspace{0.17em}}}^{\bullet -\mathit{\hspace{0.17em}}}/\bullet \mathrm{OH}/{\mathrm{e}}^{-}/{\mathrm{h}}^{+}/+\mathrm{organic}\mathrm{targets}\to \mathrm{intermediate}\mathrm{products}\to \mathrm{C}{\mathrm{O}}_2\mathit{\hspace{0.17em}}+{\mathrm{H}}_2\mathit{\hspace{0.17em}}\mathrm{O},\mathrm{etc}.$(5) ).

(1) $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5/\mathrm{PVP}+\mathrm{h}\mathit{\upsilon }\to \mathrm{photoinduced}{\mathrm{h}}^{+}\mathit{\hspace{0.17em}}+\mathit{\hspace{0.17em}}{\mathrm{e}}^{-}$(1)

(2) ${\mathrm{e}}^{-}+\mathit{\hspace{0.17em}}{\mathrm{O}}_2\mathit{\hspace{0.17em}}\to {{\mathrm{O}}_2\mathit{\hspace{0.17em}}}^{\bullet -\mathit{\hspace{0.17em}}}$(2)

(3) $5{\mathrm{e}}^{-}+\mathit{\hspace{0.17em}}{\mathrm{O}}_2\mathit{\hspace{0.17em}}+2\mathit{\hspace{0.17em}}{\mathrm{H}}_2\mathit{\hspace{0.17em}}\mathrm{O}+\to 4\bullet \mathrm{O}\mathrm{H}$(3)

(4) ${\mathrm{h}}^{+}+\mathit{\hspace{0.17em}}{\mathrm{H}}_2\mathit{\hspace{0.17em}}\mathrm{O}\to \bullet \mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}\mathit{\hspace{0.17em}}$(4)

(5) ${{\mathrm{O}}_2\mathit{\hspace{0.17em}}}^{\bullet -\mathit{\hspace{0.17em}}}/\bullet \mathrm{OH}/{\mathrm{e}}^{-}/{\mathrm{h}}^{+}/+\mathrm{organic}\mathrm{targets}\to \mathrm{intermediate}\mathrm{products}\to \mathrm{C}{\mathrm{O}}_2\mathit{\hspace{0.17em}}+{\mathrm{H}}_2\mathit{\hspace{0.17em}}\mathrm{O},\mathrm{etc}.$(5)

Conclusion and outlooks

A bird’s nest-like structure of Nb₂O₅ (Nb₂O₅/PVP) was synthesized using a hydrothermal method that introduced PVP during preparation. This unique structure acts as a micro-reactor for photocatalytic reactions, improving reaction efficiency. Additionally, PVP serves multiple functions, including altering the surface properties of Nb₂O₅, enhancing water adsorption, and increasing the yield of hydroxyl radicals in photocatalysis. It also modifies the energy band structure, reduces the rate of photogenerated charge fusion, improves photogenerated charge mobility, enhances photogenerated electron reduction, and improves the yield of superoxide radicals in photocatalysis. The addition of PVP expands the light absorption range of Nb₂O₅ from UV light to visible light, increasing light utilization efficiency. Nb₂O₅/PVP achieved photocatalytic degradation of RhB in only 10 min, and tetracycline degradation efficiency reached 90.4% in only 25 min. Therefore, we hope that researchers will widely investigate the properties of Nb₂O₅/PVP and its potential for environmental applications.

Acknowledgments

This work was supported by Chongqing Basic Science Advanced Technology Research Program (CSTB2022NSCQ-MSX1397), Science and Technology Research Program of Chongqing Municipal Education Commission (KJZD-M202301301), and funds for the introduction of talent from Chongqing Preschool Education College.

Disclosure statement

No potential conflict of interest was reported by the author(s).