Enhancing photocatalytic activity by tuning the ratio of hexagonal and orthorhombic phase Nb₂O₅ hollow fibers

Abstract

Cotton was used as template for the fabrication of a biomorphic Nb₂O₅ photocatalyst with hexagonal and orthorhombic structures. The as-prepared Nb₂O₅ was characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), specific surface area and UV-Vis diffused reflectance spectra measurements. The photocatalytic activity of the Nb₂O₅ fibers was evaluated through degradation of methylene blue (MB) in aqueous solution exposed to UV light. SEM results show that biomorphic Nb₂O₅ fibers are both hollow and have porous walls. The architectures allow a more accessible surface for the photocatalytic decomposition of dyes molecules. The photocatalytic activity of physically mixed fibers with 75% H-Nb₂O₅ and 25% O-Nb₂O₅ phases is superior to those of pure H- and O-Nb₂O₅ phases, and other samples with different H- and O-Nb₂O₅ ratios. Such superior photocatalytic activity is mainly due to an optimal balance between the adsorption of MB and the absorption of UV light. Moreover, the large size of the Nb₂O₅ hollow fibers allows them to be easily separated from the solution.

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Introduction

Fujishima and Honda¹ reported on the photoelectrochemical splitting of water on n-TiO₂ electrodes. Subsequently, semiconductor-based photocatalysis has been extensively studied for potential environmental remediation and as a new energy source. In general, the photocatalytic reactions of semiconductors involve the formation of electron–hole pairs derived from interband excitation under light irradiation. Thus, a charge migrates from electron–hole pairs to the adsorbed reactants on the semiconductor surface, resulting in a redox reaction with the surrounding media.² Numerous oxide semiconductors, such as TiO₂,^1,3 ZnO,⁴ Bi₂O₃ (ref. 5) and Ta₂O₅,⁶ have been used as photocatalysts in organic pollutant degradation and water-splitting reactions.

Among oxide semiconductors, niobium oxide (Nb₂O₅) is a promising n-type transition metal oxide⁷ with numerous significant photocatalytic activities, including selective photooxidation of amines,^8,9 photocatalytic hydrogen production¹⁰ and photodegradation of harmful organic contaminants.^11–13 Studies have shown that niobium oxide is a novel photocatalyst that can be used for the degradation of organic pollutants.^11–14 Prado and coworkers¹¹ found that Nb₂O₅ has sufficiently high photocatalytic activity for the decomposition of indigo carmine dye. Zhao et al.¹² reported that Nb₂O₅ nanorod exhibits higher photocatalytic activity than nanospheres for the decomposition of methylene blue. Recently, Qi and coworkers¹⁴ prepared Nb₂O₅ nanofibers with hexagonal (H) and orthorhombic (O) structures by electrospun method and characterized its photocatalytic activity. Results showed that the H-Nb₂O₅ nanofibers exhibit much higher activity than O-Nb₂O₅ during decomposing methyl orange (MO). Doping with metal ions,^15,16 nonmetal ions,^17,18 and mixed metal oxides¹⁹ has been conducted to enhance the photocatalytic activity of Nb₂O₅. The coexistence of anatase/rutile phases in commercial TiO₂ (Degussa P25) contributes to its excellent photocatalytic activity.^2,20 Nb₂O₅ exists in many polymorphic forms, such as H-Nb₂O₅ (hexagonal), O-Nb₂O₅ (orthorhombic) and M-Nb₂O₅ (monoclinic).⁷ However, information is lacking regarding the effect of H- and O-Nb₂O₅ ratio on photocatalytic degradation of organic dyes.

To improve photocatalytic activity, the preparation of the nanometer catalyst² has been intensively studied. Reusability of a nanosized photocatalyst is limited because of the difficulty of separating the photocatalyst from the suspension.²¹ Micrometer hollow fibers have been developed to overcome this drawback.^22,23 Cotton fibers are stable, low cost, abundant, and reproducible natural products that have a hierarchically built anatomy.²² Micrometer hollow fibers prepared using cotton as templates have large surface areas and strong organic adsorption capacities, which are favorable characteristics for photocatalytic reactions.²⁴ In the present study, micrometer Nb₂O₅ hollow fibers were obtained by calcining cotton fibers infiltrated by niobium oxalate solution for the first time. The photocatalytic properties of Nb₂O₅ hollow fibers with different ratios of H- to O-Nb₂O₅ phases were evaluated by photodegradation of methylene blue solution under UV irradiation. The physical mixed-phase Nb₂O₅ hollow fibers comprising a H- and O-Nb₂O₅ ratio of 75/25 w/w are more highly efficient than pure H-Nb₂O₅, O-Nb₂O₅ and other mixing phases.

Experimental

Synthesis

Dried and loose medical absorbent cotton (Fig. S1†) were immersed in niobium oxalate solution (3 wt%, with water as solvent) for 24 h. The cotton fibers were calcined at either 500 °C or 700 °C in air for 6 h to remove cotton templates. Biomorphic Nb₂O₅ fibers with H- and O-Nb₂O₅ phases were obtained and subsequently physically mixed. In the mixing process, the Nb₂O₅ fibers were dispersed in ethanol, ground for 30 min, and dried under 60 °C for 2 h.

Characterization

Phases of the prepared biomorphic Nb₂O₅ fibers were identified through X-ray diffraction (XRD) with Cu Kα radiation (Bruker D8 Advance, Germany). The microstructures of the biomorphic Nb₂O₅ were investigated through scanning electron microscopy (SEM, S-3400, Hitachi, Ltd., Japan). The band gap was derived from the UV-vis diffused reflectance spectra (UV3150, Shimadzu, Japan). The specific surface area of samples were determined according to Brunauer–Emmett–Teller (BET) method on liquid nitrogen adsorption at 77 K by using equipment (ASAP 2010, Micromeritics, USA).

Photocatalysis measurement

The photocatalytic activities of biomorphic Nb₂O₅ fibers were evaluated by degrading 15 mg L⁻¹ methylene blue (MB) aqueous solution under UV light (mercury lamp, 500 W; maximum emission = 365 nm). Prior to UV illumination, the samples (0.2 g) were dispersed in 200 mL MB aqueous solution and magnetically stirred in darkness for 50 min until adsorption/desorption equilibrium was reached. During irradiation, approximately 10 mL of MB degradation solution was continually extracted at 10 min intervals. Retrieved samples were centrifuged and subsequently examined using a UV-vis spectrometer (UVT6, Purkinje General, China). Absorption measurements were taken at 663 nm. The value of C/C₀ was calculated according to the calibration curve of concentration and absorption.²⁵ The photocatalytic activity of physically mixed H-Nb₂O₅/O-Nb₂O₅ phase fibers was evaluated under the abovementioned conditions for comparison.

Results and discussion

XRD patterns of cotton template-derived Nb₂O₅ catalysts calcined at 500 and 700 °C for 6 h are shown in Fig. 1. All the original components of cotton have been removed. As shown in Fig. 1a, the sample calcined at 500 °C was considered as the hexagonal Nb₂O₅ crystal (H-Nb₂O₅). Splitting of the peaks at 2θ = 28.4°, 36.7°, 50° was observed when calcination temperature was increased to 700 °C (Fig. 1b). Such splitting indicates transformation of Nb₂O₅ crystals from H-Nb₂O₅ to O-Nb₂O₅, which is consistent with the previous report.¹³ The calculated lattice constants of H-Nb₂O₅ were a = b = 0.3619 nm, c = 0.3929 nm and those of O-Nb₂O₅ were a = 0.164 nm, b = 2.9337 nm, c = 0.3931 nm. These constants were in good agreement with values form standard cards of H-Nb₂O₅ (JCPDS, PDF#28-0317, a = b = 0.3607 nm, c = 0.3925 nm) and O-Nb₂O₅ (JCPDS, PDF# 27-1003, a = 0.168 nm, b = 2.9312 nm, c = 0.3936 nm.). This result further confirms phase purity of the as-prepared samples. The XRD peaks of samples progressively narrowed and strengthened with the increase in annealing temperature, indicating the growth of the Nb₂O₅ crystallite.²⁶ To elucidate crystallite growth, the crystallite size of the samples was estimated using the Debye–Scherrer equation from line broadening of the (001) diffraction peak.²⁷ Results testified the crystallite growth of the Nb₂O₅ nanoparticles (Table 1).

Fig. 1 XRD patterns of synthesized biomorphic Nb₂O₅ hollow fibers (a) calcined at 500 °C and (b) 700 °C.

Table 1 Structural properties of calcined biomorphic Nb₂O₅ hollow fibers

Sample	Calcination temperature (°C)	Crystallite size (nm)	BET surface area (m^2 g^−1)	Average pore size (nm)
H-Nb2O5	500	18	32.8	14.1
O-Nb2O5	700	35	23.0	19.6

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SEM images of the biomorphic Nb₂O₅ fibers calcined at 500 and 700 °C are shown in Fig. 2. As shown in Fig. 2a and c, Nb₂O₅ has the same morphology as that of the cotton templates in Fig. S1† and retains the original fibrous morphology. The cotton fibers are crucial in the formation of biomorphic Nb₂O₅ hollow fibers. The niobium oxalate solution covers the outer surface of the cotton fibers during the infiltration process. Crystalline biomorphic Nb₂O₅ hollow fibers are obtained after the removal of the cotton templates during the calcination process.^22,23 The length of each biomorphic Nb₂O₅ fiber is approximately 70 μm. Magnified SEM images in Fig. 2b and d demonstrate that biomorphic Nb₂O₅ fibers are hollow with inner diameters in the range of 3 μm to 10 μm. Each hollow fiber has a wall thickness of approximately 500 nm. Pores in the wall are the results of the collapse of cotton and gases released from the decomposition of cotton during the calcination (Fig. 2e and f). This special morphology provides more accessible active sites and easier pathways for mass transferring.²⁸ Clearly, biomorphic Nb₂O₅ hollow fibers with the large dimension could be very easily recovered from the treated suspension by a less energy-consumed filtration process. The characteristics of micrometer Nb₂O₅ hollow fibers make it an ideal photocatalysis.

Fig. 2 SEM images of biomorphic Nb₂O₅ fibers, (a and b) calcined at 500 °C, (c and d) calcined at 700 °C, and (e and f) single biomorphic Nb₂O₅ hollow fibers with porous walls calcined at 700 °C.

The optical absorption properties of the biomorphic Nb₂O₅ fibers were investigated through UV-vis diffused reflectance spectra. As shown in Fig. 3, the absorption cut-off wavelength of the H-Nb₂O₅ and O-Nb₂O₅ samples are approximately 400 nm, suggesting that biomorphic Nb₂O₅ fibers can be used as potential UV light-absorbing materials. Close inspection found that the absorption edge of O-Nb₂O₅ shows a slight red shift, suggesting that the O-Nb₂O₅ could absorb longer wavelength light. Meanwhile, O-Nb₂O₅ exhibits weak UV-light absorption in the wavelength shorter than 330 nm. The difference is possibly due to the quantum-size effect and the effects of the crystalline phase.^29,30 The optical band gap of the as-synthesized biomorphic Nb₂O₅ fibers can be estimated by the Kubelka–Munk function.³¹ The band gaps (Fig. 3 inset) are approximately 3.05 and 3.19 eV for H- and O-Nb₂O₅, respectively, and these values are in agreement with those in the literature.²⁹

Fig. 3 UV-vis diffuse reflectance spectra (the inset is a plot of the modified Kubelka–Munk function vs. energy of exciting light).

The photocatalytic activity of the biomorphic Nb₂O₅ hollow fibers was evaluated by degradation of methylene blue. As shown in Fig. 4, biomorphic O-Nb₂O₅ showed low degradation rate (35%) after 50 min of irradiation, whereas biomorphic H-Nb₂O₅ showed significant activity (about 96%). Similar results of methylene blue decomposition over Nb₂O₅ nanorods under the same conditions are reported.¹² Many factors could contribute to such a notable increase in photocatalytic activity. Chai et al.³² demonstrated that the surface of H-Nb₂O₅ is covered with more Lewis acid sites than that of O-Nb₂O₅. MB is readily immobilized on the Lewis acid sites of H-Nb₂O₅ surface; photoinduced electrons and holes can directly attack these sites.^12,13,33 The surface area of a catalyst greatly affect its activity.²⁴ The larger BET surface area of H-Nb₂O₅ hollow fibers (32.8 m² g⁻¹) than that of O-Nb₂O₅ hollow fibers (23 m² g⁻¹) is also beneficial for photodecomposition of MB. Besides, Qi and coworkers proposed that the stronger oxidation property and the lower recombination rate of the e⁻/h⁺ pairs of H-Nb₂O₅ leads to its higher degradation efficiency.¹⁴ In contrast to the discrete nanoparticles, micrometer Nb₂O₅ hollow fibers could be very easily filtered from the treated solution by a common filter paper. The pore in the wall not only allows more active sites available for photocatalysis but also provides much more pathways for mass transferring during the photocatalytic process.²⁸Therefore, the easily recyclable feature and the high photocatalytic efficiency made micrometer Nb₂O₅ hollow fibers act as low-cost and feasible photocatalyst for the large-scale applications.

Fig. 4 Photocatalytic activities of biomorphic H- and O-Nb₂O₅ hollow fibers.

The overall heterogeneous photocatalysis involves three key steps as follows: (1) adsorption of reactant on the surface, (2) chemical reaction on the catalyst surface under UV irradiation, and (3) desorption of final products on the catalyst surface.³⁴ The adsorption and photodegradation rate of MB on biomorphic Nb₂O₅ hollow fibers with different ratios of H- and O-Nb₂O₅ mixed phases are shown in Fig. 5. The adsorption rate of the sample monotonously increases with decreasing O-Nb₂O₅ content, while the rate of photodegradation shows a maximum value. The biomorphic hollow fiber mixture of 25% O-Nb₂O₅ and 75% H-Nb₂O₅ (Fig. 5 and S2†) exhibits the highest activity (83%) after 20 min irradiation among all the samples. This activity, even higher than that of previously reported Nb₂O₅ nanorods (about 75%),¹² can interpreted by the following synergistic effect. The O-Nb₂O₅ with a narrower optical band gap efficiently absorbs UV light with longer wavelength, and this absorption benefits the formation of the photo-electron and hole pair, as shown in Fig. 3. However, O-Nb₂O₅ is covered with a low number of Lewis acid sites, consequently lowering the absorption and degradation of MB. Although the immobilization of MB molecules on the surface acid sites of the H-Nb₂O₅ is facilitated by the high number of Lewis acid sites, a wide optical gap suppresses the absorption of UV light. However, stronger oxidation property and the lower recombination rate of the e⁻/h⁺ pairs of H-Nb₂O₅ benefits degradation of MO.¹⁴ When excess O-Nb₂O₅ coexists with H-Nb₂O₅, which shields the Lewis acid sites of the H-Nb₂O₅ and reduce the absorption of MB molecules (Fig. 5).^35,36 At the same time, the oxidation ability of the mixture is decreased due to the decrease of H-Nb₂O₅ content. Consequently, the optimal balance of MB adsorption, the use of UV light and the oxide ability of the Nb₂O₅ with different structure results in the significantly enhanced photocatalytic activity of the mixture with optimal ratio of H- to O-Nb₂O₅ (75/25).

Fig. 5 Dependence of adsorption and photodegradation on O/H-Nb₂O₅ value in the suspending solution.

Conclusions

Micrometer bimophic H- and O-Nb₂O₅ hollow fibers with the easily recyclable feature were successfully prepared using cotton fibers as bio-templates. The hollow structure and the porous wall of Nb₂O₅ fibers provide more accessible surface areas, allowing them act as a high-efficiency photocatalyst. Besides, the ratio of phases H- to O-Nb₂O₅ greatly affects the physicochemical properties and photocatalytic activity of the biomorphic Nb₂O₅ hollow fibers by balance the adsorption of MB, the capture of UV light and the oxidation property. The biomorphic Nb₂O₅ hollow fibers comprising a H- and O-Nb₂O₅ ratio of 75/25 w/w show enhanced photocatalytic activity. These findings may lead to further research on the photocatalytic performance of Nb₂O₅ with different structures.

Acknowledgements

This study was supported by the Program for the National Natural Science Foundation of China (Grant no. 51102130) and the Program for Key Laboratory of Inorganic Function Material and Device, Chinese Academy of Sciences (Grant no. KLIFMD-2011-01).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02038c

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