Facile Synthesis of Unique Bismuth Vanadate Nano-Knitted Hollow Cage and its Application in Environmental Remediation

1 Introduction

Nowadays, ecological pollution is a roaring issue. Consequently, in the field of modern chemistry, the mineralisation of organic pollutants by photocatalytic compounds has attained huge interest. In order to make excellent use of solar energy, synthesis of competent visible-light photocatalysts has attained international recognition [1], [2], [3], [4], [5], [6], [7], [8]. Nanomaterials are very important for photocatalytic uses because they possess unique structures with increased surface area [9]. The nanomaterials show interesting physical and chemical properties because of their size, shape, orientation, and dimensionality. Therefore, they are considered to be used in various applications [10].

In the current times, bismuth vanadate (BiVO₄) has appeared as the material of interest for various scientific investigations. A dazzling yellow semiconductor pigment that is nonhazardous and possesses small bandgap to permit direct photoactivation in visible light. Nevertheless, in the past, BiVO₄ has mainly been subjugated for photocatalysis application [11], [12], [13]. The marked photocatalytic property of BiVO₄ has mainly been attributed to its crystal phase [14], [15]. Mostly, BiVO₄ exists in three forms, monoclinic sheelite, tetragonal zircon, and tetragonal sheelite [14]. In general, the monoclinic has superior photoefficiency than other tetragonal phases. Besides, the surface structure of BiVO₄ has a significant contribution to its photocatalytic action, as the photocatalytic reactions, in general, occur on the surface [16]. It has also been described earlier that monoclinic BiVO₄ has advanced photocatalytic properties than the tetragonal zircon and tetragonal sheelite phases [17]. So far, various hierarchical BiVO₄ structures have been reported [18], [19]. The synthesis of different shaped nano-sized photocatalysts is one of the key factors for high photoefficiency. Previously, various reports for the synthesis of BiVO₄ by different procedures such as solid state [20], hydrothermal process [21], combustion [22], spray pyrolysis [23], precipitation [24], and sonochemical approach [25] have been reported.

In this investigation, we account the production of hierarchical BiVO₄ nano-knitted hollow cages using an easy hydrothermal method. The synthesized samples were characterized by scanning electron microscopy (SEM), energy-dispersive x-ray spectrometer (EDX), x-ray diffraction (XRD), Fourier transform infrared (FT-IR), and Raman spectrometry. The photocatalytic action of BiVO₄ nanocages was assessed using methylene blue (MB) dye under visible-light irradiation.

2 Experimental

Bismuth vanadate nanocages were prepared using hydrothermal method. Ammonium metavanadate (1.0 mmol, purity 99.5 %; Kanto Chemical, Tokyo, Japan) was added in ammonia solution (28–30 %, Samchun Chemicals, Seoul, Korea) (20 mL) and stirred for 20 min. Bi(NO₃)₃ ⋅ 5H₂O (98 %, Samchun Chemicals, Seoul, Korea) (1.0 mmol) was separately liquefied in a mixture of ethanol (20 %, Samchun Chemicals, Seoul, Korea) and acetic acid (99.7 %, Samchun Chemicals, Seoul, Korea) in the ratio of 1:1.5, with stirring until transparent solution was obtained. Both the solutions (ammonium metavanadate and bismuth nitrate) were slowly mixed to get an orange–yellow homogeneous solution. Hydrothermal treatment at 120 °C for 20 h in an autoclave (Ilshin Autoclave Co., Seoul, Korea), without pH adjustment of the resulting solution, provided an orange precipitate. The precipitate was carefully washed using distilled water and dehydrated at 80 °C. Here, ammonia has been used for dissolution of ammonium metavanadate; it works as a capping molecule and also as a precipitating agent [26]. The role of ammonia was to increase the pH and induce the precipitation of BiVO₄ powders from a nitric acid solution containing bismuth(III) nitrate and ammonium metavanadate.

The synthesized BiVO₄ sample was also characterized by XRD (Cu Kα, λ = 1.540 Å; Rigaku, Tokyo, Japan) pattern to explain the crystalline phase. The morphological characterisation of the powders was carried out by SEM (JSM6700; JEOL, Tokyo, Japan), and the element composition of the sample was evaluated by EDX. The bandgap of the sample was measured by UV-Vis diffuse reflectance spectra (UV-DRS, Shimadzu, Kyoto, Japan).

The photocatalytic degradation of MB by BiVO₄ nanocages was performed in a Pyrex glass reactor having 400 W halide lamp with cutoff filter (λ ≥ 420 nm). The thermal consequences were avoided with the help of a fan and cooling water. In order to check the mineralisation of MB at room temperature in air, sample (0.01 g) was added into reactor holding 10 mL of MB (10 ppm) aqueous solution. The mixture was constantly stimulated for 30 min to attain adsorption–desorption equilibrium. At regular intervals, equal amount of fractions from the suspension has been collected, centrifuged and filtered, and finally analyzed. The MB degradation was evaluated using UV-Vis spectrophotometer. These BiVO₄ nanocages have been reprocessed for successive recycling experiments without any further purification.

Figure 1:

X-ray diffraction pattern of synthesized BiVO₄ nano-knitted hollow cages.

Figure 2:

Scanning electron microscopy micrographs of synthesized BiVO₄ nano-knitted hollow cages at (a) 20, (b) 50, (c) 100, and (d) 150 K magnifications.

3 Results and Discussion

From the XRD spectra of BiVO₄ sample (Fig. 1), it is comprehensible that the obtained diffraction peaks can be assigned to pristine monoclinic stage of BiVO₄ with (JCPDS Card No. 14-0688) [27], and no added adulteration can be detected. It has been reported in the literature that BiVO₄ is an excellent photocatalyst in visible-light irradiation [27]. From SEM image (Fig. 2), the synthesized BiVO₄ sample looks like homogeneous hollow cage structure where the cage is composed of aggregates of small nanobeads. The nanobeads have average length of 100 nm and width of 50 nm. Nonetheless, solitary hollow cage structures have been observed; therefore, it has been concluded as the key product. Usually, the photocatalysts with hollow structure show higher specific surface area that is suitable for the improvement of photocatalytic efficiency. In general, high surface area is considered as a vital feature that significantly increases photocatalytic degradation, owing to the fact that increased surface provides more surface for the adsorption of organic molecules and as a result facilitates the photocatalytic reaction [28].

Figure 3:

(a) Selected area for EDX, (b) energy-dispersive x-ray spectrometer spectrum of synthesized BiVO₄ nano-knitted hollow cages.

Figure 4:

Infrared spectrum of synthesized BiVO₄ nano-knitted hollow cages.

Figure 5:

Raman spectrum of synthesized BiVO₄ nano-knitted hollow cages.

Figure 6:

UV-Vis DRS spectra of synthesized BiVO₄ nano-knitted hollow cages. Plot of (αE_photon)² vs. E_photon; for the calculation of bandgap energy (inset).

The EDX of BiVO₄ sample (Fig. 3) indicates that the presence of bismuth, vanadium, and oxygen exists. Nevertheless, aluminium signal has also been observed, which has been attributed due to the SEM analysis carried out on aluminium foil. The atomic quotient of bismuth and vanadium in the as-prepared BiVO₄ was approximately found to be 1.09:1 and is in precise composition with the stoichiometric proportion of reported BiVO₄ [29]. Figure 4 shows the IR spectra of the synthesized sample. The broadband around 600–800 cm⁻¹ owing to V–O stretching was observed. This peak was in agreement with previous literature reports [11]. The synthesized sample was primarily distinguished by a large and prominent IR band near 730 cm⁻¹ with peaklets at 836, 666, and 640 cm⁻¹. The broad absorptions at low frequency (such as 729 cm⁻¹) are attributed to VO₄³⁻ [27].

Figure 7:

(a) Photocatalytic activity of synthesized BiVO₄ nano-knitted hollow cages and P25 for MB dye degradation. (b) Recycled experiments for the degradation of MB under visible light.

In the Raman spectrum of BiVO₄ (Fig. 5), band at 832 cm⁻¹ attributed to the stretching of the V–O bond. The presence of V–O bond at 827 cm⁻¹ specifies the increase in V–O bond length, which further indicates the improvement in crystalline nature [23], [30]. Figure 6 demonstrates the UV-DRS of synthesized BiVO₄ nanocages, showing prominent absorption of light in the UV and visible region. The absorption frame has been found sharp, signifying the electron transition between valence and conduction bands [31]. Such characteristic absorption peaks indicate the formation of monoclinic BiVO₄, as already authenticated with the outcome XRD spectra. The bandgap energy of the BiVO₄ nanocages calculated from the plot (insets of Fig. 4) was found to be 2.44 eV. Furthermore, in the present study, the BiVO₄ nanocages synthesized by the hydrothermal process demonstrated very significant photocatalytic activity under visible-light irradiation (Fig. 7). The concentration (C/C₀) of MB linearly declined by increase with exposure time for BiVO₄ nanocages. As observed in Figure 7a, BiVO₄ exhibited improved photocatalytic behaviour when compared with available TiO₂ (P25) sample. The mineralisation efficiency of P25 and BiVO₄ after 3 h for MB was about 11 % and 93 %, respectively. The superior photocatalytic action of BiVO₄ can be credited to the enhanced absorption property and large surface area nanocage morphology of the sample. The photostability of the synthesized sample was checked by conducting recycled experiment, as synthesized BiVO₄ was recycled five times in the same photodegradation condition. Figure 7b shows the photodegradation efficiency of BiVO₄ in the five reuse cycles. It was observed that the degradation efficiency of MB by BiVO₄ remains nearly the same even after five cycles. Therefore, we can say that BiVO₄ could be used as a stable and efficient photocatalyst for dye degradation under visible light.

4 Conclusion

Bismuth vanadate nano-knitted hollow cages have been successfully synthesized by hydrothermal method at low temperature of 120 °C. The formation of BiVO₄ sample was confirmed by XRD, EDX, FT-IR, and Raman. The synthesized sample was found to have a bandgap of 2.44 eV. Bismuth vanadate nano-knitted hollow cages showed admirable photocatalytic efficiency for MB dye removal under visible-light illumination.