Design, fabrication and characterization of photocatalyst Ni-doped BiVO4 for high effectively degrading dye contaminant

Contaminated environment from textile industries have attracted global concern owing to the traits of organic pollutions with high stability against light and chemicals attacks. How to improve the preparation process of photocatalysts and reduce the cost is a considerable requirement for the wide application of photocatalytic wastewater treatment. The Ni-doped BiVO4 (NBVO) process was improved to a facile and reliable hydrothermal method. Ni with a theoretical mass of 0.75% was added to BiVO4 (BVO), which displayed outstanding photocatalytic activity and stability. Under visible light irradiation, the decolorization rate of 0.75% NBVO to Rhodamine B (RhB) can reach 96% within 75 min, and the removal efficiency can still achieve 86% after four successive cycles. The active ingredient ∙O2 − confirmed from capture experiments played an indispensable role and was hired to explain the possible photocatalytic mechanism. In addition, the enhanced photocatalytic activity of 0.75% NBVO should be ascribed to the increase in specific surface area (beneficial for adsorption) and the decline in impedance (favorable for photocarrier migration). 0.75% NBVO as a highly efficient visible-light-driven photocatalyst has a brighter prospect for wastewater treatment in the years ahead.


Introduction
Water pollution has posed a serious threat to environmental safety and human health [1]. Moreover, the amount of wastewater discharge is increasing with the development of global industrialization [2]. Textile dyes are organic pollutants produced during industrialization, such as methylene blue, methyl orange and RhB [3][4][5][6]. RhB can cause serious food safety, agricultural pollution and health problems because of the high stability against light, temperature, chemicals and microbial attacks [7]. Therefore, the removal of organic contaminant RhB has become increasingly important [8]. Various physical, chemical and biological methods have been employed to handle RhB. However, the poor effects so far have remained due to their respective limitations [9]. The use of solar energy photocatalyst for wastewater removal is a green, environmentally friendly, convenient and safe method to treat RhB [10].
Since Honda first discovered the water decomposition trait of TiO 2 photoelectrodes [11], it has been a hot topic that the application of metal oxide semiconductors in the field of photocatalysis and photoelectron chemistry [12][13][14][15][16][17]. Nevertheless, it is a pity that various defects, including wide band gap, fast recombination of photogenerated electron-hole pairs and low hydroxyl radical (·OH) quantum yield (·OH has been frequently recognized as the major reactant responsible for the photocatalytic oxidization of organic compounds), have been discovered one after another [18,19]. As a result, many researchers have developed a large number of other photocatalysts (typical Bi 2 O 3 , ZnO, CdS and MoS 2 [20][21][22][23]) to attempt to replace TiO 2 . BVO materials have attracted tremendous attention due to their suitable energy band structure, low cost and simple preparation [24][25][26][27][28]. Unfortunately, BVO also have similar problems with TiO 2 [29]. Although a variety of modification techniques, including noble metal deposition and composite semiconductor materials [30], have been adopted to improve the photocatalytic activity, the high cost and complicated process limit the further promotion. However, Ion doping is indeed a convenient and cost-effective strategy to enhance the activity of photocatalysts, especially for transition metal ions due to their high earth abundance and obvious improvement effect [31].
To the best of our knowledge, the research on Ni-doped BiVO 4 (NBVO) removal for RhB has not been reported. Hence, we propose a simpler hydrothermal method with lower temperature to synthesize NBVO for decolorization of RhB. Simple process, green and low cost are more conducive to industrial implementation. Furthermore, the test of photoelectric performance provides powerful evidence for the improvement of NBVO photocatalytic activity. The capture experiments in this work identifies the active group of NBVO photocatalysis. Herein, the photocatalyst NBVO decontaminates organic dye wastewater as a novel attempt. This work provides a highly efficient photocatalyst for the practical application in removal of highly persistent NBVO.

Preparation of NBVO
NBVO was prepared using a facile hydrothermal method. Concretely synthesis, 2.5 mmol NH 4 VO 3 and 2.5 mmol Bi(NO 3 ) 3 ·5H 2 O were dissolved in 2 mol l −1 NaOH solution and 2 mol l −1 HNO 3 solution, respectively, and named as solution A and solution B in turn. After that, solution A was slowly added dropwise to solution B, and magnetically stirred for 60 min. Subsequently, the mixed solution was adjusted to pH=10 by 1 mol l −1 NaOH solution, and then sonicated for 10 min. Finally, the homogeneous mixed solution was poured into a Teflon-lined stainless-steel autoclave at 160°C for 8 h. The synthesized BVO was washed with distilled water and absolute ethanol, and dried at 80°C for 8 h. The obtained product was pure BVO. The key to NBVO preparation was that Ni(NO 3 ) 2 ·6H 2 O with different mass ratios were added to solution B (theoretically generated mass ratio of BVO). Finally, the resulting solution B went through the remaining procedures of BVO preparation to harvest the target product NBVO. The schematic illustration of NBVO photocatalyst synthesis was shown in figure 1.

Photocatalytic tests
The light source used a 300 W xenon lamp (CEL-HXF 300, Beijing CEAULIGHT Co., Ltd, Beijing, China) with a 400 nm cutoff filter to simulate visible light generation. The activity of the prepared photocatalyst was studied by decomposing RhB. In a typical decolorization test, 0.1 g of photocatalyst was put into 100 ml of RhB (10 mg l −1 ) solution. Before the photocatalytic reaction, the system was processed in darkness for 30 min to achieve adsorption equilibrium and homogeneous dispersion. Under light conditions, 5 ml of the test sample was centrifuged every 15 min, and the centrifuged supernatant was taken to determine the absorbance. It was worth noting that all samples were tested 3 times and averaged. Finally, the decolorization rate of RhB was calculated using equation (1) [32]: Where DR represents the decolorization rate of RhB under visible light irradiation, C 0 and C are the initial concentration and the concentration at t min, respectively.

Photoelectrochemical and electrochemical measurements
The photoelectrochemical test was carried out with electrochemical workstation (the CHI660E), and the test was carried out using the traditional three-electrode system. Taking Ag/AgCl as reference electrode and Pt sheet electrode as counter electrode, the fluorine-doped tin oxide (FTO) (working area of 1×1 cm 2 ) of the prepared catalyst was applied as working electrode [33]. The 0.5 mol l −1 Na 2 SO 4 solution was used as the electrolyte, and the visible light source was a 300 W Xe lamp light source with a filter. In this work, all the measured potentials were converted to reversible hydrogen electrodes (RHE) using the Nernst equation, where E RHE is the conversion potential relative to RHE, E Ag/AgCl is the experimentally measured potential versus Ag/AgCl (saturated KCl solution).

Results and discussion
3.1. Characterization on the Structure and Specific Surface Property Figure 2 shown the XRD spectrum of the BVO and NBVO photocatalytic material. The single peak of assynthesized BVO had high intensity and good crystallinity. Besides, all the diffraction peaks of BVO were highly consistent with the monoclinic BVO standard diffraction card (JCPDS No. 14-0688  remained the same as pure BVO, which proved that Ni doping had no effect on the composite sample. It can be seen from figure 2(b) that Ni will cause the diffraction peak to move toward a small angle, which may be caused by Ni ions entering the BVO crystal gap.
The crystallite size of BVO and NBVO was calculated using the Scherrer equation [32]: where, K is a dimensionless shape factor with a typical value of unity; λ is the x-ray wavelength of the employed XRD instrument in nm; β is the line broadening at full width at half maximum intensity (FWHM XRD ) in radian; θ is the Bragg angle in radian. β was calculated using MDI Jade 6 software (the sharpest peaks as seen in figure 2) from the lower range of diffraction angles, i.e., (110), (011), (121), (040), (200) and (002) of BVO and NBVO with the above 6 crystal faces were selected as the representative peaks for this analysis. Finally, the crystallite sizes calculated from different samples were averaged, and the results are shown in table 1. The average grain size of BVO before and after being doped did not change significantly in table 1. At the same time, it was observed that the increase in Ni content did not cause the change in grain size. Figure 3 was the FT-IR spectrum test results for 0.75% NBVO and BVO. 745 cm −1 and 833 cm −1 were the symmetric stretching vibration peak and the asymmetric stretching vibration peak of the V-O bond, respectively. The strong and broad absorption peaks at 1380 cm −1 and 3463 cm −1 may be due to the stretching vibration peaks of water molecules and hydroxyl groups adsorbed on the surface of the photocatalyst [34]. Interestingly, the characteristic diffraction peak of Ni was not found in figure 3, which may be attributed to Ni entering the 0.75% NBV lattice in the form of ions.
XPS was performed to identify the detailed chemical compositions and states of NBVO, and the measurement results were displayed in figure 4. It was confirmed in figure 4(a) that 0.75% NBVO had Bi, V, O, C and Ni elements. The appearance of the carbon signal could be ascribed to the test instrument. The characteristic peak of the binding energy around 855 eV was attributed to the peak of Ni 3+ . The lower intensity of the peak should be due to the low amount of doped Ni 3+ [35]. As shown from figure 4(b), the XPS high-resolution peaks with binding energy at 872.8 eV and 855.7 eV were the signal peaks of Ni 2p 1/2 and Ni 2p 3/2 , respectively. In addition, the XPS points with binding energy at 879.0 eV was assigned to the satellite peak of Ni 2p 1/2 , and the XPS sub-peak with the binding energy at 861.4 eV from the satellite peak of Ni 2p 3/2 . Therefore, the Ni element was present in the NBVO photocatalytic material. Figures 4(c)-(e) indicated the high-resolution XPS spectrums of V 2p, O 1s and Bi 4f, which denoted the attendance of V element in pent-valence state, Bi element in trivalence state and O element in bi-valence state, respectively [36]. It was worth noting that the XPS results were consistent with XRD, proved that BVO was successfully doped with Ni [37].   Figure 5 was the Raman characterization of BVO and 0.75% NBVO. The peak intensity values marked in the figure 5 were all caused by the vibration signal of V-O. Moreover, BVO and NBVO could correspond to each other well. This result directed that the incorporation of Ni ions did not change the structure of BVO. At the same time, the Raman outcomes also proved that BVO should be a stable substance. However, the peak intensity of the doped sample became weak, which may be due to the substitution of Ni for the stronger Bi [38]. Above results collectively demonstrated the successful preparation of the doped specimen.
The surface morphology of the prepared photocatalysts was investigated by SEM images, illustrated in figure 6. As seen from figure 6, BVO were irregular particles, which appeared to be flakes after doping. The reason for the change in microscopic morphology may be the addition of nickel nitrate to the precursor of BVO, which promoted the morphology transformation of the doped materials. EDS was used to study whether Ni ions successfully entered the BVO crystal structure. EDS results displayed that 0.75% NBVO contained B, V, O, Ni and C elements. Among them, the appearance of carbon can be related to the test equipment. Additionally, the  Figure 7 shows the N 2 adsorption-desorption isotherms and the pore size distributions (inset) of the asprepared photocatalytic materials. It was distinctly observed that the curve of BVO was similar to that of NBVO. During the progress of adsorption, desorption was enhanced without hysteresis, which was a typical characteristic of the type III isotherm adsorption curve [39,40]. As shown in table 2, the S BET and the pore volume of 0.75% NBVO sample (33.8 m 2 g −1 and 0.13 cm 3 g −1 ) were higher than that of BVO sample (24.5 m 2 g −1 and 0.08 cm 3 g −1 ), which may be beneficial for enhancing the photocatalytic activities due to provide more reaction active sites during the photocatalytic reaction [41,42]. In addition, it can be seen from the inset in figure 7 that the pore sizes of BVO and 0.75% NBVO were basically distributed in 3-10 nm, indicating that the as-synthesized both semiconductor photocatalysts were mesoporous materials.

UV-vis DRS
The optical absorption property of samples was characterized by UV-vis DRS. Figure 8 displayed the UV-vis DRS spectra of BVO and 0.75% NBVO. the BVO sample exhibited a narrower range of light absorption (565 nm), but the light absorption edge of 0.75% NBVO can be extended to 580 nm (figure 8). The band gap energy of BVO and 0.75% NBVO were 2.39 eV and 2.37 eV, respectively, and their band gap were not significantly different. Noteworthily, the band gap of Ni-doped materials was reduced and the absorption edge was red-shifted. As a result, it was beneficial to the photocatalytic activity of 0.75% NBVO.

Photocatalytic activity and reusability
The effect of 0.75% NBVO decomposing RhB under visible light illumination and UV-visible spectrum degradation variations were revealed in figure 9. The Ni-doped samples showed a much higher photocatalytic ability than BVO within 75 min ( figure 7(a)). Correspondingly, the BVO degraded only 44% RhB at 75 min. Surprisingly, the photocatalyst with a theoretical doping amount of 0.75 wt% had the best decolorization effect (96%) on RhB. In addition, it was found that the decomposition rate did not increase with increasing doping amount in figure 7(a). Besides, the highest absorption peak of RhB was blue-shifted from the initial 554 nm to 500 nm ( figure 9(b)), which indicated that RhB was degraded to small molecules due to deethylation reaction under visible light conditions. The decolorization of RhB conformed to the pseudo-first-order kinetic model, which was proved to be equation   decolorization efficiency of 0.75% NBVO for RhB decreased from the initial 96% to 86%. The experimental consequences declared that Ni doping could improve the stability and photo corrosion resistance of BVO. At the same time, it was further proved that 0.75% NBVO had an outstanding stability and long lifetime.

Photoelectric performance
The carrier mobility of BVO and 0.75% NBVO was measured by electrochemical impedance spectroscopy, and listed in figure 11. The impedance arc of BVO was greater than that of 0.75% NBVO, which meant that 0.75% NBVO had a faster migration rate of photogenerated carrier (figure (11)). In the equivalent circuit diagram of figure 11, the equivalent values of R s and R ct of BVO were 27 Ω and 848 Ω, respectively. Similarly, the equivalent values of 0.75% NBVO were 25 Ω and 322 Ω. The resistance of the electrolyte tended to be stable, and the fitted R ct resistance value of 0.75% NBVO is smaller than that of BVO. These results evidenced that the enhanced   mobility of photogenerated carriers may be caused by the reduced charge transfer resistance of 0.75% NBVO. Therefore, 0.75% NBVO exhibited better photocatalytic activity [51]. By testing the instantaneous photocurrent density (I-t) of BVO and 0.75% NBVO, the electron migration rate of the samples were further verified [52]. Figure 12 revealed the instantaneous photocurrent density of BVO and 0.75% NBVO. From figure 12, The instantaneous photocurrent densities of BVO, 0.25% NBVO, 0. 5% NBVO, 0.75% NBVO, 1.0% NBVO and 1.5% NBVO were 0.077 μA ·cm −2 , 0.1 μA ·cm −2 , 0.24 μA ·cm −2 , 0.086 μA ·cm −2 and 0.05 μA ·cm −2 , respectively. The instantaneous photocurrent density change value of the samples, which may be related to the sample's energy gain under visible light irradiation, and the migration of electrons after being excited [53]. In addition, the instantaneous photocurrent density values of NBVO were consistent with the photocatalytic activity results, which again suggested that appropriate Ni could effectively improve the photocatalytic activity of BVO. Especially, the improvement of the I-t of 0.75% NBVO had the     superiority that the recombination rate of holes and electrons became lower. Based on the results, the excellent photocatalytic activity of 0.75% NBVO was confirmed on the other hand.

Decolorization mechanism
Photocatalysts can generate superoxide radicals (·O 2 − ), hydroxyl radicals (·OH), photoelectrons [54] and holes (h + ) in decolorization reaction. Hence, in order to speculation the possible photocatalyst mechanism, EDTA-2Na, IPA and BZQ were used as trapping agents for h + , ·OH and ·O 2 − , respectively. Figure 13 showed the effect of 0.75% NBVO on the decolorization rate of RhB with the addition of different kinds and the same number of scavengers. All three capture agents had an effect on the photocatalytic activity of 0.75% NBVO ( figure 13(a)). With the addition of EDTA-2Na, IPA and BZQ, the decolorization rates of RhB within 75 min by 0.75% NBVO were 97%, 94% and 43%, respectively ( figure 13(b)). The experimental results presented that ·O 2 − was active substance in the process of photocatalytic reaction. On the basis of the above experimental results, plausible mechanism for photocatalytic decolorization of RhB by NBVO was shown in figure 14. First of all, when visible light was irradiated onto the surface of NBVO, the semiconductor was excited after absorbing light to generate electron-hole pairs. Secondly, the electrons in the valence band transited to the conduction band, and the carriers separated and migrated to NBVO surface. Doping Ni ions in the BVO lattice could form an electron capture center, resulting in a reduction in the recombination rate of electrons and holes [55]. Therefore, this is also one of the reasons why the doped samples improved the  photocatalytic activity. Eventually, electrons could interact with dissolved O 2 to generate strong oxidizing ·O 2 − , so as to achieve the purpose of degrading wastewater [56]. Based on experimental analysis and literature researches, it was concluded that the reactions occurred during the decolorization of RhB by NBVO should follows:

Conclusions
The photocatalyst NBVO with preeminent photocatalytic activity was prepared by a simple and undemanding hydrothermal method, which was applied to decompose the refractory organic dye RhB. XRD, EDS, XPS, FTIR and Raman spectroscopy confirmed that Ni was successfully doped into BVO. The NBVO photocatalytic activity had been stupendous improved. In particular, the 0.75% NBVO exhibited the upmost photocatalytic property, achieving 96% decolorization efficiency within 75 min under visible-light irradiation, which was due to the introduction of suitable Ni ions. The improved photoelectric performance of NBVO was also employed to explain the enhanced photocatalytic activity. On the one hand, 0.75% NBVO was a stable material, and the decolorization rate could still reach 86% after four cycles. On the other hand, the active radicals ·O 2 − play a crucial role in enhancing the photocatalytic performance, corresponding to the discussion of the scavengers trapping test. This work presents a simpler strategy to synthesize high efficiency visible-light-driven green photocatalyst. NBVO was used to treat organic wastewater RhB, which will be a new perspective for NBVO to handle wastewater.
Simultaneously, NBVO has the opportunity to become the novel favorite of environmental remediation materials.