Fabrication of the heterojunction catalyst BiVO4/P25 and its visible-light photocatalytic activities

A heterojunction catalyst, BiVO4/P25, was successfully fabricated using a one-step hydrothermal method. The prepared composite was characterized using XRD, XPS, Raman, FT-IR, UV–vis, SEM, HRTEM and PL. The HRTEM pictures revealed that the heterostructured composite was composed of BiVO4 and P25, and from the pictures of SEM we could see the P25 nanoparticles assembling on the surface of flower-shaped BiVO4 nanostructures. The XPS spectra showed that the prepared catalyst consisted of Bi, V, O, Ti and C. The photocatalytic activity of BiVO4/P25 was evaluated by degraded methyl blue (MB) and tetracycline under visible light illumination (λ > 420 nm), and the results showed that BiVO4/P25 composite has a better photocatalytic performance compared with pure BiVO4 and the most active c-BiVO4/P25 sample showed enough catalytic stability after three successive reuses for MB photodegradation. The enhanced photocatalytic performance could mainly be attributed to the better optical absorption ability and good absorption ability of organic contaminants.


Introduction
Photocatalysis has attracted intensive attention as a cost-effective, green chemical technology [1]. Visible light responding materials, such as BiVO 4 , ZnWO 4 and AgVO 4 were the promising semiconductors which had been previously applied to photocatalysis and have gained a great deal of attention [2]. Bismuth-based semiconductors (e.g. Bi 2 WO 6 , BiVO 4 , Bi 4 Ti 3 O 12 , Bi 2 O 2 CO 3 , BiOI, BiOBr and Bi 2 MoO 6 ) as new types of photocatalytic materials have attracted great attention [3]. solution (4 mol l 21 , 40 ml). The mixture was stirred for 20 min and sonicated for 10 min. The pH was adjusted to 9. Then, the solution was left overnight. The mixture solution was transferred to a Teflonlined stainless-steel autoclave and maintained at 1808C for 12 h. The resultant products were filtered, washed with deionized water and ethanol. Finally, the yellow product was dried at 808C for 6 h in a vacuum oven.

Synthesis of the BiVO 4 /P25 heterojunction catalysts
The BiVO 4 /P25 nanocomposite was synthesized using a one-step hydrothermal method. The steps of the method were as follows: 2.45 g of Bi(NO 3 ) 3 . 5H 2 O and 0.6 g of NH 4 VO 3 were added to 40 ml of a 4 mol l 21 HNO 3 solution and NaOH solution, respectively, which were referred to in this study as Solution A and Solution B. Then, 0.05, 0.1, 0.15 and 0.2 g of P25 were added to Solution A, and the two solutions were mixed. Magnetic stirring was conducted for 20 min, and sonication was performed for the dissolving process for another 10 min. Finally, certain concentrations of the NaOH and HNO 3 solutions were used in order to adjust the pH to 9. The mixture solution was left overnight, transferred to a Teflon-lined stainless-steel autoclave, and maintained at 1808C for 12 h. The resulting yellow powder was centrifuged and washed three times in a water and ethanol solution, and then oven dried at 808C for 6 h. In this study, the obtained yellow BiVO 4 /P25 composite with doping of 0.05, 0.1, 0.15 and 0.2 g is referred to as a-BiVO 4 /P25, b-BiVO 4 /P25, c-BiVO 4 /P25 and d-BiVO 4 /P25, respectively, for comparison purposes.

Analytical characterization
The BiVO 4 and BiVO 4 /P25 crystalline phases were measured on a SmartLab (3 KW) X-ray diffractometer, with Al Ka radiation. The morphology and composition of the BiVO 4 and BiVO 4 /P25 samples were performed using an FEI Quanta 400 FEG scanning electron microscope, along with an FEI Tecnai G2 F20 transmission electron microscope operated at 200 kV. The X-ray photoelectron spectroscopy (XPS) analyses of the BiVO 4 /P25 nanocomposite were carried out using a Thermo ESCALAB 250XI spectrometer. The optical properties of the BiVO 4 /P25 samples were estimated by a Lambda 650 UVvis spectrophotometer using the BaSO 4 as reflectance sample. A Brunauer -Emmett-Teller (BET) surface area test was performed at 77 K using a TriStar II 3020 surface area analyser.

Results and discussion
3.1. Crystal structure, morphology and composite characterization   (123), respectively, of the monoclinic crystal BiVO 4 according to the JCPDS no. 14-0688 [35]. It was observed that, with the increasing amounts of P25, no diffraction peaks appeared at 2u when compared with the planes of the rutile P25 (JCPDS no. 84-1285) [36]. It was indicated from the curves of the a-BiVO 4 /P25, b-BiVO 4 /P25, c-BiVO 4 /P25 and d-BiVO 4 /P25 that no other peaks were detected. Therefore, no other crystalline phases were observed following the P25 doping, and the doping of the P25 had not changed the crystal structure of the BiVO 4 . Figure 1b displayed the XRD patterns of the unused BiVO 4 /P25 samples and BiVO 4 /P25 samples after three cycles for degradation of MB solution. We could know from the picture, as the recycle times increased, the diffraction peaks on the (011) plane, (121) plane and (161) plane weakened, but BiVO 4 /P25 samples of 1 cycle, 2 cycles and 3 cycles still showed the characteristic diffraction peaks at 2u ¼ 18.68, 18 figure 2a shows that the pure BiVO 4 sample exhibited a smooth polyhedral morphology with a thickness of 500 nm. The thickness of 5 mm was observed to be flower-like. Figure 2c shows that the P25 nanoparticles were doped on the surface of the BiVO 4 , which made the surface rough with a higher specific surface area than that the single BiVO 4 . This resulted in improved photocatalysis and absorption. Also, the shape was maintained as before the doping process. As can be seen in rsos.royalsocietypublishing.org R. Soc. open sci. 5: 180752 figure 2c,d the P25 nanoparticles were found to be deposited on the surface of the BiVO 4 (040) crystal facet. The P25 solid coating on the surface of the BiVO 4 could be seen more clearly using a transmission electron microscope. Figure 2e,f displayed the BiVO 4 /P25 composite after three cycles of degradation of MB solution which revealed that the P25 nanoparticles were discovered on the surface of BiVO 4 and there was no change in the structure of BiVO 4 /P25. The HRTEM image (the right of figure 3) shows that the lattice fringes of BiVO 4 /P25 system and the uniform fringes with interval of 0.309 nm were in good agreement with the (2121) lattice plane of monoclinic BiVO 4 , and the interplanar distance of 0.352 and 0.223 nm agrees well with the lattice spacing at (101) plane of anatase TiO 2 and (200) plane of rutile TiO 2 . Thus, the results confirmed that the heterostructure was formed from P25 and BiVO 4 [37] .
The Raman spectrum of the single monoclinic BiVO 4 and BiVO 4 /P25 samples is presented in figure 4a. As shown in figure 4a, the Raman bands nearly at 132, 211, 325, 360, 708 and 838 cm 21 were due to vibrational modes of monoclinic BiVO 4 [38], the Raman bands of pure BiVO 4 and BiVO 4 / P25 samples have shown the same vibration peak, and with the increasing P25 nanoparticles the intensity peak decreasing. Figure   adsorbed water molecules [39]. The absorption band at 735 cm 21 in the FT-IR spectra of those samples was due to the asymmetrical and symmetrical stretching vibrations of VO 3À 4 a tetrahedron in monoclinic BiVO 4 [40].
The XPS spectra detailed in figure 5 illustrates the main surface elements of BiVO 4 /P25, which indicated that Bi, V, O, Ti and C appeared in the BiVO 4 /P25 composites. The C1s were mainly the remains of the alcohol which was used during the washing process. As can be seen in figure 5, the typical orbital of the Bi4f 7/2 and Bi4f 5/2 peaks (figure 5b) [41], and the V2p 3/2 and V2p 1/2 peaks (figure 5c) were observed with the peak locations of 158.9, 164.2, 517.2 and 529.6 eV, respectively, which were determined to be in close accordance with the Bi 3þ and V 5þ peaks in the monoclinic scheelite BiVO 4 . Figure 6a shows the UV-vis diffuse reflectance spectra of the pure BiVO 4 and BiVO 4 /P25 samples. The band gap (E g ) for a direct band gap semiconductor can be determined from the plots of (ahv) 2 as a function of hv (figure 6b) originating from the equation [12]: ahv ¼ A (hv 2 E g ) 1/2 . We calculated that the E g of BiVO 4 , a-BiVO 4 /P25, b-BiVO 4 /P25, c-BiVO 4 /P25 and d-BiVO 4 /P25 were 2.4 eV, 2.38 eV, 2.42 eV, 2.43 eV and 2.45 eV, respectively. The results revealed that the absorption edges of the BiVO 4 were approximately at 540 nm (figure 6a). When the P25 was doped on BiVO 4 , a red shift was observed. However, with the increases in the added P25, the light absorption threshold of the BiVO 4 /P25 gradually blue shifted, which indicated that the BiVO 4 which had been doped with 0.15 g of P25 did not broaden the absorption of the visible light when compared with the pure BiVO 4 , and certain blue shifts may have been inhibited by the recombination of the photon-generated carrier [42]. The surface area of the BiVO 4 /P25 was determined to be 1.2780 m 2 g -1 , while that of the pure BiVO 4 was 1.0043 m 2 g 21 . The BET surface area of the BiVO 4 /P25 was found to be larger than that of the non-doped BiVO 4 , which clearly indicated that the P25 doping could enhance the BET surface area to a great degree.

Optical analysis
Photoluminescence spectra were often employed to study the migration, transfer and trapping of the photogenerated electron -hole pairs. In semiconductor particles, less intense photoluminescence (PL) usually indicated better separation efficiency of e 2 /h þ pairs, thus better photocatalytic activity [43]. Figure 7 shows the PL spectra of the composites excited at a wavelength of 325 nm. It could be seen from the figure that the emission peaks were similar in shape and the maximum intensity was near

Photocatalytic properties
The photocatalytic performances of the BiVO 4 and the BiVO 4 /P25 composites were evaluated by the degradation efficiency of the MB solution and tetracycline solution under the visible light illumination using a xenon lamp as the light source. In the following pictures, the 'C 0 ' represents the initial absorbance, 'C t ' represents the absorbance after t times irradiation, using the (1 2 C t /C 0 )% to calculate its degradation rate of MB solution and tetracycline solution. Figure 8a details the degradation of the MB solution under different conditions. The results indicated the MB solution almost did not degrade when in dark conditions, and without any catalyst. The degradation under visible light was observed to be lower than with ultraviolet light in the presence of the BiVO 4 . Figure 8e illustrates the degradation of tetracycline solution (20 ug      In a word, the degradation rate of the BiVO 4 /P25 was the fastest when the doping amount of the P25 was 0.15 g. With the increases in the P25 doping, the adsorption of the methylene blue on the composite catalyst BiVO 4 /P25 was observed to be improved. However, the photocatalytic degradation ability had been weakened, which indicated that excessive P25 doping, and small amounts of P25 doping, were not conducive to improving the photocatalytic performance of the BiVO 4 . The excessive amount of P25 doping tended to affect the absorption of light by the BiVO 4 , and the poor effects of re-using the P25 doping was not conducive to the improvement of the performance of the BiVO 4 /P25 [44]. The optimum doping amount of the P25 was determined to be 0.15 g under this study's experimental conditions, which was found to effectively improve the photocatalytic ability of the BiVO 4 , as well as maintaining good catalytic performances after recycling three times. As figure 8f shows, its photocatalytic activity was weakened a little, but still maintains high stability after three cycles.

Photocatalytic mechanism
Based on the Results and discussion, heterostructure formed in BiVO 4 /P25 system played an important role in the efficient separation of photo-induced charge. The CB and VB potentials of BiVO 4 and P25 at the point of zero charge could be calculated by the following empirical equations (3.1) and (3.2) [45]: and 2Þ where E VB is the VB edge potential; X is the electronegativity of the semiconductor, which was the geometric mean of the electronegativity of the constituent atoms (X value of BiVO 4 and P25 are approximately 6.04 and 5.81 eV, respectively); E e is the energy of free electron on the hydrogen scale (approximately 4.5 eV); E g is the band gap energy of the semiconductor; and E CB is the CB edge potential.  BiVO 4 , a n-N junction was formed on the surface of BiVO 4 /P25 heterostructure [49]. The energy band gap of BiVO 4 /P25 heterojunction catalyst after thermodynamic equilibrium is shown in figure 9. And the BiVO 4 was considered as an intrinsic semiconductor, so the Fermi level in BiVO 4 lies in the middle of CB and VB, which was approximately equal to 1.6 eV. Generally speaking, the CB potential (E CB ) of n-type semiconductor because of the more negative potential (approx. 0.1-0.2 V) than the flat-band potential, Fermi level in P25 is 20.1 eV. When these two types of semiconductor materials were closely joined together, the heterostructure photocatalyst BiVO 4 /P25 is formed [50]. At this moment, uniformed the same Fermi level and the system was in equilibrium. After the thermodynamic equilibrium, E F turns to be 20.1 eV which was the same as the Fermi level of P25. At the same time, E CB and E VB decrease from 0.4 to 21.3 eV, 2.8 to 21.1 eV, respectively. During visible light irradiation, electrons on the surface of BiVO 4 were activated by certain energy photons [51]. Then electrons jumping from VB to CB of the BiVO 4 had left holes behind in the VB of BiVO 4 . At the same time, the electrons were quickly transferred to the CB of P25 due to the different E CB between BiVO 4 and P25. Thus, photogenerated electrons can be transferred through more pathways in the system. So that the utilization efficiency of BiVO 4 on visible light can be improved.
It is widely acknowledged that hydroxyl radicals ( †OH) and superoxide anion radicals (O 2 †) play important roles in the photocatalytic oxidation process. Therefore, photoluminescence (PL) technique was employed to study the migration, transfer and trapping of the photogenerated electron -hole pairs in the BiVO 4 /P25 system. From the PL, we could indicate better separation efficiency of e 2 /h þ pairs, the holes (h þ ) in the VB of the BiVO 4 oxidized the H 2 O into a hydroxyl radical ( †OH), which had a high oxidizing ability and could effectively oxidize the MB into small products, as well as the end products of CO 2

Conclusion
In summary, in this study, the BiVO 4 /P25 composite which had been prepared by a one-step hydrothermal method displayed a good visible light catalytic capacity and a superior absorption performance. The improved absorption ability contributed to a rough surface and large surface area due to the nanoparticle P25 adherence to the surface of the BiVO 4 plane. Also, the 0.1 g BiVO 4 /P25 nanocomposites displayed a red shift. It was determined that the addition of P25 did not change the crystals and morphology of the BiVO 4 .