Effect of Surface Site Defects on Photocatalytic Properties of BiVO4/TiO2 Heterojunction for Enhanced Methylene Blue Degradation

Combining a super photoresponsive BiVO4 catalyst to the large band gap TiO2 material seems to be a great interest in order to improve the visible-light-driven photodegradation of hazardous pollutants. BiVO4/TiO2 heterojunction composites have been synthesized via a simple one-pot hydrothermal route. Herein, we carefully highlighted the effect of BiVO4 content on the physicochemical and photocatalytic properties of solids towards the decomposition of methylene blue (MB) under solar light irradiation. )e main results revealed that the formation of the heterostructures catalyst by incorporating BiVO4 into TiO2 stabilizes the anatase phase of TiO2 by inhibiting its crystal growth and improves significantly the light absorbance of titanium dioxide.)e results showed that the best photocatalytic performance is assigned to the catalyst with 2 wt% BiVO4 loading which is higher than both pure BiVO4 and TiO2.)is improvement of photocatalytic activity is related to the synergetic effect between both materials. Furthermore, the constructed junction leads to an increase in the concentration of oxygen defects on the semiconductor surface which could create an acceptor energy level into the valence band of TiO2. Four cyclic runs for the photocatalytic degradation of MB on BiVO4/TiO2 composite revealed its stability and sustainable reusability.


Introduction
Dye effluents pose serious ecotoxic problems. Methylene blue (MB) is one of the organic dyes which are present in the textile effluents [1]. In addition to its toxicity, the presence of MB in aquatic systems could be the origin of serious damages to human health [2].
In this regard, enormous efforts have been devoted in order to get rid of these recalcitrant compounds. Photocatalytic degradation using heterojunction semiconductors is one of the most promising technologies for preventing global environmental pollution [3,4]. Several semiconductors have been analyzed in heterogeneous photocatalysis such as Fe 2 O 3 , ZnO, CdS, and ZnS [5][6][7][8][9]. Among the wide range of semiconductors, titanium dioxide (TiO 2 ) is well recognized as the most effective material in photocatalytic applications due to its strong catalytic properties [10,11].
Nevertheless, because of its wide band gap, TiO 2 presents a limited absorption in the visible light range together with a rapid recombination of photoinduced charge carriers which restricts its practical applications [12,13]. erefore, one of the greatest challenges for improving the photoactivity of TiO 2 is to increase the carrier lifetime and enhance the absorption in the visible light range. is is feasible by modifying the titanium dioxide surface by doping [14], self-doping [15], or coupling with other oxides [16,17]. Among the proposed approaches, the construction of heterojunction photocatalysts is a potential pathway because coupling TiO 2 with a narrow band gap semiconductor participates in a more powerful separation of the photogenerated electron-hole pairs [18].
As reported in the previous literature, bismuth vanadate (BiVO 4 ) has attracted substantial attention of several researchers owing to many advantages such as nontoxicity, photochromic effect, resistance to corrosion, and high photostability [19,20].
In this respect, combining both powerful photooxidation capacity of TiO 2 and strong visible light absorption provided by BiVO 4 can potentially be achieved by forming heterostructured photocatalysts based on m-BiVO 4 /TiO 2 for greater photocatalytic performances.
Song et al. [24] have shown that the construction of BiVO 4 /P25 heterostructure promotes the creation of oxygen defects on the photocatalyst surface. ese defects allow trapping the photogenerated electrons and enhance the charge separation, which ameliorates the photoactivity of catalysts.
Monfort et al. [25] synthesized BiVO 4 /TiO 2 nanocatalysts via a simple sol-gel method, and they evaluated their activity in water splitting. ey recorded the highest H 2 production for the BiVO 4 /TiO 2 framework due to the intimated contact between BiVO 4 and TiO 2 .
Zhu et al. [26] reported the elaboration of BiVO 4 /TiO 2 heterojunction containing Ti 3+ species as defect centers by a two-step hydrothermal route. e elaborated nanocomposites exhibit a photoactivity 7 times higher than pure BiVO 4 and pure TiO 2 towards the phenol degradation. is could be due to the presence of Ti 3+ which establishes an intermediate energy level, enhancing the electron transfer property.
e BiVO 4 /TiO 2 nanocomposites synthesized by the simple coupling method (sol-gel and hydrothermal methods) exhibited high photocatalytic activity for the oxidation of RhB with a yield that reached 80%. e results have provided further evidence that the BiVO 4 loading enhances considerably the decomposition of RhB dye, and this was attributed to the presence of O 2•− and OH • active species [27].
A study carried out by Rangel and coworkers [28] proved that the hydrothermal method increases the defect concentration, creating intermediate levels in the band gap energy which in turn improves the photoresponse of catalysts in the visible light range.
To the best of our knowledge, no studies on the photodegradation of methylene blue using BiVO 4 /TiO 2 heterojunction prepared via a one-step hydrothermal elaboration have been reported; hence, the originality of this investigation.
In the current study, BiVO 4 /TiO 2 heterojunction nanocomposites were synthesized using a one-pot hydrothermal method. e introduction of the BiVO 4 effect on the structural, textural, optical, and photocatalytic activities of BiVO 4 /TiO 2 nanocomposites was studied in detail. e photocatalytic performances of the coupled catalysts were investigated for the degradation of MB under solar light irradiation, and a possible photocatalytic mechanism was proposed. Furthermore, a stability and reusability study was carried out in order to ensure its sustainable use as a promising photocatalyst in the wastewater treatment field.

Materials and Methods
2.1. Materials. Titanium isopropoxide (Ti(OiPr) 4 , 97%) was purchased from Sigma-Aldrich and used as a titanium source. Nitric acid (HNO 3 , solution 65% w/w) provided by Scharlau was employed as a peptizing agent. Bismuth nitrate (Bi(NO 3 ) 3 , 99%) and ammonium metavanadate (NH 4 VO 3 , 99%) were obtained from Acros and used as the bismuth vanadate source. Ethanol (C 2 H 5 OH, 99.5%) was supplied from Sigma and used for washing solids. Sodium hydroxide (NaOH, 98%) was purchased from Aldrich and used to dissolve NH 4 VO 3 . Methylene blue (Sigma-Aldrich, ≥82%) was utilized as a model molecule in the photocatalytic reaction. All chemicals were analytical grade without further treatment.
e BiVO 4 /TiO 2 catalyst was prepared using the following protocol ( Figure 1): 0.59 g of BiNO 3 was first dissolved in an aqueous solution of nitric acid (4 mol·L −1 ) to form solution A. en, solution B was obtained by dissolving 0.18 g of NH 4 VO 3 into NaOH aqueous solution (2 mol·L −1 ). Afterward, the solution A was poured drop by drop in solution B until the formation of an intense yellow suspension. e obtained BiVO 4 suspension was subsequently added dropwise to a TiO 2 sol which was prepared as follows: a well-defined volume of titanium isopropoxide necessary to fix its concentration to 1 mol·L −1 was added to an aqueous solution of nitric acid (4 mol·L −1 ). e mixture was stirred for 24 h at 40°C. e pH of the solution was adjusted to 7 by dripping the NaOH solution (4 mol·L −1 ). e obtained mixture was then transferred to a Teflon stainless steel autoclave and heated at 180°C for 24 h. After recovering from the autoclave, the as-obtained materials were washed several times with distilled water and ethanol and left drying at 100°C for 7 h.
All composites were heat treated under oxygen flow at 500°C for 3 h.

Catalysts Characterization.
e structure and the particle sizes of as-synthesized BiVO 4 /TiO 2 composites were examined by powder X-ray diffraction (X'Pert Pro (PW3040/60) diffractometer equipped with a Cu-Kα radiation source (λ � 0.154 nm) in the scanning range of 20-80°). Raman spectra measurements were carried out on a Lab-RAM (Horiba scientific) using a green line of 563 nm Arlaser as the excitation source. e morphologies of the 2 Advances in Materials Science and Engineering BiVO 4 /TiO 2 samples were studied by scanning electron microscopy imaging with a SEM type JEOL JSM-6460LV apparatus. High-resolution transmission electron microscopy (HRTEM) was employed to observe the heterojunction formation of BiVO 4 /TiO 2 nanocomposite through HRTEM JEOL JEM-ARM 200F. e Brunauer-Emmett-Teller (BET) surface area and porosity were measured by N 2 adsorption-desorption at 77 K on an automatic Micrometritics ASAP 2020 analyzer. X-ray photoelectron spectroscopy (XPS) investigations of the catalysts were also recorded with a VG Escalab 200R spectrometer equipped with the monochromatic Mg Kα radiation (hv �1253.6 eV) X-ray source. UV-vis-NIR Varian Cary 5000 spectrometer was used in order to analyze the UV-vis spectra of the photocatalysts in the wavelength rage of 200-700 nm. e photoluminescence spectra were recorded via a Perkin Elmer Lambda S55 (LS55) spectrophotometer equipped with a xenon lamp excitation wavelength of 300 nm.

Photocatalytic Activity Test.
e photocatalytic performances of the as-prepared catalysts were examined in the degradation of methylene blue (MB) in aqueous solution and under solar light irradiation.
is reaction was then performed at room temperature. Typically, 50 mg photocatalyst was suspended into 100 mL of MB solution (10 mg·L −1 ) in a Pyrex glass vessel. In order to reach the adsorption-desorption equilibrium between MB and photocatalyst surface, the suspensions were magnetically stirred in darkness for 45 min prior to irradiation. Subsequently, the mixture was irradiated with solar light for 60 min. e luminous flux emitted by solar radiation was measured by a radiometer equipped with a detector, and it is close to 1,743 mWcm −2 . e photodegradation process was followed using a Shimadzu UV-3600 spectrophotometer at a wavelength λ max � 665 nm which is the maximum absorption of MB. e photodegradation rate was calculated according to Beer-Lambert's law [29]: where A 0 is the initial absorbance of MB, and A t is the absorbance after irradiation at time t.

Structural and Morphological Properties
3.1.1. XRD Analysis. Figure 2 illustrates the X-ray diffractograms of the BiVO 4 /TiO 2 composites prepared using hydrothermal route and calcined at 500°C. e XRD pattern of bare BiVO 4 was in good agreement with the monoclinic scheelite BiVO 4 phase according to JCPDS No. 14-0688. For pure TiO 2 , the diffraction peaks attributed to the anatase phase at 25.3°, 37.9°, 48.1°, 54.0°, showed the presence of only anatase TiO 2 which corroborates well that the combination of these two semiconductors preserves the crystalline structure of TiO 2 . It is clear that increasing the BiVO 4 leads to a reduction of the diffraction peaks intensities of samples suggesting the possible decrease of TiO 2 crystallinity. To gain further understanding on this finding, Zhang et al. [30] reported that the lattice distortion of the substrate and the presence of site defects on the surface of the matrix are the main causes of the crystallinity lessening. ese outcomes matched well with the results of the lattice parameters of samples which recorded a slight variation of the c-axis lattice parameter, as well as the unit cell volume, compared to pristine TiO 2 . It is possible to infer that the introduction of BiVO 4 by hydrothermal treatment could distort the TiO 2 network. ese observations are in good agreement with observations made by Yang et al. [31] who highlighted that interstitial doping leads to an extension of the network structure of titanium dioxide along the c-axis. Table 1 shows the average crystallite size of the assynthesized composites which were calculated by applying Scherrer's formula [32] using the line width of the (101) diffraction peak of TiO 2 . Results show that the average crystallite size of the prepared materials is between the sizes of pure TiO 2 and BiVO 4 . According to this table, it can be seen that the introduction of BiVO 4 has significantly reduced the crystallite size compared to pristine TiO 2 . ese findings are in good agreement with observations made by Song et al. [24]. In fact, the authors proved that an appropriate amount of BiVO 4 could inhibit the crystallite growth of TiO 2 . is is probably due to the fact that the crystalline growth of BiVO 4 which starts into the surface of TiO 2 is highly affected by the establishment of chemical interactions between the two materials. In fact, the Bi 3+ and V 5+ species remaining in the solution could participate in the germination process and crystal growth. ereby, a strong interaction between Bi 3+ , V 5+ species, and titanium alkoxide could explain this observation. In this case, the TiO 2 crystal growth around BiVO 4 nanoparticles could be probably influenced by the establishment of chemical bonds between the two species which lead probably to the distortion of the titanium dioxide lattice.

Raman Spectroscopy.
e determination of the vibrational transitions, the bounding states in the crystal lattice, and the local distortion of the solid can be studied with Raman spectroscopy which is considered herein as a performing technique to better discern the effect of BiVO 4 on the local structure of TiO 2 .
e Raman spectra of composites with the scope of 100-1000 cm −1 are reported in  [33]. Raman spectrum of bare TiO 2 and mixed oxides with different BiVO 4 contents show only the appearance of characteristic anatase Raman vibrations at 146.8, 197.7, 399.2, 515.6, and 641 cm −1 which correspond, respectively, to the active lattice vibrations modes E 1g , E 2g , B 1g , A 1g /B 1g , and E g [34]. No peaks corresponding to the scheelite monoclinic BiVO 4 phase were observed. ereby, the addition of BiVO 4 leads to a significant reduction of the peak intensity related to the E g active mode of anatase at 146.8 cm −1 for mixed oxides and especially at low content (2 wt% BiVO 4 /TiO 2 ) in comparison with pure TiO 2 which exhibits an intense peak. is is probably due to the presence of site defects in the surface of solids. Furthermore, the observed spectra shown in Figure 3(b) allows discerning that bismuth vanadate causes a slight shift (1.5 cm −1 ) of the main band (146.8 cm −1 ) to higher wavenumbers suggesting the increase of oxygen vacancies [35]. e highlighted conclusion from this observation is the effect of BiVO 4 in the generation of oxygen site defects which originates from the deformation of the TiO 2 lattice after its modification following BiVO 4 introduction.  Advances in Materials Science and Engineering

SEM Analysis.
e scanning electron microscopy images of pure BiVO 4 , pure TiO 2 , and 2 wt% BiVO 4 /TiO 2 samples are depicted in Figure 4. An enlarged SEM micrograph of pristine BiVO 4 (Figures 4(a) and 4(b)) shows that the latter exhibits a pinwheel-like shape. Figure 4(c) indicates that the particle distribution of the sample TiO 2 is aleatory and the grains are large with heterogeneous size since random distribution causes agglomeration of grains [36]. e 2 wt% BiVO 4 /TiO 2 nanocomposite (Figure 4(d)) has a morphology similar to pure TiO 2 but with smaller particle size. It is clearly seen that the BiVO 4 reduced the size of the titanium dioxide nanoparticles which fits well with the XRD results.

TEM Analysis.
e nanomorphology of Pt-BV/ T(X) photocatalysts was analyzed by TEM and HRTEM ( Figure 5). TEM micrographs of 2 wt% BiVO 4 /TiO 2 nanocomposite ( Figure 5(a)) exhibited a heterogeneous structure of nanoparticles which is composed of a mixture of nanospheres and small nanorods with an average size of approximately 14.62 nm. As shown by HRTEM ( Figure 5(b)), the measured spacing of nanoparticles is 0.35 nm which corresponds to the (101) crystallographic plane of TiO 2 , while the interplanar distance of 0.308 nm was indexed to the (121) planes of BiVO 4 [37,38]. HRTEM image clearly showed the formation of BiVO 4 /TiO 2 heterojunction with great interaction between BiVO 4 and TiO 2 .

N 2 Adsorption-Desorption Analysis.
A series of nitrogen adsorption-desorption isotherms at 77 K were performed as shown in Figure 6. According to the IUPAC classification [39], the as-prepared composites exhibited a type IV (a) isotherm characteristic of the mesoporous materials with several types of hysteresis loops. In fact, an H 2 (b) hysteresis loop was attributed to the bare TiO 2 . 2 wt% and 5 wt% BiVO 4 /TiO 2 and the pure BiVO 4 samples display an H 3 hysteresis loop characteristic of nonrigid aggregates of plate particles with slit-shaped pores. For the 10 wt% BiVO 4 / TiO 2 sample, an overlapping of both H 3 and H 2 (b) hysteresis loops was observed. e pore size distribution curves of materials (Figure 7) show monomodal patterns. It can be seen that increasing the BiVO 4 loading shifts the pore size distribution to lower pore diameter values except for 5 wt% loading. is could be probably due to the reduction of the intraparticle pore size [40]. e relative pressure range indicates that earlier is the onset of the loops, smaller is the diameter of the mesopores. Table 2 lists the relative pressures P/P°of the hysteresis loops onset of the as-synthesized materials. Outcomes revealed that the addition of BiVO 4 increases the relative pressure P/P°of the onset of the hysteresis loop for the catalysts except for 10 wt% loading. is fits well with the BJH pore size distribution which confirms that pure TiO 2 having the smallest P/P°value (0.73) exhibits the smallest pore diameter (169Å). e relative pressure value of 10 wt% BiVO 4 /TiO 2 (0.75) agrees well with the result of porous diameter (188). However, it does not follow the linear trend with the percentage of BiVO 4 . is could probably be due to the start of a change in the hysteresis loop from H 2b to H 3 . ereby, it is obvious to highlight the important effect of BiVO 4 on the texture and its importance in the control of the mesopores sizes and the particles morphologies. Compared to BiVO 4 , the BiVO 4 /TiO 2 composites have higher specific surface areas and larger pore volume but smaller pore size, and compared to the pure TiO 2 , the incorporation of BiVO 4 seems to improve the textural properties of the materials. ese results are in good agreement with the specific surface area (Table 2). e obtained results showed that the surface area of samples exhibits improvement after the introduction of BiVO 4 ranging from 67 m 2 g −1 for the BiVO 4 free TiO 2 to 94 m 2 g −1 for 10 wt% BiVO 4 /TiO 2 .

X-Ray Photoelectron Spectroscopy.
In order to ascertain the formation of BiVO 4 on the surface of TiO 2 and investigate the valence state of bismuth and vanadium, an XPS test was performed on the 2 wt% BiVO 4 /TiO 2 catalyst. Table 3 shows the existence of Ti, O, Bi, and V species. e characteristic peaks of Ti 4+ in the tetragonal structure (Ti 2p 3/2 and Ti 2p 1/2 ) were centered at 458.6 eV and 464 eV, respectively [41]. e summary of XPS results given in Table 3 revealed the presence of the lattice oxygen corresponding to the peaks at 530.2 eV which is inherent to the Ti-O-Ti bonds [42]. Figure 8 displays the XPS survey spectra of the sample which confirms the presence of Bi and V. As shown in Figure 8(a), two symmetric photoelectron peaks centered at 517.2 eV and 524.7 eV were assigned to V2p 3/2 and V2p 1/2 , respectively. e difference in binding energy between V2p 3/2 and V2p 1/2 is 7.6 eV proves the +5 oxidation state of vanadium. e split binding energy peaks of Bi4f 7/2 and Bi4f 5/2 (Figure 8(b)) were centered, respectively, at 159.2 eV and 164.4 eV. e distance between these two peaks is found to be 5.2 eV which is specific to Bi 3+ [30] in a typical heterojunction sample [43]. ereby, the absence of the characteristic peaks of metallic bismuth, bismuth oxide, and bismuth titanate further confirms the presence of the component bismuth in the vanadate state [44]. All of   these outcomes are testified in favor of the formation of BiVO 4 . ereafter, a heterojunction between this latter and TiO 2 is probably created.

UV-Vis Diffuse Reflectance Spectroscopy.
In order to understand the evolution of the optical properties of BiVO 4 / TiO 2 nanocomposites compared to pure TiO 2 , a series of UVvisible measurements were performed in the wavelength range 250-650 nm as shown in Figure 9. As can be seen, pure TiO 2 showed strong photoresponsiveness in the UV region estimated at 400 nm, whereas that of BiVO 4 is around 540 nm which is assigned to the band transition from the Bi 6 s orbital to V 3 d conduction band. It is worth noting that the optical absorption increases very slightly with the BiVO 4 incorporation compared to bare titanium dioxide in the visible light range, and it increases with raising the bismuth vanadate content. is finding may be attributed to the coupling effect of both semiconductors. In fact, the construction of a heterojunction between the two materials, pointed out with the XPS technique, increases the defect sites in the TiO 2 lattice creating thus intermediate energy levels between BiVO 4 and TiO 2 which is consistent with both our previous XRD and Raman results and the literature [45]. Furthermore, lattice distortion pleads in favor of the creation of more oxygen vacancies which would promote the separation of charge carriers and enhance probably in turn the photocatalytic performances of catalysts.  Advances in Materials Science and Engineering electron-hole pairs in the titanium dioxide solid. us, the photoluminescence is considered as a versatile technique to explore the defect states in the band gap of the semiconductor [46]. Notably, the photoluminescence emission originates from the recombination of photoexcited electronhole. erefore, it is well known that a better photocatalytic performance is figured by the lowest recombination rate which is represented by weaker photoluminescence intensity [26]. With the aim of better ascertain the influence of BiVO 4 incorporation on the optical properties of TiO 2 , the photoluminescence properties of catalysts were examined using an exciting wavelength at 300 nm ( Figure 10). e PL spectra of both pure TiO 2 and the BiVO 4 /TiO 2 nanocomposites were deconvoluted with Gaussian peaks after background subtraction. Careful analysis of deconvoluted PL spectrum of each sample is well fitted by five main emission peaks ( Figure 11). e peak centered at 414 nm is related to the recombination of photogenerated electron-hole pairs into the bulk TiO 2 lattice [47]. e peak at 477 nm can be interpreted as the charge transfer from Ti 3+ to the oxygen anion localized in Ti octahedral [48]. e other peaks at 452 nm and 515 nm are attributed to the recombination of generated electrons with surface oxygen defects [49]. Finally, the emission peak at 558 nm is related to defects and nonstoichiometry in the TiO 2 anatase phase generated by oxygen vacancies [50]. e analysis of the pure BiVO 4 spectrum revealed the presence of an emission peak at 500 nm corresponding to the recombination of the electrons in the conduction band of V3d orbital of BiVO 4 with the holes formed in the valence band of O2p orbital [26]. Herein,   it is clear that the higher PL intensity of the bare TiO 2 indicates a higher recombination rate. Moreover, results revealed that the PL intensity related to the recombination of electrons with surface oxygen vacancies is a very important factor that determines the photocatalytic performance of the catalyst. Indeed, it is worth noting that these defect sites are capable of trapping photogenerated electrons favoring the reduction of the recombination rate in the BiVO 4 /TiO 2 composite and enhancing the photocatalytic activity of the as-prepared solids. In this regard, Wetchakun et al. [51] reported that the surface oxygen vacancies could be considered to be the active sites of the BiVO 4 /TiO 2 material. e PL intensity decreases after the incorporation of BiVO 4 . is result could be probably due to the formation of a heterojunction which affects the charge separation properties of the photocatalysts. Indeed, we point out that the different positions of the band gap energy between BiVO 4 and TiO 2 allow the photogenerated electrons to jump from the BiVO 4 valence band to the TiO 2 conduction band [52]. However, the BiVO 4 content does not seem to be important in reducing the electron-hole pair recombination rate and only its presence is sufficient to improve the transfer feature of charge carriers.

Photocatalytic Activity
e study of the photocatalytic performances of the samples, prepared by the hydrothermal method, was evaluated by measuring the degradation of methylene blue (MB), which was used as a target pollutant, and the investigation of the BiVO 4 content's effect on their reactivities. In this respect, photocatalytic activities of samples were estimated by measuring the color removal of MB. As a comparison, the photocatalytic degradation over pure TiO 2 and BiVO 4 was performed under the same experimental conditions than the nanocomposites BiVO 4 /TiO 2 . As can be seen from Figure 12, the photocatalytic activity of pristine BiVO 4 is very low under solar light irradiation. is finding could be essentially due to its reduced specific surface area (7 m 2 g −1 ) which restricts its adsorption ability towards the decomposition of organic molecules of MB. After coupling BiVO 4 and TiO 2 , results revealed that MB degradation increases for certain bismuth vanadate amount, and the best photocatalytic conversion is attributed to the 2 wt% BiVO 4 /TiO 2 composite indicating that 99% of MB was degraded after 60 min. ereby, the photocatalytic conversion of this material is 2.5 times and 1.25 times higher than that of pure BiVO 4 and TiO 2 , respectively. Nevertheless, the photocatalytic conversion of 10 wt% BiVO 4 /TiO 2 is smaller than that of pristine TiO 2 despite its higher surface area. Accordingly, it is worth to note that a high amount of BiVO 4 appears to block the active sites [27]. e high conversion which is assigned to the low bismuth vanadate loading could be essentially due to the synergetic effect between BiVO 4 and TiO 2 after the heterojunction formation. In fact, Sajjad et al. [53] reported that the creation of a heterojunction increases the photocatalytic performances by its ability to restrict the recombination of the photoinduced electron-hole pairs. Notably, according to Raman spectroscopy analysis, we have shown that the introduction of BiVO 4 leads the generation of oxygen surface defects. ese oxygen vacancies inducing a TiO 2 lattice distortion can be able to inhibit the crystallite growth which improves the surface area of the catalysts and participate in turn in the separation of charge carriers by trapping the induced photoelectrons. Wang et al. [54] concluded that site defects are the major causes for enhancing the photocatalytic performances of catalysts towards the removal of organic dyes from wastewater. In fact, these defects especially oxygen vacancies, which are considered as a positive electric center, are able to create donor levels in the electronic structure of TiO 2 . is pleads in favor of the improvement of the absorption properties of titanium dioxide in the visible light range, on the one hand, and Advances in Materials Science and Engineering scavenging the holes generated in the valence band, on the other hand. Moreover, Lv et al. [55] have reported that the surface lattice distortion on the titanium dioxide surface participates in the degradation of target pollutants by providing energy which is able to break the chemical bonds of organic molecules such as methylene blue.

Suggested Photocatalytic Mechanism.
According to the previous results, the photocatalytic mechanism of BiVO 4 / TiO 2 heterojunction can be explained as given in Scheme 1. e suggested mechanism of the photocatalytic oxidation of methylene blue is illustrated in Scheme 2.
After thermodynamic equilibrium, when the BiVO 4 / TiO 2 composite is illuminated with photons having energy equal or greater than its band gap energy, electrons can promote from the valence band (VB) to the conduction band (CB) of BiVO 4 leaving a hole (h + ). By virtue of the joint of electric fields between two solids on the one hand and the CB edge potential of BiVO 4 which is more negative than that of TiO 2 on the other hand, the photoinduced electrons can be injected to the conduction band of TiO 2 (equation (2)) [55].
As well as, h + holes react with electron donors such as H 2 O and OH − anions adsorbed on the surface of the semiconductor and produce OH • hydroxyl groups (equations (5) and (6)) [57].
e formed radicals (OH • and O •− 2 ) will participate in the degradation of adsorbed pollutants on the surface of the catalyst. Hence, it is obvious that OH • radicals are the main oxidants in the photodecomposition of organic contaminants reactions. In fact, these species are endowed with high reactivity and strong oxidizing capability for the removal of organic targets into inorganic compounds such as CO 2 , water, and organic chain acids (equation (7)) [58].
So far, it is well recognized that oxygen vacancies (V O ) promoted by the formed heterojunction act as a hole trapping center, resulting in the effective separation of photoinduced electron-hole pairs (equation (8)) [46]:

Reusability and Stability of BiVO 4 / TiO 2 Nanocomposite
From a practical point of view, the stability and reusability study of the photocatalysts is of great importance in the field of wastewater purification. erefore, the stability and reusability of the BiVO 4 /TiO 2 nanocomposites could also be checked. In this concern, four cycles of photocatalytic MB degradation experiments via 2 wt% BiVO 4 /TiO 2 composite were carried out under identical reaction conditions, and results are depicted in Figure 13. cycles, respectively. e results show that the photocatalytic efficiency decreases but not significantly since the activity losses are 0.8%, 3.6%, and 9.4% (compared to the 1st cycle) indicating that the photocatalyst still maintains a high photodegradation capacity which reflects the high stability of the prepared nanocomposite. is result is advantageous to the recycling of BiVO 4 /TiO 2 photocatalyst and decreasing the application cost. e XRD patterns of fresh and used 2 wt% BiVO 4 /TiO 2 photocatalyst ( Figure 14) revealed that the four repeated uses do not affect the crystalline structure of anatase TiO 2 which is in good agreement with the high activity and stability of the catalyst.

Conclusion
In the present investigation, we systematically studied the effect of different BiVO 4 amounts on the structural, textural, and optical properties of BiVO 4 /TiO 2 heterojunction materials which were elaborated by the one-pot hydrothermal method. e current outcomes elucidate that the creation of heterojunction between BiVO 4 and TiO 2 induces the  formation of more site defects on the catalyst surface. Furthermore, we have demonstrated that BiVO 4 could well control the crystallite size of solids. Indeed, controlling the size of the crystallites could be of great interest in our case by the fact that this latter makes possible to stabilize the anatase phase from easy transformation to rutile. Moreover, the optical behavior of as-prepared solids was drastically modified by shifting the absorption edge of TiO 2 to the visible light range. is effect might be explained by the fact that the presence of such defects enhances the charge carriers lifetime which leads to the amelioration of the catalytic efficiency. In order to achieve a better understanding of this system, the photoactivity of BiVO 4 /TiO 2 materials was evaluated in the methylene blue degradation reaction. Results revealed that the amount of BiVO 4 in the BiVO 4 /TiO 2 nanocomposite has a significant influence on the photoactivity of photocatalysts, and the highest degradation rate catalyst can be assigned to the 2 wt% BiVO 4 /TiO 2 solid and it is equal to 99% after one hour of solar light irradiation. It is inferred to mention that such enhancement could be assigned to the controlled crystallite sizes, the interesting textural properties, and the attractive optical features of the catalysts. Furthermore, BiVO 4 /TiO 2 photocatalyst displayed significant recyclability and stability for four catalytic cycles in the methylene blue photodegradation reaction. ese results indicate that the nanocatalyst BiVO 4 /TiO 2 can be used as a promising photocatalyst for the photocatalytic treatment of industrial wastewater.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.

Authors' Contributions
Sahar Mansour was involved in conceptualization, validation, investigation, methodology, formal analysis, and writing-review and editing. Rym Akkari was involved in conceptualization, supervision, and writing-review and editing. Semy Ben Chaabene was involved in methodology, supervision, and writing-review and editing. Mongia Saïd Zina was involved in conceptualization, methodology, supervision, and writing-review and editing.