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

Authors: Sahar Mansour; Rym Akkari; Semy Ben Chaabene; Mongia Saïd Zina
Journal: Advances in Materials Science and Engineering
 (2020)

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## Abstract

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. The 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. The 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. This 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.


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## Body

## 1. 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 Fe2O3, ZnO, CdS, and ZnS [5–9]. Among the wide range of semiconductors, titanium dioxide (TiO2) 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, TiO2 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].Therefore, one of the greatest challenges for improving the photoactivity of TiO2 is to increase the carrier lifetime and enhance the absorption in the visible light range. This 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 TiO2 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 (BiVO4) has attracted substantial attention of several researchers owing to many advantages such as nontoxicity, photochromic effect, resistance to corrosion, and high photostability [19, 20].It was mentioned that BiVO4 exists mainly in three crystalline polymorphs: tetragonal scheelite (t-BiVO4), monoclinic scheelite (m-BiVO4), and tetragonal zircon structures (tz-BiVO4) [21]. In a comparative study between the different crystalline phases [22], it was pointed out that monoclinic BiVO4 (m-BiVO4) shows the highest photocatalytic activity for the degradation of organic pollutants from wastewater [23].In this respect, combining both powerful photooxidation capacity of TiO2 and strong visible light absorption provided by BiVO4 can potentially be achieved by forming heterostructured photocatalysts based on m-BiVO4/TiO2 for greater photocatalytic performances.Song et al. [24] have shown that the construction of BiVO4/P25 heterostructure promotes the creation of oxygen defects on the photocatalyst surface. These defects allow trapping the photogenerated electrons and enhance the charge separation, which ameliorates the photoactivity of catalysts.Monfort et al. [25] synthesized BiVO4/TiO2 nanocatalysts via a simple sol-gel method, and they evaluated their activity in water splitting. They recorded the highest H2 production for the BiVO4/TiO2 framework due to the intimated contact between BiVO4 and TiO2.Zhu et al. [26] reported the elaboration of BiVO4/TiO2 heterojunction containing Ti3+ species as defect centers by a two-step hydrothermal route. The elaborated nanocomposites exhibit a photoactivity 7 times higher than pure BiVO4 and pure TiO2 towards the phenol degradation. This could be due to the presence of Ti3+ which establishes an intermediate energy level, enhancing the electron transfer property.The BiVO4/TiO2 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%. The results have provided further evidence that the BiVO4 loading enhances considerably the decomposition of RhB dye, and this was attributed to the presence of O2•− 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 BiVO4/TiO2 heterojunction prepared via a one-step hydrothermal elaboration have been reported; hence, the originality of this investigation.In the current study, BiVO4/TiO2 heterojunction nanocomposites were synthesized using a one-pot hydrothermal method. The introduction of the BiVO4 effect on the structural, textural, optical, and photocatalytic activities of BiVO4/TiO2 nanocomposites was studied in detail. The 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.


## 2. 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 (HNO3, solution 65% w/w) provided by Scharlau was employed as a peptizing agent. Bismuth nitrate (Bi(NO3)3, 99%) and ammonium metavanadate (NH4VO3, 99%) were obtained from Acros and used as the bismuth vanadate source. Ethanol (C2H5OH, 99.5%) was supplied from Sigma and used for washing solids. Sodium hydroxide (NaOH, 98%) was purchased from Aldrich and used to dissolve NH4VO3. Methylene blue (Sigma-Aldrich, ≥82%) was utilized as a model molecule in the photocatalytic reaction. All chemicals were analytical grade without further treatment.


### 2.2. Elaboration of BiVO4/TiO2 Photocatalysts

BiVO4 was introduced into the TiO2 material by a hydrothermal method in order to obtain 2, 5, and 10 wt% BiVO4 loading in the final BiVO4/TiO2 composite.


#### 2.2.1. BiVO4/TiO2 Composite Synthesis by Hydrothermal Route

The BiVO4/TiO2 catalyst was prepared using the following protocol (Figure 1): 0.59 g of BiNO3 was first dissolved in an aqueous solution of nitric acid (4 mol·L−1) to form solution A. Then, solution B was obtained by dissolving 0.18 g of NH4VO3 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. The obtained BiVO4 suspension was subsequently added dropwise to a TiO2 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). The mixture was stirred for 24 h at 40°C. The pH of the solution was adjusted to 7 by dripping the NaOH solution (4 mol·L−1). The 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.Figure 1
Experimental protocol of BiVO4/TiO2 synthesis.All composites were heat treated under oxygen flow at 500°C for 3 h.


### 2.3. Catalysts Characterization

The structure and the particle sizes of as-synthesized BiVO4/TiO2 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 LabRAM (Horiba scientific) using a green line of 563 nm Ar-laser as the excitation source. The morphologies of the BiVO4/TiO2 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 BiVO4/TiO2 nanocomposite through HRTEM JEOL JEM-ARM 200F.The Brunauer–Emmett–Teller (BET) surface area and porosity were measured byN2 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. The photoluminescence spectra were recorded via a Perkin Elmer Lambda S55 (LS55) spectrophotometer equipped with a xenon lamp excitation wavelength of 300 nm.


### 2.4. Photocatalytic Activity Test

The photocatalytic performances of the as-prepared catalysts were examined in the degradation of methylene blue (MB) in aqueous solution and under solar light irradiation. This 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. The luminous flux emitted by solar radiation was measured by a radiometer equipped with a detector, and it is close to 1,743 mWcm−2. The photodegradation process was followed using a Shimadzu UV-3600 spectrophotometer at a wavelength λmax = 665 nm which is the maximum absorption of MB. The photodegradation rate was calculated according to Beer–Lambert’s law [29]:(1)Degradation efficiency%=A0−AtA0×100,where A0 is the initial absorbance of MB, and At is the absorbance after irradiation at time t.


## 2.1. Materials

Titanium isopropoxide (Ti(OiPr)4, 97%) was purchased from Sigma-Aldrich and used as a titanium source. Nitric acid (HNO3, solution 65% w/w) provided by Scharlau was employed as a peptizing agent. Bismuth nitrate (Bi(NO3)3, 99%) and ammonium metavanadate (NH4VO3, 99%) were obtained from Acros and used as the bismuth vanadate source. Ethanol (C2H5OH, 99.5%) was supplied from Sigma and used for washing solids. Sodium hydroxide (NaOH, 98%) was purchased from Aldrich and used to dissolve NH4VO3. Methylene blue (Sigma-Aldrich, ≥82%) was utilized as a model molecule in the photocatalytic reaction. All chemicals were analytical grade without further treatment.


## 2.2. Elaboration of BiVO4/TiO2 Photocatalysts

BiVO4 was introduced into the TiO2 material by a hydrothermal method in order to obtain 2, 5, and 10 wt% BiVO4 loading in the final BiVO4/TiO2 composite.


### 2.2.1. BiVO4/TiO2 Composite Synthesis by Hydrothermal Route

The BiVO4/TiO2 catalyst was prepared using the following protocol (Figure 1): 0.59 g of BiNO3 was first dissolved in an aqueous solution of nitric acid (4 mol·L−1) to form solution A. Then, solution B was obtained by dissolving 0.18 g of NH4VO3 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. The obtained BiVO4 suspension was subsequently added dropwise to a TiO2 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). The mixture was stirred for 24 h at 40°C. The pH of the solution was adjusted to 7 by dripping the NaOH solution (4 mol·L−1). The 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.Figure 1
Experimental protocol of BiVO4/TiO2 synthesis.All composites were heat treated under oxygen flow at 500°C for 3 h.


## 2.2.1. BiVO4/TiO2 Composite Synthesis by Hydrothermal Route

The BiVO4/TiO2 catalyst was prepared using the following protocol (Figure 1): 0.59 g of BiNO3 was first dissolved in an aqueous solution of nitric acid (4 mol·L−1) to form solution A. Then, solution B was obtained by dissolving 0.18 g of NH4VO3 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. The obtained BiVO4 suspension was subsequently added dropwise to a TiO2 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). The mixture was stirred for 24 h at 40°C. The pH of the solution was adjusted to 7 by dripping the NaOH solution (4 mol·L−1). The 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.Figure 1
Experimental protocol of BiVO4/TiO2 synthesis.All composites were heat treated under oxygen flow at 500°C for 3 h.


## 2.3. Catalysts Characterization

The structure and the particle sizes of as-synthesized BiVO4/TiO2 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 LabRAM (Horiba scientific) using a green line of 563 nm Ar-laser as the excitation source. The morphologies of the BiVO4/TiO2 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 BiVO4/TiO2 nanocomposite through HRTEM JEOL JEM-ARM 200F.The Brunauer–Emmett–Teller (BET) surface area and porosity were measured byN2 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. The photoluminescence spectra were recorded via a Perkin Elmer Lambda S55 (LS55) spectrophotometer equipped with a xenon lamp excitation wavelength of 300 nm.


## 2.4. Photocatalytic Activity Test

The photocatalytic performances of the as-prepared catalysts were examined in the degradation of methylene blue (MB) in aqueous solution and under solar light irradiation. This 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. The luminous flux emitted by solar radiation was measured by a radiometer equipped with a detector, and it is close to 1,743 mWcm−2. The photodegradation process was followed using a Shimadzu UV-3600 spectrophotometer at a wavelength λmax = 665 nm which is the maximum absorption of MB. The photodegradation rate was calculated according to Beer–Lambert’s law [29]:(1)Degradation efficiency%=A0−AtA0×100,where A0 is the initial absorbance of MB, and At is the absorbance after irradiation at time t.


## 3. Results and Discussion

### 3.1. Structural and Morphological Properties

#### 3.1.1. XRD Analysis

Figure2 illustrates the X-ray diffractograms of the BiVO4/TiO2 composites prepared using hydrothermal route and calcined at 500°C.Figure 2
XRD patterns of pure TiO2, pure BVO4, and BiVO4/TiO2 nanocomposites with different BiVO4 contents.The XRD pattern of bare BiVO4 was in good agreement with the monoclinic scheelite BiVO4 phase according to JCPDS No. 14–0688. For pure TiO2, the diffraction peaks attributed to the anatase phase at 25.3°, 37.9°, 48.1°, 54.0°, 55.2°, and 62.8° could be perfectly assigned to the (101), (004), (200), (105), (211), and (204) crystal diffraction planes of anatase TiO2 according to the database of JCPDS card No. 21–1272. The diffraction peaks of the as-prepared BiVO4/TiO2 solids with different contents exhibit only the characteristic peaks of the TiO2 anatase phase. For all the samples, no characteristic peaks of BiVO4 or any impurity were observed in their XRD patterns which indicate that BiVO4 is well dispersed onto the TiO2 surface. These results showed the presence of only anatase TiO2 which corroborates well that the combination of these two semiconductors preserves the crystalline structure of TiO2. It is clear that increasing the BiVO4 leads to a reduction of the diffraction peaks intensities of samples suggesting the possible decrease of TiO2 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. These 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 TiO2. It is possible to infer that the introduction of BiVO4 by hydrothermal treatment could distort the TiO2 network. These 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.Table1 shows the average crystallite size of the as-synthesized composites which were calculated by applying Scherrer’s formula [32] using the line width of the (101) diffraction peak of TiO2. Results show that the average crystallite size of the prepared materials is between the sizes of pure TiO2 and BiVO4. According to this table, it can be seen that the introduction of BiVO4 has significantly reduced the crystallite size compared to pristine TiO2. These findings are in good agreement with observations made by Song et al. [24]. In fact, the authors proved that an appropriate amount of BiVO4 could inhibit the crystallite growth of TiO2. This is probably due to the fact that the crystalline growth of BiVO4 which starts into the surface of TiO2 is highly affected by the establishment of chemical interactions between the two materials. In fact, the Bi3+ and V5+ species remaining in the solution could participate in the germination process and crystal growth. Thereby, a strong interaction between Bi3+, V5+ species, and titanium alkoxide could explain this observation. In this case, the TiO2 crystal growth around BiVO4 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.Table 1
Structural parameters of pure TiO2, pure BiVO4, and BiVO4/TiO2 nanocomposites determined from XRD data.
CatalystsXRD crystallite size (nm)a (Å)b (Å)c (Å)Unit cell volume (Å3)TiO2543.783.789.51135.882 wt% BiVO4/TiO2183.783.789.50135.935 wt% BiVO4/TiO2223.783.789.51136.2710 wt% BiVO4/TiO2313.783.789.53136.93BiVO4455.175.1011.67308.55


#### 3.1.2. Raman Spectroscopy

The 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 BiVO4 on the local structure of TiO2. The Raman spectra of composites with the scope of 100–1000 cm−1 are reported in Figure 3. Raman spectrum analysis of pristine BiVO4 (Figure 3(a)) shows the presence of six typical vibrational bands at 821, 706.3, 364.2, 331.3, 209.6, and 123.7 cm−1 which are characteristic of monoclinic scheelite phase of BiVO4 and confirming the XRD results. The intense band at 821 cm−1 is assigned to the symmetric V-O stretching mode in V units with a small shoulder at 706.3 cm−1 assigned to antisymmetric V-O vibrations. The bands appearing at 331.3 and 364.2 cm−1 are attributed to the bending vibration of V-O bonds. Likewise, two peaks located at 209.6 cm−1 and 123.7 cm−1 are assigned to the rotational and translational vibrations, respectively [33]. Raman spectrum of bare TiO2 and mixed oxides with different BiVO4 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 E1g, E2g, B1g, A1g/B1g, and Eg [34]. No peaks corresponding to the scheelite monoclinic BiVO4 phase were observed. Thereby, the addition of BiVO4 leads to a significant reduction of the peak intensity related to the Eg active mode of anatase at 146.8 cm−1 for mixed oxides and especially at low content (2 wt% BiVO4/TiO2) in comparison with pure TiO2 which exhibits an intense peak. This 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]. The highlighted conclusion from this observation is the effect of BiVO4 in the generation of oxygen site defects which originates from the deformation of the TiO2 lattice after its modification following BiVO4 introduction.Figure 3
(a) Raman spectra of materials and (b) magnified spectra of the 133–165 cm−1 region.
(a)(b)


#### 3.1.3. SEM Analysis

The scanning electron microscopy images of pure BiVO4, pure TiO2, and 2 wt% BiVO4/TiO2 samples are depicted in Figure 4. An enlarged SEM micrograph of pristine BiVO4 (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 TiO2 is aleatory and the grains are large with heterogeneous size since random distribution causes agglomeration of grains [36]. The 2 wt% BiVO4/TiO2 nanocomposite (Figure 4(d)) has a morphology similar to pure TiO2 but with smaller particle size. It is clearly seen that the BiVO4 reduced the size of the titanium dioxide nanoparticles which fits well with the XRD results.Figure 4
SEM images of the as elaborated samples: (a), (b) BiVO4, (c) TiO2, and (d) 2 wt% BiVO4/TiO2.
(a)(b)(c)(d)


#### 3.1.4. TEM Analysis

The nanomorphology of Pt-BV/T(X) photocatalysts was analyzed by TEM and HRTEM (Figure5). TEM micrographs of 2 wt% BiVO4/TiO2 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 TiO2, while the interplanar distance of 0.308 nm was indexed to the (121) planes of BiVO4 [37, 38]. HRTEM image clearly showed the formation of BiVO4/TiO2 heterojunction with great interaction between BiVO4 and TiO2.Figure 5
TEM, HRTEM, and average particle size images of 2 wt% BiVO4/TiO2 nanocomposite.
(a)(b)


### 3.2. Textural and Surface Properties

#### 3.2.1. N2 Adsorption-Desorption Analysis

A series of nitrogen adsorption-desorption isotherms at 77 K were performed as shown in Figure6. 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 H2 (b) hysteresis loop was attributed to the bare TiO2. 2 wt% and 5 wt% BiVO4/TiO2 and the pure BiVO4 samples display an H3 hysteresis loop characteristic of nonrigid aggregates of plate particles with slit-shaped pores. For the 10 wt% BiVO4/TiO2 sample, an overlapping of both H3 and H2 (b) hysteresis loops was observed. The pore size distribution curves of materials (Figure 7) show monomodal patterns. It can be seen that increasing the BiVO4 loading shifts the pore size distribution to lower pore diameter values except for 5 wt% loading. This could be probably due to the reduction of the intraparticle pore size [40].Figure 6
Nitrogen adsorption-desorption isotherm of samples.Figure 7
BJH pore size distributions of samples.The relative pressure range indicates that earlier is the onset of the loops, smaller is the diameter of the mesopores. Table2 lists the relative pressures P/P° of the hysteresis loops onset of the as-synthesized materials. Outcomes revealed that the addition of BiVO4 increases the relative pressure P/P° of the onset of the hysteresis loop for the catalysts except for 10 wt% loading. This fits well with the BJH pore size distribution which confirms that pure TiO2 having the smallest P/P° value (0.73) exhibits the smallest pore diameter (169 Å). The relative pressure value of 10 wt% BiVO4/TiO2 (0.75) agrees well with the result of porous diameter (188). However, it does not follow the linear trend with the percentage of BiVO4. This could probably be due to the start of a change in the hysteresis loop from H2b to H3. Thereby, it is obvious to highlight the important effect of BiVO4 on the texture and its importance in the control of the mesopores sizes and the particles morphologies. Compared to BiVO4, the BiVO4/TiO2 composites have higher specific surface areas and larger pore volume but smaller pore size, and compared to the pure TiO2, the incorporation of BiVO4 seems to improve the textural properties of the materials. These results are in good agreement with the specific surface area (Table 2). The obtained results showed that the surface area of samples exhibits improvement after the introduction of BiVO4 ranging from 67 m2g−1 for the BiVO4 free TiO2 to 94 m2g−1 for 10 wt% BiVO4/TiO2.Table 2
Specific surface area (SBET), pore volume, average pore diameter, and onset hysteresis loops (P/P°) of catalysts determined from N2 chemisorption at 77 K using the BET equation.
CatalystsSBET (m2g−1)Pore volume (cm3g−1)Pore size (Å)Onset of hysteresis loops (P/P°)BiVO470.053630.8010 wt% BiVO4/TiO2940.561880.755 wt% BiVO4/TiO2690.592740.792 wt% BiVO4/TiO2740.592650.78TiO2670.311690.73


#### 3.2.2. X-Ray Photoelectron Spectroscopy

In order to ascertain the formation of BiVO4 on the surface of TiO2 and investigate the valence state of bismuth and vanadium, an XPS test was performed on the 2 wt% BiVO4/TiO2 catalyst. Table 3 shows the existence of Ti, O, Bi, and V species. The characteristic peaks of Ti4+ in the tetragonal structure (Ti 2p3/2 and Ti 2p1/2) were centered at 458.6 eV and 464 eV, respectively [41]. The 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 V2p3/2 and V2p1/2, respectively. The difference in binding energy between V2p3/2 and V2p1/2 is 7.6 eV proves the +5 oxidation state of vanadium. The split binding energy peaks of Bi4f7/2 and Bi4f5/2 (Figure 8(b)) were centered, respectively, at 159.2 eV and 164.4 eV. The distance between these two peaks is found to be 5.2 eV which is specific to Bi3+ [30] in a typical heterojunction sample [43]. Thereby, 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 BiVO4. Thereafter, a heterojunction between this latter and TiO2 is probably created.Table 3
Binding energies (eV) of core levels of 2 wt% BiVO4/TiO2 sample determined from XPS data.
Bi4f7/2Bi4f5/2V2p3/2V2p1/2O1sTi2p3/2Ti2p1/2Binding energy (eV)159.2164.4517.2524.7530.2458.6464Figure 8
XPS spectra of (a) V2p and (b) Bi4f of the sample 2 wt% BiVO4/TiO2.
(a)(b)


### 3.3. Optical Properties

#### 3.3.1. UV-Vis Diffuse Reflectance Spectroscopy

In order to understand the evolution of the optical properties of BiVO4/TiO2 nanocomposites compared to pure TiO2, a series of UV-visible measurements were performed in the wavelength range 250–650 nm as shown in Figure 9. As can be seen, pure TiO2 showed strong photoresponsiveness in the UV region estimated at 400 nm, whereas that of BiVO4 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 BiVO4 incorporation compared to bare titanium dioxide in the visible light range, and it increases with raising the bismuth vanadate content. This 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 TiO2 lattice creating thus intermediate energy levels between BiVO4 and TiO2 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.Figure 9
UV-vis absorption spectra of samples.


#### 3.3.2. Photoluminescence Analysis

Photoluminescence (PL) measurements were carried out in order to investigate the migration, transfer, and trapping of the photogenerated electron-hole pairs in the titanium dioxide solid. Thus, 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 electron-hole. Therefore, 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 BiVO4 incorporation on the optical properties of TiO2, the photoluminescence properties of catalysts were examined using an exciting wavelength at 300 nm (Figure 10). The PL spectra of both pure TiO2 and the BiVO4/TiO2 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). The peak centered at 414 nm is related to the recombination of photogenerated electron-hole pairs into the bulk TiO2 lattice [47]. The peak at 477 nm can be interpreted as the charge transfer from Ti3+ to the oxygen anion localized in Ti octahedral [48]. The 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 TiO2 anatase phase generated by oxygen vacancies [50]. The analysis of the pure BiVO4 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 BiVO4 with the holes formed in the valence band of O2p orbital [26]. Herein, it is clear that the higher PL intensity of the bare TiO2 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 BiVO4/TiO2 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 BiVO4/TiO2 material. The PL intensity decreases after the incorporation of BiVO4. This 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 BiVO4 and TiO2 allow the photogenerated electrons to jump from the BiVO4 valence band to the TiO2 conduction band [52]. However, the BiVO4 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.Figure 10
Photoluminescence spectra of photocatalysts.Figure 11
Deconvoluted photoluminescence spectra of (a) TiO2, (b) 2 wt% BiVO4/TiO2, (c) 5 wt% BiVO4/TiO2, and (d) 10 wt% BiVO4/TiO2.
(a)(b)(c)(d)


## 3.1. Structural and Morphological Properties

### 3.1.1. XRD Analysis

Figure2 illustrates the X-ray diffractograms of the BiVO4/TiO2 composites prepared using hydrothermal route and calcined at 500°C.Figure 2
XRD patterns of pure TiO2, pure BVO4, and BiVO4/TiO2 nanocomposites with different BiVO4 contents.The XRD pattern of bare BiVO4 was in good agreement with the monoclinic scheelite BiVO4 phase according to JCPDS No. 14–0688. For pure TiO2, the diffraction peaks attributed to the anatase phase at 25.3°, 37.9°, 48.1°, 54.0°, 55.2°, and 62.8° could be perfectly assigned to the (101), (004), (200), (105), (211), and (204) crystal diffraction planes of anatase TiO2 according to the database of JCPDS card No. 21–1272. The diffraction peaks of the as-prepared BiVO4/TiO2 solids with different contents exhibit only the characteristic peaks of the TiO2 anatase phase. For all the samples, no characteristic peaks of BiVO4 or any impurity were observed in their XRD patterns which indicate that BiVO4 is well dispersed onto the TiO2 surface. These results showed the presence of only anatase TiO2 which corroborates well that the combination of these two semiconductors preserves the crystalline structure of TiO2. It is clear that increasing the BiVO4 leads to a reduction of the diffraction peaks intensities of samples suggesting the possible decrease of TiO2 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. These 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 TiO2. It is possible to infer that the introduction of BiVO4 by hydrothermal treatment could distort the TiO2 network. These 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.Table1 shows the average crystallite size of the as-synthesized composites which were calculated by applying Scherrer’s formula [32] using the line width of the (101) diffraction peak of TiO2. Results show that the average crystallite size of the prepared materials is between the sizes of pure TiO2 and BiVO4. According to this table, it can be seen that the introduction of BiVO4 has significantly reduced the crystallite size compared to pristine TiO2. These findings are in good agreement with observations made by Song et al. [24]. In fact, the authors proved that an appropriate amount of BiVO4 could inhibit the crystallite growth of TiO2. This is probably due to the fact that the crystalline growth of BiVO4 which starts into the surface of TiO2 is highly affected by the establishment of chemical interactions between the two materials. In fact, the Bi3+ and V5+ species remaining in the solution could participate in the germination process and crystal growth. Thereby, a strong interaction between Bi3+, V5+ species, and titanium alkoxide could explain this observation. In this case, the TiO2 crystal growth around BiVO4 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.Table 1
Structural parameters of pure TiO2, pure BiVO4, and BiVO4/TiO2 nanocomposites determined from XRD data.
CatalystsXRD crystallite size (nm)a (Å)b (Å)c (Å)Unit cell volume (Å3)TiO2543.783.789.51135.882 wt% BiVO4/TiO2183.783.789.50135.935 wt% BiVO4/TiO2223.783.789.51136.2710 wt% BiVO4/TiO2313.783.789.53136.93BiVO4455.175.1011.67308.55


### 3.1.2. Raman Spectroscopy

The 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 BiVO4 on the local structure of TiO2. The Raman spectra of composites with the scope of 100–1000 cm−1 are reported in Figure 3. Raman spectrum analysis of pristine BiVO4 (Figure 3(a)) shows the presence of six typical vibrational bands at 821, 706.3, 364.2, 331.3, 209.6, and 123.7 cm−1 which are characteristic of monoclinic scheelite phase of BiVO4 and confirming the XRD results. The intense band at 821 cm−1 is assigned to the symmetric V-O stretching mode in V units with a small shoulder at 706.3 cm−1 assigned to antisymmetric V-O vibrations. The bands appearing at 331.3 and 364.2 cm−1 are attributed to the bending vibration of V-O bonds. Likewise, two peaks located at 209.6 cm−1 and 123.7 cm−1 are assigned to the rotational and translational vibrations, respectively [33]. Raman spectrum of bare TiO2 and mixed oxides with different BiVO4 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 E1g, E2g, B1g, A1g/B1g, and Eg [34]. No peaks corresponding to the scheelite monoclinic BiVO4 phase were observed. Thereby, the addition of BiVO4 leads to a significant reduction of the peak intensity related to the Eg active mode of anatase at 146.8 cm−1 for mixed oxides and especially at low content (2 wt% BiVO4/TiO2) in comparison with pure TiO2 which exhibits an intense peak. This 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]. The highlighted conclusion from this observation is the effect of BiVO4 in the generation of oxygen site defects which originates from the deformation of the TiO2 lattice after its modification following BiVO4 introduction.Figure 3
(a) Raman spectra of materials and (b) magnified spectra of the 133–165 cm−1 region.
(a)(b)


### 3.1.3. SEM Analysis

The scanning electron microscopy images of pure BiVO4, pure TiO2, and 2 wt% BiVO4/TiO2 samples are depicted in Figure 4. An enlarged SEM micrograph of pristine BiVO4 (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 TiO2 is aleatory and the grains are large with heterogeneous size since random distribution causes agglomeration of grains [36]. The 2 wt% BiVO4/TiO2 nanocomposite (Figure 4(d)) has a morphology similar to pure TiO2 but with smaller particle size. It is clearly seen that the BiVO4 reduced the size of the titanium dioxide nanoparticles which fits well with the XRD results.Figure 4
SEM images of the as elaborated samples: (a), (b) BiVO4, (c) TiO2, and (d) 2 wt% BiVO4/TiO2.
(a)(b)(c)(d)


### 3.1.4. TEM Analysis

The nanomorphology of Pt-BV/T(X) photocatalysts was analyzed by TEM and HRTEM (Figure5). TEM micrographs of 2 wt% BiVO4/TiO2 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 TiO2, while the interplanar distance of 0.308 nm was indexed to the (121) planes of BiVO4 [37, 38]. HRTEM image clearly showed the formation of BiVO4/TiO2 heterojunction with great interaction between BiVO4 and TiO2.Figure 5
TEM, HRTEM, and average particle size images of 2 wt% BiVO4/TiO2 nanocomposite.
(a)(b)


## 3.1.1. XRD Analysis

Figure2 illustrates the X-ray diffractograms of the BiVO4/TiO2 composites prepared using hydrothermal route and calcined at 500°C.Figure 2
XRD patterns of pure TiO2, pure BVO4, and BiVO4/TiO2 nanocomposites with different BiVO4 contents.The XRD pattern of bare BiVO4 was in good agreement with the monoclinic scheelite BiVO4 phase according to JCPDS No. 14–0688. For pure TiO2, the diffraction peaks attributed to the anatase phase at 25.3°, 37.9°, 48.1°, 54.0°, 55.2°, and 62.8° could be perfectly assigned to the (101), (004), (200), (105), (211), and (204) crystal diffraction planes of anatase TiO2 according to the database of JCPDS card No. 21–1272. The diffraction peaks of the as-prepared BiVO4/TiO2 solids with different contents exhibit only the characteristic peaks of the TiO2 anatase phase. For all the samples, no characteristic peaks of BiVO4 or any impurity were observed in their XRD patterns which indicate that BiVO4 is well dispersed onto the TiO2 surface. These results showed the presence of only anatase TiO2 which corroborates well that the combination of these two semiconductors preserves the crystalline structure of TiO2. It is clear that increasing the BiVO4 leads to a reduction of the diffraction peaks intensities of samples suggesting the possible decrease of TiO2 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. These 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 TiO2. It is possible to infer that the introduction of BiVO4 by hydrothermal treatment could distort the TiO2 network. These 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.Table1 shows the average crystallite size of the as-synthesized composites which were calculated by applying Scherrer’s formula [32] using the line width of the (101) diffraction peak of TiO2. Results show that the average crystallite size of the prepared materials is between the sizes of pure TiO2 and BiVO4. According to this table, it can be seen that the introduction of BiVO4 has significantly reduced the crystallite size compared to pristine TiO2. These findings are in good agreement with observations made by Song et al. [24]. In fact, the authors proved that an appropriate amount of BiVO4 could inhibit the crystallite growth of TiO2. This is probably due to the fact that the crystalline growth of BiVO4 which starts into the surface of TiO2 is highly affected by the establishment of chemical interactions between the two materials. In fact, the Bi3+ and V5+ species remaining in the solution could participate in the germination process and crystal growth. Thereby, a strong interaction between Bi3+, V5+ species, and titanium alkoxide could explain this observation. In this case, the TiO2 crystal growth around BiVO4 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.Table 1
Structural parameters of pure TiO2, pure BiVO4, and BiVO4/TiO2 nanocomposites determined from XRD data.
CatalystsXRD crystallite size (nm)a (Å)b (Å)c (Å)Unit cell volume (Å3)TiO2543.783.789.51135.882 wt% BiVO4/TiO2183.783.789.50135.935 wt% BiVO4/TiO2223.783.789.51136.2710 wt% BiVO4/TiO2313.783.789.53136.93BiVO4455.175.1011.67308.55


## 3.1.2. Raman Spectroscopy

The 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 BiVO4 on the local structure of TiO2. The Raman spectra of composites with the scope of 100–1000 cm−1 are reported in Figure 3. Raman spectrum analysis of pristine BiVO4 (Figure 3(a)) shows the presence of six typical vibrational bands at 821, 706.3, 364.2, 331.3, 209.6, and 123.7 cm−1 which are characteristic of monoclinic scheelite phase of BiVO4 and confirming the XRD results. The intense band at 821 cm−1 is assigned to the symmetric V-O stretching mode in V units with a small shoulder at 706.3 cm−1 assigned to antisymmetric V-O vibrations. The bands appearing at 331.3 and 364.2 cm−1 are attributed to the bending vibration of V-O bonds. Likewise, two peaks located at 209.6 cm−1 and 123.7 cm−1 are assigned to the rotational and translational vibrations, respectively [33]. Raman spectrum of bare TiO2 and mixed oxides with different BiVO4 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 E1g, E2g, B1g, A1g/B1g, and Eg [34]. No peaks corresponding to the scheelite monoclinic BiVO4 phase were observed. Thereby, the addition of BiVO4 leads to a significant reduction of the peak intensity related to the Eg active mode of anatase at 146.8 cm−1 for mixed oxides and especially at low content (2 wt% BiVO4/TiO2) in comparison with pure TiO2 which exhibits an intense peak. This 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]. The highlighted conclusion from this observation is the effect of BiVO4 in the generation of oxygen site defects which originates from the deformation of the TiO2 lattice after its modification following BiVO4 introduction.Figure 3
(a) Raman spectra of materials and (b) magnified spectra of the 133–165 cm−1 region.
(a)(b)


## 3.1.3. SEM Analysis

The scanning electron microscopy images of pure BiVO4, pure TiO2, and 2 wt% BiVO4/TiO2 samples are depicted in Figure 4. An enlarged SEM micrograph of pristine BiVO4 (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 TiO2 is aleatory and the grains are large with heterogeneous size since random distribution causes agglomeration of grains [36]. The 2 wt% BiVO4/TiO2 nanocomposite (Figure 4(d)) has a morphology similar to pure TiO2 but with smaller particle size. It is clearly seen that the BiVO4 reduced the size of the titanium dioxide nanoparticles which fits well with the XRD results.Figure 4
SEM images of the as elaborated samples: (a), (b) BiVO4, (c) TiO2, and (d) 2 wt% BiVO4/TiO2.
(a)(b)(c)(d)


## 3.1.4. TEM Analysis

The nanomorphology of Pt-BV/T(X) photocatalysts was analyzed by TEM and HRTEM (Figure5). TEM micrographs of 2 wt% BiVO4/TiO2 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 TiO2, while the interplanar distance of 0.308 nm was indexed to the (121) planes of BiVO4 [37, 38]. HRTEM image clearly showed the formation of BiVO4/TiO2 heterojunction with great interaction between BiVO4 and TiO2.Figure 5
TEM, HRTEM, and average particle size images of 2 wt% BiVO4/TiO2 nanocomposite.
(a)(b)


## 3.2. Textural and Surface Properties

### 3.2.1. N2 Adsorption-Desorption Analysis

A series of nitrogen adsorption-desorption isotherms at 77 K were performed as shown in Figure6. 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 H2 (b) hysteresis loop was attributed to the bare TiO2. 2 wt% and 5 wt% BiVO4/TiO2 and the pure BiVO4 samples display an H3 hysteresis loop characteristic of nonrigid aggregates of plate particles with slit-shaped pores. For the 10 wt% BiVO4/TiO2 sample, an overlapping of both H3 and H2 (b) hysteresis loops was observed. The pore size distribution curves of materials (Figure 7) show monomodal patterns. It can be seen that increasing the BiVO4 loading shifts the pore size distribution to lower pore diameter values except for 5 wt% loading. This could be probably due to the reduction of the intraparticle pore size [40].Figure 6
Nitrogen adsorption-desorption isotherm of samples.Figure 7
BJH pore size distributions of samples.The relative pressure range indicates that earlier is the onset of the loops, smaller is the diameter of the mesopores. Table2 lists the relative pressures P/P° of the hysteresis loops onset of the as-synthesized materials. Outcomes revealed that the addition of BiVO4 increases the relative pressure P/P° of the onset of the hysteresis loop for the catalysts except for 10 wt% loading. This fits well with the BJH pore size distribution which confirms that pure TiO2 having the smallest P/P° value (0.73) exhibits the smallest pore diameter (169 Å). The relative pressure value of 10 wt% BiVO4/TiO2 (0.75) agrees well with the result of porous diameter (188). However, it does not follow the linear trend with the percentage of BiVO4. This could probably be due to the start of a change in the hysteresis loop from H2b to H3. Thereby, it is obvious to highlight the important effect of BiVO4 on the texture and its importance in the control of the mesopores sizes and the particles morphologies. Compared to BiVO4, the BiVO4/TiO2 composites have higher specific surface areas and larger pore volume but smaller pore size, and compared to the pure TiO2, the incorporation of BiVO4 seems to improve the textural properties of the materials. These results are in good agreement with the specific surface area (Table 2). The obtained results showed that the surface area of samples exhibits improvement after the introduction of BiVO4 ranging from 67 m2g−1 for the BiVO4 free TiO2 to 94 m2g−1 for 10 wt% BiVO4/TiO2.Table 2
Specific surface area (SBET), pore volume, average pore diameter, and onset hysteresis loops (P/P°) of catalysts determined from N2 chemisorption at 77 K using the BET equation.
CatalystsSBET (m2g−1)Pore volume (cm3g−1)Pore size (Å)Onset of hysteresis loops (P/P°)BiVO470.053630.8010 wt% BiVO4/TiO2940.561880.755 wt% BiVO4/TiO2690.592740.792 wt% BiVO4/TiO2740.592650.78TiO2670.311690.73


### 3.2.2. X-Ray Photoelectron Spectroscopy

In order to ascertain the formation of BiVO4 on the surface of TiO2 and investigate the valence state of bismuth and vanadium, an XPS test was performed on the 2 wt% BiVO4/TiO2 catalyst. Table 3 shows the existence of Ti, O, Bi, and V species. The characteristic peaks of Ti4+ in the tetragonal structure (Ti 2p3/2 and Ti 2p1/2) were centered at 458.6 eV and 464 eV, respectively [41]. The 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 V2p3/2 and V2p1/2, respectively. The difference in binding energy between V2p3/2 and V2p1/2 is 7.6 eV proves the +5 oxidation state of vanadium. The split binding energy peaks of Bi4f7/2 and Bi4f5/2 (Figure 8(b)) were centered, respectively, at 159.2 eV and 164.4 eV. The distance between these two peaks is found to be 5.2 eV which is specific to Bi3+ [30] in a typical heterojunction sample [43]. Thereby, 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 BiVO4. Thereafter, a heterojunction between this latter and TiO2 is probably created.Table 3
Binding energies (eV) of core levels of 2 wt% BiVO4/TiO2 sample determined from XPS data.
Bi4f7/2Bi4f5/2V2p3/2V2p1/2O1sTi2p3/2Ti2p1/2Binding energy (eV)159.2164.4517.2524.7530.2458.6464Figure 8
XPS spectra of (a) V2p and (b) Bi4f of the sample 2 wt% BiVO4/TiO2.
(a)(b)


## 3.2.1. N2 Adsorption-Desorption Analysis

A series of nitrogen adsorption-desorption isotherms at 77 K were performed as shown in Figure6. 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 H2 (b) hysteresis loop was attributed to the bare TiO2. 2 wt% and 5 wt% BiVO4/TiO2 and the pure BiVO4 samples display an H3 hysteresis loop characteristic of nonrigid aggregates of plate particles with slit-shaped pores. For the 10 wt% BiVO4/TiO2 sample, an overlapping of both H3 and H2 (b) hysteresis loops was observed. The pore size distribution curves of materials (Figure 7) show monomodal patterns. It can be seen that increasing the BiVO4 loading shifts the pore size distribution to lower pore diameter values except for 5 wt% loading. This could be probably due to the reduction of the intraparticle pore size [40].Figure 6
Nitrogen adsorption-desorption isotherm of samples.Figure 7
BJH pore size distributions of samples.The relative pressure range indicates that earlier is the onset of the loops, smaller is the diameter of the mesopores. Table2 lists the relative pressures P/P° of the hysteresis loops onset of the as-synthesized materials. Outcomes revealed that the addition of BiVO4 increases the relative pressure P/P° of the onset of the hysteresis loop for the catalysts except for 10 wt% loading. This fits well with the BJH pore size distribution which confirms that pure TiO2 having the smallest P/P° value (0.73) exhibits the smallest pore diameter (169 Å). The relative pressure value of 10 wt% BiVO4/TiO2 (0.75) agrees well with the result of porous diameter (188). However, it does not follow the linear trend with the percentage of BiVO4. This could probably be due to the start of a change in the hysteresis loop from H2b to H3. Thereby, it is obvious to highlight the important effect of BiVO4 on the texture and its importance in the control of the mesopores sizes and the particles morphologies. Compared to BiVO4, the BiVO4/TiO2 composites have higher specific surface areas and larger pore volume but smaller pore size, and compared to the pure TiO2, the incorporation of BiVO4 seems to improve the textural properties of the materials. These results are in good agreement with the specific surface area (Table 2). The obtained results showed that the surface area of samples exhibits improvement after the introduction of BiVO4 ranging from 67 m2g−1 for the BiVO4 free TiO2 to 94 m2g−1 for 10 wt% BiVO4/TiO2.Table 2
Specific surface area (SBET), pore volume, average pore diameter, and onset hysteresis loops (P/P°) of catalysts determined from N2 chemisorption at 77 K using the BET equation.
CatalystsSBET (m2g−1)Pore volume (cm3g−1)Pore size (Å)Onset of hysteresis loops (P/P°)BiVO470.053630.8010 wt% BiVO4/TiO2940.561880.755 wt% BiVO4/TiO2690.592740.792 wt% BiVO4/TiO2740.592650.78TiO2670.311690.73


## 3.2.2. X-Ray Photoelectron Spectroscopy

In order to ascertain the formation of BiVO4 on the surface of TiO2 and investigate the valence state of bismuth and vanadium, an XPS test was performed on the 2 wt% BiVO4/TiO2 catalyst. Table 3 shows the existence of Ti, O, Bi, and V species. The characteristic peaks of Ti4+ in the tetragonal structure (Ti 2p3/2 and Ti 2p1/2) were centered at 458.6 eV and 464 eV, respectively [41]. The 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 V2p3/2 and V2p1/2, respectively. The difference in binding energy between V2p3/2 and V2p1/2 is 7.6 eV proves the +5 oxidation state of vanadium. The split binding energy peaks of Bi4f7/2 and Bi4f5/2 (Figure 8(b)) were centered, respectively, at 159.2 eV and 164.4 eV. The distance between these two peaks is found to be 5.2 eV which is specific to Bi3+ [30] in a typical heterojunction sample [43]. Thereby, 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 BiVO4. Thereafter, a heterojunction between this latter and TiO2 is probably created.Table 3
Binding energies (eV) of core levels of 2 wt% BiVO4/TiO2 sample determined from XPS data.
Bi4f7/2Bi4f5/2V2p3/2V2p1/2O1sTi2p3/2Ti2p1/2Binding energy (eV)159.2164.4517.2524.7530.2458.6464Figure 8
XPS spectra of (a) V2p and (b) Bi4f of the sample 2 wt% BiVO4/TiO2.
(a)(b)


## 3.3. Optical Properties

### 3.3.1. UV-Vis Diffuse Reflectance Spectroscopy

In order to understand the evolution of the optical properties of BiVO4/TiO2 nanocomposites compared to pure TiO2, a series of UV-visible measurements were performed in the wavelength range 250–650 nm as shown in Figure 9. As can be seen, pure TiO2 showed strong photoresponsiveness in the UV region estimated at 400 nm, whereas that of BiVO4 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 BiVO4 incorporation compared to bare titanium dioxide in the visible light range, and it increases with raising the bismuth vanadate content. This 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 TiO2 lattice creating thus intermediate energy levels between BiVO4 and TiO2 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.Figure 9
UV-vis absorption spectra of samples.


### 3.3.2. Photoluminescence Analysis

Photoluminescence (PL) measurements were carried out in order to investigate the migration, transfer, and trapping of the photogenerated electron-hole pairs in the titanium dioxide solid. Thus, 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 electron-hole. Therefore, 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 BiVO4 incorporation on the optical properties of TiO2, the photoluminescence properties of catalysts were examined using an exciting wavelength at 300 nm (Figure 10). The PL spectra of both pure TiO2 and the BiVO4/TiO2 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). The peak centered at 414 nm is related to the recombination of photogenerated electron-hole pairs into the bulk TiO2 lattice [47]. The peak at 477 nm can be interpreted as the charge transfer from Ti3+ to the oxygen anion localized in Ti octahedral [48]. The 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 TiO2 anatase phase generated by oxygen vacancies [50]. The analysis of the pure BiVO4 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 BiVO4 with the holes formed in the valence band of O2p orbital [26]. Herein, it is clear that the higher PL intensity of the bare TiO2 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 BiVO4/TiO2 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 BiVO4/TiO2 material. The PL intensity decreases after the incorporation of BiVO4. This 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 BiVO4 and TiO2 allow the photogenerated electrons to jump from the BiVO4 valence band to the TiO2 conduction band [52]. However, the BiVO4 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.Figure 10
Photoluminescence spectra of photocatalysts.Figure 11
Deconvoluted photoluminescence spectra of (a) TiO2, (b) 2 wt% BiVO4/TiO2, (c) 5 wt% BiVO4/TiO2, and (d) 10 wt% BiVO4/TiO2.
(a)(b)(c)(d)


## 3.3.1. UV-Vis Diffuse Reflectance Spectroscopy

In order to understand the evolution of the optical properties of BiVO4/TiO2 nanocomposites compared to pure TiO2, a series of UV-visible measurements were performed in the wavelength range 250–650 nm as shown in Figure 9. As can be seen, pure TiO2 showed strong photoresponsiveness in the UV region estimated at 400 nm, whereas that of BiVO4 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 BiVO4 incorporation compared to bare titanium dioxide in the visible light range, and it increases with raising the bismuth vanadate content. This 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 TiO2 lattice creating thus intermediate energy levels between BiVO4 and TiO2 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.Figure 9
UV-vis absorption spectra of samples.


## 3.3.2. Photoluminescence Analysis

Photoluminescence (PL) measurements were carried out in order to investigate the migration, transfer, and trapping of the photogenerated electron-hole pairs in the titanium dioxide solid. Thus, 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 electron-hole. Therefore, 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 BiVO4 incorporation on the optical properties of TiO2, the photoluminescence properties of catalysts were examined using an exciting wavelength at 300 nm (Figure 10). The PL spectra of both pure TiO2 and the BiVO4/TiO2 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). The peak centered at 414 nm is related to the recombination of photogenerated electron-hole pairs into the bulk TiO2 lattice [47]. The peak at 477 nm can be interpreted as the charge transfer from Ti3+ to the oxygen anion localized in Ti octahedral [48]. The 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 TiO2 anatase phase generated by oxygen vacancies [50]. The analysis of the pure BiVO4 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 BiVO4 with the holes formed in the valence band of O2p orbital [26]. Herein, it is clear that the higher PL intensity of the bare TiO2 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 BiVO4/TiO2 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 BiVO4/TiO2 material. The PL intensity decreases after the incorporation of BiVO4. This 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 BiVO4 and TiO2 allow the photogenerated electrons to jump from the BiVO4 valence band to the TiO2 conduction band [52]. However, the BiVO4 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.Figure 10
Photoluminescence spectra of photocatalysts.Figure 11
Deconvoluted photoluminescence spectra of (a) TiO2, (b) 2 wt% BiVO4/TiO2, (c) 5 wt% BiVO4/TiO2, and (d) 10 wt% BiVO4/TiO2.
(a)(b)(c)(d)


## 4. Photocatalytic Activity

The 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 BiVO4 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 TiO2 and BiVO4 was performed under the same experimental conditions than the nanocomposites BiVO4/TiO2. As can be seen from Figure 12, the photocatalytic activity of pristine BiVO4 is very low under solar light irradiation. This finding could be essentially due to its reduced specific surface area (7 m2g−1) which restricts its adsorption ability towards the decomposition of organic molecules of MB. After coupling BiVO4 and TiO2, results revealed that MB degradation increases for certain bismuth vanadate amount, and the best photocatalytic conversion is attributed to the 2 wt% BiVO4/TiO2 composite indicating that 99% of MB was degraded after 60 min. Thereby, the photocatalytic conversion of this material is 2.5 times and 1.25 times higher than that of pure BiVO4 and TiO2, respectively. Nevertheless, the photocatalytic conversion of 10 wt% BiVO4/TiO2 is smaller than that of pristine TiO2 despite its higher surface area. Accordingly, it is worth to note that a high amount of BiVO4 appears to block the active sites [27]. The high conversion which is assigned to the low bismuth vanadate loading could be essentially due to the synergetic effect between BiVO4 and TiO2 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 BiVO4 leads the generation of oxygen surface defects. These oxygen vacancies inducing a TiO2 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 TiO2. This pleads in favor of the improvement of the absorption properties of titanium dioxide in the visible light range, on the one hand, and 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.Figure 12
Photocatalytic conversion of methylene blue under solar light irradiation for the different catalysts.


### 4.1. Suggested Photocatalytic Mechanism

According to the previous results, the photocatalytic mechanism of BiVO4/TiO2 heterojunction can be explained as given in Scheme 1. The suggested mechanism of the photocatalytic oxidation of methylene blue is illustrated in Scheme 2.Scheme 1
Schematic illustration of methylene blue degradation.Scheme 2
Suggested photocatalytic mechanism of methylene blue degradation.After thermodynamic equilibrium, when the BiVO4/TiO2 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 BiVO4 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 BiVO4 which is more negative than that of TiO2 on the other hand, the photoinduced electrons can be injected to the conduction band of TiO2 (equation (2)) [55].(2)BiVO4TiO2+hυ⟶BiVO4hVB++TiO2eCB−.Afterward, the injected electrons could capture the adsorbed O2 and assisting its reduction into O2•−superoxide radicals. Thus, O2•− radicals react in turn with H2O molecules and form hydroxyl ions (OH−) and hydroperoxyl radicals (HOO•) (equations (3) and (4)) [56].(3)O2+eCB−⟶O2•−,(4)O2•−+H2Oads⟶HOO•+OH−.As well as,h+ holes react with electron donors such as H2O andOH− anions adsorbed on the surface of the semiconductor and produce OH• hydroxyl groups (equations (5) and (6)) [57].(5)H2Oads+hVB+⟶H++OHads•,(6)OHads−+hVB+=OHads•,(7)MB+OHads•⟶CO2+H2O.The formed radicals (OH• and O2•−) 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 CO2, water, and organic chain acids (equation (7)) [58].So far, it is well recognized that oxygen vacancies (VO) promoted by the formed heterojunction act as a hole trapping center, resulting in the effective separation of photoinduced electron-hole pairs (equation (8)) [46]:(8)VO−+h+⟶VO.


## 4.1. Suggested Photocatalytic Mechanism

According to the previous results, the photocatalytic mechanism of BiVO4/TiO2 heterojunction can be explained as given in Scheme 1. The suggested mechanism of the photocatalytic oxidation of methylene blue is illustrated in Scheme 2.Scheme 1
Schematic illustration of methylene blue degradation.Scheme 2
Suggested photocatalytic mechanism of methylene blue degradation.After thermodynamic equilibrium, when the BiVO4/TiO2 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 BiVO4 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 BiVO4 which is more negative than that of TiO2 on the other hand, the photoinduced electrons can be injected to the conduction band of TiO2 (equation (2)) [55].(2)BiVO4TiO2+hυ⟶BiVO4hVB++TiO2eCB−.Afterward, the injected electrons could capture the adsorbed O2 and assisting its reduction into O2•−superoxide radicals. Thus, O2•− radicals react in turn with H2O molecules and form hydroxyl ions (OH−) and hydroperoxyl radicals (HOO•) (equations (3) and (4)) [56].(3)O2+eCB−⟶O2•−,(4)O2•−+H2Oads⟶HOO•+OH−.As well as,h+ holes react with electron donors such as H2O andOH− anions adsorbed on the surface of the semiconductor and produce OH• hydroxyl groups (equations (5) and (6)) [57].(5)H2Oads+hVB+⟶H++OHads•,(6)OHads−+hVB+=OHads•,(7)MB+OHads•⟶CO2+H2O.The formed radicals (OH• and O2•−) 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 CO2, water, and organic chain acids (equation (7)) [58].So far, it is well recognized that oxygen vacancies (VO) promoted by the formed heterojunction act as a hole trapping center, resulting in the effective separation of photoinduced electron-hole pairs (equation (8)) [46]:(8)VO−+h+⟶VO.


## 5. Reusability and Stability of BiVO4/TiO2 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. Therefore, the stability and reusability of the BiVO4/TiO2 nanocomposites could also be checked. In this concern, four cycles of photocatalytic MB degradation experiments via 2 wt% BiVO4/TiO2 composite were carried out under identical reaction conditions, and results are depicted in Figure 13. The 2 wt% BiVO4/TiO2 nanocomposite exhibited high efficiency and reusability; even the photocatalytic activities are 99.0%, 98.2%, 95.4%, and 89.7% after 1, 2, 3, and 4 successive cycles, respectively. The 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. This result is advantageous to the recycling of BiVO4/TiO2 photocatalyst and decreasing the application cost.Figure 13
Reusability test of 2 wt% BiVO4/TiO2 photocatalyst.The XRD patterns of fresh and used 2 wt% BiVO4/TiO2 photocatalyst (Figure 14) revealed that the four repeated uses do not affect the crystalline structure of anatase TiO2 which is in good agreement with the high activity and stability of the catalyst.Figure 14
XRD patterns of fresh and used 2 wt% BiVO4/TiO2 photocatalyst.


## 6. Conclusion

In the present investigation, we systematically studied the effect of different BiVO4 amounts on the structural, textural, and optical properties of BiVO4/TiO2 heterojunction materials which were elaborated by the one-pot hydrothermal method. The current outcomes elucidate that the creation of heterojunction between BiVO4 and TiO2 induces the formation of more site defects on the catalyst surface. Furthermore, we have demonstrated that BiVO4 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 TiO2 to the visible light range. This 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 BiVO4/TiO2 materials was evaluated in the methylene blue degradation reaction. Results revealed that the amount of BiVO4 in the BiVO4/TiO2 nanocomposite has a significant influence on the photoactivity of photocatalysts, and the highest degradation rate catalyst can be assigned to the 2 wt% BiVO4/TiO2 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, BiVO4/TiO2 photocatalyst displayed significant recyclability and stability for four catalytic cycles in the methylene blue photodegradation reaction. These results indicate that the nanocatalyst BiVO4/TiO2 can be used as a promising photocatalyst for the photocatalytic treatment of industrial wastewater.



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*Source: 6505301-2020-12-15.xml*