BiVO4 composite: A novel and promising adsorbent and photocatalyst

BiVO4 composite: A novel and promising adsorbent and photocatalyst

Materials Chemistry and Physics 240 (2020) 122238 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.el...

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Materials Chemistry and Physics 240 (2020) 122238

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Melt quenched V2O5/BiVO4 composite: A novel and promising adsorbent and photocatalyst Gurpreet Singh, Rahul Vaish * School of Engineering, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh, 175005, India

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� V2O5/BiVO4 composite was synthesized using melt-quenching followed by heattreatment route. � The sample showed promising adsorp­ tion of MB dye due to the presence of V2O5 present in it. � Heterojunction between BiVO4 and V2O5 made desorption possible. � The sample degraded MB dye under UV light due to the presence of photo­ catalytic active BiVO4 in it.

A R T I C L E I N F O

A B S T R A C T

Keywords: Melt-quench Adsorption Photocatalysis Heterojunction BiVO4

A large number of reports have been published showing negligible adsorption and promising photocatalytic properties of BiVO4 material. In this report, we have used a new and novel method for the fabrication of BiVO4, which can show significant adsorption alongwith the promising photocatalytic properties. V2O5/BiVO4 com­ posite was synthesized using melt-quenching technique followed by the specific heat-treatment at 450 � C for 15 h. The X-ray diffraction (XRD) and Raman spectroscopy techniques confirmed the presence of monoclinic BiVO4 and orthorhombic V2O5 phases in heat-treated sample. The scanning electron microscope (FE-SEM) showed the formation of rod shaped crystals of 50–300 nm sizes and the transmission electron microscope (TEM) showed the heterojunctions formation between V2O5 and BiVO4 in the heat-treated sample. The heat-treated sample adsorb e 84% of MB dye from the MB dye solution of 11.75 mg/L initial concentration within 80 min under dark. The adsorption was mainly due to the presence of V2O5 present in it. In addition to this, the 70% of the adsorbed MB dye was recovered during desorption at room-temperature using the heat-treated sample, which is not possible with V2O5 material alone. The formation of heterojunctions between V2O5 and BiVO4 made desorption possible. Moreover, during photocatalysis experiment, the heat-treated sample successfully degraded e 88% of MB dye from the MB dye solution of 11.75 mg/L initial concentration within 4 h under ultraviolet (UV)

* Corresponding author. E-mail address: [email protected] (R. Vaish). https://doi.org/10.1016/j.matchemphys.2019.122238 Received 30 March 2019; Received in revised form 6 August 2019; Accepted 28 September 2019 Available online 30 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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irradiation due to the presence of photocatalytic active crystals of BiVO4 in it. Thus, the adsorption and pho­ tocatalytic capabilities of melt-quench V2O5/BiVO4 composite made it a promising material for the removal of organic dye pollutants from the waste water coming from textile industries.

1. Introduction

2. Experimental

Water pollution is a major concern worldwide. The organic dyes present in the waste-water coming from the textile industries are continuously polluting the water resources [1–3]. Nowdays, the pho­ tocatalysis is the widely used process for the removal of organic dyes from waste-water [4–6]. TiO2, ZnO, BaTiO3, and CuO, etc are well known traditional photocatalysts [7–9]. Bismuth vanadate (BiVO4) has nowadays attracted attention due to its promising photocatalytic ac­ tivity [10,11]. BiVO4 has narrow band gap value (e2:4 –2.6 eV), due to which it can show photocatalytic activity even under the visible light irradiation [12,13]. Various researchers studied the photocatalytic na­ ture of BiVO4 photocatalyst. Yin et al. fabricated the crystals of mono­ clinic BiVO4 of 400–700 nm sizes using cetyltrimethylammonium bromide (CTAB) assisted aqueous method and successfully degraded Rhodamine B dye solution of 10 5 M concentration within 20 min under visible light irradiation [14]. Tokunaga et al. showed the higher pho­ tocatalytic activity in case of monoclinic BiVO4 phase than that of tetragonal BiVO4 phase [15]. Further, Zhang et al. synthesized the various morphologies of monocline BiVO4 phase such as nut-like, broccoli-like, and potato-like structures for photocatalytic applications [16]. One of the main limitations of BiVO4 photocatalyst is its fast re­ combinations of photogenerated electron-hole pairs [17]. Many strate­ gies such as composite formations, heterojunction formations, doping, deposition of noble metal, control of morphology have been reported in order to increase the lifetime of photogenerated charge carriers [18–20]. Out of these strategies the semiconductors with heterojunctions forma­ tions was found to be most efficient in the literature [18,21]. Several semiconductors with heterojunctions such as VO2/CuWO4 [21], Bi2WO6/Ag3PO4–Ag [18], NiO/ZnO [22], etc. showed improved pho­ tocatalytic performance due to increased lifetime of photogenerated charge-carriers. Moreover, many BiVO4 composites heterojunctions with other materials such as BiVO4/graphene [23], Bi2WO6/BiVO4 [24], BiOCl/BiVO4 [25] etc. showed the enhanced photocatalytic per­ formance than that of BiVO4 alone. Recently, BiVO4 phase has been crystallized in the glass composition of 30Bi2O3–20B2O3–50V2O5 using melt quenching and heat-treatment method, however, the authors did not find photocatalytic activity into it due to low concentration of BiVO4 crystals on the surface [26]. Like photocatalysis, the adsorption process is an another effective method used for the removal of organic dyes from the waste-water [27, 28]. The traditional well known adsorbents includes carbon-based family such as activated carbon, tire derived carbon, carbon soot etc, low cost adsorbents such as rice husk ash, clay, fly ash, etc. [29,30]. It is to be noted that BiVO4 showed negligible adsorption of dyes prior to photocatalyst in many literatures, which clearly indicate that it cannot be used as adsorbent [31,32]. Nowdays, V2O5 material has been evolved as a promising adsorbent [33]. However, the incapability of desorption at room-temperature is the main limitation of V2O5 [33]. Thus, the composite of V2O5 and BiVO4 can be expected to show both adsorption as well as photocatalytic properties. Although, there are large number of adsorbents [34–38] and photocatalytic materials [7–9] already reported in the literature, however, the present work just aims to introduce adsorption capabilities into photocatalytic active BiVO4 material by making composite with V2O5. In the present work, the V2O5/BiVO4 was fabricated from 80V2O5–20Bi2O3 using melt-quenching followed by heat-treatment method (generally used for fabrication of glass-ceramics). The adsorp­ tion and photocatalytic properties of as-prepared material were thor­ oughly investigated.

2.1. Fabrication of the material V2O5/BiVO4 composite was fabricated by using melt-quenching route, which is usually used for the fabrication of glass. In this route, the powders of vanadium pentoxide (V2O5) and bismuth oxide (Bi2O3) were taken in the agate mortar in the molar ratio of 80:20. The oxides powders were mixed and grounded properly for 30 min to get the ho­ mogeneous mixture of oxide powders. A batch of 20 g powder was taken in a platinum crucible. The crucible was then placed inside the furnace at the temperature of 1150 � C for 10 min in order to get melt. The melt was then poured on the stainless steel plate, which was maintained at the temperature of 200 � C. The melt was pressed using an electric press, which was also maintained at 200 � C. As a result, the melt quenched plate sample was obtained. Further, the melt quenched plate sample was heat-treated at 450 � C for 15 h in order to get the desired phases of V2O5 and BiVO4 in it. 2.2. Characterization The identification of phases present in the as-prepared materials (both as quenched as well as heat-treated samples) was done by using Xray diffraction (XRD) method. For this, the sample was placed in Rigaku diffractometer (Japan) using 9 kW rotating anode with Cuk α. The scanning rate was provided as 2� /min. Further, the heat-treated sample were scanned using Raman spectroscopy (HORIBA Scientific, Kyoto, Japan) in order to get more details about the types of vibrational modes of bonds present in the material. The laser of 532 nm wavelength and 25% of the laser power was used during Raman spectroscopy. The sample was scanned for the wavenumbers in the range of 50–1200 cm 1 and the scanning time was taken as 10 s. The surface morphology of heat-treated sample was visualized by using scanning electron micro­ scope (FE-SEM Inspect™ S50). Transmission electron microscope (TEM) (FEI company, USA) was used to identify the presence of heterojunctions between V2O5 and BiVO4. 2.3. Calculation of band gap The diffuse reflectance spectrum (DRS) of heat-treated sample was obtained for the wavelength range of 200–800 nm using UV–visible spectrophotometer. Further, the band gap of the sample was calculated by using Davis and Mott’s equation shown in Eq. (1) [39,40]. �r α E ¼ C E Eg 2 (1) where, α; absorbance coefficient, E; the energy of incident beam, Eg ; the band gap energy; C is the constant, and r; an another constant whose value depends upon the type of transition. The value of r is taken as 2 and 4 for indirect and direct transition, respectively. Both direct and indirect band gaps were calculated for the heat-treated sample. DRS spectra were converted into Tauc’s plots ((αE)2 vs E for direct band gap and (αE)0.5 vs E for indirect band gap). The intersection of tangent to the absorbance edge of Tauc plot on E-axis provided the value of band gap. The light source whose energy is greater than bigger value of band gap was selected for the photocatalytic experiment. 2.4. Brunauer–Emmett–Teller (BET) study The Brunauer–Emmett–Teller (BET) technique was used to measure 2

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the average pore diameter, pore volume and pore surface area of heattreated sample. BET measurements were done using an Autosorb iQ Station 2 (Quantachrome Instruments, USA) with nitogne (N2) at 77 K. 2.5. Adsorption study In order to investigate the adsorption capability of heat-treated sample, the adsorption study was done. The study was done using methylene blue (MB) dye, which is one of the known pollutants present in the wastewater coming from textile industries [41]. The chemical formula of MB is C16H18ClN3S.3H2O and the molar mass of the dye is 373.90 g/mol [42]. Prior to adsorption experiment, the calibration curve (which is a plot between the various concentrations of dye solu­ tion and the corresponding absorbance values) was obtained using UV–visible spectrophotometer. After plotting the calibration plot, the MB dye aqueous solution of initial concentration 11.75 mg/L was pre­ pared. During adsorption experiment, 0.1 g powder of heat-treated sample was put into 10 mL of MB dye solution. The adsorption experi­ ment was done in the dark and the stirring was continuously provided using the magnetic stirrer during the adsorption experiment. The test samples of 1 mL was collected after every 10 min. The absorbance of the test samples was found by using UV–visible spectrophotometer. The value of dye concentration corresponds to that absorbance was found using the calibration curve. The quantity of MB dye adsorbed (in mg/g) and the removal of MB dye (in %) were calculated as provided in Eqs. (2) and (3) [43]. Removal ​ of ​ MB ​ dye ​ ðin ​ %Þ ¼ ​

Co

Ct Co

Quantity ​ of ​ MB ​ dye ​ adsorbed ​ ðqt Þ ¼

� 100

Co

Ct m

�V

(2) (3)

where, Co ; the initial concentration of the dye solution, Ct ; the dye concentration after the finite duration, m; the mass of adsorbent (in g), and V; the volume of dye solution (in L). The desorption experiment was also performed. For this, the powder sample used in adsorption experiment was collected and was put into 10 mL of ethanol. The desorption was done for 1 h with continuous stirring. After 1 h, the 1 mL sample was collected and examined under UV–visible spectrophotometer.

Fig. 1. The X-ray diffraction (XRD) patterns of the (a) as-quenched and (b) heat-treated samples.

2.6. Photocatalytic study In order to evaluate the photocatalytic performance of the heattreated sample, the photocatalytic experiment was performed using MB dye and the dye degradation was investigated. Prior to photo­ catalytic experiment, the adsorption of the heat-treated sample was done for 24 h, so that the sample will not adsorb more dye. During the photocatalytic experiment, the 10 mL MB dye solution of 11.75 mg/L initial concentration was taken in the quartz cuvette and 0.10 g of the powder sample was put into it. The quartz cuvette was placed in the photoreactor under the irradiation of ultraviolet (UV) light of 365 nm. The light was selected according to band gap of the material and the availability of the light source. The continuous stirring was provided during photocatalytic experiment and the test samples were collected after every 1 h. These samples were tested under UV–visible spectro­ photometer for knowing absorbance value. The corresponding dye concentration was found using the calibration curve. The percentage degradation in the concentration of dye solution was found using Eq. (4) [44–46]. Degradation ​ of ​ MB ​ dye ​ ðin ​ %Þ ¼ ​

Co

Ct Co

​ � 100

Fig. 2. The Raman spectrum of the heat-treated sample.

3. Results and discussion Fig. 1(a–b) shows the X-ray diffraction (XRD) patterns of the as quenched and heat-treated samples, respectively. The sharp peaks observed in the as-quenched sample were well matched with the orthorhombic BiVO4 and the tetragonal VO2 phases in accordance with JCPDS File Nos. 00-012-0293 and 01-082-1074, respectively. On the other hand, the sharp peaks observed in XRD pattern of heat-treated

(4)

where, Co ; the initial concentration of the dye solution, and Ct ; the dye concentration after the finite duration. 3

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Fig. 3. (a) and (b) The SEM micrographs of heat-treated sample.

Fig. 4. (a) and (b) The TEM micrographs of heat-treated sample.

sample were well matched with the monoclinic BiVO4 and orthorhombic V2O5 phases in accordance with JCPDS File No. 01-075-2480 and JCPDS File No. 00-009-0387, respectively. Thus, the XRD confirmed the coexistence of V2O5 and BiVO4 phases in the heat-treated sample. Fig. 2 shows the Raman spectrum of heat-treated sample. The sharp bands 126, 212, 332, 365, and 823 cm 1 were well matched with the characteristic bands observed in BiVO4 material shown in reference [47]. Out of which, the highest intensity band at 823 cm 1 was observed mainly due to the symmetric stretching of V–O bond in BiVO4 [47]. The bands observed at 332 and 365 cm 1 could be assigned to antisymmetric and symmetric bending modes of V–O bonds in BiVO4 [47]. The bands at 126 and 212 cm 1 were the observed external modes due to the rotation and translation of structural units of BiVO4 [47]. On the other hand, the bands at 100, 144, 194, 282, 304, 405, 481, 529, 700, and 995 cm 1 were well matched with characteristics bands observed in V2O5 material shown in reference [48,49]. The external modes of vibrations, which were observed at the lower frequencies region (100, 144, and 194 cm 1) could be assigned to layered modes of V2O5 [48]. The internal modes of vibrations, which were observed at high frequency region (282, 304, 405, 481, 529, and 700 cm 1) could be assigned to stretching and bending vibrations of V–O bonds present in V2O5 [48]. The band of vanadyl mode observed at 995 cm 1 could be assigned to the stretching of shortest V–O bond in V2O5 [49]. Hence, the Raman spectrum clearly indicated the presence of BiVO4 and V2O5 phases in the heat-treated sample. The surface morphology of the heat-treated samples can be seen in Fig. 3 (a–b). The SEM micrographs clearly show the formation of rod shaped crystals of 50–300 nm size on the surface of the heat-treated sample. Fig. 4(a) shows the TEM micrograph of heat-treated powder sample. The dark regions shown in Fig. 4(a) correspond to crystals. Fig. 4 (b) clearly shows the formation of heterojunction between V2O5 and BiVO4 phases in the heat-treated sample. The d-spacing values of 0.34 and 0.30 nm can be seen in Fig. 4(a), which correspond to the highest intensity XRD peaks of V2O5 (110) and BiVO4 (112), respectively. Thus,

Fig. 5. UV–visible diffuse reflectance spectrum (DRS) of heat-treated sample. Insets show Tauc’s plots (plot between ðαEÞ2 vs E and ðαEÞ0:5 vs E for the calculation of the direct and indirect band gaps) for the heat-treated sample.

TEM confirmed the presence of V2O5/BiVO4 heterojunctions in the heattreated sample. The average pore diameter, pore volume, and pore surface area of heat-treated sample were measured to be 3.1 nm, 0.004 cc/g, and 2.278 m2/g using BET technique. BET showed the mesoporous nature of the heat-treated sample. Fig. 5 shows the DRS spectrum obtained for the heat-treated sample. The maximum absorbance can be found in the wavelength region of 280–320 nm, which was an ultraviolet region. In addition to this, the direct and indirect band gap values of the heat-treated sample were found to be 3.16 and 2.95 eV, respectively (insets of Fig. 5). The ultra­ violet source of light was selected according to band gap of the material for photocatalytic experiments. Though the band gap values of V2O5 and BiVO4 was reported mainly in the region of 2.3 eV–2.6 eV, however in the present study the more wide band gap values were observed. This was mainly due to presence of the nearly nano-sized crystals [50]. The higher values of band gaps were also observed by Sarkar and Chatto­ padhyay in case of BiVO4 nanocrystals [50]. Fig. 6(a) shows the absorbance vs wavelength plots for MB dye so­ lutions of various concentrations (1, 2, 3, 4, 5, 10, 20, and 30 mg/L). The maximum absorbance peak was found to be at 664 nm wavelength for all concentrations of dye solutions. This is the characteristic peak of MB dye solution, whose absorbance value decreased with decrease in con­ centration of MB dye in the solution. Fig. 6(b) shows the calibration curve of MB dye solution, which is a plot between various concentra­ tions of dye solution and corresponding absorbance values. The linear 4

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Fig. 6. (a) The absorbance vs wavelength plots for MB dye solutions of various concentrations and (b) the calibration curve (plot between concentration vs absorbance) of MB dye solution.

considered the chemisorption mainly occurred through Lewis acid-base reaction between MB dye and V2O5, where lewis acid sites on V2O5 (surface vanadium cations) interact with lewis base sites on MB dye (N atom present in MB dye formula) [51]. The same mechanism worked for the present case. In the present case, the 70% of the adsorbed MB dye was recovered during desorption at room-temperature using the heat-treated sample. This desorption was not possible in case of pure V2O5 [33]. Hence, it may be possible that the desorption capability mainly occurred due to heterojunctions formations between V2O5 and BiVO4 (the typical heterojunction is shown in Fig. 4(b)). The pH value is an important factor, which generally effects the adsorption process [52–54]. In the present case, the pH value of solution also showed sig­ nificant effect on the adsorption. The acidic and neutral pH favoured the adsorption of MB dye, while the basic pH decreased the MB dye adsorption as shown in Fig. 7(d). This can be explained by incorporating Lewis acid-base reaction mechanism. The acidic and neural pH favoured adsorption by increasing Lewis acid sites on V2O5, however, the basic pH possibly decreased the Lewis acid sites present on the surface of V2O5 due to which adsorption decreased at basic pH. Therefore, it is clear that the heat-treated sample could be for water-cleaning using adsorption process. The photocatalytic capability of the heat-treated sample was also evaluated. Fig. 8 shows the results of photocatalytic degradation of MB dye using the heat-treated sample under the irradiation of UV light of 365 nm. Firstly, the adsorption equilibrium was achieved by performing adsorption experiment for 24 h. After 24 h, the heat-treated sample showed no more adsorption, then it was undertaken to photocatalysis experiment. Fig. 8(a) shows the absorbance vs wavelength plots ob­ tained during photocatalytic degradation of MB dye using heat-treated sample. It can be easily seen in Fig. 8(a) The peak intensity decreased with time during photocatalysis experiment. This indicated the degra­ dation of MB dye during photocatalysis experiment using heat-treated sample. Fig. 8(b) shows the CCot vs time plots obtained for pure MB dye

Fig. 7. (a) The absorbance vs wavelength plots, (b) the quantity of dye adsorbed (qt), and the percentage removal of dye, (c) the adsorption/desorption percentage, (d) the effect of pH value on percentage removal of dye obtained during adsorption experiment using the heat-treated sample.

trend can be easily observed between concentration and absorbance. Hence, when the maximum absorbance at 664 nm of MB dye solution decreased, then the concentration of dye was also linearly decreased in the solution. Therefore, the decrease in absorbance value at 664 nm is a direct measure of decrease in concentration of MB dye in the solution. The results obtained during adsorption experiment is shown in Fig. 7. Fig. 7(a) presents the absorbance vs wavelength plots obtained during adsorption experiment using heat-treated sample. The peak intensity at 664 nm wavelength decreased with time during experiment as shown in Fig. 7(a). This clearly indicated the decrease in the concentration of MB dye in the solution due to adsorption of MB dye on heat-treated sample. The heat-treated sample adsorb e 84% of MB dye from the MB dye so­ lution of 11.75 mg/L initial concentration within 80 min under dark as shown in Fig. 7(b). The amount of dye adsorbed was found to be 0.97 mg/g of heat-treated sample as shown in Fig. 7(b). BiVO4 is well reported for photocatalytic applications [10,11,14]. Also, it can be easily seen in the literature that BiVO4 material showed negligible adsorption prior to photocatalytic experiments [10,11,14]. This clearly showed the fact, BiVO4 phase was not the reason behind the adsorption capability of heat-treated glass ceramic. Recently, Mauro et al. reported the chemisorption of MB dye on V2O5 material [51]. The authors

and heat-treated sample during photocatalytic experiment. The degra­ dation of pure MB dye under UV light without any sample in it is known as photolysis. The negligible photolysis (e7%) was observed as shown in Fig. 8(b). On the other hand, the heat-treated sample successfully degraded e 88% of MB dye from the MB dye solution of 11.75 mg/L initial concentration within 4 h under ultraviolet (UV) irradiation as shown in Fig. 8(b). This clearly indicated the promising photocatalytic activity of heat-treated sample. The crystalline phase of BiVO4 is already well known for its promising photocatalytic nature [55,56]. Thus, the photocatalytic activity of heat-treated sample was mainly due to the presence of BiVO4 crystals in it. However, in the present case, the het­ erojunctions formations between V2O5 and BiVO4 at some regions also

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Fig. 8. (a) the absorbance vs wavelength plot, (b) the CCot vs time plots obtained during photocatalytic dye degradation using the heat-treated sample under UV irradiation of 365 nm, (c–d) the effect of various scavengers and pH value on the photocatalytic efficiency, respectively using heat-treated sample.

Fig. 9. (a) The schematic of the mechanism of photocatalysis process, (b) the percentage decolourization of MB dye during photolysis, adsorption and photocatalysis processes using heat-treated sample after 1 h time.

contributed for photocatalytic activity, which is discussed in the next paragraph. Moreover, the hypsochromic shift was also observed from 664 nm to 635 nm during photocatalytic degradation of MB dye as shown in Fig. 8(a). The similar type of shift was also observed in Refs. [55,56]. This hypsochromic shift was generally found in photocatalysis through BiVO4 and was a characteristic of N-demethylation process of MB dye [57]. The hypsochromic shift also provided the indication that the MB dye was degraded not adsorbed as such shift was not observed during adsorption experiment. In order to clearly understand the mechanism behind the photo­ catalytic activity of the heat-treated sample, firstly, the identification of radicals responsible for photocatalysis was necessary. For this purpose, the radical trapping experiments were performed using various scav­ engers during photocatalysis. Three scavengers named benzoquinone (BQ), isopropanol (IPA), and ethylenediaminetetraacetic acid (EDTA) were used for capturing superoxide radical (_O2 ), hydroxyl radical (_OH), and holes (hþ), respectively. Fig. 8(c) shows the effect of addition of various scavengers on the photocatalytic degradation of MB dye using

heat-treated sample. The percentage degradation was found to be e 88%, 20%, 52%, and 37% in case of no-scavenger, BQ, EDTA, and IPA, respectively as shown in Fig. 8(c). The photocatalytic efficiency was mainly affected by the presence of BQ, which scavenge _O2 radical. Thus, _O2 is the main radical responsible for photocatalysis in the present case. Also, EDTA, and IPA also decreased photocatalytic activity, hence hþ and _OH radicals also contributed for photocatalytic activity. The pH is also a crucial factor that affects the photocatalytic activity [58,59]. The effect of pH value on photocatalytic activity is shown in Fig. 8(d). The acidic and neutral pH favour the photocatalytic activity and basic pH decreases the photocatalytic activity. The less adsorption in basic pH may be possible reason behind this observation, due to which the interaction between MB dye and catalyst become less and thus lead to low photocatalytic activity. The schematic of photocatalytic mechanism is shown in Fig. 9(a). When UV light falls on BiVO4 crystals, the electrons (e ) excite form valence band to conduction band and produce holes (hþ) in the valence band. These e and hþ move in the opposite directions and come to the 6

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to the Lewis acid-base reaction between V2O5 and MB dye. Moreover, the composite sample possessed the additional advantage of promising photocatalytic activity due to the presence of BiVO4 phase present in it. The heterojunction formation between V2O5 and BiVO4 at many places helped in increasing lifetimes of photogenerated charge carriers and hence increasing photocatalytic activity. Thus, the V2O5/BiVO4 com­ posite sample could be a promising material for the removal of organic dye pollutants from the waste water coming from textile industries. References [1] S. Mosleh, M.R. Rahimi, M. Ghaedi, K. Dashtian, S. Hajati, Sonochemical-assisted synthesis of CuO/Cu2O/Cu nanoparticles as efficient photocatalyst for simultaneous degradation of pollutant dyes in rotating packed bed reactor: LED illumination and central composite design optimization, Ultrason. Sonochem. 40 (2018) 601–610, https://doi.org/10.1016/j.ultsonch.2017.08.007. 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Fig. 10. The XRD patterns of heat-treated sample before and after adsorption/ photocatalysis experiment. The insets shows the SEM micrographs of heattreated sample before and after adsorption/photocatalysis experiment.

surface. The e reacts with adsorbed oxygen (O2) and produces _O2 radicals. The hþ reacts with adsorbed H2O or ​ OH and produces _OH radicals. These radicals (_O2 and _OH) degrade MB dye into degradation products. Moreover, in the present case, the heterojunctions formations between V2O5 and BiVO4 at some regions also contributed for photo­ catalytic activity. As the conduction band potential of BiVO4 is more negative than that of V2O5, thus at the heterojunction region, the elec­ trons move from BiVO4 to V2O5. The valence band potentials of BiVO4 and V2O5 are nearly same, hence the holes do not move. In this way, the lifetimes of photogenerated holes and electrons increase. Thus, the heterojunction formation helped in increasing photocatalytic activity. Thus, it is clear that the heat-treated sample possessed promising pho­ tocatalytic activity along with the adsorption capability. A comparison among photolysis, adsorption, and photocatalysis is shown in Fig. 9 (b). It indicated that the adsorption is much more faster process than that of photocatalysis process. However, it should be noted that in the case of adsorption, the desorption should be provided after every cycle, which is not needed in the case of photocatalysis. Thus, according to the appli­ cation, the adsorption, photocatalysis or both processes can be used for the water cleaning purpose. The chemical stability of heat-treated sample was judged by using XRD and SEM techniques. The XRD patterns and SEM micrographs of heat-treated sample (powder) after heat-treatment, after adsorption, and after photocatalysis experiment are shown in Fig. 10. It can be seen in Fig. 10 that there was no shift or disappearance of any XRD peak was observed in XRD patterns before and after adsorption/photocatalysis. Similarly, there was no change in the morphology of powder was observed before and after adsorption/photocatalysis experiments. This clearly showed that the heat-treated sample is chemical stable even after adsorption and photocatalytic experiments. 4. Conclusion V2O5/BiVO4 composite sample was successfully fabricated through melt-quenching technique followed by the specific heat-treatment at 450 � C for 15 h. The sample possessed promising MB dye adsorption due 7

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