Effects of pH on the hierarchical structures and photocatalytic performance of Cu-doped BiVO4 prepared via the hydrothermal method

Materials Science in Semiconductor Processing 35 (2015) 197–206

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Effects of pH on the hierarchical structures and photocatalytic performance of Cu-doped BiVO4 prepared via the hydrothermal method Xiaoming Gao a,n, Zihang Wang a, Feng Fu a, Wenhong Li b a Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan'an University, Yan'an, Shaanxi 716000, China b Department of Chemical Engineering, Northwest University, Xi'an, Shaanxi 710069, China

a r t i c l e i n f o

abstract

Available online 30 March 2015

Cu-doped BiVO4 with hierarchical structures was prepared by the hydrothermal method at different pH. The corresponding relationship was obtained among pH values of the precursor to phase, morphology, and photocatalytic ability of as-prepared Cu–BiVO4. The results show that, the pH value of the precursor was 5, 6, the as-prepared Cu–BiVO4 were tetragonal phase crystals with sheet-like, polyhedron-like morphologies. The phase transformation from tetragonal phase to monoclinic phase occurred between pH¼ 6 to pH¼7. Spherical morphology Cu–BiVO4 with particle sizes in the range of 5 μm could be prepared under the neutral condition (pH¼ 7), which expressed excellent photocatalytic activities for the photodegradation of phenol under visible light irradiation. Whereas leaf-like and rod-like Cu–BiVO4 with monoclinic phase could be obtained within pH¼ 8 and pH¼9 respectively. The photocatalytic activities of the as-prepared Cu–BiVO4 samples were evaluated for the photodegradation of phenol under visible light irradiation. The mechanism analysis shown that superoxide radical anion O2  was main active radicals in photocatalytic oxidation of phenol as-prepared Cu– BiVO4 samples. The reaction kinetics of the photodegradation of phenol over the Cu–BiVO4 was also established by the way of the Langmuir–Hinshewood model. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Cu-doped BiVO4 Hydrothermal preparation pH value Hierarchical structure Photocatalytic degradation kinetics

1. Introduction During the past decades, the TiO2-based photocatalysts have been extensively studied [1–3]. However, despite great efforts, the range of frequencies in which they work was mostly limited to the ultraviolet region [4–6]. Given this situation, photocatalytic properties of quite a few metal oxides different from TiO2 have been explored in order to overcome this limitation [7–9]. As a result, it was found that some vanadates show photocatalytic activities in visible range [10–15] and, among them, BiVO4 has become a promising candidate [16–19].

n

Corresponding author. Tel./fax: þ86 911 2332284. E-mail address: [email protected] (X. Gao).

http://dx.doi.org/10.1016/j.mssp.2015.03.012 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

It was well known that the photocatalytic activity of semiconductors depends strongly on three factors: adsorption behavior, photoresponse region and the separation efficiency of electron–hole pairs [19]. The adsorption behavior can be enhanced by improving the specific surface area of catalysts usually. Among this aspect, how to extend the photo-response region and improve the separation efficiency of electron–hole pairs, were important factors on the photocatalytic performance of the semiconductors catalyze [20]. The way to extend the photo-response region semiconductors catalyze was mainly doping of nonmetals or transition metals, which can also improve the separation efficiency of electron–hole pairs and increase the oxidation power of photogenerated carriers [21,22]. In general, noble metals, such as Ag [23], Pt [24], Au [25], and Pd [26], have been used as electron acceptors to separate the photoinduced hole/electron pair and promote

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interfacial charge-transfer processes. Although many efforts have been made to improve the photocatalytic activity of BiVO4 photocatalysts, there were still necessary to design novel BiVO4 based photocatalysts to further improve photocatalytic efficiencies. In addition, it was obvious that some reaction conditions such as concentration and the property of the reagent, reaction temperature and time, and reaction environment play an important role to the structure of the photocatalyst [27–29]. In particular, the pH values of the precursor solution have important effects on the crystalline phase and morphology of BiVO4 photocatalyst [27–30]. Up to date, BiVO4 samples with different photocatalytic activity and novel morphology have been obtained by tuning the parameters such as pH values. For example, Meng et al. [28] have synthesized the polyhedral, rod-like, tubular, leaf-like, and spherical morphologies BiVO4 powders by adjusting the pH values in the presence of triblock copolymer P123. After 3 h, the rod-like BiVO4 prepared hydrothermally on pH¼6 could completely degrade the Methylene Blue in visible-light irradiation. Zhang et al. [29] studied the effect of different pH values of the precursor to photocatalytic degradation of Methyl Orange over BiVO4 powders. It was found that the photodegradation activity of sheet-like BiVO4 prepared at pH¼6.9 was maximal, and after 1 h, the Methyl Orange was completely degraded in visible light irradiation. However, people have done few studies of the corresponding relationship among pH values of the precursor to crystalline phase, morphology, and photocatalytic performance. Tan et al. [27] prepared BiVO4 powders with hierarchical structures and photocatalytic performance by the means of the microwave hydrothermal method at different pH. The result shown that, at pH 7.81, the monoclinic phase BiVO4 with high specific surface area (5.16 m2/g) exhibits the best visible-light photocatalytic activity, indicating that visible-light photocatalytic activity of BiVO4 was not only related to the crystal structure, but greatly to its morphology and specific surface area. Herein, a series of Cu-doped BiVO4 with different crystalline phase and morphology was synthesized hydrothermally at different pH values. The corresponding relationship was obtained among pH values of the precursor to phase, morphology, and photocatalytic ability. The possible mechanism of photocatalytic oxidation of phenol as-prepared Cu–BiVO4 samples was also discussed. Subsequently, the reaction kinetics of the photodegradation of phenol over the Cu-doped BiVO4 was established by the way of the Langmuir– Hinshewood model.

for another 30 min. The amount of Cu-loaded was varied of 0.75 wt%. Then, the solution was transferred into and stored in a 25-mL Teflon liner stainless vessel, which was then heated at 160 1C for 8 h. After cooling, the obtained sample was centrifuged and washed with distilled water. Finally the BiVO4 sample was dried in a drying cabinet at 80 1C for 4 h. 2.2. Characterization The phase and composition of the as-prepared photocatalysts were identified by X-ray diffraction (XRD) using monochromatized Cu Kα radiation under 40 kV and 100 mA and with the 2θ ranging from 101 to 801 (Shimadzu XRD-7000). The morphologies and microstructures of the as-prepared photocatalysts were analyzed by the scanning electron microscope (SEM) (JEOL JSM-6700F). UV–vis diffuse reflectance spectra (DRS) of the as-prepared photocatalysts were recorded with an UV–vis spectrophotometer (Shimadzu UV-2550) using an integrating-sphere accessory, BaSO4 was used as a reflectance standard. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG MultiLab2000 X-ray photoelectron spectrometer with a monochromatic Al Kα source. 2.3. Enhanced visible-light-responsive photocatalytic properties Photocatalytic property of the as-prepared photocatalysts was evaluated by degradation of the phenol-containing wastewater at ambient temperature. The phenol-containing wastewater was prepared by dissolving phenol in distilled water. A 400 W metal halide with a 400 nm cut-off filter was used as the visible-light source. The reaction solution containing 10 μg as-prepared photocatalysts and 10 mL phenol-containing wastewater were continuously magnetically stirred in the dark for 1 h to assure the adsorption–desorption equilibrium between the photocatalysts and the target organic pollutant. After this period of time, the light source was turned on. During the reaction, the reaction solution was taken at given time intervals and then the as-prepared photocatalysts were separated through centrifugation. The phenol concentration was detected by a UV–vis spectrophotometer. The phenol concentration was determined by means of the 4-AAP spectrophotometric method. 3. Result and discussion 3.1. The phase purity of Cu–BiVO4 sample

2. Experimental section 2.1. Preparation All the reagents were analytical grade. The preparation of BiVO4 sample was showed in the following way: 0.01 mol of Bi(NO3)3  5H2O and 0.01 mol of NH4VO3 were dissolved in nitric acid and sodium hydroxide solution, respectively. Then 0.02 g of sodium dodecyl benzene sulfonate was added in each solution, respectively. The resulted two solutions were combined together and the mixed solution was stirred for 30 min vigorously. Then the pH value of the precursor was adjusted to different values with ammonia. Subsequently, Cu (NO3)2 was added in and the resulting mixture was stirred

The XRD patterns of the as-prepared Cu–BiVO4 samples with different pH were shown in Fig. 1. From Fig. 1, the pH of the precursor was 5 and 6, the characteristic diffraction peaks appeared obviously at 2θ¼18.11, 24.41, 30.51, 32.21, 34.71, 39.41, 48.11, 49.91, 501, 56.31 and 60.71. All the diffraction peaks could be indexed to tetragonal phase (JCPDS no. 14-0133) with lattice constants of a¼7.303, b¼6.584, c¼ 6.584, and the space group was I41/amd [141] [31–33]. The strongest diffraction peaks appeared at 2θ¼ 24.41, corresponding to the indices of (200). When the pH of the precursor was greater than or equal to 7(i.e. pH¼7, 8, 9), the characteristic diffraction peaks appeared obviously at 2θ¼191, 28.71, 28.91, 30.51, 34.81, 471, 48.11, 53.31, 501, 58.11

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f

Intensity(a.u.)

e d c b a

10

20

30

40

50

60

70

80

2θ/ °

199

crystal plane of the prominent exposure help to improve the photocatalytic efficiency of Cu–BiVO4 samples. In the acidic environment, tetragonal phase BiVO4 was thermally stable phase. But, in the weakly alkaline environment, the reaction tended to form monoclinic phase BiVO4 due to reaction thermodynamics [28,30]. In the strong alkaline environment, the characteristic peak of monoclinic phase BiVO4 decreased. It was obvious that monoclinic phase Cu–BiVO4 with crystallinity and larger grain was not conducive to growth in the strong alkaline environment. The reasons can be explained that it was relative to large amount of [OH  ] in solution caused by the excessive ammonia. In the higher concentration of [OH  ] in solution, on the one hand, there was an equation for ammonium vanadate: NH4 VO3 þ 2H þ ¼ HVO3 ↓ þH2 O þNH3

ð1Þ

Fig. 1. XRD patterns of samples prepared with different pH values. a: pH¼ 5 Cu–BiVO4, b: pH ¼6 Cu–BiVO4, c: pH¼7 Cu–BiVO4, d: pH¼7 BiVO4, e: pH¼ 8 Cu–BiVO4, f: pH ¼9 Cu–BiVO4.

The slightly soluble vanadium acid colloid was produced. So, the V presented in the solution in the form of VO3 . Subsequently, V source was washed off in the form of VO3 after reaction. At the other hand, in the higher concentration of [OH  ] in solution, Bi3 þ was likely to react in this way:

Table 1 The lattice constants of pure BiVO4 and Cu–BiVO4 series samples.

Bi3 þ þ 3OH  ¼ BiðOHÞ3 ↓

samples

pure tetragonal phase BiVO4 pH ¼ 5 Cu–BiVO4 pH ¼ 6 Cu–BiVO4 pure monoclinic phase BiVO4 pH ¼ 7 Cu–BiVO4 pH ¼ 8 Cu–BiVO4 pH ¼ 9 Cu–BiVO4

lattice constants a (Å)

b (Å)

c (Å)

7.303 7.313 7.300 5.214 5.216 5.204 5.221

6.584 6.588 6.592 5.084 5.094 5.091 5.080

6.584 6.587 6.593 11.706 11.700 11.712 11.721

and 601. All the diffraction peaks could be indexed to monoclinic phase (JCPDS no. 14-0688) with lattice constants of a¼5.214, b¼5.084, c¼11.706, and the space group was Pnca (61) [31–33]. The strongest diffraction peaks appeared at 2θ¼28.91, corresponding to the indices of (112). From Table 1, the lattice constants of pure BiVO4 were nearly identical compared to that of Cu–BiVO4 series samples. It was shown that doped Cu did not change the crystal type of BiVO4, and Cu only dispersed on the surface of the BiVO4 rather than replace the atom backbone to enter the internal lattice of BiVO4. When the pH of the precursor was 7, 8 and 9, the characteristic diffraction peaks of the tetragonal phase disappeared, and characteristic diffraction peaks of the monoclinic phase evidently appeared. In particular, the Cu– BiVO4 sample prepared in pH¼7, the intensity of the characteristic diffraction peaks of monoclinic phase was strongest and the shape of peak was sharpest. However, the pH of the precursor was more than 7, namely in the alkaline environment, the intensity of characteristic diffraction peak of monoclinic phase decreased, and the shape of peak became width. In addition, the value of I (040)/I(112) of Cu–BiVO4 sample prepared in pH¼ 7 was higher than that of pH¼8, 9. It was reported that the value of I (040)/I(112) was lower, the exposure rate of (040) crystal plane was larger [27–29]. When Cu–BiVO4 sample prepared in 7, the exposure rate of (040) crystal plane (040) was highest, and (040)

ð2Þ

The Bi(OH)3 precipitation was produced. However, Bi (OH)3 was easy to form stability bismuth oxygen polyanion precipitation. As a result, the hydrothermal synthesis reaction to produce BiVO4 could not successfully happen. Because of the above two aspects, a great quantity of Bi source and V source was run off. Therefore, pH was an important factor for the formation of product phase, which will affect the produce of crystalline phase of Cu–BiVO4. In acidic conditions, it was beneficial to form the tetragonal phase BiVO4. And monoclinic phase BiVO4 with high crystallinity was easily formatted under neutral conditions. While in the alkaline conditions, the crystalline of monoclinic phase helical was weakened, and might even form Bi(OH)3. 3.2. Visible-light-responsive properties of Cu–BiVO4 sample The UV–vis DRS patterns of the as-prepared Cu–BiVO4 samples with different pH were shown in Fig. 2. From Fig. 2, pure BiVO4 displayed significantly absorbance from ultraviolet to 525 nm region. After Cu doped, the absorbance edges of the samples extended to 550 nm. The absorbance boundary of all samples decreased sharply in the visible light region, indicating that the absorbance spectra of pure BiVO4 and Cu–BiVO4 samples were caused by interband transitions of semiconductor material. According to UV–vis diffuse reflection absorption edges of the semiconductor material, using the Eg ¼hc/ λ0 ¼ 1240/λ0 [34–36], the forbidden band width of Cu–BiVO4 samples prepared in pH¼5, pH¼6, pH¼7, pH¼8 and pH¼ 9 were 2.36 eV, 2.34 eV, 2.33 eV, 2.25 eV, 2.24 eV and 2.24 eV, respectively. For the tetragonal phase BiVO4, O2p and Bi6p orbit hybridized to form the valence band, and conduction band was formed by V3d orbit [36,37]. Thus, the absorption of tetragonal BiVO4 was caused by electron transitions from valence band of O2p and Bi6p orbital hybridization to V3d conduction band. However, for the monoclinic phase of BiVO4, Bi6p, Bi6s

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Fig. 2. The UV–vis DRS patterns of Cu–BiVO4 samples prepared at different pH values. a: pH ¼ 5 Cu–BiVO4; b: pH¼6 Cu–BiVO4, c: pH¼ 7 Cu–BiVO4, d: pH¼7 BiVO4, e: pH¼ 8 Cu–BiVO4, f: pH¼ 9 Cu–BiVO4.

and O2p orbit hybridized to form the valence band, and conduction band was formed by V3d orbit. So, the absorption of monoclinic BiVO4 was produced by the transitions electrons from valence band of Bi6p, Bi6s and O2p orbital hybridization to V3d conduction band. Compared to the tetragonal BiVO4, because the Bi6s orbit involved in O2p and Bi6p orbit hybridization, the valence band of monoclinic BiVO4 became wider, and the conduction band of monoclinic BiVO4 became smaller. So, the forbidden band width of tetragonal BiVO4 was larger than that of monoclinic BiVO4. Therefore, the forbidden band width of Cu–BiVO4 prepared in pH¼7, 8 and 9 were less than that of pH¼5 and 6. In addition, the same phase Cu–BiVO4 exhibited similar absorbance patterns, and the difference of the forbidden band width was smaller. This was the distortion degree of VO34  tetrahedral was different in the diverse acid–base environment, which resulted little difference in the electronic structures of Cu–BiVO4 samples. This can also be explained that the crystallinity and particle size of Cu–BiVO4 were different in the diverse acid–base environment. 3.3. Morphologies and microstructure of Cu–BiVO4 sample The SEM images of the as-prepared Cu–BiVO4 samples with different pH are shown in Fig. 3. Scheme 1 clearly presented the evolution process of the Cu–BiVO4 crystals by varying the pH of the precursors. It was obviously that the morphology of Cu–BiVO4 samples was highly related to the pH value of the precursor solution. In Fig. 3(a), the Cu–BiVO4 sample prepared in pH¼5, which was irregular sheet-like particles, the average diameter was about 1μm. After adding an amount of NH3  H2O, a sudden increase in pH resulted in uneven nucleation, some polyhedron-like Cu–BiVO4 was generated. Because the crystals grew along the different directions at different growth rates, as the reaction proceeded, the crystal plane with the higher growth rate would disappear while the crystal plane with the lower growth rate would increase in area [38]. So, the pH values of the precursor solution was 6, the obtained Cu–BiVO4 sample particles displayed a

polyhedron-like morphology, as shown in Fig. 3(b). The polyhedral edge lengths were in the range of about 1 μm. When another amount adding of NH3  H2O, phase transformation from the tetragonal phase to the monoclinic one took place during the recrystallization process. In the pH values of the precursor solution was 7, the bigger nucleation rate and the smaller crystal growth rate resulted in a relatively regular spherical-like monoclinic phase Cu–BiVO4. It can be seen from Fig. 3(c) that the crystal was spherical morphology with an average diameter of about 5 μm. Subsequently, the nucleation rate of the crystal was smaller than the growth rate by increasing the pH of the precursors. At pH¼8, the monoclinic spherical-shaped crystals disappeared completely and were transformed to the monoclinic irregular leaf-like (Fig. 3(d)). At pH¼9, a large amount of irregular long rod-like crystals were formed (Fig. 3(e)), which indicated that Cu–BiVO4 sample prepared at pH¼7 was single crystalline. Thus, at different preparation conditions (pH¼5, 6, 7, 8 or 9; alkaline source was NH3  H2O), the simultaneous collective self-assembly of these 2D nanoentities after the Ostwald ripening process gave rise to Cu–BiVO4 single-crystallites with a sheet-like, polyhedron-like, spherical-like, leaf-like, or rod-like morphology. Moreover, The pH values of the precursors affect not only the crystal structure, but also the morphology of Cu–BiVO4 sample. Moreover, monoclinic phase Cu–BiVO4 can be selectively prepared by adjusting the pH values of the precursors. 3.4. XPS of Cu–BiVO4 sample The XPS patterns of the pure BiVO4 sample and Cu–BiVO4 sample prepared in pH¼7 is shown in Fig. 4. Comparing to the XPS patterns of the pure BiVO4 samples, the XPS patterns of the Cu–BiVO4 sample appeared obviously Cu2p peak at 950 eV, and the entire spectrum had been enhanced greatly. The XPS patterns of Cu, Bi, V, O elements of Cu–BiVO4 are shown in Fig. 5(a)–(d). It can be seen that the binding energies of Bi4f7/2, Bi4f5/2, V2p3/2, V2p1/2, O1s were 158 eV, 164 eV, 516 eV, 532 eV, 528 eV, respectively. Cu 2p1/2 and Cu 2p3/2 were unimodality, the binding energies were 958 eV, 941 eV, respectively, and the pitch of 2p twodoublet doublet was 17 eV. It indicated that the Cu element existed in form of Cu2 þ . According to reports, the monoclinic BiVO4 crystal was composed of layered structure of Bi–V–O unit. Because the radius of Cu ions (R (Cu2 þ )¼0.0730 nm) was smaller than the radius of the bismuth ions (R (Bi3 þ )¼ 0.1110 nm), thus, the metal Cu cannot enter the internal lattice structure of BiVO4, and dispersed on the surface of BiVO4 by the form of Cu oxide. In the XRD spectra of Cu– BiVO4 samples and the pure BiVO4 samples, there was a tiny difference in 2θ¼34.3691 and 2θ¼35.2791. 3.5. The photocatalytic properties of Cu–BiVO4 sample The photocatalytic property of the as-prepared Cu-loaded BiVO4 with different pH was evaluated by comparing the degradation efficiency of phenol under visible light irradiation. The change of phenol concentration vs. illumination time is shown in Fig. 6. From Fig. 6, It can be seen that the degradation rate of phenol over pure BiVO4 sample could reach 55.97% after being illuminated for 150 min under

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201

Fig. 3. The SEM image of and Cu–BiVO4 samples at different pH values. a: pH ¼5 Cu–BiVO4; b: pH ¼6 Cu–BiVO4, c: pH¼7 Cu–BiVO4, d: pH¼8 Cu–BiVO4, e: pH ¼9 Cu–BiVO4.

visible light. While doped metal Cu to BiVO4, the degradation effect enhanced obviously. When the pH of preparation condition of Cu–BiVO4 was 5, 6, 8, 9, the degradation rate of phenol was 59.5%, 71.8%, 73.5%, 78.7%, respectively. In particular, the degradation rate of phenol over Cu–BiVO4 sample prepared in pH¼7 could reach 93.47% after being

illuminated for 150 min under visible light. It was obviously that the Cu–BiVO4 preparation environment presented neutral, the photocatalytic activity was obvious. According to the results of XRD, the Cu–BiVO4 sample prepared in pH¼ 7 were monoclinic phase BiVO4, and the characteristic diffraction peaks of the monoclinic phase were narrow, sharp, and

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Scheme 1. The Effect of different pH values on the crystalline phase and morphology of the Cu–BiVO4 sample .

Fig. 4. The XPS of (a) BiVO4 and (b) Cu–BiVO4 sample prepared at pH ¼ 7.

the peak intensity was maximum, which indicated a mature of crystallinity of Cu–BiVO4 samples, reducing of the containing defects, and increasing of the photogenerated electron hole pairs. Therefore, it expressed better photocatalytic degradation activity for phenol. On the other hand, the results of SEM, surface area and pore size analysis shown that the Cu– BiVO4 sample prepared in pH¼7 was three-dimensional spherical structure, and has larger specific surface area. Furthermore, the enhanced photocatalytic behavior of Cu– BiVO4 sample prepared in pH¼7 not only was due to its physicochemical properties such as crystal phase, surface area, appropriate band gap, and surface microstructure, but also was closely related to the photogenerated electron–hole separation. The photoluminescence (PL) spectra of semiconductors mainly resulted from the recombination of photogenerated electrons and holes [23]. Thus, PL spectra were helpful in determining the efficiency of charge carrier trapping, migration and transfer,

and were useful in understanding the recombination behavior of the electrons and holes [15,26]. Fig. 7 shows the comparison of PL spectra of pure BiVO4 and Cu–BiVO4 sample prepared in pH¼7 in the excitation wavelength of 375 nm. It was clearly observed that the PL emission spectra of two photocatalysts showed the main peaks at similar positions but with different intensities. The PL intensity of Cu–BiVO4 sample was lower than that of pure BiVO4 sample. It was well known that a low PL intensity implied a low recombination rate of the electron– hole under light irradiation [18,27]. This might attribute to the fact that the doped metal Cu could efficiently prevent the direct recombination of photogenerated carriers and enhance the interfacial charge transfer efficiency, which was of great benefit for enhancing activity in the photocatalytic reaction. To demonstrate the potential applicability of the Cu– BiVO4 sample prepared in pH ¼7, circulating runs in the degradation of phenol were carried out under visible light irradiation. As shown in Fig. 8, the Cu–BiVO4 sample prepared in pH ¼7 showed relatively stable performance under repeated use with constant degradation rate. After five recycles for the degradation of phenol, the photocatalyst did not exhibit any significant loss of activity. 3.6. Mechanism analysis of photocatalytic oxidation of phenol The typical chart of photocatalytic degradation of phenol over Cu–BiVO4 sample prepared in pH¼7 is illustrated in Scheme 2. In visible light irradiation, it was well know that BiVO4 sample absorbed photon to generate photogenerated electrons (e  ) and hole (h þ ) in the conduction band and valence band respectively. In order to photocatalytic reaction could be in progress, the recombination of the electrons and the holes must be inhibited as much as possible. In this system, the electrons could react with electron acceptors, such as O2 and OH  /H2O existed in the system, reducing

X. Gao et al. / Materials Science in Semiconductor Processing 35 (2015) 197–206

B4f7/2, 158eV

203

V2p3/2, 516eV V2p1/2, 523eV

Intensity(a.u.)

Intensity(a.u.)

B4f5/2, 164eV

156

158

160 162 164 Binding energy(eV)

166

168

170

505

510

515 520 Binding energy(eV)

525

530

O1s, 528eV

Cu2p1/2, 958eV

Intensity(a.u.)

Intensity(a.u.)

Cu2p3/2, 941eV

526

528

530 532 534 Binding energy(eV)

536

538

540

935

940

945

950 955 960 Binding energy(eV)

965

970

975

Fig. 5. The XPS of Cu–BiVO4 sample prepared at pH ¼7 (a) Bi4f; (b) V2p; (c) O1s; (d) Cu2p.

Fig. 6. The photocatalytic activity of series BiVO4. a: pH¼7 BiVO4, b: pH¼5 Cu–BiVO4, c: pH¼ 6 Cu–BiVO4, d: pH¼ 7 Cu–BiVO4, e: pH¼ 8 Cu–BiVO4, f: pH¼ 9 Cu–BiVO4.

them to form superoxide radical anion O2  and hydroxyl free radical OH. At the other hand, the doped Cu in the surface of BiVO4 sample could act as electron traps to suppress efficiently the recombination of the electrons and the holes, and photogenerated electrons and holes could be

Fig. 7. The room temperature photoluminescence (PL) spectra of of pure BiVO4 (a) and Cu–BiVO4 sample prepared in pH¼ 7 (b) (λexcitation ¼ 375 nm).

efficiently separated. Therefore, the decomposition of phenol could be finished by the oxidation of photo-generated hole, superoxide radical anion O2  and hydroxyl free radical OH directly. Based on the above discussion, the involved reactions of photocatalytic degradation of phenol over Cu–BiVO4

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pH¼7 after being illuminated for 150 min was 72.2% and 41.3%, respectively. It indicated that h þ and OH played an important role in the oxidation reaction of phenol, but the oxidation function of h þ was greater than that of OH. While adding scavenger of BQ, the degradation rate of phenol over Cu–BiVO4 sample prepared in pH¼7 after being illuminated for 150 min reduced to 16.2%, indicating that O2  was main active radicals in this reaction. 3.7. Kinetics on the degradation of phenol The Langmuir–Hinshelwood model (Eq. 10) was used to study the photocatalytic reaction kinetics of degradation of the phenol over Cu–BiVO4 sample prepared in pH¼7. [42–44] Fig. 8. Cycling runs in the photodegradation of phenol by the Cu–BiVO4 sample prepared in pH ¼ 7.

r¼

dc kK L–H c ¼ kθ ¼ dt 1 þ K L–H c

ð10Þ

Where r was the reaction rate, k was the reaction rate constant,θ was the surface coverage, KL–H was the adsorption coefficient of the reactant, and c was the reactant concentration. When c was very small, the product KL–Hc (KL–Hc o o1) was negligible with respect to unity. As a result, Eq. (11)

Scheme 2. The typical chart of photocatalytic degradation of phenol .

sample prepared in pH ¼7 could be simply described as follows: Cu–BiVO4 þhv-Cu–BiVO4 (h þ ) þCu–BiVO4 (e  )

(4)

Cu–BiVO4 (h þ )þPhenol-Cu–BiVO4 þProducts

(5)

Cu–BiVO4 (h þ )þH2O-H2Oþ OH þH þ

(6)

OHþPhenol-Products

(7)



O2 þCu  BiVO4 ðe Þ-Cu  O2  þ Phenol-Products

BiVO4 þ O2 

Fig. 9. The degradation effect of phenol over Cu–BiVO4 sample prepared in pH¼ 7 under different scavengers. a: Adding scavenger of BQ, b: adding scavenger of EDTA, c: adding scavenger of IPA.

ð8Þ ð9Þ

In order to determine the function of the active free radicals, the benzoquinone (BQ) [23,39], ethylene diamine tetraacetic acid(EDTA) [23,40] and isopropanol (IPA) [23,41] were used to capture the superoxide radical anion O2  , photo-generated hole h þ and hydroxyl free radical OH, respectively. The degradation effect of phenol over Cu–BiVO4 sample prepared in pH¼7 under different scavengers is shown in Fig. 9. From Fig. 9, when adding scavengers of IPA and EDTA, compared to 93.47% of the degradation rate of phenol over Cu–BiVO4 sample prepared in pH¼ 7 after being illuminated for 150 min without any scavengers, the degradation rate of phenol over Cu–BiVO4 sample prepared in

Fig. 10. The plot of ln(c0/c) and t.

X. Gao et al. / Materials Science in Semiconductor Processing 35 (2015) 197–206

Langmuir–Hinshewood model was follow:

can be described first-order reaction kinetics. r ffi kK LH c ¼ k1 c

ð11Þ

At the initial conditions of the photocatalytic procedure, t¼0, c¼c0, which give Eq. (12) c  0 ¼ k1 t ln ð12Þ c where c0 and c were the concentrations of phenol in solution at time 0 and t, respectively, k1 was the apparent pseudofirst-order rate constant. Fig. 10 is a plot of ln(c0/c) vs. irradiation time at various initial concentration of phenol. From Fig. 10, it was clear that the curve of time (t) and ln(c0/c) was close to a linear curve. It was shown that the degradation of the phenol over the Cu–BiVO4 follows first-order reaction kinetics. The slope of liner plot in Fig. 10 could give the apparent degradation rate constant k1. It was shown that the k1 decrease along with the increasing of the initial phenol concentration, which indicates that there was a competition between the intermediates and phenol on the surface of catalyst. Owing to the complex mechanism of photocatalytic reactions, it was difficult to develop a dependence model for the degradation rate based on the experimental parameters in the whole treatment time. Thus, the kinetic model of the photocatalytic process was usually restricted to the analysis of the initial rate of degradation, and it can be obtained from the initial slope of the curves. The extrapolation of the degradation rate to t¼ 0 avoid the possible interference from by-products. The initial degradation rate (r0) was observed to be a function of the initial concentration (c0). A linear plot of r 0 1 versus c0 1 was often obtained, which give k as the L–H rate constant and K as the Langmuir adsorption constant[42,45] 1 1 1 ¼ þ r 0 kK LH c0 k

205

ð13Þ

The kinetic parameters k and K were obtained by using linear least squares analysis (Fig. 11). The value of k and K were 0.4652 mg L  1 min  1 and 0.0695 L mg  1, respectively (R¼0.9961). Therefore, the reaction kinetics of the photodegradation of phenol over the Cu–BiVO4 on the base of the

1 30:9246 ¼ þ 2:1494 r0 c0

ð14Þ

Obviously, k》KL–H, which proves that phenol adsorption capacity was very small on the Cu–BiVO4 photocatalyst surface. cKL–H《1, the photocatalytic reaction rate can be reduced to a first-order reaction. So, the photocatalytic reaction rate was greater than adsorption rate of phenol in the surface of Cu–BiVO4, the phenol molecules adsorpted in the surface of Cu–BiVO4 can be quickly oxidized. 4. Conclusion Cu-doped BiVO4 samples with sheet-like, polyhedronlike, spherical, leaf-like and rodlike morphologies were prepared by the hydrothermal method at different pH. The corresponding relationship among pH values of the precursor to phase, morphology, and photocatalytic performance of asprepared Cu–BiVO4 was researched. It found that the spherical morphology Cu–BiVO4 with particle sizes in the range of 3 μm could be prepared under the neutral condition (pH¼7), which expressed excellent photocatalytic activities for the photodegradation of phenol under visible light irradiation. The photocatalytic activities of the as-prepared Cu–BiVO4 samples were evaluated for the photodegradation of phenol under visible light irradiation. The possible mechanism of photocatalytic oxidation of phenol as-prepared Cu–BiVO4 samples was also discussed. The result shown that superoxide radical anion O2  was main active radicals in photocatalytic oxidation of phenol as-prepared Cu–BiVO4 samples. Furthermore, the reaction kinetics of the photodegradation of phenol over the Cu-doped BiVO4 was established by the way of the Langmuir–Hinshewood model. The result of kinetics study shown that, the photocatalytic reaction rate was greater than adsorption rate of phenol in the surface of Cu– BiVO4, and the phenol molecules adsorpted in the surface of Cu–BiVO4 was quickly oxidized.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 21406188), China, and the Industrial Public Relation Project of Department of Science & Technology of Shaanxi (Grant no. 2014K10-04), China Shaanxi. References

Fig. 11. The plot of ð1=r 0 Þ and ð1=c0 Þ.

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