Porous olive-like BiVO4: Alcoho-hydrothermal preparation and excellent visible-light-driven photocatalytic performance for the degradation of phenol

Applied Catalysis B: Environmental 105 (2011) 326–334

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Porous olive-like BiVO4 : Alcoho-hydrothermal preparation and excellent visible-light-driven photocatalytic performance for the degradation of phenol Haiyan Jiang, Hongxing Dai ∗ , Xue Meng, Kemeng Ji, Lei Zhang, Jiguang Deng Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China

a r t i c l e

i n f o

Article history: Received 1 February 2011 Received in revised form 2 April 2011 Accepted 12 April 2011 Available online 20 April 2011 Keywords: Visible-light-driven photocatalyst Porous bismuth vanadate Surfactant-assisted alcoho-hydrothermal synthesis Olive-like morphology Phenol photodegradation

a b s t r a c t Bismuth vanadates with multiple morphologies and/or porous structures were prepared using the alcohohydrothermal strategy with bismuth nitrate and ammonium metavanadate as metal source, NaOH as pH adjustor, ethanol and ethylene glycol as solvent, and/or dodecylamine (DA), oleylamine (OL) or oleic acid (OA) as surfactant. The materials were characterized by means of the XRD, Raman, TGA/DSC, FT-IR, BET, SEM, TEM, XPS, and UV–vis techniques. The photocatalytic performance of the as-obtained samples was evaluated for the degradation of phenol in the presence of a small amount of H2 O2 under visible-light irradiation, and the effect of phenol concentration on the photocatalytic activity was also examined. It is found that the surfactant and pH value had a significant influence on the particle morphology and even the crystalline structure of the product. Porous olive-like monoclinic BiVO4 samples could be prepared with DA, OL or OA as surfactant at pH = 1.5 or 3.0 and alcoho-hydrothermal temperature = 100 ◦ C. With DA as surfactant at an alcoho-hydrothermal temperature of 100 ◦ C, short-rod-like monoclinic BiVO4 and porous sheet-layered spherical orthorhombic Bi4 V2 O11 were obtained when the pH value of the precursor solution was raised to 7.0 and 11.0, respectively. Among the BiVO4 samples, the porous olive-like one with a surface area of 12.7 m2 /g exhibited the best visible-light-driven photocatalytic performance for phenol degradation. It is concluded that the excellent photocatalytic activity of the porous olive-like BiVO4 sample was associated with its higher surface area and surface oxygen vacancy density, porous structure, lower bandgap energy, and unique morphology. © 2011 Elsevier B.V. All rights reserved.

1. Introduction It is generally accepted that the physicochemical property of a nano- and microscale material strongly depends upon its crystal structure, surface area, particle size, and particle morphology [1–3]. In the past decades, much attention has been paid on the controlled preparation of inorganic nano- and/or microsized materials with specific morphologies and the applications in physics and catalysis. Among these nano- and/or micromaterials, monoclinic scheelite-type BiVO4 is one of the important compounds since it can be used as an active photocatalyst due to its good visiblelight-responsive ability [3]. Several methods, such as solid-state reaction [4], co-precipitation [5], hydrothermal treatment [6–10], chemical bath deposition [11], organometallic decomposition [12], and sonochemical route [13], were developed to synthesize monoclinic scheelite-type BiVO4 nano- and/or microparticles. Up to now, BiVO4 with morphologies of microsphere [7], hollow shell [14], hyperbranch [6], nanosheet [8], microtube [2], nanoribbon

∗ Corresponding author. Tel.: +86 10 6739 6118; fax: +86 10 6739 1983. E-mail address: [email protected] (H. Dai). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.04.026

[10] or star-like nanoplate [15] have been successfully prepared. Such morphological BiVO4 materials possess unique physicochemical properties, including ferroelasticity [16], ionic conductivity [17], gas sensing [6], and photocatalytic performance [7,8]. However, most of the monoclinic BiVO4 obtained using the aforementioned methods showed nonporous structures and hence low surface areas (<4 m2 /g) [7,18]. Such a drawback would hinder its wide application in photocatalysis since surface area of a photocatalyst is closely related to the amount of surface active sites [19,20]. Moreover, the presence of porous structure in a photocatalyst is beneficial for the harvesting of incident light and the transferring of reactant molecules [21]. Therefore, the controllable preparation of high-surface-area BiVO4 photocatalysts with porous structures is a challenge. Recently, some progresses have been achieved in the generation of high-surface-area monoclinic BiVO4 materials with or without porous structures. For example, Yu and co-workers [19] synthesized ordered mesoporous monoclinic BiVO4 with a surface area of 59 m2 /g using three-dimensional ordered mesoporous silica (KIT-6) as hard template and bismuth nitrate and ammonium metavanadate as inorganic source, and observed that the mesoporous BiVO4 showed superior photocatalytic activities in catalyzing the degradation of methylene blue

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(MB) and the oxidation of NO under visible-light irradiation. By adopting the hydrothermal method and Bi(NO3 )3 ·5H2 O, V2 O5 , and K2 SO4 as starting materials, Zhou et al. [20] obtained monoclinic BiVO4 with a surface area of 15.6 m2 /g, and claimed that this material was highly active for MB degradation under visible-light illumination. It is known that most of organic pollutants can be removed via the pathways of granular activated carbon adsorption, biological treatment, ozonation, and photocatalysis [22]. Among these pathways, the photocatalytic one is a “green” process through which the toxic organic pollutants can be completely mineralized into carbon dioxide, water, and mineral acids [23]. Phenol and its derivatives in wastewater emitted from chemical industries are extremely toxic, carcinogenic, teratogenic, and refractory in nature. Therefore, it is of significance to establish a “green” and effective method to remove the phenol from wastewater. As a promising route, photocatalysis has recently been gained much attention. Previously, some photocatalysts, such as B-TiO2 [24], Bi2 WO6 [25], and Bi2 MoO6 [26], were used for the degradation of phenol. It has been reported that BiVO4 showed poor photocatalytic activities since its photoinduced electrons could hardly be captured by oxygen [27]. In the presence of an electron scavenger (e.g., Cr(VI) or H2 O2 ), however, the BiVO4 sample showed an enhanced photocatalytic activity for phenol degradation under visible-light illumination [27–29]. In recent years, organic compounds, such as oleic acid (OA), oleylamine (OL), and dodecylamine (DA), have been employed as surfactant [30,31] or stabilizing agent [32] for the making of inorganic nanoparticles [30–32]. Up to now, however, no reports have been seen in the literature on the preparation of monoclinic BiVO4 nano/microparticles with regular morphologies and/or porous structures using OA, OL or DA as surfactant. Previously, our group investigated the fabrication and physicochemical properties of a number of porous nano- or micromaterials, such as mesoporous MgO [33], mesoporous CaCO3 [34], single-crystalline BiVO4 [35], and three-dimensional ordered mesoporous Cr2 O3 [36] via the surfactant (e.g., triblock copolymer (P123), cetyltrimethylammonium bromide, sodium dodecyl sulfate, poly(N-vinyl-2-pyrrolidone) or poly(ethylene glycol))assisted hydrothermal or ultrasound-assisted nanocasting route. Recently, we have successfully generated a series of porous BiVO4 with an olive-like morphology using the alcoho-hydrothermal strategy, and found that the porous BiVO4 materials performed well in catalyzing the degradation of some organic pollutants under visible-light illumination. Herein, we report the controlled preparation, characterization, and photocatalytic performance of monoclinic BiVO4 with multiple morphologies and/or porous structures for phenol photodegradation in the presence of a small amount of H2 O2 under visible-light irradiation.

of the above mixture (a certain amount of deionized water was added if the volume of the mixture was less than 80 mL) was transferred into a 100-mL Teflon-lined stainless steel autoclave for alcoho-hydrothermal treatment at 100 ◦ C for 12 h. After being washed with deionized water and absolute ethanol 3 times and dried at 60 ◦ C overnight, the obtained solid was calcined in air at a ramp of 1 ◦ C/min from room temperature (RT) to 450 ◦ C and kept at this temperature for 4 h, thus obtaining the bismuth vanadate photocatalysts. For the sake of clear presentation, we denote the photocatalysts prepared under various conditions as BiVO–x and BiVO–y–x (x is the pH value of the precursor solution and y is the surfactant used), as described in Table 1. All of the chemicals (A.R. in purity) were purchased from Beijing Chemicals Company and used without further purification.

2. Experimental

3. Results and discussion

2.1. Catalyst preparation

3.1. Crystal phase composition

The BiVO4 photocatalysts with different morphologies and/or porous structures were prepared by adopting the alcohohydrothermal strategy with Bi(NO3 )3 ·5H2 O and NH4 VO3 as inorganic source, DA, OL or OA as surfactant, and ethanol and ethylene glycol (EG) as solvent. The typical preparation procedure was as follows: 5 mL of concentrated nitric acid (67 wt%) and 30 mmol of DA, OL or OA were added to a mixed solvent of equal amount (25 mL) of ethanol and EG under stirring. 10 mmol of Bi(NO3 )3 ·5H2 O was dissolved in the above mixed solution. Then 10 mmol of NH4 VO3 was added under stirring. The pH value was adjusted to 1.5, 3.0, 7.0 or 11.0 using a NaOH solution (2 mol/L) containing absolute ethanol and EG (volumetric ratio = 1/1). 80 mL

Fig. 1 shows the XRD patterns of the samples prepared under different conditions. It is observed that the XRD patterns of the BiVO4 samples obtained at pH = 1.5, 3.0 or 7.0 matched well with that of the monoclinic scheelite BiVO4 sample (JCPDS PDF# 14–0688) and all of the peaks could be indexed, as shown in Fig. 1(g). Furthermore, the XRD patterns of the uncalcined BiVO–1.5, BiVO–DA–1.5, BiVO–OL–1.5, and BiVO–OA–1.5 samples also reveal the formation of monoclinic scheelite BiVO4 (Fig. S1 of the Supplementary material). These results demonstrate that the BiVO–1.5, BiVO–DA–1.5, BiVO–DA–3, BiVO–DA–7, BiVO–OL–1.5, and BiVO–OA–1.5 samples were single-phase and possessed a monoclinic scheelite BiVO4 crystal structure, as confirmed by the results of laser Raman

2.2. Catalyst characterization The catalysts were characterized using the techniques of X-ray diffraction (XRD), laser Raman spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), Fouriertransform infrared (FT-IR) spectroscopy, N2 adsorption–desorption (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy. The detailed procedures were described in Supplementary material. 2.3. Photocatalytic evaluation Photocatalytic performance of the BiVO–x and BiVO–y–x samples was measured in a quartz reactor (QO250, Beijing Changtuo Sci. & Technol. Co., Ltd.) for the degradation of phenol under visiblelight irradiation using a 300-W Xe lamp with a 400-nm cutoff filter. The photocatalytic evaluation was conducted at RT as follows: 0.2 g of the BiVO–x, BiVO–y–x or commercial TiO2 (Degussa P25) sample and 0.6 mL of H2 O2 solution (30 wt%) were added to 200 mL of the phenol-containing aqueous solution (initial phenol concentration C0 = 0.1, 0.2 or 0.4 mmol/L). Before illumination, the mixed solution was ultrasonicated for 0.5 h and magnetically stirred for 3 h in the dark to ensure the establishment of the adsorption–desorption equilibrium. Then the suspension was magnetically stirred and exposed to the visible-light irradiation. 4 mL of the suspension was taken out at 30 min intervals and centrifuged to remove the photocatalyst particles for the analysis of phenol concentration. The phenol concentration (Ct ) after a certain reaction time (t) was determined by checking the absorbance of the sampled reactant suspension at the absorption band maximum (ca. 280 nm) on the aforementioned UV–vis equipment. The Ct /C0 ratio was used to evaluate the photocatalytic degradation efficiency of the sample.

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Table 1 Fabrication parameters, crystal phases, particle morphologies, BET surface areas, and bandgap energies of the as-fabricated samples. Sample code

Surfactant

pH

Crystal phase

Particle morphology

Surface area (m2 /g)

Bandgap energy (eV)

BiVO–1.5 BiVO–DA–1.5 BiVO–DA–3 BiVO–DA–7 BiVO–DA–11 BiVO–OL–1.5 BiVO–OA–1.5

– DA DA DA DA OL OA

1.5 1.5 3.0 7.0 11.0 1.5 1.5

Monoclinic BiVO4 Monoclinic BiVO4 Monoclinic BiVO4 Monoclinic BiVO4 Orthorhombic Bi4 V2 O11 Monoclinic BiVO4 Monoclinic BiVO4

Irregular Porous olive-like Porous olive-like Short-rod-like Porous sheet-layered sphere-like Porous olive-like Porous olive-like

1.7 12.7 8.0 2.4 19.2 11.2 8.7

2.51 2.48 2.49 2.51 3.07 2.49 2.49

studies (Fig. S2 of the Supplementary material). Similar XRD patterns of BiVO4 have also been recorded by other researchers [3,29,37,38]. The formation of single-phase monoclinic scheelite BiVO4 suggests that the calcination temperature (450 ◦ C) was appropriate for the complete removal of the surfactant molecules, as substantiated by the results of TGA/DSC and FT-IR investigations (Figs. S3 and S4 of the Supplementary material). When pH value of the precursor solution was raised to 11.0, however, the BiVO–DA–11 sample (Fig. 1(e)) showed an orthorhombic Bi4 V2 O11 structure (JCPDS PDF# 42–0135). With Bi(NO3 )3 ·5H2 O and NaVO3 as metal source in the presence of Na2 CO3 (pH = 9.7), Shen et al. [18] synthesized hollow spherical Bi4 V2 O11 particles via a hydrothermal treatment at 200 ◦ C for 6 h. It seems to be that the alkaline condition was more beneficial for the formation of Bi4 V2 O11 phase. The above results indicate that pH value of the precursor solution strongly influenced the crystal phase of the final product. From Fig. 1, one can also observe a small discrepancy in peak intensity of the BiVO4 samples, indicating there was the presence of a small difference in crystallinity of these samples. It is due to the different preparation conditions, such as pH value and surfactant nature. 3.2. Morphology SEM and TEM images of the BiVO4 samples obtained under different conditions are shown in Figs. 2 and 3, respectively. It is observed that the BiVO–1.5 sample fabricated at pH = 1.5 and in the absence of surfactant displayed an irregular morphology with the particle size of 0.4–2 ␮m (Fig. 2(a)). With the addition of surfactant DA, however, the obtained BiVO–DA–1.5 sample was composed of a large number of olive-like BiVO4 microentities with porous struc-

(321)

(211) (051) (240) (042) (202) (161)

(121) (040) (200) (002)

(110) (011)

Bi4V2O11

(g)

(313)

(220) (026)

Intensity (a.u.)

(113) (200)

(f)

(e)

tures (Figs. 2(b) and 3(a)). Each microentity contained a lot of mesoand macropores (diameter = 10–60 nm) distributed randomly on the surfaces. Although similarly morphological monoclinic BiVO4 microentities with or without hollow internal structures were also obtained via a sonochemical or solvo-hydrothermal route by other researchers [39–41], the olive-like BiVO4 materials with porous architectures have not been reported before. With DA as surfactant and at pH = 3.0, the obtained BiVO–DA–3 sample also retained an olive-like morphology and a porous structure (Figs. 2(c) and 3(c)); with the rise in pH value from 3.0 to 7.0, however, the morphology of the as-fabricated BiVO–DA–7 sample evolved into relatively uniform short-rod-like nanoparticles (Fig. 2(d)); taking a close-up view of the particles (Fig. 3(e)), one can see that a large number of small spherical nanoparticles (diameter = 4–8 nm) were distributed on the surfaces of the short rods. A similar phenomenon was also observed by Chen et al. [42]. When the pH value rose to 11.0, however, most of the as-generated BiVO–DA–11 sample particles displayed a spherical shape that was composed of numerous porous Bi4 V2 O11 microsheets (Figs. 2(e) and (f) and 3(g)). The results reveal that a strongly acidic precursor solution in the presence of DA favored the production of monoclinic BiVO4 with an olive-like and porous architecture. It can be seen from Figs. 2(g) and (h) and 3(i) and (k) that when the pH value of the precursor solution was 1.5, changing the surfactant (OL or OA) did not induce significant alterations in particle morphology and pore structure. This result further demonstrates the dominant role of pH value of the precursor solution on the morphology and pore structure of the product. At the same pH value, the BiVO4 samples derived in the presence of surfactant (DA, OL or OA) were totally different in morphology and pore structure from the one derived in the absence of surfactant. Obviously, the surfactant also had an important role to play in the generation of porous olive-like BiVO4 . Moreover, well-resolved lattice fringes can be clearly seen from the high-resolution TEM images (Fig. 3(b), (d), (f), (h), (j), and (l)) of the BiVO–DA–1.5, BiVO–DA–3, BiVO–DA–7, BiVO–OL–1.5, and BiVO–OA–1.5 samples. The lattice spacing (d value) of the (1 2 1) plane of these BiVO4 samples was measured to be ca. 0.31 nm, rather close to that (0.308 nm) of the standard BiVO4 sample (JCPDS PDF# 14–0688); the d value (0.31 nm) of the (1 1 3) plane of the BiVO–DA–11 (i.e., Bi4 V2 O11 ) sample was also in good agreement with that (0.312 nm) of the referenced sample (JCPDS PDF# 42–0135).

(d)

3.3. Formation mechanism

(c) (b) (a) 10

20

30

40

50

60

70

80

2 Theta (Deg.) Fig. 1. XRD patterns of the (a) BiVO–1.5, (b) BiVO–DA–1.5, (c) BiVO–DA–3, (d) BiVO–DA–7, (e) BiVO–DA–11, (f) BiVO–OL–1.5, and (g) BiVO–OA–1.5 samples.

Generally, the adsorption of additives on certain crystal surfaces plays a vital role in the shape-controlled growth for some materials [43]. Due to the van der Waals interaction of surfactant molecules adsorbed on the nanocrystal surfaces and the tendency to minimize the interfacial energy, these nanoparticles would gradually self-assemble into aggregates with a specific morphology [44,45]. In the present study, we consider that the same principle might be applicable for the surfactant (DA, OL or OA) selectively adsorbed on certain planes of the primary BiVO4 nanocrystals to control their self-assembly in a specific manner. At a low pH value (<3.0), in the

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Fig. 2. SEM images of the (a) BiVO–1.5, (b) BiVO–DA–1.5, (c) BiVO–DA–3, (d) BiVO–DA–7, (e and f) BiVO–DA–11, (g) BiVO–OL–1.5, and (h) BiVO–OA–1.5 samples.

absence of a surfactant, the initially formed BiVO4 nanoparticles could aggregate into irregularly morphological BiVO4 microparticles (Fig. 2(a)) for the minimization of total surface free energy [46] after the alcoho-hydrothermal treatment; in the presence of DA, OL or OA, however, the surfactant molecules adsorbed on the surfaces of the primary BiVO4 nanoparticles, thus preventing their further growth. The interaction of surfactant-covered nanoparticles could aggregate into an olive-like architecture, and the porous monoclinic BiVO4 microentities with an olive-like morphology (Fig. 2(b), (c),

(g), and (h)) were finally generated after the removal of the surfactant. When the pH value of the precursor solution was adjusted to 7.0, the vanadium and bismuth were present in the forms of VO3 − and BiONO3 [47], respectively; during the alcohol-hydrothermal process VO3 − and BiONO3 could react to generate BiVO4 crystal nuclei, which then grew up to produce primary BiVO4 nanoparticles and surfactant molecules adsorbed on their surfaces. These surfactant-covered nanoparticles finally self-assembled into shortrod-like BiVO4 nanoparticles (Fig. 2(d)) after the elimination of the

Fig. 3. TEM images of the (a and b) BiVO–DA–1.5, (c and d) BiVO–DA–3, (e and f) BiVO–DA–7, (g and h) BiVO–DA–11, (i and j) BiVO–OL–1.5, and (k and l) BiVO–OA–1.5 samples.

330

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B

25

dV /dD (×10-3cm3/(g nm))

Volume adsorbed (cm3/g, STP)

A

BiVO−1.5 BiVO−DΑ−1.5 BiVO−OL−1.5 BiVO−OA−1.5

20

15

10

5

0 0.0

3.0 2.5

BiVO−1.5 BiVO−DΑ−1.5 BiVO−OL−1.5 BiVO−OA−1.5

2.0 1.5 1.0 0.5 0.0

0.2

0.4

0.6

0.8

0

1.0

5

p /p 0

10

15

20

25

30

Pore size (nm)

Fig. 4. (A) N2 adsorption–desorption isotherms and (B) pore-size distributions of the as-fabricated samples.

surfactant. A further rise in pH value to 11.0, no primary BiVO4 nanoparticles were formed due to the strong complexing ability between Bi3+ and OH− ions in the precursor solution [46]. The DA molecules might interact with the formed complex to generate a more complicated species, which then organized into sheet-like entities and these sheeted entities finally aggregated and crystallized into the porous sheet-layered spherical Bi4 V2 O11 microparticles (Fig. 2(e) and (f)) after the removal of the surfactant. Hollow spherical Bi4 V2 O11 microparticles were obtained via a hydrothermal route at a pH value of 9.7 by Shen et al. [18]. The detailed formation mechanism of these mentioned morphological particles deserves further investigations. 3.4. Surface area and pore size distribution Shown in Fig. 4 are the N2 adsorption–desorption isotherms and pore-size distributions of the typical BiVO4 samples fabricated with or without surfactant. It is observed from Fig. 4(A) that the BiVO–1.5 sample exhibited a type III isotherm with a H3 hysteresis loop [48], which was related to the formation of slit-shaped pores in the aggregates of nano- or microparticles. The absence of an adsorption plateau at a relative pressure near unity suggests the presence of macropores [48,49]. Apparently, all of the BiVO4 samples had macropores that originated from the aggregation of nanoor microparticles and/or the olive-like BiVO4 microentities. The appearance of a H1 hysteresis loop in the relative pressure range of 0.1–0.9 for the BiVO–DA–1.5, BiVO–OL–1.5, and BiVO–OA–1.5

Bi 4f

163.7

B

158.4

516.5

(e)

(f) (e) (d)

(d) (c) (b) (a) 162

158

154

Binding energy (eV)

(g) (f)

Intensity (a.u.)

Intensity (a.u.)

(f)

166

529.4

O 1s 532.1

515.6

(g)

(g)

170

C

V 2p3/2

150

Intensity (a.u.)

A

samples implies the presence of mesopores [48,50], as confirmed by the SEM and TEM images of these samples (Figs. 2 and 3). The BiVO4 samples displayed wide pore-size distributions (Fig. 4(B)) and most of the mesopore sizes were in the range of 2–10 nm. From Fig. 4(B), one can also find that the dV/dD value decreased with an increase in pore size, suggesting that there might be the presence of micropores in the BiVO–DA–1.5, BiVO–OL–1.5, and BiVO–OA–1.5 samples. These micropores might originate from the olive-like BiVO4 microentities. A number of investigations have shown that surface area of a photocatalyst has a positive effect on the enhancement in photocatalytic performance [19,20]. Most of monoclinic BiVO4 catalysts reported in the literature, however, possessed a low surface area (<4 m2 /g) [7,18]. As can be seen from Table 1, the porous olive-like BiVO–DA–1.5, BiVO–DA–3, BiVO–OL–1.5, and BiVO–OA–1.5 samples derived with the addition of surfactant possessed much higher surface areas (8.0–12.7 m2 /g) than the BiVO–1.5 sample obtained without surfactant addition (1.7 m2 /g), a result due to the formation of mesoporous structure in the former samples. Obviously, the surfactant (DA, OL or OA) played a crucial role in generating high-surface-area porous BiVO4 materials. Among the BiVO4 samples, the porous olive-like BiVO–DA–1.5 sample fabricated with DA as surfactant and at pH = 1.5 showed the highest surface area (12.7 m2 /g). When pH value rose to 3.0 or 7.0, the surface area of the obtained BiVO–DA–3 or BiVO–DA–7 sample dropped significantly, indicating that the pH value of the precursor solution exerted an important impact on the surface area of the product. When the

519

(e) (d)

(c)

(c)

(b)

(b)

(a)

(a) 518

517

516

Binding energy (eV)

515

536

534

532

530

528

526

Binding energy (eV)

Fig. 5. (A) Bi 4f, (B) V 2p3/2 , and (C) O 1s XPS spectra of the (a) BiVO–1.5, (b) BiVO–DA–1.5, (c) BiVO–DA–3, (d) BiVO–DA–7, (e and f) BiVO–DA–11, (g) BiVO–OL–1.5, and (h) BiVO–OA–1.5 samples.

H. Jiang et al. / Applied Catalysis B: Environmental 105 (2011) 326–334

pH value further rose to 11.0, the obtained BiVO–DA–11 sample exhibited a high surface area of 19.2 m2 /g, but the crystal phase was changed into orthorhombic Bi4 V2 O11 . It can be concluded that the introduction of a surfactant (DA, OL or OA) and a lower pH value were beneficial for the generation of high-surface-area monoclinic BiVO4 with porous structures.

Table 2 Surface Bi/V, V4+ /V5+ , and Oads /Olatt molar ratios of the as-fabricated samples.

3.5. Metal oxidation state, oxygen species, and surface composition Fig. 5 shows the Bi 4f, V 2p3/2 , and O 1s XPS spectra of the asfabricated samples. It is observed from Fig. 5(A) that each of the BiVO4 samples as well as the Bi4 V2 O11 sample exhibited the spinorbit splitting signals of Bi 4f7/2 and 4f5/2 at BE = 158.4 and 163.7 eV, respectively, which were characteristic of Bi3+ [51]. In other words, all of the bismuth ions in the BiVO–1.5, BiVO–DA–1.5, BiVO–DA–3, BiVO–DA–7, BiVO–DA–11, BiVO–OL–1.5, and BiVO–OA–1.5 samples existed in trivalency. Each of the V 2p3/2 spectra (Fig. 5(B)) could be deconvoluted to two components at BE = 515.6 and 516.5 eV, assignable to the surface V4+ and V5+ species, respectively [52]. That is to say, there was the co-presence of V5+ (in majority) and V4+ (in minority) species in each of the as-fabricated samples. According to the electroneutrality principle, it can be deduced that all of the as-fabricated samples were oxygen-deficient (i.e., BiVO4−ı or Bi4 V2 O11−ı ) and the surface nonstoichiometric oxygen amounts (ı) depended upon the surface V4+ /V5+ molar ratios. The recording of asymmetric O 1s signal indicates the presence of different oxygen species on the surface of each sample (Fig. 5(C)). After deconvolution of the O 1s signal, the components at BE = 529.4 and 532.1 eV could be attributed to the surface lattice oxygen (Olatt 2− ) and adsorbed oxygen (Oads ) species [53,54], respectively. Since the BiVO–1.5, BiVO–DA–1.5, BiVO–DA–3, BiVO–DA–7, BiVO–DA–11, BiVO–OL–1.5, and BiVO–OA–1.5 samples were pretreated in an O2 flow at 450 ◦ C before XPS spectrum recording, the possibility of surface OH− and CO3 2− species existence was minimized. Therefore, the adsorbed oxygen species were mainly O− , O2 − or O2 2− species, which dwelled at the oxygen vacancies of the BiVO4−ı or Bi4 V2 O11−ı samples. Generally speaking, a more amount of Oads species means a higher oxygen vacancy density for an oxygen-deficient material. Table 2 summarizes the molar ratios of surface Bi/V, V4+ /V5+ , and Oads /Olatt 2− molar ratios of the asfabricated samples. The surface Bi/V molar ratios (0.97–1.07) of the BiVO–1.5, BiVO–DA–1.5, BiVO–DA–3, BiVO–DA–7, BiVO–OL–1.5, and BiVO–OA–1.5 samples and that of the BiVO–DA–11 sample were close to 1 and 2, respectively, indicating that no significant phase segregation took place in the as-fabricated samples. Moreover, in terms of the surface V4+ /V5+ and Oads /Olatt 2− molar ratios, one can see that the porous BiVO–DA–1.5, BiVO–DA–3, BiVO–OL–1.5, and BiVO–OA–1.5 samples possessed more amounts

A

Sample code

Bi/V molar ratio

V4+ /V5+ molar ratio

Oads /Olatt molar ratio

BiVO–1.5 BiVO–DA–1.5 BiVO–DA–3 BiVO–DA–7 BiVO–DA–11 BiVO–OL–1.5 BiVO–OA–1.5

0.97 1.02 1.01 1.07 1.98 0.98 1.06

0.015 0.123 0.093 0.022 0.047 0.108 0.096

0.163 0.275 0.196 0.172 0.157 0.247 0.208

of surface V4+ and Oads species than the other samples, indicating that the former samples contained more amounts of surface oxygen vacancies. It is well known that an enhancement in catalytic performance of a material is usually associated with the formation of a higher oxygen vacancy density. This was substantiated by the photocatalytic activity data shown in Section 3.7. 3.6. Optical absorption behavior Illustrated in Fig. 6 are the UV–vis DRS spectra of the asfabricated samples. The vivid yellow BiVO–1.5, BiVO–DA–1.5, BiVO–DA–3, BiVO–DA–7, BiVO–OL–1.5, and BiVO–OA–1.5 samples exhibited strong absorption in UV- and visible-light regions (Fig. 6(A)), which was characteristic of monoclinic BiVO4 [20]. It suggests that these BiVO4 materials were capable of responding to visible light. The steep absorption edge in the visible-light region of each of monoclinic BiVO4 samples reveals that the visible-light absorption was due to the bandgap transition [47]. The UV–vis DRS spectrum of the pale yellow BiVO–DA–11 sample, however, was different from those of the other samples, which was due to the difference in crystal structure [18]. The relationship between the absorption coefficient (˛) and incident photon energy (h) of a crystalline semiconductor could be described as the formula of (˛h)2 = A(h − Eg )n , where A and Eg present constant and bandgap energy, respectively. The n value depends upon the characteristics of the transition in a semiconductor. In the case of direct transition for BiVO4 , the value of n is 1 [13]. Therefore, the Eg value of each BiVO4 sample could be estimated from the intercept of the plots of (˛h)2 versus h (Fig. 6(B)), as summarized in Table 1. The bandgap energies of the BiVO–1.5, BiVO–DA–1.5, BiVO–DA–3, BiVO–DA–7, BiVO–OL–1.5, and BiVO–OA–1.5 samples were in the range of 2.48–2.51 eV, which were comparable to those (2.48–2.55 eV) of monoclinic BiVO4 reported previously [2,18]. From Table 1, one can also see that compared to the Eg values (2.51 eV) of the BiVO–1.5 and BiVO–DA–7 samples, those (2.48–2.49 eV) of the porous olive-like BiVO–DA–1.5, BiVO–DA–3,

B

0.6

14 2.49 eV 2.49 eV

(αhν )2 (eV)2

Absorbance (a.u.)

12 0.4 (e) (b) (f) (g) (c) (d) (a)

0.2

331

(e)

3.07 eV (b)

10

2.48 eV (f) (g)

8 6

(c) (d)

4

2.51 eV

2.51 eV (a)

2

0

0 200

400

600

Wavelength (nm)

800

2.0

2.5

3.0

3.5

4.0

4.5

hν (eV)

Fig. 6. (A) UV–vis diffuse reflectance spectra and (B) plots of the (˛h)2 versus h of the (a) BiVO–1.5, (b) BiVO–DA–1.5, (c) BiVO–DA–3, (d) BiVO–DA–7, (e) BiVO–DA–11, (f) BiVO–OL–1.5, and (g) BiVO–OA–1.5 samples.

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Fig. 7. Phenol concentration versus visible-light irradiation time: (

) direct pho-

tolysis in the presence of H2 O2 , () Degussa P25 in the presence of H2 O2 , ( BiVO–DA–1.5 in the absence of H2 O2 , and (

) BiVO–1.5, (

)

) BiVO–DA–1.5,

( ) BiVO–DA–3, ( ) BiVO–DA–7, ( ) BiVO–DA–11, () BiVO–OL–1.5, and ( ) BiVO–OA–1.5 in the presence of H2 O2 for the degradation of phenol (C0 = 0.2 mmol/L) aqueous solution under visible-light (≥400 nm) irradiation.

BiVO–OL–1.5, and BiVO–OA–1.5 samples were somewhat lower, a result due to the discrepancies in crystal size [13] and crystallinity [5]. This fact suggests that the porous BiVO4 samples could respond to visible light more effectively. It can also be observed that the bandgap energy of the BiVO–DA–11 sample was 3.07 eV, in good agreement with that (3.05 eV) of the Bi4 V2 O11 sample [18]. 3.7. Photocatalytic performance Photocatalytic activities of the as-fabricated samples were evaluated for the degradation of phenol in aqueous solution in the presence of a small amount of H2 O2 under visible-light illumination. For comparison purposes, we also examined the direct photolysis of phenol in the presence of H2 O2 , the degradation of phenol over the BiVO–DA–1.5 catalyst in the absence of H2 O2 , and the degradation of phenol over the commercial TiO2 (Degussa P25) nanoparticles in the presence of H2 O2 under visible-light illumination. Fig. 7 represents the phenol concentration ratios (Ct /C0 ) of different samples and that of the direct photolysis process with irradiation time. Obviously, after visible-light irradiation for 4 h, the phenol conversion was only ca. 4% over the P25 sample, whereas it almost unchanged in the experiment of phenol direct photolysis in the presence of H2 O2 , indicating that the photolysis of phenol was negligible under this condition. Similar phenomena were also reported previously [26,27]. The decrease of phenol concentration in the degradation of phenol over the BiVO–DA–1.5 catalyst in the absence of H2 O2 under visible-light irradiation was rather small (only ca. 7% of phenol conversion was achieved after 4 h of reaction), a combined result due to the poor adsorption of phenol in the aqueous solution over the BiVO4 catalyst [55] and the weak capability of oxygen to capture the photogenerated electrons [27]. The BiVO4 samples in the presence of an electron scavenger like H2 O2 , however, exhibited excellent visible-lightdriven photocatalytic performance. This indicates the presence of a synergistic effect between the photocatalyst and the electron scavenger. Wang and co-workers [55] pointed out that H2 O2 , as an efficient electron scavenger, could trap the photoinduced electrons

Fig. 8. Effect of initial phenol concentration (C0 ) on the photocatalytic performance of the BiVO–DA–1.5 sample for the degradation of phenol in the presence of the same amount of H2 O2 under visible-light (≥400 nm) irradiation.

and effectively inhibited the recombination of electron-hole pairs. From Fig. 7, one can find that the phenol conversion over the asfabricated catalysts within 4 h of reaction decreased in the order of BiVO–DA–1.5 (ca. 96%) > BiVO–OL–1.5 (ca. 91%) > BiVO–OA–1.5 (ca. 86%) > BiVO–DA–3 (ca. 83%) > BiVO–DA–7 (ca. 76%) > BiVO–1.5 (ca. 73%) > BiVO–DA–11 (ca. 43%). Compared to the orthorhombic BiVO–DA–11 (i.e., Bi4 V2 O11 ) sample, the monoclinic BiVO4 samples performed much better. A similar phenomenon was also observed in the degradation of methylene blue under visible-light irradiation [18]. Furthermore, the porous olive-like BiVO–DA–1.5, BiVO–DA–3, BiVO–OL–1.5, and BiVO–OA–1.5 catalysts were much superior in photocatalytic activity to the short-rod-like BiVO–DA–7 and irregularly morphological BiVO–1.5 samples. It should be noted that it under the irradiation of visible light, no significant amounts of intermediates were formed over our BiVO4 samples during the photocatalytic reaction processes. For example, in the case of the photocatalytic reaction of phenol over the BiVO–DA–3 sample, there was only one absorption peak at ca. 280 nm and no other absorption peaks assignable to the intermediates possibly generated in the photocatalytic reaction of phenol were detected (Fig. S5 of the Supplementary material). Similar absorption features were also observed over the other of BiVO4 samples. The results indicate that phenol was photocatalytically degraded to CO2 and H2 O over the BiVO4 samples under visible-light illumination. Therefore, the photocatalytic activity data estimated from the variations of absorbance of phenol in the solution were reliable. Such an activity measurement was also been adopted by Wang and co-workers [29]. It is well known that the crystal structure, crystallinity, morphology, and surface area of a material are important factors influencing its photocatalytic performance [56–58]. Although the BiVO–DA–11 sample possessed the highest surface area among all of the samples, it showed the worst photocatalytic performance, due to the wide bandgap energy of Bi4 V2 O11 . As revealed in the XRD analysis, each of the monoclinic BiVO–1.5, BiVO–DA–1.5, BiVO–DA–3, BiVO–DA–7, BiVO–OL–1.5, and BiVO–OA–1.5 samples possessed a similar crystallinity. It means that the big difference in photocatalytic activity of these samples was not due to the small discrepancy in crystallinity. Therefore, the high surface area and unique morphology might be the main factors governing

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the photocatalytic behavior of monoclinically crystallized bismuth vanadate. Compared to the BiVO–1.5 and BiVO–DA–7 samples, the BiVO–DA–1.5, BiVO–DA–3, BiVO–OL–1.5, and BiVO–OA–1.5 samples possessed higher surface areas (Table 1). Furthermore, higher oxygen vacancy densities and lower bandgap energies of BiVO–DA–1.5, BiVO–DA–3, BiVO–OL–1.5, and BiVO–OA–1.5 than those of BiVO–1.5 and BiVO–DA–7 might also contribute to the improved photocatalytic performance of the former catalysts. In addition, the porous structures of the BiVO–DA–1.5, BiVO–DA–3, BiVO–OL–1.5, and BiVO–OA–1.5 samples could favor the adsorption and diffusion of reactants as well as the facile accessibility of incident light to more surfaces of catalysts, thus leading to an enhancement in photocatalytic activity [59]. Fig. 8 shows the effect of phenol concentration on the photocatalytic activity of the BiVO–DA–1.5 catalyst for phenol degradation in the presence of H2 O2 under visible-light irradiation. It can be seen that at phenol concentration = 0.1 mmol/L, ca. 95% of phenol could be removed after 2.5 h of visible-light irradiation. When the H2 O2 concentration was not altered, the conversion of phenol decreased to ca. 96 and 82% after 4 h of photocatalytic reaction with the rise in phenol concentration from 0.1 to 0.2 and 0.4 mmol/L, respectively. Similar changing trends were also reported for the photocatalytic degradation of phenol in UV/TiO2 process [60] and methylene blue over TiO2 [61]. Such photocatalytic behaviors were understandable because a lower phenol concentration [62] and a higher H2 O2 concentration [60] were favorable for the photocatalytic removal of organic compounds. 4. Conclusions With bismuth nitrate and ammonium metavanadate as inorganic source, NaOH as pH adjustor, and ethanol and EG as solvent in the absence or presence of DA, OL or OA, bismuth vanadates with various morphologies and/or porous structures could be selectively prepared via a facile alcoho-hydrothermal route. It is shown that the pH value of the precursor solution and the surfactant of DA, OL or OA greatly influenced the particle morphology, porous structure, and even crystalline structure of the product. At an alcoho-hydrothermal temperature of 100 ◦ C, (i) irregular monoclinic BiVO4 was obtained in the absence of surfactant at pH = 1.5; (ii) porous olive-like monoclinc BiVO4 was generated with the addition of DA, OL or OA at pH = 1.5 or 3.0; (iii) short-rod-like monoclinic BiVO4 was fabricated in the presence of DA at pH = 7.0; and (iv) porous sheet-layered spherical orthorhombic Bi4 V2 O11 was prepared in the presence of DA at pH = 11.0. The surface areas and bandgap energies of the six monoclinically crystallized BiVO4 samples were in the range of 1.7–12.7 m2 /g and 2.48–2.51 eV, respectively; the orthorhombic Bi4 V2 O11 sample possessed a surface area of 19.2 m2 /g and a bandgap energy of 3.07 eV. The porous olive-like BiVO4 sample with a surface area of 12.7 m2 /g showed the highest visible-light-driven photocatalytic activity among the seven catalysts prepared in the present study. We believe that factors, such as high surface area and surface oxygen vacancy density, low bandgap energy, porous structure, and unique morphology, were responsible for the excellent photocatalytic performance of the porous olive-like BiVO4 sample in catalyzing the degradation of phenol in the presence of a small amount of H2 O2 under visiblelight irradiation. Acknowledgements The work described above was supported by the National Natural Science Foundation of China (Nos. 20973017 and 21077007),

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