Characterization of visible-light-driven BiVO4 photocatalysts synthesized via a surfactant-assisted hydrothermal method

Spectrochimica Acta Part A 73 (2009) 336–341

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Characterization of visible-light-driven BiVO4 photocatalysts synthesized via a surfactant-assisted hydrothermal method Aiping Zhang ∗ , Jinzhi Zhang College of Sciences, North China University of Technology, Beijing 100144, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 6 November 2008 Received in revised form 16 February 2009 Accepted 19 February 2009 Keywords: Photocatalysis Surfactant-assisted hydrothermal method Methyl orange

a b s t r a c t Phase-pure BiVO4 photocatalysts were synthesized via a surfactant-assisted hydrothermal method and were characterized with XRD, DRS, Raman, FTIR and SEM. The results indicated that the HTAB-assisted BiVO4 had a narrow band gap than the other three products (no-surfactant-assisted, PVA-assisted and PVP-assisted BiVO4 ). The addition of surfactant would greatly affect the crystal structure of BiVO4 , which can lead to different photocatalytic activities between them. Their photocatalytic activities were evaluated by the decolorization of methyl orange in aqueous solution under visible light irradiation; and the HTABassisted BiVO4 product, with well-assembled flower-like morphology, had a much higher photocatalytic activity (the photodegradation rate was about 85% in 90 min) than the other three products. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In recent years, with the extensive researches carried out in the field of decomposition of harmful organic and inorganic pollutants using photocatalytic semiconductors, many effective UV-light photocatalysts and their photocatalytic behaviors has been extensively studied due to its potential application especially in environmental remediation, though whose photosensitized spectrum occupies only a little of the whole solar energy [1–3]. While, the visible light ( > 400 nm) accounting for about half of the whole solar energy is open to exploiture, therefore, the development of visible-lightdriven photocatalysts has become one of the most challenging topics recently [4–6]. Bismuth vanadate (BiVO4 ) has long been recognized as an important semiconductor due to its unique properties such as ferroelasticity [7] and ionic conductivity [8]. It has been used for a wide range of applications including gas sensors, solid-state electrolytes, positive electrode materials for lithium rechargeable batteries, nontoxic yellow pigment for high-performance lead-free paints [9] and recently proved to be a good photocatalyst for water splitting and pollutant decomposing under visible light irradiation [10]. BiVO4 , a layered structured compound, always exists in three phases: monoclinic scheelite, tetragonal zircon, and tetragonal scheelite [11]. Up to now, several methods have been reported for the preparation of BiVO4 , such as solid-state reaction, sol–gel method, coprecipitation and metal organic decomposition, etc. However, some of these methods require high reaction tempera-

∗ Corresponding author. Tel.: +86 10 88803271; fax: +86 10 88803271. E-mail address: [email protected] (A. Zhang). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.02.028

ture, complex operating procedure, the high price of raw materials and the particle size of products is generally rather big. On the other hand, hydrothermal synthesis is a soft-chemical process that can be used to prepare advanced materials [12]. The wide range of operating temperatures available in aqueous media is the most attractive aspect of the approach for preparation of many different types of materials. Moreover, the hydrothermal conditions, such as the concentrations of precursors, pH, synthesis temperature, and synthesis time, additive species, among others, are easily controlled. In the present work, a series of BiVO4 photocatalysts were synthesized via a surfactant-assisted hydrothermal method. The effects of different surfactants on crystal structure, morphology and spectroscopic properties of BiVO4 products were discussed in details. As the efficient visible-light-driven photocatalyst, the factors affecting the photocatalytic activities for dyes degradation over different BiVO4 products are also explored. 2. Experimental 2.1. Materials and process Bismuth nitrate pentahydrate (Bi(NO3 )3 ·5H2 O) and ammonium metavanadate (NH4 VO3 ) from Beijing Chemical Company were used as received without further purification. All the other chemicals used in the experiments were analytical grade reagents, and deionized water was used for solution preparation. In a typical preparation process, 0.02 mol Bi(NO3 )3 ·5H2 O and 0.02 mol NH4 VO3 were dissolved in 20 mL of 65% (w/w) HNO3 and 20 mL 6 mol/L NaOH solutions separately, and each stirred for 2 h at room temperature. After that, these two mixtures were mixed

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together in 1:1 molar ratio and stirred for about 1 h to get a stable, salmon pink homogeneous solution. Then, the homogeneous mixture was sealed in a 50 mL Teflon-lined stainless autoclave, and allowed to heat at 180 ◦ C for 6 h under autogenous pressure. After that, the precipitate was filtered, washed with distilled water each for three times, and dried in vacuum at RT for 4 h. For the synthesis of surfactant-assisted BiVO4 , 0.25 g of hexadecyl trimethyl ammonium bromide (HTAB) or polyvinyl alcohol (PVA) or polyvinyl pyrrolidone (PVP) was diluted in the homogeneous mixture before transferring into the autoclave. The surfactant-assisted BiVO4 products were labeled as BV-HTAB, BV-PVA and BV-PVP, respectively. 2.2. Characterization X-ray powder diffraction patterns (XRDs, Puxi Co. LTD, model XD-3) were recorded in the region of 2 = 17–55◦ using Cu K␣ radiation ( = 0.15418 nm) with a step scan of 2.0◦ /min at RT using a counter diffractometer. The morphologies and microstructures of as-prepared samples were examined with scanning electron microscopy (SEM, Hitachi, model S-3500N). Optical absorbance spectra of the samples were obtained on a doubled-beam UV–vis spectrophotometer (Puxi Co. LTD, model TU-1901) equipped with an integrating sphere. UV–vis diffuse reflectance spectra (DRS) of BiVO4 were recorded by using BaSO4 as a reference and were converted from reflection to absorbance by Kubelka–Munk method [13]. The Raman spectra were recorded by a microprobe Raman (Renishaw, model H13325) system, and the excitation line was at 514.5 nm from an Ar ion laser. 2.3. Photocatalytic activity test Photocatalytic activities of samples were determined by the decolorization of methyl orange (MO) under visible light irradiation. A 500 W Xe-illuminator was used as a light source and placed about 10 cm apart from the reactor to provide visible light irradiation. Experiments were performed at ambient temperature as follows: the same amount (0.2 g) of photocatalyst was added into 100 mL of 10 mg/L MO solution. Before illumination, the solution was stirred for 10 min in the dark in order to reach the adsorption–desorption equilibrium for MO and dissolved oxygen. At different irradiation time intervals, about 5 mL of the suspensions were collected, and then centrifugalized to remove the photocatalyst particles. The concentrations of remnant MO were monitored by checking the absorbance of solutions at 464 nm during the photodegradation process.

Fig. 1. XRD patterns of BiVO4 products: (a) no-surfactant-assisted; (b) HTABassisted; (c) PVA-assisted; (d) PVP-assisted.

where D is crystalline size; k is wavelength of the X-ray radiation (0.15418 nm); K is taken as 0.89; ˇs and ˇe are peak widths at half-maximum height of the sample and the equipment broadening, respectively; and 2 = 28.90◦ . The unit-cell parameter of all the prepared samples was analyzed from Bragg equation considering the standard pattern using self-bring XRD software of the equipment. As a result, the crystalline sizes of samples were evaluated as 63, 45, 58, 54 nm for BV-NO, BN-HTAB, BV-PVA and BV-PVP, respectively. This dissimilarity may lead to the differences in their morphology and structure [15]. It is noted that the corresponding intensities and positions of a few diffraction peaks among the products are different in some cases. For example, the position of (1 2 1) and (0 4 0) reflection in BV-HTAB shows an obvious shift to higher angle comparing to the others, as shown in Fig. 2, indicating that the surfactant HTAB made a great influence on the d(1 2 1) and d(0 4 0) space of BiVO4 during the hydrothermal process. However, there is no obvious difference between the positions of the (1 2 1) and (0 4 0) reflection of BVNO, BV-PVA and BV-PVP. This may be due to the different molecule structures of these surfactants.

3. Results and discussion 3.1. Powder formation Fig. 1 shows the XRD patterns of no-surfactant-assisted BiVO4 (BV-NO), HTAB-assisted BiVO4 (BV-HTAB), PVA-assisted BiVO4 (BVPVA) and PVP-assisted BiVO4 (BV-PVP). The XRD patterns of all these samples present similar profiles and all the diffraction peaks can be well indexed as monoclinic BiVO4 (JCPDS card No. 140688, space group I2/a, unit-cell parameters a =5.195 Å, b = 11.701 Å, c = 5.092 Å, ˇ = 90.38◦ , mineral name: clinobisvanite). No other impurities such as Bi2 O3 or other organic compounds related to reactants were detected, indicating the phase purity of sample BiVO4 . The crystalline size of these powders was estimated from the Scherrer formula [14] as follows: D=

K



ˇs2 − ˇe2 cos 

(1)

Fig. 2. XRD patterns of BiVO4 products in the range of 2 = 28–32◦ : (a) no surfactantassisted; (b) HTAB-assisted; (c) PVA-assisted; (d) PVP-assisted.

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3.2. Morphologies performance The morphology of as-synthesized BiVO4 products was characterized by SEM, which is shown in Fig. 3. It is found that the surfactants used in the hydrothermal synthetic system have an

important influence on the morphology of BiVO4 . As shown in Fig. 3a, the BV-NO sample, with an amorphous structure, comprised of agglomeration of rods and slice-like particles can be easily observed. Carefully looking into the SEM image (Fig. 3b) of this BVNO sample, the thickness and the length of BiVO4 slice and rods are

Fig. 3. SEM images of BiVO4 products: (a and b) no-surfactant-assisted; (c and d) HTAB-assisted; (e and f) PVA-assisted; (g and h) PVP-assisted.

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Fig. 4. Raman (A) and FTIR (B) spectra of BiVO4 products: (a) no-surfactant-assisted; (b) HTAB-assisted; (c) PVA-assisted; (d) PVP-assisted.

widely discrepant; and the growth tendency of lamellar structures may be related to the layered feature of monoclinic BiVO4 . Fig. 3c and d give out the morphology of the BV-HTAB sample. Instead of microparticles, the BiVO4 crystallites assembled into microclusters with size to be 5–10 ␮m (Fig. 3c). By checking the detail of these microclusters, one can easily find that these microclusters are flower-like and comprised of numerous BiVO4 sub-nanoparticles, as shown in Fig. 3d. It seems that HTAB in the current hydrothermal condition would assist the formation of flower-like microclusters, as well as narrow the d(1 2 1) space by protecting the microscopic structure of BiVO4 crystallites, as indicated by the XRD analysis that the (1 2 1) peak of BV-HTAB is obviously shifted to higher angle as compared to BV-NO. In the PVA-assisted synthetic system, nanoparticles with almost spherical structure were obtained for the BV-PVA sample, as shown in Fig. 3e and f. Compared to BV-NO, the single particle of BV-PVA became smaller, with the size ranging from 70 to 150 nm (Fig. 3f), which could be owe to the special structure of the PVA micelle in the hydrothermal condition. The morphology of the BV-PVP sample is presented in Fig. 3g and h. It is obviously that this BV-PVP sample possesses a mixture of morphologies including microparticles and microclusters. The microclusters are composed of BiVO4 particles (shown in Fig. 3g), while the microparticles seem like a flat bread-like particle with unglazed surface. That is to say, the existence of PVP is disadvantageous to the formation of microcluster with assembled structure. As a consequence, surfactants greatly affect the morphology of BiVO4 , as well as the crystal structure predicted by the XRD results. 3.3. Structures performance The Raman spectra of the 170–1000 cm−1 region of samples are shown in Fig. 4A; and the results of the band component analysis [16,17] of Raman and IR spectra of these samples are listed

Fig. 5. DRS spectra of BiVO4 products: (a) no-surfactant-assisted; (b) HTAB-assisted; (c) PVA-assisted; (d) PVP-assisted.

in Table 1. It showed the same feature that each spectrum of surfactant-assisted sample was dominated by an intense Raman band near 825 cm−1 assigned to vs (V–O), and with a weak shoulder at about 703 cm−1 assigned to vas (V–O). The ıs (VO4 3− ) and ıas (VO4 3− ) modes are near 366 and 321 cm−1 , respectively, and external modes (rotation/translation) occur near 213 cm−1 . The positions of the most intense bands near 825 cm−1 were determined by the fitting to the Lorentzian peak function. An obvious shift of this Raman band to the higher wavenumber, from 825 to 829 cm−1 , reveals that the average short-range symmetry of the VO4 tetrahedra becomes less regular [18].

Table 1 Assignations of Raman and FTIR wavenumbers observed for samples (a)–(d) as shown in Fig. 4A and B. (a) BV-NO

(b) BV-HTAB

(c) BV-PVA

(d) BV-PVP

Assignation

Raman

FTIR

Raman

FTIR

Raman

FTIR

Raman

FTIR

829 706

820 730

828 709

836 774 732

827 704

836 747 700 666 554 506

825 703

836 747 700 666 518 461

640 366 322 209

470

366 325 210

554 479

366 323 212

366 321 213

Note: vs : symmetric stretching mode; vas : asymmetric stretching mode; ıs : symmetric deformation mode; ıas : asymmetric deformation mode.

vs (V–O) vas (V–O) v(Bi–O) ıs (VO4 3− ) ıas (VO4 3− ) External mode

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Fig. 4B showed FTIR spectra of samples, recorded ranging from 400 to 3500 cm−1 at RT. All the samples are mainly characterized by a broad and strong IR band near 730 cm−1 with shoulders at 836, 666 and 640 cm−1 . It reveals that spectra of all the surfactantassisted samples exhibited a split of the most intense band from 730 cm−1 to 700 and 747 cm−1 or 732 and 734 cm−1 , which may be caused by the breakage of the symmetry of VO4 tetrahedra, as can be indicated by the Raman results. It is an obvious result that all the surfactant-assisted samples exhibited stronger v(Bi–O) and ıs (VO4 3− ) bands than the BV-NO sample, which can be caused by the enhancive degree of crystallinity and decrease of defects when particles/clusters formed at surfactant-assisted hydrothermal treatment. The UV–vis DRS of samples are shown in Fig. 5. It was found that each sample showed different absorption profile in visible light region in addition to that of strong absorption in the UV-light region. The steep sharp of spectra indicated that the visible light adsorption is due to the band gap transition; and the prolonged absorption tail until about 650 nm may be result from the crystal defects formed when BiVO4 grew [19]. It was found that tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an UV absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band [20]. The band gaps of samples were calculated from the DRS. By extrapolation of the onset of the rising part to the x-axis (g , nm) of the plots (as shown in Fig. 5 by the dotted line), the band gap (Eg , eV) was determined by calculation using equation Eg = 1240/g . As shown in the spectra, their absorption onsets locate at different wavelengths, as 556 nm for the BV-NO sample, 574 nm for the BVHTAB sample, 551 nm for the BV-PVA sample and 558 nm for the

BV-PVP sample, respectively. The band gap of the BV-HTAB sample calculated from the onset of absorption edge is 2.16 eV, sizable shift in band gap to 2.27 eV has happened for the BV-NO sample, while the band gaps of the BV-PVA sample and the BV-PVP sample calculated to be 2.25 and 2.22 eV, respectively, are quite close to that of the BV-NO sample. The narrow band gap of the BV-HTAB sample indicates that this sample has the lower conduction band level, which will increase the absorption characteristic for visible light. 3.4. Photocatalytic activities The photocatalytic performance of surfactant-assisted BiVO4 powders hydrothermally synthesized were examined in terms of the photodegradation MO from an aqueous solution under visiblelight irradiation compared with using the BV-NO sample, as shown in Fig. 6. The BV-HTAB sample, with a total degradation rate of 85% in 90 min, exhibited the highest activity among these samples. However, the BV-PVP sample exhibited a less strong activity; meanwhile, the activities of the BV-PVA sample, which was composed of large-surface nanoparticles, were lower than those of the BV-HTAB sample and the BV-PVP sample. These observations indicate that the local structure, crystallinity, surface morphology, and particle shape of the different BiVO4 powders should be considered as affecting the photocatalytic performance. Moreover, the amorphous BV-NO sample gave the lowest activity (as can be seen in Fig. 6a), which was much lower than those for surfactant-assisted BiVO4 , as is commonly found. This indicates that pure crystallinity of the photocatalyst remains a factor in determining the photocatalytic activity.

Fig. 6. Changes of UV–vis spectra of BiVO4 products suspended MO solution as a function of irradiation time of visible light: (a) no-surfactant-assisted; (b) HTAB-assisted; (c) PVA-assisted; (d) PVP-assisted.

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4. Conclusion

References

A series of BiVO4 photocatalysts were synthesized via a surfactant-assisted hydrothermal method. The XRD, SEM, UV–vis, and Raman spectroscopic characterizations revealed that not only the microstructure, such as the morphology, surface texture, and grain shape, but also the local structures of the synthesized BiVO4 materials were significantly dependent on the surfactant. In the presence of different surfactants during the synthetic system, the d(1 2 1) space of the BV-HTAB sample is smaller than that of the other three photocatalysts. Compared with others, it can be found that the BV-HTAB sample has a narrow band gap and hence a more powerful absorption capability of visible light. Higher photocatalytic activity is obtained with this BV-HTAB photocatalyst. In summary, it demonstrated that the effectiveness of using surfactant such as HTAB to construct an efficient visible-light-driven BiVO4 photocatalyst; and the local structure, as well as the pure crystallinity, provided a significant effect on the photocatalytic performance.

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Acknowledgements We acknowledge the financial support from the Scientific Research Foundation of North China University of Technology (NCUT) and the Natural Science Foundation of Beijing.