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ScienceDirect Materials Today: Proceedings 5 (2018) 9447–9452

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The 10th Thailand International Metallurgy Conference (The 10th TIMETC)

Effect of pH on crystal structure and morphology of hydrothermally-synthesized BiVO4 Soriya Phiankoh, Ratiporn Munprom* Materials Engineering Department, Faculty of Engineering, Kasetsart University, 50 Ngamwongwan Rd., Chatuchak, Bangkok 10900, Thailand

Abstract Bismuth vanadate has been recently gained enormous attention due to its photocatalytic performance under visible-light irradiation. The compound can be prepared by various methods whose condition highly influences photocatalytic properties. This study investigated the effect of pH on structure and morphology of BiVO4. The BiVO4 powder was prepared using a hydrothermal method under different pH values (pH = 4, 7, 10). The samples were characterized by X-ray diffractometer (XRD) and scanning electron microscope (SEM). It was observed that phase structures and morphology of BiVO4 were pH-dependent. Additionally, different phases and morphologies strongly influenced efficiency of photocatalytic dye degradation. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference. Keywords: Hydrothermal; bismuth vanadate; pH; structure; morphology; photocatalyst

1. Introduction Industrial expansion has caused many environmental problems such as organic contaminant in water resource. These problems result in an intensive development of wastewater treatment technologies. Among all technologies, the degradation of pollutants using photocatalytic oxides is of great interest. A photocatalyst uses energy from light to accelerate a chemical process, for example a degradation process of toxic chemicals to non-toxic chemicals [1]. Early studies on photocatalysts mainly focused on TiO2 because of its low cost and photostability [2]–[4]. However, TiO2 possesses a band gap of 3.2 eV which is responsive only to ultraviolet (UV) light and limits its photocatalytic

* Corresponding author. Tel.: +66-093-408-9994; fax: +66-02-955-1811. E-mail address:[email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The 10th Thailand International Metallurgy Conference.

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Soriya Phiankoh and Ratiporn Munprom / Materials Today: Proceedings 5 (2018) 9447–9452

performance [5]. To improve efficiency, narrow band gap materials are desirable. Consequently, visible-light absorbing photocatalysts are vigorously attractive. Bismuth vanadate (BiVO4) is one of extensively-studied photocatalysts [5]–[7]. It has become a promising photocatalyst because of its visible-light responsive band gap (2.4 eV) [8], chemical stability, non-toxicity, and costeffectiveness. BiVO4 can form in three crystal structures: monoclinic scheelite-type, tetragonal scheelite-type and tetragonal zircon-type [9]–[11]. The crystal structures strongly affect photocatalytic properties. Among these three structures, monoclinic scheelite phase is the most desirable because of its high photocatalytic activity [12]. In addition to crystal structures, morphology also plays an important role on photocatalytic performance. Shape and size of BiVO4 powder are relevant to surface area which determines active sites for photocatalysis [13]. Similarly, high photocatalytic activity can be obtained when BiVO4 possesses some certain orientations because its photocatalytic properties are orientation-dependent. For example, Li et al. demonstrated that faceted BiVO4 crystals comprised of {010} and {110} orientations can improve charge separation resulting in better photocatalytic activity [14]. Therefore, tailoring morphology can enhance photocatalytic efficiency of BiVO4 [14],[15]. The BiVO4 compound can be prepared by various techniques, such as solid-state reaction, sol-gel method [16] and hydrothermal synthesis [17]. Among all, hydrothermal synthesis is widely used because it is simple and operates at low temperatures [17],[18]. It is well known that synthesizing conditions and methods are crucial to the formation of BiVO4 and can result in different phases and morphologies as well as photocatalytic properties. Therefore, herein, the effect of pH is investigated. In this study, we synthesized BiVO4 powder by a hydrothermal method under different pH values. The structure and morphology were studied by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM), respectively. Photocatalytic dye degradation of methylene blue (MB) in an aqueous solution under UV irradiation was conducted to examine the photocatalytic performance of the obtained BiVO4 powder. 2. Experimental 2.1. Sample preparation BiVO4 powder was synthesized by hydrothermal method. The chemicals used for the syntheses are bismuth nitrate pentahydrate (Bi(NO3)3.5H2O, Daejung, Korea), ammonium metavanadate (NH4VO3, Daejung, Korea), nitric acid (HNO3, Merck, Germany) and sodium hydroxide (NaOH, Daejung, Korea). All chemicals were analytical grade and were used without further purification. In a typical procedure, Bi(NO3)3.5H2O (0.01 mol) was dissolved into 4.0 mol/L HNO3 solution (40 ml), and NH4VO3 (0.01 mol) was dissolved into 4.0 mol/L NaOH solution (40 mL). The two solutions were magnetically stirred for 30 mins at room temperature and then were mixed together to form a yellowish-orange suspension. After stirring the mixture for 30 mins, the pH of the suspension was adjusted to 4.0 by adding dropwise HNO3 solution, and to 7.0 and 10.0 by adding dropwise NaOH solution. Then, the obtained suspension was stirred for another 30 mins and was heated in a Teflon-lined autoclave at a temperature of 185 ̊C for 6 h. After autoclaving, the yellow precipitate was filtered and washed with de-ionized water and ethanol (three times), and dried in an oven at 100 ̊C for an hour afterwards. 2.2. Characterizations X-ray diffraction (XRD, Phillips X’pert) analyses were carried out to determine crystalline phases of the assynthesized samples. The data was collected in the 2θ range of 20-80 ̊ using Cu Kα radiation (λ=0.15418 nm). The morphologies of the samples were analyzed by scanning electron microscope (SEM, Philips XL30). The specific surface area was measured with a Micromeritics surface analyzer. Prior to each measurement, 250 mg of the sample was degassed at 230 ̊C for 12 h. The analyses of BET surface area were carried out on nitrogen gas desorption data. 2.3. Photocatalytic dye degradation of methylene blue The photocatalytic activities of the BiVO4 samples were determined by decolorization of methylene blue (MB) under UV-light irradiation using a UV lamp as a light source. Typically, 50 mL of MB aqueous solution (5mg/L)


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added with 0.2 g of BiVO4 powder was stirred for 15 mins in darkness to obtain the adsorption–desorption equilibrium. After that, the light was illuminated and 3 mL of the suspension was collected every fifteen minutes for 135 mins. All the collected samples at different irradiation time were centrifuged at 75 rpm to separate the photocatalytic particles. The absorption spectra of the collected MB solution were recorded in the wavelength range of 300-700 nm using a UV-VIS spectrometer (UV-1700, Shimadzu). The absorption intensity at λ = 553 nm was used to determine the concentration of MB and the concentration ratio (C/C0) was calculated to represent the photodegradation efficiency of dye (where C0 is the initial concentration of MB and C is the concentration of MB at a given time). 3. Results and Discussion 3.1. Effect of pH on crystal structure

Fig. 1. XRD patterns of BiVO4 prepared by hydrothermal method under pH 4, 7 and 10.

Fig. 1 shows the XRD patterns of the BiVO4 samples hydrothermally synthesized at different pH values (pH 4, 7 and 10). Under acidic and neutral synthesizing conditions, the obtained diffraction patterns can be indexed as a single phase of monoclinic scheelite type which are matched well with the reference data (JCPDS card no. 14-0688). In addition, the diffraction peaks are sharp which indicates a good crystallinity of the samples. However, when the pH was increase to pH 10 by adding NaOH, the diffraction pattern is different from the others. It is characterized to be mixed phases of Bi2VO5.5 (JCPDS card no. 51-0032), Bi7VO13 (JCPDS card no. 44-0322), tetragonal BiVO4 structure (JCPDS card no. 48-0744), Bi8V2O17 (JCPDS card no. 44-0612), Bi2O3 (JCPDS card no. 77-2008) and V2O5 (JCPDS card no. 89-0612). This result demonstrates that it is more difficult to obtain the monoclinic scheelite phase. To form BiVO4, a hydrothermal method causes a reaction between Bi3+ and VO43- ions. However, at pH 10 under alkaline conditions, the reaction between the Bi3+ and VO43-ions hardly happened, while the Bi3+ ions can be easily combined with OH- and then form both stoichiometric and non-stoichiometric bismuth oxide compounds. The bismuth oxide compounds can further react with residual ions in the suspension to form poly-vanadate compounds. As a result, the monoclinic BiVO4 phase was vanished and the other phases appeared under basic conditions.

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3.2. Effect of pH on morphology

Fig. 2. SEM images of the BiVO4 samples prepared by hydrothermal method under different pH values: (a) 4.0; (b) 7.0; (c) 10.0.

Fig. 2 shows SEM images of the BiVO4 samples prepared by hydrothermal method under pH 4, 7 and 10. There are differences in morphologies of the BiVO4 powder depending on the pH values. At pH 4, the BiVO4 sample was rounded particles whose sizes were less than 1 μm, specifically in the range of 0.48-0.9 μm. The particles were mostly connected to form peanut-like shaped branches (Fig. 2a). As the value of pH increased to 7, we observed irregular rod-like particles (Fig. 2b). The particle sizes were 1.2-1.8 μm in length. When the concentration of OHincreased, it caused large agglomerations of small particles and form angular crystals (Fig. 2c). The accumulated particles were greater than 10 μm in size. 3.3 Photocatalytic dye degradation of methylene blue

Fig. 3. Photocatalytic activity of BiVO4 for the degradation of MB under UV irradiation.

The photocatalytic performance of the samples prepared at different pH values was shown in Fig. 3. The result reveals that pH values of the hydrothermal conditions influence the photocatalytic performance. Specifically, MB was degraded at a constant rate for monoclinic BiVO4 prepared at pH 4 and 7. On the other hands, the photodegradation rate of pH 10 sample was faster at the beginning and became slower and slower. In other words, the decolorization of MB exponentially decayed for BiVO4 prepared at pH 10. Additionally, after 135-min exposure, the monoclinic BiVO4 phase synthesized at pH 4 showed the lowest concentration of MB suggesting the highest degradation efficiency. This result is in good agreement with other previous studies finding that monoclinic BiVO4 is the most desirable phase due to its high photocatalytic activity [19]–[21].


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Not only crystal structure can influence photocatalytic properties, but different band gaps, surface area and morphologies can also alter photocatalytic performance. Based on the absorption spectra, the optical band gaps of the samples were estimated. A slight change in their band gaps can be observed. Specifically, an increase in band gaps was found when increasing pH values. In other words, the band gap of pH 4 sample was smallest. Moreover, according to the BET results, the surface area of pH 4 sample was greater the surface area of pH 7 sample as well. As a result, even though the samples prepared at pH 4 and 7 possessed the same crystal structure of monoclinic scheelite, the photocatalytic activity of pH 4 was significantly higher due to its higher surface area and smaller band gap. The BET results are in good agreement with the morphologies observed in the SEM results. The powder synthesized at pH 4 was smaller in size so the surface area was higher. Surprisingly, the BET surface area of the pH 10 sample was highest (20.89 m2g-1) although the sample was large in size. This can be explained by the SEM image of the sample prepared at pH 10 showing that the large crystals were actually the agglomeration of small particles. It is also worth noting that the surface area of the pH 10 sample was approximately eight times higher than the pH 4 sample; however, the difference in the photodegradation performance was not much significant. The higher surface may lead to the faster photodegradation rate of the pH 10 sample at the early stage; however, the rate became deteriorated because of the mixed phases. Obviously, the high performance of the pH 4 sample was resulted from the dominated phase of monoclinic scheelite. The different photodegradation efficiencies can be explained by the effect of crystal structures and morphologies altered by pH values. The variation of structure and morphology leads to differences in adsorption ability of the catalyst surfaces. 4. Conclusion The crystal structure and morphology of the hydrothermally synthesized BiVO4 powder prepared at different pH values were investigated. The different pH values lead to different formations of the samples. The monoclinic scheelite phase can be obtained in acidic and neutral conditions of syntheses. Under basic conditions, the sample was the mixed phases. The SEM images revealed that when the pH value was increased, the sizes of the powders were also increased. The results reveal that the pH values affect not only crystal structure but also morphology which are two key factors influencing photocatalytic performance. To achieve high photocatalytic activity, monoclinic scheelite BiVO4 with high surface area is suggested. Acknowledgements This research received funding from Faculty of Engineering, Kasetsart University. In addition, we gratefully acknowledge use of the facilities of Assoc. Prof. Dr. Apirut Laobuthee of Department of Materials Engineering, Kasetsart University. References [1] P. Zhou, J. Yu, and M. Jaroniec, Adv. Mat. 26 (2014) 4920–4935. [2] L. Zhang et al., Mat. Chem. and Phys. 136 (2012) 897–902. [3] K. Nakata and A. Fujishima, J. Photochem. Photobiol. C 13 (2012) 169–189. [4] S.-Y. Lee and S.-J. Park, J. Ind. Eng. Chem. 19 (2013) 1761–1769. [5] B.-X. Lei, L.-L. Zeng, P. Zhang, Z.-F. Sun, W. Sun, and X.-X. Zhang, Adv. Powder Technol. 25 (2014) 946–951. [6] R. Munprom, P. A. Salvador, and G. S. Rohrer, J. Mater. Chem. A 3 (2015) 2370–2377. [7] P. Intaphong, A. Phuruangrat, and P. Pookmanee, Integr. Ferroelectr. 2016, pp. 51–58. [8] G. Tan, L. Zhang, H. Ren, S. Wei, J. Huang, and A. Xia, ACS Appl. Mater. Interfaces. (2013) 5186–5193. [9] M. Gotić, S. Musić, M. Ivanda, M. Šoufek, and S. Popović, J. Mol. Struct. vol. 744–747 (2005) 535–540. [10] A. W. Sleight, H. -y. Chen, A. Ferretti, and D. E. Cox, Mater. Res. Bull. 14 (1979) 1571–1581. [11] H. Jiang, H. Dai, X. Meng, L. Zhang, J. Deng, and K. Ji, Chin. J. Catal. 32 (2011) 939–949. [12] L. Zhang, D. Chen, and X. Jiao, J. Phys. Chem. B 110 (2006) 2668–2673. [13] S. Dong et al., Appl. Catal. B Environ. vol. 152–153 (2014) 413–424. [14] R. Li et al., Nat. Commun. (2013) 1432. [15] T. Liu, X. Zhou, M. Dupuis, and C. Li, Phys. Chem. Chem. Phys. 17 (2015) 23503–23510. [16] X. Wang, Y. Shen, G. Zuo, F. Li, Y. Meng, and L. Hou, Mater. Res. Innov. 20 (2016) 500–503. [17] J. Liu, H. Wang, S. Wang, and H. Yan, Mater. Sci. Eng. B 104 (2003) 36–39.

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