Microwave routed hetero structural Erbium doped BiVO4

(1)

International Journal of Advanced Chemical Science and Applications (IJACSA)

_______________________________________________________________________________________________

ISSN (Print):2347-7601, ISSN (Online): 2347-761X, Volume -3, Special Issue -1, 2015 86

Microwave routed hetero structural Erbium doped BiVO

₄

with visible-light driven Photocatalytic Activity

1Subramanian Moscow, ²Kandasamy Jothivenkatachalam

1,2Department of Chemistry, Anna University – BIT campus, Tiruchirappalli, Tamilnadu – 620024, India E-mail:¹[email protected], ²[email protected]

[Received:3^rd Aug.2015; Revised:4^th Sept.2015; Accepted:

9^th Sept.2015; Available online from: 27^th Oct. 2015]

Abstract : The monoclinic – tetragonal BiVO4 (m-tBiVO4) nanoparticles was successfully synthesized by microwave heating method using domestic microwave oven for the prospective environmental applications. The structure and morphology of the synthesized catalyst were characterized by Powder X-ray diffraction (PXRD), Raman spectroscopy, scanning and transmission electron microscopy (SEM and TEM), Energy-dispersive X-ray (EDX) and UV–visible - diffuse reflectance spectroscopy (UV-vis - DRS). The PXRD pattern of the catalyst revealed that mixed phase of m-t BiVO₄ nanostructure has 5 nm in crystalline size. The SEM and TEM images exposed that the obtained m-t BiVO₄ particles are near like nuts in shape. The DRS of prepared catalyst exhibits the visible region photo-absorption and the calculated band gap 2.4 eV. The photocatalytic activities of the catalysts were evaluated by the degradation of Rhodamine B (RhB) aqueous solution under visible light irradiation. The improved photocatalytic behaviour of the m-t BiVO₄ nanostructure is due to the smaller band gap, mixed phase and nano size entities.

Key words: Semiconductor, Heat treatment, Electron microscopy, Microstructure

I. INTRODUCTION

Heterogeneous photocatalyst is considered as an important tool for organic pollutant degradation and hydrogen production [1]. TiO₂ is an efficient photocatalyst; it displays a poor performance under visible region [2]. In order to utilize solar light effectively, visible light driven photocatalysts require important attention. As a result, many efforts have been devoted to the development of other water splitting photocatalytic materials and systems that can use the visible part of the solar spectrum. [3, 4] In this sense, bismuth vanadate has been found to be a capable candidate for organic contaminants decomposition under visible-light irradiation [5, 6]. Because of its narrow band gap, nontoxicity, high stability, and sunlight utilization, BiVO₄ has drawn great attention nowadays.

BiVO₄ is a direct band gap ternary metal oxide semiconductor with a favourable band gap of 2.3−2.5 eV for solar light absorption. Furthermore, its conduction band is close to 0 V versus RHE at pH 0, as

a result of the overlap of empty Bi 6p orbitals with antibonding V 3d−O 2p states [7]. Although there have been many reports on transition metal and noble metal doped BiVO4, the effect of rare earth metal doping on the photocatalytic activity of BiVO₄ for the organic contaminants photodegradation has seldom been reported so far as we know. Therefore, using lanthanide element doped in catalysts could be an efficient way to increase the photocatalytic activity.

However, the activity of pure BiVO₄ is low due to its poor adsorptive performance and rigid to migration of photogenerated electrons from holes [8]. For the sake of improving the photocatalytic activity of m-BiVO₄, different approaches have been proposed including heterojunction structure formation [9]. To increase the photoactivity of BiVO4 systems, the coexistence of monoclinic−tetragonal heterostructure systems are anticipated to promote the separation of photoinduced electron−hole pairs [10].

Herein, BiVO4 was doped with different content of erbium (Er) and characterized by X-ray powder diffraction patterns (XRD), Raman spectrometer;scanning electron microscopy (SEM), Transmission Electron microscopy (TEM) and UV-Vis diffuse reflectance spectra (DRS) techniques. And the influence of Er doping on the structure, morphology and their visible-light-driven activities of catalysts are also discussed.

II. EXPERIMENTAL

2.1 Preparation

Int he typical procedure of microwave-assisted method, 1:1 mol Bi(NO₃)₃.5H₂O in HNO₃ and NH₄VO₃ in NaOH were dissolved and these two mixtures were mixed together in 1:1 molar ratio and stirred for about 30 min to get homogeneous solution. The stoichiometric amount of Er(NO₃)₃ was added into mixture with stirring and heated under microwave irradiation using a domestic microwave oven. Afterwards, the precipitate was filtered, washed with distilled water three times for each, and dried at 120°C for 12 h.

(2)

_______________________________________________________________________________________________

2.2 Characterization

The crystalline size and phase of the sample were recorded by powder X-ray diffractometer (PXRD) (Bruker/AXS D8 Advance, Germany) with a Cu Kα X- ray irradiation source in the range of 20-80° at room temperature. Raman shift spectrum of the Er-BiVO₄was measured using a Raman spectrometer (Bruker RFS/100, Germany). The morphology and nanostructure of the product was determined by field emission scanning electron microscope (FESEM) (Gemini Zeiss Supra 55, Germany) and transmission electron microscopy (TEM) (JEOL JEM-2100) with an accelerating voltage of 200 kV. Optical absorption spectra of the sample were obtained by double-beam UV-visible spectrophotometer equipped with an integrating sphere. (UV-2450, Shimadzu) using BaSO₄ as a reference.

2.3 Photocatalytic activity

The photocatalytic activity of the as prepared Er-BiVO₄ was evaluated tothe degradation of RhB in aqueous solution. 100 mg of the as prepared catalyst powder was dispersed in 100 ml RhB solution with a concentration of 5 mg/l was taken in photoreactor. At a given time interval, the filtrates were taken out and the intensity change of absorption peak was recorded at 553 nm by using UV-visible spectrometer.

III. RESULTS AND DISCUSSION

The phase arrangement of semiconductor materials is of great importance for its photocatalytic activity. Analysis of the XRD patterns reveals that the resulting catalyst was prepared from the microwave hydrothermal process as shown in Fig.1. As it can be noticed from the XRD patterns, the crystalline structure of Er-BiVO₄indicates the mixed phase of monoclinic and tetragonal structure of polycrystalline nature. This observation is further confirmed by the splitting of the main peaks at 2θ = 29, (121) 30.7, (040) 36.9, (220) 39.8 (211) and 47.8º (112) which is the characteristic monoclinic structure of BiVO4 (JCPDS No.14-0688) [11]. The tetragonal structure is characterised by 2θ = 24 (200) 32.7 (112) 34.7 (220) 48.4 (312) and 53.4º (411) (PDF 14-0133) [12]. The shape and intensity peaks, suggest that the catalyst is well-crystalline with size around 5 nm.

Fig.1.XRD pattern of m-t BiVO₄

Fig. 2. Ramanspectra of m-t BiVO₄

Raman spectroscopy is an effective tool for the structural characterization of materials. The formation of m-t BiVO₄ crystalline was substantiated by the results of the Raman spectra analysis (Fig.2). The m-t BiVO₄ Raman spectrum illustrates the presence of typical bands at 240, 367, 738 and 837 cm⁻¹. The most intense mode at 837 cm⁻¹ assigned to the ν1 symmetric V–O stretching vibrations mode (VO₄^3–) and with a weak shoulder at about 738 cm⁻¹ assigned to the ν3 asymmetric V–O stretching mode (VO₄^3–) [13].

The SEM measurements were carried out to study the morphology of the sample which is shown in Fig 3. The image describes that the m-t-BiVO₄ is composed of nuts like particle size around 10 nm. (Fig. 5) For erbium doping, the mixed phase particles present a clear evolution of the form of nuts shape and well-defined nanostructure morphology. As can be noticed, the combination of erbium with BiVO₄induces a change in morphology [14].

Fig.3. SEM and TEM image of m-t BiVO₄

(3)

_______________________________________________________________________________________________

The morphology and nanostructure of m-t BiVO₄ were also investigated by Transmission electron microscopy (TEM) is shown in Fig. 3. The TEM image also clearly shows that the nuts like nanostructure is comprised of well-defined lattice fringes, which indicate the particles are polycrystalline type.

In order to study the electronic states of the systems, UV–vis DRS spectra were taken (Fig. 4). The DRS of m-t BiVO4 has extended a red shift and increased absorbance-intensity in the visible- light range, which can be attributed to a charge transfer transition between the erbium ion and the BiVO₄ conduction - valence band. As the monoclinic-to-tetragonal transition is taking place, the band gaps tend to shift towards the visible light region [15].By extrapolation of the inception of the increasing part to the x-axis (λ) of the plots and the band gap (Eg) datum was determined by calculation using equation Eg=1240/ λ with 2.3 eV band gap energy.

200 300 400 500 600 700 800

Absorbance (a.u)

Wavelength (nm)

Fig.4. UV-Vis DRS spectra of m- t BiVO₄over Rhodamine B (RhB)

Fig.5. Photocatalytic activity of m-t BiVO₄ Photocatalytic oxidation of Rhodamine B (RhB) The photocatalytic activity of m-t BiVO₄was examined with RhB as a model pollutant. RhB undertook noticeable degradation upon visible light irradiation. The temporal UV-visible spectral changes of RhB solution during the photocatalytic degradation reactions over m-t BiVO₄ and the characteristic absorption of RhB at 553 nm weakens gradually which is related to a breakdown of the chromophoric ring structure of RhB to small molecules [16].The well-defined crystallinity could

minimize the defects in the crystal lattice, which leads the transfer of e^- / h⁺ pairs to the surface. In the nuts like structure with uneven morphologies of m-t BiVO₄, it is likely that more active sites are available to induce the photocatalytic reaction under visible light irradiation [17]. Moreover, the lower band gap of BiVO₄ can lead to absorption of more number of visible light photons and cumulate the photocatalytic activity [18].Since the photocatalytic degradation, the electron–hole pair is generated by the illumination of the catalyst and the mechanism may involve the photoinduced electrons in the CB react with an O₂ molecule adsorbed on the surface of the catalyst and form O^.₂^- radicals and degrade RhB. In the meantime, the photogenerated holes in the VB can straightly degrade RhB with surface h⁺_VB, an indirect reaction with OH^• radicals [19].

IV. CONCLUSION

We have successfully synthesized m-t BiVO₄ nanomaterial with near nuts like morphology via rapid microwave aided method. It was found that microwave played a key role in the formation of distinctive m-t BiVO4 with uneven surface and exhibited excellent visible-light-driven photocatalytic efficiency on photooxidation of the RhB. The improved visible-light photocatalytic activity was mainly attributed by a smaller band gap, defined crystallinity and nanostructure morphology. The strong adsorption of pollutants on the surface of m-t BiVO₄, which might allow efficient transport of electrons and reduced rate of recombination and also improved photocatalytic performance due to the hetrostructure mixed phase, the inhibited recombination between photogenerated electrons and holes. This work develops a facility, simple and promising method for the synthesis of m-t BiVO₄ hetrostructures materials in cost and time with high efficiency.

V. REFERENCES

[1] M. Anpo, M. Takeuchi, The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiationJ. Catal.216, 505–516, 2003.

[2] A. Kudo, K. Omori and H. Kato, A Novel Aqueous Process for Preparation of Crystal Form-Controlled and Highly Crystalline BiVO4 Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties, J. Am. Chem.

Soc.121, 11459–11467, 1999.

[3] K. Fujishima , A. Honda , Electrochemical Photolysis of Water at a Semiconductor ElectrodeNature, 238 , 37 – 38,1972.

[4] R. E. R. Bjorn Marsen, Eric L. Miller, D.

Paluselli, Photochemical hydrogen production, Int. J. Hydrogen Energy, 32 , 3110 – 3115, 2007.

[5] S. Navalón, A. Dhkshinamoorthy, M. Alvaro and H. García, ChemSusChem, Photocatalytic CO₂

(4)

_______________________________________________________________________________________________

reduction using non-titanium metal oxides and sulphides, 6,562–577,2013.

[6] Tan, G.; Zhang, L.; Ren, H.; Wei, S.; Huang, J.;

Xia, A. Effects of pH on the hierarchical structures and photocatalytic performance of BiVO₄ powders prepared via the microwave hydrothermal method, ACS Appl. Mater.

Interfaces 5, 5186−5193, 2013.

[7] Walsh, A.; Yan, Y.; Huda, M. N.; Al-Jassim, M.

M.; Wei, S. H.Band Edge Electronic Structure of BiVO₄: Elucidating the Role of the Bi s and V d Orbitals. Chem. Mater. 21, 547−5512009.

[8] Fan, H.; Jiang, T.; Li, H.; Wang, D.; Wang, L.;

Zhai, J.; He, D.;Wang, D.; Xie, T.. Effect of BiVO₄ crystalline phases on the photo-induced carriers behavior and photocatalytic activity.

J. Phys.Chem. C 116, 2425−2430, 2012.

[9] Long, M.; Cai, W.; Kisch, H. Visible Light Induced Photoelectrochemical Properties of n- BiVO4 and n-BiVO4/p-Co3O4. J. Phys. Chem. C, 112, 548−554, 2008.

[10] Sara Usai, Sergio Obregon, Ana Isabel

Becerro, and Gerardo Colon

Monoclinic−Tetragonal Heterostructured BiVO4

by Yttrium Doping with Improved Photocatalytic ActivityJ.Phys.Chem.C 117, 24479−24484, 2013.

[11] Tokunaga S, Kato H, Kudo A Selective Preparation of Monoclinic and Tetragonal BiVO₄ with Scheelite Structure and Their Photocatalytic PropertiesChem. Mater. 13,4624-4628, 2001.

[12] H. Liu, J. Yuan, Z. Jiang, W. Shangguan, H.

Einaga and Y. Teraoka, Roles of Bi, M and VO4 tetrahedron in photocatalytic properties of novel Bi0.5M0.5VO4 (M=La, Eu, Sm and Y) solid

solutions for overall water splitting, J. Solid State Chem., 186, 70–75, 2012.

[13] Shi, J. Zheng, and P. Wu, Preparation, characterization and photocatalytic activities of holmium-doped titanium dioxide nanoparticles. J.

Hazard. Mater. 161, 416,2009.

[14] S. Usai, S. Obregón, A. I. Becerro and G. Colón, Monoclinic–Tetragonal Heterostructured BiVO₄ by Yttrium Doping with Improved Photocatalytic Activity, J. Phys.Chem. C, 117, 24479–24484, 2013.

[15] B. Yan and X. Q. Su, chemical co-precipitation synthesis of luminescent phosphors from hybrid precursors, J. Non-Crystalline Solids 352, 3275, 2006.

[16] S. Hu, J. Zhu, L. Wu, X. Wang, P. Liu, Y. Zhang, Z. Li, Effect of Fluorination on Photocatalytic Degradation of Rhodamine B over In(OH)ySz:

Promotion or Suppression?, J. Phys.Chem. C, 115, 460, 2011.

[17] K. Jothivenkatachalam, S. Prabhu, A. Nithya and K. Jeganathan, Solar, visible and UV light photocatalytic activity of CoWO₄ for the decolourization of methyl orange, RSC Adv.4, 21221. 2014.

[18] Haoyong Yin, Yongfei Sun, Dejun Shi, XiaoxiWang Ling Wang. Controlling crystalline and morphology of monoclinic bismuth vanadate dendrite with enhanced photocatalytic activityInt.J. Phys. Sci. 64, 287-4293, 2011.

[19] C.Karunakaran, S.Kalaivani, P.

Vinayagamoorthy, Sasmita dash. Optical, electrical and visible light-photocatalytic properties of yttrium-substituted BiVO₄ nanoparticles, Mater. Sci. Semicond. Process.21,

122-131, 2014.

