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Investigation of visible light photocatalytic behavior of Bi4V2O11−δ and BIMEVOX (ME = Al, Ga) oxides
Materials Research Bulletin 45 (2010) 1250–1254
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Investigation of visible light photocatalytic behavior of Bi4V2O11 (ME = Al, Ga) oxides
d
and BIMEVOX
Vaishali Thakral, S. Uma * Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110007, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 February 2010 Accepted 7 May 2010 Available online 2 June 2010
A study of photocatalytic properties of Bi4V2O11 d for the degradation of common organic dyes such as methylene blue (MB) under visible light was carried out. Other identified bismuth based visible light photocatalysts such as Bi2GaVO7, Bi2YVO8 and Bi2AlVO7 were reinvestigated to understand their structural, and photocatalytic properties. Our experiments involving the synthesis of Bi2YVO8 indicated merely a mixture of BiYO3 and BiVO4, instead of the reported oxide containing bismuth in +3 and +5 oxidation states (J. Luan, et al., Mater. Res. Bull. 43 (2008) 3332). We have further shown the similarity of Bi2AlVO7 and Bi2GaVO7 photocatalysts with that of the metal ions substituted Bi4V2O11 d (BIMEVOX; ME = Al, Ga) based on the results of powder X-ray diffraction, UV–vis diffuse reflectance and photocatalytic studies. In general, photocatalytic behavior of Bi4V2O11 d and Bi4V2 xMxO11 d (M = Al, Ga; x = 0.4) were found to be moderate for the decomposition of MB under visible light irradiation. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides C. X-ray diffraction D. Catalytic properties D. Optical properties
1. Introduction The search for visible light active photocatalysts primarily focuses on the valence and conduction band positions of the materials. Since, the absorption of light of appropriate energy is decided by the band gap of the materials, which then is followed by the release of the powerful oxidizing holes and reductant electrons from the valence and the conduction bands respectively. Numerous oxide based semiconductors are currently being examined to discover a better alternate for TiO2 (band gap of 3.2 eV), whose photocatalytic ability is limited to UV radiation. One possible approach to identify materials with smaller band gaps has been to construct the valence band utilizing O 2p hybridized with metal s (e.g. Bi 6s) orbitals [1]. Thus, Bi3+ containing mixed metal oxides have long been viewed as potential visible light active photocatalysts [2]. In spite of this favored electronic structure, only a very few Bi3+ oxides have been identified for visible light photocatalysis. Monoclinic BiVO4 with a band gap of 2.4 eV showed high photocatalytic activity for O2 evolution under visible (l > 420 nm) light irradiation [1,3]. Subsequently, many hydrothermal synthetic methods have been reported for the synthesis of BiVO4, which is also known to decompose many organic contaminants under visible light [4–7]. Although, CaBi2O4 has been identified [8] as a very efficient visible photocatalyst for the decomposition of acetaldehyde and methylene blue solutions, its successful applicability is limited by the difficulty associated in
* Corresponding author. Tel.: +91 11 2766 2650; fax: +91 11 2766 6605. E-mail address: [email protected] (S. Uma). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.05.007
synthesizing the oxide itself [9]. Bi2WO6 [10,11], Bi2MoO6 [12] and Bi2GaTaO7 [13] exhibit visible light photocatalytic activity for the decomposition of dye solutions such as rhodamine B (RhB) and methylene blue (MB) solutions. Other mixed metal oxides such as Bi2Ti2O7 [14], LiBi4M3O4 (M = Nb, Ta) [15], Bi2MNbO7 (M = Al, Ga, In) [16], Bi2MTaO7 (M = In, Fe, Ga) [17] and BiMO4 (M = Nb, Ta) [18] have been known for the decomposition of organic contaminants under UV light irradiation. In our attempts to identify visible light photocatalysts [19,20], we were interested in investigating the yet another member of the Bi– V–O system, Bi4V2O11, well known for its oxide ion conductivity. Bi4V2O11, normally represented as Bi4V2O11 d due to oxygen deficiency, was expected to show electronic conductivity at low temperatures [21]. The experimental band gaps obtained from the UV–vis diffuse reflectance of different quenched and annealed samples were known to vary from 1.99 to 2.04 eV [22]. During the course of our investigation of Bi4V2O11 d as a photocatalyst, there were the reports of novel Bi3+ containing oxides such as Bi2YVO8 [23], Bi2GaVO7 [24] and Bi2AlVO7 [25] as visible light photocatalysts. Bi2YVO8 was reported to be a new compound containing Bi3+ and Bi5+ (Bi1.063+ Bi0.945+ Y0.983+ V1.035+ O7.952 ) ions, crystallizing in the I4/mmm space group with the lattice parameters of a = 3.9188(2) and c = 15.3105(9) A˚. Bi2YVO8 with a band gap of 2.09 eV was shown to split water under UV light and to degrade methylene blue (MB) solution under visible light irradiation [23]. Bi2AlVO7 (a = 3.99294(1) and c = 15.3469(9) A˚) and Bi2GaVO7 (a = 3.0994(6) and c = 15.2291(3) A˚) crystallizing in tetragonal crystal system with space group I4/mmm, were known to decompose MB solutions under visible light. Their band gaps were 2.06 and 2.13 eV, respectively for Bi2AlVO7 and Bi2GaVO7 [24,25]. We recognized
V. Thakral, S. Uma / Materials Research Bulletin 45 (2010) 1250–1254
1251
that the behavior of the above mentioned oxides were similar to the BIMEVOX (ME = Al, Ga) oxide series based on their structural, optical and their photocatalytic properties. In the present work, we synthesized Bi4V2O11 d and evaluated the photocatalytic activities for the decomposition of the common pollutants such as MB solutions under visible light. In addition, we systematically carried out the synthesis and characterization of the already reported Bi2YVO8, Bi2GaVO7 and Bi2AlVO7 and showed their similarity with the BIMEVOX (ME = Ga, Al) series of oxides. 2. Experimental 2.1. Synthesis The polycrystalline powder samples were prepared by conventional solid state synthesis. Bi4V2O11 d, Bi4V1.8Al0.2O11 d, Bi4V1.6Al0.4O11 d and Bi2AlVO7 were prepared by mixing and heating stoichiometric amounts of Bi2O3 (Aldrich, 99.9%), V2O5 (Aldrich, 99.6%) and Al2O3 (CDH, 99.9%) firstly at 600 8C for 12 h, reground and further calcined at 800 8C for 12 h and annealed in air. Similarly, Bi2GaVO7, Bi4V1.6Ga0.4O11 d and Bi2YVO8 compositions were also attempted using the stoichiometric amounts of Bi2O3, V2O5, Ga2O3 (Aldrich, 99.9%) and Y2O3 (Aldrich, 99.9%). Bi2GaVO7 and Bi4V1.6Ga0.4O11 d were heated at 600 8C for 12 h, reground and heated further at 800 8C for 12 h, while Bi2YVO8 was further heated to 900 8C for 24–72 h with intermediate grindings for the phase formation as mentioned in Ref. [23]. 2.2. Characterization The powder X-ray diffraction patterns of the prepared samples were recorded using BRUKER D8 high resolution diffractometer employing Cu Ka radiation. UV–vis diffuse reflectance data were collected over the spectral range 200–1000 nm using PerkinElmer Lambda 35 scanning double beam spectrometer equipped with a 50 mm integrating sphere. BaSO4 was used as a reference. The data were transformed into absorbance with the Kubelka–Munk function to estimate the band gap of the materials. 2.3. Photocatalytic experiments The photocatalytic tests were carried out using a 450 W Xenon arc lamp (Oriel, Newport, USA) along with a water filter to cut down IR radiation and glass cut off filters. Melles Griot-03FCG057 filter was used to permit only visible light (400 nm l 800 nm) radiation as desired. The experimental details of the photochemical reactor have been discussed earlier [19]. An external pyrex container of 200 ml was used for the irradiation and water circulation was carried out to avoid any thermal effects. For the photocatalytic degradation of methylene blue (MB), 0.5 g of the catalyst was added to 150 ml of aqueous solution of MB (pH 10) with an initial concentration of 15 10 6 mol/l for visible light irradiation experiments. Prior to irradiation, the suspension of the catalyst and dye solution was stirred in dark for 30 min, so as to reach the adsorption desorption equilibrium. 5 ml aliquots were pipetted out periodically from the reaction mixture. The solutions were centrifuged and the concentration of the solutions were determined by measuring the maximum absorbance (lmax = 665 nm) by using a UV–vis spectrophotometer.
Fig. 1. PXRD pattern of a-Bi4V2O11
d.
three polymorphs a, b and g. Bi4V2O11 has alternating [Bi2O2]2+ layers and (VO3.5&0.5)2- oxygen deficient perovskite slabs. aBi4V2O11 d was indexed using a face centered orthorhombic cell with the parameters a = 5.5840(3), b = 15.2218(9) and c = 5.5060(1) A˚ [22]. b-Bi4V2O11 d was indexed on a tetragonal cell with the parameters a = b = 11.285(8) and c = 15.42(1) A˚, while g-Bi4V2O11 d was indexed on a tetragonal cell in the space group I4/mmm, with the cell parameters a = b = 3.988(2) and c = 15.42(1) A˚ [26]. In the present investigation, the powder Xray diffraction of the as synthesized annealed sample of Bi4V2O11 d was found to be the a polymorph with the orthorhombic lattice parameters a = 5.5840(3), b = 15.2218(9) and c = 5.5060(3) A˚. The powder X-ray diffraction pattern is shown in Fig. 1. The powder X-ray diffraction pattern obtained for the composition of Bi2YVO8 under our experimental conditions is shown in Fig. 2. The stoichiometry of the reactants was Bi2YVO7, considering the two bismuth atoms in the oxidation state of +3. Our preparations did not yield a single phase, Bi2YVO8 as referred earlier [23], instead resulted in a mixture of BiVO4 and BiYO3. Even compositions with varying the ratio of metal ions (Bi:Y:V) such as 2:1:2, 2:2:1, 4:1:1 and 3:1:1, when reacted under similar
3. Results and discussion 3.1. Structural and phase identification Based on the success of BiVO4 as a visible light photocatalyst, we envisaged a similar behavior for Bi4V2O11 d. Bi4V2O11 exists in
Fig. 2. PXRD pattern of the product obtained by using stoichiometric Bi2YVO7 leading to a mixture of BiYO3 and BiVO4.
V. Thakral, S. Uma / Materials Research Bulletin 45 (2010) 1250–1254
1252 Table 1 Lattice parameters of Bi4V2O11 Compound
a-Bi4V2O11 d g-Bi4V2O11 d, Bi2YVO8 Bi2AlVO7 Bi2GaVO7 Bi4V1.6Al0.4O11 Bi4V1.6Ga0.4O11 a b
d d
d
and BIMEVOX (ME = Al, Ga). Space group
Lattice parameters
Reference
Aba2 I4/mmm I4/mmm I4/mmm I4/mmm I4/mmma I4/mmma
a = 5.5840(3), b = 15.2218(9) and c = 5.5060(1) A˚ a = b = 3.988(2) and c = 15.42(1) A˚ a = b = 3.9188(2) and c = 15.3105(9) A˚ a = b = 3.9294(1) and c = 15.3469(9) A˚ a = b = 3.0994(6) and c = 15.2291(3) A˚ a = b = 3.9077(6) and c = 15.2862(7) A˚b a = b = 3.9353(6) and c = 15.4216(3) A˚b
[21] [26] [22] [24] [23] Present work Present work
Symmetry may be lower than tetragonal I4/mmm. Listed lattice parameters were refined by Le Bail fit [28] using the FULLPROF program [29].
experimental conditions always resulted in known oxide phases such as BiYO3, Bi11VO19, BiVO4 and YVO4. We further noted that, the authors in Ref. [23], had mentioned about the existence of aluminium in their X-ray EDS (energy dispersive spectrum) spectra of Bi2YVO8. Subsequently, the formation of Bi2AlVO7 was reported by the same authors [25] with the similar space group and lattice parameters as that of Bi2YVO8 (Table 1). The powder X-ray diffraction pattern obtained in our study for the composition Bi2AlVO7 is shown in Fig. 3(c), and it matched with that of the pattern reported for Bi2AlVO7 [25], except for the prominent difference in the extent of splitting of the reflections marked as 110 (2u 31.70) and 116 (2u 47.89). Further we noted that the simulated X-ray diffraction pattern based on their Rietveld structural parameters did not match even with their experimentally observed pattern for Bi2AlVO7. However, we could recognize the similarity between the diffraction patterns of Bi2AlVO7 with that of the X-ray diffraction pattern of g-Bi4V2O11 d (Fig. 3(b)) reported in Ref. [27]. In Fig. 3(c) and (d), we further show the diffraction patterns of deliberately synthesized compositions of Bi4V1.8Al0.2O11 d and Bi4V1.6Al0.4O11 d. We conclude that, Bi2AlVO7 is nothing but aluminium substituted Bi4V2O11 d with the probable composition of Bi4V1.6Al0.4O11 d, with the tetragonal symmetry similar to g-Bi4V2O11 d structure. The refined lattice parameters are listed in Table 1. The aforementioned discussion led us to investigate Bi2GaVO7 reported in Ref. [24]. Here again, our experimentally observed diffraction pattern for the composition Bi2GaVO7 resembled that of the metal ion substituted Bi4V2O11 d diffraction pattern (Fig. 4). Another major discrepancy noted in the published data of Bi2AlVO7
Fig. 3. PXRD patterns: (a) simulated from positional parameters [24] of Bi2AlVO7, (b) simulated from positional parameters [31] of g-Bi4V2O11 d, (c) Bi2AlVO7, (present work), (d) Bi4V1.8Al0.2O11 d and (e) Bi4V1.6Al0.4O11 d.
and the Bi2GaVO7 was the very similar looking powder X-ray diffraction patterns, characterized with identical space group and positional parameters but with different sets of lattice parameters. While the reported tetragonal lattice parameters were 3.9294(1) and 15.3469(9) A˚ for Bi2AlVO7, the lattice parameters reported for Bi2GaVO7 were 3.0994(6) and 15.2291(3) A˚. The powder X-ray diffraction pattern obtained for our stoichiometric preparation of Bi2GaVO7 (Fig. 4(b)) was easily indexed on a tetragonal cell (a = b = 3.9942(1); c = 15.2291(9) A˚) resembling Bi4V2O11 d in the g form. For comparison, we synthesized Bi4V1.6Ga0.4O11 d, whose powder X-ray diffraction is shown in Fig. 4(c). A wide range of substitution of various cations in different oxidation states such as for example, Na+, Cu2+, Ni2+, Al3+, Ga3+, Ti4+, Nb5+ and W6+ for vanadium have been investigated in detail over the years [30–34]. Depending upon the extent of the cation substitution, the resulting compositions were stabilized either in the a or in the g Bi4V2O11 d structures [30–34]. In the present case, while Bi4V1.8Al0.2O11 d crystallized in the a form, Bi4V1.6Al0.4O11 d crystallized in the g form. On the other hand, Bi4V1.6Ga0.4O11 d crystallized in the g form of Bi4V2O11 d. 3.2. Optical properties The UV–vis diffuse reflectance spectra of the a-Bi4V2O11 d and BIMEVOX (ME = Al, Ga) synthesized under our experimental conditions are shown in Fig. 5. The optical absorption behavior of Bi4V2O11 d has known to be highly sensitive to the synthetic conditions [22], based on the amounts of the vanadium in +5 and +4 oxidation states. Essentially, the purity of V2O5 also plays a
Fig. 4. PXRD patterns: (a) simulated from positional parameters [23] of Bi2GaVO7, (b) Bi2GaVO7 (present work), and (c) Bi4V1.6Ga0.4O11 d.
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Fig. 6. Photocatalytic degradation of MB under visible light irradiation, (a) photolysis of MB in the absence of catalysts; over (b) Bi4V2O11 d, (c) observed (present work) Bi2AlVO7, (d) Bi4V1.6Al0.4O11 d, (e) observed (present work) Bi2GaVO7 and (f) Bi4V1.6Ga0.4O11 d.
3.3. Photocatalytic studies
Fig. 5. Diffuse reflectance spectra of the Bi4V2O11 d and BIMEVOX (ME = Al, Ga) phases, (a) plot of absorbance versus wavelength and (b) plot of (ahn)1/n versus hn for the calculation of band gap (in eV).
significant role in the phase formation of Bi4V2O11 d and its derivatives [22]. Thus, the band gap of Bi4V2O11 d is reported to be around 2 eV. In the present case, the as synthesized Bi4V2O11 d showed absorption in the visible range with an absorption edge of 625 nm (Fig. 5(a)) with the band gap of 1.96 eV (Fig. 5(b)). The preparations corresponding to the compositions of Bi2AlVO7 and Bi2GaVO7 also showed visible light absorption and were found to have absorption edges in the range of 650 and 575 nm, respectively (Fig. 5(a)). The band gap of Bi2YVO8 was reported to be 2.09 eV [23]. We observed that the band gap of Bi4V2O11 d matched with that of the Al substituted Bi4V2O11 d and Bi2AlVO7 oxides (Fig. 5(b)). The observed band gap of 2.14 eV for Bi2GaVO7 was in agreement with the reported value of 2.13 eV [24]. An appropriate shift in the band gap was also noticed for Bi2V1.6Ga0.4O11 d (2.03 eV), as compared to Bi4V2O11 d and their aluminium substituted analogues (Fig. 5 (b)). The valence band of Bi4V2O11 d is suggested to be composed of Bi 6s orbitals and oxygen 2p orbitals whereas the conduction band consisting of V 3d orbitals. In the case of BIMEVOXes, the conduction band consists of dominant V 3d orbitals and gallium 4s or aluminium 3s orbitals. The efficient visible absorption suggests Bi4V2O11 d and the Bi4V2 xMxO11 d (M = Al, Ga; x = 0.4) oxides are potential visible light photocatalysts.
Photodegradation of Methylene Blue was examined under the visible light irradiation (400 nm l 800 nm) over Bi4V2O11 d and Bi4V2 xMxO11 d (M = Al, Ga; x = 0.4). The photolysis of MB solution without the presence of catalysts under similar experimental conditions has also been shown in Fig. 6(a). Our investigation revealed moderate rates of decomposition of MB solutions under visible light irradiation over Bi4V2O11 d (Fig. 6(b)), Bi4V1.6Al0.4O11 d (Fig. 6(d)) and Bi4V1.6Ga0.4O11 d (Fig. 6(f)). The pseudo first order rate constants were calculated to quantify the reaction kinetics and were found to be 6.5 10 3 min 1, 3.5 10 3 min 1 and 6.9 10 3 min 1 for Bi4V2O11 d, Bi4V1.6 Al0.4O11 d and Bi4V1.6Ga0.4O11 d, respectively. A comparison of the rates of decompositions of MB solutions under visible light obtained for the bismuth based oxides such as Bi2YVO8, Bi2AlVO7 and Bi2GaVO7, suggested a uniform rate of 5 10 5 mM/s [23– 25]. The rate constants calculated for the as synthesized samples Bi2AlVO7 (Fig. 6 (c)) and Bi2GaVO7 (Fig. 6 (e)) were found to be 4.5 10 3 min 1 and 5.1 10 3 min 1, respectively. We conclude that the rates of decomposition of the Bi4V2O11 d and the substituted BIMEVOX (ME = Al, Ga) were found to be of similar orders of magnitude. 4. Conclusion We investigated Bi4V2O11 d as a potential visible light photocatalyst capable of decomposing common dye solutions such as MB moderately under visible light irradiation. We reinvestigated the already known bismuth based visible light photocatalysts such as Bi2YVO8, Bi2AlVO7 and Bi2GaVO7. Our experiments involving the synthesis of Bi2YVO8 did not result in the oxide containing bismuth in +3 and +5 oxidation states. Our results clearly point towards the possibility of Bi2AlVO7 and Bi2GaVO7 as substituted BIMEVOX (ME = Al, Ga) oxides, based on their structural, optical and photocatalytic properties. Acknowledgements The present research is supported by the Department of Science and Technology, Government of India. The authors also thank the University of Delhi for the financial support under the ‘‘Scheme to
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strengthen R & D Doctoral Research Programme’’. One of the authors, V.T. wishes to thank UGC for the junior research fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Investigation of visible light photocatalytic behavior of Bi4V2O11 (ME = Al, Ga) oxides
d
and BIMEVOX
Vaishali Thakral, S. Uma * Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110007, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 February 2010 Accepted 7 May 2010 Available online 2 June 2010
A study of photocatalytic properties of Bi4V2O11 d for the degradation of common organic dyes such as methylene blue (MB) under visible light was carried out. Other identified bismuth based visible light photocatalysts such as Bi2GaVO7, Bi2YVO8 and Bi2AlVO7 were reinvestigated to understand their structural, and photocatalytic properties. Our experiments involving the synthesis of Bi2YVO8 indicated merely a mixture of BiYO3 and BiVO4, instead of the reported oxide containing bismuth in +3 and +5 oxidation states (J. Luan, et al., Mater. Res. Bull. 43 (2008) 3332). We have further shown the similarity of Bi2AlVO7 and Bi2GaVO7 photocatalysts with that of the metal ions substituted Bi4V2O11 d (BIMEVOX; ME = Al, Ga) based on the results of powder X-ray diffraction, UV–vis diffuse reflectance and photocatalytic studies. In general, photocatalytic behavior of Bi4V2O11 d and Bi4V2 xMxO11 d (M = Al, Ga; x = 0.4) were found to be moderate for the decomposition of MB under visible light irradiation. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides C. X-ray diffraction D. Catalytic properties D. Optical properties
1. Introduction The search for visible light active photocatalysts primarily focuses on the valence and conduction band positions of the materials. Since, the absorption of light of appropriate energy is decided by the band gap of the materials, which then is followed by the release of the powerful oxidizing holes and reductant electrons from the valence and the conduction bands respectively. Numerous oxide based semiconductors are currently being examined to discover a better alternate for TiO2 (band gap of 3.2 eV), whose photocatalytic ability is limited to UV radiation. One possible approach to identify materials with smaller band gaps has been to construct the valence band utilizing O 2p hybridized with metal s (e.g. Bi 6s) orbitals [1]. Thus, Bi3+ containing mixed metal oxides have long been viewed as potential visible light active photocatalysts [2]. In spite of this favored electronic structure, only a very few Bi3+ oxides have been identified for visible light photocatalysis. Monoclinic BiVO4 with a band gap of 2.4 eV showed high photocatalytic activity for O2 evolution under visible (l > 420 nm) light irradiation [1,3]. Subsequently, many hydrothermal synthetic methods have been reported for the synthesis of BiVO4, which is also known to decompose many organic contaminants under visible light [4–7]. Although, CaBi2O4 has been identified [8] as a very efficient visible photocatalyst for the decomposition of acetaldehyde and methylene blue solutions, its successful applicability is limited by the difficulty associated in
* Corresponding author. Tel.: +91 11 2766 2650; fax: +91 11 2766 6605. E-mail address: [email protected] (S. Uma). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.05.007
synthesizing the oxide itself [9]. Bi2WO6 [10,11], Bi2MoO6 [12] and Bi2GaTaO7 [13] exhibit visible light photocatalytic activity for the decomposition of dye solutions such as rhodamine B (RhB) and methylene blue (MB) solutions. Other mixed metal oxides such as Bi2Ti2O7 [14], LiBi4M3O4 (M = Nb, Ta) [15], Bi2MNbO7 (M = Al, Ga, In) [16], Bi2MTaO7 (M = In, Fe, Ga) [17] and BiMO4 (M = Nb, Ta) [18] have been known for the decomposition of organic contaminants under UV light irradiation. In our attempts to identify visible light photocatalysts [19,20], we were interested in investigating the yet another member of the Bi– V–O system, Bi4V2O11, well known for its oxide ion conductivity. Bi4V2O11, normally represented as Bi4V2O11 d due to oxygen deficiency, was expected to show electronic conductivity at low temperatures [21]. The experimental band gaps obtained from the UV–vis diffuse reflectance of different quenched and annealed samples were known to vary from 1.99 to 2.04 eV [22]. During the course of our investigation of Bi4V2O11 d as a photocatalyst, there were the reports of novel Bi3+ containing oxides such as Bi2YVO8 [23], Bi2GaVO7 [24] and Bi2AlVO7 [25] as visible light photocatalysts. Bi2YVO8 was reported to be a new compound containing Bi3+ and Bi5+ (Bi1.063+ Bi0.945+ Y0.983+ V1.035+ O7.952 ) ions, crystallizing in the I4/mmm space group with the lattice parameters of a = 3.9188(2) and c = 15.3105(9) A˚. Bi2YVO8 with a band gap of 2.09 eV was shown to split water under UV light and to degrade methylene blue (MB) solution under visible light irradiation [23]. Bi2AlVO7 (a = 3.99294(1) and c = 15.3469(9) A˚) and Bi2GaVO7 (a = 3.0994(6) and c = 15.2291(3) A˚) crystallizing in tetragonal crystal system with space group I4/mmm, were known to decompose MB solutions under visible light. Their band gaps were 2.06 and 2.13 eV, respectively for Bi2AlVO7 and Bi2GaVO7 [24,25]. We recognized
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that the behavior of the above mentioned oxides were similar to the BIMEVOX (ME = Al, Ga) oxide series based on their structural, optical and their photocatalytic properties. In the present work, we synthesized Bi4V2O11 d and evaluated the photocatalytic activities for the decomposition of the common pollutants such as MB solutions under visible light. In addition, we systematically carried out the synthesis and characterization of the already reported Bi2YVO8, Bi2GaVO7 and Bi2AlVO7 and showed their similarity with the BIMEVOX (ME = Ga, Al) series of oxides. 2. Experimental 2.1. Synthesis The polycrystalline powder samples were prepared by conventional solid state synthesis. Bi4V2O11 d, Bi4V1.8Al0.2O11 d, Bi4V1.6Al0.4O11 d and Bi2AlVO7 were prepared by mixing and heating stoichiometric amounts of Bi2O3 (Aldrich, 99.9%), V2O5 (Aldrich, 99.6%) and Al2O3 (CDH, 99.9%) firstly at 600 8C for 12 h, reground and further calcined at 800 8C for 12 h and annealed in air. Similarly, Bi2GaVO7, Bi4V1.6Ga0.4O11 d and Bi2YVO8 compositions were also attempted using the stoichiometric amounts of Bi2O3, V2O5, Ga2O3 (Aldrich, 99.9%) and Y2O3 (Aldrich, 99.9%). Bi2GaVO7 and Bi4V1.6Ga0.4O11 d were heated at 600 8C for 12 h, reground and heated further at 800 8C for 12 h, while Bi2YVO8 was further heated to 900 8C for 24–72 h with intermediate grindings for the phase formation as mentioned in Ref. [23]. 2.2. Characterization The powder X-ray diffraction patterns of the prepared samples were recorded using BRUKER D8 high resolution diffractometer employing Cu Ka radiation. UV–vis diffuse reflectance data were collected over the spectral range 200–1000 nm using PerkinElmer Lambda 35 scanning double beam spectrometer equipped with a 50 mm integrating sphere. BaSO4 was used as a reference. The data were transformed into absorbance with the Kubelka–Munk function to estimate the band gap of the materials. 2.3. Photocatalytic experiments The photocatalytic tests were carried out using a 450 W Xenon arc lamp (Oriel, Newport, USA) along with a water filter to cut down IR radiation and glass cut off filters. Melles Griot-03FCG057 filter was used to permit only visible light (400 nm l 800 nm) radiation as desired. The experimental details of the photochemical reactor have been discussed earlier [19]. An external pyrex container of 200 ml was used for the irradiation and water circulation was carried out to avoid any thermal effects. For the photocatalytic degradation of methylene blue (MB), 0.5 g of the catalyst was added to 150 ml of aqueous solution of MB (pH 10) with an initial concentration of 15 10 6 mol/l for visible light irradiation experiments. Prior to irradiation, the suspension of the catalyst and dye solution was stirred in dark for 30 min, so as to reach the adsorption desorption equilibrium. 5 ml aliquots were pipetted out periodically from the reaction mixture. The solutions were centrifuged and the concentration of the solutions were determined by measuring the maximum absorbance (lmax = 665 nm) by using a UV–vis spectrophotometer.
Fig. 1. PXRD pattern of a-Bi4V2O11
d.
three polymorphs a, b and g. Bi4V2O11 has alternating [Bi2O2]2+ layers and (VO3.5&0.5)2- oxygen deficient perovskite slabs. aBi4V2O11 d was indexed using a face centered orthorhombic cell with the parameters a = 5.5840(3), b = 15.2218(9) and c = 5.5060(1) A˚ [22]. b-Bi4V2O11 d was indexed on a tetragonal cell with the parameters a = b = 11.285(8) and c = 15.42(1) A˚, while g-Bi4V2O11 d was indexed on a tetragonal cell in the space group I4/mmm, with the cell parameters a = b = 3.988(2) and c = 15.42(1) A˚ [26]. In the present investigation, the powder Xray diffraction of the as synthesized annealed sample of Bi4V2O11 d was found to be the a polymorph with the orthorhombic lattice parameters a = 5.5840(3), b = 15.2218(9) and c = 5.5060(3) A˚. The powder X-ray diffraction pattern is shown in Fig. 1. The powder X-ray diffraction pattern obtained for the composition of Bi2YVO8 under our experimental conditions is shown in Fig. 2. The stoichiometry of the reactants was Bi2YVO7, considering the two bismuth atoms in the oxidation state of +3. Our preparations did not yield a single phase, Bi2YVO8 as referred earlier [23], instead resulted in a mixture of BiVO4 and BiYO3. Even compositions with varying the ratio of metal ions (Bi:Y:V) such as 2:1:2, 2:2:1, 4:1:1 and 3:1:1, when reacted under similar
3. Results and discussion 3.1. Structural and phase identification Based on the success of BiVO4 as a visible light photocatalyst, we envisaged a similar behavior for Bi4V2O11 d. Bi4V2O11 exists in
Fig. 2. PXRD pattern of the product obtained by using stoichiometric Bi2YVO7 leading to a mixture of BiYO3 and BiVO4.
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1252 Table 1 Lattice parameters of Bi4V2O11 Compound
a-Bi4V2O11 d g-Bi4V2O11 d, Bi2YVO8 Bi2AlVO7 Bi2GaVO7 Bi4V1.6Al0.4O11 Bi4V1.6Ga0.4O11 a b
d d
d
and BIMEVOX (ME = Al, Ga). Space group
Lattice parameters
Reference
Aba2 I4/mmm I4/mmm I4/mmm I4/mmm I4/mmma I4/mmma
a = 5.5840(3), b = 15.2218(9) and c = 5.5060(1) A˚ a = b = 3.988(2) and c = 15.42(1) A˚ a = b = 3.9188(2) and c = 15.3105(9) A˚ a = b = 3.9294(1) and c = 15.3469(9) A˚ a = b = 3.0994(6) and c = 15.2291(3) A˚ a = b = 3.9077(6) and c = 15.2862(7) A˚b a = b = 3.9353(6) and c = 15.4216(3) A˚b
[21] [26] [22] [24] [23] Present work Present work
Symmetry may be lower than tetragonal I4/mmm. Listed lattice parameters were refined by Le Bail fit [28] using the FULLPROF program [29].
experimental conditions always resulted in known oxide phases such as BiYO3, Bi11VO19, BiVO4 and YVO4. We further noted that, the authors in Ref. [23], had mentioned about the existence of aluminium in their X-ray EDS (energy dispersive spectrum) spectra of Bi2YVO8. Subsequently, the formation of Bi2AlVO7 was reported by the same authors [25] with the similar space group and lattice parameters as that of Bi2YVO8 (Table 1). The powder X-ray diffraction pattern obtained in our study for the composition Bi2AlVO7 is shown in Fig. 3(c), and it matched with that of the pattern reported for Bi2AlVO7 [25], except for the prominent difference in the extent of splitting of the reflections marked as 110 (2u 31.70) and 116 (2u 47.89). Further we noted that the simulated X-ray diffraction pattern based on their Rietveld structural parameters did not match even with their experimentally observed pattern for Bi2AlVO7. However, we could recognize the similarity between the diffraction patterns of Bi2AlVO7 with that of the X-ray diffraction pattern of g-Bi4V2O11 d (Fig. 3(b)) reported in Ref. [27]. In Fig. 3(c) and (d), we further show the diffraction patterns of deliberately synthesized compositions of Bi4V1.8Al0.2O11 d and Bi4V1.6Al0.4O11 d. We conclude that, Bi2AlVO7 is nothing but aluminium substituted Bi4V2O11 d with the probable composition of Bi4V1.6Al0.4O11 d, with the tetragonal symmetry similar to g-Bi4V2O11 d structure. The refined lattice parameters are listed in Table 1. The aforementioned discussion led us to investigate Bi2GaVO7 reported in Ref. [24]. Here again, our experimentally observed diffraction pattern for the composition Bi2GaVO7 resembled that of the metal ion substituted Bi4V2O11 d diffraction pattern (Fig. 4). Another major discrepancy noted in the published data of Bi2AlVO7
Fig. 3. PXRD patterns: (a) simulated from positional parameters [24] of Bi2AlVO7, (b) simulated from positional parameters [31] of g-Bi4V2O11 d, (c) Bi2AlVO7, (present work), (d) Bi4V1.8Al0.2O11 d and (e) Bi4V1.6Al0.4O11 d.
and the Bi2GaVO7 was the very similar looking powder X-ray diffraction patterns, characterized with identical space group and positional parameters but with different sets of lattice parameters. While the reported tetragonal lattice parameters were 3.9294(1) and 15.3469(9) A˚ for Bi2AlVO7, the lattice parameters reported for Bi2GaVO7 were 3.0994(6) and 15.2291(3) A˚. The powder X-ray diffraction pattern obtained for our stoichiometric preparation of Bi2GaVO7 (Fig. 4(b)) was easily indexed on a tetragonal cell (a = b = 3.9942(1); c = 15.2291(9) A˚) resembling Bi4V2O11 d in the g form. For comparison, we synthesized Bi4V1.6Ga0.4O11 d, whose powder X-ray diffraction is shown in Fig. 4(c). A wide range of substitution of various cations in different oxidation states such as for example, Na+, Cu2+, Ni2+, Al3+, Ga3+, Ti4+, Nb5+ and W6+ for vanadium have been investigated in detail over the years [30–34]. Depending upon the extent of the cation substitution, the resulting compositions were stabilized either in the a or in the g Bi4V2O11 d structures [30–34]. In the present case, while Bi4V1.8Al0.2O11 d crystallized in the a form, Bi4V1.6Al0.4O11 d crystallized in the g form. On the other hand, Bi4V1.6Ga0.4O11 d crystallized in the g form of Bi4V2O11 d. 3.2. Optical properties The UV–vis diffuse reflectance spectra of the a-Bi4V2O11 d and BIMEVOX (ME = Al, Ga) synthesized under our experimental conditions are shown in Fig. 5. The optical absorption behavior of Bi4V2O11 d has known to be highly sensitive to the synthetic conditions [22], based on the amounts of the vanadium in +5 and +4 oxidation states. Essentially, the purity of V2O5 also plays a
Fig. 4. PXRD patterns: (a) simulated from positional parameters [23] of Bi2GaVO7, (b) Bi2GaVO7 (present work), and (c) Bi4V1.6Ga0.4O11 d.
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Fig. 6. Photocatalytic degradation of MB under visible light irradiation, (a) photolysis of MB in the absence of catalysts; over (b) Bi4V2O11 d, (c) observed (present work) Bi2AlVO7, (d) Bi4V1.6Al0.4O11 d, (e) observed (present work) Bi2GaVO7 and (f) Bi4V1.6Ga0.4O11 d.
3.3. Photocatalytic studies
Fig. 5. Diffuse reflectance spectra of the Bi4V2O11 d and BIMEVOX (ME = Al, Ga) phases, (a) plot of absorbance versus wavelength and (b) plot of (ahn)1/n versus hn for the calculation of band gap (in eV).
significant role in the phase formation of Bi4V2O11 d and its derivatives [22]. Thus, the band gap of Bi4V2O11 d is reported to be around 2 eV. In the present case, the as synthesized Bi4V2O11 d showed absorption in the visible range with an absorption edge of 625 nm (Fig. 5(a)) with the band gap of 1.96 eV (Fig. 5(b)). The preparations corresponding to the compositions of Bi2AlVO7 and Bi2GaVO7 also showed visible light absorption and were found to have absorption edges in the range of 650 and 575 nm, respectively (Fig. 5(a)). The band gap of Bi2YVO8 was reported to be 2.09 eV [23]. We observed that the band gap of Bi4V2O11 d matched with that of the Al substituted Bi4V2O11 d and Bi2AlVO7 oxides (Fig. 5(b)). The observed band gap of 2.14 eV for Bi2GaVO7 was in agreement with the reported value of 2.13 eV [24]. An appropriate shift in the band gap was also noticed for Bi2V1.6Ga0.4O11 d (2.03 eV), as compared to Bi4V2O11 d and their aluminium substituted analogues (Fig. 5 (b)). The valence band of Bi4V2O11 d is suggested to be composed of Bi 6s orbitals and oxygen 2p orbitals whereas the conduction band consisting of V 3d orbitals. In the case of BIMEVOXes, the conduction band consists of dominant V 3d orbitals and gallium 4s or aluminium 3s orbitals. The efficient visible absorption suggests Bi4V2O11 d and the Bi4V2 xMxO11 d (M = Al, Ga; x = 0.4) oxides are potential visible light photocatalysts.
Photodegradation of Methylene Blue was examined under the visible light irradiation (400 nm l 800 nm) over Bi4V2O11 d and Bi4V2 xMxO11 d (M = Al, Ga; x = 0.4). The photolysis of MB solution without the presence of catalysts under similar experimental conditions has also been shown in Fig. 6(a). Our investigation revealed moderate rates of decomposition of MB solutions under visible light irradiation over Bi4V2O11 d (Fig. 6(b)), Bi4V1.6Al0.4O11 d (Fig. 6(d)) and Bi4V1.6Ga0.4O11 d (Fig. 6(f)). The pseudo first order rate constants were calculated to quantify the reaction kinetics and were found to be 6.5 10 3 min 1, 3.5 10 3 min 1 and 6.9 10 3 min 1 for Bi4V2O11 d, Bi4V1.6 Al0.4O11 d and Bi4V1.6Ga0.4O11 d, respectively. A comparison of the rates of decompositions of MB solutions under visible light obtained for the bismuth based oxides such as Bi2YVO8, Bi2AlVO7 and Bi2GaVO7, suggested a uniform rate of 5 10 5 mM/s [23– 25]. The rate constants calculated for the as synthesized samples Bi2AlVO7 (Fig. 6 (c)) and Bi2GaVO7 (Fig. 6 (e)) were found to be 4.5 10 3 min 1 and 5.1 10 3 min 1, respectively. We conclude that the rates of decomposition of the Bi4V2O11 d and the substituted BIMEVOX (ME = Al, Ga) were found to be of similar orders of magnitude. 4. Conclusion We investigated Bi4V2O11 d as a potential visible light photocatalyst capable of decomposing common dye solutions such as MB moderately under visible light irradiation. We reinvestigated the already known bismuth based visible light photocatalysts such as Bi2YVO8, Bi2AlVO7 and Bi2GaVO7. Our experiments involving the synthesis of Bi2YVO8 did not result in the oxide containing bismuth in +3 and +5 oxidation states. Our results clearly point towards the possibility of Bi2AlVO7 and Bi2GaVO7 as substituted BIMEVOX (ME = Al, Ga) oxides, based on their structural, optical and photocatalytic properties. Acknowledgements The present research is supported by the Department of Science and Technology, Government of India. The authors also thank the University of Delhi for the financial support under the ‘‘Scheme to
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strengthen R & D Doctoral Research Programme’’. One of the authors, V.T. wishes to thank UGC for the junior research fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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