- Email: [email protected]
Synthesis of β-Bi2O3 towards the application of photocatalytic degradation of methyl orange and its instability
Journal of Physics and Chemistry of Solids 81 (2015) 74–78
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis of β-Bi2O3 towards the application of photocatalytic degradation of methyl orange and its instability S. Iyyapushpam, S.T. Nishanthi, D. Pathinettam Padiyan n Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 28 November 2014 Received in revised form 29 January 2015 Accepted 12 February 2015 Available online 14 February 2015
Bismuth oxide carbonate was synthesized from bismuth nitrate and potassium carbonate and then converted to phase pure β-Bi2O3 form by means of thermal decomposition. X-ray diffraction, HR-SEM, diffuse reflectance UV–vis and photocatalytic degradation studies were carried out on both the samples. Bi2O2CO3 exhibited a wide band gap of 3.406(5) eV while β-Bi2O3 had a lesser band gap of 2.589(3) eV. β-Bi2O3 degrades a higher amount of methyl orange because of its lesser band gap and its optimum loading was 0.1 g in 50 ml of 10 ppm solution. After photocatalytic degradation Bi2O2CO3 remains in the stable form whereas β-Bi2O3 changes to Bi2O2CO3. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Oxides Chemical synthesis X-ray diffraction Crystal structure Optical properties
1. Introduction
β-Bi2O3 is inert in neutral water, which is a fundamental prerequisite for its application as photocatalyst for water purification [1]. It is an important metal oxide semiconductor having a band gap of 2.58 eV which signifies that β-Bi2O3 can be used as a visible light photocatalyst. In this work, an attempt has been made to prepare β-Bi2O3 from bismuth oxide carbonate (Bi2O2CO3), since it is a kind of environmentally benign material and also widely used in medicine. However, the utilization of Bi2O2CO3 for photocatalytic degradation is less due to its wide band gap. In order to improve its photocatalytic degradation properties it is tried to reduce the band gap by annealing as well as to transfer it to phase pure β-Bi2O3 form. Recently Madhusudan et al. [2] reported the photocatalytic degradation for the mixed solutions containing methylene blue and methyl orange using Bi2O2CO3 by preparing it in the form of microstructures. Most of the reports on Bi2O2CO3 are in the form of composites or heterostructures which includes Bi2O2CO3/Bi2S3 [3], Bi2O2CO3/Bi3NbO7 [4], Bi2O2CO3/Bi2WO6 [5], Bi2O2CO3/BiOI [6], Bi2O2CO3/ZnWO4 [7], Fe3O4/Bi2O2CO3 [8], N-doped Bi2O2CO3 [9]. The reported composites are mostly studied for the degradation of rhodamine B and methylene blue. Cai et al. [10] reported the facile synthesis of β-Bi2O3/Bi2O2CO3 nanocomposite by the hydrothermal method and its degradation of methylene blue. As synthesized Bi2O2CO3 nanosheets exhibit a n
Corresponding author. Fax: þ91 4622334363. E-mail address: [email protected] (D.P. Padiyan).
http://dx.doi.org/10.1016/j.jpcs.2015.02.005 0022-3697/& 2015 Elsevier Ltd. All rights reserved.
poorer activity under visible light with only 7% degradation whereas the composite shows higher degradation activity. Liang et al. [11] reported the synthesis of Bi2O2CO3 by hydrothermal process along with their degradation activity and it degrades nearly 35% of RhB when it was prepared without the surfactant. In the present work, Bi2O2CO3 is prepared and it is converted to phase pure β-Bi2O3 form by thermal decomposition and their degradation activities are compared for the model pollutant methyl orange (MeO).
2. Experimental details All the chemicals used were of AR grade purchased from Merck and double distilled water (DD) was used throughout the process. 0.1 M of bismuth nitrate was dissolved in nitric acid and it was taken as the precursor solution. 1 M of potassium carbonate was dissolved in DD water under stirring. After obtaining a clear solution of potassium carbonate the precursor bismuth nitrate was added in drops under constant stirring and kept for drying in an oven at 80 °C. The dried sample was washed with double distilled water several times and again dried at 80 °C. The powder obtained was grounded using a mortar and pestle and used for further characterization. The samples were annealed at different temperatures to obtain the phase pure form of β-Bi2O3. The XRD patterns were recorded using PANalytical XPERT-PRO X-ray diffractometer with CuKα as incident radiation in the 2θ range of 20°–60°. The surface morphology of the samples was
S. Iyyapushpam et al. / Journal of Physics and Chemistry of Solids 81 (2015) 74–78
observed using a high resolution scanning electron microscope (HR-SEM), FEI Quanta FEG 200. The optical studies for the powder samples were carried out with a diffuse reflectance spectrophotometer (DRS), Shimadzu UV 2700 attached with integrating sphere using BaSO4 as standard. The model pollutant selected for photocatalytic degradation was methyl orange with a concentration of 10 ppm (10 mg/l) and the amount of the catalyst was varied to obtain the optimum value. 500 W xenon source (Wacom XDS 501S) was used for light irradiation and the distance between the source and sample was about 80 cm. The kinetics of dye decolorization were studied for 240 min and the residual concentration of the dye in the solution was measured for every 30 min using UV– visible spectrophotometer (Techcomp, UV 2301).
3. Results and discussion 3.1. Structural analysis The XRD pattern of the as prepared powder shown in Fig. 1a indicates the polycrystalline nature of the material. The observed 2θ values are compared with the standard values to identify the crystalline system of the material. Thirteen X-ray peaks are observed and all the peaks match well with the standard JCPDS card number 41-1488 of bismuth oxide carbonate (Bi2O2CO3) which crystallizes in the tetragonal crystal system. The peaks at 2θ of 23.8°, 25.9°, 30.2°, 32.7°, 35.2°, 39.2°, 42.2°, 46.8°, 48.8°, 51.9°, 53.2°, 54.2° and 56.8° correspond to the (0 1 1), (0 0 4), (0 1 3), (1 1 0), (1 1 2), (0 0 6), (1 1 4), (0 2 0), (0 2 2), (1 1 6), (1 2 1), (0 2 4) and (1 2 3) reflections of Bi2O2CO3 respectively. The crystallite size of the intense (0 1 3) plane of Bi2O2CO3 determined using Scherrer's formula is 38.9 nm. The cell constants and cell volume determined using the UNIT CELL software are a ¼3.8748(2) Å; c ¼13.766(1) Å and V¼ 206.69(2) Å3 respectively. The optimized condition to obtain β-Bi2O3 from Bi2O2CO3 was identified by varying the annealing temperature. When Bi2O2CO3 was kept at 300 °C it remains in the same form of Bi2O2CO3 but with the increase in crystallinity and its XRD pattern is shown in Fig. 1b. The XRD pattern of the sample annealed at 400 °C for 1 h is shown in Fig. 1c. The observed X-ray peaks are entirely different indicating the change in the crystal structure due to thermal decomposition. Eight X-ray peaks are observed and indicate that it is in the tetragonal crystal system of β-Bi2O3 (JCPDS card number 27-
75
0050). The X-ray peaks observed at 2θ of 27.9°, 31.7°, 32.7°, 46.2°, 46.9°, 54.2°, 55.4° and 57.8° correspond to the (2 0 1), (0 0 2), (2 2 0), (2 2 2), (4 0 0), (2 0 3), (4 2 1) and (4 0 2) planes of β-Bi2O3 respectively. No X-ray peak of Bi2O2CO3 is seen in the XRD pattern (Fig. 1c) which indicates that it is in the phase pure β-Bi2O3 form. The crystallite size of β-Bi2O3 for the first intense plane of (2 0 1) is 47.5 nm and it is higher than Bi2O2CO3. The cell constants and cell volume are a¼ 7.7399(4) Å; c ¼5.6410(5) Å and V¼337.93(4) Å3 respectively. Here β-Bi2O3 is successfully prepared from Bi2O2CO3 at an optimum thermal decomposition temperature of 400 °C for 1 h. Fig. 2a and b shows the HR-SEM images of Bi2O2CO3 and phase pure β-Bi2O3 respectively. Bi2O2CO3 consists of flake like particles which are agglomerated by attaching one over the other to have a look like bundle in the form of the florets of cauliflower. After its conversion to β-Bi2O3 the surface morphology of the sample also gets changed and it shows two types of particles in the image Fig. 2b with the reduction in the agglomeration. The flakes like particles placed one over the other start to separate with increase in size compared to Bi2O2CO3 due to the increase of annealing temperature. Some of the flakes are in triangular shape with some non-uniform flattened surface. The other type consists of grain like particles. This shows that the surface morphology of Bi2O2CO3 and β-Bi2O3 are entirely different because of the variation in the crystal structure. 3.2. Optical property The reflectance spectra of Bi2O2CO3 and β-Bi2O3 along with their band gap obtained using Kubelka-Munk function is shown in Fig. 3a and b respectively. For Bi2O2CO3 the reflectance increases sharply from 280 nm and then almost saturated above 600 nm indicating the presence of optical absorption in the region of 280– 600 nm whereas for β-Bi2O3 the optical absorption is in the range of 380–580 nm. The steep shape of the spectra indicates that the light absorption is due to the band gap transition. This permits the prepared β-Bi2O3 samples to respond to a wide range of the solar spectrum and it can utilize visible light for photocatalysis. The reflectance data are converted to equivalent optical absorption coefficient using Kubelka-Munk function [12]
K /S = (1 − R∞ )2 /2R∞ ≡ F (R∞ )
(1)
where K and S are Kubelka-Munk absorption and scattering coefficient respectively. F(R1) is Kubelka-Munk function, R1 ¼Rsample /Rstandard and Rstandard is the reflectance corresponding to BaSO4. The indirect band gap is determined by extrapolating the linear portion of (F(R1)hυ)1/2 vs hυ plot to F(R1)¼ 0 and the band gap value is 3.406(5) eV for Bi2O2CO3 which is equal to 364 nm. The obtained band gap of Bi2O2CO3 is nearer to the reported value of 3.3 eV by Madhusudan et al. [2]. The indirect band gap for β-Bi2O3 is 2.589(3) eV which is equal to 479 nm and it is closer to the value of 2.55 eV reported by Wang et al. [13]. The obtained band gap energy indicates the absorption of light in the visible region and hence this sample can be used as a visible light photocatalyst. The band gap of β-Bi2O3 is much lower compared to that of Bi2O2CO3 because of the change in the chemical composition and crystal structure. At the thermal decomposition temperature of 400 °C the crystal structure changes with an increase in crystallite size which in turn leads to a decrease in the band gap value. 3.3. Photocatalytic degradation
Fig. 1. XRD pattern of (a) as prepared Bi2O2CO3, (b) Bi2O2CO3 annealed at 300 °C showing increase in crystallinity and (c) after thermal decomposition at 400 °C which give phase pure β-Bi2O3.
The photocatalytic degradation activity for Bi2O2CO3 and
β-Bi2O3 are studied to know the effect of the structural and optical
property variation on the degradation efficiency of MeO. 0.1 g of
76
S. Iyyapushpam et al. / Journal of Physics and Chemistry of Solids 81 (2015) 74–78
Fig. 2. HRSEM image showing the different surface morphology displaying (a) Bi2O2CO3 as flakes and (b) β-Bi2O3 in the form of triangular shape and grain like particles.
Bi2O2CO3 was dispersed in 50 ml of a 10 ppm MeO solution and it was kept in dark for 1 h to reach adsorption desorption equilibrium between the dye and catalyst. Then it was irradiated using xenon source and its degradation activity was tested for every 30 min up to 240 min (Fig. 4). The degradation percentage of MeO is determined using the formula
% of MeO degradation = [(Abst = 0 –Abst )/(Abst = 0 )] × 100
(2)
where Abst is the MeO absorbance (463 nm) at various intervals of time t. Bi2O2CO3 degrades 43.1% of MeO with a decrease in dye concentration from 10 ppm to 5.69 ppm in 240 min of irradiation whereas β-Bi2O3 degrades 60% of MeO in the same duration. The increase of photocatalytic degradation in β-Bi2O3 is attributed to the decrease of the band gap with an increase in the optical absorption in the visible region compared to that of Bi2O2CO3. The photocatalytic degradation effect of β-Bi2O3 is tested by varying the concentration of the catalyst to identify the optimum amount of catalyst required. The same experiment was repeated with the catalyst concentration of 0.05 g and 0.15 g in 50 ml of 10 ppm of MeO. The degradation efficiency for 0.05 g and 0.15 g are 28.2% and 40.1% respectively in 240 min of irradiation. The lesser degradation efficiency obtained for 0.05 g catalyst is due to the insufficient amount of material to carry out the process of
degradation. The decreased degradation effect for 0.15 g catalyst may be due to the increase of the turbidity of the solution which blocks the irradiation for the reaction to proceed. From this it is confirmed that the optimum loading of catalyst to carry out the degradation process using β-Bi2O3 as catalyst in 10 ppm of MeO is 0.1 g. The degradation efficiency varies with the preparation methodology and the nature of the catalyst. For example Zhu et al. [14] reported on the synthesis and photocatalytic performance of Ag-loaded β-Bi2O3 microspheres prepared by hydrothermal method. They obtained 44% of degradation of rhodamine B in 210 min of irradiation and it was further improved by the addition of Ag. Qiu et al. [15] prepared β-Bi2O3 nanowires by oxidative metal vapor transport deposition technique and it degrades over 90% of orange G after 15 h of illumination. The kinetics of photocatalytic degradation are studied using the pseudo first order equation [16]
ln (co/ct ) = k app t
(3)
where kapp is the apparent first order rate constant, co is the initial concentration of MeO. The variations in ln(co/ct) as a function of irradiation time for both Bi2O2CO3 and β-Bi2O3 of different concentration are shown in Fig. 5. It is seen that the curve with time as abscissa and ln(co/ct) as vertical ordinate, is close to a linear one. The apparent first order rate constants are determined from the slope of the linear fit and the values are given in Table 1 along with
Fig. 3. Determination of the indirect band gap of (a) Bi2O2CO3 and (b) β-Bi2O3 using the Kubelka-Munk function from their corresponding reflectance spectra.
S. Iyyapushpam et al. / Journal of Physics and Chemistry of Solids 81 (2015) 74–78
77
Fig. 4. UV–vis spectra of photocatalytic degradation of MeO using (a) the wider band gap material Bi2O2CO3 and (b) β-Bi2O3 showing higher degradation.
Fig. 5. Pseudo first order kinetics plot of ln(co/ct) versus irradiation time for Bi2O2CO3 and three different catalyst concentrations of β-Bi2O3.
Fig. 6. XRD pattern of (a) Bi2O2CO3 and (b) β-Bi2O3 after photocatalytic degradation; XRD patterns of β-Bi2O3 after degradation calcined for (c) 10 min, (d) 30 min and (e) 1 h showing the arise of γ-Bi2O3.
Table 1 Apparent pseudo first order rate constant along with their correlation coefficient. S. no.
Sample
kapp (min 1)
Correlation coefficient (R2)
1 2 3 4
0.1 g Bi2O2CO3 0.05 g β-Bi2O3 0.1 g β-Bi2O3 0.15 g β-Bi2O3
0.0021(1) 0.00144(9) 0.0040(2) 0.0026(1)
0.9897 0.9872 0.9937 0.9932
their correlation coefficients. From the table it is clear that the apparent first order rate constant is higher for β-Bi2O3 with the optimum loading of 0.1 g which shows that the degradation rate is larger for the sample having greater first order rate constant. The photocatalytic degradation of MeO using Bi2O2CO3 and β-Bi2O3 powder as catalyst fits well with the first order exponential decay curve, which confirms that the degradation follows the pseudo first order reaction kinetics. The XRD pattern for the sample Bi2O2CO3 after photocatalytic degradation is shown in Fig. 6a. The stability of Bi2O2CO3 and β-Bi2O3 is analyzed from the structural property of the samples after photocatalytic degradation by means of XRD. It is noted
from the figure that Bi2O2CO3 is in the same form after photocatalytic degradation but with an increase in crystallinity. The XRD pattern of β-Bi2O3 after photocatalytic degradation is shown in Fig. 6b and it is in the form of Bi2O2CO3 due to the reaction of CO2 liberated during photocatalytic degradation with Bi2O3. Hence it was tried to convert it once again to β-Bi2O3 by keeping it in the same calcination temperature of 400 °C for 10 min and its XRD pattern is shown in Fig. 6c. It is still in the form of Bi2O2CO3 but with an increase in crystallinity compared to that of the sample after photocatalytic degradation. Then it was kept at the same calcination temperature for 30 min and its XRD pattern is shown in Fig. 6d. By comparing it with the standard JCPDS database it is noticed that the pattern consists of six additional planes than that of Bi2O2CO3 at 24.6°, 27.6°, 37.2°, 41.5°, 43.4° and 45.2° which belongs to the γ-phase of Bi2O3 (JCPDS card number 71-0467). It was again calcined at the same temperature, 400 °C for 1 h and its XRD pattern is shown in Fig. 6e. It is also in the same form as that of Fig. 6d but with the increase in crystallinity compared to 30 min calcined sample. It is concluded that the formed Bi2O2CO3 after photocatalytic degradation of β-Bi2O3 cannot be returned to its initial phase pure β form by keeping it in the calcination
78
S. Iyyapushpam et al. / Journal of Physics and Chemistry of Solids 81 (2015) 74–78
temperature which proves that the material is not a stable one.
4. Conclusions Phase pure β-Bi2O3 was prepared from Bi2O2CO3, an environmentally benign material. Conversion of Bi2O2CO3 to β-Bi2O3 increases the crystallite size which in turn decreases the band gap of β-Bi2O3. The degradation efficiency of MeO was higher for β-Bi2O3, the lower band gap material, and the optimum loading was 0.1 g in 50 ml of 10 ppm solution. The pseudo first order kinetics were best fit for the entire period of illumination with a higher correlation coefficient. Bi2O2CO3 remains in the stable form after photocatalytic degradation whereas β-Bi2O3 gets converted to Bi2O2CO3.
Acknowledgments One of the authors S.I. would like to thank CSIR, New Delhi for a Senior Research Fellowship. The authors thank SAIF, IIT Chennai for recording HR-SEM measurement.
References [1] A. Hameed, V. Gombac, T. Montini, L. Felisari, P. Fornasiero, Photocatalytic activity of zinc modified Bi2O3, Chem. Phys. Lett. 483 (2009) 254–261. [2] P. Madhusudan, J. Zhang, B. Cheng, G. Liu, Photocatalytic degradation of organic dyes with hierarchical Bi2O2CO3 microstructures under visible-light, CrystEngComm 15 (2013) 231–240. [3] W. Wang, H. Cheng, B. Huang, X. Lin, X. Qin, X. Zhang, Y. Dai, Synthesis of Bi2O2CO3/Bi2S3 hierarchical microspheres with heterojunctions and their enhanced visible light-driven photocatalytic degradation of dye pollutants, J.
Colloid Interf. Sci. 402 (2013) 34–39. [4] H. Gan, G. Zhang, H. Huang, Enhanced visible-light-driven photocatalytic inactivation of Escherichia coli by Bi2O2CO3/Bi3NbO7 composites, J. Hazard. Mater. 250–251 (2013) 131–137. [5] X. Huang, H. Chen, One-pot hydrothermal synthesis of Bi2O2CO3/Bi2WO6 visible light photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 284 (2013) 843–848. [6] L. Chen, S. Yin, S. Luo, R. Huang, Q. Zhang, T. Hong, P.C.T. Au, Bi2O2CO3/BiOI photocatalysts with heterojunctions highly efficient for visible-light treatment of dye-containing waste water, Ind. Eng. Chem. Res. 51 (2012) 6760–6768. [7] N. Tian, H. Huang, Y. Zhang, Y. He, Enhanced photocatalytic activities on Bi2O2CO3/ZnWO4 nanocomposites, J. Mater. Res. 29 (2014) 641–648. [8] G. Zhu, M. Hojamberdiev, K. Katsumata, X. Cai, N. Matsushita, K. Okada, P. Liu, J. Zhou, Heterostructured Fe3O4/Bi2O2CO3 photocatalyst: Synthesis, characterization and application in recyclable photodegradation of organic dyes under visible light irradiation, Mater. Chem. Phys. 142 (2013) 95–105. [9] C. Wang, Z. Zhao, B. Luo, M. Fu, F. Dong, Tuning the morphological structure and photocatalytic activity of nitrogen-doped (BiO)2CO3 by the hydrothermal temperature, J. Nanomater. 192797 (2014) 1–10. [10] G. Cai, L. Xu, B. Wei, J. Che, H. Gao, W. Sun, Facile synthesis of β-Bi2O3/Bi2O2CO3 nanocomposite with high visible-light photocatalytic activity, Mater. Lett. 120 (2014) 1–4. [11] H.Y. Liang, Y.X. Yang, J.C. Tang, M. Ge, Photocatalytic properties of Bi2O2CO3 nanosheets shynthesized via a surfactant-assisted hydrothermal method, Mater. Sci. Semicond. Process. 16 (2013) 1650–1654. [12] L. Yang, B. Kruse, Revised Kubelka-Munk theory I. Theory and application, J. Opt. Soc. Am. A 21 (2004) 1933–1941. [13] C. Wang, C. Shao, L. Wang, L. Zhang, X. Li, Y. Liu, Electrospinning preparation, characterization and photocatalytic properties of Bi2O3 nanofibers, J. Colloid Interface Sci. 333 (2009) 242–248. [14] G. Zhu, W. Que, J. Zhang, Synthesis and photocatalytic performance of Agloaded β-Bi2O3 microspheres under visible light irradiation, J. Alloy. Compd. 509 (2011) 9479–9486. [15] Y. Qiu, M. Yang, H. Fan, Y. Zuo, Y. Shao, Y. Xu, X. Yang, S. Yang, Nanowires of αand β-Bi2O3: phase-selective synthesis and application in photocatalysis, CrystEngComm 13 (2011) 1843–1850. [16] J. Matos, J. Laine, J. Herrmann, Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon, Appl. Catal. B: Environ. 18 (1998) 281–291.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis of β-Bi2O3 towards the application of photocatalytic degradation of methyl orange and its instability S. Iyyapushpam, S.T. Nishanthi, D. Pathinettam Padiyan n Department of Physics, Manonmaniam Sundaranar University, Tirunelveli 627012, Tamil Nadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 28 November 2014 Received in revised form 29 January 2015 Accepted 12 February 2015 Available online 14 February 2015
Bismuth oxide carbonate was synthesized from bismuth nitrate and potassium carbonate and then converted to phase pure β-Bi2O3 form by means of thermal decomposition. X-ray diffraction, HR-SEM, diffuse reflectance UV–vis and photocatalytic degradation studies were carried out on both the samples. Bi2O2CO3 exhibited a wide band gap of 3.406(5) eV while β-Bi2O3 had a lesser band gap of 2.589(3) eV. β-Bi2O3 degrades a higher amount of methyl orange because of its lesser band gap and its optimum loading was 0.1 g in 50 ml of 10 ppm solution. After photocatalytic degradation Bi2O2CO3 remains in the stable form whereas β-Bi2O3 changes to Bi2O2CO3. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Oxides Chemical synthesis X-ray diffraction Crystal structure Optical properties
1. Introduction
β-Bi2O3 is inert in neutral water, which is a fundamental prerequisite for its application as photocatalyst for water purification [1]. It is an important metal oxide semiconductor having a band gap of 2.58 eV which signifies that β-Bi2O3 can be used as a visible light photocatalyst. In this work, an attempt has been made to prepare β-Bi2O3 from bismuth oxide carbonate (Bi2O2CO3), since it is a kind of environmentally benign material and also widely used in medicine. However, the utilization of Bi2O2CO3 for photocatalytic degradation is less due to its wide band gap. In order to improve its photocatalytic degradation properties it is tried to reduce the band gap by annealing as well as to transfer it to phase pure β-Bi2O3 form. Recently Madhusudan et al. [2] reported the photocatalytic degradation for the mixed solutions containing methylene blue and methyl orange using Bi2O2CO3 by preparing it in the form of microstructures. Most of the reports on Bi2O2CO3 are in the form of composites or heterostructures which includes Bi2O2CO3/Bi2S3 [3], Bi2O2CO3/Bi3NbO7 [4], Bi2O2CO3/Bi2WO6 [5], Bi2O2CO3/BiOI [6], Bi2O2CO3/ZnWO4 [7], Fe3O4/Bi2O2CO3 [8], N-doped Bi2O2CO3 [9]. The reported composites are mostly studied for the degradation of rhodamine B and methylene blue. Cai et al. [10] reported the facile synthesis of β-Bi2O3/Bi2O2CO3 nanocomposite by the hydrothermal method and its degradation of methylene blue. As synthesized Bi2O2CO3 nanosheets exhibit a n
Corresponding author. Fax: þ91 4622334363. E-mail address: [email protected] (D.P. Padiyan).
http://dx.doi.org/10.1016/j.jpcs.2015.02.005 0022-3697/& 2015 Elsevier Ltd. All rights reserved.
poorer activity under visible light with only 7% degradation whereas the composite shows higher degradation activity. Liang et al. [11] reported the synthesis of Bi2O2CO3 by hydrothermal process along with their degradation activity and it degrades nearly 35% of RhB when it was prepared without the surfactant. In the present work, Bi2O2CO3 is prepared and it is converted to phase pure β-Bi2O3 form by thermal decomposition and their degradation activities are compared for the model pollutant methyl orange (MeO).
2. Experimental details All the chemicals used were of AR grade purchased from Merck and double distilled water (DD) was used throughout the process. 0.1 M of bismuth nitrate was dissolved in nitric acid and it was taken as the precursor solution. 1 M of potassium carbonate was dissolved in DD water under stirring. After obtaining a clear solution of potassium carbonate the precursor bismuth nitrate was added in drops under constant stirring and kept for drying in an oven at 80 °C. The dried sample was washed with double distilled water several times and again dried at 80 °C. The powder obtained was grounded using a mortar and pestle and used for further characterization. The samples were annealed at different temperatures to obtain the phase pure form of β-Bi2O3. The XRD patterns were recorded using PANalytical XPERT-PRO X-ray diffractometer with CuKα as incident radiation in the 2θ range of 20°–60°. The surface morphology of the samples was
S. Iyyapushpam et al. / Journal of Physics and Chemistry of Solids 81 (2015) 74–78
observed using a high resolution scanning electron microscope (HR-SEM), FEI Quanta FEG 200. The optical studies for the powder samples were carried out with a diffuse reflectance spectrophotometer (DRS), Shimadzu UV 2700 attached with integrating sphere using BaSO4 as standard. The model pollutant selected for photocatalytic degradation was methyl orange with a concentration of 10 ppm (10 mg/l) and the amount of the catalyst was varied to obtain the optimum value. 500 W xenon source (Wacom XDS 501S) was used for light irradiation and the distance between the source and sample was about 80 cm. The kinetics of dye decolorization were studied for 240 min and the residual concentration of the dye in the solution was measured for every 30 min using UV– visible spectrophotometer (Techcomp, UV 2301).
3. Results and discussion 3.1. Structural analysis The XRD pattern of the as prepared powder shown in Fig. 1a indicates the polycrystalline nature of the material. The observed 2θ values are compared with the standard values to identify the crystalline system of the material. Thirteen X-ray peaks are observed and all the peaks match well with the standard JCPDS card number 41-1488 of bismuth oxide carbonate (Bi2O2CO3) which crystallizes in the tetragonal crystal system. The peaks at 2θ of 23.8°, 25.9°, 30.2°, 32.7°, 35.2°, 39.2°, 42.2°, 46.8°, 48.8°, 51.9°, 53.2°, 54.2° and 56.8° correspond to the (0 1 1), (0 0 4), (0 1 3), (1 1 0), (1 1 2), (0 0 6), (1 1 4), (0 2 0), (0 2 2), (1 1 6), (1 2 1), (0 2 4) and (1 2 3) reflections of Bi2O2CO3 respectively. The crystallite size of the intense (0 1 3) plane of Bi2O2CO3 determined using Scherrer's formula is 38.9 nm. The cell constants and cell volume determined using the UNIT CELL software are a ¼3.8748(2) Å; c ¼13.766(1) Å and V¼ 206.69(2) Å3 respectively. The optimized condition to obtain β-Bi2O3 from Bi2O2CO3 was identified by varying the annealing temperature. When Bi2O2CO3 was kept at 300 °C it remains in the same form of Bi2O2CO3 but with the increase in crystallinity and its XRD pattern is shown in Fig. 1b. The XRD pattern of the sample annealed at 400 °C for 1 h is shown in Fig. 1c. The observed X-ray peaks are entirely different indicating the change in the crystal structure due to thermal decomposition. Eight X-ray peaks are observed and indicate that it is in the tetragonal crystal system of β-Bi2O3 (JCPDS card number 27-
75
0050). The X-ray peaks observed at 2θ of 27.9°, 31.7°, 32.7°, 46.2°, 46.9°, 54.2°, 55.4° and 57.8° correspond to the (2 0 1), (0 0 2), (2 2 0), (2 2 2), (4 0 0), (2 0 3), (4 2 1) and (4 0 2) planes of β-Bi2O3 respectively. No X-ray peak of Bi2O2CO3 is seen in the XRD pattern (Fig. 1c) which indicates that it is in the phase pure β-Bi2O3 form. The crystallite size of β-Bi2O3 for the first intense plane of (2 0 1) is 47.5 nm and it is higher than Bi2O2CO3. The cell constants and cell volume are a¼ 7.7399(4) Å; c ¼5.6410(5) Å and V¼337.93(4) Å3 respectively. Here β-Bi2O3 is successfully prepared from Bi2O2CO3 at an optimum thermal decomposition temperature of 400 °C for 1 h. Fig. 2a and b shows the HR-SEM images of Bi2O2CO3 and phase pure β-Bi2O3 respectively. Bi2O2CO3 consists of flake like particles which are agglomerated by attaching one over the other to have a look like bundle in the form of the florets of cauliflower. After its conversion to β-Bi2O3 the surface morphology of the sample also gets changed and it shows two types of particles in the image Fig. 2b with the reduction in the agglomeration. The flakes like particles placed one over the other start to separate with increase in size compared to Bi2O2CO3 due to the increase of annealing temperature. Some of the flakes are in triangular shape with some non-uniform flattened surface. The other type consists of grain like particles. This shows that the surface morphology of Bi2O2CO3 and β-Bi2O3 are entirely different because of the variation in the crystal structure. 3.2. Optical property The reflectance spectra of Bi2O2CO3 and β-Bi2O3 along with their band gap obtained using Kubelka-Munk function is shown in Fig. 3a and b respectively. For Bi2O2CO3 the reflectance increases sharply from 280 nm and then almost saturated above 600 nm indicating the presence of optical absorption in the region of 280– 600 nm whereas for β-Bi2O3 the optical absorption is in the range of 380–580 nm. The steep shape of the spectra indicates that the light absorption is due to the band gap transition. This permits the prepared β-Bi2O3 samples to respond to a wide range of the solar spectrum and it can utilize visible light for photocatalysis. The reflectance data are converted to equivalent optical absorption coefficient using Kubelka-Munk function [12]
K /S = (1 − R∞ )2 /2R∞ ≡ F (R∞ )
(1)
where K and S are Kubelka-Munk absorption and scattering coefficient respectively. F(R1) is Kubelka-Munk function, R1 ¼Rsample /Rstandard and Rstandard is the reflectance corresponding to BaSO4. The indirect band gap is determined by extrapolating the linear portion of (F(R1)hυ)1/2 vs hυ plot to F(R1)¼ 0 and the band gap value is 3.406(5) eV for Bi2O2CO3 which is equal to 364 nm. The obtained band gap of Bi2O2CO3 is nearer to the reported value of 3.3 eV by Madhusudan et al. [2]. The indirect band gap for β-Bi2O3 is 2.589(3) eV which is equal to 479 nm and it is closer to the value of 2.55 eV reported by Wang et al. [13]. The obtained band gap energy indicates the absorption of light in the visible region and hence this sample can be used as a visible light photocatalyst. The band gap of β-Bi2O3 is much lower compared to that of Bi2O2CO3 because of the change in the chemical composition and crystal structure. At the thermal decomposition temperature of 400 °C the crystal structure changes with an increase in crystallite size which in turn leads to a decrease in the band gap value. 3.3. Photocatalytic degradation
Fig. 1. XRD pattern of (a) as prepared Bi2O2CO3, (b) Bi2O2CO3 annealed at 300 °C showing increase in crystallinity and (c) after thermal decomposition at 400 °C which give phase pure β-Bi2O3.
The photocatalytic degradation activity for Bi2O2CO3 and
β-Bi2O3 are studied to know the effect of the structural and optical
property variation on the degradation efficiency of MeO. 0.1 g of
76
S. Iyyapushpam et al. / Journal of Physics and Chemistry of Solids 81 (2015) 74–78
Fig. 2. HRSEM image showing the different surface morphology displaying (a) Bi2O2CO3 as flakes and (b) β-Bi2O3 in the form of triangular shape and grain like particles.
Bi2O2CO3 was dispersed in 50 ml of a 10 ppm MeO solution and it was kept in dark for 1 h to reach adsorption desorption equilibrium between the dye and catalyst. Then it was irradiated using xenon source and its degradation activity was tested for every 30 min up to 240 min (Fig. 4). The degradation percentage of MeO is determined using the formula
% of MeO degradation = [(Abst = 0 –Abst )/(Abst = 0 )] × 100
(2)
where Abst is the MeO absorbance (463 nm) at various intervals of time t. Bi2O2CO3 degrades 43.1% of MeO with a decrease in dye concentration from 10 ppm to 5.69 ppm in 240 min of irradiation whereas β-Bi2O3 degrades 60% of MeO in the same duration. The increase of photocatalytic degradation in β-Bi2O3 is attributed to the decrease of the band gap with an increase in the optical absorption in the visible region compared to that of Bi2O2CO3. The photocatalytic degradation effect of β-Bi2O3 is tested by varying the concentration of the catalyst to identify the optimum amount of catalyst required. The same experiment was repeated with the catalyst concentration of 0.05 g and 0.15 g in 50 ml of 10 ppm of MeO. The degradation efficiency for 0.05 g and 0.15 g are 28.2% and 40.1% respectively in 240 min of irradiation. The lesser degradation efficiency obtained for 0.05 g catalyst is due to the insufficient amount of material to carry out the process of
degradation. The decreased degradation effect for 0.15 g catalyst may be due to the increase of the turbidity of the solution which blocks the irradiation for the reaction to proceed. From this it is confirmed that the optimum loading of catalyst to carry out the degradation process using β-Bi2O3 as catalyst in 10 ppm of MeO is 0.1 g. The degradation efficiency varies with the preparation methodology and the nature of the catalyst. For example Zhu et al. [14] reported on the synthesis and photocatalytic performance of Ag-loaded β-Bi2O3 microspheres prepared by hydrothermal method. They obtained 44% of degradation of rhodamine B in 210 min of irradiation and it was further improved by the addition of Ag. Qiu et al. [15] prepared β-Bi2O3 nanowires by oxidative metal vapor transport deposition technique and it degrades over 90% of orange G after 15 h of illumination. The kinetics of photocatalytic degradation are studied using the pseudo first order equation [16]
ln (co/ct ) = k app t
(3)
where kapp is the apparent first order rate constant, co is the initial concentration of MeO. The variations in ln(co/ct) as a function of irradiation time for both Bi2O2CO3 and β-Bi2O3 of different concentration are shown in Fig. 5. It is seen that the curve with time as abscissa and ln(co/ct) as vertical ordinate, is close to a linear one. The apparent first order rate constants are determined from the slope of the linear fit and the values are given in Table 1 along with
Fig. 3. Determination of the indirect band gap of (a) Bi2O2CO3 and (b) β-Bi2O3 using the Kubelka-Munk function from their corresponding reflectance spectra.
S. Iyyapushpam et al. / Journal of Physics and Chemistry of Solids 81 (2015) 74–78
77
Fig. 4. UV–vis spectra of photocatalytic degradation of MeO using (a) the wider band gap material Bi2O2CO3 and (b) β-Bi2O3 showing higher degradation.
Fig. 5. Pseudo first order kinetics plot of ln(co/ct) versus irradiation time for Bi2O2CO3 and three different catalyst concentrations of β-Bi2O3.
Fig. 6. XRD pattern of (a) Bi2O2CO3 and (b) β-Bi2O3 after photocatalytic degradation; XRD patterns of β-Bi2O3 after degradation calcined for (c) 10 min, (d) 30 min and (e) 1 h showing the arise of γ-Bi2O3.
Table 1 Apparent pseudo first order rate constant along with their correlation coefficient. S. no.
Sample
kapp (min 1)
Correlation coefficient (R2)
1 2 3 4
0.1 g Bi2O2CO3 0.05 g β-Bi2O3 0.1 g β-Bi2O3 0.15 g β-Bi2O3
0.0021(1) 0.00144(9) 0.0040(2) 0.0026(1)
0.9897 0.9872 0.9937 0.9932
their correlation coefficients. From the table it is clear that the apparent first order rate constant is higher for β-Bi2O3 with the optimum loading of 0.1 g which shows that the degradation rate is larger for the sample having greater first order rate constant. The photocatalytic degradation of MeO using Bi2O2CO3 and β-Bi2O3 powder as catalyst fits well with the first order exponential decay curve, which confirms that the degradation follows the pseudo first order reaction kinetics. The XRD pattern for the sample Bi2O2CO3 after photocatalytic degradation is shown in Fig. 6a. The stability of Bi2O2CO3 and β-Bi2O3 is analyzed from the structural property of the samples after photocatalytic degradation by means of XRD. It is noted
from the figure that Bi2O2CO3 is in the same form after photocatalytic degradation but with an increase in crystallinity. The XRD pattern of β-Bi2O3 after photocatalytic degradation is shown in Fig. 6b and it is in the form of Bi2O2CO3 due to the reaction of CO2 liberated during photocatalytic degradation with Bi2O3. Hence it was tried to convert it once again to β-Bi2O3 by keeping it in the same calcination temperature of 400 °C for 10 min and its XRD pattern is shown in Fig. 6c. It is still in the form of Bi2O2CO3 but with an increase in crystallinity compared to that of the sample after photocatalytic degradation. Then it was kept at the same calcination temperature for 30 min and its XRD pattern is shown in Fig. 6d. By comparing it with the standard JCPDS database it is noticed that the pattern consists of six additional planes than that of Bi2O2CO3 at 24.6°, 27.6°, 37.2°, 41.5°, 43.4° and 45.2° which belongs to the γ-phase of Bi2O3 (JCPDS card number 71-0467). It was again calcined at the same temperature, 400 °C for 1 h and its XRD pattern is shown in Fig. 6e. It is also in the same form as that of Fig. 6d but with the increase in crystallinity compared to 30 min calcined sample. It is concluded that the formed Bi2O2CO3 after photocatalytic degradation of β-Bi2O3 cannot be returned to its initial phase pure β form by keeping it in the calcination
78
S. Iyyapushpam et al. / Journal of Physics and Chemistry of Solids 81 (2015) 74–78
temperature which proves that the material is not a stable one.
4. Conclusions Phase pure β-Bi2O3 was prepared from Bi2O2CO3, an environmentally benign material. Conversion of Bi2O2CO3 to β-Bi2O3 increases the crystallite size which in turn decreases the band gap of β-Bi2O3. The degradation efficiency of MeO was higher for β-Bi2O3, the lower band gap material, and the optimum loading was 0.1 g in 50 ml of 10 ppm solution. The pseudo first order kinetics were best fit for the entire period of illumination with a higher correlation coefficient. Bi2O2CO3 remains in the stable form after photocatalytic degradation whereas β-Bi2O3 gets converted to Bi2O2CO3.
Acknowledgments One of the authors S.I. would like to thank CSIR, New Delhi for a Senior Research Fellowship. The authors thank SAIF, IIT Chennai for recording HR-SEM measurement.
References [1] A. Hameed, V. Gombac, T. Montini, L. Felisari, P. Fornasiero, Photocatalytic activity of zinc modified Bi2O3, Chem. Phys. Lett. 483 (2009) 254–261. [2] P. Madhusudan, J. Zhang, B. Cheng, G. Liu, Photocatalytic degradation of organic dyes with hierarchical Bi2O2CO3 microstructures under visible-light, CrystEngComm 15 (2013) 231–240. [3] W. Wang, H. Cheng, B. Huang, X. Lin, X. Qin, X. Zhang, Y. Dai, Synthesis of Bi2O2CO3/Bi2S3 hierarchical microspheres with heterojunctions and their enhanced visible light-driven photocatalytic degradation of dye pollutants, J.
Colloid Interf. Sci. 402 (2013) 34–39. [4] H. Gan, G. Zhang, H. Huang, Enhanced visible-light-driven photocatalytic inactivation of Escherichia coli by Bi2O2CO3/Bi3NbO7 composites, J. Hazard. Mater. 250–251 (2013) 131–137. [5] X. Huang, H. Chen, One-pot hydrothermal synthesis of Bi2O2CO3/Bi2WO6 visible light photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 284 (2013) 843–848. [6] L. Chen, S. Yin, S. Luo, R. Huang, Q. Zhang, T. Hong, P.C.T. Au, Bi2O2CO3/BiOI photocatalysts with heterojunctions highly efficient for visible-light treatment of dye-containing waste water, Ind. Eng. Chem. Res. 51 (2012) 6760–6768. [7] N. Tian, H. Huang, Y. Zhang, Y. He, Enhanced photocatalytic activities on Bi2O2CO3/ZnWO4 nanocomposites, J. Mater. Res. 29 (2014) 641–648. [8] G. Zhu, M. Hojamberdiev, K. Katsumata, X. Cai, N. Matsushita, K. Okada, P. Liu, J. Zhou, Heterostructured Fe3O4/Bi2O2CO3 photocatalyst: Synthesis, characterization and application in recyclable photodegradation of organic dyes under visible light irradiation, Mater. Chem. Phys. 142 (2013) 95–105. [9] C. Wang, Z. Zhao, B. Luo, M. Fu, F. Dong, Tuning the morphological structure and photocatalytic activity of nitrogen-doped (BiO)2CO3 by the hydrothermal temperature, J. Nanomater. 192797 (2014) 1–10. [10] G. Cai, L. Xu, B. Wei, J. Che, H. Gao, W. Sun, Facile synthesis of β-Bi2O3/Bi2O2CO3 nanocomposite with high visible-light photocatalytic activity, Mater. Lett. 120 (2014) 1–4. [11] H.Y. Liang, Y.X. Yang, J.C. Tang, M. Ge, Photocatalytic properties of Bi2O2CO3 nanosheets shynthesized via a surfactant-assisted hydrothermal method, Mater. Sci. Semicond. Process. 16 (2013) 1650–1654. [12] L. Yang, B. Kruse, Revised Kubelka-Munk theory I. Theory and application, J. Opt. Soc. Am. A 21 (2004) 1933–1941. [13] C. Wang, C. Shao, L. Wang, L. Zhang, X. Li, Y. Liu, Electrospinning preparation, characterization and photocatalytic properties of Bi2O3 nanofibers, J. Colloid Interface Sci. 333 (2009) 242–248. [14] G. Zhu, W. Que, J. Zhang, Synthesis and photocatalytic performance of Agloaded β-Bi2O3 microspheres under visible light irradiation, J. Alloy. Compd. 509 (2011) 9479–9486. [15] Y. Qiu, M. Yang, H. Fan, Y. Zuo, Y. Shao, Y. Xu, X. Yang, S. Yang, Nanowires of αand β-Bi2O3: phase-selective synthesis and application in photocatalysis, CrystEngComm 13 (2011) 1843–1850. [16] J. Matos, J. Laine, J. Herrmann, Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon, Appl. Catal. B: Environ. 18 (1998) 281–291.