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Efficient solar-driven photocatalytic performance of BiOBr benefiting from enhanced charge separation rate
Materials Letters 163 (2016) 175–178
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Efficient solar-driven photocatalytic performance of BiOBr benefiting from enhanced charge separation rate Yujun Si a, Junbo Zhong a,n, Jianzhang Li a, Minjiao Li a,b, Lei Yang a, Jie Ding a a Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Pharmaceutical Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China b Sichuan Provincial Academician (Expert) Workstation, Sichuan University of Science and Engineering, Zigong 643000, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 August 2015 Received in revised form 14 October 2015 Accepted 17 October 2015 Available online 22 October 2015
In this work, novel BiOBr photocatalyst (IL-BiOBr) with enhanced solar-driven photocatalytic activity was fabricated by a facile hydrothermal method with the assistance of ionic liquid (IL) 1-ethyl-3-methylimidazolium nitrate ([EMIm]NO3). BiOBr and IL-BiOBr were characterized by the Brunauer–Emmett– Teller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), particle size analyzer and surface photovoltage spectroscopy (SPS), respectively. The results reveal that [EMIm]NO3 increases the specific surface area, decreases the particle size, and greatly enhances separation rate of the photo-induced charge of BiOBr. The photocatalytic activity of IL-BiOBr for decolorization of rhodamine B aqueous solution was evaluated. The results illustrate that the photocatalytic performance of IL- BiOBr is more than 2.3 times of that of the reference BiOBr and the possible reason was suggested. & 2015 Published by Elsevier B.V.
Keywords: Semiconductors BiOBr Surfaces Photocatalytic performance Ionic liquid
1. Introduction As a lamellar-structured p-type semiconductor, bismuth oxybromide (BiOBr) has received increasing interest in the potential application as a visible light photocatalyst due to its medium band gap energy (2.8 eV), nontoxicity, facile preparation and chemical stability in photocatalytic process [1]. It was reported that BiOBr has unique layered structures characterized by [Bi2O2] slabs interleaved by double slabs of bromine atoms, which can effectively reduce the recombination of the photo-generated carriers and is beneficial to the photocatalytic activity [2]. However, it is still a great challenge to further boost the photocatalytic activity in order to meet the practical applications. Tremendous approaches have been developed to promote the photocatalytic activity of BiOBr, in which the hydrothermal method is more applicable due to its outstanding advantages [3]. Ionic liquids (ILs) have attracted considerable interests because of their properties. Recently, ILs have been applied to fabricate BiOBr [4–6]. However, the effect of [EMIm]NO3 on the photo-induced charge separation rate of BiOBr prepared by the hydrothermal method has been seldom addressed. The primary objective of this paper is to study the effect of [EMIm]NO3 on the photoinduced charge separation rate and the relation with the n
Corresponding author. E-mail address: [email protected] (J. Zhong).
http://dx.doi.org/10.1016/j.matlet.2015.10.095 0167-577X/& 2015 Published by Elsevier B.V.
photocatalytic activity of BiOBr.
2. Experimental section [EMIm]NO3 was purchased from Lanzhou Institute of Chemical Physics. All other chemicals (analytical grade reagents) were supplied from Chengdu Kelong Chemical Reagent Factory. IL-BiOBr was fabricated by a hydrothermal method. 5 g Bi(NO3)3 5H2O and 0.1 g [EMIm]NO3 were dissolved in 20 mL glacial acetic acid. After the mixture became clear solution, KBr aqueous solution (1.23 g KBrþ 10 mL H2O) was added dropwise to the above solution under stirring, resulting in precipitate. The above mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was maintained at 453 K for 24 h and then cooled to room temperature naturally. The product was collected by filtration, washed several times with deionized water and absolute ethanol, and then dispersed in absolute ethanol and dried at 353 K in air overnight. BiOBr was also prepared as the same procedure mentioned above without the presence of [EMIm]NO3. The specific surface area and pore size measurement were carried out on a SSA-4200 automatic surface analyzer (Builder, China). XRD patterns were performed on a DX -2600 X-ray diffractometer using Cu Kα (λ ¼0.15406 nm) radiation equipped with a graphite monochromator. The X-ray tube was operated at 40 kV and 20 mA. The UV–vis spectra of photocatalysts in the 300–850 nm range were recorded using a TU-1907 UV–vis spectrophotometer equipped
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Y. Si et al. / Materials Letters 163 (2016) 175–178
28000
Intensity
(a.u.)
24000
(102)
20000 16000
water as media and laser as light source. The SPS measurements were conducted with a home-built apparatus. The photocatalytic performance of BiOBr (50 mg) was investigated by decolorization of 50 mL rhodamine B aqueous solution (the concentration is 10 mg L 1) under the identical conditions. The light source was a 500 W xenon lamp.
(001)
(002) (003)
(101)
BiOBr
3. Results and discussion
12000
3.1. Characterization of photocatalysts
8000 (110) 4000 10
IL-BiOBr 15
20
25
30
35
40
2 Theta (degree) Fig. 1. XRD patterns of photocatalysts.
with an integrating sphere using BaSO4 as the reference. SEM images were taken with a JSM-7500F scanning electron microscope, using an accelerating voltage of 5 kV. HRTEM (Tecnai TEM G2) were used to study the microstructure of the samples using an accelerating voltage of 300 kV. The distribution of particle size was measured on a BH-9300H Bettersize particle size analyzer using
The XRD patterns of the photocatalysts are shown in Fig. 1. All of the diffraction peaks can be assigned to the tetragonal phase of the BiOBr crystal (JCPDS no. 09-0393). No other patterns can be observed, suggesting high purity of the as-prepared BiOBr samples. Furthermore, the half maximum (FWHM) of IL-BiOBr sample is wider than that of BiOBr. According to the Scherrer equation, the wider the FWHM is, the smaller the crystal size of BiOCl is. Thus, IL-BiOBr has smaller crystal size than that of BiOBr. The decreased crystal size leads to the BET surface area increase, which fits well with the result of BET surface area and distribution of the particle size. The smaller crystal size can increase the specific surface area of BiOCl and the photocatalytic reaction active sites, which can improve the photocatalytic activity. The SEM images of photocatalysts prepared are shown in Fig. 2.
Fig. 2. SEM of photocatalysts (A) BiOBr; (B) IL- BiOBr; (C) TEM image of BiOBr; (D) the HRTEM image recorded from the white framed area indicated in C.
Y. Si et al. / Materials Letters 163 (2016) 175–178
10
4
8
3
6
%
%
5
177
2
4
1
2
0
0
Particle size (um)
Particle size (um)
Fig. 3. Distribution of the particle size of photocatalyst; (A) BiOBr; (B) IL- BiOBr.
IL-BiOBr
100
Reflectance (%)
Photovoltage (mV)
0.4
0.3
0.2
75
50 BiOBr 25
IL-BiOBr 300
0.1
400
500 600 700 Wavelength (nm)
800
BiOBr
0.0
BiOBr
300
400
500
600
700
800
Wavelength (nm) Fig. 4. SPS response of photocatalysts and UV–vis diffuse reflectance spectra (inset).
Concentration of RhB (mg/L)
10
8 BiOBr
6
4 IL-BiOBr 2 0
2
4
6
8
10
Irradiation time(min) Fig. 5. Photocatalytic activities of BiOBr and IL- BiOBr.
BiOBr and IL-BiOBr both are large-scale sheet-shaped structures. The morphology in the present paper is different from the results reported in literatures [4–6], which may due to the different hydrothermal temperatures and ionic liquids. Furthermore, it is
interesting to note the mean particle size of IL-BiOBr is smaller than that of BiOBr, the small particle size can increase the specific surface area, which accords well with the results of the specific surface area (The specific surface area for BiOBr and IL-BiOBr is 7.6,13.6 m2/g, respectively). This result can further be confirmed by the results of photocatalytic activity measurements. Fig. 2c shows that the BiOBr consists of well-defined sheet-shaped structures. Fig. 2d shows a HRTEM image recorded from the white framed area indicated in Fig. 2c. The lattice fringes of 0.284 nm agree well with the spacing of the (102) plane of the BiOBr. The distribution of particle size is shown in Fig. 3. The particle size of IL-BiOBr is smaller than that of BiOBr. For the distribution of particle size, D50 is an important parameter, D50 for BiOBr and ILBiOBr is 7.81 um and 5.00 um, respectively. The size of 50% of all the BiOBr particles is below 7.81 um, while for IL-BiOBr, the value is 5.00 um, thus it is obvious that adding [EMIm]NO3 into the synthesis system greatly decreases the particle size of BiOBr. Ionic liquid can reduce the surface energy of the particles, resulting in good dispersion in the solution, thus the particles have small particle size. The small particle size leads to high specific surface area, which is beneficial to photocatalytic activity. As shown in Fig. 4, the two photocatalysts have similar UV–vis diffuse reflectance spectra, which demonstrates that [EMIm]NO3 is not effectively changing the band-gap of BiOBr and the difference in photocatalytic performance is not caused by the difference in response to light. Although two photocatalysts have similar UV– vis diffuse reflectance spectra, the SPS responses of BiOBr and ILBiOBr samples have great difference. Both BiOBr and IL-BiOBr samples display obvious SPS response from 300–500 nm and 600– 800 nm, which is assigned to the electronic transitions from the valence band to conduction band according to BiOBr energy band structure and the diffuse reflectance spectroscopy. Furthermore, strong SPS response is observed from 600–800 nm for the IL-BiOBr sample, which demonstrates that the generated electron–hole pairs can be separated effectively under the irradiation of visible light. In general, the strong SPS response corresponds to the high separation rate of photo-induced charge carriers on the basis of the SPS principle. So IL-BiOBr has higher separation rate of photoinduced charge carriers than that of BiOBr. High separation rate of photo-induced charge carriers is beneficial to the photocatalytic activity under simulated solar illumination. 3.2. Photocatalytic activity The photocatalytic activities of BiOBr and IL-BiOBr were investigated and are presented in Fig. 5.
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Y. Si et al. / Materials Letters 163 (2016) 175–178
As shown in Fig. 5, IL-BiOBr exhibits higher photocatalytic performance than BiOBr. Under simulated solar illumination, the apparent decolorization rate constant for BiOBr and IL-BiOBr is 0.078 min 1 and 0.183 min 1, respectively. The photocatalytic activity of IL-BiOBr is 2.3 times higher compared to that of the reference BiOBr. In this paper, the enhanced photocatalytic activity of IL-BiOBr may be assigned to the small particle size and high separation rate of photo-induced charge carriers.
4. Conclusions In this paper, BiOBr with efficient solar-driven photocatalytic activity was successfully prepared by a facile hydrothermal routine with the assistant of [EMIm]NO3. The results reveal that adding [EMIm]NO3 into the synthetic system increases the specific surface area, decrease the particle size and enhances the photo-induced charge separation rate. The photocatalytic experiment results demonstrate that activity of IL-BiOBr is 2.3 times of that of the reference BiOBr.
Acknowledgments This project was supported financially by the Research Fund Projects of Sichuan University of Science and Engineering (No. 2013PY03), Sichuan Provincial Academician (Expert) Workstation (No.2015YSGZZ03) and Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZY1101).
References [1] L. Li, L. Ai, C. Zhang, J. Jiang, Nanoscale 6 (2014) 4627–4634. [2] K. Li, H.B. Zhang, Y.P. Tang, D.W. Ying, Y.L. Xu, Y.L. Wang, et al., Appl. Catal. B 164 (2015) 82–91. [3] K. Dai, D.P. Li, J.L. Lv, L.H. Lu, C.H. Liang, G.P. Zhu, Mater. Lett. 136 (2014) 438–440. [4] D.J. Mao, X.M. Lü, Z.F. Jiang, J.M. Xie, X.F. Lu, W. Wei, et al., Mater. Lett. 118 (2014) 154–157. [5] J.X. Xia, J. Di, S. Yin, H.M. Li, L. Xu, Y.G. Xu, et al., Ceram. Int. 40 (2014) 4607–4616. [6] B. Chai, H. Zhou, F. Zhang, X. Liao, M.X. Ren, Mat. Sci. Semicond. Process. 23 (2014) 151–158.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Efficient solar-driven photocatalytic performance of BiOBr benefiting from enhanced charge separation rate Yujun Si a, Junbo Zhong a,n, Jianzhang Li a, Minjiao Li a,b, Lei Yang a, Jie Ding a a Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Pharmaceutical Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China b Sichuan Provincial Academician (Expert) Workstation, Sichuan University of Science and Engineering, Zigong 643000, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 August 2015 Received in revised form 14 October 2015 Accepted 17 October 2015 Available online 22 October 2015
In this work, novel BiOBr photocatalyst (IL-BiOBr) with enhanced solar-driven photocatalytic activity was fabricated by a facile hydrothermal method with the assistance of ionic liquid (IL) 1-ethyl-3-methylimidazolium nitrate ([EMIm]NO3). BiOBr and IL-BiOBr were characterized by the Brunauer–Emmett– Teller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), particle size analyzer and surface photovoltage spectroscopy (SPS), respectively. The results reveal that [EMIm]NO3 increases the specific surface area, decreases the particle size, and greatly enhances separation rate of the photo-induced charge of BiOBr. The photocatalytic activity of IL-BiOBr for decolorization of rhodamine B aqueous solution was evaluated. The results illustrate that the photocatalytic performance of IL- BiOBr is more than 2.3 times of that of the reference BiOBr and the possible reason was suggested. & 2015 Published by Elsevier B.V.
Keywords: Semiconductors BiOBr Surfaces Photocatalytic performance Ionic liquid
1. Introduction As a lamellar-structured p-type semiconductor, bismuth oxybromide (BiOBr) has received increasing interest in the potential application as a visible light photocatalyst due to its medium band gap energy (2.8 eV), nontoxicity, facile preparation and chemical stability in photocatalytic process [1]. It was reported that BiOBr has unique layered structures characterized by [Bi2O2] slabs interleaved by double slabs of bromine atoms, which can effectively reduce the recombination of the photo-generated carriers and is beneficial to the photocatalytic activity [2]. However, it is still a great challenge to further boost the photocatalytic activity in order to meet the practical applications. Tremendous approaches have been developed to promote the photocatalytic activity of BiOBr, in which the hydrothermal method is more applicable due to its outstanding advantages [3]. Ionic liquids (ILs) have attracted considerable interests because of their properties. Recently, ILs have been applied to fabricate BiOBr [4–6]. However, the effect of [EMIm]NO3 on the photo-induced charge separation rate of BiOBr prepared by the hydrothermal method has been seldom addressed. The primary objective of this paper is to study the effect of [EMIm]NO3 on the photoinduced charge separation rate and the relation with the n
Corresponding author. E-mail address: [email protected] (J. Zhong).
http://dx.doi.org/10.1016/j.matlet.2015.10.095 0167-577X/& 2015 Published by Elsevier B.V.
photocatalytic activity of BiOBr.
2. Experimental section [EMIm]NO3 was purchased from Lanzhou Institute of Chemical Physics. All other chemicals (analytical grade reagents) were supplied from Chengdu Kelong Chemical Reagent Factory. IL-BiOBr was fabricated by a hydrothermal method. 5 g Bi(NO3)3 5H2O and 0.1 g [EMIm]NO3 were dissolved in 20 mL glacial acetic acid. After the mixture became clear solution, KBr aqueous solution (1.23 g KBrþ 10 mL H2O) was added dropwise to the above solution under stirring, resulting in precipitate. The above mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was maintained at 453 K for 24 h and then cooled to room temperature naturally. The product was collected by filtration, washed several times with deionized water and absolute ethanol, and then dispersed in absolute ethanol and dried at 353 K in air overnight. BiOBr was also prepared as the same procedure mentioned above without the presence of [EMIm]NO3. The specific surface area and pore size measurement were carried out on a SSA-4200 automatic surface analyzer (Builder, China). XRD patterns were performed on a DX -2600 X-ray diffractometer using Cu Kα (λ ¼0.15406 nm) radiation equipped with a graphite monochromator. The X-ray tube was operated at 40 kV and 20 mA. The UV–vis spectra of photocatalysts in the 300–850 nm range were recorded using a TU-1907 UV–vis spectrophotometer equipped
176
Y. Si et al. / Materials Letters 163 (2016) 175–178
28000
Intensity
(a.u.)
24000
(102)
20000 16000
water as media and laser as light source. The SPS measurements were conducted with a home-built apparatus. The photocatalytic performance of BiOBr (50 mg) was investigated by decolorization of 50 mL rhodamine B aqueous solution (the concentration is 10 mg L 1) under the identical conditions. The light source was a 500 W xenon lamp.
(001)
(002) (003)
(101)
BiOBr
3. Results and discussion
12000
3.1. Characterization of photocatalysts
8000 (110) 4000 10
IL-BiOBr 15
20
25
30
35
40
2 Theta (degree) Fig. 1. XRD patterns of photocatalysts.
with an integrating sphere using BaSO4 as the reference. SEM images were taken with a JSM-7500F scanning electron microscope, using an accelerating voltage of 5 kV. HRTEM (Tecnai TEM G2) were used to study the microstructure of the samples using an accelerating voltage of 300 kV. The distribution of particle size was measured on a BH-9300H Bettersize particle size analyzer using
The XRD patterns of the photocatalysts are shown in Fig. 1. All of the diffraction peaks can be assigned to the tetragonal phase of the BiOBr crystal (JCPDS no. 09-0393). No other patterns can be observed, suggesting high purity of the as-prepared BiOBr samples. Furthermore, the half maximum (FWHM) of IL-BiOBr sample is wider than that of BiOBr. According to the Scherrer equation, the wider the FWHM is, the smaller the crystal size of BiOCl is. Thus, IL-BiOBr has smaller crystal size than that of BiOBr. The decreased crystal size leads to the BET surface area increase, which fits well with the result of BET surface area and distribution of the particle size. The smaller crystal size can increase the specific surface area of BiOCl and the photocatalytic reaction active sites, which can improve the photocatalytic activity. The SEM images of photocatalysts prepared are shown in Fig. 2.
Fig. 2. SEM of photocatalysts (A) BiOBr; (B) IL- BiOBr; (C) TEM image of BiOBr; (D) the HRTEM image recorded from the white framed area indicated in C.
Y. Si et al. / Materials Letters 163 (2016) 175–178
10
4
8
3
6
%
%
5
177
2
4
1
2
0
0
Particle size (um)
Particle size (um)
Fig. 3. Distribution of the particle size of photocatalyst; (A) BiOBr; (B) IL- BiOBr.
IL-BiOBr
100
Reflectance (%)
Photovoltage (mV)
0.4
0.3
0.2
75
50 BiOBr 25
IL-BiOBr 300
0.1
400
500 600 700 Wavelength (nm)
800
BiOBr
0.0
BiOBr
300
400
500
600
700
800
Wavelength (nm) Fig. 4. SPS response of photocatalysts and UV–vis diffuse reflectance spectra (inset).
Concentration of RhB (mg/L)
10
8 BiOBr
6
4 IL-BiOBr 2 0
2
4
6
8
10
Irradiation time(min) Fig. 5. Photocatalytic activities of BiOBr and IL- BiOBr.
BiOBr and IL-BiOBr both are large-scale sheet-shaped structures. The morphology in the present paper is different from the results reported in literatures [4–6], which may due to the different hydrothermal temperatures and ionic liquids. Furthermore, it is
interesting to note the mean particle size of IL-BiOBr is smaller than that of BiOBr, the small particle size can increase the specific surface area, which accords well with the results of the specific surface area (The specific surface area for BiOBr and IL-BiOBr is 7.6,13.6 m2/g, respectively). This result can further be confirmed by the results of photocatalytic activity measurements. Fig. 2c shows that the BiOBr consists of well-defined sheet-shaped structures. Fig. 2d shows a HRTEM image recorded from the white framed area indicated in Fig. 2c. The lattice fringes of 0.284 nm agree well with the spacing of the (102) plane of the BiOBr. The distribution of particle size is shown in Fig. 3. The particle size of IL-BiOBr is smaller than that of BiOBr. For the distribution of particle size, D50 is an important parameter, D50 for BiOBr and ILBiOBr is 7.81 um and 5.00 um, respectively. The size of 50% of all the BiOBr particles is below 7.81 um, while for IL-BiOBr, the value is 5.00 um, thus it is obvious that adding [EMIm]NO3 into the synthesis system greatly decreases the particle size of BiOBr. Ionic liquid can reduce the surface energy of the particles, resulting in good dispersion in the solution, thus the particles have small particle size. The small particle size leads to high specific surface area, which is beneficial to photocatalytic activity. As shown in Fig. 4, the two photocatalysts have similar UV–vis diffuse reflectance spectra, which demonstrates that [EMIm]NO3 is not effectively changing the band-gap of BiOBr and the difference in photocatalytic performance is not caused by the difference in response to light. Although two photocatalysts have similar UV– vis diffuse reflectance spectra, the SPS responses of BiOBr and ILBiOBr samples have great difference. Both BiOBr and IL-BiOBr samples display obvious SPS response from 300–500 nm and 600– 800 nm, which is assigned to the electronic transitions from the valence band to conduction band according to BiOBr energy band structure and the diffuse reflectance spectroscopy. Furthermore, strong SPS response is observed from 600–800 nm for the IL-BiOBr sample, which demonstrates that the generated electron–hole pairs can be separated effectively under the irradiation of visible light. In general, the strong SPS response corresponds to the high separation rate of photo-induced charge carriers on the basis of the SPS principle. So IL-BiOBr has higher separation rate of photoinduced charge carriers than that of BiOBr. High separation rate of photo-induced charge carriers is beneficial to the photocatalytic activity under simulated solar illumination. 3.2. Photocatalytic activity The photocatalytic activities of BiOBr and IL-BiOBr were investigated and are presented in Fig. 5.
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Y. Si et al. / Materials Letters 163 (2016) 175–178
As shown in Fig. 5, IL-BiOBr exhibits higher photocatalytic performance than BiOBr. Under simulated solar illumination, the apparent decolorization rate constant for BiOBr and IL-BiOBr is 0.078 min 1 and 0.183 min 1, respectively. The photocatalytic activity of IL-BiOBr is 2.3 times higher compared to that of the reference BiOBr. In this paper, the enhanced photocatalytic activity of IL-BiOBr may be assigned to the small particle size and high separation rate of photo-induced charge carriers.
4. Conclusions In this paper, BiOBr with efficient solar-driven photocatalytic activity was successfully prepared by a facile hydrothermal routine with the assistant of [EMIm]NO3. The results reveal that adding [EMIm]NO3 into the synthetic system increases the specific surface area, decrease the particle size and enhances the photo-induced charge separation rate. The photocatalytic experiment results demonstrate that activity of IL-BiOBr is 2.3 times of that of the reference BiOBr.
Acknowledgments This project was supported financially by the Research Fund Projects of Sichuan University of Science and Engineering (No. 2013PY03), Sichuan Provincial Academician (Expert) Workstation (No.2015YSGZZ03) and Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZY1101).
References [1] L. Li, L. Ai, C. Zhang, J. Jiang, Nanoscale 6 (2014) 4627–4634. [2] K. Li, H.B. Zhang, Y.P. Tang, D.W. Ying, Y.L. Xu, Y.L. Wang, et al., Appl. Catal. B 164 (2015) 82–91. [3] K. Dai, D.P. Li, J.L. Lv, L.H. Lu, C.H. Liang, G.P. Zhu, Mater. Lett. 136 (2014) 438–440. [4] D.J. Mao, X.M. Lü, Z.F. Jiang, J.M. Xie, X.F. Lu, W. Wei, et al., Mater. Lett. 118 (2014) 154–157. [5] J.X. Xia, J. Di, S. Yin, H.M. Li, L. Xu, Y.G. Xu, et al., Ceram. Int. 40 (2014) 4607–4616. [6] B. Chai, H. Zhou, F. Zhang, X. Liao, M.X. Ren, Mat. Sci. Semicond. Process. 23 (2014) 151–158.