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Optical, electrical and visible light-photocatalytic properties of hydrothermally synthesized amorphous BiVO4 nanoparticles
Materials Letters 122 (2014) 21–24
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Optical, electrical and visible light-photocatalytic properties of hydrothermally synthesized amorphous BiVO4 nanoparticles C. Karunakaran n, S. Kalaivani, P. Vinayagamoorthy Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2013 Accepted 30 January 2014 Available online 8 February 2014
Hydrothermal synthesis of BiVO4 at pH 12 yields amorphous pale yellow nanoparticles and that at pH 5 provides crystalline greenish yellow nanoparticles. The solid state impedance spectrum of crystalline BiVO4 is quasi-semicircular and the amorphous BiVO4 exhibits a quasi-linear relationship between ZIm and ZRe. The band gap of amorphous BiVO4 is larger than that of crystalline BiVO4. The band gap emission of amorphous BiVO4 is stronger than that of crystalline BiVO4. The amorphous BiVO4 nanoparticles exhibit larger visible light-photocatalytic activity than BiVO4 nanocrystals. & 2014 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Ceramics Charge transfer resistance Photoluminescence
1. Introduction Mineralization of organic pollutants through semiconductorphotocatalysis is an emerging technique for environmental remediation. Band gap-illumination of semiconductor creates electron–hole pairs, electrons in the conduction band (CB) and holes in the valence band (VB) [1]. While many of the charge carriers recombine some migrate to the crystal surface and involve in chemical reactions with the adsorbed molecules or ions resulting in photocatalysis; the VB hole takes up the adsorbed hydroxide ion or water molecule to generate hydroxyl radical, the predominant reactive oxygen species (ROS). The CB electron is picked up by the adsorbed O2 forming superoxide radical ion, which through a series of reactions yields ROS. The ROS mineralize the organics. Visible light-semiconductor photocatalyst is of interest and BiVO4 is a promising candidate [1–6]. It is an n-type intrinsic semiconductor with band gap energy of 2.4 eV. It exists in monoclinic and tetragonal crystalline forms and the former is more photocatalytically active than the latter [1,2]. Here we report hydrothermal synthesis of amorphous BiVO4 nanoparticles, which exhibit larger visible light-photocatalytic activity than monoclinic BiVO4 nanocrystals.
2. Experimental Hydrothermal synthesis: Bi(NO3)3 5H2O (1 mmol) was dissolved in 20 mL of 1 M HNO3. NH4VO3 (1 mmol) was dissolved in 20 mL of n
Corresponding author. Tel.: þ 91 94434 81590; fax: þ 91 4144238145. E-mail address: [email protected] (C. Karunakaran).
http://dx.doi.org/10.1016/j.matlet.2014.01.179 0167-577X & 2014 Elsevier B.V. All rights reserved.
0.5 M NaOH. The latter was added drop wise to the former under stirring to get a clear solution of pH 0.3–0.4. Then the pH was adjusted to 12 or 5 with the addition of 2 M NaOH. It was transferred to a 100 mL Teflon-lined stainless steel autoclave, sealed and heated at 120 1C for 12 h and allowed to cool to room temperature. The obtained samples were filtered, washed with distilled water and absolute ethanol and dried at 50 1C for 12 h in a hot air oven. Characterization: A FEI Quanta FEG 200 high resolution scanning electron microscope (HR-SEM) in high vacuum mode was used to record the HR-SEM images and the energy dispersive X-ray (EDX) spectra. A Hitachi H-7650 transmission electron microscope (TEM) was employed to obtain the TEM images and the selected area electron diffractograms (SAED). The samples were dispersed in acetone and spread on copper grids for imaging. The powder X-ray diffractograms (XRD) were recorded with a PANalytical X'Pert PRO diffractometer using Cu Kα rays at 1.5406 Ǻ with a tube current of 30 mA at 40 kV. A Shimadzu UV-2600 spectrophotometer with ISR-2600 integrating sphere attachment was used to record the UV–visible diffuse reflectance spectra (DRS). The photoluminescence (PL) spectra were obtained at room temperature with a PerkinElmer LS 55 fluorescence spectrometer. The nanoparticles were dispersed in carbon tetrachloride under sonication and the excitation wavelength was 435 nm. A CH Instrument Electrochemical Analyzer 604C was used to record the solid state electrochemical impedance spectra (IS) at room temperature in the frequency range of 0.1 MHz to 1 Hz. The disk area was 0.5024 cm2 and the thickness of amorphous and crystalline BiVO4 pellets was 1.78 and 2.50 mm, respectively. Photocatalytic study: A 150 W tungsten halogen lamp fitted into a double walled borosilicate vessel with an inlet and outlet for
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C. Karunakaran et al. / Materials Letters 122 (2014) 21–24
circulation of NaNO2 (2 M) solution to cutoff UV light was the light source used. The light intensity was measured (415 W m 2) using a Daystar solar meter (USA). Photodegradation of methylene blue was made in a wide cylindrical borosilicate glass vessel of uniform diameter (7.0 cm) placed below (13 cm) the visible light source. Freshly prepared dye solution (100 mL) with BiVO4 (0.100 g) was taken in the reaction vessel and was saturated with air using a micro-air pump. The solution was stirred continuously with a magnetic stirrer. The dye-degradation was followed spectrophotometrically at 662 nm.
3. Results and discussion Morphology: The HR-SEM images of BiVO4 nanoparticles, hydrothermally synthesized at pH 12 and 5, are displayed in Fig. 1. The images show the particulate nature of the synthesized BiVO4. They are in nanoscale and lack specific shape or structure. The HR-TEM images of both the samples are presented in Fig. 1. They confirm the synthesized materials as nanoparticles. Fig. 1 also presents the EDX spectra of the samples. They show the presence of bismuth, vanadium and oxygen. The absence of any
Fig. 1. HR-SEM (top), HR-TEM (middle) and EDX spectra (bottom) of BiVO4 nanoparticles synthesized at pH at 12 and 5.
C. Karunakaran et al. / Materials Letters 122 (2014) 21–24
other element reveals the purity of the synthesized BiVO4. The sample synthesized at pH 12 is pale yellow in color and that obtained at pH 5 is greenish yellow (Fig. 2). Amorphous phase: The SAED patterns of BiVO4, synthesized at pH 12 and 5, are presented in Fig. 2. The absence of diffraction rings in the SAED of BiVO4 (pH 12) shows the amorphous nature of the nanomaterials synthesized hydrothermally at pH 12. On the other hand, the SAED of BiVO4 (pH 5) displays a number of bright spots aligned linearly indicating the formation of well-grown monoclinic BiVO4 single-crystallites. The XRDs of BiVO4 nanoparticles synthesized at pH 12 and 5 are also displayed in Fig. 2. The XRD of BiVO4 (pH 12) shows the amorphous nature of the sample. On the other hand, the XRD of BiVO4 (pH 5) reveals its crystalline character. The observed diffraction pattern matches with that of monoclinic BiVO4 crystals (JCPDS 14-0688). Charge transfer resistance: Fig. 3 displays the room temperature Nyquist plots of amorphous and crystalline BiVO4 nanoparticles. While the latter is quasi-semicircular the former exhibits a quasilinear dependence of ZIm on ZRe. The diameter of the semicircle provides the charge transfer resistance. As there is no report on the solid state impedance spectrum of BiVO4 the present results are compared with those of three-electrode solution impedance studies, which provide information on electrode–electrolyte interface and not on solid state. However, our results are similar to those reported by Chatchai et al. [7] and Lee et al. [8]. While FTO/WO3/BiVO4/Au electrode under illumination shows a perfect semicircular Nyquist plot FTO/WO3/BiVO4 displays a half-semicircular arc and FTO/BiVO4 exhibits a quasi-linear relationship between ZIm and ZRe [7]. Under visible light illumination, BiVO4 thick film electrode shows a
23
quasi-linear ZIm–ZRe correlation [8]. However, microwave synthesized BiVO4 [9] and hydrothermally synthesized BiVO4 [10] display perfect semicircular Nyquist plots. Fig. 3 shows that BiVO4 nanoparticles synthesized at pH 12 possess very large charge transfer resistance, which is characteristic of amorphous materials. Optical properties: The DRS of amorphous and crystalline BiVO4 is presented in Fig. 3. The Kubelka–Munk (KM) plots show that the absorption edge of amorphous BiVO4 nanoparticles is blue shifted in comparison with that of BiVO4 nanocrystals. The visible-light absorption is because of excitation of electrons from the VB comprised of the hybridized Bi 6s and O 2p orbitals to the CB made of V 3d orbitals of VO4 3 [11]. The electronic excitation from the VB contributed by the O 2p orbitals to CB of V 3d orbitals in VO4 3 tetrahedron leads to the UV absorption. The direct and indirect band gaps of the nanoparticles have been obtained from the Tauc plots shown in Fig. 3 (as insets). The indirect electronic transition occurs because of the breaking of the symmetry of the VO4 3 tetrahedron by the stereoactive lone pair of Bi3 þ [12]. Plots of [F(R)hν]2 and [F(R)hν]½ versus photon energy provide the direct and indirect [12] band gaps. The direct and indirect band gaps of amorphous BiVO4 are 3.03 and 2.52 eV whereas those of crystalline BiVO4 are 2.52 and 2.33 eV, respectively. The room temperature PL spectra of amorphous BiVO4 and crystalline BiVO4 nanoparticles are displayed in Fig. 3. While the amorphous BiVO4 shows emissions at 464, 482 and 525 nm, crystalline BiVO4 emits at 483 and 532 nm. The observed emissions are in agreement with the earlier report [13]. The blue emission corresponds to the band gap emission. The green emission is likely due to crystalline defects. The intensity of band gap emission of amorphous BiVO4 is much
Fig. 2. SAED (top) and XRD with inset photograph (bottom) of BiVO4 nanoparticles synthesized at pH at 12 and 5.
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C. Karunakaran et al. / Materials Letters 122 (2014) 21–24
Fig. 3. The Nyquist plots (top left), the K–M plots (top right), the PL spectra (bottom left) and the dye degradation profiles (bottom right).
higher than that of crystalline BiVO4. This shows that charge carrier recombination is predominant in amorphous BiVO4. Photocatalytic activity: The time-profiles of visible lightdegradation of methylene blue catalyzed by amorphous and crystalline BiVO4 nanoparticles are shown in Fig. 3. The degradation profiles clearly show that the photocatalytic activity of amorphous BiVO4 is larger than that of monoclinic BiVO4. The charge transfer resistance of amorphous BiVO4 is very much larger than that of crystalline BiVO4 and hence is not the reason for the observed photocatalytic activities. The band gap of amorphous BiVO4 is larger than that of monoclinic BiVO4 and hence is not the reason for the measured photocatalytic efficiencies. The band gap emission of amorphous BiVO4 is much higher than that of crystalline BiVO4. Hence charge carrier recombination is unlikely to be the reason for the determined order of photocatalytic activity. The sizes of the amorphous and crystalline BiVO4 nanoparticles, as shown by the TEM images, do not differ significantly. Hence the size or surface area could not be the reason for the displayed photocatalytic efficiencies. A possible reason is the dispersion of the nanoparticles in the dye solution. The amorphous sample disperses effectively than the crystalline one. Effective dispersion and wet ability favor photocatalysis. 4. Conclusions Hydrothermal synthesis of BiVO4 at pH 12 provides amorphous nanoparticles and that at pH 5 gives nanocrystals. The amorphous and crystalline BiVO4 differ in their color, band gap energy, band
gap emission and solid state electrical properties. The amorphous BiVO4 is more photocatalytically active than the crystalline BiVO4.
Acknowledgments C.K. is thankful to the CSIR [21(0887)/12/EMR-II] and the DST (SR/S1/PC-41/2011) for the Grants. References [1] Navalon S, Dhakshinamoorthy A, Alvaro M, Garcia H. ChemSusChem 2013;6:562–77. [2] Tan G, Zhang L, Ren H, Wei S, Huang J, Xia A. ACS Appl Mater Interfaces 2013;5:5186–93. [3] Thalluri SM, Suarez CM, Hussain M, Hernandez S, Virja A, Saracco G, et al. Ind Eng Chem Res 2013;52:17414–8. [4] Nagabhushana GP, Nagaraju G, Chandrappa GT. J Mater Chem A 2013;1:388–94. [5] Fan H, Jiang T, Li H, Wang D, Wang L, Zhai J, et al. J Phys Chem C 2012;116:2425–30. [6] Sun S, Wang W, Zhou L, Xu H. Ind Eng Chem Res 2009;48:1735–9. [7] Chatchai P, Kishioka S-y, Murakami Y, Nosaka AY, Nosaka Y. Electrochim Acta 2010;55:592–6. [8] Lee DK, Cho I-S, Lee S, Bae S-T, Noh JH, Kim DW, et al. Mater Chem Phys 2010;119:106–11. [9] Yan Y, Sun S, Song Y, Yan X, Guan W, Liu X, et al. J Hazard Mater 2013;250– 251:106–14. [10] Fu Y, Sun X, Wang X. Mater Chem Phys 2013;131:325–30. [11] Chung C-Y, Lu C-H. J Alloys Compd 2010;502:L1–5. [12] Castillo NC, Heel A, Graule T, Pulgarin C. Appl Catal B 2010;95:335–47. [13] Liu W, Yu Y, Cao L, Su G, Liu X, Zhang L, et al. J Hazard Mater 2010;181:1102–8.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Optical, electrical and visible light-photocatalytic properties of hydrothermally synthesized amorphous BiVO4 nanoparticles C. Karunakaran n, S. Kalaivani, P. Vinayagamoorthy Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2013 Accepted 30 January 2014 Available online 8 February 2014
Hydrothermal synthesis of BiVO4 at pH 12 yields amorphous pale yellow nanoparticles and that at pH 5 provides crystalline greenish yellow nanoparticles. The solid state impedance spectrum of crystalline BiVO4 is quasi-semicircular and the amorphous BiVO4 exhibits a quasi-linear relationship between ZIm and ZRe. The band gap of amorphous BiVO4 is larger than that of crystalline BiVO4. The band gap emission of amorphous BiVO4 is stronger than that of crystalline BiVO4. The amorphous BiVO4 nanoparticles exhibit larger visible light-photocatalytic activity than BiVO4 nanocrystals. & 2014 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Ceramics Charge transfer resistance Photoluminescence
1. Introduction Mineralization of organic pollutants through semiconductorphotocatalysis is an emerging technique for environmental remediation. Band gap-illumination of semiconductor creates electron–hole pairs, electrons in the conduction band (CB) and holes in the valence band (VB) [1]. While many of the charge carriers recombine some migrate to the crystal surface and involve in chemical reactions with the adsorbed molecules or ions resulting in photocatalysis; the VB hole takes up the adsorbed hydroxide ion or water molecule to generate hydroxyl radical, the predominant reactive oxygen species (ROS). The CB electron is picked up by the adsorbed O2 forming superoxide radical ion, which through a series of reactions yields ROS. The ROS mineralize the organics. Visible light-semiconductor photocatalyst is of interest and BiVO4 is a promising candidate [1–6]. It is an n-type intrinsic semiconductor with band gap energy of 2.4 eV. It exists in monoclinic and tetragonal crystalline forms and the former is more photocatalytically active than the latter [1,2]. Here we report hydrothermal synthesis of amorphous BiVO4 nanoparticles, which exhibit larger visible light-photocatalytic activity than monoclinic BiVO4 nanocrystals.
2. Experimental Hydrothermal synthesis: Bi(NO3)3 5H2O (1 mmol) was dissolved in 20 mL of 1 M HNO3. NH4VO3 (1 mmol) was dissolved in 20 mL of n
Corresponding author. Tel.: þ 91 94434 81590; fax: þ 91 4144238145. E-mail address: [email protected] (C. Karunakaran).
http://dx.doi.org/10.1016/j.matlet.2014.01.179 0167-577X & 2014 Elsevier B.V. All rights reserved.
0.5 M NaOH. The latter was added drop wise to the former under stirring to get a clear solution of pH 0.3–0.4. Then the pH was adjusted to 12 or 5 with the addition of 2 M NaOH. It was transferred to a 100 mL Teflon-lined stainless steel autoclave, sealed and heated at 120 1C for 12 h and allowed to cool to room temperature. The obtained samples were filtered, washed with distilled water and absolute ethanol and dried at 50 1C for 12 h in a hot air oven. Characterization: A FEI Quanta FEG 200 high resolution scanning electron microscope (HR-SEM) in high vacuum mode was used to record the HR-SEM images and the energy dispersive X-ray (EDX) spectra. A Hitachi H-7650 transmission electron microscope (TEM) was employed to obtain the TEM images and the selected area electron diffractograms (SAED). The samples were dispersed in acetone and spread on copper grids for imaging. The powder X-ray diffractograms (XRD) were recorded with a PANalytical X'Pert PRO diffractometer using Cu Kα rays at 1.5406 Ǻ with a tube current of 30 mA at 40 kV. A Shimadzu UV-2600 spectrophotometer with ISR-2600 integrating sphere attachment was used to record the UV–visible diffuse reflectance spectra (DRS). The photoluminescence (PL) spectra were obtained at room temperature with a PerkinElmer LS 55 fluorescence spectrometer. The nanoparticles were dispersed in carbon tetrachloride under sonication and the excitation wavelength was 435 nm. A CH Instrument Electrochemical Analyzer 604C was used to record the solid state electrochemical impedance spectra (IS) at room temperature in the frequency range of 0.1 MHz to 1 Hz. The disk area was 0.5024 cm2 and the thickness of amorphous and crystalline BiVO4 pellets was 1.78 and 2.50 mm, respectively. Photocatalytic study: A 150 W tungsten halogen lamp fitted into a double walled borosilicate vessel with an inlet and outlet for
22
C. Karunakaran et al. / Materials Letters 122 (2014) 21–24
circulation of NaNO2 (2 M) solution to cutoff UV light was the light source used. The light intensity was measured (415 W m 2) using a Daystar solar meter (USA). Photodegradation of methylene blue was made in a wide cylindrical borosilicate glass vessel of uniform diameter (7.0 cm) placed below (13 cm) the visible light source. Freshly prepared dye solution (100 mL) with BiVO4 (0.100 g) was taken in the reaction vessel and was saturated with air using a micro-air pump. The solution was stirred continuously with a magnetic stirrer. The dye-degradation was followed spectrophotometrically at 662 nm.
3. Results and discussion Morphology: The HR-SEM images of BiVO4 nanoparticles, hydrothermally synthesized at pH 12 and 5, are displayed in Fig. 1. The images show the particulate nature of the synthesized BiVO4. They are in nanoscale and lack specific shape or structure. The HR-TEM images of both the samples are presented in Fig. 1. They confirm the synthesized materials as nanoparticles. Fig. 1 also presents the EDX spectra of the samples. They show the presence of bismuth, vanadium and oxygen. The absence of any
Fig. 1. HR-SEM (top), HR-TEM (middle) and EDX spectra (bottom) of BiVO4 nanoparticles synthesized at pH at 12 and 5.
C. Karunakaran et al. / Materials Letters 122 (2014) 21–24
other element reveals the purity of the synthesized BiVO4. The sample synthesized at pH 12 is pale yellow in color and that obtained at pH 5 is greenish yellow (Fig. 2). Amorphous phase: The SAED patterns of BiVO4, synthesized at pH 12 and 5, are presented in Fig. 2. The absence of diffraction rings in the SAED of BiVO4 (pH 12) shows the amorphous nature of the nanomaterials synthesized hydrothermally at pH 12. On the other hand, the SAED of BiVO4 (pH 5) displays a number of bright spots aligned linearly indicating the formation of well-grown monoclinic BiVO4 single-crystallites. The XRDs of BiVO4 nanoparticles synthesized at pH 12 and 5 are also displayed in Fig. 2. The XRD of BiVO4 (pH 12) shows the amorphous nature of the sample. On the other hand, the XRD of BiVO4 (pH 5) reveals its crystalline character. The observed diffraction pattern matches with that of monoclinic BiVO4 crystals (JCPDS 14-0688). Charge transfer resistance: Fig. 3 displays the room temperature Nyquist plots of amorphous and crystalline BiVO4 nanoparticles. While the latter is quasi-semicircular the former exhibits a quasilinear dependence of ZIm on ZRe. The diameter of the semicircle provides the charge transfer resistance. As there is no report on the solid state impedance spectrum of BiVO4 the present results are compared with those of three-electrode solution impedance studies, which provide information on electrode–electrolyte interface and not on solid state. However, our results are similar to those reported by Chatchai et al. [7] and Lee et al. [8]. While FTO/WO3/BiVO4/Au electrode under illumination shows a perfect semicircular Nyquist plot FTO/WO3/BiVO4 displays a half-semicircular arc and FTO/BiVO4 exhibits a quasi-linear relationship between ZIm and ZRe [7]. Under visible light illumination, BiVO4 thick film electrode shows a
23
quasi-linear ZIm–ZRe correlation [8]. However, microwave synthesized BiVO4 [9] and hydrothermally synthesized BiVO4 [10] display perfect semicircular Nyquist plots. Fig. 3 shows that BiVO4 nanoparticles synthesized at pH 12 possess very large charge transfer resistance, which is characteristic of amorphous materials. Optical properties: The DRS of amorphous and crystalline BiVO4 is presented in Fig. 3. The Kubelka–Munk (KM) plots show that the absorption edge of amorphous BiVO4 nanoparticles is blue shifted in comparison with that of BiVO4 nanocrystals. The visible-light absorption is because of excitation of electrons from the VB comprised of the hybridized Bi 6s and O 2p orbitals to the CB made of V 3d orbitals of VO4 3 [11]. The electronic excitation from the VB contributed by the O 2p orbitals to CB of V 3d orbitals in VO4 3 tetrahedron leads to the UV absorption. The direct and indirect band gaps of the nanoparticles have been obtained from the Tauc plots shown in Fig. 3 (as insets). The indirect electronic transition occurs because of the breaking of the symmetry of the VO4 3 tetrahedron by the stereoactive lone pair of Bi3 þ [12]. Plots of [F(R)hν]2 and [F(R)hν]½ versus photon energy provide the direct and indirect [12] band gaps. The direct and indirect band gaps of amorphous BiVO4 are 3.03 and 2.52 eV whereas those of crystalline BiVO4 are 2.52 and 2.33 eV, respectively. The room temperature PL spectra of amorphous BiVO4 and crystalline BiVO4 nanoparticles are displayed in Fig. 3. While the amorphous BiVO4 shows emissions at 464, 482 and 525 nm, crystalline BiVO4 emits at 483 and 532 nm. The observed emissions are in agreement with the earlier report [13]. The blue emission corresponds to the band gap emission. The green emission is likely due to crystalline defects. The intensity of band gap emission of amorphous BiVO4 is much
Fig. 2. SAED (top) and XRD with inset photograph (bottom) of BiVO4 nanoparticles synthesized at pH at 12 and 5.
24
C. Karunakaran et al. / Materials Letters 122 (2014) 21–24
Fig. 3. The Nyquist plots (top left), the K–M plots (top right), the PL spectra (bottom left) and the dye degradation profiles (bottom right).
higher than that of crystalline BiVO4. This shows that charge carrier recombination is predominant in amorphous BiVO4. Photocatalytic activity: The time-profiles of visible lightdegradation of methylene blue catalyzed by amorphous and crystalline BiVO4 nanoparticles are shown in Fig. 3. The degradation profiles clearly show that the photocatalytic activity of amorphous BiVO4 is larger than that of monoclinic BiVO4. The charge transfer resistance of amorphous BiVO4 is very much larger than that of crystalline BiVO4 and hence is not the reason for the observed photocatalytic activities. The band gap of amorphous BiVO4 is larger than that of monoclinic BiVO4 and hence is not the reason for the measured photocatalytic efficiencies. The band gap emission of amorphous BiVO4 is much higher than that of crystalline BiVO4. Hence charge carrier recombination is unlikely to be the reason for the determined order of photocatalytic activity. The sizes of the amorphous and crystalline BiVO4 nanoparticles, as shown by the TEM images, do not differ significantly. Hence the size or surface area could not be the reason for the displayed photocatalytic efficiencies. A possible reason is the dispersion of the nanoparticles in the dye solution. The amorphous sample disperses effectively than the crystalline one. Effective dispersion and wet ability favor photocatalysis. 4. Conclusions Hydrothermal synthesis of BiVO4 at pH 12 provides amorphous nanoparticles and that at pH 5 gives nanocrystals. The amorphous and crystalline BiVO4 differ in their color, band gap energy, band
gap emission and solid state electrical properties. The amorphous BiVO4 is more photocatalytically active than the crystalline BiVO4.
Acknowledgments C.K. is thankful to the CSIR [21(0887)/12/EMR-II] and the DST (SR/S1/PC-41/2011) for the Grants. References [1] Navalon S, Dhakshinamoorthy A, Alvaro M, Garcia H. ChemSusChem 2013;6:562–77. [2] Tan G, Zhang L, Ren H, Wei S, Huang J, Xia A. ACS Appl Mater Interfaces 2013;5:5186–93. [3] Thalluri SM, Suarez CM, Hussain M, Hernandez S, Virja A, Saracco G, et al. Ind Eng Chem Res 2013;52:17414–8. [4] Nagabhushana GP, Nagaraju G, Chandrappa GT. J Mater Chem A 2013;1:388–94. [5] Fan H, Jiang T, Li H, Wang D, Wang L, Zhai J, et al. J Phys Chem C 2012;116:2425–30. [6] Sun S, Wang W, Zhou L, Xu H. Ind Eng Chem Res 2009;48:1735–9. [7] Chatchai P, Kishioka S-y, Murakami Y, Nosaka AY, Nosaka Y. Electrochim Acta 2010;55:592–6. [8] Lee DK, Cho I-S, Lee S, Bae S-T, Noh JH, Kim DW, et al. Mater Chem Phys 2010;119:106–11. [9] Yan Y, Sun S, Song Y, Yan X, Guan W, Liu X, et al. J Hazard Mater 2013;250– 251:106–14. [10] Fu Y, Sun X, Wang X. Mater Chem Phys 2013;131:325–30. [11] Chung C-Y, Lu C-H. J Alloys Compd 2010;502:L1–5. [12] Castillo NC, Heel A, Graule T, Pulgarin C. Appl Catal B 2010;95:335–47. [13] Liu W, Yu Y, Cao L, Su G, Liu X, Zhang L, et al. J Hazard Mater 2010;181:1102–8.