- Email: [email protected]
Self-assembly of highly crystalline spherical BiVO4 in aqueous solutions
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 4505–4509
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Self-assembly of highly crystalline spherical BiVO4 in aqueous solutions Tao Yang, Dingguo Xia College of Environmental & Energy Engineering, Beijing University of Technology (BUT-CEEE), Chaoyang District, 100022 Beijing, PR China
a r t i c l e in fo
abstract
Article history: Received 25 September 2008 Received in revised form 30 June 2009 Accepted 11 August 2009 Communicated by H. Fujioka Available online 19 August 2009
Spherical bismuth vanadate particles are self-assembled from aqueous Bi(NO3)3 and NH4VO3 solutions by adjusting pH and tuning the amount of surfactant sodium dodecyl sulfate (SDS) via facile hydrothermal method. The BiVO4 samples were characterized by X-ray diffraction (XRD) and the peaks suited well with the pure phase monoclinic scheelite BiVO4. Field emission scanning electron microscopy (SEM) showed the average size of the spherical particles was 5 mm and the assembling stages in the hydrothermal synthesis process were recorded. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) revealed the nanoparticles were single crystal. FT-IR spectroscopy test results demonstrated there was no SDS left in the samples. The mechanism of the self-assembling has also been proposed. & 2009 Elsevier B.V. All rights reserved.
PACS: 81.10.Dn Keywords: B1. BiVO4 A2. Hydrothermal A2. Microstructure A2. Self-assemble
1. Introduction Facile assembly methods for synthesizing extended threedimensional architectures with building blocks of rare metal or composite oxide are always a major limitation in nanoparticle inorganic chemistry. Morphology, proper parameters such as size distribution and specific area and purity of the composite oxide nanoparticles, the stability of the particles are partly controllable to some extent to date [1–5]. Composite oxide bismuth vanadate is of great interest due to their versatile properties. It is a visible light responsive photocatalyst to split water into oxygen and to degradate some organic pollutants in the visible light zone [6]. Besides, it shows ferroelastic characteristics and is a possible cathode catalyst in solid oxide fuel cell [7]. BiVO4 is also a high quality pigment used in the automobile painting and house decoration industry because of its useful luminescent and environment-friendly properties. In the three structure types of bismuth vanadate–zircon tetragonal, scheelite monoclinic, and scheelite tetragonal, only the monoclinic scheelite BiVO4 has the best properties for the functions stated above [8–10]. These properties and functions strongly depend on the crystal form and morphologies, which is related to the synthesis method. A lot of work have been done to research on the synthesis of this compound, and many methods are developed and many crystal morphologies are acquired
purposely [11–20]. These methods have advantage and disadvantage aspects on preparation of different particles. The present work deals with the self-assemble process and the characterization of the fine spherical BiVO4 with monoclinic scheelite crystal type. In our synthesis conditions, all the elements were evenly dispersed in aqueous solutions and self-assembled to highly crystalline BiVO4 spontaneously and simultaneously at the effect of the template sodium dodecyl sulfate (SDS). The easy control of the temperature, pH and reaction time of the hydrothermal synthesis facilitates the reacting process of BiVO4 formation and crystal evolution, which is the biggest advantages of the hydrothermal methods compared with the traditional solid state methods. The crystal growth of the assembling stages from single-crystalline nanometer BiVO4 to fine spherical particles assisted with surfactant SDS in the hydrothermal synthesis process was recorded on the FE-SEM images. The influences of the surfactant and pH on the specific area, size distribution, microstructure and morphology were studied. However, the controlling of the three-dimensional architecture and the corresponding mechanism of the growth of the nanoparticles (NPs) need intensive endeavors to understand clearly. One possible mechanism of self-assembling process was proposed.
2. Experimental 2.1. Preparation
Corresponding author. Tel.: +86 010 67396158.
E-mail address: [email protected] (D. Xia). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.08.006
The self-assemble synthesis of BiVO4 A was performed starting from stoichiometric amounts of Bi(NO3)3 5H2O and NH4VO3.
ARTICLE IN PRESS 4506
T. Yang, D. Xia / Journal of Crystal Growth 311 (2009) 4505–4509
Table 1 The parameters and the numbers of FE-SEM images of the three samples.
BiVO4 A BiVO4 B BiVO4 C
TemratureTemperature and time
SDS amount (g)
SEM images
200nm
433K, 20 h
0.23
A
5mm
433K, 20 h
2.3
10mm
433K, 20 h
4.6
B1, B2, B3 and B4 C
1400 1200 1000 Intensity
Sample Average size
800 600 400 200 0
3. Results and discussion 3.1. X-ray characterization The diffraction peaks of the BiVO4 A and B fit well with the standard Joint Committee on Powder Diffraction Standards (JCPDS) card of the monoclinic scheelite BiVO4 with space group I2/a and no other peaks for impurities were detected in XRD patterns. The peak intensity of BiVO4 B is stronger than that of BiVO4 A, which shows that the BiVO4 B may have better crystallinity than that of BiVO4 A. For the BiVO4 C, the diffraction peaks can be assigned to the JCPDS card of the tetragonal scheelite BiVO4 with space group I41/amd (blue hollow square in Fig. 1C) though there were other strong peaks detected in XRD that can be assigned to monoclinic scheelite BiVO4. The XRD analysis results show the BiVO4 C was a mixture structure of monoclinic and tetragonal scheelite and the amount of SDS is a key to synthesize pure monoclinic scheelite BiVO4.
20
30
40 50 2-Theta-scale
10
20
30
40 50 2-Theta-Scale
60
70
80
1600 1400 1200 1000 800 600 400 200 0
2.2. Characterization
60
70
80
1000 800 Intensity
The crystals were characterized by X-ray diffraction using a German Bruker D-8 Advance Diffract Meter with Cu Ka radiation ˚ The patterns were recorded between 101 (wavelength 1.54178 A). and 801 (2y), at increments of 0.021 and counting time of 0.4 s per step. The specific area of the samples was determined by a German Coulter SA 3100 Surface Area and Pore Size Analyzer. The morphology of the samples was observed in a Jeol JSM-6500F SEM. The microstructure of BiVO4 A was investigated by a Jeol MCL J-2010 TEM with an accelerating voltage 300 kV. The infrared spectra were recorded on a US Digilab FTS 7000 spectrometer using dry KBr disks of small amounts of BiVO4.
10
1800
Intensity
Initially, Bi(NO3)3 5H2O was dispersed in 50 ml pure water and the hydrolysate was white floccule. And NH4VO3 was dissolved in 50 ml 40 1C pure water at the stir of a magnetic bar. After NH4VO3 resolved and the solution cooled down to room temperature, the mixture of hydrolysate of Bi(NO3)3 was added slowly. Then the pH of the aqueous part was adjusted to 7 using 0.10 M ammonia solution. The brownish yellow mixture was added with 0.23 g SDS and transferred into the autoclave made of polytetrafluorethylene and sealed tight in the self-made boiler. The reaction was performed at 160 1C for 20 h. The BiVO4 B was prepared exactly the same as that of BiVO4 A but the SDS was 2.3 g. The sample of BiVO4 C was prepared with SDS 4.6 g in the brownish yellow mixture and the reaction conditions was also the same. The three kind of hydrothermal-treated mixtures were filtrated and washed with pure water for several times after the filtrate became colorless and transparent. After that, the powders were dried in air for 2 h at 60 1C. The reaction parameters were listed in Table 1.
600 400 200 0 10
20
30
40 50 2-Theta-scale
60
70
80
Fig. 1. The XRD patterns of the BiVO4 A, B and C synthesized by hydrothermal method.
3.2. The effect of the surfactant on the morphology of the compound Fig. 2 is the SEM images of BiVO4 A, B and C. The morphology of the BiVO4 samples is quite different. The BiVO4 A obtained at 160 1C for 20 h in the presence of 0.23 g SDS, which is its critical micelle concentration(CMC), exhibits well-developed crystal edges and facets with highly homogeneous morphology. The average size of the particles is 200 nm (Fig. 2A). For a CMC of SDS
ARTICLE IN PRESS T. Yang, D. Xia / Journal of Crystal Growth 311 (2009) 4505–4509
in the reaction (2.3 g), the small pieces evolved dramatically at the direction of the SDS micelles and almost all of them conglomerated into big spherical shape after 20 h (Fig. 2B1). Fig. 2B2 and B3 is a typical view of a self-assembling ball of the BiVO4 and the morphology evolved into surprisingly fine sphere and the diameter is 5 mm. Fig. 2B4 is a magnification image of the center of the sphere in Fig. 2B3, from which we can confirm that the nanoparticles self-assembled and inlaid into each other to form such a sphere. As a comparison, the BiVO4 obtained at the excessive amount of SDS are a mixture of crystallites of tetragonal and monoclinic scheelite and exhibits rod-shaped morphology (Fig. 2C). Generally, the size of particles was increasing at the influence of SDS amount [15,21]. It was concluded that the micelle concentration of SDS has strong influence on the particle’s
4507
morphology and shape of the BiVO4 in the self-assemble process. 3.3. TEM Fig. 3 is the TEM and SAED patterns of the nanometer BiVO4 A. TEM shows a very interesting microstructure of the particles. At the edge of the particles, lots of bumps can be observed. It is generic on all the particles of BiVO4 A and the average size of the bump is measured to be 2 nm. The SAED patterns suggested the nanoparticles are single-crystalline. The bumps cannot maintain their shape when the SDS amount extended, which is suggested that the conglomeration firstly took place at the bumps and then the bumps wedged into each other and the nanoparticles evolved to fine spheres.
Fig. 2. The SEM images of BiVO4 A, B and C.
ARTICLE IN PRESS 4508
T. Yang, D. Xia / Journal of Crystal Growth 311 (2009) 4505–4509
3.4. FT-IR analysis Fig. 4 shows the FT-IR spectra of samples BiVO4 A, B and C. The spectra of samples A and B are both characterized by a very broad and strong band at 729 cm 1 but the band of the sample C is much smaller and shifted to 671 cm 1.It is resulted from the bending vibration of the tetragonal VO4. The IR band at around 1635 and 2440 cm 1 corresponds to vibrations of adsorbed H2O and CO2 molecules. There are no typical bands between 500 and
3000 cm 1 that could be assigned to the SDS which indicates the surfactant has been removed completely out of the samples after its function as template in the self-assemble process.
4. The mechanism of the self-assemble process The mechanism of the nucleation and growth of crystal is a complex process of chemical reactions and self-assembly. Based on the facts of the crystal evolution process, the primary hypothesis of the growth and self-assembly mechanism and process of the BiVO4 has been suggested. All the ions were evenly dispersed in aqueous solution at the presence of SDS. And the conjectured chemical reactions involve the next 3 stages: Stage 1: Bi(OH)32Bi3++3OH NH4VO3-NH+4+VO3 Stage 2: VO3 +OH -VO34 +H+ NH+4+OH -H2O Stage 3: Bi3++VO34 -BiVO4k
Fig. 3. TEM and SAED patterns of BiVO4 A. The bar is 20 nm.
H++OH -H2O In the above 3 stages, the last one Bi3++VO34 -BiVO4 is the crucial step of the nucleation process. The ions of Bi3+ were covered with three times of dodecyl sulfate anion at the effect of static forces and the long hydrophobic chain is towards the outside. The SDS/water droplets act as confined-space
90 Relative Transmittance
84 82 80 78 76 74 72 70 68
500 1000 1500 2000 2500 3000 3500 4000 Wavenumber cm-1
85 80 75 70 65 60 55 50
500 1000 1500 2000 2500 3000 3500 4000 Wavenumber cm-1
85 Relative Transmittance
Relative Transmittance
86
80 75 70 65 60 55 50 45 500 1000 1500 2000 2500 3000 3500 4000 Wavenumber cm-1 Fig. 4. FT-IR spectra of BiVO4 A, B and C.
ARTICLE IN PRESS T. Yang, D. Xia / Journal of Crystal Growth 311 (2009) 4505–4509
microreactors, greatly inhibiting BiVO4 nucleus growth in the early stages of synthesis and delicately balancing the nucleation rate and crystal growth rate. Fundamentally, it is the cooperation effects of spacial position block of the long CH2 chain and the neutralization of the static charges of the Bi3+. And after 20 h of reaction, all the particles were 200750 nm crystal. And at the critical point of SDS concentration, the selfassembly phenomenon occurred. This self-conglomeration process was conjectured that the small NPs inlaid into each other to form fine spheres at the direction of the SDS templates, which needs our further investigation with in-situ SEM. 5. Conclusions We discussed the influence of the surfactant on the morphology and microstructure of monoclinic scheelite BiVO4, recorded the self-assembly process by SEM images and introduced a practical strategy for assembling pure vanadate spheres at low temperature. And this approach will therefore be an essential part for pure vanadic salts synthesis technology. Meanwhile, it also shows how the SDS molecule templates can be used to direct the components of the BiVO4 nanoparticles into larger, functional ensembles. Understanding the mechanisms and processes of selfassembly is an indispensable step in the nanotechnology, and subsequently designing more complicated systems with novel functions. Acknowledgements This work is supported by the Beijing Natural Science foundation (Grant no.207001). Funding Project for Academic
4509
Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality and the National 973 Program of China (Grant no. 2002CB211807). Additional support is from the National Outstanding Youth Fund (Project no. 10125523 to Z.W.); and the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-SW-N11, KJCX2-SWH12-02).
References [1] R.L. Whetten, J.T. Khoury, M.M. Alvarez, et al., Adv. Mater. 8 (1996) 428. [2] R. Elghanian, J.J. Storhoff, R.C. Mucic, R.L. Letsinger, C.A. Mirkin, Science 277 (1997) 1078. [3] J.J. Storhoff, R. Elghanian, R.C. Mucic, et al., J. Am. Chem. Soc. 120 (1997) 11959. [4] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607. [5] A.P. Alivisatos, K.P. Johnsson, X. Peng, T.E. Wilson, C.J. Loweth, et al., Nature 382 (1996) 609. [6] Shigeru Kohtani, Junpei Hiro, Akihiko Kudo, et al., Catal. Commun. 6 (2005) 185. [7] J.C. Boivin, G. Mairesse, Chem. Mater. 10 (1998) 2870. [8] S. Kohtani, M. Koshiko, A. Kudo, et al., Appl. Catal. B: Environ. 46 (2003) 573. [9] A. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (1999) 11459. [10] Saimi Tokunaga, Hideki Kato, Akihiko Kudo, Chem. Mater. 13 (2001) 4624. [11] A.R. Lim, S.H. Choh, M.S. Jang, J. Phys.: Condens. Matter 7 (1995) 7309. [12] Jianqiang Yu, Akihido Kudo, Adv. Funct. Mater. 16 (2006) 2163. [13] A. Galembeck, O.L. Alves, Thin Solid Films 365 (2000) 90. [14] Marcia C. Neves, Tito Trindade, Thin Solid Films 406 (2002) 93. [15] M. Gotic, S. Music, M. Ivanda, et al., J. Mol. Struct. 744 (2005) 535. [16] A.K. Bhattacharya, K.K. Mallick, A. Hartridge, Mater. Lett. 30 (1997) 7. [17] J. Yu, A. Kudo, Chem. Lett. 34 (2005) 850. [18] D.H. Franklin, I.E. Washs, J. Phys. Chem. 95 (1991) 5031. [19] M.T. Weller, Inorganic Materials Chemistry, Oxford University Press, Oxford, UK, 1994. [20] H. Harada, C. Hosoki, A. Kudo, J. Photochem. Photobiol. A: Chem. 141 (2001) 219. [21] T. Yang, D.G. Xia, G. Chen, Y. Chen, Mater. Chem. Phys. 114 (2009) 69.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Self-assembly of highly crystalline spherical BiVO4 in aqueous solutions Tao Yang, Dingguo Xia College of Environmental & Energy Engineering, Beijing University of Technology (BUT-CEEE), Chaoyang District, 100022 Beijing, PR China
a r t i c l e in fo
abstract
Article history: Received 25 September 2008 Received in revised form 30 June 2009 Accepted 11 August 2009 Communicated by H. Fujioka Available online 19 August 2009
Spherical bismuth vanadate particles are self-assembled from aqueous Bi(NO3)3 and NH4VO3 solutions by adjusting pH and tuning the amount of surfactant sodium dodecyl sulfate (SDS) via facile hydrothermal method. The BiVO4 samples were characterized by X-ray diffraction (XRD) and the peaks suited well with the pure phase monoclinic scheelite BiVO4. Field emission scanning electron microscopy (SEM) showed the average size of the spherical particles was 5 mm and the assembling stages in the hydrothermal synthesis process were recorded. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) revealed the nanoparticles were single crystal. FT-IR spectroscopy test results demonstrated there was no SDS left in the samples. The mechanism of the self-assembling has also been proposed. & 2009 Elsevier B.V. All rights reserved.
PACS: 81.10.Dn Keywords: B1. BiVO4 A2. Hydrothermal A2. Microstructure A2. Self-assemble
1. Introduction Facile assembly methods for synthesizing extended threedimensional architectures with building blocks of rare metal or composite oxide are always a major limitation in nanoparticle inorganic chemistry. Morphology, proper parameters such as size distribution and specific area and purity of the composite oxide nanoparticles, the stability of the particles are partly controllable to some extent to date [1–5]. Composite oxide bismuth vanadate is of great interest due to their versatile properties. It is a visible light responsive photocatalyst to split water into oxygen and to degradate some organic pollutants in the visible light zone [6]. Besides, it shows ferroelastic characteristics and is a possible cathode catalyst in solid oxide fuel cell [7]. BiVO4 is also a high quality pigment used in the automobile painting and house decoration industry because of its useful luminescent and environment-friendly properties. In the three structure types of bismuth vanadate–zircon tetragonal, scheelite monoclinic, and scheelite tetragonal, only the monoclinic scheelite BiVO4 has the best properties for the functions stated above [8–10]. These properties and functions strongly depend on the crystal form and morphologies, which is related to the synthesis method. A lot of work have been done to research on the synthesis of this compound, and many methods are developed and many crystal morphologies are acquired
purposely [11–20]. These methods have advantage and disadvantage aspects on preparation of different particles. The present work deals with the self-assemble process and the characterization of the fine spherical BiVO4 with monoclinic scheelite crystal type. In our synthesis conditions, all the elements were evenly dispersed in aqueous solutions and self-assembled to highly crystalline BiVO4 spontaneously and simultaneously at the effect of the template sodium dodecyl sulfate (SDS). The easy control of the temperature, pH and reaction time of the hydrothermal synthesis facilitates the reacting process of BiVO4 formation and crystal evolution, which is the biggest advantages of the hydrothermal methods compared with the traditional solid state methods. The crystal growth of the assembling stages from single-crystalline nanometer BiVO4 to fine spherical particles assisted with surfactant SDS in the hydrothermal synthesis process was recorded on the FE-SEM images. The influences of the surfactant and pH on the specific area, size distribution, microstructure and morphology were studied. However, the controlling of the three-dimensional architecture and the corresponding mechanism of the growth of the nanoparticles (NPs) need intensive endeavors to understand clearly. One possible mechanism of self-assembling process was proposed.
2. Experimental 2.1. Preparation
Corresponding author. Tel.: +86 010 67396158.
E-mail address: [email protected] (D. Xia). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.08.006
The self-assemble synthesis of BiVO4 A was performed starting from stoichiometric amounts of Bi(NO3)3 5H2O and NH4VO3.
ARTICLE IN PRESS 4506
T. Yang, D. Xia / Journal of Crystal Growth 311 (2009) 4505–4509
Table 1 The parameters and the numbers of FE-SEM images of the three samples.
BiVO4 A BiVO4 B BiVO4 C
TemratureTemperature and time
SDS amount (g)
SEM images
200nm
433K, 20 h
0.23
A
5mm
433K, 20 h
2.3
10mm
433K, 20 h
4.6
B1, B2, B3 and B4 C
1400 1200 1000 Intensity
Sample Average size
800 600 400 200 0
3. Results and discussion 3.1. X-ray characterization The diffraction peaks of the BiVO4 A and B fit well with the standard Joint Committee on Powder Diffraction Standards (JCPDS) card of the monoclinic scheelite BiVO4 with space group I2/a and no other peaks for impurities were detected in XRD patterns. The peak intensity of BiVO4 B is stronger than that of BiVO4 A, which shows that the BiVO4 B may have better crystallinity than that of BiVO4 A. For the BiVO4 C, the diffraction peaks can be assigned to the JCPDS card of the tetragonal scheelite BiVO4 with space group I41/amd (blue hollow square in Fig. 1C) though there were other strong peaks detected in XRD that can be assigned to monoclinic scheelite BiVO4. The XRD analysis results show the BiVO4 C was a mixture structure of monoclinic and tetragonal scheelite and the amount of SDS is a key to synthesize pure monoclinic scheelite BiVO4.
20
30
40 50 2-Theta-scale
10
20
30
40 50 2-Theta-Scale
60
70
80
1600 1400 1200 1000 800 600 400 200 0
2.2. Characterization
60
70
80
1000 800 Intensity
The crystals were characterized by X-ray diffraction using a German Bruker D-8 Advance Diffract Meter with Cu Ka radiation ˚ The patterns were recorded between 101 (wavelength 1.54178 A). and 801 (2y), at increments of 0.021 and counting time of 0.4 s per step. The specific area of the samples was determined by a German Coulter SA 3100 Surface Area and Pore Size Analyzer. The morphology of the samples was observed in a Jeol JSM-6500F SEM. The microstructure of BiVO4 A was investigated by a Jeol MCL J-2010 TEM with an accelerating voltage 300 kV. The infrared spectra were recorded on a US Digilab FTS 7000 spectrometer using dry KBr disks of small amounts of BiVO4.
10
1800
Intensity
Initially, Bi(NO3)3 5H2O was dispersed in 50 ml pure water and the hydrolysate was white floccule. And NH4VO3 was dissolved in 50 ml 40 1C pure water at the stir of a magnetic bar. After NH4VO3 resolved and the solution cooled down to room temperature, the mixture of hydrolysate of Bi(NO3)3 was added slowly. Then the pH of the aqueous part was adjusted to 7 using 0.10 M ammonia solution. The brownish yellow mixture was added with 0.23 g SDS and transferred into the autoclave made of polytetrafluorethylene and sealed tight in the self-made boiler. The reaction was performed at 160 1C for 20 h. The BiVO4 B was prepared exactly the same as that of BiVO4 A but the SDS was 2.3 g. The sample of BiVO4 C was prepared with SDS 4.6 g in the brownish yellow mixture and the reaction conditions was also the same. The three kind of hydrothermal-treated mixtures were filtrated and washed with pure water for several times after the filtrate became colorless and transparent. After that, the powders were dried in air for 2 h at 60 1C. The reaction parameters were listed in Table 1.
600 400 200 0 10
20
30
40 50 2-Theta-scale
60
70
80
Fig. 1. The XRD patterns of the BiVO4 A, B and C synthesized by hydrothermal method.
3.2. The effect of the surfactant on the morphology of the compound Fig. 2 is the SEM images of BiVO4 A, B and C. The morphology of the BiVO4 samples is quite different. The BiVO4 A obtained at 160 1C for 20 h in the presence of 0.23 g SDS, which is its critical micelle concentration(CMC), exhibits well-developed crystal edges and facets with highly homogeneous morphology. The average size of the particles is 200 nm (Fig. 2A). For a CMC of SDS
ARTICLE IN PRESS T. Yang, D. Xia / Journal of Crystal Growth 311 (2009) 4505–4509
in the reaction (2.3 g), the small pieces evolved dramatically at the direction of the SDS micelles and almost all of them conglomerated into big spherical shape after 20 h (Fig. 2B1). Fig. 2B2 and B3 is a typical view of a self-assembling ball of the BiVO4 and the morphology evolved into surprisingly fine sphere and the diameter is 5 mm. Fig. 2B4 is a magnification image of the center of the sphere in Fig. 2B3, from which we can confirm that the nanoparticles self-assembled and inlaid into each other to form such a sphere. As a comparison, the BiVO4 obtained at the excessive amount of SDS are a mixture of crystallites of tetragonal and monoclinic scheelite and exhibits rod-shaped morphology (Fig. 2C). Generally, the size of particles was increasing at the influence of SDS amount [15,21]. It was concluded that the micelle concentration of SDS has strong influence on the particle’s
4507
morphology and shape of the BiVO4 in the self-assemble process. 3.3. TEM Fig. 3 is the TEM and SAED patterns of the nanometer BiVO4 A. TEM shows a very interesting microstructure of the particles. At the edge of the particles, lots of bumps can be observed. It is generic on all the particles of BiVO4 A and the average size of the bump is measured to be 2 nm. The SAED patterns suggested the nanoparticles are single-crystalline. The bumps cannot maintain their shape when the SDS amount extended, which is suggested that the conglomeration firstly took place at the bumps and then the bumps wedged into each other and the nanoparticles evolved to fine spheres.
Fig. 2. The SEM images of BiVO4 A, B and C.
ARTICLE IN PRESS 4508
T. Yang, D. Xia / Journal of Crystal Growth 311 (2009) 4505–4509
3.4. FT-IR analysis Fig. 4 shows the FT-IR spectra of samples BiVO4 A, B and C. The spectra of samples A and B are both characterized by a very broad and strong band at 729 cm 1 but the band of the sample C is much smaller and shifted to 671 cm 1.It is resulted from the bending vibration of the tetragonal VO4. The IR band at around 1635 and 2440 cm 1 corresponds to vibrations of adsorbed H2O and CO2 molecules. There are no typical bands between 500 and
3000 cm 1 that could be assigned to the SDS which indicates the surfactant has been removed completely out of the samples after its function as template in the self-assemble process.
4. The mechanism of the self-assemble process The mechanism of the nucleation and growth of crystal is a complex process of chemical reactions and self-assembly. Based on the facts of the crystal evolution process, the primary hypothesis of the growth and self-assembly mechanism and process of the BiVO4 has been suggested. All the ions were evenly dispersed in aqueous solution at the presence of SDS. And the conjectured chemical reactions involve the next 3 stages: Stage 1: Bi(OH)32Bi3++3OH NH4VO3-NH+4+VO3 Stage 2: VO3 +OH -VO34 +H+ NH+4+OH -H2O Stage 3: Bi3++VO34 -BiVO4k
Fig. 3. TEM and SAED patterns of BiVO4 A. The bar is 20 nm.
H++OH -H2O In the above 3 stages, the last one Bi3++VO34 -BiVO4 is the crucial step of the nucleation process. The ions of Bi3+ were covered with three times of dodecyl sulfate anion at the effect of static forces and the long hydrophobic chain is towards the outside. The SDS/water droplets act as confined-space
90 Relative Transmittance
84 82 80 78 76 74 72 70 68
500 1000 1500 2000 2500 3000 3500 4000 Wavenumber cm-1
85 80 75 70 65 60 55 50
500 1000 1500 2000 2500 3000 3500 4000 Wavenumber cm-1
85 Relative Transmittance
Relative Transmittance
86
80 75 70 65 60 55 50 45 500 1000 1500 2000 2500 3000 3500 4000 Wavenumber cm-1 Fig. 4. FT-IR spectra of BiVO4 A, B and C.
ARTICLE IN PRESS T. Yang, D. Xia / Journal of Crystal Growth 311 (2009) 4505–4509
microreactors, greatly inhibiting BiVO4 nucleus growth in the early stages of synthesis and delicately balancing the nucleation rate and crystal growth rate. Fundamentally, it is the cooperation effects of spacial position block of the long CH2 chain and the neutralization of the static charges of the Bi3+. And after 20 h of reaction, all the particles were 200750 nm crystal. And at the critical point of SDS concentration, the selfassembly phenomenon occurred. This self-conglomeration process was conjectured that the small NPs inlaid into each other to form fine spheres at the direction of the SDS templates, which needs our further investigation with in-situ SEM. 5. Conclusions We discussed the influence of the surfactant on the morphology and microstructure of monoclinic scheelite BiVO4, recorded the self-assembly process by SEM images and introduced a practical strategy for assembling pure vanadate spheres at low temperature. And this approach will therefore be an essential part for pure vanadic salts synthesis technology. Meanwhile, it also shows how the SDS molecule templates can be used to direct the components of the BiVO4 nanoparticles into larger, functional ensembles. Understanding the mechanisms and processes of selfassembly is an indispensable step in the nanotechnology, and subsequently designing more complicated systems with novel functions. Acknowledgements This work is supported by the Beijing Natural Science foundation (Grant no.207001). Funding Project for Academic
4509
Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality and the National 973 Program of China (Grant no. 2002CB211807). Additional support is from the National Outstanding Youth Fund (Project no. 10125523 to Z.W.); and the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-SW-N11, KJCX2-SWH12-02).
References [1] R.L. Whetten, J.T. Khoury, M.M. Alvarez, et al., Adv. Mater. 8 (1996) 428. [2] R. Elghanian, J.J. Storhoff, R.C. Mucic, R.L. Letsinger, C.A. Mirkin, Science 277 (1997) 1078. [3] J.J. Storhoff, R. Elghanian, R.C. Mucic, et al., J. Am. Chem. Soc. 120 (1997) 11959. [4] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607. [5] A.P. Alivisatos, K.P. Johnsson, X. Peng, T.E. Wilson, C.J. Loweth, et al., Nature 382 (1996) 609. [6] Shigeru Kohtani, Junpei Hiro, Akihiko Kudo, et al., Catal. Commun. 6 (2005) 185. [7] J.C. Boivin, G. Mairesse, Chem. Mater. 10 (1998) 2870. [8] S. Kohtani, M. Koshiko, A. Kudo, et al., Appl. Catal. B: Environ. 46 (2003) 573. [9] A. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (1999) 11459. [10] Saimi Tokunaga, Hideki Kato, Akihiko Kudo, Chem. Mater. 13 (2001) 4624. [11] A.R. Lim, S.H. Choh, M.S. Jang, J. Phys.: Condens. Matter 7 (1995) 7309. [12] Jianqiang Yu, Akihido Kudo, Adv. Funct. Mater. 16 (2006) 2163. [13] A. Galembeck, O.L. Alves, Thin Solid Films 365 (2000) 90. [14] Marcia C. Neves, Tito Trindade, Thin Solid Films 406 (2002) 93. [15] M. Gotic, S. Music, M. Ivanda, et al., J. Mol. Struct. 744 (2005) 535. [16] A.K. Bhattacharya, K.K. Mallick, A. Hartridge, Mater. Lett. 30 (1997) 7. [17] J. Yu, A. Kudo, Chem. Lett. 34 (2005) 850. [18] D.H. Franklin, I.E. Washs, J. Phys. Chem. 95 (1991) 5031. [19] M.T. Weller, Inorganic Materials Chemistry, Oxford University Press, Oxford, UK, 1994. [20] H. Harada, C. Hosoki, A. Kudo, J. Photochem. Photobiol. A: Chem. 141 (2001) 219. [21] T. Yang, D.G. Xia, G. Chen, Y. Chen, Mater. Chem. Phys. 114 (2009) 69.