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Preparation, crystallization and properties of Bi2WO6 nanoparticles
Journal Pre-proof Preparation, crystallization and properties of Bi2 WO6 nanoparticles Zhenqian Zhang (Conceptualization) (Methodology) (Software) (Supervision) (Writing - review and editing), Yongzhou Lin (Data curation) (Writing - original draft) (Software) (Formal analysis) (Investigation) (Resources) (Validation) (Writing - original draft), Fang Liu (Visualization) (Investigation)
PII:
S0927-7757(20)30085-6
DOI:
https://doi.org/10.1016/j.colsurfa.2020.124493
Reference:
COLSUA 124493
To appear in:
Colloids and Surfaces A: Physicochemical and Engineering Aspects
Received Date:
14 November 2019
Revised Date:
18 January 2020
Accepted Date:
20 January 2020
Please cite this article as: Zhang Z, Lin Y, Liu F, Preparation, crystallization and properties of Bi2 WO6 nanoparticles, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124493
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Preparation, crystallization and properties of Bi2WO6 nanoparticles
Zhenqian Zhang*; Yongzhou Lin; Fang Liu School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, Changzhou University, Jiangsu Changzhou 213164, China
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Corresponding author: Zhang Zhenqian; E-mail: [email protected]. Tel: +8651986330096; Fax: +8651986330095
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Graphical abstract
Abstract: Bismuth tungstate (Bi2WO6)-sodium polyacrylate (PAANa) latex particles were prepared by
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inverse miniemulsion and hydrothermal treatment. The effects of Bi(NO3)3/Na2WO4 dosage ratio and different hydrothermal treatment conditions on the particle size and morphology of Bi2WO6-PAANa
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latex particles were studied. The results show that the stability of the inverse microemulsion droplets was effectively improved with the increase of the soluble salt in the inverse microemulsion system, and the effect of different hydrothermal treatment on the particle size of the colloidal particles was not significant. During the formation of Bi2WO6 nanoparticles, PAANa latex particles could become a reaction site for forming Bi2WO6 nanoparticles, and Bi2WO6 nanoparticles were dispersed in the polymer. The effects of
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different hydrothermal treatment conditions on the size, crystal form and light absorption properties of Bi2WO6 nanocrystals were studied. The results show that Bi2WO6 nanoparticles with orthorhombic tungsten-rhenium ore structure could be prepared under different hydrothermal treatment conditions; moreover, the temperature at which the hydrothermal treatment forms the lowest Bi 2WO6 nanocrystals was 130 °C. But at pH 9, the product was no longer pure phase. The prepared nanocrystals simultaneously exhibited strong ultraviolet-visible absorption characteristics and luminescence intensity. Finally, the
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Bi2WO6 nanoparticles with photoluminescence properties could be successfully prepared by the inverse
miniemulsion method.
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Key words: Inverse miniemulsion; Hydrothermal treatment; Bi2WO6; Nanoparticle
1 Introduction
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In recent years, with the increasing energy crisis and environmental pollution, people have begun to seek an energy source for environmentally sustainable use. Among them, tungstate has the
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perfect performance on luminous efficiency, and anti-radiation damage capability and luminescence, which attracts the attention on solar energy utilization [1-4]. Bismuth tungstate (Bi2WO6) belongs to
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an important class of semiconductor oxides and has excellent physical properties and chemical properties, such as: ferroelectric
[5],
piezoelectricc
[6],
pyroelectric
[7],
photoluminescence
[8],
nonlinear dielectric induction [9], photocatalysis [10, 11] and others Bi2WO6 has an appropriate band gap and excellent light absorption characteristics for visible light [12]. The formation methods of Bi2WO6 which have been reported so far include high temperature 2
solid phase synthesis
[13]
, microwave assisted heat method
[14]
, ion exchange method
[15]
, sol-gel
method [16], co-precipitation method [17], solvothermal method [18, 19], electro-spinning method [20,21], ultrasonic spray pyrolysis hydrothermal method
[22],
[25, 26]
ultrasonic chemical method
[23],
sucrose template method
[24],
and so on. The Bi2WO6 nanocrystals obtained from the different
preparation methods result in the different structures, particle sizes, morphology and other properties. For example, the smaller the particle size is; the shorter for electrons and holes migrate
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to the surface. However, the Bi2WO6 nanocrystals prepared by most of the above preparation
methods have large particle size and broad dispersion, and the Bi2WO6 nanocrystals obtained have the shape such as a bird's nest, a solid ball flower, an irregular sheet and so on. Yang et al
[27]
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synthesized spherical Bi2WO6 nanoparticles with an average particle size of about 85 nm by
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hydrothermal method, and the band gap was 2.93 eV. R Kalai Selvan et al prepared Bi2WO6 [23],
so that
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nanoparticles with strong electrochemical properties by ultrasonic chemical method
Bi2WO6 could be used as a suitable negative electrode material for supercapacitors. Liu et al used a
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flux growth method to synthesize a single crystal of the bismuth layered-perovskite ferroelectric oxide Bi2WO6, and exhibited rich optical properties and large spectral response, making it a
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candidate material for mid-IR NLO [28].
In general, the stable liquid/liquid dispersion system composed of submicron (50-500nm)
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droplets is called "miniemulsion", and the corresponding polymerization is called "miniemulsion polymerization" [29]. Zhang et al prepared CdFe2O4-polymer nanoparticles by inverse miniemulsion, the films prepared by spin coating at lower temperature showed good spinel structure, and the CdFe2O4-polymer films also showed high reactivity
[30].
Therefore, the miniemulsion as an ideal
method has received a great attention on nano materials preparation [31]. 3
In this paper, Bi2WO6-Poly sodium acrylate (PAANa) latex particles were prepared by inverse miniemulsion and then are hydrothermal treated. Compared with other methods
[32],
the Bi2WO6
was synthesized in the process of miniemulsion polymerization; crystal Bi2WO6 was obtained from the hydrothermal treatment at the relative lower temperature, and the prepared nanoparticles had the advantages of uniform dispersion and small in size. The effects of the bismuth nitrate/ sodium tungstate dosage ratio (Bi(NO3)3/Na2WO4, Bi/W) and the hydrothermal treatment conditions on
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particles size, particles distribution and morphology in the process of colloid formation were investigated. The effects of temperature, time and pH of hydrothermal treatment on the stability of
colloidal particles were under consideration. The size, crystal form and light absorption properties
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of Bi2WO6 nanocrystals were also discussed.
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2 Experimental
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2.1 Materials
Sodium acrylate (NaAA): chemically pure, Saan Chemical Technology (Shanghai) Co., Ltd.
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N-heptane (CH): analytical grade, Shanghai Lingfeng Chemical Reagent Co., Ltd. Sorbitan monooleate (Span-80): analytical grade, Sinopharm Chemical Reagent Co., Ltd. Bismuth nitrate
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pentahydrate (Bi(NO3)3•5H2O): analytically pure, Sinopharm Chemical Reagent Co., Ltd. Nitrogen diisobutyronitrile (AIBN): analytical grade, Shanghai Test Sihewei Chemical Co., Ltd. Sodium
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tungstate dihydrate (Na2WO4•2H2O): analytical grade, Sinopharm Chemical Reagent Co., Ltd. Anhydrous ethanol: analytically pure, Sinopharm Chemical Reagent Co., Ltd. Other reagents are commercially available and used directly. 2.2 Preparation of Bi2WO6-PAANa Preparation of Bi(NO3)3miniemulsion: 0.150 g of Bi(NO3)3•5H2O was weighed and dissolved 4
in 8.000 g of 10% (mass concentration) diluted nitric acid to prepare an aqueous phase; 2.100 g of Span-80 was dissolved in 40 g of N-heptane and stirred for 10 min to prepare an oil phase. The aqueous phase was slowly poured into the oil phase for magnetic stirring for 15 min to prepare a Bi(NO3)3 pre-emulsion. It was then placed in an ultrasonic cell pulverizer for 5 min at 5 °C to obtain a stable Bi(NO3)3 miniemulsion. Preparation of Na2WO4-NaAA miniemulsion: A certain amount of Na2WO4•2H2O and 2.000
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g of NaAA were weighed and dissolved in 8.000 g of deionized water, and uniformly stirred on a
thermostatic magnetic stirrer for 15 min to completely dissolve to prepare an aqueous phase. 2.100 g of Span-80 was dissolved in 40 g of N-heptane and stirred for 10 min to prepare an oil phase. The
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aqueous phase was slowly poured into the oil phase for magnetic stirring for 15 min to prepare a
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pre-emulsion, which was then placed in an ultrasonic cell pulverizer for 5 min at 5 °C to obtain a
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Na2WO4-NaAA miniemulsion.
Preparation of Bi2WO6-PAANa latex: The mixture of the Bi(NO3)3 miniemulsion and the
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Na2WO4-NaAA miniemulsion prepared above was added 0.120 g of AIBN, stirred evenly at 65 °C under N2 protection and then reacted 4 h to gain Bi2WO6-PAANa latex. Several components of latex
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particles were prepared as described above. Table 1 shows the experimental formula for preparing latex particles by changing the amount of Na2WO4•2H2O added.
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Hydrothermal treatment of Bi2WO6-PAANa latex: A 30 mL latex was poured into a 100mL
Teflon-lined-autoclave. Table 2 is the specific experimental formula for the hydrothermal reaction by adjusting the pH value, the hydrothermal treatment temperature and the hydrothermal treatment time by using the latex particles prepared by Exp No. 4 in Table 1 as the experimental basis. After completing treatment, the Bi2WO6-PAANa latex samples were naturally cooled to room temperature. 5
Table 1 Experimental Formulation for Preparation of Bi2WO6-PAANa Latex Particles Exp No.
Bi/W* (g/g)
NaAA (g)
H2O (g)
Span-80 (g)
N-heptane (g)
AIBN (g)
1
0.150/0.026
2.000
8.000×2
2.100×2
40×2
0.120
2
0.150/0.051
2.000
8.000×2
2.100×2
40×2
0.120
3
0.150/0.077
2.000
8.000×2
2.100×2
40×2
0.120
4
0.150/0.102
2.000
8.000×2
2.100×2
40×2
0.120
*Dosage ratio of Bi(NO3)3·5H2O to Na2WO4·2H2O
Temperature (ºC)
pH
Time (h)
5
0.150/0.102
120
7
12
6
0.150/0.102
130
7
12
7
0.150/0.102
160
7
8
0.150/0.102
180
7
9
0.150/0.102
160
7
10
0.150/0.102
160
7
11
0.150/0.102
160
12
0.150/0.102
160
13
0.150/0.102
160
14
0.150/0.102
160
12 12 6
15
7
18
7
24
4
12
9
12
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2.3 Characterization
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Bi/W (g/g)
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Exp No.
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Table 2 Experimental Formulation of Bi2WO6 Nanoparticles Prepared by Hydrothermal Treatment
The diluted droplets (diluted with a drop of 5 mL of N-heptane) or the Z-average particle sizes
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(Dz) and intensity distribution (PDI) of the latex particles were tested by a British Mastersizer-2000 laser particle size analyzer.
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The diluted sample (diluted a drop of emulsion sample with 5 mL of n-heptane) was added
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dropwise to a copper mesh and dried at room temperature for 24 h, and the morphology of the particles was observed by a Japanese JEM-2100 high-resolution transmission electron microscope. After different hydrothermal treatment conditions, the sample was centrifuged, washed several
times and then dried, repeated treatment to remove PAANa to obtain Bi2WO6 nanoparticles, and phase structure was determined by X-ray powder diffractometer manufactured by Bruker Corporation of the USA. 6
UV absorption characteristics were tested using an ultraviolet-visible spectrophotometer from Shimadzu, Japan. The photoluminescence properties of the samples were tested using a fluorometer (LS45) instrument with a wavelength accuracy of ±1 nm. 3 Results and discussion 3.1 Effect of Na2WO4 dosage and formation process on particle size of synthetic latex particles In the miniemulsion, the soluble salt can be used as a co-stabilizer to delay the Ostwald ripening so that the fine emulsion prepared in Exp No. 1-4 exhibits no delamination. Fig.1 is a graph
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[29],
showing the particle size and distribution of Bi2WO6-PAANa latex particles synthesized in the experimental formulation Exp No. 4. Table 3 shows the effect of varying the amount and formation
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of Na2WO4 on the particle size of the synthesized Bi2WO6-PAANa latex particles. Fig.2 is a
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schematic diagram of the reaction of preparing Bi2WO6-PAANa latex particles by inverse
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miniemulsion method.
Table 3 Particle size and distribution of Bi2WO6-PAANa latex particles Bi/W (g/g)
1 2 3 4 5 6 7 8 9
0.150/0.026 0.150/0.051 0.150/0.077 0/0.102 0.150/0 0.150/0.102 0.150/0.102 0.150/0.102 0.150/0.102
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Temperature (C) 65 65 65 ---65 65 65
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Sample No.
7
Time (h)
Dz (nm)
PDI
4 4 4 ---1 3 4
431.4 403.2 368.7 255.0 229.4 235.2 260.8 357.5 339.3
0.204 0.181 0.236 0.234 0.157 0.222 0.214 0.216 0.202
Fig.1 Particle size and distribution of Bi2WO6-PAANa latex particles during formation (4: Na2WO4-NaAA miniemulsion; 5: Bi(NO3)3 miniemulsion; 6: Mixed miniemulsion; 7:
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Preliminary formation of Bi2WO6-PAANa nanoparticles after 1 h of reaction; 8: Partially
formation of Bi2WO6-PAANa nanoparticles after 2 h of reaction; 9: Final formation of Bi2WO6PAANa nanoparticles after 4 h of reaction.)
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As could be seen from Table 3, all the samples in the table were latex particles prepared by
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mixing and dispersing a Na2WO4-NaAA miniemulsion with a Bi(NO3)3 miniemulsion. With the
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increase of the amount of Na2WO4 added, the particle size data of the latex particles using the DLS test sample showed a trend of decreasing, indicating that the stability of the monomer droplets
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gradually became better as the amount of soluble salt increased. Shinoda et al [33] and Florence et al [34] studied that the amount of added inorganic salt would slightly decrease the lipophilic-hydrophilic
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balance (HLB), but improve the stability of the emulsion. In Fig. 1 and Table 3, the Dz of Na2WO4-NaAA droplets (Sample No. 4) was 255.0 nm, and
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the PDI was 0.234. The Dz of the Bi(NO3)3 droplet (Sample No. 5) was 229.4 nm, and the PDI was 0.157. The mixed miniemulsion (Sample No. 6) was with the mixture of Na2WO4-NaAA miniemulsion (Sample No. 4) and Bi(NO3)3 miniemulsion (Sample No. 5). The mixed miniemulsion had a Z-average particle size (Dz) of 235.2 nm and a PDI of 0.222. The Bi(NO3)3 and Na2WO4NaAA droplets were separately stable due to the repressed Ostwald ripening at room temperature 8
[35]
. When the polymerization time reached 1 h, the initial formation of Bi2WO6-PAANa colloidal
particles (Sample No. 7) had a Dz of 260.8 nm and a PDI of 0.214. It indicates that some pseudostable droplets may be absorbed by the relative stable polymerized latex particles at 65 °C. This process is carried out though a phase transfer way, due to the improved water solubility in hydrocarbon at high temperature
[36-38].
Some Bi2WO6 were synthesized by the reaction between
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Bi(NO3)3 and Na2WO4 in absorbing latex particles. As the polymerization time simultaneously increased to 3 h, the particle size of Bi2WO6-PAANa colloidal particles (Sample No. 8) increased
to 357.5 nm, and PDI became 0.216. The increased size of Bi2WO6-PAANa colloidal particles was
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result of the absorbing process to a fully degree. When the polymerization time was increased to 4
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h, the Bi2WO6-PAANa colloid (Sample No. 9) had a Dz of 339.3 nm and a PDI of 0.202. Therefore,
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the mixture of Bi(NO3)3miniemulsion and Na2WO4-NaAA miniemulsion are successful to form mixed to form Bi2WO6 contained PAANa latex particles.
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In the process of forming latex particles, stable latex particles are formed and droplet size becomes smaller, which effectively improves the stability of Bi2WO6-PAANa colloid
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formation process of Bi2WO6-PAANa latex particles is shown in Fig. 2.
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[39].
The
Fig. 2 Schematic diagram of the formation of Bi2WO6-PAANa latex prepared by inverse miniemulsion method 3.2 Effect of different hydrothermal treatment conditions on particle size of synthesized latex Fig. 3 is a graph showing the particle size and distribution of latex particles at different pH values, temperatures and times. Table 4 is a table of particle size and distribution data tested by DLS
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after different hydrothermal treatment conditions.
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Fig. 3 Particle size and distribution of latex particles formed under different hydrothermal treatment conditions
Table 4 Particle size and distribution of latex particles formed by hydrothermal treatment Sample No.
Bi/W (g/g)
Temperature (ºC)
pH
Time (h)
Dz (nm)
PDI
10
0.150/0.102
120
7
12
384.6
0.144
11
0.150/0.102
130
7
12
396.1
0.285
10
12
0.150/0.102
160
7
12
403.6
0.186
13
0.150/0.102
180
7
12
423.8
0.313
14
0.150/0.102
160
7
6
412.0
0.235
15
0.150/0.102
160
7
15
395.5
0.225
16
0.150/0.102
160
7
18
406.2
0.215
17
0.150/0.102
160
7
24
416.5
0.246
18
0.150/0.102
160
4
12
379.1
0.110
19
0.150/0.102
160
9
12
404.5
0.193
As shown in Fig. 3 and Table 4, with the increase of pH value, hydrothermal treatment
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temperature and hydrothermal treatment time, the particle size of the 10 groups of samples did not change much, indicating that the hydrothermal treatment had little effect on the particle size of the
synthesized latex particles. When the ratio of Bi(NO3)3/Na2WO4 was 0.150/0.102, the latex particle
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Dz was 339.3 nm and the PDI was 0.202 before hydrothermal treatment; however, the
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hydrothermally treated latex particles had larger particle size than the unhydro-treated latex particles, which attributed to trace agglomeration of latex particles.
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3.3 Morphology of particles before and after hydrothermal treatment Fig. 4 is the TEM image of Bi2WO6-PAANa latex particles prepared by formula Exp No.4 and
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Bi2WO6 nanoparticles obtained from hydrothermal treatment by formula Exp No. 7.
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Fig. 4 TEM image of Bi2WO6-PAANa latex particles (a: Before hydrothermal treatment (Exp No.
c: PAANa free)
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4 in Table 1); b: After hydrothermal treatment (Exp No. 7 in Table 2)) and Bi2WO6 nanoparticles;
Fig. 4a shows the TEM image of Bi2WO6-PAANa latex particles prepared without
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hydrothermal treatment, the particle size of the latex particles was about 320 nm, and the particle
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size of the Bi2WO6-PAANa latex particles was slightly smaller than that measured by DLS, the
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possible reason was that the sample needed to be dried at a certain temperature before testing the TEM. Fig. 4b shows the Bi2WO6-PAANa latex particles observed after hydrothermal treatment. Fig.
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4c shows that the Bi2WO6 nanoparticles were PAANa free and exhibited a dispersed state, which size was 10-20 nm. Therefore, PAANa latex particles could be used as nanoreactors for forming
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Bi2WO6 nanoparticles. However, Yang’s research groups have done Bi2WO6 nanoparticles with aggregation phenomenon
[11].
The present work is making Bi2WO6 nanoparticles with good
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dispersion effect, which is very different from Yang’s work, and this is also the major contribution of our work.
3.4 XRD analysis of Bi2WO6 nanoparticles Fig. 5 is a Bi2WO6-PAANa latex particle prepared from Experimental Formula Exp No. 5-14, which was post-treated to obtain an XRD pattern of Bi2WO6 nanoparticles. Table 5 shows the grain 12
PDF # 73-1126
(200)
b
(139) (420)
(313) (226)
(220)
180 °C
180 °C
160 °C
160 °C
130 °C
130 °C
120 °C
120 °C Before hydrothermal treatment
Before hydrothermal treatment 30
40
50
60
80 20
70
25
(113)
d
(139) (420)
(313) (226)
(220)
24 h
35
40
24 h 18 h
15 h
15 h
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18 h
-p
(113)
PDF # 73-1126 (200)
c
30
2θ / °
2θ / °
(200)
20
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(200)
a
(113)
(113)
size and structure of Bi2WO6 nanoparticles synthesized under different treatment conditions.
12 h
12 h
6h
6h
80 20
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(139) (420)
(313) (226)
Bi2WO6 Bi0.875W0.125O1.6875
pH = 7
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pH = 4
pH = 4
30
40
50 2θ / °
60
70
80 20
35
25
30 2θ / °
Fig. 5 XRD patterns of Bi2WO6 crystals Table 5 Grain size and structure of Bi2WO6 nanoparticles synthesized 13
40
pH = 9
pH = 7
20
30 2θ / °
(200)
PDF # 73-1126 PDF # 85-1268
25
(113)
70
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pH = 9
60
(220)
e
50 2θ / °
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40
(200)
30
(113)
20
35
40
under different treatment conditions Treatment conditions Sample No.
D (nm)
Structure
pH
Time (h)
10
120
7
12
--
Amorphous
11
130
7
12
17.6
Orthorhombic system
12
160
7
12
18.3
Orthorhombic system
13
180
7
12
18.5
Orthorhombic system
14
160
7
6
18.2
Orthorhombic system
15
160
7
15
18.5
Orthorhombic system
16
160
7
18
20.1
Orthorhombic system
17
160
7
24
21.9
18
160
4
12
20.2
19
160
9
12
20.3
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Temperature (ºC)
Orthorhombic system
Orthorhombic system Orthorhombic system
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The Bi2WO6-PAANa latex particles were prepared by inverse miniemulsion, and the XRD
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patterns of Bi2WO6 crystals were obtained under different hydrothermal treatment conditions， and then post-treated. The peaks of all the spectra in the figure showed characteristic diffraction peaks
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at about 2θ=28°, 32°, 47°, 55°, 58°, 75°, and 78°, which were consistent with the characteristic diffraction peak position of the orthorhombic barium tungstate standard card PDF=73-1126. It was
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indicated that Bi2WO6 nanocrystals could be formed after hydrothermal treatment. It could be clearly observed from Fig. 5(a) that the sample which had not been hydrothermally treated had no
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diffraction peak in its spectrum, it was indicated that Bi2WO6 crystal could not be formed without
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hydrothermal treatment. Among them, when the hydrothermal treatment temperature was 120 °C, a diffraction peak appears at 2θ = 28°, and the intensity of the diffraction peak was weak, it was indicated that the product under this condition was amorphous and no Bi2WO6 crystal was formed. When the temperature rose to 130 °C, a weak diffraction peak could be seen, it was indicated that the temperature of 130 °C was the lowest temperature at which the system formed Bi2WO6 nanocrystals; at this time, the crystal structure growth was incomplete and the crystallinity was low. 14
The results showed that as the hydrothermal treatment temperature increased, the intensity of the diffraction peaks increased and the crystallinity increased. According to the Debye-Scherrer formula [30],
the average grain size under hydrothermal treatment temperature was about 18.1 nm. It could
be observed from the (131) crystal plane in Fig. 5(b) that the intensity of the diffraction peak relatively increased relatively rapidly, it showed that the growth trend of Bi2WO6 crystal became obvious with the increase of hydrothermal treatment temperature, and the crystal grains grow
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gradually. As shown in Fig. 5(c) and 5(d), as the hydrothermal treatment time was gradually increased, the intensity of the diffraction peak of the Bi2WO6 nanocrystals on the (131) crystal plane
was continuously enhanced, and the half-height width was gradually narrowed. When the
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hydrothermal treatment time was 24 h, the half-height width of the (131) crystal plane was the
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narrowest, and the average grain size under hydrothermal treatment time was about 19.4 nm, it
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showed that the increase of hydrothermal treatment time leads to the crystallization of the product gradually deepening, the crystallinity was increased, and the grain size was enlarged. In Fig. 5(e), it
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could be observed that the product prepared at pH 4 and 7 was pure Bi2WO6 crystal. When the pH value was adjusted to 9, impurity peaks appeared on the spectra, the impurity peaks were
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Bi0.875W0.125O1.6875, which was consistent with the standard card PDF=85-1268. It could be seen from Fig. 5(f) that the pure phase Bi2WO6 crystal was prepared at pH = 7, and the diffraction peak
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at the (131) crystal plane was sharp and clear, the product crystallinity was high, and no other peak appears, it was indicated that a product with good purity could be obtained at pH = 7 and the obtained grain size was 19.6 nm in Table 5. 3.5 Ultraviolet visible light analysis and photoluminescence analysis of Bi2WO6 nanoparticles. Fig. 6 show the UV-visible analysis of Bi2WO6 nanoparticles prepared under different 15
25
hydrothermal treatment conditions (Table 2) to characterize their light absorption characteristics and
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photoluminescence.
25
b1 20
6h 12 h 15 h 18 h 24 h
15 10
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(αhν)2 [(cm-1·eV)2]
Abs (a.u.)
6h 12 h 15 h 18 h 24 h
-p
b
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300
350
400 450 500 Wavelength (nm)
600
300
d
350
400 450 500 Wavelength (nm)
550
2.5
3.0
3.5
5.0
4.0
4.5
5.0
(αhν)2 [(cm-1·eV)2]
pH = 4 pH = 7 pH = 9
15 10 5 0 2.0
600
2.5
3.0
3.5 hν (eV)
e
24 h
Intensity (a.u.)
160 °C
Intensity (a.u.)
4.5
c1
180 °C
130 °C
4.0
hν (eV)
20
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0 2.0
25
pH = 4 pH = 7 pH = 9
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c
Abs (a.u.)
550
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250
5
16
18 h
15 h 12 h 6h
f pH = 9
Intensity (a.u.)
pH = 7
350
400
450
500
550
600
-p
Wavelength (nm)
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pH = 4
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Fig.6 UV and PL diagram of Bi2WO6 nanoparticles
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As shown in Fig. 6 (a-c), the UV-visible characterization of Bi2WO6 nanoparticles prepared by four different hydrothermal treatment temperatures, five different hydrothermal treatment times and
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three different pH values was carried out. It was thus known that the synthesized Bi2WO6 nanoparticles had strong absorption in the visible light region and the ultraviolet light region, and
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all components had steep absorption boundary phenomena in the ultraviolet visible spectrum, it was indicated that the Bi2WO6 nanoparticle product itself was not caused by the transition of the impurity
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level in the absorption of visible light but by the electronic transition of the band gap energy level structure. According to the Tauc equation
[20],
the forbidden band width of Bi2WO6 nanoparticles
could be calculated from the ultraviolet-visible spectrogram data as shown in Fig. 6 (a1), 6 (b1) and 6 (c1). It could be seen from Fig. 6a and a1 that when the hydrothermal treatment temperature was 120 °C, the absorption boundary of the spectral line was blue-shifted, it was suggested that there 17
was no formation of Bi2WO6 crystal under this condition, and the product was still in an amorphous state. It was indicated that the crystal growth of Bi2WO6 was gradually complete with the increase of hydrothermal treatment temperature, the crystallinity was continuously increased. It could be seen from Fig. 6(b) and (b1) that the products prepared under different hydrothermal treatment times had no significant difference in light absorption, and a certain concentration of the system, as the hydrothermal treatment time increased, the grain size of the product gradually grew; it indicated
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that the prolongation of hydrothermal treatment time had a certain influence on the forbidden band width. Fig. 6c and c1 showed that when the pH value was 9, the line was red-shifted, and the forbidden band width of the product was slightly weakened, the possible cause was due to the fact
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that the product was not pure phase.
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As shown in Fig. 6 (d-f), the atoms on the luminescent substrate in the Bi2WO6 nanocrystals
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were excited by the effective collision of the excitation light particles, and the ionized free electrons had a certain energy, which would cause excitation ionization after colliding with other atoms. When [40].
The
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the excited or ionized atoms returned to the steady state, luminescence occurs
photoluminescence spectra of Bi2WO6 nanoparticles synthesized under different hydrothermal
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treatment conditions at 300 nm excitation wavelength were shown. It could be observed that all products had a blue-violet luminescence peak at 424 nm, possibly due to charge transfer transitions
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in the WO42- ion [41]. At the same time, the peak appearing at 486 nm might be caused by defects or impurities in the product itself. It could also be seen from the figure that the luminescence intensity of the product was the highest at T = 180 °C, t = 24 h or pH = 9. The XRD pattern of the bound product could be obtained, with the increase of temperature, time or pH value, the intensity of the diffraction peak of Bi2WO6 nanocrystals on the (131) plane was gradually enhanced, the crystal 18
plane would grow preferentially, and the crystal form would gradually develop and complete. The proportion of atoms on the surface of the nanocrystals was increased, the number of collisions between the extra nuclear electrons of the luminescent atom and the excitation light particles is increased, and the luminescence intensity was enhanced. 4 Conclusions On basis of the Bi2WO6 nanoparticles preparation reported before [1-28], in this paper, the
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Bi2WO6-PAANa latex particles were successfully prepared by inverse miniemulsion and then hydrothermal treated. The crystal size and light absorption properties of the products were studied. By adjusting the ratio of Bi(NO3)3/Na2WO4, the stability of the droplets in the inverse miniemulsion
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system could be controlled. During the formation of Bi2WO6 nanoparticles, the latex particles could
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maintain a stable particle size under different hydrothermal treatment conditions. It could be seen
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from the TEM image that the Bi2WO6 nanoparticles could be synthesized in the reaction site of the latex particles and in a dispersed state. However, the hydrothermal treatment caused trace
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aggregation of some of the micelles, and the particle size of the hydrothermally treated latex particles was as light increase. XRD results showed that the orthorhombic tungsten-type Bi2WO6
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could be prepared by temperature, time and pH of different hydrothermal treatments, and the minimum hydrothermal treatment temperature of Bi2WO6 nanocrystals was 130 °C. When the pH
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of the hydrothermal treatment was 9, the product was no longer a pure phase. UV-Vis and PL analysis showed that the Bi2WO6 nanoparticles prepared under different hydrothermal treatment conditions exhibited light absorption properties to ultraviolet visible light, and with the gradual increase of hydrothermal treatment temperature, hydrothermal treatment time and pH value, the crystal form development was gradually improved, and the luminescence intensity was enhanced. 19
We assume that the described preparation method of Bi2WO6 achieves Bi2WO6 nanoparticles with better dispersive effect, and will carry out a new field of material synthesis.
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Credit author statement:
Zhenqian Zhang: Conceptualization, Methodology, Software; Supervision; Writing- Reviewing
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and Editing.
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Yongzhou Lin: Data curation, Writing- Original draft preparation; Software; Formal analysis;
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Investigation; Resources; Validation; Writing - Original Draft.
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Fang Liu: Visualization, Investigation.
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PII:
S0927-7757(20)30085-6
DOI:
https://doi.org/10.1016/j.colsurfa.2020.124493
Reference:
COLSUA 124493
To appear in:
Colloids and Surfaces A: Physicochemical and Engineering Aspects
Received Date:
14 November 2019
Revised Date:
18 January 2020
Accepted Date:
20 January 2020
Please cite this article as: Zhang Z, Lin Y, Liu F, Preparation, crystallization and properties of Bi2 WO6 nanoparticles, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124493
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Preparation, crystallization and properties of Bi2WO6 nanoparticles
Zhenqian Zhang*; Yongzhou Lin; Fang Liu School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, Changzhou University, Jiangsu Changzhou 213164, China
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Corresponding author: Zhang Zhenqian; E-mail: [email protected]. Tel: +8651986330096; Fax: +8651986330095
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Graphical abstract
Abstract: Bismuth tungstate (Bi2WO6)-sodium polyacrylate (PAANa) latex particles were prepared by
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inverse miniemulsion and hydrothermal treatment. The effects of Bi(NO3)3/Na2WO4 dosage ratio and different hydrothermal treatment conditions on the particle size and morphology of Bi2WO6-PAANa
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latex particles were studied. The results show that the stability of the inverse microemulsion droplets was effectively improved with the increase of the soluble salt in the inverse microemulsion system, and the effect of different hydrothermal treatment on the particle size of the colloidal particles was not significant. During the formation of Bi2WO6 nanoparticles, PAANa latex particles could become a reaction site for forming Bi2WO6 nanoparticles, and Bi2WO6 nanoparticles were dispersed in the polymer. The effects of
1
different hydrothermal treatment conditions on the size, crystal form and light absorption properties of Bi2WO6 nanocrystals were studied. The results show that Bi2WO6 nanoparticles with orthorhombic tungsten-rhenium ore structure could be prepared under different hydrothermal treatment conditions; moreover, the temperature at which the hydrothermal treatment forms the lowest Bi 2WO6 nanocrystals was 130 °C. But at pH 9, the product was no longer pure phase. The prepared nanocrystals simultaneously exhibited strong ultraviolet-visible absorption characteristics and luminescence intensity. Finally, the
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Bi2WO6 nanoparticles with photoluminescence properties could be successfully prepared by the inverse
miniemulsion method.
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Key words: Inverse miniemulsion; Hydrothermal treatment; Bi2WO6; Nanoparticle
1 Introduction
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In recent years, with the increasing energy crisis and environmental pollution, people have begun to seek an energy source for environmentally sustainable use. Among them, tungstate has the
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perfect performance on luminous efficiency, and anti-radiation damage capability and luminescence, which attracts the attention on solar energy utilization [1-4]. Bismuth tungstate (Bi2WO6) belongs to
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an important class of semiconductor oxides and has excellent physical properties and chemical properties, such as: ferroelectric
[5],
piezoelectricc
[6],
pyroelectric
[7],
photoluminescence
[8],
nonlinear dielectric induction [9], photocatalysis [10, 11] and others Bi2WO6 has an appropriate band gap and excellent light absorption characteristics for visible light [12]. The formation methods of Bi2WO6 which have been reported so far include high temperature 2
solid phase synthesis
[13]
, microwave assisted heat method
[14]
, ion exchange method
[15]
, sol-gel
method [16], co-precipitation method [17], solvothermal method [18, 19], electro-spinning method [20,21], ultrasonic spray pyrolysis hydrothermal method
[22],
[25, 26]
ultrasonic chemical method
[23],
sucrose template method
[24],
and so on. The Bi2WO6 nanocrystals obtained from the different
preparation methods result in the different structures, particle sizes, morphology and other properties. For example, the smaller the particle size is; the shorter for electrons and holes migrate
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to the surface. However, the Bi2WO6 nanocrystals prepared by most of the above preparation
methods have large particle size and broad dispersion, and the Bi2WO6 nanocrystals obtained have the shape such as a bird's nest, a solid ball flower, an irregular sheet and so on. Yang et al
[27]
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synthesized spherical Bi2WO6 nanoparticles with an average particle size of about 85 nm by
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hydrothermal method, and the band gap was 2.93 eV. R Kalai Selvan et al prepared Bi2WO6 [23],
so that
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nanoparticles with strong electrochemical properties by ultrasonic chemical method
Bi2WO6 could be used as a suitable negative electrode material for supercapacitors. Liu et al used a
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flux growth method to synthesize a single crystal of the bismuth layered-perovskite ferroelectric oxide Bi2WO6, and exhibited rich optical properties and large spectral response, making it a
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candidate material for mid-IR NLO [28].
In general, the stable liquid/liquid dispersion system composed of submicron (50-500nm)
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droplets is called "miniemulsion", and the corresponding polymerization is called "miniemulsion polymerization" [29]. Zhang et al prepared CdFe2O4-polymer nanoparticles by inverse miniemulsion, the films prepared by spin coating at lower temperature showed good spinel structure, and the CdFe2O4-polymer films also showed high reactivity
[30].
Therefore, the miniemulsion as an ideal
method has received a great attention on nano materials preparation [31]. 3
In this paper, Bi2WO6-Poly sodium acrylate (PAANa) latex particles were prepared by inverse miniemulsion and then are hydrothermal treated. Compared with other methods
[32],
the Bi2WO6
was synthesized in the process of miniemulsion polymerization; crystal Bi2WO6 was obtained from the hydrothermal treatment at the relative lower temperature, and the prepared nanoparticles had the advantages of uniform dispersion and small in size. The effects of the bismuth nitrate/ sodium tungstate dosage ratio (Bi(NO3)3/Na2WO4, Bi/W) and the hydrothermal treatment conditions on
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particles size, particles distribution and morphology in the process of colloid formation were investigated. The effects of temperature, time and pH of hydrothermal treatment on the stability of
colloidal particles were under consideration. The size, crystal form and light absorption properties
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of Bi2WO6 nanocrystals were also discussed.
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2 Experimental
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2.1 Materials
Sodium acrylate (NaAA): chemically pure, Saan Chemical Technology (Shanghai) Co., Ltd.
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N-heptane (CH): analytical grade, Shanghai Lingfeng Chemical Reagent Co., Ltd. Sorbitan monooleate (Span-80): analytical grade, Sinopharm Chemical Reagent Co., Ltd. Bismuth nitrate
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pentahydrate (Bi(NO3)3•5H2O): analytically pure, Sinopharm Chemical Reagent Co., Ltd. Nitrogen diisobutyronitrile (AIBN): analytical grade, Shanghai Test Sihewei Chemical Co., Ltd. Sodium
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tungstate dihydrate (Na2WO4•2H2O): analytical grade, Sinopharm Chemical Reagent Co., Ltd. Anhydrous ethanol: analytically pure, Sinopharm Chemical Reagent Co., Ltd. Other reagents are commercially available and used directly. 2.2 Preparation of Bi2WO6-PAANa Preparation of Bi(NO3)3miniemulsion: 0.150 g of Bi(NO3)3•5H2O was weighed and dissolved 4
in 8.000 g of 10% (mass concentration) diluted nitric acid to prepare an aqueous phase; 2.100 g of Span-80 was dissolved in 40 g of N-heptane and stirred for 10 min to prepare an oil phase. The aqueous phase was slowly poured into the oil phase for magnetic stirring for 15 min to prepare a Bi(NO3)3 pre-emulsion. It was then placed in an ultrasonic cell pulverizer for 5 min at 5 °C to obtain a stable Bi(NO3)3 miniemulsion. Preparation of Na2WO4-NaAA miniemulsion: A certain amount of Na2WO4•2H2O and 2.000
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g of NaAA were weighed and dissolved in 8.000 g of deionized water, and uniformly stirred on a
thermostatic magnetic stirrer for 15 min to completely dissolve to prepare an aqueous phase. 2.100 g of Span-80 was dissolved in 40 g of N-heptane and stirred for 10 min to prepare an oil phase. The
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aqueous phase was slowly poured into the oil phase for magnetic stirring for 15 min to prepare a
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pre-emulsion, which was then placed in an ultrasonic cell pulverizer for 5 min at 5 °C to obtain a
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Na2WO4-NaAA miniemulsion.
Preparation of Bi2WO6-PAANa latex: The mixture of the Bi(NO3)3 miniemulsion and the
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Na2WO4-NaAA miniemulsion prepared above was added 0.120 g of AIBN, stirred evenly at 65 °C under N2 protection and then reacted 4 h to gain Bi2WO6-PAANa latex. Several components of latex
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particles were prepared as described above. Table 1 shows the experimental formula for preparing latex particles by changing the amount of Na2WO4•2H2O added.
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Hydrothermal treatment of Bi2WO6-PAANa latex: A 30 mL latex was poured into a 100mL
Teflon-lined-autoclave. Table 2 is the specific experimental formula for the hydrothermal reaction by adjusting the pH value, the hydrothermal treatment temperature and the hydrothermal treatment time by using the latex particles prepared by Exp No. 4 in Table 1 as the experimental basis. After completing treatment, the Bi2WO6-PAANa latex samples were naturally cooled to room temperature. 5
Table 1 Experimental Formulation for Preparation of Bi2WO6-PAANa Latex Particles Exp No.
Bi/W* (g/g)
NaAA (g)
H2O (g)
Span-80 (g)
N-heptane (g)
AIBN (g)
1
0.150/0.026
2.000
8.000×2
2.100×2
40×2
0.120
2
0.150/0.051
2.000
8.000×2
2.100×2
40×2
0.120
3
0.150/0.077
2.000
8.000×2
2.100×2
40×2
0.120
4
0.150/0.102
2.000
8.000×2
2.100×2
40×2
0.120
*Dosage ratio of Bi(NO3)3·5H2O to Na2WO4·2H2O
Temperature (ºC)
pH
Time (h)
5
0.150/0.102
120
7
12
6
0.150/0.102
130
7
12
7
0.150/0.102
160
7
8
0.150/0.102
180
7
9
0.150/0.102
160
7
10
0.150/0.102
160
7
11
0.150/0.102
160
12
0.150/0.102
160
13
0.150/0.102
160
14
0.150/0.102
160
12 12 6
15
7
18
7
24
4
12
9
12
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2.3 Characterization
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Bi/W (g/g)
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Exp No.
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Table 2 Experimental Formulation of Bi2WO6 Nanoparticles Prepared by Hydrothermal Treatment
The diluted droplets (diluted with a drop of 5 mL of N-heptane) or the Z-average particle sizes
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(Dz) and intensity distribution (PDI) of the latex particles were tested by a British Mastersizer-2000 laser particle size analyzer.
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The diluted sample (diluted a drop of emulsion sample with 5 mL of n-heptane) was added
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dropwise to a copper mesh and dried at room temperature for 24 h, and the morphology of the particles was observed by a Japanese JEM-2100 high-resolution transmission electron microscope. After different hydrothermal treatment conditions, the sample was centrifuged, washed several
times and then dried, repeated treatment to remove PAANa to obtain Bi2WO6 nanoparticles, and phase structure was determined by X-ray powder diffractometer manufactured by Bruker Corporation of the USA. 6
UV absorption characteristics were tested using an ultraviolet-visible spectrophotometer from Shimadzu, Japan. The photoluminescence properties of the samples were tested using a fluorometer (LS45) instrument with a wavelength accuracy of ±1 nm. 3 Results and discussion 3.1 Effect of Na2WO4 dosage and formation process on particle size of synthetic latex particles In the miniemulsion, the soluble salt can be used as a co-stabilizer to delay the Ostwald ripening so that the fine emulsion prepared in Exp No. 1-4 exhibits no delamination. Fig.1 is a graph
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[29],
showing the particle size and distribution of Bi2WO6-PAANa latex particles synthesized in the experimental formulation Exp No. 4. Table 3 shows the effect of varying the amount and formation
-p
of Na2WO4 on the particle size of the synthesized Bi2WO6-PAANa latex particles. Fig.2 is a
re
schematic diagram of the reaction of preparing Bi2WO6-PAANa latex particles by inverse
lP
miniemulsion method.
Table 3 Particle size and distribution of Bi2WO6-PAANa latex particles Bi/W (g/g)
1 2 3 4 5 6 7 8 9
0.150/0.026 0.150/0.051 0.150/0.077 0/0.102 0.150/0 0.150/0.102 0.150/0.102 0.150/0.102 0.150/0.102
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Temperature (C) 65 65 65 ---65 65 65
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Sample No.
7
Time (h)
Dz (nm)
PDI
4 4 4 ---1 3 4
431.4 403.2 368.7 255.0 229.4 235.2 260.8 357.5 339.3
0.204 0.181 0.236 0.234 0.157 0.222 0.214 0.216 0.202
Fig.1 Particle size and distribution of Bi2WO6-PAANa latex particles during formation (4: Na2WO4-NaAA miniemulsion; 5: Bi(NO3)3 miniemulsion; 6: Mixed miniemulsion; 7:
ro of
Preliminary formation of Bi2WO6-PAANa nanoparticles after 1 h of reaction; 8: Partially
formation of Bi2WO6-PAANa nanoparticles after 2 h of reaction; 9: Final formation of Bi2WO6PAANa nanoparticles after 4 h of reaction.)
-p
As could be seen from Table 3, all the samples in the table were latex particles prepared by
re
mixing and dispersing a Na2WO4-NaAA miniemulsion with a Bi(NO3)3 miniemulsion. With the
lP
increase of the amount of Na2WO4 added, the particle size data of the latex particles using the DLS test sample showed a trend of decreasing, indicating that the stability of the monomer droplets
na
gradually became better as the amount of soluble salt increased. Shinoda et al [33] and Florence et al [34] studied that the amount of added inorganic salt would slightly decrease the lipophilic-hydrophilic
ur
balance (HLB), but improve the stability of the emulsion. In Fig. 1 and Table 3, the Dz of Na2WO4-NaAA droplets (Sample No. 4) was 255.0 nm, and
Jo
the PDI was 0.234. The Dz of the Bi(NO3)3 droplet (Sample No. 5) was 229.4 nm, and the PDI was 0.157. The mixed miniemulsion (Sample No. 6) was with the mixture of Na2WO4-NaAA miniemulsion (Sample No. 4) and Bi(NO3)3 miniemulsion (Sample No. 5). The mixed miniemulsion had a Z-average particle size (Dz) of 235.2 nm and a PDI of 0.222. The Bi(NO3)3 and Na2WO4NaAA droplets were separately stable due to the repressed Ostwald ripening at room temperature 8
[35]
. When the polymerization time reached 1 h, the initial formation of Bi2WO6-PAANa colloidal
particles (Sample No. 7) had a Dz of 260.8 nm and a PDI of 0.214. It indicates that some pseudostable droplets may be absorbed by the relative stable polymerized latex particles at 65 °C. This process is carried out though a phase transfer way, due to the improved water solubility in hydrocarbon at high temperature
[36-38].
Some Bi2WO6 were synthesized by the reaction between
ro of
Bi(NO3)3 and Na2WO4 in absorbing latex particles. As the polymerization time simultaneously increased to 3 h, the particle size of Bi2WO6-PAANa colloidal particles (Sample No. 8) increased
to 357.5 nm, and PDI became 0.216. The increased size of Bi2WO6-PAANa colloidal particles was
-p
result of the absorbing process to a fully degree. When the polymerization time was increased to 4
re
h, the Bi2WO6-PAANa colloid (Sample No. 9) had a Dz of 339.3 nm and a PDI of 0.202. Therefore,
lP
the mixture of Bi(NO3)3miniemulsion and Na2WO4-NaAA miniemulsion are successful to form mixed to form Bi2WO6 contained PAANa latex particles.
na
In the process of forming latex particles, stable latex particles are formed and droplet size becomes smaller, which effectively improves the stability of Bi2WO6-PAANa colloid
Jo
ur
formation process of Bi2WO6-PAANa latex particles is shown in Fig. 2.
9
[39].
The
Fig. 2 Schematic diagram of the formation of Bi2WO6-PAANa latex prepared by inverse miniemulsion method 3.2 Effect of different hydrothermal treatment conditions on particle size of synthesized latex Fig. 3 is a graph showing the particle size and distribution of latex particles at different pH values, temperatures and times. Table 4 is a table of particle size and distribution data tested by DLS
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na
lP
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-p
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after different hydrothermal treatment conditions.
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Fig. 3 Particle size and distribution of latex particles formed under different hydrothermal treatment conditions
Table 4 Particle size and distribution of latex particles formed by hydrothermal treatment Sample No.
Bi/W (g/g)
Temperature (ºC)
pH
Time (h)
Dz (nm)
PDI
10
0.150/0.102
120
7
12
384.6
0.144
11
0.150/0.102
130
7
12
396.1
0.285
10
12
0.150/0.102
160
7
12
403.6
0.186
13
0.150/0.102
180
7
12
423.8
0.313
14
0.150/0.102
160
7
6
412.0
0.235
15
0.150/0.102
160
7
15
395.5
0.225
16
0.150/0.102
160
7
18
406.2
0.215
17
0.150/0.102
160
7
24
416.5
0.246
18
0.150/0.102
160
4
12
379.1
0.110
19
0.150/0.102
160
9
12
404.5
0.193
As shown in Fig. 3 and Table 4, with the increase of pH value, hydrothermal treatment
ro of
temperature and hydrothermal treatment time, the particle size of the 10 groups of samples did not change much, indicating that the hydrothermal treatment had little effect on the particle size of the
synthesized latex particles. When the ratio of Bi(NO3)3/Na2WO4 was 0.150/0.102, the latex particle
-p
Dz was 339.3 nm and the PDI was 0.202 before hydrothermal treatment; however, the
re
hydrothermally treated latex particles had larger particle size than the unhydro-treated latex particles, which attributed to trace agglomeration of latex particles.
lP
3.3 Morphology of particles before and after hydrothermal treatment Fig. 4 is the TEM image of Bi2WO6-PAANa latex particles prepared by formula Exp No.4 and
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Bi2WO6 nanoparticles obtained from hydrothermal treatment by formula Exp No. 7.
11
Fig. 4 TEM image of Bi2WO6-PAANa latex particles (a: Before hydrothermal treatment (Exp No.
c: PAANa free)
ro of
4 in Table 1); b: After hydrothermal treatment (Exp No. 7 in Table 2)) and Bi2WO6 nanoparticles;
Fig. 4a shows the TEM image of Bi2WO6-PAANa latex particles prepared without
-p
hydrothermal treatment, the particle size of the latex particles was about 320 nm, and the particle
re
size of the Bi2WO6-PAANa latex particles was slightly smaller than that measured by DLS, the
lP
possible reason was that the sample needed to be dried at a certain temperature before testing the TEM. Fig. 4b shows the Bi2WO6-PAANa latex particles observed after hydrothermal treatment. Fig.
na
4c shows that the Bi2WO6 nanoparticles were PAANa free and exhibited a dispersed state, which size was 10-20 nm. Therefore, PAANa latex particles could be used as nanoreactors for forming
ur
Bi2WO6 nanoparticles. However, Yang’s research groups have done Bi2WO6 nanoparticles with aggregation phenomenon
[11].
The present work is making Bi2WO6 nanoparticles with good
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dispersion effect, which is very different from Yang’s work, and this is also the major contribution of our work.
3.4 XRD analysis of Bi2WO6 nanoparticles Fig. 5 is a Bi2WO6-PAANa latex particle prepared from Experimental Formula Exp No. 5-14, which was post-treated to obtain an XRD pattern of Bi2WO6 nanoparticles. Table 5 shows the grain 12
PDF # 73-1126
(200)
b
(139) (420)
(313) (226)
(220)
180 °C
180 °C
160 °C
160 °C
130 °C
130 °C
120 °C
120 °C Before hydrothermal treatment
Before hydrothermal treatment 30
40
50
60
80 20
70
25
(113)
d
(139) (420)
(313) (226)
(220)
24 h
35
40
24 h 18 h
15 h
15 h
re
18 h
-p
(113)
PDF # 73-1126 (200)
c
30
2θ / °
2θ / °
(200)
20
ro of
(200)
a
(113)
(113)
size and structure of Bi2WO6 nanoparticles synthesized under different treatment conditions.
12 h
12 h
6h
6h
80 20
lP f
(139) (420)
(313) (226)
Bi2WO6 Bi0.875W0.125O1.6875
pH = 7
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pH = 4
pH = 4
30
40
50 2θ / °
60
70
80 20
35
25
30 2θ / °
Fig. 5 XRD patterns of Bi2WO6 crystals Table 5 Grain size and structure of Bi2WO6 nanoparticles synthesized 13
40
pH = 9
pH = 7
20
30 2θ / °
(200)
PDF # 73-1126 PDF # 85-1268
25
(113)
70
ur
pH = 9
60
(220)
e
50 2θ / °
na
40
(200)
30
(113)
20
35
40
under different treatment conditions Treatment conditions Sample No.
D (nm)
Structure
pH
Time (h)
10
120
7
12
--
Amorphous
11
130
7
12
17.6
Orthorhombic system
12
160
7
12
18.3
Orthorhombic system
13
180
7
12
18.5
Orthorhombic system
14
160
7
6
18.2
Orthorhombic system
15
160
7
15
18.5
Orthorhombic system
16
160
7
18
20.1
Orthorhombic system
17
160
7
24
21.9
18
160
4
12
20.2
19
160
9
12
20.3
ro of
Temperature (ºC)
Orthorhombic system
Orthorhombic system Orthorhombic system
-p
The Bi2WO6-PAANa latex particles were prepared by inverse miniemulsion, and the XRD
re
patterns of Bi2WO6 crystals were obtained under different hydrothermal treatment conditions， and then post-treated. The peaks of all the spectra in the figure showed characteristic diffraction peaks
lP
at about 2θ=28°, 32°, 47°, 55°, 58°, 75°, and 78°, which were consistent with the characteristic diffraction peak position of the orthorhombic barium tungstate standard card PDF=73-1126. It was
na
indicated that Bi2WO6 nanocrystals could be formed after hydrothermal treatment. It could be clearly observed from Fig. 5(a) that the sample which had not been hydrothermally treated had no
ur
diffraction peak in its spectrum, it was indicated that Bi2WO6 crystal could not be formed without
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hydrothermal treatment. Among them, when the hydrothermal treatment temperature was 120 °C, a diffraction peak appears at 2θ = 28°, and the intensity of the diffraction peak was weak, it was indicated that the product under this condition was amorphous and no Bi2WO6 crystal was formed. When the temperature rose to 130 °C, a weak diffraction peak could be seen, it was indicated that the temperature of 130 °C was the lowest temperature at which the system formed Bi2WO6 nanocrystals; at this time, the crystal structure growth was incomplete and the crystallinity was low. 14
The results showed that as the hydrothermal treatment temperature increased, the intensity of the diffraction peaks increased and the crystallinity increased. According to the Debye-Scherrer formula [30],
the average grain size under hydrothermal treatment temperature was about 18.1 nm. It could
be observed from the (131) crystal plane in Fig. 5(b) that the intensity of the diffraction peak relatively increased relatively rapidly, it showed that the growth trend of Bi2WO6 crystal became obvious with the increase of hydrothermal treatment temperature, and the crystal grains grow
ro of
gradually. As shown in Fig. 5(c) and 5(d), as the hydrothermal treatment time was gradually increased, the intensity of the diffraction peak of the Bi2WO6 nanocrystals on the (131) crystal plane
was continuously enhanced, and the half-height width was gradually narrowed. When the
-p
hydrothermal treatment time was 24 h, the half-height width of the (131) crystal plane was the
re
narrowest, and the average grain size under hydrothermal treatment time was about 19.4 nm, it
lP
showed that the increase of hydrothermal treatment time leads to the crystallization of the product gradually deepening, the crystallinity was increased, and the grain size was enlarged. In Fig. 5(e), it
na
could be observed that the product prepared at pH 4 and 7 was pure Bi2WO6 crystal. When the pH value was adjusted to 9, impurity peaks appeared on the spectra, the impurity peaks were
ur
Bi0.875W0.125O1.6875, which was consistent with the standard card PDF=85-1268. It could be seen from Fig. 5(f) that the pure phase Bi2WO6 crystal was prepared at pH = 7, and the diffraction peak
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at the (131) crystal plane was sharp and clear, the product crystallinity was high, and no other peak appears, it was indicated that a product with good purity could be obtained at pH = 7 and the obtained grain size was 19.6 nm in Table 5. 3.5 Ultraviolet visible light analysis and photoluminescence analysis of Bi2WO6 nanoparticles. Fig. 6 show the UV-visible analysis of Bi2WO6 nanoparticles prepared under different 15
25
hydrothermal treatment conditions (Table 2) to characterize their light absorption characteristics and
ro of
photoluminescence.
25
b1 20
6h 12 h 15 h 18 h 24 h
15 10
re
(αhν)2 [(cm-1·eV)2]
Abs (a.u.)
6h 12 h 15 h 18 h 24 h
-p
b
lP
300
350
400 450 500 Wavelength (nm)
600
300
d
350
400 450 500 Wavelength (nm)
550
2.5
3.0
3.5
5.0
4.0
4.5
5.0
(αhν)2 [(cm-1·eV)2]
pH = 4 pH = 7 pH = 9
15 10 5 0 2.0
600
2.5
3.0
3.5 hν (eV)
e
24 h
Intensity (a.u.)
160 °C
Intensity (a.u.)
4.5
c1
180 °C
130 °C
4.0
hν (eV)
20
Jo 250
0 2.0
25
pH = 4 pH = 7 pH = 9
ur
c
Abs (a.u.)
550
na
250
5
16
18 h
15 h 12 h 6h
f pH = 9
Intensity (a.u.)
pH = 7
350
400
450
500
550
600
-p
Wavelength (nm)
ro of
pH = 4
re
Fig.6 UV and PL diagram of Bi2WO6 nanoparticles
lP
As shown in Fig. 6 (a-c), the UV-visible characterization of Bi2WO6 nanoparticles prepared by four different hydrothermal treatment temperatures, five different hydrothermal treatment times and
na
three different pH values was carried out. It was thus known that the synthesized Bi2WO6 nanoparticles had strong absorption in the visible light region and the ultraviolet light region, and
ur
all components had steep absorption boundary phenomena in the ultraviolet visible spectrum, it was indicated that the Bi2WO6 nanoparticle product itself was not caused by the transition of the impurity
Jo
level in the absorption of visible light but by the electronic transition of the band gap energy level structure. According to the Tauc equation
[20],
the forbidden band width of Bi2WO6 nanoparticles
could be calculated from the ultraviolet-visible spectrogram data as shown in Fig. 6 (a1), 6 (b1) and 6 (c1). It could be seen from Fig. 6a and a1 that when the hydrothermal treatment temperature was 120 °C, the absorption boundary of the spectral line was blue-shifted, it was suggested that there 17
was no formation of Bi2WO6 crystal under this condition, and the product was still in an amorphous state. It was indicated that the crystal growth of Bi2WO6 was gradually complete with the increase of hydrothermal treatment temperature, the crystallinity was continuously increased. It could be seen from Fig. 6(b) and (b1) that the products prepared under different hydrothermal treatment times had no significant difference in light absorption, and a certain concentration of the system, as the hydrothermal treatment time increased, the grain size of the product gradually grew; it indicated
ro of
that the prolongation of hydrothermal treatment time had a certain influence on the forbidden band width. Fig. 6c and c1 showed that when the pH value was 9, the line was red-shifted, and the forbidden band width of the product was slightly weakened, the possible cause was due to the fact
-p
that the product was not pure phase.
re
As shown in Fig. 6 (d-f), the atoms on the luminescent substrate in the Bi2WO6 nanocrystals
lP
were excited by the effective collision of the excitation light particles, and the ionized free electrons had a certain energy, which would cause excitation ionization after colliding with other atoms. When [40].
The
na
the excited or ionized atoms returned to the steady state, luminescence occurs
photoluminescence spectra of Bi2WO6 nanoparticles synthesized under different hydrothermal
ur
treatment conditions at 300 nm excitation wavelength were shown. It could be observed that all products had a blue-violet luminescence peak at 424 nm, possibly due to charge transfer transitions
Jo
in the WO42- ion [41]. At the same time, the peak appearing at 486 nm might be caused by defects or impurities in the product itself. It could also be seen from the figure that the luminescence intensity of the product was the highest at T = 180 °C, t = 24 h or pH = 9. The XRD pattern of the bound product could be obtained, with the increase of temperature, time or pH value, the intensity of the diffraction peak of Bi2WO6 nanocrystals on the (131) plane was gradually enhanced, the crystal 18
plane would grow preferentially, and the crystal form would gradually develop and complete. The proportion of atoms on the surface of the nanocrystals was increased, the number of collisions between the extra nuclear electrons of the luminescent atom and the excitation light particles is increased, and the luminescence intensity was enhanced. 4 Conclusions On basis of the Bi2WO6 nanoparticles preparation reported before [1-28], in this paper, the
ro of
Bi2WO6-PAANa latex particles were successfully prepared by inverse miniemulsion and then hydrothermal treated. The crystal size and light absorption properties of the products were studied. By adjusting the ratio of Bi(NO3)3/Na2WO4, the stability of the droplets in the inverse miniemulsion
-p
system could be controlled. During the formation of Bi2WO6 nanoparticles, the latex particles could
re
maintain a stable particle size under different hydrothermal treatment conditions. It could be seen
lP
from the TEM image that the Bi2WO6 nanoparticles could be synthesized in the reaction site of the latex particles and in a dispersed state. However, the hydrothermal treatment caused trace
na
aggregation of some of the micelles, and the particle size of the hydrothermally treated latex particles was as light increase. XRD results showed that the orthorhombic tungsten-type Bi2WO6
ur
could be prepared by temperature, time and pH of different hydrothermal treatments, and the minimum hydrothermal treatment temperature of Bi2WO6 nanocrystals was 130 °C. When the pH
Jo
of the hydrothermal treatment was 9, the product was no longer a pure phase. UV-Vis and PL analysis showed that the Bi2WO6 nanoparticles prepared under different hydrothermal treatment conditions exhibited light absorption properties to ultraviolet visible light, and with the gradual increase of hydrothermal treatment temperature, hydrothermal treatment time and pH value, the crystal form development was gradually improved, and the luminescence intensity was enhanced. 19
We assume that the described preparation method of Bi2WO6 achieves Bi2WO6 nanoparticles with better dispersive effect, and will carry out a new field of material synthesis.
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Credit author statement:
Zhenqian Zhang: Conceptualization, Methodology, Software; Supervision; Writing- Reviewing
-p
and Editing.
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Yongzhou Lin: Data curation, Writing- Original draft preparation; Software; Formal analysis;
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Investigation; Resources; Validation; Writing - Original Draft.
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ur
na
Fang Liu: Visualization, Investigation.
20
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