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Hydrothermal synthesis of aluminum-doped zincophosphate large single crystal with different morphology
Accepted Manuscript Hydrothermal synthesis of Aluminum-doped Zincophosphate large single crystal with different morphology W.T. Shi, D.L. Sun, J.Q. Chen, L. Sun, S.W. Chu, Z. Xi, Yu-Jia Zeng, X.T. Xu, S.C. Ruan PII: DOI: Reference:
S0022-0248(18)30576-1 https://doi.org/10.1016/j.jcrysgro.2018.11.004 CRYS 24838
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
17 September 2018 31 October 2018 9 November 2018
Please cite this article as: W.T. Shi, D.L. Sun, J.Q. Chen, L. Sun, S.W. Chu, Z. Xi, Y-J. Zeng, X.T. Xu, S.C. Ruan, Hydrothermal synthesis of Aluminum-doped Zincophosphate large single crystal with different morphology, Journal of Crystal Growth (2018), doi: https://doi.org/10.1016/j.jcrysgro.2018.11.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrothermal synthesis of Aluminum-doped Zincophosphate large single crystal with different morphology W.T. Shia, D.L. Suna, J.Q. Chena, L. Suna, S.W. Chua, Z. Xib, Yu-Jia Zenga, X.T. Xua, S.C. Ruana,* a
Shenzhen Key Laboratory of Laser Engineering, Shenzhen University, Shenzhen, 518060, P. R. China
b
Electron Microscopy Center of Shenzhen University, Shenzhen University, Shenzhen, 518060, P. R. China
Abstract
Large single crystals of Aluminum-zincohposphate (AlZnPO4-DFT) with different morphology have been hydrothermally synthesized by using ethylenediamine (EDA) as structure-directing agent. The effects of crystallization conditions and gel compositions on products were investigated in detail. The pure phase with DFT structure could be obtained at 170 °C, while the competitive phase would appear when the crystallization temperature rises to 190 °C. The quality of AlZnPO4-DFT crystals enhances firstly and then deteriorates with the extension of crystallization duration from 20 h to 50 h. Moreover, the gel composition not only affects the size of the product, but also affects the morphology and quality of the product.
Key words: A1. Crystal morphology; A2. Hydrothermal crystal growth; A2. Single crystal growth; 1
B1. Phosphates. 1. Introduction Since the first report of zincophosphates with zeolites topology by Nenoff et al. [1], considerable efforts have been made to synthesize novel zincophosphates and other porous materials with zeolite topology for their diverse structure with different chemical composition and promising applications in many fields [2-7]. In the family of metal phosphate materials, DFT topology is famous for its special three-dimensional structure. AlZnPO4-DFT crystallizes in tetragonal system and the framework of the porous-material consists of three types of channels with 8-rings parallel to each of the crystalline axis [6, 8]. The size of 8-rings channels along [001] is 4.1 Å × 4.1 Å, and the channels along [100] and [010] directions have similar characteristics with 8-rings (1.8 Å × 4.7 Å). Hitherto, some materials with DFT structure have been synthesized such as ZnPO4-EU1(ZnPO) [5], CoZnPO4-II [6], DAF-2 (CoPO) [8], UiO-20 (MgPO) [9] and so on. However, most of synthesis methods involving overlong crystallization duration and the final products with rough surface and small size, which highly limiting their application [5, 6]. It is well known that large zeolite single crystals have attracted great attention due to their potential in the structure analysis, preparation of host-guest materials, studies on the mechanism of crystal growth and optical applications [10-13]. Up to now, many different synthetic methods have been used to synthesize large single crystals by controlling the balance of nucleation rate and growth rate through the using of assist-amine or two-step crystalization route. Unfortunately, the final products tend to be powders with small size [14-17]. Morphology is another key factor that affecting the properties of zeolite and their potential applications [18-21]. Thus, the synthesis of large zeolite single crystals with different morphology is highly urgent. 2
In this paper, large single crystals of AlZnPO4-DFT with different morphology have been hydrothermal synthesized by carefully adjusting the synthesis parameters. The effects of synthesis parameters were investigated in detail, including crystallization temperature and duration, content of HF acid, Ethylenediamine (EDA), phosphoric acid and molar Al2O3/ZnO ratio.
2. Experimental 2.1. Synthesis Aluminum isopropoxide (98 wt%, Aldrich), zinc acetate dihydrate (99 wt%, Aldrich) and orthophosphoric acid (85 wt%, Aldrich) were used as sources of aluminum, zinc and phosphorus, respectively. EDA (99 wt%, Aldrich) was used as the organic template. The typical synthesis procedure was as follows: (1) the aluminum tri-isopropoxide and zinc acetate dihydrate were first dissolved in distilled water and stirred at a rate of 600r/min for 6 h; (2) the diluted orthophosphoric acid and HF acid solution was added slowly into the gel solution and stirred for 2 h; (3) the EDA was added into the mixed zinc aluminophosphate gel and stirred for 2 h; (4) the gel formed from the reaction mixture was sealed into a Teflon-lined stainless-steel autoclave and heated to 170-190 °C under autogenous pressure for 20-50 h; (6) the solid products were filtered with distilled water under ultrasonic cleaning equipment after crystallization, then dried at 80 °C. 2.2. Characterization As-synthesized AlZnPO4-DFT crystals were identified by X-ray powder diffraction (XRD) using a D8 ADVANCE with Cu target at room temperature. The morphology and size of the crystals were studied by scanning electron microscopy (model: SU-70) and optical microscope, respectively. 3
3. Results and discussion In order to synthesize large and transparent single crystals with different morphology, a series of experiments have been carried out to investigate the influence of the synthesis parameters on the size and quality of AlZnPO4-DFT crystals, such as crystallization conditions and gel compositions.
3.1. Effect of the crystallization temperature and duration The gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O and the crystallization duration of 40 h are employed to investigate the influence of crystallization temperature. Fig. 1 shows the SEM images of as-synthesized products under different crystallization temperature. An octahedron-like single crystal with uniform size (50 µm × 40 µm) could be obtained with the crystallization temperature of 170 °C (see Fig. 1a). The corresponding XRD pattern (Fig. 2b) matches well with the simulated XRD pattern of DFT (Fig. 2a) and previously reported crystals with DFT structure [6], indicating the synthesis of pure phase of AlZnPO4-DFT crystals. By increasing the crystallization temperature up to 180 °C, the size of AlZnPO4-DFT crystal increases to 180 µm × 150 µm (see Fig. 1b and Fig. 2c). With further increasing the crystallization temperature up to 190 °C, the competitive phase with rod-like shaped morphology is obtained (Fig. 1c and Fig. 2c). Thus, the crystallization temperature is set to 180 °C for synthesizing large AlZnPO4-DFT single crystals. In order to investigate the influence of crystallization duration on the size and purity of products, the crystallization temperature is set at 180 °C with the gel composition of 1.0Al2O3: 4
0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O. The crystallization duration varies from 20 h to 50 h. Fig. 3 is the SEM images of as-synthesized AlZnPO4-DFT crystals under different crystallization duration. The size of AlZnPO4-DFT crystals increases firstly and then decreases with the extension of crystallization duration. The size of AlZnPO4-DFT single crystals reaches maximum (220 µm × 200 µm) when the crystallization duration is 40 h (see Fig. 3c). Under this circumstance, it is favor to the growth of crystal rather than nucleation, which would result in the formation of large size single crystals. The similar phenomenon has been reported by Fu et al. [22] and Xu et al. [23]. However, as the crystallization duration is extended to 50 h, the size of the crystal is reduced and pores appear on the surface (see Fig. 3d). The similar phenomenon of the size changes over the crystallization duration has been reported by previous researchers [24]. What deserves to be mentioned is that only the size and quality of samples are influenced by the crystallization duration in this system.
3.2. Gel composition In order to study the effect of gel compositions on the synthesis of AlZnPO4-DFT crystals, the synthesis condition were selected as follows: crystallization temperature 180 °C, crystallization duration 40 h.
3.2.1. Effect of the HF content It is believed that F- has a tendency to form complexes with the initial reactant species [25, 26]. In the presence of aluminum and phosphorus, the formed complexes are (AlF6)3− and (PF6) –, which would consume aluminum and phosphorus in the solution to reduce the degree of supersaturation. 5
Since the hydrolysis of (AlF6)
3−
and (PF6) – can reduce the degree of supersaturation and supply
nutrients by releasing Al and P during crystallization, the HF are widely used as mineralizer for synthesizing large-size molecular sieve crystals [22, 23, 27-29]. The influence of HF acid on the formation of products has been carried out by using gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: xHF: 300H2O, where x varies from 1.1 to 1.7. Experimental products all presented octahedron-like crystals (50 µm × 40 µm), accompanied by slight deterioration of the quality when x value equals 1.1 (see Fig. 4a). As the x value rises to 1.3, large size octahedron-like crystals (220 µm × 200 µm) with perfect surface could be synthesized (see Fig. 4b). With further increase of x value to 1.5, the size of the product was significantly reduced and the quality was distinctly decreased (see Fig. 4c). As x increases to 1.7, the products consist entirely of polyhedral crystals with rough surface (see Fig. 4d). Thus, the value of x is fixed at 1.3 for our further investigation.
3.2.2. Effect of the EDA content According to the previous researches, the structure-directing agent is very important for synthesizing crystals [6, 9, 17, 19, 30]. In order to explore the effect of EDA on the synthesis of AlZnPO4-DFT single crystals, the initial gel of 1.0Al2O3: 0.56ZnO: yEDA: 1.0P2O5: 1.3HF: 300H2O were used in this study; the range of y is ranged from 1.6 to 2.2. Octahedron-like AlZnPO4-DFT crystals with regular size (50 µm × 30 µm) and rough surface could be obtained when the y is 1.6 (see Fig. 5a). As the value of y equals to 1.8, the size of crystal increases to 120 µm × 100 µm (see Fig. 5b). With further increasing of y to 2.0, large size crystal (220 µm × 200 µm) with smooth surface can be obtained (see Fig. 5c). Nevertheless, as the y value rises to 2.2, the 6
size of crystal decreases and its surface deteriorates significantly (see Fig. 5d). Thus, the content of EDA only affects the size and quality of the products, rather than its morphology. And the size of crystal reaches a maximum when the value of y is equal to 2.0.
3.2.3. Effect of the H3PO4 content The influence of P content on the size and quality of AlZnPO4-DFT single crystal has been investigated under the gel composition 1.0Al2O3: 0.56ZnO: 2.0EDA: zP2O5: 1.3HF: 300H2O, where z varies from 0.87 to 1.26. The Fig. 6 shows pictures of crystals synthesized at different z value. When z is equal to 0.87, small-sized (100 µm × 80 µm) octahedral crystals with DFT structure can be obtained (see Fig. 6a). As the z value increases to 1.0, the morphology of the crystal does not change but the size increases to 220 µm × 200 µm (see Fig. 6b). There is no significant change in the morphology and size of the crystal when z = 1.13 (see Fig. 6c). However, with further increasing the value of z to 1.26, the quality of crystal deteriorates and the size of AlZnPO4-DFT crystal (120 µm × 100 µm) decreases dramatically. At the same time, the (110) face appears in AlZnPO4-DFT crystal (see Fig. 6d). Combing the effects of the content of EDA and P on the synthesis of AlZnPO4-DFT crystals, we speculate that these two additions affect the size and quality of AlZnPO4-DFT crystals by adjusting the initial gel pH value.
3.2.4. Effect of molar Al2O3/ZnO ratio The influence of the molar Al2O3/ZnO ratio on the size and quality of AlZnPO4- DFT crystals has been investigated in the system mAl2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O, m varies from 0.5 to 1.0. Fig. 7 shows the SEM images of as-synthesized products under different 7
molar Al2O3/ZnO ratio. It can be observed that the molar Al2O3/ZnO ratio not only affects the morphology of products, but also affects the size and quality. When m = 0.5, products all presented large size single crystals with square bifrustum morphology, the morphology is similar with the CoZnPO4-II crystals synthesized by Lu and co-works (see Fig. 7a) [6]. The corresponding powder XRD pattern is shown in Fig. 8b, and the structure of crystals with square bifrustum morphology is known as DFT topology by comparison with the simulated result (see Fig. 8). By increasing m up to 0.63, the size of crystal decreases and the quality deteriorates dramatically (see Fig. 7b). When the m = 0.76, the morphology of crystals changes from square bifrustum to polyhedron (see Fig. 7c). By increasing m up to 0.88, it can be observed that small sized (40 µm × 30 µm) octahedron-like crystals appear (see Fig. 7d). As the molar Al2O3/ZnO ratio m rises to 1.0, pure octahedron-like crystals with high quality could be synthesized (see Fig. 7e). Thus, the preferable molar Al2O3/ZnO ratio m to synthesize large single crystal is 0.5 and 1.0. The results of SEM and corresponding XRD patterns were taken together to investigate the relationship between the morphology of crystal and molar Al2O3/ZnO ratio. In general, the changes in aspect ratio of crystals can be reflected by the changing in the relative intensity of the (001) and (100) plane in the XRD patterns [30-33]. There is only a change in the content of aluminum during the transformation of the crystal morphology from square bifrustum to octahedron. So we speculate that the aggregation tendency of Al ions may change the surface energy of different crystal faces. As shown in the Fig. 7a and e, the average aspect ratio of crystals increased from 0.1 to 0.9, which indicated that the growth rate of AlZnPO4-DFT along c-direction was accelerated with increase of Al ions. It agrees well with the fact that the diffraction intensity of the (002) plane of the square bifrustum morphology (aspect ratio = 0.1) is much higher than that 8
of the octahedron morphology (aspect ratio = 0.9) (see Fig. 8b and c). The similar phenomenon that aspect ratio of crystal vary with the relative intensity of (001) and (100) plane has been reported many times [30-33]. 3.3. Characterization of large size AlZnPO4-DFT single crystals with different morphology Based on the above discussion, the suitable parameters to synthesize AlZnPO4-DFT single crystals are summarized as follows: the crystallization temperature is 180 °C and duration is 40 h, the initial gel composition of (a) 0.5Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O; (b) 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O, corresponding to the synthesis of square bifrustum morphology and octahedron-like crystals, respectively (see Fig. 7a and 7e). The optical micrograph is shown in Fig. 9, which demonstrates transparency of as-synthesized crystals.
4. Conclusions In summary, well-shaped and optically transparent AlZnPO4 large single crystals with DFT topology have been successfully synthesized by optimizing the following synthetic parameters: the crystallization temperature and duration, the content of HF acid, EDA, phosphoric acid and the molar Al2O3/ZnO ratio. The hydrofluoric acid is beneficial to the growth of large single crystals. The size of crystals is affected drastically by the content of phosphoric acid and structure-directing agent, which may attribute to the variation of pH value. Moreover, perfect large single crystals with square bifrustum morphology can be obtained by adjusting the molar Al2O3/ZnO ratio.
Acknowledgments This research was supported by the National Key Research and Development Program of China
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(No. 2016YFA0401100), National Natural Science Foundation of China (No. 61575129, 61705134), Shenzhen Science and Technology Project (No. JCYJ20160328144942069), China Postdoctoral Science Foundation (2017M612723) and Postgraduate Innovation Development Fund Project of Shenzhen University (PIDFP-ZR2018019).
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Caption to Figures Fig. 1 SEM micrographs of as-synthesized samples under different crystallization temperatures: (a) 170 °C; (b) 180 °C; (c) 190 °C. Fig. 2 Powder XRD patterns of as-synthesized samples under different crystallization temperatures: (a) simulated data for DFT structure; (b) 170 °C; (c) 180 °C; (d) 190 °C. Fig. 3 SEM micrographs of the crystals synthesized from the gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O at 180 °C under different crystallization durations: (a) 20 h (insert shows the corresponding partial enlargement); (b) 30 h; (c) 40 h; (d) 50 h. Fig. 4 SEM micrographs of the crystals synthesized from the gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: xHF: 300H2O: (a) x = 1.1; (b) x = 1.3; (c) x = 1.5 (insert shows the corresponding XRD pattern); (d) y =1.7. Fig. 5 SEM micrographs of the crystals synthesized from the gel composition of 1.0Al2O3: 0.56ZnO: yEDA: 1.0P2O5: 1.3HF: 300H2O: (a) y = 1.6; (b) y = 1.8; (c) y = 2.0; (d) y = 2.2. Fig. 6 SEM micrographs of the crystals synthesized from the gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: zP2O5: 1.3HF: 300H2O: (a) z = 0.87; (b) z = 1.0; (c) z = 1.13; (d) z = 1.26 (insert shows the corresponding partial enlargement). Fig. 7 SEM micrographs of the crystals synthesized from the gel composition of mAl2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O: (a) m = 0.5; (b) m = 0.63; (c) m = 0.76; (d) m = 0.88 (insert shows the corresponding partial enlargement); (e) m = 1.0. Fig. 8 Powder XRD patterns of crystals: (a) simulated from single-crystal data (DFT structure); (b) with square bifrustum morphology; (c) with octahedron morphology. Fig. 9 Optical image of large single crystals synthesized at 180°C for 40 h using gel composition 12
of (a) 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O and (b) 0.5Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O, corresponding to octahedron morphology and square bifrustum morphology, respectively.
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Fig. 1, by Shi et. al.
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Fig. 2, by Shi et. al.
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Fig. 3, by Shi et. al.
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Fig. 4, by Shi et. al.
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Fig. 5, by Shi et. al.
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Fig. 6, by Shi et. al.
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Fig. 7, by Shi et. al.
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Fig. 8, by Shi et. al.
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Fig. 9, by Shi et. al.
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Highlights ►Large single crystals with DFT topology were synthesized hydrothermally. ►The detailed influences of synthesis conditions on products were investigated. ►Morphological changes from octahedron to square bifrustum were achieved. ►We proved XRD peak intensities of AlZnPO4-DFT correlate with its morphology. ►Decrease of aspect ratio increases of (001) relative intensity.
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S0022-0248(18)30576-1 https://doi.org/10.1016/j.jcrysgro.2018.11.004 CRYS 24838
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
17 September 2018 31 October 2018 9 November 2018
Please cite this article as: W.T. Shi, D.L. Sun, J.Q. Chen, L. Sun, S.W. Chu, Z. Xi, Y-J. Zeng, X.T. Xu, S.C. Ruan, Hydrothermal synthesis of Aluminum-doped Zincophosphate large single crystal with different morphology, Journal of Crystal Growth (2018), doi: https://doi.org/10.1016/j.jcrysgro.2018.11.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrothermal synthesis of Aluminum-doped Zincophosphate large single crystal with different morphology W.T. Shia, D.L. Suna, J.Q. Chena, L. Suna, S.W. Chua, Z. Xib, Yu-Jia Zenga, X.T. Xua, S.C. Ruana,* a
Shenzhen Key Laboratory of Laser Engineering, Shenzhen University, Shenzhen, 518060, P. R. China
b
Electron Microscopy Center of Shenzhen University, Shenzhen University, Shenzhen, 518060, P. R. China
Abstract
Large single crystals of Aluminum-zincohposphate (AlZnPO4-DFT) with different morphology have been hydrothermally synthesized by using ethylenediamine (EDA) as structure-directing agent. The effects of crystallization conditions and gel compositions on products were investigated in detail. The pure phase with DFT structure could be obtained at 170 °C, while the competitive phase would appear when the crystallization temperature rises to 190 °C. The quality of AlZnPO4-DFT crystals enhances firstly and then deteriorates with the extension of crystallization duration from 20 h to 50 h. Moreover, the gel composition not only affects the size of the product, but also affects the morphology and quality of the product.
Key words: A1. Crystal morphology; A2. Hydrothermal crystal growth; A2. Single crystal growth; 1
B1. Phosphates. 1. Introduction Since the first report of zincophosphates with zeolites topology by Nenoff et al. [1], considerable efforts have been made to synthesize novel zincophosphates and other porous materials with zeolite topology for their diverse structure with different chemical composition and promising applications in many fields [2-7]. In the family of metal phosphate materials, DFT topology is famous for its special three-dimensional structure. AlZnPO4-DFT crystallizes in tetragonal system and the framework of the porous-material consists of three types of channels with 8-rings parallel to each of the crystalline axis [6, 8]. The size of 8-rings channels along [001] is 4.1 Å × 4.1 Å, and the channels along [100] and [010] directions have similar characteristics with 8-rings (1.8 Å × 4.7 Å). Hitherto, some materials with DFT structure have been synthesized such as ZnPO4-EU1(ZnPO) [5], CoZnPO4-II [6], DAF-2 (CoPO) [8], UiO-20 (MgPO) [9] and so on. However, most of synthesis methods involving overlong crystallization duration and the final products with rough surface and small size, which highly limiting their application [5, 6]. It is well known that large zeolite single crystals have attracted great attention due to their potential in the structure analysis, preparation of host-guest materials, studies on the mechanism of crystal growth and optical applications [10-13]. Up to now, many different synthetic methods have been used to synthesize large single crystals by controlling the balance of nucleation rate and growth rate through the using of assist-amine or two-step crystalization route. Unfortunately, the final products tend to be powders with small size [14-17]. Morphology is another key factor that affecting the properties of zeolite and their potential applications [18-21]. Thus, the synthesis of large zeolite single crystals with different morphology is highly urgent. 2
In this paper, large single crystals of AlZnPO4-DFT with different morphology have been hydrothermal synthesized by carefully adjusting the synthesis parameters. The effects of synthesis parameters were investigated in detail, including crystallization temperature and duration, content of HF acid, Ethylenediamine (EDA), phosphoric acid and molar Al2O3/ZnO ratio.
2. Experimental 2.1. Synthesis Aluminum isopropoxide (98 wt%, Aldrich), zinc acetate dihydrate (99 wt%, Aldrich) and orthophosphoric acid (85 wt%, Aldrich) were used as sources of aluminum, zinc and phosphorus, respectively. EDA (99 wt%, Aldrich) was used as the organic template. The typical synthesis procedure was as follows: (1) the aluminum tri-isopropoxide and zinc acetate dihydrate were first dissolved in distilled water and stirred at a rate of 600r/min for 6 h; (2) the diluted orthophosphoric acid and HF acid solution was added slowly into the gel solution and stirred for 2 h; (3) the EDA was added into the mixed zinc aluminophosphate gel and stirred for 2 h; (4) the gel formed from the reaction mixture was sealed into a Teflon-lined stainless-steel autoclave and heated to 170-190 °C under autogenous pressure for 20-50 h; (6) the solid products were filtered with distilled water under ultrasonic cleaning equipment after crystallization, then dried at 80 °C. 2.2. Characterization As-synthesized AlZnPO4-DFT crystals were identified by X-ray powder diffraction (XRD) using a D8 ADVANCE with Cu target at room temperature. The morphology and size of the crystals were studied by scanning electron microscopy (model: SU-70) and optical microscope, respectively. 3
3. Results and discussion In order to synthesize large and transparent single crystals with different morphology, a series of experiments have been carried out to investigate the influence of the synthesis parameters on the size and quality of AlZnPO4-DFT crystals, such as crystallization conditions and gel compositions.
3.1. Effect of the crystallization temperature and duration The gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O and the crystallization duration of 40 h are employed to investigate the influence of crystallization temperature. Fig. 1 shows the SEM images of as-synthesized products under different crystallization temperature. An octahedron-like single crystal with uniform size (50 µm × 40 µm) could be obtained with the crystallization temperature of 170 °C (see Fig. 1a). The corresponding XRD pattern (Fig. 2b) matches well with the simulated XRD pattern of DFT (Fig. 2a) and previously reported crystals with DFT structure [6], indicating the synthesis of pure phase of AlZnPO4-DFT crystals. By increasing the crystallization temperature up to 180 °C, the size of AlZnPO4-DFT crystal increases to 180 µm × 150 µm (see Fig. 1b and Fig. 2c). With further increasing the crystallization temperature up to 190 °C, the competitive phase with rod-like shaped morphology is obtained (Fig. 1c and Fig. 2c). Thus, the crystallization temperature is set to 180 °C for synthesizing large AlZnPO4-DFT single crystals. In order to investigate the influence of crystallization duration on the size and purity of products, the crystallization temperature is set at 180 °C with the gel composition of 1.0Al2O3: 4
0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O. The crystallization duration varies from 20 h to 50 h. Fig. 3 is the SEM images of as-synthesized AlZnPO4-DFT crystals under different crystallization duration. The size of AlZnPO4-DFT crystals increases firstly and then decreases with the extension of crystallization duration. The size of AlZnPO4-DFT single crystals reaches maximum (220 µm × 200 µm) when the crystallization duration is 40 h (see Fig. 3c). Under this circumstance, it is favor to the growth of crystal rather than nucleation, which would result in the formation of large size single crystals. The similar phenomenon has been reported by Fu et al. [22] and Xu et al. [23]. However, as the crystallization duration is extended to 50 h, the size of the crystal is reduced and pores appear on the surface (see Fig. 3d). The similar phenomenon of the size changes over the crystallization duration has been reported by previous researchers [24]. What deserves to be mentioned is that only the size and quality of samples are influenced by the crystallization duration in this system.
3.2. Gel composition In order to study the effect of gel compositions on the synthesis of AlZnPO4-DFT crystals, the synthesis condition were selected as follows: crystallization temperature 180 °C, crystallization duration 40 h.
3.2.1. Effect of the HF content It is believed that F- has a tendency to form complexes with the initial reactant species [25, 26]. In the presence of aluminum and phosphorus, the formed complexes are (AlF6)3− and (PF6) –, which would consume aluminum and phosphorus in the solution to reduce the degree of supersaturation. 5
Since the hydrolysis of (AlF6)
3−
and (PF6) – can reduce the degree of supersaturation and supply
nutrients by releasing Al and P during crystallization, the HF are widely used as mineralizer for synthesizing large-size molecular sieve crystals [22, 23, 27-29]. The influence of HF acid on the formation of products has been carried out by using gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: xHF: 300H2O, where x varies from 1.1 to 1.7. Experimental products all presented octahedron-like crystals (50 µm × 40 µm), accompanied by slight deterioration of the quality when x value equals 1.1 (see Fig. 4a). As the x value rises to 1.3, large size octahedron-like crystals (220 µm × 200 µm) with perfect surface could be synthesized (see Fig. 4b). With further increase of x value to 1.5, the size of the product was significantly reduced and the quality was distinctly decreased (see Fig. 4c). As x increases to 1.7, the products consist entirely of polyhedral crystals with rough surface (see Fig. 4d). Thus, the value of x is fixed at 1.3 for our further investigation.
3.2.2. Effect of the EDA content According to the previous researches, the structure-directing agent is very important for synthesizing crystals [6, 9, 17, 19, 30]. In order to explore the effect of EDA on the synthesis of AlZnPO4-DFT single crystals, the initial gel of 1.0Al2O3: 0.56ZnO: yEDA: 1.0P2O5: 1.3HF: 300H2O were used in this study; the range of y is ranged from 1.6 to 2.2. Octahedron-like AlZnPO4-DFT crystals with regular size (50 µm × 30 µm) and rough surface could be obtained when the y is 1.6 (see Fig. 5a). As the value of y equals to 1.8, the size of crystal increases to 120 µm × 100 µm (see Fig. 5b). With further increasing of y to 2.0, large size crystal (220 µm × 200 µm) with smooth surface can be obtained (see Fig. 5c). Nevertheless, as the y value rises to 2.2, the 6
size of crystal decreases and its surface deteriorates significantly (see Fig. 5d). Thus, the content of EDA only affects the size and quality of the products, rather than its morphology. And the size of crystal reaches a maximum when the value of y is equal to 2.0.
3.2.3. Effect of the H3PO4 content The influence of P content on the size and quality of AlZnPO4-DFT single crystal has been investigated under the gel composition 1.0Al2O3: 0.56ZnO: 2.0EDA: zP2O5: 1.3HF: 300H2O, where z varies from 0.87 to 1.26. The Fig. 6 shows pictures of crystals synthesized at different z value. When z is equal to 0.87, small-sized (100 µm × 80 µm) octahedral crystals with DFT structure can be obtained (see Fig. 6a). As the z value increases to 1.0, the morphology of the crystal does not change but the size increases to 220 µm × 200 µm (see Fig. 6b). There is no significant change in the morphology and size of the crystal when z = 1.13 (see Fig. 6c). However, with further increasing the value of z to 1.26, the quality of crystal deteriorates and the size of AlZnPO4-DFT crystal (120 µm × 100 µm) decreases dramatically. At the same time, the (110) face appears in AlZnPO4-DFT crystal (see Fig. 6d). Combing the effects of the content of EDA and P on the synthesis of AlZnPO4-DFT crystals, we speculate that these two additions affect the size and quality of AlZnPO4-DFT crystals by adjusting the initial gel pH value.
3.2.4. Effect of molar Al2O3/ZnO ratio The influence of the molar Al2O3/ZnO ratio on the size and quality of AlZnPO4- DFT crystals has been investigated in the system mAl2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O, m varies from 0.5 to 1.0. Fig. 7 shows the SEM images of as-synthesized products under different 7
molar Al2O3/ZnO ratio. It can be observed that the molar Al2O3/ZnO ratio not only affects the morphology of products, but also affects the size and quality. When m = 0.5, products all presented large size single crystals with square bifrustum morphology, the morphology is similar with the CoZnPO4-II crystals synthesized by Lu and co-works (see Fig. 7a) [6]. The corresponding powder XRD pattern is shown in Fig. 8b, and the structure of crystals with square bifrustum morphology is known as DFT topology by comparison with the simulated result (see Fig. 8). By increasing m up to 0.63, the size of crystal decreases and the quality deteriorates dramatically (see Fig. 7b). When the m = 0.76, the morphology of crystals changes from square bifrustum to polyhedron (see Fig. 7c). By increasing m up to 0.88, it can be observed that small sized (40 µm × 30 µm) octahedron-like crystals appear (see Fig. 7d). As the molar Al2O3/ZnO ratio m rises to 1.0, pure octahedron-like crystals with high quality could be synthesized (see Fig. 7e). Thus, the preferable molar Al2O3/ZnO ratio m to synthesize large single crystal is 0.5 and 1.0. The results of SEM and corresponding XRD patterns were taken together to investigate the relationship between the morphology of crystal and molar Al2O3/ZnO ratio. In general, the changes in aspect ratio of crystals can be reflected by the changing in the relative intensity of the (001) and (100) plane in the XRD patterns [30-33]. There is only a change in the content of aluminum during the transformation of the crystal morphology from square bifrustum to octahedron. So we speculate that the aggregation tendency of Al ions may change the surface energy of different crystal faces. As shown in the Fig. 7a and e, the average aspect ratio of crystals increased from 0.1 to 0.9, which indicated that the growth rate of AlZnPO4-DFT along c-direction was accelerated with increase of Al ions. It agrees well with the fact that the diffraction intensity of the (002) plane of the square bifrustum morphology (aspect ratio = 0.1) is much higher than that 8
of the octahedron morphology (aspect ratio = 0.9) (see Fig. 8b and c). The similar phenomenon that aspect ratio of crystal vary with the relative intensity of (001) and (100) plane has been reported many times [30-33]. 3.3. Characterization of large size AlZnPO4-DFT single crystals with different morphology Based on the above discussion, the suitable parameters to synthesize AlZnPO4-DFT single crystals are summarized as follows: the crystallization temperature is 180 °C and duration is 40 h, the initial gel composition of (a) 0.5Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O; (b) 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O, corresponding to the synthesis of square bifrustum morphology and octahedron-like crystals, respectively (see Fig. 7a and 7e). The optical micrograph is shown in Fig. 9, which demonstrates transparency of as-synthesized crystals.
4. Conclusions In summary, well-shaped and optically transparent AlZnPO4 large single crystals with DFT topology have been successfully synthesized by optimizing the following synthetic parameters: the crystallization temperature and duration, the content of HF acid, EDA, phosphoric acid and the molar Al2O3/ZnO ratio. The hydrofluoric acid is beneficial to the growth of large single crystals. The size of crystals is affected drastically by the content of phosphoric acid and structure-directing agent, which may attribute to the variation of pH value. Moreover, perfect large single crystals with square bifrustum morphology can be obtained by adjusting the molar Al2O3/ZnO ratio.
Acknowledgments This research was supported by the National Key Research and Development Program of China
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(No. 2016YFA0401100), National Natural Science Foundation of China (No. 61575129, 61705134), Shenzhen Science and Technology Project (No. JCYJ20160328144942069), China Postdoctoral Science Foundation (2017M612723) and Postgraduate Innovation Development Fund Project of Shenzhen University (PIDFP-ZR2018019).
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Caption to Figures Fig. 1 SEM micrographs of as-synthesized samples under different crystallization temperatures: (a) 170 °C; (b) 180 °C; (c) 190 °C. Fig. 2 Powder XRD patterns of as-synthesized samples under different crystallization temperatures: (a) simulated data for DFT structure; (b) 170 °C; (c) 180 °C; (d) 190 °C. Fig. 3 SEM micrographs of the crystals synthesized from the gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O at 180 °C under different crystallization durations: (a) 20 h (insert shows the corresponding partial enlargement); (b) 30 h; (c) 40 h; (d) 50 h. Fig. 4 SEM micrographs of the crystals synthesized from the gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: xHF: 300H2O: (a) x = 1.1; (b) x = 1.3; (c) x = 1.5 (insert shows the corresponding XRD pattern); (d) y =1.7. Fig. 5 SEM micrographs of the crystals synthesized from the gel composition of 1.0Al2O3: 0.56ZnO: yEDA: 1.0P2O5: 1.3HF: 300H2O: (a) y = 1.6; (b) y = 1.8; (c) y = 2.0; (d) y = 2.2. Fig. 6 SEM micrographs of the crystals synthesized from the gel composition of 1.0Al2O3: 0.56ZnO: 2.0EDA: zP2O5: 1.3HF: 300H2O: (a) z = 0.87; (b) z = 1.0; (c) z = 1.13; (d) z = 1.26 (insert shows the corresponding partial enlargement). Fig. 7 SEM micrographs of the crystals synthesized from the gel composition of mAl2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O: (a) m = 0.5; (b) m = 0.63; (c) m = 0.76; (d) m = 0.88 (insert shows the corresponding partial enlargement); (e) m = 1.0. Fig. 8 Powder XRD patterns of crystals: (a) simulated from single-crystal data (DFT structure); (b) with square bifrustum morphology; (c) with octahedron morphology. Fig. 9 Optical image of large single crystals synthesized at 180°C for 40 h using gel composition 12
of (a) 1.0Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O and (b) 0.5Al2O3: 0.56ZnO: 2.0EDA: 1.0P2O5: 1.3HF: 300H2O, corresponding to octahedron morphology and square bifrustum morphology, respectively.
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Fig. 1, by Shi et. al.
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Fig. 2, by Shi et. al.
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Fig. 3, by Shi et. al.
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Fig. 4, by Shi et. al.
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Fig. 5, by Shi et. al.
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Fig. 6, by Shi et. al.
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Fig. 7, by Shi et. al.
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Fig. 8, by Shi et. al.
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Fig. 9, by Shi et. al.
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Highlights ►Large single crystals with DFT topology were synthesized hydrothermally. ►The detailed influences of synthesis conditions on products were investigated. ►Morphological changes from octahedron to square bifrustum were achieved. ►We proved XRD peak intensities of AlZnPO4-DFT correlate with its morphology. ►Decrease of aspect ratio increases of (001) relative intensity.
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