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Bottom-up synthesis of aluminophosphate nanosheets by hydrothermal process
Advanced Powder Technology xxx (2017) xxx–xxx
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Original Research Paper
Bottom-up synthesis of aluminophosphate nanosheets by hydrothermal process Takayuki Ban ⇑, Shota Iriyama, Yutaka Ohya Department of Chemistry and Biomolecular Science, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan
a r t i c l e
i n f o
Article history: Received 29 June 2017 Received in revised form 27 September 2017 Accepted 17 October 2017 Available online xxxx Keywords: Aluminophosphate Nanosheet Hydrothermal synthesis Bottom-up process Aqueous solution process
a b s t r a c t Hydrothermal treatment of aqueous mixtures of boehmite (AlOOH), phosphoric acid, and tetramethylammonium hydroxide provided two types of layered aluminophosphates having tetramethylammonium ion (TMA+) as an interlayer cation, despite the fact that TMA+ ion acts as a structure-directing agent of microporous AlPO4 materials and less likely leads to a lamellar structure. The layered aluminophosphates were formed in amorphous gels, so that they were obtained as precipitates. Upon dispersing the precipitates to water under agitation, the layered aluminophosphates were transferred to the aqueous phase, resulting in transparent aqueous sols. Because they had bulky interlayer cation TMA+, it is likely that in the sols, layered aluminophosphates were exfoliated, providing aluminophosphate nanosheets. Moreover, their characterization results suggest that the layered compound formed at 170 °C consisted of aluminophosphate layers with kanemite-like structure. Furthermore, the prolonged hydrothermal treatment at 170 °C led to the formation of microporous ATT-type AlPO4 crystals. Under the synthesis condition employed in this study, layered aluminophosphates were formed at early stage, and then structurally converted to microporous AlPO4 crystals. Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Metalate nanosheets are two-dimensional materials. There is some possibility that novel chemical and physical properties emerge due to their highly anisotropic shapes. Upon exfoliating layered metalates consisting of negatively charged metalate slabs and interlayer cations, the metalate slabs are obtained as metalate nanosheets. Conventionally, metalate nanosheets have been prepared by the ion exchange of interlayer cations for bulky tetraalkylammonium ions, which causes the swelling and exfoliation of the layered metalates in aqueous solution [1–3]. The resulting metalate nanosheets have so large lateral size that their aqueous sols have interesting properties such as liquid crystalline properties [4,5]. Unlike the conventional method, we have synthesized metalate nanosheets by bottom-up process, which utilizes the acid-base reactions between metallic acid and tetraalkylammonium hydroxide in aqueous solutions [6–10], although the lateral size of the resulting nanosheets is not so large. The applications of metalate nanosheet thin films have been studied extensively [11–16]. Thin films of metalate nanosheets
⇑ Corresponding author.
are fabricated by Layer-by-Layer method and Langmuir-Blodgett method. Bottom-up synthesis of metalate nanosheets has some possibility that continuous monolayer thin films of metalate nanosheets are fabricated by depositing nanosheets directly on chemically modified substrate surface. However, the bottom-up syntheses, which we have studied, were conducted in basic aqueous solutions. Since basic solutions cause corrosion and dissolution of many types of substrates, bottom-up synthesis of metalate nanosheets in acidic or neutral aqueous solutions is desirable for thin film fabrication. Moreover, we previously reported the bottom-up synthesis of titanate nanoflakes in ionic liquid solvents [17]. However, the synthesis conditions providing titanate nanoflakes were limited, because OH ion reacted with ionic liquid molecules. Also for the investigation of the influence of ionic liquid solvent on morphology of metalate nanosheets obtained by bottom-up process, synthesis in acidic or neutral solvents is preferable. It is known that layered aluminophosphates and aluminum phosphates with different crystal structures are hydrothermally synthesized in acidic or neutral solutions [18–24]. So, we envisaged that layered aluminophosphates or aluminophosphate nanosheets would be synthesized in acidic or neutral solutions by bottom-up process. However, there were some problems.
E-mail address: [email protected] (T. Ban). https://doi.org/10.1016/j.apt.2017.10.013 0921-8831/Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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Although tetraalkylammonium ions such as TMA+ ion are used for our bottom-up process of metalate nanosheets, they have ability to act as a structure-directing agent of crystalline microporous aluminum phosphates AlPO4. Furthermore, although the metalate nanosheets that we have synthesized by bottom-up process included only one type of cation, aluminophosphate nanosheets include two types of cations, i.e. Al3+ and P5+, so that the formation reactions would be more complex. So, the objective of this study is to examine if aluminophosphate nanosheets are synthesized in acidic or neutral aqueous solutions by bottom-up process. 2. Experimental procedure 2.1. Synthesis Typical synthesis was conducted, as follows: 30 mmol of boehmite (AlOOH; CATAPAL C1, Sasol Chemicals) was dispersed in 18.6 mL of distilled water. 85% phosphoric acid (30 mmol of H3PO4) was added to the suspension. The suspension was stirred for 1 day. Then, 25% tetramethylammonium hydroxide (N(CH3)4OH; TMAOH) solution (30 mmol of TMAOH) was added to the suspension. The suspension was further stirred for 1 day. The resulting mixture was used as a reaction gel, was about 35 g in weight, had a molar ratio of 1 AlOOH:1 H3PO4:1 TMAOH:50 H2O, and was 7.5 in pH. The reaction gel was hydrothermally treated for 1–14 days at 80–170 °C, resulting in the formation of gel-like precipitates. The precipitates were collected by certification at 9000 rpm for 15 min. The precipitates were washed, as follows: they were dispersed in 200 mL of distilled water, were stirred for 1 day, and then were collected by centrifugation. The supernatant and washed precipitates were characterized. This washing process was repeated four times. 2.2. Characterization X-ray diffraction (XRD) measurements were performed on a Rigaku Ultima IV diffractometer with a monochromatic CuKa irradiation. XRD patterns were recorded at a scan rate of 2° min 1 in the 2h range of 2–70°. Powders or thin films prepared from aqueous suspensions of precipitates or aqueous colloids were used as a sample. The thin film samples were prepared by evaporating the suspensions or colloids on a glass substrate under the ambient condition. Transmission electron microscopy (TEM) images were captured using a JEOL JEM-2100 model at an accelerating voltage of 200 kV. The samples were prepared by evaporating a drop of aqueous suspension of samples on a Cu grid supported with a Formvar thin film. Hydrophilic treatment was conducted for the Cu grid before use. Wavelength-dispersive type X-ray fluorescence analysis (WDXRF) was conducted on a Bruker-AXS S8 TIGER-MA model for estimating Al/P molar ratio in the precipitates before and after washing. The chemical composition was evaluated by digital scan screening analysis using a fundamental parameter software. Measurement were made for the powdery sample placed on a polypropylene film, which is the bottom of a sample holder.
aqueous suspensions of the precipitates. Some peaks appeared at low diffraction angles (Fig. 1). Before the hydrothermal treatment, only peaks assigned to pseudo-boehmite was observed. Upon hydrothermally treating at 80–170 °C, peaks with d-spacing of 1.42 and 0.71 nm appeared. For the sample prepared at 170 °C, peaks with d-spacing of 2.36, 1.16, and 0.77 nm were also observed. Their d-spacings had a relation of 1:1/2:1/3, indicating high orientation of formed crystals. Because the peaks attributed to high orientation were observed at low diffraction angles, it is inferred that two types of layered aluminophosphates were formed at 80 and 170 °C. Hereafter, the compounds with a basal spacing of 1.42 and 2.36 nm are called ‘‘layered aluminophosphate A” and ‘‘layered aluminophosphate B,” respectively. Moreover, the basal spacings of the layered aluminophosphates were as large as 1.42 and 2.36 nm, suggesting the presence of tetramethylammonium (TMA+) ion in the interlayer. Next, the aqueous mixtures of AlOOH, H3PO4 and TMAOH were hydrothermally treated at 170 °C for different periods. The formed precipitates were collected by centrifugation, and then evaporated under the ambient condition. For the resulting powders, XRD measurements were made (Fig. 2). When the hydrothermal period was shorter than 7 days, the formed crystalline phases were only layered aluminophosphates. The hydrothermal treatment for further longer periods decreased the amount of the layered aluminophosphates and provided the formation of AlPO4-12-TAMU (ATT-type AlPO4), which is one of microporous aluminophosphates synthesized using TMA+ as a structure-directing agent [25]. After 14 days of hydrothermal treatment, the sample contained only AlPO4-12TAMU. Since tetraalkylammonium ion acts as a structuredirecting agent of zeolites, hydrothermal treatment of aqueous mixtures of AlOOH, H3PO4 and tetraalkylammonium ion ordinarily leads to the formation of microporous AlPO4 crystals. However, these results suggest that under the synthesis condition employed in this study, layered aluminophosphates were formed at early stage, and then structurally converted to microporous AlPO4. When layered materials with a bulky interlayer cation such as TMA+ are dispersed in water, the swelling and exfoliation of the layered materials occur, resulting in the formation of nanosheets. However, in this study, such layered materials were obtained as
3. Results and discussion 3.1. Synthesis of aluminophosphate nanosheets The aqueous mixtures of AlOOH, H3PO4 and TMAOH were hydrothermally treated for 1 day at different temperatures. XRD measurements were conducted for the thin films prepared from
Fig. 1. XRD patterns of the thin films fabricated from the precipitates (a) before hydrothermal synthesis and after hydrothermal synthesis at (b) 80 °C, (c) 120 °C and (d) 170 °C for 1 day.
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obtained as precipitates. Upon dispersing the precipitates to water under agitation, the layered aluminophosphates were transferred to aqueous phase, resulting in the sols of aluminophosphates. Since they had bulky interlayer cation TMA+, it is likely that in the sols, the layered aluminophosphates were exfoliated, providing aluminophosphate nanosheets. 3.2. Characterization of aluminophosphate nanosheets
Fig. 2. XRD patterns of the precipitates obtained by hydrothermal synthesis at 170 °C for (a) 1 day, (b) 3 days, (c) 7 days, and (d) 14 days.
precipitates. The reasons why the layered aluminophosphates with a bulky interlayer cation were formed as precipitates were examined. The precipitates obtained by hydrothermal treatment at 170 °C for 1 day were dispersed in water and then stirred for 1 day. This process was repeated four times. The resulting supernatants were transparent. When a laser light was irradiated to them, the trace of the light passage was observed due to Tyndall effect, indicating colloidal nature of the supernatants (Fig. 3). Moreover, XRD measurements were made for the thin films prepared by evaporating the supernatants on a glass substrate (Fig. 4a) and for the powders prepared by evaporating the precipitates under the ambient condition (Fig. 4b). The aluminophosphate phases including in the supernatants and precipitates obtained by each washing are also listed in Table 1. The supernatants obtained by the first and second washings included layered aluminophosphate A, while layered aluminophosphate B was present in the supernatants obtained by the second, third and fourth washings. However, for the supernatant of obtained by the second washing, the amount of layered aluminophosphate A was changed sample by sample. Furthermore, before the fourth washing, layered aluminophosphate B was included in the precipitates; however, after the fourth washing, the precipitates were amorphous (Fig. 4b). These results suggest that since layered aluminophosphates A and B were formed in an amorphous phase, they were
Fig. 3. Appearance of the supernatants obtained by the washing process of the precipitates formed by hydrothermal treatment at 170 °C for 1 day.
Many types of layered aluminophosphates and layered aluminum phosphates are known, such as [Al3P4O16]3 , [AlP2O8]3 , [Al2P3O12Hx](3 x) , [Al(HPO4)2(H2O)2] , [Al4P5O20H]2 , [AlHP3O10] , and [AlPO4(OH)] [18–24]. So, the structure types of layered aluminophosphates A and B obtained in this study were investigated. First, the P/Al ratios of layered aluminophosphates A and B were roughly estimated. The precipitates formed by hydrothermal treatment at 170 °C for 1 day were washed 4 times. The precipitates obtained by each washing process were heated at 800 °C. The P/ Al molar ratios of the resulting powders were estimated by XRF (Table 1). Their weights were also measured. From these values, the ratio of Al species in the precipitates after each washing process against the used AlOOH was evaluated, and the molar ratio P/Al of the supernatant obtained by each washing was also estimated (Table 1). The supernatant obtained by the first washing included a large amount of amorphous phase, so that the amorphous phase probably influenced the P/Al ratio. The supernatant obtained by the second washing included layered aluminophosphates A and B, and had a P/Al ratio of about 2. The supernatant obtained by the third washing included only layered aluminophosphate B. Its P/Al ratio was decreased to about 1 (1.3 ± 0.4). Since the coexistence of an amorphous phase and the small amount of the precipitates leads to some errors in the P/Al ratios, the accurate chemical compositions of layered aluminophosphates A and B could not be evaluated. However, these results suggest that layered aluminophosphate A had a P/Al ratio larger than 2, while the P/Al ratio of layered aluminophosphate B was close to 1. Moreover, these results indicate that the yield of layered aluminophosphate B was smaller than 9% on Al species basis. This low yield was possibly related to the inertness of AlOOH. Next, TEM images were taken for layered aluminophosphates A and B in the supernatants obtained by the washing of the precipitates formed by hydrothermal treatment at 170 °C for 1 day. In the TEM images of the samples prepared by evaporating the supernatants of the first and second washings, many thin particles with a lateral size ranging from several nanometers to about 20 nm were observed. The TEM images of the aluminophosphate layers in the supernatant obtained by the second washing were shown in Fig. 5a and b. We guessed that the very thin particles were layered aluminophosphate A. In their high-resolution images, lattice fringes were seen with an interval of about 0.5 nm (Fig. 5b). Moreover, for the samples prepared by evaporating the supernatants of the third and fourth washings, very thin particles with a lateral size of 5–10 nm were observed. The TEM images of the aluminophosphate layers in the supernatant obtained by the fourth washing were shown in Fig. 5c and d. We guessed that the very thin particles were layered aluminophosphate B. In their high-resolution images, lattice fringes were observed at an interval of about 0.24 and 0.19 nm (Fig. 5d). The selected area electron diffraction (SAED) showed diffraction rings and spots with d-spacing of 0.24, 0.19, and 0.14 nm. Some SAED patterns showed that the lattice planes with d-spacing of 0.19 nm are normal to the ones of d = 0.14 nm (Fig. 5e). Furthermore, the amorphous precipitates were seen as particles with a size of about 50 nm. AFM measurement is required for examining if the layered aluminophosphates were present as a monolayer nanosheet in the sols. However, the small lateral size of the layered aluminophos-
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Fig. 4. XRD patterns of (a) the supernatants and (b) the precipitates obtained by the washing. For the supernatants, XRD measurements were made for the thin films prepared by evaporating the supernatants on a glass substrate.
Table 1 Aluminophosphate phases# included in the precipitates and supernatant obtained by each washing, the molar ratio P/Al of the precipitates and the supernatant, and the ratio of Al3+ amount in the precipitates to that in the starting gels. Precipitates
Before washing 1st washing 2nd washing 3rd washing 4th washing
Supernatant
Phases#
P/Al ratio
Al3+ in ppt./used AlOOH
Phases#
P/Al ratio
A + B + amor. B + amor. (+A) B + amor. B + amor. Amor.
1.56 1.31 1.21 1.20 1.12
57% 26% 23% 20% 17%
– A + amor. A+B B B + amor.
– 1.76 ± 0.06 2.0 ± 0.7 1.3 ± 0.4 1.6 ± 0.7
#: ‘‘A”, ‘‘B”, and ‘‘amor.” represent layered aluminophosphates A and B and an amorphous phase, respectively.
Fig. 5. TEM images of aluminophosphate layers in the supernatants obtained by (a, b) the second and (c, d) the fourth washing processes. We guess that they are layered aluminophosphates A and B, respectively. Panel (e) is the SAED pattern of image d.
phates and the coexistence of an amorphous phase made AFM measurements harder. As mentioned above, it is certain that the layered aluminophosphates were exfoliated in aqueous sols,
because they had a bulky interlayer cation TMA+. Although we could not elucidate that the thus obtained nanosheets were a monolayer, they were highly water-dispersible.
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On the basis of the results of XRD, XRF, and TEM, we inferred that layered aluminophosphate B had kanemite-like aluminophosphate layers [AlPO4(OH)] , which is called AlPO-ntu (Fig. 6a). The synthesis of AlPO-ntu by using NH2(CH2)nCH3 (n = 3, 5, and 7) and 1-phenylethylamine as a template have been reported until now [23,24]. In AlPO-ntu structures, [AlPO4(OH)] layers are stacking along the b-axis. The amines formed a bi-layer in the interlayer, providing a basal spacing ranging from 1.9 to 2.7 nm [23,24]. Layered aluminophosphate B had a basal spacing of 2.36 nm, which is a probable value if TMA+ ion forms a bi-layer in the interlayer, because TMA+ ion is 0.4–0.5 nm in diameter [26,27]. Moreover, the diffractions with d-spacing of 0.24, 0.19, and 0.14 nm in the SAED patterns are assignable to the ones from the (2 0 2), (4 0 0), and (0 0 4) planes, respectively.
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For layered aluminophosphate A, there is no definitive evidence for its crystal structure; however, we supposed that layered aluminophosphate A might have [AlHP3O10] layers (Fig. 6b). Layered aluminophosphates AlH2P3O102H2O [21] and Al(NH4)HP3O102H2O [22] have [AlHP3O10] layers stacking along the a-axis. Their basal spacings are 0.79 nm and 0.76 nm, respectively, while the basal spacing of layered aluminophosphate A was 1.42 nm. Since TMA+ ion is 0.4–0.5 nm in diameter [26,27], these differences in basal spacing are reasonable. Furthermore, the lattice fringes with an interval of about 0.5 nm in the TEM images are possibly assigned to the (0 1 0) plane. 3.3. Synthesis conditions providing aluminophosphate nanosheets The influence of experimental conditions on the products was examined. The reaction gels were prepared by mixing AlOOH, H3PO4, and TMAOH. For typical synthesis, AlOOH and H3PO4 were mixed, and then TMAOH was added to the mixture, as outlined in the Experimental Procedure section. So, the influence of the mixing order of AlOOH, H3PO4, and TMAOH was investigated. First, the reaction gel was prepared by adding H3PO4 to a mixture of AlOOH and TMAOH. Hydrothermal treatment of this reaction gel at 170 °C also provided layered aluminophosphate B and a small amount of layered aluminophosphate A. However, when reaction gel was prepared by adding AlOOH to a mixture of H3PO4 and TMAOH, hydrothermal treatment of the reaction gel at 170 °C brought about the formation of AlPO-14 (AFN-type AlPO4). These results suggested that the reaction of AlOOH with acid (H3PO4) or base (TMAOH) during the preparation of reaction gels led to the formation of layered aluminophosphates, which have some relation to the inertness of AlOOH. Next, for investigating the influence of pH of reaction gels, the pH of the reaction gels was changed from 7.5 to 6.1 by adding HCl to the reaction gels at an HCl/TMAOH mole ratio of 0.5. Hydrothermal treatment at 170 °C provided AlPO-20 (SOD-type AlPO4), which is one of microporous AlPO4 materials synthesized using TMA+ as a structure-directing agent [28]. Thus, although the change in pH of reaction gels by the addition of HCl was small, layered aluminophosphates were not obtained, meaning that the pH range of the reaction gels providing layered aluminophosphates was limited. The use of NaOH instead of TMAOH was also investigated. Upon hydrothermal treating the reaction gel including NaOH, AlPO-5 (AFI-type AlPO4) was formed, indicating that TMA+ ion plays an important role in the formation of layered aluminophosphates A and B. 4. Conclusions
Fig. 6. Structures of (a) kanemite-like [AlPO4(OH)] Hydrogen atoms are omitted.
and (b) [AlHP3O10]
layer.
Until now, we reported that metalate nanosheets were synthesized by the acid-base reactions between metallic acid and TMAOH or tetrabutylammonium hydroxide in basic aqueous solutions. In this study, the application of this bottom-up synthesis of metalate nanosheets to the synthesis of aluminophosphate nanosheets was examined. Unlike previous bottom-up synthesis of metalate nanosheets, crystalline aluminophosphates or aluminum phosphates are hydrothermally synthesized in acidic or neutral aqueous solutions. Moreover, there was possibility that TMA+ acts as a structure-directing agent of microporous aluminum phosphates. However, in this study, layered aluminophosphates with an interlayer TMA+ ion were hydrothermally synthesized from aqueous mixtures of AlOOH, H3PO4, and TMAOH by bottom-up process. The layered aluminophosphates were formed in amorphous precipitates. Upon dispersing the precipitates to water under agitation, the layered aluminophosphates were transferred to aqueous
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phase, resulting in the transparent sols of aluminophosphates. Since the layered aluminophosphates had bulky interlayer cation TMA+, it is likely that in the sols, the layered aluminophosphates were exfoliated, providing aluminophosphate nanosheets. If metalate nanosheets were synthesized in acidic or neutral aqueous solution by bottom-up process, such synthesis method would enable the fabrication of continuous monolayer nanosheet thin films by directly depositing nanosheets on positively charged substrate surface. Since layered aluminophosphates were formed in amorphous gels, the bottom-up synthesis in this study can not be utilized to such thin film fabrication. However, the results in this study showed some possibility that the bottom-up synthesis of other metalophosphates can be applied to such thin film fabrication. Furthermore, the prolonged hydrothermal treatment provided microporous AlPO4 (ATT type) crystals. Thus, it is likely that under the synthesis conditions employed in this study, layered aluminophosphates were formed, and then structurally converted to crystalline microporous AlPO4. Such results would also provide important insight into the formation mechanism of microporous AlPO4 using tetraalkylammonium ions as a structure-directing agent.
Acknowledgement This study was supported by KAKENHI (Grant-in-Aid for Scientific Research 17K06014) from Japan Society for the Promotion of Science (JSPS).
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Bottom-up synthesis of aluminophosphate nanosheets by hydrothermal process Takayuki Ban ⇑, Shota Iriyama, Yutaka Ohya Department of Chemistry and Biomolecular Science, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan
a r t i c l e
i n f o
Article history: Received 29 June 2017 Received in revised form 27 September 2017 Accepted 17 October 2017 Available online xxxx Keywords: Aluminophosphate Nanosheet Hydrothermal synthesis Bottom-up process Aqueous solution process
a b s t r a c t Hydrothermal treatment of aqueous mixtures of boehmite (AlOOH), phosphoric acid, and tetramethylammonium hydroxide provided two types of layered aluminophosphates having tetramethylammonium ion (TMA+) as an interlayer cation, despite the fact that TMA+ ion acts as a structure-directing agent of microporous AlPO4 materials and less likely leads to a lamellar structure. The layered aluminophosphates were formed in amorphous gels, so that they were obtained as precipitates. Upon dispersing the precipitates to water under agitation, the layered aluminophosphates were transferred to the aqueous phase, resulting in transparent aqueous sols. Because they had bulky interlayer cation TMA+, it is likely that in the sols, layered aluminophosphates were exfoliated, providing aluminophosphate nanosheets. Moreover, their characterization results suggest that the layered compound formed at 170 °C consisted of aluminophosphate layers with kanemite-like structure. Furthermore, the prolonged hydrothermal treatment at 170 °C led to the formation of microporous ATT-type AlPO4 crystals. Under the synthesis condition employed in this study, layered aluminophosphates were formed at early stage, and then structurally converted to microporous AlPO4 crystals. Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Metalate nanosheets are two-dimensional materials. There is some possibility that novel chemical and physical properties emerge due to their highly anisotropic shapes. Upon exfoliating layered metalates consisting of negatively charged metalate slabs and interlayer cations, the metalate slabs are obtained as metalate nanosheets. Conventionally, metalate nanosheets have been prepared by the ion exchange of interlayer cations for bulky tetraalkylammonium ions, which causes the swelling and exfoliation of the layered metalates in aqueous solution [1–3]. The resulting metalate nanosheets have so large lateral size that their aqueous sols have interesting properties such as liquid crystalline properties [4,5]. Unlike the conventional method, we have synthesized metalate nanosheets by bottom-up process, which utilizes the acid-base reactions between metallic acid and tetraalkylammonium hydroxide in aqueous solutions [6–10], although the lateral size of the resulting nanosheets is not so large. The applications of metalate nanosheet thin films have been studied extensively [11–16]. Thin films of metalate nanosheets
⇑ Corresponding author.
are fabricated by Layer-by-Layer method and Langmuir-Blodgett method. Bottom-up synthesis of metalate nanosheets has some possibility that continuous monolayer thin films of metalate nanosheets are fabricated by depositing nanosheets directly on chemically modified substrate surface. However, the bottom-up syntheses, which we have studied, were conducted in basic aqueous solutions. Since basic solutions cause corrosion and dissolution of many types of substrates, bottom-up synthesis of metalate nanosheets in acidic or neutral aqueous solutions is desirable for thin film fabrication. Moreover, we previously reported the bottom-up synthesis of titanate nanoflakes in ionic liquid solvents [17]. However, the synthesis conditions providing titanate nanoflakes were limited, because OH ion reacted with ionic liquid molecules. Also for the investigation of the influence of ionic liquid solvent on morphology of metalate nanosheets obtained by bottom-up process, synthesis in acidic or neutral solvents is preferable. It is known that layered aluminophosphates and aluminum phosphates with different crystal structures are hydrothermally synthesized in acidic or neutral solutions [18–24]. So, we envisaged that layered aluminophosphates or aluminophosphate nanosheets would be synthesized in acidic or neutral solutions by bottom-up process. However, there were some problems.
E-mail address: [email protected] (T. Ban). https://doi.org/10.1016/j.apt.2017.10.013 0921-8831/Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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Although tetraalkylammonium ions such as TMA+ ion are used for our bottom-up process of metalate nanosheets, they have ability to act as a structure-directing agent of crystalline microporous aluminum phosphates AlPO4. Furthermore, although the metalate nanosheets that we have synthesized by bottom-up process included only one type of cation, aluminophosphate nanosheets include two types of cations, i.e. Al3+ and P5+, so that the formation reactions would be more complex. So, the objective of this study is to examine if aluminophosphate nanosheets are synthesized in acidic or neutral aqueous solutions by bottom-up process. 2. Experimental procedure 2.1. Synthesis Typical synthesis was conducted, as follows: 30 mmol of boehmite (AlOOH; CATAPAL C1, Sasol Chemicals) was dispersed in 18.6 mL of distilled water. 85% phosphoric acid (30 mmol of H3PO4) was added to the suspension. The suspension was stirred for 1 day. Then, 25% tetramethylammonium hydroxide (N(CH3)4OH; TMAOH) solution (30 mmol of TMAOH) was added to the suspension. The suspension was further stirred for 1 day. The resulting mixture was used as a reaction gel, was about 35 g in weight, had a molar ratio of 1 AlOOH:1 H3PO4:1 TMAOH:50 H2O, and was 7.5 in pH. The reaction gel was hydrothermally treated for 1–14 days at 80–170 °C, resulting in the formation of gel-like precipitates. The precipitates were collected by certification at 9000 rpm for 15 min. The precipitates were washed, as follows: they were dispersed in 200 mL of distilled water, were stirred for 1 day, and then were collected by centrifugation. The supernatant and washed precipitates were characterized. This washing process was repeated four times. 2.2. Characterization X-ray diffraction (XRD) measurements were performed on a Rigaku Ultima IV diffractometer with a monochromatic CuKa irradiation. XRD patterns were recorded at a scan rate of 2° min 1 in the 2h range of 2–70°. Powders or thin films prepared from aqueous suspensions of precipitates or aqueous colloids were used as a sample. The thin film samples were prepared by evaporating the suspensions or colloids on a glass substrate under the ambient condition. Transmission electron microscopy (TEM) images were captured using a JEOL JEM-2100 model at an accelerating voltage of 200 kV. The samples were prepared by evaporating a drop of aqueous suspension of samples on a Cu grid supported with a Formvar thin film. Hydrophilic treatment was conducted for the Cu grid before use. Wavelength-dispersive type X-ray fluorescence analysis (WDXRF) was conducted on a Bruker-AXS S8 TIGER-MA model for estimating Al/P molar ratio in the precipitates before and after washing. The chemical composition was evaluated by digital scan screening analysis using a fundamental parameter software. Measurement were made for the powdery sample placed on a polypropylene film, which is the bottom of a sample holder.
aqueous suspensions of the precipitates. Some peaks appeared at low diffraction angles (Fig. 1). Before the hydrothermal treatment, only peaks assigned to pseudo-boehmite was observed. Upon hydrothermally treating at 80–170 °C, peaks with d-spacing of 1.42 and 0.71 nm appeared. For the sample prepared at 170 °C, peaks with d-spacing of 2.36, 1.16, and 0.77 nm were also observed. Their d-spacings had a relation of 1:1/2:1/3, indicating high orientation of formed crystals. Because the peaks attributed to high orientation were observed at low diffraction angles, it is inferred that two types of layered aluminophosphates were formed at 80 and 170 °C. Hereafter, the compounds with a basal spacing of 1.42 and 2.36 nm are called ‘‘layered aluminophosphate A” and ‘‘layered aluminophosphate B,” respectively. Moreover, the basal spacings of the layered aluminophosphates were as large as 1.42 and 2.36 nm, suggesting the presence of tetramethylammonium (TMA+) ion in the interlayer. Next, the aqueous mixtures of AlOOH, H3PO4 and TMAOH were hydrothermally treated at 170 °C for different periods. The formed precipitates were collected by centrifugation, and then evaporated under the ambient condition. For the resulting powders, XRD measurements were made (Fig. 2). When the hydrothermal period was shorter than 7 days, the formed crystalline phases were only layered aluminophosphates. The hydrothermal treatment for further longer periods decreased the amount of the layered aluminophosphates and provided the formation of AlPO4-12-TAMU (ATT-type AlPO4), which is one of microporous aluminophosphates synthesized using TMA+ as a structure-directing agent [25]. After 14 days of hydrothermal treatment, the sample contained only AlPO4-12TAMU. Since tetraalkylammonium ion acts as a structuredirecting agent of zeolites, hydrothermal treatment of aqueous mixtures of AlOOH, H3PO4 and tetraalkylammonium ion ordinarily leads to the formation of microporous AlPO4 crystals. However, these results suggest that under the synthesis condition employed in this study, layered aluminophosphates were formed at early stage, and then structurally converted to microporous AlPO4. When layered materials with a bulky interlayer cation such as TMA+ are dispersed in water, the swelling and exfoliation of the layered materials occur, resulting in the formation of nanosheets. However, in this study, such layered materials were obtained as
3. Results and discussion 3.1. Synthesis of aluminophosphate nanosheets The aqueous mixtures of AlOOH, H3PO4 and TMAOH were hydrothermally treated for 1 day at different temperatures. XRD measurements were conducted for the thin films prepared from
Fig. 1. XRD patterns of the thin films fabricated from the precipitates (a) before hydrothermal synthesis and after hydrothermal synthesis at (b) 80 °C, (c) 120 °C and (d) 170 °C for 1 day.
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obtained as precipitates. Upon dispersing the precipitates to water under agitation, the layered aluminophosphates were transferred to aqueous phase, resulting in the sols of aluminophosphates. Since they had bulky interlayer cation TMA+, it is likely that in the sols, the layered aluminophosphates were exfoliated, providing aluminophosphate nanosheets. 3.2. Characterization of aluminophosphate nanosheets
Fig. 2. XRD patterns of the precipitates obtained by hydrothermal synthesis at 170 °C for (a) 1 day, (b) 3 days, (c) 7 days, and (d) 14 days.
precipitates. The reasons why the layered aluminophosphates with a bulky interlayer cation were formed as precipitates were examined. The precipitates obtained by hydrothermal treatment at 170 °C for 1 day were dispersed in water and then stirred for 1 day. This process was repeated four times. The resulting supernatants were transparent. When a laser light was irradiated to them, the trace of the light passage was observed due to Tyndall effect, indicating colloidal nature of the supernatants (Fig. 3). Moreover, XRD measurements were made for the thin films prepared by evaporating the supernatants on a glass substrate (Fig. 4a) and for the powders prepared by evaporating the precipitates under the ambient condition (Fig. 4b). The aluminophosphate phases including in the supernatants and precipitates obtained by each washing are also listed in Table 1. The supernatants obtained by the first and second washings included layered aluminophosphate A, while layered aluminophosphate B was present in the supernatants obtained by the second, third and fourth washings. However, for the supernatant of obtained by the second washing, the amount of layered aluminophosphate A was changed sample by sample. Furthermore, before the fourth washing, layered aluminophosphate B was included in the precipitates; however, after the fourth washing, the precipitates were amorphous (Fig. 4b). These results suggest that since layered aluminophosphates A and B were formed in an amorphous phase, they were
Fig. 3. Appearance of the supernatants obtained by the washing process of the precipitates formed by hydrothermal treatment at 170 °C for 1 day.
Many types of layered aluminophosphates and layered aluminum phosphates are known, such as [Al3P4O16]3 , [AlP2O8]3 , [Al2P3O12Hx](3 x) , [Al(HPO4)2(H2O)2] , [Al4P5O20H]2 , [AlHP3O10] , and [AlPO4(OH)] [18–24]. So, the structure types of layered aluminophosphates A and B obtained in this study were investigated. First, the P/Al ratios of layered aluminophosphates A and B were roughly estimated. The precipitates formed by hydrothermal treatment at 170 °C for 1 day were washed 4 times. The precipitates obtained by each washing process were heated at 800 °C. The P/ Al molar ratios of the resulting powders were estimated by XRF (Table 1). Their weights were also measured. From these values, the ratio of Al species in the precipitates after each washing process against the used AlOOH was evaluated, and the molar ratio P/Al of the supernatant obtained by each washing was also estimated (Table 1). The supernatant obtained by the first washing included a large amount of amorphous phase, so that the amorphous phase probably influenced the P/Al ratio. The supernatant obtained by the second washing included layered aluminophosphates A and B, and had a P/Al ratio of about 2. The supernatant obtained by the third washing included only layered aluminophosphate B. Its P/Al ratio was decreased to about 1 (1.3 ± 0.4). Since the coexistence of an amorphous phase and the small amount of the precipitates leads to some errors in the P/Al ratios, the accurate chemical compositions of layered aluminophosphates A and B could not be evaluated. However, these results suggest that layered aluminophosphate A had a P/Al ratio larger than 2, while the P/Al ratio of layered aluminophosphate B was close to 1. Moreover, these results indicate that the yield of layered aluminophosphate B was smaller than 9% on Al species basis. This low yield was possibly related to the inertness of AlOOH. Next, TEM images were taken for layered aluminophosphates A and B in the supernatants obtained by the washing of the precipitates formed by hydrothermal treatment at 170 °C for 1 day. In the TEM images of the samples prepared by evaporating the supernatants of the first and second washings, many thin particles with a lateral size ranging from several nanometers to about 20 nm were observed. The TEM images of the aluminophosphate layers in the supernatant obtained by the second washing were shown in Fig. 5a and b. We guessed that the very thin particles were layered aluminophosphate A. In their high-resolution images, lattice fringes were seen with an interval of about 0.5 nm (Fig. 5b). Moreover, for the samples prepared by evaporating the supernatants of the third and fourth washings, very thin particles with a lateral size of 5–10 nm were observed. The TEM images of the aluminophosphate layers in the supernatant obtained by the fourth washing were shown in Fig. 5c and d. We guessed that the very thin particles were layered aluminophosphate B. In their high-resolution images, lattice fringes were observed at an interval of about 0.24 and 0.19 nm (Fig. 5d). The selected area electron diffraction (SAED) showed diffraction rings and spots with d-spacing of 0.24, 0.19, and 0.14 nm. Some SAED patterns showed that the lattice planes with d-spacing of 0.19 nm are normal to the ones of d = 0.14 nm (Fig. 5e). Furthermore, the amorphous precipitates were seen as particles with a size of about 50 nm. AFM measurement is required for examining if the layered aluminophosphates were present as a monolayer nanosheet in the sols. However, the small lateral size of the layered aluminophos-
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Fig. 4. XRD patterns of (a) the supernatants and (b) the precipitates obtained by the washing. For the supernatants, XRD measurements were made for the thin films prepared by evaporating the supernatants on a glass substrate.
Table 1 Aluminophosphate phases# included in the precipitates and supernatant obtained by each washing, the molar ratio P/Al of the precipitates and the supernatant, and the ratio of Al3+ amount in the precipitates to that in the starting gels. Precipitates
Before washing 1st washing 2nd washing 3rd washing 4th washing
Supernatant
Phases#
P/Al ratio
Al3+ in ppt./used AlOOH
Phases#
P/Al ratio
A + B + amor. B + amor. (+A) B + amor. B + amor. Amor.
1.56 1.31 1.21 1.20 1.12
57% 26% 23% 20% 17%
– A + amor. A+B B B + amor.
– 1.76 ± 0.06 2.0 ± 0.7 1.3 ± 0.4 1.6 ± 0.7
#: ‘‘A”, ‘‘B”, and ‘‘amor.” represent layered aluminophosphates A and B and an amorphous phase, respectively.
Fig. 5. TEM images of aluminophosphate layers in the supernatants obtained by (a, b) the second and (c, d) the fourth washing processes. We guess that they are layered aluminophosphates A and B, respectively. Panel (e) is the SAED pattern of image d.
phates and the coexistence of an amorphous phase made AFM measurements harder. As mentioned above, it is certain that the layered aluminophosphates were exfoliated in aqueous sols,
because they had a bulky interlayer cation TMA+. Although we could not elucidate that the thus obtained nanosheets were a monolayer, they were highly water-dispersible.
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On the basis of the results of XRD, XRF, and TEM, we inferred that layered aluminophosphate B had kanemite-like aluminophosphate layers [AlPO4(OH)] , which is called AlPO-ntu (Fig. 6a). The synthesis of AlPO-ntu by using NH2(CH2)nCH3 (n = 3, 5, and 7) and 1-phenylethylamine as a template have been reported until now [23,24]. In AlPO-ntu structures, [AlPO4(OH)] layers are stacking along the b-axis. The amines formed a bi-layer in the interlayer, providing a basal spacing ranging from 1.9 to 2.7 nm [23,24]. Layered aluminophosphate B had a basal spacing of 2.36 nm, which is a probable value if TMA+ ion forms a bi-layer in the interlayer, because TMA+ ion is 0.4–0.5 nm in diameter [26,27]. Moreover, the diffractions with d-spacing of 0.24, 0.19, and 0.14 nm in the SAED patterns are assignable to the ones from the (2 0 2), (4 0 0), and (0 0 4) planes, respectively.
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For layered aluminophosphate A, there is no definitive evidence for its crystal structure; however, we supposed that layered aluminophosphate A might have [AlHP3O10] layers (Fig. 6b). Layered aluminophosphates AlH2P3O102H2O [21] and Al(NH4)HP3O102H2O [22] have [AlHP3O10] layers stacking along the a-axis. Their basal spacings are 0.79 nm and 0.76 nm, respectively, while the basal spacing of layered aluminophosphate A was 1.42 nm. Since TMA+ ion is 0.4–0.5 nm in diameter [26,27], these differences in basal spacing are reasonable. Furthermore, the lattice fringes with an interval of about 0.5 nm in the TEM images are possibly assigned to the (0 1 0) plane. 3.3. Synthesis conditions providing aluminophosphate nanosheets The influence of experimental conditions on the products was examined. The reaction gels were prepared by mixing AlOOH, H3PO4, and TMAOH. For typical synthesis, AlOOH and H3PO4 were mixed, and then TMAOH was added to the mixture, as outlined in the Experimental Procedure section. So, the influence of the mixing order of AlOOH, H3PO4, and TMAOH was investigated. First, the reaction gel was prepared by adding H3PO4 to a mixture of AlOOH and TMAOH. Hydrothermal treatment of this reaction gel at 170 °C also provided layered aluminophosphate B and a small amount of layered aluminophosphate A. However, when reaction gel was prepared by adding AlOOH to a mixture of H3PO4 and TMAOH, hydrothermal treatment of the reaction gel at 170 °C brought about the formation of AlPO-14 (AFN-type AlPO4). These results suggested that the reaction of AlOOH with acid (H3PO4) or base (TMAOH) during the preparation of reaction gels led to the formation of layered aluminophosphates, which have some relation to the inertness of AlOOH. Next, for investigating the influence of pH of reaction gels, the pH of the reaction gels was changed from 7.5 to 6.1 by adding HCl to the reaction gels at an HCl/TMAOH mole ratio of 0.5. Hydrothermal treatment at 170 °C provided AlPO-20 (SOD-type AlPO4), which is one of microporous AlPO4 materials synthesized using TMA+ as a structure-directing agent [28]. Thus, although the change in pH of reaction gels by the addition of HCl was small, layered aluminophosphates were not obtained, meaning that the pH range of the reaction gels providing layered aluminophosphates was limited. The use of NaOH instead of TMAOH was also investigated. Upon hydrothermal treating the reaction gel including NaOH, AlPO-5 (AFI-type AlPO4) was formed, indicating that TMA+ ion plays an important role in the formation of layered aluminophosphates A and B. 4. Conclusions
Fig. 6. Structures of (a) kanemite-like [AlPO4(OH)] Hydrogen atoms are omitted.
and (b) [AlHP3O10]
layer.
Until now, we reported that metalate nanosheets were synthesized by the acid-base reactions between metallic acid and TMAOH or tetrabutylammonium hydroxide in basic aqueous solutions. In this study, the application of this bottom-up synthesis of metalate nanosheets to the synthesis of aluminophosphate nanosheets was examined. Unlike previous bottom-up synthesis of metalate nanosheets, crystalline aluminophosphates or aluminum phosphates are hydrothermally synthesized in acidic or neutral aqueous solutions. Moreover, there was possibility that TMA+ acts as a structure-directing agent of microporous aluminum phosphates. However, in this study, layered aluminophosphates with an interlayer TMA+ ion were hydrothermally synthesized from aqueous mixtures of AlOOH, H3PO4, and TMAOH by bottom-up process. The layered aluminophosphates were formed in amorphous precipitates. Upon dispersing the precipitates to water under agitation, the layered aluminophosphates were transferred to aqueous
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phase, resulting in the transparent sols of aluminophosphates. Since the layered aluminophosphates had bulky interlayer cation TMA+, it is likely that in the sols, the layered aluminophosphates were exfoliated, providing aluminophosphate nanosheets. If metalate nanosheets were synthesized in acidic or neutral aqueous solution by bottom-up process, such synthesis method would enable the fabrication of continuous monolayer nanosheet thin films by directly depositing nanosheets on positively charged substrate surface. Since layered aluminophosphates were formed in amorphous gels, the bottom-up synthesis in this study can not be utilized to such thin film fabrication. However, the results in this study showed some possibility that the bottom-up synthesis of other metalophosphates can be applied to such thin film fabrication. Furthermore, the prolonged hydrothermal treatment provided microporous AlPO4 (ATT type) crystals. Thus, it is likely that under the synthesis conditions employed in this study, layered aluminophosphates were formed, and then structurally converted to crystalline microporous AlPO4. Such results would also provide important insight into the formation mechanism of microporous AlPO4 using tetraalkylammonium ions as a structure-directing agent.
Acknowledgement This study was supported by KAKENHI (Grant-in-Aid for Scientific Research 17K06014) from Japan Society for the Promotion of Science (JSPS).
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