9958
J. Phys. Chem. B 1997, 101, 9958-9963
Structure of Porous Aggregates of the Ammonium Salt of Dodecatungstophosphoric Acid, (NH4)3PW12O40: Unidirectionally Oriented Self-Assembly of Nanocrystallites Takeru Ito, Kei Inumaru, and Makoto Misono* Department of Applied Chemistry, Graduate School of Engineering, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan ReceiVed: May 1, 1997; In Final Form: August 1, 1997X
It was demonstrated that porous aggregates of (NH4)3PW12O40 nanocrystallites were formed by epitaxial selfassembly, when dodecatungstophosphoric acid was precipitated by ammonium hydrogen carbonate in an aqueous solution. Scanning electron microscopy combined with nitrogen and argon adsorption measurements showed that nanocrystallites of (NH4)3PW12O40 formed aggregates of ca. 0.3-1 µm in size, which are porous with a nearly uniform micropore size of 0.6-1.3 nm in diameter. The shape of these aggregates changed from spherical to symmetric dodecahedral as the precipitation temperature increased. Electron diffraction, transmission electron microscopy, and X-ray powder diffraction demonstrated that the nanocrystallites which form the aggregates had the same crystal orientation in each aggregate, leaving pores between the nanocrystallites, and they were connected epitaxially to each other to variable extents, depending on the precipitation temperature. The dodecahedral aggregates, for which the epitaxial interconnections grow significantly, can be regarded as microporous single crystals. The results were compared with the aggregates of Cs salts which were previously shown to be randomly oriented and exhibit shape-selective catalysis.
Introduction Self-assembly of molecules1,2 and particles3-7 has attracted much attention as a strategy to design new materials with dimensions of the order of 100 nm or above. In the molecular self-assembly, noncovalent bonds such as hydrogen bonds connect molecules to form structurally well-defined aggregates.1,2 These materials are interesting because of their potential application as electronic devices1,8 and as models of biological energy transfer system.9 As for the self-assembly of particles, three-dimensional periodicity in the liquid phase has been known as “colloidal crystals”, in which electrostatic repulsive force plays an important role.10 Particles of latex and silica are known to form such ordered structures.11 Fixation of the colloidal crystal array into a polymer and its application as optical devices have been reported.3 The self-assembly of nanocrystallites which are stabilized by organic molecules has also been extensively investigated. For example, Murray et al. reported “superlattice” formation by self-assembly of CdSe nanocrystallites.4 Chemseddine et al. investigated the CdS nanocluster self-assembly process by scanning tunneling microscopy and ultraviolet and visible spectroscopy.5 Mirkin et al. reported self-assembly of Au nanocrystals coated by DNA molecules by utilizing their molecular-recognizable nature.6 In these self-assembled particles, although the location of primary particles is ordered, the orientation of the particles is usually random. Matijevic´ and Scheiner have reported the preparation of R-Fe2O3 consisting of particles with uniform shape.12 Recently, it was also reported that R-Fe2O3 formed polycrystalline pseudocubic particles in which rodlike crystallites had the same crystal orientation.13 It had been pointed out that the study of microporous materials with well-defined structure is a promising strategy for understanding catalysis at the molecular level.14 Cs salts of a heteropolyacid, H3PW12O40, which is known as solid “superacid” catalysts,15 form random aggregates of ultrafine crystallites * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, October 15, 1997.
S1089-5647(97)01483-1 CCC: $14.00
(ca. 10 nm in diameter), having well-controlled porosity in the spaces between the crystallites.16 It was possible to develop a shape-selective solid “superacid” by taking advantage of the controlled microporosity of Cs2.2H0.8PW12O40.17 To design the pore structure in this case, it is important to control the aggregation process of nanocrystallites. Previously we reported the formation of a novel type of porous aggregate (ca. 0.5-6 µm in size) of an ammonium salt of dodecatungstophosphoric acid, (NH4)3PW12O40, in which nanocrystallites were unidirectionally oriented and were connected with each other by epitaxial interfaces.18 These large aggregates have symmetrical shape and high porosity and could grow to a variable extent. In this paper, the structure of these aggregates and the effect of preparation conditions, particularly the temperature, are discussed in more detail, focusing on the self-assembly of (NH4)3PW12O40 nanocrystallites. Experimental Section Preparation of Materials. H3PW12O40 was purchased from Nippon Inorganic Color & Chemical Co., Ltd., and used after extraction with diethyl ether and recrystallization from water. (NH4)3PW12O40 was prepared at controlled temperatures as follows: An aqueous solution of NH4HCO3 (0.055 mol dm-3) was added dropwise to ca. 30 cm3 of an aqueous solution of H3PW12O40 (0.025 mol dm-3) with vigorous stirring to form a white colloidal solution, which contains well-dispersed (NH4)3PW12O40 precipitates. Usually the rate of NH4HCO3 addition was ca. 0.40 cm3 min-1, and the time taken for the addition of NH4HCO3 was ca. 90 min. During this procedure, the H3PW12O40 solution was kept at a constant temperature in the range of 273-368 K, which we call the “precipitation temperature” hereafter. The resulted solutions were aged usually for 30 min at the same temperature. Then they were cooled to room temperature and were dried at 328 K with a vacuum rotary evaporator to obtain the white powder of (NH4)3PW12O40. Hereafter, these (NH4)3PW12O40 are designated by NH4-t, where t indicates the precipitation temperature (for example, NH4-368 means (NH4)3PW12O40 precipitated at 368 K). In some cases, © 1997 American Chemical Society
Structure of (NH4)3PW12O40
J. Phys. Chem. B, Vol. 101, No. 48, 1997 9959
Figure 1. SEM images of (NH4)3PW12O40: (a) (NH4)3PW12O40 precipitated at 368 K, (b) the same sample as a with lower magnification, (c) (NH4)3PW12O40 precipitated at 273 K, and (d) the same sample as c with lower magnification.
a freeze-drying method was employed: The colloidal solution was transferred dropwise into a flask which was cooled by liquid N2. The solution was quickly frozen on the internal wall of the flask. Then the frozen solution was dried under vacuum, maintaining the solid state by using a freeze-dryer FD-80 (EYELA). To study the stability of the samples, treatments under vacuum and in the presence of water vapor were carried out. In the former case, the samples were evacuated at 573 K for 2 h. In the latter case, the sample powder (ca. 0.3 g) was placed in a Pyrex tube and heated at 573 K for 2 h in a helium flow (150 cm3 min-1) which contained 5% water vapor. SEM, TEM, and Other Measurements. The images of scanning electron microscopy (SEM) were taken with an S-900 FE-SEM (Hitachi). Transmission electron microscopy (TEM) and electron diffraction (ED) measurements were carried out using a JEM-4000FX II microscope (JEOL). Powder X-ray diffraction (XRD) was measured with an MXP3 diffractometer (Mac Science) using Cu KR radiation. The diffraction line widths of the samples were obtained after the subtraction of the instrumental width, which was determined using the line width of well-crystallized Si powder. Simulation of the XRD pattern was carried out using a Cerius2 diffraction program module (Byosim/Molecular Simulation Inc.), in which relative intensities of the diffraction peaks were calculated by taking into account the structure factors and polarization factors. N2 adsorption isotherms were measured at 77 K using an ASAP2000 (Micromeritics). The samples were usually heated at 573 K for 2 h under vacuum prior to the measurements. Ar isotherms used in the micropore analysis by the Horva´thKawazoe method19 were taken at 87 K (liquid Ar) after evacuation at 573 K with Omnisorp 100 (Coulter). Results SEM Observation. Figure 1 shows the SEM images of NH4368 and NH4-273. When the salt was precipitated at 368 K
(NH4-368, Figure 1a), aggregates having a symmetric dodecahedral shape were observed. The SEM image with lower magnification (Figure 1b) shows that all aggregates had the identical dodecahedral shape, having different sizes (0.5-2 µm). On the other hand, NH4-273 formed spherical aggregates as shown in Figure 1c. The SEM image with lower magnification (Figure 1d) showed only spherical particles (ca. 0.1-0.5 µm in size). The sample preparation in this study comprises three steps. To be discussed in a later section about which step most influences the final structure, a freeze-drying method was applied as described in the Experimental Section for the colloidal solution of NH4-368. SEM showed that dodecahedral aggregates very similar to those shown in Figure 1 were formed also in this case, demonstrating that these aggregates were already formed before the evaporation or freeze-drying. It was also confirmed that the dodecahedral shape was little affected by the change of the addition rate of NH4HCO3 solution (ca. 0.2-40 cm3 min-1) and of the aging time (ca. 0-30 min). When the spherical aggregates of NH4-273 were treated in water at 368 K, little change was observed in SEM with respect to the shape of the aggregates. N2 Adsorption at 77 K. The surface area of the dodecahedral aggregates of NH4-368 was 65 m2 g-1. This is much greater than what is expected from the SEM image; the apparent outer surface area of dodecahedra seen in SEM was estimated to be less than 4 m2 g-1. These results demonstrate that the aggregates shown in Figure 1 are highly porous and may be regarded as consisting of fine crystallites. The spherical aggregates of NH4-273 (Figure 1c) are also porous, since the BET surface area was 116 m2 g-1, while the surface area estimated from SEM was less than 5 m2 g-1. Hereafter, the particles seen in SEM are called “aggregates” and the apparent fine particles “nanocrystallites”. Table 1 lists the BET surface areas and sizes of nanocrystallites. The latter is denoted hereafter by dBET, which was
9960 J. Phys. Chem. B, Vol. 101, No. 48, 1997
Ito et al.
TABLE 1: Properties of (NH4)3PW12O40 Precipitated at Various Temperatures precipitation tempa/K 368 328 298 273
BET surface area/(m2 g-1)
dBETb/nm
65 79 104 116
15 12 9 9
micropore volc/(10-2 cm3 g-1) 2.7 3.2 4.1 4.3
SMPd/(m2 g-1) >83 >98 >126 >132
dN2e/nm
