Formation of Mesoporous BaSO4 Microspheres with a Larger Pore Size via Ostwald Ripening at Room Temperature

Mesoporous microspheres with a larger pore size (diameter of pore: ca. 35 nm, Barrett-Emmett-Teller (BET) surface area: 25.6 m/g), the radial arrangement of irregular BaSO4 nanorods, were successfully synthesized by the radiolysis of an aqueous solution of K2S2O8, Ba(NO3)2, and disodium ethylenediaminetetraacetate with an irradiation time of 1000 min (dose rate: 20 Gy/min) at room temperature. It was confirmed that the mesoporous microspheres with a larger pore size evolved from the mesoporous microspheres with a smaller pore size (diameter of pore: ca. 4 nm, BET surface area: 49.6 m/g), which were generated at the early stage of the irradiation course and were mainly constructed by quasi-spherical nanoparticles, via Ostwald ripening at room temperature. In the current materials synthesis and nanodevice fabrication, complex architectures constructed by the self-assembly and highly ordered organization of oneand two-dimensional (1D, 2D) nanoscale building blocks have been of intense interest because of their uniqueproperties andpotential applications, andare of importance in understanding the regularity of self-assembly with artificial buildingblocks.Asa result of rapid advancements in synthetic strategies, highlyorganized1Dand2Dbuildingblocksofmetals,metal oxides, ,3 organic-inorganic hybrid materials, minerals, and soon have been synthesized.Among the numerous synthetic methods, Ostwald ripening is simple and powerful, and has been widely applied. However, as to the materials with very low equilibrium solubility over a wide range of pH values, the molecular redissolution-crystallization events are suppressed to a great extent. This makes it difficult to construct the complex nanostructures based on these materials by Ostwald ripening. Therefore, it becomesnecessary to extend the application rangeof Ostwald ripening in the construction of complex architectures. Furthermore, if this process takesplacewith a relatively rapid rate at room temperature, it will be more practical. Barium sulfate (BaSO4), which is commonly known as Barite and inert inmany chemical reactions, has beenwidely used inmany areas such as catalyst carriers, adsorbents, contrast agents in the field of radiology, fillers and additives in polymers, and so forth. Besides, BaSO4 has been extensively used to investigate the precipitation and crystallization processes. So far, many amazing BaSO4 particles with complex architectures have been synthesized, for example, fiber bundles/zones, shuttlecocks, flowers, peanuts, peaches, etc. As one of the typical materials with a very low equilibrium solubility over a wide range of pH values, complex BaSO4 nanostructures were seldom constructed by Ostwald ripening. Consequently, there still remains a great challenge. In recent years, porous materials, especially mesoporous materials, have attracted much attention for their large specific surface area, tailored pore size and structure, surface properties, and therefore their wide range of potential applications in catalysis, adsorption, and so on. As to BaSO4, after Li et al. reported the excellent performance of the agglomerate BaSO4 nanotubes in supporting sulfates for methane activation at lower temperature, mesoporous BaSO4 nanomaterials began to attract notice. It is believed that mesoporous BaSO4 nanomaterials will be widely used in the future. Obviously, the syntheses of mesoporous BaSO4 nanomaterials with different morphologies and controllable pore size are important and necessary. To the best of our knowledge, there is no report on the preparation of mesoporous BaSO4 microsphere with a larger pore size. γ-Irradiation is a powerful method in the syntheses of nanoparticles and inorganic-polymer nanocomposites. In our previous work, for the first time, we successfully synthesized BaSO4 nanoparticles by the controlled-release effect of γ-irradiation, which subsequently formed“solid”microspheres via aggregation. Herein, we report the radiolytic synthesis of mesoporous BaSO4 microspheres with a larger pore size, the radial assemblies of irregular nanorods, in aqueous solution.As far asweknow, this is the first report about this kind of complex BaSO4 nanostructure. The research results indicate that Ostwald ripening can be effectively used to construct the complex nanostructures of the materialswithvery lowequilibriumsolubilityover awide rangeof pH values. The typical aqueous solution containing 4 mmol/L Ba(NO3)2, 4 mmol/L K2S2O8, and 8 mmol/L disodium ethylenediaminetetraacetate (EDTA) was prepared. After being bubbled with highpurityN2 under anaerobic conditions, the solutionwas irradiated in the field of a Coγ-ray source for a definite time at the location whose dose rate was fixed at 20 Gy/min. Then, white precipitates were obtained. Morphology of the products was observed with a scanning electronmicroscope (SEM,Hitachi S-4800), a transmission electronmicroscope (TEM, JEOLJEM-200CX), and ahighresolution TEM (HRTEM, Hitachi 9000). To investigate the inner structure, theobtainedpowderwasdispersed inwaterand sonicated at room temperature for 1 h to get the sample for characterization.X-raydiffraction (XRD) patternwas recordedonan X0 Pert PRO MPD diffractometer with Cu KR radiation, X-ray photoelectron spectrum (XPS) was collected on a Kratos Axis Ultra spectrometer with monochromatized Al KR radiation, and N2 adsorption-desorption isotherms were determined on a Micromeritics ASAP-2010 apparatus. As is well-known, when the diluted aqueous solution is irradiated by γ-rays, the water molecules absorb the irradiation energy and generate many reactive species, such as eaq , H, and •OH (eq 1): H2O sf irradiated eaq , H, •OH,H3Oþ, ::: ð1Þ Then, the oxidative •OH is eliminated by EDTA, with a rate constant of 4.0 10 L 3mol 3 s, and the reducing species, *Corresponding author. Fax: þ86-10-62759191. Tel: þ86-10-62765915. E-mail: [email protected]. Communication Crystal Growth & Design, Vol. 10, No. 9, 2010 3839 especially eaq , reduce S2O8 2ions to SO4 2ions (eq 2), with a rate constant of 1.2 10 L 3mol 3 s. S2O8 2þ eaq f SO4 þ SO4 • ð2Þ Thus, the controlled release of SO4 2and the following generation of BaSO4 could be realized, which favors the generation of uniform BaSO4 microspheres. 17 Figure 1A presents the SEM image of the sample prepared at an irradiation time of 1000 min. It can be seen that the product is composed of microspheres with a diameter of 2-4 μm, besides a few fragments. From the related SEM image in higher magnification (inset, Figure 1A), it can be clearly found that a lot of pores, with a diameter ranging from 20 to 60 nm, are evenly distributed on the surface of the microspheres. Moreover, in the N2 adsorption-desorption isotherm experiment, there appears an obvious hysteresis loop (curve a, Figure 2) associatedwith the filling of the mesopores due to capillary condensation, and the Barrett-Emmett-Teller (BET) surface area is calculated to be 25.6m/g,which is large forhigh-densitymaterials. Furthermore, the pore size profile of the sample (curve a, inset of Figure 2) shows that most of the pores are mesoporous and there is a single peak at about 35 nm, which is consistent with the SEM result. All of the above results indicate the existence of a mesoporous structure with a larger pore size in the microspheres. The corresponding XPS analysis (Figure SI-1, Supporting Information) shows that the binding energies of Ba 3d, S 2p, Figure 1. SEM images (A and E), TEM images (B and C), and HRTEM image (D) of the sample synthesized at an irradiation time of 1000 min. The insets in (A) and (C) show the image at higher magnification and the SAED pattern of the corresponding product, respectively. The dose rate is 20 Gy/min, and the concentration of EDTA is 8 mmol/L. Figure 2. N2 adsorption (solid)-desorption (open) isotherms of the samples synthesized at different irradiation times: (a) 1000 min, (b) 250 min. The inset shows the pore size distribution of the corresponding samples. The dose rate is 20Gy/min, and the concentration of EDTA is 8 mmol/L. Figure 3. XRD patterns of the samples synthesized under different conditions: (a) irradiation time: 1000 min, [EDTA] = 8mmol/L; (b) irradiation time: 250 min, [EDTA] = 8 mmol/L; (c) irradiation time: 1000min, [EDTA]=0.8mmol/L; (d) irradiation time: 1000min, [EDTA]=0.08mmol/L; (e) irradiation time: 1000min, [EDTA]=0. The dose rate is 20 Gy/min. 3840 Crystal Growth & Design, Vol. 10, No. 9, 2010 Chen et al. and O 1s are 779.76, 168.40, and 531.18 eV, respectively, close to the values of BaSO4 reported in the literature. 19 Furthermore, the analysis result suggests the presence of Ba, S, and O in a ratio of 1.00:0.95:4.02, close to the stoichiometry of BaSO4 within experimental error. Thus, the generation of BaSO4 can be demonstrated. The related XRD pattern (curve a, Figure 3), which is consistent with the orthorhombic BaSO4 structure, further confirms the generation of BaSO4. The TEM images of the fragments presented in Figure 1B,C clearly show that themesoporousmicrosphereswith a larger pore size are constructed by numerous irregular nanorods with a diameter ranging from40 to 120 nm. This is further confirmedby the SEM image of the cross-section of a microsphere (Figure 1E). Moreover, the SEM image (Figure 1E) shows that the microspheres are formed via the radial self-assembly of the irregular nanorods, and there are interstitial spaces available among these nanorods, which provide connected channels for mass exchange between the inner space of BaSO4 microspheres and the outer solution. The selected area electron diffraction (SAED) pattern related to a small fragment (inset, Figure 1C) can be indexed to the [010] zone axis of orthorhombic BaSO4, suggesting that a single nanorod is a single crystal of predominantly grown along the [001] direction. The typical HRTEM image of a nanorod shown in Figure 1D exhibits clear lattice fringes with d spacing of 0.27 nm, which corresponds to (002) reflection of orthorhombic BaSO4, further confirming that the nanorods are single crystals grown along the c-axis. However, the alignment of the rodlike nanocrystals is not parallel, and there is a certain angle between them (Figure 1B,C), leading to the radial arrangement of the nanorods. It is the arrangementmodeand the irregular shape that causes the generation of the mesoporous structure with a larger pore size. In the syntheses of BaSO4 particles, amino-carboxylate additives play important roles. As one of the important amino-carboxylate additives, EDTA is found to affect the morphology of BaSO4 particles in thiswork. In the absence ofEDTA, the obtained product is made of well-crystallized Barite crystals according to the XRD analysis (curve e, Figure 3). The TEM image (Figure SI-2A, Supporting Information) shows that the product is composed of rectangular tablets with different sizes. The SAED analysis of the edge of a particle (inset, Figure SI-2A, Supporting Information) indicates that the obtained particles are single-crystal Barite. The SEM image (Figure SI-2B, Supporting Information) further shows that the crystals are pillow-like, similar to the shape reported in the literature. When the concentration of EDTA increases, the obtained BaSO4 particles are gradually transformed from pillow shape to microsphere, from single crystals to the aggregation of nanorods, and from nonporous to mesoporous (Figures 1A and SI-2B-D, Supporting Information). At the same time, the XRD diffraction peaks are broadened with an increase in the concentration of EDTA (curves a and c-e, Figure 3), suggesting that the building blocks of the Barite particles become small gradually. In the literature, because of a lower concentration and narrower concentration range of EDTA, similar morphology transformation was not found, while at the higher concentration of EDTA (ca. 8mmol/L), quasi-spherical nanoparticles with an average size of 16 nm were obtained. The generation of nanoparticles was ascribed to the adsorption of EDTA on the surface of BaSO4 nuclei, which retards the growth of BaSO4 nuclei and favors the formation of nanoparticles. Herein, the formation of BaSO4 nanoparticles, which subsequently form microspheres via aggregation, may be due to a similar reason. Recently, we obtained BaSO4 microspheres with a diameter of ca. 700 nm via increasing the pH value, leading to a stronger interaction between Ba2þ and EDTA. It can be seen that EDTA plays an important role in the formation of BaSO4 microspheres. BesidesEDTA, the irradiation time could also affect themorphology of the BaSO4 microspheres. In the present work, the dose rate was fixed at 20 Gy/min. At an irradiation time of 250 min, microspheres were formed (Figure 4A). From the morphologies of the surface (inset, Figure 4A) and the cross-section (Figure 4B) of some microspheres, it seems that they are “solid”. The TEM image of the fragments and the related SAEDanalysis (Figure 4C) indicate that the obtainedmicrospheres consist of well-crystallized quasi-spherical nanoparticles. Nevertheless, the results of N2 Figure 4. SEM (A, B, D) and TEM (C) images of the samples synthesized at different irradiation times: (A-C) 250 min, (D) 450 min. Insets: (A andD) the image of the corresponding product at highermagnification; (C) the SAEDpattern of the corresponding product. The dose rate is 20 Gy/min, and the concentration of EDTA is 8 mmol/L. Communication Crystal Growth & Design, Vol. 10, No. 9, 2010 3841 adsorption-desorption isotherm experiments (curves b, Figure 2) suggest that the microspheres are actually mesoporous, but the diameter of the pores is only about 4 nm, which is much smaller than that at an irradiation time of 1000 min and not easy to be found by SEM.TheBET surface area is calculated to be 49.6m/g, which is appreciably larger than that at an irradiation timeof1000 min. This implies that the size of the building block of the former is smaller than that of the latter. The widths of the XRD diffractionpeaks areobviously larger than those at the irradiation time of 1000 min (curves a and b, Figure 3), also suggesting that the size of nanoparticles increases by increasing the irradiation time. This is consistent with the analysis result of the BET surface area. The visual proof comes from the analysis of the fragments, which shows that the size of the building block of themesoporous microspheres with a smaller pore size (Figure 4C) is obviously smaller than that at an irradiation time of 1000min (Figure 1B,C). When the irradiation time increases to 450 min, there appears to be an obvious porous structure, with a pore diameter of about 10 nm, on the surface of most microspheres (Figure 4D). Thus, it can be concluded that the mesoporous BaSO4 microsphereswitha larger pore size should evolve fromthemesoporous microspheres with a smaller pore size. In the evolution process, the quasi-spherical nanoparticles (the building block of themesoporousmicrosphereswith a smaller pore size) were transformed to the nanorods (thebuilding blockof themesoporousmicrospheres with a larger pore size). This evolution may be the result of ripening, etching, orγ-ray action. To clarify the evolution pathway, the water-washed mesoporous microspheres with a smaller pore size (irradiation time: 250min) were dispersed in water and irradiated for 1000 min (dose rate: 20 Gy/min). There is no obvious mesoporous structure with a larger pore size on the surface of most microspheres (not shown here). In addition, during the irradiation course, the size of the BaSO4 nanoparticles became larger. This cannot be ascribed to the action of etching only. Therefore, the ripening of BaSO4 nanoparticles should play a key role in the formation of mesoporous structure with a larger pore size. To confirm our surmise, the mesoporous BaSO4 microspheres with a smaller pore size obtained at an irradiation time of 250min were ripened in the irradiated mother solution. The SEM images of themicrospheres and cross-section of amicrosphere (Figure SI-3, Supporting Information) clearly show that themesoporousmicrospheres with a smaller pore size are transformed to mesoporous oneswith a larger pore size, similar to that obtained at the irradiation timeof 1000min (Figure 1A). In otherwords, the reconstruction of BaSO4 microspheres can be easily realized via a simple ripening process. Therefore, if the conditions are suitable, Ostwald ripening can be effectively used to construct complexBaSO4 nanostructures with a relatively rapid rate at room temperature, and it makes the process more practical. As to the specific species leading to themorphology transformation, it is worthy of further investigation. In summary, themesoporousBaSO4microsphereswith a larger pore size are successfully synthesized by γ-irradiation. Indeed, the formation process can be divided into two steps as follows (Scheme 1). (1) When the aqueous solution containing S2O8 2andBa2þ ions is irradiated under anaerobic conditions, Ba2þ ions are precipitated by the control-released SO4 2ions, leading to the formation ofBaSO4 nanoparticles in the presence of EDTA, which are subsequently aggregated to form microspheres, while pillow-like BaSO4 microcrystals are generated in the absence of EDTA. In this step, the concentration of EDTA and the controlled release of SO4 2ions are important to the formation of the microspheres. (2) At the early stage of the irradiation course, the obtained microspheres are mainly constructed by quasispherical nanoparticles and have amesoporous structure with a smaller pore size. Because a smaller nanoparticle has a higher surface energy, a part of the BaSO4 nanoparticles, especially the smaller,may be gradually dissolved in the course of irradiation, resulting in a higher solubility of BaSO4. Then, BaSO4 is precipitated on the surfaces of the larger nanoparticles again, and a new redissolution-crystallization balance can be achieved. Indeed, this is an Ostwald ripening process. In the process, some species may preferentially adsorb on the surfaces parallel to the [001] axes of BaSO4 nanoparticles, leading to the formation of irregular nanorods. The radial arrangement and the irregular shape cause the generation of the mesoporous structure with a larger pore size. Obviously, Ostwald ripening can be effectively used to construct complex BaSO4 nanostructures with a relatively rapid rate at room