Synthesis of BaMoO4 Nestlike Nanostructures Under a New Growth

May 21, 2008 - Valeria M. Longo , Laécio S. Cavalcante , Elaine C. Paris , Júlio C. Sczancoski , Paulo S. Pizani , Maximo Siu Li , Juan Andrés , El...
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Synthesis of BaMoO4 Nestlike Nanostructures Under a New Growth Mechanism Zhijun Luo,† Huaming Li,*,† Huoming Shu,‡ Kun Wang,† Jiexiang Xia,† and Yongsheng Yan†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2275–2281

School of Chemistry and Chemical Engineering, Jiangsu UniVersity, Zhenjiang 212013, P. R. China, and Department of Chemistry, Hainan Normal UniVersity, Haikou 571158, P. R. China ReceiVed October 4, 2007; ReVised Manuscript ReceiVed February 10, 2008

ABSTRACT: Novel BaMoO4 nestlike nanostructures assembled with single-crystal nanosheets have been successfully synthesized by using PVP (K30) as capping reagents under hydrothermal conditions. Detailed proofs indicated that the process of crystal growth was dominated by a crystallization-dissolution-recrystallization-self-assembly growth mechanism. The morphology of BaMoO4 evolved from several micro-compressed decahedrons to two-dimensional (2-D) nanoplates and to three-dimensional (3-D) nestlike nanostructures. The compressed decahedrons with rough surfaces synthesized in a very short time should have larger numbers of lattice defects which induce its state to be metastable. Room-temperature photoluminescence (PL) spectra also reflected the evolution of intrinsic lattice and morphology. Both the concentration of PVP aqueous solutions and the concentration of initial reagents play important roles in the formation of the BaMoO4 nestlike nanostructure.

1. Introduction Synthesis of micro- and nanoscale inorganic materials with special size, morphology, and hierarchy has stimulated intensive interest because of their importance in basic scientific research and potential technological applications of such materials. Therefore, the synthesis of two-dimensional (2-D) and threedimensional (3-D) micro- and nanostructures with wellcontrolled morphology and architectures is important for uncovering their morphology-dependent properties and for achieving their practical applications.1–3 Many recent efforts have been devoted to the morphological control and spatial patterning of various materials, which is a crucial step toward realization of functional nanosystems. Generally, hierarchical superstructures can be formed through the evolution of zeroor one-dimensional (0-D or 1-D) primary crystals via the oriented attachment process, Ostwald ripening process, or both by using surfactants as capping reagents.4–10 Small primary particles may aggregate in an oriented fashion to produce a larger single crystal, in which the adjacent nanoparticles are self-assembled by sharing a common crystallographic orientation and docking of these particles at a planar interface. The evolution of crystals from 3-D dendrites to 1-D rods and to 0-D particles with prolonged aging time has also been reported.11 Recently, molybdates and tungstate have attracted much interest because of their luminescent behavior, structural properties, and potential application.12–14 Among the molybdates materials, BaMoO4 with a scheelite structure is an important material in electrooptics due to its production of green luminescence and in electro-optical applications including solid-state lasers and optical fibers.15,16 Various methods have been employed to synthesize metal molybdates, such as solid-state reaction, electrochemical methods, the Chochralski technique, hydrothermal method, solvothermal synthesis, and coprecipitation.17–21 Among these methods, the surfactant-assisted wet-chemical approach is one of the effective methods for the synthesis of inorganic materials at lower temperature. However, * To whom correspondence should be addressed. Phone: +86 511 88791800; fax: +86 511 88791708; e-mail: [email protected]. † Jiangsu University. ‡ Hainan Normal University.

solid-state reaction often needs high temperature and rigorous reaction conditions.22,23 A variety of BaXO4 (X ) Mo, W) hierarchical superstructures assembled from the nanobelts and nanowires have been synthesized in a catanionic reverse-micelle system. By adjusting the anionic and catanionic surfactants mixing ratio, BaXO4 (X ) Mo, W) nanobelts, nanowire, and penniform superstructures can be easily synthesized.24–26 Spindle arrays, bundle-like, and brushlike BaMoO4 nanostructures were also obtained in reverse micelles with a solvothermal method.19 Nanofibers were synthesized by using CTAB as capping reagents.17 Recently, our group also reported a facile synthesis of BaWO4 with different morphologies under microwave irradiation by using PVP as capping reagents.27 Nestlike structure is a very interesting hierarchical superstructure. Nestlike structures have been reported previously for a variety of materials including Y2O3, CaCO3, and zeolite, etc.28–30 In this paper, we present a hydrothermal method for the synthesis of uniform nestlike BaMoO4 crystals assembled with single-crystal nanosheets. We have observed an unusual morphology evolution process in which nanoscale crystals derived from the microscale crystals by using PVP as capping reagents. Detailed proofs indicated that the process of crystal growth was dominated by a crystallization-dissolution-recrystallization-self-assembly growth mechanism. The interaction between PVP and nestlike BaMoO4 crystal was investigated by the FT-IR spectra, which indicated that the van der Waals attractions between PVP and BaMoO4 crystal should be responsible for the morphology evolution. Room temperature photoluminescence (PL) spectra of the synthesized BaMoO4 crystals revealed that the optical properties could be modulated through different hydrothermal treatment times.

2. Experimental Section All chemicals are of analytical grade and were used without further treatments. In a representative synthesis route, appropriate PVP was first dissolved in 20 mL of deionized water with vigorous stirring. Then, the appropriate Na2MoO4 · 2H2O aqueous solution was added. After that, the appropriate BaCl2 · 2H2O aqueous solution was added dropwise to the above solution with vigorous stirring. The mixture was placed in a 30 mL Teflon-sealed autoclave and maintained at 140 °C for 24 h. To investigate the intermediates of the nestlike BaMoO4, the synthesis

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Figure 2. FT-IR spectra of pure BaMoO4 crystal (a) and nestlike BaMoO4 crystal (b).

Figure 1. (a) The XRD pattern of products synthesized by hydrothermal treatment at 140 °C in the presence of PVP aqueous solutions (0.05 g/mL) with different reaction times and (b) EDS results of the nestlike BaMoO4. was stopped at different stages during the synthesis process. When the autoclave was taken out of the oven, it was cooled to room temperature as soon as possible by water. After the sample was cooled to room temperature, white precipitation was collected by centrifugation and washed several times with deionized water and absolute ethanol. The washed precipitation was dried in a vacuum oven at 50 °C for 12 h. X-ray diffraction (XRD) analysis was carried out on a Bruker D8 diffractometer with high-intensity Cu KR radiation (λ ) 1.54 Å). FTIR spectra analysis was carried out on Thermo Electron Corporation Nicolet Nexus 470. Transmission electron microscopy (TEM) images were taken with a JEOL 2100 transmission electron microscope operated at 200 kV. The field-emission scanning electron microscopy (FESEM) measurements were carried out with a field-emission scanning electron microscope (JEOL, 7500B) equipped with an energy-dispersive X-ray spectroscope (EDS) operated at an acceleration voltage of 10 kV. The samples used for TEM and FESEM were prepared by dispersing some products in ethanol, then placing a drop of the solution onto a copper grid or the surface of Al column and letting the ethanol evaporate slowly in air. Room temperature photoluminescence spectra (PL) were recorded on a Varian Cary Eclipse fluorescence spectrophotometer. All the measurements were carried out at room temperature.

3. Results and Discussion 3.1. Synthesis of Nestlike BaMoO4 Crystals. The XRD patterns of the products synthesized at different reaction times are shown in Figure 1a. All peaks can be indexed to the tetragonal phase of BaMoO4 with lattice constants a ) 5.58 Å and c ) 12.82 Å, which is in agreement with the standard data from JCPDS card No. 29-0193. No impurity peaks were detected

in the experimental range. It is obvious that the crystallization of BaMoO4 crystal is improved with different trends by prolonging the reaction time. Compared with the standard diffraction pattern, the highest intensity of peaks for product synthesized at 140 °C for 20 min comes from the (004) plane rather than the (112) plane. After 1 h reaction, the highest intensity of peaks is from the (112) plane; however, the intensity of (004) plane is still higher than that of the (200) plane. With the reaction time increased to 24 h, the intensity of the (004) plane becomes the lowest in three planes. These results imply that the products at different stages of the reaction may have a differently preferred growth direction, which may result in the transition of morphology of BaMoO4 crystal. EDS analysis on the nestlike BaMoO4 crystals suggests that the sample contains only the elements of Ba, Mo, and O. The Al signal in the EDS comes from the Al base (Figure 1b). The molar ratio is about 1:1.05:4, which comes from the standard stoichiometric composition. The interaction between PVP and nestlike BaMoO4 crystal was investigated by the FT-IR spectra. Figure 2 shows the FTIR spectra of pure BaMoO4 crystal (a) and the nestlike BaMoO4 crystal (b) synthesized by using PVP. The FT-IR spectra of sample (a) and sample (b) all show a main absorption band at ∼3460, ∼2358, ∼1654, ∼960, and ∼791 cm-1. Among them, the bands at ∼3460 and ∼2358 cm-1 should come from a small quantity of H2O and CO2. The nestlike BaMoO4 contains more H2O than that of the normal BaMoO4 powder due to the capillarity. The nestlike nanostructure has many gaps, which are not easily dried. The same absorption band suggests that the interaction between PVP and nestlike BaMoO4 crystal should be van der Waals interaction. FESEM shows that the as-prepared products are mainly composed of many uniform nestlike BaMoO4 crystals, which are in fact built by small 2-D nanosheets (Figure 3a). These BaMoO4 nestlike nanostructure crystals with a diameter from 5 to 10 µm were obtained by hydrothermal treatment at 140 °C for 24 h in the presence of PVP aqueous solutions (0.05 g/mL). The hatch of these nests is square, and the length of the sides is about 5 µm. Figure 3c,d shows that the nestlike nanostructure is made up of numerous rectangle nanoplates, whose length is about 2.5 µm and width is about 1 µm. From the high magnification FESEM image (Figure 3b), one can see that the

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Figure 3. SEM images of the BaMoO4 nestlike nanostructures consisting of numbers of nanosheets synthesized by hydrothermal treatment at 140 °C for 24 h in the presence of PVP aqueous solutions (0.05 g/mL), where the concentration of initial reagents [Ba2+] ) [MoO42-] ) 0.05 M. (a) The low magnification SEM image; (b) the high magnification SEM image; (c) top view SEM image of nestlike nanostructure; (d) TEM image of nanosheet moved off after sonication; the inset in (d) shows the corresponding electron diffraction (SAED) and the electron diffraction spots are marked in black dots.

thicknesses of the nanosheets is less than 100 nm. These very thin nanosheets are responsible for the breadth of the XRD peaks. Figure 3d shows a broken sheet of BaMoO4 nestlike nanostructure crystals obtained by ultrasonic treatment for several minutes. The selected-area electron diffraction (SAED) pattern confirms its single-crystalline structure with tetragonal symmetry. The SAED of this part should be attributed to the [001] zone; that is, the plane that has the largest area is the (002) plane. As is known, the growth rate of a crystal face is usually related to its surface energy. The fastest crystal growth will occur in the direction perpendicular to the face with the highest surface energy in order to reduce higher energy surfaces. Finally, these faces with the highest surface energy will vanish in the final shape.31 In the presence of PVP, the (002) plane should be absorbed by the PVP via van der Waals attractions. Because of the selective adsorption of PVP to the (002) plane, the surface energy of (002) plane can be reduced and will become more exposed in area. 3.2. Growth Mechanism and Factors Influencing the Formation of the Nestlike BaMoO4 Crystals. The crystal growth mechanisms in solution are so complicated that the actual crystallization mechanism remains an open question. The Ostwald ripening, selective polymer adsorption, and oriented attachment, etc., were adopted to account for the process of crystal growth. Recently, tungstate and molybdate with novel nanostructures can be achieved via the “oriented attachment” mechanism.25,26 On the basis of time-dependent experiments, we do not think the “oriented attachment” mechanism can entirely explain the formation of the nestlike BaMoO4 crystal. From the SEM observations, it can be concluded that nestlike

nanostructures can be obtained via a crystallization-dissolution-recrystallization-self-assembly growth mechanism. The nucleation-dissolution-recrystallization growth process was also observed by Qian et al. in the synthesis of t-selenium nanotubes.32 We found that the Ostwald ripening process dominated the crystal growth process at the initial stage. Figure 4a shows some tiny equiaxial nanoparticles that were collected before being transferred to the Teflon-sealed autoclave. As is well-known, at the initial stage tiny crystalline nuclei form in a supersaturated solution that acts as the centers of crystallization. Then the crystal growth follows, and bigger particles grow at the expense of smaller crystals, as described by the Gibbs-Thompson equation. Figure 4b shows the morphology of the products synthesized at 140 °C or 20 min under the hydrothermal conditions. We get some compressed decahedrons, which like an octahedron are truncated two tops and a small number of plate crystals. When the reaction was increased to 1 h, the product was mainly composed of nestlike nanostructures. However, the nestlike nanostructure was not the exclusive morphology in the products, as a small number of dissolved partially compressed decahedrons were also found (Figure 4c). From Figure 4c, we find that two different parts with different morphologies and structures, part of nestlike and part of decahedron, are integrated into a special structure. From the closer SEM image (Figure 4d), the joint of the two different structures was carefully observed. The two different structures are not loosely congregated but grow into a whole crystal. It seems that the nanosheets are grown on the surface of

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Figure 4. SEM images of the products synthesized at 140 °C in the presence of PVP aqueous solution (0.05 g/mL) at different times. (a) Before hydrothermal treatment; (b) 20 min; (c, d, e) 1 h; (f) 24 h.

decahedron. In another SEM image, several nanosheets growing on the surface of decahedron, which looks like the nanosheets are inserted into the surface. At the same time, large numbers of nanosheets are observed on the surface of decahedron and in the concave of nest as well (Figures 4e and 5a). After hydrothermal treatment for 24 h, large numbers of nestlike BaMoO4 crystals were obtained and no other morphology was observed (Figure 4f). We also observed the surface of decahedron carefully. The surface of decahedron is so rough that many cracks can be found (Figure 5b). The rough surface of decahedron was probably ascribed to the gradually dissolution of the decahedron crystals, followed by the recrystallization process. The reason why the two different morphologies can be connected is that many BaMoO4 heaves on the rough surfaces act as the nucleation sites; that is, these heaves with a side length of ∼100 nm can serve as seeds for further growth to form nanosheets.33 The thickness of nanosheets connected with the rough surface of the decahe-

dron is about 100 nm (Figure 5c). In the recrystallization process, as the concentrations of the Ba2+ and MoO42- become sufficiently high, they aggregate on these heaves of the rough surface of decahedron through homogeneous nucleation while the dissolution is going. There are some similarities between this process and the formation of nanoarray.34,35 Many tiny nanoplates were formed via the recrystallization process, followed by the assembly of these building blocks into 3-D nestlike nanostructure. On the basis of the above discussion, it has been suggested that the formation of nestlike BaMoO4 crystals may result from the crystal growth mechanism “crystallization-dissolution-recrystallization-self-assembly”. The process of the morphology evolution of BaMoO4 nestlike nanostructure is summarized in Scheme 1. Our experimental results indicated that PVP as a capping reagent, the hydrophilic polymer, played an important role in the process of assembling nanosheets into nestlike nanostruc-

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Figure 5. (a) SEM images of the products synthesized at 140 °C for 1 h in the presence of PVP aqueous solutions (0.05 g/mL); (b) SEM image of the surface of decahedron synthesized at 140 °C for 20 min in the presence of PVP aqueous solutions (0.05 g/mL); (c) enlarged image of panel (a).

Scheme 1. Schematic Illustration of the Formation and Morphology Evolution of BaMoO4 Nestlike Nanostructure in the Whole Synthetic Process

tures. Figure 6 shows the morphologies of the products synthesized at 140 °C in the presence of PVP aqueous solutions with different concentrations. Figure 6a shows the morphology for the products synthesized by hydrothermal treatment at 140 °C for 24 h in the aqueous solutions with 0.005 g/mL PVP. In this case, some polyhedrons assembled into a disk. With an increase of the concentration of PVP to 0.025 g/mL, products consist of several nanosheets bundled in the middle part, in which the thicknesses of the nanosheets is also about 100 nm (Figure 6b). This indicates that the appropriate concentration of PVP aqueous solutions is vital for the formation of nestlike BaMoO4 crystals. The concentration of initial reagents is another factor influencing the formation of nestlike BaMoO4 crystal. It is obvious that the formation of nestlike nanostructure is possible if the supersaturation exceeds a certain “critical” level. When the concentration of all the initial reagents is 0.02 M, 2-D nanosheets were obtained and these nanosheets

aggregated in parallel, together with a few tiny nanoplates (Figure 7a,b). When the concentration of initial reagents was increased from 0.02 to 0.2 M and other reaction parameters are similar to the product shown in Figure 3, nestlike BaMoO4 crystals were also obtained (Figure 7c). Compared with the product synthesized on the concentration of initial reagents of 0.05 M, the diameter of these nestlike BaMoO4 crystals decreased from 10 µm to 4 µm and the side length of hatches decreased from 5 µm to 1.5 µm. The nanosheets that construct the nestlike structure was also decreased. However, the thickness of the nanosheets is about 100 nm, which is similar to the product synthesized on the concentration of initial reagents of 0.05 M. Different molybdenian reagents were tested to check their different effects on the synthesis. The results indicated that the use of (NH4)6Mo7O24 · 4H2O resulted in the decrease of the thickness of the nanosheets from ∼100 nm to ∼50 nm (Figure 7d). 3.3. Photoluminescent Properties. Room temperature photoluminescence (PL) spectra of the synthesized BaMoO4 crystals with different reaction times are also investigated, and the results are shown in Figure 8. With the excited wavelength at 240 nm, the spectra of all samples show that the strong and broad green emission peak is at 526 nm, and the red emission peak is at 584 nm, and 605 nm. The sample (a) (nestlike nanostructure) synthesized at 140 °C for 24 h exhibits the highest intensity of PL at 526 nm in three samples; however, the lowest is at 605 nm. The sample (c) (decahedron) synthesized at 140 °C for 20 min exhibits the highest intensity of PL at 605 nm in three samples, and the lowest at 526 nm. With the reaction time increased, the PL in the green region becomes stronger but weaker in the red region. The emission originates from the

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Figure 6. SEM images of products synthesized by hydrothermal treatment at 140 °C for 24 h in the presence of PVP aqueous solutions with different concentrations. (a) 0.005 g/mL; (b) 0.025 g/mL.

Figure 7. SEM images of products synthesized by hydrothermal treatment at 140 °C for 24 h at different concentration of initial reagents (a, b) 0.02 M; (c) 0.2 M. (d) SEM images of products synthesized by using (NH4)6Mo7O24 · 4H2O as molybdenian reagents and the reaction condition is the same as Figure 3.

intrinsic molybdate group. Before the crystallization, the structure is a mixture of MoOx (x ) 3 and 4) intercalated by Ba atoms. The green luminescence of BaMoO4 is ascribed to the intrinsic MoO42- and the red luminescence is ascribed to the defect MoO3 group. If the crystallization is completed, only MoO4 clusters exist.22,36 Therefore, we can conclude that sample (a) has better crystallization than sample (c); that is, BaMoO4 crystals synthesized for 24 h should be crystallized much better than that synthesized in a short time. So the compressed decahedron with rough surfaces should has large numbers of lattice defects. The lattice defect energy may induce the micronscale BaMoO4 crystals to evolve into nanoscale BaMoO4 crystals.11,37

4. Conclusions In summary, novel BaMoO4 nestlike nanostructures consisting of single-crystal nanosheets have been successfully synthesized by using PVP (K30) as capping reagents under hydrothermal conditions. Our experimental results indicate that the formation of nestlike nanostructures is dominated by a crystallizationdissolution-recrystallization-self-assembly growth mechanism. The factors influencing the formation of nestlike nanostructure were investigated and indicated that the appropriate concentration of initial reagents and PVP is vital for the formation of nestlike nanostructure. Room-temperature photoluminescence (PL) spectra show that the optical properties could be modulated

Synthesis of BaMoO4 Nestlike Nanostructures

Figure 8. Room temperature photoluminescence spectra of the samples obtained at 140 °C under different reaction times. (a) 24 h; (b) 1 h; (c) 30 min.

through different hydrothermal treatment times. The decahedron with several micrometers side length synthesized in a very short time should be in a metastable state which is bound to evolve into a stable state. Acknowledgment. The authors extend special thanks to Prof. Hongwu Tong, Weidong Zhou, and Bin Xu (Analysis Center, Yangzhou University) for kindly supporting the FESEM and XRD measurements. The present work is supported by the National Natural Science Foundation of China (No. 20676057) and Jiangsu University Scientific Research Funding (No. 04JDG044).

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