Shape- and Size-Controlled Synthesis of Calcium Molybdate

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Shape- and Size-Controlled Synthesis of Calcium Molybdate Doughnut-Shaped Microstructures Wenshou Wang,†,‡ Yongxing Hu,† James Goebl,† Zhenda Lu,† Liang Zhen,‡ and Yadong Yin*,† Department of Chemistry, UniVersity of California, RiVerside, California 92521, and School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China ReceiVed: June 24, 2009; ReVised Manuscript ReceiVed: August 7, 2009

We report a solution-phase rapid-injection-based route for the synthesis of calcium molybdate (CaMoO4) microstructures with well-defined shape and size. This ternary metal oxide material has been widely used as phosphors, scintillator, and laser materials for a long time because of its attractive luminescence property. Complex three-dimensional structures, mainly doughnut-shaped microparticles with highly controllable size and aspect ratio, can be obtained at 100 °C by controlling the reaction conditions including the concentration of reactants, molar ratio between reactants, molybdenum source, pH, and the volume ratio of the mixed solvents. Other shapes such as spindles, nanosheets, and microspheres can also be produced by pushing the reaction conditions away from those optimal for doughnuts. X-ray diffraction, scanning electron microscopy, transmission electronic microscopy, and X-ray energy dispersive spectrometer were used to characterize the obtained samples. The photoluminescence spectra of the CaMoO4 doughnut microstructures reveal a strong and broad emission with a maximum at 512 nm, which is blue-shifted by ∼20 nm compared to the value for the bulk samples. The advantages of this synthetic route over the previously reported ones include the precise control over particle size and shape, simple synthetic procedure, relatively low reaction temperature, and high reproducibility of the process. 1. Introduction The synthesis and self-organization of micro- and nanoscale inorganic materials with special morphology, size, and hierarchy have attracted considerable attention in the past few decades because of their importance in basic scientific research and potential technological applications.1-3 Hierarchical inorganic micro-/nanostructures, constructed by using various nanomaterials as building blocks, may provide an effective strategy for the systematic study of structure-property relationships and improve the physical and chemical properties of the nanoscale materials with simple configurations.4-6 At present, rational control over the morphology, crystalline structure, and size of inorganic materials has commanded the attention of many research groups worldwide and efforts have focused on mastering the synthetic routes to afford a host of novel and diverse nano- and microstructured materials.7 Various synthetic methods, such as vapor-phase growth8 and solution-phase techniques,9 have been devoted to the controlled synthesis of inorganic materials with specific morphologies. Although vapor-phase synthesis is a powerful technique for preparing nano- and microstructures with controlled shapes and sizes, this method is generally expensive, has difficulty producing uniform samples, and requires complicated reaction equipment, making it unsuitable for large-scale production for industrial applications. In comparison, solution-phase synthetic methods have many advantages, including relatively low reaction temperatures, convenience in handling, inexpensive reaction instruments, and ease in procedural control, making them very promising for the large-scale synthesis of materials. Many research efforts have * To whom correspondence should be addressed. E-mail: yadong.yin@ ucr.edu. † University of California. ‡ Harbin Institute of Technology.

been focused on the shape- and size-controlled solution-based synthesis of Ag, Au, and Pt nanostructures with various morphologies,3a,10 while Qian et al. developed a simple hydrothermal route for the selectively controlled synthesis of 1D and various 3D CdS spherical nanostructures11 and ZnS hierarchical structures with various morphologies.12 Compared to these noble metals and binary systems, the synthesis of nanostructures of ternary metal oxides has been rarely studied because of difficulties in attaining compositional and structural control of ternary and quaternary compounds. Therefore, it will be of fundamental and technological interest to develop facile and effective methods for the preparation of ternary metal oxide micro-/nanostructures with fine shape and size control. Recently, several efforts devoted to the synthesis of ternary metal oxide microstructures with controlled morphologies have produced promising results. Lin et al. reported a phase- and shapecontrolled synthesis of rare earth orthoborates via a hydrothermal method13 and Wang et al. developed a hydrothermal process for the shape-controlled synthesis of Bi2WO6 with complex morphologies.14 Despite these hydrothermal strategies, the direct synthesis of ternary metal oxide microstructures with designed chemical components and controlled morphologies is still considerably more difficult. Alkali-earth metal molybdates have very important applications in various fields that involve sensor and detector, luminescent devices, optical fibers, scintillator materials, catalysis, and lithium ion batteries.15-17 They represent an important group of ternary metal oxide, which contains large bivalent cations (MMoO4, ionic radius >0.99 Å, M ) Ca, Ba, or Sr) and exists in the so-called scheelite structure form, in which the Mo atom adopts a tetrahedral coordination.18 Scheelitetype molybdates of the same divalent metal are reciprocally soluble over the entire compositional range, which results in a rich family of solid state solution compounds.19 Most of previous

10.1021/jp9059278 CCC: $40.75  2009 American Chemical Society Published on Web 08/24/2009

Synthesis of CaMoO4 Doughnut-Shaped Microstructures approaches to the preparation of these materials require high temperatures and harsh reaction conditions, such as the Czochralski method,20 the coprecipitation method,21 the combustion route,22 and the conventional solid-state reaction strategy.23 As a result, the alkali-earth metal molybdates prepared by these methods are relatively large and have inhomogeneous morphology and composition because MoO3 tends to vaporize at high temperature. With the aim to improve the quality of these materials, recent research works have been focused on the synthesis of alkali-earth metal molybdates with sizes at the nanometer and/or micrometer scales.24-28 Among the alkali-earth metal molybdates, calcium molybdate (CaMoO4) has been widely used as phosphors, scintillator, and laser materials for a long time because of its attractive luminescence property.29 Most of the previous efforts in CaMoO4 preparation have been directed toward the synthesis of single crystals30 and thin films,31 with a few recent exceptions for the production of CaMoO4 nanoparticles,32 whiskers or rodshaped particles,33 and microcrystals.34,35 Most of these prior methods developed for micro- and nanoparticles do not have the capability to tightly control the size and shape of the products. In this work, we report a facile solution-phase route based on the concept of rapid-injection to prepare CaMoO4 doughnut-shaped microstructures with fine shape and size control. The effects of experimental conditions including concentration of reactants, molar ratio between reactants, Mo source, pH value, and the volume ratio of the mixed solvents on the morphology and size of CaMoO4 microstructures have been investigated systematically. In addition to the doughnutshaped microparticles, complex three-dimensional hierarchical structures such as spindles, microspheres with wool roving textures, and nanosheets can also be obtained by varying the reaction conditions. The doughnut microstructures can be further manipulated by etching in acidic solution, which dissolves the center and results in the formation of microscale ring doughnuts. Our measurement of the photoluminescence property of the CaMoO4 doughnut microstructures reveals a strong and broad emission with a maximum at 512 nm, which is blue-shifted by ∼20 nm from the value obtained for bulk samples. Comparing to the previously reported routes to CaMoO4 microstructures, our method has a number of advantages including the precise control over particle size and shape, simple synthetic procedure and easy setup, mild reaction conditions (ambient pressure and relatively low reaction temperature), and high reproducibility of the process. 2. Experimental Section 2.1. Chemicals. Calcium chloride, ammonium molybdates, sodium molybdates, sodium hydroxide, acetic acid, sodium dodecyl sulfate (SDS), ethylene glycol (EG), and N,N-dimethylformamide (DMF) were purchased from Fisher Scientific. All the chemical reagents were of analytical grade and were used as received without further purification. 2.2. Synthesis. The hierarchical CaMoO4 structures were synthesized by using an aqueous solution-phase reaction at 100 °C. A (NH4)6Mo7O24/H2O stock solution was prepared by dissolving (NH4)6MoO7O24 (5 mmol) in distilled water (25 mL) at room temperature. In a typical procedure, a mixture of CaCl2 (5 mmol), SDS (5 mmol), and distilled water (25 mL) was heated to about 100 °C under vigorous stirring, forming a transparent solution. A 2.5 mL sample of (NH4)6Mo7O24/H2O stock solution was injected rapidly into the hot mixture and the reaction solution turned white immediately. The molar ratio of CaCl2 to (NH4)6Mo7O24 was fixed at 10. The resulting mixture

J. Phys. Chem. C, Vol. 113, No. 37, 2009 16415 was further heated for 30 min to form hierarchical CaMoO4 doughnut structures. CaMoO4 doughnut microstructures can also be prepared without using SDS, but this results in poor monodispersity. The morphology and size of the doughnut structures could be tuned by controlling the concentration of precursors, molar ratio between the precursors, mixed solvents, and pH value. The products were then collected by centrifugation, washed several times with distilled water and absolute ethanol to removal any residuals, and finally dried in air at 65 °C for 14 h. 2.3. Characterization. The crystal structure of the CaMoO4 products was measured on a Bruker D8 Advance X-ray diffractometer (XRD) with Cu KR radiation (λ ) 1.5418 Å). Morphology and size of the CaMoO4 products were characterized by using an XL 30 scanning electron microscope (SEM) equipped with an X-ray energy dispersive spectrometer (EDS) and a Tecnai T12 transmission electron microscope (TEM). Photoluminescence (PL) properties were measured on a Spex Fluorolog-3. 3. Results and Discussion The reaction between Mo7O246- and Ca2+ proceeds slowly at room temperature. Precipitation of CaMoO4 particles with irregular shapes appears after mixing the precursors in water for ∼5 days. The following two steps are believed to be responsible for the formation of CaMoO4: Mo7O246- + 8OH) 7MoO42- + 4H2O (reaction 1) and Ca2+ + MoO42- ) CaMoO4 (reaction 2). Performing the reaction at high temperature (100 °C) significantly increases the reaction rate so that particles form within minutes. However, slow heating of the precursor mixture from room temperature leads to uncontrolled nucleation and growth, and yields product with broadly distributed sizes. To enhance the uniformity of the particles, we use the concept of “rapid injection” that has been widely used for nanostructure synthesis.36 Injection of (NH4)6Mo7O24/H2O stock solution quickly into the boiling CaCl2 solution induces an instant nucleation event that is fast enough to separate in time from the growth step, thus producing relatively monodisperse samples. 3.1. Structural Characterization of CaMoO4 Doughnut Structures. The rapid injection process at relatively high temperature produces a uniform population of microparticles with an interesting doughnut shapesflattened spheres each containing a concavity on its surface, as shown in the typical scanning electron microscopy (SEM) images in Figure 1. The microparticles have an average diameter of ∼2.1 µm and thickness of ∼0.7 µm. No other morphologies can be observed, indicating a high yield of these CaMoO4 doughnut microstructures. The higher magnification SEM image shown in Figure 1c reveals that each doughnut is composed of many layers of nanoscale sheet-like structures. These nanosheets are arranged at progressively increasing angles to the radial axis and are highly directed to form arrays in a regular fashion. As shown in Figure 1d, the side view of an individual doughnut structure supports the conclusion that such a microstructure is composed of densely packed nanosheets with an average thickness of about 30 nm. The EDS spectrum shows strong peaks from Ca, Mo, and O (Figure 1e). Because the sample was coated with a conducting layer of Au-Pd alloy for optimal SEM observation, these two metals were also detected in the EDS measurement. Additional signals also include Si, which was used as the supporting substrate. Quantitative analysis shows that the atom ratio of Ca:Mo:O is about 1:1:4, giving a stoichiometric composition of CaMoO4.

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Figure 1. (a) Low-magnification and (b) high-magnification SEM images of a typical sample of as-prepared CaMoO4 doughnut structures. (c, d) SEM images of individual doughnut structures viewed from face and side. (e) A typical EDS spectrum of the CaMoO4 doughnut microstructures.

Figure 2. TEM images of typical as-prepared CaMoO4 doughnut microstructures at various magnifications. The inset in panel a is a highmagnification image of the edge of a doughnut.

The morphology and size of the products were further investigated by transmission electron microscope (TEM). As shown in Figure 2, the contrast at the center of the particles is lower than that of the edges, confirming the doughnut structures. Close inspection of the edge of the doughnuts indicates that the nanosheets are very thin and therefore relatively transparent to the electron beam (Inset in Figure 2a). Figure 3 shows the X-ray diffraction (XRD) pattern of typical CaMoO4 doughnut microstructures. All of the observed diffraction peaks can be perfectly indexed to those of the tetragonal

Figure 3. XRD pattern of the as-prepared CaMoO4 doughnut microstructures.

phase of CaMoO4 with cell constants of a ) 5.22 Å and c ) 11.42 Å (JCPDS No. 85-1267). The strong and sharp diffraction peaks indicate good crystallinity in the as-synthesized products. No peaks of other impurity phases are detected in this pattern,

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Figure 5. XRD patterns of the as-prepared CaMoO4 products obtained by using different concentrations of precursors. The curves of a, b, c and d correspond to the samples shown in panels a, b, d, and e of Figure 4, respectively.

Figure 4. SEM images of the as-prepared CaMoO4 products obtained by using different concentrations of precursors with fixed ratio (NH4)6Mo7O24:CaCl2 ) 1:10, while keeping other reaction conditions the same as in a typical synthesis. The concentrations of CaCl2 are (a) 0.04, (b) 0.12, (c) 0.2, (d) 0.4, and (e, f) 0.8 M. The amount of CaCl2 is 1, 3, 5, 10, and 20 mmol for a, b, c, d, and e, respectively.

suggesting that CaMoO4 products with high phase purity can be easily obtained by this synthesis. 3.2. Formation Process of CaMoO4 Doughnut Microstructures. At 100 °C, the reaction between CaCl2 and (NH4)6Mo7O24 in aqueous solution is very fast. The system becomes turbid immediately after the (NH4)6Mo7O24/H2O stock solution is injected into the CaCl2 solution. To shed light on the formation process of the CaMoO4 doughnut microstructures, two sets of comparison experiments were performed by using various precursor concentrations and different molar ratios of the reactants. In the first experiment, adjusting the concentration of the reactants was found to dramatically affect the formation of the CaMoO4 doughnut microstructures. We fixed the molar ratio of (NH4)6Mo7O24 to CaCl2 at 1:10 for all the experiments. Figure 4 shows the SEM images of the CaMoO4 products obtained by using different concentrations of the reactants while keeping other reaction conditions the same as in a typical synthesis (Note: for the experimental conditions of 0.4 and 0.8 M CaCl2, the concentration of (NH4)6Mo7O24/H2O stock solution is 0.4 M). When the concentrations of precursors are low (0.04 M CaCl2), the obtained CaMoO4 sample is composed of largescale nanosheets with a thickness of ∼20-40 nm and a diameter of ∼1.5 µm (Figure 4a). Most of the nanosheets tend to be aggregated together. Doughnut-shaped particles dominate when the amount of precursor is increased, with average diameters increasing from ∼1.5 µm to ∼2.1, ∼2.3, and ∼2.7 µm, and average thicknesses increasing from ∼300 nm to ∼700, 930, and 1200 nm for reactions with 0.12, 0.2, 0.4, and 0.8 M CaCl2. It is important to note that all the doughnut microstructures with the different thicknesses shown in Figure 4b-e are composed of many layers of nanosheets with thicknesses of tens of nanometers. It is evident from Figure 4 that varying the concentration of the reaction reagent provides systematic control of the thickness and diameter of the CaMoO4 microstructures. Calculation of the ratios between the thickness and the diameter

Figure 6. SEM images of the as-prepared CaMoO4 products obtained with different molar ratios of (NH4)6Mo7O24/CaCl2 while keeping other reaction conditions the same as in a typical synthesis: (a) 1/100, (b) 3/50, (c) 1/10, and (d) 1/5. The amount of (NH4)6Mo7O24 is 0.05, 0.3, 0.5, and 1 mmol for panels a, b, c, and d, respectively.

of the products clearly suggests that the increase in thickness is faster than that of the diameter upon increasing the concentrations of the precursors. XRD patterns of the CaMoO4 products obtained by using different concentrations of the reactants are shown in Figure 5, which again can be perfectly indexed to the tetragonal phase of CaMoO4. EDS analysis of the selected samples also shows that they are composed of Ca, Mo, and O with an atomic ratio of ∼1:1:4 (Supporting Information, Figure S1). We have also varied the amount of (NH4)6Mo7O24 while keeping the concentration of CaCl2 fixed at 0.2 M. Inconsistent with the previous cases, the thickness of the CaMoO4 increases from 370 to 970 nm, whereas the diameter increases from 1.6 to2.2µmwithgraduallyincreasingthemolarratioof(NH4)6Mo7O24/ CaCl2 from 1/100 to 1/5 (Figure 6). Interestingly, a large change of the CaCl2 concentration from the optimal value while keeping the amount of (NH4)6Mo7O24 fixed produces aggregated CaMoO4 particles although most of them are still in the doughnut shape. It is therefore reasonable to conclude that the concentration of (NH4)6Mo7O24 is the determining factor of the shape evolution. On the basis of the SEM studies and experimental process, we believe that the formation of CaMoO4 doughnut microstructures involves a two-step growth process, which has also been

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observed in the formation of doughnut structures of orthoborates, orthovanadates, calcium carbonate, and zinc oxide.13,37 When the (NH4)6Mo7O24/H2O stock solution is quickly injected into the CaCl2 aqueous solution at 100° C under vigorous stirring, Mo7O246- ions are transformed into MoO42- ions according to reaction 1. The reaction between MoO42- and Ca2+ produces CaMoO4 species which then nucleate upon supersaturation and finally grow into particles. Although it is difficult to reveal the exact mechanism due to the difficulty in catching the intermediate products during such a fast reaction, we can still conclude from the above observations that the axial growth is preferred at higher concentrations of MoO42- to produce thick doughnuts, while lateral growth is dominant at lower MoO42- concentration to yield nanosheets or thin doughnuts. At high reactant concentrations, the monomer concentration in the solution is higher, which increases the chemical potential of the system and promotes the growth of the nanosheets, which tend to stack face-to-face to decrease surface energy by reducing exposed areas.37 Although many hierarchical microstructures with oriented growth and assembly of nanosheets have been reported, the detailed mechanism for the formation of complex inorganic microstructures still remains a mystery and has rarely been discussed.38 Several factors, such as crystal-face attraction, electrostatic and dipolar fields associated with the aggregate, van der Waals forces, hydrophobic interactions, and hydrogen bonds, may have various effects on the growth process.39 Zeng et al. proposed an edge-to-edge dendritic growth mechanism for the formation of cabbagelike microspheres of layered hydroxide zinc carbonate in a hydrothermal reaction, although no solid experimental proof was provided in these prior works.38h,i Considering the concentration-dependent change of morphology from thin stacks of nanosheets to thick doughnuts, we believe the growth of CaMoO4 microstructures might be better described by using a model that was initially proposed by Sujimoto et al. for the formation of highly uniform peanuttype hematite particles.40 The peanut particles were found to be polycrystals consisting of much smaller subcrystals with elongated shapes. With the support of detailed analyses of the growth process and internal structures and computer simulation, the authors have concluded that the peanut-like shape is formed by repetitive surface nucleation and the subsequent growth of crystallites in the spaces of the elongated crystallites densely grown on both ends of each ellipsoidal secondary particle, so that the growth directions of the elongated crystallites are tilted away from the revolution axis of the ellipsoidal secondary particles. Although this model was initially developed for the transition from elongated subcrystals to peanut-shaped microparticles, the main concepts can still be adopted for explaining the formation of microscale doughnuts. In our case, the subcrystals are CaMoO4 nanosheets which are two-dimensional anisotropic structures. At the early stage of the reaction, thin nanosheets form as the nuclei whose flat surfaces serve as nucleation sites for additional nanosheet growth. Due to the variation in the concentration of precursors, the outer edge of the new nanosheets grows more extensively and becomes relatively thicker than that close to the nucleation sites. This is similar to the formation of truncated core-shaped subcrystals at the outer surface of hematite peanuts.40b,c Nucleation of new nanosheets occurs not only on the “external” surfaces that are exposed to the bulk solution, but also on the “internal” surfaces that are sandwiched between nanosheets. As a result, the particles grow preferentially in the direction parallel to the original nanosheets (along the lateral direction), and form flat

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Figure 7. Typical SEM images of the as-prepared CaMoO4 products synthesized by using different volumes of 6 M NaOH aqueous solution: (a, b) 0.17, (c, d) 0.24, (e, f) 0.32, (g) 0.53, and (h) 0.67 mL.

shapes. Because the surface nucleation rate on the external surfaces is higher than that on the internal surfaces,40c the particles also show considerable growth in the direction perpendicular to the original nanosheets, especially at a high concentration of precursors. This axial growth leads to the formation of doughnut shapes. As discussed in the later sections, the increase in the effective precursor concentration usually leads to a relatively higher axial growth rate and accordingly a gradual transition from flat doughnuts to fat ones and eventually spherical shapes. While it seems to be consistent with the model of the prior systems, the proposed growth mechanism still needs to be carefully verified in future works. 3.3. Size and Morphology Control by Changing the Basicity. According to reaction 1, the transformation of Mo7O246- to MoO42- ions is strongly pH dependent. It is therefore expected that the reaction kinetics will be enhanced if it is performed at high pH conditions. We have studied the influence of pH to the size and morphology of the products by injecting (NH4)6Mo7O24/H2O stock solution containing different amounts of NaOH into the CaCl2 solution while keeping the other conditions constant. As shown in Figure 7a-f, the initial increase of pH apparently promotes the formation of more MoO42- and enhances the overall growth rates so that doughnuts become larger and thicker, with increasing average diameters from 2.1 to 6.1 µm. The thicknesses of the doughnuts also increase dramatically. When 0.32 mL of NaOH (6 M) is added to the system, the growth rate along the axial direction is comparable to that of lateral growth, leading to the formation of spherical balls with observable central cavities (Figure 7e,f). The axial growth eventually exceeds the lateral growth upon



Figure 8. (a, b) SEM images and (c) XRD pattern of the CaMoO4 products obtained by using Na2MoO4 as the Mo source.

further increasing the amount of NaOH. Careful inspection of the products suggests an ellipsoidal shape, as shown in Figures 7g. The overall size of the particles, however, starts to decrease because of the enhanced nucleation events at significantly higher MoO42- concentration. The dominant axial growth is evidenced by the formation of spindle-shaped particles at even higher addition of NaOH to the system (Figure 7h). XRD and EDS results demonstrate that the samples obtained with different amounts of NaOH are phase purity CaMoO4 (Supporting Information, Figures S2 and S3). 3.4. Using Na2MoO4 as the Mo Precursor. We found that the gradual release of MoO42- from (NH4)6Mo7O24 is critical to the formation of doughnut structures. Replacing (NH4)6Mo7O24 with Na2MoO4 (0.2 M aqueous solution) as the Mo source while keeping the other reaction conditions the same results in the formation of spindle-like microstructures together with some superstructures, as shown in Figure 8. The spindlelike structures have lengths of 5-10 µm and diameters of 1-4 µm in the middle (Figure 8a,b). XRD diffraction peaks shown in Figure 8c suggest that the spindle-like structures are the tetragonal phase of CaMoO4. This spindle-like structure is consistent with previous observations that the axial growth of the particles is preferred when the concentration of MoO42- is relatively high. While the concentration of MoO42- ions can be increased by adding NaOH to the solution of (NH4)6Mo7O24 (reaction 1), directly adding MoO42- to the reaction produces a similar condition that favors the formation of spindle-shaped particles. Comparing the two types of Mo sources, using (NH4)6Mo7O24 as the precursor might have the advantage of relatively slow release of MoO42- ions so that the reaction kinetics favors the generation of CaMoO4 doughnut structures. 3.5. Size and Morphology Control by Using Binary Mixed Solvents. The morphology and size of CaMoO4 products can also be tuned by using binary solvents. Figure 9 shows the SEM images of CaMoO4 samples synthesized in a mixed solvent at decreasing volume ratios of distilled water/EG while keeping other reaction conditions constant. The diameter of the doughnut microstructures decreases whereas the thickness increases gradually by increasing EG in the mixed solvents. Compared with the doughnut microstructures with a diameter of 2.1 µm and a thickness of 0.7 µm obtained in distilled water (Figure

9a,b), the products obtained in mixed water/EG solvents with volume ratios of 3:2, 2:3, and 1:4 show decreasing diameters from 2.0 to 1.8 and 1.3 µm, and increasing thicknesses from 0.9 to 1 and 1.1 µm, corresponding to a decreasing aspect ratios of diameter/thickness from 3 to 1.8 and 1.2. When only EG is used as solvent, microspheres with an average diameter of about 1.1 µm coexisting with some nanospheres with diameters of 300-600 nm are obtained (Figure 9i,j). Interestingly, the microspheres, with an aspect ratio close to 1.0, still possess central cavities as marked by white arrows in Figure 9j. Apparently, the reduced amount of water restricts the overall growth of the particles and at the same time limits the anisotropic growth, yielding smaller and more spherical samples. By combining pH control and use of mixed solvent, it should allow us to tune the diameter and thickness of CaMoO4 doughnut microstructures in a wide range (from ∼1 to ∼6 µm). Not surprisingly, the samples prepared by using mixed solvents can all be indexed to phase pure tetragonal CaMoO4, as clearly suggested by XRD analyses (Supporting Information, Figure S4). It is also observed that the thickness of the nanosheet subunits decreases when the concentration of EG increases, suggesting possible binding of EG molecules on the growing crystal surfaces. In addition to water/EG, other binary solvents can also be used in the synthesis. For example, when DMF is mixed with water and used as the reaction solvent, another interesting morphology evolution of CaMoO4 can be observed. Figure 10 shows SEM images of CaMoO4 samples synthesized in various solutions of distilled water and DMF at various volume ratios while keeping other reaction conditions constant. When 5 mL of DMF is used making the volume ratio of water/DMF 4:1, what was obtained is shown in Figure 10a, revealing that the CaMoO4 products consist of monodispersed doughnut microstructures with an average diameter of about 3.0 µm and thickness of about 2.0 µm. It is clear that the diameter and thickness increase significantly when the binary mixed solvents of water/DMF is used. Panels c and d of Figure 10 show typical SEM images of the sample prepared with the volume ratio of water/DMF of 3:2, indicating that nearly monodispersed doughnut microstructures with narrow diameter and thickness distributions are the main products besides a few small doughnut

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Figure 9. SEM images of CaMoO4 samples prepared by using water/ ethylene glycol solutions with different volume ratios: (a, b) water only, (c, d) 3:2, (e, f) 2:3, (g, h) 1:4, and (i, j) ethylene glycol only. The volume of EG is 0, 10, 15, 20, and 25 mL for panels a, c, e, f, and i, respectively.

Figure 10. SEM images of CaMoO4 samples prepared by using binary mixed solvent of water/N,N-dimethylformamide with different volume ratios: (a, b) 4:1, (c, d) 3:2, (e, f) 2:3, (g, h) 3:7, and (i, j) 1:4. The volume of N,N-dimethylformamide is 5, 10, 15, 17.5, and 20 mL for panels a, c, e, f, and i, respectively.

structures. The diameters and the thickness of the CaMoO4 doughnut microstructures are about 3.2 and 3.0 µm, respectively. Although the diameter increases little, the thickness increases dramatically with increasing the volume of DMF in the binary mixed solvents, leading to the formation of sphere-like doughnut microstructures. Further decreasing the volumes ratio of water/ DMF to 2:3, all the obtained CaMoO4 products are in the form of architectural spherules with wool Roving texture and a diameter of about 5.5 µm (Figure 10e). Although each microsphere is still constructed of densely packed nanosheets (Figure 10f), the nanosheets are arranged in a random fashion without the formation of central cavities. These microstructures are obviously different from the morphology of the abovementioned doughnut microstructures. At a volume ratio of water/ DMF of 3:7, the obtained samples were found to contain CaMoO4 hierarchical microspheres with broad diameter distributions ranging from 0.7 to 1.1 µm (Figure 10g). The microsphere is composed of loosely packed nanosheets with a thickness of ∼51 nm (Figure 10h). When the volumes ratio of water/DMF is further decreased to 1:4, the obtained samples are composed of nanospheres with much broader diameter

distributions in the range of 130-420 nm, which is also constructed of loose nanoflakes (Figure 10i,j). It is worthy to point out that the products are composed of an aggregate of nanoparticles if DMF is used as the only solvent. Although there is a large variation in morphology, the EDS analyses prove that all the samples prepared with water and DMF as mixed solvents are still single-phase compounds of CaMoO4 (Supporting Information, Figure S5). The above two examples of using binary mixed solvents suggest a general and convenient method for controlling the size and morphology of the CaMoO4 microparticles. Although the exact contribution of mixed solvents still needs to be systematically investigated, we believe the complexation of the ions with the solvent molecules may alter the kinetics of the nucleation and growth.41-43 EG is a polar protic solvent that tends to solvate anions such as Mo7O246- via hydrogen bonding. However, replacing water with EG as a solvent reduces the overall solvation power and slows down the transformation of Mo7O246- to MoO42- in reaction 1, therefore leading to the gradual decrease in particle size. The selective adsorption of solvent molecules to the particular growing crystallographic



Figure 12. Photoluminescence spectra of the as-prepared CaMoO4 doughnut microstructures. Curves a, b, and c correspond to the samples shown in panels c, d, and e of Figure 4, respectively.

Figure 11. SEM and TEM images showing the morphology evolution of CaMoO4 doughnut microstructures after etching by HAc solution for (a, b) 0, (c, d) 25, and (e, f) 60 min.

facets, suggested by the reduced thickness of the nanosheet subunits at higher EG concentrations, may also contribute to the change of the final morphology of the products.44,45 On the other hand, DMF is a dipolar aprotic solvent that preferentially solvates positively charged species via its negative dipole.46 In this case, replacing water with DMF may effectively increase the concentration of Mo7O246- in water, thus enhancing the growth rate, and eventually promote the formation of the microparticles with larger sizes. Consistent with the previous observation that axial growth rate increases more than that of lateral growth at a higher precursor concentration, the microparticles possess near-spherical shapes. When DMF becomes the dominant solvent, the effective concentration of Mo7O246is so high that many more nucleation events occur, leading to the dramatic decrease in particle size. Careful inspection indicates that these small particles are spheres composed of nanosheets with a thickness comparable to those large ones, suggesting again the relatively high precursor concentration in the growth step and the weak binding of the DMF molecules on the crystal facets. 3.6. Etching of CaMoO4 Doughnut Microstructures. Chemical etching can also be used to change the morphology of the doughnut microstructures. We noticed that CaMoO4 particles can be slowly dissolved in acetic acid within 2 h. As shown in Figure 11, the samples harvested at different stages of etching still display a doughnut structure. The TEM image of the original doughnuts does not show obvious contrast between the edge and the center (Figure 11b). After etching in acetic acid solution for 25 min, the particles remain doughnut shaped with only a slight decrease in thickness and diameter (Figure 10c). Instead of etching the material isotropically to uniformly reduce the particle size, the acetic acid selectively removes some of the thin nanosheets so that the packing density of the nanosheets is reduced. In addition, the etching process seems to be more

effective around the central cavity. As shown in Figure 10d, the cavities become larger and more distinctive after etching. Figure 11d shows a strong contrast between the dark edge and the pale center, indicating the hollowness at the center of the doughnuts. When the etching time is prolonged to 60 min, the doughnut shape is still retained, as shown in Figure 11e. The corresponding TEM image indicates that the surface of the doughnuts becomes very rough, and the central cavity is further enlarged, leading to the shape transition from filled doughnuts to ring doughnuts (Figure 11f). Further increasing the etching time to 1.5 h, however, yields only irregular aggregated nanosheets. 3.7. Photoluminescence of CaMoO4 Doughnut Microstructures. CaMoO4 has been widely used in industry such as scintillator detectors because of its interesting luminescent property. The doughnut-shaped microstructures produced in this work have shown photoluminescence that is different from that of both bulk samples and nanoparticles of CaMoO4. Figure 12 shows the room temperature PL emission spectra measured from 320 to 560 nm for as-synthesized CaMoO4 samples by using an excitation at 290 nm. Curves of a, b, and c in Figure 12 correspond to CaMoO4 doughnut microstructures with increasing thicknesses, whose morphologies are shown in panels c, d, and e of Figure 4, respectively. All samples show a strong emission with a maximum centered at 512 nm, which is blue-shifted by ∼20 nm compared to the value reported for bulk crystals (∼530 nm).29a,47,48 This is consistent with the prior observation of a size-dependent blue-shift of PL for CaMoO4 samples with decreased crystal sizes.47 The blue-shifts in our case, however, are not as significant as that reported for nanocrystals with diameters below 30 nm (480 nm), suggesting the relatively large domain size (>30 nm) of the nanocrystals contained in the doughnut microstructures.19 The intensity in PL emission is enhanced by increasing the thickness of CaMoO4 doughnut microstructures. It is generally accepted that the PL emission of MMoO4 is mainly attributed to the charge-transfer transitions between the O 2p orbitals and the Mo 4d orbitals within the [MoO42-] complex.15a,50 Molecular orbital calculations performed for the [MoO42-] complex show that the ground state (1A1) has a one-electron configuration. The lowest excited electron configuration associated with the metal d-orbital contains four states 1T1, 3T1, 1T2, and 3T2.51 On the basis of molecular orbital theory, the excitation and emission bands of CaMoO4 can be ascribed to the transition from the 1A1 ground

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state to the high vibration level of 1T2 and from the low vibration level of 1T2 to the 1A1 ground state within the MoO42- complex.51 The progressive enhancement in PL intensity for doughnut microstructures with increasing thicknesses may be attributed to the reduction of surface defects and the higher uniformity in particle size and shape for fully grown samples.49 4. Conclusions In summary, we have demonstrated a simple solution-phase route for the shape- and size-controlled synthesis of CaMoO4 hierarchical doughnut-shaped microstructures. Rapid injection at 100 °C enables the production of uniform samples. The effects of reaction conditions including concentration of reactants, molar ratio between reactants, Mo source, pH value, and the volume ratio of the mixed solvents on the morphology of CaMoO4 were investigated systematically. Results show that the initial concentration of reactants and the molar ratio between reactants play a crucial role in governing the final size and shape of the CaMoO4 doughnut microstructures. Further studies indicate that the morphology of CaMoO4 changes from doughnut microstructures to microspheres, microdisks, and final spindle-like microcrystals with increasing the amount of NaOH in the reaction system. Various morphologies of CaMoO4, such as hierarchical microspheres and architectural spherules with different sizes, can also be fine-tuned by using binary mixed solvents such as water/ethylene glycol or water/DMF. By carefully controlling the etching process in acidic solution, CaMoO4 doughnut microstructures with hollow centers can be obtained. The synthesis procedure reported here is highly reproducible and can be used to synthesize CaMoO4 doughnuts with tailored sizes and aspect ratios. The room temperature photoluminescence measurements of the CaMoO4 doughnut microstructures show a strong emission band centered at about 512 nm, which is blue-shifted from the value for bulk samples due to the significantly reduced crystal domain size in doughnuts. Acknowledgment. Y.Y. thanks the University of California, Riverside for startup support. W.W. gratefully thanks the fellowship support by the China Scholarship Council (CSC) (No. 20083019). Supporting Information Available: Figure S1 showing EDS spectra of the as-prepared CaMoO4 products obtained by using different concentrations of precursors, Figure S2 showing XRD patterns of the as-prepared CaMoO4 products synthesized by using different volumes of 6 M NaOH aqueous solution, Figure S3 showing EDS spectra of the as-prepared CaMoO4 products prepared by using different volumes of 6 M NaOH aqueous solution, Figure S4 showing XRD patterns of the asprepared CaMoO4 products obtained by using water/ethylene glycol solutions with different volume ratios, and Figure S5 showing EDS spectra of the as-prepared CaMoO4 products prepared by using binary mixed solvent of water/N,N-dimethylformamide with different volume ratios. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Cao, Y. W. C.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536. (b) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547. (c) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673. (2) (a) Fan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700. (b) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 10, 1025. (c) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Dong, X. P.; Feng, J. W.; Ruan, M. L.; Li, Y. S. Angew. Chem., Int. Ed. 2005, 44, 5083.

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