Facile Synthesis and Luminescent Properties of Novel Flowerlike

Mar 4, 2009 - After reaction for 2 h, underdeveloped flowerlike nanostructures were .... when the morphology of the crystal changed from tetragonal fl...
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J. Phys. Chem. C 2009, 113, 4856–4861

Facile Synthesis and Luminescent Properties of Novel Flowerlike BaMoO4 Nanostructures by a Simple Hydrothermal Route Yong-Song Luo, Wei-Dong Zhang, Xiao-Jun Dai, Yang Yang, and Shao-Yun Fu* Key Laboratory for Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ReceiVed: December 15, 2008; ReVised Manuscript ReceiVed: January 22, 2009

Flowerlike nanostructures of metals or compounds show interesting physical properties. In this article, novel flowerlike BaMoO4 nanostructures were successfully synthesized via a simple hydrothermal route. The assynthesizedproductswerestudiedbyX-raypowderdiffraction,scanningelectronmicroscopy,Brunauer-Emmett-Teller, and transmission electron microscopy. The results showed that the nucleation and growth of the flowerlike nanostructures were governed by a nucleation-dissolution-recrystallization growth mechanism. The formation of the three-dimensional flowerlike BaMoO4 nanostructures was strongly dependent on the concentration of cetyltrimethylammonium bromide (CTAB). Control experiments were carried out to investigate various influencing factors on the morphology of the products. Different morphologies of BaMoO4 nanostructures were synthesized through adjusting CTAB concentration, temperature, and the volume ratio of N,Ndimethylacetamide to H2O. Luminescent properties of the flowerlike and multilayered BaMoO4 nanostructures consisting of nanosheets were studied, and the flowerlike BaMoO4 nanostructures showed a strong green emission, indicating that the flowerlike BaMoO4 nanostructures have great potential to be applied in luminescent areas. 1. Introduction Much effort has been directed to the synthesis of hierarchical nanostructures because of their unique properties that are not conceivable for micrometric structures.1-4 Remarkable progress has been made for the synthesis of inorganic nanostructures with controlled morphologies, sizes, and so forth, since these parameters are key in determining their electrical and optical properties.5-8 Hierarchical nanostructures can be obtained through the evolution of zero- or one-dimensional primary crystals via the self-assembly method. 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. So far, series of the design and controllable synthesis of hierarchical nanostructures have been successfully reported using this process.9-15 Recently, there has been great interest in developing molybdates because of their interesting luminescent and structural properties, potential application in phosphors, optical fibers, pigments, ionic conductors, and so forth.16-18 Among the molybdates materials, BaMoO4 with a scheelite structure is an important material in electro-optics areas because of its production of green luminescence and potential in electro-optical applications.19-22 In this regard, dendritic BaMoO4 microcrystals were fabricated via a microemulsion-mediated route.23 BaMoO4 nestlike nanostructures were also synthesized by a hydrothermal method.24 Moreover, BaMoO4 nanobelts and penniform patterns were achieved in catanionic reverse micelles.25 Flowerlike structure is a highly desirable hierarchical nanostructure, which has been reported previously for a variety of materials including Co, MoS2, Cu2O, and so forth.26-30 For instance, roselike nanoflowers consisting of two-dimensional (2D) ZnO nanosheets * Corresponding author. [email protected].

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were deposited in chemical bath using layered basic acetate dehydrate for enhancing light conversion efficiency to electricity in dye-sensitized solar cells (DSCs).26 Three-dimensional (3D) flowerlike MoS2 nanostructures were fabricated by a thermal evaporation process, and they exhibited good capability in field emission due to the existence of the open edges in their nanopetals.27 Flowerlike BaMoO4 nanostructures should also possess interesting properties as a result of its peculiar morphology, and synthesis of such BaMoO4 nanostructures is of great interest. However, it remains a challenge to develop facile methods for synthesis of flowerlike BaMoO4 nanostructures. In this paper, we demonstrate a convenient hydrothermal route to prepare BaMoO4 3D flowerlike nanostructures, which are selfassembled from tiny BaMoO4 nanoplatelets. The morphology evolution process is studied through adjusting cetyltrimethylammonium bromide (CTAB) concentration, temperature, and the volume ratio of N,N-dimethylacetamide (DMAc) to H2O. A similar microstructure of BaMoO4 consisting of nanosheets with a thickness of 30-60 nm was reported via a different synthetic route through a simple reflux method under microwave irradiation.31 The same authors also reported the luminescent properties of BaMoO4 nestlike nanostructures consisting of small 2D nanosheets, which were synthesized using polyvinyl pyrolydone (PVP K30) as capping reagents under hydrothermal conditions.24 In this study, the luminescent properties of the flowerlike BaMoO4 nanostructures are examined and compared with that of multilayered BaMoO4 nanostructures consisting of nanosheets similar to that reported previously.24 2. Experimental Section All the chemicals were of analytical grade and utilized without further purification. In a typical synthesis route, 1 mmol of hydrated BaCl2 · 2H2O was dissolved in 35 mL of DMAc-H2O solutions at a volume ratio of DMAc/H2O ) 85%:15%, followed

10.1021/jp811038f CCC: $40.75  2009 American Chemical Society Published on Web 03/04/2009

Hydrothermal Synthesis of BaMoO4 Nanoflowers

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Figure 1. (a) Low-magnification SEM image of the BaMoO4 nanoflowers; (b) enlarged SEM image of the BaMoO4 nanoflowers; (c) SEM image of an individual BaMoO4 nanoflower; (d) powder XRD pattern of the as-synthesized BaMoO4 nanoflowers.

by the addition of CTAB (1 mmol, molecular weight ) 364.46). The obtained solution was sonicated in an ultrasonic water bath for 30 min. Then, 1 mmol of (NH4)6MoO24 · 4H2O was added to the above solution. The mixture was placed in a 40 mL Teflon-sealed autoclave and maintained at 180 °C for 12 h. After cooling to room temperature, the product was separated from the solution by centrifugation, washed several times with alcohol, and then dried in air at 70 °C. To investigate the intermediates, the BaMoO4 products were also obtained at different reactive stages during the synthesis process. The phase purity of the products was characterized by X-ray diffraction (XRD) using a X-ray diffractometer with Cu KR radiation (λ) 1.5418 Å). Scanning electron microscope (SEM) images and X-ray energy dispersive spectroscopy (EDS) analyses were obtained using a HITACHI S-4300 microscope (Japan). Transmission electron microscope (TEM) and highresolution transmission electron microscopy (HRTEM) observations were carried out on a JEOL JEM-2010 instrument in bright field and on a HRTEM JEM-2010FEF instrument (operated at 200 kV). The surface area was measured using a Micromeritics (NOVA 4200e) analyzer. The nitrogen adsorption and desorptionisothermswereobtainedat77K.TheBrunauer-Emmett-Teller (BET) surface area was calculated from the linear part of the BET plot. Room temperature photoluminescence (PL) spectra were recorded on an F-4600 (Hitachi) spectrophotometer. 3. Results and Discussion Flowerlike BaMoO4 nanostructures were synthesized by the reaction between Ba2+ and MoO42- ions in a H2O/CTAB/DMAc three-phase system at the appropriate temperature of 180 °C. Figure 1a-c shows the SEM images of the BaMoO4 nanoflowers at low, medium, and high magnification, respectively. From the SEM observations, it can be seen that the BaMoO4 product contains numerous flowerlike aggregates, single flowers have a diameter ranging from 2 to 4 µm, and almost all of them show the same morphology (Figure 1a,b). In addition, each flower is made up of many thin nanopetals, and the petals are plateletlike (Figure 1c), indicating that the significant modified reaction activities of different crystal planes resulted from the specific three-phase chemical conditions. Careful examination reveals that the BaMoO4 nanopetals are 1-2 µm in size, and

about 10 nm in thickness. The XRD pattern of the flowerlike BaMoO4 nanostructures is displayed in Figure 1d. All of the diffraction peaks can be indexed to the tetragonal phase of BaMoO4 with a lattice constant of a ) 5.58 Å and c ) 12.82 Å, which is consistent with the values in the standard card (JCPDS card No. 29-0193). No impurity peaks were detected, indicating the formation of pure products. The as-prepared products were also determined by EDS analysis under N2 atmosphere. The EDS result shown in the inset of Figure 1d demonstrates that the as-prepared sample contains Ba, Mo, and O, and the atomic ratio of Ba, Mo, and O is nearly 1:1:4. The similar XRD pattern for other shaped products such as multilayered platelets is also obtained as shown in the Supporting Information, Figure S1, confirming the formation of BaMoO4 products. To obtain further information about the microstructure of these flowerlike BaMoO4 nanostructures, HRTEM analysis was performed. In this image (Figure 2a), the edge and center of the flower show a strong brightness contrast, further confirming their flower nature. Figure 2b exhibits small parts cracked from flowers, which verifies that the flowers are self-organized from small nanoplatelets. Figure 2c shows the typical HRTEM image of an individual BaMoO4 nanopetal. The clearly resolved lattice fringes are 0.28 nm, indicating that the BaMoO4 nanoflower is single crystal in nature. The enlarged image (Figure 2d) of the area labeled in Figure 2c by a rectangle further reveals the single-crystal structure of BaMoO4 nanopetal. The selected area electron diffraction (SAED, see the inset of Figure 2d) can be indexed to a pure tetragonal phase (sheelite). It also indicates that the sheet is a scalelike “single-crystal” with the (002) plane, as the 2D exposed surface and the formation process of each petal should be via an oriented attachment. This result is in accordance with the HRTEM image of an individual petal, which demonstrates a single crystal structure with a lattice interplanar spacing of 2.8 Å, corresponding to the (200) plane of tetragonal BaMoO4. Moreover, the slight mismatchings are also observed for the nanopetals as revealed in the image. It is worth mentioning that the slight mismatching cannot be entirely avoided in HRTEM, since there are unequal sizes and/or uneven surfaces in these zero dimensional subunit particles.

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Figure 2. (a) TEM image of an individual BaMoO4 nanoflower; (b) TEM image of the fragmentized part of one nanopetal; (c) HRTEM image of the fringe part of a nanopetal; (d) enlarged image of the area marked by a rectangle in panel c, and the inset is the SAED result of the nanosheet.

Figure 3. Nitrogen adsorption-desorption and pore-size distribution isotherm for the obtained flowerlike BaMoO4 product.

Nitrogen adsorption-desorption measurements were conducted to characterize the BET surface area and internal pore size distribution. The recorded adsorption and desorption isotherms for the flowerlike BaMoO4 nanostructures show a significant hysteresis (Figure 3). The BET surface area for the as-obtained samples calculated from the linear part of the BET plot is about 2.757 m2/g, which is much higher than that (1.335 m2/g) of the multilayered BaMoO4 nanostructures consisting of multilayered platelets (Supporting Information, Figure S2). This is attributed to the fact that the latter has comparatively compact structure, while the former has a flower-like structure growing radiallyfromthecoresofthenanoplatelets.Barrett-Joyner-Halenda (BJH) calculations for the pore size distribution, derived from desorption data, reveal a narrow distribution for the flowerlike BaMoO4 hierarchical nanostructures centered at 3.09 nm (inset of Figure 3). The mesopores of the as-prepared flowerlike BaMoO4 nanostructures may result from the interstitial voids between adjoining subparticles within the aggregates. In order to understand the morphological evolution, BaMoO4 nanostructures attained at different growth stages were carefully examined by SEM observation. The obvious evolutionary stages are shown in Figure 4. Figure 4a shows tiny particles that were collected before being transferred to the Teflon-sealed autoclave. After reaction for 30 min, products with few platelets are

observed (Figure 4b). After reaction for 2 h, underdeveloped flowerlike nanostructures were formed (Figure 4c). When the reaction time is increased to 6 h, the 3D flowerlike congeries appear as a result of self-assembly, as presented in Figure 4d. Upon gradual evolution of the BaMoO4 nanostructures, welldefined nanoflowers are produced after a reaction time of 12 h. Most of the obtained products are perfect nanoflowers as shown in Figure 4e, and almost no impurities can be observed. The process of the shape transition from nanoparticles to nanoplatelets and then to nanoflowers is summarized in Figure 5. On the basis of the results discussed above, we believe that the formation of BaMoO4 nanoflowers can be rationally expressed as a kinetically controlled nucleation-dissolutionrecrystallization mechanism (see Figure 5). First, when the reaction was performed in the solution-phase system before being transferred to the Teflon-sealed autoclave, it directly gave fine BaMoO4 particles, which were formed in the solution through a homogeneous nucleation process. Furthermore, the small BaMoO4 particles grow to large particles via a process known as Ostwald ripening32 as the aging process continued. Second, when the reaction was carried out at 180 °C for 30 min, due to anisotropic crystal structure, there was an intrinsic tendency for nucleation growth along the planar direction,33 and partial BaMoO4 nanoparticles started to dissolve in the solution and further grow into plateletlike nanocrystals through oriented aggregation. After a longer reaction time, the plateletlike products gradually grew into larger sheetlike structures by recrystallization process. Finally, the sheetlike nanocrystals gradually evolved to 3D flowerlike nanostructures through the self-assembly of nanocrystals. Moreover, in this experiment, CTAB plays a key role in the formation of the as-synthesized product. As is well-known, CTAB is an ionic compound, which ionizes completely in water. In the synthetic procedure of nanomaterials including BaMoO4, the CTAB was used as a capping agent and/or a “soft” template in the synthesis of mesostructured materials to form spherical, cylindrical micelle, or even higher-order phases depending on the solution conditions.34-36 In our system, the CTAB is used as a “soft” template. We speculate that during the addition of an amount of CTAB to the reaction solution, many active sites will be

Hydrothermal Synthesis of BaMoO4 Nanoflowers

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Figure 4. Time-dependent morphological evolution of the BaMoO4 products at different growth stages: (a) 0 min, (b) 30 min, (c) 2 h, (d) 6 h, and (e) 12 h.

Figure 5. Schematic illustration of the formation and shape evolution of BaMoO4 nanoflowers in the whole synthetic process.

Figure 6. Typical SEM images of BaMoO4 products synthesized under different volume ratios of DMAc to H2O: (a) ∞ (namely, in the absence of water), (b) 11, (c) 6, (d) 4, (e) 1, and (f) 0 (namely, in the absence of DMAc).

produced around the circumference of BaMoO4 nuclei (the BaMoO4 nanoparticles formed earlier) in the hydrothermal conditions. In addition, detailed experiments revealed that the composition of solution, especially the value of ω (volume ratio of DMAc to H2O), is another important factor and could also be considered to influence the growth process of BaMoO4 products. Figure 6 shows the SEM images of the as-synthesized products obtained at the different values of ω, respectively. It is clear

that the morphology of the products critically depends on this volume ratio. The SEM images reveal that the morphologies of the as-synthesized products changed gradually upon decreasing the ω value. For ω ) 11, nanostructures with sizes of 2-3 µm consisting of multilayered nanoplatelets are observed in the products (Figure 6b). Each platelet is about 25 nm in thickness. When the ω value is decreased to 6, nanoflowers with a diameter of about 2-4 µm are the exclusive products (Figure 6c). The morphology of the multilayered platelet BaMoO4 nanostructures

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Figure 7. Room-temperature PL spectra of the samples of BaMoO4 nanoflowers and multilayered platelets.

consisting of nanosheets obtained in the present study is similar to the morphology of the nestlike nanostructures reported previously.24 The morphology of the flowerlike nanostructures is different from that of their nestlike nanostructures,24 not only in their morphologies but also in their sizes. However, when the value of ω was decreased to 0, namely, synthesis of BaMoO4 crystal in the absence of DMAc, the experimental result indicated that multilayered nanosheets with a diameter of about 2-4 µm were obtained (Figure 6f). Thus, it can be inferred that the molar ratio of DMAc to H2O (or the water content) is another important factor for the formation of the products with various morphologies. Further control experiment studies also verify that the final morphologies of the products are strongly affected by the reaction concentration of the CTAB. We repeated the synthesis in the absence of CTAB, and the resultant product of BaMoO4 was amorphous with aggregates seen as shown by the SEM image in Figure S3a. This result further indicates that CTAB plays an important role in controlling the morphology of the product. Moreover, temperature also affects the morphology of BaMoO4 product. Figure S3b represents the products obtained at a relatively low temperature (160 °C), the image reveals that no nanoflowers can be observed under low temperature conditions. PL properties of the flowerlike BaMoO4 nanostructures were studied, and the results are shown in Figure 7 at the excited wavelength of 240 nm. For the purpose of comparison, the PL properties of the multilayered platelets of BaMoO4 were also studied. The spectra of both the samples show that the emission peaks are centered at around 530 and 618 nm, respectively. These peaks are similar to those reported previously and can be mainly attributed to the charge-transfer transitions within the MoO42- complex.37-39 It can be seen that the nanoflowers exhibit a strong luminescent emission and have a much higher green emission intensity at 530 nm, but a slightly lower red emission intensity at 618 nm compared with the multilayered platelets of BaMoO4. The components of the PL emission bands are linked to specific atomic arrangements.40 Molybdenum ideally has the tendency to bond with four oxygen atoms, but there are various coordination numbers for Mo in the structure before it reaches the ideal configuration. Before crystallization, a mixture of MoOx clusters (x ) mostly 3 and 4) intercalated by Ba atoms exists in the structure. This mixture of the clusters is responsible for the PL in the red region. When complete crystallization is reached, only MoO4 clusters exist in the structure, and the PL

Luo et al. in the red region vanishes while the PL in the green region appears.40 In the present study, the flowerlike BaMoO4 nanostructures and the multilayered BaMoO4 platelets were produced at the same reaction temperature for the same reaction time. They show similar XRD patterns, indicating the similar crystallization degrees. The only difference between the two products is with the volume ratio of DMAc to H2O, leading to different morphologies. Figure 7 shows that the morphology has no influence on the peak locations but affects the green emission intensity strongly, and the flowerlike morphology of BaMoO4 corresponds to the higher green emission intensity. It is thus inferred that the morphology of the flowerlike nanostructures is more beneficial than the morphology of the multilayered platelets to the PL emission in the green region. Similar interesting observations about the dependency of the PL intensity have been reported previously on the morphology of other micro- and nanostructures.41-45 The effect of the microscopic morphology of silica gels was clearly observed on their PL properties.41 Also, the effect of the morphology of Cu2O micronanostructures prepared by electrodeposition on the roomtemperature PL properties was reported.42 In another work, the flower-shaped ZnO nanostructure has the highest surface area compared to the urchin-shaped and butterfly-shaped morphologies of ZnO architectures.43 It was observed that the PL intensity for the flower-shaped product corresponds to the highest PL intensity at around 375 nm. It was thus suggested that the optical properties of ZnO nanostructures are sensitive to their morphologies. The room-temperature emission spectra of PbMoO4 were investigated, and the relative PL intensity between 400 and 300 nm was intensified when the morphology of the crystal changed from tetragonal flake into nanobelts.44 The dependency of the PL intensity of Y2O3:Eu nanocrystals was studied on the surface area of nanocrystals.45 It was observed that the PL intensity increased with increasing surface area. It has been shown in Figure 3 and Figure S2 that the specific surface area of the flowerlike morphology of BaMoO4 is much higher than that of the multilayered morphology of BaMoO4. This is probably one major reason the flower BaMoO4 nanostructures have a higher PL intensity compared to the multilayered BaMoO4 nanostructures. 4. Conclusions In conclusion, novel flowerlike BaMoO4 nanostructures have been successfully synthesized via a simple hydrothermal route. The nucleation-dissolution-recrystallization mechanism is proposed for the organization of the flowerlike BaMoO4 nanostructures. The luminescent properties of the flowerlike BaMoO4 nanostructures have been studied, and it was observed that the flowerlike BaMoO4 nanostructures showed a strong green emission with a much higher intensity than that of the multilayered BaMoO4 platelets. It was thus concluded that the rational design of the BaMoO4 nanostructures could be of significance, and that flowerlike nanostructures have great potential in luminescent applications. Moreover, the preparation method reported here is not limited only to the synthesis of BaMoO4 hierarchical nanostructures, but can also be extended to other related inorganic nanostructures. Acknowledgment. We appreciate the financial support of the Beijing Municipal Natural Science Foundation (Nos. 2082023 and 2091004). Supporting Information Available: XRD pattern, nitrogen adsorption-desorption, and pore-size distribution isotherm of

Hydrothermal Synthesis of BaMoO4 Nanoflowers the multilayered platelets of BaMoO4, and SEM images of BaMoO4 products synthesized at different conditions are provided in detail. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (2) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (3) Fang, X. S.; Zhang, L. D. J. Mater. Sci. Technol. 2006, 22, 1. (4) Shen, G. Z.; Chen, D. J. Am. Chem. Soc. 2006, 128, 11762. (5) Mann, S. Angew. Chem., Int. Ed. 2000, 39, 3392. (6) Alivisatos, A. P. Science 1996, 271, 933. (7) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (8) Zhang, H.; Yang, D. R.; Li, D. S.; Ma, X. Y.; Li, S. Z.; Que, D. L. Cryst. Growth Des. 2005, 5, 547. (9) Zeng, H. B.; Liu, P. S.; Cai, W. P.; Cao, X. L.; Yang, S. K. Cryst. Growth Des. 2007, 7, 1092. (10) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244. (11) Cheng, Y.; Wang, Y. S.; Chen, D. Q.; Bao, F. J. Phys. Chem. B 2005, 109, 794. (12) Lu, Q. Y.; Gao, F.; Komameni, S. J. Am. Chem. Soc. 2004, 126, 54. (13) Lee, E. J. H.; Ribeiro, C.; Longo, E.; Leite, E. R. J. Phys. Chem. B 2005, 109, 20842. (14) Wang, Y.; Zhu, Q. S.; Zhang, H. G. Chem. Commun. 2005, 41, 5231. (15) Lu, L. H.; Kobayashi, A.; Kikkawa, Y.; Tawa, K.; Ozaki, Y. J. Phys.Chem. B 2006, 110, 23234. (16) Ryu, J. H.; Choi, B. G.; Yoon, J. W.; Shim, K. B.; Machi, K.; Hamada, K. J. Lumin. 2007, 124, 67. (17) Gong, Q.; Qian, X. F.; Ma, X. D.; Zhu, Z. K. Cryst. Growth Des. 2006, 6, 1821. (18) Eda, K.; Uno, Y.; Nagai, N.; Sotani, N.; Chen, C.; Whittingham, M. S. J. Solid State Chem. 2006, 179, 1453. (19) Yu, S. H.; Liu, B.; Mo, M. S.; Huang, J. H.; Liu, X. M.; Qian, Y. T. AdV. Funct. Mater. 2003, 13, 639. (20) Saito, N.; Sonoyama, N.; Sakata, T. Bull. Chem. Soc. Jpn. 1996, 69, 2191. (21) Porto, S. P. S.; Scott, J. F. Phys. ReV. 1967, 157, 716.

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