Room-Temperature Synthesis and Luminescent Properties of Single

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Room-Temperature Synthesis and Luminescent Properties of Single-Crystalline SrMoO4 Nanoplates Yan Mi, Zaiyin Huang,* Feilong Hu, Yanfen Li, and Junying Jiang College of Chemistry and Ecological Engineering, Guangxi UniVersity for Nationalities, Nanning 530006, People’s Republic of China ReceiVed: July 30, 2009; ReVised Manuscript ReceiVed: October 18, 2009

Two-dimensional (2D) free-standing single-crystalline SrMoO4 nanoplates of obvious quasi-square shape have been controllably synthesized using a facile liquid approach at room temperature. The structural characterizations of the nanoplates were investigated in detail by means of XRD, FESEM, and HRTEM. The results indicate that the 2D quasi-square SrMoO4 nanoplates can be formed through the oriented connection and self-assembly process. Meanwhile, controlled experiments show that the concentration of cetyltrimethylammonium bromide (CTAB), water content, reaction time, and concentration of Sr2+ and MoO42- play important roles in the formation of the nanoplates. Moreover, the ultraviolet-visible (UV-vis) absorbance spectra reveal a characteristic optical band gap of 3.12 eV. Most interestingly, such SrMoO4 nanoplates present a strong and broad blue PL emission, which exhibits a noted blue shift compared to previous reports, indicating that the square-shaped SrMoO4 nanostructures have great potential to be applied in luminescent and optoelectronic devices. 1. Introduction Much effort has been directed toward the synthesis of nanomaterials with controlled shapes and ordered morphologies due to their unique properties that are not conceivable in bulk structures.1 In particular, low-dimensional nanostructures, such as nanocubes, nanoplates, nanobelts, nanorods, nanowires, and nanotubes have attracted intense interest for their distinctive geometries and novel physical and chemical properties, and thus are particularly attractive building blocks for nanoscale electronic and photonic applications.2 Metal tungstates and molybdates are two important families of inorganic materials that have received considerable attention since the discovery of their interesting luminescent and structural properties for potential application in various fields, such as in the fields of photoluminescence (PL),3 optical fibers,4 scintillator materials,5 humidity sensors,6 magnetic properties,7 catalysis,8 et al. Among the metal tungstates and molybdates, SrMoO4 with a scheelite structure is one of the most important materials in optoelectronic areas due to its emission of blue and green luminescence.9 For example, Liu et al.10 reported that SrMoO4 crystallites with various morphologies show a difference PL performance that the spindle-shape SrMoO4 crystallites with a higher aspect ratio resulted in better PL properties. Under this circumstance, much effort has been devoted to controlling its morphology, and thus various micro/nanostructural SrMoO4, such as nanowires, flower-like mesocrystals, ellipsoidal rods, peanuts, dumbbells, peaches, spherules, bipyramids, rice-like nanostructures, and films10,11 have been successfully synthesized by different methods and techniques in recent years. Twodimensional (2D) nanostructures with one restricted dimension are highly desirable and may possess some specific properties. For instance, bismuth molybdate nanoplates synthesized by a simple hydrothermal process exhibited good photocatalytic activities for degradation of organic contaminants.12 Silver nanoplates obtained via a photoreduction method showed much stronger enhancement efficiency than that of silver nanoparticles

in surface enhanced Raman scattering (SERS) of hyaluronan (HA).13 In this regard, 2D SrMoO4 nanoplates should also possess interesting properties as a result of their peculiar morphology. However, to the best of our knowledge, 2D SrMoO4 nanostructures were only synthesized on the surface of substrates or in the interstices of some three-dimensional (3D) networks.14 Therefore, it remains a significant challenge to develop facile routes for the fabrication of free-standing SrMoO4 nanoplates. Simple chemical reactions in reverse micelles and microemulsions have been shown to be powerful in controlling the size and shape of inorganic nanocrystals.15 Reverse microemulsions are colloidal “nano-dispersions” of water in oil stabilized by a surfactant and cosurfactant film localized at the water-oil interface.16 The morphology and size of water pool can be rationally controlled by adjusting the experimental conditions. Thus, these nanoscale water pools can provide ideal spatially constrained microreactors for the formation of controlled size and morphology of inorganic nanoparticles under room temperature and ambient pressure without any special equipment. Specifically, inorganic nanocrystals of various materials, including metal semiconductors, as well as inorganic salt have been successfully synthesized in reverse microemulsion media so far.17 We have also previously successfully prepared BaMoO4 nano-octahedra by microemulsion method.18 In this work, we present the synthesis of free-standing quasisquare SrMoO4 nanoplates with side lengths of ∼200 nm and thicknesses of ∼12 nm by a facile reverse microemulsion approach at room temperature in a controlled way. It has been shown that rational control of shape and size of nanocrystals can be readily achieved by altering the experimental parameters. Furthermore, detailed growth process and luminescent properties of the structural SrMoO4 nanoplates were also studied further. The PL spectra of the as-prepared SrMoO4 nanoplates display a notable blue shift compared with that of previous works, which illustrated that these nanoplates have potential applications in novel optoelectronic devices. The present synthesis method

10.1021/jp907328v CCC: $40.75  2009 American Chemical Society Published on Web 11/04/2009

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might be exploited to convenient synthesis of a family of molybdate nanoplates. 2. Experimental Section 2.1. Synthesis of SrMoO4 Nanoplates. All chemical reagents (sodium molybdate (Na2MoO4 · 2H2O)), strontium chloride (SrCl2 · 6H2O), cetyltrimethylammonium bromide (CTAB), n-butanol (CH3(CH2)3OH), and n-heptane (CH3(CH2)8CH3) were analytical grade, purchased from Xilong Chemical Industrial Co. and were used without further purification. The water used throughout this work was deionized water. The reaction was carried out in a 50 mL glass beaker. The strontium molybdate nanostructures were fabricated in the cetyltrimethylammonium bromide (CTAB)/water/n-butanol/ n-heptane reverse microemulsion liquid system. In a typical procedure, 1.0 mL of 0.05 M SrCl2 · 6H2O aqueous solution and 1.0 mL of 0.05 M Na2MoO4 · 2H2O aqueous solution were added into two separated solutions containing 12 mmol of CTAB, 8 mL of n-butanol and 20 mL of n-heptane under vigorous stirring at room temperature, respectively. After about 10 min vigorous stirring, two optically transparent microemulsion solutions were formed. Subsequently, dropwise addition of the microemulsion, which contained Sr2+, was added into another microemulsion which containing MoO42- and kept under stirring for another 30 min. Afterward, the resulting solution was settled for 24 h at room temperature. The white product was separated by centrifugation, washed with acetone, deionized water, and absolute ethanol several times, and then dried in a vacuum at room temperature. 2.2. Characterizations. The phase purity of the products was characterized by X-ray diffraction (XRD) using an X-ray diffractometer (the Philips X’Pert) with Cu KR radiation (λ ) 1.5406 Å). Field emission scanning electron microscopy (FESEM) images and X-ray energy dispersive spectroscopy (EDX) analyses were obtained using a Sirion 200 FEG microscope, and the as-synthesized products were dispersed on the Si substrate. Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) observations were carried out on a JEM-200CX instrument (operated at 200 kV). The UV-vis absorption spectrum was measured on a Cary 5E UV-vis-IR spectrometer. Room temperature photoluminescence (PL) spectra were recorded on an FLS920 PL spectrophotometer with a Xe lamp as the light source. 3. Results and Discussion 3.1. Morphology and Microstructure. The XRD pattern of the as-synthesized samples settled for 24 h at room temperature is displayed in Figure 1a. All of the diffraction peaks can be indexed to the scheelite-type tetragonal structure, I41/a(88) space group of SrMoO4 with a lattice constant of a ) 5.35 Å and c ) 11.93 Å, which is in good agreement with the standard card (JCPDF Card No. 08-0482). The strong and sharp peaks indicated that the as-prepared products are highly crystallized and structurally ordered at long-range.19 No impurity peaks were detected, indicating the formation of pure products. To determine the elemental composition of the as-synthesized samples, EDX was applied. The typical EDX spectrum of the samples was shown in the inset of Figure 1a demonstrates that the assynthesized sample is composed of Sr, Mo, and O (Si comes from the substrate), and the atomic ratio for Sr, Mo, and O is close to 1:1:4, in agreement with the expected stoichiometry of the SrMoO4 phase. A typical field-emission scanning electron microscope (FESEM) image of the SrMoO4 nanoplates is shown in Figure 1b at

Figure 1. (a) Power XRD pattern of the as-synthesized SrMoO4 nanoplates, and the inset is the EDX spectrum of the nanoplates. (b) A typical SEM image of the as-synthesized SrMoO4 nanoplates. (c) TEM image of the quasi-square SrMoO4 nanoplates. (d) TEM image of an individual quasi-square SrMoO4 nanoplate. (e) The corresponding SAED pattern of the individual nanoplate. (f) HRTEM image of the individual nanoplate.

low magnification, from which it can be seen that the asprepared SrMoO4 nanostructures were mainly quasi-square nanoplates (indicates the high yield of such nanoplates). Each nanoplate has a side length of about 200 nm. To obtain further information about the nanostructure of these quasi-square SrMoO4 nanoplates, TEM, HRTEM, and selected area electron diffraction (SAED) analysis were performed. Figure 1c shows the typical TEM image of the SrMoO4 particles which reacted at room temperature for 24 h. The strong brightness contrast in the image indicated that the as-prepared products were square-shape nanoplates, which have a typical side length of ∼200 nm and thickness of ∼12 nm, respectively. An image of a randomly chosen single SrMoO4 nanoplate is shown in Figure 1d. The difference of brightness contrast and rough surface presents that these nanoplates are self-assembled from small and ultrathin irregular plates.14 This was further confirmed by the ultrathin irregular plates cracked from single quasi-square nanoplate marked by a white rectangle in Figure 1c. Figure 1e presents the SAED pattern of a single quasisquare SrMoO4 nanoplate shown in Figure 1d. The presence of the sharp diffraction spots rather than an amorphous ring is suggestive of the predicted formation of single crystalline SrMoO4. And this SAED pattern can be indexed to pure SrMoO4 crystals of a tetragonal scheelite structure. It also demonstrates that the nanoplate is a plate-like single-crystal with the (002) plane as the 2D exposed surface and the formation process of each nanoplate should be via an oriented attachment. This result is in accordance with the typical HRTEM image (Figure 1f) of an individual quasi-square SrMoO4 nanoplate shown in Figure 1d. The HRTEM image further indicates that the observed quasisquare SrMoO4 nanoplates are single crystalline with no defects or dislocations. The 2D lattice fringes reveal that the singlecrystalline quasi-square SrMoO4 nanoplates possess interplanar spacing of about 0.27 nm, corresponding to the (200) plane of tetragonal SrMoO4. 3.2. Formation Process of the Quasi-Square SrMoO4 Nanoplates. For better understanding of the formation process of the nanoplates, reaction products obtained at different growth stages were carefully examined by TEM observations. The obvious evolutionary stages are shown in Figure 2. First, nearly

Synthesis and Properties of SrMoO4 Nanoplates

Figure 2. Products prepared at different reaction time: (a), (b), (c), and (d) correspond to 5 min, 30 min, 3 h, and 24 h, respectively.

spherical nanoparticles were formed in the water pools after the mixing of two microemulsions for 5 min, which is shown in Figure 2a. The SAED pattern illustrates that these nanoparticles are single crystalline SrMoO4 structures. When the reaction time was prolonged to 30 min under stirring at room temperature, ultrathin irregular plates were observed (Figure 2b). While the solution was settled still for 3 h, coarsening nanoplates appear as a result of ultrathin irregular plates self-assemble in the restricted space as shown in Figure 2c. After gradual ripening of the SrMoO4 nanostructures, well-defined quasi-square SrMoO4 nanoplates were constructed when the reaction duration increased to 24 h, as presented in Figure 2d. The process of the shape evolution from nanoparticles to ultrathin irregular plates, and then to quasi-square nanoplates is schematically illustrated in Figure 3, which will be helpful to understand the above content. On the basis of the shape evolution discussed above, we believe that the formation of the quasi-square SrMoO4 nanoplates can be rationally explained by a kinetically controlled oriented connection and self-assembly process (See Figure 3). (I) The reaction between the Sr2+ and MoO42- starts when the two microemulsions were mixed together, and it directly gave tiny SrMoO4 spherical particles as a consequence of homogeneous nucleation in the system. (II) The spherical particles oriented connected between adjacent particles. When structurally similar surfaces of particles approach, there will be a driving force to form chemical bonds between atoms of opposing surfaces so as to achieve full coordination.20 It is known that there exists an interface between two randomly conjoined single crystal nanoparticles. Thermodynamically, these two conjoined particles will rotate or fine-tune to share a common crystallographic orientation, and bonding these particles at a planar interface so that the interface disappears and the overall energy decreases.21 In principle, if only the nanoparticles in the solution are active enough and the time is sufficiently long, then such oriented connection growth would probably take place in nature.22 It is presented here that the single crystalline SrMoO4 nanoparticles will grow into ultrathin irregular plates in the solution through oriented attachment. After the formation of ultrathin plates, the main framework of the quasi-square nanostructure has been built. (III) In order to further decrease the overall energy, the ultrathin plates started to self-assemble to quasi-square SrMoO4 nanoplates in the restricted water pools which stabilized by the surfactant of CTAB by the following two manners. For the first one, the ultrathin plates coalesce each other through side-by-side means to enlarge its planar area (A). For the second one, the ultrathin plates attach by layer-by-layer route (B). Combinations of different ultrathin plates result in

J. Phys. Chem. C, Vol. 113, No. 49, 2009 20797 slabs of different thicknesses.20 The corresponding SAED pattern (Figure 1e) indicates that (200) was the bonding plane. At last, well-defined quasi-square SrMoO4 nanoplates were obtained by further ripening process. 3.3. Influence Factors. 3.3.1. Concentration of CTAB. In this experiment, the surfactant CTAB plays a key role in the formation of the as-synthesized product. In the synthetic procedure of nanomaterials, the CTAB is used as a capping agent and/or a “soft” template in the synthesis of mesostructured materials to form spherical and cylindrical micelle and so forth depending on the system condition.23 In our system, the CTAB is used as a capping agent and a “soft” template. We inferred that the reaction between Sr2+ and MoO42- was performed in the individual restrict water pools of the reverse microemulsion which was stabilized by the CTAB. During the addition of an amount of CTAB to the microemulsion, the CTAB molecules adsorbed around the water pools, which made the water pools flat. 3.3.2. Water Content Factor. In addition, detailed experiments revealed that the composition of solution, especially the value of ω (water content), is another important factor and could also considerably influence the growth process of SrMoO4 nanostructures. Figure 4 shows the SEM images of the asprepared products obtained at the different values of ω, respectively. The SEM images demonstrate that the morphologies of the as-synthesized products changed upon increasing the ω value. From ω ) 2, irregular plates with sizes of ∼100 nm are observed in the products (Figure 4a). When the ω value is increased to 10, flat microspindles with a length of 1 µm and a diameter of 200 nm are observed (Figure 4b). Microspindles with a length of 1 µm and a diameter of 500 nm were observed, when the ω value was further increased to 20 (Figure 4c). Further, when the SrMoO4 crystal was synthesized in pure water, a microspindle with a length of 5 µm and a diameter of 2-4 µm was formed (Figure 4d). Thus, it is clear that the morphology of the products critically depends on the water content factor. 3.3.3. Concentration of Reactants. The concentration of the reactants (Sr2+ and MoO42-) are also strongly affected by the final morphologies of the products which are verified further by the control experiment studies. We repeat the synthesis at a lower and higher concentration, respectively. At a lower concentration (0.01M), the products are nanospindles about 500 nm in length and about 100 nm in diameter. At a higher concentration (0.1M), the products are flat nanospindles having a length of 1 µm and a diameter of 100 nm (not shown here). 3.4. Optical Measurements. 3.4.1. UV-Wis Absorption Spectroscopy Analyses. Figure 5 shows the UV-vis absorption spectra of quasi-square SrMoO4 nanoplates at room temperature for 24 h. It is known that the relationship between the absorption coefficient (R) near the absorption edge and the optical band gap energy (Egap) for direct interband obey the following formula:24

(hVa)2 ) A(hV - Egap) where A is the parameter that relates to the effective masses associated with the valence and conduction bands, and hV is the photon energy. Hence, the optical band gap for the absorption edge can be obtained by extrapolating the linear portion of the plot (hVR)2 - hV to R ) 0 from Figure 5. The optical absorption in the edge region can be well-fitted using a relation (hVR)2 ≈ hV - Egap), as shown in the inset, which

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Figure 3. Schematic illustration of the formation and shape evolution of quasi-square SrMoO4 nanoplates in the whole synthetic process: (I) Nucleation and crystallization; (II) Oriented connection and self-assembly (A: Side-by-side, B: layer-by-layer); (III) Ripening.

Figure 4. Typical SEM images of SrMoO4 products synthesized under different water content ω. Parts (a), (b), (c), and (d) are related to ω ) 2, 10, 20, and ∞, respectively.

Figure 5. Optical absorption spectrum of the quasi-square SrMoO4 nanoplates at room temperature. Inset: the determination of the band gap.

indicates that the as-synthesized quasi-square SrMoO4 nanoplates have a band gap of 3.12 eV. This result presents that the quasi-square SrMoO4 nanoplates exhibits a notable blue shift (to ∼0.86 eV) compared to the SrMoO4 powders.25 This implies that the quasi-square SrMoO4 nanoplates display a notable quantum-confinement effect. It has also reported that the observed behavior could also be related to other factors, such as the degree of structural order-disorder in the lattice, the energy difference between the valence and conduction band, preparation method, morphology, shape, temperature, and processing time of this material. 3.4.2. PL Measurements. The optical properties of these quasi-square SrMoO4 nanoplates are being studied. Figure 6 shows the PL spectra of the quasi-square SrMoO4 nanoplates. As shown in Figure 6, with the excited wavelength at 360 nm, the quasi-square SrMoO4 nanoplates exhibited a broad (∼300 nm) and strong PL emission band in the blue and purple wavelength ranges, and the peak is located at about 440 nm, which shows a noted blue shift comparable with that of the previously reported rice-like SrMoO4 nanostructure,10 SrMoO4

Figure 6. Room-temperature PL spectra of the quasi-square SrMoO4 nanoplates (excited at a wavelength of 360 nm) and the results by Gaussian multipeak fitting (red line: measured result).

powders,25 and SrMoO4 thin films that were prepared at room temperature or annealed at high temperature.11f-i In Figure 6, the emission profile of this sample can be associated with the contribution of various components. Thus, in order to estimate the contribution of each individual component, it was necessary to deconvolute the PL spectra. The deconvolution of the PL spectrum was performed by the PeakFit program (4.05 version) using the Gauss area function. The PL profiles were better adjusted by the addition of three peaks. The P1 peak is positioned in the purple wavelength region; the P2 is situated in the blue wavelength region; and the P3 peak is localized in the green wavelength region. The PL emission process of molybdates with scheelite-type tetragonal structure is not completely understood yet; therefore, several hypotheses have been reported in the literature to explain the possible mechanisms responsible for the emission process of the metal molybdates. For example, Wu et al.26 pointed out that the 1T2-1A1 electronic transitions within the [MoO4] tetrahedron groups are responsible for the blue PL emission. Chen and Gao11i argued that the green emission is a result of the intrinsic luminescent behavior of the [MoO4] group. These authors maintained that the transition of green luminescence is due 3T1, 3T2 f 1A1 transition in the tetrahedral molybdates group. Luo et al.9e claimed that the components of the PL emission bands are linked to specific atomic arrangements, and the morphology also strongly affects the emission intensity. Liu et al.10 showed that the PL behavior of SrMoO4 can be modified by the crystallite’s morphology. Sczancoski et al.25 asserted that the PL properties are also related to preparation conditions and excitation wavelengths of samples. In our present work, the samples are not only structurally ordered but also possess good crystallinity, we suppose that each component represents a different type of electronic transition in the [MoO4] group, which can be associated with the morphology, surface defects, structural arrangement, and excitation wavelength of samples. Therefore, the strong and broad PL emission band indicated that the 2D quasi-square SrMoO4

Synthesis and Properties of SrMoO4 Nanoplates nanoplates have great potential to be applied in luminescent areas. A more detailed investigation of the properties of this sample is underway. 4. Conclusions In summary, 2D free-standing quasi-square SrMoO4 nanoplates have been successfully synthesized via a simple liquid route at room temperature. The oriented connection and selfassembly process is proposed for the organization of the quasisquare SrMoO4 nanoplates. UV-vis absorption spectroscopy revealed a characteristic optical band gap of 3.12 eV, which shows a notable blue shift compared to the SrMoO4 powders. The luminescence properties of the quasi-square SrMoO4 nanoplates have been studied. It was observed that the 2D quasisquare SrMoO4 nanoplates showed a strong and broad PL emission band peaked at 440 nm (blue emission) under the excitation of 360 nm, displaying an obvious blue shift compared to that of previous reports, which can be associated with the different types of electronic transitions in the [MoO4] group. Therefore, the present quasi-square SrMoO4 nanoplates are envisaged to become prime candidates for future applications in nanoscale optoelectronic devices. The synthesized method developed here will be valuable to the syntheses of other related inorganic nanostructures. Acknowledgment. The work is financially supported by the Natural Science Foundation of China (Grant No. 20963001); Guangxi Natural Science Foundation of China Nos. 0575030 and 0832078; and Innovation Project of Guangxi Graduate Education No. 2009106080817M364. References and Notes (1) (a) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (b) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; McDermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (c) Fang, X. S.; Zhang, L. D. J. Mater. Sci. Technol. 2006, 22, 1. (d) Li, L.; Yang, Y.; Huang, X.; Li, G.; Zhang, L. J. Phys. Chem. B 2005, 109, 12394. (e) Wang, X. D.; Song, J. H.; Wang, Z. L. Mater. Chem. 2007, 17, 711. (f) Fang, X. S.; Bando, Y.; Gantam, U. K.; Ye, C. H.; Golberg, D. J. Mater. Chem. 2008, 18, 509. (2) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (c) Fang, X. S.; Bando, Y.; Liao, M. Y.; Gantam, U. K.; Zhi, C. Y.; Dierre, B.; Liu, B. D.; Zhai, T. Y.; Sekiguchi, T.; Koide, Y.; Golberg, D. AdV. Mater. 2009, 21, 2034. (d) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Liber, C. M. Nature 2001, 409, 66. (e) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Wang, Y. H.; Wu, Y. C. AdV. Funct. Mater. 2005, 15, 63. (f) Kovtyukhova, N. I.; Mallouk, T. E. Chem.sEur. J. 2002, 8, 4355. (3) (a) Kozma, P.; Bajgar, R.; Kozma, P. Radiat. Phys. Chem. 2002, 65, 127. (b) Sun, L.; Guo, Q.; Wu, X.; Luo, S.; Pan, W.; Huang, K.; Lu, J.; Ren, L.; Cao, M.; Hu, C. J. Phys. Chem. C 2007, 111, 532. (4) (a) Rushbrooke, J. G.; Ansorge, R. E. Nucl. Instrum. Methods Phys. Res. Sect. A 1989, 280, 83. (b) Wang, H.; Medina, F. D.; Zhou, Y. D.; Zhang, Q. N. Phys. ReV. B 1992, 45, 10356. (5) (a) Wang, H.; Medina, F. D.; Liu, D. D.; Zhou, Y. D. J. Phys.: Condens. Mater. 1994, 6, 5373. (b) Tanaka, K.; Miyajima, T.; Shirai, N.; Zhang, Q.; Nakata, R. J. Appl. Phys. 1995, 77, 6581. (6) Qu, W.; Wlodarski, W.; Meyer, J.-U. Sens. Actuators B 2000, 64, 76.

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