Template-Free Fabrication of CdMoO4 Hollow Spheres and Their

Template-Free Fabrication of CdMoO4 Hollow Spheres and Their Morphology-Dependent Photocatalytic Property Lin Zhou, Wenzhong Wang,* Haolan Xu, and Songmei Sun State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3595–3601

ReceiVed January 21, 2008; ReVised Manuscript ReceiVed June 5, 2008

ABSTRACT: The controllable synthesis of CdMoO4 hollow microspheres has been successfully realized in a large scale by a facile precipitation reaction between Cd(CH3COO)2 and Na2MoO4 in the absence of any templates or additives. The as-prepared hollow spheres consist of small nanoparticles with a size of ca.100 nm. By adjusting the reaction condition, hollow spheres with different morphologies and microstructures could be obtained. The growth mechanism was investigated on the basis of the results of time-dependent experiments. The band gap estimated from the main absorption edges of the UV-vis diffuse reflectance spectrum is 3.31 eV. The photocatalytic activity experiment indicated that the CdMoO4 hollow microspheres prepared at 40 °C exhibited a higher photocatalytic activity in the photocatalytic decolorization of Rhodamine-B aqueous solution under the UV light illumination than commercially available TiO2. Notably, the photocatalytic property of CdMoO4 samples is strongly dependent on their morphologies and microstructures. The present work might afford some guidance for the rationally controllable synthesis of other photocatalytic materials and the as-grown architectures show good application potential. 1. Introduction Hollow nanostructures with higher specific surface area, lower density, and better permeation, represent a particularly attractive class of material owing to their unique properties and promising applications in material science and micro/nanodevices, ranging from artificial cells, drug delayed release, to acoustic insulators, photonic crystals, catalysis, and as lightweight filler materials and chemical reactors etc.1 To date, methods to manipulate these nanostructures usually include the use of various hard templates (e.g., polymer latex, monodispersed silica, and reducing metal nanoparticles)2 or soft templates (e.g., supramolecular, ionic liquids, surfactants, and micelles),3 which involve the adsorption of nanoparticles or polymerization on modified polymeric or inorganic template surface and subsequent removal of the templates by calcinations or dissolution with solvents. These methods often bring difficulties related to materials compatibility, high cost and complex synthetic procedures, which may prevent them from potential applications. Very recently, a number of template-free approaches employing novel mechanisms, such as the Kirkendall effect4 and Ostwald ripening,5 have been developed for the synthesis of hollow spheres. For example, Zeng et al. have prepared TiO2 hollow spheres5a by the merging of crystallites located in the inner space with those on the surface, based on Ostwald ripening. Silver cages4b and ZnO dandelion structures4c were synthesized through a Kirkendall effect. However, the controllable organization of nanocrystals into hollow structures by a facile, template-free, onestep solution route remains a significant challenge. As important materials in the electrooptical industry, metal molybdates have been intensively studied over past few years because of their potential applications in photoluminescence, optical fibers, and scintillators.6–10 Among them, cadmium molybdate (CdMoO4) is an interesting semiconductor that crystallizes in the scheelite structure and is isostructural to CaMoO4 and PbMoO4. Recent study found that CdMoO4 exhibits unique properties, such as electronic excitation with * Corresponding author. Tel.: 86-21-5241-5295. Fax: 86-21-5241-3122. E-mail: [email protected].

VUV synchrotron radiation,11 pressure-induced phase transformations,12 and 111Cd and 113Cd spin-lattice relaxation.13 A lot of effort has been devoted to the shape-controlled synthesis of CdMoO4 micro- and nanocrystals. For example, CdMoO4 nanoparticles were prepared by a low-temperature hydrothermal process.14 Qian et al.15 reported the synthesis of single-crystal octahedra of CdMoO4 via a microemulsion-mediated route. More recently, Zhen et al.16 obtained CdMoO4 hollow microspheres composed of nanorods by a template-free method with the assistance of NaCl at room temperature. In comparison with the large amount of research on the synthesis, the corresponding properties of CdMoO4 have been rarely reported. As a widegap semiconductor with a gap of 3.25 eV, CdMoO4 is expected to be a photocatalyst under UV irradiation. To date, however, studies on the photocatalytic property of CdMoO4 have been limited, as has that of the relationship between its morphology and properties. The photocatalytic activity of semiconductor oxides was found to be strongly affected by the crystalline size and microstructure of photocatalysts.17 Our previous work reveals that Bi2WO6 micro- and nanostructures with various morphologies show markedly shape-associated photocatalytic property.18 The Cu2O octahedra with exposed {111} crystal surfaces was also found to be of much higher photocatalytic activity than Cu2O cubes with exposed {100} crystal surfaces.19 Herein, we present the synthesis of hollow CdMoO4 microspheres via a fast, low-temperature, one-step solution route in the absence of any template or additive. The as-grown spheres are constructed of numerous nanoparticles and the formation of the hollow interior structure is governed by Ostwald ripening mechanism, based on the morphology evolution investigation. The size of the nanoparticles could be controlled to obtain hollow spheres with different microstructures by varying the reaction conditions. The CdMoO4 hollow spheres prepared at 40 °C possessed higher photocatalytic activity than that of commercially available TiO2, whereas the fact is that there are few samples whose photocatalytic properties are comparable with that of commercial TiO2 under UV illumination. In particular, the as-synthesized products

10.1021/cg800077h CCC: $40.75  2008 American Chemical Society Published on Web 08/29/2008

3596 Crystal Growth & Design, Vol. 8, No. 10, 2008

Figure 1. XRD pattern of the as-prepared hollow microspheres.

exhibited interesting morphology-dependent photocatalytic property, showing good application potential for these unique structures. 2. Experimental Section All chemicals were used as received from Shanghai Chemical Company with analytical-grade purity and used without purification. In a typical procedure, 0.002 mol of cadmium acetate dihydrate (Cd(CH3COO)2 · 2H2O), 0.002 mol of sodium molybdate dehydrate (Na2MoO4 · 2H2O) were dissolved in 30 mL of deionized water, respectively. The sodium molybdate solution was then added dropwise into the cadmium acetate solution. White precipitates formed slowly. The mixture was heated at 40 °C for 6 h in an open system under continuous vigorous stirring. The final products were centrifuged, washed with deionized water, and absolute ethanol for several times, and then dried at 60 °C for 10 h in air.

Zhou et al. The powder X-ray diffraction (XRD) patterns of the as-synthesized samples were measured on a D/MAX 2250V diffractometer (Rigaku, Japan) using monochromatized Cu KR (λ ) 0.15418 nm) radiation under 40 kV and 100 mA and scanning with the 2θ ranging from 20 to 80°. The morphologies and microstructures of as-prepared samples were examined with transmission electron microscopy (TEM, JEOL JEM-2100F; accelerating voltage: 200 kV) and scanning electron microscopy (SEM, JSM-6700F). High-resolution transmission electron microscopy (HRTEM) images were obtained by JEM 2100F field emission transmission electron microscope operated at an accelerating voltage of 200 kV. The specimens used for TEM studies were dispersed in absolute ethanol by ultrasonic treatment. The sample was then dropped onto a copper grid coated with a holey carbon film and dried in air. UV-visible diffuse reflectance spectra of the samples were obtained on an UV-vis spectrophotometer (Hitachi U-3010) using BaSO4 as reference. The Brunauer-Emmett-Teller (BET) surface area was estimated using adsorption data in a relative pressure range from 0.05 to 0.3. The photocatalytic activities of the CdMoO4 samples were evaluated by the photocatalytic decolorization of a model pollutant Rhodamine-B (RhB) under UV light. A 500 W high-pressure Hg lamp was used as UV light source. Experiments were performed at ambient temperature as follows: In each run, 50 mg of catalyst was added into 100 mL RhB solution (10-5 mol/L). Before illumination, the photocatalyst was stirred for 30 min in the dark in order to reach the adsorption-desorption equilibrium between the RhB and the photocatalyst. The suspension was then stirred and exposed to UV light irradiation. The concentrations of the RhB were monitored by checking the absorbance at 553 nm during the photodegradation process using a Hitachi U-3010 UV-vis spectrophotometer.

3. Results and Discussion 3.1. Structure and Morphology. The crystal structure of CdMoO4 hollow microspheres sample was revealed by X-ray diffraction (XRD). Figure 1 shows the XRD pattern of the

Figure 2. SEM images of the CdMoO4 sample prepared at 40 °C for 6 h: (a) overall product morphology; (b) detailed view of an individual sphere; (c) high-magnification SEM image of the surface of the sphere; (d) two cracked spheres, showing the hollow interior structure.

Template-Free Fabrication of CdMoO4 Hollow Spheres

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Figure 3. (a) TEM image of an individual subunit (nanoparticle) of the hollow sphere. Inset: the corresponding SAED pattern of the nanoparticle; (b) HRTEM images taken from the rectangle part of Figure 3b.

samples obtained by a mild aqueous route at 40 °C. All the peaks could be well-indexed to a pure tetragonal phase (I41/a space group) of CdMoO4 with lattice constant a ) 5.148 Å, c ) 11.183 Å, which are consistent with the literature and the values given in the standard card (JCPDS No. 07-0209). No peaks of impurities were detected from this pattern. The strong and sharp peaks indicate that the as-obtained product are highly crystallized. The morphology and microstructures were studied by scanning electron microscopy (SEM). The low-magnification SEM image shown in Figure 2a clearly reveals that there exists a great deal of microspheres with diameters ranging from 3 to 5 µm. As shown by the high magnification images in Figure 2b, the surface of the spherical stucture is not smooth but composed of numerous nanoparticles. Close observation shows that these nanoparticles are grown in high density with a size of ca. 100 nm (Figure 2c). Although on the nanometer scale the superstructure looks random, the whole crystal appears to be a quite regular “solid” sphere. However, a more careful obervation reveals that it is hollow inside. Figure 2d shows two partially broken or cracked spheres, which clearly indicates the hollow interior structure of these microspheres. The transmission electron microscopy (TEM) investigation gave more details of morphological and structural features. Figure 3a displays a typical TEM image of a single subunit (nanoparticle), with the corresponding selected area electron diffraction (SAED) pattern shown in the inset. The well-aligned sharp diffraction spots indicate that the nanoparticle is single-crystalline. Shown in Figure 3b is a high-resolution transmission electron microscopy (HRTEM) image, which was recorded from the area marked by a rectangle (in Figure 3a). The clear lattice fringe confirms the high crystallinity and single crystalline nature of the nanoparticle. The measured d spacings are 0.561 and 0.365 nm, which are in good agreement with the ideal values of (002) and (011) planes of tetragonal CdMoO4, respectively. 3.2. Growth Mechanism. For a substantially view of the growth mechanism of the CdMoO4 hollow microspheres, the time-dependent evolution process was monitored. Figure 4 shows the SEM images of the products that were obtained at 40 °C at different growth stages. The product obtained after 10 min contains solid spheres (3-5 µm in diameter), and their surfaces are densely covered by a great deal of small nanoparticles with a diameter of ca. 60 nm, as shown in images a and

b in Figure 4. When the reaction time was 30 min, the diameter of the solid spheres hardly changed, but the surface becomes rougher. The subunits (nanoparticles) grow larger and show clear edges (images c and d in Figure 4). Upon prolonging the reaction time to 3 h, some microspheres with hollow interior could be observed (Figure 4e), indicating that the hollow structures begin to form from the interior of aggregated particles. By increasing the reaction time from 3 to 5 h, and then to 6 h, the shells of the aggregated spherical particles become thinner, as shown in images e and f in Figure 4 and Figure 2d. The appearance of two kinds of intermediate product reveals that there are two growth stages during the whole process. One refers to the aggregation and growth of the nanoparticles, the other refers to the formation of the hollow interior structures. Recently, a new solid-solution-solid process called as Ostwald ripening, in which large crystallites grow at the expense of small ones through the diffusion of ions, atoms, or molecules, has been reported by Zeng et al. for the fabrication of a range of different hollow oxide nanostructures, such as TiO2,5a Cu2O,20 and ZnS.5c In this process, large crystallites are essentially immobile while the small ones are undergoing mass relocation through dissolving and regrowing, which creates the interior space within the original aggregates. From our experimental results, it is reasonable to presume that the formation of CdMoO4 hollow microspheres is based on the Ostwald ripening mechanism, and the evolution process is illustrated in Figure 5. At the first stage, tiny CdMoO4 nanoparticles were quickly produced when the MoO42- was added into the solution containing Cd2+ and spontaneously aggregated to form large spheres to minimize their surface energy:

Cd(CH3COO)2+Na2MoO4 f CdMO4 V +2Na(CH3COO) (a) During the continuous heating process, these nanoparticles grew larger gradually. As a solid sphere composed of numerous nanoparticles, the particles in the inner core can be visualized as smaller spheres with higher curvature compared to those on the outer surface. Therefore, at the second stage, they could dissolve and merge into particles on the outer surface because of the higher surface energy, resulting in the formation of hollow interior structures. This process is different from that reported by Zhen et al.16 In their experiment, NaCl with a suitable concentration is necessary to serve as an electrolyte and thus


Zhou et al.

Figure 4. SEM images of CdMoO4 sample obtained at various reaction stages: (a, b) 10 min, (c, d) 30 min, (e) 3 h, and (f) 5 h.

Figure 5. Schematic illustration of the proposed formation mechanism of CdMoO4 hollow spheres.

modify the ζ potential of spherical CdMoO4 species generated at the initial stage to construct a core-shell structure, followed by the formation of hollow CdMoO4 microspheres. 3.3. Effect of Reaction Temperature. It is found that the reaction temperature plays an important role in determining the morphology of the final product, thus the effect of reaction temperature was further investigated. A series of CdMoO4 samples were prepared from the precursor suspensions at the temperature of 60 and 80 °C for 6 h in open system, respectively, keeping other experimental conditions unchanged. The cresponnding XRD patterns of the products are shown in Figure 6, from which a pure tetragonal phase of CdMoO4 can be readily assigned. By varying the reaction temperature, however, the microstructures of the products are different. The corresponding SEM images are shown in Figuge 7a-d. At 60 °C, the dominant product are microspheres with a diameter of about 3 µm (Figure 7a). From the magnified image shown in the inset of Figure 7a, we can see that the sphere is constructed by lots of nanoparticles (250-300 nm in size). The hollow structure could be resolved from a cracked sphere (Figure 7b). When the reaction temperature was increased to 80 °C, the size of the spheres is nearly the same but the size of the small particle become larger (ca. 400 nm in size), as shown in Figure 7c. The BET surface area of samples were estimated to be ca. 1.86, 1.03, and 0.83 m2/g

Figure 6. XRD patterns of the CdMoO4 products obtained under different reaction temperatures.

for the samples prepared at 40, 60, and 80 °C, respectively. These results demonstrated that hollow CdMoO4 microspheres with different microstructure can be selectively fabricated by simply changing the experimental condition. Although the main strucuture of the hollow spheres are similar, the subunits (nanoparticles) grow larger as the reaction temperature increase. This phenomenon could be ascribed to that the Ostwald ripening


Crystal Growth & Design, Vol. 8, No. 10, 2008 3599

Figure 7. SEM images of the products prepared at (a, b) 60 °C for 6 h and (c, d) 80 °C for 6 h.

absorption edge obeys Urbach’s rule. The optical absorption coefficient near the band edge follows the equation (Rhν)2 ) A(hν - Eg)

Figure 8. UV-visible diffuse reflectance spectra of the CdMoO4 samples prepared under different reaction temperatures for 6 h.

is more efficient at higher temperatures. Moreover, following the increase of reaction temperature, the solubility of precursor increased, which will accelerate the growth of the subunits. 3.4. Optical Property and Photocatalytic Activity. The optical absorption property and the migration of the lightinduced electrons and holes of a semiconductor, which are relevant to the electronic structure feature, are known as the key factors in determining its photocatalytic activity.21 Figure 8 shows the UV-visible diffuse reflectance spectra of the different CdMoO4 samples (40, 60, 80 °C). The spectra do not have much difference among the CdMoO4 powders prepared at different reaction temperature. All samples show the light absorption in the UV region, implying the products may have photocatalytic activity under UV light irradiation. The steep absorption edge confirms that the band gap is due to the intrinsic transition of the nanomaterials but not due to the transition from impurity level. On the basis of the structural studies of Itoh et al.22a and Errandonea et al.,22b we could consider that the band gap for CdMoO4 is of the direct type and that the fundamental

(b)

for a direct-bandgap materials,23 where R, h, ν, Eg, and Α are absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. This relationship gives the band gap (Eg) by extrapolating the straight portion of (Rhν)2 against hν plot to the point R ) 0, which is about 3.31 eV for CdMoO4 obtained under different conditions. This value is a litter bigger than the result in literatures (3.25 eV),11 which may be attributed to the size effect. The electronic structure of CdMoO4 has been recently reported by Abraham et al.24 on the basis of the DFT calculations. It was found that that the O 2p states have significant contributions throughout the main portion of the valence bands. The contributions of the Cd 4d states are concentrated at the bottom of the valence bands and hybridize to a small extent throughout the valence bands. The conduction bands are mainly composed of Mo 4d states. The band gap is located at the Γ point at the center of the Brillouin zone, confirming that CdMoO4 have a direct band gap. The charge transfer upon photoexcitation is thus supposed to occur from Cd 4d and O 2p hybrid orbitals to Mo 4d orbitals. Although the hybridization of the Cd 4d and O 2p levels has almost no contribution to narrowing the band gap of CdMoO4 because Cd 4d orbitals is below O 2p, their hybridation makes the VB largely dispersed, which favors the mobility of photoexcitated holes. This kind of electronic structure was also observed in other materials, such as BiVO4,25 Bi2WO6,26 and Zn2V3O8,27 which is beneficial to photocatalytic oxidation of organic pollutants and photocatalytic water splitting. As a widely used dye, RhB with a major absorption band at 553 nm is chosen as a model pollutant to evaluate the


Zhou et al.

an increase in surface area not only because the photocatalytic reaction usually takes place on the catalyst surface but also the efficiency of the electron-hole separation is promoted.21b,29 Moreover, the unique hollow structure could be helpful in maintaining the higher active surface area. On the other hand, the beneficial effect of textural meso- and macroporosity on photocatalysis has been reported recently.17b,30 We speculate that the presence of pores among the nanoparticles could provide more efficient transport channels for the transmission and diffusion of the reactant and product molecules during the photocatalytic reaction as well. Figure 9. Changes of UV-visible spectra of typical CdMoO4 hollow spheres (prepared at 40 °C) suspended in RhB solution as a function of irradiation time.

Figure 10. Photocatalytic degradation of RhB over as-prepared samples and TiO2: (a) 40 °C, (b) TiO2, (c) 60 °C, (d) 80 °C, (e) without catalyst under UV light irradiation and (f) without UV.

photocatalytic activities of the CdMoO4 samples. Temporal evolution of the spectral changes taking place during the photodegradation of RhB mediated by typical CdMoO4 spheres (40 °C, 6 h) under UV light is displayed in Figure 9. The absorption of the dye solution decreased gradually under UV light irradiation and the major absorption peak shifted to 525 nm step by step, indicating the degradation of the dye and the removal of ethyl groups.28 A comparison experiment showed that RhB did not degrade in the dark with the presence of CdMoO4 photocatalysts. On the other hand, the blank test revealed that the degradation of RhB was very slow when illuminated by UV light in the absence of photocatalysts. These results confirmed that the catalyst and UV light are indispensable to the photodegradation of RhB. For comparison, the same photodegradation experiments were also performed by using different samples, as shown in Figure 10. Commercial TiO2 (Shanghai Chemical Company, anatase phase, ca. 100 nm in size) was also used as a reference to evaluate the photocatalytic performance of our product. One can see that the loss of RhB over CdMoO4 hollow spheres prepared at 40 °C was more rapid than that in the case of TiO2. It is notable that the reaction temperature has a significant effect on the degradation rate. An increase of reaction temperature results in the deceleration of the degradation of RhB. The sample prepared at 40 °C shows the highest photocatalytic activity. Considering the phase structure and band gaps of the samples are nearly the same, the difference of photocatalytic property could be ascribed to the difference of their morphologies and microstructures. First, from the SEM and XRD results, with the decrease in the reaction temperature, the crystallinity of CdMoO4 hardly changed, but the size of the subunits became smaller, which leads to the higher surface area. Previous studies found that the activity of a photocatalyst would improve with

4. Conclusion In summary, we have demonstrated that CdMoO4 hollow spheres can be fabricated in large-scale via a facile, efficient, one-step aqueous route. The key process does not use hard/soft templates or additives. The spheres were further found to be constructed by a great deal of nanoparticles. An Ostwald ripening mechanism is proposed to be accounted for the template-free formation of these hollow structures via timedependent observation. It was found that the reaction temperature plays an important role in determining the morphology of the final product. The size of the subunits (nanoparticles) could be modified from 100 to 400 nm by simply adjusting the reaction temperature from 40 to 80 °C. Furthermore, the asprepared product exhibit significant morphology-depended photocatalytic activity for the degradation of RhB under UV light irradiation. The present work will be not only helpful in systematically explore fabrication of hollow architectures but also provides new insights into morphology-controllable design of photocatalytic materials for their applications. Acknowledgment. Financial support from the Chinese Academy of Sciences and Shanghai Institute of Ceramics under the program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. We also thank the Fund from the Innovation Research of the Shanghai Institute of Ceramics and the Key Project from the National Natural Science Foundation of China (50732004).

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Template-Free Fabrication of CdMoO4 Hollow Spheres and Their

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