Facile Synthesis of CePO4 Nanowires Attached to CeO2 Octahedral

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J. Phys. Chem. C 2008, 112, 16452–16456

Facile Synthesis of CePO4 Nanowires Attached to CeO2 Octahedral Micrometer Crystals and Their Enhanced Photoluminescence Properties Guicun Li,* Kun Chao, Hongrui Peng, Kezheng Chen, and Zhikun Zhang Key Laboratory of Nanostructured Materials, College of Materials Science and Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, People’s Republic of China ReceiVed: May 19, 2008; ReVised Manuscript ReceiVed: August 18, 2008

CePO4 nanowires attached to CeO2 octahedral micrometer crystals have been synthesized on a large scale using a higher molar ratio of Ce3+ and PO43- by a facile one-step hydrothermal method. The influences of synthetic parameters, such as reaction time, ratios, and types of reagents, on the morphologies and crystal structures of the resulting products have been investigated. The formation process of CePO4 nanowires attached to CeO2 octahedral micrometer crystals is time dependent: CePO4 nanowires are formed first, and then CeO2 grows into octahedral micrometer crystal. It is found that the photoluminescence properties of CePO4 nanowires attached to CeO2 octahedral micrometer crystals are enhanced strongly in comparison with pure CePO4 nanowires due to the synergic effects of CePO4 nanowires and CeO2 octahedral micrometer crystals. Introduction There has been considerable interest in design and fabrication of functional composite materials for their applications because of the synergic effects of different components on the interfaces.1-3 Nanoscale luminescent materials have been a subject of rapid development in nanoscale phosphors and biomedical applications.4-10 Among them, rare earth phosphate is of importance in biomedical applications due to the high chemical stability, high quantum yield, and expected low toxicity.6 In recent years, cerium phosphate (CePO4) is of interest as a promising ultraviolet (UV) luminescent material owing to the 4f-5d emissions of Ce3+.11,12 A variety of methods, such as solution reaction,13-15 microassisted reaction,16 microemulsion reaction,17,18 sol-gel,19,20 and combustion reaction,21 have been developed to synthesize rare earth phosphate nanostructures including nanoparticles, nanorods, and nanowires, for their luminescent properties and potential applications. Recently, considerable efforts have been made on enhancement of rare earth phosphate luminescent properties. Tang et al.11 reported the synthesis of Ce(HPO4)2 · nH2O nanotubes via a lowtemperature solution method and Ce3+/Ce4+ hybrid phosphate nanotubes by subsequent annealing. The desired Ce3+/Ce4+ hybrid phosphate nanotubes exhibited a strong blue luminescence because of a charge transfer between Ce4+ electron donation centers and Ce3+ luminescence centers, although the doping of transparent Ce4+ ions in luminescence materials has been prohibited due to the competitive absorption in UV region. Many research groups22-25 have fabricated core-shell nanostructures of rare earth phosphate to improve the luminescence possibly due to the elimination of surface trapping states and the suppression of the energy quenching in energy-transfer processes. Ceria (CeO2) has a band gap of 3.2 eV26 and is of interest because of its novel physichemical properties and potential applications in UV-shielding,27 luminescence,28 solar cell,29 fuel cell,30 biomedical treatment,31 chemical-mechanical polishing for microelectronics,32 catalyst, and sensor.33,34 In contrast to * To whom correspondence should be addressed. Phone: 86-53284022869. Fax: 86-532-84022869. E-mail: [email protected].

Ce3+, Ce4+ has no 4f electron, indicating that CeO2 is a promising photoluminescence (PL) host material because of strong light absorption through the charge transfer from O2- to Ce4+.35 However, enhancing the PL properties CePO4 nanowires based on the energy transfer from Ce4+ (CeO2) to Ce3+ (CePO4) remains challenging to researchers. Herein, we report a new method to improve the PL properties of CePO4 nanowires. CePO4 nanowires attached to CeO2 octahedral micrometer crystals are synthesized on a large scale using a higher molar ratio of Ce3+ and PO43- by a facile one-step hydrothermal method. In comparison with pure CePO4 nanowires, the PL properties of CePO4 nanowires on CeO2 octahedral micrometer crystals are enhanced strongly due to the synergic effects of CePO4 nanowires and CeO2 octahedral micrometer crystals. Experimental Section Synthesis of CePO4 Nanowires Attached to CeO2 Octahedral Micrometer Crystals. In a typical synthesis, 0.87 g of Ce(NO3)3 · 6H2O and 0.13 g of (NH4)2HPO4 were dissolved in 40 mL of distilled water, respectively. The Ce(NO3)3 aqueous solution was added dropwise to the (NH4)2HPO4 aqueous solution to form a opalescent colorless colloidal solution with vigorous stirring. Then, an appropriate amount of hydrochloric acid was added dropwise to the mixture to adjust the pH value to about 1. The resulting solution was poured into a 100-mL stainless steel autoclave with a Teflon liner. The autoclave was sealed and maintained at 180 °C for 24 h and then air-cooled to room temperature. Finally, the resulting product was collected and washed with distilled water and anhydrous alcohol several times and then dried at 60 °C for 10 h. Characterization. The morphologies, sizes, and composition of the resulting products were characterized by field-emission scanning electron microscopy (FE-SEM, JSM 6700F) equipped with an energy-dispersive X-ray spectrum (EDS, INCAx-sight), transmission electron microscopy (TEM, JEM 2000EX), and high-resolution transmission electron microscopy (HRTEM, JEOL 2010). The crystal structures of the resulting products were characterized by powder X-ray diffraction (XRD, Rigaku D-max-γA XRD with Cu KR radiation, λ ) 1.54178 Å). PL spectra were recorded with a fluorescence spectrophotometer

10.1021/jp804567t CCC: $40.75  2008 American Chemical Society Published on Web 10/01/2008

Facile Synthesis of CePO4 Nanowires

Figure 1. XRD patterns of pure CePO4 nanowires (A) and CePO4 nanowires attached to CeO2 octahedral micrometer crystals (B).

(FL, Hitachi F-4500). Dispersions of the resulting products in water were measured in standard quartz cuvettes at room temperature. Results and Discussion Pure CePO4 nanowires can be synthesized by hydrothermal method using equal molar ratios of Ce3+ and PO43- or excessive PO43- (see Figure S1 of Supporting Information), which is similar to the results reported in literature.12,14,24 For the synthesis of CePO4 nanowires attached to CeO2 octahedral micrometer crystals, a higher molar ratio of Ce3+ and PO43- was used to form CePO4 and CeO2. Parts A and B of Figure 1 present XRD patterns of pure CePO4 nanowires and CePO4 nanowires attached to CeO2 octahedral micrometer crystals, respectively. As shown in Figure 1A, all the diffraction peaks of pure CePO4 nanowires synthesized with the molar ratio of Ce3+ and PO43) 1 can be indexed to pure monoclinic phase CePO4 with lattice constants a ) 6.80 Å, b ) 7.02 Å, c ) 6.47 Å, and β )103.46°, which are well consistent with the literature values (JCPDS No. 32-0199). As the molar ratio of Ce3+ and PO43- increases to 2, most of the diffraction peaks in Figure 1B can be assigned to face-centered cubic (fcc) CeO2 with lattice constant a ) 5.41 Å (JCPDS No. 34-0394). In addition, a small amount of monoclinic phase CePO4 exists in the products as indicated by an asterisk. The diffraction peaks of CeO2 are very sharp and strong, indicating that CeO2 have high crystallinity in comparison with that of CePO4. Typical SEM and TEM images of CePO4 nanowires attached to CeO2 octahedral micrometer crystals synthesized at 180 °C for 24 h are represented in Figure 2 (the molar ratio of Ce3+ and PO43- is 2:1). Low-magnification SEM images in Figure 2A and Figure S2 of Supporting Information reveals that the products are composed of a large quantity of CeO2 octahedral micrometer crystals and a small quantity of CePO4 nanowires. The diameters of CePO4 nanowires are in the range of 15-35 nm, which is similar to that in Figure S1 of Supporting Information. The separate CePO4 nanowires are several tens of micrometers in length. In a high-magnification SEM image in Figure 2B, one can observe that CeO2 has perfect octahedral shapes with uniform edge length of about 600 nm-1 µm and most of the surfaces of CeO2 octahedral micrometer crystals are very smooth. Interestingly, some CePO4 nanowires are attached to the surfaces of CeO2 octahedral micrometer crystals to form the interfaces of CePO4/CeO2 (see Figure S3 of Supporting Information). In addition, some scotches can be seen clearly due to the removing of CePO4 nanowires parallel to the

J. Phys. Chem. C, Vol. 112, No. 42, 2008 16453 surfaces of CeO2 octahedral micrometer crystals as indicated by arrows, suggesting that CePO4 nanowires are embedded in CeO2 octahedral micrometer crystals during the growth process of CeO2, which can be further confirmed the EDS results taken from an individual octahedral micrometer crystal (the molar ratio of Ce and P is about 10: 1, see Figure S4 of Supporting Information). Figure 2C shows a typical TEM image of CePO4 nanowires attached to CeO2 octahedral micrometer crystals, suggesting the octahedral shapes of CeO2. The electron diffraction (ED) pattern in Figure 2C of CeO2 octahedral micrometer crystal can be indexed to fcc CeO2, in agreement with that in Figure 1B, indicating that each octahedral micrometer crystal with the surfaces bounded by {111} facets is single crystalline. HRETM images of CePO4 nanowires attached to CeO2 octahedral micrometer crystals are shown in Figure 2D. The lattice planes with d spacing of 0.271 and 0.518 nm correspond to (200) planes of CeO2 and (1j01) planes of CePO4, respectively, revealing that CePO4 nanowires are penetrated into CeO2 octahedral micrometer crystals. To investigate the formation mechanism of CePO4 nanowires attached to CeO2 octahedral micrometer crystals, time-dependent experiments have been made as follows. When the reaction is carried out for 1 h, all the products are pure CePO4 nanowires with diameters of 10-15 nm (Figure 3A) and length ranging from several hundreds of nanometers to several micrometers. As the reaction time increases to 3.5 h, it is found that the diameters of pure CePO4 nanowires increase to 15-35 nm (Figure 3B), which is similar to that synthesized with an equal molar ratio of Ce3+ and PO43- reported before. After 5 h, it is interesting that a lot of CeO2 octahedral micrometer crystals with edge length of about 300 nm are formed in addition to CePO4 nanowires (Figure 3C). Clearly, some CePO4 nanowires adhered to the surfaces of CeO2 octahedral micrometer crystals (see Figure S5 of Supporting Information). When the reaction time increases to 12 h, the edge length of CeO2 octahedral micrometer crystals in Figure 3D grows up to about 500 nm, and the number of CePO4 nanowires on the surfaces (see FIgure S5 of Supporting Information) increases compared with that in Figure 3C. A CePO4 nanowire runs through the entire CeO2 octahedral micrometer crystal is clear as indicated by arrows. It seems that the length of CePO4 nanowires is shorter than that in parts A and B of Figure 3, further proving that one tip of CePO4 nanowire is embedded in CeO2 octahedral micrometer crystal to form the interfaces of CePO4/CeO2. These results reveal that the formation of CePO4 nanowires attached to CeO2 octahedral micrometer crystals is time-dependent: CePO4 nanowires are formed first, and then CeO2 grows into octahedral micrometer crystal. When equivalent Cl- and NO3- are used to take the place of PO43- in the reaction system, CeO2 with different morphologies are obtained. As shown in Figure 4A, spherical CeO2 particles with diameters of 0.5-1 µm are prepared in the NH4Cl and HCl solution. Also, some CeO2 nanorods grow on the surfaces of CeO2 particles to form an urchinlike structure. The diameters and length of CeO2 nanorods are in the range of 100-120 nm and 1 µm, respectively. As the reaction is carried out in NH4NO3 and HNO3 solution, some CeO2 octahedral micrometer crystal are aggregated together to form irregular structures. Moreover, the sizes of CeO2 crystals range from several hundred of nanometers to several micrometers, and its surfaces are rough compared with that in Figure 2. It is clear that PO43- plays an important role in the formation of CePO4 nanowires attached to CeO2 octahedral micrometer crystals.

16454 J. Phys. Chem. C, Vol. 112, No. 42, 2008

Li et al.

Figure 2. SEM and TEM images of CePO4 nanowires attached to CeO2 octahedral micrometer crystals synthesized with the molar ratio of Ce3+ and PO43- ) 2:1 at 180 °C for 24 h. (A) Low-magnification SEM image; (B) High-magnification SEM image; (C) TEM image; (D) HRTEM image. The inset in Figure 2C shows a typical ED pattern taken from a CeO2 octahedral micrometer crystal.

Figure 3. SEM images of the products synthesized with the molar ratio of Ce3+ and PO43- ) 2:1 at 180 °C for different reaction times: (A) 1, (B) 3.5, (C) 5, (D) 12 h.

According to the time-dependent experiments, it is supposed that the formation of CePO4 nanowires attached to CeO2 octahedral micrometer crystals is related to the higher molar ratio of Ce3+ and PO43-. The reaction may be expressed as follows + Ce3+ + HPO24 f CePO4 V + H

6H2O + 4Ce

3+

(1) +

+ O2 f 4CeO2 V + 12H

(2)

A schematic illustration of the formation process of CePO4 nanowires attached to CeO2 octahedral micrometer crystals,

based on the SEM and TEM results, is presented in Figure 5. At the early stage of the reaction, Ce3+ can first react with HPO42- to form monoclinic-phase CePO4 nanowires by homogeneous nucleation (eq 1). As the PO43- is depleted, CeO2 is formed by the oxidation reaction between the excessive Ce3+ and O2 dissolved in the solution (eq 2). The formation of CeO2 octahedral micrometer crystals can be ascribed to the anisotropic growth of fcc CeO2 because the order of surface energies is (111) < (100) < (110). With the reaction proceeding, CeO2 octahedral micrometer crystals continue to grow and some adjacent CePO4 nanowires can be embedded in CeO2 to form

Facile Synthesis of CePO4 Nanowires

J. Phys. Chem. C, Vol. 112, No. 42, 2008 16455

Figure 4. SEM images of the products synthesized with different reactants at 180 °C for 24 h. (A) NH4Cl and HCl; (B) NH4NO3 and HNO3.

Figure 5. Schematic illustration of the formation process of CePO4 nanowires attached to CeO2 octahedral micrometer crystals.

CePO4 nanowires attached to CeO2 octahedral micrometer crystals, which can be observed in high-magnification SEM image (see Figures S2 and S5 of Supporting Information). The high molar ratio of Ce3+ and PO43- is important for the formation of CePO4 nanowires attached to CeO2 octahedral micrometer crystals. Figure 6 shows PL spectra of pure CePO4 nanowires and CePO4 nanowires attached to CeO2 octahedral micrometer crystals. The excitation spectrum of CePO4 nanowires and CePO4 nanowires attached to CeO2 octahedral micrometer crystals monitored at the emission wavelength of 345 nm are very different (Figure 6A). CePO4 nanowires (line 1 in Figure 6A) only has one excitation peak at 277 nm corresponding to the transitions from the ground state 2F5/2 of Ce3+ to the excited Ce3+ 5d states,12,22 but CePO4 nanowires attached to CeO2 octahedral micrometer crystals (line 2 in Figure 6A) exhibit two excitation peaks at 230 and 277 nm. In comparison with line 1 in Figure 6A, the intensity of the excitation peak at 277 nm is higher. The excitation peak at 230 nm may be attributed to the charge transfer from O2- to Ce4+ of CeO2. Upon excitation at 230 nm, pure CePO4 nanowires (line 1 in Figure 6B) have no luminescence (not excited), but CePO4 nanowires attached to CeO2 octahedral micrometer crystals (line 2 in Figure 6B) exhibit strong ultraviolet emission peaks centered at 332 and 345 nm arising from 5d f 2F5/2 and 5d f 2F7/2 transitions due to spin orbit splitting of the 4f1 ground state of Ce3+,11,12 indicating that an energy transfer from Ce4+ to Ce3+ occurs. Upon excitation at 277 nm, it is found that CePO4 nanowires attached to CeO2 octahedral micrometer crystals (line 2 in Figure 6C) display broader and stronger UV emission than pure CePO4 nanowires (line 1 in Figure 6C) and that the blue emission from Ce4+ in the range of 400-500 nm is not found,11,28 demonstrating the charge transfer between Ce4+ electron donation centers and Ce3+ luminescence centers through the interface of CePO4

Figure 6. PL spectra of pure CePO4 nanowires and CePO4 nanowires attached to CeO2 octahedral micrometer crystals. (A) Excitation spectrum (monitored at the emission wavelength of 345 nm); (B) emission spectrum (excited at the wavelength of 230 nm); (C) emission spectrum (excited at the wavelength of 277 nm); lines 1 and 2 in Figure 6 show PL spectra of CePO4 nanowires and CePO4 nanowires attached to CeO2 octahedral micrometer crystals, respectively.

and CeO2. The PL results reveal that the strong absorption of CeO2, especially in the short UV range, improve the energy

16456 J. Phys. Chem. C, Vol. 112, No. 42, 2008 transfer from Ce4+ to Ce3+, resulting in the enhanced UV emission of CePO4 nanowires. Conclusion In summary, CePO4 nanowires attached to CeO2 octahedral micrometer crystals have been synthesized on a large scale using higher molar ratio of Ce3+ and PO43- by a facile one-step hydrothermal method. Their PL properties are stronger than that of pure CePO4 nanowires due to the synergic effects of CePO4 and CeO2. The time-dependent formation process of CePO4 nanowires attached to CeO2 octahedral micrometer crystals has been investigated. The enhanced PL properties of CePO4 nanowires may open up new applications in the assembly of nanoscale UV optical devices. Acknowledgment. The research was supported by the National Natural Science Foundation of China (NSFC 50702028), People’s Republic of China. Supporting Information Available: SEM image of CePO4 nanowires. Low and high-magnification SEM images and EDS spectrum of CePO4 nanowires attached to CeO2 octahedral micrometer crystals. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhang, Y.; Ichihashi, T.; Landree, E.; Nihey, F.; Iijima, S. Science 1999, 287, 1719. (2) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 61. (3) Thiel, S.; Hammerl, G.; Schimehl, A.; Schneider, C. W.; Mannhart, J. Science 2006, 313, 1942. (4) Pellegrino, T.; Kudera, S.; Liedl, T.; Javier, A. M.; Manna, L.; Parak, W. J. Small 2005, 1, 48. (5) Dubertret, B.; Skouride, P.; Norris, D. J.; Noireaus, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 298, 1759. (6) Meiser, F.; Cortez, C.; Caruso, F. Angew. Chem., Int. Ed. 2004, 43, 5954. (7) Li, Q.; Yam, V. W. W. Angew. Chem., Int. Ed. 2007, 46, 3486. (8) Heer, S.; Ko¨mpe, K.; Gu¨del, H. U.; Haase, M. AdV. Mater. 2004, 16, 2102. (9) Wang, F.; Xue, X.; Liu, X. Angew. Chem., Int. Ed. 2008, 47, 906.

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