Synthesis of Rare Earth Ions-Doped ZnO Nanostructures with Efficient

Aug 18, 2009 - Characteristic photoluminescence of rare earth ions has been observed for these doped nanocrystals. Effective doping has also been mani...
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J. Phys. Chem. C 2009, 113, 16439–16444

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Synthesis of Rare Earth Ions-Doped ZnO Nanostructures with Efficient Host-Guest Energy Transfer Shulin Ji, Liangliang Yin, Guodong Liu, Lide Zhang, and Changhui Ye* Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: July 22, 2009

Rare earth (RE) ions (Tb3+, Dy3+, and Er3+) are incorporated into ZnO nanostructures by a facile isocrystalline core-shell (ICS) protocol. Characteristic photoluminescence of rare earth ions has been observed for these doped nanocrystals. Effective doping has also been manifested by dramatic splitting and enhancement of intra-4f transitions in photoluminescence excitation spectra. Efficient energy transfer from ZnO host to guest RE ions has been revealed through the characteristic emissions of RE ions by direct excitation of ZnO host. The ICS protocol is universal in doping rare earth ions into ZnO nanocrystals with a spherical shape and shows a great potential for a variety of applications. Introduction Methods for introducing new optical, electronic, and magnetic properties to semiconductor nanocrystals are attracting a great deal of interest as there are growing prospects for technological applications of these materials in the fields of spintronics,1 photocatalysis,2 and so on.3-6 An effective method for manipulating the physical properties of semiconductors involves impurity doping. Among host materials, ZnO, which has a wide band gap of 3.37 eV at room temperature, a large exciton binding energy of 60 meV, and excellent electronic and piezoelectric properties and is inexpensive and environmentally benign, has been identified as a promising candidate.7 In the past decade, ZnO nanocrystals doped with transition metal ions have been intensively studied to obtain diluted magnetic semiconductors. Effective doping has been aimed at realizing this purpose; however, the problem of dopant ions being excluded during nanocrystal synthesis has to be tackled.8,9 Many methods have been developed to dope ZnO nanomaterials10-13 however, a facile and general synthetic approach is still lacking for ZnO nanocrystals in a quantum confinement regime. Rare earth ions that have partially filled 4f shells, if incorporated into suitable matrixes, their intra-4f optical transitions become possible because of splitting induced by the crystal field of the matrix.14 RE ion-doped ZnO has been studied widely for industrial applications in the fields of photoluminescence (PL),15-17 laser,18 fiber amplifier,19 bioassay, solar cells, and so on. Er3+-doped semiconductors are expected to be used in optical communication systems because intra-4f transition of Er3+ with a photoemission at a wavelength of 1.54 µm is within a wavelength range of a minimum loss for silica optical fibers, and Er3+-doped ZnO systems have been studied extensively.15,19,20 Besides infrared luminescence, visible emissions of Er3+-doped ZnO were also investigated.21,22 Tb3+-doped ZnO has been studied a great deal as a green luminescence material17,23 and a local structural probe.23,24 However, doping of ZnO with Dy3+ ions for optical study has rarely been reported.17,25 Though some controversies still exist (for instance, whether RE ions could be incorporated into the semiconductor nano* To whom correspondence should be addressed. E-mail: chye@ issp.ac.cn.

crystals or not26,27 and whether efficient energy transfer from the ZnO host to RE3+ could occur28-30), many important results have been reported in the literature. Liu et al.31,32 prepared Tb3+doped ZnO nanocrystals by reaction between Zn-O-Tb precursors and LiOH in ethanol and observed relaxation of charge carriers from excited states of ZnO hosts to RE dopants. Pereira et al.24 showed that Tb3+ ions could be incorporated in nanosized ZnO using colloidal synthetic procedures and by forming a shell of ZnSe outside the ZnO core. We have reported effective doping of Tb3+ in ZnO nanocrystals by an isocrystalline core-shell (ICS) protocol.33 In this study, we systematically investigated the doping effects of RE ions (Tb3+, Dy3+, and Er3+) in ZnO nanostructures of both spherical and sheet morphologies. The effective inner doping and efficient energy transfer from ZnO to RE ions demonstrated that the ICS protocol is a general approach to incorporate RE ions in ZnO nanocrystals with a spherical shape. Experimental Section Materials. All chemicals were analytical-grade reagents and were used without further purification. RE3+-Doped ZnO Nanocrystals. The synthetic procedure of doped ZnO spherical nanocrystals and later outer growth of ZnO shells were described elsewhere.33 Briefly, RE3+(4.0 at. %)doped ZnO core nanocrystals with an average size of 7-8 nm were prepared by a sol-gel method with the molar ratio of LiOH · H2O/Zn2+ at 2, and the obtained RE3+-doped ZnO core nanocrystals were dispersed in 100 mL of ethanol and stirred for several hours to form uniform suspensions, followed by alternating dropwise addition under constant stirring of 50 mL of ethanol of zinc acetate and lithium hydroxide. The mixture was allowed to grow at room temperature for 15 h and then n-heptane was added. The precipitate was vacuum-dried and annealed in air at different temperatures for 2 h. RE3+-Doped ZnO Nanosheets. RE nitrates were prepared by first dissolving RE oxides in dense nitric acid at 60 °C and then heating at 100 °C. Subsequently, RE nitrate (0.32 mmol) and Zn(NO3)2 · 6H2O (7.68 mmol) were dissolved in 100 mL of deionized water by stirring, followed by slow addition of 100 mL of a 0.8 M aqueous solution of urea. After the mixture was stirred for a few minutes, the clear mixture was transferred to

10.1021/jp906501n CCC: $40.75  2009 American Chemical Society Published on Web 08/18/2009

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Figure 1. XRD patterns of doped ZnO nanocrystals (labeled as RE1) and nanosheets (labeled as RE2) annealed at 400 °C.

a 60 mL Teflon-lined stainless steel autoclave that was then put in an oven and heated at 180 °C for 5 h. After cooling down, precipitates in the autoclave were carefully washed using deionized water and ethanol and then dried in vacuum at 100 °C, followed by annealing in air at different temperatures for 2 h. For comparison with core-shell nanocrystals, the obtained 400 °C air-annealed nanosheets were dispersed into ethanol and subject to the same outer growth procedure of ZnO shells as used in core-shell nanocrystal growth. Characterization. Crystal structures of the as-synthesized samples were examined by powder X-ray diffraction (XRD) using a Philips diffractometer (X’pert Pro) with Cu KR radiation. Optical absorption spectra were obtained using a UV-vis spectrometer (Shimadzu SolidSpec-3700) with an integral sphere detector. Microstructures and composition were analyzed with a JEM-2010 transmission electron microscope (TEM) equipped with an Oxford INCA energy dispersive spectrometer (EDS). PL and photoluminescence excitation (PLE) spectra were measured by a Fluorolog-3-TAU spectrometer (Jobin Yvon) using a 450 W Xe lamp as the continuous light source. Results and Discussion Structural Characterizations of RE-ZnO Spherical Nanocrystals and Nanosheets. Figure 1 shows XRD patterns of REdoped ZnO nanocrystals (labeled as RE1) and nanosheets (labeled as RE2) after annealing in air at 400 °C for 2 h. All the peaks can be indexed to wurtzite ZnO. XRD patterns of RE-ZnO nanocrystals under different heat treatments reveal that 400 °C is an optimal annealing temperature for RE-ZnO nanocrystals [see Figure S1 in the Supporting Information]. For

Ji et al. RE-ZnO nanosheets, directly obtained were zinc hydroxide carbonates (JCPDS No. 72-1100) as revealed by the vacuumdried curve in Figure S1(c) and the absorption spectrum [Figure S3(a)]. Hydroxide carbonates were usually used as precursors for doping RE elements in metal oxides.34,35 Nanosheets under annealing above 400 °C turned into wurtzite ZnO (JCPDS No. 89-0510) but higher temperature annealing (higher than 500 °C) led to Tb4O7 phase separation from the ZnO wurtzite phase though a reducing atmosphere was used [Figure S1(c)]. No significant shifts in lattice constants compared with those of undoped ZnO under same conditions were observed. All other RE-ZnO nanosheet samples had similar XRD patterns. We chose the 400 °C annealed samples synthesized from both routes for further study to exclude the disturbance from RE-oxides. RE-ZnO nanocrystals after outer growth of ZnO shells were studied by TEM as shown in Figure 2 and Figure S2. Particles with sizes of ∼9 nm were synthesized and an interface separating the core from the shell in all investigated particles was absent, which is strong evidence of epitaxial growth of an isocrystalline shell on the surface of a doped core. The representative morphology of RE-ZnO nanosheets is given in Figure 3 (Tb-ZnO). Nanosheet structures were revealed in a low-magnification image [Figure 3a] and a high-magnification image [Figure 3b], revealing some pores on the surface resulted from release of gas during pyrolysis but the selected area electron diffraction (SAED) pattern in the inset of Figure 3b manifests the high-crystalline quality of ZnO. The spacing between (0002) planes of ZnO in HRTEM [Figure 3c] matches well with the XRD data. Figure 3d gives the EDS of (4% Tb)ZnO nanosheets, which shows the existence of terbium ions with a concentration close to 4%. Enhanced Optical Properties. The absorption spectra of the RE-ZnO samples are displayed in Figure 4. Except for precursors of nanosheets (absorption spectrum shown in Figure S3(a)) that are zinc hydroxide carbonates confirmed by XRD, all annealed samples show ZnO band-edge absorption and characteristic absorption of RE ions. Compared with undoped ZnO nanosheets, RE-ZnO nanosheets (labeled as RE2) have obvious band-edge red shifts. As for RE-ZnO core-shell nanocrystals, this phenomenon is negligible except for Tb-ZnO. In our early report, we realized the contribution of terbium doping to optical absorption in the visible range and attributed it to the existence of Tb4+ ions.33 The absorption was understood to be caused by charge transfer (CT) transition from oxygen to higher valent Tb4+ ions centered at around 27400 cm-1 with a full-width at half-maximum (fwhm) of about 10900 cm-1 as observed in glasses.36 Higher valence Tb4+ ions come from the starting materials Tb7O12 which we used (XRD pattern not shown) to synthesize Tb-ZnO nanosheets and Tb(CH3COO)3 · 4H2O to synthesize Tb-ZnO nanocrystals. Figure S3(b) shows the

Figure 2. (a) TEM image, (b) HRTEM image, and (c) EDS pattern of (4% Dy)-ZnO nanocrystals after outer growth of ZnO shells annealed at 400 °C. Inset in (a) is the corresponding electron diffraction pattern.

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Figure 3. (a) Low-magnification TEM image, (b) high-magnification TEM image, (c) HRTEM image, and (d) EDS pattern of (4% Tb)-ZnO nanosheets annealed at 400 °C. Inset in (b) is the SAED pattern taken along the [10 1j0] zone axis.

Figure 4. Absorption spectra of RE-ZnO core-shell nanocrystals (labeled as RE1), RE-ZnO nanosheets (labeled as RE2), and undoped ZnO nanosheets annealed at 400 °C. Inset is characteristic spectra of Dy-ZnO (transitions from ground-state 6H15/2 to excited states) and ErZnO (transitions from ground-state 4I15/2 to excited states) samples in a long wavelength range, shifted for clarity. Curves are normalized at 300 nm.

absorption spectrum of our Tb7O12 reagent. More Tb4+ ions existed in nanosheets than in nanocrystals; therefore, a bandedge red shift was more significant in Tb-ZnO nanosheets. PL spectroscopy is a powerful technique for revealing the energy structure of materials and is frequently used in doping analysis of RE elements into hosts because of their characteristic intense intra-4f transitions in PL. Figure 5 shows PL spectra under above band gap excitation (325 nm) of 4% Tb, Dy, and Er-ZnO nanocrystals before and after outer growth of ZnO shells. Characteristic intra-4f transition emissions of RE ions, near band-edge emission (NBE) associated with exciton recombination,37 and defect-related emission of ZnO were present in each curve. It is noteworthy that 325 nm excitation does not

correspond to any particular intra-4f transitions of RE ions studied; therefore, their characteristic emissions must come from energy transfer from ZnO host. Moreover, the intensity ratio of the rare earth ion-related peaks to the NBE peak increases dramatically for the core-shell sample after normalization at NBE emission in each subfigure. It implies improved RE ions doping by outer growth of isocrystalline ZnO. Red shift of the NBE peak of core-shell structures to those of core crystals is due to crystal growth from 7 to about 9 nm by coating an outer shell. To confirm the contribution of ZnO host to observed characteristic intra-4f transition emissions, PLE spectra monitoring the marked peaks in Figure 5 are given in Figure 6. Absorption peaks of RE ions corresponding to transitions from ground state (7F6 for Tb3+, 6H15/2 for Dy3+, and 4I15/2 for Er3+) to excited states are clearly seen in core-shell samples and show significant splitting and enhancement compared with those of core samples, which again manifests effective inner doping.33 Besides these peaks, a peak at around 280 nm in the vacuum ultraviolet (VUV) range and another one at about 368 nm appears too. Actually, curves in Figure 6 are normalized at the VUV peak. The origins of these two peaks are related to the ZnO host which will be explained later. The red shift of the VUV peak of core-shell structures to those of core crystals is due to crystal growth as described above. As shown in Figure 7a, under 325 nm excitation undoped ZnO nanocrystals show NBE emission and defect-related emission and their intensity ratio is much smaller than the doped ones in Figure 5. In other words, defect-related emission is quenched by doping to some extent. Monitoring the defect-related emission of undoped ZnO nanocrystals at 488, 560, and 620 nm, respectively, all PLE spectra in Figure 7b show VUV peak and 368 nm absorption. The 368 nm absorption is directly related to the formation of excitons, and the dissociation and trapping of these photogenerated excitons at excited state of the defects contribute to the visible luminescence.38 Consequently, the observed quenching

Figure 5. PL spectra under above band gap excitation (325 nm) of (a) (4% Tb)-ZnO, (b) (4% Dy)-ZnO, and (c) (4% Er)-ZnO nanocrystals before and after outer growth of ZnO shells. Curves are normalized at the NBE peak.

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Figure 6. PLE spectra monitoring: (a) 542 nm emission of (4% Tb)-ZnO, (b) 572 nm emission of (4% Dy)-ZnO, and (c) 552 nm emission of (4% Er)-ZnO nanocrystals before and after outer growth of ZnO shells. Curves are normalized at VUV peak.

Figure 7. PL spectra (a) under excitations of 325, 280, and 368 nm and PLE spectra (b) monitoring 488, 560, and 620 nm of undoped ZnO nanocrystals. The inflections at around 650 nm are caused by an error in the spectrometer.

Figure 8. PL spectra of (a) (4% Tb)-ZnO core-shell nanocrystals under 278, 368, and 487 nm excitations, (b) (4% Dy)-ZnO core-shell nanocrystals under 280, 365, and 451 nm excitations, and (c) (4% Er)-ZnO core-shell nanocrystals under 287, 363, and 457 nm excitations. The inflections at around 650 nm are caused by an error in the spectrometer.

of defect-related emission by doping is the result of competition for trapping of photogenerated excitons from ZnO host to excited states of the defects and the RE ions. PL spectra of doped core-shell samples under 368 nm excitation proved this point (Figure 8). Overlapped with the wide defect-related emission band, intra-4f transitions of RE ions appear, in agreement with Chen’s results.39,40 With 368 nm excitation, the more intense emission from intra-4f transitions of RE ions of core-shell structures compared to those of core crystals is more evidence for effective doping in core-shell structures as more photogenerated excitons of ZnO host are trapped by excited states of RE ions. Under VUV excitation as shown in Figure 8, however, NBE emission of ZnO host and intra-4f transitions of RE ions are almost invisible, whereas defect-related emission dominates. In other words, VUV peak excitation leads primarily to wide

defect-related emission band of ZnO host and that is the reason why we normalize PLE curves in Figure 6 at this peak. Its origin is not clear and may be due to high-energy excited states of ZnO. PL under excitation directly into excited states of Tb3+ (5D4), Dy3+ (4I15/2), and Er3+ (4F5/2) ions by 487, 451, and 457 nm radiations are also recorded as shown in Figure 8. Intense intra-4f transition emissions are acquired and a broad defectrelated emission band is simultaneously quenched mostly. It is the result of effective doping and good crystallinity of the host material. With respect to the mechanism of mentioned defect-related emission, exact origin is still under debate. Oxygen vacancies are among the most likely candidates for the recombination centers involved in the visible luminescence of ZnO.41,42 The oxygen vacancies in our samples are formed in the synthesis

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Figure 9. PL spectra under 325 nm excitation and PLE spectra monitoring: (a) 542 nm emission of (4% Tb)-ZnO, (b) 572 nm emission of (4% Dy)-ZnO, and (c) 552 nm emission of (4% Er)-ZnO nanosheets. The inflections at around 650 nm are caused by an error in spectrometer.

Figure 10. PL spectra of (a) (4% Tb)-ZnO nanosheets under 280, 371, and 486 nm excitations, (b) (4% Dy)-ZnO nanosheets under 277, 378, and 451 nm excitations, and (c) (4% Er)-ZnO nanosheets under 277, 368, and 456 nm excitations. The inflections at around 650 nm are caused by an error in the spectrometer.

Figure 11. PL (solid line) and PLE (dotted line) spectra of (a) (4% Tb)-ZnO, (b) (4% Dy)-ZnO, and (c) (4% Er)-ZnO nanosheets after outer growth of ZnO shells. The inflections at around 650 nm are caused by an error in spectrometer.

process under the atmosphere in solvent lack of oxygen and later annealing in air is not effective to eliminate all of them. Although numerous studies appeared in the literature, the exact energy level relative to conduction band (or valence band) was not fixed, and luminescence in the range of 480-570 nm was attributed to this transition. To investigate possible structure related doping effects, REZnO nanosheets have also been prepared by a commonly used method34,35 which involves transformation of directly precipitated hydroxide carbonates into oxides by annealing. PL and PLE spectra of undoped ZnO nanosheets are displayed in Figure S4. Compared with those of Figure 7, it is clearly seen that defect-related emission is red-shifted from ∼560 nm to ∼580 nm, and NBE emission under both 325 nm and VUV peak excitations is enhanced a little. PLE spectra also show the same two absorption peaks. Figure 9 shows the PL spectra under 325 nm excitation and the corresponding PLE spectra of the doped nanosheets. Characteristic emissions of RE ions are almost invisible

compared with those of doped core-shell nanocrystals in Figure 5. Besides, from comparison of PLE with those of doped core-shell nanocrystals in Figure 6, intra-4f transition absorptions are more or less merged and weaker than band-edge absorption which contributes to almost all defect-related emissions as will discussed below. Under band-edge excitation as shown in Figure 10, a broad band of defect-related emission is predominant while RE ion emissions are neglectable, quite different from the case of doped core-shell nanocrystals. So no competitive trapping of photogenerated excitons of ZnO host between excited states of the defects and RE ions happens and RE ions may possibly reside in a random, disordered environment, not well-defined sites within the ZnO lattice.29 Presence of transitions from higher excitated states of RE ions under VUV peak excitation in Figure 10 supports this point. Besides, even excitation directly into excited states of Tb3+ (5D4), Dy3+ (4I15/2), and Er3+ (4F5/2) ions by 486, 451, and 456 nm gives weak intra-4f transition emissions, which is also a sign of poor crystallographic

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surroundings. However, this disorder may not be in the bulk of the nanosheets since the crystallinity is good as seen from Figure 3 and Figure S5. Instead, it may come from the less favorable coordinating sites on the surface of the nanosheets as discussed in the following section. It is noteworthy that, under VUV peak excitation, PL of (4% Tb)-ZnO nanosheets is different from those of (4% Dy)-ZnO and (4% Er)-ZnO nanosheets as significant emission peaks of Tb3+ locate on the broad emission band of ZnO. It is due to overlapping of the VUV peak of ZnO with a CT transition band from oxygen to higher valent Tb4+ ions centered at around 27400 cm-1 with a fwhm of about 10900 cm-1 as discussed before. Figure 11 presents the PL and PLE spectra of doped nanosheets after outer growth of ZnO shells by a method similar to that used in core-shell nanocrystal growth. No characteristic emissions of RE ions can be observed under 325 nm excitation or band-edge excitation (except for 4F7/2-4I15/2 transition of Er3+), and only weak intra-4f transition emissions are obtained under excitation directly into excited states of Tb3+ (5D4), Dy3+ (4I15/2), and Er3+ (4F5/2) ions. Besides, intra-4f transition absorptions in PLE are more or less merged and relatively weak compared to band-edge absorption. These observations imply that a nanosheet may not be a good host for encapsulation of RE ions. Another possibility is that the ICS protocol is not as effective for nanosheets as for nanocrystals. It is well-known that on their surface nanocrystals contain more uncoordinated dangling bonds that can trap RE ions, whereas fewer dangling bonds on the surface of nanosheets exist. In addition, TEM images show that ZnO nanocrystals reside on part of the nanosheet surface randomly and no conformal coating was achieved (Figure S5). Apparently, both these possibilities account for the role of ZnO nanocrystals as a better host for RE ions. Conclusions RE ions (Tb3+, Dy3+, and Er3+) are incorporated into ZnO nanostructures by a facile isocrystalline core-shell (ICS) protocol. Significant splitting and enhancement of intra-4f transition absorptions, an increase of ZnO band-edge absorption in PLE, and improvement of characteristic emissions of RE ions are observed in these core-shell spherical nanocrystals compared with the core-only ones. So inner doping and efficient energy transfer from ZnO host to guest RE ions are realized. These core-shell nanocrystals are ideal candidates in the application of light-emitting devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 10874183), Anhui Provincial Key Laboratory Special Fund, and the Presidential Scholarship Special Fund. We thank Dr. M. Fang for valuable discussions. Supporting Information Available: Figures showing the structural characterization and optical absorption of nanosheets and the XRD of nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. J. Am. Chem. Soc. 2003, 125, 13205.

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