Solution-Based Doping of Manganese into Colloidal ZnO Nanorods

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J. Phys. Chem. C 2008, 112, 8832–8838

Solution-Based Doping of Manganese into Colloidal ZnO Nanorods Yang Guo, Xuebo Cao,* Xianmei Lan, Cui Zhao, Xiudong Xue, and Yingying Song Key Lab of Organic Synthesis of Jiangsu ProVince and Department of Chemistry, Suzhou UniVersity, Suzhou, Jiangsu 215123, P. R. China ReceiVed: January 6, 2008; ReVised Manuscript ReceiVed: March 26, 2008

This manuscript describes the low-temperature, solution-based doping of Mn2+ ions into colloidal ZnO nanorods, and the yield of the products is in a gram scale. The structures and chemical compositions of the products were characterized by XRD, XPS, EDS, and FT-IR spectroscopy. The results demonstrate that Mn2+ ions were successfully incorporated into the lattice position of Zn2+ ions in ZnO. The concentration of Mn2+ ions (in molar %) in the products can be controlled in the range of 1.25∼5%. The surfaces of Mn-doped ZnO nanocrystals have very rich hydroxyl groups, which enhance their solubility in many polar and nonpolar solvents. TEM and FESEM were used to characterize the morphology of ZnO and Mn-doped ZnO nanocrystals, and they revealed that both the undoped and doped ZnO nanocrystals are composed of uniform nanorods with a diameter of 8 nm and a length of 95 nm. The doping of Mn2+ ions has significant influences on the optical properties of ZnO nanorods. UV-vis absorption spectroscopy measurements reveal that the doping of Mn2+ lead to a red shift of the absorption edge of ZnO nanorods. Undoped ZnO nanorods exhibit a pure excitonic emission centered at 384 nm, whereas Mn-doped ZnO nanorods only show a red emission that is assigned to the Mn2+ 4T(G) ligand-field excited state. 1. Introduction Since colloidal nanocrystals possess uniform morphologies, very small sizes, and narrow size distributions, they usually exhibit strong quantum confinement effects and unique optical, electrical, magnetic, and catalytic properties.1–3 They are also rather stable and keep the dispersivity well in the solution, which is largely different from the severe aggregation of other nanomaterials. This good solubility will facilitate the fundamental and technical applications of the nanocrystals as well as the assembly of the nanocrystals into distinct nanostructures.4,5 Of various colloidal nanocrystals including dots, rods, and wires, the rods show the unique superiority in acting as building blocks for the construction of two-dimensional (2D) or threedimensional (3D) nanoarchitectures.6–8 Zinc oxide (ZnO) is an environmentally friendly semiconductor, which can be grown into a wide variety of nanostructures (e.g., rods, wires, belts, etc.). These nanostructures show broad applications in the fields of catalysis, field-effect transistors, resonators, gas sensors, solar cells, and so on.9 For the purpose of introducing new properties into this interesting semiconductor, attempts have been made to combine it with impurity atoms or other nanomaterials such as quantum dots, metal nanoparticles, carbon nanotubes, and so on.10–12 In particular, since Dietl et al. predicted that the introduction of Mn impurity into ZnO would get a novel semiconductor with room-temperature ferromagnetism,13 efforts have been devoted to preparing welldefined Mn-doped ZnO nanostructures and applying them in spin-based electronics devices.14–21 Besides the generation of novel magnetic properties, the introduction of Mn2+ ions can also tune the electrical conductivity and band gap of ZnO nanocrystals. Previously, the growth of Mn-doped ZnO nanocrystals was mainly by means of molecular beam epitaxy and thermal evaporation.14–20 For example, well-aligned Mn-doped ZnO * Corresponding author. E-mail: [email protected].

nanorods on sapphire substrate were prepared through the coevaporation of pure zinc and MnCl2 powders at the temperature of 600 °C and the pressure of 50 torr;17 single-crystalline Mn-doped ZnO nanowires that exhibit a room-temperature ferromagnetism were prepared through the evaporation of ZnO and MnCl2 powders at 600∼700 °C.19 Solution-based synthesis represents another important strategy for the preparation of Mndoped ZnO nanocrystals.22–26 Relative to vapor phase methods, solution-based synthesis has the benefits of simplicity, lower reaction temperatures, and soluble materials whose surface functionalization can be investigated. Hence, solution-based synthesis should be the most efficient strategy to prepare colloidal, uniform Mn-doped ZnO nanorods. However, in previous reports about the preparation of Mn-doped ZnO nanocrystals via the solution route, the products were mainly composed of isotropic dots or polydispersed wires,22–26 which is disadvantageous to their assembly into ordered nanostructures for devices. To date, there are no reports on colloidal Mn-doped ZnO nanorods with a narrow size distribution and highly crystallinity. Herein, we present a low-temperature, solution-phase strategy for the successful preparation of uniform, colloidal Mn-doped ZnO nanorods with controlled Mn content (in molar %) of 1.25, 2.5, and 5%. During the preparation, pure ZnO nanorods were first grown through the decomposition of zinc acetate in the methanol solution of potassium hydroxide, and then manganese acetate was supplemented to provide Mn2+ ions to incorporate into ZnO nanorods. This method allows the preparation of Mn-doped ZnO nanorods in a gram scale, and the products have a uniform rod-shaped morphology. Especially, the surface of the as-prepared Mndoped nanorods have rich surface hydroxyl groups, which means that they own excellent solubility and can be processed into nanostructures or devices more readily.

10.1021/jp800106v CCC: $40.75  2008 American Chemical Society Published on Web 05/21/2008

Solution-Based Doping of Mn into Colloidal ZnO Nanorods

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2. Experimental Section Preparation of Colloidal Mn-Doped ZnO Nanorods. All the reagents were of analytical grade (purchased from Shanghai Chemical Reagents Co.) and used as received. The preparation of Mn-doped ZnO nanorods underwent a two-step process: first the growth of ZnO nanorods and subsequently the incorporation of Mn2+ ions into the nanorods. For the synthesis of undoped ZnO nanorods, 14.75 g of zinc acetate and 7.4 g of potassium hydroxide were dissolved with 60 and 32 mL of methanol, respectively. Then, the two solutions were mixed and gave white colloids immediately. The colloids were ripened at 70 °C at least three days to allow them to reach a narrow size distribution. Thus, undoped colloidal ZnO nanorods were obtained. For the preparation of Mn-doped ZnO nanorods, 0.17 g of potassium hydroxide and manganese acetate with a set weight were supplemented to the resulting ZnO colloids, and the colloids would change their color from the initial white to brown. The resulting brown colloids were also ripened at 70 °C for 24 h. The solids were separated from the solution by centrifugalization, washed several times with distilled water and absolute alcohol to remove any impurities, and dried at 60 °C at vacuum for three days. Finally, about 5.1 g of products were harvested. Characterization. The phase compositions of the products were measured by an X’Pert PRO SUPER rA rotation anode X-ray diffractometer with Ni-filtered Cu KR radiation (λ ) 1.54178 Å). The surface structures of the products were studied by a Vgescalab MK II X-ray photoelectron spectrometer (XPS) using Mg KR radiation (hV ) 1253.6 eV) with a resolution of 1.0 eV. The morphologies of the products were observed by a FEI Tecnai G20 transmission electron microscope (TEM) and a Hitachi S-4700 field emitting scanning electron microscope (FESEM). High-resolution transmission electron microscopy (HRTEM) images of the products were taken at 200 kV with a JEOL-2010 TEM. Energy-dispersive X-ray (EDAX) measurements were performed with the energy-dispersive X-ray spectrometer attached on the Hitachi S-4700 FESEM. Fourier transform infrared (FT-IR) spectra were recorded on a Bio-Rad FTS 575C instrument. Optical absorption spectra were measured by a Shimadzu 3150 UV-vis-near-infrared spectrophotometer. Photoluminescence spectra were performed on an Edinburgh FLS-920 Steady-State/Lifetime spectrofluorometer with Xe lamp as the excitation source. Hysteresis loops of the products were measured at room temperature by a BHV-55 vibrating sample magnetometer (VSM). 3. Results and Discussions Figure 1 displayed the XRD patterns of undoped ZnO colloids and ZnO doped with different levels of Mn2+ ions. The concentration (in molar %) of Mn2+ ions in the products was 1.25%, 2.5%, and 5%, as determined by EDAX. In all cases, only wurtzite structures of ZnO were detected (JCPDS files, No. 80-0075) and no signals of manganese oxides appear in the figure, which indicates that phase-pure doped nanocrystals were obtained by this synthetic scheme. The strong and sharp diffraction peaks demonstrate that the products are well crystalline. The abnormal intensity of (002) plane of ZnO relative to the standard data suggested that a preferential growth occurred along this direction and the products might occupy elongated shapes. Although no manganese oxides were detected, the results of XRD are not sufficient to certify the doping of Mn2+ ions at the lattice position of Zn in ZnO because manganese oxides may be small enough not to be detected in XRD measurements.

Figure 1. XRD patterns of undoped ZnO and those doped with different levels of Mn.

To distinguish that Mn2+ ions in the products exist in the form of dopants to substitute the lattice position of Zn or in the form of isolated manganese oxides, we used XPS technique to gain insight into the chemical composition and surface structure of the products (the binding energies obtained in the XPS analysis were corrected by referring the C 1s to 284.6 eV). Figure 2a is the XPS survey spectrum of Mn-doped ZnO, in which all of the peaks can be ascribed to Zn, Mn, O, and C elements.24 Parts b-d of Figure 2 displayed the high-resolution XPS spectra of Mn, O, and Zn species, respectively. In the Mn 2p spectrum (Figure 2b), the binding energy of Mn 2p3/2 is about 640.1 eV, which demonstrates the presence of bivalent Mn ions in the products.27–29 It is noticed that the binding energy of Mn 2p3/2 for Mn-doped ZnO shifts to the low-energy side compared with the 640.7 eV of Mn 2p3/2 for commercial MnO powders, which is related to the insertion of Mn2+ ions into the lattice position of Zn of ZnO. In the Zn 2p spectrum (Figure 2c), the peaks of Zn 2p3/2 of undoped ZnO and Mn-doped ZnO are centered at 1021.6 and 1020.4 eV, respectively. Park et al. have pointed out that this large shift of binding energy is due to the substitution of parts of lattice Zn in ZnO by Mn2+ ions and the formation of Zn-Mn bonding structure.16 Since the electronegativity of Mn (χMn ) 1.55) is smaller than that of O (χO ) 3.44), Zn atoms bonded to the Mn atoms will contribute to the shift of the binding energy of Zn 2p to a lower energy relative to the binding of Zn atoms to O atoms. Consequently, XPS data of both Zn 2p3/2 and Mn 2p3/2 provide evidence for the incorporation of Mn2+ ions into ZnO. In the O 1s spectrum (Figure 2d), the profile is asymmetric and can be fitted to two symmetrical peaks, which indicate two different kinds of O species in the sample. The O 1s peaks at 530.0 eV can be assigned to lattice oxygen in ZnO, whereas 532.6 eV should arise from the surface hydroxyl group of the products rather than absorbed water because the samples for XPS measurements were washed at least three times with absolute alcohol and dried at vacuum at 60 °C for several days.30–33 The ratio of the hydroxyl oxygen to the lattice oxygen is as high as 0.45, which

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Figure 2. (a) XPS survey spectra of Mn-doped ZnO nanocrystals. (b) Binding energy spectrum of Mn 2p. (c) Binding energy spectra of Zn 2p3/2 of undoped ZnO and ZnO doped with 2.5% Mn. (d) Binding energy spectrum of O 1s.

Figure 3. FT-IR spectra of Mn-doped ZnO nanorods and commercial ZnO nanoparticles.

indicates that there are very rich hydroxyl groups on the surface of Mn-doped ZnO. The high content of hydroxyl groups should be associated with the synthetic route that they were prepared through the decomposition of metal precursors in the methanol solution of potassium hydroxide. Besides XPS results, FT-IR measurements also confirm the existence of large amounts of hydroxyl groups on the surface of Mn-doped ZnO nanorods, as learned from their much broader absorption band at about 3400 cm-1 than that of commercial ZnO nanoparticles (Figure 3). It should be noted that, due to the rich surface hydroxyl groups, these Mn-doped ZnO colloids can be easily dispersed into many polar and nonpolar solvents (e.g., water, alcohol, CHCl3, etc.), and the dispersions show good stability (Supporting Information). This property is usually absent for the nanocrystals prepared by conventional vapor methods that tend to aggregate severely in the solution. The enhanced solubility means that they can be assembled into nanoarchitectures or be processed into devices more readily. In addition, the surface hydroxyl will provide functional groups

to react with functional organic molecules with optical or electrical properties (e.g., dyes, cluster compounds), which may generate novel organic-inorganic hybrids. Figure 4 showed the results of electron microscopy characterizations of undoped ZnO colloids. From the TEM (Figure 4a) and FESEM (Figure 4b) images, it can be seen that the products were completely composed of monodispersed nanorods. The statistical diameter and length of the nanorods were 8 and 95 nm, respectively. Figure 4c showed the representative HRTEM image of colloidal ZnO nanorods; the clear lattice fringes reveal that the nanorods are highly crystalline and free of defects such as twin structures, stacking faults, or dislocations. Under HRTEM observations, it was found that this perfect crystal structure was maintained well along the whole rod. The resolved interplanar spacing of 2.62 Å corresponds to the separation between the (002) crystallographic planes of hexagonal ZnO. In combination with the result of SAED (Figure 4d), the nanorods were determined to be grown along the [001] direction. This result is also consistent with the abnormal intensity of (001) plane observed in their XRD pattern. Figure 5 shows the results of electron microscopy characterizations of ZnO nanorods doped by Mn ions, in which parts a-b, c-d, and e-f correspond to the TEM and FESEM image of the products with Mn concentration (in molar %) of 1.25%, 2.5%, and 5%, respectively. In all cases, the products are composed of uniform nanorods with the lengths and diameters close to that of pure ZnO. Other shapes, such as irregular particles, long wires, and plates, were not seen during TEM observations. That is, the difference of the concentration of manganese precursors introduced had little influence on the morphology of the products. These results demonstrated that colloidal nanorods of Mn-doped ZnO can be prepared in a large scale and an extremely high yield via the present route. HRTEM image and SAED pattern of Mn-doped ZnO nanorods are shown in Figure 5g and 5h, respectively. They revealed that the nanorods were also single-crystalline in structure and grown

Solution-Based Doping of Mn into Colloidal ZnO Nanorods along [001] direction. However, by comparison with the HRTEM images of Figure 5g and Figure 4c, it is learned that the smoothness of the surface of Mn-doped ZnO nanorods is somewhat inferior to that of pure ZnO nanorods. Obviously, the destruction of the perfect surfaces of the nanorods is associated with the incorporation of Mn2+ ions into them. In addition, the calculations of lattice parameters based on the SAED pattern and HRTEM image reveal that the lattice constants of Mn-doped ZnO nanorods is slightly larger (about 0.09 Å for constants c) than that of undoped ZnO nanorods, which is because Mn2+ ions have a larger radius than Zn2+ ions (RMn2+) 80 pm; RZn2+ ) 74 pm). Figure 6a shows the optical absorption spectra of nanorods of pure ZnO and ZnO doped by different levels of Mn2+ ions. All the samples exhibit a strong absorption in the ultraviolet region (3.2∼3.5 eV), while the difference between pure ZnO and Mn-doped ZnO is that red shift is observed for the latter. Previously, it is proposed that size effects have little or no influence on the band structure of ZnO nanocrystals with diameters greater than 7.0 nm.34,35 Consequently, herein the red shift indicates the possible influences of Mn doping on the band gap of ZnO. Theoretically speaking, the doping of Mn into ZnO will lead to the shift of the absorption edge of ZnO to shorter wavelength because of the larger Eg of MnO relative to that of ZnO (4.2 eV vs 3.37 eV).23,25,36 However, contrary phenomenon is observed in the present Mn-doped ZnO nanorods and some previous reports.37–39 The shift of the absorption edge to the longer wavelength side has been attributed to the strong exchange interactions between the d electrons of Mn and the s and p electrons of the host band. The band gap values of nanorods of ZnO and Mn-doped ZnO are derived based on the well-established equation

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R ) A(hV - Eg)1/2/hV

(1)

where R, Eg, and A are the absorption coefficient, band gap, and constant, respectively. By extrapolating the linear region in the plots of (RhV)2 versus hV (Figure 6b), the band gap value of undoped ZnO nanorods is about 3.29 eV, whereas the values of ZnO nanorods doped with 1.25%, 2.5%, and 5% Mn are about 3.27 eV, 3.26 eV, and 3.25 eV, respectively. Consequently, there is about 0.02∼0.04 eV red shift after the inclusion of Mn2+ ions into ZnO nanorods. Photoluminescence studies of the undoped and Mn-doped ZnO nanorods at room temperature reveal that they exhibit interesting features, as shown in Figure 7. Undoped ZnO nanorods only feature a narrow emission band centered at 384 nm that is attributed to the radiative recombination of a hole in the valence band and an electron in the conduction band (excitonic emission), whereas the defect-related emission (green or yellow emission) is not observed. As observed by HRTEM, colloidal ZnO nanorods have a rather perfect crystal structure. Consequently, it is not surprising that undoped ZnO nanorods exhibit a pure excitonic emission. Upon the inclusion of Mn2+ ions, the UV emission band of ZnO nanorods is diminished and a new broad peak centered at 626 nm appears. The disappearance of UV emission band in Mn-doped ZnO nanorods is because Mn is a strong quencher of the band-edge luminescence of ZnO.22,25,40,41 The red emission in Mn-doped ZnO nanorods can be assigned to the 4T1 f 6A1 transition in Mn2+ ions. This phenomenon has also been observed in many Mn-doped II-VI semiconductor nanocrystals, including Mn2+:ZnO,22,25 Mn2+:ZnS,42,43 Mn2+:CdS,44 and Mn2+:ZnSe.45 It is also noted that the intensity of the red emission band is reduced drastically as

Figure 4. TEM (a) and FESEM (b) images of undoped ZnO nanocrystals. The products are composed of uniform nanorods. (c) Representative HRTEM image of undoped ZnO nanorods. (d) Corresponding SAED pattern.

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Figure 5. TEM and FESEM images of ZnO nanorods doped with different levels of Mn: (a, b) with 1.25% Mn; (c, d) with 2.5% Mn; and (e, f) with 5% Mn. In all cases, the products are composed of uniform nanorods, like that of undoped ZnO. (g) Representative HRTEM image of ZnO nanorods doped with Mn. The arrows indicate the area destructed. (h) Corresponding SAED pattern.

the concentration of Mn2+ ions in the products increases. Generally, at a high Mn2+ concentration, the manganese atoms have a tendency to form clusters around oxygen.46,47 The formation of these clusters will cause the passivation of

the surface of the nanorods and the reduction in manganese photoluminescence. Due to the absence of the superconducting quantum interference device (SQUID) equipment to learn the magnetization

Solution-Based Doping of Mn into Colloidal ZnO Nanorods

J. Phys. Chem. C, Vol. 112, No. 24, 2008 8837 temperature via the decomposition of Zn and Mn precursors in the methanol solution of potassium hydroxide. The substitution of parts of lattice Zn of ZnO by Mn2+ ions is confirmed by XRD and XPS techniques. The nanorods are well crystalline and have a narrow size distribution. Their surfaces are surrounded by large amounts of hydroxyl groups, which results in their good solubility and dispersivity in water and alcohol. The concentration of Mn2+ ions in the products can be controlled at the level of 1.25%, 2.5%, and 5% (in molar %), respectively. The doping of Mn2+ ions can tune the band gap and photoluminescent properties of ZnO nanorods. The absorption edge of Mn-doped ZnO nanorods shift about 0.02∼0.04 eV to the lower energy side relative to that of undoped ZnO. Undoped ZnO nanorods exhibit a pure excitonic emission centered at 384 nm, whereas Mn-doped ZnO nanorods show a red emission that is assigned to the 4T1 f 6A1 transition in Mn2+ ions. These uniform Mn-doped ZnO nanorods should have potential applications in novel phosphors, in dilute magnetic semiconductors, and as building blocks in the construction of some 2D and 3D nanostructures. The investigations of their applications are ongoing by us. Acknowledgment. This work was financially supported by the Key Lab of Organic Synthesis of Jiangsu Province (P. R. China), the Education Department of Jiangsu Province, and the National Natural Science Foundation of China (20601020).

Figure 6. (a) Absorption spectra of undoped ZnO nanorods and those doped with different levels of Mn. (b) Plots of (RhV)2 against hV of undoped and Mn-doped ZnO nanorods (derived from Figure 5a). The samples for UV-vis absorption measurements were prepared through dispersing the colloids into absolute alcohol.

Figure 7. PL spectra of undoped ZnO nanorods and those doped with different levels of Mn (excitation wavelength 375 nm). The samples for PL measurements were the dry solids.

behaviors of Mn-doped ZnO nanorods at low temperature, we only measure the room-temperature magnetic properties of the products with a vibrating sample magnetometer (VSM). The nanorods doped with 1.25%, 2.5%, and 5% Mn all present paramagnetic behaviors without hysteresis (the curves are not shown). Normally, magnetic materials may show a better signal for ferromagnetic ordering at low temperature. Consequently, it is expected that at low temperature the as-prepared nanorods may also exhibit such a behavior.14,48 4. Conclusions In summary, we have synthesized colloidal undoped ZnO nanorods and Mn-doped ZnO nanorods in a gram scale at low

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