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Aqueous Synthesis of Mn- and Co-Doped ZnO Nanorods Bharati Panigrahy,† M. Aslam,*,‡ and D. Bahadur*,† Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India, and Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India ReceiVed: March 10, 2010; ReVised Manuscript ReceiVed: May 28, 2010
Herein, we report the optical and magnetic properties of Mn- and Co-doped ZnO nanorods fabricated via a simple one-step aqueous-based chemical method. Interestingly, SEM results reveal a uniform size distribution of the nanorods throughout the substrate. The UV emission band of doped ZnO nanorods reveals a red shift from 382 to 384.5 nm, indicating a band-edge bending due to the dopants. The defect-related band centered at 600 nm is suppressed (ID/IUV ) 1-0.35) considerably in doped nanorods, revealing the quenching of surface defects present in the nanostructures. XRD, XPS, Raman spectra, and EDS data demonstrate a successful incorporation of TM dopants in ZnO nanorods. Localized SAED patterns taken using nanoprobe size reveals that the nanorods are single crystals, grown along the c-axis [0002] direction. A systematic evalution of the enhancement in ferromagnetism (M ) 0.15 × 10-2 to 1.3 × 10-2 emu/g) is found in modified doped ZnO nanorods. 1. Introduction Diluted magnetic semiconductors (DMSs) have attracted extensive scientific interests because of their potential applications in spintronics and optoelectronics.1-3 Most of these systems were derived from various wide-band-gap semiconductors doped with transition-metal (TM) ions. Among the reported DMSs, the wide-band-gap ZnO are particularly interesting because they exhibit pronounced optical properties, controllable magnetic properties, and have a Curie temperature (Tc) well above the room temperature (RT).2,4 The coexistence of magnetic, electric, and optical properties increases the potential of TM-doped ZnO (ZnO:TM) to be a multifunctional material. However, the controversy over the existence of ferromagnetic ordering has been reported by many research groups for the same functional oxide material.5,6 In particular, 3d transition-metal-doped ZnO shows broad applications in the fields of catalysis, field effect transistors, resonators, solar cells, and so on.7,8 Currently, a tremendous amount of research activity is going on to use one-dimensional (1D) nanostructures, which are considered to be well-defined building blocks for the fabrication of various nanoscale devices.9 Among a wide variety of nanostructures, such as nanobelts, nanorods, nanoflowers, nanodots, nanocages, and nanoribbons, the rods show unique superiority because of having a high surface-to-volume ratio and, hence, a large surface area.10 There are many reports on doped ZnO thin films and spherical nanoparticles;5,11-15 however, only a few have been published on doped 1D nanostructures.6,16 A diversity in various aspects, such as UV and broad-band emission, and magnetization behavior are influenced by a variety of synthesis procedures and constituent elements. For Mn- and Co-doped ZnO, RT ferromagnetism is observed for thin films,17-19 nanorods grown by chemical vapor deposition,16,20 and in cornlike nanostructures synthesized by the rheological phase reaction-precursor method.21 On the contrary, antiferro* To whom correspondence should be addressed. E-mail:
[email protected] (D.B.),
[email protected] (M.A.). † Department of Metallurgical Engineering & Materials Science. ‡ Department of Physics.
magnetism and ferrimagnetism was reported in doped ZnO nanostructure synthesized by an aqueous sol-gel method.22 To understand these conflicting magnetic properties of 1D nanostructures, it is essential to have a careful control over the synthesis conditions. Among the various synthesis procedures, chemical synthesis approaches, such as seed- or templateassisted growth, electrodeposition, and hydrothermal routes are the simple and most efficient strategies to prepare uniform TMdoped ZnO nanorods as compared to the discussed physical deposition techniques (complex procedure, high cost, sophisticated instrumentation, and rigid experimental conditions). There are a few studies focused on the chemical growth of such nanostructures, which mostly requires special experimental conditions, such as electric fields,23 ultrasound treatment,11 or autoclaves.24 However, in previous reports about the preparation of TM-doped ZnO nanostructures via the solution route, the products were mainly composed of isotropic dots or polydispersed wires, having an unclear and daisy morphology.25,26 Moreover, synthesis of doped ZnO nanorods having a narrow size distribution and high crystallinity still remains a significant challenge. Here, we succeeded in establishing an easy approach toward the synthesis of long and uniform ZnO:TM nanorods in aqueous solution under ambient conditions. The present aqueous-based chemical synthesis method is fast, facile, simple, economical, and environmentally benign. The selected area diffraction pattern (SAED) confirms that ZnO nanorods are c-axis (0002) oriented. Introduction of Mn2+ and Co2+ ions can tune the photoluminescence properties of ZnO nanorods (ID/IUV ) 1-0.35). Further, the XPS data reveal that the dopants Mn and Co exist in the 2+ oxidation state. Raman studies reveal the red shift of the wurzite conformation peak (E2, 437 cm-1) due to the introduction of the transition-metal ions. Furthermore, the doped and undoped ZnO nanorods exhibit interesting room-temperature ferromagnetic behavior, which enhances linearly due to the dopant ions (M ) (0.15-1.3) × 10-2 emu/g).
10.1021/jp102163b 2010 American Chemical Society Published on Web 06/21/2010
Aqueous Synthesis of Mn- and Co-Doped ZnO Nanorods 2. Experimental Section Zinc chloride (ZnCl2, 99.99%, Aldrich), manganese chloride tetrahydrate (MnCl2 · 4H2O, 99%, S. D. Fine-Chem Ltd.), cobalt chloride hexahydrate (CoCl2 · 6H2O, 99%, Aldrich), and hexamethylene tetramine (HMTA, C6H12N4, Thomas Baker) were used as precursors. All chemicals were analytical grade reagents and used as reactants without further purification. The reaction solutions of Zn1-xMnxO and Zn1-xCoxO (x ) 0.05) were prepared by mixing 0.01 M of both zinc chloride dihydrate and HMTA and specific concentrations of manganese chloride tetrahydrate and cobalt chloride hexahydrate with Milli-Q water in separate containers. Precursors are completely dissolved in water, and then all the solutions were mixed in a beaker. The substrates were rinsed ultrasonically with acetone and Milli-Q water for 5 min each and dried in an air atmosphere. Before the precipitation begins, clean substrates were vertically dipped into the reaction solution and the beaker containing the solution was kept inside a heating oven at 85-90 °C for 1.5 h. The deposited substrate was then taken out of the solution, copiously rinsed with Milli-Q water, and dried at room temperature. The identification and purity of the phase was tested by X-ray diffraction (XRD) studies using a Philips powder diffractometer PW3040/60 with Cu KR radiation (λ ) 1.54 Å). The morphology and composition of the samples were examined by Hitachi S-3400 N scanning electron microscope (SEM) combined with energy-dispersive spectroscopy (EDS) and an ICP-AES 8440 Plasmalab, Labtam. Transmission electron microscopic (TEM) images and selected area diffraction patterns (SAED) were obtained with a CM 200, Philips transmission electron microscope at an accelerating voltage of 200 kV. The Raman scattering measurements were performed on a Lab RAM HR 800 Micro laser Raman system at backscattering geometry using the 519 nm line of an Ar+ laser as an excitation source. The XPS analyses were performed on a Thermo VG Scientific MultiLab, ESCA Probe using Mg KR (hν ) 1253.6 eV) as the exciting source for identification of the elements and their oxidation state. Photoluminescence (PL) spectra were taken at room temperature following excitation with a He-Cd laser (λ ) 325 nm). The magnetic properties of the samples were measured by a physical property measurement system (Quantum Design PPMS).
J. Phys. Chem. C, Vol. 114, No. 27, 2010 11759 NH3 + H2O f NH+ 4 + OH
Zn(Cl2)0.95(TMCl2)0.05 f (Zn0.95TM0.05)2+ + 2Cl(Zn0.95TM0.05)2+ + 2OH- f (Zn0.95TM0.05)(OH)2 (Zn0.95TM0.05)(OH)2 f ZnO:TM + H2O The solubility limit of different transition metals in ZnO has been analyzed by many researchers. Mandal et al. has studied the solubility limits of Co, Mn, Fe, and Ni in ZnO nanoparticles and reported that, among transition metals, Co and Mn are the most soluble in ZnO.27 TM solubility has been reported to be higher in ZnO thin films (33%)28 grown under nonequilibrium conditions, such as a pulsed laser deposition. However, it is much lower in the low-temperature synthesis procedure.29 Viswanatha et al. has reported that the solubility limit drastically reduced to as low as ∼1% for the Mn-doped ZnO nanocrystals.11 From the above discussion, we found that the solubility of TM ions in thin film is higher compared with the nanostructures prepared by chemical synthesis procedures. It has been reported that the solubility limit increases with increasing temperature (basically in physical deposition procedures) as, at hightemperature, the diffusion of the dopant ion from the vapor into the ZnO crystals is easier. Here, in our low-temperature system, which is thermodynamically stable and is an aqueous synthesis approach, the solubility of Mn and Co in ZnO nanorods ranges from 1 to 2%, which is much less as compared with the thin films. Typical XRD patterns of ZnO and ZnO:TM nanorods with 5 mol % of transition-metal ion doping are presented in Figure 1a. The XRD analysis reveals that Zn1-xTMxO samples have a
3. Results and Discussion The main objective of our research is to synthesize ZnO:TM (Mn, Co) nanorods by a one-step chemical synthesis approach without using any surfactant to assist the synthesis. In brief, a zinc cation (Zn2+) from zinc chloride, TM2+ from TM chlorides, and a hydroxide ion (OH-) from HMTA favorably react in aqueous solution to form a zinc TM hydroxide quasi-precursor. This decomposes by heating at temperatures of 85-90 °C to form ZnO:TM nuclei, which nucleate and grow in a specific pattern to form ZnO:TM nanostructures. The reaction mechanism for the formation of ZnO:TM can be explained as
C6H12N4 + 6H2O f 6HCHO + 4NH3
Figure 1. (a) XRD patterns taken from the pure ZnO and ZnO:TM (5 mol %) nanorods. The inset shows the XRD pattern of the three most intense peaks [(100), (002), and (101)] of the TM-doped ZnO samples, showing shifting of the center of diffraction. (b) Plots of ∆W cos θ versus sin θ (∆W is the width of the Bragg peak at angle θ).
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single-phase hexagonal wurtzite structure, indicating possible doping throughout the nanorods. It should be noted that no TM clusters or other oxides of TM ions or mixed oxide phases are observed in the XRD pattern. The three most intense peaks of the XRD pattern of ZnO:TM samples (inset of Figure 1a) clearly show shifting of the diffraction peaks slightly toward lower angles by Mn2+ substitution and a higher angle shift by Co2+ substitution in ZnO lattices, indicating an expansion or reduction of the unit cell, respectively. This is consistent with the fact that the ionic radii of Mn2+ and Co2+ are 0.8 and 0.72 Å, respectively, whereas that of Zn2+ is 0.74 Å.30 The shifting of XRD lines strongly suggests that Mn2+ and Co2+ are successfully substituted into the ZnO structure at the Zn2+ site. Again, we calculated the stress via Scherrer’s formula from the linear dependence of ∆W cos θ versus sin θ31
∆W cos θ ) kλ/L + η sin θ where ∆W is the line width (fwhm) of the diffraction peak at angle θ and η is the strain coefficient. Figure 1b shows the experimental dependences of ∆W cos θ versus sin θ for the most pronounced peaks in the XRD diagrams, which were analyzed by linear fits. The differences in the slopes are presumably due to significant strain contribution to the diffraction line widths. The higher magnitude of slope by Mn and Co doping (5 mol %) leads to an increase in strain as compared with the pure ZnO. As the ionic radius of Mn2+ (0.8 Å) is higher and Co2+ (0.72 Å) is lower than Zn2+ (0.74 Å), the incorporation of Mn and Co in the ZnO alters the lattice parameters in concordance with the lattice strain. From this analysis, we can conclude that doping of TM ions generates stress on the host lattice. Figure 2 shows SEM images of pure, Mn (5 mol %) and Co (5 mol %) doped ZnO nanorods. The length and diameter of the pure ZnO nanorods were around 5 µm and 85 ( 10 nm, respectively, with an aspect ratio of nearly 50 (Figure 2a). The average length of the doped nanorods is the same as that of pure nanorods, whereas the diameters are 150 and 60 nm for ZnO:Mn and ZnO:Co nanorods, respectively (Figure 2 b,c). Straumal et al. has reported that the overall solubility of TM drastically increases with the decreasing size of the particle.32 Consistent with the above statement, it can be noted from the SEM data that the sizes of the nanorods were increased with Mn doping and decreased with Co doping as compared with the pure ZnO. As the ionic radius of Mn2+ is higher and that of Co2+ is lower than that of Zn2+, accordingly, we have the variations in the size of the microstructures. TEM images reveal good quality nanorods having a smooth surface without any amorphous outer layers or nanoparticle impurities. The SAED pattern confirms that the nanorods are single crystals and are grown along the c-axis [0002] direction. The EDS data and ICP of Mn- and Co-doped ZnO nanorods have been analyzed. With consideration of the accuracy of the EDS analysis, we can conclude that there are 1.4% and 2% atomic ratios of manganese and cobalt doping in the ZnO nanorods, respectively, corresponding to the 5% of the respective initial cationic precursor. In comparison, ICP-AES analysis, which is considered to be more accurate, reveals 1.14 atom % ((0.021) and 1.69 atom % ((0.031) of Mn and Co are incorporated into the host lattice, respectively, which is in good agreement with the EDS analysis (Supporting Information, Table 1). X-ray mapping of the SEMEDS analysis of doped ZnO samples strongly suggests that transition-metal ions are almost uniformly distributed in the samples. We confirm the presence of doped ions by XPS study, as discussed below.
Figure 2. SEM images of (a) pure ZnO, (b) ZnO:Mn, and (c) ZnO: Co nanorods. In all cases, the products are composed of uniform nanorods. The inset of (a) shows that the SAED pattern confirms the single-crystalline and [0002] directional growth of the nanorods.
Identification of chemical states is done by determining the line energies (binding energy) in XPS spectra. XPS measurements give further evidence for the incorporation of doped ions into the host lattice. Figure 3 shows the XPS spectra of (a) Zn 2p, (b) O 1s, (c) Mn 2p, and (d) Co 2p positions of pure and 5% Mn- and Co-doped ZnO nanorods. Two strong peaks centered on 1022.5 and 1045.6 eV (Figure 3a), which are in agreement with the binding energies of Zn 2p3/2 and Zn 2p1/2, respectively, with a spin-orbital splitting of 23.1 eV confirm that Zn is present as Zn2+. The asymmetric O 1s peak in the surface was coherently fitted by three nearly Gaussian components, centered at 530.7, 532, and 533.2 eV, respectively, as shown in Figure 3b. The first peak on the low binding energy
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Figure 4. Room-temperature micro-Raman spectra of pure ZnO and ZnO:TM nanorods.
peak is associated with O2- ions that are in oxygen-deficient regions within the matrix of ZnO.34 The higher binding energy at 533.2 eV usually corresponds to absorbed H2O or O2 onto the surface.35 Figure 3c shows that one broad peak appears at 640.8 eV, which indicates the existence of Mn as the Mn2+ state.36 The Co 2p3/2 and Co 2p1/2 core levels are observed at 779.2 and 794.7 eV (Figure 3d), respectively, with a spin-orbital splitting of 15.5 eV, indicating that Co ions in Co-doped ZnO samples may have a valence of +2.37 Moreover, the presence of Co as metal cluster in the Co-doped sample can be ruled out because the differences of the 2p3/2 and 2p1/2 for the metallic state should be 15.05 eV.38 The intensity of the Mn 2p and Co 2p XPS spectra is very weak due to the low level of Mn and Co doping. These results demonstrate that substitution of Mn2+ and Co2+ ions in the host ZnO lattice can be done via the present route without any impurity oxide phase. Raman scattering spectra studies were carried out on pure and TM-doped ZnO nanorods to investigate the dynamics of the crystal lattices. ZnO, with a wurtzite structure, belongs to the C46V (P63mc) symmetry group with two formula units per primitive cell. The optical phonon mode at the Γ-point of the Brillouin zones can be represented as39
Γopt ) A1 + 2B1 + E1 + 2E2
Figure 3. XPS spectra of the (a) Zn 2p3/2 and 2p1/2 regions and (b) O 1s region of pure ZnO and the (c) Mn 2p3/2 region and (d) Co 2p3/2 and 2p1/2 regions of ZnO:TM nanorods recorded at room temperature.
side of the O 1s spectrum can be attributed to the O2- ions on the wurtzite structure of the hexagonal Zn2+ ion array, which are surrounded by zinc atoms with their full complement of nearest-neighbor O2- ions.33 The intermediate binding energy
In ZnO, the bond between the atoms have both ionic-covalent forces, so the long-range Coulomb field leads to the splitting of the A1 and E1 vibrations into longitudinal (LO) and transverse optical (TO) phonon modes. Nonpolar E2 modes are Ramanactive, whereas B1 modes are Raman-inactive. Figure 4 shows the room-temperature micro-Raman spectra of pure ZnO and ZnO:TM samples. In the pure ZnO sample (x ) 0), the predominant peak at 437.5 cm-1 is attributed to the nonpolar optical phonon modes of ZnO (high-frequency E2 mode) and the band at 578 cm-1 corresponds to the LO mode of A1 symmetry. In addition to these first-order Raman modes, the spectra also show a band at 332 cm-1 that corresponds to the second-order phonon of the low-frequency E2. In our sample, except the high-frequency E2 mode, all other modes are very weak. The intensity of the first-order E2 mode reduces after doping of Mn and Co in ZnO. This peak also red shifted after doping. This shifting of Raman modes can be ascribed to the local stress arising as a result of a change in the nanorod size with incorporation of Mn2+ and Co2+ ions into the Zn2+ lattice sites. With a change in the size of the nanorods, disorder of surface atoms, and thus distortion, is generated, which can result in different local stresses (also discussed by using the Scherrer’s formula).40
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Figure 5. Room-temperature photoluminescence (PL) spectra: comparison of pure ZnO and ZnO:TM nanorods. The inset shows the magnified UV emission spectral region.
Photoluminescence studies for pure ZnO and ZnO:TM nanorods at room temperature reveal a weak UV emission peak centered at around 382 nm and a broad yellow-orange emission band ranging from 550 to 730 nm, as shown in Figure 5. The UV emission is attributed to the radiative recombination of a hole in the valence band and an electron in the conduction band.41,42 This peak is found to be shifting continuously toward lower energies by the substitution of Mn and Co ions (inset of Figure 5). The shift of the UV emission peak to the longer wavelength side has been attributed to the strong exchange interactions between the “d” electrons of the doping ion and the “s” and “p” electrons of the host band. For the doped ZnO samples, the broad emission peaks become very weak as compared with the pure ZnO, indicating the presence of Mn2+ and Co2+ in the sample as the intensity reduces considerably after doping. The doped cations provide competitive pathways for recombination, which results in quenching of the broad yellow-orange emission. Similar reduction effects of luminescence have been observed previously in nanocrystals.43 The doping of the TM ions in pure ZnO minimizes the concentration of defects that resided on the surface of the nanorods. The origin of ferromagnetism in pure and doped semiconductor oxide materials has been controversial, while structural defects and magnetic impurities has often been invoked to explain magnetic ordering.12,44-46 It has been discussed that the magnetic behavior is highly dependent on the synthesis conditions.47 For the same synthesis condition (sol-gel method), Codoped ZnO samples show RT ferromagnetism48 in one case, whereas paramagnetism49 in an other. Further, Mn- and Codoped ZnO nanowires resulted in paramagnetism in the Mn case and RT ferromagnetic behavior in the case of Co doping.50,51 Figure 6 shows the magnetization (M) versus magnetic field (H) curve of the pure and doped (5 mol %) ZnO nanorods at different applied magnetic fields, which reveals that all the samples have ferromagnetic features at RT. A clear hysteresis curve is observed with a coercivity field Hc ∼ 30 Oe for pure ZnO nanorods (zoomed image as an inset). The temperature dependence of the magnetization at the applied magnetic field of 2000 Oe is shown as an inset of Figure 6. The ferromagnetic ordering in the nanorods has no interruption up to 320 K, which means that there is no sign of a Curie temperature in our samples. The mechanism responsible for the observed ferromagnetic-like behavior at room temperature is still unclear and remains controversial. However, detailed structural characterization minimizes the possibility of ferromagnetism due to any impurity phases in our samples. Therefore, the room-temperature
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Figure 6. Hysteresis loop (M-H) of pure ZnO and ZnO:TM (5 mol %) nanorods at room temperature. The insets show the M-H and M-T curves of pure ZnO.
ferromagnetism of ZnO:TM nanorods could originate from longrange TM2+-TM2+ ferromagnetic coupling mediated by itinerant electrons. Hence, it can be concluded that the detected ferromagnetism arises from the homogeneous doping of Mn and Co ions into the ZnO crystal lattice. 4. Conclusions In summary, high-quality single crystals of pure and TM (Mn, Co)-doped nanorods have been synthesized by a facile, lowcost, one-step aqueous-based chemical approach. The nanorods are single crystals grown along the c-axis [0002] direction. It is found that the structural, optical, and magnetic properties are sensitively dependent on the incorporation of Mn2+ and Co2+ ions in the Zn2+ lattice site. The subsequent XRD peak shift confirms the effectiveness of the Mn and Co doping. The doping of Mn2+ and Co2+ ions can tune the photoluminescence properties of ZnO nanorods. Undoped ZnO nanorods exhibit a pure excitonic emission centered at 382 nm, whereas Mn- and Co-doped ZnO nanorods show a red shift in the UV emission, which indicates a shift in the band gap to the lower-energy side relative to that of undoped ZnO. Also, the broad-band emission peak centered around 600 nm in the yellow-orange region decreases considerably after doping. The incorporation of Mn2+ and Co2+ ions in the Zn2+ lattice site generates local stress that strongly influences the Raman bands. The doping of Mn and Co ions in ZnO nanorods showed an enhancement of roomtemperature ferromagnetism. We believe that the homogeneous distribution of dopant ions and the substitution of the TM cations by host cations are responsible for the magnetization enhancement. Acknowledgment. Financial support from the Nano Mission of the Department of Science and Technology (DST), Government of India, is greatly acknowledged. The authors are grateful to the Centre for Research in Nanotechnology & Science (CRNTS) for the TEM facility. Supporting Information Available: ICP-AES and EDS data for the Co- and Mn-doped nanowires (Table 1) and superimposed magnetization curves for the mixtures of CoO and MnO powders with ZnO pristine nanorods. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Nat. Mater. 2005, 4, 173.
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