Effect of Cobalt Doping on Structural, Optical, and ... - ACS Publications

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Effect of Cobalt Doping on Structural, Optical, and Magnetic Properties of ZnO Nanoparticles Synthesized by Coprecipitation Method Vijayaprasath Gandhi,† Ravi Ganesan,*,† Haja Hameed Abdulrahman Syedahamed,‡ and Mahalingam Thaiyan§ †

Department of Physics, Alagappa University, Karaikudi-630 004, Tamil Nadu, India PG and Research Department of Physics, Jamal Mohamed College, Tiruchirappalli-620 020, Tamil Nadu, India § Department of Electrical and Computer Engineering, Ajou University, Suwon 443-749, South Korea ‡

S Supporting Information *

ABSTRACT: Zn1−xCoxO (x = 0, 0.05, 0.10, and 0.15) nanoparticles (NPs) were synthesized by a coprecipitation method. The crystalline sizes of synthesized samples were calculated from the powder XRD patterns, which were found to decrease with the increase of cobalt content. The FT-IR spectra confirmed the Zn−O stretching bands at 468, 456, 452, and 461 cm−1 for the respective ZnO NPs. SEM images demonstrated the distinct flowerlike morphology. The photoluminescence spectra of all the samples exhibited a broad emission in the visible range. XPS studies were carried out for Zn0.90Co0.10O NPs. The carriers (donors) bound on the Co sites were observed from the micro-Raman spectroscopic studies. The pure and Co-doped ZnO NPs showed significant changes in the M−H loop where the diamagnetic behavior of ZnO changes to ferromagnetic nature when doping with Co. Oxygen vacancies and zinc interstitials were found to be the main reasons for room-temperature ferromagnetism in the Codoped ZnO NPs with the support of the results obtained from the EPR, photoluminescence, and micro-Raman studies.

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INTRODUCTION

Among these synthetic routes, the coprecipitation method has been popularly adopted to synthesis ZnO NPs due to its low cost, superior uniformity, and high yield of nanoparticles. In Co-doped ZnO nanoparticles, the doping of Co2+ yields increased magnetization values because Co2+ in the half-filled 3d shell has five spins, which normally gives a maximum dipole moment value of 3 μB/Co ion.16 In the present work, pure and Co2+-doped ZnO NPs were synthesized by the coprecipitation method. Their structural, optical, and magnetic properties are studied, and the obtained results are discussed in detail.

Nanoparticles of transition-metal oxides have been investigated by many researchers in recent years.1,2 The structural, optical, and magnetic properties of metal oxide nanoparticles are of particular interest for practical applications. Recently, oxidebased diluted magnetic semiconductors (DMS) such as transition-metal-doped semiconductors with room-temperature ferromagnetism (RTFM) have been studied for their advanced applications in spintronic devices. Transition-metal-doped semiconductors have attracted a lot of attention due to their potential applications, such as UV detectors, field-effect transistors, short wavelength lasers, high sensitive chemical sensors, and nonlinear varistors.3−5 ZnO nanoparticles (NPs) are the wurtzite-phase n-type semiconductors having a wide direct band gap of ∼3.37 eV, which causes an enormous attraction in commercial applications. The transition metal doping in semiconductor ZnO facilitates the generation of carrier mediated ferromagnetism.6 Many reports addressed room-temperature ferromagnetic behavior of transition metal [Fe, Mn, Ni, Co, Cr] doped semiconductor oxides,7−9 and the behavior of ferromagnetism is caused mainly by intrinsic defects or impurity phases or ferromagnetic precipitates.10,11 Several methods are available for the synthesis of ZnO nanoparticles, such as a chemical or physical method,12 hydrothermal process,13 sol−gel method,14 and coprecipitation method.15 © XXXX American Chemical Society

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EXPERIMENTAL SECTION Synthesis of pure and Co-doped ZnO was carried out using analytical grade zinc acetate dihydrate [Zn(CH3COO)2·2H2O], cobalt acetate dihydrate [Co(CH3COO)2·2H2O], and sodium hydroxide (NaOH) in as-received condition. In the synthesis process, a required amount of zinc acetate was completely dissolved in deionized water and a required amount of aqueous NaOH solution was added dropwise to the mixture. Later, the solution with the white precipitate was stirred at room temperature for 30 min and then kept at 80 °C for 5 h. Thereafter, the white precipitate was washed several times with Received: December 3, 2013 Revised: March 18, 2014

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dx.doi.org/10.1021/jp411848t | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. (a) Powder X-ray diffraction patterns of (A) ZnO, (B) Zn0.95Co0.05O, (C) Zn0.90Co0.10O, and (D) Zn0.85Co0.15O nanoparticles. (b) (002) peak shifting toward lower 2θ value.

It is also observed that the diffraction peaks’ intensities were decreased for the Co-doped ZnO, which indicates that the dopant Co2+ ions (ionic radius: 0.58 Å) are substituted in the inner lattice of Zn2+ ions (ionic radius: 0.60 Å).17 Since the ionic radius of Co2+ is close to that of Zn2+, the changes in the FWHM are in accordance with the crystallite size, which is calculated from the Debye−Scherrer’s formula18

double distilled water and ethanol. Finally, the precipitate was dried at 120 °C for an hour. The obtained ZnO samples were annealed at 300 °C in air for 2 h. Thus, a nanopowder of ZnO NPs was collected and used for further studies. For the synthesis of Zn1−xCoxO (x = 0.05, 0.10, and 0.15) nanoparticles, the calculated amount of cobalt acetate in water was mixed with zinc acetate solution separately. The required amount of aqueous NaOH solution was added drop by drop to the homogeneous mixture to get a white precipitate with a pale pink color. A similar procedure adopted for the preparation of pure ZnO NPs was followed for the synthesis of Co-doped ZnO NPs. The structural analysis of the synthesized samples is carried out using a powder X-ray diffractometer (PANalytical X′Pert Pro) with a Cu−Kα radiation source (wavelength: 1.5418 Å). The samples were functionally characterized by Fourier transform infrared (FT-IR) spectroscopy using a Thermo Nicolet 380 with the KBr pellet method at room temperature in the range of 4000−400 cm−1. The morphology of the samples was examined using an FEI - QUANDA 200F Field Emission SEM (FESEM) operating at 30 kV. Energy-dispersive X-ray analysis was performed using an inbuilt EDS (model: AMETEK). The XPS measurements were performed with an XPS instrument (Carl Zeiss) equipped with an Ultra 55 FESEM with EDS, and all the spectra were recorded under ultrahigh vacuum with Al Kα excitation at 250 W. The magnetic measurements were carried out using a vibrating sample magnetometer (Lake Shore model-7404). The electron paramagnetic resonance (EPR) measurement was conducted with a Bruker EMX Plus spectrometer using an X band (9.78 GHz) at room temperature.

D=

0.9λ → β cos θ

(1)

where D is the size of the particle, λ is the wavelength of Cu Kα radiation (1.5418 Å), β is the full width at half-maximum intensity, and θ is the peak position. The average crystallite size of pure ZnO NPs is calculated as 35 nm, and it is found to be decreased with the cobalt concentration and represented in Table 1. The reduction in the crystallite size is mainly due to Table 1. Crystallite Size and Microstrain of Pure and CoDoped ZnO Nanoparticles concentration of cobalt (x)

crystallite size (nm)

microstrain × 10−3 lines−2/m4

0 0.05 0.10 0.15

35 33 31 29

1.028 1.078 1.161 1.244

the distortion in the host ZnO lattice by the foreign impurities (i.e., Co2+) that decrease the nucleation and subsequent growth rate of ZnO NPs. The substitution of Co2+ in an interstitial position would affect the concentration of the interstitial Zn, oxygen, and Zn vacancies. The observation of small changes of 2θ values in diffraction peaks and the peak broadening is due to the increase of microstrain,19 and the line broadening may be due to the size and microstrain20 of nanoparticles. The structural change obtained from the diffraction peaks illustrates the incorporation of Co2+ ions into the ZnO lattice, which indicates that the crystal lattice has no obvious change by Co doping. The lower shift in the peak position is observed in Figure 1b. The microstrain can be calculated using the formula18

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RESULTS AND DISCUSSION X-ray diffraction patterns of Zn1−xCoxO (x = 0, 0.05, 0.10, 0.15) NPs are shown in Figure 1a. The d-spacing of peaks are well-matched with standard data (JCPDS-89-0510), and the corresponding structure of the sample is hexagonal wurtzite. No other secondary phases, such as Co clusters or cobalt oxides, could be observed from the XRD patterns. In order to study the effect of Co doping, a careful analysis of the position of XRD peaks is carried out, and especially the prominent (002) peak is shifted toward a lower 2θ value, as shown in Figure 1b.

ε= B

β cos θ → 4

(2)



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Table 1 shows the comparison of crystallite size and strain with the increasing of dopant concentration. The lattice parameters for hexagonal wurtzite structured pure and Codoped ZnO NPs are calculated from the following equation18 1 4 (h2 + hk + k 2) l2 = + → 3 d2 a2 c2

The volume of the unit cell increases with the increase of Co doping level, which is displayed in Table 2. According to Vegard’s law, higher doping levels could increase the volume of the unit cell.22 The incorporation of Co2+ ions into the ZnO lattice could be easily identified from the variations of lattice constants. It is evident that the fwhm gradually increases with increasing the doping of Co, which, in turn, decreases the crystallite size up to the higher content of Co. The relations between various structural parameters are given in Figure 2. The FTIR spectra of the synthesized pure and Co-doped ZnO NPs are shown in Figure 3. All the samples exhibited

(3)

where a and c are the lattice constants; h, k, and l are the Miller indices; and dhkl is the interplanar spacing. This interplanar spacing can be calculated from Bragg’s law.21 The pure and Codoped ZnO NPs crystalline peaks of (100) and (002) planes are used for the calculation of lattice parameters. The changes in a and c parameters are observed due to the incorporation of Co dopant, as shown in Table 2. Table 2. Bond Length and Cell Volume of Pure and CoDoped ZnO Nanoparticles lattice parameters (Å) concentration of cobalt (x)

a

c

bond length (l) nm

cell volume Å3

0 0.05 0.10 0.15

3.242 3.245 3.248 3.251

5.194 5.201 5.201 5.209

1.8917 1.8937 1.9761 1.9789

47.27 47.44 47.50 47.69

Figure 3. FTIR spectra of (A) ZnO, (B) Zn0.95Co0.05O, (C) Zn0.90Co0.10O, and (D) Zn0.85Co0.15O nanoparticles.

The bond length of pure and doped ZnO has been calculated using the following formula18 l=

⎛1 ⎞2 a2 + ⎜ − u⎟ ∗ c 2 → ⎝2 ⎠ 3

absorption bands at 3472, 2347, 1613, 1382, 872, 576, and 446 cm−1. The absorption bands at 576 and 446 cm−1 are attributed to the Zn−O stretching in the ZnO lattice.23,24 The absorption peak that appeared at 3472 cm−1 is attributed to O−H stretching vibrations of H2O (Table 3). The peak around 1613 cm−1 is due to H−O−H bending vibration, which is assigned to a small amount of H2O in ZnO NPs.25 The absorption peak observed at 2347 cm−1 is due to the existence of CO2 molecules in air.26 The medium to weak band at 872 cm−1 is assigned to the metal−oxygen vibration frequency due to the changes in the microstructural features by the addition of Co into the Zn−O lattice.

(4)

where a and c are lattice constants u=

a2 + 0.25→ 3c 2

(5)

where u is a positional parameter. The volume of the unit cell for the hexagonal system has been calculated using the following relation18 V = 0.866∗a 2 ∗c→

(6)

Figure 2. Relations between (a) crystallite size and strain with Co concentration and (b) cell volume and bond length versus concentration of Co. C



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represents the images of Zn0.95Co0.05O, Zn0.90Co0.10O, and Zn0.85Co0.15O NPs, respectively. The incorporation of Co ions in Zn lattice sites could influence the changes of the flowerlike morphology into bounded grains. The small particles are agglomerated and bound to the spherical shape due to the doping of Co. The chemical compositions of synthesized ZnO NPs with different Co concentrations were measured by EDS spectra and are shown in Figure 4E−H. These spectra indicated the presence of only Zn, O, and Co. The obtained NPs are made up of only these elements, which shows that the Co2+ ions are substituting the Zn2+ ions in the Zn matrix. In pure ZnO NPs, only zinc and oxygen ions are detected, as shown in Figure 4E. Figure 5 displays the emission spectrum of pure and Codoped ZnO NPs using an excitation wavelength of 325 nm. The photoluminescence (PL) emission is observed for all the samples covering from a short wavelength of 350 nm to a long wavelength of 550 nm. The seven different peaks that appeared are well-fitted with a Gaussian function for all the PL spectra of the samples, and the solid lines represent the linear combination of seven Gaussian peaks covering the entire range. The UV emission peaks of lowest wavelength are observed at 380 nm (3.27 eV) and 392 nm (3.17 eV), which correspond to the near-band emission (NBE) of ZnO.30−32 The peak observed at 407 nm is attributed to a violet emission of ZnO NPs. The peaks at 419, 440, and 488 nm correspond to the blue emission, while the peak that arose at 520 nm represents green emission. Many researchers have reported that the observation of visible emissions is related to intrinsic defects in ZnO, which include Zn vacancies (VZn), O vacancies (VO), interstitial Zn (Zni), interstitial O (Oi), and substitution of O at a Zn position (OZn).33−36 Most of the reports about emission properties of ZnO showed a higher intensity of UV or green emissions compared to the blue emissions.37−39 However, Figure 5A shows that the violet emission intensity is higher than UV and green emissions. The violet emission at 407 nm is

Table 3. FTIR Peak Assignments of Co-Doped ZnO Nanoparticles wavenumber (cm−1) assignments O−H stretching H−O−H bending vibration acetate group stretching O−H asymmetric stretching weak vibrations of ZnO Zn−O stretching

x=0

x = 0.05

x = 0.10

x = 0.15

3472 1613

3430 1540

3417 1527

3447 1548

1382 1034

1440 1061

1429 1061

1443 1052

850 576, 468

863 573, 456

877 572, 452

863 568, 461

It is observed from IR spectra for the doped samples that the peak found at ∼3440 cm−1 is assigned to the −OH mode in H2O molecules. The presence of these bands in synthesized nanoparticles may be due to the adsorption of atmospheric water content. The bands around ∼1040 cm−1 are shoulders with asymmetric stretching of resonance interaction between vibration modes of oxide ions in the nanocrystals.27 The absorption band of Zn−O stretching is exhibited at 468, 456, 452, and 461 cm−1 for ZnO, Zn0.95Co0.05O, Zn0.90Co0.10O, and Zn0.85Co0.15O NPs, respectively. These absorptions are wellmatched with the reported literature values.28,29 The hydroxyl groups and acetate groups are adsorbed on the ZnO nanoparticles’ surface. When ZnO samples are calcined above 600 °C, these groups are easily removed from ZnO. FTIR results also indicated that Co is occupying the Zn position in the ZnO matrix as there is no peak found in the spectra that corresponds to Co, which coincides with the XRD results. Figure 4A−H shows the morphology and chemical composition of annealed NPs investigated by SEM and EDS analyses. The pure ZnO (Figure 4A) shows a flowerlike morphology, and tiny ZnO nanoparticles are filled in the inner and intermediate spaces of the nanoflowers. Figure 4B−D

Figure 4. Morphology of (A) ZnO, (B) Zn0.95Co0.05O, (C) Zn0.90Co0.10O, and (D) Zn0.85Co0.15O NPs. Composition of (E) ZnO, (F) Zn0.95Co0.05O, (G) Zn0.90Co0.10O, and (H) Zn0.85Co0.15O NPs. D



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Figure 5. PL emissions spectra of (A) ZnO, (B) Zn0.95Co0.05O, (C) Zn0.90Co0.10O, and (D) Zn0.85Co0.15O NPs using the excitation wavelength at 325 nm.

probably due to the radiative defects of Zni and VZn related to the interface traps existing at the grain boundaries of ZnO. The blue emission shown at 440 nm is probably caused by two defect’s level, either transition from Zni to the valence band or transition from the bottom of the conduction band to the Oi level.40 The emission peak observed at 488 nm (Figure 5A) can be attributed to the transition between the oxygen vacancy and interstitial oxygen.41 The green emission peak at 520 nm is ascribed to the recombination of electrons in singly ionized oxygen vacancy with photoexcited holes in the valence band.40 The green emission peak proposed in ZnO may probably be due to the transition from Zni levels to the VZn level. The green emission peak at 520 nm, which has the lowest intensity of emission in the spectrum, coincides with the energy interval from the bottom of the conduction band (2.37 eV).40 The emission spectra of Co-doped ZnO NPs with different concentrations (x = 0.05, 0.10, and 0.15) are shown in Figure 5B−D (see Table 4), using the excitation wavelength of 325 nm. The variation of emission properties can be clearly seen from these figures for increased concentration of Co in ZnO. While increasing the doping concentration, the UV near band emissions are red-shifted when compared with that of the pure ZnO. The red shift of the UV emission could also be as a result of reduction of the energy gap due to the band tailing effect.42 These shifts are occurring due to the dopant Co2+ ions substituted in Zn2+ ions. It is interesting to note that an additional green emission peak appeared at 540 nm for the doped samples and also the intensity of the green emission

Table 4. Gaussian Decomposed Photoluminescence Emission Values of Co-Doped ZnO NPs Co concentration (x)

NBE (nm)

violet emission (nm)

0 0.05 0.10 0.15

380, 392 380 381 387

407 401 408 410

blue emission (nm)

green emission (nm)

419, 415, 438, 419,

520 522 520, 540 520, 540

440, 434, 464, 432,

488 455, 489 489 489

peaks is increased with respect to the increasing doping level of Co2+. It has been reported that the green emission peaks are due to the oxygen vacancies and transition of a photogenerated electron from the conduction band to a deeply trapped hole.43 The XPS measurement aims to investigate the oxidation states of ions present in the samples. Figure 6A shows the compositional oxidation peaks of Zn0.90Co0.10O NPs. The peaks corresponding to Zn are shown in Figure 6B. The two peaks located at 1020.7 and 1044.1 eV correspond to the binding energies of core levels 2p3/2 and 2p1/2, respectively. The observed energy difference is found to be 23.4 eV, which is matching with the previous report.44 The valence states of cobalt in the system are shown in Figure 6C, and the Co peaks are to be fitted into two Gaussian peaks having different binding energy positions. It clearly shows that Co 2p peaks are located at 779.2 eV for 2p3/2 and 794.4 eV for 2p1/2 electrons. It has been reported that the value of the difference in binding energies is 15.2 eV for Co 2p ions.45 E



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Figure 6. XPS spectra of (A) Zn0.90Co0.10O wide survey, (B) Zn 2p, (C) Co 2p, and (D) O 1s of Co-doped ZnO NPs.

The oxygen 1s spectrum in Figure 6D shows a single peak that can be fitted into three Gaussian peaks having different binding energy positions. The peak located at 529.6 eV in the O 1s spectrum is attributed to O2− ions in the Zn−O bonding of the wurtzite hexagonal ZnO structure,46 and the other two peaks at 531.1 and 532.3 eV can be ascribed to the Zn−OH in ZnO.47 This indicated that there are more oxygen vacancies in the samples.42 In the XPS spectrum of Zn0.90Co0.10O, the peaks of Co 2p3/2 and Co 2p1/2 appeared much closer to that of Co2+ ions. This reveals that Co2+ ions are present in the ZnO lattice. Therefore, we conclude that the doping of Co into ZnO increases the hole carrier concentration of the material. Free carriers play an important role in establishing the ferromagnetism in ZnO-based DMS.48 Raman spectroscopy is a versatile technique for detecting dopant incorporation, defects, and lattice disorder in a host lattice.49 In the case of doped nanoparticles, the surface is modified due to charge transfer between the host and the dopant, which changes the optical Raman spectra.50 Figure 7 shows micro-Raman spectra of the NPs at RT in the range of 128−800 cm−1. For the undoped ZnO, the sharpest and strongest peak at 439 cm−1 can be assigned to the highfrequency branch of the E2H mode of ZnO. The peaks at 158, 331, and 580 cm−1 are assigned to the first- and second-order vibration modes of ZnO.51 The blue shift is obtained, and the corresponding peak values are 519, 524, and 528 cm−1 for doped ZnO samples, respectively. In comparison of Raman spectra of undoped and doped ZnO, there is a remarkable vibration centered at near 530 cm−1, which can be assigned to the local vibration mode related to Co that is bound with the

Figure 7. Micro-Raman spectra of (A) ZnO, (B) Zn0.95Co0.05O, (C) Zn0.90Co0.10O, and (D) Zn0.85Co0.15O NPs.

donor defects. These donor defects can be doubly occupied oxygen vacancies, and zinc interstitials, which are the typical shallow donor in ZnO.52−54 Magnetization versus magnetic field (M−H) curves for Zn1−xCoxO (x = 0, 0.05, 0.10, and 0.15) samples, measured at 300 K with a maximum applied field of ±1.5 T, are used to identify the effect of Co doping on magnetic properties of ZnO NPs. Figure 8 shows that all the samples exhibited ferromagnetic behavior except the pure one with x = 0. The existence of diamagnetic behavior for pure ZnO is also referred F



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source is the magnetism of micro-Co clusters, which is another phase of cobalt oxide or zinc cobalt spinel. CoO2, Co3O4, and ZnCo3O4 are antiferromagnetic in nature,58 and hence, they cannot be the source of ferromagnetism in our samples. Therefore, the possibility for the observed magnetism is due to the Co clusters. However, the corresponding signal of the Co cluster could not be observed by XRD as mentioned above. Sundaresan et al. observed that the metal oxide NPs, such as CeO2, Al2O3, ZnO, In2O3, and SnO2, exhibit room-temperature ferromagnetism presumably due to the exchange interaction between localized spin moments resulting from the oxygen vacancies at the surface of nanoparticles.59 Barick et al. reported the defect mediated ferromagnetism in self-aggregated ZnO and transition-metal-doped ZnO nanostructures.60 There is also an emerging consensus that ferromagnetic behavior in transition-metal-doped ZnO is correlated with defects such as oxygen or zinc vacancies.52,61−64 In light of the above literature, the existence of weak ferromagnetism is explained as follows. From the EDS spectra of Co-doped ZnO NPs, the atomic percentage of oxygen is found to be increased with increasing cobalt concentration and also the atomic percentage of zinc is found to be decreased. From the photoluminescence spectra of the pure and Codoped ZnO, the wavelengths for the blue and green emissions are increased as compared to that of the pure ZnO NPs. This shows that there are an increased number of oxygen vacancies and zinc vacancies in the Co-doped ZnO NPs. Moreover, XPS peaks located at 529.6, 531.1, and 532.3 eV can be ascribed to the oxygen vacancies in the cobalt-doped ZnO NPs. In addition, the micro-Raman spectra reveal that the donor defects can be doubly occupied oxygen vacancies and zinc interstitials in ZnO. With the support of our experimental results, we conclude that oxygen vacancies and zinc interstitials are the main reasons for ferromagnetism in the Co-doped ZnO NPs. The qualitative literature suggests that the origin of RTFM in these systems is strongly dependent on the synthesis route; however, regarding the general understanding of RTFM in Co-

Figure 8. Magnetic behavior of (A) ZnO, (B) Zn0.95Co0.05O, (C) Zn0.90Co0.10O, and (D) Zn0.85Co0.15O nanoparticles.

to in the literature.55 There is an increase in magnetic moment (emu/g) with an increase in concentration of cobalt, and the saturation magnetization is not observed up to the maximum applied field of 15 kOe, as shown in Figure 8. The magnetization values observed at 15 kOe are 0.031, 0.069, and 0.092 emu/g for Zn0.95Co0.05O, Zn0.90Co0.10O, and Zn0.85Co0.15O NPs, respectively. The changes in the M−H loop can be explained on the basis of the magnetic contribution from the orientation of the strong exchange interaction in the d−d couple with cobalt ions. An increase of Co doping concentration increases the linear behavior of the M−H loop. It indicates that the cobalt−cobalt superexchange dominates at higher Co doping concentrations at room temperature.56 In the Zn0.95Co0.05O and Zn0.90Co0.10O samples, there is no indication for a strong ferromagnetic coupling between the transitionmetal cations (Co2+) at this microscopic level. Hence, one can rule out the possibility of strong ferromagnetic behavior of Codoped ZnO, and it can result in weak ferromagnetic or superparamagnetic behavior.57 However, another possible

Table 5. Overview of the Reported Experimental Results Considering the Magnetic Behavior of Co-Doped ZnO Nanoparticles preparation method sol−gel method simple hydrolysis and condensation method autocombustion method sol−gel-based chemical method hydrothermal polymerizable precursor method hydrothermal method simple solution route coprecipitation method hydrothermal method solvothermal technique facile biomineralization process solvothermal method sol−gel technique refluxing chemical process

reason for magnetization RTFM due to secondary phase formation of Co metallic cluster, or metal oxide CoO and oxygen vacancies.65 The maximum attainable magnetization decreases for high Co concentrations, most likely because of antiferromagnetic coupling of Co atoms occupying neighboring Zn lattice sites.66 The localization of carriers at low temperature (below 50 K) reduces the dopant−donor hybridization and weakens the FM interaction between the magnetic ions.67 Ferromagnetism depending upon Co concentration and the oxygen vacancies.68 A small portion of Co(II) species are contributing to the ferromagnetism.69 The specific ferromagnetism seems to change with the particle size and calcination temperature at 873 K.70 Co impurities contribute to the magnetism in ZnO:Co particles.71 Ferromagnetism is dependent on the temperature.72 Structural defects, which depend sensitively on atmosphere and annealing temperature.73 Co3+ enrichment on the particle surface and the coexistence of Co2+/Co3+ in the particle, which may lead to the superexchange or a double-exchange mechanism between the Co2+ and Co3+.74 Inconsistent with a scenario involving FM from the segregation of secondary phases, such as metallic Co, suggesting that the observed high-temperature FM is an intrinsic property.75 The chains of silk-fibroin peptide could promote more Co ions to enter into the ZnO structure, leading to the enhancement of ferromagnetic property of Co-doped ZnO.76 Ferromagnetic characteristics at room temperature, and the detected ferromagnetism could arise from the homogeneous doping of Co2+ into the ZnO crystal lattice.77 FM caused by the formation of defects, such as oxygen vacancy and Zn interstitials.78 FM is due to the presence of defects in the host ZnO lattice.79 G



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Figure 9. EPR spectra of (A) Zn0.95Co0.05O, (B) Zn0.90Co0.10O, and (C) Zn0.85Co0.15O NPs measured at RT.

This leads to the evolutions of coexisting components of oxygen vacancies and Zn interstitials. Moreover, from Raman spectra of doped ZnO, there is a remarkable vibration centered near 530 cm−1, which can be assigned to the local vibration mode related to Co that is bound with the donor defects. These donor defects can be assigned to doubly occupied oxygen vacancies and zinc interstitials.

doped ZnO NPs, large numbers of literatures have been reported and a number of theories have been proposed. Table 5 shows the existence of RTFM in the Co-doped ZnO NPs synthesized by the different methods. The room-temperature EPR spectra of the Co-doped ZnO NPs are shown in Figure 9. Relative changes are found in the signal line width (increasing trend) and intensity (decreasing trend) with increasing Co content. In these types of compounds, the cobalt atoms substitute with the zinc atoms and the neutral charge state is Co2+ (a 3d7 configuration).68 For the Zn0.95Co0.05O, Zn0.90Co0.10O, and Zn0.85Co0.15O samples, the spectra are composed of an intense line at ∼2990 G with the value of the g factor of about 2.24. A small shift in the broad signal of the Co-doped ZnO NPs toward higher magnetic fields with increased doping concentration clearly indicates that the dopant atoms are influential and affect the local disorder on the ferromagnetic exchange interaction. The Zn interstitial and oxygen vacancies are the predominant defects in the doped samples. The EPR signals can be assigned to oxygen vacancies and Zn interstitials, from low to high field with the corresponding values of the g factor.12,80 Among them, Zn interstitials are one of the predominant defects in the doped ZnO nanoparticles. This is in accord with the above analysis of the preferential formation of Zn interstitials since the Zn0.95Co0.05O, Zn0.90Co0.10O, and Zn0.85Co0.15O NPs exhibit dominant blue emissions at 455, 464, and 480 nm, respectively. Such a Zn interstitial-dominant EPR signal verifies one of the origins of RTFM in the Co-doped samples. The PL peaks observed at 520, 522, and 540 cm−1 for the respective Co-doped ZnO samples correspond to the green emissions due to the existing oxygen vacancies. Both green and blue emissions are relatively strong with suitable excitations and also with the stabilities of oxygen vacancies and Zn interstitials.

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CONCLUSION Zn1−xCoxO NPs were successfully prepared by a coprecipitation method. The hexagonal (wurtzite) crystalline structure was identified for pure and doped ZnO through XRD analysis and confirmed that Co2+ ions were incorporated into the lattice site of Zn2+ ions in the ZnO matrix and further confirms that there is no existence of a CoO (Co cluster) peak. The presence of all functional groups was observed from FTIR analysis, and this study confirmed that Co2+ is occupying a Zn2+ position. Compositional analysis (EDS) ascertained the existence of Zn, O, and Co in the synthesized materials. The UV near band emission wavelength was found to be red-shifted for Co-doped ZnO compared to pure ZnO. From vibrating sample magnetometer study for pure and doped ZnO NPs, pure ZnO exhibits diamagnetic behavior, whereas Co doping in ZnO makes the samples ferromagnetic at room temperature, and magnetization values are found to be increased with the increase of Co concentration in ZnO. Though several reasons surfaced for the change of magnetic behavior in the literature, we believe that, in our case, the doping in ZnO creates the oxygen vacancies at interstitial sites of Zn and also zinc interstitials play an important role for the change from diamagnetic to ferromagnetic behavior, which is evidenced from our experimental results with the support of the literature. H



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ASSOCIATED CONTENT

S Supporting Information *

Co-doped ZnO NPs ferromagnetic behavior loop and inset picture represented by M (emg/g) vs Co %. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91-04565-225206. Fax: +91-04565-225202. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G. Ravi and G. Vijayaprasath have gratefully acknowledged the UGC (ref. No. F. 41-933/2012 (SR)), India, for providing financial support to carry out this work. The authors acknowledge V. Shanmugam, CECRI, Karaikudi, for valuable suggestions to interpret EPR results.

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