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Evidence of Ferromagnetic Signal Enhancement in Fe and Co CoDoped ZnO Nanoparticles by Increasing Superficial Co Content 3+

Jailes Joaquin Beltran Jimenez, C.A. Barrero, and Alex Punnoose J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp501933k • Publication Date (Web): 20 May 2014 Downloaded from http://pubs.acs.org on May 25, 2014

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The Journal of Physical Chemistry

Evidence of Ferromagnetic Signal Enhancement in Fe and Co co-Doped ZnO Nanoparticles by Increasing Superficial Co3+ Content

J.J. Beltrán1, 2, C. A. Barrero1 and A. Punnoose2* 1

Grupo de Estado Sólido, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia UdeA, Calle 70 No 52-21, Medellín, Colombia. 2

Department of Physics, Boise State University, Boise, Idaho 83725-1570, USA.

_________________________________________________________________________ * Corresponding Author. Department of Physics, Boise State University, 1910 University Drive, Boise, ID 83725-1570, (208) 426-2268. E-mail: [email protected].

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ABSTRACT In spite of the various theoretical and experimental efforts performed to understand the origin of ferromagnetism in Fe and Co co-doped ZnO, there are still serious controversies in the reported data. While theoretical studies predicted the relative spin alignment and location of Co2+ and Fe2+ as the main source of magnetism, experimental studies have reported Co2+ and superficial Fe3+. In this work, we performed a careful experimental study on Zn1-2xFexCoxO (x = 0, 0.01, 0.03 and 0.05) nanoparticles prepared by a sol-gel method and have found a new interesting results. We detected only Fe3+ ions located in tetrahedral-core and pseudo-octahedral-surface sites. The Co ions displayed 2+ and 3+ oxidation states, with Co2+ ions in high spin state located mostly in the tetrahedralcore sites, while Co3+ in low spin states located presumably in pseudo-octahedral-surface sites. We detected isolated Fe3+ ions and weakly ferromagnetic coupled Co2+ ions. The most important finding is that the saturation magnetization (Ms) did not depend on the magnetic interactions involving the high spin Co2+ or Fe3+; but Ms and Co3+ concentration increased systematically with x indicating that multi-valent ionic states may be playing a crucial role in the observed ferromagnetism. .

Keywords: Room temperature ferromagnetism, XPS, EPR, Mössbauer spectroscopy.


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I. INTRODUCTION ZnO is a wide band-gap semiconductor that has gained a great deal of attention because of its potential applications in nano-electronics, especially spintronic devices, due to the ability to change its optical and magnetic properties by doping with suitable ions.1 Recently, the presence of multi-valent ions, in the ZnO lattice has been considered as an important alternative strategy to achieve ferromagnetism with a Curie temperature (Tc) above room temperature (RT).2 In fact, when ZnO is co-doped with transition metal (TM) impurity ions, the different d electrons of these ions can tailor the position and occupancy of the Fermi energy and its magnetic behavior.3 Because of these reasons, intense research is focused recently on understanding the magnetic interactions in ZnO co-doped with different TM ions such as Co and Ga,4 Co and In,5 Co and Ni,6 Co and Cu,7–10 Co and Cr,11 Co and Mn,12,13 Mn and Ni,14 Fe and Cu,15–19 Fe and Al20 and Fe and Ni.21 Some of these studies have showed transition from paramagnetic (PM) to ferromagnetic (FM) state at RT when additional dopant was introduced into a single TM doped ZnO.4,5,7 Other studies have observed an enhancement in the FM properties in co-doped samples in comparison to doping with only one cation;6,8–11,19,20 while other studies have shown PM behavior,14 or reduction in the FM signal with co-doping16 or that the presence of impurity phases is presumably causing the observed RT ferromagnetism.17,18 These studies have also documented co-doping as a technique for feasible and potential means to introduce additional carrier density8,10 or large number of effective defects into the host material,12,13 which are essential for the optimization and stabilization of RTFM ground state,4–6 while other have reported a reduction in the carrier concentration with the insertion of an additional element in TM doped ZnO.19,22 Another interesting ZnO system is that co-doped with Fe and Co, which has been investigated as nanoparticles23,24 and also as thin films,25–27 3 ACS Paragon Plus Environment


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with both forms showing RT ferromagnetism, although a good agreement on the physical origin of the magnetism is lacking.

Theoretical studies of Fe and Co co-doped ZnO nanowires by Ghosh et al.28have shown that spin alignments of TM ions depended on their location so that FM, ferrimagnetic (FIM) or antiferromagnetic (AFM) state can occur. The exchange mechanisms can be RKKY or direct exchange interactions involving carriers. Park and Min15 mentioned that the double exchange mechanism is not possible in bulk Zn1x(FeCo)xO

because there is no indication of charge transfer between Fe and Co, and so only

2+ oxidations states are possible for both TM ions. They also invoked a RKKY type of exchange. The theoretical studies by Karmakar et al.23 also agreed with this, that the oxidation states of both Fe and Co are expected to be 2+ in bulk ZnO, and that Fe and Co can interact. However, in their experimental studies they do not detect the presence of Fe2+, but only Fe3+, suggesting that this state oxidation is possible for Fe ions located at the surface of the nanoparticles. The exact mechanism of magnetic interaction depends upon the interaction between Fe3+ and Co2+ and the position of defects relative to them. FIM state is expected if the spins are antiparallel and superexchange mechanism plays role, and FM if the spins are parallel and double exchange is the mechanism.23 Dinesha et al.24 proposed that the observed ferromagnetism in their Fe and Co co-doped ZnO nanoparticles are mainly due to an RKKY exchange involving free delocalized carriers and the localized d spins on the Co2+only, and that Fe3+ do not contribute to this signal. In our previous work we reported that the simultaneous presence of Fe and Co greatly improves the FM signal of ZnO nanoparticles in comparison to the presence of only one of these cations.29 Fe2+ could not be detected and Fe3+ ions were not participating in the magnetic ordering. This result 4 ACS Paragon Plus Environment

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was interpreted in a very different way, in comparison to previous studies, by assuming a charge transfer involving Fe3+ and Co2+ mixed valence ions providing a reservoir to facilitate stronger RT ferromagnetism in this co-doped ZnO sample. Perhaps, it is interesting to mention that in none of these mentioned works, there is neither experimental evidence nor first principle calculations that take into account the presence of Co3+. From this literature review we noticed that the reported experimental and theoretical studies on Fe and Co co-doped ZnO are still controversial, and therefore it is of great importance to study some key experimental issues as most completely as possible, such as: (i) demonstration of the absence of both spurious phases and clustering of dopants, (ii) clear determination of the preferable sites, high or low spin character, and oxidation states of the TM ions, (iii) preferable location of the TM ions, either at the surface, at the interior or in the whole nanoparticles, (iv) identification and characterization of defects; (v) proper characterization of the electronic, optical, crystallographic, and magnetic properties, and their possible relations; and (vi) find out which of the TM ions are involved in the magnetic ordering, either both ions, or only one, or none of them. The present work is aimed shed light into these issues by performing a careful quantitative experimental investigation on the nanoparticles and therefore contributing to the understanding on the origin of the ferromagnetism in these samples.

II. EXPERIMENTAL DETAILS Nanocrystalline powders of Zn1-2xFexCoxO (x = 0, 0.01, 0.03 and 0.05) were prepared by the polymeric precursor method based on the modified Pechini process.30 Required amounts of Zn(NO3)2·6H2O, Co(NO3)2·6H2O, Fe(NO3)3·9H2O and citric acid (CA), with a {[Zn]+[Fe]+[Co]}:[CA] molar relation of 1:3, were dissolved in 200 mL of 5 ACS Paragon Plus Environment


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distilled water, by keeping the beaker on hot plate with a magnetic stirrer. The mixture was gradually heated at 70 °C under strong stirring and then NH3 aqueous solution was introduced dropwise to maintain a pH of 5 and afterwards ethylene glycol (EG) with EG/CA ratio of 5 was added to the solution. This mixture was kept at a temperature between 90 and 95 °C under constant stirring, to evaporate water and then the temperature was raised between 100 and 110 °C, to promote the polyesterification reaction. In this last step the solution became more viscous and finally turned into wet gel. The gel thus formed was dried at 200 °C in air for 4 h to get rid of volatile components, resulting in a black solid mass. In order to investigate the effect of annealing time on the magnetic, structural and optical properties of Fe-Co co-doped ZnO nanoparticles, this black powder precursor was divided into two portions and annealed in a tubular furnace at 550 °C (heating rate: 5 °C/min) in air atmosphere for 1 h and 3h, to obtain two sets of samples for this work. To obtain more insight into possible Fe and Co impurity phases which might have formed under these synthesis conditions, ZnFe2O4 and Co3O4 were also prepared following identical synthesis procedures.

X-ray diffraction (XRD) patterns were collected using a Phillips X’Pert X-ray diffractometer with a Cu-Kα source (λ= 1.5406Å) in Bragg-Brentano geometry. The XRD patterns were fitted using the Rietveld method to obtain crystal lattice parameters and average crystallite size. RT optical absorption spectra in the ultraviolet and visible light wavelength were measured using an Evolution 600 UV-Vis spectrophotometer (Thermo Scientific) fitted with an integrating sphere diffuse reflectance accessory. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Physical Electronics VersaProbe II Scanning Microprobe. This system uses a focused 6 ACS Paragon Plus Environment

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monochromatic Al Kα X-ray (1486.7 eV) source and a spherical section analyzer at a base pressure of 12h) accumulation time on the samples with the highest Fe-Co concentration, enhancing the detection limit considerably. As shown in Figure S4, Zn0.90Fe0.05Co0.05O sample annealed at 550 oC for 1h did not reveal any phase other than ZnO; in contrast, XRD pattern of the sample with same nominal concentration, but annealed for 3h showed 3 additional peaks located at 44.5, 59.1 and 64.8 degrees indicating a phase concentration of ~0.6 %. These peaks matches well either with diffraction planes (440), (511) and (440) of ZnCo2O4 and/or Co3O4.42,59 Considering the fact that the Zn 2p3/2 peak position in the XPS results of this sample showed a slight shift to lower BE, this phase is attributed to ZnCo2O4. The more pronounced increase in the EPR signal intensity, area and Hr with decreasing temperature in Zn0.90Fe0.05Co0.05O sample annealed for 3h (see Supporting Information, Figure S3) in comparison to the same sample annealed for 1h may be related to the presence of Zn(Co1-xFex)2O4 nanoparticles as a weak impurity phase, taking also into account the RT 57Fe Mössbauer results also (see below). The integral intensity of the EPR


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signal corresponding to Co2+ varied with temperature following Curie law, with θ values of -2.61 and -1.08 for Zn0.90Fe0.05Co0.05O sample annealed for 1 and 3h, respectively. The lower θ value for the sample annealed for 3h can also be attributed to the formation of Fe doped ZnCo2O4 as mentioned above.

E. RT 57Fe Mössbauer spectroscopy: Typical RT 57Fe Mössbauer spectra of Zn12xFexCoxO

samples (x =0.03 and 0.05) annealed at 550 °C for1 and 3 h are shown in Figure

9 and the derived hyperfine parameters for these samples along with those for reference ZnFe2O4 are summarized in Table 2. The spectrum of ZnFe2O4 displayed a well-defined doublet whose hyperfine parameters are in good agreement with those reported in literature.60 Mössbauer spectra of all Fe-Co co-doped ZnO samples consisted of only doublets, and no sextets could be observed within the detection limit of the technique, thus suggesting that at RT the Fe3+ ions do not order magnetically. Moreover, these results also suggest that the presence of FM impurities such as metallic iron, FeCo clusters, iron oxides (Fe2O3, and Fe3O4), CoFe2O4 or ZnFe2O4 nanoparticles60,61 can be ruled out. The spectra for these samples were properly fitted with two doublets. The necessity of a second doublet is clearly seen by the wide shape of the spectra at the tails located at the lower velocities. The introduction of a second doublet improves the fitting considerably, both visually and statistically. In the spectrum of Zn0.90Fe0.05Co0.05O sample annealed at 550 during 3h, the introduction of a third doublet was necessary, to improve the fitting. The hyperfine parameters (δ=0.40 mm/s, ∆=0.58 mm/s and A= 6 %) of this additional doublet 3 (D3) matches well with Fe3+ in octahedral environment. These values lie between the quadrupole splitting values reported for CoFe2O4 and ZnFe2O4 nanoparticles60,61 Therefore, we have



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attributed this doublet D3 to Fe3+ in . From Figure 9 it can be seen

that the spectra of all the other samples exhibit considerable symmetry, which suggest that impurity phases arising from Fe ions are not present in these samples. From the analysis of the Mössbauer spectra, we found that doublet (D1) shows center shifts, δ, in the range of 0.25 mm/s

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