microwave-assisted nonaqueous sol−gel chemistry for highly concentrated zno-based magnetic...

Published: January 18, 2011

r 2011 American Chemical Society 1484 dx.doi.org/10.1021/jp108050w | J. Phys. Chem. C 2011, 115, 1484–1495

ARTICLE

pubs.acs.org/JPCC

Microwave-Assisted Nonaqueous Sol-Gel Chemistry for HighlyConcentrated ZnO-Based Magnetic Semiconductor NanocrystalsIdalia Bilecka,† Li Luo,† Igor Djerdj,‡ Marta D. Rossell,† Marko Jagodi�c,§,|| Zvonko Jagli�ci�c,*,§,^

Yuji Masubuchi,# Shinichi Kikkawa,# and Markus Niederberger*,†

†Laboratory for Multifunctional Materials, Department of Materials, ETH Z€urich, Wolfgang-Pauli Strasse 10, 8093 Z€urich, Switzerland‡Ru{er Bo�skovi�c Institute, Bijeni�cka 54, 10000 Zagreb, Croatia§Institute of Mathematics, Physics and Mechanics, Jadranska 19, 1000 Ljubljana, Slovenia

)EN-FIST Centre of Excellence, Dunajska 156, 1000 Ljubljana, Slovenia^Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova 2, 1000 Ljubljana, Slovenia#Faculty of Engineering, Hokkaido University, Sapporo 060-8268, Japan

bS Supporting Information

ABSTRACT: Various transition metal (TM) doped zinc oxidenanoparticles with the composition TMxZn1-xO (TM = V, Mn,Fe, Co, and Ni; x = 0.01-0.3) were prepared by a microwave-assisted nonaqueous sol-gel route in benzyl alcohol within afewminutes. The high doping levels in the range 20-30 atom%achieved for Co and Fe provide a promising opportunity tostudy the magnetic properties of such potential diluted magneticsemiconductors. However, only Fe0.2Zn0.8O was ferromagneticat room temperature. The Co-doped sample showed Curie-Weiss behavior up to a doping level of 30 atom %. According toX-ray absorption fine structure (XAFS) measurements, at high doping levels the Fe-doped ZnO samples contain an increasingfraction of Fe3þ ions (in addition to Fe2þ), whereas Co is predominantly in the oxidation state of þ2. Clustering of Fe ions intoamorphous ferromagnetic Fe3O4 within the ZnO host and the magnetic interactions between the Fe3O4 regions is a possibleexplanation for the ferromagnetic properties.

1. INTRODUCTION

The study of diluted magnetic semiconductors (DMS) is afascinating, but also controversially discussed, research area.1

The expectation that such materials can be applied in spin-tronics,2-5 greatly improving traditional electronic and photonicdevices, represents a strongmotivation to study such systems. Onthe other hand, there is a major scientific problem of explainingand understanding the contradicting magnetic results obtainedfrom compositionally similar materials.6 Transition metal dopedzinc oxide is an example of such a diluted magnetic semicon-ductor, which has attracted a lot of scientific interest, but hasalso resulted in inconsistent data with respect to the magneticproperties.7-9 Especially in cases of low doping levels, the ferro-magnetism observed is many orders of magnitude smaller thanthe expected spin-only saturationmoments, and thus it is difficultto exclude the influence of traces of impurities.8 Therefore, oneway to approach a solution to this problem is to synthesizeconcentrated magnetic semiconductors, i.e., to reach dopinglevels higher than 10 atom%. Although such high concentrationsmight better be referred to as “solid solution formation”, wewill talk about “doping” throughout the paper, because this istypically the term used in the literature.

In the following, we will focus our discussion on nanoparti-culate materials rather than thin films,5,6,10,11 because we presenthere the synthesis of transition metal doped ZnO nanoparticles.

To provide homogeneous distribution of the dopant ionsin substitutional sites without segregation and to minimize theformation of clusters or secondary phases, low-temperatureliquid-phase routes represent the most promising synthesisstrategy. Early examples of extended solid solution synthesis ofwurtzite ZnO-MO (M = Mg, Co, Ni) date back to the work ofJayaram and Rani.12 Although the authors call the process asoft chemical route, postsynthetic heat treatment in the range300-800 �Cwas required. According to X-ray diffraction (XRD),single-phase solid solutions were obtained at 500 �C for “doping”levels up to 25mol %MgO, 20mol %NiO, and 60mol %CoO.12

Liquid-phase syntheses of transition metal doped ZnO nanopar-ticles that directly lead to crystalline nanomaterials withoutpostsynthetic annealing have also been reported.13-16 Especiallythe synthesis in organic solvents seems to be particularly suitable

Received: August 25, 2010Revised: December 17, 2010

1485 dx.doi.org/10.1021/jp108050w |J. Phys. Chem. C 2011, 115, 1484–1495

The Journal of Physical Chemistry C ARTICLE

for the low-temperature synthesis of doped ZnO nanoparticlesand nanostructures.17-24 A more detailed overview of theseapproaches can be found elsewhere.25,26

Wet chemical methods clearly offer the highest flexibility interms of controlling particle size, shape, and surface chemistry,while providing high compositional homogeneity-all crucialparameters in determining the magnetic properties. The combi-nation of liquid-phase synthesis with microwave chemistryrepresents a versatile tool to control the morphological proper-ties of nanomaterials.27 Recently, we reported that microwave-assisted nonaqueous sol-gel chemistry can be used for theefficient synthesis of nanocrystalline metal oxides, includingZnO, within just a few minutes.28,29

In this work we show that microwave chemistry is suitable forthe high concentration doping of ZnO. In this context it isinteresting to mention that analogous approaches in benzylalcohol, but using conventional heating in an autoclave, alwaysresulted in doping levels lower than 3 atom %.22,30 With theuse of microwave-assisted liquid-phase synthesis approaches, notonly a large variety of dopants (V, Mn, Fe, Co, Ni) can beincorporated in the ZnO lattice, but also high concentrations ofdopants up to 30 atom % can be achieved in selected cases. Thedopants were chosen on the basis of reports in the literature thatproposed all of them induce room-temperature ferromagnetismin ZnO.6,8,10,25,31-35 The crystal structure, the morphology, themagnetic properties, and the local structure around the transi-tion metal ions are described on the basis of data from X-raydiffraction (XRD) combined with Rietveld refinement, trans-mission electron microscopy (TEM), energy-dispersive X-ray(EDX) spectroscopy, and SQUID magnetometry. In addition, adetailed study using X-ray absorption spectroscopy (XAS)makesit possible to correlate the atomic structure with the magneticproperties of Co2þ- and Fe2þ-doped ZnO at different dopantconcentrations. In spite of the high doping levels, the Co-dopedZnO does not exhibit ferromagnetism even at a low temperatureof 2 K, whereas in the case of Fe-doped ZnO room-temperatureferromagnetism appears.

The paper is organized as follows: In section 2, we discuss thesynthesis, structural, and morphological characterization as wellas the magnetic properties of ZnO nanoparticles doped withjust one concentration of V (20 atom %), Mn (20 atom %), Fe(20 atom %), Co (20 atom %), and Ni (5 atom %). The givenconcentrations in parentheses refer to the molar concentrationused in the initial reaction mixture. In section 3, we present adetailed study of the Co- and Fe-doped ZnO nanoparticles withvarying dopant concentrations from 2 to 45 atom % togetherwith their characterization by XRD/Rietveld, UV-vis, and XAS.In addition, the magnetic properties are presented and discussedin the context of the data obtained from the structural analysis.As a possible explanation of the observed magnetic properties inthe highly Fe-doped samples, we propose a simple physicalpicture based on magnetically interacting amorphous Fe3O4

regions inside the ZnO host.

2. TRANSITION METAL (TM) DOPED ZnO (TM = V, Mn,Fe, Co, and Ni)

2.1. Synthesis. The experimental parameters for the micro-wave-assisted synthesis of pure and transition metal doped ZnOnanoparticles are summarized in Figure 1a. The initial dopantconcentration was varied in the range 1-30 mol %. The colorchange of nanoparticle dispersions in benzyl alcohol (Figure 1b)

from white (pure ZnO) to black (V:ZnO), blue (Co:ZnO),brown (Fe:ZnO), yellow-green (Ni:ZnO), and orange (Mn:ZnO) gives a first indication of the successful incorporation ofthe dopants in the ZnO lattice.2.2. Characterization. Figure 2 shows the XRD patterns

of (a) ZnO, (b) VxZn1-xO, (c) CoxZn1-xO, (d) FexZn1-xO,(e) NixZn1-xO, and (f) MnxZn1-xO nanoparticles with initialdopant concentrations of x = 0.2 for V, Co, Fe, and Mn andx = 0.05 for Ni. Independent of the dopant, all the diffractionpatterns are in good agreement with the reference pattern(ICDD PDF No. 36-1451) of ZnO with the wurtzite structurein the hexagonal space group P63mc (186). The lack of anyadditional reflections proves that all the samples are single phasewithout any crystalline byproduct within the detection limit ofXRD. In order to quantify the effects of the dopant on the lattice,a phase analysis using the Rietveld method has been performed.The lattice parameters were calculated using the FullProf pro-gram.36 Table 1 presents the refined parameters as a functionof the dopant. The obvious line broadening points to crystallite

Figure 1. (a) Reaction scheme with precursors, solvent, temperature,and irradiation time (MW, microwave) and compositions of theproducts. (b) Photographs of nanoparticle dispersions in benzyl alcoholwith different colors depending on the composition.

Figure 2. X-ray diffraction patterns of (a) ZnO, (b) V0.2Zn0.8O, (c)Co0.2Zn0.8O, (d) Fe0.2Zn0.8O, (e) Ni0.05Zn0.95O, and (f) Mn0.2Zn0.8O.The stoichiometry (i.e., dopant concentration) is based on the initialconcentrations in the reaction solution.



sizes on the nanoscale. In the Rietveld refinement we applied aspherical harmonics model, which assumed slight anisotropy inthe line broadening and which yielded a better least-squares fit.The directional averaged crystallite sizes (Table 1) were esti-mated to be around 20 nm for pure ZnO and 10 nm for all thedoped materials, indicating that TM doping inhibited the growthof the ZnO nanoparticles.Figure 3 displays TEM images for the different nanoparticles

corresponding to (a) ZnO, (b) VxZn1-xO, (c) CoxZn1-xO, (d)FexZn1-xO, (e) NixZn1-xO, and (f) MnxZn1-xO nanoparticleswith initial dopant concentrations of x = 0.2 for V, Co, Fe, andMn and x = 0.05 for Ni. The morphology of the nanoparticles israther irregular, ranging from nearly spherical in the case of pureZnO to rather ill-defined for Co, Ni, and Mn and triangular for Vand Fe doping. The average particle sizes were found to be ca. 20nm for undoped ZnO and 10 nm for the doped compounds.These values are in good agreement with the XRD data. Theobservation that doping alters the crystal size and shape can beexplained by the fact that often the surface energy for differentexposed crystal planes is dependent on the dopant and itsconcentration.37

Selected high resolution TEM (HRTEM) images of Co-doped ZnO with 20 atom % nominal doping concentration areshown in Figure S1 (Supporting Information). The particles are

characterized by irregular shapes in the range 10-20 nm. Thewell-developed lattice fringes underline the high crystallinity ofthe particles. Energy-filtered transmission electron microscopy(EFTEM) elemental maps for the same sample give evidencethat the dopant is homogeneously distributed in the sample(Figure S2, Supporting Information). However, since these mapsare two-dimensional projections of a three-dimensional volume,it is difficult to obtain information about the location of thedopant, i.e., whether the dopant is located inside the particles orjust on their surface.EDX spectra confirmed the presence of the doping elements in

the sample. The EDX results are summarized in Table 1. Thefinal concentration of the dopant is generally smaller than in theinitial reaction solution, although the discrepancy is not the samefor all the elements. Whereas there is a good match between thenominal and final Ni concentrations and a reasonable agreementfor V, Co, and to some extent Fe, the doping withMn seems to beless efficient. Quantitative analysis, averaged over three differentspots, gave concentrations (in atom %) of 16% for V (initial20%), 15% for Co (initial 20%), 13% for Fe (initial 20%), and 5%for Ni (initial 5%). The concentration of Mn was found to beonly 6% (initial 20%), revealing that incorporation of Mn ionsoccurs only to a significantly lower extent.2.3. Magnetic Properties. Figure 4 shows the temperature-

dependent susceptibilities of VxZn1-xO, CoxZn1-xO, FexZn1-xO,NixZn1-xO, and MnxZn1-xO nanoparticles with initial dopantconcentrations of x = 0.2 for V, Co, Fe, and Mn and x = 0.05for Ni. All samples except the Fe-doped ZnO show Curie orCurie-Weiss-like behavior from room temperature down to200 K or lower. The susceptibilities were measured in zero-field-cooled (ZFC) and field-cooled (FC) regimes. Only negligiblesplitting between ZFC and FC curves was observed below 20 K.Inverse susceptibilities (χ-1) of FC runs are shown in the inset ofFigure 4. The Ni- and V-doped samples behave paramagneticallyin the whole temperature range. The χ-1(T) curves for thesesamples can be fitted by lines intercepting 0 K. The Co- andMn-doped samples show Curie-Weiss magnetic behavior. TheCurie constants (C) were estimated from the slope of the linesthat best fit themeasured data for temperatures above 200 K. Theobtained C and Curie temperature θ values are listed in Table 2.Negative θ values indicate an antiferromagnetic (AFM) couplingbetween the TM magnetic moments.

Table 1. Chemical Composition According to the InitialConcentration, Dopant Concentration Measured by EDX,Crystallite Sizes fromRietveld Refinement, Lattice Constants,and Unit Cell Volume

Rietveld refinement

sample

EDX concn

(atom %)

size

(nm) a (Å) c (Å) vol (Å3)

ZnO - 19 3.2502(3) 5.2075(5) 47.641(1)

Co0.2Zn0.8O 15 9 3.251(2) 5.211(5) 47.676(3)

Fe0.2Zn0.8O 13 9 3.261(1) 5.201(1) 47.903(3)

Mn0.2Zn0.8O 6 7 3.255(7) 5.221(5) 47.917(7)

Ni0.05Zn0.95O 5 12 3.2511(2) 5.2081(6) 47.673(1)

V0.2Zn0.8O 16 12 3.252(2) 5.193(1) 47.486(2)

Figure 3. TEM overview images of (a) ZnO, (b) V0.2Zn0.8O, (c)Co0.2Zn0.8O, (d) Fe0.2Zn0.8O, (e) Ni0.05Zn0.95O, and (f) Mn0.2Zn0.8O.The stoichiometry (i.e., dopant concentration) is based on the initialconcentrations in the reaction solution.

Figure 4. Temperature dependence of magnetic susceptibilities χ andinverse susceptibilities χ-1 (inset) measured in a dc field H = 1000 Oe.In the main graph data of all samples are shown, whereas in the inset theFe:ZnO is omitted as its χ (T) is far from Curie-Weiss-like behavior.



Theoretically, the Curie constant C can be described as C =Nμeff

2/(3kB), whereN is the number of magnetic atoms per massof the sample, μeff is the effective moment per atom, and kB is theBoltzmann constant. Assuming that C is determined only by TMions in the common oxidation state of þ2, we can estimate theTM concentration from magnetic measurements by comparingthe measured and theoretical C values. For Co2þ ions thetheoretical value of the effective magnetic moment is μeff ≈4.8μB

38 and thus the theoretical CpureTM of Co2þ is 0.049 emuK/g. From our susceptibility measurements we obtained C =0.0051 emu K/g, resulting in an estimation of the percentage ofCo2þ in the Co-doped ZnO amounting to 10 wt % (=0.0051/0.049). Finally, we calculated the atomic concentration x in thechemical formula of CoxZn1-xO, from the obtained weightpercent. The atomic concentrations of Mn-, Ni-, and V-dopedsamples were estimated using the same procedure and are listedin Table 2. In all calculations, we considered only the valencestate 2þ, although ions with other valences could also contributeto the measured Curie constant. Due to the fact that the valenceof 2þ corresponds to the highest effective magnetic moment, thecalculated atomic concentrations should be understood as thelower limit for x. Whereas in the case of Co and Mn the obtainedresults agree well with EDX data, only half of the Ni ions and onlya minor part of the V ions are detected by the magneticmeasurements. This can be explained by the fact that Ni andespecially V are presumably mainly present in another oxidationstate than þ2.We cannot apply the same analysis for the susceptibility data of

the Fe-doped sample, because it shows quite a different behavior.Even at a temperature above 200 K the susceptibility does notfollow the Curie-Weiss temperature dependence. This observa-tion means that the magnetic interactions are already effective atroom temperature.To get deeper insight into the nature of the magnetic inter-

actions, we performed isothermal magnetization measurementsat 2 K and at room temperature for all samples. At roomtemperature all samples, except the Fe-doped ZnO, follow alinear M(H) dependence (not shown here), while at 2 K themagnetization curves roughly follow the Brillouin function(Figure 5). All M(H) data at 2 K of the non-Fe-doped samplescan successfully be fitted with a sum of a Brillouin function BJ(with total spin J between 0.7 and 5, which is in reasonableagreement with the expected values for V2þ, Ni2þ, Co2þ, andMn2þ) and a small linear term kH that phenomenologicallydescribes a contribution of AFM coupled magnetic moments:

M ¼ MSBJ þ kH

The saturation magnetization MS and the magnetization atmaximal magnetic fieldH = 50 kOe are comparable to the values

expected for samples containing transition metals in the oxida-tion states þ2 and concentrations as estimated from the struc-tural and compositional investigations. However, the valuesare in agreement with our results obtained from temperature-dependent susceptibility measurements, and the small values oftotal spin J's rule out any ferromagnetic interactions. An excep-tion is Fe-doped ZnO, which cannot be described with the sameapproach as the other samples. A steep increase of magnetizationin small magnetic fields at 2 and 300 K and a hysteresis loopwith coercivity Hc = 250 Oe at 2 K provide strong evidence forferromagnetism.

3. CO- AND Fe-DOPED ZnO: VARIATION OF THEDOPANT CONCENTRATION

3.1. Rietveld Analysis of the XRD Data. Figure 6a showstypical powder XRD patterns of the CoxZn1-xO nanoparticleswith nominal Co doping concentrations of x = 0.02, 0.1, 0.2, 0.3,0.4, and 0.45. Up to a nominal doping concentration of 20 atom% for Co, the reflections can be assigned to ZnO with thewurtzite structure without any indication of other crystallineimpurity phases. In the XRD patterns corresponding to xg 0.3,weak reflections of Co3O4 (ICDD PDF No. 42-1467, markedwith asterisks (*)) appear and gradually gain intensity withincreasing Co doping, suggesting a saturation limit. TheFexZn1-xO samples, on the other hand, seem to be phase pureup to a dopant concentration of 40 atom % (Figure 6b). For theCo doping as well as for the Fe doping, the XRD patterns exhibitan increase of the full width at half-maximum (fwhm) of thediffraction peaks with increasing dopant concentration, pointing

Table 2. Theoretical and Experimental Values Obtained by Fitting the Susceptibility Data with the Curie or Curie-Weiss Law:Measured Curie Constants (Cmeas), Measured Curie Temperatures (θ), Calculated Curie Constants (CpureTM) for the Pure TMIons Shown in Parentheses without ZnO, and Calculated wt % and atom % of TM Ion from the Ratio Cmeas/CpureTM

a

sample Cmeas (emu K/g) θ (K) CpureTM (emu K/g)

Cmeas/CpureTM

(wt %) [atom %]

20 atom % Co:ZnO 5.1� 10-3 -67 49 � 10-3 (Co2þ) 10 [13.7]

20 atom % Mn:ZnO 3.1� 10-3 -46 79 � 10-3 (Mn2þ) 3.9 [5.7]

5 atom % Ni:ZnO 0.47� 10-3 2 22 � 10-3 (Ni2þ) 2.1 [2.9]

20 atom % V:ZnO 0.10� 10-3 -1 35 � 10-3 (V2þ) 0.29 [0.5]aThe valence state of the ions used in the calculation is given in parentheses.

Figure 5. Magnetization curves measured at 2 K of V0.2Zn0.8O,Co0.2Zn0.8O, Fe0.2Zn0.8O, Ni0.05Zn0.95O, and Mn0.2Zn0.8O (the stoi-chiometry is based on the initial concentrations in the reaction solution).The inset shows the magnetization curves at small magnetic fields. Ahysteresis in the Fe0.2Zn0.8O sample is clearly visible.



to a decrease of the crystallite size upon incorporation of thedopant and/or an increase of the microstrain.39 It is worthmentioning at this point that Knut et al. recently reported thatXRD in some cases does not reveal the presence of secondaryphases.40 This statement is in agreement with our results thatother characterization techniques (see below) indicate the pre-sence of secondary phases that are not visible in the XRDpatterns.The lattice parameters and the evolution of the unit cell

volume of the Co- and Fe-doped ZnO in dependence of theinitial dopant concentration are displayed in Figure 7. For theundoped ZnO nanoparticles, the lattice constants are a = 3.250 Åand c = 5.207 Å (cf. Table 1), agreeing well with the reportedvalues of 3.249 Å and 5.205 Å (ICDD PDF No. 36-1451). Incomparison to Fe, the effect of Co doping on the latticeparameters and on the cell volume is much smaller. There isno clear trend visible with increased incorporation of Co in theZnO lattice. However, considering that the ionic radius of tetra-hedrally coordinated Co2þ (0.58 Å) is close to that of Zn2þ

(0.60 Å) and assuming substitutional accommodation of Co2þ,only small changes are expected.In the case of Fe doping, however, a completely different

behavior was found. Whereas the a axis monotonically increasesup to a doping level of 40 atom %, the c axis becomes shorterup to a doping level of 20 atom %, followed then by a rapidelongation of the c axis at a doping level of 40 atom %. Theanomalous behavior of the 40%-doped sample strongly indicatesthat at this doping level the solid solution limit has already beenexceeded. The unit cell volume continuously increases, whichcan be explained by looking at the ionic radii. The substitution ofthe smaller Zn2þ ion by the larger Fe2þ (0.63 Å in tetrahedral

environment) is expected to result in an expansion of the unit cellvolume. On the other hand, it is more difficult to explain theobserved anisotropic change in lattice parameters. One reasoncould be that the incorporation of Fe2þ or Fe3þ in ZnO hasopposite effects on the strain and on the peak shifts, which makesthe interpretation of such observations in mixed Fe2þ/Fe3þ

systems rather speculative.41 From extended X-ray absorptionfine structure (EXAFS) measurements we know (see below)that, with higher doping levels, also the amount of Fe3þ (withrespect to Fe2þ) increases.3.2. UV-Vis Measurements. UV-vis absorption spectra

were recorded from stable transparent (i.e., highly diluted)dispersions of undoped, Co-doped, and Fe-doped ZnO nano-crystals (Figure 8). Undoped ZnO exhibits an apparent absorp-tion edge at about 380 nm corresponding to a band gap of 3.3 eV,in agreement with reports in the literature.42 The Co-doped ZnOnanoparticle dispersion changes its color from colorless to darkblue with increasing doping level. The spectrum for Co-dopedZnO shows three absorption bands at around 567, 613, and653 nm (Figure 8a). The origin of the peaks is well documentedand is due to d-d absorption levels of the Co2þ ions in atetrahedral crystal field,43-46 indicating that the Co ions sub-stitute Zn2þ in the wurtzite structure. The tetrahedral coordina-tion and the þ2 oxidation state of Co are also confirmed by theEXAFS results (see below). Furthermore, the intensity of theabsorption related to substitutional Co2þ continuously increaseswith the doping level, giving evidence for the growing number ofsubstitutional Co ions in the nanocrystals.In comparison with the pure ZnO, the Co-doped samples

show a red shift with increasing dopant concentration. Such adecrease in the optical band gap absorption edge has been

Figure 6. XRD patterns of (a) Co-doped and (b) Fe-doped ZnO nanoparticles with different nominal doping levels (* = Co3O4).

Figure 7. Variation of a and c cell parameters and unit cell volume with doping level (Co, blue squares; Fe, red squares). Error bars represent thestandard deviation in the Rietveld refinement.



interpreted as mainly due to the sp-d exchange interactionsbetween the band electrons and the localized d electrons of themetal ions substituting the Zn ions.16,39,47 Recently, it wasproposed that in the case of Mn-doped ZnO clustering effectscould also lower the optical band gap energy.48

The spectra of Fe-doped ZnO samples exhibit a strongabsorption edge at about 380 nm. The absorption edge shiftsto shorter wavelengths (blue shift) as the concentration of thedopant increases from 2 to 10%, the opposite of the behaviorobserved for Co doping, but in agreement with previousreports.49,50 Perales-Perez et al. attributed the blue shift to thequantum confinement effect and the decreasing crystal size withincreasing Fe doping level. However, size effects in ZnO nano-crystals larger than 7 nm, which is the case for our samples, arenot to be expected.49 O'Brien et al. also found a blue shift upondoping of ZnO nanocrystals with Fe.51 In this context it isinteresting to note that these authors reported a red shift for Cdas dopant, whereas Mg, Mn, and Fe doping produced a blue shiftin the absorption onset.51

For doping levels x g 0.15 the absorption spectra exhibitadditional features. The absorption band is red-shifted, and theoptical spectra show an extra absorption in the wavelength regionof 400-600 nm. This absorption band is typical for ferrites andcan be attributed to the formation of Fe3O4

52 or ZnFe2O4.53 It

was reported before that Fe implantation into ZnO can lead tothe formation of crystallographically oriented ZnFe2O4 nano-particles inside single crystalline ZnO.54 The UV-vis results arethus consistent with the solubility limit observed by the EXAFSmeasurements (see below), but lower than that from XRD data.Doping of ZnO with transition metals offers potentially aversatile way of tailoring the optical properties either via thechoice of the dopant for a red or a blue shift or by adjusting thedopant concentration for a linear variation in the bandgap.55

3.3. X-ray Absorption Fine Structure. The X-ray absorptionnear-edge structure (XANES) spectra (Figure 9a) of the FeK-edge of FeO, Fe2O3, and Fe3O4 and of the various Fe-dopedZnO samples exhibit a main peak around 7128 eV originatingfrom the 1s to 4p transition and a visible pre-edge peak around7112 eV, which can be assigned to the 1s to 3d-4p hybridizedstate. The pre-edge positions in Fe = 2 and 5% are slightly lowerby ∼1 eV than those in the other samples and similar to that inthe Fe3O4 reference. The pre-edge position of the Fe K-edgeXANES spectrum typically shifts to higher energy upon changingthe valence from Fe2þ to Fe3þ. The shift of the pre-edge peakindicates that Fe in the products is a mixture of ferrous (Fe2þ)and ferric (Fe3þ) ions, and the contribution of Fe3þ increases

with increasing the doping concentration from 2 to 30%.Unfortunately, the exact Fe2þ-to-Fe3þ ratio is not accessible byEXAFS. This result is important with respect to the magneticproperties, because higher doping levels do not necessarily leadto the incorporation of more magnetic ions within the ZnOlattice due to changes in the initial oxidation state. The intensityof the pre-edge peak is related to the Fe site symmetry. Higherintensity is obtained in tetragonal rather than octahedral coordi-nation due to an increased hybridization between the 3d and 4pstates in the lower symmetric site. The intensity of the pre-edgepeak is larger in the doped samples than in Fe2O3, which hasFe3þ ions only on octahedral sites. This indicates that part ofthe Fe ions have tetrahedral coordination. There is an obviousshoulder at 7115 eV in the Fe-doped ZnO sample with Fe = 2and 5%. Its intensity decreases with an increase in the dopinglevel. The complete disappearance of the shoulder peak fordoping concentration equal to or larger than 15% suggests thatthe local environment around the Fe ion changes for these highdoping levels. The value of 15% Fe corresponds to the solidsolution limit estimated by UV-vis spectroscopy, but is a bitlower than what is obtained from the lattice parameter change inFigure 7 extracted from XRD data.Radial distribution functions (RDFs) are shown in Figure 9b.

Around the Fe ion in the Fe = 2% sample, the RDF is similar tothe one around Zn in the ZnO reference, but different from thosein the iron oxide references. The similarity indicates that Fe ionreplaces Zn ion in the ZnO lattice. However, the second nearestneighboring peaks in the products decrease in their intensitywith increasing Fe doping. The changing in the RDFs might beattributed either to the disordering of the local structure aroundthe Fe ions on the tetrahedral site in the ZnO lattice, or to theformation of Fe3O4-like clusters, as found in Fe-doped galliumoxynitride.56 The broadening in the RDFs and disappearing ofthe shoulder peak can indicate that the substitutional limit in theFe-doped ZnO prepared by nonaqueous sol-gel method isaround 15%, which is much larger than that reported by othersynthesis methods.57

XANES spectra of Co-doped ZnO and the references CoO,LiCoO2, and Co3O4 show the main absorption peak around7724 eV, originating from the 1s to 4p transition, and an obviouspre-edge peak assigned to the transition from 1s to 3d-4phybridized orbital at 7707 eV (Figure 10a). The pre-edge peakshifts toward higher energy from CoIIO to LiCoIIIO2 reference.The peak position of each Co-doped ZnO is close to that of theCoIIO reference, indicating that the most of Co ions are divalent.In addition, the intensity of the pre-edge peak in Co-doped ZnO

Figure 8. UV-vis spectra of (a) ZnO and Co-doped ZnO nanoparticle dispersions, and (b) Fe-doped ZnO nanoparticle dispersions in ethanol withdifferent nominal dopant concentrations.



is much higher than that of the references. The intensity is closelyrelated to the Co site symmetry, as discussed for Fe-doped ZnO.The higher intensity indicates that the Co ions have tetragonalcoordination in the products. The shoulder is also observed in allproducts, indicating that the local structure around Co does notchange significantly in the ZnO lattice up to 30% Co doping.

Figure 10b shows RDFs around Co in Co-doped ZnO and inthe references. The distributions for the Co-doped ZnO samplesare comparable to that of Zn in ZnO, but different from those forthe Co references. The RDFs of all Co-doped ZnO samples arenearly the same. Therefore, a high amount of Co, up to 30%,might be incorporated into the ZnO lattice. However, the peak

Figure 9. (a) Normalized XANES spectra and (b) Fourier transform of the Fe K-edge EXAFS for the samples and references. The vertical dashed linerepresents the pre-edge position in the Fe2O3 reference.

Figure 10. (a) Normalized XANES spectra and (b) Fourier transform of Co K-edge EXAFS for the samples and references. The vertical dashed linesrepresent the pre-edge position of CoO reference.



profiles for the Co-doped ZnO samples become broader towardshorter lengths, which is particularly true for the second nearestneighboring peak. As Co3O4 has shorter bond lengths, some ofthe cobalt ions may be present in octahedral environment, eitherin Co3O4-like clusters or in the Zn

2þ site of ZnO. According toXAS, the Co doping limit is around 30% in the ZnO lattice, whichis higher than what is proposed by XRD and much higher thanthat prepared by conventional methods.58

3.4. Magnetic Properties. By comparing the ZnO samplesdoped with different TM atoms, we have seen that only theFe-doped sample showed ferromagnetic (FM) properties. Forthis reason our further investigations were focused on themagnetic properties of Fe-doped samples with different dopingconcentrations.In Figure 11 the temperature-dependent susceptibilities of five

(nominal 2, 5, 10, 15, and 20%) Fe-doped samples are shown.The susceptibilities of the 2 and 5% Fe doped samples follow theCurie-Weiss law above 200 K. The obtained parameters are C =0.0048 emu K/g, θ =-7 K for 2% doped sample and C = 0.0016emu K/g, θ = -63 K for 5% doped sample. Negative Curie-Weiss temperatures indicate a prevailing AFM interaction.Assuming again that C is determined only by Fe2þ or Fe3þ ions(they have very similar μeff values of≈5.4μB and 5.9μB per Fe forFe2þ and Fe3þ, respectively) with an average μeff ≈ 5.6μB perFe atom, and using the same procedure as above for otherTM atoms, we calculated the Fe2þ/Fe3þ concentration as only0.8 wt % (1.1 atom %) and 2.5 wt % (3.6 atom %) for 2 and5 atom % doped samples, respectively. Again, magnetically we donot see all the TM ions that should be incorporated into the ZnOstructure according to the nominal concentration. An almost perfectCurie-like magnetic behavior in the case of 2% doped samplemeansthat we deal with very weak interacting magnetic moments.By increasing the Fe ion concentration the χ(T) curves

become less and less Curie-Weiss-like. This observation meansthat FM interactions between magnetic moments start to appearwith increasing Fe2þ and/or Fe3þ concentration.The isothermal magnetization curves M(H) measured at

300 K are shown in Figure 12. While theM(H) for 2, 5, and 10%Fe doped samples are linear up to 50 kOe, the curves for 15 and20% Fe doped samples are apparently “S”-shaped with a steepincrease of the magnetization at small magnetic fields. This isclear evidence of FM order at room temperature.From the room-temperatureM(H) curve of the 20% Fe doped

sample we subtracted the linear contribution assigned mainly to

the paramagnetism of isolated Fe ions on Zn sites and endedup with a “S”-shaped curve saturating above ∼10 kOe at MS =0.36 emu/g (Figure 12). We associate this ferromagnetic-likebehavior with the Fe3O4 nanoclusters suggested by the UV-vismeasurements. The obtained MS is equivalent to 0.03μB per Featom, which is in the same range as values reported by otherauthors for lower doping levels (10 atom %) but larger particles(2-30 nm).59 From the fit of the ferromagnetic curve with theLangevin function we found themagnetic moment of the clustersto be approximately 12500μB, which is equivalent to 1.16 �10-16 emu. By taking the saturation magnetization of bulkmagnetite Mbulk = 92 emu/g,60 we calculated the mass of aFe3O4 cluster to be m = 1.3� 10-18 g. Considering a density ofFe3O4 of 5.2 g/cm3 and supposing spherical particles, weestimated the diameter of the nanoclusters to be D = 8 nm.By comparing the room temperature susceptibility obtained

from the linear part of the M(H) curve, and the theoreticalsusceptibility calculated from peff(Fe

2þ) = 5.4μB, we estimatedthe atomic concentration x in the chemical formula FexZn1-xO tobe 0.084 (i.e., 8.4 atom %). This value is slightly smaller than13 atom % obtained from EDX spectra. From the saturationmagnetization of the “S”-curve we found the mass of Fe3O4 to be4 mg per 1 g of sample, which adds only an additional 0.2 atom %of Fe.At this point two questions arise: Why are the Fe3O4 clusters

with a calculated diameter of 8 nm invisible to XRD? How couldsuch a cluster fit into a 10 nm size ZnO nanoparticle? Areasonable answer to the first question would be that theFe3O4 clusters are amorphous in nature. There is a considerableamount of literature about ferromagnetic amorphous ironoxides.61 However, the saturation magnetization of amorphousiron oxides is smaller than that for the crystalline phase,61,62

which would then consequently yield a cluster diameter evenlarger than 8 nm. The following physical picture accounts for theapparently too large Fe3O4 nanocluster: There are small amor-phous ferromagnetic Fe3O4 regions within the ZnO host. Themagnetic interaction between the Fe3O4 regions within a singleZnO particle and between Fe3O4 regions from neighboring ZnOparticles (keep in mind that in the dried powder the nanoparti-cles are agglomerated) combine into a single entity that effec-tively behaves as a larger ferromagnetic cluster (Figure 13a).Doped or undoped zinc oxide nanoparticles often aggregate in acrystallographically oriented fashion, which is exemplarily shown

Figure 11. Susceptibility curves for samples with different Fe concen-trations.

Figure 12. Magnetization curves for samples with different Fe concen-trations at room temperature. Open circles are the FM part ofmagnetization for the sample with 20 atom % Fe. The linear term wassubtracted. The dashed line is a Langevin fit to the FM part of the signal.



in theHRTEM image inFigure 13b for Fe-dopedZnO(10 atom%).The cluster size obtained from the magnetic measurements isthus considerably larger than the actual size of the individualFe3O4 regions inside the ZnO nanoparticles (and also larger thanthe crystal size of ZnO). A similar explanation was proposed for1.0-1.5 nm diameter rare-earth iron garnets.63

Comparing the EXAFS data of the Fe- and Co-doped ZnOsamples with the magnetic properties of these materials, severalfindings must be considered. With increasing dopant concentra-tion, the amount of Fe3þ in the ZnO host increases, whereas Comainly remains in theþ2 oxidation state. The oxidation states aswell as the changes in the RDFs at higher doping levels give someindication that the iron ions start to aggregate into Fe3O4-likeregions, whereas for Co such a tendency is less pronounced.Furthermore, the solid solution limit is around 15 atom % in thecase of Fe and as high as 30 atom % for cobalt doping.As already discussed in section 2.3 and Figure 5, the 20% Co

doped sample does not show ferromagnetic behavior. To com-plete the picture, we also studied the influence of other Coconcentrations on the magnetic properties (Figure 14). For allconcentrations the magnetization curves at room temperature arelinear with the magnetic field with no evidence of ferromagnetism,although the highest Co concentration is 30% and thus much

larger than the Fe-doped sample with FM response at roomtemperature (20 atom%). Clearly the dopant concentration is notthe main factor in determining the magnetic properties.

4. CONCLUSION

We presented a simple and time-efficient synthesis route totransition metal doped ZnO nanoparticles involving a micro-wave-assisted nonaqueous sol-gel approach. A major advantageof this method is that high doping levels up to 30 atom % can beachieved within a reaction time of just a few minutes, which isinteresting with respect to the study of the magnetic properties.Using the same synthesis approach for the preparation of samplescovering a wide range of doping levels allows the exclusionof magnetic effects stemming from variations in the synthesisprotocol. This is a critical issue, as the results obtained in thisstudy do not agree with previous investigations performed in ourgroup. Whereas we reported room-temperature ferromagnetismfor Co-doped ZnO prepared by a solvothermal route in benzylalcohol,22 all the Co-doped ZnO samples synthesized in themicrowave reactor followed only Curie-Weiss behavior.

The V- and Ni-doped ZnO samples showed no magneticinteractions, while the Co- and Mn-doped samples were char-acterized by Curie-Weiss magnetic behavior and antiferromag-netic coupling between the dopants. Both results are in con-tradiction to as well as in agreement with literature data. In somecases, the authors observed or predicted ferromagnetism in sam-ples with similar concentrations of dopants at room tempera-ture,64-66 whereas other authors reported the absence offerromagnetism.7,67 Antiferromagnetic coupling in Co- andMn-doped ZnO was experimentally observed by Iusan et al.68

Their ab initio calculations indicated a tendency for the forma-tion of Co clusters, giving rise to an antiferromagnetic exchangeinteraction between the Co atoms. In our case, we did not findany indication for metallic Co clustering, i.e., formation ofCo-Co bonds. X-ray absorption spectra reveal that some Coions may be present in octahedral environment in Co3O4-likeclusters. Co3O4 is an antiferromagnet with a Neel temperature of30 K and is a paramagnet above that temperature.69 Co3þ ionsare in a diamagnetic state in the spinel structure69 and thus do notcontribute to the paramagnetic signal. This may be the reason thecalculated concentration of cobalt ions from the magneticmeasurements, where we assumed all cobalt in the Co2þ state,is considerably smaller than the cobalt concentration obtainedfrom the structural investigations. It is interesting to compare ourresults with a recent study by Knut et al.40 They also detectedCo3þ in a sample of 15 atom % Co doped ZnO, pointing to thepresence of a secondary phase, which, however, was not visiblein XRD.

We found that only the Fe-doped samples show room-temperature ferromagnetism at sufficiently high doping levels.Several studies on Fe-doped ZnO report ferromagnetism,59,70

whereas others do not.71 As amatter of fact, doping levels as smallas 3% were reported to result in room-temperature ferro-magnetism.70 One of the main problems in comparing resultsfrom several groups lies in the fact that completely differentsynthesis routes were applied. Therefore, a systematic study ofthe influence of the iron doping level on the magnetic propertiesis only conclusive if the samples are prepared by the samemethod, as is the case here. The samples with 2, 5, and 10 atom% iron exhibit paramagnetism at 300 K, while the doping levelsof 15 and 20% lead to ferromagnetism at room temperature.

Figure 13. (a) Fe3O4 regions in ZnO nanoparticles and the ferro-magnetic cluster, which yields the calculated (magnetic) particle size.(b)HRTEM image of a cluster of crystallographically oriented Fe-dopedZnO (10 atom %) nanoparticles. The corresponding power spectrum(shown as an inset) confirms the common orientational relationship ofthe particles.

Figure 14. Magnetization curves for samples with different Co con-centrations at room temperature.



A possible reason for ferromagnetismwas found in the analysis ofXAS data, which gives some indication that at high doping levelsinteracting Fe3O4-like regions form within the ZnO lattice thatmight be responsible for FM ordering at room temperature.

Our study shows that the preparation and the characterizationof diluted magnetic semiconductors with room-temperatureferromagnetism using ZnO as host is a complicated endeavor.In addition to high doping levels, also the oxidation state ofthe dopant must be controlled. The arrangement of the dopantswithin the host lattice is a key factor, but it is also difficult toanalyze. Although the use of several characterization techniquesis essential, they can also produce slightly different results. In anycase, the clustering behavior of dopants is nearly impossible tocontrol, unless one uses techniques such as ion implantation.72

Whatever the experimental problems are, as long as the originof ferromagnetism is not understood on an atomic level, it isdifficult to develop a rational synthesis to diluted magneticsemiconductors.

5. EXPERIMENTAL SECTION

5.1.Materials. Zn(II) acetate (99.99%),Mn(II) acetate (98%),Co(II) acetate (99.995%), V(V) oxytriisopropoxide, and benzylalcohol (99.8%, anhydrous) were obtained fromAldrich, whereasFe(II) acetate (95%) and oleic acid (99.0%) were purchasedfrom Acros Organics and Fluka, respectively. All chemicals wereused as received without further purification.5.2. Syntheses and Dispersions. ZnO and doped ZnO

nanoparticles were synthesized using a microwave-assisted non-aqueous sol-gel method in a CEMDiscover reactor operating ata frequency of 2.45 GHz. The typical synthesis of the ZnOnanoparticles was performed by adding 1 mmol of Zn(II) acetate(0.1835 g) in 5 mL of benzyl alcohol in a glovebox under Aratmosphere (O2 and H2O < 0.1 ppm). Doping was achieved byadding dopant precursor (ranging from 1 to 30 mol %) to thereactionmixture. In all cases, the reactionmixture was transferredinto a 10 mL glass tube and sealed with a Teflon cap. Duringmicrowave heating, the reaction mixture is stirred with a stir bar.The heat treatment was performed at 160 �C for 3 min. Theprecipitate was separated from the liquid phase by centrifugation,washed with ethanol and diethyl ether, and dried at 60 �Covernight.5.3. Characterization. X-ray powder diffraction (XRD) pat-

terns for as-synthesized powders were measured in reflectionmode with Cu KR radiation on a Philips PW1800 diffractometerequipped with secondary monochromator. The structural andmicrostructural parameters were extracted using Rietveld refine-ment with the program FullProf.36 The Rietveld refinement ofpowder XRD patterns was performed starting from the structuralmodel of ZnO wurtzite. The Thompson-Cox-Hastings pseudo-Voigt with axial divergence asymmetry function was selected asthe profile fitting function. The scale parameter, the backgroundto a fourth-order polynomial, the lattice parameters, the fullwidth at half-maximum (fwhm), the atomic coordinates, theanisotropic Lorentzian size broadening (spherical harmonic),and the isotropic temperature parameters were refined. Finally,the occupation factors for Fe and Co atoms were refined as well.Transmission electron microscopy (TEM) and selected area

electron diffraction (SAED) were performed on a Philips CM30ST (LaB6 cathode, operated at 300 kV, point resolution 2 Å),while energy-dispersive X-ray spectroscopy (EDX) was doneusing a Philips CM200-FEG microscope (200 kV, a spherical

aberration constant of the objective lens Cs 1/4 1.35 mm). Thehigh resolution TEM images (HRTEM) and the energy-filteredtransmission electron microscopy (EFTEM) elemental maps ofCo-doped ZnO were obtained using a JEOL 2200FS TEM/STEM microscope operated at 200 kV and equipped with anin-column Omega-type energy filter. The conventional three-window technique was used to obtain element specific images.The samples for TEM characterization were prepared such thatone drop of the dispersion of as-synthesized powder in chloro-form using oleic acid as surfactant was deposited onto a coppergrid covered by an amorphous carbon film.The optical properties of the products were studied by a

Perkin-Elmer Lambda 900 UV-vis spectrophotometer. Thesamples were prepared such that as-synthesized powders weredispersed in ethanol.Magnetic properties were investigated with aQuantumDesign

MPMS XL-5 SQUID magnetometer, equipped with a 50 kOemagnet. Magnetic susceptibilities were measured between 2 and300 K in an external magnetic field of H = 1000 Oe. Magnetiza-tion curves were obtained at T = 2 K and T = 300 K. All magneticdata are corrected for the diamagnetic contribution of the innershell electrons obtained from Pascal’s tables.73

X-ray absorption of Fe, Co, and Zn K edges were measured intransmission mode at the beamline 9C in the Photon Factory,KEK, Tsukuba, Japan. A small amount of the sample powderwas sandwiched between Scotch tape. FeO, Fe2O3, Fe3O4, CoO,Co3O4, LiCoO2, and ZnO were used as references for Fe, Co,and Zn, respectively. The EXAFS data were analyzed using theprogram REX-2000.74 Details of these procedures are given inref 75.

’ASSOCIATED CONTENT

bS Supporting Information. HRTEM images and EFTEMelemental maps of Co-doped ZnO (20 atom %). This material isavailable free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected] (Z.J.); [email protected] (M.N.).

’ACKNOWLEDGMENT

Financial support from ETH Z€urich (ETH-07 09-2) and theSwiss National Science Foundation (Project No. 200021_124632) is gratefully acknowldedged. This work was conductedunder the approval of the Photon Factory Advisory Committee(Proposal No. 2010G024). We thank the Electron MicroscopyCenter of the Swiss Federal Laboratories for Materials Scienceand Technology (EMPA), D€ubendorf, for the use of their TEMfacilities.

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microwave-assisted nonaqueous sol−gel chemistry for highly concentrated zno-based magnetic...

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