Surface Magnetism of Cobalt-Doped Anatase TiO2 Nanopowders

Article pubs.acs.org/JPCC

Surface Magnetism of Cobalt-Doped Anatase TiO2 Nanopowders A. Ye. Yermakov,† G. S. Zakharova,†,‡ M. A. Uimin,† M. V. Kuznetsov,‡ L. S. Molochnikov,¶ S. F. Konev,§ A. S. Konev,† A. S. Minin,† V. V. Mesilov,† V. R. Galakhov,*,† A. S. Volegov,† A. V. Korolyov,† A. F. Gubkin,† A. M. Murzakayev,§,∥ A. D. Svyazhin,† and K. V. Melanin† †

M. N. Miheev Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, 620137 Yekaterinburg, Russia Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620990 Yekaterinburg, Russia ¶ Federal State Budget Educational Institution of Higher Education, Ural State Forest Engineering University, 620100, Yekaterinburg, Russia § Federal State Autonomous Educational Institution of Higher Education, Ural Federal University, named after the first President of Russia B. N. Yeltsin, 620002, Yekaterinburg, Russia ∥ Institute of Electrophysics, Ural Branch of the Russian Academy of Sciences, Yekaterinburg, Russia ‡

ABSTRACT: Cobalt-doped anatase Ti1−xCoxO2 (0 < x ≤ 0.04) nanopowders (with a particle size of 30−40 nm) were produced by the hydrothermal synthesis method. Morphology, structure, and thermal stability of the synthesized compounds were examined using transmission electron microscopy, infrared spectroscopy, and X-ray diffraction analysis. Using X-ray photoelectron spectroscopy, cobalt ions are shown to have an oxidation state of 2+, with titanium ions having a tetravalent state of Ti4+. In the as-prepared state, all investigated compounds of Ti1−xCoxO2 are paramagnetic, with the value of paramagnetic susceptibility growing in proportion to cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals that doping TiO2 with cobalt (up to 2%) is accompanied by a significant increase in the concentration of F+ centers. Further growth of the cobalt content results in a relatively wide line (nearly 600 Oe) in the spectrum, with a gfactor of about 2.005, demonstrating exchange-coupled regions being formed, the fraction of which increases with cobalt content, while the intensity of F+-center signals is reduced appreciably. Annealing of Ti0.96Co0.04O2 in vacuum at 1000 K is shown to have resulted in the substantial localization of cobalt atoms in the subsurface layers, resulting in an approximately 3-fold increase in the Co atoms content on the surface of nanoparticles as compared with that in the bulk. This is shown to be accompanied by appearance of spontaneous magnetization at room temperature, the value of which depends on the cobalt content in TiO2 nanopowders. The value of magnetic moment per Co atom decreases monotonically down to a value of ≃1 μB with cobalt content increasing. A core−shell model proposed to be the most adequate for describing the magnetic properties of TiO2:Co after the reducing annealing. A hypothesis is put forward suggesting that the defect surface enriched with Co atoms and vacancies is described with itinerant type magnetism, allowing for the delocalized nature of electrons near vacancies.

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INTRODUCTION Titanium dioxide based semiconductor nanocrystals are remarkable materials to be used as active elements of nanotechnology applications due to their improved physical and chemical properties as compared to their bulk material analogue. Realization of the ferromagnetic state of semiconductor materials doped with a few percent of a magnetic impurity offers a promising way of solving a variety of applied problems. Nanocrystalline titanium dioxide TiO2 is of special interest for the reason that defects and doping can induce significant modifications to electronic and optical properties and can ensure effective usage in photocatalysis and photoelectrochemical solar cells, in sensors, spintronics, and medical applications.1 As a result of various reducing treatments, changing a defect-rich structure (for example, increasing concentration of vacancies in the anion sublattice) dramatically © 2016 American Chemical Society

modifies the electronic and magnetic properties of titanium dioxide. A significant contribution of the surface states in nanocrystalline TiO2 might heavily influence the spatial distribution of defects and doping components in nanoparticles. It has been commonly assumed2,3 that the surface of TiO2 nanoparticles appears to be the most preferable localization of 3d impurities. Considering the complicated competing exchange interactions between 3d ions, a variety of spin structures can be realized with the involvement of defects (e.g., vacancies with an unpaired electron), which are capable of changing significantly the physical and chemical properties of the material both on the surface and in the bulk. Cobalt has Received: October 15, 2016 Revised: November 29, 2016 Published: November 30, 2016 28857

DOI: 10.1021/acs.jpcc.6b10417 J. Phys. Chem. C 2016, 120, 28857−28866

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The Journal of Physical Chemistry C

dioxide after the reducing vacuum heat treatment. It is strongly indicated to investigate the effect of the treatments on localization of defects and cobalt atoms over the nanoparticles surface and to study the surface magnetic states.

proved to be an effective doping element significantly modifying the electronic and magnetic properties of titanium dioxide.4−7 The TiO2:Co-based systems demonstrate ferromagnetic behavior with the Curie temperature above room temperature. According to the papers,8−11 defects observable in titanium dioxide, preferentially in the anion sublattice, play a key role in such behavior. The defectsfor instance, F+-centers with an unpaired delocalized electroncan serve as a mediator system to realize exchange interactions of various types. Considering the elevated cobalt content and increased vacancies concentration in the anion sublattice on the nanoparticles surface, and keeping in mind the delocalized nature of an unpaired electron (e.g., around a vacancy), one can expect the itinerant type magnetism to appear, which is characteristic of 3d metals with itinerant electrons.12−16 At the same time, in the core of nanoparticles (matrix + Co ions), the paramagnetic state might become realized that has the localized magnetic moment, which magnitude is determined by the cobalt valence state. The “core−shell” model of a particle appears to be the most adequate to describe all the magnetic properties. By analogy with topological insulators,17,18 such magnetic systems can be attributed to the topological paramagnetics. In this regard, it is worth mentioning that nanoparticles with significant surface contribution are characterized with a fundamental feature that the electronic surface properties differ strongly from the bulk properties, even in the absence of defects. Therefore, it is possible to expect that the electron properties of a surface in nanocrystalline state will determine the properties of nano-objects. However, there are very few experimental data available that prove the validity of the “core-shell” structure model, where 3d impurities would be mainly localized on the surface of nanoparticles and the particle volume would contain noninteracting paramagnetic centers. Moreover, local alterations in the crystallographic field are possible, which are caused by the disturbance of the local symmetry of atoms in nanocrystalline titanium dioxide and by microstrains being formed on the surface of TiO2 particles. Therefore, a model has still not been developed to describe a magnetic state for nanocrystalline titanium oxides with 3d impurities, taking into consideration the possibility of inhomogeneous distribution both of 3d impurities and oxygen vacancies. To construct such a model, experimental data about the oxides structure in a wide range of cobalt content are needed to be obtained immediately after synthesis, and data on the evolution of the oxides structure under further heat treatments are also required. Usually, in order to attain the ferromagnetic state in the oxides with 3d elements, annealing in vacuum or hydrogen would be carried out. The annealing of the bulk state in vacuum should not result in significant reduction of TiO2; however, in the nanocrystalline state, which has lower chemical Ti−O bond energy, especially on the surface, the reducing annealing can be quite effective. It was shown19 that TiO2:Co solid solution in bulk with Co content about 3−4 at. % can be obtained. Annealing in a reducing atmosphere is conducive to forming TiO2:Co-based solid solution but, at the same time, is able to cause its decomposition: for example, cobalt oxides may be formed or even a pure metal cobalt may occur at higher Co contents (more than 3%). It is for this reason that the structure of nanocrystalline TiO2:Co has to be monitored thoroughly at every stage of its production. The purpose of this research is to investigate the structure and magnetic properties of cobalt-doped nanoscale titanium

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EXPERIMENTAL SECTION Nanocrystalline cobalt-doped materials TiO2:Co (Co content varies up to 4.5 at. %) were obtained by the hydrothermal method.20 The following materials were used as starting materials in this experiment: the Merck 15% solution of titanium chloride (III) in a 10% HCl solution and “chemically pure” hydrate of cobalt(II) chloride and ammonium hydroxide. The experimental samples were synthesized as follows: CoCl2· 6H2O powder was dissolved in TiCl3 according to the predetermined molar ratio, then NH4OH was added dropwise under the constant stirring to get pH = 9.2. The reaction mass was placed in an autoclave at 430 K and left for 24 h, then cooled naturally. The resulting product was filtered, washed with water to get the neutral reaction of washing water and dried in the air at 320 K. Spectromass 2000 (inductively coupled plasma mass spectrometry, ICP-MS) was used to determine the cobalt and titanium content in the samples. The transmission electron microscope (TEM) (JEM 2100 (JEOL)) was used to determine the morphology of the synthesized compounds. The method of scanning transmission electron microscopy (STEM) provided by built-in energy dispersive X-ray (EDX) microanalyzer was used to perform the elemental chemical analysis and the distribution of elements in the sample. PerkinElmer FT-IR “Spectrum One-B” spectrometer was used along with automatic diffuse reflection console to record the IR spectra of solid samples. X-ray diffraction analysis of samples was performed on a Shimadzu XRD-7000 diffractometer in Cu Kα radiation (λ = 1.5418 Å). Different scanning calorimetry (DSC) and thermal gravimetric (TG) analyzes were performed with a “TA Instruments” SDT Q10 thermal analyzer with the samples heat rate 10 K/min from room temperature to 1170 K in argon. X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB MkII electron spectrometer. The vacuum in the spectrometer analyzer chamber was maintained at 10−8 Pa during measurement. Mg Kα X-ray exciting radiation (1256.3 eV) was applied. The scanned energy step was 0.1 eV. The spectrometer was calibrated using an Au foil as a reference sample (the binding energy of the Au f7/2 core level was 84.0 eV). The sample charge was evaluated by the C 1s carbon spectra (284.6 eV) with respect to the natural hydrocarbon contaminants on the surface. The survey spectra of powders were recorded in the range 0−1000 eV with a 0.5 eV step to assess the qualitative estimation of elemental powders composition. Electron paramagnetic resonance (EPR) measurements were performed on the Bruker ELEXSYS 580 pulse spectrometer in a stationary mode. The sample powder was placed in a special quartz tube of 4 mm in diameter. The sample volume ranged from 2 to 5 mm3. The spectra were registered in a stationary mode at room temperature with Super High-Q rectangular resonator. The frequency of the microwave field was about 9.40 GHz. The interval of induction change of the constant magnetic field B ranged from 480 to 6000 G. The level of microwave power was 4.7 mW and modulation amplitude was 1 G. 28858


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0.04) and the results obtained by means of coprecipitation (x ≤ 0.05).20 The Ti1−xCoxO2 unit cell parameters were calculated using the FULLPROF-2010 analysis method (see Table 1).

Cobalt Kα1,2 X-ray emission spectra (XES) of Co metal, CoO, and Ti0.965Co0.035O2 were measured at room temperature using a laboratory X-ray tube spectrometer. The Pd-anode Xray tube was powered with U = 25 kV and I = 40 mA. The unmonochromatized beam had spot size of about 10 mm in diameter on a sample. The emitted beam was focused according to the Johann scheme21 using a bent (R = 1900 mm) quartz (1340) 40 × 10 × 0.2 mm single-crystal analyzer. All the experimental spectra were obtained in the dispersive geometry in which the energy range of about 100 eV is recorded in a single shot by a position-sensitive detector. The intensity of the collected spectra in the region of the full width at halfmaximum (fwhm) of the Kα1 line was about 104 counts. The magnetic properties of the titanium dioxide nanoparticles were investigated by Quantum Design MPMS-5XL SQUID magnetometer in the fields up to 5 T at a temperature ranging from 2 to 300 K. Magnetization at room temperature was measured on the Faraday balance in the fields up to 1.2 T. To obtain the spontaneous magnetization in the TiO2:Co nanopowders, the samples were annealed in vacuum of 10−3 Pa at 1000 K for 0.5 h.

Table 1. Lattice Parameters of TiO2 Doped by Co chemical composition

a (Å)

c (Å)

TiO2 Ti0.985Co0.015O2 Ti0.979Co0.021O2 Ti0.96Co0.04O2 Ti0.955Co0.045O2

3.7805 3.788(1) 3.788(1) 3.789(1) 3.791(1)

9.4839 9.486(3) 9.491(2) 9.489(2) 9.490(1)

It was found that the TiO2 lattice parameters under doping with Co-ions are comparable with the results obtained by Fleischhammer et al.19 and Crisan et al.:22 doping titanium dioxide with Co-ions is accompanied by a significant increase of the lattice parameter a; while parameter c remains mostly unchanged. Evidently, the increase in Co ions concentration in the titanium oxide matrix leads to an increase in the cell unit volume. By the Debye−Scherrer method, the average size of coherent scattering region (CSR) was determined. It was found that the CSR size of the powder particles not containing cobalt is approximately 30 nm. Doping titanium dioxide with Co ions results in a slight increase of the CSR size, and for Ti0.96Co0.04O2 composition, the CSR size increases up to 36 nm approximately. The results are in good agreement with TEM observation. TEM observation has showed that titanium oxide particles, both in the initial state and after annealing, are of an elongated shape, being 60−80 nm in length and 20−25 nm in width (Figure 2a). There is a little change in the nanoparticle size after annealing at 1000 K (Figure 2b).

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RESULTS AND DISCUSSION Structural Characterization. According to the chemical analysis, the composition of the air-dried powders can be described by the general formula Ti1−xCoxO2, where 0 < x ≤ 0.045. The X-ray diffraction patterns of the samples (Figure 1)

Figure 2. Electron micrographs of the Ti0.965Co0.035O2 initial state sample immediately after the synthesis (a) and the state after annealing at 1000 K in vacuum (b).

Figure 1. X-ray diffraction patterns of anatase titanium dioxide powder doped with 1.5 atom. % Co (1), 2.3 at. % Co (2), 4 at. % Co (3), and 4.5 at. % Co (4). For TiO2, doped with 4.5 atomic % Co, the rutile phase (R) weak reflections appeared.

Studying IR spectra of Co-doped titanium dioxide nanopowder (the spectra are not shown here), it was revealed that the valence vibrations of the titanium−oxygen bonds in TiO6 octahedron occur in the frequency range of 500−700 cm−1.23 The weak absorption bands observed at 2340 and 2375 cm−1 are attributed to CO2 molecules adsorbed on the developed surface of the sample. Bending vibrations of OH-bonds of water molecules appear as an absorption band at 1621 cm−1. The broad intense band with a peak at 3462 cm−1 indicates the presence of OH-groups coordinated with titanium ions. DSC and TG analysis (DSC and TG curves are not shown) carried out on a sample with 2 at. % Co showed that heating is accompanied by weight losses that occurred in two steps, being

indicate the formation of anatase titanium dioxide (A) (JCPDS No. 21-1272); no additional diffraction peaks attributed to impurity phases are observed. The rutile phase reflexes are only observed starting at a Co content of 4.5 at. %. This allows one to regard the obtained compounds as solid solutions, where titanium ions are substituted with anatase cobalt ions, at least up to 4 at. % of Co. Therefore, the maximal content of Co2+ doping ion in the anatase titanium oxide matrix was found to be 4 at. % per one formula unit. There is a good agreement between the results on the homogeneity area of Ti1−xCoxO2 substitutional solid solution synthesized in the hydrothermal conditions (x ≤ 28859


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The Journal of Physical Chemistry C complete at about 770 K. The weight loss was caused by the sample dehydration and by the release of a trace amount of adsorbed CO2 molecules. Total dehydration of the sample during heating leads to a complete crystallization of the sample, as evidenced by the weak exothermic effect observed on the DSC curve with a maximum at 696 K. Further heating of nanopowder is accompanied by a TiO2 (A) to TiO2 (R) phase transition, which is described by the exothermic effect with a maximum at 1111 K. The weight loss data allow to calculate the initial composition (Ti0.98Co0.02O1.98) of synthesized nanopowders, which turned out to be correlating with the Ti0.98Co0.02O1.98 ·0.1H2O-based hydrate formula. XPS Measurements. Figure 3a shows Ti 2p X-ray photoelectron spectra of two samples of TiO2:Co. The maximum of Ti 2p3/2 spectrum (458.5 eV) corresponds to tetravalent titanium, as for TiO2. Figure 3b shows oxygen 1s spectra. The spectra are represented by two lines at 529.8 and 531.8 eV. The first line corresponds to the structural positions of oxygen in the TiO2 lattice;24 the second line corresponds to hydroxyl OH groups located on the surface of the particles.24−26

Information about the oxidation state of cobalt ions in TiO2:Co powders can be obtained from the Co 2p spectra shown in Figure 3c. Also the spectra are shown for metallic cobalt and CoO single crystal. Annealing the TiO2:Co powders at 1000 K leads to an increase of the spectrum intensity. In addition to the two spin−orbit components, Co 2p3/2 and Co 2p1/2, there are satellites observed in the spectra, which are determined by the effects of the electron charge transfer from the 2p ligand shell (in this case, it refers to oxygen) onto the 3d cobalt shell. In the spectra obtained for the oxides with trivalent cobalt ions (LiCoO2), the distance between the satellite and the main Co 2p3/2-peak is 9.5 eV;27,28 and in the spectra for the oxides with Co2+ ions, such as CoO, this value is about 4.5 eV.27,28 The Co 2p spectra of the TiO2:Co nanopowders are similar to the CoO spectrum; the distance between the main peak and the satellite maximum is 4.5 eV. This can be realized if cobalt is in the tetrahedral interstitials of the titanium dioxide. On the defect-rich surface of the TiO2 nanoparticle, cobalt atoms may be situated in the surrounding with highly distorted bonds and with a local symmetry close to the tetrahedral environment.29 The following reason for the appearance of tetrahedral sites is due to the local structural transformations, such as A (anatase) → R (rutile) phase, which are observed with increasing Co content or as a result of reducing heat treatments: for example, see Figure 1 illustrating the rutile phase appearing at a Co content of 4.5 at. %. Note that the XPS method is surface sensitivity since photoelectrons are formed in thin layers of about 5 nm near the surface of materials. Table 2 shows the atomic concentrations Table 2. Atomic Contents of Nanocrystalline Powder Ti0.965Co0.035O2 Determined by XPS Spectra before and after Annealing sample

Ti (at. %)

O (at. %)

Co (at. %)

as-prepared annealed (1000 K, 0.5 h)

26.6 23.4

70.7 68.7

2.7 7.9

of the basic elements (Ti, O, Co) according to the X-ray photoelectron measurements of the TiO2:Co powders carried out before and after annealing at 1000 K. The absolute accuracy of the measured atomic concentrations is not high; however, the relative changes of atomic concentrations under varying powder treatment are determined quite accurately and indicate the growth of Co content on the surface of particles after annealing. In accordance with the X-ray photoelectron analysis results, cobalt concentration has almost tripled after annealing at 1000 K in the subsurface regions. EDX Measurements. Analyzing these samples by EDX method, the Co concentration growth was definitely confirmed in the near surface areas of the nanoparticles upon annealing in vacuum at 1000 K. Parts a−d of Figure 4 show the EDX data obtained for the sample of Ti0.965Co0.035O2 in the as-prepared state. The as-prepared sample manifests a fairly uniform distribution of titanium, oxygen and cobalt (Figure 4b−d). Significant localization of Co atoms in some areas of the titanium dioxide matrix is shown after annealing at 1000 K in vacuum (Figure 5a−d). Preferably, cobalt atoms are localized at the surface of the nanoparticles (Figure 5d), substantially increasing Co atoms concentration in the near surface regions as compared to the bulk.

Figure 3. (a) Ti 2p X-ray photoelectron spectra of the Ti0.965Co0.035O2 nanopowder before (1) and after annealing in vacuum at 1000 K for 0.5 h (2); (b) O 1s X-ray photoelectron spectra of the Ti0.965Co0.035O2 nanopowder before (1) and after annealing at 1000 K for 0.5 h (2); (c) Co 2p X-ray photoelectron spectra for the TiO2:Co powder before (1) and after vacuum annealing (2). For comparison, the spectra of metallic cobalt and of a single crystal of cobalt oxide CoO are shown. 28860


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conditions. Cobalt is rather to be located in the TiO2 lattice in the octahedral sites or, probably, in the positions with a tetrahedral environment due to the reasons we discussed before. Thus, based on the analysis of the XPS and EDX data set, a preliminary conclusion can be made that the observable distribution of Co atoms is most likely to be related to surface nanoparticle Co-atoms localization. The analysis of the above given TEM data (Figure 2) shows that the size of the nanoparticles (of the order of 60−80 nm) is similar to the cobalt region accumulation observable. EPR Studies. EPR spectroscopy is an enough reliable way to identify paramagnetic centers in the initial state; and it can be assumed that what may be recorded are vacancies or other defects (e. g., interstitial Ti-atoms) or the states of titanium (or cobalt) atoms in the tetrahedral environments, which may appear due to nonequilibrium the defect specific surface area.29−31 The defects of various nature and the magnetic moment carriers, e. g., Ti3+, Fe2+, Fe3+, and Co2+, present on the nanoparticle surface, are prerequisite to the exchange interactions being realized, which lead to the appearance of spontaneous magnetic moment. Figure 6 and the inset display the EPR spectra of the anatase TiO2 samples in the initial state without doping and after

Figure 4. STEM image of the region taken to perform the EDX element analysis of Ti0.965Co0.035O2 powders in the initial state after the synthesis (a), STEM element mapping for O (b), Ti (c) and Co (d).

Figure 6. EPR spectra of the TiO2 samples doped with 4.5 at. % Co, registered room temperature immediately after synthesis (4) and after annealing at 1000 K in vacuum (5). The inset shows the EPR spectra for magnetic fields in the range from 3350 and 3750 G of the undoped TiO2 (1) sample and doped with 3.5% Co (2) and 2 at. % Co (3).

doping with Co (2 at. % Co, 3 at. % Co, and 4.5 at. % Co). All the samples demonstrate an intense peak with an effective gfactor (g = 1.997) and width of about 50 G. It may also be suggested that there is a contribution from Ti vacancies (with a narrow line ≃5 G) and with g = 1.998, which signal is reported to have been observed in ref 32. In our opinion, the major contribution into the observed peak is made by oxygen vacancies (F+ centers) with g = 2.003.33,34 Because of the various localizations (surface or bulk) of these vacancies in the nanoparticles, the respective EPR signals are described by similar but not identical g-factors, overlapping each other and giving the broadened line observed. However, in our case, the width of line is almost an order of magnitude greater and is about 50 G. Thus, far it would be premature to

Figure 5. STEM image of the region taken to perform the EDX element analysis of Ti0.965Co0.035O2 powders after annealing at 1000 K for 0.5 h (a), STEM element mapping for O (b), Ti (c), and Co (d).

It is determined that cobalt localization is heterogeneous throughout the matrix volume; some areas are more preferred ones, where the separation process is mostly distinguished. Still, there is no accurate answer why these areas preferably accumulate cobalt. The presence of cobalt oxide and pure cobalt is not detected in the samples by the diffraction and XPS methods. Moreover, it is mostly unlikely for cobalt oxide to be formed on the surface of TiO2 nanoparticles in the reducing 28861


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The Journal of Physical Chemistry C make any final conclusion on the nature of the line with the effective g-factor g = 1.997 or to attribute this line either to Ti vacancies or oxygen vacancies. However, we believe that in this case the signal was mostly observed from vacancies in the anion sublattice. This approach provides more consistent explanations to the EPR data obtained from the samples of titanium dioxide doped with cobalt. Doping the sample with cobalt should lead to a noticeable increase of the vacancy peak, which is due to the fact that nonisovalent substitution of Ti4+ with Co2+ under the electrical neutrality condition results in the appearance of vacancies at oxygen sites and, hence, the signal should increase from the vacancies. Indeed, doping with up to 2% of Co results in an appreciable intensity increase of the signal from the vacancies which can still be considered isolated compare the curves (spectra 1 and 2 in the inset) of the initial sample with those of the doped sample (2 at. % of Co). Further doping of TiO2 up to 3.5% of Co2+ leads to a significant intensity decrease of the signal from vacancies more than half the intensity (curve 3 in the inset of Figure 6). The intensity decrease may be caused by a reduced number of isolated, noninteracting vacancies in the sample, which are responsible for the signal. The increase in Co2+ content in the sample up to 4.5% is accompanied by the appearance of an additional broad signal (ΔH ≃ 600 G) with g = 2.005, which is superimposed by the narrow signal from the vacancies (Figure 6, curve 4). The appearance of this wide signal possibly indicates the exchange interaction between the paramagnetic centers and may reflect formation of clusters of closely spaced Co2+ ions in the sample or the occurrence of magnetic structures where Co2+ ions interact with each other with vacancies involved. Involvement of vacancies into such structure is the reason for the observed decrease of the narrow signal intensity with g = 1.997. Possibly, the narrow signal intensity decrease (with g = 1.997) in the sample with 3.5% of Co2+ as compared to the sample with 2% of Co2+ is also due to the formation of such structure. The changes in narrow signal intensity is well detected in spectrum, and registration of symbatical emergence of the broad signal in the EPR spectrum of this sample can be hampered due to its low intensity. Annealing the sample at 1000 K in vacuum for 0.5 h leads to the formation of a broad EPR signal typical for a ferromagnetic (FM) behavior with a long-range magnetic order (Figure 6, curve 5), where all vacancies are most likely to be involved in the exchange interaction. Accordingly, one could argue that vacancies already exist in the initial state. Doping with small Co contents is accompanied by an increase in the vacancy concentration, and at a higher content of 3d metal, the vacancy might be involved in the exchange processes, showing either the formation of a shortrange magnetic order, or even a long-range order at a higher concentration of Co, for example. The signal from the vacancies, depending on the degree of their involvement in the exchange processes, may vary greatly and be absent principally due to the participation of all vacancies in the exchange processes responsible for the appearance of the spontaneous magnetic moment in the system, for example, after annealing at 1000 K. At the same time, the available EPR data are not sufficient to conclude that the magnetic ordering of any typeantiferromagnetic (AFM) or ferromagnetic (FM)is formed. Regardless of the type of magnetic order, the EPR spectra can be of the same shape qualitatively. Of paramount

importance in the context of this work is only the presence of paramagnetic centers (e.g., vacancy with unpaired electron) and their involvement in exchange processes, which are likely to be realized at the increased content of Co in TiO2 or after a reduction treatment. Magnetic Properties. For the titanium dioxide powders immediately after synthesis, the plotted magnetization curves are represented by a linear field function typical for paramagnetic materials; and by the slope of the curve, the effective magnetic moment of cobalt ions can be determined on the assumption that the cobalt ions are noninteracting paramagnetic ions. For the samples with cobalt content varying in the range of 1.5 to 4.5 at. % of Co, the magnetic moment of Co ions amounts to 4 μB approximately and, within the measurement error, does not depend on the content within the range. This value appears to be somewhat larger than the effective spin moment (3.87 μB) for S = 3/2 but is less than 4.8 μB, which is the experimentally determined value mostly reported in literature. After the samples are annealed at 1000 K in vacuum for 0.5 h (Figure 7), the behavior of the magnetization curve changes drastically, and the ferromagnetic contribution is clearly observed, which is seen to be depending on the cobalt content.

Figure 7. Magnetization curves of the TiO2:Co samples with various cobalt concentrations (1.5 at. % Co, 3.3 at. % Co, 4 at. % Co) after annealing at 1000 K for 0.5 h.

Figure 7 shows that at a higher cobalt content (4 and 4.5 at. %) the specific magnetization is a weak function of the cobalt content as opposed to the samples with the cobalt content of 1.5 and 3.5 at. %. Figure 8 shows clearly that the as-prepared Ti0.96Co0.04O2 samples exhibit primarily a paramagnetic behavior of the temperature dependence of magnetic susceptibility χ(T), which is characteristic of the Curie−Weiss law at temperatures below 100 K. A relatively small negative Néel temperature was estimated from the extrapolation of inverse susceptibility as a function of temperature which characterize the antiferromagnetic interactions (short-range order or clusterization) in the system. At low temperatures (2 K), the magnetization curve is well enough described by the Brillouin function with spin equal to S = 3/2, typical for the high-spin state of Co2+. 28862


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the absolute value of the magnetic moment of cobalt may vary widely. A few explanations may be offered to the decrease in the magnetic moment with increasing cobalt content in a TiO2 sample. One of the explanations is a possibility of antiferromagnetic interactions at higher Co content. However, a significant antiferromagnetic contribution should lead to a change in the behavior of the magnetization curve. Assuming that a high (above room temperature) Néel temperature is observed for the antiferromagnetic phase and that there is a relatively small magnitude of the anisotropy field, there should be a linear behavior of magnetization as a function of magnetic field with no saturation observed for the antiferromagnetics. However, the magnetization curve of antiferromagnetic is primarily determined by the magnitude of the negative exchange interaction between the magnetic moment carriers, which in our case are cobalt atoms. It follows from magnetic data in the fields up to 5 T, that the behavior of the magnetization curve shows no paraprocess involved or hard magnetization observed under high fields at room temperature. Hence, we can make a conclusion that the decrease in magnetization is caused by some other reason not related to the appearance of antiferromagnetism. A contribution from the low-spin states of cobalt may also be the cause for somewhat lower magnetization and the decrease in the magnetic moment of cobalt with Co content increasing. However, analysis of the literature data35 proves that the ion of Co2+ does not have the low-spin state in the compounds under consideration. Finally, another reason may be in the localization of cobalt, which preferably takes place on the surface where the vacancies with a partially delocalized electron may also be observed. That may also be responsible for the itinerant character of magnetism. Indeed, in literature, there are convincing data on the presence of oxygen vacancies to be observed both in the initial nanocrystalline samples immediately after synthesis and after reduction annealing.8,36−38 The presence of vacancies in the nanocrystalline state, especially in the near surface layers, is justified thermodynamically, even without the effect of reduction annealing, and their concentration may significantly exceed the equilibrium concentration of vacancies reported for the bulk state at a given temperature.39 Reduction annealing may lead to vacancies concentration increase. At the same time, the vacancies might be redistributed if their concentration is above the equilibrium at annealing temperature. Localization of 3d impurities (which is cobalt in our case) on the surface has received considerably less or little coverage in literature. It should be pointed out that the surface of nano-objects is also a perfect site for sinking of different types of impurities,40−42 especially in view of the presence of defects, particularly, vacancies on the surface. It has been commonly accepted that oxygen vacancies are located near the cobalt atoms. One fundamental peculiarity is also worth mentioning that the vacancies tend to move the Fermi level into the conduction band and, thus, the impurity states, being placed inside the gap, may be of metallic nature.40 Obviously, this could cause a considerable effect on the nature and mechanism of magnetism in such systems. Availability of the itinerant type magnetism may occur at all the cobalt contents, but more effectively at higher cobalt concentrations. Within this approach, given the delocalized nature of charge carriers, the magnetic moment of Co may vary within a wide range. Therefore, the observed values of the

Figure 8. Temperature dependence of the magnetic susceptibility of the Ti0.96Co0.04O2 sample. The inset shows the reduced Brillouin function (solid curve) and the experimental curve (points), measured at 2 K.

Hence, forming small areas of short-range magnetic order with spontaneous magnetization should not be ruled out either: for example, they may be formed by the magnetic structures that have frustrated exchange bonds in such areas. These results confirm qualitatively the conclusions drawn on the basis of the EPR spectra analysis, from which follows that, as cobalt content in a sample is increased, the areas of enhanced exchange interaction with a certain type of magnetic ordering in the nearest neighborhood are likely to be formed (for example, see Figure 8, curve 4). Table 3 presents the magnetic moment of cobalt atoms at room temperature, calculated for the samples with various Table 3. Magnetic Moments of the Cobalt Atoms at Room Temperature Obtained for Samples with Different Cobalt Concentrations after Annealing in Vacuum at 1000 K for 0.5 h Co content (at. %)

magnetic moments (μB)

1.5 2.1 2.5 3.3 3.5 4.0 4.5

1.7 1.3 1.4 1.3 1.2 1.2 1.0

cobalt contents after annealing at 1000 K for 0.5 h, assuming that all the cobalt atoms in titanium dioxide are involved in the formation of spontaneous magnetic moment. One can see that, unlike the previously mentioned behavior of the effective magnetic moment remaining at about 4 μB in the initial condition, after annealing of the samples at 1000 K in vacuum, the magnetic moment per cobalt atom at room temperature varies from approximately 1.7 μB to about 1 μB as the cobalt content grows to reach 4.5 at. %. One can only be sure about the clearly pronounced tendency of reducing the atomic magnetic moment per cobalt atom, rather than its absolute value. Strictly speaking, in the ferromagnetic state of cobalt, the magnetic moment depends on the nature of ferromagnetism, in the formation of which cobalt is involved, so 28863


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The Journal of Physical Chemistry C magnetic moment ranging from 1.6 to 1 μB per cobalt atom can be hypothetically explained within the itinerant magnetism model. However, the physical models presented here call for experimental justification and further comprehensive examination. Co Kα1,2 X-ray Emission Spectra. Core-to-core X-ray emission spectroscopy is a widely used probe of local spin moments (LSM) of the 3d transition elements.43,44 Sensitivity of the emission spectra to the number of unpaired 3d electrons results from the exchange interaction between a hole left in a core level in final state of the emission process and electrons of the 3d shell.45 In the case of Kα spectra (2p → 1s transition), compounds with a larger LSM of an element under investigation reveal broader Kα1 and Kα2 lines44,46 due to larger exchange splitting of 2p3/2 and 2p1/2 sublevels, respectively. The local nature of intra-atomic exchange interaction makes this method applicable to any compound irrespective of the type of long-range magnetic order. If there are several species of an investigated element with different LSMs in a compound, the resulting spectrum will be a weighted average over all the species. The Kα1,2 XES spectra of cobalt in the TiO2 matrix before and after annealing are shown in Figure 9. For the simplicity of visual inspection, the spectra have been normalized by Kα1 line height and aligned at the position of Kα1 peak maximum of metallic cobalt.

nanoparticles in the range of 30 to 40 nm were synthesized hydrothermally. In the initial state after synthesis, the nanopowders are of anatase structure; and when the cobalt content has reached 4.5 at. %, traces of the rutile phase appear. 2 The synthesis has resulted in the formation of cobalt solid solution in TiO2 matrix. Magnetic properties of such solid solutions are described well by the Curie law with the magnitude of Co magnetic moment related to the spin state S = 3/2. 3 Analysis of the EPR spectra has proved the existence of vacancies and their influence on the magnetic state of TiO2:Co. EPR spectra demonstrate that doping TiO2 with cobalt (up to 2%) is accompanied by a significant increase in the concentration of F+ centers (vacancy with unpaired electron). The amount of magnetically ordered regions with short-range order increases as cobalt content increases, with the concentration of free vacancies being significantly reduced as a result of their involvement in exchange interactions. 4 After vacuum annealing at 1000 K, along with the minor paramagnetic contribution from the matrix comprising localized Co2+ ions, the spontaneous magnetization contribution that is caused by cobalt localization and magnetic ordering on the surface which also contains defects in the anion sublattice. 5 It can be assumed that the ferromagnetic ordering results from the interaction of delocalized electrons near the vacancies and the nearest neighbor cobalt atoms due to itinerant type interaction. Finally, the core−shell model of a particle with defects and magnetic moment carriers localized in the shell provides a more consistent explanation to the experimentally observed structural and magnetic properties of Co-doped TiO2 nanoparticles. The subsurface localization of Co ions should be extensively examined.

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AUTHOR INFORMATION

Corresponding Author

*(V.R.G.) E-mail: [email protected]. ORCID

V. R. Galakhov: 0000-0002-3654-3993 Notes

The authors declare no competing financial interest.

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Figure 9. Co Kα1,2 spectra of Ti0.965Co0.035O2 nanopowder in the initial state and after annealing at 1000 K for 0.5 h. For comparison, the spectrum of CoO is shown.

ACKNOWLEDGMENTS This work was supported by the Russian Scientific Foundation, Grant No. 16-12-10004. We are grateful to Dr. N. N. Schegoleva for help and useful comments.

The spectrum of as-prepared sample coincides with that of CoO in the bulk sample. This result is in agreement with the presence of Co2+ ions in the TiO2 matrix with the local spin magnetic moment S = 3/2. For the annealed sample, significant narrowing of the Kα1 and Kα2 emission lines is observed, suggesting a corresponding decrease in the average LSM of the Co atoms after annealing. This result supports the abovementioned reduction of the average magnetic moment of cobalt atoms obtained from the magnetometry measurements.

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REFERENCES

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CONCLUSIONS 1 The nanocrystalline TiO2:Co samples with a cobalt content up to 4.5 at. % with an average size of 28864


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