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Advances in Condensed Matter Physics
Volume 2016 (2016), Article ID 1562596, 7 pages
http://dx.doi.org/10.1155/2016/1562596
Research Article

Electronic and Magnetic Properties Studies on Mn and Oxygen Vacancies Codoped Anatase TiO2

1College of Physics and Material Science, Henan Normal University, Xinxiang 453007, China
2Henan Key Laboratory of Photovoltaic Materials, Xinxiang 453007, China
3Key Laboratory of Artificial Micro- and Nano-Structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China
4College of Electronic and Electrical Engineering, Henan Normal University, Henan 453007, China

Received 11 June 2016; Accepted 23 August 2016

Academic Editor: Jörg Fink

Copyright © 2016 Zhongpo Zhou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The electronic and magnetic properties of Mn and oxygen vacancies codoped anatase TiO2 were investigated. The calculated results showed that the TiO2 codoped with Mn and oxygen vacancies have a magnetic moment value of 3.415  per Ti31MnO63 supercell. Furthermore, Ti31MnO63 gets the lowest energy with a geometrical optimization where the Mn ions locate at the nearest-neighbor sites of the oxygen vacancy. And experimental results indicated the magnetism is associated with the defects of Mn ions and oxygen vacancies induced by the Mn doping, which is consistent with the calculation results.

1. Introduction

Diluted magnetic semiconductors (DMSs), which are desirable for spintronic applications, have been extensively investigated for their magnetic and transport properties. Oxygen vacancies (s) are important intrinsic defect in TiO2 which can be manipulated by oxygen gas pressure in the process of the fabrication, and they could alternate the materials’ properties significantly [110]. The origin of the observed ferromagnetism has been considered to be related closely to the instead of the cation vacancies reported by Wang et al. [11]. The magnetization enlarges as content increases in oxygen-deficient TiO2 () films observed by Yoon et al. [8]. And the carrier is n-type and the resistivity increases upon cooling [12] according to the transport measurements. Several theoretical investigations employed by first principle have been reported, most of which focus on cation vacancy in TiO2 bulk materials [1328]. However, less attention has been drawn to the doped anatase TiO2 and -Mn codoped anatase TiO2 in those researches.

There are three questions that should be figured out: (1) what is the electronic orbital composition of a -induced state in detail for anatase TiO2? (2) What is the electronic orbital geometry for the -Mn induced states in -Mn codoped anatase TiO2? (3) What is the most stable structure for -Mn codoped anatase TiO2? It is necessary to bring further insight into the origin of the ferromagnetism in the TiO2 with the transition-metal doping, where the s play an important role in promoting long-range ferromagnetic order [29]. In this paper, we investigated the electronic and magnetic properties of Mn and s codoped anatase TiO2 (Ti31MO63) using the first-principles method based on the density functional theory (DFT). The experimental results are consistent with that of the first principles calculations, and the magnetism is associated with the defect electrons induced by the Mn doping. The connections between the doped Mn ions and s are discussed to explain the ferromagnetism observed in these materials.

2. Experiments and Model Calculations

The undoped TiO2 films were fabricated by sol-gel spin coating. Step one: Mn-doped TiO2 sol-gel solution preparation. In this step, titanium isoproxide (99%), absolute ethanol and hydrochloric acid were used as starting materials. Firstly, 10 mL titanium isoproxide was dissolved in 70 mL absolute ethanol solution. The mixture was stirred for 5 min. Then 2 mL of 2 mol/L hydrochloric acid was added to the mixture and was stirred for 5 min. And then 30 mL absolute ethanol solution and proper weight of MnCl2 (to obtain a doping concentration of 1 mol%, 3 mol%, 5 mol%, 7 mol%, and 9 mol% resp.) were added to the mixture and then were stirred for 24 h. Subsequently, the solution was kept stirred for 2 h with a heating magnetic stirrer until the sol-gel formed. Step two: the preparation of the Mn-doped anatase TiO2 films using a spin coating method. Firstly, the fluorine-doped tin oxide (FTO) substrates were cleaned in de-ion water and in absolute ethanol solution for 5 min with ultrasonic cleaner, respectively. Then, the anatase TiO2 films were prepared using TiO2 sol-gel by the spin coating method. The spin rate was 400 rpm for 10 s and then was extended to 3000 rpm for 25 s. This spin coating procedure was repeated for five times to get the ideal film thickness of 800 nm. After coating, these samples were treated at 450°C in atmosphere for 2 h and cooled down naturally. All the reagents of acetone, absolute ethanol, titanium isoproxide, manganese dichloride, and hydrochloric acid were of analytical grade without further purification. The surface morphologies of TiO2 films were observed by the scanning electron microscopy (SEM, S-4800). X-ray photoelectron spectroscopy (XPS, VG Multilab 2000x) was utilized to qualify and the chemical compositions and identify electronic structures. The binding energy of the XPS spectra was calibrated with the reference to the C 1 s peak at 284.6 eV. The magnetic properties were investigated using a vibrating sample magnetometer (VSM) equipped in the physical property measurement system (PPMS-9, Quantum Design).

First-principles calculations based on spin-polarized density-functional theory and projector augmented wave (PAW) pseudopotential technique are performed as implemented within the Vienna Ab-Initio Simulation Package (VASP) [30, 31]. The generalized gradient approximation (GGA-PBE) for the wave functions is used with a cutoff of 400 eV to model the exchange and correlation functional [32]. The calculations have been carried out for two cases: (1) an oxygen atom is substituted by a ; (2) a titanium (Ti) atom and an oxygen atom are substituted by a Mn atom and a . The Monkhorst-Pack scheme k-points grid sampling was set to be 2 × 2 × 5 for the 95-atom anatase supercell, Ti31MnO63 (corresponding to doping content of Mn ~ 3.13%). The valence electrons configuration for the O, Ti, and Mn are 2s2 2p4, 3s2 3p6 3d2 4s2, and 3d5 4s2, respectively. All the atomic positions are fully optimized until the atom forces drop below the value 0.02 eV/Å. The DFT method has been proven to be one of the most accurate methods for the computation of the electronic structure of solids [47].

3. Results and Discussions

3.1. Electronic Properties and Magnetism of a Doped TiO2

According to the crystal field theory, due to the hybridization of Ti–O, the Ti 3d orbits split into two parts, the (, , ) and (, ) states. Geometrically, orbitals have lobes pointing between oxygen atoms, while orbitals point toward oxygen. The O 2p orbit split into and states. O 2p and the (, ) of Ti 3d devoted to the valence band ( devote to the top of valance band), while the conduction band was contributed by the and of Ti 3d ( devotes to the bottom of conduction band) [33]. In the ionic limit, each Ti gives four electrons to two oxygen atoms, resulting in the nominal charges of Ti4+ and O2− [12]. When the is introduced into the lattice, each is assumed to donate two electrons. For Figure 1(b) (I), the TDOS of -doped anatase TiO2, there is a spin-split lower around the Fermi level illustrating the existence of magnetism. Such a state is stabilized in the spin-polarized DFT calculation (with a lower free energy of 0.0042 eV than the result of non-spin-polarized DFT calculation). The calculated net magnetic moment is about 0.533 . Figure 1(a) (II, III, and IV) shows the orbital decomposed density of states for three Ti 3d electrons nearest to the . Three Ti atoms are labeled as Ti-II, Ti-III, and Ti-IV, with Ti-III and Ti-IV being equivalent with respect to the vacancy. It can be seen from the figure that, for Ti-II atom, the exchange spin-splitting at the bottom of the conduction band is mainly derived from the one orbital () and two orbitals ( and ), while the exchange spin-splitting at the bottom of the conduction band is mainly derived from the one orbital () and two orbitals ( and ) for Ti-II and Ti-IV atom. The orbitals overlap between the three Ti atoms nearest to the . The hybridization between these three Ti 3d electron states leads to the electrons of Ti 3d spin-polarized. The charge distribution is illustrated in Figure 1(b). It can be seen that the state spread over all three Ti atoms adjacent to the vacancy and the local orbitals are the mixture of and The charge distribution is similar to the charge distribution of the hybrid state in [12]. In the calculations, the results are obtained in a Ti32O63 anatase supercell with lowest total free energy for different sites. The two electrons donated by one are shared by the three Ti ions with up-spin forming Ti3+ ion to get the lowest energy. It is guessed that the appearance of the net magnetic moment in anatase TiO2 may come from the larger crystal lattice distortion compared to rutile TiO2.

Figure 1: (a) Charge distribution of -doped anatase TiO2. (b) TDOS of -TiO2 and the orbitally decomposed density of states for three Ti 3d electrons nearest to . The olive and black dashed curves represent the bands; the red, blue, and magenta solid curves represent the bands, respectively.
3.2. Electronic Properties and Magnetism of -Mn Codoped TiO2

Magnetic properties of Ti31MnO63 were investigated using the first-principles methods based on the DFT. The lowest energies are achieved after geometrical optimization of Ti31MnO63 for each -Mn codoping models, where doped Mn ions are located at the nearest-neighbor site of . The result is similar to that of the report done recently [34].

In order to understand the nature of ferromagnetism, the electronic properties and magnetism of -Mn codoped TiO2 are investigated in detail. The lowest energy is obtained in the spin-polarized DFT calculation. Figure 2 shows the spin-resolved total density of states for the Ti31MnO63. The calculations show ferromagnetism with clear majority-spin defect states localized in the band gap for -Mn codoped TiO2. The major magnetic moment originates from the d electron of Mn ions. For -Mn codoped TiO2, the valence electrons configuration of doped Mn atom is 3d5. And the calculated net magnetic moment is about 3.415 , which is mainly contributed by doped Mn ion (calculated net magnetic moment is about 3.323 ).

Figure 2: Spin-resolved total density of states for the Mn codoped TiO2 in combination with an oxygen vacancy and spin- and orbital-resolved PDOS for the doped Mn atom.

The isosurface plot of magnetization density (majority-spin minus minority-spin) for Ti31MnO63 supercell and the orbital decomposed density of states projected on Mn ion are shown in Figure 3. Spin density distribution of Ti31MnO63 supercell is shown in Figure 3(a). It can be noticed that the spin densities are mainly distributed on the doped Mn ion. It holds the spin with up direction. The values of calculated band lengths for -Mn, -Ti (III), and -Ti (IV) are 1.8073 Å, 2.09822 Å, and 2.16686 Å, respectively. The lowest energy is achieved after geometrical optimization of Ti31MnO63, where one Mn ion is located at the nearest-neighbor site of .

Figure 3: Isosurface plot of magnetization density (majority-spin minus minority-spin) for (a) Ti31MnO63 supercell; (b) the orbitally decomposed density of states projected on Mn ion.

The orbital decomposed density of states projected on Mn ion is presented in Figure 3(b). There is a complete exchange spin-splitting of Mn 3d appearing within the Fermi level. The results indicate that there are and states of Mn 3d electrons with up spin within the Fermi level. The valence electrons configuration on the Mn is 3s2 3p6 3d5. The orbital decomposed density of states projected on Ti II 3d electrons and Ti IV 3d electrons nearest to for (a) Ti31MnO63 supercell is noticed in Figure 4. As it can be seen, there is almost no spin-splitting of Ti II 3d electrons and Ti IV 3d electrons nearest to within the Fermi level. The results indicate that there are no electron states of Ti 3d electrons appearing within the Fermi level, which is consistent with the result of Figure 3. That just says that, due to the difference in the electronegativity and valence between Ti4+ ion and Mn2+ ions, the effect of the substitution is that no electron is donated to the TiO2 lattice instead of two. Two electrons denoted by one are captured by doped Mn ion and two electrons donated by the doped Mn2+ ion will be captured by five O ions nearest. As a result, the total net magnetic moment of -Mn codoped TiO2 is 3.415 .

Figure 4: The orbital decomposed density of states projected on Ti II 3d electrons and Ti IV 3d electrons nearest to for Ti31MnO63 supercell.
3.3. Experiment Results

The calculation results are confirmed by experimental data obtained for Mn-doped TiO2 films. The Mn-doped TiO2 films (with 1%, 3%, 5%, 7%, and 9% doping content) were grown by prepared by sol-gel spin coating. Figure 5(a) demonstrates the SEM images of the sample, which suggests that the film is smooth and the size of particles is about 10 nm.

Figure 5: (a) SEM image of Ti0.97Mn0.03O2 film; (b) XPS spectra of the core-level signal of Ti-2p and (c) Mn-2p of Ti0.97Mn0.03O2 film; (d) magnetic hysteresis curves of the Mn-doped TiO2 films with 1%, 3%, 5%, 7%, and 9% doping content measured at 300 K.

The binding state of Ti and Co in the as-synthesized cobalt incorporated TiO2 is analyzed with XPS. XPS analysis is performed after sputtering with Ar+. The Ti-2p and Co-2p core levels spectra of the Ti0.97Mn0.03O2 doped sample are shown in Figures 5(b) and 5(c). Figure 5(b) displays the Ti-2p core level XPS spectrum for Ti0.97Al0.03O2 film sample. The open circles are the experimental data, the blue line is the background, the red line is the sum, and the peaks related to Ti4+ are fitted with magenta lines, where Ti 2p3/2 core level spectrum was fitted with combined Gaussian-Lorentzian functions. The main peak of Ti 2p3/2 situates around at 458.5 eV and a low intensity minor peak at 464.2 eV is attributed to Ti 2p1/2, close to the binding energies of the core-level of Ti4+ ion, which indicates the existence of Ti4+. No signal of Ti3+ was detected. The Ti3+ ions were also not observed in other Mn-doped films, which is not like the case of Nb- or Fe-doped TiO2 films, due to the difference in the electronegativity and valence between Mn ion, Nb ion, and Fe ion [35, 36]. Figure 5(c) shows the Mn-2p core-level XPS spectrum for Ti0.97Mo0.03O2 films. The open circles are the experimental data, the blue line is the background, the red line is the sum, and the peaks related to Mn2+ ion are fitted with magenta lines. The main peak of Mn 2p3/2 situates around at 641.2 eV and a low intensity minor peak at 653.6 eV is attributed to Mn 2p1/2, respectively, close to the binding energies of the core level of Mn2+ ion and lower to the binding energies of the core-levels of Mn3+ ion and Mn4+ ion, which indicates the existence of Mn2+.

Figure 5(d) indicates the magnetic hysteresis (M-H) curves of Mn-doped samples (with 1%, 3%, 5%, 7%, and 9% doping content) at room temperature. All the Mn-doped samples showed ferromagnetism at room temperature (RT). The saturation magnetization MS increases rapidly as the Mn content increases from 1% to 9%, alike other reports [37]. The magnetic moment per doped Mn atom exhibits more or less constant values for the 1% to 9% samples, up to the present experimental data.

Combining the calculation and experimental results presented above, we introduce a defect electron based model for the observed ferromagnetism. The magnetic moment associated with a Mn2+//Ti4+ complex is spread over two or more neighboring Ti sites. These moments are aligned ferromagnetic. The major contribution to the magnetic moment of our Mn-doped TiO2 films comes from the unpaired d electron of Mn2+, consistent with our first-principles calculation.

4. Conclusions

Electronic and magnetic properties of Ti31MnO63 have been studied combined with experiments and the first-principles calculations. Magnetic moment value is 3.415 per supercell for the -Mn codoped anatase TiO2. The lowest energy is achieved after geometrical optimization of Ti31MnO63, where doped Mn atoms are located at the nearest-neighbor site of . Four electrons donated by the the doped Mn2+ ion are shared by the nearest five O ions. The major magnetic moment originates from the d electron of Mn. Ferromagnetic behavior in -Mn doped TiO2 is corresponding to the strong Mn d-shell contribution and s. The calculation results are confirmed by experimental data of Mn-doped TiO2 films.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work is supported by the NSFC nos. 11404100, 11175135, 10904116, and 11304083, the Post-Doctoral Research Foundation of Henan Province no. 01026500204, and the Scientific Research Foundation for PhD of Henan Normal University nos. 01026500257 and 01026500121. This work is also supported by the High Performance Computing Center of Henan Normal University.

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