- Email: [email protected]
TiO2 superlattice
Materials Chemistry and Physics xxx (2016) 1e4
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Magnetic and optical properties of WO3/TiO2 superlattice Tian Jiao a, Junrong Jiao b, * a b
College of Mechanical Engineering, Sichuan University of Science & Engineering, Zigong 643000, China School of Materials Science and Technology, Taiyuan University of Science and Technology, Taiyuan 030024, China
h i g h l i g h t s We found the magnetic phenomenon in the WO3/TiO2 thin film. The electrons transfer from TiO2 and WO3 induce the occurrence of ferromagnetism. The electrons transfer between TiO2 and WO3 shift the absorption region to visible light.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2015 Received in revised form 3 February 2016 Accepted 17 February 2016 Available online xxx
The electronic structure and properties of WO3/TiO2 superlattice were investigated by the calculation and experiment. The O electrons transfer the spin angular momentum from the unoccupied Ti eg states electrons to W eg states electrons and let Ti and W electrons spin splitting, which were the reason of ferromagnetism occurrence, while the electrons transfer from the CB of TiO2 to the CB of WO3. Coupling TiO2 and WO3 greatly reduce the recombination rate of electron and hole and shift the absorption region to visible light because of the electrons transfer between TiO2 and WO3. © 2016 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Ab initio calculations Magnetic properties Optical properties
1. Introduction Spuerlattices are materials with narrow-band-gap and wideband-gap, which can improve the effective electron quantity and activity [1e3]. Coupling nanometer TiO2 with narrow band gap semiconductor (CdS、MgO、WO3、Fe2O3) may apparently improve the active of TiO2 and induce the ferromagnetism occurrence. Researchers [4e6] have reported that coupled TiO2 with WO3 (WO3/TiO2) can decrease the photo-induced hole-electrons recombination rate and the light absorption region transfer to visible light. Coey's group [7] discovers the high-temperature ferromagnetism of HfO2 thin film and firstly proposes the anisotropic high-temperature d0 ferromagnetism. Yoon's group [8] finds that the oxygen vacancies surrounding Ti2þ and Ti3þ ions potentially can give rise to magnetism of TiO2-d films. The roomtemperature ferromagnetism of SmCo5/Co films are induced by strong interaction of Sm and Co [9]. The Fe2O3/TiO2 films [10]
* Corresponding author. E-mail address: [email protected] (J. Jiao).
possess higher photocatalysis efficiency than pure TiO2 films and the Fe2O3/TiO2 composite particles [11] possess room-temperature ferromagnetism which comes mainly from Fe2O3 phase. Choi et al. [12] proved the existence of uniaxial magnetic anisotropy in epitaxial Fe/MgO films on GaAs (001). However, the superlattice materials all have magnetic phases and it is of great significance to develop new type superlattice magnetic materials. Herein, we investigated the micro-structure, optical and magnetic properties of WO3/TiO2 thin films by the coupling method of experiment and calculation. We found the ferromagnetism of WO3/TiO2 thin films and explain the occurrence reason of ferromagnetism. 2. Calculations and experiments The calculation method of pure TiO2 and WO3/TiO2 superlattice are consistence with the calculation method described in Ref. [13], but the parameter setting is not exactly same. The CASTEP code was used to calculation, the electron wave function was expanded in plane waves up to cut-off energy of 450 eV. The Monkhorst-Pack scheme k-points grid sampling was set to be 4 4 5 for the
http://dx.doi.org/10.1016/j.matchemphys.2016.02.051 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Jiao, J. Jiao, Magnetic and optical properties of WO3/TiO2 superlattice, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.051
2
T. Jiao, J. Jiao / Materials Chemistry and Physics xxx (2016) 1e4
spectroscopic analysis at 508 nm using a UV spectrometer (UV1801, China).
Table 1 The band gap of anatase TiO2 and WO3 with different U values. U (eV) Eg of TiO2 (eV) Eg of WO3 (eV)
0.00 2.14 1.48
3.00 2.55 2.08
5.00 2.83 2.47
7.00 3.05 2.71
8.00 3.18 2.84
8.50 3.22
9.00 3.29
unit cell and 8 8 10 for the supercell. As displayed in Table 1, the U ¼ 8.50 eV for Ti 3d and U ¼ 7.00 eV for W 5d were selected in subsequent calculations, because of the bandgap of TiO2 and WO3 were 3.22 eV and 2.71 eV, respectively, which is consistent with the experiment ones [14,15]. The 2 2 1 superlattice of TiO2 was built (Fig. 1a) for pure TiO2 calculation. The lattice plane of TiO2 and WO3 were (002) and (101) respectively, in the WO3/TiO2 crystal model [6]. As shown in Fig. 1, the crystal model b, c and d were named W4T4、W4T6 and W4T8, respectively, because of the atom number of the W and Ti. The a, b, a, b, g of W4T4、W4T6 and W4T8 are the same, and a ¼ 0.536028 nm, b ¼ 0.446862 nm, a ¼ b ¼ 90.0000 , g ¼ 100.148 , but c of W4T4、W4T6 and W4T8 are different, c ¼ 1.48067 nm for W4T4, c ¼ 1.83149 nm for W4T6, c ¼ 2.26029 nm for W4T8. All the models were geometrically optimized, then total density of states (TDOS) and partial density of states (PDOS) were calculated and analyzed. The WO3/TiO2 thin films were prepared by magnetron sputtering method. At firstly, spurting Ti on 3 sheets cleaned ultraviolet quartz glasses for 15 min, then the sheet glasses with coating were calcined at 500 C for 0.5 h. Secondly, spurting W on the glasses with coating for 5 min, 10 min and 15 min, respectively. Then the glasses were calcined at 500 C for 0.5 h. The WO3/TiO2 thin films based on silicon slices were prepared by the same method. The WO3/TiO2 thin films were named W5T15, W10T15 and W15T15 because of the W and Ti spurting time, respectively. The crystallite phases of the samples were identified by XRD on X'pert Philips using Cu Ka (l ¼ 0.15406 nm) radiation operating at 40 kV and 30 mA at a rate of 0.03 /s. The diffraction data were recorded between 15 and 80 . The magnetic properties of WO3/ TiO2 were characterized by VSM (Versalab, Quantum DeSign, USA) at room temperature. The absorption spectra were determined by
3. Results and discussion According to the crystal field theory, due to the hybridization of TieO and WeO, the Ti 3d and W 5d orbits split into two parts, the t2g (dxy, dxz, dyz) and eg (dz2 , dx2 y2 ) states, meanwhile the O 2p orbit split into pp and ps states. O 2p and the t2g (dxz, dyz) of Ti 3d and W 5d devoted to the valence band (VB) (pp devote to the top of valance band), while the conduction band (CB) was contributed by the dxy and eg of Ti 3d and W 5d (dxy devote to the bottom of conduction band) [16]. Fig. 2 shows the TDOS and PDOS of TiO2 and the TDOS and PDOS of WO3/TiO2, respectively. Comparing with pure TiO2 (3.22 eV), the W4T4 (1.45 eV) and W4T6 (0.62 eV) have more narrow band gap, and the W4T8 has not obviously forbidden band. These illustrate that coupling TiO2 with WO3 can decrease the energy required for electron transfer and shift the light absorption region from ultraviolet to visible light. As shown in Fig. 2a, there is no splitting between the spin-up and spin-down states, which confirms that TiO2 has not magnetism. For the TDOS of WO3/TiO2 (Fig. 2b (I), c (I) and d (I)), there is a spin-split around the Fermi Level illustrating the existence of magnetism. For the PDOS of W 5d, O 2p and Ti 3d (Fig. 2b (II, III and IV), 2c (II, III and IV), 2d (II, III and IV)), there are also exchange splitting around the Fermi level between the spin-up and spindown states, and the magnetic moment was mainly devoted by W and Ti atoms. The bottom of CB (dxy) move to a low energy region and the top of VB (pp) move to a high energy region with the increasing of TiO2. This is the reason of why there is not obviously forbidden band in W4T8. The XRD patterns for the pure and coextruded films are displayed in Fig. 3. The diffraction peaks of each sample can be indexed to anatase phase with space group I41/amd (141) and the tungsten oxide phase with space group P21/n and P4/nmm, which indicate that the W has not complete oxidation in W10T15 and W15T15 samples for calcined 0.5 h. Researchers [17e19] found the ferromagnetism of TiO2 films and indicated that the ferromagnetism of TiO2 thin films were
Fig. 1. The 2 2 1 superlattice model of TiO2(a) and superlattice crystal model of WO3/TiO2. The number ratio of W and Ti atoms were 4:4 (b), 4:6 (c), 4:8 (d).
Please cite this article in press as: T. Jiao, J. Jiao, Magnetic and optical properties of WO3/TiO2 superlattice, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.051
T. Jiao, J. Jiao / Materials Chemistry and Physics xxx (2016) 1e4
3
Fig. 2. The TDOS for TiO2 (a I), and PDOS for Ti 3d electrons (a II) and its nearest-neighboring O 2p electrons (a III). The TDOS for W4T4, W4T6 and W4T8 (b I, c I, d I), and PDOS for W 5d electrons (b II, c II, d II), O 2p electrons (b III, c III, d III) and Ti 3d electrons (b IV, c IV, d IV). The W, O and Ti locate at the interface of WO3 and TiO2. Spin-up/down states are plotted along the positive/negative ordinates with the Fermi Level (Ef) shown by a vertical line. The position of Ti, O and W atoms were marked in Fig. 1.
Fig. 3. XRD spectra of pure TiO2 (a), W5T15 (b), W10T15 (c) and W15T15 (d) samples.
mainly devoted by the oxygen vacancies which were formed in the preparing process. But in our experiment, the XRD results show that the Ti were oxidized completely, and we did not find the existence of ferromagnetism of pure TiO2 film. So the behavior of ferromagnetism was not detected. The hysteresis loops of WO3/ TiO2 samples which indicate the coupled samples having the behavior of magnetism are displayed in Fig. 4a. The magnetic moment of W15T15 sample was much higher than that of W5T15 and W10T15 samples, which because of there are a great quantity of oxygen vacancies in W15T15 sample (as shown in Fig. 3). The oxygen vacancies [17e19] are considered to be F-center (oxygen vacancies) mediated bound magnetic polaron which can increases the magnetic moment. As shown in Fig. 4b, the absorption spectra of doped samples shift to visible light region, and the absorbance intensifies with increase of WO3 layer thickness. The oxygen vacancies in W15T15, which can induce the impurity bands occur in forbidden band,
Fig. 4. The hysteresis loops (a) of WO3/TiO2 samples and the diffuse reflectance UVevis DRS spectra (b) of pure TiO2 and WO3/TiO2 samples.
Please cite this article in press as: T. Jiao, J. Jiao, Magnetic and optical properties of WO3/TiO2 superlattice, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.051
4
T. Jiao, J. Jiao / Materials Chemistry and Physics xxx (2016) 1e4
bound magnetic polaron. The composite system can extensively apply as information transfer, new type light control superlattice devices, sewage disposal and so on. Acknowledgments The authors are pleased to acknowledge the guidance of Dr. Changwei Gong and Yuming Tian, the financial supported provided for this research by the National Natural Science Foundation of China (Grant No. 11204199) and the National Research Foundation for the Doctoral Program of Higher Education of China (Grant No.20111415120002). Fig. 5. The electrons transfer mode of WO3/TiO2 film.
were also contributed to the absorption spectra shift [20]. These results were consistent with other research ones [19]. The band gaps of the TiO2 and WO3/TiO2 samples were usually estimated from the absorption thresholds wavelength according to the following equation:
a¼
Kðhv EgÞ1=n hv
where a was the absorbance, K was a constant, n equals 2 for indirect transition and 1/2 for the direct transition [21]. The band gap of TiO2 was evaluated to be 3.32 eV (372 nm) from the plot of (ɑhv)n versus energy (hv), not shown in this paper, since TiO2 was considered as an indirect semiconductor. Similarly, the band gaps of W5T15, W10T15 and W15T15 samples were estimated to be 2.75 eV (451 nm), 2.62 eV (474 nm) and 2.51 eV (495 nm), respectively. As displayed in Fig. 5, the electrons on CB of TiO2 transfer to lower energy orbit CB of WO3 in the WO3/TiO2 superlattice. The electrons transfer mode greatly reduced the energy required for electrons transfer and the recombination rate of electron and hole, and shift the absorption region to visible light. When the O electrons are excited and transfer to CB, the spin angular momentum of unoccupied Ti eg states electrons transfer to O electrons and the unoccupied Ti eg states electrons spin splitting when O electrons leave. The O electrons have spin memory and transfer the spin angular momentum to W eg states electrons while the electrons transfer from the CB of TiO2 to the CB of WO3, and the spin angular momentum lead to the spin splitting of W eg states electrons. The spin splitting of Ti and W electrons were the reason of ferromagnetism occurrence in WO3/TiO2 system. This is consistent with calculation results, the O mainly transfer the spin angular momentum instead of devoting the magnetic moment. 4. Conclusion Coupling TiO2 and WO3 greatly reduce the energy required for electrons transfer and the recombination rate of electron and hole, and shift the absorption region to visible light because of the hole and electrons transfer between TiO2 and WO3. The O electrons transfer the spin angular momentum from the unoccupied Ti eg states electrons to W eg states electrons and let Ti and W electrons spin splitting while the electrons transfer from the CB of TiO2 to the CB of WO3. The spin splitting of Ti and W electrons were the reason of ferromagnetism occurrence in WO3/TiO2 system and the O mainly transfer the spin angular momentum instead of devoting the magnetic moment. The oxygen vacancies in WO3/TiO2 thin films also contributed to ferromagnetism because of mediated
References [1] K. Wang, R.F. Chen, C.W. Chen, R.C.C. Ward, Patterning effects on magnetic reversal properties in epitaxial-grown Laves phase DyFe2/YFe2 superlattice, J. Magn. Magn. Mater 377 (2015) 295e297. [2] Min Luo, Guanxia Yu, Lijuan Xia, Calculation of conductance for triangular multi-barrier structure in a constant electric field, Superlattice Microstruct. 83 (2015) 168e175. [3] Jai-Lin Tsai, Qi-Shao Luo, Po-Ran Chen, Yun-Ting Tseng, Magnetic properties and microstructure of FePt/MoC/CrRu films, J. Magn. Magn. Mater 382 (2015) 335e341. [4] Ying Li, Pei-Chi Hsu, Shen-Ming Chen, Multi-functionalized biosensor at WO3TiO2 modified electrode for photoelectrocatalysis of norepinephrine and riboflavin, Sensors Actuators B 174 (2012) 427e435. [5] M.R. Bayati, F. Golestani-Fard, A.Z. Moshfegh, et al., A photocatalytic approach in micro arc oxidation of WO3-TiO2 nano porous semiconductors under pulse current, Mater. Chem. Phys. 128 (2011) 427e432. [6] J. He, Q.Z. Cai, D. Zhu, et al., In-situ preparation of WO3/TiO2 composite film with increased photo quantum efficiency on titanium substrate, Curr. Appl. Phys. 11 (2011) 98e100. [7] M. Venkatesan, C.B. Fitzgerald, J.M.D. Coey, Thin films: unexpected magnetism in a dielectric oxide, Nature 430 (2004) 630. [8] S.D. Yoon, Y. Chen, A. Yang, et al., Magnetic semiconducting anatase TiO2d grown on (100) LaAlO3 having magnetic order up to 880K, J. Magn. Magn. Mater 309 (2007) 171e175. [9] P. Chowdhurya, M. Krishnana, Harish C. Barshiliaa, et al., Structural and magnetic properties of SmCo5/Co exchange coupled nanocomposite thin films [J], J. Magn. Magn. Mater. 342 (2013) 74e79. [10] H. Liu, H.K. Shon, X.A. Sun, et al., Preparation and characterization of visible light responsive Fe2O3-TiO2 composites[J], Appl. Surf. Sci. 257 (2011) 5813e5819. [11] Suiyuan Chen, Yikun Zhang, Weili Han, et al., Synthesis and magnetic properties of Fe2O3-TiO2 nano-composite particles using pulsed laser gas phase evaporation-liquid phase collecting method[J], Appl. Surf. Sci. 283 (2013) 422e429. [12] Jun Woo Choi, Hyung-jun Kim, Kyung-Ho Kim, et al., Uniaxial magnetic anisotropy in epitaxial Fe/MgO films on GaAs(001), J. Magn. Magn. Mater. 360 (2014) 109e112. [13] Chang wei Gong, Jun Rong Jiao, Jia Heng Wang, Wei Shao, Structural, optical and magnetic properties of W-doped TiO2: theory and experiment, Phys. B 457 (2015) 140e143. [14] S.M. Baizaee, N. Mousavi, First-principles study of the electronic and optical properties of rutile TiO2, Phys. B 404 (2009) 2111e2116. [15] Soyeon An, Sunghoon Park, Hyunsung Ko, Chongmu Lee, Fabrication of WO3 nanotube sensors and their gas sensing properties, Ceram. Int. 40 (2014) 1423e1429. [16] QiLi Chen, Bo Li, Guang Zheng, KaiHua He, AnShou Zheng, First-principles calculations on electronic structures of Fe-vacancy-codoped TiO2 anatase (101) surface, Phys. B 406 (2011) 3841e3846. [17] S.D. Yoon, Y. Chen, A. Yang, et al., Oxygen-defect-induced magnetism to 880 K in semiconducting anatase TiO2-d films, J. Phys. Condens. Matter. 18 (2006) L355eL361. [18] C. Sudakar, P. Kharel, R. Suryanarayanan, et al., Room temperature ferromagnetism in vacuum-annealed TiO2 thin films, J. Magn. Magn. Mater 320 (2008) L31eL36. [19] A. Hassini, J. Sakai, J.S. Lopez, N.H. Hong, Magnetism in spin-coated pristine TiO2 thin films, Phys. Lett. A 372 (2008) 3299e3302. [20] Xiaoye Xina, Tao Xuc, Jiao Yina, Lan Wanga, Chuanyi Wanga, Management on the location and concentration of Ti3þ in anatase TiO2 for defects-induced visible-light photocatalysis, Appl. Catal. B Environ. 176e177 (2015) 354e362. [21] L. Cui, F. Huang, M. Niu, L. Zeng, J. Xu, Y. Wang, A visible light active photocatalyst: nano-composite with Fe-doped anatase TiO2 nanoparticles coupling with TiO2(B) nanobelts, J. Mol. Catal. A Chem. 326 (2010) 1e7.
Please cite this article in press as: T. Jiao, J. Jiao, Magnetic and optical properties of WO3/TiO2 superlattice, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.051
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Magnetic and optical properties of WO3/TiO2 superlattice Tian Jiao a, Junrong Jiao b, * a b
College of Mechanical Engineering, Sichuan University of Science & Engineering, Zigong 643000, China School of Materials Science and Technology, Taiyuan University of Science and Technology, Taiyuan 030024, China
h i g h l i g h t s We found the magnetic phenomenon in the WO3/TiO2 thin film. The electrons transfer from TiO2 and WO3 induce the occurrence of ferromagnetism. The electrons transfer between TiO2 and WO3 shift the absorption region to visible light.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2015 Received in revised form 3 February 2016 Accepted 17 February 2016 Available online xxx
The electronic structure and properties of WO3/TiO2 superlattice were investigated by the calculation and experiment. The O electrons transfer the spin angular momentum from the unoccupied Ti eg states electrons to W eg states electrons and let Ti and W electrons spin splitting, which were the reason of ferromagnetism occurrence, while the electrons transfer from the CB of TiO2 to the CB of WO3. Coupling TiO2 and WO3 greatly reduce the recombination rate of electron and hole and shift the absorption region to visible light because of the electrons transfer between TiO2 and WO3. © 2016 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Ab initio calculations Magnetic properties Optical properties
1. Introduction Spuerlattices are materials with narrow-band-gap and wideband-gap, which can improve the effective electron quantity and activity [1e3]. Coupling nanometer TiO2 with narrow band gap semiconductor (CdS、MgO、WO3、Fe2O3) may apparently improve the active of TiO2 and induce the ferromagnetism occurrence. Researchers [4e6] have reported that coupled TiO2 with WO3 (WO3/TiO2) can decrease the photo-induced hole-electrons recombination rate and the light absorption region transfer to visible light. Coey's group [7] discovers the high-temperature ferromagnetism of HfO2 thin film and firstly proposes the anisotropic high-temperature d0 ferromagnetism. Yoon's group [8] finds that the oxygen vacancies surrounding Ti2þ and Ti3þ ions potentially can give rise to magnetism of TiO2-d films. The roomtemperature ferromagnetism of SmCo5/Co films are induced by strong interaction of Sm and Co [9]. The Fe2O3/TiO2 films [10]
* Corresponding author. E-mail address: [email protected] (J. Jiao).
possess higher photocatalysis efficiency than pure TiO2 films and the Fe2O3/TiO2 composite particles [11] possess room-temperature ferromagnetism which comes mainly from Fe2O3 phase. Choi et al. [12] proved the existence of uniaxial magnetic anisotropy in epitaxial Fe/MgO films on GaAs (001). However, the superlattice materials all have magnetic phases and it is of great significance to develop new type superlattice magnetic materials. Herein, we investigated the micro-structure, optical and magnetic properties of WO3/TiO2 thin films by the coupling method of experiment and calculation. We found the ferromagnetism of WO3/TiO2 thin films and explain the occurrence reason of ferromagnetism. 2. Calculations and experiments The calculation method of pure TiO2 and WO3/TiO2 superlattice are consistence with the calculation method described in Ref. [13], but the parameter setting is not exactly same. The CASTEP code was used to calculation, the electron wave function was expanded in plane waves up to cut-off energy of 450 eV. The Monkhorst-Pack scheme k-points grid sampling was set to be 4 4 5 for the
http://dx.doi.org/10.1016/j.matchemphys.2016.02.051 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Jiao, J. Jiao, Magnetic and optical properties of WO3/TiO2 superlattice, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.051
2
T. Jiao, J. Jiao / Materials Chemistry and Physics xxx (2016) 1e4
spectroscopic analysis at 508 nm using a UV spectrometer (UV1801, China).
Table 1 The band gap of anatase TiO2 and WO3 with different U values. U (eV) Eg of TiO2 (eV) Eg of WO3 (eV)
0.00 2.14 1.48
3.00 2.55 2.08
5.00 2.83 2.47
7.00 3.05 2.71
8.00 3.18 2.84
8.50 3.22
9.00 3.29
unit cell and 8 8 10 for the supercell. As displayed in Table 1, the U ¼ 8.50 eV for Ti 3d and U ¼ 7.00 eV for W 5d were selected in subsequent calculations, because of the bandgap of TiO2 and WO3 were 3.22 eV and 2.71 eV, respectively, which is consistent with the experiment ones [14,15]. The 2 2 1 superlattice of TiO2 was built (Fig. 1a) for pure TiO2 calculation. The lattice plane of TiO2 and WO3 were (002) and (101) respectively, in the WO3/TiO2 crystal model [6]. As shown in Fig. 1, the crystal model b, c and d were named W4T4、W4T6 and W4T8, respectively, because of the atom number of the W and Ti. The a, b, a, b, g of W4T4、W4T6 and W4T8 are the same, and a ¼ 0.536028 nm, b ¼ 0.446862 nm, a ¼ b ¼ 90.0000 , g ¼ 100.148 , but c of W4T4、W4T6 and W4T8 are different, c ¼ 1.48067 nm for W4T4, c ¼ 1.83149 nm for W4T6, c ¼ 2.26029 nm for W4T8. All the models were geometrically optimized, then total density of states (TDOS) and partial density of states (PDOS) were calculated and analyzed. The WO3/TiO2 thin films were prepared by magnetron sputtering method. At firstly, spurting Ti on 3 sheets cleaned ultraviolet quartz glasses for 15 min, then the sheet glasses with coating were calcined at 500 C for 0.5 h. Secondly, spurting W on the glasses with coating for 5 min, 10 min and 15 min, respectively. Then the glasses were calcined at 500 C for 0.5 h. The WO3/TiO2 thin films based on silicon slices were prepared by the same method. The WO3/TiO2 thin films were named W5T15, W10T15 and W15T15 because of the W and Ti spurting time, respectively. The crystallite phases of the samples were identified by XRD on X'pert Philips using Cu Ka (l ¼ 0.15406 nm) radiation operating at 40 kV and 30 mA at a rate of 0.03 /s. The diffraction data were recorded between 15 and 80 . The magnetic properties of WO3/ TiO2 were characterized by VSM (Versalab, Quantum DeSign, USA) at room temperature. The absorption spectra were determined by
3. Results and discussion According to the crystal field theory, due to the hybridization of TieO and WeO, the Ti 3d and W 5d orbits split into two parts, the t2g (dxy, dxz, dyz) and eg (dz2 , dx2 y2 ) states, meanwhile the O 2p orbit split into pp and ps states. O 2p and the t2g (dxz, dyz) of Ti 3d and W 5d devoted to the valence band (VB) (pp devote to the top of valance band), while the conduction band (CB) was contributed by the dxy and eg of Ti 3d and W 5d (dxy devote to the bottom of conduction band) [16]. Fig. 2 shows the TDOS and PDOS of TiO2 and the TDOS and PDOS of WO3/TiO2, respectively. Comparing with pure TiO2 (3.22 eV), the W4T4 (1.45 eV) and W4T6 (0.62 eV) have more narrow band gap, and the W4T8 has not obviously forbidden band. These illustrate that coupling TiO2 with WO3 can decrease the energy required for electron transfer and shift the light absorption region from ultraviolet to visible light. As shown in Fig. 2a, there is no splitting between the spin-up and spin-down states, which confirms that TiO2 has not magnetism. For the TDOS of WO3/TiO2 (Fig. 2b (I), c (I) and d (I)), there is a spin-split around the Fermi Level illustrating the existence of magnetism. For the PDOS of W 5d, O 2p and Ti 3d (Fig. 2b (II, III and IV), 2c (II, III and IV), 2d (II, III and IV)), there are also exchange splitting around the Fermi level between the spin-up and spindown states, and the magnetic moment was mainly devoted by W and Ti atoms. The bottom of CB (dxy) move to a low energy region and the top of VB (pp) move to a high energy region with the increasing of TiO2. This is the reason of why there is not obviously forbidden band in W4T8. The XRD patterns for the pure and coextruded films are displayed in Fig. 3. The diffraction peaks of each sample can be indexed to anatase phase with space group I41/amd (141) and the tungsten oxide phase with space group P21/n and P4/nmm, which indicate that the W has not complete oxidation in W10T15 and W15T15 samples for calcined 0.5 h. Researchers [17e19] found the ferromagnetism of TiO2 films and indicated that the ferromagnetism of TiO2 thin films were
Fig. 1. The 2 2 1 superlattice model of TiO2(a) and superlattice crystal model of WO3/TiO2. The number ratio of W and Ti atoms were 4:4 (b), 4:6 (c), 4:8 (d).
Please cite this article in press as: T. Jiao, J. Jiao, Magnetic and optical properties of WO3/TiO2 superlattice, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.051
T. Jiao, J. Jiao / Materials Chemistry and Physics xxx (2016) 1e4
3
Fig. 2. The TDOS for TiO2 (a I), and PDOS for Ti 3d electrons (a II) and its nearest-neighboring O 2p electrons (a III). The TDOS for W4T4, W4T6 and W4T8 (b I, c I, d I), and PDOS for W 5d electrons (b II, c II, d II), O 2p electrons (b III, c III, d III) and Ti 3d electrons (b IV, c IV, d IV). The W, O and Ti locate at the interface of WO3 and TiO2. Spin-up/down states are plotted along the positive/negative ordinates with the Fermi Level (Ef) shown by a vertical line. The position of Ti, O and W atoms were marked in Fig. 1.
Fig. 3. XRD spectra of pure TiO2 (a), W5T15 (b), W10T15 (c) and W15T15 (d) samples.
mainly devoted by the oxygen vacancies which were formed in the preparing process. But in our experiment, the XRD results show that the Ti were oxidized completely, and we did not find the existence of ferromagnetism of pure TiO2 film. So the behavior of ferromagnetism was not detected. The hysteresis loops of WO3/ TiO2 samples which indicate the coupled samples having the behavior of magnetism are displayed in Fig. 4a. The magnetic moment of W15T15 sample was much higher than that of W5T15 and W10T15 samples, which because of there are a great quantity of oxygen vacancies in W15T15 sample (as shown in Fig. 3). The oxygen vacancies [17e19] are considered to be F-center (oxygen vacancies) mediated bound magnetic polaron which can increases the magnetic moment. As shown in Fig. 4b, the absorption spectra of doped samples shift to visible light region, and the absorbance intensifies with increase of WO3 layer thickness. The oxygen vacancies in W15T15, which can induce the impurity bands occur in forbidden band,
Fig. 4. The hysteresis loops (a) of WO3/TiO2 samples and the diffuse reflectance UVevis DRS spectra (b) of pure TiO2 and WO3/TiO2 samples.
Please cite this article in press as: T. Jiao, J. Jiao, Magnetic and optical properties of WO3/TiO2 superlattice, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.051
4
T. Jiao, J. Jiao / Materials Chemistry and Physics xxx (2016) 1e4
bound magnetic polaron. The composite system can extensively apply as information transfer, new type light control superlattice devices, sewage disposal and so on. Acknowledgments The authors are pleased to acknowledge the guidance of Dr. Changwei Gong and Yuming Tian, the financial supported provided for this research by the National Natural Science Foundation of China (Grant No. 11204199) and the National Research Foundation for the Doctoral Program of Higher Education of China (Grant No.20111415120002). Fig. 5. The electrons transfer mode of WO3/TiO2 film.
were also contributed to the absorption spectra shift [20]. These results were consistent with other research ones [19]. The band gaps of the TiO2 and WO3/TiO2 samples were usually estimated from the absorption thresholds wavelength according to the following equation:
a¼
Kðhv EgÞ1=n hv
where a was the absorbance, K was a constant, n equals 2 for indirect transition and 1/2 for the direct transition [21]. The band gap of TiO2 was evaluated to be 3.32 eV (372 nm) from the plot of (ɑhv)n versus energy (hv), not shown in this paper, since TiO2 was considered as an indirect semiconductor. Similarly, the band gaps of W5T15, W10T15 and W15T15 samples were estimated to be 2.75 eV (451 nm), 2.62 eV (474 nm) and 2.51 eV (495 nm), respectively. As displayed in Fig. 5, the electrons on CB of TiO2 transfer to lower energy orbit CB of WO3 in the WO3/TiO2 superlattice. The electrons transfer mode greatly reduced the energy required for electrons transfer and the recombination rate of electron and hole, and shift the absorption region to visible light. When the O electrons are excited and transfer to CB, the spin angular momentum of unoccupied Ti eg states electrons transfer to O electrons and the unoccupied Ti eg states electrons spin splitting when O electrons leave. The O electrons have spin memory and transfer the spin angular momentum to W eg states electrons while the electrons transfer from the CB of TiO2 to the CB of WO3, and the spin angular momentum lead to the spin splitting of W eg states electrons. The spin splitting of Ti and W electrons were the reason of ferromagnetism occurrence in WO3/TiO2 system. This is consistent with calculation results, the O mainly transfer the spin angular momentum instead of devoting the magnetic moment. 4. Conclusion Coupling TiO2 and WO3 greatly reduce the energy required for electrons transfer and the recombination rate of electron and hole, and shift the absorption region to visible light because of the hole and electrons transfer between TiO2 and WO3. The O electrons transfer the spin angular momentum from the unoccupied Ti eg states electrons to W eg states electrons and let Ti and W electrons spin splitting while the electrons transfer from the CB of TiO2 to the CB of WO3. The spin splitting of Ti and W electrons were the reason of ferromagnetism occurrence in WO3/TiO2 system and the O mainly transfer the spin angular momentum instead of devoting the magnetic moment. The oxygen vacancies in WO3/TiO2 thin films also contributed to ferromagnetism because of mediated
References [1] K. Wang, R.F. Chen, C.W. Chen, R.C.C. Ward, Patterning effects on magnetic reversal properties in epitaxial-grown Laves phase DyFe2/YFe2 superlattice, J. Magn. Magn. Mater 377 (2015) 295e297. [2] Min Luo, Guanxia Yu, Lijuan Xia, Calculation of conductance for triangular multi-barrier structure in a constant electric field, Superlattice Microstruct. 83 (2015) 168e175. [3] Jai-Lin Tsai, Qi-Shao Luo, Po-Ran Chen, Yun-Ting Tseng, Magnetic properties and microstructure of FePt/MoC/CrRu films, J. Magn. Magn. Mater 382 (2015) 335e341. [4] Ying Li, Pei-Chi Hsu, Shen-Ming Chen, Multi-functionalized biosensor at WO3TiO2 modified electrode for photoelectrocatalysis of norepinephrine and riboflavin, Sensors Actuators B 174 (2012) 427e435. [5] M.R. Bayati, F. Golestani-Fard, A.Z. Moshfegh, et al., A photocatalytic approach in micro arc oxidation of WO3-TiO2 nano porous semiconductors under pulse current, Mater. Chem. Phys. 128 (2011) 427e432. [6] J. He, Q.Z. Cai, D. Zhu, et al., In-situ preparation of WO3/TiO2 composite film with increased photo quantum efficiency on titanium substrate, Curr. Appl. Phys. 11 (2011) 98e100. [7] M. Venkatesan, C.B. Fitzgerald, J.M.D. Coey, Thin films: unexpected magnetism in a dielectric oxide, Nature 430 (2004) 630. [8] S.D. Yoon, Y. Chen, A. Yang, et al., Magnetic semiconducting anatase TiO2d grown on (100) LaAlO3 having magnetic order up to 880K, J. Magn. Magn. Mater 309 (2007) 171e175. [9] P. Chowdhurya, M. Krishnana, Harish C. Barshiliaa, et al., Structural and magnetic properties of SmCo5/Co exchange coupled nanocomposite thin films [J], J. Magn. Magn. Mater. 342 (2013) 74e79. [10] H. Liu, H.K. Shon, X.A. Sun, et al., Preparation and characterization of visible light responsive Fe2O3-TiO2 composites[J], Appl. Surf. Sci. 257 (2011) 5813e5819. [11] Suiyuan Chen, Yikun Zhang, Weili Han, et al., Synthesis and magnetic properties of Fe2O3-TiO2 nano-composite particles using pulsed laser gas phase evaporation-liquid phase collecting method[J], Appl. Surf. Sci. 283 (2013) 422e429. [12] Jun Woo Choi, Hyung-jun Kim, Kyung-Ho Kim, et al., Uniaxial magnetic anisotropy in epitaxial Fe/MgO films on GaAs(001), J. Magn. Magn. Mater. 360 (2014) 109e112. [13] Chang wei Gong, Jun Rong Jiao, Jia Heng Wang, Wei Shao, Structural, optical and magnetic properties of W-doped TiO2: theory and experiment, Phys. B 457 (2015) 140e143. [14] S.M. Baizaee, N. Mousavi, First-principles study of the electronic and optical properties of rutile TiO2, Phys. B 404 (2009) 2111e2116. [15] Soyeon An, Sunghoon Park, Hyunsung Ko, Chongmu Lee, Fabrication of WO3 nanotube sensors and their gas sensing properties, Ceram. Int. 40 (2014) 1423e1429. [16] QiLi Chen, Bo Li, Guang Zheng, KaiHua He, AnShou Zheng, First-principles calculations on electronic structures of Fe-vacancy-codoped TiO2 anatase (101) surface, Phys. B 406 (2011) 3841e3846. [17] S.D. Yoon, Y. Chen, A. Yang, et al., Oxygen-defect-induced magnetism to 880 K in semiconducting anatase TiO2-d films, J. Phys. Condens. Matter. 18 (2006) L355eL361. [18] C. Sudakar, P. Kharel, R. Suryanarayanan, et al., Room temperature ferromagnetism in vacuum-annealed TiO2 thin films, J. Magn. Magn. Mater 320 (2008) L31eL36. [19] A. Hassini, J. Sakai, J.S. Lopez, N.H. Hong, Magnetism in spin-coated pristine TiO2 thin films, Phys. Lett. A 372 (2008) 3299e3302. [20] Xiaoye Xina, Tao Xuc, Jiao Yina, Lan Wanga, Chuanyi Wanga, Management on the location and concentration of Ti3þ in anatase TiO2 for defects-induced visible-light photocatalysis, Appl. Catal. B Environ. 176e177 (2015) 354e362. [21] L. Cui, F. Huang, M. Niu, L. Zeng, J. Xu, Y. Wang, A visible light active photocatalyst: nano-composite with Fe-doped anatase TiO2 nanoparticles coupling with TiO2(B) nanobelts, J. Mol. Catal. A Chem. 326 (2010) 1e7.
Please cite this article in press as: T. Jiao, J. Jiao, Magnetic and optical properties of WO3/TiO2 superlattice, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.051