Effects of Co doping on properties of ilmenite NiTiO3 Ceramics - Tran Tat Dat

Vietnam Journal of Science and Technology 56 (1A) (2018) 119-126 EFFECTS OF Co DOPING ON PROPERTIES OF ILMENITE NiTiO3 CERAMICS Tran Tat Dat 1 , Pham Phi Hung 1 , Dang Ngoc Toan 2 , Dang Duc Dung 1 , Cao Xuan Quan 3 , Luong Huu Bac 1, * 1 School of Engineering Physics, HUST, 1 Dai Co Viet, Ha Noi, Viet Nam 2 Institute of Research and Development, R.809, K7/25 Quang Trung, Da Nang, Viet Nam 3 Vietnam Metrology Institute, 8 Hoang Quoc Viet, Ha noi, Viet Nam * Email: bac.luonghuu@hust.edu.vn Received: 15 August 2017; Accepted for publication: 15 February 2018 ABSTRACT Pure and Co-doped NiTiO3 nanocrystalline powders were prepared by the sol-gel method. The effect of Co 2+ doping on structural, optical and magnetic properties of NiTiO3 was investigated by X-ray diffraction, UV-vis absorption, Raman spectroscopy and vibration samples magnetometer. It was found that all fabricated samples were in single phase with rhombohedral structure. The presence of Co modifies the optical properties of the NiTiO3 nanocrystalline powders. Co doping in NiTiO3 resulted in decreasing of optical bandgap from 2.34 to 1.91 eV. The undoped and Co-doped samples showed weak ferromagnetism at room temperature. Keywords: NiTiO3, Co-doped NiTiO3, ilmenite, sol-gel, optical. 1. INTRODUCTION NiTiO3 (NTO) is an important and interesting member of the ATiO3 family which holds tremendous potential for a wide range of applications such as visible-light photocatalyst [1], solid oxide fuel cells [2], gas or glucose sensor [3, 4], and spin electronic devices with magnetoelectric effect [5]. NiTiO3 belongs to the ilmenite type structure with both Ni and Ti processing octahedral coordination and the alternating cation layers occupied by Ni 2+ and Ti 4+ alone [6]. The bulk NiTiO3 are antiferromagnetic materials with a Neel temperature of 22 K [7]. The Ti ions in 3d 0 state of NiTiO3 dominate the ferroelectric polarization, whereas Ni ions (having partially filled orbitals) are considered to contribute to the antiferromagnetic properties of NiTiO3. However, the investigation of NiTiO3 ceramic in nanostructure exhibited weak ferromagnetism. Chao Xin et al. showed that NiTiO3 the antisymmetric Dzyaloshinskii–Moriya (DM) interaction leads to weak ferromagnetism [8]. The structure of this compound was initially predicted to be rhombohedral with R-3 symmetry, and then was confirmed by recent studies with X-ray diffraction investigations on the epitaxial film. At high temperatures and pressures, NiTiO3 within R-3 phase transforms to a denser R3c phase through a cation reordering process [8]. Tran Tat Dat, Pham Phi Hung, Dang Ngoc Toan, Dang Duc Dung, Cao Xuan Quan, Luong Huu Bac 120 Recently, many investigations on this material are focused on the visible light photocatalyst to discolor the organic dyes [9–11]. It was found that TiO2 is the most photocatalyst materials which have been used in research and practical application. However, TiO2 material has a large bandgap energy with a value of 3.2 eV for anatase phase and 3.0 eV for rutile phase which limits its practical application under the condition of natural solar light because the sunlight only has several percents of UV light. NiTiO3 materials are heavily colored, the reflection spectra were dominated by the broad intense absorption in the visible region around 600 nm and have the band gap energy of 2.18 eV which may be suitable for photocatalysis shift to the visible region. Therefore, NiTiO3 can be considered as an inorganic pigment for yellow color due to its good stability, highly visible opacity and high solar reflection in near-infrared radiation [12]. There are many reports of doping and other modifications to increase the visible absorption of NiTiO3 and/or magnetic properties of NiTiO3 [13–15]. In this work, Co doped NiTiO3 was synthesized by sol-gel method. The systematic investigation the effect of Co doping on the structural, optical and magnetic properties have been carried out. It showed that the Co dopants modified the optical properties of NiTiO3 with changing color from yellow to green and decreasing optical bandgap from 2.34 eV (undoped sample) to 1.91 eV (10 mol.% Co doped sample). 2. MATERIALS AND METHODS The NiTiO3 and Co-doped NiTiO3 (Ni1-xCoxTiO3, x = 0.02, 0.04, 0.06, 0.08, and 0.1 and they were named as NTO-2, NTO-4, NTO-6, NTO-8 and NTO-10) nanocrystalline powders were synthesized using the sol-gel technique. The raw materials used consist of tetraisopropoxytitanium (IV) (C12H28O4Ti), nickel nitrate (Ni(NO3)2.6H2O), and cobalt nitrate (Co(NO3)3.9H2O). The citric acid (C6H8O7) with CM = 1.5 mol/L solution was selected as a solvent. The experimental procedure for the NiTiO3 and Co-doped NiTiO3 samples was as follows. Firstly, the tetraisopropoxytitanium (IV) was dissolved in citric acid solution at 70 o C. A transparent homogeneous sol was formed after stirring vigorously for 2 h. Then, the nickel nitrate was introduced with mol of Ni equal to mol of Ti for fabricating of NiTiO3. The addition amounts of cobalt nitrate were added to the solution for preparing Co-doped NiTiO3 samples. The solutions were stirred around 3-4 h. After that, the ethylene glycol was added. The solutions were kept stirring around two hours then were heated around 120 o C to prepare dry gels. The dry gels were ground and calcined from 900 o C for 3 hours. The crystalline structures of the samples were characterized by X-ray diffraction (XRD). The vibrational and rotational modes of samples were characterized by Raman spectroscopy. The optical properties were studied by UV-Vis spectroscopy. The magnetic properties were characterized by vibration samples magnetometer (VSM-DMS 880, Digital Measurement Systems – USA) at room temperature. 3. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction spectra of NiTiO3 and Co-doped NiTiO3 samples. All peaks and relative intensity of Co-doped NiTiO3 samples were indexed as single ilmenite structure (JCPDS card No. 33-0960, space group R-3, rhombohedral crystal). The impurity phases could not be found even in logarithms scale. The lattice parameter of the fabricated samples was calculated and shown in Fig. 1 (b). The result indicated that the lattice parameter trended to expand as increasing the Co concentration dopants. The angle between adjacent unit cell axes of the rhombohedral phase increased with low Co doping concentration (2 mol.%) and Effect of Co doping on properties of ilmenite NiTiO3 ceramics 121 then decreased with increasing of Co doping concentration. Although the lattice parameters changed via Co doping, the phase of fabricated ceramics still kept the rhombohedral phase. The expansion of lattice parameter can be explained by the different radius of ion Ni 2+ and Co 2+ . The radius of Ni 2+ ions is 0.69Å which were smaller than that of Co 2+ ions 0.745 Å, therefore, Co ion substituted on the Ni site made the expansion of the NiTiO3 lattice [11]. Lin et al. reported that the distorted structure of NiTiO3 via Ag ions replacement to Ni ions resulted from vacancies [12]. Thanh et al. reported that the distortion of structure resulted from competition between difference the radius of the host with substitution ions and/or radius of oxygen vacancies and anion oxygen [13]. In other words, the substitution of Co ions in NiTiO3 materials resulted in distorted structure. The crystalline size of particle estimated by Scherrer equation was 52 nm with undoped sample and 60 nm with 10 mol.% Co doped sample. 20 30 40 50 60 70 In te n s it y ( a .u .) 2(deg.) (3 0 0 ) (2 1 4 ) (0 1 8 ) (1 1 6 ) (0 2 4 ) (2 0 2 ) (1 1 3 )(1 1 0 ) (1 0 4 ) (0 1 2 ) NTO-10Co NTO-8Co NTO-6Co NTO-4Co NTO-2Co NTO Figure 1. XRD patterns of NiTiO3 with different Co doping concentration Figure 2 (a) shows the optical absorption spectroscopy of NiTiO3 and Co-doped NiTiO3 with various Co concentrations at room temperature. The absorption spectra of NiTiO3 samples showed three main absorption region from 300 nm – 550 nm, 600 nm – 1000 nm and 1000 nm – 1800 nm. Our results are in agreement with recently reported for optical properties of NiTiO3 materials where the absorbance peaks resulted from charge transfer from Ni 2+ to Ti 4+ cause of spin splitting of Ni ions under crystal field [16]. NiTiO3 ceramic exhibited the broad intense absorption in the visible region with a range of wavelength from 300 nm to 550 nm. This absorption made the NiTiO3 had heavy yellow color. However, the Co substitution for Ni-site in NiTiO3 resulted in one more absorption peak around 600 nm. The absorption intensity at this wavelength increased with increasing of Co concentration. This band could be attributed to 4A2 (F) → 4T1 (F) and 4A2 (F)→4T1 (P) transitions, indicating the presence of tetrahedral Co2+ species. Due to this absorption, the color of Co-doped NiTiO3 changed gradually from yellow to green color. Recently, theoretical calculation predicted that there are two bonding features between Co and Ti atoms but only one between Ni and Ti atoms which may be relative to the Co-Ti and Ni-Ti bond characteristics in the respective ilmenite system [17]. Therefore, we suggested that Co atom replaced for Ni atom in ilmenite structure resulted in inducing new transition. The optical band gap energy (Eg) were estimated by using the Wood and Tauc method, where Eg value is associated with the absorbance and photon energy by the following equation (h) ~ (h-Eg) n , where  is the absorbance coefficient, h is the Planck constant,  is Tran Tat Dat, Pham Phi Hung, Dang Ngoc Toan, Dang Duc Dung, Cao Xuan Quan, Luong Huu Bac 122 the frequency. The optical band gap Eg and a constant n associated with different types of electronic transition (n = 1/2, 2, 3/2, or 3 for direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions, respectively). The optical band gap values were estimated from extrapolating linear fitting from the (h)2 as a function of photon energy (h) plot. For NiTiO3 materials, the largest band gap is expected to relate to the direct electronic transition between the upper edge of O 2p valence band and the lower edge for Ti 3d conduction band. Thus, the NiTiO3 samples exhibited the direct band gap values of 2.34 eV. The results were well consistent with observation for direct band gap which have been reported in literature [18,19]. The Co doped NiTiO3 materials resulted in decreasing in optical band gap from 2.34 eV for pure NiTiO3 to 1.91 eV for 10 mol.% Co dopant. The dependence of the optical band gap in direct transition was plotted as function of Co concentration dopants in NiTiO3 materials which was shown in the Fig. 2 (b). The modification optical band gap of NiTiO3 materials was recently reported for metal (Ag-, Mo-) or non-metal (N-) doped NiTiO3 materials [20]. Our result was also consistent with recent observation of a reduction in optical band gap in transition metal such as Mn-, Cr-doped lead-free ferroelectric Bi0.5Na0.5TiO3 materials, and Fe- and Ni-doped Bi0.5K0.5TiO3 materials which resulted from the spin splitting of transition metal under crystal field to make effect optical band gap [21–23]. Therefore, we suggested that the reduction of optical band gap energy in NiTiO3 materials via Co-dopants resulted from the localization new state of Co ions. 200 400 600 800 1000 1200 1400 1600 1800 2000 10Co 8Co 6Co 4Co 2CoIn te n s it y ( a .u .) Wavelength (nm) 0Co 0 2 4 6 8 10 1.6 1.8 2.0 2.2 2.4 2.6 E g ( e V ) Co concentration (mol.%) Figure 2. UV-vis spectra of NiTiO3 with different Co doping concentration. Figure 3 shows the Raman scattering of NiTiO3 and Co-doped NiTiO3 samples at room temperature. The theoretical calculation predicted that the optical normal modes of vibrations at the Brillouin zone center have the following symmetries 5Ag + 5Eg + 4Au + 4Eu where 5Ag+ 5Eg are ten active Raman modes, and 4Au+4Eu are inference active modes, and Au+Eu are two modes inactive in both Raman and inference modes [24]. Therefore, ten Raman active modes 5Ag+5Eg are expected with each Eg mode being twofold degenerated to E 1 g+E 2 g along with the eight IR active modes 4Au+4Eu [24]. Preciado et al. predicted that the band located at 720 cm -1 corresponds to the Ti-O-Ti vibration of the crystal structure [25]. Vijayalakshmi et al. pointed out that the bands located at 617 cm -1 and 690 cm -1 originate from the stretching of Ti-O and bending of O-Ti-O bonds while the contribution at 547 cm -1 results from Ni-O bonds [26]. The vibration modes are localization at 631.9 and 760.5 cm -1 which resulted from stretching vibrations of TiO6 and octahedral vibrations in the region 500-830 cm -1 [27]. In addition, Preciado et al. pointed out that Eg mode at 227.6 cm -1 can be considered as the asymmetric Effect of Co doping on properties of ilmenite NiTiO3 ceramics 123 breathing vibration of the oxygen octahedral and the ones at 290.2 and 434.3 cm -1 can be described by the twist of oxygen octahedral due to vibrations of the Ni and Ti atoms parallel to the xy plane, while the Eg modes at 463.4 and 609.7 cm -1 are assigned to the asymmetric breathing and twist of the oxygen octahedral with the cationic vibrations parallel to the xy plane [24]. Our result indicated that the ten Raman active modes observed in NiTiO3 and Co-doped NiTiO3 sample confirmed the rhombohedral structure in agreement with recently reported in literature [24,27,28]. Furthermore, the role of Co ions in the lattice vibration modes was shown in Fig. 3 (b) where the selected Eg and Ag modes were magnified in the range of 200-260 cm -1 and 310-380 cm -1 . The results were clearly indicated that the Raman activity modes trended to shift to lower frequencies as increasing the Co concentration dopants. The shifted in Raman vibration modes in Co-doped NiTiO3 samples suggested to strong relation with the distorted structure cause of substitution of Co ions at Ni-site in NiTiO3 crystal. 150 300 450 600 750 900 10Co 8Co 6Co 2Co In te n s it y ( a .u .) Wavenumber (cm -1 ) 0Co 4Co (a) 200 220 240 260 320 340 360 380 10Co 8Co 6Co 4Co 2CoI n te n s it y ( a .u .) Wavenumber (cm -1 ) 0Co (b) Figure 3. Raman spectra of NiTiO3 with different Co doping concentration. To study the magnetic properties of Co doped NiTiO3 samples, room temperature M–H loops were recorded and showed in Fig. 4. -10000 -5000 0 5000 10000 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 M a g n e ti s a ti o n ( e m u /g ) Magnetic Field (Oe) NTO NTO-2Co NTO-4Co NTO-6Co NTO-8Co NTO-10Co -10 -5 0 5 10 -10 -8 -6 -4 -2 0 2 4 6 8 10 M a g n e ti s a ti o n ( m e m u /g ) Magnetic Field (kOe) NTO NTO-2Co NTO-4Co NTO-6Co NTO-8Co NTO-10Co Figure 4. (a) The M-H curves of pure and Co doped NiTiO3 and (b) the M-H curves after subtraction of the paramagnetic signal. It is observed that all the undoped and Co-doped samples show paramagnetic nature at room temperature. Increasing Co doping concentration resulted in increasing the paramagnetic signals. It could be attributed to the competition between ferromagnetic and paramagnetic Tran Tat Dat, Pham Phi Hung, Dang Ngoc Toan, Dang Duc Dung, Cao Xuan Quan, Luong Huu Bac 124 contributions to the magnetism of the samples. The clear evidence for ferromagnetism hysteresis loop is shown in the Fig. 4 (b) after subtracting the paramagnetic component. The coercive field (Hc) and remanence magnetization (Mr) of NiTiO3 were estimated around 80 Oe and 1 memu/g, respectively, which were solid evidence for ferromagnetism behavior at room temperature. Recently, both theoretical and experimental investigation demonstrated that the oxide ceramics had the week ferromagnetism because they contained defects in their lattice. At room temperature, the undoped and Co-doped materials exhibited week temperature ferromagnetism which can explain by the presence of vacancies and/or from the exchange interactions between localized electron spin moments and oxygen vacancies at the surfaces of nanocrystalline powders. 4. CONCLUSIONS In this study, highly crystalline Co-doped NiTiO3 powders were successfully synthesized via sol-gel method. The presence of Co modifies the optical properties of the NiTiO3 nanocrystalline powders. 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