Characterization and magnetic properties of SrTi1−xNixO3 nanoparticles prepared by hydrothermal method

Physica B 504 (2017) 31–38

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Characterization and magnetic properties of SrTi1−xNixO3 nanoparticles prepared by hydrothermal method

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Attaphol Karaphuna, Sitchai Hunpratubb, Sumalin Phokhab, Thanin Putjusoc, ⁎ Ekaphan. Swatsitanga,d, a

Integrated Nanotechnology Research Center, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand Department of Physics, Faculty of Science, Rajabhat Udon Thani University, Udon Thani 41000, Thailand c Rajamangala University of Technology Rattanakosin Wang Klai Kangwon Campus, Prachuap Khiri Khan 77110, Thailand d Nanotec-KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage, Khon Kaen 40002, Thailand b

A R T I C L E I N F O

A BS T RAC T

Keywords: Ni-doped SrTiO3 Hydrothermal method Annealing effect Magnetic properties

SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) nanoparticles were prepared by the hydrothermal method. All asprepared samples were annealed at 800 °C for 3 h in argon to study the annealing effect on their magnetic properties. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray absorption near edge spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM) were used to study the crystalline structure, morphology, oxidation state and magnetic properties of samples. XRD results indicate a cubic perovskite structure of all samples with the impurity phase of SrCO3 in all as-prepared samples and Ni metal in annealed samples of x=0.10 and 0.15. SEM and TEM images confirmed a cubic shape for all samples with decreasing average particle sizes from 136.8 ± 4.7 to 126.2 ± 6.9 nm for annealed samples upon Ni doping. XANES results revealed the existence of Ni metal in sample of x=0.05 with the oxidation state of +2 for Ni ion in a SrTi0.95Ni0.05O3 sample. XPS results indicated the promotion of oxygen vacancies. VSM results revealed a paramagnetic behavior at room temperature of all asprepared samples. Ni-doped samples exhibited ferromagnetic behavior after annealing in argon with the Curie temperature (TC) above 380 K for a sample with x=0.05 as shown by field cooling (FC) and zero-field cooling (ZFC) measurements. The room temperature ferromagnetism (RT-FM) of ferromagnetic samples was suggested to be originated from Ni metal and F-center exchange (FCE) mechanism due to the promotion of oxygen vacancies in the perovskite structure.

1. Introduction

strontium titanate (SrTiO3), with paramagnetic behavior [6], is the most interesting, because TM-doped SrTiO3 can show an incredible change in many properties, leading to a wide range of applications in many fields such as photocatalysts [7–9], microwave devices [10], dynamic random access memory (DRAM) [11], and anode materials for lithium-ion batteries [12]. Recently, a few studies on RT-FM in TMdoped SrTiO3 have been reported, for example in SrTi1−xFexO3 [13–15] and SrTi1−xMnxO3 [16]. It is reported that the magnetic properties of these materials is attributed to the presence of oxygen non-stoichiometry, which is strongly required for a net magnetic moment through the FCE mechanism [17]. In general, SrTiO3- based materials can be synthesized by various methods such as sol-gel [17], hydrothermal [12,18–20], solid state reaction [21], polymeric precursor method [8], pulsed laser decomposition [21,22] and electrospinning [23]. From these techniques, the hydrothermal method is an efficient one for the synthesis of SrTiO3 due to its simplicity, low cost and effectiveness in

The magnetic properties at room temperature (RT) and over a wide temperature range of various metal oxides such as ZnO, TiO2, SnO2 and CeO2 [1–4], including oxide-diluted magnetic semiconductor (ODMS) materials, by substitution into their structures of an appropriate transition metal (TM) are still interesting topics for researchers worldwide, owing to the advantages for the flexibility of crystal structure distortion, leading to RT-FM with Curie temperature (TC) above room temperature. In some of these materials, it has been reported that ferromagnetic behavior depends not only on the effect of TM doping, but also on defects, especially oxygen vacancies (VO) induced in the structure. In addition, the role of VO on ferromagnetic ordering in O-DMS materials is usually reported to be due to the FCE mechanism [5]. Among these materials, the wide direct band gap of 3.4 eV in

⁎

Corresponding author at: Integrated Nanotechnology Research Center, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. E-mail address: [email protected] (E. Swatsitang).

http://dx.doi.org/10.1016/j.physb.2016.10.012 Received 23 April 2016; Received in revised form 20 August 2016; Accepted 10 October 2016 Available online 11 October 2016 0921-4526/ © 2016 Elsevier B.V. All rights reserved.

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Ni cations in the perovskite structure of as-prepared and annealed SrTi0.95Ni0.05O3 samples were investigated by XANES measurements performed in the transmission mode using the synchrotron light of beam line 5 at the Synchrotron Light Research Institute, Nakhon Ratchasima, Thailand. The surface chemicals of as-prepared and annealed SrTi0.95Ni0.05O3 samples were studied by XPS (AXIS ULTRA DLD, Kratos analytical, Manchester, UK). The magnetization measurements of all samples were performed at room temperature

Fig. 1. XRD patterns of the as-prepared SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15)

obtaining products with high crystallinity and homogeneity, with the controllability of a fine particle of nanosize and having various shapes [13]. However, there are few reports on annealing studies of the magnetic properties of Ni-doped SrTiO3 nanoparticles prepared by the hydrothermal method. Therefore, it is of great interest to study the magnetic properties of Ni-doped SrTiO3 nanoparticles as a promising ferromagnetic material for future applications. In this work, we report the preparation of SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) nanoparticles by the hydrothermal method. The crystallinity, structure and morphology of all samples were characterized by XRD, SEM and TEM. The oxidation state of Ni cations in as-prepared and annealed SrTi0.95Ni0.05O3 samples were investigated by XANE, whereas those of Sr and Ti cations with oxygen vacancies in their structures were investigated by XPS. The magnetic properties of all samples were investigated using VSM.

Fig. 2. XRD patterns of the annealed SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) samples.

Table 1 Phase, estimated crystallite size (DXRD), lattice parameter (a), average particle size by SEM (DSEM), and TEM (DTEM) of the as-prepared SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) samples.. Parameter

2. Experimental Phase DXRD (nm) a (Å) DSEM (nm) DTEM (nm)

SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) nanoparticles were prepared by the hydrothermal method. Sr(CH3CO2)2 (99%, Aldrich), Ni(NO3)2·6H2O (99.99%, Kanto chemical) and TiO2 (99%, Aldrich) were used as the starting materials. In the preparation process, Sr(CH3CO2)2 and Ni(NO3)2·6H2O at stoichiometric amounts were dissolved in 40 ml of deionized water and stirred using a magnetic stirrer at room temperature for 15 min until a clear solution was obtained, and then a stoichiometric amount of TiO2 was added into this solution. The colloidal solution was stirred at room temperature for a further 30 min, followed by the addition of KOH pellets to obtain a solution of 10 M concentration. The mixture was continuously stirred using a magnetic stirrer at room temperature for a further 12 h and was transferred to a Teflon-lined stainless steel autoclave for hydrothermal treatment at 220 °C for 24 h. At the end of the process, the system was left to cool down to room temperature. The product was washed several times with deionized water and dried in an oven at 80 °C for 5 h. In order to study the effect of annealing on the magnetic properties of samples, the as-prepared SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) samples were annealed at an appropriate temperature of 800 °C for 3 h in argon (Ar) according to our previous work [13]. The crystallinity and structure of all as-prepared and annealed samples were characterized by XRD (PW3040 Philips X-ray diffractometer with Cu-Kα radiation, λ =0.15406 nm, Netherlands). The morphologies of these samples were studied by SEM (Hitachi S3400, Japan) and TEM (TECNAI G2 20, USA). The oxidation state of

SrTi1−xNixO3 samples x=0

x=0.05

x=0.10

x=0.15

Cubic 37.6

Cubic 33.2

Cubic 32.5

Cubic 31.3

3.9050 108.0 ± 17.3 112.8 ± 2.3

3.9024 100.9 ± 12.5 109.5 ± 3.5

3.9011 96.9 ± 8.6

3.9004 90.2 ± 10.3 93.0 ± 2.7

105.9 ± 4.7

Table 2 Phase, estimated crystallite size (DXRD), lattice parameter (a), average particle size by TEM (DTEM), saturation magnetization (Ms) remanent induction (Mr) and coercive force (Hc) of the annealed SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) samples. Parameter

Phase DXRD (nm) a (Å) DTEM (nm) Ms (emu/ g) Mr (emu/ g) Hc (Oe)

32

SrTi1−xNixO3 samples x=0

x=0.05

x=0.10

x=0.15

Cubic 42.9 3.9054 136.8 ± 4.7 –

Cubic 37.5 3.9047 136.4 ± 7.3 0.185

Cubic 34.6 3.9036 132.6 ± 5.1 1.153

Cubic 32.0 3.9024 126.2 ± 6.9 1.729

0.024

0.150

0.420

106.58

159.41

206.53

–

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Fig. 3. SEM micrographs of the as-prepared SrTi1−xNixO3 samples, (a) x=0, (b) x=0.05, (c) x=0.10 and (d) x=0.15.

respectively. This is due to the replacement of a larger Ti4+(0.605 Å) ion by a smaller Ni2+(0.550 Å) ion, resulting in lattice distortion and the overall decreasing of crystallite size.

using VSM (Versa Lab VSM, Quantum Design) in the magnetic field range of ± 10 kOe. The magnetizations of samples with x=0.05 were also measured at different low temperatures in the same range of magnetic field. The dependence of magnetization on temperature was also measured for the sample with x=0.05 in ZFC and FC modes at 1000 Oe.

3.2. SEM and TEM analysis

3. Results and discussion

The morphology of all as-prepared SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) samples are shown by SEM micrographs in Fig. 3(a) - (d). It is obvious that all samples consist of agglomerated cubic-like nanoparticles with decreasing average particle size from 108.0 ± 17.3 to 90.2 ± 10.3 nm upon Ni doping. TEM bright field images with the corresponding selected area electron diffraction (SAED) patterns of asprepared and annealed samples are shown in Fig. 4 and Fig. 5, respectively. Fig. 4 confirms the cubic-like shape of agglomerated nanoparticles with average particle sizes in the range of 112 ± 2.3 – 93.0 ± 2.7 nm, correspond to the results observed by SEM. Similarly, all annealed samples shown in Fig. 5 consist of agglomerated cubic-like nanoparticles with average particle sizes decrease from 136.8 ± 4.7 to 126.2 ± 6.9 nm upon the increase of Ni content. The insets in Fig. 4 and Fig. 5 show the spotty ring SAED patterns of all samples, indicating a polycrystalline structure formation of nanoparticles.

3.1. XRD analysis The XRD patterns of the as-prepared and annealed SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) samples are shown in Fig. 1 and Fig. 2, respectively. The major diffraction peaks of all samples correspond to those of a cubic perovskite structure (JCPDS: 35–0734). However, an impurity phase of SrCO3 (JCPDS: 05–0418) is found in all as-prepared samples, and Ni metal (JCPDS: 04–0850) is detected in annealed samples with Ni doping of x=0.10 and 0.15, as shown in Fig. 1 and Fig. 2, respectively. The estimated crystallite sizes (DXRD) of all samples are calculated from the X-ray line broadening of the (110) plane using Scherrer's equation with DXRD = kλ /(βcosθ ), where DXRD is the crystallite size, λ is the X-ray wavelength, k is a constant (shape factor, 0.98), θ is the diffraction angle and β is the full width at half maximum [24], and they are found to be 37.6, 33.2, 32.5 and 31.3 nm for as-prepared samples of x=0, 0.05, 0.10 and 0.15, respectively. In the case of annealed samples, heat treatment can increase the crystallite size to 42.9, 37.5, 34.6 and 32.0 nm for samples of x=0, 0.05, 0.10 and 0.15, respectively. It is obvious that the estimated DXRD values of as-prepared and annealed samples decrease with increasing Ni content. In addition, the diffraction peak of (110) is also used to determine the lattice parameter (a) of each sample. The values of DXRD and a are listed in Table 1 and Table 2 for as-prepared and annealed samples, respectively. Similarly, the values of a for as-prepared and annealed samples decrease from 3.9050 to 3.9004 Å and 3.9054–3.9024 Å with the increase of Ni content as clearly seen in Table 1 and Table 2,

3.3. XANES analysis The XANES spectrum of the SrTi0.95Ni0.05O3 sample and those of standards Ni metal of different oxidation states are shown Fig. 6. All XANES spectra at the Ni K-edge were measured in transmission mode at room temperature. In Fig. 6, the edge positions between 8333 and 8346 eV are close to those of the Ni metal and Ni2+ standard, indicating a valence state of +2 for Ni ions and the existence of Ni metal in this sample. This confirmation for the existence of Ni indicates that XANES technique is more sensitive to surface of the sample. 33

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nm

Fig. 4. TEM images with the corresponding inset of SAED patterns of the as-prepared SrTi1−xNixO3 samples. (a) x=0, (b) x=0.05, (c) x=0.10 and (d) x=0.15.

force (Hc) increase with increasing Ni content as summarized in Table 2. The RT-FM observed in annealed SrTi0.95Ni0.05O3 sample is suggested to originate from the promotion of VO in sample, as confirmed by XPS results, supporting the VO mediated ferromagnetic exchange mechanism between two neighboring Ni2+ cations based on FCE coupling [5]. A part from this, the ferromagnetism can result from a trace of well-known ferromagnetic material of Ni metal found in this sample as confirmed by the XANES results. In addition, those samples with x=0.1 and 0.15 show higher Ms values, which are possibly related to the more promotion of VO and the secondary magnetic phases of Ni metal in the samples as can be obviously seen from the XRD results in Fig. 2. The M-H curves of the annealed SrTi0.95Ni0.05O3 sample obtained from VSM measurements at different temperatures are shown in Fig. 10. The results show FM clearly, with a slight decrease in Ms measured from 50 K to 300 K, due to the thermal fluctuations causing the randomization of the polarization direction. Fig. 11 shows the temperature dependence of magnetization (M-T curve) for the annealed SrTi0.95Ni0.05O3 sample measured under a constant external magnetic field H of 1000 Oe between 50 and 380 K in the ZFC and FC modes, the results of which are similar to those of Fe-doped SrTiO3 perovskite crystal [14]. At low temperature (~75 K), the obtained curves of magnetization in FC and ZFC modes as shown in Fig. 11 are different because in FC mode the magnetic moments are forced to align themselves to the external field, while in the ZFC mode they are free to orientate, resulting in the gradually decrease of the magnetization for FC mode and for ZFC mode, it increases first and then decreases with increasing temperature. In addition, the magnetization

3.4. XPS analysis The high resolution XPS spectra of Sr and Ti cations with that of O for as-prepared and annealed SrTi0.95Ni0.05 O3 samples are shown in Fig. 7(a)–(c) and Fig. 7(d)–(f), respectively. The peak position of binding energy for each line of Sr 3d, Ti 2p and O 1s levels of both samples are summarized in Table 3. Fig. 7(a) and (d) show the binding energy peak positions of the Sr 3D-doublet lines attributed respectively to Sr 3d5/2 and Sr 3d3/2 in as-prepared and annealed samples, corresponding to Sr2+ ion [25]. The binding energy peak positions of the Ti 2p3/2 and Ti 2p1/2 for as-prepared and annealed SrTi0.95Ni0.05O3 samples are shown in Fig. 7(b) and (e), respectively. These peaks correspond to Ti4+ ion [25]. Fig. 7(c) and (f) show the XPS spectrum of O 1s for which the main peaks at 527.5 eV and 530.0 eV correspond to oxygen in the crystal lattice (OL) and hydroxyl oxygen (OH), respectively. After annealing, it is clearly seen in Fig. 7(f) that the spectrum of oxygen vacancy (VO) is recorded at 531.2 eV [26], indicating the existence of oxygen vacancies in the annealed sample. 3.5. VSM analysis The magnetic field dependence of the specific magnetization (M-H curves) obtained from VSM measurements at room temperature for all as-prepared samples are shown in Fig. 8. All samples exhibit paramagnetic behavior, due to ordering of spins [27,28]. Additionally, a weak RT-FM can be observed in SrTi0.95Ni0.05O3 sample after annealing in argon at 800 °C as shown in Fig. 9. It is obvious in Fig. 9 that the saturation magnetization (Ms), remanent induction (Mr) and coercive 34

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Normalized absorption (a.u.)

Fig. 5. TEM images with the corresponding inset of SAED patterns of annealed SrTi1−xNixO3 samples. (a) x=0, (b) x=0.05, (c) x=0.10 and (d) x=0.15.

1.5

curve in FC mode shows no evidence of secondary phase down to 50 K. However, the Curie temperatures (TC) of Ni (TC ∼ 627 K) and Neel temperatures (TN) of NiO (TN ∼ 523 K) cannot be detected due to the limit of the instrument. Generally, the TC of a sample can be determined by setting the temperature derivative of the magnetization to zero i.e. dMFC (T )/ dT = 0 [29]. In this case, the magnetization curves shown in Fig. 11 indicate that the TC of the annealed SrTi0.95Ni0.05O3 sample is above 380 K.

1.0

4. Conclusion

3.0

Ni K-edge

2.5

Ni foil NiO as prepared SrTi0.95Ni0.05O3

2.0

annealed SrTi0.95Ni0.05O3-δ

In summary, SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) nanoparticles can be successfully prepared by the hydrothermal method in 10 M KOH solution at 220 °C for 24 h. The structure of all as-prepared and annealed samples is a cubic type perovskite as revealed by the XRD results. XRD results indicate that there is an impurity phase of SrCO3 in all as-prepared samples and it is disappeared after annealing at 800 °C in Ar. Obviously, Ni metal is detected in annealed samples of x=0.10 and 0.15 as evidenced by the XRD results. The morphologies of all products observed by SEM micrographs and TEM images confirm a cubic-like shape of agglomerated nanoparticles with decreasing average

0.5

0.0 8300

8320

8340

8360

8380

8400

8420

8440

8460

8480

Energy (eV) Fig. 6. XANES spectra of the standards Ni metal, as-prepared and annealed SrTi0.95Ni0.05O3 samples.

35

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eV Sr 3d5/2 : 131.196 131.1 eV Sr 3d5/2 : 132.959 eV 132.0 eV

Sr 3d5/2 : 132.264 eV 132.2 eV eV Sr 3d3/2 : 133.994 133.9 eV

(a)

Sr 3d5/2

Sr 3d3/2

Sr 3d3/2

136

135

134

133

132

131

130

129

137

136

Binding Energy (eV) Ti 2p3/2 : 457.476 eV 457.4 eV ev Ti 2p1/2 : 463.083 463.0 eV

466

464

(b)

Ti 2p3/2

462

534

532

133

132

131

130

Ti 2p3/2

(e)

Ti 2p1/2

460

458

O 1s

530

134

Ti 2p3/2 : 458.450 eV 458.4 eV 464.1 eV eV Ti 2p1/2 : 464.112

456

454

468

466

528

462

460

O 1s : 528.749 eV 528.7 eV O 1s : 530.042 eV 530.0 eV O 1s : 531.283 eV 531.2 eV

(c)

526

464

458

456

454

Binding Energy (eV)

Binding Energy (eV) O 1s : 527.594 eV 527.5 eV O 1s : 530.019 eV 530.0 eV

135

Binding Energy (eV)

Ti 2p1/2

468

(d)

Sr 3d5/2

536

524

Binding Energy (eV)

534

532

O 1s

530

528

(f)

526

524

Binding Energy (eV)

Fig. 7. XPS spectra of as-prepared ((a)-(c)) and annealed ((d)–(f)) SrTi0.95Ni0.05O3 samples.

annealed SrTi0.95Ni0.05O3 sample is above 380 K as revealed by the temperature dependence of magnetization. It is suggested that the ferromagnetism in all annealed Ni-doped SrTiO3 samples is essentially related to Ni metal and oxygen vacancies in the perovskite structure due to the FCE mechanism.

particle sizes (DTEM) from 112.8 ± 2.3 to 93.0 ± 2.7 and 136.8 ± 4.7 to 126.2 ± 6.9 nm for as-prepared and annealed samples, respectively. All as-prepared and annealed SrTiO3 samples exhibit paramagnetic behavior at room temperature, whereas all annealed Ni-doped SrTiO3 samples display ferromagnetic behavior. Although the XRD results show no evidence of Ni metal in annealed SrTi0.95Ni0.05O3 sample, XANES results indicate the existence of Ni metal in this sample. This sample clearly exhibits the ferromagnetic nature at room temperature with Ms value of 0.185 emu/g, which is approximately 3 times less than that of SrTi0.95Fe0.05O3 sample in our previous work. Moreover, TC of

Conflict of interests The authors declare that there is no conflict of interests regarding the publication of this paper. 36

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0.015

Element

Sr

3d 5/2 3d 3/2 Ti 2p 3/2 2p 1/2 O (O 1s)

As-prepared sample

Magnetization (emu/g)

Table 3 Binding energy, oxidation state of metal ions and oxygen in the as-prepared and annealed SrTi 0.95Ni0.05 O3 samples. Annealed sample

Binding energy (eV)

Oxidation state

Binding energy (eV)

Oxidation state

131.1 132.9 457.4 463.0 527.5 530.0

+2 +2 +4 +4 OL OH

132.2 133.9 458.4 464.1 528.7 530.0 531.2

+2 +2 +4 +4 OL OH Oxygen vacancy

x=0.15

0.010

x=0.10 x=0.05

0.005

x=0 0.000

-0.005

-0.010

-0.015 -10000

-5000

0

5000

10000

Magnetic Field (Oe) Fig. 8. Room temperature M-H curves of the as-prepared SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) samples.

Fig. 9. Room temperature M-H curves of the annealed SrTi1−xNixO3 (x=0, 0.05, 0.10 and 0.15) samples.

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Fig. 10. M-H curves at various temperatures of annealed SrTi0.95Ni0.05O3 sample..

Fig. 11. M-T curves at 1000 Oe of annealed SrTi0.95Ni0.05O3 sample.

Acknowledgments This work was financially supported by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission and the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of the Center of Excellence Network. References [1] T. Fukumura, H. Toyosaki, Y. Yamada, Magnetic oxide semiconductors, Semicond. Sci. Technol. 20 (2005) S103–S111. [2] Y.Q. Wang, S.L. Yuan, L. Liu, P. Li, X.X. Lan, Z.M. Tian, J.H. He, S.Y. Yin, Ferromagnetism in Fe-doped ZnO bulk samples, J. Magn. Magn. Mater. 320 (8) (2008) 1423–1426. [3] S. Phokha, S. Pinitsoontorn, S. Maensiri, Room-temperature ferromagnetism in Codoped CeO2 nanospheres prepared by the polyvinylpyrrolidone-assisted hydro-

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