Colossal dielectric permittivity in hydrogen-reduced rutile TiO2 crystals

Journal of Alloys and Compounds 692 (2017) 375e380

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Colossal dielectric permittivity in hydrogen-reduced rutile TiO2 crystals Jinglei Li a, Fei Li a, *, Xuhui Zhu a, Dabin Lin b, Quanfu Li c, Weihua Liu c, Zhuo Xu a, ** a Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Centre for Dielectric Research, Xi'an Jiaotong University, Xi'an 710049, China b Laboratory of Thin Film Techniques and Optical Test, Xi'an Technological University, Xi'an 710032, China c School of Electronics and Information Engineering, School of Microelectronics, Xi'an Jiaotong University, Xi'an 710049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 July 2016 Received in revised form 25 August 2016 Accepted 4 September 2016 Available online 7 September 2016

(Nb þ In) co-doped rutile TiO2 (TINO) ceramics have received considerable attention, due to their high dielectric permittivity (on the order of 10,000) and low loss factor (<0.05) over a broad temperature/ frequency range. The high dielectric permittivity of TINO has been attributed to a special defect-dipole structure. Here, we observed a similar dielectric behavior in hydrogen-reduced rutile TiO2 crystals, where the colossal dielectric permittivity of ~30,000 and the low loss factor of ~0.05 exist in the temperature range of 30e480 K (frequency at 10~105 Hz). Based on the investigations on the phase microstructures, the elemental valences, impedance spectroscopy, nonlinear IeV behavior and dielectric response, the colossal dielectric permittivity of the hydrogen-reduced rutile TiO2 crystals was thought to be associated with the electronic relaxation polarization mechanism. This research further revealed that the weak-binding electron is the most critical factor for the high dielectric permittivity in rutile TiO2 system, while a special defect-dipole structure may not be a preliminary requirement. © 2016 Elsevier B.V. All rights reserved.

Keywords: Colossal dielectric permittivity Hydrogen-reduced TiO2 single crystal Electronic relaxation polarization

1. Introduction Materials with colossal dielectric permittivity (CP, i.e., dielectric permittivity is larger than 1000) are in the focus of interest, not only for academic research but also for the development of modern electronics [1]. The CP materials, such as CaCu3Ti4O12 (CCTO) [2e4], doped-NiO [5], La15/8Sr1/8NiO4 [6], Ba(Fe0.5Nb0.5)O3 [7], K0$3MoO3 [8], have received considerable attention, but these materials have not yet be used in practical applications due to their high dielectric loss. Recently, a remarkable dielectric behavior was reported in a (Nb þ In) co-doped rutile TiO2 (TINO) systems [7], which possessed a high dielectric permittivity (>104) as well as a low dielectric loss (<0.05) over a very broad temperature range from 80 to 450 K. This new finding led to considerable investigations on the TINO system for clarifying the underlying CP mechanisms. Currently, the major debate is whether the CP of TINO originates from “intrinsic” or “extrinsic” effects. From intrinsic respect, Hu et al. [9] proposed that the colossal dielectric permittivity in TINO was an intrinsic

* Corresponding author. ** Corresponding author. E-mail address: [email protected] (F. Li). http://dx.doi.org/10.1016/j.jallcom.2016.09.044 0925-8388/© 2016 Elsevier B.V. All rights reserved.

property, being associated with a special defect-dipole structure. From the extrinsic respect, however, Song et al. [10] investigated the Ti0.9In0.05Nb0.05O2 single crystal and attributed the CP of TINO to a mechanism of the surface barrier layer capacitor (SBLC); meanwhile, by studying the IeV behaviors for the grain and grain boundary, Li et al. [11e14] and Liu [15] et al. demonstrated that the grain boundary capacitance (GBC) effect played an important role in the colossal dielectric permittivity of TINO ceramics. Crandles [16] et al. suggested that the dielectric response were strongly affected by the electrodes. Here, we investigated the hydrogen-reduced rutile TiO2 crystals, where oxygen vacancies exist while the special defect-dipoles induced by Nb5þ and In3þ do not exist. Compared to TINO, interestingly, we observed a similar dielectric behavior in the hydrogenreduced rutile TiO2 crystals, where both the high dielectric permittivity (~30,000) and the low dielectric loss (~0.05) were observed over a broad temperature range (30e480 K). Based on the analysis of the phase microstructures and the elemental valences, the colossal dielectric permittivity of the hydrogen-reduced rutile TiO2 crystals was attributed to the electronic relaxation polarization mechanism. This research may as well shed light on the recognition of CP behavior in other metallic oxides.

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2. Experimental procedures

3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the hydrogenreduced TiO2 crystals. The peak with 2q of 63 corresponds to the (002) plane of the rutile TiO2 crystal, revealing that all crystals are in rutile phase and no secondary phase presents. Fig. 2 shows the Raman spectra of the hydrogen-reduced rutile TiO2 crystals. The dotted lines (143 cm1, 241 cm1, 446 cm1, and 614 cm1) indicate the Raman vibration mode of rutile phase. Multi-phonon peak near 241 cm1 is induced by the internal stress/strain and partial reduction [17]. There are three Raman active fundamental modes in rutile TiO2: B1g (143 cm1), Eg (446 cm1), and A1g (614 cm1). The 143 cm1 (B1g) [18] Raman peak is an OeTieO bond bending mode, the 614 cm1(A1g) relates to TieO stretch mode while the 446 cm1 (Eg) mode is due to oxygen atom liberation along the c-axis out of phase [19]. The colors of the crystals change from the colorless transparency to dark blue with increasing the annealing temperature, as shown in the inset of Fig. 1, which is thought to be associated with the F-type color center (generated by the combination of oxygen vacancy and the electron from Ti3þ ions) in the rutile TiO2 structure [20]. In order to verify the existence of oxygen vacancy and the Ti3þ ions, Fig. 3(a) presents the O 1s XPS spectra for TO-850 crystal. It can be seen that the O1s state contained three binding energy components, a low binding energy peak (denoted as OL), a middle binding energy peak (denoted as OM), and a high binding energy peak (denoted as OH), which centered nearly at 530.15, 531.37 and 532.37 eV, respectively. The OL and OM are attributed to the oxygen atoms at the intrinsically sites and oxygen vacancy, respectively. The OH is assigned to chemisorbed oxygen, which is closely related to the hydroxyl groups, due to the chemisorbed water [21e23]. During the annealing process, the loss of oxygen atoms requires the reduction of Ti4þ for charge compensation. The reduction of Ti4þ was certified by the XPS experiments, as shown in Fig. 3(b), where

(002)

Intensity (a.u.)

TO-900 TO-850 TO-800 TO

10

20

30

40

50

60

70

2 theta (degrees) Fig. 1. The XRD powder patterns of TO, TO-800, TO-850, and TO-900. The insets are the macroscopic pictures.

A1g Intensity (a.u.)

The [001]-oriented rutile TiO2 crystals (abbreviated as TO, bought from Hefei Kejing Materials Technology Corporation, P. R. China) were placed into a 1-inch quartz tube furnace (Lindberg/ Blue M, TF55035C-1, Thermo Electron Corporation) for an annealing process. The TO samples were annealed in the temperature of 800, 850 and 900  C, respectively, for 30 min under 100 sccm flow of H2. Then, the samples were cooled to room temperature at a cooling rate of 5  C/min under protection of H2. After annealing, the silver electrodes were pasted to the crystals and fired at 600  C for 0.5 h. The hydrogen-reduced rutile TiO2 crystals annealed at 800, 850 and 900  C were abbreviated as TO-800, TO-850 and TO-900, respectively. The phases of the TiO2 samples were characterized by the X-ray diffraction (XRD, D/MAX 2400, Japanese) and the Raman spectra (Raman, HR800, France, the 541.32 nm line of Arþ laser was used as an excitation source). The X-ray photoelectron spectroscopy (XPS, AXIS ultra DLD, and England) and electron spin resonance (ESR, JESX330, Japan) experiments were used to analyze the valent states of various elements recorded with a monochromatic Al Ka radiation. The corresponding fitting results were given by Casaxps software using the Gaussian method with an addition of a Shirley background. An Agilent 4294 A facility was used to measure the dielectric response with respect to frequency. The electric properties at ultralow temperature were measured by an Agilent 4980 A in PPMS (Physical Property Measurement System) stove and a Novocontrol broadband dielectric spectrometer with an alpha-A high performance frequency analyzer in the frequency range of 102e107 Hz over the temperature range of 123e773 K.

B1g Multiphonon

143 241 200

TO TO-800 TO-850 TO-900

Eg

446 400

614 600

Wave number (cm-1)

800

1000

Fig. 2. Raman spectra of the raw and hydrogen-reduced TiO2 single crystals obtained in the range of 100e1000 cm1.

the peak of Ti3þ is detected. In Fig. 3(b), the spin-orbit splitting of 5.6 eV corresponds to that of the rutile TiO2 (5.7 eV) [24], indicating the presence of Ti4þ ions. The existence of oxygen vacancy and Ti3þ can induce the weak binding electrons in the rutile TiO2 crystals. As shown in Fig. 4, the coefficient g in ESR spectra of TO-850 was found to be 2.13. This value of g is associated with the paramagnetic resonances of Ti3þ with 3d [1] electrons trapped on the lattice [25e29], indicating the existence of weak binding electrons in the TO-850. Fig. 5 shows the frequency dependence of the dielectric permittivity and loss for the hydrogen-reduced TiO2 crystals. Over a wide frequency range from 40 to 105 Hz, the permittivity and loss of the hydrogen-reduced TiO2 crystals are ~30,000 and ~0.05, respectively. According to the XPS and ESR experiments, the high dielectric permittivity was thought to originate from the weak binding electrons. At the frequency higher than 106 Hz, the dielectric permittivity significantly drop to ~100 accompanied by the maximum of loss factor, indicating the relaxation frequency of the weak binding electrons is around 106 Hz. Furthermore, it can be seen that the CP plateau of the hydrogen-reduced TiO2 crystal is enlarged by increasing the annealing temperature. For the non-

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377

10

5

TO TO-800 TO-850 4 TO-900

10

3

10

2

10

1

10

Dielectric loss

Dielectric permittivity

10

0

10

10

10

10

10

10

10

Frequency (Hz) Fig. 5. Dielectric permittivity and loss with respect to frequency of TO, TO-800, TO850, and TO-900 crystals, measured at room temperature.

TiO2 crystals. The dielectric permittivity up to 105 and loss under 0.1 are quite stable with respect to the temperature from 300 to 30 K, then following by a sharp reduction at the temperature of ~20 K. As shown in Fig. 6(c), the peak of loss factor shifts to higher temperature with increasing frequency, indicating that the reduction of the dielectric permittivity is accompanied by a dielectric relaxation process, which can be described by Eq. (1):

tpeak ¼ t0 exp½  U=ðKB TÞ

Fig. 3. XPS data (open circles) of O 1s (a) and Ti 2p (b) for TO-850 sample.

annealed rutile TiO2 crystal, the dielectric permittivity is quite high at relatively low frequencies (<102 Hz) but it drastically decreases to ~102 at the frequencies higher than 103 Hz. The high dielectric permittivity at low frequencies for the non-annealed TiO2 crystal can be attributed to the space charges accumulated around some line defects or plane defects in crystal lattice. Fig. 6 gives the temperature dependence of dielectric permittivity and loss from 300 K to 10 K for the hydrogen-reduced rutile

Derivative symbol Integral line

where tpeak ¼ 1/fPeak is the relaxation time at certain temperature (reciprocal to the relaxation frequency), t0 the frequency constant (relaxation time at infinite temperature), U the activation energy, KB the Boltzmann constant, and T the absolute temperature. By fitting Eq. (1), the activation energy U is calculated to be 24.5 meV, being on the same order to the impurity ionization energy of typical semiconductors. This indicates that the low temperature dielectric relaxation behavior of the hydrogen-reduced rutile TiO2 is possibly associated with a frozen-out effect of impurity charges, i.e., the weak-binding electrons of hydrogen-reduced rutile TiO2 were “frozen” below 20 K [30e32]. Fig. 7 shows the impedance spectroscopy (IS) of TO-850, where an equivalent circuit (conductance and capacitance in series) is presented in inset, due to the hydrogen reduced single crystals do not possess any secondary phase. The real part Z0 and imaginary part Z00 of the complex impedance are given as follows:

ZðuÞ0 ¼ G0

.  G20 þ u2 C20

Intensity [a.u.]

ZðuÞ00 ¼ uG0

g=2.13

3.0

2.5

2.0

1.5

1.0

g value Fig. 4. ESR spectra of TO-850 measured at room temperature. The cyan and blue line were the derivative symbol and the integral line, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(1)

.  G20 þ u2 C20

(2)

(3)

where, G0 and C0 are the conductance and capacitance of single crystals, respectively, u the angular frequency, being equal to 2pf. The IS is plotted with Z0 as the abscissa and Z00 as the ordinate. The position of impedance spectroscopy arc depends on the angular frequency u. When u ¼ 0, Z(0)0 ¼ 1/G0, Z(0)00 ¼ 0, and at the maxima arc point, the relationship u ¼ G0/C0 holds, Zmax00 ¼ 1/2G0. By fitting the data, G0 and C0 of TO-850are 7.7  109 S and 1.22  108 F, respectively. In order to provide more clues for the CP mechanism of hydrogen-reduced TiO2 crystals, Fig. 8 illustrates the non-linear IeV behaviors of TO-850. The nonlinear behavior of current density (J) to the electric field (E) is given by the equation J ¼ kEa, where the nonlinear coefficient a gives the degree of non-linearity, the constant k depends on the microstructure. Here, the nonlinear

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Fig. 6. Temperature dependence of dielectric permittivity (a) and loss (b) of TO-850. (c) The enlarge image of (b) in low temperature range. The inset in (b) is the temperature dependence of corresponding dielectric loss peak.

difference surface work function around the interface, resulting in a bias effect and nonlinear IeV behavior [35e37]. In order to elucidate whether the existence of surface barrier layer capacitor (SBLC) or not, caused by interface effect between TO-850 with oxygen vacancy and the silver electrode, i.e., metal and semi-conductor contact, Fig. 9(a) shows the dielectric permittivity and loss versus frequency of TO-850 at different temperature. In low frequency range (f < 104 Hz), the dielectric permittivity ascends with increasing the temperature. Meanwhile the dielectric loss show strong temperature dependent Maxwell-Wagner relaxation behaviors with temperature range from 123 K to 483 K. Colossal

Fig. 7. Impedance spectroscopy (IS) analysis of TO-850 measured at room temperature. The solid circle symbols are experimental data. The open circle symbols are indicated the test frequency. The insets are the equivalent circuit diagram and an enlarged IS view in high frequency range.

coefficient increase from 2.4 to 9.3 around breakdown voltage region, suggesting existence of the potential barrier. Meanwhile, TO850 has much higher breakdown voltage when compared to that of TINO ceramics (~100 V/cm) [12,14], which may be attributed to the concentration of oxygen vacancy. The similar IeV behaviors were also observed in CCTO and TINO ceramics, and argued to be related to the insulating grain boundaries and conductive grains [33,34]. Nevertheless, due to lack of grain boundaries in single crystal, the nonlinear IeV behavior of TO-850 cannot be explained by this respect, which is thought to be associated with the interface between the surface of sample and the silver electrode, i.e., metal and semi-conductor contact. The surface energy bands bend as the

Fig. 8. Current density-electric field (IeV) characteristics of the TO-850. The inset image is its nonlinear IeV coefficient, a ¼ d[log(I)]/d[log(V)].

J. Li et al. / Journal of Alloys and Compounds 692 (2017) 375e380

M* ¼ 1=ðε0  iε00 Þ ¼ ε0

379

.  .  2 2 ε02 þ ε00 þ iε00 ε02 þ ε00 ¼ M 0 þ M 00 (4)

where M 0 ¼ ε0 =ðε02 þ ε00 2 Þ and M 0 ¼ ε00 =ðε02 þ ε00 2 Þ are the real and imaginary part of complex modulus, respectively. Presented in Fig. 9(b) is frequency dependence of the real and imaginary parts of electrical modulus of TO-850 with temperature range from 123 to 483 K. The real part of the modulus shows a steplike decrease with increasing the temperature range from 102e104 Hz, being accompanied by relaxation behavior of imaginary part of complex modulus. The maximum imaginary part of the complex modulus shifts to higher frequency with increasing temperature, suggesting an increase of dipole density and a faster polarization process:

f peak ¼ f 0 exp½U=ðKBTÞ

(5)

where fpeak the frequency of maximum imaginary part modulus at certain temperature (relaxation frequency), f0 the frequency constant (relaxation frequency at infinite temperature), U the activation energy for the dielectric relaxation, KB the Boltzmann constant, and T the absolute temperature. By fitting the relaxation data, the value U is found to be 1.09 eV, shown in Fig. 9(c). Compared with other titanium oxides with oxygen-octahedra structure, such as BaTiO3, Bi4TiO12, and SrTiO3, the relaxation activation energy is around 1.0 eV [38,39], which is coincide with the relaxation activation energy found in this material system. Here, the activation energy of 1.09 eV is associated with the interface effect between TO-850 and the silver electrode, which is different from the previous activation energy of 24.5 meV associated with the typical oxygen vacancy ionization energy in crystal lattices. 4. Conclusions In summary, we systematically studied the hydrogen-reduced rutile TiO2 crystals with oxygen vacancies and Ti3þ ions on phase structure, valence state, impedance spectroscopy, dielectric response, nonlinear IeV behavior, and SBLC with Maxwell-Wagner relaxation. These crystals exhibited colossal dielectric permittivities (~104), which were thought to be associated with the weakbinding electrons generated from the oxygen vacancies. It is worthy to note that the weak-binding electrons may also present in Nb þ In co-doped TiO2 ceramics, since indium is a volatile element (may volatilize during the solid state reaction) and the excess Nb5þ can induce the weak-binding electrons in rutile TiO2 structure. Thus, it is possible that the colossal dielectric response of the Nb þ In co-doped TiO2 ceramics is also highly related to the weakbinding electrons. Fig. 9. (a) Dielectric permittivity and loss versus frequency of TO-850. (b) The real and imaginary parts of electric modulus versus frequency of TO-850. (c) The temperature dependence of the relaxation frequency fpeak. The solid circle symbols are the relaxation frequency fpeak with respect to temperature from (b). The solid line is the best fitting result.

permittivity accompanied by a strong Maxwell-Wagner (M-W) relaxation mode could be interpreted by SBLC mechanism. To obtain insight of this phenomenon, electrical modulus M*, which is effective to analyze the SBLC, is introduced, based on Eq. (4):

Acknowledgement This work was supported by: National Natural Science Foundation of China (Grant nos. 51572214 and 51372196), Shaanxi Provincial Natural Science Foundation of China (Grant nos. 2015JQ5135 and 2015JM5185 and 2015JM5199), Key Scientific Project of Shaanxi Provincial Education Department No.15JS033, the 111 Project (B14040), International Science & Technology Cooperation Program of China under Grant no. 2015DFA51100 and the Fundamental Research Funds for the Central Universities. References [1] P. Lunkenheimer, S. Krohns, S. Riegg, S.G. Ebbinghaus, A. Reller, A. Loidl, Colossal dielectric constants in transition-metal oxides, Eur. Phys. J. Spec. Top.

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