Structural and Optical Properties of SrTiO₃-Based Ceramics for Energy and Electronics Applications

Abstract

A series of Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10) samples were fabricated using cold compaction, followed by sintering in a (95% N₂ + 5% H₂) reducing atmosphere. We studied the crystal structure and optical properties of Sr_1−xDy_xTi_1−yNb_yO_3−δ using X-ray diffraction (XRD) with Rietveld refinement, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet−visible−near-infrared (UV-VIS-NIR) spectroscopy. The sintered Sr_1−xDy_xTi_1−yNb_yO_3−δ had a tetragonal structure (I4/mcm space group). In the sintered samples, Ti ions existed as a mixture of Ti³⁺ and Ti⁴⁺, and Nb ions existed as a mixture of Nb⁴⁺ and Nb⁵⁺. The band-gap energies decreased with increasing Dy/Nb concentrations. The incorporation of Ti and Nb ions, the formation of both Ti³⁺ and Nb⁴⁺ ions, and the reduction in band-gap energies are likely highly effective for increasing the electron concentration and the corresponding electrical conductivity. Sr_1−xDy_xTi_1−yNb_yO_3−δ with high electrical conductivity is suitable for energy and electronics applications.

1. Introduction

A perovskite-type metal oxide can be represented by the general formula ABO₃ (A: alkaline earth metal, B: transition metal). Strontium titanate (SrTiO₃) at ambient temperature and pressure has a cubic crystal structure (space group Pm$\overline{3}$m, lattice parameter a = 3.905 Å). As the temperature decreases, SrTiO₃ undergoes a cubic-to-tetragonal and antiferrodistortive phase transition at approximately 100–110 K [1]. In addition, cubic SrTiO₃ consists of corner-shared TiO₆ octahedra surrounding Sr²⁺, and the Sr²⁺ is bonded to twelve equivalent O²⁻ ions [2,3,4]. The electronic band structure of SrTiO₃ consists of a conduction band formed by triply degenerate Ti 3d orbitals and a valence band formed by an O 2p orbital [2]. Pure SrTiO₃ is an electronic insulator at room temperature [5] but can become an n-type semiconductor by doping donor impurities such as Y, Sm, Nb, and La. Oxygen vacancies can be introduced by heating in a reducing atmosphere or low oxygen partial pressure [1], thereby generating electrons and charged ions. Hence, SrTiO₃ is regarded as a mixed electronic–ionic semiconductor.

SrTiO₃ possesses several interesting properties, including a high thermoelectric power factor, high chemical and thermal stability, low dielectric loss, a high dielectric constant, high anisotropy, a high melting point (2080 °C), and the generation of photogenerated charge carriers [4,5,6,7,8]. Thus, these characteristics enable a wide range of applications, including thermoelectric materials, anode materials for solid oxide fuel cells (SOFCs), hydrogen sensors, photocatalysts, capacitors, hydrogen storage, solar cells, optical sensors, field effect transistors, random access memory, oxygen sensors, environmental remediation, synaptic emulators, dielectric substrates, and memristors [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24].

As typical application examples, SrTiO₃ possesses a high electron effective mass and Seebeck coefficient due to its degenerate conduction band, leading to excellent thermoelectric performance [9,25,26]. Thermoelectric power generation is a promising technology for addressing global warming and climate change by converting waste heat into electricity. The energy conversion efficiency of thermoelectric materials is measured by the dimensionless figure-of-merit, ZT = (σα²/κ)T, where σ, α, κ, and T represent the electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature, respectively. Consequently, increasing the values of σ and α as well as lowering the value of κ value are necessary to enhance the ZT value. In addition, La-doped SrTiO₃ has garnered considerable interest as an anode material for SOFCs because of its high electrical conductivity in reducing environments, excellent dimensional and chemical stability during redox cycling, and remarkable resistance to sulfur and coking [11]. SOFCs can help to produce electrical energy and reduce environmental pollution associated with conventional fossil fuels. SrTiO₃ can also be used as a photocatalyst to construct photocatalyst panel reactors for evaluating the feasibility of photocatalytic water splitting technology, including the largest panel reactor built for hydrogen production [4,5,13,14].

The electrical conductivity of SrTiO₃ increases with donor doping of higher valence cations on Sr and/or Ti sites. Therefore, to further extend the application of SrTiO₃ by increasing electrical conductivity, in this work, trivalent Dy³⁺ ions are doped into the Sr²⁺ ion site and pentavalent Nb⁵⁺ ions are doped into the Ti⁴⁺ site. Dy/Nb co-doping can enhance the electron concentration in accordance with the following Kröger−Vink equations:

$${\mathrm{D}\mathrm{y}}_2{\mathrm{O}}_3+2{\mathrm{T}\mathrm{i}}_{\mathrm{T}\mathrm{i}}^{\times }\stackrel{\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}{\mathrm{O}}_3}{\to }2{\mathrm{D}\mathrm{y}}_{\mathrm{S}\mathrm{r}}^{•}+{2\mathrm{O}}_{\mathrm{O}}^{\times }+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{2\mathrm{T}\mathrm{i}}_{\mathrm{T}\mathrm{i}}^{\prime }$$

(1)

$${\mathrm{N}\mathrm{b}}_2{\mathrm{O}}_5+2{\mathrm{T}\mathrm{i}}_{\mathrm{T}\mathrm{i}}^{\times }\stackrel{\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}{\mathrm{O}}_3}{\to }2{\mathrm{N}\mathrm{b}}_{Ti}^{•}+{4\mathrm{O}}_{\mathrm{O}}^{\times }+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{2\mathrm{T}\mathrm{i}}_{\mathrm{T}\mathrm{i}}^{\prime }$$

(2)

In the present study, we investigated the crystal structure and optical properties of Dy and Nb co-doped SrTiO₃, Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10), using SEM, XRD Rietveld refinement, XPS, and optical band-gap energy calculations. XRD Rietveld refinement was performed using FullProf software to retrieve detailed structural parameters. XPS was utilized to examine the oxidation states of the constituting elements (Sr, Dy, Ti, Nb, and O) in Sr_1−xDy_xTi_1−yNb_yO_3−δ. UV-Vis-NIR diffuse reflectance spectra were obtained to investigate the optical band-gap energy.

2. Experimental Section

Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10) powders were synthesized using a solid-state reaction. The starting powders used were SrCO₃ (99.9%), TiO₂ (99.9%, 2 μm), Dy₂O₃ (99.9%), and Nb₂O₅ (99.9%, 1 μm). All the raw materials were purchased from Kojundo Chemical Laboratory Co., Ltd(Osaka, Japan). The starting materials were weighed in specific proportions. A mixture of the weighed starting powders was ball-milled for 8 h at 200 rpm using a ZrO₂ ball as a grinding media. The ball-milled powders were heated to 1300 °C at a heating rate of 5 °C min⁻¹ in air, kept at this temperature for 6 h, and then furnace-cooled naturally to room temperature. The resulting powders were milled for 4 h at 350 rpm using a planetary mill (Fritsch Pulverisette 6, Idar-Oberstein, Germany) and ZrO₂ balls. The planetary-milled powders were passed through a 100-mesh sieve and then cold-pressed at 100 MPa to prepare green pellets. The pellets were heated to 1300 °C at a heating rate of 5 °C min⁻¹ in a (95% N₂ + 5% H₂) reducing atmosphere, maintained at that temperature for 10 h, and then furnace-cooled to room temperature. The fabricated Sr_0.95Dy_0.05Ti_0.95Nb_0.05O_3−δ, Sr_0.95Dy_0.05Ti_0.90Nb_0.10O_3−δ, Sr_0.90Dy_0.10Ti_0.95Nb_0.05O_3−δ, and Sr_0.90Dy_0.10Ti_0.90Nb_0.10O_3−δ samples were designated as D5/N5, D5/N10, D10/N5, and D10/N10, respectively.

The microstructural properties of the sintered samples were examined using a scanning electron microscope (SEM; Hitachi SU8010, Tokyo, Japan). The porosity of the sintered samples was measured using Archimedes’ principle. The crystal structure of the sintered samples was analyzed by X-ray diffraction (XRD) at room temperature using an XRD system (X’Pert PRO, PANalytical, Almelo, the Netherlands) with Cu K_α radiation (λ = 1.5406 Å). The crystal structure was further analyzed using XRD Rietveld refinement with the FullProf software. XPS (K−alpha, Thermo VG, Oxford, UK) was employed to examine the surface elemental composition and valence states of the constituent elements in the sintered samples. The optical band-gap of the sintered samples was estimated from the diffuse reflectance spectra measured using a UV−Vis−NIR spectrophotometer (Agilent, Varian Cary 5000, Santa Clara, CA, USA).

3. Results and Discussion

Figure 1 shows the SEM images of Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10). The introduction of Dy and Nb ions slightly decreases the bulk density of Sr_1−xDy_xTi_1−yNb_yO_3−δ, resulting in 94% and 90% of the theoretical density for the D5/N5 and D10/N10 samples, respectively. Most pores are located at the grain boundaries, which can act as electron scattering centers. As a result, the pores reduce the electrical conductivity (σ), which can be explained by the effective medium theory as follows [27]:

σ = [(3f − 1)σ₀]/2

(3)

where f is the fraction of material excluding pores, and σ₀ is the electrical conductivity of pore-free material. Furthermore, the doped Dy and Nb ions reduce the grain size of Sr_1−xDy_xTi_1−yNb_yO_3−δ, with grain sizes of 1.3 μm and 0.8 μm for the D5/N5 and D10/N10 samples, respectively. The grain size is estimated using the linear intercept method from the SEM images. The reduction in grain size results from the inhibition of grain boundary mobility due to the pinning effect and solute dragging effect [28,29].

The standard file of tetragonal SrTiO₃ (ICSD # 65140) is employed for the Rietveld refinement. Figure 2 displays the calculated XRD refined profiles of Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10) along with the measured diffraction profiles. The diffraction peaks are indexed as a tetragonal structure with the I4/mcm space group. Detailed crystallographic information extracted from the refinements and reliability factors (R values), including weighted-profile R factor (R_wp), expected R factor (R_exp), and goodness of fittings (GOF χ = R_wp/R_exp), is summarized in

Table 1. Crystallographic data and reliability factors of Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10).

	D5/N5 sample	D5/N10 sample	D10/N5 sample	D10/N10 sample
Crystal structure	Tetragonal	Tetragonal	Tetragonal	Tetragonal
Space group	I4/mcm	I4/mcm	I4/mcm	I4/mcm
Lattice parameter
a (Å)	5.5241	5.5352	5.5220	5.5336
c (Å)	7.8243	7.8375	7.8241	7.8374
α = β = γ (°)	90.0	90.0	90.0	90.0
Volume (Å3)	238.76	240.13	238.58	239.99
Reliability factor
Rexp (%)	7.14	6.23	7.77	6.29
Rexp (%)	4.17	4.19	4.45	4.41
χ2	2.94	2.21	3.05	2.03

. The lattice parameter decreases with an increasing Dy concentration, whereas it increases with an increasing Nb concentration. The lattice parameters (a, c) of the D5/N5, D5/N10, D10/N5, and D10/N10 samples are (5.5241 Å, 7.8243 Å), (5.5352 Å, 7.8375 Å), (5.5220 Å, 7.8241 Å), and (5.5336 Å, 7.8374 Å), respectively. This is due to the fact that the ionic radius of Dy³⁺ (1.08 Å) is smaller than that of Sr²⁺ (1.44 Å), while the ionic radius of Nb⁵⁺ (0.64 Å) is larger than that of Ti⁴⁺ (0.605 Å) [30,31]. The lattice parameters obtained here are similar to those of previously reported SrTiO₃ [32]. The values of R_wp, R_exp, and χ² are in the ranges of 6.23–7.77, 4.17–4.45, and 2.03–3.05, respectively, indicating highly credible Rietveld refinement.

The three-dimensional crystal structures of Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10) are generated using the VESTA program and displayed in Supplementary Figure S1. There are two types of oxygen atoms, O(1) and (O2). The bond distances between the constituting atoms of Sr_1−xDy_xTi_1−yNb_yO_3−δ are provided in

Table 2. Bond distances between the constituting atoms in Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10).

	D5/N5 sample	D5/N10 sample	D10/N5 sample	D10/N10 sample
Sr−O(1) distance (Å)	2.6458	2.6630	2.6139	2.6522
Sr−O(2) distance (Å)	2.7620	2.7676	2.7610	2.7668
Ti−O(1) distance (Å)	1.9561	1.9594	1.9560	1.9645
Ti−O(2) distance (Å)	1.9606	1.9630	1.9645	1.9637
Ti−Ti distance (Å)	3.9061	3.9140	3.9046	3.9128

. Generally, the bond distances of Sr−O(1), Sr−O(2), Ti−O(1), Ti−O(2), and Ti−Ti for the D5/N5 sample are smaller than those for the D5/N10 sample, whereas the bond distances of Sr−O(1), Sr−O(2), Ti−O(1), Ti−O(2), and Ti−Ti for the D10/N10 sample are larger than those for the D10/N5 sample. This is due to the fact that Dy³⁺ (1.08 Å) has a smaller ionic radius compared to Sr²⁺ (1.44 Å), while Nb⁵⁺ (0.64 Å) has a larger ionic radius than Ti⁴⁺ (0.605 Å) [30,31]. The atomic coordinate, isotropic thermal parameter (B_iso), and occupancy factor extracted from the Rietveld refinements for Sr_1−xDy_xTi_1−yNb_yO_3−δ are summarized in Supplementary Table S1. The atomic coordinates of Sr, Ti, and O(1) atoms in Sr_1−xDy_xTi_1−yNb_yO_3−δ are scarcely affected by the compositions, specifically (0, 0.50, 0.25), (0, 0, 0), and (0, 0, 0.25), respectively. On the other hand, the atomic coordinates of the O(2) atom are dependent on the compositions, i.e., (0.2281, 0.7281, 0.0000), (0.2304, 0.7304, 0.0000), (0.2220, 0.7220, 0.0000), and (0.2284, 0.7284, 0.0000) for the D5/N5, D5/N10, D10/N5, and D10/N10 samples, respectively. Furthermore, the occupancy factors of the constituting atoms for all the fabricated samples are 1.0000.

The oxidation states and binding energies of the constituting elements (Sr, Dy, Ti, Nb, and O) in Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10) are investigated using XPS. The survey spectra of Sr_1−xDy_xTi_1−yNb_yO_3−δ in the binding energy range of 0–1350 eV show the presence of the constituting elements and their binding energies (Figure 3). No notable differences are observed among the survey spectra with varying compositions. The binding energies and oxidation states of the Sr, Dy, Ti, Nb, and O elements are obtained from high-resolution XPS scan spectra. In addition to the characteristic peaks of the constituting elements, the C 1s peaks are observed at ~285 eV, indicating the presence of carbon- and oxygen-containing contaminants that typically adsorb onto the sample surface from the atmosphere [33,34,35]. The adsorbed oxygen may interfere with the O in Sr_1−xDy_xTi_1−yNb_yO_3−δ.

The high-resolution XPS scan spectra of the Sr element in the D5/N5 and D10/N10 samples are shown in Figure 4a. The XPS spectra of Sr 3d are composed of two peaks, Sr 3d_3/2 and Sr 3d_5/2 [36,37,38]. The binding energies of the Sr 3d_3/2 and Sr 3d_5/2 peaks are summarized in Supplementary Table S2. The difference between the binding energies of the Sr 3d_3/2 and Sr 3d_5/2 peaks is 1.8 eV, irrespective of the composition. These results indicate that the Sr ions are in the +2 oxidation state. The high-resolution XPS spectra of the Dy element in the D5/N5 and D10/N10 samples are shown in Figure 4b. The symmetrical Dy spectra, centered at ∼1296 eV, correspond to Dy 3d_5/2 [39,40], suggesting that the Dy ions are in the +3 oxidation state.

The high-resolution XPS spectra of the Ti element in the D5/N5 and D10/N10 samples are shown in Figure 4c. The XPS spectrum of the D5/N5 sample exhibits characteristic doublet peaks centered at 462.4 and 456.6 eV, which correspond to the Ti³⁺ 2p_1/2 and Ti³⁺ 2p_3/2 peaks, respectively. The binding energies of the Ti 2p_1/2 and Ti 2p_3/2 peaks and the energy separations between these peaks are listed in Supplementary Table S2. The high-resolution spectra of the Ti ions are deconvoluted to examine their oxidation states. The second set of doublet peaks corresponds to the oxidation state of Ti³⁺ [30]. The Ti³⁺ ions are formed by the partial reduction of Ti⁴⁺ to Ti³⁺ during sintering in a reducing atmosphere, compensating for the excess electrons introduced by the Dy/Nb co-doping. The relative amounts of the Ti³⁺ and Ti⁴⁺ ions can be estimated by calculating the ratio of each area of the Ti³⁺ and Ti⁴⁺ peaks to the total area of the Ti 2p_1/2 and Ti 2p_3/2 peaks. The relative amount of Ti³⁺ increases with an increasing Dy/Nb concentration, reaching 12% and 20% for the D5/N5 and D10/N10 samples, respectively (Supplementary Table S3). These findings indicate that the Ti ions are in a mixed oxidation state (+3 and +4), with the major oxidation state being +4 [30,41,42,43,44,45,46]. The formation of trivalent Ti³⁺ can generate additional electrons.

The high-resolution XPS spectra of the Nb element in the D5/N5 and D10/N10 samples are shown in Figure 4d. The XPS spectra of Nb 3d consist of Nb 3d_5/2 and Nb 3d_3/2 spin−orbit doublets [30]. In the D5/N5 sample, the binding energies of Nb⁴⁺ 3d_3/2 and 3d_5/2 peaks are located at 208.5 and 205.7 eV, respectively, while those of Nb⁵⁺ 3d_3/2 and 3d_5/2 peaks are located at 209.4 and 206.7 eV, respectively (Supplementary Table S2). The Nb ions exist as a mixture of Nb⁴⁺ and Nb⁵⁺. The relative amounts of the Nb⁴⁺ and Nb⁵⁺ ions are presented in Supplementary Table S3. As the Dy/Nb concentration increases, the relative amount of Nb⁴⁺ rises from 8% in the D5/N5 sample to 10% in the D10/N10 sample. The formation of both Ti³⁺ and Nb⁴⁺ ions in Sr_1−xDy_xTi_1−yNb_yO_3−δ can increase the electron concentration of the system, in accordance with the Kröger−Vink equations discussed previously in Equations (1) and (2), thereby enhancing electrical conductivity. The electrical conductivity (σ) can be expressed as σ = neμ, where n is the electron concentration, e is the electronic charge, and μ is the electron mobility. The electron concentration and electrical conductivity of Sr_1−xDy_xTi_1−yNb_yO_3−δ are provided in our previous report [47].

The high-resolution XPS spectra of the O element in the D5/N5 and D10/N10 samples are shown in Figure 4e. The O 1s spectra of the D5/N5 sample can be decomposed into two distinct peaks at 531.1 eV and 529.3 eV, which are associated with surface-adsorbed or non-lattice oxygen and lattice oxygen bonded with titanium (Ti−O), respectively [48,49]. The O 1s corresponds to an oxidation state of −2.

To investigate the optical band-gap energy of Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10), the UV−Vis−NIR diffuse reflectance spectra are obtained, as shown in Figure 5a. The diffuse reflectance substantially increases with increasing wavelength in the wavelength of 350 to 400 nm. It has been reported that SrTiO₃ is an indirect semiconductor [50,51]. Therefore, the band-gap energy (E_g) of Sr_1−xDy_xTi_1−yNb_yO_3−δ can be calculated using the relation (αhν)^1/2 = A(hν − E_g) [52], where α is the absorption coefficient, ν is the photon frequency, h is the Planck constant, E_g is the optical band-gap energy, and A is a proportionality constant. The band-gap energy (E_g) of all the samples is obtained by extrapolating the linear part to (αhν)^1/2 = 0, as represented in Figure 5b. The band-gap energies slightly decrease with increasing Dy/Nb concentrations, e.g., 2.9 and 2.7 eV for the D5/N5 and D10/N10 samples, respectively (

Table 3. Band-gap energy of Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10).

	D5/N5 sample	D5/N10 sample	D10/N5 sample	D10/N10 sample
Band-gap energy (eV)	2.9	2.8	2.8	2.7

). Furthermore, the band-gap energies of the prepared Sr_1−xDy_xTi_1−yNb_yO_3−δ are smaller than those of previously reported pristine SrTiO₃ (~3.2 eV) [52,53]. The narrowing of band-gap energy is beneficial for enhancing the electron concentration due to the increased excitation of electrons from the valence to conduction band.

4. Conclusions

Sr_1−xDy_xTi_1−yNb_yO_3−δ (0.05 ≤ x, y ≤ 0.10) samples were fabricated using cold compaction, followed by sintering in a (95% N₂ + 5% H₂) reducing atmosphere. Sr_1−xDy_xTi_1−yNb_yO_3−δ had a tetragonal structure (I4/mcm space group). Ti ions in the sintered samples existed as a mixture of Ti³⁺ and Ti⁴⁺, and Nb ions existed as a mixture of Nb⁴⁺ and Nb⁵⁺. The major valences of Ti and Nb ions were +4 and +5, respectively. The incorporation of Ti and Nb ions into SrTiO₃ reduced the band-gap energies. The incorporation of Ti and Nb ions, the formation of both Ti³⁺ and Nb⁴⁺ ions, and the reduction in band-gap energies can effectively increase the electron concentration and its corresponding electrical conductivity. We believe that Sr_1−xDy_xTi_1−yNb_yO_3−δ with high electrical conductivity can be utilized in energy and electronics applications.