Advances in Materials Science and Engineering

Volume 2014 (2014), Article ID 602504, 7 pages

http://dx.doi.org/10.1155/2014/602504

Research Article

Effect of Na Content on the Physical Properties of Powders

Tassanee Tubchareon,¹ Soipatta Soisuwan,² Somchai Ratanathammaphan,³ and Piyasan Praserthdam¹

¹Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand

²Department of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi 20131, Thailand

³Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand

Received 29 May 2013; Accepted 26 November 2013; Published 30 January 2014

Academic Editor: Jun Liu

Copyright © 2014 Tassanee Tubchareon et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The different Na contents (0 ≤ Na ≤ 0.35, based on mole of NaOH) of doped Ba0.5Sr0.5TiO₃ (BST) powders synthesized via sol-gel process were studied. The substitution of Na⁺ ions into a partial A-site of BST powders provided the reduction in vacancy defects as confirmed by electron paramagnetic resonance (EPR) and UV-visible spectroscopy. Photoluminescence (PL) spectra appeared in violet, blue, and green emissions. The phase structure, oxygen deficiency, and titanium deficiency of BST powders were further investigated as a function of Na content. X-ray diffraction (XRD) result was found that low Na content (0 ≤ Na ≤ 0.15) exhibited the tetragonal structure, while it was transformed to the cubic phase when high Na content. Moreover, X-ray photoelectron spectroscopy (XPS) result revealed that the partial oxidation of Ti³⁺ ions to Ti⁴⁺ ions was observed at Na content lower than 0.05 mole, while more addition of Na content resulted in the increasing of the oxygen and the titanium deficiency. Furthermore, the result indicated the oxygen deficiency significantly formed at the A-site of Sr atoms more than that of Ba atoms.

1. Introduction

The Ba0.5Sr0.5TiO₃ (BST) powders exhibit excellent electrical properties and have been extensively used in various electronic applications such as semiconductor, dynamic random access memory (DRAM), ceramic capacitors, pyroelectric sensors, and microwave devices [1, 2].

The metal doped titanate-based perovskite has been widely reported such as Na⁺ [3], K⁺ [3], Mg²⁺ [3–5], Ca²⁺ [4], Ni²⁺ [5, 6], Cu²⁺ [7], Pb²⁺ [8, 9], Ga³⁺ [3], Fe³⁺ [5, 6], Pr³⁺ [6], La³⁺ [6, 10], and Nd³⁺ [11]. The metal doped perovskite materials can substitute into A- and/or B-site, relying on the ionic radii of metal dopant. Typically, acceptor-dopant is the creation of vacancy site, that is, cation vacancy and anion vacancy. The oxygen vacancy is a kind of anion vacancy that can be generated as well as the recombination of electrons and holes defect [12]. Moreover, it plays an important role in the structurally order-disorder of perovskite materials [13]. In our previous study, it was found that the oxygen deficiency significantly affected the A-site of Sr atoms more than that of Ba atoms and the modified A/B-site of BST powders influenced the phase structure [3]. Therefore, this work investigates the modified A-site of BST powders under the study of the influence of Na content on the physical properties of BST powders. Since the solid state technique of preparation has met a restriction concerning high temperature reaction and nonhomogeneity, lower temperature preparation is required. Hence, the sol-gel technique has been introduced in our experiments. The Na-doped BST powders were characterized by TG, XRD, XPS, PL, UV-vis, and EPR.

2. Experimental

2.1. Material Synthesis

All the starting materials used in this work are commercially available and are used without further purification. Barium hydroxide octahydrate [Ba(OH)₂·8H₂O] (98%, Aldrich), strontium nitrate [Sr(NO₃)₂] (99%, Aldrich), titanium (IV) isopropoxide [C₁₂H₂₈O₄Ti] (97%, Aldrich), sodium hydroxide [NaOH] (Merck), and deionized water were used as the starting materials for BST powders synthesis by the sol-gel method. The starting solution (50 mL) contained the Ba + Sr precursors and Ti precursor at (Ba + Sr) : Ti ratio of 1 : 1, and Ba : Sr ratio is equal to 0.5 : 0.5. To concentration of sodium hydroxy doped BST powders were to be at 0, 0.025, 0.05, 0.15, 0.25, and 0.35, respectively. The abbreviations of BST powders in this work are described as BST, where is mole of sodium hydroxide doped BST powders.

In preparation solution, the raw materials of Ba, Sr, and Na precursor were dissolved in deionized water, and then the homogeneous solution was stirred for 1 h at mild heating to completely dissolve the metallic precursor. The solution was then stirred at room temperature for 2 h, and then Ti precursor was continuously added drop-wise and continuously stirred until a white precipitate appeared. The BST solution was then stirred overnight at room temperature to completely mix. The solution was washed three times in methanol and the as-synthesized BST product was finally dried in an oven at 110°C and calcined in stagnant air at 750°C for 5 h.

2.2. Materials Characterizations

All samples characterizations were measured at room temperature. The as-synthesized (dried-gel) BST powders were characterized by thermogravimetric analyzer (TG). The thermal analysis was performed by SDT Q600 instrument under air flow of 400 mL min⁻¹ and heating rate of 5°C min⁻¹. The calcinations BST powders were examined by XRD, XPS, PL, UV-vis, and EPR. X-ray diffraction (XRD) was performed by SEIMENS D5000 using and Ni filter. X-ray photoelectron spectroscopy (XPS) was examined by AMICUS with X-ray source. The C1s is used as reference peak at 285.0 eV. Photoluminescence (PL) was measured by Perkin Elmer LS 55 and was excited by Xenon lamp source at 325 nm. The ultraviolet-visible spectroscopy (UV-vis) was examined with Perkin Elmer LAMDA 650 which was measured between 200 and 900 nm. The electron paramagnetic resonance (EPR) was studied by JEOL model JES-RE2X with DPPH used for value calibration.

3. Results and Discussions

3.1. Effect of Na Contents on the Thermal Properties of BST Powders

Thermal analysis was carried out under air flow in order to investigate the thermal properties of dried-gel with various Na contents as shown in Figure 1.

Figure 1: The TG curves of dried-gel BST powders with various Na contents.

The two stepwise losses of weight exhibited at temperatures 400°C and 600°C for Na contents up to 0.15 mole, that is, BST0.025, BST0.05, and BST0.15. This can be explained that the physical and the chemical desorption of water were presented at the temperature below 400°C [14], in which the carbonate species were possibly decomposed at higher temperature in range of 400–600°C [15, 16], and no significant weight changes were eventually observed after the temperature 650°C. In case of Na contents up to 0.25 and 0.35 moles (BST0.25 and BST0.35), the differences in thermal decomposition patterns which possessed constant loss rate in the experimental range of room temperature to 1000°C are apparently presented in Figure 1. However the thermal response of dried-gel for both patterns did not show the significant effect on the formation of perovskite crystalline structure as will be mention later.

3.2. Effect of Na Contents on the Phase Structure of BST Powders

The structure of BST powders was achieved via the sol-gel process as depicted in Figure 2. The incorporation of the stranger Na⁺ ions in BST structure was ascribed to the shifted position of the main peak . Moreover, the substitution of Na⁺ ions into A-site of BST crystalline can be verified by the reduction in the lattice parameter [3] as illustrated in Table 1. The decreasing of lattice parameter probably occurred since the smaller ionic radii of Na⁺ ion (0.139 nm) were partially occupied in the A-site of Ba²⁺ ion (0.161 nm) and/or Sr²⁺ ion (0.144 nm). Consequently, the phase structures of BST perovskite were observed as a function of Na contents. The XRD patterns of BST0 showed the cubic structure of Ba0.5Sr0.5TiO₃ which referred to Joint Committee on Powder Diffraction Standards (JCPDS) card number 39–1395. Considering low Na content (0–0.15 mole), BST0.025, BST0.05, and BST0.15 showed slightly asymmetric peak of , indicating the presence of the tetragonal distortion [3, 17]. As a result, it was evident that the tetragonality ( ratio) of BST powders was approximately 1.004. Thus, the B-site cation shifted away from the center of BO₆ octahedral due to the local charge imbalance, resulting in the tetragonal distortion [18]. On the other hand, at high Na content, that is, BST0.25 and BST0.35, the Na⁺ ions were randomly occupied into the A-site of BST powders and the BO₆ octahedral distortion was therefore decreased. As the result of the local charge balance, thus causing the phase transformation from tetragonal to cubic [18].

Table 1: Crystallite size, lattice parameter, and indirect band gap of BST powders with various Na contents.

Sample
Crystallite size

(nm)
Lattice parameter

(nm)
Indirect band gap (eV)

BST0
30
0.3954
—
2.80

BST0.025
29
0.3934
0.3950
3.15

BST0.05
30
0.3931
0.3946
2.90

BST0.15
28
0.3927
0.3944
3.18

BST0.25
40
0.3934
—
3.20

BST0.35
54
0.3939
—
2.90

Figure 2: The XRD patterns of BST powders with various Na contents.

3.3. Effect of Na Contents on the Vacancy Defects of BST Powders

The ordered structure of titanate-based perovskite can be explained as (TiO₆)-() clusters, in which their cation and vacant of A- or B-site are arranged in the order that is close to the perfect lattice crystalline. These clusters are only significant when the absences of A- or B-site of metal vacancies occurred, whereas the disordered structure suggests that the metal-dopant is randomly distributed in the perovskite structure. The disordered structure can be explained as ()-(TiO₆) clusters where the oxygen vacancies can possibly occur in three states: (i) neutral oxygen vacancy (), (ii) singly ionized oxygen vacancy (), and (iii) fully ionized oxygen vacancy (). Moreover, the degree of the ordered-disordered structure can be interpreted by the optical absorption edge [19]. In Figure 3, it can be observed that the absorbance spectra dramatically decreased at the wavelength of ca. 380 nm. The indirect band gap of Na-doped BST powders was slightly higher than the BST0 as seen in Table 1. The increase in the band gap was attributed to the reduction in the lattice defect [20] and the vacancies site [21].

Figure 3: The UV-vis spectra of BST powders with various Na contents.

Furthermore, the ordered-disordered structure was also examined by the photoluminescence behavior. According to typically the PL light emissions [22–25], the shallow defect indicated more structural order of perovskite materials, showing in violet and blue light emissions. Contrarily, the deep defect pointed to the structural disorder, presenting in green, yellow, orange, and red light emissions. Figure 4 determines the PL spectra of BST powders with various Na contents. In the violet light emissions, the first peak is attributed to the optical band gap of BST powders [3, 26]. This peak rather agreed with the optical band gap of UV-vis result. The second peak at 2.95 eV was assigned to the electron transferring in TiO₆ [21, 27]. The third peak emitting in the blue light (2.80 eV) was related to the fully ionized oxygen vacancy () [28, 29]. Furthermore, for the green light emissions, the fourth peak (2.55 eV) corresponded to the charge-transfer of singly ionized oxygen vacancy () [21, 27, 30] and the last one (at 2.33 eV) was assigned to the charge-transfer vibronic excitons (CTVE) of Ti–O–Ti in perovskite [28, 30].

Figure 4: The PL spectra of BST powders with various Na contents.

In addition, the electron paramagnetic resonance (EPR) is one of the most widely used techniques to investigate the defect complex with one unpaired electron. Figure 5 illustrates the EPR spectra of BST powders with different Na contents. It can be clearly seen that BST0 exhibited the strongest intensity of the EPR signal at the value of 2.004, while other BST powders modified by Na⁺ ions obviously provided the decreased g-signal intensity. This signal was associated with the titanium vacancy, that is, or [31, 32]. The result was in a good agreement with our previous work [3], claiming that the Na⁺ ions which modified BST powder caused the decrease in the g-signal intensities. Moreover, low intensity of EPR signal indicated lower content of defect. This result corresponded to the UV-vis result. Regarding, higher in doping level, the symmetry of EPR spectra peak obviously decreased, relying on the Na contents.

Figure 5: The EPR signal of BST powders with various Na contents.

In this work, all samples were observed the green emissions in PL spectra. It can be implied that the formation of oxygen deficiency reasonably occurred in BST powders. Therefore, it was necessary to consider the amount of oxygen on the surface by means of XPS. The deconvolution of O 1s and Ti 2p with Gaussian function for all samples is presented in Figures 6 and 7, respectively. According to the curve fitting analysis, the O 1s spectra peak was composed of three oxygen species. The first species observed at ca. 528.7 and 529.8 eV [33–35] was attributed to the lattice oxygen (OL). Secondly, the chemisorption oxygen (OC) appeared at ca. 531.4 and 532.4 eV [33, 34]. For the last one, the oxygen in hydroxyl (OH), observed at about 533.5 eV [34]. Moreover, the presence of Ti⁴⁺ ion exhibited the XPS spectra of two positions located at ca. 458.5 and 464.3 eV, while the spectra of the defect Ti³⁺ ion that might occur via the loss of oxygen site were apparently found at ca. 457.8 and 462.4 eV [36, 37]. From Table 2, it is clearly revealed that low Na contents (BST0.025 and BST0.05) suppressed both of the (OC + OH)/OL ratio and the Ti³⁺/Ti⁴⁺ ratio. It is indicated that the oxygen and the titanium deficient formations reduced due to the occurrence of the oxidation of titanium leading to the change in the valence state from Ti³⁺ to Ti⁴⁺ ion. At higher Na contents (BST0.15, BST0.25, and BST0.35), the increase in the oxygen and the titanium deficient formation was achieved because of the increase of the chemisorption oxygen.

Table 2: The atomic ratio calculations of O 1s, Ti 2p, Sr 3d, and Ba 3d.

Samples
(OC + OH)/OL
Ti³⁺/Ti⁴⁺
/SrO
/BaO

BST0
0.56
0.22
0.62
0.82

BST0.025
0.36
0.17
0.58
0.68

BST0.05
0.40
0.15
0.87
0.56

BST0.15
0.56
0.22
0.73
0.59

BST0.25
0.63
0.26
1.08
0.52

BST0.35
0.69
0.22
1.05
0.57

Figure 6: The fitting XPS spectra of O 1s deconvolution with Gaussian function.

Figure 7: The fitting XPS spectra of Ti 2p deconvolution with Gaussian function.

As mentioned above, the B-site cation was fluctuated in titanium ions as a result of oxygen vacancies. In order to provide a further insight into the fluctuation in A-site, the surface chemistry of Sr 3d and Ba 3d was essentially investigated. From the curve fitting of Sr 3d spectra peak, it was found that the Sr 3d consisted of two strontium species. The SrO oxide appeared at ca. 132.4 and 134.4 eV, while the SrO1−δ suboxide was observed at about 133.2 and 135.1 eV [38], as seen in Figure 8. Figure 9 presented two barium species, that is, the BaO oxide was found at ca. 778.7 and 794.2 eV, while the suboxide was observed at about 780.2 and 795.2 eV [39–41]. As reported in Table 2, it was revealed that Na-doped BST powders with increasing the Na contents affected the increased /SrO ratio, whereas decreased /BaO ratio was obtained. The oxygen deficiency significantly affected the A-site of Sr atoms more than that of Ba atoms [3]. This was probably due to the strength of Ba–O chemical bonds (561.9 ± 13.4 kJ/mol) which are higher than those of O–Sr bonds (426 ± 6.3 kJ/mol) [42].

Figure 8: The fitting XPS spectra of Sr 3d deconvolution with Gaussian function.

Figure 9: The fitting XPS spectra of Ba 3d deconvolution with Gaussian function.

4. Conclusions

The various moles of sodium hydroxide (0, 0.025, 0.05, 0.15, 0.25, and 0.35) doped BST powders were investigated. The Na⁺ ions were partially substituted into A-site of BST powders. It was revealed that the Na-doped BST powders obviously caused the decreased weight loss and titanium vacancy defects. The five peaks of PL spectra were found to be at 3.15, 2.95, 2.80, 2.55, and 2.33 eV. These can be assigned to the band gap, the electron transfer in TiO₆, the fully ionized oxygen vacancy (), the charge-transfer of singly ionized oxygen vacancy (), and the charge-transfer vibronic excitons in BST powders, respectively. The presence of Na content affected the phase structure. At low Na content (0–0.15), the appearance of tetragonal structure was observed because of slightly asymmetric peak of from XRD pattern. Besides, at higher in Na content, the Na⁺ ions were randomly occupied into the A-site of BST powder, leading to the phase structure evolved from tetragonal to cubic structure. Moreover, the oxygen and the titanium deficiency were dependent on the Na content. At low Na content (0–0.05), both of the oxygen and the titanium deficiency were reduced due to the oxidation of a small fraction of Ti³⁺ ion to Ti⁴⁺ ion, whereas those increased with more addition of Na content. Furthermore, this work can confirm that the Sr–O was lower in the strength of chemical bond than the Ba–O, indicating the A-site of Sr atom formed the oxygen deficient more than A-site of Ba atom.

Conflict of Interests

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

Acknowledgments

The financial support from Mektec Manufacturing Corporation (Thailand) Ltd. and The Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University (RES5605300086-AM) are greatly appreciated.

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