Electrocatalytic Properties of Calcium Titanate, Strontium Titanate, and Strontium Calcium Titanate Powders Synthesized by Solution Combustion Technique
Calcium titanate (CaTiO3), strontium titanate (SrTiO3), and strontium calcium titanate (SrxCa1−xTiO3) are widely recognized and utilized as dielectric materials. Their electrocatalytic properties, however, have not been extensively examined. The aim of this research is to explore the electrocatalytic performance of calcium titanate, strontium titanate, and strontium calcium titanate, as potential sensing materials. Experimental results revealed that CaTiO3, SrTiO3, and Sr0.5Ca0.5TiO3 powders synthesized by the solution combustion technique consisted of submicrometer-sized particles with specific surface areas ranging from 4.19 to 5.98 m2/g. Optical bandgap results indicated that while CaTiO3 and SrTiO3 had bandgap energies close to 3 eV, Sr0.5Ca0.5TiO3 yielded a lower bandgap energy of 2.6 eV. Cyclic voltammetry tests, measured in 0.1 M sodium nitrite, showed oxidation peaks occurring at 0.58 V applied voltage. The highest peak current was observed in Sr0.5Ca0.5TiO3 powder. The superior electrocatalytic performance of Sr0.5Ca0.5TiO3 might be attributed to lower bandgap energy, which consequently facilitates higher electron transfer. Electrocatalytic performance of Sr0.5Ca0.5TiO3 was subsequently reexamined in a wider concentration range of sodium nitrite. The results revealed that the material responded linearly to nitrite solution in the range of 0.1 mM to 0.1 M and exhibited sensitivity ranging from 3.117 to 0.040 μA/mM, in the entire tested nitrite concentrations. The results suggest that Sr0.5Ca0.5TiO3 could also be used for nitrite detection.
Calcium titanate, strontium titanate, and strontium calcium titanate are perovskite-structured ceramics often exploited in fabrication of electronic devices. In general, perovskite-structured titanate, such as CaTiO3, SrTiO3, BaTiO3, and FeTiO3, have relatively wide bandgap energies ranging between 2.8 and 3.6 eV [1–6]. Bandgap energies can be modified to suit the intended application through doping . Perovskite materials, owing to their high stability, good electrical response, and bandgap energy capability to be tailored, have been developed for utilization in photocatalytic applications as in wastewater treatment, which involved antibiomicrobial action and water splitting [8–10]. Other known usages of perovskite materials include the production of solar cells and gas sensing as well as biosensing devices [11, 12]. Since nonenzymatic sensors used in food chemical or human fluid detection are generally produced from semiconductor materials with bandgap energy values ranging from 2.3 to 3.4 eV, considering perovskite’s bandgap energy characteristics, the materials have great potential for electrochemical sensors [13, 14].
Food chemical contaminant and toxicants are of great importance. For example, the food additive nitrite is a commonly used chemical in curing meat and usually found in processed foods such as sausage, bacons, and drinks. Consumption of nitrite in high concentrations can adversely affect human health. Nitrite is known as a potential carcinogenic substance [15, 16]. It has been reported that nitrite can reduce oxygen exchange in haemoglobin and oxygen-carrying capacity with adverse health effects especially in pregnant women and infants [17, 18].
Nitrite detection via the electrochemical technique using nonenzymatic sensors requires materials with high electrocatalytic activities. Enhancement of electrocatalytic activities can be achieved through chemical composition control, microstructure refining, and material electronic structure tailoring [19, 20].
Doping is one of the processes to improve perovskite’s electrochemical performance through structure alteration. Distortion of the material structure often occurs as a result of the introduction of dopants into the crystal lattice sites. This is also accompanied by an increase of charged ions . Microstructure control also enhances electrocatalytic activity. Fine and porous particulates with an increased surface area also create more active sites for reactions and consequently results in improved sensing performance [22, 23]. According to Zhu et al., refined microstructure also facilitates mobility of electrons between sensing materials and substance for detection [24, 25].
This study, therefore, aims at studying electrocatalytic performance of calcium titanate, strontium titanate, and strontium calcium titanate synthesized by the solution combustion technique. Relationships among chemical compositions, microstructure, bandgap energies, and electrocatalytic properties of the materials were also examined.
2. Materials and Methods
Calcium titanate, strontium titanate, and strontium calcium titanate were synthesized by the solution combustion technique. The initial reagents consisting of calcium nitrate (Deajung), strontium nitrate (Deajung), and titanium dioxide (Sigma-Aldrich) in nitric acid (Univar) were mixed to attain an aqueous solution of 0.54 M concentration. Initiation of combustion was facilitated by glycine (Deajung), which was added to the prepared aqueous solution, at a temperature of about 400°C. The powders obtained from the solution combustion process were subsequently calcined at 900°C for 3 hours.
Chemical composition and microstructural analysis of the calcined powder were conducted using an X-ray diffractometer (Philips, X’Pert) and a scanning electron microscope (FEI, Quanta 450), respectively. ImageJ software was employed in particle size analysis, whereas Brunauer–Emmett–Teller (Micrometrics, 3 Flex) was utilized in the measurement of specific surface area of the powders.
Optical bandgap energy measurement was conducted using a UV-Vis spectrophotometer (UV-1700). The measurements of optical transmission were conducted within the wavelength ranging between 300 and 900 nanometers. The value of optical bandgap is expressed as follows :where α is the absorption coefficient, Eg is the energy (eV), h is Planck’s constant (Js), and υ is the wave frequency.
Electrocatalytic activities of the powders were determined using a Metrohm AutoLab Potentiostat (PGSTAT302N). Cyclic voltammetry technique was performed on the working electrode containing the synthesized powder and multiwalled carbon nanotube (MWCNT) within the voltage range of −0.8 to 0.8 V, at the scan rate of 0.01 V/s. Pt was used as the counterelectrode, while Ag/AgCl as the reference electrode. The working electrode was activated in 0.1 M sodium hydroxide solution. Detection of nitrite was conducted in sodium nitrite solution with the concentration range of 0.1 to 100 mM. The working electrode was prepared by mixing the synthesized powders with the multiwalled carbon nanotubes at the ratio 1 : 1 by weight and subsequently underwent the hydrothermal process at 120°C for 5 hours. The mixture was drop onto the 1 cm by 1 cm stainless steel substrate.
3. Results and Discussion
3.1. Chemical Composition
Upon calcination, the synthesized powders were tested for their chemical compositions using the X-ray diffraction technique. The Sr-Ti-O powders exhibit the peaks corresponding to strontium titanate (JCPDS 082-0230), as shown in Figure 1. For the Ca-Ti-O powders, the compositional analysis showed a single phase corresponding to calcium titanate, CaTiO3 (JCPDS 082-0229). Strontium calcium titanate, SrxCa1−xTiO3 (JCPDS 089-8032), was observed in the Sr-Ca-Ti-O powders. Low intensity peaks corresponding to anatase titanium dioxide (JCPDS 084-1285) were observed in the calcium titanate and strontium calcium titanate powder.
X-ray diffraction patterns were also utilized in the semiquantitative analysis. Fraction of the main phase (SrxCa1−xTiO3 (x = 0, 0.5, and 1)) to the total powder content, defined as F, was determined as follows:where F is the weight faction of the primary phase to the total powder, Imix is the integrated intensity of pure phase and secondary phase, Ipure is the integrated intensity of pure phase, Aimpurity is the mass attenuation factor of titanium dioxide (anatase), and Apurity is the mass attenuation factor of SrTiO3 or SrxCa1−xTiO3.
The semiquantitative analysis revealed that content of the secondary phase contained in the synthesized powder was minimal. The titanium dioxide present in the synthesized powder did not exceed 1.25 wt.%. The trivial quantity of the secondary phase seemed to have not contributed to significant electrocatalytic activity alterations.
3.2. Microstructure and Specific Surface Area
Particle size and morphology of the powders generally influence specific surface area, which control reaction sites involved in the catalytic activity. In this study, the average particle sizes were determined by an image analysis technique, while the specific surface areas (SSAs) were measured by BET. Images from a scanning electron microscope revealed that the powders contained equiaxial particles with sizes in submicrometer ranges. Base on the image analysis, average particle sizes of all powders were within the range of 313 and 358 nm (Figure 2). Assuming that the particles are well dispersed, nonporous, and spherical in shape, the relationship between specific surface area and particle size can be expressed by the following equation :where D is the average particle diameter, ρ is the density of powder, and SSA is the specific surface area.
Using equation (3), the specific surface area of the powders base on the particle size obtained from image analysis ranging would be in the range between 2.76 and 4.40 m2/g for particle sizes ranging from 313 to 358 nm. However, in this study, specific surface area of the powders measured by the BET technique ranged from 4.19 to 5.98 m2/g (Table 1). The marginally higher specific surface areas obtained from measurements indicated that the particles might be slightly porous in nature, which is a common observation in powders synthesized by the solution combustion technique [28, 29].
3.3. Optical Bandgap
Results from bandgap energy measurements, as shown in the tauc plot in Figure 3, indicated that bandgap energies of synthesized powders ranged between 2.59 and 3.01 eV, with the lowest values found in Sr0.5Ca0.5TiO3. The bandgap energies of CaTiO3 and SrTiO3 are in close proximity, with the value of around 3 eV (Table 2). The values obtained in this study were comparable with those reported by Osterloh and Alammar et al. [30, 31].
In general, doping introduces extra energy levels between the bandgap of the host. This resulted in the decrease of total bandgap energy . In this study, the combination of Sr and Ca created an impure energy band, leading to the reduction of bandgap energy. The findings were being in agreement with the results reported by Wu et al., which indicated that doping perovskite-structured metal titanate by ions of strontium, barium, and calcium facilitated minimization of bandgap energy .
3.4. Electrocatalytic Activity
Cyclic voltammetry was employed in the determination of electrocatalytic activities of the powders. Oxidation peaks occurring as a result of the MWCNT reaction were evident at 0.18 V, while oxidation peaks resulting from the reaction between powder and nitrite occurred at 0.58 V (Figure 4). MWCNT oxidation peaks obtained in the study were comparable with those reported by Fayemi et al. .
Potential oxidation reactions are expressed by the following equations:
Yatsunami et al. reported a similar observation indicating that detection of nitrite can be achieved using perovskite-structured oxides such as SmBO3 (B = Cr, Mn, Fe, and Co) as sensing materials .
In this study, oxidation peak currents ranged from 0.038 to 0.081 mA, with the greatest values found in Sr0.5Ca0.5TiO3. Enhanced electrocatalytic activity was attributed to low bandgap energy. Narrow bandgap facilitates generation of electrical charge carriers, which consequently results in current enhancement.
Voltammetry measurements of Sr0.5Ca0.5TiO3 were also performed in a wider concentration range of sodium nitrite. Current densities resulting from the reaction between powder and nitrite are plotted against concentration of sodium nitrite (Figure 5). Measured oxidation current densities were taken in the range between 0.1 mM and 0.1 M. Calibration curves show linear relationship between current density (J) and nitrite concentration (C) for various nitrite concentration ranges: 0.1–1 mM, 1–10 mM, and 10–100 mM, as shown in Figure 6. Fairly good linearity, represented by high R2 values, was evident. R2 values obtained from the current experiment ranged from 0.94 to 0.96, which were far higher than the generally accepted value of 0.75 .
Sensitivities of the detection are generally determined from the slope of the calibration curve. The equations representing the linear relationship and sensitivity are shown in Table 3. The sensitivity values obtained in this study were in the range between 3.12 and 0.04 μA·M−1·cm−2, which were in the same range with the value observed by Salimi et al. The overall results, hence, suggested that that Sr0.5Ca0.5TiO3 had potential as a nitrite detection material .
Calcium titanate, strontium titanate, and strontium calcium titanate powders were successfully synthesized by the solution combustion technique. The effects of chemical compositions on microstructure, bandgap energies, and electrocatalytic properties of the synthesized powders were examined. It was found that particle size and specific surface areas were not significantly affected by chemical compositions. However, bandgap energy was significantly reduced in strontium calcium titanate. The reduced bandgap energy contributed to enhanced electrocatalytic activity. At the applied voltage close to 0.6 V, the oxidation reaction between synthesized particles and nitrite occurred, revealing the potential capability of the materials for exploitation in the fabrication of enzyme-less sensors for nitrite detection.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the financial support of Kasetsart University Research and Development Institute (KURDI), the equipment support of the KU Department of Materials Engineering, and the valuable and fruitful dialogues and suggestions of Dr. Gasidit Panomsuwan.
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