Facile and Eco-friendly One Step Low Temperature Synthesis of Very Large Scale ATiO3 (A = Ca, Sr, Ba and Cd)

Highlights

Development of a low-cost methodology for the hydrothermal synthesis of titanates with varying size and morphology

Low temperature general synthesis protocol for 4 titanates

Variation of reactant phase from solid to liquid and their effect of morphology

Hollow rectangle shape morphology of CaTiO3

Enhancement in the surface area of BaTiO3

Scope of applications in various industry

Introduction

Nanostructured materials such as nanoparticles, nano cubes, nanorods, nanotubes and nanowires are under intensive investigation due to their size and shape dependent properties and potential application in the all field of technology due to their thermal, electrical, mechanical and optical properties [1-5]. Among the nanostructured materials, perovskite with general formulae ABO3 constitute important classes of materials in solid state chemistry due their wide applicability in ferroelectrics, dielectric, piezoelectric nonlinear optics properties with their technology importance for fabricating the different class of devices [6-10]. On the commercial scale, these oxide powders are produced at high- temperature which’s chemical in homogeneity, reactivity problems and very little control on the size, shape and agglomeration. Due to these above problems, there is demand for the low temperature synthesis route to yield high-purity fine powders with controlled size and morphology for miniaturization of these materials to the nanometer scale for the next generation of electronics.

All the titanates are useful class of materials due to their industrial application in various technologies. Among them BaTiO3, SrTiO3, and CaTiO3 has more importance. BaTiO3 has ferroelectric property, high dielectric constant and positive temperature coefficient of resistivity (PTCR) used as multilayered ceramic capacitors (MLCCs), embedded decoupling capacitors (EDC), thermistor, waveguide modulators, IR detectors, microwave absorbers, dynamic random access memory and field - effect transistors [11-15]. SrTiO3 is an important n-type semiconductor with band gap between 3.2 – 3.4 eV have high dielectric constant, thermal and chemical stability. It has a wide application in field of thin-film capacitors, nonlinear optics, optical memories, oxygen gas sensors, photocatalyst and photoelectrode for splitting of water, solar cell and electrooptic modulators [16-19]. CaTiO3 crystallizes in orthorhombic crystal system with space group (Pbnm). It has various applications such as component of capacitors, immobilization of nuclear wastes, mineralogy, microwave dielectric applications (as resonators and filters), luminescent material and biomedical area [20-23]. CdTiO3 crystallizes in non- ferroelectric ilmenite rhombohedral crystal system. Below the 1000 0C annealing temperature is an intelligent material due to their application in optical fiber and sensor [24, 25]. It has also piezoelectric, pyroelectric, magneto restrictive and photostrictive properties [26-28].

There are various reports for synthesizing such titanates by solid state, sol – gel, hydrothermal, microwave hydrothermal method, spray pyrolysis, microemulsion, polymeric precursor and coprecipitation [29-37]. There are few reports for general methodology of the MTiO₃ nanoparticles [38-41]. There are reports of the synthesis of SrTiO3 and BaTiO₃ at low temperature however there is no any such report on CaTiO₃ and CdTiO₃ at low temperature [42-46]. There is no general approach reported for synthesizing the above four compounds by a single step on large scale.

In this letter, we report the synthesis of ATiOâ?? (A = Ca, Sr, Ba, and Cd) perovskite materials using two different titanium sources: titanium dioxide and titanium isopropoxide. The resulting compounds were thoroughly characterized using powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer– Emmett–Teller (BET) surface area analysis, and UV-Visible diffuse reflectance spectroscopy (DRS).

Synthesis and Characterstics

ATiO₃ nanoparticles were synthesized by hydrothermal route. In typical synthesis process 3.43 m mol of A(NO3)2 was dissolve in 20 ml of double distilled water followed addition of 3.43 m mol of TiO2 (Titanium isopropoxide also). The above solution was stirred for 30 min and then 0.6 mol NaOH dissolved in 40 ml of water were added and stirred for 3 h. The obtained milky white solution was transferred to teflon beaker and putted inside the stainless- steel hydrothermal bomb and placed in oven at 180°C for 16 h. The schematic diagram for the synthesis of ATiO₃ nanoparticles of different morphology is given in scheme 1.

Scheme 1: General Procedure for the Synthesis of ATiO₃

Powder X-ray diffraction studies (PXRD) were carried out using Ni filtered Cu-Kα radiation. Normal scans were recorded with a step size of 0.02° and step time of 1 s. The Kα2 reflections were removed to obtain accurate lattice constants. Field emission scanning electron microscopy (FESEM) was carried out by FEI quanta 3D FEG – FESEM operated at 10 kV. The pellet had been coated with gold. Transmission Electron Microscopy (TEM) studies were carried out using a Tecnai G2 20 electron microscope operated at 200 KV. TEM specimens were prepared by dispersing the sample in ethanol by ultrasonic treatment, dropping onto a porous carbon film supported on a copper grid, and then drying in air. Nitrogen adsorption–desorption isotherms were recorded at liquid nitrogen temperature (77 K) using a Nova 2000e (Quantachrome Corp.) equipment and the specific area was determined by the Brunauer–Emmett–Teller (BET) method. The powder sample was degassed at 150 ºC for 6 h prior to the surface area measurements. Diffuse-reflectance spectra (DR) spectra were recorded on Shimadzu UV-2450 spectrophotometer where the baseline was fixed using a barium sulfate reference.

Result and Discussion

The aim behind this study is to develop a low-cost methodology for the synthesis of important titanates with different morphology at very low temperature. Here we have developed a common one step synthesis protocol at very low temperature (180 ºC) to synthesize titanates at gram scale (~ 1g) level by hydrothermal route. Here we fixed the hydrothermal reaction time, temperatures, reactant and base concentration. We varied the one of the reactant (Ti source) phase from solid to liquid to see change in the morphology. Using a NaOH base, we obtained pure phase of the nanocrystalline titanates ATiO3 (Ca, Sr, Ba, Cd). Figures 1 (A, B, C, D) show the powder X-ray diffraction patterns of CaTiO3, SrTiO3, BaTiO3 and CdTiO3 respectively using both TiO2 and titanium isopropoxide as Ti source. The PXRD patterns confirm the formation of monophasic nature for all the eight titanates. All the observed diffraction patterns were index on the basis of their crystal structures. The lattice parameters were refined and it observed that in case of titanium isopropoxide as Ti source the values was lower as compared to TiO2 as Ti source due to decrease in the particles size of corresponding titanates.

Figure 1: Powder x-ray Diffraction Pattern of (A) CaTiO3(B) SrTiO3(C) BaTiO3(D) CdTiO3

Field emission scanning electron microscopy was carried out to check the morphology of these synthesized titanates. Figure 2 and 3 show the field emission scanning electron micrograph of titanates. Figure 2a shows cube shape morphology of CaTiO₃ obtained using TiO₂ whereas in case of titanium isopropoxide (Figure 2b), rectangle hollow shape morphology was obtained. This result clearly indicates that with change in source phase from solid (TiO2) to liquid (titanium isopropoxide) the morphology was changed. This hollow morphology type’s materials are very interesting for various applications such as drug delivery and catalysis [47-49]. The SrTiO₃ (Figure 2c and 2d) the particles are spherical in nature having two different sizes for the two different titanium sources. For the titanium isopropoxide source, the particles are smaller as compared to titanium dioxide. In the case of TiO₂ source some of these nanoparticles (30 nm size) are aggregated and form cube shaped structures. The BaTiO₃ nanoparticles size distribution (Figure 3b) is very uniform in case of titanium isopropoxideas Ti source and it is found to be very small in size (further confirm by TEM) as compared to TiO2 sources. The BaTiO₃ obtained from TiO₂ (Figure 3a) shows the formation of mixed morphology i.e. spherical and cube. For CdTiO3, sheet like morphology was obtained in both the cases. The size of sheet is smaller and broken in case of liquid phase sources. These sheets like structures are very useful for the formation of 2-D materials for practical applications.

Figure 2: FESEM Micrograph of (a, b) CaTiO₃ (c, d) SrTiO₃ using TiO₂ and Ttitanium Isopropoxide as Ti Source Respectively

Figure 3: FESEM Micrograph of (a, b) BaTiO₃ (c, d) CdTiO₃ using TiO₂ and Titanium Isopropoxide as Ti Source Respectively

Transmission electron microscopy (Figure 4) was carried out to confirm the particle size of SrTiO₃ and BaTiO₃ nanoparticles. The SrTiO₃ nanoparticles (Figure 4a and 4b) are spherical in nature having average particles size of 30 nm and 12 nm for titanium dioxide and titanium isopropoxide respectively. There are very few reports of SrTiO3 having particles of size less than 15 nm but none of the reports claim synthesis at gram scale level [50, 51]. The BaTiO₃ nanoparticles are uniformly distributed and spherical in nature in case of titanium isopropoxide as titanium sources (Figure 4d). The average diameter of the particles was found to be 5 nm. In case of metal titanates obtained from TiO₂ source showed mixed distribution of particle size (Figure 4c). There are only some reports in literature of BaTiO₃ of such small size nanoparticles however the synthesis method not showed a general protocol [42- 46, 51].

Figure 4: TEM Micrograph of (a, b) SrTiO₃ (c, d) BaTiO₃ using TiO₂ and Titanium Isopropoxide as Ti Source Respectively

N₂ adsorption-desorption measurement was carried out for surface area measurements of the nanoparticles (Table 1). It was observed that as titanium source changed from solid phase (TiO₂) to liquid phase (titanium isopropoxide) the surface area of the titanates enhanced along with decrease in particle size. In case of CaTiO₃ and BaTiO₃, the surface area is more enhanced due to the hollow shape particles (CaTiO₃) and small in BaTiO3 in case of titanium isopropoxide as Ti sources. Figure 5 shows the diffuse reflectance spectra of the all the titanates. It was observed that as titanium source changed from TiO₂ to titanium isopropoxide the band gap of the titanates increased due to decrease in particle size. All the titanates have band gap in the UV and close to the reported value. The optical properties (band gap) of nanomaterials are size and morphology dependent [52, 53]. For several cases, it is observed that a change in size and morphology can alter the band gap. The band gap of all the titanates is given in Table 1.

Figure 5: Diffuse Reflectance Spectra of(A) CaTiO3(B) SrTiO3(C) BaTiO3(D) CdTiO3

                                                                   Table 1: Surface Area and Band Gap of Synthesized Titanates

Conclusions

We report a general and cost-effective one-step hydrothermal protocol for the low-temperature synthesis of CaTiO₃, SrTiO₃, BaTiO₃, and CdTiO₃ nanoparticles. The morphology and particle size are strongly influenced by the titanium precursor, with liquid- phase titanium isopropoxide producing significantly smaller and more uniformly distributed nanoparticles than TiO₃; in particular, BaTiO₃ nanoparticles as small as ~5 nm were obtained. The size and morphology can be further tuned by varying hydrothermal parameters such as reaction time, temperature, base, and reactant concentration. The successful synthesis of ultra-small titanate nanoparticles at low temperatures and in large quantities demonstrates the scalability of this approach and highlights its potential for commercial production of nanostructured metal titanates.

Acknowledgements

PKP and BK thanks ANRF for providing support through the grant no SUR/2022/004717. The authors also thank IIT Delhi India for some of reaction and characterization facility.

Author Contributions

Pramendra Kumar Pandey and Revati Raman Chaubey: Synthesis and draft writing, Gyandeshwar Kumar Rao, Suman Srivastava and Arvind Kumar Singh: Characterization, Bharat Kumar: Concept, synthesis modification, final draft writing and final submission. All the authors reviewed the manuscript.

Funding

Funding is not applicable

Declarations

Clinical Trial

Clinical trial is not applicable in the manuscript.

Consent to Publish Declaration

Not applicable

Ethics and Consent to Participate Declarations:

Not applicable

Competing Interests:

The authors declare no competing interests.

Data Availability

The authors declare that the data supporting the findings of this study are available within the paper file. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.

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