Influence of synthesis procedures on the preparation of strontium titanate nanoparticles and photocatalytic application for methylene blue degradation

SrTiO3 is a well-known photocatalyst with various applications, such as antibacterial agents, self-cleaning surfaces, and water and air conditioning. With the increased environmental pollution, SrTiO3 is one of the most studied perovskite photocatalysts, exhibiting pronounced photocatalytic activity for removing chemical pollutants and water splitting. In the present work, pure Strontium titanate (ST) nanoparticles were successfully prepared using high-energy ball milling and Pechini techniques and characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Fourier Transform Infrared spectroscopy (FTIR), scanning and transmission electron microscopy, respectively. Structural parameters were evaluated by Rietveld refinement analysis from XRD data, which confirmed the cubic system of SrTiO3 with Pm-3 m space group. Scanning electron microscope results showed that ST1 samples consisted of agglomerated and irregular-shaped structures between 20 and 40 nm, and in ST2, the particles were round-shaped and had an average size of 150 nm. The obtained nanoparticles were used for photocatalytic methylene blue (MB) degradation, and synthesis methods' influence on catalytic activity was investigated. The photocatalytic studies examining the decoloration of MB dye reveal the function of smaller particles in increasing the rate of reactions. The degradation rate constant of MB on the ST1 (Pechini-synthesized sample) and ST2 (high energy ball milled sample) is 0.0145 and 0.0112 min−1, respectively. The better photocatalytic activity of the ST1 demonstrated 93% degradation of dye under the solar light simulator. The photocatalytic reaction data provided well a first-order kinetic model.


Introduction
Dye pollutants in industrial wastewater are one of the most critical challenges facing researchers in environmental protection today. Wastewater from textiles, paper, and other industries, including non-degradable dyes, causes severe ecological problems. In addition, non-degradable dye residues entering the ecosystem are a source of aesthetic pollution, eutrophication, and deterioration in an aquatic environment. With the evolution of science and technology, dye pollution control methods are very diverse today. Low energy consumption, uncomplicated operation, ability to completely decompose organic pollutants and other benefits have caused extensive attention and research on photocatalytic technology [1][2][3][4][5][6][7][8][9].
Since the degradation efficiency of photocatalytic degradation technology depends on the performance of photocatalysts, it is crucial to study and develop catalytic materials with high photocatalytic activity. Among the various wide bandgap semiconductors, SrTiO 3 , a well-known perovskite with an indirect bandgap of 3.25 eV and a direct bandgap of 3.75 eV [10], has attracted much attention to its diverse applications in rechargeable batteries, oxygen gas sensors, photocatalysts, and dye-sensitized solar cells photoelectrodes [11][12][13][14][15][16]. SrTiO 3 is thought to have advantages due to its multi-cationic oxidizing activity, its versatility in tailoring its chemical and physical properties, and a more significant number of photocatalytic sites. However, this material is quite limited for applications using sunlight as it can only use less than 3-5% of the UV radiation the solar spectrum provides [3]. Large-scale use of UV radiation is limited as it makes the process costly and environmentally unfriendly. The studies in which metal-dopped or bare SrTiO 3 is activated under UV-VIS light as a photocatalyst have become remarkable in recent years [2,17]. Thus, the present research has focused on preparing photocatalysts that can degrade dyes under sunlight irradiation. Generally, SrTiO 3 could be prepared by ceramic (solid-state, high energy ball milling) and various wet chemical routes such as coprecipitation, sol-gel, Pechini, hydrothermal, solvothermal, sonochemical, and microemulsion [18][19][20][21][22][23][24][25][26]. Among the numerous synthesis methods, Pechini is one of the practical production techniques for preparing nanopowders due to the relatively simple, homogeneously dispersed gel precursor formation and low-temperature process. The synthesis of SrTiO 3 via sol-gel, Pechini, and alkoxide methods was previously reported [18,[27][28][29][30][31][32][33][34][35][36]. In most cases, moisture-sensitive Ti precursors are used that must be protected from rapid hydrolysis. In a limited number of studies, SrTiO 3 nanoparticles have been reported to be synthesized from air-sensitive titanium precursors using triethanolamine as a chelating agent or additive [25,[33][34][35]. This study also points to the first-time synthesis of SrTiO 3 nanoparticles from titanium-triethanolaminato isopropoxide precursor, which can be used as a photocatalyst.
SrTiO 3 (ST1) is prepared via a facile Pechini method in the present study. The starting materials, titanium triethanolaminato isopropoxide complex and strontium nitrate, are air-stable, and the resulting oxide product SrTiO 3 can be efficiently prepared at the nanoscale. It also allows low-temperature working (compared to the conventional solid-state method) and commercially available inexpensive starting materials. As a comparison, SrTiO 3 (ST2) nanoparticles were prepared by a highenergy ball milling process. In addition, SrTiO 3 catalysts were tested in the MB degradation reaction, and the results were evaluated and given comparatively. The influence of different synthesis methods on photocatalytic activity was investigated.

Synthesis of ST1
Strontium and titanium citrate solution was prepared by mixing Sr(NO 3 ) 2 (3.17 g, 15.0 mmol), titanium (triethanolaminato) isopropoxide (4.746 g, 15.0 mmol, 80 wt% in isopropanol) and citric acid (67.5 mmol) in 17 mL of ethylene glycol. The molar ratio can be expressed as strontium/titanium /citric acid/ethylene glycol = 1: 1: 4.5: 20. A clear yellow solution was obtained after the mixture was stirred at 80 °C for 2 h. The solution was heated in a vacuum oven at 120 °C for 24 h. The resulting dry powder was heated in two steps: first, it was treated at 400 °C for 4 h to remove organic residues, then at 800 °C for 4 h at a rate of 10 °C/min, cooled to room temperature. The sample was washed with 5 mL acetic acid solution (10% v/v) to remove the unwanted SrCO 3 phase. Finally, pure SrTiO 3 powder was dried at 135 °C for 2 h. The flow chart of the Pechini method is given in Fig. 1.

Synthesis of ST2
The powders of SrCO 3 (10 g, 67.70 mmol) and TiO 2 (5.40 g, 67.70 mmol) were stirred in 50 mL of 2-propanol for 30 min. The mixture of powders was dried at 80 °C for 2 h in a vacuum oven. The dried powders were milled for 4 h at room temperature using a Retsch PM100 type planetary ball milling system equipped with a tungsten carbide jar (250 mL) and ten balls (77.58 g) with a diameter of 10 mm. The milling speed was set at 300 rpm and was stopped for 5 min for milling every 25 min to cool the system. The powder mass/ ball mass ratio was used as 1:5. The milled powders were heat-treated at 1000 °C for 4 h with a heating rate of 10 °C/min and then slowly cooled at room temperature.

Photocatalytic degradation of methylene blue
The photocatalytic activities of STiO 3 nanopowders were determined by the photodegradation of MB solution under the solar light simulator. Reaction tests were carried out at room temperature and in a reaction vessel. 10 -5 M, 1 L methylene blue solution (3.19 mg/L) was kept in the dark for 30 min to get equilibrium. The photocatalytic reaction was carried out by adding 800 mg of catalyst into the solution and irradiating it with a sun simulator. The solution was mixed continuously throughout the experiment, and every 30 min, 3 mL aliquots were taken from the reaction media and centrifuged for 5 min. Afterward, the concentration of the solutions was measured by a UV-Visible spectrophotometer. The experiment was also carried out without a catalyst.

X-ray diffraction analysis and rietveld refinement
The crystal structure and phase of the synthesized samples are evaluated by XRD. The diffraction patterns of SrTiO 3 powders synthesized by high-energy ball milling and Pechini processes are illustrated in Fig. 2. The XRD results demonstrated well-defined diffraction peaks, and major peaks were indexed (JCPDS No. 79-0176) cubic structure with Pm-3 m (#221) space group in both ST1 and ST2. As shown in  . 71-239). In preparing SrTiO 3 with many methods in the literature, the formation of SrCO 3 impurity is inevitable. SrCO 3 can remain an intermediate step for the final product and a pollutant after product formation. SrCO 3 is challenging to avoid because of CO 2 's carbon contamination from the air. It has been reported that it is removed by washing with the appropriate acid when it occurs as a by-product [29,36,[40][41][42]. In this study, the diluted acetic acid  solution was used to eliminate the SrCO 3 impurity. XRD diffraction pattern of the ST1 sample before and after acetic acid treatment was given in Fig. 3.
Since the single phases of SrTiO 3 were obtained by calcination at 800 °C (Pechini process-after acetic acid treatment) and 1000 °C (high energy ball milling), these samples were selected for Rietveld refinement. The pseudo-Voigt function was used with standard Debye-Scherrer geometry to define the peak shape in the refinement. The refined parameters are scale factor, shift on the 2θ axis, specimen displacement, background, atomic coordinates, profile shape parameters, isotropic displacements, and Caglioti half-width parameters (U, V, and W). The observed, calculated, and difference in XRD patterns of ST1 and ST2 nanoparticles were interpreted in Fig. 4. Rietveld refinement analysis results showed a good correlation between the observed, experimental, and calculated XRD patterns. The refinement and unit cell parameters are compatible with the data obtained from earlier investigations [43,44]. A summary of the refinement and unit cell parameters for ST1 and ST2 is given in Table 1. The average crystallite size was estimated based on FWHM values using

Thermal and FTIR analysis
The thermogravimetric analysis curves of ST1 (dried at 120 °C) and ST2 (as-prepared at 80 °C) powders under a nitrogen atmosphere are shown in Fig. 5. TG curves of samples showed significant differences according to the synthesis method. TG curve of ST1 showed three-step degradation from 205 °C to 1080 °C. The first step is to remove water and light volatiles as isopropanol occurred at temperatures below 205 °C with a weight loss of 12%. The second is the dehydration and destruction of organic fractions (ethylene glycol-citric acid polyester chains) between 205 °C and 563 °C [45,46]. Subsequently, the sample weight was almost constant after 700 °C with degradation of 1% up to 800 °C. ST1 nanoparticles were obtained calcination at 800 °C and after washing and drying. At temperatures of 800 °C and above, there was a 15% decrease in weight, possibly due to SrCO 3 and CO 2 gas evaluation. In the case of ST2 at temperatures lower than 900 °C, the sample did not show any significant change. Above 900 °C, a weight loss of 15% occurred with the decomposition of SrCO 3 into CO and SrO 2 . Finally, between 985 °C and 1200 °C, the residue remains about 80 wt% of the sample. The FTIR spectra of ST1 samples were heated at 120 °C and calcined at 400 °C are presented in Fig. S1. The peaks around 3400 cm −1 and 1600 cm −1 are due to OH groups' O-H stretching and bending vibrations [22,47]. The spectrum of the powder precursor (as prepared at 120 °C) exhibits significant peaks at 2951 cm −1 (C-H anti-symmetric stretching), 2882 cm −1 (C-H symmetric stretching) [48,49] and 1741 cm −1 assigned to -C=O stretching vibrations in the bidentate carboxylate group. By calcinating the ST1 sample at 400 °C,  [50,51]. These peaks disappeared after calcination at 800 °C and above temperatures. The FTIR spectra of ST1 and ST2 nanoparticles are given in Fig. S2. The main peaks at 560 and 563 cm −1 can be assigned to stretching vibrations of Ti-O bonds in TiO vı octahedra. The peaks at around 450 cm −1 were due to the TiO ıı bending vibrations for SrTiO3 [23,45,52].

Scanning and transmission electron microscopy
The ST1 and ST2 nanoparticles' morphology was studied by SEM analysis, as shown in Fig. 6a-c. To further investigate the morphology of ST1 nanoparticles, TEM analysis was performed. Fig. 6b shows the TEM image of the ST1 nanostructure. ST1 samples comprised agglomerated and irregular-shaped structures with sizes between 20 and 40 nm. As seen in the SEM image of ST2, the particles were round-shaped and had an average size of 150 nm. Pechini method satisfied the formation of smaller particles than the high-energy ball milling. EDX analysis was performed to confirm the formation of ST1 and ST2 nanoparticles. Details of EDX spectra of the nanostructures are shown in Fig. S3.

Photocatalytic activity
The photocatalytic activity of the as-prepared ST1 and ST2 nanoparticles was investigated by the degradation of MB under the solar light simulator. The custom-made photoreactor (40 × 40 × 36 cm 3 ) consisting of a glass beaker, a magnetic stirrer, and an OSRAM Ultra Vitalux sun simulator lamp was installed, as shown in Fig. 7. The solution was stirred continuously using a magnetic stirrer speed of 250 rpm, and a fan was fitted for ventilation and displacement of excess heat. The MB degradation reaction was carried out for 180 and 240 min, and the UV-visible absorption spectra taken at different times were given in Fig. 8. The maximum absorbance at 664 nm, the characteristics of MB, decreases with irradiation time. Fig. 9a shows the photocatalytic degradation of the MB by using ST1 and ST2 nanoparticles as a function of time. The photocatalytic activity of ST1 was 93% decomposition at 180 min, while ST2 was 80% under solar light. The dye degradation was 26% at 150 min without catalyst. Time-dependent UV-Vis absorption spectra of MB dye degradation without catalyst were given in Fig. S4. The degradation percentage depends on the synthesis method, and ST1 exhibits the highest degradation efficiency.
According to Lente's technique [53], the non-linear least squares fitting was achieved to the first-order kinetic exponential curve: A is the concentration of MB dye taken from the relative concentration C/C 0 , k is the first-order rate constant (min −1 ), X is the amplitude of the process, and E is the endpoint. Prof. Dr. Gábor Lente at http:// lenteg. ttk. pte. hu/ Kinet Fit. html kindly provides the Excel file to calculate the kinetic parameters. Table 2 lists the first-order rate constants, the standard deviation of parameter k, and the linear correlation coefficient R 2 for ST1 and ST2. According to Fig. 9b, photocatalytic degradation of MB in aqueous ST1 and ST2 suspensions follows first-order kinetics, and photodegradation rate constants (k) values of ST1 and ST2 are calculated as 0.0145 and 0.0112 min −1 , respectively. The better activity of the ST1 is attributed to the smaller crystallite and particle size, which makes it a potential candidate for photocatalytic degradation of pollutant dyes. It is known that the size of nanoparticles has a substantial influence on the photocatalytic properties due to the variation of the surface area and the number of active sites [54,55]. The smaller particle size of nanoparticles would influence a larger surface area to enrich photocatalytic activity. Table 3 illustrates the comparative photocatalytic setup and training of some SrTiO 3 samples reported in the literature. Using cheap and non-air-sensitive starting materials in the Pechini method is advantageous. Although the catalyst obtained by the high-energy ball milling process has low catalytic activity, it is remarkable that its synthesis is industrially simple and feasible. It can be observed that photocatalyst ST1 exhibits a reasonable degradation rate compared to other irradiation studies.

Conclusion
In the present study, nanoscale strontium titanate was successively prepared by two different methods. XRD studies of the ST1 and ST2 have shown that nanoparticles were single-phase and had a cubic structure with an average crystallite size of 26,73 nm and 55,94 nm, respectively. Traces of SrCO 3 impurities are present in the XRD of Pechini synthesized ST1 that was removed by washing with acetic solution, whereas a pure phase without any by-product was obtained in the ball milling process. Pechini's method satisfied the formation of smaller particles than the highenergy ball milling.
This study shows that the photocatalytic activity of ST1 is remarkable depending on the particle size. ST1 indicated more efficient catalytic performance due to smaller crystallite and particle size. The photocatalytic results revealed that the Pechini synthesized strontium titanate nanoparticles showed a pronounced catalytic activity towards the decolorization of MB dye. The results show a relationship between the morphology and catalytic activity of SrTiO 3 . It can be   concluded that the production method is also effective in the catalytic performance of the SrTiO 3 photocatalyst.