Abstract:
Mechanically activated strontium titanate (SrTiO3) powders with various iron (III)oxide (Fe2O3) doping levels (1.5, 3, and 6 wt%) were prepared by solid state method. Due to the possibility of iron ion incorporation in SrTiO3 onto Sr2+ and/or Ti4+ sites a detailed analysis of the influence of dopants on the microstructure, morphology, optical properties and photocatalytic activity was conducted. The XRD analysis showed that iron was incorporated into the lattice of SrTiO3 particles. In the case of a lower concentration of dopant (1.5 wt%), there was a significant relative contribution of the substitution of Sr2+ ions by Fe3+ ions. In contrast, substituting Ti4+ ions with Fe3+ ions dominated samples with an increased concentration. Optical measurements indicated a shiftof the absorption edge to higher values of wavelengths where the lowest value of the band gap (Eg=1.85 eV) was for the longest activation time (120 min) and the highest weight percentages of dopant (6 wt%). All samples have degraded tetracycline (TC) where degradation increases with longer irradiation time and dopant concentration. The highest degradation at 43 % was for 120 min activated sample with 6 wt% of Fe2O3.
Keywords: Mechanical activation; Fe-SrTiO3 system; Ceramics; Antibiotic removal; Visible light photocatalysis.
Сажетак: Механички активирани npaxoeu cmponyujym mumanama (SrTiO;) ca различитим тежинским процентима 2eodiche триоксида (Fe:03) (1,5, 3 и 6 mexc.%) припремелени су методом чврстог стаъа. 3602 могуйе инкорпорациуе она гвожйа не местима Sr" шили Ti4+ jona y SrTiO; структури, извршена je детальна анализа ymuyaja donanma на микроструктуру, mopeonoeujy, оптичка ceojemea и фотокаталитичку активност. Рендгено дифракциона анализа (XRD) je показала да ce гвожйе уградило y решетку SrTiO; честица. Y случа)у ниже konyenmpayuje donanma (1,5 mexc.%), долази до знача/ног релативног доприноса cyncmumyyuje Sr" jona са Fe·· jonuma. Насупрот томе, замена Ti4+ jona ca Fe3+ jonuma je доминирала y узорцима ca вишим konyenmpayujama допанта. Оптичка мереъа су показала da ce ueuya ancopnyuje помера ка вишим вредностима таласних дужина, при чему je dobujena нанижа вредност ширине забраюене зоне (Ее= 1.85 eV) била за узорак активиран 120 мин и ca najeehum тежинским процентом допанта (6 mexc.%). Ceu узорци су показали способност dezpadayuje тетрациклина (TC) roja ce повейавала ca дужином времена зрачеюъа и KonyeHmpayujom допанта. На/виша вредност oeepadayuje je била 43 % за узорак 120 мин активиран и ca 6 mexc.% РезОз.
Къучне речи: Механичка axmusayuja, Fe-SrTiO; систем, керамике, уклаъаюе антибиотика, фотокатализа видльивим светлом.
1. Introduction
In recent years, removing organic pollutants from the environment has become the number one topic. The use of antibiotics has become increasingly widespread for therapeutic purposes in medicine and veterinary. However, antibiotics are more difficult to decompose, so they harm the environment and humans, the consequence of which could be the emergence of increasingly resistant bacterial strains. Research has shown that antibiotics used in veterinary medicine have poor absorption in animal tissue, 40-90 % are excreted as metabolites through urine and feces [1, 2]. Antibiotics can enter the aquatic environment which affects aquatic microorganisms, raising further concerns [3]. Li et al. [4] reported that concentrations in the aquatic environment are at levels of ng/l, µg/l, ng/g, and µg/g. Tetracycline (TC) is one of the most commonly used antibiotics with bad contraindications when it is present in higher concentrations in the human body, such as nephropathy, photosensitivity, arthropathy, etc. [5, 6]. The presence of TC in surface and ground water has been reported by Liu et al. [7], due to the release of more than 70 % of TC antibiotics in active form into the environment [8]. Taking everything into account, it is a priority to further develop more effective methods for removing antibiotics from the human environment, which should be sufficiently effective, environmentally friendly, and affordable. So far, many methods have been used for removing antibiotics from aquatic environments such as biodegradation, electrochemical methods and photocatalysis [9-11]. The titanium dioxide (TiO2) photocatalytic process is one of the most used technologies because of its advantages such as its ability to operate in ambient conditions, where this mechanism includes the activation of semiconductor (TiO2) via sunlight or artificial light [12]. Most technologies adopt tandem approaches which increase the operation's complexity and the price of equipment [13]. To save costs and to improve degradation efficiency it is necessary to make a system for catalytic degradation that can simultaneously utilize microwave (MW) and light energy [14]. One excellent photocatalyst is n-type semiconductor strontium titanate (SrTiO3) (Eg=3.40 eV) with a perovskite structure [15-17]. SrTiO3 can be combined with TiO2 to create a heterojunction-type photocatalyst, it has good properties such as easy defect modulation and physicochemical stability [18]. Regarding this Cao et al. [19] synthesized SrTiO3-TiO2 heterostructure nanofibers from TiO2 nanofibers using an in situ hydrothermal process. Also, a SrTiO3-TiO2 non-homogeneous structure was constructed to enhance solar water decomposition but it was found that the catalytic activity of this system was relatively low under photocatalytic conditions [20, 21].
A frequent topic of investigation to extend the photoactivity to visible light regions is an examination of the influence of doping with various transition metals into semiconductors with wide band gaps such as SrTiO3 and TiO2 [22, 23]. Doping can be used to modify the crystal structure of a system, adjusting its properties and band gap values to increase photocatalytic efficiency. It was reported that doping with Ru3+, Fe3+, Cr3+, Mn3+ and Pb2+ into TiO2 and SrTiO3 leads to the creation of localized levels into a forbidden band which generates an optical absorption shiftto visible light [24]. Metal-doped SrTiO3 has been very interesting for research in the area of photocatalytic water splitting [25] in the visible light region as well as in photocatalytic degradation of organic pollutants but it is just a small number of reports. This is a consequence that only in a few cases by metal doping localized levels can be formed in the forbidden band mix or overlap sufficiently with the valence band top formed by O2p or the conduction band formed by Ti3d in TiO2 and SrTiO3 to continuously transfer photoexcited carriers to reactive sites [22, 26]. The visible-light-induced photocatalytic performances of Ag+- and Pb2+-doped SrTiO3 have pointed this out [22]. Srivastava et al. [27] revealed that after doping with La the photocatalytic activity of SrTiO3 was increased 1.38 times that of pure SrTiO3. At the same time, transition metals such as Fe have aroused interest for examination due to their electron properties [28], where energy levels of Fe3+ are close to that of Ti4+ [29] and Fe can act as an inhibitor of electron-hole pair recombination [30]. TiO2 doped with V, Cr, Mn, Fe, Ni, and other transition metals via high energy ion implantation [31], flame spray pyrolysis technique [32], and sol-gel method [33] can generate effective photocatalytic reactivity for the decomposition of organic compounds and nitrogen monoxide under visible light irradiation. The fabrication process of a system such as SrTiO3 as a photocatalyst still needs to be investigated to optimize its photocatalytic performance.
In the present work, a typical perovskite oxide SrTiO3 was chosen as a host structure and doped with Fe cation by using a solid state reaction in the presence of mechanical activation. The synthesized Fe-doped SrTiO3 was demonstrated to have higher photocatalytic activity than pure SrTiO3 for the degradation of TC which was selected as a model of water pollutant. A systematic investigation of the influence of dopant concentration and time of mechanical activation on the microstructure, optical properties and photocatalytic activity of Fe-doped SrTiO3 for the degradation of TC was done.
2. Materials and Experimental Procedures
As the starting materials, high-purity commercial SrTiO3 (99 % purity, mean particle size ≤ 5 µm) and Fe2O3 (Sigma-Aldrich, purity ≥99 %) powders were used. The mechanical activation was conducted by ball-milling of 20 g of the powder mixture in a 45 cm3 tungsten carbide jar, with 5 mm diameter tungsten carbide balls (the powder mixture-to-ball mass ratio was 1:20), in a planetary micro mill (PULVERISETTE 7 premium line, FRITSCH, Germany). The powders were mechanically activated in the air for 10, 30 and 120 min. Unmilled powders denoted as 0 min mechanical activation were included for comparison. Mechanically activated samples non-activated and activated were labeled as STOFeI0, STOFeII0, STOFeIII0, STOFeI10, STOFeII10, STOFeIII10, STOFeI30, STOFeII30, STOFeIII30, STOFeI120, STOFeII120, STOFeIII120, according to the corresponding weight percentages of dopant (I, II and III for 1.5, 3 and 6 wt%, respectively and 10, 30 and 120 to indicate the activation time in min).
The morphologies of non-activated and mechanically activated Fe-doped SrTiO3 powders were analyzed by a scanning electron microscope (SEM; RaitheLine Plus, Germany).
A laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., UK) (PSA) was used for the determination of particle size distribution. Samples were mixed with distilled water, and an aqueous suspension was obtained after 10 min ultrasonic disaggregation.
The powders were examined using X-ray diffraction (XRD; Philips XË pert X-ray diffractometer), which was performed using Ni-filtered Cu-Kα radiation at room temperature with 2θ angles ranging from 10 to 90o.
UV-Vis diffuse reflectance spectroscopy was performed using Fiber Optic Reflectance Sphere (1.5" Dia, Edmund optics) coupled with UV/VIS/NIR source (30W Deuterium/5W Tungsten Lamp, Edmund optics) and TE-cooled CCD fluorescence spectrometer (Glacier X, BWTEK, Plainsboro, NJ, USA). LabsphereSpectralon standard was used as a reference. The band gap energy of powders was determined from diffuse reflectance spectra using the Kubelka-Munk transformation.
The photocatalytic degradation of tetracycline solution (2 mg L-1) was performed using an irradiation source of G2V Pico Solar Simulator with AM1.5G spectral match. The tetracycline degradation tests were conducted under vigorous magnetic stirring, using 50 mg L-1 of the Fe-SrTiO3 samples. Comparative photocatalytic tests were completed using 50 mg L-1 of pure SrTiO3 nanoparticles. After ensuring the adsorption-desorption equilibrium in darkness (30 min) the light was turned on, and 3 mL aliquots of supernatant were acquired at designed time intervals. The supernatant was filtered using an LLG-Syringe filter (0.45 µm), centrifuged at 5000 rpm for 5 min, and absorption spectra were recorded using a UV spectrophotometer (Maya 2000 pro, Ocean Optics, wavelength range 230-400 nm). The absorption band situated at 357 nm was used to determine the percentage of tetracycline photodegradation over a period of time (up to 2h) by comparing the absorption of the filtered supernatant with that of the initial tetracycline solution.
3. Results and Discussion
To analyze the morphology of Fe-SrTiO3 samples, doped with different weight percentages of Fe2O3 precursors and for different times of mechanical activation, SEM images were obtained. As shown in all pictures of Fig. 1 the morphologies of samples consist of particles with polygonal shapes where larger particles of SrTiO3 are dominant with the presence of smaller ones originating from Fe2O3. From the SEM images, it can be seen that the SrTiO3 and Fe2O3 particles were well-alloyed so contact was likely to form with an increase in the time of activation. In the non-activated and 10 min activated Fe-doped SrTiO3 powders the presence of segregation was noticed (Fig. 1 a-f). Mechanical activation during 30 min (Fig. 3 g-i) leads to particle comminution when part of the segregation was crushed, however, Fe2O3 particles remain present. Particles showed a tendency to agglomerate due to increased surface activity which is a consequence of mechanical activation (Fig. g-i). Further activation (120 min) leads to a more homogeneous morphology of powders with the presence of comminution of the starting powder particles and agglomerate (Fig. 1 j-l).
The particle size distribution of Fe-doped non-activated and activated SrTiO3 powders is shown in Fig. 2. Curves have a bimodal particle size distribution for all three dopant concentrations and all times of mechanical activation. The bimodal distribution of particles is a consequence of the presence of a group of very finely divided, and on the other hand, much larger particles in the samples. Larger particles in non-activated powders originate from SrTiO3 particles, while for longer activation times they may be the result of the presence of agglomerates, as indicated by SEM analysis (Fig. 1). Namely, mechanical activation induced higher energy in the system so it is accumulated on the surface of powder particles and as a result, agglomeration can be observed [34]. It is very difficult to establish the trend in particle size distribution due to the Fe doping, because of the irregular shape of particles and the wide range of distribution particle size of powders as shown by SEM analysis (Fig. 1).
Generally, it is noticed that the displacement of the maximum of the distribution curve towards smaller particle sizes is most pronounced for samples with 6 wt% Fe2O3 for 30 and 120 min activation except for sample activated 10 min (Fig. 2). The average particle size is in the range of 8.52 µm
XRD diffractograms of non-activated and activated doped Fe-SrTiO3 powders correspond to the cubic perovskite structure of SrTiO3, with the presence of Fe2O3 in the nonactivated strontium titanate sample with 6 wt% of dopant (Fig. 3). These structures are matched well with the standard form the crystallographic database JCPDS No. 35-0754 for SrTiO3-cubic and No.86-0550 for Fe2O3 [35, 36].
Comparison of (110) diffraction peaks in the range (2θ=31-34o) of samples activated 30 and 120 min, for 1.5, 3 and 6 wt% of dopant, reveals that the diffraction peak position for STOFeII30 and STOFeII120 samples are moved towards higher values of the angles (Fig. 4a and b). This could indicate the substitution of Sr cations for Fe knowing that the ionic radius of Fe3+ (0.064 nm) is much smaller than that of Sr2+ (0.144 nm) [37], which further leads to structure distortions and results in shrinkage of the unit cell [38]. On the other hand, for samples STOFeIII30 and STOFeIII120 (110) diffraction peaks are directed toward lower angle values, which indicate the incorporation of Fe ions onto Ti sites, where Fe3+ has a higher value of ionic radii compared to radii of Ti4+ ion (0.060 nm) [39]. In the case of this substitution (Fe3+→Ti4+), there is an increase in the unit cell (Fig. 4a and b). It can be said that increasing Fe contents in SrTiO3 results in changes in the size of the unite cell due to the incorporation of dopant on A and/or B positions in the structure as expected from the ionic size consideration. Since the structure of cubic perovskite-type SrTiO3 stabilized by 6-fold coordination of Ti cations (octahedron) and 12-fold coordination of Sr cations, it is expected that Fe cations occupy predominantly octahedrally coordinated positions, such as Ti4+ in Fe- SrTiO3 [24], and this was observed in the case of higher dopant concentrations (6 wt% Fe2O3) (Fig. 4a and b). A comparison of the (110) diffraction peaks for all three dopant concentrations of samples 30 and 120 min activated indicates changes in the relative peak intensity associated witha structural disorder in the doped lattice [40].
A UV-Vis study was carried out to identify the effect of the influence of Fe dopant and mechanical activation on strontium-titanate optical properties. UV-Vis spectroscopy is used to determine the band gap (Eg) that indicates the energy difference between the valence zone (filled with electrons) and the conduction zone (without electrons), where the band gap is a quantity that is related to the electrical conductivity of the material. In SrTiO3, the higher valence zones have 18 electrons in dominant O (2p) states, hybridized with Ti and Sr states [41], while the conduction zones are built from Ti (3d) t2g and eg states at lower energies. At higher energies of the conduction zone, Sr (4d) t2g and eg states dominate, and at the highest energy values (higher than 15 eV) free electrons dominate [42].
The diffuse reflectance spectra of the obtained mechanically activated samples with different concentrations of dopant are shown in Fig. 5a. The absorption edge of non-activated doped powder (STOFeI0) is at a wavelength of ~541 nm (Fig. 5a), but it shifts (red shift) to a higher value of wavelength with increasing activation time and dopant concentration, so the sample STOFeIII120 has the highest value of absorption edge at ~561 nm. With an increase in the time of mechanical activation and the amount of dopant in the system the reflection curves additionally become shorter for the longest activation time (120 min) it is the shortest (Fig. 5a). This indirectly indicates that samples activated 120 min has the highest absorbance values, in visible light region 400-700 nm [43], as a consequence of the increase in Fe ion concentration in the system and activation time. In the work of Wu et al. [44] it was reported that the implementation of Fe3+ into TiO2 leads to the generation of an impurity energy level between the conduction and valence band of TiO2, where under visible light illumination, Fe3+ plays a role in extending light absorption into the visible region.
The measured spectra R=f(λ) were transformed according to the Shuster-Kubelka- Munk theory, all for quantitative assessment [45]. The ratio of adsorption coefficient, K and scattering, S can be called 'absorbance' or Kubelka-Munk function, F(R∞) (Eq. 1):
(ProQuest: ... denotes formula omited.) (1)
where R∞ is the reflectance obtained through the ratio of the total intensity of reflected light from the sample and the standard [45]. The graphs presented in Fig. 5 b-d shows the modified form of the Kubelka-Munk function as a function of the photon energy (hv) of the initial radiation for doped non-activated, 30 and 120 min activated powders. The values of energies of the band gaps were determined using the Tauc plot method [46], where the average bang gap was estimated from the intercept of the linear portion of the [F(Rα)(hv)]2/3 vs. hv plots on the hv axis as shown in Fig. 5 (b-d) for directly forbidden transition. Comes et al. [47], by using the Tauc formula for a dipole forbidden direct gap of Fe-3d → Ti-3d transition, determined a band gap value for Fe-doped SrTiO3.
The determined values of band gaps decreased with the increasing activation time and concentration of dopant (Fig. 6), a deviation was observed in the form of an increase in Eg value for the sample STOFeI10. Pure SrTiO3 is transparent in the visible light region but when it is doped with Fe a certain amount of visible absorption occurs. Fe3+enteres the lattice onto Ti4+ sites (forming linkage Ti-O-Fe in two neighboring octahedrons) [24] and brings with it oxygen-ion vacancies to preserve lattice neutrality but at the same time iron-vacancy complex introduces an additional set of energy levels [48]. A b initio calculations of vacancy defects in SrTiO3 show a decrease in Eg with an increase in oxygen vacancy concentration to 8.3 % [49]. It can be said that the reduction of the value of Eg is also influenced by the increase in the concentration of oxygen vacancies, which area consequence of the application of mechanical activation in addition to their formation due to Fe doping [34, 50]. Combining doping and mechanical activation leads to a lowering of the Eg value (Fig. 6). The difference in band gap energy values between STOFeI120 and STOFeIII120 samples was 0.07 eV (that difference between: STOFeI0 and STOFeIII0 samples is 0.01, STOFeI10 and STOFeIII10 samples is 0.03, STOFeI30 and STOFeIII30 samples is 0.05), which indicates a direct influence of the Fe-dopant. In the case of a sample with different activation times but the same amounts of dopant, these difference in Eg value was also observed, indirectly suggesting the simultaneous influence and mechanical activation time (that difference for samples: STOFeI0 and STOFeI120 are 0.05, STOFeII0 and STOFeII120 are 0.07, STOFeIII0 and STOFeIII120 are 0.09 (Fig. 6)). The lowest value of band gap (Eg=1.85 eV) was registered with the longest time for 120 min and the highest concentration of dopant (STOFeIII120), while this value for undoped powder activated for 120 min was 3.19 eV [34]. The appearance of a mutual deviation of Eg values due to the presence of a dopant in the powder could be a consequence of a disturbance in the periodicity of Ti4+ ions, as the XRD analysis showed (Fig. 4b) [51]. Iron doping onto titanium ion sites reduces the value of the band gap, due to introducing some impurity states between the conduction and valence band of TiO2, narrowing an Eg of about ~1.1 eV [24]. From the impurity energy level of Fe, the energy state of 3d electrons could be excited to the conduction band of TiO2 so it appears absorption in the visible light region [24]. SrTiO3 has a similar electronic band structure as TiO2, so the presence of the isolated energy levels created by Fe3+and Fe2+dopants in the forbidden band of SrTiO3 was observed [52]. The occurrence of absorption would result from the electron excitation from two isolated energy levels to the conduction band of SrTiO3, representing a reductant-to-band charge transfer (RBCT) [24]. On the other hand, Lin et al. show metal-tometal (TiIV-O-CuI→TiIII-O-CuII) charge transfer as a class of visible light-absorbing chromophores [53], so the eventual absorption that would appear in the Fe-SrTiO3 system would be a consequence visible light excitation of TiIV-O-FeII→TiIII-O-FeIII beside RBCT [24]. In Fe-SrTiO3 films, the influence of Fe ions on the valence band is attributed to the presence of Fe3+ and Fe2+ ions, which contribute in different ways to the valence band, especially within the in-gap region [54]. Neither state is observed at the Fermi level, but Fe2+ ions form a rather broad feature located in the binding energy range below 0.5 eV, while Fe3+ ions form a sharp peak just above the valence band peak [54]. Resonance photoemission indicated the contribution of the Ti state in the gap, which increases with the amount of Fe dopant, and this effect was attributed to the hybridization between the electronic states of Ti and Fe in the valence band, as well as Ti ions associated with Fe ions forming complexes with oxygen vacancies [54].
The photocatalytic efficiency of Fe-doped SrTiO3 system was evaluated based on tetracycline degradation during the irradiation period of 2h. Fig. 7 shows the UV-Vis absorption spectra of tetracycline degradation in the presence of different weight percentages of Fe2O3 dopant in SrTiO3 for 120 min activation. The pure tetracycline exhibits two strong absorption bands at around 276 and 358 nm, in Fig. 7 the second peak is presented due to the better visibility of the effect of the Fe-SrTiO3 system as photocatalyst. The absorption of pure TC at 358 nm originated from aromatic rings B-D, including the developed chromophores [55], while the absorption centered at 276 nm is attributed to the ring A and dimethylamino group [56]. As can be noted from Fig. 7 the addition of STOFeI120, STOFeII120 or STOFeIII120 samples into tetracycline solution under darkness induced a red shiftfrom 358 nm to: 363 nm (STOFeI120), 365 nm (STOFeII120) and 369 nm (STOFeIII120). The presence of irradiation induced a blue shiftof the band centered at 358 nm for all three samples, where the most dominant shiftwas observed for the STOFeIII120 sample (Fig. 7c).
Fig. 8 shows the change in the tetracycline and concentrations of dopant during the testing period of 2h. As demonstrated by Fig. 8 photolysis showed tetracycline degradation where with irradiation the photocatalytic degradation of tetracycline reached about 36, 39, and 43 % for STOFeI120, STOFeII120 and STOFeIII120 samples, respectively after 2h. The sample STOFeIII120 shows the highest degree of degradation of tetracycline (Fig. 8), also in the middle region from 0 min to 100 min it can see a more intense trend of photocatalytic activity of all samples in which the highest degree of TC degradation is shown the sample STOFeIII120, STOFeII120 and STOFeI120, respectively. This implies that metal-doped photocatalysts had more notable catalytic efficacy than undoped materials [57]. Namely, previous research has shown that doping with metallic elements in SrTiO3/TiO2 causes lattice distortion which results in the formation of low-potential traps for photogenerated electron and wholes which increase the active sites of the material, reduced Eg, broadens spectra response interval and increase the degradation efficiency of TC [58]. This is consistent with our observed decrease in the band gap values (Fig. 6). Scaffeta et al. [59] showed a greater absorption in the Fe3+-doped film attributed to a combination of defects-induced states or excitations into localized unfilled Fe 3d states below the bottom of the Ti 3d conduction band and pointed out that such absorption could have a great impact on photocatalytic activity. The lower photocatalytic activity of the STOFeI120 and STOFeII120 samples (Fig. 8) may be because catalysts may prove insufficient photocatalytic reaction sites to treat tetracycline effectively [60] as well as the presence of lower dopant concentrations. Also, the accumulation of dopant products could interfere with the active substances within the system, resulting in a decreased efficiency of tetracycline degradation, since Fe2O3 is known to be insoluble in water. This is consistent with the PSA analysis, which indicated the presence of even larger particles, which were assumed to originate from leftFe2O3 (Fig. 2d).
4. Conclusion
A series of Fe-doped SrTiO3 perovskites with varying ratios of Fe2O3 and distinct structures and morphologies were synthesized by solid-state reaction in the presence of mechanical activation. The SEM image of the composite powders showed that SrTiO3 and Fe2O3 particles contacted well with increasing activation time and dopant concentration. Curves have a bimodal particle size distribution for all three dopant concentrations and all activation times as shown by PSA analysis because of the irregular shape of particles and the wide range of particle size distribution of powders. XRD analysis revealed the formation of cubic Fe-SrTiO3 without impurity Fe phase except for non-activated samples. Increasing Fe2O3 contents in SrTiO3 results in changes in the size of the unite cell due to the incorporation of Fe dopant on Sr2+ and/or Ti4+ positions in the structure as expected from the ionic size consideration. Samples activated for 30 and 120 mins showed a shiftof 110 peaks, which indicated that for the present dopant concentration of 3 wt% Fe2O3, Fe is dominantly incorporated onto Sr sites, while for a higher concentration of 6 wt% Fe2O3, Fe ions are dominantly incorporated onto Ti site. Combining doping and mechanical activation leads to a lowering of the Eg value. The appearance of a mutual deviation of Eg values due to the presence of a dopant in the powder is a consequence of a disturbance in the periodicity of Ti4+ ions, as the XRD analysis showed. When Fe ions occupy Ti sites, at the same time, bonds with oxygen vacancies are formed to maintain the electroneutrality of the system. The decrease in the value of Eg is also influenced by the increase in the concentration of oxygen vacancies, which are a consequence of the application of mechanical activation in addition to their formation due to Fe doping. The lowest value of band gap (Eg=1.85 eV) was registered with the longest time for 120 min and the highest concentration of Fe2O3 dopant (STOFeIII120). The results further exhibited that the STOFeIII120 sample had the highest degradation of TC at 43 %, indicating the best photocatalytic activity compared to the other two. All three Fe-doped samples revealed better photocatalytic activity than undoped SrTiO3, proving that Fe doping could improve the photocatalytic activity of SrTiO3.
Based on everything presented, it can be said that by doping SrTiO3 with a certain amount of dopant, such as iron, and applying mechanical activation, the evolution of the morphology and structure can be controlled, which can significantly affect changes in optical properties and photocatalytic activity. It was found that under optimal conditions (certain dopant concentration and time of mechanical activation) it is possible to obtain a Fe-doped SrTiO3 as a material with good photocatalytic efficiency in the area of TC degradation, which enables important progress in the field of photocatalytic application.
Acknowledgments
Funds for the realization of this work are provided by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia, according to the agreements related to the realization and financing of scientific research work at the Institute of Technical Sciences of SASA, Serbia (Contract number: 451-03-66/2024-03/200175). The author would like to acknowledge Dr. Smilja Marković from the Institute of Technical Sciences of SASA, Serbia for her assistance with PSA measurements and Prof. Dr. Jelena Rogan from Faculty of Technology and Metallurgy, University of Belgrade due to enable working on a device for mechanical activation.
ORCID numbers:
Dr. Jelena Živojinović, https://orcid.org/0000-0002-2977-7134
Dr. Adriana Peleš Tadić, https://orcid.org/0000-0002-4970-5306
Dr. Darko Kosanović, https://orcid.org/0000-0002-0819-8539
Dr. Ivana Dinić, https://orcid.org/0000-0002-0909-8230
Dr. Marina Vuković, https://orcid.org/0000-0001-5824-3484
Dr. Nina Obradović, https://orcid.org/0000-0002-7993-293X
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*) Corresponding author: [email protected] (Dr. Jelena Živojinović)
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Abstract
Mechanically activated strontium titanate (SrTiO3) powders with various iron (IDoxide (Fe>03) doping levels (1.5, 3, and 6 wt%) were prepared by solid state method. Due to the possibility of iron ion incorporation in SrTiO; onto Sr" and/or Ti" sites a detailed analysis of the influence of dopants on the microstructure, morphology, optical properties and photocatalytic activity was conducted. The XRD analysis showed that iron was incorporated into the lattice of SrTiO; particles. In the case of a lower concentration of dopant (1.5 wt%), there was a significant relative contribution of the substitution of Sr" ions by Fe" ions. In contrast, substituting Ti" ions with Fe· ions dominated samples with an increased concentration. Optical measurements indicated a shift of the absorption edge to higher values of wavelengths where the lowest value of the band gap (Eg=1.85 eV) was for the longest activation time (120 min) and the highest weight percentages of dopant (6 wt%). All samples have degraded tetracycline (TC) where degradation increases with longer irradiation time and dopant concentration. The highest degradation at 43 % was for 120 min activated sample with 6 wt% of Fe203.