Visible Light-driven Effective Photocatalytic Degradation of the Persistent Organic Pollutant Using Cobalt-doped Strontium Titanate

Article information

Korean J. Met. Mater.. 2024;62(10):803-819

Publication date (electronic) : 2024 October 5

doi : https://doi.org/10.3365/KJMM.2024.62.10.803

Shanmugam Mahalingam ¹, Abinaya Srinivasan ², Senthil Bakthavatchalam ², Chandramohan Chandramohan ³, Karnan Ramachandiran ⁴, Sathish Kumar Paneerselvam ⁵, Junghwan Kim ¹^,

¹Department of Materials System Engineering, Pukyong National University, Busan 48513, Republic of Korea

²Department of Chemistry, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Ramapuram, Chennai 600089, Tamil Nadu, India

³Department of Community Health Sciences, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabia

⁴PG and Research Department of Zoology, Raja Serfoji Government College (Autonomous), Thanjavur 613 005, Tamil Nadu, India

⁵Department of Chemistry, SAS, VIT, India

- Shanmugam Mahalingam: Research professor, Abinaya Srinivasan: Research Scholar, Senthil Bakthavatchalam: Assistant Professor, Chandramohan Govindasamy: Professor, Ramachandiran Karnan: Research Scholar, Sathish Kumar Paneerselvam: Assistant Professor, Junghwan Kim: Associate professor

^*Corresponding Author: Junghwan Kim Tel: +82-51-629-6372, E-mail: junghwan.kim@pknu.ac.kr

Received 2024 August 7; Accepted 2024 August 27.

Abstract

Residual antibiotics in natural aquatic environments pose a critical threat to humans and other organisms. However, most sewage treatment plants fail to remove them. Photocatalytic nanomaterials can efficiently destroy these persistent organic pollutants in wastewater. In this study, we developed a series of cobalt-doped SrTiO₃ (Co-STO) catalysts with different doping amounts (3, 5, 7, and 9 wt%) for the effective photocatalytic degradation of ciprofloxacin (CIP). The nanostructures were characterized using X-ray diffraction, field-emission scanning electron microscopy with energy dispersive X-ray analysis, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-visible diffuse reflectance spectroscopy, and Brunauer–Emmett–Teller N₂ adsorption isotherms. The Co-STO particles have a mesoporous diameter of ~30.8 nm, and the Co-doped nanostructures have a rhombohedral hopper-like shape. Co-doping decreased the bandgap of pure STO from 3.61 to 3.42 eV, which enabled it to absorb visible light. Among the catalysts, 7 wt% Co-STO showed the highest CIP degradation activity (90.6%) during 120 min of visible-light irradiation. Radical scavenging experiments revealed that superoxide (O₂^•-) is the primary reactive species during degradation. These Co-doped nanostructures have potential applications in the remediation of hazardous pollutants in pharmaceutical wastewater. Moreover, the crystal and energy band structure, density of states, and Bader charge of these molecules were analyzed.

Keywords: Perovskite; drug degradation; anthropogenic waste degradation; photocatalysis; visible light irradiation

1. INTRODUCTION

Unused antibiotics have emerged as trace refractory contaminants in various natural aquatic environments. These pollutants can lead to the development of bacterial resistance to antibiotics, hormonal disruptions, and the formation of cancerous tumors. Traditional water treatment methods, such as coagulation or filtration, cannot effectively remove antibiotics from wastewater [1]. Consequently, pharmaceutical industrial waste poses a significant threat to water resources and endangers both ecosystems and human health. This is especially true for persistent organic pollutants (POPs), which have emerged as a pressing environmental issue. Antibiotics are prevalent in sewers and surface water in ng/L to μg/L concentrations. Typically, the toxicity of POPs depends on their oxidation state and solubility, which can facilitate their rapid infiltration into surface water and groundwater [2]. Most POPs, in addition to high toxicity, mutagenic properties, and carcinogenic risks, exhibit low biodegradability, and this makes their removal from aquatic ecosystems particularly challenging [3,4].

For example, ciprofloxacin (CIP), a popular second-generation fluoroquinolone, is widely used in both human and veterinary medicine because of its extensive antibacterial activity and favorable oral absorption characteristics [5]. It is one of the most frequently prescribed antibiotics and, thus, is detectable at varying concentrations in wastewater, hospitals, and the pharmaceutical sector [6]. When an antibiotic is ingested by humans or animals, a portion of it is metabolized, while the remaining compounds are excreted into the environment. The residue persists in its pharmacological form, mainly because of its high hydrophilicity and bioaccumulation, and is detectable across various environmental matrices. However, most sewage treatment plants fail to efficiently remove antibiotics, such as CIP, which necessitates alternative removal strategies [7,8].

Photocatalytic methods, specifically heterogeneous photocatalysis, are promising approaches for the removal of pharmaceutical contaminants. Advanced oxidation processes (AOPs) using a photocatalyst can degrade POPs within the reaction medium under light exposure [9]. Researchers are exploring nanomaterials, [10] especially perovskite nanostructures, such as SrTiO₃, BaTiO₃, and CaTiO₃, for advanced photocatalysis because of their stability, low toxicity, and cost-effectiveness [11-14]. SrTiO₃ (STO) is a promising material for pollutant degradation; however, its wide bandgap limits its effectiveness under UV light [15]. To overcome this, researchers have doped STO with transition metals, such as Mn, and successfully reduced the bandgap and enhanced photocatalytic activity [16]. Ce/N co-doping further decreased the bandgap of STO and improved its photocatalytic efficiency [17]. These findings highlight the potential of tailored modifications for optimizing the photocatalysis of perovskite materials.

Co-doping STO with Bi³⁺/Fe³⁺ ions enhanced photocatalytic hydrogen production under both UV and visible light [18]. Doping STO with elements such as Na, K, Ca, Bi, Sb, Pb, and Ag and atoms such as Ru, Rh, Ir, N, and Ag improves its visible-light photocatalytic activity [19-24]. Doping STO with rare earth elements (e.g., La, Ce, Pr, and Nd) also enhances its visible-light photocatalytic activity [25-26]. Similarly, doping oxides such as ZnO, TiO₂, and LaMnO₃ with Co ions increased their photocatalytic activity, the onset potential for oxygen reduction, and current density, to values higher than those of oxides doped with other transition metals [27-29]. Recently, Mishra et al. discovered that 3% Co doping significantly enhanced charge transfer and the recombination capability of STO during water splitting [30-32].

Consequently, we aimed to investigate Co doping at the Ti sites of STO to improve its optical properties and photocatalytic activity.

In this study, we investigated the photocatalytic degradation activities of cobalt-doped strontium titanate [SrTi_(1−x) Co_xO₃, where x = 0, 0.1, 0.3, 0.5, 0.7, and 0.9] in degrading CIP under visible-light irradiation under ambient conditions. This onepot synthetic approach is cost-efficient, sustainable, and simple to operate. The reusability of the most active catalyst was assessed over five degradation cycles. Then, scavenger trapping experiments were performed to identify the reactive species responsible for the photocatalytic processes. Additionally, density functional theory (DFT) was used to simulate the energy band structure and density of states for 7% Co-STO.

2. MATERIALS AND METHODS

2.1. Materials

Cobalt nitrate hexahydrate (Co (NO₃)₃.6H₂O), strontium nitrate (Sr (NO₃)₂.6H₂O), titanium isopropoxide (TTIP) (Ti (OCH(CH₃)₂)₄), sodium hydroxide (NaOH), isopropyl alcohol (C₃H₈O), benzoquinone (C₆H₄O₂), ethylenediaminetetraacetic acid (EDTA; C₁₀H₁₆N₂O₈), ethanol (C₂H₅OH), and double distilled water (D2 water) were purchased from Merck. Ciprofloxacin (C₁₇H₁₈FN₃O₃) was purchased from Cipla India.

2.2. Preparation of STO and Co-doped STO

Typically, 1 mL of TTIP was slowly added to 25 mL of ethanol while stirring with a magnetic stirrer at room temperature. An aqueous solution (25 mL) containing 5 mmol Sr (NO₃)₂.6H₂O was added to the ethanolic TTIP solution. Co (NO₃)₃.6H₂O was then added, and the mixture was stirred well for 2 h to obtain a homogenous suspension. The pH was increased to 13 by adding 0.1 M NaOH. The suspension was then transferred to a Teflon-lined stainlesssteel autoclave for hydrothermal treatment. The solution was mixed and placed in an autoclave at 150 °C for 24 h. The resultant solution was filtered, and the precipitate was rinsed with distilled water and ethanol and dried at 80 °C for 6 h to obtain Co-STO. The same procedure was followed for the synthesis of 3, 5, 7, and 9 wt% Co-STO. Figure S1 shows a schematic of the synthesis of the Co-STO nanostructures.

2.3. Characterization

Powder X-ray diffraction (PXRD) was employed with Cu Kα radiation at an accelerating voltage of 30 kV (PANalytical/XPert3-Powder) to characterize the crystal structures of both pure STO and Co-doped STO. Field-emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) was performed using a MIRA3 TESCAN instrument (TESCAN KOREA, Seoul, South Korea) to obtain EDX spectra and elemental maps. The morphology of 7% Co-STO was examined via high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) using a JEM-F200 instrument (JEOL Ltd., Tokyo, Japan) at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) with a monochromatic Al Kα source at a spot size of 400 μm and a pass energy of 40 eV using an AXIS SUPRA instrument (KRATOS Analytical Ltd., Stretford, UK). The surface areas and pore sizes were analyzed using a Quantachrome/Autosorb-iQ analyzer. A Perkin Elmer LAMBDA 950 UV-visible spectrophotometer was used to validate the optical properties of the synthesized samples.

2.4. Photocatalytic assessment

A Xe lamp (λ = 420 nm, 86 W) was used as the visible light source. When a 20 ppm CIP solution was added to a glass beaker containing Co-STO, a desorption/adsorption equilibrium was formed. Then, the suspension was placed in the dark for 20 min. The suspension was exposed to visible light with a 15-cm gap between the sample and the light source. A suspension of 3 mL was extracted and centrifuged at 20-min time intervals. The concentration of CIP in the supernatant was determined using a UV-Vis spectrophotometer. The percentage degradation was estimated using Eq. 1.

(1) Degradation(%) = Co-CC0×100

where C₀, C, and T represent the initial concentration, final concentration, and time of degradation of the CIP drug solution, respectively.

2.5. Scavenging activity

Scavenging agents are typically used to investigate the photocatalytic degradation pathway. We used EDTA-2Na, isopropyl alcohol (IPA), diphenylamine (DPH), and benzoquinone (BQ) as scavengers for h⁺, ^•OH, e^-, and ^•O₂^-, in particular. The corresponding scavenger (1 mmol) and catalyst (50 mg) were mixed in 10 mL of water. The suspension was ultrasonicated for 15 min and dried in an oven at 60 °C until the water was completely evaporated. The desiccated blend was applied to a glass plates, which were used to monitor the degradation of CIP during visible-light irradiation.

2.6 Computational Methodology

Calculations were performed using the QUANTUM ESPRESSO package, an open-source integrated suite of software designed for the geometric optimization of atomic structures and electronic property computations [31]. In this study, BURAI software was utilized as a graphical user interface for the Quantum Espresso package. A 2 × 2 × 2 supercell containing 8 Ti and 27 Sr atoms was used to model the structures of pure STO, as depicted in Figure 1. For 7% Cobalt doping, the super cell comprised 1 Co, 7 Ti, and 27 Sr atoms, [32] using VESTA tool [33]. The crystal structure optimization and electronic properties, including band structure and density of states (DOS) of the aforementioned materials, were analyzed using the generalized gradient approximation (GGA) method, [34] implemented through BURAI [35].

Fig. 1.

X-ray diffraction patterns of (bottom to top) pure SrTiO₃ (STO) and 3, 5, 7, and 9 wt% cobalt-doped STO (Co-STO) nanostructures.

3. RESULTS AND DISCUSSION

3.1. XRD

Figure 1 shows the XRD patterns of the 3–9% Co-STO and pure STO perovskites. The main peaks at 22.6°, 24°, 32.1°, 39.6°, 46.2°, 57.3°, and 67.3° correspond to the crystallographic planes with Miller indices of (100), (101), (110), (111), (200), (211), and (220), respectively. These values are characteristic of the perovskite structure of cubic STO, as evidenced by JCPDS No. 89-3697 [36]. However, the XRD peak intensities of the 3–9% Co-STO catalysts increased as the amount of Co increased in STO, with the 9% Co-STO showing the highest peak intensities. This also confirms the ability of the STO lattice to accommodate different amounts of Co. The increase in peak intensities can be attributed to changes in surface energy and internal tension caused by the presence of Co during the formation of STO.

Furthermore, Co ions gradually fill both the interstitial and regular positions of the STO ions, as the Co doping levels increase from 3 to 9%. In particular, 9 wt% Co would have filled the empty interstitial spaces that would otherwise be unoccupied [37,38]. The Debye–Scherrer equation (Eq. 2) was applied to compute the crystallite size of pure STO using the (110) plane, which was determined to be ~46.6 nm. Notably, the sizes of the crystallites in the Co-doped STO nanostructures decreased as the amount of Co doping increased. The crystallite sizes of 3, 5, 7, and 9% Co-STO were determined to be 42.5, 38.6, 30.8, and 33.7 nm, respectively. This could be mainly attributed to the formation of Co–O–SrTi on their surfaces, which may have hindered crystal growth. Table 2 lists the crystallite sizes of the pure and doped STO nanostructures.

Table 2.

Crystalline size, R² values, and bandgap values of pure and doped STO nanostructures

(2) D=kλβCosθ

3.2. Morphological analyses of STO and Co-STO nanostructures using FE-SEM/EDX mapping

Figure 2 shows the SEM images of pure STO and the Co-STO nanostructures. The pure STO crystals (Figures 2 a-b) have indistinct rhombohedral shapes with minimal clustering. 3% Co-STO exhibited tiny particles (Figures 2 c-d), whereas 5% Co-STO demonstrated smaller rhombohedral hopper-like crystals (Figure 2 f). Notably, a significant disparity was observed between the crystal surfaces (Figures 2 e-f) because the edges and corners of these crystals were not as clearly visible as those in the 3% Co-STO crystals (Figure 2d). The 7% Co-STO crystals also exhibited rhombohedral hopperlike structures (Figures 2 g-h), albeit more deteriorated than those observed for the 5% Co-STO crystals. However, the 9% Co-STO crystals were agglomerated compared to the previous samples (Figures 2 i-j). Figure 2 j clearly shows the presence of clusters on the crystal surfaces, which could be attributed to the increase in Co concentration from 7 to 9% in the titanate crystal.

Fig. 2.

Scanning electron microscopy images of (a, b) pure STO, (c, d) 3% Co-STO, (e, f) 5% Co-STO, (g, h) 7% Co-STO, and (i, j) 9% Co-STO at 3-mm and 1-mm scales, respectively.

The various shapes observed in the images suggest that the hydrothermal synthesis method can be used to precisely control the formation of the Co-STO crystals. Notably, an increase in Co²⁺ concentration promotes the formation of crystals by affecting the nucleation rate.

The elemental compositions of the catalyst were confirmed by EDX analysis (Figures S2 and S3). Figure S2 k unambiguously demonstrates the presence of Sr, Ti, and O in the catalysts, while Figures S3 l-o clearly indicate the presence of Co. Moreover, the elemental mapping confirmed the uniform distribution of Ti, Sr, O, and Co elements, respectively, in the 7% Co-STO nanostructure. Table 1 shows the elemental composition, atomic percentage, and mass percentage of the 7% Co-STO.

Table 1.

Elemental composition of the 7% Co-STO nanostructure

3.3. HR-TEM with SAED of 7% Co-STO

The TEM images of 7% Co-STO (Figure 3) demonstrated numerous small rhombohedra with a distinct lamellar texture, which is consistent with its SEM results. Moreover, the crystal lattices exhibited a high degree of order and were tightly packed (Figure 3d). The average particle size of the 7% Co-STO was calculated to be 17.3 nm using ImageJ software. The d-spacing was ~3.236 Å, indicating equidistance between the (110) planes of rutile TiO₃ (Figure 3e). The SAED pattern of the sample (Figure 3f) confirmed the presence of numerous bright spots, indicating the highly crystalline nature of the particles. This indicates the successful intercalation of Co, which was achieved on the surface of the STO using the hydrothermal method.

Fig. 3.

Transmission electron microscopy images of the 7% Co-STO nanostructures at the scales of (a) 100 nm, (b) 50 nm, (c) 20 nm, (d) 10 nm, and (e) 2 nm, and (f) shows the corresponding selected area diffraction pattern.

3.4. Optical properties and surface area of 7% Co-STO

The UV-visible absorption spectra of the 7% Co-STO nanocubes are shown in Figure 4. Notably, these nanocubes absorb more visible light, and hence, their photocatalytic performance may be improved by visible-light exposure. The energy bandgap (E_g) is a crucial optical feature that determines the semiconducting properties of nanocomposites. Therefore, the bandgaps of all Co-STO nanostructures were determined by Tauc plots (Eq. 3; Figure 4).

Fig. 4.

(a) UV-visible diffuse reflectance spectra and (b) the corresponding Tauc plots of 3, 5, 7, and 9% Co-STO nanostructures. (c) N₂ adsorption-desorption isotherms and (d) the corresponding pore diameter curves of 7% Co-STO.

(3) hv=A(hv-Eg)12

where h is the Planck’s constant, v is the frequency of light, and α is the absorption coefficient. The energy bandgap values for pure STO, 3% Co-STO, 5% Co-STO, 7% Co-STO, and 9% Co-STO are 3.61, 3.54, 3.49, 3.42, and 3.44 eV, respectively. Table 2 presents a comparison of the bandgap values for pure and doped STO nanostructures.

An N₂ adsorption-desorption method was used to evaluate the physical surface area of the photocatalyst. All the hv A hv E nanostructures exhibited a Type IV adsorption isotherm with a H3 hysteresis loop within a measured pressure range of 0.5−1.0 (Figure 4), indicating the presence of mesoporous structures. The Barrett–Joyner–Halenda pore size distribution plots (Figure 4b) revealed mesopore sizes in the range of 2–200 nm. The Brunauer–Emmett–Teller surface area of the 7% Co-STO nanocomposite was found to be 11.35 cm³/g. Moreover, the average pore diameter, pore volume, and pore width were 15.3 nm, 0.0281 cc/g, and 2.769 nm, respectively.

3.5. XPS analysis of the 7% Co-STO

XPS was used to examine the surface chemical composition and valence states of the elements in the 7% Co-STO composite (Figure 5). The survey XPS spectrum (Figure 5a) indicated the presence of C, O, Co, Ti, and Sr in CO-Sr₃/TiO₃. The C 1s XPS spectrum (Figure 5b) was deconvoluted into three distinct peaks at 282.5, 283.9, and 286.4 eV, corresponding to C–C, C–O and C=O bonds, respectively [39]. The O 1s XPS spectrum (Figure 5c) was deconvoluted into three distinct peaks at 527.4, 529.0, and 533.5 eV, corresponding to the lattice O and OH groups, respectively [40]. The Co 2p XPS spectrum (Figure 5d) was deconvoluted into a spin-orbit doublet at 778.2 and 792.9 eV, corresponding to Co 2p_1/2 and Co 2p_3/2, respectively, which confirms the presence of both Co²⁺ and Co³⁺ [41]. The highresolution Sr 3d XPS spectrum (Figure 5e) was deconvoluted into a doublet at 130.4 and 132.0 eV, corresponding to Sr 3d_5/2 and Sr 3d_3/2, respectively. This 1.6 eV split is consistent with the presence of Sr²⁺ (SrTiO₂) [44,45]. The Ti 2p XPS spectrum (Figure 5f) was deconvoluted into a doublet at 456.1 and 461.9 eV, corresponding to Ti 2p_3/2 and Ti 2p_1/2, with a doublet split of 5.8 eV, which confirms the presence of Ti⁴⁺ [42-44].

Fig. 5.

(a) Survey, (b) C 1s, (c) O 1s, (d) Co 2p, (e) Sr 3d, and (f) Ti 2p X-ray photoelectron spectra of 7% Co-STO.

3.6. Photocatalytic degradation

Figure 9 (a) demonstrates the photocatalytic degradation process of CIP using nanostructures of different concentrations under visible light. Notably, the 7% Co-STO nanostructures achieved an impressive 90.6% degradation of CIP within just 120 min, outperforming the other concentrations. Figure 6b shows the photocatalytic degradation of CIP in the absence of a catalyst and in the presence of pure STO and all the Co-doped STO nanostructures under visible-light irradiation. Among the catalysts, 7% Co-STO exhibited the highest degradation (90.6%) of CIP in just 120 min. The CIP degradation activities of the catalysts were in the following order: pure STO (51%) < 3% Co-STO (58%) < 5% Co-STO (69.5%) < 9% Co-STO (81.2%) < 7% Co-STO (90.6%). Figure 6c shows the kinetic plots [ln(C/C_o) vs. time] of the different catalysts. The reaction constant (k) for the 7% Co-STO catalyst (0.9675) was the highest among the catalysts. Figure 6d demonstrates the percentage of degradation of CIP using pure STO and different Co-loaded STO catalysts.

Fig. 9.

(a, b) Photocatalytic CIP degradation activities of 7% Co-STO in the presence of different radical scavengers at pH 7. (c) Cycling stability of 7% Co-STO and (d) total organic carbon mineralization of CIP.

Fig. 6.

(a) Photocatalytic degradation of CIP over 7% Co-STO synthesized nanostructures. (b) Photocatalytic degradation of ciprofloxacin (CIP) in the absence and presence of pure STO and all Co-doped STO nanostructures. (c) Pseudo-first-order photocatalytic degradation kinetic plots in the absence and presence of pure STO and all Co-doped STO nanostructures. (d) degradation (%) of CIP for all samples.

Table 3.

Comparison of the photocatalytic CIP degradation efficiencies of our photocatalysts with previously reported photocatalysts.

3.7. Factors that influence photocatalytic degradation

3.7.1. pH

The pH of the medium has a complex effect on the rates of photocatalytic degradation, depending on the type of pollutant and the semiconductor zero charge point (P_CZ) or the electrostatic interactions between the organic molecule and the catalyst surface (Leu and Zhang, 2007). When the adsorption of the potential-determining ions (h⁺ and OH^-) is equal, the corresponding pH value is known as the zerocharge point (P_CZ). As the P_CZ is approached, the rates of adsorption and reactions typically increase. The pK_a values of CIP and TiO₂ are 6.09 and 6.80, respectively [55]. We analyzed the photocatalytic activities of 7% Co-STO under acidic (pH 4), neutral (pH 7), and basic (pH 10) conditions (Figure 7a) and observed better catalyst adsorption, larger interfacial charge transfer, and higher affinity and load balance between the drug and the photocatalyst 7% Co-STO at pH 7. However, the degradation rates at both pH 4 and 10 were remarkably similar, suggesting that pH variation does not significantly impact the degradation efficiency.

Fig. 7.

Factors that influence the photocatalytic degradation of CIP: (a) pH, (b, c) temperature, (d) catalyst concentration, and (e) pollutant concentration.

3.7.2. Temperature

The rate of degradation increased with increasing temperatures (Figure 7b). The temperature of the bulk phase affects several parameters, such as vapor pressure, viscosity, gas solubility, and surface tension. Increasing the temperature increases the vapor pressure of the solvent. Consequently, the cavitation bubbles would contain a higher concentration of water vapor, which reduces the forceful collapse of the cavitation bubbles; this phenomenon is called the cushion effect. This leads to a decrease in the temperature at which collapse occurs, resulting in the generation of a smaller number of ^•OH radicals. Conversely, an increase in temperature leads to a decrease in viscosity and surface tension, which reduces the minimum level of intensity required to create cavitation. Therefore, we presumed that an increase in temperature would result in a higher quantity of cavitation bubbles, which would stimulate the generation of ^•OH and ^•OOH free radicals, thereby significantly increasing the oxidation capacity of the catalyst.

Simultaneously, higher temperatures increase the mass transfer of various species in the bulk solution, thereby facilitating the reactions of ^•OH radicals and other oxidative species. This increases the reaction rate between the radicals and pollutants. For chemicals that do not easily evaporate, increasing the overall temperature typically increases their degradation rate [56]. The temperature dependence of chemical reactions typically adheres to the Arrhenius rule (Eq. 4):

(4) ln k1 = -Ea/RT + ln A

where E_a is the apparent activation energy (kJ mol^-1), R is the ideal gas constant, and A is the pre-exponential factor (min^-1).

Figure 7c shows the effect of reaction temperature on CIP degradation in the presence of 7% Co-STO. As expected, increasing the reaction temperature expedited CIP degradation. This can be attributed to the rapid degradation of PMS into reactive radicals due to thermal activation. The activation energy (E_a) was estimated using the Arrhenius equation by plotting the natural logarithm of the rate constant (ln k_app) vs. the reciprocal of temperature (1/T). The E_a values for the degradation of CIP in the presence of 3, 5, 7, and 9% Co-STO catalysts were 53.99, 57.288, 62.569, and 58.992 kJ mol^-1, respectively. These values are consistent with those of other cobalt-based systems (47–70 kJ mol^-1) [57]. However, the computed E_a values were significantly higher than those obtained by diffusion-controlled reactions (10–13 kJ mol^-1). This indicates that the observed reaction rates for this degradation process are primarily influenced by the chemical reaction rate occurring on the surfaces of Co-STO, rather than the rate of mass transfer.

3.7.3. Catalyst dosage

Catalyst dose is a crucial factor in photocatalytic degradation. Hence, we measured the effects of various catalyst doses under the ideal conditions identified by experimental design, rather than studying the dosage effect as an independent parameter. Figure 7d shows the percentages of CIP removed at various catalyst doses (30, 40, 50, and 60 mg) using the same amount of solution (100 mL). The photocatalytic degradation efficiency initially increased with increasing catalyst dosages until it reached a certain threshold and then declined. The initial increase could be attributed to an increase in the catalyst concentration and, consequently, the number of catalytic active sites, which in turn increases the generation of free radicals that are responsible for degradation. However, exceeding the critical quantity of the catalyst leads to turbidity in the solution, which obstructs visible-light irradiation and diminishes the degradation efficiency. Furthermore, the catalyst particles agglomerate at large dosages during photodegradation, which leads to a decrease in surface area, negatively affecting the degradation of the drug [58].

3.7.4. Pollutant dosage

The impact of the baseline drug concentrations on the percentage of drug removal was examined by reacting a catalyst dose of 0.01 g with 100 mL (20 ppm) of the drug solution for 120 min. A tradeoff was observed between CIP photodegradation and its initial concentration (Figure 7e). A high initial concentration of the drug led to the adsorption of more drug molecules on the catalyst surface, which may have suppressed drug degradation as the adsorbed layer prevented direct interactions of the drug molecules with h⁺ or ^•OH radicals. Another plausible explanation for this outcome could be that a significant portion of light is typically absorbed by the drug molecules at high drug concentrations. The decrease in OH^- and O₂^-• concentrations hinders the catalytic activity. Another potential factor is the generation of byproducts during the degradation of the original therapeutic compounds. Furthermore, the degradation rate was significantly higher at low CIP concentrations and gradually decreased as the initial CIP concentration increased [59]. The CIP degradation percentages at 10, 20, 30, and 40 ppm initial concentrations after 120 min were 85.79, 90.60, 76.49, and 71.34%, respectively. This suggests that the initial concentration of the contaminant is the primary factor that influences the degradation percentage.

3.8. Proposed mechanism for the photodegradation of CIP by 7% Co-STO

Figure 8 shows our proposed photodegradation pathway for CIP in the presence of the 7% Co-STO nanocatalyst under visible-light irradiation (Xe lamp). Light irradiation creates photoelectrons, which are then transferred to the conduction band of STO by overcoming the Schottky barrier on the metal/semiconductor surface. This process generates ^•OH and O₂^-• species, which react with electrons. The valence band level suggests the presence of Ag in the structure of STO, which enables the electrons in the valence band to migrate to the Fermi level (E_f) of the Co nanostructures, thus creating holes. These holes interact with the -OH ions to form ^•OH radicals, which in turn react with CIP and degrade it.

Fig. 8.

Proposed photocatalytic degradation mechanism of CIP with 7% Co-STO nanocomposite under visible-light irradiation.

A potential reason for the decrease in degradation capacity could be the passivation of surface defects caused by drug adsorption on the photocatalyst surface. This could explain the lower photocatalytic activity of both pure and doped STO. The sharp edges observed in 7% Co-STO impede this shielding effect, leaving available active sites for catalysis. Hence, the photocatalytic degradation efficiency can be enhanced by increasing the number of sharp edges on the surface. This could explain the excellent CIP degradation activity of 7% Co-STO, in addition to its minimal band gap and small crystallite size. The reactions in the proposed catalytic pathway are shown below (Eqs. 5–11).

(5) Co + λ visible → Co*

(6) Co* + SrTiO3 → SrTiO3 (e- CB)

(7) SrTiO3 (e- CB) + O2 → O·2-

(8) O·2- + 2H+ + SrTiO3 (e- CB) → H2O2

(9) H2O2 + SrTiO3 (e- CB) →OH· + OH-

(10) SrTiO3 (h+ VB)+OH-→•OH

(11) CIP + OH· →Products

3.9. Scavenging studies, total organic carbon analysis, and recyclability test

Radical scavenging experiments were performed to identify the primary reactive species produced during CIP photodegradation (Figures 9a-b). DPH, IPA, EDTA-2Na, and BQ were used to scavenge e^-, ^•OH, h⁺, and O₂^•- species, respectively. All four chemicals inhibited CIP degradation at an initial pH of 7 (Figure 9a). However, BQ showed the strongest inhibitory effect among the scavengers. The extent to which the CIP degradation rate decreased in the presence of a scavenger revealed its inhibitory effect on the corresponding active species. The CIP degradation rate in the presence of 7% Co-STO without the use of a radical trapping agent was 90.6%. However, the addition of IPA, EDTA-2Na, DPH, and BQ to the abovementioned reaction mixture decreased the degradation rates to 85.2%, 81.7%, 77.4%, and 30.8%, respectively. The fact that BQ had the highest effect on the degradation rate of the 7% Co-STO suggests that O₂^•- was the main active species.

The reusability of the 7% Co-STO photocatalyst was evaluated for five consecutive cycles (Figure 9c). At the conclusion of one degradation cycle, a mixture of ethanol and BQ was added to the catalyst, which was then dried and reused for another photodegradation cycle. The catalyst showed a CIP degradation capacity of 87.60 ± 0.50% during the fifth cycle, which indicates its excellent reusability.

The total organic carbon percentage in a mixture of 7% Co-STO and CIP was only 73% (Figure 9d) after 120 min of visible-light irradiation, even though the photocatalytic efficiency reached 90.6%. This could be explained by the presence of intermediates, which are mineralized over a longer reaction time.

3.10. Stability test

The photocatalysts were assessed by powder XRD and SEM after five consecutive runs to determine their stability (Figures 10a and b, respectively). No noticeable decline in stability was observed in the XRD measurements or SEM micrographs after five runs. This indicates that the 7% Co-STO photocatalyst is suitable for prolonged use and practical CIP degradation applications.

Fig. 10.

(a) X-ray diffraction and (b) scanning electron microscopy images of 7% Co-STO before and after five consecutive photocatalyst degradation runs.

3.11 DFT study

Computational methods and theoretical processes were used to analyze the properties of pure STO and Co-doped STO models with a doping level of 7%. This computational technique has effectively been utilized to investigate the electrical and structural characteristics of diverse materials. Two theoretic models were developed using a typical 40 atom 2 × 2 × 2 with cubic symmetry. Figure 1 displays the depiction of the Pure STO and Co-STO nanostructures. Figure 11 (a & b) depicts the correspondence between the green, red, yellow, blue, and turquoise/blue balls with the Sr, O, Ti, and Co atoms, respectively. Here, the Co atom is surrounded by six O atoms, resulting in the formation of cubic clusters.

Fig. 11.

(a and b) Crystal structure of 7% Co-STO (Green: Sr, red: O, Yellow Ti and Blue Co), using VESTA tool, DOS date of STO (c) and Co-STO (d) and Charge carrier analysis of STO (e) and Co-STO (f)

Table 4 illustrates the many structural properties, including the unit cell, symmetry, and atomic locations. The structural refinements were conducted utilizing the cubic structure in the Pm3−m symmetry for various concentrations of Co dopant in the Ti-site, employing an initial model of STO. Table 4 shows that the crystal structure and lattice parameters of our theoretical results have a low mean percentage error compared to the experimental data. This indicates that our calculations are consistent with the experiments. In addition, when comparing the crystalline properties of the pristine and Co-doped STO models, it was noticed that the cell parameters of the cubic polymorphs (STO and Co-STO) expanded by approximately 2.13% following the Co doping.

Table 4.

Wyckoff positions, fractional atomic coordinates, and occupancy for different atoms using XRD data and EDX recorded for the Co-STO (SrTiCo_{(x = 0.03, 0.05, 0.07, 0.09)}O₃

Here, we present the theoretical discoveries regarding the electronic properties of the investigated perovskite compounds. The configuration of electrons has a vital role in determining the band structure, density of states (DOS), and charge density. Figure 11 (c-d) displays an analysis of the band structure and projected density of states (DOS). The valence band (VB) of STO was determined to have an energy range of 0 to 38 eV. The conduction band (CB) was determined to have a range of 0 eV to -39 eV. The calculated direct band gap energy was determined to be 3.63 eV, suggesting that the electronic excitation is not direct. The valence band (VB) of Co-STO was found to span an energy range from 0 to 36 eV and the conduction band (CB) was found to extend from 0 eV to -36 eV. The direct band gap energy calculated for this material was 3.58 eV. The inclusion of Co²⁺ dopants causes slight alterations in the bandgap energy (3.58 eV) of Co-STO due to the existence of localized states in the conduction band area, which originate from the intermediate electronic level. The dopant forms clusters that affect the electrical density of the crystal at various scales, including short, medium, and long range, due to the violation of symmetry. Therefore, the findings illustrate how the dopant affects the semiconductor by giving it new and unique properties.

Furthermore, by comparing the conduction bands (CB) of both the STO and 7% Co-STO models, we are able to examine the properties of the charge carriers (electrons) by assessing the curvature of the conduction band minimum (CBM). The charge balance model (CBM) for STO models is distinguished by a wide-ranging characteristic as given in Fig 11 e. Conversely, in the Co-STO models, the CBM at the same place displays a distribution that resembles a parabola, with a distinct lowest point. The correlation between the effective mass of charge carriers and the curvature of the band leads us to infer that the rate of electron-hole recombination varies between the STO and Co-STO models.

In this context, a broader band can help augment the effective mass of the stimulated electrons, resulting in a reduction of their mobility. On the other hand, a clearly defined parabolic band can be associated with a reduction in effective mass and an enhancement in electron mobility. Therefore, the obtained results for the band structure profiles of STO and Co-STO indicate that Co-doping improves electron mobility, thus making Co-STO a favorable choice for electro-optical applications.

In addition, Fig 11 f provides a succinct summary of the atomic contribution analysis for both the valence band (VB) and conduction band (CB). The pattern displayed is distinct and easily recognizable, and it is directly associated with the local clusters centered on the Co, Sr, Ti, and O atoms. In this setting, the main impact on the VB area comes from the 2p (p_x, p_y, p_z) orbitals of the oxygen anions, with a little contribution from the Co and Sr orbitals in both the STO and Co-STO models. In contrast, the conduction band (CB) mostly utilized unoccupied valence orbitals (3dxz, 3dxy, 3dyz, 3dz₂, 3dx₂-y₂) that originated from the titanium (Ti) atom. There was a small contribution from Co²⁺ that was blended with oxygen atomic orbitals. This emphasizes the participation of titanium (Ti) and cobalt (Co) clusters. These results confirm the impact of the Co-doping method on the control of CBM distribution, which can be attributed to the mobility of electrons within the electronic structure, as previously described.

Table 5 exhibits the Bandgap values from the Computational and experimental techniques and confirms that the experimental technique shows a lower bandgap than the computational values [60].

Table 5.

Band gap analysis using computational techniques

4. CONCLUSION

In this study, we developed a series of Co-doped SrTiO₃ (STO) catalysts to enhance the visible-light photocatalytic degradation activity of pure STO in degrading ciprofloxacin (CIP), a popular antibiotic and a persistent organic pollutant that is harmful to humans and the environment. A one-pot hydrothermal approach was used to synthesize STO and 3 wt%, 5 wt%, 7 wt%, and 9 wt% Co-STO. The Co-doped nanostructures had a rhombohedral hopper-like shape with an average micropore size of ~30.8 nm. Among the four Co-doped STO catalysts, the 7% Co-STO exhibited the highest CIP degradation activity (90.6%) after 120 min of visiblelight irradiation. Radical scavenging experiments confirmed that superoxide (O₂^•-) was the most active species during photocatalytic degradation. These results highlight the potential of 7% Co-STO as an efficient photodegradation catalyst for pharmaceutical contaminants in wastewater.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (grant number 2021R1C1C1014039) and the Basic Science Research Program through the NRF of Korea funded by the Ministry of Education (grant number 2022R1A6A1A03051158).

This work was also supported by the Global Joint Research Program funded by the Pukyong National University (202411820001).

This project was supported by Researchers Supporting Project number (RSPD2024R712), King Saud University, Riyadh, Saudi Arabia.

SUPPLEMENTAL MATERIAL

Fig S1.

Schematic representation of Synthesis of Cobalt doped Strontium Titanate nanostructures

kjmm-2024-62-10-803-Supplementary-Fig-S1.docx

Fig S2.

EDAX pattern of pure STO (k) nanostructures

kjmm-2024-62-10-803-Supplementary-Fig-S2.docx

Fig S3.

EDAX pattern of 3% (l), 5% (m), 7% (n) and 9% Co-STO (o) nanostructures

kjmm-2024-62-10-803-Supplementary-Fig-S3.docx

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Element	Count	Mass%	Atom%	Chemical formula
O K	37387.98	18.87	51.29	O
Ti K	45675.72	19.35	17.58	Ti
Co K	3406.15	1.91	1.41	Co
Sr L	49429.91	59.87	29.72	Sr
TOTAL		100.00	100.00

Atom	Wyckoff Position	Atomic coordinates			Occupation
Atom	Wyckoff Position	x	Y	z	Occupation
x = 0	Pm-3m		Pure STO
Sr	1a	0	0	0	1
Ti	1b	0.5	0.5	0.5	1
O	3c	0.5	0.5	0	3
x = 0.03	Pm-3m		3% Cob alt STO
Sr	1a	0	0	0	1
Ti/Co	1b	0.5	0.5	0.5	0.97/0.03
O	3c	0.5	0.5	0	3
x = 0.05	Pm-3m		5% Cob alt STO
Sr	1a	0	0	0	1
Ti/Co	1b	0.5	0.5	0.5	0.95/0.05
O	3c	0.5	0.5	0	3
x = 0.07	Pm-3m		7% Cobalt STO
Sr	1a	0	0	0	1
Ti/Co	1b	0.5	0.5	0.5	0.93/0.07
O	3c	0.5	0.5	0	3
x = 0.09	Pm-3m		9% Cobalt STO
Sr	1a	0	0	0	1
Ti/Co	1b	0.5	0.5	0.5	0.91/0.09
O	3c	0.5	0.5	0	3

Co-STO concentration (%)	Band gap (Energy/eV) (Computational techniques)	Band gap (Energy/eV) (Experimental techniques)
Pure STO	3.80	3.61
7 % Co-STO	3.72	3.42

Nanostructure	Crystallite size (nm)	R2 value	Bandgap (eV)
STO	46.6	0.9136	3.61
3% Co-STO	42.5	0.9387	3.54
5% Co-STO	38.6	0.9499	3.49
7% Co-STO	30.8	0.9776	3.42
9% Co-STO	33.7	0.9621	3.44

S. No.	Photocatalyst	Bandgap of the photocatalyst (eV)	% Degradation of the drug	Time (min)	References
1.	Y₂O₃/MgO	3.4	66.5 (CIP)	120	46
2.	TiO₂/MoS₂	2.71	93 (CIP)	60	47
3.	rGO-CuS-ZnS	NA	86.65 (Ofloxacin)	140	48
4.	Biochar@BiFeO₃	2.2	77.8 (CIP)	140	49
5.	CuFe₂O₄-70%/rGO/HNT	1.9	98 (CIP)	60	50
6.	Zinc ferrite titania	1.2	99.9 (CIP)	90	51
7.	ZnO	3.7	50 (CIP)	120	52
8.	Magnetite	NA	67 (CIP)	30	53
9.	CeO₂-Ag/AgBr	2.56	93 (CIP)	120	54
10.	7% Co-STO	3.56	90.6 (CIP)	120	This work