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) CoxO3, 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.
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].
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.
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.
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.
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.
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 (Eg) 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).
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 2p1/2 and Co 2p3/2, 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 3d5/2 and Sr 3d3/2, 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 2p3/2 and Ti 2p1/2, with a doublet split of 5.8 eV, which confirms the presence of Ti4+ [42-44].
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/Co) 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.
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 (PCZ) 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 (PCZ). As the PCZ is approached, the rates of adsorption and reactions typically increase. The pKa 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.
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):
where Ea 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 (Ea) was estimated using the Arrhenius equation by plotting the natural logarithm of
the rate constant (ln kapp) vs. the reciprocal of temperature (1/T). The Ea 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 Ea 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 (Ef) 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.
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).
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.
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.
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 Pm
3
−
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.
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 (px, py, pz) 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].