Phenol, as a vital industrial chemical product, is well known for its bio-treatment of recalcitrant and acute toxicity. The ubiquitous presence of phenol in wastewater and its associated environmental risks have raised significant public health concerns^[1]. Phenol is continuously discharged into aquatic environments through various anthropogenic activities. Its presence in storm and wastewater effluent poses a major obstacle to the widespread adoption of water recycling^[2−3]. Moreover, it can directly harm ecosystems and endanger human health through the contamination of drinking water sources, including surface and groundwater^[4]. Consequently, developing effective strategies for the removal of persistent phenol from wastewater effluent is crucial to minimize pollution risks.

Currently, numerous efforts have been devoted to developing suitable methods for removing phenol from wastewater effluent^[5−12]. In recent years, photocatalytic degradation of phenol has emerged as a prominent research area. As an advanced oxidation technology, photocatalytic degradation has garnered significant attention due to its ability to efficiently degrade organic compounds in wastewater without generating secondary pollution^[7−11]. However, conventional photocatalysts like TiO₂ and ZnO are unable to fully utilize visible light, which constitutes approximately 45% of the solar spectrum, and thus fail to meet the rapidly increasing energy demand. This limitation has spurred extensive research into the development and exploration of efficient visible-light-driven photocatalysts to utilize solar energy more effectively.

Perovskite oxide semiconductors (ABO₃) have shown great potential as photocatalysts, due to their excellent absorption of solar energy, particularly in the visible light spectrum, and their high quantum efficiency^[13−16]. The term “perovskite” refers to the crystal structure of calcium titanate (CaTiO₃), discovered by Gustav Rose in 1839 and named in honor of Lev Perovski^[17]. In perovskite structures, the A-site typically accommodates rare earth or alkali metals with larger ionic radii, while the B-site is occupied by transition metals with smaller ionic radii.

According to the literature, doping, as a modification method, has been shown to enhance the catalytic properties of perovskites. By introducing metal dopants at either the A or B sites, the lattice structure can be modified, indirectly influencing and improving the activity and mobility of lattice oxygen, which in turn enhances catalytic performance^[18−24]. For example, Wang et al.^[18] synthesized BiCo_xFe_1−xO₃ by doping Fe into BiCoO₃, demonstrating a significant increase in magnetization due to Co substitution for Fe ions, while DTA confirmed the preservation of ferroelectric properties. Fan et al.^[19] prepared La_1−xCe_xMnO₃ perovskites with varying Ce doping ratios for catalytic ozonation of phenol and found that La_0.9Ce_0.1MnO₃ exhibited superior catalytic performance and good reusability. Huang et al.^[20] reported the synthesis of LaCu_xFe_1−xO₃ with different Fe doping ratios for the removal of organic pollutants, achieving optimal performance with 0.4 Fe doping at a gelation temperature of 80 ℃ and calcination temperature of 700 ℃, resulting in a 91.64% COD removal rate in cellulosic ethanol wastewater. Bouchal et al.^[21] prepared via a sol-gel method with different concentrations of Bi nitrate and examined as a photocatalyst for RhB degradation under sunlight, and its antioxidant and antibacterial activities were examined. The results showed that the highest degradation rate was exhibited by 15% Bi-doped BaBiO₃ for the degradation of RhB under solar radiation. According to the antibacterial activity study, the addition of Bi enhanced the antibacterial activity of the resulting material against both Gram-positive and Gram-negative microorganisms.

Based on the previous experimental findings, the results indicated LaCoO₃ perovskite catalyst exhibited a certain photocatalytic activity for phenol, and the degradation rate reached 52.56% with 4 h irradiation. To further improve the performance of LaCoO₃ catalysts, this study focused on doping Ba into the A-site of LaCoO₃ to synthesize La_1−xBa_xCoO₃ catalysts. The physical and chemical properties of these A-site doped materials were characterized and analyzed to understand the effect of doping on their catalytic behavior. Furthermore, the photocatalytic degradation of phenol under visible light irradiation was investigated to evaluate the efficacy of the doped perovskites.

1 Experimental section 1.1 Materials

Lanthanum nitrate (La(NO₃)₃·6H₂O, AR, 99%), cobalt nitrate (Co(NO₃)₂·6H₂O, AR, 99%), barium nitrate (Ba(NO₃)₂, AR, 99%), phenol (AR), citric acid (AR, ≥99.5%), ammonia (AR, 25%), ethanol (AR, ≥99.7%). The phenol solution was prepared into distilled water.

1.2 Catalyst preparation

The specific preparation method of modified LaCoO₃ catalyst is citric acid complexation-hydrothermal synthesis combined method and as follows: (ⅰ) Solution preparation: 80 mL of deionized water was added to a beaker. La(NO₃)₃·6H₂O, Ba(NO₃)₂, Co(NO₃)₂·6H₂O, and citric acid were accurately weighed and dissolved in the water, maintaining the stoichiometric ratio of La_1−xBa_xCoO₃ (x = 0.2, 0.3, 0.4, 0.5, 0.6). The molar ratio of n(La³⁺+Ba²⁺)∶ n(Co²⁺)∶ n(citric acid) was kept at 1∶1∶1; (ⅱ) pH adjustment and complexation: The solution was stirred continuously at a uniform speed on a magnetic stirrer for 1 h to ensure complete complexation of the metal ions with citric acid. Meanwhile ammonia solution was added dropwise to the stirred solution to adjust the pH to approximately 8.5. This alkaline environment facilitated the complete precipitation of the metal ions as hydroxides; (ⅲ) Hydrothermal reaction: The mixture was poured into a teflon stainless steel container and reacted in an oven at 150 ℃ for 20 h; (ⅳ) Washing and drying: After the hydrothermal reaction, the resulting precipitate was removed from the teflon stainless steel container, centrifuged to separate it from the liquid, and washed thoroughly with deionized water to remove any residual reactants. The precipitate was then dried in an oven at 120 ℃; (ⅴ) Calcination: The dried precipitate was ground into a fine powder and calcined at 750 ℃ for 2 h. This high-temperature treatment removed any remaining organic matter and facilitated the formation of the final perovskite crystal structure; (ⅵ) Labeling: The synthesized catalysts were labeled as Ba-0.2, Ba-0.3, Ba-0.4, Ba-0.5 and Ba-0.6, corresponding to their respective Ba doping ratios (x values in La_1−xBa_xCoO₃). The specific synthesis flowchart was shown in Fig.1.

1.3 Catalyst characterization

X-ray powder diffraction (XRD): Crystalline phases and crystal structure of the synthesized catalysts were determined using a Rigaku D/max-2500 X-ray diffractometer. The diffractometer was operated with Cu Kα radiation (λ = 0.15418 nm) generated at 40 kV and 100 mA. Diffraction patterns were collected in the 2θ range of 5° to 90° with a scanning rate of 0.02 (°)·s⁻¹; Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS): Surface morphology, particle size, and elemental composition of the catalysts were analyzed using a LEO-435VP scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (EDS). The microscope was operated at an accelerating voltage of 20 kV; Fourier transform infrared spectroscopy (FT-IR): The functional groups present in the catalysts were identified using a Nicolet 380 FTIR spectrometer. Spectra were acquired in transmission mode over the wavenumber 4000 to 400 cm⁻¹ with a resolution of 4.0 cm⁻¹; N₂ adsorption-desorption isotherms: Specific surface area was determined by applying the Brunauer-Emmett-Teller (BET) model to the N₂ adsorption isotherms measured at −196 ℃ using a Micromeritics Tristar 3000 instrument. Pore size distributions were calculated using the Barrett-Joyner-Halenda (BJH) model applied to the desorption branch of the isotherms. Prior to analysis, samples were outgassed under vacuum at 300 ℃ for 3 h; X-ray photoelectron spectroscopy (XPS): Surface chemical com-position and oxidation states of elements in the catalysts were investigated using a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). High-resolution spectra were acquired with pass energy of 20 eV. The binding energy scale was calibrated using the C 1s peak at 284.8 eV. Peak fitting and deconvolution were performed using CasaXPS software with a Shirley background subtraction; Magnetic property measurement system (MPMS): Magnetic properties of the powdered catalysts were measured using a Quantum Design MPMS-XL SQUID magnetometer. The magnetization was measured as a function of temperature (−271.15~26.85 ℃) and applied magnetic field (−5 to 5 T).

1.4 Photocatalytic activity tests

The photocatalytic activity of the synthesized La_1−xBaₓCoO₃ catalysts was evaluated by monitoring the degradation of phenol in aqueous solution (100 mL, 10 mg·L⁻¹) under visible light irradiation. All experiments were conducted at a constant temperature of 20 ℃ using a jacketed beaker with continuous water circulation to maintain the temperature. The solution was continuously stirred magnetically and ventilated throughout the experiment. A PLS-SXE300 xenon lamp equipped with a cutoff filter was used as the visible light source.

Experimental procedure: (Ⅰ) Dark adsorption: 100 mg of the synthesized photocatalyst was dispersed in 100 mL of the phenol solution in the jacketed beaker. The dispersion was stirred in the dark for 40 min to ensure adsorption-desorption equilibrium between phenol and the catalyst surface; (Ⅱ) Photocatalytic reaction: The dispersion was then irradiated with visible light. At predetermined time intervals of 30 min, 2 mL of the specified dispersion was withdrawn from the reaction mixture and filtered through a 0.22 µm syringe filter to remove the catalyst particles; (Ⅲ) UV-Vis spectrophotometry: The concentration of phenol in the filtered aliquots was determined by measuring the absorbance at 270 nm using a UV-2550 spectrophotometer. A calibration curve was constructed using standard phenol solutions to relate absorbance to concentration; (Ⅳ) Control experiments: Control experiments were performed under identical conditions but in the absence of visible light irradiation and without the addition of the photocatalyst; (Ⅴ) Degradation efficiency calculation: The photocatalytic degradation efficiency (D) was calculated using the following equation (1):

$ \begin{array}{c}D=\dfrac{\left({C}_{0}-{C}_{t}\right)}{{C}_{0}}\times 100\text{%}\end{array} $

(1)

where C₀ is the initial concentration of phenol, and C_t is the concentration at time t.

2 Results and discussion 2.1 Structure characterization

The phase structure of prepared samples were analyzed by XRD, and the XRD pattern of catalysts with different Ba doping ratios were compared with the standard spectra (in Fig.2). It can be found that the characteristic diffraction peaks of LaCoO₃ in the 2θ of 23.79°, 26.18°, 32.90°, 40.54°, 47.41°, 58.77°, 69.07° and 78.78° were attributed to (110), (111), (200), (211), (220), (222), (400) and (420) lattice planes (JPCDS No.32-0480)^[25], respectively. XRD results showed that LaCoO₃ was successfully synthesized by citric acid complexation-hydrothermal synthesis combined method. Notably, no additional peaks corresponding to impurities or secondary phases were observed, confirming the phase purity of the synthesized catalysts. Furthermore, Fig.2 demonstrated a gradual increase in the intensity of diffraction peaks with increasing Ba doping ratio, and the Ba-0.6 showed the strongest diffraction peaks. This observation suggested that Ba successfully substituted La in the perovskite lattice. Due to the larger ionic radius of Ba compared to La, the lattice parameters expanded, leading to a slight shift of the XRD pattern towards lower angles.

The morphology and microstructure of modified LaCoO₃ samples were studied by SEM. The distribution of elements within the samples was further analyzed using EDS spectroscopy. SEM images of the Ba-0.2, Ba-0.3, Ba-0.4, Ba-0.5, Ba-0.6 and LaCoO₃ catalysts were presented in Fig.3(a–f), respectively. These images revealed a general trend of increasing average particle size with increasing Ba doping ratio. Notably, the Ba-0.6 catalyst exhibited the largest particle size, ranging from 20 to 50 nm, with a certain degree of agglomeration observed between the particles, which further illustrated that the Ba doping could increase the catalyst particle size. The smaller size was beneficial to the transfer of photo-generated electrons and enhancement of the photocatalytic performance^[26]. The EDS was employed to analyze the elemental composition of the modified LaCoO₃ catalysts. The EDS spectrum of the Ba-0.5 catalyst in Fig.4, revealed the presence of La, Ba, Co, O and C. The atomic percentages of these elements were determined to confirm the successful incorporation of Ba into the perovskite lattice. Table 1 presented the elemental ratios, which closely approximate the expected stoichiometry of (La+Ba)∶Co∶O = 1∶1∶3 for the perovskite ABO₃ structure. This finding confirms the successful doping of Ba into the LaCoO₃ lattice. Notably, the Ba-0.5 catalyst exhibited an elemental ratio closest to the ideal 1∶1∶3 stoichiometry among the five synthesized catalysts, suggesting a higher degree of purity. This enhanced purity may contribute to the superior photocatalytic performance observed for the Ba-0.5 catalyst.

The FT-IR spectroscopy was employed to identify the functional groups in the catalysts. The FT-IR spectra of the catalysts with different Ba doping ratios were shown in Fig.5, and the corresponding peak assignments were listed in Table 2. The peak at approximately 417 cm⁻¹ corresponded to the bending vibration of O—Co—O in the perovskite ABO₃ structure, while the peak at about 550 cm⁻¹ was attributed to the stretching vibration of Co—O^[27]. The strong absorption peak at 602 cm⁻¹ further confirmed the presence of Co—O stretching vibrations in the ABO₃ phase^[28]. A weak absorption peak observed at about 850 cm⁻¹ suggests the presence of carbonate species^[29], likely originating from residual carbon and oxygen impurities introduced during the synthesis process using citric acid as the complexing agent. Notably, the Co—O stretching vibration peak at 550 cm⁻¹ gradually shifted to lower wavenumbers with increasing Ba doping ratio, indicating a weakening of the Co—O bond covalency and an increase in ionic character upon Ba incorporation^[30]. This observation can be attributed to lattice strain induced by lattice distortion due to the larger ionic radius of Ba compared to La. The FT-IR results, in conjunction with XRD and other characterization data, provided further evidence for the successful doping of Ba into the LaCoO₃ crystal structure.

The textural properties of the modified LaCoO₃ catalysts, as determined by N₂ adsorption-desorption isotherms, were summarized in Table 3. The specific surface areas of the synthesized samples were relatively low, likely due to the presence of citric acid during synthesis. As shown in Table 3, the specific surface area of the catalysts decreased with increasing Ba doping ratio. Among the prepared catalysts, Ba-0.2 exhibited the highest specific surface area (23.95 m²·g⁻¹), along with the largest pore volume and pore size. This decrease in textural properties with increasing Ba content may be attributed to the increased energy required for interdiffusion between grains in the presence of Ba(NO₃)₂^{[31, 32]}. The presence of Ba ions could hinder the direct contact between crystal particles and impede crystal growth, leading to a reduction in both specific surface area and pore size^[33]. There were research findings indicating that specific surface area of photocatalyst was related with its photocatalytic performance. A larger specific surface area was beneficial for the contact between the catalyst and the reaction solution, thereby improving the photocatalytic performance of the catalyst.

The N₂ adsorption-desorption isotherms and pore size distributions of the LaCoO₃ catalysts with different Ba doping ratios were presented in Fig.6. According to the IUPAC classification, all catalysts exhibited Ⅳ type isotherms, characteristic of mesoporous materials. The presence of H3 hysteresis loops in the catalysts, with closure points in the p/p₀ between 0.1 and 0.2, indicated the presence of slit-shaped pores or plate-like particles. Furthermore, the pore size distributions revealed that all the catalysts possessed mesopores with sizes ranging from 2 to 50 nm. The gradual decrease in adsorption volume with increasing Ba doping ratio suggested a corresponding decrease in the specific surface area of the catalysts. This observation was consistent with the SEM analysis, which showed an increase in particle size and agglomeration with higher Ba content, leading to a reduction in the available surface area.

XPS was employed to investigate the elemental composition and valence states of the La_1−xBa_xCoO₃ (x=0.2, 0.3, 0.4, 0.5, 0.6) catalysts. The XPS survey spectra (Fig.7(a)) confirmed the presence of La, Co, Ba, O and C, consistent with the EDS analysis. The carbon signal (C 1s) was used as a reference for binding energy calibration.

The La 3d spectra in Fig.7(b) exhibited two characteristic peaks, La 3d_5/2 and La 3d_3/2 separated by approximately 4 eV due to spin-orbit coupling. Each of these peaks was further resolved into two components, arising from the transfer of electrons from the oxygen valence band to the empty La 4f orbital during the ionization process^[34]. The La 3d XPS spectra confirmed the presence of La in the trivalent oxidation state^[35], indicating that it did not participate in redox reactions as an active site. The Ba 3d spectra (Fig.7(c)) displayed two peaks, the binding energy centered at 795.1 and 779.65 eV ascribed to Ba 3d_3/2 and Ba 3d_5/2 respectively, with a separation of 15.4 eV, consistent with literature values^[36]. Each peak was further resolved into two components, separated by approximately 3 eV, corresponding to Ba in the bivalent oxidation state^[37−40]. The Co 2p spectra (Fig.7(d)) exhibited four peaks at 778.9, 781.6, 793.9, and 796.8 eV, attributed to Co³⁺ and Co²⁺ species, respectively. The peak at 790.0 eV was assigned to the satellite peak of Co²⁺, while the peak at 805.1 eV was ascribed to the satellite peak of Co species in Co₃O₄^[41−42].

Finally, the O 1s spectra (Fig.7(e)) was deconvoluted into three peaks located at 528.9, 531.3 and 532.8 eV, corresponding to lattice oxygen, oxygen vacancies, and surface-adsorbed oxygen, respectively^[43]. The presence of oxygen vacancies was noteworthy as it could significantly influence the catalytic activity of the perovskite materials.

The magnetic properties of the La_1−xBa_xCoO₃ catalysts were investigated by measuring their magnetic hysteresis loops at room temperature (Fig.8). As La and Co were known to exhibit magnetic behavior, the observed strong linear M-H curves passing through the origin are indicative of paramagnetic behavior in these catalysts^[44−46]. The modified LaCoO₃ catalyst displayed a relatively strong paramagnetism, with an increasing trend observed as the Ba doping ratio increased. Notably, the Ba-0.5 and Ba-0.6 catalysts exhibited the highest paramagnetic susceptibility among the samples, suggesting a potential correlation between Ba content and the density of unpaired electrons responsible for paramagnetism. While the observed paramagnetism may be associated with enhanced catalytic activity, it was not the sole determining factor, as other factors such as surface area and crystal structure also play crucial roles. Nevertheless, the strong paramagnetic nature of the catalysts presents an opportunity for their efficient recovery and recycling using magnetic separation techniques, thereby reducing costs and promoting the sustainability of the photocatalytic process.

2.2 Photocatalytic performance

The photocatalytic activity of LaCoO₃ and La_1−xBa_xCoO₃ catalysts with different Ba doping ratios were assessed by degradation of phenol under visible light irradiation. As shown in Fig.9(a), the degradation rate of phenol initially increased and then decreased as the Ba doping ratio increased, indicating an optimal Ba content beneficial for photocatalytic performance. The Ba-0.5 catalyst exhibited the highest photocatalytic activity, achieving the phenol degradation rate of 83.50% after 6 h irradiation at 20 ℃. In contrast, the Ba-0.2 catalyst showed the lowest degradation rate (28.09%), while the Ba-0.3, Ba-0.4 and Ba-0.6 catalysts achieved degradation rates of 36.92%, 54.07% and 63.40%, respectively, under the same conditions. To elucidate the role of the Ba-0.5 photocatalyst in phenol degradation and to differentiate the contributions of adsorption and photocatalysis, a comparative experiment was conducted with and without visible light irradiation. As depicted in Fig.9(b), in the absence of light, the Ba-0.5 catalyst adsorbed approximately 10% of phenol, and this adsorption capacity had no change over 6 h. Moreover, the degradation rate of phenol significantly increased to over 80% under visible light irradiation. The results demonstrated that the Ba-0.5 catalyst exhibited high photocatalytic activity towards phenol, and the adsorption played a minor role in the overall degradation process.

2.3 The degradation kinetics curve of phenol

To further corroborate the observed photocatalytic activities, the degradation kinetics of phenol by the La_1−xBa_xCoO₃ catalysts were investigated. The linear relationship between ln(C₀/C) and irradiation time (t) was given in Fig.10(a), indicated that the photodegradation of phenol follows pseudo-first-order kinetics. This kinetic model suggested that the rate of phenol degradation was directly proportional to its concentration, with the photocatalyst acting as a constant factor. The rate constant (k) for each catalyst could be determined from the slope of the linear fit in Fig.10(a)^[47].

$ \begin{array}{c}r=k{C}_{A}\end{array} $

(2)

$ \begin{array}{c}r=-\dfrac{d\left({C}_{A}\right)}{dt}\end{array} $

(3)

$ \begin{array}{c}-\dfrac{d\left({C}_{A}\right)}{dt}=k{C}_{A}\end{array} $

(4)

$ \begin{array}{c}-{\displaystyle\int }_{{C}_{A,0}}^{{C}_{A}}\dfrac{1}{{C}_{A}}={\displaystyle\int }_{{t}_{0}}^{t}k\cdot dt\end{array} $

(5)

$ \therefore \text{ln}\left({C}_{0}\text{/}C\right)={kt} $

(6)

The rate constants (k) for phenol degradation by the different La_1−xBa_xCoO₃ catalysts, derived from the slopes of the linear fits in Fig.10(a), were presented in Fig. 10(b). The Ba-0.5 catalyst exhibited the highest rate constant (0.0045 min⁻¹), demonstrating superior photocatalytic activity compared to the other catalysts. The rate constants for Ba-0.2, Ba-0.3, Ba-0.4 and Ba-0.6 were 0.0009, 0.0010, 0.0017 and 0.0023 min⁻¹, respectively. Notably, the Ba-0.5 catalyst exhibited a rate constant approximately 5, 4.5, 2.7 and 2.0 times higher than that of Ba-0.2, Ba-0.3, Ba-0.4 and Ba-0.6, respectively. This significant enhancement in photocatalytic activity at a La∶Ba ratio of 1∶1 (Ba-0.5) was consistent with the observed trend in phenol degradation efficiency, further confirming the optimal doping ratio for maximizing the catalytic performance of LaCoO₃.

2.4 Catalyst reusability

The reusability of the catalyst was a critical factor in assessing its overall effectiveness. To this end, the stability of the Ba-0.5 catalyst was evaluated by recyclability test. The photocatalyst was recycled by filtration, washing and drying for further evaluation. The recovered catalyst (100 mg) was introduced into a 100 mL phenol solution (10 mg·L⁻¹), and its degradation rate was measured using absorbance spectroscopy. As depicted in Fig.11(a), although the degradation rate of phenol displayed a slightly decrease over five cycles, it consistently remained above 70%. This result demonstrates the Ba-0.5 catalyst maintains substantial photocatalytic activity after repeated use. XRD characterization and comparative analysis were then performed on Ba-0.5 before and after five repeated experiments, as shown in Fig.11(b). There was no obvious change could be found in the XRD patterns compared to the used and fresh Ba-0.5 catalyst, suggesting that the crystal structure of Ba-0.5 was well-preserved after five cycles.

2.5 Photocatalytic mechanism

To elucidate the photocatalytic mechanism of the modified Ba-0.5 catalyst in phenol degradation^{[6, 8, 48−49]}, the roles of various active radicals in the phenol decomposition process were investigated. Four trapping agents isopropanol (IPA), p-benzoquinone (BQ), ammonium oxalate (AO), and silver nitrate (AgNO₃) were introduced into the phenol solution under consistent experimental conditions, the agents served as scavengers for hydroxyl radicals (•OH), superoxide radicals (${\text{•}{\mathrm{O}}}_{\mathrm{2}}^- $), holes (h⁺), and photogenerated electrons (e⁻), respectively. The results of these trapping experiments were illustrated in Fig.12.

The perovskite-type LaCoO₃ photocatalyst was known to exhibit catalytic activity under visible light. Upon irradiation with a xenon lamp, the surface of the LaCoO₃ catalyst absorbs photons with energy exceeding its bandgap. This absorption led to the formation of electrons (e⁻) in the conduction band and holes (h⁺) in the valence band. These charge carriers subsequently reacted with water and oxygen molecules in the system, generating highly reactive species such as hydroxyl radicals (•OH) and superoxide radicals (${\text{•}{\mathrm{O}}}_{\mathrm{2}}^- $) ^[50−59]. These radicals could then interact with phenol molecules adsorbed on the catalyst surface, breaking down the organic structure and transforming the phenol into non-polluting small molecules, effectively achieving phenol degradation. The mechanism was as follows:

$ \mathrm{\ LaCoO}_{\mathrm{3}}\mathrm{+}\mathit{h\nu\to}\mathrm{e}^-\mathrm{(LaCoO}_{\mathrm{3}}\mathrm{)+h}^{\mathrm{+}}\mathrm{(LaCoO}_{\mathrm{3}}\mathrm{)} $

$ \mathrm{\ h}^{\mathrm{+}}\mathrm{+H}_{\mathrm{2}}\mathrm{O}\mathit{\to}\mathrm{\text{•}OH+H}^{\mathrm{+}} $

$ \mathrm{\ h}^{\mathrm{+}}\mathrm{+OH}^-\mathit{\to}\mathrm{\text{•}OH} $

$ \mathrm{e}^-\mathrm{+O}_{\mathrm{2}}\mathit{\to}\mathrm{\text{•}O}_{\mathrm{2}}^- $

$ \mathrm{H}_{\mathrm{2}}\mathrm{O+\text{•}O}_{\mathrm{2}}^-\mathit{\to}\mathrm{HO}_{\mathrm{2}}\mathrm{\text{•}+OH}^- $

The results demonstrated that after 6 h of reaction, the phenol degradation rate was 83.5% in the system without a trapping agent. In contrast, the degradation rates were 26.4%, 61.3%, 65.7% and 80.9% in the systems containing IPA, BQ, AO and AgNO₃, respectively. This indicated that photo-generated electrons (e⁻) had a negligible effect on phenol degradation, while hydroxyl radicals (•OH), superoxide radicals (${\text{•}{\mathrm{O}}}_{\mathrm{2}}^- $), and holes (h⁺) played varying roles in the process. Notably, •OH radicals were the primary contributors to phenol degradation, with ${\text{•}{\mathrm{O}}}_{\mathrm{2}}^- $ and h⁺ having relatively minor influences. These reactive species interacted with phenol molecules adsorbed on the catalyst surface, leading to the decomposition of phenol into smaller molecules through three distinct degradation pathways, ultimately achieving photocatalytic degradation of the organic pollutant. The proposed photocatalytic degradation mechanism of phenol was depicted in Fig.13(a), and the La_1−xBa_xCoO₃ mechanism diagram was shown in Fig.13(b).

2.6 Synthesis mechanism of La_1−xBa_xCoO₃

The La_1−xBa_xCoO₃ catalyst was prepared by the citric acid complexation-hydrothermal combined method, and the synthesis mechanism was elucidated through a series of assumptions and rigorous deductions.

Complexation of citric acid with metal ions:

$ \mathrm{C}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{8}} \mathrm{O}_{ \mathrm{7}} \mathrm{+La}^{ \mathrm{3+}} \mathrm{\to LaC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{5}} \mathrm{O}_{ \mathrm{7}} \mathrm{+3H}^{ +} $

$ \mathrm{C}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{8}} \mathrm{O}_{ \mathrm{7}} \mathrm{+Co}^{ \mathrm{2+}} \mathrm{\to CoC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{6}} \mathrm{O}_{ \mathrm{7}} \mathrm{+2H}^{ \mathrm+} $

$ \mathrm{C}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{8}} \mathrm{O}_{ \mathrm{7}} \mathrm{+Ba}^{ \mathrm{2+}} \mathrm{\to BaC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{6}} \mathrm{O}_{ \mathrm{7}} \mathrm{+2H}^{ \mathrm+} $

Hydrolysis reaction:

$ \mathrm{LaC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{5}} \mathrm{O}_{ \mathrm{7}} \mathrm{\to LaC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{3}} \mathrm{O}_{ \mathrm{6}} \mathrm{+H}_{ \mathrm{2}} \mathrm{O} $

$ \mathrm{2LaC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{3}} \mathrm{O}_{ \mathrm{6}} \mathrm{\to La}_{ \mathrm{2}} \mathrm{C}_{ \mathrm{12}} \mathrm{H}_{ \mathrm{6}} \mathrm{O}_{ \mathrm{12}} $

$ \mathrm{CoC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{6}} \mathrm{O}_{ \mathrm{7}} \mathrm{\to CoC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{4}} \mathrm{O}_{ \mathrm{6}} \mathrm{+H}_{ \mathrm{2}} \mathrm{O} $

$ \mathrm{2CoC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{4}} \mathrm{O}_{ \mathrm{6}} \mathrm{\to Co}_{ \mathrm{2}} \mathrm{C}_{ \mathrm{12}} \mathrm{H}_{ \mathrm{8}} \mathrm{O}_{ \mathrm{12}} $

$ \mathrm{BaC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{6}} \mathrm{O}_{ \mathrm{7}} \mathrm{\to BaC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{4}} \mathrm{O}_{ \mathrm{6}} \mathrm{+H}_{ \mathrm{2}} \mathrm{O} $

$ \mathrm{2BaC}_{ \mathrm{6}} \mathrm{H}_{ \mathrm{4}} \mathrm{O}_{ \mathrm{6}} \mathrm{\to Ba}_{ \mathrm{2}} \mathrm{C}_{ \mathrm{12}} \mathrm{H}_{ \mathrm{8}} $

Oxidation-reduction creaction:

$ \mathrm{La}_{ \mathrm{2}} \mathrm{C}_{ \mathrm{12}} \mathrm{H}_{ \mathrm{6}} \mathrm{O}_{ \mathrm{12}} \mathrm{+9O}_{ \mathrm{2}} \mathrm{\to La}_{ \mathrm{2}} \mathrm{O}_{ \mathrm{2}} \mathrm{CO}_{ \mathrm{3}} \mathrm{+CO}_{ \mathrm{2}} \mathrm{+H}_{ \mathrm{2}} \mathrm{O} $

$ \mathrm{La}_{ \mathrm{2}} \mathrm{O}_{ \mathrm{2}} \mathrm{CO}_{ \mathrm{3}} \mathrm{\to La}_{ \mathrm{2}} \mathrm{O}_{ \mathrm{3}} \mathrm{+CO}_{ \mathrm{2}} $

$ \mathrm{3Co}_{ \mathrm{2}} \mathrm{C}_{ \mathrm{12}} \mathrm{H}_{ \mathrm{8}} \mathrm{O}_{ \mathrm{12}} \mathrm{+28O}_{ \mathrm{2}} \mathrm{\to 2Co}_{ \mathrm{3}} \mathrm{O}_{ \mathrm{4}} \mathrm{+36CO}_{ \mathrm{2}} \mathrm{+12H}_{ \mathrm{2}} \mathrm{O} $

$ \mathrm{Ba}_{ \mathrm{2}} \mathrm{C}_{ \mathrm{12}} \mathrm{H}_{ \mathrm{8}} \mathrm{O}_{ \mathrm{12}} \mathrm{+9O}_{ \mathrm{2}} \mathrm{\to 2BaO+12CO}_{ \mathrm{2}} \mathrm{+4H}_{ \mathrm{2}} \mathrm{O} $

Decomposition reaction:

$ \mathrm{La}_{ \mathrm{2}} \mathrm{O}_{ \mathrm{3}} \mathrm{+Co}_{ \mathrm{3}} \mathrm{O}_{ \mathrm{4}} \mathrm{+BaO\to LaBaCoO}_{ \mathrm{3}} \mathrm{+CoO} $

3 Conclusions

A-site Ba-doped La_1−xBa_xCoO₃ materials were successfully synthesized using a citric acid complexation-hydrothermal synthesis combined method. The physicochemical structures and properties of the resulting samples were characterized using XRD, SEM, BET and XPS. Among the synthesized materials, La_0.5Ba_0.5CoO₃ exhibited the highest phenol degradation rate (83.5%), following pseudo-first-order kinetics.

Various characterization techniques confirmed the successful incorporation of Ba into the LaCoO₃ crystal structure. While the morphology remained largely unchanged, a slight degree of agglomeration was observed in the modified catalyst. The specific surface area of the catalysts was relatively small and decreased with increasing Ba doping. Magnetization, as assessed by M-H curves, appeared to correlate with catalytic activity, with stronger magnetic intensity associated with better phenol photodegradation. The strong paramagnetism of the catalyst suggested good recyclability and potential cost savings. Notably, the Ba-0.5 catalyst retained high photocatalytic activity after five cycles, with XRD analysis confirming minimal structural changes. The XPS characterization results showed that Ba-0.5 had the highest oxygen content, and its oxygen vacancies made it positively charged, leading to an extension of the positron lifetime, reducing the recombination of electron-hole pairs, and enhancing the photocatalytic activity. Additionally, the magnetic properties of the photo-catalyst were also related to its photocatalytic activity. A photocatalyst with certain magnetic properties could better adsorb the light source required for photocatalytic reactions, thereby improving photocatalytic activity. Furthermore, magnetism could also help in the recovery and reuse of the photocatalyst, reducing catalyst loss, and ultimately improving the economic and environmental friendliness of photocatalysis. The MPMS characterization results showed that Ba-0.5 has high magnetism, which helped to enhance its catalytic activity. In conclusion, Ba-0.5 had the highest photocatalytic activity.

The photocatalytic mechanism study suggested that electron-hole pairs convert surface-adsorbed oxygen into highly active free radicals (•OH and ${\text{•}{\mathrm{O}}}_{\mathrm{2}}^- $). Photogenerated holes migrate to the catalyst surface, where they participated in redox reactions with adsorbed phenol. The degradation of phenol was primarily driven by •OH radicals, with ${\text{•}{\mathrm{O}}}_{\mathrm{2}}^- $ and h⁺ playing a supporting role. Under visible light irradiation, phenol was ultimately degraded into carbon dioxide, water, and small inorganic molecules through three distinct pathways.