Photocatalytic Dye Decomposition over CaMnO_3−δ and Pr_0.5Ca_0.5MnO₃: A Combined XPS and DFT Study

Abstract

In the field of environmental sustainability, the development of highly efficient photocatalytic under a wide wavelength range with band engineering is regarded as a promising strategy to enhance photocatalytic dye degradation. Here, we report on CaMnO_3−δ and Pr_0.5Ca_0.5MnO₃ perovskite materials prepared by a sol-gel combustion method. From X-ray photoelectron spectroscopy (XPS), the particle surfaces of both compounds are oxygen deficient, while the surface hydroxyl and carbonyl groups’ adsorption on the surface of Pr_0.5Ca_0.5MnO₃ particles is more pronounced. FT-FIR spectroscopy has been used to investigate the covalent bonds and oxygen vacancy characteristics. Photocatalytic activities were investigated by the degradation of methylene blue and methyl orange under UV light. It was observed that both dye molecules are more degraded over CaMnO_3−δ. The underlying mechanisms behind the photoexcitation and degradation process are established via the Spin-polarized Density Functional Theory (DFT).

1. Introduction

Today, the serious environmental problems due to the growth of fossil fuel consumption have attracted special attention to developing efficient and nontoxic materials to assist in solving these issues. Photocatalysis is a promising technology in the field of clean energy applications to prevent organic pollutants from potentially causing environmental degradation. Many efforts have been made on various materials to develop new semiconductor photocatalysts [1,2] and find out the photocatalytic mechanism. Among the studied materials, rare earth ABO₃ perovskite compounds with unusual physical and chemical properties have shown photophysical properties due to the polaron formations inside the crystal lattice [3,4]. The effects of doping and nanosized crystalline are effective parameters in these physical and chemical properties. The perovskite structure consists of oxygen octahedra, where the B cation (Mn ion) is the atom in the center of the octahedron, and the A cation (Pr and Ca ions) is the atom outside the octahedron. The position of oxygen atoms around the transition metal cations B in the ABO₃ perovskites determines the exciting properties. The transfer of electrons between the B-sites is not direct transfer but through the intervention of the oxygen atoms surrounding the transition metal atoms in the B-site [5,6]. The electronic and magnetic properties of the perovskite change by the distortion of the octahedral reflects the importance of the mixed-valence states of the transition metal at B-site and the corner-shared octahedral BO₆ in these materials [7,8,9]. Moreover, the small ionic radii of the A cations cause BO₆ octahedral tilting. This tilting turns the cubic lattice structure to the lower symmetry orthorhombic crystal structure [10,11]. In perovskite compounds, octahedral tilting influences the electronic structure, electron or hole transport, and dielectric properties [12,13].

The perovskite-like materials such as tantalate, titanate, ferrite, and manganites have exhibited visible light photocatalytic activity because of the exclusive electronic properties correlated with the crystal structures [14,15,16]. The optimized bandgap in such materials, the doping concentration of the divalent element, explains and enhances photocatalytic performance and the separation of charge carriers. These compounds represent the bandgap values of the produced visible-light absorption as well as the UV region [17]. The potentials of optimizing the bandgap and the lattice distortion to capture charge carriers presented in such materials affect the efficiency of photocatalysts [18]. A simple member of the manganite compounds, CaMnO₃, represents the bandgap of about 1.6 eV between the O 2p valence band and Mn 3d conduction bands. The Mn 3d orbitals split into the triply degenerated Mn t_2g and doubly degenerated Mn e_g states originating from the crystal field splitting [6,19]. Hence, there are two relative electron transitions in the energy range of 1 eV to 6 eV. The UV transition at the higher energies of 4–5 eV corresponds to the transitions between the O 2p states and the minority-spin states, while the lower energies are assigned to the transitions between the O 2p and majority-spin states. These transitions are of interest to the polaron physics of manganites. In addition, manganese-containing compounds can be considered promising candidates for functional water oxidation [20,21,22,23] inspired by the natural photosynthesis process in which the Ca₂Mn₃O₈, CaMn₂O_4, and CaMnO₃ clusters are identified as the catalytic site for the four-electron involved water oxidation [24].

Doping CaMnO₃ by the trivalent ions (Pr) inserts the extra electrons into the antibonding e_g states of the Mn 3d orbitals. These electrons form polarons and cause octahedral distortion [10]. Therefore, octahedral distortion affects the conduction band distributions as well as the valance band top [25]. Oxygen vacancy is another approach to modifying the conduction band distribution of electronic states [26]. Water oxidation and O₂ reduction depend on the photoinduced reactions and potential levels of the valence and conduction bands with respect to the oxidation and reduction of potential levels. Thus, doping and deficiencies in the lattice offer great potential for band structure engineering and consequently designing new photocatalysts. The photocatalytic properties and the relative mechanism for the transitions, particularly at lower energies (visible region), draw the attention to sunlight. The effects of UV transitions in perovskites corresponded to the transition between the O 2p states and minority-spin states of the conduction band are still missing. Here, oxygen deficiencies, as well as doping agents, have been used to prepare the CaMnO₃ manganite structure with Mn³⁺ and Mn⁴⁺ coexistence to investigate the mechanism of the photocatalytic activities in the UV radiation region.

In this work, nanosized CaMnO_3−δ (CMO) and Pr_0.5Ca_0.5MnO₃ (PCMO) were characterized by FT-FIR spectroscopy and XPS and the photocatalytic activity in the UV region for the decomposition of methyl orange (MO) and methylene blue (MB) was investigated. To better understand the photocatalytic behavior of ABO₃ perovskites, the band structures of the compounds were discussed concerning the photocatalytic activities. To shed light on the experimental finding, first-principle calculations based on the density functional theory (DFT) were carried out to assess the influences of oxygen vacancy on the electronic density of states (DoS).

2. Materials and Methods

2.1. Preparation of Samples

The CMO and PCMO nanoparticles were prepared by gel combustion method. Calcium nitrate tetrahydrate Ca(NO₃)₂·4H₂O (99%), manganese nitrate tetrahydrate Mn(NO₃)₂·4H₂O (99.5%), praseodymium nitrate hexahydrate Pr(NO₃)₃·6H₂O (99.9%) and gelatin were used. In order to prepare the primary sol, appropriate amounts of nitrates were dissolved in distilled water, stirring at room temperature for 20 min. Then, the gelatin solution was added to the sol and stirred at 60 °C for 2 h. The final gel was obtained by heating the sol at 90 °C. Finally, the brownish gel was dried at 200 °C for 5 min. The nanoparticles were prepared after calcining the samples at 900 °C for 5 h. The preparation methods, as well as the structural analysis by X-ray diffraction and Rietveld refinement, have been published recently [27].

2.2. Characterizations and Photocatalysis Experiments

The structural, microstructural, electrocatalytic, and in situ investigations of CMO and PCMO nanopowders have been previously reported [27,28]. Here, the infrared optical density O_d for both compounds was obtained in the 150–700 cm⁻¹ using a far-infrared Fourier spectrometer (FT-FIR). The powders were pressed into pellets under a vacuum followed by finely milling and mixing with CsI in the ratio of 1:100 in weight.

X-ray Photoelectron Spectroscopy (XPS) was performed in a custom-designed system with an Al-Kα X-ray source (1486.6 eV), steps of 0.1 eV and 20 eV pass energy. Chemical compositions of particles have been investigated using core-level photoemission spectra from Ca 2p, Pr 3d, Mn 2p, and O 1s regions collected in normal emission at room temperature. The binding energies were referenced to Au-4f at 84 eV.

In order to collect the photoemission spectra, the monochromator and exit slit was set to c_ff = 2.25 and 111 μm, respectively. The step size for Ca 2p and O 1s spectra was 20 meV. All spectra were collected using pass energy of 20 eV and a dwell time of 100 ms. The intensities have been scaled and normalized with reference to impinging photon flux. A blend of linear and Shirley-type backgrounds was subtracted. Experiments have been conducted according to the protocol given in Ref. [29].

The photocatalytic reaction in the ultraviolet region was performed with a 200 W HBO Mercury short-arc lamp as an ultraviolet light source with a peak irradiance at 365 nm and intensity of 50 mW/cm² at the sample position. The concentration of MB and MO dyes was chosen as 5 ppm. The amount of photocatalyst was 50 mg in 50 mL of deionized water. The solution was stirred in darkness for 30 min to complete the adsorption–desorption equilibrium between the dye and the catalyst. The solution temperature was kept at 25 °C throughout the experiment. After darkness, solutions were exposed to light. Aliquots were taken at the time interval of 20 min. The solution was then centrifuged, and its absorption spectrum was recorded by UV-Visible spectrometer.

2.3. Theoretical Method

Calculations were performed based on the Spin-polarized Density Functional Theory (DFT) [30]. The exchange-correlation functional is approximated with the HSE06 functional [31] to obtain a proper description of the Mn 3d orbitals, as implemented in the Vienna Ab initio Simulation Package (VASP) [32,33]. The typical value of 0.2 is employed as a mixing factor. We used 2 × 2 × 2 supercells of the primitive perovskite cell for CaMnO₃, corresponding to 20 atoms. The total energy was sampled on a well-converged 4 × 4 × 4 k-point grid together with projector-augmented wave theory [34] and a 520 eV plane-wave cutoff. The total energy is converged within 1 × 10⁻⁵ eV per supercell. For Pr_0.5Ca_0.5MnO₃, a larger supercell has been considered with the CE-type order, corresponding to 80 atoms, sampled on a 2 × 2 × 2 k-point grid. Oxygen vacancy in the perovskite is assessed in the low-vacancy limit, one O vacancy per supercell. The structures were allowed to relax until the convergence of the forces on the atom were lower than 1 × 10⁻² eV Å⁻¹. The minimum distance between O vacancies considered as at least 10 Å minimized the fictitious interactions across periodic boundaries [6].

3. Results and Discussion

3.1. Structural and Microstructural Properties

The XRD patterns of the single CMO and PCMO are shown in Figure 1. We identified the orthorhombic space group of Pnma (no. 62) for the crystal structure. While the ionic radius of Pr³⁺ (1.13 Å) is close to that of Ca²⁺ (1.12 Å), the lattice constants enlarge with an increase in Pr content, as has already been published [27]. The lattice expansion is because of the Mn–O bond length increases caused by electron insertion into the antibonding Mn e_g orbitals. The surface morphology has been investigated by scanning electron microscopy (SEM), and a representative SEM image for the micrographs of undoped and Pr-doped CaMnO₃ is shown in Figure 2. The samples were composed of nanoparticles with an average particle size of 70 nm. Phase identification, lattice parameters, and microstructural analysis were published in previous work [27].

3.2. Spectroscopic Analysis

In our previous work [27], X-ray absorption spectroscopy of the Mn-L edge gave the Mn valence of the undoped calcium manganite about 2.95, confirming the oxygen deficiency in the structure, which is very common in Ca-rich manganites. The structural properties of the samples obtained by Rietveld refinement indicated that Mn–O bond length increases with the increase of e_g electron occupation of the antibonding Mn (e_g)–O (2p) levels. Here, FT-FIR spectroscopy has been used to investigate the covalent bond characteristics and the position of oxygen vacancies. The infrared optical densities of polycrystalline Ca_1−xPr_xMnO₃ (x = 0.00 and 0.50) at 300 K are shown in Figure 3. The numerous peaks in the spectra relating to the infrared active transverse optical TO modes show strong deviations from a cubic symmetry to orthorhombic in both compounds. Two strong, broad peaks are contributing several smaller peaks at 414 cm⁻¹ and 596 cm⁻¹ have been observed for Pr_0.5Ca_0.5MnO₃. This is due to the lifting of the e_g degeneracy (Jahn–Teller distortion) because of the strong elongation of the Mn–O(1) bond and consequently splitting one IR absorption band into two adjacent bands [35]. The spectra shown in Figure 3 appear qualitatively similar in structure, exhibiting three main groups centered around 200, 400, and 600 cm⁻¹. The low-energy modes are the bending band and are expected to be sensitive to the MnO₆ octahedra tilting distortions. On the other hand, the high-energy modes (centered around 600 cm⁻¹) are thought to involve mainly stretching vibrations of MnO₆ octahedra. The frequencies of these modes are expected to be directly related to the interatomic distances of Mn–O bonds. Therefore, the structure of the bending band and Mn–O bonds in CMO with oxygen vacancies should appear significantly reduced compared to the fine structure of PCMO.

The phonon modes below 270 cm⁻¹ for all samples correspond to mixed vibrations of Ca/Pr atoms and octahedral [35]. Because the strong orthorhombic lattice is strongly distorted, none of the modes observed above 280 cm⁻¹ can be considered as purely bending or stretching, as these modes considerably depend on the changes of both the Mn–O–Mn bond angles and the Mn–O bond lengths [36]. The phonon modes between 280 and 350 cm⁻¹ correspond to the motions in which the Mn displacements are comparable with O atoms. For higher frequencies, the displacements of Mn and Ca/Pr atoms and phonons involve mostly the motions of oxygen atoms [35,36]. As shown in Figure 3 for CaMnO_3−δ, the peaks relating to the vibration of apical and in-plane oxygen are indexed with (1) and (2), respectively. For the modes at 354, 368, 396, 514, and 560 cm⁻¹, the in-plane oxygen vibrations dominate. For those at 430, 460, and 640 cm⁻¹, the motions of the oxygen atoms in apical sites play the main role [35]. The infrared absorption spectrum of polycrystalline CaMnO₃ reported and thoroughly discussed by Fedorov et al. [35] is similar to our results for CaMnO_3−δ with a significant difference. The absorption bands corresponding to the vibration of apical oxygen atoms are so close to the results reported, while the vibration of Mn–O(2) considerably shifts compared to the results obtained for CaMnO₃ which was free from oxygen vacancies. This may be due to the fact the oxygen vacancies are more or less localized in in-plane sites and consequently can affect the ion conductivity and the photocatalytic properties.

Figure 4a shows the survey XPS spectra obtained for CaMnO_3−δ and Pr_0.5Ca_0.5MnO₃. As shown in this figure, no other impurity elements were observed. Figure 4b,c show the doublet XPS spectrum of Ca 2p at binding energies around 345.5 and 349 eV assigning to Ca 2p_3/2 and Ca 2p_1/2, respectively. Ca²⁺ exhibits a binding energy (BE) shift toward higher energies with increasing Pr content due to the changes in the nearest neighbors of Ca atoms and, consequently, the electronic structure of the Ca atoms. In the case of the Ca 2p_3/2 component, it can be seen that the peak is broadened in Pr_0.5Ca_0.5MnO₃ compared to undoped CaMnO_3−δ. This can be due to the formation of CaCO₃ and/or CaO at the oxide surface due to Ca segregation [37]. The narrow scan spectrum of the oxygen 1s core level of samples is shown in Figure 4d,e. The oxygen peaks corresponding to O 1s can be resolved into two components at around 529 and 532.1 eV. This doublet peak of O 1s agrees with the earlier reports on perovskite oxides [37,38,39]. This doublet peak corresponds to the chemical shifts in the oxygen core level arising out of two kinds of chemical bonding. The lower binding energy component is assigned to the oxygen in the perovskite lattice (metal-oxygen bonds). The next component at around 531.9 eV can be associated with CaO and/or CaCO₃ formed at the surface due to Ca segregation. In PCMO, the peak at BE energies of 532 eV has a higher intensity than the 529 eV line compared to CMO, indicating that Pr substitution in Ca sites helps to more occurrence probability of surface Ca/Pr–O bonds and Ca segregation [37].

The mixed-valence of surface manganese in perovskite manganites can be determined by analyzing the manganese doublet spectra corresponding to the spin-orbit split of manganese 2p peaks (Mn 2p_3/2 and Mn 2p_1/2) around 642.1 eV and 653.5 eV, respectively (Figure 5). Quantitative deconvolution and curve fitting results for Mn 2p_3/2 give evidence for the existence of mixed-valence states of manganese. Here, the higher binding energy component at 642.6 eV relates to Mn⁴⁺, and the other component at 641.5 eV is assigned to Mn³⁺ [40]. The mixed-valence Mn ratios of Mn⁴⁺/Mn³⁺ of all samples are approximately determined as 1.37 and 0.92 for CaMnO_3−δ and Pr_0.5Ca_0.5MnO₃, respectively. By comparing this ratio obtained by XPS from the surface of CaMnO_3−δ nanoparticles from one side and Mn valence estimated from XAS [27], it is observed that the surface of particles is more oxygen-deficient than the bulk. However, the ratio of Mn⁴⁺/Mn³⁺ for both surface and bulk of Pr_0.5Ca_0.5MnO₃ particles is near one, which is consistent with the Mn valence obtained by XAS. These shreds of evidence indicate that the surface of Pr_0.5Ca_0.5MnO₃ particles is more stable than CaMnO_3−δ, consistent with in-situ HRTEM investigations reported before [28]. The third peak in Figure 5 (green line) corresponds to the satellite structure observed in about 5 eV higher binding energy than the Mn 2p_3/2 clearly associated with ligand 2p to Mn 3d charge transfer [41]. Note that the satellite component comes from Mn⁴⁺, as shown in Figure 5b. In the case that the interaction between the 2p core hole and the correlated 3d valence electrons is sufficiently strong, satellites are present in the photoemission spectra accompanying the main lines.

3.3. Photocatalytic Degradation Analysis

The photodegradation of MB and MO is given in Figure 6, which was carried out by degrading MB and MO in an aqueous solution under irradiation of a 200W HBO Mercury short-arc lamp as an ultraviolet light source. The decoloration rate of MB in the presence of CMO reached 43% at 180 min, which was close to 41% obtained for PCMO. However, the MO degradation performance of CMO and PCMO was limited to 23% and 21% within three hours, respectively.

Photocatalytic oxidation of organic pollutants follows Langmuir–Hinshelwood kinetics in which only the first-order form (−ln(C₀/C) = k_appt) is accounted for when the reactant concentration is very small [42]. In this equation, k_app is the apparent first-order reaction constant, and C₀ and C are the reactant concentrations at the initial and later times, respectively. The photocatalytic activities of CMO and PCMO were evaluated by comparing the k_app obtained from the plots of −ln(C₀/C) against irradiation time (insets of Figure 5) and listed in

Table 1. Apparent first-order reaction constants obtained for MO and MB, and the band structure parameters determined by ab initio studies.

Material	Particle Size (nm) [27]	Specific Surface Area (cm2/mg)	kapp for MB (10−3 min−1)	kapp for MO (10−3 min−1)
CMO	70	81.63	3.98	2.56
PCMO	64	49.96	3.42	1.81

.

CMO and PCMO exhibited apparent rate constants of 3.98 × 10⁻³ min⁻¹ and 3.42 × 10⁻³ min⁻¹ for MB decomposition, respectively, showing higher activities than the values of 2.56 × 10⁻³ min⁻¹ and 1.81 × 10⁻³ min⁻¹ obtained for MO degradation. In addition, the photocatalytic activity of nanosized CMO is slightly better than PCMO.

Here, the main factors, including specific surface area, amount of chemisorbed oxygen at the surface of the particles, band structure, and electron-hole pair recombination rate, play significant roles in the photocatalytic activities of CMO and PCMO samples. The specific surface area has been obtained in our previous report [27] and given in Table 1. The higher specific surface area of CMO in comparison with PCMO, can provide a higher density of active sites [43,44]. In addition, the diffusion rate of the photogenerated electron-hole pair must be longer than the particle size to avoid recombination [45]. It should be noted that the recombination rate depends on the crystal phase and doping level. Nevertheless, enhanced photo-catalytic performance through the high surface-area-to-volume ratio of nanostructured PCMO can be achieved without detriment to the rates of charge carrier recombination in the composites. Consequently, the recombination rate of the carriers on the surface of the photocatalyst decreases with the decrease in particle size.

In addition to the surface area, the amount of chemisorbed oxygen usually associated with the mixed-valence states of the transition metal ion B is correlated with the photocatalytic activity [46,47,48]. It should be noted that the surface measured under UHV conditions can be restored by annealing at about 120 °C in 0.1 mbar O₂.

Molecular oxygen has a great tendency to be adsorbed on a surface vacancy site, and consequently, a surface-adsorbed O ad-atom is formed. As shown in Figure 4b, the content of chemisorbed oxygen observed on the surface of PCMO particles is considerably higher than CMO, while the amount of oxygen vacancies created on the surface and bulk of CMO is much more pronounced. This may be due to the fact that in the ABO₃ perovskites, the AO-terminated facets showed stronger binding to the adsorbed oxygen [49]. In addition, our FT-FIR results about CMO show that the oxygen vacancies are more or less localized in BO₂ sites rather than AO. Thus, the amount of chemisorbed oxygen more constructively affects the photocatalytic activity of PCMO in comparison with CMO.

While all parameters play a role in photocatalytic activities, the activity can be described dominantly through the factors such as geometry and the electronic structure of the perovskites. The potential levels configuration of the reduced conduction band and the oxidized valence band have to be compared with the O₂/O₂⁻ and the OH/H₂O potential levels, respectively [50].

To clarify the photodegradation mechanism of MB by CMO under UV light, several scavengers were used. Generally, during the photodegradation of dyes, different reactive species, such as OH and O₂⁻, are generated in addition to the e⁻/h⁺ pair. For example, the free electrons reduce the dissolved oxygen, resulting in the formation of superoxide ions, while the holes may react with H₂O and OH⁻ to produce hydroxyl radicals [51]. The scavengers used in this work are EDTA for holes, K₂S₂O₈ and AgNO₃ as electron scavengers, sodium azide (NaN₃) for singlet oxygen (¹O₂), DMSO for OH_bulk, sodium iodide (NaI) for OH_ads, and tert-butanol as a free OH radical scavenger [52]. If the photodegradation of MB by the catalyst is performed because of any of the reactive species, the reaction is slowed down or inhibited in the presence of the corresponding scavenger [52]. For the sake of comparison, the MB degradation in the absence of a catalyst under light exposure was investigated for possible self-degradation of MB. Moreover, the MB degradation by CMO without using a scavenger was carried out. Figure 7 shows the variation of C/C₀ for MB as a function of exposure time by adding different scavengers into the photocatalytic system. As this figure shows, no degradation in the absence of the photocatalyst for MB under light irradiation was observed. It means that MB is a photo-stable dye during our experiments. Moreover, this figure shows that the dye was degraded 43% within 180 min in the absence of any scavenger. Adding NaI, tert-butanol, DMSO, and EDTA had no considerable influence on the photodegradation process. Thus OH radicals and holes are not the main active species in MB photodegradation. As Figure 7 shows, the degradation efficiency of MB over CMO significantly decreases with the addition of NaN_3, indicating that ¹O₂ is the main active species during the photocatalytic degradation process. Since the photocatalytic degradation efficiency decreases in the presents of K₂S₂O₈ and AgNO₃, electrons play a supplementary role. A possible mechanism for the degradation based on our radical scavenger results is as follows. Under light irradiation, electrons are excited from the valance band (VB) to the conduction band (CB) of CMO nanoparticles. Electrons react with dye and dissolved oxygen molecules. The excited electrons reduce the dissolved oxygen, resulting in the formation of singlet oxygen ions. These active singlet oxygen ions and electrons degrade MB dye, which was adsorbed on the surface of nanoparticles. Note that the interfacial modification and composition manipulation by coating provides an efficient way for stabilizing and improving photocatalytic activity [53,54].

3.4. Electronic Structure of CaMnO_3−δ and Pr_1−xCa_xMnO₃

In order to obtain the potential levels of the conduction band Mn t_2g↓ and e_g↑, the theoretical calculation of the electronic band structures of CMO and PCMO was carried out with the HSE06 functional. CaMnO₃ is the simple member of the Pr_1−xCa_xMnO₃ manganite. This compound has an orthorhombic perovskite structure. Mn and O ions form a network of corner-sharing MnO₆ octahedra with a formal valence of 4+ for the Mn cations. Thus, the Mn atoms have a 3d³ configuration. It means three electrons in the t_2g orbitals and a strong crystal field splitting with the empty e_g states. Ca donates its two valence electrons to the valence band of the O 2p character. In the CaMnO₃, the size of the Ca ions is sufficiently small so that the MnO₆ octahedra tilt increases the ionic attraction. The orientation of the tilt axis is determined by a bond angle force at the oxygen bridge.

The calculated DoS of stoichiometric CaMnO₃ is shown in Figure 8a. The valance band is dominated by O p states (red) with the contribution of Mn d orbitals (green and yellow) at the bottom part of the valence band. Above the valence band, Mn d and Ca d states (blue) form the conduction band. The empty Mn d states are associated with different spin orientations according to their relative electron population, namely majority Mn e_g states (yellow) from 2 eV to 4 eV and minority Mn t_2g (green) and Mn e_g stats (yellow) from 4 eV to 7 eV.

In CaMnO₃, each O atom at apical and in-plane sites is surrounded by the two Mn atoms. The Mn–O bond lengths are similar in this structure, so O cages are not Jahn–Teller distorted.

The formation of an oxygen vacancy leaves two Mn ions under coordination, and as suggested by FT-FIR analysis, the O-vacancies are located at in-plane sites. The DoS for the supercell with the oxygen vacancy in the neutral charge state is shown in Figure 8b. As shown in this figure, the oxygen vacancy creates a deep level in the bandgap. This state is a vacancy-assisted polaron that originates from the majority spin direction of Mn e_g states. The Fermi level shifts upward to the top part of the mid-gap states. More vacancy can separate more states from the bottom part of the Mn e_g state in the conduction band to the top part of the valance band, as shown in Figure 8b.

In Pr_0.5Ca_0.5MnO₃, which is oxygen stoichiometric as reported previously, each Mn ion is sixfold coordinated with O atoms which form an octahedron cage around the Mn ion. However, in the half-doped system, Pr_0.5Ca_0.5MnO₃, the Zener polaron forms, which is characterized by an electron shared by two ferromagnetically coupled Mn neighbors. Characteristic of a Zener polaron are two neighboring Mn sites, both having a Jahn–Teller expansion along the axis of the pair. In Pr_0.5Ca_0.5MnO₃, the lower Jahn–Teller band is itself split into two, of which only one is occupied. The origin of the splitting of the lower Jahn–Teller band is due to the formation of an antibond with the bridging oxygen ion [55,56].

The DoS for the half-doped system is shown in Figure 8c. The filled majority Mn e_g states are located on top of the valence band, and the empty majority Mn e_g states are located at the bottom part of the conduction band. The oxygen vacancy adds electrons in the unoccupied majority Mn e_g states in the half-doped system and shifts them down to the top of the valence band. As shown in Figure 8d, the vacancy can separate more states from the majority Mn e_g state in the conduction band.

As shown in Figure 8, photoexcited electrons can be placed in the Mn 3d t_2g↓ conduction bands under UV light irradiation in both CaMnO_3−δ and Pr_0.5Ca_0.5MnO₃. The Mn 3d t_2g↓ conduction band level is more negative than the O₂/O₂⁻ reduction level, and the hybridized O 2p and Mn e_g↑1valence band levels are more positive than H₂O/OH⁻ oxidation level [51].

The calculated absorption spectra for CaMnO₃ and Pr_0.5Ca_0.5MnO₃ are shown in Figure 9. The Pr_0.5Ca_0.5MnO₃ spectrum exhibits a considerable shift towards higher energies and a loss of absorption intensity between 2 eV and 4 eV compared to CaMnrO₃. The intensity reduction in the Pr_0.5Ca_0.5MnO₃ is attributed to the empty Mn e_g states, which lower from CaMnO₃ to Pr_0.5Ca_0.5MnO₃. The Mn e_g states become occupied by Pr doping, and they become unable to be used for optical excitation. Thus, this analysis sheds light on the experimentally obtained spectra of doped and undoped CaMnO₃.

4. Conclusions

In this work, the photocatalytic dye decomposition over the perovskites CaMnO_3−δ and Pr_0.5Ca_0.5MnO₃ under UV irradiation has been studied. The kinetics of photocatalytic oxidation of organic pollutants showed that CaMnO_3−δ was slightly more active than Pr_0.5Ca_0.5MnO₃. The effects of surface oxygen vacancies and electronic structure on photocatalytic degradation have been investigated by XPS and DFT. The surface oxygen vacancies were found not to be a pivotal factor for improving the photocatalytic properties as long as the O-vacancies occupied the BO₂ positions. The content of chemisorbed oxygen observed on the surface of PCMO particles is considerably higher than CMO, while the amount of oxygen vacancies created on the surface and bulk of CMO is much more pronounced. XPS studies showed the ratio of Mn⁴⁺/Mn³⁺ for both surface and bulk of Pr_0.5Ca_0.5MnO₃ particles. Based on the theoretical calculation of the electronic structure, the photoexcitation of the electrons from the hybridized O 2p and Mn e_g↑ valance band to the Mn e_g↑ and Mn e_g↓ were responsible for the O₂ reduction under UV irradiation. This work may be useful for designing new Mn-based oxide photocatalysts.