Study of methylene blue removal and photocatalytic degradation on zirconia thin films modified with Mn-Anderson polyoxometalates

Abstract

Recalcitrant pollutants are challenging to degrade during water treatment processes. Methylene blue (MB), a cationic dye, is particularly resistant to degradation and is environmentally persistent. Heterogeneous photocatalysis has emerged as a suitable strategy for removing such pollutants from water. In this work, ZrO₂ thin films were modified with Anderson-type Mn-polyoxometalate (MnPOM) ((NH₄)₃[MnMo₆O₂₄H₆]), and the efficiency of MB removal from water was studied. ZrO₂ was synthesized by a sol–gel method, with thin films deposited using the doctor blade method, and ZrO₂ thin films were modified using chemisorption method. The synthesized materials were characterized using SEM, EDX, UV-Vis diffuse reflectance spectroscopy and FTIR. The adsorption kinetics and isotherms for MB were studied for both bare ZrO₂ and ZrO₂/MnPOM composites. Optical characterization showed a band gap energy of 4.02 eV for bare ZrO₂, while the ZrO₂/MnPOM composite exhibited a band gap of 3.7 eV. Furthermore, ZrO₂ showed lower MB removal capacity (∼8%) than ZrO₂/MnPOM thin films (∼29%). The isothermal adsorption studies indicated that MB adsorption onto both bare ZrO₂ and ZrO₂/MnPOM followed the Langmuir adsorption model (q_m = 20.6 mg g⁻¹ for ZrO₂ and q_m = 62.9 mg g⁻¹ for ZrO₂/MnPOM). Furthermore, the adsorption kinetics of MB were well described by a pseudo-second-order model. Photocatalytic testing under UV irradiation showed an apparent rate constant (k_ap) of 2 × 10⁻³ min⁻¹ for bare ZrO₂ and a value of k_ap 5.4 × 10⁻³ min⁻¹ for ZrO₂/MnPOM after 100 minutes. TD-DFT calculations revealed an LMCT interaction between the ZrO₂ nanoparticle and the MnPOM, which likely contributes to the enhanced photocatalytic activity of the ZrO₂/MnPOM composite.

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1. Introduction

Exponential population growth, industrialization, and unsustainable agricultural practices have intensified the global issue of water pollution, leading to significant environmental impacts on both human health and ecosystems.^1,2 Industries such as textiles, cosmetics, food processing, and pharmaceuticals are major contributors to freshwater pollution, primarily due to their high water consumption and the discharge of dyes into effluents without efficient water treatment.^3,4 Over the past decade, a special group of compounds called recalcitrant compounds have emerged as a critical target for environmental research owing to their stability and resistance to degradation by conventional treatment methods.^5,6 MB, a common cationic dye, is of particular concern due to its environmental persistence, toxicity, carcinogenicity and mutagenicity.⁷ Conventional strategies for water treatment for addressing such pollutants include (i) adsorption, (ii) filtration, (iii) thermal separation, (iv) coagulation, (v) biological treatment, (vi) chemical oxidation and (vii) photochemical oxidation.⁸ Advanced oxidation processes (AOPs) represent green strategies for the treatment of contaminated samples containing recalcitrant compounds due to their high oxidative capacity. These processes are non-selective and versatile and can be combined with other technologies. However, one of the drawbacks of AOPs is their substantial energy requirement, particularly for UV irradiation.⁹ The physicochemical phenomenon of AOPs relies on the generation of reactive oxygen species (ROS) on the catalyst surface. These ROS are highly reactive and capable of degrading a wide range of recalcitrant chemical compounds.^10,11 Heterogeneous catalytic processes can be considered as AOPs when a semiconductor is used as a catalyst. Specifically, the semiconductor can generate ROS upon absorbing radiation with energy exceeding its band gap value.^12,13 Among oxide semiconductors, TiO₂ and ZnO are the most studied photocatalysts in the field, due to their activity and stability.^14,15 However, other semiconductors such as zirconia (ZrO₂) have great potential to be used as photocatalysts due to their special and favorable chemical and physical properties (e.g., acid–base properties and high redox properties, high density, high resistance to fracture, thermal stability, photochemical stability, and high refraction index).¹⁶ ZrO₂ has a stable bandgap, transparency in the visible and near-infrared regions of the spectrum, and low photon energy, which reduces the probability of non-radiative decay due to multiphoton relaxation.¹⁷ Furthermore, due to its high ion exchange capacity and redox activity, ZrO₂ exhibits a great number of oxygen vacancies on its surface.¹⁸ However, the energy band gap of ZrO₂ is 5.0 eV, thus, this semiconductor in its pure phase is only photoactive under UV irradiation (∼250 nm).^19–21 The strategies to tune the energy band gap value towards lower energy values include: (i) doping, (ii) changing the defects number, and (iii) the use of quantum dots.^22,23 The change in the number of defects can increase the specific surface area and the adsorption capacity of organic molecules.²⁴ After the modification of ZrO₂ with a polymorphic phase (e.g. monoclinic m-ZrO₂, tetragonal t-ZrO₂ and cyclic c-ZrO₂), the ions diffuse through interstitial sites inside the ZrO₂ lattice changing the conduction and valence bands values, reducing the band gap value.²⁵ Furthermore, reports indicate that use of the heterostructures of ZrO₂ with other materials (metals and non-metals) is another strategy to optimize its photocatalytic efficiency.²⁶ The doping process is another typical strategy, Chen et al. fabricated the Mn-doped catalyst ZrO₂/TiO₂ for the photodegradation of rhodamine B. In this report, a removal of higher than 90% after 50 minutes of UV irradiation was observed.²⁷ Akilandeswari et al. reported the synthesis of ZrO₂ doped with Mn, showing a band gap value of 2.46 eV. After the doping process, the doped ZrO₂ showed a methylene violet removal of 95% under visible irradiation. Reddy et al. fabricated nanoparticles of Mg-doped t-ZrO₂ to remove methylene orange using visible irradiation, reporting 83.7% removal.²⁸ Another alternative to improve the photocatalytic properties of ZrO₂ is the use of heterostructures. In this sense, due to their redox properties polyoxometalates (POM) are an interesting option to modify the ZrO₂ surface. POMs have empty d-orbitals, which become an electron acceptor region. This characteristic does not change upon doping, thus the presence of POMs might reduce the photocatalyst recombination process. Furthermore, the POM can increase the electronic transfer rates.^29,30 Sampurnam et al. reported the photocatalytic activity of hybrid ZrO₂ polyaniline–polyoxometalate ternary nanocomposites in the MB photodegradation under visible radiation.³¹ Most of the reported studies have been carried out in a homogeneous phase. The main drawback of this application is the requirement of additional steps to recover the catalyst after the photocatalytic process. Different studies have demonstrated that immobilization (e.g. coatings, layers, and thin films) of POMs on adequate substances (e.g., ZnO, TiO₂, and ZrO₂) is an appropriate method to obtain catalysts in a solid phase with a high surface area, porosity and, adequate optical properties to develop photocatalytic applications.^32,33 Diaz et al. reported a theoretical and experimental study of a TiO₂ photocatalysis modified with Anderson POMs (containing different central metals). The TiO₂/POM was more efficient in MB removal from water than bare TiO₂.^34,35 Most studies on photocatalytic POM applications include classical POMs (e.g., Keggin, Dawson, and Lindquist). Furthermore, there are few reports about Anderson-type POM deposited on ZrO₂. The reported information indicates that a synergistic effect between the different POMs and ZrO₂ is present for this nanocomposite.^36–38 In this work, ZrO₂ thin films doped with Anderson-type MnPOM (ZrO₂/MnPOM) were synthesized and fully characterized and also the effect of the modifications on the photo-catalytic properties was studied. Finally, to support all the observed experimental results, DFT calculations were performed over a model system, to explain the observed catalytic activity.

2. Experimental

2.1 Materials

The following reagents were used without further purification: Zr(NO₃)₄·5H₂O (Sigma-Aldrich, 99%), urea (Sigma-Aldrich 99.5%), glucose (Sigma-Aldrich >99.5%), citric acid (Sigma-Aldrich >99.5%), (NH₄)₆Mo₇O₂₄·4H₂O (Merck, 99%), Mn(NO₃)₂·4H₂O (Sigma-Aldrich, 98%) and peroxymonosulfuric acid (Sigma-Aldrich 98%).

2.2. Synthesis of used compounds

ZrO₂ powder was synthesized by a sol–gel method.³⁹ First, 5 g (12.5 mmol) of Zr(NO₃)₄·5H₂O and 0.15 mg (2.5 mmol) of urea were mixed in water at 50 °C and pH = 5. Then, 2.1 g (11.7 mmol) of glucose (Sigma-Aldrich >99.5%) and 0.25 g (1.2 mmol) of citric acid (Sigma-Aldrich >99.5%) were added. This mixture was stirred for 4 days at 300 rpm until a homogeneous suspension was obtained. After that, the suspension was dried at 423 K for 12 hours in an oven. Finally, the sintering process was carried out at 873 K for 6 hours in a muffle. The mass ratio of the used reagents was Zr(NO₃)₄·5H₂O : urea : glucose : citric acid (1 : 0.03 : 0.42 : 0.05). The yield of the process was 85%. The MnPOM was synthesized using 2.5 g (2.0 mmol) of (NH₄)₆Mo₇O₂₄·4H₂O (Merck, 99%) and 0.9 g (4.0 mmol) of Mn(NO₃)₂·4H₂O (Sigma-Aldrich, 98%), which were dissolved in 100 mL of distilled water by a co-precipitation method. The pH of this solution was adjusted to 4.0–5.0 using peroxomonosulfuric acid (H₂SO₅). The mixture was stirred at room temperature for 24 hours to allow for complete oxidation, followed by filtration and drying at 333.15 K for 24 hours. The yield of the process was 90%. The synthesized MnPOM has the chemical formula [MnMo₆O₂₄H₆]³⁻.⁴⁰

2.3. Thin film deposition and characterization

We deposited ZrO₂ thin films using the Doctor Blade technique according to the procedure reported earlier.⁴¹ The addition of the MnPOM over the ZrO₂ surface was carried out by a chemisorption process. Therefore, the thin films were immersed in a MnPOM suspension of 0.5 g (0.042 mmol) MnPOM in 100 mL of distilled water for 24 hours at 25 °C and stirring at 100 rpm. Then, the modified thin films were cleaned with distilled water and dried at 333.15 K. To ensure proper adherence of the nanoparticles to the thin films, a thermal treatment was performed in an oven at 523 K for 30 minutes. The obtained samples were characterized by UV-vis reflectance diffuse (ERS), FT-IR, SEM and EDX assay.

2.4. Adsorption and photocatalytic reactions

In the kinetic adsorption study, the thin films were immersed in a batch reactor (100 mL) with a solution of MB (10 mg L⁻¹). The system was stirred in darkness and the adsorption process was monitored by UV-vis spectrophotometry (at λ = 665 nm) until the adsorption/desorption equilibrium was reached. The process was carried out at different MB concentrations (20, 30, 40, 50 and 60 mg L⁻¹). In the photocatalytic test, after the adsorption/desorption equilibrium was reached, the system was irradiated with a UV tubular lamp manufactured by PHILIPS® (arc length 161 mm, tube diameter 16 mm, 7 W, 15 μW cm⁻² at 1 m, emission maximum of 260 nm). The photocatalytic process was monitored by UV-vis spectrophotometry (at λ = 665 nm) every 20 minutes. Finally, the Langmuir–Hinshelwood (L–H) kinetic model was used to describe the photocatalytic process.

2.5. Computational details

Density functional (DFT) and time-dependent DFT (TDDFT) calculations were performed using the Amsterdam density functional package (ADF 2019.01) using the zeroth order regular approximation (ZORA) Hamiltonian with scalar corrections.^42,43 Slater type orbitals (STO) basis set using triple-ζ accuracy⁴⁴ with a single polarization function was employed (TZP). All optimizations were performed using analytical energy gradient techniques and the generalized gradient approximation (GGA) method with nonlocal exchange and correlation corrections within the BP86 functional.^45,46 The conductor-like screening model (COSMO) was used to model water as a solvent. Furthermore, to study the stability of ZrO₂/MnPOM, the Morokuma–Ziegler bonding energy decomposition analysis^43,47,48 was carried out. In this scheme, the interaction energy, ΔE_Int, is divided into the ΔE_elec + ΔE_Pauli + ΔE_orb + ΔE_disp components. (i) ΔE_elec can be related to the classical electrostatic interaction, calculated by superposition of the unperturbed fragment densities of the molecular geometry; (ii) ΔE_Pauli, represents the repulsive interactions between the fragments, as two electrons with the same quantum numbers cannot occupy the same region in the space and (iii) ΔE_orb is the stabilizing orbital interaction term.⁴⁹

3. Results and discussion

3.1. FTIR characterization of thin films

Fig. 1 shows the FTIR spectra of the bare ZrO₂ and ZrO₂/MnPOM thin films. The band located at 3000–4000 cm⁻¹ corresponds to –OH bond stretching of the film surface adsorbed water molecules. Also, the signal located at 1630 cm⁻¹ corresponds to the bending of water –OH.^50–52 The signal located at 2380 cm⁻¹ can be assigned to the Zr–OH bond stretching as previously reported.^53,54 The bare ZrO₂ spectrum shows a broad band at 660 cm⁻¹. This peak can be assigned to the stretching of the Zr–O bond.^54,55 The peak located at 1414 cm⁻¹ is assigned to the nitrates from the precursor used during the POM's synthesis.^56–59 The peaks located between 1000 cm⁻¹–500 cm⁻¹ are associated with the vibration of the MnPOM bonds. The characteristic signals located at 980 cm⁻¹; 660 cm⁻¹ and 560 cm⁻¹ are assigned to both symmetric and anti-asymmetric stretching bands of Mo–O–Mo bonds, which correspond to MnPOM. The signal located at 815 cm⁻¹ is assigned to the stretching of the Mn–O bond, while the band located at 949 cm⁻¹ is attributed to the bending of the Mn–OH bond. Finally, the peak located at 848 cm⁻¹ is assigned to the bending of the Mn–O bond.^60,61

Fig. 1 FT-IR spectra of the bare ZrO₂ and ZrO₂/MnPOM thin films.

3.2. Morphological characterization of thin films

The catalytic activity of any material is highly dependent on the shape, size, and size distribution of the particles. The morphological characteristics of ZrO₂ thin films are an important result before the development of catalytic applications. Fig. 2(a) shows SEM images of the thin films synthesized in this work. The bare ZrO₂ thin films consist of non-uniform microcrystals. In some regions, agglomerates can be observed; additionally, the SEM image shows holes in the ZrO₂ film. This result is consistent with other reports on the use of the sol–gel synthesis method for ZrO₂ films.⁴⁹ After surface modification, the agglomeration increased and, the surface showed laminated agglomerates. These results could be beneficial for photocatalytic application, as photogenerated charge pairs can be separated between particles within agglomerates, reducing the recombination rates.⁵⁰Fig. 2(b) shows the EDX assay for ZrO₂ and ZrO₂/MnPOM thin films, and Table 1 lists the elemental composition of the analyzed thin films. The shown result is in agreement with a 12% MnPOM chemisorption, as previously reported for similar modified materials.⁵⁰

Fig. 2 SEM images of (a) ZrO₂, (b) ZrO₂/MnPOM thin films, and (c) EDX images of ZrO₂/MnPOM thin films.

Table 1 EDX results of the synthesized thin films

Thin film	% Zr	% O	% Mo	% Mn
ZrO2	69.90	25.62	—	—
ZrO2/MnPOM	55.19	24.84	10.62	7.65

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3.3. Optical thin film characterization

Fig. 3(a) shows UV-vis DRS of bare ZrO₂ and ZrO₂/MnPOM thin films. The bare ZrO₂ did not show any signal in the visible electromagnetic spectrum, this is in concordance with its higher energy band gap value. The signal in the spectrum (333 nm) corresponds to an electronic excitation from the valence band to the conduction band of the semiconductor due to the charge transfer transition O²⁻ (2P → Zr (A_1g)).⁴² The ZrO₂/MnPOM thin film shows a red-shift in the optical activity with signals between (450 nm–590 nm) and (600–800 nm). This result suggests that ZrO₂/MnPOM can absorb visible radiation, improving the optical properties compared with the bare semiconductor. Fig. 1(b) shows the Kubelka–Munk function for the data shown in Fig. 1(a). The band gap (E_g) of catalysts can be calculated from this figure.^44,50 Thus, the bare ZrO₂ has an E_g value of 4.02 eV. This transition can be attributed to the structural extrinsic states due to particle size, surface defects, or traps of the material.^42,43 The E_g value reported for ZrO₂ is in the range of 3.5–5.1 eV, depending on the synthesis technique, thus our data are in agreement with those reported previously.^45,46 The ZrO₂/MnPOM shows an E_g value of 3.70 eV, this reduction in the energy transition can be associated with ligand-to-metal charge transfer (LMCT) that involves the Anderson-type POM structure.⁴⁸ This observed change in the UV-vis spectrum might affect the photocatalytic properties of the material.

Fig. 3 (a) Diffuse reflectance spectra of thin films (b) Kubelka–Munk function and E_g value estimation.

3.4. Isothermal adsorption study

An important step before the photodegradation process of the pollutant is to reach the adsorption–desorption equilibrium state. This process is not commonly studied during photocatalytic activity studies. However, Zr-based sorbents have been known for their removal capacity of different pollutants.⁶² In this work, we studied the removal of MB on bare ZrO₂ and ZrO₂/MnPOM thin films. We applied the isothermal models of Langmuir (1), Freundlich (2), Temkin (3) and Dubinin–Radushkevich (4) according to:^59,60

q_e = K_FC_e^1/n (2)

q_e = B_T ln(A_TC_e) (3)

where C_e is the MB concentration (mg L⁻¹), q_e is the MB adsorbed on the catalyst (mg g⁻¹) in the adsorption/desorption equilibrium state, q_e is the maximum adsorption capacity, K_L is the Langmuir constant, K_F and n represent the intensity and the maximum adsorption capacity of Freundlich isotherm, respectively. B and K_T are the constants of the Temkin isotherm model and quantify the heat and the binding energy of the adsorption, respectively. Finally, the parameter β (mol² kJ⁻²) is the Dubinin–Radushkevich constant, ε relates the Polanyi potential with the C_e and the saturation capacity of the catalyst (q_m).⁶¹ Furthermore, we used the correlation coefficient (R²) and the average relative error (ARE) to determine a suitable fitting:where q_e is the experimental value, q_f is the fitting value and n is the total of data.⁶²Fig. 4 shows the isothermal fitting results and Table 2 lists the physical–chemical parameters of the fitting.

Fig. 4 Isothermal fitting results for the thin films of (a) ZrO₂ and (b) ZrO₂/MnPOM.

Table 2 Physical–chemical fitting results of the isothermal models

Isothermal model	Parameter	ZrO2	ZrO2/MnPOM
Freundlich	K F (mg g^−1)/(mg L^−1)^n	7.37	1.90
	n	3.46	1.90
	R ^2	0.865	0.925
	ARE (%)	8.5	8.6
Langmuir	q m (mg g^−1)	20.57	62.89
	K L (L mg^−1)	0.285	0.118
	R ^2	0.995	0.977
	ARE (%)	3.4	4.9
Temkin	B T	4.02	15.4
	A T	3.84	1.09
	R ^2	0.892	0.963
	ARE (%)	5.3	9.5
Dubinin–Radushkevich	β (mol^2 kJ^−2) × 10^−6	2.1	2.8
	q m (mg g^−1)	18.17	44.35
	R ^2	0.984	0.970
	ARE (%)	4.3	9.2

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The experimental data show an L-shaped isotherm indicating that the MB adsorption process has a strong competition between the water molecules and/or other molecules containing metal–oxygen (M–O) bonds for the active sites on the ZrO₂ surface.⁶⁰ This behavior has been previously reported for other semiconductors (e.g., TiO₂, ZnO).^32,63 According to the results listed in Table 2, the Langmuir model adequately describes the ZrO₂ thin-film adsorption data (higher R² and smaller ARE), indicating a monolayer adsorption. This may indicate that the adsorption takes place at specific sites and that all the adsorption sites have equal affinity for the adsorbate. It might be further assumed that the adsorption on one site does not influence the adsorption on an adjacent site.⁶⁴ However, after the modification of the semiconductor by chemisorption of MnPOM, it is observed that the monolayer behavior adsorption of the MB changes to a multilayer adsorption (on the surface of ZrO₂/MnPOM).⁶⁵ Therefore, Langmuir's kinetic coefficients were higher for ZrO₂/MnPOM than for bare ZrO₂, suggesting a higher affinity for MB ions in the solution on ZrO₂/MnPOM than for bare ZrO₂ thin films. This behavior could be due to the interaction between POM metal–oxygen anion nanoclusters and the MB molecules.⁶⁶

3.5. Kinetic adsorption study

To model the kinetic data, two models were applied: pseudo-first-order (6) and, pseudo-second-order (7), according to these equations:

ln (q_t − q_e) = ln(q_e) − k₁t (6)

where q_t is the MB adsorbed on the catalyst's surface (mg g⁻¹). q_e is the maximum adsorption capacity (mg g⁻¹), and k₁ (min⁻¹) and, k₂ (g mg⁻¹ min⁻¹) are the rate constants to the pseudo-first and pseudo-second order, respectively. Fig. 5 shows the kinetic fitting of MB adsorption on the catalyst's thin films and Table 3 lists the physical chemical parameters of the fitting.

Fig. 5 Kinetic fitting results of MB adsorption on both thin films: (a) ZrO₂ and (b) ZrO₂/MnPOM.

Table 3 Kinetic adsorption model fitting results for MB adsorption onto thin films

Model	Parameters	ZrO2	ZrO2/MnPOM
1^st order	q e (mg g^−1)	28.3	34.8
	k 1 (min^−1)	0.109	0.151
	R ^2	0.978	0.991
	ARE (%)	5.8	3.3
2^nd order	q e (mg g^−1)	28.5	33.5
	k 2 (g mg^−1 min^−1) × 10^−3	0.150	0.209
	R ^2	0.983	0.996
	ARE (%)	6.5	2.5

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Recently, adsorption kinetics has been a relevant part of understanding the sorption processes, particularly in water treatment processes. However, it is notable that these processes have been less frequent and less explored in photocatalytic applications. The kinetic mechanism of MB adsorption in the catalysts can be studied indirectly using the kinetic models mentioned above. Results show that thin films reach the adsorption/desorption equilibrium after (25 min to 30 min).⁶⁷ The bare ZrO₂ thin film showed a lower removal capacity (∼8%) than ZrO₂/MnPOM thin films (∼29%), indicating a greater affinity of MB for ZrO₂/MnPOM than for bare ZrO₂. This might be due to the anionic properties of the POMs assisting the sorption of the cationic dyes (e.g., MB).^29,65 The pseudo-second-order model was appropriate to describe the experimental data for the adsorption process (higher R² and smaller ARE, see Table 3). During MB adsorption on the thin film surface, chemisorption is considered as the main chemical interaction. The adsorption rate is proportional to the MB concentration and behaves similarly to the first-order kinetics in terms of its adsorption rate. In this context, effective electrostatic interactions play a significant role in the adsorption phenomenon. The k₂ values of the pseudo-second-order model show a larger value for ZrO₂/MnPOM compared to bare ZrO₂, indicating that both the adsorption capacity and the adsorption rate increased after the modification of the ZrO₂ thin films.

3.6. Photocatalytic study

In the kinetic degradation study, the Langmuir–Hinshelwood model was applied⁶⁸ as shown below.

ln [MB]_t = ln [MB]₀ − k_ap(t) (8)

where [MB]_t is the MB concentration as a function of time irradiation; (t) is the time irradiation and the k_ap is the rate constant of the process (min⁻¹). Fig. 6 shows the MB concentration decay under UV irradiation onto bare ZrO₂ and ZrO₂/MnPOM thin films. Bare ZrO₂ thin films reached 18% of MB photodegradation, whereas ZrO₂/MnPOM thin films reached 43% of MB photodegradation. The Langmuir–Hinshelwood model fitting shows that bare ZrO₂ thin films have a k_ap value (2 × 10⁻³ min⁻¹) smaller than ZrO₂/MnPOM (5.4 × 10⁻³ min⁻¹). Therefore, the adsorbed POM could act as an electron scavenger both on the surface and in the bulk of the material, reducing the recombination rate, and thus, increasing the photocatalytic properties.^32,69 The previously shown band gap reduction observed for ZrO₂/MnPOM could be responsible for the increased photocatalytic activity. Similar results have been reported before for ZrO₂ modified with POMs, as shown in Table 4.^32,50,70

Fig. 6 MB concentration decay under UV irradiation for free MB and onto bare ZrO₂ and ZrO₂/MnPOM thin films.

Table 4 Photocatalytic efficiency of different systems under UV irradiation

Semiconductor	Pollutant/irradiation time	Degradation efficiency (%)	Ref.
a Thin films. b Graphene oxide.
ZnO-Ag	MB/180 min	35	71
BCTi^b/powder	MB/180 min	90	72
CoPOM/TiO2	MB/300 min	50	73
SnO2/Fe	RB^a/180 min	55	74
ZrO2/MnPOM	MB/100 min	43	This work

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The recyclability of the catalysts is an important parameter for developing photocatalytic applications. Fig. 7a shows the results of the stability tests for bare ZrO₂ and ZrO₂/MnPOM thin films. The ZrO₂ films did not change significantly their MB adsorption capacity even after the third cycle, suggesting that the thin film deposition method was suitable. Fig. 7 shows that the MB adsorbed amount onto recycled ZrO₂/MnPOM thin films reduced gradually after each photocatalytic cycle from 28% to 15%. Furthermore, the same trend was observed in the photocatalytic test. Fig. 7b shows a slight decrease in photocatalytic degradation after three cycles from 43% to 31%. The loss in photocatalytic activity might be associated with the amount of the catalyst diminution through the cycles. This result is similar to the one reported by Tang et al., where they reported the obstruction of the adsorption by surface residuals and the mass loss during catalyst recovery in the bisphenol A photodegradation using POMs/TiO₂ photocatalysts after 4 photocatalytic cycles.⁷⁵

Fig. 7 Recyclability tests for photocatalytic degradation of MB onto ZrO₂ and ZrO₂/MnPOM thin films. (a) Adsorption (b) photocatalytic degradation.

3.7. Computational modeling

It has been previously shown that ZrO₂/CrPOM has a 43% degradation under UV irradiation.⁵⁰ In this report, a slightly smaller degradation capability was obtained for ZrO₂/MnPOM. These slight observed differences can be supported with the FMO localization, the global reactivity indexes and the EDA analysis. Therefore, a reduced model considering a ZrO₂ nanoparticle representation and one MnPOM was considered. The EDA analysis shows similar results to those previously reported. Furthermore, it is observed that the most active compounds and Mn, have the largest stabilization energy on the ZrO₂ surface. This corroborates that the most stable molecule has the best catalytic conversion over MB. The observed stabilization is mostly due to the electrostatic contribution, which correlates with the large charge of the studied POM. As shown in Fig. 8, the HOMO and the LUMO show large contributions to the d_z² orbital. The reactivity indexes show the MnPOM has similar reactivity in terms of chemical potential and chemical hardness than the CrPOM (which shows the best photocatalytic activity), see Table 5. Also, MnPOM shows a large value of electrophilicity compared to the CoPOM and CuPOM.⁵⁰

Fig. 8 Frontier molecular orbitals for the ZrO₂/MnPOM.

Table 5 Bonding energy decomposition analysis Morokuma–Ziegler EDA, HOMO, LUMO, GAP and reactivity indexes in eV

	ΔEPauli	ΔEorb	ΔEelect	ΔEdisp	ΔEtot	HOMO	LUMO	GAP	μ	η	ω
MnPOM	14.26	−40.10 (36)	−70.64 (63)	−0.42	−96.90	−5.39	−3.55	1.84	−4.47	0.92	10.84

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Finally, to provide a better rationalization of the photocatalytic activity for the MnPOM, TD-DFT calculations were performed. As described before, due to the addition of the MnPOM to the, there is observed an LMCT between the nanoparticle and POM, see Table 6. This transition involves orbitals that are located over the ZrO₂ nanoparticle towards the MnPOM nanoparticle. Furthermore, larger energy transitions have the opposite direction (from the MnPOM towards the ZrO₂ nanoparticle), data not shown. This charge separation induced by the presence of the MnPOM in the structure would explain the enhanced photocatalytic activity.

Table 6 Theoretical TD-DFT transitions observed in the UV-vis spectra. The extinction coefficient (f), orbitals involved and the oscillator strength are also reported for each transition

Compound	Λth	f	Contribution			%
ZrO2/MnPOM	330	1.20 × 10^−3	HOMO−1	→	LUMO	89

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4. Conclusions

A novel photocatalytic system based on ZrO₂/POMs was synthesized and characterized, exploring the incorporation of manganese (Mn³⁺) as a transition metal within the molecular structure of POMs. Optical results show a red shift in the optical activity of the MnPOM-modified material, the band gap value was lower for ZrO₂ (3.70 eV) than the band gap value of ZrO₂/MnPOM (3.70 eV). Modeling of MB adsorption on thin films revealed that the Langmuir model was suitable to describe the adsorption process on both materials. Modified ZrO₂ thin films were more efficient in the photocatalytic degradation of MB. Specifically bare ZrO₂ thin films show a k_ap value (2 × 10⁻³ min⁻¹) smaller than that of ZrO₂/MnPOM (5.4 × 10⁻³ min⁻¹). Results showed a significant improvement in the adsorption capacity after modification of ZrO₂ with MnPOM, providing a positive effect on photochemical, photophysical and photocatalytic properties. This increase in efficiency can be attributed to improvement in the adsorption capacity and optical properties of the material after modification with MnPOM. Furthermore, DFT calculations suggested that charge separation induced due to the presence of MnPOM in the structure would explain the enhanced photocatalytic activity.

Data availability

The data supporting this article have been included within the main document; no additional data are presented.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

C. D.-U., W. V., would like to thank Universidad del Atlántico. The authors would like to acknowledge ANID/FONDAP/1523A0006; FONDECYT 1241917, 1231194, the Anillos de Ciencia y Tecnología ACT210057 and ANID Postdoctoral 3240414.

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