Synthesis of RE³⁺ (RE = Ho, Tb, Pr)-Doped Alumina Ceramic Coatings by Plasma Electrolytic Oxidation of Aluminum: Investigation of Photocatalytic Performance

Abstract

Porous, crystalline gamma-Al₂O₃ coatings with a thickness of (6 ± 0.5) μm and a uniform distribution of rare earth (RE) dopants are synthesized by plasma electrolytic oxidation of aluminum at a current density of 150 mA/cm² in a boric acid and borax (BB) solution containing added RE oxide particles (Ho₂O₃, Tb₄O₇, and Pr₆O₁₁) at concentrations of 1, 2, and 4 g/L. The concentration of RE oxide particles in the BB solution determines the amount of RE elements incorporated into the coatings but does not significantly affect their surface morphology, crystal structure, or light absorption properties. The coatings exhibit high absorption in the middle/near-ultraviolet region, characteristic of Al₂O₃. Typical 4f-4f transitions of Ho³⁺, Tb³⁺, and Pr³⁺ are observed in the photoluminescence spectra. Photocatalytic evaluations using methyl orange degradation under simulated solar irradiation show that RE doping significantly enhances photocatalytic efficiency. Peak degradation efficiencies are achieved at a concentration of 4 g/L for all RE oxides. After 8 h of irradiation, maximum degradation reaches 88%, 92%, and 85% with pseudo-first-order rate constants (k_app) of about 0.274 h⁻¹, 0.339 h⁻¹, and 0.232 h⁻¹ for coatings synthesized in BB with 4 g/L Ho₂O₃, Tb₄O₇, or Pr₆O₁₁, respectively. In comparison, the pristine Al₂O₃ coating achieves only about 50% degradation (k_app ≈ 0.087 h⁻¹). Photoluminescence indicates that RE³⁺ ions serve as effective charge-carrier traps, suppressing electron–hole pair recombination. RE-doped Al₂O₃ coatings demonstrate exceptional structural stability and reusability over six cycles, highlighting their potential for sustainable wastewater remediation.

1. Introduction

The effects of organic pollutants from industrial discharge and domestic effluents on the environment are now a major global concern [1]. Metal oxide photocatalysis is among the most advanced remediation techniques, using sunlight to mineralize organic pollutants into harmless byproducts [2,3]. Metal oxides demonstrate photocatalytic efficacy in the mineralization of organic pollutants, a performance supported by the synergistic combination of their thermodynamic stability, tunable electronic band structures, and optimized charge-carrier dynamics. Additionally, their inherent resistance to photocorrosion and broad spectral absorption profiles enable sustained catalytic turnover under diverse irradiation conditions [4]. While TiO₂ and ZnO remain the benchmark photocatalysts due to their superior quantum yields, research has expanded to a diverse range of alternative oxides, including WO₃, ZrO₂, Nb₂O₅, Fe₂O₃, Al₂O₃, CuO, Ta₂O₅, etc. [5]. Among these metal oxides, Al₂O₃ (alumina) is a versatile ceramic known for its robust physicochemical properties, such as high hardness, refractory stability (melting point about 2072 °C), and strong chemical inertness [6]. Its high dielectric strength and volume resistivity make it an excellent electronic insulator, while its wide optical transparency from ultraviolet (UV) to near-infrared (NIR) is complemented by a high refractive index [7]. As a result, Al₂O₃ is a key material in structural, microelectronic, and biomedical engineering. Despite these advantages, Al₂O₃ is traditionally excluded from primary photocatalytic applications due to its wide electronic band gap (7 eV to 9 eV) [8]. This large gap restricts photon absorption to the deep-UV range, effectively rendering the material inactive under the visible light spectrum, which comprises the majority of solar radiation. Furthermore, its insulating nature results in negligible charge-carrier density and high recombination rates compared to transition-metal oxide semiconductors such as TiO₂ or ZnO. However, the material’s high specific surface area, tailored surface acidity or basicity, and thermal durability make it an exceptional catalytic support [9,10,11]. By integrating Al₂O₃ into heterostructured composite systems, researchers can leverage its structural stability to enhance the dispersion and photocatalytic efficiency of narrower-gap semiconductors.

In recent years, the plasma electrolytic oxidation (PEO) process has emerged as a highly effective electrochemical surface treatment for fabricating immobilized photocatalysts [12,13,14]. This technique applies a high-voltage potential to a metallic substrate submerged in an aqueous electrolyte, exceeding the dielectric breakdown threshold [15,16,17]. The resulting localized micro-discharges promote the growth of a robust, highly porous, ceramic-like oxide layer. The intrinsic porosity of the PEO layer greatly increases the specific surface area, providing a high density of active sites for the adsorption and subsequent degradation of organic pollutants. The extreme localized temperatures and pressures generated by the micro-discharges facilitate the formation of highly crystalline oxide phases. These crystalline structures generally exhibit superior photocatalytic efficiency compared to amorphous forms. Unlike suspended powder systems, PEO coatings offer exceptional interfacial adhesion to the substrate, eliminating the energy-intensive post-treatment filtration steps required to recover powdered catalysts from aqueous media.

Al₂O₃ coatings synthesized by PEO on aluminum substrates possess an inherent, though baseline, level of photocatalytic activity (PA), typically demonstrated by the oxidative degradation of organic pollutants [18]. However, to achieve higher photocatalytic efficiency, functionalization of the Al₂O₃ matrix with extrinsic semiconductors is required. Conventional enhancement strategies involve the direct incorporation of semiconductor nanoparticles such as ZnO [19], TiO₂ [20,21], WO₃ [22], Co₃O₄ [22], MnO₂ [23], or CuO [23] from the electrolyte into the developing Al₂O₃ matrix, which modifies the electronic band structure of the composite coating and induces a shift in the optical absorption edge. By narrowing the effective bandgap, this shift extends the spectral response from the UV into the visible light region, substantially increasing the utilization of the solar spectrum. Furthermore, these incorporated particles function as charge separation mediators, providing alternative pathways for photo-generated species and facilitating the spatial separation of electron–hole pairs.

This research investigated the synthesis of rare earth (RE) ion-doped Al₂O₃ coatings by PEO on aluminum substrates. The process used an electrolyte enriched with RE oxide particles to modify the coating matrix. The primary focus was to evaluate the photocatalytic efficacy of these coatings in degrading methyl orange (MO).

MO is a recalcitrant synthetic anionic azo dye frequently detected in industrial effluents [24]. Its high water solubility facilitates accumulation in aquatic ecosystems, while its complex chemical structure, characterized by stable azo bonds (–N=N–), makes it resistant to conventional biological and chemical remediation. Given the potential environmental toxicity of MO, developing highly efficient degradation methods is critical. This study contributes to the ongoing development of sustainable and economically viable photocatalytic systems for advanced wastewater treatment.

RE ions have become one of the key dopants for enhancing the performance of photocatalytic systems, especially in environmental remediation [25,26,27]. Their impact on the physicochemical properties of host materials is mainly due to their unique electronic configurations, with partially filled 4f orbitals. Incorporating RE ions into a semiconductor lattice leads to several important improvements in the photocatalytic process. RE doping introduces intra-band states that reduce the band gap energy (E_g) of the host material and extend the optical absorption threshold from the UV into the visible light spectrum, increasing solar energy utilization. RE ions act as localized trapping sites for photo-generated electrons or holes. By separating the charge carriers, these ions suppress the recombination of electron–hole pairs, extending carrier lifetimes and increasing the likelihood of interfacial redox reactions. In addition to electronic modification, RE doping can change the surface morphology, enhancing the chemisorption of organic pollutants and ensuring that target contaminants remain close to the reactive oxygen species (ROS) generated at the surface.

The existing literature reveals a notable gap in the investigation of photocatalytic performance in RE ion-doped Al₂O₃ systems. To date, research has been predominantly limited to our recent findings, which demonstrate that incorporating Eu²⁺ and Ce³⁺ ions into Al₂O₃ coatings during PEO of aluminum significantly enhances PA compared to undoped Al₂O₃ [28]. These doped coatings, synthesized by PEO in electrolytes containing Eu₂O₃ and CeO₂ precursors, represent a novel advancement in optimizing the functional properties of Al₂O₃-based photocatalysts.

In this study, RE-doped Al₂O₃ coatings were synthesized using an environmentally friendly PEO process by suspending RE oxide particles (Ho₂O₃, Tb₄O₇, and Pr₆O₁₁) in a borax-boric acid electrolyte. RE³⁺ ions (Ho³⁺, Tb³⁺, and Pr³⁺) were incorporated into the Al₂O₃ matrix. This work systematically evaluates how RE³⁺ dopant concentration affects the photocatalytic performance of the resulting coatings. This approach provides a robust framework for engineering light-responsive functionalities on aluminum substrates, enabling their application in environmental remediation.

2. Materials and Methods

High-purity aluminum rectangular samples (25 mm × 10 mm × 0.125 mm, 99.999%, Alfa Aesar) were used as the starting material for the PEO process. Substrate preparation included ultrasonic degreasing in acetone, followed by drying with a warm air stream. The samples were then masked with a chemically resistant insulating resin, leaving a defined active surface area of 15 mm × 10 mm exposed to the electrolyte. The PEO process was carried out in a double-walled glass electrolytic cell specifically designed for continuous water cooling and thermal stability. The electrode geometry featured a tubular stainless-steel cathode positioned concentrically around the aluminum anode samples, which were placed at the center of the cell to ensure a uniform electric field distribution [29]. PEO was performed using a direct current source (Consort EV261) at a working current density of 150 mA/cm² for 10 min. The electrolyte solution was prepared by adding RE oxide particles (Ho₂O₃, Tb₄O₇, and Pr₆O₁₁, 99.9% purity, Pangea International, primary particle size < 80 nm, granularity < 300 nm) at concentrations of 1 g/L, 2 g/L, and 4 g/L to a boric acid and borax water solution (BB, 0.1 M H₃BO₃ and 0.05 M Na₂B₄O₇·10H₂O). A continuously agitated magnetic stirrer at 250 rpm was used to mix the electrolyte in the cell, ensuring even distribution of the RE oxide particles. The electrolyte temperature was maintained at (15 ± 1) °C. After PEO treatment, samples were rinsed with distilled water to remove residual electrolyte components and prevent their accumulation during drying.

The surface morphology and elemental composition of the synthesized coatings were characterized using a scanning electron microscope (SEM, JEOL 840A, Tokyo, Japan) integrated with an energy dispersive X-ray spectroscopy (EDS, Oxford INCA, Abingdon, UK) system. Structural phase analysis was performed using X-ray diffraction (XRD; Rigaku Ultima IV, Tokyo, Japan) in a Bragg–Brentano configuration. The system used Ni-filtered Cu Kα radiation (λ = 1.54178 Å) operated at 40 kV and 40 mA. To determine the crystalline phases, diffraction patterns were recorded over a 2θ range of 15° to 75°, with a scanning step of 0.050° and an acquisition rate of 2°/min.

The absorption profiles of the synthesized PEO coatings were characterized using diffuse reflectance spectroscopy (DRS). Measurements were performed with a UV-Vis spectrophotometer (Shimadzu UV-3600, Tokyo, Japan) equipped with an internal integrating sphere assembly. Barium sulfate (BaSO₄) served as the reference standard to establish the reflectance baseline.

Room-temperature photoluminescence (PL) emission spectra were recorded using a spectrofluorometer (Horiba Jobin Yvon Fluorolog FL3–22, Edison, NJ, USA). Excitation was provided by a 450 W xenon arc lamp, with the incident wavelength range from 250 nm to 600 nm, isolated by a double-grating monochromator. The resulting emission was resolved through a secondary double-grating monochromator over a spectral range of 350 nm to 750 nm. Signal detection was performed using a Hamamatsu 928 P side-on photomultiplier tube. The obtained emission spectra were corrected for the measuring system’s spectral response and the xenon lamp’s spectral distribution.

The photocatalytic performance of the synthesized coatings was evaluated by degrading MO under simulated solar irradiation. An aqueous solution (8 mg/L, 10 cm⁻³) was introduced into a double-walled glass reactor maintained at 20 °C using a circulating water-cooling system [30]. Each sample (1.5 × 1 cm²) was placed on a perforated holder 5 mm above the reactor base to ensure consistent magnetic stirring of the solution. To isolate the photocatalytic effect, control experiments were conducted for 8 h under both dark conditions (to assess adsorption) and irradiation without a catalyst (to assess photolysis). Both processes resulted in negligible changes in MO concentration. Before light exposure, the system was equilibrated in the dark for 1 h to achieve adsorption–desorption equilibrium on the catalyst surface.

The photocatalytic reactions were initiated using a 300 W OSRAM ULTRA-VITALUX UV-A lamp (OSRAM GmbH, Munich, Germany), which provided simulated sunlight at the sample surface from a distance of 25 cm. The concentration of MO was monitored at regular intervals of 2 h using a UV-Vis spectrophotometer (Thermo Electron Nicolet Evolution 500, Thermo Fisher Scientific, Altrincham, UK) at the maximum absorption wavelength of 464 nm. Photocatalytic efficiency was calculated using the formula: (C_o − C)/C_o (%), where C_o is the initial MO concentration and C is the MO concentration after irradiation.

The role of ROS in the photocatalytic degradation of MO was elucidated through in situ quenching experiments. To identify the primary oxidative species, selective scavengers were added to the reaction matrix at a controlled concentration of 1·10^–4 M relative to the initial concentration of MO. Specifically, oxalic acid was used as a scavenger for photogenerated holes (h⁺), p-benzoquinone served as the quenching agent for superoxide radical anions (^●O²⁻), and tert-butanol was used to sequester hydroxyl radicals (^●OH). Except for the inclusion of quenching agents, all parameters remained consistent with established photocatalytic protocols.

The cyclic stability and reusability of the synthesized coatings were evaluated over six consecutive photocatalytic degradation cycles of MO. These experiments were conducted on the most photoactive sample. To maintain consistency between trials, specimens were thoroughly rinsed with deionized water and air-dried after each cycle.

3. Results and Discussion

3.1. Morphology, Chemical, and Phase Composition of RE-Doped Al₂O₃ Coatings

Figure 1a shows the galvanostatic potential–time transients during the PEO of aluminum in BB and in composite electrolytes containing 4 g/L Ho₂O₃, Tb₄O₇, and Pr₆O₁₁ particles. Particle inclusion had a negligible effect on the potential–time behavior. The process begins with a highly linear potential increase accompanied by vigorous gas evolution, corresponding to the growth of a compact, amorphous Al₂O₃ barrier layer typical of conventional anodizing [31]. During this initial stage, the total current density is dominated by the ionic component, which typically exceeds the electronic current density by two to three orders of magnitude. As the anodic film thickens, electrons are injected into the oxide conduction band. Accelerated by the intense electric field, these charge carriers trigger impact ionization, resulting in electron avalanches [32]. The onset of the PEO process occurs when this avalanche electronic current reaches a critical threshold at the dielectric breakdown potential, where the electric field strength exceeds the dielectric strength of the passive film. This transition is marked by a deviation from linearity in the potential–time curve and is physically manifested by the appearance of distributed micro-discharges across the substrate surface. As a result, the electronic current density becomes the primary contributor to the total current density. The final stage is characterized by stabilization of the anodic potential (around 500 V). In this steady-state regime, the electronic current density remains dominant, sustaining the plasma state necessary for the synthesis of complex ceramic coatings.

To evaluate the quality of the coatings, Figure 1b shows photos of the aluminum samples before and after the formation of the PEO coatings synthesized in BB and in BB with the addition of 4 g/L Ho₂O₃, Tb₄O₇, and Pr₆O₁₁ particles. As shown in the photos, the untreated aluminum has a typical metallic, reflective surface. After PEO treatment, the samples display a uniform, matte ceramic appearance confirming the macroscopic uniformity and successful formation of the coatings without visible defects or peeling.

Characteristic SEM micrographs of the coatings synthesized in BB and in BB with the addition of 4 g/L Ho₂O₃, Tb₄O₇, and Pr₆O₁₁ particles are presented in Figure 2. All surfaces demonstrate the morphological characteristics of PEO coatings on aluminum substrates processed in BB [22,23]. During PEO, high-energy micro-discharges generate intense localized heat that melts both the substrate and the dielectric oxide layer. This thermal surge coincides with anodic reactions that evolve oxygen gas, which becomes trapped as expanding bubbles within the molten phase. Driven by high-pressure gradients, the mixture of molten material and gas is propelled through micro-arc channels toward the electrolyte, where it rapidly quenches and solidifies upon contact. This process creates a distinct microstructure characterized by solidified melt regions and a network of micropores left by evacuated gas bubbles, effectively determining the coating’s final morphology. A 10-min processing duration produced uniformly dense layers with a mean thickness of (6 ± 0.5) μm. Notably, the addition of these RE oxide particles into the BB does not cause significant modifications to the fundamental surface topography and thickness.

EDS elemental mapping of coatings synthesized in a BB with the addition of 4 g/L Ho₂O₃, Tb₄O₇, or Pr₆O₁₁ particles (Figure 3a) revealed a uniform spatial distribution of Al, O, and the respective RE elements throughout the composite layer, confirming successful incorporation of RE particles without significant agglomeration. Quantitative analysis of three independent samples and four distinct spatial locations per concentration (Figure 3b) showed that the levels of Ho, Tb, and Pr increased proportionally with the concentration of Ho₂O₃, Tb₄O₇, or Pr₆O₁₁ particles in the BB. The low standard deviations, represented by error bars, further underscore the high spatial uniformity of the elements.

Figure 4 shows the XRD patterns of coatings synthesized in the BB, both without and with 4 g/L Ho₂O₃, Tb₄O₇, or Pr₆O₁₁ particles. The observed diffraction peaks correspond primarily to the gamma-Al₂O₃ phase (ICCD card no.: 00-10-0425), a metastable phase whose formation is kinetically favored by the rapid quenching of molten alumina as it exits micro-discharge channels and contacts the cool electrolyte during the PEO process [33]. While Al peaks are detected from the underlying substrate, no discrete reflections for Ho, Tb, or Pr species are visible. This suggests that these RE species are present in concentrations below the XRD detection limit. Given the large mismatch in ionic radii between the massive RE ions and the small Al³⁺ host site, it is highly likely that they do not substitute directly into the gamma-Al₂O₃ lattice. Instead, they are expected to exist as highly dispersed species or to be localized at the grain boundaries and defect sites of the host matrix.

3.2. Photocatalytic Efficient of RE-Doped Al₂O₃ Coatings

The photocatalytic performance in MO degradation of pure and RE-doped Al₂O₃ coatings synthesized by PEO in BB with varying concentrations of Ho₂O₃, Tb₄O₇, and Pr₆O₁₁ particles is shown in Figure 5. To ensure statistical reliability, the reported photocatalytic efficiency values represent the arithmetic mean of five independently prepared samples for each dopant concentration. The experimental data indicate that incorporating RE ions significantly enhances the photocatalytic efficiency of the Al₂O₃ matrix. While the pristine Al₂O₃ coatings exhibited a photocatalytic efficiency of approximately 50% after 8 h of irradiation, the RE-doped variants demonstrated superior degradation kinetics. The peak degradation efficiencies were consistently observed at a concentration of 4 g/L for all RE oxides. After the 8-h irradiation period, the maximum degradation reached 88%, 92%, and 85% for coatings synthesized in BB with the addition of 4 g/L Ho₂O₃, Tb₄O₇, or Pr₆O₁₁, respectively.

The photocatalytic degradation kinetics of MO were modeled using the linearized Langmuir-Hinshelwood expression for pseudo-first-order reactions:

$$ln\left({\displaystyle \frac{C_o}{C}}\right)=k_{app}t$$

(1)

where k_app is the apparent pseudo-first-order rate constant. The k_app values and their corresponding linear correlation coefficients (R²) were obtained from the slopes of the ln(C_o/C) versus irradiation time plots (Figure 5d–f). The results show that the composite coatings synthesized in BB with the addition of 4 g/L Ho₂O₃, Tb₄O₇, or Pr₆O₁₁ achieved k_app values of about 0.274 h⁻¹, 0.339 h⁻¹, and 0.232 h⁻¹, respectively. These values indicate a significant increase in photocatalytic efficiency compared to the pure Al₂O₃ coating, which had a much lower k_app value of approximately 0.087 h⁻¹ demonstrating that the incorporation of RE effectively accelerates the degradation kinetics.

Since SEM and XRD analyses confirmed that the PEO coatings exhibit uniform morphology, thickness, and phase composition across all samples, the observed disparities in photocatalytic performance are attributed to the incorporation of RE ions into the Al₂O₃ matrix. The degree of doping is governed by the concentration of Ho₂O₃, Tb₄O₇, and Pr₆O₁₁ particles in BB. The resulting enhancement in photocatalytic efficiency is likely driven by modulation of charge-carrier recombination kinetics or sensitization of the wide-bandgap Al₂O₃ host to the visible light spectrum.

PL spectroscopy is an essential diagnostic tool for characterizing and optimizing photocatalytic materials [34,35]. While photocatalysis drives chemical transformations and PL involves radiative relaxation, both are competitive pathways originating from the same initial electronic excitation. Therefore, the efficiency of both processes depends on the underlying spatiotemporal dynamics of photo-excited charge carriers. PL emission occurs when a photo-excited electron recombines radiatively with a hole, emitting a photon. In semiconductor diagnostics, high PL intensity corresponds to increased electron–hole recombination rates, indicating that charge carriers are lost to photon emission rather than participating in chemical reactions. Suppressed PL intensity reflects improved charge separation efficiency. When radiative recombination is inhibited, it often indicates that carriers successfully migrate to the catalyst surface for redox reactions. Analytically, PL intensity serves as an indirect proxy for photocatalytic potential. Because radiative recombination and surface-mediated redox reactions are mutually exclusive decay channels, a reduced PL signal is often a hallmark of high-performance photocatalysts, indicating that charge carriers remain available to drive chemical reactions.

RE ions are essential components in high-performance optical materials due to their unique PL profiles [36]. These characteristics, including narrow-band emission and prolonged excited-state lifetimes, arise from their distinctive electronic configurations, specifically the partially filled 4f subshells. Luminescence in RE-doped systems is primarily governed by two distinct electronic mechanisms: 4f ⟶ 4f and 4f ⟶ 5d transitions [37,38]. In most trivalent RE³⁺ ions, radiative relaxation occurs via intra-configurational 4f transitions. Because these 4f orbitals are spatially shielded by the filled 5s² and 5p⁶ outer shells, they remain largely insulated from the external crystalline electric field. As a result, these transitions produce sharp, atomic-like emission spectra with peak positions that show minimal dependence on the host lattice. In contrast, certain RE ions exhibit inter-configurational 4f ⟶ 5d transitions. Unlike the 4f electrons, the 5d orbitals are parity-allowed and spatially exposed, making the resulting luminescence highly sensitive to the local coordination environment and site symmetry.

The PL excitation and emission spectra of RE-doped Al₂O₃ coatings synthesized by PEO in BB with varying concentrations of Ho₂O₃, Tb₄O₇, and Pr₆O₁₁ particles are shown in Figure 6. Under 453 nm excitation (Figure 6a), three emission bands are observed in the Ho-doped Al₂O₃ coatings. The strongest green PL emission band between 525 and 560 nm is attributed to the ⁵F₄, ⁵S₂ ⟶ ⁵I₈ transitions of Ho³⁺. The other two red bands, with significantly lower PL intensity, correspond to the Ho³⁺ transitions ⁵F₅ ⟶ ⁵I₈ (between 620 nm and 670 nm) and ⁵F₄, ⁵S₂ ⟶ ⁵I₇ (between 735 nm and 780 nm) [39]. The PL excitation spectra monitored at 544 nm, the wavelength of the most intense peak in the PL emission spectra, reveal two primary features. A broad band spanning 250 nm to 325 nm originates from the O 2p ⟶ Ho 4f charge transfer process [40]. A series of sharp, characteristic peaks between 325 nm and 500 nm result from the direct excitation of Ho³⁺ ions from the ⁵I₈ ground state to higher energy levels within the 4f manifold. Among these transitions, the ⁵I₈ ⟶ ⁵G₆, ⁵F₁ transitions, with a maximum at 453 nm, show the highest intensity, identifying them as the optimal excitation channel for the observed green luminescence. PL bands in both the excitation and emission spectra increase with the rising concentration of Ho₂O₃ particles in the BB.

The PL emission and excitation spectra of Tb-doped Al₂O₃ coatings are shown in Figure 6b. The PL emission spectra excited at 260 nm feature several major emission bands, attributed to the typical 4f ⟶ 4f transitions of Tb³⁺ ions from the excited level ⁵D₄ to the lower levels ⁷F_J (J = 3, 4, 5, and 6) [41]. Among these transitions, the green emission corresponding to ⁵D₄ ⟶ ⁷F₅ at 540 nm is the most dominant. The excitation PL spectra of Tb-doped Al₂O₃ coatings monitored at 540 nm consist of a broad band in the range from 250 nm to 300 nm, originating from the spin-allowed 4f⁸ ⟶ 4f⁷5d¹ transition of Tb³⁺ ions, and several small PL bands in the spectral range 300–450 nm corresponding to 4f ⟶ 4f transitions of Tb³⁺ ions [41]. The PL intensity of Tb³⁺ bands in both the excitation and emission spectra showed a positive correlation with the increasing concentration of Tb₄O₇ particles in the BB.

Figure 6c shows the PL emission spectra excited at 447 nm (³H₄ ⟶ ³P₂ transition of Pr³⁺) and the corresponding PL excitation spectra monitored at 642 nm (³P₀ ⟶ ³F₂ transition of Pr³⁺) for the Pr-doped Al₂O₃ coatings. The PL emission spectra, recorded in the 500–750 nm range, display characteristic bands originating from the 4f-4f intra-configurational transitions of Pr³⁺ [42]. These signals result from the relaxation of the ³P₀ and ³P₁ excited states to the lower-lying ³H_J (J = 4, 5, 6) and ³F_J (J = 2, 3, 4) manifolds. Notably, the red emission centered at 642 nm (³P₀ ⟶ ³F₂) is the dominant transition in the spectra. The PL excitation spectra show three distinct bands attributed to direct electronic excitation from the ³H₄ ground state to the ³P_J (J = 0, 1, 2) upper states. A positive correlation was observed between the Pr-doping concentration and the PL response. The PL intensity of the Pr³⁺ bands increases proportionally with the concentration of Pr₆O₁₁ particles in the BB, confirming the successful integration of Pr ions into the Al₂O₃ matrix.

The PL measurements presented in Figure 6 confirm that RE³⁺ is the predominant oxidation state for RE ions incorporated into the Al₂O₃ matrix. To elucidate the influence of these trivalent dopants on the photocatalytic efficacy of the Al₂O₃ coatings, it is necessary to assess their impact on the radiative transitions of the host lattice. Al₂O₃ coatings synthesized in BB exhibit the most intense PL under excitation at 260 nm [28]. Figure 7 shows the PL emission spectra, excited at 260 nm, for RE-doped Al₂O₃ coatings synthesized by PEO in BB, with varying concentrations of Ho₂O₃, Tb₄O₇, and Pr₆O₁₁ particles. The PL emission spectrum of pure Al₂O₃ coating is characterized by broad bands in the visible region, attributed to the presence of F⁺ and F centers [43]. Incorporation of Ho³⁺ ions induce pronounced quenching of these intrinsic Al₂O₃ PL bands (Figure 7a). A systematic, concentration-dependent attenuation of the PL emission intensity is observed, suggesting that Ho³⁺ centers act as efficient non-radiative recombination centers or charge-carrier traps. By establishing competitive charge-transfer pathways, these dopants facilitate the spatial separation of photo-generated electron–hole pairs, thereby suppressing radiative recombination and diminishing the cumulative PL yield. A similar trend is observed in Pr³⁺-doped Al₂O₃ coatings. The spectra reveal a high-intensity band between 350 nm and 450 nm, corresponding to the 5d-4f transitions of Pr³⁺, and multiple lower-intensity bands associated with characteristic 4f-4f transitions [44]. As the concentration of incorporated Pr³⁺ increases, the intrinsic Al₂O₃ PL emission is suppressed, while the specific Pr³⁺ transition bands increase slightly in intensity. These findings indicate that Pr³⁺ ions similarly inhibit the radiative recombination of the host matrix, effectively modulating the electronic environment of the Al₂O₃ bands. Tb³⁺ ions exhibit intense PL that spectrally overlaps with the intrinsic emission of Al₂O₃. This complicates direct deconvolution of the host PL response, but the consistent behavior of Ho³⁺ and Pr³⁺ suggests that Tb³⁺ has a similar effect on charge carrier dynamics.

The distinct splitting and intensity of the RE³⁺ emission bands (Figure 6) serve as an internal probe of the local crystal field within the PEO-grown Al₂O₃ matrix. The high sensitivity of the 4f-5d transitions in Tb³⁺ and Pr³⁺ to their coordination environment indicates that the RE ions occupy sites that effectively facilitate electronic interaction with the host lattice, thereby optimizing the charge-trapping process necessary for enhanced photocatalysis.

The DRS results, which illustrate the optical absorption characteristics of the synthesized coatings, are shown in Figure 8. The absorption profile is mainly determined by the wide band gap of the Al₂O₃ matrix, resulting in dominant, broad absorption features in the mid-ultraviolet spectral region [28]. Depending on its crystalline phase, pure Al₂O₃ has a broad band gap that typically ranges from 7.0 eV to 8.8 eV [8]. In contrast, PEO-synthesized Al₂O₃ coatings in BB electrolyte exhibit a significantly smaller optical band gap of approximately 4.6 eV [28]. Defect-induced electronic states (DS) in the forbidden energy region are responsible for this notable decrease compared to the stoichiometric standard [45]. The presence of oxygen vacancies, interstitial aluminum ions, and high-density grain boundaries introduces intermediate energy levels between the valence and conduction bands, enabling lower-energy electronic transitions.

The incorporation of Ho³⁺ dopants causes subtle changes in these optical properties (Figure 8a). Specifically, a distinct, low-intensity absorption band appears at approximately 452 nm, attributed to the ⁵I₈ ⟶ ⁵G₆, ⁵F₁ electronic transitions within the Ho³⁺ ions. Additionally, the increased absorption observed between 200 nm and 250 nm is assigned to the electronic transition from the O²⁻ (2p) orbitals to the Ho³⁺ (4f) orbitals (charge transfer band—CTB) [46]. While the incorporation of Tb³⁺ ions into the Al₂O₃ matrix causes negligible changes to the host’s absorption profile (Figure 8b), the incorporation of Pr³⁺ dopants induces a prominent absorption band in the 200–270 nm spectral range (Figure 8c). This feature is attributed to the parity-allowed 4f² ⟶ 4f¹5d¹ electronic transitions of Pr³⁺ ions.

Based on the preceding results, it can be inferred that the primary role of incorporated RE³⁺ ions in the Al₂O₃ matrix is to inhibit the recombination of photo-generated electron–hole pairs. This is supported by the observation that adding RE oxide particles to the BB produces no discernible impact on the morphology, crystal structure, or visible-light absorption properties of the coatings. A proposed mechanism for the improved photocatalytic performance of RE-doped Al₂O₃ coatings in MO degradation is detailed below and illustrated schematically in Figure 9. Upon excitation by ultraviolet radiation, the Al₂O₃ coatings generate electrons (e⁻) and holes (h⁺). Photo-generated electrons from the Al₂O₃ are effectively sequestered by the partially filled 4f orbitals of the RE ions (RE³⁺ + e⁻ ⟶ RE²⁺) [47]. This confinement mitigates the rapid recombination of electron–hole pairs, a primary kinetic bottleneck in photocatalysis. Adsorbed oxygen molecules subsequently scavenge these trapped electrons to form superoxide radicals ^●O₂⁻ (RE²⁺ + O₂ ⟶ RE³⁺ + ^●O₂⁻). Concurrently, holes from the Al₂O₃ valence band react with adsorbed water or hydroxide ions to produce hydroxyl radicals ^●OH (h⁺ + H₂O ⟶ H⁺ + ^●OH; h⁺ + OH⁻ ⟶ ^●OH). These ROS are the primary agents for the oxidative degradation of organic pollutants. Furthermore, the incorporation of RE ions induces structural defects, specifically oxygen vacancies. These vacancies facilitate electron transfer between the reactants and the photocatalyst and are intrinsically linked to the active sites in heterogeneous photocatalytic reactions [48]. Oxygen species (O₂⁻ and OH⁻) can be adsorbed at these vacancy sites and converted into ^●OH and ^●O₂⁻, further accelerating the photocatalytic degradation process [48].

To clarify the role of ROS in MO photo-degradation, in situ radical scavenging experiments were conducted. The observed inhibitory effects indicate that ^●OH and ^●O₂⁻ radicals are the primary ROS responsible for the degradation process (Figure 10). These highly potent, non-selective oxidants promote the oxidative cleavage of the MO molecular framework, ultimately leading to complete mineralization into benign end products.

The choice between suspended and immobilized photocatalysts involves a trade-off between interfacial kinetics and engineering feasibility. Suspended powders provide superior surface area and higher initial reaction rates but are hindered by particle aggregation, limited light penetration due to turbidity, and high energy costs for post-treatment separation. Additionally, loose nanoparticles present significant inhalation and environmental toxicity risks. In contrast, immobilized catalysts, though sometimes limited by mass transfer, eliminate the need for complex recovery steps, enable continuous-flow processing, and reduce bioavailability hazards, making them more suitable for industrial-scale environmental remediation. A key factor in the industrial viability of immobilized photocatalysts is their physicochemical stability, which determines both operational lifespan and overall cost-effectiveness. To assess the long-term performance of RE-doped Al₂O₃ photocatalysts, the recyclability of the most efficient coating synthesized in BB with the addition of 4 g/L Tb₄O₇ particles was tested over six consecutive degradation cycles. As shown in Figure 11a, the PA demonstrated remarkable stability throughout the experiment. Post-experimental analysis showed no structural breakdown (Figure 11b), and the crystalline structure remained unchanged (Figure 11c).

4. Conclusions

This study demonstrated the synthesis of rare earth (RE) doped gamma-Al₂O₃ coatings on aluminum substrates using a one-step plasma electrolytic oxidation (PEO) process. After adding Ho₂O₃, Tb₄O₇, and Pr₆O₁₁ particles to a borax and boric acid electrolyte, the following conclusions were drawn:

• The PEO process produced porous, crystalline coatings dominated by the gamma-Al₂O₃ phase. Although the concentration of RE dopants increased proportionally with their concentration in the electrolyte, the fundamental surface morphology and crystal structure of the alumina matrix remained unchanged, ensuring consistent physical properties across all samples.
• The incorporation of RE³⁺ ions significantly improved the degradation efficiency of methyl orange (MO) under simulated solar irradiation. The optimal concentration for all RE oxides was 4 g/L. At this level, Tb-doped coatings achieved the highest degradation efficiency (92%), followed by Ho-doped (88%) and Pr-doped (85%) coatings, all substantially outperforming undoped Al₂O₃ (~50%).
• Photoluminescence spectroscopy confirmed that RE³⁺ ions (Ho³⁺, Tb³⁺, and Pr³⁺) were successfully incorporated into the lattice. The observed quenching of the intrinsic Al₂O₃ emission bands indicates that these RE ions act as effective charge-carrier traps. This mechanism suppresses the radiative recombination of photo-generated electron–hole pairs, thereby increasing the availability of charge carriers for redox reactions.
• Radical scavenging experiments identified ^●OH and ^●O₂⁻ radicals as the primary reactive oxygen species (ROS) responsible for the mineralization of MO. The RE dopants facilitate the formation of these ROS by sequestering electrons and promoting the creation of structural defects such as oxygen vacancies.
• The RE-doped Al₂O₃ coatings exhibited exceptional mechanical and chemical stability, maintaining high photocatalytic activity over six consecutive cycles without structural degradation or phase changes.

In summary, RE doping via PEO transforms wide-bandgap Al₂O₃ coatings into a high-performance, reusable, and stable photocatalyst. This approach provides a promising and environmentally friendly solution for advanced remediation of recalcitrant organic pollutants in industrial wastewater.