Photoluminescent and Photocatalytic Properties of Eu³⁺-Doped MgAl Oxide Coatings Formed by Plasma Electrolytic Oxidation of AZ31 Magnesium Alloy

Abstract

The synthesis of Eu³⁺-doped MgAl oxide coatings containing MgO and MgAl₂O₄ was accomplished through plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate electrolyte with the addition of Eu₂O₃ particles in various concentrations. Their morphological, structural, and above all, photoluminescent (PL) and photocatalytic activity (PA) were thoroughly investigated. PL emission spectra of MgAl oxide coatings feature characteristic emission peaks, which are ascribed to f–f transitions of Eu³⁺ ions from excited level ⁵D₀ to lower levels ⁷F_J. The charge transfer state of Eu³⁺ or direct excitation of the Eu³⁺ ground state ⁷F₀ into higher levels of the 4f-manifold are both attributed to the PL peaks that appear in the excitation PL spectra of the obtained coatings. The fact that the transition ⁵D₀ → ⁷F₂ (electrical dipole transition) in Eu³⁺-doped MgAl oxide coatings is considerably stronger than the transition ⁵D₀ → ⁷F₁ (magnetic dipole transition) indicates that Eu³⁺ ions occupied sites with non-inversion symmetry. Because of oxygen vacancy formation, the Eu³⁺-doped MgAl oxide coatings had a higher PA in the degradation of methyl orange than the pure MgAl oxide coating. The highest PA was found in Eu³⁺-doped MgAl oxide coating formed in an aluminate electrolyte with 4 g/L of Eu₂O₃ particles. The PA, morphology, and phase of Eu³⁺-doped MgAl oxide coatings did not change after several consecutive runs, indicating outstanding stability of these photocatalysts.

1. Introduction

Magnesium alloys are commonly used in the aircraft, automotive, military, medical, and electronic industries [1,2,3]. However, because of their high chemical reactivity, magnesium alloys are prone to corrosion, limiting their use in corrosive environments [4,5]. Surface modification is widely recognized as the most prevalent method for improving a magnesium alloy’s surface properties. A coating layer can enhance the corrosion resistance of the metallic substrate by acting as a barrier between the substrate and the environment [6,7,8]. Recently, many research groups have been investigating the plasma electrolytic oxidation (PEO) process as a surface-protective treatment for magnesium alloys [9,10,11,12,13,14].

PEO is a high-efficiency, low-priced, and environmentally friendly process for producing multicomponent oxide coatings with numerous functions on lightweight metals or metallic alloys [15,16,17]. The incidence of micro-discharging on the metal surface, which is complemented by gas evolution, indicates that the PEO process is connected with plasma creation [18,19,20]. Many processes such as light and heat emission and electrochemical and plasma-chemical reactions occur at the micro-discharge sites as a result of the elevated local temperature and pressure. These processes are responsible for altering the structure, content, and morphology of the oxide coatings that are produced.

Despite numerous articles investigating PEO of magnesium alloys [21,22,23,24], there is a lack of data describing the formation of PEO coatings on magnesium alloys with intended application as photocatalytic or as photoluminescent materials [25]. In this study, we showed that MgO and MgAl₂O₄ phases are present in coatings formed by PEO of AZ31 magnesium alloy in aluminate electrolyte. MgAl₂O₄ has been identified as a matrix component for the preparation of very efficient photoluminescent materials [26,27], whereas both MgO and MgAl₂O₄ have been recognized as efficient materials in the photocatalytic degradation of organic pollutants due to high concentration of surface vacancies and other defects [28,29]. MgO- and MgAl₂O₄-photoluminescent and photocatalytic active materials have previously been synthesized in powder form, limiting their practical applications, particularly in the field of photocatalysis, due to the high cost of photocatalyst separation and recovery from suspension. Additionally, the easy aggregation of photocatalyst particles renders them unsuitable for use in flow systems.

The main goal of presented research was to synthesize Eu³⁺-incorporated oxide coatings by PEO on AZ31 magnesium alloy in an aluminate electrolyte with the addition of Eu₂O₃ particles and to examine their photoluminescent and photocatalytic capabilities. Our recent investigation has showed that Eu³⁺-doped PEO-based coatings on different substrates (Ti, Zr, Nb, Hf, Gd, Y, Zn) can be formed in electrolyte containing Eu₂O₃ [30,31,32,33,34,35]. Among various trivalent rare earth ions, the Eu³⁺ ion is most promising among light-emitting materials, exhibiting intense PL red emission under UV excitation due to the ⁵D₀ → ⁷F₂ transition [36]. In addition, positive effects of Eu³⁺ ion doping on the photocatalytic capabilities of several materials have been described in the literature [37,38,39]. The only partially populated 4f orbital structure of Eu³⁺ ions is favorable for electron trapping, which boosts photocatalytic activity by decreasing charge carrier recombination.

2. Materials and Methods

In this research, an AZ31 magnesium alloy (96% Mg, 3% Al, 1% Zn, Alfa Aesar) plate was used as a substrate. The substrates were cut in rectangular pieces with dimensions of 25 mm × 10 mm × 0.81 mm and then ultrasonically cleaned with acetone, warm air stream dried, and covered with insulating resin, leaving only the 15 mm × 10 mm active surface exposed to the electrolyte. More details about the experimental setup can be found in [40]. PEO coatings were formed in an aluminate electrolyte (5 g/L NaAlO₂) with Eu₂O₃ particles added in concentrations up to 8 g/L. PEO was done for 10 min at 150 mA/cm² constant current density. The electrolyte was kept at a constant temperature of 20 ± 1 °C. Following the PEO procedure, the samples were rinsed with distilled water and air dried.

The structural and elemental analyses of the PEO coatings were carried out utilizing scanning electron microscope (SEM, JEOL 840A, Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS, Oxford INCA, Abingdon, UK). A Rigaku Ultima IV diffractometer (XRD, Tokyo, Japan) equipped with a CuKα radiation source was used for phase identification of PEO coatings. The ultraviolet–visible diffusion reflectance spectra (DRS) were collected by a Shimadzu UV-3600 spectrophotometer (Tokyo, Japan) coupled with an integrating sphere. The spectrofluorometer (Horiba Jobin-Yvon Fluorolog FL3-22, Edison, NJ, USA) using a xenon lamp as radiation source was used to measure excitation and emission photoluminescence (PL) spectra at room temperature.

Under simulated solar radiation, photodegradation of methyl orange (MO) was investigated to probe the photocatalytic activity (PA) of formed coatings. In a 6.8 cm diameter open cylindrical thermostated pyrex glass reactor, a solution containing 8 mg/L of MO was kept in the dark for one hour to achieve absorption–desorption equilibrium. The concentration of MO remained nearly constant, indicating that MO adsorption was negligible. Following that, samples were exposed to the light of a 300 W incandescent lamp (Solimed BH Quarzlampen, Leipzig, Germany) placed 25 cm above the top surface of the solution. On the steel wire holder, the samples were placed 5 mm above the reactor bottom. PA was estimated by monitoring MO decomposition in selected time intervals after exposure to irradiation. A spectrophotometer (UV-Vis Thermo Electron Nicolet Evolution 500, Sittingbourne, England) was used to measure the MO concentration using the maximum MO absorption peak at 464 nm.

Free hydroxyl radicals (^•OH) have high potential rate to photodegrade MO and other organic pollutants. To identify possible creation of ^•OH on irradiated PEO coatings, PL measurements were carried out using terephthalic acid, because it is observed that it reacts with ^•OH, thus producing extremely fluorescent 2-hydroxyterephthalic acid with a PL maximum positioned at 425 nm [41]. The PEO coatings were placed in reactor with 10 mL of 5 × 10⁻⁴ mol/L terephthalic acid, which was diluted in aqueous solution of 2 × 10⁻³ mol/L NaOH. After the irradiation, PL spectra of the solution in which the reaction took place were collected utilizing the spectrofluorometer under the excitation wavelength of 315 nm.

3. Results and Discussion

3.1. Morphology, Chemical, and Phase Composition of Eu-Doped MgAl Oxide Coatings

Figure 1 shows the top-surface morphology of coatings made in aluminate electrolyte with the addition of Eu₂O₃ particles. Obtained coatings are laced with a number of micro-discharging canals of changing diameter and areas formed by the freezing of molten materials between them. It can be also observed that the addition of Eu₂O₃ particles to aluminate electrolyte does not influence the surface morphology.

High temperatures which are inherent to PEO processing cause the melting of magnesium substrate, which enters the micro-discharging channels where it reacts with penetrated electrolyte. Products of these reactions, formed inside of micro-discharges, are ejected towards the oxide/electrolyte surface, where they promptly solidify upon contacting colder electrolyte. The repetition of these steps on a vast number of discrete locations where micro-discharging occurs results in thickening of the oxide coating. After 10 min of PEO, the formed coatings are roughly 22 μm thick and weakly dependent on the Eu₂O₃ particles’ concentration in the aluminate electrolyte (Figure 2).

Table 1. EDS analysis of coatings in Figure 1.

Sample	Eu2O3 (g/L)	Atomic (%)
		O	Mg	Al	Eu
Figure 1a	0	65.98	15.78	18.24
Figure 1b	2	65.22	15.76	18.85	0.18
Figure 1c	4	65.35	16.02	18.21	0.41
Figure 1d	6	65.43	15.67	18.17	0.74
Figure 1e	8	64.96	16.44	17.67	0.92

presents the elemental composition of the coatings in Figure 1. The dominant elements in the coatings are Mg, Al, O, and Eu. The observed amount of Eu in coatings surges as the concentration of Eu₂O₃ in electrolyte increases. EDS elemental mapping reveals that observed elements are rather uniformly distributed in obtained coatings (Figure 3).

The XRD patterns of pure Eu₂O₃ particles, AZ31 magnesium substrate, and formed coatings displayed in Figure 4. Observed diffraction patterns are characterized by XRD peaks that correspond to cubic phases of MgO (JCPDS card No. 79-0612) and MgAl₂O₄ (JCPDS card No. 77-0435). Diffraction peaks from the substrate are significant due to X-ray penetration through oxide coating and its reflection off the substrate. Despite the fact that elemental analysis confirmed the presence of Eu in the coatings, XRD analysis revealed no XRD maxima which can be attributed to Eu-containing crystalline phases.

The melting point of the Eu₂O₃ particles is around 2400 °C, which is considerably less than the estimated local temperature at the micro-discharging sites during PEO of AZ31 magnesium alloy [42]. Inside the micro-discharge channels, molten Eu₂O₃ particles can react with other components (originating either from substrate or from electrolyte) and participate in the formation of mixed-oxide coatings. The lack of diffraction peaks associated with Eu is most likely due to the uniform distribution of Eu throughout the surface coatings. It can also be observed that low Eu content in obtained coatings does not considerably change the crystalline structure of MgO and MgAl₂O₄.

3.2. PL of Eu³⁺-Doped MgAl Oxide Coatings

The PL of MgAl oxide coatings with incorporated Eu is a combination of PL inherent to MgAl and Eu ions. PL excitation and emission spectra of MgAl oxide coating obtained in aluminate electrolyte are presented in Figure 5. The emission spectrum shows three dominant PL bands under ultraviolet excitation. The PL emission band centered at around 325 nm can be attributed to F⁺ centers in MgO [40], while the PL band from 360 nm to 600 nm can be attributed to structural defects in the coating, such as Mg vacancies and interstitials [43,44]. The band with a maximum near 725 nm can be related to F⁺ centers in MgAl₂O₄ [45].

The PL emission spectra of oxide coatings formed in aluminate electrolyte with varying concentration of Eu₂O₃ is presented in Figure 6a. Obviously, PL emission spectra show two discrete regions: the first one contains a typical broad PL band of MgAl oxide coatings, whereas the second one contains well-defined PL emission bands intrinsic to Eu³⁺ 4f–4f transitions from the excited ⁵D₀ level to the lower ⁷F_J levels (J = 0, 1, 2, 3, and 4) superimposed on PL of MgAl oxide coatings [36]. The PL bands are positioned at about 580 nm (⁵D₀ → ⁷F₀), 592 nm (⁵D₀ → ⁷F₁), 614 nm (⁵D₀ → ⁷F₂), 659 nm (⁵D₀ → ⁷F₃), and 708 nm (⁵D₀ → ⁷F₄). The intensity of the PL emission of 4f–4f transitions of Eu³⁺ increases as the concentration of Eu³⁺ in MgAl oxide coatings rises, indicating that the concentration of Eu³⁺ which is necessary for concentration quenching has not yet been reached.

When PL excitation spectra are monitored at 614 nm (this wavelength corresponds to the most intense observed Eu³⁺ transition), an intense wide-ranging PL band in the region from 250 nm to 350 nm is observed (Figure 6b), which is followed by a series of narrow low-intensity bands (Figure 6c). Because the contribution of PL caused by optical transitions of MgAl oxide coatings is negligible, the broad PL band appears as a result of electron transfer between the fully occupied 2p orbital of O²⁻ and the incompletely occupied 4f orbital of the Eu³⁺ [46]. Sharp bands, as shown in Figure 6c, mostly correspond to direct excitation of the Eu³⁺ ground state ⁷F₀ into higher levels of the 4f-manifold [47]. The ⁷F₀ → ⁵D₂ transition at 469 nm is the most intense of these excitation transitions and is used to record the PL emission spectra (Figure 6d).

When PL emission spectra are monitored under 469 nm excitation, typical narrow bands corresponding to the intra-configurational 4f–4f transitions of Eu³⁺ are observed. The intensity of the observed narrow band increases when the concentration of Eu³⁺ in MgAl oxide coatings increases, but the peak positions remain constant. Another interesting observation is the very high intensity of hypersensitive electric dipole transition ⁵D₀ → ⁷F₂ at 614 nm compared to magnetic dipole transition ⁵D₀ → ⁷F₁ at 592 nm. The electric dipole transition ⁵D₀ → ⁷F₂ (ΔJ = ±2) is strongly dependent on changes in the environment around Eu³⁺ ions, while the magnetic dipole transition ⁵D₀ → ⁷F₁ (ΔJ = ±1) does not depend on the surrounding environment [36]. In other words, when Eu³⁺ ions are incorporated into non-inversion symmetry lattice sites, then ⁵D₀ → ⁷F₂ electric dipole transition becomes dominant. In contrast, if Eu³⁺ ions are positioned at the inversion symmetry sites then the ⁵D₀ → ⁷F₁ magnetic dipole transition becomes most prominent. In MgAl oxide coatings, the observed intensity of magnetic dipole transition is much lower than the corresponding intensity of electric dipole transition, suggesting that Eu³⁺ ions are positioned at non-inversion symmetry lattice sites.

The asymmetric ratio R between the transitions ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ provides information about the surroundings of incorporated Eu³⁺ ions. R values were calculated employing the equation [48]:

$$R=\frac{I\left({}^5{\mathrm{D}}_0 \to { }^7{\mathrm{F}}_2\right)}{I\left({}^5{\mathrm{D}}_0 \to { }^7{\mathrm{F}}_1\right)}$$

(1)

where I(⁵D₀ → ⁷F₂) and I(⁵D₀ → ⁷F₁) are the integrated intensities for the transitions ⁵D₀ → ⁷F₂ (600 nm to 635 nm) and ⁵D₀ → ⁷F₁ (585 nm to 600 nm), respectively. In MgAl oxide coatings, a high R value indicates an environment with a low degree of symmetry around the Eu³⁺ ions. Figure 7 clearly shows that an increase in Eu³⁺ concentration in the coatings results in higher R values, implying that increased concentration of Eu³⁺ in oxide coating results in lattice distortion and decreased symmetry around the Eu³⁺ ions. Because of differences in ionic radii between Eu³⁺ (95 pm), Mg²⁺ (66 pm), and Al³⁺ (51 pm), after replacing the Mg²⁺ or Al³⁺ ions, Eu³⁺ ions will create oxygen vacancies and cause the lattice distortion due to the size mismatch [49,50,51].

3.3. PA of Eu³⁺-Doped MgAl Oxide Coatings

PA and optical absorption properties of pristine and Eu³⁺ doped MgAl oxide coatings formed by PEO processing of AZ31 in aluminate electrolyte with varying concentration of Eu₂O₃ are presented in Figure 8. As a result of the large bandgap between MgO [43] and MgAl₂O₄ [52], the broad absorption band in the mid-UV region is observed for MgAl oxide coatings. Although the MgAl coating’s absorption band is detrimental for photocatalytic applications utilizing sunlight as the irradiation source, a high concentration of different types of oxygen vacancies and other defects generated in MgAl during the PEO processing results in good PA of MgAl coating. Because of the high concentration of surface vacancies, the PA of pure MgAl coating in MO degradation is considerably higher than that of commonly used TiO₂ [30], MgO [25], ZrO₂ [53], and Nb₂O₅ [54] coatings formed by PEO on titanium, AZ31 magnesium alloy, zirconium, and niobium substrates, respectively. Surface vacancies, which determine electron transfer between reactants and photocatalysts, were found to be closely related to active centers of heterogeneous photocatalytic reactions [55]. The PA of Eu³⁺-doped MgAl oxide coatings is significant in comparison with pristine MgAl oxide coating (Figure 8a). Because the incorporation of Eu³⁺ ions into MgAl oxide coatings has no effect on their optical absorption in comparison to pristine MgAl oxide coating (Figure 8b), increased PA can be related to the interaction of Eu³⁺ ions and MgAl oxide coatings. The incorporation of Eu³⁺ ions into MgAl oxide coatings during the PEO result in the formation of oxygen vacancy charge-trapping centers, which can lower the recombination rate of electrons and holes. As a result, Eu³⁺-doped MgAl oxide coatings have higher PA than pure MgAl oxide coatings.

It can be clearly seen that PA is dependent on the concentration of Eu₂O₃ particles which are added to aluminate electrolyte, i.e., Eu³⁺ in MgAl oxide coatings. The PA of the Eu³⁺-doped MgAl oxide coating formed in aluminate electrolyte with the addition of 4 g/L Eu₂O₃ is the highest, and it begins to decrease as the concentration of Eu₂O₃ particles increases. As a result, the most likely cause of PA decrease is that higher concentration of Eu³⁺ ions in MgAl oxide coatings increases the concentration of recombination centers for electron–hole pairs. Tests in the dark show that MO self-degradation is negligible.

Photo-generated electron–hole pairs formed on photocatalytic surfaces produce various active oxygen species such as the superoxide anion radical (^•O²⁻), hydrogen peroxide (H₂O₂), singlet oxygen (¹O₂), and the hydroxyl radical (^•OH) [56]. ^•OH radicals are primarily responsible for organic pollutants’ degradation [57]. PL measurements performed in terephthalic acid on the surface of coatings formed in aluminate electrolyte with and without the addition of 4 g/L Eu₂O₃ particles are employed to test the formation of ^•OH radicals. The quantity of produced ^•OH radicals directly correlates with the intensity of PL [41]. Longer illumination times increase the PL intensity of both MgAl and Eu³⁺-doped MgAl coatings, but the latter exhibits higher PL intensity at 425 nm emission (Figure 9). This observation is not surprising, since Eu³⁺-doped MgAl coating shows higher PA.

One of the most important requirements for the photocatalyst’s applicability is its stability, which regulates the catalyst’s lifetime and operating costs. The PA after five consecutive runs using the most photocatalytically active formed coating is shown in Figure 10a. The sample was rinsed with water and dried between the runs. Figure 10a demonstrates that the PA did not decrease. Additionally, the morphology (Figure 10b) and phase structure (Figure 10c) did not change after five runs, proving the reliability and effectiveness of the Eu³⁺-doped MgAl oxide coatings formed by PEO as photocatalysts.

4. Conclusions

In summary, Eu³⁺-doped MgAl coatings were formed by plasma-electrolytic oxidation of AZ31 magnesium alloy in aluminate electrolyte which contains Eu₂O₃ particles. The coatings are crystallized and composed of MgO and MgAl₂O₄. DRS spectra of coatings exhibit a wide absorption band in the mid-UV region.

The emission PL spectra of Eu³⁺-doped MgAl coatings display narrow emission bands, which are attributed to Eu³⁺ ion ⁵D₀ → ⁷F_J transitions. The excitation PL spectra of MgAl coatings doped with Eu³⁺ have a strong broad band which is related to the electron transfer transition from the 2p orbital of O²⁻ ions to the 4f orbital of Eu³⁺ ions followed by weak, well-defined peaks corresponding to direct excitation of the Eu³⁺ ground state ⁷F₀ into higher levels of the 4f-manifold. The electrical dipole transition ⁵D₀ → ⁷F₂ is significantly stronger than the magnetic dipole transition ⁵D₀ → ⁷F₁, indicating the presence of Eu³⁺ ions in an asymmetric environment.

The PA of MgAl coatings doped with Eu³⁺ is determined by the concentration of Eu₂O₃ particles in aluminate electrolyte, i.e., Eu³⁺ incorporated into MgAl coatings, with 4 g/L being the optimum Eu₂O₃ concentration in the aluminate electrolyte. Because the incorporation of Eu³⁺ particles into MgAl coatings has no significant effect on the absorption properties of formed coatings when compared to pure MgAl oxide coating, the increased PA of Eu³⁺-doped MgAl coatings is due to the formation of oxygen vacancy charge-trapping centers, which reduces the recombination rate of electrons and holes.