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Article

MgAl Oxide Coatings Modified with CeO2 Particles Formed by Plasma Electrolytic Oxidation of AZ31 Magnesium Alloy: Photoluminescent and Photocatalytic Properties

by
Stevan Stojadinović
1,2,* and
Nenad Radić
3
1
Faculty of Physics, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
2
Faculty of Forestry, University of Belgrade, Kneza Višeslava 1, 11000 Belgrade, Serbia
3
IChTM-Department of Catalysis and Chemical Engineering, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
*
Author to whom correspondence should be addressed.
Metals 2024, 14(3), 366; https://doi.org/10.3390/met14030366
Submission received: 27 February 2024 / Revised: 18 March 2024 / Accepted: 19 March 2024 / Published: 21 March 2024
(This article belongs to the Topic Materials and Surface Treatment Processes Used for Engineering Applications)
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Abstract

:
MgAl oxide coatings composed of MgO and MgAl2O4 phases were doped with CeO2 particles via plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy in a 5 g/L NaAlO2 water solution. Subsequently, particles of CeO2 up to 8 g/L were added. Extensive investigations were conducted to examine the morphology, the chemical and phase compositions, and, most importantly, the photoluminescent (PL) properties and photocatalytic activity (PA) during the photodegradation of methyl orange. The number of CeO2 particles incorporated into MgAl oxide coatings depends on the concentration of CeO2 particles in the aluminate electrolyte. However, the CeO2 particles do not significantly affect the thickness, phase structure, or surface morphology of the coatings. The PL emission spectrum of MgAl oxide coatings is divided into two bands: one in the 350–600 nm range related to structural defects in MgO, and another much more intense band in the 600–775 nm range attributed to the F+ centres in MgAl2O4. The incorporated CeO2 particles do not have a significant effect on the PL intensity of the band in the red spectral region, but the PL intensity of the first band increases with the concentration of CeO2 particles. The PA of MgAl/CeO2 oxide coatings is higher than that of pure MgAl oxide coatings. The MgAl/CeO2 oxide coating developed in aluminate electrolyte with a concentration of 2 g/L CeO2 particles exhibited the highest PA. The MgAl/CeO2 oxide coatings remained chemically and physically stable across multiple cycles, indicating their potential for applications.

1. Introduction

Plasma electrolytic oxidation (PEO) is an eco-friendly electrochemical surface treatment that produces highly stable oxide coatings on a variety of metals (Mg, Al, Ti, Ta, Nb, Zr) and their alloys with a high crystallinity, substrate adhesion, and physical, chemical, and thermal stability [1,2,3,4]. PEO necessitates a high anodic voltage (several hundred volts) to promote the local dielectric breakdown of the growing oxide film, resulting in numerous short-lived micro-discharges formed continuously over the metal electrode surface [5]. The breakdown process entails intricate electrochemical, thermal, and plasma processes that incorporate both metal and electrolyte components into coatings.
Magnesium and magnesium alloys are desirable materials for practical applications due to their low density; high strength-to-weight ratio; excellent dimensional stability, biodegradability, and biocompatibility; large hydrogen storage capacity; high specific capacity for batteries; good electromagnetic shielding; high machinability; and so on [6,7]. The foremost drawback of magnesium-based materials is their poor resistance to corrosion, which limits their application [8]. Magnesium and its alloys’ surfaces are commonly modified to improve their corrosion resistance and to create functional coatings suitable for various applications [9].
Lately, PEO has become increasingly popular as a method for producing multifunctional coatings and improving the surface characteristics of magnesium and its alloys [10,11,12,13,14,15,16,17,18]. Some studies have shown that oxide coatings formed on magnesium alloys can be used in photocatalytic applications [19,20,21,22,23]. The present study focuses on the investigation of the photoluminescent (PL) and photocatalytic properties of coatings formed via PEO of AZ31 magnesium alloy in aluminate electrolyte with the addition of CeO2 particles at different concentrations. PEO formed coatings on AZ31 magnesium alloys in aluminate electrolytes containing MgO and MgAl2O4 phases [22]. Both of these phases have found application in photocatalysis [24,25] and as hosts for photoluminescence materials [26,27] due to the presence of different types of oxygen vacancies and other defects.
Adding CeO2 particles to the electrolyte causes their incorporation into the coatings during PEO in magnesium alloys, which improves their corrosive properties and resistance to wear [28,29]. This is critical in engineering applications involving oxide coatings on magnesium alloys. The main idea behind this work was to create MgO/MgAl2O4/CeO2 coatings in order to improve the photocatalytic and PL properties of a single MgO/MgAl2O4 coating and optimize the amount of CeO2 particles in the electrolyte. The properties of CeO2, such as its low toxicity, strong oxygen storage capacity, high chemical stability, and ability to display dual oxidation states of cerium, Ce3+/Ce4+, have drawn a lot of attention in the field of photocatalysis due to the formation of abundant oxygen vacancies in CeO2, which serve as active electron trap centres, inhibiting recombination of photogenerated electron/hole pairs [30,31,32,33,34].
The photocatalytic and PL properties of the MgO/CeO2 and MgAl2O4/CeO2 systems have received little attention in the literature [35,36,37], with no data available for the MgO/MgAl2O4/CeO2 system. Our study has, for the first time, shown that PEO applied to magnesium alloys can generate MgO/MgAl2O4/CeO2 coatings suitable for photocatalytic and PL applications. Consequently, the MgO/MgAl2O4/CeO2 coatings formed by PEO were examined using SEM/EDS, XRD, Raman spectroscopy, and DRS to investigate the effect of CeO2 on the morphology, chemical composition, crystal structure, and absorption properties of MgO/MgAl2O4/CeO2. The PL of the MgO/MgAl2O4/CeO2 coatings was thoroughly investigated, as the incorporation of CeO2 into MgO/MgAl2O4 coatings increases the PL intensity by more than an order of magnitude. The photocatalytic efficiency of the coatings was evaluated through the degradation of methyl orange (MO) dye. MO is an example of a typical azo-anionic dye, which is hard to degrade, hazardous to the environment, and potentially dangerous to human health if it finds its way into soil and water resources.

2. Materials and Methods

The starting material for the preparation of PEO coatings was a rectangular sample (25 mm × 10 mm × 0.81 mm) of AZ31 magnesium alloy (96% Mg, 3% Al, 1% Zn, Alfa Aesar, Ward Hill, MA, USA). Ultrasonic cleaning using acetone and subsequent drying with a warm air stream were included in the sample preparation process for PEO. After this, the samples were coated with an insulating resin, ensuring that the electrolyte only made contact with the 15 mm × 10 mm active surface.
The electrolytic cell was made of double-walled glass and cooled with water (refer to Figure 10 in ref. [22]). A magnetic stirrer was employed to mix the electrolyte in the electrolytic cell, ensuring the even distribution of CeO2 particles. A tubular stainless-steel cathode was positioned around the anode samples of AZ31 magnesium alloy, which were used and positioned in the centre of the electrolytic cell. The electrolyte solution was prepared by adding CeO2 particles at concentrations of 1 g/L, 2 g/L, 4 g/L, and 8 g/L into a water solution containing 5 g/L of NaAlO2. The PEO processes were conducted using a DC power source (Consort EV261) at a constant current density of 150 mA/cm2 for 10 min. The electrolyte temperature was maintained at (20 ± 1) °C. Following the PEO process, samples were rinsed with distilled water to prevent the accumulation of electrolyte components during the drying process.
The morphology, thickness, elemental, and phase analyses of the PEO coatings were performed using a scanning electron microscope (SEM, JEOL 840A, Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS, Oxford INCA, Abingdon, UK), X-ray diffraction (XRD, Rigaku Ultima IV, Tokyo, Japan), and Raman spectroscopy (TriVista 557 Raman system, S&I GmbH, Germany). UV-Vis diffuse reflectance spectra (DRS) were employed, utilizing a Shimadzu UV-3600, Tokyo, Japan, to analyse the optical properties of the PEO coatings. To acquire room temperature PL excitation and emission spectra, a spectrofluorometer (Horiba Jobin Yvon, Fluorolog FL3-22, Edison, NJ, USA) was utilized with a 450 W xenon lamp as the excitation source.
To assess the photocatalytic activity (PA) of the coatings, the photodegradation of MO, serving as a model compound for organic pollution, was carried out at 20 °C under simulated artificial solar radiation. A photocatalytic reactor featuring double-walled glass with water cooling was utilized (see Figure 1 in ref. [38]). The samples were positioned on the stainless-steel holder, 5 mm above the bottom of the photocatalytic reactor. A magnetic stirrer that was positioned underneath the holder was used to mix the 10 cm3 solution of MO. The MO concentration was 8 mg/L at first. The samples were exposed to illumination from a 300 W light source (OSRAM ULTRA-VITALUX UV-A, Munich, Germany) positioned 25 cm above the solution’s upper surface. To achieve an adsorption–desorption equilibrium, the initial MO solution and samples were left in the dark for an hour prior to illumination. PA was assessed by monitoring the decomposition of MO following an appropriate duration of light exposure. The maximum absorption peak of MO at 464 nm was measured utilizing a UV-Vis spectrometer (Thermo Electron Nicolet, Evolution 500, Cambridge, UK). The absorbance was transformed into MO concentration utilizing a standard curve that exhibited a linear correlation between concentration and absorbance at this wavelength.

3. Results and Discussion

Figure 1 illustrates the voltage–time characteristics observed during the anodization process of AZ31 magnesium alloy in a solution containing 5 g/L NaAlO2, both with and without the addition of 8 g/L CeO2 particles. As shown, the inclusion of CeO2 particles did not produce any noticeable impact on the voltage–time curves, with two distinct regions being identifiable. The first region is linked to classical anodization and the formation of a thin dense oxide layer, characterized by an almost linear rise in voltage. The second region is correlated with dielectric breakdown of the formed compact oxide layer, indicated by a noticeable deviation from the linearity of the voltage–time curve and the occurrence of numerous micro-discharges.
Figure 2 displays the top view and cross-section SEM micrographs of the coating created in 5 g/L NaAlO2 with different concentrations of added CeO2 particles. Changing the concentration of CeO2 particles had no discernible effect on the surface morphology or coating thickness. All coatings share a common morphology, defined by the presence of molten regions dispersed throughout the surface created when the molten oxide heats up, melts, and then cools down in contact with the surrounding electrolyte and pores originating from gas bubbles released during the PEO [39]. The coatings formed after 10 min of PEO are about (22 ± 1) μm thick.
The results of the integrated EDS analysis of the coatings are given in Table 1 (the relative errors are less than 5%). The coatings’ chemical constituents are Mg, Al, O, and Ce. The electrolyte is the main source of Al. Small amounts of Ce are present in the coatings, which increases with the concentration of CeO2 particles in the electrolyte.
The XRD patterns of PEO coatings formed in 5 g/L NaAlO2 with different concentrations of CeO2 particles, along with the XRD patterns of pure CeO2 particles and the AZ31 magnesium alloy substrate, are displayed in Figure 3a. The XRD pattern of CeO2 particles reveals peaks at 2θ values of 28.7, 33.2, 47.6, 56.4, 59.2, 69.5, 76.8, and 79.2 degrees, corresponding to the (111), (200), (220), (311), (222), (400), (331), and (420) crystalline planes of the cubic fluorite structure of CeO2 (JCPDS Card No. 75-0162). The formation of MgO (JCPDS card No. 79-0612) and MgAl2O4 (JCPDS card No. 77-0435) phases, as a result of an interaction between the AZ31 substrate and the electrolyte components, is confirmed by the XRD pattern of the PEO coating formed in 5 g/L NaAlO2 [22]. The significant diffraction peaks observed from the substrate are a result of X-ray penetration through the porous oxide layer and subsequent reflection from the substrate. The diffraction peaks of CeO2 can be clearly seen together with the diffraction peaks arising from MgO and MgAl2O4 in the XRD patterns of the PEO coatings formed in 5 g/L NaAlO2 with the addition of a high concentration of CeO2 particles (4 g/L and 8 g/L). This is primarily due to the low concentration of evenly distributed CeO2 particles throughout the surface coatings [22,23].
Raman measurements were conducted (Figure 3b) to verify the presence of CeO2 particles in the PEO coatings formed in a solution containing 5 g/L NaAlO2 with the addition of lower concentrations of CeO2 particles (1 g/L and 2 g/L). The F2g mode of the cubic fluorite structure of CeO2, identified as the prominent band in the Raman spectrum of CeO2 particles around 465 cm−1, is attributed to the symmetrical vibration of oxygen atoms surrounding Ce4+ [40]. All coatings formed in a solution containing 5 g/L NaAlO2 with the inclusion of CeO2 particles exhibit this mode in their Raman spectra, suggesting the integration of CeO2 particles into the coatings.
PEO facilitates the inclusion of electrolyte particles into coatings in three distinct forms: partially reactive, reactive, and inert forms [41]. The two main factors that determine the mode of incorporation are the melting point and the particle size. An inert mode of incorporation typically applies to particles with high melting points, such as CeO2 (approximately 2400 °C) [42], which is also applicable in our case.
PL excitation and emission spectra of the MgO/MgAl2O4 coatings are shown in Figure 4. The PL emission spectrum excited at 265 nm (Figure 4a) is characterized by a strong emission band in the red region with a maximum of about 720 nm, related to F+ centres in MgAl2O4 [43], and a broad band in the range of 350 nm to 600 nm, which is associated with oxygen vacancies (e.g., F, F+, F2, and F22+ centres) mostly in MgO [44,45]. Upon excitation at 340 nm (Figure 4b), two PL bands with peak positions at about 410 nm and 660 nm can be observed in the PL emission spectrum, which are related to oxygen vacancies in MgO [21].
The incorporation of CeO2 particles into MgO/MgAl2O4 coatings does not notably impact the photoluminescence (PL) intensity of the band peaking around 720 nm under 265 nm excitation. However, it does lead to a significant increase in the PL intensity of the broad band, with a maximum of around 410 nm (Figure 5a). The PL emission spectra, excited at 340 nm, further reveal that the PL intensity of the band peaking around 410 nm increases with the concentration of CeO2 particles incorporated into the MgO/MgAl2O4 coatings (Figure 5b). The ratio of PL intensity of coatings formed in a solution containing 5 g/L NaAlO2 with and without 8 g/L CeO2 particles is approximately 20. In addition to this PL band, a PL band with a maximum at around 520 nm, as well as PL bands in the red region with a weak intensity, can be observed in the PL emission spectra (Figure 5a,b).
The PL excitation spectra of MgO/MgAl2O4/CeO2 coatings monitored at 720 nm consist of one intense band with a maximum of around 265 nm (Figure 5c). The content of CeO2 in the MgO/MgAl2O4/CeO2 coatings does not affect the PL intensity of this band, which is in agreement with the corresponding PL emission spectra in Figure 4a. PL excitation spectra of MgO/MgAl2O4/CeO2 coatings monitored at 410 nm and 520 nm consist of at least three bands at about 265 nm, 315 nm, and 340 nm (Figure 5c,d). Among these excitation transitions, the one at 340 nm is the most intense.
The increase in the PL intensity of MgO/MgAl2O4/CeO2 coatings compared to pure MgO/MgAl2O4 is due to the creation of oxygen vacancies as a result of the incorporation of CeO2, because the PL originating from CeO2 particles is negligible. Bands with maxima at around 410 nm and 520 nm are attributed to F+ and F centres, respectively [46,47,48].
Figure 6a illustrates how the concentration of CeO2 particles in the electrolyte affects the MO photodegradation efficiency using the formed coatings. C0 is the initial concentration of MO, and its concentration at time t is C. For every CeO2 concentration, three samples were examined, and the mean values are displayed in Figure 6a. Samples collected under identical conditions have a very high reproducibility (within 3%) for the PA. The concentration of CeO2 particles added to the electrolyte affects the PA of the MgO/MgAl2O4/CeO2 coatings, which is significantly higher than that of MgO/MgAl2O4 coatings. The highest PA of MgO/MgAl2O4/CeO2 coatings was achieved with the addition of 2 g/L of CeO2 particles.
The first-order kinetic Langmuir Hinshelwood model (Figure 6b) provides a good description of the photocatalytic degradation of MO:
l n C o C = k a p p t
The table in Figure 6b presents the first-order kinetic constant kapp, along with the corresponding standard squared deviation (σ) and linear correlation coefficient (R2). The value of kapp was determined through non-linear least squares fitting conducted across the entire experimental time range [49]. As the concentration of CeO2 particles in the aluminate electrolyte increased up to 2 g/L, the degradation rate constant kapp increased from 0.0809 h−1 to 0.1273 h−1. The sensitivity of MO degradation to the content of CeO2 in MgO/MgAl2O4/CeO2 coatings was confirmed by a decrease in the degradation rate constant with the increase in the concentration of CeO2 in the aluminate electrolyte up to 8 g/L.
CeO2 particles have a very low PA in organic dye degradation due to the rapid recombination of photogenerated electron/hole pairs [50]. Because the concentration of CeO2 particles in the formed MgO/MgAl2O4/CeO2 coatings is so low, the contribution of CeO2 particles to the total PA of these coatings is negligible. Since the morphology, thickness, and phase structure of all the MgO/MgAl2O4/CeO2 coatings are essentially the same (Figure 2 and Figure 3), CeO2 particles contribute to the increasing PA of MgO/MgAl2O4 coatings primarily by extending their optical absorption range or by decreasing the prompt recombination of photogenerated electron/hole pairs.
The UV–Vis DRS spectra of CeO2 particles and the formed coatings are shown in Figure 7. A broad absorption band in the mid-UV region is typical for MgO/MgAl2O4 formed in an aluminate electrolyte [22]. The used CeO2 particles have an absorption band edge at approximately 440 nm. Due to the low concentration of CeO2 in the formed coatings, the shift in the absorption curves towards the visible region is insignificant, especially for low concentrations of CeO2 particles in the electrolyte (1, 2, and 4 g/L). This indicates that the increased PA of MgO/MgAl2O4/CeO2 coatings compared to MgO/MgAl2O4 coatings is due to a decrease in photogenerated electron/hole recombination rate as a result of MgO/MgAl2O4 and CeO2 coupling.
PL and PA measurements indicate that the high concentration of various types of oxygen vacancies and other defects is related to the significant PA of MgO/MgAl2O4/CeO2 coatings. The formation of oxygen vacancies during PEO introduces defect states within the material’s bandgap, facilitating photogenerated charge carrier separation and causing an increase in the PA of MgO/MgAl2O4/CeO2 coatings formed in aluminate electrolyte with the addition of CeO2 particles in relation to MgO/MgAl2O4 coatings formed in a pure aluminate electrolyte. The PA of MgO/MgAl2O4/CeO2 coatings varies with the concentration of CeO2 particles in the aluminate electrolyte. The MgO/MgAl2O4/CeO2 coating which was formed in the aluminate electrolyte with 2 g/L CeO2 particles had the highest PA. As the concentration of CeO2 in the aluminate electrolyte continues to increase, the PA decreases because CeO2 particles serve as photoinduced electron capture centres [51].
Ten consecutive photocatalytic tests were conducted on the most active photocatalyst in order to investigate the potential application of MgO/MgAl2O4/CeO2 coatings in photocatalysis. Figure 8 shows the recycling test of MO photodegradation along with the morphology and composition before and after 10 runs. The morphology, composition, and PA did not change, indicating that the produced photocatalyst exhibited a high degree of chemical and physical stability.

4. Conclusions

PEO of AZ31 magnesium alloy in an aluminate electrolyte with the addition of CeO2 particles at different concentrations was utilized to create MgO/MgAl2O4/CeO2 coatings. To examine the morphology, crystal structure, chemical composition, and optical and PL properties of the formed coatings, various techniques, including SEM/EDS, XRD, Raman spectroscopy, XPS, DRS, and PL, were employed. The photodegradation of MO under simulated sunlight was employed to evaluate the photocatalytic potential of MgO/MgAl2O4/CeO2 coatings.
The results can be summarized as follows:
  • The surface morphology, thickness, phase structure, and light-harvesting characteristics of MgO/MgAl2O4/CeO2 coatings are not significantly affected by the addition of CeO2 particles to the aluminate electrolyte.
  • As a result of the incorporation of CeO2 in the coatings during PEO, oxygen vacancies are created, which accounts for the increase in the PL intensity of MgO/MgAl2O4/CeO2 coatings over pure MgO/MgAl2O4 coatings, as the PL originating from CeO2 particles is barely noticeable.
  • The content of CeO2 particles in the aluminate electrolyte, i.e., the amount of CeO2 particles incorporated within MgO/MgAl2O4 coatings, determines the PA of the MgO/MgAl2O4/CeO2 coatings. The decrease in the photogenerated electron/hole recombination rate resulting from MgO/MgAl2O4 and CeO2 coupling is linked to the increased PA of MgO/MgAl2O4/CeO2. The MgO/MgAl2O4/CeO2 coating formed in aluminate electrolyte with the addition of 2 g/L CeO2 particles exhibits the highest PA.
  • The PA, morphology, and composition of the formed photocatalysts did not alter after multiple PA cycles, indicating their chemical and physical stability, which is a crucial requirement for any potential applications.

Author Contributions

Conceptualization, S.S.; methodology, S.S. and N.R.; validation, S.S., investigation, S.S. and N.R.; writing—original draft preparation, S.S. and N.R.; writing—review and editing, S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Education, Science, and Technological Development of the Republic of Serbia (grants 451-03-65/2024-03/200162 and 451-03-68/2023-14/200026) and by the European Union Horizon 2020 Research and Innovation program under the Marie Sklodowska-Curie grant agreement no. 823942 (FUNCOAT).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Kaseem, M.; Fatimah, S.; Nashrah, N.; Ko, Y.G. Recent progress in surface modification of metals coated by plasma electrolytic oxidation: Principle, structure, and performance. Prog. Mater. Sci. 2021, 117, 100735. [Google Scholar] [CrossRef]
  2. Simchen, F.; Sieber, M.; Kopp, A.; Lampke, T. Introduction to plasma electrolytic oxidation—An overview of the process and applications. Coatings 2020, 10, 628. [Google Scholar] [CrossRef]
  3. Tsai, D.-S.; Chou, C.-C. Review of the soft sparking issues in plasma electrolytic oxidation. Metals 2018, 8, 105. [Google Scholar] [CrossRef]
  4. Sikdar, S.; Menezes, P.V.; Maccione, R.; Jacob, T.; Menezes, P.L. Plasma electrolytic oxidation (PEO) process—Processing, properties, and applications. Nanomaterials 2021, 11, 1375. [Google Scholar] [CrossRef]
  5. Clyne, T.W.; Troughton, S.C. A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals. Int. Mater. Rev. 2019, 64, 127–162. [Google Scholar] [CrossRef]
  6. Yang, Y.; Xiong, X.; Chen, J.; Peng, X.; Chen, D.; Pan, F. Research advances of magnesium and magnesium alloys worldwide in 2022. J. Magnes. Alloys 2023, 11, 2611–2654. [Google Scholar] [CrossRef]
  7. Tan, J.; Ramakrishna, S. Applications of magnesium and its alloys: A review. Appl. Sci. 2021, 11, 6861. [Google Scholar] [CrossRef]
  8. Esmaily, M.; Svensson, J.E.; Fajardo, S.; Birbilis, N.; Frankel, G.S.; Virtanen, S.; Arrabal, R.; Thomas, S.; Johansson, L.G. Fundamentals and advances in magnesium alloy corrosion. Prog. Mater. Sci. 2017, 89, 92–193. [Google Scholar] [CrossRef]
  9. Gray, J.E.; Luan, B. Protective coatings on magnesium and its alloys—A critical review. J. Alloys Compd. 2002, 336, 88–113. [Google Scholar] [CrossRef]
  10. Zhao, C.; Wang, X.; Yu, B.; Cai, M.; Yu, Q.; Zhou, F. Research progress on the wear and corrosion resistant plasma electrolytic oxidation composite coatings on magnesium and its alloys. Coatings 2023, 13, 1189. [Google Scholar] [CrossRef]
  11. Darband, G.B.; Aliofkhazraei, M.; Hamghalam, P.; Valizade, N. Plasma electrolytic oxidation of magnesium and its alloys: Mechanism, properties and applications. J. Magnes. Alloys 2017, 5, 74–132. [Google Scholar] [CrossRef]
  12. Whitea, L.; Koo, Y.; Neralla, S.; Sankar, J.; Yun, Y. Enhanced mechanical properties and increased corrosion resistance of a biodegradable magnesium alloy by plasma electrolytic oxidation (PEO). Mater. Sci. Eng. B 2016, 208, 39–46. [Google Scholar] [CrossRef]
  13. Usmaniya, N.; Pillai, S.R.; Edalacheruvu, L.; Palanivel, M.; Chennampalli, P.; Vaithiyanathan, P.; Parfenov, E.; Lingamaneni, R.K.; Nagumothu, R. Effect of polycaprolactone coating on the corrosion and biological characteristics of plasma electrolytic oxidised ZM21 magnesium alloy. Surf. Coat. Technol. 2023, 471, 129915. [Google Scholar] [CrossRef]
  14. Fattah-alhosseini, A.; Molaei, M.; Nouri, M.; Babaei, K. Antibacterial activity of bioceramic coatings on Mg and its alloys created by plasma electrolytic oxidation (PEO): A review. J. Magnes. Alloys 2022, 10, 81–96. [Google Scholar] [CrossRef]
  15. Kumara, S.; Katyala, P.; Chaudhary, R.N.; Singh, V. Assessment of factors influencing bio-corrosion of magnesium-based alloy implants: A review. Mater. Today Proc. 2022, 56, 2680–2689. [Google Scholar] [CrossRef]
  16. Kröger, N.; Kopp, A.; Staudt, M.; Rusu, M.; Schuh, A.; Liehn, E.A. Hemocompatibility of plasma electrolytic oxidation (PEO) coated Mg-RE and Mg-Zn-Ca alloys for vascular scaffold applications. Mater. Sci. Eng. C 2018, 92, 819–826. [Google Scholar] [CrossRef]
  17. Husak, Y.; Olszaniecki, J.; Pykacz, J.; Ossowska, A.; Blacha-Grzechnik, A.; Waloszczyk, N.; Babilas, D.; Korniienko, V.; Varava, Y.; Diedkova, K.; et al. Influence of silver nanoparticles addition on antibacterial properties of PEO coatings formed on magnesium. Appl. Surf. Sci. 2024, 654, 159387. [Google Scholar] [CrossRef]
  18. Patrascu, I.; Ducu, M.C.; Negrea, A.D.; Moga, S.G.; Plaiasu, A.G. Overview on plasma electrolytic oxidation of magnesium alloys for medical and engineering applications. IOP Conf. Series Mater. Sci. Eng. 2022, 1251, 012001. [Google Scholar] [CrossRef]
  19. Tang, M.; Li, G.; Li, W.; Liu, H.; Zhu, L. Photocatalytic performance of magnesium alloy microarc oxides. J. Alloys Compod 2013, 562, 84–89. [Google Scholar] [CrossRef]
  20. Li, W.; Tang, M.; Zhu, L.; Liu, H. Formation of microarc oxidation coatings on magnesium alloy with photocatalytic performance. Appl. Surf. Sci. 2012, 258, 10017–10021. [Google Scholar] [CrossRef]
  21. Stojadinović, S.; Tadić, N.; Radić, N.; Grbić, B.; Vasilić, R. MgO/ZnO coatings formed on magnesium alloy AZ31 by plasma electrolytic oxidation: Structural, photoluminescence and photocatalytic investigation. Surf. Coat. Technol. 2017, 310, 98–105. [Google Scholar] [CrossRef]
  22. Stojadinović, S.; Radić, N.; Vasilić, R. ZnO particles modified MgAl coatings with improved photocatalytic activity formed by plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate electrolyte. Catalysts 2022, 12, 1503. [Google Scholar] [CrossRef]
  23. Stojadinović, S.; Radić, N.; Vasilić, R. Photoluminescent and photocatalytic properties of Eu3+-doped MgAl oxide coatings formed by plasma electrolytic oxidation of AZ31 magnesium alloy. Coatings 2022, 12, 1830. [Google Scholar] [CrossRef]
  24. Supin, K.K.; Saji, A.; Chanda, A.; Vasundhara, M. Effects of calcinations temperatures on structural, optical and magnetic properties of MgO nanoflakes and its photocatalytic applications. Opt. Mater. 2022, 132, 112777. [Google Scholar]
  25. Nitha, T.V.; Britto, S. MgAl2O4 nanospinel: Green synthesis, characterization and effective heterogeneous catalyst for the photocatalytic degradation of carbol fuchsin dye and synthesis of 2-aryl substituted benzoxazole derivatives. Inorg. Chem. Commun. 2024, 159, 111776. [Google Scholar] [CrossRef]
  26. Kiran, N.; Baker, A.P.; Wang, G.-G. Synthesis and luminescence properties of MgO: Sm3+ phosphor for white light-emitting diodes. J. Mol. Struct. 2017, 1129, 211–215. [Google Scholar] [CrossRef]
  27. Kumar, K.G.; Bhargav, P.B.; Aravinth, K.; Ahmed, N.N.; Balaji, C. Photoluminescence and electrochemical performance evaluation of Eu3+ doped MgAl2O4 phosphors for LED and energy storage applications. Ceram. Int. 2022, 48, 36038–36045. [Google Scholar] [CrossRef]
  28. Mohedano, M.; Blawert, C.; Zheludkevich, M.L. Silicate-based plasma electrolytic oxidation (PEO) coatings with incorporated CeO2 particles on AM50 magnesium alloy. Mater. Des. 2015, 86, 735–744. [Google Scholar] [CrossRef]
  29. Lim, T.S.; Ryu, H.S.; Hong, S.-H. Electrochemical corrosion properties of CeO2-containing coatings on AZ31 magnesium alloys prepared by plasma electrolytic oxidation. Corros. Sci. 2012, 62, 104–111. [Google Scholar] [CrossRef]
  30. Pan, J.; Wang, S.; Chen, A.; Chen, Y.; Wang, M.; Chen, Y. Visible-light-active mesoporous ceria (CeO2) nanospheres for improved photocatalytic performance. J. Alloys Compd. 2022, 898, 162895. [Google Scholar] [CrossRef]
  31. Kusmierek, E. A CeO2 semiconductor as a photocatalytic and photoelectrocatalytic material for the remediation of pollutants in industrial wastewater: A review. Catalysts 2020, 10, 1435. [Google Scholar] [CrossRef]
  32. Pudukudy, M.; Jia, Q.; Yuan, J.; Megala, S.; Rajendran, R.; Shan, S. Influence of CeO2 loading on the structural, textural, optical and photocatalytic properties of single-pot sol-gel derived ultrafine CeO2/TiO2 nanocomposites for the efficient degradation of tetracycline under visible light irradiation. Mater. Sci. Semicond. Process. 2020, 108, 104891. [Google Scholar] [CrossRef]
  33. Habib, I.Y.; Burhan, J.; Jaladi, F.; Lim, C.M.; Usman, A.; Kumara, N.T.R.N.; Tsang, S.C.E.; Mahadi, A.H. Effect of Cr doping in CeO2 nanostructures on photocatalysis and H2O2 assisted methylene blue dye degradation. Catal. Today 2021, 375, 506–513. [Google Scholar] [CrossRef]
  34. Salimi, K. Self-assembled bio-inspired Au/CeO2 nano-composites for visible white LED light irradiated photocatalysis. Colloids Surf. A 2020, 559, 124908. [Google Scholar] [CrossRef]
  35. Wang, S.; Gao, H.; Li, J.; Wang, Y.; Chen, C.; Yu, X.; Tang, S.; Zhao, X.; Sun, G.; Li, D. Comparative study of the photoluminescence performance and photocatalytic activity of CeO2/MgAl2O4 composite materials with an n-n heterojunction prepared by one-step synthesis and two-step synthesis methods. J. Phys. Chem. Solids 2021, 150, 109891. [Google Scholar] [CrossRef]
  36. Matsukevich, I.; Kulinich, N.; Kulbitskaya, L.; Kuznetsova, T.; Popkov, V.; Chebanenko, M.; Moskovskikh, D.; Kuskov, K.; Romanovski, V. Mesoporous nanocomposites based on CeO2 and MgO: Preparation, structure and photocatalytic activity. J. Chem. Technol. Biotechnol. 2023, 98, 2497–2505. [Google Scholar] [CrossRef]
  37. Hoque, K.A.; Kumer, A.; Chakma, U.; Chowdhury, A.-N. Facile Synthesis of computationally designed MgAl2O4/CeO2/Cu2O and MgAl2O4/CeO2/Ag2O smart heterojunction photocatalysts for aqueous organic pollutants degradation. ECS Trans. 2022, 107, 13785–13796. [Google Scholar] [CrossRef]
  38. Stojadinović, S.; Radić, N.; Perković, M. Highly efficient ZrO2 photocatalysts in the presence of UV radiation synthesized in a very short time by plasma electrolytic oxidation of zirconium. Opt. Mater. 2023, 146, 114608. [Google Scholar] [CrossRef]
  39. Rakoch, A.G.; Monakhova, E.P.; Khabibullina, Z.V.; Serdechnova, M.; Blawert, C.; Zheludkevich, M.L.; Gladkova, A.A. Plasma electrolytic oxidation of AZ31 and AZ91 magnesium alloys: Comparison of coatings formation mechanism. J. Magnes. Alloys 2020, 8, 587–600. [Google Scholar] [CrossRef]
  40. Zimou, J.; Nouneh, K.; Hsissou, R.; El-Habib, A.; El Gana, L.; Talbi, A.; Beraich, M.; Lotfi, N.; Addou, M. Structural, morphological, optical, and electrochemical properties of Co-doped CeO2 thin films, Mater. Sci. Semicond. Process. 2021, 135, 106049. [Google Scholar] [CrossRef]
  41. Lua, X.; Mohedano, M.; Blawerta, C.; Matykina, E.; Arrabal, R.; Kainer, K.U.; Zheludkevich, M.L. Plasma electrolytic oxidation coatings with particle additions–A review. Surf. Coat. Technol. 2016, 307, 1165–1182. [Google Scholar] [CrossRef]
  42. Orsetti, F.R.; Bukman, L.; Santos, J.S.; Nagay, B.E.; Rangel, E.C.; Cruz, N.C. Methylene blue and metformin photocatalytic activity of CeO2-Nb2O5 coatings is dependent on the treatment time of plasma electrolytic oxidation on titanium. Appl. Surf. Sci. Adv. 2021, 6, 100143. [Google Scholar] [CrossRef]
  43. Pathak, N.; Ghosh, P.S.; Gupta, S.K.; Mukherjee, S.; Kadam, R.M.; Arya, A. An insight into the various defects induced emission in MgAl2O4 and their tunability with phase behavior: Combined experimental and theoretical approach. J. Phys. Chem. C 2016, 120, 4016–4031. [Google Scholar] [CrossRef]
  44. Choudhuryn, B.; Basyach, P.; Choudhury, A. Monitoring F, F+, and F22+ related intense defect emissions from nanocrystalline MgO. J. Lumin. 2014, 149, 280–286. [Google Scholar] [CrossRef]
  45. Pathak, N.; Ghosh, P.S.; Gupta, S.K.; Kadam, R.M.; Arya, A. Defects induced changes in the electronic structures of MgO and their correlation with the optical properties: A special case of electron–hole recombination from the conduction band. RSC Adv. 2016, 6, 96398. [Google Scholar] [CrossRef]
  46. Zhang, J.; Zhang, L. Intensive green light emission from MgO nanobelts. Chem. Phys. Lett. 2002, 363, 293–297. [Google Scholar] [CrossRef]
  47. Li, M.; Wang, X.; Li, H.; Di, H.; Wu, X.; Fang, C.; Yang, B. Preparation of photoluminescent single crystalline MgO nanobelts by DC arc plasma jet CVD. Appl. Surf. Sci. 2013, 274, 188–194. [Google Scholar] [CrossRef]
  48. Uchino, T.; Okutsu, D.; Katayama, R.; Sawai, S. Mechanism of stimulated optical emission from MgO microcrystals with color centers. Phys. Rev. B 2009, 79, 165107. [Google Scholar] [CrossRef]
  49. Lente, G. Facts and alternative facts in chemical kinetics: Remarks about the kinetic use of activities, termolecular processes, and linearization techniques, Curr. Opin. Chem. Eng. 2018, 21, 76–83. [Google Scholar] [CrossRef]
  50. Fauzi, A.A.; Jalil, A.A.; Hassan, N.S.; Aziz, F.F.A.; Azami, M.S.; Hussain, I.; Saravanan, R.; Vo, D.-V.N. A critical review on relationship of CeO2-based photocatalyst towards mechanistic degradation of organic pollutant. Chemosphere 2022, 286, 131651. [Google Scholar] [CrossRef] [PubMed]
  51. Jing, L.; Qu, Y.; Wang, B.; Li, S.; Jiang, B.; Yang, L.; Fu, W.; Fu, H.; Sun, J. Review of photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity. Sol. Energy Mater. Sol. Cells 2006, 90, 1773–1787. [Google Scholar]
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Figure 1. Voltage–time curves during anodization in 5 g/L NaAlO2 without and with 8 g/L of CeO2 particles.
Figure 1. Voltage–time curves during anodization in 5 g/L NaAlO2 without and with 8 g/L of CeO2 particles.
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Figure 2. (a) Top view and (b) cross-section micrographs of coatings formed in 5 g/L NaAlO2 by adding CeO2 particles in concentrations of (i) 0 g/L; (ii) 1 g/L; (iii) 2 g/L; (iv) 4 g/L; (v) 8 g/L.
Figure 2. (a) Top view and (b) cross-section micrographs of coatings formed in 5 g/L NaAlO2 by adding CeO2 particles in concentrations of (i) 0 g/L; (ii) 1 g/L; (iii) 2 g/L; (iv) 4 g/L; (v) 8 g/L.
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Figure 3. (a) XRD patterns and (b) Raman spectra of coatings formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles.
Figure 3. (a) XRD patterns and (b) Raman spectra of coatings formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles.
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Figure 4. PL excitation and emission spectra of MgO/MgAl2O4 coatings: (a) λex = 265 nm, λem = 720 nm; (b) λex = 340 nm, λem = 410 nm.
Figure 4. PL excitation and emission spectra of MgO/MgAl2O4 coatings: (a) λex = 265 nm, λem = 720 nm; (b) λex = 340 nm, λem = 410 nm.
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Figure 5. PL spectra of MgO/MgAl2O4/CeO2 coatings formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles: (a) emission spectra excited at 265 nm; (b) emission spectra excited at 340 nm; (c) excitation spectra monitored at 720 nm; (d) excitation spectra monitored at 410 nm; (e) excitation spectra monitored at 520 nm.
Figure 5. PL spectra of MgO/MgAl2O4/CeO2 coatings formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles: (a) emission spectra excited at 265 nm; (b) emission spectra excited at 340 nm; (c) excitation spectra monitored at 720 nm; (d) excitation spectra monitored at 410 nm; (e) excitation spectra monitored at 520 nm.
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Figure 6. (a) PA and (b) first-order kinetic plots of coatings formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles.
Figure 6. (a) PA and (b) first-order kinetic plots of coatings formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles.
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Figure 7. DRS spectra of CeO2 particles and coatings formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles added.
Figure 7. DRS spectra of CeO2 particles and coatings formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles added.
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Figure 8. (a) MO photodegradation recycling experiment; (b) SEM micrographs before and after 10 cycles; and (c) XRD patterns before and after 10 cycles of a coating formed in 5 g/L NaAlO2 + 2 g/L CeO2.
Figure 8. (a) MO photodegradation recycling experiment; (b) SEM micrographs before and after 10 cycles; and (c) XRD patterns before and after 10 cycles of a coating formed in 5 g/L NaAlO2 + 2 g/L CeO2.
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Table 1. Integrated EDS analysis of coatings in Figure 2a formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles added.
Table 1. Integrated EDS analysis of coatings in Figure 2a formed in 5 g/L NaAlO2 with varying concentrations of CeO2 particles added.
CeO2 (g/L)Atomic (%)
OMgAlCe
065.4414.0220.54/
165.5314.7919.630.05
264.5215.7619.610.11
464.8415.9119.020.23
864.4315.8519.320.40
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Stojadinović, S.; Radić, N. MgAl Oxide Coatings Modified with CeO2 Particles Formed by Plasma Electrolytic Oxidation of AZ31 Magnesium Alloy: Photoluminescent and Photocatalytic Properties. Metals 2024, 14, 366. https://doi.org/10.3390/met14030366

AMA Style

Stojadinović S, Radić N. MgAl Oxide Coatings Modified with CeO2 Particles Formed by Plasma Electrolytic Oxidation of AZ31 Magnesium Alloy: Photoluminescent and Photocatalytic Properties. Metals. 2024; 14(3):366. https://doi.org/10.3390/met14030366

Chicago/Turabian Style

Stojadinović, Stevan, and Nenad Radić. 2024. "MgAl Oxide Coatings Modified with CeO2 Particles Formed by Plasma Electrolytic Oxidation of AZ31 Magnesium Alloy: Photoluminescent and Photocatalytic Properties" Metals 14, no. 3: 366. https://doi.org/10.3390/met14030366

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