Ce^3+/Eu²⁺ Doped Al₂O₃ Coatings Formed by Plasma Electrolytic Oxidation of Aluminum: Photoluminescence Enhancement by Ce³⁺→Eu²⁺ Energy Transfer

1. Introduction

Ce³⁺ and Eu²⁺ ions have found widespread use as activators in luminescent materials because their parity allows 4f¹→4f⁰5d¹ and 4f⁷→4f⁶5d¹ electric dipole transitions, respectively. Outer orbitals at 5d states of Ce³⁺ and Eu²⁺ ions are sensitive to crystal-field splitting and nephelauxetic effects [1,2], therefore, energy positions of excitation and emission bands of these ions can be tuned by changing the host matrix. Ce³⁺ ion also acts as an effective sensitizer due to its effective absorption [3]. Numerous studies demonstrated the importance and methods for enhancing the photoluminescence (PL) via an efficient energy transfer (e.g., [4,5]). The energy transfer from the Ce³⁺ sensitizer to the Eu²⁺ activator has been reported for some Ce³⁺/Eu²⁺ doped host lattices because of the spectral overlap between Ce³⁺ emission band and Eu²⁺ excitation band [6,7,8,9]. The aim of this work is to investigate PL properties of Ce³⁺/Eu²⁺ doped Al₂O₃ coatings created by plasma electrolytic oxidation (PEO) of pure aluminum and an energy transfer from Ce³⁺ to Eu²⁺ ions.

Recent investigations have shown that PEO process of aluminum is capable of preparation of Ce³⁺ and Eu²⁺ doped Al₂O₃ coatings in electrolyte enriched with CeO₂ and Eu₂O₃ particles, respectively [10,11]. As ionic radii of Al³⁺ (0.53 Å), and Eu³⁺ (0.95 Å) and Ce⁴⁺ (0.97 Å) [12] significantly differ, the substitution of Al³⁺ by either Eu³⁺ or Ce⁴⁺ produces oxygen vacancies that in turn distort the crystal lattice. The excess O²⁻ ions give their electrons to the lattice (2O²⁻ − 4e⁻→O₂), which are then captured by Ce⁴⁺ and Eu³⁺, leading to their reduction to Ce³⁺ and Eu²⁺, respectively [13].

Thus, the interactions of CeO₂ or Eu₂O₃ particles and Al₂O₃ coatings via the PEO process cause the reduction of Ce⁴⁺ to Ce³⁺ ions and Eu³⁺ to Eu²⁺ ions, enabling their efficient incorporation into Al₂O₃. Our assumption was that Al₂O₃:Ce³⁺/Eu²⁺ coatings could be formed if the PEO of aluminum is performed with an electrolyte containing CeO₂ and Eu₂O₃ particles.

2. Materials and Methods

Pure aluminum foil (99.9%) with 0.25 mm thickness was used as a substrate in PEO. Before PEO, the substrate was cut into plates (25 × 10 mm²), cleaned ultrasonically in acetone and ethanol, rinsed with distilled water, dried in a warm air stream and sealed with insulation resin leaving a working area of ~15 × 10 mm². PEO was carried out in a double-walled glass cell, with water cooling. The temperature of the electrolyte was kept at the constant (20 ± 1) °C during the PEO. Aluminum samples were used as an anode while the cathode was a cylinder of stainless steel. An aqueous solution of 0.1 M H₃BO₃ + 0.05 M Na₂B₄O₇ was employed as a supporting electrolyte. CeO₂ and Eu₂O₃ particles in different concentrations were added to the electrolyte. The magnetic stirrer allowed for the homogeneous particle distribution in the glass cell. The coatings were formed using a constant current density regime of 150 mA/cm² for 10 min. Following PEO, the samples were rinsed in distilled water and dried in warm air.

The surface morphology of formed coatings was examined by scanning electron microscope (SEM, Tokyo, Japan) JEOL 840A and surface elemental distribution were determined by the integrated energy-dispersive x-ray spectroscopy (EDS, Oxford, UK). Phase composition of created PEO coatings was analyzed by X-ray diffraction (XRD, Tokyo, Japan), using a Rigaku Ultima IV diffractometer over a scanning range of 20°–80° with a 0.05° step size and 2°/min scanning speed. The Kratos AXIS Supra photoelectron spectrometer using a monochromatic Al K_α source with the energy of 1486.6 eV was used for obtaining the X-ray photoelectron spectra (XPS). The base pressure in the analysis chamber was 5 × 10⁻⁸ Pa.

Room temperature PL spectral measurements were acquired by a Horiba Jobin Yvon Fluorolog FL3-22 spectrofluorometer (Edison, NJ, USA), with a 450 W xenon lamp as the excitation source, coupled to a double grating monochromator in a wavelength range 220–600 nm. A 290–850 nm double grating monochromator and a side-on Hamamatsu 928P photomultiplier tube (PMT) allowed for recording of the PL emission spectra. The spectra were corrected for the spectral distribution of the excitation lamp and the measuring system’s spectral response.

3. Results and Discussion

3.1. PEO Coatings’ Morphology, Phase and Chemical Composition

PEO is a high-voltage anodizing process of some metals (Al, Mg, Ta, Ti, etc.) and their alloys, combined with the plasma formation, as indicated by the presence of micro-discharges on the metal surface, and followed by the gas evolution [14]. High pressure (~10² MPa) and temperature (10³ K to 10⁴ K) at the micro-discharge sites induce numerous processes (electrochemical, thermodynamical, and plasma-chemical) responsible for structural, compositional, and morphological modifications of the obtained oxide coatings. The PEO formed oxide coatings usually have crystalline phases with constituents originating from both the electrolyte and the metal. During the PEO of aluminum, Al₂O₃ layer grows at the oxide/electrolyte and aluminum/oxide interfaces as a consequence of a strong electric field (~10⁷ V/cm) induced relocation of O²⁻/OH⁻ and Al³⁺ ions across the oxide [15]. Moreover, melted aluminum and electrolyte components get drawn into the micro-discharge channels. From the channels, the oxidized aluminum gets ejected to the coating surface contacting the electrolyte. In that manner, the coating thickness increases around the channels. During the PEO, the electrophoretic effect drives CeO₂ and Eu₂O₃ particles towards the anode. Local temperature at the micro-discharge sites is higher than the melting points of CeO₂ and Eu₂O₃ particles (~2400 °C), enabling the molten particles to react with Al₂O₃ and form Ce and Eu ions doped Al₂O₃ coatings.

SEM images of coating surfaces, formed in supporting electrolyte by PEO, with the addition of CeO₂ and Eu₂O₃ particles in various concentrations, are shown in Figure 1a. PEO coating displays a porous morphology with pores of varying diameter and shape, which appear at the sites of the micro-discharge channels because of the gas evolution erupting the molten oxide material during the PEO process [16]. The surface morphology of the formed coatings does not significantly change with the addition of CeO₂ and Eu₂O₃ particles in supporting electrolyte. Elemental mapping by EDS shows that the main constituents of the coatings are uniformly distributed O, Ce, and Eu from the electrolyte and Al from the substrate (Figure 1b).

Integrated EDS analysis of all coatings created in the experiment is given in

Table 1. EDS analysis of plasma electrolytic oxidation (PEO) coatings coalesced in supporting electrolyte with different concentrations of added CeO₂ and Eu₂O₃ powders.

Concentration of Particles in Supporting Electrolyte	Atomic (%)
	O	Al	Ce	Eu
4 g/L CeO2	65.25	34.62	0.13	-
4 g/L Eu2O2	65.19	34.55	-	0.26
4 g/L CeO2 + 0.5 g/L Eu2O3	64.89	34.95	0.10	0.06
4 g/L CeO2 + 1 g/L Eu2O3	64.73	35.06	0.11	0.09
4 g/L CeO2 + 2 g/L Eu2O3	64.39	35.35	0.11	0.16
4 g/L CeO2 + 4 g/L Eu2O3	64.79	34.82	0.12	0.27
0 g/L CeO2 + 0.5 g/L Eu2O3	64.95	34.98	-	0.07
4 g/L CeO2 + 0.5 g/L Eu2O3	64.90	34.94	0.10	0.06
8 g/L CeO2 + 0.5 g/L Eu2O3	64.99	34.73	0.21	0.07
12 g/L CeO2 + 0.5 g/L Eu2O3	64.97	34.65	0.32	0.06

. The content of Ce and Eu in the coatings increases with increased concentration of CeO₂ and Eu₂O₃ particles in supporting electrolyte, respectively.

XRD patterns of created coatings are presented in Figure 2. Diffraction peaks corresponding to the gamma phase of Al₂O₃ (reference ICCD card No. 10-0425) and Al substrate (reference ICCD card No. 89-4037) are observed in XRD patterns. Obviously, the formation of gamma Al₂O₃ during the PEO is favored by the rapid solidification of molten alumina due to the contact with surrounding low-temperature electrolyte [17]. We were not able to detect any peaks corresponding to CeO₂ or Eu₂O₃ particles as well as any other Ce or Eu species, probably due to low concentration of incorporated Ce and Eu elements into Al₂O₃ coatings (Table 1).

For further investigation of the chemical nature and oxidation state of Ce and Eu in created coatings, we recorded a high-resolution Ce 3d and Eu 3d core-level spectra of Ce and Eu doped Al₂O₃ coating created in supporting electrolyte with the addition of 4 g/L CeO₂ + 4 g/L Eu₂O₃ particles (Figure 3). The peak deconvolution of the high-resolution XPS peaks of Ce 3d shows that the level of Ce ion is composed of four Gaussian peaks, i.e., two pairs of doublets (3d_5/2 at 881.5 eV and 885.3 eV, 3d_3/2 at 899.5 eV and 903.9 eV), characteristic for Ce³⁺ oxidation state [18]. The Eu 3d core level spectrum consists of two doublets: (i) The Eu 3d_5/2 (at 1135.3 eV) and Eu 3d_3/2 (at 1165.3 eV) peaks, attributed to Eu³⁺ oxidation state, and (ii) at lower binding energies at ca. 1125.1 eV and 1155.2 eV, from the Eu²⁺ oxidation state [19]. These results indicate that interaction between CeO₂ and Eu₂O₃ particles with Al₂O₃, under environmental conditions set by PEO, causes the reduction of Ce⁴⁺ ions to Ce³⁺ ions and the reduction of some of Eu³⁺ to Eu²⁺ ions in Al₂O₃ host (e.g., [10,11] and references therein).

3.2. PL of Al₂O₃:Ce³⁺ and Al₂O₃:Eu²⁺ Coatings

The PL excitation and emission spectra of Ce³⁺ and Eu²⁺ singly doped Al₂O₃ coatings are presented in Figure 4. The PL excitation spectrum of Al₂O₃:Ce³⁺ coating (Figure 4a) exhibited the broad band in the range from 250 to 340 nm and peaked at about 285 nm, corresponding to the direct excitation from the Ce³⁺ ground state 4f to the field-splitting levels of 5d state [20]. The corresponding PL emission spectrum excited at 285 nm shows one broad emission band peaked at about 340 nm, attributed to the transitions of 5d excited state to the ²F_7/2 and ²F_5/2 ground states [20].

Figure 4b presents PL excitation and emission spectra of Al₂O₃ coating doped with Eu ions. The PL spectra show the characteristic Eu²⁺ broad band excitation and emission [21]. Although XPS indicates that Eu incorporated into Al₂O₃ is also in the 3+ oxidation state, typical f-f transitions of Eu³⁺ ions have not been identified in PL excitation and emission spectra, not even under the 395 nm excitation corresponding to the ⁷F₀→⁵L₆ transition of Eu³⁺. The PL excitation spectrum shows a large absorption band ranging from 250 to 330 nm, with center at 260 nm, attributed to the 4f⁶5d¹ multiplet excited states of Eu²⁺ ions. Under the excitation of 260 nm the PL emission spectrum shows two distinct bands reaching maxima at about 405 and 500 nm, caused by the 4f⁶5d¹→4f⁷ (⁸S_7/2) transition of Eu²⁺ ions. This transition is structurally sensitive to the local environment around the Eu²⁺ in Al₂O₃ [21]. The gamma Al₂O₃ has two different sites for Al³⁺ ions. The appearance of two emission bands for a single transition of Eu²⁺ in Al₂O₃:Eu²⁺ is thus attributed to the Eu²⁺ substituting the Al³⁺ ions at both crystallographic sites.

3.3. PL of Al₂O₃:Ce³⁺/Eu²⁺ Coatings

Figure 4 shows the spectral overlapping of the Ce³⁺ PL emission to the Eu²⁺ PL excitation in Al₂O₃ between 300 and 400 nm, which indicates that the energy transfer from a sensitizer Ce³⁺ to an activator Eu²⁺ is possible. To verify the energy transfer from Ce³⁺ to Eu²⁺ in Al₂O₃:Ce³⁺/Eu²⁺ coatings, the PL emission spectra excited at 260 and 285 nm of coatings formed in supporting electrolyte with the addition of 4 g/L CeO₂ and different concentrations of Eu₂O₃ are shown in Figure 5. The PL emission spectra of Al₂O₃:Ce³⁺/Eu²⁺ coatings consist of the emission band peaking at about 345 nm, assigned to the 4f⁰5d¹→4f¹ transition of Ce³⁺ ion, and two emission bands peaking at about 405 and 500 nm, attributed to the 4f⁶5d→4f⁷ transition of Eu²⁺ ion. The intensity of the PL band of Ce³⁺ decreases with the increasing concentration of Eu₂O₃ in supporting electrolyte, i.e., content of incorporated Eu in Al₂O₃ coatings (Table 1), but the intensity of PL bands of Eu²⁺ increases. With the higher Eu²⁺ doping content, the Ce³⁺ emission practically disappears, and only the Eu²⁺ emission remains in the PL spectra of Al₂O₃:Ce³⁺/Eu²⁺ coatings. These results indicate that the energy transfer from Ce³⁺ to Eu²⁺ is confirmed. The intensity of PL bands of Eu²⁺ not only increases due to Ce³⁺→Eu²⁺ energy transfer, but also due to the increase of the Eu content in the Al₂O₃ coatings.

Ce³⁺→Eu²⁺ energy transfer in Al₂O₃:Ce³⁺/Eu²⁺ coatings created in supporting electrolyte with the addition of 0.5 g/L Eu₂O₃ and different concentrations of CeO₂ is verified as well (Figure 6). The intensity of PL bands originating from Ce³⁺ and Eu²⁺ increases with increasing concentration of CeO₂ in the supporting electrolyte, i.e., content of incorporated Ce³⁺ in Al₂O₃ coatings (Table 1). The intensity of PL bands of Eu²⁺ increases due to Ce³⁺→Eu²⁺ energy transfer, but the intensity of the PL band of Ce³⁺ increases due to the increase of the Ce³⁺ content in the Al₂O₃ coatings.

The energy transfer efficiency (η_T) from Ce³⁺ to Eu²⁺ can be calculated by the formula η_T = 1 – I_S/I_S0, where I_S and I_S0 are the PL intensities of the Ce³⁺ emissions with and without the presence of Eu²⁺, respectively [22]. The energy transfer efficiencies from Ce³⁺ to Eu²⁺ are calculated from the spectra in Figure 5 and presented in Figure 7. The energy transfer efficiency increases with increasing Eu²⁺ concentration, indicating that the energy transfer from Ce³⁺ to Eu²⁺ is effective under middle UV excitation. The energy transfer Ce³⁺→Eu²⁺ can also be observed via the shortening of the Ce³⁺ emission decay times with the increasing Eu²⁺ concentration, as demonstrated in Ref. [7].

In order to identify the energy transfer mechanism from a Ce³⁺ sensitizer to an Eu²⁺ activator, the equation of exchange interaction and electric multipolar interactions proposed by Dexter and Reisfeld was used [3]:

$$\frac{I_{\mathrm{S}0}}{I_{\mathrm{S}}}\propto C^{\frac{n}{3}}.$$

(1)

From Equation (1) the dominant multipolar interaction can be identified, where C is the total concentration of Ce³⁺ and Eu²⁺ ions, and n = 6, 8, 10 values correspond to dipole–dipole, dipole–quadropole and quadropole–quadropole interactions, respectively. In Figure 8, the plots of C^n/3 vs. I_S0/I_S are presented. The best fit is with n = 6, i.e., the energy transfer Ce³⁺→Eu²⁺ is primarily due to the electric dipole–dipole interaction.

The critical transfer distance for electric dipole–dipole interactions equal to [8]:

$$R_C^6[Å]=6·\frac{{10}^3}{L^4}·\frac{{\displaystyle \int }I\left(Ce\right)·I\left(Eu\right)dE}{{\displaystyle \int }I\left(Ce\right)dE·{\displaystyle \int }I\left(Eu\right)dE},$$

(2)

where I(Ce) and I(Eu) are the intensities of Ce³⁺ emission and Eu²⁺ excitation spectra, respectively, and L is the largest energy point at which the spectral overlap occurs. As can be extracted from the graph in Figure 9, that point is equal to 3.985 eV, the term with integrals is equal to 0.3596 eV⁻¹, and thus the critical distance was estimated to be 8.6 Å.

3.4. CIE Chromaticity Analysis

The Commission International de I’Eclairage (CIE) 1931 (x,y) coordinates of Al₂O₃:Ce³⁺/Eu²⁺ coatings created in supporting electrolyte with the addition different concentration of CeO₂ and Eu₂O₃ were estimated by the JOES software, v. 2.8 [23], and the results are presented in Figure 10 and

Table 2. CIE x, y coordinates, color purity and dominant wavelength of Al₂O₃:Ce³⁺/Eu²⁺, coatings doped with various concentrations of CeO₂ and Eu₂O₃ particles and excited at 260 and 285 nm.

Label	Concentration of Particles		λex (nm)	x	y	Color Purity	λdominant (nm)
	CeO2	Eu2O3
a	4	0	260	0.10644	0.08603	1	467
b	4	0.5	260	0.20944	0.18067	0.606957	471
c	4	1	260	0.21295	0.21447	0.501821	477
d	4	2	260	0.24027	0.25217	0.374879	483
e	4	4	260	0.25693	0.27666	0.285429	488
f	0	0.5	260	0.21503	0.19208	0.550572	472
g	8	0.5	260	0.2022	0.17773	0.606957	471
h	12	0.5	260	0.21743	0.19386	0.550572	472

. From the diagram, the evident is the shift from pure blue towards the white color by increasing Eu²⁺ concentration. There is also a small shift in the same direction by changing the excitation from 260 to 285 nm. The emission color of the samples with constant Eu²⁺ is almost independent on the Ce³⁺ concentration. Thus, the emission color of Al₂O₃:Ce³⁺/Eu²⁺ can be fine-tuned from pure blue to white by increasing Eu²⁺ concentration.

4. Conclusions

In summary, we have successfully prepared Al₂O₃ coatings doped with Ce³⁺ and Eu²⁺ ions by the PEO process and their PL properties have been investigated in detail. PL emission spectra of Al₂O₃:Ce³⁺/Eu²⁺ are a sum of PL originating from 5d–4f transitions of Ce³⁺ and Eu²⁺ ions. Due to the spectral overlapping of the Ce³⁺ PL emission with the Eu²⁺ PL excitation in Al₂O₃, an energy transfer from Ce³⁺ sensitizer to Eu²⁺ activator occurs. The energy transfer mechanism is dominant by electric dipole–dipole interaction.