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 (~10
2 MPa) and temperature (10
3 K to 10
4 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, Al
2O
3 layer grows at the oxide/electrolyte and aluminum/oxide interfaces as a consequence of a strong electric field (~10
7 V/cm) induced relocation of O
2−/OH
− and Al
3+ 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 CeO
2 and Eu
2O
3 particles towards the anode. Local temperature at the micro-discharge sites is higher than the melting points of CeO
2 and Eu
2O
3 particles (~2400 °C), enabling the molten particles to react with Al
2O
3 and form Ce and Eu ions doped Al
2O
3 coatings.
SEM images of coating surfaces, formed in supporting electrolyte by PEO, with the addition of CeO
2 and Eu
2O
3 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 CeO
2 and Eu
2O
3 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. The content of Ce and Eu in the coatings increases with increased concentration of CeO
2 and Eu
2O
3 particles in supporting electrolyte, respectively.
XRD patterns of created coatings are presented in
Figure 2. Diffraction peaks corresponding to the gamma phase of Al
2O
3 (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 Al
2O
3 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 CeO
2 or Eu
2O
3 particles as well as any other Ce or Eu species, probably due to low concentration of incorporated Ce and Eu elements into Al
2O
3 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 Al
2O
3 coating created in supporting electrolyte with the addition of 4 g/L CeO
2 + 4 g/L Eu
2O
3 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 Ce
3+ 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 Eu
3+ oxidation state, and (ii) at lower binding energies at ca. 1125.1 eV and 1155.2 eV, from the Eu
2+ oxidation state [
19]. These results indicate that interaction between CeO
2 and Eu
2O
3 particles with Al
2O
3, under environmental conditions set by PEO, causes the reduction of Ce
4+ ions to Ce
3+ ions and the reduction of some of Eu
3+ to Eu
2+ ions in Al
2O
3 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 Ce
3+ and Eu
2+ singly doped Al
2O
3 coatings are presented in
Figure 4. The PL excitation spectrum of Al
2O
3:Ce
3+ 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 Ce
3+ 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 Al
2O
3 coating doped with Eu ions. The PL spectra show the characteristic Eu
2+ broad band excitation and emission [
21]. Although XPS indicates that Eu incorporated into Al
2O
3 is also in the 3+ oxidation state, typical f-f transitions of Eu
3+ ions have not been identified in PL excitation and emission spectra, not even under the 395 nm excitation corresponding to the
7F
0→
5L
6 transition of Eu
3+. The PL excitation spectrum shows a large absorption band ranging from 250 to 330 nm, with center at 260 nm, attributed to the 4f
65d
1 multiplet excited states of Eu
2+ 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 4f
65d
1→4f
7 (
8S
7/2) transition of Eu
2+ ions. This transition is structurally sensitive to the local environment around the Eu
2+ in Al
2O
3 [
21]. The gamma Al
2O
3 has two different sites for Al
3+ ions. The appearance of two emission bands for a single transition of Eu
2+ in Al
2O
3:Eu
2+ is thus attributed to the Eu
2+ substituting the Al
3+ ions at both crystallographic sites.
3.3. PL of Al2O3:Ce3+/Eu2+ Coatings
Figure 4 shows the spectral overlapping of the Ce
3+ PL emission to the Eu
2+ PL excitation in Al
2O
3 between 300 and 400 nm, which indicates that the energy transfer from a sensitizer Ce
3+ to an activator Eu
2+ is possible. To verify the energy transfer from Ce
3+ to Eu
2+ in Al
2O
3:Ce
3+/Eu
2+ coatings, the PL emission spectra excited at 260 and 285 nm of coatings formed in supporting electrolyte with the addition of 4 g/L CeO
2 and different concentrations of Eu
2O
3 are shown in
Figure 5. The PL emission spectra of Al
2O
3:Ce
3+/Eu
2+ coatings consist of the emission band peaking at about 345 nm, assigned to the 4f
05d
1→4f
1 transition of Ce
3+ ion, and two emission bands peaking at about 405 and 500 nm, attributed to the 4f
65d→4f
7 transition of Eu
2+ ion. The intensity of the PL band of Ce
3+ decreases with the increasing concentration of Eu
2O
3 in supporting electrolyte, i.e., content of incorporated Eu in Al
2O
3 coatings (
Table 1), but the intensity of PL bands of Eu
2+ increases. With the higher Eu
2+ doping content, the Ce
3+ emission practically disappears, and only the Eu
2+ emission remains in the PL spectra of Al
2O
3:Ce
3+/Eu
2+ coatings. These results indicate that the energy transfer from Ce
3+ to Eu
2+ is confirmed. The intensity of PL bands of Eu
2+ not only increases due to Ce
3+→Eu
2+ energy transfer, but also due to the increase of the Eu content in the Al
2O
3 coatings.
Ce
3+→Eu
2+ energy transfer in Al
2O
3:Ce
3+/Eu
2+ coatings created in supporting electrolyte with the addition of 0.5 g/L Eu
2O
3 and different concentrations of CeO
2 is verified as well (
Figure 6). The intensity of PL bands originating from Ce
3+ and Eu
2+ increases with increasing concentration of CeO
2 in the supporting electrolyte, i.e., content of incorporated Ce
3+ in Al
2O
3 coatings (
Table 1). The intensity of PL bands of Eu
2+ increases due to Ce
3+→Eu
2+ energy transfer, but the intensity of the PL band of Ce
3+ increases due to the increase of the Ce
3+ content in the Al
2O
3 coatings.
The energy transfer efficiency (
ηT) from Ce
3+ to Eu
2+ can be calculated by the formula
ηT = 1 –
IS/
IS0, where
IS and
IS0 are the PL intensities of the Ce
3+ emissions with and without the presence of Eu
2+, respectively [
22]. The energy transfer efficiencies from Ce
3+ to Eu
2+ are calculated from the spectra in
Figure 5 and presented in
Figure 7. The energy transfer efficiency increases with increasing Eu
2+ concentration, indicating that the energy transfer from Ce
3+ to Eu
2+ is effective under middle UV excitation. The energy transfer Ce
3+→Eu
2+ can also be observed via the shortening of the Ce
3+ emission decay times with the increasing Eu
2+ concentration, as demonstrated in Ref. [
7].
In order to identify the energy transfer mechanism from a Ce
3+ sensitizer to an Eu
2+ activator, the equation of exchange interaction and electric multipolar interactions proposed by Dexter and Reisfeld was used [
3]:
From Equation (1) the dominant multipolar interaction can be identified, where
C is the total concentration of Ce
3+ and Eu
2+ ions, and
n = 6, 8, 10 values correspond to dipole–dipole, dipole–quadropole and quadropole–quadropole interactions, respectively. In
Figure 8, the plots of
Cn/3 vs.
IS0/
IS are presented. The best fit is with
n = 6, i.e., the energy transfer Ce
3+→Eu
2+ is primarily due to the electric dipole–dipole interaction.
The critical transfer distance for electric dipole–dipole interactions equal to [
8]:
where
I(
Ce) and
I(
Eu) are the intensities of Ce
3+ emission and Eu
2+ 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.