- Email: [email protected]
Down-conversion photoluminescence of ZrO2:Er3+ coatings formed by plasma electrolytic oxidation
Accepted Manuscript Down-conversion photoluminescence of ZrO2:Er3+ coatings formed by plasma electrolytic oxidation Stevan Stojadinović, Nenad Tadić, Rastko Vasilić PII: DOI: Reference:
S0167-577X(18)30340-9 https://doi.org/10.1016/j.matlet.2018.02.126 MLBLUE 23952
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
15 January 2018 19 February 2018 26 February 2018
Please cite this article as: S. Stojadinović, N. Tadić, R. Vasilić, Down-conversion photoluminescence of ZrO2:Er3+ coatings formed by plasma electrolytic oxidation, Materials Letters (2018), doi: https://doi.org/10.1016/ j.matlet.2018.02.126
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Down-conversion photoluminescence of ZrO2:Er3+ coatings formed by plasma electrolytic oxidation Stevan Stojadinović*, Nenad Tadić, Rastko Vasilić University of Belgrade, Faculty of Physics, Studentski trg 12-16, 11000 Belgrade, Serbia *Corresponding author. Tel:+381-11-7158161; Fax:+381-11-3282619 E-mail address: [email protected] Abstract Plasma electrolytic oxidation of zirconium in alkaline solution containing Er 2O3 powder was used for preparation of ZrO2:Er3+ coatings. Photoluminescence (PL) emission spectra of ZrO2:Er3+ excited by ultraviolet irradiation are composed of broad PL band associated with ZrO2 host and sharp bands corresponding to f–f transitions of Er3+. The strongest green PL emission band of Er3+ in the range from 540nm to 580nm is assigned to 4S3/24I15/2 transition. The PL excitation spectra of ZrO2:Er3+ characterize broad band from 250nm to 350nm associated with charge transfer state of Er3+ and the series of peaks in the range from 350nm to 530nm which are associated with 4f transitions of the Er3+ from ground state 4I15/2 to higher levels. Obtained results allowed the identification of down-conversion PL mechanism.
Keywords: Plasma electrolytic oxidation; ZrO2; Er3+; Luminescence; Phosphors.
1. Introduction ZrO2 has been widely used as a highly efficient host matrix for trivalent rare-earth ions for the fabrication of photoluminescent materials because of its low phonon frequency (about 470cm1) as well as excellent chemical, photo-chemical and photo-thermal stability, high refractive index, wide optical band gap, high transparency in the visible and near infrared region, etc. [1,2]. Trivalent rare-earth ions are characterized by a partially filled 4f shell that is 1
well shielded by 5s2 and 5p6 orbitals. The f-f emission transitions of trivalent rare-earth ions yield sharp lines in both absorption and emission spectra. Among trivalent rare
2
3. Results and discussion Top view and cross-sectional SEM micrographs (Fig. 1) reveal a typical structure of PEO coatings featuring a number of diversely sized and shaped pores, cracks, and regions resulting from the rapid cooling of molten material [9]. Thickness of the coatings increases approximately 0.3μm/min from 6.7μm to 14.4μm for the coatings formed between 5min and 30min. It is also clear that thicker coatings have higher surface roughness. Zr, O, and Er elements were identified by EDS, which all have rather uniform distribution throughout the coatings. Concentration of Er is close to the detection limit of our EDS system, so we used XRF measurement to obtain Er/Zr ratio (Table 1). Content of Er increases with PEO time due to the increase in the thickness of the coatings. Also, longer PEO processing time leads to increased size and strenghth of micro-discharges [8], resulting in higher incorporation of Er at micro-discharging sites. XRD patterns of used Er2O3 powder and Er doped ZrO2 coatings are shown in Fig. 2a. All peaks in the XRD pattern of Er2O3 powder can be indexed to cubic structure of Er2O3. The average Er2O3 crystallite size was estimated to be 16 nm by Holder-Wagner method. Diffraction peaks corresponding to monoclinic phase of ZrO2 are observed in XRD patterns, but diffraction peaks related to Er species were not detected. Due to the mismatch of ionic radii and the charge imbalance between Zr 4+ (89 pm) and Er3+ (103 pm), the substitution of Zr4+ with Er3+ ions causes the lattice distortion and Er doping can be evidenced by the shift of m-ZrO2 plane diffraction peaks, but we were not able to detect it (Fig. 2b), possibly due to low concentration of uniformly dispersed Er all over the coatings’ surface. PL of rare-earth doped ZrO2 originates from ZrO2 host and incorporated rare-earth ions. Wide PL emission band in the visible region characterizes pure ZrO2 coatings (Fig. 3a). It is generally proposed that PL of ZrO2 originates from optical transitions in PL centers which are defect centers related to oxygen vacancies [10]. The principal spectral maximum in 3
PL emission spectra is positioned at around 490 nm, while its PL excitation spectra equivalent is at around 280nm. Some authors also point out that residual titanium might be responsible for PL of ZrO2 [11], which is not the case in our work because the coatings were formed on high purity titanium-free Zr substrate. In ZrO2 coatings formed by PEO processing many oxygen vacancy defects are present and for this reason PL intensity increases with PEO time [10]. Another important parameter affecting PL is surface roughness of coatings [12]. PL intensity depends on surface roughness, i.e., increased surface roughness provides larger interaction area for excitationof the coatings. The evolution of PL emission spectra excited at 280nm and PL excitation spectra monitored at 490nm of ZrO2:Er3+ are shown in Fig. 3b. The PL emission is a sum of PL originating from ZrO2 host and f–f transitions of Er3+ ions incorporated into coatings. The strongest green PL emission band between 540nm and 580nm is attributed to 4S3/24I15/2 transition of Er3+ [13]. PL excitation spectra monitored at 548nm, i.e. at the wavelength of the most intense peak in PL emission spectra (Fig. 3b), are shown in Fig. 3c. The PL excitation spectra can be divided into two regions: the broad charge transfer excitation band in the region from 250nm to 350nm [14] and a series of peaks in the range from 350nm to 535nm which are associated with 4f transitions of the Er3+ ions from ground state 4I15/2 to higher levels [15]. PL emission spectra excited at 378nm (4I15/24G11/2 transition) are shown in Fig. 3d. PL spectra consist of violet transition 2H9/24I15/2 with maximum at 408nm, green transitions 2H11/24I15/2 and 4S3/24I15/2 with maxima at 526nm and 548nm, respectively, and red transition 4F9/24I15/2 with maximum at 660nm [15]. With the increase of coatings’ thickness shape of PL emission and excitation spectra and peak positions remain practically unchanged, but PL becomes more intense due to the increased concentration of oxygen vacancy defects and Er3+ ions in formed coatings. 4
Fig. 3e shows the energy level diagram and proposed mechanism of charge transfer pathways involved in the PL of ZrO2:Er3+. Excitation may take place in two processes, either by transferring electrons to the charge transfer band or by electron jumping to the higher levels of Er3+. After the excitation, excited electrons are transferred to the lowest excited levels of Er3+ ions (2H9/2, 2H11/2, 4S3/2, and 4F9/2) by non-radiative transition, with subsequent radiative emission to the ground state, 4I15/2.
4. Conclusions For the first time we have shown that ZrO2:Er3+ coatings can be prepared by PEO of zirconium in electrolyte containing Er 2O3 powder. PL emission spectra of ZrO2:Er3+ are a sum of PL originating from ZrO2 host and f–f transitions of Er3+ incorporated into coatings. PL excitation spectra, monitored at 548nm (4S3/24I15/2 transition of Er3+) are characterized by a broad charge transfer excitation band (from 250nm to 350nm) and a series of peaks in the range from 350nm to 535nm which are associated with 4f transitions of the Er3+ from ground state 4I15/2 to higher levels.
Acknowledgements This work is supported by the Ministry of Education, Science, and Technological Development of the Republic of Serbia under project No. 171035.
References [1] P. Salas, C. Angeles, J.A. Montoya, E. De la Rosa, L.A. Dıaz-Torres, A. Martınez, M.A. Romero-Romo, J. Morales, Synthesis, characterization and luminescence properties of ZrO 2: Yb3+-Er3+ nanophosphor, Opt. Mater. 27 (2005) 1295–1300.
5
[2] L.X. Lovisa, V.D. Araújo, R.L. Tranquilin, E. Longo, M.S. Li, C.A. Paskocimas, M.R.D. Bomio, F.V. Motta, White photoluminescence emission from ZrO 2 co-doped with Eu3+, Tb3+ and Tm3+, J. Alloys Compd. 674 (2016) 245–251. [3] L. Cao, Y. Zhou, W. Xiang, D. Yin, X. Liang, G. Gu, J. Li, Upconversion luminescence of cerium-stabilized high temperature phase zirconia phosphors with a high Er 3+ doping concentration, RSC Adv. 5 (2015) 107857–107863. [4] L.A. Díaz-Torres, E. De la Rosa-Cruz, P. Salas, C. Angeles-Chavez, Concentration enhanced red upconversion in nanocrystalline ZrO2:Er under IR excitation, J. Phys. D: Appl. Phys. 37 (2004) 2489–2495. [5] A. Martínez-Hernández, J. Guzmán-Mendoza, T. Rivera-Montalvo, D. Sánchez-Guzmán, J.C. Guzmán-Olguín, M. García-Hipólito, C. Falcony, Synthesis and cathode luminescence characterization of ZrO2:Er3+ films, J. Lumin. 153 (2014) 140–143. [6] M. Hattori, M. Ozawa, Surface charge and properties of rare earth metal oxide particles in water, Proceedings of International Symposium on EcoTopia Science 2007, ISETS7 (2007) 953–954. [7] S. Stojadinović, J. Radić-Perić, R. Vasilić, M. Perić, Spectroscopic investigation of direct current (DC) plasma electrolytic oxidation of zirconium in citric acid, Appl. Spectrosc. 68 (2014) 101–112. [8] S. Stojadinović, N. Radić, B. Grbić, S. Maletić, P. Stefanov, A. Pačevski, R. Vasilić, Structural, photoluminescent and photocatalytic properties of TiO 2:Eu3+ coatings formed by plasma electrolytic oxidation, Appl. Surf. Sci. 370 (2016) 218–228. [9] S. Cengiz, Y. Gencer, The characterization of the oxide based coating synthesized on pure zirconium by plasma electrolytic oxidation, Surf. Coat. Technol. 242 (2014) 132–140.
6
[10] S. Stojadinović, R. Vasilić, N. Radić, B. Grbić, Zirconia films formed by plasma electrolytic oxidation: photoluminescent and photocatalytic properties, Opt. Mater. 40 (2015) 20–25. [11] V. Kiisk, L. Puust, K. Utt, A. Maaroos, H. Mändar, E. Viviani, F. Piccinelli, R. Saar, U. Joost, I. Sildos, Photo-, thermo- and optically stimulated luminescence of monoclinic zirconia, J. Lumin. 174 (2016) 49–55. [12] K.S. Shim, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, J.S. Bae, K.H Kim, Crystallinity and surface roughness dependent photoluminescence of Y 1−xGdxVO4:Eu3+ thin films grown on Si (100) substrate, Thin Solid Films 517 (2009) 5137–5140. [13 X. Zhang, D. Xu, G. Zhou, X. Wang, H. Liu, Z. Yu, G. Zhang, L. Zhu, Color tunable upconversion emission from ZrO2:Er3+,Yb3+ textile fibers, RSC Adv. 6 (2016) 103973–103980. [14] M.R.N. Soares, T. Holz, F. Oliveira, F.M. Costa, T. Monteiro, Tunable green to red ZrO2:Er nanophosphors, RSC Adv. 5 (2015) 20138–20147. [15] S.A. Yan, J.W. Wang, Y.S. Chang, W.S. Hwang, Y.H. Chang, Synthesis and luminescence properties of Ln3+ (Ln3+=Er3+,Sm3+)-doped barium lanthanum tungstate BaLa2 WO7 phosphors, Opt. Mater. 34 (2011) 147–151.
7
Figure Captions: Figure 1. (a) SEM micrographs of coatings formed at various stages of PEO process and EDS maps of coating formed for 30min. Figure 2. XRD pattern of Er2O3 powder and XRD patterns of coatings formed at various stages of PEO; (a) 2 theta range from 10° to 80°; (b) 2 theta range from 27° to 32°. Figure 3. (a) Evolution of PL spectra of ZrO2; (b) Evolution of PL spectra of ZrO2:Er3+; (c) Evolution of PL excitation spectra of ZrO2:Er3+ monitored at 548nm; (d) Evolution of PL emission spectra of ZrO2:Er3+ excited at 379nm; (e) Schematic representation of energy level diagrams and proposed mechanism of charge transfer pathways involved in the PL of ZrO2:Er3+.
8
9
10
11
Table Caption: Table 1. Er/Zr ratio obtained by XRF analyses at various stages of PEO process.
PEO time (min)
5
10
20
30
Er (wt. %)
0.26
0.67
0.97
1.48
Zr (wt. %)
99.74
99.33
99.03
98.52
Er/Zr (10-3)
2.61
6.74
9.79
10.02
12
ZrO2:Er3+ coating is formed by plasma electrolytic oxidation of zirconium.
PL emission spectra excited by UV light exhibit well pronounced bands inherent to ZrO2 and Er3+ ions.
The strongest PL emission band is attributed to 4S3/24I15/2 transition of Er3+.
PEO time is an important factor affecting PL intensity.
13
S0167-577X(18)30340-9 https://doi.org/10.1016/j.matlet.2018.02.126 MLBLUE 23952
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
15 January 2018 19 February 2018 26 February 2018
Please cite this article as: S. Stojadinović, N. Tadić, R. Vasilić, Down-conversion photoluminescence of ZrO2:Er3+ coatings formed by plasma electrolytic oxidation, Materials Letters (2018), doi: https://doi.org/10.1016/ j.matlet.2018.02.126
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Down-conversion photoluminescence of ZrO2:Er3+ coatings formed by plasma electrolytic oxidation Stevan Stojadinović*, Nenad Tadić, Rastko Vasilić University of Belgrade, Faculty of Physics, Studentski trg 12-16, 11000 Belgrade, Serbia *Corresponding author. Tel:+381-11-7158161; Fax:+381-11-3282619 E-mail address: [email protected] Abstract Plasma electrolytic oxidation of zirconium in alkaline solution containing Er 2O3 powder was used for preparation of ZrO2:Er3+ coatings. Photoluminescence (PL) emission spectra of ZrO2:Er3+ excited by ultraviolet irradiation are composed of broad PL band associated with ZrO2 host and sharp bands corresponding to f–f transitions of Er3+. The strongest green PL emission band of Er3+ in the range from 540nm to 580nm is assigned to 4S3/24I15/2 transition. The PL excitation spectra of ZrO2:Er3+ characterize broad band from 250nm to 350nm associated with charge transfer state of Er3+ and the series of peaks in the range from 350nm to 530nm which are associated with 4f transitions of the Er3+ from ground state 4I15/2 to higher levels. Obtained results allowed the identification of down-conversion PL mechanism.
Keywords: Plasma electrolytic oxidation; ZrO2; Er3+; Luminescence; Phosphors.
1. Introduction ZrO2 has been widely used as a highly efficient host matrix for trivalent rare-earth ions for the fabrication of photoluminescent materials because of its low phonon frequency (about 470cm1) as well as excellent chemical, photo-chemical and photo-thermal stability, high refractive index, wide optical band gap, high transparency in the visible and near infrared region, etc. [1,2]. Trivalent rare-earth ions are characterized by a partially filled 4f shell that is 1
well shielded by 5s2 and 5p6 orbitals. The f-f emission transitions of trivalent rare-earth ions yield sharp lines in both absorption and emission spectra. Among trivalent rare
2
3. Results and discussion Top view and cross-sectional SEM micrographs (Fig. 1) reveal a typical structure of PEO coatings featuring a number of diversely sized and shaped pores, cracks, and regions resulting from the rapid cooling of molten material [9]. Thickness of the coatings increases approximately 0.3μm/min from 6.7μm to 14.4μm for the coatings formed between 5min and 30min. It is also clear that thicker coatings have higher surface roughness. Zr, O, and Er elements were identified by EDS, which all have rather uniform distribution throughout the coatings. Concentration of Er is close to the detection limit of our EDS system, so we used XRF measurement to obtain Er/Zr ratio (Table 1). Content of Er increases with PEO time due to the increase in the thickness of the coatings. Also, longer PEO processing time leads to increased size and strenghth of micro-discharges [8], resulting in higher incorporation of Er at micro-discharging sites. XRD patterns of used Er2O3 powder and Er doped ZrO2 coatings are shown in Fig. 2a. All peaks in the XRD pattern of Er2O3 powder can be indexed to cubic structure of Er2O3. The average Er2O3 crystallite size was estimated to be 16 nm by Holder-Wagner method. Diffraction peaks corresponding to monoclinic phase of ZrO2 are observed in XRD patterns, but diffraction peaks related to Er species were not detected. Due to the mismatch of ionic radii and the charge imbalance between Zr 4+ (89 pm) and Er3+ (103 pm), the substitution of Zr4+ with Er3+ ions causes the lattice distortion and Er doping can be evidenced by the shift of m-ZrO2 plane diffraction peaks, but we were not able to detect it (Fig. 2b), possibly due to low concentration of uniformly dispersed Er all over the coatings’ surface. PL of rare-earth doped ZrO2 originates from ZrO2 host and incorporated rare-earth ions. Wide PL emission band in the visible region characterizes pure ZrO2 coatings (Fig. 3a). It is generally proposed that PL of ZrO2 originates from optical transitions in PL centers which are defect centers related to oxygen vacancies [10]. The principal spectral maximum in 3
PL emission spectra is positioned at around 490 nm, while its PL excitation spectra equivalent is at around 280nm. Some authors also point out that residual titanium might be responsible for PL of ZrO2 [11], which is not the case in our work because the coatings were formed on high purity titanium-free Zr substrate. In ZrO2 coatings formed by PEO processing many oxygen vacancy defects are present and for this reason PL intensity increases with PEO time [10]. Another important parameter affecting PL is surface roughness of coatings [12]. PL intensity depends on surface roughness, i.e., increased surface roughness provides larger interaction area for excitationof the coatings. The evolution of PL emission spectra excited at 280nm and PL excitation spectra monitored at 490nm of ZrO2:Er3+ are shown in Fig. 3b. The PL emission is a sum of PL originating from ZrO2 host and f–f transitions of Er3+ ions incorporated into coatings. The strongest green PL emission band between 540nm and 580nm is attributed to 4S3/24I15/2 transition of Er3+ [13]. PL excitation spectra monitored at 548nm, i.e. at the wavelength of the most intense peak in PL emission spectra (Fig. 3b), are shown in Fig. 3c. The PL excitation spectra can be divided into two regions: the broad charge transfer excitation band in the region from 250nm to 350nm [14] and a series of peaks in the range from 350nm to 535nm which are associated with 4f transitions of the Er3+ ions from ground state 4I15/2 to higher levels [15]. PL emission spectra excited at 378nm (4I15/24G11/2 transition) are shown in Fig. 3d. PL spectra consist of violet transition 2H9/24I15/2 with maximum at 408nm, green transitions 2H11/24I15/2 and 4S3/24I15/2 with maxima at 526nm and 548nm, respectively, and red transition 4F9/24I15/2 with maximum at 660nm [15]. With the increase of coatings’ thickness shape of PL emission and excitation spectra and peak positions remain practically unchanged, but PL becomes more intense due to the increased concentration of oxygen vacancy defects and Er3+ ions in formed coatings. 4
Fig. 3e shows the energy level diagram and proposed mechanism of charge transfer pathways involved in the PL of ZrO2:Er3+. Excitation may take place in two processes, either by transferring electrons to the charge transfer band or by electron jumping to the higher levels of Er3+. After the excitation, excited electrons are transferred to the lowest excited levels of Er3+ ions (2H9/2, 2H11/2, 4S3/2, and 4F9/2) by non-radiative transition, with subsequent radiative emission to the ground state, 4I15/2.
4. Conclusions For the first time we have shown that ZrO2:Er3+ coatings can be prepared by PEO of zirconium in electrolyte containing Er 2O3 powder. PL emission spectra of ZrO2:Er3+ are a sum of PL originating from ZrO2 host and f–f transitions of Er3+ incorporated into coatings. PL excitation spectra, monitored at 548nm (4S3/24I15/2 transition of Er3+) are characterized by a broad charge transfer excitation band (from 250nm to 350nm) and a series of peaks in the range from 350nm to 535nm which are associated with 4f transitions of the Er3+ from ground state 4I15/2 to higher levels.
Acknowledgements This work is supported by the Ministry of Education, Science, and Technological Development of the Republic of Serbia under project No. 171035.
References [1] P. Salas, C. Angeles, J.A. Montoya, E. De la Rosa, L.A. Dıaz-Torres, A. Martınez, M.A. Romero-Romo, J. Morales, Synthesis, characterization and luminescence properties of ZrO 2: Yb3+-Er3+ nanophosphor, Opt. Mater. 27 (2005) 1295–1300.
5
[2] L.X. Lovisa, V.D. Araújo, R.L. Tranquilin, E. Longo, M.S. Li, C.A. Paskocimas, M.R.D. Bomio, F.V. Motta, White photoluminescence emission from ZrO 2 co-doped with Eu3+, Tb3+ and Tm3+, J. Alloys Compd. 674 (2016) 245–251. [3] L. Cao, Y. Zhou, W. Xiang, D. Yin, X. Liang, G. Gu, J. Li, Upconversion luminescence of cerium-stabilized high temperature phase zirconia phosphors with a high Er 3+ doping concentration, RSC Adv. 5 (2015) 107857–107863. [4] L.A. Díaz-Torres, E. De la Rosa-Cruz, P. Salas, C. Angeles-Chavez, Concentration enhanced red upconversion in nanocrystalline ZrO2:Er under IR excitation, J. Phys. D: Appl. Phys. 37 (2004) 2489–2495. [5] A. Martínez-Hernández, J. Guzmán-Mendoza, T. Rivera-Montalvo, D. Sánchez-Guzmán, J.C. Guzmán-Olguín, M. García-Hipólito, C. Falcony, Synthesis and cathode luminescence characterization of ZrO2:Er3+ films, J. Lumin. 153 (2014) 140–143. [6] M. Hattori, M. Ozawa, Surface charge and properties of rare earth metal oxide particles in water, Proceedings of International Symposium on EcoTopia Science 2007, ISETS7 (2007) 953–954. [7] S. Stojadinović, J. Radić-Perić, R. Vasilić, M. Perić, Spectroscopic investigation of direct current (DC) plasma electrolytic oxidation of zirconium in citric acid, Appl. Spectrosc. 68 (2014) 101–112. [8] S. Stojadinović, N. Radić, B. Grbić, S. Maletić, P. Stefanov, A. Pačevski, R. Vasilić, Structural, photoluminescent and photocatalytic properties of TiO 2:Eu3+ coatings formed by plasma electrolytic oxidation, Appl. Surf. Sci. 370 (2016) 218–228. [9] S. Cengiz, Y. Gencer, The characterization of the oxide based coating synthesized on pure zirconium by plasma electrolytic oxidation, Surf. Coat. Technol. 242 (2014) 132–140.
6
[10] S. Stojadinović, R. Vasilić, N. Radić, B. Grbić, Zirconia films formed by plasma electrolytic oxidation: photoluminescent and photocatalytic properties, Opt. Mater. 40 (2015) 20–25. [11] V. Kiisk, L. Puust, K. Utt, A. Maaroos, H. Mändar, E. Viviani, F. Piccinelli, R. Saar, U. Joost, I. Sildos, Photo-, thermo- and optically stimulated luminescence of monoclinic zirconia, J. Lumin. 174 (2016) 49–55. [12] K.S. Shim, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, J.S. Bae, K.H Kim, Crystallinity and surface roughness dependent photoluminescence of Y 1−xGdxVO4:Eu3+ thin films grown on Si (100) substrate, Thin Solid Films 517 (2009) 5137–5140. [13 X. Zhang, D. Xu, G. Zhou, X. Wang, H. Liu, Z. Yu, G. Zhang, L. Zhu, Color tunable upconversion emission from ZrO2:Er3+,Yb3+ textile fibers, RSC Adv. 6 (2016) 103973–103980. [14] M.R.N. Soares, T. Holz, F. Oliveira, F.M. Costa, T. Monteiro, Tunable green to red ZrO2:Er nanophosphors, RSC Adv. 5 (2015) 20138–20147. [15] S.A. Yan, J.W. Wang, Y.S. Chang, W.S. Hwang, Y.H. Chang, Synthesis and luminescence properties of Ln3+ (Ln3+=Er3+,Sm3+)-doped barium lanthanum tungstate BaLa2 WO7 phosphors, Opt. Mater. 34 (2011) 147–151.
7
Figure Captions: Figure 1. (a) SEM micrographs of coatings formed at various stages of PEO process and EDS maps of coating formed for 30min. Figure 2. XRD pattern of Er2O3 powder and XRD patterns of coatings formed at various stages of PEO; (a) 2 theta range from 10° to 80°; (b) 2 theta range from 27° to 32°. Figure 3. (a) Evolution of PL spectra of ZrO2; (b) Evolution of PL spectra of ZrO2:Er3+; (c) Evolution of PL excitation spectra of ZrO2:Er3+ monitored at 548nm; (d) Evolution of PL emission spectra of ZrO2:Er3+ excited at 379nm; (e) Schematic representation of energy level diagrams and proposed mechanism of charge transfer pathways involved in the PL of ZrO2:Er3+.
8
9
10
11
Table Caption: Table 1. Er/Zr ratio obtained by XRF analyses at various stages of PEO process.
PEO time (min)
5
10
20
30
Er (wt. %)
0.26
0.67
0.97
1.48
Zr (wt. %)
99.74
99.33
99.03
98.52
Er/Zr (10-3)
2.61
6.74
9.79
10.02
12
ZrO2:Er3+ coating is formed by plasma electrolytic oxidation of zirconium.
PL emission spectra excited by UV light exhibit well pronounced bands inherent to ZrO2 and Er3+ ions.
The strongest PL emission band is attributed to 4S3/24I15/2 transition of Er3+.
PEO time is an important factor affecting PL intensity.
13