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Sm3+ doped Al2O3 coatings formed by plasma electrolytic oxidation of aluminum
Journal of Luminescence 192 (2017) 110–116
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence of Sm2+ / Sm3+ doped Al2O3 coatings formed by plasma electrolytic oxidation of aluminum
MARK
⁎
Stevan Stojadinović , Nenad Tadić, Rastko Vasilić University of Belgrade, Faculty of Physics, Studentski trg 12-16, 11000 Belgrade, Serbia
A R T I C L E I N F O
A B S T R A C T
Keywords: Plasma electrolytic oxidation Al2O3 Sm2+ Sm3+ Photoluminescence
Sm doped Al2O3 coatings are formed by plasma electrolytic oxidation of aluminum substrate in supporting electrolyte with addition of various concentrations of Sm2O3 particles. Photoluminescence (PL) emission spectra of Sm doped Al2O3 coatings exhibit three distinct regions: the first region is related to Al2O3 PL band with a maximum positioned at about 410 nm, the second region in the range from 540 nm to 670 nm features sharp emission bands related to f–f transitions of Sm3+ ions from excited level 4G5/2 to lover levels 6HJ (J = 5/2, 7/2, 9/2), while the third region in the range from 670 nm to 750 nm is characterized by sharp emission bands corresponding to 5D0 → 7FJ (J = 0, 1, 2) transitions of Sm2+ ions. PL excitation spectrum monitored at 410 nm exhibits maximum at about 260 nm, while PL excitation spectrum monitored at the wavelength of the most intense PL of Sm3+ ions (transition 4G5/2 → 6H5/2 at 599 nm) can be divided into two regions. The broad band region from 240 nm to 300 nm with a maximum at about 250 nm is associated with the electron transfer transition from 2p orbital of O2- ions to 4 f orbital of Sm3+ ions, while the series of peaks ranging from 300 nm to 550 nm corresponds to direct excitation of the Sm3+ ground state 6H5/2 into higher levels. PL excitation spectrum obtained by monitoring the 688 nm (5D0 → 7F0) emission from Sm2+ ions shows three broad bands centered around 285 nm, 340 nm, and 480 nm, which arise from the 4f6 → 4f55d1 transitions of Sm2+ ions. Evolution of PL emission spectra excited at 250 nm indicates that at the beginning of PEO all incorporated samarium ions are present as Sm2+, while prolonged PEO process leads to oxidation of Sm2+ ions and their conversion to Sm3+ ions.
1. Introduction In the past few decades photoluminescence of rare-earth ions in different host matrices has been of great interest for researchers from both scientific and technological communities. Due to large transparency from ultraviolet to near infrared region, photochemical stability, high melting point, chemical inertness, dimensional stability and good mechanical strength Al2O3 is one of the best host matrices for rare-earth ions [1–6]. Sm2+ and Sm3+ ions are among the most interesting rareearth elements with promising PL properties in the orange-red visible region, but very scarce information is available on the preparation and PL investigation of pure Al2O3 doped with Sm ions. Plasma electrolytic oxidation (PEO) is a high-voltage anodizing process that produces stable oxide coatings on the surface of lightweight metals (Al, Mg, Zr, Ti, etc.) or metal alloys [7]. PEO process is usually coupled with local formation of numerous microdischarges continuously spread over the PEO coating surface, accompanied by gas evolution. High temperature and pressure at microdischarging sites allow the formation of coatings composed not only of principal ⁎
Corresponding author. E-mail address: sstevan@ff.bg.ac.rs (S. Stojadinović).
http://dx.doi.org/10.1016/j.jlumin.2017.06.043 Received 19 April 2017; Received in revised form 15 June 2017; Accepted 19 June 2017 Available online 21 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
substrate oxides but also of more complex compounds, which involve species present in the electrolyte. For this reason we assumed that conditions existing at microdischarging sites allow the incorporation of Sm from electrolyte into the oxide coatings and that obtained oxide coatings may have interesting PL properties. The aim of this work is to examine the possibility of the formation of Sm doped Al2O3 coatings by PEO process of aluminum in the electrolyte containing Sm2O3 particles and to investigate PL properties of obtained coatings. 2. Experimental details Aluminum samples (99.9% purity) were used as working electrodes in the experiment. Before PEO, samples were degreased in acetone using ultrasonic cleaner and dried in a warm air stream. The samples were sealed with insulation resin leaving only surface of 15 mm × 10 mm accessible to the electrolyte. The experimental setup used for PEO is described in Ref. [8]. Water solution of 0.1 M boric acid (H3BO3) + 0.05 M borax (Na2B4O7·10H2O) was used as a supporting electrolyte. High purity Sm2O3 powder was added to supporting electrolyte in
Journal of Luminescence 192 (2017) 110–116
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Table 1 EDS analysis of PEO coatings in Fig. 1. Sample
Fig. Fig. Fig. Fig.
1a 1b 1c 1d
PEO time [min]
1 3 5 10
Atomic [%] O
Al
Sm
60.04 61.57 65.73 64.65
39.82 38.23 33.92 34.89
0.14 0.20 0.35 0.46
Fig. 1. Time variation of voltage during PEO of aluminum in supporting electrolyte without and with addition of 2 g/L Sm2O3 powder.
concentrations up to 4 g/L. During the PEO, current density was set to 150 mA/cm2 and temperature of the electrolyte was maintained at (18 ± 1) °C. After the PEO, samples were rinsed in distilled water to prevent additional deposition of electrolyte components during drying. Scanning electron microscope (SEM) JEOL 840 A equipped with energy dispersive x-ray spectroscopy (EDS) was used to characterize morphology and chemical composition of formed oxide coatings. The crystallinity of oxide coatings was analyzed by X-ray diffraction (XRD), using a Rigaku Ultima IV diffractometer in Bragg-Brentano geometry, with Ni-filtered CuKα radiation (λ = 1.54178 Å). Diffraction data were acquired over the scattering angle 2θ from 20° to 70° with a step of 0.020° and acquisition rate of 2°/min. PL spectral measurements were taken on a Horiba Jobin Yvon Fluorolog FL3-22 spectrofluorometer at room temperature, with a Xe lamp as the excitation light source. The obtained spectra were corrected for the spectral response of the
Fig. 3. XRD patterns of coatings formed at various stages of PEO in supporting electrolyte with addition of 2 g/L Sm2O3 particles.
Fig. 2. SEM micrographs of coatings formed at various stages of PEO process in supporting electrolyte with addition of 2 g/L Sm2O3 particles: (a) 1 min; (b) 3 min; (c) 5 min; (d) 10 min.
111
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Fig. 4. SEM micrographs of coatings formed by PEO for 10 min in supporting electrolyte with addition of various concentrations of Sm2O3 particles: (a) 0.0 g/ L; (b) 0.5 g/L; (c) 1 g/L; (d) 2 g/L; (e) 4 g/L.
early stage, the process is similar to conventional anodization in which relatively compact barrier oxide film grows at the aluminum/oxide and oxide/electrolyte interfaces as a result of migration of Al3+ and O2−/ OH− ions across the oxide assistated by a strong electric field (∼ 107 V/m) [9]. Uniform film thickening is terminated by dielectric breakdown characterized by deflection from linearity of voltage versus time curve. After the breakdown, a large number of small microdischarges appear, evenly distributed over the whole sample surface. Under breakdown conditions aluminum is melted from the substrate, it enters the microdischarge channels and reacts with electrolyte components. Reaction products are ejected from active microdischarge channels onto the coating surface where they rapidly solidify in contact with the low temperature electrolyte and in that way increase the coating thickness around the channels [10]. Finally, microdischarge channels get cooled and the reaction products are deposited onto its walls. This process repeats itself at a number of discrete locations over the coating surface, leading to increase in the coating thickness. Fig. 2 shows the surface morphology evolution of coatings formed by PEO of aluminum with addition of 2 g/L Sm2O3 powder to water based boric acid + borax electrolyte. The coatings show a typical structure for PEO coatings formed on aluminum [11], with a number of
Table 2 EDS analysis of PEO coatings in Fig. 3. Sample
Fig. Fig. Fig. Fig. Fig.
2a 2b 2c 2d 2e
Sm2O3 (g/L)
0.0 0.5 1.0 2.0 4.0
Atomic (%) O
Al
Sm
62.90 63.84 65.44 64.65 65.30
37.10 36.06 34.27 34.90 33.92
/ 0.10 0.29 0.46 0.77
measuring system and spectral distribution of the Xe lamp. 3. Results and discussion Typical voltage versus time curves during anodization of aluminum in supporting electrolyte without and with addition of 2 g/L Sm2O3 powder is shown in Fig. 1. There is no significant influence of particle addition on voltage response during PEO. From the beginning of anodization the voltage sharply increases with time to about 440 V. In this 112
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Fig. 5. EDS maps of coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles.
melting point around 2335 °C, which is much lower than plasma electron temperature during PEO of aluminum [14] and molten Sm2O3 particles can react inside the microdischarge channels with other components from both electrolyte and substrate to form mixed-oxide coatings. The influence of Sm2O3 particles concentration in supporting electrolyte on surface morphology of coatings formed after 10 min of PEO is shown in Fig. 4. The surface morphology is not significantly influenced by changing the concentration of Sm2O3 particles in supporting electrolyte. The main reason for this is the low concentration of Sm2O3 particles incorporated into coatings (Table 2) which are uniformly dispersed all over the coatings’ surface (Fig. 5). The content of Sm detected in coatings increases with Sm2O3 particle concentration in the supporting electrolyte. The XRD patterns of coatings formed in supporting electrolyte with addition of various concentrations of Sm2O3 powder are shown in Fig. 6. Again, the coatings are partially crystallized and mostly composed of gamma Al2O3. PL emission spectra of Sm doped Al2O3 coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles are shown in Fig. 7. Unambiguously, PL emission spectra exhibit three distinct regions. The first region is composed of the characteristic broad PL band of Al2O3 with maximum at about 410 nm. PL of Al2O3 originates from optical transitions in PL centers which are defect centers related to oxygen vacancies [15]. The second region in the range from 540 nm to 670 nm features sharp emission bands related to f–f transitions of Sm3+ from excited level 4G5/2 to lover levels 6HJ (J = 5/2, 7/2, 9/2) [16]. The bands are positioned at about 563 nm (4G5/2 → 6H5/2), 599 nm (4G5/2 → 6H7/2), and 645 nm (4G5/2 → 6H9/2), respectively. The third region in the range from 670 nm to 750 nm is characterized by sharp emission bands centered at about 688 nm, 703 nm, and 728 nm corresponding to 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2 transitions of Sm2+, respectively [17]. Fig. 8 shows PL excitation spectra monitored at 410 nm, 599 nm, and 688 nm, i.e., at the wavelength of the most intense PL in each of three regions in emission PL spectra. PL excitation spectrum monitored
Fig. 6. XRD patterns of coatings formed by PEO for 10 min in supporting electrolyte with addition of varying concentration of Sm2O3 particles.
pores and regions resulting from the rapid cooling of molten material. Results of the EDS analyses of surface coatings in Fig. 2 are given in Table 1. The main elements of the coatings are Al, O, and Sm, and it is clearly observable that Sm content in the coatings increases with PEO time. The XRD patterns of coatings formed after various PEO times are shown in Fig. 3. During the first minute of PEO processing only amorphous Al2O3 is formed. Diffraction peaks indexed to gamma Al2O3 phase indicate crystallization of amorphous Al2O3 after about 3 min of PEO processing. Incorporation of particles into coatings during PEO process is possible through the electrophoretic and micro-discharging mechanisms [12]. In our experiment Sm2O3 particles are dispersed in the electrolyte at pH around 8.5. The isoelectric point of Sm2O3 particles is around 8.3 indicating that surface of Sm2O3 particles are negatively charged [13] and can move toward the aluminum anode. Sm2O3 particles have 113
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Fig. 9. The influence of concentration of Sm2O3 particles in supporting electrolyte on PL emission spectra of coating formed by PEO for 10 min: (a) excited at 250 nm; (b) excited at 340 nm.
and the series of sharp peaks in the range from 300 nm to 550 nm. The wide PL band is associated with charge transfer state of Sm3+, which originates from electronic transition between the completely filled 2p orbital of O2− ions (2p6) and empty 4 f orbital of Sm3+ ions (4f6) [18]. On the other hand, observed sharp peaks correspond to direct excitation of Sm3+ ground state 6H5/2 to higher levels, where the most intense peak at 404 nm is assigned to the electronic transition 6H5/2→4F7/2 [19]. PL excitation spectrum obtained by monitoring at 688 nm shows three broad bands centered around 285 nm, 340 nm, and 480 nm originating from the 4f6 → 4f55d1 transitions of Sm2+ ions [20]. The influence of concentration of Sm2O3 particles on PL is shown in Fig. 9. Obviously, PL intensity of Sm3+ emission bands (Fig. 9a) and Sm2+ emission bands (Fig. 9b) increases with the Sm2O3 particle concentration in the supporting electrolyte because the content of Sm incorporated into coatings increases (Table 2). The highest PL is observed for the sample formed in supporting electrolyte with addition of 4 g/L of Sm2O3 particles. The evolution of PL emission spectra of coatings formed in the supporting electrolyte with addition 4 g/L Sm2O3 particle is shown in Fig. 10, suggesting that both Sm3+ and Sm2+ emission bands of Sm doped Al2O3 coatings increase with PEO processing time. PL emission spectra excited at 250 nm (Fig. 10a), i.e., at the wavelength of the most intense peak in PL excitation spectra of Sm3+ (Fig. 8) indicates that at the beginning of PEO all incorporated Sm ions are present as Sm2+. After about 3 min from the beginning of PEO, bands related to Sm3+ ions can be observed in PL emission spectrum indicating that some of Sm2+ ions are oxidized and converted to Sm3+ ions. Further PEO processing leads to an increase in conversion of Sm2+ ions into Sm3+ ions, confirming the conversion of Sm2+ to Sm3+ as a consequence of elevated temperatures [21,22] induced by the presence
Fig. 7. PL emission spectra of coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles.
Fig. 8. PL excitation spectra of coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles.
at 410 nm exhibits maximum at about 260 nm that is in accordance with PL emission spectral measurement. PL excitation spectrum monitored at the wavelength of the most intense PL of Sm3+ ions (transition 4 G5/2 → 6H5/2 at 599 nm) can be divided into two regions: the broad band region from 240 nm to 300 nm with a maximum at about 250 nm 114
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at about 250 nm (associated with the electron transfer transition from 2p orbital of O2− ions to 4 f orbital of Sm3+ ions) and the series of sharp peaks in the range from 300 nm to 550 nm (assigned to the direct excitation of the Sm3+ ground state 6H5/2 into higher levels). When PL excitation spectrum is monitored at 688 nm (Sm2+ transition 5D0 → 7F0) it shows three broad bands centered around 285 nm, 340 nm, and 480 nm, arising from the 4f6 → 4f55d1 transitions of Sm2+ ions. (5) Evolution of emission PL spectra excited at 250 nm indicates that at the beginning of PEO all incorporated samarium ions are present as Sm2+, while prolonged PEO processing time leads to oxidation of Sm2+ ions and their conversion to Sm3+ ions. This may be related to the appearance of large microdischarges with prolonged PEO processing time, which allocate enough energy for oxidation of Sm2+ ions. 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] Y. Onishi, T. Nakamura, S. Adachi, Solubility limit and luminescence properties of Eu3+ ions in Al2O3, J. Lumin. 176 (2016) 266–271. [2] P. Nayar, X.Y. Zhu, F. Yang, M. Lu, G. Lakshminarayana, X.P. Liu, Y.F. Chen, I.V. Kityk, Fabrication and characterization of highly luminescent Er3+:Al2O3 thin films with optimized growth parameters, Opt. Mater. 60 (2016) 57–61. [3] Z. Zhu, D. Liu, H. Liu, G. Li, J. Du, Z. He, Fabrication and luminescence properties of Al2O3:Tb3+ microspheres via a microwave solvothermal route, J. Lumin. 132 (2012) 261–265. [4] T.P. Mokoena, E.C. Linganiso, H.C. Swart, V. Kumar, O.M. Ntwaeaborwa, Cooperative luminescence from low temperature synthesized α-Al2O3: Yb3+ phosphor by using solution combustion, Ceram. Int. 43 (2017) 174–181. [5] Y. Gui, Q. Yang, Y. Shao, Y. Yuan, Spectroscopic properties of neodymium-doped alumina (Nd3+:Al2O3) translucent ceramics, J. Lumin. 184 (2017) 232–234. [6] R. Martínez-Martínez, E. Álvarez, A. Speghini, C. Falcony, U. Caldiño, White light generation in Al2O3:Ce3+:Tb3+:Mn2+ films deposited by ultrasonic spray pyrolysis, Thin Solid Films 518 (2010) 5724–5730. [7] S. Stojadinović, R. Vasilić, M. Perić, Investigation of plasma electrolytic oxidation on valve metals by means of molecular spectroscopy – a review, RSC Adv. 4 (2014) 25759–25789. [8] S. Stojadinović, N. Radić, B. Grbić, S. Maletić, P. Stefanov, A. Pačevski, R. Vasilić, Structural, photoluminescent and photocatalytic properties of TiO2:Eu3+ coatings formed by plasma electrolytic oxidation, Appl. Surf. Sci. 370 (2016) 218–228. [9] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews, Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions, Electrochim. Acta 49 (2004) 2085–2095. [10] A.L. Yerokhin, V.V. Lyubimov, R.V. Ashitkov, Phase formation in ceramic coatings during plasma electrolytic oxidation of aluminium alloys, Ceram. Int. 24 (1998) 1–6. [11] S. Stojadinović, R. Vasilić, N. Radić, N. Tadić, P. Stefanov, B. Grbić, The formation of tungsten doped Al2O3/ZnO coatings on aluminum by plasma electrolytic oxidation and their application in photocatalysis, Appl. Surf. Sci. 377 (2016) 37–43. [12] X. Lu, M. Mohedano, C. Blawert, E. Matykina, R. Arrabal, K.U. Kainer, M.L. Zheludkevich, Plasma electrolytic oxidation coatings with particle additions – a review, Surf. Coat. Technol. 307 (2016) 1165–1182. [13] M. Ozawa, M. Hattori, Ultrasonic vibration potential and point of zero charge of some rare earth oxides in water, J. Alloy. Compd. 408–412 (2006) 560–562. [14] J. Jovović, S. Stojadinović, N.M. Šišović, N. Konjević, Spectroscopic characterization of plasma during electrolytic oxidation (PEO) of aluminium, Surf. Coat. Technol. 206 (2011) 24–28. [15] S. Stojadinovic, R. Vasilic, Z. Nedic, B. Kasalica, I. Belca, Lj Zekovic, Photoluminescent properties of barrier anodic oxide films on aluminum, Thin Solid Films 519 (2011) 3516–3521. [16] P. Haritha, C.S. Dwaraka Viswanath, K. Linganna, P. Babu, C.K. Jayasankar, V. Lavín, V. Venkatramu, Nanocrystalline Sm3+-doped Lu3Ga5O12 garnets: an intense orange-reddish luminescent material for white light emitting devices, J. Lumin. 179 (2016) 533–538. [17] S. Sakirzanovas, A. Katelnikovas, D. Dutczak, A. Kareiva, T. Justel, Synthesis and Sm2+/Sm3+ doping effects on photoluminescence properties of Sr4Al14O25, J. Lumin. 131 (2011) 2255–2262. [18] L. Yang, X. Yu, S. Yang, C. Zhou, P. Zhou, W. Gao, P. Ye, Preparation and luminescence properties of LED conversion novel phosphors SrZnO2:Sm, Mater. Lett. 62 (2008) 907–910. [19] A.D.J. David, G.S. Muhammad, V. Sivakumar, Synthesis and photoluminescence properties of Sm3+substituted glaserite-type orthovanadates K3Y[VO4]2 with
Fig. 10. Evolution of PL emission spectra of coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles: (a) excited at 250 nm; (b) excited at 340 nm.
of locally high temperatures microdischarges during the PEO processing. 4. Conclusions Based on our experimental results we can conclude that: (1) PEO of aluminum in electrolyte containing Sm2O3 particles is a suitable technique for the synthesis of Sm doped Al2O3 coatings. (2) Morphology, phase, and chemical compositions of formed oxide coatings depend on PEO time and concentration of Sm2O3 particles in the electrolyte. The main elemental components of PEO coatings are Al, O, and Sm, which are uniformly dispersed all over the coatings’ surface. The content of Sm incorporated into coatings increases with PEO time as well as with Sm2O3 particle concentration. The coatings are crystallized and mostly composed of gamma phase of Al2O3. (3) PL spectra of formed PEO coatings feature well pronounced bands native to Al2O3, Sm2+ and Sm3+ ions. PL emission band of Al2O3 has maximum at about 410 nm and it is related to oxygen vacancy defects. PL emission spectra of Sm3+ features sharp bands in the range from 540 nm to 670 nm related to f–f transitions from excited level 4G5/2 to lover levels 6HJ (J = 5/2, 7/2, 9/2), while the emission PL spectra of Sm2+ features sharp bands in the range from 670 nm to 750 nm corresponding 5D0 → 7FJ (J = 0, 1, 2) transitions. (4) PL excitation spectrum of Sm doped Al2O3 coatings monitored at 410 nm exhibits broad maximum at about 260 nm, while when monitored at 599 nm (Sm3+ transition 4G5/2 → 6H5/2) it consists of a broad band in the range from 240 nm to 300 nm, with a maximum 115
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SRPES, XPS and STM, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc. 2016.11.145. [22] G.U. Zhenan, d-f and f-f transition bands of praseodymium and samarium ions in silica glasses, J. Non-Cryst. Solids 80 (1986) 429–434.
monoclinic structure, J. Lumin. 177 (2016) 104–110. [20] Q. Zeng, N. Kilah, M. Riley, The luminescence of Sm2+ in alkaline earth borophosphates, J. Lumin. 101 (2003) 167–174. [21] Q. Xu, S. Hu, W. Wang, Y. Wang, H. Ju, J. Zhu, Temperature-induced structural evolution of Sm nanoparticles onAl2O3 thin film: an in-situ investigation using
116
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence of Sm2+ / Sm3+ doped Al2O3 coatings formed by plasma electrolytic oxidation of aluminum
MARK
⁎
Stevan Stojadinović , Nenad Tadić, Rastko Vasilić University of Belgrade, Faculty of Physics, Studentski trg 12-16, 11000 Belgrade, Serbia
A R T I C L E I N F O
A B S T R A C T
Keywords: Plasma electrolytic oxidation Al2O3 Sm2+ Sm3+ Photoluminescence
Sm doped Al2O3 coatings are formed by plasma electrolytic oxidation of aluminum substrate in supporting electrolyte with addition of various concentrations of Sm2O3 particles. Photoluminescence (PL) emission spectra of Sm doped Al2O3 coatings exhibit three distinct regions: the first region is related to Al2O3 PL band with a maximum positioned at about 410 nm, the second region in the range from 540 nm to 670 nm features sharp emission bands related to f–f transitions of Sm3+ ions from excited level 4G5/2 to lover levels 6HJ (J = 5/2, 7/2, 9/2), while the third region in the range from 670 nm to 750 nm is characterized by sharp emission bands corresponding to 5D0 → 7FJ (J = 0, 1, 2) transitions of Sm2+ ions. PL excitation spectrum monitored at 410 nm exhibits maximum at about 260 nm, while PL excitation spectrum monitored at the wavelength of the most intense PL of Sm3+ ions (transition 4G5/2 → 6H5/2 at 599 nm) can be divided into two regions. The broad band region from 240 nm to 300 nm with a maximum at about 250 nm is associated with the electron transfer transition from 2p orbital of O2- ions to 4 f orbital of Sm3+ ions, while the series of peaks ranging from 300 nm to 550 nm corresponds to direct excitation of the Sm3+ ground state 6H5/2 into higher levels. PL excitation spectrum obtained by monitoring the 688 nm (5D0 → 7F0) emission from Sm2+ ions shows three broad bands centered around 285 nm, 340 nm, and 480 nm, which arise from the 4f6 → 4f55d1 transitions of Sm2+ ions. Evolution of PL emission spectra excited at 250 nm indicates that at the beginning of PEO all incorporated samarium ions are present as Sm2+, while prolonged PEO process leads to oxidation of Sm2+ ions and their conversion to Sm3+ ions.
1. Introduction In the past few decades photoluminescence of rare-earth ions in different host matrices has been of great interest for researchers from both scientific and technological communities. Due to large transparency from ultraviolet to near infrared region, photochemical stability, high melting point, chemical inertness, dimensional stability and good mechanical strength Al2O3 is one of the best host matrices for rare-earth ions [1–6]. Sm2+ and Sm3+ ions are among the most interesting rareearth elements with promising PL properties in the orange-red visible region, but very scarce information is available on the preparation and PL investigation of pure Al2O3 doped with Sm ions. Plasma electrolytic oxidation (PEO) is a high-voltage anodizing process that produces stable oxide coatings on the surface of lightweight metals (Al, Mg, Zr, Ti, etc.) or metal alloys [7]. PEO process is usually coupled with local formation of numerous microdischarges continuously spread over the PEO coating surface, accompanied by gas evolution. High temperature and pressure at microdischarging sites allow the formation of coatings composed not only of principal ⁎
Corresponding author. E-mail address: sstevan@ff.bg.ac.rs (S. Stojadinović).
http://dx.doi.org/10.1016/j.jlumin.2017.06.043 Received 19 April 2017; Received in revised form 15 June 2017; Accepted 19 June 2017 Available online 21 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
substrate oxides but also of more complex compounds, which involve species present in the electrolyte. For this reason we assumed that conditions existing at microdischarging sites allow the incorporation of Sm from electrolyte into the oxide coatings and that obtained oxide coatings may have interesting PL properties. The aim of this work is to examine the possibility of the formation of Sm doped Al2O3 coatings by PEO process of aluminum in the electrolyte containing Sm2O3 particles and to investigate PL properties of obtained coatings. 2. Experimental details Aluminum samples (99.9% purity) were used as working electrodes in the experiment. Before PEO, samples were degreased in acetone using ultrasonic cleaner and dried in a warm air stream. The samples were sealed with insulation resin leaving only surface of 15 mm × 10 mm accessible to the electrolyte. The experimental setup used for PEO is described in Ref. [8]. Water solution of 0.1 M boric acid (H3BO3) + 0.05 M borax (Na2B4O7·10H2O) was used as a supporting electrolyte. High purity Sm2O3 powder was added to supporting electrolyte in
Journal of Luminescence 192 (2017) 110–116
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Table 1 EDS analysis of PEO coatings in Fig. 1. Sample
Fig. Fig. Fig. Fig.
1a 1b 1c 1d
PEO time [min]
1 3 5 10
Atomic [%] O
Al
Sm
60.04 61.57 65.73 64.65
39.82 38.23 33.92 34.89
0.14 0.20 0.35 0.46
Fig. 1. Time variation of voltage during PEO of aluminum in supporting electrolyte without and with addition of 2 g/L Sm2O3 powder.
concentrations up to 4 g/L. During the PEO, current density was set to 150 mA/cm2 and temperature of the electrolyte was maintained at (18 ± 1) °C. After the PEO, samples were rinsed in distilled water to prevent additional deposition of electrolyte components during drying. Scanning electron microscope (SEM) JEOL 840 A equipped with energy dispersive x-ray spectroscopy (EDS) was used to characterize morphology and chemical composition of formed oxide coatings. The crystallinity of oxide coatings was analyzed by X-ray diffraction (XRD), using a Rigaku Ultima IV diffractometer in Bragg-Brentano geometry, with Ni-filtered CuKα radiation (λ = 1.54178 Å). Diffraction data were acquired over the scattering angle 2θ from 20° to 70° with a step of 0.020° and acquisition rate of 2°/min. PL spectral measurements were taken on a Horiba Jobin Yvon Fluorolog FL3-22 spectrofluorometer at room temperature, with a Xe lamp as the excitation light source. The obtained spectra were corrected for the spectral response of the
Fig. 3. XRD patterns of coatings formed at various stages of PEO in supporting electrolyte with addition of 2 g/L Sm2O3 particles.
Fig. 2. SEM micrographs of coatings formed at various stages of PEO process in supporting electrolyte with addition of 2 g/L Sm2O3 particles: (a) 1 min; (b) 3 min; (c) 5 min; (d) 10 min.
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Fig. 4. SEM micrographs of coatings formed by PEO for 10 min in supporting electrolyte with addition of various concentrations of Sm2O3 particles: (a) 0.0 g/ L; (b) 0.5 g/L; (c) 1 g/L; (d) 2 g/L; (e) 4 g/L.
early stage, the process is similar to conventional anodization in which relatively compact barrier oxide film grows at the aluminum/oxide and oxide/electrolyte interfaces as a result of migration of Al3+ and O2−/ OH− ions across the oxide assistated by a strong electric field (∼ 107 V/m) [9]. Uniform film thickening is terminated by dielectric breakdown characterized by deflection from linearity of voltage versus time curve. After the breakdown, a large number of small microdischarges appear, evenly distributed over the whole sample surface. Under breakdown conditions aluminum is melted from the substrate, it enters the microdischarge channels and reacts with electrolyte components. Reaction products are ejected from active microdischarge channels onto the coating surface where they rapidly solidify in contact with the low temperature electrolyte and in that way increase the coating thickness around the channels [10]. Finally, microdischarge channels get cooled and the reaction products are deposited onto its walls. This process repeats itself at a number of discrete locations over the coating surface, leading to increase in the coating thickness. Fig. 2 shows the surface morphology evolution of coatings formed by PEO of aluminum with addition of 2 g/L Sm2O3 powder to water based boric acid + borax electrolyte. The coatings show a typical structure for PEO coatings formed on aluminum [11], with a number of
Table 2 EDS analysis of PEO coatings in Fig. 3. Sample
Fig. Fig. Fig. Fig. Fig.
2a 2b 2c 2d 2e
Sm2O3 (g/L)
0.0 0.5 1.0 2.0 4.0
Atomic (%) O
Al
Sm
62.90 63.84 65.44 64.65 65.30
37.10 36.06 34.27 34.90 33.92
/ 0.10 0.29 0.46 0.77
measuring system and spectral distribution of the Xe lamp. 3. Results and discussion Typical voltage versus time curves during anodization of aluminum in supporting electrolyte without and with addition of 2 g/L Sm2O3 powder is shown in Fig. 1. There is no significant influence of particle addition on voltage response during PEO. From the beginning of anodization the voltage sharply increases with time to about 440 V. In this 112
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Fig. 5. EDS maps of coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles.
melting point around 2335 °C, which is much lower than plasma electron temperature during PEO of aluminum [14] and molten Sm2O3 particles can react inside the microdischarge channels with other components from both electrolyte and substrate to form mixed-oxide coatings. The influence of Sm2O3 particles concentration in supporting electrolyte on surface morphology of coatings formed after 10 min of PEO is shown in Fig. 4. The surface morphology is not significantly influenced by changing the concentration of Sm2O3 particles in supporting electrolyte. The main reason for this is the low concentration of Sm2O3 particles incorporated into coatings (Table 2) which are uniformly dispersed all over the coatings’ surface (Fig. 5). The content of Sm detected in coatings increases with Sm2O3 particle concentration in the supporting electrolyte. The XRD patterns of coatings formed in supporting electrolyte with addition of various concentrations of Sm2O3 powder are shown in Fig. 6. Again, the coatings are partially crystallized and mostly composed of gamma Al2O3. PL emission spectra of Sm doped Al2O3 coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles are shown in Fig. 7. Unambiguously, PL emission spectra exhibit three distinct regions. The first region is composed of the characteristic broad PL band of Al2O3 with maximum at about 410 nm. PL of Al2O3 originates from optical transitions in PL centers which are defect centers related to oxygen vacancies [15]. The second region in the range from 540 nm to 670 nm features sharp emission bands related to f–f transitions of Sm3+ from excited level 4G5/2 to lover levels 6HJ (J = 5/2, 7/2, 9/2) [16]. The bands are positioned at about 563 nm (4G5/2 → 6H5/2), 599 nm (4G5/2 → 6H7/2), and 645 nm (4G5/2 → 6H9/2), respectively. The third region in the range from 670 nm to 750 nm is characterized by sharp emission bands centered at about 688 nm, 703 nm, and 728 nm corresponding to 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2 transitions of Sm2+, respectively [17]. Fig. 8 shows PL excitation spectra monitored at 410 nm, 599 nm, and 688 nm, i.e., at the wavelength of the most intense PL in each of three regions in emission PL spectra. PL excitation spectrum monitored
Fig. 6. XRD patterns of coatings formed by PEO for 10 min in supporting electrolyte with addition of varying concentration of Sm2O3 particles.
pores and regions resulting from the rapid cooling of molten material. Results of the EDS analyses of surface coatings in Fig. 2 are given in Table 1. The main elements of the coatings are Al, O, and Sm, and it is clearly observable that Sm content in the coatings increases with PEO time. The XRD patterns of coatings formed after various PEO times are shown in Fig. 3. During the first minute of PEO processing only amorphous Al2O3 is formed. Diffraction peaks indexed to gamma Al2O3 phase indicate crystallization of amorphous Al2O3 after about 3 min of PEO processing. Incorporation of particles into coatings during PEO process is possible through the electrophoretic and micro-discharging mechanisms [12]. In our experiment Sm2O3 particles are dispersed in the electrolyte at pH around 8.5. The isoelectric point of Sm2O3 particles is around 8.3 indicating that surface of Sm2O3 particles are negatively charged [13] and can move toward the aluminum anode. Sm2O3 particles have 113
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Fig. 9. The influence of concentration of Sm2O3 particles in supporting electrolyte on PL emission spectra of coating formed by PEO for 10 min: (a) excited at 250 nm; (b) excited at 340 nm.
and the series of sharp peaks in the range from 300 nm to 550 nm. The wide PL band is associated with charge transfer state of Sm3+, which originates from electronic transition between the completely filled 2p orbital of O2− ions (2p6) and empty 4 f orbital of Sm3+ ions (4f6) [18]. On the other hand, observed sharp peaks correspond to direct excitation of Sm3+ ground state 6H5/2 to higher levels, where the most intense peak at 404 nm is assigned to the electronic transition 6H5/2→4F7/2 [19]. PL excitation spectrum obtained by monitoring at 688 nm shows three broad bands centered around 285 nm, 340 nm, and 480 nm originating from the 4f6 → 4f55d1 transitions of Sm2+ ions [20]. The influence of concentration of Sm2O3 particles on PL is shown in Fig. 9. Obviously, PL intensity of Sm3+ emission bands (Fig. 9a) and Sm2+ emission bands (Fig. 9b) increases with the Sm2O3 particle concentration in the supporting electrolyte because the content of Sm incorporated into coatings increases (Table 2). The highest PL is observed for the sample formed in supporting electrolyte with addition of 4 g/L of Sm2O3 particles. The evolution of PL emission spectra of coatings formed in the supporting electrolyte with addition 4 g/L Sm2O3 particle is shown in Fig. 10, suggesting that both Sm3+ and Sm2+ emission bands of Sm doped Al2O3 coatings increase with PEO processing time. PL emission spectra excited at 250 nm (Fig. 10a), i.e., at the wavelength of the most intense peak in PL excitation spectra of Sm3+ (Fig. 8) indicates that at the beginning of PEO all incorporated Sm ions are present as Sm2+. After about 3 min from the beginning of PEO, bands related to Sm3+ ions can be observed in PL emission spectrum indicating that some of Sm2+ ions are oxidized and converted to Sm3+ ions. Further PEO processing leads to an increase in conversion of Sm2+ ions into Sm3+ ions, confirming the conversion of Sm2+ to Sm3+ as a consequence of elevated temperatures [21,22] induced by the presence
Fig. 7. PL emission spectra of coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles.
Fig. 8. PL excitation spectra of coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles.
at 410 nm exhibits maximum at about 260 nm that is in accordance with PL emission spectral measurement. PL excitation spectrum monitored at the wavelength of the most intense PL of Sm3+ ions (transition 4 G5/2 → 6H5/2 at 599 nm) can be divided into two regions: the broad band region from 240 nm to 300 nm with a maximum at about 250 nm 114
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at about 250 nm (associated with the electron transfer transition from 2p orbital of O2− ions to 4 f orbital of Sm3+ ions) and the series of sharp peaks in the range from 300 nm to 550 nm (assigned to the direct excitation of the Sm3+ ground state 6H5/2 into higher levels). When PL excitation spectrum is monitored at 688 nm (Sm2+ transition 5D0 → 7F0) it shows three broad bands centered around 285 nm, 340 nm, and 480 nm, arising from the 4f6 → 4f55d1 transitions of Sm2+ ions. (5) Evolution of emission PL spectra excited at 250 nm indicates that at the beginning of PEO all incorporated samarium ions are present as Sm2+, while prolonged PEO processing time leads to oxidation of Sm2+ ions and their conversion to Sm3+ ions. This may be related to the appearance of large microdischarges with prolonged PEO processing time, which allocate enough energy for oxidation of Sm2+ ions. 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] Y. Onishi, T. Nakamura, S. Adachi, Solubility limit and luminescence properties of Eu3+ ions in Al2O3, J. Lumin. 176 (2016) 266–271. [2] P. Nayar, X.Y. Zhu, F. Yang, M. Lu, G. Lakshminarayana, X.P. Liu, Y.F. Chen, I.V. Kityk, Fabrication and characterization of highly luminescent Er3+:Al2O3 thin films with optimized growth parameters, Opt. Mater. 60 (2016) 57–61. [3] Z. Zhu, D. Liu, H. Liu, G. Li, J. Du, Z. He, Fabrication and luminescence properties of Al2O3:Tb3+ microspheres via a microwave solvothermal route, J. Lumin. 132 (2012) 261–265. [4] T.P. Mokoena, E.C. Linganiso, H.C. Swart, V. Kumar, O.M. Ntwaeaborwa, Cooperative luminescence from low temperature synthesized α-Al2O3: Yb3+ phosphor by using solution combustion, Ceram. Int. 43 (2017) 174–181. [5] Y. Gui, Q. Yang, Y. Shao, Y. Yuan, Spectroscopic properties of neodymium-doped alumina (Nd3+:Al2O3) translucent ceramics, J. Lumin. 184 (2017) 232–234. [6] R. Martínez-Martínez, E. Álvarez, A. Speghini, C. Falcony, U. Caldiño, White light generation in Al2O3:Ce3+:Tb3+:Mn2+ films deposited by ultrasonic spray pyrolysis, Thin Solid Films 518 (2010) 5724–5730. [7] S. Stojadinović, R. Vasilić, M. Perić, Investigation of plasma electrolytic oxidation on valve metals by means of molecular spectroscopy – a review, RSC Adv. 4 (2014) 25759–25789. [8] S. Stojadinović, N. Radić, B. Grbić, S. Maletić, P. Stefanov, A. Pačevski, R. Vasilić, Structural, photoluminescent and photocatalytic properties of TiO2:Eu3+ coatings formed by plasma electrolytic oxidation, Appl. Surf. Sci. 370 (2016) 218–228. [9] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews, Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions, Electrochim. Acta 49 (2004) 2085–2095. [10] A.L. Yerokhin, V.V. Lyubimov, R.V. Ashitkov, Phase formation in ceramic coatings during plasma electrolytic oxidation of aluminium alloys, Ceram. Int. 24 (1998) 1–6. [11] S. Stojadinović, R. Vasilić, N. Radić, N. Tadić, P. Stefanov, B. Grbić, The formation of tungsten doped Al2O3/ZnO coatings on aluminum by plasma electrolytic oxidation and their application in photocatalysis, Appl. Surf. Sci. 377 (2016) 37–43. [12] X. Lu, M. Mohedano, C. Blawert, E. Matykina, R. Arrabal, K.U. Kainer, M.L. Zheludkevich, Plasma electrolytic oxidation coatings with particle additions – a review, Surf. Coat. Technol. 307 (2016) 1165–1182. [13] M. Ozawa, M. Hattori, Ultrasonic vibration potential and point of zero charge of some rare earth oxides in water, J. Alloy. Compd. 408–412 (2006) 560–562. [14] J. Jovović, S. Stojadinović, N.M. Šišović, N. Konjević, Spectroscopic characterization of plasma during electrolytic oxidation (PEO) of aluminium, Surf. Coat. Technol. 206 (2011) 24–28. [15] S. Stojadinovic, R. Vasilic, Z. Nedic, B. Kasalica, I. Belca, Lj Zekovic, Photoluminescent properties of barrier anodic oxide films on aluminum, Thin Solid Films 519 (2011) 3516–3521. [16] P. Haritha, C.S. Dwaraka Viswanath, K. Linganna, P. Babu, C.K. Jayasankar, V. Lavín, V. Venkatramu, Nanocrystalline Sm3+-doped Lu3Ga5O12 garnets: an intense orange-reddish luminescent material for white light emitting devices, J. Lumin. 179 (2016) 533–538. [17] S. Sakirzanovas, A. Katelnikovas, D. Dutczak, A. Kareiva, T. Justel, Synthesis and Sm2+/Sm3+ doping effects on photoluminescence properties of Sr4Al14O25, J. Lumin. 131 (2011) 2255–2262. [18] L. Yang, X. Yu, S. Yang, C. Zhou, P. Zhou, W. Gao, P. Ye, Preparation and luminescence properties of LED conversion novel phosphors SrZnO2:Sm, Mater. Lett. 62 (2008) 907–910. [19] A.D.J. David, G.S. Muhammad, V. Sivakumar, Synthesis and photoluminescence properties of Sm3+substituted glaserite-type orthovanadates K3Y[VO4]2 with
Fig. 10. Evolution of PL emission spectra of coating formed by PEO for 10 min in supporting electrolyte with addition of 4 g/L Sm2O3 particles: (a) excited at 250 nm; (b) excited at 340 nm.
of locally high temperatures microdischarges during the PEO processing. 4. Conclusions Based on our experimental results we can conclude that: (1) PEO of aluminum in electrolyte containing Sm2O3 particles is a suitable technique for the synthesis of Sm doped Al2O3 coatings. (2) Morphology, phase, and chemical compositions of formed oxide coatings depend on PEO time and concentration of Sm2O3 particles in the electrolyte. The main elemental components of PEO coatings are Al, O, and Sm, which are uniformly dispersed all over the coatings’ surface. The content of Sm incorporated into coatings increases with PEO time as well as with Sm2O3 particle concentration. The coatings are crystallized and mostly composed of gamma phase of Al2O3. (3) PL spectra of formed PEO coatings feature well pronounced bands native to Al2O3, Sm2+ and Sm3+ ions. PL emission band of Al2O3 has maximum at about 410 nm and it is related to oxygen vacancy defects. PL emission spectra of Sm3+ features sharp bands in the range from 540 nm to 670 nm related to f–f transitions from excited level 4G5/2 to lover levels 6HJ (J = 5/2, 7/2, 9/2), while the emission PL spectra of Sm2+ features sharp bands in the range from 670 nm to 750 nm corresponding 5D0 → 7FJ (J = 0, 1, 2) transitions. (4) PL excitation spectrum of Sm doped Al2O3 coatings monitored at 410 nm exhibits broad maximum at about 260 nm, while when monitored at 599 nm (Sm3+ transition 4G5/2 → 6H5/2) it consists of a broad band in the range from 240 nm to 300 nm, with a maximum 115
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SRPES, XPS and STM, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc. 2016.11.145. [22] G.U. Zhenan, d-f and f-f transition bands of praseodymium and samarium ions in silica glasses, J. Non-Cryst. Solids 80 (1986) 429–434.
monoclinic structure, J. Lumin. 177 (2016) 104–110. [20] Q. Zeng, N. Kilah, M. Riley, The luminescence of Sm2+ in alkaline earth borophosphates, J. Lumin. 101 (2003) 167–174. [21] Q. Xu, S. Hu, W. Wang, Y. Wang, H. Ju, J. Zhu, Temperature-induced structural evolution of Sm nanoparticles onAl2O3 thin film: an in-situ investigation using
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