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Structural and luminescence characterization of porous anodic oxide films on aluminum formed in sulfamic acid solution
Applied Surface Science 255 (2008) 2845–2850
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural and luminescence characterization of porous anodic oxide films on aluminum formed in sulfamic acid solution S. Stojadinovic a,*, R. Vasilic b, I. Belca a, M. Tadic a, B. Kasalica a, Lj. Zekovic a a b
Faculty of Physics, Belgrade University, Studentski trg 12-16, 11000 Belgrade, Serbia Faculty of Applied Ecology, Singidunum University, Bul. Kralja Aleksandra 79, 11000 Belgrade, Serbia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 July 2008 Received in revised form 11 August 2008 Accepted 11 August 2008 Available online 20 August 2008
Atomic force microscopy (AFM) and luminescence methods (galvanoluminescence and photoluminescence) were used to characterize porous oxide films obtained by aluminum anodization in sulfamic acid solution. For the first time we measured weak galvanoluminescence during aluminum anodization in sulfamic acid and found strong influence of sample’s surface pretreatment as well as anodic conditions on luminescence intensity. AFM analysis showed that the pore arrangement of porous oxide films formed in sulfamic acid by two-step anodization process at a constant voltage of 15–30 V is relatively irregular. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Porous oxide films Galvanoluminescence Photoluminescence Aluminum Sulfamic acid
1. Introduction Thin oxide films on aluminum are inexhaustible source of interest for researchers in various scientific and technological areas: physics, chemistry, electronics, etc. In the last decades the scientists have been focused on the fabrication self-organized alumina nanostructures by the anodization of aluminum in various electrolytes. This is a result of application of porous oxide films, with ordered pores of dimensions ranging from submicrometer to nanometer range, as template in nanotechnology for nanotubes [1], nanowires [2], solar cells [3], micro-optical elements [4], photonic crystals [5], etc. Since the application of two step procedure results in selforganized porous structures on anodized aluminum [6], this technique become promising for preparing such ordered porous structures due to its low cost and relative easy use. The procedure involves two separate anodization processes. The first anodization process consists of a long period of anodization forming the ordered porous structure. After the removal of the oxide, an array of highly ordered dimples is formed on aluminum. These dimples act as initiation sites for growing highly ordered porous structure. Typical electrolytes forming these porous structures are aqueous
* Corresponding author. Tel.: +381 11 2630152; fax: +381 11 3282619. E-mail address: [email protected] (S. Stojadinovic). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.08.023
solutions of oxalic, sulfuric and phosphoric acid. Dimensions and regularity of pores are determined by a type of electrolyte [7,8], quality of aluminum wafer [9,10] and anodic conditions (temperature and composition of electrolyte, voltage of anodization) [11,12]. The study of the luminescence of porous oxide films used as templates for synthesizing new luminescence materials is very important, because their luminescence properties certainly affect the luminescence properties of materials based on porous oxide films. In recent years, much attention has been paid to the various galvanoluminescence (GL) and photoluminescence (PL) methods, which enable in situ characterization of porous oxide films formed in various electrolytes. Galvanoluminescence or electroluminescence is a common name for light appearing at one of the electrodes in an electrolytic solution during anodization. During the past several years, the authors conducted a number of investigations of the GL characteristics of porous oxide films obtained in organic electrolytes (oxalic acid and malonic acid) [13,14] and inorganic electrolytes (phosphoric acid, chromic acid and sulfuric acid) [15–18]. We have showed that the nature and intensity of GL depend on many factors such as type of electrolytes (organic or inorganic), surface pretreatment and anodizing conditions. Surface pretreatment of aluminum samples (surface preparation and annealing) has a significant influence on GL obtained in inorganic electrolytes. Consequently, GL intensity might be an indicator of
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aluminum surface quality and roughness. Furthermore, GL methods can be used for the determination of the oxide film thickness, growth rate, refraction index, optical constants of aluminium, etc. [19–21]. In organic electrolytes GL is excited by collision of electrons, injected into the oxide film at the electrolyte–oxide interface and accelerated by high electric field (nearly 107 V/cm), with luminescence centers (carboxylate ions) inside the oxide film. In porous oxide films, the light is emitted from the barrier part of porous films, i.e. at the bottom of the pores, in the high field region. If an anodic oxide film formed on aluminum is illuminated with UV radiation, the emission of visible light takes place. This phenomenon is termed photoluminescence. Many authors have investigated PL, but explanations of its nature and mechanism are still incomplete. The intensity of emitted PL radiation and the shapes of PL excitation and emission spectra depend on many factors: the nature of the electrolytes (organic or inorganic) and the conditions of anodizing process [22], the thickness of the oxide films and its additional treatment (electrolytic or thermal) [23], the
wavelength of the incident radiation [24], etc. PL spectral measurement can be used for determination of the refractive index, thickness and porosity of porous oxide films [25–28]. Despite enormous number of articles related to aluminum anodization in various electrolytes, there is a lack of data of the structural and luminescence properties of porous oxide films formed by aluminum anodization in sulfamic acid solution. In this paper, we have investigated the structural features of porous oxide film obtained by two-step aluminum anodization in sulfamic acid solution, in order to examine possibility for fabrication of wellordered nanoporous alumina. In addition, we have presented the results of galvanoluminescence and photoluminescence measurements. 2. Experimental In the experiment, anodic oxide films were formed on high purity cold-rolled aluminum (99.999% Goodfellow). The anodic oxidation process was carried out in an electrolytic cell with flat
Fig. 1. AFM images of porous oxide films formed in 0.3 M sulfamic acid by two-step anodization procedure at 1 8C at (a) 15 V, (b) 20 V, (c) 25 V, and (d) 30 V.
S. Stojadinovic et al. / Applied Surface Science 255 (2008) 2845–2850
glass windows. Platinum wires were used as cathodes. During anodization, the electrolyte circulated through the chamber– reservoir system, and the control temperature sensor was situated immediately by the sample. The temperature of the electrolyte was maintained during anodization to within 0.1 8C. The electrolyte was prepared by use of double distilled, deionized water and PA grade sulfamic acid (J.T. Baker). Anodizing was carried out at different current densities in the range from 10 mA/cm2 to 20 mA/ cm2 (galvanostatic regime), different voltages in the range from 15 V to 30 V (potentiostatic regime) and different temperatures of electrolyte in the range from 1 8C to 30 8C. The aluminum samples were annealed for 5 h at various temperatures (350 8C and 450 8C) to remove mechanical stresses and recrystallize the aluminum. The surface of aluminum was prepared for anodization in three ways: (a) electropolished in a mixed solution of perchloric acid and ethanol (1:4 in volume) under a constant voltage of 18 V for 1 min, rinsed with ethanol and dried; (b) chemically cleaned in the bath consisting of 20 g/l chromium trioxide and 35 ml/l concentrated phosphoric acid at 80 8C for 5 min followed by rinsing in distilled water and dried; (c) just degreased in ethanol by using ultrasonic cleaner. The morphology of the aluminum and anodized aluminum samples were characterized using an atomic force microscope (AFM; Veeco Instruments, model Multimode V). PL spectral measurements were taken on a Horiba Jobin Yvon Fluorolog FL3-22 spectrofluorometer with a Xe lamp as the excitation light source at room temperature. Our preliminary experiments showed that very weak GL appears during aluminum anodization in sulfamic acid. Low GL intensity at a level very near to noise requires sensitive experimental equipment and noise suppression measurement techniques. GL measurements were performed utilizing a monochromatic measuring systems consisting of a large-aperture achromatic lens, an optical monochromator of a rather high luminosity (Zeiss SPM-2) and a very sensitive cooled (at approximately 40 8C) photomultiplier (RCA J1034 CA) [14]. 3. Results and discussion 3.1. AFM characterization of porous oxide films Fig. 1 shows AFM images of the top surface morphology of porous oxide films, where the films were anodized in 0.3 M sulfamic acid solution by the two-step anodization at the temperature of 1 8C under potentials of (a) 15 V, (b) 20 V, (c) 25 V, and (d) 30 V. Stable anodization in sulfamic acid solution was usually difficult to maintain over 30 V due to the occurrence of pitting on the surface. Before anodization, the aluminum surface was electropolished, which is considered as main factor for the preparation of ordered pores [9]. The structural analyses using AFM were performed in tapping mode in air and the scan rate was kept at 1 Hz. The circular pores are distributed on the entire surface, and their size depends on the voltage of anodization and is higher for higher voltage. Porous oxide films formed in sulfamic acid solution by the two-step anodization do not exhibit ideally hexagonal configuration, uniform pore size and highly ordered pore arrangement over a large area, as they do when anodized in some other electrolytes (oxalic, sulfuric and phosphoric acid). For that reason these porous structures are not a promising candidate as templates in nanotechology.
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Fig. 2. Voltage–time and GL intensity–time dependence during galvanostatic anodization of aluminum in 0.1 M sulfamic acid ( j = 20 mA/cm2, tel = 21 8C, l = 570 nm).
solution. In the early stage of anodization (formation of barrier part of the film), GL intensity is much higher then in steady state regime (bulk porous film formation), where GL almost disappears. We have already seen similar GL properties during aluminum anodization in phosphoric, sulfuric and chromic acid [15–18], so we can conclude that they can be ascribed to the nature of aluminum samples, e.g., internal and surface impurities in samples. Further results should confirm this presumption. The effect of surface pretreatment on GL intensity is shown in Fig. 3. Only degreased samples give the highest GL intensity, chemical cleaning remarkably reduces intensity, while after electropolishing GL is almost immeasurable. We can explain this result by correlation between concentrations of ‘‘flaws’’ as sources of GL and the intensity of luminescence (‘‘flaws’’ is general term for microfissures, cracks, local regions of different compositions and impurities, etc. [29]). Fig. 4 shows aluminum surface after various pretreatments. Degreasing gave a roughness higher than 0.1 mm caused mainly by the rolling process of aluminum sheets (Fig. 4a). Chemical cleaning in a boiling mixture of phosphoric and chromic acids removes various kinds of surface contaminants, while electropolishing gave the flattest surface (Fig. 4b and c). Another pretreatment factor that affects GL intensity is annealing temperature of a sample. Fig. 5 shows that higher annealing temperature results in higher GL intensity. Annealing at different temperatures has different influence on the state of sample’s surface, number of defects, crystal grains and their orientation, in other words concentration of ‘‘flaws’’.
3.2. GL properties of porous oxide films Fig. 2 shows typical voltage vs. time and GL intensity vs. time characteristics during anodization of aluminum in sulfamic acid
Fig. 3. Effect of the surface pretreatment of aluminum samples on GL intensity during galvanostatic anodization in 0.1 M sulfamic acid (j = 20 mA/cm2, tel = 21 8C, l = 570 nm).
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Fig. 6. GL spectrum during anodization of aluminum in 0.1 M sulfamic acid (j = 20 mA/cm2, tel = 21 8C).
wavelength. Obtained GL spectrum was corrected for the measuring system’s spectral response. There is a wide GL band in the wavelength from 400 nm to 700 nm with two spectral peaks at about 430 nm and 600 nm. Aluminum oxide has the energy gap between the conduction and valence bands in the oxide of 6.5 eV and therefore there are not any electrons in the conduction band [30]. The electrons, which take part in galvanoluminescence during anodizing, are injected from the electrolyte into the conduction band of anodic alumina. Some of the electrons may fall from conduction band to impurity levels in band gap emitting galvanoluminescence. That is why we have observed strong influence of the surface pretreatment of aluminum on GL intensity. The broad GL peaks can be explained
Fig. 4. AFM images of aluminum surface after various pretreatments: (a) degreasing; (b) chemical cleaning; (c) electropolishing.
GL spectrum of porous oxide films formed in sulfamic acid solution is shown in Fig. 6. GL spectrum was recorded in an early stage of anodization. Only a single datum was taken per sample corresponding to the maximum of GL intensity for a certain
Fig. 5. Effect of the annealing of aluminum samples on GL intensity during galvanostatic anodization of aluminum in 0.1 M sulfamic acid (j = 20 mA/cm2, tel = 21 8C, l = 570 nm).
Fig. 7. Effect of anodization conditions on GL intensity: (a) influence of current density (0.1 M sulfamic acid, t = 21 8C, l = 570 nm); (b) influence of temperature of electrolyte (0.1 M sulfamic acid, j = 20 mA/cm2, l = 570 nm).
S. Stojadinovic et al. / Applied Surface Science 255 (2008) 2845–2850
by considering scattering of the impurity levels in energy. According to Shimizu and Tajima [31] ‘‘flaws’’ are the main generators of luminescence during aluminum anodization in inorganic electrolytes. These impurities act as preferable places for radiative recombination of electrons injected into conducting band of oxide and multiplied in avalanching process. We have already proved [32] that the concentration of flaws as well as GL intensity depend on aluminum sample’s pretreatment and that the ‘‘flaws’’ responsible for the GL in the inorganic electrolytes are various surface defects such as: impurities, crystalline islands of gamma alumina remained on the surface even after the surface pretreatment of the sample, etc. During aluminum anodization, porous oxide grew at the oxide/electrolyte interface because of the outward migration of aluminum ions and their reaction with oxygen-containing electrolytes species at aluminum/oxide. At the same time, the oxide at the oxide/ electrolyte interface was dissolved due to field-stimulated interaction of electrolyte species with the oxide [33]. In porous oxide films, the light was emitted from the barrier part of porous films and the GL intensity increased with anodization voltage (Fig. 2). When all impurities completely moved from the barrier to porous part of oxide films, GL disappears. Two GL spectral peaks obtained in sulfamic acid are observed in other inorganic electrolytes, which form barrier and porous oxide films [15– 17,34]. This fact points to the same origin of GL in all inorganic electrolytes. The results of the investigation of the influence of anodizing conditions on the GL intensity are showed in Fig. 7. GL is more intense for higher current density (Fig. 7a) and lower temperature of electrolyte (Fig. 7b). According to van Geel et al. [35] GL intensity is proportional to current density for constant thickness (determined by anodization voltage). This seems understandable
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Fig. 9. PL emission spectra of porous oxide films with normalized intensities formed in sulfuric acid, sulfamic acid and malonic acid, taken under excitation by the 325 nm line of Xe lamp.
because higher anodization current (ionic + electronic) means higher electronic current and higher number of electrons in conductive zone accelerated in strong electric field (107 V/cm) inside oxide film which results in higher GL intensity. 3.3. PL properties of porous oxide films Fig. 8 shows emission and corresponding excitation spectra of porous oxide films formed by aluminum anodization in sulfamic acid solution. There are wide PL bands present in the range from 300 nm to 600 nm. The PL intensity and peak positions of emission and excitation spectra change with excitation and emission wavelength, respectively. PL spectra of porous oxide formed in sulfamic acid have the same shape as PL spectra of porous oxide films in sulfuric acid [36], and differ from PL spectra of porous oxide films formed in organic electrolytes (oxalic and malonic acid) [24]. Fig. 9 shows PL spectra of porous oxide films formed in malonic acid, sulfuric acid and sulfamic acid, taken under excitation by the 325 nm line of Xe lamp. The PL intensity of porous oxide film formed in oxalic acid is much higher than that of porous oxide films formed in sulfuric acid and sulfamic acid [36]. The same shape of PL spectra of oxide films formed in sulfuric acid and sulfamic acid indicates the same types of luminescence centers in these electrolytes. There are two peaks for both emission and excitation PL spectra of oxide films formed in sulfamic acid. Huang et al. identified two types of PL bands [36]. One is the a-band with the emission center at about 460 nm, which is connected with oxygen vacancies in porous oxide films. The other is b-band and attributed to radiative recombination of carriers in the isolated hydroxyl groups at the surface of the pore wall, whereas the photogeneration of carriers takes place in oxygen vacancies in the pore wall. 4. Conclusion
Fig. 8. (a) PL emission spectra of porous oxide films formed in sulfamic acid; (b) PL excitation spectra of porous oxide films formed in sulfamic acid.
For the first time we have showed that the anodization of aluminum in sulfamic acid is followed by weak luminescence from thin oxide film. This GL, mainly in visible spectral range, is measurable in the first stage of anodization, during barrier layer formation, but in steady state GL becomes almost immeasurable. Properties of GL measured in sulfamic acid are strongly influenced by the sample’s pretreatment. Namely, GL intensity is highest for degreased samples, lower for chemically cleaned samples and almost disappears for electropolished samples. This result confirms our presumption that ‘‘flaws’’ (generated by surface and internal impurities) are responsible for GL. Also, GL intensity
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depends of the anodic conditions and is higher for higher current density and lower temperature of electrolyte. Porous oxide films formed in sulfamic acid show a wide PL bands in the range from 300 nm to 600 nm. The PL intensity and peak positions of emission and excitation spectra change with excitation and emission wavelength, respectively. The same shape of PL spectra porous oxide films formed in sulfamic acid and sulfuric acid indicates the same type of PL centers in these electrolytes. AFM analysis of porous oxide films formed in sulfamic acid has showed that two-step anodization procedure at a constant voltage of 15–30 V does not result in a regular porous alumina structure. Acknowledgements The authors would like to express their appreciation to Serbian Ministry of Science for financial support of the project 141017. References [1] J.S. Suh, J.S. Lee, Appl. Phys. Lett. 75 (1999) 2047. [2] X.S. Peng, J. Zhang, X.F. Wang, Y.W. Wang, L.X. Zhao, G.W. Meng, L.D. Zhang, Chem. Phys. Lett. 343 (2001) 470. [3] R. Karmhag, T. Tesfamichael, G. Niklasson, M. Nygren, Solar Energy 68 (2000) 329. [4] M. Saito, M. Kirihara, T. Taniguchi, M. Miyagi, Appl. Phys. Lett. 55 (1989) 607. [5] B. Wang, G.T. Fei, M. Wang, M.G. Kong, L.D. Zhang, Nanotechnology 18 (2007) 365601. [6] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [7] H. Masuda, F. Hasegawa, S. Ono, J. Electrochem. Soc. 144 (1998) L127. [8] O. Jessennsky, F. Muller, U. Gosele, Appl. Phys. Lett. 72 (1998) 1173. [9] P. Bocchetta, C. Sunseri, R. Masi, S. Piazza, F. Di Quarto, Mater. Sci. Eng. C 23 (2003) 1021. [10] P. Bocchetta, C. Sunseri, G. Chiavarotti, F. Di Quarto, Electrochim. Acta 48 (2003) 3175.
[11] M.A. Kashi, A. Ramazani, J. Phys. D: Appl. Phys. 38 (2005) 2396. [12] L. Ba, W.S. Li, J. Phys. D: Appl. Phys. 33 (2000) 2527. [13] I. Belca, B. Kasalica, Lj. Zekovic, B. Jovanic, R. Vasilic, Electrochim. Acta 45 (1999) 993. [14] S. Stojadinovic, I. Belca, M. Tadic, B. Kasalica, Z. Nedic, Lj. Zekovic, J. Electroanal. Chem. 619/620 (2008) 125. [15] S. Stojadinovic, Lj. Zekovic, I. Belca, B. Kasalica, Electrochem. Commun. 6 (2004) 427. [16] S. Stojadinovic, Lj. Zekovic, I. Belca, B. Kasalica, D. Nikolic, Electrochem. Commun. 6 (2004) 708. [17] S. Stojadinovic, I. Belca, Lj. Zekovic, B. Kasalica, D. Nikolic, Electrochem. Commun. 6 (2004) 1016. [18] B. Kasalica, S. Stojadinovic, Lj. Zekovic, I. Belca, D. Nikolic, Electrochem. Commun. 7 (2005) 735. [19] Lj.D. Zekovic, V.V. Urosevic, Thin Solid Films 78 (1981) 279. [20] Lj.D. Zekovic, V.V. Urosevic, Thin Solid Films 86 (1981) 347. [21] Lj.D. Zekovic, V.V. Urosevic, B.R. Jovanic, Appl. Surf. Sci. 11/12 (1982) 90. [22] Z. Li, K. Huang, J. Phys.: Condens. Matter 19 (2007), 2126203. [23] Y. Li, G.H. Li, G.W. Meng, L.D. Zhang, F. Phillipp, J. Phys.: Condens. Matter 13 (2001) 2691. [24] G.H. Li, Y. Zhang, Y.C. Wu, L.D. Zhang, J. Phys.: Condens. Matter 15 (2003) 8663. [25] Lj.D. Zekovic, V.V. Urosevic, B.R. Jovanic, Thin Solid Films 105 (1983) 169. [26] Lj.D. Zekovic, V.V. Urosevic, B.R. Jovanic, Thin Solid Films 109 (1983) 217. [27] Lj.D. Zekovic, V.V. Urosevic, B.R. Jovanic, Thin Solid Films 139 (1986) 109. [28] Lj.D. Zekovic, B.R. Jovanic, Lj. Ristovski, G. Davidovic-Ristovski, V.V. Urosevic, Thin Solid Films 157 (1988) 59. [29] K. Shimizu, G.E. Thompson, G.C. Wood, Electrochim. Acta 27 (1982) 245. [30] Y. Li, H. Shimada, M. Sakairi, K. Shigyo, H. Takahashi, M. Seo, J. Electrochem. Soc. 144 (1997) 866. [31] K. Shimizu, S. Tajima, Electrochim. Acta 25 (1980) 259. [32] B. Kasalica, I. Belca, S. Stojadinovic, M. Sarvan, M. Peric, Lj. Zekovic, J. Phys. Chem. C 111 (2007) 12315. [33] V.P. Parkhutik, V.I. Shershulsky, J. Phys. D: Appl. Phys. 25 (1992) 1258. [34] S. Stojadinovic, I. Belca, B. Kasalica, Lj. Zekovic, M. Tadic, Electrochem. Commun. 8 (2006) 1621. [35] W.Ch. van Geel, C.A. Pistorius, B.C. Bouma, Philips Res. Rep. 12 (1957) 465. [36] G.S. Huang, X.L. Wu, G.G. Siu, P.K. Chu, Solid State Commun. 137 (2006) 621.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural and luminescence characterization of porous anodic oxide films on aluminum formed in sulfamic acid solution S. Stojadinovic a,*, R. Vasilic b, I. Belca a, M. Tadic a, B. Kasalica a, Lj. Zekovic a a b
Faculty of Physics, Belgrade University, Studentski trg 12-16, 11000 Belgrade, Serbia Faculty of Applied Ecology, Singidunum University, Bul. Kralja Aleksandra 79, 11000 Belgrade, Serbia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 July 2008 Received in revised form 11 August 2008 Accepted 11 August 2008 Available online 20 August 2008
Atomic force microscopy (AFM) and luminescence methods (galvanoluminescence and photoluminescence) were used to characterize porous oxide films obtained by aluminum anodization in sulfamic acid solution. For the first time we measured weak galvanoluminescence during aluminum anodization in sulfamic acid and found strong influence of sample’s surface pretreatment as well as anodic conditions on luminescence intensity. AFM analysis showed that the pore arrangement of porous oxide films formed in sulfamic acid by two-step anodization process at a constant voltage of 15–30 V is relatively irregular. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Porous oxide films Galvanoluminescence Photoluminescence Aluminum Sulfamic acid
1. Introduction Thin oxide films on aluminum are inexhaustible source of interest for researchers in various scientific and technological areas: physics, chemistry, electronics, etc. In the last decades the scientists have been focused on the fabrication self-organized alumina nanostructures by the anodization of aluminum in various electrolytes. This is a result of application of porous oxide films, with ordered pores of dimensions ranging from submicrometer to nanometer range, as template in nanotechnology for nanotubes [1], nanowires [2], solar cells [3], micro-optical elements [4], photonic crystals [5], etc. Since the application of two step procedure results in selforganized porous structures on anodized aluminum [6], this technique become promising for preparing such ordered porous structures due to its low cost and relative easy use. The procedure involves two separate anodization processes. The first anodization process consists of a long period of anodization forming the ordered porous structure. After the removal of the oxide, an array of highly ordered dimples is formed on aluminum. These dimples act as initiation sites for growing highly ordered porous structure. Typical electrolytes forming these porous structures are aqueous
* Corresponding author. Tel.: +381 11 2630152; fax: +381 11 3282619. E-mail address: [email protected] (S. Stojadinovic). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.08.023
solutions of oxalic, sulfuric and phosphoric acid. Dimensions and regularity of pores are determined by a type of electrolyte [7,8], quality of aluminum wafer [9,10] and anodic conditions (temperature and composition of electrolyte, voltage of anodization) [11,12]. The study of the luminescence of porous oxide films used as templates for synthesizing new luminescence materials is very important, because their luminescence properties certainly affect the luminescence properties of materials based on porous oxide films. In recent years, much attention has been paid to the various galvanoluminescence (GL) and photoluminescence (PL) methods, which enable in situ characterization of porous oxide films formed in various electrolytes. Galvanoluminescence or electroluminescence is a common name for light appearing at one of the electrodes in an electrolytic solution during anodization. During the past several years, the authors conducted a number of investigations of the GL characteristics of porous oxide films obtained in organic electrolytes (oxalic acid and malonic acid) [13,14] and inorganic electrolytes (phosphoric acid, chromic acid and sulfuric acid) [15–18]. We have showed that the nature and intensity of GL depend on many factors such as type of electrolytes (organic or inorganic), surface pretreatment and anodizing conditions. Surface pretreatment of aluminum samples (surface preparation and annealing) has a significant influence on GL obtained in inorganic electrolytes. Consequently, GL intensity might be an indicator of
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aluminum surface quality and roughness. Furthermore, GL methods can be used for the determination of the oxide film thickness, growth rate, refraction index, optical constants of aluminium, etc. [19–21]. In organic electrolytes GL is excited by collision of electrons, injected into the oxide film at the electrolyte–oxide interface and accelerated by high electric field (nearly 107 V/cm), with luminescence centers (carboxylate ions) inside the oxide film. In porous oxide films, the light is emitted from the barrier part of porous films, i.e. at the bottom of the pores, in the high field region. If an anodic oxide film formed on aluminum is illuminated with UV radiation, the emission of visible light takes place. This phenomenon is termed photoluminescence. Many authors have investigated PL, but explanations of its nature and mechanism are still incomplete. The intensity of emitted PL radiation and the shapes of PL excitation and emission spectra depend on many factors: the nature of the electrolytes (organic or inorganic) and the conditions of anodizing process [22], the thickness of the oxide films and its additional treatment (electrolytic or thermal) [23], the
wavelength of the incident radiation [24], etc. PL spectral measurement can be used for determination of the refractive index, thickness and porosity of porous oxide films [25–28]. Despite enormous number of articles related to aluminum anodization in various electrolytes, there is a lack of data of the structural and luminescence properties of porous oxide films formed by aluminum anodization in sulfamic acid solution. In this paper, we have investigated the structural features of porous oxide film obtained by two-step aluminum anodization in sulfamic acid solution, in order to examine possibility for fabrication of wellordered nanoporous alumina. In addition, we have presented the results of galvanoluminescence and photoluminescence measurements. 2. Experimental In the experiment, anodic oxide films were formed on high purity cold-rolled aluminum (99.999% Goodfellow). The anodic oxidation process was carried out in an electrolytic cell with flat
Fig. 1. AFM images of porous oxide films formed in 0.3 M sulfamic acid by two-step anodization procedure at 1 8C at (a) 15 V, (b) 20 V, (c) 25 V, and (d) 30 V.
S. Stojadinovic et al. / Applied Surface Science 255 (2008) 2845–2850
glass windows. Platinum wires were used as cathodes. During anodization, the electrolyte circulated through the chamber– reservoir system, and the control temperature sensor was situated immediately by the sample. The temperature of the electrolyte was maintained during anodization to within 0.1 8C. The electrolyte was prepared by use of double distilled, deionized water and PA grade sulfamic acid (J.T. Baker). Anodizing was carried out at different current densities in the range from 10 mA/cm2 to 20 mA/ cm2 (galvanostatic regime), different voltages in the range from 15 V to 30 V (potentiostatic regime) and different temperatures of electrolyte in the range from 1 8C to 30 8C. The aluminum samples were annealed for 5 h at various temperatures (350 8C and 450 8C) to remove mechanical stresses and recrystallize the aluminum. The surface of aluminum was prepared for anodization in three ways: (a) electropolished in a mixed solution of perchloric acid and ethanol (1:4 in volume) under a constant voltage of 18 V for 1 min, rinsed with ethanol and dried; (b) chemically cleaned in the bath consisting of 20 g/l chromium trioxide and 35 ml/l concentrated phosphoric acid at 80 8C for 5 min followed by rinsing in distilled water and dried; (c) just degreased in ethanol by using ultrasonic cleaner. The morphology of the aluminum and anodized aluminum samples were characterized using an atomic force microscope (AFM; Veeco Instruments, model Multimode V). PL spectral measurements were taken on a Horiba Jobin Yvon Fluorolog FL3-22 spectrofluorometer with a Xe lamp as the excitation light source at room temperature. Our preliminary experiments showed that very weak GL appears during aluminum anodization in sulfamic acid. Low GL intensity at a level very near to noise requires sensitive experimental equipment and noise suppression measurement techniques. GL measurements were performed utilizing a monochromatic measuring systems consisting of a large-aperture achromatic lens, an optical monochromator of a rather high luminosity (Zeiss SPM-2) and a very sensitive cooled (at approximately 40 8C) photomultiplier (RCA J1034 CA) [14]. 3. Results and discussion 3.1. AFM characterization of porous oxide films Fig. 1 shows AFM images of the top surface morphology of porous oxide films, where the films were anodized in 0.3 M sulfamic acid solution by the two-step anodization at the temperature of 1 8C under potentials of (a) 15 V, (b) 20 V, (c) 25 V, and (d) 30 V. Stable anodization in sulfamic acid solution was usually difficult to maintain over 30 V due to the occurrence of pitting on the surface. Before anodization, the aluminum surface was electropolished, which is considered as main factor for the preparation of ordered pores [9]. The structural analyses using AFM were performed in tapping mode in air and the scan rate was kept at 1 Hz. The circular pores are distributed on the entire surface, and their size depends on the voltage of anodization and is higher for higher voltage. Porous oxide films formed in sulfamic acid solution by the two-step anodization do not exhibit ideally hexagonal configuration, uniform pore size and highly ordered pore arrangement over a large area, as they do when anodized in some other electrolytes (oxalic, sulfuric and phosphoric acid). For that reason these porous structures are not a promising candidate as templates in nanotechology.
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Fig. 2. Voltage–time and GL intensity–time dependence during galvanostatic anodization of aluminum in 0.1 M sulfamic acid ( j = 20 mA/cm2, tel = 21 8C, l = 570 nm).
solution. In the early stage of anodization (formation of barrier part of the film), GL intensity is much higher then in steady state regime (bulk porous film formation), where GL almost disappears. We have already seen similar GL properties during aluminum anodization in phosphoric, sulfuric and chromic acid [15–18], so we can conclude that they can be ascribed to the nature of aluminum samples, e.g., internal and surface impurities in samples. Further results should confirm this presumption. The effect of surface pretreatment on GL intensity is shown in Fig. 3. Only degreased samples give the highest GL intensity, chemical cleaning remarkably reduces intensity, while after electropolishing GL is almost immeasurable. We can explain this result by correlation between concentrations of ‘‘flaws’’ as sources of GL and the intensity of luminescence (‘‘flaws’’ is general term for microfissures, cracks, local regions of different compositions and impurities, etc. [29]). Fig. 4 shows aluminum surface after various pretreatments. Degreasing gave a roughness higher than 0.1 mm caused mainly by the rolling process of aluminum sheets (Fig. 4a). Chemical cleaning in a boiling mixture of phosphoric and chromic acids removes various kinds of surface contaminants, while electropolishing gave the flattest surface (Fig. 4b and c). Another pretreatment factor that affects GL intensity is annealing temperature of a sample. Fig. 5 shows that higher annealing temperature results in higher GL intensity. Annealing at different temperatures has different influence on the state of sample’s surface, number of defects, crystal grains and their orientation, in other words concentration of ‘‘flaws’’.
3.2. GL properties of porous oxide films Fig. 2 shows typical voltage vs. time and GL intensity vs. time characteristics during anodization of aluminum in sulfamic acid
Fig. 3. Effect of the surface pretreatment of aluminum samples on GL intensity during galvanostatic anodization in 0.1 M sulfamic acid (j = 20 mA/cm2, tel = 21 8C, l = 570 nm).
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Fig. 6. GL spectrum during anodization of aluminum in 0.1 M sulfamic acid (j = 20 mA/cm2, tel = 21 8C).
wavelength. Obtained GL spectrum was corrected for the measuring system’s spectral response. There is a wide GL band in the wavelength from 400 nm to 700 nm with two spectral peaks at about 430 nm and 600 nm. Aluminum oxide has the energy gap between the conduction and valence bands in the oxide of 6.5 eV and therefore there are not any electrons in the conduction band [30]. The electrons, which take part in galvanoluminescence during anodizing, are injected from the electrolyte into the conduction band of anodic alumina. Some of the electrons may fall from conduction band to impurity levels in band gap emitting galvanoluminescence. That is why we have observed strong influence of the surface pretreatment of aluminum on GL intensity. The broad GL peaks can be explained
Fig. 4. AFM images of aluminum surface after various pretreatments: (a) degreasing; (b) chemical cleaning; (c) electropolishing.
GL spectrum of porous oxide films formed in sulfamic acid solution is shown in Fig. 6. GL spectrum was recorded in an early stage of anodization. Only a single datum was taken per sample corresponding to the maximum of GL intensity for a certain
Fig. 5. Effect of the annealing of aluminum samples on GL intensity during galvanostatic anodization of aluminum in 0.1 M sulfamic acid (j = 20 mA/cm2, tel = 21 8C, l = 570 nm).
Fig. 7. Effect of anodization conditions on GL intensity: (a) influence of current density (0.1 M sulfamic acid, t = 21 8C, l = 570 nm); (b) influence of temperature of electrolyte (0.1 M sulfamic acid, j = 20 mA/cm2, l = 570 nm).
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by considering scattering of the impurity levels in energy. According to Shimizu and Tajima [31] ‘‘flaws’’ are the main generators of luminescence during aluminum anodization in inorganic electrolytes. These impurities act as preferable places for radiative recombination of electrons injected into conducting band of oxide and multiplied in avalanching process. We have already proved [32] that the concentration of flaws as well as GL intensity depend on aluminum sample’s pretreatment and that the ‘‘flaws’’ responsible for the GL in the inorganic electrolytes are various surface defects such as: impurities, crystalline islands of gamma alumina remained on the surface even after the surface pretreatment of the sample, etc. During aluminum anodization, porous oxide grew at the oxide/electrolyte interface because of the outward migration of aluminum ions and their reaction with oxygen-containing electrolytes species at aluminum/oxide. At the same time, the oxide at the oxide/ electrolyte interface was dissolved due to field-stimulated interaction of electrolyte species with the oxide [33]. In porous oxide films, the light was emitted from the barrier part of porous films and the GL intensity increased with anodization voltage (Fig. 2). When all impurities completely moved from the barrier to porous part of oxide films, GL disappears. Two GL spectral peaks obtained in sulfamic acid are observed in other inorganic electrolytes, which form barrier and porous oxide films [15– 17,34]. This fact points to the same origin of GL in all inorganic electrolytes. The results of the investigation of the influence of anodizing conditions on the GL intensity are showed in Fig. 7. GL is more intense for higher current density (Fig. 7a) and lower temperature of electrolyte (Fig. 7b). According to van Geel et al. [35] GL intensity is proportional to current density for constant thickness (determined by anodization voltage). This seems understandable
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Fig. 9. PL emission spectra of porous oxide films with normalized intensities formed in sulfuric acid, sulfamic acid and malonic acid, taken under excitation by the 325 nm line of Xe lamp.
because higher anodization current (ionic + electronic) means higher electronic current and higher number of electrons in conductive zone accelerated in strong electric field (107 V/cm) inside oxide film which results in higher GL intensity. 3.3. PL properties of porous oxide films Fig. 8 shows emission and corresponding excitation spectra of porous oxide films formed by aluminum anodization in sulfamic acid solution. There are wide PL bands present in the range from 300 nm to 600 nm. The PL intensity and peak positions of emission and excitation spectra change with excitation and emission wavelength, respectively. PL spectra of porous oxide formed in sulfamic acid have the same shape as PL spectra of porous oxide films in sulfuric acid [36], and differ from PL spectra of porous oxide films formed in organic electrolytes (oxalic and malonic acid) [24]. Fig. 9 shows PL spectra of porous oxide films formed in malonic acid, sulfuric acid and sulfamic acid, taken under excitation by the 325 nm line of Xe lamp. The PL intensity of porous oxide film formed in oxalic acid is much higher than that of porous oxide films formed in sulfuric acid and sulfamic acid [36]. The same shape of PL spectra of oxide films formed in sulfuric acid and sulfamic acid indicates the same types of luminescence centers in these electrolytes. There are two peaks for both emission and excitation PL spectra of oxide films formed in sulfamic acid. Huang et al. identified two types of PL bands [36]. One is the a-band with the emission center at about 460 nm, which is connected with oxygen vacancies in porous oxide films. The other is b-band and attributed to radiative recombination of carriers in the isolated hydroxyl groups at the surface of the pore wall, whereas the photogeneration of carriers takes place in oxygen vacancies in the pore wall. 4. Conclusion
Fig. 8. (a) PL emission spectra of porous oxide films formed in sulfamic acid; (b) PL excitation spectra of porous oxide films formed in sulfamic acid.
For the first time we have showed that the anodization of aluminum in sulfamic acid is followed by weak luminescence from thin oxide film. This GL, mainly in visible spectral range, is measurable in the first stage of anodization, during barrier layer formation, but in steady state GL becomes almost immeasurable. Properties of GL measured in sulfamic acid are strongly influenced by the sample’s pretreatment. Namely, GL intensity is highest for degreased samples, lower for chemically cleaned samples and almost disappears for electropolished samples. This result confirms our presumption that ‘‘flaws’’ (generated by surface and internal impurities) are responsible for GL. Also, GL intensity
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depends of the anodic conditions and is higher for higher current density and lower temperature of electrolyte. Porous oxide films formed in sulfamic acid show a wide PL bands in the range from 300 nm to 600 nm. The PL intensity and peak positions of emission and excitation spectra change with excitation and emission wavelength, respectively. The same shape of PL spectra porous oxide films formed in sulfamic acid and sulfuric acid indicates the same type of PL centers in these electrolytes. AFM analysis of porous oxide films formed in sulfamic acid has showed that two-step anodization procedure at a constant voltage of 15–30 V does not result in a regular porous alumina structure. Acknowledgements The authors would like to express their appreciation to Serbian Ministry of Science for financial support of the project 141017. References [1] J.S. Suh, J.S. Lee, Appl. Phys. Lett. 75 (1999) 2047. [2] X.S. Peng, J. Zhang, X.F. Wang, Y.W. Wang, L.X. Zhao, G.W. Meng, L.D. Zhang, Chem. Phys. Lett. 343 (2001) 470. [3] R. Karmhag, T. Tesfamichael, G. Niklasson, M. Nygren, Solar Energy 68 (2000) 329. [4] M. Saito, M. Kirihara, T. Taniguchi, M. Miyagi, Appl. Phys. Lett. 55 (1989) 607. [5] B. Wang, G.T. Fei, M. Wang, M.G. Kong, L.D. Zhang, Nanotechnology 18 (2007) 365601. [6] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [7] H. Masuda, F. Hasegawa, S. Ono, J. Electrochem. Soc. 144 (1998) L127. [8] O. Jessennsky, F. Muller, U. Gosele, Appl. Phys. Lett. 72 (1998) 1173. [9] P. Bocchetta, C. Sunseri, R. Masi, S. Piazza, F. Di Quarto, Mater. Sci. Eng. C 23 (2003) 1021. [10] P. Bocchetta, C. Sunseri, G. Chiavarotti, F. Di Quarto, Electrochim. Acta 48 (2003) 3175.
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