Applied Surface Science 258 (2012) 4995–5000 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Effect of annealing on photoluminescence and optical properties of porous anodic alumina ﬁlms formed in sulfuric acid for solar energy applications Mondher Ghrib a,∗ , Rachid Ouertani a , Monir Gaidi a , Najoua Khedher a , Mohamed Ben Salem b , Hatem Ezzaouia a a Photovoltaic Laboratory Research and Technology Centre of Energy, Borj-Cedria Science and Technology Park, BP 95, 2050 Hammam-Lif, Tunisia b L3M, Department of Physics, Faculty of Sciences of Bizerte, 7021 Zarzouna, Tunisia a r t i c l e i n f o Article history: Received 10 March 2011 Received in revised form 9 December 2011 Accepted 13 December 2011 Available online 5 January 2012 Keywords: Porous oxide ﬁlms Annealing Photoluminescence Ellipsometry Reﬂectivity a b s t r a c t Photoluminescence and optical properties of porous oxide ﬁlms formed by two-step aluminum anodiza- tion at a ﬁxed current 200 mA have been investigated. It was found that the crystallographic structure depend strongly on the annealing temperature. X-ray diffraction (XRD) reveals an amorphisation of the porous oxide ﬁlms after annealing. This evolution has been conﬁrmed by Raman spectroscopy measure- ment. Spectroscopic ellipsometry (SE) in the UV–vis and near infra red (IR) spectra shows that refraction index n increases and the extinction coefﬁcient k decreases with annealing temperature. This observa- tion has been conﬁrmed with reﬂectivity measurements. As a consequence the reﬂectivity reaches 97% when porous alumina ﬁlms were annealed at 650 ◦ C. Photoluminescence (PL) measurements show two PL peaks in the emission and excitation spectra. The ﬁrst emission peak is centered at 460 nm (-band) and the second (-band) shifts from 500 to 525 nm, depending on excitation wavelength. For excitation spectra, one spectral peak is located at 271 nm and the second shifts to longer wavelengths with increas- ing emission wavelength. The results indicate the existence of two PL centers. One is associated with oxygen adsorption at the pore wall and oxygen vacancies inside the alumina. The other is related to the adsorption of water and/or OH groups at the surface of the pore wall and to structure defects and sulfur inclusion inside the ﬁlms. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Porous oxide ﬁlms obtained by aluminum anodization in various electrolytes exhibit many interesting properties and have enormous technological and industrial implication [1]. In recent years, scientiﬁc interest has been focused on anodic alumina membranes with highly ordered nanochannel arrays formed in oxalic, sulfuric and phosphoric acid [2]. Because of the regularity of cell–pore structure these ﬁlms are widely used as templates for nanosize structures, such as electronic, optoelectronic and magnetic devices [3–8]. Kennedy et al. [9] have made some advances on aluminum mir- rors protected with Al 2 O 3 using various substrates. The approach of this work is to elaborate new reﬂector materials based on porous alumina that shows high reﬂectance compared to that exhibited by conventional aluminum foil. ∗ Corresponding author. E-mail addresses: mondherghrib@yahoo.fr (M. Ghrib), rachid.ouertani@crten.rnrt.tn (R. Ouertani). Most investigators have focused their attention to photolumi- nescence (PL) of porous oxide ﬁlms formed in oxalic acid [10–20]. The number of articles studying the photoluminescence properties of porous aluminum oxide ﬁlms has increased. Little has been done on the PL of porous oxide ﬁlms formed in sulfuric and phosphoric acid [10,13,20]. The intensity of emitted PL radiation and the shapes of PL excitation and emission spectra depend on many factors such as the wavelength of the incident radiation [13], the nature of the electrolytes (organic or inorganic), the conditions of anodizing process [21,22], the thickness of the oxide ﬁlms and the thermal treatment conditions [10]. Many models have been presented to describe the PL mechanisms and the nature of PL centers of the porous anodic oxide ﬁlms on aluminum [33]. Two main opinions are presented to explain the nature of PL centers of porous oxide ﬁlms formed in organic electrolytes. The ﬁrst one makes clear that PL centers are related to oxygen vacancies [7]. The second opinion suggests that the PL could be attributed to oxalic impurities incorporated in oxide ﬁlms during the anodization [6]. The origin of the PL from porous anodic ﬁlms formed in inorganic electrolytes (sulfuric acid and phosphoric acid) is still unclear. In the last case the PL emission was simply considered to have the same origin as in ﬁlms formed in oxalic acid [9]. Huang et al. identiﬁed two types of 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.12.056