Structural and Fluorescence Studies of Polycrystalline α‐Al₂O₃ Obtained From Sulfuric Acid Anodic Alumina

Abstract

In the present study, robust α‐Al2O3 specimens (144 µm thick) are obtained by the heat treatment at 1400 °C of free‐standing sulfuric acid anodic alumina. Scanning electron microscopy analysis show that the as‐anodized anodic alumina films possess well‐ordered porous structure with the pore diameter of about 10.2 nm. After heat treatment at 1400 °C the samples lose their porous structure and certain crystallites with average size of 5–6 µm can be observed. These crystallites can be visualized by fluorescence imaging. According to differential scanning calorimetry data accomplished by X‐ray analysis, the first step of crystallization occurs at around 967 °C, producing γ‐Al2O3. The second one takes place at around 1194 °C, which corresponds to the formation of α‐Al2O3. For the as‐anodized and samples treated at temperatures below 1200 °C the band at 420 nm can be attributed to the emission of OH‐related and other impurities centers. The fluorescence at 460 nm relates to emission of oxygen defect centers, such as F and F2 centers, and sharp bands at 678 and 693 nm indicate the formation of highly crystalline alumina. For α‐Al2O3, its fluorescence is caused by both surface defects and oxygen defect centers. The fluorescence spectroscopy can be applied as a cheap, fast, nondestructive method for the identification of amorphous alumina crystallization.

Tuning the Photo-Luminescence Properties of WO3 Layers by the Adjustment of Layer Formation Conditions

“…The reasons for different behaviour of sample EtOH-140 and correlation between PL properties, photoelectrochemical performance and morphological features of WO3 films formed under different conditions are discussed in the next section. It is necessary to mention that blue photoluminescence band in the 400-440 nm region also could be attributed to optical transition in the isolated -OH groups formed at the WO 3 coating surface [53,54]. This correlates well with FTIR results, because W-OH stretching and bending vibrations were observed at 3400 cm −1 and 1625 cm −1 wavenumbers, respectively.…”

“…It is necessary to mention that blue photoluminescence band in the 400–440 nm region also could be attributed to optical transition in the isolated -OH groups formed at the WO 3 coating surface [ 53 , 54 ]. This correlates well with FTIR results, because W-OH stretching and bending vibrations were observed at 3400 cm −1 and 1625 cm −1 wavenumbers, respectively.…”

Understanding the thermal conductivity variations in nanoporous anodic aluminum oxide

“…The Raman spectra of the three sets of samples (Fig. 5) conrm both the different phase transitions observed by XRD and the gas evolution detected by TGA-DSC mass spectroscopy, which were comparable to those discussed in previous studies on similar samples, 44,50 and added some further details in the crystallization route followed.…”

Chromaticity coordinate vector principle for charge-transfer-type thermochromic material design: case of Fe/Cr-(co)doped α-Al2O3host

“…The intensities of the luminescent double frequency peaks at 1364.4 cm À1 and 1394.6 cm À1 were reduced and shifted to a lower frequency with an increase in the doping level of Fe 3+ , which may be contributed by the lattice expansion and internal quenching of the Fe3d-O2p hybrid orbitals. 61,63 The Raman peaks of the Fe,Cr-co-doped samples were compared with that of the solely Fe-doped samples with similar band assignments (Fig. 2(b)).…”