The effects of electrolytes and pore widening on the photoluminescence properties of porous alumina membranes

The PL spectra of the porous alumina membranes formed in 10 wt%

sulfuric acid solution, 0.3 M oxalic acid solution, and 0.1 M phosphoric acid solution are shown in Fig. 4-10. It can be seen that under an excitation of the 365 nm using a Xe lamp, the membrane formed in oxalic acid displays a strong blue PL peak at 452 nm, while those formed in sulfuric acid solution, and phosphoric acid solution only have a weak emission around 443 nm. The PL properties of porous alumina membranes prepared in sulfuric acid solution and phosphoric acid solution are very similar to that of dense alumina.

Therefore, we believe that the sulfuric and phosphoric impurities have less contribution, even no help to the emission band. These results also indicate that the electrolyte has a large influence on the light-emitting property of porous alumina membranes.

400 500 600 700

Fig. 4-10 PL spectra of the aluminum sheet and porous alumina membranes prepared in different acid solution, 1: annealed aluminum; 2: 10 wt% sulfuric acid; 3:

400 500 600 700

0.3 M oxalic acid; 4: 0.1 M phosphoric acid. The insert is an enlargement of the curve 4.

400 500 600 700

Intensity (a.u.)

Wavelength (nm)

1 2 3 4

Fig. 4-11 PL spectra of porous alumina membranes prepared in oxalic acid solution, 1: as prepared sample; 2: pore widening 30 minutes; 3: pore widening 60 minutes; 4: pore widening 120 minutes.

The effect of pore widening by chemical etching is investigated. Figure 4-11 reveals that as the etching time of membranes immersed in phosphoric acid solution increases, the PL intensity shows a tendency to decrease.

Figures 4-12 (a)-(d) show that the surface morphologies of porous alumina membranes with the different level of etching in phosphoric acid solution.

Based on the model for the porous alumina structure researched by Ono et al.

[78], the pore wall of the porous alumina membranes consisted of an inner oxide layer composed of pure alumina and an outer oxide layer. The outer oxide could also divide into two parts, an outermost oxide close to the

[79] suggested a distribution of anions in the duplex oxide layers of the porous alumina wall when the alumina was anodized in phosphoric acid solution. Anions and anion incorporated compounds were gathered in the intermediate part of the outer oxide. In this work, the intensity of the emission band decreases slightly when the etching time reaches thirty minutes. The SEM image (Fig. 4-12 (b)) shows the outermost oxide of alumina wall is removed at this moment. As the etching time increases to an hour, the blue shift of PL emission appears obviously from 455 nm to 445 nm, as indicated in Fig. 4-13. In the meanwhile, the intensity of the emission band declines gradually. These phenomena occur because of a large consumption of the most anions or impurities in the intermediate oxide over this period. When the most parts of the porous alumina are removed (Fig. 4-12 (d)), the intensity of the emission band drops abruptly and the peak has a slight blue shift to 443 nm behaved as the dense alumina. From Fig. 4-13, the distance from the pore center to the pore wall increases with the pore widening time. It can be seen that a large blue shift occurs when the intermediate part of alumina is removed. That means the PL contribution of oxalic impurities whose emission peak centered at 470 nm disappear mainly at this region.

According to the results, it can be suggested that only oxalic impurities could be emission centers in the porous alumina membranes. The distribution of oxalic impurities in this work is in agreement with pervious researches and variations in amount of oxalic impurities would have a strong influence on the emission intensity.

Fig. 4-12 SEM images of porous alumina membranes were formed in oxalic acid solution with different etching time: (a) 0 minute, (b) 30 minutes, (c) 60 minutes, and (d) 120 minutes.

0 30 60 90 120

440 445 450 455 460

Distance from the pore center (nm)

Pore widening time (min)

10 20 30 40 50 60

Peak position (nm)

Fig. 4-13 The variations in the PL peak position and the distance from the center of the nanopore at the different pore widening time.

(a) (b)

(c) (d)

4.4 Summary

Porous alumina membranes were produced by adopting a two-step anodization process in sulfuric acid solution, oxalic acid solution, and phosphoric acid solution.

X-ray analyses show that the prepared alumina membranes are amorphous phase. As indicated by the PL emission spectra, the intensity of PL band increases with increasing thickness of alumina membranes determined from SEM figures. The emission wavelength shifts from 443 nm to 443 nm and 470 nm, and finally located around 452 nm as the thickness of porous alumina membranes increases. A part of the explanation for these may lie in the fact that there are two emission centers caused by oxygen vacancies and oxalic impurities in the PL band. According to deconvolution of the PL spectra, both centers contribute greatly to the PL emission band. Finally, the effects of electrolytes and pore widening on PL properties of alumina membranes are also investigated. The alumina membrane prepared only in oxalic acid solution has a strong blue emission band. According to experiments of pore widening, a distribution of oxalic impurities in the porous alumina wall can be found to correspond to the previous researches. This study has taken a step in the direction of defining the relationship between the thickness and the photoluminescence in porous alumina membranes as well as the effects of process conditions.

Chapter 5 Fabrication and properties of PbS