Study of long-term changes in visible-light transmittance of porous alumina membranes

The paper presents a study of the optical properties of porous alumina membranes obtained by electrochemical anodizing in electrolytes based on sulfuric and orthophosphoric acids. The pore size was analyzed by SEM. The degradation of transmittance of membranes after one year and six years storage was studied. The change one year later in optical properties was explained by the adsorption of water molecules. Six years later, an increase in throughput is observed for all membranes. At present, there are no theoretical models that can explain obtained experimental results and additional studies in this area are necessary.


Introduction
Dielectric matrices based on porous anodic alumina (PAA) are an important material for photonic crystals [1][2][3]. Due to the unique ordered honeycomb structure of the array of channels located along the normal to the film surface [4,5], spatial modulation of the dielectric constant can be realized on a scale comparable to the wavelength of light. Earlier studies of the optical properties [6,7] showed that PAA is transparent in the visible wavelength range, while they well screen IR radiation [8][9][10]. By the shape of transmission spectrum, it is possible to determine the pore diameter and their uniformity in size qualitatively.
These studies were confirmed by thermal imaging measurements [11,12]. Dielectric porous materials with such properties are widely used in many fields of science and technology [4,5]. To date, no scientific papers devoted to the study of the degradation of the optical properties of PAA membranes are published. Only rare studies on a similar topic are presented in literature [13,14].
Nowadays the degradation of optical properties over time is of great interest. In this work, we investigated PAA membranes with different geometric parameters obtained in electrolytes based on sulfuric and orthophosphoric acids.

Experiment
To study the changes in optical properties over time, PAA membranes with significantly different geometric parameters were prepared. The membranes were obtained by electrochemical anodizing in a single-chamber cell under the potentiostatic regime in electrolytes based on sulfuric (at U=25...30 V) and orthophosphoric (at U=100...120 V) acids. As a starting material, we used aluminum foil (thickness of 40 µm) pre-purified in acetone and isopropyl alcohol. Each sample, for the reliability of the results, was fixed in a fluoroplastic frame with the same hole. The optical properties of the obtained samples were studied with an interval of 1 year and 6 years using a PE-5400UF spectrophotometer operating in the visible spectral range (190-1000 nm). During the entire time, the membranes were stored in separate identical sealed packages. A Tescan scanning electron microscope was used to study the morphology of PAA membranes.

Results and discussion
The analysis of SEM images of PAA ( Figure 1) synthesized in electrolytes based on sulphuric and orthophosphoric acids showed that that the pore diameters in samples differ by almost 10 times (d por (H 2 SO 4 ) ~ 17-20 nm, d por (H 3 PO 4 ) ~ 180-220 nm). The membrane thickness also differed: 15 µm for H 2 SO 4 and 8 µm for H 3 PO 4 .
As can be seen from   Initially, the semi-transparency of thin PAA membranes can be explained by several mechanisms of light scattering [7]: a) at the grain boundaries and at the boundaries of grains with pores, b) on inclusions of another phase, c) on the membrane surface roughness.
The latter component, diffuse light scattering on the membrane roughness, is negligible since PAA membranes are obtained by electrochemical anodizing, after pretreatment of the surface. As a result the size of the inhomogeneities becomes comparable to the size of the pores.
Grain boundaries and pores are the most important causes of light scattering in PAA membranes. The light scattering associated with the birefringence effect at the grain boundaries is comparable to the light scattering at the pores only for membranes with very low porosity. Therefore, for porous membranes, the scattering at the pores significantly influence the transmission of light. The smaller the pores are formed in membranes, the wider the transmission range in the visible region can be reached.
It was found that, due to air humidity, over time, water is adsorbed on a porous surface. Despite the important role of light scattering by pores in the transmittance of porous membranes, the adsorbed water molecules also make their contribution. It was found that in the case of PAA with a large pore diameter, water molecules penetrate inside pores and change their shape, thereby reducing light scattering and increasing the transparency of membranes. For PAA with a small pore diameter (Figure   a   b c  (Figure 2, b, c), water molecules are adsorbed on the surface, forming a continuous thin layer that reduces the transparency of the membrane.
Six years later, an increase in throughput is observed for all membranes. The possible causes of observed phenomenon can be the change in some reflective features inside the pores, the change in the roughness and the fractal properties of the pore surface, as well as electron density redistribution. However, at present, there are no theoretical models that can explain obtained experimental results and additional studied in this area are necessary.

Conclusions
At intervals of 1 year, the optical properties of PAA membranes were studied and their changes were analyzed. Water is adsorbed on the porous surface over time due to the humidity of the air. Despite the fact that the light scattering at the pores plays a crucial role in the transmission of light in porous membranes, the adsorbed water molecules also influence this process. In the case of large pores (in this paper ~ 200 nm), water molecules, penetrating inside, begin to change their shape, thereby reducing the light scattering. For PAA with a small pore diameter, water molecules are adsorbed on the surface, forming a solid thin layer that reduces the transparency of the membrane.