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NANOSYSTEMS: PHYSICS, CHEMISTRY, MATHEMATICS, 2019, 10 (2), P. 154-157
Effect of anodizing voltage and pore widening time on the effective refractive index
of anodic titanium oxide
N. A. Sapoletova1, S.E. Kushnir1'2*, K. S. Napolskii1'2
1 Department of Chemistry, Lomonosov Moscow State University, 119991, Moscow, Russia 2Department of Materials Science, Lomonosov Moscow State University, 119991, Moscow, Russia
*kushnir@elch.chem.msu.ru
PACS 42.25.Gy, 82.45.Aa, 82.45.Cc DOI 10.17586/2220-8054-2019-10-2-154-157
The unique optical properties of porous anodic titanium oxide (ATO) make it a promising material for solar energy conversion, sensorics, and opto-electronics. The optical path length and effective refractive index (neff) of ATO can be tuned by chemical etching of pore walls. However, precise control of these optical parameters is still challenging due to the lack of data on the effect of pore widening time on the neg. Here, a detailed study of the influence of anodizing voltage and pore widening time on the neg of the ATO films was performed. Analysis of reflectance spectra of ATO synthesized at 35 - 50 V shows that pore widening in 3 wt. % H2O2 aqueous solution allows one to control the neg at values ranging from 1.54 to 1.84. The data required for the prediction of the thickness, neg, and optical path length of the ATO films from anodizing and etching conditions are obtained.
Keywords: anodic titanium oxide, anodization, porous materials, film, refractive index, optical materials and properties.
Received: 5 April 2019 Revised: 15 April 2019
1. Introduction
Anodizing is one of the more promising methods for the production of porous TiO2 films at room temperature [1]. Porous anodic titanium oxide (ATO) films demonstrate high efficiency in water photoelectrolysis [2], photocatalysis [3,4], gas sensing [5], and can be used as carriers for controlled drug release [6-8]. In addition, ATO films have potential for application in dye-sensitized solar cells [9], refractive index sensors [10,11], smart color windows, and electronic displays [12] owing to a high refractive index (nTio2 = 2.6 at A = 600 nm [13]) and semiconductor properties of titanium oxide.
Optical path length of ATO films, the product of the film thickness and refractive index, is one of the key parameters that has great importance for optical applications. The thickness of ATO is directly proportional to charge density consumed during anodizing [14, 15], whereas control of the refractive index of ATO more challenging. Taking into account a small diameter of the pores in ATO films, which is much lower than the wavelength of visible light, the effective refractive index (neff) of ATO is a function of film porosity within a framework of the effective medium model. It is worth noting that the porosity and the neff values for anodic oxides change during the anodizing process due to chemical etching of the pore walls [16]. To the best of our knowledge, there are no systematic data on the neff of ATO as a function of anodizing voltage and the duration of chemical etching.
Here, we study the dependence of the thickness-to-charge density ratio and the neff of ATO films on anodizing voltage. Special attention is paid to the variation of the neff caused by chemical etching of pore walls in the anodizing electrolyte and aqueous H2 O2 solution.
2. Materials and methods
Prior to anodizing, titanium foils (0.15 mm thick, 99.6 % purity) were electrochemically polished in a mixture of 99.5 wt. % acetic acid and 65 wt. % perchloric acid with a volume ratio of 9:1. Electrochemical polishing was performed over 4 min under square-wave applied voltage (40 V for 10 s and 60 V for 10 s) at a temperature below 25 °C as described elsewhere [15].
To prepare porous anodic titanium oxide films, Ti was anodized at a constant voltage between 30 - 60 V in an ethylene glycol electrolyte containing 0.3 wt. % NH4F, 0.66 wt. % CH3COONa, and 2 wt. % H2O at 30 °C. The electrolyte solution was prepared by adding aqueous NH4F to sodium acetate dissolved in ethylene glycol. The electrolyte was stirred at 480 RPM using an overhead stirrer. Experiments were performed in a two-electrode electrochemical cell with the distance of 2 cm between Ti electrodes. The anodized area and the total charge density consumed during anodizing of each sample was 1.13 cm2 and 3.36 C-cm-2, respectively. To study pore
Effect of anodizing voltage and pore widening time on the effective refractive index.
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widening effect on the neff of the ATO in electrolyte, some of the prepared films were stored in electrolyte solution for 2 - 6 hours after anodizing. Finally, the ATO films were washed with ethanol and then air-dried. Some as-synthesized ATO films were aged in 3 wt. % H2O2 aqueous solution for 7.5 - 30 minutes under stirring.
Morphological characterization of the ATO films was performed by scanning electron microscopy (SEM) using LEO Supra 50 VP instrument. Perkin Elmer Lambda 950 spectrophotometer was used to record the reflectance spectra at incident angles of 8, 30, 45, 60, and 65° in a wavelength range of 650 - 890 nm.
3. Results and discussion
Optical reflectance spectra of the ATO films prepared at different anodizing voltages (U) are shown in Fig. 1a. In the case of the ATO obtained at U = 35 - 50 V, intense Fabry-Perot oscillations of the reflectivity can be clearly seen in the spectra, resulting in high thickness uniformity for these samples at least across the irradiated area (4 x 4 mm2). The Fabry-Perot oscillations in the spectra of the samples, prepared at anodizing voltages outside the 35 - 50 V range, are weak or completely lacking, manifesting non-uniform thickness. Thus, the range of 35 - 50 V was chosen to study the dependence of the neff of the ATO films on the pore widening duration.
Fig. 1. Optical properties of ATO films prepared at various anodizing voltages. (a) Reflectance spectra recorded at incident angle of 8°. (b) The effective refractive index at 700 nm as a function of anodizing voltage
The positions of Fabry-Perot oscillations in the spectra recorded at different incident angles were used for calculating the neff and sample thickness (h) using a protocol described previously [17]. The neff increases from 1.78 to 1.84 with growth of anodizing voltage from 35 to 50 V (Fig. 1b). Thickness-to-charge density ratio (h/q) lies in the range of 0.88 - 1.00 ^m^cm2 •C-1.
Typical SEM images of the ATO are shown in Fig. 2 on the example of the sample prepared at 35 V. ATO film consists of vertically aligned nanotubes contacting each other. The average distance between the centers of neighboring nanotubes (Dint) increases linearly from 79 to 111 nm with an increase in the anodizing voltage from 35 to 50 V; the proportionality constant Dint/U is 2.23 ± 0.03 nm V-1, that is inside the range of corresponding value observed earlier for ethylene glycol based electrolytes (1.8 - 3.0) [14]. According to SEM data, the thickness of the samples prepared at U = 35 - 50 V is in the range of 2.9 - 3.2 ^m, and thickness-to-charge density ratio lies in the range of 0.86 - 0.95 ^m-cm2•C-1. It is worth noting that these values are in good agreement with spectral data.
The dependence of the effective refractive index of ATO films on the duration of chemical etching is shown in Fig. 3. Chemical etching of nanotube walls in F--containing electrolyte solution leads to the growth of the porosity of the ATO, resulting in a decrease in the neff (Fig. 3a). The rate of pore widening in electrolyte solution is slow enough: change of the neff after several hours of etching is lower than 0.11 independently of anodizing voltage. A stronger (up to 0.28) decrease in the neff is observed in the aqueous H2O2 solution (Fig. 3b). The rate of pore widening is substantially faster only during the first 15 minutes of etching. Further etching leads to a minor change in the neff followed by the detaching of ATO film after 30 min. Pore widening in H2O2 solution increases the difference between the neff of the layers obtained at 35 and 50 V (Aneff) from 0.06 to 0.14. The trend of the Aneff increase during pore widening is in good agreement with one reported for the anodic alumina films prepared in oxalic [18] and sulfuric acid electrolytes [16]. Most likely, the decrease in etching rate (decreasing of the neff) after 7.5 min in H2O2 is caused by layered structure of ATO pore walls. The inner layer, contacting with
156 N.A. Sapoletova, S.E. Kushnir, K. S. Napolskii
Fig. 2. SEM images of the ATO film prepared at U = 35 V: cross section (a), bottom (b) and top view (c)
Fig. 3. Dependence of the effective refractive index at 700 nm on pore widening time for the ATO films prepared at 35 - 50 V: in the electrolyte (a) and in the 3 wt. % H2O2 aqueous solution (b)
electrolyte during anodizing, contains more impurities from electrolyte, that makes it less stable than the deeper layer consisting mainly of titania [19]. Obtained quantitative data on thickness and the neff of ATO will be in demand for the design of materials for antireflection coatings, memristive elements [20], photonic crystals [21], and solar cells.
4. Conclusions
High uniformity for the ATO film thickness was observed for the samples obtained in ethylene glycol electrolyte (0.3 wt. % NH4F, 0.66 wt. % CH3COONa, 2 wt. % H2O) at anodizing voltages ranging from 35 to 50 V. The values of film thickness-to-charge density ratio vary from 0.86 to 1.00 ^m^cm2 •C-1 according to analysis of the reflectance spectra and SEM images. The neff at 700 nm of the as-prepared ATO films is in the range of 1.78 -1.84, which is higher than in the case of anodic alumina and bulk Al2O3. Pore widening in H2O2 solution allows one to achieve the lower bound of the neff of 1.54 and to increase the difference between the neff of the layers obtained at 35 and 50 V from 0.06 to 0.14. Chemical etching in aqueous H2O2 solution is an effective method for tuning the porosity of ATO films, saving their planarity and uniformity.
Acknowledgements
This work is supported by the Russian Science Foundation under grant No. 17-73-10471. Some experiments were carried out using the scientific equipment purchased by M. V. Lomonosov Moscow State University Program of Development.
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