The aim of this paper is to investigate the properties and photocatalytic activity of nanostructured TiO2 layers. The glancing angle deposition method with DC sputtering at low temperature was applied for deposition of the layers with various columnar structures. The thin-film structure and surface morphology were analyzed by XRD, SEM, and AFM analyses. The photocatalytic activity of the films was determined by the rate constant of the decomposition of the Acid Orange 7. In dependence on the glancing angle deposition parameters, three types of columnar structures were obtained. The films feature anatase/rutile and/or amorphous structures depending on the film architecture and deposition method. All the films give the evidence of the photocatalytic activity, even those without proved anatase or rutile structure presence. The impact of columnar boundary in perspective of the photocatalytic activity of nanostructured TiO2 layers was discussed as the possible factor supporting the photocatalytic activity.
Titanium dioxide is one of the most investigated materials in the last decades. Three forms of TiO2 can be found in nature: anatase, rutile, and brookite. Its properties like a high refractive index, chemical and mechanical stability, and/or photocatalytic activity [
One of the most attractive areas concerning titanium dioxide is its photocatalytic activity. The photocatalytic effect of TiO2 material is based on generation of electron-hole pairs due to the UV irradiation of TiO2. The generated pairs diffuse to the surface of the material and participate in the surface reaction leading to the decomposition of adsorbed matter [
A lot of articles have been published about the relation between the thin-film structure and the photocatalytic activity [
One of the most important factors affecting the photocatalytic efficiency is the lifetime of the electron/hole pairs. The excitons generated during the UV irradiation of TiO2 layers dissipate the stored energy within few nanoseconds by the recombination. Doping of the thin-film surface leads to enhancement of the excitons lifetime (as well as photocatalytic activity) because of a Schottky’s barrier creation [
TiO2 photocatalytic films can be prepared by various methods of which SOL-GEL, PECVD, and/or PVD belong to the most frequently used. One of the well-available PVD technologies enabling the deposition of thin films with columnar structure is the so-called glancing angle deposition. (GLAD) [
Principles of glancing angle deposition [
The presence of columnar structure and architecture type of columns strongly affect functional properties of thin films [
TiO2 films were deposited in a classical PVD apparatus equipped with a movable substrate nonheatable holder. Changing the angle of the holder and/or its rotation enables the control of the film columnar structure. The detailed description of the experimental setup has been described elsewhere [
The ultimate pressure inside the chamber was about 1 Pa; typical values of discharge current and voltage were 200 mA and 450 V, respectively. Argon with the admixture of oxygen was used as a working gas mixture. The oxygen was flowed either constantly of 2 sccm (reactive gas constant flow—RGCF) or in a pulsed mode (reactive gas pulsing process—RGPP). The period of the pulsation was 45 s with the
The amount of the injected oxygen during the
The surface morphology of thin films was investigated by the atomic force microscopy (AFM) and scanning electron microscopy (SEM). Nanoscope 3 MORTIMORT machine was used for the AFM. Surfaces of all the samples were scanned by the contactless regime using the FastScan method with resonation frequency 50 kHz. The software Gwyddion was used for the evaluation of the arithmetic roughness calculation. The values of the arithmetic roughness were evaluated due to the comparison of AFM data matrix by the middle plane interlay. The values of the arithmetic roughness indicate the arithmetic deviation from a mean value of point heights.
The JEOL-JSM 740 scanning electron microscope operated with accelerating potential 5 kV and working distance of 14.4 mm.
The thin-film structure was investigated by the X-ray diffraction. Due to a low value of the thin-film thickness, the special diffractometer X’Pert Pro made by the PANalytical company was used with CoK
For the determination of the ratio of specific surface
The photocatalytic activity was derived from the decomposition speed of aqueous solution of the Orange 7 (sodium salt of sulphonated azo dye), assuming the Langmuir-Hinshelwood law for degradation according to the pseudo-first-kinetic order. The experimental method and determination of the apparent kinetic constant
The photocatalytic activity of the deposited films was also compared with the films prepared from commercially available Evonik/Degussa Aeroxide P25 photocatalyst. Samples in form of this powder were prepared by sedimentation on soda lime glass substrates. The sedimentation was performed by placing 0.2 mL/cm2 of aqueous suspension of known catalyst concentration on the substrate, followed by drying. After the drying process under laboratory condition, the deposited layers were heated to 300°C for 30 minutes to achieve the better adhesion of the particulate layer on the glass support. Such prepared layers contain 0.5 mg of P25 per cm2, with layer thickness of 5 microns. The efficiency of Degussa Aeroxide P25 is shown only for the comparison of TiO2 films made by PVD GAD with highly efficient photocatalyst.
Four types of the thin-film structure were prepared in dependency on GLAD parameters (a deposition angle
Both the RGCF and RGPP methods were applied for deposition of each type of the structure, presented in Figure
SEM analyses of chosen thin-film edges.
The thin films called dense layers (Figures
The surface morphology was investigated by SEM and AFM analysis. The SEM pictures of surfaces of particular samples are shown in Figure
SEM surface photos.
The surface of dense layers (both RGCF and RGPP: Figures
To quantify the differences in the thin-film surface structure, the arithmetic roughness measurement by AFM was evaluated and the arithmetic roughness for was calculated for all the samples. Results are summarized in Figure
Roughness in dependency on layers structure.
The lowest value of the arithmetic roughness was observed at the dense layer made by RGCF—1.6 nanometers. The layer with zigzag columns made by RGPP showed the highest value of the arithmetic roughness—3.9 nanometers. Higher values of the arithmetic roughness observed at the layers with the oriented structure (incline, zigzag, and helical columns) are clearly visible in comparison with the nonoriented structured layers (dense layers). This is in a good agreement with the SEM surface analysis shown in Figure
The pulsation of the reactive gas is known to reduce the target poisoning resulting in higher sputtering yield. The higher deposition rate during the RGPP in combination with moderate sputtering of the substrate during the oxygen off-phase might be the reason of generally higher surface roughness of samples prepared by RGPP in comparison with those deposited by RGCF.
High specific surface is a general assumption for effective catalytic reaction. Jong [
Dependency of adsorption on relative pressure during Kr adsorption.
Specific surfaces of TiO2 layers.
The normalized specific surface
To the investigation of crystalline quality, low-angle X-ray diffraction analysis was realized on the individual representatives for each type of the structure and both deposition methods. In Figure
Results of low-angle XRD for layers made by RGCF.
Results of low-angle XRD for layers made by RGPP.
It is obvious from Figures
Mostly the presence of the anatase structure (angle values—29.9°, 57.3°, 74.9°) was proved by the low-angle XRD for incline and zigzag column architecture type (both RGCF and RGPP), especially anatase peaks relevant to the angle 29.9°. Rutile phase is proved by the peak with low intensity at the angle value equal to 51.6°.
The normalized kinetic constants
Photocatalytic activities in dependency on thin-film structure.
GLAD parameters relevant to specific structure.
The dark absorption measurement was part of the photocatalytic activity analysis. All tested samples were immersed in the AO7 solution in the dark for 6 hours. The AO7 concentration changes were evaluated due to UV/VIS spectroscopy, and no differences between the initial and final concentration were observed, even for films with different values of the specific surface area (Aeroxide TiO2 P25 Degussa). Based on these results, the contribution of dark absorption to the AO7 concentration decrease was excluded.
All tested samples were irradiated by UV for the same time of 6 hours and only the initial and final concentrations were analyzed. According to our experiences with this type of experiments and results presented in [
Figure
As mentioned above, the photocatalytic activity is connected with presence of anatase or anatase/rutile crystalline fractions (Figures
On the other hand, the films with helical-columns structure evidence substantial photoactivity eventhough they did not prove any crystalline phases. The dense layers should exceed the photocatalytic activity of the thin films with helical-column structure due to the positive impact of the crystalline anatase/rutile phases [
The high value of the specific surface area connected to the layer with helical columns made by RGPP should be one of the factors explaining the comparable photocatalytic activity of amorphous helical-column layers with dense layers proving the presence of anatase and rutile. On the other hand, from Figure
As already mentioned above, the photocatalytic effect of the TiO2 is connected with anatase/rutile structure and amorphous phase is swallowed to be detrimental in pure TiO2 photocatalyst. On the other hand Horakova et al. [
TiO2 thin films were prepared by the GLAD PVD method. The films evidenced nanostructured columnar structure. They are characterized by higher surface roughness and substantial higher specific surface in comparison with films prepared by conventional PVD. All the films evidence photocatalytic activity even those with amorphous character. Boundaries between the oriented structures are explained to be a reason of the photocatalytic activity of amorphous TiO2 layers. The photocatalytic activity was observed at the thin film with zigzag columnar structure made by RGPP reaching 70% efficiency of Aeroxide TiO2 P25 Degussa.
This work was supported by MSMT, Project NANOPIN 1M0577, by GA AV CR, Project No. KAN101120701, and by the project CZ.1.05/2.1.00/03.0086 funded by European Regional Development Fund.