Wetting Angles on Illuminated Ta₂O₅ Thin Films with Controlled Nanostructure

Abstract

Ta 2 O 5 thin films with different nanostructure and surface roughness have been prepared by electron evaporation at different angles between the evaporation source and the substrates. Large variation of refraction indexes (n) from 1.40 to 1.80 were obtained by changing the geometry of evaporation and/or by annealing the evaporated films at increasing temperatures up to 1000°C to make them crystalline. Very flat and compact thin films (n ) 2.02) were also obtained by assisting the growth by bombardment with O 2 + ions of 800 eV kinetic energy. A similar correlation has been found between the wetting contact angle of water and the roughness of the films for the evaporated and evaporated + annealed samples, irrespective of their procedure of preparation and other microstructural characteristics. When the films were illuminated with UV light of h > E g ) 4.2 eV (E g , band gap energy of Ta 2 O 5 ), their surface became superhydrophilic (contact angle < 10°) in a way quite similar to those reported for illuminated TiO 2 thin films. The rate of transformation into the superhydrophilic state was smaller for the crystalline than for the amorphous films, suggesting that in Ta 2 O 5 the size of crystal domains at the surface is an important parameter for the control of this kinetics. Changes in the water contact angle on films illuminated with visible light were also found when they were subjected to implantation with N 2 + ions of 800 eV kinetic energy. The origin of this photoactivity is discussed in terms of the electronic band gap states associated with the nitrogen-implanted atoms. The possibility of preparing antireflective and self-cleaning coatings of Ta 2 O 5 is discussed.

Atomistic details of oxide surfaces and surface oxidation: the example of copper and its oxides

“…The different results obtained by these experiments can be due to the texture and microstructure of the copper film 276 , by the surface roughness or by defects present at the surface. Indeed, the surface roughness of metal oxide surfaces has been shown to have a direct influence on their wetting properties towards water, which in turn could have a direct influence on the formation of the native oxide 296 . In particular, very smooth copper surfaces are hydrophobic while rough surfaces (∼ 5 nm-high roughness) are hydrophilic 297 .…”

Synthesis, characterization, and photoactivity of InTaO4 and In0.9Ni0.1TaO4 thin films prepared by electron evaporation

“…Another interesting feature in these spectra is the existence of some losses in the gap region, as indicated by the asterisk included in the figure. In a previous publication on InTaO 4 thin films prepared by chemical vapor deposition, 11 similar states were detected with a higher intensity. In the present case the exact energy of these states cannot be determined accurately because their spectroscopic features overlap with the band corresponding to the elastic peak.…”

“…From these spectra, band gap energies have been determined by extrapolation to zero the edge jump at around 5 eV. 17 The obtained values, collected in Table I, indicate that the three oxides have a wide band gap that ranges from around 4.2 eV for Ta 2 O 5 to 4.6 for InTaO 4 and In 0.9 Ni 0.1 TaO 4 . Another interesting feature in these spectra is the existence of some losses in the gap region, as indicated by the asterisk included in the figure.…”

“…C-V curves were recorded for Ta 2 O 5 , InTaO 4 , and Ni 0.1 In 0.9 TaO 4 thin films annealed at 800 and 1000°C. Figure 6 shows, as an example, the curves measured at 150 kHz and 100 Hz corresponding to the InTaO 4 thin films annealed at these two temperatures.…”

“…2,3 Another less well known property of this material is its ability to act as a photocatalyst, [4][5][6] although its practical use is restricted by the fact that photons of very short wave length ͑i.e., Ͻ300 nm͒ are required to photoinduce the generation of electron-hole pairs suitable to produce this type of photoactivation processes. Recently, new mixed oxides of the type InTaO 4 and In 0.9 Ni 0.1 TaO 4 ͑i.e., Ni-doped InTaO 4 ͒ have been claimed to be photoactive with visible light ͑i.e., with photons of Ͼ400 nm͒. 7 Owing to the large interest of these processes, different publications have recently reported about the synthesis, characterization, and analysis of these mixed oxides, particularly when they are prepared in the form of thin films.…”