Plasma-Enhanced ALD of TiO[sub 2] Thin Films on SUS 304 Stainless Steel for Photocatalytic Application

Abstract

Plasma-enhanced atomic layer deposition ͑PE-ALD͒ of TiO 2 thin films using Ti͑NMe 2 ͒ 4 ͓tetrakis͑dimethylamido͒ Ti͔ and O 2 plasma were prepared on stainless steel to show the self-cleaning effect. The TiO 2 thin films deposited on stainless steel have high growth rate, large surface roughness, and low impurities. The film deposited below 200°C was amorphous, while the films deposited at 300 and 400°C showed anatase and rutile phases, respectively. The contact angle measurements on crystalline PE-ALD TiO 2 thin films exhibited superhydrophilicity after UV irradiation. The TiO 2 thin film with anatase phase deposited at 300°C showed the highest photocatalytic efficiency, which is higher than on Activ glass or thermal ALD TiO 2 films. We suggest that anatase structure and large surface area of TiO 2 thin film on stainless steel are the main factors for the high photocatalytic efficiency.Since photoinduced water splitting on titanium dioxide ͑TiO 2 ͒ electrode was discovered, 1 TiO 2 has been regarded as one of the most promising photocatalytic materials due to its high photocatalytic activity, high chemical stability, low toxicity, and low cost. As a photocatalyst, when UV light with higher energy than the bandgap of TiO 2 is illuminated, electron-hole pairs are generated in the TiO 2 films. The photogenerated holes in the valence band and the electrons in the conduction band diffuse to the TiO 2 surface and they produce highly energetic hydroxyl radicals ͑ • OH͒ and superoxide radical anions ͑O 2 •− ͒ which can oxidize organic molecules on the TiO 2 surface. 2-4 In addition, TiO 2 photocatalyst kills bacteria 5,6 and induces a hydrophilic surface after UV irradiation. 7,8 Thus, TiO 2 photocatalysts are widely used in various fields, such as window glass, ceramic tiles, and wall papers utilizing its self-cleaning, 9,10 antifogging, 7,8 antibacterial activities, 5,6 and water or air purification. 11,12 Two different tetragonal phases of TiO 2 , anatase and rutile, are commonly used in photocatalysis. The structures of both anatase and rutile consist of chains of TiO 6 octahedra, but lattice structures of two crystals are obviously different. 13 The differences in lattice structure cause different mass densities ͑3.894 g/cm 3 for anatase and 4.250 g/cm 3 for rutile͒ and electronic bandgaps ͑3.3 eV for anatase and 3.1 eV for rutile͒. The anatase phase has a more negative conduction bandedge potential ͑higher potential energy of photogenerated electrons͒ than the rutile phase, so it is well known that the photocatalytic activities of the anatase phase are superior to those of the rutile phase. 3 Thus, in terms of photocatalytic efficiency, obtaining a suitable crystalline phase and high specific area are important factors.For practical applications, it is necessary to deposit TiO 2 thin films on various solid substrates such as glass, ceramic tiles, and steel. There are several reports about the TiO 2 photocatalyst on glass 9,14-17 and ceramic tiles 18,19 to give the surface of these materials a self-cleaning effect u...

Titanium dioxide thin films by atomic layer deposition: a review

“…The films deposited at 50 and 150 o C showed 3.5 and 2% of carbon impurities [384]. Lee et al reported very high GPC values around 2 Å/cycle even around 100-200 o C, however neither the carbon content, nor pulse saturation was commented [747,749]. For the Ti(NMe2)4/H2O plasma process the GPC-vs.-temperature trend was very similar to that of the thermal process [384].…”

“…The poor crystallization tendency is not necessarily related to incomplete surface reactions and consequent high carbon and/or nitrogen content, as the films are reported to be of high purity [384,634,636]. Regarding the O2 plasma-based process, formation of the rutile structure was seen for films deposited on stainless steel substrates at 400 o C, while on silicon substrates mixed-phase anatase-rutile structure formed at 300 o C and above [746,747,749]. Xie et al compared H2O, H2O plasma and O2 plasma as the oxygen source for Ti(NMe2)4; the highest level of carbon impurities was found for the Ti(NMe2)4/O2 plasma process, which correlated with the poor crystallization tendency upon annealing [384].…”

“…Hence, in most of the cases obtaining crystalline as-deposited films becomes difficult, or the processes may not be decomposition free in the temperature region where crystalline films are obtained. For the Ti(NMe 2 ) 4 /H 2 O, Ti(NMe 2 ) 4 /H 2 O plasma and Ti(NMe 2 ) 4 /O 2 plasma processes growth of amorphous films has been reported to take place below 250 °C [384,634,636,746,747,749]. The anatase phase has been seen to appear at 250 °C, while at 300 °C rutile inclusions have been potentially observed [635,746,749].…”

“…x [561,683,[735][736][737][738][739][740][741][742][743]; [384,646,744]; [384,646,651,741,[745][746][747][748][749][750][751][752][753][754][755][756][757][758][759][760][761];…”

“…The data illustrated in color in one of the figures, is shown in black in all the other figures. The data was retrieved from the following references: Ti(NMe2)4/H2O [384,634,636,637,639,642,643,674,767], Ti(NMe2)4/H2O2 [729], Ti(NMe2)4/O3 [735,736,742,767,801], Ti(NMe2)4/H2O plasma [384,744], Ti(NMe2)4/O2 plasma [384,651,[745][746][747]749], Ti(NEt2)4/H2O [767], Ti(NEt2)4/O3 [767], Ti(NEtMe)4/H2O [767], Ti(NEtMe)4/O3 [767].…”