Anatase TiO2 deposited at low temperature by pulsing an electron cyclotron wave resonance plasma source

Abstract

Photocatalytic surfaces have the potentiality to respond to many of nowadays societal concerns such as clean H2 generation, CO2 conversion, organic pollutant removal or virus inactivation. Despite its numerous superior properties, the wide development of TiO2 photocatalytic surfaces suffers from important drawbacks. Hence, the high temperature usually required (> 450 °C) for the synthesis of anatase TiO2 is still a challenge to outreach. In this article, we report the development and optimisation of an ECWR-PECVD process enabling the deposition of anatase TiO2 thin films at low substrate temperature. Scanning of experimental parameters such as RF power and deposition time was achieved in order to maximise photocatalytic activity. The careful selection of the deposition parameters (RF power, deposition time and plasma gas composition) enabled the synthesis of coatings exhibiting photocatalytic activity comparable to industrial references such as P25 Degussa and Pilkington Activ at a substrate temperature below 200 °C. In addition, to further decrease the substrate temperature, the interest of pulsing the plasma RF source was investigated. Using a duty cycle of 50%, it is thus possible to synthesise photocatalytic anatase TiO2 thin films at a substrate temperature below 115 °C with a deposition rate around 10 nm/min.

Low-temperature deposition of self-cleaning anatase TiO2 coatings on polymer glazing via sequential continuous and pulsed PECVD

“…In a former article, it was established that, providing a fine tuning of plasma deposition parameters, TiO2 thin films growth occurs following a transition from amorphous to anatase along the thickness [37]. During these studies, the ECWR plasma source has shown its ability to readily deposit anatase layers with high power input [38] and at a high deposition rate [37] (≈10 nm.min -1 ). Also, it is worth mentioning that the proposed process provides very good uniformity of coating thickness (+/-5 % thickness on 5x5 cm 2 surface) and excellent crystalline homogeneity.…”

“…Although the set up descibed in a previous work enabled to deposit anatase TiO2 at Tsub max < 120°C [37], it has been sligthly tweaked in the perspective of lowering the deposition temperature even further. As such, copper substrate holder was used and temperature evolutions were compared with the one obtained previously on a stainless steel substrate holder.…”

“…Hence, pulsed waves plasma have been investigated to further decrease the temperature of the coatings while maintaining a partially crystallized structure [36]. In a recently published paper [37], we demonstrated the great ability of pulsed wave (PW) Electron Cyclotron Wave Resonance (ECWR) plasma technology to deposit crystalline TiO2 at low temperature. Based on fine tuning of the duty cycle (DC = TPlasma On/(TPlasma On + TPlasma Off)) and the gas composition, one could deposit anatase TiO2 at Tsub max < 120°C with a growth rate of 10 nm.min -1 .…”

Unveiling a critical thickness in photocatalytic TiO2 thin films grown by plasma-enhanced chemical vapor deposition using real time in situ spectroscopic ellipsometry

“…Prior to any UV illumination, the samples were immersed in the [MB] = 1 µmol l −1 solution then stored under dark condition for one night in order for the adsorption/desorption to reach equilibrium. The 365 nm UV lamps (Herolab UV-8 SL, 365 nm, 2 × 8 W) were then placed at a distance around 8 cm so the measured irradiance was set at 3 mW cm −2 [26,48].…”

“…(26,29) Based on XRD results, previous works and literature, we can assume that the bright column ends are associated with crystallization of the film, while the darker contrast can be associated with amorphous-like TiO2. (19,29,33,41) The column size is expected to be much larger than the crystal size, hence the use of the term "polycrystalline columns" through the article. Top-view SEM pictures have been exploited with ImageJ software to extract the density and average area of bright columns arising at the surface and the results are plotted in Figure 5.…”