Nitrogen Plasma Treatment on Titanium Dioxide Films Applied in Self-cleaning Glass

Abstract

This study reports the feasibility of nitrogen plasma post-annealing on titanium dioxide (TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) films applied in self-cleaning glass. Self-cleaning glass forms a photocatalytic reaction on the TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films coated on the glass during sunlight, making the glass hydrophilic, and using rainwater to remove dust away from the glass. Self-cleaning function are assessed by measuring the water contact angle on TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films. A low water contact angle means that the TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> surface has good hydrophilic properties. Previous study on TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films showed the oxygen vacancies in TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films are created during the nitrogen plasma treatment process to improve its surface hydrophilic. Our team has produced TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films which titanium films were thermally oxidized at different heating temperatures. It was hydrophilic after plasma annealing. However, thermal oxidation methods need more process steps to produce titanium oxide. Titanium films have to be coated first, followed by thermal oxidation process step. In this study, TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films were directly deposited by in-line sputtering with a TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> target. Three TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films with different thickness of 5, 15 and 50 nm were obtained. After that, the TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films were post-annealed in nitrogen plasma. Nitrogen plasma treatment contributes to increase of hydrophilic of TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . The structure, visible transmittance, contact angles and photoluminescence (PL) spectrum of the as-deposited and nitrogen plasma treatment TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films were compared. Experiment results indicate applied plasma treatment on TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films indeed influences the surface roughness and reducing water contact angle. The water contact angle of all as-deposited TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films is over than 50 degrees. The water contact angle reduces to less than 30 degrees after nitrogen plasma treatment. The films thickness of 5 nm corresponds to lesser contact angle of TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films among all samples, where the films contact angle decreased from 51.81 (without plasma treatment) to 24.91 degrees. PL measurements were performed before and after nitrogen plasma treatment. PL spectrum shows that the PL intensity of the plasma treated TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> is slightly decreased. Previous report implies that PL intensity decrease implies that the TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> surface has high photocatalytic activity. Improving hydrophilic property for TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films after nitrogen plasma treatment could be ascribed to 1. removal of organic contaminants; and 2. oxygen vacancies. The optimized sample thickness is 5nm for nitrogen plasma treatment. The water contact angle, and the average visible transmittance for nitrogen plasma treatments TiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films (5nm sample) is 24.91 degrees, and 96.75%, respectively, which can be applied to self-cleaning glass. This paper contributes to industry of energy saving buildings and green materials.