Nano-zone News


TECHNICAL PROPOSAL FOR
THE DURABILITY STUDY OF
TITANIUM OXIDE
NANO COATING ON GLASS


Prepared by:
Prof. YEUNG Lam Lung Department of Chemistry
HK University of Science and Technology

Nano TiO2 Solution provided by:
Nano-Zone Eco Environments Limited
Namely: TDX-HT

1. Background


Photocatalysis is a common word nowadays as various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysis, titanium oxide (TiO2) is almost the only material suitable for industrial use at present as its photoactivity is known for approximately for 60 years and its photocatalytic function has been investigated extensively. TiO2 is a semiconducting material with band gap energy of 3.2 eV. If this material is irradiated (h) with photons of the energy > 3.2eV, electrons will be promoted from the valence to the conduction band generating holes (h+) which subsequently react with water to produce highly reactive hydroxyl radicals (•OH)

TiO2 + h → h+ + e-
h+ + H2O → •OH + H+

Both the holes and the hydroxyl radicals are very strong oxidants with reactivity similar to fluorine (the most powerful oxidant) that can oxidize almost all organic contaminants. The followings are the advantages of using TiO2 as the photocatalyst:

  • It is an efficient and low-cost material for photocatalysis.
  • The oxidation reaction is fast at mild operating conditions (at ambient temperature and atmospheric pressure).
  • A wide spectrum of organic contaminants can be converted to water and CO2
  • No chemical reactants must be used and no side reactions are produced.

Another property of TiO2 surface irradiated with ultraviolet light is the formation of super-hydrophilic surface. The wetting of a solid with water, where air is the surrounding medium, is dependent on the relation between the interfacial tensions. The ratio between these tensions determines the contact angle between a water droplet on a given surface. A contact angle of 0o means complete wetting whereas 180o corresponds to complete non-wetting. Cleaning procedures based on low contact angle are known since the discovery of soap as it can reduce the surface tension of water which causes low contact angle. Super-hydrophilic surface has the property of attracting water resulting water lying flat on surface in sheets with very low contact angle. Even the irradiation of ultra-violet ceased, the super-hydrophilic behavior of the TiO2 surface will be retained for approximately 2 days! Grease, dirt and other staining materials can easily be swept away with a stream of water as shown in the following diagram (Left: ordinary uncoated tile. Right: hydrophilic tile coated by TiO2)


The aforementioned two functions i.e. photocatalytic and super-hydrophilicity of TiO2 irradiated with ultraviolet light, have opened up various novel applications of TiO2, which can be summarized in the following diagram. Compared with traditional oxidation cleaning processes, TiO2 with ultraviolet light has many advantages such as ease of setup and operation at ambient temperatures, environmental friendly as only carbon dioxide and water produced after oxidation of organic pollutants, no need for post-processes, low operation cost because of self-cleaning property and consequently no detergent is needed.


2. Objective of the Study


Nano TiO2 solution provided by Nano-Zone Eco Environments Limited, namely TDX-HT, can be directly applied to glass surface either by spraying or brushing onto the surface. This solution penetrates and forms semi-permanent bond with the glass surface. After air drying, a thin film of nano sized TiO2 coating will be formed on the glass surface. The objectives of this study will be focused on:

  • 1. characterization of the TiO2 film formed on the glass surface by using TDX-HT solution
  • 2. hydrophilicity of the TiO2 film under UV light
  • 3. durability of the TiO2 film
  • 4. weathering resistance of TiO2 film

3. APPROACH AND METHODOLOGY


Characterization of the TiO2 Film 3 pieces of glasses with a dimension of 18 x 30 cm coated with TiO2 film using TDX-HT solution will be examined. The TiO2 film will be characterized by X-ray Diffraction (XRD) employing CuK as the radiation source. The morphology and microstructures of the TiO2 film will be examined by using a scanning electron microscope (SEM). The light transmittance and absorbance of the TiO2 film will be measured by Ultraviolet-Visible (UV-Vis) spectroscopy.

Hydrophilicity Test of the TiO2 Film It is known that there will be marked change in the water wettability of TiO2 surface before and after UV light irradiation. When UV is irradiated onto a TiO2 surface, a highly hydrophilic surface will be generated. This surface exhibits both antifogging and self-cleaning properties. In this study, the photoinduced hydrophilicity of the TiO2 film of the 3 pieces of glasses, in term of water contact angle (CA), defining as the angle between the solid surface and the tangent line of the liquid phase at the interface of the solid liquid gas phases, will be evaluated using an automatic contact angel system.


Durability Test of the TiO2 Film In the assessment of the durability of the TiO2 film, a sample glass coated with TiO2 film using TDX-HT solution will be placed under a column of water falling freely from a height of 33 cm, according to the a standard procedure published by Carlson, P.J. et al. The water flow rate will be fixed at 870 ml/min and the total exposure time will be 1 min, 5 min and 10 min. After each exposure, glass sample will be shaken to remove excess water and will be dried at ambient temperature under vacuum for 2 hours. The mass of each sample will be determined. The residual mass of the surface film after each washing step will be calculated by subtracting the mass of the clean, dried support materials. The above entire process will be repeated on a glass sample with no deposition of TiO2 film as a reference sample for comparison with TiO2 film coated samples.

After each washing, the TiO2 film will be characterized by XRD and UV-Vis. Hydrophilicity of the TiO2 film will also be evaluated by using the water contact angle test.

Weathering Resistance Test of TiO2 Film Weathering tests of a sample glass coated with TiO2 film using TDX-HT solution will be carried out a condition chamber with the following temperatures and humilities for 48 hours. Temperature: 50oC and 60oC Relative humidity: 90% and 95% After each test, the change of TiO2 film will be examined by using SEM.