UV Induced Wetting of 2 by Water

Transcription

1 UV Induced Wetting of 2 by Water: Removal of Surface Contaminants Tykhon Zubkov, 1 Dirk Stahl, 2 Tracy L. Thompson, 3 Dimitar Panayotov, 4 liver Diwald, 5 John T. Yates, Jr. 6 1 Department of Chemistry, Ball State University, Muncie, IN Leica Microsystems Lithography GmbH, Jena, Germany 3 GraphTech Intl, Ltd., Parma, H Department of Chemistry, VirginiaTech, Blacksburg, VA Institut für Materialchemie der TU-Wien, Bereich, Physikalische Chemie, Wien, Austria 6 Department of Chemistry, University of Virginia, Charlottesville, VA 22904

2 2 coatings offer two unique properties for efficient and continuous decontamination: 1. Strong photooxidation power to degrade surface contaminants. 2. Hydrophilicity induced by UV light helps to wash away the oxidation products from exterior surfaces. (+ anti-fogging effect) Taken from Taken from

3 UV-Induced Hydrophilicity of 2 Before UV irradiation 2 is somewhat hydrophobic. Contact angle is higher After UV irradiation 2 is hydrophilic. Contact angle is very low

4 Proposed Model #1 UV-induced increase in H-groups causes hydrophilicity? Sakai et al., J. Phys. Chem. B 107 (2003) 1029 Mechanism: UV UV irradiation causes causes conversion from from bridging to to isolated -H -H groups. The The total total number of of H-groups increases. More More hydrophilic surface.

5 Proposed Model #2 Photocatalytic removal of hydrophobic contaminants from hydrophilic surface? White et al., J. Phys.Chem. B 107, , 2003 H Water Water binds binds strongly to to the the 2 (110) 2 (110) surface surface with with or or without the the surface surface H-groups present H H H H H Water Water binding binding to to the the 2(110) surface surface is is weak weak when when a hydrophobic adsorbate covers covers the the surface surface H H

6 Goal Get insight into the phenomenon of UV-induced hydrophilicity using 1. well characterized atomically-clean 2 surface (110) 2 Prepared in ultra-high vacuum (UHV) 2. wetting with water under controlled clean conditions (ultrapure water and gases).

7 Apparatus for Wetting Studies

8 Apparatus for Wetting Studies 2 (110) Crystal Mount Sessile Droplet Method

9 2 Sample Characterization Low Energy Electron Diffraction Tells if the surface is well-ordered Auger Electron Spectroscopy Tells if the surface has contaminants y Before wetting experiments The The surface surface is is not not eroded eroded by by water water droplets After wetting experiments dn/de (arb. units) Clean sample C < ML x2.5 x2.5 C ~ 0.05 ML E (ev) Non-volatile contamination introduced by bywater droplets is is very very little little

10 Hydrophilicity of Clean 2 2 (110) surface 1. Prepared as atomically-clean in UHV 2. Very pure 2 or Ar ( %) is admitted into the chamber until the pressure is 1 atm. 3. A droplet of DI water is placed onto the 2 sample. ( Sessile droplet ) 4. The picture is taken Result: The surface is fairly hydrophilic. Water contact angle is Addition of H 2 2 improves the wetting by 5-10 In In the the absence of of ambient contamination, true true contact angle of of water on on atomically-clean 2 (110) 2 must be be < 20 20

11 UV-Induced Hydrophilicity Needs xygen UV-irradiation in H2 saturated Ar atmosphere 80mW /cm2 (hν 3eV), 45min => photons/cm2 UV-irradiation in H2 saturated (Ar( 90% )/2( 10% ) atmosphere 80mW/cm 2 (hν 3eV), 5min => photons/cm2

12 Wetting of UV-Irradiated 2 (110) UV-Induced Hydrophilicity Needs xygen Region of Uncertainty

13 UV Irradiation of a Water Droplet on 2 Continuously exposing a water droplet on the 2 (110) surface to UV under the atmosphere of Ar + 10% 2 The The wetting wetting occurs occurs abruptly after after an an induction period period (delay (delay time) time)

14 Controlled Additions of Hexane to 2 40 CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 Hexane Vapor Effect on the UV-Induced Wetting of 2 (110) Contact Angle (degrees) ppm hexane 120 ppm hexane 360 ppm hexane Region of uncertainty me (s) The The wetting wetting delay delay time time increases with with increased gas gas phase phase hydrocarbon concentration as as a result result of of increased hydrocarbon flux flux at at the the surface.

15 Controlled Additions of Hexane to 2 Long delay before the the wetting occurs No No delay: Wetting starts immediately

16 Photooxidation of Hexane on 2 (110): Interpretation of the Induction Period of Wetting The The critical critical wetting wetting condition occurs occurs when when the the steady steady state state surface surface coverage of of adsorbed hydrocarbons approaches zero zero at at and and beyond beyond the the water waterdroplet perimeter as as a result result of of 2 -mediated 2 photooxidation.

17 Model of Sudden Hydrophilic Effect due to Hydrocarbon Photooxidation on 2

18 Adding Water to a Droplet Pre-exposed to UV Contact Angle for Increased H 2 Droplet Size After Slight UV Irradiation water droplet slightly exposed to UV increasing the size of the pre-exposed droplet by adding more water Result: complete wetting Water droplet shields the surface underneath from the 2 supply. The process of hydrocarbon photooxidation is slow under the water droplet. Beyond the water droplet perimeter, 2 is readily available. In there, the photooxidation of the adsorbed hydrocarbons is fast.

19 Sudden vs. Gradual Contact Angle Changes During Hydrocarbon Photooxidation on 2

20 Photooxidation Model of Photo-Hydrophilicity White et al., J. Phys.Chem. B 107, , 2003 ur ur data data are are consistent with with the the model, model, in in which which 2 as 2 as surfaces are are considered inherently hydrophilic, and and the the UV-induced wetting wetting is is a result result of of the the photocatalytic oxidation of of hydrophobic contaminants

21 Profound Effect of Adsorbates on 2 Hydrophilicity Hydrophilicity of of an an adsorbate monolayer strongly affects the the macroscopic wetting of of 2 by 2 by water Trimethylacetic acid (TMAA) Adsorption at 200 K Warming up to 298 K Trimethylacetate on 2 (110) surface presents fully hydrophobic end Pyruvic acid Pyruvate on 2 (110) surface presents partially hydrophilic end

22 Hydroxyl Model of Photo-Hydrophilicity (revisited): Lack of UV Effect on -H Groups on 2 IR spectra of 2 (polycrystalline, P-25) with adsorbed H 2 : effect of UV irradiation in vacuum in 2 UV UV irradiation of of H 2 / 2 2 does 2 not not generate isolated -H groups (to (to within ±3%)

23 Covering the 2 Surface with Extra H Groups H-covered 2 (110) surface was prepared by: 1. Annealing to 900 K 2. Adsorbing H 2 in UHV H H H H According to J.M. White et al.,j. Phys. Chem. B 107 (2003) 9029

24 Covering the 2 Surface with Extra H Groups Result: H-covered 2 (110) surface exhibits similar macroscopic wetting by water as the non-hydroxilated surface (20-30 ). hydroxilated non-hydroxilated Implication: The The hypothesis about the the UV-induced production of of H-groups as as the the primary cause of of UV-induced hydrophilicity of of 2 is 2 is under question.

25 Conclusions 1. The first observation of sudden onset of wetting when the droplet is continuously exposed to UV irradiation. 2. The wetting delay increases with increased gas phase hydrocarbon concentration. Contact Angle (degrees) Hexane Vapor Effect on the UV-Induced Wetting of 2 (110) ppm hexane Region of uncertainty T = K F hν = 5x10 18 photons cm -2 s -1 ( ev) P hν = W cm ppm hexane 360 ppm hexane me (s) 3. Explanation: A water droplet shields the 2 surface underneath from 2, which slows down self-cleaning. 4. ur results suggest that photooxidative removal of hydrocarbon adsorbates is a PRIMARY mechanism of UV-induced hydrophilicity of 2 Acknowledgements PPG Industries and DARPA / AR