Liquid Solid
2.3 Photocatalytic Coatings
The second class of self-cleaning surfaces to be discussed differ from otu -Effect coatings in that they are hydrophilic rather than hydrophobic, and do not rely s
L s
olely on the flow of water to wash away dirt. These coatings chemically break ight, a process known as ‘photocatalysis’, although of course
down dirt when exposed to l
it is the coating not the incident light that acts as a catalyst. While the Lotus-Effect was inspired by the self-cleaning properties of plant leaves,hydrophilic coatings have parallels with photosynthesis, using sunlight to drive chemistry. Despite the commercialisation of a hydrophilic self-cleaning coating in a number of products, the field is far from mature; investigations into the fundamental mechanisms of self-cleaning and characterisations of new coatings are regularly published in the primary literature.
In 2001 Pilkington Glass announced the development of the first self-cleaning windows, Pilkington Activ , and in the following months several other major glass companies released similar products, including PPG’s Sunclean . As a result, glazing is perhaps the largest commercialisation of self-cleaning coatings to date. All of these windows are coated with a thin transparent layer of titanium dioxide (titania or TiO2), a coating which acts to clean the window in sunlight through two distinct properties: photocatalysis causes the coating to chemically break down organic dirt adsorbed onto the window, while hydrophilicity causes water to form ‘sheets’ rather than droplets – contact angles are reduced to very low values in sunlight (the coating becomes ‘super-hydrophilic’),and dirt is washed away. Titania has become the material of choice for self-cleaning windows, and hydrophilic self-cleaning surfaces in general, because of its favourable physical and chemical properties. Not only is titania highly efficient at photocatalysing dirt in sunlight and reaching the
superhydrophilic state, it is also non-toxic, chemically inert in the absence of light, inexpensive, relatively easy to handle and deposit into thin films and is an established household chemical – it is used as a pigment in cosmetics and paint and as a food additive. The latter point may explain the rapid transition of self-cleaning titania surfaces from the research laboratory to the marketplace. The mechanisms of the self-cleaning processes that occur on titania surfaces have been thoroughly investigated over the past decade,[85] and although research continues to describe the exact mechanism for the destruction of specific pollutants,[86–88] a basic model has gained wide acceptance. A thorough discussion of the theory of photocatalysis and super-hydrophilicity is beyond the scope of this article, hence only a brief summary follows. Greater detail can be found in one of several review articles on the subject.[85,89–91] A semiconductor under normal conditions, titanium dioxide absorbs light with energy equal to or greater than its band gap energy, resulting in excited charge carriers: an electron, e-, and a hole, h+ (Fig. 2-31). Although the fate of most of these charge carriers is rapid recombination, some migrate to the surface.
There, holes cause the oxidzation of adsorbed organic molecules while electrons eventually combine with atmospheric oxygen to give the superoxide radical, which quickly attacks nearby organic molecules. The result is a cleaning of the surface by
‘cold combustion’, the conversion of organic molecules to carbon dioxide and water (and other products if heteroatoms are present) at ambient temperatures. This process is remarkably effective and clean; e.g. the total decomposition of stearic acid [CH3(CH2)16CO2H] in the presence of atmospheric oxygen to CO2 and H2O, A wide range of solid-, liquid- and gas-phase organic pollutants can be broken down in this way, including aromatics, polymers, dyes and surfactants,[85] although a much smaller range of inorganic materials have been successfully decomposed on titania.
Photocatalysis is usually tested by monitoring the destruction of a model pollutant.
Figure 2-31 Upon irradiation of TiO2 by ultra band gap light, the semiconductor undergoes photo-excitation. The electron and the hole that result can follow one of several pathways: (a) electron–hole recombination on the surface; (b) electron–hole recombination in the bulk reaction of the semiconductor; (c) electron acceptor A is
resulting in oxygen vacancies. These can be filled by adsorbed ater, resulting in surface hydroxide groups that make the wetted surface more reduced by photogenerated electrons; and (d) electron donor D is oxidised by photogenerated holes.
Super-hydrophilicity in TiO2 is also a light-induced property. [92] Holes produced by photo-excitation of the semiconductor oxidise lattice oxygen at the surface of the material,
w
favou
2
rable compared to the dry surface, lowering the static contact angle to almost 0。 after irradiation. [93] Both self-cleaning properties of TiO2 are therefore governed by the absorption of ultra band gap light and the generation of electron/hole pairs. The band gap of bulk anatase TiO2 is 3.2 eV, corresponding to light of wavelength 390 nm – near-ultraviolet (UV) light. The physical and chemical properties of TiO depend greatly on its form and the method of preparing the sample. Several polymorphs of TiO2 are known, the most significant of which are rutile and anatase.
The positions of the conduction and valence bands relative to key redox potentials cause pure anatase to be very photoactive (photocatalytic and super-hydrophilic) while pure rutile is less so.[85,94] As highlighted by a recent Royal Society report, nanoscale (dimensions in the range 1–10 nm) and micro- or macro-scale titania show distinctly different properties. Semiconductors with physical dimensions of the order of the wavelengths of the electrons they contain display properties not observed in the bulk solid. Such materials are often referred to as nanoparticles, nanocrystals or quantum dots. Bulk or macroscale powder titania is a brilliant white solid, non-toxic and widely used as a pigment in paint, cosmetics and food, while nanoscale titania particles are used in sunscreens and their absorption properties.
Refer
[1] A. Dupré, Theorié mécanique de la chleur, Paris, 1869, p.368.
[2] T. oung, Phil Trans. R. Soci. Lond. 1805, 95, 65.
[3] A. S. Michaeles, S. W. Dean, Jr., J. Phys. Chem. 1962, 66, 34.
aily, Proc. 2nd Int. Congr. Surf. Act., 1957, 3, 189.
ies of Matter, 8th
, 1418.
[9] bility and Adhesion, Advances in Chemistry
ashington, DC, 1964, p.
, Polymer Science and
[11
[12 H. Kaelble, J. Adhesion 1970, 2, 50.
[14 and
80, p. 53.
hem. Prod. Res. Dev. 1978, 17, 3.
afa, J. Colloid Interface Sci. 1980, 78,
[17
, ences
Y
[4] G. L. J. B
[5] J. H. Poynting, J. J. Thompson, A Textbook of Physics: Propert ed., Charles Griffin, London, 1920.
[6] L. A. Girifalco, R. J. Good, J. Phys. Chem. 1957, 61, 904.
[7] R. J. Good, L. A. Girifalco, G. Kraus, J Phys. Chem. 1957, 62 [8] R. J. Good, L. A. Girifalco, J. Phys. Chem. 1960, 64, 561.
F. W. Fowkes, in Contact Angle, Wetta
Series 43, R. F. Gould, Ed., American Chemical Society, W 99.
[10] F. W. Fowkes, in Adhesion and Adsorption of Polymers
Technology, vol. 12A, L. H. Lee, Ed., Plenum Press, New York, 1980, p. 43.
] D. K. Owens, R. C. Wendt, J. Appl. Polym. Sci. 1969, 13, 1741.
] D.
[13] S. Wu, J. Polym. Sci., Part C 1971, 34, 19.
] S. Wu, in Adhesion and Adsorption of Polymers, Polymer Science Technology, vol. 12A, L. H. Lee, Ed., Plenum Press, New York, 19
[15] F. M. Fowkes, M. A. Mostafa, Int. Eng. C [16] F. M. Fowkes, D. C. McCarthy, M. A. Most
200.
] C. J. van Oss, M. K. Chaudhury, R. J. Good, Chem. Rev. 1988, 88, 927.
[18] C. J. van Oss, L. Ju, M. K. Chaudhury, R. J. Good, J. Colloid Interface Sci. 1989
128, 313.
kuni, E. Kojima, A. Kitamura, M.
388, 431.
[24] R. J. Good, C. J. van Oss, in Modern Approaches to Wettability, M. E. Schrader and G. I. Loeb, Eds., Plenum
[25] R. J. Good, in Contact Angle, Wetting, and Adhesion, K. L. Mittal, Ed., VSP, xxx 1993, p. 3.
[26] H. W. Fox, W. A. Zisman, J. Colloid Sci. 1950, 5, 514.
] H. W. Fox, W. A. Zisman, J. Colloid Sci. 1952, 7, 109.
[28] H. W. Fox, W. A. Zisman, J. Colloid Sci. 1952, 7, 428.
] D. G. LeG
[30] D. G. LeGrand, G. L. Gaines, Jr., J. Colloid Interface Sc [31] S. Wu, J. Colloid Interface Sci. 1969, 31, 153.
[32] S. Wu, J. Macromol. Sci. 1974, C10, 1.
[33] H. Schonhorn, F. W. Ryan, J. Phys. Chem. 1966, 70, 3811.
[34] A. K. Rastogi, L. E. St. Pierre, J. Colloid Interface Sci. 1969, 31, 168.
[35] R. Wang, K. Hashimoto, A. Fujishima, M. Chi Shimohigoshi, T. Watanabe, Nature 1997,
[36] W. Chen, A. Y. Fadeev, M. C. Hsieh, D. Õner, J. Youngblo Langmuir 1999, 15, 3395.
[37] L. Feng, S. Li, Y. Li,; H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, D. Zhu,
Adv. Mater. 2002, 14, 1857.
[38] R. Blossey, Nat. Mater. 2003, 2, 301.
] A. Lafuma, D. Quéré, Nat
[39 . Mater. 2003, 2, 457.
artz, Langmuir 2000, 16, 2957.
, 1723.
1.
osition 2002, 8, 47.
3.
11,
[49 ima, K. Abe, K. Hashimoto, T. Watanabe, Thin Solid Films 2000, 376,
[51 a, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir
[52 rangoni, Ann. Phys. Chem. 1871, 143, 337.
ies vol 43) (Washington, DC: American Chemical Society) 1964 [40] A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Adv. Mater. 1999, 11,
1365.
[41] J. T. Woodward, H. Gwin, D. K. Schw
[42] T. Sun, W. Song, L. Jiang, Chem. Commun. 2005
[43] T. Nishino, M. Meguro, K. Nakamae, M. Matsushita, Y. Ueda, Langmuir 1999, 15, 432
[44] T. Onda, S. Shibuichi, N. Satoh, K. Tsujii, Langmuir 1996, 12, 2125.
[45] Y. Wu, H. Sugimura, Y. Inoue, O. Takai, Chem. Vap. Dep
[46] K. Tsujii, T. Yamamoto, T. Onda, S. Shibuchi, Angew. Chem. Int. Ed. Engl. 1997, 36, 1011.
[47] K. Tadanaga, N. Katata, T. Minami, J. Am. Ceram. Soc. 1997, 80, 321
[48] A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Adv. Mater. 1999, 1365.
] A. Nakaj 140.
[50] J. Bico, C. Marzolin, D. Quéré, Europhys. Lett. 1999, 47, 220.
] M. Miw 2000, 16, 5754.
] C. Ma
[53] P. G. De Gennes, Rev. Mod. Phys. 1985, 57, 827.
[54] E. G. Shafrin, W. A. Zisman, Contact Angle, Wettability and Adhesion (Advances in Chemistry Ser
pp 145
[55] C. Neinhuis, W. Barthlott, Ann. Bot. 1997, 79, 667.
[57
[58 ner, R. Fürstner, W. Barthlott, C. Neinhuis, J. Exp. Bot. 2003, 54, 1.
6, 3, 213.
ir 2004, 20, 7665.
.
ucoules, J. P. S. Badyal, Langmuir 2003,
Mert, Science 2003, 299, 1377.
. Zhang, C. C. Han,
[68 Lau, J. Bico, K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I.
[56] C. Neinhuis, W. Barthlott, Planta 1997, 202, 1.
] R. Blossey Nat. Mater. 2003, 2, 301.
] P. Wag
[59] T. Wagner, C. Neinhuis, W. Barthlott, Acta Zool. 199 [60] W. Lee, M. K. Jin, W. C. Yoo, J. K. Lee, Langmu [61] X. Gao, L. Jiang, Nature 2004, 432 36
[62] A. R. Parker, C. R. Lawrence, Nature 2001, 414 33 [63] R. Blossey Nat. Mater. 2003, 2, 301.
[64] M. Morra, E. Occhiello, F. Garbassi, Langmuir 1989, 5, 872.
[65] I. Woodward, W. C. E. Schofield, V. Ro
Milne, G. H. McKinley, K. K. Gleason, Nano. Lett. 2003, 3, 1701.
[69] L. Feng, S. Li,Y. Li, H. Li, L. Zhang, J. Zhai,Y. Song, B. Liu, L. Jiang, D. Zhu, Adv. Mater. 2002, 14, 1857.
[70] H. Li, X. Wang, Y. Song, Y. Liu, Q. Li, L. Jiang, D. Zhu, Angew. Chem. Int. Ed.
2001, 40, 1743.
[71] L. Feng, S. Li, H. Li, J. Zhai, Y. Song, L. Jiang, D. Zhu, Angew. Chem. Int. Ed.
2002, 41, 1221.
[72] C. Neinhuis, W. Barthlott, New Phytologist 1998, 138, 91.
[73 , Europhys. Lett. 2000, 52, 165.
[75 . Phys. Chem. 1949, 53, 1466.
2002, 206, 41.
, 40, 546.
and Adhesion, Chemical Society, Washington DC, 1964,
[84
[85 unte, J. Photochem. Photobiol., A: Chem., 1997, 108,1 . Photochem Photobiol., A:
04, 50, 1017.
hys.,
[89 Martin, W. Y. Choi and D. W. Bahnemann, Chem. Rev.,
[91 , K. Hashimoto and T. Watanabe, TiO2 Photocatalysis Fundamentals ] S. Herminghaus
[74] R. N. Wenzel, Ind. Eng. Chem. 1936, 28, 988.
] R. N. Wenzel, J
[76] A. Lafuma and D. Quéré, Nat. Mater. 2003, 2, 457.
[77] J. Bico, U. Thiele and D. Quéré, Colloids Surf. A [78] N. Patankar, Langmuir 2003, 19, 1249.
[79] A. Marmur, Langmuir 2003, 19, 8343.
[80] A. B. D. Cassie, S. Baxter Trans. Faraday Soc. 1944
[81] R. E. Johnson, R. H. Dettre, in Contact angle, Wettability Advances in Chemistry Series, American
vol. 43, pp. 112–135.
[82] D. Öner, T. J. McCarthy, Langmuir 2000, 16, 7777.
[83] Weimin Liu J. Am. Chem. Soc. 2005, 127, 15670 ] Jiang L. et al. Angew Chem. Int. Ed. 2004, 43, 357 ] A. Mills and S. Le H
[86] A. Sobczynski, L. Duczmal and W. Zmudzinski, J Chem., 2004, 213, 225
[87] H. Ibrahim and H. de Lasa, React. Kinet. Catal., 20
[88] C. H. Kwon, H. Shin, J. H. Kim, W. S. Choi and K. H. Yoon, Mater. Chem. P 2004, 86, 78
] M. R. Hoffman, S. T.
1995, 95, 69.
[90] A. Hagfeldtt and M. Gratzel, Chem. Rev., 1995, 95, 49.
] A.Fujishima
and Applications, BKC, Tokyo, 1999.
[92 Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M.
, 160, 213. ] R. Wang, K.
Shimohigoshi and T. Watanabe, Nature, 1997, 338, 431
[93] A. Mills, A. Lepre, N. Elliot, S. Bhopal, I. P. Parkin and S. A. O’Neill, J.
Photochem. Photobiol., A: Chem., 2003
[94] B. Sun and P. G. Smirniotis, Catal. Today, 2003, 88, 49.
Chapter 3 Experimental
3.1 Part A
3.1.1 Materials
1. 2,2-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine) propane(BA-a)
：Shikoku Chemicals Corp.,Japan
O O
N N
2. TiO2 nanoparticles , AEROXIDE® TiO2 P25, were purchased from the Degussa Corporation. The size of nanoparticle is a 21nm
3. Tetrahydrofurane ( THF )：J.T. Baker，100％：FW=72.11，d=0.886 g/cm3，
b.p.=66℃
4. Aluminum Alloys: Pan-Folks Corporation,Taiwan:96%
5. Ethanol：TEDIA，99.9％：FW=46.07，d=0.789 g/cm3，b.p.= 78.4℃
6. Acetone：LEDA，99.5％：FW=58.08，d=0.79 g/cm3，b.p.=56.29℃
Roughness Process by Sandblasting with Vairous Microscale
Aluminum substrate
Polybenzoxazine Benzoxazine Monomer/nanoparticles
Composite
Thermal curing
nanoparticles
Figure 3.1. Flow diagram of the processing of superhydrophobic films by sandblasting.
3.1.2 Fabrication of Superhydrophobic Surfaces by Sandblasting
Super-hydrophobic coating on a Al substrate was performed through a two-step process. Firstly, Aluminum specimens with a size of 5 cm ╳ 5 cm ╳ 0.1 cm were cut from a rolled aluminum sheet(96.0%). Then they were etched by sandblasting with varuous roughness. The roughness of Al surface to was changed as 0.4 , 0.8 , 1 , 2 , 4 , and 6 µm to control the super- hydrophobic property of Al surface.
Secondly ,the BA-a benzoxazine (0.5 g) was mixed with nanoparticles (0.5 g) in tetrahydrofuran (THF) (10 mL). After keeping the solutions in ultrasound for 30min, the mixture was spin-coated on a Al sheet (50 × 50 × 1 mm) at 1500 rpm for 45 s and then cured in an oven at 240 °C for 1 h. The flow diagram of the film processing is shown in Figure 3.1
3.2 Part B
3.2.1 Materials
1. 2,2-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine) propane(BA-a)
：Shikoku Chemicals Corp.,Japan
O O
N N
2. Silica nanoparticles, Tokusil 233G, were kindly provided by Oriental Silicas Corp. The nanoparticle is a precipitate hydrated silica with a ultimate particle size of 22 nm.
3. TiO2 nanoparticles , AEROXIDE® TiO2 P25, were purchased from the Degussa Corporation. The nanoparticle is a 21nm
4. Tetrahydrofurane ( THF )：J.T. Baker，100％：FW=72.11，d=0.886 g/cm3，b.p.=66℃
5. Aluminum Alloys: Pan-Folks Corporation,Taiwan:96%
6. Ethanol：TEDIA，99.9％：FW=46.07，d=0.789 g/cm3，b.p.= 78.4℃
7. Acetone：LEDA，99.5％：FW=58.08，d=0.79 g/cm3，b.p.=56.29℃
Roughness Process by Sandblasting
Figure 3.2. Flow diagram of the processing of superhydrophobic films with photocatalytic coatings
3.2.2 Photocatalytic Coatings on Superhydrophobic Surfaces
Super-hydrophobic coating on a Al substrate was performed through a four-part process(○1 ○2 ○3 ○4 ). Firstly, Aluminum specimens with a size of 5 cm × 5 cm × 0.1 cm were cut from a rolled aluminum sheet(96.0%). Then they were etched by sandblasting with the roughness is 6 µm.
Secondly ,the BA-a benzoxazine (0.5 g) was mixed with nanoparticles (TiO2
and SiO2)(0.5 g) in tetrahydrofuran (THF) (10 mL) respectively. After keeping the solutions in ultrasound for 30min, the mixture was spin-coated on a Al sheet (50 × 50 × 1 mm) at 1500 rpm for 45 s and then cured in an oven at 240 °C for 1 h.
Subsequently, the polybenzoxazine hybrid surface(○2 ○4 ) was modified with the
pure BA-m polybenzoxazine film. BA-m benzoxazine solution having 0.05%
concentrations was spin-coated onto a rough surface for 45 s at 1500 rpm then been cured at 240 °C for 60 mins.The flow diagram of the film processing is shown in Figure 3.2
3.3 Part C
Preparation of PVPh/PMMA Random and Block Copolymers and Blends.
The detailed synthesis procedures of PVPh-r-PMMA and PVPh-b-PMMA copolymers have been reported previously[1]. Table 3-1 lists the characterizations of PVPh, PMMA, and PVPh/PMMA random and block copolymers. Various binary PVPh/PMMA blend compositions were prepared by solution-casting. A THF solution containing 5 wt % polymer was stirred for 6-8 h and then cast onto a wafer. The solution was left to evaporate at 60°C for 1 day and dried in vacuum at room temperature for 2 days. The thermal treatment was carried out by placing the as-prepared polymer film in a vacuum oven at 180 °C for 24 h and then quenching to ambient temperature
Table 3-1. Formulations and thermal properties of PVPh-co-PMMA copolymers and PVPh92-r-PMMA8 92 16000 1.67 173.7 362.1 PVPh10-b-PMMA90 10 37000 1.15 143.1 373.3 PVPh30-b-PMMA70 30 16000 1.11 155.0 372.0 PVPh44-b-PMMA56 44 16000 1.15 163.4 370.5 PVPh55-b-PMMA45 55 30000 1.10 164.5 368.0
PVPh75-b-PMMA25 75 22000 1.13 176. 7 365.0
PVPh10/PMMA90 10 124.4 359.9
PVPh30/PMMA70 30 135.5 353.1
PVPh50/PMMA50 50 147.8 351.8
PVPh70/PMMA30 70 159.2 345.9
PVPh90/PMMA10 90 166.8 343.6
PVPh 100 20000 186.8 1.07 372.1
3.4 Experimental Equipments