Method for photocatalytically rendering a surface of a substrate superhydrophilic, a substrate with superhydrophilic photocatalytic surface, and method of making thereof

• US 6,013,372 A
• Filed: 09/19/1997
• Issued: 01/11/2000
• Est. Priority Date: 03/20/1995
• Status: Expired due to Term

• Current Assignee: Toto Limited
• Original Assignee: Toto Limited
• Inventors: Fujishima, Akira, Hashimoto, Kazuhito, Hayakawa, Makoto, Watanabe, Toshiya, Kitamura, Atsushi, Kojima, Eiichi, Machida, Mitsuyoshi, Chikuni, Makoto, Norimoto, Keiichiro
• Primary Examiner(s): CLEVELAND, MICHAEL B
• Application Number:
  US08/933,886
• Time in Patent Office: 844 Days
• Field of Search: 427/299, 427/164, 427/165, 427/126.3, 106/287.16, 428/411.1, 428/446, 428/688, 428/689, 502/240, 134/1
• US Class Current: 428/411.1
• CPC Class Codes: A47G 1/02 Mirrors used as equipment hand, pocket, or shaving mirrors A45D42/00; optical viewing means for vehicles B60R1/00
• CPC Class Codes: C03C 17/009 Mixtures of organic and inorganic materials, e.g. ormosils and ormocers
• CPC Class Codes: C03C 17/23 Oxides C03C17/02 takes precedence
• CPC Class Codes: C03C 17/2456 Coating containing TiO2
• CPC Class Codes: C03C 17/256 Coating containing TiO2
• CPC Class Codes: C03C 17/3417 all coatings being oxide coatings
• CPC Class Codes: C03C 17/36 at least one coating being a metal
• CPC Class Codes: C03C 17/3663 specially adapted for use as mirrors
• CPC Class Codes: C03C 2217/211 SnO2
• CPC Class Codes: C03C 2217/212 TiO2
• CPC Class Codes: C03C 2217/213 SiO2
• CPC Class Codes: C03C 2217/229 Non-specific enumeration
• CPC Class Codes: C03C 2217/23 Mixtures
• CPC Class Codes: C03C 2217/251 Al, Cu, Mg or noble metals
• CPC Class Codes: C03C 2217/71 Photocatalytic coatings
• CPC Class Codes: C03C 2217/75 Hydrophilic and oleophilic coatings
• CPC Class Codes: C03C 2218/113 by sol-gel processes
• CPC Class Codes: C03C 2218/32 After-treatment
• CPC Class Codes: C04B 2111/00827 Photocatalysts; materials containing photocatalysts to avoid staining by air pollutants C04B2111/2061
• CPC Class Codes: C04B 33/00 Clay-wares monolithic refractories or refractory mortars C04B35/66; porous products C04B38/00
• CPC Class Codes: C04B 41/009 : characterised by the material treated
• CPC Class Codes: C04B 41/4537 : by the sol-gel process
• CPC Class Codes: C04B 41/5022 : with vitreous materials composition of vitreous glazes and enamels C03C; ceramic pigments C09C1/0009
• CPC Class Codes: C04B 41/5035 : Silica
• CPC Class Codes: C04B 41/5041 : Titanium oxide or titanates
• CPC Class Codes: C04B 41/505 : Tin oxide
• CPC Class Codes: C04B 41/5116 : Ag or Au
• CPC Class Codes: C04B 41/5122 : Pd or Pt
• CPC Class Codes: C04B 41/5127 : Cu, e.g. Cu-CuO eutectic
• CPC Class Codes: C04B 41/52 : Multiple coating or impregnating multiple coating or impregnating with the same composition or with compositions only differing in the concentration of the constituents, is classified as single coating or impregnation
• CPC Class Codes: C04B 41/522 : Multiple coatings, for one of the coatings of which at least one alternative is described
• CPC Class Codes: C04B 41/89 : for obtaining at least two superposed coatings having different compositions
• CPC Class Codes: C08K 3/22 : of metals
• CPC Class Codes: C09D 5/1618 : inorganic
• CPC Class Codes: C09D 7/61 : inorganic
• CPC Class Codes: C09K 3/18 : for application to surfaces to minimize adherence of ice, mist or water thereto rendering particulate materials free flowing, in general, e.g. making them hydrophobic B01J2/30; Thawing or antifreeze materials for application to surfaces used in liquids for heat-transfer, heat-exchange or heat-storage or for the production of heat or cold other than by combustion, e.g. radiator liquids, C09K5/00
• CPC Class Codes: F28F 13/18 : by applying coatings, e.g. radiation-absorbing, radiation-reflecting; by surface treatment, e.g. polishing
• CPC Class Codes: F28F 2245/02 : hydrophilic
• CPC Class Codes: G02B 1/18 : Coatings for keeping optical surfaces clean, e.g. hydrophobic or photo-catalytic films G02B1/16 takes precedence
• CPC Class Codes: Y10T 428/31504 : Composite [nonstructural laminate]

Abstract

The surface of a substrate is coated with an abrasion-resistant photocatalytic coating comprised of a semiconductor photocatalyst. Upon irradiation by a light having a wavelength of an energy higher than the bandgap energy of the photocatalyst, water is chemisorbed onto the surface in the form of hydroxyl groups (OH^--) whereby the surface of the photocatalytic coating is rendered highly hydrophilic. In certain embodiments, the surface of a mirror, lens, or windowpane is coated with the photocatalytic coating to exhibit a high degree of antifogging function. In another embodiment, an article or product coated with the photocatalytic coating is disposed outdoors and the highly hydrophilic surface thereof is self-cleaned as it is subjected to rainfall. In a still another embodiment, an article is coated with the photocatalytic coating and, when the article is soaked in, rinsed by or wetted with water, fatty dirt and contaminants are readily released without resort to a detergent.

Claims

1. A composite with a hydrophiliflable surface, comprising:

a substrate having a surface; and

a photocatalytic layer comprising a photocatalyst, said layer being bonded to the surface of said substrate wherein said photocatalyst operates after photoexcitation thereof to render the surface of said composite hydrophilic such that the surface presents, a water wettability of less than about 20°

in terms of the contact angle with water.

2. A composite according to claim 1, said composite further comprising molecules of water adsorbed onto the surface of said photocatalytic layer after photoexcitation of the photocatalyst.

3. A composite according to claim 2, wherein water is physically adsorbed onto the surface of said photocatalytic layer after photoexcitation of the photocatalyst.

4. A composite according to claim 2, wherein hydroxyl groups are bonded to the surface of said photocatalytic layer after photoexcitation of the photocatalyst.

5. A composite according to claim 1, wherein said photocatalytic layer, upon photoexcitation thereof, is resistant to adhesion by deposits and contaminants when contacted with an aqueous substance.

6. A composite according to claim 1, wherein said photocatalytic layer, after photoexcitation thereof, is resistant to the formation of water droplets on the surface of said layer.

7. A composite according to claim 1, wherein said photocatalyst operates after photoexcitation thereof to render the surface of said composite hydrophilic such that the surface presents a water wettability of less than about 10°

in terms of the contact angle with water.

8. A composite according to claim 7, wherein after photoexcitation the surface of said photocatalytic layer presents a water wettability of less than about 5°

in terms of the contact angle with water.

9. A composite according to claim 1, wherein the surface of said layer is further coated with a hydrophilic protective layer that does not interfere with the hydrophilifiability of the photocatalytic layer.

10. A composite according to claim 1, wherein said photocatalyst comprises an oxide selected from the group consisting of TiO₂, ZnO, SnO₂, SrTiO₃, WO₃, Bi₂ O₃ and Fe₂ O₃.

11. A composite according to claim 1, wherein said photocatalyst comprises the anatase form of titania.

12. A composite according to claim 1, wherein said photocatalyst comprises the rutile form of titania.

13. A composite according to claim 1, wherein said photocatalytic layer further comprises SiO₂.

14. A composite according to claim 13, wherein said photocatalyst comprises TiO₂.

15. A composite according to claim 1, wherein said photocatalytic layer comprises a coating in which particles of said photocatalyst are uniformly dispersed.

16. A composite according to claim 1, wherein said photocatalytic layer further comprises silicone having organic groups bonded to silicon atoms.

17. A composite according to claim 16, wherein said photocatalyst comprises TiO₂.

18. A composite according to claim 16, wherein after photoexcitation organic groups bonded to silicon atoms are substituted at least in part with hydroxyl groups to thereby render the surface of said layer hydrophilic.

19. A composite according to claim 1, further comprising a non-decomposable material interleaved between said substrate and said photocatalytic layer.

20. A composite according to claim 1, wherein said substrate includes alkaline metal ions or alkaline-earth metal ions, and wherein a thin film for preventing said ions from diffusing from said substrate into said photocatalytic layer is interleaved between said substrate and said photocatalytic layer.

21. A composite according to claim 20, wherein said thin film comprises silica.

22. A composite according to claim 1, wherein the thickness of said photocatalytic layer is less than about 0.2 micrometers.

23. A composite according to claim 1, wherein said photocatalytic layer further comprises a metal selected from the group consisting of Ag, Cu and Zn.

24. A composite according to claim 1, wherein said photocatalytic layer further comprises a metal selected from the group consisting of Pt, Pd, Rh, Ru, Os and Ir.

25. A composite according to claim 1, wherein said substrate is a plastic material.

26. A composite according to claim 1, wherein said substrate is a plastic material that is deformable at temperatures below sintering temperatures for titania.

27. A composite according to claim 1, wherein said substrate is the body of a motor vehicle.

28. A composite according to claim 1, wherein said substrate is glass.

29. A composite according to claim 1, wherein said substrate is a mirror.

30. A composite according to claim 1, wherein said substrate is a lens.

31. A composite according to claim 1, wherein said photocatalytic layer is substantially transparent.

32. A composite according to claim 1, wherein said photocatalytic layer, after photoexcitation, is capable of sustaining hydrophilicity for a period of at least three weeks without further photoexcitation of the photocatalytic layer.

33. A composite according to claim 1, where in said photocatalyst operates after photoexcitation thereof to render said hydrophilifiable surface hydrophilic such that it presents a water wettability of less than about 3°

in terms of the contact angle with water.

34. A composite according to claim 1, wherein said photocatalyst operates after photoexcitation thereof to render said hydrophilifiable surface hydrophilic such that it presents a water wettability of about 0°

in terms of the contact angle with water.

35. A composite according to claim 1, wherein, following photoexcitation with UV light for 1 day at an intensity of 0.004 mW/cm² the surface of said composite is hydrophilic such that it has a water wettability of less than about 10°

in terms of the contact angle with water regardless of how long the sample was kept in the dark prior to said irradiation.

36. A composite according to claim 1, wherein, following photoexcitation with UV light for 2 days at an intensity of 0.03 mW/cm² the surface of said composite is hydrophilic such that it has a water wettability of less than about 10°

in terms of the contact angle with water regardless of how long the sample was kept in the dark prior to said irradiation.

37. A composite according to claim 1, wherein, if said composite is kept in the dark for 8 days following a 2 day period of photoexcitation with UV light at an intensity of 0.03 mW/cm² the surface of said composite is hydrophilic such that it has a water wettability of less than about 10°

in terms of the contact angle with water regardless of bow long the sample was kept in the dark prior to said irradiation.

38. A composite according to claim 1, wherein, immediately after said composite has been kept in the dark for 3 weeks following photoexcitation with UV light for 1 days at an intensity of 0.5 mW/cm² the surface of said composite is hydrophilic such that it has a water wettability of less than about 10°

in terms of the contact angle with water regardless of how long the sample was kept in the dark prior to said irradiation.

39. A composite according to claim 1, wherein said hydrophilifiable surface has a pencil hardness of HB or greater.

40. A composite according to claim 31 wherein said photocatalytic layer is colorless.

41. A composite according to claim 1, wherein said composite is substantially incapable of promoting redox reactions in materials contacting said hydrophilifiable surface.

42. A composite according to claim 1, wherein said photocatalytic layer is less than 100 nm thick.

43. A composite according to claim 1, wherein said photocatalytic layer comprises SnO₂.

44. A composite according to claim 43, wherein said photocatalytic layer comprises titania.

45. A composite according to claim 1, wherein said composite is selected from the group consisting of toilet bowls, bath tubs, and wash basins.

46. A composite according to claim 1, wherein said composite is an interior member of a building.

47. A composite according to claim 1, wherein said composite is a window pane, said window pain has an interior side, and said photocatalytic surface is on said interior side of said window pane.

48. A composite according to claim 1, wherein said composite is selected from the group consisting of kitchenware, tableware, sinks, cooking ranges, kitchen hoods, and ventilation fans.

49. A composite according to claim 27, wherein after photoexcitation said hydrophilifiable surface presents a water wettability of less than about 10°

in terms of the contact angle with water.

50. A composite according to claim 28, wherein after photoexcitation said surface presents a water wettability of less than about 5°

in terms of the contact angle with water.

51. A composite according to claim 29, wherein after photoexcitation said surface presents a water wettability of loss than about 5°

in terms of the contact angle with water.

52. A composite according to claim 31, wherein after photoexcitation said surface presents a water wettability of less than about 5°

in terms of the contact angle with water.

53. A composite according to claim 16, wherein said photocatalytic layer is no more than about 0.2 um thick.

54. A method of manufacturing a composite with a hydrophilifiable surface, comprising the steps of:

providing a substrate having a surface; and

,

coating the surface of said substrate with a photoreactive layer comprising a photocatalyst, wherein said photocatalyst operates after photoexcitation thereof to render the than about 20°

in terms of the contact angle with water.

55. A method of manufacturing according to claim 54, wherein said photocatalyst operates after photoexcitation thereof to render the surface of said composite hydrophilic such that the surface presents a water wettability of less than about 10°

in terms of the contact angle with water.

56. A method according to claim 55, wherein after photoexcitation the surface of said photocatalytic layer presents a water wettability of less than about 5°

in terms of the contact angle with water.

57. A method according to claim 54, wherein said step of coating comprises the substeps of:

(a) coating the surface with a thin film of amorphous titania; and

,

(b) heating said thin film at a temperature less than the softening point of the substrate to transform said amorphous titania into crystalline titania.

58. A method according to claim 57, wherein prior to said substep (a) the substrate is coated with a thin film of silica to prevent alkaline network-modifier ions from diffusing from the substrate into said photoreactive layer.

59. A method according to claim 57, wherein said substep (a) is carried out by applying onto the surface a solution of an organic compound of titanium, followed by subjecting said compound to hydrolysis and dehydration polymerization to form said thin film of amorphous titania over the surface.

60. A method according to claim 59, wherein said organic compound of titanium is selected from the group consisting of an alkoxide of titanium, a chelate of titanium and an acetate of titanium.

61. A method according to claim 57, wherein said substep (a) is carried out by applying onto the surface a solution of an inorganic compound of titanium, followed by subjecting said compound to hydrolysis and dehydration polymerization to form said thin film of amorphous titania over the surface.

62. A method according to claim 61, wherein said inorganic compound of titanium is TiCl₄ or Ti(SO₄)₂.

63. A method according to claim 57, wherein said substep (a) is carried out by sputtering.

64. A method according to claim 54, wherein said step of coating comprising the substeps of:

(a) applying onto the surface of the substrate a suspension comprising crystalline titania particles dispersed in a precursor of amorphous silica; and

,

(b) subjecting said precursor to hydrolysis and dehydration polymerization to thereby form on said surface a photoreactive layer of titania particles bound by amorphous silica.

65. A method according to claim 64, wherein said precursor is selected from the group consisting of tetraalkoxysilane, silanol, polysiloxane having an average molecular weight of less than 3000, and mixtures thereof.

66. A method according to claim 54, wherein said step of coating, comprising the substeps of:

(a) applying onto the surface of the substrate a suspension comprising particles of silica dispersed in a solution of an organic compound of titanium;

(b) subjecting said compound to hydrolysis and dehydration polymerization to form a thin film of amorphous titania in which particles of silica are dispersed; and

,

(c) heating said film at a temperature less than the softening point of the substrate to transform amorphous titania into crystalline titania to thereby form a photoreactive coating of photocatalytic titania in which particles of silica are dispersed.

67. A method according to claim 66, wherein said organic compound of titanium is selected from the group consisting of an alkoxide of titanium, a chelate of titanium and an acetate of titanium.

68. A method according to claim 54, wherein said step of coating comprises the substeps of:

(a) applying onto the surface of the substrate a suspension comprising particles of crystalline titania and particles of silica; and

,

(b) heating said substrate at a temperature less than the softening point thereof to bond said particles to said substrate and to sinter said particles with each other to thereby form a photoreactive coating comprising sintered particles of titania and silica.

69. A method according to claim 54, wherein said step of coating comprises the substeps of:

(a) applying onto the surface of the substrate a suspension comprising particles of the anatase form of titania and particles of tin oxide; and

,

(b) heating said substrate at a temperature of less than 900°

C. to bond said particles to said substrate and to sinter said particles with each other to thereby form a photoreactive coating comprising sintered particles of titania and tin oxide.

70. A method according to claim 54, wherein said step of coating comprises the substeps of:

(a) applying onto the surface of the substrate a suspension comprising particles of tin oxide dispersed in a solution of an organic compound of titanium;

(b) subjecting said compound to hydrolysis and dehydration polymerization to thereby form a thin film of amorphous titania in which particles of tin oxide are dispersed; and

,

(c) heating said thin film at a temperature of less than 900°

C. to transform amorphous titania into crystalline titania to thereby form a photoreactive coating of photocatalytic titania in which particles of tin oxide are dispersed.

71. A method according to claim 54, wherein said step of coating comprises the substeps of:

(a) applying onto the surface of the substrate a coating composition comprising particles of a photocatalyst and a film-forming element of uncured or partially cured silicone or a precursor thereof;

(b) curing said film-forming element to form a silicone coating in which particles of the photocatalyst are uniformly dispersed, said silicone coating having organic groups bonded to silicon atoms of silicone molecules; and

,

(c) subjecting the photocatalyst to photoexcitation so that said organic groups are substituted at least in part with hydroxy groups.

72. A method of manufacturing a composite with a hydrophilifiable surface as in claim 54, wherein said photocatalytic layer is no more than about 0.2 um thick and comprises silicone or a precursor of silicone.

77. A coating composition according to claim 70, wherein said photocatalyst is titania, the composition further comprising a precursor of amorphous silica which is capable of forming a coating of amorphous silica when cured.

78. A coating composition according to claim 77, wherein said titania is uniformly dispersed in said precursor of amorphous silica.

79. A coating composition according to claim 77, wherein said precursor of amorphous silica is selected from the group consisting of tetraalkoxysilane, silanol, polysiloxane having an average molecular weight of less than 3000, and mixtures thereof.

73. A coating composition for use in forming a photocatalytically hydrophilifiable coating on a surface, said coating composition comprising a photocatalyst operable after photoexcitation thereof to render the surface of said coating hydrophilic on the order of less than about 20°

in term of the contact angle with water.

74. A coating composition in accordance with claim 73, wherein said photocatalyst is operable after photoexcitation thereof to render the surface of said coating hydrophilic on the order of less than about 10°

in terms of the contact angle with water.

75. A coating composition according to claim 73, wherein said photocatalyst is operable after photoexcitation thereof to render the surface of said coating hydrophilic on the order of less than about 5°

in terms of the contact angle with water.

76. A coating composition according to claim 73, wherein said photocatalyst is titania or a precursor of titania which is capable of forming a coating of photocatalytic titania upon curing and calcination.

80. A coating composition according to claim 76, wherein said photocatalyst is a precursor of titania, and said composition further comprises silica.

81. A coating composition according to claim 80, wherein said silica is uniformly dispersed in said precursor of amorphous titania.

82. A coating composition according to claim 80, wherein said precursor of amorphous titania is selected from the group consisting of an alkoxide of titanium, a chelate of titanium and an acetate of titanium.

83. A coating composition according to claim 76, wherein said photocatalyst is titania, and said composition further comprises silica.

84. A coating composition according to claim 83, wherein said coating composition comprises a suspension comprising particles of said titania and particles of said silica.

85. A coating composition according to claim 76, wherein said photocatalyst is titania, and said composition further comprises tin oxide.

86. A coating composition according to claim 85, wherein said coating composition comprises a suspension comprising particles of said titania and particles of said tin oxide.

87. A coating composition according to claim 76, wherein said photocatalyst is a precursor of titania, and said composition further comprises tin oxide.

88. A coating composition according to claim 87, wherein said tin oxide is uniformly dispersed in said precursor of amorphous titania.

89. A coating composition according to claim 76, wherein said photocatalyst is titania, and said composition further comprises uncured or partially cured silicone or a precursor thereof which is capable of forming a coating of silicone when cured.

90. A coating composition according to claim 89, wherein said titania is uniformly dispersed in said uncured or partially cured silicone or a precursor thereof.

91. A method for rendering the surface of a substrate hydrophilic, comprising the steps of:

providing a substrate coated with a photocatalytic layer comprising a photocatalyst; and

,

subjecting said photocatalyst to photoexcitation to thereby render the surface of said layer hydrophilic, wherein the surface of said layer presents a water wettability of less than about 20 °

in terms of the contact angle with water.

92. A method according to claim 91, wherein the surface of said layer presents a water-wettability of less than about 10°

in terms of the contact angle with water.

93. A method according to claim 91, wherein the contact angle with water is less than about 5°

.

94. A method according to claim 91, wherein molecules of water are absorbed onto the surface of said layer after photoexcitation of the photocatalyst.

95. A method according to claim 94, wherein water is physically adsorbed onto the surface of said photocatalytic layer after photoexcitation of the photocatalyst.

96. A method according to claim 94, wherein hydroxyl groups are bonded to the surface of said photocatalytic layer after photoexcitation of the photocatalyst.

97. A method according to claim 91, wherein said layer further comprises silica or silicone.

98. A method according to claim 97, wherein said layer comprises silicone in which organic groups bonded to silicon atoms are, after photoexcitation, substituted at least in part with hydroxyl groups to thereby render the surface of said layer hydrophilic.

99. A method according to claim 91, wherein said step of subjecting said photocatalyst to photoexcitation is carried out outdoors.

100. A method according to claim 91, wherein said substrate is a plastic material.

101. A method according to claim 91, wherein said substrate is a plastic material that is deformable at temperatures below sintering temperatures for titania.

102. A method according to claim 91, wherein said substrate is the body of a motor vehicle.

103. A method according to claim 91, wherein said substrate is glass.

104. A method according to claim 91, wherein said substrate is a mirror.

105. A method according to claim 91, wherein said substrate is a lens.

106. A method according to claim 91, wherein said photocatalytic layer is substantially transparent.

107. A method according to claim 91, wherein said photocatalytic layer, after photoexcitation, is capable of sustaining hydrophilicity for a period of at least three weeks without further photoexcitation of the photocatalytic layer.

111. A method according to claim 100, wherein said layer comprises silicone in which organic groups bonded to silicone atoms are, upon excitation, substituted at least in part with hydroxyl groups to thereby render the surface of said layer hydrophilic.

108. A method for preventing adherent moisture condensate or water droplets from growing on a surface, said method comprising the steps of:

providing a substrate coated with a photocatalytic layer comprising a photocatalyst; and

subjecting said photocatalyst to photoexcitation to thereby render the surface of said layer hydrophilic, wherein the surface of said lay presents a water wettability of less than about 20°

in terms of the contact angle with water, and whereby adherent moisture condensate and water droplets are caused to spread over the surface of said layer.

109. A method according to claim 108, wherein the surface is not fogged or blurred by adherent moisture.

110. A method according to claim 108, wherein said layer further comprises silica or silicone.

112. A method according to claim 108, wherein said step of subjecting said photocatalyst to photoexcitation is carried out outdoors.

113. A method according to claim 109, wherein the substrate is substantially transparent.

114. A method according to claim 109, wherein said substrate is glass.

115. A method according to claim 109, wherein said substrate is a mirror.

116. A method according to claim 109, wherein said substrate is a lens.

117. A method according to claim 108, wherein said step of subjecting said photocatalyst to photoexcitation is carried out until the water wettability of the surface of said coating becomes less than about 10°

in terms of the contact angle with water.

118. A method according to claim 117, wherein the contact angle with water is less than about 5°

.

119. A method for cleaning a substrate, comprising the steps of:

providing a substrate coated with a photocatalytic layer comprising a photocatalyst;

subjecting said photocatalyst to photoexcitation to thereby render the surface of said layer hydrophilic, wherein the surface of said layer presents a water wettability of less than about 20°

in terms of the contact angle with water; and

,

contacting said substrate with water, whereby deposits and contaminants adhering on the surface of said layer are washed away by water, and whereby contaminants in said water are prevented from adhering to said surface.

120. A method according to claim 119, wherein the surface of said layer presents a water-wettability of less than about 10°

in terms of the contact angle with water.

121. A method according to claim 120, wherein the contact angle with water is less than about 5°

.

122. A method according to claim 119, wherein said step of contacting said substrate with water is carried out by subjecting said substrate to rainfall.

123. A method according to claim 119, wherein said layer further comprises silica or silicone.

124. A method according to claim 123, wherein said layer comprises silicone in which organic groups bonded to silicon atoms are, after photoexcitation, substituted at least in part with hydroxyl groups to thereby render the surface of said layer hydrophilic.

125. A method according to claim 119, wherein said step of subjecting said photocatalyst to photoexcitation is carried out outdoors.

126. A method according to claim 119, wherein the step of subjecting said photocatalyst to photoexcitation is carried out with sunlight.

127. A method according to claim 119, wherein the step of subjecting said photocatalyst to photoexcitation is carried out with an electric lamp selected from the group consisting of fluorescent lamps, incandescent lamps, metal halide lamps, and mercury lamps.

128. A method according to claim 119, wherein said substrate is the body of a motor vehicle.

129. A method according to claim 119, wherein said photocatalytic layer is substantially transparent.

130. A method according to claim 119, wherein said photocatalytic layer, after photoexcitation, is capable of sustaining hydrophilicity for a period of at least three weeks without further photoexcitation of the photocatalytic layer.

131. A method for rendering bydrophilic a surface of a substrate in need of hydrophilification, comprising the steps of:

providing a substrate coated with a photocatalytic layer comprising a photocatalyst; and

subjecting said photocatalyst to photoexcitation to thereby render the surface of said photocatalytic layer hydrophilic, wherein the surface of said layer presents a water wettability of legs than about 20°

in terms of the contact angle with water.

132. A composite with a hydrophilifiable surface, comprising;

a substrate having a surface; and

a photocatalytic layer comprising a photocatalyst, said layer being bonded to the surface of the substrate, wherein the surface of said layer is further coated with a protective layer that does not interfere with the hydrophilifiability of the photocatalytic layer.

133. A composite with a hydrophilifiable surface, comprising:

a substrate having a surface; and

a photocatalytic layer comprising a photocatalyst, said layer being bonded to the surface of the substrate, wherein said substrate includes alkaline metal ions or alkaline-earth metal ions, and wherein a thin film for preventing said ions from diffusing from said substrate into said photocatalytic layer is interleaved between said substrate and said photocatalytic layer.