Mesoporous-silica films, fibers, and powders by evaporation

Abstract

This invention pertains to surfactant-templated nanometer-scale porosity of a silica precursor solution and forming a mesoporous material by first forming the silica precursor solution into a preform having a high surface area to volume ratio, then rapid drying or evaporating a solvent from the silica precursor solution. The mesoporous material may be in any geometric form, but is preferably in the form of a film, fiber, powder or combinations thereof. The rapid drying or evaporation of solvent from the solution is accomplished by layer thinning, for example spin casting, liquid drawing, and liquid spraying respectively. Production of a film is by layer thinning, wherein a layer of the silica precursor solution is formed on a surface followed by removal of an amount of the silica precursor solution and leaving a geometrically thinner layer of the silica precursor solution from which the solvent quickly escapes via evaporation. Layer thinning may be by any method including but not limited to squeegeeing and/or spin casting. In powder formation by spray drying, the same conditions of fast drying exists as in spin-casting (as well as in fiber spinning) because of the high surface-area to volume ratio of the product. When a powder is produced by liquid spraying, the particles or micro-bubbles within the powder are hollow spheres with walls composed of mesoporous silica. Mesoporous fiber formation starts with a similar silica precursor solution but with an added pre-polymer making a pituitous mixture that is drawn into a thin strand from which solvent is evaporated leaving the mesoporous fiber(s).

1. A method of making mesoporous silica materials, comprising the steps of
 (a) combining a silica precursor with an aqueous solvent, an acid and a surfactant having an ammonium cation into a silica precursor solution,    (b) templating the silica precursor with the surfactant and obtaining the mesoporous material from the templated silica precursor,    (c) forming said silica precursor solution into a preform; and    (d) rapidly evaporating said aqueous solvent from said preform for obtaining the mesoporous material, wherein the improvement comprises: 
 (i) providing said aqueous solvent in an amount resulting in complete hydrolysis and providing said acid in an amount maintaining a hydrolyzed precursor and avoiding gelation or precipitation; and  
 (ii) providing said surfactant and said silica precursor in a mole ratio that is above a lower mole ratio that produces a non-porous silica phase and below an upper mole ratio that produces a lamellar phase.  
   
     
     
       2. The method as recited in  claim 1 , wherein said lower mole ratio is about 0.05. 
     
     
       3. The method as recited in  claim 1 , wherein said upper mole ratio is about 0.3. 
     
     
       4. The method as recited in  claim 1 , wherein said acid is added in an amount resulting in a pH of said silica precursor solution of from about 1 to about 4. 
     
     
       5. The method as recited in  claim 4 , wherein said pH is about 2. 
     
     
       6. The method as recited in  claim 1 , wherein the step of forming includes diluting with an alcohol. 
     
     
       7. The method as recited in  claim 6 , wherein said alcohol is ethanol. 
     
     
       8. The method as recited in  claim 1 , wherein said aqueous solvent, said acid, and said surfactant are premixed before combining with said silica precursor. 
     
     
       9. The method as recited in  claim 1 , wherein said mesoporous material is in a geometric form selected from the group consisting of fiber, powder, and film. 
     
     
       10. The method as recited in  claim 1 , wherein said forming is spin-casting. 
     
     
       11. The method as recited in  claim 1 , wherein said forming is spraying. 
     
     
       12. The method as recited in  claim 1 , further comprising adding a pre-polymer or a polymer to said silica precursor solution making a pituitous mixture. 
     
     
       13. The method as recited in  claim 1 , wherein said forming is drawing. 
     
     
       14. The method as recited in  claim 1 , wherein said forming is squeegeeing. 
     
     
       15. The method as recited in  claim 1 , further comprising the step of adding a metal compound to the silica precursor solution. 
     
     
       16. The method as recited in  claim 15 , wherein said metal compound is selected from the group consisting of metal halide, metal nitrate, and combinations thereof. 
     
     
       17. The method as recited in  claim 16 , wherein said metal halide is a metal chloride. 
     
     
       18. The method as recited in  claim 16 , wherein said metal is selected from the group of aluminum, iron and combinations thereof. 
     
     
       19. The method as recited in  claim 1 , wherein said silica precursor is an alkoxide silica precursor or a tetrachlorosilane. 
     
     
       20. The method as recited in  claim 1 , wherein said aqueous solvent amount is characterized by a ratio of said aqueous solvent to said silica precursor of about 7. 
     
     
       21. The method as recited in  claim 1 , wherein said acid amount is characterized by a ratio of said acid to said silica precursor of about 0.1. 
     
     
       22. The method as recited in  claim 1 , further comprising adding a swelling agent to the silica precursor solution. 
     
     
       23. The method as recited in  claim 22 , wherein said swelling agent is 1,3,5-thimethylbenzene. 
     
     
       24. The method as recited in  claim 1 , further comprising the step of calcining the mesoporous material. 
     
     
       25. A method of making a mesoporous silica film, comprising the steps of
 (a) combining a silica precursor with an aqueous solvent, an acid and a surfactant having an ammonium cation into a silica precursor solution,    (b) templating the silica precursor with the surfactant and obtaining the mesoporous material from the templated silica precursor,    (c) forming said silica precursor solution into a preform; and    (d) rapidly evaporating said aqueous solvent from said preform for obtaining the mesoporous material, wherein the improvement comprises: 
 (i) said silica precursor is tetraethoxysilane;  
 (ii) providing said aqueous solvent in a superstoichiometric amount and providing said acid in an amount maintaining a hydrolyzed precursor and avoiding gelation or precipitation;  
 (iii) providing said surfactant and said silica precursor in a mole ratio that is above a lower mole ratio that produces a non-porous silica phase and below an upper mole ratio that produces a lamellar phase; and  
 (iv) said forming includes diluting with an alcohol.  
   
     
     
       26. The method as recited in  claim 26 , further comprising adding a pre-polymer or a polymer to said silica precursor solution making a pituitous mixture. 
     
     
       27. The method as recited in  claim 26 , wherein said rapidly evaporating is by spin-casting. 
     
     
       28. A method of making mesoporous silica materials, comprising the steps of
 ( a )  combining a silica precursor with an aqueous solvent, an acid and a surfactant having an ammonium cation into a silica precursor solution,      ( b )  templating the silica precursor with the surfactant and obtaining the mesoporous material from the templated silica precursor,      ( c )  forming said silica precursor solution into a preform; and      ( d )  rapidly evaporating said aqueous solvent from said preform for obtaining the mesoporous material, wherein the improvement comprises:  
 ( i )  providing said aqueous solvent in an amount resulting in complete hydrolysis and providing said acid in an amount maintaining a hydrolyzed precursor and avoiding gelation or precipitation; and    
 ( ii )  providing said surfactant and said silica precursor are in a mole ratio that is above a lower mole ratio that produces a non - porous silica phase and below an upper mole ratio that produces a lamellar phase.   
   
     
     
       29. The method as recited in  claim 28 , wherein said lower mole ratio is about  0 . 05 . 
     
     
       30. The method as recited in  claim 28 , wherein said upper mole ratio is about  0 . 3 . 
     
     
       31. The method as recited in  claim 28 , wherein said acid is added in an amount resulting in a pH of said silica precursor solution of from about  1  to about  4 . 
     
     
       32. The method as recited in  claim 31  wherein said pH is about  2 . 
     
     
       33. The method as recited in  claim 28 , wherein the step of forming includes diluting with an alcohol. 
     
     
       34. The method as recited in  claim 33 , wherein said alcohol is ethanol. 
     
     
       35. The method as recited in  claim 28 , wherein said aqueous solvent, said acid, and said surfactant are premixed before combining with said silica precursor. 
     
     
       36. The method as recited in  claim 28 , wherein said mesoporous material is in a geometric form selected from the group consisting of fiber, powder, and film. 
     
     
       37. The method as recited in  claim 28 , wherein said forming is spin- casting.   
     
     
       38. The method as recited in  claim 28 , wherein said forming is spraying. 
     
     
       39. The method as recited in  claim 28 , further comprising adding a pre- polymer or a polymer to said silica precursor solution making a pituitous mixture.   
     
     
       40. The method as recited in  claim 28 , wherein said forming is drawing. 
     
     
       41. The method as recited in  claim 28 , wherein said forming is squeegeeing. 
     
     
       42. The method as recited in  claim 28 , further comprising the step of adding a metal compound to the silica precursor solution. 
     
     
       43. The method as recited in  claim 42 , wherein said metal compound is selected from the group consisting of metal halide, metal nitrate, and combinations thereof. 
     
     
       44. The method as recited in  claim 43 , wherein said metal halide is a metal chloride. 
     
     
       45. The method as recited in  claim 43 , wherein said metal is selected from the group of aluminum, iron and combinations thereof. 
     
     
       46. The method as recited in  claim 28 , wherein said silica precursor is an alkoxide silica precursor or a tetrachlorosilane. 
     
     
       47. The method as recited in  claim 28 , wherein said aqueous solvent amount is characterized by a ratio of said aqueous solvent to said silica precursor of about  7 . 
     
     
       48. The method as recited in  claim 28 , wherein said acid amount is characterized by a ratio of said acid to said silica precursor of about  0 . 1 . 
     
     
       49. The method as recited in  claim 28 , further comprising adding a swelling agent to the silica precursor solution. 
     
     
       50. The method as recited in  claim 49 , wherein said swelling agent is  1 , 3 , 5 - thimethylbenzene.   
     
     
       51. The method as recited in  claim 28 , further comprising the step of calcining the mesoporous material. 
     
     
       52. A method of making a mesoporous silica film, comprising the steps of
 ( a )  combining a silica precursor with an aqueous solvent, an acid and a surfactant having an ammonium cation into a silica precursor solution,      ( b )  templating the silica precursor with the surfactant and obtaining the mesoporous material from the templated silica precursor,      ( c )  forming said silica precursor solution into a preform; and      ( d )  rapidly evaporating said aqueous solvent from said preform for obtaining the mesoporous material, wherein the improvement comprises:  
 ( i )  said silica precursor is tetraethoxysilane;    
 ( ii )  providing said aqueous solvent in a superstoichiometric amount and providing said acid in an amount maintaining a hydrolyzed precursor and avoiding gelation or precipitation;    
 ( iii )  providing said surfactant and said silica precursor in a mole ratio that is above a lower mole ratio that produces a non - porous silica phase and below and upper mole ratio that produces a lamellar phase; and    
 ( iv )  said forming includes diluting with an alcohol.   
   
     
     
       53. The method as recited in  claim 52 , further comprising adding a pre- polymer or a polymer to said silica precursor solution making a pituitous mixture.   
     
     
       54. The method as recited in  claim 52 , wherein said rapidly evaporating is by spin- casting.   
     
     
       55. A method of making a mesoporous film on a substrate, the method comprising the steps of:
 ( a )  combining a silica precursor with an aqueous solvent, an acid catalyst and an ammonium cationic surfactant into a precursor solution;      ( b )  dispensing said precursor solution onto the substrate;      ( c )  forming a film by evaporation of the solvent in less than  5  minutes; and      ( d )  heating the film on the substrate to a temperature sufficient to decompose the surfactant, thereby producing a mesoporous film on the substrate.     
     
     
       56. The method of  claim 55  wherein the precursor solution is a silica precursor solution and wherein the surfactant and the silica precursor solution are in a mole ratio that is above a lower mole ratio that produces a non- mesoporous silica phase and below an upper mole ratio that produces a lamellar phase.   
     
     
       57. The process of  claim 55 , wherein the film exhibits an index of refraction between  1 . 16  and that of silica. 
     
     
       58. A process to form mesostructured films, comprising:
 ( a )  preparing a precursor sol containing a soluble source of silica, an aqueous solvent, an ammonium cationic surfactant and an acid catalyst; and      ( b )  depositing the precursor sol on a substrate wherein evaporation of solvent and water is less than  5  minutes causes the formation of said mesostructured films on the substrate surface.     
     
     
       59. The process of  claim 58  wherein the aqueous solvent and the catalyst are provided in amounts that maintain a hydrolyzed precursor sol while avoiding gelation or precipitation. 
     
     
       60. The process of  claim 58  wherein the soluble source of silica is a silica precursor alkoxide or tetrachlorosilane and wherein the surfactant and the soluble source of silica are in a mole ratio that is above a lower mole ratio that produces a non- porous silica phase and below an upper mole ratio that produces a lamellar phase.   
     
     
       61. The process of  claim 58 , wherein the ammonium cationic surfactant further includes alkyl triethylammonium chloride or bromide surfactants with different chain lengths. 
     
     
       62. The process of  claim 58 , further comprising the step of calcining said film at  450 ° C. 
     
     
       63. The process of  claim 58 , wherein the precursor sol is deposited on a substrate by spin coating. 
     
     
       64. The process of  claim 58 , wherein said soluble source of silica is an alkoxide silica precursor or tetrachlorosilane. 
     
     
       65. The process of  claim 58 , wherein the films exhibit an index of refraction between  1 . 16  and that of silica. 
     
     
       66. A process to form a mesoporous structure, comprising:
 ( a )  preparing a precursor sol containing a soluble source of silica, an alcohol and water solvent, an ammonium cationic surfactant, and an acid catalyst, wherein said solvent is provided in an amount resulting in complete hydrolysis and said acid catalyst is in an amount to maintain a hydrolyzed precursor and to avoid gelation or precipitation in said precursor sol;      ( b )  forming the precursor sol into a preform;      ( c )  evaporating said solvent from the preform at a rate that forms a mesostructured material; and      ( d )  calcining the mesostructured material to form a mesoporous structure.     
     
     
       67. The process of  claim 66 , wherein said precursor sol contains alcohol which is a byproduct of hydrolysis, and said mesoporous structure is a film. 
     
     
       68. The process of  claim 66 , wherein said preform is a droplet, said alcohol is a byproduct of hydrolysis, and said sol is spray dried to form a powder. 
     
     
       69. The process of  claim 66 , wherein said drying is preformed in less than  5  minutes. 
     
     
       70. The process of  claim 66 , wherein said precursor sol contains dilutant alcohol, and wherein the mesoporous structure is a film. 
     
     
       71. The process of  claim 66 , wherein the mesoporous structure is a film and wherein the film exhibits an index of refraction of between  1 . 16  and that of silica. 
     
     
       72. The process of  claim 66 , wherein the said precursor sol contains alcohol which is a byproduct of hydrolysis, and wherein said mesostructure is a film. 
     
     
       73. The process of  claim 72 , wherein the film exhibits an index of refraction of between  1 . 16  and that of silica. 
     
     
       74. The process of  claim 66 , wherein said preform is a droplet, wherein said alcohol is a byproduct of hydrolysis, and wherein said precursor sol is spray dried. 
     
     
       75. The process of  claim 66 , wherein said evaporating is performed in less than  5  minutes. 
     
     
       76. The process of  claim 66 , wherein said soluble source of silica includes a silica alkoxide precursor or tetrachlorosilane. 
     
     
       77. A process to form a mesoporous structure, comprising:
 ( a )  preparing a precursor sol containing a soluble source of silica, an alcohol and water solvent, an ammonium cationic surfactant, and an acid catalyst, wherein said solvent is provided in an amount resulting in complete hydrolysis and said acid is in amount to maintain a hydrolyzed precursor and to avoid gelation or precipitation in said precursor sol;      ( b )  forming the precursor sol into a preform;      ( c )  evaporating said solvent from the preform at a rate that forms a mesostructured material, wherein said mesostructured material contains surfactant; and      ( d )  calcining the mesostructured material to form a mesoporous structure.     
     
     
       78. A process to form a mesostructure, comprising:
 ( a )  preparing a precursor sol containing a soluble source of silica, water and alcohol solvent, an ammonium cationic surfactant and an acid catalyst; and      ( b )  evaporating said solvent in less than  5  minutes to cause the formation of a mesostructure, wherein said mesostructure contains surfactant.     
     
     
       79. The process of  claim 78 , wherein the mesostructure is a film, and wherein the film exhibits an index of refraction of between  1 . 16  and that of silica. 
     
     
       80. A process to form a mesostructure, comprising:
 ( a )  preparing a precursor sol containing a soluble source of silica, a water and alcohol solvent, an ammonium cationic surfactant and an acid catalyst, and      ( b )  evaporating said solvent in less than  5  minutes to cause the formation of a mesostructure.     
     
     
       81. The process of  claim 80 , wherein said solvent is evaporated in less than  1  minute. 
     
     
       82. The process of  claim 80 , wherein said solvent is evaporated in less than  10  seconds. 
     
     
       83. The process of  claim 82 , wherein the mesostructure is a film, and wherein the film exhibits an index of refraction of between  1 . 16  and that of silica. 
     
     
       84. The process of  claim 80 , wherein the said precursor sol contains both dilutant alcohol and alcohol which is a byproduct of hydrolysis, and wherein said mesostructure is a film. 
     
     
       85. The process of  claim 80 , wherein said preform is a droplet, said alcohol is a byproduct of hydrolysis, and said sol is spray dried. 
     
     
       86. The process of  claim 80 , wherein the ammonium cationic surfactant further includes alkyl triethylammonium chloride or bromide surfactants with different chain lengths.