Porous tin oxide films
Initial film layers prepared from tin(II) chloride spontaneously generate open cavities when the initial film layers are thermally cured to about 400° C. using a temperature ramp of 1° C./minute to 10° C./minute while exposed to air. The openings of the bowl-shaped cavities have characteristic dimensions whose lengths are in a range of 30 nm to 300 nm in the plane of the top surfaces of the cured film layers. The cured film layers comprise tin oxide and have utility in gas sensors, electrodes, photocells, and solar cells.
1. A method, comprising:
forming an initial solution comprising i) a tin(II) salt, ii) an organic solvent capable of dissolving water, and iii) water;
aging the initial solution with agitation at a solution temperature and for a period of time sufficient to generate a film-forming solution, the film-forming solution capable of forming a cured film layer having a top surface comprising independent open cavities;
depositing the film-forming solution on a substrate, thereby forming an initial film layer disposed on the substrate, the initial film layer having a temperature T1 less than a maximum curing temperature T2; and
increasing the temperature of the initial film layer from T1 to T2 at a rate effective in forming the open cavities spontaneously while contacting the initial film layer with an oxygen-containing atmosphere, thereby forming a layered structure comprising the cured film layer;
wherein
the cured film layer comprises a tin oxide selected from the group consisting of tin(II) oxide (SnO), tin(IV) oxide (SnO 2 ), and combinations thereof,
the open cavities have respective characteristic dimensions whose lengths are in the range of 30 nm to 300 nm at the top surface of the cured film layer, the characteristic dimensions having a mean length, and
80% to 100% of the characteristic dimensions of the open cavities are within 10% of the mean length.
2. The method of claim 1 , wherein the method comprises adding to the film-forming solution, before said depositing, a separately prepared and aged auxiliary film-forming solution formed using i) an auxiliary metal salt comprising an ion of a metal other than tin, ii) an organic solvent capable of dissolving water, and iii) water, thereby forming a combined film-forming solution, and
depositing the combined film-forming solution on the substrate, thereby forming the initial film layer disposed on the substrate.
3. The method of claim 2 , wherein the auxiliary metal salt comprises an ion of a metal selected from the group consisting of Pd, Zn, Ga, In, Pt, Ce, W, Cu, and combinations thereof.
4. The method of claim 2 , wherein the auxiliary metal salt is an indium(III) salt.
5. The method of claim 4 , wherein the indium(III) salt is selected from the group consisting of indium(III) chloride, indium(III) bromide, indium(III) iodide, indium(III) nitrate, and combinations thereof.
6. The method of claim 4 , wherein the cured film layer comprises a tin:indium molar ratio between 50:50 and 100:0 based on total moles of tin and indium present in the cured film layer.
7. The method of claim 1 , wherein T1 is between 0° C. and 75° C., and T2 is between 300° C. and 800° C.
8. The method of claim 1 , wherein the rate of said increasing the temperature is between 1° C./minute and 10° C./minute.
9. The method of claim 1 , wherein said aging the initial solution is performed at a solution temperature between 0° C. and 75° C.
10. The method of claim 1 , wherein said aging the initial solution is performed for a period of time greater than 1 hour and less than 51 hours.
11. The method of claim 1 , wherein said increasing the temperature of the initial film layer comprises heating the initial film layer using a first temperature ramp from T1 to an intermediate temperature greater than T1, followed by a hold time greater than 1 minute at the intermediate temperature, followed by a second temperature ramp from the intermediate temperature to T2, wherein T2 is greater than the intermediate temperature, and wherein the first temperature ramp and the second temperature ramp have independent rates between 1° C./minute and 10° C./minute.
12. The method of claim 1 , wherein the tin(II) salt is selected from the group consisting of tin(II) chloride, tin(II) bromide, tin(II) iodide, and combinations thereof.
13. The method of claim 1 , wherein the tin oxide of the film layer is substantially tin(IV) oxide.
14. The method of claim 1 , wherein the top surface of the substrate comprises a patterned conductive metal layer disposed on an insulating layer.
15. The method of claim 1 , wherein the film layer has a thickness between 1 nm and 600 nm.
16. The method of claim 1 , wherein the openings constitute at least 10% of the volume of the film layer.
17. The method of claim 1 , wherein the solution has a tin(II) salt concentration of 0.01 to 1.0 g/mL.
18. The method of claim 1 , wherein the water is used in an amount of 1 to 4 molar equivalents based on total moles of the tin(II) salt.
19. The method of claim 1 , wherein the organic solvent has a boiling point above 75° C. at one atmosphere pressure.
20. The method of claim 1 , wherein the organic solvent is an alcoholic solvent.