Array of titanium dioxide nanostructures for solar energy utilization
An array of titanium dioxide nanostructures for solar energy utilization includes a plurality of nanotubes, each nanotube including an outer layer coaxial with an inner layer, where the inner layer comprises p-type titanium dioxide and the outer layer comprises n-type titanium dioxide. An interface between the inner layer and the outer layer defines a p-n junction.
1. A titanium dioxide nanostructure for solar energy utilization, the nanostructure comprising:
a nanotube comprising an outer layer coaxial with an inner layer, the inner layer comprising p-type titanium dioxide and the outer layer comprising n-type titanium dioxide, an interface between the inner layer and the outer layer defining a p-n junction.
2. The nanostructure of claim 1 , wherein the inner layer comprises a thickness of between about 5 nm and 40 nm.
3. The nanostructure of claim 1 , wherein the inner layer comprises titanium dioxide doped with nitrogen.
4. The nanostructure of claim 3 , wherein the nitrogen is present in an amount that varies along a radial direction.
5. The nanostructure of claim 4 , wherein the amount of nitrogen decreases in the radial direction away from a center of the nanotube.
6. The nanostructure of claim 3 , wherein the nitrogen is present in an amount that is substantially uniform along a radial direction.
7. The nanostructure of claim 3 , wherein the nitrogen is present in both substitutional and interstitial sites of the titanium dioxide.
8. The nanostructure of claim 7 , wherein a majority of the nitrogen is present in the substitutional sites.
9. The nanostructure of claim 1 , wherein the outer layer comprises substantially undoped titanium dioxide.
10. The nanostructure of claim 9 , wherein the outer layer comprises a thickness of between about 5 nm and 40 nm.
11. The nanostructure of claim 1 , wherein the nanotube has an inner diameter of between about 10 nm and 80 nm.
12. An array of titanium dioxide nanostructures for solar energy utilization, the array comprising:
a plurality of nanotubes, each nanotube comprising an outer layer coaxial with an inner layer, the inner layer comprising p-type titanium dioxide and the outer layer comprising n-type titanium dioxide, an interface between the inner layer and the outer layer defining a p-n junction.
13. The array of claim 12 , wherein the plurality of nanotube structures exhibit a light absorption of visible light by shifting from 400 nm to 625 nm.
14. A method of making an array of titanium dioxide nanostructures for solar energy utilization, the method comprising:
creating an array of nanotubes comprising titanium dioxide;
exposing the array of nanotubes to a gas comprising a dopant element; and
controllably doping each of the nanotubes to form an inner layer comprising p-type titanium dioxide and an outer layer coaxial with the inner layer that comprises n-type titanium dioxide, where an interface between the inner layer and the outer layer defines a p-n junction.
15. The method of claim 14 , wherein creating the array of titanium dioxide nanotubes comprises anodizing a titanium foil.
16. The method of claim 15 , wherein the anodizing is carried out at a voltage between 10 V and 100 V.
17. The method of claim 16 , wherein the voltage is between about 40 V and 60 V.
18. The method of claim 14 , wherein the anodizing is carried out at an anodizing temperature of between about −20° C. and about 20° C.
19. The method of claim 14 , wherein the dopant element is nitrogen.
20. The method of claim 14 , wherein exposing the array to the gas comprises flowing the gas through the array at a flow rate of between about 2 milliliters/hour and 10 liters/hour.
21. The method of claim 14 , further comprising heating the array at an annealing temperature of between about 100° C. and 700° C. during the exposure to the gas.
22. The method of claim 21 , wherein the annealing temperature is at least about 500° C.
23. The method of claim 14 , wherein the gas is NH 3 .