Method for Reducing the Bandgap of Titanium Dioxide

Abstract

This invention describes a new method for reducing the bandgap of titanium dioxide by forming solid solutions with other dioxides that a) have either rutile or anatase crystal structure, b) exhibit either metallic or semiconducting characteristics and c) maintain stable 4+ valence during high temperature processing as well as during cooling to room temperature.

Claims

1. Solid solutions of TiO.sub.2 with MO.sub.2 where MO.sub.2 has the following characteristics; a) has a crystal structure of either rutile or anatase, b) is either metallic conductor or semiconductor and c) maintains their stable 4+ valence during processing.

2. In claim #1 MO.sub.2 is MoO.sub.2.

3. In claim #2 solid solutions of TiO.sub.2 and MoO.sub.2 are processed with the sol-gel method using organometallic compounds of and Mo.sup.4+ and fired at elevated temperatures in CO.sub.2CO gas mixtures with their CO.sub.2/CO ratio of between 10.sup.4 and 1.

Description

BRIEF SUMMARY OF FIGURES

[0009] FIG. 1: The band gap of the solid solution between TiO.sub.2 and MoO.sub.2.

[0010] The bandgap is determined by the optical diffuse scattering method. In the FIGURE a straight line is drawn to connect the bandgap of pure titanium dioxide and that of pure manganese dioxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0011] The present invention will now be described in detail. The solid solution between TiO.sub.2 and MoO.sub.2 is the only pair amenable to the conventional ceramic powder processing method. (1x)TiO.sub.2-xMoO.sub.2 with x between 0.1 and 0.6 is synthesized by mixing TiO.sub.2 and MoO.sub.2 powers. Subsequently the mixture is placed in a platinum crucible. Then the crucible is inserted in a muffle tube furnace and heated to 1,200 C. for 10 to 100 hours in flowing gas mixture of CO.sub.2 and CO. The ratio of CO.sub.2 to CO is maintained 10:1. After firing at the temperature the crucible is cooled to room temperature while maintaining the gas flow. X-ray fluorescence analyses indicate that the resulting solid solution lost a significant amount of MoO.sub.2. XRD analyses indicate that the resulting solid solution has a rutile crystal structure.

[0012] In order to minimize the volatility of MoO.sub.2 at elevated temperatures, an alternative method is also employed to obtain uniform mixtures of titanium(IV) and molybdenum(IV) ions. In the method aqueous solutions of titanium(IV) oxalate and oxy-molybdenum(IV) oxalate are mixed at a desired proportion. Then water is allowed to evaporate while the solution is continuously stirred to obtain dry cake. Subsequently the cake is ground and fired in air at 500 C. while the oxygen partial pressure of the effluent gas is monitored continuously. When the decomposition of the oxalate mixture approaches completion, the oxygen partial pressure starts to increase sharply. At this point the flow of gas is switched from air to 10:1 CO.sub.2 to CO gas mixture and the temperature is raised to 700 C. After firing at the temperature for a few hours the crucible is cooled to room temperature while maintaining the gas flow. XRD analyses indicate that the resulting solid solution has a rutile crystal structure.

[0013] Since the photo-anode of a photo-catalytic decomposition system requires a thin layer, 5 microns thick, the sol-gel method is also employed to synthesize (1x)TiO.sub.2-xMoO.sub.2. Titanium butoxide, Ti(IV)(O-Bu).sub.4, and Molybdenum butoxide, Mo(IV)(O-Bu).sub.4 are mixed at a desired proportion and allowed to form a sol in the presence of acetic acid and using acetylacetone as a chelating agent. The resulting sol is spin-coated on a metallic substrate, such as gold or platinum. After the film is dried, the coated substrate is heated in air to 400 C. while the oxygen partial pressure of effluent gas is continuously monitored. When the decomposition of the film approaches completion, the oxygen partial pressure starts to increase sharply. At this point the flow of gas is switched from air to 10:1 CO.sub.2 to CO gas mixture and the temperature is raised to 700 C. After firing at the temperature for a few hours the substrate is cooled to room temperature while maintaining the gas flow. Their band gaps are determined from the optical diffuse scattering measurements and are shown in FIG. 1.

[0014] The solid solutions between TiO.sub.2 and VO.sub.2 are synthesized as follows. The solutions of Titanium(IV) isopropoxide and Vanadium(IV) butoxide are mixed at a proper proposition to form (1x)TiO.sub.2-xVO.sub.2 with x between 0.1 and 0.6. The mixed solution is then spray-coated on a substrate, either platinum or stainless steel. After drying the film is fired in air at 600 C. for a few hours. While the film is fired at the temperature, the oxygen partial pressure of the effluent gas is monitored continuously. When the oxygen partial pressure of the effluent gas hits 10.sup.5 atm, the air flow is shut off. During the rest of the time, the oxygen partial pressure is maintained between 10.sup.3 and 10.sup.6.5 atm. The coated substrate was cooled to room temperature while the oxygen partial pressure is reduced from 10.sup.6.5 to 10.sup.40 atm. linearly with decreasing temperature. The films are found by XRD to have mixed phases of rutile and anatase. The band gaps of the films are determined by the optical diffuse scattering method and the results are similar to those in FIG. 1.

REFERENCES CITED

[0015] 1. A Fujishima and K. Honda, Nature, 238, 37 (1972) [0016] 2. Chapters 15 & 16 in Photocatlysis ed. by K. Kaneko and I. Okura (200) [0017] 3. H. C. Cassy and M. B. Panish, in Heterostructure Laser, pub. by Academic Press, New York (1978) [0018] 4. D. E. Scaife, Solar Energy 25, 41 (1980) [0019] 5. M. Anpo, Pure Appl. Chem., 72(9), 1787-92 (2000) [0020] 6. I. Song, Defect Structure and DC Electrical Conductivity of TiO.sub.2NbO.sub.2 Solid Solution, Ph. D. Dissertation, Case Western Reserve University (1990)