Abstract:
A gas ionizer includes a photocatalyst activated with an electric field to emit electrons. The photocatalyst is also illuminated with an ultraviolet light source. The ionized gas is passed through a chamber between the photocatalyst and the ultraviolet light source. The photocatalyst may be titanium oxide.

Description:
This application for patent claims priority to U.S. Provisional Patent Applications Ser. Nos. 60/891,927, 60/941,858, and 60/844,761 which are hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     A source of gas ions is needed for commercial and residential air handling units. It is known that the presence of gas ions may improve the health and attitude of people exposed to this ion source. It is also known that this gas ion source not produce or contain ozone as this is generally considered hazardous to health. Most air handling equipment has high flow rates and pushes large volumes of air. 
     U.S. provisional application Ser. No. 60/844,761 (the “&#39;761 application”), which is hereby incorporated by reference herein, disclosed the use of carbon nanotubes operating in a field emission mode as a source of ions operating at atmospheric pressure. 
     The &#39;761 application describes how gas ions are formed at atmospheric pressure by placing a carbon nanotube film on one or both electrodes and then biasing these electrodes while gas is flowing between them (see FIG. 3 of the &#39;761 application). The bias between the electrodes can be in DC mode or in AC mode. Even a series of electrodes can be used (see FIG. 4 of the &#39;761 application). In order for the carbon nanotubes to emit electrons, the electric field applied to the carbon nanotube layer is on the order of 1 V/micron. This requires small gaps or high electrical potentials on the electrode surfaces. For example, a 1 mm gap requires a voltage of 1000V on the electrodes. This is acceptable for small flow rates such as needed in analytical equipment, but will not work well for applications requiring high gas flow rates. 
     It is also well known that titanium dioxide (also referred to as titanium oxide, titania or TiO 2 ) can be used to decontaminate air (see, Tracy L. Thompson and John T. Yates. Jr., “Surface science Studies of the Photoactivation of TiO 2 —New Photochemical Processes,” Chem. Rev. Vol. 106, pp. 4428-4453, (2006); and S. Banerjee et al., “Physics and chemistry of photocatalytic titanium dioxide: Visualization of bactericidal activity using atomic force microscopy,” Current Science, Vol. 90, p. 1378, May 2006). TiO 2  is known as a photocatalyst, especially the anatase phase of this material.  FIG. 1  shows one mechanism how the titania photocatalyst works. When titanium oxide is exposed to ultraviolet rays, electron and hole pairs are created in the titanium oxide material. These charges diffuse to the surface of the titania. Active species such as oxygen radicals are generated on the surface. The active oxygen radicals oxidize organic contaminants including acetaldehyde (cigarette smell) and biologicals in almost the same way as combustion: converting contaminants into harmless water and carbon dioxide. Other active radicals (both positive-charged and negative-charged) are also formed that may attack other contaminants (not shown). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates how a titania photocatalyst functions; 
         FIG. 2A  illustrates a rectangular column structured titania oxide photocatalyst of anatase type; 
         FIG. 2B  illustrates a photocatalyst treated by a slurry coating method using a binder; 
         FIG. 3  illustrates an embodiment of the present invention; 
         FIG. 4  illustrates another embodiment of the present invention; 
         FIG. 5  illustrates another embodiment of the present invention; and 
         FIG. 6  illustrates another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Titania can have several physical forms. Some forms are round or spherical and some forms are columnar with sharp ends or edges.  FIGS. 2A and 2B  show examples of these two forms.  FIG. 2A  is a scanning electron microscope image of titania that is in a columnar structure that has sharp edges.  FIG. 2B  is a scanning electron microscope image of a form of this material that has more spherical particles. The forms that are columnar may be suitable for field emission of electrons, similar to how carbon nanotubes are used to field emit electrons at atmospheric pressure as described in the &#39;761 application. Other patents describe the use of carbon nanotubes for field emitters in vacuum applications (RE38,223 and RE38,561). Metal oxides such as zinc oxide (ZnO) have been proposed as field emitters for vacuum applications (see, C. X. Xu and X. W. Sun, Appl. Phys. Lett., Vol. 83, p. 3806, November, 2003, “Field emission from zinc oxide nanopins”). Zinc oxide, Si and similar materials are low band gap semiconductors and thus can be used as field emitters. Other metal oxides and diamond materials are considered as wide band gap semiconductors (2.5-7.0 electron volts wide). Metal oxides such as tin oxide (SnO 2 ), with a band gap of about 3.6 electron volts, and titanium oxide (TiO 2 ) with a band gap of about 3.0-3.2 electron volts, are considered wide band gap semiconductors. Prior art teaches that they would not make good field emitters unless they are made very small, as it would be difficult to get a charge from the conducting electrode that the semiconductor is attached to the surface of the wide band gap semiconductor. As an example, the patents by Kumar (U.S. Pat. Nos. 5,536,193, 5,199,918 and 5,341,063) teach that the distance between the injection surface and the emission tip be closer than the mean free path of electrons in the emission tip material, which is expected to be on the order of 20-50 angstroms and certainly in the range of 10-100 angstroms. Some inventors propose methods around this by coating a conducting field emitter structure with nanometer-sized particles of insulator materials such as titania, silica, etc. (see U.S. Pat. No. 6,342,755) or mixtures of insulating and conducting materials. But even in this example, the mixture is characterized as a mixture of “electron emitting materials and insulating materials.” The &#39;755 patent teaches that the insulating materials are not included in the list of electron emitting materials. The Kumar patents do teach field emitters from wide band gap materials, but only if they are protruding above a conducting matrix on the order of the electron mean free path in the materials as described above. 
     One reference mentions the use of titanium oxide as a field emitter (“Fabrication and Field Emission Characteristics of Highly Ordered Titanium Oxide Nanodot Arrays”, Po-Lin Chen, Wen-Jun Huang, Jun-Kai Chang, Cheng-Tzu Kuo, and Fu-Ming Pan, Electrochem. Solid-State Lett., Volume 8, Issue 10, pp. H83-H86 (2005)) In this example, nanometer-sized dots of titania are coated onto a conductor; they used a p-doped Si substrate. Chen et al. also teach that the TiO 2  film be thermally annealed in a vacuum environment at 450° C. for 2 hours to introduce oxygen defects and vacancies to promote oxygen diffusion in order to reduce the electrical resistance of the titanium oxide film and thus improve the field emission properties. Furthermore, the size of the particles is on the order of 10 nm-100 nm. Tatarenko et al. (“Novel nanoscale field emission structures: Fabrication technology, experimental, and calculated characteristics,” N. I. Tatarenko, et al., J. Vac. Sci. Technology B., Vol. 17, March 1999, p. 647) describes a similar experiment where the growth of the TiO 2  layer was only 200 nm thick. U.S. Pat. No. 6,806,630 also describes a field emitter in which the surface of the emitter is coated with a TiO 2  film. Birecki et al. teach specifically that the thickness of the TiO 2  layer be between 2 to 8 nm thick, with 5 nm being the optimal thickness. Because the size or thickness of the titania is so small, electrons are able to tunnel through the insulating layer or hop across the surface of the insulating layer from the conducting contact and emit. For micron-size particles, this would not be possible. The particles in  FIG. 2  on the left are microsize and thus would not be considered electron emitters as taught by Chen et al., Tatarenko et al., and definitely by Birecki et al. and Kumar. 
     This disclosure combines the shape of the columnar structure of the titania shown in the left image of  FIG. 2  with the photocatalytic behavior of this material and its negative electron affinity to make a field emitter that may be used at atmospheric pressure and in harsh environments as an enhanced photocatalyst or as an ion generator or both. This approach has several advantages: 
     1) Unlike carbon or metallic emitters, metal oxide materials are chemically stable—they are already oxidized. They would be stable emitters in air environments or other highly-oxidizing environments. 
     2) Many wide band gap materials such as diamond and metal oxides (titanium oxide is a good example) are known to have low or negative electron affinities (see Kumar for discussion and definition of negative electron affinity). This means that once an electron is in the conduction band of the material and the electron is able to diffuse to the surface, there is little or no energy to hold on to the electron at the surface. 
     3) Titanium oxide is used as a photocatalyst for cleaning air and water in many applications. It is easy to form and is inexpensive. 
     In the embodiments described below, titanium oxide is used to generate ions in an atmosphere of gas that is at 1 mTorr or higher pressure and specifically for gas at standard atmospheric pressure. The embodiments are also used to make a titanium oxide film or a photocatalyst that has higher activity and is more effective at cleaning contaminants from an atmosphere. 
     Referring to  FIG. 3 , an embodiment of this invention is to use titanium oxide as a field emitter  304  to create ions. The carbon nanotube film described in the &#39;761 application is replaced by a wide band gap material such as a titanium oxide film. Specifically, in FIG. 3 of the &#39;761 application, the “CNT coating” is replaced with a titanium oxide coating. This titanium oxide coating may be the columnar structure shown in  FIG. 2  of this disclosure, since this has a form factor that is conducive to field emission of charges. 
     Again referring to  FIG. 3 , another embodiment of this invention is to use titanium oxide as a “photo-activated” field emitter  304  or photocathode. A UV light source  301  is illuminating the photocatalyst layer  304 . In this example, titanium oxide is used, but other photocatalyst materials may also be used. The surface of the lamp is coated with a transparent conductive film  302 . An electrical bias is placed between the electrode  302  on the light source and the electrode  305  (conductive film) of the photocatalyst layer. The magnitude polarity of the electrical may be adjusted to optimize performance. In one embodiment, UV  303  light that has a wavelength shorter than 380 m (near UV spectrum) or with a photonic energy that is higher than the bandgap of the photocatalyst material illuminates the photocatalyst  304  at the same time an electric field is applied to the surface of the titanium oxide. The photon energy of the UV light  303  to activate the photocatalyst  304  may be below the energy that is typically used to generate ozone. Because the UV light  303  generates electron-hole pairs in the material  304 , electrons may drift to the surface  305  of the titanium oxide  304  and be pulled from the material by the applied electric field, depending on the polarity of the electric field. In the same manner, by reversing the polarity of the electric field, holes may migrate to the surface and create a positively charged ion in the atmosphere as a result of an electron exchange with a neutral atom at the surface of the material. Because the charges may already be in all excited state as a result of the UV illumination, the tunneling barrier to emitting the charge may be smaller or non-existent in the case that the titanium oxide surface has negative electron affinity. The electric field may be either DC or AC. If DC, the counter electrode potential may be either positive or negative with respect to the titanium oxide or the conductor  305  on which the titanium oxide  304  sits. If AC, it may be modulated between positive and negative values with respect to the titanium oxide. Combinations of AC and DC are also possible in that AC modulation may be on top of a DC bias. The frequency of modulation and the level of the bias or electric field may be adjusted to optimize the performance of the titanium oxide film  304  both as an ion generator or a more active photocatalyst while minimizing the generation of ozone or both. There are other applications where ozone generation is desired, thus the conditions of operating the device may be changed to optimize ozone generation for these applications, including changing the energy of the UV light. An alternative embodiment is to have the UV light source as the substrate  306  on which the photocatalyst is coated. In this case, the photocatalyst is back-side illuminated. In this case, layer  305  is also a transparent conductor. Both sides may be light sources ( 306  and  302 ). 
       FIG. 4  illustrates an alternative embodiment where there are two separate photocatalyst layers on substrates on either side of the UV light source, which take advantage of the fact that the light source may emit light in more than one direction. 
       FIG. 5  is another embodiment in which the electrode is not on the lamp but is a grid  502  suspended above (or below as depicted in this figure). The grid  502  may be semitransparent to allow the light  503  from the lamp  501  to pass through. The lamp  501  may be any shape. The photocatalyst layer  504  and grid layer  503  are shown flat but this is not required. They could be wavy or undulating, they could be in a cylindrical configuration (example photocatalyst on an inner cylinder facing the grid as an outer cylinder). It may be more important to maintain the gap between the grid  503  and the photocatalyst layer  504 . Other configurations are possible, including configurations in which the titanium oxide is on a conducting or insulating layer and the bias is created by placing two grids on opposite sides of the titanium layer (see  FIG. 6 ). It is also possible to have the light source near perpendicular to the plane of the photocatalyst such that the volume of air moving across the photocatalyst is maximized. 
     The ion intensity may be adjusted by changing the intensity of the light used, by changing the wavelength of the light used, by modifying the polarity and magnitude of the applied electrical field to the photocatalyst, by changing the frequency of the applied electric field or by changing one of more of these variables at the same time. The frequency of the applied electrical field may be as high as the megahertz range. The ion source may be switched on and off by either switching the light source on and off or by switching the applied electric field on and off or both. Fast light sources may also be used to create fast ion sources since there are some laser-based light sources and LEDs that may be switched on and off quickly. 
     One feature described here is the use of columnar structure titanium oxide as a field emitter 
     Another feature described here is the use of a photocatalyst as a field emitter. 
     Another feature described here is the use of a photocatalyst activated by a light source and combined with an applied electric field as a source of charged particles to generate ions and to enhance the activity of the photocatalyst at breaking down or decomposing chemical compounds.