Nanostructures and Porous Silicon: Activity and Phase Transformation in Sensors and Photocatlytic Reactors James Gole, Stephen Lewis, Sharka Prokes, and Andrei Federov School of Physics, Georgia Tech   Porous interfaces are transformed within the framework of nanotechnology to develop highly efficient sensors, interactive support surfaces, active battery electrodes, and nanophotocatalytic frameworks. A rapid, reversible, and sensitive porous silicon (PS) gas sensor, based upon a uniquely formed highly efficient electrical contact to a nanopore covered microporous array, is modified to introduce selectivity and for the purpose of creating a novel heterogeneous photocatalytic reactor. Photoluminescence induced metallization is used to obtain a highly efficient electrical contact as we demonstrate the detection of HC1, NH3 , CO and NO at the ppm level. The device, which operates at a bias voltage which can be as low as 1-10 mV, resulting from a low resistance, 20Ω, contact, also forms the basis for more efficient electroluminescent devices. Electroless gold and tin treatments selectively modify the impedance response of the device to considerably improve detection for NH3 , CO and NO. Through FFT analysis, a gas response can now be acquired and filtered on a drifting baseline, further increasing sensitivity. These sensor suites are now being extended to develop microreactors in which nanoscale silica² and titania³ based quantum dot (QD) photocatalysts will be placed within the pores of PS and excited using PS electroluminescence or photoluminescence to excite visible light absorbing QDs. Highly efficient light absorbing tintania-based QDs have been developed. Using a nanoscale exclusive synthesis route, we directly treat TiO2 nanocolloids and, in seconds, at room temperature, we produce nitrogen doped, stable, and environmentally benign TiO 2-xNx photocatalysts whose optical response, now not limited to the ultraviolet, can be tuned across the entire visible region. A tunability throughout the visible is found to depend upon the degree of nanoparticle agglomeration and upon the ready ability to seed these nanoparticles with metal (metal ions) including Pt, Co, and Ni. This metal ion seeding also leads to uniqe efficient phase transformations, including that of anatase to rutile TiO2 , at room temperature. The visible light absorbing photocatalysts readily photograde methylene blue and gaseous ethylene. They can be transformed from liquids to gels and, in addition, can be placed on the surfaces of sensor and microreactor based configurations 1) to produce an improved photocatalytically induced solar based sensor response, and 2) with a goal to facilitate catalytically induced disinfection of airborne pathogens. In contrast to the nitridation process which is facile at the nanoscale, we find little or no direct nitridation of micrometer sized anatase or rutile TiO2 powders at room temperature. Thus, we illustrate an example of how a traversal to the nanoscale can vastly improve the efficiency for producing important submicron particles.