From nanostructures to porous silicon: sensors and photocatalytic reactors

Abstract

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 a novel heterogeneous photocatalytic reactor. Utilizing photoluminescence induced electroless metallization as a means of obtaining a highly efficient electrical contact we demonstrate the detection of HC1, NH3, CO and NO at the ppm level. The response of this device, which operates with limited sensitivity at a bias voltage of 1–10 mV, is rapid and reversible. The form of metallization used to establish a low resistance, 20 Ω, contact also provides the basis for more efficient electroluminescent devices. After an electroless gold treatment, the impedance response of the device to ammonia increases by 2.5 times, while the CO and NO responses are unchanged. With an electroless tin coating, the room temperature responses to NH3, NO, and CO are all amplified. With an 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 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. Visible light absorbing titania-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 TiO2–xNx photocatalysts whose optical response, now not limited to the ultraviolet, can be tuned across the entire visible region. This synthesis, which can be simultaneously accompanied by metal atom seeding, can be accomplished through the direct nitration of anatase TiO2 nanostructures with alkyl ammonium salts. Tunability throughout the visible depends on the degree of TiO2 nanoparticle agglomeration and the influence of metal seeding. No organics are incorporated into the final TiO2–xNx products. These visible light absorbing photocatalysts readily photodegrade methylene blue and gaseous ethylene. They can be transformed from liquids to gels and 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 a 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 demonstrate an example of how a traversal to the nanoscale can vastly improve the efficiency for producing important submicron particles. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)