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, NH 3 , 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 NH 3 , 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 TiO 2 nanocolloids and, in seconds, at room temperature, we produce nitrogen doped, stable, and environmentally benign TiO 2-x N x 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 TiO 2 nanostructures with alkyl ammonium salts. Tunability throughout the visible depends on the degree of TiO 2 nanoparticle agglomeration and the influence of metal seeding. No organics are incorporated into the final TiO 2-x N x 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 TiO 2 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.

Unexpected Photocatalytic Activity of Simple and Platinum Modified Tubular SiO2 for the Oxidation of Oxalic Acid to CO2

“…According to the published data, the surface defects as well as the surface silanol groups play important roles in determining the activity in catalytic reactions [18]. Carboxyl groups have been shown to form stable linkages with the surface hydroxyl groups to form esters.…”

A pulsed system frequency analysis for device characterization and experimental design: Application to porous silicon sensors and extension

“…It must be noted, however, that the gas pulsing technique requires a gas flow system with macroscopic components including valves and, in the absence of a calibrated array, a series of reference gases. Any consideration of the use of this method to improve the performance of low-cost MEMS gas sensors for normal field use must take into account not only the inherent size of a final system but also the cost of implementing the gas pulsing method as it improves sensitivity, selectivity, and reversibility [2,[4][5][6]. Of significant importance is that the gas pulsing method can be used to eliminate false positives.…”

“…Current fabrication procedures and the application of the PS gas sensor to the detection of a number of gases are outlined in detail elsewhere [2,[4][5][6]. The basic experimental configuration for testing the PS sensor, designed for rapid and easily repeatable experiments, is illustrated in Fig.…”

“…The basic experimental configuration for testing the PS sensor, designed for rapid and easily repeatable experiments, is illustrated in Fig. 1 [2,[4][5][6][7][8].…”

Photoelectrochemical properties of porous silicon based novel photoelectrodes

“…It is known that electrochemically etched porous silicon surface has silicon oxyhydride‐like fluorophors strongly bound to it. The fluorophors represent localized centres that would be similar in form to the localized surface states acting as shallow donor states 38–39. The schematic in Figure 8 shows that electron transfer can be facilitated from the conduction band of oxynitride through localized surface states of PS whereas holes can be transferred to the N 2p levels thereby increasing the current in the external circuit.…”