Nanostructure-enhanced interfaces for sensors and microreactors
Assessing personal air quality and its remediation, as well as monitoring harmful contaminants, offer numerous potential applications. In the workplace, these include the detection of hazardous materials such as SO2 and H2S near petrochemical plants, volatile organic compounds (VOCs) in coating operations, and NOx in heating, ventilation, and air conditioning equipment. In law enforcement, authorities want to detect methamphetamine production and its noxious by-products, including NH3, HCl, PH3, benzene, and formaldehyde.
These applications require sensor arrays that can simultaneously monitor multiple gases and address a wide a range of analytes. To address this problem, we are modifying hybrid nanoporous/microporous-silicon interfaces (see Figure 1).1,2 Sensors that take advantage of enhanced nanostructure activity, and microreactors that use visible-light-accessible nanoscale photocatalysts (quantum dots),3–5 could eventually be combined in a complimentary framework.
Sensor arrays to detect multiple analytes have focused largely on metal-oxide systems, primarily related to tin oxide (SnO2). Building combinational SnO2-based arrays can be complicated, however,6 because several e-beam lithography steps with a wide variety of dopants may be needed.6 Further, these SnO2-based sensors operate at elevated temperatures,7 between 100 and 600°C, where the detection of a particular gas is oxide-temperature dependent. A sensor must be independently heated and thermally isolated from the remainder of a device (using trenches) to maintain its selected temperature. Operating a temperature-dependent sensor in an elevated-temperature environment—for example, in flue gas—is not feasible.
In contrast, the conductometric porous-silicon gas sensor (PSGS) uses a combination of photoluminescence-induced (PL-induced) metallization and e-beam deposition to form unique, low-resistance (<100Ω) contacts to a hybrid nanoporous/ microporous scaffold.1,2,8
The advantages of the PSGS include the following: simple operation; high sensitivity; short recovery time; operation at room temperature or at a single, readily accessible, temperature with an insensitivity to temperature drift; operation at temperatures to 80°C even in elevated-temperature environments; ease of coating with gas-selective nanostructures to form sensor arrays; low fabrication cost; low power consumption and operating cost; and rapid assessment of false positives, using a controlled gas-pulsing fast-Fourier-transform technique.9
The enhanced activity of nanoscale structures can profoundly affect the design of MEMS/NEMS interaction, reaction, and sensing technology, providing new approaches to create hybrid devices with greatly enhanced sensitivity and selectivity. Our laboratories have explored the introduction of nanostructures—in the form of metals, metal oxides, and catalytic nanoparticles—at nanoporous/microporous interfaces (see Figure 1), and their subsequent enhancement of interaction to promote increased conversion efficiency, transduction, and/or selectivity. We have also developed facile, high-yield, nanoscale exclusive synthesis processes for the novel interactive nanostructures needed for this interface modification. In developing these hybrid interfaces, we seek to identify the microscale and nanoscale phenomena that govern these novel materials conversions. The resulting principles can improve not only advanced catalysis but also marking, tagging, and chemical sensing.
With the PSGS we have obtained sensitivities in the moderate-to-high ppb range for gases such as NH3, NOx, CO, H2S, and SO2. A linear response occurs within, at most, two seconds of sensor exposure.2 Rather than waiting for the response to saturate before a measurement is taken (which would take several minutes), the slope of the sensor's rising resistance (see Figure 2) can be measured in seconds.
Incorporating quantum dot metal and metal-oxide coatings (see Figures 1 and 3) can allow arrays of gas sensors to discriminate gas mixtures of different compositions.2 This has been demonstrated for treatment with with an electroless metal (gold) source and with a metal oxide (tin oxide) formed from an electroless tin source. These introduced nanostructures enhance the ability of the PSGS to detect certain gases: particularly NH3 for gold and CO for tin oxide. Moreover, the interaction of tin/tin oxide nanostructures with the nanoporous/microporous interface greatly increases the SnO2 sensitivity, so that the tin oxide-coated PSGS is able to detect CO at a ppm level at room temperature.2,8
We are investigating the use of nanoscale-based nitrogen doping and magnetic-metal seeding of semiconductor-based photocatalysts, including SixO,10 TiO2 (TiO2–xNx),3,5 and ZrO2.11 The eventual goal is to create visible-light-absorbing microreactors that can be used in solar-pumped sensors. In these configurations, the photocatalyst could be used in a pre-filter format to break down higher-molecular-weight compounds into more reliably sensed products. Reducing large molecules into smaller constituents can obviate the problem that arises when they adsorb in specific configurations on a sensor, precluding their detection.
Other catalysts start with highly efficient titania-based nanocolloids, produced in a nanoscale exclusive synthesis at room temperature. These can be nitrided in seconds to produce nitrogen-doped, stable, and environmentally benign TiO2–xNx photocatalysts (see Figure 4), whose optical response can be tuned across the entire visible region. This wide tunability depends upon the degree of nanoparticle agglomeration and the ability to seed these nanoparticles with metals (metal ions) including Pd and additional active particles including Co, Ni, and Cu. In addition, using magnetic particles and proper preparation, it has been possible to create seeded, photocatalytic, nanostructure-modified interfaces that display surface-enhanced infrared-detection capability for minor constituents. The coating techniques are being extended to additional metals, metal oxides, and other materials, with the intention to provide selectivity and sensitivity for a number of devices.
James Gole, professor of Physics and Mechanical Engineering at Georgia Tech, an APS and AAAS Fellow, and affiliate of Pacific Northwest National Laboratory, has authored over 230 papers and over 20 patents. He studies physical phenomena that fall at the interface between chemical and condensed matter physics and material science, and his research is oriented to MEMS and NMEMS applications. In addition, he has submitted several papers to SPIE conferences.
