The development of low cost, scalable, renewable energy technologies is one of today's most pressing scientific
challenges. We report on progress towards the development of a photoelectrochemical water-splitting system that will
use sunlight and water as the inputs to produce renewable hydrogen with oxygen as a by-product. This system is based
on the design principle of incorporating two separate, photosensitive inorganic semiconductor/liquid junctions to
collectively generate the 1.7-1.9 V at open circuit needed to support both the oxidation of H2O (or OH-) and the
reduction of H+ (or H2O). Si microwire arrays are a promising photocathode material because the high aspect-ratio
electrode architecture allows for the use of low cost, earth-abundant materials without sacrificing energy-conversion
efficiency, due to the orthogonalization of light absorption and charge-carrier collection. Additionally, the high surfacearea
design of the rod-based semiconductor array inherently lowers the flux of charge carriers over the rod array surface
relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the
solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts. Arrays of Si
microwires grown using the Vapor Liquid Solid (VLS) mechanism have been shown to have desirable electronic light
absorption properties. We have demonstrated that these arrays can be coated with earth-abundant metallic catalysts and
used for photoelectrochemical production of hydrogen. This development is a step towards the demonstration of a
complete artificial photosynthetic system, composed of only inexpensive, earth-abundant materials, that is
simultaneously efficient, durable, and scalable.
Inorganic semiconductors are promising materials for driving photoelectrochemical water-splitting reactions. However,
there is not a single semiconductor material that can sustain the unassisted splitting of water into H2 and O2. Instead, we
are developing a three part cell design where individual catalysts for water reduction and oxidation will be attached to
the ends of a membrane. The job of splitting water is therefore divided into separate reduction and oxidation reactions,
and each catalyst can be optimized independently for a single reaction. Silicon might be suitable to drive the water
reduction. Inexpensive highly ordered Si wire arrays were grown on a single crystal wafer and transferred into a
transparent, flexible polymer matrix. In this array, light would be absorbed along the longer axial dimension while the
resulting electrons or holes would be collected along the much shorter radial dimension in a massively parallel array
resembling carpet fibers on a microscale, hence the term "solar carpet". Tungsten oxide is a good candidate to drive the
water oxidation. Self-organized porous tungsten oxide was successfully synthesized on the tungsten foil by anodization.
This sponge-like structure absorbs light efficiently due to its high surface area; hence we called it "solar sponge".
Linear sensor arrays made from small molecule/carbon black composite chemiresistors placed in a low headspace
volume chamber, with vapor delivered at low flow rates, allowed for the extraction of chemical information that
significantly increased the ability of the sensor arrays to identify vapor mixture components and to quantify their
concentrations. Each sensor sorbed vapors from the gas stream to various degrees. Similar to gas chromatography,
species having high vapor pressures were separated from species having low vapor pressures. Instead of producing
typical sensor responses representative of thermodynamic equilibrium between each sensor and an unchanging vapor
phase, sensor responses varied depending on the position of the sensor in the chamber and the time from the beginning
of the analyte exposure. This spatiotemporal (ST) array response provided information that was a function of time as
well as of the position of the sensor in the chamber. The responses to pure analytes and to multi-component analyte
mixtures comprised of hexane, decane, ethyl acetate, chlorobenzene, ethanol, and/or butanol, were recorded along each
of the sensor arrays. Use of a non-negative least squares (NNLS) method for analysis of the ST data enabled the correct
identification and quantification of the composition of 2-, 3-, 4- and 5-component mixtures from arrays using only 4
chemically different sorbent films and sensor training on pure vapors only. In contrast, when traditional time- and
position-independent sensor response information was used, significant errors in mixture identification were observed.
The ability to correctly identify and quantify constituent components of vapor mixtures through the use of such ST
information significantly expands the capabilities of such broadly cross-reactive arrays of sensors.
The vapor classification performance of arrays of conducting polymer composite vapor detectors has been evaluated as a function of the number and type of detectors in an array. Quantitative performance comparisons were facilitated by challenging a collection of detector arrays with vapor discrimination tasks that were sufficiently difficult that at least some of the arrays did not exhibit perfect classification ability for all of the tasks of interest. For nearly all of the discrimination tasks investigated in this work, classification performance either increased or did not significantly decrease as the number of chemically different detectors in the array increased. Any given subset of the full array of detectors, selected because it yielded the best classification performance at a given array size for one particular task, was invariably outperformed by a different subset of detectors, and by the entire array, when used in at least one other vapor discrimination task. Arrays of detectors were nevertheless identified that yielded robust discrimination performance between compositionally close mixtures of 1-propanol and 2-propanol, n-hexane and n-heptane, and meta-xylene and para-xylene, attesting to the excellent analyte classification performance that can be obtained through the use of such semi-selective vapor detector arrays.
Arrays of conducting polymer composite vapor detectors have been evaluated for performance in the presence of the nerve agent simulants dimethylmethylphosphonate (DMMP) and diisopropylmethylphosponate (DIMP). Limits of detection for DMMP on unoptimized carbon black-organic polymer composite vapor detectors in laboratory air were estimated to be 0.047-0.24 mg m-3. These values are lower than the EC50 value for the nerve agents sarin (methylphosphonofluoridic acid, (1-methylethyl) ester) and soman, which have been established as equals 0.8 mg m-3. Arrays of these vapor detectors were easily able to resolve signatures due to exposures to DMMP from those due to DIMP or due to a variety of other test analytes in a laboratory air background. In addition, DMMP at 27 mg m-3 could be detected and differentiated from the signatures of the other test analytes in the presence of backgrounds of potential interferents in the background ambient, including water, methanol, benzene, toluene, diesel fuel, lighter fluid, vinegar and tetrahydrofuran, even when these interferents were present in much higher concentrations than that of the DMMP or DIMP being detected.
Thin films of carbon black-organic polymer composites have been deposited across two metallic leads, with sorption of vapors producing swelling-induced resistance changes of the detector films. To identify and classify vapors, arrays of such vapor sensing elements have been constructed in which each element of the array contains a different polymer as the insulating phase and a common conductor, carbon black, as the conducting phase. The differing gas-solid partition coefficients for the various polymers of the detector array produce a pattern of differential resistance changes that is used to classify vapors and vapor mixtures. The performance of this detector array system towards 2,4-dinitrotoluene, the predominant signature in the vapor phase above land mines, in the presence high concentrations of water or of acetone has been evaluated.
Thin films of carbon black-organic polymer composites have been deposited across two metallic leads, with swelling- induced resistance changes of the films signaling the presence of vapors. To identify and classify vapors, arrays of such vapor sensing elements have been constructed. Each element contained a different organic polymer as the insulating phase. The differing gas-solid partition coefficients for the various polymers of the detector array produced a pattern of resistance changes that was used to classify vapors and vapor mixtures. The performance of this system towards DNT, the predominant signature in the vapor phase above land miens, has been evaluated in detail, with robust detection demonstrated in the laboratory in less than 5 s in air at DNT levels in the low ppb range.
Thin films of carbon black-organic polymer composites have been deposited across two metallic leads, with swelling- induced resistance changes of the films signaling the presence of vapors. To identify and classify vapors, arrays of such vapor-sensing elements have been constructed, with each element containing a different organic polymer as the insulating phase. The differing gas-solid partition coefficients for the various polymers of the sensor array produce a pattern of resistance changes that can be used to classify vapors and vapor mixtures. This type of sensor array has been shown to resolve all organic vapors that have been analyzed, and can even resolve H2O from D2O. Blends of poly(vinyl acetate) and poly(methyl methacrylate) have been used to produce a series of sensor that response to vapors with a change in resistance of a magnitude that is not simply a linear combination of the responses of the pure polymers. These compatible blend composite detectors provided additional analyte discrimination information relative to a reference detector array that only contained composites formed using the pure polymer phases. Vapor signatures from chemicals used in land mine explosives, including TNT, DNT, and DNB, have been detected in air in short sampling time and discriminated from each other using these sensor arrays.
Response data were collected for a carbon black-polymer composite electronic nose array during exposure to homologous series of alkanes and alcohols. At a fixed partial pressure of odorant in the vapor phase, the mean response intensity of the electronic nose signals varied significantly for members of each series of odorants. However, the mean response intensity of the electronic nose detectors, and the response intensity of the most strongly-driven set of electronic nose detectors, was essentially constant for members of a chemically homologous odorant series when the concentration of each odorant in the gas phase was maintained at a constant fraction of the odorant's vapor pressure. Because the thermodynamic activity of an odorant at equilibrium in a sorbent phase is equal to the partial pressure of the odorant in the gas phase divided by the vapor pressure of the odorant, and because the activity coefficients are similar within these homologous series of odorants for sorption of the vapors into specific polymer films, the data imply that the trends in detector response can be understood based on the thermodynamic tendency to establish a relatively constant concentration of sorbed odorant into each of the polymeric films of the electronic nose at a constant fraction of the odorant's vapor pressure. This phenomenon provides a natural mechanism for enhanced sensitivity to low vapor pressure compounds, like TNT, in the presence of high vapor pressure analytes, such as diesel fuel. In a related study to evaluate the target recognition properties of the electronic nose, a statistical metric based on the magnitudes and standard deviations along Euclidean projections of clustered array response data, was utilized to facilitate an evaluation of the performance of detector arrays in various vapor classification tasks. This approach allowed quantification of the ability of a fourteen-element array of carbon black-insulating polymer composite chemiresistors to distinguish between members of a set of nineteen solvent vapors, some of which vary widely in chemical properties (e.g. methanol and benzene) and others of which are very similar (e.g. n-pentane and n-heptane). The data also facilitated evaluation of questions such as array performance as a function of the number of detectors in the system.
We describe herein the construction of a simple, low-power, broadly responsive vapor sensor. Carbon black-organic polymer composites have been shown to swell reversibly upon exposure to vapors. Thin films of carbon black-organic polymer composites have been deposited across two metallic leads, with swelling-induced resistance changes of the films signaling the presence of vapors. To identify and classify vapors, arrays of such vapor-sensing elements have been constructed, with each element containing the same carbon black conducting phase but a different organic polymer as the insulating phase. The differing gas-solid partition coefficients for the various polymers of the sensor array produce a pattern of resistance changes that can be sued to classify vapors and vapor mixtures. This type of sensor array has been shown to resolve common organic solvents, including molecules of different classes as well as those within a particular class.
Recent developments in ring-opening metathesis polymerization (ROMP) have enabled the synthesis of poly-cyclooctatetraene (poly-COT), a material which is isostructural to polyacetylene. This liquid-phase polymerization method allows facile construction of interfaces, films, and devices with polyacetylene-like materials. The ROMP method also allows the preparation of soluble, yet highly conjugated polyacetylene analogs from substituted cyclooctatetraenes (R-COT). The redox characteristics of R-COT polymers were investigated at electrodes modified with thin polymer films. Voltammetric methods were used to characterize the redox response, band gap, electrochemical doping, and cis-trans isomerization properties of these polyenes. We have applied poly-COT technology to the fabrication of Schottky diodes and photoelectrochemical cells, by forming poly-COT films on semiconductor surfaces. The resultant semiconductor/organic-metal interfaces behave more ideally than semiconductor contacts with conventional metals, in that changes in the work function of the conducting polymer exert a large and predictable effect on the electrical properties of the resulting Schottky diodes. Transparent films of the solution-processible polymer poly- trimethylsilyl-cyclooctatetraene (poly-TMS-COT) have been cast onto n-silicon substrates and doped with iodine to form surface barrier solar cells. These devices produce photovoltages that are much larger than can be obtained from n-silicon contacts with conventional metals.
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