Patent Application
20060213251
Not applicable.
The invention relates to multi-layer hydrogen sensors which include a carbon nanotube layer.
Ongoing efforts towards a hydrogen-based transportation economy have stimulated an increased need for compact, reliable, low cost, low power consumption hydrogen sensors for monitoring and safety. These are also required for spacecraft powered by hydrogen fuel cells where long-duration space flights can last for years and potentially decades.
Single walled carbon nanotubes (SWNTs) have exhibited a charge transport sensitivity to their chemical environment, making them attractive for chemical sensing applications. In one paper (Y. Lu, J. Li, H. T. Ng, C. Binder, C. Partridge, M. Meyyapan, Chem. Phys. Lett. 391, 344 (2004), a group of researchers showed that by palladium loading the nanotube networks, the devices acquired a ppm level sensitivity for detecting methane, a gas for which the devices were otherwise insensitive. Lu et al. discloses sputter deposition of Pd onto nanotube powder followed by “shaking” and ultrasonic dispersion in purified water and subsequent drop drying across interdigitated microelectrodes. The reproducibility of devices made by such a method leaves much to be desired. For example, the amount of Pd loading was poorly controlled, as was the density of the nanotubes in the drop-dried networks bridging the electrodes of the device.
A multi-layer hydrogen sensor comprises a carbon nanotube comprising layer, and an ultra-thin metal or metal alloy layer disposed on the carbon nanotube comprising layer, wherein an electrical resistance of the layered sensor increases upon exposure to H2. The carbon nanotube comprising layer preferably consists essentially of single wall nanotubes (SWNTs).
As used herein, the phrase “ultra-thin” corresponds to a thickness of from about 0.5 nm to about 10 nm. The ultra-thin layer can be a layer having uniform thickness, or be a sub-percolation layer of metal defined herein as a plurality of discrete, nanoscale metal islands, not in themselves electrically connecting by metal to one another, or a plurality of nanoscale metal islands wherein at least a portion of the nanoscale metal islands are electrically connected other islands by metal. In the case where the metal islands are not electrically connected to form a low resistance electrical path from one end of the sensor to the other, the thickness of the metal islands can be on the order of hundreds of nuns, or more, since such an arrangement will not shunt the carbon nanotube comprising active sensing layer.
The thickness of the ultra-thin metal or metal alloy layer is preferably from 0.5 to 5 nm thick. The ultra-thin metal or metal alloy layer can be selected from Ni, Pd, Pt, Ti, Ag and W, or mixtures thereof. In a preferred embodiment, the ultra-thin metal or metal alloy layer comprises Pd.
The thickness of the carbon nanotube comprising layer is preferably from 4 to 60 nm, such as from 4 to 10 nm. The interface between the carbon nanotube comprising layer and the ultra-thin metal or metal alloy layer is preferably characteristic of an evaporated interface or other low energy deposition process.
In a related embodiment that can provide integrated microsensors formable generally using conventional integrated circuit processing steps, the sensor further comprises an integrated circuit substrate, wherein the sensor is disposed on the substrate. At least one electronic device can be disposed on the substrate, the electronic device coupled to an output of the sensor.
A method of forming a layered hydrogen sensor comprises the steps of providing a substrate, forming an active sensor region on the substrate, the active sensor region comprising a carbon nanotube comprising layer disposed on or under an ultra-thin metal or metal alloy layer, and forming contacts to the active sensor region on either side of the active sensor region. The ultra-thin metal or metal alloy layer can be formed using an evaporation process, or other low energy deposition process.
The forming step can comprise forming the carbon nanotube comprising layer on a porous support layer, placing the carbon nanotube comprising layer on the porous support layer on the substrate, and then removing the support layer. The support layer can comprises a porous membrane. The nanotube comprising layer on the support layer can be formed using the steps of dispersing a plurality of nanotubes into a solution, the solution including at least one surface stabilizing agent for preventing the nanotubes from flocculating out of suspension, applying the solution to the porous support, and removing the solution, wherein the nanotubes are forced onto a surface of the porous support.
A fuller understanding of the present invention and the features and benefits thereof will be obtained upon review of the following detailed description together with the accompanying drawings, in which:
It has been discovered that although electronic transport across pure SWNT films is relatively insensitive to the presence of H2, electronic transport across the SWNT films become sensitive to H2 if thin layers of certain metals, such as Pd, are present on the SWNT layer. A multi-layer H2 sensor according to the invention comprises a carbon nanotube comprising layer, and a ultra-thin metal or metal alloy layer in contact with the nanotube layer. An electrical resistance of the layered sensor increases upon exposure to H2. The metal or metal alloy layer is preferably selected from the group consisting of Ni, Pd and Pt, or mixtures thereof. However, other metals or metal alloys that provide good sensitivity may also be used, including, but not limited to, Ti, Ag, and W. As defined herein, sensitivity is defined as the change in resistance of the layered sensor (AR) divided by the initial resistance prior to exposure to the H2-containing ambient (R). Sensors according to invention provide detection of ppm levels of hydrogen gas (H2) down to at least 10 ppm, and can be conveniently operated at room temperature.
In a preferred embodiment, the carbon nanotube comprising layer consists essentially of single wall nanotubes (SWNTs). SWNTS are preferred because the inner layers in multi-walled nanotubes (MWNTs) are protected by the outer layer and will not respond to hydrogen interaction with the outer layer. Thus, the initial R will be larger and the AR upon hydrogen exposure smaller upon exposure H2 for MWNTs, and thus lower sensitivity as compared to SWNTs. Although the invention is described using the preferred SWNTs, MWNTs, or mixtures of SWNTs and MWNTs may also be used, although these embodiments will generally yield a reduced device sensitivity.
As noted above, the thickness of the ultra-thin layer corresponds to a thickness of from about 0.5 nm to about 10 nm and can be embodied as a layer having uniform thickness, or a sub-percolation layer comprising a plurality of metal islands, where some or all the metal islands can be connected by metal. The thinner the metal or metal alloy layer, the higher the resulting sensitivity of the sensor since the active sensing nanotube layer is shunted by the metal or metal alloy layer generally disposed thereon or disposed thereunder. As noted above, in the case where the metal islands are not electrically connected to form a low resistance electrical path from one end of the sensor to the other, the thickness of the metal islands can be on the order of hundreds of nms, or more, since such an arrangement will not shunt the carbon nanotube comprising active sensing layer. Moreover, if maximum sensitivity is not required, a thicker metal or metal alloy layer, such as 10 nm to 200 nm, or more, may be used with the invention.
Conductance or resistance of the coated SWNT layer sensor can be conveniently measured. In a preferred embodiment, conductance is measured by measuring the change in current between two electrodes which contact opposing sides of the metal-coated SWNT sensor upon exposure to H2 to determine the presence of H2, and once calibrated, the concentration of H2.
An important advantage of the invention relates to reusability. Sensors according to the invention have been found to quickly recover their initial film conductance upon exposure to air or other ambient having little or no H2. For example, it has been found that most of the initial film conductance can be recovered in as little as 30 seconds in air.
Another attractive feature of sensors according to the invention is the low power consumption, which allows batteries to be used to provide power for sensor measurements. Power consumption can be on the order of 0.25 mW. This low power requirement is in part due to the lack of need for heating the sensor as must be done for catalytic bead and semiconducting oxide sensors. This power requirement could be reduced still further by pulsed operation. Moreover, sensors according to the invention have a simple structure, and can be measured using simple measurement arrangements as described below.
The interface between the SWNT layer and the nanoscale metal or metal alloy layer is preferably characteristic of an evaporated interface, or other low energy deposition process that does not significantly damage the nanotube layer. This finding was based on measurements that indicated that sputter deposition of metals onto the nanotubes may result in damage to the nanotube layer, while thermal evaporation of metals does not result in appreciable damage. Sputtered species generally have energies significantly higher than evaporated species, likely causing damage to the nanotube layer and potentially significant penetration depths.
Methods other than evaporation and sputtering may be used to deposit the ultra-thin metal or metal alloy layer, including, but not limited to, electrochemical or electroless deposition. For example, metal deposition could be performed electrochemically, utilizing the electrical conductivity of the nanotubes to act as the electrode upon which the electrodeposition takes place. Alternatively electroless deposition may also be employed with the nanotubes as the template upon which the deposition occurs.
A preferred method of forming thin (2 to 60 nm, such as 7 to 25 nm) SWNT films which have highly uniform thickness for use with the invention is described in commonly owned U.S. patent application Ser. No. 10/622,818 entitled “Transparent electrodes from single wall carbon nanotubes” filed on Jul. 18, 2003 ('818) and published on Oct. 7, 2004 as Published App. No. 20040197546. The transparent aspect of the nanotube films provided is generally not utilized in the present invention. However the high level of electrical interconnectedness of the nanotubes in the films generated using the process described in '818 which provides very high electrical conductivity, maximizes the sensitivity of sensors according to the invention to H2 because nanotubes not electrically connected with other nanotubes do not contribute to the sensitivity of the device. The uniform SWNT film thickness provided using the process described in '818 moreover provides a uniformity of metal association with the SWNT film, thereby maximizing the sensitivity of the device, defined as noted above as the change in resistance AR (upon exposure to H2) divided by the initial resistance (R) of the metallized SWNT film (ΔR/R).
Briefly, a dilute surfactant-suspension of SWNTs is vacuum filter onto a mixed cellulose ester (MCE) filtration membrane (0.1 μm pore size, Millipore). The SWNTs are preferably purified pulsed-laser-vaporization grown SWNTs. However, SWNTs can be grown by any method. For example, moderately dense SWNTs can be grown on a surface as an inter-connected network as disclosed by Snow E S, Novak J P, Campbell P M, Park D. Appl Phys Lett 2003; 82:2145. The nanotubes deposit as a thin film on the membrane. Washing with purified water removes the surfactant. Once dried, the membrane with the SWNT film attached is cut to the desired size (e.g. 3 mm×8 mm), wetted with pure water again, and the film side placed against the substrate (e.g. 600 nm thermal SiOx/Si wafer) to which the SWNT film transfer is to be made. The substrate and membrane are sandwiched between wicking filter paper situated between metal plates and pressure is applied via spring clamps. Once the assembly is nearly dry (accelerated by oven heating at 95° C.) the film and membrane adhere sufficiently to the substrate to permit transfer to an acetone vapor bath. Acetone condensing onto the MCE membrane in the vapor bath dissolves it away, leaving only the SWNT film adhered to the substrate. These films are preferably subsequently baked in inert atmosphere at a fairly high temperature, such as 600° C., to desorb residual contaminants and charge transfer nanotube dopants.
Sensor 100 includes contact electrodes 110 and 120 disposed on top of the nanotube comprising layer 105. Contact electrodes can be deposited using either sputtering or thermal evaporation. However, as noted above, evaporation is preferred. Nanotube comprising layer 105 is preferably a SWNT film having a highly uniform thickness from 7 to 25 nm as described in '818. However, the invention is not limited to SWNT layers described in '818.
Although the resulting arrangement is not shown in
Although sensor devices described above are described as being macroscopic, such as sensor 100 shown in
In another related embodiment, integrated sensing systems according to the invention can be conveniently mounted in locations of interest, such as near H2 sources and associated H2 supply lines. Such systems provide continuous, automatic and real time (or near real-time) detection of H2 in the surrounding environment. The conductance measurements provided by sensor 200 are communicated to a processor (not shown). Processor can include associated non-volatile memory, such as for storing data manipulation algorithms, calibration data, or predetermined user programmable setpoints. Upon detection of more than a predetermined level of H2 indicating possible danger, the system can provide a visual display or an audible alarm (not shown).
Systems according to the invention can be positioned at several locations along a H2 supply line which provides fuel to an electrochemical generator, such as a PEM fuel cell. In a preferred embodiment, the system includes a valve which when closed turns off the supply of H2 to the electrochemical generator. Upon the detection of H2 above a predetermined level using sensors according to the invention, a sequence of events can be initiated to close the valve.
Although not shown, sensors according to the invention can be part of sensor arrays which detect not only H2, but other species as well. The other sensors in the array can also be thin film sensors, electrochemical sensors, or other sensor types.
The present invention is further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of the invention in any way.
Sample exposure to gasses including H2 was performed at room temperature and atmospheric pressure in a quartz flow tube with electrical feed-throughs for voltage and current leads. The gasses were fed via mass flow controllers to maintain a total flow of 450 sccm of either pure nitrogen, 500 ppm H2 in N2, or a mixture of the two to obtain reduced concentrations of H2, or air. Electrical measurements were performed using an HP4156B source-meter. Excitation voltages ranged ±0.5 V For I-V measurements and were held at 0.5 V for current-time measurements.
Over the concentration range investigated, there was an essentially linear dependence of ΔR/R on H2 concentration, with a slope of about 0.04%/ppm of H2. The detection limit of the 25 nm films for measurement times of about 10 minutes following exposure for sensors according to the invention was found to be about 10 ppm H2 at room temperature.
It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.
The U.S. government may have certain rights to the invention based on AFOSR grant number F49620-03-1-0370; NSF(CTS-0301178), NASA Kennedy Space Center Grant NAG 10-316, ONR (N00014-98-1-02-04), and NSF DMR 0400416.
