KR20050039016A - Hydrogen sensor using palladium coated carbon nanotube - Google Patents

Abstract

The present invention relates to a carbon nanotube hydrogen sensor coated with palladium, and more particularly, palladium, which is a catalytic metal that selectively adsorbs and dissociates hydrogen, is coated on a surface of a carbon nanotube and positioned between electrodes of the hydrogen sensor. For example, palladium dissociates hydrogen molecules into hydrogen atoms, where the dissociated hydrogen atoms are adsorbed on the surface of the carbon nanotubes, whereby electrons move from hydrogen to carbon nanotubes, reducing the hole concentration of the carbon nanotubes and simultaneously The present invention relates to a palladium-coated carbon nanotube hydrogen sensor capable of detecting hydrogen gas effectively and with high sensitivity by reducing its electrical conductivity.

Description

Hydrogen sensor using palladium coated carbon nanotube}

The present invention relates to a carbon nanotube hydrogen sensor coated with palladium, and more particularly, palladium, which is a catalytic metal that selectively adsorbs and dissociates hydrogen, is coated on a surface of a carbon nanotube and positioned between electrodes of the hydrogen sensor. For example, palladium dissociates hydrogen molecules into hydrogen atoms, where the dissociated hydrogen atoms are adsorbed on the surface of the carbon nanotubes, whereby electrons move from hydrogen to carbon nanotubes, reducing the hole concentration of the carbon nanotubes and simultaneously The present invention relates to a palladium-coated carbon nanotube hydrogen sensor capable of detecting hydrogen gas effectively and with high sensitivity by reducing its electrical conductivity.

Hydrogen, which is widely used or suggested for industrial and scientific applications, is highly flammable and may explode when the minimum explosive limit concentration (4.65% by volume) is exceeded. There is a growing interest in sensors. Hydrogen sensors include ammonia, ethanol, methanol, aniline, hydrogen chloride industry, hydroforming, hydrocarcking and hydrorefining. It is used in a wide range of industries, such as the reduction of semiconductors and ores, such as the steel industry, automobiles, nuclear power plants, power facilities, medical fields, and space industries. Recently, hydrogen has been used as a clean energy source for fuel cells and internal combustion engines. This attention has increased the importance and the need to develop inexpensive, reliable and compact safe hydrogen sensors for hydrogen concentration control and leak hydrogen detection.

The commercially developed hydrogen sensor is a method of measuring the change of electrical characteristics of the device according to the hydrogen concentration. The most widely used hydrogen sensors are catalytic combustible (CC) or hot wire sensors.

The CC sensor is composed of two beads of platinum / iridium wire in the form of a Wheatstone bridge and heated to 600 to 800 ° C. One of the two beads is coated with an active catalyst and the other is not coated. It is a principle that detects hydrogen concentration by measuring the change of electrical resistance when the bead temperature rises by the heat of oxidation of flammable gas. However, the CC sensor is not suitable for oxygen-poor atmosphere or explosion limit (4.65% by volume) or higher, and because the CC sensor uses an oxidation reaction, the same reaction can occur for hydrocarbons. , It is contaminated with halogenated hydrocarbons or poisoned with silicon, lead and the like. In addition, since the operating temperature is 600 ℃ or more, the sensor itself causes a fire or explosion, and power consumption is also a disadvantage.

Secondly, the metal oxide semiconductor (MOS) sensor is widely used. It mainly uses Fe, Zn, Sn oxides or mixtures thereof and operates at a temperature of 150 to 350 ° C. Oxygen atoms are adsorbed on the surface of the MOS to achieve an equilibrium concentration of oxide ions in the surface layer. When the basic resistance or conductivity of MOS sensor is corrected in air and CO, H ₂ S or hydrocarbon molecules are contacted, they are adsorbed on the surface to change the oxygen balance and change the electrical resistance of MOS material. However, the MOS sensor described above has to be calibrated once every 3 months in the case of commercially available sensors, and the response time is long (3 to 5 minutes), and the sensor itself may cause ignition or explosion. There are disadvantages such as not being used together.

Recently, interest in oxide semiconductor hydrogen sensor of Pd MOS structure has increased, and the oxide semiconductor hydrogen sensor of Pd MOS structure has a structure of palladium / oxide / semiconductor. The reason why palladium (Pd) is used in the hydrogen sensor is that palladium has an excellent catalytic activity of selectively adsorbing hydrogen molecules and dissociating them into hydrogen atoms. When the dissociated hydrogen atom diffuses through the palladium and is adsorbed at the palladium / oxide interface, it uses a principle of increasing the electrical conductivity by decreasing the Schottky barrier height between the oxide and the semiconductor by polarization. The first Pd MOS sensor is Lundstrom's proposed Pd / SiO ₂ / Si MOS field effect transistor (I) structure [I. Lundstrom et. al., J. Appl. Phys., 46, 3876 (1975). However, the sensor of the Pd MOS FET structure has a disadvantage that the manufacturing process is complicated and the manufacturing cost is high because the three-pole structure including the gate electrode.

On the other hand, since the characteristics of the oxide layer affect the hydrogen detection ability, much effort has been made to improve the characteristics of the oxide layer. Studies to solve this problem use ZnO [A. Dutta et. al., Materials Science Engineering, B14, 31 (1992)], using TiO ₂ [L. Yadava et. al., Solid state Electron., 33, 1229 (1990), Akbar et. al., US5439580 (1995).

Another approach is to form a bipolar structure that eliminates the influence of an oxide layer as a Schottky barrier diode of a bipolar structure. For example, Pd / CdS structure [M. C. Steele et. al., Appl. Phys. Lett., 28, 687 (1976)], Pd / InP structure [H. Chen et. al., US 2002/0182767, (2003)], Pd / ZnO structure [K. Ito et. al., Surface Sci., 86, 345 (1982)], Pd / GaAs structures [W. Liu et. al., US 6160278, (2000). However, the above sensors have limitations in terms of sensitivity or response time.

In addition, research on the hydrogen sensor using an electrode that serves as a catalyst and an ion conductive solid electrolyte is also active. Examples of the ion-conductive solid electrolyte used for the hydrogen sensor of the above-described frame structure for measuring the electrical conductivity changes from a change in the oxygen vacancies of the hydrogen absorption of SrTiO ₃ [Jonda, et. al., US 6513364 (2003)], structures using BaCe _1-xy L _x M _y O _3-x [N. Taniguchi et. al., US 2003/0024813 (2003)], a structure for measuring voltage change using Na (H ₃ O) Zr ₂ Si _x P _3-x ) O ₁₂ [K. Areekattuthazhayil et. al., US 6073478 (2000)] and structures using oxides of perovskite structures such as SrCe _0.95 Yb _0.05 O _3-x [K. Kunihiro et. al., US5445725 (1995), and their operating temperatures are higher than 200 ° C.

Another type of hydrogen sensor reported is to utilize the change in electrical properties when forming metal-hydrogen compounds. LaNi ₅ [Z. Jennifer et. al., US 6539774 (2003), rare earth metals [DJ Frank et. al., US6265222 (2001), Sc, Y, La [B. Gautam et. al., US6006582 (1999)], Ni _x Zr _1-x (Y. Cheng et. al., US5886614 (1999), etc., and a structure using an electrode having catalytic activity. Has the disadvantage of long response time.

Palladium causes changes in physical properties when the hydrogen is selectively adsorbed, for example, changes in mass, volume, electrical resistance, optical constant, and work function, and is used as a hydrogen sensor. However, the hydrogen sensor using the characteristics of palladium is difficult to measure the gas concentration in real time because the response time is long from a few seconds to a few minutes, and chemical adsorption occurs on the surface of the palladium when poisoned by active gases such as hydrocarbons, oxygen, water and carbon monoxide. Interfering with the adsorption of hydrogen has a problem that the sensitivity of the sensor is lowered. In addition, these hydrogen sensors have a problem that the power consumption is high because the operating temperature must be heated.

In order to overcome the limitations of the bulk palladium hydrogen sensor, hydrogen sensor using nanowire of palladium or nanowire of metal (Cu, Au, Ni, Pt, etc.) forming an stable metal-hydrogen compound or an array thereof Was developed [R. M. Penner et. al., US 2003/0079999 (2003). The hydrogen sensor has the advantages of smaller power consumption, smaller size, and faster response time (several tens of msec) than the bulk palladium element, but the minimum detectable hydrogen concentration is 0.4% and the hydrogen concentration less than that can be detected. There is a limit that cannot be.

Accordingly, the present inventors have made efforts to solve the above problems, and when the carbon nanotubes coated with palladium nanoparticles, porous palladium layers or dense palladium layers are mounted between the electrodes of the hydrogen sensor, the hydrogen sensor may operate at room temperature. As a result, power consumption is small, small sensor devices can be manufactured, response time is fast by several msec, real-time hydrogen concentration measurement and leakage monitoring are possible, and trace hydrogen concentration in ppm unit can be measured, even in mixed gas atmosphere. The present invention was completed by confirming that a reliable hydrogen sensor capable of measuring hydrogen concentration selectively.

Accordingly, an object of the present invention is to provide a hydrogen sensor to which carbon nanotubes coated with palladium that can detect hydrogen gas with high sensitivity are applied.

The present invention features a hydrogen sensor where palladium-coated carbon nanotubes are positioned between electrodes.

The present invention will be described in detail as follows.

According to the present invention, palladium, which is a catalytic metal that selectively adsorbs and dissociates hydrogen, is coated on a surface of a carbon nanotube and placed between electrodes of a hydrogen sensor, whereby palladium dissociates hydrogen molecules into hydrogen atoms. Adsorbed on the surface of the carbon nanotubes, electrons move from hydrogen to carbon nanotubes, reducing the hole concentration of the carbon nanotubes and at the same time reducing the electrical conductivity of the carbon nanotubes, thereby effectively detecting hydrogen gas with high sensitivity. The present invention relates to a carbon nanotube hydrogen sensor coated with palladium, and according to the present invention, the hydrogen sensor can operate at room temperature, and thus, power consumption is low, a small sensor device can be manufactured, and the response time is several msec. Measurement and leak monitoring are possible, and trace hydrogen concentrations in ppm can be measured. In addition, there is an effect that can selectively measure the hydrogen concentration in the mixed gas atmosphere.

The hydrogen sensor of the present invention is characterized by a structure in which carbon nanotubes coated with palladium, a catalytic metal that selectively adsorbs and dissociates hydrogen in the form of nanoparticles, porous palladium layers, or dense palladium layers, are positioned between two electrodes. 1A shows a hydrogen sensor applying nanotubes coated with palladium nanoparticles, and FIG. 1B shows a hydrogen sensor applying nanotubes coated with a palladium layer. In the hydrogen sensor, palladium acts as a catalyst for selectively adsorbing hydrogen molecules and dissociating them into hydrogen atoms. At this time, the dissociated hydrogen atoms are adsorbed on the surface of the carbon nanotubes, thereby reducing the concentration of holes in the carbon nanotubes. Allow the electrical resistance to increase.

2A and 2B show the results of theoretically calculating the energy when hydrogen is adsorbed to the carbon nanotubes in a molecular state and in an atomic state. When hydrogen is adsorbed in the carbon nanotubes in the molecular state, as the distance between the carbon nanotubes and the hydrogen molecules near as shown in Figure 2a it can be seen that it does not adsorb in the molecular state because it continuously increases without energy minima. . On the other hand, when the hydrogen atoms dissociated in the carbon nanotubes are adsorbed on the carbon nanotubes as shown in FIG. 2B, energy minima appear at a specific distance (1.113 kPa), and this distance becomes the bonding distance between the carbon nanotubes and the adsorbed hydrogen atoms. The calculation results show that the carbon nanotubes are more easily adsorbed when hydrogen is present in the atomic state. Therefore, it can be seen that palladium which dissociates hydrogen molecules into hydrogen atoms is necessary as a catalyst.

Since semiconducting carbon nanotubes are generally p-type semiconductors whose holes are charge carriers, changes in electrical properties of carbon nanotubes can be predicted by calculating the transfer and distribution of charges when hydrogen is adsorbed using computational simulation. .

The results of calculating the electronic structure when the hydrogen atoms are adsorbed on the carbon nanotubes by using computer simulation are shown in the accompanying drawings. 3 is a diagram showing the charge distribution when the carbon nanotubes and the hydrogen atoms are subtracted from the total charge distribution when one hydrogen atom is adsorbed on the carbon nanotubes, and the hydrogen atoms are adsorbed on the carbon nanotubes. It shows that the amount of charge increases. The fact from the calculation of the electronic state using the simulation is that when hydrogen is adsorbed on the surface of carbon nanotubes, 0.192 electrons are transferred from hydrogen to carbon nanotubes. As a result, the electrons transferred from the hydrogen atoms to the carbon nanotubes decrease the hole concentration in the carbon nanotubes, and as a result, the electrical resistance of the carbon nanotubes increases.

Theoretically, the prediction of the increase in electrical resistivity when hydrogen atoms are adsorbed on carbon nanotubes, and palladium, which is known to be capable of dissociating hydrogen molecules into hydrogen atoms, led to the present invention.

In other words, when the palladium nanoparticles are coated on the surface of the carbon nanotubes, the palladium nanoparticles dissociate the hydrogen molecules into hydrogen atoms, and the dissociated hydrogen atoms are easily adsorbed onto the carbon nanotubes, thereby increasing the electrical resistivity of the carbon nanotubes, thereby acting as a sensor. Will be done.

Carbon nanotubes that can be used in the present invention may be selected from single-walled carbon nanotubes, multi-walled carbon nanotubes and carbon nanotube bundles, etc., and can be coated with the palladium nanoparticles or palladium layer. The palladium layer may be any palladium layer, such as a porous palladium layer or a denser palladium film layer.

Palladium nanoparticles or palladium layers coated on carbon nanotubes are advantageous in that they have a large specific surface area for effective adsorption and dissociation of hydrogen molecules, so it is preferable to coat porous layers containing nanoparticles or nanopores, but in the case of palladium film layers In addition, since the hydrogen atoms can move to the carbon nanotubes by hydrogen diffusion, it can be utilized in the hydrogen sensor of the present invention.

The method of coating the surface of the carbon nanotubes with a palladium nanoparticle or a palladium layer can be prepared by vapor phase, liquid phase, and solid phase methods, and is not limited to palladium nanoparticles prepared by a specific method.

Synthesis of palladium nanoparticles is carried out by a reduction reaction in an aqueous solution, or by dissolving a surfactant such as diisooctyl sodium sulfosuccinate or sodium dodecyl sulfate (SDS) in an oil such as isooctane or heptane. To prepare a w / o emulsion, and then react to prepare palladium nanoparticles.

Synthetic nanoparticles can be added to a solution in which carbon nanotubes are dispersed and coated, or the palladium nanoparticles can be attached to carbon nanotubes simultaneously with particle synthesis.

It is also possible to effectively modify the surface of the carbon nanotubes through functional groups or the like to charge them, and to modify the surface of the palladium nanoparticles with opposite charges to effectively coat the surface by electrostatic attraction. The aqueous surfactant may be anionic surfactant such as polymethyl methacryl ammonium salt, polyacrylic ammonium salt, polyacryl amine salt, polyacrylic sodium salt, cationic surfactant such as polyethyl imine, etc. Nonionic surfactants such as polyoxyethylene nonylphenyl ether can be used.

Alternatively, as the non-aqueous surfactant, sodium dodecyl sulfate, diisooctyl sodium sulfosuccinate, naphthalenedicarboxylic acid or naphthalenedicarboxyl metal salt, etc. can be used as a dispersant to control the coating amount of the palladium particles.

The coating method of the palladium layer is to coat a palladium layer of a certain thickness on the surface of the carbon nanotube using conventional vacuum evaporation, or electroless plating using a palladium raw material such as PdCl ₂ , Sol-Gel method, etc. can be used.

The present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following Examples.

Example

First, to prepare a dissolving PdCl ₂ in water, 0.82 weight% solution, and added to NaBH ₄ as a reducing agent was applied an ultrasonic wave at least about one hour dissolution two times (43% by weight relative to the PdCl ₂₎ in the PdCl ₂ molar amount of palladium metal It was reduced to prepare a nanoparticle aqueous solution.

The palladium nanoparticles were separated from the aqueous solution of nanoparticles, placed in a ceramic container, and heat-treated at 400 to 600 ° C. for 1 hour at a temperature increase rate of 10 ° C./min to obtain final palladium nanoparticles.

The carbon nanotubes were dispersed in N, N- dimethylformamide (DMF) at a concentration of 25 μg / ml, and the solution obtained by dispersing the palladium nanoparticles prepared above in DMF at a concentration of 2.5 μg / ml was used in a volume ratio. Mixing at 1: 1 and adding ultrasonic waves for 1 hour to prepare a carbon nanotube dispersion solution coated with palladium.

In order to fabricate the electrode of the sensor element, an oxide layer is formed on a silicon substrate by a general thermal oxidation method, and a 400 nm thick gold thin film is deposited on the silicon substrate by vacuum evaporation, and then lift-off using a mask. ) Two electrode patterns parallel to each other were formed. An electrode pattern manufactured by this method is shown in FIG. 4.

2 μl of the palladium-coated carbon nanotube dispersion solution prepared above was dropped and spinned at a rotational speed of 800 rpm to form a palladium-coated carbon nanoparticle on the electrode pattern prepared by the method described above. The tube was positioned to construct a sensor element.

Electron micrographs of the gas detection part of the hydrogen sensor device manufactured in the present invention are shown in the accompanying drawings, Figs. 5a and 5b and it can be seen that one or several palladium-coated carbon nanotubes are connected between the two electrodes. Can be.

Experimental Example

In order to evaluate the electrical characteristics of the hydrogen sensor manufactured according to the above embodiment, the current change was measured while changing the voltage at regular intervals. The current change characteristic curve according to the voltage change was obtained by varying the voltage by 0.04 V in the range of -2 V to +2 V. The results are shown in FIG. The IV characteristic curve of the hydrogen sensor manufactured in the present invention showed semiconductor characteristics, and the current was 1 × 10 ⁻⁸ A when the applied voltage was 2 V.

In order to measure the hydrogen detection performance of the hydrogen sensor manufactured in the present invention, the current change was measured while flowing hydrogen gas at a rate of 10 ml / min while applying a constant voltage of 2 V with a hydrogen sensor in a closed space. . As a result, as shown in FIG. 7, when a constant voltage of 2 V is applied, the current is maintained at about 1 × 10 ^-8 A, and when hydrogen is flowed, the current rapidly drops to 1 × 10 ^-10 A or less. This is in good agreement with what was previously described in theoretical calculations and predictions. As a result, hydrogen molecules dissociate into palladium as hydrogen atoms, and the dissociated hydrogen atoms are adsorbed on the surface of carbon nanotubes. This is because the concentration of holes in the nanotubes is reduced, thereby reducing the electrical conductivity of the carbon nanotubes, which are p-type semiconductors.

Since the current value in the hydrogen atmosphere is reduced to 1/100 in the absence of hydrogen, the hydrogen sensor of the present invention has a sensitivity that is 10 times higher than that of a conventional hydrogen sensor and the operating temperature is room temperature. ) And the operating environment performance is very good.

As described above, in the case of a hydrogen sensor in which a surface of a carbon nanotube is coated with palladium nanoparticles or a porous or dense palladium layer and connected between two electrodes, palladium dissociates hydrogen molecules to hydrogen atoms, and As hydrogen atoms are adsorbed on the surface of carbon nanotubes, electrons move from hydrogen to carbon nanotubes to reduce the hole concentration of carbon nanotubes and consequently to detect hydrogen gas effectively by using the principle of reducing the electrical conductivity of carbon nanotubes. have.

In the case of the hydrogen sensor of the present invention, since it operates at room temperature without a separate heater device, the power consumption is small, and the change in conductivity according to the adsorption of hydrogen gas (△ I / I) is 1/100, so it is applied as a high sensitivity hydrogen sensor. Applications include petrochemical industry, food industry, semiconductor, steel industry, automobiles, nuclear power plants, power equipment, medical field, aerospace industry, fuel cells, etc.

1A is a schematic diagram of a hydrogen sensor using carbon nanotubes coated with palladium nanoparticles of the present invention, and FIG. 1B is a schematic diagram of hydrogen sensor using carbon nanotubes coated with a porous or dense palladium layer of the present invention.

Figure 2a is a graph showing the change in energy according to the distance when the hydrogen molecules (H ₂ ) cited in the SP Chan et al. Research paper is adsorbed on the carbon nanotubes, Figure 2b is a hydrogen atom (H) to the carbon nanotubes It is a graph calculated by computer simulation of energy change with distance when adsorbed.

3 is a diagram showing the result of calculating the charge distribution using computer simulation when hydrogen atoms are adsorbed on carbon nanotubes.

4 is an electron micrograph of an electrode pattern formed on a substrate as manufactured according to an embodiment.

5a and 5b are electron micrographs of the gas detection portion of the hydrogen sensor of the present invention.

6 is a graph showing the current change characteristics according to the voltage change of the hydrogen sensor of the present invention.

7 is a graph showing the hydrogen detection characteristics of the hydrogen sensor of the present invention.

<Description of the symbols for the main parts of the drawings>

1 substrate 2 electrode

3: carbon nanotube 4: palladium nanoparticles

5: palladium layer

Claims (5)

Hydrogen sensor characterized in that the palladium-coated carbon nanotubes are located between the electrodes. The hydrogen sensor of claim 1, wherein the carbon nanotubes are selected from single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon nanotube bundles. The hydrogen sensor of claim 1, wherein the palladium is palladium nanoparticles. The hydrogen sensor of claim 1, wherein the palladium is a porous palladium layer. The hydrogen sensor of claim 1, wherein the palladium is a palladium film layer.