ROOM TEMPERATURE HYDROGEN SENSOR BASED ON NANO-MICRO INTEGRATION FOR SPACE EXPLORATION

PI: S. Seal (University of Central Florida)

Co-PI: H.J. Cho (University of Central Florida) S. Shukla (University of Central Florida)

L. Ludwig (Kennedy Space Center) P. Zhang (University of Central Florida)

S. Deshpande (University of Central Florida) C. Drake (University of Central Florida)

Abstract

We are developing the sol-gel derived nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) sensor, in the form of thin film/nanowires/nanofibers, for room temperature hydrogen (H2) sensing application, for NASA, under the atmospheric conditions existing on the surface of the Earth. The nanocrystalline thin film sensor is incorporated into the microelectromechanical system (MEMS) device to achieve high H2 sensitivity and selectivity with minimum detection and recovery time at room temperature. Effect of various test parameters such as the air pressure, the H2 concentration as well as the MEMS design parameters such as the finger spacing and the number of fingers on the room temperature H2 sensing characteristics of the present sensor has been demonstrated. The present nano-micro integrated sensor shows giant room temperature H2 sensitivity (S=103-105) with high selectivity over CO. The current H2 detection and recovery time at room temperature lie within the range of 100-250 sec and 150-200 sec respectively. New technological solutions for further reducing the response and the recovery time of the present nano-micro integrated sensor have been proposed. Sensor tests are underway to test the H2 sensitivity of the present nano-micro integrated sensor under the atmospheric conditions existing on the surface of Mars and Moon.

Introduction

Hydrogen (H2) is the most abundant element in the universe and one of the most abundant on Earth. Due to the rapid consumption of the fossil fuels, much attention has been paid towards H2 as an economical non-conventional energy source for the diversified industrial applications. For example, solid oxide fuel cell (SOFC) technology uses gaseous H2 for the generation of power and heat. H2 powered cars and buses are already in normal transit service in some of U.S. cities. Liquid H2 has been used by NASA for launching the space-shuttles. As summarized in Figure 1, H2 also finds applications in electronic, metallurgical, pharmaceutical, nuclear fuel, food and beverages, as well as glass and ceramic industries. Every day, millions of pounds of H2 are used by hundreds of industries around the world. Due to the realization of the potential use of H2 energy further interest has grown into the production of large quantity of different forms of H2, the enhancement of H2 storage capacity, and the development of safe transportation system for H2.

Depending on the quantities required, H2 can be transported by road tanker or pipeline. North America alone has at least 700 km H2 pipeline system. Pipelines for liquid H2 have also been built by NASA for direct delivery of H2 to the space vehicle at the launch pad. However, due to its very small size, H2 is the most susceptible for the leakage through

the pipelines; typically about 1-3 % of H2 in the existing systems is always lost, mostly through the joints in the pipes. If handled carelessly, H2 is as dangerous for transport, storage and use as many other fuels. As a result, safety remains a top priority in all the aspects of H2 energy and has been the prime motivation for the present work.

In the recent years, nanotechnology has emerged as an attractive field for the development of novel materials having unusual properties, which have provided different pathways to solve many unresolved issues in various other fields. We strongly believe that the application of nanotechnology to H2 sensors would help in advancing the science and the technology related to the development of sensor materials.

Background

Different experimental (metal-oxide-semiconductor (MOS)-based, catalytic resistor, acoustic wave and pyroelectric) and commercial sensors (catalytic combustion, electrochemical, semiconductor, and thermal conductivity) based on different principles are currently available but with major drawbacks as outlined in Figure 1. The very low sensitivity of these sensors at room temperature to low concentrations of H2 has been invariable associated with the poor response and recovery time, which insists further investigation in these areas. In addition to this, the poor H2 selectivity, which is a severe problem at room temperature, has been another major but pending issue, which needs to be delved in great detail. Particularly, attention must be paid in improving the H2 selectivity by a novel approach without sacrificing the room temperature H2 sensitivity and the response and recovery time of the sensor. Moreover, many of the experimental as well as commercially available sensors use the nanocrystalline materials, which are susceptible to changes in their physical properties (such as nanocrystallite size) if operated at highly temperature, which may reduce the potential life of the sensor. Such sensors, hence, must be operated at lower temperatures, where further research is still awaited.

In Table 1, we summarize the list of companies, which manufacture and sell the H2 sensor devices. Various characteristics of these H2 sensor devices have also been shown for comparison. It appears to us that, these commercially available sensor devices are claimed to sense H2 at room temperature within the concentration limits of 200 ppm-2%, which large enough for any practical application. Response time quoted for these sensor devices is less than 10 sec, but it is related to high H2 concentration as high as 90% and not for ppm-level H2 concentration. Although the sensor devices are claimed to operate at room temperature, the recovery time often is associated with higher temperature (70 oC). Moreover, the room temperature H2 sensitivity values for these devices are often not quoted. In addition to this, there are only few manufacturers, which guarantee the cross-sensitivity to other poisonous gases. No claims are made regarding the suitability of these sensor-deices for sensing H2 on other planetary conditions, which is an essential requirement for NASA. It is suggested that, the commercial sensors currently available in the market are designed only to meet the atmospheric conditions on the earth’s surface. Modifying the sensor material properties to meet the NASA’s over all requirements is imperative.

Since our efforts are mainly focused on improving the semiconductor oxide based sensors, the current status of this particular class of sensor has been summarized in Table 2. Various forms of sensors such as thin films, random network of nanowires and

WHY ARE HYDROGEN SENSORS NEEDED?

Hydrogen Applications

Metallurgical Industries • Reducing Atmosphere • Steel Making • Copper Brazing

Chemical and Petroleum Industries • Production of Ammonia and

Methanol • Sulfur Removal • Catalytic Converter

Pharmaceutical Industries • Production of sorbitols

used in cosmetics, adhesives, surfactants, vitamins a and C.

Food and Beverages Industries • Hydrogenating Liquid

Oils Converting them into Semisolid Materials

Aerospace Industries • Launching Space

Shuttles • Power Life Supports and

Computers • Produce Drinkable

Water as Byproduct

Glass and Ceramic Industries • Manufacturing Float

Glass to Prevent Oxidation of Large Tin Bath.

Electronic Industries • Carrier Gas for Active

Trace Elements as Arsine and Phospine for Manufacturing Semi-Conducting Layers in Integrated Circuits.

Nuclear Fuel Industries • Protective Atmosphere for

Fabrication of Fuel Rods

Hydrogen Production • Crude Oil, Coal,

Natural Gas • Nuclear • Solar, Hydro, Wind

Wave, Geothermal • Wood, Organic Waste,

Biomass

Hydrogen Transportation • Specially Designed Tanker

Trucks • Pipelines

Hydrogen Storage • Metal Hydrides • Liquid Hydrides • Carbon Nanotubes • Compressed Hydrogen

Figure 1. Chart summarizing the need for an immediate development of hydrogensensors based on innovative approaches for overcoming the limitations of the currentsensor technology and meeting the requirements of the hydrogen based industries.

STATUS OF HYDROGEN SENSORS

Exiting Hydrogen Sensors Experimental

• MOS Structures • Catalytic Resistors • Acoustic Wave • Pyroelectric

Commercial • Catalytic Combustion • Electrochemical • Semiconductor • Thermal Conductivity

Precincts • H2O Sensitive • Film Damage at High Hydrogen

Concentrations • Hydrogen Induced Drift • Insensitive to Low H2 Concentrations • Poor Response at Room Temperature • Limited Hydrogen Detection Range • Poor Selectivity • Cannot Operate in Helium and in Vacuum • Large Size

Table 1. Summary of characteristics of commercially available H2 sensors.

Company

Sensor

H2 Range

Temperature

Range (oC)

Humidity

%

Power

Response/ Recovery

(sec)

Fuel Cell-Sensor

-

0-5000 ppm 0-10000 ppm

80

<95

1 W

12 V DC

<10 sec

90% Concentration

H2 Scan LLC

Pd/Ni Thin Film

0.5%-10%

70 (Gas

Temp.)/1 atm 0-40

(Operating)

<95

At 40 oC

2 W

12-24 V DC

In sec

Depending on Concentration

Neodym

Technologies

MOS

<2%

80

10-95

At 40 oC

1 W

12 V DC

4 (Recovery 10) Alarm Point 2000 ppm

RKI

Instruments

MOS with Molecular

Sieve+Transmittor

2000 ppm-

2%

-9 to 43

5-95

11-30 V DC

20 sec for

90% Concentration

Applied Nanotech. Inc.

Pd Nanoparticles

0.5-2%

In Volume

110

-

microwatts

<10 sec

(Recovery <10 sec at

70 oC)

Arrgh! Manufacturing

Inc.

-

1%

-10 to 40

-

12-48 V DC

-

Industrial Sci.

Corp.

-

-

60

<99

3 V DC

-

Enmet Co.

-

200-2000

ppm

-

-

-

-

Nanotubes, as well as aligned or single nanowires of semiconductor oxides have been developed for the H2 sensing application. It seems that, the maximum H2 sensitivity at room temperature has been reported for the thin film form of the sensor with the response and the recovery time in few minutes. On the other hand, a single nanowire type sensor exhibits the response time in few seconds but with extremely low H2 sensitivity. The random network of nanowires, having large porous structure, may be a good choice for selecting the appropriate form of the sensor, which may compromise the H2 sensitivity to some extent for improving the response time. Recently, the single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) have been synthesized and utilized for the H2 (and other gases as well) sensing application. Some of the typical gas sensing results reported in the literature for these new gas sensing materials are tabulated in Table 3 and can be compared with the gas sensing properties of semiconductor oxides gas sensors, Table 2. Three different forms of carbon nanotubes such as single, parallely aligned, and

random network have been investigated for the gas sensing. Comparison reveals that, relative to the semiconductor oxides gas sensors, the carbon nanotubes exhibit very low gas sensitivity (<2) and high detection time (5-2700 sec). Moreover, the recovery time associated with the carbon nanotubes based gas sensor has been few hours.

Table 2. Typical gas sensing results reported recently for the various forms of

semiconductor oxides and our results

Sensor Material

Sensor Form

Synthesis

Method

Operating

Temperature(oC)

Gas

(Amount)

Sensitivity (Rair/Rgas)

Response

Time (Sec)

C2H5OH

(250 ppm)

2

SnO2

Single Nanobelt

Vapor Phase Evaporation

200

NO2

(0.5 ppm)

30

Few Seconds

SnO2/Pd

Single

Nanowire

Thermal

Evaporation

200

H2 (?)

2.5

2.5

Pd

Single

Nanowires

Electrochemical

Deposition

25

H2 (5 %)

3.5

75

(mSec)

In2O3

Random

Network of Nanowires

Carbothermal

Reduction

370

C2H5OH (1000 ppm)

30

10

ZnO

Random

Network of Nanowires

Thermal

Evaporation

300

C2H5OH

(200 ppm)

50

15

TiO2

Nanotubes

Array

Anodization

290

H2

(1000 ppm)

10000

200

In2O3-SnO2SEAL

Thin Film

Sol-Gel

20

H2

(900 ppm)

110000

100

Table 3. Typical gas sensing results reported recently for the various forms of carbon nanotubes.

Sensor Material

Sensor Form

Synthesis

Method

Operating

Temperature (oC)

Amount of Gas

Sensitivity (Rair/Rgas)

Response

Time (Sec)

SWCNT-

Pd

Single Tube

Patterned

CVD Growth

RT

H2

(400 ppm)

2

5-10

SWCNT

Parallely Aligned

PECVD

165

NO2

(100 ppb)

1.28

2700

SWCNT-

PABS

Random Network

Arc

Discharge

32

NH3 (100 ppm)

0.25

60

25

1.1

SWCNT -Pd

Random Network

Arc Discharge

250

H2 (0.5-2.0

%)

1.2

120

MWCNT

Random Network

Modified PECVD

25

NH3 (200 ppm)

0.7

180

CNT

Random Network

PECVD

165

NO2

(100 ppb)

0.56

Few

Minutes

Experimental

(a) H2 Sensing Tests The MEMS devices, which utilize an oxidized Si-wafer (Si-SiO2) as the platforms, were patterned with four interdigitated (Au) electrodes, using thermal evaporation, photolithography, and wet chemical etching techniques. The MEMS devices were designed with the different number of fingers (8 and 20) and different finger spacing (10 μm and 20 μm). The tin-isopropoxide solution in iso-propanol and toluene, corresponding to the concentration of 0.23 M of tin-isopropoxide, was used with the addition of calculated amount of indium(III)-isopropoxide to obtain the thin films of SnO2-6.5 mol% In2O3 via a sol-gel dip-coating process. The dried gel films were sputtered with a thin Pt-layer for 10 sec using a sputter-coater. The coated-MEMS devices were dried at 150 oC for

15-30 min in air. The dip-coating, the sputtering, and the drying processes were repeated to obtain a desired film thickness. Finally, the Pt-sputtered dried gel films were fired at 400 oC in air for 1 h and utilized for the characterization and H2 sensing tests. The coated and calcined MEMS devices were wire-bonded to an integrated circuit chip and installed in the 32 pin socket assembly, which was in turn placed centered over the sensor test-board designed using the LPKF CircuitCAM 4.0 software and cut using the LPKF Boardmaster 4.0 software on a single-sided copper clad prototype boards. All H2 sensing tests were conducted in the dynamic test condition at room temperature (22 oC with the relative humidity of 35-50%). In this type of sensor-testing, the air-pressure within the test-chamber was reduced and maintained at a desired level using the turbo-pumps. A mixture of appropriate amounts of nitrogen (N2) and H2 was admitted into the test-chamber through the respective mass-flow-controllers. The N2 (15000 ppm) was used as a carrier-gas. The amount of H2 in ppm was calculated using the ratio of number of moles of H2 admitted into the test-chamber per minute to the total number of moles of gas molecules (that is, the summation of number of moles of N2, H2, and air) within the test-chamber. Thus, in the dynamic test condition, a desired amount of H2 was continuously blown into the test-chamber per minute and simultaneously pumped out of the test-chamber throughout the test-duration. Thus, the dynamic test condition simulates the condition, which may be encountered in an actual service application, for example, H2 leakage through a pipe line. (b) Nanowires/Nanofibers Sensor Material Development

Two different techniques, electrospinning and thermal evaporation, have been utilized to synthesize the nanocrystalline SnO2-based nanowires/nanofibers. In the electrospinning Figure 2, tin (II) chloride (SnCl2) precursor was dissolved completely in a highly concentrated polymeric solution, which was then taken in a syringe placed on a syringe pump, that fed the syringe tip with the polymeric solution at a constant speed. A Cu-plate covered with Al-foil was placed in front of the syringe at a distance of 10 cm and a high voltage (15 kV) was applied in between the Cu-plate and the syringe tip. The

-+ 0-30 kV

Syringe Pump Syringe

Taylor Cone

Jet

Cu-Plate Covered with Al-Foil

High Voltage Power Supply

Polymer Solution

Metallic Syringe Tip

Bending Instability of Jet

Bending Instability of Jet

-+ 0-30 kV

Syringe Pump Syringe Metallic Syringe Tip

Taylor Cone

Jet

Cu-Plate Covered with Al-Foil

High Voltage Power Supply

Polymer Solution

Figure 2. Schematic of electrospinning process for nanowires/ nanofibers formation.

polymer fibers were drawn from the syringe tip, which were subsequently deposited on the Al foil (or the Si-SiO2 substrate). Since, the polymer fibers contain the Sn-precursor, low temperature calcination temperature burnt off the polymer leaving behind the inorganic SnO2-based nanowires/nanofibers. In the thermal evaporation technique, Figure 3, a high temperature furnace was utilized. The Sn-precursor powder and the Si-SiO2 substrate (with Pt catalyst) were taken into the Al2O3 crucible, which was placed in the center of the furnace. Argon (Ar) gas was blown continuously into the furnace. The furnace was ramped to 900 oC, held at that temperature for 5 h, then cooled naturally to room temperature.

Ar InAr Out

Furnace

Precursor PowderSubstrate

Ar InAr Out

Furnace

Precursor PowderSubstrate

Figure 3. Schematic diagram describing the thermal evaporation process fornanowires/nanofibers formation.

Results and Discussion (a) Room Temperature H2 Sensing Characteristics of Nano-Micro Integrated Sensor As shown in Figures 4 and 5, the present nano-micro integrated sensor shows very high H2 sensitivity at room temperature as high as 103-104. Moreover, the sensor being insensitive to CO at room temperature, it exhibits very high H2 selectivity over CO. As shown in Figure 6, the response time of the present sensor lies within the range of 100-250 sec and the recovery time lies within the range of 150-200 sec. The variation in the response kinetics of the present nano-micro integrated sensor is presented in Figure 7. Improved response kinetics with increasing H2 concentration within the range of 100-15000ppm is noted. Within this range room temperature H2 sensitivity lies within the range of 3-105. As demonstrated in Figure 8, the room temperature H2 sensitivity of the present sensor is almost insensitive to the air pressure level within the range of 50-760 Torr. The sensor is also sensitive to H2 at room temperature in the helium (He) atmosphere, Figure 9. The room temperature response kinetics of the present nano-micro integrated sensor is superior for smaller finger spacing, Figure 10.

Figure 5. Room temperature H2 selectivity over CO.

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

0 2000 4000 6000 8000 10000 12000

Sens

or R

esis

tanc

e (Ω

)

Air

H2

CO Air

104

105

106

107

108

109

Time (sec)0 4000 8000 12000

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

0 2000 4000 6000 8000 10000 12000

Sens

or R

esis

tanc

e (Ω

)

Air

H2

CO Air

104

105

106

107

108

109

Time (sec)0 4000 8000 12000

Time (sec)

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1000 2000 3000 4000 5000 6000 7000

H2

Air

H2H2 H2 H2 H2

Air AirAir Air AirSe

nsor

Res

ista

nce

(Ω)

106

107

108

109

2000 40000 6000

Time (sec)

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1000 2000 3000 4000 5000 6000 7000

H2

Air

H2H2 H2 H2 H2

Air AirAir Air AirSe

nsor

Res

ista

nce

(Ω)

106

107

108

109

2000 40000 6000

Figure 6. Low room temperature response (100-250 sec) and recovery time(150-200 sec). (Note: Data points are separated by 50 sec time interval.)

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

0 4000 8000 12000 16000

Sens

or R

esis

tanc

e (Ω

)

Air

H2 H2

Air

Time (sec)

(b)

107

105

109

4000 8000 12000 1600001.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

0 4000 8000 12000 16000

Sens

or R

esis

tanc

e (Ω

)

Air

H2 H2

Air

Time (sec)

(b)

107

105

109

4000 8000 12000 160000

Figure 4. Room temperature H2 (900 ppm) sensing with high sensitivity.

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

0 2000 4000 6000 8000 10000 12000

He Atmosphere

H2

AirSens

or R

esis

tanc

e (Ω

)

UV-ON

UV-OFF

H2

104

105

106

4000 8000 120000

107

108

109

Time (sec)

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

0 2000 4000 6000 8000 10000 12000

He Atmosphere

H2

AirSens

or R

esis

tanc

e (Ω

)

UV-ON

UV-OFF

H2

104

105

106

4000 8000 120000

107

108

109

Time (sec)

Figure 9. Room temperature H2 sensing in He atmosphere.

B B

1.0E-01

1.0E+01

1.0E+03

1.0E+05

1.0E+07

0 2000 4000 6000 8000

Sens

itivi

ty

Time (sec)

10-1

101

103

105

107

0 2000 4000 6000 8000

H2

Air

1.0E-01

1.0E+01

1.0E+03

1.0E+05

1.0E+07

0 2000 4000 6000 8000

Sens

itivi

ty

Time (sec)

10-1

101

103

105

107

0 2000 4000 6000 80001.0E-01

1.0E+01

1.0E+03

1.0E+05

1.0E+07

0 2000 4000 6000 8000

Sens

itivi

ty

Time (sec)

10-1

101

103

105

107

0 2000 4000 6000 8000

H2

Air

Figure 8. Room temperature H2 sensing for different air pressure levels(50-760 Torr).

Figure 7. Room temperature H2 sensing for different H2 concentration levels.

0.1

1

10

100

1000

10000

100000

1000000

0 2000 4000 6000 8000 10000 12000

100

300

500700

15000

100

102

104

106

0 2000 4000 6000 8000 10000 12000

Time (sec)

Sens

itivi

ty (R

air/R

gas)

Air

900

H20.1

1

10

100

1000

10000

100000

1000000

0 2000 4000 6000 8000 10000 12000

100

300

500700

15000

100

102

104

106

0 2000 4000 6000 8000 10000 12000

Time (sec)

Sens

itivi

ty (R

air/R

gas)

Air

900

H2

0.1

1

10

100

1000

10000

100000

0 2000 4000 6000 8000 10000 120000 2000 4000 6000 8000 10000 1200010-1

101

103

105

Time (sec)

Sens

itivi

ty (R

air/R

gas)

10 μm

H2

Air

20 μm

0.1

1

10

100

1000

10000

100000

0 2000 4000 6000 8000 10000 120000 2000 4000 6000 8000 10000 1200010-1

101

103

105

Time (sec)

Sens

itivi

ty (R

air/R

gas)

10 μm

H2

Air

20 μm

Figure 10. Comparison of room temperature H2 response kinetics for twodifferent finger spacing.

(b) Nanowire/Nanofiber Sensor Material Development The nanocrystalline SnO2based fibers deposited on the MEMS device via electrospinning technique are shown Figure 11; while, that derived using the thermal evaporation process are shown in Figure 12. Determining the room temperature H2 sensing characteristics of the present nanofibrous sensor is under investigation.

1 μm

(b)

500 μm

(a)

1 μm

(b)

1 μm

(b)

500 μm

(a)

500 μm

(a)

Figure 11. SEM images of SnO2 nanowires synthesized via electrospinning. In (a),microelectromechanical system (MEMS) device with interdigitated gold (Au)electrodes is seen.

2 μm2 μm

Figure 12. SnO2 nanowires synthesized via thermal evaporation technique.

Future Work

- Utilize nanoelectromechanical system (NEMS) design (instead of MEMS) for reducing the room temperature response and recovery time below 60 sec.

- Utilize the nanofibrous SnO2-based sensor for the room temperature H2 sensing.

- Utilize the thin film as well as nanofibrous sensor to sense H2 under the

atmospheric conditions existing on the surface of Moon and Mars.

- Develop a prototype H2 sensor device for NASA operating at room temperature.

March 2006