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A passive wireless hydrogen surface acoustic wave sensor based on Pt-coated ZnO

nanorods

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2010 Nanotechnology 21 095503

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 21 (2010) 095503 (6pp) doi:10.1088/0957-4484/21/9/095503

A passive wireless hydrogen surfaceacoustic wave sensor based on Pt-coatedZnO nanorodsYa-Shan Huang 1, Yung-Yu Chen 2 and Tsung-Tsong Wu 1, 3

1 Institute of Applied Mechanics, National Taiwan University, Taipei 106, Taiwan2 Department of Mechanical Engineering, Tatung University, Taipei 104, Taiwan

E-mail: [email protected]

Received 28 September 2009, in nal form 21 January 2010Published 8 February 2010

Online at stacks.iop.org/Nano/21/095503

AbstractUsing a passive wireless sensor to detect hydrogen can reach the goals of reducing cost andincreasing the lifetime since the sensor can work without batteries. In this paper, a passivewireless hydrogen SAW sensor operating at room temperature has been achieved by combininga SAW tag and a resistive hydrogen sensor. The SAW tag is fabricated on a 128 YX-LiNbO 3substrate and its central frequency is 433 MHz. The resistive hydrogen sensor with thePt-coated ZnO nanorods as the sensing lm has the advantages of high stability, goodrepeatability and simple fabrication. The ZnO nanorods are synthesized by using the aqueoussolution method and the Pt coating is employed as a catalyst for the hydrogen detection. Theproperty of the resistive hydrogen sensor is examined before combining with the SAW tag.Results show that the resistance changes caused by the variations of relative humidity andtemperature are negligible. Finally, the hydrogen SAW sensor is congured and measuredwirelessly for various hydrogen concentrations at room temperature. The difference of therelative return loss caused by the hydrogen concentration variation is obvious and recognizable.All responses show that the proposed hydrogen sensor not only has good repeatability and highsensitivity but is capable of passive wireless detection.

(Some gures in this article are in colour only in the electronic version)

1. Introduction

Using wireless sensors to monitor the environment andmaintain human safety is highly convenient because theyeliminate the circuitry problem. Most wireless sensors areactive systems, which have a limited life because they arebattery-powered. Passive wireless sensors not only keep theadvantages of wireless but can also work without batteries. Intimes of energy shortage, the development of passive sensorsis expected. In addition, clean energies including hydrogenbecome more attractive in an energy crisis. Hydrogen gasis colorless, tasteless and ammable. When concentrationsof hydrogen are between 4% and 75%, hydrogen is subjectto combustion and an explosion risk. During the process of hydrogen gas storage and transportation, monitoring for leaks

3Address for correspondence: Room 411, Institute of Applied Mechanics,National Taiwan University, No 1, Sector 4, Roosevelt Road, Taipei 106,

Taiwan.

of hydrogen gas is important for environmental protectionand human safety. Hence, the passive wireless hydrogensurface acoustic wave (SAW) sensor has been developed forits convenience and low cost.

The SAW-based radio-frequency identication (RFID)system, which can transmit identication information wire-lessly without an additional power supply, was proposed forthe rst time in 1972 [ 1]. Steindl et al [2] showed therelation between the reected signal characteristics, such asphase and return loss (RL), and the impedance across areector. They also provided a wide range of remote sensingopportunities by using a SAW RFID system. Afterwards therewere many investigations into the impedance-loaded SAWsensor in various applications, such as for measurements of temperature [ 3], humidity [4], pressure [ 5], tire friction [ 6],water content in sandy soil [7], and wheel rotation and key-click functionality of a personal computer mouse [ 8], as wellas Holter electrocardiograms [ 9].

0957-4484/10/095503+06 $30.00 2010 IOP Publishing Ltd Printed in the UK1
http://dx.doi.org/10.1088/0957-4484/21/9/095503mailto:[email protected]://stacks.iop.org/Nano/21/095503http://stacks.iop.org/Nano/21/095503mailto:[email protected]://dx.doi.org/10.1088/0957-4484/21/9/095503


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Nanotechnology 21 (2010) 095503 Y-S Huang et al

Figure 1. Conguration of the passive wireless hydrogen SAWsensor.

For monitoring hydrogen, a palladium/platinum resistivesensor was presented [10]. High temperature aqueous systemswere employed to dissolve hydrogen and measured theresistance difference of palladium. Then, Favier et al [11]fabricated a hydrogen sensor using mesoscopic palladium wirearrays with gaps in the wire arrays in the nanoscale as a sensinglm. The circuits of the wire arrays were turned open in theabsence of hydrogen. Recently, Wang et al [12] used multipleZnO nanorods with different catalyst coatings as the sensinglayers to detect hydrogen. Tien et al [13] further demonstratedthat the Pt-coated ZnO nanorod is a good selective layer forsensing hydrogen.

In this paper, a passive wireless hydrogen SAW sensoris proposed. As shown in gure 1, the SAW sensor wascongured by combining a SAW tag and a resistive hydrogensensor. The sensing lm of the resistive sensor is the Pt-coated

ZnO nanorods and was synthesized by using the aqueoussolution method. The SAW tag is based on a 128 YX-LiNbO 3substrate and was fabricated via the micromachining process.After combining the SAW tag and the resistive hydrogensensor, the device performances of the proposed sensor weremeasured by direct connection or wireless access.

2. Sensor design and fabrication

2.1. Fabrication of SAW tag

As shown in gure 2, the SAW tag consists of three normalinterdigital transducers (IDTs) on a piezoelectric substrate.

The middle IDT is to convert the electrical signal from anantenna into a SAW via the reverse piezoelectric effect, andthe others act as reectors. One of the reectors is connectedto a resistive hydrogen sensor; the other is a reference. TheSAW tag was designed by the coupling-of-modes model andfabricated on a 128 YX-LiNbO 3 substrate because of its large

piezoelectric coupling, low propagation loss and near-isotropy.The central frequency of the SAW tag is 433 MHz. Thewavelength of the middle IDT, , is 8.8 m, the aperture is75 and the number of electrode pairs is 20. The delay lengthbetween the middle IDT and the rst reector is 3978 .97 mand the delay length between the middle IDT and the secondreector is 5968 .455 m. The electrode pair number of eachreector is 30. A 150 nm thick aluminum lm was depositedon 128 YX-LiNbO 3 by sputtering. After lithography andetching, the electrodes were nished.

2.2. Fabrication of resistive hydrogen sensor

The substrate of the resistive hydrogen sensor is 7740 glass forits insulating properties. The electrode material of the resistivehydrogen sensor is gold (Au) due to its low resistivity. Theperiod of the electrode is 50 m, the aperture is 8000 m andthe number of pairs is 122. Chromium (Cr) was deposited asan adhesion layer between the glass wafer and Au. Cr and Auwere evaporated on the wafer by an electron beam evaporator.The thicknesses of the Cr and Au lms are about 100 and1500 A. After lithography and etching, the electrodes werenished.

In this study, the Pt-coated ZnO nanorods are employed asthe sensing lm to detect hydrogen because of its large surface-

to-volume ratio. There are many methods for the growth of ZnO nanorods in the literature [1418 ], and we chose theaqueous solution method [16] in this study. First, a ZnO lmwith a thickness of about 500 A was deposited on the electrodesof the resistive sensor by sputtering. Then, the Pt-coated ZnOnanorods were synthesized and the procedure is as follows:rst, the ZnO-based chips were immersed in a mixed aqueoussolution of zinc nitrate hydrate (0.025 M) and methenamine(0.025 M) at 90 C for 6 h. Then, the chips were washed indeionized water and then dried by using nitrogen gas. Finally,the Pt coating with a thickness of about 100 A was depositedonto the ZnO nanorods as a catalyst by sputtering. The SEM

Figure 2. Illustration of SAW tag.

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Figure 3. SEM images of the Pt-coated ZnO nanorods: (a) top view,(b) side view.

images of the Pt-coated ZnO nanorods are shown in gure 3.One is a top view and the other is a lateral view. The diameterand height of the ZnO nanorods are 123.4 and 1303.9 nm,respectively.

2.3. Construction of sensing system

The sensing system consists of a reader unit and a SAW

sensor, which contains a resistive hydrogen sensor, a SAW tag,matching circuit and antenna (TriCOME, 433 MHz antenna).Currently, the reader unit is composed of a network analyzerand an antenna (Kinsun, 433 MHz antenna). A GPIB (general-purpose interface bus) interface links the reader unit and acomputer together. The request signal is transmitted from theantenna connected at port 1 of the network analyzer. Afterthe SAW tag receives the request signal, a response signalinvolving the sensing information is sent back to the reader.Therefore, we can utilize computer software, programmed byusing LabView, to analyze the S 11 parameter and estimate thehydrogen concentration.

The gas ow system for testing the nished SAW sensorconsists of an acrylic chamber, ow meters and hydrogen gascylinders of different concentrations. The SAW tag was put

Figure 4. Repeatability of the resistive hydrogen sensor.

into the acrylic chamber, whose volume is 140 cm 3. The owmeters were used to control the ow rate of the input gas. Toavoid any gas leak, the seams of the acrylic chamber wereplugged with silicone. The chamber was drilled for two holesto let gas in and out. The gas cylinder was linked to the owmeter to control the ow rate. The mass ow rate into thechamber was set to be 500 sccm (standard cubic centimetersper minute).

3. Measurement results

3.1. Measurement of resistive hydrogen sensor

3.1.1. Repeatability. First, the resistive hydrogen sensorwas measured at room temperature in order to evaluate therepeatability of the Pt-coated ZnO nanorods. At rst, it takes5 min to purge the chamber. Then, air and hydrogen gas of different concentrations ow into the chamber alternately. Thisperiod takes 30 min. Figure 4 shows the real-time responseof the sensor to a hydrogen concentration of 1500 ppm. Thevertical axis is the normalized resistance change (NRC), whichis dened as R R0

R0 100%, where R0 is the initial resistance

and R is the measured resistance while hydrogen gas owsinto the chamber. Results show that the resistance of theresistive sensor varies alternately with hydrogen concentration.As hydrogen gas ows into the chamber, the resistance of thesensor starts to decrease. The average normalized resistancechange is about 60%; moreover, the reaction and recovery of the sensor are obvious, and the repeatability of the resistivehydrogen sensor is found to be good.

3.1.2. Sensitivity. After repeating the measurementprocedure as mentioned above, the NRC caused by differenthydrogen concentrations can be measured. The resultsare shown in gure 5. The NRC caused by hydrogenconcentrations of 200 ppm, 500 ppm, 1500 ppm, 2500 ppm

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Figure 5. Sensitivity of the resistive hydrogen sensor.

Figure 6. Responses of the resistive hydrogen sensor toward variousRH.

and 40 000 ppm are 20.8%, 78.7%, 90.2%, 92.1% and 95.3%,

respectively. The resistive hydrogen sensor with the Pt-coatedZnO nanorods shows a high sensitivity at low concentrationwhile operating at room temperature.

3.1.3. Thermal and humidity tests. In practical situations,interferences from humidity or temperature uctuations couldaffect the hydrogen sensing of the resistive sensor. The sensorresponses to the humidity and temperature variations should beinvestigated. Figure 6 shows the NRC as a function of relativehumidity. The maximum shift of the NRC is 2.2% when therelative humidity is 86%. As mentioned above, the differencein the NRC is 20.8% when the hydrogen gas concentration is200 ppm. The interference from humidity is negligible.

Figure 7 is the result under a thermal test, whosetemperature varies from room temperature to 55 C. The

Figure 7. Responses of the resistive hydrogen sensor towarddifferent temperature.

resistance change and temperature variation are found to havea similar trend. Moreover, the difference in the NRC is 4%as the temperature reaches 55 C. Therefore, the interferencefrom the temperature uctuation can also be ignored.

3.2. Measurement of hydrogen SAW sensor

As is well known, the reectivity of a reector depends on theimpedance of the external sensor. In other words, when onereector of the SAW tag is connected to a resistive hydrogen

sensor and the other reector is free for reference, the hydrogenconcentration may be described by the relative return lossbetween the two reectors. Therefore, a hydrogen SAW sensorwas congured by connecting the resistive hydrogen sensorto the SAW tag and examined experimentally by direct andwireless measurements.

3.2.1. Direct measurement. Figure 8 shows the real-timeresponse of the hydrogen SAW sensor which was connecteddirectly to a network analyzer. Air was used to purgethe chamber rst. Then, the SAW sensor was exposed to40000 ppm hydrogen gas and air alternately. The averagechange of the relative return loss is 0.6 dB. The reaction andrecovery of the sensor can be observed. The results indicatethat the reectivity of the reector connected with a resistivehydrogen sensor does respond to the resistance variation of theresistive hydrogen sensor with hydrogen concentration.

3.2.2. Wireless measurement. By repeating the measurementprocedure as mentioned above, the differences of relative RLof the hydrogen SAW sensor caused by different hydrogenconcentrations were measured wirelessly. The real-timeresponse of the sensor to 40 000 ppm hydrogen is shown ingure 9 ( N = 1). The result exhibits a signicant noise dueto the wireless measurement. To suppress the noise effect, thedata was rened by the simple moving average (SMA) method.The SMA method is the unweighted mean of the previous N

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Figure 8. Repeatability of the passive wireless hydrogen SAWsensor.

Figure 9. Wireless sensing responses of the hydrogen SAW sensor.

data points. For instance, a 20-point simple moving averageof closing price is the mean of the previous 20 points closingprices. If the 20 prices are P M , P M 1 , . . . , and P M 19 , theformula is

SMA =P M + P M 1 + + P M 19

20 . (3.1)

The result of implementing the SMA method with N = 20 isalso shown in gure 9.

From the rened data, the differences of the relativeRL caused by various hydrogen concentrations at 200 ppm,500 ppm, 1500 ppm, 2500 ppm and 40 000 ppm are found

Figure 10. Sensitivity of the passive hydrogen SAW sensor viawireless measurement.

clearly to be 0.04 dB, 0.53 dB, 0.60 dB, 0.69 dB and 0.80 dB,respectively. The relation between the hydrogen concentrationand the difference in relative RL is drawn in gure 10. Fromthe results, we claim that the hydrogen SAW sensor is capableof detecting hydrogen wirelessly and has a good sensitivity.

4. Conclusions

A passive wireless hydrogen SAW sensor has been success-fully realized by combining a SAW tag and a resistive hydro-gen sensor. The device performances of the fabricated SAWsensor were measured by wireless access. The measurementresults show that the resistive sensor is sensitive and repeatablefor detecting hydrogen while operating at room temperatureand can avoid the inuence of temperature uctuation andhumidity. Finally, the proposed SAW sensor is shown to becapable of sensing hydrogen passively and wirelessly.

Acknowledgment

The authors are grateful for the nancial support of thisresearch from the National Science Council, Taiwan throughthe grant NSC 93-2218-E-002-052.

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