Highly Sensitive Rayleigh Wave Hydrogen Sensors with WO3Sensing Layers at
Room Temperature*
WANG Cheng(王成),FAN Li(范理),ZHANG Shu-Yi(张淑仪)**,YANG Yue-Tao(杨跃涛),
ZHOU Ding-Mao(周丁卯),SHUI Xiu-Ji(水修基)
Lab of Modern Acoustics,Institute of Acoustics,Nanjing University,Nanjing210093
(Received14July2011)
Rayleigh wave hydrogen nsors bad on128∘YX-LiNbO3substrates with WO3nsing layers operating at room temperature are studied.The experimental results indicate that the WO3layers obtained by a sol-gel method have much higher nsitivities becau the nsing layers produced by the sol-gel method have small grains and high roughness and porosity.It is also confirmed that in the sol-gel method,keeping WO3solutions at low temperature and/or decreasing the viscosity of the solutions can decrea the grain sizes and increa the hydrogen-absorbability of the nsing layer.Under the optimized preparation conditions,the high nsitivity of the hydrogen nsors at room temperature is obtained,in which1%hydrogen in natural air induces the frequency shift of72kHz
at the operating frequency of124.2MHz.
PACS:07.07.Df,81.20.Fw,43.35.Yb,43.38.Rh DOI:10.1088/0256-307X/28/11/110701
Surface acoustic wave(SAW)hydrogen nsors have attracted a great deal of attention so far,in which the nsors have achieved high nsitivity as the n-sors were often operated at high temperature,such as higher than100∘C.[1−4]However,in the experi-ments,a heater and a thermostat were required,which induced the nsors to be more complicated and un-favorable for miniaturization,and limited their appli-cation at room temperature.Furthermore,the heater can induce extra power loss and risks of fire and ex-plosion.
Hydrogen nsors operating at room temperature are greatly desirable.However,the nsitivities of the SAW nsors at room temperature are always very low.Several groups ud different types of piezoelectric substrates and nsitive layer materials to improve the nsitivities.For example,a SAW hydrogen nsor made of a piezoelectric substrate of64∘YZ-LiNbO3and a nsing layer of Pt/WO3 has a frequency shift of only2.4kHz at the hydro-gen concentration of4%as the operating frequency at43.6MHz.[5]Moreover,veral SAW nsors, such as graphene-like nano-sheets/36∘YX-LiTaO3,[6] polyaniline/WO3/ZnO/64∘YX-LiNbO3,[7]highly or-dered polyaniline nano-fibers/36∘YX-LiTaO3[8]and
polypyrrole nanofibers/ZnO/36∘YX-LiTaO3,[9]have also been utilized to increa the nsitivities of the hydrogen nsors at room temperature.However,the nsitivities are still quite low even at lower than 20kHz at1%hydrogen.
In addition,Huang et al.[10]put forward SAW hy-drogen nsors bad on128∘YX-LiNbO3substrates with ZnO nano-rod layers coated by Pt catalytic films as the nsing layers and obtained a frequency shift of about26kHz at0.6%hydrogen at room temperature with the operating frequency of145MHz.Meanwhile, Atashbar et al.[11]reported a SAW hydrogen n-sor bad on an AlN/64∘YX-LiNbO3substrate with polyaniline nano-fibers as the nsing layer and ob-tained a frequency shift of34.6kHz at1%hydrogen at room temperature with the operating frequency of 108.2MHz.In the last two cas,the nsitivities of the nsors were incread,but improvements still need to be made to increa the nsitivity further by using some new techniques.
It is well known that WO3can absorb/react with hydrogen at room temperature to form H x WO3 (0.3<x<0.5)structures.[12,13]Therefore,WO3films are always lected as the nsing layers in hydrogen nsors.Several papers have described the fabrica-tion of WO3thin films,which are typically made by evaporation,[14]sputtering,[15]anodization,[16]electro-deposition[17]and the sol–gel method.[18,19]
In this work,we prent a kind of Rayleigh wave hydrogen nsor bad on128∘YX-LiNbO3substrates with Pt/WO3films as the nsing layers.Becau the 128∘YX-LiNbO3substrates have a high electrome-chanical coupling coefficient k2and a velocity-shift coefficient k11m.A large k11m can induce the cen-tral frequency to have a larger shift for surface per-turbations,which is easier to obrve in the output inter-digital transducer(IDT).[20]IDTs with the elec-trode pairs of80,periodicity of32µm and aperture of2.5mm operating at frequency about125MHz are ud to transmit and receive the Rayleigh waves.
The WO3films deposited by the sol-gel method
*Supported by the National Natural Science Foundation of China under Grant No11174142,and the National Basic Rearch Program of China under Grant No2012CB921504.
**Correspondence author.Email:***************形容寒冷的诗句
c○2011Chine Physical Society and IOP Publishing Ltd
论文注释怎么加are lected as the nsing layers becau of the ea of forming films that are homogenous,have high rough-ness and porosity,low crystallinity,and are low cost. The WO3solution is prepared by reac七年级下册政治
ting metallic tungsten powder5g dissolved in20mL hydrogen per-oxide(H2O2).The resulting product,W-peroxy acid, is then esterified by reacting with20mL alcohol,in the process the solution is heated up to about80∘C and stirred for about an hour to form a peroxyester-W derivative and then prerved in a refrigerator. Table1.Preparation condition of WO3nsing layer
Fabricating
Sensor A Sensor B conditions
Reacting condition Clod esterification Open esterification
Prerving
5∘C10∘C
temperature
Spinning velocity2500rotations/min2500rotations/min Spinning time25s25s
Layer thickness350nm350nm
Operating
∼10∘C∼10∘C temperature
Relative humidity∼35%∼35%
In order to obtain the optimized nsitive layers of WO3,two esterification process are performed.In one process,the mixture sol of W-peroxy acid with 20mL alcohol is put in a clod flask and the sol is prerved in a refrigerator with a temperature of5∘C, which is denoted as nsor A;in the other process,the mixture is put in an open flask and kept at the tem-perature of10∘C,which is more viscous and denoted as nsor B.Then the spinning method is adopted for depositing the nsitive layers of the sol-gel.The fabrication conditions of the WO3sol-gel layers are listed in Table1.Finally,thin Pt films about30nm are deposited on WO3nsing layers by using the rf magnetron sputtering method as catalyst films,which dissociate H2molecules to H atoms.
Frequency counter
Amplifier
Power Supply
Hydrogen Generator Hydrogen Sensor Hydrogen Concentration Calibrator
Network Analyzer
Reacting Chamber Fig.1.Experimental system for nsitivity measurement.
In the study of the nsitivity of the hydrogen n-sors,a hydrogen gas generator,which can produce a constant hydrogen flow at the rate of210mL/min,is ud to generate hydrogen to be mixed with natu-ral air in different ratios in a chamber.Meanwhile, a hydrogen concentration electrochemical detector is ud to measure the concentration of hydrogen.A barometer,a flowmeter and a thermometer are ud to monitor the changes of the gas pressure,flow rate and temperature in the chamber.
Fig.2.Frequency shift of nsor A under different H2 concentrations at room temperature:(a)measured data at H2concentrations of0.3%,0.5%,1.0%,2.0%and3.0%;
(b)frequency shift of nsor(sample A)versus H2concen-
tration.
Fig.3.Frequency shift of nsor B at H2concentrations of1.0%and2.0%.
Several properties and the nsitivities of the n-sors are characterized at room temperature as follows:
(1)As the WO3layer with the Pt catalytic film coated on a glass substrate contacts hydrogen, H x WO3forms and induces the color of the nsing layer to change from light yellow to dark blue depen-dending on the hydrogen concentration increa.This is a simple method for mi-quantitatively displaying the existence and concentration of hydrogen.
(2)For quantitatively measuring hydrogen concen-tration,the frequency shift of the Rayleigh wave n-sor is measured,the experimental system is shown in Fig.1.The nsor is connected to an amplifier with high gain to form a clod-loop oscillator and placed
in a transparent chamber for obtaining an isolated en-vironment.A frequency counter is ud to measure the frequency and/or frequency shift of the oscilla-tor.Meanwhile,the hydrogen concentratio
n is varied by controlling the hydrogen flow in the chamber and measured by the hydrogen concentration detector.
0 Fig.4.Inrtion loss and conductance variations of the nsor versus hydrogen concentration at room tempera-ture (nsor A).
(n m )
豆角焖面做法(n m )
0Sensor A
10200
0.20.4
0.60.8
1.0
0.2
0.4
0.6
0.8
Sensor B
0.2
0.2
0.4
0.4
0.6
0.6
0.8
0.8
1.0 1.0
4020060(m m )
(m m )
(m m
)(m m
)Fig.5.Morphology of the WO 3layer of nsors A and B by an AFM.
In the experiments,the stability and noi of the experimental system is measured first of all.The re-sults of the measurements indicate that the tempera-ture changes of the nsors are about 0.06∘C during the experiment process.Bad on the temperature coeffi-cient of the substrate,−76ppm/∘C,and the operating frequency,125MHz,the frequency variation induced by the temperature fluctuation is about 570Hz.On the other hand,the background nois of the nsors are also measured,which shows a frequency fluctua-tion of about 1.2kHz at room temperature (∼10∘C).
Meanwhile,different gas,such as pure argon and nitrogen,which will not react with the nsing film,are introduced into the experimental chamber to ver-ify that the operating frequency is not changed by the impact of the introduced gas.
Fig.6.XRD spectra of the WO 3nsing layers A and B.
Then the frequency shifts of the nsors are mea-sured after exposure to various concentrations of hy-drogen balanced in natural air for about 5min,which ensures that the hydrogen fully reacts with the nsing film.The recovery time is also about 5min to ensure that the hydrogen is relead from the nsing film of the nsor.
The frequency shifts of nsor A measured by the clod-loop oscillator are shown in Fig.2.Figure 2(
a)shows that the frequency shifts of about 3kHz,8kHz,72kHz,207kHz and 222kHz are obtained while the hydrogen concentrations mixed in natural air are in-cread as 0.3%,0.5%,1.0%,2.0%and 3.0%,respec-tively.It must be noted that in the 0.3%H 2ca,the frequency shifts at about 3kHz may be not pre-ci due to the fact that it is less than 3times of the background noi (about 1.2kHz),but which may dis-play the existence of H 2.Figure 2(b)shows that the frequency shifts with the H 2concentration increasing and then approaches saturation as the H 2concentra-tion increas to quite high.The phenomena illustrate that as WO 3reacts with hydrogen to form H x WO 3,an inver ,H x WO 3decompos to WO 3and H 2,is accompanied.When both process are in dynamic equilibrium,the H 2adsorption by the ns-ing film approaches saturation.
Figure 3shows the frequency respon of nsor B,in which the frequency shift is only about 2kHz when the nsor is expod in 1%hydrogen.Obviously,the nsitivity of nsor A is much higher than that of nsor B.Therefore,in the preparation of the sols,the chemical contents,preparation conditions and pro-cess,and environments are very crucial,and which must be rigorously controlled.
(3)On the other hand,the nsor can be con-nected directly with a network analyzer to measure the frequency(shift)and also the inrtion loss varia-tions.In addition,in order to compare the nsitivi-tie
s of the SAW nsors with that of other methodolo-gies,a system compod of a voltmeter and a micro-galvanometer is t up to measure the electric conduc-tance variations of the nsors.上海高楼排名列表
The variations of the inrtion loss and conduc-tance of the nsors are measured in different hydro-gen concentrations.The measured results of nsor A are shown in Fig.4,in which,as the hydrogen concen-trations are0.5%,1%,2%and3%,the inrtion loss variations are0.14dB,0.89dB,18.41dB and25.23dB, and the conductance variations are0.009µS,0.017µS, 0.214µS and0.260µS,respectively.From the above results,it is obvious that for the hydrogen concentra-tion of0.3%,a frequency shift of the SAW nsor can be obrved,but cannot be nd in the variations of the inrt loss and conductance.
Generally,the measuring method bad on fre-quency shift posss advantages in terms of mea-suring very low hydrogen concentrations becau the frequency shifts reflect both changes of mass and also conductance effects.Meanwhile,the loop oscillation system has a high precision and resolution.Neverthe-less,the network analyzer has much lower nsitivity for measuring the inrtion loss.Therefore,the varia-tions of the inrt loss and conductance have nsitiv-ity lower than that of the frequency shift for nsing hydrogen.
To understand the effect of the structures of the nsing layers on the nsitivity of the nsors,the morphologies of WO3films of both nsors A and B are obtained by an atomic force microscope(AFM)as shown in Fig.5.It can be en that the sizes of the nanograins of nsor A are only about40nm,while the sizes in nsor B are about200–400nm.Therefore,the smaller grains can improve the nsitivity of the ns-ing layers becau of the larger surface dimension(or surface to volume ratio)to contact with hydrogen.As described above,different preparation process will induce sols with different viscosties,and a lower con-rving temperature will decrea the viscosity and as-mble velocity of the sol,and then decrea the sizes of grains.
On the other hand,for characterizing the crystal structures of the nsing layers,nsing layers A and B are prepared on glass substrates under the same con-ditions corresponding to nsors A and B.Figure6 shows the XRD curves of the WO3films of samples A and B,respectively.The XRD of layer A has a non-crystalline diffraction spectrum,but that of layer B ems to indicate a trend of crystallization.It can be expected that the amorphous structures of the nsing layers have better adsorbability or reactive capacities toward hydrogen than that of the crystalline structure.
In summary,room-temperature Rayleigh wave hy-drogen nsors have been fabricated and studied,in which the nsors are compod of128∘YX-LiNbO3 substrates with WO3films obtained by
陶土样大便the sol-gel method and covered with a Pt catalyst as the n-sitive layers.The frequency shifts and variations of the inrtion loss and electric conductance of the n-sors corresponding to different hydrogen concentra-tions are measured at room temperature.Comparable to the last two measuring methodologies,the method bad on frequency shift of SAW nsors have some advantages;especially,it has high nsitivity for lower concentrations of hydrogen.As a result,the fabrica-tion process of the WO3nsing layers is optimized, by which the high nsitivity of the frequency shift of72kHz expod in1%hydrogen diluted in natural air at the operating frequency about125MHz in room temperature is obtained.
References
[1]Ippolito S J,Kandasamy S,Kalantar-zadeh K,Wlodarski
W,Galatsis K,Kiriakidis G,Katsarakis N and Suchea M 2005Sens.Actuators B111-112207
[2]Ippolito S J,Kandasamy S,Kalantar-Zadeh K and Wlo-
darski W2005Sens.Actuators B108553
[3]Ippolito S J,Kandasamy S,Kalantar-zadeh K,Wlodarski
W and Holland A2006Smart Mater.Struct.15S131 [4]Fechete A C,Wlodarski W,Kalantar-Zadeh K,Holland A
S,Antoszewski J,Kaciulis S and Pandolfi L2006Sens.Ac-tuators B118362
三英战吕布读后感[5]Jakubik W P2007Thin Solid Films5158345
[6]Arsat R,Breedon M,Shafiei M,Spizziri P G,Gilje S,Kaner
R B,Kalantar-zadeh K and Wlodarski W2008Chem.Phys.
Lett.467344
[7]Sadek A Z,Wlodarski W,Shin K,Kaner R B and Kalantar-
zadeh K2008Synth.Met.15829
[8]Arsat R,Yu X F,Li Y X,Wlodarski W and Kalantar-zadeh
K2009Sens.Actuators B137529
[9]Mashat L A,Tran H D,Wlodarski W,Kaner R B and
Kalantar-zadeh K2008Sens.Actuators B134826
[10]Huang F C,Chen Y Y and Wu T T2009Nanotechnology
20065501
[11]Atashbar M Z,Sadek A Z,Wlodarski W,Sriram S,
Bhaskaran M,Cheng C J,Kaner R B and Kalantar-zadeh K2009Sens.Actuators B13885
都江堰旅游[12]Schjrmer O F,Wittwer V,Baur G and Brand G1977J.
Electrochem.Soc.124794
[13]Wittwer V,Schjrmer O F and Schlotter P1978Solid State
Commun.25977
[14]Porqueras I and Bertran E2000Thin Solid Films3778
[15]Shanak H,Schmitt H,Nowoczin J and Ziebert C2004Solid
State Ionics17199
[16]Ohtsuka T,Goto N and Sato N1990Electroanal.Chem.
287249
[17]Steveson K J and Hupp J T1999Electrochem.Solid-State
Lett.2497
[18]Cronin J P,Tarico D J,Agrawal A and Zhang L1994US
patent277986
[19]Cronin J P,Tarico D J,Tonazzi J C,Agrawal A and
Kennedy S R1993Sol.Energy Mater.Sol.Cells29371 [20]Ou H C and Zaghloul M2010IEEE Electron.Device Lett.
31518
