The investigation of hydrogen gas sensing properties of SAW gas sensor based on palladium surface modified SnO2 thin film

The investigation of hydrogen gas sensing properties of SAW gas sensor based on palladium surface modified SnO2 thin film

Materials Science in Semiconductor Processing 60 (2017) 16–28 Contents lists available at ScienceDirect Materials Science in Semiconductor Processin...

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Materials Science in Semiconductor Processing 60 (2017) 16–28

Contents lists available at ScienceDirect

Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

The investigation of hydrogen gas sensing properties of SAW gas sensor based on palladium surface modified SnO2 thin film

MARK



Liu Yang, Chenbo Yin , Zili Zhang, Junjing Zhou, Haihan Xu College of Mechanical and Power Engineering, Nanjing Tech University, Nanjing 210009, PR China

A R T I C L E I N F O

A BS T RAC T

Keywords: Surface acoustic wave Hydrogen gas sensor Palladium surface modification SnO2 thin film Magnetron sputtering method

The sol-gel method and the magnetron sputtering method were used to deposit the SnO2 thin films on 128° YX LiNbO3 piezoelectric substrate for fabricating different kinds of delay-line surface acoustic wave (SAW) hydrogen gas sensors. To improve hydrogen gas sensing performance of SnO2 sensing films, the bi-layer structure sensitive films was used, which was composed of one pure SnO2 layer and one highly dispersed palladium nanoparticle layer. This bi-layer structure evidently enhanced the hydrogen gas sensing properties of SnO2 thin films. The microstructure, surface morphology and composition of as-prepared SnO2 sensitive films were analyzed by XRD, FESEM and XPS. The oscillator circuit with SAW sensor as resonate was designed to transduce the response of sensitive film into the frequency shift of oscillator. One precise temperature control system was used to ensure the temperature stability of SAW device. The Pd-surfaced-modified SnO2 thin film deposited by magnetron sputtering method has the highest frequency shift of 115.9 kHz to 2000 ppm hydrogen gas at 175 °C. The influence of thickness on the morphology of Pd nanoparticles and hydrogen gas sensing performance was studied. The size of Pd nanoparticle increases with thickness of Pd films and too thick Pd layer will reduce the hydrogen gas sensing performance of SnO2 films.

1. Introduction Hydrogen (H2) is extensively used in various industries such as chemical industry, food, semiconductor manufacturing, metallurgical field [1,2]. Additionally, it also serves as a clean, abundant and high efficiency energy carrier used in military, spacecraft launch and automotive vehicle, and so on [3,4]. However, hydrogen gas also has some disadvantages, such as easy to leak, and inflammable and explosive performance, because of its ultra-small molecule size and high chemical activity [5–7]. Furthermore, the pure hydrogen gas is an odorless and tasteless gas and undetectable to humans [8]. Therefore, it is essential to develop high-performance H2 gas sensors for precisely detecting hydrogen concentrations and ensuring the security in using of hydrogen energy. Among various types of hydrogen gas sensors, the surface acoustic wave (SAW) gas sensors are very attractive due to their advantages including remarkable sensitivity, fast response and recovery speed, minimal power requirement, low price, small size, good reliability, remote wireless operation ability and compatibility with digital system [9–12]. For surface acoustic wave device, the great majority of energy is concentrated near the surface of piezoelectric crystal within a few wavelengths. This makes SAW highly sensitive to any change of the



physical or chemical properties on the surface of piezoelectric substrate, such as electrical conductance, elastic moduli, mass loading and permittivity of sensitive film [13]. The velocity and amplitude of surface acoustic wave will be modified when the physical and chemical properties of sensitive films change as the target gas adsorbs into or desorbs out from the sensitive films of SAW gas sensors [14]. Then, we can evaluate the concentration of target gas according to these changes of velocity and amplitude. However, direct measurement of the velocity change or the amplitude change is unpractical. The velocity change can cause the variation of phase delay, so some interface architectures are used to measure the phase delay variation of SAW device directly or transform it into frequency shift, such as oscillator circuit, phase detector and phase-locked loop circuit [15]. These promising properties of SAW gas sensor have attracted more and more attention. For example, Wei Luo et al. [16] used a new aqueous and non-corrosion sol-gel method to deposit SnO2 film for H2S SAW gas sensor with frequency response of about 92 kHz to 27.4 ppm H2S at 60 °C. Cheng Wang et al. deposited In2O3 films on the 128° YX LiNbO3 piezoelectric substrate by RF diode sputtering method to fabricate SAW hydrogen gas sensor. This sensor obtained the frequency shift of 11.83 kHz towards H2 concentration of 400 ppm at room temperature [17]. It must be noted that the sensitivity and selectivity of SAW sensors

Corresponding author. E-mail address: [email protected] (C. Yin).

http://dx.doi.org/10.1016/j.mssp.2016.11.042 Received 8 July 2016; Received in revised form 29 November 2016; Accepted 30 November 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.

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are largely determined by the properties of the sensitive films [18]. In the past several decades, many kinds of sensitive materials, such as SnO2, WO3, TiO2, ZnO, Pd, and organic polymer, etc. were used as the sensitive films to detect hydrogen gas [4,19–22]. Among these materials, SnO2 is the most popular n-type metal-oxide semiconductor material used as gas sensing films to detect reducing gases (H2, CO, CH4, SO2, H2S, C2H5OH, etc.) and oxidizing gases (NO2, O2, etc.). SnO2 gas sensors have many advantages such as fast-response speed, low cost and high sensitivity [23–25]. However, the pure SnO2 gas sensors also suffer from the bad selectivity, high operation temperature and so on [26]. Thus, some SnO2 gas sensing materials were doped with metal oxides (CuO, ZnO, WO3, TiO2, In2O3, Cr2O3, etc.) and noble metal (Pt, Pd, Ag, Au, Ru, etc.) to improve its sensitivity, low-temperature operation performance and selectivity to specific target gas [27–29]. These promising properties make SnO2 serve as a suitable sensitive layer for hydrogen surface acoustic wave gas sensors. In our previous work [30], the sol-gel method was used to prepare the SnO2 sensitive films, and Pd, Fe and PEG-400 were added to improve the hydrogen gas sensing properties of SnO2 films. The response magnitude of these films could increase to 100. In order to further improve its hydrogen gas sensing performance, the magnetron sputtering method was used to deposit SnO2 films and Pd layer on its surface. By adjusting the fabrication processes, the sensor with Pdsurface-modified SnO2 films had the highest response magnitude of 4636 to 2000 ppm H2 [31]. Hence, it is a promising way to fabricate the SAW hydrogen gas sensor with excellent performance by using this bilayer structure film. Hence, it is very necessary to investigate the hydrogen gas sensing properties, the gas sensing mechanism and the dominant factor of these SAW hydrogen gas sensors based on Pdsurface-modified SnO2 sensitive films. In this work, Rayleigh wave delay-line SAW hydrogen gas sensors were fabricated with the 128° YX LiNbO3 serving as piezoelectric substrate. The sol-gel method and DC magnetron sputtering method were employed to deposit SnO2 films and thin Pd layers on the surface of SnO2 films. As illustrated in Table 1, the designations of Ssol-gel, SDC, SPd-sol-gel and SPd-DC are used to represent these four kinds of SnO2 sensitive films. In order to further study the influence of the Pd film thickness on the size and shape of Pd nanoparticles and the hydrogen gas sensing performance of SnO2 SAW sensors, the SnO2 films with different thickness of Pd films were prepared by magnetron sputtering method. The microstructure, surface morphology and composition of as-prepared SnO2 sensitive films were analyzed by XRD, FESEM and XPS. The uniform and porous SnO2 films with highly distributed Pd particles were observed. The oscillator circuit with good performance was designed to test the gas sensing properties of prepared SAW hydrogen gas sensors. The Pd-surface-modified SnO2 films deposited by magnetron sputtering method (SPd-DC) shows excellent performance with the frequency shift of 115.9 kHz towards 2000 ppm hydrogen gas at 175 °C.

sputtering method. Fig. 1(a) and (b) show the resistive hydrogen gas sensor with the entire sensor size of approximately 2×2×0.35 mm. The structure and the fabrication processes of micro-circuit on the silicon wafer substrates of resistive hydrogen gas sensors were detailed in our previous work [30]. Fig. 1(c) and (d) show the two-port SAW delay-line device, which is connected to TO-8 chip package. The 128° YX LiNbO3 piezoelectric substrate was adopted for its high electromechanical coupling coefficient, high velocity and pure Rayleigh wave mode [32,33]. Due to the good antioxidant property of gold electrodes, the 150 nm Au layer was deposited on substrate as the interdigital transducers (IDTs). The input and output transducers were both composed of 20 finger pairs with finger width (d) of 10 µm and the wavelength (λ=4d) of 40 µm, respectively. The aperture width was 50 λ=2000 µm. The distance between the input and output IDTs was 4 mm (100 λ). When the sensitive films on SAW substrate were deposited by magnetron sputtering method, the pure SnO2 target and Pd target were sputtered by DC (DC power of 28 W) and RF (RF power of 30 W, 13.56 MHz) power sources, respectively. The pure SnO2 layer was deposited firstly. Before the deposition process began, one plastic shadow mask was covered on the IDTs section. This shadow mask would be removed after deposition process to remove the SnO2 film on IDTs section. Subsequently, the SnO2 films were annealed at temperature of 500 °C in air for 2 h. Then, some sensors were modified by sputtering Pd on the surface of the SnO2 thin film for 30 s by RF magnetron sputtering method. When SnO2 thin films were deposited by sol-gel method, the gel was spin-coated on the substrates. The annealing and Pd surface modifying processes of these films were the same as above. These fabrication processes were detailed in our previous work [31]. The Pd films with different thickness were prepared by RF magnetron sputtering method at 30 W with depositing times of 15 s, 30 s, 2 min and 4 min, respectively. After Pd layers had been deposited on SnO2 surface, these films were sintered at 400 °C for 1 h to make Pd films agglomerate into Pd nanoparticles. These samples were denoted as SDC15, SDC30, SDC120 and SDC240, respectively. 2.2. Film characterization The morphologies of the synthesized SnO2 thin films were analyzed by Field Emission Scanning Electron Microscopy (FESEM, Hitachi SU8010). The crystal structures and phases of the synthesized SnO2 thin films were studied by X-ray diffraction (XRD, Rigaku Smart Lab) using CuKα radiation over a 2θ range from 10° to 80°. The XPS study of the oxidation states of the elements in the surface layers of Pd-surfacemodified SnO2 films was performed using a spectrometer “PHI 5000 VersaProbe” (ULVAC-PHI, Japan) which incorporated a rotating anode AlKα (hv=1486.6 eV). The binding energies (BEs) were calibrated using the C1s peak at 284.8 eV as a reference. 2.3. Gas sensing properties measurement

2. Experimental techniques The gas sensing performances of the prepared SAW delay-line hydrogen gas sensors and resistive hydrogen gas sensors were measured in a homemade closed steel test chamber. The dry air and dry nitrogen gas were used as carrying gas. The experimental measurement system used to test the performance of hydrogen gas sensor is shown in

2.1. Preparation of hydrogen gas sensors and SnO2 thin films The delay-line SAW hydrogen gas sensors and the resistive hydrogen gas sensors were fabricated by sol-gel method and magnetron Table 1 The sensing materials and preparation methods for preparing four kinds of sensing films. Sample

Sensing materials

Preparation method

Ssol-gel SDC SPd-sol-gel SPd-DC

SnO2 SnO2 Pd + SnO2 Pd + SnO2

Pure SnO2 layer deposited by sol-gel method Pure SnO2 layer deposited by DC magnetron sputtering method Pd-surface-modified layer deposited by RF magnetron sputtering method, SnO2 layer deposited by sol-gel method Pd-surface-modified layer deposited by RF magnetron sputtering method, SnO2 layer deposited by DC magnetron sputtering method

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Fig. 1. Two kinds of hydrogen gas sensors based on SnO2 films: (a) the chemi-resistive hydrogen gas sensor, (b) the micrograph of the chemi-resistive hydrogen sensor, (c) the delay-line SAW hydrogen gas sensor, and (d) the micrograph of IDTs.

Fig. 2. The test chamber was connected to a gas flow system with two groups of mass-flow controllers (MFCs) to control the flow rates of the hydrogen gas and carrying gas, respectively. To test the resistive hydrogen gas sensor, the pins of sensor were linked to an Agilent 34972 A data acquisition/ switch unit via the binding posts of the test chamber. To test the SAW delay-line hydrogen gas sensor, the SAW device was mounted on a test socket and coaxial-cable was used to link the SAW device with the radio-frequency unit outside the test chamber to form a closed-loop oscillator. One coupling device was put into the closed-loop oscillator circuit to transmit oscillator signal to the Agilent

53181 A frequency counter. The SAW device can be directly connected to network analyzer to measure the magnitude-frequency and phasefrequency characteristics and the insertion loss of SAW device. A pulse heating system was designed to precisely control the working temperature of sensitive films so that the sensors could work at a constant temperature. The gas sensing properties of all sensors were investigated in 100 ppm, 200 ppm, 500 ppm, 1000 ppm and 2000 ppm of hydrogen at temperatures ranging from RT to 275 °C. The various concentrations of hydrogen gases were prepare by mixing the pure hydrogen gas with carrying gas. At the same time, the flow rate to test

Fig. 2. The test system for measuring gas-sensing properties of different SAW gas sensors.

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Fig. 3. XRD patterns of (a) four kinds of SnO2 films on 128° YX LiNbO3 substrate and (b) clean 128° YX LiNbO3 piezoelectric substrate.

films are too thin and the low substrate temperature and low RF power in the sputtering process inhibit the formation of Pd nanoparticles. Moreover, when depositing time increases and Pd layer is sintered at suitable temperature, the greater Pd nanoparticles will form on SnO2 surface. Therefore, the Pd films with different thickness were prepared by RF magnetron sputtering method at 30 W with different depositing times. Then, these films were sintered to make different the size and shape of Pd nanoparticles. These Pd nanoparticles are large enough for FE-SEM to recognize. Fig. 5 shows the FE-SEM images of SDC15, SDC30, SDC120 and SDC240. There is no distinguishable Pd nanoparticle occurring in the FE-SEM images of SDC15 and SDC30. As the depositing time increases to 2 min, lots of Pd nanoparticles form on the surface of SnO2 films. In additional, the size of these Pd nanoparticle increases with the depositing time. The Pd nanoparticles on SDC120 and SDC240 both have highly dispersed and discontinuous morphology. To confirm the existence of the Pd component on the surface of Pdsurface-modified SnO2 films (SPd-sol-gel and SPd-DC), XPS was used to analyze the composition and the chemical state of the elements that existed in the SPd-DC. As shown in Fig. 6(a), the main contribution to O 1 s spectra is that of oxygen bound to Sn4+ at 530.9 eV and the peak appearing at 532.2 eV indicates an additional binding environment assigned to oxygen adsorption on the material surface [34]. Fig. 6(b) displays Sn 3d spectra with two peaks of 486.8 eV and 495.4 eV, which indicates the surface tin solely in the Sn4+ oxidation state. The Pd 3d region, Fig. 6(c), demonstrates the existence of Pd element and the surface layer of the Pd/SnO2 system containing two types of Pd atoms: the peaks of 335.3 eV and 340.6 eV corresponding to metallic Pd (Pd°) and the peaks of 336.6 eV and 341.9 eV corresponding to PdO (Pd2+) [35]. Moreover, the existence of PdO also indicates that part of Pd nanoparticles can be oxidized in air at room temperature.

chamber was kept at 1000 sccm. Before the hydrogen-sensing experiment, the closed-loop oscillator circuit must work in stable condition. Then the hydrogen gases mixed with carrying gas in different ratios were delivered into the test chamber. In this work, the response of delay-line SAW hydrogen gas sensor is defined as frequency shift Δf = fH–f0, where f0 and fH are the center frequency of SAW sensor in dry air and in different concentrations of hydrogen gases, respectively. The response time (tres.) and recovery time (trec.) are defined as the time required for the resistance (or center frequency of SAW gas sensor) of sensor to reach 90% of the final value.

3. Results and discussion 3.1. XRD characterization, FE-SEM micrograph and XPS analysis Fig. 3(a) illustrates the XRD diffraction patterns of prepared pure SnO2 and Pd-surface-modified SnO2 films deposited by two kinds of methods on 128° YX LiNbO3 piezoelectric substrate. The diffraction peaks reveal that these four kinds of SnO2 films have the tetragonal crystal structure, which match well with the Joint Committee of Power Diffraction Standard (JCPDS) card (No. 41–1445) of rutile SnO2. Furthermore, there are three peaks of 128° YX LiNbO3 substrate in these four XRD diffraction patterns, the reason of which is that the Xray can penetrate the thin SnO2 films and show the diffraction peaks of 128° YX LiNbO3 piezoelectric substrate. To confirm that three peaks come from 128° YX LiNbO3 piezoelectric substrate, the XRD diffraction pattern of clean 128° YX LiNbO3 substrate was investigated and shown in Fig. 3(b). In addition, there is no obvious difference between the XRD peaks of pure SnO2 and Pd-surface-modified SnO2 films. No diffraction peaks related to Pd, PdO or PdO2 are observed from patterns, which is because that the highly dispersed Pd particles on the surface of SnO2 layer have little influence on the crystallographic structure of SnO2 films. The broadened diffraction peaks suggest that the films are composed of polycrystalline SnO2 particles. Furthermore, the peaks of SnO2 films deposited by sol-gel method are broader than that of SnO2 deposited by magnetron sputtering method, which indicates the crystallinity deterioration. The FE-SEM images of four kinds of SnO2 films deposited on 128° YX LiNbO3 substrate are shown in Fig. 4. The deposited films are polycrystalline and have loose and porous structure. This structure indicates that these films have the high surface-to-volume ratio, which can improve its hydrogen gas sensing properties. Comparing the images of the SnO2 films deposited by magnetron sputtering method with that of the SnO2 films deposited by sol-gel method, the former shows columnar structure with larger grain size and the latter has grainy structure with smaller grain size. Meanwhile, there is no observable difference between the FE-SEM images of pure and Pdsurface-modified SnO2 film, the reason is that the Pd layer on SnO2

3.2. The electrical characteristics and the oscillator circuit of SAW devices As shown in Fig. 1(c), the sound absorbing rubber was used in SAW sensors to absorb the reflected acoustic wave and to diminish its effect on the IDTs [36]. The magnitude-frequency and phase-frequency characteristics of SAW hydrogen gas sensor with sensitive film and sound absorbing rubber at 25 °C in dry air are given in Fig. 7. The insertion loss of SAW device is about 8.4 dB at the center frequency of 99.8 MHz. In addition, the SAW device shows typical band-pass filter characteristics. As stated earlier, there are several kinds of circuits for detecting the changes of wave in SAW gas sensor. The network analyzer usually is used to detect these changes in laboratory due to its convenience and simple electrical circuit architecture. However, the huge bulk and expensive price of network analyzer make it not suitable for the SAW sensor used outside the laboratory. The cheap detecting method with 19

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Fig. 4. FE-SEM images of SnO2 films on 128° YX LiNbO3 substrate: (a) and (b) pure SnO2 film deposited by magnetron sputtering method, (c) and (d) Pd-surface-modified SnO2 film deposited by magnetron sputtering method, (e) and (f) pure SnO2 film deposited by sol-gel method, and (g) and (h) Pd-surface-modified SnO2 film deposited by sol-gel method.

SAW sensors may vary in wide range, the amplifier with gain higher than necessary is chosen to guarantee the start-up of oscillation. However, because that the large loop gain may generate higher-order harmonic signal and possibly give rise to an undesirable phenomenon called mode hopping, an adjustable attenuator should be put into the loop circuit to adjust the loop gain [37].

small size electrical circuit will be more suitable for the large-scale commercial applications of SAW sensor in our daily life. Therefore, the SAW oscillator circuit was designed to transduce the velocity change of Rayleigh Wave into the frequency shift of oscillator circuit. As shown in Fig. 8, this oscillator circuit is composed of a RF low noise amplifier, a phase shifter, an adjustable attenuator and one SAW sensor serving as frequency-selector. Considering that the insertion losses of different

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Fig. 5. FE-SEM images of SnO2 films on 128° YX LiNbO3 substrate with different thicknesses of Pd films: (a) Pd film deposited with 15 s (SDC15), (b) Pd film deposited with 30 s (SDC30), (c) Pd film deposited with 2 min (SDC120), and (d) Pd film deposited with 4 min (SDC240).

the increase of carrier concentration. The initial resistance of Pdsurface-modified SnO2 film deposited by magnetron sputtering is always higher than that of other three types SnO2 films at temperature range of 25–275 °C, which is due to the catalysis of Pd in oxygen adsorption process. The initial resistance of Pd-surface-modified SnO2 film deposited by sol-gel method is smaller than that of Pd-surfacemodified SnO2 film deposited by magnetron sputtering method, which is the result of that the former is thicker than the latter and the Pd particle on surface cannot enhance the oxygen adsorption of internal SnO2. The initial resistances of Pd-surface-modified SnO2 films increase with temperature in low temperature range and then decrease as the temperature increases continuously. The Pd nanoparticles can increase the dissociation rate of oxygen and produce more adsorbed oxygen, and this catalytic action of Pd can be enhanced with increase of temperature [38]. Thus, the initial resistance of Pd-surface-modified SnO2 obtained by DC magnetron sputtering will increase with temperature at low temperature range. However, as the temperature increases continuously, the high temperature makes the adsorbed oxygen easy to desorb from SnO2 film, which will result in the decrease of the adsorbed oxygen and initial resistance [39]. The response curves of the SAW sensors to 2000 ppm hydrogen gas at 175 °C are shown in Fig. 12, the Ssol-gel, SDC, SPd-sol-gel and SPd-DC have frequency shifts of 12.3 kHz, 4.6 kHz, 73.6 kHz, and 115.9 kHz, respectively. Compared with the other SAW sensors, SPd-DC shows the fastest response time ( < 1 s) and the lowest recovery time (about 583 s). This indicates that Pd surface modifying process makes great influence on the gas sensing performance of SnO2 film. Moreover, the frequency shifts of four types of SAW sensors towards the hydrogen gas in the concentration range from 100 to 2000 ppm at 175 °C are shown in Fig. 13(a). All sensors show negative frequency shifts. Compared with the absolute frequency shifts of Ssol-gel and SDC, SPd-sol-gel and SPdDC show dramatic improvement in response. It is obvious that the growth rates of the responses of SPd-sol-gel and SPd-DC slow down as H2 concentration increases continuously, which may be due to that the reaction between high concentration hydrogen gas and the lattice

3.3. The gas sensing characteristics of SAW hydrogen gas sensors Fig. 9 shows that the initial frequencies of the SAW devices without and with different sensitive films at different operation temperatures. Obviously, the frequencies of all SAW devices decrease with rising temperature due to the negative frequency-temperature coefficient of 128° YX LiNbO3 piezoelectric substrate and the resistance change of sensitive layers being subordinate in the influence on the frequency of SAW devices. In addition, Fig. 9 also shows that 128° YX LiNbO3 piezoelectric substrate has a relatively large temperature coefficient. Therefore, the center frequency of SAW device will change inversely with operation temperature. In the practical application, to diminish the influence of temperature, the SAW devices can be designed as dual delay-line structure. However, the resistance of sensitive film on the SAW gas sensors also changes with temperature. Therefore, the dual delay-line structure can’t completely eliminate the influence of temperature on SAW gas sensors. In laboratory tests, the working temperature of SAW gas sensors can be precisely controlled by temperature control system. Therefore, in this work, one pulse heating system was used to precisely control the working temperature of SAW gas sensors and eliminate the influence of temperature. Moreover, gas sensing properties of sensors were tested after the working temperature reached steady state. Fig. 10 shows the temperature stability of the SAW oscillator. The SAW device was placed into sealing test chamber and the flow rate of dry air was kept at 1000 sccm. Then, precise temperature control system was operated to make the temperature of SAW device change from 25 °C to 150 °C and hold on 150 °C for 20 min. As the temperature stabilizes after 10 min, the change of frequency shift of SAW device is small enough and shows promising stability of 7 ppm. Therefore, the temperature control system used in this work can make sure the temperature stability of SAW gas sensors and eliminate the influence of temperature on frequency shift. Fig. 11 shows the variation of initial resistances of different SnO2 films with the operation temperatures in dry air. The initial resistances of two kinds of pure SnO2 films decrease with the temperature due to 21

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Fig. 6. XPS spectra of Pd-surface-modified SnO2 film deposited by magnetron sputtering method: (a) O 1 s energy region; (b) Sn 3d energy region; (c) Pd 3d region; (d) typical XPS spectra.

which makes the number of oxygen species reacting with hydrogen gas reduce. Hence, the frequency shift of SPd-DC towards 1000 ppm H2 reduces as the temperature higher than 175 °C. As shown in Fig. 14(a) and (b), the response times of SPd-sol-gel and SPd-DC are shorter than these of Ssol-gel and SDC towards every concentration of hydrogen gas at the operation temperature of 25–275 °C. However, the recovery times of SPd-sol-gel and SPd-DC are longer than these of Ssol-gel and SDC towards every concentration of hydrogen gas at the operation temperature of 25–275 °C, as shown in Fig. 14(c) and (d). The response times of four types of sensors all decrease with hydrogen concentration, while the recovery times of them increase with hydrogen concentration. The response times and recovery times of all sensors decrease with operation temperature rising. When SAW gas sensors work at optimization temperature of 175 °C, the response times of SPd-DC, SPd-sol-gel, SDC and Ssol-gel are 1 s, 12 s, 25 s and 24 s and the recovery times of them are 583 s, 512 s, 342 s and 312 s, respectively. The response time is determined by the diffusion rate of hydrogen gas in sensing films, the dissolution and spillover rate on Pd nanoparticles, and the time of reaction with adsorption oxygen. The recovery time is determined by the diffusion rate of oxygen gas in sensing films, the dissolution and spillover rate on Pd nanoparticles, and the desorption rate of the generated water molecule. According to previous report [40], Knudsen diffusion can be used to describe the diffusion process of hydrogen gas and oxygen gas in SnO2 films prepared in our work. And the Knudsen diffusion coefficient can be expressed as Dk=4r(2RT/πM)1/2/3, where the variables of r, T and M are radius of pores, operation temperature and molecule weight. R is the universal gas constant. Thus, the diffusion rate of hydrogen gas is faster than that of oxygen gas at the same condition. This may be one reason that the response times of SPd-

Fig. 7. The magnitude-frequency and phase-frequency characteristics of SAW hydrogen gas sensor with sensitive layer and sound absorbing rubber at 25 °C in dry air.

oxygen of SnO2 in deep position of film makes less contribution to response. Fig. 13(b) shows the frequency shifts of four types of SAW sensors towards 1000 ppm hydrogen gas at the temperature of 25– 275 °C. Ssol-gel and SDC show higher frequency shifts as the operation temperature increases. The frequency shifts of SPd-sol-gel and SPd-DC increase as the operation temperature increases from 25 to 175 °C and then decrease as the operation temperature rises continuously, which is in accordance with the changing trend of initial resistance. When the operation temperature is higher than optimal operation temperature, the adsorption oxygen on Pd-surface-modified SnO2 films will reduce,

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Fig. 8. The SAW oscillator circuit: (a) and (b) the picture of SAW oscillator circuit prototype, (c) the circuit diagram of SAW oscillator circuit.

Fig. 10. The temperature stability of testing device: the center frequency of SAW device without sensitive layer in dry air as the operation temperature changes from 25 °C to 150 °C and keeps at 150 °C for 20 min.

Fig. 9. Initial frequencies of the SAW sensors with different sensitive layers in dry air at various operation temperatures. DC, SPd-sol-gel, SDC and Ssol-gel are shorter than their recovery times. Moreover, the chemisorbed water, which was produced in response process, may inhibit the formation of oxygen adsorbates in recovery process, and then lead to the slow recovery process [41]. In order to further study the influence of Pd film thickness and oxygen gas on hydrogen sensing performance of SnO2 SAW sensor, the hydrogen gas sensing properties of these different Pd-surface-modified SnO2 films were investigated in dry air and dry nitrogen gas. Fig. 15(a) shows the initial resistances of SnO2 films with different thicknesses of Pd layers at various operation temperatures in dry air. In dry air, the initial resistances of all Pd-surface-modified SnO2 films increase at first

and then decrease as operation temperature increases continuously. Fig. 15(b) shows the initial resistances of SnO2 films with different thicknesses of Pd layers at various operation temperatures in dry nitrogen gas. In dry nitrogen gas, the initial resistances of all Pdsurface-modified SnO2 films increase with operation temperature. This is because that the high temperature reduces the remaining adsorbed water. The frequency shifts of different Pd-surface-modified SnO2 SAW sensors towards 1000 ppm H2 at various operation temperatures in dry air and dry nitrogen gas are shown in Fig. 16. In dry air, the responses of SDC15 and SDC30 increase at first and then decrease as operation temperature increases continuously. In dry nitrogen gas, the responses 23

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orders of magnitude larger than the latter, respectively. This phenomenon indicates that the initial resistance of Pd-surface-modified SnO2 film is mainly determined by adsorbed oxygen and the adsorbed oxygen is important in the SnO2 SAW sensor sensing process towards hydrogen gas. The high temperature and too thick Pd layer both will restrain the oxygen absorption and decrease the response of Pdsurface-modified SnO2 SAW sensor in dry air. 3.4. Gas response mechanism explanation In this work, SAW gas sensors always work at constant temperature and constant pressure. According to the perturbation theory, the major effects, which lead the negative frequency shift of SAW gas sensors, are mass loading effect, elastic loading effect and acoustoelectric loading effect [42]. The relationship between the center frequency changes (Δf) of SAW sensor and these perturbation parameters can be defined as

Fig. 11. Initial resistances of different SnO2 films in dry air at various operation temperatures.

⎛ ⎛ 4μ ⎞ ⎛ μ + λ ⎞ ⎞ Δf ΔV =k = −C m f0 Δ(ρs ) + Ce f0 hΔ ⎜⎜ ⎜ 2 ⎟ × ⎜ ⎟ ⎟⎟ f0 V0 ⎝ ⎝ V0 ⎠ ⎝ μ + 2λ ⎠ ⎠ −

⎞ K2 ⎛ 1 Δ⎜ ⎟ 2 ⎝ 1 + (V0 Cs/ σs)2 ⎠

(1)

Where Δf is the frequency shift, f0 is the initial center frequency, ΔV and V0 are the velocity shift and unperturbed velocity of SAW propagating on the piezoelectric substrate of sensor, k is the fractional film coverage (the ratio of the length of the sensitive film to the centerto-center distance between IDTs), Cm is the coefficient of mass sensitivity, ρs is the mass surface density (mass per unit area) of the sensitive film, Ce is the coefficient of elasticity sensitivity, h is the thickness of the sensitive films, μ and λ are the shear and bulk modulus of elasticity, K2 is the electromechanical coupling coefficient, σs is the sheet conductivity of the film (σs=σh, where σ is the film bulk conductivity), and Cs is the capacitance per unit length of the SAW substrate material (Cs=εs+ε0, where εs and ε0 are the permittivities of the substrate and free space, respectively). According to the Eq. (1), the gas molecules adsorption on the surface of sensitive film will increase the mass, and then the center frequency of SAW sensor will decrease. The gas adsorption on the surface of sensitive film also may change the elasticity modulus. In our previous work [31], the resistance changes of Pd surface modified SnO2 films and pure SnO2 films in different concentration H2 all decrease as sensitive films exposed to hydrogen gas, which also lead to decrease of center frequency of SAW gas sensors. Moreover, the dominant response mechanism of the SAW sensors with Pd-surface-modified SnO2 sensitive films can be identified based on the above experimental phenomena. As shown in Fig. 13(b), SAW sensors’ frequency shifts to H2 gas

Fig. 12. Response profiles of the SAW gas sensors with different sensitive films towards 2000 ppm hydrogen gas at 175 °C.

of all Pd-surface-modified SnO2 SAW sensors increase with operation temperature. Fig. 15(a) and Fig. 16 also suggest that the suitable thickness of Pd layer can increase the initial resistance and the response of SnO2 films. However, the initial resistances and the responses of SnO2 films decrease as the thickness of Pd layer increases continuously, which is due to that the thick Pd film will make the size of Pd nanoparticles increase and then inhibit the catalytic activity of Pd nanoparticles at hydrogen gas sensing process [38]. Comparing the initial resistances and the responses of Pd-surface-modified SnO2 SAW sensors in dry air with those in dry nitrogen gas, the former is several

Fig. 13. The frequency shifts of SAW hydrogen gas sensors: (a) towards various concentrations of H2 gas in dry air at 175 °C and (b) towards 1000 ppm H2 at operation temperature of 25–275 °C.

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Fig. 14. The response times of SAW hydrogen gas sensors: (a) towards different concentrations of H2 gas at 175 °C and (b) towards 1000 ppm H2 at temperature of 25–275 °C and the recovery times of SAW hydrogen gas sensors: (c) towards different concentrations of H2 gas at 175 °C and (d) towards 1000 ppm H2 at temperature of 25–275 °C.

pure SnO2 SAW sensor, it is difficult to distinguish the dominant response mechanism between elastic effect and acoustoelectric coupling effect in this work. The hydrogen gas sensing process involves two reaction steps: the step of sensitive films exposing to hydrogen gas and the step of sensitive films exposing to dry air. For the pure SnO2 sensitive films, the O2 molecules directly adsorb on the surface of SnO2 and capture electrons from film as SnO2 sensitive films exposed to dry air. When SnO2 sensitive films are exposed to hydrogen gas, H2 molecules directly react with the oxygen species adsorption on the surface of SnO2 and release the electrons into the SnO2 film. For the Pd-surface-modified SnO2 sensitive films, the hydrogen gas sensing processes and mechanism are illustrated in Fig. 17. Fig. 17(a) shows the sensitive film in vacuum condition, and there is no gas

increase with operation temperature. Furthermore, it is important to note that the optimal operating temperatures of four kinds of SAW sensors are very high. Moreover, the physical adsorption is not valid and the chemisorption is dominant in the whole response process at high temperature. Thus, the mass effect is not the dominant response mechanism. Meanwhile, as shown in Fig. 13(a), the SAW sensor with Pd-surface-modified SnO2 films shows huge improvement in the response to any concentration of hydrogen gas in comparison to pure SnO2 SAW sensor. In addition, the Pd-surface-modified process could not mightily improve the physical adsorption of sensitive films, namely no evident improvement in mass effect and elastic effect. Therefore, it may safely draw the conclusion that the acoustoelectric coupling effect acts as the dominant response mechanism for the SAW hydrogen gas sensor with Pd-surface-modified SnO2 sensitive films. However, for

Fig. 15. Initial resistances of SnO2 films with different thicknesses of Pd-surface-modified layers at various operation temperatures in (a) dry air and (b) dry nitrogen gas.

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Fig. 16. The frequency shifts of different Pd-surface-modified SnO2 SAW sensors towards 1000 ppm H2 at various operation temperatures in (a) dry air and (b) dry nitrogen gas.

adsorbing on the sensitive film. When sensitive film is in dry air, the oxygen molecules in air adsorb on SnO2 surface as shown in Fig. 17(b). The oxygen molecules will firstly adsorb on Pd particles and dissociate into atomic oxygen. The atomic oxygen will capture electrons from SnO2 films and become O2−, O−, and O2−, leading to the increase of the electron depleted layer and initial resistances. These oxygen species (O2−, O−, and O2−) subsequently spillover to the SnO2 surface via the inter-grain boundaries [43]. Then, the velocity of SAW wave and the center frequency of SAW gas sensor increase. In this oxygen absorption process, the Pd nanoparticles make more oxygen molecules quickly dissociate and diffuse on the SnO2 surface. The catalytic mechanism of Pd nanoparticles can be described as follows:

O2(g) + Pd → 2O(Pd) (dissociation)

(2)

O2(g) + O (Pd) → O(surf) (SnO2) + 2O(Pd) (spillover)

(3)

(5)

H + SnO2 → Had (SnO2)(spillover)

(6)

Had (SnO2) +

O−ad(SnO2)



OH−ad(SnO2)

(7)

OH−ad(SnO2) + Had (SnO2) → H2 O (g) ↑ + (SnO2) + e− 2Had (SnO2) +

2− Oad (SnO2)

→ H2 O (g) ↑ + (SnO2) +

2e−

(8) (9)

According to these equations, this reduction reaction process can release electrons into the conduction band of SnO2 and reduce the depletion layer around SnO2 particle. Consequently, the conductivity of films increases, which slows down the velocity of SAW wave and reduces the center frequency of SAW gas sensor. The Pd-surfacemodified layer enormously enhances the resistance change of SnO2 sensitive film at gas sensing process, so that the SAW sensor with Pdsurface-modified SnO2 film has a huge promotion in hydrogen sensing properties in comparison to the SAW sensor with pure SnO2 film. We know that the adsorption and dissociation process of oxygen and hydrogen molecules only occur on the surface of SnO2 grains, so only the Pd catalytic cluster on the surface can improve the reaction between adsorbed oxygen and hydrogen species. In addition, the adsorbed gas species must migrate through the inter-gains contact boundaries in spillover process. Thus, the Pd catalyst particles must be well dispersed on the SnO2 surface so that they can effectively control the hydrogen gas sensing processes overall SnO2 surface. In this work, the magnetron sputtering method was used to deposit Pd catalyst layer. This method can make the Pd catalyst nanoparticles homogeneously dispersed on the surface with small particle size. This is one reason that the hydrogen gas sensors with Pd-surface-modified SnO2 sensitive films have excellent hydrogen sensing performance. Furthermore, as shown in Figs. 12 and 13, the Pd-surface-modified SnO2 sensitive film deposited by sol-gel method shows lower response magnitude than that

When the sensitive films are exposed to H2 gas, the H2 molecules can react with adsorbed oxygen species as shown in Fig. 17(c). Pd nanoparticles play an important role in this reduction reaction process. The hydrogen molecules firstly react with the oxygen species on Pd particles. Then, hydrogen molecules dissociate into atomic hydrogen on the Pd nanoparticles and spillover to the SnO2 surface around Pd nanoparticles to react with oxygen species. This reduction reaction process between adsorbed oxygen and hydrogen gas will generate H2O gas. In this reaction process, the Pd nanoparticles on SnO2 surface reduce the adsorption energies for H2 gas on the SnO2 surface [44]. The low adsorption energies will make more H2 molecules quickly adsorb on Pd nanoparticles and dissociate into atomic hydrogen, which makes H2 and O2 on the Pd-surface-modified SnO2 surface easy to react [45]. This reaction process can illustrate as follows:

H2 (g) + O (Pd) → H2 O (g) ↑ + (Pd)

H2 (g) → H + H (dissociation)

(4)

Fig. 17. The hydrogen gas sensing processes and mechanism of the Pd-surface-modified SnO2 films.

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of Pd-surface-modified SnO2 sensitive film deposited by magnetron sputtering method, the reason is that the large thickness of SnO2 deposited by sol-gel method weakens the Pd catalytic effect to the SnO2 grain in deep section.

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4. Conclusion The delay-line SAW device based on 128° YX LiNbO3 piezoelectric substrate was fabricated for SAW hydrogen gas sensor. Pure SnO2 layers and bi-layer structure SnO2 layers prepared by sol-gel method and DC magnetron sputtering method were used as the sensitive films of SAW gas sensor. The Pd nanoparticles deposited by RF magnetron sputtering method homogeneously distributed on the surface of SnO2 layers. The SAW hydrogen gas sensors were test in the homemade test system. The Pd nanoparticles improve the performance of SnO2 films and lower the optimal operation temperature. The SAW gas sensor with Pd-surface-modified SnO2 sensitive film deposited by magnetron sputtering method (SPd-DC) shows highest frequency shift of 115.9 kHz to 2000 ppm hydrogen gas at 175 °C. Additionally, this SAW sensor shows extremely fast response and recover speed of 1 s and 512 s, respectively. The Pd catalytic nanoparticle increases the amount of oxygen species and hydrogen gas adsorption on SnO2 surface, leading to the decrease of initial electrical conductivity in dry air and the increase of electrical conductivity in hydrogen gas. Therefore, the responses of SnO2 SAW hydrogen gas sensors can be dramatically enhanced by using the Pd-surface-modified SnO2 bi-layer as sensitive film. In addition, the influence of the Pd film thickness on the morphology of Pd nanoparticles and hydrogen gas sensing performance was studied. The size of Pd nanoparticle increases with thickness of Pd films. Too thick Pd films will degrade the hydrogen gas sensing performance of SnO2 films. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 50875122, and 51575255), China, Postgraduate Scientific Research and Innovation Projects of Jiangsu Province (KYLX_0752), China and Science and Technology Support Plan of Jiangsu Province (BE2011187), China. Professor Chunhai Cao at Department of Electronic Science & Engineering of Nanjing University made suggestions for this project and supplied several analysis instruments to our research team. References [1] A.Z. Sadek, W. Wlodarski, Y.X. Li, W. Yu, X. Li, X. Yu, K. Kalantar-zadeh, A ZnO nanorod based layered ZnO/64° YX LiNbO3 SAW hydrogen gas sensor, Thin Solid Films 515 (2007) 8705–8708. [2] Ahmad I. Mohammad Abu Haija, Sadiqa Ayesh, Ahmed, S. Katsiotis Marios, Selective hydrogen gas sensor using CuFe2O4 nanoparticle basedthin film, Appl. Surf. Sci. 369 (2016) 443–447. [3] Shahruz Nasirian, Hossain Milani Moghaddam, Polyaniline assisted by TiO2:sno2 nanoparticles as a hydrogen gas sensor at environmental conditions, Appl. Surf. Sci. 328 (2015) 395–404. [4] Yeongjin Lim, Yunjeong Lee, Jeong-Il Heo, Heungjoo Shin, Highly sensitive hydrogen gas sensor based on a suspended palladium/carbon nanowire fabricated via batch microfabrication processes, Sens. Actuators B 210 (2015) 218–224. [5] Thanittha Samerjai, Nittaya Tamaekong, Chaikarn Liewhiran, Anurat Wisitsoraat, Adisorn Tuantranont, Sukon Phanichphant, Selectivity towards H2 gas by flamemade Pt-loaded WO3 sensing films, Sens. Actuators B 157 (2011) 290–297. [6] I. Fasaki, M. Suchea, G. Mousdis, G. Kiriakidis, M. Kompitsas, The effect of Au and Pt nanoclusters on the structural and hydrogen sensing properties of SnO2 thin films, Thin Solid Films 518 (2009) 1109–1113. [7] Cristian Viespe, Constantin Grigoriu, SAW sensor based on highly sensitive nanoporous palladium thin film for hydrogen detection, Microelectron. Eng. 108 (2013) 218–221. [8] Hossain Milani Moghaddam, Shahruz Nasirian, Hydrogen gas sensing feature of polyaniline/titania (rutile) nanocomposite at environmental conditions, Appl. Surf. Sci. 317 (2014) 117–124. [9] Changbao Wen, Changchun Zhu, Yongfeng Ju, Hongke Xu, Yanzhang Qiu, A novel NO2 gas sensor using dual track SAW device, Sens. Actuators A 159 (2010) 168–173.

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