Hydrogen Gas Sensing Using Palladium-Graphene Nanocomposite Material Based on Surface Acoustic Wave

We report the fabrication and characterization of surface acoustic wave (SAW) hydrogen sensors using palladium-graphene (PdGr) nanocomposite as sensing material. The Pd-Gr nanocomposite as sensing layer was deposited onto SAW delay line sensorbased interdigitated electrodes (IDTs)/aluminum nitride (AlN)/silicon (Si) structure. The Pd-Gr nanocomposite was synthesized by a chemical route and deposited onto SAW sensors by air-brush spraying. The SAW H 2 sensor using Pd-Gr nanocomposite as a sensing layer shows a frequency shift of 25 kHz in 0.5%H 2 concentration at room temperature with good repeatability and stability. Moreover, the sensor showed good linearity and fast response/recovery within ten seconds with various H 2 concentrations from 0.25 to 1%. The specific interaction between graphene and SAW transfer inside AlN/Si structures yields a high sensitivity and fast response/recovery of SAWH 2 sensor based on Pd-Gr/AlN/Si structure.


Introduction
In order to meet the demands of future hydrogen economy, there are various types of hydrogen (H 2 ) sensors, which use different mechanisms (e.g., resistance based, optical based, catalyst based, electrochemical based, thin and thick film based, chemochromic based, Schottky based, MEMS-based, and surface acoustic wave (SAW) based) to detect H 2 gas [1].A method for precisely detecting hydrogen with high sensitivity and selectivity and fast response is required for the development of hydrogen energy economy as well as environmental protection and human safety.Among these sensors, the SAW sensor has advantages that allow for remote wireless operation and a high potential in passive sensors [1,2].
Among various sensing materials and noble metal catalysts for H 2 sensors, palladium (Pd) nanoparticles (NPs) are well known and popular for H 2 sensing detectors due to their high sensitivity, good selectivity, and ability to operate at room temperature.In addition, Pd NPs expand their volume to a few percent during H 2 absorption/desorption, which can easily cause nonstructural stability in sensors [3].In order to mitigate these problems, recently, graphene has been shown to have excellent properties as a potential material to support Pd NP catalyst or form Pd-graphene (Pd-Gr) nanocomposites [3] and has shown attractive results for H 2 sensing in terms of high sensitivity and stability.
However, applying these Pd-Gr composites for H 2 detection is still limited to a resistivity type [3,4].The SAW gas sensor has been widely investigated due to several attractive advantages including high sensitivity, small size, reliability, and wireless ability [5,6].In addition, graphene is promising to develop SAW sensors because of its planar 2D crystal structure and specific sensitivity with electric field and acoustic wave [7][8][9][10][11][12].Most of the previously used SAW devices are based on traditional bulk substrates such as quartz, LiNbO 3 , and LiTaO 3 due to their high electromechanical coupling coefficient, but their poor temperature stability leads to reliability problems [7][8][9][10][11][12].In fact, the temperature stability of the SAW sensor is a significant problem in designing a sensor that avoids the interference of ambient temperature with actual sensing characteristics [13].The AlN films were proven to benefit SAW applications because of the high acoustic velocity, superior temperature, chemical stability, and compatibility with the MEMS process [13].In addition, compared to the temperature coefficient of frequency (TCF) of traditional bulk substrates such as LiNbO 3 and LiTaO 3 (upper −40 ppm/ ∘ C), that of AlN/Si (−30 ppm/ ∘ C) is significantly smaller, which can reduce the influence of the outside temperature [13].
Recently, graphene was reported as superior for sensing materials with advantages of high sensitivity and fast response [14].In addition to these advantages, defective/functionalized graphene and graphene-based nanocomposite were applied in sensing field with many kinds of sensing mechanisms [15][16][17][18].Nevertheless, graphene-based SAW sensors have been limited by few reports [7][8][9][10][11][12], especially the SAW sensor using piezoelectric thin films as AlN and graphene, in which the interaction between graphene and AlN interface has never been reported yet.Hence, in this work, we tried developing a SAW H 2 sensor based on Pd-Gr composite using AlN thin films; the properties of the SAW H 2 sensor were investigated and discussed in detail in this paper.

Experimental Section
High-quality polycrystalline piezoelectric (002)-oriented AlN thin films, 2 m thick (ℎ), were deposited on (100)-Si wafers by a pulsed reactive magnetron sputtering system [13].The two-port SAW delay lines based on IDT/AlN/Si were constructed using conventional photolithography and lift-off process.Gold (Au) with thickness of 100 nm prepared by RF sputtering was used for IDT.We used IDTs of 50 finger pairs with an electrode period () of 10 m.The aperture was 100 = 4000 m.The IDT-IDT gap was 5 mm.The wavelength ( = 4) was 40 m.
Graphene oxide (GO) was prepared from extra pure graphite powder (Merck, 99.99%) according to Hummers method.Pd-Gr nanocomposites were synthesized by a simple, one-step process using 25 mL of a GO aqueous solution (with fixed concentration of 1 mg/mL) and 25 mL of DI water containing 0.25 mg/mL concentrations of palladium chloride (PdCl 2 , Aldrich, 99%) [3].First, 25 mL of the GO solution was mixed with 25 ml PdCl 2 , followed by adding 500 L hydrazine monohydrate (N 2 H 4 ⋅H 2 O, Aldrich, 65 wt.%) as a reduction agent with rigorous stirring.The resulting 50 mL stable suspension, which was black, was used to fabricate SAW sensors.In brief, Pd-Gr composites were deposited on an AlN/Si substrate on the selected area via a mask by airbrush spraying from 2 mL above the suspension.The AlN/Si substrate was heated on a hot plate at 200 ∘ C during spraying.
Hydrogen gas sensing characteristics of the SAW sensor were tested for various hydrogen concentrations at room temperature (25 ∘ C) and relative humidity (RH) level of 30%.The sensor was mounted inside an enclosed environmental chamber.The characteristics of the two-port SAW sensor were measured with an Agilent 8802A Network Analyzer.The frequency shift of the SAW sensor was recorded by a frequency counter (Agilent 53181A) connected to a laptop via a General-Purpose Interface Bus (GPIB) card.A computerized mass flow controller system was used to vary the concentration of H 2 in synthetic air (0.25%, 0.5%, 0.75%, and 1%).The gas mixture was delivered to the chamber at a constant flow rate of 50 sccm (standard cubic centimeters per minute).The gas exposure time was fixed for each pulse of H 2 gas and the cell was purged with synthetic air between each pulse to allow the surface of the sensor to recover to atmospheric conditions.The crystalline characteristics of the Pd-Gr composite on the AlN/Si structure were investigated using X-ray diffraction (XRD) with CuK radiation (1.54178 Å).The surfaces of the thin films were characterized using a JEOL model 7600F field emission scanning electron microscope (FE-SEM).The Raman spectra were collected by a HORIBA Raman.

Result and Discussion
Figure 1(a) shows the SEM images of pure AlN/Si substrate.The surface of AlN thin film, which was prepared by pulsed reactive magnetron sputtering system, was smooth and consisted of grain size of 50 nm.Figure 1(b) shows the SEM images of Pd-Gr nanocomposite depositing on the AlN/Si substrate.The Pd NPs were well isolated and decorated on the graphene flakes with a high density of Pd NPs. Figure 1(c) shows the TEM image of as-synthesized Pd-Gr nanocomposites.The Pd NPs with size of around 10 nm are well dispersed and wrapped into graphene flakes as shown in Figures 1(b) and 1(c).The Pd-Gr formed like core-shell structures and Pd lattice constant was 0.23 nm. Figure 1(d) shows selected area electron diffraction (SAED) of Pd-Gr composites.
Figure 2 shows the XRD patterns of the Pd-Gr/AlN/Si structure.The peaks of the graphene in the XRD spectra did not clearly appear due to high intensity ratio of AlN peaks/Pd-Gr composites peaks.Other peaks appearing at 2 = 36.08∘ and 76.47 ∘ indicated (002) and (004) planes of the AlN film (JCPDS 00-065-0831).Among the peaks related to the hexagonal structure of the AlN film, the strongest peak appeared at 2 = 36.08∘ , which proves that the poly-AlN film had the (002) dominant oriented plane [13].The full width at half maximum (FWHM) of the rocking curve at the (002) peak of the AlN films was 0.19 ∘ .The small FWHM values of the preferred (002) peak of the AlN thin film indicated the high quality of the films, which is very important for estimation of piezoelectric properties of AlN [13].The peak at 2 = 40.44∘ (JCPDS 01-087-0645) corresponding to the Pd(111) plane confirmed the presence of Pd in the structure.
Figure 3 shows Raman spectra of the pure AlN/Si and Pd-Gr/AlN/Si structure.The peak at 521 cm −1 indicates Si substrate.The peak at 653 cm −1 is correlated to the first-order vibrational mode of  2 (high) of AlN thin film [19].The Raman spectrum of Pd-Gr composite consists of two major sharp peaks: G and D peaks.The features of those peaks are the indicators of the quality of graphene such as doping and strain.The G peak at 1590 cm −1 corresponds to the first-order scattering of the  2g mode.A prominent D peak at 1330 cm −1 is related to defects present [3].attenuation of the structures was increased by 3.03 dB.These changes may originate from the change in the mass loading and high conductivity in the coated Pd-Gr nanocomposite.In fact, the Pd-Gr nanocomposite coating degrades resonant phenomenon in a SAW delay line [5,6].The results of the research can indicate a possible effect of the high conductivity of the Pd-Gr that degrades the acoustic transmission, as it is shown in Figures 4(b) and 4(c) (around 3 dB between acoustic and electromagnetic signals), which has negative effects, such as a strong noise in the resonant frequency [5].
Figure 4(c) shows the frequency responses of the twoport SAW delay line coated with Pd-Gr nanocomposite with 0.5% H 2 at room temperature (25 ∘ C).The frequency down shifts of 25 kHz at 0.5% hydrogen concentration were observed in Figure 4(c).The attenuation of the structures was increased by 0.29 dB.The hydrogen atoms are decomposed to hydrogen ions (protons) and free electrons by the Pd catalytic reaction even at room temperature.These free electrons were transferred to the p-type graphene, reduced conductivity in the Pd-Gr composite [3], interacted with the electric field accompanying the propagating SAW [12], and modified the SAW properties, resulting in increased attenuation and a shift down resonant frequency in the SAW delay line [2,[5][6][7][8][9].In Figure 4(c), the mass loading effect is insignificant in the frequency shift due to H 2 interaction; therefore, a SAW sensor, with acoustic transmission degraded, is used to detect conductivity changes due to the interaction of Pd-Gr composite with H 2 .In our previous work [3], a resistive sensor based on Pd-Gr composite was used to measure conductivity changes of Pd-Gr, showing better results than the SAW sensor.Therefore, the resistive sensor detected the minimum H 2 concentration of 0.2 ppm [3], in contrast with the present work in which the lowest concentration detected by means of SAW sensor was 0.25% (2500 ppm).time is defined as the time required for the H 2 sensor to reach 90% of the frequency shift (Δ) when the sensor is exposed to a given concentration of hydrogen.The recovery time is defined as the time needed to recover 90% of the initial baseline after hydrogen turn-off.The response/recovery time of SAW sensor estimated from responses in Figure 5(a

Conclusion
We demonstrated the SAW H 2 sensor using Pd-Gr composite based on AlN/Si structure.The SAW H 2 sensor has higher sensitivity and fast response because of the accompanying effect of the graphene/AlN interface.In addition, graphene has a planar 2D crystal structure and has specific sensitivity with electric field accompanying the propagating SAW inside the AlN thin film.The SAW hydrogen sensor shifted to a maximum at 30 kHz in a 1% hydrogen concentration and showed a linear response in the range of 0.25 to 1% hydrogen at room temperature.The SAW sensor has fast response/recovery time of 1/9 seconds with 1% H 2 at room temperature.In addition, the sensor has good repeatability in both H 2 cycle testing and different response to various H 2 concentrations.

Figure 4 (
Figure1(a) shows the SEM images of pure AlN/Si substrate.The surface of AlN thin film, which was prepared by pulsed reactive magnetron sputtering system, was smooth and consisted of grain size of 50 nm.Figure1(b)shows the SEM images of Pd-Gr nanocomposite depositing on the AlN/Si substrate.The Pd NPs were well isolated and decorated on the graphene flakes with a high density of Pd NPs.Figure1(c)shows the TEM image of as-synthesized Pd-Gr nanocomposites.The Pd NPs with size of around 10 nm are well dispersed and wrapped into graphene flakes as shown in Figures1(b) and 1(c).The Pd-Gr formed like core-shell structures and Pd lattice constant was 0.23 nm.Figure1(d) shows selected area electron diffraction (SAED) of Pd-Gr composites.Figure2shows the XRD patterns of the Pd-Gr/AlN/Si structure.The peaks of the graphene in the XRD spectra did not clearly appear due to high intensity ratio of AlN peaks/Pd-Gr composites peaks.Other peaks appearing at 2 = 36.08∘ and 76.47 ∘ indicated (002) and (004) planes of the AlN film (JCPDS 00-065-0831).Among the peaks related to the hexagonal structure of the AlN film, the strongest peak appeared at 2 = 36.08∘ , which proves that the poly-AlN film had the (002) dominant oriented plane[13].The full width at half maximum (FWHM) of the rocking curve at the (002) peak of the AlN films was 0.19 ∘ .The small FWHM values of the preferred (002) peak of the AlN thin film indicated the high quality of the films, which is very important for estimation of piezoelectric properties of AlN[13].The peak at 2 = 40.44∘ (JCPDS 01-087-0645) corresponding to the Pd(111) plane confirmed the presence of Pd in the structure.Figure3shows Raman spectra of the pure AlN/Si and Pd-Gr/AlN/Si structure.The peak at 521 cm −1 indicates Si substrate.The peak at 653 cm −1 is correlated to the first-order vibrational mode of  2 (high) of AlN thin film[19].The Raman spectrum of Pd-Gr composite consists of two major sharp peaks: G and D peaks.The features of those peaks are the indicators of the quality of graphene such as doping and strain.The G peak at 1590 cm −1 corresponds to the first-order scattering of the  2g mode.A prominent D peak at 1330 cm −1 is related to defects present[3].Figure 4(a) shows schematic of the fabricated sensor with coating Pd-Gr composites.The SAW sensor of pure AlN/Si without coating Pd-Gr composite has resonant frequency of 128.78 MHz and attenuation of −23.73 dB as shown in Figure 4(b).Figure 4(c) shows the frequency responses of the two-port SAW delay with coating Pd-Gr nanocomposite.
Figure1(a) shows the SEM images of pure AlN/Si substrate.The surface of AlN thin film, which was prepared by pulsed reactive magnetron sputtering system, was smooth and consisted of grain size of 50 nm.Figure1(b)shows the SEM images of Pd-Gr nanocomposite depositing on the AlN/Si substrate.The Pd NPs were well isolated and decorated on the graphene flakes with a high density of Pd NPs.Figure1(c)shows the TEM image of as-synthesized Pd-Gr nanocomposites.The Pd NPs with size of around 10 nm are well dispersed and wrapped into graphene flakes as shown in Figures1(b) and 1(c).The Pd-Gr formed like core-shell structures and Pd lattice constant was 0.23 nm.Figure1(d) shows selected area electron diffraction (SAED) of Pd-Gr composites.Figure2shows the XRD patterns of the Pd-Gr/AlN/Si structure.The peaks of the graphene in the XRD spectra did not clearly appear due to high intensity ratio of AlN peaks/Pd-Gr composites peaks.Other peaks appearing at 2 = 36.08∘ and 76.47 ∘ indicated (002) and (004) planes of the AlN film (JCPDS 00-065-0831).Among the peaks related to the hexagonal structure of the AlN film, the strongest peak appeared at 2 = 36.08∘ , which proves that the poly-AlN film had the (002) dominant oriented plane[13].The full width at half maximum (FWHM) of the rocking curve at the (002) peak of the AlN films was 0.19 ∘ .The small FWHM values of the preferred (002) peak of the AlN thin film indicated the high quality of the films, which is very important for estimation of piezoelectric properties of AlN[13].The peak at 2 = 40.44∘ (JCPDS 01-087-0645) corresponding to the Pd(111) plane confirmed the presence of Pd in the structure.Figure3shows Raman spectra of the pure AlN/Si and Pd-Gr/AlN/Si structure.The peak at 521 cm −1 indicates Si substrate.The peak at 653 cm −1 is correlated to the first-order vibrational mode of  2 (high) of AlN thin film[19].The Raman spectrum of Pd-Gr composite consists of two major sharp peaks: G and D peaks.The features of those peaks are the indicators of the quality of graphene such as doping and strain.The G peak at 1590 cm −1 corresponds to the first-order scattering of the  2g mode.A prominent D peak at 1330 cm −1 is related to defects present[3].Figure 4(a) shows schematic of the fabricated sensor with coating Pd-Gr composites.The SAW sensor of pure AlN/Si without coating Pd-Gr composite has resonant frequency of 128.78 MHz and attenuation of −23.73 dB as shown in Figure 4(b).Figure 4(c) shows the frequency responses of the two-port SAW delay with coating Pd-Gr nanocomposite.

Figure 5 (Figure 5 :
Figure 5: (a) Transient response of H 2 sensor with various H 2 concentrations and (b) the linearity of the sensor at room temperature.
[12] 1/9 seconds with 1% H 2 at room temperature.Figure5(b)shows sensor response at various H 2 concentrations; the resonant frequency of the structure without Pd-Gr composite (control device) did not demonstrate any change because of the stability of the AlN films in the presence of hydrogen.The small shift of SAW sensor without Pd-Gr composite (1-2 kHz) was caused by the moisture effect created by different levels of H 2 gas concentration in the chamber.However, the negligible shift was in the error range of testing measurement.In contrast, the resonant frequency of SAW sensors with the coated Pd-Gr composite was clearly shifted.These changes may originate from the change in the mass loading and conductivity in the coated Pd-Gr composite as H 2 sensing layer.In comparison with previous works as pure graphenebased SAW H 2 sensor on LiTaO 3 substrate[8](frequency shift of 6 kHz, response/recovery time of 75/500 seconds with 1% H 2 ) or Pt/ZnO-based SAW H 2 sensor on AlN/Si substrate[2](frequency shift of 55 kHz, response/recovery time of few minutes with 1% H 2 ), our SAW H 2 sensor of Pd-Gr/AlN/Si structure in this work (frequency shift of 30 kHz, response/recovery time of 1/9 seconds with 1% H 2 ) had advantages of higher sensitivity and faster response.Thanks to the advantages of graphene as high electron mobility and specific graphene/AlN interface, the SAW H 2 sensor in this work has very fast response and recovery time[12].