Fast Response and High Sensitivity ZnO/glass Surface Acoustic Wave Humidity Sensors Using Graphene Oxide Sensing Layer

We report ZnO/glass surface acoustic wave (SAW) humidity sensors with high sensitivity and fast response using graphene oxide sensing layer. The frequency shift of the sensors is exponentially correlated to the humidity change, induced mainly by mass loading effect rather than the complex impedance change of the sensing layer. The SAW sensors show high sensitivity at a broad humidity range from 0.5%RH to 85%RH with < 1 sec rise time. The simple design and excellent stability of our GO-based SAW humidity sensors, complemented with full humidity range measurement, highlights their potential in a wide range of applications.


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found that a GO layer thicker than 500-600 nm can render the device inoperable with drastically deteriorated resonant spectrum 1 and diminished resonance as shown in Figure S2.
The GO layer thickness is thus limited to 200-300 nm for all the sensors used for this work. Figure S1. The thickness profiles of GO measured by profilometer. Figure S2. The thickness of GO layer (a) and (b) the transmission spectrum of the SAW device before and after a 700-800nm GO layer deposited.

The effect of surface conductivity on the resonance
Moisture adsorbed on the surface induces not only a mass loading to a device, but may also change the conductivity of the surface layer. 2 Generally, the change of surface conductivity will shift both the acoustic resonant frequency and attenuation of the acousto-electric interaction, i.e. the transmission signal amplitude. It is known that within a narrow conductivity window, they are strongly correlated indicated by the following where K 2 is the electromechanical coupling coefficient, σ sh is the surface conductivity of the sensitive film, C S is the capacitance per unit length of the surface, α is the attenuation, k is the wave number, and α/k is the acoustic-electric interaction attenuation per wave number. Figure   S3 shows the calculated acoustic velocity shift and attenuation as a function of surface conductivity change, 3,5,6 clearly demonstrating the existence of a narrow surface conductivity window in which the velocity and attenuation of the acoustic waves are strongly correlated to the surface conductivity. 3,4 Furthermore, the SAW devices used in this work have resonant frequency of about 140 and 225 MHz, far away from the ionic and dipole relaxation frequency of water molecules, where it is typically in the range of Hz to hundreds of kHz. 7 We believe this is the main reason that the humidity-induced conductivity change has little effect on the resonant frequency of our SAW humidity sensors. Figure S3. Acoustic velocity shift and acousto-electric attenuation as a function of surface conductivity of a thin film on the ZnO piezoelectric layer. The electromechanical coupling coefficient K 2 is assumed to be constant in these calculations.

The mass loading effect
Assume the hydrophilicity for ZnO and GO is similar, then the increased surface mass density obtained can be treated as the increased sensing areas. For comparison, we have plotted the surface mass density ratio of D5-D10 with respect to that of sample D1 with clean surface in Figure S4. D5 with the GO in between the IDTs has the same sensing area as that of D1.

Frequency and GO thickness effects on response to humidity
The resonant frequency has a significant effect on sensitivity of the SAW sensors. Figure   S5a shows the frequency shift as a function of humidity for the two sensors with different frequencies but a similar GO layer (Group B, GO thickness 100-130 nm). When the relative humidity is changed from 10%RH to 85%RH in a 10%RH interval, the frequency shift of the two devices show nonlinear characteristics as observed in Figure S5a. After re-plotting them 5 in Figure S5b, it is clear that the frequency responses to humidity are approximately exponential with excellent linearity (0.9763<R 2 <0.9774). The thickness of the GO layer has a significant effect on the response speed. A thicker GO layer increases both the rise time and fall time as shown in Figure S6, especially that with 200-300 nm GO layer. This is because water molecules need more time to penetrate into and escape from the GO layer. The sensitivity and response times increase rapidly with the GO thickness. Figure S6 shows the frequency response of the sample D9, D10 and D11 with the same f r~2 25 MHz but different GO thicknesses between the humidity 80%RH and 10%RH, clearly showing the rapid increase of the response time with the GO thickness.