Ammonia gas sensors based on ZnO/SiO₂ bi-layer nanofilms on ST-cut quartz surface acoustic wave devices

Highlights

• We demonstrate an ammonia gas sensor based on ZnO/SiO₂ bi-layer nanofilms on ST-cut quartz surface acoustic wave devices.

• The ZnO/SiO₂ bi-layer nanofilms are prepared by combining a sol–gel process and a spin-coating technique.

• The ZnO/SiO₂ bi-layer nanofilms inherit the porous structure of SiO₂ film.

• The gas sensing property of ZnO/SiO₂ nanofilm is dependent on its sheet conductivity value.

• The sensor exhibits the ability to detect ammonia gas in a concentration as low as 5 ppm, and shows an excellent reproducibility and stability.

Abstract

Surface acoustic wave (SAW) ammonia gas sensors based on ZnO/SiO₂ bi-layer nanofilms on ST-cut quartz surface acoustic wave devices were fabricated and characterized. The ZnO and SiO₂ layers were coated onto SAW resonators by combining a sol–gel process and a spin-coating technique. The SEM and AFM results revealed the ZnO/SiO₂ bi-layer films had porous structures. The gas sensing results showed that the sensitivity of sensors was dependent on the value of sheet conductivity of the sensing films. As a result, the bi-layer nanofilms were much more sensitive than the single layer films due to their appropriate sheet conductivity, and the absolute response value was dependent on the thickness of the top ZnO layer. The sensor based on the bi-layer nanofilm with 60 nm top ZnO layer showed the best gas sensing property. It exhibited a frequency shift of 2000 Hz in 30 ppm ammonia gas with good repeatability and stability.

Introduction

High performance and low cost gas sensors are required for many industry applications, such as environment pollution detection, toxic gas leakage, detection of explosives, combustible, flammable and toxic gases, oxygen depletion, detection and warning systems for domestic and military bases, battlefields, petrochemical processing, mining, tunneling and offshore industries, automotive industry, petrochemical industry, etc. [1], [2], [3], [4]. Ammonia is one of the most dangerous industrial gases, and it is mainly generated in manufacturing of nitrogenous fertilizers, industrial refrigerant, and manure from agriculture and wildlife. It could cause poisoning and harming of people as well as explosion once there is a leakage of ammonia even in a low concentration level, such as ppm or even ppb [5]. Among different types of ammonia sensors available, such as electrochemical sensors [6], semiconductor sensors [5], [7], [8], [9], [10], surface acoustic wave (SAW) sensors [11], [12], [13] have their advantages of high sensitivity, high speed, good reliability, high accuracy and low cost. The high sensitivity associated with the SAW sensor results from the fact that most of the wave energy is concentrated on the SAW device surface within one or two wavelengths, therefore, any surface perturbations in the physical or chemical properties on the SAW device surface, such as mass loading, conductivity, temperature, and pressure, etc., will have a significant impact on the propagating waves. A SAW gas sensor can be based on the change of conductivity [14]. A typical SAW gas sensor is made up of an oscillator with a SAW resonator [15], which is based on a piezoelectric substrate on which inter digitated transducers (IDTs) with reflectors are fabricated to excite and reflect the surface acoustic waves (SAWs), and the corresponding oscillator circuits including phase shift circuit, power supply circuit and matching network which are used to maintain the starting condition for oscillation.

In order to detect low concentration levels of hazard gases, various types of thin films and nanostructured materials, such as TiO₂ [16], [17], ZnO [18], [19], [20], SnO₂ [21], [22], WO₃ [23] and polymers [24], [25], [26], have been applied in the SAW devices due to their high sensitivity to changes in the different gas/vapor atmospheres. Among them, ZnO and SiO₂ are commonly used as sensing materials because of their chemical sensitivity, high thermal and chemical stabilities, amenability to doping, non-toxicity and low cost [27], [28]. ZnO/SiO₂ nanocomposites are commonly used in photocatalysis [29], photoluminescence [30] and humidity sensing [31]. However, the gas sensing property of ZnO/SiO₂ bi-layer thin films has rarely been investigated. The mechanism of using the bi-layer gas sensing nanofilm in a SAW sensing system can be based on changes in conductivity of a bi-layer thin sensor structure [32]. In the case of a single layer film sensing system, it has been addressed previously by various authors [14], [32], [33] the relationship between the change of SAW velocity (Δν) versus the sheet conductivity (σ_s) of the film can be described as,

$$\frac{\Delta \nu }{{\nu }_0}\approx -\frac{K^2}{2}\frac{{\sigma }_s^2}{{\sigma }_s^2+{\nu }_0^2{C}_s^2}$$

where C_s is the surface capacity, σ_s = σ_h is the sheet conductivity, K is an electromechanical coefficient, ν₀ is the unperturbed SAW velocity. The plot of Eq. (1) is shown in Fig. 9. As is shown, if the sheet conductivity (σ_s) is too low or high, Δν changes little despite a huge change in σ_s. The sensor, then, will have no significant response when exposed to gas. But for a bi-layer film sensing system, the conductivity of the film may fall between those of the two layers which compose the bi-layer film. When both layers are very thin in comparison to SAW wavelength and Debye screening length [33], the sheet conductivity of the bi-layer film can be adjusted to an appropriate value by changing the thickness of two layers. Thus, Δν will have a relatively great change despite little change in σ_s. As a result, the sensor shows a high response. Moreover, the ZnO and SiO₂ layers have high and low sheet conductivities, respectively. The bi-layer films composed of these two layers, then, may have appropriate sheet conductivity by adjusting the thickness of the two layers. Based on this principle, in this paper, we proposed to use the ZnO/SiO₂ bi-layer nanofilms with different thicknesses of top ZnO layer on a standard quartz SAW device in order to form a highly sensitive ammonia gas sensor.