Oxygen and hydrogen sensing effects of supersonic cluster beam deposited zinc oxide films

Abstract

This project was carried out for investigating the gas sensing properties of nanocluster-assembled highly porous zinc oxide (ZnO) films prepared using the Supersonic Cluster Beam Deposition (SCBD) technique. The study has two main parts. They are focused on investigating the oxygen (O₂) and hydrogen (H₂) sensing properties of the material and the devices made of it. For thin film fabrication, SCBD method is a physical vapor deposition (PVD) technique which is quite distinctive from many other PVD techniques. An SCBD process starts from generating micro-plasma pulses on the surface of a metal rod target. The target is placed in a discharge chamber at reduced pressure. Atoms ablated from the metal target re-condense to form nanoclusters. The nanoclusters are concurrently carried by an argon gas flow to move towards the exit of the discharge chamber. The gas is extracted from the exit to enter an expansion chamber, where the gas experiences an expansion. Streamlines are formed and collimated with a set of aerodynamic lenses. The nanoclusters in the streamlines are forced to concentrate close to the axis of the gas flow. They finally enter a deposition chamber pumped to reach a high vacuum condition. In this chamber, the nanoclusters are deposited on a substrate to form a coating. Due to the low kinetic energy of the nanoclusters, the impact on the substrate is so light that the structural features of the nanoclusters are retained and diffusion of atoms does not happen. This facilitates the formation of a nanocluster-assembled highly porous coating on the substrate. Furthermore, the clusters size could be controlled with some techniques, and the film thickness can be adjusted according to the deposition time. For producing an oxide film, some amount of oxygen gas can be mixed into the carrier gas. Post-oxidation process at elevated temperature is often required to improve the crystallization of the nanoclusters and the linkage among them, and also to enrich the oxygen content in the deposits. In the first part of the study for investigating the oxygen sensing properties of SCBD ZnO-based sensors, all the tests were conducted at ambient temperature. The measurement was done in a way that a film sensor was first put to be stabilized in an environment containing a certain amount of oxygen content. A light source was then turned on to generate above-bandgap photons to illuminate the sensor. The real-time change of the electrical resistance of the sensor was monitored over a specific duration of time. The light source was then turned off and the real-time resistance was continued to be recorded. The time dependence of the current collected throughout the process was analyzed and correlated to the oxygen concentration in the detected region. Moreover, measurements based on different combinations of sensor structure and the condition of detected region were employed. The purposes are to reveal the influences of the sensor structure on the sensor output, and to assess the applicability of the measurement schemes in environments close to actual circumstances. The combinations covered in the study include: (i) flow of dry inert gas like argon (Ar) or nitrogen (N₂) containing a certain amount of detected gas, namely oxygen (O₂) in the study; (ii) flow of dry inert gas containing O₂ with the film sensor covered with a water-proof superhydrophobic polymer coating, made of a commercial spray cylinder product called NeverWet (Rust-Oleum). (iii) above 90%-humidity O₂-containing gaseous environment detected with the polymer-coated film sensor, (iv) in water containing a broad range of dissolved oxygen detected using a sensor made of the polymer-coated ZnO film with the light source and all electrical parts enclosed in a cavity. Results of measurements show that in all cases the sensors can generate detectable signals at room temperature if the sensors are illuminated with photons of energy above the bandgap of zinc oxide. The sensor response drops with increasing oxygen content in the detected region. Addition of a polymer coating on the film surface reduces the sensor response. The best post-oxidation condition for obtaining the best compromise between sensing properties and stability is found. The real-time dependence of the light-induced current response exhibit rather systematic trends associated with the change of oxygen concentration. It can be correlated with the oxygen concentration in the detected region in accordance with the magnitude of the sensor response, response time and recovery time. A model was proposed to give more fundamental interpretation of the observed trends. The model is based on photo-assisted electrical effect, redox reactions among oxygen, detected gas species and the ZnO film. The model incorporates the contributions from photo generation and recombination of electron-hole pairs, surface sorption of oxygen species, trapping and release of conduction electrons through interaction with the surface sorbed oxygen species, and transport via the defect states in the porous oxide material structure. The study associated with the measurements in water may lead to further develop of an immersion-type dissolved oxygen sensor. A practical method for quantitative determination of oxygen concentration based on the fitting parameters to the real-time resistive response and expected asymptote was proposed and tested, instead of going through long-time measurement to reach real equilibrium. In the second part of the study for investigating the H2 sensing properties of SCBD ZnO film sensors,the as-deposited ns-ZnO film is composed of nanoclusters of an average diameter ~5 nm embedded in an amorphous matrix. The post-oxidization condition leading to optimum gas sensing properties was determined to be 500°C for 1 h. With this condition, the nanocluster size increases to 13 nm. They are loosely connected to give a high porosity of 73%. To specify this extraordinary high porosity and roughness, we name the material as ZnO "nanosponge (ns-ZnO)". Furthermore, the ns-ZnO film was decorated with a palladium (Pd) coating to enhance the capability of catalytic dissociation of hydrogen so as to enhance the sensor response to the gas [58]. The overall sensor structure is thereby denoted as Pd/ns-ZnO. Measurements of resistive response of the Pd/ns-ZnO film sensor versus H₂ concentration contained in an inert carrier gas like Ar or N₂ was carried out at two different temperatures, namely 20°C and 80°C, respectively. For each temperature, the sample was also set to be under or without UV illumination. The effect of operation temperature can be illustrated using an example. For 2% H₂ in the sample gas, the sensor response and response time detected at 20°C are 82 and 1 s, respectively. On the other hand, at a higher operation of 80°C, the sensor response increases by 43 times, whereas the response and recovery times are considerably shortened to 0.3 s and 18 s. A moderate operation like 80°C is thereby preferred to use, because the sensor behaves better, and less moisture can stay on the film surface such that the change in surrounding humidity affects much less on the sensor's output. Also, the temperature is low to prevent from significant post-annealing effect leading to instability material structure and subsequent drift of the gas sensing signal. Compared with published data, the H₂ sensing properties of our sensors are superior to most others made of metal oxide nanomaterials operating at temperatures > 200°C in terms of showing higher sensor response and shorter response time. The effect of applying UV illumination is to improve the stability of the output in cyclic tests. A theoretical model based on the mechanisms introduced in the previous model for O₂ sensing was used to interpret the H₂ sensing properties in a more fundamental manner. The influences of increasing the operation temperature and UV illumination are discussed.