Ozone Sensors Working at Room Temperature Using Zinc Oxide Nanocrystals Annealed at Low Temperature ^†

Abstract

We focus on ozone (O₃) detection at ambient temperature by Zinc Oxide (ZnO) nanoparticles (NPs) deposited on Si/SiO₂ by spin coating from colloidal solutions as sensitive layers for air quality monitoring. We establish that at room temperature using continuous Ultra-Violet (UV) light irradiation enhances the sensing responses. Three annealing temperatures of ZnO films were performed to compare the sensing properties. These sensors present repeatable responses towards O₃ with fast responses for concentrations as low as 35 ppb with processes compatible with most of flexible substrates.

1. Introduction

Chemical gas sensors are applied in many areas, such as agriculture, medical diagnosis and detection of environmental pollutants [1,2,3]. The naturally present ozone (O₃) in the atmosphere is harmful even at low concentrations (<200 ppb). It leads to health problems such as inflammation or respiratory tract [4]. It well known that Metal Oxide Semiconductor (MOX) sensors require high operating temperature (>200 °C) to accelerate molecule chemical reactivity in the adsorption and desorption process. We aimed to fabricate MOX sensors on flexible substrate to fit shapes on smart object for O₃ monitoring. However, most of flexible substrates do not resist to temperature higher than 120 °C. As far as we know, no study reports O₃ MOX sensors using Zinc Oxide (ZnO) nanoparticles (NPs) with annealing temperature lower than 150 °C. Furthermore, to detect low O₃ concentrations at room-operating temperature, we replaced heating excitation by continuous Ultra-Violet (UV) Light-Emitting Diode (LED) for UV illumination (λ = 390 nm). Indeed, photogenerated carriers improve the layer conductivity [5].

2. Materials and Methods

Our gas sensor consists of Ti/Pt interdigitated electrodes (5 and 100 nm, respectively) deposited on Si/SiO₂ by magnetron sputtering. To optimize the sensitive layer performances, three solutions based on ZnO NPs with or without ethanolamine (EA) were deposited during 30 s at either 1500 or 2000 rpm/min. Thus, we prepared five different sensitive layers with resulting layer thicknesses measured by a Veeco Dektak 6M stylus profiler (Veeco, USA) (see

Table 1. Five sensitive layers with resulting thickness based on ZnO NPs in solutions at 10 or 30 mg/mL in isopropanol, with or without EA.

S1	S2	S3	S4	S5
30 mg/mL	30 mg/mL	10 mg/mL	10 mg/mL	30 mg/mL
without EA	with EA	with EA	with EA	with EA
0%	0.2%	0.2%	0.2%	0.2%
2000 rpm/min	2000 rpm/min	2000 rpm/min	1500 rpm/min	1500 rpm/min
100 ± 10 nm	90 ± 10 nm	80 ± 10 nm	180 ± 15 nm	205 ± 15 nm

).

Previous studies have shown that 0.2% volume of EA, a short-chain surfactant, gives aggregate-free solution [6,7]. ZnO NPs were deposited by spin coating on rigid substrates to compare the sensing performances for three post-annealing temperatures: 80 °C, 150 °C and 300 °C during 30 min, respectively. With a boiling point of 82.6 °C for isopropanol, solvent molecules were completely evaporated for drying temperature reaching 150 °C. The EA surfactant molecules stayed adsorbed on the surface of NPs at least up to the boiling point at 170 °C. A 0.1 V dc voltage was applied to the sample while the electrical resistance was monitored using a Keithley Model 2450 SourceMeter source measure unit (SMU) Instrument (Keithley, USA). Dry air was used as both the reference and the carrier gas, maintaining a constant total flow of 500 standard cubic centimeters per minute (SCCM) via mass flow controllers. Oxidizing oxygen using a UV pen-ray lamp, resulting in a generation of O₃ output level from 35 to 165 ppb.

3. Results and Discussion

By High-Resolution Transmission Electron Microscope (HR-TEM) JEOL 3010, ZnO NPs, ZnO NPs drop casted from a diluted solution on a mesh-coated carbon film (Figure 1a), presented a homogeneous size (diameter about 5 nm) and shape dispersions [6,7]. By scanning electron microscope (SEM) a JEOL JSM 6320F, analysis of spin-coated ZnO thin films highlights the formation of uniform and densely packed agglomerates (Figure 1b).

Figure 2 presents O₃ detection with sensitive layer without and with EA for 30 mg/mL of ZnO nanocrystals annealed at 80 °C. For comparable thickness (~100 nm) and same concentration (30 mg/mL) of ZnO, the sensor responses with EA are slightly lower than the one without EA due to grafting and isolating EA properties [7].

Figure 3 exhibits higher amplitude response for S5 and good repeatability for low O₃ concentrations without sensor saturation.

Figure 4 demonstrates that sensitive layers gave already good O₃ detection at 25 °C even with a low post-annealing temperature (80 °C) which is compatible with most of flexible substrates.

To compare the O₃ detection at room temperature and high classical temperature detection (275 °C), measurements have been realized on S4 post-annealed at 300 °C. No detection was possible in dark at room temperature due to the high layer resistivity. Figure 5 revealed that the amplitude is higher at 275 °C in dark and under UV light than at room temperature. However, these temperatures cannot be used on all flexible substrates.

Based on our studies on Kapton substrate showing NO₂ detection in dark at lower temperature than for O₃ detection by ZnO NPs [8], we are now focusing our attention on NO₂ detection by these samples in the same conditions with UV light and/or temperature excitations.

4. Conclusions

This work reported an ambient temperature way to detect ozone molecules by sensors based on thin-film ZnO NPs structures synthetized by hydrothermal process. Gas sensing measurements showed the great potential of ZnO NPs, which display a fast response, short recovery time, at room temperature under UV-light irradiation. This versatile methodology provides an alternative way to manufacture gas sensor devices, operating at room temperatures, which is compatible with most of flexible substrates. Electrical resistance measurements at room temperature under UV irradiation showed that ZnO thin films were sensitive even at low O₃ concentration (35 ppb).