The Effect of Monolayer Graphene on the UV Assisted NO₂ Sensing and Recovery at Room Temperature ^†

Abstract

In the present study, UV light induced desorption of nitrogen dioxide (NO₂) on pristine graphene based gas sensor is used to improve the sensing performance. Compared to the sample without UV light exposure, the response is 12%, 18% and 21% for NO₂ concentrations of 1, 3 and 5 ppm. In addition, the recovery could be speeded up by UV irradiation. The sensor shows good behavior of repeatability when tested for 1 ppm of NO₂ in 3 cycles. The graphene sensor with UV irradiation has a higher sensitivity to NO₂ than to other gases under higher concentrations.

1. Introduction

Nitrogen dioxide (NO₂) is a pollutant with negative effects on soil, plants, animals and human health. It causes acid rain and photochemical smog. It is necessary to develop highly sensitive and inexpensive NOx gas sensors capable of detecting low concentrations of NO₂ [1]. Metal oxides have been used as sensing materials in solid-state gas sensors. However, these sensors work only at high temperatures due to the high activation energy of reactions with gas molecules [2]. Gas sensors based on graphene is suitable for NO₂ sensor, mainly due to its two-dimensional single atomic layer thickness, large specific surface area, excellent temperature and electrical tolerance properties [3]. In this work, we demonstrate the impact of UV exposure on graphene sensor recovery for NO₂ gas detection.

2. Experimental

The interdigitated electrodes were fabricated by a lift off process. Chromium and gold layers with thickness of 30 and 120 nm were deposited on glass. Graphene films were grown on a copper (Cu) substrate by chemical vapor deposition (CVD). Transfer of graphene from Cu to the interdigitated electrodes was done by standard transfer process. The sensor and measurement setup are shown in Figure 1. The NO₂ concentration was controlled by Flexstream Module Instrument that modulates the flow rate of air through the NO₂ permeation tube to get ppm level, and a mechanical pump was used to remove the residual gas. The gas sensing response was measured by monitoring the change in resistance of the graphene film.

3. Results and Discussion

Figure 2a,b display the comparison of the dynamic response with and without UV irradiation for NO₂ concentration from 1 to 5 ppm. As shown in

Table 1. Gas sensing properties of UV light and without.

	Without UV Light		With UV Light
NO2 (ppm)	Response (%)	Recovery (%)	Response (%)	Recovery (%)
1	−14.35	21	−12.33	100
3	−11.57	38	−18.25	100
5	−7.7	51	−21.8	100

. The results show a significant effect of UV irradiation on the sensor response: 12, 18 and 21% for concentrations of 1, 3 and 5 ppm. This can be explained by the UV irradiation induced molecular desorption from the graphene surface. As shown in Figure 3, the sensor was exposed to 1 ppm NO₂ with UV irradiation for 3 cycles. A good repeatability is evident from the cycle variation of response within 0.4%. To investigate the selectivity, the sensor was also tested for 10 ppm NH₃, 10 ppm acetone, air and 1 ppm NO₂. The sensor exhibits a higher response for NO₂ compared to other gas, as shown in Figure 4.

4. Conclusions

In summary, UV irradiation is shown to improve the sensor recovery during NO₂ detection in a chemiresistive sensor with pristine graphene as the sensing material. The sensor recovers to the original value in a few seconds due to the pump and UV light induced residual gas desorption. The response change is 12, 18 and 21% with UV irradiation for NO₂ concentrations of 1, 3 and 5 ppm with UV exposure. In addition, this effect enables the sensor to provide good repeatability and high selectivity to different gases.