Resistive humidity sensors based on nanocellulose films for biodegradable electronics

Abstract

Nanocellulose (NC) is a promising modern material suitable for use in electronics. This material is biodegradable, and thus, if used in electronic devices, will not require disposal and will decompose naturally. An interesting feature of nanocellulose is its hygroscopicity, which makes it applicable for the manufacture of humidity sensors. In this study, we synthesized nanocellulose-based humidity sensors with a weight of humidity-sensitive layer from 0.3 to 3.6 mg. The following static and dynamic characteristics of the obtained sensors were measured: sensitivity, response, hysteresis, repeatability, response and recovery time, short and long-term stability. It was determined that at a frequency of 100 Hz, the maximum sensitivity was observed in the sample with NC mass of 1.8 mg (0.215 (%RH)^-1), and at 1000 Hz, in the sample with NC mass of 0.5 mg (0.155 (%RH)^-1). Thus, with increasing frequency of test signal, the sensitivity of the sensors decreases. These same samples (with NC mass of 1.8 mg at 100 Hz and 0.5 mg at 1000 Hz) showed the highest values of sensor response — 1.99·10⁶ and 5.43·10⁴, respectively. Same as with sensitivity, when frequency increases, sensor response decreases. For both frequencies, the sample with NC mass of 0.4 mg showed the lowest value of hysteresis - 0.04 and 0.12% at 100 and 1000 Hz, respectively. It was also found that the sample with NC mass of 0.3 mg has the shortest response time of 42 s. With increasing of NC weight, the response time increases about 20-fold and recovery time - by 2 orders of magnitude. The highest short-term stability was demonstrated by the sample with NC weight of 0.5 mg: deviations from the arithmetic mean were 8 and 7.8% at test frequencies of 100 and 1000 Hz, respectively. The worst short-term stability was demonstrated by the sample with NC mass of 3.3 mg with the deviation of 31.7 and 39.2% at the same frequencies. It was also determined that such sensors need to be further researched to improve long-term stability.
Therefore, the measurement results demonstrate that, in terms of sensitivity and response, the optimal mass of NC film is 1.8 mg at the test frequency of 100 Hz. This sample also shows the best long-term stability. From the point of view of recoverability and sensor speed, the sample with NC weight of 0.3-0.5 mg is preferable.