The Impact of ZIF-8 Particle Size Control on Low-Humidity Sensor Performance

Abstract

An accurate humidity measurement is essential in various industries, including product stability, pharmaceutical and food preservation, environmental control, and precise humidity management in experiments and industrial processes. Crafting effective humidity sensors through precise material selection is crucial for detecting minute humidity levels across various fields, ultimately enhancing productivity and maintaining product quality. Metal-organic frameworks (MOFs), particularly zeolitic imidazolate frameworks (ZIFs), exhibit remarkable properties and offer a wide range of applications in catalysis, sensing, and gas storage due to their structural stability, which resembles zeolites. The previous research on MOF-based humidity sensors have primarily used electrical resistance-based methods. Recently, however, interest has shifted to capacitive-based sensors using MOFs due to the need for humidity sensors at low humidity and the resulting high sensitivity. Nevertheless, further studies are required to optimize particle structure and size. This study analyzes ZIF-8, a stable MOF synthesized in varying particle sizes, to evaluate its performance as a humidity sensor. The structural, chemical, and sensing properties of synthesized ZIF-8 particles ranging from 50 to 200 nanometers were examined through electron microscopy, spectroscopic, and electrochemical analyses. The fabricated copper electrodes combined with these particles demonstrated stable and linear humidity sensing capabilities within the range of 3% to 30% relative humidity (RH).

Keywords: MOF; ZIF-8; capacitive sensor; humidity sensor; particle size.

Figure 1

Schematic of humidity sensor fabrication using ZIF-8 on the interdigit-patterned Cu electrode/glass substrate.

Figure 2

(a–c) SEM images and (d) XRD graphs of ZIF-8 at different sizes.

Figure 3

Structural analysis of ZIF-8 by (a,d) TEM, (b) STEM, (c,e,f) EDX analysis.

Figure 4

Analysis of the (a) C 1s, (b) Zn 2p, (c) N 1s, and (d) O 1s bonds in ZIF-8 by XPS analysis.

Figure 5

(a) Graphs of capacitive humidity measurements using different sizes of ZIF-8 under 500 Hz, (b) Graphs of capacitive humidity measurements in a low-humidity area using ZIF-8 with a particle size of 200 nm under 100Hz.