Synthesis and characterizationof zinc stannate nanomaterials for humidity sensing applications

Abstract

Humidity sensors have drawn huge attention and became extensively important integrated device to use in our daily life for monitoring the environmental moisture for human comfort. Humidity sensors are also useful for online monitoring of the automotive, medical, construction, semiconductor, meteorological and food processing industries. The deployment of any sensor in the detection of actual humidity and environmental monitoring under various conditions demands for the quality control, high sensitivity and selectivity, fast response/recovery times, low hysteresis, high stability, serviceability in wider humidity range and temperature, miniaturization and economic viability, etc. The opportunities to achieve these features by sensors based on bulk materials under real time monitoring conditions are hindered by the unpredictable humidity levels present in the environment. However, chemically and thermally stable metal oxide nanostructures, known to provide larger surface-to-volume ratio for interaction with the environment are observed to be suitable candidates for humidity sensing with high sensitivity, fast response than that of bulk materials. In recent years, metal oxide nanostructures such as ZnO, SnO2, TiO2, In2O3, and WO3 are utilized for the fabrication of variety of chemical sensors. But the presence of single cation site and high temperature operation restricts its much needful requirement. Zinc stannate, a multifunctional ternary metal oxide material, exists as two types of oxide with different Zn/Sn/O ratios (ZnSnO3 and Zn2SnO4), which have potential applications in lead-free ferroelectrics, transparent conductors, photo catalysts, dye-sensitized solar cells, gas sensors, and lithium ion batteries (LIBs). Recently, ZnSnO3 nanostructures using various synthesis routes have drawn attention for obtaining better crystallites. However, the synthesis of composite metal oxides is particularly challenging in water due to variation in precursors reactivity. To overcome this here we report the synthesis of Zinc Stannate nanoparticles utilizing wet chemical synthesis method with variation in synthesis temperature which helps to get crystallites after annealing at different temperature. For evaluation of structural and morphological properties of prepared ZnSnO3 samples, X-ray diffraction (XRD) and scanning electron microscopy (FE-SEM) are implemented. To analyse the thermal behaviour TGA-DSC and for surface porosity Brunauer–Emmett–Teller (BET) and the Barrett–Joyner–Halenda (BJH) methods, respectively are used. The humidity sensing of all the samples were performed on indigenous developed humidity sensor testing setup within a wide range. Annealing effect has prominent role over the physical and sensing properties of the as prepared samples. XRD and FE-SEM confirms the sequential growth of ZnSnO3 crystallites into much uniform crystalline cubes of nearly 100nm with implementation of annealing temperature from 250°- 500°C. TGA and differential TGA study over the as prepared samples shows overall 19% weight loss due to removal of water molecules within 0-600°C temperature range after which no such loss is visible suggests the optimal thermal treatment temperature. Similarly, on carrying out the N2 adsorption-desorption study over the ZnSnO3 samples shows surface area of 17.18 ,19.99 and 15.55 m2/g for samples prepared at a stirring temperature of 65°C, 85°C and 100°C respectively. On implementation of the as prepared and different annealed samples towards humidity sensing in a range of 08-97% RH value, better sensitivity factor of around 700-4500% is observed in case of 500°C annealed ZnSnO3 samples. The hysteresis error calculation for the 500°C annealed samples showed much lower value i.e. about 0.000003 at 97% RH and practically none of the sample shows significant hysteresis value over the applied humidity range promoting it device usage. To investigate the isotherm behaviour of ZnSnO3 samples and to confirm the water adsorption-desorption mechanism which takes place in humidity sensing, Freundlich isotherm modelling of all the samples using relative deviation in resistance (R.D.R) against applied relative humidity was done. The results inferred that 500°C annealed samples show 87-97% deviation in resistance value from the initial value at ambient along with higher adsorption strength and adsorption capacity value of 2.5 and 9.5 respectively. The linear fitting of experimental results clearly shows presence of two adsorption regions in all the samples supporting the plausible humidity sensing mechanism and ZnSnO3 turns out to be excellent humidity sensing material.