Hydrothermal Synthesis of Fe2O3 for the Humidity Sensing Application

Fe2O3 nanorods have been successfully synthesized by a hydrothermal method using stainless steel Teflon autoclave. The solution of FeCl3 and Urea was transferred to Teflon-lined stainless-steel autoclave and then maintained the temperature 180oC for 24 hrs. As prepared Fe2O3 powder were characterized by X-ray Diffraction (XRD) and the result shows the formation of Hematite phase of Fe2O3 after calcination at 400 o C of 2h. Field Emission Scanning Electron Microscopy (FESEM) was used to study the morphology of synthesized Fe2O3 powder and the result indicates that bunch of nanorods were formed with width 44 nm and length of 220 nm. Also, Fourier Transform Infrared spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) techniques were used to characterize synthesized Fe2O3 nanorods. To study the humidity sensing performance of Fe2O3 nanorods, sensor was synthesized as interdigitated electrode coated with sensing material on top of the electrode. The present study investigated good humidity sensing performance of Fe2O3 nanorods at room temperature in humidity (RH) range of 11%-97% at DC condition.


Introduction
Metal oxides are the promising material for the various applications in the field of electrical and optical device development. Also, to control environmental conditions, metal oxide nanostructures are extensively used as Humidity Sensor [1][2][3]. As a result, it is a challenging task of design a good humidity sensor. The performance of humidity sensor is improved by taking efforts in betterment of sensor performance parameters such as sensing elements, structure design, principle of mechanism and fabrication technologies [4]. Metal oxides have wide ranges of properties, including large band-gap, transitions from insulators to metallic and superconducting nature. Among these metal oxides, the iron oxide (Fe 2 O 3 ) has significantly used in various application because of its stability, high electrical conductivity, large band gap, low cost and environmentally friendly [5][6][7][8]. The α-Fe 2 O 3 is an n-type semiconductor with a thermodynamically stable with a band gap of 2.10eV. As a result, a variety of nanostructures of iron oxide like nanoparticles, nanorods, nanocubes, nanorings, nanonails, nanodiscs and nanoflowers by using different synthesis route has been reported to enhance material properties. These nanostructures of α-Fe 2 O 3 are extremely promising and attractive materials for their wide range of applications, such as catalysis, gas sensors, magnetic recording materials, lithium rechargeable batteries, biosensor, pigments and paints [9][10][11][12][13]. In a present work, nanorods of α-Fe 2 O 3 were synthesized by hydrothermal method. The synthesized α-Fe 2 O 3 nanorods were characterized by XRD, FESEM, XPS and FTIR. And further humidity sensing performance of α-Fe 2 O 3 nanorods were examined at room temperature.

Experimental 2.1 Materials
The purity of all chemicals was of analytical grade and there is no subsequent purification process. Ferric chloride (FeCl 3 6H 2 O) was procured from Sigma-Aldrich, India and Urea was procured from Qualigens Fine Chemicals, India.

Preparation of Iron Oxide nanorods.
In this work, Hematite α-Fe 2 O 3 nanorods were synthesized by hydrothermal method. In the typical experiment, 0.1M solution of FeCl 3 was prepared in 120ml distilled water, and then urea was added into the solution with continuous magnetic stirring for 30 min. The prepared solution was transferred to Teflon-lined stainlesssteel autoclave and then maintained the temperature 180 0 C for 24hrs. The resulting precipitated was filtered, washed several times by double distilled water and dried at 60 0 C for 6hrs.
The final precursor was calcinated at 400 0 C for 2hrs to form final product of Hematite α-Fe 2 O 3 nanorods.

Humidity sensing measurement. 2.3.1 Preparation of sensor electrode.
The sensor consist of an interdigitated electrode coated with sensing material on top of the electrode as shown in Fig.1.(a) The electrode consists of five pairs of Cu tracks .The width and gap between two successive tracks is about 1 mm.

Humidity sensing measurement.
In the present study, humidity sensingperformance of α-Fe 2 O 3 nanorods were examined at room temperature (27 o C) using indigenously developed humidity sensing set-up as shown in Fig.1.(b). In the typical experimental method, humidification inside the chamber was developed by saturated salt solution (K 2 SO 4 ) by adding distilled water drop by drop in salt K 2 SO 4 at room temperature. The variations in electrical current (nA) with different RH level were manually recorded by digital picoammeter (SES, Model: DPM-111).

Characterization of α-Fe 2 O 3 nanorods
X-ray diffraction (XRD) analysis were performed with a Bruker D8 Advance X-ray diffractometer with CuK-alpha radiation to identification of phase and crystallite size. The XRD patterns of synthesized nanorod powder shown in Fig.2

Humidity Sensing Study of α-Fe 2 O 3 nanorods
To study the humidity-sensing performance based on of α-Fe 2 O 3 nanorods, the current (nA) in the sensors was measured as a function of Relative Humidity (10 % RH to 99% RH) at D.C condition and the results are depicted in Fig.6. The current in the sensor was affected by change in Relative Humidity The current in the sensor changes by two orders of magnitude from 29 nA to 2320 nA RH increases from 11% RH to 99% RH. The change in the current value with RH is attributed to the adsorption of water molecules onto the surface of α-Fe 2 O 3 nanorods.

Conclusion
α-Fe 2 O 3 nanorods have been successfully synthesized by a hydrothermal method by calcinating precursor at 400 o C for 2h and their humidity sensing properties are investigated. The synthesized material was characterized by XRD, FESEM, XPS and FTIRto know the phase and morphology of material. Humidity sensors based on α-Fe 2 O 3 nanorods exhibit high and linear response within the whole relative humidity (RH) range of 11%-99% RH at an operating at DC conditions. The corresponding current (nA) changes by approximately two orders of magnitude within the entire humidity range from 11% to 99% RH.