Humidity sensing characteristics of manganese oxide prepared by sol-gel method

Abstract

Electrolytic manganese dioxide (EMD), which has been commonly used as a cathode material in drycell batteries, catalyst and water puri®cation material, is a typical example of a multifunctional material. Xu and Miyazaki [1, 2], found that EMD derivatives could be utilized for humidity sensor materials [1], and developed a resistance-type humidity sensor with a high sensitivity and selectivity [2]. Recently, a new potential-type humidity sensor consisting of EMD solid electrolyte with two electrodes of different metals, such as Pt=Cu or Pt=Al, was explored [3, 4]. The humidity sensing properties were strongly dependent on the features of manganese oxides such as crystalline structure and powder morphology, so further research was needed for the development of an ef®cient humidity sensing system. The sol±gel process based on inorganic polymerization reaction from molecular precursors has provided a new approach to the preparation of inorganic materials [5]. This process has allowed better control of the characteristics of oxide powders. Bach et al. [6] reported the synthesis of manganese oxides via a sol±gel route in detail. Le Goff et al. [7, 8] investigated the structural and electrochemical properties of sol±gel-derived manganese oxides and revealed that these oxides were promising rechargeable cathodic materials for lithium batteries. In this study, the potential-type humidity sensing characteristics of manganese oxide samples prepared by a sol±gel method were evaluated in comparison with an International Common EMD sample no. 14 (IC no. 14 EMD). The sol±gel-derived manganese oxide powder was prepared by the following method, as reported by Bach et al. [6]. A 0.25 M aqueous potassium permanganate solution was mixed with an organic reducing agent, such as fumaric acid solution. The molar ratio of potassium permanganate to fumaric acid was ®xed at 3. Under this condition, a darkbrown transparent gel of KMnO2 was rapidly formed at room temperature. The gel was ®ltered and dried in air at 90±100 8C, giving a black xerogel powder. This KMnO2 xerogel was treated with a 2.5 M sulphuric acid solution for 5 h at room temperature to remove potassium. After the sulphuric acid treatment, the resulting manganese oxide powder was washed by decanting, neutralized with an ammoniacal aqueous solution, ®ltered and dried in air at 90±100 8C. The potassium content of the sample was analysed using a ame spectrophotometer (Shimadzu AA6400F). The crystal structure was characterized by X-ray diffraction (XRD); Mac Science M03X-HF) using CuKa radiation, and the Brunauer±Emmet± Teller (BET) surface area was measured with an adsorption porosimeter (Micromeritics ASAP 2000). The potential-type humidity sensor constituting a galvanic-type cell system was used in this work. The electrochemical cell system is in the form of Pt=MnOx=Al, where Pt and Al act as the cathode and anode, respectively, and MnOx (manganese oxides) act as the solid electrolyte. To fabricate a sensor element, each of the manganese oxide samples under examination was ground in an agate mortar and wetted with ethanol, then the resulting paste was coated onto a mullite tube (1.0 mm in diameter) with Pt and Al ®lament electrodes (0.25 mm in diameter). The electrodes were embedded in the coated MnOx layer to ensure that an electrical contact was made. The thickness of the MnOx layer was approximately 0.3 mm. The sensor element was placed in a test chamber where the humidity was controllable, as shown in Fig. 1. The electromotive force (EMF) by the galvanic cell-type sensor was measured at room temperature by means of an ADVANTEST-TR2114 digital voltmeter (internal impedance . 10 U), and the impedance of the sensor element containing MnOx was of the order of 10 U [4], which is well within the meaningful range of measuring impedance in this case. A sample of sol±gel-derived manganese oxide produced an almost amorphous XRD pattern, as shown in Fig. 2. The potassium content within this