Preparation and properties of a humidity sensor based on LiCl-doped porous silica

Abstract

The importance of humidity control has been recognized in many fields, such as the electronics industry, food manufacturing processes, hospitals and, in everyday life [1]. A wide variety of ceramic, polymeric and composite sensors are being produced to serve these applications. With the development of silicon micromachining and on-chip integration technology, miniaturization of sensors is increasingly required. Recent progress in ceramic deposition technologies, mainly by the continuous advancement of microelectronics, provides new possibilities for the improvement of ceramic active elements in the field of chemical sensors. The sol–gel method is very attractive for the preparation of very uniform ceramic powders with high purity and a very homogeneous distribution of the components on the atomic scale. Also several ceramic oxides have recently been studied in thin film form prepared by the sol–gel process [2, 3]. This method starts with liquid precursors and converts the liquid into useful solids through chemical reactions at low temperatures. Some research on sol–gel-derived silica films has been done and the results provide some useful guidelines for obtaining successful films [4]. The sol–gel silica films were possibly used as humidity sensors because of their large pores, but had a high intrinsic resistance [5]. So, in this report, LiCl is employed as a dopant to modify the humidity sensitivity of the porous silica thin film. In addition, preparation by the dip-coating method and properties of the materials are described. Reagent grade tetraethyl orthosilicate (TEOS), ethyl alcohol (C2H5OH) and deionized water (H2O) were used to prepare a silica sol solution. The optimized composition of the solution is TEOS: C2H5OH:H2O ˆ 1:4:4(mol). LiCl was dissolved in deionized water to form another solution. These two solutions were mixed by magnetic stirring for 30 min. Dilute hydrochloric acid (HCl) at above 1% in molar ratio was added into the mixed solution as a catalyst when the TEOS hydrolized. Four specimens, with the proportions of TEOS:LiCl ˆ 100:0, 90:10, 80:20, and 70:30 (mol), respectively, were studied in this report. Al2O3 plates, with a Ag– Pd integrated electrode, were used as substrates. After cleaning, the substrates were immersed in the solution for 15 s and then withdrawn at a rate of 2 cm miny1. Coated samples were air-dried for a least 24 h and then calcined in air at over 400 8C. The thickness of the silica thin films with or without LiCl is 0.1–0.2 μm. The microstructural characteristics of the films were studied by using an Amray 100 type scanning electron microscope. Several saturated salt aqueous solutions were used to provide ambient atmospheres with humidities from 11 to 95%RH. The temperature was kept at 25 8C throughout the measurement process. The resistance of the samples at different relative humidities were measured with a HP 4274A impedance analyser. Fig. 1 indicates the typical microstructure of the silica film with 20% LiCl treated at 400 8C. It shows that the film is very uniform. At the same time, it can be observed that the sol–gel-derived silica films have a lot of pores. This result suggests that the films could be suitable for adsorbing water under ambient atmospheric conditions, that is to say, they are sensitive to relative humidity. Fig. 2 shows the humidity sensitivity of the films at different %RH at 1 KHz. It is found that the silica films with LiCl have a lower impedance than the pure silica ones, and the impedance of the dried gel films (not treated at an elevated temperature) is less than that of those films calcined at 400 8C. With increasing LiCl content, the conductance of the films increases by one to three orders of magnitude at different humidity regions. The impedance–humidity characteristic of pure silica films indicate that the gel sample is sensitive to humidity at the higher %RH region while that treated at 400 8C shows no sensitivity. Because the silica gel has many surface Si–OH hydroxyl groups [6], which adsorb water vapour physically through hydrogen bonding, this physisorbed water layer is localized by hydrogen bonding of a single water molecule to two surface hydroxyls, and a proton may be transferred from a