Application of a cubic-like mesoporous silica film to a surface photovoltage gas sensing system
Introduction
Nitrogen oxides (NOx) generated by combustion are harmful to one's health, and their concentrations in the environment have been regulated. Therefore, a highly sensitive, highly responsive, and portable device for monitoring these gases are urgently required [1], [2], [3], [4], [5], [6], [7], [8].
The surface photovoltage (SPV) semiconductor characterization technique [1], [9] has a great potential for satisfying the requirements of such gas sensors. The basic principle of this characterization technique is based on the semiconductor surface voltage properties of the metal–insulator–semiconductor (MIS) structure [9]. By measuring semiconductor photocurrent, this SPV system sensitively detects the variation in surface voltage, which is induced by physical adsorption of the target gases and depends mainly on the interactions between the target gases and the surface of the sensitive layer [1]. Therefore, the enhancement of the sensor performance requires a refinement of the metal and insulator layer of the MIS. Especially, the capacitance of the insulator layer depends on the gas adsorption performance, and the improvement of gas adsorption performance leads directly to the enhancement of SPV gas sensor characteristics.
The large surface area created by mesoporous materials [10], [11], [12], [13], [14] enables an improvement in the gas adsorption properties of SPV devices. In particular, during the past decade, these mesoporous materials have continued to progress through innovative synthesis methods [10], [11], [12], [13] which incorporate the surfactant self-assembly template system into the sol–gel synthesis method [10], [11], [12], [13]. The innovative mesoporous materials produced by the method have uniform pore size, pore channel alignment in two or three dimensions, and large surface area [10], [11], [12], [13]. Furthermore, this sol–gel synthesis method can be applied to the production of films [15], [16], [17], [18], [19], [20].
Large surface area and bi-continuous mesopores make a mesoporous silica film, the best candidate for the gas adsorption insulator layer of the SPV gas sensor. This paper reports, for the first time, a possible application of a self-ordered cubic-like mesoporous silica film to an NO gas sensor, the film being synthesized by the spin coating method and by the use of nonionic poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO100–PPO65–PEO100) type triblock copolymer surfactant as a structure-directing agent. The NO gas sensor is based on the SPV characterization system. The mesoporous silica film is assembled as an insulator gas adsorption layer of the MIS structure. The properties, efficacy, and potential performance of the SPV NO gas sensor incorporating the cubic-like mesoporous film are examined.
Section snippets
Synthesis of mesoporous silica film
SBA-16-type mesoporous silica film was adopted as the gas adsorption insulator layer, because its three-dimensional structure provides open pores on its top surface as shown in Fig. 1 [19], [20]. This kind of thick film was produced by the dip coating method [19], and has a cubic structure and a thickness larger than 1 μm. Furthermore, for attaining gas accessibility into mesopores and capacitance for SPV measurement, thin film rather than thick film is desired. Therefore, we modify the dip
Film structure
Fig. 4 shows The X-ray diffraction (Material Analysis and Characterization Science Co., Ltd.: XRD, M03XHF22) pattern and transmission electron micrograph (HITACHI, Ltd.: TEM, HF-2000) images of SBA-16 mesoporous silica. Fig. 4(b) shows the XRD pattern of SBA-16 mesoporous silica powder. Six well-resolved peaks are observed in this figure. These peaks have a d spacing ratio of 1:1/√2:1/2:1/√6:1/√10:1/√12:1/√14 and can be indexed as (1 1 0), (2 0 0), (2 1 1), (3 1 0), (2 2 2) and (3 2 1) reflections
Conclusions
A spin-coated film composed of cubic-like mesoporous silica SBA-16 has been successfully fabricated and incorporated into a SPV gas sensor system. The sensing properties for NO gas (100 ppm) have been investigated by the CB curve. The large bias shift in the CB curve and the well-reproducible responses observed in the measurement can be explained by changes in dielectric constant and charge in the cubic-like mesoporous silica layer. The changes in dielectric constant and charge are due to the
Acknowledgements
This work was funded in part by a grant from New Energy and Industrial Technology Development Organization (NEDO).
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