Elsevier

Sensors and Actuators B: Chemical

Volume 176, January 2013, Pages 460-466
Sensors and Actuators B: Chemical

Formaldehyde gas sensor based on silver-and-yttrium-co doped-lithium iron phosphate thin film optical waveguide

https://doi.org/10.1016/j.snb.2012.10.037Get rights and content

Abstract

Several formaldehyde gas sensors were fabricated using silver-and-yttrium-co doped-lithium iron phosphate (LiFe1−0.01xY0.005xAg0.005xPO4)-thin-film-based optical waveguides (OWGs). LiFe1−0.01xY0.005xAg0.005xPO4 powders with different concentrations of the dopants Ag and Y were prepared in high purity via a one-step hydrothermal method, and characterized using their X-ray diffraction patterns and energy dispersion spectra. The obtained powders were subsequently utilized in a spin-coating procedure for the fabrication of LiFe1−0.01xY0.005xAg0.005xPO4 thin films.

The obtained thin films were characterized by refractive index, thickness, attenuation, and sensing properties to formaldehyde at room temperature. The response of LiFe1−0.01xY0.005xAg0.005xPO4-thin-film/tin-diffused-glass-based optical waveguide sensors to formaldehyde was higher than their responses to other VOCs. For a set of formaldehyde concentrations ranging from 1 ppb to 1000 ppm, the sensitivity of the sensors increased with a decrease in the dopant concentrations. Moreover, the ability to be used repeatability as well as the selectivity of the sensors was tested using lower gas concentrations; at the lower concentrations (below 250 ppm), the sensor only responsive to formaldehyde gas.

Introduction

The evaluation of indoor air quality is a serious environmental issue that needs to be addressed urgently. Formaldehyde (CHOH) is one of the harmful volatile organic compounds (VOCs) emitted by building materials, interior decoration materials, wood furniture, carpets, and so on. Formaldehyde gas irritates human eyes and noses at concentrations higher than 200 ppb, causes breathing difficulties above concentrations of 5 ppm, and has been known to be a carcinogen since 1980 [1], [2]. The World Health Organization (WHO) has established a concentration of 0.08 ppm, averaged over 30 min, as a standard for long-term exposure to formaldehyde vapors [3], while the Chinese Environmental Protection Agency (EPA) established 0.06 ppm as the standard [4].

Several sensors have been reported for formaldehyde detection. These include enzyme-based biosensors [5], metal-oxides-based gas sensors [6], piezoelectric sensors [7], and optical fiber sensors [8]. Among the various kinds of sensors, optical chemical sensors have been shown to be safe for use in an explosive environment, in addition to be applicable in unusual or extreme conditions [9]. The optical waveguide (OWG) sensor [10], [11] is of particular interest owing to its high sensitivity, fast response time, low cost, and intrinsically safe detection.

A simple planar OWG consists of a substrate, a thin top layer (waveguide layer) with a refractive index greater than that of the substrate (onto this, a sensitive covering material is deposited to detect the analyte), and the covering material (usually air). In this study, LiFePO4 co doped with Ag and Y (LiFe1−0.01xY0.005xAg0.005xPO4) was used as the sensing element for a formaldehyde gas sensor. LiFePO4, which has an olivine structure, has attracted great interest as the cathode material in rechargeable lithium-ion batteries because of its high energy density, low cost, low toxicity, excellent thermal stability, and safety [12], [13], [14]. So far, a number of experimental studies on LiFePO4 have appeared that have improved its electrochemical properties through doping with other elements [15], [16], prepared LiFePO4 thin film electrodes [17], and used it lithium-ion sensors [18]. However, there is almost no report on its optical properties as well as the gas sensing applications.

After enormous experiment confirm that, LiFePO4 thin film exhibited excellent optical transparency [19], [20], [21], easy fabricated, high refractive index, good response to pollutant gases, and was a qualitative candidate for optical waveguide gas sensor.

In semiconductor gas sensor, the gas-sensitivity has been improved by introducing dopants or decreasing particle size of sensing materials [22]. Previously [20], [21], the sensitivity and selectivity of LiFePO4 thin film OWG were improved by doping Ag and Y, separately. In this study, in order to get more sensitive material, both Ag and Y were selected as dopants.

Herein, we report the development of a LiFe1−0.01xY0.005xAg0.005xPO4-thin-film/tin-diffused glass-based OWG sensor and its application in formaldehyde gas detection. Our research focused on the effect that the concentrations of the dopants, Ag and Y, used to fabricate the LiFe1−0.01xY0.005xAg0.005xPO4 thin film, had on the gas sensing properties of the sensor with respect to formaldehyde gas.

Section snippets

Preparation of LiFe1−0.01xY0.005xAg0.005xPO4 powders

LiFe1−0.01xY0.005xAg0.005xPO4 powders were synthesized by a hydrothermal method. Iron (II) sulfate heptahydrate (analytically pure), phosphoric acid (85 wt%; analytically pure) and lithium hydroxide monohydrate (analytically pure) were mixed in a molar ratio of 1:1:3 [23]. In order to simplify the synthesis process, AgNO3, Y (NO3)3·6H2O and 0.1 g of ascorbic acid were added to form LiFe1−0.01xY0.005xAg0.005xPO4 (x = 0.5, 1.0, 2.0). The resulting solution was placed in a hydrothermal reactor having

Characterization of the LiFe1−0.01xY0.005xAg0.005xPO4 powders

Fig. 2 shows the XRD patterns of the LiFe1−0.01xY0.005xAg0.005xPO4 powders. All diffraction peaks are shown to follow the standard crystal structure patterns (JCPDS Nos. 40-1,499; LiFePO4) [12]. Diffraction peaks for Ag and Y were not observed because of their low concentration (≤3 wt%). The fact that the peaks were so similar to the standard ones for LiFePO4 suggested the absence of significant amounts of any by-products in the samples. In addition, the XRD patterns confirmed the existence of

Conclusion

In this study, we report a highly sensitive optical waveguide (OWG) sensor for detecting formaldehyde gas. LiFe1−0.01xY0.005xAg0.005xPO4-film/tin-diffused-glass-based OWG sensors were developed and used in the detection of volatile organic compounds (VOCs). The responses of these sensors to formaldehyde gas were higher compared to other VOCs. The sensitivity of these sensors decreased with an increase in the concentration of the dopants, Y and Ag as well as with an increase in the attenuation

Acknowledgement

The authors would like to acknowledge the National Natural Science Foundation of China for the support of this project under the 20965008 and 21265020 grants.

Patima Nizamidin Doctor, Student of College of Chemistry and Chemical Engineering, Xinjiang University, mainly engaged in nano-thin film and optical waveguide gas sensor.

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    Patima Nizamidin Doctor, Student of College of Chemistry and Chemical Engineering, Xinjiang University, mainly engaged in nano-thin film and optical waveguide gas sensor.

    Abliz Yimit Doctor, graduate from Yokohama National University, Japan, in 2003 year. Now work at College of Chemistry and Chemical Engineering, Xinjiang University, mainly engaged in nano-thin film and optical waveguide gas sensor.

    Adalat Abdurrahman associate professor. Now work at College of Chemistry and Chemical Engineering, Xinjiang University,mainly engaged in nano-thin film and optical waveguide gas sensor.

    Kiminori Itoh Doctor, received his Dr. Eng. degree in Industrial Chemistry from the University of Tokyo, Japan, in 1980, and is now a Professor at the Institute of Environmental Science and Technology, Yokohama National University. Mainly engaged in the chemical application of optical waveguides and the measurement of complicated systems, e.g. soil bacteria, human bodies, and global climate.

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