NiO thin-film formaldehyde gas sensor

doi:10.1016/S0925-4005(01)00898-X

Sensors and Actuators B: Chemical

Volume 80, Issue 2, 20 November 2001, Pages 106-115

https://doi.org/10.1016/S0925-4005(01)00898-X Get rights and content

Abstract

The suitability of both pure and Li-doped NiO as a thin-film resistive gas sensor for formaldehyde has been investigated. Pure NiO had a linear formaldehyde sensitivity of 0.825 mV ppm⁻¹ while that for 0.5 at.% Li-doped NiO was 0.488 mV ppm⁻¹ at 600°C. These gas-sensing materials also showed similar sensitivity for methanol and acetone as well as a reduced sensitivity for toluene and ethanol. Chloroform was a poison for these gas-sensing materials. Due to resistive noise, the detection limit for formaldehyde was found to be ≈40 ppm.

Introduction

Solid oxide gas sensors use two properties of doped ceramic oxide, thin-film, semiconductors: (1) their solid-state electronic conductivities and (2) their surface catalytic properties as oxidation catalysts. This technology has brought the alcohol sensor used in electronic breath analyzer equipment, hydrocarbon sensor used in many apartments in Japan to detect natural gas leaks and the carbon monoxide (CO) sensor used in many homes to monitor CO levels in the winter months when kerosene space heaters and natural gas burners are in use. Formaldehyde, HCHO, is a known carcinogen found in (1) pathology laboratories due to vapors of formalin solutions (40 wt.% formaldehyde, 12.5 wt.% methanol and 47.5 wt.% water) used to preserve tissues, (2) industrial chemical processes and (3) buildings as a results of condensation polymerizations used for building materials, paint and carpets. The hazardous formaldehyde levels in air are mandated on a “long term exposure” basis, e.g. averaged over 1–2-week-period. The following governing agencies have established limits of long-term exposure: 3 ppm by OSHA, 1 ppm by NIOSH and 0.1 ppm by AGCHI-residential. This long term averaging is not reflective of the extreme transients in toxic exposure in pathological laboratory workers who use formalin solutions. Detection at these mandated levels have been historically achieved by the use of colored adsorption techniques. Alternative techniques have been used which apply liquid–gas adsorption technology but with an even higher detection minimum. Both of these techniques involve a stringent semi-batch air-sampling procedure followed by a batch calorimetric analysis of the sample, making reporting of extreme exposure significantly delayed. Presently, there are no continuous electronic sensors available for the detection of formaldehyde gas [1]. This work details the development of a formaldehyde gas sensor from a new NiO thin-film solid-oxide sensing element with appropriate electrical and catalytic properties.

Section snippets

Thin-film synthesis and deposition

Dip coating solutions were prepared by adding a metered amount of nickel acetylacetonate powder (supplied by P. Faltz, Bauer, 172 E. Aurora Street, Waterbury, CT 06708. Certified purity of 99%.) to an acetylacetone solvent (Sigma Chemical Co., St. Louis, MO 63178) for a resultant 3.5 mM metal stock solution. This stock solution was then heated to the solution’s boiling point (acetylacetone boils at 139°C under 746 mm Hg of pressure, the pressure in Salt Lake City; ranging from 108 to 132°C

Undoped NiO

Initially, experiments were performed to attempt to measure the NiO sensor’s response to pulsed exposure of formaldehyde; however, we found that the baseline signal was varying with time. To track this variation, the sensor was exposed to flowing air at 600°C for several days. The results are shown in Fig. 3. Here, we see a somewhat noisy signal with a roughly exponential decay in the shunt voltage over a 16-h period. This and longer decay periods are typical of the period necessary for

Catalysis

A catalyst is an agent, which causes reactions that are thermodynamically favored but kinetically slow, to take place at faster rates. Both metal clusters and oxides are used as catalysts for oxidation reactions. For example, in automobile exhaust catalysis Pt, Rh and Pd metallic clusters are used on a substrate of Al₂O₃. In Claus catalysis, γ-Al₂O₃ is used alone without metals as the catalysis. For the oxidation of formaldehyde, several oxides, metals and metal–oxide systems have been

Conclusions

The experimental data provided show that NiO is an effective sensor for gases that undergo oxidation reaction on its catalytic surface. Li doping allows the conductivity of the NiO semiconductor to be increased so that lower applied voltages can be used and still maintain sensor sensitivity. The NiO sensor is most sensitive to methanol, formaldehyde and acetone and is poisoned by chloroform. Amongst the sensitive gases the sensor is not selective. This sensor has detection limits for methanol,

Acknowledgements

Funding for this work was provided buy a University of Utah Technology Innovation Grant. The authors would like to thank Dr. Anil Virkar for proofreading this manuscript.

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NiO thin-film formaldehyde gas sensor

Abstract

Introduction

Section snippets

Thin-film synthesis and deposition

Undoped NiO

Catalysis

Conclusions

Acknowledgements

Phys. Chem. Solids

J. Phys. Chem. Solids

Electrical properties of NiO

Phys. Rev.

Vapor–liquid equilibria for HCHO–MeOH–H2O

Ind. Eng. Chem.

Conduction in Li-substituted transition metal oxides

J. Chem. Phys.

Dielektrische leitfahigkeit von nikeloxyd

Z. Physik Chem. (Leipzig)

Electrical conduction in NiO at high temperature

Phys. Soc. Jpn.

High temperature defect structure and electrical properties of nickelous oxide

J. Chem. Phys.

Electrical conduction in MgO

J. Phys. Chem.

Thermodynamic and transport properties of nickelous oxide

J. Electrochem. Soc.

Ionic and electronic defect structures of pure and Cr-doped NiO

Z. Phys. Chem. N.F.

Oxides that show metal insulator transitions at nil temperature

Bell Syst. Technol. J.

Semiconductor behavior of NiO

Z. Elektrochem.

Electrical conduction of single crystal NiO

Phys. Stat. Sol.

Cation self-diffusion and electrical conductivity in NiO

J. Chem. Phys.

Vapor–liquid equilibria for HCHO–MeOH–H₂O