Zn-doped γ-Fe2O3 sensors for flammable gas detection: Effect of annealing on sensitivity and stability

https://doi.org/10.1016/j.jiec.2010.12.016Get rights and content

Abstract

In this study, we investigated the effect of synthesis conditions and processing methods on the stability of gas sensors made of flame-synthesized Zn-doped γ-Fe2O3 particles. Nanocrystalline Zn-doped γ-Fe2O3 particles were synthesized by flame spray pyrolysis using either H2/Air or H2/O2 coflow diffusion flames. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller surface-area-measurement (BET) were employed to characterize the particles. Gas sensors were fabricated by applying the as-synthesized and annealed particles on interdigitated electrodes. High-temperature flame (H2/O2) generated nanometer-sized particles; lower temperature flame (H2/Air) generated micrometer-sized particles. The sensors made from as-synthesized particles showed a gas sensing sensitivity that was 20 times higher than the literature value. The sensors made of microparticles lost their sensing ability after three days of aging, but sensors made of nanoparticles did not show significant change after aging. XPS results showed significant Zn enrichment on the surface of as-synthesized particles. However, Zn concentration on the surface of particles decreased significantly after annealing. The results showed that sensors made of nanoparticles have higher gas sensing signal, and higher resistance towards aging than sensors made of microparticles. In addition, the annealing process is apparently related to the solid state diffusion of the Zn dopant.

Introduction

Gas sensors based on semiconducting metal oxides have been one of most investigated devices of gas sensors. They have aroused the attention from many researchers interested in gas sensing due to the low cost, ease of fabrication, simplicity of their use, and large numbers of detectable gases [1a,1b].

Today, many companies provide this type of sensors, such as Figaro, FIS, MICS, UST, CityTech, Applied-Sensors, NewCosmos, etc. And their applications range from combustible or toxic gas alarms to air intake controls in automobile, to glucose biosensors [2].

Even though many different metal oxides have been investigated as gas sensor materials, gamma-iron oxide (γ-Fe2O3) is an n-type semiconductor and one of the most commonly used materials in research. Tao et al. [3] reported that gas sensitivity of γ-Fe2O3 to alcohol was greatly improved by doping Y2O3 through sol–gel process. Jing [4] promoted the sensitivity and selectivity of γ-Fe2O3 towards acetone, ethanol, methane, and hydrogen by doping it with zinc. Jiao et al. [5] enhanced the selectivity and stability of gas sensor to acetone, H2, CO and CH4 by using SnO2/Fe2O3 multilayer thin film. Yamazoe [6] proposed two types of sensitization mechanism for various dopant materials; chemical sensitization, electronic sensitization. In this study, the γ-Fe2O3 was doped with 15 mol% zinc by flame spray pyrolysis which provides the opportunity to control particle size of sensor material that would be an important factor in sensitivity and stability of gas sensor.

Gas sensors should provide long-term performance, even at high operation temperature and corrosive media. In general, any gas sensing device should exhibit a stable and reproducible signal for the period of at least 2–3 years [7]. The requirement means that stability of gas sensor is one of most important factors determining the practical uses of gas sensors.

Many parameters affect the stability of a gas sensor; grain size of sensing material (synthetic method), reducing gas, operating temperature, relative humidity, annealing process, film thickness, deposition method and etc. [8].

In this study we focus on some of these parameters (grain size of sensing material, reducing gas, operating temperature, annealing process) to determine the effect of synthesis conditions and processing methods on the stability of gas sensor made of flame-synthesized Zn-doped Fe2O3 particles before and after 3-day aging. And we find out how the annealing process affects the sensing material with different analyses from previous work [9].

Gamma-Fe2O3 based gas sensor detects gases via variations in their resistances. Oxygen electron vacancies operate as donors and transfer oxygen gas to the negative charged oxygen adsorbates, which play important role in detecting inflammable gases such as acetone, H2 and CO. Oxygen adsorbates, such as O, is known to cover the surface of semiconductive metal oxides in air and eventually the variation in surface coverage of O dominates the sensor resistance [10], [11].

Sensor selectivity and rate of target gas adsorption are influenced by reactions between the sensing material surface and the target gas. These reactions are in turn, affected by catalysts on the material surface, ambient conditions, and sensor temperature. When thermal energy is supplied to Fe2O3 nanoparticles, the free charge carriers (electrons for Fe2O3) increase and cause a decrease in resistance. Then synthetic air is supplied, the free charge carriers are absorbed by O2 gas. So the surface of Fe2O3 acts as an electric potential barrier which decreases conduction between particles. Reduction gas or other combustible gases remove oxygen adsorbates on the Fe2O3 surface. This causes the free charge carriers captured in O2 gas to move into Fe2O3 nanoparticles; which weakens the electric potential barrier and increases particle conduction.

These processes can be explained by following equations.S+e+12O2SOsSOs+R(H2,C3H6O)RO+e+Swhere S denotes a surface adsorption site, e is a free electron, R is a reducing gas, and S  Os is an oxygen adsorbates. And reversible equation (1) is a function of temperature and oxygen partial pressure [12].

As a result, the rate of adsorption and desorption is key factor to deciding the sensor's sensitivity to various gases. Since target gas concentration is a major factor in the rate of adsorption, an accurate method of calculating target gas concentration is needed. Using partial pressures and the saturation vapor pressure of the target gas (acetone), the acetone gas was introduced into the main 4 SLM (Standard Liter per Minute) flow of synthetic air via a bubbler. This assumption is valid for small flow rates because the gas can get the sufficient residence time in the bubbler to forms separate bubbles only at small flow rates and at large flow rates, a gas jet is formed i.e. less than 100 SCCM (Standard Cubic Centimeter per Minute). And H2 gas was injected into the main 4 SLM flow of synthetic air directly. Synthetic air was used in the experiments to minimize the influence of humidity.

Section snippets

Flame spray pyrolysis (FSP) apparatus

Zn-doped Fe2O3 sensor materials were prepared by flame spray pyrolysis method using H2 diffusion flame with O2 or air support. Fig. 1 shows a schematic for the flame spray pyrolysis apparatus. The precursor solution was injected at a steady flow rate into the atomizer vessel using a syringe pump (Cole Parmer, Vernon Hills, IL). The H2 fuel gas flew through the vessel and carried the precursor droplets generated by atomizer into the flame through the furnace. The droplets underwent solvent

Particle size and morphology

Fig. 7 shows representative TEM images of Zn-doped Fe2O3. Particle sizes of Zn-doped Fe2O3 formed in H2/O2 diffusion flame are much smaller than ones formed in H2/Air flame. That is, high-temperature flame generated nanometer-sized particles; lower temperature flame generated micrometer-sized particles.

Signal of gas sensor

Fig. 8 shows the sensor signals towards acetone of Zn-doped γ-Fe2O3 particles synthesized using H2/Air and H2/O2 flames. Both sensors showed quick-time response ability and significant

Conclusion

Zn-doped Fe2O3 particles synthesized by flame spray pyrolysis in a high temperature flame were proved to be more effective in detecting acetone vapor and more resistant towards aging than particles made in a low temperature flame. TEM images showed that high-temperature flame generated nanometer-sized particles. Annealing processing deteriorated the gas sensing ability, which can be explained by XPS analysis; solid state diffusion of Zn into the core. However, it improved the stability of gas

Acknowledgements

The author is greatly indebted to faculty members of Department of Mechanical Engineering in Korea Military Academy for their valuable words of encouragement and advices. And I really appreciate for Mira's devotion that makes me better. Financial support for this research was provided by Texas Engineering Experiment Station and Texas A&M University.

References (14)

  • N. Barsan et al.

    Sens. Actuators B: Chem.

    (2007)
    Y.J. Shin et al.

    J. Ind. Eng. Chem.

    (2010)
  • H.P. Yang et al.

    Biosens. Bioelect.

    (2007)
  • S.W. Tao et al.

    Sens. Actuators B: Chem.

    (1999)
  • Z.H. Jing

    Mater. Sci. Eng. Struc. Mater. Propert. Microstruc. Proc.

    (2006)
  • Z. Jiao et al.

    Mater. Res. Bull.

    (2000)
  • N. Yamazoe

    Sens. Actuators B: Chem.

    (1991)
  • G. Korotcenkov

    Mater. Sci. Eng. B: Solid State Mater. Adv. Tech.

    (2007)
There are more references available in the full text version of this article.

Cited by (13)

  • Gas sensors based on Pd-decorated and Sb-doped SnO<inf>2</inf> for hydrogen detection

    2022, Journal of Industrial and Engineering Chemistry
    Citation Excerpt :

    Therefore, developing long-term and efficient real-time gas sensors for monitoring H2 has become more critical. Towards this perspective, to deal with increasing requirements for gas sensing materials, various metal-oxide semiconductor materials including TiO2 [5], SnO2 [6–8], ZnO [9], In2O3 [10,11], NiO [12], WO3 [13], Fe2O3 [14], and CeO2 [15] have been fully developed and applied for gas detection applications. Interestingly, SnO2 [16,17] has been successfully developed and widely used for monitoring various gases, such as CO [18], acetone [19], NO2 [20], and ethanol [21] owing to its unique physical and chemical properties.

  • Improvement in CO<inf>2</inf> sensing characteristics using Pd nanoparticles decorated La<inf>2</inf>O<inf>3</inf> thin films

    2017, Journal of Industrial and Engineering Chemistry
    Citation Excerpt :

    In this case, at low temperature, the sensor response is restricted by the speed of the chemical reaction, and at higher temperature, it is restricted by the speed of diffusion of gas molecules. At some intermediate temperature, the speed values of the two processes become equal, and at that point, the sensor response reaches its maximum [27]. Thus, in the present case, the optimum operating temperature for La2O3 and Pd-La2O3 thin films are 523 K at which La2O3 and Pd-La2O3 sensor response attains it’s peak value.

  • Self-heating effects on the toluene sensing of Pt-functionalized SnO<inf>2</inf>–ZnO core–shell nanowires

    2017, Sensors and Actuators, B: Chemical
    Citation Excerpt :

    Gas sensors should provide a long-term performance, exhibiting a stable and reproducible signal for the period of at least 2–3 years. This means that stability of gas sensors is one of the most important factors determining the practical uses of gas sensors [44]. However, one of the most serious problems of metal oxide-based gas sensors is their low stability, including the drift phenomena.

  • Flame aerosol synthesis of nanostructured materials and functional devices: Processing, modeling, and diagnostics

    2016, Progress in Energy and Combustion Science
    Citation Excerpt :

    Mn (Mn2O3, ZnMn2O4, Mn/ZnO, MnOx/ZnO, SrMO3, LaMnO3±δ, Pd/LaMnO3±δ, Pd/YMnO3±δ, La1- xAgxMnO3±δ, La1-xCexMnO3, La(Mn, Pd)O3, and species as before with Li, Ti) [46,50,51,55,63,92,119,154–162], Fe (Fe2O3, Fe3O4, FeO, FePO4, ZnFe2O3, Fe/SnO2, Ag/Fe2O3/Fe3O4, Au/Fe2O3/Fe3O4, AuAg/Fe2O3/Fe3O4, LaFeO3±δ, Pd/LaFeO3±δ, Pd/YFeO3±δ, Nd:Co:Fe2O3, NdFeB alloy, and species as before with Li, Mg, Si, Ti) [51,63,90,96,104,119–121,160,163–175], Co (CoO, Co3O4, Co3O4NiO, Co3O4ZrO2, LaCoO3, Ru/Co3O4ZrO2, Pd/LaCoO3±δ, xAg/LaCoO3, La1-xAgxCoO3, La1-xCexCoO3+δ, La1-xEuxCoO3+δ, and species as before with Na, Al, Fe, Ti) [46,48,63,81,97,119,139,167,168,176–187],

  • Development of metal-loaded mixed metal oxides gas sensors for the detection of lethal gases

    2015, Journal of Industrial and Engineering Chemistry
    Citation Excerpt :

    For example, our living environment is exposed to various toxic gases. The indoor pollution of volatile organic compounds (VOCs), such as benzene, toluene, acetaldehyde, and formaldehyde, has been recognized as the principal cause of many diseases including asthma, emphysema, allergies, and cancer, and received more and more public concern these days [1–5]. Hydrocarbons, oxidized hydrocarbons, nitrogen oxides and sulfur oxides are common air pollutants in everyday living.

View all citing articles on Scopus
View full text