Gas sensing properties of LangmuirBlodgett polypyrrole film investigated by surface acoustic waves

1
Gas sensing properties of Langmuir-Blodgett
polypyrrole film investigated by surface acoustic
waves
2
Introduction

• Solid-state sensors
• Organic sensing coating
• LB technique
• SAW gas response

3
Theoretical background
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Experimental

• Substrate 128o YX-LiNbO3
• Al IDTs 1200
• fo 42 MHz, BW 2.8 MHz, IL 20 dB, N 10,
  aperture 4 mm, ?IDTs 8.5 mm, and ?film 4 mm
• Test cell 250 ml, temperature stability ?0.5oC
  for 5h, and flow rate 500 ml/min

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• Device delay line
• Reference and carrier gas dry air
• IL maximum of the frequency transmission curve
  at the fundamental frequency
• Operating temperature RT

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Results and discussion

• Sensitivity for NH3 6 ? 10-3 ppm/ppm
• The selectivity of PPy toward NH3 is ascribed to
  its strong dipole moment
• NH3 sensitivity of LB PPy gt NH3 sensitivity of
  electrochemically PPy

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• LOD 46 ppm NH3
• PPy sensor exhibits an increasing response as the
  concentration is raised, but the curve is not
  linear
• Better sensitivity has been measured at low
  concentrations (lt1000 ppm)

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• The thinner films provide faster response times
  and weaker responses
• The bulk effects for PPy film play an important
  role in SAW sensing mechanisms
• The response time is independent on gas
  concentration
• The desorption of NH3 from PPy is a slower
  process than its adsorption

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• Response shift from average value ? ?10 for 1
  NH3
• Response shift from average value ? ?6 for 1
  NH3 (excluding the first two exposures)
• The sort-term repeatability of the SAW phase
  response toward NH3 is excellent

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• The variations of the phase response were within
  ?4 over a mouth, mainly caused by the drift of
  the baseline phase signal
• The long-term repeatability is quite enough

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• The steady state of the response decreases with
  the increasing amount of RH
• The presence of water vapor in the ambient air
  negligibly shows the SAW phase NH3 response

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• The LB PPy films are irreversibly damaged at
  Tgt80oC
• Degradation effects on NH3 response are observed
  when the LB PPy film is exposed to hazardous
  ambient air of elevated NOx gas concentration in
  wet air

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• The NH3 gas is successfully discriminated as one
  component of the mixture of interfering gases
• The SAW detector can clearly distinguish a
  tailored amount of NH3 in the gas-mixture

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• The SAW velocity and IL decrease upon NH3
  exposure
• The electrical conductivity of PPy film decreases
  upon NH3 exposure
• The elastic loading is the dominant mechanism for
  SAW velocity response of PPy film toward NH3