Investigation on Novel Poly 3hexylthiopheneZnO Nanocomposite Thin Films Gas Sensor

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Investigation on Novel Poly (3-hexylthiophene)-ZnO
Nanocomposite Thin Films Gas Sensor

• C. Baratto, G. Faglia, M. Ferroni, G.
  Sberveglieri
• Department of Chemistry and Physics , C.N.R. -
  INFM University of Brescia, Italy
• M.Z. Atashbar, E. Hrehorova
• Department of Electrical and Computer
  Engineering, Western Michigan University USA

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Outline

• Introduction
• Use of conducting polymer as gas sensors
• Preparation and characterization
• Test as gas sensor
• Conclusions

3
Introduction I

• P3HT regioregular conducting polymer
• p-type semiconductor-could be used in application
  like electronic nose
• Band gap 2.14 eV, good solubility, thermal and
  environmental stability
• Easily prepared at low temperatures
• Used for electronic applications
• LEDs, solar cells, photovoltaic applications
• Gas sensing application as selective layers

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Introduction II

• P3HT reported as a sensor for ppb range
  concentration of hydrazine (N2H4),
  monomethyhydrazine (MMH) vapors, and N2O, and
  PH detection
• Reacts with strong acid and bases
• Acids decrease R, while bases increase R
• Gas interacts with the organic semiconductor and
  modulates the carrier population of the
  semiconductor
• Introduction of ZnO nanoparticles to enhance
  selectivity of gas sensor towards gases (NO2).
• L. Diane et al. Anal. Chem 68, 817-822,1996
• H.Fukuda et al. Sens Act. B, 108, 414-417,
  2005

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Sensor preparation

• 23 w/v P3HT dissolved in chloroform23 w/v ZnO
  nanopowders (50-70 nm) dispersed in chloroform
• Mixed at 7030,5050 and 1000 ratio P3HTZnO
• Solution were casted on substrates.
• Thickness about 20 nm (pure P3HT)
• Deposited on Al2O3 substrates with Pt IDCs and
  heater. Spacing between fingers 150 ?m

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FTIR

• All the films contains typical vibration peaks of
  P3HT
• at 3055 cm-1aromatic CH stretching
• 2916 cm-1 and 2857 cm-1 for CH2 stretching of
  aliphatic side chain
• 1509 cm-1 and 1455 cm-1 stretching vibrations of
  the thiophene ring
• 1376 cm-1CH3 deformation vibration
• 820 cm-1 C-H out-of-plane vibration
• 726 cm-1 methyl rocking,
• The presence of ZnO resulted in widening of the
  peak at 820 cm-1 and introduction of new peaks at
  796 cm-1 and 748 cm-1

• T.-A Chen et.al. J. Am. Chem. Soc., 117, 233,
  (1995).
• J. E. Hernandez at.al. J. Phys. Chem. B, 105
  8339-8344, (2001).

P3HT peaks
ZnO presence
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SEM TEM

• Uniform layer without cracks
• Darker agglomerates due to ZnO aggregation
• ZnO is randomly distributed

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Film degradation after thermal treatment

• Degradation of the layer after prolonged thermal
  treatment at 50C.

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Electrical characterization

• Volt amperomentric technique at constant bias
  (1V)
• Gases of interest for environmental monitoring
  application
• NO2 (100ppb- 5 ppm),
• CO (500ppm),
• NH3(25ppm)
• ethanol (500ppm)
• Acetone (20-100 ppm)
• Working temperature 20C - 50C.
• 50 RH synthetic air
• Relative response
• oxidizing gases ?I/I (Igas-Iair)Iair
• reducing gases ?R/R (Rair-Igas)Igas

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Conducting polymer-sensing mechanism

• The band bending at the surface is modulated by
  adsorbed species like NO2
• NO2 adsorbs over the surface of organic
  semiconductor, trapping an electron
• The number of holes is increased and the
  conductance semiconductor increases.

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Effect of working temperature

• Baseline current decrease with increasing ZnO
  content
• Response to NO2, NH3
• No response to ethanol and CO
• Only partial reversibility at RT
• Recovery times decrease at 50C

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Tests at working temperature50C

• No response to acetone
• The addiction of ZnO nanoparticles
• enhance response to NO2 with respect to P3HT
  samples
• Decrease recovery times to 20 minutes
• slow recovery times are due to presence of
  strongly adsorbing sites

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Response to NO2 at 50C

• Data are fitted by a power law
• RM0XM1

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Lower NO2 concentrations
Camilla Baratto In effetti il grafico di destra
era riferito al 9010. Ma a 20C. Lho cambiato-
in questo caso il coefficiente non cambia tra i
due, la diff di risposta e bassissima

• New batch of sensor produced from 3 month old
  solution for test with lower NO2 concentrations
• There is a solution aging effect on the starting
  baseline
• currently under investigations some samples are
  not reported because of high resistance
• No response to humidity

WT50C
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Conclusions

• P3HT-ZnO nanocomposites proved good p-type
  sensors for NO2
• No interference from reducing gases like CO,
  ethanol, acetone and humidity.
• Low working temperature (50C)

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Future work

• Investigate the influence of film thickness on
  baseline resistance
• Aging of polymer with temperature
• Investigate the effect of preparation procedure
  on heat resistance of P3HT thin films
• Long term stability tests
• Effect of UV light on response and recovery
  times.