Effects of In2O3 modification of sprayed multiwalled carbon nanotubes for pH-sensing applications

Improvements in the pH sensitivity of multiwalled carbon nanotubes (MWCNTs) on modification with In2O3 were investigated. Sensing layers comprising pH extended-gate field-effect transistor (pH-EGFET) sensors were fabricated by spraying MWCNTs onto glass substrates. Then an indium layer was deposited by RF sputtering and the resulting composite film was annealed at different temperatures in the presence of oxygen. The pH sensitivity depended on the annealing temperature. Raman and X-ray spectroscopy analyses revealed that the chemical composition of thermally treated MWCNTs and the In2O3 surface depended on the annealing temperature. The pH sensitivity of In2O3 films increased as the annealing temperature increased from 300 to 500 °C. The MWCNT layer dominated the sensing characteristics for composite samples annealed at high temperature (≥300 °C). No significant improvement in the sensitivity of In2O3-coated MWCNTs film was observed after annealing at low temperature (~200 °C). High annealing temperatures improved the In2O3 crystallinity and enhanced the MWCNT sensitivity.