Effect of nitrogen-vacancy defects in graphitic carbon nitride on hydrogen adsorption in quartz-crystal microbalance sensor for hydrogen gas

A graphitic carbon nitride (g-C3N4) with nitrogen-vacancy defects was synthesized by calcination of a mixture of melamine and potassium hydroxide (KOH). The nitrogen/carbon ratio (N/C) of the g-C3N4 film decreased with the addition of KOH, indicating that the KOH addition could control the nitrogen-vacancy defects in the g-C3N4. The g-C3N4 films with nitrogen-vacancy defects were formed on a quartz crystal substrate. The change in the resonant frequency of the crystal was measured before and after the introduction of hydrogen gas, utilizing the quartz crystal microbalance (QCM) method. This approach was employed to characterize the hydrogen sensor effectively. The QCM-type sensor with the defective g-C3N4 film had a different frequency response to the hydrogen than the sensor with pristine g-C3N4 film synthesized without KOH addition. That is, the resonant frequency of the defective g-C3N4 decreased with the introduction of hydrogen, while that of pristine g-C3N4 was observed to increase. The reduced frequency of the defective g-C3N4 is probably due to the formation of chemical bonds with hydrogen (C-H) as observed in the FT-IR measurements. Our findings show that the introduction of nitrogen-vacancy defects into the g-C3N4 significantly impacts the characteristics of QCM-type hydrogen sensors. Moreover, controlling the amount of nitrogen vacancies introduced has a significant effect on the frequency response to the hydrogen gas. The QCM-type sensor utilizing g-C3N4 is anticipated to find applications in a range of gas sensors in the future, contingent upon the progress of research focused on controlling the precise number of nitrogen-vacancy defects.