Counterintuitive sensing mechanism of ZnO nanoparticle based gas sensors

Zinc oxide nanoparticles are prepared by calcining zinc hydrocarbonate precursors at 300–700 °C (ZnO300–700), and corresponding gas sensing property are tested at 300 °C by using formaldehyde as the probe. Although the nanoparticle sizes are found to gradually increase with calcination temperature, the sensor measurements reveal the size-independent behavior that ZnO500 and ZnO300 have the highest and lowest responses, respectively. Spectroscopic characterization further reveals nonstoichiometric compositions of ZnO nanoparticles: ZnO300 has the largest excess oxygen (oxygen interstitial, Oi), whereas ZnO500 has the largest excess zinc (oxygen vacancy, VO and/or zinc interstitial, Zni). Accordingly, a new sensing mechanism is proposed for ZnO nanoparticle sensors. Excess zinc favors chemisorption of oxygen onto the nanoparticle surface, leading to reacting with more formaldehyde molecules to get a high signal. On the contrary, excess oxygen inhibits free oxygen to be chemisorbed onto the nanoparticle surface, and thus decreases the gas response. Finally, this new sensing mechanism is verified by testing gas response of ZnO500 nanoparticles annealed at different atmospheres.