Highly sensitive and lower detection-limit NO2 gas sensor based on Rh-doped ZnO nanofibers prepared by electrospinning

As a kind of toxic gas, NO2 can cause damages to the environment and threaten the health of human beings. To develop highly sensitive NO2 gas sensor is necessary. Here, the Rh-doped ZnO nanofibers were prepared by electrospinning. The Rietveld refinements results of XRD patterns have proved the successful doping of Rh into the crystal lattice of ZnO. The XPS spectra reveals the existence of large amounts of surface oxygen vacancy defects, which are beneficial to the enhancement of the sensitivity. The morphology and crystal lattice of the nanofibers were investigated by SEM and HRTEM, respectively. The highly sensitive NO2 gas sensor based on 1% Rh-doped ZnO nanofibers, with the experimental detection limit of 50 ppb (or 18.6 ppb, theoretical signal-to-noise ratio > 3) was fabricated. At the optimal operating temperature of 150 degrees C, its response values (Rg/Ra) are 1.04 and 36.17, with the corresponding response time of 45 s and 32 s, towards 50 ppb and 10 ppm NO2, respectively. The sensor also exhibits good selectivity, repeatability and linearity between the relative humidity and the response values. The doping of Rh introduces large amounts of surface oxygen vacancy defects. The defects are more favored adsorption sites for NO2. The reasons for the ultrasensitivity of the sensor are as followings: (1) The binding energy between NO2 and ZnO with surface oxygen vacancy defects is about 1.5 times stronger than that for adsorption on the stoichiometric surface; (2) The surface oxygen vacancies of ZnO can have a reaction with NO2 to produce NO inducing the charge transfer; (3) The catalytic activities of Rh towards NO2 and H2O.