Fig. 1. (Color online) Schematic illustration on growing BiOBr nanostructures under ultrasonic treatment.

Fig. 2. (Color online) (a) Spin coating photoresist on PET substrate. (b) UV exposure under interdigital electrode patterned mask. (c) Develop the exposed PET substrate. (d) Thermal evaporation 60 nm Au electrode. (e) Acetone stripping to form interpolation gold electrode.

Fig. 3. (Color online) (a) SEM image of Bi powder. (b, c) SEM images of BiOBr nanostructure under different magnifications. (d) Typical XRD pattern of BiOBr nanostructure. (e) Raman spectrum of BiOBr nanostructure excited at 532 nm. (f) HRTEM images of BiOBr nanostructure，the down-left insets are the lattice fringes.

Fig. 4. (Color online) (a) XPS survey scan of BiOBr nanostructure. (b–d) High-resolution XPS spectra of Bi 4f, Br 3d and O 1s of BiOBr, respectively.

Fig. 5. (Color online) (a) Schematic illustration of a humidity sensor based on BiOBr nanostructure. (b) Time-dependent variation of relative current change of device under various concentrations of relative humidity. (c) Relative current change of sensor under different humidity relative to 0% humidity. (d) Response and recovery of the humidity sensor exposed to 90%. (e) Relative current changes of the humidity sensor under cyclic run between ambient humidity and 90% RH. (f) For the influence of other gases on the performance of humidity sensor, the relative humidity of the test environment is 40% RH, and the gas concentration is 100 ppm.

Fig. 6. (Color online) (a) The application of a humidity sensor in human respiratory monitoring is intelligent mask. (b) Photos of humidity sensors. (c) Response of the sensor under various breathing modes. (d–f) The smart mask's response under different breathing rates.