SOURCE:

Faculty: Physical Sciences
Department: Physics And Industrial Physics

CONTRIBUTORS:

Ebede H. Nkiruka
Nwokoye A. O. C.

ABSTRACT:

In this study two samples of ZnO nanoparticles were synthesized by solvothermal technique using sodium lauryl sulphate (SLS) and linear alkylbenzenesulphonate (LAS or LABS) as capping agents. The solvothermal synthesis was completed at 1900C using zinc acetate precursor and diethylene glycol solvent. The ZnO nanoparticles produced with LABS was of better quality and was used for the fabrication of nanostructured ZnO capacitive RH sensor. The interdigitated electrodes (IDEs) for the sensors were designed and printed on Printed Circuit Board (PCB) by screen printing technique. The nanostructured ZnO capacitive relative humidity sensors were fabricated by depositing the ZnO nanoparticles on the IDEs using spin coating technique. The ARDUINO NANO ATMEGA 328 microcontroller programmed at 0 V (digital low) and 5 V (digital high) was used to process the signals (stimuli) from DHT11 and the nanostructured ZnO capacitive relative humidity sensors in the electronic sensor circuit. The sensors were calibrated using the RH standards of saturated solutions of lithium Chloride (LiCl), Potassium carbonate (K2CO3), potassium chloride (KCl), potassium sulphate (K2SO4) and sodium chloride (NaCl). A model equation was derived and used to calibrate the sensors to take care of the RMSE. The sample with SLS showed a prominent Raman peak at 426 cm−1 while the sample with LABS showed a prominent peak at 424 cm-1 implying that the samples contained ZnO nanoparticles with stable wurtzite (hexagonal) crystal structure. The ZnO nanoparticles produced with SLS has multipoint BET surface area of 284.286 m2/g, BJH adsorption pore diameter of 2.411 nm and BJH cumulative adsorption volume of 0.181 cm3/g. The LABS sample had BET surface area of 288.421 m2/g, BJH cumulative adsorption volume of 0.184 cm3/g and BJH adsorption pore diameter of 2.433 nm. TGA analysis revealed that ZnO nanoparticles produced with SLS degraded at 325 0C while ZnO nanoparticles produced with LABS degraded at 420 0C. Z-average particle size PSD analysis for ZnO nanoparticles produced with SLS was 354 nm and 715.2 nm for the sample with LABS. The diameter of the predominant ZnO nanoparticles in SLS sample was 722.3 nm occupying 68.5 % of the entire volume whereas the diameter of the predominant ZnO particles in LABS sample was 981.1 nm occupying 97.3 % of the entire volume. The results of the characterization indicates that the ZnO nanoparticles in both samples have good surface areas, mesopores characteristic structure for excellent sensing, good thermal stability and good size quality and size distribution by volume and intensity. Each of the IDEs has sensing area of 46.948 mm2. Three sensors including unannealed nanostructured ZnO capacitive RH sensor and nanostructured ZnO capacitive RH sensors annealed at 150 0C and 200 0C were fabricated. The root-mean square error (RMSE) associated with the sensors were 8.367, 8.344 and 7.170 for the unannealed sensor, sensor annealed at 150 0C and sensor annealed at 200 0C respectively. It was observed that the performance of the sensor improved with increase in annealing temperature. The sensitivity (S) of the sensors to 1 %RH change in relative humidity were 0.161 for unannealed sensor, 0.868 for sensor annealed at 150 0C and 1.031 for sensor annealed at 200 0C. The calibrated sensors can be used to measure any relative humidity variation from 0 %RH to 100 %RH with very negligible error.