Influence of nanoparticle size on the characterization of ZnO thin films for formaldehyde sensing at room temperature

Thin films of zinc oxide (ZnO) were deposited on glass substrates using chemical bath deposition (CBD) by varying the deposition time from one to three hours. The structural, morphological, optical, and responsiveness of the ZnO thin films to formaldehyde gas at room temperature were studied with respect to the nanoparticle grain sizes. The average grain sizes of the ZnO nanoparticles deposited for 1, 2, and 3 h were 19.6, 25.1, and 31.5 nm, while the average crystallite sizes were 73.07, 88.28, and 95.25 nm, respectively. The band-gap energies of the ZnO thin films deposited for 1, 2, and 3 h were found to be 3.27, 3.28, and 3.29 eV, respectively. Energy dispersive X-ray (EDX) analysis of the synthesized ZnO nanoparticles confirmed their purity, and water contact angles indicated that longer deposition durations with larger grains enabled them to be more hydrophobic. The formaldehyde gas-sensing experiments showed the ZnO thin films deposited for larger grain sizes had a better gas response at room temperature than the rest. The maximum responses of the samples deposited for 1, 2, and 3 h were 5.5, 11.0, and 17.8, respectively, at 400 parts per million (ppm) of formaldehyde gas. Additionally, it was observed that the samples deposited for 1, 2, and 3 h could detect as low as 100, 25, and 12 ppm of formaldehyde, respectively.