A simple optical fiber relative humidity (RH) sensor was proposed and demonstrated using a dumbbell-shaped inline Mach-Zehnder interferometer (IMZI) as a probe. The humidity sensor was constructed by coating the tapered waist region of the IMZI with ZnO nanowires. Based on an optical interferometric technique, the response of the sensor probe was examined by changing the RH within a range of 35%-60% RH. A wavelength shift of 0.490 nm was achieved for a humidity variation from 35% to 60% RH. It was also found that the sensitivity of IMZI-based sensor improved from 0.0002 to 0.020 nm/%RH with the ZnO coating.

ZnO nanoparticles (NPs) have been synthesized on porous silica microparticles, namely sand microparticles using the sol–gel technique. The ZnO NPs grown on the multifaceted surface of porous silica microparticles were applied as photocatalyst for the degradation of methylene blue (MB) in aqueous solution. The enhanced rate constant observed was due to two reasons. Firstly, the multifaceted surface of the sand substrate provided fixed space for growth space which prevented agglomeration of the ZnO NPs photocatalyst, thus maintaining a large surface area. Secondly, the presence of nanopores on the sand surface provided adsorption sites for MB molecules to be in the vicinity of the photocatalyst. The photocatalytic activity was significantly enhanced where photodegradation efficiency of supported ZnO NPs reached 71.7% compared to 48.2% for unsupported ZnO NPs under UV light irradiation within 150 min. The values of k increased from 4.3 × 10⁻³ min⁻¹ to 8.6 × 10⁻³ min⁻¹ for unsupported and supported ZnO NPs, respectively.

Ag and Ni/ZnO photocatalyst nanostructures were successfully synthesized by a sol–gel method. In this work, the photocatalyst sample was systematically studied based on several factors affecting the performance of photocatalyst, which are size, morphology, band gap, textural properties and the number of active sites presence on the surface of the nanocatalyst. X-ray diffraction revealed that Ag/ZnO nanomaterials experienced multiple phases, meanwhile for Ni/ZnO the phase of nanomaterials were pure and single phase for stoichiometry less than 5%. Field emission scanning electron microscope (FESEM) showed almost all of the synthesized nanomaterials possessed a mixture of nanorods and spherical-like shape morphology. The Ag/ZnO showed high photocatalytic activity, producing at least 14th trials of nanocatalyst reusability on degradation of methyl orange under UV irradiation. Interestingly, this phenomenon was not observed in larger surface area of Ni/ZnO nanomaterials which supposedly favour photocatalytic activity, but instead producing poor photocatalytic performance. The main reasons were studied and exposed by temperature-programmed desorption of carbon dioxide (TPD–CO ₂ ) which showed that incorporation of Ag into ZnO lattice has enhanced the number of active sites on the surface of the nanocatalyst. Whereas incorporation of Ni in ZnO has lowered the number of active sites with respect to undoped ZnO. Active sites measurement is effective and significant, providing opportunities in developing an intensive study as an additional factor.