The influence of TiO2 seed layer on the growth of ZnO nanorods for UV sensor application / Nur Amierah Mohd Asib

In this work, ZnO nanorod arrays were fabricated at a low temperature by a simple and low cost method of solution-immersion on TiO2 seed layer coated glass substrate. The TiO2 seed layers were prepared via sol-gel spin-coating technique. This work consist of two fabrication steps; (a) preparation of TiO2 seed layer on glass substrate, (b) synthesization of ZnO nanorods on TiO2 seed layer. The optimization of fabrication parameters, which are mainly related to preparation processes for the seed layer and ZnO nanorods, were studied. Titanium (IV) butoxide was used as starting material for deposition of TiO2 seed layer on glass substrate. The seed layer was prepared at molar concentrations of 0.05-1.00 M. 0.25 M of TiO2 showed optimum average surface roughness with highest crystallinity and UV absorption properties. Then, the effect of number coating layers (1-9 layers) which performed by using spin-coating technique was investigated. The highest surface roughness, UV absorption properties and crystallinity with the smallest value of FWHM was observed for seven coating layers. The effects of annealing temperatures (350-550˚C) and times (30-150 min) also were studied. The seed layer annealed at 450˚C for 60 minutes possessed highest surface roughness with enhanced crystallinity and UV absorption properties. FESEM result showed that the TiO2 seed layer composed of small compact particles with porous nature and the thickness was measured about 222.5 nm. For the second step of fabrication, Zn2+ solution were varied from 0.002-0.060 M. Well aligned, uniform, dense ZnO nanorods with prominent (0 0 2) peak and have good UV absorbance was found at 0.040 M. In addition, TEM images prove the successful formation of ZnO nanorods (without hollow). Next, the effects of immersion temperatures (75-95˚C) and times (2-6 h) were observed. It was found that ZnO grown at 90˚C for 4 h have smallest size with dense distribution of nanorods and have good UV absorption properties. Moreover, the effects of annealing temperatures (400-660˚C) and times (30-150 min) were investigated. Smallest size with good distribution of nanorods was also observed when ZnO was annealed at 500˚C for 60 min. FESEM result showed that the average diameter size and length of optimum ZnO nanorods was about 116.7 nm and 3.6 μm. The growth mechanisms of bare TiO2, bare ZnO and TiO2:ZnO were also discussed in this work. Lastly, three type of UV sensors were fabricated using different samples of bare TiO2, bare ZnO and TiO2:ZnO. These samples were tested using UV measurement system and their performances were compared and discussed. The responsivities of the bare TiO2, bare ZnO and TiO2:ZnO based UV sensors were recorded at 7.76×10-5 A/W, 2.22×10-7 A/W and 1.70×10-1 A/W, respectively. Meanwhile the sensitivities of the bare TiO2, bare ZnO and TiO2:ZnO based UV sensors were calculated and reported at 1.5, 1.0 and 9.2, respectively. It was proved that the best performance of UV sensor was observed for TiO2:ZnO nanorods thin film with the increasing of responsivity and sensitivity about 99% and 89.13%, respectively compared to bare ZnO. Hence, a new attempt of utilizing TiO2 as seed layer to grow nanorods and to be used as based material in fabrication of UV sensor was successful, as nanorods grown on the TiO2 seed layer exhibit superior properties of packed nanorods with good crystallinity quality and improved UV absorption properties which can produce highly response, sensitive, reproducible and stable UV sensor.