Seed-layer-free deposition of well-oriented ZnO nanorods thin films by SILAR and their photoelectrochemical studies

Morphological forms of ZnO nanostructures play a vital role in deciding properties such as high internal surface area, efficient light scattering and harvesting, lowest charge transfer resistance, etc. which are important for photoelectrochemical (PEC) performance. Herein successful deposition of well oriented ZnO nanorods thin films over fluorine doped tin oxide (FTO) coated glass substrate is achieved by using simple, soft solution and scalable method known as successive ionic layer adsorption and reaction (SILAR). For the first time a compact ZnO layer over large area is deposited in one step synthesis approach, without any assistance of seed layer, by using hydrazine hydrate as a source of hydroxyl ions. The plausible growth mechanism of the morphological variation (alignment and orientation) happening with increasing SILAR cycles and its consequence on PEC performance are discussed in detail. All ZnO thin films show wurtzite crystal structure, however variations in their texture coefficients were found with SILAR cycles, which turns out to be a major aspect for PEC application. Anodic shift was observed in flat band potential values with increment in number of SILAR cycles. The ZnO thin films deposited for 120 cycles showed preferential orientation along (0002) plane and showed better PEC performance with photocurrent of 0.19 mA/cm2 (1 V) and maximum photo conversion efficiency of 0.084% at 0.45 V. On the other hand, film deposited for 60 (photocurrent of 0.11 mA/cm2 (1 V); efficiency of 0.055%) and 180 cycles (photocurrent of 0.15 mA/cm2 (1 V); efficiency of 0.063%) demonstrated inferior PEC performance.