On the growth of zinc oxide nanowires towards photoelectrochemical applications

Abstract

Zinc oxide is regarded as an attractive semiconductor alternative to the most commonly used silicon and GaAs owing to its abundancy, thermodynamic stability and the large variety of morphologies it can be grown into. Among these morphologies, nanowires (NWs) have gathered vast attention as an ideal research platform for new and enhanced functionalities of ZnO. One of these functionalities is the integration of ZnO into the renewable production of hydrogen from water splitting. In this thesis, the growth of ZnO NWs, its doping and surface functionalization are studied with the aim of developing highly efficient photoelectrochemical (PEC) water splitting photoanodes.

Before delving in the functional application, a growth study of the ZnO NWs was necessary to understand the factors that control this growth. For this, a seed mediated chemical bath deposition (CBD) approached was explored in detail and adopted. Control over NW growth was obtained by tuning the seed layer deposition with two different techniques (atomic layer deposition and sol-gel processing) and by controlling the CBD parameters. This study demonstrated that NW diameter, length, growth orientation and crystallinity can be controlled by this approach.

To modulate the optoelectronic properties, ZnO NWs were doped with two different transition metals, copper and cobalt. A detailed study of the optoelectronic properties of these doped-ZnO NWs revealed that the introduction of cobalt into the ZnO lattice considerably improved the optoelectronic properties of ZnO. This enhancement was induced by the introduction of traps states in the bandgap of ZnO prompted by the interaction between the sp orbitals of ZnO and the d orbitals of Co. In particular, a 1% nominal doping yielded the most promising results of this study.

Further improvement of the ZnO properties towards PEC water splitting was achieved by functionalizing the surface of the NWs with iridium and a metal-organic framework the zeolitic imidazolate framework-8 (ZIF-8). The successful integration was demonstration by electron microscopic analysis that showed the control of this conformal surface functionalization. The integration of Co-doping and ZIF-8 functionalization resulted in a large enhancement of the PEC performance of the ZnO NWs, doubling the photogenerated current and the stability over time while also increasing the incident photon-to-current efficiency from 11% for ZnO NWs to 75% in the blue and ultraviolet region for ZnO:Co@ZIF-8 core-shell NWs.