Low-Cost ZnO nanostructures: controlled synthesis and applications

Abstract

Advances in nanotechnology is strongly dependent on research innovative nanostructured materials in nanostructure form. Multifunctional metal oxides have recently emerged as smart materials with a wide range of controllable properties leading to innovative device concepts. In particular, zinc oxide (ZnO) presents an unique combination of interesting properties such as non-toxicity, wide band-gap, high exciton binding energy at room temperature, piezoelectric behavior, high physical, chemical and mechanical stabilities [1]. Research on ZnO is an old story. In 1957, the New Jersey Zinc Company published a book entitled Zinc Oxide Rediscovered to promote the material s properties worthy to be further investigated [2]. Since then, ZnO have gained growing importance in the material science. In the last decade, ZnO nanostructures have been the focus of intensive studies, being an almost ideal systems both from a fundamental point of view (ease of nanostructuration, energy band bending, intrinsic and extrinsic defects, combination of polar and non polar planes, ) and from the applications (energy harvesting devices, sensors, biomedical devices, lighting, ). Many methods, both physical and chemical, have been employed for the synthesis of nanostructured ZnO; however most of the synthesis methods involve expensive experimental set up and sophisticated equipment. One of the basic requirements for nanomaterials to become industrially viable is the low production cost alongside the possibility to supply large amount of non-toxic material. In this regard, wet-chemical synthesis approaches are very appealing, having the advantages of easy use, low cost, reduced process temperatures and potential for scaling up. On the other hand, these kinds of synthesis have the shortcoming of lack of a good control and reproducibly. Overcoming these limitations and ensuring a high degree of reliability represent important challenges for modern material science. Among the wet-chemical approaches, chemical bath deposition (CBD) has emerged as the simplest and more versatile method for the synthesis of ZnO nanostructures. It is a chemical process, similar to an electroless deposition of solid phase from an aqueous solution. In this thesis, the attention is reserved to ZnO nanorods (NRs) and nanowalls (NWLs). ZnO NRs grown by CBD are among the most promising semiconducting nanostructures currently investigated for a wide range of applications. ZnO NWLs represent a new form of nanostructure with very large surface area, easy to synthesize by CBD on Al-covered substrate. Currently, some investigations have been focused on the microscopic mechanisms leading to the formation of ZnO NRs and NWLs, however some outstanding issues remain to be solved. A clearer comprehension of the growth mechanism in CBD and a better control of the synthesis are fundamental issues for taking full advantage of this technique. The work described in this thesis aims to deeper understand the mechanisms underlying the formation of ZnO NRs and NWLs under CBD conditions, attempting to clarify the role of synthetic parameters. Discerning their effects leads to enhance the control over the growth process. The potentialities of ZnO NRs and NWLs with optimized morphologies as critical components in technological solar cell, photocatalysis and sensing applications are also investigated.

[1] A. Ko³odziejczak-Radzimska, T. Jesionowski, Materials 7(4), 2833 (2014). [2] Brown H. Zinc Oxide Rediscovered. New York (1957).