Functionalization of zinc oxide nanostructures

Abstract

ZnO is nowadays a key technological material and a promising building block for functional devices. The lack of a center of symmetry in the wurtzite structure lend to an extremely interesting material, which exhibits both semiconducting and piezoelectric behavior together with a large exciton binding energy. Moreover ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. Although ZnO properties are much interesting themselves, the functionalization of ZnO nanostructures – by means of surface functionalization as well as the creation of heterostructures combining different materials (organic / inorganic, metals, semiconductors, insulators…) – paves the way to the creation of novel functional materials that exhibit unique new properties, or the enhancement of the pre-existent ones.

This thesis deals with the functionalization of ZnO nanostructures, tetrapods (TP) and nanorods (NR) in particular, which have been grown by our group at IMEM Institute through a non-catalyzed CVD technique. My research, in particular, focused on different kinds of functionalization in order to tailor the properties of the resulting material towards different applicative fields, namely gas sensing devices, photocatalytic systems and photovoltaic cells. Different materials, belonging to three specific classes, have been chosen to tailor the properties of ZnO nanostructures: (1) cadmium sulphide (II-VI semiconductor), (2) magnetite (magnetic materials), (3) phthalocyanine and porphyrin (organic semiconductors).

More in detail, the cadmium sulphide (CdS) functionalization of ZnO TP is aimed to create an heterojunction for photoenergy conversion applications. Although the use of CdS to extend the fraction of visible light to be collected by the heterostructure is not new, the present work focuses on a new synthetic approach. This is required since, despite of the huge variety of literature-proposed methods for CdS synthesis, none of them can be considered optimal for ZnO functionalization. In fact, on the one hand, CdS aqueous-based synthesis requires alkaline or acidic environments that invariably etch the ZnO surface; on the other, high temperature syntheses in organic solvents require the presence of ligands or surfactant to effectively control the CdS dimension. Unfortunately such polymers / molecules would likely be trapped at the interface limiting the heterojunction performance. Hence, an in–situ deposition of CdS on ZnO TP, without the need of any surface passivating agent, is presented. The CdS functionalization allows to extend the ZnO TP light absorption into the visible range, and the formation of a type–II heterojunction promotes exciton dissociation at the interface and electron transfer from CdS to the ZnO conduction band. Thus the composite material can be employed as photoanode in photovoltaic devices or photo electro–chemical cells, as well as in photocatalytic systems (degradation of organic pollutants, photo-induced water splitting), and gas sensing applications. The following section briefly extends the CdS deposition to ZnO NR, which are promising nanostructures for photovoltaic applications, since they show excellent transport properties and allow a continuous path for the electrons, as opposite to ZnO TP which shows hopping/percolative transport.

The second part of the Thesis, deals with the functionalization ZnO TP with magnetite nanoparticles. Small superparamagnetic magnetite nanoparticles, with mean dimensions up to 10 nm, have been synthesized in both aqueous solution and organic solvents. All the prepared nanoparticles have good magnetic properties: i.e. superparamagnetic behavior with saturation magnetizations above 50 A m2 Kg–1. These nanoparticles have been used as building blocks for the functionalization of ZnO tetrapods: this results in the creation of a coupled material which still possess all ZnO own attractive features, e.g. surface reactivity, strong UV–emission, piezoelectricity etc., together with added magnetism. The magnetic response of the coupled compound is ferromagnetic but, considering the very small remanence (MR), the Fe3O4–ZnO coupled nanostructures, dispersed in a liquid, don’t aggregate because of magnetic dipolar interactions: this paves the way for future use in photocatalytic systems, as well as biomedical applications.

Finally, the last section deals with the functionalization of ZnO nanorods with organic molecules: the aim of this work is to create and characterize a hybrid photoanode to be used in photovoltaic applications. One of the challenging issues in excitonic photovoltaic cells deals with the optimization of the absorbing layer in order to collect a vast fraction of the incident light. Owing to the limited absorption width of organic molecules and polymers, only a small fraction of the solar flux can be harvested by a single-layer bulk heterojunction photovoltaic cell. In addition to the aforementioned issues, as a matter of fact, ZnO solar cells have shown relatively low overall conversion efficiencies when compared with TiO2-based systems. The limited performance in ZnO-based DSSC may be explained by the instability of ZnO in acidic dye and the slow electron-injection kinetics from dye to ZnO. Considering these open issues, on the one side, one promising strategy to overcome both the limited absorption and the low exciton diffusion in organic materials, is to couple organics with inorganic semiconductor nanostructures (e.g. ZnO). On the other, ZnO functionalization has been carried out by means of molecular beam deposition (SuMBD), which is an attractive growth technique, alternative to wet impregnation, that doesn’t affect ZnO surface, on the contrary, by varying the kinetic energy of the beam, it is possible to promote chemical bonds at the interface. Driven by the high directionality of SuMBD deposition, the goal is to deposit two distinct molecules, with convenient absorption range, on the surface of a single ZnO NR, in order to collect a wider spectrum of incident light. Photovoltaic, however, is not the only applicative field where sensitized ZnO NR can be employed in. According to literature, either porphyrin, ZnO and the resulting hybrid material have been well established as materials for singlet oxygen production, hence they can be used in photocatalytic systems as well as in nano-medicine.