Monitoring the Formation of CuO Nanowires using In-situ High Temperature X-ray Diffraction

The formation of CuO nanowires is monitored using in-situ X-ray diffraction measurements. Nanowires are formed on heating a Cu foil to temperatures above 500 °C in air. In-situ studies allow the changes in phase fractions (Cu → Cu2O → CuO) to be monitored in almost real time.

1 Introduction

Nanostructured semiconductors have a wide variety of potential applications ranging from photovoltaic devices (e.g. solar cells) to gas / liquid sensors. To be used on a commercial scale, fast and reproducible methods for nanostructure formation must be developed. In this context, in-situ measurements allow detailed information to be obtained during formation which could allow for new and better methods to be developed.

The interest in nanostructured semiconductors stems from their wide-ranging potential applications. Their size on the nanoscale can result in enhanced properties; for example, the increased surface area of nanoparticles means that in photovoltaic devices a higher photon absorption rate is observed, while in sensor applications absorption of the analyte species is more likely.

Cuprous oxide (Cu2O) and cupric oxide (CuO) are the two naturally occurring oxide forms of copper which can grow as nanoscale crystals. Both display semiconducting properties and their band gaps (2.0 and 1.2 eV, respectively) mean that they can be applied in various fields. Of particular interest are nanowires of the two oxide forms. While both oxides can form nanowires, CuO nanowires can be produced more easily and are the subject of this study (Cu2O nanowires must be grown by reduction of CuO nanowires).

Several methods are possible for the growth of CuO nanowires, but the simplest relies on only heating a pure Cu foil in air to a sufficiently high temperature for a certain length of time. The temperature at which nanowire growth occurs is in the region from 300 – 800 °C, and the time required is typically a few hours. Generally, it has been reported that higher temperatures result in larger nanowire diameters, while longer annealing times result in an increase in the length of the wires.

During heating it is expected that Cu2O first forms as a layer on the foil surface and from this layer CuO nanowires grow when held at higher temperatures for a longer period of time. The goal of the present study is to find the optimum conditions for the growth of CuO nanowires with regards to temperature and annealing time. This information can then be used to optimize the process so that nanowires can be pro-duced in the rapid and efficient manner that is required for the commercialization of nanowire devices.

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