Elsevier

Electrochimica Acta

Volume 55, Issue 14, 30 May 2010, Pages 4353-4358
Electrochimica Acta

Evidence and analysis of parallel growth mechanisms in Cu2O films prepared by Cu anodization

https://doi.org/10.1016/j.electacta.2009.10.031Get rights and content

Abstract

We have studied the preparation of Cu2O films by copper anodization in a 0.1 M NaOH electrolyte. We identified the potential range at which Cu+ dissolution takes place then we prepared films with different times of exposure to this potential. The morphology, crystalline structure, band gap, Urbach energy and thickness of the films were studied. Films prepared with the electrode unexposed to the dissolution potential have a pyramidal growth typical of potential driven processes, while samples prepared at increasing exposure times to dissolution potential present continuous nucleation, growth and grain coalescence. We observed a discrepancy in the respective film thicknesses calculated by coulometry, atomic force microscopy and optical reflectance. We propose that anodic Cu2O film formation involves three parallel mechanisms (i) Cu2O nucleation at the surface, (ii) Cu+ dissolution followed by heterogeneous nucleation and (iii) Cu+ and OH diffusion through the forming oxide and subsequent reaction in the solid state.

Introduction

Cu2O is a p-type semiconductor with a band gap around 2 eV that has recently aroused interest because of its potential applications in random access memories [1], low-cost solar cells [2], and gas sensors [3] among other devices. The electrochemical fabrication of Cu2O has been usually reported by electrodeposition onto transparent conductive oxides [2], [4], [5], [6], [7].

Copper electrochemistry and Cu2O growth in alkaline media have been extensively studied [8], [9], [10], [11], [12]: the main anodic peak observed around U = −400 mV (vs silver/silver chloride electrode, henceforth SSC) in the potentiodynamic curves is associated to the formation of Cu2O. Additionally, at potentials negative with respect to Cu2O formation, the development of Cu+ hydrous species and OH adsorption has been described. Copper (I) soluble species, namely (Cu2O2H)aq can also be produced. The Cu2O growth starts from the reconstructed OH-populated surface [11]. Nevertheless only few works focus on the preparation of thick Cu2O anodic films and although soluble Cu+ formation has been repeatedly suggested, we did not found reports with experimental evidence of copper dissolution and its effects on film properties.

In a recent contribution [13] we reported an anodization method of Cu0 in alkaline media that allows the preparation of Cu2O films as thicker as 100 nm. We studied the electronic properties of the films and proposed an improved electronic band diagram of the Cu|Cu2O|electrolyte interface. Herewith, a Cu+ dissolution potential range forming Cu+ species in solution before the Cu2O growth potential range has been identified by current enhancement with the addition of a complexing agent for Cu+ and then, several films with different exposure times to the dissolution potential have been prepared. The contribution of this exposure to the film properties is analyzed in terms of optical, microstructural and thickness changes. The band gap and Urbach parameter were modulated as achieved. Moreover, the consideration of this dissolution effect brings new information on the growth mechanism of Cu2O films.

Section snippets

Experimental details

Cu2O layers were prepared onto polycrystalline Cu disks in a 0.1 M NaOH electrolyte by an electrochemical routine described elsewhere [13]. Briefly, the electrochemical procedure involves a series of potential steps that lead to different processes, i.e. the reduction of the native oxide, the formation of an OH-adsorbed submonolayer, a dissolution stage and oxide formation. The growth was performed using a PGSTAT 12 Autolab potentiostat in the three electrode configuration, using a homemade

Morphology

Fig. 2a presents 2 μm × 2 μm AFM images of the different films prepared at several exposure times. In the absence of a dissolution pathway, grains have a pyramidal shape with a wide size distribution. When the electrode is largely exposed to the dissolution potential, the grains evolve to a more irregular shape and grains become more definite and their size increases with the exposure time. Fig. 2b shows in more detail the morphology of the films td = 0 s and td = 1000 s. In the image corresponding to td =

Conclusions

We prepared Cu2O films by Cu anodization using an electrochemical program that includes electrode exposition to Cu+ dissolution potential at selected times. The films were crystalline Cu2O, with an energy band gap that was tailored from 2.07 to 2.21 eV. From the analysis of the film morphology and thickness and from the current transients during the film growth we propose that the Cu2O film growth mechanism includes concurrent processes: a pure electrochemical step, an heterogeneous nucleation

Acknowledgements

FCB recognizes IPN-México for COTEPABE License; APP acknowledges financial support through an UB-Collaboration Grant at the Physical Chemistry Department. OCA acknowledges a UB-Visiting Professor Aid. The support of the Scientific Technical Services of the University of Barcelona is kindly recognized, particularly the Units of X-ray Diffraction, Molecular Spectroscopy and Nanometric Techniques. This work was partially financed by the Project CTQ-2007-68101-C02-01 from the Ministerio de

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