Evidence and analysis of parallel growth mechanisms in Cu2O films prepared by Cu anodization
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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On leave from: CICATA-IPN Altamira, Km 14.5 Carr. Tampico-Pto Industrial, 89600 Altamira, Mexico.