The electrochemical reduction of O2 in a dimethyl sulfoxide DMSO solution onto a glassy carbon substrate at 423 K has been studied. Electrochemical potentiodynamic studies show that the reduction process is controlled by O2 diffusion and reveal a two-step mechanism. The first one corresponds to an electrochemical step which gives the superoxide ion that, in a second chemical step, forms the peroxide ions through a disproportion reaction. The proposed mechanism can be used for the formation of semiconductor CdO thin films. Here, results for CdO film formation, obtained in a direct way without the presence of CdOH2 The metal oxides and peroxides are materials of great interest in current research. This is because, while some of them have low conductance, many others usually called TCOs for transparent con- ducting oxides can exhibit high electrical conductivity and optical transparency and can be grown efficiently as thin films. Moreover, they are extensively used for a variety of applications such as pho- tovoltaic solar cells, 1 phototransistors, 2 photodiodes, 3 and gas sensors. 4 Different methods can be used for their synthesis. 5-10 A common preparation method of these materials is the chemical bath deposition CBD 11,12 in an aqueous solution. However, this tech- nique has the disadvantage of forming deposits that include, besides the desired metal-oxide phase, the presence of metallic hydroxides, which implies the need for a postdeposition thermal treatment. 13 Alternatively, the electrodeposition of these materials can be carried out in an organic solution starting from the electrochemical reduc- tion of molecular oxygen. In fact, the electrochemical reduction of O2 in aprotic solvents has been known since 1965. 14-18 These studies have included the effect of the solvent as well as the employed metallic substrate. 19 However, an adequate comprehension of the oxygen reduction mechanism at high temperature is of great impor- tance when dealing with the formation of metallic oxide or peroxide thin films by electrochemical methods. concentration was obtained by a cathodic pulse developed at 0.75 V onto a GC electrode in an O2-saturated DMSO solution. The concentration of O2 was calculated from the difference between the total charge and the capacitive charge, both obtained by the integra- tion of i/t responses in the O2/LiCl/DMSO and LiCl/DMSO solu- tions, respectively. Considering a two-electron mechanism, the mo- lecular oxygen concentration was 0.48 mM at 423 K. The potentiodynamic studies as well as the film deposition experiments were made in darkness using a conventional three-electrode electro- chemical cell connected to an Autolab PGSTAT30 potentiostat. The GC electrode was mechanically polished with 0.03 m alumina, washed with deionized water, and afterward dried by an argon flow. A platinum wire separated from the GC by a glass frit was used as a counter electrode and a Ag/AgClsat electrode was used as a refer- ence one E Ag/AgClsat 0 = + 0.192 V vs normal hydrogen electrode. For the CdO synthesis, the films were prepared at 423 K by a cathodic pulse developed at 1.00 V on glass electrodes covered with SnO2:F fluorine-doped tin oxide, FTO from 20 mM CdCl2 Aldrich 99.9% and 0.10 M LiCl in the O2-saturated DMSO solu- tion. The oxygen content was adjusted according to the same proce- dure indicated previously. The properties of the CdO films were studied by X-ray diffraction XRD spectroscopy with a Philips PW3710 diffractometer using Cu K radiation. The morphology and topology of the films were obtained by scanning electron mi- croscopy SEM using a JEOL JSM-5900LV instrument and atomic force microscopy AFM Digital Instruments NanoScope IIIA series employed in the tapping mode at a scan rate of 10.0 m/s, re- spectively. The effect of the thermal treatment on the optical prop- erties of the CdO films was obtained at room temperature by a Shimadzu UV-160A double-beam spectrophotometer using a clean FTO glass as a reference. For this, the films were annealed at 623 K for 2 h under oxygen flux in a Lindberg/Blue M Tube Furnace.
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Sep 2, 2024
Martin Bojinov + 3
Open Access
