DETERMINATION OF NANOMETRIC THICKNESSES OF Ti/TiO2 ELECTRODES

LUCÍA AVALLE; RITA D. BONETTO; JORGE C. TRINCAVELLI

Rosario, Argentina

Congreso; 10th Inter-American Congress of Electron Microscopy (CIASEM 2009). 1st Congress of the Argentine Society of Microscopy (SAMIC 2009).1st Congress of the Argentine Society of Microscopy (SAMIC 2009).1st Congress of the Argentine Society of Microscopy (SAMIC; 2009

CIASEM, the Inter-American Committee of Societies for Electron Microscopy

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:ES-AR;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> The rate of electrocatalytic reactions such as hydrogen or oxygen evolution, both offundamental importance in fuel cell technology, strongly depends on the material and on the nanostructure of the electrode[1, 2]. Previous studies have shown an improvement in Ti/TiO2 electrocatalytic properties with Pt doping of the oxide [3]. Current-potential curves obtained for titanium oxide films electrochemically grown after Pt deposition, indicate that the oxygen evolution reaction takes place simultaneously with the oxide growth. Thus, the kinetics of oxide growth depends on the amount of previously electrodeposited platinum on the bare electrode surface, changing the catalytic properties as well as the oxide thickness.Conventional electrochemical techniques, such as Capacitance Curves (CC), Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV), as well as ellipsometry, are usual for the characterization of the semiconductor/electrolyte interface [4], although they have a low lateral resolution (of the order of the mm). The aim of this work is to develop an alternative method based on electron probe microanalysis (EPMA) to determine, with better lateral resolution (of the order of the µm), TiO2 film thicknesses in the nanometric range, where the thickness changes related to Pt doping take place. The titanium oxide was grown at 50 mV/s up to four different final potentials Vform (1, 2, 3 and 4 V vs. Ag/AgCl reference electrode) in 0.010 M HClO4 and KClO4 by CV.The present method was performed by using the intensity of the characteristic O-Ka lineobtained by electron probe microanalysis. In Fig. 1a) the O-Ka x-ray generation region for a TiO2 pure sample is shown. If the oxide film is much thinner than the penetration depth d3 of the electrons able to ionize the oxygen atoms, the O-Ka signal will be much lower. For increasing thicknesses (d1 or d2), the intensity of this peak will rise due to the increase of the ionization volume, up to a saturation value when the film thickness is equal to d3 or greater (d4).For each of the four TiO2 films studied, around four X-ray spectra were taken at different points (Fig. 1b) by using an EDAX DX Prime spectrometer attached to a Philips SEM505 electron microscope operated at 4 kV. This voltage was chosen after performing Monte Carlo simulations by means of the PENELOPE code system [5] at different incidence energies Eo, to achieve the minimum interference between Ti-L lines and the O-Ka peak. The simulations at Eo=4 keV for different TiO2 film thicknesses on Ti substrate totalized 25 hours of calculation.In Fig. 2a) these results are presented along with the fitting curve.As can be seen from Fig. 2b), the experimental intensity corresponding to the O-Ka maximum plotted against the formation voltage follows the same trend than the simulated data. It is important to emphasize that both curves have different abscissas: Dx and Vform; for this reason it is necessary to find a relation between them or to corroborate some relationships available in the literature for similar systems [4]. Both experimental and simulated intensities have a different scale value, which was found using the asymptotic trends in Figs. 2a) and 2b). After rescaling, film thicknesses of 48 and 75 nm were obtained for Vform=1 and 2 V, respectively, which are consistent with the first rough observation realized on the sample edges by SEM.The results obtained were satisfactory; nevertheless, some efforts in sample preparation and spectra acquisition must be done in order to improve thickness homogeneity and decrease the experimental errors in the calibration curve. In addition, thickness measurements for some of the samples should be obtained near the central region with a direct technique, i.e. scanning electron microscopy. For this last method, a cross section of the sample should be polished very carefully in order to avoid material drag. On the other hand, a good spectral processing should be carried out to obtain a more accurate calibration curve; to this end, atomic and instrumental parameters related to the Ti-L lines and the spectrometer efficiency must be well known.The method presented here will allow the verification of results obtained by means of othertechniques with lower lateral resolution. Bearing in mind the behavior of the simulated values (Fig. 2a), it is possible to foresee that the application range of this method will be in the region of the nanometer and tens of nanometer.References[1] G. A. Somorjai, “Introduction to surface chemistry and catalysis”; 1994, Wiley, NY, U.S.[2] M. Chaudhury and A. V. Pocius (Eds.), “Surfaces, Chemistry and Applications”; 2002,Elsevier B.V., Amsterdam, The Nederlands.[3] L. Avalle, E. Santos and V. Macagno, Electrochim. Acta 39 (1994) 1291-1295.[4] O. E. Linares Pérez, V. C. Fuertes, M. A. Pérez and M. López Teijelo, Electrochem.Commun. 10 (2008) 433–437.[5] F. Salvat, J. Fernández-Varea and J. Sempau, PENELOPE-a code system for Monte Carlo simulation of electron and photon transport. France: OECD/NEA Data Bank, Issy-les-Molineaux, France, 2003.The Extended Abstract (EA) will be evaluated by the Reviewers Committee to be accepted for publication in the CD-ROM Proceeding. This Proceeding will be published by Acta Microscopica (ISSN: 0798-4545), wich is the official Journal of the CIASEM. Thus the EA may need corrections, modifications or it may be considered not appropriated for publication in the CD. The final version should be ready before October 1st, to be included in the CD-ROM Proceedings.