The use of Ols charge referencing for the X-ray photoelectron spectroscopy of Al/Si, AI/Ti and Al/Zr mixed oxides

Abstract

X-ray photoelectron spectroscopy (XPS) is an analytical spectroscopy which is used to determine the oxidation state or chemical environment and to quantify elements in the surface region [1]. This technique uses low power X-rays to excite surface atoms which then emit photoelectrons. The resultant spectrum of photoelectron energy versus intensity has peaks with energies which are characteristic of the element and the orbital from which the electron is emitted. In losing an electron, the atom, and by extrapolation the analyte, becomes positively charged. If the analyte is a metal, charge neutralization can be achieved by earthing, but in the case of semiconductors and insulators, sample charging is a problem. Various charge neutralization techniques can be used but total control of the process cannot always be accomplished and a small correction to the photoelectron energy has to be applied [2]. Correcting the photoelectron energy is referred to as charge referencing and the procedure depends on being able to identify a feature in the photoelectron energy spectrum for which the exact energy in an uncharged state is known. The difference between the charged and uncharged position is the amount of charge which can be subtracted from all the other photoelectron peaks to give their true positions. Many charge referencing methods have been employed with varying degrees of success. External referencing involves the addition of a standard, usually a precious metal, by evaporation or doping [2]. The preferred method is to use an internal reference. The most widely applied method is to use the Cis peak arising from adsorbed hydrocarbon contamination [3]. This contamination originates from airborne sources and is a common feature in all X-ray photoelectron spectra of samples that have been exposed to air. Metal oxides exhibit wide variations in their electron transfer properties, surface redox potentials and surface acidities. In XPS studies of oxidic compounds, the authors have noted significant variation in the reported Cis spectra [4]. Similar variations in C ls spectra have been observed by Gross et al. [5] in studies of Cu—Zn--Al oxide catalysts. Both the position and the shape of the Cls peak change according to the preparation method and the subsequent sample history, for example, calcination and reduction; however, the complexity of the peak shape makes it difficult to unequivocally assign the Cls peak position to a known hydrocarbon with a well defined binding energy. This introduces a degree of uncertainty when attempting to compare the spectra from different samples. During a study of mixed oxides of alumina, silica, titania and zirconia, we examined the XPS of materials prepared by the same alkoxide method and calcined to the same temperature in air. The Cis spectra showed systematic differences which indicated varying surface reactivity to the hydrocarbon components present in the atmosphere. This led us to address the problem of C ls charge referencing in oxidic materials. The oxides of Al, Si, Ti and Zr and mixed oxides (with A1203:M02 molar ratios from 95:5 and 70:30) were prepared by the alkoxide sol-gel route. The appropriate alkoxides were mixed in solution and hydrolysed using a 50% excess of water [6]. All samples were dried in air at 333 K and then calcined for 6 h at 873 K. The XPS analysis was performed using a Kratos XSAM 800 spectrometer fitted with a dual anode (Mg/Al) X-ray source and a multichannel detector. The spectrometer was calibrated using the Ag3d5/2 line at 367.9 eV and the AgMVV line at 901.5 eV. MgKa radiation (1253.6 eV) was the exciting source (120 W) and spectra were collected in the high resolution mode (1.2 eV) and fixed analyser transmission (FAT). Kratos DS800 software, running on a DEC pdpl 1/23 computer, was used for data acquisition and analysis. XPS results reported here are given in terms of binding energy which is simply defined as; binding energy (BE) = X-ray energy — photoelectron kinetic energy. In a recent study [7] of the oxides formed on Al— Si alloys, charge referencing using the Cls peak could not be used to analyse the spectra because of the complexity of the peak shape and the resultant difficulty in accurately assigning a known hydrocarbon feature. An alternative charge referencing strategy was therefore attempted using the Ols peak position from a thin film Al oxide and a binding energy of 532.7 eV, previously reported for thin film alumina [8]. Charge referencing is not required for a thin film oxide because electron tunnelling and other conduction processes effectively neutralize sample charging. Using this method, an analysis of the Cls peak was successfully achieved and the results showed that the hydrocarbon contamination was involved in various surface reactions. Fig. la shows the unreferenced Cls spectra