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J. Phys. Chem. C 2009, 113, 3502–3511
On the Relationship between Nonstoichiometry and Passivity Breakdown in Ultrathin Oxides: Combined Depth-Dependent Spectroscopy, Mott-Schottky Analysis, and Molecular Dynamics Simulation Studies Chia-Lin Chang,*,† Subramanian K. R. S. Sankaranarayanan,† Mark H. Engelhard,‡ V. Shutthanandan,‡ and Shriram Ramanathan† HarVard School of Engineering and Applied Sciences, HarVard UniVersity, Cambridge, Massachusetts 02138, and W.R. Wiley EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: September 22, 2008; ReVised Manuscript ReceiVed: December 16, 2008
Understanding the relationship between nonstoichiometry and physical properties of ultrathin oxides is of great importance from both scientific and technological aspects. A specific example includes the onset of passivity breakdown in an ultrathin oxide film in aqueous medium leading to the onset of corrosion. In this work, using the model system of ultrathin oxide of alumina on aluminum synthesized by natural oxidation and photon-assisted oxidation processes, we demonstrate a direct correlation between passivity and quality of the oxide film quantitatively. Depth-dependent high-resolution X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and nuclear reaction analysis (NRA) have been performed to characterize the physical and chemical properties of the oxide films, while detailed impedance measurements and Mott-Schottky studies have been performed to understand electronic transport. Combined NRA and TEM analysis reveal an 18% increase in oxygen density (for oxide films with near identical thicknesses ∼3.8 nm) in the case of photon-assisted oxidation. The denser oxide film results in a ∼34% more blockage of chloride ions transport as indicated by XPS analysis. Mott-Schottky measurements on these oxide films indicates a 43% reduction of defect levels for UV-synthesized alumina when compared to native one, suggestive of chloride ion transport via oxygen vacancies. Additionally, molecular dynamics simulations have been performed to provide insights into the structure of the oxides at the atomic level to correlate with the experimental measurements. These simulations employ dynamic charge transfer between atoms and are used to investigate nanoscale oxides grown on Al (100) surfaces because of atomic and molecular oxygen. Oxidation using molecular and atomic oxygen resulted in an amorphous oxide scale with self-limiting thickness of ∼16 and 22 Å, respectively, at 300 K. Structural and dynamic correlations indicate significant charge transfer to exist in the oxide film in both the cases. The oxide growth in both the cases occurs due to the inward oxygen and outward cation diffusion. The calculated in-plane and out-of-plane atomic diffusivities are 40-70% higher in case of atomic oxidation. In the presence of atomic oxygen, the O/Al ratio is more uniform and varies from 1.37 at the oxide-gas interface to 1.30 at the metal-oxide interface, whereas that formed by natural oxidation was substoichiometric and oxygen deficient with O/Al values varying from 1.27 (oxide-gas interface) to 1.05 (metal-oxide interface) at room temperature. The simulation results are consistent with the reported experimental investigations. I. Introduction An ultrathin layer of amorphous alumina, also referred to as a native oxide layer, usually forms on aluminum surfaces upon exposure to oxygen or air at room temperature. With a thickness that can grow to about 4-5 nm over long periods of time,1 this type of aluminum oxide can form a continuous film on an aluminum surface to offer a natural shield against corrosion.1 The rate at which a metallic object corrodes is highly dependent on the type of environment to which it is exposed as well as the physical structure and chemical composition of the metal itself.2,3 A common form of corrosion for metals such as aluminum is pitting, which refers to the local breakdown of the oxide films. * To whom correspondence should be addressed. E-mail: clchang@ fas.harvard.edu. † Harvard University. ‡ Pacific Northwest National Laboratory.
Although pitting is a local phenomenon, the pit initiation kinetics is controlled by the transport of ionic species. Thus, the nature of defects present in the passive oxide film will impact passivity against corrosive environments that prevents the underlying metals’ collapse. In aqueous environment, corrosion pit initiation on aluminum typically requires aggressive halogen ions, such as Cl-, F-, and SO4-. Proposed mechanisms of chloride ion transport through the oxide film include (a) oxygen vacancies,4,5 (b) water channels,6,7 and (c) localized oxide film thinning or dissolution.8-10 These are mainly dependent on the defect nature (for example, nonstoichiometry) present in passive oxide films formed on the metal surface. Nonstoichiometric alumina, for example, can have a much smaller bandgap of 3 eV compared to that of bulk stoichiometric alumina (∼9 eV).11,12 This is attributed to defects, i.e., higher number of localized electronic states. Reducing the number of localized electronic states is expected to improve the resistance to corrosion.11,13,14
10.1021/jp808424g CCC: $40.75 2009 American Chemical Society Published on Web 02/06/2009
Nonstoichiometry and Passivity Breakdown in Ultrathin Oxides The microstructure of the oxide film might also play a prominent role in determining its corrosion resistance. Zavadil et al. have investigated the relationship between nanostructure and the initiation of pit and pore formation in the passive oxide.2,3,15 Their studies suggest nucleation and growth of nanoscale voids at the Al/AlO interface prior to stable pitting.2 Cation and anion vacancy saturation at the metal/oxide interface supported by considerable lateral diffusion of these point defects was speculated to be the origin of these voids. However, they find that that these voids are not the vacancy condensate structures as postulated in the point defect model applied to pit initiation.2,3 Additionally, the absence of Cl- within these pores as observed by Zavadil et al. demonstrated that they may not be precursors for microcorrosion cells as postulated by electrokinetic models.3 Similarly, other kinds of open volume defects, such as monoand divacancies, vacancy clusters, or dislocations, may also affect the quality of the oxide films.16-18 Analysis of the defect structures of the aluminum oxide films have been carried out by Somieski et al. using positron spectroscopy techniques.18 Alumina films generated by natural oxidation were found to have high densities of open volume defects, mainly consisting of a few aggregated vacancies.18 Similarly, Xu et al. studied microdefects in Al2O3 films grown on iron and nickel aluminide substrates and observed divacancies, vacancy clusters, and microvoids in the oxide scales; the size and distribution of which varied significantly between deposited amorphous Al2O3 and thermally grown R-Al2O3 as well as the type of substrate.17 Although these defects can significantly influence the ability of the oxide films to act as protective surface coatings,19 a detailed mechanistic understanding of the processes correlating the nanometer scale morphological changes in the passive oxide and the pit initiation is still not available. Furthermore, following pit initiation, salt coupled with hydroxyl ions and chloride ions at oxide/metal interface can stabilize the pit growth, assist metal dissolution, and result in the collapse of the passivation film. Creation of artificial passive oxide layers, by exposure to ozone20 and electron enhanced oxidation21-23 of aluminum, has been observed to improve the quality of passivating layer and provide increased corrosion resistance against NaCl solution. In case of electron enhanced oxidation, the electrostatic field produced across the depth of the oxide film causes an enhancement in ion migration through the film, leading to a rapid and more uniform oxide film growth. This oxidation rate enhancement was rationalized by the creation of the defect sites or by partial reduction of Al3+ to Al0 in the oxide film.24,25 Another investigation of electron-stimulated oxidation of Si by Xu et al. showed a resonance in the dependence of the oxidation rate on the primary beam energy, which corresponded to the onset of the lowest dissociative electron attachment (DEA) transition.26 Therefore, electron attachment to an adsorbed O2 precursor molecule leading to the formation of a temporary negative ion and subsequent dissociation were proposed for the mechanism of electron-stimulated oxidation. On the other hand, the oxidation of aluminum at 300 K with ozone was found to proceed with an 8.5 times higher initial sticking coefficient compared to oxygen.27 The oxide thicknesses were found to have 2-2.3 times higher limiting thickness. The lower activation energy barrier leads to rapid dissociation of the ozone molecules on the surface possibly through the production of chemically active atomic oxygen. Ozone-induced oxidation was proposed by Popova et al. to occur by means of a mechanism involving preferential direct formation of Al2O3 clusters.27 In contrast, the formation of a chemisorbed
J. Phys. Chem. C, Vol. 113, No. 9, 2009 3503 O phase, followed by its slow transformation into oxide clusters, was observed in case of molecular oxidation of Al. The higher electron affinity of the ozone molecule compared to molecular oxygen was also proposed to explain the enhancement in the kinetics of Al oxidation. Although the different oxidation mechanisms discussed above are found to have a significant effect on the structure and quality of the oxide films, the relationship between the oxide structures, in particular oxygen nonstoichiometry and passivity breakdown in ultrathin oxides, has not been studied. In a previous investigation, we have demonstrated that photon irradiation during oxide growth or on existing native oxides can also result in significant improvement in corrosion resistance as observed using electrochemical impedance spectroscopy (EIS) study.28 The effect of UV photons during oxidation of metals as well as on ultrathin oxide synthesis has been studied theoretically and experimentally.28-34 The rate of oxygen incorporation can be dramatically enhanced compared to natural oxidation29,30 leading to a significant improvement in the quality of oxide films. UV photons interact with molecular oxygen leading to creation of activated atomic oxygen and helps overcome the activation barrier for chemisorption.35 Further, the ionic currents within the growing oxide film are also enhanced by the UV-light-induced high-field migration, which comprises the electric field effect. Possible mechanisms leading to the enhancement in oxide quality have also been ascribed to the reduction in oxygen vacancies resulting from the creation of activated oxygen species that are highly reactive31,33 and an increased oxide film density.32 Insights into the atomistic mechanism of oxide film growth and the differences in oxide structure and morphology in the presence of activated oxygen species such as atomic oxygen can be obtained using molecular dynamic (MD) simulations.36 Most of the previous theoretical studies utilizing MD simulations have focused on understanding the oxidation processes of metals such as aluminum by using fixed atomic charges.37-42 The fixed charge potential model allows for easy implementation in efficient MD algorithms; however, it has some shortcomings.43 Multiple oxidation states cannot be modeled using the fixed charge model. For example, aluminum can form different oxide compounds such as Al2O3 and AlOx, where the charge on the ions is a function of the oxygen/aluminum ratio.41 During the oxidation of metals, the charges induced on these atoms are environment dependent. The charges on metal atoms change continuously from a zero value in a fully metallic region to their valency-determined maximum value in the stoichiometric oxide. The fixed charge models cannot be used to study the heterogeneous structure such as the interface between a metal and its oxide. Therefore, a transferable potential model which can switch between the metallic interactions in the metal region and the one dominated by ionic interactions in the oxide regions is required.43,44 The charge transfer ionic potential (CTIP) approach allows the environment-dependent charges on the atoms to be dynamically deduced. It was initially proposed by Rappe and Goddard45 and later by Streitz and Mintmire,46 who developed the potential model capable of modeling oxidation of Al substrates. This potential model was utilized by Ogata and Campbell39 and Campbell et al.41 in their MD simulations of 800 nm diameter aluminum nanocluster as well as by Hasnaoui et al. to investigate the oxidation of low-index aluminum surfaces at low temperatures.37 The growth kinetics was found to follow a direct logarithmic law. Additionally, Hasnaoui et al. found the oxide structure to exhibit a tetrahedral environment (AlO4) in the oxide
3504 J. Phys. Chem. C, Vol. 113, No. 9, 2009 interior and mixed tetrahedral and octahedral (AlO6) environments in the outer oxide regions for oxide thicknesses of ∼2 nm. In another investigation, they found the oxidation rate and oxide limiting thicknesses to be independent of the crystallographic orientation.38 Modifications to the potential model developed by Streitz and Mintmire46 has been proposed by Zhou et al.47 This model was used in one of our previous investigation to study the oxidation kinetics and nanoscale passive oxide growth on pure Al and Ni-Al alloy surfaces.48 In this article, we present a systematic experimental and modeling study of the physical and electrochemical properties of near-surface properties of ultrathin alumina film synthesized by natural oxidation and photon-assisted oxidation. Comparison of physical, chemical, and passive state properties between ultrathin native oxide on aluminum and an oxide of comparable thickness synthesized under photon irradiation have been carried out. Although this study is presented in the framework of corrosion protection coatings, the experiments and analysis presented here are of relevance to the chemistry of oxide surfaces and in particular, the role of point defects in transport across ultrathin oxides. By use of a combination of synthesis and electrochemical measurements, we highlight the role of oxygen nonstoichiometry on ion transport as well as electrochemical phenomena such as impedance response in aqueous media. We also quantify relevant parameters of interest through experiments such as the Mott-Schottky analysis. These studies have been performed on ultrathin aluminum oxide, and the results have been explained further through atomistic simulations. To the best of our knowledge, this is the first report on investigation of physical properties of ultrathin alumina synthesized under photon irradiation and a systematic comparison with native oxide films. We study the problem from a combination of experimental and simulation approaches: X-ray photoelectron spectroscopy (XPS) to investigate depth-dependent compositional profiling, transmission electron microscopy (TEM), and nuclear reaction analysis (NRA) to investigate structure and density, EIS and Mott-Schottky analysis to investigate oxide resistance to electron transport and defect concentrations, and finally variable charge molecular dynamics simulations to compare oxide structure and morphology at near-atomic length scales. We present a holistic picture of the fundamentals associated with role of point defects in alumina films and their role in enabling passivity. The combined experimental and simulation study provides fundamental insights into the nature of defects in ultrathin oxides and their resulting impact on functional properties. We believe that the results presented henceforth will be of interest to physical chemists interested in oxide surfaces as well as those studying interactions of oxide surfaces with aqueous media and ion transport through oxides. II. Experimental Details A. Sample Synthesis. Aluminum thin films were deposited on ultralow resistivity (100) silicon wafers by sputtering from an aluminum target at a pressure of 2m Torr of argon. The base pressure of the sputtering chamber was approximately 4 × 10-9 Torr, and the thickness of the aluminum films was nearly 150 nm. Every silicon wafer, prior to aluminum deposition, was dipped in dilute hydrofluoric acid for two minutes, then rinsed in deionized water, and dried in nitrogen to remove the native oxide and enable optimum electrical contact between the aluminum film and substrate. After aluminum deposition, the sample was transferred into the load-lock for photoactivated oxidation. A UV photon source has been custom designed and built into the load-lock to perform
Chang et al. oxidation studies under controlled atmospheres and as a function of temperature. Various photon exposures, ranging from 1 to 120 min, were performed at room temperature. Moreover, natural oxidation (i.e., with no exposure to UV light) at room temperature with identical oxygen pressures and times was performed on control samples. B. Electrochemical Characterization. The electrochemical corrosion test cell was filled with 0.5 M NaCl solution and included three electrodes: a reference electrode (SCE), a counter electrode (graphite rod), and a working electrode (WE, 147nm aluminum on silicon wafer). Prior to EIS measurements, Porthole was attached on each sample for an exact 3-cm2 circular opening to face the 0.5 M NaCl solution electrolyte. Before the measurements were taken, the electrolyte was deaerated in 99.99% nitrogen gas for 1 h. To minimize the effect of native SiO2 on the electrical connection of the working electrode, any undeposited area (1 cm2 on the edge of each sample) was cleaned with dilute HF (50:1) and deionized water. EIS spectra on multiple samples were acquired at open circuit potential to minimize the effect of chemistry on the oxide film in the solution using our Solartron EIS system in the frequency range of 100 kHz to 0.01 Hz. The amplitude of the applied alternating current potential was 40 mV for all measurements. All measurements were performed on multiple samples to ensure reproducible results. Tafel analysis was performed to obtain linear polarization measurements for each sample after the EIS measurements. A scan rate of 1 mV/s was adopted for the potential applied to the test electrode. The 2-h native alumina and 2-h UV alumina samples with an exposed area of 1 cm2 in 0.1 M NaCl solution were investigated further for Mott-Schottky studies. Impedance scans were performed under potentiostatic conditions with applied potentials ranging from -1.4 to -0.75 V vs SCE. Twenty minutes was allowed to deaerate oxygen following which the samples were maintained at each potential for one hour prior to acquiring the impedance spectra. Impedance were recorded and characterized over the frequency range from 100 Hz to 10 mHz using 10 mV rms potential stimulus. C. XPS. The XPS experiments were performed using Physical Electronics Quantum 2000 Scanning ESCA Microprobe. This system uses a focused monochromatic aluminum KR X-ray (1486.7 eV) source and a spherical section analyzer. The X-ray beam used was a 100-W, 100-µm diameter beam rastered over a 1.3-mm by 0.2-mm area on the sample. The X-ray beam was incident normal to the sample, and the photoelectron detector was at 45° off-normal. McCafferty and Wightman49 have performed angle-dependent XPS measurements to determine the concentration of surface hydroxyl groups on aluminum. Both the calculated oxide thickness and the calculated number of surface hydroxyl groups vary at takeoff angle
