Scattering of F atoms and anions on a TiO2(1 1 0) surface
Introduction
Titanium dioxide (TiO2) has a wide range of applications ranging from use in heterogeneous catalysis, as a photo-catalyst, in solar cells, gas sensors, pigments etc. This has stimulated a large number of theoretical and experimental investigations of its characteristics. There exist now a rather wide range of surface science studies on the properties of various pristine defect free or defected TiO2 surfaces and their interaction with metal atoms and various molecules. Several fairly recent books and reviews exist on this subject [1], [2], [3], [4], [5], [6].
In this paper we will focus on electron transfer processes in fluorine interaction with TiO2(1 1 0). Electron transfer processes play an important part in surface reactivity. Resonant transfer processes have been extensively investigated in the interaction of atoms and ions with metal surfaces [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30] and clusters [31], [32] in ion scattering studies and for this case are, with a few exceptions fairly well understood. Similarly, considerable progress has been achieved in the description of two electron Auger transfer processes [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. This is not the case of dielectric surfaces where significantly fewer studies have been performed [44]. Recent years have witnessed a considerable activity centred on wide band gap ionic insulator surfaces like MgO and alkali halides; experimental and theoretical work [45], [46], [47], [48] performed has led to a fairly good understanding of the specific mechanisms that govern the charge transfer dynamics that is quite distinct from that of metals. In the case of metals charge transfer is generally described in a delocalised, jellium model type, free electron description of the solid [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Resonant electron transfer processes are quite well understood in this framework and recently more sophisticated treatments have been presented [24], [27], [28], [29]. In the case of ionic solids on the other hand the strongly localised nature of alternating positive and negative charges prompted a different molecular type description involving a non-resonant electron transfer [45], [47] process. Here the Madelung potential plays an important role inducing specific level shifts of atomic states near the surface [49].
From the above perspective the TiO2(1 1 0) surface presents a particular interest as opposed to MgO, as an oxide with covalent characteristics. Its electronic structure has been investigated extensively (see e.g. [4], [53], [56]). The conduction band is mostly Ti(3d) derived. The occupied states corresponding mainly to O(2p) exhibit a significant degree of covalency. The ionic-covalent characteristics of this surface has been investigated theoretically. On the average the Ti charge is close to +2.6 (formal oxidation state is +4) and the oxygen is close to −1.3, as compared to formal oxidation state of −2 [49] (somewhat different numbers were given previously in [53], reflecting difficulties of defining this quantity). It was thus interesting to investigate the characteristics of electron transfer on TiO2(1 1 0) and ascertain if they would be closer to observations on metals or ionic wide band gap insulators studied previously.
In performing this study we also measure energy losses of scattered ions and atoms. Studies of energy losses are an important area and have attracted considerable attention. From a practical point of view this subject is of interest in a number of areas such as semiconductor doping using ion beams or ion beam cancer treatment. In all these cases an accurate knowledge of the penetration depth of ions in matter is necessary. Many studies relate to charged particle penetration of bulk matter [54], [55], [56], [57] and for polycrystalline materials. In the case of monocrystalline solids the situation is complicated by channeling effects. For surface scattering, the situation is intricate because regions far from the first atomic layer of the surface, where the electronic density decreases, may contribute to energy losses [58], [59], [60], [61], [62], [63], [64], [65], [66], [67]. The trajectory lengths depend on the scattering conditions and very different trajectory types can be found. Recent work has also focused on the difference of stopping between metallic surfaces and those of insulating ionic solids, where discreet energy losses have been observed, related to the existence of the large band gap as for alkali halides [44], [68], [69]. In this context the intermediate case of TiO2 is interesting. Furthermore since energy losses are usually trajectory dependent, these can provide some additional information with regard to electron transfer characteristics.
In the following the results of these experiments will be presented and discussed.
Section snippets
Experiment
Our experiments were performed on a setup described in detail elsewhere [70]. The setup allows us to perform time of flight (TOF) scattered ion and atom energy loss spectroscopy, Auger electron spectroscopy (AES). Fluorine anions are produced in a discharge source, mass selected and steered into the main UHV chamber. The pressure in the UHV chamber is of typically 2 × 10−8 Pa.
To study electron transfer processes we perform measurements of ion and neutral particle yield in a given scattering
Azimuthal scattering patterns
Fig. 2a presents a general view of an azimuthal scan (count rate for ions and neutrals as a function of crystal azimuth) for 4 keV incident F− in specular scattering conditions. This is a composite of two independent scans, hence the scale break at about 40°. The scans show a series of well defined peaks corresponding to the various crystallographic orientations, with the most intense structures observed along the main channelling directions: 0° corresponding to the (0 0 1) direction and 90° to
Scattering and energy losses
In our previous paper [71] on He+ scattering on TiO2 we presented a discussion of some of the features in the azimuthal distributions based on classical binary collision code Marlowe, [72]. For He, also strong increases in the scattering intensity were observed for channelling along the main directions [0 0 1] and [0-1 1] and along secondary channels at the same angles as in Fig. 1. A discussion of these features related to scattering on the Ti/O plane and to the presence of the bridging oxygen
Concluding remarks
In F− scattering on TiO2, strong variations in the overall intensity of scattered particles as a function of crystal azimuthal orientation are observed and are due to channelling effects. The energy losses do not show strong variations as a function of crystal azimuth, except for the case of scattering along the (0°) direction corresponding to scattering between the bridging oxygen atom rows. Because the overall characteristics and magnitude of the energy losses are not very different from the
Acknowledgements
The authors are sad to say that their friend and colleague Ana Rita Canario passed away after a sudden illness. This work was supported by the SeCyT (Argentina) – ECOS (France) Program (N° A07E01).
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Towards an ab initio description of the charge transfer between a proton and a lithium fluoride surface: A quantum chemistry approach
2013, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and AtomsCitation Excerpt :In this work we focus on a prototypical non-adiabatic effect, the charge transfer between a projectile and a crystal. Resonant charge transfer between ions and metal surfaces (see, e.g., [4–7] and references in [8]) as well as ion neutralization and negative ion conversion of neutral atoms in front of wide band gap insulator surfaces have been extensively studied in many theoretical and experimental works (see, e.g., [9–16]). Most of the theoretical models in the latter case, however, are based on the HF approximation, i.e. single configuration, calculations which, by definition, cannot account for correlation effects and are known to poorly describe excited electronic states of a system.
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Deceased.