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Scanning tunnelling microscopy: studies of Fe, Cr and SO2 adsorption on rutile TiO2 (110)
Başlık:
Scanning tunnelling microscopy: studies of Fe, Cr and SO2 adsorption on rutile TiO2 (110)
Yazar:
Limb, Adam Jon, author.
ISBN:
9780355978094
Yazar Ek Girişi:
Fiziksel Tanımlama:
1 electronic resource (196 pages)
Genel Not:
Source: Dissertation Abstracts International, Volume: 76-08C.
Özet:
Scanning tunnelling microscopy (STM) has been used to determine the growth structures of metallic adsorbates and the reactivity of molecular adsorbates on TiO2(110) - 1x1 as a function of coverage and surface temperature. The nucleation and growth of vapour deposited Fe on TiO2(110) - 1x1 has been studied at room temperature using STM. An explanation is suggested here for the observed self-limitation of two-dimensional metal clusters that has been reported in the literature for transition metals supported on oxide surfaces. High resolution STM imaging of initial nucleation (surface coverage <0.05 ML) results in the identification of individual Fe atoms, following a deconvolution of the tip structure from the data. There is a lack of mobility observed for these Fe atoms which appear to show a preference for surface oxygen binding sites. These are 3-fold hollow sites associated with in-plane and bridging substrate oxygen. On the basis of charge transfer observed in XPS studies, we identify these surface oxygen based Fe atoms as Fe2+. Arrays comprising of 2,3,4, and 6 monomer units have been observed, forming the apparent 2D clusters. Further exposure of the surface to the Fe flux results in the formation of 3D clusters after a surface coverage of 0.15 ML. This is consistent with the kinetically limited Volmer-Weber growth mode: the two-dimensional islanding (2D-I) growth mode. Continued dosing up to and beyond substrate saturation yields 3D clusters with typical dimensions of ~40 A diameter and ~10 A height. Overlayer thermal stability studies have been performed. Upon annealing up to 700 K, the 3D clusters coalesce to form larger clusters (~150 A diameter and ~ 40 A height), revealing a reduced substrate beneath. The nucleation and growth of vapour deposited Cr on TiO2(110) - 1x1 at room temperature has also been studied using STM. The results are compared with STM studies of Fe and Cu. Cr atoms are found to be stable on the surface and are coordinated with both the substrate oxygen and the exposed Ti centres. 3D cluster growth is observed after a surface coverage of 0.8 ML which is again consistent with the 2D-I growth mode. Thermal stability of an overlayer of 3D clusters upon annealing to 800 K reveals markedly different reactivity to that observed for Fe. On the basis of results previously published the Cr overlayer appears to form a mixed oxide phase. The reactivity of SO2 on TiO2(110) -1x1 has been studied by variable temperature (VT) STM following initial adsorption at 120 K and 298 K. Initial adsorption at 120 K results in the formation of SO32- species. Saturation is achieved at 0.5 ML, after dosing ~6 L of SO2 at 120 K, forming a 2x1 ordered overlayer that has been observed by STM and LEED. A 0.05 ML coverage of SO32- remains on the surface following an increase in surface temperature to 300 K. Upon armealing to 420 K a coverage of 0.025 ML of SO42- is observed. Initial adsorption of SO2 at 298 K results in a coverage of 0.05 ML of SO32-, which suggests the involvment of surface oxygen defects.
Notlar:
School code: 1543
Tüzel Kişi Ek Girişi:
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Yer Numarası | Demirbaş Numarası | Shelf Location | Lokasyon / Statüsü / İade Tarihi |
---|---|---|---|
XX(683845.1) | 683845-1001 | Proquest E-Tez Koleksiyonu | Arıyor... |
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