The surface science of titanium dioxide - Niser

More documents

Recommendations

Info

84 U. Diebold / Surface Science Reports 48 (2003) 53±229 2.2.1.4.3. Oxygen vacancies created by other means. Oxygen vacancies can also be created by bombardment with electrons. TiO 2 is the classic example for a maximum-valencycompound material where electron-stimulated desorption occurs via the Knotek±Feibelman process [135]. Bombardment with energetic electrons creates a core hole in the Ti3p level. With a certain probability, this hole is filled through an inter-atomic Auger process from a neighboring O atom. If two (instead of the usual one) valence electrons are emitted during the Auger decay, the oxygen anion becomes positivelycharged. The previouslyattractive Madelung potential changes into a repulsive one, and an O ‡ ion is emitted [136]. This process has a threshold energythat correlates with, but is not exactly located at, the Ti3p edge as discussed in detail in [137]. Such electron-stimulated defects behave somewhat different than thermallycreated ones [138]. It is generallyassumed that electron bombardment results in ejection of bridging oxygen atoms, but direct evidence from STM studies points towards more complicated structures [139]. The high current and high field provided byan STM tip has been used to create protrusions and craterlike depressions structures [140]. Irradiation with high-energyelectrons (300 keV) induced a TiO as well as an intermediate TiO 2 -II phase [141]. Defects can also be created byirradiation with UV light, but nothing is known about their structure [142]; as is generallythe case, the cross-section for photon-stimulated desorption is much less as compared to electron-stimulated desorption. Sputtering with rare gas ions reduces the surface oxygen content. Usually, the long-range order of the surface is lost, and the LEED pattern disappears. Spectroscopic measurements as well as adsorption experiments indicate the defects are more complex, involving more than one atom, and are partiallysubsurface [138]. There are indications that sputtering does not completelyrandomize the surface but results in a surface with short-range order that is changed from a twofold to a fourfold symmetry [143]. Generally, sputter-induced damage can be removed easilybyannealing in UHV [74]. 2.2.1.4.4. Line defects. STM images of UHV-annealed surfaces (which exhibit a (1 1) LEED pattern) often show dispersed bright stands, typically several tens of AÊ ngstroms long. Theyare distributed across terraces and have a tendencyto grow out of step edges onto the lower terrace (see Fig. 16a). The strands are centered on top of bright rows of the lower terrace (on top of the fivefold coordinated Ti atoms). STM often shows a bright spot at the end. A double-strand structure is resolved in high-resolution images (Fig. 16a). As shown in Fig. 16, these strands are precursors for the (1 2) reconstruction. Conflicting geometric models have been proposed for this reconstruction. These are discussed in Section 2.2.2. The presence of such dispersed strands is sample-dependent. Li et al. heated samples cut from the same specimen to different temperatures in a furnace in order to achieve different levels of bulk reduction (see Fig. 5). After sputtering and annealing at 973 K for 10 min, strands were present on dark blue samples. Less reduced samples that exhibit a lighter color did not show anystrands [144]. The reduction state of the crystal may not only in¯uence the density but also the geometric structure of the strands [76]. Investigations with numerous TiO 2 samples in this author's laboratoryhave shown that small amount of bulk impurities (well below the detection limit of commonlyused surface analytical techniques) can also cause strands on the surface. This is in addition to the bright strings caused byCa segregation discussed in the next section. 2.2.1.4.5. Impurities. Commercial TiO 2 single crystals are generally quite clean. A common impurity that has been investigated on TiO 2 (1 1 0) is calcium. It tends to segregate to the surface upon hightemperature annealing [145±148]. Typically, Ca can be depleted from the near-surface region in a few
U. Diebold / Surface Science Reports 48 (2003) 53±229 85 Fig. 16. (a) STM image of strings growing out of the upper terrace. These strings are centered at the ®vefold coordinated Ti rows of the lower terrace. Theyare precursors of the (1 2) reconstruction. The surface was annealed at 1020 K for 1 min (8:5nm 12:3 nm, sample bias: ‡0.7 V, tunneling current: 0.3 nA). (b) Variable current scan of the (1 2)-ordered strings on the surface annealed at 1150 K (70 nm 70 nm, sample bias ‡2.0 V, tunneling current 0.3 nA). The vertical axis in both images is parallel to the [0 0 1] direction. From Onishi and Iwasawa [123]. # 1994 Elsevier. sputter/annealing cycles. For high coverages, it forms a well-ordered overlayer which can clearly be observed in LEED [145±147]. Zhang et al. reported a 6 0 3 1 structure in LEED and [0 0 1]-oriented, bright strands in STM. The formation of a CaTiO 3 -like surface compound was tentativelyproposed. This structure was questioned byNoÈrenberg and Harding [147], who reported a p(3 1) reconstruction and antiphase boundaries which could give rise to the LEED pattern observed in [145]. Model calculations suggest that Ca substitutes ®vefold coordinated surface Ti atoms. Two vacancies form at the sites of the threefold coordinated O atoms located next to the Ca atom. Ordering is energeticallyfavorable, and the relaxed structure is predicted to be buckled [147]. The Ca-segregated surface is very¯at which points to a rather low surface free energy. Such a ¯attening takes place also in other segregated oxide systems [149]. Ca segregation also appears to affect the formation of CSPs (see next section). It has been reported that plasma treatment is ef®cient for the removal of Ca at low temperatures [150]. The presence of H on nominallyclean surfaces has been discussed in the context of STM of O vacancies, above. In addition to Ca, some Mg segregation is sometimes observed with LEIS. Persistent Al impurities were detected with static SIMS [151]. This technique is verysensitive to certain elements. The Al probablyresulted from the polishing procedure. SSIMS measurements have also shown that the most common impurityin samples from different vendors is K [152]. All alkali and earth alkali impurities can be removed to a large extent bysputtering/annealing cycles. A persistent impurity, which was onlyapparent in STM images, was attributed to V contamination [153]. 2.2.1.4.6. Crystallographic shear planes. As pointed out above, the oxygen loss through thermal annealing that occurs in the bulk of TiO 2 crystals can lead to CSPs. An overview of CSPs and their
Page 1 and 2: Surface Science Reports 48 (2003) 5
Page 3 and 4: U. Diebold / Surface Science Report
Page 9 and 10: Table 1 Bulk properties of titanium
Page 19 and 20: The second concept, autocompensatio
Page 31: U. Diebold / Surface Science Report
Page 43 and 44: No experimental data on relaxations
Page 47 and 48: organic molecules, see Section 5.2.
Page 57 and 58: 3.2. Reduced TiO 2 surfaces U. Dieb
Page 61 and 62: predicting a varietyof properties o
Page 65 and 66: Table 6 Summary of growth results o
Page 67 and 68: Mo/rutile (1 1 0) Ti 3‡ and Ti 2
Page 83 and 84:
U. Diebold / Surface Science Report
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
change of Au from a noble metal in
Page 97 and 98:
Page 99 and 100:
Table 8 (Continued ) Substrate Tech
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Table 16 Surveyof the adsorption an
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Page 137 and 138:
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
Page 151 and 152:
Page 153 and 154:
Page 155 and 156:
The `other' TiO 2 structure, anatas
Page 157 and 158:
epitaxial growth of superconductors
Page 159 and 160:
6.5. Going beyond single crystal an
Page 161 and 162:
Page 163 and 164:
Page 165 and 166:
Page 167 and 168:
Page 169 and 170:
Page 171 and 172:
Page 173 and 174:
Page 175 and 176:
Page 177:
show all

The surface science of titanium dioxide - Niser

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?