No abstract

CONTENTS 1. Introduction 3888 2. Clean Surfaces 3888 2.1. TiO 2 (110) 3888 2.2. TiO 2 (011) 3891 2.3. TiO 2 (100) 3892 2.4. TiO 2 (001) 3893 2.5. TiO 2 (111) 3893 2.6. Stepped Surfaces 3893 2.6.1. TiO 2 (210) 3893 2.6.2. TiO 2 (771) and TiO 2 (870) 3893 3. Inorganic Nonmetallic Adsorbates 3894 3.1. Oxygen 3894 3.1.1. r-TiO 2 (110) at Room Temperature 3894 3.1.2. r-TiO 2 (110) at Low Temperature 3895 3.1.3. h-TiO 2 (110) 3900 3.2. Hydrogen Adsorbates 3901 3.2.1. Water 3902 3.2.2. Atomic Hydrogen 3905 3.3. Carbon Adsorbates 3907 3.3.1. CO 2 3907 3.3.2. CO 3909 3.3.3. C 60 3909 3.4. Silicon 3912 3.5. Nitrogen Adsorbates 3912 3.5.1. NO 2 3912 3.5.2. NO 3913 3.5.3. N 2 O 3913 3.5.4. N 2 3913 3.5.5. NH 3 3914 3.6. Sulfur Adsorbates 3915 3.6.1. SO 2 3915 3.6.2. S 3915 3.7. Chlorine 4. Organic Adsorbates 4.1. Organic Acids 4.1.1. TiO 2 (110) 4.1.2. TiO 2 (011) 4.1.3. TiO 2 (001) 4.1.4. TiO 2 (111) 4.2. Alcohols 4.2.1. TiO 2 (110) 4.2.2. Other TiO 2 Surfaces 4.3. Aldehydes, Ketones, and Acid Anhydrides 4.4. Amine, Azo, and Pyridine Compounds 4.5. Aromatic Hydrocarbons 4.6. Organometallic Compounds 5. Metals 5.1. Group 1 5.1.1. TiO 2 (110) 5.1.2. TiO 2 (100) 5.1.3. TiO 2 (441) 5.2. Group 2 5.2.1. TiO 2 (110) 5.2.2. TiO 2 (011) 5.2.3. TiO 2 (100) 5.2.4. TiO 2 (001) 5.3. Group 4 5.4. Group 5

Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO 2 ð110Þ in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.The rutile TiO 2 ð110Þ surface, which we use as a model photocatalytic system here, is displayed as a ball model in Fig. 1A where the reduction of one oxygen atom of O 2 ðgÞ to one bridging oxide species (O 2− b ) is accomplished by oxidation of the two Ti 3þ sites associated with O b -vac to Ti 4þ (3), on the basis of a purely ionic model. (Formal charges are written in reactions 1 and 2 to highlight the redox processes involved.)The interaction of O 2 with OH b , on the other hand, is still a matter of controversy. Following the reaction of these species at temperatures ≤240 K, water is seen to desorb at ∼310 K in temperature programmed desorption (TPD) spectra (3, 4). Henderson et al. (3) concluded that this water evolution is a consequence of the formation of oxygen adatoms (O ad ) at the surface as follows:where the two Ti 3þ species provide the two electrons necessary to reduce one oxygen atom of O 2 ðgÞ to H 2 OðgÞ (3). In stark contrast to the TPD results, previous calculations find H 2 O 2 to be by far the most stable product (5). Moreover, on the basis of these calculations, water desorption is not expected up to the highest temperature computed, 350 K (5). This discrepancy provided the initial motivation for the present work. Results and DiscussionWe use STM to provide an additional experimental test of the picture that has emerged thus far. Fig. 1B shows a surface containing both O b -vac and OH b , alongside the same surface in Fig. 1C after it was exposed to 90 Langmuirs (L) O 2 at 300 K (1 L ¼ 1.33 × 10 −6 mbar · s, 1 mbar ¼ 100 Pa). A number of small, bright spots can be seen on the Ti 5c sites (bright rows) in the latter image. The histogram of the height distribution of these bright spots, shown in Fig. 1D, indicates that these bright spots are almost entirely due to one final product.It should be noted that at lower O 2 exposures we see a number of different types of species on Ti 5c rows that are likely to arise from terminal hydroxyls (OH t ) and other metastable species such as O 2 H. These latter results ...