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Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101)

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Science  30 Aug 2013:
Vol. 341, Issue 6149, pp. 988-991
DOI: 10.1126/science.1239879

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Oxide Chemistry Below the Surface

Although metal oxides, such as titanium dioxide (TiO2), are used for catalytic oxidation reactions and photocatalysis, the O2 does not react directly with substrates. Vacancies in the surface region of the TiO2 rutile phase can transfer a negative charge to adsorbed O2 to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO2 particles—subsurface vacancies form. Setvin et al. (p. 988) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O2 into a peroxo species and a bridging O2 dimer.

Abstract

Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed, transformed, and, in conjunction with theory, identified the nature of O2 molecules on the (101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope tip. O2 adsorbed as superoxo (O2) at fivefold-coordinated Ti sites was transformed to peroxo (O22–) and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called bridging dimer also formed when O2 directly reacted with VOs at or below the surface.

Molecular O2 interacts weakly with fully oxidized metal oxides. When excess electrons are present, it adsorbs as an anion in either superoxo (O2), peroxo (O22–), or dissociated (2 × O2–) form. The negative charge can be provided by intrinsic defects [e.g., oxygen vacancies (VOs) or cation interstitials] in a reduced oxide, by doping, or through photoexcitation. Such adsorbed oxygen plays a key role in several technological processes; for example, in oxidation reactions promoted by heterogeneous catalysts, in gas sensors, or in photocatalysis (1, 2). Because of the complexity of technical materials, multiple oxidation states, and the background of lattice oxygen, a molecular-level understanding of O2 adsorption is just emerging (3, 4). Scanning tunneling microscopy (STM) studies on macroscopic, single-crystalline samples with flat surfaces and controlled defects allow a direct view of adsorbed species. In conjunction with density functional theory (DFT) calculations, these studies provide fundamental insights into a rich surface chemistry (39). However, experimental studies have not focused on anatase that makes up nanophase titania, nor have they addressed the role of subsurface VOs that are prevalent in this material (10, 11). Here, we show that manipulations with the scanning tunneling microscope tip can be used to (i) locally create surface VOs, (ii) alter the charge state of adsorbed O2, and (iii) react adsorbed O2 with subsurface VOs to produce an interstitial (O2)O species (or bridging O2 dimer) that has been predicted theoretically.

Our sample was an anatase mineral single crystal, naturally doped with 1.1 atomic % niobium [see the supplementary materials (12)], an efficient electron donor. Although the thermodynamically stable rutile phase [especially the (110) surface of single crystals] has been extensively used as a model surface (3, 4, 13), nanophase TiO2, which is often used in applications, usually consists of anatase (14). Anatase is also frequently referred to as photocatalytically more active, and several studies (1517) have shown that substantial amounts of O2 are present on the surface of photoirradiated anatase, but not on rutile. This could be caused by the different response to photoexcitation, notably by long-lived photoexcited electrons present in anatase but not in rutile (18), which provide the charge for oxygen adsorption, or by the specific surface properties of anatase (2, 19).

In contrast to TiO2 rutile (110), VOs are not stable at the surface of anatase (10, 11). When surface VOs are created by electron bombardment, they move into the bulk at temperatures as low as 200 K (11). In DFT calculations for an anatase slab with a subsurface VO (which provides negative charge), O2 adsorbs molecularly at fivefold-coordinated surface (Ti5c) atoms (Fig. 1A) as a peroxo (O22–) and a superoxo (O2) species at low and high coverages (20), respectively. Ti interstitials, which lead to dissociation of O2 at rutile (110) (5), have the same effect on anatase (101) (21).

Fig. 1 Reaction between an adsorbed peroxide, O22–, and a subsurface oxygen vacancy at TiO2 anatase (101).

(A) Initially, an (O22–)ads molecule (orange) is adsorbed at a fivefold-coordinated surface Ti atom (black). The O (purple) next to the subsurface VO is strongly relaxed toward the surface and returns to its lattice site in (B). (C) An O (light blue) from the bottom of the first TiO2 layer fills the vacancy site, leading to a VO in an unstable position. In a final, combined event (D and E), the vacancy diffuses to the surface and is filled by the O2 molecule. This chain of events heals the VO, and the O2 is incorporated into the surface as a bridging dimer, (O2)O. (F) The potential energy profile is shown in (G). See also movie S1 (12).

A peroxide (O22–)ads adsorbed in the vicinity of a subsurface VO (Fig. 1A) induces substantial structural relaxation, suggesting the existence of an energetically more favorable configuration. In first-principles molecular dynamics (FPMD) simulations (at 220 K), we observed the cascade of events shown in Fig. 1, B to E. In the resulting structure, the O2 takes the position of a twofold-coordinated O (O2c) atom in a bridging, side-on η2 configuration; in Kröger-Vink notation, we refer to this species as an (O2)O. The (O2)O retains a bond length of 1.46 Å, characteristic of O22– (20). This species (often referred to as interstitial or bridging O2) has been predicted consistently in theoretical calculations of oxygen in or on anatase (22, 23) and has been proposed to be an important intermediate in the photocatalytic splitting of H2O.

If such an (O2)O species exists, it should also form when an O2 directly reacts with a surface VO. To test this hypothesis, we prepared surface VOs on anatase (101) (Fig. 2). In previous work, we created such VOs by bombarding TiO2 with electrons (11, 24). Here, we find that identical defects can be generated by the scanning tunneling microscope tip. Figure 2 shows STM images after scanning with high bias voltage and tunneling current. The bright spots within a well-defined area (Fig. 2A) are the same VOs as the ones generated by electron irradiation (11). After dosing with O2 (Fig. 2), some of the VOs were replaced by double spots located at O2c sites. Their appearance agrees well with calculated STM images of the (O2)O configuration (fig. S4). The (O2)O species did not form at temperatures below 20 K, but it readily appeared when O2 was dosed above 40 K, with a sticking coefficient S near unity. This result roughly fits the DFT-derived activation energy of 47 meV (see Fig. 1G).

Fig. 2 Interaction of O2 with surface oxygen vacancies (VO) on TiO2 anatase (101).

In reduced anatase, VOs are normally present within the bulk. In (A), these were pulled to the surface locally by scanning the center with a high sample bias of +5.2 V. In the high-resolution images of the same area before (B) and after (C) dosing 0.15 Langmuir (L) O2 at 45 K, the arrows point out VOs that reacted with O2, forming the (O2)O configuration shown in Fig. 1F. IT, tunneling current; T, temperature.

When we dosed a surface without such artificially created surface VOs, we also see the same (O2)O configuration, albeit quite rarely (see supplementary materials). Most often, we observed two different O2 species (Fig. 3). One of them, denoted (O2)extr, is related to the Nb dopant that preferably replaces a surface Ti6c atom (fig. S2 and table S2). These dopants are distributed unevenly at the surface, with an average concentration of 0.5% of a monolayer [(ML), defined as the density of surface Ti5c atoms] and a local variation between 0.1 and 1% ML. In STM, these impurities exhibit a typical triangular shape (Fig. 3A and fig. S1). Upon O2 exposure at 105 K, bright dimers were found at the position of these extrinsic dopants; they adsorbed with S ≈ 0.1. In DFT calculations (fig. S3), the adsorption of an O2 molecule at a Ti5c atom is preferred by 0.12 to 0.15 eV when next to a Nb6c atom.

Fig. 3 Scanning tunneling microscope tip–induced conversion of adsorbed O2.

Sequence of STM images of the same area of an anatase (101) surface after exposure to 20 L O2 at 105 K. (A) Various configurations of O2: at regular sites, (O2)ads; at the position of lattice oxygen, (O2)O; and at the dopants, (O2)extr. The dopants are likely Nb; one of them is marked with a dashed circle. After increasing the sample bias voltage to +3.2 V (B), the (O2)extr is converted to two (O2)O species and the (O2)ads to an intermediate species, likely a peroxo ion, (O22–)ads (C). An additional scan with Vsample = +3.3 V (D), converts each (O22–)ads into one (O2)O species (E). (F) Overview of the tip-induced processes.

In addition to (O2)extr, a bright species, labeled (O2)ads, formed with with S ≈ 10−4 to 10−2 when anatase (101) was exposed to O2 at 100 K. (The higher values were observed on a more reduced crystal.) The (O2)ads is located at regular Ti5c surface atoms. Its concentration increases linearly with O2 exposure (fig. S5). The highest coverage observed was 5% ML. For higher exposures, STM imaging becomes difficult; the saturation coverage could be considerably higher.

The (O2)ads can be converted into other species with the scanning tunneling microscope tip. Figure 3A shows several (O2)ads and (O2)extr. During scanning at Vsample = +3.2 V (Fig. 3B), horizontal streaks indicate that the adsorbates were modified by the presence of the tip. (The slow scan direction was from bottom to top.) The (O2)ads became more dimerlike (Fig. 3C), and the dark contours indicate that this new species carried a more negative charge. When subjecting these intermediate species to a slightly higher voltage (Fig. 3D), a second conversion took place; see Fig. 3E. The resulting species is the (O2)O that also formed when an O2 molecule directly reacted with a surface VO. (See supplementary materials for more experimental evidence.) Also note that the (O2)extr was already converted into two (O2)O species during the first high-voltage scan (Fig. 3, B and C) and that two (O2)O were always produced per one (O2)extr, whereas only one (O2)O species resulted from each (O2)ads (Fig. 3F). For example, the two neighboring (O2)O configurations indicated in the initial image (Fig. 3A) resulted from previously scanning an (O2)extr at high bias. The tip-induced conversion was reproducible on different samples and with different tips at a minimum Vsample of +3.3 ± 0.1 V.

From these results, we derive a complete picture of the reaction of O2 with a reduced TiO2 anatase (101) surface. When O2 impinges on the cold (~100 K) sample, it is weakly adsorbed. Stronger adsorption occurs when electrons are transferred from the surface to the molecule. The Nb5+ dopants with nearby electrons are populated first, resulting in (O2)extr species. A molecule can also extract an electron from TiO2 at a terrace site to form an (O2)ads. Adsorbed O2 species have a high binding energy (20) and are immobile in STM.

Although (O2)O species form on as-dosed surfaces with subsurface VOs, their concentration is low. The spontaneous healing process depicted in Fig. 1 happens rarely. VOs are more stable in the subsurface region than on the surface by ~0.4 eV (10). The activation energy for a VO to hop from the surface to the first subsurface layer ranges from 0.6 to 1.2 eV, and the barrier for the reverse process is higher by ~0.4 eV (11). In the bulk, VOs diffuse with a much lower barrier of ~0.2 eV; thus, they tend to avoid the surface. Although an adsorbed, negatively charged O2 reverses the energy balance (Fig. 1), the bulk VOs seldom come close enough to the selvedge for the healing process to take place.

We pulled VOs to the adsorbate-free surface with a sufficiently negative scanning tunneling microscope tip. Figure S10 shows that this process is clearly field-induced. The field likely reaches into the semiconductor (tip-induced band-bending) and pushes away electrons that are more or less localized around the VO. This ionizes and destabilizes the vacancy, similar to the effect triggered by draining of electrons via localization at the O22– in the FPMD simulations. The threshold bias voltage for this process is 4.5 eV; the process becomes efficient at 5.2 V. The field-induced migration of intrinsic defects within TiO2 is already used in novel memory devices, but the nature of the mobile species is controversial (25). Our results show that VOs in TiO2 can be manipulated by high electric fields.

The tip-induced transformations of the adsorbed O2 species are summarized in Fig. 3F. We propose that, initially, superoxide (O2)ads forms at regular Ti5c sites. These are transformed into peroxide ions; the increase in band-bending observed after the first tip-induced conversion step suggests that the O2 becomes more negatively charged. A similar, thermally activated transformation occurs if the sample is heated to temperatures between 200 and 300 K (fig. S11). DFT predicts similar adsorption configurations for (O2)ads and (O22–)ads, except for a slightly longer bond length in the peroxide ion (1.33 versus 1.48 Å) (20). By hybrid functional calculations (26), we found a barrier of ~0.3 eV to transform (O2)ads into the more stable (O22–)ads species. Once the (O22–)ads is formed, somewhat higher bias voltages (and/or prolonged exposure to the field) help the adsorbed molecule to merge with a subsurface VO, resulting in the bridging dimer at a lattice site, (O2)O. For (O2)extr near the Nb impurity, the situation is different. (O2)extr is dissociated by the tip with a threshold voltage of 1.6 to 2.5 V, smaller than the 3.3 ± 0.1 V needed for the (O2)ads → (O2)O conversion. Each of the resulting adatoms merges with an O2c atom to form an (O2)O species, consistent with the DFT predictions (23).

Our results clearly show the importance of subsurface O vacancies in TiO2 anatase. They also exemplify how the electric field—for example, from a scanning tunneling microscope tip—can be used as an effective tool to control the charge state of photocatalytically active species. The field can induce transformations of adsorbed O2 that, by merging with a subsurface VO, ultimately lead to the formation of the (O2)O interstitial. This is by far the most stable O2 species on the anatase surface. (O2)O is also an important intermediate in the photooxidation of water (23), which suggests that it could contribute to the higher photocatalytic activity of anatase relative to rutile. Thus, manipulating and controlling the (O2)O interstitial might be a key to furthering the development of more active O-rich TiO2 photocatalysts for water oxidation (27).

Supplementary Materials

www.sciencemag.org/cgi/content/full/341/6149/988/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S11

Tables S1 and S2

References (2830)

Movies S1 and S2

References and Notes

  1. See supplementary materials on Science Online.
  2. Acknowledgments: The experimental work was supported by the Austrian Science Fund (FWF; project F45) and the European Research Council Advanced Grant “OxideSurfaces.” The theoretical work was supported by the U.S. Department of Energy (DOE)–office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under award DE-FG02-12ER16286. We used resources of the National Energy Research Scientific Computing Center (DOE contract DE-AC02-05CH11231) and the Terascale Infrastructure for Groundbreaking Research in Science and Engineering High-Performance Computer Center at Princeton University.
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