Bowker M., Davies P.R. (Eds.) Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis

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An A

defect-free region (the clean-off area) was created by scanning across the

surface at þ3 V before exposing to water, as shown in Figure 8.4. According to the

assignment of Schaub et al. [15] exposure to water should have little effect in the clean-

off area as there would be no O

-vacs with which to react. However, exposing the

surface to water in fact replenishes the A

defects in the clean-off area and reduces the

number of A

defects (Figure 8.4c). If water dissociates in O

-vacs then the number of

-vacs is expected to decrease and the number of OH

species to increase, thus A

defects were reassigned as OH

and A

defects reassigned as O

-vacs. This also

means that approximately þ3 V scans and tip pulses remove hydrogen from OH

While the mechanism for this hydrogen desorption has not yet been elucidated, it

may be related to an empty state identiﬁed by Onda et al. [28] approximately 2.4 eV

above E

Returning to Figure 8.4 because the STM images were recorded from the same

area before and after exposure to water, it can be seen that some of the new OH

take

the positions of the reacting O

-vacs. Thus, Bikondoa et al. [16, 17] imaged water

dissociating in the O

-vacs forming one OH

in place of the O

-vacs and another OH

elsewhere, consistent with the mechanism depicted in Figure 8.1. STM measure-

ments at low temperature show that water dissociates at least down to approximately

187 K [19], a conclusion conﬁrmed by high-resolution electron energy loss

spectroscopy [29].

Surprisingly, the number of OH

in positions previously taken by O

-vacs always

appears slightly higher than elsewhere. Accordingto the mechanism in Figure 8.1, an

equal number of OH

species should appear at the O

-vacs and at a distance from

them. Wendt et al. [18, 19] suggest that when the water molecules dissociate, the two

resulting OH

lie initially in pairs, adjacent in the [0 0 1] direction. As individual OH

from the pair are difﬁcult to resolve with STM, each OH

pair therefore appears as a

single feature, which would account for the apparent anomaly. This interpretation is

supported by the observation of three apparent sizes of type-A defects in some STM

images [17– 20, 30]. The largest of these is assigned to OH

pairs, the next largest to

isolated OH

, and the smallest to O

-vacs, an assignment corroborated in recent STM

simulations of O

-vacs and OH

[30, 31].

Further evidence for the OH

pairs is given in an STM movie recorded at

approximately 187 K. The movie shows the OH

pairs separating across the O

rows

via proton exchange with water molecules [19]. Key frames from the movie are shown

in Figure 8.5. This water-assisted OH

diffusion mechanism is supported by

calculations, which show that the barrier to diffusion is lowered by the exchange

with water. The same diffusion mechanism is also observed for isolated OH

and

because of the misassignment of OH

and O

-vacs by Schaub et al. [15], this water-

mediated diffusion of OH

was incorrectly reported as oxygen-mediated diffusion of

-vacs [32, 33] via a mechanism inconsistent with subsequent isotope studies [34].

Figure 8.6 summarizes our current knowledge of the appearance of point defects

in STM images. The most prevalent point defects on sputtered/annealed TiO

(1 1 0)

1 1 surfaces have been identiﬁed as O

-vacs, OH

, and OH

pairs and these are

shown in a ball model together with an STM image decorated with a number of all

three types of defects.

224

8 Point Defects on Rutile TiO

(1 1 0): Reactivity, Dynamics, and Tunability

Figure 8.4 Sequential (285 Å 250 Å) STM

images of TiO

(1 1 0). (a) Sputtered/annealed

TiO

(1 1 0) with approximately 3.5 and 5.5% ML

and A

defects, respectively. The blue crosses

denote A

defects that are also present in (b),

the red circles denote A

defects, and the yellow

circles denote A

defects that are removed to

form the image in (b). (b) Following a þ3V

scan. The yellow lines indicate the approximate

boundaries beneath which the þ3 V scan was

applied. Blue crosses and red circles denote A

and A

defects, respectively. (c) Following

exposure to 0.1 L water. Blue crosses and filled

red circles, respectively, denote A

and A

defects that were present in (b). Open red circles

indicate positions where A

defects were

present in (b) but not in (c). Red crosses denote

new A

species that reside where A

defects

were positioned in (b) and black crosses denote

new A

species appearing elsewhere. The

images are duplicated for clarity in (d), (e), and

(f). (Modified with permission from Ref. [16].)

8.3 Water Dissociation at Oxygen Vacancies and the Identification of Point Defects

225

The unambiguous assignment of OH

and O

-vacs in STM images also paved the

way for the same point defects to be identiﬁed in NC-AFM images. Most NC-AFM

images of TiO

(1 1 0) show bright rows alternating with dark rows and with dark

depressions appearing on the bright rows. Early interpretations of the NC-AFM

contrast were based on geometrical considerations so that the bright rows were

assigned to O

rows and the depressions to O

-vacs [35]. The assignment of the bright

rows to O

rows was backed up by adsorbing a probe molecule (formic acid) [36] and

by checking the alignment of the bright rows with coexisting strands of the 1 2

reconstruction [37].

Pang et al. exploited information accessible from STM by scanning an area with

NC-AFM then rescanning the same area with STM within minutes [38]. The

experiment was revisited by simultaneously recording STM and NC-AFM images,

an example being shown in Figure 8.7 (Pang et al., unpublished work). In both

experiments, the positions of OH

and O

-vacs in the STM images correlate

with dark depressions in the NC-AFM images, thus showing that both OH

and

-vacs give rise to dark depressions in NC-AFM. The positions of these OH

species

and O

-vacs indicate that the bright rows in the NC-AFM image must correspond to

rows.

Figure 8.5 Sequential images of TiO

(1 1 0) showing the splitting

of an OH

pair mediated by a water molecule at 187 K.OH

groups

are labeled with white circles and water on Ti

sites is labeled with

black squares. Solid grid lines intersect the O

sites whereas

dashed gridlines are along Ti

rows. (Modified with permission

from Ref. [19].)

226

8 Point Defects on Rutile TiO

(1 1 0): Reactivity, Dynamics, and Tunability

In the true surface topography, OH

are protrusions and O

-vacs are depressions.

As such, the appearance of both OH

and O

-vacs as depressions in the NC-AFM

image is signiﬁcant because it means that NC-AFM images do not reproduce the

true surface topography, as is often assumed. In further support of this, Pang et al.

recorded images in the same area with different contrasts, between which there

was presumably some change to the tip (Figure 8.8) [38]. In Figure 8.8a, type-A

defects appear as dark spots in the bright O

rows as they do in Figure 8.7b

(Contrast I). However, in Figure 8.8b, the same type-A defects appear as bright

spots between bright rows, which means the bright rows must be assigned to Ti

rows (Contrast II).

Similar contrast inversions were reported by Lauritsen et al. [39] who also

reproduced the tip change in simulations. The tip change was modeled as a

switch between a tip with a positive potential and one with a negative potential.

The positive tip gives images with Contrast I while the negative tip produces images

with Contrast II.

Figure 8.6 (a) Schematic models of the three most prevalent

types of point defects on sputtered/annealed TiO

(1 1 0) surfaces.

(b) (120 Å)

perspective STM image showing the same defects.

The image is color contoured so that Ti

rows appear green,

rows appear blue, O

-vacs appear green, OH

yellow, and

pairs appear red.

8.3 Water Dissociation at Oxygen Vacancies and the Identification of Point Defects

227

A third contrast regime, which appears more rarely, has also been reported in

which bright rows are imaged with bright spots on them [40, 41]. Fukui and

Iwasawa [40] assigned the bright rows to O

rows and the bright spots to OH

based

on geometrical considerations. This assignment was supported by simulations that

employed a charge-neutral tip [41].

Figure 8.7 Simultaneously recorded

80 Å 150 Å STM and NC-AFM images of

TiO

(1 1 0). (a) A current map represents

the STM image. Because the tip is oscillating

during NC-AFM measurements, the time-

averaged current is recorded. The current

map is color contoured so that OH

appear

as broad, red spots and O

-vacs appear as

narrow, yellow spots. Arrowheads point at

three O

-vacs and circles are drawn over

three OH

, respectively. (b) The simultaneously

recorded NC-AFM image with the arrow-

heads superimposed in blue and the circles

superimposed in red, both indicate dark

depressions. (From Pang et al., unpublished

work.)

Figure 8.8 115 Å 185 Å NC-AFM images of TiO

(1 1 0).

(a) Before the tip change. (b) After the tip change. Purple arrow-

heads indicate coincident type-A defects. (Modified with

permission from Ref. [38].)

228

8 Point Defects on Rutile TiO

(1 1 0): Reactivity, Dynamics, and Tunability

8.4

Dissociation at Oxygen Vacancies

Like water, the interaction of O

with TiO

also has implications for photocatalysis. As

with water again, reactions of O

with TiO

are also important because O

will form

part of the environment in many TiO

applications.

It has been known for some time that the spectroscopic signature of O

-vacs can be

healed by exposure to O

[42–46]. In addition, Epling et al. [47] show that temperature-

programmed desorption (TPD) spectra of water and ammonia are perturbed when

the surface is predosed with O

. This implies that oxygen is left on the surface in some

form when O

-vacs are healed by O

, As such, Epling et al. proposed that one O

-vac is

healed per O

molecule with the other O atom being adsorbed at a Ti

site (O

), a

dissociation mechanism supported by theoretical calculations [48, 49].

Bikondoa et al. followed the dissociation of O

using STM [16, 17]. Any convolution

of the O

reaction with O

-vacs and that with OH

was avoided by removing OH

from an area of the surface by scanning at þ3 V. The same area was then

imaged before and after exposure to approximately 0.6 L O

at room temperature,

as shown in Figure 8.9. Following O

exposure, the four O

-vacs highlighted in

Figure 8.9a are healed and four bright features appear nearby on the bright Ti

rows.

This suggests that O

dissociates in the O

-vacs, one O atom ﬁlling the vacancy and

the other forming O

on a nearby Ti

site. Wendt et al. [18] performed similar

experiments at low temperature, showing that O

dissociates in O

-vacs at least down

to 120 K.

Detailed analysis of the O

positions following dissociation at room temperature

shows that most O

are found one lattice constant (in the [0 0 1] direction) away

from the O

-vac that is ﬁlled, the rest being immediately adjacent and two lattice

constants away [50]. As there is little thermal diffusion of O

on TiO

(1 1 0), the

separation of O

from the closest positions to the reacting O

-vacs was attributed

to the energy released during the exothermic dissociation of O

(calculated at

3.5 eV [18]) in a similar way to that observed, for example, for Cl

dissociation

on TiO

(1 1 0) [51].

8.5

Alcohol Dissociation at Oxygen Vacancies

The reactivity of TiO

with alcohols is important in a number of technological

applications. For example, alcohols can be used as energy carriers in renewable

sources and they are also employed as model pollutants so that environmental

cleaning strategies can be tested.

TPD and static secondary ion mass spectrometry (SSIMS) data suggest that

methanol dissociatively adsorbs at O

-vacs and molecularly at the Ti

sites [52, 53].

There is also some evidence that methanol also dissociates at other sites apart from

-vacs, presumably Ti

sites [53–55]. Similar conclusions have been reached for a

series of short-chain (C2–C8) aliphatic alcohols [56–58].

8.5 Alcohol Dissociation at Oxygen Vacancies

229

Figure 8.10b shows an STM image of the TiO

(1 1 0) surface after a low exposure to

methanol at approximately 300 K [59]. Bright spots are found in place of O

-vacs.

These bright spots were attributed to pairs of methoxy/OH

, by analogy with the OH

pairs formed from water dissociation [18–20]. After dosing more methanol, further

bright spots appear together with a number of darker spots (Figure 8.10c). The darker

spots are assigned to OH

that have separated from the methoxy/OH

pairs across

the O

rows via proton exchange with molecularly adsorbed methanol, through

essentially the same mechanism as described for water in Section 8.3.

Figure 8.9 Reaction of O

with O

-vacs on

TiO

(1 1 0). (a) 100 Å 120 Å STM image of

TiO

(1 1 0) following a þ3 V scan to remove

. The O

-vacs are marked with blue crosses

except for four O

-vacs that do not appear in

(b) that are instead marked with purple crosses.

(b) STM image with the same size and scan

parameters as (a), following exposure to 0.6 L

-vacs that coincide with those in (a) are

indicated with blue crosses. Bright spots that

appearonthebrightrows,closetothepositionof

the O

-vacs in (a) are indicated with yellow

crosses. The images are duplicated for clarity in

reaction. Oxygen originating from the exposed

is colored yellow. The O

is positioned

diagonally adjacent to the O

-vac after Du et al.

[50]. (Modified with permission from Ref. [16].)

230

8 Point Defects on Rutile TiO

(1 1 0): Reactivity, Dynamics, and Tunability

As it is known that high tip bias (approximately þ3 V) can remove OH

[16–18],

Zhang et al. performed a high bias scan in order to distinguish the dark and bright

spots. The darker spots were removed but the brighter spots remained, thus

conﬁrming that the darker species are OH

and the brighter spots therefore methoxy.

As the methoxy groups take the positions of the O

-vacs, this indicates that the

OH bond is broken rather than the CH

OH bond.

A similar series of STM experiments were performed for 2-butanol with almost

identical results, evidence again being given for ROH cleavage at O

-vacs [60]. In

addition to this, for both methanol and 2-butanol, bright spots appeared on the bright

sites. Zhang et al. did not discuss these features in the case of methanol [59] but it

seems likely that the bright spots arise from either methanol adsorbed at Ti

sites or

methoxy dissociated at Ti

sites. For 2-butanol, one of the bright spots was seen

Figure 8.10 STM images of the same area

before and after exposure of methanol on

TiO

(1 1 0) at 300 K. The dosing pressure is

constant in all images so the dose time is

proportional to the exposure. (a) Before

exposure to methanol. (b) After 80 s expo-

sure to methanol. (c) After 110 s exposure to

methanol. (d) The same area as (c) following

a þ3 V scan. Yellow circles show the position

of O

-vacs, blue circles show the bright features

on O

-vacs, red squares mark the darker spots.

Green arrows point at bright spots on the

bright Ti

rows. (Modified with permission

from Ref. [59].)

8.5 Alcohol Dissociation at Oxygen Vacancies

231

ﬁlling an O

-vac in sequential STM images (Figure 8.11). This sequence was

therefore interpreted as a 2-butanol molecule initially adsorbed at a Ti

site

eventually dissociating in an O

-vac.

As the adsorption behavior of ROH on TiO

(1 1 0) is similar, where R ¼H, CH

,or

2-butyl, Zhang et al. [60] suggest that all alcohols may behave as follows: (i) the ROH

bond breaks at the O

-vac sites with RO ﬁlling the O

-vac and (ii) ROH adsorbs

molecularly at Ti

sites and facilitates diffusion of OH

formed from dissociation at

-vacs.

8.6

Diffusion of Oxygen Vacancies and Surface Hydroxy

The surface diffusion of defects and adsorbates is of obvious importance in

heterogeneous catalysis, as this process brings the reactants together. Understanding

the dynamics of molecules on oxide surfaces is also a key step toward the realization

of working molecular electronics. We note here that diffusion of O

-vacs really means

diffusion of O

into the vacancy, which leaves another O

-vac in the position vacated

by the O

. Similarly, diffusion of OH

occurs by diffusion of the H atom.

Zhang et al. [61] recorded STM images between 350 and 423 K to investigate the

diffusion of O

-vacs. Figure 8.12 shows two sequential STM images taken at 400 K,

with a difference image being shown in Figure 8.12c. The difference image clearly

shows that the O

-vacs diffuse along the O

rows. Diffusion was never observed

across the rows in the temperature range investigated. Hopping rates for O

-vacs

along the rows were determined at seven temperatures so that an Arrhenius plot

could be performed. This yielded a diffusion barrier of E

¼1.15 eV that closely

matches the value given by their DFT calculations (E

¼1.03 eV) [61]. Calculations

also show that it is energetically unfavorable for O

-vacs to be positioned adjacent to

or close to another O

-vac along the O

row [48, 61, 62]. This is reﬂected in the spatial

distribution of O

-vacs in STM images. If O

-vacs are positioned randomly over the

surface, statistically a 10% density of vacancy sites should lead to 10% of the O

-vacs

existing as pairs, yet O

-vac pairs have not been reported [61].

Figure 8.11 Sequential STM images showing

a 2-butanol molecule initially adsorbed on a

site dissociating at an O

-vac. (a) Before

dissociation, the 2-butanol molecule is

indicated with a green arrowhead. (b) After

the reaction, the 2-butoxy takes the position

of the O

-vac (red arrowhead) and the OH

sits adjacent (red cross). (c) Schematic

representation of the dissociation process.

(Reprinted with permission from Ref. [60].

232

8 Point Defects on Rutile TiO

(1 1 0): Reactivity, Dynamics, and Tunability

As for OH

diffusion, Zhang et al. [20, 63] also show that diffusion occurs along the

[0 0 1] direction, distinct from the [1



1 0] direction water-assisted diffusion reported by

Wendt et al. [19]. This is demonstrated in Figure 8.13. Furthermore, by carefully

analyzing the positions of the OH

it was shown that the initial hop away from OH

pairs is usually taken by the OH

formed from the hydrogen, which splits off from the

OH fragment that ﬁlls the O

-vac, a preference that holds at least from 300 to 372 K.

Therefore, in contrast to previous expectations, the two OH

that form from

dissociation of a water molecule appear to be inequivalent. Zhang et al. speculate

that this is due to a different charge distribution around each OH

; the O

-vac

involves two nominally Ti

3 þ

ions so that when water dissociates, the OH fragment

Figure 8.12 64 Å 69 Å STM images of a

TiO

(1 1 0) surface recorded at 400 K with O

vacs present. (a) and (b) are sequential images

recorded 2 min apart. The Ti

rows appear red

and the O

rows appear blue. A schematic

model of the surface is shown to scale above

parts of the image in (a) and (b). Ti atoms are

shown red, and oxygen blue with the O

rows

shown a lighter blue. (c) Difference image,

the image in (b) being subtracted from that

in (a). Yellow protrusions indicate the original

positions of O

-vacs whereas blue depressions

indicate their positions in (b). (Reprinted with

American Physical Society.)

Figure 8.13 STM images of a TiO

(1 1 0)

surface recorded at 381 K with OH

present. (a)

and (b) are sequential images recorded 3 min

apart. The white arrows in (a) markthe positions

of two remaining O

-vacs. (c) Difference image,

in which (a) is subtracted from (b). The dark

spots represent the initial hydrogen positions in

(a) whereas the bright spots show their final

positions in (b). The black arrows show the

hopping directions. (Reprinted with permission

Chemical Society.)

8.6 Diffusion of Oxygen Vacancies and Surface Hydroxy

233