Bowker M., Davies P.R. (Eds.) Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis
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onset onward. Owing to a substantially improved description in terms of real-space
distribution of the electronic states associated with the CB onset, the agreement with
the experiment concerning the simulated corrugation and the tip–surface separation
results is notably improved. In fact, the simulations for the clean TiO
2
(1 1 0) surface
at the considered experimental tunneling conditions of I ¼0.08 nA, V
B
¼1.5 V,
suggest a tip–surface distance of roughly 480 pm that is physically significant even
in the absence of any scaling parameter. Of course, the consideration previously
reported on the causes of the underestimated tunneling currents [4] also applies
in these cases. On these grounds, the agreement can be considered as semiquan-
titative. Interestingly, because of the improved description of the surface electronic
structure, HSE03 simulations with explicit inclusion of the tips unequivocally assign
to H
2
O a smaller STM protrusion with respect to the spot associated with OH
br
,
in close agreement with experimental data for the same tunneling conditions
(Figure 4.12).
To conclude, TiO
2
(1 1 0) indeed represents a key model for understanding the
surface chemical reactivity of metal oxides. So far, it has posed major challenges to
theory in terms of both chemical reactivity and STM appearance. Despite an intrinsic
incapacity to correctly account for the energy localization of defect-induced states
in the bandgap, GGA-based approaches to the surface reactivity have so far been
able to account for most of the experimental findings related to chemical reactivity
of pristine defect sites of TiO
2
(1 1 0). At the present stage, the main challenge is
Figure 4.12 Left: experimental line profile for
TiO
2
(1 1 0) along ½1 10 in the presence of
pristine defects (O
vac
,OH
br
) and undissociated
water molecule for V ¼1.5 V, I ¼0.08 nA (80K).
Right: calculated density contour (top, 1.5 V,
4 10
8
eÅ
3
) and current contour (bottom:
W tip, V ¼1.5 V, I ¼0.08 nA) HSE03 scan lines.
The modeled baseline for the clean TiO
2
(1 1 0)
surface is at approximately 5 Å for both density
and current contours (unpublished work).
114
j
4 Theory of Scanning Tunneling Microscopy and Applications in Catalysis
represented by the surface chemical reactivity in the presence of molecular oxygen,
O
2
(g) [43], although some recent results [83] suggest that interstitial Ti atoms in the
immediate subsurface region (a possibility that was previously not considered by any
ab initio investigation [42, 44]) should play a major role with respect to O
2
(g) reactivity
on TiO
2
(1 1 0). Concerning the simulated appearance of TiO
2
(1 1 0) in the presence of
pristine defects, also the tendency of GGA to excessively delocalize electronic charge,
turns out not to be a major problem when properties related to the onset of the CB
edge are investigated such as in the vast majority of positive bias STM applications.
The use of hybrid-DFT functionals such as B3LYP and HSE03 is found to correctly
account for the energy localization of the electronic states associated with pristine
defects. In addition, the simulated STM appearance is notably improved when
relying on a HSE03 description of the surface electronic structure. Despite these
encouraging findings, the increased computational cost associated with hybrid-DFT
descriptions of the surface poses major practical problems to viable modeling of
multistep reactions on TiO
2
(1 1 0), an aspect that urges the theoretical community to
explore and/or suitably develop the use of nimbler alternative approaches capable
of correctly accounting for the surface properties. In this perspective, we note that to
the best of our knowledge, at the present stage the actual capacities of linear scaling
N-order DFT methods [84, 85] to model the surface reactivity of TiO
2
(1 1 0) are still
unexplored.
4.6
Outlook
In this chapter, we have reviewed some of the key processes and systems in catalysis
research, which have been analyzed by STM. If one message is relevant for all
considered systems, it is that electronic structure is usually dominant in the images:
what one measures, with an STM, has therefore frequently no immediate bearing
on the position of atoms. Given that a key feature of catalytic processes is the
balance between electronic structure changes from adsorption and electronic
structure changes correlating with reactions, it is thus quite often impossible to
attain a thorough understanding of the images and reactions without high-level
theory.
Here, we pointed to the problem of theoretical representation, in particular, in two
aspects of theory: (i) the existence of highly mobile atoms at the surface such as
hydrogen, which are usually not considered in the atomistic models and (ii) the
importance of bandgaps and relative energy levels of electronic states, which is often
distorted in local density approximations. In both respects, a quick fix to the problem
is not very likely. However, as both theory and experiment continue to be developed
and applied in common research projects, it can be expected that the actual
understanding of the processes involved in reaction on model catalysts will sub-
stantially improve over the next 10 years. After all, the ability to trace reactions and to
account for the position and charge state of each reactant is already a realization of
what seemed 20 years ago a fiction rather than fact.
4.6 Outlook
j
115
Acknowledgments
We are grateful to EPSRC UK for the financial support (GT: EP/C541898/1; HL:
EP/E062490/1). WAH acknowledges support from the Royal Society.
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4 Theory of Scanning Tunneling Microscopy and Applications in Catalysis
5
Characterization and Modification of Electrode
Surfaces by In Situ STM
Dieter M. Kolb and Felice C. Simeone
5.1
Introduction
Structure–reactivity relations are among the key issues both in electrocatalysis and
in heterogeneous catalysis [1, 2]. Hence, determining the structure of an electrode
surface, which shows a given catalytic activity, is an important goal in electrochem-
istry, and likewise, tailoring the structure of electrode surfaces for maximum catalytic
output has been a long-lasting desire. The routine use of single-crystal electrodes with
structurally well-characterized surfaces laid the basis of structure–reactivity studies
in electrochemistry [3]. However, while well-prepared flat single-crystal surfaces,
preferably with the three low-index crystallographic orientations, tremendously
increased our understanding of structure effects in adsorption reactions, it was
quite clear that truly catalytic reactions will occur preferentially on surface defects.
The latter, however, will escape detection by diffraction techniques, the commonly
employed method to determine the structure of single-crystal surfaces. In this
respect, scanning tunneling microscopy (STM) like its related scanning probe
techniques plays an important role for catalysis because of their inherent ability to
image surfaces in real space rather than in k-space. Under very special conditions,
surfaces may be imaged while the reaction of interest is ongoing. This is limited
to reactions where at least one partner gives sufficient contrast to be detected by
STM [4]. A typical example of this is metal deposition.
In the following section, we focus on imaging single-crystal electrode surfaces that
are of relevance to electrocatalysis. We will first deal with flat, defect-free terraces as
well as with more real surfaces with monoatomic high steps as the most common
active sites. We will then explore various strategies for nanostructuring surfaces,
for example, by repetitive oxidation–reduction cycles (ORCs).
Soon after the invention of the STM as a tool for imaging surfaces in real space,
it was discovered that the microscope could also be used (or misused) for surface
manipulations, that is, for nanostructuring of surfaces [5]. The extremely close
vicinity of the STM tip and the sample surface required by the tunnel process
j
119
inherently leads to an overlap of both electric double layers and hence to a tip
influence on the surface processes under study. This seemingly undesirable draw-
back of tunnel microscopy is frequently employed for nanostructuring with hitherto
unprecedented precision. While imaging bare and adsorbate-covered electrode
surfaces, even with atomic resolution, has become a routine procedure, measuring
electrochemical activities, that is, reaction currents, with a similarly high lateral
resolution, still needs to be achieved. In this respect, scanning electrochemical
microscopy (SECM) shows promise in accomplishing this goal in the near future.
Since SECM is not the subject of this chapter, the interested reader is referred to
a recent review [6].
5.2
In Situ STM: Principle, Technical Realization and Limitations
5.2.1
Principle Considerations for In Situ Operation
The principle of STM is well described in this book as well as in many other
monographs [7–9]. In brief, a fine metal tip is brought into close proximity of the
surface under study, typically 0.5–2.0 nm, so that the electrons can tunnel from one
side to the other when a voltage U
T
is applied between the tip and the sample
(Figure 5.1). Then, the tip is scanned across the surface with either the tunnel current
kept constant via a feedback circuit (constant current mode) or the height of the tip
kept constant (constant height mode) [8]. In the first case, which is the commonly
used one, the surface topography is reflected in the voltage applied to the z-piezo
Figure 5.1 Schematic diagram showing the principle of STM.
120
j
5 Characterization and Modification of Electrode Surfaces by In Situ STM
(z being the direction normal to the surface) in order to keep the tunnel distance s
constant. In the second case, the surface topography can be obtained from the
variation of the tunnel current. The exponential dependence of the tunnel current
on the tunnel gap s is the reason for the extreme height sensitivity of the STM, which
allows detecting height variations in the 0.01 nm range and below. The lateral
resolution, of course, is considerably lower, the exact value depending on the tip
shape. The image of single (nonperiodic) events, such as monoatomic high steps, is
usually smeared out by tip shape convolution over 1–2 nm. Nevertheless, periodic
structures with 0.3 nm distances like that from individual atoms of a single-crystal
surface can be clearly imaged [10].
One has to keep in mind that STM images show contours of constant tunnel
probability rather than height contours directly. Nevertheless, for simple cases such
as a metal surface, both quantities are closely related to each other. The dependence
of the tunnel current I
T
on the tunnel voltage U
T
and on other parameters is given
for the ideal case in Eq. (5.1):
I
T
¼ R
1
0
U
T
expðA
ffiffiffiffiffi
f
T
p
sÞ; ð5:1Þ
where f
T
is the tunnel barrier, R
0
¼12.90 kW is the resistance of a point contact,
and A ¼10.25 eV
1/2
nm
1
[11].
The joint density of states (JDOS) for the electrons in the tip and the sample enter
the equation for I
T
, and hence in principle, STM images contain some chemical
information about the imaged species. For all practical purposes, however, it is fair to
state that STM does not yield chemical information because such information is very
indirect and often heavily masked by topographic effects. This statement holds
primarily for electrochemical systems, and also for a great deal of the UHV studies,
although there are a few very beautiful examples presented in the literature that
demonstrate a clear chemical contrast for different atomic species of bimetallic
surfaces [12–14]. Since for electrochemical systems, potential limitations (e.g., due to
hydrogen evolution or tip oxidation) severely restrict the application of scanning
tunneling spectroscopy (STS), chemical information can hardly be derived using an
STM. Additional methods, such as cyclic voltammetry, in situ SXRD or even ex situ
XPS, and Auger electron spectroscopy, are often crucial for a safe assignment of
features in the STM image.
Although the tunnel barrier f
T
varies with the tip–sample distance s, it resembles
the local work function of the surface for the large s, but the numbers for an
electrochemical environment are substantially different from those for UHV con-
ditions. For metal electrodes in aqueous solution and s > 1 nm, tunnel barriers range
between 1.0 and 2.0 eV, f
T
¼1.5 eV being a typical value [15]. However, while for
metals in UHV or in air and s < 0.5 nm, the tunnel barrier increases almost linearly
from zero at point contact to a constant maximum value [16], f
T
in situ shows a
very characteristic, potential-dependent variation with s, from which structure
information about the electrochemical interface normal to the metal surface can be
extracted [17]. A result from the so-called distance tunneling spectroscopy, I
T
¼f(s)
at U
T
¼const, is given in Figures 5.2 and 5.3. From a simple visual inspection of
the various exponential decays of I
T
with increasing tip–sample distance s for
5.2 In Situ STM: Principle, Technical Realization and Limitations
j
121
different el ectrode potentials, it becomes evident that the tunnel barrie r markedly
depends on the electrode potential. A quantitative evaluat ion of f
T
via Eq. (5.2)
f
T
ðsÞ¼
h
2
8m
dlnI
T
ds
2
ð5:2Þ
for three different electrode potentials is shown in Figure 5.3 [18, 19]. The three
cases refer to potential value s, positive and negative of the potential of zero charge
(pzc) and at the pzc. An oscillatory behavior of f
T
is observed on either side of the
pzc, that is, when t he ions of the supporting electrolyte build up the solution side of
the electric double layer. With the help of ab initio DFTcalculations, the maxima and
minima of the tunnel b arrier could b e r elated to pos itive and negative excess charge
densities and hence to the positions of ions in the double layer [ 18, 19]. At the pzc,
that is, in the absence of any excess charge, and hence in essence in the sole
presence of water, the barrier height vari es with distance from the surface like for
metal/air interfaces. These results emphasize once more the importance of ions in
determining the electrochemic al interface properties.
For in situ investigations of electrode surfaces, that is, for the study of electrodes
in an electrochemical environment and under potential control, the metal tip
inevitably also becomes immersed into the electrolyte, commonly an aqueous
solution. As a consequence, electrochemical processes will occur at the tip/solution
interface as well, giving rise to an electric current at the tip that is superimposed
on the tunnel current and hence will cause the feedback circuit and therefore
the imaging process to malfunction. The STM tip nolens volens becomes a fourth
electrode in our system that needs to be potential controlled like our sample by a
bipotentiostat. A schematic diagram of such an electric circuit, employed to
combine electrochemical studies with electron tunneling between tip and sample,
is provided in Figure 5.4. To reduce the electrochemical current at the tip/solution
Figure 5.2 Tunnel current as a function of tip–sample distance for
Au(1 1 1) in 0.1 M H
2
SO
4
for three different electrode potentials
(vs. SCE): Positive of ( ), negative of (– –), and at the
pzc (—). s
0
¼0 refers to the tip position at the closest approach
that could be experimentally achieved. (Reproduced with
permission from Ref. [19].)
122
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5 Characterization and Modification of Electrode Surfaces by In Situ STM
interface sufficiently enough to let the tunnel current control the feedback circuit of
the microscope, two different actions have to be taken [20]: (a) the tip potential may
be chosen such that it is close to its rest potential and (b) the area of the tip exposed
to the electrolyte has to be reduced as much as possible by an appropriate isolation
(see Section 5.2.2.1).
Figure 5.3 Tunnel barrier f
T
as a function of tip–sample
distance for Au(1 1 1) in 0.1 M H
2
SO
4
for three different
potentials (vs. SCE). s ¼0 refers to the surface plane of Au(1 1 1).
f
T
(s) for Au(1 1 1) in air is also shown for comparison. For details
see Refs [18, 19].
5.2 In Situ STM: Principle, Technical Realization and Limitations
j
123