21510
J. Phys. Chem. C 2010, 114, 21510–21515
Mechanism of Electron-Induced Hydrogen Desorption from Hydroxylated Rutile TiO2 (110)
D. P. Acharya,† C. V. Ciobanu,‡ N. Camillone III,§ and P. Sutter*,†
Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973, United States,
DiVision of Engineering, Colorado School of Mines, Golden, Colorado 80401, United States, and Chemistry
Department, BrookhaVen National Laboratory, Upton, New York 11973, United States
ReceiVed: August 2, 2010; ReVised Manuscript ReceiVed: October 7, 2010
The mechanism of hydrogen desorption from rutile TiO2(110)-(1 × 1) was studied by injecting electrons
with controlled energy and flux into single surface hydroxyls (OH) in cryogenic scanning tunneling microscopy
(STM). Desorption proceeds without a clear threshold already at much lower energies than reported previously.1
Our analysis identifies a transfer of H atoms from the TiO2 surface to the STM tip, triggered by vibrational
heating due to inelastic electron tunneling, as the desorption mechanism. The reversible H-atom transfer
between sample and tip can be used as a tool to discriminate OH from other surface species on TiO2 and to
control the density and configuration of OH by selective removal and redeposition of H atoms on the oxide
surface.
Introduction
Photocatalysts are widely used in water and air treatment,2
organic waste remediation,2,3 and have the potential to become
key materials for the renewable conversion of solar energy to
fuels, for example, via water splitting4 or greenhouse gas
reforming.5 The (110) surface of rutile titanium dioxide is often
seen as the prototype model system for studying surface
chemistry and photocatalysis on metal oxides. Extensive
experimental and theoretical work has been focused on understanding the properties of atomic-scale defects giving rise to
reactivity, such as metal interstitials (Tii6) and oxygen vacancies
(VO,br7,8). The simplest adsorbed species, a single hydrogen atom
whose chemisorption on the bridging oxygen (Obr) rows on
TiO2(110) generates a bridge-bonded hydroxyl (OHbr), has
received attention because it is expected to participate as an
intermediate in a variety of photocatalytic reactions, including
water splitting,9 the decomposition of organic molecules,2,3,5,10
and the hydrogenation of CO2.10
Whereas the primary mechanism of OHbr formation on
reduced TiO2 surfaces prepared in vacuum - via the reaction
of H2O with VO,br - is well-documented,9,11 the desorption of
H from OHbr remains less well understood.1,12 Various scanning
tunneling microscopy (STM) studies have shown that H
desorption from hydroxylated TiO2 can be induced by the STM
tip, either during imaging at elevated bias or by the application
of voltage pulses,1,13 similar to STM tip-stimulated H-desorption
from Si(100)14 and Ge(111).15 This finding not only provides a
means for identifying OHbr among other surface species with
similar STM contrast (e.g., VO,br), but it also opens up the
possibility of using the atomically precise injection of charge
carriers at well-defined energy and current into single OHbr
species to gain a fundamental understanding of the H desorption
mechanism.
Here, we report the analysis of the controlled, STM tipinduced desorption of individual H atoms from OHbr on
TiO2(110) at cryogenic temperatures (77 K). The extreme
* Corresponding author.
†
Center for Functional Nanomaterials, Brookhaven National Laboratory.
‡
Division of Engineering, Colorado School of Mines.
§
Chemistry Department, Brookhaven National Laboratory.
stability of the tunneling gap in low-temperature STM has been
exploited primarily for the manipulation of atoms and molecules
on metal surfaces, but the potential of atomically precise
manipulation for studies of elementary processes on oxides has
remained largely unexplored. The present study is a step in this
direction. Injecting charge carriers with variable energy and dose
into individual OHbr, we find that voltage pulses remove single
H atoms from Obr, transferring them to the STM tip. The yield
of this single-H removal from TiO2(110) depends strongly on
both the electron energy and tunneling current, but the STMinduced desorption proceeds without a clear onset voltage
already at a sample bias as low as 1.3 V. The dependence on
the carrier injection rate (i.e., tunneling current) shows that the
H-removal involves both one- and two-electron processes,
without detectable (H, D) isotope effect. By considering several
possible mechanisms for the electron-stimulated desorption, we
conclude that vibrational heating by inelastic electron tunneling
is the mechanism responsible for H desorption. Besides
understanding the nonthermal, electron-stimulated desorption
of an adsorbate ubiquitous on the TiO2(110) model photocatalyst, establishing the conditions for STM tip-induced H desorption has practical implications for experiments on TiO2 surfaces.
Our findings suggest that conditions exist in which the H atoms
in OHbr species are selectively removed without affecting other
adsorbates or surface defects. Hence, STM-induced desorption
can be used, for example, as a means to identify OHbr discriminating it from other adsorbates with similar STM
contrast - and to control the local OHbr population in STM
studies by selectively desorbing H from extended surface areas.
Methods
Our experiments were performed in a low-temperature STM
system (Createc), liquid nitrogen cooled to operate in cryogenic
ultrahigh vacuum (UHV, T ) 77 K, P < 10-11 Torr). Two
different types of rutile TiO2(110) single crystals (Princeton
Scientific; Commercial Crystal Laboratories) were used, following preparation in UHV by several cycles of Ar+ sputtering
and annealing to 910 K. Electrochemically etched W tips,
electron bombardment annealed in UHV, were used for imaging
and hydrogen atom manipulation.
10.1021/jp107262b  2010 American Chemical Society
Published on Web 11/19/2010
Hydroxylated Rutile TiO2 (110)
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21511
Figure 1. Controlled desorption of individual H atoms from OHbr species on TiO2(110). (a) to (c) - STM images (upper row) and corresponding
schematic structures (lower row). (a) Pair of OHbr (center, dash-dotted oval), surrounded by single OHbr (dashed circles) and VO,br. Note the continuous
electron density across the nearest-neighbor OHbr, consistent with the 1D electronic hybridization suggested in.24 (b) Same field of view after a
voltage pulse (V ) +1.7 V, I ) 0.5 nA) on one of the OHbr, causing the removal of its H atom. (c) Same area after a second pulse, desorbing the
remaining H atom. Schematics illustrate the sequential removal of single H atoms from OHbr. (d) STM height profiles along [11j0] and [001]
directions of OHbr pair (blue), single OHbr (red), and adsorbate-free surface (green). Imaging parameters: V ) +1.27 V, I ) 0.53 nA. Image sizes:
4 × 4 nm2.
STM imaging was performed in constant current mode with
positive sample bias under conditions that did not affect the
surface hydroxyls, typically V ) 1.3 V and I < 0.5 nA.
H-desorption from the surface was stimulated by pulses to higher
voltage over individual OHbr, or alternatively by scanning at
elevated bias to remove H from larger areas of hydroxylated
TiO2(110). Bias pulses were applied either with tunneling
feeback enabled or disabled, and both scenarios gave similar
results. Unless stated otherwise, the experiments presented here
were performed with feedback loop on. The feedback ensured
that a well-defined, constant tunneling current was maintained
throughout each voltage pulse, and it also prevented very high
current densities at the tip apex and sample surface for longer
periods of time.18 This mode of pulsing implies that the tip
height remained constant during the electron injection process
until H-atom desorption from the sample was induced. During
each voltage pulse, the current-time (I-t) signal was recorded.
A sharp drop in tunneling current accompanied the H-desorption,
allowing us to precisely determine the desorption time. The
overall pulse duration was varied, depending on the desorption
yield under the chosen voltage and current conditions, from tens
of milliseconds to over 1 s. After pulsing, the area of interest
was scanned under standard (noninvasive) imaging conditions
to confirm that H-desorption had indeed been induced.
In addition to experiments with native hydroxyl populations,
generated by exposure of the reduced crystal to a low water
vapor background in the UHV preparation chamber, the TiO2
surface was also hydroxylated with pure isotopes by exposure
to atomic hydrogen or deuterium, produced by cracking H2 or
D2 gas (99.998% purity, P ) 10-6 Torr) by a hot W filament
placed close to the sample surface. STM imaging showed
hydroxyl coverages up to 25% of the available Obr sites at the
surface, compared to the highest hydroxyl coverage achievable
by this method, ∼70%.16
Results and Discussion
The basic principle of the STM-induced desorption of single
Obr-bound H atoms is illustrated in Figure 1. The STM image
of part a of Figure 1 shows alternating bright and dark rows
consisting of 5-fold coordinated Ti5f atoms and 2-fold coordi-
nated Obr atoms, respectively.7 For the desorption experiment,
a large flat surface terrace was selected and a small region was
imaged with high resolution to exactly identify the location of
all OHbr within the scan area. The STM tip was then positioned
above a H atom, and a voltage pulse (positive sample bias) was
applied for a short interval of time while recording the time
dependent tunneling current, I(t). Following the pulse, the same
area was imaged to assess the resulting changes to the surface.
By positioning the STM tip above a single OHbr and applying
a suitable voltage pulse, the corresponding H atom can be
removed from the TiO2 surface with high fidelity and excellent
control. At low voltage, desorption is a local effect involving
only the H atom beneath the apex of the STM tip. The selectivity
achieved in this way is demonstrated in Figure 1 by sequentially
desorbing H atoms from two OHbr in nearest-neighbor sites in
the same Obr row.
To determine the mechanism of STM-induced desorption, we
have statistically analyzed a large number of desorption events
for several pulse voltages (V) and different tunneling currents
(I), probing the desorption of H(D) atoms from OHbr (ODbr).
For each set of parameters (V, I), a histogram of the number of
successful events as a function of desorption time, N(t), was
plotted and the average desorption time (τ) was extracted by
fitting an exponential relation, N ) ∼exp[-t/τ].17 Figure 2
shows examples of this analysis, as well as individual I(t) traces
(inserts), for H desorption with pulse voltage +1.7 V. The
desorption times could be identified clearly by spikes in the
I(t) traces. Generally, a brief initial spike in the tunneling current
was quickly compensated by the feedback loop and a constant
current was maintained until a second spike, a sharp decrease
in current, marked the desorption of the H (or D) atom. The
average desorption time and tunneling current can be used to
calculate the desorption yield, Y ) (e/Iτ).17
We have used measurements of the desorption yield in
different, well-defined scenarios to identify the mechanism of
H-atom desorption from OHbr. Possible mechanisms include a
weakening of the O-H bond by the applied electric field,18,19
the direct quantum tunneling of the H atom from the surface to
the tip;20 desorption by electron attachment;21 and local heating,
that is, the vibrational excitation of the O-H bond by inelastic
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J. Phys. Chem. C, Vol. 114, No. 49, 2010
Acharya et al.
Figure 2. Signatures of single STM-induced H desorption events, and statistical analysis of desorption times. (a) Histogram of the distribution of
H desorption times corresponding to voltage pulses with V ) +1.7 V and I ) 0.5 nA. The red line is a fit to an exponential distribution for
statistically independent events.17 The insert shows an example of the individual I(t) traces used to compile the histogram. (b) Analysis of desorption
times and single I(t) trace for V ) +1.7 V and I ) 1.0 nA. (c) Same for V ) +1.7 V and I ) 5 nA.
Figure 3. Analysis of electric field effect and tunneling yield. (a) Desorption yield for different tip-sample separations, obtained by stabilizing the
STM tip with tunneling conditions V ) +1.27 V; I ) 0.5 nA above a single OHbr, disabling the feedback loop, then retracting the tip by ∆z (insert)
and pulsing with V ) +2.0 V. The desorption yield remains constant for different tip-sample separations. (b) Double-logarithmic plot of desorption
yield, Y, as a function of tunneling current, I. Filled circles and squares are measured yields for single D and H desorption, respectively, at different
pulse voltages. Lines are power-law fits to the experimental data, with exponents as given in the text.
tunneling.14 Our measurements indicate that the latter is
responsible for STM-induced H desorption at low voltages
(e1.7 V).
Electric field effects in STM are difficult to identify unambiguously. Any variation in the tip-sample separation, z, for
instance, not only changes the electric field, E ) V/z, in the
tunneling junction but also causes an exponential reduction in
the tunneling current, I ) ∼e-az, that is, in the flux of injected
charge carriers. Changing the bias voltage, V, on the other hand,
also alters the energy of the tunneling electrons. In our study,
the most direct evidence for the absence of significant electric
field effects is given by the fact that local voltage pulses can
selectively desorb individual H atoms from nearest-neighbor
pairs of OHbr (Figure 1). Whereas electron tunneling in STM
is determined by the electronic orbitals at the apex of the probe
tip and is thus localized at the atomic scale (giving STM its
high spatial resolution), the electric field in the tunneling junction
is determined by the nanometer-scale tip shape and varies more
slowly away from the tip apex.22,23 Hence, the electric field
experienced by two nearest-neighbor OHbr is nearly the same.
Yet, in moderate voltage pulses (+1.7 V), H is only removed
from the hydroxyl subjected to electron injection from the tip.
The conclusion that electric field effects play a minor role in
the STM induced H-desorption from TiO2(110) is corroborated
by direct measurements of the desorption yield in +2 V bias
pulses as a function of tip-sample separation. Part a of Figure
3 shows data obtained by changing the magnitude of the electric
field at the OHbr site via controlled retraction of the STM tip.
Following the stabilization of the tip at a fixed height above a
single OHbr, the feedback loop was disabled, the tip retracted
by different amounts ∆z, and a +2 V pulse was applied between
sample and tip. As in Figure 2, the desorption yield was
determined from the elapsed time, indicated by an abrupt
decrease in tunneling current in the recorded I(t) accompanying
the H-desorption. Because the tunneling impedance increases
exponentially with increasing tip-sample distance, the current
decreases sharply at higher separation. Desorption still occurs
but it takes a longer time at reduced current. For +2 V pulses,
however, the total injected electron dose required for desorption
(i.e., the yield Y), remains constant and is independent of the
tip-sample distance. Given this insensitivity to changes in the
electric field, we conclude that field-induced effects, such as
bond softening, are not the primary H-desorption mechanism.
Quantum tunneling of H atoms from the sample to the tip
can be excluded as a desorption mechanisms, because a strongly
mass-dependent tunneling rate would give rise to significant
differences in desorption yield for H and D, which we do not
observe (below). Bikondoa et al. have suggested that the
injection of tunneling electrons into an unoccupied wet-electron
state could be responsible for the STM-induced H-desorption.1
Indeed, the bias voltages at which we observe desorption
coincide quite closely with the energy of a wet electron state
involving a saturation coverage of OHbr on TiO2(110) (∼1.5 V
above the Fermi level, EF24). In this state, the orbitals of the
individual OHbr hybridize to give rise to one-dimensional
extended states with strong electronic coupling between neighboring OHbr. The STM image contrast of a nearest neighbor
OHbr pair (part a of Figure 1), a single protrusion narrowly
confined along [11j0] but continuously covering both OHbr in
the [001] direction, is consistent with the delocalized nature of
Hydroxylated Rutile TiO2 (110)
unoccupied states of neighboring OH. If electron addition to
this state were responsible for H desorption, one would expect
at sufficiently high current the concerted desorption of multiple
H atoms bound to nearest-neighbor Obr. We did observe the
desorption of multiple H atoms, but it occurred only during
pulses to high bias voltage (V > +2.5 V). Under these conditions,
bias pulses were found to desorb multiple H- atoms within 2-3
atomic spacings in both the [001] and [11j0] directions. In
contrast, Figure 1 shows that STM is capable of selectively
desorbing one H atom from a nearest-neighbor pair of OHbr
using low-voltage (1.7 V) pulses. The complete absence of
multiple desorption events of electronically coupled neighboring
H atoms along the same Obr row in the relevant energy range
provides strong evidence against desorption driven by electron
attachment into the suggested wet electron state.1
We find that desorption by vibrational excitation due to energy
transfer from tunneling electrons (i.e., inelastic tunneling) is the
most likely mechanism for H desorption from Obr. In contrast
to vibrational ladder climbing processes on metals, the same
process on wide-bandgap oxides, such as TiO2, is complicated
by less efficient vibrational de-excitation and the resulting longer
vibrational lifetimes. Short vibrational lifetimes on metals (<1
ps) imply that the ladder climbing process involves the lowest
possible number of excitations.21 On semiconductors, vibrational
lifetimes involving H adsorbates can be as long as 1 ns (H-Si
stretch on H-terminated Si14), that is, fall in the range of typical
electron arrival intervals in STM. This overlap in time scales
can give rise to more complex behavior, in which different
numbers of excitations can be involved in desorption. We have
analyzed this behavior by considering the current-dependent
desorption yield, Y.
For coherent n-electron processes, the desorption yield
exhibits a power-law dependence Y ∝ I(n - 1) on the rate of
electron injection (i.e., tunneling current I). Plots of the
experimentally determined desorption yield as a function of
tunneling current for different pulse voltages and desorbing
species (H, D) are shown in part b of Figure 3. For pulse voltage
V ) 1.7 V, we find values nH ) (1.6 ( 0.2) and nD ) (1.7 (
0.2) for H and D desorption respectively suggesting contributions from both 1- and 2-electron processes. For smaller pulse
voltages (V ) 1.3 V) the yield was very low, and thousands of
individual events were recorded to achieve sufficient statistics.
The value of n for D-atom desorption at 1.3 V is again (1.7 (
0.2). The yields for H- and D-desorption at V ) 1.7 V are nearly
identical, that is, no isotope effect is detectable. Additional yield
data at higher pulse voltage (+2 V), obtained by converting
the points of part a of Figure 3 using the measured I(z)
characteristics, suggest that the desorption yield becomes
independent of tunneling current, and hence n ) 1, at higher
voltages.
An analysis of the onset of tunneling conduction at positive
sample bias in STM current-voltage spectra shows the conduction band edge in our samples about 0.4 eV above the Fermi
energy. Hence, at a sample bias of 1.7 V electrons are injected
with 1.3 eV excess energy above the conduction band minimum
of TiO2 (part a of Figure 4). Similarly, electrons tunneling at
1.3 V have 0.9 eV excess energy. Comparison of these values
with the measured energy of the O-H stretch vibration of OHbr
on TiO2(110) (pω ) ∼0.45 eV25) suggests the energy diagram
shown in part b of Figure 4, that is, desorption by excitation of
two vibrational quanta pω - by one or two electrons - for
both H and D. Despite the larger error bars on the data obtained
for +2 V pulse voltages, we tentatively conclude that there is
a transition from a mixed one- and two-electron stimulated
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21513
Figure 4. Schematics of vibrational excitation by inelastic tunneling
on a wide-bandgap oxide, and of the ladder-climbing causing H/D
desorption. (a) Band diagram for electron injection from the STM tip
into empty states of the sample at bias voltage V (energy eV, where e
denotes the electron charge). EC: conduction band minimum; EV:
valence band maximum; EFt,s: Fermi level of tip, sample. pω denotes
the vibrational quantum of the O-H stretch for OHbr on TiO2(110).
(b) Schematic of the double potential well with bound states on the
TiO2 sample (left) and tungsten STM tip (right). The blue and red levels
correspond to the vibrational ground and excited states of D and H on
Obr, and the height of the transfer barrier (Et) is consistent with the
observed one- and two-electron induced desorption.
desorption at low bias to a single-electron process for pulse
voltages of +2 V, that is, excess electron energies of 1.6 eV or
higher above the conduction band minimum. Within the
framework of vibrational excitation by inelastic electron scattering we intuitively expect the order of the process (n) to
decrease with increasing incident electron energy; however,
additional work will be required to further analyze the crossover
between these two regimes.
Part b of Figure 4 shows a second bound state for the H/D
atoms on the tungsten STM tip. Because individual atoms are
desorbed (i.e., recombinative desorption via the formation of
H2 does not take place), the energetically preferred final state
after desorption should indeed involve H (or D) not in vacuum,
but bound to the tip. To verify that STM voltage pulses indeed
transfer atoms to the tip, we performed the following experiment: i) desorption of a single H atom from OHbr on TiO2(110)
by pulsing at positiVe sample voltage, ii) imaging to verify
successful H desorption, iii) displacement of the tip to a different
location on the sample, and iv) pulsing at negatiVe sample
voltage in an attempt to redeposit the H atom onto the TiO2
surface.
Figure 5 shows the result of this manipulation in a sequence
of empty-state STM images where individual H atoms were
picked up by the tip and redeposited onto the surface using
vertical manipulation by voltage pulses at 77 K. Brighter
protrusions (part a of Figure 5) represent individual OHbr,
whereas a fainter protrusion, marked by a dashed loop, is a Obr
vacancy. The STM tip was precisely positioned on top of a H
atom, labeled ‘1’ in part a of Figure 5, and a voltage pulse (V
) +2.0 V and I ) 1 nA) was applied for 50 ms. After the
pulse, the area was imaged to confirm that a single H atom has
been removed from the surface (part b of Figure 5). A second
H atom was picked up in the same way outside the field of
view. In the following, the tip was positioned on the clean area
near two remaining OHbr and a negative pulse of (V ) -2.7 V,
I ) 1 nA) was applied. The subsequent STM image (part c of
Figure 5) confirms redeposition of two H atoms onto free Obr
sites. The process was repeated and another H atom was
removed (parts c and d of Figure 5) and redeposited on the
surface. The resulting four-atom line is shown in parts e and f
of Figure 5. The atom transfer from sample to tip was much
more efficient than from tip to sample, and higher bias voltages
21514
J. Phys. Chem. C, Vol. 114, No. 49, 2010
Figure 5. H atom manipulation by transfer between sample and STM
tip at 77 K. Larger protrusions represent OHbr. A faint protrusion
elongated along [11j0], indicated by a dotted loop, represents an Obr
vacancy. (a) Initial state. (b) Two H atoms, one outside the field of
view and one labeled ‘1’ in (a), were picked up by the tip using positive
sample bias pulses, V ) +2.0 V, I ) 1.0 nA. (c) The H atoms were
redeposited on the surface by a negative sample bias pulse (-2.70 V,
1.0 nA) at the location marked ‘x’. (d) H-atom ‘2’ was picked up again,
using a positive sample bias pulse (+2.0 V, 1.0 nA). (e) The H-atom
was redeposited at the left upper corner, thus forming a staggered line
of OHbr species on adjacent bridging oxygen rows. Imaging parameters:
V ) +1.2 V, I ) 0.3 nA. (f) Schematic of the final arrangement of the
OHbr species, following STM manipulation.
were required to achieve the redeposition onto TiO2. This finding
is consistent with the strong binding (i.e., adsorption energy)
of H on tungsten (-2.8 eV26).
The difference between the behavior reported in ref 1 showing an onset of H-desorption at a high sample bias of +2.6
V - and the finite desorption probability already at low bias
(+1.3 V) found here is quite striking and calls for an explanation. The key to understanding the different behaviors is implicit
in part b of Figure 3. The previous work on STM stimulated
H-desorption at room-temperature employed small tunneling
currents (∼0.3 nA) and either short (300 ms) voltage pulses or
scans at elevated bias. The enhanced stability of the present
experiments carried out at cryogenic temperatures allowed the
application of much higher currents for longer times. As shown
in part b of Figure 3, the H desorption yield depends on both
current and voltage. A combination of low bias and small current
implies a low yield, that is, longer exposures would be required
to trigger desorption than were employed in the previous roomtemperature experiments. Because of the rise in yield with
increasing voltage, H-desorption can occur even at low current
and short exposure time if a sufficiently large bias is applied.
This explains the high apparent onset voltage seen in the
previous work. At the higher electron doses feasible in cryogenic
STM, however, H-desorption proceeds without a clear threshold
already at low voltages.
Rutile TiO2(110) has been used extensively as a transition
metal oxide model system for studying surface chemistry and
photocatalysis, primarily by STM. Our finding that H atoms
can be removed from Obr sites on this surface at substantially
lower bias voltages than reported previously has important
implications for STM studies of reactions on TiO2(110), and
possibly other transition metal oxide surfaces. We find that the
moderate (V, I) conditions causing STM induced H desorption
Acharya et al.
Figure 6. Controlled OHbr removal by by atom-by-atom bias pulses
and by elevated-bias scanning. (a)-(c) Different stages of atom-byatom removal of H from OHbr, using voltage pulses of (V ) 1.7 V, I
) 1.0 nA). (d) Empty state STM image of TiO2(110) with a high
coverage of bridging hydroxyls. (e) Same field of view after H atom
desorption by a high-bias scan (V ) 2.0 V, I ) 0.61 nA) in the region
indicated by dashed square. Imaging parameters for all panels: T ) 77
K, V ) +1.27 V, I ) 0.61 nA.
leave other important adsorbates (e.g., H2O) and surface defects
(e.g., VO,Br) largely unaffected. Hence our findings demonstrate
a means for distinguishing OHbr from other adsorbates with
similar STM contrast, and for selectively controlling the
population of hydroxyls on the surface. Such control over the
hydroxyl population by STM manipulation can be performed
by individual voltage pulses on single OHbr or on several OHbr
chosen from a larger array (parts a-c of Figure 6). Alternatively,
scanning at elevated bias (which can also be performed reliably
at room temperature) can be used to remove all H atoms from
Obr sites in larger sample areas (parts d and e of Figure 6). The
desorption mechanism in both single pulses or in scanning at
elevated bias is H transfer to the tip, as confirmed by the
controlled redeposition of H atoms following the removal of a
large number of atoms from the surface. Whereas the arrangement of multiple H-atoms on the tip is unknown, our observations suggest that they reside close to the tip apex and can thus
be transferred back to the sample. The desorption of many H
atoms in high-bias scans on hydroxylated TiO2(110) raises
interesting questions on the placement and storage of large
amounts of H on the W probe tip. Addressing these questions
will require additional work on the controlled transfer of H
between the tip and sample at cryogenic temperatures.
Conclusions
In conclusion, we have addressed long-standing questions on
the mechanism of hydrogen atom desorption from bridging
hydroxyls on rutile TiO2(110) by using atomically precise charge
injection from the probe tip in cryogenic STM. Systematic
measurements of the yield of the tunneling electron induced
desorption of individual H atoms at 77 K show that desorption
occurs already at much lower voltages than reported previously.1
There is no well-defined threshold voltage for this process, but
the desorption yield scales strongly with both tunneling bias
and current. The desorption mechanism is identified as an atom
transfer to the STM tip, triggered by excitation of the O-H
stretch vibration by inelastic tunneling of one or two electrons.
Hydroxylated Rutile TiO2 (110)
The highly efficient removal of H atoms from the surface at
moderate conditions can be used as a tool to identify OHbr on
TiO2(110), to distinguish hydroxyls from other adsorbates and
defects on this surface. Our findings also open new avenues
for controlling the population of OHbr species on hydroxylated
TiO2(110), by selectively removing individual or groups of H
atoms, and by redepositing H atoms with atomic-scale precision
from the STM tip onto the surface. This capability adds an
important new tool to the study of metal oxide surface chemistry
by STM.
Acknowledgment. Work performed under the auspices of
the U.S. Department of Energy under contract No. DE-AC0298CH1-886. Supported by the Office of Basic Energy Sciences,
Chemical Imaging Initiative, FWP CO-023.
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