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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
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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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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 (
