Tip-Induced Control of Charge and Molecular Bonding of Oxygen

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Tip-Induced Control of Charge and Molecular Bonding of Oxygen Atoms on the Rutile TiO2 (110) Surface with Atomic Force Microscopy Yuuki Adachi,† Huan Fei Wen,† Quanzhen Zhang,† Masato Miyazaki,† Yasuhiro Sugawara,† Hongqian Sang,‡,§ Ján Brndiar,∥ Lev Kantorovich,‡ Ivan Š tich,∥,⊥ and Yan Jun Li*,† Downloaded via UNIV OF SOUTHERN INDIANA on July 24, 2019 at 05:43:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Applied Physics, Osaka University, Yamada-oka 2-1, Suita 565-0871, Japan Department of Physics, School of Natural and Mathematical Sciences, King’s College London, The Strand, London, WC2R 2LS, United Kingdom § Institute for Interdisciplinary Research, Jianghan University, Wuhan 430056, China ∥ CCMS, Institute of Physics, Slovak Academy of Sciences, 84511 Bratislava, Slovakia ⊥ Department of Natural Sciences, University of Ss. Cyril and Methodius, 91701 Trnava, Slovakia ‡

S Supporting Information *

ABSTRACT: We study a low-temperature on-surface reversible chemical reaction of oxygen atoms to molecules in ultrahigh vacuum on the semiconducting rutile TiO2(110)-(1 × 1) surface. The reaction is activated by charge transfer from two sources, natural surface/subsurface polarons and experimental Kelvin probe force spectroscopy as a tool for electronic charge manipulation with single electron precision. We demonstrate a complete control over the oxygen species not attainable previously, allowing us to deliberately discriminate in favor of charge or bond manipulation, using either direct charge injection/removal through the tip-oxygen adatom junction or indirectly via polarons. Comparing our ab initio calculations with experiment, we speculate that we may have also manipulated the spin on the oxygens, allowing us to deal with the singlet/triplet complexities associated with the oxygen molecule formation. We show that the manipulation outcome is fully governed by three experimental parameters, vertical and lateral tip positions and the bias voltage. KEYWORDS: bond manipulation, charge manipulation, spin manipulation, noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, oxygen adatom and molecule, rutile TiO2(110)-(1 × 1) surface surfaces,13 image and manipulate atoms14,15 and molecules,16 and detect their charge state.17,18 Kelvin probe force spectroscopy (KPFS), due to its charge control with single electron precision,12,19−23 allows for a precise transition to a different charge state of the on-surface species signaled by the appearance of a jump in the frequency shift Δf vs voltage parabola. Indeed, KPFS was recently used to activate the O2 molecules on anatase TiO2 substrate12 or trigger an on-surface chemical reaction by supplying one-electron quanta to a molecule on an insulating substrate.21 Here we study oxygen atomic and molecular species on a rutile TiO2 surface and show that AFM/KPFM methods can provide results deemed previously unattainable, such as, for instance, make two oxygen atoms bind to a molecule by depriving them of two unlike-spin

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ormation and activation of molecular oxygen is one of the most important chemical processes of immense interest and practical importance.1,2 Activation of molecular oxygen proceeds by electron transfer and conversion of nonreactive neutral atomic and molecular species in triplet states to reactive singlets. The sensitiveness of a chemical reaction to spin would lead to an additional manipulation channel as the reaction product will not only depend on the total charge but may also depend on the spin of the electrons entering the chemical reaction. In addition, when this reaction happens on substrates, the latter usually also plays a role.1 More specifically, molecular oxygen species on the rutile TiO2 surface3 were investigated using various experimental techniques,4−11 including scanning tunneling microscopy (STM).5,6 STM has a difficulty in keeping atomic charges due to the flowing current,6,12 therefore it cannot be used to activate oxygen species on a semiconducting rutile TiO2 surface3 in a controlled way. Atomic force microscopy (AFM), as a viable alternative, has been used to provide atomic-scale images of © 2019 American Chemical Society

Received: March 6, 2019 Accepted: June 10, 2019 Published: June 10, 2019 6917

DOI: 10.1021/acsnano.9b01792 ACS Nano 2019, 13, 6917−6924

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Cite This: ACS Nano 2019, 13, 6917−6924

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Figure 1. (a) AFM image of the rutile TiO2 (110)-(1 × 1) surface after exposure to oxygen gas. O and Ti atom rows on TiO2 (110) are observed, respectively, as bright and dark by a tip in the neutral mode.26 Negative adatom pairs Oad2−−Oad2− and Oad−−Oad− are observed on Ti rows. Imaging parameters: constant Δf mode, Vbias = 0 V, 4.5 × 4.5 nm2; scale bar in (a) corresponds to 0.5 nm. Inset: AFM line profiles measured above the four types of adsorbed oxygen species as indicated by different colors. (b) DFT calculated total energies, relative to Oad2−−Oad2−, of various double oxygen adatom species as the function of their total charge (in terms of the electron charge, e): Oad2−−Oad2− (4e), Oad2−−Oad− (3e), O23− (3e), Oad−−Oad− (2e), peroxide molecule O22− (2e), and superoxide molecular ion O2− (1e). The corresponding spin multiplicity (singlet (S), doublet (D), and triplet (T)) is shown next to each point. Note that Oad−−Oad− is energetically much less stable than the molecule O22− and that Oad−−Oad− in triplet and singlet states differ by only about 20 meV in our DFT calculations. The lower energy point for Oad2−−Oad2− corresponds to two oxygen adatoms Oad2− separated by two lattice distances.

Charge and Bond Manipulation of Two Oxygen Atoms using KPFS. The most important KPFS manipulations of charge, bond, and the one tentatively assigned to spin are summarized in Figure 2a−i, focusing on the most stable Oad2−−Oad2− species. By placing the tip above the oxygen species and swiping the bias voltage, electrons are injected (removed) to (from) the tip,9,12,22 and three outcomes of such charge manipulation are observed: (A) dissociation/association transition between Oad2−−Oad2− and the oxygen peroxide ion O22− via singlet-to-singlet charge injection/removal (Figure 2a−c); (B) presumably singlet-to-triplet charge transition Oad2−−Oad2− → Oad−−Oad− not accompanied by the formation of the molecule upon electrons removal (Figure 2d−f); and (C) singlet-to-doublet recharging of only one oxygen adatom of the pair (Figure 2g−i) by forming the species Oad2−−Oad−. Characterization of the species before and after manipulation is based on DFT optimized structures (Figure 2j−m) and their AFM line scans (Figure 2c,f,i). These outcomes, discussed below, are controlled by placing the tip appropriately in KPFS experiments laterally above the oxygen pair, which suggests that the precise tip positioning allows to discriminate in favor of manipulating bond, charge, and also determines the spin multiplet of the product. DFT calculated total energies presented in Figure 1b suggest different types of electron tunneling between the tip and oxygens when Δf jumps: (i) quantitatively in terms of the number of tunneling electrons (one in C and two in A/B) and (ii) qualitatively, as two electrons tunnel either in parallel as unlike-spins (A) or sequentially as like-spins (B). Reversible Bond Formation between Two Oxygen Atoms via Charge Manipulation. Next, we demonstrate that the outcome (A) (Figure 2a−c) can be achieved reversibly in both directions. Here the bond formation of O22− from two adatoms in Oad2−−Oad2− is either initiated or, reversely, the pair Oad2−−Oad2− turns into the peroxide O22−, depending, respectively, on whether two electrons are removed or injected. The initial charge state Oad2−−Oad2− shown in Figure 3a was confirmed by AFM imaging before the manipulation (see Figure S4). After AFM imaging (Figure 3a), the tip was

electrons or render them long-lived and nonreactive by presumably removing two like-spin electrons.

RESULTS AND DISCUSSION Oxygen Species on Rutile TiO2 (110) Surface. Rutile TiO2 (110)-(1 × 1) surface contains two-fold-coordinated protruding O atoms and five-fold-coordinated Ti atoms that are alternatively aligned,3 see also Supporting Information (SI), sections S1−S3 and S9. In practice, the sample preparation in UHV leads to creation of point defects that produce excess electrons, which are localized as polarons24,25 and provide a natural source of electronic charge. Figure 1a shows an atomically resolved AFM image of this surface exposed at room temperature to oxygen, which is known to dissociate and adsorb as adatoms.7−9 Different types of stable bright features above the Ti rows, most importantly the nearest oxygen pairs Oad2−−Oad2− and less frequent Oad−−Oad−, are seen9 in Figure 1a, distinguished by their line scans (inset). Note that oxygen adatoms are spontaneously negatively charged due to a charge transfer from surface polarons,24 with both Oad2− atoms appearing as big bright spots in AFM images and at the ground state as confirmed by density functional theory (DFT) calculations (Figure 1b). The much rarer appearance of Oad−−Oad− is in line with the computed relative energies of Oad2−−Oad2− and Oad−−Oad− (Figure 1b), which favor the former by 2.66 eV. Presence of the high-energy Oad−−Oad− states can only be explained by local shortage of the polaronic defects, which would otherwise charge them to the energetically more favorable Oad2−−Oad2− states, and their triplet nature, which protects them against spontaneous transformation to the singlet state. More distant oxygen adatoms are also observed (see Figure S3). We also believe that asymmetrically charged Oad2−−Oad− species9 may also be present on the surface, although we have not seen them. Note that the Oad2−−Oad2− (singlet), Oad2−−Oad− (doublet), and Oad−−Oad− (triplet) are different spin multiplets, and their charge state can clearly be distinguished by their contrast in the AFM image. 6918


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Figure 3. (a−e) Consecutive AFM images of two kinds of oxygen adsorption species in the reversible manipulation sequence. Imaging parameters: constant Δf mode, Vbias = 0 V, 1.25 × 2.55 nm2, scale bar in (e) corresponds to 0.3 nm. (f−i) The results of KPFS applied between images (a and b), (b and c), (c and d) as well as (d and e).The feedback loop was switched off during the measurements, and the sample bias was ramped from zero to a certain negative (f, h) or positive (g, i) voltage and ramped back to zero. The light blue lines show forward and pink lines backward KPFS spectra. The tip position is indicated by a circle inside images (a−d). The insets in (f−i) show enlarged KPFS curves within dashed boxes placed around the regions of frequency jumps: between −2.5 V and −2.68 V (f), 0.55 and 0.8 V (g), −2.65 V and −2.4 V (h), and 0.9 and 1.1 V (i).

Figure 2. (a−c) AFM images before (a) and after (b) KPFS manipulation of Oad2−−Oad2− with the tip positioned symmetrically in the middle and the corresponding line profiles (c). The double bright spot became a single one, indicating formation of an oxygen molecular species, O22−. (d−f) AFM images before (d) and after (e) KPFS manipulation of Oad2−−Oad2− with the tip positioned slightly off the middle position between the two atoms and the corresponding line profiles (f). (g−i) AFM images before (g) and after (h) KPFS manipulation of Oad2−−Oad2− with the tip positioned above one oxygen atom and the corresponding line profiles (i), indicating a change in the charge state from Oad2− to Oad− of only the oxygen atom that is under the tip. (j−m) DFT optimized structures are shown in each case: (j) Oad2−−Oad2−; (k) Oad2−−Oad−; (l) Oad−−Oad−; and (m) O22− adsorbed at the on-top Ti5c site. Note that there is also another adsorption site, the bridge site, see SI, section S9. Dashed horizontal lines depict the differences in heights of the oxygen adatoms. (A), (B), and (C) denote the three key manipulation scenarios, see text.

species, which is Oad2−−Oad2−. Hence, a complete association− dissociation cycle was successfully achieved: Oad2−−Oad2−→ O22−→ Oad2−−Oad2−. In both elementary charge manipulation events, two electrons tunnel between the tip and surface causing a single Δf jump in each case. Note that here only a single Δf jump was observed because the two electrons are removed from two oxygen atoms (see also SI, section S10)20 within a time that is too short to be resolved experimentally. The performed manipulation cycle can be repeated as shown in the succession Figure 3c → 3h → 3d → 3i → 3e. Note that the bias voltages where the Δf jumps occurred vary, influenced primarily by the tip−surface distance. These experimental results, fully supported by our DFT calculations (SI, section S11), strongly suggest that the consecutive transformation of the observed contrast is due to reversible bond formation between two oxygen adatoms. Breaking the Bond in O22− Remotely via Tip-Induced Surface Polaron. In Figure 4a → 4f → 4b → 4g → 4c, we show the same reversible Oad2−−Oad2−→ O22−→ Oad2−−Oad2− manipulation cycle from Figure 3, but this time we supply the charge to the O22− molecule remotely via surface polarons with the tip positioned on a titanium row far away from the manipulated molecular species. The initial charge state (Oad2−−Oad2−) shown in Figure 4a was confirmed by AFM imaging before the manipulation (see Figure S5). The KPFS measurement shown in Figure S6 indicates that O22− is intrinsically less negatively charged than the Oad2−−Oad2−

brought above the middle of Oad2−−Oad2−, and the bias was ramped from zero in the negative direction until the steep jump in Δf appeared and then back to zero. Figure 3b is the AFM image performed at 0 V of the same scan area immediately after that, showing that the double bright spots attributed to Oad2−−Oad2− changed into a single bright spot associated with O22−. Then the tip was brought above the O22− spot, and the bias was ramped from zero toward the positive direction until a steep Δf jump, indicating that electrons tunnel from the tip back to the oxygen species. After that, the bias was ramped back to zero and an AFM image was taken (Figure 3c). It shows that the O22− species changed back into a double bright spot identical to the Oad2−−Oad2− we had started from. DFT calculations support this assignment (Figure 1b): After two extra electrons are removed from Oad2−−Oad2−, the most energetically favorable O22− species with two extra electrons is formed, while when these electrons are injected back into O22−, it dissociates into the most favorable four extra electrons 6919


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Figure 4. (a−e) Consecutive AFM images of two types of oxygen adsorption species (in the dashed box) performed before (after) each manipulation event. Imaging parameters: constant Δf mode, Vbias = 0 V, 1.5 × 8.0 nm2, scale bar in (e) corresponds to 0.5 nm. (f−i) KPFS spectra obtained between images (a and b), (b and c), (c and d) as well as (d and e). The feedback loop was switched off during the KPFS measurements, and the sample bias was ramped from 0 V to a certain negative (positive) voltage and then back to 0 V. The light blue lines show forward and the pink the backward spectra. The insets in (f and h) (between −3.0 V and −2.6 V) show enlarged KPFS curves within dashed boxes placed around the regions of frequency jumps. The light green lines in (g and i) indicate that no jump in frequency shift was observed during the voltage ramp. The tip position is indicated by a circle inside images (a−d). (j−m) Shematic images of removing electron using tip (j, l) and attaching electron via surface polarons (k, m).

the tip was placed just slightly away from the middle position between the two oxygen adatoms. As is discussed in Figure S8, we observed a double jump in Δf when the bias is swiped into a negative value and back, yielding a spin triplet Oad−−Oad−. In this process, two electrons tunneled to the tip consecutively, without the molecule O22− being formed. We presume this process might be reversible. Consider now case (C) (Figure 2g−i) corresponding to the tip positioned directly above one of the two oxygen adatoms. It is seen, after the bias was swiped in the negative direction, that one of the spots attributed to the oxygen adatom directly underneath the tip has been greatly reduced, indicating a change in its charge and spin state from Oad2− (closed-shell singlet) to Oad− (open-shell doublet, Figure 2g → 2h), yielding the asymmetric species Oad2−−Oad− as supported by the line scan (Figure 2i). No bonding between the two atoms is observed, and the second oxygen atom remained doubly charged; this is due to a much longer distance between it and the tip greatly reducing the tunneling probability. Hence, only a single electron tunneling to the tip happens. Swiping the bias into positive voltages recharges the Oad2−−Oad− back into its original Oad2−−Oad2− state (see Figure S7). Exploration of the Manipulation Parameter Space. The described experiments correspond to “large” tip heights when the cycle hysteresis loop covers negative and positive voltages (Figure 3). When the tip height is reduced, the center position of the KPFS loop moves toward negative voltages and its width narrows (see Figure 5a,d). Hence, we can distinguish two more tip height regimes: intermediate, when the loop is contained completely within negative voltages (yellow and green curves), and small (blue), when the loop is either very narrow or disappears completely.27,28 Noting an extremely narrow window of the tip heights at which successful manipulation is observed and that the closest approach (blue

supporting the observed sequence. Compared to Figure 3, this manipulation sequence was apparently performed at a larger tip−sample distance and possibly with a blunter tip as the individual atoms in the Oad2−−Oad2− pair cannot be resolved. In that respect, the nearby oxygen atom adsorbed on the neighboring Ti row can be used to identify indirectly the manipulated species as being indeed the Oad2−−Oad2− pair in Figure 4a,c and an O22− peroxide molecule adsorbed in the ontop Ti position in Figure 4b. It is also apparent that the charge state of the nearby oxygen atom changes in the course of the manipulation. We speculate that this can be due to one electron being transferred into this atom rather than into the tip. Interestingly, when we repeated this cycle of charge manipulation, a slightly different sequence of states was observed as shown in Figure 4c → 4h → 4d → 4i → 4e. It is seen in Figure 4e that the pair of adatoms Oad2−−Oad2− separated by two lattice constants was found after the positive direction bias ramp. Note that this configuration is known to be energetically more stable than the nearest-neighbor oxygen adatom pair6 (Figure 1b). Notably, we did not observe this configuration more frequently than the nearest-neighbor two oxygen adatoms configuration during our reversible bond formation experiments, and we speculate that this is due to the fact that there is an energy barrier for one oxygen adatom to diffuse out of the other in the nearest-neighbor pair. Even though there is another species present next to the oxygen pair, importantly, this manipulation still directly proves that the observed processes are entirely due to tunneling electrons and do not correspond to any atomic (e.g., vertical) manipulation.15 Hence, we have demonstrated that a surface polaron can be controlled via the KPFS tip and that electron can be supplied to the O22− also remotely via the titania substrate. Charge Manipulation: Discharging and Recharging the Oxygen Pair. In the intermediate case (B) (Figure 2d−f), 6920


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Figure 5. (a−c) The tip is positioned in the middle between two oxygen atoms. (d−f) The tip is positioned above one oxygen atom. (a and d) KPFS spectra taken at different tip heights, shown relative to the spectrum in blue, that corresponds to the closest tip heights considered. (b and e) KPFS spectra taken at intermediate heights. (c and f) Δf time evolution taken at specific voltages, as indicated by dashed colored lines in (b and e), respectively. Note that the tip−sample distances in panels (a−c) and (d−f) are different so that the frequency shifts cannot be directly compared. See also Figure S9 for KPFS spectra with the tip positioned in the half distance between the middle of the two oxygen atoms and the on-top adatom position.

reorganization energy Ereorg, which can be measured by KPFS. From Figure 5a,d, it follows that Ereorg decreases with the tip−sample distance and vanishes in the near-contact regime. This in turn means that in this regime, the geometries, that is, essentially the heights of the Oad2−/Oad− or 2Oad2−/ O22− species above the surface, must be equal (see Figure 2j− m). Similarly to the symmetric tip position (Figure 5a−c), in the case of an asymmetric tip position, the location and width of the hysteresis loops are too very sensitive to the tip height (Figure 5d). Interestingly, at very small tip−sample distances and large negative voltages, the O22− is observed. This is due to tunneling probabilities to both atoms becoming significant at very small heights. Finally, we also observe telegraph-like oscillations in Δf at the voltages that fall within the hysteresis loop window (Figure 5e,f), although here the upper state may correspond to either Oad2−−Oad− or O22−, the latter being more likely at smaller tip−sample distances. Potential Energy Surfaces Diagram. The whole association−dissociation cycle that accompanies the charge

curves) must correspond to a near-contact regime, we presume that in all cases considered, the tip is within 2−3 Å from the oxygen species and hence plays chemically crucial role in the processes (see Figure 5a,d and SI section S11B, especially Figure S13, for more details). Now we discuss intermediate distances for case (A) with the tip being above the center of the oxygen atom pair. In Figure 5c, the time evolution of Δf is shown, which was taken at some intermediate height at several values of the bias voltage (Figure 5b). If at small (pink line) and large (light blue) negative voltages Δf remains constant, corresponding, respectively, either to Oad2−−Oad2− or O22−, at an intermediate voltage chosen within the hysteresis loop (light green line), Δf jumps between these two values and exhibits telegraph-like oscillations.20,21 This indicates that the electron tunneling between the tip and the oxygens happens in both directions all the time the system enters the electronic coupling regime, resulting in association−dissociation processes following one another. At large tip−sample distances, the net energy change for the oxidation and reduction of the oxygen species, eVjump− and eVjump+, is related21 to a 6921


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Figure 6. (a) PES of the pair of oxygen atoms in different charge states with two (blue, yellow), three (green), and four (red) extra electrons. The blue PES with two extra electrons was calculated, while the other ones are sketches based on the calculated total energies (Figure 1b) in their minima. The color-coding follows that of Figure 1b and corresponds to the tip position as indicated in the upper left inset. (b) Schematic of the manipulation parameter space showing the areas of vertical and lateral tip positions at which charge and bond manipulations are observed. A region of charge oscillations is also shown (yellow dashed box).

zero-spin molecular state, O22−. This hypothesis is corroborated by the fact that stable Oad−−Oad− species are indeed observed on the surface and that they would only be stable if they are in the triplet spin state. Note that the calculated triplet pair Oad−−Oad− is only by 20 meV more favorable than the singlet one (see also Figure 1b). Finally, the reversible singleelectron transition corresponding to the tip positioned above an oxygen atom (case (C)) is shown by the dashed light blue line. Here only one electron tunnels from the atom under the tip, as the other O atom is much further away. Figure 6b reveals schematically the entire manipulation parameter space probed in our experiments. We see that for charge and bond manipulation events, the tip must be positioned closer to the middle point between two oxygens, otherwise only charge/spin manipulation occurs. Also, we indicated the region of intermediate heights when the reactant-product oscillations are observed.

manipulation for not too short tip−sample distances and the symmetric position of the tip (case (A)) is pictured in Figure 6a. The red curve corresponds to the potential energy surface (PES) of Oad2−−Oad2− with four extra electrons. The ground state of Oad2−−Oad2− corresponds to the pair of nearest doubly charged oxygen atoms in the singlet spin state at a distance of around 3 Å. When the bias voltage is ramped in the negative direction, within the resolution of the microscope,20 two electrons tunnel to the tip at the same time. An intermediate state that is formed here must be a discharged pair of atoms Oad−−Oad− in the singlet spin state, at practically the same distance. This state, which must be facilitated by the close proximity of the tip (see SI section S11B for more details) corresponds to a shallow minimum on the blue PES. The molecular peroxide state, O22−, with two oxygen adatoms separated by only 1.4 Å, also in the spin singlet state, is the blue PES global minimum lying much lower. As long as the metastable state is reached after the electron transition, a small calculated energy barrier of 0.16 eV is overcome, and the two oxygen adatoms form the peroxide molecule. Similarly, when the bias is ramped in the opposite direction, two electrons are injected back into the molecule, and the system jumps onto the red PES. As the oxygen molecule with four extra electrons is highly unstable, the system immediately goes into the ground state Oad2−−Oad2−, thereby completing the cycle. The other excitations shown in Figure 6a correspond to the various charge/spin manipulations occurring with other lateral tip positions. When the tip is positioned slightly off the middle distance (case (B)) between the two oxygen atoms (and hence the tunnelling rates that exponentially depend on the distance between the O atoms and the tip are different), two sequential one-electron transitions occur. After the first electron from the nearest oxygen tunnels to the tip, an asymmetric Oad2−−Oad− species is formed (shown in green). After the second electron tunnels from the second oxygen (this requires applying a higher negative bias), the formation of the molecule does not happen (Figure 2d,e). This may be explained by the fact that in this complicated process, a triplet spin state, Oad−−Oad−, must be formed (yellow) that is spin protected from forming the

CONCLUSIONS By combining AFM/KPFS capabilities, we fully explored the parameter space for bond and charge manipulation performed on a pair of nearest oxygen atoms on the rutile TiO2 (110)-(1 × 1) surface. Electrons from two very different sources have been exploited and experimentally controlled: electrons initially trapped in polarons and electrons tunneling with single-electron precision through the tip-oxygen adatom junction via KPFS manipulation. Depending on the precise lateral and vertical position of the tip and the applied bias voltage, we performed controlled reversible bond formation and rupture between two oxygen adatoms via the KPFS charging−recharging process. The suggested spin manipulation, which accompanies the charge manipulation, renders the oxygen species either reactive (when in the spin singlet state) or chemically inactive (triplet state).12 Depending on the precise tip location, the electrons can be made to tunnel either as a single electron or as an electron pair; the latter may happen either sequentially or simultaneously. In the twoelectron tunnelling case, the tunnelling electrons could be 6922


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ASSOCIATED CONTENT

either of the same or opposite spins. These different charge manipulation scenarios, cases (A), (B), and (C), were all induced by positioning the tip relative to the oxygen atom(s) and thereby modulating the electron tunnelling probabilities. By KPFS charge manipulation, we have been able to modify the charge on the oxygen species in a systematic manner from 4e to 2e (Oad2−−Oad2−, Oad2−−Oad−, Oad−−Oad−, O22−). A remarkable experimental feature was introduced by controlling remotely the oxygen species with the tip positioned far away from the manipulated species, where the charge is injected into the manipulated species indirectly via a surface polaron. We expect that similar procedures could be applied to other species and possibly other surfaces, where controlled chemical reactions initiated by charge manipulation could be performed.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b01792. Details of identification of the tip mode; the initial charge state of oxygen adatom shown Figures 3 and 4; KPFS measurements over atomic and molecular species; demonstration of a discharging−recharging cycle performed on a single oxygen adatom in a pair; demonstration of a two-step discharging performed on a double oxygen pair; the tip positioned in the half distance between the middle of the two oxygen atoms and on-top adatom position; peroxide and superoxide molecule adsorption sites; number of electrons tunneling between the tip and surface and the final oxygen species in the discharging manipulation experiment; details of DFT simulations. (PDF)

METHODS Experimental Details. The experiments were carried out using a low-temperature ultrahigh-vacuum (UHV) AFM system. The deflection of the cantilever was measured using the optical beam deflection method. The base pressure was lower than 5.0 × 10−11 Torr. The temperature of the AFM unit was kept at liquid nitrogen temperature (78 K). The AFM/KPFS measurements were performed in the frequency modulation (FM) detection mode. The cantilever was oscillated at resonance frequency, keeping the oscillation amplitude constant. We used an iridium (Ir)-coated Si cantilever (Nano sensors SD-T10L100, f 0 = 800 kHz, A = 500 pm). Metal Ir tips provide stable AFM imaging compared to the bare Si tip. The tip was initially annealed to 600 K and then cleaned by Ar+ ion sputtering to remove the contamination before experiments. Pure metallic Ircoated tip corresponds to the neutral (not charged) mode, see SI section S2 and Figure S2 for more details. The rutile TiO2 (110) 1 × 1 sample was prepared by sputtering and annealing to 900 K in several cycles. The sample was exposed to oxygen at room temperature for ∼0.5 L and then transferred to the measurement chamber precooled to 78 K. The atom tracking method was used to compensate the thermal drift between the tip and surface during the measurements.29 The dc bias voltage was applied to the sample to compensate the contact potential difference (CPD) between the tip and surface, hence the CPD reflects the work function variation. The AFM imaging was performed using constant Δf mode at Vbias= 0 V to avoid the tunneling current to flow. DFT Calculations. To model the possible ground states of various oxygen species, we considered a six-layer stoichiometric TiO2 rutile slab in periodic boundary conditions with the upper three layers of atoms allowed to relax. Calculations of the energies and forces were performed using DFT with projector augmented-wave pseudopotentials30 along with hybrid31 exchange−correlation functional and 400 eV plane wave cutoff as implemented in the VASP code.32 Atomic forces were converged to better than 0.01 eV/Å. The diffusion barrier for the oxygen species was calculated using the nudged elastic band technique.33 Four polarons in the slab were created by placing four hydrogen atoms on its surface (also see SI section S11A, where another method that gives equivalent results is also considered). After that, two oxygen atoms or an oxygen molecule were placed on the surface and the whole system relaxed. By carefully preparing initial geometries (prior to geometry relaxation), one can make the required number of polarons to get localized on the oxygen species upon geometry relaxation. In this way, all the required oxygen species, Oad2−−Oad2−, Oad2−−Oad−, Oad−−Oad−, and O22− and Oad− could be prepared. Since all the described systems contain the same number of atoms and electrons, their energies are strictly comparable. In general, the model used is similar to that used previously to describe manipulation of charge on oxygen atom adsorbed on rutile TiO2.9 Further details can be found in SI section S11.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Yuuki Adachi: 0000-0002-9723-1520 Huan Fei Wen: 0000-0002-2972-9669 Lev Kantorovich: 0000-0001-9379-6834 Ivan Š tich: 0000-0003-3338-8737 Yan Jun Li: 0000-0001-7845-326X Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (JSPS) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) by MEXT/JSPS KAKENHI grant numbers (JP16H06327, JP16H06504, and JP17H01061) and Osaka University International Joint Research Promotion Program (J171013014, J171013007, J181013006). Via our membership of the UK’s HEC Materials Chemistry Consortium, this work used the ARCHER UK National Supercomputing Service funded by EPSRC (EP/L000202) as well as the UK Materials and Molecular Modelling Hub, THOMAS, which is partially funded by EPSRC (EP/P020194). This work was also financed by the APVV-0759-15 and VEGA 2/0123/ 18 projects. We also gratefully acknowledge use of the MASAMUNE-IMR supercomputer system at CCMS/IMR, Tohoku University, Japan. H. S. would like to thank the support from the National Natural Science Foundation of China (grant no. 21603086) and China Scholarship Council (grant no. 201608420186). REFERENCES (1) Schweitzer, C.; Schmidt, R. Physical Mechanisms of Generation and Deactivation of Singlet Oxygen. Chem. Rev. 2003, 103, 1685− 1757. (2) Bugg, T. D. Dioxygenase Enzymes: Catalytic Mechanisms and Chemical Models. Tetrahedron 2003, 59, 7075−7101. (3) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (4) Matthiesen, J.; Wendt, S.; Hansen, J. Ø.; Madsen, G. K.; Lira, E.; Galliker, P.; Vestergaard, E. K.; Schaub, R.; Lægsgaard, E.; Hammer, B.; Besenbacher, F. Observation of All the Intermediate Steps of a 6923


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