Visualizing Elementary Reactions of Methanol by Electrons and Holes

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Visualizing Elementary Reactions of Methanol by Electrons and Holes on TiO2(110) Surface Shijing Tan, Hao Feng, Yongfei Ji, Qijing Zheng, Yongliang Shi, Jin Zhao, Aidi Zhao, Jinlong Yang, Yi Luo, Bing Wang, and Jian Guo Hou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09784 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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The Journal of Physical Chemistry

Visualizing Elementary Reactions of Methanol by Electrons and Holes on TiO2(110) Surface Shijing Tan,†,‡,|| Hao Feng,†,|| Yongfei Ji,‡,|| Qijing Zheng,† Yongliang Shi,† Jin Zhao,† Aidi Zhao,† Jinlong Yang,† Yi Luo,†,‡* Bing Wang,†* J.G. Hou†*

† Hefei

National Research Center for Physical Sciences at the Microscale and Synergetic Innovation

Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ Department

of Theoretical Chemistry and Biology, School of Engineering Sciences in Chemistry,

Biotechnology and Health, Royal Institute of Technology, Stockholm S-10691, Sweden

||These

authors contributed equally.

*Corresponding authors: [email protected], [email protected], [email protected]

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ABSTRACT: Direct visualization and comparison of the elementary reactions induced by electrons and holes are of importance for finding a way to conduct chemical reactions and reaction sequences in a controllable manner. As a semiconductor, TiO2 provides a playground to perform the measurements, and more, the information can be useful for design of high performance TiO2-based catalysts and photocatalysts. Here, we present our investigation on the elementary reactions of CH3OH on TiO2 surface through visualization of specific elementary steps by highly controllable electron- and holeinjection using scanning tunneling microscopy. The distinct sequential routes and their kinetics, namely breaking C-O and O-H bonds by electrons and breaking O-H and C-H bonds by holes, respectively, have been experimentally identified and well elucidated by density functional theory calculations. Our nonlocal h-injection experimental and theoretical results suggest that the delocalized holes in the TiO2 substrate should be responsible for the temperature-dependent h-route reactions. The locally triggered e-route reaction is associated with the fact that the location of the unoccupied hybridization states is much higher than the conduction band onset. Our findings resolve the long standing debate about the intermediate species and reaction mechanism in photocatalytic oxidation of CH3OH. Our proposed protocol offers a powerful means to study elementary reactions induced by electrons and holes on semiconductor surface in general.

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INTRODUCTION The elementary reactions induced by electrons or holes act as the key and initial steps that control the entire reaction sequences in photocatalytic and electrochemical processes.1,2 A good understanding of these elementary reactions has important implication on the design of high performance catalysts and photocatalysts. One of the good examples is TiO2-based catalysts and photocatalysts that hold promise for decades.3-7 On the other hand, since alcohols are commonly used as starting materials in the chemical syntheses of value-added products,8,9 it has been highly demanded to selectively break or make chemical bonds and to conduct reaction sequences of alcohols in a controllable manner,10-13 especially by combination of photocatalysis.4-6, 14-19 To achieve this, a challenging task is to precisely determine how a specific chemical bond can be activated by electrons and holes to initiate the reaction sequences. The high spatial resolution and superior manipulation ability of scanning tunneling microscopy (STM) have made it as an important tool to visualize chemical reactions on surfaces.20-28 The studied molecules are often placed on conducting substrates, for which the reaction mechanisms can be mainly attributed to the vibrational or electronic excitations of molecules by inelastic tunneling electrons,20-26 or electron-attachment processes,27,28 or the effect of electric field.29,30 However, a big advantage of semiconductor substrate, like TiO2, has not been fully explored. In this case, the type of injected carriers can be selected by the bias polarity of the STM tip, enabling electrons into the conduction band and holes into the valence band, respectively. This unique capability will thus provide a means for us to precisely and visually study the elementary reactions induced by electrons and holes, which is demonstrated here by systematically studying a simple, yet important, polyatomic molecule, methanol (CH3OH) on rutile TiO2(110) surface. The fact that methanol contains the commonly 3



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concerned chemical bonds, such as C-O, O-H and C-H bonds, has also made the choice more generally relevant. Moreover, it might provide an answer to one of the unsettled problems concerning the role of photogenerated carriers on sequential reactions of CH3OH on TiO2 surface.31-37 By controlling various experimental conditions, such as bias voltages, currents, temperature, and local and nonlocal injection of carriers, we have unambiguously identified the distinct stepwise electron-induced C-O and O-H bond breaking, as well as hole-induced O-H and C-H bond breaking processes, revealing the specific pathways and kinetics involved. The possible intermediates and reaction mechanisms of CH3OH in oxidation by holes or reduction by electrons have also been completely mapped out. The observed reaction steps have been further verified by the density functional theory (DFT) calculations. Our measurements have indicated that the injected electrons or holes have the ability to effectively break specific chemical bonds of CH3OH.

EXPERIMENTAL SECTION Sample Preparation. The rutile TiO2(110) sample (Princeton Scientific Corp.) was prepared by repeated cycles of Ar+ ion sputtering (2000 eV) and annealing to 900 K with a Ta-foil heater behind the sample. Methanol (Aldrich, 99.99%) was purified by several freeze-pump-thaw cycles using liquid nitrogen. The methanol was deposited directly to the TiO2 surface through a dedicated tube in the chamber. The outlet of the tube was about 5 mm from the TiO2 surface. The coverage of methanol was directly determined from the STM images,33 which is about 0.03 Monolayer in this study. STM Measurement. The STM experiments were conducted with a low temperature STM (Omicron, Matrix controller) in an ultrahigh vacuum system with a base pressure less than 3  1011 Torr. The measurements were mainly performed at 80 K, and from 80-180 K for the temperature 4


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dependent measurement. An electrochemically etched polycrystalline tungsten tip was used in the experiment. The pulses over CH3OH molecules were applied with the feedback loop off, in order to monitor the current jump in the I-t curves, where in the nonlocal and scanning mode experiments the feedback loop is on to keep a constant current injection. Calculations. We performed spin-polarized GGA calculations with PBE38 exchange-correlation functional and PAW39 pseudo-potential implemented in the Vienna ab initio simulation package (VASP)40-43 with a dipole corrections. A five tri-layer of TiO2 slab with (4×2) unit cell is used with Γ point only and a plane-wave basis cutoff of 400 eV. A 15 Å thick area of vacuum is inserted between neighboring slabs. During the optimization, the lower 2 layers of the TiO2 slab is fixed while the other atoms are relaxed until the force on each atom is less than 0.02 eV/Å. The orbital energy alignment between CH3OH and TiO2 is calculated using HSE functional,44 which includes the long-range screening effect. We have considered both of the free (delocalized) and the trapped hole.5, 45 When we considered the effects of holes, we just have the center layer fixed and other atoms were allowed to relax in dealing with the possibly caused artificial surface states on the top of the valence band (VB).45 Due to the selfinteraction error,46 the hole orbital tends to delocalize in DFT calculation. To overcome this problem, we used GGA+U method47 with U(d) = 4.2 eV for Ti d states and different U(p)’s for O p states in this work to investigate the oxidation of methanol by both of the free and the trapped hole. A test calculation of the PDOS of methanol was performed with HSE hybrid functional. The peak position of the highest occupied molecule orbital (HOMO) of methanol in GGA calculation is about 0.2 eV higher than that in HSE calculation. Different U(p)’s for O p states were tested. The peak position of the HOMO descend as the U(p) increases. The agreement with the HSE calculations is greatly 5



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enhanced in GGA+U method when U(p) = 5.0 or 6.0 eV is applied. We adopted U(p) = 6.0 eV in the current calculations since it gives good accordance with the experimental results. We used an OH group to inject a hole into the system which can remain free or trapped, which avoid the introduction of the electron-hole pair.45 To trap the hole at a specific surface oxygen, that oxygen was initially shifted towards the vacuum by 0.5 Å before the spin-polarized optimization; the free hole, on the other hand, can be obtained by doing a spin-unpolarized optimization followed by a spin-polarized optimization. The transition state was calculated with the nudged elastic band method with climbing images (CNEB),48 and the site projected magnetic moments of oxygen atom in CH3OH (or CH3O) were calculated to trace the transfer of the hole during the reaction.

RESULTS AND DISCUSSION The tip bias in our experiment is set with respect to the TiO2 substrate. In this case, under the negative bias the electrons are injected into the substrate (Figure 1a), while under the positive bias the electrons are extracted from the substrate, i.e., the injection of holes (Figure 2a). The energy of injected carriers is with respect to the Fermi energy (EF) of the reduced TiO2 surface. The EF typically locates below the conduction band minimum (CBM) by about 0.4 eV according to the measured dI/dV spectra for the TiO2(110) surface in previous studies.49,50 Considering our sample preparation conditions are similar to those in previous studies, in the following discussion we adopt the location of the EF below the CBM by 0.4 eV. In our samples, the concentration of oxygen vacancies (VO’s) is about 0.08 ML (1 ML = 5.2 × 1014 cm2). At low temperatures (80-180 K) CH3OH molecules are adsorbed mainly at the five-coordinate Ti atoms (Ti5c) and only a small fraction at the VO’s (Supporting information Figure

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S1), and the CH3OH molecules at Ti5c can remain intact and immobile under the typical imaging conditions of 1.0 V and 10 pA.31-34 Reactions under the Electron Injection: E-Route Reactions. By applying a pulse at 2.6 V (Figure 1a), the CH3OH molecule “m1” was first changed to a fuzzy species (Figure 1b,c), and further changed to a smooth and less protruded species by the second pulse (Figure 1d). Such structure changes are also reflected by the sharp jumps in the current-time (I-t) curves with feedback loop off (Figure 1f,g). The fuzzy species and the smooth species can be assigned to the products of hydroxyl (OHt) and oxygen adatom (Oad) at the Ti5c sites, respectively, in comparison with the products produced in similar manipulation of H2O.51 When a higher voltage of 3.0 V was applied to the CH3OH molecule “m2”, it was directly changed to the smooth species of Oad (Figure 1d,e). In the I-t curve, two successive current jumps were also obtained, implying that the process goes through two steps (Figure 1h). The reaction routes induced by the injected electrons (e-route) can be schematically summarized in Figure 1i. The first reaction step is to remove CH3 through the breaking of C-O bond, while the second step breaks the O-H bond of the produced OHt.

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Figure 1. Electron induced CH3OH dissociation (e-route). (a) Schematic drawing of electron injection over the CH3OH adsorbed on TiO2(110)-(11) surface using the STM tip at a negative bias voltage. (b)-(e) Consecutively acquired STM images of the individually adsorbed CH3OH under different negative voltage pulses (electron-injection), two pulses of 2.6 V over “m1”, one pulse of 3.0 V over “m2”. Scale bar: 1 nm. (f)-(h) Corresponding I-t curves recorded during applying pulses over “m1” and “m2”, respectively. All images are acquired at 1.0 V and 10 pA, at 80 K. (i), Schematic drawing of the two-step dissociation of CH3OH in e-route: step I, C-O bond breaking; step II, O-H bond breaking. Reactions under the Hole Injection: H-Route Reactions. By applying a pulse at +2.7 V (Figure 2a), the CH3OH molecule “m1” was first changed to an expanded intermediate species, labeled as “int” (Figure 2b,c), and further changed to two protrusions at the neighboring Obr sites by the second pulse (Figure 2d). In another set of images, when higher voltage pulses of +3.0 V were applied to molecules “m2” and “m3”, respectively, both of them were directly changed to paired protrusions (Figure 2g,h). In the case of “m3”, there is an additional species near the paired protrusion in the product. The recorded I-t curves also indicate a two-step process by the injected holes (h-route) (Figure 2e,f,i). The images of the products are quite different from those in the e-route reactions. Instead, the features are very similar to the ones obtained in photocatalytic reactions of CH3OH under UV light irradiation.3134

In photocatalytic reactions, the expanded intermediate species have been assigned to a complex of

CH3O (CH3Ot) at the Ti5c site and hydroxyl (OHb) at the Obr site, and the two neighboring protrusions to a pair of OHb’s. Apparently the hole induced CH3OH dissociation follows the same stepwise O-H and C-H bond breaking process, which finally produces a pair of OHb species and a CH2O (Figure 2j). Note that OHb is immobile at a low temperature,51 while the product of CH2O is quite mobile and 8


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shows site-dependent adsorption configurations.52,53 Under higher voltages in h-route reactions, a fraction of CH2O may depart from the surface, which is also similar to what was observed in the photocatalytic reaction of CH3OH.32,33

Figure 2. Hole induced CH3OH dissociation (h-route). (a) Schematic drawing of hole injection using a STM tip at positive bias. (b)-(d) STM images showing the changes of “m1” by consecutively applying two +2.7 V pulses. (e)-(f) Corresponding I-t curves recorded during applying the two pulses. (g)-(i) Another set of STM images showing the changes of “m2” and “m3” by applying +3.0 V pulses and the corresponding I-t curves. All images are acquired at 1.0 V and 10 pA, at 80 K. Scale bars: 1 nm. (j) Schematic drawing of the two-step dissociation of CH3OH in h-route: step I, O-H bond breaking

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by transferring the H atom to an adjacent Obr; step II, C-H bond breaking by transferring the H atom to the adjacent Obr site at the opposite side.

Reactions under Alternate Injections of the Electron and the Hole. We find that the intermediate species in e-route and h-route reactions can undergo different processes by alternating injections of electron and hole, as shown in Figure 3. An intermediate species of OHt (Figure 3a2) produced by a negative pulse of 2.8 V can be changed to an OHb with an Oad nearby after applying a positive pulse of +3.3 V (hole injection) (Figure 3a3). In another word H atom is transferred from the OHt to the neighboring Obr site and the final product of Oad shifts to an adjacent Ti5c site, indicating that the OHt is reactive with the hole. After applying another pulse of 2.8 V over the OHb, the H atom is removed (Figure 3a4), confirming the fact that injected electrons could remove the H atom.54,55 It is interesting to note that after applying the pulse of +3.3 V (Figure 3a2), the two nearby CH3OH molecules are changed to expanded species as the results of nonlocally injected holes (labeled by “int” in Figure 3a3 and 3a4). It will be discussed below (see Figure 4).

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Figure 3. Reaction routes of CH3OH under alternate injections of electrons and holes. (a1)-(a4) A set of consecutively acquired STM images after electron injection (2.8 V) over the molecule and then hole injection (+3.3 V) over the intermediate species of OHt. Note that after applying the pulse of +3.3 V (hole injection), the other two molecules nearby are also changed to expanded species (labeled by “int”). (b1)-(b4) A set of consecutively acquired STM images after hole injection (+2.7 V), lateral manipulation using the tip (0.5 V and 500 pA), and then electron injection (2.8 V) over the molecule or the separated species of CH3O. (c1)-(c4) A set of consecutively acquired STM images after hole injection (+2.7 V), lateral manipulation using the tip (0.5 V and 500 pA), and then hole injection (+2.7 V) over the molecule or the separated species of CH3O. The green arrows in (b2) and (c2) show 11



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the move direction of the tip during manipulations. In each lower panel, the schematic models, corresponding to the marked areas in (a1), (b1), and (c1), show the configuration changes during the manipulations. Scale bars: 1 nm. All images are acquired at 1.0 V and 10 pA, at 80 K. (d) Schematic drawings, showing the remaining species in each steps after the injections of electrons and holes alternately. Figure 3b1-b4 and 3c1-c4 give two sets of images measured under alternate injections of holes and electrons. The expanded species produced by the injected holes are separated into two parts by lateral manipulations of the tip (Figure 3b2-b3 and 3c2-c3), which also confirm our assignments that the expanded species are the complex of OHb and CH3O.31,33 It can be seen that the separated CH3O undergoes quite different reaction routes with respect to the electron injection and the hole injection. The CH3O reacts with the electron to break the C-O bond and produce an Oad on the surface (Figure 3b4), but the reaction with the hole breaks the C-H bond to produce an OHb on the surface (Figure 3c4). In the former case, the CH3 group should depart from the surface as a CH3 radical, and in the latter case, a gaseous CH2O species depart from the surface. These observations are consistent with the sequential reactions for CH3OH with injected electrons or holes (Figure 1 and 2). As summarized by the schematic drawings in Figure 3d, the products occurred in different steps are highly distinguishable. For instance, under a single pulse of electron injection over a CH3OH molecule, once the reaction happens we can always see a fuzzy species (product of OHt in e-route step I) or a smooth but dim species (product of Oad in e-route step II). And vice versa, we can always see an expanded species (product of “int” in h-route step I) or a paired species in Ob rows (product of OHb pair in h-route step II, sometimes with CH2O nearby) under a single pulse of electron injection over a CH3OH molecule. No any exception was observed in measuring about hundreds of molecules. The use 12


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of alternate electrons and holes injection not only provides the insightful information about the effects of electrons and holes, but also helps to identify species in different reaction steps. For the assignments of the species in every step, we have considered the previous STM characterizations of the products in photocatalytic reactions of CH3OH31-33 and the species of OHt,51,56 CH2O,52,53 and Oad.57-59 On the basis of the previous identified characteristics of these species, we believe that our assignments should be very reliable. Here, the relatively simple structure of CH3OH that contains only C-H, O-H, and C-O bonds is also helpful for the judgement. Although the inelastic electron tunneling spectroscopy (IETS) can be used to measure vibrational spectra of chemical bonds for certain species adsorbed on metal surfaces,60,61 it is still difficult to perform such measurements on the TiO2 substrate because of its wide bandgap and too low current in the concerned bias region (typical within several tens to hundreds meV). Reactions under Nonlocal Electron and Hole Injections. The reactions discussed so far were triggered by carriers injected from the STM tip that was directly placed over the molecules (local injections). We now examine the situation that the electrons and holes are injected from a location off the molecules, i.e. nonlocal injections. Figure 4 shows two sets of results from the measurements performed at 80 K and 180 K, respectively, by nonlocal injections of holes. It is striking to see that the molecules off the injection site up to several nanometers can still be dissociated. It is noticed that all of reacted CH3OH at 80 K become the expanded species (Figure 4b), but many of reacted CH3OH at 180 K show up as paired OHb species and some of them with CH2O nearby (Figure 4d). It indicates that the h-route reactions are temperature-dependent. With even higher positive voltages, such nonlocal h-route reaction events can be found in a place with the lateral distance up to about 50 nm away from the hole-injection site (Supporting information Figure S2). The holes nonlocally injected 13



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at relatively high voltages can transport across the surface. A recent theoretical work shows that the holes may transfer forward and reverse between CH3OH and TiO2 during the hole energy relaxation.62 In contrast, for the electron injection at a negative voltage lower than 3.3 V (the electronic bandgap energy of TiO2),63 one cannot find any nonlocal reaction event, indicating that the e-route reaction can only be triggered by the local electron injection. However, when the nonlocal electron injection at a negative voltage higher than 3.3 V took place, the h-route reaction events can be observed, but the eroute events are still silent (Supporting information Figure S3). In comparison with the reactions induced by locally injected electrons, the silence of the e-route events in the nonlocal electroninjections can be ascribed to the decay of the injected electrons to the CBM, while these relaxed electrons do not have the ability to trigger the reaction of CH3OH. This behavior is quite different from the relaxed holes. The use of relatively high bias voltages could inject electrons into the high-lying unoccupied states of the TiO2 CB, and the injected electrons may radiatively decay and emit photons, which is similar to the process in the inverse photoemission spectroscopy (IPES) excited by incident low-energy electrons.64 Once the photons have an energy over the TiO2 bandgap, the absorbed photons by TiO2 may further generate the electron-hole pairs, as the common process for the photo-excitation of the electron-hole pairs. At the electron-injection voltage of 4.2 V, the radiative decay of the injected electrons may emit the photons with high enough energy to generate the holes in the TiO2 VB to conduct the h-route reactions. In previous studies it was suggested that the nonlocally injected hot electrons could transport across the surfaces to react with the molecules at a site away from the injection site.65-69 The applied electric field between the tip and the surface was also attributed to the reaction of molecules on the surface,30,70 which could even acts as the catalyst for chemical reactions.29 In our experiment, the effect of electric 14


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field can be ruled out since the observed reactions does not show visible dependence on the field strength through changing the tip-substrate distance (Supporting information Figure S2), but displays distinct dependence on the energy of injected carriers (see below in Figure 5). Based on our experimental results, it allows us to conclude that the nonlocally-injected holes can transport across the surface up to tens of nm to react with CH3OH molecules, as illustrated in Figure 4e.

Figure 4. Nonlocal h-route reactions at 80 K and 180 K. (a)-(b), STM images of CH3OH before and after applying positive pulses (+3.0 eV, 400 pA, 4 s with feedback loop on) at the site marked by the 15



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cross. The blue circles mark the reacted CH3OH in the expanded shape. The dashed lines indicate the distance of the reacted molecules away from the hole injection site. The images are acquired with 1.0 V and 10 pA, at 80 K. (c)-(d) Similar nonlocal experiment performed at 180 K with a positive pulse (+2.8 eV, 400 pA, 4 s). The paired black arrows denote the OHb pairs, and the red arrows denote the adsorbed CH2O nearby. Scale bars: 2 nm. (e) Schematic drawing of the nonlocal hole injection, hole transport across the TiO2 surface, and hole transfer to an adsorbed CH3OH. Dependence of Reactions on Voltage, Current, and Temperature. Figure 5 gives the results obtained under various experimental parameters. The image set in Figure 5a-d shows the e-route reaction events before and after scanning at 3.1 V and 1000 pA, while the image set in Figure 5e-h shows the h-route reaction events before and after scanning at +3.0 V and 20 pA. The final products in the magnified images (Figure 5d and 5h) are the same as the results shown above.

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Figure 5. Dependence of reaction probability on voltage and current. (a)-(c) Comparison of the changes for the adsorbed CH3OH molecules before and after electron-injection by scanning at 3.1 V and 1000 pA (image in (b)). Scale bar: 2 nm. (d) Magnified images from the marked areas in (a) and (c), for a direct comparison of the typical changes between them. (e)-(g) Comparison of the changes for the adsorbed CH3OH molecules before and after hole-injection scanning at +3.0 V and 20 pA (image in (f)). Scale bar: 2 nm. (h) Magnified images from the marked areas in e and g, for a direct comparison of the typical changes between them. (i) Plots of reaction fractions as a function of the applied scanning voltages, measured with currents of 100 and 1000 pA for negative voltages (e-route), and 2 and 20 pA for positive voltages (h-route) at 80 K, respectively. The data for 100 pA in e-route 1 t

is enlarged by a factor of 5. (j) Log-log plot of  ln(1  P ) as a function of current, measured for final products at 3.0 and 2.9 V for e-route, and +2.8 and +3.0 V for h-route reactions, respectively. (k) Plot of

e

+2.8 V for

 h . The error bars give the standard deviation of the experimental data from three to five

and

 h as a function of temperature. The scanning conditions are 2.9 V for  e and

runs. We have measured reaction fraction, P = n/N, where n is the number of the reacted molecules, and N the total number of originally adsorbed CH3OH molecules (typically, N= ~140 in a ~30  30 nm2 frame, with a scanning speed of 200 s per frame) at different voltages. The fractions of reacted molecules are plotted as a function of voltages in Figure 5i. For the e-route, the reaction fraction is strongly dependent on the e-injection current. At 1000 pA an onset voltage of about 2.4 V is observed, and above this threshold the fraction P increases almost linearly. However, at 100 pA the fraction becomes as small as P ~ 1.2% even at 3.2 V, and at 50 pA almost no reaction event is observable. For h-route, the rapid increase of fraction P occurs at about +3.0 V under relatively small h-injection 17



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currents of 2 and 20 pA. According the EF location (below the CBM by 0.4 eV) and the electronic bandgap of 3.3 eV for TiO2(110),63 the location of the valence band maximum (VBM) with respect to the EF is at 2.9 eV. Hence, the holes can be injected to the TiO2 VB only if their energy is larger than 2.9 eV. It is observed that our measured hole-injection voltage to trigger the rapid increase of the reaction fraction is at about +3.0 V (Figure 5i), in good agreement with the location of the VBM. When the energy of the injected holes is larger than this value, say, at 3.3 eV, the reaction fraction gets to about 90%, and tends to be saturated. This behavior means that once the holes are injected to the TiO2 VB, they may react with CH3OH. While the reaction may occur for the injected holes during their relaxation,62 the reaction can also occur after their relaxation to the VBM, which is evidenced by the rapid increase of the reaction fraction at the voltage of about 2.9 V. In Figure 5i, the relatively small fraction around +2.8 V may be most likely attributed to the broadening of the VBM. Apparently, the dependence of e-route and h-route reactions on voltages and currents are quite different. 1 t

Using the kinetic equation, dn/dt  (N-n)I, we can obtain the relation  ln(1  P )  I  , which connects the reaction rate with the injection current I and the carrier injection time t. Here α gives the order of the reaction in carriers. The validity of this equation has been verified by the linear dependence of –ln(1P) on t at a constant current (Supporting information Figure S4). Figure 5j shows the log-log 1 t

plot of  ln(1  P ) against I for e-route and h-route reactions with final products, respectively. In our measurements, the scanning speed of 200 s per ~30  30 nm2 frame was used, and the CH3OH coverage is about 0.03 ML, which leads to the estimated effective time teff ~28.5 s, for the tip locating over adsorbed CH3OH molecules in one frame scanning. From linear fitting of the data, we obtain 𝛼𝑒 ~1.9 and 𝛼ℎ~1.4 at 80 K. These values indicate that about two electrons or holes are needed in e-

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route and h-route reactions, referring as a two-step process. However, the large deviation of

 h from

the value of 2 implies the possible involvement of other factors in the h-route reactions. The Role of Thermal Activation. The first possible factor is the nonlocality of h-route reactions. During the scanning, the nonlocal injected holes may already trigger some reaction events, resulting in a longer effective time than that estimated. At the lower voltage of about +2.8 V, such effect should become less significant (Figure 5i). Another factor could be the thermal activation, as shown by the temperature dependence in Figure 3. To be more precise, we plot the dependence of the measured

e

and

 h as a function of temperature in Figure 5k (Supporting information Figure S5). It is seen that

e

is around 2, showing no dependence on temperature for e-route. However,

 h decreases

obviously with the increase of temperature, even reducing the value to about 1 at 180 K. The thermal activation should thus be an important factor in the h-route reaction. Calculated Results. We have calculated the density of states (DOSs) of the system under investigations using HSE hybrid functional (Supporting information Figure S6). The unoccupied hybridization states in the range from 2.3 to 4.3 eV should be responsible for the electron attachment of CH3OH, where the unoccupied hybridization states contribute to the anti-bonding orbitals that distribute over the C-O bond. This result can explain the C-O bond breaking in the first-step e-route reaction, also in agreement with the measured threshold voltage to trigger the e-route reaction (Figure 5i). In our case for the single CH3OH molecule, the lowest unoccupied molecular orbital (LUMO) is calculated to be approximately at 5 eV above the EF (Supporting Information Figure S6a). Our experimental results show that the e-route reaction events occur at an onset voltage of 2.4 V above the EF, which can be more reasonably associated to the unoccupied hybridized states (Un-HSs) locating at 2.3~4.3 eV, rather than the LUMO for individual CH3OH. Previous results show the existence of the 19



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wet electron state at about 2.3 eV above the EF for the 1 ML CH3OH. Previous results show the existence of the wet electron state at about 2.3 eV above the EF for the 1 ML CH3OH.71-73 We believe that such wet electron states observed for the 1 ML CH3OH may not be applicable to explain the observed onset voltage for the individual CH3OH in our experiment, since the wet electron states show dependence on coverage, in comparison with the energy position changing from 2.4 to 4.5 eV with the coverage from 1 ML to 0.5 ML for water.74 Based on the previous calculations,28,50 we here used a commonly adopted 5-trilayer slab model. The weak DOS of hybridization states can explain why a large current (>50 pA) is always requested in the e-route. The first-step C-O bond breaking can thus be ascribed to the electron-attachment mechanism.27,28 Our results here suggest that the injected electrons should be higher than the hybridization states in order to trigger the C-O bond breaking of CH3OH.

Figure 6. Calculated potential energy surfaces and the magnetic moment changes. (a) and (b) Potential energy surfaces of the CH3OH reactions with a delocalized (free) hole or a trapped hole at the TiO2(110) surface, respectively, through breaking the O-H bond. The structural models for each 20


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reaction coordinates can be found in the cartoon shown in Supplementary Information. (c) Potential energy surface of the dissociation of •CH3O radical through C-H bond breaking. The structural models of the transition state (TS) are shown in the insets. (d)-(f) Corresponding changes of the magnetic moment of the oxygen atom in CH3OH or •CH3O (Om) and the Obr at TiO2(110) surface. Considering the fact that in the h-route reaction the H atoms always transfer to surface Obr to form OHb rather than departure from the surface, we can propose the mechanism of proton-coupled hole transfer (PCHT)73,75 process in the first-step O-H bond breaking for the h-route reaction. In order to get insight into the PCHT process, we calculated the reaction potentials for O-H and C-H bond breaking by considering the reaction of CH3OH with either the delocalized or the surface trapped holes at the Obr site using GGA+U method.45,76,77 The calculated potentials are shown in Figure 6a-c (Supporting information Figure S7). Meanwhile, the site-projected magnetic moments of the Obr and the O atom in CH3OH molecule (Om) are recorded to track the location of hole in the reaction path, as shown in Figure 6d-f, correspondingly. The processes of the O-H bond breaking are both exothermic in the presence of hole, but the energy barrier (0.59 eV) for the CH3OH reaction with the trapped hole is much larger than that (0.09 eV) for the reaction with the delocalized hole (Figure 6a, b). The results strongly suggest that the delocalized hole is more favorable to initiate the O-H bond breaking than the trapped hole. The O-H bond breaking through PCHT produces a ●CH3O radical and an OHb. In the following step, the produced ●CH3O radical can exothermically dissociate to produce a CH2O and another OHb through breaking one of the C-H bonds (Figure 6c). This process just has a negligible energy barrier (0.02 eV). Although this barrier might be underestimated, the second-step C-H bond breaking from the ●CH3O radical is much feasible through thermally-driven activation, which may well explain the temperature-dependence and the nearly one-hole process at an elevated temperature 21



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of 180 K in the h-route reactions, as shown in Figure 5k. The recorded changes of magnetic moments can well support the concerted PCHT process (Figure 6d-f). As we have shown above, in h-route reactions, the breaking of O-H and C-H bonds is directly related to the photocatalytic reactions of CH3OH. One of the unsettled problems in the field is that whether the breaking of O-H and C-H bonds is thermal activation or photocatalytic processes.7,31-37,78,79 With the ability to distinguish every step of the bond breaking, we are now able to resolve the debate. Our results clearly demonstrate that both mechanisms are relevant, but give only a half-truth. The actual picture is that the O-H bond breaking is hole induced alone, while the C-H bond breaking can be assisted by thermal activation along the h-route reaction, especially, the nonlocal h-route reaction. We here revisit the assignment of the intermediate expanded “int” species to the complex of CH3O and OHb based on the images and the tip manipulations. There is a different assignment of the expanded species to a complex of CH2O and a pair of OHb.32 In fact, the expanded species have various symmetric and asymmetric shapes in the STM images, which makes it difficult to determine the exact configurations. As discussed above, if the ●CH3O reduces to ground state CH3O anion,36 the complex might be CH3O and OHb; if the ●CH3O proceeds the C-H bond breaking, the complex can be CH2O and an OHb pair. In a recent theoretical study on the photocatalytic reaction of CH3OH,36 a similar proton-coupled electron transfer mechanism has been adopted, where the first-step O-H bond breaking through transfer of hole to CH3OH. The excited-state ●CH3O radical is also suggested. In consistent with the theoretical suggestion, our findings for the first time experimentally uncover the entire photocatalytic processes of CH3OH dissociation.

CONCLUSIONS 22


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In conclusion, we have provided a complete atomistic picture for the elementary redox reactions of methanol on TiO2. The selective bond-breaking phenomena by the electrons and holes have been identified. It is found that in the initial reaction step a hole prefers to break the O-H bond, and an electron to break the C-O bond, respectively. In the consecutive step, the produced CH3O species by the hole exist as the radical with strong tendency to undergo C-H bond breaking. Our nonlocal hinjection experimental and theoretical results suggest that the delocalized holes in the TiO2 substrate should be responsible for the h-route reactions, which are strongly linked to the current studies of photo-catalytic reactions and are helpful for the understanding of the photo-oxidation processes of molecules. The locally triggered e-route reaction is associated with the fact that the location of the unoccupied hybridization states is much higher than the CB onset, preventing the photo-excited electrons around the CB onset to reach them. Nevertheless, the occurrence of the unoccupied hybridization states is useful information for the design of molecular syntheses activated by the lowenergy electron technique for practical applications.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. S1-S7 give additional experimental and theoretical calculation details, including the adsorption behavior of CH3OH at TiO2(110) surface and the energy diagram (S1); nonlocal hole injection induced reaction of CH3OH (S2); nonlocal electron injection induced reaction of CH3OH (S3); verify the validity of the kinetic equation in CH3OH reactions (S4); The  e and  h obtained at different temperatures using the scanning mode (S5); The calculated partial DOS and the unoccupied anti-bonding orbitals of an adsorbed CH3OH (S6); comparison of calculated partial DOSs using different methods (S7). 23



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AUTHOR INFORMATION *Corresponding authors: Email: [email protected], [email protected], [email protected]. Author contributions ||S.J.T.,

H.F., and Y.F.J. contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology of China (2016YFA0200603 and 2017YFA0303500), the National Natural Science Foundation of China (21421063, 21633007, and 21790350), the Chinese Academy of Science (2016HSC-IU003), Anhui Initiative in Quantum Information Technologies (AHY090000), and Swedish Research Council (VR). The Swedish National Infrastructure for Computing (SNIC) was acknowledged for computer time. S.J.T acknowledges the support by CAS Pioneer Hundred Talents Program C.

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Figure 1. Electron induced CH3OH dissociation (e-route). (a) Schematic drawing of electron injection over the CH3OH adsorbed on TiO2(110)-(1×1) surface using the STM tip at a negative bias voltage. (b)-(e) Consecutively acquired STM images of the individually adsorbed CH3OH under different negative voltage pulses (electron-injection), two pulses of -2.6 V over “m1”, one pulse of -3.0 V over “m2”. Scale bar: 1 nm. (f)-(h) Corresponding I-t curves recorded during applying pulses over “m1” and “m2”, respectively. All images are acquired at -1.0 V and 10 pA, at 80 K. (i), Schematic drawing of the two-step dissociation of CH3OH in e-route: step I, C-O bond breaking; step II, O-H bond breaking. 153x80mm (300 x 300 DPI)



Figure 2. Hole induced CH3OH dissociation (h-route). (a) Schematic drawing of hole injection using a STM tip at positive bias. (b)-(d) STM images showing the changes of “m1” by consecutively applying two +2.7 V pulses. (e)-(f) Corresponding I-t curves recorded during applying the two pulses. (g)-(i) Another set of STM images showing the changes of “m2” and “m3” by applying +3.0 V pulses and the corresponding I-t curves. All images are acquired at -1.0 V and 10 pA, at 80 K. Scale bars: 1 nm. (j) Schematic drawing of the twostep dissociation of CH3OH in h-route: step I, O-H bond breaking by transferring the H atom to an adjacent Obr; step II, C-H bond breaking by transferring the H atom to the adjacent Obr site at the opposite side. 122x135mm (300 x 300 DPI)


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Figure 3. Reaction routes of CH3OH under alternate injections of electrons and holes. (a1)-(a4) A set of consecutively acquired STM images after electron injection (-2.8 V) over the molecule and then hole injection (+3.3 V) over the intermediate species of OHt. Note that after applying the pulse of +3.3 V (hole injection), the other two molecules nearby are also changed to expanded species (labeled by “int”). (b1)(b4) A set of consecutively acquired STM images after hole injection (+2.7 V), lateral manipulation using the tip (-0.5 V and 500 pA), and then electron injection (-2.8 V) over the molecule or the separated species of CH3O. (c1)-(c4) A set of consecutively acquired STM images after hole injection (+2.7 V), lateral manipulation using the tip (-0.5 V and 500 pA), and then hole injection (+2.7 V) over the molecule or the separated species of CH3O. The green arrows in (b2) and (c2) show the move direction of the tip during manipulations. In each lower panel, the schematic models, corresponding to the marked areas in (a1), (b1), and (c1), show the configuration changes during the manipulations. Scale bars: 1 nm. All images are acquired at -1.0 V and 10 pA, at 80 K. (d) Schematic drawings, showing the remaining species in each steps after the injections of electrons and holes alternately. 165x133mm (300 x 300 DPI)



Figure 4. Nonlocal h-route reactions at 80 K and 180 K. (a)-(b), STM images of CH3OH before and after applying positive pulses (+3.0 eV, 400 pA, 4 s with feedback loop on) at the site marked by the cross. The blue circles mark the reacted CH3OH in the expanded shape. The dashed lines indicate the distance of the reacted molecules away from the hole injection site. The images are acquired with -1.0 V and 10 pA, at 80 K. (c)-(d) Similar nonlocal experiment performed at 180 K with a positive pulse (+2.8 eV, 400 pA, 4 s). The paired black arrows denote the OHb pairs, and the red arrows denote the adsorbed CH2O nearby. Scale bars: 2 nm. (e) Schematic drawing of the nonlocal hole injection, hole transport across the TiO2 surface, and hole transfer to an adsorbed CH3OH. 103x143mm (300 x 300 DPI)


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Figure 5. Dependence of reaction probability on voltage and current. (a)-(c) Comparison of the changes for the adsorbed CH3OH molecules before and after electron-injection by scanning at -3.1 V and 1000 pA (image in (b)). Scale bar: 2 nm. (d) Magnified images from the marked areas in (a) and (c), for a direct comparison of the typical changes between them. (e)-(g) Comparison of the changes for the adsorbed CH3OH molecules before and after hole-injection scanning at +3.0 V and 20 pA (image in (f)). Scale bar: 2 nm. (h) Magnified images from the marked areas in e and g, for a direct comparison of the typical changes between them. (i) Plots of reaction fractions as a function of the applied scanning voltages, measured with currents of 100 and 1000 pA for negative voltages (e-route), and 2 and 20 pA for positive voltages (h-route) at 80 K, respectively. The data for 100 pA in e-route is enlarged by a factor of 5. (j) Log-log plot of 1/tln(1-P) as a function of current, measured for final products at -3.0 and -2.9 V for e-route, and +2.8 and +3.0 V for h-route reactions, respectively. (k) Plot of αe and αh as a function of temperature. The scanning conditions are -2.9 V for αe and +2.8 V for αh. The error bars give the standard deviation of the experimental data from three to five runs. 145x116mm (300 x 300 DPI)



Figure 6. Calculated potential energy surfaces and the magnetic moment changes. (a) and (b) Potential energy surfaces of the CH3OH reactions with a delocalized (free) hole or a trapped hole at the TiO2(110) surface, respectively, through breaking the O-H bond. The structural models for each reaction coordinates can be found in the cartoon shown in Supplementary Information. (c) Potential energy surface of the dissociation of •CH3O radical through C-H bond breaking. The structural models of the transition state (TS) are shown in the insets. (d)-(f) Corresponding changes of the magnetic moment of the oxygen atom in CH3OH or •CH3O (Om) and the Obr at TiO2(110) surface. 153x92mm (300 x 300 DPI)


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Visualizing Elementary Reactions of Methanol by Electrons and Holes

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