by Atomic Force Microscopy at 78 K

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C: Physical Processes in Nanomaterials and Nanostructures

Characterization and Reversible Migration of Subsurface Hydrogen on Rutile TiO(110) by Atomic Force Microscopy at 78 K 2

Quanzhen Zhang, Huan Fei Wen, Yuuki Adachi, Masato Miyazaki, Yasuhiro Sugawara, Rui Xu, Zhihai Cheng, and Yan Jun Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05744 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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The Journal of Physical Chemistry

Characterization and Reversible Migration of Subsurface Hydrogen on Rutile TiO2(110) by Atomic Force Microscopy at 78 K Quanzhen Zhang,† Huan Fei Wen,† Yuuki Adachi,† Masato Miyazaki,† Yasuhiro Sugawara,† Rui Xu,‡ Zhi Hai Cheng,‡ Yan Jun Li†* †

Department of Applied Physics, Graduate School of Engineering, Osaka University, 2-1

Yamada-oka, Suita, Osaka 565-0871, Japan ‡

Low-Dimensional and Surface Physics, Department of Physics, Renmin University of China,

LiGongLou 714, Beijing 100872, P. R. China

Corresponding Author: Yan Jun Li* Address: Department of Applied Physics, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Telephone number: + 81-6-6879-7854

ACS Paragon Plus Environment

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E-mail: [email protected]


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ABSTRACT: In this study, we have systematically characterized and reversibly manipulated the subsurface hydrogen (Hsub) on rutile TiO2(110)-(1 × 1) by a combination of non-contact atomic force microscopy, scanning tunneling microscopy and Kelvin probe force microscopy at 78 K. Four different configurations of the Hsub, including the monomer, dimer, trimer and tetramer, are clearly characterized and discriminated by simultaneous atomic force microscopy and scanning tunneling microscopy measurement. Specifically, by using Kelvin probe force microscopy, the local contact potential difference of the subsurface hydrogen is mapped with atomic resolution. In addition to multi-channel characterization, we unprecedentedly demonstrate ability to reversibly migrate the Hsub between the subsurface and surface layers, which is electrically activated by switching the polarities of voltage pulse. The dominant underlying mechanism of the reversible migration process of the Hsub is tentatively explained by the inelastic electron tunneling effect and the local electric filed in the tunneling junction. Our study opens up an unprecedented playground for the systematic investigation and deliberate manipulation approach of the subsurface defects and may potentially have an overriding effect on revolutionizing the investigation of the catalytic reactions based on transition metal oxides.


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INTRODUCTION Point defects are of paramount importance in governing the properties and technological applications of the nanoscale materials, such as the catalytic activity of the catalysts supported on reducible oxides, the electric conductivity of the ferroelectrics and the magnetic properties of the metal-free magnetic materials.1-3 As such, a large amount of experimental and theoretical works have been performed to characterize and manipulate the configuration, distribution and charge states of the point defects at the atomic scale level.4-8 Specifically, subsurface defects in the nearsurface region have recently drawn extraordinary research interest experimentally and theoretically, which is due to its dramatic modification on the local surface properties and the overriding effects on the adsorbates.4,9-11 However, compared to surface defects, the systematic investigation and deliberate manipulation of the subsurface defects still remains an outstanding experimental challenge so far. Rutile TiO2(110), as the most stable and best characterized surface, has been intensely investigated since 1970s and widely employed as an active catalyst in the prototypical catalytic reactions.12 The introduction of intrinsic and extrinsic point defects, including the surface and subsurface defects, can transform the stoichiometric TiO2 from an insulator to a partially reduced n-type semiconductor and dramatically modify its electronic properties and photocatalytic activity.13-16 The importance of the TiO2 surface defects have been fully established through abundant experimental and theoretical works. For example, the electronic structure and excess electrons distribution around the surface defects have been clarified by scanning probe microscopies (SPM) at various low temperatures.16-18 In addition, the surface defects can be diffused,19-21 desorpted22,23 and re-deposited at the atomic scale level in a controllable way.24 Moreover, the surface defects, acting as the reaction intermediates and abundant surface electron


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donors, can also mediate the water molecules dissociation23 and oxygen adatoms adsorption and dissociation on the surface.25-27 In addition to the surface defects, the Ti interstitials in the near surface region and the subsurface oxygen vacancy in TiO2 have been systematically investigated and demonstrated to facilitate the oxygen adsorption28,29 and enhance the electrical conductivity.9 However, on the other hand, subsurface hydrogen (Hsub), as a fundamental and predominant impurity in the near surface regions of metal and metal oxide,30,31 is yet to be fully investigated, possibly due to the limited analytical methodologies to reveal the subsurface defects with atomic resolution. Specifically, the Hsub in rutile TiO2(110) can strongly affect the local photocatalytic activity and chemical reactivity,10 while surprisingly only few available literatures have discussed the Hsub to data. For example, the experimental and theoretical works have demonstrated that the adsorbed hydrogen atoms can easily diffuse into the TiO2 bulk through a thermodynamically spontaneous process with a low activation barrier and remain the stable Hsub specie in a wide temperature range.32-34 Moreover, the residence interstitial locations and the volumetric maps of the tip-sample interaction force of the Hsub have already been revealed by an interplay between simultaneous non-contact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM) measurement in constant frequency shift mode.35,36 However, in these works, crosstalk of the topographic and tunneling current signals simultaneously recorded by nc-AFM and STM in constant frequency shift mode makes the interpretation complicated and lack of credibility. In addition, several other important information about the Hsub remains a largely unexplored topic, such as the local electronic properties at the single defect level and the possibility of reversible transformation between surface and subsurface layers in a controlled manner, which may have an overriding effect on revolutionizing the investigation and applications of the rutile TiO2.


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Simultaneous multi-channel SPM measurement offers an unprecedented technique to characterize and investigate the surface chemistry, and specifically solve the elusive challenges of the investigation of the subsurface defects. For example, the complementary information of the interaction force and local density of states (LDOS) of the Si atoms have been provided by simultaneous nc-AFM and STM measurements in constant height mode.37 The interplay between nc-AFM and Kelvin probe force microscopy (KPFM) offers the electrostatic force and local contact potential difference (VLCPD) of the charged subsurface impurities in TiO2(110).11 Moreover, multiple signals of the frequency shift (∆f), time-averaged tunneling current () and the VLCPD of the TiO2 surface have been recorded simultaneously using the nc-AFM, STM and KPFM without bias voltage feedback technique.31 On the other hand, the SPM, besides the benchmark capability of probing the local structural and electronic properties of the surface with atomic resolution and providing multiple channels of physical information, has been widely employed as a versatile and efficient tool to chemically identify the surface atomic species with precise measurement of the short-range force,38,39 to measure40,41 and manipulate42,43 the charge states of the adsorbed atoms and molecules with single-electron sensitivity, to mechanically manipulate the individual atoms laterally44,45 and vertically46,47 at the atomic scale level, and to precisely measure the lateral and vertical forces exerted on single adsorbates during the atom or molecule displacement with piconewton-scale force resolution.48 Among the adsorbates manipulation techniques, the electrically-driven manipulation of the adsorbed atoms or molecules induced by the inelastic electron tunneling (IET) fascinates the scientific community recently. Its underlying manipulation mechanism is distinctly different from the conventionally SPM mechanical manipulations, which is exclusively based on the tip-sample interaction force.49,50 The IET technique can induce the vibrational,49 electronic51 and vibronic52


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excitations of the target adsorbates, depending on the applied voltage pulse. A large varieties of manipulation process can be triggered by the IET, such as lateral hopping,53 rotation,54 dissociation,55 and desorption56 of the individual atoms, molecules and even nanostructures. In addition, the lateral movement direction of the adsorbates can be precisely determined by controlling the electron injection site,57 the polarity of the voltage pulse,58 and the local electric field in the tunneling junction.59,60 Most importantly, from a methodological point of view, the IET can be developed into an available method to manipulate the subsurface impurities and defects under the top layer of the surface, which drastically exceeds the capacity of the conventional SPM manipulation utilizing the mechanical tip-sample interaction force.61,62 In this work, we have systematically characterized and manipulated the subsurface defect on rutile TiO2(110)-(1 × 1) surface using the multi-channel SPM technique including the simultaneous nc-AFM/STM and nc-AFM/KPFM measurement in constant height mode at 78 K. Here, we propose that, due to the highly localized configuration and reversible migration behavior, the subsurface defect is most possible to be the subsurface hydrogen atoms. We find that four different configurations of the Hsub are clearly characterized and distinguished by nc-AFM/STM measurement. In particular, the VLCPD of the subsurface hydrogen is mapped by KPFM with atomic resolution. Besides the capability of characterizing the Hsub by nc-AFM/STM/KPFM, we also demonstrate that the Hsub can be reversibly migrated between the surface and subsurface layers. The movement direction of the Hsub, such as upward or downward migration, can be strictly determined by controlling the polarities of the voltage pulse.

METHODS


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Our experiment was performed with a home-built nc-AFM under ultrahigh vacuum condition (< 5 × 10-11 Torr) at low temperature of 78 K. An optical beam deflection (OBD) method was employed to detect the deflection of the cantilever. Simultaneous measurements of the frequency shift ∆𝑓 and time-averaged tunneling current were performed using the dual-channel nc-AFM and STM approach in constant height mode to avoid the crosstalk between ∆𝑓 and signals.37 The DC bias voltage was applied to the sample and the signal was detected from the tip. The frequency modulation KPFM, based on nc-AFM with bias voltage feedback, was used to measure the ∆𝑓 and VLCPD images simultaneously.40 As a sensor, an Ir-coated Si cantilever (Nanosensors SD-T10L100, spring constant k = 1500 N/m, quality factor Q = 24500, resonant frequency f0= 800 kHz) was oscillated with a constant oscillation amplitude (500 pm). Conventional preparation of annealing and Ar ion sputtering was used to remove the contamination of the tip apex. A commercial rutile TiO2(110) crystal (Furuuchi Chemical Corporation) was used as a sample without any extrinsic doping, and prepared by several circles of Ar ion sputtering (22 μA, 1 keV, 1 × 10-6 Torr, 15 min) and annealing (970 K, 30 min). The clean sample was cooled to room temperature and transferred into the observation chamber of the nc-AFM at 78 K. All the images in this work were measured in constant height mode and the atom tracking technique was used to compensate the thermal drift.

RESULTS AND DISCUSSION Figure 1 shows four sets of frequency shift (|∆f|) and time-averaged tunneling current (||) images of the rutile TiO2(110)-(1 × 1) surface measured by nc-AFM and STM simultaneously in constant height mode at 78 K. Here, in the nc-AFM (|∆f|) images, the bright and dark rows correspond to the alternating 2-fold coordinated bridge oxygen rows (O2c) and the 5-fold


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coordinated titanium rows (Ti5c), respectively. Inversely, in the corresponding STM (||) images, the bright and dark rows correspond to the Ti5c and O2c, respectively, due to the dominated electronic effects.12 Usually, the surface defects, such as the oxygen vacancy (Ov) and hydroxyl (OH) can be clearly observed and distinguished in the nc-AFM image, as shown in Figure S1. In Figure 1, the surface defects are recorded as dark holes in the nc-AFM images (hole mode) and as bright spots in STM images, respectively. Besides that, some extra bright spots can be clearly distinguished on the O2c rows in the STM images, while imperceptible in the corresponding ncAFM images, as the dashed blue circles indicated in Figure 1. It suggests that these specific species are not on the topmost surface layer, and the tip-sample interaction force is completely screened in nc-AFM measurement. Here we stress that the experimental challenge exists in directly identifying the surface and subsurface species by STM and nc-AFM measurement, and we presume that, based on the following discussed experimental results, these highly localized bright extra spots are most possible to be the subsurface hydrogen atoms (Hsub) in the subsurface layer of rutile TiO2(110). In our study, four different configurations of the Hsub are clearly characterized and distinguished, including the Hsub monomer in Figure 1b, the dimer in Figure 1d, the trimer in Figure 1f and the tetramer in Figure 1h. Note that, among these Hsub configurations, the Hsub dimer is found to be the most dominant specie, as shown in Figure S2, which is in good agreement with previous work.36 Specifically, we note that the characterization of Hsub is independent of the tip apex polarity (see the Hsub characterization by the simultaneous STM and nc-AFM image in protrusion mode in Figure S3). In addition, the Hsub species show various intensity of brightness even with the same configuration, such as in Figure 1d and 1h, and here the contrast difference is tentatively attributed to the Hsub lying at different subsurface layers, as the STM can detect the electronic signal up to several atomic layers beneath the surface.10 We speculate that these Hsub


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species are formed due to surface hydrogen atoms migration during sample preparation. Some experimental works in combination with theoretical calculations have demonstrated that the surface hydrogen atoms can easily migrate into the bulk with a rather low energy barrier when increasing the temperature,32-34 and the Hsub can remain a more stable subsurface specie than in the under-coordinated sites on the surface, due to the formation of hydrogen bond with other atoms inside the cavity.34 In this work, we note that the Hsub with four kinds of configurations has long lifetime to be characterized for several hours and no spontaneous configuration change can be observed during characterization even with decreased tip-sample distance (see Figure S4), which is perfectly consistent with the theoretical calculation.34 Particularly, the employment of constant height scanning approach in this study completely avoids the crosstalk between the electrostatic force and tunneling current channels induced by the active feedback, which can provide more accurate physical information of the local geometric and electronic properties of the Hsub compared to previous works.35,36 Our experimental measurement systematically reveals four different configurations of the Hsub, and demonstrates the unprecedented capability of the dual-channel ncAFM/STM characterization approach for the subsurface defects.


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Figure 1. Characterization of the subsurface hydrogen (Hsub) on rutile TiO2(110)-(1 × 1) by ncAFM and STM simultaneously in constant height mode. (a, b), (c, d), (e, f), (g, h), Four sets of simultaneously measured frequency shift |∆𝑓| (nc-AFM, hole mode, 2.5 × 6 nm2) and timeaveraged tunneling current || (STM, empty state, 2.5 × 6 nm2) images of rutile TiO2(110)-(1 × 1) surface, respectively. The dashed blue circles indicate the Hsub monomer in (b), dimer in (d), trimer in (f) and tetramer in (h), respectively. O2c: 2-fold coordinated bridging oxygen row. Ti5c: 5-fold coordinated titanium row. Imaging parameters: f0 = 813 kHz, A = 500 pm, Q = 24500, T = 78 K, V = + 0.8 V.

We now focus on the investigation of the electronic property of the Hsub using the multi-channel nc-AFM/STM and nc-AFM/KPFM measurement approach in constant height mode. Figure 2a,b show the simultaneous nc-AFM and STM images of the Hsub tetramer on rutile TiO2(110)-(1 × 1) measured in constant height mode. It is clearly observed in the STM image that the Hsub tetramer


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resides on the O2c row with bright contrast, which is in perfect agreement with the experimental results in Figure 1. We note that the nc-AFM image in Figure 2a is in the protrusion mode now, which indicates that the characterization of Hsub is independent of the tip polarity (also see Figure S3). Also, the Hsub tetramer can be slightly observed in the nc-AFM image in Figure 2a, which is possibly due to the smaller tip-sample distance. After the simultaneous nc-AFM and STM characterization of the Hsub tetramer, the KPFM with bias voltage feedback measurement is performed to characterize the same area, as shown in Figure 2c. Interestingly, in the KPFM image of Hsub tetramer, the dumbbell-like feature is observed rather than the tetramer configuration. Moreover, we perform the identical multi-channel measurement on the Hsub dimer, as shown in Figure S5. The dumbbell-like configuration can still be observed, even with different tip-sample distance (see Figure S6). It is pointed that KPFM can enable the direct measurement of local contact potential difference (VLCPD) at the atomic level,31 and further theoretical calculation such as density functional theory (DFT) is necessary to verify that if the local potential measurement by KPFM can reflect the charge distribution of the Hsub species.63


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Figure 2. Characterization of the Hsub tetramer by nc-AFM/STM/KPFM measurement of the same area in constant height mode. (a, b) Simultaneously measured |∆𝑓| (protrusion mode, 2.5 × 5 nm2) and || (empty state, 2.5 × 5 nm2) images of rutile TiO2(110)-(1 × 1) surface. (c) Corresponding experimental VLCPD images on the same area as shown in (a, b), respectively. Imaging parameters: f0 = 813 kHz, A = 500 pm, Q = 24500, T = 78 K, V = + 0.8 V.

The local geometric configuration and electronic property of the Hsub on rutile TiO2(110)-(1 × 1) have been systematically investigated and clearly clarified using the multi-channel SPM measurement methodology in our study. Now, we extent our capability to deliberately manipulate the Hsub by the SPM tip in a controllable manner.


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Figure 3. Downward migration of the Hsub trimer between subsurface layers. (a, b) Simultaneously recorded |∆𝑓| (hole mode, 2.5 × 6 nm2) and || (empty state, 2.5 × 6 nm2) images of rutile TiO2(110)-(1×1) surface in constant height mode, respectively. (c, d) Corresponding |∆𝑓| (hole mode) and || (empty state) images of the same area as shown in (a, b) after the positive voltage pulse. Blue dot in (b) means the positive voltage pulse site. Imaging parameters: f0 = 813 kHz, A = 500 pm, Q = 24500, T = 78 K, V = + 0.8 V.

Figure 3 shows the downward migration of the Hsub trimer between subsurface layers activated by the positive voltage pulse. In Figure 3b, the Hsub trimer is clearly recorded in the STM image, as indicated by the blue dashed circle. Then, the tip is positioned precisely above the center of the Hsub trimer (blue dot in Figure 3b) and the feedback loop is interrupted. We try the positive voltage pulse from 1 V, which is slightly beyond the scanning bias voltage (0.8 V), and gradually increase the voltage until successive migration event is observed. After each voltage pulse with a duration of up to 10 s, the same area is characterized to check if any migration is triggered. In this experiment, we notice that the Hsub trimer completely disappear in the STM image when a 2.5 V voltage pulse is applied, as shown in Figure 3d. Here we speculate that the Hsub trimer migrate into the deeper subsurface layers, which exceeds the detecting limitation of the STM. In addition, we


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also successfully manipulate the surface hydrogen atom (Hs) to the subsurface layer by the positive voltage pulse, which is in line with the previous experiment,35 as shown in Figure S7. We suggest the underlying migration mechanism of the surface and subsurface hydrogen atom is tentatively explained by the injection of inelastic tunneling electrons accompanied with the strong local electric field in the tunneling junction. During the positive voltage pulse, the SPM tip acts as the electron source with high local electron density and injects a small fraction of the inelastic tunneling electrons into the Hsub. The inelastic tunneling electrons can transfer energy from the tip to Hsub. Meanwhile, the strong local electric field can be automatically formed accompanied with tunneling electrons injection, which is confined between the tip apex and the target Hsub. With the combined effect of IET and local electric field during the positive voltage pulse, the Hsub trimer can be activated to migrate into the deeper subsurface layers, which is speculated to be a new energetically favorable state. Our work demonstrates that the IET accompanied with local electric field effect offers an efficient approach for the manipulation and migration of the subsurface defects.


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Figure 4. Reversible migration of the Hsub between subsurface and surface layers by switching the polarities of the voltage pulse. (a, b) Simultaneously measured |∆𝑓| (hole mode , 4 × 8 nm2) and || (empty state, 4 × 8 nm2) images of rutile TiO2(110)-(1×1) surface in constant height mode. (d, e) Simultaneous |∆𝑓| (natural mode) and || images (empty state) of the same area as shown in (a, b) after the negative voltage pulse. (g, h) Simultaneously recorded |∆𝑓| (natural mode) and || (empty state) images of the same area as shown in (d, e) after the positive voltage pulse. (c, f, i) Corresponding schematic models of the reversible switching of the Hsub. Red and blue dots in (b) and (d) indicate the negative and positive voltage pulse sites, respectively. Hs: surface hydrogen atom. Imaging parameters: f0 = 813 kHz, A = 500 pm, Q = 24500, T = 78 K, V = + 0.8 V.

Inspired by the one-orientation migration (downward migration) of the surface and subsurface hydrogen activated by the positive voltage pulse, now we try to explore a more complicated migration process of the Hsub, namely, a reversible migration between subsurface and surface hydrogen by switching the polarities of the voltage pulse. as shown in Figure 4. Here we demonstrate that the Hsub can be upward transformed from subsurface to surface layers by the negative voltage pulse (in Figure 4a-f), and downward migrated from surface to subsurface layers activated by the positive voltage pulse (in Figure 4d-i). More specifically, as shown in Figure 4a and 4b, the surface defects, including OH and Ov, are characterized as dark holes in nc-AFM image (hole mode) and as bright spots in STM image. We note that, depending on the contrast in STM image, the two brighter spots of the surface defects in Figure 4b can be clearly distinguished as OH (surface hydrogen Hs) and others as oxygen vacancy (Ov).20,25 One Hsub dimer can be clearly observed with brightest contrast in Figure 4b, as the model presented in Figure 4c. As the Hsub can downward migrate to the deeper subsurface layers activated by the positive voltage pulse as


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demonstrated in Figure 3, now the negative voltage pulse is employed to transform the Hsub from the subsurface to the topmost surface layer. As the red dot indicated in Figure 4b, a voltage pulse of -3.5 V with a duration of 10 s is applied precisely above the upper Hsub. The tunneling current is monitored as a function of time during the pulse, as shown in Figure 5b. A sudden jump in current trace is observed after some time, which is indicative of the successful upward migration of the Hsub from the subsurface to the surface layer, as schematic model shown in Figure 4f.53 After that, the same area is characterized again without changing any scanning parameters. Note that the nc-AFM image in Figure 4d changes to neutral mode now, and, besides the two surface hydrogen (Hs), one additional bright spot on the O2c row is clearly observed on the right side of the nc-AFM image, and only one brighter spot is left in the original Hsub dimer position as shown in Figure 4e. We notice that the bright spot on the right side of the nc-AFM image in Figure 4d shows quite similar configuration with the left Hs. Hence, we strongly propose that the right bright spot in Figure 4d is one hydrogen atom lying on the O2c row, which is upward migrated from the subsurface Hsub dimer (Figure 4f). In addition, the fresh surface hydrogen atom is speculated to be precisely located above the left Hsub (Figure 4f), resulting the brighter contrast in nc-AFM and STM images, as clearly presented in Figure 4d. Moreover, we demonstrate that, not only the single Hsub but also the Hsub dimer can also be integrally upward migrated from subsurface to surface by precisely applying the negative voltage pulse in the middle of the Hsub dimer, which is presented in Figure S8. The upward migration of the Hsub is experimentally demonstrated for the first time, and we propose here that the migration mechanism is tentatively attributed to the injection of inelastic tunneling electrons from the subsurface polaron in substrate64 and the strong local electric field in the tunneling junction. After the upward migration, a positive voltage pulse of 2 V is applied on the fresh surface hydrogen to activate the downward migration, as the blue dot indicated


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in Figure 4d. The tunneling current is monitored (Figure 5c), and a sudden jump in current trace during the positive pulse is observed after some time, which indicates the successful downward migration of the fresh surface hydrogen atom (Figure 4i). The same area is characterized again by nc-AFM and STM without changing any scanning parameters. The fresh Hs disappears from the nc-AFM image (Figure 4g), and the Hsub dimer is recreated as presented in Figure 4h. The reversible migration process of the Hsub between subsurface and surface has been successfully demonstrated. In addition, we clearly notice that it needs higher voltage to induce upward migration than that inducing downward migration of the hydrogen atoms, indicating the higher energy barrier in the upward migration process than that in the downward migration, which is in perfect agreement with the previous theoretical calculation.35 Note that, in this experiment, the surface hydrogen can be downward migrated rather than desorbed as reported in previous work.22 Here, we propose that the strong local electric field confined between the tip and sample can compel the surface hydrogen to preferably downward migration rather than desorption. Based on our experimental result. we tentatively propose that the manipulation mechanism of the reversible downward and upward migration of the hydrogen atoms is dominantly attributed to the injection of the inelastic tunneling electrons with high energy from the tip or from the subsurface polaron in the substrate to the target Hsub during the positive and negative voltage pulse, respectively. Meanwhile, the local electric field can be automatically formed in the tunneling junction and the electric field direction can be deliberately controlled by changing the polarities of the voltage pulse. As a result of the combined activation of IET effect and local electric field, the Hsub can be electrically manipulated between the energetically favorable states in a controllable and reversible manner.


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Figure 5. Statistical analysis of the Hsub and the voltage pulse effect. (a) I-V spectrum measured on top of the Hsub as indicated in the inset STM image. The black dot in (a) means the tip position during the spectrum. (b) and (c) Tunneling current recorded as a function of time during the negative and positive voltage pulse on subsurface and surface hydrogen atoms, respectively. (d) Quantum yield as a function of the negative voltage pulse. Imaging parameters of the inset in (a): f0 = 813 kHz, A = 500 pm, Q = 24500, T = 78 K, V = + 0.8 V.

To complement the interpretation of the reversible migration of the Hsub, a typical I–V spectrum recorded precisely on the top of the Hsub is presented in Figure 5a. It shows that the tunneling current can be detected for negative (positive) voltage ≤ -2 V (≥ 2 V), which corresponds to the threshold of the tunneling electrons channel. In addition, according to the tunneling current and the time the sudden jump takes of each effective negative voltage pulse, the quantum yield Y is deduced as a function of voltage pulse Vs, via58 Y = e/Iτ


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Where e is the elementary charge, I is the tunneling current during effective voltage pulse, and τ is the time it takes to a sudden jump, as shown in Figure 5d. The manipulation yield is strongly dependent on the pulse voltage. Specifically, note that, a sudden jump at about -3.5 V in the I-V spectrum in Figure 5a is observed, which is quite consistent with the sudden increase in the manipulation yield from -3 to -3.5 V in Figure 5d. It suggests that the electrically-driven migration of the subsurface defect is a much more complicated process, and it deserves further investigation, such as the Kelvin probe force spectroscopy (KPFS) technique.

CONCLUSIONS In this study, we have systematically characterized and deliberately manipulated the subsurface hydrogen Hsub on rutile TiO2(110)-(1 × 1) by nc-AFM/STM/KPFM in constant height mode at 78 K. Four different configurations of the Hsub, including the monomer, dimer, trimer and tetramer, are clearly observed and distinguished using the simultaneous nc-AFM and STM measurement. In addition, the local contact potential difference of the Hsub is mapped by KPFM with atomic resolution. In addition to characterizing the local geometric and electronic properties of the Hsub, we have also demonstrated our ability to reversibly manipulate the Hsub between subsurface and surface layers by controlling the polarity of the voltage pulse. The reversible downward and upward migrations of the Hsub can be triggered by a positive and negative voltage pulse precisely applied above the target Hsub, respectively. The underlying mechanism of the reversible migration process of the Hsub is tentatively explained by the injection of inelastic tunneling electrons from the tip or from the subsurface polaron in substrate and the local electric field confined in the tunneling junction. Our study provides systematic investigation and unprecedented manipulation approach for the subsurface defect with atomic resolution and may potentially have an overriding


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effect on revolutionizing the investigation and applications of the catalytic reactions based on the metal oxides.

ASSOCIATED CONTENT Supporting Information Topographic nc-AFM image of rutile TiO2(110)-(1 × 1) surface; dominant Hsub dimer specie recorded by nc-AFM and STM; simultaneous nc-AFM and STM characterization of the Hsub dimer; characterization of the Hsub dimer and tetramer; simultaneous nc-AFM and KPFM characterization of the Hsub dimer; downward migration of the surface hydrogen atom; symmetrically upward migration of the Hsub dimer.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Quanzhen Zhang: 0000-0003-3423-5863 Yan Jun Li: 0000-0001-7845-326X Huan Fei Wen: 0000-0002-2972-9669 Yuuki Adachi: 0000-0002-9723-1520

Notes The authors declare no competing financial interest.

ACKNOLEDGEMENTS


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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 of Japan (JP16H06327, JP16H06504, JP17H01061). This work was also supported by Osaka University’s International Joint Research Promotion Program (J171013014, J171013007, Ja19990011), and also supported by the National Natural Science Foundation of China (NSFC) (No. 61911540074) and JSPS-NSFC (J191053055). We also thank the helpful discussion with Lev Kantorovich and Hongqian Sang from department of physics, King' s College London, U. K, and Ivan Štich and Ján Brndiar from CCMS, Slovak Academy of Sciences, Slovakia.

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by Atomic Force Microscopy at 78 K

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