Multi-Channel Exploration of O Adatom on TiO₂(110) Surface by Scanning Probe Microscopy

Abstract

We studied the O₂ dissociated state under the different O₂ exposed temperatures with atomic resolution by scanning probe microscopy (SPM) and imaged the O adatom by simultaneous atomic force microscopy (AFM)/scanning tunneling microscopy (STM). The effect of AFM operation mode on O adatom contrast was investigated, and the interaction of O adatom and the subsurface defect was observed by AFM/STM. Multi-channel exploration was performed to investigate the charge transfer between the adsorbed O and the TiO₂(110) by obtaining the frequency shift, tunneling current and local contact potential difference at an atomic scale. The tunneling current image showed the difference of the tunneling possibility on the single O adatom and paired O adatoms, and the local contact potential difference distribution of the O-TiO₂(110) surface institutively revealed the charge transfer from TiO₂(110) surface to O adatom. The experimental results are expected to be helpful in investigating surface/interface properties by SPM.

1. Introduction

Scanning probe microscopy (SPM) has developed as a powerful tool for exploring the surface properties and surface dynamic process at an atomic scale on a semiconductor or insulator [1,2,3,4,5,6,7,8,9,10]. For example, atomic manipulation has been realized, and surface chemical reactions have been observed with atomic resolution by atomic force microscopy (AFM). Based on AFM, Kelvin probe force microscopy (KPFM) was developed to characterize the contact potential difference (CPD) between the substrate and cantilever tip. CPD originates from the difference of the work functions and is specifically referred to as the local CPD (LCPD) in atomic-resolution KPFM [11,12]. To date, different modes of KPFM have been successfully used to simultaneously measure surface structures and LCPD, and the surface potential of TiO₂(110) was measured [13,14,15,16,17,18,19]. Local density of states (LDOS) gives significant information of the electronic structure of the surface, which is measured by scanning tunneling microscopy (STM). Combining AFM/STM techniques has been developed to investigate the surface structure and LDOS [20,21,22]. Simultaneous measurement of tunneling current and LCPD is useful to explore the surface properties and surface reaction process, but it cannot be achieved by the conventional KPFM due to the regulation of DC bias voltage. To simultaneously characterize the electronic structure and LCPD, we previously proposed a method to achieve the frequency shift (∆f), average tunneling current (<I_t>) and LCPD by the KPFM without DC bias voltage feedback, and this method was successfully performed on the rutile TiO₂(110) surface [19]. Hence, based on AFM, multi-channel exploration has the potential to explore the charge transfer of the interface and to unravel physical features for understanding the surface catalytic process.

Oxygen molecular interaction with TiO₂ has attracted much attention and has become a prototypical model due to its importance in many surface reactions. Molecular oxygen acts as the main oxidizing reagent in many catalytic reactions and is used as an electron scavenger, which is believed to facilitate surface reactions. The adsorption and dissociation of O₂ on the rutile TiO₂(110) surface at low and room temperatures have been well characterized by STM experimental and theoretical study, where the oxygen molecular adsorbed on the Ti_5c site or the surface bridging oxygen vacancies (O_v) and the oxygen adatom (O_ad) along the fivefold-coordinated Ti_5c site is formed [23,24]. Importantly, it is believed that chemical adsorption and dissociation of O₂ on the TiO₂ surface accompanied by the charge transfer and the activation of O₂ dissociation are key factors in the reaction process, which is importantly related to their charge state. Therefore, multi-channel exploring of the state of O_ad on TiO₂(110) surface is useful in understanding its fundamental mechanism.

In this study, we first showed the dissociation state of O_ad on the TiO₂(110) surface under the different O₂-exposed temperatures by AFM, and then we characterized the O_ad by simultaneous AFM/STM, which would provide complementary information in recording the topographic and tunneling current signals. Finally, simultaneous measurements of frequency shift, tunneling current and LCPD images on the O-TiO₂(110) surface are taken by SPM. The tunneling current contrast and surface potential difference were analyzed to explore the charge transfer between the O_ad and TiO₂ surface. The experimental results and methods are useful in characterizing the other nanomaterials.

2. Experimental Details

Experiments were performed with a home-built no contact (NC)-AFM system under ultrahigh vacuum conditions (3 × 10⁻¹¹ Torr) at 78 K, which was operated in frequency modulation (FM). An AFM cantilever was oscillated at a constant amplitude and at its resonant frequency by automatic gain control (AGC). AFM/STM simultaneous measurements can be carried out in two ways, and force signal and tunneling current are recorded in the separated channel. When the frequency shift was operated as the feedback signal, topographic and <I_t> images were simultaneously recorded and <I_t> contained crosstalk of tip motion. When measurements were taken in the constant height mode, frequency shift and <I_t> could be simultaneously obtained. In this mode, two signals were independently measured, so they did not contain the artificial signal. The comparison of two operation modes will be shown in the results section. Figure 1 shows the experimental setup for simultaneous measurements of topography, <I_t> and LCPD. Topography and <I_t> images were obtained by AFM, and LCPD images were recorded by FM-KPFM in the constant height mode. The equation for V_LCPD was derived as follows; details can be found in Ref. [18,19]:

$$V_{\mathrm{LCPD}}=V_{\mathrm{DC}}-\mathrm{sgn}({\alpha }_{\mathrm{m}})\frac{V_{\mathrm{ac}}}{4}\frac{\left|\Delta f_{\mathrm{m}}\right|}{\left|\Delta f_{2\mathrm{m}}\right|}\frac{T(f_2{}_{\mathrm{m}})}{T(f_{\mathrm{m}})}$$

(1)

Here, V_DC is the dc voltage. The parameter sgn(α_m) is known by the phase difference between V_ac and |∆f_m|. |∆f_m| and |∆f_2m| are the f_m and f_2m components of ∆f, respectively. The T(f_m) and T(f_2m) are the transfer functions of the phase-locked-loop (PLL).

The signal frequency shift was obtained by the phase locked loop (PLL) and divided into two parts. One was applied to adjust the tip–sample interaction with a band elimination filter (BEF), and the other was connected to FM-KPFM by feeding it into the lock-in amplifiers. An ac bias voltage was obtained by an oscillator and acted as the reference signal. The f_m and f_2m components of the frequency shift were detected by two lock-in amplifiers. As shown in Figure 1, <I_t> was recorded in a separate channel from the tip to cancel the crosstalk, and bias voltage was applied to the sample.

The commercial Ir-coated cantilever (Nanosensors SD-T10L100, f₀ ~800 kHz) was used in the current study. The cantilever tip was first degassed at approximately 650 K for 30 min and then cleaned by Ar ion bombardment to remove the contaminants, prior to the measurements. Features of the surface structure were related to the charge states of the tip apex, and a stable tip was essential to accurately characterize the surface structure and properties in the experiment [25,26]. The imaging mode became stable in AFM experiments when the metal-coated Si cantilever was employed in the experiments [27,28,29].

The TiO₂(110) sample surface (provided by Furuuchi Chemical Corporation, Hyogo, Japan) was prepared by several cycles of Ar ion sputtering and subsequent annealing at 1000 K for 20 min. After that, the freshly cleaned surface was exposed to O₂ in the preparation chamber and then transferred into the observation chamber. AFM images were taken after the sample temperature decreased to 78 K.

3. Results and Discussion

We first introduced the surface structure model of rutile O-TiO₂ (110)-(1 × 1) and O₂ dissociation state at room temperature (RT) and 400 K. Figure 2a shows a ball model of the rutile O-TiO₂ (110)-(1 × 1) surface, which consists of alternating Ti_5c rows and sixfold-coordinated Ti_6c rows surrounded by in-plane threefold-coordinated O_3c rows and bridging twofold-coordinated O_2c rows. The single O_ad (O_ad: light green ball) formed by O₂ dissociation at the O_v site indicated that one O atom healed O_v and the other located at the Ti_5c site, and paired O_ad resulted from O₂ dissociation at the Ti_5c site.

As reported in the previous literature [30,31,32], atomic contrast in the AFM image depended on the tip apex polarity, and surface defects were used as markers to distinguish the imaging mode. Hole and protrusion modes usually appeared in the imaging modes. When the tip apex was positively charged, the O_2c row was bright on the image due to the larger attractive force between the tip and the negative O_2c row. Surface defects were imaged as dark holes, which is called the hole mode [28,29]. When the tip apex was negatively charged, the contrast was inverted compared with that in the hole mode, and H atoms appeared as brighter spots than the O_V defects, which is called the protrusion mode. Figure 2b shows the topography image of the O-TiO₂(110)-(1 × 1) surface recorded in the hole mode. According to the experiment, bridging O_2c and Ti_5c rows were imaged as bright and dark features, respectively, and the bright spots on the Ti_5c rows are O_ad. As introduced before, the single O_ad (denoted by the white dotted circle) was attributed to O₂ dissociation at the O_v site, and paired O_ad separated by three lattice distances (denoted by the white dotted oval circle) was the result of O₂ dissociation at the Ti_5c site.

Figure 3 shows two AFM topographic images of the rutile O-TiO₂(110)-(1 × 1) surface exposed to O₂ at RT and 400 K, respectively, and the corresponding line profiles along the O_ad. The contrast is the same as Figure 2b. O_2c and Ti_5c rows are imaged as bright and dark features, respectively, and the bright spots on the Ti_5c rows are O_ad. Here, paired O_ad separated by one lattice constant is denoted as the P-O_ad(1). As shown in Figure 3a,b and Figure 2b, single O_ad, P-O_ad(1), P-O_ad(2) and P-O_ad(3) are observed when O₂ is exposed to the TiO₂ surface at room temperature (RT). As introduced before, single O_ad was attributed to O₂ dissociation at the O_v site, and paired O_ad was the result of O₂ dissociation at the Ti_5c site. In our results, a single O adatom was the distinctly dominant state of O_ad, when the exposure temperature was at RT. P-O_ad(2) is the second preferred state. Density functional theory (DFT) showed the P-O_ad(2) configuration was the most preferred structure at RT, and further O_ad diffusion (P-O_ad(2) to P-O_ad(3)) was hindered by a barrier of 1.3 eV [24]. In addition, the separation of P-O_ad(1) to P-O_ad(2) was exothermic by 0.4 eV theoretically. P-O_ad(1) and P-O_ad(3) configurations were observed in our experiments, but they were rare because separation of O_ad is the result of a balance between Coulombic repulsion of two O_ad and is thermally driven, and P-O_ad(1) and P-O_ad(3) configurations can be generated.

When the sample was exposed to O₂ conditions beyond 350 K, dissociated O₂ could overcome the diffusion barrier forming the P-O_ad(3) structure [33]. The number of paired O_ad distinctly increased on the O₂-exposed surface at 400 K, and the states of the paired O_ad were mainly P-O_ad(3) and P-O_ad(5) configurations, shown in Figure 3c,d. Under high temperatures for the O₂-exposed surface (>400 K), the Ti interstitials (Ti_int) can diffuse from the bulk to the near-surface region via an interstitial diffusion mechanism [34]. The concentration and distribution of Ti_int in the near-surface region can vary significantly, depending on the level of bulk reduction. Current experiments suggest that the excess charge on these paired O_ad is mainly provided by the Ti_int rather than Ov in determining the adsorption behavior when the O₂-exposed surface temperatures went beyond 400 K. Our results demonstrate that O₂ dissociatively adsorbed on the rutile TiO₂(110)-(1 × 1) surface when the temperature of O₂ exposure was at or beyond RT, and results are consistent with the conventional STM observations [24,33]. Next, we explored the O_ad on TiO₂ surface by AFM/STM.

The four images in Figure 4 are obtained in the same area of the O-TiO₂(110) surface. Figure 4a and 4b experimentally show simultaneously recorded topographic (Z) and <I_t> images recorded in the constant frequency shift mode. In the topographic image (Figure 4a), the atomic contrast is the same as that in Figure 3a, in that the bright and dark rows are the O_2c and Ti_5c rows, respectively. A bright spot marked by the dashed white circle is O_ad. Figure 4b demonstrates the corresponding tunneling current image. Usually, the empty state is imaged at positive sample bias voltage in STM, so O_2c and Ti_5c rows are imaged as dark and bright rows, respectively, where the contrast of O_2c and Ti_5c rows is reversed compared with the topographic image. The dark spot marked by the white dashed circle is O_ad, and this is different from the conventional STM image of O-TiO₂(110), which is caused by the crosstalk of tip motion. When the tip moves on the O_ad site, the additional attractive force acts in the tip–sample interaction, and the tip has to retract in order to keep a constant frequency shift. Thus, tunneling current dramatically decreases, and the contrast of O_ad becomes a dark spot in <I_t> image. The bright spot marked by an oval circle is due to subsurface defects, which was not probed in Figure 4a, ever reported by our group or other groups [19,22]. Figure 4c is the ∆f image and Figure 4d is the corresponding tunneling current image, recorded in the constant height mode. Compared with Figure 4a,b, the image contrast is the same, except the O_ad in Figure 4d. O_ad is imaged as bright spot in the <I_t> image due to elimination of the crosstalk between topography and <I_t> signals in the constant height mode. A bright spot and two weak bright spots (denoted by the white square) are observed in Figure 4d, and they are O_ad and subsurface defects. It indicates the subsurface defect is not repulsive to O_ad. Therefore, AFM/STM is a useful technique to investigate the interaction of the adsorbate and substrate, and constant height operation is necessary. Next, we investigated the O_ad by AFM/STM/KPFM.

Figure 5 shows frequency shift, tunneling current and local contact potential difference images with atomic resolution and corresponding line profiles along the single O_ad. In the experiment, the measurement was performed in the constant height mode to prevent crosstalk between the signals of the frequency shift and tunneling current. In the Δf image shown in Figure 5a, O_2c and Ti_5c rows are simultaneously observed as bright rows with super high resolution, and O_ad is imaged as the bright spot. In the <I_t> image (see Figure 5b), the contrast was the same as in Figure 5a, except O rows are imaged as dark, which is consistent with the previous studies by conventional STM in that the conduction band of TiO₂ is dominated by Ti 3d states, and bright features are usually assigned to the empty Ti 3d states of Ti_5c ions under positive sample bias voltage, even though they lie lower than the bridging O_2c rows [35]. The tunneling current value on O_ad was higher than that on the Ti_5c rows, and the current difference between the O_ad and Ti_5c row was about 0.375 nA. The contrast in STM images depends on the different contributions of Ti 3d and O 2p states and their different decay as a function of the tip–sample separation [36]. O_ad is higher than O_2c and Ti_5c rows in surface geometry, and O_ad finally appeared as bright on the image. It was clearly observed that there was some depression around the O_ad, and this was due to the decreased numbers of empty states near the conduction band for tunneling, which resulted from the negatively charged O_ad [24]. This phenomenon was more pronounced in paired O_ad, as shown in the following tunneling current line profile. In the V_LCPD image (see Figure 5c), the imaging contrast was the reverse of that in the image, except for the O_ad. The relative value of V_LCPD between the Ti_5c and O_2c sites was approximately 28 mV from the line profile (not shown here), and this was in good agreement with our previous studies [18,19]. V_LCPD had a higher value on the O_ad than on the proximate Ti_5c rows, and the relative value of CPD between the O_ad and Ti_5c row was about 75 mV. It intuitively suggests the electrons transferred from TiO₂ to O_ad.

As shown in Figure 6, line profiles are plotted along the paired O_ad. The current difference between the O_ad and adjacent Ti_5c row was about 0.5 nA. The depression between the paired O_ad was very pronounced due to the combined effect in the corresponding line profile. The surface potential distribution was also different around the single O adatom and paired O_ad. The potential difference was 52 mV, as shown in Figure 6b. This indicates the charge state of O_ad is different, and excess electrons around the paired O_ad are shared by the paired O_ad resulting in that the relative value of CPD between the O_ad and Ti_5c row is lower. Therefore, multi-channel exploration demonstrates the powerful ability to explore surface properties with SPM.

4. Conclusions

We studied the O₂ dissociated state under different O₂-exposed sample surfaces with high resolution, and we investigated the electron charge transfer between the adsorbed O and TiO₂ substrate with multi-channel exploration. We observed the interaction between O_ad and the subsurface, and we confirmed the electron charge transfer from the TiO₂ surface to the adsorbed O upon O₂ dissociation on the TiO₂(110) surface by tunneling current and local contact potential difference. Our results demonstrated that multi-channel exploration was able to obtain the surface structures and charge transfers between the adsorbate and substrate, and this is expected to be useful for investigating the surface properties and charge transfer phenomena at the interface.