Hexagonal Boron Nitride Cover on Pt(111): A New Route to Tune

Letter pubs.acs.org/NanoLett

Hexagonal Boron Nitride Cover on Pt(111): A New Route to Tune Molecule−Metal Interaction and Metal-Catalyzed Reactions Yanhong Zhang,† Xuefei Weng,‡ Huan Li,‡ Haobo Li,† Mingming Wei,† Jianping Xiao,† Zhi Liu,§ Mingshu Chen,*,‡ Qiang Fu,*,† and Xinhe Bao† †

State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China ‡ State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, P.R. China § Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: In heterogeneous catalysis molecule−metal interaction is often modulated through structural modifications at the surface or under the surface of the metal catalyst. Here, we suggest an alternative way toward this modulation by placing a two-dimensional (2D) cover on the metal surface. As an illustration, CO adsorption on Pt(111) surface has been studied under 2D hexagonal boron nitride (h-BN) overlayer. Dynamic imaging data from surface electron microscopy and in situ surface spectroscopic results under near ambient pressure conditions confirm that CO molecules readily intercalate monolayer h-BN sheets on Pt(111) in CO atmosphere but desorb from the h-BN/Pt(111) interface even around room temperature in ultrahigh vacuum. The interaction of CO with Pt has been strongly weakened due to the confinement effect of the h-BN cover, and consequently, CO oxidation at the h-BN/Pt(111) interface was enhanced thanks to the alleviated CO poisoning effect. KEYWORDS: hexagonal boron nitride (h-BN), intercalation, Pt(111), CO oxidation, confinement effect

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We suggest that surface reactivity of a metal catalyst can be altered by placing an adlayer of two-dimensional (2D) atomic crystal on top of the metal surface (Scheme 1c). The 2D atomic crystals present strong in-plane bonding but weak out-of-plane interaction.19 When covering a metal surface with a 2D atomic sheet, molecules can intercalate the 2D cover because of its weak interaction with the metal, and metal-catalyzed reactions may happen within the confined space. Taking advantage of the strong confinement effect inside the 2D space, placing 2D atomic sheet on metal surface can be regarded as a new route to tune the molecule−metal interaction and surface reactivity (Scheme 1). Recent results demonstrate that molecules such as O2, CO, and H2O can intercalate graphene sheets grown on metals,20−30 and adsorption of the intercalated molecules on Ru(0001) and Pt(111) surfaces has been strongly weakened by the graphene cover.21−23,31 Surface reactions on metals were enhanced under the graphene cover.28,31−33 The single sheet of hexagonal boron nitride (h-BN), often called “white graphene”, is a structural analogue of graphene.34,35 Intercalation of bulk h-BN and

n heterogeneous catalysis many catalytic reactions are governed by the Sabatier principle, in which the maximum activity relies on the intermediate adsorption strength of reaction molecules with catalysts.1,2 A grand challenge toward rational design of advanced catalyst is to tune the molecule− catalyst interaction and make it “just right”.3−5 Taking COinvolved reactions on platinum group metal (PGM) catalysts as an example, strong CO adsorption on the PGM surfaces at low temperatures, for example, around room temperature (RT), often causes blockage of surface sites and prevents surface adsorption of other reactants, which is widely known as the CO poisoning effect.6,7 To optimize the surface reactivity, it is highly desirable to tune the binding energy of CO on PGM. The CO poisoning effect can be reduced by introducing other elements at the subsurface regions or alloying the PGM surfaces with the second component (Scheme 1a, b).8−12 Previous works demonstrate that Pt-skin surfaces containing subsurface 3d transition metals bond weakly with CO, which facilitates surface reactions such as CO oxidation and water gas shift (WGS) reactions.13−16 Another route to the surface modification is to alloy the PGM surface with other metals. For example, adding anoxophile transition metal at Pt surface is an effective way to develop CO-tolerant electrocatalyst in H2−O2 fuel cells.17,18 © 2015 American Chemical Society

Received: March 29, 2015 Revised: April 20, 2015 Published: April 21, 2015 3616

DOI: 10.1021/acs.nanolett.5b01205 Nano Lett. 2015, 15, 3616−3623

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(a) Alloy formation at the metal surface; (b) introduction of subsurface elements under the metal surface; (c) placing a 2D atomic crystal cover on the metal surface. CO oxidation was taken as an example of the surface reaction

Figure 1. (a−b) In-situ LEEM images recorded during h-BN growth on Pt(111) (p = 5.6 × 10−9 Torr; T = 800 °C; video is given in Supporting Information). (a) 1000 s; (b) 2200 s. The start voltage (STV) is 3 V. The red lines mark the nucleation location. (c) STM image of the h-BN/ Pt(111) surface (50 nm × 45 nm), and defects such as grain boundary and vacancies can be distinguished on the h-BN surface. (d) μ-LEED pattern (50 eV) from the h-BN/Pt(111) surface region. (e−g) In-situ LEEM images (STV = 2.25 V) acquired from a h-BN/Pt(111) surface when exposed to 5.0 × 10−8 Torr CO at RT (video is given in Supporting Information). (e) 0 s; (f) 54 s; (g) 104 s. (h) μ-LEED pattern (50 eV) of the h-BN/ Pt(111)surface exposed in 5.0 × 10−8 Torr CO. (i) I−V curves from the pristine h-BN/Pt(111) surface, the CO-treated surface, and this COintercalated h-BN structure kept in UHV at RT for 3 h.

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RESULT AND DISCUSSION Growth of h-BN on Pt(111). h-BN overlayers were grown on Pt(111) using chemical vapor deposition (CVD). In-situ LEEM imaging shows that monolayer h-BN islands nucleate at a few surface sites and then expand anisotropically on the surface with the growth dominated by downhill direction across the Pt(111) steps (Figure 1a,b and Supporting Information Figure S1a−c and Video 1). In contrast, both downhill and uphill growth of graphene islands were observed on Pt(111).42 It has been shown that strong interaction of the 2D atomic crystals with the metal substrate causes the downhill growth of graphene or h-BN overlayers.43−45 Therefore, the observed downhill growth of h-BN on Pt(111) indicates the stronger interaction of h-BN with Pt(111) than graphene. With prolonged CVD growth, large h-BN islands form and eventually a full h-BN (1 ML h-BN) layer can be obtained (Supporting Information Figure S1d−f), suggesting self-limiting growth of one monolayer h-BN on metals under ultrahigh vacuum CVD (UHV-CVD) conditions.34 The h-BN islands contain high density wrinkles after cooling down to RT (Supporting Information Figure S1g).46 Using scanning tunneling microscopy (STM), we can see domain boundaries and vacancies in the h-BN structure (Figure 1c). As discussed previously, these defects including wrinkles, domain boundaries, and vacancies play an important role in the potential molecule intercalation processes.30−32,47,48 Microregion low energy electron diffraction (μ-LEED) measurements made on the different domains show that there is only one orientation of h-BN overlayer with respect to the Pt(111) surface (Figure 1d) and well-defined moiré superstructure of the h-BN structure can be seen from the STM images (Supporting Information

epitaxial h-BN overlayers with molecules have been experimentally demonstrated,36−40 which suggests the possibility of having chemical reactions under the h-BN layers. Despite the structural similarity between graphene and h-BN, they both exhibit quite different properties. h-BN has polarity because of the strong ionicity of B−N bonds and its interaction with molecules and metals should be stronger than graphene. Furthermore, h-BN presents higher chemical and thermal stability than graphene in reactive environments, which is a big advantage for its use in chemical reactions.41 These structural characteristics make h-BN layer to act as a better promoter for metal-catalyzed reactions than graphene. In the present work, epitaxial h-BN overlayers grown on Pt(111) were used as the model surfaces, and in situ surface techniques including low energy electron microscopy (LEEM), infrared reflection absorption spectroscopy (IRRAS), and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) were applied to study CO adsorption and CO oxidation on the h-BN/Pt(111) surface. We show that CO molecules can intercalate the h-BN overlayers. More interestingly, CO molecules confined under the h-BN cover desorb from the Pt(111) surface around RT, which confirms that CO adsorption strength on Pt has been significantly weakened. Consequently, CO oxidation can happen more easily at the hBN/Pt(111) interface at relatively low temperature, which is otherwise hindered by the CO poisoning effect on the clean Pt(111) surface. These results suggest that metal-catalyzed reactions can be promoted by placing a 2D cover on the metal surface. 3617


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Figure 2. In-situ AP-XPS Pt 4f (a), N 1s (b), and B 1s (c) spectra recorded from the 1 ML h-BN/Pt(111) surface in 1 × 10−6, 0.001, 0.01, 0.1, and 0.5 Torr CO at RT. The Pt 4f7/2 peaks can be deconvoluted into bulk component, surface component, and Pt surface species bonded with CO, which are marked by the dashed lines. (d) In-situ IR spectra from 1 ML h-BN/Pt(111) surface exposed to different CO atmospheres at RT. (e) Dependence of the intensity of the top-site CO peak at 2095 cm−1 (marked in (d) by the dashed line) with the CO partial pressure. The sample was kept in each CO atmosphere for 10 min, and it takes 2 min to acquire one IR spectrum.

interface, in situ AP-XPS and IRRAS measurements under near ambient pressure conditions are conducted. Figure 2a shows that each XPS Pt 4f7/2 peak contains one bulk Pt component at 71.2 eV and one surface Pt component at 70.9 eV31,51 on the 1 ML h-BN/Pt(111) surface kept in UHV as well as in CO atmosphere with the pressure lower than 0.1 Torr. In 0.5 Torr CO, the surface component diminishes, whereas two new Pt 4f7/2 components located at 72.2 and 71.6 eV appear. These two peaks are assigned to surface Pt atoms adsorbed with CO.31,52 At the same time, additional O 1s peaks at 532.8 and 531.2 eV appear, which are attributed to CO adsorbed at top and bridge sites of Pt(111) surface (Supporting Information Figure S5).52 These data provide direct spectroscopic evidence that CO molecules can diffuse to the h-BN/Pt interface and adsorb on the Pt(111) surface underneath the hBN cover. Accompanied with the changes in Pt 4f and O 1s spectra, N 1s peak shifts from 397.5 to 397.2 eV and B 1s from 189.6 to 189.3 eV (Figure 2b and c). The negative binding energy shifts reveal that the chemical environments of N and B atoms in h-BN have been altered by the interfacial intercalation, and these atoms are now free from interaction with the metal surface.53 IRRAS was also applied to study CO adsorption on the 1 ML h-BN/Pt(111) surface at RT (Figure 2d). Visible signals of gas phase CO and surface CO adsorbed on Pt start to appear in 0.1 Torr CO. Figure 2e displays the intensity of the 2095 cm−1 peak characteristic for the stretching vibration of on-top CO54 as a function of CO pressure, which shows that 0.1 Torr is the threshold pressure for CO penetration into the 1 ML h-BN/ Pt(111) interface, similar to the AP-XPS results. It should be noted that the AP-XPS O 1s spectra (Supporting Information Figure S5) and IRRAS spectra (Figure 2d) did not show any CO signal adsorbed on the surface in CO atmosphere with pressure lower than 0.1 Torr, which excludes the possibility of CO adsorption on the h-BN surface and are consistent with the weak CO adsorption (0.02 eV) on graphitic BN structure at RT.55 CO Desorption from the h-BN/CO/Pt(111) Surface. After demonstrating CO intercalation at the h-BN/Pt(111)

Figure S1h−i), which all confirm the epitaxial growth of h-BN on Pt(111).49 Compared to multiple orientations of graphene overlayers on Pt(111),42 the single orientation of h-BN on Pt(111) again substantiates the stronger interaction of h-BN with Pt(111) than graphene. CO Intercalation under h-BN Islands. The as-prepared hBN islands on Pt(111) were exposed to 5.0 × 10−8 Torr CO at RT, and real-time LEEM imaging was made simultaneously (Video 2 in Supporting Information). Upon the CO exposure, the bare Pt(111) surface region gets slightly darker due to quick CO adsorption (Supporting Information Figure S2). At the same time, significant contrast change occurs at the h-BN islands, which starts from the island edge and propagates to the island center (Figure 1e−g). Such a contrast change phenomenon has also been observed in molecule intercalation of graphene islands.22,23,43 A typical μ-LEED pattern taken from the CO-treated h-BN islands is shown in Figure 1h. The satellite diffraction spots characteristic for the surface moiré superstructure49 (marked in Figure 1d) disappear upon the CO exposure, which indicate decoupling of the h-BN overlayer from the metal surface. For the LEEM intensity versus incident electron energy (I−V) curve acquired from the CO-treated h-BN islands two sharp dips at 2.5 and 5.3 eV were observed instead of the shallow dip at 6.7 eV from the pristine h-BN islands (Figure 1i). The sharp dip structures of the I−V curve between 0 and 10 eV serve as fingerprints of intercalation at graphene/metal and h-BN/metal interfaces.21,22,50 Besides, moiré patterns of h-BN islands disappear after the CO exposure (STM, Supporting Information Figure S3) due to the levitation effect of intercalated CO molecules.21,27 On the basis of the LEEM, μ-LEED, and STM results, we conclude that CO molecules can intercalate h-BN islands at RT. CO Intercalation under Full h-BN Layer. Under the similar CO exposure condition no CO intercalation was observed on the full h-BN layer (1 ML h-BN/Pt(111)). However, CO intercalation could occur on the surface after exposure to 1 bar CO at RT (Supporting Information Figure S4). To elucidate CO intercalation at the 1 ML h-BN/Pt(111) 3618


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Figure 3. (a−c) In-situ LEEM images (STV = 2.25 V) acquired from a CO-intercalated h-BN/Pt(111) surface kept in UHV for different time (video is given in Supporting Information). (a) 0 s; (b) 3500 s; (c) 7000 s. (d) μ-LEED pattern (50 eV) of the h-BN/CO/Pt(111) surface when keeping in UHV for 2 h. (e) LEEM intensity of the selected area from CO-intercalated h-BN islands versus desorption time in UHV at 35, 45, and 55 °C. The inset shows the Arrhenius plot of the CO desorption rate versus the reciprocal of the desorption temperature. (f) IR intensity of the top-site CO adsorption peak at 2095 cm−1 as a function of heating temperature in UHV. IR spectra were recorded from a CO-intercalated 0.6 ML h-BN/Pt(111) surface and a CO-saturated Pt(111) surface.

Figure 4. (a) LEEM image intensity of a h-BN island as a function of exposure time in 5 × 10−8 Torr CO at RT showing the repeated CO intercalation process. AP-XPS N 1s (b) and O 1s (c) spectra from a 0.5 ML h-BN/Pt(111) surface at 60 °C when exposed to 1 × 10−5 Torr CO and then pumped back to UHV for three cycles. I, pristine h-BN/Pt(111) surface; II, 1 × 10−5 Torr CO; III, UHV; IV, 1 × 10−5 Torr CO; V, UHV; VI, 1 × 10−5 Torr CO. (d) Schematics of CO intercalation of BN layer in CO atmosphere and CO desorption from the h-BN/Pt interface in UHV. Red ball, O; yellow ball, C; blue ball, Pt.

adlayer adsorbed on Pt(111) after evacuating CO for 2 h (Supporting Information Figure S6). CO desorption from the h-BN/CO/Pt(111) surface was also done at the temperatures of 35 °C (Supporting Information Figure S7), 45 °C (Supporting Information Figure S8), and 55 °C, respectively (Supporting Information Figure S9). Quantitative analysis of in situ LEEM videos produces three LEEM intensity versus time (I−t) curves recorded from the h-BN islands, which are nearly linear (Figure 3e). Altman and coworkers56 have reported that LEEM image intensity (I) of a CO-exposed Pt(111) surface presents a linear dependence on CO coverage (θCO): I = α·θCO (α is a constant). Accordingly, the linear character of the I−t curves displayed in Figure 3e suggests a constant CO desorption rate under each desorption condition, which is independent of θCO and, thus, characteristic

interface, we further studied CO desorption from the COintercalated h-BN/Pt(111) surface (h-BN/CO/Pt(111)). As depicted in Figure 3a−c and Video 3 in Supporting Information, the LEEM image brightness of the h-BN islands increased once the CO supply was cut off. After keeping the surface in UHV at RT for 7000 s, all h-BN islands became bright. μ-LEED patterns taken from the h-BN islands show that the satellite spots from the moiré superstructure reappear (Figure 3d). Meanwhile, the line shape of the I−V curve resembles that of the pristine h-BN/Pt(111) surface (Figure 1i). These results confirm that the intercalated CO molecules can desorb from the h-BN/CO/Pt(111) surface in UHV at RT and the h-BN overlayer resumes coupled with the Pt surface. In contrast, the LEED pattern and I−V curve acquired from the Pt(111) surface region still show the strong features of CO 3619


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Nano Letters for the zero-order desorption kinetics.57,58 The fitting of the linear segments yields the following desorption rate constants (Kdes): 0.00078 s−1 at 35 °C, 0.0039 s−1 at 45 °C, and 0.02167 s−1 at 55 °C. Desorption barrier (Edes) can be derived from the Arrhenius plot of the rate constants versus reciprocal of the desorption temperature, which is 1.45 ± 0.15 eV (inset of Figure 3e). The CO desorption barrier on Pt(111) has been previously determined to be ranged from 1.67 to 1.91 eV.59 Edes measured at the h-BN/Pt(111) surface is much less than that on the Pt(111) surface. These results demonstrate that CO interaction with the Pt surface has been significantly weakened by the h-BN cover. Temperature-programmed CO desorption from Pt(111) and h-BN/Pt(111) surfaces was studied by in situ IRRAS. A 0.6 ML h-BN/Pt(111) surface was treated in 5 Torr CO at RT for 10 min in a high pressure cell. After evacuating the cell to UHV, the sample was stepwise heated up and IRRAS spectra were recorded at elevated temperatures (Supporting Information Figure S10a). The 2095 cm−1 peak intensity was plotted against the heating temperatures (Figure 3f), and the most intensive change was observed between RT and 75 °C. The control experiment was done on the clean Pt(111) surface, in which the 2095 cm−1 peak intensity keeps on decreasing from 50 to 175 °C (Figure 3f and Supporting Information Figure S10b). This spectroscopic study again confirms that CO desorbs more quickly and intensively from the h-BN/CO/Pt(111) surface with the feature of the zero-order desorption, which is in stark contrast to the CO/Pt(111) surface. Reversible CO Intercalation and CO Desorption at the h-BN/Pt(111) Interface. We found that CO intercalation and CO desorption at h-BN/Pt(111) interface is reversible. CO intercalation under h-BN islands was made in 5.0 × 10−8 Torr CO at RT (Supporting Information Figure S11a−c), and then the intercalated CO molecules were desorbed by annealing in UHV at 100 °C. After cooling down to RT, the surface was exposed to 5.0 × 10−8 Torr CO for the next intercalation process. This cycle can be repeated for many times (Supporting Information Figure S11). For example, the changes of LEEM intensity (from the selected region marked in Supporting Information Figure S11a) with CO exposure time were displayed in Figure 4a for the first three cycles. The CO intercalation process occurs more and more quickly since more h-BN island edges have been opened and CO penetration through the edge sites becomes promoted.26 This reversibility was further supported by AP-XPS experiments. A 0.5 ML h-BN/Pt(111) sample was kept alternatively in 1 × 10−5 Torr CO and in UHV at 60 °C, and XPS N 1s and O 1s spectra were simultaneously acquired (Figure 4b,c). In the CO atmosphere (stage-II, stage-IV, and stage-VI), the N 1s spectra have the main peak position at 397.2 eV and two strong O 1s peaks at 532.8 and 531.2 eV appear, which all indicate CO intercalation of the h-BN structure. Under UHV condition (stage-I, stage-III, and stage-V) the N 1s peaks move back to 397.5 eV, the same as that of the pristine h-BN/Pt(111) surface, and the O 1s peaks intensity decreases to almost half which is from CO adsorbed on the bare Pt surface. The reversible CO intercalation of h-BN/Pt(111) interface can be illustrated in Figure 4d. When the h-BN/Pt(111) surface is exposed to CO atmosphere, CO molecules can penetrate into the h-BN/Pt interface. As soon as CO is pumped away the intercalated CO molecules desorb from the h-BN/Pt(111) interface around RT. It should be noted that the N 1s intensity keeps unchanged during the CO intercalation and CO

desorption processes. Combined with the STM results (Supporting Information Figure S3), we can infer that the hBN structure has not been damaged by these interfacial processes. CO Oxidation Underneath h-BN Cover. The feasible and repeatable CO intercalation of h-BN cover suggests that COinvolving catalytic reactions may be performed at the h-BN/ Pt(111) interface. Furthermore, in situ AP-XPS investigations demonstrate that oxygen can intercalate the 1 ML h-BN overlayers in 0.1 Torr O2 (Supporting Information Figure S12), similar to the oxygen intercalation at h-BN/Ru(0001) interface.39 Therefore, CO oxidation reaction (8 Torr CO and 4 Torr O2) was further tested on the h-BN/Pt(111) surfaces at 398 K, and the BN coverage was controlled by growth time with the growth rate of 0.1 ML per 3 min. CO2 formation rate was measured by a gas chromatograph (GC).60 The turnover frequencies (TOF/molecules·site−1·sec−1) were calculated with the assumption that there are 1.5 × 1015 Pt atoms on per square centimeter close-packed Pt(111) surface. The reaction data (Figure 5a) show that the surface reactivity has been improved by h-BN overlayers and the promotion effect was the most significant when the h-BN growth time was less than 6 min.

Figure 5. (a) TOF of CO2 formation on h-BN/Pt(111) surfaces at 398 K with various h-BN coverages. The h-BN coverage was controlled by the growth time, and rate is around 0.1 ML per 3 min. CO oxidation was conducted in the batch reactor with 8 Torr CO and 4 Torr O2 for 10 min, and reaction gas was analyzed by GC. (b) Arrhenius plots of TOF of CO2 formation on Pt(111) and 1 ML hBN/Pt(111) surfaces in the temperature range of 475−575 K.

The reaction kinetics were further studied by carrying out the CO oxidation on both clean Pt(111) surface and 1 ML h-BN/ Pt(111) surface in the temperature range of 475−575 K. To avoid any deactivation induced by the high pressure reactions, each reaction was performed on the newly prepared Pt(111) and 1 ML h-BN/Pt(111) surfaces. The Arrhenius plots of TOF of CO2 formation versus the reciprocal of reaction temperatures are shown in Figure 5b. The apparent reaction activation energy on the 1 ML h-BN/Pt(111) surface was derived to be 72 kJ/mol, which is lower than that on the Pt(111) surface (91 kJ/mol). The reaction rate on the h-BN/Pt(111) surface is slightly higher than that on the Pt(111) surface in the low temperature regime, similar to the finding in Figure 5a. Because the reaction was conducted at the full h-BN layer, which covers the whole surface of the Pt crystal, the observed catalytic reactions must occur on Pt surface under the h-BN cover. Discussion. The demonstrated CO intercalation and CO oxidation processes on the h-BN/Pt(111) surfaces suggest that 3620


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intercalated CO molecules levitate the h-BN cover from the metal surface making it free-standing. On the other hand, the hBN cover exerts a strong confinement effect on CO adsorption on Pt(111) such that CO desorbs from the h-BN/Pt interface around room temperature and the calculated CO adsorption energy decreases from 1.99 to 1.48 eV with the presence of hBN cover. The space between the h-BN cover and the Pt surface can act as 2D nanoreactor, and the Pt-catalyzed CO oxidation occurs with lower apparent barrier due to the alleviated CO poisoning effect. This work confirms that the hBN cover can be used as a promoter to tune molecule−metal interaction and enhance the metal-catalyzed reactions.

the space between BN cover and metal surface can act as 2D nanoreactor (Scheme 1c). The confinement effect from the top h-BN cover weakens CO adsorption strength on Pt surface, which alleviates the CO poisoning effect and enhances the Ptcatalyzed CO oxidation, particularly at low temperatures. Compared to graphene, the h-BN cover has exerted a much stronger confinement effect because we observe the complete CO desorption from Pt surface under the BN islands near RT (Figure 3) but the complete CO desorption from the graphene/Pt(111) occurs around 100 °C.31 To elucidate the different effect of BN and graphene, density functional theory (DFT) calculations have been conducted to study interaction of the 2D covers with Pt(111) as well as CO adsorption on Pt under the covers. The optimized graphene-Pt(111) distance is 3.20 Å and that at the BN-Pt(111) interface is 3.16 Å. 3/7 ML CO coverage with a ratio of 2:1 for CO adsorbed at top and bridge sites has been adopted in accordance with the experimental results (Supporting Information Figure S13). With the Pt(111) surface covered by graphene, the averaged CO adsorption energy decreases from 1.99 to 1.57 eV. The adsorption energy can be further reduced to 1.48 eV under hBN. The stronger confinement effect of h-BN than graphene has been clearly revealed by the DFT calculation, which is consistent with the experimental finding. As shown above the thickness of the space under the two covers is similar and thus the geometric confinement effect on CO adsorption should be trivial. Instead, we suggest that it is the electronic interaction of the 2D cover with the metal surface that determines the confinement effect. The polar h-BN sheet may interact more strongly with a metal than the nonpolar graphene, which has been substantiated by the growth data shown in Figure 1. The much stronger interaction of the 2D cover with the metal, the more significant confinement effect inside the nanoreactor. On the basis of this consideration, the confinement effect can be controlled by using various 2D atomic crystals, such as doped graphene, 2D oxides, and 2D chalcogenides,35 which enables us to tune the molecule−metal interaction and surface reactivity in a wide range. It should be noted that the h-BN coverage can influence the surface chemistry of CO at the h-BN/Pt(111) interface. As demonstrated above, CO intercalates h-BN nanoislands in 10−8 Torr CO, whereas 0.1 Torr CO is needed to see the CO intercalation of the full h-BN layer. The similar dependence of the CO intercalation pressure on the coverage of the 2D cover has been observed for CO intercalation of graphene/Pt(111) interface (Supporting Information Figure S14). For CO desorption, we demonstrate that CO confined under h-BN islands can desorb completely at RT (Figure 3), whereas desorption of CO molecules intercalated under the full h-BN layer happens continuously from RT to 250 °C (Supporting Information Figure S15). Overall, both CO intercalation and CO desorption are impeded on the full h-BN, in which molecule diffusion mainly happens through domain boundaries and needs overcome higher barrier compared to the channel of h-BN island edges on the submonolayer h-BN/Pt(111) surface. Therefore, h-BN or graphene adlayer structure consisting of highly dispersed monolayer islands should be preferred to have enhanced reactions under the covers.

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METHODS

In-Situ LEEM Characterization. LEEM experiments were carried out in an UHV Elmitec LEEM-III system, which consists of a preparation chamber, a main chamber for imaging, and a vacuum ultraviolet laser source (λ = 177.3 nm).61 Samples were heated by tungsten filament heating and electron bombardment at the backside, which temperatures were measured by a W−Re thermocouple welded at the sample stage and calibrated by an infrared thermometer (Land Cyclops 100). Ammonia borane (NH3−BH3) in a steel container was heated up to 130 °C and the vapor produced in the container can be leaked into the main chamber for h-BN growth. Clean Pt(111) surface was exposed to the vapor with partial pressure of 10−9−10−8 Torr at 800 °C, followed by quick cooling to RT. The growth process was monitored by in situ LEEM, and h-BN coverage was controlled by the gas exposure. Catalytic Reaction Test. Another UHV system was equipped with Auger electron spectroscopy (AES) and LEED. h-BN overlayers were grown on the Pt(111) surface by the same process adopted in the LEEM system. The coverage and quality of h-BN layers were monitored by AES and LEED. A high pressure cell (1.2 L in volume) was attached to the bottom of the UHV chamber, which was used for in situ IRRAS measurement and catalytic reaction test. High-purity CO and O2 were introduced into the high-pressure cell after being purified by a liquid nitrogen trap, and CO oxidation was conducted in the cell with 8 Torr CO and 4 Torr O2.60 The CO2 formation rate was measured by using GC connected to the reaction cell or monitoring the pressure change of the reaction gas with a baratron nanometer. In-Situ AP-XPS Study. AP-XPS experiments were performed at the beamline of Advanced Light Source, and the specially designed photoemission spectrometer can work under near-ambient pressure conditions.62,63 Then, 5 × 10−7 Torr borazine (B3N3H6) was input into the analysis chamber with the sample temperature at 780 °C. The growth of BN on the Pt(111) surface was in situ monitored by recording the N 1s spectra along with the elapse of growth time. High-purity CO and O2 were leaked into the analysis chamber. Pt 4f and B 1s spectra were acquired with the photon energy of 275 eV. O 1s and N 1s spectra were recorded at the photon energy of 735 eV. The binding energy was calibrated by referring to the Fermi edges. In all the three surface analysis systems, the Pt(111) surface was prepared by cycled Ar+ sputtering (2.0 keV, 7.5 × 10−6 Torr Ar, 10 min), oxidation (530 °C, 1.3 × 10−6 Torr O2, 5 min), and annealing in UHV at 800 °C until a clean surface was obtained.

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CONCLUSION We have demonstrated that CO molecules readily intercalate hBN islands on Pt(111) in 10−8 Torr CO and penetrate into the 1 ML h-BN/Pt(111) interface in 0.1 Torr CO at RT. The 3621


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ASSOCIATED CONTENT

S Supporting Information *

LEEM images, STM images, LEEM videos, AP-XPS spectra, I− V curves, IRRAS spectra, structural models, coverage effect graph, and additional references. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (Nos. 21222305, 21373208, and 21321001), Ministry of Science and Technology of China (Nos. 2011CB932704, 2013CB933100, and 2013CB834603), and the Key Research Programme of the Chinese Academy of Science (Grant No. KGZD-EW-T05). The Advanced Light Source and beamlines 11.0.2 and 9.3.1 are supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, and Chemical Sciences Division of the U.S. Department of Energy under contracts No. DE-AC0205CH11231.

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Hexagonal Boron Nitride Cover on Pt(111): A New Route to Tune

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