CO Induced Single and Multiple Au Adatoms Trapped by Melem Self-Assembly

HUANG Lili, SHAO Xiang

Department of Chemical Physics, CAS Key Laboratory of Urban Pollutant Conversion, Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, P. R. China.

Abstract: The controllability of metal adatoms has been attracting ever-growing attention because the metal species in particular single-atom metals can play an important role in various surface processes, including heterogeneous catalytic reactions. On the other hand, organic self-assembly films have been regarded as an efficient and versatile bottom-up method to fabricate surface nanostructures, whose functionality and periodicity can be highly designable. In this work, we have developed a novel strategy to steer the generation and distribution of metal adatoms by combining the surface self-assemblies with exposure to small inorganic gaseous molecules. More specifically, we have prepared a honeycomb structure of melem (triamino-s-heptazine)on the Au(111) surface based on a well-structured hydrogen bonding network. The achieved melem self-assembly contains periodic hexagonal pores having diameters as large as around 1 nm. More importantly, the peripheries of the nanopores are decorated with heterocyclic N atoms that can probably form strong interactions with the metal species. Upon exposing the melem self-assembly to a CO atmosphere at room temperature, a fair number of Au adatoms were produced and trapped inside the nanopores encircled by the melem molecules. Single or clustered Au vacancies were concomitantly formed that were also trapped by the melem pores and stabilized by the surrounding molecules, as confirmed by high-resolution scanning tunneling microscopy (STM) images.Both types of added species showed positive correlations with the CO exposure and saturated at around 0.01 monolayer.In addition, owing to the large pore size, as well as the presence of multiple docking sites inside the nanopores, more than one Au adatom can reside in a melem nanopore; they can be distributed in a variety of configurations for bi-Au (two Au adatoms) and tri-Au (three Au adatoms) species, whose population can be manipulated with the CO exposure. Moreover,control experiments demonstrated that these CO-induced Au species, including the adatoms and vacancies, can survive annealing treatments up to the temperature at which the melem molecules start to desorb, indicating a substantial thermal stability. The formed Au species may hold great potential for serving as active sites for surface reactions. More interestingly,the bi-Au and tri-Au species have moderate Au-Au intervals, and can be potentially active for certain structurally sensitive bimolecular reactions. Considering all these aspects, we believe that this work presents a fresh approach to utilizing organic self-assembly films and has demonstrated a rather novel strategy for preparing various single-atom metal species on substrate surfaces.

Key Words: Au adatom; CO; Melem; Self-assembly; STM

1 Introduction

The active metal species are at the center of concerns in heterogeneous catalysis researches1–3. In particular, the single atomic metal species are attracting more and more attention recently due to a number of unique characteristics4–8. Native adatoms on metal surfaces delegate a special group of such monoatomic metal species which are frequently involved in various surface processes including aggregation, diffusion, and reactions9–13. In principle, the native adatoms can be manipulated via the orientation, composition as well as the temperature of the surfaces9,14,15. However, it is usually beyond the controllability for a selected surface under fixed temperature conditions. Under such circumstances, modifying or reconstructing the surfaces via either a grafted molecular adlayer or certain pretreatments can provide a versatile strategy to steer the distribution as well as the properties of these metal adatoms16–19.

As a practically efficient surface modifier, organic selfassembly films hold a number of advantages in the flexibility for designable functionality and simplicity for fabrication20–22. A large number of organic self-assembly films have been investigated on various surfaces, which have presented very rich periodic structures with designable distances and geometry23–25.Moreover, the binding strengths for the incorporated functional groups with the metal species can be well selected ranging from strong coordinative bonding to weak van de Waals interactions26,27. This actually demonstrates a prevailing characteristic in catalytic applications that a good catalyst normally has a moderate interaction with the reactant molecules.Aside from the surface grafted organic films, the inorganic adsorbate can also exert significant influence on the surface adatoms either electronically or geometrically. For instance, the strongly surface-bound oxygen or hydroxyls can directly build up energy traps for capturing the single atomic metals28–30. On the other hand, the weakly adsorbed CO molecules may also induce surface reconstructions after large number of impinging events particularly under high pressure atmosphere31–33.

In our previous work we have actually combined the above two strategies together. By exposing the melamine assembly film on Au(111) to a CO atmosphere, we have been able to manipulate the creation of Au adatom and Au vacancy species which are trapped by the nanopores encircled by the selfassembled melamine molecules34. However, in that case the Au adatoms were only physically blocked by the surrounding melamine molecules without substantial strength. The relatively low stability were considered improper to serve as sustainable active sites for real catalysis. An amendatory way is to introduce more proper anchoring sites into the molecular building blocks which has considerable binding strength with these Au adatoms.Therefore in this work, we have considered the trimerization product of melamine, melem (triamino-s-heptazine), which can also form similar honeycomb self-assembly structure but with larger pores concomitantly decorated with heterocyclic N (h-N)atoms at the sides35–39. Our scanning tunneling microscopy(STM) experiments clearly show that exposing the melem film to CO atmosphere can efficiently generate quite a number of Au adatoms inside the nanopores of melem, whose concentration and stability are greatly enhanced in comparison with those on melamine films. Moreover, there are multiple Au atoms residing with versatile geometries in the melem pores, whose appearance frequency can be manipulated via the exposure amount of CO.These single or multiple Au atom species can potentially provide unique active sites for surface reactions.

2 Experimental

The experiments were performed on a commercial lowtemperature STM (LT-STM, Createc Co., Germany) which is housed in a UHV chamber with base pressure of 1 × 10-10mbar(1 mbar = 100 Pa). The atomically flat Au(111) surface was prepared by repeated cycles of Ar+sputtering and annealing. The melem molecules (Ambinter, 90+%) were thoroughly degassed(～300 °C) inside a Knudsen-cell type evaporator (Createc Co.,Germany) for 10 h in vacuum before deposition. During deposition the substrate was kept at room temperature (RT). But in order to prepare a pure honeycomb melamine film the samples were annealed to around 380 K for half an hour. CO (Air product Co., 99.999%) was introduced to the sample surface by background feeding through a variable leak valve. The partial pressure of CO was controlled at 1.0 × 10-7–1.0 × 10-6mbar and the samples were kept at RT. For all STM experiments the electrochemically etched tungsten tips were used. And the STM images were acquired with constant current mode at liquid nitrogen (LN2) temperature.

3 Results and discussion

Scheme 1 Molecular structure of melem (a) and the possible intermolecular hydrogen bonding patterns (b–d).

The self-assembly of melem has been investigated before on several substrates including Au(111), Ag(111) and HOPG35-39.As shown in Scheme 1, the molecule has a triangular shaped planar structure. It possesses three amino endgroups (denoted as D) and six on-side heterocyclic nitrogen atoms (denoted as A),through which the intermolecular hydrogen bonding (HB) pairs can be formed and the corresponding assembly structures can be constructed on inert surfaces. Honestly, all the predictable HB patterns (Scheme 1b–d) have been observed in the assemblies of melem35,39. We want to point out that in the cases of Scheme 1c,d, wherein the [D1-A2][A2-D1] and [D1-A1][D2-A2] HB pairs are formed, all the on-side nitrogen atoms are either occupied or protected hence unavailable for accepting other species.Whereas for Scheme 1b, wherein the [D1-A1][A1-D1] HB pairs are formed, the A2-N atoms would be completely free and may provide the docking positions for the metal species. With this in mind, we will mainly focus on the assembled honeycomb structure formed by the [D1-A1][A1-D1] HB pairs. The experimental results are shown in below.

As already addressed in our previous report, a nearly pure phase of honeycomb structure of melem (termed as H-melem)can be achieved by controlling the melem coverage during deposition in combination with subsequent thermal annealing35.As shown by the STM image in Fig. 1a, each triangular shaped feature corresponds to one melem molecule lying flat on the Au(111) surface. The inserted ball model in Fig. 1a shows how the melem molecules interconnect each other by the [D1-A1][A1-D1] HB pairs and spread into periodic honeycomb structure. After exposing the H-melem film to CO atmosphere,there were many new protrusions observed in the hexagonal pores, as highlighted by the inserted STM image in Fig. 1b.These added species have a round shape and a diameter around 0.5 nm. Moreover, the number of these added bright spots(ABSs) increased along with the CO exposure, as seen in Fig.1b–f, and gradually reached a saturation of ～0.011 ML(monolayer) after the exposure rose to above 1200 L (Langmuir,1 L = 1 × 10–6Torr·s, 1 Torr ≈ 133 Pa). Here we define one monolayer as one added species per unit cell of the Au(111)surface. All these aspects of the CO-induced ABS species here are consistent with the CO-induced Au adatoms on the melamine assembly films over Au(111) surface34. Therefore, we tentatively assign them as the Au adatoms generated upon CO exposure and trapped inside the hexagonal pore of melem molecules.

Fig. 1 STM images of (a) the pure honeycomb structure of melem on Au(111) and after exposing to different amounts of CO for(b) 100 L, (c) 400 L, (d) 800 L, and (e) 1200 L at RT.

Fig. 2 Variously different residing positions for Au adatom positions inside a melem pore as proposed (a) and as observed under STM (b–h).

The nanopores encircled by melem molecules in the H-melem structure are measured as ～1 nm in diameter. As depicted in Fig.2a, each hexagonal pore has six equivalent h-N atoms exposed to the inside. In addition, the center of the pore is also available for Au adatom but without any connection to the surrounding molecules. Therefore, in the melem pore there would be totally seven possible positions for holding the Au adatoms. In fact, all these predicted configurations have been observed in our STM experiments, including six on-side (Fig. 2b–g) and one center(Fig. 2h) configurations. Such versatility was not observed in melamine self-assembly since its pore size is much smaller (～0.6 nm) and no free nitrogen atoms are available for binding the Au adatom. We notice that the on-side Au atoms looked almost the same bright as the melem molecules whereas the center Au looked rather dimmer. This is probably due to the chemical bonding of the h-N atom which has modified the electronic property of the on-side Au atoms. On the other hand, the centered Au is only interacting with the substrate without the interference of the melem molecules hence may keep the original electronic property. During the scanning, we never observed any diffusion/hopping events of the on-side Au adatoms. Moreover,annealing experiments (not shown here) demonstrated that these Au adatoms can be stabilized until the temperature (at around 200 °C) when melem began to desorb. Both evidences manifest the relatively high stability of the Au adatoms trapped inside the melem assembly, which can be attributed to the strong interactions formed between melem and Au adatom. In contrast,the Au adatoms trapped by the hexagonal pore of melamine were frequently observed hopping from one pore to another during STM scanning, dictating a much weaker stability.

Considering the interaction between Au atom and the melem molecule, the h-N atom has a lone pair of electrons occupying the sp2hybridized orbital while Au has relatively large electronegativity. Therefore, the on-side Au atoms may be negatively charged when bonding to the h-N sites hence display different characteristics as those free of such interactions.Previously on melamine assembly films we expounded our proposition of the CO-induced Au adatom by their biasdependent changes of the topographic heights34. Here we resorted to the similar experimental strategy. As shown in Fig. 3,there are two separated Au adatoms in the selected STM image.Along with the bias decreasing from 2.0 to 1.0 V and further to 0.1 V, the height of the measured Au adatom changed from around 72 to 83 pm and further to 100 pm. However, the biasrelated contrast changes are in reversed direction in comparison with the Au adatoms on melamine films, possibly due to distinct charging state of the Au species in this study. One may notice that the second Au adatom in Fig. 3 shows a little different brightness as compared to the measured one. Actually, such brightness fluctuations of the melem-trapped Au species were very commonly observed on our samples, but their bias-related contrast changes more or less follow the same tendency (not shown here). For this topographic variations of the Au species we propose several potential explanations. One possible reason may be that the absolute positions of the Au atoms relative to the melem molecules and the substrate are slightly different from one to another. A second possible explanation may be the molecular adsorbates on some of the Au species. Nevertheless,this proposition still lack enough experimental supports since we did not observe any tip-induced desorption or hopping events.Finally the contrast variations may indicate that the Au species can have varied sizes, particularly when the concentration of the Au adatoms are raised to a high value. Further elucidation of the detailed structure of the Au added species would demand more systematic in-situ experiments of molecular adsorption in combination with detailed theoretical simulations, which will be conducted in our future explorations.

Fig. 3 Topographic dependence of the Au adatom on imaging bias.

Fig. 4 STM images of four different residing configurations of two Au atoms trapped in one melem pore.

Aside from the well-resolved singly dispersed Au adatoms(termed as s-Au), there were also many split ABSs species which can be assigned as two Au adatoms (termed as bi-Au, and multi-Au if the number is larger than 2) trapped simultaneously inside a same melem pore. As illustrated in Fig. 2a, the hexagonal pore of melem is as large as 1 nm in diameter and possesses up to seven docking sites for Au atoms, which makes it feasible to accommodate more than one Au adatoms. In fact, in our experiments of high CO exposure, we frequently observed two or more Au adatoms trapped in the melem pores. Fig. 4a–c show the STM images of the bi-Au atoms residing at othor-, meta-,and para- positions of the hexagonal melem pore. The corresponding tentative models are shown in Fig. 4e–g. Fig. 4d,h show another special configuration composed of one on-side Au atom and one centered Au. But the centered Au atom is not exactly residing at the center position but slightly shifts closer to this N-captured Au atom, possibly due to the attractive Au–Au interactions.

The formation of the bi- and multi-Au species has close relationship with the CO exposure, or in other words, with the total number of the Au adatoms. As can be seen in Fig. 5, along with the increase of CO exposure, the generated s-Au species were gradually increased (marked by the black circles), whereas concomitantly the number of the bi-Au species (marked by blue circles) also grew steadily. For instance, when CO was dosed for 800 L, almost 75% of the Au adatoms were singly dispersed in the melem pores. However, when CO exposure increased to around 1200 L, almost half of the generated Au adatoms were residing with a certain bi-Au configurations. On the largeexposure sample surface one can even observe multi-Au species as pointed out by the white arrow in Fig. 5e. To stepping away from the annoying issues of possible impurities caused by large CO exposure, we did not go further to try extreme exposure conditions. Nevertheless, the current results already clearly demonstrate that the distribution of various types of Au species can be readily manipulated by the CO dosage.

Fig. 5 STM images showing the increased appearance frequency of the bi-Au and multi-Au species inside the melem pores along with the increase of CO exposure (a, 100 L; b, 400 L; c, 800 L; d, 1200 L).

Different from the singly dispersed Au adatoms which may activate single molecules, the multi-Au species can be of potential interests for bimolecular surface reactions that demand two active sites with specific geometry. Previously Chen et al.40studied the dehydrogenation esterification between ethylene and acetic acid on bimetal model catalyst. They found that the Au dimers on Pt(100) gave much higher activity than those on Pt(111), which was attributed to the more proper Au–Au distance in the former case. Such ensemble effect have also been reported in many other bimolecular reactions3,41,42. Similarly, the observed bi-Au species here may also hold great potentials for catalyzing some specific bimolecular reactions. The related investigations will be conducted in the near future.

Finally, we would briefly discuss the formation mechanism of the Au adatom species on the melem self-assembly film under CO exposure. It is well known that CO adsorption can induce significant surface reconstructions for active metals43–45. For coinage metal surfaces including Au and Cu, when CO exposure is large enough, for instance under high pressure regime, the surface can also be changed by the impinging CO molecules31–33,46.Reasonably, we propose in our case, even though under low pressure (lower than 10-6mbar) and relatively low exposure, the impinged CO molecules still have the certain capability to lift the Au atoms from original terrace positions. The formed transient CO-Au species may diffuse on the surface and release CO at some point. When the Au surface is free of any other adsorbates, the neat consequence of the above process would be ignorable since the diffusing Au adatoms will quickly heal the vacant positions at room temperature. However, in the presence of organic molecules such as melamine and melem in our cases,the diffusion of the Au adatoms may be blocked or trapped by the molecules, particularly in the current case of melem which contains functional groups for binding the Au adatom. As a result, both the Au adatoms and the Au vacancies are preserved even after CO desorbs, as shown by the schematic model in Fig. 6c.Such proposition is supported by our observation of an equivalent number of Au vacancies stabilized by the films of melamine on Au(111)34. Here in the current case of melem, we also observed Au vacancies which were trapped in the hexagonal melem pores, as evidenced by the STM image and the corresponding height profile in Fig. 6a and 6b, respectively. The only difference is the nonequivalent number of the Au vacancies relative to Au adatoms as well as the nonequivalent shapes of the Au vacancies themselves. A plausible reason can be that the hexagonal melem pore is large enough to be impinged by multiple CO molecules hence allows the formation of Au vacancy clusters of more than one Au atoms. Moreover, the formed Au vacancy cluster can be well stabilized by the surrounding melem molecules after CO gets released.Considering the versatility of the Au vacancy clusters, the nonequivalence of the number of the observed Au adatom and vacancy species can be understood.

Fig. 6 The proposed generation mechanism of Au adatoms upon exposing the melem film on Au(111) to a CO atmosphere.

4 Conclusions

In conclusion, we have successfully utilized the synergistic effects of CO and melem self-assembly film on Au(111) to create and stabilize Au adatom species. Owing to the multiple heterocyclic N atomic sites which are exposed to the inside of the nanopores of melem, the CO-induced Au adatoms are well stabilized and dispersed with increased versatility. These results demonstrate that the organic self-assembly can really provide a superior method to modify the surface and steer new surface functionalities.