Transition metals of Pt and Pd on the surface of topological insulator Bi 2 Se 3

Transition metal catalysts supported on topological insulators are predicted to show improved catalytic properties due to the presence of topological surface states, which may float up to the catalysts and provide robust electron transfer. However, experimental studies of surface structures and corresponding catalytic properties of transition metal/topological insulator heterostructures have not been demonstrated so far. Here, we report the structures, chemical states, and adsorption behaviors of two conventional transition metal catalysts, Pt and Pd, on the surface of Bi 2 Se 3 , a common topological insulator material. We reveal that Pt forms nanoparticles on the Bi 2 Se 3 surface. Moreover, the interaction between Pt and surface Se is observed. Furthermore, thermal dosing of O 2 onto the Pt/Bi 2 Se 3 heterostructure leads to no oxygen adsorption. Detailed scanning tunneling microscopy study indicates that Pt transforms into PtSe 2 after the thermal process, thus preventing O 2 from adsorption. For another transition metal Pd, it exhibits approximate layer-island growth on Bi 2 Se 3 , and Pd–Se interaction is also observed. Our work provides significant insights into the behaviors of transition metals on top of a common topological insulator material and will assist in the future design of catalysts built with topological materials.


INTRODUCTION
2][3] The low-dissipation electron transport in topological surface states (TSSs) provides stable and fast charge transfer channels for catalytic reactions, offering new possibilities for designing highly efficient catalysts, as well as promising new catalytic mechanisms. 4,5A number of recent experiments have unveiled outstanding catalytic performance in different topological materials.7][8] Pt-based topological semimetals not only exhibit hydrogen evolution performance close to that of commercial Pt/C but also possess a low Pt mass ratio, thus lowering the catalyst cost. 9,10Very recently, topological insulator Bi 2 Se 3 has been shown to be an efficient catalyst for oxidative carbonylation of amines with a high yield of 99% even at room temperature, much lower than the temperatures required for most noble metal catalysts. 11opological materials can also boost catalysis when served as supporting substrates for traditional catalysts since it has been suggested that TSSs may float to the overlayer and influence the electronic structures of catalysts. 12As a result, enhancement of CO and O 2 adsorption has been theoretically predicted on gold-covered Bi 2 Se 3 , and facilitation of CO oxidation by TSSs was predicted. 13imilarly, single-layer ZnSe/Bi 2 Se 3 heterostructure has been predicted to exhibit optimal hydrogen adsorption energy, leading to promising catalytic performance for hydrogen evolution reaction  15 The HER catalytic ability of different transition metal catalysts may be enhanced or suppressed, depending on the initial adsorption strength between metals and hydrogen.However, compared with a fair amount of theoretical research, the experimental progress of catalysts supported on topological materials remains very limited.Surface structures and catalytic properties of transition metals on topological material substrates have not been determined so far, greatly hindering the study of catalytic properties of conventional catalysts influenced by topological effects.
Here, we study the structures of transition metals on the surface of topological insulator Bi 2 Se 3 and the interaction between them by various microscopic and spectroscopic techniques.The topographic and atomic structures of deposited Pt on Bi 2 Se 3 were imaged by scanning tunneling microscopy (STM).Chemical states of Pt on Bi 2 Se 3 were characterized by x-ray photoemission spectroscopy (XPS), and charge interaction between them was evaluated by high-resolution electron energy loss spectroscopy (HREELS).Oxygen (O 2 ) adsorption was tested on the Pt/Bi 2 Se 3 heterostructure by introducing O 2 in ultrahigh vacuum (UHV), and structures and chemical states after O 2 dosing were studied.In addition, topographic structures of deposited Pd on Bi 2 Se 3 were discussed.Our work discloses the surface structures and properties of transition metals on a topological insulator material and may provide insights into catalyst design based on quantum materials in the future.

RESULTS AND DISCUSSION
Bi 2 Se 3 consists of quintuple layers with intralayer Se-Bi-Se-Bi-Se atomic sequences and weak interlayer van der Waals (vdW) interactions, which ensures easy cleavage between layers and obtaining a clean surface.Bi 2 Se 3 single crystal was cleaved in a UHV environment with a base pressure of ∼5 × 10 −9 mbar in this study.After cleavage, the sample was directly transferred to the STM chamber for characterization without breaking UHV. Figure 1(a) demonstrates a clean Bi 2 Se 3 surface with a step height of 9.7 Å, corresponding to the step height of a quintuple layer. 16he atomic resolution STM image [inset in Fig. 1(a)] illustrates the hexagonal atomic lattice with a periodicity of 4.1 Å, consistent with Se-termination on the surface. 16We conducted Pt deposition at room temperature on Bi 2 Se 3 using a Pt e-beam evaporator installed on the UHV chamber.After deposition, Pt nanoparticles were observed randomly and nearly homogeneously distributed on the surface [Fig.1(b)].Apparent heights of the Pt nanoparticles distribute in the range from 2.8 to 7.3 Å with most particles demonstrating a height of ∼4.4 Å [Fig.1(c)].The evolution of surface morphologies after different Pt deposition times suggests that Pt may undergo the Volmer-Weber growth (island growth) mode on Bi 2 Se 3 , similar to the growth of Au on Bi 2 Se 3 17 (see details in the supplementary material, Fig. S1).When Pt is deposited, due to lack of surface diffusion (Fig. S2) and possible reactions between Pt and Se, 18,19 Pt atoms and clusters may bond to the substrate immediately.This results in the growth of random Pt nanoparticles on the Bi 2 Se 3 surface.In addition, we also note that the lattice mismatch between Pt and Bi 2 Se 3 is large, ∼33%.This could be another reason that leads to the island growth.
To understand the interaction between Pt and Bi 2 Se 3 , XPS evaluation of the chemical states of Pt and Se was carried out.As shown in Fig. 2(a), Pt core levels demonstrate binding energies at 71.8 and 75.1 eV, corresponding to the Pt 4f 7/2 and 4f 5/2 orbitals, respectively.These binding energies are ∼0.6 eV higher than those of metal Pt (71.2 and 74.5 eV), 20 indicating the apparent loss of electrons of the deposited Pt.Meanwhile, two sets of peaks appear in the Se 3d spectra [Fig.2(b)].The major set exhibits binding energies at 53.4 and 54.3 eV, corresponding to Se 3d 5/2 and 3d 3/2 orbitals in Bi 2 Se 3 , respectively 21 [inset in Fig. 2(b)].The minor set of binding energies takes place at 54.2 and 55.0 eV, which are ∼1.4 eV lower than the binding energies of elemental Se. 20 Combined with the higher binding energies of Pt, it indicates Pt demonstrates strong interaction with surface Se after deposition. 22The XPS peaks of the minor set of Se can be attributed to the Pt-Se interaction.
HREELS was employed to further explore the charge interaction between Pt and Bi 2 Se 3 by measuring the collective modes of Bi 2 Se 3 .In the energy loss spectrum of the as-cleaved Bi 2 Se 3 [Fig.2(c)], a sharp peak at 21.7 meV and a broad peak at 74 meV were observed, corresponding to the characteristic optical phonon mode and bulk plasmon mode of Bi 2 Se 3 , respectively. 23Note that Bi 2 Se 3 is typically n-type doped with a substantial amount of carriers in the bulk.After Pt deposition, an energy shift of the bulk plasmon mode was clearly observed, resulting in the new plasmon excitation  at a higher energy of 92 meV.This energy shift can be attributed to increased carrier density (at least in the region near the surface as probed by HREELS) in Bi 2 Se 3 , 24 indicating strong electron doping from the deposited Pt.Moreover, the low-energy peak at 21.7 meV, which corresponds to the A 1g phonon mode of Bi 2 Se 3 , was shifted to 21.2 meV after Pt deposition (Fig. S3), possibly resulting from combined effects of lattice constant change at the Bi 2 Se 3 surface and electron-phonon coupling.
As an enhancement of O 2 adsorption was predicted on Bi 2 Se 3 supported transition metals, 13,15 we introduced O 2 into the UHV chamber to examine the oxygen adsorption properties of the Pt/Bi 2 Se 3 heterostructure.To ensure a reasonable amount of dosing, we applied thermal dosing of O 2 with a pressure at ∼5 × 10 −6 mbar, which is in between high and low pressures for O 2 adsorption on Pt according to a previous study. 25After dosing at 250 ○ C for 10 min, XPS did not show oxygen traces in the O 1s spectrum [Fig.3(a)].Moreover, binding energies of Pt 4f peaks appear at 72.0 and 75.3(c)], 0.1 eV higher than those after Pt deposition [Fig.2(b)] and close to the binding energies of PtSe 2 . 26The binding energies' shift of both Pt and Se indicates that it is changed toward PtSe 2 after the thermal process during the dosing.Thus, it is highly possible that the binding sites of Pt are fully occupied by Se, resulting in no active sites for O 2 adsorption.
To better understand the Pt/Bi 2 Se 3 heterostructure after thermal dosing of O 2 , we conducted STM to image the topographic and atomic structures.Interestingly, instead of randomly distributed nanoparticles, Pt appears as orientated two-dimensional (2D) triangular islands on the surface [Fig.4(a)].Some of the islands are more pronounced with bigger heights of ∼5.0 Å, which is consistent with the height of monolayer PtSe 2 . 27While some other islands appear to have the central parts more immersed in Bi 2 Se 3 , leaving noticeable triangular edges with a smaller height of ∼2.0 Å on the surface.This phenomenon suggests that Pt undergoes interdiffusion into Bi 2 Se 3 .Atomic scale STM images were carried out to resolve the atomic structures.Figure 4(b) shows the atomic arrangements of a lower island and the hexagonal lattices illustrate the periodicity of 3.8 Å, close to the lattice constant of PtSe 2 of 3.75 Å. 27 The zoomed-in STM image of a higher island displays a moiré pattern (periodicity of 25.7 Å) [Fig.4(c)], which suggests a 2D stacking between the triangular island and substrate.We also conducted a parallel experiment in which we heated the Pt/Bi 2 Se 3 heterostructure in UHV (same heating condition with that of O 2 thermal dosing but without O 2 ).We observed that the surface demonstrates similar triangular islands (Fig. S4) with those in Fig. 4(a) after heating.This suggests that it is the heating that makes the transformation of Pt nanoparticles to PtSe 2 islands.
In addition to Pt, we also studied another well-known transition metal catalyst, Pd, on the surface of Bi 2 Se 3 .Rather than nanoparticles, the deposited Pd exhibits random shape 2D islands with a typical height of ∼4.4 Å on the surface [Fig.5(a)].On the 2D islands, three-dimensional (3D) growth of bulky clusters was observed.This indicates an approximate Stranski-Krastanov layerplus-island growth mode 28,29 of Pd on Bi 2 Se 3 .The different growth modes of Pt and Pd on Bi 2 Se 3 may result from different degrees of interaction between the transition metals and Bi 2 Se 3 (see details in Fig. S5).Similar to Pt, XPS spectra of Pd show ∼0.6 eV higher binding energies (335.8 and 341.1 eV) [Fig.5(b)] than those of metal Pd, 20 indicating the Pd-Se interaction.Two sets of binding energies were again observed in the Se 3d spectra [Fig.5(c)]: the set at lower binding energies corresponds to Bi 2 Se 3 , and the set at the higher binding energies of 53.7 and 54.5 eV is related to Pd-Se interaction on the surface.
The common interaction feature of Pt and Pd with Bi 2 Se 3 is consistent with the results of previous studies, which reported the interfacial interaction between Bi 2 Se 3 and a series of contact metals 17,30 (Au, Ir, Pd, etc.).Given that this hinders the adsorption capability of transition metals as we observed in the O 2 adsorption test for Pt on Bi 2 Se 3 , depositing a relatively thicker layer of transition metal may help screen the properties of interfacial interaction and, thus, unveil intrinsic adsorption properties of transition metals.In the meantime, the evolution of TSSs and their effectiveness as a function of the thickness of the deposited layer could be traced in future studies, although TSSs are reported to be relatively robust to surface doping at low impurity concentration. 31

CONCLUSIONS
In summary, we demonstrated the structures and chemical states of transition metal Pt and Pd on the surface of topological insulator Bi 2 Se 3 .Pt exhibited randomly distributed nanoparticles and significant interaction with surface Se after deposition.Thermal dosing of O 2 did not result in any oxygen adsorption on Pt, which can be attributed to the complete occupation of Pt binding sites by Se.Atomic resolution STM images suggested the transformation of Pt to PtSe 2 after the thermal process.In contrast to Pt nanoparticles, Pd demonstrated an approximate 2D-3D layer-island growth mode on the Bi 2 Se 3 surface.Pd also demonstrated apparent interaction with surface Se after deposition.Our work shed light on the interaction between traditional transition metal catalysts and a common topological insulator material and will guide the rational design of future topological catalysts.

SUPPLEMENTARY MATERIAL
Experimental details of single crystal growth and cleavage, deposition, O 2 dosing and characterizations, and additional experimental results can be found in the supplementary material.

FIG. 1 .
FIG. 1. Bi 2 Se 3 and Pt deposition.(a) An STM topographic image of the surface of as-cleaved Bi 2 Se 3 .Inset: an atomic resolution STM image of the as-cleaved Bi 2 Se 3 surface.V b = 1.3 V and I t = 1.0 nA.(b) An STM topographic image of the surface of as-cleaved Bi 2 Se 3 after Pt deposition for 7 min.V b = −1.0V and I t = 1.0 nA.Inset: a zoomed-in STM image of a few Pt nanoparticles.(c) Height distribution of the deposited Pt nanoparticles.

FIG. 2 .FIG. 3 .
FIG. 2. Spectroscopic measurements before and after Pt deposition on Bi 2 Se 3 .[(a) and (b)] Pt 4f and Se 3d XPS spectra after Pt deposition on Bi 2 Se 3 , respectively.Inset in (b) shows Se 3d XPS spectra of the as-cleaved Bi 2 Se 3 .(c) HREELS spectra of the as-cleaved Bi 2 Se 3 and after Pt deposition.
3 eV [Fig.3(b)], shifted to higher energies by 0.2 eV compared with those of Pt just after deposition [Fig.2(a)].The major set of Se binding energies (from Bi 2 Se 3 ) remains at 53.4 and 54.3 eV.The minor set of Se displays binding energies at 54.3 and 55.1 eV [Fig.

FIG. 4 .FIG. 5 .
FIG. 4. STM images of Pt after thermal dosing of O 2 .(a) A topographic STM image of Pt after the thermal process.[(b) and (c)] Atomic resolution and zoomed-in STM images of two triangular islands with typical heights of ∼2.0 and ∼5.0 Å, respectively.V b = −1.0V and I t = 1.0 nA.