Exploring the Surface Chemical Reactivity of Single Crystals of Binary

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Exploring the Surface Chemical Reactivity of Single Crystals of Binary and Ternary Bismuth Chalcogenides Antonio Politano, Silvia Nappini, Federica Bondino, Elena Magnano, Ziya S. Aliev, Mahammad B. Babably, Andrea Goldoni, Gennaro Chiarello, and Evgueni V. Chulkov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp506444f • Publication Date (Web): 25 Aug 2014 Downloaded from http://pubs.acs.org on August 26, 2014

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The Journal of Physical Chemistry

Exploring the Surface Chemical Reactivity of Single Crystals of Binary and Ternary Bismuth Chalcogenides A.Politano1,*, M. Caputo1,2, S. Nappini3, F. Bondino3, E. Magnano3, Z. S. Aliev4,5,6, M. B. Babanly4,7, A. Goldoni2, G. Chiarello1,8, and E. V. Chulkov4,9,10

1

Università degli Studi della Calabria, Dipartimento di Fisica, 87036 Rende, Italy

2

Elettra Sincrotrone Trieste, Strada Statale 14 km 163.5, I-34149 Trieste, Italy

3

Istituto Officina dei Materiali (IOM)-CNR, Laboratorio TASC, Area Science ParkBasovizza, Strada Statale 14, Km.163.5, I-34149 Trieste, Italy 4

Donostia International Physics Center (DIPC), 20080 San Sebastian, Spain

5

Baku State University, General and Inorganic Chemistry Department, Z. Khalilov, 23, AZ1143, Baku, Azerbaijan 6

Institute of Physics, ANAS, AZ1143 Baku, Azerbaijian

7

Institute of Catalysis and Inorganic Chemistry, ANAS, H.Javid, ave.31, AZ1143 Baku, Azerbaijian 8

Consorzio Interuniversitario di Scienze Fisiche per la Materia (CNISM), Via della Vasca Navale, 84, 00146 Roma, Italy 9

Departamento de Física de Materiales and Centro Mixto CSIC-UPV/EHU, Facultad de Ciencias Químicas, Universidad del País Vasco, Apdo. 1072, 20080 San Sebastián— Donostia, Spain 10

Tomsk State University, Pr. Lenin 36, 634050 Tomsk, Russian Federation

*Corresponding author: Dr. Antonio Politano Tel: +39-0984-496107; Fax +39-0984-494401 ACS Paragon Plus Environment


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e-mail: [email protected]

Abstract The chemical reactivity of single crystals of binary and ternary bismuth chalcogenides toward oxygen, water, and carbon monoxide has been investigated by X-ray photoemission spectroscopy by using synchrotron radiation. We find that single crystals of Bi2Se3 are inert toward oxygen in the temperature range 300-550 K. The protection against surface oxidation at high temperature is important for preserving the thermoelectric efficiency in applications for power generation. By contrast, water adsorbs at room temperature on Bi2Se3. Finally, we find a different reactivity toward carbon monoxide in PbBi6Te10 at room temperature with respect to other bismuth chalcogenides.

Keywords: Thermoelectric materials, topological insulators, synchrotron radiation, X-ray photoemission spectroscopy

Introduction

Bismuth chalcogenides are excellent thermoelectric materials 1-2. In the 300–500 K temperature range, the value of their dimensionless thermoelectric figure-of-merit (ZT) can be higher than one1. Thus, these materials are suitable for cooling3, thermoelectric solar cells 4, waste heat recovery units 5, and power generation6. Unfortunately, the thermoelectric performance is affected by surface oxidation 7, which strongly limits their use. To overcome ACS Paragon Plus Environment

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this problem, surface protection by costly capping layers or by acid ligands 8 is often mandatory. Moreover, nowadays bismuth chalcogenides are attracting renewed interest because they exhibit time reversal symmetry and non-trivial topological order 9-10 and, consequently, they are promising materials for applications in spintronics

11-12

and plasmonics

13-14

. These ap-

plications require high surface stability toward oxidation and corrosion in ambient pressure. However, controversial results exist about the chemical reactivity of bismuth chalcogenides. Some authors have shown that the degradation of their properties as topological insulators arise from surface oxidation at room temperature. As an example, it has been reported that Bi2Se3 single crystals get additional n-type doping after exposure to the ambient gases (oxygen and water)15, thereby reducing the relative contribution of topological surface states in the total conductivity. Other researchers have concluded that Se vacancies are responsible for the enhanced oxidation rate of bismuth atoms16 at room temperature. Benia et al. 17 by using photoemission spectroscopy (PES) have found that the adsorption of water on the Bi2Se3 surface induces a band bending, which shifts the Dirac point, causing the emergence of Rashba-split quantum well states. Although the surface of a topological insulator is not chemically inert, their topological states remain still protected. Such finding has been used to explain the aging effect observed whenever Bi2Se3 crystals are left in vacuum for long time. It should also be noticed that all studies regarding the chemical reactivity of bismuth chalcogenides have been carried out at room temperature. However, in many realistic applications such systems operate at temperature well above room temperature. High temperature may enhance surface chemical reactivity and drive surface phenomena18-25. Despite the imACS Paragon Plus Environment


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portance of such information for engineering of more efficient thermoelectric applications for power generation from heat sources26, no studies exist on the reactivity of bismuth chalcogenides at high temperature (350-550 K). Moreover, the reactivity of the bismuth chalcogenides toward CO deserves a particular attention as it is known to be one of the aging agents of topological insulators27. ARPES experiments have shown that upon CO exposure, the Dirac point moves to higher binding energies, indicating a strong downward bending of the bands near the surface

27

, with the

emergence of large tunable Rashba spin splitting of surface states 28. Herein, we report on X-ray photoemission spectroscopy (XPS) investigation with synchrotron radiation on the chemical reactivity of: 1) Bi2Se3 toward oxygen in the 300-550 K temperature range; 2) Bi2Se3 toward water at room temperature; 3) Bi2Se3, Bi2Te3, GeBi2Te4 and PbBi6Te10 toward carbon monoxide at room temperature.

Experimental

Single crystalline ingots of Bi2Se3, GeBi2Te4 and PbBi6Te10 were grown from melt by the vertical Bridgman–Stockbarger method. The polycrystalline samples which synthesized from the starting elements of a high purity grade (not less than 99.999%) were placed in the conical-bottom quartz ampoules, which were sealed under a vacuum better than 10− 5 Pa. Before growing process, the ampoules were held in the “hot” zone (~80 K higher than melting point) of two-zone tube furnace for 12 h for a complete melting of the composition. Then, the charged ampoule moves from the “hot” zone to the “cold” zone with the required rate 1.0 mm per hr. The temperatures of the “cold” zone were about 100 K lower than melting point. The grown crystal was checked by XRD using a Bruker D8 ADVANCE diffractometer with Cu–Kα radiation. By using EVA and TOPAS V3.0 software the unit cell parameters are calculated and these parameters are in good agreement with the literature data. The grown crystals consisted of one or several large single crystalline blocks. As received, ACS Paragon Plus Environment

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fresh surfaces are readily available by cleaving the crystals along its natural cleavage plane, so to obtain (0001)-oriented surfaces. .Angle-resolved PES measurements, carried out previously on these samples, show the presence of a well-defined Dirac cone, as reported elsewhere29-32. All investigated samples have been characterized with atomic force microscopy, low-energy electron diffraction, Xray electron diffraction and with transport measurements. XPS experiments have been carried out at the BACH beamline of the Elettra synchrotron, Trieste by using a Scienta R3000 hemispherical analyzer with an energy resolution ranging from 0.1 to 0.2 eV. XPS spectra have been acquired in normal incidence at different temperatures, ranging from room temperature to 550 K. Gases have been dosed by leak valves. The cleavage of samples of bismuth chalcogenides has been made in ultra-high vacuum conditions, with a base pressure of 5·10-10 mbar.

3.1 Oxygen interaction with Bi2Se3 samples

Figure 1 shows the XPS spectra of 4f-Bi (panel a) and 3d-Se (panel b) core levels of Bi2Se3 after exposure to 2000 L of O2 at progressively increasing temperature, in the range 300-550 K. The large exposure to oxygen does not affect core levels of Bi and Se. Oxide components are absent in the XPS spectra. The lack of oxygen on the surface is confirmed by the missing 1s-O line.

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Figure 1. XPS spectra of (a) 3d-Se and (b) 4f-Bi core levels of Bi2Se3 acquired after dosing 2000 L of O2 at different temperatures, ranging from 300 to 550 K. The photon energy is 334 eV. As many chalcogenides (IV–VI, II–VI compounds), Bi2Se3 is expected to experience surface oxidation in the presence of oxygen and water. However, they have a layered structure, which is typically associated with low reactivity, e.g., in the case of GaSe and InSe 33-34. Nonetheless, even in inert layered compounds, surface oxidation is usually observed by heating in oxygen at temperature higher than 470 K35-36. ACS Paragon Plus Environment

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The robustness toward surface oxidation found in Bi2Se3 (Figures 1 and 2) conflicts with the results of Ref. 37. Therein, a rapid surface oxidation of Bi2Se3 has been observed upon exposure to ambient gases (oxygen and water) at room temperature. The evident discrepancy with our results can be explained by claiming a different crystalline quality of investigated samples. The presence of Se vacancies enhances surface oxidation16 and, therefore, samples with a different amount of defects have dissimilar surface chemical reactivity. Thus, the cleaved (0001) surfaces of single-crystalline surfaces of Bi2Se3 are inert to oxidation at room temperature and this is a consequence of the absence of surface defects. As a matter of fact, a well-defined 1s-O line can be recorded only for samples with defects implanted by ion bombardment (Supplementary Information, Figure S1).

Figure 2. XPS spectra for 1s-O for Bi2Se3 exposed at room temperature to 2000 L of O2 (blue curve) and 1000 L of H2O (red curve). The photon energy is 654 eV. Both measurements and exposures have been performed at room temperature.



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In Figure 3 we compare the effects of a prolonged O2 exposure at room temperature to Bi and Se core levels. No change with respect to the pristine Bi2Se3 is noticeable. Similar results have been obtained also for samples exposed one month in air (Supporting Information, Figure S1).

Figure 3. Effects of a prolonged (105 L) O2 exposure at room temperature on (a) 4f-Bi and 3p-Se core levels; (b) 3d-Se core levels. A Shirley background38 has been subtracted from raw XPS spectra. The resulting spectra have been fitted with Voigt functions. The photon energy is 334 eV.


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3.2 Reactivity to water The Bi2Se3 sample has been exposed to 1000 L of water at room temperature. The top spectrum of Figure 2 shows the presence of 1s-O peak, indicating that water interacts with Bi2Se3. Surface reactivity toward water is thus responsible for the aging of Bi2Se3 samples in vacuum. Water interacts with systems characterized by electron quantum confinement in surface states 39 or quantum well states 40-41, leading to charge transfer and to an increase of the adsorption energy.

3.3 Reactivity toward CO Exposing the surface of the topological insulator Bi2Se3 to carbon monoxide results in strong shifts of the features observed in ARPES42 and, moreover, a Rashba splitting of topological surface states28. Upon CO exposure, the Dirac point moves to higher binding energies, indicating an increasingly strong downward bending of the bands near the surface27. However, our results contrast with previous results reporting CO-induced effects on the band structure of Bi2Se3. As a matter of fact, we find that carbon monoxide does not interact with bismuth chalcogenides (Bi2Se3, Bi2Te3, and GeBi2Te4). Likely, the adsorption of CO on Bi2Se3 reported in Ref. 27 is a consequence of the doping of the sample with Ca, which is known to be reactive toward CO 43. Figure 4 shows the wide XPS spectra of Bi2Se3, Bi2Te3 and GeBi2Te4 samples exposed to 50 L of CO at room temperature. No features arising from C or O are noticeable in photoemission spectra.



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Figure 4. Wide spectrum of GeBi2Te4 (blue line), Bi2Te3 (green line), and Bi2Se3 (red line) after exposure to 50 L of CO. The absence of 1s-C and 1s-O lines unambiguously indicates that CO does not adsorb on both surfaces. The XPS spectra have been acquired with a photon energy of 654 eV.

Different results have been obtained by exposing the ternary compound PbBi6Te10 to CO. The presence of both O-1s and C-1s core levels in XPS spectra in Figure 5 indicates that the PbBi6Te10 sample reacts toward CO. The different behavior of this sample with respect to other bismuth-based chalcogenides (GeBi2Te4 and Bi2Se3) can be related to the presence of Pb atoms. However, the analysis of the Bi-4f and Pb-4f core levels (Figures 6 and 7) suggests that CO molecules bind both to Bi and Pb, since in the CO-exposed sample both Bi-4f and the Pb-4f


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core levels show a shoulder whose binding energy is higher of 1.21±0.07 and 0.74±0.02 eV, respectively. We also notice that the MNN Te peak is shifted by 2 eV. A shift in the energies of the Auger electrons is a fingerprint of an occurring charge transfer 44. The kinetic energies of the Auger electrons shift by large amounts of 2–6 eV. These shifts are to lower kinetic energy with respect to the pure metal and are largest for the oxides and least for the semiconductors45. The changes in energy, shape, and intensity of MNN Te Auger peak is in agreement with the usually strong charge transfer following the adsorption of CO, also accompanying a subsequent rearrangement of chemical bonds 46-47. The PbBi6Te10 sample is constituted by three blocks (Bi2Te3-PbBi2Te4-Bi2Te3) with three different surface terminations (see Ref.

32

for more details). The surface band structure is

quite complicate since it depends on the surface termination with quintuple (5L) or septuple (7L) layers. This system is characterized by the presence of buried topological surface states which are strongly protected against surface perturbations. Topological surface states have also been suggested to facilitate surface reactions by serving as an effective electron bath. As shown for CO oxidation on gold-covered Bi2Se3 48, topologically protected surface states significantly enhance the adsorption energy of both CO and O2 molecules, by promoting different directions of static electron transfer. An enhancement of surface reactivity induced by topological surface states is also observed for Pd/Bi2Te3 49. The topological surface states from Bi2Te3 act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively.



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We have demonstrated that topological surface states of PbBi6Te10 make favorable CO adsorption (the adsorption energy of CO is therefore negative) with respect to topological surface states in Bi2Se3 and GeBi2Te4. The concept of topological surface states as an electron bath may lead to tailor innovative catalysts beyond the conventional d-band theory of heterogeneous catalysis. These findings may pave the way for novel applications in surface science, gas sensing and catalysis.

Figure 5. XPS survey for as-cleaved PbBi6Te10 (brown curve) and the same surface exposed to CO (blue curve). The photon energy is 830 eV.


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Figure 6. 4f-Bi and 4f-Pb core levels in as-cleaved (brown curve) and CO-exposed (blue curve) PbBi6Te10. The photon energy is 370 eV.

Figure 7. Fit procedure of 4f7/2-Bi (panel a) and 4f7/2-Pb core levels (panel b). A Shirley background38 has been subtracted from XPS spectra in Figure 5. The resulting spectra have been fitted with Voigt functions.

4 Conclusions We have found that single crystal of Bi2Se3 are inert toward oxygen, even for oxidation temperature as high as 550 K. This finding allows the use of binary chalcogenides grown by ACS Paragon Plus Environment


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Bridgman method as thermoelectric applications for power generation. The weak but noticeable chemical reactivity of Bi2Se3 toward water is responsible for the aging effects observed by many groups in samples left in vacuum for long time. Finally, we have demonstrated that the adsorption energy for CO in PbBi6Te10 is lower than in other bismuth-based chalcogenides.

Acknowledgments We acknowledge partial support from the Basque Country Government, Departamento de Educación, Universidades e Investigación (Grant No. IT-756-13), the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609 C02-01), the Ministry of Education and Science of Russian Federation (Grant No. 2.8575.2013), the Russian Foundation for Basic Research (Grant No. 13-02-12110_oﬁ_m), the Science Development Foundation under the President of the Republic of Azerbaijan, Grant No. EIF-2011-1(3)-82/69/4-M-50, and the Italian Ministery of Education, University and Research (Grant FIRB-Futuro in Ricerca No. RBFR128BEC_002). We also acknowledge technical support from F. Salvador and P. Bertoch (IOM-CNR).

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48. Chen, H.; Zhu, W.; Xiao, D.; Zhang, Z., CO Oxidation Facilitated by Robust Surface States on AuCovered Topological Insulators. Phys. Rev. Lett. 2011, 107, 056804. 49. He, Q. L.; Lai, Y. H.; Lu, Y.; Law, K. T.; Sou, I. K., Surface Reactivity Enhancement on a Pd/Bi2Te3 Heterostructure Through Robust Topological Surface States. Sci. Rep. 2013, 3, 2497.



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Exploring the Surface Chemical Reactivity of Single Crystals of Binary

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