In-Situ Scanning Tunneling Microscopy Research Articles

In-Situ STM Study of Sodium Deposition Onto Au(111) in [Mppip][Tfsi]:Initial Stages, Nucleation, Cluster Growth, and Alloy Formation

The development of battery systems fabricated from abundant materials that meet the highest safety requirements is a major challenge, while the demand for efficient energy storage is continuously increasing.[1] Sodium metal batteries might be an adequate substitute for lithium-ion batteries. In comparison to lithium, sodium is more abundant, is a strong reducing agent, and has a less negative standard electrode potential.[2] Investigations of the deposition and the dissolution behavior of sodium are of utmost importance since dendritic growth is amongst the key factors limiting the lifetime and safety of sodium metal batteries.[3] In-situ scanning tunneling microscopy (STM) combined with cyclic voltammetry allows observing topographical changes in real-time and to determine potential regions of related electrochemical processes. Single-crystal surfaces are commonly used in fundamental studies of metal deposition due to their (i) well-defined structure, (ii) ease and reproducibility of preparation, and (iii) stability in the electrochemical environment.[4] However, metallic sodium is extremely reactive. In particular, Na reacts violently with water and molecules containing acidic hydrogen atoms, but even with commonly used battery electrolytes as organic carbonates.[5] Fortuitously, ionic liquids (ILs) are promising solvents in sodium metal batteries.[6] Due to their unique properties, they are appropriate for preparing electrolytes in investigations of the deposition and dissolution behaviour of metals. They are known for their relatively wide electrochemical stability windows for both sodium and lithium deposition, as well as for low diffusion rates, which facilitate real-time deposition studies.[7] In this work, the initial stages of sodium underpotential deposition (UPD) onto a Au(111) electrode surface from the ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([MPPip][TFSI]) is investigated using in-situ STM and classical electrochemical measurements.Four subsequent stages in the UPD process of sodium on Au(111) can be discerned. (i) Nucleation of sodium starts around 1.1 V vs. Na/Na+ at the so-called elbows of the reconstructed Au(111) surface. (ii) Small mono-atomically high islands grow around the nuclei at a slightly more negative electrode potential of 1.0 V. (iii) The islands slowly coalesce into smooth layers after lowering the potential to 0.5 V (Figure 1), resulting in several clusters with defined steps. (iv) More islands grow on top of the existing layer and the preferred deposition mode changes from smooth layer formation to 3D-growth, resulting in cauliflower-like structures, especially at the step edges of the clusters. The deposition of several layers of sodium in the UPD regime can be explained by the alloy formation of sodium and sodiophilic gold.[8] It is worth mentioning that this deposition behaviour shows several similarities to that of lithium UPD on Au from ILs, including island formation and multiple layer growth.[7,9] So far, nucleation at the reconstruction elbows has not been reported for lithium, but it has already been observed during deposition of other metals such as nickel, cobalt, and palladium both for electrodeposition[10] and under UHV conditions.[11,12] References [1] B. L. Ellis, L. F. Nazar, Curr. Opin. Solid State Mater. Sci. 2012, 16, 168–177.[2] J. Zheng, S. Chen, W. Zhao, J. Song, M. H. Engelhard, J. Zhang, ACS Energy Lett. 2018, 3, 315–321.[3] Z. W. Seh, J. Sun, Y. Sun, Y. Cui, ACS Cent. Sci. 2015, 1, 449–455.[4] M. A. Schneeweiss, D. Kolb, Chemie unserer Zeit 2000, 34, 72–83.[5] K. Pfeifer, S. Arnold, J. Becherer, C. Das, J. Maibach, H. Ehrenberg, S. Dsoke, ChemSusChem 2019, 12, 3312–3319.[6] R. Wibowo, L. Aldous, E. I. Rogers, S. E. Ward Jones, R. G. Compton, J. Phys. Chem. C 2010, 114, 3618–3626.[7] C. A. Berger, M. U. Ceblin, T. Jacob, ChemElectroChem 2017, 4, 261–265.[8] S. Tang, Z. Qiu, X. Y. Wang, Y. Gu, X. G. Zhang, W. W. Wang, J. W. Yan, M. Sen Zheng, Q. F. Dong, B. W. Mao, Nano Energy 2018, 48, 101–106.[9] L. H. S. Gasparotto, N. Borisenko, N. Bocchi, S. Zein El Abedin, F. Endres, Phys. Chem. Chem. Phys. 2009, 11, 11140–11145.[10] M. Kleinert, H. F. Waibel, G. E. Engelmann, H. Martin, D. M. Kolb, Electrochim. Acta 2001, 46, 3129–3136.[11] D. D. Chambliss, R. J. Wilson, S. Chiang, Phys. Rev. Lett. 1991, 66, 1721–1724.[12] C. S. Casari, S. Foglio, F. Siviero, A. Li Bassi, M. Passoni, C. E. Bottani, Phys. Rev. B - Condens. Matter Mater. Phys. 2009, 79, 1–25. Figure 1

Au(111) in [Mppip][Tfsi] in Dependence of Water Content: Revealing the Initial Stages of Cathodic Corrosion Using in-Situ STM

Cathodic corrosion was first observed by Fritz Haber at the end of the 19th century,[1] and was nearly forgotten for a long time, but has recently returned as an important research topic.[2,3] The prospects to form nanoparticles of noble metals and to restructure metal surfaces, in particular, stirred researchers’ interest.[4,5] Moreover, cathodic corrosion may hinder other processes such as metal deposition that occur at highly negative potentials. As ionic liquids have desirable properties of being stable over a relatively wide potential window coupled with low ion mobility, these electrolytes are often used for studies of processes at very low electrode potentials, for example, the deposition of metals with low reduction potentials such as lithium or sodium.[6,7] Unfortunately, if an electrode is cathodically corroded at potentials positive of the onset of metal deposition, this is of little use. Therefore, it is crucial to understand and thus prevent the initial stages of cathodic corrosion in such systems. In-situ scanning tunneling microscopy (STM) was used to study cathodic corrosion of a Au(111) surface in the hydrophobic ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([MPPip][TFSI]) with various water contents. It has been observed that the onset potential of cathodic corrosion is highly dependent on the presence and the amount of water. This notable role of water has been found already for cathodic corrosion of Au in aqueous methanolic alkali metal hydroxide electrolytes:[8] the higher the water content, the more positive the onset potential. Cathodic corrosion of Au(111) begins at the so-called elbows of the herringbone reconstruction, which can lead to a characteristic pattern of equidistant holes. In hydrophilic ionic liquids, a similar process has already been observed.[9] In contrast, for the hydrophobic ionic liquid [MPPip][TFSI], the pits did not merge with time, instead, the number of pits slowly increased. Furthermore, the number of pits formed during this process was observed to increase with the increase of water content. Further studies are planned to elucidate the dependence of preparation procedure, as well as experimental constituents and contaminants on the cathodic corrosion of Au(111) in the ionic liquid [MPPip][TFSI]. Such studies can help to understand if cathodic corrosion can be induced by the ionic liquid itself or only by contaminants such as water. It is known that adsorbed hydrogen plays a major role in the process[3] and it has been reported that in the absence of water cathodic corrosion is not taking place.[10] On the other hand, other studies proposed that the presence of water is not obligatory if hydrogen can be formed in another way than water reduction, for example by reduction of the IL cation.[11] References [1] F. Haber, Zeitschrift für Anorg. Chemie 1898, 16, 438–449.[2] Y. I. Yanson, A. I. Yanson, Low Temp. Phys. 2013, 39, 304–311.[3] T. J. P. Hersbach, M. T. M. Koper, Curr. Opin. Electrochem. 2021, 26, 100653.[4] T. J. P. Hersbach, V. A. Mints, F. Calle-Vallejo, A. I. Yanson, M. T. M. Koper, Faraday Discuss. 2016, 193, 207–222.[5] M. M. Elnagar, J. M. Hermann, T. Jacob, L. A. Kibler, Electrochim. Acta 2021, 372, 137867.[6] C. A. Berger, M. U. Ceblin, T. Jacob, ChemElectroChem 2017, 4, 261–265.[7] R. Wibowo, L. Aldous, E. I. Rogers, S. E. Ward Jones, R. G. Compton, J. Phys. Chem. C 2010, 114, 3618–3626.[8] M. M. Elnagar, T. Jacob, L. A. Kibler, Electrochem. Sci. Adv. 2021, accepted.[9] A. V. Rudnev, M. R. Ehrenburg, E. B. Molodkina, A. Abdelrahman, M. Arenz, P. Broekmann, T. Jacob, ChemElectroChem 2020, 7, 501–508.[10] M. M. Elnagar, J. M. Hermann, T. Jacob, L. A. Kibler, ECS Meet. Abstr. 2020, MA2020-01, 1011–1011.[11] F. Lu, X. Ji, Y. Yang, W. Deng, C. E. Banks, RSC Adv. 2013, 3, 18791. Figure 1

In-Situ STM Studies on Li- and Na- Intercalation Batteries

The processes of SEI formation and of intercalation are still not well understood on the molecular level. Scanning probe techniques are capable of analysing these processes under in-situ conditions. The SEI-formation on graphitic electrodes operated as an Li+-ion battery anode in a standard 1 M LiPF6 EC/DMC (1 : 1) electrolyte has been studied using in-situ electrochemical scanning tunnelling microscopy (EC-STM)1. Two different modes of in situ study were applied, one, which allowed to follow topographic and crystallographic changes (solvent co-intercalation, graphite exfoliation, SEI precipitation on the HOPG basal plane) of the graphite electrode during SEI-formation, and the second, which gave an insight into the SEI precipitation on the HOPG basal plane in real time. From the in situ EC-STM studies, not only conclusions about the SEI-topography could be drawn, but also about the formation mechanism and the chemical composition, which strongly depend on the electrode potential. It was shown that above 1.0 V vs. Li/Li+ the SEI-formation is still reversible, since the molecular structure of the solvent molecules remains intact during an initial reduction step. During further reduction, the molecular structures of the solvents are destructed, which causes irreversible charge losses. The reversible intercalation of solvated Na-ions into graphite and the concomitant formation of ternary Na-graphite intercalation compounds (GICs) is studied by EC-STM and other in-operando techniques2. Linear ethylene glycol dimethyl ether homologes (“glymes”) Gx with x+1 O-atoms were used as solvents, where x is 1-4. The intercalation mechanism of Na+(Gx)y-complexes was investigated with the focus on phase transitions and diffusion rates of the Na+(Gx)y-complexes inside the graphite lattice. For the four shortest glymes (G1 to G4), it is found by XRD that an intermediate stage 2 Na-GIC (NaC48) is formed upon partial sodiation of the graphite electrode. At full sodiation a stage 1 Na-GIC (NaC18, 112 mAh g-1) is obtained for G1, G2 and G4, while the G3-system is also forming a stage 1 Na-GIC but with less Na incorporated (NaC30, 70 mAh g-1). Phase transitions of a battery electrode upon ion-intercalation are visualised by STM on the atomic scale for the first time. In addition, local diffusion rates of the intercalated species inside the electrode were determined, a unique approach to determining kinetic effects in batteries on the atomic scale. The formation of a solid electrolyte interphase (SEI) is observed in EC-STM. Lukas Seidl, Sladjana Martens, Jiwei Ma, Ulrich Stimming, Oliver Schneider "In-Situ Scanning Tunneling Microscopy of the SEI Formation on Graphite Electrodes for Li-Ion Batteries" Nanoscale, 2016, 8, 14004;Seidl, N. Bucher, E. Chu, S. Hartung, S. Martens, O. Schneider and U. Stimming, "Intercalation of solvated Na-ions into graphite", Energy Environ. Sci.,2017, 10, 1631.

Graphene/Rh(111) Structure Studied Using In-Situ Scanning Tunneling Microscopy**Supported by the National Natural Science Foundation of China under Grant No 51402026, the Basic Research Program of Jiangsu Province under Grant No BK20130236, and the High Technology Research Key Laboratory of Changzhou under Grant No CM20133007.

Scanning tunnel microscopy (STM) is performed to verify if an Rh ‘nails’ structure is formed accompanying the graphene growing during chemical vapor deposition. A structure of a graphene island in an Rh vacancy island is used as the start. While the graphene island is removed by oxygenation, the variations of the Rh vacancy island are imaged with an in-situ high-temperature STM. By fitting with our model and calculations, we conclude that the best fit is obtained for 0% Rh, i.e., for the complete absence of nails below graphene on Rh(111). That is, when graphene is formed on Rh(111), the substrate remains flat and does not develop a supporting nail structure.

Insights into the SEI Formation Mechanism By in-Situ Scanning Tunneling Microscopy and Electrochemical Quartz Crystal Microbalance Studies

The strongly increasing worldwide energy demand threatens to lead to a critical shortage of fossil fuel resources as well as a strong climate change in the next decades. The changeover to renewable energy sources like sun and wind and a complete transition in the transport sector to electromobility are potential solutions for the existing problems and thus under intensive discussion nowadays [1]. For this reason, strong efforts have been undertaken for the further development and improvement of electric energy storage devices such are batteries [2,3]. Li+-ion batteries as the front-runners especially in the area of electromobility undergo intensive research for improving efficiency, lifetime and safety [4]. These properties are crucially affected by the properties of the formed SEI (Solid Electrolyte Interphase) on the battery electrodes [5]. In order to optimize the battery performance, a complete understanding of the formation mechanism, morphology and chemistry of this interphase is inevitable. Therefore, the formation of the SEI has been in the focus of recent research [6, 7]. In this study two powerful tools, namely In-Situ Scanning Tunneling Microscopy and the Electrochemical Quartz Crystal Microbalance (EQCM) Technique, have been applied to gain further insights into the SEI formation and its morphology. The STM studies were building on earlier work by Inaba et al. [8]. Highly oriented pyrolytic graphite (HOPG) has been chosen as the model electrode for intercalation of Li+-ions from commercially available and alternative electrolytes. Both in-situ and ex-situ imaging was carried out. It could be demonstrated that the initially formed SEI is reversible, i.e. it dissolves when increasing the potential, while at lower potentials it becomes irreversible. Also the insertion of solvated ions could be visualized. The STM studies were complemented by EQCM measurements. EQCM with bare Au electrode coated quartz resonators have been shown to provide information on SEI formation [7]. In our studies the electrodes were modified in order to be more representative of real battery anodes and to represent more closely the experimental conditions applied in STM. The conclusions from the results of these measurements for the SEI formation in different electrolytes are discussed in this contribution. . [1] Esbenshade J. L. et al., Effect of Mn and Cu addition on lithiation and SEI formation an model anode electrodes, Journal of The Electrochemical Society 161 (2014) A513 [2] Armand M. et al., Novel weakly coordinating heterocyclic anions for use in lithium batteries, Journal of Power Sources 178 (2008) 821 [3] Goodenough J. B. et al., Rechargeable batteries: challenges old and new, J. Solid State Electrochem. 16 (2012) 2019 [4] Amereller M. et al., Electrolytes for lithium and lithium ion batteries: From synthesis of novel lithium borates and ionic liquids to development of novel measurement methods, Progress in Solid State Chemistry 42 (2014) 39 [5] Balbuena P. B. et al., Lithium-Ion Batteries: Solide-Electrolyte Interphase, Imperial College Press (2004), ISBN 1-86094-362-4 [6] Gourdin G. et al., Spectroscopic Compositional Analysis of Electrolyte during Initial SEI Layer Formation, J. Phys. Chem. C 118 (2014) 17383 [7] Tavassol H. et al., Solvent Oligomerization during SEI Formation on Model Systems for Li-Ion Battery Anodes, J. Electrochem. Soc. 159 (2012) A730 [8] Inaba M. et al., Electrochemical Scanning Tunneling Microscopy Observation of Highly Oriented Pyrolytic Graphite Surface Reactions in an Ethylene Carbonate-Based Electrolyte Solution, Langmuir 12 (1996) 1535

ChemInform Abstract: An in situ Scanning Tunneling Microscopy Study of Ag Electrodeposition on Au(111).

In situ scanning tunneling microscopy (STM) was used to study the electrodeposition of silver on Au(111) electrodes from sulfuric acid solution. The process was followed in both the underpotential and overpotential regions. STM images of underpotentially deposited silver revealed a series of ordered adlayer structures of increasing coverage with decreasing potential, namely (√3×√3)R30°, a distorted hexagonal stripe pattern, a (3×3) and, finally, a close-packed (1×1) layer. The bulk deposition was also monitored as a function of the electrode potential. At low overpotentials, a layer-by-layer growth up to at least 10 monolayers was detected. Additionally, a 3D cluster growth took place at the step edges. Finally, deposition of a thick silver layer resulted in the leveling of substrate defects and a very flat surface was obtained.

Self-Assembled Monolayer of Polyoxometalate on Gold Surfaces: Quartz Crystal Microbalance, Electrochemistry, and in-Situ Scanning Tunneling Microscopy Study

A new kind of inorganic self-assembled monolayer (SAM) was prepared by spontaneous adsorption of polyoxometalate anion, AsMo11VO404-, onto a gold surface from acidic aqueous solution. The adsorption process, structure, and electrochemical properties of the AsMo11VO404- SAM were investigated by quartz crystal microbalance (QCM), electrochemistry, and scanning tunneling microscopy (STM). The QCM data suggested that the self-assembling process could be described in terms of the Langmuir adsorption model, providing the value of the free energy of adsorption at −20 KJ mol-1. The maximum surface coverage of the AsMo11VO404- SAM on gold surface was determined from the QCM data to be 1.7 × 10-10 mol cm-2, corresponding to a close-packed monolayer of AsMo11VO404- anion. The analysis of the voltammograms of the AsMo11VO404- SAM on gold electrode showed three pairs of reversible peaks with an equal surface coverage of 1.78 ×10-10 mol cm-2 for each of the peaks, and the value was agreed well with the QCM data. In-situ STM image demonstrated that the AsMo11VO404- SAM was very uniform and no aggregates or multilayer could be observed. Furthermore, the high-resolution STM images revealed that the AsMo11VO404- SAM on Au(111) surface was composed of square unit cells with a lattice space of 10−11 Å at +0.7 V (vs Ag|AgCl). The value was quite close to the diameter of AsMo11VO404- anion obtained from X-ray crystallographic study. The surface coverage of the AsMo11VO404- SAM on gold electrode estimated from the STM image was around 1.8 × 10-10 mol cm-2, which was consistent with the QCM and electrochemical results.

In-Situ Scanning Tunneling Microscopy of Ir(100) Electrodes with Adsorbed Nitric Oxide and Carbon Monoxide

A Summary Report to The Electrochemical Society for a 1998 Department of Energy Summer Research Fellowship.

An in situ scanning tunneling microscopy study of Ag electrodeposition on Au(111)

In situ scanning tunneling microscopy (STM) was used to study the electrodeposition of silver on Au(111) electrodes from sulfuric acid solution. The process was followed in both the underpotential and overpotential regions. STM images of underpotentially deposited silver revealed a series of ordered adlayer structures of increasing coverage with decreasing potential, namely (√3×√3)R30°, a distorted hexagonal stripe pattern, a (3×3) and, finally, a close-packed (1×1) layer. The bulk deposition was also monitored as a function of the electrode potential. At low overpotentials, a layer-by-layer growth up to at least 10 monolayers was detected. Additionally, a 3D cluster growth took place at the step edges. Finally, deposition of a thick silver layer resulted in the leveling of substrate defects and a very flat surface was obtained.

In-Situ Scanning Tunneling Microscopy Imaging of the Reductive Desorption Process of Alkanethiols on Au(111)

The reductive desorption process of self-assembled monolayers of 1-hexadecanethiol, 1-propanethiol, and 3-mercaptopropionic acid on Au(111) has been studied in 0.5 M KOH solution by in-situ scanning tunneling microscopy (STM) and cyclic voltammetry. In-situ STM images of the monolayers at the potentials between −0.2 V and the reduction potentials of each thiols show the pits that are commonly seen in STM images of thiol self-assembled monolayers. A drastic morphological change takes place in the STM image around the peak potential in a cyclic voltammogram for the reductive desorption of adsorbed thiols. The images indicate that 3-mercaptopropionic acid molecules diffuse away from the surface after the reduction because of its higher solubility, while 1-propanethiol and 1-hexadecanethiol molecules stay in the vicinity of the surface forming aggregates. The partial recovery of the 1-hexadecanethiol monolayer after the anodic scan, suggested by cyclic voltammograms, is confirmed by STM, whereas 1-propanethio...

Corrosion of Alkanethiol-Covered Cu(100) Surfaces in Hydrochloric Acid Solution Studied by in-Situ Scanning Tunneling Microscopy

The surface structure and corrosion of alkanethiol-covered Cu(100) surfaces in 1 mM HCl was studied by in-situ scanning tunneling microscopy (STM) and complementary electrochemical measurements. The octanethiol (OT) and hexadecanethiol (HDT) monolayers were prepared by spontaneous adsorption from ethanolic solution onto an electropolished single crystal substrate and then immersed into 1 mM HCl solution at potentials between −0.16 and −0.26 V vs Ag/AgCl (KCl sat.). Samples prepared in this way exhibit a well-defined surface morphology, where atomically smooth Cu terraces, which are found also on thiol-free Cu(100), are covered by monoatomically high Cu islands and by pits. Keeping the potential in this regime causes slow Cu roughening via the formation of additional Cu monolayer islands and pits. This surface restructuring is probably caused by exchange of Cu or Cu−thiolates with the electrolyte. The onset of Cu corrosion is shifted anodically to potentials in the range −0.12 to −0.10 V, reflecting the inhibition by the thiol layer. Copper corrosion proceeds via the formation of many small etch pits and pronounced surface roughening. At higher potentials and/or after prolonged etch times only few larger pits are observed. Local Cu corrosion within these pits proceeds via a similar mechanism as on bare Cu(100), suggesting that the inhibiting thiol layer is completely removed at these places.

Atomic Level Characterization of the Iodine-Modified Au(111) Electrode Surface in Perchloric Acid Solution by in-Situ STM and ex-Situ LEED

In-situ scanning tunneling microscopy (STM) and ex-situ low-energy electron diffraction (LEED) have been employed to characterize an iodine-coated Au(111) electrode surface in perchloric acid solution in the absence of iodide ions. Iodine forms a well-ordered monolayer with two distinguished sets ofincommensurate lattices (phases) : a centered rectangular c(p x √3R-30°) phase and a rotated hexagonal (rot-hex) phase in the range of electrode potentials between 0.3 and 1.4 V vs a reversible hydrogen electrode (RHE). Both lattices become more compressed with increasing electrode potential. The (√3 x √3)R30° phase was found to have the most open structure for the iodine monolayer on the Au(111) surface. Uniaxial compression of the c(p x √3R -30°) phase (p decreases from 3 to ca. 2.5) resulting from an increase in electrode potential was successfully observed by LEED. In the range of the electrode potential between 0.5 and 1.2 V vs RHE, LEED and in-situ STM measurements indicate a small change in the p value around 2.5. Isometrical electrocompression of the rot-hex-I phase was successfully monitored using in-situ STM by following changes in Moire patterns resulting from an increase in electrode potential. A detailed characterization of the rot-hex-I phase was achieved with the aid of computer- simulated STM images. The reversible phase transition between c(p x √3R-30°) and rot-hex occurred at potentials more positive than ca. 1.3 V vs RHE. Atomic size vacancies were observed at the early stages ofelectrooxidation of the rot-hex-iodine adlayer.

In-Situ Scanning Tunneling Microscopy of Semiconductor(n-TiO2)/Liquid Interfaces. A Role of Band Bending in Semiconductors

Abstract It is found in this study that electron tunneling from the conduction band in the semiconductor to the metal tip is possible if the electrode potential of the semiconductor is negative with respect to the flat band potential. The result clearly reveals the role of band bending in semiconductors for electron tunneling at the semiconductor/liquid interface.