Electrochemical study of bismuth chalcogenides

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Bismuth chalcogenides have been studied as thermoelectric materials, layered materials and topological insulators. They have shown considerable promise in many areas, including but not limited to energy conversion, surface catalysis and quantum computing. However, there is still a long way to go before some of the most exciting applications become practical. One important constraint is their stability in air or solution. Although the studies of bismuth chalcogenides after exposure to air or water have led to different conclusions, the majority of work shows that air or water can modify their surface chemistry and electronic properties.
Here, X-ray photoelectron spectroscopy was used to measure in-situ cleaved, air exposed, electrochemically modified and thiol functionalized single-crystal Bi2Te0.9Se2.1 samples. The results not only demonstrated the ability to control their surface chemistry in an electrochemical environment but also confirmed the feasibility of thiol functionalization. Pourbaix diagrams were used to identify probable products for electrochemical oxidation and reduction. For thiol functionalization, the difference between 1,5’-pentanedithiol and 1-pentanethiol was also discussed.
Next, electrochemical scanning tunnelling microscopy experiments were carried out for different bismuth chalcogenides in pH 3 electrolyte. The images for freshly cleaved samples showed atomically smooth terraces and steps with height equal to the expected quintuple layer thickness (1 nm). After electrochemical oxidation, clear surface dissolution accompanied by pit formation was observed. With increasing oxidation time, these pits only expanded in the lateral direction, keeping a pit depth of about 1 nm. Three probable dissolution modes were discussed to explain the positive correlation between the pit coverage and the oxidation time.
Electrochemical impedance measurements were carried out for freshly cleaved, pre-oxidized and subsequently reduced single-crystal Bi2Se3 samples in pH 7 electrolyte. Electrochemical oxidation significantly increased the Bi2Se3 charge transfer resistance whereas electrochemical reduction had the ability to lower the transfer resistance to its original value. Electrochemical scanning tunnelling microscopy experiments were also carried out in pH 7 electrolyte to link surface morphology with impedance data. The images and Pourbaix diagrams suggested that the increase of charge transfer resistance may be attributed to bismuth oxides.
Metal electrodeposition on bismuth chalcogenides was also studied with the same set-up. A preliminary study of Ag deposition was recorded. Reversible epitaxial growth of bismuth films on single-crystal Bi2Se3 surface was observed.
Date of Award28 Sep 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SponsorsChina Scholarship Council
SupervisorWalther Schwarzacher (Supervisor) & Natasa Vasiljevic (Supervisor)

Keywords

  • EC-STM
  • bismuth chalcogenides
  • electrochemical modification

Cite this

'