Surface chemistry of liquid bismuth under oxygen and water vapor studied by ambient pressure X-ray photoelectron spectroscopy

Plasmonic Bismuth Nanoparticles: Thiolate Pyrolysis Synthesis, Size-Dependent LSPR Property, and Their Oxidation Behavior

“…On the basis of the difference in binding energy (BE), the measured peaks can be classified into two sets of 4f 7/2 and 4f 5/2 doublet peaks. The two peaks with BEs at 156.7 eV (4f 7/2 ) and 162.0 eV (4f 5/2 ) can be attributed to zero-valent bismuth, while the double ones with relatively high BEs at 158.7 eV (4f 7/2 ) and 164.0 eV (4f 5/2 ) arise from trivalent bismuth, which is predominately derived from the surface native oxide species (amorphous Bi 2 O 3 or Bi–O bonds) ,,,,, and partly from the Bi–S components due to the surface coating of thiol molecules . The molar ratio of Bi 3+ /Bi 0 oxidation states in the outer surface of Bi NPs (160 nm) is calculated about 69.4:30.6 according to the 4f 7/2 XPS peak area ratio.…”

“…As displayed in Figure S6, a quite weak S 2s XPS signal is detectable, indicating the presence of surface Bi–S components (i.e., bismuth thiolate); at the same time, there are two peaks with BEs of 529.4 and 531.0 eV in the O 1s region, which are consistent with the characteristic positions for Bi–O (Bi 2 O 3 ) ,, and surface O–H hydrated species, , respectively. The oxidation of Bi to Bi 2 O 3 , that is, 4Bi(s) + 3O 2 (g) → 2Bi 2 O 3 (s), is a thermodynamically favorable reaction due to its negative Gibbs free energy . Accordingly, Bi is easily oxidized and even forms a shell layer of native oxide on the surface as exposed to an oxygen-containing environment, especially upon electric or laser heating. − ,,,− However, thiols are a kind of robust reducing agents and capping ligands ,, and thus are able to not only reduce surface bismuth oxide to zero-valent Bi but also protect Bi NPs from reoxidation by forming a thiol-adsorbed layer on the surface (Figure b), thus generating Bi NPs with high quality that is favorable for the investigations on their intrinsic physical and chemical properties.…”

“…The oxidation of Bi to Bi 2 O 3 , that is, 4Bi(s) + 3O 2 (g) → 2Bi 2 O 3 (s), is a thermodynamically favorable reaction due to its negative Gibbs free energy . Accordingly, Bi is easily oxidized and even forms a shell layer of native oxide on the surface as exposed to an oxygen-containing environment, especially upon electric or laser heating. − ,,,− However, thiols are a kind of robust reducing agents and capping ligands ,, and thus are able to not only reduce surface bismuth oxide to zero-valent Bi but also protect Bi NPs from reoxidation by forming a thiol-adsorbed layer on the surface (Figure b), thus generating Bi NPs with high quality that is favorable for the investigations on their intrinsic physical and chemical properties.…”

“…Therefore, it can be rationally argued that Bi NPs are oxidized more easily when they begin to melt, which means that the oxidation reaction is more likely to take place at the solid–liquid phase transition in a fast, complete way. However, an incomplete oxidation of bismuth with slow speed and controllable level may be achieved by circumventing its occurrence at the solid–liquid transition point (i.e., the melting point temperature). ,,, For instance, the oxidation reaction could be confined only to the top surface of molten Bi liquid droplets at 300 °C at the low oxygen concentration, from which sub-nanometer thick 2D monolayers of Bi 2 O 3 were synthesized . Additionally, it should be pointed out that the oxidation temperature may decrease for Bi NPs of relatively small sizes because with a further decrease in the particle size, their surface reactivity would be enhanced and their melting temperature would decrease as well. , …”

“…Similar to noble-metal Au and Ag NPs, bismuth NPs (Bi NPs) are found to exhibit LSPR optical bands and extinction spectra highly sensitive to their grain sizes, shapes, and surrounding dielectric media [especially the solvents of different relative dielectric constants (ε)]. − , For spherical NPs, an increase in the size of Bi NPs would cause a red shift of LSPR bands, doublet or multiple resonance bands, enhanced light scattering, and bandwidth broadening in the ultraviolet–visible–near-infrared (UV–vis–NIR) extinction spectra. − , Unlike Au and Ag NPs, however, the synthesis of monodisperse Bi NPs with tunable average sizes is rather difficult, ,− so that a comprehensive experimental investigation concerning the size effect on their LSPR optical properties is significantly limited. On the other side, the oxidation of metallic Bi would produce thin films or particulates of bismuth oxide (Bi 2 O 3 ), − which is known as a versatile material that exhibits rich polymorphism and phase-dependent physical and chemical properties. , Recently, it has been found that controlling the surface oxidation level of liquid Bi droplets could successfully make atomically thin Bi 2 O 3 monolayer sheets, opening up a novel route to 2D materials of nonlayered oxides. , In this context, further investigations and mechanistic insights are required in order to reveal and regulate the oxidation behaviors of liquid or solid Bi NPs as well as the polymorphism in so-formed Bi 2 O 3 .…”