Unusual Demetalation and Ordered Adsorption of a Pyridine-Appended Zinc Phthalocyanine at Metal–Electrolyte Interfaces Studied by in Situ Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy

Abstract

A combination of cyclic voltammetry, in situ electrochemical scanning tunneling microscopy (EC-STM), and ex situ X-ray photoelectron spectroscopy (XPS) studies was employed to investigate the electrochemical behavior and to characterize the structure of adsorbed layers of a redox-active pyridine-appended zinc phthalocyanine (ZnPc) at Cu(100)/electrolyte interfaces. Additional cyclic voltammograms (CVs) of a HOPG electrode in phthalocyanine- and iodide-containing electrolyte showed four redox peak pairs related to the phthalocyanine macrocycle. Focusing on the reduction processes, the four observed cathodic waves were assigned to four single electron transfer steps causing four successive reductions of the phthalocyanine. Due to its narrow potential window, the CV of Cu(100) in the same electrolyte covers only the first three reduction features in the acidic solution superimposed by the CuI formation/dissolution processes. Surprisingly, the XPS data from the iodide and phthalocyanine covered Cu(100) surface, however, revealed a demetalation of the molecules, that is, a replacement of the central zinc atom of adsorbed ZnPc + by hydrogen atoms resulting in the formation of the corresponding free base phthalocyanine H2Pc+. These H2Pc+ molecules form highly ordered adlayers on the iodide-modified Cu(100) surface with the coexistence of different phases, that is, one with a square-shaped unit cell enclosing an angle of (91 ± 2)° and a rhombic one with an angle of (66 ± 2)°, and intermolecular distances of 2.0 ± 0.1 nm and 2.3 ± 0.1 nm, respectively. The main symmetry axes of the H2Pc+ adlayer run parallel neither to the commensurate direction of the iodide layer underneath nor to the close-packed atomic rows of the copper substrate. Detailed molecular models are proposed which are in agreement with the patterns experimentally observed with EC-STM.

Stabilizing CuPc Coordination Networks on Ag(100) by Ag Atoms

“…Finally, the density of adatoms (counted from the model) in the rhombic structure is 0.80 atoms per nm 2 almost twice as large as the one in the square one being 0.45 atoms per nm 2 . Interestingly, similar square and rhombic structures were observed recently by Phan et al for adsorption of 3-pyridyloxy-appended ZnPc on iodine precovered Cu(100) 40 observed in electrolyte and anchored in place by molecule ligands not by the presence of adatoms.…”

Single-Molecule Observation of Redox Reactions Enabled by Rigid and Isolated Tripodal Molecules

“…Electrochemical scanning tunneling microscopy (EC-STM) consists of the combination of STM and Electrochemical control which enables us to directly observe solid–liquid interfaces at the molecular scale − and track redox reactions at single-molecule junctions − by means of the tunneling current and an atomically sharp tip. In particular, scanning the tip over the electrode has the advantage of providing spatial information on the electronic state of the solid–liquid interfaces at the nanometer scale, such as the redox reactions of relatively large molecules ,,− and single molecules of proteins. − …”

On the role of applied potential in adsorption of dibenzyl viologen molecules on HOPG surface

“…Tuy nhiên, sự hấp phụ của chúng trên bề mặt graphene thường không đồng đều, do vậy ko thể kiểm soát được mức độ pha tạp. Trong khi đó, quá trình tự sắp xếp của phân tử hữu cơ là sự hấp phụ và hình thành cấu trúc xác định trên bề mặt nên có thể dễ dàng điều khiển được mức độ pha tạp graphene [14][15][16][17][18].…”