Perspective on the Interfacial Reduction Reaction

Abstract

Chemical reactions involving oxidation and reduction processes at interfaces may vary from those in conventional liquid-phase or solid-phase reactions and could influence the overall outcome. This article primarily features a study on metal−ligand coordination at the solid−liquid interface. Of particular mention is the spontaneous reduction of Cu(II) to Cu(I) at a solid−liquid interface without the need of any extraneous reducing agent, unlike in the liquidphase reaction whereby no reduction of Cu(II) to Cu(I) took place. As a consequence of the interfacial reduction reaction (IRR), thin films of Cu-TCNQ (tetracyanoquinodimethane) and Cu-HCF (hexacyanoferrate) were successfully deposited onto a thiol-functionalized Au substrate via a layer-by-layer (LbL) method. IRR is anticipated to be useful in generating new functional and stimuli-responsive materials, which are otherwise difficult to achieve via conventional liquid-phase reactions.

Emergent Interface in Heterostructured Thin Films of Cu(II) and Cu(I) Coordination Polymers

“…We have recently discovered spontaneous reduction of Cu(II) to Cu(I) at solid−liquid interface−interfacial reduction reaction (IRR), which successfully led to fabrication of a high-quality thin film of pristine Cu(I)-TCNQ on a functionalized Au substrate by LbL involving ethanolic solutions of Cu II (OAc) 2 and TCNQ. 28 IRR was consistently observed during LbL growth involving ethanolic solutions of Cu II (OAc) 2 and (K 4 [Fe(CN) 6 ]) leading to successful fabrication of a high-quality thin film of Cu(I)−hexacyanoferrate. 27 Thus, in course of the growth of Cu(I)-TCNQ thin film on top of Cu(II)-BTC thin film, at every immersion step of LbL into TCNQ solution, Cu(II) was getting converted into Cu(I) by IRR, which finally resulted in formation of welldefined AB-type heterostructured thin film.…”

“…Pristine and heterostructured thin films of Cu-BTC and Cu-TCNQ on functionalized Au substrates were successfully fabricated by standard protocols established in our group (see the Supporting Information for experimental details and Figure S1). , Our data are consistent with earlier reports on the interfacial growth of pristine Cu-BTC and Cu-TCNQ thin films whereby in each cycle of the LbL growth, multilayers of respective CP were getting deposited at the solid–liquid interface. − In the present study, we have changed the ligand to sequentially deposit Cu-TCNQ on top of Cu-BTC and vice versa, thereby leading to generation of AB- and BA-type heterostructured thin films, and thus, it has been conventionally named LbL growth. It is worthwhile to mention that upon dipping the Cu-BTC thin film into a methanolic solution of TCNQ for a prolonged time, complete conversion of Cu-BTC to Cu-TCNQ can take place, and as a result, it remained really difficult to grow Cu-TCNQ thin film on top of the Cu-BTC thin film by LbL with a defined interface.…”

“…Such observations are consistent with previous reports on thin films of CPs, specifically, heterostructures whereby orientation of the initially deposited layers determines the final orientation of thin films. , What was responsible for such an unusual growth of a Cu­(I)-CP thin film on top of a Cu­(II)-CP thin film leading to a well-defined heterostructure? We have recently discovered spontaneous reduction of Cu­(II) to Cu­(I) at solid–liquid interface–interfacial reduction reaction (IRR), which successfully led to fabrication of a high-quality thin film of pristine Cu­(I)-TCNQ on a functionalized Au substrate by LbL involving ethanolic solutions of Cu II (OAc) 2 and TCNQ . IRR was consistently observed during LbL growth involving ethanolic solutions of Cu II (OAc) 2 and (K 4 [Fe­(CN) 6 ]) leading to successful fabrication of a high-quality thin film of Cu­(I)–hexacyanoferrate .…”

“…As for the BA-type heterostructured thin film, Cu­(II) remained Cu­(II) due to binding with non-cyanide, carboxylate (COO − ) ligand facilitating the growth of a Cu­(II)-BTC thin film on top of a Cu­(I)-TCNQ thin film. The concomitant oxidation of protic solvents like ethanol/methanol to acetaldehyde/formaldehyde (or acetic acid/formic acid) was assigned to be the source of electron for the IRR of Cu­(II) to Cu­(I). , …”

Deciphering the Electrochemical Behaviors of the Electrode‐Electrolyte Coupling toward Advanced Electrochemical Energy Storage Device

“…[34–36] As the potential steps increasing to 0.1 V, there is no observable difference that can be found in the Raman spectra, demonstrating that the electrochemical process only involves the ionic absorption at the lower potential. While at the potential steps of 0.2 V, the Raman spectra feature the signal of [Fe(CN) 6 ] 3– at 2135.7 cm –1 (ν 1 , A 1g ), [ 34–35 ] and the signal shows the enhanced trend as the increase of potential, which corresponds to the transformation of [Fe(CN) 6 ] 4– to [Fe(CN) 6 ] 3– . Note that the peak of [Fe(CN) 6 ] 3– exhibits the strong intensity even at the discharge potential of 0.5 V, confirming that [Fe(CN) 6 ] 3– ions still distribute in EDL due to the higher Q d arising from the oxides of [Fe(CN) 6 ] 4– ions.…”