Uncovering the Role of Countercations in Ligand Exchange of WSe₂: Tuning the d-Band Center toward Improved Hydrogen Desorption

Abstract

The role of countercations that do not bind to core nanocrystals (NCs) but rather ensure charge balance on ligand-exchanged NC surfaces has been rarely studied and even neglected. Such a scenario is unfortunate, as an understanding of surface chemistry has emerged as a key factor in overcoming colloidal NC limitations as catalysts. In this work, we report on the unprecedented role of countercations in ligand exchange for a colloidal transition metal dichalcogenide (TMD), WSe2, to tune the d-band center toward the Fermi level for enhanced hydrogen desorption. Conventional long-chain organic ligands, oleylamine, of WSe2 NCs are exchanged with short atomic S2– ligands having countercations to preserve the charge balance (WSe2/S2–/M+, M = Li, Na, K). Upon exchange with S2– ligands, the charge-balancing countercations are intercalated between WSe2 layers, thereby serving a unique function as an electrochemical hydrogen evolution reaction (HER) catalyst. The HER activity of ligand-exchanged colloidal WSe2 NCs shows a decrease in overpotential by down-shift of d-band center to induce more electron-filling in antibonding orbital and an increase in the electrochemical active surface area (ECSA). Exchanging surface functionalities with S2– anionic ligands enhances HER kinetics, while the existence of intercalated countercations improves charge transfer with the electrolyte. The obtained results suggest that both anionic ligands and countercationic species in ligand exchange must be considered to enhance the overall catalytic activity of colloidal TMDs.

Colloidal Inorganic Ligand-Capped Nanocrystals: Fundamentals, Status, and Insights into Advanced Functional Nanodevices

“…For example, negative ζ-potential was observed for colloidal stripped WSe 2 NCs. 95 In this special case, the surface negative charge was contributed by surface-rich Se atoms from NC synthesis, and the lack of surface ligands was evidenced by FTIR spectrum.…”

“…In another investigation, WSe 2 NCs underwent a ligand stripping process followed by capping with S 2− . 95 However, in this special case, WSe 2 NCs contained a negative charge contributed by surface-rich Se atoms from NC synthesis, and this unusual phenomenon interfered the judgment of subsequent S 2− capping by common ζ-potential measurement. In order to eliminate such an interference, ζpotential titration was applied, and the gradual increasing negative charge over incremental concentration of S 2− confirmed S 2− capping of WSe 2 NCs.…”

“…However, due to the strong oxidizing ability of NO + and the presence of strong acidic reagents in the stripping process, this treatment is only applicable for the relatively stable NCs, such as Fe 2 O 3 , FePt, Bi 2 S 3 , and NaYF 4 , and further modification relies on the alteration of cations by the mild and unreactive ones, such as Me 3 OBF 4 , Et 3 OBF 4 (Meerwein’s salt) , and triphenylcarbenium tetrafluoroborate (Ph 3 CBF 4 ), for ligand stripping of less stable CdSe NCs. Given as one piece of evidence, the measured redox potential of Ph 3 CBF 4 (−0.11 V vs ferrocene in acetonitrile) is much smaller than that of NOBF 4 (0.87 V). , It should also be noted that the stripped NCs dispersed in solvents are not always positively charged. For example, negative ζ-potential was observed for colloidal stripped WSe 2 NCs .…”

“…Given as one piece of evidence, the measured redox potential of Ph 3 CBF 4 (−0.11 V vs ferrocene in acetonitrile) is much smaller than that of NOBF 4 (0.87 V). 95,247 It should also be noted that the stripped NCs dispersed in solvents are not always positively charged. For example, negative ζ-potential was observed for colloidal stripped WSe 2 NCs.…”

Ruthenium-Doping-Induced Amorphization of VS4 Nanostructures with a Rich Sulfur Vacancy for Enhanced Hydrogen Evolution Reaction in a Neutral Electrolyte Medium

“…Therefore, researchers have devoted themselves to finding an alternative to Pt metal, and in this regard, it has been observed that various transition-metal-based catalysts such as phosphide, sulfide, and selenide could be considered as alternatives . Recently, various sulfide-based materials have been reported for their excellent HER activity like MoS 2 , WS 2 , VS 2 , and various sulfides of Fe, Ni, and Co. − Among all catalysts, the sulfide-based catalyst VS 2 and MoS 2 - and WS 2 -based catalysts have proven to be effective candidates for HER because of their optimum hydrogen adsorption energies. VS 2 possesses a Gibbs free energy for hydrogen adsorption (Δ G H ) of −0.16 eV, which is much smaller than those of MoS 2 (Δ G H = 1.89–1.91 eV) and very close to that of Pt (Δ G H = −0.09). , By taking this Gibbs free energy point of view, another sulfide of V with higher S content (VS 4 ) has gained greater attention because of its fascinating electrochemical applications like lithium- and sodium-ion batteries and many other battery applications. , Although VS 4 has been used for various electrochemical applications, its water splitting activity has not been explored much to date.…”