Nanoblackberries of {W₇₂Fe₃₃} and {Mo₇₂Fe₃₀}: Electrocatalytic Water Reduction

The reversible self-assembly of a {Mo₇₂Fe₃₀} cluster into nanoblackberries in a dilute solution of the relevant crystalline compound [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂{H₂Mo₂O₈(H₂O)}(H₂O)₉₁]·150H₂O ({MoFe}) was demonstrated by Liu, Müller, and their co-workers as a landmark discovery in the area of polyoxometalate chemistry. We have described, in the present work, how these ∼2.5 nm nano-objects, {M (M = W, Mo) can be self-assembled into nanoblackberries irreversibly, leading to their solid-state isolation as the nanomaterials Fe₃[W₇₂Fe₃₀O₂₅₂(CH₃COO)₂(OH)₂₅(H₂O)₁₀₃]·180H₂O ({WFe}) and Na₂[Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₄(OH)₁₆(H₂O)₁₀₈]·180H₂O ({MoFe}), respectively (NM stands for nanomaterial). The formulations of these one-pot-synthesized nanoblackberries of {WFe} and {MoFe} have been established by spectral analysis including Raman spectroscopy, elemental analysis including ICP metal analysis, volumetric analysis (for iron), microscopy techniques, and DLS studies. The thermal stability of the tungsten nanoblackberries {WFe} is much higher than that of its molybdenum analogue {MoFe}. This might due to the extra three ferric (Fe³⁺) ions per {W₇₂Fe₃₀} cluster in {WFe}, which are not part of the {W₇₂Fe₃₀} cluster cage but are placed between two adjacent clusters (i.e., each cluster has six surrounding 0.5Fe³⁺) to form this self-assembly. The isolated blackberries behave like an inorganic acid, a water suspension of which shows pH values of 3.9 for {WFe} and 3.7 for {MoFe} because of the deprotonation of the hydroxyl groups in them. We have demonstrated, for the first time, a meaningful application of these inexpensive and easily synthesized nanoblackberries by showing that they can act as electrocatalysts for the hydrogen evolution reaction (HER) by reducing water. We have performed detailed kinetic studies for the electrocatalytic water reduction catalyzed by {WFe} and {MoFe} in a comparative study. The relevant turnover frequencies (TOFs) of {WFe} and {MoFe} (∼0.72 and ∼0.45 s^-1, respectively), the overpotential values of {WFe} and {MoFe} (527 and 767 mV, respectively at 1 mA cm^-2), and the relative stability issues of the catalysts indicate that {WFe} is reasonably superior to {MoFe}. We have described a rationale of why {WFe} performs better than {MoFe} in terms of catalytic activity and stability.