Single Crystals of α-MoO3-Intercalated {Ni(H2O)6}2+ and Electrocatalytic Water Reduction: Toward a Class of Molybdenum Bronzes.

We have successfully intercalated {NiII(H2O)6}2+ into the α-MoO3 layer, leading to the isolation of green single crystals of [MoVI2O6(CH3COO){NiII(H2O)6}0.5]·H2O (1). The homogeneous electrochemistry of 1 in its aqueous solution exhibits electrocatalytic hydrogen evolution reaction (HER) with concomitant electrochemical deposition of [HMo3VIMoVO12(CH3COO){NiII(H2O)5(OH)}] (2). Compound 2, a new molybdenum bronze, acts as an efficient and stable heterogeneous electrocatalyst for water reduction to molecular hydrogen. This work represents the first paradigm of a molybdenum bronze intercalating a transition metal-aqua ion.

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 ({Mo72Fe30}cryst) 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, {M72Fe30} (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 ({W72Fe33}NM) and Na2[Mo72Fe30O252(CH3COO)4(OH)16(H2O)108]·180H2O ({Mo72Fe30}NM), respectively (NM stands for nanomaterial). The formulations of these one-pot-synthesized nanoblackberries of {W72Fe33}NM and {Mo72Fe30}NM 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 {W72Fe33}NM is much higher than that of its molybdenum analogue {Mo72Fe30}NM. This might due to the extra three ferric (Fe3+) ions per {W72Fe30} cluster in {W72Fe33}NM, 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 {W72Fe33}NM and 3.7 for {Mo72Fe30}NM 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 {W72Fe33}NM and {Mo72Fe30}NM in a comparative study. The relevant turnover frequencies (TOFs) of {W72Fe33}NM and {Mo72Fe30}NM (∼0.72 and ∼0.45 s-1, respectively), the overpotential values of {W72Fe33}NM and {Mo72Fe30}NM (527 and 767 mV, respectively at 1 mA cm-2), and the relative stability issues of the catalysts indicate that {W72Fe33}NM is reasonably superior to {Mo72Fe30}NM. We have described a rationale of why {W72Fe33}NM performs better than {Mo72Fe30}NM in terms of catalytic activity and stability.

Efficient homogeneous electrocatalytic hydrogen evolution using a Ni-containing polyoxometalate catalyst.

NiCl2·6H2O ([Ni(H2O)6]2Cl2) per se does not show electrocatalytic hydrogen evolution reaction activity (HER) in an acidic aqueous medium as well as in neutral water. Interestingly, when [Ni(H2O)6]2+ is present in a polyoxovanadate matrix, for example, in the compound K2[Ni(H2O)6]2[V10O28]·4H2O (1), it exhibits homogeneous electrocatalytic HER activity in an acidic aqueous solution with a turn over frequency of 2.1 s-1 and an effective low overpotential of 127 mV at pH 2.3. Compound 1 is the first nickel-containing polyoxometalate catalyst for hydrogen production via homogeneous electrocatalytic proton reduction without its decomposition under electrochemical conditions of HER.

Deciphering the Ultrafast Nonlinear Optical Properties and Dynamics of Pristine and Ni-Doped CsPbBr3 Colloidal Two-Dimensional Nanocrystals.

While the unabated race persists in achieving record efficiencies in solar cells and other photonic/optoelectronic devices using lead halide perovskite absorbers, a comprehensive picture of the correlated third-order nonlinear optical (NLO) properties is yet to be established. The present study is aimed at deciphering the role of dopants in multiphoton absorption properties of intentionally engineered CsPbBr3 colloidal nanocrystals (NCs). The charge separation of the plasmon-semiconductor conduction band owing to the hot electron transfer at the interface was demystified using the dynamics of the bleached spectral data from femtosecond (fs) transient absorption spectroscopy with broadband capabilities. The NLO properties studied through the fs Z-scan technique revealed that Ni-doped CsPbBr3 NCs exhibited strong third-order NLO susceptibility of ∼10-10 esu. The exotic photophysical phenomena in these pristine and Ni-doped CsPbBr3 colloidal two-dimensional (2D) NCs reported herein are believed to provide the avenues to address the critical variables involved in the structural differences and their correlated optoelectronic properties.