Search for: All records

Thermally regenerative electrochemical cycles and thermogalvanic cells harness redox entropy changes (ΔSrc) to interconvert heat and electricity with applications in heat harvesting and energy storage. Their efficiencies depend on ΔSrc because it relates directly to the Seebeck coefficient, yet few approaches exist for controlling the reaction entropy. Here, we demonstrate the design principle of using highly charged molecular species as electrolytes in thermogalvanic devices. As a proof-of-concept, the highly charged Wells-Dawson ion [P2W18O62]6– exhibits a large ΔSrc (−195 J mol–1 K–1) and a Seebeck coefficient comparable to state-of-the-art electrolytes (−1.7 mV K–1), demonstrating the potential of linking the rich chemistry of polyoxometalates to thermogalvanic technologies. 

We report a low-temperature colloidal synthesis of WSe₂nanocrystals from tungsten hexacarbonyl and diphenyl diselenide in trioctylphosphine oxide (TOPO). We identify TOPO-substituted intermediates, W(CO)₅TOPO andcis-W(CO)₄(TOPO)₂by infrared spectroscopy. To confirm these assignments, we synthesize aryl analogues of phosphine-oxide-substituted intermediates, W(CO)₅TPPO (synthesized previously, TPPO = triphenylphosphine oxide) andcis-W(CO)₄(TPPO)₂andfac-W(CO)₃(TPPO)₃(new structures reported herein). Ligation of the tungsten carbonyl by either the alkyl or aryl phosphine oxides results in facile labilization of the remaining CO, enabling low-temperature decomposition to nucleate WSe₂nanocrystals. The reactivity in phosphine oxides is contrasted with syntheses containing phosphine ligands, where substitution results in decreased CO labilization and higher temperatures are required to induce nanocrystal nucleation. 

We present a method for thephotochemical conversion of the inverse spinel iron oxides in which the mixed-valent magnetite phase (Fe 3 O 4 ) is accessed from the maghemite phase (γ-Fe 2 O 3 ) via a stable, colloidal nanocrystal-to-nanocrystal transformation. Anaerobic UV-irradiation of colloidal γ-Fe 2 O 3 nanocrystals in the presence of ethanol as a sacrificial reductant yields reduction of some Fe 3+ to Fe 2+ , resulting in a topotactic reduction of γ-Fe 2 O 3 to Fe 3 O 4 . This reduction is evidenced by the emergence of charge-transfer absorption and increased d -spacing in UV-irradiated nanocrystals. Redox titrations reveal that ∼43% of Fe in < d > = 4.8 nm nanocrystals can be reduced with this method and comparison of optical data indicates similar reduction levels in < d > = 7.3 and 9.0 nm nanocrystals. Addition of excess acetaldehyde during photoreduction shows that the extent of reduction is likely pinned by the hydrogenation of acetaldehyde back to ethanol and can be increased with the use of an alkylborohydride sacrificial reductant. Photochemical reduction is accompanied by increased magnetization and emergence of magnetic features characteristic of Fe 3 O 4 . Overall, this work provides a reversible, post-synthetic strategy to obtain Fe 3 O 4 nanocrystals with well-controlled Fe 2+ compositions.