Precise control of the oxidation state of transition-metal oxides, such as copper, is important for high selectivity of CO reduction in an aqueous condition to compete with the reduction of water. The phase of copper oxide nanofibers was controlled by predictive synthesis, which controls the nanoscale gas-solid reaction by considering thermodynamics and kinetics. The driving force of the phase transformation between the different oxidation states of copper oxide is calculated by comparing the Gibbs free energy of each of the oxidation states. From the calculation, the kinetically processable window for the fabrication of CuO in which monophase CuO can be fabricated in a reasonable reaction time scale is discovered. Herein, we report the monophase CuO nanofiber photocathode, which photoelectrochemically converted CO into methanol with over 90% selectivity in an aqueous electrolyte, and a hierarchical structure is developed to optimize the photoactivity and stability of the electrode. Our work suggests a rational design of the calcination strategy for precisely controlling the oxidation states of transition metals that can be applied to various applications in which the phase of the materials plays an important role.
Cu acetate/PAN nanofibers were transformed into porous C nanofibers with doped N and Cu particles, via O2 partial pressure-controlled calcination. N atoms next to Cu trigger the CO2RR by increasing the amount of CO* on the Cu, lowering the energy needed for CO dimerization.
Controlled
phase conversion in polymorphic transition metal dichalcogenides
(TMDs) provides a new synthetic route for realizing tunable nanomaterials.
Most conversion methods from the stable 2H to metastable 1T phase
are limited to kinetically slow cation insertion into atomically thin
layered TMDs for charge transfer from intercalated ions. Here, we
report that anion extraction by the selective reaction between carbon
monoxide (CO) and chalcogen atoms enables predictive and scalable
TMD polymorph control. Sulfur vacancy, induced by anion extraction,
is a key factor in molybdenum disulfide (MoS2) polymorph
conversion without cation insertion. Thermodynamic MoS2–CO–CO2 ternary phase diagram offers a processing
window for efficient sulfur vacancy formation with precisely controlled
MoS2 structures from single layer to multilayer. To utilize
our efficient phase conversion, we synthesize vertically stacked 1T-MoS2 layers in carbon nanofibers, which exhibit highly efficient
hydrogen evolution reaction catalytic activity. Anion extraction induces
the polymorph conversion of tungsten disulfide (WS2) from
2H to 1T. This reveals that our method can be utilized as a general
polymorph control platform. The versatility of the gas–solid
reaction-based polymorphic control will enable the engineering of
metastable phases in 2D TMDs for further applications.
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