Morphological Tuning of Nanoporous Metals Prepared with Conversion Reaction Synthesis via Thermal Annealing

Abstract

Conversion reaction synthesis, namely, reacting an organolithium reducing agent with a metal chloride to produce a metal/LiCl nanocomposite and then removing lithium chloride with a polar organic solvent, is an efficient and scalable way to fabricate a variety of three-dimensional, bicontinuous nanoporous transition metals. Thermal annealing was investigated as a means to tune the morphology of these nanoporous metals. X-ray diffraction studies were used to investigate the effect of thermal annealing on the size and strain of phases in Cu/ LiCl, Co/LiCl, and Fe/LiCl nanocomposites, while electron microscopy and nitrogen adsorption studies were used to study the porosity and surface properties of the resulting nanoporous metals after LiCl was removed from the annealed nanocomposites. Annealing metal/LiCl nanocomposites resulted in the controlled growth of the metal nanoparticles, the rate of which depended on the diffusivity of the metal. It was observed that annealing nanocomposites produced more robust nanoporous metals with increased porosity under certain conditions. Overall, it was found that annealing the as-formed nanocomposites rather than the isolated nanoporous metals provides finer control of the conversion synthesis process and allows for the design of more optimized pore structures and thus enhanced suitability for catalytic and electrochemical applications.

Ultrafine Nanoporous Gold via Thiol Compound-Mediated Chemical Dealloying

“…To this end, the pore size evolution of NPG during the dealloying process with and without the L-cysteine additive was carefully investigated (Figures 5 and S1). As a result, pore sizes were exponentially dependent on the etching time for the formation of both C-NPG and Cys-NPG (Figure 5a), consistent with the isothermal grain growth in polycrystalline materials, 25 which is governed by the following equation 11,26…”

“…To this end, the pore size evolution of NPG during the dealloying process with and without the l -cysteine additive was carefully investigated (Figures and S1). As a result, pore sizes were exponentially dependent on the etching time for the formation of both C-NPG and Cys-NPG (Figure a), consistent with the isothermal grain growth in polycrystalline materials, which is governed by the following equation , where d ( t ) is the pore size at etching time t ; the surface diffusion coefficient , k 0 and K are constants, R is the gas constant, T is the etching temperature; E is the activation energy required for coarsening, and n is the coarsening exponent that reflects the NPG formation mechanism. The derived slope values (1/ n ) from fitting the In­[ d ( t )] vs In­( t ) were found to be ∼0.30 and ∼0.26 for C-NPG and Cys-NPG, respectively (Figure b), signifying their same formation process as discussed above.…”

Morphological Control of Nanoporous Copper Formed from Conversion Reaction Synthesis

“…Similar justifications have previously been made for dealloying reactions. 18,30 To measure the activation energy for Cu diffusion, we employed a series of thermal annealing experiments. 18 The nanocomposites were isothermally annealed at 100, 200, or 300 °C for 1 h, with the resulting diffraction patterns and their fits shown in Figures S4−S11.…”

“…18,30 To measure the activation energy for Cu diffusion, we employed a series of thermal annealing experiments. 18 The nanocomposites were isothermally annealed at 100, 200, or 300 °C for 1 h, with the resulting diffraction patterns and their fits shown in Figures S4−S11. The crystallite size obtained from diffraction data was found to systematically vary with the annealing temperature.…”

“…Lattice strain values (Table S2) represent the maximum displacement of the lattice or the maximum strain moment (ε 0 ). In our prior work on size−strain analysis for CRS-derived metal/salt nanocomposites, 18 it was found that the salt lattice strain is likely due to the chemical substitution of metal ions in the salt phase that (1) produce vacancies to maintain charge neutrality and (2) cause lattice expansion or contraction as a result of differences in the ionic radii of Li + and M X+ (M = Fe, Co, and Cu; X = 1, 2, and 3). Due to the systematic decrease of strain in the bromide and chloride salts after thermal annealing, we attribute strain in the salt phase of the Cu-NP nanocomposites to the same mechanism.…”