Dual-Step Reduction of Copper and Formation Mechanism of Cu Pseudo-Icosahedral Microcrystals

Abstract

Pseudo-icosahedral Cu microcrystals have been synthesized in a solvothermal system containing CuSO 4 •5H 2 O as the precursor, polyvinylpyrrolidone (PVP) as a reductant/capping agent, and dimethylformamide as the solvent. The structural and morphological evolutions over the reaction time are investigated, which enable us to establish a novel formation mechanism of pseudo-icosahedral crystals of Cu. The first crystalline phase that appeared in the solution is Cu 4 SO 4 (OH) 6 •H 2 O in the form of microflakes. The microflakes are reduced and decomposed to Cu 2 O nanocrystallites, which assemble with PVP into spherulites. The Cu 2 O crystals are further reduced to Cu nanocrystallites, which aggregate with PVP again into spherical particles. An extraordinary phenomenon is that 20 separated (111) triangular plates form on each particle during surface recrystallization of Cu, and their locations match to the final facets of the pseudo-icosahedron. The plates extend to cover the whole surface of the sphere, forming a pseudo-icosahedral shell, followed by extension of the recrystallization from the surface to the core. This reversed crystal growth process increases the thickness of the plates until all the Cu nanocrystallites in the core are consumed. It is found that, during the surface recrystallization of polycrystalline spheres, the particles preferentially select the (111) planes of the face-centered cubic structure of Cu as the exposed faces because these planes have the minimum surface binding energy. The particles then try to keep as small as possible the specific surface area, and a pseudo-icosahedral shape consisting of 20 (111) plates, which has a specific surface area, about 10% lower than that of an octahedron, formed by eight (111) plates. Accordingly, the overall surface free energy of a pseudo-icosahedron is about 10% lower than that of an octahedron. The formation of tetrahedra as building units of icosahedra, as predicted previously, has not been observed. The formation of ideal icosahedra, the so-called perfect Platonic solid, and the formation of twin defects between neighboring (111) plates are not possible. The newly proposed formation mechanism of pseudo-icosahedra Cu sheds light on the understanding of formation of many other polyhedral crystals.

Growth mechanism and microstructures of Cu2O/PVP spherulites

“…The synthesis conditions of this work were almost identical to the conditions used for growth of Cu icosahedra, 18 except in the present work the precursor CuSO 4 ·5H 2 O was replaced by Cu(NO 3 ) 2 ·H 2 O. In the system containing sulfate, the first crystalline phase was monoclinic Cu 4 SO 4 (OH) 6 ·H 2 O as observed in 1.5 h sample.…”

“…Shortly after, Cu 2+ cations are reduced by PVP to form Cu 2 O nanocrystallites. During this time, NO 3 − anions, unlike SO 4 2− , 18 do not form crystalline phase with Cu 2+ . Their decomposition can also accelerate the process of Cu 2+ to Cu 2 O.…”

“…34 Spherical aggregates of nanocrystallites of Cu and PVP underwent surface recrystallisation into pseudo-icosahedral shells. 18 This surface recrystallisation of polycrystalline spheres seems to be quite common. 35 However, such a surface recrystallisation process was never found in any type of spherulites.…”

“…In our recent research of the formation of Cu pseudo-icosahedral microcrystals using CuSO 4 ·5H 2 O as precursor, polyvinylpyrrolidone (PVP) as a reductant/capping-agent and dimethylformamide (DMF) as the solvent, Cu 2 O/PVP composite spherulites were detected as an intermediate phase. 18 The Cu 2 O nanocrystallites in these spherulites were further reduced into Cu nanocrystallites, therefore these spherulites were not preserved. Cu 2 O is a promising semiconductor and photocatalyst in reduction of CO 2 , 19 dye degradation, 20 H 2 production, 21 etc .…”

Icosahedral supracrystal assembly from polymer-grafted nanoparticles via interplay of interfacial energy and confinement effect

“…2C indicates the coexistence of FCC and BCC packing symmetries in this sample (detailed analysis of the SAXS profiles can be found in the Supplementary Materials). Icosahedral supracrystals often consist of 20 deformed tetrahedral domains of FCC ordering ( 45 , 46 ). Moreover, Talapin and co-workers ( 42 ) showed the FCC packing of nanocrystals detected in icosahedral morphology according to SAXS.…”