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3.2.2.3.1 Controllable Synthesis of Cu 2 O with Different Polyhedral Structures A number of studies have described the
synthesis of Cu 2 O nanocubes. Guo and Murphy [119] prepared Cu 2 O nanocubes with edge lengths of approximately 450 nm by
mixing a solution of CuSO 4 , cetyltrimethylammonium bromide (CTAB) surfactant, sodium ascorbate, and NaOH at a reaction
temperature of 55°C. Wang et al. [104] synthesized uniform crystalline Cu 2 O cube in high yields by reducing the copper-citrate
complex solution with glucose. A series of shape evolutions for Cu 2 O particles from the transient species such as multipod and
star-shaped particles to cubic crystals have been captured based on transmission electron microscope (TeMic) and scanning
electron microscope (SeMic) observations. It was concluded that a higher growth rate along the [111] direction induces
shrinking of the eight {111} facets, while six {100} facets remain to form Cu 2 O cubes because of their lower growth rate. Kim
et al. [120] heated a solution of ethylene glycol at 140°C and added NaCl, Cu(NO 3 ) 2 , and poly(vinyl pyrrolidone) (PVP) to
synthesize Cu 2 O nanocubes with an average edge length of 410 nm. Chloride ions were found to play a pivotal role in the
formation of nanocubes, and only polycrystalline spheres were generated in the absence of chloride ions.
Reports primarily on the synthesis of octahedral Cu 2 O nanocrystals are also available. Zhang et al. [107] successfully pre-
pared monodispersed submicron-sized Cu 2 O octahedra in large quantities assisted by the capping reagent PVP-K30
(MW = 58,000). The mechanism of Cu 2 O octahedra formation can be explained as the cooperative effect coming from the
growth units of the anion coordinative polyhedra theoretical model and polymer selective adsorption. He et al. [108] prepared
monodispersed cuprous oxide octahedron nanocrystals with sizes smaller than 100 nm under controlled conditions. The method
is based on the reduction of copper nitrate in Triton X-100 water-in-oil (w/o) microemulsions by γ-irradiation. The average
edge length of the octahedron-shaped nanocrystals varies from 45 to 95 nm as a function of the dose rate. Guo et al. [109]
described a simple electrochemical route for the controlled synthesis of a Cu 2 O microcrystal from perfect octahedra to mono-
dispersed colloid spheres via the adjustment of electrodeposition potential without the introduction of any template or surfactant.
High yields (~100%) of perfect Cu 2 O octahedra and monodispersed colloid spheres were obtained. In their electrochemical
synthesis system, Cu(OH) 4 2− exists in the following equilibrium condition 3.1:
(3.1)
) 4 2
2
+
Cu OH
(
↔+
Cu
4
OH
Cu 2+ can be slowly reduced to Cu 2 O in an alkaline solution under a certain potential range (from −0.5 to −0.6 V). Also, the OH ions
might be selectively adsorbed on (111) facets of Cu 2 O crystals, and it may slow the growth rates along the [111] direction,
which results in the final morphology of octahedra with (111) facets. As a comparison, when the electrodeposition potential was
changed to −0.4 V for 15 min, no particles could be obtained. It can be concluded that the reduction potential of Cu(OH) 4 2− /Cu 2 O
can be estimated at −0.4V versus Ag/AgCl (saturated KCl), which is slightly higher than the potential range employed (from
−0.5 to −0.6 V). Thus, Cu(OH) 4 2− will gradually reduce in this case. However, changing the electrodeposition potential to −0.7 V
will improve the reaction kinetics and result in an increase in the reaction rates. Thus, it is hard to adjust the growth of certain
facets of Cu 2 O via OH adsorption. Therefore, monodispersed Cu 2 O colloid spheres are finally produced. In addition, the
potential-dependent adsorption of OH on the preformed Cu 2 O facets {100} and {111} is probably a key factor. When the
potential is moved to a more negative one, fewer OH ions are adsorbed on the Cu 2 O surface because the surface concentration
of OH decreases, leading to the formation of spherical particles without preferential growth on certain facets.
The synthesis of polyhedral Cu 2 O with high-index facets is of great interest since it may exhibit higher chemical activities in
practical applications. Leng et al. [110] reported a solution-based approach for the preparation of unusual polyhedral 50-facet
Cu 2 O microcrystals, whose surfaces are enclosed by high-index {311} facets in addition to low-index {100}, {110}, and {111}
facets. The formation of the 50 facets can be geometrically viewed as the truncation of all 24 vertices of a small rhombicuboc-
tahedron with 26 facets (Fig. 3.5). during growth in solutions, the anisotropic growth rates along the [100], [110], and [111]
directions might be responsible for the formation of this morphology. The Miller index of the 24 nearly isosceles trapezoids could
be assigned to the {311} planes based on geometrical analysis, and this was verified by simulated models using the WinXmorph
software and supported by TeMic and electron diffraction (ed) observations. Sun et al. [121] described the synthesis of highly
symmetric multifaceted polyhedral 50-facet polyhedral Cu 2 O crystals with high-index {522} facets and polyhedral architectures
Cu 2 O crystals with high-index {544} and {211} planes via a template-free complex-precursor solution route. Later, they [122]
presented another route for the synthesis of highly symmetric polyhedron-aggregated multifaceted Cu 2 O homogeneous structures
composed of a cubic core and many abridged polyhedral building blocks with different index facets (including high-index {522}
facets and low-index {111}, {100}, and {110} facets). Wang et al. [123] developed a facile wet chemical method to synthesize
truncated concave octahedral Cu 2 O microcrystals mainly enclosed by { hhl } high-index facets using an aqueous solution of
Cu(CH 3 COO) 2 , sodium dodecyl sulfate (SdS) surfactant, NaOH, and d-(+)-glucose reductant. In this system, SdS was found to
be crucial for the formation of the concave Cu 2 O microcrystals, and the crystal growth rate also affected the morphology of Cu 2 O
microcrystals. Wang et al. [124] described a facile strategy for crystal engineering of Cu 2 O polyhedrons with high-index facets
developed in the system of CuSO 4 /NaOH/ascorbic acid. Novel micron-sized Cu 2 O 74-facet polyhedrons with the exposure of
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