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(a)
(b)
1 µm
figure 3.6 SeM ic image (a) and the corresponding simulated structure (b) of a single74-facet Cu 2 O crystal: squares for {100}, hexagons
for {110}, truncated triangles for {111}, long quadrangles for {211} and isosceles trapezoids for {744}. Reproduced by permission from
Ref. [124]. © 2013, The Royal Society of Chemistry.
(Fig. 3.7l). Because there are 6 (100) facets in a 50-facet Cu 2 O crystal, 24 {522} facets are observed in a 50-facet Cu 2 O crystal.
When the NaOH concentration further increased to 8 M, another type of 50-facet Cu 2 O crystal exposing {211} facets was obtained
(Fig. 3.7m and n). The new exposing {211} facets shown as long hexagons adjacent to squares facets also grow around every single
(100) facet (Fig. 3.7o). Finally, when NaOH concentration reached 9.5 M, a well-defined 74-facet Cu 2 O polyhedron with exposing
{744} facets was produced (Fig. 3.7p and q). The newest {744} facets given by isosceles trapezoids grew between a single (100)
facet and 4 newer {211} facets (Fig. 3.7r), so there are 24 {744} facets and 24 {211} facets in a single 74-facet Cu 2 O crystal.
3.2.2.3.2 Controllable Synthesis of Cu 2 O with Hollow Structures The general method for chemical synthesis of hollow
nanostructures is template strategy, using either a hard or a soft template. In addition, templateless synthesis through self-
assembly of nanoparticles (NPs) is also achievable for forming hollow materials [125]. The use of templates that serve as cores
is for the subsequent growth of shells. The cores need to be removed by dissolution, etching, or thermal treatment. Templateless
synthesis by self-construction, which is based on the ability of spherical nanocrystallites to stabilize certain crystallographic
planes as they undergo two-dimensional (2d) and then 3d aggregation, represents another mechanism for fabricating hollow
nanostructures [118]. Zhang et al. [113] reported a simple liquid reduction approach to prepare a Cu 2 O hollow microsphere film
and hollow nanosphere powder with Cu(OH) 2 nanorods as the precursor and ascorbic acid as the reductant at 60°C. When a
Cu(OH) 2 nanorod array film grown on a copper foil was used as a precursor, a Cu 2 O thin film made up of hollow microspheres
with average diameter of 1.2 mm was successfully prepared. When the Cu(OH) 2 nanorods were scraped from the copper foil
and then used as the precursor, Cu 2 O hollow nanosphere powder with average diameter of 270 nm was obtained. Xu et al. [115]
synthesized the nanosized Cu 2 O/poly (ethylene glycol 400) (PeG-400) composite hollow spheres (HSs, 50-80 nm in diameter)
with mesoporous shells of approximately 15-20 nm. In the hollow nanostructures, the polymer content is about 18.1 wt.%, and
the mean sizes of the component nanocrystals and the pore diameter are about 5 and 3.8 nm, respectively. In the fabrication
process, PeG-400 molecules self-assemble to form micelles, which act as templates for the formation of hollow structures. PeG
also acts as a reducing agent, solvent, and complexing agent. The formation of mesoporous structures is due to the oriented
aggregation of composite NPs. Xu et al. [116] successfully prepared multishelled structures as well as a single-crystalline shell
wall Cu 2 O by two stages. In the first stage, Cu 2 O NPs (2-5 nm) underwent a 2d-oriented attachment under the direction of
CTAB multilamellar vesicles to form the primary well-crystallized porous-shelled hollow spheres, which serve as the base
framework for the single-crystalline multishelled hollow spheres (MHSs). In the second stage, densification of this porous shell
wall via Ostwald ripening took place to achieve compact multishelled hollow spheres with a single-crystalline shell wall.
Remarkably, the formation of the porous shell wall is important for the formation of the multishelled hollow spheres for
providing exchange channels for reactants. Pang and Zeng [93] successfully fabricated monodispersed Cu 2 O spheres with
diameters of 130-135 nm. Using the Cu 2 O spheres thus prepared as solid precursors, uniform hollow CuS and CuSe derivatives
have also been synthesized. More importantly, a range of 2d and 3d superlattices of Cu 2 O, CuS, and CuSe solid/hollow spheres
have been assembled for the first time. Xu et al. [117] assembled single-crystalline hollow Cu 2 O nanocubes with well-defined
shape and tunable size (50-200 nm) (Fig. 3.8) by a PeG-200-assisted precursor hydrolysis process in aqueous solution at room
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