Designated-Tailoring on {100} Facets of Cu2O Nanostructures: From Octahedral to Its Different Truncated Forms

A facile template-free controlled synthesis of Cu2O architectures from octahedral to its different truncated forms is successfully achieved. It is found that the precursor formation temperature is crucial to the designated-tailoring on the {100} facets of Cu2O crystals, which can modify the ratio (R) between the growth rates along the 〈100〉 and 〈111〉 directions, leading to the formation of the initial structures with different shapes. The multiple morphologies can be evolved from these varied initial structures via the synergic effect of oriented attachment and ripening mechanism. This template-free complex precursor-based solution route has provided an innovative approach to design the {100} facets with different sizes to further enrich the current morphologies of Cu2O crystals.

Cu 2 O has a cuprite structure with no free internal parameters.The structure is a cubic close packing of copper atoms, in which the coordination of the copper atoms is twice that of the oxygen atoms in a tetradecahedral unit cell [26][27][28].A useful way that the anisotropic Cu 2 O architectures could be formed in the solution condition is kinetically modifying the growth rates of different crystallographic planes of the Cu 2 O by adopting appropriate reaction conditions [26,29], which can lead to the variation of the ratio (R) between the growth rates along the 100 and 111 directions.Reference has demonstrated that the Wulff shape, such as an octahedron with four pairs of {111} facets or a cube with three pairs of {100} facets, can be easily formed [26].However, the formation process of Cu 2 O octahedra or cubes, and their truncated forms is primarily limited to additivesassisted solution synthesis.For example, Siegfried and Choi have fabricated cubes, octahedra, and their truncated forms via an additives-assisted electrochemical deposition  [30,31].Kuo and Huang have reported Cu 2 O Crystals with morphological evolution from cubic to octahedral structures by an SDS-(sodium dodecyl sulfate-) assisted solution synthesis [25].Yang and Liu have achieved hexapod structures to octahedra and their truncated forms via a Na 2 (C 4 H 4 O 6 ) (sodium tartrate)-assisted hydrothermal route [32].To our best knowledge, a template-free synthesis route for elucidating the shape evolution from octahedra to their different truncated forms was seldom reported, and it still remains a great challenge to systemically manipulate the sizes of truncated facets.Herein, we develop a facile and low-cost template-free complex-precursor aqueous solution synthesis method for designed-tailoring on {100} facets with different sizes to achieve the shape evolution of Cu 2 O crystals from octahedral to its different truncated forms.The growth process and mechanism of Cu 2 O crystals are discussed in detail.

Experimental Section
2.1.Preparation Procedures.All chemicals used in our experiment were of analytical grade and used without further purification.The synthetic procedure was described as follows: 0.825 g of CuCl 2 powder was dissolved in 50 mL deionized water using a beaker under a constant stirring at different temperature (25  C, sample S3).A precipitate was produced when a NaOH solution (3 M, 10 mL) was added dropwise to the above solution.In our paper, we describe the temperature of adding NaOH solution into the reaction system as the precursor formation temperature.After being stirred for 5 min, D-glucose powder (0.2 g) was added into the dark precursor with a constant stirring for 15 min at 70 • C. The precipitate gradually turned brick red and then was allowed to cool to room temperature naturally.Afterward, the obtained particles were centrifuged at 5000 rpm for 1 min (XIANYI TG16-WS centrifuge).The precipitates were centrifuged twice more in deionized water and anhydrous ethanol, respectively and finally were dried at 70 • C for 12 hours in a vacuum oven.

Characterization.
The crystal phase of as-prepared products was characterized by an X-ray diffractometer (Bruker-AXS D8 ADVANCE) using Cu Kα radiation (λ = 1.54 Å) in the range (20 • ∼ 80 • ).The morphology of the powders was investigated by field emission scanning electron microscope (FESEM) using JEOL (JSM-7000F) at an accelerating voltage of 20 kV.EDX analysis was obtained with an Oxford INCA Energy dispersive X-ray detector installed on the JEOL JSM-7000F.The transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) analysis images were performed on a JEOL JEM-2100 transmission electron microscope operating at an accelerating voltage of 200 kV.Sample for the TEM analysis was prepared by ultrasonic dispersion for 30 s with ethanol (1.5 mL) in a 2 mL centrifuge tube.Then, the products were dropped onto a carbon-coated copper grid and dried in air before TEM analysis.The crystal structure of Cu 2 O was drawn by using the Diamond 3.2 program.The ultraviolet-visible (UV-Vis) spectra were measured by a UV/vis/NIR spectrophotometer (Hitachi U-4100).

Results and Discussion
3.1.X-Ray Diffraction.The phase structure and purity of the products were examined by X-ray diffraction (XRD) characterization.Figure 1 shows the XRD patterns of the asprepared products obtained at different precursor formation temperature (samples S1-S3).All the diffraction peaks of these three samples are indexed according to the standard cubic structure of Cu 2 O (space group : Pn3m, lattice constant a = 0.427 nm, JCPDS file no.05-0667).No peaks of impurities such as copper or cupric oxide were detected, suggesting the high purity of the as-obtained products.Moreover, it is found that the ratio between the intensity of the ( 111) and ( 200) peak is higher than the standard value (3.25 versus 2.50 (sample S1), 3.06 versus 2.50 (sample S2), and 3.05 versus 2.50 (sample S3)).It is known that the facets with a slower growth rate will be exposed more on the crystal surface and exhibit relatively stronger diffraction intensity in the corresponding XRD pattern [33,34].Hence, it can be concluded that the products are abundant in {111} facets and the {100} facets might be exposed gradually in the final morphology due to the ratio between the intensity of the ( 111) and (200) peak decrease.

EDX Measurement.
The chemical composition and purity of the products were examined by energy-dispersive X-ray spectroscopy.Figure 2 displays the electron energy dispersive X-ray (EDX) spectra of the as-prepared products obtained at different precursor formation temperature (samples S1-S3).It can be found that no peaks of impurities such as sodium and chlorine are detected, indicating the high purity of the as-obtained products.Only copper and oxygen are detected in the spectrum, and their atomic ratio is close to 2 : 1, confirming that the obtained products are Cu 2 O crystals.Figure 3 shows representative FESEM images of the products obtained at different formation temperature of the cupreous complex precursor (samples S1-S3).From Figure 3(a), it can be seen that octahedral crystals are formed as the precursor formation temperature was 25    the crystallographic planes denoted by blue color are similar to the {111} family, and the red areas (squares) are the {100} family.Based on the above analysis, it is believed that the shape evolution of Cu 2 O crystals from octahedral to its different truncated forms can be achieved via controlling the precursor formation temperature.
3.4.Growth Mechanism.Our template-free complex precursor aqueous solution synthesis method is based on the reduction of CuCl 2 /NaOH/H 2 O solution-phase system with D-glucose powder.The formation of Cu 2 O crystals can be suggested as follows [37]: (1) Cu 2+ ions can coordinate with excess OH − ions to generate [Cu(OH) 4 ] 2− complexes precursors [38].Appropriate amount of D-glucose serves as a weak reducer at relatively high reaction temperature and can ensure the purity of products, which is in a manner similar to the reduction mechanism of reported D-glucose-based synthesis of Cu 2 O crystals [17].
In this experiment, it has been found that the properties of complex precursors can be strongly affected by the precursor formation temperature.The XRD patterns of the precursors obtained at different formation temperature are shown in Figure 4. Figure 4(a) is the typical XRD pattern of the precursors obtained at the formation temperature of 25 • C. It can be seen that all the diffraction peaks of the products are indexed according to the standard orthorhombic structure of Cu(OH) 2 (JCPDS file no.80-0656), indicating that these precursors are pure Cu(OH) 2 .The result is corresponding to the reaction mechanism.Figure 4(b) displays the XRD pattern of the precursors obtained at the formation temperature of 55 • C. It is found that both the Cu(OH) 2 and CuO (JCPDS file no.80-1268) are coexisted in the precursors.It is proposed that the Cu(OH) 2 were decomposed in higher reaction temperature, leading to the formation of CuO crystals.As the precursor formation temperature was increased to 70 • C, the XRD pattern of the precursors (nanobelts) was given in the Figure 4(c), and it also has the diffraction peaks of both Cu(OH) 2 and CuO.Hence, the precursors prepared at different temperature have different phase structure, and it is proposed that the formation of Cu 2 O architectures with different morphologies may be related to the characteristics of the complex precursors synthesized in different reaction conditions (1)-( 3).The varied precursors formed in different temperatures can modify the reduction process (3), which might affect the competition between kinetics and thermodynamics during the reduction of precursors, nucleation, and growth of Cu 2 O crystals.
Figure 5 is the typical TEM images of the precursors obtained at different formation temperature.Figures 5(a 6(c)).Therefore, it is proposed that the formation of Cu 2 O architectures with different morphologies may be related to the properties of complex precursors synthesized in different formation temperature.
To shed light on the formation mechanism of these Cu 2 O crystals with different shapes (samples S1-S3), their growth process has been followed by examining the products obtained at different reaction stages.Figure 7 shows the FESEM images of the products obtained at different reaction temperature as the precursor formation temperature was 25 • C. In this case, when the reaction temperature was 55 • C, the FESEM images of products are shown in Figure 7(a), and the insert is the corresponding TEM image.It can be found that both octahedra particles and precursors (nanowires) are coexisted in this product.With the reaction temperature increased to 60 • C, the amount of precursors was decreased (Figure 7(b)), and entire octahedra particles were formed as the reaction temperature was 68 • C (Figure 7(c)).When the precursor formation temperature was 55 • C, the intermediated products obtained at different reaction temperature are displayed in Figure 8. Figure 8(a) is the products obtained as the reaction temperature was 55 • C, and the insert is the corresponding TEM image.It was observed that both octahedral and precursors (nanorods) were synthesized.However, truncated octahedral crystals were formed as the reaction temperature was 60 • C (Figure 8(b)), and the precursors still adsorbed onto the truncated octahedral products, indicating that the reaction was not over at this time.When the reaction temperature was increased to 68 • C, truncated octahedral Cu 2 O crystals were entirely prepared (Figure 8(c)).Further enhancing the reaction time to 70 • C for some minutes, the well-defined products can be synthesized (Figure 3(b)).When the precursor formation temperature was 70 • C, the morphologies of intermediated products obtained at different reaction time (5 s, 10 s, 15 s, and 30 s) are shown in Figure 9. Figure 9(a) is the products obtained as the reaction time was 5 s, and the insert is the corresponding TEM image.
Figure 11: A schematic illustration of the proposed particle growth mechanism and reaction pathways that lead to the formation of Cu 2 O crystals with different polyhedral shapes.As the essence of our synthesis, the precursors with different properties can be reduced by C 6 H 12 O 6 to form Cu 2 O particles, which subsequently aggregate to form different initial structures.Subsequently, these initial structures will grow into various polyhedral forms through the synergic effect of oriented attachment and ripening mechanism, which can modify the ratio (R) between the growth rates along the 100 and 111 directions.
From the results, we can see that truncated octahedral Cu 2 O crystals and nanobelts (precursors) were fabricated in the initial stage and the amount of nanobelts was decreased along with the reaction time increase (Figures 9(a)-9(d)), which suggests that the precursors were consumed continuously.Finally, truncated octahedra with larger {100} sections were formed (Figure 3(c)).Based on the above discussion, it is found that the polycrystalline precursors obtained at lower formation temperature (25 • C and 55 • C) can be reduced to octahedral structures in the initial stage, while single crystalline precursors prepared at higher formation temperature (70 • C) are prone to generating 14-facet initial structures.Hence, the precursor formation temperature is a crucial parameter to determine the shape-evolution of Cu 2 O architectures.However, the exact relation between precursors and shapes of Cu 2 O crystals is worth further investigation.
In Cu 2 O crystal lattice, due to the difference in electronegativities between Cu and O atoms, the {100} facets of Cu 2 O are predominated by Cu or O atoms only (Figures 10(a) and 10(b)), leading to the electrically neutral state of {100} facets usually [39].While the {111} facets are formed by both Cu and O atoms [26] and surface Cu atoms with dangling bonds on {111} facets can make them being positive charged (Figures 10(c)), {111} facets can be protected by negative charged molecules or ions [39].The effect of anions used on the shape-controlled synthesis of crystals has been demonstrated [40], so it is believed that the presence of Cl − ions can affect the shapes and sizes of the products in our experiment.It is proposed that the interaction strength between Cl − ions and {111} facets might be not very strong at relatively lower reaction temperature, which results in the {111} facets (positive charged) to be easily protected by Cl − ions (negative charged).An octahedral form enclosed with four pairs of triangle {111} facets (Figures 7(a) and 8(a)) was obtained when the R value between the growth rates along the 100 and 111 directions reached to 1.73 [25,26,41].However, when the precursor formation temperature was higher (55 • C), the surface energies of {111} facets could be to some extent enhanced along with the reaction temperature increase, thus the ratio R would be decreased (1.15R < 1.73), so the initial octahedral shape gradually evolved to its truncated forms (Figures 8(a ratio (R) between the growth rates along the 100 and 111 directions reached to 1.0 ∼ 1.15 [25,26,41], a truncated octahedral shape (6 {100} and 8 {111} facets) was formed (Figure 3(c)).In our template-free complex-precursor-based solution route, a growth mechanism of these Cu 2 O crystals is proposed.After addition of D-glucose powder into the reaction system, small Cu 2 O seeds would be generated at relatively higher reaction temperature.In order to minimize the overall energy of the reaction system, seed particles tended to aggregate together (oriented attachment process), and the aggregates subsequently evolved into well-defined initial structures via a ripening mechanism [17,38].The initial structures with different shapes can be formed at different precursor formation temperature, which are shown in Figure 7(a), Figure 8(a), and Figure 9.In the present work, the morphology of these initial Cu 2 O architectures is determined by the characteristic of precursors formed at different reaction temperature, which can modify the R values.As the reaction time increased, particles were continuously adsorbed onto these structures to allow further growth (ripening process) to generate Cu 2 O crystals with different shapes.A schematic illustration of the proposed particle growth mechanism and reaction pathways is listed in Figure 11.

UV-Vis Measurement.
Figure 12 shows the UV-vis spectra of Cu 2 O products with different shape including octahedron (sample S1), truncated octahedron with different sizes of {100} facets (sample S2 and S3).The optical absorption edge of the octahedral Cu 2 O crystals generates at about 643 nm, and we can calculate the band gap according to the absorption edge position [16], and the calculated band gap energy of sample S1 is about 1.93 eV.The optical absorption edge of the truncated octahedron with small section {100} facets (sample S2) occurs at about 630 nm, corresponding to the band gap energy of 1.97 eV.The optical absorption edge of the truncated octahedron with larger section {100} facets (sample S3) occurs at about 623 nm, corresponding to the band gap energy of 1.99 eV.All the band gaps are smaller than that those bulk Cu 2 O (∼ 2.17 eV).These absorption band positions are larger than those of the previousy reported nanocubes and nano-octahedra [25].Moreover, the blueshift (643 nm (sample S1) → 630 nm (sample S2) → 623 nm (sample S3)) has been observed along with the section areas of {100} facets increase.

Conclusions
Shape-controlled synthesis of Cu 2 O architectures from octahedral to its different truncated forms is successfully achieved via a facile template-free aqueous synthesis route.It is found that the precursor formation temperature is crucial to designated-tailoring of the {100} facets of Cu 2 O crystals, which can modify the ratio (R) between the growth rates along the 100 and 111 directions, leading to the formation of the initial structures with different shapes.
The multiple morphologies can be evolved from these varied initial structures via the synergic effect of oriented attachment and ripening mechanism.Based on the relatively clear understanding of shape evolution and corresponding growth mechanism, this template-free complex precursorbased solution route has provided an innovative approach to design the {100} facets with different sizes to further enrich the current morphologies of Cu 2 O crystals.Additionally, the UV-vis spectra suggest that the blue-shift has been observed along with the section areas of {100} facets increase.

3. 3 .
Morphological Observation.The morphology and size of the Cu 2 O architectures were observed by field-emission scanning electron microscope.The precursor formation temperature was a crucial prerequisite for the formation of octahedral Cu 2 O crystals with different truncated forms.

Figure 3 (
b) is the FESEM image of the sample S2, which was obtained as the precursor formation temperature was 55 • C. The products are truncated octahedral morphologies, and each particle has three pairs of squares and four pairs of hexagons.The edges of squares are about 220 nm in sizes, and the long edges of hexagon are about 450 nm.When the precursor formation temperature was increased to 70 • C (sample S3), different truncated octahedra with larger square sections were synthesized.Figure3(c)shows the typical FESEM image of sample S3.There are also two kinds of truncated sections, including squares and hexagons in these products, but the areas of the squares (440 nm × 440 nm) are larger than those of sample S2, while the sizes of the hexagons relatively decrease.The inserts as shown in Figures3(a)-3(c) are schematic illustrations of the corresponding samples.According to Steno's law, the angles between two corresponding facets on the crystals are constant, thus the crystallographic planes can be well defined[35,36].Hence,

Figure 7 :
Figure 7: SEM images of the products obtained at different reaction temperature as the precursor formation temperature was 25 • C ((a): 55 • C, (b) 60 • C, and (c) 68 • C).The insert of (a) is the corresponding TEM images.

Figure 8 :
Figure 8: SEM images of the products obtained at different reaction temperature as the precursor formation temperature was 55 • C ((a): 55 • C; (b) 60 • C; (c) 68 • C).The insert of (a) is the corresponding TEM images.

Figure 9 :Figure 10 :
Figure 9: SEM images of the products obtained at different reaction time as the precursor formation temperature was 70 • C ((a): 5 s, (b) 10 s, (c) 15 s, and (d) 30 s).The insert of (a) is the corresponding TEM images.