Journal Information

Article Information

Morphology-modulated Synthesis of Cu2O Hexapods

Expand AllCollapse All

The development of morphology-controlled synthesis methods is an important issue in inorganic materials science.1,2 Because copper(I) oxide (Cu2O) has a cubic crystal system, it exhibits various morphologies.3,4 Cu2O is a non-stoichiometric defect p-type semiconductor with a direct bandgap of 2.2 eV and has been used as a photocatalyst for the degradation of dye molecules and for water splitting.57

Simple closed morphologies, such as cubic, octahedral, and rhombic dodecahedral morphologies, are formed in the crystal habit to minimize the total surface free energy under thermodynamic equilibrium.810 Cubic, octahedral, and rhombic dodecahedral Cu2O microcrystals have six {100} facets, eight {111} facets, and twelve {110} facets, respectively.1115 Because, the {100} facets of Cu2O microcrystals have different atomic arrangements from those of the {111} and {110} facets, the photocatalytic activities of Cu2O microcrystals are strongly dependent on the morphology of the Cu2O microcrystals.5,6

However, branched morphologies, such as hexapods, octapods, dodecapods, and dendrites, are formed through kinetic branching growth as a consequence of the non-equilibrium reaction conditions; these morphologies do not achieve the minimum total surface free energy.1621 Thus, the final morphologies of inorganic oxides are affected both by crystal habit formation and by branching growth. Most of the hexapodal Cu2O microcrystals have unsmoothed surfaces on their branched arms.13,2226 However, relatively little is known about the hexapodal Cu2O crystals, in which their arms covered with clean and smoothed cubic or octahedral surfaces.

In the present paper, we present preparation methods for cubic, octahedral, and hexapodal Cu2O microcrystals. Cu2O microcrystals with six cubic arms and six square-pyramidal arms were also prepared by an additional hydrothermal reaction in which the Cu2O hexapods were used as templates. The crystal growth mechanism of the various morphologies of Cu2O products is discussed.

Scheme 1 shows the method used to prepare the cubic, octahedral, hexapodal, six-cubic-arm, and six-square-pyramidal-arm Cu2O microcrystals. The cubic (or octahedral) Cu2O microcrystals were prepared through reduction with D-(+)-glucose (or hydrazine) of copper in an alkaline aqueous solution, respectively, as shown in Figs. 1(a) and (b). In our earlier work, we found that the Cu2O crystal habit changed from cubic to beveled cubic, rhombicuboctahedral, and 50-facets polyhedral with increases in the NaOH concentration. When neutral D-(+)-glucose was used as a reductant, cubic Cu2O crystals were formed at the lowest concentration of NaOH under thermodynamic equilibrium condition.10 In this work, we also found that octahedral Cu2O crystals were obtained at the lowest concentration of NaOH by using a hydrazine as a reductant. Because hydrazine is a stronger and more ionic reductant compared to the D-(+)-glucose, the octahedral Cu2O crystals as another thermodynamically stable morphology are formed instead of cubic Cu2O crystals. The choice of reductant plays an important role in the formation of cubic and octahedral Cu2O microcrystals. The results show that the hydroxide ions (OH) selectively added to the {100} or {111} facets of the Cu2O crystal seed during crystal when D-(+)-glucose or hydrazine, respectively, was used as the reductant. Because the hydrothermal reaction was used to achieve thermodynamic equilibrium, Cu2O microcrystals with a closed morphology, such as cubes or octahedra, were formed as part of the crystal habit.


Illustration of the method used to prepare (a) cubic (sample 1), (b) octahedral (sample 2), (c) hexapodal (sample 3), (d) six-cubic-arm (sample 4), and (e) six-square-pyramidal-arm (sample 5) Cu2O microcrystals.


SEM images of the (a) cubic (sample 1) and (b) octahedral (sample 2) Cu2O products prepared using D-(+)-glucose and hydrazine under hydrothermal reaction conditions, respectively. (c) SEM image of the hexapodal (sample 3) Cu2O products prepared using D-(+)-glucose in a microwave-assisted reaction.


Because the microwave-assisted reaction was only carried out for 2 min, the kinetic branching growth mechanism was involved in the formation of Cu2O hexapods, as shown in Fig. 1(c). The mean length of each Cu2O pod was 1.5 µm. It exhibited rough surfaces with irregular end spheres. N,N,N′,N′-tetramethylethylenediamine (TMEDA) was used as a chelating agent for the formation of copper-TMEDA complex. Without TMEDA chelating agent, the Cu2O hexapods with thin and sharp arms were formed.22 TMEDA may affect the release rate of copper ion from the copper-TMEDA complex. Thus, TMEDA plays an important role in the formation of regular Cu2O hexapods with thick arms. The regular Cu2O hexapods with thick arms are adequate for the preparation of the six-cubic-arm and six-squarepyramidal-arm Cu2O microcrystals from Cu2O hexapods as templates. The XRD patterns of the cubic, octahedral, and hexapodal Cu2O products matched the cubic crystal structure of Cu2O (JCPDS 05-0667, a = 0.4269 nm), as shown in Fig. 2. The XRD intensity ratios of (200) to (111) are 0.62 and 0.35 for cubic and octahedral for Cu2O products, respectively. The characteristics of (100) planes resembles those of (200) planes. It indicates that cubic and octahedral Cu2O products are covered by the {100} and {111} surfaces, respectively.


XRD patterns of the (a) cubic (sample 1), (b) octahedral (sample 2), and (c) hexapodal (sample 3) Cu2O products. (d) The XRD pattern and Miller indices of Cu2O (JCPDS 05-0667) are shown for comparison.


The Cu2+ ions react with TMEDA in solution to form a copper-TMEDA complex, [Cu(TMEDA)2]2+. The following reaction mechanism is proposed for the formation of Cu2O products via dehydration and subsequent reduction of the Cu(OH)42− by D-(+)-glucose from the copper-TMEDA complex:

2 Cu2+ (aq) + 4 TMEDA (aq) → 2 [Cu(TMEDA)2]2+ (aq) 2 [Cu(TMEDA)2]2+ (aq) + 4 OH (aq) + C6H12O6 (aq) → Cu2O (s) + 4 TMEDA (aq) + C6H12O7 (aq) + 2H2O (l)

Fig. 3 shows the preparation of the six-cubic-arm and six-square-pyramidal-arm Cu2O microcrystals from Cu2O hexapods as templates. These Cu2O products were prepared by an additional hydrothermal reaction involving the reduction of a copper solution with D-(+)-glucose or hydrazine using the Cu2O hexapods as a template. Because the hydroxide ions (OH) selectively added to the {100} facets of the Cu2O crystal under reduction by D-(+)-glucose, as shown in Fig. 3(a). The surfaces of the arms became smooth as additional Cu2O crystals filled along the {100} facets of the Cu2O hexapods.

Similarly, the Cu2O microcrystals with six square-pyramidal arms were obtained after additional reduction with hydrazine solution, as shown in Fig. 3(b). As expected, the square-pyramidal arms were formed by filling of the Cu2O crystal along the {111} facets of each arm. With increasing filling along the {111} facets of hexapodal Cu2O crystals, octahedral crystals with some voids at the center of the {111} facets were also formed, as shown in the white circled region in Fig. 3(b). The XRD patterns of samples 4 and 5 confirmed that the Cu2O products were prepared without any impurities, as shown in Fig. 4.


SEM images of the (a) six-cubic-arm Cu2O products (sample 4) prepared using D-(+)-glucose and (b) six-square-pyramidal-arm Cu2O products (sample 5) prepared using hydrazine.


XRD patterns of the (a) six-cubic-arm (sample 4) and (b) six-square-pyramidal-arm (sample 5) Cu2O products. (c) The XRD pattern and Miller indices of Cu2O (JCPDS 05-0667) are shown for comparison.


Thus, Cu2O products with six cubic arms and six squarepyramidal arms were prepared by the consecutive reaction method, which first involved the branching growth mechanism, followed by the crystal habit formation mechanism. This consecutive reaction method can provide new insights into the synthesis of extended and unique morphologies in which both branched shapes and closed shapes are combined.

In conclusion, the choice of reductant plays an important role in the formation of cubic and octahedral Cu2O microcrystals through crystal habit formation. Cu2O hexapods were obtained through branching growth during microwave-assisted reaction. Six-cubic-arm and six-square-pyramidal-arm Cu2O products were obtained from the Cu2O hexapods through additional crystal habit formation. The consecutive synthesis method that is branching growth followed by crystal habit formation provides unique branched morphologies such as six-cubic-arm and six-square-pyramidal-arm Cu2O products.


CuCl2·H2O (Sigma-Aldrich, St. Louis, MO, USA), Cu (CH3COO)2·H2O (Sigma-Aldrich), NaOH (Sigma-Aldrich), polyethylene glycol (PEG, Mw 20,000, Sigma-Aldrich), N,N,N′,N′-tetramethylethylenediamine (TMEDA, 99%, Sigma-Aldrich), hydrazine (Sigma-Aldrich), and D-(+)-glucose (Sigma-Aldrich) were used as-received.

The D-(+)-glucose (or hydrazine) and TMEDA were used as a reductant and a chelating agent of Cu2+ ions, respectively. The experimental conditions for the preparation of various Cu2O crystals are summarized in Table 1. For the preparation of cubic (sample 1) or octahedral (sample 2) Cu2O products, 5 mL of a 0.044 M D-(+)-glucose (or 0.017 mL hydrazine) solution was added to 10 mL of a 0.11 M Cu(CH3COO)2·H2O solution, respectively, and 5 mL of 1.2 M NaOH solution was added to the resultant solution. The final solution was incubated for 60 min at 70 °C.


Brief summary of the experimental conditions used in this work

Sample number Seed Copper source Reductant Chelating agent Synthetic method Morphology SEM
1 No CuCl2 D-(+)-glucose No Hydrothermal Cube Fig. 1(a)
2 No CuCl2 Hydrazine No Hydrothermal Octahedron Fig. 1(b)
3 No CuCl2 D-(+)-glucose TMEDA Microwave-assisted Hexapods Fig. 1(c)
4 Yes Cu(CH3COOH)2 D-(+)-glucose No Hydrothermal Six-cubic-arm Fig. 3(a)
5 Yes Cu(CH3COOH)2 Hydrazine No Hydrothermal Six-square-pyramidal-arm Fig. 3(b)

The Cu2O hexapods (sample 3) were synthesized via a microwave-assisted reaction. Fifty milliliters of a 0.01 M D-(+)-glucose solution and 2.4 g PEG were added to 10 mL of a 0.2 M CuCl2·H2O solution, followed by the addition of 0.25 mL TMEDA; 89.75 mL of water was subsequently added to the solution in a 500 mL beaker. The final solution was placed in a commercial microwave oven (Magic MWO-230 KD, 2.45 GHz, 800 W, Seoul, Korea) and irradiated for 2 min. When the microwave irradiation process was completed, the solution was quenched in ice-water.

The six-cubic-arm Cu2O microcrystals (sample 4) were prepared by adding 5.0 mL of 1.2 M NaOH solution to 5.0 mL of 0.22 M Cu(CH3COO)2·H2O solution in a 70 mL beaker, followed by the addition of 5.0 mL 0.044 M D-(+)-glucose solution and 0.015 g of the Cu2O template. The final solution was incubated for 60 min at 70 °C. For the preparation of Cu2O microcrystals with six square-pyramidal arms (sample 5), 5.0 mL of 1.2 M NaOH solution was added to 5.0 mL of 0.040 M Cu(CH3COO)2·H2O solution in a 70 mL beaker and 0.017 mL hydrazine was subsequently added, along with 0.015 g of the Cu2O template. The final solution was incubated for 60 min at 70 °C. All Cu2O products were collected by centrifugation at 4000 rpm for 10 min, washed with ethanol, and then dried for 24 h in a vacuum oven at room temperature.

The structures and morphologies of the Cu2O products were analyzed by powder X-ray diffraction (XRD, PAN-alytical X'Pert-PRO MPD, Almeldo, The Netherlands) using Cu Kα radiation and by scanning electron microscopy (SEM, Hitachi S-4300, Tokyo, Japan), respectively.


The authors acknowledge financial support from the National Research Foundation of Korea (NRF-2018R1D1A1B07040714).



C. Burda X. Chen R. Narayanan M. A. El-Sayed Chem. Rev.20051051025 [CrossRef]


Z. Wu S. Yang W. Wu Nanoscale201681237 [CrossRef]


S. Sun X. Zhang Q. Yang S. Liang X. Zhang Z. Yang Prog. Mater. Sci.201896111 [CrossRef]


S. Sun Z. Yang RSC adv.201443804 [CrossRef]


C. H. Kuo M. H. Huang J. Phys. Chem. C200811218355 [CrossRef]


H. Xu W. Wang W. Zhu J. Phys. Chem. B200611013829 [CrossRef]


P. E. de Jongh D. Vanmaekelbergh J. Kelly J. Chem. Commun.19991069


Q. Hua D. Shang W. Zhang K. Chen S. Chang Y. Ma Z. Jiang J. Yang W. Huang Langmuir201127665 [CrossRef]


X. Zhao Z. Bao C. Sun D. Xue J. Cryst. Growth2009311711 [CrossRef]


B. E. Yeo Y. S. Cho Y. D. Huh CrystEngComm2017191627 [CrossRef]


L. Gou C. J. Murphy Nano Lett.20033231 [CrossRef]


M. J. Siegfried K. S. Choi J. Am. Chem. Soc.200612810356 [CrossRef]


D. Wang M. Mo D. Yu L. Xu F. Li Y. Qian Cryst. Growth Des.20033717 [CrossRef]


H. C. Song Y. S. Cho Y. D. Huh Mater. Lett.2008621734 [CrossRef]


J. Yang X. Wan S. Tie S. Lan X. Gao Solid State Sci.2020104106203 [CrossRef]


Y. J. Lee Y. D. Huh Mater. Res. Bull.2011461892 [CrossRef]


K. Chen C. Sun S. Song D. Xue CrystEngComm2014165257 [CrossRef]


J. Xu D. Xue Acta Mater.2007552397 [CrossRef]


J. Xue W. Liang X. Liu Q. Shen B. Xu CrystEngComm2012148017 [CrossRef]


J. Y. Ho M. H. Huang J. Phys. Chem. C200911314159 [CrossRef]


L. Tang J. Lv S. Sun X. Zhang C. Kong X. Song Z. Yang New J. Chem.2014384656 [CrossRef]


Y. S. Cho Y. D. Huh Bull. Kor. Chem. Soc.2013343101 [CrossRef]


X. Zhang Y. Xie F. Xu D. Xu H. Liu Can. J. Chem.2004821341 [CrossRef]


P. Li L. Liu D. Qin C. Luo G. Li J. Hu H. Jiang W. Zhang J. Mater. Sci.2019542879


T. Aditya J. Jana N. K. Singh A. Pal T. Pal ACS Omega201721968 [CrossRef]


X. L. Luo M. J. Wang D. S. Yang J. Yang Y. S. Chen J. Ind. Eng. Chem.201532313 [CrossRef]