Cu2O nanocrystals with various morphology: Synthesis, characterization and catalytic properties

Article abstract of DOI:10.1016/j.cclet.2017.01.022

Cu₂O nanocubes, octahedra, spheres and truncated rhombic dodecahedral were prepared and their structural, morphological, and electronic properties were investigated by X-ray diffraction analysis, X-ray absorption near edge structure, scanning electron microscope and transmission electron microscope and X-ray absorption near edge structure. Cu₂O nanocrystals were successfully employed to catalyze the 1,3-dipolar cycloaddition reaction for the synthesis of 1,4-disubstituted triazoles. Cu₂O nanocubes and octahedral showed the superior catalytic performance in the cycloaddition reaction. These results reveal that crystal-plane engineering of oxide catalysts is a useful strategy for developing efficient catalysts for organic reaction.

Full text of DOI:10.1016/j.cclet.2017.01.022

Accepted Manuscript
Title: Cu2O nanocrystals with various morphology: Synthesis,
characterization and catalytic properties
Authors: Mojtaba Bagherzadeh, Narges-alsadat Mousavi,
Mojtaba Amini, Sanjeev Gautam, Jitendra Pal Singh, Keun
Hwa Chae
PII:
S1001-8417(17)30045-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2017.01.022
CCLET 3971
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
18-11-2016
10-1-2017
18-1-2017
Please cite this article as: Mojtaba Bagherzadeh, Narges-alsadat Mousavi, Mojtaba
Amini, Sanjeev Gautam, Jitendra Pal Singh, Keun Hwa Chae, Cu2O nanocrystals
with various morphology: Synthesis, characterization and catalytic properties, Chinese
Chemical Letters http://dx.doi.org/10.1016/j.cclet.2017.01.022
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Original article
Cu2O nanocrystals with various morphology: Synthesis, characterization and
catalytic properties
a
Mojtaba Bagherzadeh a, , Narges-alsadat Mousavi , Mojtaba Amini*,b, Sanjeev Gautam c, Jitendra Pal
Singh d, Keun Hwa Chae*,d
a Chemistry Department, Sharif University of Technology, Tehran, P.O. Box 11155-3615, Iran
bDepartment of Chemistry, Faculty of Science, University of Maragheh, Maragheh, Iran
cDr. SSBhatnagar, University Institute of Chemical Engineering & Technology (SSB UICET), Panjab University Chandigarh 160-014, India
dAdvanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
 Corresponding authors.
E-mail addresses: bagherzadeh@sharif.edu; mamini@maragheh.ac.ir
ABSTRACT
Cu2O nanocubes, octahedra, spheres and truncated rhombic dodecahedral were prepared and their structural, morphological, and electronic properties
were investigated by X-ray diffraction analysis, X-ray absorption near edge structure, scanning electron microscope and transmission electron
microscope and X-ray absorption near edge structure. Cu2O nanocrystals were successfully employed to catalyze the 1,3-dipolar cycloaddition reaction
for the synthesis of 1,4-disubstituted triazoles. Cu2O nanocubes and octahedral showed the superior catalytic performance in the cycloaddition reaction.
These results reveal that crystal-plane engineering of oxide catalysts is a useful strategy for developing efficient catalysts for organic reaction.
Keywords:
Nanocrystal
Morphology
Cu2O
Azide-alkyne cycloaddition
1. Introduction
In recent years, there is a growing interest to the controlled synthesis of Cu2O nanocrystals with uniform morphology not only for the
development of synthetic strategies, but also for the comprehensive study of the effect of the crystal plane on the surface chemistry and
catalytic properties of Cu2O catalysts [1-6]. Furthermore, Cu2O nanocrystals are relatively easy to make, inexpensive, readily available,
safe, and has low toxicity and good environmental acceptability, which favors the investigation of their properties and applications in
catalysis [6-10].
Huang et al. reported the correlation between the morphology and catalytic activity of Cu2O nanocrystals in propylene oxidation,
CO oxidation and the photocatalytic degradation of dyes [11-18]. These and many other examples clearly illustrate the importance of
shape and morphology control to the efficient application of Cu2O nanocrystals [19-21].
The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) as most famous click reaction are efficient method for the synthesis of
1,4-disubstituted 1,2,3-triazoles that have tremendous applications in many areas such as polymers, drug discovery, and advanced
material science, etc. [22-28]. In recent years, our group have reported some work on catalytic application of copper-based catalysts for
azide-alkyne cycloaddition and achieved some meaningful results [29-31]. However, due to the complicated interaction between Cu
species with ligands or supports, the Cu-based catalysts are not ideal model catalysts to study the relationships between structures and
activities. Therefore, in this work, uniform Cu2O nanocrystals, including rhombic dodecahedrons, spheres, octahedrons and cubes were
synthesized according to well-established procedures with some modification [13, 32]. We also report detailed structural
characterization and the crystal-plane effects on the catalytic properties in the 1,3-dipolar cycloaddition of organic azides and alkynes.
The catalytic performances of Cu2O nanocrystals normalized to the percentage of the exposed Cu atom were followed the order d-
Cu2O> o-Cu2O~ c-Cu2O> s-Cu2O.
2. Results and discussion
The Cu2O nanocubes, octahedra, spheres and truncated rhombic dodecahedra were synthesized in aqueous solutions following
previously reported procedures [13, 32]. Exact amounts of CuCl2 solution, SDS surfactant, NaOH solution, and NH2OH.HCl as a
reducing reagent were added in the sequence listed, and the mixture was aged for 1 h at 40 °C for growth the Cu2O nanocrystals.
X-ray diffraction (XRD) was used to investigate the structures and phase purities of Cu2O nanocrystals. Fig. 1 shows typical
diffraction patterns of the Cu2O nanopowders with no other crystalline impurities of CuO or metallic Cu, which fairly match with that
of cubic bulk Cu2O (JC-PDS file No. 78–2076) [33].
———

In order to probe the 3d character of Cu in the Cu2O samples, the Cu L-edge XANES spectra were collected from all Cu2O
nanocrystals with cubic, octahedral, spheres and truncated rhombic dodecahedral structures. According to Fig. 2, it is clear that all
samples shows two distinct peaks at ~935.6 eV and ~955.6 eV which are assigned to the transitions of Cu2p3/2 (L3) and 2p1/2 (L2)
electrons into the empty d-states, respectively. The difference among L3 and L2, which is determined by the spin–orbit coupling, has
value of ~20 eV. The post- L3-edge structures at ~938.6, 944.5 and 951.6 eV are the characteristics of Cu2O and relevant with previous
reports [34]. Similarly, post L2-edge structure at 958.0 eV in Cu2O is reported in previous published reports [34]. It is seen that shape
and intensity of various spectral features of o-Cu2O and c-Cu2O resembles to that of reported Cu2O [34]. Thus, it appears that only c-
Cu2O represents Cu+1 states. Drastic changes for these parameters for s-Cu2O and d-Cu2O reflect the possibility of different oxidation
state of Cu. It is reported that Cu2 p3/2 peak appear at 1 eV and 2.5 eV lower photon energy for Cu metal and CuO than that of Cu2O
except the significant change of energy [34]. In this case, no energy shift is observed for this edge, hence, copper ions has +1 oxidation
state in s-Cu2O and d-Cu2O samples too. It is shown that intensity of Cu2p3/2 peak is associated with density of states of d-character at
the bottom of conduction band, distorted by strong core-hole potential [34]. Thus modifications of d-character seem to be associated
with low intense Cu2p3/2 peaks of s-Cu2O and d-Cu2O. The structures at 938.6 and 958.0 eV also exhibit intensity variations. To study
the intensity variation of these structures, XANES spectra is de-convoluted in Fig. S1 in Supporting information as shown in Fig. 2b.
The intensity of spectral feature at 938.6 eV is almost thrice for c-Cu2O, s-Cu2O and d-Cu2O to that of o-Cu2O. The intensity for c-
Cu2O is maximum for structure appearing at 958.0 eV and almost same for rest of the samples.
Fig. 3 and Fig. 4 show the SEM and TEM images of typically synthesized Cu2O nanocubes with sizes of 500–600 nm, octahedral
with sizes of 250–350 nm, rhombic dodecahedra with sizes of 0.7–1.1 μm and spheres with sizes of 0.8–1.3 μm respectively. Cubes
and octahedral are more uniformly-sized, whereas rhombic dodecahedra and spheres have larger size distributions.
To compare the catalytic performance of all Cu2O nanocrystals, cycloaddition reaction of benzyl chloride, phenyl acetylene and
NaN3 to synthesize 1-benzyl-4-phenyl-1,2,3-traizole were investigated in present of water as the standard “green” solvent without any
additive under air at 70 °C. The specific cycloaddition reaction rate of benzyl chloride, phenyl acetylene and NaN3 normalized to the
percentage of the exposed Cu atoms for various Cu2O nanocrystals followed the order d-Cu2O > o-Cu2O ~ c-Cu2O > s-Cu2O (Table 1).
It took 120 min of reaction for the Cu2O spheres to attain a product yield of 91%, but when Cu2O nanocubes and octahedra were used,
the product yield was improved to an excellent yield of 97% at 120 min. Remarkably, the use of Cu2O rhombic dodecahedra rhombic
dodecahedra as a catalyst significantly reduced the time of reaction to just 90 min with a yield of 97%. The above results clearly
confirm that the morphology of Cu2O nanocrystals controls their catalytic activity.
The morphology effect of Cu2O nanocrystals on the product yield was also studied (Fig. 5). Results clearly demonstrated that on the
basis of per unit mass of catalyst, the crystal-plane-controlled catalytic performance of Cu2O nanocrystals in the 1,3-dipolar
cycloaddition follows the order octahedral> cubes> rhombic dodecahedra~ spheres. The above results clearly confirm that the
morphology of Cu2O nanocrystals controls their catalytic activity.
To illustrate the scope of this method, we extended the cycloaddition reaction of a wide variety of terminal alkynes and organic
halides in presence of d-Cu2O, o-Cu2O and c-Cu2O. In most cases similar results were obtained (Table 2). First, various benzyl
chlorides/bromides and phenyl acetylenes with the substitution of electron withdrawing and electron donating groups on the phenyl
ring were investigated and did not have any appreciable influence on the outcome of the reaction (entries 1–17). Furthermore, steric
hindrance of the ortho substituents on the benzyl chloride had little effect on the reaction yields (entries 2 and 3). As can be seen from
the Table 1, in case of deactivated alkynes such as propargyl alcohol and 2-methyl-3-butyn-2-ol, when benzyl bromide was used under
the similar reaction conditions, the corresponding products were obtained in higher yields than the case of benzyl chloride (entries 7, 8
and 17, 18). On the other hand, t-butyl bromide as a deactivated halide reacted nicely with various alkynes to obtain the corresponding
1,4-disubstituted triazoles in excellent yields (entries 13-15).
The recyclability of catalyst is an important and critical topic in the field of heterogeneous catalysis. Therefore the recyclability of
the d-Cu2O, c-Cu2O and o- Cu2O as catalysts was also surveyed (Fig. 6). After completing one run of reaction, the reaction mixture
was centrifuged and another cycle of the reaction was carried out using the same particles. The results revealed that catalysts could be
reused at least four times without appreciable dramatic yield loss.
Expectedly, in presence of the recovered d-Cu2O, the reaction rate slightly decreased as compared to fresh d-Cu2O (Fig. 7).
The SEM images of the recycled catalysts after the fourth cycle show some morphological and surface changes and therefore these
changes and also small losses of catalyst mass (10%−15% of the total mass of catalyst) during washing procedure may be caused in the
low diminution in the activity from the first to the fourth cycle (Fig. 8).
To further confirm the structures and phase purities of recycled Cu2O nanocrystals, XRD patterns of the recycled nanocrystals were
recorded (Fig. 9). The XRD patterns of the recycled d-Cu2O and c- Cu2O showed only peaks of the Cu2O nanopowders with no
impurities, but recycled o-Cu2O indicated mixed phases of Cu2O and CuO, probably arising from slight oxidation of CuI during
cycloaddition reaction.

To investigate heterogeneous nature of the d-Cu2O, c-Cu2O and o- Cu2O nanocrystals, the cycloaddition of benzyl chloride, NaN3
and phenylacetylene after hot filtration was done and it was observed that no product is formed after separation of nanocrystals and
therefore they act heterogeneously in the reaction. Also, ICP analysis was employed to be sure that no leaching of copper took place
during cycloaddition reaction.
3. Conclusion
For the first time, Cu2O nanocubes, octahedra, rhombic dodecahedra and icosahedra have been used as catalysts in the 1,3-dipolar
cycloaddition reaction of benzyl chloride, phenyl acetylene and NaN3 for the generation of 1,4-disubstituted triazoles. The catalytic
performance of Cu2O nanocrystals on the basis of the exposed Cu atoms followed the order d-Cu2O> o-Cu2O~c-Cu2O> s-Cu2O. A
wide variety of triazoles with excellent yields have been synthesized, demonstrating that o-Cu2O and c-Cu2O are broadly useful and
highly efficient catalysts for the azide-alkyne cycloaddition reaction. Also these nanocrystals are recyclable catalysts at least four
times.
4. Experimental
Chemicals: Anhydrous copper(II) chloride, sodium hydroxide, SDS, and hydroxylamine hydrochloride were purchased from Aldrich
and Merck. All chemicals were used as received without further purification.
Cu2O nanocrystal synthesis: For the synthesis of Cu2O nanocrystals with cubic, octahedral, spheres and truncated rhombic
dodecahedral structures, 5 mL of 0.1 mol/L CuCl2 solution and 0.87 g of SDS powder were respectively added to four beakers
containing 89.2, 83.2, 78.2, and 69.2 mL of deionized water with vigorous stirring. After complete dissolution of SDS powder, 1.8 mL
of 1.0 mol/L NaOH solution was added. Finally, 4.0, 10.0, 15.0 and 24.0 mL of 0.1 mol/L NH2OH.HCl were quickly injected into
beakers respectively. The solutions were kept in the water bath for 1 h at 40 °C to growth of nanocrystals. The product was collected
by centrifugation, washed by excessive water and ethanol for several times to remove unreacted chemicals and SDS surfactant, and
finally dried in vacuum at 50 °C.
General procedure for azide–alkyne cycloaddition: To a solution of Cu2O (1 mg) in H2O (2 mL) were added alkyne (0.5mmol), the
organic halide (0.55 mmol) and NaN3 (0.55 mmol). The reaction mixture was warmed to 70 °C continuing stirring with monitoring by
TLC until total conversion of the starting materials. After addition of water (3 mL), the resulting mixture was extracted with EtOAc
(2× 10 mL). The collected organic phases were dried with anhydrous CaCl2 and the solvent was removed in vaccum to give the
corresponding triazoles, which did not require any further purification.
Acknowledgment
M. Bagherzadeh and M. Amini thank the Iranian National Science Foundation (INSF), Sharif University of Technology and
University of Maragheh for financial supports of this work.
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Fig. 1. XRD patterns of the Cu2O nanocrystals synthesized with various morphologies
Fig. 2. (a) The Copper L-edge XANES spectra of the Cu2O nanocrystals synthesized with various morphologies; (b) reflects the intensity variations of structures
appearing at 938.6 and 958 eV.
Fig. 3. SEM images of different morphologies of c-Cu2O (a), o-Cu2O (b), s-Cu2O (c), d-Cu2O (d).
Fig. 4. TEM images of different morphologies of c-Cu2O (a), o-Cu2O (b), s-Cu2O (c), d-Cu2O (d).
Fig. 5. The morphology effect of Cu2O nanocrystals for the synthesis of 1-benzyl-4-phenyl-1,2,3-traizole

Fig. 6. Recycling studies of d-Cu2O, c-Cu2O and o- Cu2O catalysts in azide–alkyne cycloaddition
Fig. 7. Reaction kinetic plots by using the fresh and recovered d-Cu2O catalyst, respectively.
Fig. 8. SEM images of a) d-Cu2O; b) o- Cu2O and c) c- Cu2O after azide–alkyne cycloaddition
Fig. 9. XRD patterns of the recycled Cu2O nanocrystals

Table 1. Comparison of catalytic potential of different Cu2O nanocrystals for the synthesis of 1-benzyl-4-phenyl-1,2,3-traizole
Entry
Catalyst
Exposed Cu atoms
(%)
Amount used (mg)
Time (min)
Yield (%)
1
2
3
4
c-Cu2O
o-Cu2O
s-Cu2O
d-Cu2O
28.49
33.45
14.08
18.60
1.2
1
120
120
120
90
97
97
91
97
2.4
1.8
Table 2. Cycloaddition of benzyl halides with terminal alkynes in the presence of d-Cu2O, o-Cu2O and c-Cu2O
Entry
R1
R2
X
R3
Yield (%)a
d-Cu2Ob
97
o-Cu2Oc
97
c-Cu2Od
97
1
2
Ph
4-Me-Ph
2-Me-Ph
4-NO2-Ph
Ph
H
H
H
H
H
H
H
H
H
H
H
Ph
H
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Br
Br
Br
Br
Br
Br
Ph
Ph
97
96
95
3
Ph
93
90
88
4
Ph
95
94
94
5
4-MeO-Ph
4-Me-Ph
(HO)CH2
HOCH(CH3)(CH2)2
Ph
97
96
95
6
Ph
96
96
92
7
Ph
88
81
83
8
Ph
85
79
76
9
Ph
97
97
97
10
11
12
13
14
15
16
17
Ph
4-MeO-Ph
4-Me-Ph
Ph
97
96
97
Ph
97
97
97
Ph
91
87
85
CH3(CH2)3
CH3(CH2)3
CH3(CH2)3
Ph
Ph
96
94
93
4-MeO-Ph
4-Me-Ph
(HO)CH2
HOCH(CH3)(CH2)2
93
92
94
95
90
88
93
91
92
Ph
90
88
85
a Isolated yield; bReaction time= 90 min; cReaction time = 120 min