Controllable synthesis of Cu2O hierarchical nanoclusters with high photocatalytic activity

Article abstract of DOI:10.1039/c4ra06119e

Cu₂O nanocrystals with different morphologies and sizes were controllably synthesized by simply adjusting the reaction temperature and duration. The results suggested that Cu₂O nanocrystals obtained at 120 °C were hierarchical nanoclusters (NCs) with an average size of 25 nm and the NCs were made up of many ultrafine grains with the sizes of 2-7 nm. Such a structure facilitated the generation and transfer of carriers, leading to a high visible-light photocatalytic activity. Further analysis indicated that the Cu₂O nanocrystals with smaller size exhibited much better photocatalytic activities in degrading methyl orange (MO) under visible-light irradiation. The best result appeared in the NCs of 25 nm, which could degrade the MO solution (100 ml, 10 mg L^-1) completely in 12 minutes with a dosage of 10 mg. That work provides a good method for enhancing the photocatalytic activity of Cu₂O and other photocatalysts.

Full text of DOI:10.1039/c4ra06119e

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Controllable synthesis of Cu2O hierarchical
nanoclusters with high photocatalytic activity†
Cite this: RSC Adv., 2014, 4, 42892
Lun Zhang,ab Pengzhan Ying, Bing Yu,d Ling Wu,a Jieru Wang,e Xiuquan Gu,c
c
*
b
d
b
*
Shanliang Chen, Rui Zhou and Zhonghai Ni
Cu2O nanocrystals with different morphologies and sizes were controllably synthesized by simply adjusting
the reaction temperature and duration. The results suggested that Cu2O nanocrystals obtained at 120 ꢀC
were hierarchical nanoclusters (NCs) with an average size of 25 nm and the NCs were made up of many
ultrafine grains with the sizes of 2–7 nm. Such a structure facilitated the generation and transfer of
carriers, leading to a high visible-light photocatalytic activity. Further analysis indicated that the Cu2O
nanocrystals with smaller size exhibited much better photocatalytic activities in degrading methyl orange
(MO) under visible-light irradiation. The best result appeared in the NCs of 25 nm, which could degrade
the MO solution (100 ml, 10 mg Lꢁ1) completely in 12 minutes with a dosage of 10 mg. That work
provides a good method for enhancing the photocatalytic activity of Cu2O and other photocatalysts.
Received 23rd June 2014
Accepted 4th September 2014
DOI: 10.1039/c4ra06119e
www.rsc.org/advances
his coworkers14 reported on the preparation of hollow Cu2O
sphere and octahedron, which exhibited good photocatalytic
1. Introduction
activities. Nevertheless, it is still a huge challenge to realize the
large-scale application of Cu2O due to both a high loss or
consumption and a long degradation time.13,14,23,29 Thus, it is
necessary to enhance both the photocatalytic activities and
stabilities of Cu2O catalysts by doping, surface modication or
decreasing the grain sizes.
Recently, cuprous oxide (Cu2O), a well-known p-type semi-
conductor with a band gap of 1.9–2.2 eV,1–3 has attracted
worldwide interest due to its potential applications in many
elds, such as solar energy conversion,1 photocatalysts,3 lubri-
cants,4 gas sensing,5 magnetic memory6 and lithium ion
batteries.7 Herein, it should be pointed out that Cu2O crystals
are an excellent visible-light photocatalyst applied in water
splitting and organic contamination degradation.3,8–15 It is well
known that the activity of a photocatalyst is dependent on its
size, crystal structure and surface state. Up to now, there are a
great number of reports on the synthesis of Cu2O micro and
nanostructures with different morphologies including spheres,
cubes, octahedron, dodecahedrons, wires and tubes via various
chemical routes.16–28 Zhang et al.13 synthesized Cu2O poly-
hedrons with an average size of ꢂ2 mm and found that 96% of
the methyl orange (MO) solution (100 ml, 15 mg Lꢁ1) could be
degradated by 0.05 g of that catalyst during 3 hours. Feng and
As far as we know, there were a great number of reports on
the Cu2O photocatalysts modied by Au, Ag and reduced gra-
phene oxides (rGO) nanoparticles etc.15,30–35 It was because that
the photocatalytic activities could be enhanced by suppressing
the carriers (electrons and holes) recombination rate. The easier
separation of carriers is, the better photocatalytic activity will
be. However, the researches on decreasing Cu2O sizes were still
limited due to a huge challenge existed in controllable synthesis
of Cu2O crystals especially the sizes. Naturally, the smaller sizes
of Cu2O crystals are, the higher surface area and photocatalytic
activity will be. However, it was considered a big challenge to
controllably synthesize the Cu2O nanoparticles with sizes lower
than 40 nm. Especially, the synthesis of ultrane Cu2O nano-
crystallines with a good crystallinity and a single-phase struc-
ture was considered to be rather difficult.29,36,37
Herein, we focus on the controllable synthesis of Cu2O
nanoclusters (NCs) with small sizes via a facile technique by
using sodium borohydride (NaBH4) as the reducing agent and
poly(vinylpyrrolidone) (PVP, K30) as the dispersing agent. It has
been demonstrated that the photocatalytic activities of Cu2O
NCs could be enhanced greatly by optimizing their morphol-
ogies simply. Five different shaped Cu2O nanocrystals with
average sizes ranging from 25 to 155 nm were prepared by just
changing the reaction duration and the reaction temperatures
aCollege of Sciences, China University of Mining and Technology, Xuzhou City, 221116,
P.R. China
bSchool of Chemical Engineering and Technology, China University of Mining and
Technology, Xuzhou City, 221116, P.R. China. E-mail: nizhonghaicumt@gmai.com;
Fax: +86-0516-83883927; Tel: +86-0516-83883927
cSchool of Material Science and Engineering, China University of Mining and
Technology, Xuzhou City, 221116, P.R. China. E-mail: ypz3889@sina.com; Fax:
+86-0516-83995653; Tel: +86-0516-83995653
dAdvanced Analysis
& Computation Center, China University of Mining and
Technology, Xuzhou City, 221116, P.R. China
eDepartment of Materials Science and Engineering, Zhejiang University, Hangzhou
City, 310009, P.R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra06119e
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from 110 to 152.8 ꢀC. Of all the as-synthesized samples, the or 3 min (10 mg Lꢁ1) and centrifuged immediately to remove
Cu2O NCs with size of 25 nm exhibited the best visible-light solids. And then the absorbance of MO aqueous solution was
photocatalytic ability. Compared with other bigger sizes of characterized by a 752N UV-visible spectrometer at 464 nm. C0
Cu2O catalysts, the MO solution could be degraded completely and Ct represent the initial absorbance and the real time
and rapidly using lower amount of this sample.
absorbance of MO solution under irradiation, respectively.
2. Experimental procedure
3. Results and discussion
3.1 Structure of different shaped Cu2O nanocrystals
All of the chemical reagents used in the experiment were
analytical grade and used as received without further Fig. 1 shows the low and high magnication SEM images, TEM
purication. In a typical synthesis, 0.4 g cupric acetate (Cu images and size distribution histograms of various shaped
(CH3COO)2$H2O) was dissolved in 30 ml N,N-dimethylforma- Cu2O nanocrystals obtained at different temperature. One
mide (DMF) with continuous stirring. Then 0.165 g PVP and explanation about calculation of particle sizes should be made
0.04 g NaBH4 were added into the above solution and kept at before discussion. That is, the average particle size of each
110 ꢀC in an oil bath for about 18 minutes until numerous sample was calculated by collecting four hundreds of Cu2O
orange particles appeared. Aer that, it was cooled naturally to particles from the TEM images. Generally speaking, S1–S4
room temperature and then the nal product was separated by obtained at 110–140 ꢀC as shown in Fig. 1(A1)–(D1) exhibit
centrifugation and cleaned several times by ltration with quasi-spherical structure with particle sizes ranging from 25 to
plenty of deionized water and ethanol. Finally, the product was 58 nm while S5 prepared by DMF reuxed (Fig. 1(E1)) takes on
dried at 60 ꢀC for 4 h in a vacuum oven. The obtained sample at polyhedron structure with an average particle size of 155 nm.
this temperature was named sample 1 (S1). Only changing the
Fig. 1(A1)–(A3) and S1 (ESI†) present the SEM and TEM
ꢀ
ꢀ
ꢀ
oil bath temperature to 120 C, 130 C and 140 C, the sample images of S1 with an average particle size of 25 nm, suggesting
2–4 (S2–S4) were achieved correspondingly. Sample 5 (S5) was that monodispersed particles are not many while aggregation is
prepared as follows by adjusting reaction temperature and time. very obvious. There are many “aggregation units” with sizes of
First, the oil bath temperature was increased to the boiling 40–65 nm as shown in Fig. S1 (ESI†). And each “aggregation
point of DMF (152.8 ꢀC), and the mixed solution was reuxing at unit” looks like “single particle” under low magnication
that temperature for about 70 min by constant stirring. Then (Fig. 1(A3)). It is well known that the smaller particle size is, the
the temperature was reduced to 130 ꢀC and the as-prepared more instability is. Fig. 1(A4) shows that the content of particles
solution was reacting continuously for another 30 min until a lower than 20 nm is 41%. Therefore, it is considered that maybe
large number of erythrinus particles emerged. The following so many ultrane particles caused aggregation. Further aggre-
procedure was the same with that of S1.
gation probably leaded to forming bigger single particle. That is
X-ray diffraction (XRD) analysis was carried out using a why there are also many particles (21% of the total) bigger than
diffractometer (D8 ADVANCE, Bruker, Germany) with Cu-Ka 35 nm (Fig. 1(A4)).
˚
l ¼ 1.5418 A radiation operating at 40 kV and 30 mA. Field
Among the ve samples, S2 with an average particle size of
emission scanning electron microscopy (FESEM) images were 25 nm exhibits uniform size distribution and good dispersal
obtained on a microscope (S4800, Hitachi, Japan) at an accel- according to Fig. 1(B1)–(B3) and S2 (ESI†). Compared with S1,
erating voltage of 5 kV. Transmission electron microscopy the particles with sizes smaller than 20 nm decreased as shown
(TEM) and high-resolution TEM (HRTEM) images were in Fig. 1(B4), which maybe results in better dispersal. With the
collected on a microscope (Tecnai G2 F20, FEI, America) at an temperature climbing to 130 ꢀC, the average size of S3 increases
accelerating voltage of 200 kV. The Fourier transform infrared to 50 nm (relevant information shown in Fig. 1(C1)–(C4)).
(FTIR) spectra of the samples were examined in the form of KBr Moreover, Fig. 1(D1)–(D4) displays some particles of S4 become
pellets with a spectrometer (Nicolet iS5, in the range of 350– bigger continuously with an average particle size of 58 nm when
8000 cmꢁ1, America).
the temperature goes up to 140 ꢀC. Moreover, it is analyzed that
The photocatalytic activities of various shaped Cu2O nano- polyhedron Cu2O has been formed because Quasi-spherical
crystals were investigated by using MO as a model pollutant structure is destroyed by the gas bubbles produced during
molecule at room temperature. It is because that MO has good DMF being reuxed. In summary, the particle size becomes
stability under visible light irradiating.13,15,31 Different samples bigger as the temperature increases. It is because the reaction
(0.01 g) were dispersed respectively in MO aqueous solution speed is accelerated at higher temperature and the disperse
(100 ml, with two kinds of concentrations of 10 mg Lꢁ1 and 40 ability of PVP is probably weakened.
mg Lꢁ1) under the visible light irradiation provided by a 150 W
High-resolution TEM (HRTEM) as shown in Fig. 2 was used
tungsten–halogen lamp (Beijing Institute of Optoelectronic to investigate the inner ultrane structure of S1 and S2, in order
Technology) with illuminance of 66 600 Lux. The distance to study the inuence of morphology on the photocatalytic
between the lamp and the reaction solution was 18 cm. The activity that will be discussed latter. HRTEM image (Fig. 2(a))
mixture solution was rstly stirred for 30 min in dark to adsorb shows obviously that the “aggregation unit” is made up of a few
MO adequately. Then, the beaker with above solution was 10–20 nm particles, which causes the specic surface area of S1
placed in the visible light source. An amount (3 ml) of the to decrease greatly. Moreover, Fig. 2(b) suggests that S1 belongs
suspension was removed from the beaker every 10 min (40 mg Lꢁ1
)
to common Cu2O nanocrystal (“integral particle”) without
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Fig. 1 Low (columns 1) and high (columns 2) magnification SEM images, TEM images (columns 3) and size distributions (columns 4) of different
shaped Cu2O nanocrystals. A–E corresponding to sample S1–S5 obtained at 110 ꢀC, 120 ꢀC, 130 ꢀC, 140 ꢀC and by DMF refluxed, respectively.
further ne structure. However, Fig. 2(c) exhibits that S2 has a “aggregation units” and “common structure” will result in the
special nanostructure and belongs to hierarchical nanoclusters falling of photocatalytic activity of S1 and the former is thought
(NCs). And the NCs are made up of ultrane grains with the to be the main factor. On the other hand, photocatalytic ability
sizes of 2–7 nm. Due to high surface energy and intrinsic cubic of S2 should be better.
crystal structure, the smaller grains have a high tendency to
Fig. 3 and S3 (ESI†) show the typical (XRD) patterns of the
regrow together, forming bigger NCs. The inserts of Fig. 2(b) different shaped Cu2O nanocrystals. All the diffraction peaks of
and (c) correspond to the white marked regions, which reveal cuprous oxide can be indexed to the (110), (111), (200), (220) and
the well-dened S1 and S2. The lattice fringes of the common (311) facets of a cubic phased Cu2O (JCPDS no. 05-0667), both in
nanocrystal and NCs have interplanar spacing of 0.247 nm and peak position and relative intensity. As can be seen, each Cu2O
0.240 nm, respectively, which is in agreement with the cubic sample has good crystallinity, which is consistent with the
Cu2O (111) planes.15,28 Therefore, it is considered that HRTEM analysis as shown in Fig. 2(b) and (c). Among samples
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Fig. 2 HRTEM images of Cu2O nanocrystals (S1) obtained at 110 ꢀC (a) and (b) and Cu2O NCs (S2) got at 120 ꢀC (c). The inserts show the marked
regions.
of S1–S4, there are no obvious changes in the width of XRD There are four peaks in the FTIR spectrum of each sample. The
diffraction peaks. However, compared with S5 having relative Cu–O vibration was detected at 632 cmꢁ1 and the two peaks at
bigger particle size, the peaks of S1–S4 are broadened. It is 3447 cmꢁ1 and 1629 cmꢁ1 corresponding to the stretching and
because Cu2O nanocrystals with smaller size has wider peak, bending vibrations of the O–H groups from the unavoidable
which was also reported by Y. Sui and his coworkers.38 In adsorbed water, while the peak at 1400 cmꢁ1 was attributed to
addition, no other impurities are found in all of the samples, the absorption of carbon dioxide in the air.28,39 Therefore, it can
which is also consistent with the FT-IR spectra as will be dis- be concluded that no other impurities appeared in each sample,
cussed according to Fig. 4.
which is in accordance with the XRD analysis.
The FTIR spectra were used to examine the purity of different
shaped Cu2O nanocrystals as shown in Fig. 4 and S4 (ESI†).
3.2 Photocatalytic activity
The relationships between the irradiation time and the absor-
bance at 464 nm of the MO aqueous solution were recorded
using various shaped Cu2O nanocrystals as shown in Fig. 5 and
S5 (ESI) and Table S1 (ESI).† As can be seen, no changes are
observed in the two kinds of concentrations of MO under visible
light irradiation in the absence of catalysts (magenta line shown
in Fig. 5(a) and (b)). Overall, the photocatalytic activities of
different shaped Cu2O nanocrystals weaken as their sizes
increase. The order of decomposition rate of ve samples is as
follows: S2 (120 ꢀC) > S1 (110 ꢀC) > S3 (130 ꢀC) > S4 (140 ꢀC) > S5
(DMF reuxed). And the photocatalytic activities of quasi-
spherical Cu2O nanocrystals (black, red, green, blue line) are
much better than that of polyhedral Cu2O (cyan line) as shown
in Fig. 5(a) and (b). The Cu2O NCs (S2), the best catalyst, could
completely degradate the MO solution with the initial concen-
trations of 10 mg Lꢁ1 and 40 mg Lꢁ1 in 12 and 65 minutes.
The pseudo-rst order reaction was observed for different
shaped Cu2O nanocrystals as shown in Fig. 5(c) and (d). The
order of the pseudo-rst order rate constants (k) is also follows:
S2 > S1 > S3 > S4 > S5 (shown in Table S2 (ESI†)). The highest k of
S2 calculated are 0.2033 (10 mg Lꢁ1) and 0.0053 (40 mg Lꢁ1),
respectively. The corresponding k values of S5, 0.0533 and
0.0017, are the lowest. The high k value is well known to reect
high photocatalytic activity.15,40 Here, photocatalytic activities of
Cu2O crystals towards MO degradation were summarized and
listed in Table S3 (ESI†) on the basis of previous literatures. It
can be seen clearly that the dosage of S2 is reduced greatly and
also does the degradation time compared with other reported
Cu2O crystals.
Fig. 3 XRD patterns of different shaped Cu2O nanocrystals.
3.3 Analysis of photocatalytic mechanism
It is considered that there are four main factors which lead to S2
Fig. 4 FT-IR spectra of various shaped Cu2O nanocrystals.
with remarkable photocatalytic activity. Firstly, S2 has ultrane
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Fig. 5 Photocatalytic activities and kinetic plots of various shaped Cu2O nanocrystals under visible light irradiation by using different initial
concentrations of MO solution ((a) and (c) 10 mg Lꢁ1, (b) and (d) 40 mg Lꢁ1). The (c) and (d) kinetic plots are based on the data in (a) and (b).
particle, high size uniformity and good dispersal as shown in greatly. On the other hand, oxygen vacancies in Cu2O
Fig. 1(B1)–(B4). However, photocatalytic activity of S1 weakens undoubtedly can act as potential wells to trap either one or two
obviously due to serious aggregation of particles and that of S3– electrons, helping electron–hole pair separation and hence
S5 also reduces because of bigger particles.
increasing the photocatalytic activity.13 Moreover, it is worthy to
Secondly, S2 has ultrane structure as shown in HRTEM point out that photocatalytic activity of hierarchical NCs (S2) is
images, which causes it to have strong MO adsorption ability much better than that of “integral particle” with the similar size,
and remarkable photocatalytic activity (Fig. 5(a) and (b)). The which can be proved from the degradation results of Cu2O
mechanism of MO photodegradation with Cu2O as catalyst is nanocrystals (S1), TiO2–Cu2O with size of 20–40 nm (ref. 34) and
schematically illustrated in Fig. 6. And the nal degradation Ag–Cu2O of 60 nm (ref. 35) listed in Table S3 (ESI†). For
products are CO2, H2O and organic compounds.9,41 Previous comparison, the MO adsorption ability and specic surface area
researches indicate that MO adsorption on Cu2O surface is the of polyhedral Cu2O nanocrystals of 155 nm decrease enor-
pre-procedure in photocatalysis reaction.13,14,23,30–35 It is due to mously and so does the photocatalytic activity.
small size effect that both specic surface area and atoms at the
Thirdly, high separation of the photogenerated carriers
surface of Cu2O NCs increase sharply. As a result of partial (electron–hole pairs) creates excellent photocatalytic activity of
coordination, the surface atoms are not stable and will combine S2. It is reported that size of semiconductor nanocrystal is
other encountered atoms quickly for stabilization, which make usually smaller than the thickness of spatial electron layer,41
the photocatalytic active sites and adsorption function increase which will make the carriers diffuse Cu2O NCs surface quickly.
Fig. 6 Schematic diagram of photodegradation mechanism with Cu2O and MO as catalyst and pollutant, respectively.
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Fig. 7 Schematic diagram of the quantum size effect from Cu2O NCs.
Then the holes of valence band (h+, VB) will be trapped with
H2O and electrons of conduction band (eꢁ, CB) will react with
O2 easily. Moreover, diffusion time of the carriers can be
expressed with a formula t ¼ d2/k2D (where d is the particle size,
k is a constant, and D is the diffusion coefficient of the
carriers).42,43 On the basis of the equation, it can be concluded
that the smaller particle size is, the shorter time of carriers
diffusing from inner to outer will be. As a result, the electron–
hole pairs are easier to separate and meanwhile they recombi-
nation become harder. Thus the photocatalytic activity is
enhanced. As listed in Table S3 (ESI†), the vast difference of
degradation rate between S2 and the porous sphere Cu2O of
ꢂ300 nm (ref. 14) attribute to rapid transformation and high
separation of carriers also mentioned in other articles.15
Acknowledgements
Support from the National Natural Science Foundation of China
(21176246 and 11204371) and the Priority Academic Program
Development of Jiangsu Higher Education Institutions are
gratefully acknowledged.
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