Temperature-Induced Stacking to Create Cu2O Concave Sphere for Light Trapping Capable of Ultrasensitive Single-Particle Surface-Enhanced Raman Scattering

Article abstract of DOI:10.1002/adfm.201801868

The fabrication of bowl or concave particles with “asymmetric centers” has drawn considerable attentions, in which multiple scattering occurs inside the particles and the ability of light scattering is distinctly enhanced. However, the limited variety of templates, the uncontrollable dimensions such as the size of concavity and the complex growth process have posed serious limitations to the reproducible construction of concave particles with desired geometries and their light-trapping properties. Herein, a “temperature-induced stacking” strategy is proposed to create controllable concavity Cu₂O spheres for the first time. Different sizes of F68 micelles can be formed through aggregation under different reaction temperatures, which can serve as soft template to tailor concave geometries of Cu₂O spheres. The as-prepared Cu₂O concave sphere (CS) can serve as single-particle (SP) surface-enhanced Raman scattering (SERS) substrate for highly repeatable and consistent Raman spectra. The unique cavity of Cu₂O CS entraps light effectively, which also enhances the scattering length owing to multiple light scattering. Combined with slightly increased surface area and charge-transfer process, Cu₂O CS exhibits remarkable single-particle SERS performance, with an ultralow low detection limit (2 × 10^?8 mol L^?1) and metal comparable enhancement factor (2.8 × 10⁵).

Full text of DOI:10.1002/adfm.201801868

FULL PAPER
Nanostructures
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Temperature-Induced Stacking to Create Cu2O Concave
Sphere for Light Trapping Capable of Ultrasensitive
Single-Particle Surface-Enhanced Raman Scattering
Xiaoxia Li, Yang Shang, Jie Lin, Anran Li, Xiaotian Wang, Bin Li, and Lin Guo*
1. Introduction
The fabrication of bowl or concave particles with “asymmetric centers”
has drawn considerable attentions, in which multiple scattering occurs
inside the particles and the ability of light scattering is distinctly
enhanced. However, the limited variety of templates, the uncontrollable
Over the past decades, fabrication of ani-
sotropic particles, such as misshapen,
bowl, concave, patchy, and Janus parti-
cles with “asymmetric centers” has drawn
considerable attentions.[1–6] Their reduced
morphological symmetries have enabled
possibly new or enhanced properties.[1–6]
For bowl-like or concave particles,
multiple scattering would occur within the
particles, which in turn distinctly enhance
the ability of light scattering.[7–9] Up to
now, these structures, especially noble
metals and metal oxides, have exhibited
a great advantage in optical field, such
as surface-enhanced Raman scattering
(SERS),[9–16] liquid crystals,[17,18] water
splitting,[8,19] and imaging.[20] Due to the
asymmetric geometry of concave particles,
a template is often introduced to control
their growth. Several groups[1,7–11,19–22]
have reported that the use of monolayer
of polystyrene spheres and silica colloids
as hard templates for most concave mate-
rials fabrication. However, the tedious
template-removing process and the
dimensions such as the size of concavity and the complex growth
process have posed serious limitations to the reproducible construction
of concave particles with desired geometries and their light-trapping
properties. Herein, a “temperature-induced stacking” strategy is proposed
to create controllable concavity Cu2O spheres for the first time. Different
sizes of F68 micelles can be formed through aggregation under different
reaction temperatures, which can serve as soft template to tailor concave
geometries of Cu2O spheres. The as-prepared Cu2O concave sphere (CS)
can serve as single-particle (SP) surface-enhanced Raman scattering
(SERS) substrate for highly repeatable and consistent Raman spectra. The
unique cavity of Cu2O CS entraps light effectively, which also enhances
the scattering length owing to multiple light scattering. Combined with
slightly increased surface area and charge-transfer process, Cu2O CS
exhibits remarkable single-particle SERS performance, with an ultralow
low detection limit (2 × 10−8 mol L−1) and metal comparable enhancement
factor (2.8 × 105).
involvement of toxic organic solvent, are two obvious disadvan-
tages for the hard template-assisted concave material fabrica-
tion method.
X. X. Li, Dr. Y. Shang, J. Lin, Dr. A. R. Li, Prof. X. T. Wang,
Prof. L. Guo
Beijing Advanced Innovation Center for Biomedical Engineering
Key Laboratory of Bio-Inspired Smart Interfacial Science
and Technology
Ministry of Education
School of Chemistry
Beihang University
Beijing 100191, P. R. China
Fortunately, several novel templates have been reported to
synthesize concave structures.[4,11,21,23–25] Lou and co-workers[4]
reported an emulsion-induced interface anisotropic assembly
route to create bowl-like polydopamine particles, in which poly-
dopamine seeds were generated on the 1,3,5-trimethylbenzene
droplet. Wang and co-workers[11] fabricated Au nanocups with
rough outer surface and faceted inner surface by using PbS
nano-octahedra as the sacrificial templates, where Au grew
along the PbS nano-octahedra facets. Based on confined-space
thermal dewetting, Yin and co-workers[20] recently reported an
original approach for the synthesis of Au nanocups. Although
these routes have been proposed, the exploration of new strate-
gies for creating concave structures is still at preliminary stage,
especially for tunable concavity, which has significant impact
on their light-trapping properties.
E-mail: guolin@buaa.edu.cn
Dr. Y. Shang
Key Laboratory of Micro-Nano Measurement-Manipulation and Physics
Ministry of Education
School of Physics and Nuclear Energy Engineering
Beihang University
Beijing 100191, China
Dr. B. Li
College of Chemistry and Molecular Engineering
Qingdao University of Science and Technology
Qingdao 266042, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201801868.
For triblock copolymer (TCP), especially [poly(ethylene
oxide)–poly(propylene oxide)–poly(ethylene oxide); PEOx–
PPOy–PEOx], TCP micelles with external hydrophilic PEO
DOI: 10.1002/adfm.201801868
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Figure 1. Scheme illustration of the aggregation of F68 micelles with the
increasing of temperature from 15 to 45 °C.
and internal hydrophobic PPO will form initially in aqueous
solution.[26–30] By increasing the temperature of TCP solution,
the hydrogen bond between PEO and water can be broken,
inducing the aggregation and segregation of TCP micelles
in water.[26–30] This phase segregation is a very common phe-
nomenon for TCP.[26–30] Take F68 (PEO78–PPO30–PEO78, at
80.0 g L−1) as an example (Figure S1, Supporting Information),
dynamic light scattering (DLS) has verified that monomer F68
spherical micelles with a diameter of ≈4.5 nm are formed at
15 °C (Figure S1a, Supporting Information). It is interesting
to note that, by increasing the temperature, monomer F68
micelles would aggregate to be spherical-like structures with
diameters of 350, 650, and 1100 nm at 25, 35, and 45 °C,
respectively (Figure 1 and Figure S1b–d, Supporting Informa-
tion). Thus far, studies of TCP have been restricted to produce
mesoporous materials by using monomer micelles.[31–35] So,
if those “bigger” micelles are used, could “bigger” pores be
produced?
Based on this interesting phenomenon of F68, we propose a
strategy to create controllable concavity Cu2O spheres. Different
sizes of F68 micelles could be formed through aggregation
under different reaction temperatures (RT), which can serve
as soft template to tune the size of concavity of Cu2O spheres.
We define the aggregation of F68 micelles with the increased
temperature as temperature-induced stacking (TIS). This is the
first time that the concept of TIS has been applied. We further
exhibit the use of the as-prepared Cu2O concave sphere (CS)
as single-particle (SP) SERS substrate. Cu2O concave sphere
shows an ultralow limit of detection (LOD) (2 × 10−8 mol L−1)
and metal comparable enhancement factor (EF) (2.8 × 105),
which is amongst the better performing semiconductor SERS
substrate, rivalled even that of the noble metal substrate
without “hot spot.”
Figure 2. Scheme illustration of the TIS process for the formation of
Cu2O CSs.
moment, the pH of IS was 9.77 derived from the excess NH3.
With the increased temperature from 15 to 45 °C, the gradually
turbid IS (inset of Figure S3, Supporting Information)
suggested that F68 micelles aggregated and segregated in water.
DLS (Figures S2 and S3, Supporting Information) verified that
the size of F68 micelles was slightly increased compared to that
of pure F68 aqueous solution (Figure S1, Supporting Informa-
tion). By increasing the temperature of IS to be 35 °C (step 1
in Figure 2), the dissociation of the hydrogen bond between
PEO and water would induce F68 micelles to gradually dehy-
drate and aggregated into a compact structure, namely, TIS
(step 2 in Figure 2).[36] The corresponding DLS (Figure S2, Sup-
porting Information) demonstrated that the diameter of F68
was 730 nm at the moment. With ascorbic acid (AA) solution
instilled to the IS, the deep blue solution (Cu(NH3)42+) quickly
+
+
turned into limpid (Cu(NH3)2 ) [Equation (1)]. Cu(NH3)2
instantaneously transformed into yellow precipitates (Cu2O)
[Equations (2) and (3)]. When the reaction finished, the pH
decreased to 7.20. Those processes can be described by the
following equations[37]
2Cu
(
NH3
)
2+ + C6H8O6 = C6H6O6 + 2NH3 + 2Cu
(
NH3
)
+ + 2NH+4
4
2
(1)
Cu
(
NH3
)
+ + OH−  CuOH + 2NH3
(2)
2
2CuOH = Cu2O+ H2O
(3)
The fact that Cu2O CSs rapidly formed in duration of 5 min
suggested that the “bigger” F68 micelles acted as the soft tem-
plate. Meanwhile, due to surface tension, the anisotropic epi-
taxial growth that the Cu2O nucleated on the large F68 micelles
resulted in the formation of cavity geometries, which have
been widely explained for Janus structures.[38–40] In solvent, the
2. Results and Discussion
In our case, an initial solution (IS) was first prepared by mixing
2+
Cu(NH3)4 with F68 (80.0 g L−1) aqueous solution. At that
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surface energy of F68 (σF68-solvent) was larger than the surface
energy of Cu2O nuclei (σCu2O-solvent). When Cu2O nucleated on
the surfaces of F68 micelles (the surface of F68 micelles could be
deemed as plane in a small area), the interfacial energy between
Cu2O and F68 (σCu2O-F68) is very low due to the bond of Cu in
Cu2O and O in PEO group atoms.[37] It is difficult for Cu2O
nuclei to spread on the surface of F68 due to the negative total
spheres, concave spheres, and finally to hemispheres, respec-
tively (Figure 4). SEM revealed the quite uniform sizes of the
Cu2O particles with the diameter of ≈1.4 0.2 µm and relatively
smooth surface (Figure S7, Supporting Information). At 15 °C,
the individual F68 micelles did nothing useful during the for-
mation of Cu2O, resulting in its sphere structure (Figure 4a).
When increasing the temperature to 25 °C and the small-scale
F68 micelles would decorate Cu2O spheres in different direc-
tions. Finally, multiconcave spheres with an average of four
cavities were generated. Those cavities had smaller diameter
(500 150 nm) and shallower depth (100 50 nm) (Figure 4b).
Only one cavity was created on most Cu2O spheres at 35 °C
(Figure 4c). When further increasing the temperature to 45 °C,
the larger F68 micelles would result in larger cavities, even
hemispheres can be obtained (Figure 4d). Also, the cubic Cu2O
phase (JCPDS No. 05-0667, information from CD-ROM) of
those four samples was verified by X-ray Diffraction (XRD)
characterization (Figure S8, Supporting Information).
Because of their similar diameter (d2 = 1.4 0.2 µm) to laser
spot size (d1 = 1.58 µm), the four Cu2O spheres (in Figure 4)
can serve as SP-SERS for highly repeatable and consistent
Raman spectra (Figure 5a and Figures S10–S15 and details
in the Supporting Information, “Single-particle SERS perfor-
mance of Cu2O spheres”). When Cu2O materials were acted as
SERS substrates, chemical enhancement (CE) effect was always
taken into consideration.[43,44] CE induced by charge transfer
(CT) played the crucial role at the semiconductor–molecule
interface, where amplification of the molecular polarization
tensor occurs[45–48] and thus enhanced the Raman scattering
of molecules. Herein we used crystal violet (CV) as probe
molecule. The a′′ vibration mode (1615 cm−1) of CV was more
enhanced than other vibration modes, which was a nontotally
symmetric mode and only came from the CT process.[49–51] The
surface energy variation[6,41,42] (Δσ = σCu2O-solvent + σCu2O-F68
–
σF68-solvent < 0). This in turn leads to the anisotropic growth of
Cu2O nuclei on the surface of F68 (step 3 in Figure 2). Con-
sequentially, the energy barrier of the growth of Cu2O in the
heterogeneous direction was much higher than that in the homo-
geneous directions, which further led to the anisotropic growth
of the initial F68 micelles. Thus, the Janus nanocomposites
had been created by Cu2O nuclei anisotropic growth on the F68
micelles (step 4 in Figure 2 and Figure S4, Supporting Informa-
tion). Finally, the cavity geometry was formed after F68 removal
through ethanol washing for several times (step 5 in Figure 2).
A proper concentration of F68 (80.0 g L−1) was necessary for
the formation of cavity with the ideal size. In the absence of
F68, only Cu2O complete spheres were obtained (Figure S5a,
Supporting Information). When the concentration of F68
was halved (40.0 g L−1), some Cu2O spheres with small cavity
appeared in the Cu2O complete spheres (Figure S5b, Sup-
porting Information). Insufficient F68 micelles would result
in smaller diameter of cavities and nonuniform cavities on
Cu2O spheres. If increasing the concentration of F68 by twice
(160.0 g L−1), Cu2O hemispheres would appear due to the larger
size of F68 micelles (Figure S5c, Supporting Information).
Furthermore, a key for the construction of concave structure
was choosing the surfactant that could results in the com-
mencement of TIS process. By replacing the F68 with P123
(a TCP could TIS, PEO20–PPO70–PEO20), we can obtain Cu2O
spheres with cavity. However, the different chain length and
stacking mode of P123 would have resulted in a different dia-
meter and cavity depth (Figure S6a, Supporting Information).
However, when F68 was replaced with polyvinylpyrrolidone
(a surfactant that dissolve in H2O), only Cu2O complete spheres
were produced (Figure S6b, Supporting Information) due to the
disappeared TIS process.
Overview scanning electron microscopy (SEM) (Figure 3a)
revealed that the sizes of the Cu2O CSs typically range from 1.4
to 1.6 µm. Magnified SEM image (Figure 3b) clearly showed
that the Cu2O CSs possessed relatively smooth surface and an
eccentric cavity with a diameter of ≈700 50 nm. Figure 3c
exhibited the depth of cavity (h) is ≈300 30 nm, and the fact
that no channels or pores were found in transmission electron
microscopy (TEM) image demonstrated the F68 did not serve
as pore-forming agent during the rapid reduction process.[37]
The measured lattice spacing in the high-resolution (HR)
TEM image (Figure 3d) was ≈0.24 nm, fitting well with the
(111) crystal plane of the cubic Cu2O. The selected area elec-
tron diffraction (SAED) pattern (inset in Figure 3d) of the Cu2O
CS exhibited discontinuous bright diffraction rings, revealing
polycrystalline structure nature.[37]
Due to the increased size of F68 micelles (4.5, 400, 730,
and 1300 nm at 15, 25, 35, and 45 °C, respectively, Figure S2,
Supporting Information), different concavity of Cu2O spheres
could be obtained from complete spheres, to multiconcave
Figure 3. a,b) SEM and c) TEM images of Cu2O CSs. d) HRTEM image
of the framed area in (c). The inset of (d) illustrates the corresponding
SAED patterns.
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Figure 4. Scheme illustration and the corresponding SEM images of Cu2O spheres with controllable concavity obtained under different reaction
temperatures: a) 15, b) 25, c) 35, and d) 45 °C, scale bar = 1 µm.
classical CE was derived from resonant Raman scattering effect
between the excitation wavelength and charge-transfer complex
(semiconductor and probe molecule).[49–51] To verify the interac-
tion between Cu2O CSs and CV, ultraviolet-visible absorption
spectra of CV adsorbed on Cu2O CSs were collected to compare
with pristine Cu2O CSs and CV (Figure S10, Supporting
Information). A new band from 650 to 725 nm appeared,
suggesting that a CT process happened between Cu2O CSs and
CV molecules.
A concentration of 2 × 10−5 m CV was used to prevent the
supersaturated adsorption (Figure S11, Supporting Infor-
mation). SP-SERS spectra of 2 × 10−5 m CV adsorbed on the
four Cu2O spheres placed on marked silicon (Figure S12,
Supporting Information) were measured from 20 different
regions (Figure S13, Supporting Information). The strongest
vibration peak at 1615 cm−1 of CV was used as reference for the
spectra normalization, and the signal intensity was averaged
for final analysis. From the view of the opposite of the cavity
(the opposite side of the sphere) (Figure S12e–h, Supporting
Information), SEM images showed consistent, perfect spherical
morphology for all Cu2O spheres. With light directly irradiated
onto the opposite side of the spheres of all samples, all the irra-
diation areas were not different due to the similar morphology
of all samples. It also meant that the number of CV molecules
adsorbed on the irradiation area was almost identical (details
in the Supporting Information, “Surface areas of the four
spheres”). Thus, it is understandable that the peak intensities of
CV on the opposite side of the other spheres were very close to
complete sphere (Figure 5c and Figure S13a2–d2, Supporting
Information). Therefore, Raman scattering enhancement con-
tributed by CE should be almost identical for all samples.
Interestingly, the intensities of the Raman peak differed sig-
nificantly if light directly shined onto the cavity of multicon-
cave-, concave-, and hemispheres (Figure 5c and Figures S12b–d
and S13b2–d2, Supporting Information). For multiconcave
sphere, the peak intensity was enhanced 40% compare to
the complete sphere. For hemisphere, the peak intensity was
doubled compare to the complete sphere. For the CS, the
peak intensity was sixfold as compare to the complete sphere.
Figure 5. a) Schematic diagram of the light trapping by using single
Cu2O CS particle as SERS substrate. b) Calculated electric field (|E|/|E0|)
distributions of a single CS. c) A comparison of the Raman intensities
of CV molecule (2 × 10−5 m) at peak of 1615 cm−1 adsorbed on single
complete sphere, multi-CS, CS, and hemisphere (the left columns), and
the corresponding downward particles (the right columns).
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Brunauer–Emmett–Teller (BET) N2 adsorption–desorption
profiles (Figure S9 and details in the Supporting Information,
“Surface areas of the four spheres”) showed that the specific
surface areas (SSA) of complete-, multiconcave, concave-, and
hemispheres were 7.23, 8.51, 9.63, and 15.22 m2 g−1, respec-
tively. The SSA was calculated by dividing the surface area
(SA) by mass (M), where M was the product of density (ρ) and
volume (V). ρ was identical since that the four Cu2O spheres
were obtained through same system. V could be easily derived
from the SEM images (Figure 4). Thus, under light directly
irradiated to the cavity, irradiation areas of complete-, multi-
concave-, concave-, and hemispheres were 3.08, 3.74, 4.61, and
3.40 × 106 nm2, respectively. For the multiconcave sphere, the
increased loading of the CV molecules due to the increased
irradiation area may dominate the enhancement of peak inten-
sity. For the CS and hemisphere, although the loading of the
CV molecules was enhanced caused by the increased irradia-
tion areas, it was not enough to enhance the peak intensities
for sixfold and twofold. Thus, it inferred that the markedly
enhanced Raman performances mainly attributed to unique
concave geometries.
as the different SERS performances.[55,56] The observation of
distribution |E|/|E0| of CS and hemisphere inside the cavity
demonstrates that light trapping occurs within the cavity.[55]
Especially, the distribution of |E|/|E0| of CS is found in the
cavity enclosed by walls, suggesting a considerable improve-
ment of light trapping ability inside the cavity of CS. The cavity
enhances the scattering length owing to multiple light scat-
tering inside the unique concave structures.[7–9] Furthermore,
for the CS, the effective enhancement sites partly coincide with
the molecular adsorption sites. For the hemisphere, although
the light trapping occurs on the cavity, the most effective SERS
enhancement area concentrates at the top of the molecular
adsorption sites. The maximum intensity |E|/|E0| of CS is 1.8,
while that of hemisphere is 1.2. As the SERS signals were
roughly proportional to (|E|/|E0|)4, CS obviously has stronger
SERS enhancement than hemisphere. Therefore, combined
with the increased SA, the Raman scattering signal is enhanced
to around sixfold for CS. For hemisphere, the weaker light
trapping with slightly increased SA coenhances the Raman
scattering signal to around twofold. In contrast, for the mul-
ticoncave sphere, the maximum intensity of |E|/|E0| appeared
over where the cavity forms. The shallow and small cavity
cannot effectively entrap light; and the enhanced Raman inten-
sity is attributed to their increased surface area.
It is well documented that concave-like structure possessed
strong light scattering ability, thereby greatly improving light
trapping.[7–9,52–54] 3D finite-difference time domain (3D-FDTD)
simulation (Figure 5b and Figure S15, Supporting Informa-
tion) was conducted to investigate the electric field (|E|/|E0|)
distributions around the four Cu2O spheres, which is essen-
tial to understand the mechanism of the light trapping as well
The excellent light-trapping properties endowed Cu2O CS
with outstanding SERS performances. As a result, an ultralow
LOD for 2 × 10−8 m of CV and Rhodamine 6G (R6G) could
be reached by using Cu2O CS as substrates (Figure 6a,c and
Figure 6. a,c) Raman spectra collected for single Cu2O CS at four different concentrations (2 × 10−5, 2 × 10−6, 2 × 10−7, and 2 × 10−8 m) of CV and R6G
molecule, respectively. b,d) The statistical Raman intensities of CV and R6G molecule at peak of 1615 and 610 cm−1, acquired by recording spectra at
20 stochastic spots at different locations across from two substrates.
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measurement and DLS were measured using Malvern Nano ZS90, UK.
Raman spectra were acquired with a Jobin Yvon Raman spectrometer
model HR800. Laser 647 and 514.5 nm lines from an Ar–Kr ion laser
were used as excitation source. This laser power was density was
≈109 W m−2 (laser power: ≈2 mW; laser spot: 1.58 µm2) to the surface of
the sample. Ultraviolet-visible absorption spectra were recorded by UV–
vis–NIR spectrophotometer (UV-3600, Shimadzu). Specific surface areas
were measured by using at least 0.1 g samples at −196 °C through BET
nitrogen adsorption–desorption (NOVA 2200e, Quanthachrome, USA).
Figures S16 and S17, Supporting Information). The relation-
ship between the Raman intensities and concentrations for CV
and R6G molecule fitted relatively well with the linear curve
fitting model (Figure 6b,d). EF was calculated based on the
magnification of Raman intensity compared with that on bare
substrate (details in the Supporting Information, Calculation of
EF). The conservative EF was calculated to be around 2.8 × 105
for CV (peak at 1615 cm−1) and 2.5 × 105 for R6G (peak at
610 cm−1) adsorbed on single Cu2O CS, which is amongst
the better performing SERS substrate as reported (Table S1,
Supporting Information), rivalled even that of the noble metal
substrate without “hot spot.”[12,57,58]
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
3. Conclusions
Acknowledgements
In conclusion, Cu2O spheres with controllable concavity can be
obtained through a TIS process. Different sizes of F68 micelles
could be formed through aggregation under different reaction
temperatures, which can serve as soft template to tune the
concave geometries of Cu2O spheres. The FDTD simulation
exhibits the high distribution |E|/|E0| of CS inside the cavity.
Additionally, the highest intensity of |E|/|E0| was found in the
cavity enclosed by walls, suggesting that the unique cavity of
Cu2O CS entraps light effectively, which also enhances the
scattering length owing to multiple light scattering. This light
trapping and slightly increased SA coenhances the Raman scat-
tering signal to around sixfold. Combined with charge-transfer
process, Cu2O CS shows a remarkable SP-SERS performance
with ultralow LOD (2 × 10−8 mol L−1) and a metal comparable
EF (2.8 × 105). As a novel approach for designing new functional
materials, this synthetic strategy, is not just limited in the syn-
thesis of concave structures, it would inspire more researches
on the fabrication of a new generation of SERS substrate.
X.L. and Y.S. contributed equally to this work. X.L., Y.S., and L.G.
conceived the study and cowrote the paper. X.L. and Y.S. designed and
performed the experiments and analyzed the data. J.L. assisted in SERS
measurement. A.L. helped in FDTD simulations. X.W. and B.L. discussed
the SERS mechanism and F68 aggregation. The authors thank Yew Von
Lim and Mei Er Pam for polishing the paper. This project was supported
by the National Natural Science Foundation of China (21601009 and
51532001), National Basic Research Program of China (2014CB931802),
and the International Postdoctoral Exchange Fellowship Program.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
concave structures, cuprous oxide, F68, light trapping, surface-enhanced
raman scattering (SERS)
Received: March 14, 2018
Revised: May 21, 2018
Published online:
4. Experimental Section
Synthesis of Cu2O Concave Spheres: All of the chemical reagents used
in this experiment were of analytical grade without further purification.
2+
Cu(NH3)4 aqueous solution was first prepared by adding ammonia
solution (NH3·H2O, 14.0 m) into copper chloride (CuCl2, 0.20 m)
aqueous solution to keep molar ratio of NH3 to Cu2+ at 10: 1. An IS was
prepared by mixing Cu(NH3)42+ solution (2.25 mL) and 14.0 mL triblock
copolymer Pluronic F68 (PEO78–PPO30–PEO78, MW 8400, 80.0 g L−1)
together into a 100 mL glass flask. Then kept IS under stirring for 1 h
in water bath at 35 °C (calibrated by Lauda Ecoline staredition RE 106).
Subsequently, 3.4 mL AA (0.60 m) was added dropwise into the above
mixture. A yellow turbid formed instantly. The mixture was aged for
5 min under constant stirring. The resulted orange-yellow precipitate
was collected by centrifuging and washed with ethanol for several times
to remove the F68 and finally dried under vacuum at 60 °C for 4 h.
Synthesis of Cu2O Complete, Multiconcave Sphere, Hemisphere and
Spheres: The reaction temperature was changed to 15, 25, and 45 °C
while keeping other conditions identical.
Characterization: The powder XRD of the samples was characterized
by the Rigaku Rotaflex Dmax2200 diffractometer with Cu Kα radiation
(λ = 1.54056 Å). The morphologies of the samples were observed using
Hitachi S-4800 SEM with an accelerating voltage of 10 kV. TEM and
high-resolution transmission electron microscopy (HRTEM) images
were recorded by JEOL JEM-2100F, with an accelerating voltage of
200 kV. pH value was measured by pH meter (PHS-3C). Zeta-potential
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