Significantly enhanced photocatalytic performance of mesoporous C@ZnO hollow nanospheres via suppressing charge recombination

Article abstract of DOI:10.1016/j.cplett.2018.12.013

Mesoporous C@ZnO hollow nanospheres were prepared by carbon sphere template. The influence of defects and surface state on photocatalytic performance was studied. The photocatalytic activity of C@ZnO nanospheres without treatment is the worst, that of the sample via pickling and annealing is secondary, and the pickling sample is the best. The improved performance can be attributed to the reduction of defects, which suppress the charge recombination. This work helps us better understand the important effects of defects on photocatalytic properties, and provides us with a feasible way to improve the UV photocatalytic activity of ZnO and other metal oxide.

Full text of DOI:10.1016/j.cplett.2018.12.013

Accepted Manuscript
Research paper
Significantly enhanced photocatalytic performance of mesoporous C@ZnO
hollow nanospheres via suppressing charge recombination
Kexin Zhu, Changqing Jin, Zengyun Jian, Yongxing Wei, Ruihua Nan, Chi
Zhang, Lin Hu
PII:
S0009-2614(18)31007-8
DOI:
https://doi.org/10.1016/j.cplett.2018.12.013
CPLETT 36147
Reference:
Chemical Physics Letters
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26 September 2018
9 November 2018
6 December 2018
Please cite this article as: K. Zhu, C. Jin, Z. Jian, Y. Wei, R. Nan, C. Zhang, L. Hu, Significantly enhanced
photocatalytic performance of mesoporous C@ZnO hollow nanospheres via suppressing charge recombination,
Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett.2018.12.013
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Significantly enhanced photocatalytic performance of mesoporous
C@ZnO hollow nanospheres via suppressing charge recombination
Kexin Zhu, Changqing Jin*, Zengyun Jian, Yongxing Wei，Ruihua Nan, Chi Zhang, Lin Hu
School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021,
PR China， E-mail: eaglejin@xatu.edu.cn
Abstract: Mesoporous C@ZnO hollow nanospheres were prepared by carbon sphere
template. The influence of defects and surface state on photocatalytic performance
was studied. The photocatalytic activity of C@ZnO nanospheres without treatment is
the worst, that of the sample via pickling and annealing is secondary, and the pickling
sample is the best. The improved performance can be attributed to the reduction of
defects, which suppress the charge recombination. This work helps us better
understand the important effects of defects on photocatalytic properties, and provides
us with a feasible way to improve the UV photocatalytic activity of ZnO and other
metal oxide.
Keywords:ZnO; Hollow spheres; Templates; Defects; Photocatalys; Nanocomposites.
1

1. Introduction
With rapid development of urbanization and industrialization, a tremendous series
of environmental pollution problems have been brought out. A great deal of attention
has been thus paid to convert contaminants into non-toxic, harmless, and /or useful
products [1]. As a sustainable advanced oxidation technology, semiconductor
photocatalysis has been explored to oxidize or reduce environmental pollution [2-6].
ZnO has been proved as one of the most potential photocatalysts for water
purification [2]. However, photocatalysts need to have higher activities to be
economically competitive as a practical water treatment technology. This can be
achieved by enhancing the quantum efficiency. The recombination and
separation/migration processes of photo-generated carriers are crucial factors to
improve the quantum efficiency [7]. Charge recombination reduces the number of
electron-hole pairs by emitting light or generating phonons, which is ineffective for
the quantum efficiency [7]. Efficient carrier separation avoids any interior/surface
charge recombination and improves photocatalytic performance. To improve the
quantum efficiency and photocatalytic performance, it is therefore important to
enhance the charge separation and/or to suppress the charge recombination. During
the past decades, many methods (e.g., noble metal loading [8], heterostructure [9],
plasmonic structures [10], Graphenic carbon coatings [5], etc.) have been developed
to improve the carrier transfer as well as their separation. To date, little effort has been
made to improve the quantum efficiency by suppressing the charge recombination.
Metal oxide semiconductor (MOS) nanostructures involve large specific surface area,
2

surface area and surface energy, which lead to more surface defects. The defects are
recombination centers of photogenerated carriers [11], which reduce of quantum
efficiency and photocatalytic activity.
In this paper, we show that mesoporous C@ZnO hollow spheres are prepared by a
template technology, and that quantum efficiency and photocatalytic activity could be
controlled by reducing defects. Furthermore, significantly enhanced photocatalytic
performance via suppressing charge recombination was explored.
2. Experimetal section
2.1 Synthesis of mesoporous C@ZnO hollow spheres
The preparation of mesoporous C@ZnO hollow spheres was carried out according
to the method reported in [12, 13], using carbon spheres as the template zinc acetate
as the main raw material. Firstly, the carbon spheres were prepared from glucose
solution (65 mL, 0.55 mol/L) under hydrothermal conditions at 180 °C for 9 h.
Secondly, the suspension (0.171mol/L Zn(AC)2·2H2O, 0.30 g carbon spheres and 60
mL ethanol solution) ultrasound for 30 minutes. After magnetically stirred for 12 h at
room temperature, the mixture was washed with ethanol there times and dried at
80 °C for 4 h. The final products were calcined in air at 550 °C for 4 h to form
C@ZnO hollow structure, which were named S0. S0 powder was immersed in a 0.1
mol/L hydrochloric acid solution under ultrasonication for 3 minutes, subsequently
washed and dried, which is designated as S1. S1 was annealed at 250 °C for 1 hour
and the resulting sample was designated as S2.
2.2 Photocatalytic activity
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The photocatalytic activities were studied by using Rhodamine B (RhB) as a
contrast pollutant [8, 9]. The suspension consists of 50 mg of the photocatalyst and 50
mL of the RhB aqueous solution (10 mg/L), which were stirred in the dark for 30 min.
Subsequently, the room-temperature photocatalytic performance was tested under a
high-pressure mercury lamp (CHF-XM-300W).
The degradation efficiency could be expressed as
, where
η  (C0 Ct ) C0 100%
C0 is the initial concentration of RhB and Ct is the concentration after t minutes of
light exposure. In order to quantify the degradation rate, we use the pseudo first-order
kinetic equation
time) [9, 14].
(K is the kinetic rate constant, t is the irradiation
ln(C0 Ct )  Kt
3. Results and discusstion
Figure 1 shows X-ray diffraction patterns. All diffraction peaks could be indexed
as the wurtzite ZnO, and no impurity phase is found. The diffraction intensity of S1 is
sharper than that of S0, which can be attributed to the reduction of surface defects by
an acid cleaning process. Then the neat lattice structure increases, the X-rays
scattering reduces, and the diffraction intensity increases. After annealing, the
diffraction intensity decreases, indicating the surface atoms reconstruct, and that the
neat and clear surface structure were disrupted. TEM and SEM images of S0 (as
shown in inset of Figure 1) indicate a mesoporous spherical shell structure, which is
assembled by nanoparticles.
The surface composition and chemical state of nanospheres were analyzed by
X-ray photoelectron spectroscopy (XPS) (Figure 2). The characteristic peaks of Zn, O
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and C can be observed in Figure 2a. The C element comes from carbon sphere
templates, which is not completely removed by calcination and proved by previous
work [13].
Figure 2b-c shows the high resolution XPS spectra of the Zn 2p and C 1s. Figure
2b shows the binding energy of the Zn 2p of the samples is slightly different, which
may be corresponding to the difference of surface defects and states. For Zn 2p1/2, the
binding energy of S0, S1 and S2 is respectively 1045.1, 1044.8 and 1045.1 eV [8,15].
For Zn 2p3/2, the binding energy is respectively 1022.0, 1021.8 and 1022.0 eV [8,15].
The spin splitting Zn 2p energy was 23.0 eV, indicating that the chemical valence of
Zn is Zn2+ [16]. Figure 2c shows the C 1s spectrum of S1. The typical C 1s peak can
be fitted by two approximate Gauss distributions, and the binding energy of 284.8 and
288.5eV is assigned to C-C and C=O respectively [17, 18].
Figure 3 shows high resolution XPS spectra of O 1s, which can be fitted by two
symmetrical Gauss function curves, centered at ~530.2 and ~531.6 eV, respectively.
The peak at 530.2 eV corresponds to the crystal lattice oxygen ions (Olatt) in the form
of ZnO, which is relatively stable. The peak at 531.6 eV correlates with the adsorbed
−
oxygen ions (Ox ) caused by oxygen vacancy (VO), oxygen interstitial (Oi), and
oxygen antisite (OZn) in the ZnO matrix [19], which is related to the surface defects.
−
For S0, S1, and S2, the area ratios of Ox and Olatt are respectively calculated to be
0.871, 0.755, and 1.222, which means difference of the surface defects and state.
Compared with S0, the surface defects of S1 reduce and those of S2 increase, which is
consisted with the results of XRD.
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UV–vis spectra of RhB on samples after 30 min dark adsorption/desorption and t
minutes of light exposure (as described earlier) are shown in Figure 4a-c. The spectral
intensity in the UV-Vis is proportional to the concentration of Rhodamine B. The
intensity decreases with time, reflecting the decrease of RhB concentration with time.
Figure 4d shows the degradation efficiency of S1, S2 and S0 decrease in sequence.
The degradation efficiency of S1 and S2 can reach 97.16% and 96.06% in 30 minutes,
which is significantly higher than S0. Figure 4e shows the K of S0, S1 and S2 are
0.0978, 0.117 and 0.106 min-1, respectively. The results indicate the processing
technology would enhance the photocatalytic activity of nanospheres by changing
surface state and defects.
In order to further confirm that the change of photocatalytic performance mainly
comes from the change of surface state and defect rather than the change of specific
surface area and mesoporous size distribution. Nitrogen adsorption–desorption
isotherm measurements were utilized to investigate sample structure, surface area and
pore size distribution. The isotherm of the samples exhibits a hysteresis loop,
associated with the filling up and emptying of mesopores by capillary condensation
(Figure 5). This phenomenon clearly indicates that the samples exhibit a large
structural porosity. The BET surface area of S0, S1 and S2 was calculated as 147.3,
162.9 and 156.5 m2 g-1, respectively. Using the BJH method, the desorption branch of
the nitrogen isotherm indicates that the pore size distribution of S0, S1 and S2 is
respectively 1.6–32.5 nm, 0.9-42.3 nm and 1.1-38.9 nm (Table 1). Comparing S0, S1
and S2, the specific area and the average mesopores size of the mesoporous spheres
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first increase then decrease, which further confirms that acid washing and subsequent
annealing change the microstructure and surface state. The large surface area also
provides numerous active sites for surface contact reactions. The large mesopores size
of the samples are advantageous for molecular adsorption–desorption and diffusion.
The large surface area and mesopores size result in the excellent photocatalytic
performance. However, a small change in the surface area and mesopores size does
not bring significant changes in photocatalytic performance.
Table 1 the surface area and pore size distribution by BET analysis
Sample surface area pore size distribution average pore size
number
S0
(cm3/g)
147.3
162.9
156.5
(nm)
(nm)
16.5
19.8
17.8
1.6-32.5
0.9-42.3
1.1-38.9
S1
S2
In order to further investigate the relationship between the defects and
photocatalytic performance, we focused on the key processes affecting the quantum
efficiency and photocatalytic reaction: charge recombination. Surface and interior
defects are recombination centers of photogenerated carriers, which reduce of
quantum efficiency. Compared with S1, S0 and S2 have more surface defects, as is
proved by XPS results. Thus, the photocatalytic performance of S1 is better than that
of S0 and S2 by suppressing charge recombination. Compared with S0, S2 has more
surface defects. However, annealing would reduce the internal defects of S2. The sum
defects of S2 are less than S0. Thus, photocatalytic performance of S2 is significantly
better than that of S0.
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4. Conclusions
Mesoporous C@ZnO hollow nanospheres were synthesized by carbon sphere
templates. The sample without treatmeat has the poor photocatalytic performance,
while the pickling sample and the sample via pickling and annealing have excellent
performance. Pickling or annealing would reduce surface or internal crystal defects
that are recombination centers of photogenerated carriers, which is an effective
method to improve the photocatalytic performance by suppressing charge
recombination.
Acknowledgements
This work was supported by the President Foundation of Xi’an Technological of
China (Grant No. XAGDXJJ18006), the Natural Science Basic Research Plan in
Shaanxi Province of China (Grant Nos. 2018JM509 and 2018JQ1092), the National
Natural Science Foundation of China (Grant Nos. 11404251, 11704301 and
51671151), and Shaanxi Provincial Education Department Program (18JK0394).
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Figure 1 X-ray diffraction patterns of the samples, insets a and b show the TEM and
SEM images of S0.
Figure 2 Wide XPS spectra (a), high resolution Zn 2p (b) and C 1s (c) spectra
Figure 3 High resolution spectra of the O 1s spectra of S0 (a), S1(b) and S2 (c).
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Figure 4 UV–vis spectra changes of RhB of S0 (a), S1(b) and S2 (c), photocatalytic
degradation efficiencies of samples (d), Kinetics curves of samples (e).
Figure 5 Nitrogen adsorption–desorption isotherm and pore size distribution (inset) of
S0 (a), S1(b) and S2 (c).
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Graphical Abstract
(a) X-ray diffraction patterns of samples, TEM (a) and SEM (b) pictures inserted, (b)
photocatalytic kinetic curve of mesoporous C@ZnO hollow nanospheres via surface
treatment process
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Highlights
Mesoporous C@ZnO hollow nanospheres were prepared by carbon sphere template.
Enhanced photocatalytic performance can be realized by suppressing charge
recombination.
Pickling process reduces surface defects and improves photocatalytic performance.
Annealing reduces internal defects and improves photocatalytic performance.
13