Facile and single-step route towards ZnO@C core-shell nanoparticles as an oxygen vacancy induced visible light active photocatalyst using the thermal decomposition of Zn(an)2(NO3)2

Article abstract of DOI:10.1039/c6ra14108k

Zinc oxide-carbon (ZnO@C) core-shell nanoparticles were synthesized using a facile and single-step method, which involved the thermal degradation of a zinc aniline nitrate complex in methanol. The formation of ZnO and carbon was observed during the early stages of synthesis (>200 °C), whereas a further increase in the temperature determines the level of the carbon coating. Transmission electron microscopy confirmed that the ZnO@C nanoparticles obtained at 600 °C were ～100 nm in size with a uniform 5-20 nm thick carbon coating. The nano-coating of carbon on ZnO along with the presence of oxygen vacancies promoted its photocatalytic activity under visible light with higher efficiency for the photodegradation of rhodamine B than bare ZnO. The two probable pathways for the visible activity and the enhanced photodegradation capacity of ZnO@C core-shell nanoparticles are also discussed. The synthesized ZnO@C core-shell nanoparticles exhibited very good stability and recyclability, highlighting their potential use as an efficient visible light driven photocatalyst for pollutant degradation.

Full text of DOI:10.1039/c6ra14108k

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Facile and single-step route towards the ZnO@C core-shell
nanoparticles as oxygen vacancy induced visible light active
Received 00th January 20xx,
Accepted 00th January 20xx
photocatalyst using the thermal decomposition of Zn(an) (NO3)2
2
complex†
DOI: 10.1039/x0xx00000x
www.rsc.org/
Sandesh Y. Sawant and Moo Hwan Cho*
Zinc oxide-carbon (ZnO@C) core-shell nanoparticles were synthesized using a facile and single-step method, which
involved the thermal degradation of a zinc aniline nitrate complex in methanol. The formation of ZnO and carbon was
observed at the early stages of synthesis (> 200 °C), whereas a further increase in the temperature determines the level of
the carbon coating. Transmission electron microscopy confirmed that the ZnO@C nanoparticles obtained at 600 °C were
~100 nm in size with a uniform 5-20 nm thick carbon coating. The nano-coating of carbon on ZnO along with the presence
of oxygen vacancies promoted its photocatalytic activity under visible light with higher efficiency for the photodegradation
of rhodamine B than bare ZnO. The two probable pathways for the visible activity and the enhanced photodegradation
capacity of ZnO@C core-shell nanoparticles are also discussed. The synthesized ZnO@C core-shell nanoparticles exhibited
very good stability and recyclability, highlighting their potential use as an efficient visible light driven photocatalyst for
pollutant degradation.
other hand, its applications are still limited because of its
inherent defects involving a wide band gap, which restricts the
absorbing wavelength just in the ultraviolet light range and
Introduction
st
In the 21 century, water pollution is the major environmental
issue and a leading cause of diseases and deaths of thousands
of people daily. The effluent from the textile industry, which
results in low light utilization efficiency. Photo-corrosion is also
one of the obstacles impeding the practical maximization of
photocatalytic performance. To overcome these disadvantages
1
contains various organic dyes, is one of the major point source
of water pollution and significant cause of eutrophication and
perturbation in aquatic ecosystems. Among the various
techniques 2 of water pollution remediation, such as
8
several methods including noble metal deposition, creation of
9
10
structural defects and semiconductor coupling have been
developed. The photocatalyst composite with different
carbon-based materials including graphene, carbon
nanotubes, fibers, and fullerene provides an efficient means of
overcoming these drawbacks with enhanced photocatalytic
3
4
adsorption,
flocculation
and
reverse
osmosis,
5
,6
heterogeneous photocatalysis has been considered as a
promising economical technique owing to its advantages of
complete organic carbon mineralization and ambient
operating conditions.
Zinc oxide (ZnO) with exceptional electrical, optical and
piezoelectric properties is an important semiconductor
material for applications in the areas of optics, solar cells and
11
activity. Because of the high cost and complex preparation
process of these carbon materials, researchers have developed
the methods to fabricate ZnO-carbon (ZnO@C) composites
using the thermal degradation of carbon precursors in the
1
2-14
presence of zinc oxide or zinc salt.
If the surface of the
nanostructure ZnO is modified with carbon, the good
properties of ZnO and carbon will be integrated into the
hybrids, which is advantageous for overcoming some of the
intrinsic defects of ZnO. Thermal degradation of the zinc
complex is a promising simple methodology that requires less
expensive and simple instrumental facilities to fabricate
gas sensors, and it has also considered as a promising7
photocatalyst owing to its direct and wide band gap of 3.3 eV.
Over the past few years, various ZnO based materials with
different shapes and sizes have been synthesized and
attempted for the photodegradation of organic dyes. On the
1
5
16
ZnO@C composites. Ma et al. and Cho et al. reported the
synthesis of ZnO@C composites using citric acid and vitamin C
as the carbon source, respectively. Most papers reported the
School of Chemical Engineering, Yeungnam University, Gyeongsan-si, Gyeongbuk
7
5
12-749, South Korea. E-mail: mhcho@ynu.ac.kr; Fax: +82-53-810-4631; Tel: +82-
3-810-2517
17-19
20
multi-step synthesis or use of commercial ZnO.
Xue et al.
†
Electronic Supplementary Information (ESI) available: XPS survey spectra of ZnO
and ZnO@C-2, XRD pattern of ZnO@C composite, Pseudo-first order kinetics plots performed the single stage synthesis of ZnO@C composite
of RhB degradation using ZnO and ZnO@C, and Uv-visible absorption spectra of
rhodamine B after photodegradation. See DOI: 10.1039/x0xx00000x
using zinc citrate as the carbon source, which exhibited
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efficient photocatalytic activity for methylene blue (MB) Thermo Fisher Scientific Escalab 250) with monochromatized
degradation under UV irradiation. AlKα excitation (h = 1486.6 eV). Electrochemical impedance
The synthesis of 1D ZnO@C core-shell structures and their spectroscopy (EIS) was performed in a three electrode cell
field emission studies have been reported by several with a 0.2 M Na SO aqueous solution as the electrolyte using
To the best of the authors’ knowledge, there a potentiostat (VersaSTAT 3, Princeton Research, USA). The
2
4
2
1,22
researchers.
are no reports available on the synthesis of the ZnO@C core- working electrodes were prepared by coating a paste of the
shell nanoparticles and their applications as photocatalyst. This sample with 10 wt% Nafion as the binder on a FTO glass
paper reports the facile single stage synthesis of ZnO@C core- electrode using the doctor-blade method. A Ag/AgCl (3M KCl)
shell nanoparticles via the thermal degradation of zinc-aniline and platinum gauge were used as the reference and counter
nitrate complex. The obtained ZnO@C core-shell nanoparticles electrodes, respectively.
were utilized further as a photocatalyst for the degradation of Photodegradation of rhodamine B under Visible light
rhodamine B (RhB) under visible light irradiation.
In a typical process, the catalyst (500 mg /L) was suspended in
the dye solution (5 ppm) using an ultrasonication treatment
for 10 min. The adsorption-desorption equilibrium of the dye
on the catalyst was achieved by stirring the suspension in the
Experimental
Synthesis of ZnO@C core-shell nanoparticles
dark for 30 min. Under ambient conditions and stirring, the
solution was exposed to visible light placed 12 cm away from
the solution. A 400 W visible light lamp with an intensity of 31
Zinc nitrate hexahydrate was procured from Duksan chemicals
Company, South Korea. Aniline and RhB were purchased from
Sigma Aldrich. All chemicals were used as received. In a typical
procedure, zinc nitrate hexahydrate (2 mmol) was dissolved in
methanol (2 mL) followed by the addition of aniline (4 mmol)
in a porcelain crucible. The resulting zinc aniline nitrate
2
mW/cm and λ > 500 nm obtained from 3M USA was used as
the light source for the photocatalytic studies. A sample
solution was taken at certain time intervals during the
experiments, and centrifuged to remove the catalyst
completely. The solution was analyzed by UV-Vis
spectrophotometer (Optizen 2120UV; Mecasys Co., Ltd., South
(
2 3 2
Zn(an) (NO ) ) complex was heated to 500, 600 and 700 °C in
a muffle furnace to obtain ZnO@C-1, ZnO@C-2 and ZnO@C-3,
respectively. For a comparative study, the synthesis of bare
ZnO was carried out in a similar manner without aniline.
Characterization
Korea). The percentage degradation is reported as C/C
0
, where
C and C are the absorbance of the dye solution after each
0
irradiated time interval and the initial absorbance dye solution
at the ʎmax, respectively.
X-ray diffraction (XRD, PANalytical, X’pert PRO-MPD,
1
Netherland) with Cu Kα (λ = 1.54056 Å) radiation was used for
crystallographic phase identification of the sample. Field
emission scanning electron microscopy (FE-SEM, S-4200;
Hitachi, Ltd.) and transmission electron microscopy (TEM, JEM-
Results and discussion
Characterization of ZnO@C core-shell nanoparticles
2100 JEOL) with an accelerating voltage of 15 and 200 kV, Though the formation of complex was clearly intimated by the
respectively, were used to examine the morphology of the quick precipitation, it was further confirmed using XRD and
samples. The optical properties of the materials were analyzed ATR-IR analysis. The crystallographic changes occurred in the
by UV-VIS-NIR diffuse reflectance spectrophotometer zinc nitrate salt was recorded by XRD pattern of Zn(an)
2 3 2
(NO )
(
VARIAN, Cary 5000, USA) and photoluminescence complex (Fig. S1†) showing the numbers of comparatively less
spectroscopy (PL) using a wavelength of 325 nm and a power intense peak instead of sharp peaks of zinc nitrate
of 50 mW (Kimon, 1 K, Japan), respectively. Attenuated total hexahydrate. The transmittance band observed at 515 and 425
-
1
2 3 2
reflection – infrared (ATR-IR) was performed on a Thermo cm in ATR-IR spectrum of Zn(an) (NO ) complex (Fig. S2†)
Scientific Nicolet Smart iTR iS10. Raman spectra were collected attributed to Zn–O and Zn–N bond and also evidenced the
using the XploRa Plus instrument (Horiba, Jobin Yvon, formation of complex. Fig. 1(a) shows the XRD pattern of the
Germany) coupled to an optical microscope. A laser having a as-prepared samples from the thermal degradation of the
wavelength of 532 nm was used for excitation. The samples
were investigated with a grating having 1200 grooves/mm and
using a HORIBA Scientific CCD detector (1024 × 256-pixel, TE
air cooled). With this configuration the wavenumber and
-
1
wavelength resolution are approx. 3.26 cm /CCD pixel and
.09 nm/CCD pixel, respectively. The calibration of
wavenumber shifting was also performed before the sample
0
-
1
characterization using silicon wafer (< 1cm ). The peak fitting
of recorded Raman spectra using Lorentzian function showed
the maximum error of 0.05. The elemental composition of the Fig. 1 XRD pattern of the as-obtained ZnO and ZnO@C core-
samples was determined using a CHN/O analyzer (Perkin- shell nanoparticles (a) XRD of the samples obtained at
Elmer, Optima 8300). The chemical states of the samples were different temperatures. (b) Enlarged area of 31° to 37° 2θ in
characterized by X-ray photoelectron spectroscopy (XPS, (a).
2
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3 2
(NO ) complex at different temperatures, which shows
ARTICLE
Zn(an)
the characteristic XRD peaks of ZnO corresponding to the
100), (002), (101), (102), (110), (103), (112), and (201) planes
of the typical wurtzite ZnO structure (JCPDS 36-1451), while no
other peaks from Zn(NO and ZnO impurities were
2
(
3
)
2
2
2
3
observed. The mean crystallite size calculated from the X-ray
line broadening of the 002 peak using the Scherrer's equation
were 59, 32, 33 and 47 nm for ZnO, ZnO@C-1, ZnO@C-2 and
ZnO@C-3, respectively. Although, all samples were nano-
crystalline in nature, the ZnO prepared without complex
formation showed the highest crystalline size, whereas the
crystalline size increased with increasing temperature. Similar
2
4
observations were also noted by Zhang et al. in case of a
ZnO@C composite, where carbon plays an important role in
XRD peak broadening. No peak was observed in the 20-30° 2θ
range, possibly due to the lower contribution of carbon as well
as its amorphous nature. Hence, the obtained samples were
analyzed further by elemental (CHN) analysis for determining
the carbon content. The carbon and hydrogen contents (wt %)
of the ZnO@C-1, ZnO@C-2 and ZnO@C-3 were 0.410 (C),
Fig. 3 FESEM images of (a) ZnO, (b) ZnO@C-1, (c) ZnO@C-2
and (d) ZnO@C-3.
ZnO structure. Fig. 2b presents the typical Raman spectra
recorded for ZnO and ZnO@C core-shell nanoparticles. The
0.043 (H); 0.181 (C), 0.027 (H); and 0.024 (C), 0.002 (H),
-
1
more intense peaks at
~
100 and
modes of wurtzite ZnO, respectively, due to
the vibration of the zinc sub lattice in ZnO and oxygen
~440 cm were assigned to
respectively. In contrast, carbon and hydrogen were not
detected in ZnO.
The distinguished peaks of ZnO for the shoulder of Zn–O
bending and Zn–OH vibrations were observed in all samples at
low
high
the E
2
and E
2
6
-1
vibrations. The transition at 334 cm was attributed to a ZnO
high
low
-
1
multiple-phonon-scattering process, E
2
– E
2
. The mode at
~
500 and 900 cm , respectively, in the ATR-IR spectra (Fig.
-
1
30
6
,25,26
3
84 cm was associated with the A1 (TO) phonon frequency.
The peak observed for the E1LO mode at ~ 580 cm was
attributed to the formation of defects, such as oxygen vacancy,
zinc interstitial, or their complexes. The inset in Fig. 2b shows
the peak shift from higher frequency to lower frequency (red
shift) of about 5 cm , which also confirms the presence of
2a).
In the case of ZnO@C-1 and ZnO@C-2, the sharp
peaks at 2902 and 2981 cm were assigned to the stretching
vibration of the C–H bond, which also confirms the presence of
the carbon structure.
400 cm were attributed to the C–O and C=C stretching
-
1
-
1
3
1
2
7,28
The peaks observed at 1050 and
-
1
1
-
1
vibrations, respectively. The absence of such peaks in the case
of ZnO is due to the lack of aniline, as the carbon precursor
during synthesis. In case of ZnO@C-3, however, the absence of
such peaks reflects the absence or presence of very minute
quantities of carbon due to its oxidation at such a high
3
2-34
oxygen vacancies and the incorporation of the carbon.
The
presence of carbon in the case of ZnO@C-1 and ZnO@C-2 was
also confirmed by its characteristic peaks at 1355 (D-band) and
-
1
1
575 cm (G-band) associated with defects or disorder due
3
2
mainly to sp hybridization and stretching of the sp bonded
carbon atoms, respectively.
calcination temperature (700
°C). The intensity of the peaks,
3
5,36
The D- and G-band,
representing the presence of carbon in ZnO, also decreased
-
1
representing the presence of carbon were not observed in
case of ZnO and ZnO@C-3, whereas the intensity of the peaks
decreased with increasing the calcination temperature, which
is also in accordance with the ATR-IR results.
with increasing calcination temperature. A peak at 3400 cm ,
which is generally assigned to the –OH stretching mode and is
present in the current spectra, might be due to the carbon
2
9
bonded –OH groups or surface Zn–OH groups. Raman
spectroscopy has been used to examine the carbon
incorporation and defects, mainly the oxygen vacancies in the
The morphology of the obtained materials was investigated by
FE-SEM. The ZnO synthesized from the thermal degradation of
zinc nitrate hexahydrate salt showed the formation of
diamond shaped particles with an average size of 10 μm and
possessing a six-faced pyramidal structure at both sides (up
and down). The formation ZnO with similar morphological
features from the thermal decomposition of zinc nitrate
3
7
hexahydrate has been reported previously. ZnO obtained
from the thermal degradation of Zn(an) (NO complex
resulted in the formation of nanoparticles, which might be due
to the sudden decomposition of the complex at 180 C into
2
3 2
)
~
°
Fig. 2 (a) ATR-IR and (b) FT-Raman spectra of the as-obtained ZnO and carbon. The thick layer of carbon on the ZnO
ZnO and ZnO@C core-shell nanoparticles. The inset in 2b nanoparticles can be visualized by FE-SEM (Fig. 3b) for ZnO@C-
shows the enlarged area of Raman shift from 400 to 500 cm-1 1, whereas in the case of ZnO@C-2, agglomerated ZnO
of 2b.
nanoparticles with a smooth surface were observed (Fig. 3c).
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morphology in FE-SEM, possessed a shell of carbon (inset of
Fig. 4b). Fig. 3c clearly shows the nanosized coating of carbon
layer on the ZnO particle. The coating of the carbon material
on ZnO varied in thickness from 5-20 nm, which resulted from
the oxidation of the excess carbon material with a higher
calcination temperature, e.g. 600 °C. The interaction of ZnO
core with the carbon shell at their interfaces with a
disturbance in the ZnO lattice (Fig. 4d), was also helpful in the
easy transfer of excited electrons from ZnO to carbon, which
3
8
extended the time for electron hole pair recombination.
These stacking faults and lattice defects could be attributed to
the substitution of larger C anions for the O sites in the ZnO
1
6,39
lattice.
The spacing between the lattice of the ZnO core
was 0.26 nm (Fig. 4d), which corresponds to the distance
2
4
between the (002) planes of ZnO crystal lattice. Fig. 4e
depicts the typical SAED pattern of ZnO. Elemental mapping by
TEM also reflects the formation of the ZnO@C core-shell
structure (Fig. f, g and h).
In semiconductor materials, a reduction of the band gap and
the response of a catalyst to visible light can be achieved
either by the formation of oxygen vacancies or carbon
4
0,41
doping.
In both cases, optical characterization provides
crucial information regarding its electronic interaction with
light, which is a more important factor in the case of
photocatalysis. The optical properties of the obtained samples
were analyzed by UV-Visible DRS and PL spectroscopy.
Surprisingly, ZnO obtained from zinc nitrate hexahydrate
showed the lowest band gap as compared with the other
synthesized ZnO@C core-shell nanoparticles (Fig. 5a and b).
The red shift observed in the absorbance of ZnO under UV-
Visible light can be explained by oxygen vacancies in the ZnO
due to the shortage of oxygen in the environment during the
decomposition of nitrate ions in zinc nitrate hydrate and its
24,42,43
large grain size.
The shoulder peak observed at ~500 nm
in the absorbance spectrum of ZnO (Fig. 5a) compared to the
Fig. 4 TEM images of ZnO@C-2 showing (a) agglomerated
ZnO@C-2 nanoparticles, (b) close view of a cluster of ZnO@C-
2
nanoparticles coated with the carbon shell (as shown in
inset), (c) individual ZnO@C-2 particle with a clear view of
carbon shell, (d) HR-TEM image representing the ZnO core,
carbon shell and their interface, (e) SAED pattern of ZnO@C-2,
and elemental mapping showing the distribution of (f) zinc, (g)
oxygen and (h) carbon.
FESEM did not show any changes in the morphological
features when the calcination temperature was increased from
600 to 700 °C (ZnO@C-3, Fig. 3d). ZnO@C-2 was examined
further by TEM analysis based on its higher photocatalytic
activity. Similar to that observed by FE-SEM, TEM of ZnO@C-2
also showed the agglomerated form of ZnO nanoparticles (Fig.
Fig. 5 Optical properties of the obtained ZnO and ZnO@C
core-shell nanoparticles (a) UV-Visible absorbance spectra, (b)
corresponding Tauc plot and (c) room temperature PL spectra
4a), a separated bunch of a few ZnO particles and individual
(Inset in Fig. 3c shows a the plot of the IDL/INBE ratios of the
ZnO particles (Fig. 4b and c). A closer view showed that the
ZnO nanoparticles, which reflect the smooth surface
different synthesized ZnO@C core-shell nanoparticles).
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other samples also reflects the formation of extensive oxygen interstitials. Although the overall defects in ZnO@C-2 decrease
3
7
vacancies. The trend of the band gap energy of obtained due to the higher calcination temperature, oxygen vacancies
sample was as ZnO < ZnO@C-1 < ZnO@C-2 < ZnO@C-3. As were dominant over oxygen interstitial, which might be due to
shown in the Tauc plot (Fig. 5b), all samples exhibited lower the utilization of surface oxygen during the oxidation of
binding energies (≤ 3.1 eV) than typical ZnO (3.3 eV) and were carbon. In the case of ZnO@C-2, the surface oxygen is utilized
expected to show photocatalytic activity in the visible region of for the oxidation of carbon, which leads to the dominance of
light. Fig. 5c presents the room temperature PL spectra of the oxygen vacancies over oxygen interstitials and a decrease in
as-synthesized ZnO and ZnO@C core-shell nanoparticles the carbon concentration. Further increase in the calcination
obtained at different temperatures. As shown in the PL spectra temperature (700
°C) results no or negligible amount of
(
Fig. 5c), all samples exhibited two major peaks, first at 390 carbon in ZnO@C-3, which tends to decrease in the oxygen
~
nm, which was assigned to the near band gap emission (NBE) vacancies because of the higher availability of oxygen on the
resulting from the recombination of free charge carriers, ZnO surface. The appearance of the band in the near IR region
whereas the second at
~
530 nm broad deep-level (DL)
(~770 nm) for ZnO@C-2 and ZnO@C-3 was due either to ZnO
47,48
emission, also called green emission, was mainly because of defects or the second-order feature of NBE.
the radial-recombination of a photo-generated hole with a The detail structural features of the ZnO and ZnO@C-2 were
singly ionized charge state of the specific defect (oxygen examined by XPS. The peaks representing the presence of Zn,
4
4,45
vacancies) in the ZnO crystal.
defects due to oxygen interstitials (as observed by peak in red survey spectra of ZnO and ZnO@C-2 (Fig. S3a†). The two peaks
orange region and positioned at 610 nm) were more observed in the high resolution XP spectra of Zn 2p for ZnO
dominant than oxygen vacancies. The intensities of the peak (Fig. S3b†) and ZnO@C-2 (Fig. S3c†) were attributed to the
In the case of ZnO@C-1, the O and C (in case of ZnO@C-2) can be seen clearly from the
~
2
+
2+
for oxygen vacancies increases with increasing calcination spin-orbit splitting, Zn Zn2p3/2
temperature compared to the intensities of peak for oxygen 1045 eV) states. The high resolution O1s spectrum of ZnO
interstitials. A plot of the ratio of intensities of peak DL (in (Fig. 6a) and ZnO@C-2 (Fig. 6b), de-convoluted into three
case of ZnO@C-1, peak at 610 nm was considered) and NBE, peaks using the XPS peak fitting program, clearly indicates the
i.e., IDL/INBE (inset of Fig. 5c) provides a clear understanding of presence of the different oxygen bonding in the respective
the crystalline structure of the obtained photocatalysts. ZnO samples. The major band ( 530.8 eV) observed in both the O
obtained from zinc nitrate (without aniline) showed the 1s spectra was attributed to the O ions on the wurtzite
(~1022 eV) and Zn Zn2p1/2
(
~
4
6
~
~
2
-
2
+
highest IDL/INBE ratio depicting the larger number of defects, structure of the hexagonal Zn ion array, which indicates the
mainly oxygen vacancies, which were also correlated with the Zn–O bonds in ZnO crystal. On the other hand, the peak
DRS results. In the case of the ZnO@C core-shell nanoparticles, located at higher binding energy (
the ratio was decreased further with increasing calcination to the chemisorbed oxygen, such as H
temperature, i.e., from 500 to 700 C, suggesting the peak at the intermediate binding energy (
~532.5 eV) could be assigned
2
O, CO
3
and so on. The
~531.5 eV) was
°
enhancement in the crystallinity of the ZnO. The attributed mainly to the presence of the oxygen vacancies in
characterization suggests that the ZnO@C-1 obtained at 500 the ZnO crystal. In the case of ZnO, the relative intensity or the
°
C contains a comparatively larger amount of carbon and a area of the peak defining the oxygen vacancies was higher
higher defective structure associated mainly with oxygen than the peak associated with chemisorbed oxygen. In
contrast, a reverse trend was noticed for ZnO@C-2. This
suggests that oxygen vacancies are prominent in the case of
ZnO, whereas the larger amount of chemisorbed oxygen in
ZnO@C-2 might be due to the presence of the more oxygen
bonded to the carbon layer. Fig. 6c shows the high resolution
C1s spectrum of ZnO@C-2 showing the presence of the carbon
with various chemical states. The major peak at 284.6 eV was
3
24
assigned to the sp hybridized C–C bond. The peaks observed
at relatively lower and higher binding energies, i.e., at 282.6
and 288.9, were attributed mainly to the presence of Zn–C
~
3
9
bonds and carbonate species, respectively. The broad band
2
4,49
at 286.2 eV depicts the presence of the O–C–O complex.
XPS provides further support of the doping of carbon with a
ZnO crystal and also a clearer view on the oxygen vacancies in
both samples.
Fig. 7 presents a schematic representation of the different
stages involved in the current photocatalyst synthesis, as
observed by different characterization techniques and
literature support. The Zn(an)
< 180 C under atmospheric pressure and exothermic
degradation occurred with further increases in temperature.
2 3 2
(NO ) complex was stable up to
Fig. 6 High-resolution XPS (a) O 1s spectrum of ZnO, (b) O 1s
spectrum of ZnO@C-2 and (c) C 1s spectrum of ZnO@C-2.
°
5
0
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Fig. 7 Schematic diagram of different stages involved in the
synthesis of ZnO@C core-shell nanoparticles using the
Fig. 8 Photocatalytic degradation of RhB using ZnO and
different Zn@C core-shell nanoparticles under visible light.
Zn(an)
Although the synthesis of the ZnO@C photocatalysts was
carried out at > 500 C, the formation of ZnO nanoparticles
and carbon occurred at 180 °C due to exothermic
degradation of the complex. XRD analysis of the product
recovered at 200 C (Fig. S4†) revealed the characteristics
2 3 2
(NO ) complex.
°
~
°
peaks of the typical wurtzite ZnO structure (JCPDS 36-1451).
The product obtained at this early stage was completely black
in color, showing the presence of a larger amount of carbon,
which is described as ZnO@C composite. The product obtained
at different calcination temperatures from 500 to 700
showed the decreasing carbon content was due to oxidation to
CO , which resulted in the ZnO@C core-shell structure (at 600
C). The thermal degradation of zinc nitrate in methanol allows
°C
2
°
the formation of micron sized diamond shaped ZnO particles.
The comparatively slower decomposition of zinc nitrate allows
nucleation and agglomeration to form the diamond shaped
Fig. 9 Effect of the catalyst amount on photodegradation of
RhB using ZnO@C-2 under visible light.
adsorption and a decrease in the production of photo-charges.
The excess of carbon deposited on the ZnO may also lead to
poor contact between the photocatalyst and dye, which
resulted in a decrease in photocatalytic activity. The order of
2 3 2
micro particles, whereas in case of the Zn(an) (NO ) complex,
thermal decomposition51,52of the complex resulted in
nanoparticles formation.
Photocatalytic activity
Based on a literature review and detailed characterization, the the photocatalytic activity for the degradation of RhB under
synthesized ZnO@C core-shell nanoparticles were expected to similar conditions was ZnO@C-2 > ZnO@C-3 >ZnO@C-1 > ZnO,
behave as a visible light active photocatalyst. Therefore, the which further confirms that the loading or thickness of the
photocatalytic degradation of RhB was performed in the carbon shell plays a crucial role in the photocatalytic
5
3
presence of visible light as a test reaction (Fig. 8). As shown in properties. After the photodegradation of RhB, no new
Fig. 8, photolysis has no pronounced effect on the degradation absorption band was observed in its UV-visible spectra (Fig.
of RhB (> 9%) under visible light, whereas ZnO showed the 45% S5†), indicating the complete degradation of RhB. Fig. 9
of degradation of RhB under similar conditions. Although, ZnO describes the effect of the catalyst loading on the
possesses a lower band gap, it showed less photocatalytic photocatalytic degradation of RhB under similar conditions. As
activity for RhB degradation due mainly to the larger particle expected, the decrease in the catalyst amount from 500 mg/L
size, which provides fewer catalytic sites and less interactions to 250 mg/L and further to 100 mg/L leads to a decrease in the
with RhB. The carbon content or shell thickness on the photodegradation of RhB from 98 to 88 % and further to 77%,
photocatalyst is a crucial factor to enhance its photocatalytic respectively (Fig. 9). In contrast, an increase in catalyst loading
activity. Among the ZnO@C photocatalysts synthesized,
ZnO@C-2 exhibited the highest photocatalytic activity and
showed 98% degradation of RhB after 150 min irradiation. The
lower photocatalytic activity of ZnO@C-1 (82% degradation of
RhB after 150 min) might be due to the presence of a large
amount of carbon on ZnO, which leads to a blockage of optical
(
750 mg/L) allowed faster photodegradation of RhB (90 min)
with similar efficiency (93%, Fig. 9).
Generally, the pseudo-first order kinetic model used to
understand the kinetic properties of the photodegradation
process suggests that the reaction takes place at the interface
6
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which was
~7.5 times higher than the ZnO (0.0034), compared
to the other photocatalysts. No photocatalytic system is
practically applicable without recyclability. Therefore, the
recyclability study of ZnO@C-2 was performed for up to four
successive cycles after regenerating it by centrifugation
followed by drying at 200 °C for 3 h. As shown in Fig. 10b,
ZnO@C-2 exhibited very good stability and showed
th
photocatalytic degradation of RhB up to ~96% after the 4
cycle without any significant decrease in activity. The overall
study showed that the resulting ZnO@C core-shell
nanoparticles (ZnO@c-2) could be recycled easily and reused
multiple times for the photocatalytic degradation of organic
pollutants under visible light irradiation.
Fig. 10 (a) Pseudo-first order rate constant for the
degradation of RhB using ZnO and ZnO@C core-shell
nanoparticles (500 mg/L) and (b) recyclability of ZnO@C-2 for
photodegradation of RhB under visible light.
Based on the detail characterization and experimental
evidence, the possible mechanism for the photodegradation of
RhB using ZnO@C core-shell nanoparticles under visible light is
shown in Fig. 11. Generally, the deposited carbon shell
included two types of carbon species, one is the interfacial
carbon attached chemically to ZnO and other is surface
carbon. The highest occupied molecular orbital of amorphous
carbon exists in the intermediate position between the
conduction band and valence band of ZnO on the energy level
diagram. Therefore, the interfacial carbon species are believed
to play an important role in the more efficient charge
separation and reducing the probability of photogenerated
electron–hole recombination (Path Ib, Fig. 11). The prolonged
charge separation leads to the production of more active free
radicals, which are responsible for the oxidation of RhB to
Fig. 11 Schematic diagram of the proposed mechanism for
the photodegradation of RhB using ZnO@C core-shell
nanoparticles (ZnO@C-2) under visible light.
2 2
mineralized products, such as CO and H O. In this case, the
absorption of visible light was attributed mainly to the lower
band gap achieved by the presence of oxygen vacancies in the
ZnO core (Path Ia, Fig. 11). The presence of a surface carbon
layer with the appropriate thickness can also behave as a
sensitizer and absorb the visible light. In this case the excited
electrons transferred directly to the conduction band of the
ZnO (Path II, Fig. 11) and the generated electron-hole pairs
were utilized further for the degradation of RhB. The efficient
transfer of charge carriers in the ZnO@C-2 core-shell
nanoparticles over bare ZnO was also confirmed by EIS. Fig. 12
shows the EIS Nyquist plots of ZnO@C-2 and ZnO in the dark
and under visible light. The arc radius of the EIS Nyquist plot,
which reflects the interface layer resistance occurring at the
surface, of the ZnO@C-2 electrode was smaller than that of
the ZnO in the dark and under visible light. In particular, the
interaction of the ZnO core with the carbon shell improves the
transfer of photogenerated charge carriers and is responsible
for the higher photoactivity of ZnO@C-2.
Fig. 12 Nyquist impedance plots of the ZnO and ZnO@C-2 in
the dark and visible light.
between the catalyst and organic pollutants. The data
obtained for the photocatalytic degradation of RhB using the
synthesized photocatalysts at different time intervals was
fitted to a pseudo-first order kinetic model (eq. 1) and the
resulting rate constants are shown in Fig. 10a.
Conclusions
ln(Cₒ/C) = kt
(1)
Zn@C core-shell nanoparticles were synthesized using a
where Cₒ and C are the concentration of RhB at time 0 and t, simple, single-step process of thermal degradation of
respectively; t and k are the reaction time and rate constant, Zn(an) (NO complex. The formation of ZnO nanoparticles
2
3 2
)
respectively. A plot of ln(Cₒ/C) vs. t (Fig. S6†) shows that the was observed at lower temperatures, whereas a further
photodegradation of RhB follows pseudo-first order kinetics. increase in the temperature helps decorate the ZnO particles
As ZnO@C-2 possessed the highest photocatalytic ability, it with a carbon shell with the oxidation of excess carbon. The
showed the highest pseudo-first order rate constant (0.0254), ZnO@C core-shell structure obtained at 600 °C showed a
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2
5 A. Kolodziejczak-Radzimska, E. Markiewicz and T.
Jesionowski, J. Nanomater., 2012, Doi
0.1155/2012/656353.
uniform nanosized coating of the carbon layer with efficient
photocatalytic activity for RhB degradation under visible light
irradiation. The interaction of carbon with ZnO at the ZnO-
carbon interface along with oxygen vacancies plays an
1
2
6 Z. Dai, K. Liu, Y. Tang, X. Yang, J. Bao and J. Shen, J. Mater.
Chem., 2008, 18, 1919-1926.
important role in their higher photocatalytic activity and 27 S. Y. Sawant, R. S. Somani, A. B. Panda and H. C. Bajaj, Mater.
Lett., 2013, 94, 132-135.
lowering the band gap to the visible active region. The present
method shows the facile and fast synthesis of highly stable and
recyclable visible active photocatalysts for pollutant removal.
2
2
3
3
3
8 S. Y. Sawant, R. S. Somani, M. H. Cho and H. C. Bajaj, Rsc
Adv., 2015, 5, 46589-46597.
9 S. Y. Sawant, R. S. Somani, S. S. Sharma and H. C. Bajaj,
Carbon, 2014, 68, 210-220.
0 E. Karber, T. Raadik, T. Dedova, J. Krustok, A. Mere, V. Mikli
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Graphical abstract:
Zinc oxide/carbon core-shell nanoparticles, synthesized using thermal decomposition of zinc aniline
nitrate complex, showed the excellent visible-light driven photocatalytic activity.