- Email: [email protected]
ZnO composite oxide with enhanced photocatalytic activity
Accepted Manuscript Title: Utilizing peroxide as precursor for the synthesis of CeO2 /ZnO composite oxide with enhanced photocatalytic activity Author: Zijian Lv Qin Zhong Man Ou PII: DOI: Reference:
S0169-4332(16)30141-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.280 APSUSC 32512
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-8-2015 1-11-2015 31-1-2016
Please cite this article as:http://dx.doi.org/10.1016/j.apsusc.2016.01.280 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Utilizing peroxide as precursor for the synthesis of CeO2/ZnO composite oxide
2
with enhanced photocatalytic activity
3
Zijian Lv, Qin Zhong*, Man Ou
4
School of Chemical Engineering, Nanjing University of Science and Technology,
5
Nanjing, Jiangsu 210094, PR China
6
Nanjing AIREP Environmental Protection Technology Co., Ltd, Nanjing, Jiangsu
7
210091, PR China
8
Corresponding author: Qin Zhong. Email: [email protected].
9
Tel/Fax number: +86 2584315517
cr
us
Abstract
an
10
ip t
1
A facile synthesis method of CeO2/ZnO composite oxides with higher oxygen vacancy
12
concentration was developed by a two-step precipitation method, in which peroxide was used as
13
precursor. The photocatalytic activities of the catalysts under UV irradiation were studied in
14
degradation of methylene blue (MB). All CeO2/ZnO photocatalysts exhibited higher photocatalytic
15
performance than pure ZnO, and 1%CeO2/ZnO showed highest photocatalytic activity among the
16
prepared catalysts. It was confirmed that the synergistic effect of CeO2 and oxygen vacancy caused
17
the improved photocatalytic activity. Furthermore, the mechanism was investigated by introducing
18
different additives, and it was found that the hydroxyl radicals played a crucial role in degradation
19
process.
20
Keywords: CeO2/ZnO composite oxide; oxygen vacancy; photocatalyst; methylene blue
21
1. Introduction
Ac ce p
te
d
M
11
22
Semiconductor metal oxide plays an important role on the effective degradation of the organic
23
pollutants in water and has been widely investigated [1]. Zinc oxide is one of the most promising
24
third-generation semiconductors due to its wide direct bandgap of 3.37 eV and a large exciton 1
Page 1 of 22
binding energy of 60 meV at room temperature [2]. It has been extensively studied owing to its
2
non-toxic nature, excellent physical and chemical stability coupled with low cost [2-4]. Additionally,
3
due to the good photoelectric conversion properties and rapid photoresponse [5], the utilization of
4
ZnO for photocatalytic degradation of organic pollutants in water has acquired more attention [6-8].
5
Nevertheless, fast recombination of photogenerated electron-hole pairs in ZnO greatly reduces its
6
quantum efficiency and seriously limits its photocatalytic activity [9].
us
cr
ip t
1
Up to now, various methods have been proposed to enhance the photocatalytic activity of ZnO
8
[10], including metal ion doping, surface modification by noble metals, and coupling with other
9
semiconductors or other complex oxides [11-14]. As a major compound in the rare earth family,
10
cerium oxide has been applied as a useful ultraviolet absorbent [15], and it can be suitable candidate
11
for doping or composite oxide. Besides, it has been reported that Ce4+ can act as electron trapping
12
sites to suppress the recombination of electron-hole pairs and then to increase photocatalytic activity
13
[16]. Li and coworkers have synthesized CeO2/ZnO nanofibers possess a higher photocatalytic
14
activity than pure CeO2 or ZnO [17]. Furthermore, intrinsic defects (oxygen vacancy, interstitial zinc
15
and so on) in ZnO play important roles in some fundamental properties such as photoluminescence,
16
electrical conductivity and others [18, 19]. Therefore, the control of oxygen vacancy in ZnO is also a
17
key factor for adjusting optical properties. Uekawa and coworkers reported the preparation of the
18
zinc peroxide nanoparticles and obtained ZnO nanoparticles by decomposition of ZnO2 [20], and the
19
relationship between the oxygen vacancy and the decomposition condition was studied as well [21].
20
Zeng group synthesized the ZnO nanostructures by a facile method to control the concentration of
21
oxygen vacancies, and it was found that the defect states inside ZnO can be controlled and have great
22
impact on its optical properties [2]. Though there have been many studies focus on these two aspects,
Ac ce p
te
d
M
an
7
2
Page 2 of 22
1
the study of both have not been reported yet. In this article, ZnO and CeO2/ZnO composite oxides have been synthesized by a two-step
3
precipitation method. The photocatalytic activities of prepared catalysts were evaluated by the
4
photodecolorization of methylene blue (MB) under UV irradiation. The photocatalytic performance
5
of 1%CeO2/ZnO was further investigated in the presence of different additives, and the
6
corresponding mechanism was also discussed.
7
2. Experimental Section
8
2.1. Fabrication of CeO2/ZnO composite oxide
an
us
cr
ip t
2
All the chemicals were of analytical grade and used without further purification. CeO2/ZnO
10
composite oxides were prepared by a co-precipitation method, which was modified according to the
11
literature procedure reported by Zeng et al [2]. 2.98 g Zn(NO3)2·6H2O was dissolved in 60 mL
12
ethanol at 50 ℃ for 30 min (Solution A), then appropriate amount of ammonia (molar ratio of
13
ammonia and Zn2+ should be more than 4:1) was dropped into the solution until it changed from
14
turbid to transparent. A certain mole ratio (0.05, 0.025, 0.01, 0.005) of Ce(NO3)3·6H2O was dissolved
15
in 10 mL deionized water, then 0.5 mL ammonia was added and stirred for 20 min (Solution B).
16
Solution B was added in solution A by inches and the resulting solution was stirred for another 15
17
min under room temperature. After that, 20 mL H2O2 was added into the solution and it became
18
milky immediately. The yellow product was separated from the solution by a centrifuge at 4000 rpm
19
for 5 min after vigorous stirring for 4 h. The precipitate was washed with ethanol three times, and
20
then dried at 60 ℃ for 12 h. The as-obtained powder was calcinated in air for 2 h at 500 ℃.
21
2.2. Analytical methods
22
Ac ce p
te
d
M
9
X-ray powder diffraction (XRD) measurements were carried out using a XD-3 diffractometer 3
Page 3 of 22
(Beijing Purkinje General Instrument Co., China). The tube voltage was 35 kV, and the current was
2
20 mA. The XRD patterns were taken in the 2θ range of 5~80° at a scan speed of 8°/min. The crystal
3
morphology was characterized by a high resolution transmission electron microscope (HRTEM)
4
(JEM-200CX, 200kV). Diffuse reflectance spectra (DRS) were recorded using a Shimadzu UV-2600.
5
Raman spectra were measured on LABRAM 800 with laser excitation of 514 nm. The chemical
6
states of catalysts were characterized by X-ray photoelectron spectra (XPS) in a Thermo-VG
7
Scientific Escalab 250 spectrometer with monochromatized Al Kα excitation.
8
2.3. Photocatalytic experiments
an
us
cr
ip t
1
The photocatalytic activity of ZnO and CeO2/ZnO catalysts was measured by monitoring the
10
degradation of methylene blue (MB) solution under UV irradiation in a photocatalytic reaction
11
system, using a 500 W high-pressure mercury lamp as light source. In a typical experiment, 50 mL
12
aqueous solution of MB (20 mg/L) was placed in a quartz tube, and then 50 mg photocatalysts were
13
added. In prior to illumination, the solution was stirred in dark for 30 min in order to attain the
14
adsorption/desorption equilibrium between the dye and the catalysts. During the illumination, 5 mL
15
solution was withdrawn at an interval of 20 min and centrifuged at 4000 rpm for 5 min to remove the
16
catalysts before analysis. The concentration of remnant dye was measured by a UV-vis
17
spectrophotometer at the wavelength of 664 nm corresponding to maximum adsorption wavelength
18
of MB. The decolorization (%) can be calculated as follows:
Ac ce p
te
d
M
9
Decolorization(%) =
19
C × 100(%) C0
20
Where C0 is the initial concentration of dye and C is the dye concentration after degradation.
21
3. Results and Discussion
22
3.1. Characterization of the catalysts 4
Page 4 of 22
1
(Fig. 1)
Fig. 1 shows the X-ray diffraction patterns of the pure ZnO and CeO2/ZnO with different
3
composite ratio. The XRD patterns of the pure ZnO exhibited the typical hexagonal wurtzite
4
structures (JCPDS No. 36-1451) [22]. The strong peaks at 2θ = 31.7°, 34.4° and 36.2° can be
5
attributed to (100), (002), (101) lattice planes, respectively. The sharp diffraction peaks in the XRD
6
patterns indicated that the synthesized catalysts were well crystallized [5]. The peak located at 2θ =
7
28.6 was related to the (111) plane of CeO2, and the intensity of this peak was enhanced as the
8
composite ratio increased while the intensity of other peaks were weakened. Such a result indicated
9
that CeO2 deteriorated the crystallinity of ZnO.
an
us
cr
ip t
2
(Fig. 2)
M
10
To obtain more details about the structures and compositions of the catalysts, TEM and HRTEM
12
measurements were carried out. Fig. 2(a) and (b) displays the TEM images and HRTEM images of
13
ZnO and 1%CeO2/ZnO catalysts, respectively. From Fig. 2(a), we can find that the synthesized ZnO
14
particles were not of uniform size and vary from 50 to 100 nm. And the lattice spacing is about 0.24
15
nm, corresponding to the (101) planes of wurtzite ZnO. As for 1%CeO2/ZnO catalyst, there are small
16
particles of approximately 10 nm size scattered around the ZnO particles as shown in Fig. 2(b). The
17
lattice spacing in small particles is about 0.30 nm, it can be ascribed to the (111) planes of CeO2.
18
Thus the small particles might be CeO2 which were formed during the synthesis procedure. In
19
addition, the lattice spacing of ZnO was increased to 0.27 nm. It indicated that except the particles
20
dispersed in the surrounding of ZnO, there might be a part of Ce4+ would substitute into the Zn2+
21
sites or interstitial in ZnO lattice. This was consistent with the XRD result.
22
Ac ce p
te
d
11
(Fig. 3(a) and (b)) 5
Page 5 of 22
The optical properties of the catalysts were probed by UV-visible diffuse reflectance
2
spectroscopy. From Fig. 3(a), we can find that ZnO and CeO2/ZnO only responds to ultraviolet light,
3
and all the reflectance spectra showed a similar shape. It can be seen from Fig. 3(a) that the optical
4
absorption of CeO2/ZnO catalysts in the UV region was enhanced as the composite ratio increased.
5
The bandgap energy was deduced by extrapolating the linear region of the plot to intersect the photo
6
energy axis [2], as depicted in Fig. 3(b). We find that the calculated energy gap for ZnO and
7
CeO2/ZnO (from 0.5% to 5%) is about 3.15, 3.14, 3.14, 3.15 and 3.16 eV, respectively. It has been
8
reported that the bandgap narrowing is closely related to oxygen vacancy concentration [2, 5].
9
Therefore, the observed phenomenon can be attributed to the increase of oxygen vacancy
cr
us
an
concentration.
M
10
ip t
1
(Fig. 4)
11
Raman spectra of ZnO and CeO2/ZnO catalysts with different composite ratio are shown in Fig.
13
4. The peak (A) at 434 cm-1 was assigned to E2 (high) mode of the Raman active mode, a
14
characteristic peak for the wurtzite hexagonal phase of ZnO [23, 24]. And there was a new peak (B)
15
centered at about 460 cm-1, which was attributed to the F2g vibrational mode of CeO2 [25, 26].
16
Meanwhile, the intensity of peak B was enhanced as the composite ratio of CeO2 increased. As
17
reported in the previous literature [27], a vibration mode at about 580 cm-1 (C) could be assigned to
18
the contribution of oxygen vacancies (E1-LO), which resulted from the decomposition of the
19
peroxide. Simultaneously, peak C could also be ascribed to the vacancy-interstitial (Frenkel-type)
20
oxygen defects in CeO2 [19]. It was reported that the relative intensity of peak A and C can be used
21
to characterize the concentration of oxygen vacancy [2, 8]. In this article, the relative intensity of
22
peak A, B and peak C was used as an indicator of the oxygen vacancy concentration, since the
Ac ce p
te
d
12
6
Page 6 of 22
intensity of peak C was associated with both ZnO and CeO2. It can be concluded from the spectra
2
that pure ZnO possess similar oxygen vacancy concentration in comparison to 1%CeO2/ZnO and
3
0.5%CeO2/ZnO, while the other two catalysts possess a lower concentration. Such a result was
4
revealed by XPS later.
ip t
1
(Fig. 5)
6
To gain more insight into the oxygen vacancies in the ZnO and CeO2/ZnO catalysts, the
7
chemical states of O were investigated by XPS, and the XPS spectra for O1s are shown in Fig. 5. For
8
ZnO, there were two components of the observed peak which are attributed to two kinds of chemical
9
states. The peak located at 530.0 eV was attributed to the crystal lattice oxygen in ZnO, and the peak
10
located at 531.6 eV was associated with chemical adsorbed oxygen on the catalyst surface [28, 29].
11
And for CeO2/ZnO catalysts, there was a new peak centered at 528.7 eV which was attributed to the
12
crystal lattive oxygen in CeO2 [30]. The coexistence of two crystal lattice oxygen demonstrated that
13
CeO2 coexisted with ZnO. It is believed that the intensity of adsorbed oxygen peak is connected to
14
the variation in the concentration of oxygen vacancies [2, 5]. Therefore, changes in the intensity of
15
this component may be related to the concentration of oxygen vacancy. The area ratio of adsorbed
16
oxygen peak to crystal lattice oxygen peak was calculated and used to evaluate the variation of
17
oxygen vacancy. As show in Table 1, the ratio increased firstly and then decreased as the composite
18
ratio of CeO2 increased. It suggested that the oxygen vacancies in the CeO2/ZnO composite oxides
19
were reduced as the composite ratio increased.
Ac ce p
te
d
M
an
us
cr
5
(Table 1)
20 21 22
3.2. Evaluation of photocatalytic activity (Fig. 6 (a) and (b)) 7
Page 7 of 22
The photocatalytic activities of pure ZnO and CeO2/ZnO composite oxides were evaluated by
2
the degradation of MB (20mg/L) under UV irradiation. The results are shown in Fig. 6(a). In order to
3
eliminate the affect of adsorption in experiment, the adsorption ability of the catalysts for MB was
4
characterized in dark. After 30 min stirred in dark, the adsorption/desorption equilibration was
5
almost established. It can be observed from the spectra that there was no significant change in MB
6
concentration when the reaction has been carried out without photocatalyst after 80 min. Thus the
7
degradation of dye due to direct photolysis could be neglected.
us
cr
ip t
1
For comparison, P25 TiO2 was used as a reference. It can be seen that the P25 TiO2 exhibited a
9
slightly higher adsorptive performance than the as-prepared samples. Nevertheless, when UV light
10
was turned on to irradiate the reaction system, P25 TiO2 showed inferior degradation performance
11
than the CeO2/ZnO composite oxide in the earlier time. It indicated that the photocatalytic
12
degradation of MB was accelerated in the presence of CeO2/ZnO catalysts. All CeO2/ZnO
13
photocatalysts exhibited higher photocatalytic efficiency than pure ZnO. Obviously, the introduction
14
of CeO2 resulted in improved photodegradation performances. And the 1%CeO2/ZnO photocatalyst
15
exhibited the highest degrading ability of MB dye over all as-prepared catalysts. Fig. 6(b) shows the
16
UV-vis spectra taken over performing time in the process of MB decolorization by 1%CeO2/ZnO
17
catalyst. It could be found that absorption maximum wavelength blue-shifts gradually from 664 nm
18
to 612 nm during the UV light irradiation, indicating that the degradation of MB in this work was
19
ascribed to N-demethylated process, which had been reported by Hidaka et al [31].
20
3.3. Photocatalytic decoloration mechanism
Ac ce p
te
d
M
an
8
21
(Fig. 7)
22
It is well known that Ce4+ ions can act as the trapping site to suppress the recombination of 8
Page 8 of 22
photogenerated electrons and holes [32], so that the number of photogenerated electron-hole pairs
2
will increase and ultimately accelerate the photodegradation (Eq. (2)). The oxygen vacancy defect in
3
the photocatalysts can act as electron donors to form charged oxygen vacancy and trap the
4
photogenerated holes [33] (Eq. (3)). And the reaction of charged oxygen vacancy with hydroxyl ion
5
(OH-) will generate the ·OH group (Eq. (4)). Obviously, the photocatalytic reaction was boosted by
6
the synergistic effect of CeO2 and oxygen vacancy. The possible photocatalytic reaction mechanism
7
is proposed as follows: ZnO + hν → e- + h+
9
Ce4+ + e- → Ce3+
10
Vo+ + h+ → Vo2+
11
Vo2+ + OH- → Vo+ + ·OH
(1)
M
an
8
us
cr
ip t
1
(2)
(3)
(4)
To further understand the role ·OH group played, the degradation experiments of MB dye over
13
1%CeO2/ZnO catalyst were carried out by adding different additives. Different additives of
14
2-propanol (IPA, ·OH scavenger), methanol (ME, h+ scavenger), benzoquinone (BQ, ·O2- scavenger)
15
and ammonium hydroxide (AH, OH- source) were introduced. As shown in Fig. 7, the phtotocatalytic
16
degradation efficiecncies of the catalyst significantly decrease in the prensence of IPA and ME, while
17
the addition of BQ shows less influence in the degradation process. On the contrary, it shows
18
improved photocatalytic activity by adding AH. ·OH group was trapped by IPA, while holes were
19
trapped by ME and indirectly affected the formation of ·OH group. The introduction of AH increased
20
the OH- concentration in solution and promoted the generation of ·OH group. These results indicated
21
that hydroxyl radicals are the predominant active species, while ·O2- does not play a major role in
22
degradation of dye.
Ac ce p
te
d
12
9
Page 9 of 22
1
4. Conclusion CeO2/ZnO composite oxides with enhanced photocatalytic activities were successfully prepared.
3
The XRD, TEM characterizations confirmed that the prepared catalysts were highly crystalline.
4
From the XPS and Raman results, it can be concluded that the synergistic effect of CeO2 and oxygen
5
vacancy concentration lead to the enhancement of photocatalytic activity. As a result, CeO2/ZnO
6
composite oxides exhibited enhanced photocatalytic activities under UV irradiation than pure ZnO
7
did, and 1%CeO2/ZnO catalyst showed the highest photocatalytic activity. The corresponding
8
mechanism was proposed to explain the higher photocatalytic activities of the catalysts, and hydroxyl
9
radicals were confirmed to be the dominant active species in the photocatalytic process. The study
10
demonstrated that the CeO2/ZnO composite oxides may be an ideal system for practical application
11
in environmental purification.
12
Acknowledgements
te
d
M
an
us
cr
ip t
2
This work was financially supported by the Assembly Foundation of The Industry and
14
Information Ministry of the People's Republic of China 2012 (543), the National Natural Science
15
Foundation of China (51408309), Science and Technology Support Program of Jiangsu Province
16
(BE2014713), Natural Science Foundation of Jiangsu Province (BK20140777), Scientific Research
17
Project of Environmental Protection Department of Jiangsu Province (2013003), Industry-Academia
18
Cooperation Innovation Fund Projects of Jiangsu Province (BY2014004-10), Science and technology
19
project of Nanjing (201306012).
20
References
21
[1] C.J. Chang, C.Y. Lin, M.H. Hsu, Enhanced photocatalytic activity of Ce-doped ZnO nanorods under UV and
22
visible light, Journal of the Taiwan Institute of Chemical Engineers, 45 (2014) 1954-1963.
Ac ce p
13
10
Page 10 of 22
[2] X. Li, J. Song, Y. Liu, H. Zeng, Controlling oxygen vacancies and properties of ZnO, Current Applied Physics,
2
14 (2014) 521-527.
3
[3] A. Chelouche, T. Touam, D. Djouadi, A. Aksas, Synthesis and characterizations of new morphological ZnO and
4
Ce-doped ZnO powders by sol–gel process, Optik - International Journal for Light and Electron Optics, 125 (2014)
5
5626-5629.
6
[4] M. Faisal, A.A. Ismail, A.A. Ibrahim, H. Bouzid, S.A. Al-Sayari, Highly efficient photocatalyst based on Ce
7
doped ZnO nanorods: Controllable synthesis and enhanced photocatalytic activity, Chemical Engineering Journal,
8
229 (2013) 225-233.
9
[5] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai, Oxygen Vacancy Induced Band-Gap
10
Narrowing and Enhanced Visible Light Photocatalytic Activity of ZnO, ACS Applied Materials & Interfaces, 4
11
(2012) 4024-4030.
12
[6] C. Wu, H. Guo, S. Cui, H. Li, F. Li, Influence of Ce doping on structure, morphology, and photocatalytic
13
activity of three-dimensional ZnO superstructures synthesized via coprecipitation and roasting processes,
14
Proceedings of the Institution of Mechanical Engineers, Part N: Journal of Nanoengineering and Nanosystems,
15
(2014).
16
[7] A. Di Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental
17
remediation, Journal of Hazardous Materials, 211–212 (2012) 3-29.
18
[8] J. Becker, K.R. Raghupathi, J. St. Pierre, D. Zhao, R.T. Koodali, Tuning of the Crystallite and Particle Sizes of
19
ZnO Nanocrystalline Materials in Solvothermal Synthesis and Their Photocatalytic Activity for Dye Degradation,
20
The Journal of Physical Chemistry C, 115 (2011) 13844-13850.
21
[9] T. Xu, L. Zhang, H. Cheng, Y. Zhu, Significantly enhanced photocatalytic performance of ZnO via graphene
22
hybridization and the mechanism study, Applied Catalysis B: Environmental, 101 (2011) 382-387.
Ac ce p
te
d
M
an
us
cr
ip t
1
11
Page 11 of 22
[10] D. Zhang, X. Pu, H. Li, Y.M. Yu, J.J. Shim, P. Cai, S.I. Kim, H.J. Seo, Microwave-assisted combustion
2
synthesis of Ag/ZnO nanocomposites and their photocatalytic activities under ultraviolet and visible-light
3
irradiation, Materials Research Bulletin, 61 (2015) 321-325.
4
[11] N. Mary Jacob, G. Madras, N. Kottam, T. Thomas, Multivalent Cu-Doped ZnO Nanoparticles with Full Solar
5
Spectrum Absorbance and Enhanced Photoactivity, Industrial & Engineering Chemistry Research, 53 (2014)
6
5895-5904.
7
[12] Priyanka, V.C. Srivastava, Photocatalytic Oxidation of Dye Bearing Wastewater by Iron Doped Zinc Oxide,
8
Industrial & Engineering Chemistry Research, 52 (2013) 17790-17799.
9
[13] D. Jassby, J. Farner Budarz, M. Wiesner, Impact of Aggregate Size and Structure on the Photocatalytic
an
us
cr
ip t
1
Properties of TiO2 and ZnO Nanoparticles, Environmental Science & Technology, 46 (2012) 6934-6941.
11
[14] C. Peverari, A.M. Pires, R.R. Gonçalves, O.A. Serra, Synthesis, structural and morphological characterization
12
of CeO2-ZnO nanosized powder systems from Pechini's method, Eclética Química, 30 (2005) 59-64.
13
[15] S. Yabe, T. Sato, Cerium oxide for sunscreen cosmetics, Journal of Solid State Chemistry, 171
14
[16] W. Zhang, Z. Zhong, Y. Wang, R. Xu, Doped Solid Solution: (Zn0.95Cu0.05)1−xCdxS Nanocrystals with High
15
Activity for H2 Evolution from Aqueous Solutions under Visible Light, The Journal of Physical Chemistry C, 112
16
(2008) 17635-17642.
17
[17] C. Li, R. Chen, X. Zhang, S. Shu, J. Xiong, Y. Zheng, W. Dong, Electrospinning of CeO2–ZnO composite
18
nanofibers and their photocatalytic property, Materials Letters, 65 (2011) 1327-1330.
19
[18] D.C. Look, J.W. Hemsky, J.R. Sizelove, Residual Native Shallow Donor in ZnO, Physical Review Letters, 82
20
(1999) 2552-2555.
21
[19] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Correlation between photoluminescence
22
and oxygen vacancies in ZnO phosphors, Applied Physics Letters, 68 (1996) 403-405.
7-11.
Ac ce p
te
d
M
10
12
Page 12 of 22
[20] N. Uekawa, J. Kajiwara, N. Mochizuki, K. Kakegawa, Y. Sasaki, Synthesis of ZnO Nanoparticles by
2
Decomposition of Zinc Peroxide, Chemistry Letters, 30 (2001) 606-607.
3
[21] N. Uekawa, N. Mochizuki, J. Kajiwara, F. Mori, Y.J. Wu, K. Kakegawa, Nonstoichiometric properties of zinc
4
oxide nanoparticles prepared by decomposition of zinc peroxide, Physical Chemistry Chemical Physics, 5 (2003)
5
929-934.
6
[22] D. Jung, Syntheses and characterizations of transition metal-doped ZnO, Solid State Sciences, 12 (2010)
7
466-470.
8
[23] R. Cuscó, E. Alarcón-Lladó, J. Ibáñez, L. Artús, J. Jiménez, B. Wang, M.J. Callahan, Temperature dependence
9
of Raman scattering in ZnO, Physical Review B, 75 (2007) 165202.
an
us
cr
ip t
1
[24] M.E. Koleva, N.N. Nedyalkov, P.A. Atanasov, N. Fukata, M. Dutta, Optical properties of Ag-ZnO
11
nanostructures, in, 2015, pp. 94470E-94470E-94477.
12
[25] S.Y. Wang, N. Li, L.F. Luo, W.X. Huang, Z.Y. Pu, Y.J. Wang, G.S. Hu, M.F. Luo, J.Q. Lu, Probing different
13
effects of surface MOy and Mn+ species (M = Cu, Ni, Co, Fe) for xMOy/Ce0.9M0.1−xO2−δ catalysts in CO oxidation,
14
Applied Catalysis B: Environmental, 144 (2014) 325-332.
15
[26] W.H. Weber, K.C. Hass, J.R. McBride, Raman study of CeO2 Second-order scattering, lattice dynamics, and
16
particle-size effects, Physical Review B, 48 (1993) 178-185.
17
[27] M. Rajalakshmi, A.K. Arora, B.S. Bendre, S. Mahamuni, Optical phonon confinement in zinc oxide
18
nanoparticles, Journal of Applied Physics, 87 (2000) 2445-2448.
19
[28] H. Bai, Z. Liu, D.D. Sun, Hierarchical ZnO/Cu "corn-like" materials with high photodegradation and
20
antibacterial capability under visible light, Physical Chemistry Chemical Physics, 13 (2011) 6205-6210.
21
[29] S.M. Park, T. Ikegami, K. Ebihara, Effects of substrate temperature on the properties of Ga-doped ZnO by
22
pulsed laser deposition, Thin Solid Films, 513 (2006) 90-94.
Ac ce p
te
d
M
10
13
Page 13 of 22
[30] X. Yao, Q. Yu, Z. Ji, Y. Lv, Y. Cao, C. Tang, F. Gao, L. Dong, Y. Chen, A comparative study of different doped
2
metal cations on the reduction, adsorption and activity of CuO/Ce0.67M0.33O2 (M = Zr4+, Sn4+, Ti4+) catalysts for NO
3
+ CO reaction, Applied Catalysis B: Environmental, 130–131 (2013) 293-304.
4
[31] T. Zhang, T. Oyama, A. Aoshima, H. Hidaka, J. Zhao, Photooxidative N-demethylation of methylene blue in
5
aqueous TiO2 dispersions under UV irradiation, Journal of Photochemistry and Photobiology A: Chemistry, 140
6
(2001) 163-172.
7
[32] M. Rezaei, A. Habibi-Yangjeh, Simple and large scale refluxing method for preparation of Ce-doped ZnO
8
nanostructures as highly efficient photocatalyst, Applied Surface Science, 265 (2013) 591-596.
9
[33] C. Wu, L. Shen, H. Yu, Q. Huang, Y.C. Zhang, Synthesis of Sn-doped ZnO nanorods and their photocatalytic
cr us
an
properties, Materials Research Bulletin, 46 (2011) 1107-1112.
M
10
ip t
1
te Ac ce p
12
d
11
14
Page 14 of 22
1 2
ip t
3 4
cr
5
us
6
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Ac ce p
8
te
d
M
an
7
Fig. 1 XRD patterns of pure ZnO and CeO2/ZnO catalysts of different composite ratio
23 15
Page 15 of 22
1 2
ip t
3 4
7 8 9 10
Ac ce p
6
te
d
M
an
us
cr
5
Fig. 2 TEM images and the corresponding HRTEM images of (a) pure ZnO; (b) 1%CeO2/ZnO
11 12 13 14 16
Page 16 of 22
1 2
ip t
3 4
cr
5
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Fig. 3 UV-visible diffuse reflectance spectra (a) and the energy band gap (b) of pure ZnO and CeO2/ZnO catalysts
te
8
Ac ce p
7
d
M
an
us
6
26 17
Page 17 of 22
1 2
ip t
3 4
8 9 10 11 12 13
Fig. 4 Room temperature Raman spectra of ZnO and CeO2/ZnO catalysts
te
7
Ac ce p
6
d
M
an
us
cr
5
14 15 16 17 18
Page 18 of 22
1 2
ip t
3 4
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Fig. 5 O1s XPS spectroscopic spectra of different catalysts
te
7
Ac ce p
6
d
M
an
us
cr
5
26 19
Page 19 of 22
1 2
ip t
3
an
us
cr
4
M
5
Fig. 6 (a) Time-course variation of C/C0 of MB under UV irradiation over various catalysts and
7
(b) The UV-vis spectra of MB solution by 1%CeO2/ZnO in different performing time
10 11 12 13 14
te
9
Ac ce p
8
d
6
15 16 17 18 20
Page 20 of 22
1
an
us
cr
ip t
2
3
Fig. 7 Photodecolorization behavior of MB over 1%CeO2/ZnO composite oxide with different additives under UV
M
4 5
irradiation
9 10 11
ZnO
0.5%CeO2/ZnO
Lattice Oxygen Adsorbed Oxygen
1 0.35
1 0.40
Area Ratio 1%CeO2/ZnO 2.5%CeO2/ZnO
te
Chemical States of O
1 0.36
1 0.29
5%CeO2/ZnO 1 0.21
Ac ce p
7 8
Table 1 Details of the XPS peak information.
d
6
Notes: The area ratio of lattice oxygen was deemed to be 1.
Graphical Abstract
21
Page 21 of 22
ip t cr us an
1
Highlights
M
2 3
The CeO2/ZnO composite oxide is synthesized with peroxide as a precursor.
5
The excellent photocatalytic activity is due to the synergistic effect of CeO2 and oxygen vacancy.
6
The hydroxyl radicals are confirmed to be the dominant active species.
te
Ac ce p
7
d
4
22
Page 22 of 22
S0169-4332(16)30141-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.280 APSUSC 32512
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-8-2015 1-11-2015 31-1-2016
Please cite this article as:
Utilizing peroxide as precursor for the synthesis of CeO2/ZnO composite oxide
2
with enhanced photocatalytic activity
3
Zijian Lv, Qin Zhong*, Man Ou
4
School of Chemical Engineering, Nanjing University of Science and Technology,
5
Nanjing, Jiangsu 210094, PR China
6
Nanjing AIREP Environmental Protection Technology Co., Ltd, Nanjing, Jiangsu
7
210091, PR China
8
Corresponding author: Qin Zhong. Email: [email protected].
9
Tel/Fax number: +86 2584315517
cr
us
Abstract
an
10
ip t
1
A facile synthesis method of CeO2/ZnO composite oxides with higher oxygen vacancy
12
concentration was developed by a two-step precipitation method, in which peroxide was used as
13
precursor. The photocatalytic activities of the catalysts under UV irradiation were studied in
14
degradation of methylene blue (MB). All CeO2/ZnO photocatalysts exhibited higher photocatalytic
15
performance than pure ZnO, and 1%CeO2/ZnO showed highest photocatalytic activity among the
16
prepared catalysts. It was confirmed that the synergistic effect of CeO2 and oxygen vacancy caused
17
the improved photocatalytic activity. Furthermore, the mechanism was investigated by introducing
18
different additives, and it was found that the hydroxyl radicals played a crucial role in degradation
19
process.
20
Keywords: CeO2/ZnO composite oxide; oxygen vacancy; photocatalyst; methylene blue
21
1. Introduction
Ac ce p
te
d
M
11
22
Semiconductor metal oxide plays an important role on the effective degradation of the organic
23
pollutants in water and has been widely investigated [1]. Zinc oxide is one of the most promising
24
third-generation semiconductors due to its wide direct bandgap of 3.37 eV and a large exciton 1
Page 1 of 22
binding energy of 60 meV at room temperature [2]. It has been extensively studied owing to its
2
non-toxic nature, excellent physical and chemical stability coupled with low cost [2-4]. Additionally,
3
due to the good photoelectric conversion properties and rapid photoresponse [5], the utilization of
4
ZnO for photocatalytic degradation of organic pollutants in water has acquired more attention [6-8].
5
Nevertheless, fast recombination of photogenerated electron-hole pairs in ZnO greatly reduces its
6
quantum efficiency and seriously limits its photocatalytic activity [9].
us
cr
ip t
1
Up to now, various methods have been proposed to enhance the photocatalytic activity of ZnO
8
[10], including metal ion doping, surface modification by noble metals, and coupling with other
9
semiconductors or other complex oxides [11-14]. As a major compound in the rare earth family,
10
cerium oxide has been applied as a useful ultraviolet absorbent [15], and it can be suitable candidate
11
for doping or composite oxide. Besides, it has been reported that Ce4+ can act as electron trapping
12
sites to suppress the recombination of electron-hole pairs and then to increase photocatalytic activity
13
[16]. Li and coworkers have synthesized CeO2/ZnO nanofibers possess a higher photocatalytic
14
activity than pure CeO2 or ZnO [17]. Furthermore, intrinsic defects (oxygen vacancy, interstitial zinc
15
and so on) in ZnO play important roles in some fundamental properties such as photoluminescence,
16
electrical conductivity and others [18, 19]. Therefore, the control of oxygen vacancy in ZnO is also a
17
key factor for adjusting optical properties. Uekawa and coworkers reported the preparation of the
18
zinc peroxide nanoparticles and obtained ZnO nanoparticles by decomposition of ZnO2 [20], and the
19
relationship between the oxygen vacancy and the decomposition condition was studied as well [21].
20
Zeng group synthesized the ZnO nanostructures by a facile method to control the concentration of
21
oxygen vacancies, and it was found that the defect states inside ZnO can be controlled and have great
22
impact on its optical properties [2]. Though there have been many studies focus on these two aspects,
Ac ce p
te
d
M
an
7
2
Page 2 of 22
1
the study of both have not been reported yet. In this article, ZnO and CeO2/ZnO composite oxides have been synthesized by a two-step
3
precipitation method. The photocatalytic activities of prepared catalysts were evaluated by the
4
photodecolorization of methylene blue (MB) under UV irradiation. The photocatalytic performance
5
of 1%CeO2/ZnO was further investigated in the presence of different additives, and the
6
corresponding mechanism was also discussed.
7
2. Experimental Section
8
2.1. Fabrication of CeO2/ZnO composite oxide
an
us
cr
ip t
2
All the chemicals were of analytical grade and used without further purification. CeO2/ZnO
10
composite oxides were prepared by a co-precipitation method, which was modified according to the
11
literature procedure reported by Zeng et al [2]. 2.98 g Zn(NO3)2·6H2O was dissolved in 60 mL
12
ethanol at 50 ℃ for 30 min (Solution A), then appropriate amount of ammonia (molar ratio of
13
ammonia and Zn2+ should be more than 4:1) was dropped into the solution until it changed from
14
turbid to transparent. A certain mole ratio (0.05, 0.025, 0.01, 0.005) of Ce(NO3)3·6H2O was dissolved
15
in 10 mL deionized water, then 0.5 mL ammonia was added and stirred for 20 min (Solution B).
16
Solution B was added in solution A by inches and the resulting solution was stirred for another 15
17
min under room temperature. After that, 20 mL H2O2 was added into the solution and it became
18
milky immediately. The yellow product was separated from the solution by a centrifuge at 4000 rpm
19
for 5 min after vigorous stirring for 4 h. The precipitate was washed with ethanol three times, and
20
then dried at 60 ℃ for 12 h. The as-obtained powder was calcinated in air for 2 h at 500 ℃.
21
2.2. Analytical methods
22
Ac ce p
te
d
M
9
X-ray powder diffraction (XRD) measurements were carried out using a XD-3 diffractometer 3
Page 3 of 22
(Beijing Purkinje General Instrument Co., China). The tube voltage was 35 kV, and the current was
2
20 mA. The XRD patterns were taken in the 2θ range of 5~80° at a scan speed of 8°/min. The crystal
3
morphology was characterized by a high resolution transmission electron microscope (HRTEM)
4
(JEM-200CX, 200kV). Diffuse reflectance spectra (DRS) were recorded using a Shimadzu UV-2600.
5
Raman spectra were measured on LABRAM 800 with laser excitation of 514 nm. The chemical
6
states of catalysts were characterized by X-ray photoelectron spectra (XPS) in a Thermo-VG
7
Scientific Escalab 250 spectrometer with monochromatized Al Kα excitation.
8
2.3. Photocatalytic experiments
an
us
cr
ip t
1
The photocatalytic activity of ZnO and CeO2/ZnO catalysts was measured by monitoring the
10
degradation of methylene blue (MB) solution under UV irradiation in a photocatalytic reaction
11
system, using a 500 W high-pressure mercury lamp as light source. In a typical experiment, 50 mL
12
aqueous solution of MB (20 mg/L) was placed in a quartz tube, and then 50 mg photocatalysts were
13
added. In prior to illumination, the solution was stirred in dark for 30 min in order to attain the
14
adsorption/desorption equilibrium between the dye and the catalysts. During the illumination, 5 mL
15
solution was withdrawn at an interval of 20 min and centrifuged at 4000 rpm for 5 min to remove the
16
catalysts before analysis. The concentration of remnant dye was measured by a UV-vis
17
spectrophotometer at the wavelength of 664 nm corresponding to maximum adsorption wavelength
18
of MB. The decolorization (%) can be calculated as follows:
Ac ce p
te
d
M
9
Decolorization(%) =
19
C × 100(%) C0
20
Where C0 is the initial concentration of dye and C is the dye concentration after degradation.
21
3. Results and Discussion
22
3.1. Characterization of the catalysts 4
Page 4 of 22
1
(Fig. 1)
Fig. 1 shows the X-ray diffraction patterns of the pure ZnO and CeO2/ZnO with different
3
composite ratio. The XRD patterns of the pure ZnO exhibited the typical hexagonal wurtzite
4
structures (JCPDS No. 36-1451) [22]. The strong peaks at 2θ = 31.7°, 34.4° and 36.2° can be
5
attributed to (100), (002), (101) lattice planes, respectively. The sharp diffraction peaks in the XRD
6
patterns indicated that the synthesized catalysts were well crystallized [5]. The peak located at 2θ =
7
28.6 was related to the (111) plane of CeO2, and the intensity of this peak was enhanced as the
8
composite ratio increased while the intensity of other peaks were weakened. Such a result indicated
9
that CeO2 deteriorated the crystallinity of ZnO.
an
us
cr
ip t
2
(Fig. 2)
M
10
To obtain more details about the structures and compositions of the catalysts, TEM and HRTEM
12
measurements were carried out. Fig. 2(a) and (b) displays the TEM images and HRTEM images of
13
ZnO and 1%CeO2/ZnO catalysts, respectively. From Fig. 2(a), we can find that the synthesized ZnO
14
particles were not of uniform size and vary from 50 to 100 nm. And the lattice spacing is about 0.24
15
nm, corresponding to the (101) planes of wurtzite ZnO. As for 1%CeO2/ZnO catalyst, there are small
16
particles of approximately 10 nm size scattered around the ZnO particles as shown in Fig. 2(b). The
17
lattice spacing in small particles is about 0.30 nm, it can be ascribed to the (111) planes of CeO2.
18
Thus the small particles might be CeO2 which were formed during the synthesis procedure. In
19
addition, the lattice spacing of ZnO was increased to 0.27 nm. It indicated that except the particles
20
dispersed in the surrounding of ZnO, there might be a part of Ce4+ would substitute into the Zn2+
21
sites or interstitial in ZnO lattice. This was consistent with the XRD result.
22
Ac ce p
te
d
11
(Fig. 3(a) and (b)) 5
Page 5 of 22
The optical properties of the catalysts were probed by UV-visible diffuse reflectance
2
spectroscopy. From Fig. 3(a), we can find that ZnO and CeO2/ZnO only responds to ultraviolet light,
3
and all the reflectance spectra showed a similar shape. It can be seen from Fig. 3(a) that the optical
4
absorption of CeO2/ZnO catalysts in the UV region was enhanced as the composite ratio increased.
5
The bandgap energy was deduced by extrapolating the linear region of the plot to intersect the photo
6
energy axis [2], as depicted in Fig. 3(b). We find that the calculated energy gap for ZnO and
7
CeO2/ZnO (from 0.5% to 5%) is about 3.15, 3.14, 3.14, 3.15 and 3.16 eV, respectively. It has been
8
reported that the bandgap narrowing is closely related to oxygen vacancy concentration [2, 5].
9
Therefore, the observed phenomenon can be attributed to the increase of oxygen vacancy
cr
us
an
concentration.
M
10
ip t
1
(Fig. 4)
11
Raman spectra of ZnO and CeO2/ZnO catalysts with different composite ratio are shown in Fig.
13
4. The peak (A) at 434 cm-1 was assigned to E2 (high) mode of the Raman active mode, a
14
characteristic peak for the wurtzite hexagonal phase of ZnO [23, 24]. And there was a new peak (B)
15
centered at about 460 cm-1, which was attributed to the F2g vibrational mode of CeO2 [25, 26].
16
Meanwhile, the intensity of peak B was enhanced as the composite ratio of CeO2 increased. As
17
reported in the previous literature [27], a vibration mode at about 580 cm-1 (C) could be assigned to
18
the contribution of oxygen vacancies (E1-LO), which resulted from the decomposition of the
19
peroxide. Simultaneously, peak C could also be ascribed to the vacancy-interstitial (Frenkel-type)
20
oxygen defects in CeO2 [19]. It was reported that the relative intensity of peak A and C can be used
21
to characterize the concentration of oxygen vacancy [2, 8]. In this article, the relative intensity of
22
peak A, B and peak C was used as an indicator of the oxygen vacancy concentration, since the
Ac ce p
te
d
12
6
Page 6 of 22
intensity of peak C was associated with both ZnO and CeO2. It can be concluded from the spectra
2
that pure ZnO possess similar oxygen vacancy concentration in comparison to 1%CeO2/ZnO and
3
0.5%CeO2/ZnO, while the other two catalysts possess a lower concentration. Such a result was
4
revealed by XPS later.
ip t
1
(Fig. 5)
6
To gain more insight into the oxygen vacancies in the ZnO and CeO2/ZnO catalysts, the
7
chemical states of O were investigated by XPS, and the XPS spectra for O1s are shown in Fig. 5. For
8
ZnO, there were two components of the observed peak which are attributed to two kinds of chemical
9
states. The peak located at 530.0 eV was attributed to the crystal lattice oxygen in ZnO, and the peak
10
located at 531.6 eV was associated with chemical adsorbed oxygen on the catalyst surface [28, 29].
11
And for CeO2/ZnO catalysts, there was a new peak centered at 528.7 eV which was attributed to the
12
crystal lattive oxygen in CeO2 [30]. The coexistence of two crystal lattice oxygen demonstrated that
13
CeO2 coexisted with ZnO. It is believed that the intensity of adsorbed oxygen peak is connected to
14
the variation in the concentration of oxygen vacancies [2, 5]. Therefore, changes in the intensity of
15
this component may be related to the concentration of oxygen vacancy. The area ratio of adsorbed
16
oxygen peak to crystal lattice oxygen peak was calculated and used to evaluate the variation of
17
oxygen vacancy. As show in Table 1, the ratio increased firstly and then decreased as the composite
18
ratio of CeO2 increased. It suggested that the oxygen vacancies in the CeO2/ZnO composite oxides
19
were reduced as the composite ratio increased.
Ac ce p
te
d
M
an
us
cr
5
(Table 1)
20 21 22
3.2. Evaluation of photocatalytic activity (Fig. 6 (a) and (b)) 7
Page 7 of 22
The photocatalytic activities of pure ZnO and CeO2/ZnO composite oxides were evaluated by
2
the degradation of MB (20mg/L) under UV irradiation. The results are shown in Fig. 6(a). In order to
3
eliminate the affect of adsorption in experiment, the adsorption ability of the catalysts for MB was
4
characterized in dark. After 30 min stirred in dark, the adsorption/desorption equilibration was
5
almost established. It can be observed from the spectra that there was no significant change in MB
6
concentration when the reaction has been carried out without photocatalyst after 80 min. Thus the
7
degradation of dye due to direct photolysis could be neglected.
us
cr
ip t
1
For comparison, P25 TiO2 was used as a reference. It can be seen that the P25 TiO2 exhibited a
9
slightly higher adsorptive performance than the as-prepared samples. Nevertheless, when UV light
10
was turned on to irradiate the reaction system, P25 TiO2 showed inferior degradation performance
11
than the CeO2/ZnO composite oxide in the earlier time. It indicated that the photocatalytic
12
degradation of MB was accelerated in the presence of CeO2/ZnO catalysts. All CeO2/ZnO
13
photocatalysts exhibited higher photocatalytic efficiency than pure ZnO. Obviously, the introduction
14
of CeO2 resulted in improved photodegradation performances. And the 1%CeO2/ZnO photocatalyst
15
exhibited the highest degrading ability of MB dye over all as-prepared catalysts. Fig. 6(b) shows the
16
UV-vis spectra taken over performing time in the process of MB decolorization by 1%CeO2/ZnO
17
catalyst. It could be found that absorption maximum wavelength blue-shifts gradually from 664 nm
18
to 612 nm during the UV light irradiation, indicating that the degradation of MB in this work was
19
ascribed to N-demethylated process, which had been reported by Hidaka et al [31].
20
3.3. Photocatalytic decoloration mechanism
Ac ce p
te
d
M
an
8
21
(Fig. 7)
22
It is well known that Ce4+ ions can act as the trapping site to suppress the recombination of 8
Page 8 of 22
photogenerated electrons and holes [32], so that the number of photogenerated electron-hole pairs
2
will increase and ultimately accelerate the photodegradation (Eq. (2)). The oxygen vacancy defect in
3
the photocatalysts can act as electron donors to form charged oxygen vacancy and trap the
4
photogenerated holes [33] (Eq. (3)). And the reaction of charged oxygen vacancy with hydroxyl ion
5
(OH-) will generate the ·OH group (Eq. (4)). Obviously, the photocatalytic reaction was boosted by
6
the synergistic effect of CeO2 and oxygen vacancy. The possible photocatalytic reaction mechanism
7
is proposed as follows: ZnO + hν → e- + h+
9
Ce4+ + e- → Ce3+
10
Vo+ + h+ → Vo2+
11
Vo2+ + OH- → Vo+ + ·OH
(1)
M
an
8
us
cr
ip t
1
(2)
(3)
(4)
To further understand the role ·OH group played, the degradation experiments of MB dye over
13
1%CeO2/ZnO catalyst were carried out by adding different additives. Different additives of
14
2-propanol (IPA, ·OH scavenger), methanol (ME, h+ scavenger), benzoquinone (BQ, ·O2- scavenger)
15
and ammonium hydroxide (AH, OH- source) were introduced. As shown in Fig. 7, the phtotocatalytic
16
degradation efficiecncies of the catalyst significantly decrease in the prensence of IPA and ME, while
17
the addition of BQ shows less influence in the degradation process. On the contrary, it shows
18
improved photocatalytic activity by adding AH. ·OH group was trapped by IPA, while holes were
19
trapped by ME and indirectly affected the formation of ·OH group. The introduction of AH increased
20
the OH- concentration in solution and promoted the generation of ·OH group. These results indicated
21
that hydroxyl radicals are the predominant active species, while ·O2- does not play a major role in
22
degradation of dye.
Ac ce p
te
d
12
9
Page 9 of 22
1
4. Conclusion CeO2/ZnO composite oxides with enhanced photocatalytic activities were successfully prepared.
3
The XRD, TEM characterizations confirmed that the prepared catalysts were highly crystalline.
4
From the XPS and Raman results, it can be concluded that the synergistic effect of CeO2 and oxygen
5
vacancy concentration lead to the enhancement of photocatalytic activity. As a result, CeO2/ZnO
6
composite oxides exhibited enhanced photocatalytic activities under UV irradiation than pure ZnO
7
did, and 1%CeO2/ZnO catalyst showed the highest photocatalytic activity. The corresponding
8
mechanism was proposed to explain the higher photocatalytic activities of the catalysts, and hydroxyl
9
radicals were confirmed to be the dominant active species in the photocatalytic process. The study
10
demonstrated that the CeO2/ZnO composite oxides may be an ideal system for practical application
11
in environmental purification.
12
Acknowledgements
te
d
M
an
us
cr
ip t
2
This work was financially supported by the Assembly Foundation of The Industry and
14
Information Ministry of the People's Republic of China 2012 (543), the National Natural Science
15
Foundation of China (51408309), Science and Technology Support Program of Jiangsu Province
16
(BE2014713), Natural Science Foundation of Jiangsu Province (BK20140777), Scientific Research
17
Project of Environmental Protection Department of Jiangsu Province (2013003), Industry-Academia
18
Cooperation Innovation Fund Projects of Jiangsu Province (BY2014004-10), Science and technology
19
project of Nanjing (201306012).
20
References
21
[1] C.J. Chang, C.Y. Lin, M.H. Hsu, Enhanced photocatalytic activity of Ce-doped ZnO nanorods under UV and
22
visible light, Journal of the Taiwan Institute of Chemical Engineers, 45 (2014) 1954-1963.
Ac ce p
13
10
Page 10 of 22
[2] X. Li, J. Song, Y. Liu, H. Zeng, Controlling oxygen vacancies and properties of ZnO, Current Applied Physics,
2
14 (2014) 521-527.
3
[3] A. Chelouche, T. Touam, D. Djouadi, A. Aksas, Synthesis and characterizations of new morphological ZnO and
4
Ce-doped ZnO powders by sol–gel process, Optik - International Journal for Light and Electron Optics, 125 (2014)
5
5626-5629.
6
[4] M. Faisal, A.A. Ismail, A.A. Ibrahim, H. Bouzid, S.A. Al-Sayari, Highly efficient photocatalyst based on Ce
7
doped ZnO nanorods: Controllable synthesis and enhanced photocatalytic activity, Chemical Engineering Journal,
8
229 (2013) 225-233.
9
[5] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai, Oxygen Vacancy Induced Band-Gap
10
Narrowing and Enhanced Visible Light Photocatalytic Activity of ZnO, ACS Applied Materials & Interfaces, 4
11
(2012) 4024-4030.
12
[6] C. Wu, H. Guo, S. Cui, H. Li, F. Li, Influence of Ce doping on structure, morphology, and photocatalytic
13
activity of three-dimensional ZnO superstructures synthesized via coprecipitation and roasting processes,
14
Proceedings of the Institution of Mechanical Engineers, Part N: Journal of Nanoengineering and Nanosystems,
15
(2014).
16
[7] A. Di Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental
17
remediation, Journal of Hazardous Materials, 211–212 (2012) 3-29.
18
[8] J. Becker, K.R. Raghupathi, J. St. Pierre, D. Zhao, R.T. Koodali, Tuning of the Crystallite and Particle Sizes of
19
ZnO Nanocrystalline Materials in Solvothermal Synthesis and Their Photocatalytic Activity for Dye Degradation,
20
The Journal of Physical Chemistry C, 115 (2011) 13844-13850.
21
[9] T. Xu, L. Zhang, H. Cheng, Y. Zhu, Significantly enhanced photocatalytic performance of ZnO via graphene
22
hybridization and the mechanism study, Applied Catalysis B: Environmental, 101 (2011) 382-387.
Ac ce p
te
d
M
an
us
cr
ip t
1
11
Page 11 of 22
[10] D. Zhang, X. Pu, H. Li, Y.M. Yu, J.J. Shim, P. Cai, S.I. Kim, H.J. Seo, Microwave-assisted combustion
2
synthesis of Ag/ZnO nanocomposites and their photocatalytic activities under ultraviolet and visible-light
3
irradiation, Materials Research Bulletin, 61 (2015) 321-325.
4
[11] N. Mary Jacob, G. Madras, N. Kottam, T. Thomas, Multivalent Cu-Doped ZnO Nanoparticles with Full Solar
5
Spectrum Absorbance and Enhanced Photoactivity, Industrial & Engineering Chemistry Research, 53 (2014)
6
5895-5904.
7
[12] Priyanka, V.C. Srivastava, Photocatalytic Oxidation of Dye Bearing Wastewater by Iron Doped Zinc Oxide,
8
Industrial & Engineering Chemistry Research, 52 (2013) 17790-17799.
9
[13] D. Jassby, J. Farner Budarz, M. Wiesner, Impact of Aggregate Size and Structure on the Photocatalytic
an
us
cr
ip t
1
Properties of TiO2 and ZnO Nanoparticles, Environmental Science & Technology, 46 (2012) 6934-6941.
11
[14] C. Peverari, A.M. Pires, R.R. Gonçalves, O.A. Serra, Synthesis, structural and morphological characterization
12
of CeO2-ZnO nanosized powder systems from Pechini's method, Eclética Química, 30 (2005) 59-64.
13
[15] S. Yabe, T. Sato, Cerium oxide for sunscreen cosmetics, Journal of Solid State Chemistry, 171
14
[16] W. Zhang, Z. Zhong, Y. Wang, R. Xu, Doped Solid Solution: (Zn0.95Cu0.05)1−xCdxS Nanocrystals with High
15
Activity for H2 Evolution from Aqueous Solutions under Visible Light, The Journal of Physical Chemistry C, 112
16
(2008) 17635-17642.
17
[17] C. Li, R. Chen, X. Zhang, S. Shu, J. Xiong, Y. Zheng, W. Dong, Electrospinning of CeO2–ZnO composite
18
nanofibers and their photocatalytic property, Materials Letters, 65 (2011) 1327-1330.
19
[18] D.C. Look, J.W. Hemsky, J.R. Sizelove, Residual Native Shallow Donor in ZnO, Physical Review Letters, 82
20
(1999) 2552-2555.
21
[19] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Correlation between photoluminescence
22
and oxygen vacancies in ZnO phosphors, Applied Physics Letters, 68 (1996) 403-405.
7-11.
Ac ce p
te
d
M
10
12
Page 12 of 22
[20] N. Uekawa, J. Kajiwara, N. Mochizuki, K. Kakegawa, Y. Sasaki, Synthesis of ZnO Nanoparticles by
2
Decomposition of Zinc Peroxide, Chemistry Letters, 30 (2001) 606-607.
3
[21] N. Uekawa, N. Mochizuki, J. Kajiwara, F. Mori, Y.J. Wu, K. Kakegawa, Nonstoichiometric properties of zinc
4
oxide nanoparticles prepared by decomposition of zinc peroxide, Physical Chemistry Chemical Physics, 5 (2003)
5
929-934.
6
[22] D. Jung, Syntheses and characterizations of transition metal-doped ZnO, Solid State Sciences, 12 (2010)
7
466-470.
8
[23] R. Cuscó, E. Alarcón-Lladó, J. Ibáñez, L. Artús, J. Jiménez, B. Wang, M.J. Callahan, Temperature dependence
9
of Raman scattering in ZnO, Physical Review B, 75 (2007) 165202.
an
us
cr
ip t
1
[24] M.E. Koleva, N.N. Nedyalkov, P.A. Atanasov, N. Fukata, M. Dutta, Optical properties of Ag-ZnO
11
nanostructures, in, 2015, pp. 94470E-94470E-94477.
12
[25] S.Y. Wang, N. Li, L.F. Luo, W.X. Huang, Z.Y. Pu, Y.J. Wang, G.S. Hu, M.F. Luo, J.Q. Lu, Probing different
13
effects of surface MOy and Mn+ species (M = Cu, Ni, Co, Fe) for xMOy/Ce0.9M0.1−xO2−δ catalysts in CO oxidation,
14
Applied Catalysis B: Environmental, 144 (2014) 325-332.
15
[26] W.H. Weber, K.C. Hass, J.R. McBride, Raman study of CeO2 Second-order scattering, lattice dynamics, and
16
particle-size effects, Physical Review B, 48 (1993) 178-185.
17
[27] M. Rajalakshmi, A.K. Arora, B.S. Bendre, S. Mahamuni, Optical phonon confinement in zinc oxide
18
nanoparticles, Journal of Applied Physics, 87 (2000) 2445-2448.
19
[28] H. Bai, Z. Liu, D.D. Sun, Hierarchical ZnO/Cu "corn-like" materials with high photodegradation and
20
antibacterial capability under visible light, Physical Chemistry Chemical Physics, 13 (2011) 6205-6210.
21
[29] S.M. Park, T. Ikegami, K. Ebihara, Effects of substrate temperature on the properties of Ga-doped ZnO by
22
pulsed laser deposition, Thin Solid Films, 513 (2006) 90-94.
Ac ce p
te
d
M
10
13
Page 13 of 22
[30] X. Yao, Q. Yu, Z. Ji, Y. Lv, Y. Cao, C. Tang, F. Gao, L. Dong, Y. Chen, A comparative study of different doped
2
metal cations on the reduction, adsorption and activity of CuO/Ce0.67M0.33O2 (M = Zr4+, Sn4+, Ti4+) catalysts for NO
3
+ CO reaction, Applied Catalysis B: Environmental, 130–131 (2013) 293-304.
4
[31] T. Zhang, T. Oyama, A. Aoshima, H. Hidaka, J. Zhao, Photooxidative N-demethylation of methylene blue in
5
aqueous TiO2 dispersions under UV irradiation, Journal of Photochemistry and Photobiology A: Chemistry, 140
6
(2001) 163-172.
7
[32] M. Rezaei, A. Habibi-Yangjeh, Simple and large scale refluxing method for preparation of Ce-doped ZnO
8
nanostructures as highly efficient photocatalyst, Applied Surface Science, 265 (2013) 591-596.
9
[33] C. Wu, L. Shen, H. Yu, Q. Huang, Y.C. Zhang, Synthesis of Sn-doped ZnO nanorods and their photocatalytic
cr us
an
properties, Materials Research Bulletin, 46 (2011) 1107-1112.
M
10
ip t
1
te Ac ce p
12
d
11
14
Page 14 of 22
1 2
ip t
3 4
cr
5
us
6
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Ac ce p
8
te
d
M
an
7
Fig. 1 XRD patterns of pure ZnO and CeO2/ZnO catalysts of different composite ratio
23 15
Page 15 of 22
1 2
ip t
3 4
7 8 9 10
Ac ce p
6
te
d
M
an
us
cr
5
Fig. 2 TEM images and the corresponding HRTEM images of (a) pure ZnO; (b) 1%CeO2/ZnO
11 12 13 14 16
Page 16 of 22
1 2
ip t
3 4
cr
5
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Fig. 3 UV-visible diffuse reflectance spectra (a) and the energy band gap (b) of pure ZnO and CeO2/ZnO catalysts
te
8
Ac ce p
7
d
M
an
us
6
26 17
Page 17 of 22
1 2
ip t
3 4
8 9 10 11 12 13
Fig. 4 Room temperature Raman spectra of ZnO and CeO2/ZnO catalysts
te
7
Ac ce p
6
d
M
an
us
cr
5
14 15 16 17 18
Page 18 of 22
1 2
ip t
3 4
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Fig. 5 O1s XPS spectroscopic spectra of different catalysts
te
7
Ac ce p
6
d
M
an
us
cr
5
26 19
Page 19 of 22
1 2
ip t
3
an
us
cr
4
M
5
Fig. 6 (a) Time-course variation of C/C0 of MB under UV irradiation over various catalysts and
7
(b) The UV-vis spectra of MB solution by 1%CeO2/ZnO in different performing time
10 11 12 13 14
te
9
Ac ce p
8
d
6
15 16 17 18 20
Page 20 of 22
1
an
us
cr
ip t
2
3
Fig. 7 Photodecolorization behavior of MB over 1%CeO2/ZnO composite oxide with different additives under UV
M
4 5
irradiation
9 10 11
ZnO
0.5%CeO2/ZnO
Lattice Oxygen Adsorbed Oxygen
1 0.35
1 0.40
Area Ratio 1%CeO2/ZnO 2.5%CeO2/ZnO
te
Chemical States of O
1 0.36
1 0.29
5%CeO2/ZnO 1 0.21
Ac ce p
7 8
Table 1 Details of the XPS peak information.
d
6
Notes: The area ratio of lattice oxygen was deemed to be 1.
Graphical Abstract
21
Page 21 of 22
ip t cr us an
1
Highlights
M
2 3
The CeO2/ZnO composite oxide is synthesized with peroxide as a precursor.
5
The excellent photocatalytic activity is due to the synergistic effect of CeO2 and oxygen vacancy.
6
The hydroxyl radicals are confirmed to be the dominant active species.
te
Ac ce p
7
d
4
22
Page 22 of 22