CeO2 composite photocatalyst heterostructure and photocatalytic degradation of methyl red

Journal Pre-proof Study on the construction of YMnO3 /CeO2 composite photocatalyst heterostructure and photocatalytic degradation of methyl red Yanping Wang, Heng Tian

PII:

S0030-4026(19)31422-6

DOI:

https://doi.org/10.1016/j.ijleo.2019.163524

Reference:

IJLEO 163524

To appear in:

Optik

Received Date:

1 July 2019

Accepted Date:

2 October 2019

Please cite this article as: Wang Y, Tian H, Study on the construction of YMnO3 /CeO2 composite photocatalyst heterostructure and photocatalytic degradation of methyl red, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163524

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Study on the construction of YMnO3/CeO2 composite photocatalyst heterostructure and photocatalytic degradation of methyl red

Yanping Wanga), Heng Tiana)

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a) Department of Chemical and Chemical Engineering, Hefei Normal University, Hefei, Anhui, China, 230601 * Corresponding author:

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Yanping Wang

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e-mail: [email protected]

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Abstract: The present investigation reports, the novel chemical synthesis of the CeO2, YMnO3 and YMnO3/CeO2 oxides using sonochemistry method and their

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physicochemical characterization. Phase structure and functional group analysis indicates that the CeO2, YMnO3 and YMnO3/CeO2 oxides has good crystallization with

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a polycrystalline structure and only a few NO3- impurities. Scanning electron microscope (SEM) reveals that a nanoscale ball shape with a size range of 30-100 nm consist of the CeO2, YMnO3 and YMnO3/CeO2 oxides. The optical absorption band gap of the CeO2, YMnO3 and YMnO3/CeO2 oxides was estimated to be 3.03, 1.03 and 2.51 eV, respectively. Photoluminescence, electrochemical and photocatalytic experiments indicates that the YMnO3/CeO2 oxides exhibits high photocatalytic activity due to the

strong absorption capacity, fluorescence quenching, low interfacial resistance, high photocurrent density and low charge recombination rate.

Key words: Sonochemistry method; Phase structure; Functional group; Photocatalytic activity; Fluorescence quenching

1. Introduction

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Semiconductor photocatalysts have been the subject of intensive research throughout the entire world for their exception physicochemical properties and widely applications in environmental purification, solar energy conversion and wastewater

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treatment. [1-5] Nowadays, there is a continuously increasing research interest in

developing semiconductor photocatalysts with better photocatalytic activity, which

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depend not only on the surface morphology or compositions of the photocatalysts but

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also on the synthetic routes of the photocatalysts. [6, 7] To our best knowledge, the composite photocatalyst combine the advantages of two or multi- single phase photocatalysts are expected to exhibit enhanced photocatalytic activity than that of

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single phase photocatalysts. Therefore, it is important to develop a novel composite photocatalyst and study its photocatalytic activity.

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Ceria (CeO2) and manganese acid yttrium (YMnO3) are two common

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photocatalysts for photocatalytic degradation of organic dyes. [8-14] However, single CeO2 or YMnO3 used as photocatalysts for photocatalytic degradation of organic dyes have many drawbacks including low visible photocatalytic activity, low charge carrier mobility and high recombination of photon-generated carriers. [15] Recently, Wang et al. [15] reported the NiO/YMnO3 nanocomposites photocatalysts prepared by a simple two phase recombination route exhibits high photocatalytic activity for degradation of

the methyl red dye under visible light irradiation. Cao et al. [16] synthesized the p–n junction YMnO3/SrTiO3 composite photocatalysts by a simple wet chemical route exhibits high photocatalytic activities and robust stabilities for degradation of the methyl red dye under visible light irradiation. And inspired by that, a novel photocatalyst of YMnO3/CeO2 composite can be synthesized to study its photocatalytic activities. However, there is no report so far of the YMnO3/CeO2 composite synthesized by a sonochemistry method and investigation the photocatalytic activity for degradation

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of the organic dyes under visible light irradiation . In this paper, we present a novel sonochemistry synthesis method for the synthesis

of YMnO3/CeO2 composite photocatalysts. The CeO2, YMnO3 and YMnO3/CeO2

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oxides were characterized using powder X-ray diffraction (XRD), Fourier transform

infrared (FTIR), field emission scanning electron microscopy (FE-SEM), UV–vis

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spectroscopy, photoluminescence spectroscopy, electrochemical workstation, and 721

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spectrophotometer. The obtained results have demonstrated that the YMnO3/CeO2 oxides exhibits high photocatalytic activity for degradation of the methyl red dye under visible light irradiation. As a representative of type II p–n junction composite

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photocatalysts, the photocatalytic mechanism for the YMnO3/CeO2 oxides was studied.

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2. Materials synthesis and characterization

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2.1 Prepared of the CeO2, YMnO3 and YMnO3/CeO2 oxides The CeO2, YMnO3 and YMnO3/CeO2 oxides were synthesized via sonochemistry

method. Stoichiometric amounts of cerium nitrate hexahydrate or yttrium nitrate hexahydrate and manganese nitrate (nY:nMn=1:1) were dissolved in the distilled water. Subsequently, A certain stoichiometric amounts of citric acid was added to the solution. After the solution is fully mixed by stirring, it is transferred to a pyrex vessel. The

ultrasonic processing of above solution was performed by a ultrasonic apparatus with a power output of 100 W and steady frequency of 20 kHz for 1 h. After sonification, the obtained solution was dried at 120 ℃ for 48 h and calcined at 700 ℃ for 5 h in air. To prepare the YMnO3/CeO2 oxides with nYMnO3:nCeO2=1:1, the synthesized YMnO3 oxides is dispersed in the beaker. Soon afterwards, the cerium nitrate hexahydrate was added to the beaker. The mixed solutions was transferred to a pyrex vessel. The next step is the same as the synthesis of the CeO2 or YMnO3 oxides.

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2.2 Material characterization The phase structure and functional group of the CeO2, YMnO3 and YMnO3/CeO2

oxides were characterized by a Shimadzu X-ray powder diffraction (XRD) and a Bruker

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IFS 66 v/S spectrometer. The morphology of the CeO2, YMnO3 and YMnO3/CeO2 oxides were examined by a Hitachi S-4800 field-emission scanning electron

oxides

were

studied

by

a

UV1800

UV–vis

double-beam

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YMnO3/CeO2

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microscope (SEM). UV-visible diffuse reflectance spectra of the CeO2, YMnO3 and

spectrophotometer with a wavelength range of 190 – 1100 nm and integrating sphere attachment. The electrochemical properties of the CeO2, YMnO3 and YMnO3/CeO2

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oxides were performed on a CST 350 electrochemical workstation. The photocatalytic activity of the CeO2, YMnO3 and YMnO3/CeO2 oxides were measured by using methyl

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red (MR) as a target dye. The photocatalytic experimental process and relevant

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experimental parameters are similar to literature [15]. According to the equation (1), the degradation percentage (D%) of the CeO2, YMnO3 and YMnO3/CeO2 oxides are calculated. D% 

Ct  C0  100% C0

(1)

Where Ct and C0 are the instantaneous and initial concentration of the MR dye, respectively.

3. Results and discussion

4000

CeO2

3500

YMnO3

30

40

2 deg)

50

60

70

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20

(311)

(a)

0

(220)

(111)

500

(200)

(b)

1000

(114)

1500

(104) (202)

2000

(110) (111) (004) (112)

2500

YMnO3/CeO2



(221) (215) (222) (312) (108)



(c)

(212) (115) (300) (302) (214) (116)

3000

(102)

Intensity(arb.units)

3.1 Phase structure analysis

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Figure 1 X-ray diffraction pattern of the (a) CeO2, (b) YMnO3 and (c) YMnO3/CeO2

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oxides.

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The crystalline phases formed during CeO2, YMnO3 and YMnO3/CeO2 oxides preparation were identified by XRD. Figure 1 shows the XRD patterns of the CeO2,

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YMnO3 and YMnO3/CeO2 oxides. CeO2 has diffraction peaks at 2θ = 28.589, 33.107, 47.439, and 56.396° corresponding to the (111), (200), (220), and (311) planes

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indicating high degree of crystallinity (JCPDS No. 43–1002). The planes at (102), (110), (111), (004), (112), (104), (202), (114), (212), (115), (300), (302), (214), (116), (221),

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(215), (222), (312), and (108) are ascribed to hexagonal phase of YMnO3 (JCPDS No. 25-1079). The XRD diffraction peaks of the YMnO3/CeO2 oxides matches well with the cubic phase of CeO2 and the hexagonal phase of YMnO3. The result indicates that the CeO2, YMnO3 and YMnO3/CeO2 oxides can be successfully synthesized by a sonochemistry method.

3.2 Functional group analysis 220 200 160

60

CeO2

40

YMnO3

Ce-O

80

H-O-H

100

YMnO3/CeO2

20 0 4000

3500

3000

2500

2000

1500

Wave number (cm-1)

1000

500

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120

Mn-O Y-O Mn-O-Mn

-

NO 3

140

O-H

Transmittance (a.u.)

180

Figure 2 FTIR spectra of the CeO2, YMnO3 and YMnO3/CeO2 oxides.

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The FTIR spectra of as-synthesized the CeO2, YMnO3 and YMnO3/CeO2 oxides

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are presented in Figure 2. In the spectrum of the CeO2, YMnO3 and YMnO3/CeO2 oxides, the prominent bands at 3456 and 1640 cm−1 are typical of the stretching and

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bending modes of adsorbed water, [17, 18] respectively. For the YMnO3 and YMnO3/CeO2 oxides, the absorption peak at 1380 cm-1 can be assigned to the NO3-.

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[19] This confirms incomplete removal of NO3-, even after cerium dioxide was added to the precursor solution of yttrium manganate. The absorption peak at 599, 508, 450,

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and 423 cm-1 can be ascribed to the Mn-O of stretching vibration, [16, 20, 21] Y-O of stretching vibration, [16, 20, 21] the Ce-O stretching vibration, [22] and Mn-O-Mn of

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bending vibration, [16, 20, 21] respectively.

3.3 Surface morphology analysis

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Figure 3 SEM images of the CeO2, YMnO3 and YMnO3/CeO2 oxides.

Scanning electron micrographs of the CeO2, YMnO3 and YMnO3/CeO2 oxides are shown in Figure 3(a), (b), and (c), respectively. The micrographs reveal a nanoscale ball

shape with a size range of 30-100 nm. As can be seen from Figure 3(b), the YMnO3 oxides shows the aggregate particles with inhomogeneous particle size. It can be seen that the YMnO3/CeO2 oxides have a uniform particle size distribution due to introduce the CeO2 precursor. [16]

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3.4 Optical properties and optical band gap values

Figure 4 (a) UV-Vis diffuse spectra and (b) UV-Vis absorption spectra of the CeO2, YMnO3 and YMnO3/CeO2 oxides. The Eg values of the (c) CeO2, (d) YMnO3 and (e),

(f) YMnO3/CeO2 oxides.

The UV–visible diffuse reflectance spectra of the CeO2, YMnO3 and YMnO3/CeO2 oxides are shown in Figure 4(a). A low reflectance in λ<350 nm can be observed for the CeO2 oxides. On the contrary, a continuous increase in reflectance in λ>350 nm for the CeO2 oxides. For the YMnO3 and YMnO3/CeO2 oxides, a continuous increase in reflectance in λ>800 nm can be found up to 1100 nm. It can be seen that the

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reflectance of YMnO3/CeO2 oxides is higher than that of YMnO3 oxides. Figure 4(b) shows the UV-Vis absorption spectra of the CeO2, YMnO3 and YMnO3/CeO2 oxides. For the CeO2 oxides, the strong absorption capacity only in λ<420 nm can be observed.

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The result indicates that the CeO2 oxides exhibits a ultraviolet photocatalytic activity.

The YMnO3 and YMnO3/CeO2 oxides shows strong absorption capacity in the

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wavelength range of 200-1100 nm, implying that the two samples possess excellent

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ultraviolet and visible photocatalytic activity. The stronger absorption capacity for the YMnO3/CeO2 oxides meants the sample have a highest photocatalytic activity. The optical bandgap energy (Eg) values of the CeO2, YMnO3 and YMnO3/CeO2

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oxides can be obtained by the UV-visible absorption spectra and equation (2).  h  A(h  Eg )n

(2)

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where α, һ, , A and n is the absorption coefficient, the Plank constant, the light

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frequency, a constant, and the nature of transition, respectively. n=1/2 and 2 for the direct gap (CeO2) and indirect gap semiconductor (YMnO3), respectively. The plot (αһ)1/2 versus һ of the CeO2, YMnO3 and YMnO3/CeO2 oxides as shown in Figure 4(c)-(f). The Eg value of the CeO2, YMnO3 and YMnO3/CeO2 oxides is 3.03, 1.03 and (0.82 or 2.51) eV, respectively. According to literature [16], the Eg value of the YMnO3/CeO2 oxides is 2.51 eV.

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3.5 Photoluminescence properties

Figure 5 (a) Excitation and (b) emission spectra of the CeO2, YMnO3 and YMnO3/CeO2

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oxides.

Figure 5(a) shows the excitation spectra of the CeO2, YMnO3 and YMnO3/CeO2

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oxides with the emission wavelength at 470 nm. An obviously excitation peak at 280

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nm can be found for the CeO2 and YMnO3 oxides. For the YMnO3/CeO2 oxides, no excitation peaks can be observed. Figure 5(b) shows the emission spectra of the CeO2, YMnO3 and YMnO3/CeO2 oxides with the excitation wavelength at 280 nm. A strong emission peak at 470 nm can be observed for the the CeO2 and YMnO3 oxides. The emission peak located at 470 nm for the CeO2 oxides can be assigned to the hopping from different impurity levels to the VB. [23] However, the emission peak located at

470 nm for the YMnO3 oxides can be ascribed to the NO3-. Fluorescence quenching was observed for the YMnO3/CeO2 oxides due to the electron hole pair recombination rate decreases. The lowest emission intensity for the YMnO3/CeO2 oxides implying that

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the sample exhibits a highest photocatalytic activity.

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Figure 6 CIE diagram of the CeO2 oxides.

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The CIE diagram of the CeO2 oxides as shown in Figure 6. The color coordinates (x, y) of the CeO2 oxides can be obtained by the CIE1931 chromaticity software and

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the emission spectra. As can be seen from Figure 6, the color coordinates (x, y) of the CeO2 oxides is (0.1555, 0.2179). The result indicates that the the CeO2 oxides exhibits a blue emission.

3.6 Electrochemical properties

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Figure 7 (a) EIS spectra and (b) Photocurrent response of the CeO2, YMnO3 and

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YMnO3/CeO2 oxides. Mott–Schottky (M–S) plots of (c) CeO2 and (d) YMnO3 oxides.

The measurement of electrochemical properties of semiconductor materials has

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great advantages in predicting their photocatalytic activity. The electrochemical properties of the CeO2, YMnO3 and YMnO3/CeO2 oxides were measured by a electrochemical workstation. Figure 7(a) shows the EIS spectra of the CeO2, YMnO3

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and YMnO3/CeO2 oxides. For all samples, a single semicircle at high frequency and an

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oblique line at low frequency can be observed. The semicircle and oblique line can be ascribed to the charge-transfer resistance [24] and Warburg impedance, [25, 26] respectively. The interfacial resistance for the CeO2, YMnO3 and YMnO3/CeO2 oxides is obtained as 660, 400, and 300 Ω, respectively. It is noted that the YMnO3/CeO2 oxides exhibits lowest charge-transfer resistance due to the smallest semicircle’s diameter than that of other samples. It can be seen that the YMnO3/CeO2 oxides shows

a highest photocatalytic activity for photocatalytic degradation of organic dyes under visible light irradiation. Figure 7(b) displays the photocurrent response of the CeO2, YMnO3 and YMnO3/CeO2 oxides measured for five on-off cycles under visible light irradiation. The photocurrent density of the YMnO3 oxides is higher than that of CeO2 oxides. The result confirms that the photocatalytic activity of the YMnO3 oxides for photocatalytic degradation of organic dyes under visible light irradiation is higher than that of CeO2

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oxides. It is noted that the YMnO3/CeO2 oxides exhibits highest photocurrent density. The result indicates that the YMnO3/CeO2 oxides have a highest photocatalytic activity

for photocatalytic degradation of organic dyes under visible light irradiation. To

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estimate the flat band potential (VFB) of the CeO2 and YMnO3 oxides, Figure 7(c) and

(d) displays the Mott–Schottky plots of the CeO2 and YMnO3 oxides in Na2SO4 (pH=8)

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electrolyte solution, respectively. A positive and negative slope for the CeO2 and

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YMnO3 oxides, respectively, is n-type and p-type semiconductor. The estimated VFB value was -0.786 and 0.924 V vs. SCE for the CeO2 and YMnO3 oxides, respectively. According to the equation (3), the potential at normal hydrogen electrode (NHE) of the

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CeO2 and YMnO3 oxides can be obtained. [16] V(NHE)=VFB+0.059*8+0.242

(3)

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The VFB values of the CeO2 and YMnO3 oxides is -0.072 and 1.638 V vs. NHE at

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pH =0, respectively. The conduction band (CB) potential of the CeO2 oxides and valence band (VB) potential of the YMnO3 oxides is -0.072 and 1.638 V, respectively, due to the difference between the CB potential of the CeO2 oxides and VB potential of YMnO3 oxides and VFB potential can be ignored. The VB potential of the CeO2 oxides and CB potential of the YMnO3 oxides is found to 2.958 and 0.608 V, respectively,

3.7 Photocatalytic performance

110 90 80 70

40

Dark Without catalyst CeO2

30

YMnO3

20

YMnO3/CeO2

60 50

10 0

0

20

40

60

80

100 120 140 160 180 200

Time (min)

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Degradation percentage (%)

100

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Figure 8 Degradation percentage of the CeO2, YMnO3 and YMnO3/CeO2 oxides.

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To compare with the literature [15, 16], methyl red (MR) was used as the degradation dye to study the photocatalytic activity. Degradation percentage of the

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CeO2, YMnO3 and YMnO3/CeO2 oxides as shown in Figure 8. The MR dye only have a slightly degraded without light irradiation and without YMnO3/CeO2 photocatalysts,

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it showed a 3.6 and 5.1 % degradation percentage for 180 min, respectively, which is meant the MR dye is stable dye and its scarcely self-decomposition in the absence of

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YMnO3/CeO2 oxides. [15] For the CeO2, YMnO3 and YMnO3/CeO2 oxides, the degradation percentage for photocatalytic degradation of MR dye under visible light

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irradiation increases with increasing the irradiation or reaction time. The degradation percentage of the CeO2, YMnO3 and YMnO3/CeO2 oxides for 240 min is about 45 %, 62 % and 99 %, respectively. The degradation percentage of the YMnO3/CeO2 oxides is highest than that of other samples due to the strong absorption capacity in the range from ultraviolet to visible, fluorescence quenching, low interfacial resistance, and high photocurrent density.

80

60 1st

3rd

2nd

4th

40

20

0

0

100

200

300

400

Time (min)

500

600

700

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Degradation percentage (%)

100

Figure 9 Photocatalytic degradation of the MR dye over YMnO3/CeO2 oxides for

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reusing four times under visible light irradiation.

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The stability of photocatalyst determines whether the semiconductor material can be used in the field of photocatalysis. The recycling degradation experiment of the

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YMnO3/CeO2 oxides for photocatalytic degradation of the MR dye under visible light irradiation was performed to study the stability of the YMnO3/CeO2 oxides. As can be

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seen from Figure 9, the degradation percentage of the YMnO3/CeO2 oxides did not decrease significantly for reuse four times. The result indicates that the YMnO3/CeO2

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oxides exhibits an excellent stability for photocatalytic degradation of the MR dye

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under visible light irradiation.

3.8 Photocatalytic mechanism

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Figure 10 Photocatalytic mechanisms of type II p–n junction YMnO3/CeO2 oxides.

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To study the photocatalytic mechanisms of the YMnO3/CeO2 oxides, the CB and

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VB potentials of the CeO2 and YMnO3 oxides can be calculated by the equations (4) and (5). [27-29] ECB = X − Ee − 0.5Eg

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EVB = ECB + Eg

(4)

(5)

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Where, ECB, EVB, Ee, and Eg is CB potential, VB potential, the energy of free electrons on the hydrogen scale (4.5 eV), and optical absorption band gap, respectively.

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X of the CeO2 and YMnO3 oxides were estimated as 5.56 [30] and 5.76 V [15], respectively. The CB potential of the CeO2 and YMnO3 oxides is found to be -0.455

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and 0.745 V, respectively. The VB potential of the CeO2 and YMnO3 oxides is 2.575 and 1.775 V, respectively. The calculated CB and VB potentials of the CeO2 oxides is smaller than that of the experimental results. However, the contrary is the result of the YMnO3 oxides. And yet for all that, this difference does not affect to analyze the photocatalytic mechanism of the YMnO3/CeO2 oxides by the band arrangement theory. Figure 10 shows the photocatalytic mechanisms of type II p–n junction YMnO3/CeO2

oxides. In the YMnO3/CeO2 oxides, the CeO2 possess a more negative CB and a more positive VB compared with the the YMnO3 oxides. This result indicates that the band gap positions of YMnO3 and CeO2 oxides are consistent with Type II heterojunction. [31-33] When the p–n YMnO3/CeO2 heterojunctions are formed and are kept in the dark, electrons (e-) will move from CeO2 to YMnO3 due to the more negative CB of CeO2. Subsequently, a built-in electric field is constructed in the junction area by the depletion layer (CeO2 side) and accumulation layer (YMnO3 side) realizes an

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equilibrium state. When the YMnO3 oxides in the p-n heterojunctions is excited by visible light, the built-in field promotes the separation of the photogenerated electron

hole pairs in the YMnO3 oxides and facilitates the electrons (e-) flowing towards CeO2.

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However, the electrons (e-) cannot migrate to the YMnO3 oxides due to the more negative CB of YMnO3 oxides. These electrons in the CB of the YMnO3 oxides capture

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the O2 in the photocatalytic reaction process to form superoxide radicals (O2−). The

e−+O2→•O2−

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relevant reaction process can be described as follows: (6)

•O2−+H2O2→•OH + OH−+O2

(7)

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At the same time, the photogenerated holes (h+) on VB of the YMnO3 oxides will react with OH−/H2O to form hydroxyl radical (H2O). The detail photocatalytic reaction

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process can be also described as follows:

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h+ + OH− → •OH

h+ + H2O → •OH + H+ •OH + dye → degradation products

(8) (9) (10)

These photocatalytic reactions can restrain the recombination of the electron (e-) – hole (h+) pairs in the photocatalytic system.

4. Conclusion The CeO2, YMnO3 and YMnO3/CeO2 oxides can be successfully synthesized by a sonochemistry method. The phase structure and functional group of the CeO2, YMnO3 and YMnO3/CeO2 oxides were confirmed by XRD and FTIR. No direct relationship was concluded between photocatalytic activity with either particle size or optical absorption band gap of the CeO2, YMnO3 and YMnO3/CeO2 oxides. The study revealed that synergistic effect among absorption capacity, fluorescence emission intensity,

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interfacial resistance, photocurrent density and charge recombination rate indicated that these are effective factors on the photocatalytic activity of photocatalysts. The higher

photocatalytic activity of the YMnO3/CeO2 oxides is attributed to the strong absorption

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capacity, fluorescence quenching, low interfacial resistance, high photocurrent density

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and low charge recombination rate.

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Acknowledgements

The subject is supported from Key Project of Provincial Natural Science Research

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of University of Anhui Province of China (Grant No. KJ2017A932).

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