TiO2 nanotube arrays and enhanced photocatalytic oxidative desulfurization performance

Accepted Manuscript Synthesis and characterization of CeO2/TiO2 nanotube arrays and enhanced photocatalytic oxidative desulfurization performance Xiaowang Lu, Xiazhang Li, Junchao Qian, Naiming Miao, Chao Yao, Zhigang Chen PII:

S0925-8388(15)31704-7

DOI:

10.1016/j.jallcom.2015.11.148

Reference:

JALCOM 36003

To appear in:

Journal of Alloys and Compounds

Received Date: 15 June 2015 Revised Date:

19 November 2015

Accepted Date: 21 November 2015

Please cite this article as: X. Lu, X. Li, J. Qian, N. Miao, C. Yao, Z. Chen, Synthesis and characterization of CeO2/TiO2 nanotube arrays and enhanced photocatalytic oxidative desulfurization performance, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.11.148. 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.

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H2O2 H2O2+commerical CeO2

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Desulfurization Rate(%)

100

60

H2O2+TiO2 nanotuble arrarys H2O2+CeO2/TiO2 nanotuble arrarys

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dark

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20

0

-50

0

50

100

150

200

250

300

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irradiaton time (mins)

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Well-aligned CeO2/TiO2 nanotube arrays photocatalyst was successfully prepared by anodization and microwave homogeneous synthesis technique. CeO2 /TiO2 nanotube arrays exhibited excellent photocatalytic activity, which can photo- oxidize and remove more than 90% of sulfur compounds in model oil in 5h.

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ACCEPTED MANUSCRIPT Synthesis and characterization of CeO2/TiO2 nanotube arrays and enhanced photocatalytic oxidative desulfurization performance Xiaowang Lua,b,c* , Xiazhang Lib, Junchao Qiand, Naiming Miaoa,Chao Yaob, Zhigang Chen d,a* a.School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013 China;

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b.School of Material Science and Engineering, Changzhou University, Changzhou 213164 China; c.Key laboratory of advanced metallic materials of Changzhou city, Changzhou 213164 China . d.School of Chemistry and Bioengineering, Suzhou University of Science and Technology,

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Suzhou 2150009,China.

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Abstract: Well-aligned CeO2/TiO2 nanotube arrays photocatalyst was successfully prepared by anodization and microwave homogeneous synthesis technique. The nanocomposites spectroscopy(RS),

were

characterized

scanning

electron

electron

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spectrometry(EDS), transmission reflection

spectroscopy

by

(DRS),

X-ray

X-ray

diffraction(XRD),

microscopy(SEM), microscopy(TEM),

photoelectron

Raman

energy-dispersive UV-vis

diffuse

spectroscopy

(XPS),

photocurrent-time measurements, and electrochemical impedance spectroscopy(EIS).

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The TiO2 nanotube arrays showed enhanced optical properties with significant red

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shift after the introduction of CeO2 nanoparticles. Photocatalytic activities of the prepared samples were examined by the photocatalytic oxidation of benzothiphene (BT) under visible light irradiation. The CeO2/TiO2 nanotube arrays photocatalyst showed higher photocatalytic activity than that of TiO2 nanotube arrays. It was found that more than 90% of sulfur compounds in model oil were photocatalytic oxidized Corresponding author Address: 1. School of Material Science and Engineering, Jiangsu University,Tel:+86-0519-86330092, Fax: +86-0519-86330066,E-mail: [email protected] (X.Lu) 2. School of Chemistry and Bioengineering, Suzhou University of Science and Technology China. Tel: +86-0519-86330092, Fax: +86-0519-86330066 E-mail: [email protected] (Z.Chen). 1

ACCEPTED MANUSCRIPT and removed by using CeO2/TiO2 nanotube arrays as photocatalyst. Key words: CeO2/TiO2 nanotube arrays; Photocatalytic oxidation; Desulfurization; Synergistic effect

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1. Introduction Air pollution caused by the emission of sulfur oxides (SOx) is one of the most serious environmental problems in the world. The motor vehicle exhaust gas is the

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major source of sulfur oxides (SOx) formed via the burning of the gasoline and diesel fuel including thiophenic sulfur and its derivatives, which can cause acid rain,

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atmospheric haze and photochemical smog[1,2]. Hydrogenation desulfurization is the predominant technique employed to reduce the sulfur content in fuel in modern industry, which reckons on high pressure, high temperature and high hydrogen

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consumption. Moreover, it is difficult to remove sulfur containing organic compounds from fuels, especially thiophene sulfur in gasoline [3,4]. To save energy and decrease costs, several non-HDS technologies have been developed to produce low sulfur gasoline.

Such

desulfurization[7,8],

as

adsorptive

oxidative

desulfurization[5,6],

desulfurization[9,10]

and

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extractive

of

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content

biodesulfurization[11,12]. Oxidative desulfurization (ODS) is considered to be one of the most promising desulfurization processes due to its mild operating conditions and no consumption of H2. However, the traditional ODS technology operates at temperatures between 60 – 80

. At these temperatures, the oxidation of desired fuel

constituents, such as alkenes and aromatics consume part of the oxidant and decreases the overall octane rating of the fuel [13]. Photocatalytic oxidative desulfurization

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ACCEPTED MANUSCRIPT technology is essentially an advanced version of ODS involved with photocatalyst and light source favoring the oxidation rate, which can be conducted at ambient temperature, atmospheric pressure and high efficiency [14]. For instance, Dedual et al. reported

the

effective

photocatalytic

oxidative

desulfurization

of

a

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[13]

thiophene-containing solution using TiO2. Wang et al. [15] prepared the TiO2/g-C3N4 composites directed to the photocatalytic oxidative desulfurization. Zhao et al. [16]

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reported the photocatalytic oxidation desulfurization of model diesel over

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phthalocyanine/La0.8Ce0.2NiO3 photocatalyst. Entezari et al. [17] synthesized nickel nanoparticles modified C/TiO2@MCM-41 composites for the photocatalytic oxidative desulfurization.

As an important semiconductor material, Titanium dioxide (TiO2) has been widely

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used as the photocatalyst due to its chemical and biological inertness, high stability against photocorrosion, non-toxicity, low cost [18,19]. Recently, one-dimensional TiO2 nanotube arrays have been widely studied due to their highly ordered array

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structure, high specific surface area, unidirectional charge transfer and transportation

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features, which exhibit promising photocatalytic performances in solar cells, water splitting and photocatalytic environmental purification. However, their poor utilization for visible light as well as the high recombination rate of photoexcited electron-hole pairs limit their practical applications [20,21]. To overcome these limitations, the transition metal ion doping(such as Fe[22], Ni[23]), nonmetal doping(such as C[24], N[25], S[26]), noble metals deposition(such as Pt [27], Au[28] Ag[29] ), other narrow band gap semiconductors coupling(such as Cu2O[30],

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ACCEPTED MANUSCRIPT WO3[31], Fe2TiO5[32], CdS[33])

have already been used to improve the

photocatalytic properties of TiO2 nanotube arrays. Coupling TiO2 nanotube arrays with narrow bandgap semiconductor is considered to be one of the most effective

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methods to improve the photocatalytic activity of TiO2 nanotube arrays. Recently, Ji et al. [30] prepared Cu2O/TiO2 heterostructure nanotube arrays by the anodization and the electro-deposition technique, and found that the obtained nanocomposite exhibited

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high photocatalytic activity in conversion of CO2 to methanol. Momeni et al. [31]

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synthesized hierarchical WO3-TiO2 nanotube arrays by single-step electrochemical anodization shown that WO3-TiO2 nanotube arrays exhibited better catalytic activity in a photoelectrochemical cell for hydrogen production than pure TiO2 nanotube arrays. Liu et al. [32] fabricated pseudobrookite Fe2TiO5 ultrathin layers grown on

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vertically aligned TiO2 nanotube arrays by an electrochemical deposition enhancing the conduction and utilization of photogenerated charge carriers. Wang et al. [33] successfully deposited CdS quantum dots on TiO2 nanotube arrays by successive

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ionic layer adsorption and reaction method, which showed excellent photoactivity for

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both H2 generation and Rh B degradation under visible light. Cerium oxide (CeO2) is one of the relatively inexpensive rare earth metal oxides,

which has attracted much attention due to the special electron orbital structure, the unique optical, redox properties and high oxygen storage capability[34,35]. It has been found that well-coupled CeO2-TiO2 nanocomposites can produce a special electron transfer process that increases the yield of the electron–hole pairs and improves the photocatalytic activity[36,37]. Recently, Li et al. [38] prepared CeO2

4

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modified

anodized

TiO2

nanotube arrays

nanocomposite by

electrochemical method, which showed largely enhanced charge storage capacity and good stability. Yu et al. [39] synthesized TiO2 nanotube arrays decorated with CeO2

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photoelectrode through the electro-deposition technique and found that it exhibted higher photoelectrocatalytic activity in the degradation of methyl orange. However, to the best of our knowledge, there is rare report on the synthesis of the CeO2/TiO2

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nanotube arrays nanocomposite by microwave assisted method and their application

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toward photocatalytic oxidation desulfurization under visible light irradiation. Herein, in this work, we report a new approach to prepare CeO2/TiO2 nanotube arrays nanocomposite.

Highly dispersed CeO2 nanoparticles on orifices and the

surface of the TiO2 nanotube arrays were synthesized by anodization and microwave

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homogeneous method. The CeO2/TiO2 nanotube arrays photocatalyst shows high activity on removal of BT in model oil and the induced CeO2 significantly enhance the photoactivity of TiO2 nanotube arrays. This novel CeO2/TiO2 nanotube arrays

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photocatalyst demonstrates potential applications in industrial oil purification.

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2. Experimental 2.1 Material

Titanium foil (0.1 mm, 99.9%) was purchased from Baoji Titanium Industry Co.,

Ltd., China. Acetone, ethanol, nitric acid, hydrofluoric acid, ammonium fluoride, ethylene glycol, Cerium nitrate(Ce(NO3)3·6H2O), hexamethylene tetramine (HMT) 30wt% H2O2 solution, n-octane, commercial CeO2 were supplied from Sinopharm Chemical. Reagent Co. Ltd., China. benzothiophene (BT) was purchased from

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ACCEPTED MANUSCRIPT Sigma-Aldrich. And all these chemicals were of analytical reagent grade. 2.2. Preparation of photocatalysts The fabrication approach of CeO2 /TiO2 nanotube arrays have been illustrated in

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Fig. 1. Firstly, the titanium foil was cut into strips with 1 cm width and 3 cm length, then the titanium strips were burnished by thin sand papers, ultrasonically cleaned with acetone, ethanol and deionized water, subsequently, the titanium strips were

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polished in the mixture of hydrofluoric acid, nitric acid and deionized water at the

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ratio of 1:1:1 for 10 seconds, then wished with deionized water and followed by drying under a nitrogen stream. Secondly, the titanium strips (effective length 2 cm) were immersed in electrolyte a mixed ethylene glycol solution (200 mL) of ammonium fluoride (0.5 wt %) and deionized water (5 wt %) and subjected to a

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constant potential (60 V) at room temperature in a two-electrode configuration with titanium foil as the working electrode and platinum mesh as the counter electrode for 30 min. Thirdly, the strips were washed with deionized water and sonicated to remove

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the surface deposit then dried at 60

for 1 h and cooled down naturally. The well aligned TiO2 nanotube

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annealed at 500

in the oven for 10 h. Finally, these strips were

arrays were obtained.

The TiO2 nanotube arrays obtained in the above step were used as both substrate

and reactant for the fabrication of CeO2/TiO2 nanocomposites. In general, CeO2 /TiO2 nanotube arrays were prepared according to the following procedure. 10 mmol Ce(NO3)3·6H2O and 50 mmol hexamethylene tetramine HMT

dispersed in 100 mL

of aqueous to form a mixture solution. Then, a piece of TiO2 nanotube arrays strip was

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ACCEPTED MANUSCRIPT added to the solution. The flask was placed in a microwave chemical reactor (MCR-3, Gongyi City Yuhua Instrument Co., Ltd.China). Subsequently, the microwave chemical reactor was turned on and kept the microwave frequency of 2450MHZ, the for 20 min. After the microwave heated

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output power of 400W, temperature of 75

reaction, the strip was drawn from the flask and washed with deionized water, and then dried at 80

for 10 h. Finally, CeO2 /TiO2 nanotube arrays have been

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synthesized.

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2.3 Characterizations

X-ray diffraction (XRD) measurement was performed on a Rigaku X-ray diffractometer with Cu Kα radiation (Rigaku, D/max-RB,λ=0.15406nm), operated at 40 kV and 100 mA (scanning step:0.05◦ per second). Raman spectra were measured at

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room temperature by a Thermo Fisher Scientific DXR Raman spectrophotometer, and the excitation laser wave length was 532 nm using a laser power level of about 5 mW. The morphology observation was performed on the Zeiss Supra55 scanning electron

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microscope (SEM) operated at an accelerating voltage of 5 kV. Energy-dispersive

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spectroscopy (EDS) (Oxford INCA) was utilized to have an analysis of the chemical composition of the products. Transmission electron microscopy (TEM) analysis was conduced on a JEOL JEM-2100 microscope coupled with a Gatan 832 CCD operated at an accelerating voltage of 200 kV. The samples for TEM measurements were powders scraped off from the substrates, which were suspended in ethanol and supported onto a holey carbon film on a Cu grid.–X-ray photoelectron spectroscopy (XPS) analysis was carried out using an ESCALAB 250 spectrometer (Thermo Fisher

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ACCEPTED MANUSCRIPT Scientific) with the mono Al Kα radiation (1486.6 eV) under a pressure of 2×10−9 Torr. Ultraviolet–visible diffuse reflectance spectroscopy (UV-vis DRS) was recorded on a spectrophotometer (Shimadzu, UV-2450) equipped with an integrating sphere, in

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which BaSO4 was used as the reflectance sample. The electrochemical analysis was carried out on an electrochemical workstation (CHI660C, CHI Shanghai Co., Inc. China) in a standard three-electrode configuration

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with a Pt foil as the counter electrode, Ag/AgCl (in saturated KCl) as the reference

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electrode and the prepared strips as the working electrode. Photocurrent was investigated by the electrolyte which was KOH (1M) aqueous solution and irradiation proceeded by a 300 W xenon lamp equipped with an ultraviolet cut off filter. The electrochemical impedance spectroscopy (EIS) measurements were performed at bias

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voltages 0.5 V versus RHE, in the frequency range of 0.1 Hz–100k Hz with oscillation potential amplitudes of 0.01V at room temperature. 2.4 Photocatalytic reaction

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Photocatalytic activity of photocatalyst was evaluated by the photooxidation of

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model oil. Model oil was prepared by dissolving benzothiophene (BT) in n-octane with a corresponding sulfur content of 200 ppm. Photocatalytic reaction was used a photochemical reaction instrument (GHX-2, Yangzhou science and technology city instrument co., Ltd., China). The reaction system consisted of a 300 W xenon lamp with an ultraviolet cut off filter as the light source, a circulator bath as to keep the constant temperature, a 100mL glass reactor containing model oil and magnetic stirrer.

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ACCEPTED MANUSCRIPT In a typical reaction, a piece of stripe and 90mL model oil were added into the reactor. Solution was formed with constant stirring, and then the mixed solution was placed in the dark for 30 min to establish adsorption-desorption equilibrium.

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Subsequently, 10 mL H2O2 was added into the reaction system. Then the suspension was irradiated with the xenon lamp. Two milliliters solution was collected every 30 min and centrifuged for 5 min at 12000 r/min. Then, 5 µL upper clear layer solution

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was injected into an ultraviolet fluorescence sulfur measuring instrument (THA2000S,

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Taizhou Jinhang Analysis Instrument Co., Ltd., China). The content of sulfur of the model fuel oil was detected by ultraviolet fluorescence sulfur measuring instrument. The desulfurization rate η (%) was calculated according to the following formula: η= (1-C/C0) ×100%

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Where C0 is the sulfur content of the initial solution at t = -30, and C is the sulfur content of the solution after catalytic at time t. 3. Results and discussion

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3.1. XRD analysis

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Fig. 2 shows the XRD patterns of TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays. Three kinds of diffraction peaks can be identified from the XRD patterns in Fig. 2. The diffraction peaks at 2θ= 38.45°, 40.24°, 53.14° and 70.80°correspond respectively to the (002), (101), (102) and (103) crystal planes of the Ti substrate(JCPDS file: 44–1294). The diffraction peaks at 2θ= 25.37°, 48.12°, 53.97°, 55.10°, 62.74° and 68.79° correspond to the (101), (200), (105), (211), (211) and (116) crystal planes of the anatase TiO2 (JCPDS file: 21-1272). The

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ACCEPTED MANUSCRIPT presence of CeO2 in the sample after deposition can be confirmed by the characteristic reflection peaks at 28.68°, 33.38°, 47.84° and 56.48, which are assigned to (111), (200), (220) and (311) crystal planes of the cubic CeO2 (JCPDS file: 34-0394). Aside

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from the diffraction peaks of Ti substrate, TiO2 and CeO2, no other material is detected in the as-prepared sample. 3.2. Raman spectral analysis

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Raman spectroscopy by dispersion is an effective method for studying the TiO2

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mixed oxide structures due to the high dispersion properties of TiO2 phases [40]. Therefore, it is possible to observe the CeO2 associated with TiO2 on the anatase structure with this technique. The Raman spectra (Fig. 3) show the obtained phases in the samples. The results show the characteristic signals for the tetragonal phase of

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TiO2 (anatase) around 153 cm−1, 380–408 cm−1, 527 cm−1 and 640–650 cm−1 [41]. The CeO2 fluorite type phase is detected only in the spectrum of the Ce/TiO2 nanotube arrays. The band at 465 cm−1 corresponds to the cubic phase of the CeO2 fluorite type

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phase. Moreover, the band at 585 cm−1 is attributed to the oxygen vacancies in the

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CeO2 lattice [42].

3.3. SEM and EDS analysis Fig.4 (a) and Fig.4 (b) exhibit the typical scanning electron microscopy (SEM)

images of the TiO2 nanotube arrays by electrochemical method. It can be observed that TiO2 nanotube arrays consist of uniform nanotubes with an average length of 5 µm, a mean pore diameter of 60 nm, and a wall thickness of about 30 nm (as shown in Fig. 4(a)). The orifices and the side surfaces of these nanotubes are composed of

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ACCEPTED MANUSCRIPT relatively smooth surfaces without any nanoparticles. However, when TiO2 nanotube arrays are dipped into an aqueous solution contains Ce(NO3)3·6H2O and HMT under microwave treatment, the CeO2/TiO2 nanotube arrays are obtained. As shown in Fig.

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4(c) and Fig. 4 (d), the highly dispersed CeO2 nanoparticles are supported on orifices and the surface of the TiO2 nanotube arrays. The energy dispersive spectrometer (EDS) spectrum of Ce/TiO2 nanotube arrays is shown in Fig. 4 (e). Ti, Ce, O elements are

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detected with the absence of other elements, suggesting Ce in the form of CeO2 have

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been successfully mixed with TiO2. 3.4. TEM analysis

Additionally, Fig. 5(a), (b) show the transmission electron microscopy (TEM) images of the fragments of CeO2/TiO2 nanotube nanocomposite obtained through the

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sonication treatment in the presence of acetone solution. The surface of TiO2 nanotube is rather rough, and the CeO2 nanoparticles are attached onto the surface of the TiO2 nanotube. The observed lattice spacing of 0.35nm in the Fig. 5(b) corresponds to (101)

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plane of anatase TiO2, proving the existence of TiO2. The observed fringe with the

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distance of 0.31 nm of the wall correspond to (111) planes of CeO2. 3.5. XPS analysis

In order to further study the composition of CeO2 /TiO2 nanotube arrays, X-ray photoelectron spectroscopy (XPS) is also conducted. Fig. 6(a) shows the Ti 2p core-level spectrum of TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays. The binding energy at 458.7 and 464.4 eV are attributed to Ti 2p3/2 and Ti 2p1/2 respectively in the TiO2 nanotube arrays [43]. Compared to TiO2 nanotube arrays, the

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ACCEPTED MANUSCRIPT binding energy of CeO2 /TiO2 nanotube arrays increases slightly. The shift may be due to the different electronic interaction between titanium and cerium, which indicates that Ti3+ species are formed in the CeO2 /TiO2 nanotube arrays [44]. The Fig. 6(b) is

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used identify Ce 3d XPS peaks, where v and u indicate the spin–orbit coupling 3d5/2 and 3d3/2 respectively. The peaks of v and u are assigned to Ce IV (3d94f2) O (2p4) state, the peaks of v2 and u2 are assigned to Ce IV (3d94f1) O (2p5) state and the peaks

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of v3 and u3 are assigned to Ce IV (3d94f 0) O (2p6) state. These six peaks are the

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characteristic peaks of Ce4+. The peaks labeled as v1 and u1 are attributed to Ce III (3d94f2)O(2p5) state .Therefore, a mixture of Ce3+/Ce4+ oxidation states exists on the surface of the sample[35]. Fig. 6(c) shows that the O 1s core level is composed of at least two components. The O 1s signal at about 529.6 eV is assigned to the lattice

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oxygen (O2-) for TiO2 and CeO2 according to the literature, while the signal at 532.0 eV is probably due to the oxygen in surface hydroxyl groups [35,41]. There is an increase in the intensity of peak at 532 eV in CeO2 /TiO2 nanotube arrays. These

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oxygen ions could be described as "O-" species with respect to the sites where the

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coordination number of oxygen ions is smaller than in a regular site [45]. It can be considered that the existence of this peak for ceria is due to defects in the subsurface. The higher concentration of Ce3+ and more oxygen vacancies may promote photocatalysis activity.

3.6 Photocurrent-time analysis Photocurrent measurements are carried out for TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays to evaluate the electronic interaction between TiO2 nanotube

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ACCEPTED MANUSCRIPT arrays and CeO2 /TiO2 nanotube arrays. As shown in Fig. 7, steady and prompt photocurrent generation is obtained during on and off cycles of illumination. The changes of both “on” and “off” currents are nearly vertical, which indicate that charge

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transport in this as-prepared sample proceeds very quickly. Under visible light irradiation, both samples show photocurrent responses. The photocurrent of the CeO2 /TiO2 nanotube arrays electrode is about 2.2 times higher than that of the pure TiO2

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nanotube arrays electrode. It is well known that the photoelectrochemical

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performances are influenced by not only the excitation and separation of electron-hole pairs but also the transport of charge carriers, and the final photocurrent density are determined by the synergistic function of these two factors[46], which are beneficial to the enhancement of the photocatalytic activity in CeO2 /TiO2 systems.

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3.7 EIS analysis

Fig. 8 shows impedance plots of TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays. The test data figure shows the semicircle structure, therefore the electrical

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performance of the electrode can be used Nyquist Equivalent circuit analysis.

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Equation (1) represents parallel circuit impedance (Z) composed of resistor (R) and capacitor (C): Z=

R ωCR 2 −j 1 + ωCR 1 + ωCR 2

Where ω=2πf (f

(1)

is Frequency)

Formula (2) shows the relationship between the Z ' (real part) and Z " (imaginary Part) of the semicircle with the radius R/2. ( Z ' − R / 2) 2 + Z " = ( R / 2) 2

(2) 13

ACCEPTED MANUSCRIPT The relative sizes of circular arc radius corresponded to the charge transfer resistance and separation efficiency of electron-hole in Nyquist plots[47]. As shown in Fig. 8, the Nyquist plots show that the diameter of the semicircle for CeO2 /TiO2

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nanotube arrays in the high-to-medium frequency region is much smaller than that of TiO2 nanotube arrays(The values of the ohmic resistance and charge transfer resistance are 10Ω and 147Ω for CeO2 /TiO2 nanotube arrays, which are significantly

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lower than those of TiO2 nanotube arrays(6.5 Ωand 297Ω)),the smaller circular arc

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radius of impedance spectra implies the better separation efficiency of electron-hole as well as the faster of the carrier transmission rate[48]. 3.8 DRS analysis

The UV-vis diffuse reflectance spectra of TiO2 nanotube arrays and CeO2 /TiO2

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nanotube arrays are investigated using a UV-vis spectrometer. Compared with TiO2 nanotube arrays, the CeO2 /TiO2 nanotube arrays show a broader absorption in the visible region (wavelength = 400–800 nm), the increased absorption intensity and the

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absorption edge is slightly red-shifted (Fig. 9a).When the absorption intensity

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increases, the formation rate of electron-hole pairs on the photocatalyst surface also increases, leading to the higher photocatalytic activity[49].The red-shift of the absorption threshold may be relative to the inter-function of the photo-activity TiO2 surface with the heterojunction formed by TiO2 and CeO2 networks via chemical bonds[50,51]. In addition, the oxygen vacancies in composite are considered to be major reasons for the observed photocatalytic activity enhancement, which could play a role as electron acceptor in improving charge transport and shift the Fermi level of

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ACCEPTED MANUSCRIPT composite toward the conduction band. Such a shift of the Fermi level can facilitate the charge separation at the semiconductor–electrolyte interface [34,52,53].The band gap energy of a semiconductor can be calculated by the following formula:

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α hv = A(hv − Eg ) n /2 (3) Where α, h, ν, Eg and A are absorption coefficient, Planck constant, light frequency, band gap energy and a constant, respectively. Among them, n is determined by the

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type of optical transition of a semiconductor (n =1 for direct transition and n =4 for

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indirect transition). For anatase TiO2 and CeO2 /TiO2 nanotube arrays, the values of n are both 4 [42]. Therefore, as seen in Fig.9(b), Eg of anatase TiO2 nanotube arrays is estimated to be 3.2 eV, which is consistent with the literature reported according to a plot of (αhν)1/2 versus energy (hν). However, Eg of CeO2 /TiO2 nanotube arrays is

(Fig.9(c)).

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determined from a plot of (αhν)1/2 versus energy (hν) and is found to be 2.72 eV

3.9 Photocatalytic activity

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The photocatalytic activity of commercial CeO2, TiO2 nanotube arrays and CeO2

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/TiO2 nanotube arrays are evaluated by photocatalytic oxidation desulfurization in model oil under visible light. Fig.10 shows the comparison of visible-light-driven photocatalytic desulfurization activity for commercial CeO2(0.1g), TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays. It can be seen that the removal rate of BT in model oil gradually increases as the reaction proceeds and reaches the balance stage for all photocatalysts. With the absence of photocatalyst, the desulfurization rate by H2O2 is only about 24 % after reaction for 5h, which indicates that H2O2 can generate

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ACCEPTED MANUSCRIPT hydroxyl radicals (·OH) and oxidize the groups of BT in little amount, but the oxidation ability of H2O2 is extremely limited[15,54].–CeO2 /TiO2 nanotube arrays exhibit higher photocatalytic activity than commercial CeO2 and pure TiO2 nanotube

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arrays, which can photo-oxidize and remove more than 90% of sulfur compounds in model oil in 5h, suggesting that the introduction of CeO2 can enhance the photocatalytic efficiency of pure TiO2 nanotube arrays.–The enhanced photocatalytic

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performance for the CeO2 /TiO2 nanotube arrays can be ascribed to following reasons.

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At first, ordered nanotube array architecture with high specific surface area, which provides larger reaction interface area and fast intraparticle molecular transfer [55,56]. Secondly, the CeO2 addition can greatly enhance the surface oxygen vacancies, which could easily capture electrons and yield surface oxygen radicals with excellent

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reduction capability [57]. Finally, Ce4+ ion can capture electron and are transformed to Ce3+ ions [58], which has one electron in the 4f orbital and this electron is transferred to adsorb oxygen to form superoxide radical, superoxide radicals interact

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with the BT and degrade it [59]. In addition, the electrons trapped in Ce4+ sites are

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subsequently transferred to the adsorbed O2 by oxidation process, which can prevent the recombination of photogenerated electrons and holes [36]. Therefore, the CeO2 /TiO2 nanotube arrays photocatalyst exhibit the excellent photocatalytic activity due to the synergistic effect of CeO2 and TiO2. The stability of photocatalyst is important for practical application. The repeated photocatalytic oxidative desulfurization of the model oil experiment on the–CeO2 /TiO2 nanotube arrays is examined and shown in Fig. 11. After three consecutive runs,

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ACCEPTED MANUSCRIPT the photocatalytic oxidative desulfurization efficiency still reaches 82.6% after 3 cycles in 5 h. Therefore, the–CeO2 /TiO2 nanotube arrays indicate the outstanding recyclability under visible light.

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Upon the irradiation of visible light, the CeO2 /TiO2 composite absorb an amount of photon energy, then electrons and holes are separated effectively, subsequently, the hole interact with H2O molecule to form hydroxyl radicals (·OH) as well as the

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electrons reacts with H2O2 molecule to form ·OH[15,54]. whereas Ce4+ ion can capture electron and are transformed to Ce3+ ions, and subsequently interact with

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adsorbed oxygen to form superoxide radical( ⋅O −2 )[59]. With the existence of ·OH and ⋅O −2 , BT is easily oxidized to benzothiophene sulfone in boundary of water and oil phase, then benzothiophene sulfone is removed from oil phase he reaction process is

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shown as follows

(4)

h + + H 2O  →⋅OH+H +

(5)

e− + H 2O2  →⋅OH+HO-

(6)

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CeO 2 / TiO 2 + hv  → h + + e−

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Ce 4 + + e −  → Ce3+

(7)

Ce3+ + O 2  → Ce 4 + + O 2− +

OH

oxidation

S

_

+ S

O2

(8)

O

S O

oxidation O

S O

(9)

(10)

4. Conclusions 17

ACCEPTED MANUSCRIPT In summary, well-aligned CeO2/TiO2 nanotube arrays photocatalyst was successfully synthesized by anodization and microwave homogeneous synthesis method. CeO2 /TiO2 nanotube arrays exhibited excellent photocatalytic activity, which

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can photo- oxidize and remove more than 90% of sulfur compounds in model oil in 5h. The ordered nanotube array architecture, efficient separation of photogenerated electron–hole pairs, fast charge transfer at the interface, high concentration of oxygen

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vacancies, and narrowing bandgap contributed to the excellent photocatalytic activity.

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The CeO2/TiO2 nanotube arrays open up a window for environmental application in future as a promising photooxidative-desulfurization catalyst.

Acknowledgements

This work was supported by the Natural Science Foundation of China through

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Grant NSFC21277094, Young Talent fund of Changzhou University through Grant No.JQ201003, Jiangsu Technology Support Program (BE2014100, BE2014103), Jiangsu International Cooperation Project Project

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Cooperation

(BZ2015040), Huai’an International (HAC2014014)

and

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the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) were gratefully acknowledged.

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Fig. 1. Schematic illustration of the fabrication process for CeO2 /TiO2 nanotube

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arrays

Fig. 2. XRD patterns of TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays

Fig. 3. Raman spectra of TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays

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Fig. 4. SEM images (a),(b) of TiO2 nanotube arrays and (c),(d) CeO2 /TiO2 nanotube

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arrays. (e) EDS spectrum of CeO2 /TiO2 nanotube arrays

Fig. 5. TEM images (a),(b) of CeO2 /TiO2 nanotube arrays

Fig. 6. (a) Ti 2p (b) Ce 3d and (c) O 1s core level spectra collected for TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays

arrays

Photocurrent profiles of TiO2 nanotube arrays and CeO2 /TiO2 nanotube

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Fig. 8. Impedance plots of TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays

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Fig. 9. (a) Diffuse reflection spectra of nanotube arrays and CeO2 /TiO2 nanotube

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arrays; (b) (c) estimated band gap of photocatalyst by plots of (αhv)1/2 vs photon energy

Fig. 10. Comparsion of visible-light-driven photocatalytic desulfurization activity for commerical CeO2,TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays

Fig. 11. Cycling runs of CeO2 /TiO2 nanotube arrays on the desulfurization rate

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Fig. 1. Schematic illustration of the fabrication process for CeO2 /TiO2 nanotube arrays

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T

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T:Ti substrate A:Anatase C:CeO2 A

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(TiO2)

TiO2 arrary CeO2/TiO2 arrary

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(CeO2) (TiO2) 465 585 (TiO2)

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Fig. 3. Raman spectra of TiO2 nanotube arrays and CeO2 /TiO2 nanotube arrays

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0.35nm TiO2(101)

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Intensity(a.u.)

TiO2 nanotube arrays CeO2 /TiO2 nanotube arrays

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Ti2p3/2

458.4 458.7

Ti2p1/2 464.2

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3. 4. 5.

CeO2/TiO2 nanotube array photocatalist was successfully synthesized. The highly dispersed CeO2 nanoparticles are supported on the TiO2 nanotube arrays. CeO2 /TiO2 nanotube arrays exhibited excellent photocatalytic activity . The better photocatalytic activity due to the synergistic effect of CeO2 and TiO2. The photocatalyst may have a potential application for environmental protection.