Origin of enhanced photocatalytic activity of F-doped CeO2 nanocubes

Accepted Manuscript Title: Origin of enhanced photocatalytic activity of F-doped CeO2 nanocubes Author: Hui Miao Gui-Fang Huang Jin-Hua Liu Bing-Xin Zhou Anlian Pan Wei-Qing Huang Guo-Fang Huang PII: DOI: Reference:

S0169-4332(16)30300-2 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.122 APSUSC 32640

To appear in:

APSUSC

Received date: Revised date: Accepted date:

16-11-2015 11-2-2016 14-2-2016

Please cite this article as: H. Miao, G.-F. Huang, J.-H. Liu, B.-X. Zhou, A. Pan, W.-Q. Huang, G.-F. Huang, Origin of enhanced photocatalytic activity of F-doped CeO2 nanocubes, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.122 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.

Origin of enhanced photocatalytic activity of F-doped CeO2 nanocubes Hui Miao, Gui-Fang Huang∗, Jin-Hua Liu, Bing-Xin Zhou, Anlian Pan, Wei-Qing Huang# Department of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082, China

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Guo-Fang Huang

Changsha Nanfang Vocational College, Changsha 410208, China

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CeO2 nanoparticles are synthesized using a low-temperature solution combustion method and

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subsequent heat treatment in air. It is found that F-doping leads to smaller particle size and the formation of CeO2 nanocubes with higher percentage of reactive facets exposed. The band gap is

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estimated to be 3.16 eV and 2.88 eV, for pure CeO2 and fluorine doped CeO2 (F-doped CeO2) nanocubes, respectively. The synthesized F-doped CeO2 nanocubes exhibit much higher photocatalytic activities than commercial TiO2 and spherical CeO2 for the degradation of MB dye

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under UV and visible light irradiation. The apparent reaction rate constant k of MB decomposition over the optimized F-doped CeO2 nanocubes is 9.5 times higher than that of pure

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CeO2 and 2.2 times higher than that of commercial TiO2. The enhanced photocatalytic activity of

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F-doped CeO2 nanocubes originates from the fact that F-doping induces the small size, the highly reactive facets exposed, the intense absorption in the UV-visible range and the narrowing of the

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band gap. This research provides some new insights for the synthesis of the doping of the foreign atoms into photocatalyst with controlled morphology and enhanced photocatalytic activity. Keywords: F-doped, CeO2, photocatalytic activity, solution combustion method, nanomaterials

1 .Introduction

There is an urgent need for developing efficient photocatalyst since they can eventually lead to pollution control and renewable energy[1, 2]. Since the report on photoelectrochemical splitting of water over n-TiO2 electrodes in 1972[3], TiO2, the typical and most promising photocatalyst, has been the focus in this field owning to its unique photocatalytic efficiency, low cost, ∗

.Corresponding author. E-mail address: [email protected] #.Corresponding author. E-mail address: [email protected] . Corresponding author. E-mail address: [email protected]

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nontoxicity, and high stability. However, the inefficient use of solar energy and the rapid charge recombination largely limit its practical application. Thus, great research effort has been devoted to improve the solar energy conversion efficiency and enhance the photocatalytic activity by modifying

the

traditional

photocatalysts[4-8]

or

developing

efficient

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photocatalysts[9-11].

other new

CeO2, one of the most abundant and inexpensive rare earth materials, shows some properties

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like TiO2 such as wide band gap, nontoxicity, and high stability, and has been considered as an alternate material for the photocatalysis application due to the appropriate redox potential of the

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Ce4+/Ce3+ couple and strong light absorption in the UV region[12-14]. Unfortunately, the large band gap (3.2 eV) results in the little visible light absorption and hinders its widespread

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application in photocatalysis.

To extend its light absorption into the visible region and thus enhance the photoactivity,

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various metalcations, such as Fe[15-17], Co[18] and Y[19, 20], have been doped into the CeO2 lattice to narrow its band gap. The obtained superior photocatalytic performances of the metal

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ion-doped CeO2 is mainly ascribed to a decrease in band gap energy, an increase in specific

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surface area[1, 15] and/or oxygen vacancies[19]. Compared with metal cation dopants, nonmetal dopants may be more appropriate for the enhancement of photocatalytic activity of metal

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oxides[21]. Mao and his coworks found that the N-doped CeO2 nanoparticles possess much higher enhanced visible-light sensitivity and photocatalytic activity compared to pure CeO2[22]. It is also

found that C-N co-doped nano-CeO2 synthesized by the solvothermal method shows enhanced activity for the degradation of acid orange under UV and visible light irradiation[23]. It is known that photocatalytic processes basically involve the excitation of a valence electron

into the conduction band followed by the reduction of surface adsorbed species by the photoexcited electrons and the oxidation by the excited holes[24]. Efficient photocatalysis therefore largely depends on the separation of the photogenerated electrons and holes and their subsequent migration to the surface before recombination. Understanding of the electronic structure of pure CeO2 and F-doped CeO2 could provide new insights and understanding on the

mechanisms of photocatalytic activity enhancement by doping nonmetal ions into the lattice of semiconductors, thus provide design guidelines for new efficient photocatalysts. Many researchers

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have been devoted to theoretical studies to understand the enhanced photocatalytic activity in photocatalyst[25-27]. Based on DFT investigations, it has been found that N-doping leads to an add-on shoulder on the edge of the valence band of TiO2, and the visible-light photocatalytic activity originates from the localized N 2p levels near the valence band[28]. Fe-doping into TiO2

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may induce the shift of the absorption edge into the visible-light range with the narrowing of the band gap[29]. DFT calculations show that the exposed (101) facet of TiO2nanobelts may yield

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enhanced charge separation and reactivity due to trapping of photogenerated electrons by chemisorbed molecular O2 on the (101) facet and facilitating the generation of superoxide

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radical[30].

As well known, control over the morphology of semiconductor photocatalysts has never been

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ceased because it is a crucial parameter determining the photocatalytic performance of photocatalyst. Metal or nonmetal ions doped into CeO2 are usually used as a way to enhance the

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photocatalytic performance of CeO2. This work reports the effect of F doping on the morphology, particle size, microstructures and photocatalytic activity of CeO2 nanoparticles. Noteworthily,

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appropriate F-doping can promote the formation of small CeO2nanocubesbounded by the highly

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reactive (100) facets. Moreover, F doping in CeO2 results in the narrowing of band gap, which can increase the efficiency of the F-doped CeO2 as photocatalysts. This work demonstrates that

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F-doping is an effective means to improve the photocatalytic efficiency of CeO2 and provide new insights for the synthesis of photocatalysts enhanced by doping ions into the lattice of semiconductors.

2. Experimental section

2.1 Preparation of samples

All the chemicals are of analytical grade and used as received without further purification. Pure CeO2 and F-doped CeO2 nanoparticles are synthesized by the solution combustion method using citric acid as fuel. In a typical synthesis procedure, a certain amount of sodium fluoride, 2.1 g cerium nitratehexahydrate and 1.6 g citric acid monohydrate are dissolved in 20 mL distilled water with thorough stirring. The mass ratio of sodium fluoride to cerium nitrate ranges from 1/10 to 2/5. Then, 2 mL concentrated nitric acid is added into the above solution. The resultant

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homogeneous solution is transferred into a crucible, which is then put into a muffle furnace and heated at 300 ℃ for about half an hour.Finally, the synthesized samples are washed with distilled water and alcohol and annealed at 500 ℃ in air for 2h. The prepared samples doped with

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different nominal mass ratio of sodium fluoride are denoted as F(x)-CeO2, and x is 1/10, 3/10, 1/3, and 2/5, respectively. As a comparison, pure CeO2 is obtained using the same method in the

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absence of sodium fluoride.

2.2 Characterizations

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The prepared samples’ crystal structures are characterized by using power X-ray diffraction (XRD, Siemens D-5000 diffractometer with Cu Kα irradiation). The morphological details of the

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prepared samples are probed by an S-4800 field emission scanning electron microscopy (FESEM). The elemental analysis of prepared nanoparticles is detected by environmental scanning electron

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microscope (FEI QUANTA 200). The optical absorption spectra of the samples are recorded using a UV/VIS spectrometer (UV-2450, Shimadzu). According to the absorption spectra, we can obtain the curves of (ahv)2 with the changing of hv, and then draw a tangent to get the band gap of the

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samples.

2.3 Photocatalytic degradation of MB

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The photocatalytic activity of the samples are evaluated by the degradation of methylene blue (MB,10mg/L,80mL) with 30 mg of the photocatalysts under irradiation of 300 W UV or a low-power 50 W compact fluorescent lamp. Before irradiation, solutions suspended with photocatalysts are sonicated in the dark for 20 minutes to ensure the adsorption-desorption equilibrium of MB on the surface of photocatalysts. During the irradiation, 5 mL suspension is withdrawn at regular time intervals and centrifuged to remove the photocatalysts. The photodegradation efficiency is monitored by measuring the absorbance of the solution samples at the characteristic absorption wavelength of MB with a UV-vis spectrophotometer at room temperature. The degradation efficiency can be evaluated by the function Ct/C0×100%, where C0 is the initial concentration of MB and Ct is the concentration after degradation. The photocatalytic activity of commercial TiO2 nanoparticle is also evaluated as a reference to compare with that of the synthesized photocatalysts.

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3. Results and discussion 3.1 Morphology and structure characterization The morphology, size and crystal-facet-controlled fabrication of semiconductor materials

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have attracted considerable attention since their photocatalytic properties can be further enhanced or optimized by tailoring the surface atomic structures. The typical SEM images of pure CeO2 and

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F-doped CeO2 are displayed in Fig. 1. It can be clearly seen that pure CeO2 (Fig. 1(a)) possesses

irregular spherical morphology with an average diameter of about 100 nm, and some of them

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agglomerates into larger particles. Whereas F(1/3)-CeO2 (Fig. 1(b)) is found to be approximately nanocubes with 50 nm in dimensions. It is considered that the (100) terminated surface of CeO2 is

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more reactive and catalytically important than (110) and (111) facets[31]. Thus, the formation of nanocubes is favorable to enhance the photoactivity since the nanocubes have more potential to be

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bounded by the reactive (100) facets. It is known that the highly reactive facets usually diminish rapidly during the growth process because of their high surface energies. Based on the SEM observation, the possible growth process for CeO2 nanocubes can be described as a growth and

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etching mechanism. With the reaction carrying on, the rapid nucleation and growth of CeO2 occur

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in solution and tend to grow into spherical morphology and agglomerate into larger particles to

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reduce the surface energy[32,33]. As sodium fluoride is added into the reaction solution, HF will be released and etch the surface of CeO2, which promote the formation of nanocubes by concurrently etching.

To further investigate the distribution of F in the F-doped CeO2, SEM elemental mapping

measurement is carried out as shown in Fig. 2. The experimental result confirms that the F(1/3)-CeO2 nanoparticles contains only Ce, O and F elements, no other elements are observed in

the sample. From the elemental mapping it is confirmed that F is distributed homogeneously throughout the F(1/3)-CeO2 samples, indicating that F-doped CeO2 has been homogeneously doped with F and no secondary phase is present. XRD is used to investigate the phase structure of the as-prepared pure CeO2 and F-doped CeO2 photocatalysts. Fig. 3 illustrates the XRD patterns of the CeO2 and F-doped CeO2 photocatalysts. It is observed that all the diffraction peaks of pure CeO2 are correspond to the

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surface-centered cubic fluorite-type CeO2 structure (JCPDS NO. 34-0394). Moreover, the XRD patterns of F-doped CeO2 only show the main characteristic peaks of CeO2, and no characteristic reflections for F or F containing phases are observed even at the highest F doping concentration, indicating that F as a dopant in CeO2 does not change the crystalline phase and is highly dispersed

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in CeO2. However, the diffraction peaks of F-doped CeO2 are obviously widen compared to that of pure CeO2. The widen of diffraction peaks for F-doped CeO2 can be ascribed to the smaller

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crystallite size. In addition, it is worth noting that the intensity ratios for various peaks change obviously. The intensity ratio of the (200) to (111) peaks for pure CeO2 is 0.32, which is similar to

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that for the powder intensity ratio (0.35), while the intensity ratio of (200) to (111) peaks for F-doped CeO2 increase with F-doping concentration. The higher intensity ratio of (200) to (111)

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for F(1/3)-CeO2 nanocubes confirms that these F-doped CeO2 crystals are primarily composed of (200) crystalline facets[34]. This is consistent with the above SEM observations. In the three

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low-index facets of CeO2 fluorite cubic structure, the surface energy of (100) are much higher than that of (111)[31]. The percentage of highly reactive facets exposed in F-doped CeO2 is higher than

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F-doped CeO2.

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that of pure CeO2, which may contribute greatly to the enhanced photocatalytic activity of

3.2 Photocatalytic behaviors for the degradation of MB

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It is well known that the abundant solar light is inexhaustible and unlimited supply of energy

in nature. Enhancing semiconductor’s photocatalytic capability greatly under both UV and visible light irradiation has become an imperative topic to solve the worldwide energy shortage and environment pollution with highly utilization the solar energy[35,36]. Before investigating the photocatalytic performance of the samples, the light absorption property of pure CeO2 and F(1/3)-

CeO2 photocatalysts is studied by UV–vis diffuse reflectance spectroscopy, and the absorption spectra are shown in Fig. 4(a). From the Fig. 4(a), it can be seen that F-doping obviously affects light absorption characteristics of CeO2. The spectra of the F(1/3)-CeO2 samples show a stronger

absorption in the UV-vis range and a red shift in the band gap transition. It is considered that the rate of the photocatalytic reaction is proportional to (IaФ)n (n=1 for low light intensity and n=1/2 for high light intensity), where Ia is the photo numbers absorbed by photocatalyst per second and Ф is the efficiency of the band gap transition[37]. The increasing in IaФ resulting from intensive

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absorbance in the UV region and the visible light response suggests that F- doping increases the number of photogenerated electrons and holes to participate in the photocatalytic reaction and will enhance the photocatalytic activity of F-doped CeO2 photocatalyst. As shown in Fig. 4(a), F-doped CeO2 photocatalyst leads to the absorption edge shift toward

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longer wavelengths. It can be seen from Fig. 4(a) that the absorption edge of the pristine CeO2 is about 380 nm. Nevertheless, the F(1/3)-CeO2 exhibits the absorption edges at around 550 nm.

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This clearly indicates a decrease in the band gap energy of CeO2 photocatalyst owing to F doping.

The band gap energy of pure CeO2 and F(1/3)-CeO2 can be estimated according to the

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Kubelka–Munk function from a plot of (αhν)2 versus photon energy (hν) as showed in Fig. 4(b). The band gap Eg estimated from the intercept of the tangents to the plots are approximately 3.16

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eV for CeO2 and 2.88 eV for F(1/3)-CeO2, respectively. The narrowing of band gap for F-doped CeO2 samples provides a possibility of enhancing the visible-light photoactivity of CeO2.

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The photocatalytic activity of CeO2 is tested by chosen MB as a typical organic pollutant. To reach the adsorption equilibrium, the samples are mixed with the MB solution and sonicated in the

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dark for 20 min prior to the photodegradation process. Fig. 5 shows the typical absorption spectra

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of the aqueous solution of MB in the presence of 30 mg of pure CeO2 and F(1/3)-CeO2, respectively, in dark and with UV light irradiation for various durations. As shown in Fig. 5, the

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characteristic absorption peaks of MB decrease in dark after 20 min sonication, indicating the adsorption of dyes. The decrease of absorbance is recorded for measurement of the decrease of MB concentration. Fig. 5(a) shows that only about 5% of MB is adsorbed over pure CeO2 in the

dark for 20 min, while 12% of MB is adsorbed in the presence of F(1/3)-CeO2(Fig. 5(b)). The higher adsorption capacity suggests that F(1/3)-CeO2 provides more active sites to absorb MB, which may facilitate the separation of electron–hole pairs during the photochemical reaction and lead to the superior photocatalytic activity. The characteristic absorption peaks of MB further decrease as the exposure time increases due to the degradation of MB (Fig. 5). It is clear that the photocatalytic degradation rate of MB over F(1/3)-CeO2 is much higher than that over pure CeO2

under UV light irradiation. For comparison, the degradation efficiency of MB with different photocatalysts under UV light irradiation is presented in Fig. 6. It is noted that in the presence of photocatalysts, the MB

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concentration is decreased steadily with increasing irradiation time. The MB is degraded by 46% and 85%, respectively, after 15 min irradiation for the pure CeO2 and commercial TiO2 samples. It can be clearly seen that F-doped CeO2 exhibits higher photocatalytic activity than that of pure CeO2. With increasing the F-doping content, the photocatalytic activity of F-doped CeO2 increases.

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Interestingly, 92.1% of the MB is degraded after only 6 min irradiation in the presence of

F(1/3)-CeO2, displaying the highest photocatalytic activity of F(1/3)-CeO2. Further increasing the

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F-doping content will lead to the slight decrease of photocatalytic activity.

To quantitatively understand the reaction kinetics of MB degradation in our

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experiments, the kinetic data curves for MB photocatalytic degradation with photocatalysts is illustrated in Fig. 7. Fig. 7 shows that the relationship between ln(C0/Ct) (C0 and Ct are the initial

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concentration and the concentrations of MB after irradiation t min, respectively) and irradiation time is almost linear. According to Langmuir-Hinshelwood model (-ln(Ct/C0)=kt), the apparent

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reaction rate constant k of MB decomposition over F(1/3)-CeO2 is estimated to be about 0.314 min-1, which is much larger than that of pure CeO2 (0.033 min-1) and commercial TiO2 (0.144

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min-1). The photocatalytic activity of F(1/3)-CeO2 nanocubes is 9.5 times higher than that of pure

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CeO2 and 2.2 times higher than that of commercial TiO2. Besides the photocatalytic activity, stability is another key parameter to evaluate

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photocatalysts for their practical application in environmental protection. In order to investigate the recyclability and stability of F-doped CeO2, recycle experiments for F(1/3)-CeO2 is carried out under the indentical condition, as shown in Fig. 8. It is clear thatthe photocatalytic activities of F(1/3)-CeO2 shows little decrease after three cycling runs, indicating the good stability. As well known, solar light is an inexhaustible supply of energy in nature. However, only a

small fraction (less than 4%) of solar radiation is in the UV region, while visible light is far more abundant (46%), thus enhancing the photocatalytic activity of semiconductors under visible light irradiation is important to highly utilize solar energy. Therefore, the samples’ photocatalytic activities under visible light irradiation are also investigated. From Fig. 9, it can be seen that the decrease in the concentration of MB with pure CeO2 photocatalyst is negligible under visible light irradiation, indicating that there is little visible light response for CeO2 due to the relatively large band gap of CeO2. Interesting, F(1/3)-CeO2 exhibits enhanced photocatalytic activity under visible

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light irradiation. The enhanced visible light photocatalytic activities can be attributed to the narrowing of band gap due to F-doping as shown in Fig.4. These experimental results indicate that the synthesized CeO2 nanocubes exhibit higher photocatalytic activities than commercial TiO2 and spherical CeO2 for the degradation of MB dyes

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under UV and visible light irradiation. Generally, the overall photocatalytic reaction involves three

major steps: (i) absorption of light by a semiconductor to generate electron–hole pairs, (ii) charge

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separation and migration to the surface of the semiconductor, and (iii) the reactant adsorption and

surface reactions[38]. Based on the results, the enhanced photocatalytic activities of F-doped CeO2

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can be partially attributed to the smaller particle size and the highly reactive facets exposed as shown in Fig. 1 and Fig. 3. It is well known that the photoreaction mainly proceeds on the surface

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of photocatalyst. The small size of F-doped CeO2 bounded by the reactive (100) facets exposed can provide more reactive sites at the surface for adsorbing MB and the photocatalytic reaction,

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and thus enhance the photocatalytic activity. Moreover, the intense absorption in the UV-visible range and a red shift in the band gap transition of F-doped CeO2 as shown in Fig.4 suggest that

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more photons can be effectively absorbed by F-doped CeO2 and therefore induce more

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photogenerated electrons and holes that can participate in the photocatalytic reactions. Consequently, the high photocatalytic activity of F-doped CeO2 can be ascribed to several

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beneficial effects produced by F-doping: enhancement of photo absorption, presence of small size of F-doped CeO2 bounded by the reactive (100) facets exposed, and so on. This work demonstrates that the synthesized F-doped CeO2 photocatalysts exhibit high UV

and visible light photocatalytic activity by tailoring the shape and surface structure of CeO2. The

facile synthetic route may provide a general simple approach for synthesizing efficient solar light photocatalysts.

4. Conclusion In conclusion, we have demonstrated a facile and efficient process for fabricating F-doped CeO2 nanoparticles by low-temperature solution combustion method using citric acid as fuel. F-doping in CeO2 induces the smaller crystallite size and highly reactive facets exposed. The synthesized F(1/3)-CeO2 is approximately nanocubes with 50 nm in dimensions and bounded by the reactive (100) facets. It is interesting that the F-doped CeO2 nanoparticles exhibit excellent

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photocatalytic activity for the organic contaminants degradation under UV-light and visible light irradiation. The rate constant of MB degradation over F(1/3)-CeO2 nanoparticles (0.314 min-1) is much faster than pure CeO2 (0.033 min-1) sample and commercial TiO2 (0.145 min-1) under UV-light irradiation. The enhanced photocatalytic activities can be attributed to the small size and

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the reactive facets exposed, as well as the intense absorption in the UV-visible range and a red

novel photocatalyst with highly efficient for photocatalytic application.

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Acknowledgments

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shift in the band gap transition of F-doped CeO2. This work provides a simple strategy to design

This work is supported by the Changsha Science and Technology Plan Projects, China (Grant No. K1403067-11).

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(b)

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(a)

Fig.1. SEM images of as-prepared CeO2 nanoparticles (a) and F(1/3)-CeO2 nanoparticles(b).

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(Ce)

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(O) (F)

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Fig.2. Mapping of F(1/3)-CeO2 sample.

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(111)

pure CeO2

F(1/3)-CeO2

F(1/10)-CeO2

F(2/5)-CeO2

20

30

40

50

60

70

ip t

80

an

2Theta(deg)

us

cr

(420)

(331)

(400)

(222)

(311)

(220)

(200)

Intensity(a.u.)

F(3/10)-CeO2

Ac ce p

te

d

M

Fig.3. XRD patterns of as-prepared CeO2 and F(1/3)-CeO2 samples.

Page 14 of 21

F(1/3)-CeO2

F(1/3)-CeO2

-3

400

500

2.2

600

2.4

2.6

an

Wavelength(nm)

us

300

cr

2

2

Intensity

ip t

Pure CeO2

(αhν) (ev nm )

Pure CeO2

(a)

2.8

3.0

3.2

3.4

hν(eV) (b)

Fig.4. (a) UV-Vis absorption spectra of Pure CeO2 and F(1/3)-CeO2, (b) Plots of

Ac ce p

te

d

M

(αhν)2 versus photon energy (hν) for pure CeO2 and F(1/3)-CeO2

Page 15 of 21

700

500

ip t cr

Intensity(a.u.) 600

us

500

blank 0min 3min 6min 9min 12min 15min 18min

600

700

Wavelength(nm)

an

Wavelength(nm)

(a)

(b)

M

Fig.5. The typical absorption spectra of the aqueous solution of MB in the presence of

te

d

pure CeO2 (a) and F(1/3)-CeO2 (b)

Ac ce p

Intensity(a.u.)

blank 0min 3min 6min 9min 12min 15min 18min

Page 16 of 21

0.8

Pure CeO2

F(1/3)-CeO2

F(1/10)-CeO2

F(2/5)-CeO2

F(3/10)-CeO2

TiO2

cr

0.4 0.2

0

3

6

9

12

15

18

an

0.0

us

Ct/C0

0.6

ip t

1.0

Time(min)

M

Fig.6. Comparisons of photocatalytic activities of different samples for the

Ac ce p

te

d

photodegradation of MB under UV light irradiation.

Page 17 of 21

Pure CeO2

3.5 3.0

F(1/3)-CeO2 TiO2

-1

2.0

min k=0.130

1.5

-1

k=0.085

1.0 0.5 0.0

ip t

-1

44min k=0.1

F(2/5)-CeO2

2.5

-1

9min k=0.23

min

us

ln(C0/Ct)

-1

F(1/10)-CeO2 k=0.314min F(3/10)-CeO2

4.0

cr

4.5

-1

k=0.033min

0

3

6

9

12

18

21

an

Time(min)

15

Ac ce p

te

d

M

Fig.7. Kinetics of the MB degradation for different photocatalysts.

Page 18 of 21

1.0

1st 2nd 3rd

ip t

0.8

cr

0.4 0.2

3

6

9

12

15

18

21

24

an

0.0

us

Ct/C0

0.6

27

Time (min)

M

Fig.8. Cycling runs of F(1/3)-CeO2 sample in the photodegradation of MB under UV

Ac ce p

te

d

light irradiation.

Page 19 of 21

1.0 Pure CeO2 F(1/3)-CeO2

ip t

0.2

ln(C0/Ct)

k=0.0896min-1

Ct/C0

0.9

Pure CeO2 F(1/3)-CeO2 0.0

2

0

3

1

an

1

Time (h) (a)

k=0.0149min-1

2

3

Time (h) (b)

M

Fig.9. Comparisons of photocatalytic activities of different samples for the photodegradation of MB under visible light irradiation and the kinetic curves of

te

d

samples.

Graphical abstract

Ac ce p 1.0 0.8

Pure CeO2

F(1/3)-CeO2

F(1/10)-CeO2

F(2/5)-CeO2

F(3/10)-CeO2

TiO2

0.6

Ct/C0

0

us

0.8

cr

0.1

0.4 0.2 0.0

0

3

6

9

12

15

18

Time(min)

Page 20 of 21

Highlights F-doped CeO2 nanocubes with higher percentage of reactive facets exposed is synthesized.

Ac ce p

te

d

M

an

us

cr

F-doped CeO2 nanocubes exhibit high photocatalytic activities.

ip t

F-doping in CeO2 results in the narrowing of band gap.

Page 21 of 21