Tb2O3 nanotubes

Accepted Manuscript Title: Photocatalytic degradation mechanisms of CeO2 /Tb2 O3 nanotubes Author: Narayanasamy Sabari Arul Devanesan Mangalaraj Tae Whan Kim PII: DOI: Reference:

S0169-4332(15)01072-7 http://dx.doi.org/doi:10.1016/j.apsusc.2015.04.206 APSUSC 30299

To appear in:

APSUSC

Received date: Revised date: Accepted date:

24-10-2014 27-4-2015 28-4-2015

Please cite this article as: N.S. Arul, D. Mangalaraj, T.W. Kim, Photocatalytic degradation mechanisms of CeO2 /Tb2 O3 nanotubes, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.04.206 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.

*Highlights (for review)

Research Highlights

 CeO2/Tb2O3 nanotubes have been synthesized using the surfactant free co-precipitation

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method.

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 HRTEM images, XPS spectra, and EDAX profiles showed that the as-synthesized

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samples were CeO2/Tb2O3 nanotubes.

 Photocatalytic activity of the synthesized catalysts was evaluated by degrading

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Methylene blue under visible light irradiation.

 Estimated rate constants for the CeO2 nanoparticles and the CeO2/Tb2O3 nanotubes were

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0.0134 and 0.0317 min-1, respectively.

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te

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 Photodegradation efficiency of CeO2/Tb2O3 nanotubes was 93% after 75 min.

Page 1 of 21

*Manuscript

Photocatalytic degradation mechanisms of CeO2/Tb2O3 nanotubes

Narayanasamy Sabari Arul,1,2,3 Devanesan Mangalaraj,2,a) and Tae Whan Kim1,a) 1

Department of Electronic Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong2

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gu, Seoul 133-791, Republic of Korea Department of Nanoscience and Technology, Bharathiar University, Coimbatore-641 046, India

Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, 100715,

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3

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Republic of Korea.

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Abstract

CeO2/Tb2O3 nanotubes (NTs) have been synthesized using the surfactant free co-

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precipitation method. High resolution transmission electron microscopy (HRTEM) images, X-ray spectroscopy (XPS) spectra, and energy dispersive X-ray (EDAX) profiles showed that

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the as-synthesized samples were CeO2/Tb2O3 NTs. The photocatalytic activity of the synthesized catalysts was evaluated by degrading Methylene blue (MB) under visible light

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irradiation. The fitting of the absorbance maximum as a function of time showed that the photodegradation of the MB followed pseudo-first-order reaction kinetics. The estimated rate constants for the CeO2 NPs and the CeO2/Tb2O3 NTs were found to be 0.0134 and 0.0317

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min-1, respectively. The photodegradation efficiency of CeO2/Tb2O3 nanotubes was 93% after 75 min, which was found to be higher than those of CeO2 NPs (66%).

Keywords: CeO2/Tb2O3, Nanotubes, Transmission electron microscopy, Methylene Blue, Photocatalyst PACS number: 81.07.De, 68.37.Lp, 82.80.Pv

a)

Authors to whom correspondence should be addressed. Electronic address: [email protected] and [email protected]

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1. Introduction The promising applications of semiconductor photocatalysts in wastewater treatment for environmental remediation have attracted a great deal of interest because of their utilization of solar energy [1, 2]. Among the various kinds of oxide semiconductor photocatalysts, cerium-

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oxide (CeO2) materials have shown a promising photocatalytic activity for the elimination of environmental pollutants [3], and they have possible applications in oxygen sensors, catalysts,

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gas sensors, and electrolyte materials for solid oxide fuel cells [4-8].

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One-dimensional (1D) CeO2 nanostructures have been particularly attractive because of interest in investigations of both fundamental physical properties and potential applications in

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optoelectronic, electrochemical and mechanical devices operating at low powers [9, 10]. CeO2 semiconductors have limited photocatalytic activity in the visible-light region because of their

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large energy band gap of 3.2 eV [11]. Besides, with suitable matching of the energy band levels, CeO2 combined with other semiconductors can dramatically increase the photocatalytic

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efficiency due to an increase in the charge separation and to an extension of the energy range for photoexcitation [12, 13]. However, the photocatalytic degradation mechansims of

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heterojunction CeO2/Tb2O3 nanotubes synthesized by using the precipitation method have not yet been clarified. Some studies concerning the photocatalytic activity of BiVO4/CeO2 nanocomposites prepared by coupling a homogenous precipitation method with a

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hydrothermal technique have been performed [14]. Furthermore, the photocatalytic degradation of the Methylene Blue (MB) for the CeO2 composites was much higher than that of CeO2 under visible-light illumination [15-17]. This paper reports data for the photocatalytic degradation mechansims of CeO2/Tb2O3 nanotubes formed by using a surfactant-free precipitation method. High-resolution transmission electron microscopy (HRTEM) measurements were performed to characterize the structural properties of the formed CeO2 nanoparticles (NPs) and composite CeO2/Tb2O3

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nanotubes (NTs). X-ray photoelectron spectroscopy (XPS) measurements were carried out to investigate the stoichiometry and the electronic properties of the CeO2/Tb2O3 NTs. Ultraviolet-visible (UV-vis) absorption measurements were performed to compare the

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photocatalytic behaviors of CeO2 NPs with those of CeO2/Tb2O3 NTs.

2. Experimental

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2.1. Chemicals

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Cerium (III) nitrate (Ce(NO3)3.6H2O), terbium (III) nitrate (Tb (NO3)2.6H2O), and ammonium hydroxide (NH4OH) was purchased from Alfa Acer. All chemicals were of

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analytical grade. De-ionized water is used throughout all of our experiments. 2.2. Synthesis of CeO2 NPs and CeO2/Tb2O3 NTs catalysts

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CeO2 NPs and CeO2/Tb2O3 NTs were synthesized by using a chemical precipitation method [18, 19]. Initially, 0.5 M of Cerium(III) nitrate (Ce (NO3)3.6H2O) was added drop

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wise to the required amount of Terbium (II) nitrate (Tb (NO3)3.6H2O) solution and stirred vigorously at room temperature. The pH was maintained at 10 by using an ammonium

ce pt

hydroxide (NH4OH) solution. The resultant colloidal solution was sealed in a vessel to make a precipitate, which was aged at room temperature for two days. The obtained precipitate was separated by centrifuging at 6000 rpm and washed several times with ethanol and de-ionized

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water to remove the impurities. Finally, the colloidal precipitate was dried at 90oC for over night and then subjected to further characterization. A similar procedure was adopted to obtain the CeO2 NPs.

2.3. Characterization techniques The crystallinity and the phase purity of the formed products were determined by using a PAN Analytical X’Pert Pro diffractometer using Cu Kα radiation (λ = 0.15406 nm). A scanning rate of 0.05os-1 was applied to record the XRD patterns. HRTEM observations were

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captured by using a JEM JEOL-2100F system operating at an accelerating voltage of 200 kV. The XPS measurements were performed by using the Kratos Analytical ESCA-3400 system with an MgKα X-ray source, energy of 1253.6 eV, and an operating voltage of at 10 kV. The charging effect was corrected by referencing the data to the binding energy of the carbon C 1s

spectrophotometer-3600

at

room

temperature

under

ambient

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core level peak at 285.9 eV. UV-vis absorption spectra were obtained by using a Shimadzu conditions.

The

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photodegradation properties were observed by using a SHIMADZU-3600 spectrophotometer

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to measure the absorption of the MB dye at 663 nm.

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3. Results and Discussion

Fig. 1 shows the XRD patterns of CeO2 NPs and CeO2/Tb2O3 composites NTs. The

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diffraction peaks at 28.8o, 33.3o, 47.6o, 56.4o, 59.1o, 69.3o, 76.7o, 79.1o, and 88.3o can be indexed as the (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes of the

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cubic fluorite CeO2 crystal according to the Joint Committee Powder Diffraction Standards (JCPDS) PDF file no. 34-0394 [20]. The diffraction peaks at 28.8o, 33.3o, 47.9o, 51.1o, and

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56.8o for the CeO2/Tb2O3 composite nanotubes correspond to (222), (400), (440), (600), and (622) planes of monoclinic Tb2O3 (JCPDS PDF file no. 86-2478) [21], while those of CeO2 can be readily indexed to the cubic fluorite CeO2. The XRD pattern of the CeO2/Tb2O3 NTs

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clearly matches with the polycrystalline structures of CeO2 and Tb2O3, indicating the formation of composite CeO2/Tb2O3 NTs without any other impurity. The HRTEM images in Fig. 2 show the morphologies of the CeO2 nanoparticles and the CeO2-Tb2O3 nanotubes. The selected-area electron diffraction pattern for the CeO2 nanoparticles demonstrates the polycrystalline nature of the cubic fluorite CeO2, as shown in the inset of Fig. 2(a). Figures 2(b) and 2(c) show the formation of CeO2/Tb2O3 nanotube-like

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morphologies with a length of 300 nm and a width of 20 nm. Figure 2(d) clearly shows two different crystal structures on the composite surfaces of the CeO2-Tb2O3 nanotubes. XPS measurements were conducted to analyze the elemental compositions and the chemical-bonding environments of CeO2 nanoparticles and CeO2-Tb2O3 nanotubes, as shown

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in Fig. 3. The wide survey-spectra, shown in Fig. 3(a) reveal the presence of Ce, Tb, and O in the synthesized CeO2 nanoparticles and in the synthesized CeO2/Tb2O3 NTs. The Ce 3d core

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spectra depict the presence of ν and u, indicative of the spin-orbit couplings of the Ce 3d5/2

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and 3d3/2 levels, respectively, as shown in Fig. 3(b). The peaks assigned to CeO2, denoted by ν', ν'', and ν ''', are attributed to mixtures of Ce IV (3d9 4f2) O (2p4), Ce IV (3d9 4f1) O (2p5),

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and Ce IV (3d9 4f0) O (2p6) [22]. The same peak assignment is applied to ‘u’ mixtures and the ν'/u' doublet, which is attributed to the photon emission from Ce3+ cations [23]. The Ce 3d

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spectra of both samples indicate the chemical state of Ce in CeO2. Figure 3(c) shows the O1s spectra for CeO2 nanoparticles and CeO2-Tb2O3 nanotubes. The full width at half maximum of

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the O1s peak of the CeO2/Tb2O3 NTs is broader than that of the CeO2 nanoparticles, indicating that the surfaces of the crystalline CeO2-Tb2O3 nanotubes are more affluent in

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oxygen species [24]. The O1s peaks at 528.2 and 530.6 eV are attributed to the lattice oxygen, O2-, of CeO2, and the peak at 532.2 eV is related to the chemisorbed OH group [25, 26]. Figure 2(d) shows the Tb 4d core-level spectrum of CeO2/Tb2O3 NTs. The most intense core

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level of terbium is the Tb 3d level with a high binding energy between 1230 and 1290 eV. Because the kinetic energy of the photoemitted electrons is relatively very low for a commercial Al K X-ray source (1486.6 eV), analysis of the Tb 3d core level is very difficult. Thus, the next most intense level, the Tb 4d core level, was used for the analysis. The Tb 4d core-level spectrum exhibits both the Tb3+ ions in Tb2O3 and the Tb4+ ions in TbO2 at 149.1 and 156.1 eV, respectively, as shown in Fig. 3(d) [27]. Moreover, the broadness of the peak at

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156.1 eV confirms the presence of Tb4+ ions, indicating that the concentration of Tb4+ ions is much higher than that of Tb3+ ions. The photocatalytic activities of the CeO2 NPs catalyst and the CeO2/Tb2O3 NTs catalyst were evaluated by using the photocatalytic decolorization of a model pollutant, MB

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commercial dye [28], using a 50 W Xenon lamp under visible light illumination. A 10 mg of CeO2 NPs and CeO2/Tb2O3 NTs photocatalyst were taken and dispersed separately in 15 ml of

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0.3-mM MB dye. Before illumination, the suspension was vigorously stirred for 10 min in a

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dark to attain absorption-desorption equilibrium between the MB dye and the photocatalyst. A blank experiment showed no photocatalytic activity in the absence of the photocatalysts under

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UV irradiation. The efficiencies of the degradation processes with the photocatalysts were evaluated by using an UV-vis spectrophotometer to monitor the dye decolorization at the

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maximum absorption wavelength of the MB dye.

Figures 4(a) and 4(b) represent the absorption spectra for various irradiation time

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intervals for the MB aqueous solutions containing CeO2 NPs and CeO2/Tb2O3 NTs catalyst. The main absorption peak at 663 nm gradually decreases with increasing irradiation time

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from 0 to 75 min. However, the photodegradation rate of the MB dye solution with CeO2/Tb2O3 NTs is faster than that with CeO2 NPs. The absorption peak of the MB completely disappears when the UV exposure time reaches 75 min. The photodegradation

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efficiencies of the MB dye mediated with and without photocatalysts under visible light irradiation are shown in Fig. 4(c). The degradation magnitude of the MB dye under visible irradiation without a photocatalyst is almost negligible, as shown in Fig. 4(c). The plot of ln (Ao/At) as a function of the irradiation time (t) for the MB dye with a photocatalyst is shown in Fig. 4(d). The degradations in the MB dye photocatalyzed by CeO2 NPs and by CeO2/Tb2O3 NTs follow a pseudo-first-order equation (1) as follows;

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ln(Ao/At) = kt,

(1)

where Ao is the final concentration of the MB dye, At is the initial concentration of the MB dye, and k is the apparent rate constant for the pseudo-first-order reaction [29]. The visible

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light driven from the photocatalytic degradation rate of the CeO2/Tb2O3 nanotubes is 93% after 75 min, which is found to be superior to those of the CeO2 NPs (66%). The apparent rate

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constants for the CeO2 NPs and the CeO2/Tb2O3 NTs are estimated from the slopes of plots of

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ln(Ao/At) versus t and are found to be 0.0134 and 0.0317 min-1, respectively. Fig. 4(e) shows the recyclable test of the CeO2/ Tb2O3 NTs for 1-4 cycles in degrading MB dye under visible

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light irradiation. These results indicate that the CeO2/ Tb2O3 NTs clearly exhibit an excellent stability with less than 10% decrease from its initial activity to final photodegradation process.

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Fig. 5 displays the plot for (αhυ)2 versus hυ of CeO2 NPs and CeO2/Tb2O3 NTs. The

(2)

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αhν = C (hν – Eg)n,

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optical energy band gap values are estimated by using the equation (1) as follows;

where α is the absorption co-efficient, hν is photon energy, C is the constant, n is ½ (for directly allowed transition), and Eg is the band gap of the material. The estimated band gap

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values for CeO2 NPs and CeO2/Tb2O3 NTs are found to be 3.24 and 2.92 eV, respectively. The absorption of CeO2/Tb2O3 NTs exhibited a broad visible-light absorption, which might be beneficial for the generation of electron-hole pairs under visible-light irradiation, resulting in the enhanced photocatalytic activity [30]. Fig. 6 shows the proposed photocatalytic degradation mechanism of CeO2/Tb2O3 NTs. The band edge positions of the conduction band (CB) and valence band (VB) of the semiconductor can be determined using the equation (3) [31];

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EVB = χ – Ee + 0.5 Eg,

(3)

where χ is the absolute electronegativity of the semiconductor (χCeO2 = 5.57 eV and χTb2O3 =

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5.33 eV), Ee is the energy of the free electrons on the hydrogen scale (ca. 4.5 eV), EVB is the VB edge potential, and Eg is the energy band gap of the CeO2 (3.24 eV) or Tb2O3 (3.8 eV)

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[32]. The CB position can be deduced by ECB = EVB - Eg. The estimated CB and the VB of the

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CeO2 are 2.71 and -0.55 eV, respectively, and they are more beneficial than those of the Tb2O3 (2.69 and -1.304 eV). In addition, the work function of the Tb2O3 (3.1 eV) [33], being

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smaller than that of the CeO2 (4.69 eV) [34], enables the transfer of electrons from the Tb2O3 to the CeO2, leading to a superior charge carrier concentration in the nanotubes [35].

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The photogenerated holes from CeO2 migrate to the VB of Tb2O3 during a photodegradation process, while the photogenerated electrons in Tb2O3 transfer to the

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CB of CeO2. Besides, the holes in the VB of Tb2O3 readily oxidize OH- species or H2O● producing reactive hydroxyl (●OH) radicals, which are further involved in the

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photodegradation of the EY [16, 17]. This reaction can be given by,

h+VB + OH- → OH●.

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(4)

In the meantime, the gathered electrons in the CB of CeO 2 can be migrated to

O2 molecules absorbed on the surface of the product and yield superoxide anion (O 2●-), the equation is given by;

e-CB + O2 → O2●-.

(5)

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Finally, the formed ●OH and O2●- act as an effective oxidizing agent in degrading MB dye and represented in the equation as follows;

(OH ●, O2●-) + EY dye → Degradation of the MB dye.

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(6)

These results indicate that CeO2/Tb2O3 NTs hinder the recombination of photo-

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induced charge carries and promote the generation of more OH ● radicals, resulting in an

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enhancement of the photocatalytic degradation efficiency for the CeO2/Tb2O3 NTs.

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4. Conclusions

CeO2/Tb2O3 NTs were synthesized by using chemical precipitation method on a large

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scale. HRTEM images showed that the length and the width of the CeO2/Tb2O3 NTs were approximately 400 and 50 nm, respectively. XPS spectra confirmed the valence states of Ce

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3d, Tb 4d, and O1s in the prepared samples. The photocatalytic degradation efficiency of CeO2/Tb2O3 nanotubes is 93% after 75 min, which is found to be superior to those of CeO2

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NPs (66%). The excellent photocatalytic activity of the CeO2/Tb2O3 NTs catalyst was attributed to the high absorption capability and enhanced lifetimes of the electron-hole pairs for the effective degradation of the MB dye. These results indicate that CeO2/Tb2O3 NTs have

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great potential for applications as photocatalysts.

Acknowledgments

This research work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-016467).

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Figure captions

Fig. 1. XRD patterns of CeO2 NPs and CeO2/Tb2O3 NTs. Fig. 2. HRTEM images of (a) 0.5 M of CeO2, (b) low, (c, d) high bright-field TEM images of

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CeO2/Tb2O3 NTs. The insets of (a, d) present the SAED patterns of CeO2 NPs and CeO2/Tb2O3 NTs.

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Fig. 3. XPS spectra of 0.5-M pure CeO2 NPs and CeO2/Tb2O3 NTs (a) wide spectra, (b) Ce 3d,

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(c) O 1s, (d) Tb 4d.

Fig. 4. UV-Vis absorbance spectra of (a) CeO2 NPs and (b) CeO2/Tb2O3 NTs in an MB dye

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solution with various irradiation times. (c) Photocatalytic degradation rate of MB dye (blank), MB dye solution with CeO2 NPs, and CeO2/Tb2O3 NTs under various

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UV-Vis light irradiation times, (d) plot of In (Ao/At) as a function of the irradiation time for CeO2 NPs and CeO2/Tb2O3 NTs, and (e) recyclable test of CeO2/ Tb2O3

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NTs in degrading MB dye under visible light irradiation. Fig. 5. Plot of (αhυ)2 versus hυ of CeO2 NPs and CeO2/Tb2O3 NTs.

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Fig. 6. Proposed photocatalytic degradation mechanisms of the CeO2/Tb2O3 NTs during the

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degradation of MB dye.

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