CeO2 Nanorod–TiO2 Nanotube Hybrid Nanostructure

Yang Yang, Xiaohui Wang,w Changku Sun, and Longtu Li

State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering,Tsinghua University, Beijing 100084, China

CeO2 nanorods with a polycrystalline structure were preparedby galvanostatic electrodeposition using TiO2 nanotube arraysas templates. The heterogeneous nucleation and subsequentgrowth of CeO2 nanorods are an electric field-assisted process,which depend upon the current density and electrolyte concen-tration. At low electric field intensity, amorphous nanoparticlesare formed; whereas, at high electric field intensity, a layer ofdense film is deposited (caused by the overgrowth of CeO2). Byusing an optimum electrochemical condition, namely currentdensity of 0.1 mA/cm

2and Ce(Cl)3 solution concentration of 0.5

mmol/L, moderate electric field intensity can be obtained togrow the CeO2 nanorod–TiO2 nanotube hybrid nanostructure.

I. Introduction

THE discovery of TiO2 nanotube arrays electrochemically an-odic grown in fluorinated electrolyte was reported by

Grimes and colleagues,1 and since then related issues have at-tracted considerable attention from numerous researchers due tothe controllability, high efficiency, and repeatability of anodizat-ion. Related works were focused on investigating the anodicgrowth mechanism, optimizing the morphology, and exploitingthe potential applications of TiO2 nanotube arrays.

2–5 The ver-tically well-aligned nanotubular structure of TiO2 nanotube ar-rays makes them have various novel properties such as electronfield emission and photoluminescence.6 Using the unique nano-tubular structure of TiO2 nanotube arrays, on the other hand,one-dimensional nanocomposites can be synthesized by tem-plate-assisted electrodeposition.7,8 CeO2 is a promising materialthat has been widely used in modern technologies, including in-dustrial catalyst,9 gas sensors,10 and solid oxide fuel cells.11 Ithas been found that a well-coupled CeO2–TiO2 composite couldproduce a special electron transfer process, which facilitates theseparation of the electron–hole pairs and improve the photo-catalytic activity.12 In addition, TiO2 nanotube arrays have ahighly ordered nanotubular structure, which can be used as ahost to support other nanoscale materials. Recently, the prep-aration of low-dimensional CeO2 nanomaterials and CeO2/TiO2

nanoparticles have been reported13–15; however, there is still norelated report on the fabrication of CeO2 nanorod–TiO2 nano-tube hybrid arrays by electrodeposition using TiO2 nanotubearrays as templates. In this work, the preparation of CeO2

nanorod–TiO2 nanotube hybrid arrays by galvanostatic elec-trodeposition is demonstrated for the first time.

II. Experimental Details

Titanium foils (99.5% purity) were used as substrates to growanodic TiO2 nanotube arrays in 1M H3PO4 containing 0.5 wt%HF electrolyte. The potentiostatic anodization was conducted at20 V for 30 min using a platinum foil as a counter electrode. Theanodic TiO2 nanotube arrays were annealed at 4001C for 3 h toconvert the amorphous TiO2 to anatase phase. Thereafter, TiO2

nanotube arrays grown on Ti substrates were used as templates toelectrodeposit CeO2 nanorods. The electrodeposition experimentswere conducted in Ce(Cl)3 solution using the galvanostatic mode.The microstructures were observed using a field-emission scan-ning electron microscope (FESEM, LEO-1530, LEO, Oberkoc-hen, Germany). The cross-section images and crystal lattice stripeimage of the specimens were obtained using a JEOL-2010 high-resolution transmission electron microscope (HRTEM, JEOL,Tokyo, Japan). The crystal structures of the specimens were de-termined by X-ray diffraction (XRD) on a D/max-RB diffracto-meter (CuKa radiation; Rigaku, Rotafles, Tokyo, Japan). Ramanspectra were recorded using a confocal microscopic Raman spec-trometer (RM2000, Renishaw, Gloucestershire, U.K.) with anexciting line of 633 nm. X-ray photoelectron spectroscopy (XPS)measurement was carried out using an AXIS Ultra spectrometer(Kratos, Manchester, U.K.), with AlKa radiation (binding en-ergy51486.7 eV). The binding energy for the samples was cal-ibrated by setting the measured BE of C1s to 284.8 eV.

III. Results and Discussion

Figure 1(a) shows the FESEM image of TiO2 nanotube arraysanodic grown on Ti substrates. The as-prepared TiO2 nanotubesshow a highly ordered nanotubular structure with an averagediameter of 150 nm. Different current densities of 0.08, 0.1, and0.15 mA/cm2 were used in the galvanostatic electrodeposition.The experiments were conducted in the Ce(Cl)3 electrolyte so-lution with a concentration of 0.5 mmol/L. Figures 1(b)–(d)show the FESEM images of the as-prepared specimens obtainedby electrodeposition at different current densities for 1 h. Figure1(b) shows the FESEM image of CeO2 nanorods prepared at acurrent density of 0.1 mA/cm2. CeO2 nanorods with an averagediameter of 100 nm were electrodeposited on the TiO2 nano-tubes. The CeO2 electrodeposits have a rodlike structure and allthe CeO2 nanorods assemble into an array structure. Seen fromFigs. 1(c) and (d), it is also clear that lower and higher currentdensities cannot grow CeO2 nanorod arrays. At a current den-sity of 0.08 mA/cm2, some nanoparticles with an average size of30 nm were tightly embedded with TiO2 nanotubes. At a currentdensity of 0.15 mA/cm2, the template was covered by a layer ofdense film without any nanotubular structure and nanorods.This layer of dense film was composed of nanoparticles with anaverage diameter of 100 nm. In order to investigate the influenceof electrolyte concentration on the formation of CeO2 nanorods,Ce(Cl)3 solutions with a concentration of 0.5, 10, and 100 mmol/L were used at a current density of 0.1 mA/cm2 for 1 h. Figures1(e) and (f) show the FESEM images of the specimens preparedin Ce(Cl)3 solutions with a concentration of 10 and 100 mmol/L,

O. Varghese—contributing editor

This work was supported by National Science fund for distinguished young scholars(Grant No. 50625204), Science Fund for Creative Research Groups (Grant No. 50621201)National Natural, Science Foundation of China (Grant No. 50921061), and by the Ministryof Sciences and Technology of China through the National Basic Research Program ofChina (973 Program 2009CB623301) and through the 863-program under Grant2006AA03Z0428, also supported by Samsung Electro-Mechanics Co. Ltd.

wAuthor to whom correspondence should be addressed. e-mail: wxh@mail.tsinghua.edu.cn

Manuscript No. 27630. Received March 3, 2010; approved April 22, 2010.

Journal

J. Am. Ceram. Soc., 93 [9] 2555–2559 (2010)

DOI: 10.1111/j.1551-2916.2010.03892.x

r 2010 The American Ceramic Society

2555

respectively. It can be seen from Fig. 1(e) that some nanorodsand nanoparticles were deposited on top of the TiO2 nanotubes.Both nanorods and nanoparticles have the same average diam-eter of 100 nm, which is similar with the inner diameter of theTiO2 nanotube. But the nanorods grow poorly and no highlyordered nanorod arrays can be formed in 10 mmol/L Ce(Cl)3solution. It is also clear that the undergrowth situation of elec-trodeposits is aggravated in the 100 mmol/L Ce(Cl)3 solution. Alarge amount of nanoparticles with an average diameter of 100nm cover the surface of TiO2 nanotube arrays.

Seen from the TEM image of CeO2 nanorod as shown inFig. 2(a), the diameter of CeO2 nanorod is approximately 100nm. HRTEM observation was also performed on the specimen,as shown in Figs. 2(b) and (c). The d spacing estimated statis-tically from the lattice fringe is 0.32 nm, which is ascribed toCeO2 (111). The polycrystalline structure of the CeO2 nanorodalso can be seen from HRTEM images. The growth direction ofouter nucleus is slightly different from the inner part in thenanorod. And the diameter of the outer nucleus is approxi-mately 5 nm. It is obvious that the outer nuclei are newlyformed, which have not grown into a CeO2 nanorod. Underthe guidance of electric field, the nuclei in CeO2 nanorods willgrow in an identical direction with enough growth duration.

In order to investigate the growth mechanism of CeO2 nano-rod arrays, XRD, Raman, and XPS were performed on theCeO2 nanorod–TiO2 nanotube hybrid arrays obtained. Figure3(a) shows the XRD pattern of the specimen with the Lorentz-ian-fitted CeO2 (111) diffraction peak (marked as dashed line) inthe inset. The obvious widening of the CeO2 (111) diffractionpeak is due to the nanoscale size effect of CeO2 nanorods. Ac-cording to Scherrer’s equation, Dhkl5 0.89l/bcosy, the crystal-lite sizes of the CeO2 nanorods were calculated to beapproximately 7 nm, which was similar with the value calcu-lated from HRTEM image. Figure 3(b) shows the Raman spec-trum of the specimen. The phonon modes for anatase TiO2

nanotubes can be found at 146, 396, 515, and 634 cm�1. Addi-tionally, a phonon mode for CeO2 can also be observed at 455cm�1, which is corresponding to the CeO2 F2g vibration mode.16

The Lorentzian-fitted CeO2 F2g Raman spectrum is marked as adashed line in the inset of Fig. 3(b). The typical CeO2 F2g vi-bration mode in fluorite structure is usually viewed as a sym-metric breathing vibration mode of O2� ions around each cationand is sensitive to oxygen lattice disorder.17,18 In bulk CeO2, theF2g vibration mode centers at about 465 cm�1.19 But in the CeO2

nanorods obtained, this mode shows a strong red shift to 455cm�1, which is due to the asymmetry with the increased line-width in nanoscale CeO2.

20 Figures 3(c) and (d) show the XPSnarrow scan spectra of Ce 3d and O 1s core level peaks, respec-tively (the Lorentzian-fitted XPS spectra were marked as dashedline in the figures). Seen from Fig. 3(c), three groups of doubletsf 0, f 1, and f 2 can be found, which are induced by the Ce 3d3/2–3d5/2 spin–orbit split.

21 The relative intensities of these doubletsshow a typical evidence of the formation of CeO2: a high f 0

signal, together with an f 1 peak that is less intense than the f 2

peak.22,23 Additionally, two other doublets at 884.67 and 903.08eV represent the existence of Ce2O3, which is induced by thesurface states in the surface of CeO2 nanorods. The existence ofsurface states can also be proved by the O 1s XPS spectra asshown in Fig. 3(d). Two fitted binding energies peaks can befound centered at 528.14 and 529.92 eV. The major peak of thelower binding energy was assigned to the lattice oxygen (Ce–Obond) in CeO2 nanorods, while the smaller peak of a higherbinding energy was assigned to the hydroxyl group or absorbedoxygen in the surface of the CeO2 nanorods obtained. The rel-ative intensity of the higher binding energy (absorbed oxygen) isclose to that of the lower binding energy (lattice oxygen) in CeO2

nanorods, which is a typical evidence for the existence of surfacestates in the surface of CeO2 nanorods. The XRD and XPS an-alyses were also performed on the specimens without the for-mation of CeO2 nanorods in order to investigate the growthmechanism of CeO2 nanorods during electrodeposition. Figure3(e) shows the XRD patterns of the CeO2 nanoparticles elec-trodeposited specimens before and after heat treatment at5001C. Before heat treatment (red line), only the diffractionpeak of anatase phase TiO2 nanotubes templates can be found.After heat treatment, in the specimen at 5001C (black line),

Fig. 1. (a) Field-emission scanning electron microscopy image of TiO2 template anodic grown at 20 V for 30 min; (b) CeO2 nanorods with a diameter of100 nm obtained by galvanostatic electrodeposition at 0.1 mA/cm2; (c) nanoparticles with a diameter of 30 nm were absorbed and embedded with TiO2

nanotube at 0.08 mA/cm2; (d) a layer of dense film was formed at 0.15 mA/cm2; (e) the specimens prepared in 10 mmol/L Ce(Cl)3 solution; (f) thespecimens prepared in 100 mmol/L Ce(Cl)3 solution.

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beside the anatase (101) diffraction peak, a rutile (100) diffrac-tion peak also appears due to the phase transformation in theTiO2 nanotubes templates. Additionally, CeO2 (111) and (200)diffraction peaks also appear after heat treatment. Figure 3(f)shows the XPS spectra of the Ce-containing amorphous nano-particles electrodeposited specimens before and after heat treat-ment at 5001C. The bonding configuration in nanoparticlesbefore heat treatment is mainly composed of Ce31-related bind-ing energy peaks together with much lower Ce41-related bindingenergy peaks. Whereas after heat treatment at 5001C, the Ce31-related XPS peak intensity weakens and Ce41-related XPS peakintensity is much stronger. Associated with the XRD discussion,it is clear that the as-prepared nanoparticles are composed of aCe31-dominated amorphous phase and a part of poorly crys-tallized Ce41, and after heat treatment, the amorphous nano-particles crystallized into fluorite structure of CeO2.

Deduced from the FESEM observation and microstructureanalyses, the electrodeposition process of CeO2 nanorods on TiO2

nanotube arrays can be concisely depicted as shown in Fig. 4.And during the electrodeposition process, there are some chem-ical reactions responsible for the formation of CeO2 nanorods

2H2Oþ 2e� ! H2 þ 2OH� (1)

O2 þ 2H2Oþ 4e� ! 4OH� (2)

Ce3þ þ 3OH� ! CeðOHÞ3 (3)

2CeðOHÞ3 ! Ce2O3 þ 3H2O (4)

2Ce2O3 þO2 ! 4CeO2 (5)

At the initial stage of electrodeposition process, Ce31 ionswere oriented moving toward the cathode under an electric fieldeffect. Meanwhile at the cathode surface, a large number ofhydroxyl groups (OH�) were produced due to the electrolysis ofwater (reactions (1) and (2)). These OH� groups had a highconcentration on the surface of cathode. Hence, Ce31 combinedwith those OH� groups to form Ce(OH)3 nanoparticles (reac-tion (3)). Because of the surface absorption effects of TiO2

nanotubes, the amorphous Ce(OH)3 nanoparticles were ab-sorbed on top of the nanotubes (process B1 as shown inFig. 4). The heterogeneous nucleation and the subsequentgrowth of CeO2 nanorods on the top of TiO2 nanotubes wereinduced by dewatering of Ce(OH)3 (reaction (4)) and reoxidiza-tion process (reaction (5)) under the effects of electric field (pro-cess B2). With prolonging the electrodeposition time, moreCeO2 nuclei formed and combined with the already-formedCeO2 nanorod. Then, highly ordered CeO2 nanorod arrayswere formed (process B3). The nucleation and growth of CeO2

nanorods are electric field-assisted processes that depend uponcurrent density and field intensity in the electrolyte. At a lowcurrent density, the field intensity in the electrochemical systemis low and not enough to support the electric field-assistedgrowth of CeO2 nanorods, which results in the formation ofamorphous nanoparticles (process A). Whereas at high currentdensity, the field intensity is so high that a layer of dense film isdeposited (caused by the overgrowth of CeO2). The concentra-tion of Ce(Cl)3 solution has a great influence on the the growth ofCeO2 nanorods. At a current density of 0.1 mA/cm2, high

Fig. 2. (a) The transmission electron microscopy image of CeO2 nanorod; (b) and (c) high-resolution transmission electron microscopy images of CeO2

nanorod. The d spacing estimated from the lattice fringe is 0.32 nm, which is ascribed to CeO2 (111).

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Fig. 3. (a) X-ray diffraction pattern of CeO2 nanorod–TiO2 nanotube hybrid arrays with a Lorentzian-fitted CeO2 (111) diffraction peak in the inset;(b) Raman spectra of CeO2 nanorod–TiO2 nanotube hybrid arrays with Lorentzian-fitted CeO2 F2g Raman spectrum in the inset; (c) XPS spectrum ofCe 3d peaks in CeO2 nanorods; (d) XPS spectrum of O 1s peaks in CeO2 nanorods (dashed line denotes Lorentzian-fitted XPS spectra); (e) XRD patternof Ce-containing nanoparticles embedded with TiO2 nanotube arrays: red line denotes the specimen before heat treatment; and black line denotes thespecimen after heat treatment at 5001C; (f) XPS spectra of Ce 3d peaks in Ce-containing nanoparticles embedded with TiO2 nanotube arrays: the blackline denotes the specimen before heat treatment; and the red line denotes the specimen after heat treatment at 5001C.

Fig. 4. Schematic diagram of the growth process of CeO2 nanorod arrays.

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Ce(Cl)3 concentration (c) leads to high electrolyte conductivity(s) and low electric field intensity, which cannot support theelectric field-assisted growth of CeO2 nanorods. In order to in-vestigate the photocatalytic activity of the as-prepared CeO2–TiO2 nanocomposites, we will carry out systematic studies on thecatalytic property of CeO2–TiO2 nanostructured hybrid materi-als in our future works.

IV. Conclusion

In summary, CeO2 nanorod–TiO2 nanotube hybrid arrays havebeen grown by galvanostatic electrodeposition. The electricfield-assisted growth of CeO2 nanorods can be controlled bychoosing different electrochemical conditions. Both current den-sity and electrolyte concentration have direct impacts on theformation of CeO2 nanorods. The CeO2 nanorods obtained arepolycrystalline with an identical nucleus orientation.

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