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Electrochimica Acta 55 (2009) 480–484

Contents lists available at ScienceDirect

Electrochimica Acta

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etal nanoparticle chains embedded in TiO2 nanotubes prepared by one-steplectrodeposition

ei Zhu, Guanzhong Wang ∗, Xun Hong, Xiaoshuang Shen, Dapeng Li, Xing Xieefei National Laboratory for Physical Sciences at Microscale, and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026,eople’s Republic of China

r t i c l e i n f o

rticle history:eceived 13 July 2009eceived in revised form 28 August 2009ccepted 29 August 2009

a b s t r a c t

Arrays of Ni nanoparticle chains embedded in TiO2 nanotubes (TiO2/Ni) were fabricated by one-stepelectrodeposition in anodic aluminum oxide membranes. The formation of spacing Ni nanoparticles orcontinuous nanorods in the TiO2 nanotubes depends on the deposition potential, the potential waveform,and the pH value in the electrolyte. The growth mechanism is attributed to the generation of H2 bubbles

vailable online 4 September 2009

eywords:iO2 nanotube arraysi nanoparticle chainslectrodeposition

and their periodical evolution outside of the TiO2 nanotubes.© 2009 Elsevier Ltd. All rights reserved.

AO membrane2 bubbles

. Introduction

Nanotubular titania (TiO2) materials are widely studied dueo their excellent physical and chemical performance, and theirotential applications in solar cells, photocatalysis, and photo-lectrolysis of water [1–9]. Recently, TiO2 nanotubes have alsoeen used as templates to prepare metal/TiO2 core/shell struc-ures. Compared with empty nanotubes, these nanostructures havedvantages in the application requiring separation and transportf carriers by reducing the recombination of electrons and holesenerated by band gap excitation [10–14].

The structure of metal nanoparticles embedded in nanotubess also desirable for plasmonic waveguides and applications inlectromagnetic field enhanced spectroscopies such as surface-nhanced Raman scattering, infrared, or fluorescence [15–19].owever, this structure is rarely reported. Keating and co-worker

abricated Au and Ag nanoparticle chains embedded in SiO2 nan-tubes by selectively etching striped metal nanowires preparedy multiple-step electrodeposition [16]. The particle size, place-ent, and spacing could be adjusted with different deposition time.

owever, a sacrificial metal is always necessary, which adds dif-culty to the electrodeposition process and limits the range ofpplication. Knez et al. reported an approach to the preparation ofetal nanoparticle chains encapsulated in nanotubes by thermal

∗ Corresponding author. Tel.: +86 551 3603323; fax: +86 551 3606266.E-mail address: gzwang@ustc.edu.cn (G. Wang).

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.08.059

annealing of metal nanowires in nanotubes [20,21]. This method issuitable for synthesis of noble metal nanoparticle chains embeddedin nanotubes with tunable particle sizes and spacings. However,it is impractical to employ this technique in large-scale industrialfabrication because it is too costly and time-consuming.

As a low-cost and high rate method, electrodeposition has beenutilized widely to synthesis metal or semiconductor nanowires andnanotubes [22–26]. However, there is no report on the discontin-uous nanostructures yet. In this paper, we present the preparationof Ni nanoparticle chains embedded in TiO2 nanotubes by elec-trodeposition and whether Ni nanoparticle chains or continuousnanorods can be formed in TiO2 nanotubes, and the length ofthe particles and the particle spacings could be modulated by theapplied potential, the potential waveform, and the pH value inthe electrolyte. We suggest that the H2 bubbles’ generating andevolving outside the TiO2 nanotubes periodically are responsiblefor formation of this structure.

2. Experimental

2.1. Preparation of Ni nanoparticle chains embedded in TiO2nanotubes

The Ni nanoparticle chains embedded in TiO2 nanotube arrayswere prepared by a one-step electrodeposition process. The elec-trolyte was prepared by dissolving TiF4 (98%, Alfa Aesar) andNiCl2·6H2O (98%, SCRS) in 100 mL stirring deionized water. Theconcentration of TiF4 and NiCl2 is 0.04 and 0.08 M, respectively.

mica Acta 55 (2009) 480–484 481

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Fig. 1. Ni nanoparticle chains embedded in TiO2 nanotubes electrodeposited in elec-trolyte containing 0.04 M TiF and 0.08 M NiCl ·6H O at −1.50 V (vs Ag/AgCl) for

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W. Zhu et al. / Electrochi

he stirring time is 30 min. The pH of the solution was main-ained at about 1.8. AAO membranes (Whatman Anodisc 47) weresed as the templates. The diameter and length of the channelsre about 250 nm and 60 �m, respectively, according to our SEMbservation. The electrodeposition was performed with a stan-ard three-electrode system (CHI 620B). An Ag/AgCl electrode inaturated KCl solution was used as the reference electrode and alatinum coil was used as the counter electrode. The electrode-osition potential was varied from −1.30 to −1.70 V at roomemperature. Before electrodeposition, a thin layer of Au about0 nm was sputtered on the back of the membranes as the workinglectrode.

.2. Characterization

Before characterization of the samples, the AAO membranesere removed with 1 M NaOH (96%, SCRC) at 40 ◦C for 30 min.

he morphology of the samples was characterized using field-mission scanning electron microscopy (FE-SEM, JEOL 6700F) andigh-resolution transmission electron microscopy (HRTEM, JEOLodel 2010). X-ray diffraction (XRD), selected area electron diffrac-

ion (SAED) and energy dispersive spectroscopy (EDS) were usedo determine the crystal structure and chemical composition ofhe samples. X-ray photoelectron spectroscopy (XPS) was used tonalyze the bonded states of Ti atoms.

. Results and discussion

Fig. 1 shows the typical top view field-emission scanninglectron microscopy (FESEM) image and side view backscattered

lectron (BSE) image of the as-grown Ni filled TiO2 nanotubes afteremoving the AAO membranes. The secondary electron image (SEI)mage in Fig. 1(a) shows that the TiO2 nanotubes are highly ordered

ith an average outer diameter of about 250 nm. The BSE image,hown in Fig. 1(b) with enhanced atomic contrast [15], reveals that

4 2 2

10 min. (a) Top view SEI image of the nanotube arrays. (b) Side view BSE image ofthe Ni nanoparticle chains in nanotubes.

ig. 2. (a) TEM image of the Ni nanoparticle chains embedded in a TiO2 nanotube: the inset is the enlarged image of the marked part, which shows the Ni nanoparticle (darkart) and TiO2 wall (gray part). (b) SAED pattern of the marked part in (a). (c) EDS spectrum of the marked part in (a). (d) XPS spectrum of the sample shows Ti 2p core levels.

482 W. Zhu et al. / Electrochimica Acta 55 (2009) 480–484

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Ni2+ + 2e− → Ni (2)

2H+ + 2e− → H2↑ (3)

TiF4 + (4 − x)OH− → Ti(OH)4−xFx + (4 − x)F−

ig. 3. BSE images of the samples prepared at different potentials and square potenotentiostatic −1.70 V and (d) square potential with 10 s pulse width. The scale bar

he Ni nanoparticle chains embedded in the nanotubes. The lengthf the Ni chains in TiO2 nanotubes is dependent on the depositionime. For 10 min deposition, the length is about 10 �m, and longerhains could be obtained by increasing the deposition time.

The sample shown in Fig. 1 was also inspected with transmis-ion electron microscopy (TEM), energy dispersive spectroscopyEDS) and X-ray photoelectron spectroscopy (XPS). The TEM imageshown in Fig. 2(a) and its inset indicate the morphology of Nianoparticles embedded in TiO2 nanotubes. The Ni nanoparticleshould be cylindrical (or disc) like because they are formed by fillinghe nanotubes. As shown in Fig. 2(a), the diameter of the parti-les (discs) is about 200 nm, the length of the particles (thicknessf the discs) is about 150 nm and the spacing of the particles isbout 150 nm, while the TiO2 wall is amorphous with a thicknessf about 25 nm. As shown in Fig. 2(b), the selected area electroniffraction (SAED) pattern reveals that the Ni particles have a poly-rystalline face-centered cubic (fcc) structure. Fig. 2(c) is the EDSpectrum of the area marked in Fig. 2(a), which shows that the Nind Ti contents are about 73.47 and 10.35 atom%, respectively. Theu-related peaks in the spectrum originate from the copper grids.ig. 2(d) shows the XPS spectrum of the nanotubes embedded withi nanoparticles. The doublet spectral line of Ti 2p can be charac-

erized by a binding energy of 458.3 (2p3/2) and 464.1 eV (2p1/2),hich match closely with the Ti4+ spectra in bulk TiO2 [27].

Fig. 3 shows the side view BSE images of the Ni nanorods oranoparticle chains embedded in TiO2 nanotubes prepared at dif-

erent potential or square potential waveform. At a lower potentialf −1.30 V or square potential pulses between −1.50 and −0.50 Vpulse width 2 and 1 s), only continuous Ni nanorods embedded iniO2 nanotubes were formed, as shown in Fig. 3(a) and (b) (pulseidth 2 s). However, at a higher potential of −1.70 V or squareotential pulses between −1.50 and −0.50 V (pulse width 10 and

s), most of the TiO2 nanotubes are embedded with Ni nanoparticlehains, as shown in Fig. 3(c) and (d) (pulse width 10 s). Moreover,ig. 3 shows that the lengths of the Ni particles and the spacingsetween two adjacent particles are related to the deposition poten-ial or the square potential pulses.

aveforms. (a) Potentiostatic −1.30 V; (b) square potential with 2 s pulse width; (c)m. Insets in (b) and (d) are the applied square potential waveforms.

Besides the deposition potential and the potential waveform,the pH of the electrolyte could also influence the formation of Ninanoparticle chains. As shown in the side view BSE image in Fig. 4,only continuous Ni nanorods were formed in TiO2 nanotubes whilethe pH of the electrolyte was adjusted to about 2.3 with Na2CO3(99%, SCRC). This result is different to the sample prepared at lowerpH value of about 1.8 (Fig. 1).

According to FESEM, XPS, and TEM analysis, a growth mecha-nism is suggested to explain the Ni nanoparticle chains formed inTiO2 nanotubes, which is schematic illustrated in Fig. 5. The chem-ical reactions involved in the deposition process are described asfollows [28,29]:

2H2O → H+ + 2OH− (1)

Fig. 4. Side view BSE image of the sample prepared at higher pH value of about 2.3.The deposition potential was maintained at −1.50 V.

W. Zhu et al. / Electrochimica Acta 55 (2009) 480–484 483

ible for the formation of Ni nanoparticle chains embedded in TiO2 nanotubes.

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Fig. 5. Schematic illustration of the growth mechanism which is respons

i(OH)4−xFx + xOH− → Ti(OH)4 + xF− → TiO2 (4)

We have discussed the growth mechanism of Ni/TiO2 core/shellanorod arrays in AAO membranes in our previous publication [13].iO2 could be deposited on the inner surface of the channels to formanotubes (Reaction (4)), and Ni could be deposited in the TiO2anotubes to form core/shell structure (Reaction (2)). In this work,2 bubbles (Reaction (3)) play an important role for the formationf Ni nanoparticle chains in TiO2 nanotubes. At higher potential,2 could be produced rapidly, hence some of the H2 that couldot diffuse out of the membrane in time would form bubbles andccupy the space of the channels. In this case, Ni deposition couldccur only in the gap between H2 bubbles and channel walls [30].hen H2 bubbles became bigger enough, they would detach from

he deposition surface and evolve outside the TiO2 nanotubes. Inhis case, Ni would be deposited in the channels to form nanopar-icles until new H2 bubbles were formed. With the formation of H2ubbles periodically, Ni nanoparticle chains were formed in TiO2anotubes. However, at lower potential, higher pH value, or squareotential waveform with shorter pulse width, H2 could diffuse outf the membrane in time, which did not hinder the formation ofhe continuous Ni nanorods (Fig. 5). Because only part of a TiO2anotube could be embedded with Ni, its length depending on theeposition time, the rest of the nanotube still remained empty (seeig. S2 in the Supplementary Materials). After removing the AAOembrane and ultrasonic treating for 3 min, these two parts could

e easily broken near their joints due to the surface tension inducedy solvent evaporation [31].

In our previous publication, the samples prepared at −1.50 and1.70 V seemed to be Ni/TiO2 core/shell nanorods according to the

op and side view SEI images [13]. In fact, Ni in the TiO2 nanotubeshould be Ni nanoparticle chains instead of continuous Ni nanorodsccording to the side view BSE images as shown in Figs. 1 and 3c,hich was confirmed by the TEM investigation (Fig. 2a). The sum

ength of Ni nanoparticle chains excluding the spacing betweenwo adjacent particles still increases with the potential applied inhe electrodeposition, which is about 7 and 12 �m for the samplerepared at −1.50 and −1.70 V, respectively.

Besides Ni, other metal nanoparticle chains, such as Co, couldlso be embedded in TiO2 nanotubes with this method. Fig. 6 shows

he side view BSE image of the Co embedded in TiO2 nanotubes pre-ared in electrolyte containing 0.04 M TiF4 and 0.08 M CoCl2·6H2Ot −1.90 V. Furthermore, we have fabricated semiconductor (suchs CdS) particles embedded in TiO2 nanotubes by the similar elec-rodeposition process.

Fig. 6. BSE image of Co embedded in TiO2 nanotubes fabricated by one-step elec-trodeposition.

4. Conclusions

In conclusion, Ni nanoparticle chains embedded in TiO2 nan-otubes have been fabricated in AAO membranes by a one-stepelectrodeposition technique. The formation of Ni nanoparticlechains in TiO2 nanotubes is attributed to the generation of H2 bub-bles and their periodical evolution outside the TiO2 nanotubes. Thelengths of the Ni particles and the spacings between two adjacentparticles could be modulated by the applied potential or squarepotential waveform. This approach opens a new possibility to pre-pare various nanoparticle chains embedded in nanotubes withtunable magnetic, electrical, and optical properties.

Acknowledgements

This work was supported by Natural Science Foundation ofChina (Grant Nos. 10574122, 50772110, 50721091), the NationalBasic Research Program of China (2006CB922000, 2007CB925202,2009CB939901).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.electacta.2009.08.059.

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