Materials Letters 100 (2013) 11–14

Contents lists available at SciVerse ScienceDirect

Materials Letters

0167-57

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/matlet

Hydrothermal synthesis of Zinc oxide (ZnO) nanotubes and itselectrophoretic deposition on nickel filter

Tugba _Ipeksac- a,b, Figen Kaya a, Cengiz Kaya a,n

a Metallurgical and Materials Engineering Department, Yıldız Technical University, _Istanbul, Turkeyb TUB_ITAK MAM, Chemistry Institute, Kocaeli, Turkey

a r t i c l e i n f o

Article history:

Received 23 January 2013

Accepted 26 February 2013Available online 7 March 2013

Keywords:

Nanoparticles

Ceramics

Hydrothermal synthesis

Zinc oxide

Electrophoretic deposition

7X/$ - see front matter & 2013 Elsevier B.V.

x.doi.org/10.1016/j.matlet.2013.02.099

esponding author. Tel.: þ90 2123834713; fa

ail addresses: cengizkaya66@gmail.com, ceng

a b s t r a c t

Multi-walled ZnO nanotubes were synthesized by a simple hydrothermal treatment of nanowire-rod

like ZnO powders without use of any catalysts, plate or substrates. ZnO nanotube structures were

synthesized at a low temperature of 180 1C in an aqueous solution including nanowire-rod like ZnO

powders and hydrogen peroxide. The synthesized ZnO nanotubes were coated on 3D nickel filters by

electrophoretic deposition (EPD) technique. It is shown that the obtained multi-walled ZnO nanotubes

show antibacterial behavior and have variable inner diameters of 5–17 nm, outer diameters of

10–22 nm and 8–9 layers. It is suggested from the results obtained that formation of zinc oxide

nanotubes from nanowire-rod like ZnO structures can be explained with Kirkendall-type diffusion.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Nano-sized structures have gained significant scientific andtechnological interest due to their unique physical, chemical andoptical properties [1]. Nanotubes have more interest than otherstructures because of their high porosity and large specific surfacearea [2]. One of the most important nanotubes is ZnO and it is awell known direct-band gap semiconductor with large excitonbinding energy (60 meV) at room temperature and one of theexcellent semiconductor photocatalysts with wider direct bandgap (3.37 eV) than TiO2 [1,3,4]. Furthermore, some metal oxidestructures, such as ZnO, have been found to inhibit bacterialgrowth [5]. It has been found that toxicity of nanoparticles tomicroorganisms is generally higher than larger particles of thesame material [6]. Also, zinc oxide has various applications, suchas in optoelectronics, sensors, lasers, UV detectors and solar cells[7,8]. Up to now, many different techniques including sol–gel,hydrothermal synthesis, thermal decomposition, polymer-assistedprecipitation have been studied to synthesize ZnO structures[1–3,9]. However, the hydrothermal method has been proved themost suitable procedure because of its simplicity and ability tocontrol particle shape and size by controlling the proses parameters,such as time, temperature and starting chemicals [9].

The electrophoretic deposition (EPD) method has a wide range ofapplications, such as coatings, thin films, solid oxide fuel cells,structural components and composites [10]. Factors, such as viscosity

All rights reserved.

x: þ90 2123834661.

izk@yildiz.edu.tr (C. Kaya).

of suspension, deposition time, stability of suspension, applied voltageand concentration of solid in suspension are important parameters inEPD that affect the coating properties [10,11].

Although many papers were published on the synthesis of ZnOparticles/rods, there are limited research studies on ZnO tube forma-tion and the growth mechanism of tube-like ZnO structures inaqueous solution, which is still not completely understood [1,9,12].Different mechanisms for the formation of ZnO nanotubes have beenproposed [12], including the Kirkendall-type diffusion process [13,14].

Therefore, in the present paper, for the first time to date, multi-walled ZnO nanotubes were synthesized and characterized by anefficient and practical hydrothermal synthesis method without usingany template and the obtained ZnO nanotubes were successfullycoated onto nickel-based filters by EPD. Based on the results obtained,a formation mechanism for ZnO nanotubes is also suggested.

2. Experimental work

The previously synthesized nanowire-rod like ZnO powders [15]and hydrogen peroxide (H2O2) were used as starting materials. First,2 g nanowire-rod like ZnO powders were added to 100 mL of H2O2

and then magnetically stirred at room temperature for 1 h. Secondly,the prepared solution was placed into a 230 mL Teflon-linedstainless steel autoclave, which was sealed and maintained at180 1C for 24 h and then cooled down to room temperature. Thewhite-colored solution was centrifuged, washed three times withethanol and after that washed three times with distilled water. Theobtained white precipitates were dried in an air furnace.

Fig. 1. TEM images of the obtained ZnO nanotubes: the cross section of the ZnO nanotubes (a, b); the general structure of the tubes showing the length, shape and the

average diameter of the tubes (c, d).

Fig. 2. XRD pattern of the obtained ZnO nanotubes.

T. _Ipeksac- et al. / Materials Letters 100 (2013) 11–1412

The colloidal suspension of the obtained ZnO nanotubes wasprepared for the coating experiments of nickel based filter. 0.3 g ZnOnanotube powders were added into 30 mL of ethanol, then ball milledfor 1 h with 5 mm zirconium balls. After that, 0.005 g of polyethyle-neimine (PEI) was added into the solution and ultrasonically stirredfor 0.5 h. The pH of the solution was adjusted to 9–10 using NaOHsolution (10 M). The EPD experiments were conducted using adeposition time of 45 s and an applied voltage of 72 V with anelectrode separation of 20 mm.

Antibacterial effect of ZnO nanotubes against S. aureus bacteriawas examined under UV light using the MTT technique (usingthe 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide(MTT) assay) [16]. ZnO nanotube powders were suspended in distilledwater (in a range between 1 and 1000 mg/mL) and anti-bacterialassay is given as a comparative depending on the concentration ofbacteria at the end of the 24 h. Cell viability values were determinedquantitatively using a microplate reader (Fluoroskan Ascent, ThermoLabsystems, Helsinki, Finland) at 570 nm.

3. Results

Fig. 1 shows the detailed TEM microstructures of the as-preparedZnO nanotubes after hydrothermal synthesis. Fig. 1(a) shows the

Fig. 3. Optical images of un-coated (a) and ZnO nanotube-coated nickel filters (b), SEM image of ZnO nanotube-coated nickel filters (c, d).

Fig. 4. Antibacterial effect of ZnO nanotubes on S. aureus bacteria.

T. _Ipeksac- et al. / Materials Letters 100 (2013) 11–14 13

cross-sectional structure of the ZnO nanotube with inner and outerdiameters of 5 nm and 10 nm, respectively with a near sphericalgeometry. The multi-walled structure of the ZnO nanotubes isclearly seen in the TEM image in Fig. 1(a and b). The lengths ofthe nanotubes are also determined to be between 300–450 nm, asshown in Fig. 1(c and d).

The formation mechanism of ZnO nanotubes is not completelyunderstood, however it is believed that the mechanism of ZnOnanotubes from nanowire-rod like ZnO powders can be explainedwith the Kirkendall effect [12,13]. In the Kirkendall effect, diffu-sion of atoms cause oversaturation of lattice voids. It is consideredthat this oversaturation cause condensation of more voids(Kirkendall voids) close to the interface. Therefore, these Kirken-dall voids changes properties of the interface and force it to formmulti-walled nanotubes [14] (see also the graphical abstract).Now, more experimental studies and HR-TEM observations areunderway to clarify the factors that affect formation of nanotubes.

Fig. 2 shows the XRD pattern of the obtained ZnO nanotubesafter the hydrothermal process which are match up with thestandard data of zinc oxide (JCPDS card no. 36-1451). All theoverlapping diffraction peaks with the standard values for bulkZnO indicate that the prepared nanotubes are zinc oxide (ZnO) asshown in Fig. 2. The peak intensities and widths indicate that theobtained ZnO nanotubes are in highly crystalline form, but some

sulfur containing compounds has remained in the sample. Theresidues can removed by washing with methanol.

Fig. 3 shows the SEM images of ZnO nanotube-coated nickelfilter by EPD. Fig. 3(a) shows the image of un-coated 3D nickel-based filter while Fig. 3(b) shows the image of ZnO nanotube-coated nickel filter using 72 V for 45 s during EPD. A generalcoated filter image is shown in Fig. 3(c) with the presence of nocracks on the surface. But as the deposition time is increasedbeyond 45 s, the surface coating layer contains many micro-cracks around the corners in particular, as shown in Fig. 3(d).

Depending on the concentration of ZnO nanotubes, bacterialgrowth at the end of the 24th hour, decreased by approximatelyfour-fold, as shown in Fig. 4. For example, the number of bacteriawas determined to be 1,400,000 for a ZnO concentration of50 mm/mL, while the number of bacteria was decreased to400,000 for a ZnO concentration of 1000 mm/mL, as shown inFig. 4.

4. Conclusions

It is shown that multi-walled ZnO nanotubes with controlledsize and shape can be prepared by a simple hydrothermal methodwithout the use of any catalysts, surfactant or substrates. It is alsoshown that the obtained multi-walled ZnO nanotubes have highcrystallinity with variable inner diameters of 5–17 nm, outerdiameters of 10–22 nm and layer numbers up to nine with astrong antibacterial effect. The final ZnO nanotubes are consid-ered to be suitable candidates for antibacterial applications, suchas filters in air conditioning systems and other engineeringapplications in electronics or storage purposes.

Acknowledgments

Financial support from TUBITAK-COST under the contractnumber 109R007 and the YTU Scientific Research Fund is greatlyacknowledged.

T. _Ipeksac- et al. / Materials Letters 100 (2013) 11–1414

References

[1] Li Y, Liu C, Zou Y. Growth mechanism and characterization of ZnO nano-tubessynthesized using the hydrothermal-etching method. Chem Pap 2009;63:698–703.

[2] Tong Y, Liu Y, Shao C, et al. Growth and optical properties of facetedhexagonal ZnO nanotubes. J Phys Chem B 2006;110:14714–8.

[3] Wang L, Yang X, Han Y, Zhang R, Yang Y. Synthesis and characterization ofZnO nanoparticles in the method of precipitation. Key Eng Mater 2011;474-476:1725–9.

[4] Rusdi R, Rahman AA, Mohamed NS, Kamarudin N, Kamarulzaman N.Preparation and band gap energies of ZnO nanotubes, nanorods and sphericalnanostructures. Powder Technol 2011;210:18–22.

[5] Sawai J, Shoji S, Igarashi H. Hydrogen peroxide as an antibacterial factor inzinc oxide powder slurry. J Ferment Bioeng 1998;86:521–2.

[6] Tam KH, Djurisic AB, Chan CMN, et al. Antibacterial activity of ZnO nanorodsprepared by a hydrothermal method. Thin Solid Films 2008;516:6167–74.

[7] Yamamoto O. Influence of particle size on the antibacterial activity of zincoxide. Int J Inorg Mater 2001;3:643–6.

[8] Suchanek WL. Systematic study of hydrothermal crystallization of zinc oxide(ZnO) nano-sized powders with superior UV attenuation. J Cryst Growth2009;312:100–8.

[9] Wang C, Mao B, Wang E, et al. Solution synthesis of ZnO nanotubes via atemplate-free hydrothermal route. Solid State Commun 2007;141:620–3.

[10] Besra L, Liu M. A review on fundamentals and applications of electrophoretic

deposition (EPD). Prog Mater Sci 2007;52:1–61.[11] Randall CA, Van Tassel J, Matsko M, Bowen CP. Electric field processing of

ferroelectric particulate ceramics and composites. IEEE Appl Ferro1996;1:189–92.

[12] Wei A, Sun XW, Xu CX, et al. Growth mechanism of tubular ZnO formed inaqueous solution. Nanotechnology 2006;17:1740–4.

[13] Chang Y, Lye ML, Zeng HC. Large-scale synthesis of high-quality ultralongcopper nanowire. Langmuir 2005;21:3746–8.

[14] Fan HJ, Gosele U, Zacharias M. Formation of nanotubes and hollow nano-particles based on kirkendall and diffusion processes: a review. Small2007;3:1660–71.

[15] Ipeksac T. Synthesis and applications of various metal oxide nanotubes. MScThesis, Yildiz Technical University, Istanbul; 2012.

[16] Allahverdiyev AM, Abamor ES, Bagirova M, et al. Antileishmanial effect ofsilver nanoparticles and their enhanced antiparasitic activity under ultravio-

let light. Int J Nanomed 2011;6:2705–14.