TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

43518. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Synthesis of Copper Doped Zinc Oxide (ZnO:Cu) Nano-Centipedes via Vapor Deposition Method

Özlem Altıntaş Yıldırım¹,², Yuzi Liu², Amanda K. Petford Long²,³

¹Selçuk University, ²Argonne National Laboratory, ³Northwestern University - Türkiye, USA

Abstract Synthesis of copper doped zinc oxide (ZnO:Cu) nano-centipede structure was performed via a facile vapor-liquid-solid method using a mixture of seed ZnO:Cu nanoparticles and graphene nano powder. Analytical characterization of ZnO:Cu centipede structures were performed using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive x-ray analysis. It was found that skeleton of the centipede structure is approximately 50 m of length and is composed of nanowire leg with ~20 m length. In addition, ZnO nanowire legs have single crystalline nature and Cu atoms are substitutionally incorporated into ZnO lattice. 1. Introduction Metal oxide nanoparticles have attracted considerable attention in many scientific and technological applications due to their unique properties originating from the nanoscale particle sizes [1-3]. Zinc oxide (ZnO) is one of the well-known semiconductor metal oxides, which has a wide direct band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature. Reduction in size to the nano scale, novel structural, optical and magnetic properties can be attained for ZnO nanomaterials. For example, these unique properties give ZnO several advantages in spintronic devices due to room temperature ferromagnetism behavior when they are doped with small amounts of transition metals such as cobalt, manganese, and copper (Cu) [4]. The magnetic properties of the Cu-doped ZnO (ZnO:Cu) systems strongly depend on (i) amount of dopant atoms, (ii) size and morphology of the nanostructures and (iii) the local distribution of Cu atoms into ZnO structure [5-7]. A significant parameter to study is the way in which the Cu atoms are incorporated into the nanowires; for example whether they are in clusters, or whether they

substitute for Zn atoms leading to distortion of the oxide crystal lattice. Therefore, there has been a strong interest in the synthesis of homogeneously distributed and substitutionally incorporated Cu atoms into ZnO nanostructures with well-controlled size and shape. A variety of techniques like precipitation [8], sol–gel [9], solid state reaction method [10] and flame spray pyrolysis [11] have been already used in the synthesis of ZnO:Cu nanostructures. In this study, we want to explore synthesis of ZnO:Cu centipede structure with well distributed Cu atoms at atomic scale by using two step synthesis method: first, synthesis of seed nanoparticles by precipitation method and then, the growth of these seed nanoparticles with vapor-solid-liquid technique.

2. Experimental Methods and Materials Characterization

2.1. Materials

ZnO:Cu nano-centipede structures were synthesized using ZnO:Cu nanoparticles as a seed layer. For the synthesis of seed nanoparticles, zinc acetate dihydrate (C4H6O4Zn.2H2O, 99.5%; Fluka), copper (II) acetate monohydrate ((CH3COO)2-Cu.H2O, 99%, Merck), polyvinyl pyrrolidone ((C6H9NO)n, PVP, MW~55000, Aldrich) and sodium hydroxide (NaOH pellets, 97%, Merck) were used. Ethylene glycol (C2H6O2, EG, 99%, Merck) and ultra-pure deionized (DI) water were used as solvents. For the growth of nano-centipede from the seed nanoparticles graphene nano-plate aggregates were used. All reagents were used without further purification. 2.2. Experimental methods

For the growth of nano-centipede structure, a seed layer of 1.25 at.% ZnO:Cu nanoparticles with ~20 nm particle size were prepared via a simple room temperature precipitation technique [5]. Growth of

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

436 IMMC 2016 | 18th International Metallurgy & Materials Congress

nano-centipede structure from the seed nanoparticles was carried out by vapor-liquid-solid (VLS) method. A 1:1 weight ratio mixture of ZnO:Cu nanoparticles and graphene nano-platelets was ground together, placed in a quartz boat, and then inserted into a horizontal tube furnace. P-type boron-doped silicon prime wafers were used as substrates. Prior to growth, cleaning of the wafers were done in an ultrasonicater with 2-proponal, acetone and DI-water, respectively. The wafer surfaces were coated using electrodeposition with 5 nm Cr (0.036 nm/s) and 10 nm Au (0.167 nm/s) thin films. To collect the VLS products, the silicon wafers were placed at downstream position in the furnace. VLS growth of nano-centipedes was performed under a carrier gas mixture containing high purity Ar (98 vol.%) and O2 (2 vol.%) which was kept flowing into the tube furnace with 120 sccm (standard cubic centimeter per minute). The tube furnace was adjusted to 1000 °C and kept at the same temperature for 2 h. Then, the furnace was cooled to room temperature over a 3h period. At the end of process, a white deposit layer on the surface of substrates were obtained. 2.3. Materials Characterization

The phases of the reaction products were identified by using X-ray diffraction (XRD) using Cu K radiation ( =1.54 Å) and an x-ray source operating voltage of 40 kV. Scans were performed in the range of 28–90° at a scanning rate of 0.01°/sec. Detailed crystal structure investigation was performed with Rietveld refinement analyses, conducted using the General Structural Software [12] with an EXPGUI graphical user interface [13]. The re nements were done using the P63mc space group with a Zn atom at (1/3, 2/3, 0) and an oxygen atom at (1/3, 2/3, 0.3817).

The size and morphology of the ZnO nanoparticles and nano-centipedes were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The energy dispersive x-ray (EDX) and elemental mapping analysis were also performed. The SEM samples were examined without any conductive coating layer. The representative TEM samples were prepared simply by drying out an ultrasonically-dispersed aqueous suspension on holey carbon-coated copper grids. 3. Results and Discussion The synthesis method of ZnO:Cu centipede structure includes two steps. First, synthesis of 1.25 at% Cu incorporated ZnO nanoparticles via simple room temperature precipitation technique and then growth of nano-centipede via a VLS deposition technique,

using the ZnO:Cu nanoparticles to act as seeds for the nano-centipede structure growth. For the growth of nano-centipedes, a mixture of ZnO:Cu nanoparticles-graphene powders was used. The graphene powder was used as a catalyst to decrease the vaporization temperature of the ZnO.

Figure 1. (a) XRD diffractograms of ZnO:Cu nanoparticles. The results of Rietveld-refinement simulations for the nanoparticles and JCPDS data for wurtzite ZnO are also included. (b) Low magnification TEM image of seed ZnO:Cu nanoparticles. The inset in (b) shows HRTEM image of a selected particle. An XRD pattern from the precursor ZnO:Cu nanoparticles is shown in Figure 1(a). All diffraction peaks belong to the crystalline ZnO with the hexagonal wurtzite structure (JCPDS card no: 14-1451). The lattice parameter (c) and average crystallite size were determined from XRD analyses as 0.516 nm and 21.1 nm by the Rietveld refinement analyses, respectively. The lattice parameter (c) of ZnO:Cu nanoparticles is slightly lower than that of

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

43718. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

undoped ZnO nanoparticle, 0.526 nm [5]. This shows substitutionally incorporation of Cu ions into ZnO crystal structure. Figure 1(b) shows a low magnification TEM image of the ZnO:Cu nanoparticles. The average particle size was determined as 22.6 (±3.4) nm, which is in agreement with the crystallite size determined from XRD analyses. ZnO:Cu nanoparticles have narrow size distribution which is important during growth of centipede to achieve more homogeneous vaporization of seed particles. The high-resolution TEM (HRTEM) image of a selected single ZnO:Cu nanoparticle (inset Figure 1(b)) shows that nanoparticles have inter-connected hexagonal morphology and clear (0002) lattice fringes with a spacing of 0.26 nm, which means the single crystal ZnO:Cu nanoparticles have highly crystalline structure. In addition, Cu addition does not result any distortion or defect creation of ZnO crystal.

Figure 2. (a) XRD patterns of ZnO:Cu nanostructures. (b) Low magnification SEM image of a centipede structure and (c) high magnification SEM images of nanowires of the centipede leg. Figure 2(a) shows an XRD pattern of 1.25 at.% Cu doped ZnO nanostructures prepared with VLS technique. The XRD diffractograms of ZnO:Cu nanostructures also match with JCPDS card data of ZnO having wurtzite type crystal structure. Note that, there are some additional diffraction peaks coming from the (004) planes of the Si substrate and catalyst Au coating film layer. These data indicate no trace for presence of any additional phase within the intrinsic detection limit of XRD for phase analyses as a result of lower dopant amount of Cu than the solubility limit in ZnO [14]. ZnO is a native n-type material [15] and incorporated Cu atoms are mainly substitutionally sitting on Zn sites into the ZnO crystal. For ZnO:Cu nano-centipede structure, lattice parameter (c) was determined as 0.522, which is also lower than that of undoped ZnO nanoparticles. This suggest that after VLS growth of nanoparticles, Cu is also substitutionally incorporated into ZnO lattice. Fig. 2(b) shows the typical morphology of ZnO:Cu nanostructures. A centipede structure with nearly 50

m skeleton length and ~20 m leg length is shown in Fig. 2(b). The high magnification view of legs indicates that they are consisted of core-shell nanowires (Fig. 2(c)). The diameter of leg nanowire is determined as ~150 nm. Figure 3 shows TEM micrograph, EDX result and Zn-Cu elemental mapping images of ZnO:Cu nanowires structure. Low magnification TEM image (Fig. 3(a)) indicates that the surface of ZnO:Cu nanowire seems clear and any amorphous layer is not observed. HRTEM image (Fig. 3b) shows that absence of visible defects or precipitate of second phase meaning that Cu atoms are well-incorporated into the ZnO lattice. Fringes of individual planes are clearly visible in the micrograph indicate the single crystalline nature of the ZnO:Cu nanowires preserving the growth direction along the c-axis. The measured distance between adjacent fringes is about 0.26 nm, corresponding to the (0002) plane of ZnO. The representative selected area of electron diffraction (SAED) pattern taken from the HRTEM images shows that the growth direction is [0001] (inset in Figure 3b). An approximate chemical composition of ZnO:Cu nanowires was determined with in-situ X-ray energy dispersive spectroscopy (EDS) analysis (Fig. 3 (c)). EDX data indicated that the particles consist of Zn, O and Cu elements and atomic proportion of Zn:O elements are close to stoichiometric ratio of 1:1. In addition, Cu amount is

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

438 IMMC 2016 | 18th International Metallurgy & Materials Congress

determined as 1.14 at.% from the tip region of the Cu-doped ZnO nanowires and there is little Cu loss during growth of centipede structure from the 1.25 at. % Cu doped ZnO seed particles.

Figure 3. (a) Low magnification TEM (b) HRTEM micrograph, and (c) EDX spectrum of ZnO:Cu nano-centipedes. The inset in (b) shows SAED pattern of the tip region of nanowires.

4. Conclusion

As a summary, ZnO:Cu nano-centipede structure was synthesized via two step reaction mechanism. Firstly, seed ZnO:Cu nanoparticles with narrow size distribution and 22.6 (±3.4) nm average particle size were prepared by using room temperature precipitation method. Then, nano-centipede structure was grown from the seed nanoparticles via VLS technique. Results of this study indicate that the suggested synthesis method provides a simple

approach for synthesizing ZnO:Cu nanostructures with distinct morphologies and homogeneous Cu distribution which are advantageous for the spintronic applications. Acknowledgments

This work is supported by SU through grant no. BAP- 15401123. OAY thanks the Scientific and Technological Research Council of Turkey (TUBITAK) for support by the national scholarship program for PhD students and also by the METU-ÖYP Program. AKPL acknowledges support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Use of Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357.

References

C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo, D. Wang, Nano Lett 7 1003-1009 (2007)

L. Schmidt-Mende, J.L. MacManus-Driscoll, Materials Today 10 40-48 (2007)

Y.B. Li, Y. Bando, D. Golberg, Appl Phys Lett 84 3603-3605 (2004)

S.J. Pearton, W.H. Heo, M. Ivill, D.P. Norton, T. Steiner, Semicond Sci Tech 19 R59-R74 (2004)

Ö. Alt nta Y ld r m, C. Durucan, Ceram Int 42 3229-3238 (2016)

Z. Zhang, J.B. Yi, J. Ding, L.M. Wong, H.L. Seng, S.J. Wang, J.G. Tao, G.P. Li, G.Z. Xing, T.C. Sum, C.H. Alfred Huan, T. Wu, The Journal of Physical Chemistry C 112 9579-9585 (2008)

G.Z. Xing, J.B. Yi, J.G. Tao, T. Liu, L.M. Wong, Z. Zhang, G.P. Li, S.J. Wang, J. Ding, T.C. Sum, C.H.A. Huan, T. Wu, Adv Mater 20 3521-3527 (2008)

O.A. Yildirim, C. Durucan, J Mater Res 27 1452-1461 (2012)

M. Fu, Y. Li, S. wu, P. Lu, J. Liu, F. Dong, Appl Surf Sci 258 1587-1591 (2011)

R. Elilarassi, G. Chandrasekaran, J Mater Sci: Mater Electron 21 1168-1173 (2009)

P.S. Shewale, V.B. Patil, S.W. Shin, J.H. Kim, M.D. Uplane, Sensors and Actuators B: Chemical 186 226-234 (2013)

A. C. Larson, R.B.V. Dreele, LANSCE, MS-H805 Los Alamos National Laboratory NM 87545 (1994)

B. Toby, J Appl Crystallogr 34 210-213 (2001) S. Singhal, J. Kaur, T. Namgyal, R. Sharma,

Physica B: Condensed Matter 407 1223-1226 (2012) S.B. Zhang, S.H. Wei, A. Zunger, Phys Rev B

63 075205 (2001)