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doi:10.1016/j.ph

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Physica E 33 (2006) 331–335

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From Zn microspheres to hollow ZnO microspheres:A simple route to the growth of large scale metallicZn microspheres and hollow ZnO microspheres

Aurangzeb Khana,�, Wojciech M. Jadwisienczakb, Martin E. Kordescha

aDepartment of Physics and Astronomy and CMSS program, Ohio University, 251B Clippinger, Athens OH 45701, USAbSchool of Electrical Engineering and Computer Science, Ohio University, Athens OH 45701, USA

Received 6 February 2006; received in revised form 27 March 2006; accepted 27 March 2006

Available online 5 June 2006

Abstract

Large scale metallic Zn microspheres and hollow ZnO microspheres are synthesized by thermal evaporation and vapor transport by

heating a ZnO/graphite mixture at 1000 1C. Firstly, metallic Zn microspheres are fabricated with diameters in the range of 1–10 mm. The

Zn microspheres are then annealed at 600 1C in air, which form hollow semiconducting ZnO microspheres. EDX and XRD spectra reveal

that the oxidized material is indeed ZnO. Room temperature photoluminescence spectra of the oxidized material show a sharp peak at

�380 nm and a wider broad peak centered at �490 nm. This growth mechanism is discussed and further investigated for other metallic

and metal oxide microstructures.

r 2006 Elsevier B.V. All rights reserved.

PACS: 78.55.Fv; 78.55.�m; 78.40.Me

Keyword: Microspheres; Zinc; Zinc oxide; Oxidation; Photoluminescence

1. Introduction

Nanostructured materials have attracted great interest inrecent years due to their unique electrical, catalytic,mechanical and optical properties which are different fromtheir corresponding bulk counterparts [1–3]. Considerableresearch work is done on the synthesis of nanowires,nanotubes, nanobelts and nanorods [4–14]. In addition toone-dimensional microstructures, other kinds of micro-structures are synthesized such as flower like SiO2, MgOfish ribbons and SnO2 nanoribbons networks [15–17].

In recent years much attention has focused on ZnOmaterial because of its wide band gap (3.37 eV at roomtemperature) with low lasing threshold, high excitonbinding energy (60meV) and potential for nanoscaleoptoelectronic devices [18–21]. ZnO thin film, nanowires,nanobelts and nanorods are widely studied [5,7,13].

e front matter r 2006 Elsevier B.V. All rights reserved.

yse.2006.03.159

ing author. Tel.: +740 597 1259; fax: +740 593 0433.

ess: khan@helios.phy.ohiou.edu (A. Khan).

Due to their large surface area and low density, hollowmicrospheres are of particular interests compared to theircorresponding bulk materials. The possible applications ofthese microstructures may be in filters, coatings, capsuleagents for drug delivery, or templates for functional-engineered composite materials [22].The synthesis of microspheres of both metallic and semi-

conducting materials would also enable their use as buildingblocks for new photonic crystals and other model systems forlight scattering, which are of fundamental importance to boththeoretical and experimental sciences [23]. New approacheshave been made to preparing semiconducting hollow spheresover the last few years [24–33]. Recently, Xia et al. [34] havesynthesized ZnO microballs using a pyrolysis method. Themicroballs are made of small crystallites, while the hollowspheres we have synthesized are made of small nanorods andhave higher surface to volume ratio which is important for gassensors applications.Gao et al. [35] and Fan et al. [22] reported the synthesis

and growth mechanism of micro- and nano-cages from

ARTICLE IN PRESSA. Khan et al. / Physica E 33 (2006) 331–335332

ZnO/SnO2/graphite powders and Zn powders, respectively,however further details are needed to explore the path ofmaking hollow nano and microspheres of semiconductingmaterials and to understand their growth process.

We report a simple and successful mechanism ofsynthesizing metallic Zn microspheres and ZnO hollowmicrospheres. We believe that the as developed route willbe helpful in the future fabrication of both Zn and ZnOspherical micro- and nano-sized structures in an easy andinexpensive way.

2. Experimental

Metallic Zn microspheres are synthesized by thermalevaporation and vapor transport method by heating themixture of high purity ZnO (99.0% min and �325 mesh)and graphite (99.0% min and �300 mesh) powders in an25mm wide and 1.2m long quartz tube placed horizontallyin a conventional furnace. Silicon substrates are kept at thedownstream towards one end of the tube at nearly roomtemperature. Ar gas is used at 30–60 sccm to transportvapors in the tube towards the substrates. The ZnO powderand graphite mixture is placed in a quartz crucible (2:1weight ratio) kept in the middle of furnace tube at 1000 1C.

After 10–20min of operation a gray material is depositedon Si substrates and all over the area around substrates.The deposited material is loosely bound to the substratesand independent of the type of substrate. The samples arethen annealed at 600 1C for 20–30min in air which oxidizedto produce hollow ZnO microspheres. Both the as grownand oxidized samples are characterized with energydispersive X-ray spectroscopy (EDX) connected to scan-

Fig. 1. (a, b) Low magnification SEMmicrographs of the as grown Zn microsp

(d) Diameter profile of the as grown Zn microspheres.

ning electron microscope (SEM) [JOEL JSM 5300], X-raydiffractometer (XRD) [Rigaku Geigerflex, 2000 Watts]with CuKa (1.54 A) as the incident radiation. Photolumi-nescence (PL) spectra of selected ZnO samples were studiedwith He–Cd laser (Kimon Electric Model 1M1301C) in thetemperature range between 10 and 300K. The emitted lightwas collected by a quartz lens on the entrance slit of thespectrograph monochrometer (ISA model HR-320) oper-ated in Czerny–Turner configurations with different holo-graphic gratings. The optical signal was detected by aPrinceton Instruments back-illuminated charge-coupleddevice (CCD camera model TEA-CCD-512TK) with aUV/AR coating and controlled by a computer.

3. Results and discussions

Fig. 1 shows the SEM micrographs of as synthesized Znmicrospheres collected on the Si substrate at roomtemperature. Fig. 1(a) and (b) are the low magnificationSEM images of the microspheres deposited on thesubstrate. It is important to note that these microspheresare very loosely bound to the substrates and the growthprocess seems to be independent of the substrates. Fig. 1(c)is the high magnification image of the microspheres havingan average diameter of about 5 mm. The image shows nofurther structure and the microspheres shape is almostuniform. The distribution of the microsphere diameters areshown in Fig. 1(d). It is clear that the majority of thespheres have a diameter lying in the range of 4–6 mm. Thegrowth mechanism of the Zn spheres is proposed to be thereduction of the ZnO powders by graphite, vapor forma-tion and recombination of Zn vapors to form a liquid

heres. (c) High magnification SEM image of the as grown Zn microspheres.

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Fig. 2. (a) Low magnification SEM micrograph of the oxidized hollow Zn

microspheres annealed at 600 1C in air for 30min. (b) High magnification

image of individual hollow sphere showing that the outer shell (wall) is

made of ZnO nanorods.

2000

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Zn

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Fig. 3. EDX spectra of the (a) as grown metallic Zn microspheres and (b)

ZnO microspheres after annealing at 600 1C in air.

Fig. 4. XRD spectra of the (a) metallic Zn microspheres and (b) ZnO

microspheres after annealing at 600 1C in air.

A. Khan et al. / Physica E 33 (2006) 331–335 333

droplet with subsequent cooling of the Zn vapors. The Zndroplets are then transported by the Ar gas towards thesubstrate kept at 27 1C.

Fig. 2 shows SEM images of the microspheres afterannealing at 600 1C for 30min in air. The microstructuresare oxidized and transformed to hollow ZnO microspheres.Fig. 2(a) shows a low magnification image of hollow ZnOspheres. During the annealing process, the outer surface ofthe Zn spheres quickly oxidizes forming ZnO shell aroundthe sphere and is stabilized this way, whereas the interiorZn overpressure, and cracks the ZnO outer shell leaving thesphere as Zn vapors. This way the solid Zn microspherestransform into ZnO hollow microspheres. The Zn vaporswhich come out of the microspheres oxidize and form otherZnO nanostructures such as nanorods in the oxygenambient, on the surface of the hollow microspheres whichprovides good epitaxial conditions. It is seen in Fig. 2(b)that outer shell of the single ZnO microsphere is composedof small ZnO nanorods. It is important to note that thehollowness of the structures totally depends on theannealing conditions. A similar mechanism of formingZnO nanocages was briefly explained by Gao et al. [35].

Fig. 3 depicts the energy dispersive X-rays spectra of theas-synthesized and annealed samples. Fig. 3(a) shows theEDX spectra of as-grown materials where all peakscorrespond to metallic Zn and no oxygen is evident. Fig.3(b) is the EDX spectra after annealing the sample at600 1C for 30min. The peaks corresponding to Zn and theO are clearly visible indicating that the annealed structuresare indeed made of Zn and O.

In order to verify the Zn and ZnO structures, furtherXRD is done for selected samples as is shown in Fig. 4.Fig. 4(a) shows a typical XRD powder pattern of the asgrown materials where all peaks can be indexed to hexagonalstructured Zn with lattice constants a ¼ b ¼ 0:2664 nm,c ¼ 0:4943 nm. These values are in accordance with theICDD data. There are no peaks coming from any otherphases or material. XRD of the oxidized sample is shown inFig. 4(b). All peaks are assigned to hexagonal wurtzitestructure of ZnO with the cell constants a ¼ b ¼ 0:325 nmand c ¼ 0:521 nm, respectively, indicating that the annealedproduct is pure ZnO. No peaks related to metallic Zn or

other phases of ZnO which means that the annealingcondition were sufficient to oxidize the Zn microspheres intowurtzite ZnO.To investigate the luminescence properties of hollow

ZnO microspheres, PL experiments were conducted withan excitation photon energy (325 nm He–Cd laser) greaterthan that of ZnO band gap at room temperature and at10K. Fig. 5(a) shows the room-temperature PL spectrum,which consists of two emission bands: an ultravioletemission at 380 nm and double peak green emission band

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Fig. 5. PL spectra of ZnO microspheres synthesized at 600 1C for 30min

in air excited with HeCd laser and measured at (a) 300K and (b) 10K.

A. Khan et al. / Physica E 33 (2006) 331–335334

centered at �490 nm, respectively. The UV emissioncorresponds to the near band edge emission of the wideband gap of ZnO due to the excitonic recombination [18].The presence of UV emission at room temperatureindicates that the material is indeed ZnO and may bepolycrystalline. The green emission band dominating thespectrum at room temperature is typically attributed to thenon-stochiometric composition of ZnO [18,36] and alsolikely to surface states [37]. The last one is likely if oneconsiders the huge surface-to-volume ratio of thin ZnOnanowires. We believe that this is a case for our samplessince the hollow ZnO microspheres are densely decoratedwith ZnO nanowires (see Fig. 2(b)) which significantlyincrease the sample area and make the observed PLemission surface sensitive. In addition, since the oxidationprocess of Zn microspheres occurred at relatively lowtemperature (600 1C) the point defect density in ZnOnanocrystals attached to ZnO microspheres is probablyhigher than in crystals grown at higher temperature [30].

We believe that these arguments can account for the greenband emission observed in the hollow ZnO microspheresdecorated with ZnO nanowires. Fig. 5(b) shows the lowtemperature PL spectrum of the same hollow ZnO micro-spheres sample measured at 10K. It is seen that the PLspectrum is dominated by the excitonic emission peak (seeinset Fig. 5(b)) containing free exciton (FX at �3.37 eV)and bound exciton (DX at �3.36 eV) peaks. The donor–-acceptor recombination peak (DAP) at �3.31 eV arefollowed by the first and higher order LO replicas of theFX and DX lines with an energy separation of �70meV.The first-order LO-phonon replica of the main boundexciton line falls at 3.29 eV. However, due to the linebroadening, the peaks corresponding to each individualbound exciton could not be resolved very well. Theintensity ratio between the green emission band and theUV excitonic emission band is significantly changed at10K indicating the dominance of excitonic emission overthe defect related recombination. The spectral position andFWHM of the green emission band remain relativelyunchanged over investigated temperature range. The PLresults recorded for the hollow ZnO microspheres indicatethat the simple oxidation process of the Zn microspheresproduces hollow ZnO microspheres retaining all featurescharacteristic for its bulk counterpart.

4. Conclusion

In summary, metallic Zn and hollow ZnO microspheresare synthesized via thermal evaporation and vapor trans-port method. The size of the microspheres is in rangebetween 1 and 10 mm. This approach can pave the way foreasy and inexpensive large scale production of bothmetallic Zn and semiconducting ZnO hollow spheres.Due to their large surface to volume ratio, these structuresmay have useful applications such as in gas sensors.

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