07) 4459–4462www.elsevier.com/locate/matlet

Materials Letters 61 (20

A simple and cheap way to produce porous ZnO ribbons and theirphotovoltaic response

Feifei Wang a,b, Ruibin Liu a,b, Anlian Pan a,b, Sishen Xie a, Bingsuo Zou a,b,⁎

a Institute of Physics, Chinese Academy of Sciences, Beijing 100080, Chinab Micro-Nano Technologies Research Center and National Key Lab of CBSC, Hunan University, Changsha 410082, China

Received 9 December 2006; accepted 9 February 2007Available online 16 February 2007

Abstract

A simple and cheap method is proposed to achieve porous ZnO ribbons by oxidation of ZnS ribbons in the air. ZnS has a fully transformationto ZnO at an annealing temperature of 700°C from energy dispersive X-ray spectra and X-ray diffraction patterns. Scanning electron microscopyimages indicate that ZnO ribbons keep the original shapes of ZnS, but produce some ordered and uniform pores on their surfaces. Thephotovoltage spectrum of ZnO/N3 indicates such dye-porous ZnO ribbons may be used in the dye-sensitized solar cells. The porous ZnO ribbonsmay also find potential applications in catalyst, sensor, and molecular selection. This technique to produce porous ribbons may also be applied toprepare other porous metal oxide ribbons.© 2007 Elsevier B.V. All rights reserved.

Keywords: Semiconductors; Porosity

1. Introduction

Nowadays, lots of effort has been put into the fabrication ofone-dimensional (1D) nanostructures for their wide applicationsin the field of optical, electronic, magnetic, sensor, etc. [1–4].Among them, ZnO has attracted quite a large amount of attentionfor its unique properties and 1D structure. Control of spatialvariation along or within 1D nanostructures may impact theirproperties. For example, gas sensor using porous structure, whichhas a very high surface-to-volume ratio, has a higher sensibilitythan that based on single-crystalline materials [5,6]. Furthermore,porous structures are widely used in catalyst, solar cell, andmolecular cognition and selection. So some attention has beenpaid on the preparation of porous structures [6–11]. It is veryimportant to find a simple and inexpensive method to prepareporous semiconductor structure. It is found [12–14] that using onekind of semiconductor nanostructures as precursor, another kindof semiconductor can be synthesized by taking some chemical

⁎ Corresponding author. Micro-Nano Technologies Research Center andNational Key Lab of CBSC, Hunan University, Changsha 410082, China.Tel.: +86 10 82649021; fax: +86 10 62556598.

E-mail address: zoubs@aphy.iphy.ac.cn (B. Zou).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.02.021

processing. The product made by such processing can almostpreserve the precursor morphology and some of them possess aporous structure. In addition, metal oxide nanowires can besimply produced by oxidation of metal nanowires in air [15–17].According to that, we think annealing in the air may become agood and simple way to convert 1D compound semiconductor toits corresponding porous metal oxide. Furthermore, the porousnanoribbons should be popular for better electronic conductionand mechanic stability than the nanowires.

In this paper, porous ZnO ribbonswere synthesized by oxidationof ZnS ribbons at 700 °C for an hour, and the preparationmethod isvery simple and easy to control compared with somemethods. TheZnS powder and CdS powder (less than 0.05% molecular ratio),used as source materials, is much cheaper than ZnSe ever used inRef. [15]. So this method is very suitable to obtain porous ZnOstructure. Furthermore, the photoresponse of dye adsorbed porousZnO ribbons was also studied.

2. Experimental

The synthesis of porous ZnO ribbons includes two steps. First,ZnS ribbons with smooth surfaces were grown on silicon wafers.ZnS ribbons were synthesized in an electrical furnace with a

4460 F. Wang et al. / Materials Letters 61 (2007) 4459–4462

horizontal quartz tube (35 mm in diameter, 120 cm in length) byphysical evaporation of ZnS and CdS (less than 0.05%) powderusing Au catalysts. Silicon wafers were first ultrasonically cleanedin acetone and then sputter-coatedwith a thin (∼10 nm) layer ofAufilm. A mixed powder of ZnS and CdS was put into the center of ahorizontal tube furnace, and several clean silicon wafers coatedwith Au filmwere laid downstream of the gas flow. High purity Hewas introduced to the quartz tube to eliminate the oxygen inside itbefore heating. After about an hour, the furnace was quickly heatedup to a temperature of 1100 °C andmaintained that temperature foran hour while the carrier gas was introduced. After that, the powercircuit was switched off and the furnace was cooled down to roomtemperature naturally. Finally, white product was observed on thesurface of silicon wafers, which was about 6–10 cm away from thesource materials. Second, ZnS ribbons were transformed to porousZnO ribbons. The ZnS ribbons were placed in a crucible, whichwere loaded in an electricmuffle furnace. The oxidation of ZnSwascarried out at 700 °C for an hour in ambient air.

Fig. 1. SEM images of (a1) and (a2): the ZnS ribbons, (b1) to (b3): the ZnO ribbons arespectively.

The morphology of the as-synthesized sample was charac-terized by scanning electron microscopy (SEM, Hitachi, S-4200). The crystal structure was analyzed by X-ray diffraction(XRD, Japan, Rigaku, D/MAX-2400) with graphite-monochro-matized Cu Kα radiation (λ=1.54178 Å). The photovoltagemeasurements were carried out on an assembled photovoltagespectrometer using two-electrode system with light sourcemonochromator-lock-in detection techniques (150 W Xe lamp,Zolix SBP 300 spectrometer, Stanford SR830 lock-in amplifier).All measurements were carried out at room temperature.

3. Results and discussion

The sample morphology was characterized by SEM. Fig. 1(a) and (b)show the SEM images of ZnS precursor and ZnO product ribbons. It canbe seen from the images that the morphological integrity of the ribbonswas kept after oxidation at 700 °C for an hour. The typical width of theribbons varies from 2 μm to 7 μm and the length is over 100 μm onaverage. However, the SEM images at higher magnification shown in

t magnification scales and (c1) and (c2): EDX patterns of ZnS and ZnO ribbons,

Fig. 3. The photovoltage spectra of ZnO and ZnO/N3.

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Fig. 1(a2) and (b2) reveal that these ribbons with smooth surfaces aretransformed into ribbons with uniform pores and high pore density. Themorphology of the porous ZnO ribbons is shown more clearly in a high-magnification SEM image of Fig. 1(b3). The ribbons contain almosthomogenously distributed through-pores measuring about 50 nm indiameter. The elemental compositions of these ribbons were investigatedusing in situ energy-dispersion X-ray (EDX) spectra. Fig. 1(c1) and (c2)are the EDX spectra of ZnS precursors and ZnO products. The EDXspectra confirm the full transformation fromZnS toZnO. The spectrum ofZnS precursors only show the signals from the zinc and sulfur, while afteroxidation, the EDX spectrum of the product show signals from zinc andoxygen, and no signal from sulphur element. The Si signal in thespectrum of the oxidized sample is from the substrate. The appearance ofthe Si element in the EDX spectrum of ZnO ribbons while not in that ofZnS ribbons may be attributed to the lighter molecular weight of O ascompared to that of S [14].

The transformation from ZnS to ZnO is also verified by XRDanalysis. Fig. 2(a) and (b) show the XRD patterns of ZnS and ZnOribbons, respectively. All other diffraction peaks in Fig. 2 (a), expectthose come from Au catalysts, can be indexed to hexagonal wurtzitephase of ZnS (JCPDS: 36–1450). After oxidation, no peaks of otherphases, but only those from hexagonal wurtzite phase of ZnO (JSPDS:36–1451) were detected.

Owing to the high surface-to-volume ratio, these porous ZnOribbons may be used in dye-sensitized solar cell (DSSC), catalyst,environmental engineering, and sensor systems. Fig. 3 is thephotovoltage spectra of the ZnO ribbons and the porous ZnO/N3(RuL2(NCS)2) structure. The ZnO/N3 structure was obtained byimmersing the ZnO ribbons in N3 ethanol solution for 10 h and let it

Fig. 2. X-ray diffraction patterns of the ZnS (a) and ZnO (b).

dry naturally. The photovoltage spectrum of ZnO ribbons only shows abroad photovoltage band at 300–425 nm, which is due to the transitionof carriers from the valence band to the conduction band. Except for theband from ZnO, the photovoltage spectrum of ZnO/N3 shows twoadditional bands at about 400 nm and 515 nm. And the two bands aresimilar to the photoresponse of N3 [18]. It is found by comparing thephotovoltage spectra of ZnO and ZnO/N3 that the dye-sensitized ZnOribbons can extend the photoresponse to almost the over visible range.This result indicates that the porous ZnO/N3 ribbons may be a goodmaterial for dye-sensitized solar cells.

4. Conclusions

In summary, using ZnS ribbons as the precursor, nanos-tructured porous ZnO ribbons have been synthesized. Theanalyses of EDX spectra and XRD patterns show that ZnS wasfully transformed to ZnO. The formed ZnO basically preservesthe morphology of ZnS while leaves the surface of ZnO withlarge quantities of homogeneous nanopores. The photovoltagespectrum of porous ZnO/N3 shows ideal photovoltageresponses in the visible region, and the porous ZnO may havea potential application in DSSC. The transformation from ZnSto ZnO suggests a promising route for fabricating freestandingporous structures for sensor, photocalyst, and solar cell. Thisapproach may also be extended to converting other metalsulphides to their corresponding metal oxides.

Acknowledgments

The authors thank the financial supports of NSFC of China(90606001), National 973 project (2002CB713802), project(grant No.705040) of MOE of China and 985 fund of HNU.

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