ARTICLE IN PRESS

Journal of Crystal Growth 310 (2008) 3645– 3648

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

� Corr

Univers

E-m

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

Solvothermal synthesis and characterization of ZnSe nanoplates

Juan Yang a,b,�, Guoxiu Wang a, Hao Liu a, Jinsoo Park a, Xinglong Gou a, Xiaonong Cheng b

a Materials Chemistry Research Group, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2512, Australiab School of Materials Science & Engineering, Jiangsu University, Zhenjiang Jiangsu 212013, China

a r t i c l e i n f o

Article history:

Received 20 March 2008

Received in revised form

7 May 2008

Accepted 7 May 2008

Communicated by P. Rudolphwurtzite structured ZnSe by heat-treatment in Ar atmosphere. The as-prepared ZnSe was composed of

Available online 15 May 2008

PACS:

81.05.Dz

81.10. Dn

63.22.Np

Keywords:

A1. Nanostructures

A2. Hydrothermal crystal growth

B1. Zinc compounds

B2. Semiconducting II–VI materials

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.jcrysgro.2008.05.004

esponding author at: School of Materials Sc

ity, Zhenjiang Jiangsu 212013, China. Tel.: +86

ail address: yangjuan6347@ujs.edu.cn (J. Yang

a b s t r a c t

ZnSe nanoplates were synthesized by a solvothermal method using ethylenediamine (EN) as the

liganding solvent. The crystal structures, morphologies and optical properties of the precursor and ZnSe

products were systematically characterized. Results reveal that precursor ZnSe(en)1/2 with a layered

structure was initially obtained through solvothermal synthesis, which can be converted into hexagonal

stacked nanoplates. The field emission scanning electron microscope (FESEM) and transmission electron

microscope (TEM) results reveal that every nanoplate was made up of plenty of tiny single crystals. The

room temperature ultraviolet–visible (UV/vis) measurements indicate that the bandgap of the obtained

ZnSe is 2.7 eV and a large blue shift about 1.3 eV was observed in precursor ZnSe(en)1/2.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

As one of the important II–IV group semiconductors, ZnSe,with a room-temperature bulk bandgap of 2.7 eV, has attractedintense attention in recent years because of the potentialapplications in light-emitting diodes, photodetectors, full-colordisplays and room-temperature excitonic devices [1,2]. Thesynthesis of nanocrystalline ZnSe powders with tunable phase,morphology and size provides alternative variables in tailoring itsphysical and chemical properties [3–7]. Therefore, a tremendouseffort has been made to control the shape and crystal structure ofZnSe nanocrystals, and various methods have been employed forthe synthesis of ZnSe-based nanostructures [3,4,8–10], especiallyone-dimensional ZnSe nano- or microstructures [9,11,12].

Among the numerous methods developed for the controllablesynthesis of metal chalcogenide materials, solvothermal reactionin a homogeneous mixed solution has been proven to be aconvenient approach to prepare semiconductor nanocrystals,especially to control the morphology and crystal structure of theresulting products by adjusting the composition of the mixedsolution [13–19]. Up to now, II–IV semiconductor nanorods

ll rights reserved.

ience & Engineering, Jiangsu

51188780195.

).

[14,17,18], nanobelts [9], nanoflowers [16], nanowires [13,16],nanotrees [16], microspheres [10,15] and hollow microspheres[7,8] have all been successfully synthesized via a facial solvother-mal approach.

In this paper, ZnSe nanoplates were synthesized via a mixedsolvothermal process and subsequent thermal treatment, using amixed solution of ethylenediamine (EN) and de-ionized water(DIW) as the solvents and N2H4 �H2O as the reducing agent. Theresults showed that wurtzite structured ZnSe nanoplates can beobtained, and the formation mechanism is also discussed.

2. Experimental procedure

Analytical-grade reagents including zinc sulfate heptahydrate(ZnSO4 �7H2O) and selenium (Se) were purchased from Sigma-Aldrich. EN and hydrazine hydrate (N2H4 �H2O) were used asreceived without further purification.

ZnSO4 �7H2O (0.75 mmol) was dissolved in DIW (4.5 mL) toform a transparent solution. Anhydrous EN (9.4 mL) was added tothe solution. After stirring for 10 min, Se (0.75 mmol) andN2H4 �H2O (1.9 mL) were added. Then, the solution was trans-ferred into a 25-mL stainless steel Teflon-lined autoclave, whichhas been filled to 65% of its capacity. The autoclave was taken outof the furnace after the reaction had been carried out at 180 1C for

ARTICLE IN PRESS

100

95

90

85

80

Wei

ght (

%)

J. Yang et al. / Journal of Crystal Growth 310 (2008) 3645–36483646

20 h and cooled to room temperature. The pink precipitates werecollected, washed with distilled water and absolute alcohol threetimes, followed by drying in vacuum at 60 1C for 4 h. To obtainZnSe, the as-prepared precursor was calcined in a tube furnace at500 1C under an argon atmosphere for 4 h.

The crystal structure and phase of the synthesized productswere characterized by a Philips 1730 powder X-ray diffractometer(XRD) with Cu Ka radiation (l ¼ 1.5406 A). The morphology andmicrostructures were observed by a field emission scanningelectron microscope (FESEM, JEOL JSM-6700F). High-resolutionTEM (HRTEM) was conducted on a JEOL 2011 transmissionelectron microscope (TEM) at an accelerating voltage of 200 kV.Infrared (IR) spectra of the samples were measured on NicoletAvatar 360FTIR spectrometer. Thermogravimetric analysis (TGA)of the precursor was recorded on a TGA-Q500 thermogravimeter,and ultraviolet–visible (UV/vis) spectra were recorded on aShimadzu UV-1700 spectrophotometer at room temperature.

100 200 300 400 500 600Temperature [°°C]

Fig. 2. TGA curve of the ZnSe precursor.

3. Results and discussion

Fig. 1(a) shows the XRD pattern of the pink product obtainedfrom the solvothermal reaction. It can be seen that there is a strongorganic amine-related peak at around 10.161, corresponding to a d

spacing of 8.708 A. Taking account of the thickness of inorganic ZnSeframework (2.415 A), the interlayer distance can be calculated to be6.293 A, which is similar to the data reported in Ref. [20] (6.26 A). Toobtain information on the organic composition in the layered ZnSeprecursor, a TGA experiment was carried out, and the result isshown in Fig. 2. The TGA curve revealed about 17% weight loss fromabout 200 to 400 1C, which is consistent with the theoreticalcontent of EN in ZnSe(en)1/2 compound (17.2%).

Fig. 1(b) shows the XRD pattern of the sample obtained bythermal treatment of the precursor at 500 1C in an Ar atmospherefor 4 h, which is different from that of Fig. 1(a), indicating that thelayered structure is totally changed by the heat-treatment. All thereflection peaks in Fig. 1(b) can be readily indexed to hexagonal,wurtzite ZnSe (JCPDS card no. 80-0008) and no other impuritypeaks were detected.

The general morphology of the obtained ZnSe nanoplates wasobserved by a FESEM. Fig. 3(a) presents a typical FESEM image,and it can be seen that the obtained ZnSe shows plate-like

Fig. 1. X-ray diffraction patterns of (a) ZnSe precursor and (b) ZnSe nanoplates.

Fig. 3. FESEM images of ZnSe nanoplates. The image shown in the inset is a higher

magnification image of part A in (b).

aggregations, which are made up of several nanoplates stackedtogether. The length of the plate is about 10 mm. The thickness ofthe plates can be determined in Fig. 3(b), which is a higher

ARTICLE IN PRESS

J. Yang et al. / Journal of Crystal Growth 310 (2008) 3645–3648 3647

magnification image of Fig. 3(a). The nanoplates have a uniformthickness of about 100 nm. We noticed that each nanoplate iscomposed of innumerable particles in a random arrangementwith an individual crystal size of about 10–20 nm.

The stacked nanoplates can be dispersed by ultrasonicvibration, and the monodispersed nanoplates were observed inHRTEM, which provides further insight into their crystal struc-tures. Fig. 4(a) shows a typical TEM image, illustrating that thenanoplates are well dispersed and have a width of about 300 nm.The surfaces of ZnSe nanoplates are rough, suggesting that thecrystal is not perfect. Fig. 4(b) shows the HRTEM image recordedfrom the edge of one of the nanoplates in Fig. 4(a). By carefulanalysis, it can be found that the fringe pattern is not uniform andcan be divided into many regions, such as A, B, C, D areas markedin Fig. 4(b). It clearly reveals that the whole nanoplate is not asingle crystal and is composed of many tiny single crystalsgrowing simultaneously. The size of an individual tiny singlecrystal is estimated to be about 10–20 nm, which is consistentwith the FESEM observation. The inset is the correspondingselected area electron diffraction (SAED) pattern recorded fromthe marked region C, indicating that the selected area is a well-crystallized single crystal. The fringe spacings shown in the imageare about 0.35 and 0.33 nm, which agree well with the (10 0) and(0 0 2) lattice planes, respectively.

Fig. 4. (a) TEM image of ZnSe nanoplates and (b) HRTEM image and SAED pattern

(inset) of ZnSe nanoplate.

Infrared spectroscopy (IR) measurement of the as-synthesizedZnSe and its precursor was performed to identify the role of theEN in the solvothermal synthesis. The IR spectra of the precursorin Fig. 5(a) exhibit vibrational bands corresponding to ENmolecules. Bands above 3000 cm�1 are assigned to the stretchingvibrations of N–H, and bands at about 2934 and 2868 cm�1 can beassigned to the C–H vibration. The band at about 1588 cm�1 is thescissors vibration of NH2, and the band at 1029 cm�1 can beassigned to C–N. Those bands around 1080 and 1352 cm�1 arecaused by the vibration of CH2. The spectrum is different fromthat of the pure liquid-state EN reported in the literature [11],indicating that an interaction exists between EN and ZnSe.According to the previous report [20], this kind of spectrum ischaracteristic of trans-EN which functions as a bridged ligand andwill be aligned in an orderly way in the precursor. After removingthe EN molecules, wurtzite ZnSe was obtained, and the IRspectrum is shown in Fig. 5(b). Compared to the precursor, almostall the characteristic bands of EN have disappeared, indicating thesuccessful removal of EN by heat-treatment.

Based on these observations, a growth mechanism of the ZnSenanoplates is proposed. In the reaction, EN is a bidentate ligand andcan coordinate with Zn2+ to form a relatively stable Zn2+ complex asshown in Fig. 6 [11]. The stability of the complex is expected todecrease with the increase of the temperature. In the reactionsystem, Se is reduced by N2H4 �H2O to form Se2�, and Se2� ions willthen react with the above complex to form a layered structure withEN molecules orderly intercalated between the slabs, which issimilar to the structure shown in Fig. 6. From the structural point ofview, the adopted way in which the Zn and Se atoms wereconnected in the inorganic layers of the precursors is almost thesame as that of one atomic slice cut from the hexagonal wurtzitestructure despite the slight distortions in bond angles and bondlengths [20]. In the precursor, since the layered structure isstabilized by the interlayer EN molecules, it is reasonable that thelayered structures would collapse upon the removal of the organicintercalant. After being heat-treated at 500 1C in Ar atmosphere, ENmolecules would be removed initially from some place near crystaldefects or the edge of the layered precursors, and then the nakedinorganic slabs would become unstable. To minimize the systemenergy, the layered structures were collapsed by layer-by-layercoupling between the ZnSe slabs, hundreds of inorganic slabswere stacked together and reorganized to form wurtzite structured

Tran

smitt

ance

(%)

4000 3500 3000 2500 2000 1500 1000Wavenumber (cm-1)

a

b

3239

.12

3116

.54

2934

.85

2868

.28

1588

.05

1352

.49

1080

.97

1029

.87

Fig. 5. IR spectra of (a) the ZnSe precursor and (b) ZnSe nanoplates.

ARTICLE IN PRESS

Fig. 6. Schematic representation of the formation of ZnSe(en)1/2 and ZnSe nanoplates.

0.24

0.22

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04200 400 600 800 1000

Wavelength (nm)

Abs

orba

nce

pre cursorZn Se

Fig. 7. UV/vis spectra of ZnSe(en)1/2 and ZnSe nanoplates at room temperature.

J. Yang et al. / Journal of Crystal Growth 310 (2008) 3645–36483648

plate-like products. The thickness of the plates might be determinedby the numbers of the inorganic slabs stacked. Plates were obtainedwith uniform thickness, indicating that an appropriate conditionwas offered in our experiment to guarantee the uniformity in thelayer-by-layer coupling process.

Fig. 7 shows the optical absorption properties of ZnSe and itsprecursor. The UV/vis spectrum of the precursor has a strongabsorption peak at about 310 nm (4 eV). While for ZnSe, anabsorption peak at about 459 nm was detected corresponding to abandgap of 2.7 eV, which is in accordance with the value reported inthe literatures [9,19]. Compared with ZnSe, the precursor ZnSe(en)1/

2 exhibited a large blue shift (about 1.3 eV). This phenomenon,which might be caused by the interlayer organic molecules, has alsobeen found in the ZnSe(pda)1/2 and ZnTe(en)1/2 systems. It might beexplained by quantum confinement effect (QCE) [21,22].

4. Conclusion

ZnSe nanoplates were prepared by a solvothermal methodusing EN and DIW as a mixed solvent and subsequent heat-

treatment in Ar atmosphere. The obtained precursor, ZnSe(en)1/2,showed a layered structure with an interlayer distance of 8.708 A.After removing the interlayer EN molecules, the layered structurewas collapsed and rearranged to form plate-like aggregates, whichwas confirmed by FESEM observation. The HRTEM analysisrevealed that a single plate is composed of many tiny singlecrystals. The UV/vis spectra indicated that the bandgap of theobtained ZnSe was 2.7 eV.

Acknowledgments

This work was financially supported by the Australian ResearchCouncil (ARC) through an ARC Discovery Project (DP0559891). Wethank Prof. X.Y. Kong for performing TEM and HRTEM analysis.

References

[1] T.Y. Zhai, H.Z. Zhong, Z.J. Gu, A.D. Peng, H.B. Fu, Y. Ma, Y.F. Li, J.N. Yao, J. Phys.Chem. C 111 (2007) 2980.

[2] S.Y. Liu, W.C.H. Choy, L. Jin, Y.P. Leung, G.P. Zheng, J.B. Wang, A.K. Soh, J. Phys.Chem. C 111 (2007) 9055.

[3] A.B. Panda, S. Acharya, S. Efrima, Y. Golan, Langmuir 23 (2007) 765.[4] H. Gong, H. Huang, L. Ding, M.Q. Wang, K.P. Liu, J. Crystal Growth 288 (2006)

96.[5] Y. Jiao, D.B. Yu, Z.R. Wang, K. Tang, X.Q. Sun, Mater. Lett. 61 (2007) 1541.[6] H.N. Wang, F.L. Du, Cryst. Res. Technol. 41 (2006) 323.[7] Q. Peng, Y.J. Dong, Y.D. Li, Angew. Chem. Int. Ed. 42 (2003) 3027.[8] X. Wang, X. Chen, H. Zheng, J. Jin, Z. Zhang, Appl. Phys. A Mater. Sci. Proc. 80

(2005) 511.[9] W.T. Yao, S.H. Yu, X.Y. Huang, J. Jiang, L.Q. Zhao, L. Pan, J. Li, Adv. Mater. 17

(2005) 2799.[10] H. Gong, H. Huang, M.Q. Wang, K.P. Liu, Ceram. Int. 33 (2007) 1381.[11] Z.X. Deng, C. Wang, X.M. Sun, Y.D. Li, Inorg. Chem. 41 (2002) 869.[12] S.L. Xiong, J.M. Shen, Q. Xie, Y.Q. Gao, Q. Tang, Y.T. Qian, Adv. Funct. Mater. 15

(2005) 1787.[13] J.S. Jang, U.A. Joshi, J.S. Lee, J. Phys. Chem. C 111 (2007) 13280.[14] H.B. Chu, X.M. Li, G.D. Chen, W.W. Zhou, Y. Zhang, Z. Jin, J.J. Xu, Y. Li, Cryst.

Growth Des. 5 (2005) 1801.[15] Y.C. Zhang, G.Y. Wang, X.Y. Hu, J. Alloys Compd. 437 (2007) 47.[16] W.T. Yao, S.H. Yu, S.J. Liu, J.P. Chen, X.M. Liu, F.Q. Li, J. Phys. Chem. B 110 (2006)

11704.[17] H.N. Wang, Z.Y. Guo, F.L. Du, Mater. Chem. Phys. 98 (2006) 422.[18] Y.D. Li, H.W. Liao, Y. Ding, Y. Fan, Y. Zhang, Y.T. Qian, Inorg. Chem. 38 (1999)

1382.[19] J. Du, L.Q. Xu, G.F. Zou, L.L. Chai, Y.T. Qian, Mater. Chem. Phys. 103 (2007) 441.[20] Z.X. Deng, L.B. Li, Y.D. Li, Inorg. Chem. 42 (2003) 2331.[21] B. Gawe", W. Łasocha, M. Zieba, J. Alloys Compd. 442 (2007) 77.[22] X.Y. Huang, J. Li, J. Am. Chem. Soc. 122 (2000) 8789.