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the 2 range between 5 and 70. Transmission electronmicroscopy (TEM) was applied to determine the mor-phology and particle size of as-prepared products. Theimages were taken with a Hitachi H-800 transmissionelectron microscope. Differential scanning calorimetric(DSC) and thermogravimetric (TG) analysis was con-ducted on a Shimadzu TA-50 thermal analyzer betweenroom temperature and 500 C in flowing N2 atmosphere.Infrared (IR) analysis of these samples were conductedon a Magna IR-750FT Spectrometer ranging from 400 to4000 cm1 at room temperature with the samples mulledin KBr water. Absorption spectra were collected at roomtemperature on a Shimadzu ultraviolet-visible (UV-VIS)absorption diode array spectrometer using 1-cm Quartzcuvettes. Samples were prepared by dispersing ZnSenanocrystallites in methanol. The luminescence spectrawere measured on a Hitachi 850 fluorescence spectrom-eter with a Xe lamp at room temperature.

III. RESULTS AND DISCUSSION

Zinc powder was ready to react with selenium in eth-ylenediamine to form an orange product, which failed tobe identified from XRD pattern [Fig. 1(a)] on JCPDScards. IR analysis was applied to determine the existenceof ethylenediamine. The absorption peaks ranging from440 to 3400 cm1 in Fig. 2(a) are mainly attributed toethylenediamine. The elemental analysis gave a compo-sition ZnSeC2 N2 H8 . It is possible that zinc seleniumforms a stable complex with ethylenediamine, since zincis ready to form zinc chalcogenides with sulfur or sele-nium in strong donor solvents.20,21 DSC plot of this com-plex showed a strong endothermic peak at 239.7 Ccorresponding to the decomposition of the complex. TheTG plot showed 30% weight loss (calculated value,29.4%) for this complex corresponding to the loss of ethylenediamine.

As treated at 300 C in flowing N2 atmosphere, thiscomplex was converted to a yellow powder which couldbe identified to be hexagonal ZnSe from XRD pattern[Fig. 1(b)]. On the other hand, as the complex was treatedin dilute HCl solution (pH 1), it was converted to ared-brown product, which could also be identified to bewurtzite ZnSe [Fig. 1(c)]. After refinement, the calcu-lated cell constants a 3.98 , c 6.53 were closeto the reported value.22 Therefore, there were two waysto prepare ZnSe nanoparticles after the complex was ob-tained by a solvothermal treatment. They can be dia-gramed as

ZnSeC2H8N2

ZnSe+ C2H8N2 (1)

ZnSeC2H8N2 H

ZnSe+ C2H10 N2 2+ . (2)

ZnSe nanoparticles could be produced by pyrolysis of complex as indicated by the DSC. This complex decoposed at about 237.9 C until 280 C. In this preparatiprocess, 300 C was applied to obtain pure ZnSe. Tother way is treatment of the complex in acid solutioThis complex is stable in neutral aqueous solutions, bdecompose at low pH. In acid solution, protonizationethylenediamine results in ZnSe as showed with Eq. (An aqueous solution with a much lower pH has not be

FIG. 1. XRD patterns for (a) the complex prepared by reaction tween stoichiometric zinc and selenium in ethylenediamine, (b) Zprepared by pyrolysis of the complex at 300 C, (c) ZnSe preparedprotonization of the initial product in dilute HCl solution (pH 1).

FIG. 2. IR spectra of the samples: (a) the complex, (b) the prodobtained by pyrolysis of the complex at 300 C, (c) the product pared by protonization of the complex.

J.H. Zhan et al.: A solvothermal route to wurtzite ZnSe nanoparticles

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applied for it could make ZnSe decompose as shown byEq. (3). In this preparation process, an acid solution(pH 1) was applied.

ZnSe + 2H+ Zn2+ + H2Se (3)

IR analysis of the as-formed products was carried outto investigate attachment of ethylenediamine to ZnSe[Fig. 2(b,c)]. The absorption peaks at about 3400 and1600 cm1 , which correspond to the OH stretching vi-bration [ (OH)] and HOH bending vibration[ (OH2)] respectively, could be due to absorption of H2 Oabsorbed in these samples. In fact, the absorption of wa-ter is very common for powder samples with high surfacearea, which have been exposed to atmosphere. Theseresults suggest the products prepared by pyrolysis andhydrolysis of the complex are pure ZnSe nanocrystallitesfree from ethylenediamine.

The particle size and morphology of as-formed prod-ucts were investigated by TEM and the typical imageswere shown on Fig. 3. TEM observation indicated thatthe preparation methods had a considerable effect on themorphology of the resulted products. The pyrolysis-formed ZnSe nanoparticles have a spherical shape[Fig. 3(a)]. The particle size shown by the TEM image islarger than that estimated from the XRD pattern withDebyeScherer formula (about 18 nm). This differencemay be attributed to the agglomeration of these particles.On the other hand, the protonization-formed ZnSe nano-particles have a laminar shape [Fig. 3(b)]. These thinplatelets may stack on each other to form a big particle as

shown by Fig. 3(c). The small thickness may cause thebreadth of these XRD reflection peaks. The different for-mation mechanisms could be responsible for the differ-ence in morphology. The elemental composition of the

as-formed ZnSe was of 50:50 atomic ratio as confirmby atomic absorption on a Perkin-Elmer 1100B atomabsorption spectrophotometer. So the preparation rouby pyrolysis or protonization had no obvious effect the composition but on the morphology.

The solvent ethylenediamine plays an important roin the formation of hexagonal wurtzite ZnSe nanopaticles. As mentioned in Ref. 14, solvothermal treatmof zinc and selenium leads to formation of cubic stillenanoparticles. As soon as the nucleus of hexagonal Zncrystallites forms, it cannot convert to cubic structueven though it is treated solvothermally in pyridine 180 C for 6 days. If the reaction route is changed wZnCl2 + Se + Na, the final product is the same.

The UV-VIS absorption spectra of the ZnSe nanocrytals prepared by protonization are shown in Fig. 4. Tonset of the absorption spectrum at about 470 nm corsponds to the band gap of ZnSe. The second transitionabout 320 nm may be assigned to the second excited stwithin each crystallite.23 The photoluminescence (PL)spectrum, using an excitation at 233 nm with a 290-nfilter, shows a sharp PL peak at 310 nm (4.0 eV) withshoulder peak at 330 nm as shown in Fig. 5. These sults indicate that these ZnSe crystallites do not shownear band-edge emission but an emission near the secotransition. A similar fluorescence was also observedthe ZnSe crystallites prepared by pyrolysis.

IV. CONCLUSION

A solvothermal method combined with pyrolysis aprotonization has been successfully used to prepare Znnanocrystallites. XRD results indicated that the finproducts were of wurtzite structure. The morphology

FIG. 3. TEM images of ZnSe nanoparticles: (a) by pyrolysis of the complex at 300 C, (b) by protonization of the complex, (c)laminar shape.


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the final product was influenced by the post-treatingmethods. Spherical and laminar nanoparticles were ob-tained through pyrolysis and protonization of the com-plex respectively. An ultraviolet emission has beenobserved in these ZnSe nanocrystals.

ACKNOWLEDGMENTS

Financial support from the National Natural ScienFunds of China and National Outstanding Youth Fundgratefully acknowledged.

REFERENCES

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FIG. 4. UU-VIS absorption spectrum of ZnSe crystallites.

FIG. 5. Photoluminescence spectrum of ZnSe crystallites.


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