A reduction approach to prepare ZnSe nanocrystallites

JIANG Hai-qing(姜海青)^{1, 2}, CHE Jun(车 俊)², LI Zhi-min(李智敏)¹, YAO Xi(姚 熹)²

1. School of Technical Physics, Xidian University, Xi’an 710071, China;

2. Key Laboratory of Electronic Material Research, Ministry of Education,

Xi’an Jiaotong University, Xi’an 710049, China

Received 10 April 2006; accepted 25 April 2006

 Abstract: ZnSe nanocrystallites powders were successfully prepared by reduction method with H₂SeO₄ or H₂SeO₃ as selenium source and Zn(Ac)₂·H₂O as zinc source. The reaction mechanism was also discussed through XRD and DTA/TG analysis. In the process, ZnSeO₄·H₂O or ZnSeO₃·H₂O crystallites precipitated in organic solvent was prepared at first. Then the crystallites were reduced with carbon monoxide atmosphere. The XRD analysis reveals that the structure of ZnSe phase is sphalerite (cubic ZnS). The XPS results show that the atomic ratio of Se to Zn is 1?1 approximately. The TEM shows that the size of ZnSe crystallite is 30-50 nm. This method of synthesis of ZnSe nanocrystallites has the potential application in the preparation of optical composite thin films from sol-gel process.

Key words: ZnSe; nanocrystallites; reduction method; carbon monoxide

1 Introduction

The wide band gap Ⅱ-Ⅵ semiconductor materials ZnSe which have the unique chemical and physical properties have been extensively used in optical material, sun energy material and sensor material, especially in blue-green diode lasers. In recent years, much attentions [1-10] were paid on the preparation methods, performances and applications of ZnSe nanocrystallites and thin solid films, and a lot of important accomplishments have been obtained. Nowadays, researchers were focused on the improvement of properties and the preparation technology of thin films and nanocrystallites.

WANG et al [1] synthesized ZnSe using ZnCl₂, selenium powder and aluminium foil as the starting materials under 1 mol/L alkali condition. SU and CHOK [2] synthesized ZnSe in aqueous conditions using ZnCl₂, selenoure as the zinc and selenium source, respectively. NODA et al [4] prepared ZnSe films by MOCVD system on GaAs substrate, using diethylzinc(DEZn) and diethylselenide(DESe) as source materials with the carrier gas NF₃. LOKHANDE et al [4] prepared ZnSe films through the chemical bath deposition method(CBD) using selenoure as selenium source and ZnSO₄ as zinc source. Selenoure will releases Se^2- ions in an alkaline medium which react with zinc ions. ZnSe will deposit on the surface of substrates. Then the films were washed with deionized water and dried under N₂ atmosphere. LI and NOGAMI [7] prepared ZnSe/SiO₂ thin films. They prepared ZnSe/SiO₂ thin composite films using H₂SeO₄ and ZnCl₂ as source materials by sol-gel process adding ZnSeO₄·H₂O into TEOS sol-gel system under nitrogen and hydrogen reduction atmosphere. But our research work revealed that selenium element would escape from the sample heavily because of the reaction between selenium and hydrogen at 215 ℃, which is the reason of emergence of ZnO phase and H₂Se in their samples. In this paper, we successfully synthesized ZnSe nanocrystallites using relatively cheap H₂SeO₄ or H₂SeO₃ as selenium source and Zn(Ac)₂·2H₂O as zinc source in the carbon monoxide atmosphere at 500 ℃. The synthesis process of ZnSe can solve the problems of emergence of ZnO phase and it is very convenient to use in the process of preparing the composite thin films.

2 Experimental

The white ZnSeO₄·H₂O or ZnSeO₃·H₂O crystallites precipitated in organic solvent were prepared firstly by mixing 0.1 mol/L ethanol solution of H₂SeO₄ or H₂SeO₃ with the ethanol solution of 0.1 mol/L Zn(Ac)₂·H₂O at the atomic ratio of Se to Zn 1?1. For promoting the solubility of Zn(Ac)₂·H₂O in ethanol solvent, 0.2 mL NH₃·H₂O was added in the ethanol solution. The ZnSeO₄·H₂O or ZnSeO₃·H₂O crystallites can be obtained after stirring the mixture for 2 h and dried in revolving evaporator at low pressure at 60 ℃ for 3 h. Finally, ZnSeO₄·H₂O or ZnSeO₃·H₂O crystallites in quartz crucible were reduced under carbon monoxide  atmosphere at 500 ℃. To ensure the partial pressure of selenium element, small amounts of metal selenium were also sealed in quartz tube stove. In the reduction course, oxygen must be removed as far as possible at low temperature. That is the important step to ensure the samples without the emergence of ZnO phase. For preparing the samples of ZnSeO₄·H₂O or ZnSeO₃·H₂O for DTA-TGA analysis, 0.1 mol/L Zn(Ac)₂·H₂O water solution and 0.1 mol/L H₂SeO₄ or H₂SeO₃ water solution were prepared firstly, then they were mixed without adding NH₃·H₂O so as to decrease the influence of NH₃·H₂O.

The phase structures of samples were characterized by XRD (Rigaku D/MAX-2400, Cu K_α). DTA-TG (TA- 2000) analysis was used to record the thermal effect in the process of heat-treatment. The atomic ratio of Se/Zn was analyzed by XPS(VG ESCALAB MKⅡ,MgK_α). The size of ZnSe crystallite was investigated by TEM(JEM-200cx).

3 Results and discussion

3.1 DTA-TG analysis

3.1.1 DTA-TG analysis of ZnSeO₄·H₂O

The DTA-TG analysis of ZnSeO₄·H₂O prepared by the above-mentioned method was performed from room temperature to 700 ℃ with the heating rate of 10 ℃/min under air atmosphere. Although that condition is not consistent with that of ZnSe prepared, it can give us useful instruction in the preparation of ZnSe. The results of DTA-TG are shown in Fig.1. The curves indicate that there are three endothermic peaks. An endothermic peak at about 103 ℃ was associated with the losing of physical adsorption water. The endothermic peak at about 216 ℃ was associated with the losing of chemical adsorped water. The third endothermic peak at about 675 ℃ was associated with the decomposing reaction of ZnSeO₄. The TG curve reveals that the mass ratio of ZnO/SeO₃ is 39.3% which is associated with the mass

Fig.1 DTA-TG curves of ZnSeO₄·H₂O

loss from 76.61% to 29.56% at 674.51 ℃.

3.1.2 DTA-TG analysis of ZnSeO₃·H₂O

The DTA-TG analysis of ZnSeO₃·H₂O prepared by above-mentioned method was performed from room temperature to 600 ℃ under the condition same as the sample of ZnSeO₄·H₂O. The results of DTA-TG are shown in Fig.2. The curves indicate that ZnSeO₄·H₂O has an endothermic peak at 213.5 ℃ and an exothermic peak at 435.7 ℃.The endothermic peak at 213.5 ℃ was associated with the losing of chemical adsorbed water. The exothermic peak at 435.7 ℃ was associated with the creation of new phase of ZnSeO₃.

Fig.2 DTA-TG curves of ZnSeO₃·H₂O

3.2 XRD analysis

3.2.1 XRD analysis of ZnSeO₄·H₂O

The phase structures of ZnSeO₄·H₂O with different heat-treatment temperatures are shown in Fig.3. The ZnSeO₄·H₂O was heat-treated at 100, 200, 300, 320 and 410 ℃ for 1 h under air condition, respectively. The XRD results indicate that ZnSeO₄·H₂O  was compara- tively stable under 200 ℃, which is consistent with the results of DTA-TG curves. With the increasing of temperature, ZnSeO₄·H₂O becomes amorphous at 300 ℃, which means a new phase formed in this period. ZnSeO₄ will form at 410 ℃. The XRD pattern of the sample (not shown in Fig.3) heat-treated at 650 ℃ indicated the emergence of ZnO wurtzite phase. Even though there are some differences between that under air condition and that under carbon monoxide atmosphere, the results could give us some valuable hints. The temperature of reaction between ZnSeO₄ and carbon monoxide is much lower than the temperature of decomposing reaction of ZnSeO₄. The reaction equation between ZnSeO₄ and carbon monoxide also is

ZnSeO₄+4CO == ZnSe+4CO₂

Fig.3 XRD patterns of ZnSeO₄·H₂O heat-treated at different temperatures for 1 h

The XRD pattern of ZnSe nanocrystallites synthesized at 500 ℃ under carbon monoxide atmosphere using ZnSeO₄·H₂O is shown in Fig.4. The XRD pattern indicates that the phase structure of ZnSe is sphalerite (cubic ZnS) without the appearance of ZnO phase.

Fig.4 XRD pattern of ZnSe using ZnSeO₄·H₂O crystallites synthesized at 500 ℃ under carbon monoxide atmosphere

3.2.2 XRD analysis of ZnSeO₃·H₂O

The structure change of ZnSeO₃·H₂O with different heat-treatment temperatures is shown in Fig.5. The ZnSeO₃·H₂O was heat-treated at 100, 213, 300 and 438 ℃ for 1 h under air condition, respectively. The XRD results indicate that ZnSeO₃·H₂O is stable under 200 ℃, which is consistent with the results of DTA-TG curves. With the increasing of temperature, ZnSeO₃·H₂O will transform to ZnSeO₃ above 300 ℃. The XRD pattern of ZnSe nanocrystallites synthesized at 500 ℃ under carbon monoxide atmosphere using ZnSeO₃·H₂O is shown in Fig.6. The XRD pattern indicates that the structure ZnSe is sphalerite (cubic ZnS) without the existence of ZnO phase.

Fig.5  XRD patterns of ZnSeO₃·H₂O heat-treated at different temperatures for 1 h

Fig.6 XRD pattern of ZnSe using ZnSeO₃·H₂O crystallites synthesized at 500 ℃ under carbon monoxide atmosphere

3.3 XPS analysis of ZnSe

The XPS results of ZnSe prepared from ZnSeO₄·H₂O and ZnSeO₃·H₂O are shown in Fig.7. The XPS results also indicate that the atomic ratio of Se to Zn of the two samples is 1?1 approximately.

3.4 TEM analysis of ZnSe nanocrystallites

The TEM images of ZnSe nanocrystallites are shown in Fig.8. It can be seen that the size of ZnSe

Fig.7 XPS spectra of ZnSe synthesized from ZnSeO₄·H₂O(a) and ZnSeO₃·H₂O(b)

Fig.8 TEM images of ZnSe synthesized from ZnSeO₄·H₂O(a) and ZnSeO₃·H₂O(b)

nanocrystallites is 30-50 nm.

4 Conclusions

ZnSe nanocrystallites without the emergence of ZnO phase has been synthesized by the reduction method with H₂SeO₄ or H₂SeO₃ as selenium source and Zn(Ac)₂·H₂O as zinc source under carbon monoxide atmosphere. The size of ZnSe crystallite is 30-50 nm. This method has potential applications for preparing ZnSe/SiO₂ composite thin films.

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(Edited by YUAN Sai-qian)

Foundation item: Project(2002CB613305) supported by the National Basic Research Program of China; Project supported by International Cooperation Research Project of China-Israel of the Ministry of Education of China

Corresponding author: JIANG Hai-qing; Tel:+86-29-82668679; Fax:+86-29-82668794; E-mail: hqjiang@mail.xidian.edu.cn