Electrodeposition mechanism of ZnSe thin film in aqueous solution
来源期刊:Rare Metals2017年第10期
论文作者:Jun-Li Xu Wei-Ying Gong Wei Wang Hao Meng Xia Zhang Zhong-Ning Shi Geir-Martin Haarberg
文章页码:816 - 820
摘 要:ZnSe is one of the important and excellent Ⅱ-Ⅵsemiconductor materials, which has direct transition band structure. In this paper, ZnSe thin films were prepared by an electrochemical deposition method, and the formation mechanism of ZnSe was studied systematically. Voltammetry and chronoamperometry combined with X-ray diffraction(XRD) and Raman techniques were used to analyze the deposition processes. It is found that the substrate and deposition potentials have a great influence on the phase composition of deposited thin film, and Zn substrate is beneficial to the preparation ZnSe films. Strong selenium-substrate interaction results in the formation of selenium compounds involving electrode materials. The addition of Zn(Ⅱ) source can affect the reduction potential of Se, and results in the change of reducing mechanism of Se(0) from Se(Ⅳ). Se(0) formed from H2Se because the formation of H2Se is more active than forming Se(0)directly from Se(Ⅳ), and H2Se can recombine with the substrate material, forming selenium-substrate compounds more quickly.
稀有金属(英文版) 2017,36(10),816-820
Jun-Li Xu Wei-Ying Gong Wei Wang Hao Meng Xia Zhang Zhong-Ning Shi Geir-Martin Haarberg
School of Science, Northeastern University
School of Materials and Metallurgy,Northeastern University
Department of Materials Science and Engineering,Norwegian University of Science and Technology
收稿日期:18 June 2015
基金:financially supported by the National Natural Science Foundation of China (Nos.51574071. 51322406 and 21501023);the Fundamental Research Funds for the Central Universities(No.140205001);
Jun-Li Xu Wei-Ying Gong Wei Wang Hao Meng Xia Zhang Zhong-Ning Shi Geir-Martin Haarberg
School of Science, Northeastern University
School of Materials and Metallurgy,Northeastern University
Department of Materials Science and Engineering,Norwegian University of Science and Technology
Abstract:
ZnSe is one of the important and excellent Ⅱ-Ⅵsemiconductor materials, which has direct transition band structure. In this paper, ZnSe thin films were prepared by an electrochemical deposition method, and the formation mechanism of ZnSe was studied systematically. Voltammetry and chronoamperometry combined with X-ray diffraction(XRD) and Raman techniques were used to analyze the deposition processes. It is found that the substrate and deposition potentials have a great influence on the phase composition of deposited thin film, and Zn substrate is beneficial to the preparation ZnSe films. Strong selenium-substrate interaction results in the formation of selenium compounds involving electrode materials. The addition of Zn(Ⅱ) source can affect the reduction potential of Se, and results in the change of reducing mechanism of Se(0) from Se(Ⅳ). Se(0) formed from H2Se because the formation of H2Se is more active than forming Se(0)directly from Se(Ⅳ), and H2Se can recombine with the substrate material, forming selenium-substrate compounds more quickly.
Keyword:
ZnSe; Thin film; Electrodeposition; Mechanism; Raman spectra;
Author: Jun-Li Xu,e-mail: jlxu@mail:neu.edu.cn;
Received: 18 June 2015
1 Introduction
ZnSe is a promising photovoltaic material because of its optical band gap energy (2.7 eV) and the high absorption coefficient for efficient energy conversions
Electrolytes used for the ZnSe electrodeposition can be pided into three major types:aqueous solutions,organic solutions
Chandramohan et al.
However,it was proposed that Se was reduced to Se2-state,forming H2Se,which was highly reactive.H2Se immediately reacted with Zn2+which was adsorbed on the surface of the substrate and formed ZnSe
Moreover,a wide range of studied experimental conditions was examined in order to determine the best way of electrosynthesis of the material,such as pH,reagent concentration,deposition potential,temperature and substrates.The substrates used were Ag,Ti,indium tin oxide (ITO),glass carbon,stainless steel,A1 and Cu.In this paper,Cu and Zn were used as the substrates for the electrodeposition of ZnSe.The aim of the present studies was to investigate the mechanism of ZnSe electrodeposition,and a new mechanism was proposed according to our results.
2 Experimental
Electrochemical tests were carried out in an acidic solution containing ZnSO4 and SeO2.The concentrations of ZnSO4and SeO2 were 0.200 and 0.002 mol·L-1,respectively,and pH was adjusted to 2 by sulfuric acid addition.All chemicals were analytical grade and used directly without further purification.The cyclic voltammograms (CV) and chronoamperometric measurements were taken using a potentiostat (CHI660).For all the electrochemical measurements,a conventional three electrode system was used.Zinc tablet or copper tablet was used as working electrode.A platinum wire was used as the counter-electrode,and a platinum wire served as the pseudo-reference electrode.Prior to the measurements,the working electrodes were cleaned as follows.The copper and zinc plates (about3 cm2 in geometric area) were polished with emery paper,rinsed with deionized water,then immersed for 30 min in98%alcohol solution and thoroughly rinsed with water.
The products were characterized by X-ray diffractometer (XRD,MAC Science X'Pert PRO) with Cu Kαradiation (λ=0.15406 nm).Scanning electron microscopy(SEM) images were obtained using a Zeiss ultra plus field emission scanning electron microscopy (FESEM) apparatus.The Raman spectra of the products were recorded at ambient temperature on a 633-nm He-Ne laser with a liquid helium cooling system (Jobin-Yvon LabRAM HR800UV).
3 Results and discussion
Typical CV curves of aqueous solutions containing different solutes using a copper electrode at pH=2 are shown in Fig.1.As seen in Fig.1(1),a reduction peak at about-0.60 V and a small reduction peak at-0.75 V are observed in 0.002 mol·L-1 SeO2 solution.With the addition of ZnSO4 to SeO2 solution,the reduction peaks move to the positive direction as seen from Fig.1(2).The reduction peak at-0.60 V is shifted to-0.30 V,and the second reduction peak at-0.75 V appears at-0.60 V.
Electrodeposition experiments were performed for 2 h at different potentials on Cu substrate to characterize the reduction reaction.XRD was used to characterize the phase of the deposits,and the results are shown in Fig.2.As shown in Fig.2,there are Se,CuSe and Cu2Se phases in the deposits along with peaks for the Cu substrate.No ZnSe or Zn is detected in the deposited coating even when the deposition process was carried out at-1.00 V.Cu2Se is also detected for the electrodeposition of ZnSe when using copper as cathode by Skyllas Kazacos and Miller
Fig.1 CV curves with a copper electrode in different aqueous electrolytic baths at 25℃and scan rate of 100 mV·s-1
Fig.2 XRD patterns of films deposited at different potentials for 2 h on copper cathode in ZnSO4-SeO2 solution
As there are Cu2Se and CuSe in the deposits and no Cu source is contained in the solutions,it can be deduced that the deposited Se may react with the Cu substrate,forming Cu2Se and CuSe.Another possibility is that selenium atoms diffuse into substrate and then form alloys.To prepare ZnSe,Cu substrate was changed to Zn substrate and the electrodeposition processes were carried out at-0.6 V in the same electrolyte composition.Figures 3 and 4 show XRD pattern and Raman spectrum of the obtained thin films on Zn substrates.As shown in Fig.3,the diffraction peaks at 2θ=27.60°,45.68°,84.03°,91.32°are attributed to the (111),(220),(422),(511) planes of cubic ZnSe phase,respectively,which is confirmed using the standard JCPDS data card (No.01-6920).Se and Zn are also detected in the deposits.Compared with XRD results of copper substrate (Fig.2),the appearance of Zn in XRD pattern (Fig.3) should be attributed to the Zn substrate.Se is caused by the reduction of Se(Ⅳ) to Se(0) on Zn cathode.For the Raman spectrum shown in Fig.4,two peaks at252 and 235 cm-1 are observed.As the spectrum of pure ZnSe consists of a longitudinal optical (LO) phonon at253 cm-1 and the transverse mode (TO) at about 204 cm-1
Fig.3 XRD pattern of electrodeposited ZnSe film obtained at-0.6 V on Zn substrate in ZnSO4-SeO2 solution
Fig.4 Raman spectrum of electrodeposited film obtained at-0.6 V on Zn substrate in ZnSO4-SeO2 solution
Figure 5 shows SEM images of ZnSe deposited on Zn substrate at-0.20 and-0.60 V versus Pt for 2 h.It is clearly seen that the deposited thin films are composed of nanoparticles which are homogenous and well cover the Zn substrate.Moreover,during the electrodeposition process,it is observed visually that the cathode becomes dark when using copper as cathode with prolonged electrodeposition time at-0.60 V,while it turns to golden yellow color first and then changes to red when using zinc as cathode.According to Refs.
Se obtained via the first process 1 is the gray type of the selenium,which is recognized as electroinactive,while the latter step leads to red Se,which is the only electroactive form of Se(0)
Fig.5 SEM images of ZnSe deposited at a static potential for 2 h:a-0.20 V and b-0.60 V versus Pt
From the above results,it can be concluded that ZnSe can be obtained with the use of Zn substrate,while Cu2Se is formed when Cu was used as substrate.It is deduced that the formation mechanism for ZnSe in this experimental condition can be described as following:
As pointed out above,ZnSe is formed by the reaction of reduced Se with Zn substrate.ZnSO4 does not seem to be essential for the formation of ZnSe.To verify this conclusion,electrodeposition processes were carried out using solutions which only contained Se source.Figure 6shows the Raman result of the deposits obtained in0.002 mol·L-1 SeO2 acidic solution.The peaks at 145and 252 cm-1 show the existence of ZnSe in the deposit,and this confirms the proposed ZnSe formation mechanism.However,the intensity of ZnSe characteristic peaks are smaller compared with the Raman result of deposits obtained at-0.6 V on Zn substrate in ZnSO4-SeO2solution (Fig.4).This indicates that the addition of ZnSO4 can accelerate the formation of ZnSe.As shown in Fig.1,the addition of Zn(Ⅱ) source results in the positive transfer of the reduce reaction,and it changes the reducing mechanism of Se(0).
Fig.6 Raman spectrum of electrodeposited film obtained at-0.6 V on Zn substrate in SeO2 acidic solution
4 Conclusion
ZnSe thin films were successfully deposited on Zn substrate using electrodeposition method.Strong seleniumsubstrate interaction results in the formation of selenium compounds involving electrode materials.The presence of Zn(II) in the electrolytic bath is not essential for the deposition of ZnSe when using zinc cathode.However,the addition of Zn(Ⅱ) source results in the positive transfer of the reduction reaction.It makes a change in the reducing mechanism of Se(0),from direct reduction from Se(Ⅳ) to form H2Se first,and then form Se(0),which is more active than reducing Se(Ⅳ) to Se(0) directly.
参考文献
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