DOI: 10.1021/cg900775a
CuI Crystal Growth in Acetonitrile Solvent by the Cycle-Evaporation Method
2009, Vol. 9 3825–3827
Jianguo Pan,* Shuying Yang, Yuebao Li, Lei Han, Xing Li, and Yuejie Cui State Key Laboratory Base of Novel Functional Materials and Preparation Science, Faculty of Materials Science and Engineering, Ningbo University, Ningbo, Zhejiang 315211, P.R. China Received February 25, 2009; Revised Manuscript Received July 26, 2009
ABSTRACT: Acetonitrile solvent was applied to grow CuI crystals by the cycle-evaporation method with Cu as the reductant. The growth temperature was 70 °C. The process was performed in a nitrogen atmosphere. Clear, millimeter-scaled CuI single crystals were obtained. Moreover, UV-visible spectra, IR spectra, and XRD were used to investigate the redox processes in the solution. The crystal was characterized by X-ray powder diffraction and differential thermal /thermogravimetry analysis. Cuprous iodide (CuI) crystal has attracted much interest because of its many particular features, such as larger band gaps, electroensitivity, and photosensitivity.1-3 Cuprous iodide has three crystalline phases: R, β, and γ.4,5 The high-temperature cubic R-phase and hexagonal β-phase are Cuþ ion conductors.6,7 The low-temperature γ-phase is also of cubic structure and is a p-type semiconductor of large band gap. In recent years, γ-CuI has attracted steadily growing interest because of its ultrafast scintillation property with a decay time of about 90 ps at room temperature .7,8 As is known, it is the fastest inorganic scintillation crystal at present. The challenge of growing the γ-CuI crystal is great. The melting point of cuprous iodide is 602 °C and γ-CuI exists only below 350 °C, so we can hardly grow γ-CuI crystal from the melt. On the other hand, cuprous iodide is a water insoluble solid (pKsp=11.96), so we cannot grow γ-CuI crystal from the aqueous solution, either. Several growth methods have been reported, such as decomplexation method,9 vapor deposition method,10 flux method,11 hydrothermal method,12 but it is difficult to obtain large optical quality single crystal. The solution growth method is the most versatile method for growing a large optical-quality single crystal. Herein, we report a novel method to grow a γ-CuI crystal by the cycle-evaporation acetonitrile in a nitrogen atmosphere. As is known, acetonitrile is usually used as a good organic solvent, and there is a good solubility of CuI in acetonitrile at room temperature owing to solvent effect. On the other hand, because of strong solvent effect, the CuI-acetonitrile crystal that recrystallized into CuI power in air quickly is obtained when acetonitrile is volatilized at room temperature. The solubility curve at different temperatures is shown in Figure 1, which is measured by means of traditional weight analysis from 30 to 75 °C. The solubility of CuI in acetonitrile decreases gradually with increasing temperature, which indicates the solvent effect weaken with increasing temperature. We found CuI crystal can be produced on top 45 °C. Depending on the solubility of CuI in acetonitrile, the bulk CuI crystal has been obtained by slow evaporation method on top 45 °C. The solution of CuI in acetonitrile is unstable in air. The effect of oxygen on the stability of solution has been studied. When we added CuI powder to acetonitrile, the color of solution turned to be yellow gradually. Herein, UV-visible spectra (TU-1901 UVvis spectrophotometer, Beijing pgeneral) were used to investigate the properties of the solution. Figure 2 shows UV-vis spectra of the samples in the wavenumber of 200-600 nm. As indicated in
Figure 1. Solubility of CuI in acetonitrile at different temperatures.
Figure 2. UV-vis spectra of the samples in the wavenumber of 200-600 nm: (a) CuI in acetonitrile (black solid line); (b) CuI in acetonitrile after a few days (red solid line); (c) NaI in acetonitrile (blue solid line); (d) I2 in acetonitrile (dark cyan solid line).
*Correspondingauthor. E-mail:
[email protected]. Phone: 860574-87600793. Fax: 86-0574-87600734.
spectra a and c in Figure 2, the set of bands and spectra of features of CuI in acetonitrile agrees fairly well with UV-vis data for NaI in acetonitrile. Therefore, two absorption peaks at 225 nm and 244 nm are attributed to the I- absorption. Figure 2b shows the absorption spectra of the solution of CuI in acetonitrile after a few days, which have four absorption peaks at 225, 244, 292, and
r 2009 American Chemical Society
Published on Web 08/17/2009
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Figure 3. Typical images of CuI crystals prepared at 60 °C by vaporizing acetonitrile: (a) in an air atmosphere; (b) in a nitrogen atmosphere.
Figure 4. FTIR spectra of the samples in the wavelength of 4004000 cm-1: (a) the black substance in the solution; (b) CuO power.
Figure 6. Schematic drawing of the crystal growth system. 1, CuI powder; 2, stirring device; 3, heating device; 4, Pt wire; 5, a seed crystal; 6, glass instrument; 7, temperature regulating device; 8, Cu sheet.
Figure 5. X-ray powder diffractogram of the black substance in the solution.
362 nm. The peaks of 292 and 362 nm show that I2 is separated out partially in the solution (Figure 2d). Furthermore, when we added Cu to a solution of CuI in acetonitrile, the solution turned clear. We concluded that Cu and I2 turned into CuI. According to Figure 2, I- in CuI solution should be oxidized to I2 gradually. Furthermore, we found that the black substances were deposited in the solution a few days later and that the crystal obtained
Figure 7. Photograph of a CuI crystal prepared by the cycleevaporation method.
by slow evaporation is black. Typical images of CuI crystals prepared with evaporation at 60 °C are shown in Figure 3a. The clear crystal of CuI shown in Figure 3b has been obtained in a nitrogen atmosphere at 60 °C, which indicates black substances
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Crystal Growth & Design, Vol. 9, No. 9, 2009
Figure 8. X-ray powder diffractogram of grown CuI crystal.
Figure 9. DTA/TG curves of of grown CuI crystal.
have been produced because of oxygen. Figure 4 is FTIR spectra (a Shimadzu FTIR-8900 infrared spectrophotometer with a pressed KBr disk) of the substances deposited in the solution. As given in spectra a and b in Figure 4, it is obvious that the position of these IR absorption peaks for the remaining substances is very similar to that of CuO. Furthermore, as revealed in Figure 5, although the XRD results (XRD, using a Bruker D8 Focus X-ray diffractometer equipped using Cu KR radiation) are not good because of the bad morphology, the results show that the main diffraction peaks are readily indexed to CuO (JCPDS No. 48-1548, space group: C2/c). According to Figures 4 and 5, it is possible that the substances deposited in the solution are admixture that may contain CuO. On the basis of the investigation, we speculated the reaction as follows Cuþ þ I - þ O2 f CuO þ I2 A more detailed investigation about the chemical mechanism in the solution is being carried out. Because of redox reactions, the process of growing CuI crystals was performed in nitrogen atmosphere with Cu as the reductant.
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CuI crystals were grown by the cycle-evaporation method. The growth temperature was 70 °C. The schematic drawing of the crystal growth apparatus is shown in Figure 6. This process was carried out in a glass instrument heated by water bath, CuI powder was placed in the above part. The glass instrument was filled with CuI saturated solution of acetonitrile. A platinum (Pt) wire with a seed crystal was placed in the solution. With acetonitrile vaporizing and the CuI powder dissolving, the seed crystal grows gradually. Figure 7 shows photography of the CuI crystal grown by the cycle-evaporation method, the size of which has been up to 3 mm. The typical XRD pattern (XRD, using a Bruker D8 Focus X-ray diffractometer equipped using Cu KR radiation) of CuI crystals obtained at 70 °C is shown in Figure 8. The XRD results show that all the diffraction peaks are readily indexed to cubic phase CuI (JCPDS No. 06-0246, space group: F43m). All of the results are consistent with the results of CuI crystals in the literature. The TG and DTA curves (with a Seiko EXSTAR 6300 thermal analyzer calibrated by Al2O3, heating and cooling rates of 10 °C /min) of CuI crystal are shown in Figure 9. In the curve of the DTA, there are three sharp peaks at 377.90, 405.72, and 606.00 °C. The peaks at 377.90 and 405.72 °C can be assigned to the phase transition. The CuI crystal obtained by the cycleevaporation acetonitrile in nitrogen atmosphere is the low-temperature γ-phase (below 377.90 °C). It is seen that the crystal has no loss of weight below 606.00 °C. However, a major weight loss is observed above 606.00 °C, which is attributed to decomposition of CuI crystal. In conclusion, millimeter-scaled optical-quality CuI crystals were obtained with using acetonitrile as solvent and Cu as the reductant at 70 °C in nitrogen atmosphere by the cycle-evaporation method, which is a novel organic solvent to grow CuI crystal. Further studies on the optimization of the growth conditions are in progress. Acknowledgment. This work is partially supported by the Zhejiang Provincial natural Science Foundation (Y406277), Ningbo Municipal Natural Science Foundation (2009A610015), and the K.C.Wang magna Fund in Ningbo University.
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