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Research Article, J Nanomater Mol Nanotechnol Vol: 3 Issue: 3

Preparation of Copper Selenide Platelets

Angshuman Pal, Ionel Halaciuga and Dan V. Goia*
Center for Advanced Materials Processing, Department of Chemistry and Biomolecular Science, Clarkson University, USA
Corresponding author : Paul J. Higgins
Center for Cell Biology & Cancer Research, Albany Medical College, 47 New Scotland Avenue, Albany, New York 12208, USA
Tel: 518-262-5168; Fax: 518-262-5669
E-mail: [email protected]
Received: July 16, 2013 Accepted: August 02, 2013 Published: August 05, 2013
Citation: Simone TM, Higgins PJ (2012) Low Molecular Weight Antagonists of Plasminogen Activator Inhibitor-1: Therapeutic Potential in Cardiovascular Disease. Mol Med Ther 1:1 doi:10.4172/2325-9701.1000102

Abstract

Preparation of Copper Selenide Platelets

Copper selenide (CuxSe) particles were prepared by heating copper carbonate and selenous acid in diethylene glycol in the presence of various dispersing agents. The impact of reaction conditions on the structural and morphological properties of the precipitated particles were assessed by X-ray diffraction (XRD) and electron microscopy. Although all additives promoted anisotropic growth, it was found that polyvinyl alcohol (PVA) was particularly effective in generating dispersed high aspect ratio CuxSe platelets. The optimum reaction conditions for the formation of hexagonal copper selenide were 190°C and a Cu: Se molar ratio of 1 to 1.2.

Keywords: Platelets; Copper selenide; PVA

Keywords

Platelets; Copper selenide; PVA

Introduction

Copper selenide is a p-type semiconductor often used in solar cells [1], optoelectronics [2], gas sensors [3], supersonic conductors [4], thermoelectric converters [5], photothermal therapy [6] and electro-conductive structures [7]. High purity crystals of copper selenide are particularly suitable in building long-lasting efficient photovoltaic cells [8]. In the electronic industry, anisotropic particles of copper selenide (platelets) are of great interest because of their potential in the miniaturization of electronic components. Indeed, by using platelets with controlled crystal orientation instead isometric particles, it is easier to construct thinner layers free of structural defects. Copper selenide displays a large number of stoichiometric ratios and crystallographic phases of either cuprous (Cu2Se or Cu2- xSe) or cupric (CuSe or Cu3Se2) ions, each having distinct properties. The morphology, composition, and crystal structure of chemically precipitated particles are highly dependent on the preparation conditions. Since the early works dealing with the preparation of copper selenide nanostructures [9,10], new methods with superior capabilities in controlling the properties of CuxSe particles have been developed [11,12]. In particular, their size and shape can be effectively tailored through an appropriate selection of reactants, reductant, solvent, and reaction conditions [12]. It has been shown that adsorption of additives (molecules or ions) onto particular crystal facets can lead to different growth rates in preferential directions and formation of anisotropic particles. However, the inability to predict the effect of a capping agent in the case of a particular crystal structure makes the identification of an effective phase tuner a 'trial-and-error' effort. For this reason, it is often advantageous to select precipitation systems that favor anisotropic growth even in the absence of additives. The polyol process is one such example. In this case, the mobility of ions in the crystalline lattice is increased by carrying out reactions at elevated temperatures and even slight differences in energy between various facets can trigger anisotropic growth. Introducing additives in a process already having a propensity for generating such structures, further improves the chances of obtaining anisotropic particles. Various versions of the polyol process have been successfully used to date for the preparation of many such structures [13]. The ability of polyols to prevent severe particle aggregation in dispersions of higher concentration also makes this process particularly attractive for developing precipitation systems suitable for industrial purposes [14,15]. In this study we reveal a modified polyol process capable of generating dispersed high aspect ratio CuxSe platelets. We show that the reactants nature and ratio, the reaction temperature, and the nature of the dispersing agent play a critical role in the nucleation and ensuing particle growth. The optimized precipitation process represents a facile eco-friendly chemical route to copper selenide particles with controlled morphology.

Experimental

Materials
Copper carbonate (CuCO3. Cu(OH)2), PVP (polyvinyl pyrrolidone), and PVA(polyvinyl alcohol) were purchased from Sigma-Aldrich. Diethylene glycol (DEG) was obtained from Pharmco-aaper (Brookfield, CT). Selenous acid (H2SeO3), d-sorbitol, and dextran were purchased from Alfa Aesar. All compounds were used as received.
Preparation of copper selenide particles
The precipitation experiments were conducted in a 500 cm3 4-neck round bottom flask placed in an electrically heated mantle. The stirring assembly consisted of a 1.9 x 6.0 cm glass paddle attached to a glass rod inserted through the center neck of the flask. The temperature variation during the reaction was controlled using a thermocouple connected to a programmable heating mantle. After introducing 250 cm3 DEG into the flask, the copper carbonate powder (2.0 g) and selenous acid solution (2.0 g) were added (in this order) under vigorous stirring. The content of the flask was heated at a rate of 1.0°C/min to the set temperature. After 2 hours of stirring, the reaction mixture was cooled and the black precipitate was separated and washed 5 times with 400 cm3 of deionized (DI) water. The particles were next rinsed with alcohol and dried in a forced air circulation oven at 80 °C. The calculated reaction yield based on the mass of the dry solids recovered was ≥99%. The effect of the final temperature and dispersing agent were evaluated using this recipe, which corresponded to a Cu:Se molar ratio of 1:1.2. The amount of dispersing agent added represented in all cases 25% of the weight of the precipitated copper selenide. The experimental conditions are summarized in Table 1.
Table 1: Experimental conditions used for the preparation of CuxSe platelets.
Characterization
The purified dry solids were inspected by transmission (TEM, JEM-2010) and scanning (FESEM, JEOL-7400) electron microscopy. The crystal structure of the particles was evaluated by X-ray diffraction (XRD) using a Bruker-AXS D8 Focus instrument. The scanning step width and period were 0.01° and 3s respectively. The values for the source, sample, and detector slits were 2, 0.6, and 1 mm.

Results and Discussion

The use of basic carbonate as source of Cu2+ species was found to be essential for the formation of dispersed precipitated CuSe particles. In contrast to other stable anions, the carbonate decomposes in polyols at high temperatures [24,25] yielding a lower ionic strength in the dispersion medium. The enhanced electrostatic repulsive forces between the precipitated particles facilitate the formation of dispersions with higher solids concentration. The Se2- ions needed for the formation of CuSe structure are generated in situ at high temperature as a result of the transfer of electrons from the diethylene glycol molecules to the SeO3 2- ions. The redox transformation and the following precipitation event are schematically captured by Eqs. 1 and 2.
H2SeO2 + 6e- → Se2-
Cu2+ +Se2- → CuSe
A complicating factor in the studied process is the ability of polyol molecules to reduce Cu2+ to Cu+ (and eventually to metallic copper if the reaction temperature is high enough [16]). The cuprous ions can form Cu2Se or other mixed compositions (CuxSey) if Cu2+ ions are present. Therefore, the key for obtaining the CuSe structure is to prevent the reduction of Cu2+ species while reducing the selenous ions to Se2-. Since the redox potentials associated with the electron transfer from DEG to cupric and selenous ions vary with pH, this parameter is crucial in controlling the composition of the precipitated solids. When selenous acid only was added in DEG, the pH of the colorless solution was highly acidic (~1.0). Upon heating, the reduction of selenous ions started at ~170°C, as indicated by the formation of a black precipitate. The dispersion of copper carbonate in polyol has a significantly higher pH (4.4). In these conditions, the reduction of Cu2+ to Cu+ occurs at ~165°C, as indicated by the change of the green color of the carbonate to the typical ochre color of cuprous oxide. If, however, the dispersion of copper carbonate is acidified to pH 1.2 (the value when both selenium and copper precursors are present), the reduction of Cu2+ ions does not start until the temperature reaches 195°C. This suggests that the 170-190°C range may be optimum for the formation of particles with predominant CuSe structure.
At 170°C (experiment S1), the reaction was too slow and the conversion to copper selenide was not completed in 2 hours. This was substantiated by the XRD analysis, which revealed that the precipitated solids still contained a significant amount of unreacted copper carbonate (Figure 1a). The FESEM analysis revealed the presence of heavily aggregated particles with broad size distribution and irregular shape (Figure 1b). When the reaction temperature was increased to 190°C (experiment S2), all copper carbonate was consumed. The XRD data (Figure 1c) in this case indicated the presence of only hexagonal CuSe (JCPD 00-034-0171) and cubic Cu2Se (JCPD 01-088-2043) phases. When inspected by electron microscopy, the precipitated particles were well developed, albeit aggregated, crystalline platelets (Figure 1d). As one would expect, changes in the reactants molar ratio should bring changes in the pH of the precursors' mixture, which would likely affect the structure/ composition of the precipitated solids. When the [Cu]/[Se] ratio was changed from 1.0/1.2, to 1.0/1.0, and 1.0/0.8, the [CuSe]/[Cu2Se] (calculated using the intensity ratio of the diffraction peaks of CuSe at 28.3 and Cu2Se at 27.0 degrees) decreased from 3.37, to 0.85, and 0.18 respectively (Table 2). The effect can be attributed to the increase in the pH of the dispersion from 1.2 to 1.5, which is associated with drop in the temperature at which Cu+ ions are generated. An even more acidic pH at a larger excess of selenous acid ([Cu]/[Se] ratio 1:1.4) did not increase the fraction of CuSe in the final solids.
Figure 1: XRD pattern (a) and SEM image (b) of solids precipitated at 170 °C indicating the presence of copper carbonate (peaks identified with red dots). X-Ray diffractogram (c) and SEM image (d) of particles obtained at 190 °C (green ovals depict Cu2Se peaks, blue dots depict CuSe).
Table 2: Effect of [Cu]/[Se] molar ratio on the structure of the precipitated solids.
Once the conditions yielding the highest fraction of CuSe were identified, we investigated the effect of four dispersing agents on the aspect ratio, dispersion, and uniformity of the platelets. Their effect on particle morphology is presented in figure 2a-d. Thick, irregular, and aggregated low aspect ratio platelets were formed in the case of d-sorbitol, dextran, and PVP (experiments S3-S5). In contrast, in the case of PVA (experiment S6) the electron microscopy analysis revealed the presence of well-developed thin hexagonal platelets (Figure 2d). The selected area electron diffraction pattern performed on a single hexagonal platelet (Figure 3) confirmed that the CuSe crystalline structure was predominant.
Figure 2:FESEM images of particles obtained with d-sorbitol (a) dextran (b) PVP (c), and PVA (d).
Figure 3: High resolution transmission electron micrograph and electron diffractogram (insert) of a CuSe platelet.

Conclusion

Thin and well-dispersed high aspect ratio copper selenide platelets were prepared by boiling copper carbonate and selenous acid in diethyleneglycol. The pH of the dispersion, the ratio of precursors, and the reaction temperature were identified as key factors controlling the shape and composition of particles. A Cu/Se molar ratio of 1:1.2 and 190°C were the best conditions for the formation of high aspect ratio dispersed platelets with a predominant CuSe structure. Among the dispersant investigated, PVA was found to have a particularly beneficial effect on platelets uniformity.

Acknowledgement

The authors are grateful to Umicore (Olen, Belgium) for the financial support provided for this research.

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