Surfactant involved in Copper Sulfide Nanocrystallites Synthesis

CLAUDIA MARIA SIMONESCU 1*, VALENTIN aERBAN TEODORESCU 2, CAMELIA CÃPÃÞÎNÃ3 1University Politehnica Bucharest, Faculty of Applied Chemistry and Materials Science, Department of Inorganic Technology and Environmental Protection, 1-7 Polizu Str., 011061, Bucharest, Romania 2 National Institute for Material Physics, P.O.Box. Mg-7, Bucharest-Magurele, 77125, Romania 3 University “Constantin Brâncuoi”, Department of Environmental Engineering, 3 Genova Str., 210152, Targu-Jiu, Gorj, Romania

In the last time, methods in which were involved surfactants, were widely used to obtain nanocrystals.Surfactans have an important role in the formation of nanoparticles due to the compartmentalization offered by host surfactant assemblies [19].They assure several types of well organized assemblies which provide specific size, geometrical control and stabilization to particulate assemblies formed within the organized surfactant assemblies.Among the host surfactant assemblies used for the formation of nanoparticle species it can be mentioned the following: the aqueous micellar solutions, reverse micelles, microemulsion, vesicles, monolayers, Langmuir-Blodgett films and bilayer lipid membranes [19].
Nanoparticles of CuS with various morphologies have been prepared using surfactant assemblies and they have been well characterized and studied [20][21][22].In these studies, anionic surfactants, cationic surfactants and their mixtures were used.It was established that surfactants had an important role in copper sulfide nanoparticles obtained.
In this article, we described a facile aqueous synthesis of CuS nanocrystals of various sizes and shapes.The effect of an anionic surfactant like sodium -bis(2-ethylhexyl) sulfosuccinate (Na-AOT) on the size and on the shape of the nanoparticles was established.The present strategy is an environmental friendly method.

Experimental part
Copper acetate monohydrate (Cu(ac) 2 ) and thioacetamide (CH 3 CSNH 2 ) were used as copper and sulfur sources.There were made two sets of experiments.In one of them was used only copper acetate monohydrate and thiocetamide, and in the other, copper acetate, thioacetamide and an anionic surfactant sodium-bis(2-* email: claudiamaria_simonescu@yahoo.com;Tel: (+40) 021 4023825 ethylhexyl) sulfosuccinate (Na-AOT).The quantity of surfactant was varied.
In the first experiment 0.5 g of Cu(ac) 2 (2.5 mmoles) was firstly dissolved in 100 mL deionized water at room temperature.In another Berzelius glass, 0.2 g of thioacetamide (2.5 mmoles) was dissolved in 100 mL deionized water at room temperature.The Cu(ac) 2 solution was slowly dropped into the thiacetamide solution, along with the continuous stirring.The reaction mixture was stirred at 30 o C. The solution turned golden brown as soon as the first drop of Cu(ac) 2 solution was added.The solution's colour deepened with more addition of Cu(ac) 2 solution.The brown solution turned light brown over a period of 30 min, and a black precipitate was formed.After another 10 min. of stirring at 30 o C solution turned colourless, and precipitate was crowded.This mixture was maintained at this temperature and stirring for five hours.At the end, this mixture was cooled at room temperature and the precipitate was filtered.After repeatedly washed with distilled water, the powder of final product obtained was dried at room temperature.
In the second experiment 0.5 g of Cu(ac) 2 (2.5 mmoles) was dissolved in 100 ml deionized water.The aqueous mixture of surfactant solution (0.2 g of thioacetamide (2.5 mmoles) and 0.022 g of sodium-bis(2-ethylhexyl) sulfosuccinate (Na-AOT) (0.05 mmoles) were dissolved in 100 mL deionized water.The aqueous Cu(ac) 2 solution was added to the thiacetamide and surfactant solution dropwise at 30 o C with continuous stirring.When the last drop of Cu(ac) 2 solution was added the solution turned light yellow.After 10 min of stirring at 30 o C, the solution turned yellow earth, and after other 5 min of stirring this solution turned brown.This solution turned dark brick.After 12 h of stirring the solution turned green.
In the third experiment were used the same quantities of copper acetate, thioacetamide, and instead of 0.02 g of surfactant was used 0.22 g of surfactant ((0.5 mmoles).The reaction conditions were the same as in the experiment number two.The last drop of Cu(ac) 2 solution added makes the solution to turn light yellow.After 20 min of stirring at 30 o C the solution turned yellow earth, and other 10 min. of stirring this solution turned brown.This solution turned in a dark brick colored solution.After 10 h of stirring the solution turned green.
In the fourth experiment 0.5 g of Cu(ac) 2 (2.5 mmoles) was dissolved in 100 mL deionized water, and in other container, 0.2 g of thioacetamide (2.5 mmoles) and 1.11 g of sodium-bis(2-ethylhexyl) sulfosuccinate (Na-AOT) (2.5 mmoles) were dissolved in 100 mL deionized water.After the all Cu(ac) 2 was added by dropping to the solution containing thioacetamide and sodium-bis(2-ethylhexyl) sulfosuccinate (Na-AOT), the solution which resulted was a yellow solution.This solution turned into a brown solution after 30 min of stirring at 30 o C.After 10 h of stirring at 30 o C this brown solution turned into a green solution.
In the fifth experiment 0.5 g of Cu(ac) 2 (2.5 mmoles), 0.2 g of thioacetamide (2.5 mmoles) and 2.22 g of sodiumbis(2-ethylhexyl) sulfosuccinate (Na-AOT) (5 mmoles) were used.The whole quantity of Cu(ac) 2 was dissolved in 100 mL of deionized water.The mixture formed by thioacetamide and sodium-bis(2-ethylhexyl) sulfosuccinate was dissolved in 100 mlL of deionized water.The yellow solution obtained after dropping the Cu(ac) 2 solution over the solution containing thioacetamide and surfactant turned into a brown solution after 10 min of stirring at 30 o C.This brown solution turned green after 4 h of stirring at 30 o C.
The black precipitate obtained in the first experiment was characterized by IR Spectroscopy, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED).The elemental chemical analyses were performed by an Electronics SPD 1200A ICP emission analyzer with a pump flow of 1.85 mL .min -1 and a flow rate of the auxiliary gas (Ar 99.99%) of 0.5 L .min -1 .The IR spectra were recorded on a FT-IR 620 (Jasco, Japan) spectrometer in the 400-4000 cm -1 range using KBr pellets.Powder X-ray diffraction (XRD) was used to characterize the sample.Data were collected on the Shimadzu XRD6000 with CuKα radiation (λ = 1.54178Å).A scan rate of 0.05°s -1 was applied to record the patterns in the 2θ range of 20-80°.Transmission electron microscopy (TEM) was applied to determine the morphology of the prepared products by a specific methods for crystalline powders study.Powder was dispersed in alcohol and then it was deposited on a TEM grid with carbon support.Selected area electron diffraction (SAED) patterns were obtained from polycrystalline particles aggregates.TEM study was performed using a Jeol 200CX electron microscope.
In the case of the green solutions containing CuS nanocrystals, a drop of this solution was dried on a holey carbon grid and investigated using the same electron microscope.UV-VIS absorption spectra of the nanocrystals in water were recorded using a V-530 UV-VIS Spectrophotometer.

Results and discussions
The elemental chemical analyses of the powder obtained from copper acetate and, thioacetamide system (1:1) at 30 o C showed that the powder is copper monosulfide.
The IR spectrum presents one large band which is characteristic CuS.This shape of spectrum showed that there are no impurities, e.g., acetate, or other impurities, which could be detected in the samples.
One of the UV-VIS Spectrum of CuS nanocrystals from solution samples is presented in figure1a.All solution samples have the same UV-VIS spectrum.
The intense broad band which appeared at 600 nm may be assigned as transitions from d xy , d z 2 and d xy , d yz pair to σ -anti-bonding and half -filled d x

2
-y 2 level, in an octahedral tetragonal distorted configuration characteristic for Cu(II).This band absorption in the near IR region corresponds to the pure covellite phase.Absence of shoulder at ~450 nm corresponding to the Cu 2 S phase indicates that in these solutions the CuS covellite is the only phase presented [21].
One of the XRD patterns of a copper monosulfide prepared is shown in figure 1b.All the copper monosulfide peaks in the pattern correspond to the reflections of hexagonal phases (ASTM File No. 77-877) and the pattern indicates that the prepared product is crystalline.In figure 2a is presented a typical TEM image of CuS nanocrystals obtained from reaction of Cu(ac) 2 (2.5 mmoles) with 0.2 g of thioacetamide (2.5 mmoles).The nanocrystals are relatively bigger with dimensions between 20 and 60 nm.Nanocrystals are plate, probably with a discoidal morphology, and they are welded in relatively compact aggregates.Figure 2b presents the TEM image of CuS nanocrystals obtained from the same system, but in presence of 0.022 g of sodium-bis(2-ethylhexyl) sulfosuccinate (Na-AOT).It can be seen from this picture that in this case the nanocrystals are dispersed, and their dimensions are between 10 and 30 nm.
From these two pictures it can be concluded that the presence of the surfactant influences the size and the shape of CuS nanocrystals.In this case, the presence of surfactant leads to the CuS nanocrystals with lower dimensions.
Figure 2c shows the SAED pattern corresponding to the sample exposed in figure 2a.Only reflexions of the hexagonal CuS phase are present.The SAED patterns for all the analyzed samples are similar.
The concentration of Na-AOT influences the size and the shape of the CuS nanocrystals.Thus we obtained CuS nanocrystals with dimensions between 10 and 30 nm in the presence of 0.022 g of surfactant.When we used 0.22 g of surfactant, the nanocrystals dimensions are 10 nm, approximately.From figure 3a, it can be seen that, in this case, we obtained ring-shaped or globular aggregates, probably due to the surfactant presence.
The same shape and dimension of copper sulfide nanocrystals was obtained in case of using 1.11 g of sodium-bis(2-ethylhexyl) sulfosuccinate (Na-AOT) (2.5 mmoles).
When it was used 2.22 g of surfactant we obtained dispersed nanocrystals included in surfactant (fig.3b).

Conclusions
We obtained CuS nanocrystals of various sizes and shapes by a simple method involving the reaction of copper acetate with thioacetamide in presence/absence of one surfactant sodium-bis(2-ethylhexyl) sulfosuccinate (Na-AOT).The reaction carried in absence of surfactant yielded 20-60 nm nanocrystals while that in presence of surfactant yielded 10-30 nm nanocrystals.The particle size of CuS decreases with the surfactant concentration.One bigger quantities of surfactant leads to obtain dispersed nanocrystals included in surfactant.
This method is one cost-effective and environmentally friendly technique for CuS nanocrystals obtained.

Fig. 1 .
Fig. 1. a) UV-VIS spectrum of sample from solution with 0.22 g surfactant; b) the XRD pattern of CuS nanocrystals obtained from copper acetate:thioacetamide system

Fig. 3 .
Fig. 3. a) TEM image of the sample from solution with 0.22g surfactant; b) TEM image of the sample from solution with 2.22 g of surfactant