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J. Phys. Chem. 1996, 100, 5868-5873
Synthesis and Characterization of Copper Sulfide Nanoparticles in Triton-X 100 Water-in-Oil Microemulsions Santosh K. Haram, Anand R. Mahadeshwar, and Sharad G. Dixit* Applied Chemistry DiVision, UniVersity Department of Chemical Technology, UniVersity of Bombay, Matunga, Bombay 400 019, India ReceiVed: August 16, 1995; In Final Form: December 3, 1995X
Nanoparticles of copper sulfide have been synthesized by reacting a copper ammonia complex with an equimolar thiourea solution in Triton-X 100/cyclohexane water-in-oil microemulsions. The presence of an exceptionally sharp and blue-shifted peak at 475 ( 2 nm in the UV-vis spectrum reveals the formation of quasi-monodispersed, size-quantized particles. Using absorption spectroscopy, the formation of a chalchocite (Cu2S) phase is inferred. The peak position in the absorption spectra was found to be independent of net micellar water content as well as aging effect. It is attributed to the formation of a Cu(I) thiourea complex on the surface of the particles, which are hydrogen bonded to the polyoxyethylene (POE) chain of Triton-X 100 (TX-100). The role of thiourea in the tailoring of particles to the micellar periphery was confirmed by synthesizing the nanoparticles using other S2- agents like H2S, and Na2S. The role of the POE chain in mediating the adsorption was brought out by carrying out the reaction in microemulsions of POE containing surfactants other than TX-100. The kinetics of agglomeration has been studied and fits well in second-order rate law. A further enhancement in the solution stability of the particles was achieved by means of sodium hexametaphosphate.
Introduction In recent years, immense research interest has been developed worldwide in studying semiconductor quantum well crystallites displaying size dependent optical and electronic properties when their size is restricted below the excitonic size.1 This privilege of manipulating the electronic properties of the material by just controlling its size has great potential importance in the fields of photocatalysis2-6 and optoelectronics with nonlinear optical properties.7-11 In this scenario, size quantized particles of CdS,12-14 CdSe,15 PbS,16 and ZnS17 have been well tested and understood. However, little attention has been paid toward relatively cheap, equally valuable18-25 nonlinear devices and still less hazardous materials like copper sulfides. Though its synthesis has been tried in aqueous sols,26 monolayers,27 bilayer lipid membranes,28,29 LB films,30 and water-in-oil (w/o) microemulsions of bis(2-ethylhexyl) sulfosuccinate sodium salt, AOT,31 except in the case of aqueous sols, these particles have been poorly characterized. In view of this, a systematic study has been undertaken in the synthesis and characterization of nanoparticles of copper sulfide. The (w/o) microemulsion of nonionic surfactants is mainly Triton X-100 (polyoxyethylene teroctylphenyl ether). Unlike typical ionic surfactants, its hydrophilic counterpart, polyoxyethylene (POE) has a chain length longer than the hydrophobic part. So, the polar interior resembles the normal micelles in aqueous solution rather than the reverse form of the ionic surfactants.32,33 The kinetics of flocculation has been studied. The stability of the particles has been improved by means of a stabilizing agent like sodium hexametaphosphate (HMP). Experimental Procedure Chemicals. All the chemicals used were of extrapure grade. Copper acetate, ammonia solution (25%), thiourea, cyclohexane * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5868$12.00/0
(99%), and 2-methylpropan-1-ol were procured from SD's Fine Chemicals, India. Sodium hexametaphosphate was purchased from Loba Chemicals, India. Triton-X 100 was purchased from UBI Chem., England. The nonyl phenyl ether surfactant series with average POE chain lengths of 4, 7, and 9.5 (namely NP-4, NP-7, and NP-9.5) was procured from Unitop Chemicals, India. Sodium dodecyl sulfate (SDS) was procured from Sigma. All the chemicals were used as received without further purification. Synthesis of Nanoparticles. Preparation of Microemulsions. The synthesis of particles has been carried out in the (w/o) microemulsions of the nonionic surfactants TX-100, NP-4, NP7, and NP-9.5 and the ionic surfactant SDS. In all the cases, the solutions were prepared (typically 0.2 mol/L) well above their respective critical micellar concentrations. In the case of the nonionic surfactants, the solutions were prepared in cyclohexane, while in the case of SDS, toluene was used as a solvent. Typically, 0.036 mL of copper ammonia complex (pH 11) and thiourea (0.6 M) each were emulsified separately in 5 mL of a 0.2 mol/L nonionic surfactant solution in cyclohexane placed in a 15 mL stoppered centrifuge tube. 2-Methylpropan1-ol (10% v/v) was then added as a cosurfactant in order to stabilize the microemulsions. This procedure leads to formation of a microemulsion with a net reactant concentration of 2.16 × 10-3 mol/L and a ratio of moles of water to moles of surfactant (w) equal to 2. Microemulsions with higher w were prepared by adding the appropriate volume of distilled water at a constant surfactant concentration. The SDS microemulsion (SDS ) 0.07 mol/L, w ) 10) was prepared in toluene with benzyl alcohol as a cosurfactant using the prescription reported earlier.34 At the same time, corresponding control solutions were prepared by emulsifying the appropriate amount of distilled water in the surfactant solution. All the microemulsions were homogenized by sonicating them for 2 min each. Formation of Particles. The reaction between the copper ammonia complex and thiourea was initiated by mixing the two microemulsions containing corresponding reactants. Soon they exchange their core contents via fusion and redispersion processes. This leads to the formation of initially greenish © 1996 American Chemical Society
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turbidity which was found to settle down after about 6 h in the form of a black precipitate, leaving behind a golden yellow supernatant solution having nanoparticles of copper sulfide. The supernatant was then separated by simple filtration and used for further analysis. After few days, the particles from the supernatant solution were found to be agglomerated in the form of black turbidity at the bottom of the tube accompanied by fading of the color of the solution. The stability of the particles in the solution was enhanced by adding (typically 0.072 mL of a 0.1-0.6 M solution/ 10 mL of microemulsion ) HMP as a stabilizing agent. Characterization of the Nanoparticles. The size dependent optical properties of such particles provide a very convenient way to detect them and pursue their growth. Therefore, a characterization of the particles has been carried out with a UVvis spectrophotometer (GBC UV/VIS 911A, Australia). In order to study the kinetics of flocculation and hence the stability of the nanoparticles, 2 mL of the solution was withdrawn at stipulated time intervals and the measurements were carried out against an adequate control solution. Results and Discussion Formation of Copper Sulfide Nanoparticles. It is reasonable to assume that Cu2+ ions would react with S2- ions generated by hydrolysis of thiourea20,21,35 as per following reaction mechanism36
Cu2+ + 4NH3 h Cu(NH3)42+
(1)
NH3 + H2O h NH4+ + OH-
(2)
(NH2)2CS + 2OH- h S2- + 2H2O + H2CN2
(3)
Cu2+ + S2- f CuS(s)
(4)
The role of ammonia in this reaction is twofold. It controls the rate of the reaction by limiting the availability of free Cu2+ required for the reaction and also provides an alkaline environment for the hydrolysis of thiourea. However, in the first report on chemical deposition of copper sulfide thin films using a copper ammonia complex/thiourea bath, the formation of a Cu1.8S phase has been inferred.37 This could be possibly due to the well established equilibrium38
[Cu(NH3)4]2+ + Cu0 h 2[Cu(NH3)4]1+
(5)
The formation of Cu1+ ions could be attributed to the reduction of Cu2+ ions by the sulfide ions present in the bath. Thus, the presence of a series of CuxS phases is expected depending upon the experimental conditions. Mixing of the two inverse microemulsions containing the copper ammonia complex and thiourea, respectively, leads to an immediate formation of yellow turbidity which turns into green and after a few hours gets settled down in the form of a black precipitate, leaving a golden yellow supernatant behind. The absorption spectrum recorded on such a solution is depicted in Figure 1e. For comparison, the spectra were also recorded on the reactants, individually as well in the different combinations which are depicted in Figure 1a-d. It reveals that the peak at 475 nm in the spectrum (Figure 1e) is due to a reaction product, copper sulfide. It is neither due to the unreacted reactants (refer to Figure 1a-c) nor due to some side reaction between thiourea and ammonia (refer to Figure 1d). Moreover, the hump at 630 nm due to the Cu ammonia complex is
Figure 1. Absorption spectra recorded on TX-100 microemulsions/ cyclohexane (0.2 mol/L, w ) 2) containing (a) 4.32 × 10-3 mol/L copper ammonia complex, (b) 4.32 × 10-3 mol/L thiourea, and (c) 0.036 mL of 25% ammonia solution and (d) after adding 0.036 mL of 25% NH3 solution to part b and (e) after mixing the microemulsions from parts a and b.
completely absent after its reaction with thiourea, indicating completion of the reaction. Phase of the Copper Sulfide Nanoparticles. The mineralogy of the Cu-S system is quite interesting. It has many known stable phases from chalcocite (Cu2S) to sulphur-rich covellite (CuS). Fortunately, each stable phase has its own characteristic optical property; e.g., covellite (CuS) has a characteristic broad absorption band in the near-IR region (∼920 nm) which decreases on increasing the sulfur content (i.e. from covellite to digenite (Cu1.8S) to djurleite (Cu1.96S)). This absorption is completely absent in the chalcocite (Cu2S) phase. So, the absorption spectrum of copper sulfide is indeed very handy in the determination of its phase. Moreover, it has been reported that the overall structure of the spectrum is not affected by the size of the particles.26 On the basis of the above mentioned facts, the absorption spectrum recorded on copper sulfide nanoparticles was compared with the one reported by Silvester et al.26 From that, it is inferred that the phase of copper sulfide obtained is Cu2S (chalcocite) rather than CuS (covellite), as it does not show the characteristic absorption of covellite at 920 nm. Further comparison of this absorption spectrum with spectra reported on bulk39 and a thin film40 of Cu2S shows distinct changes; the spectrum of the bulk, which onsets at 1022 nm (band gap 1.21 eV), gets replaced by a sharp blue shifted peak at 475 nm (2.603 eV) in the case of Q-Cu2S prepared in the microemulsion environmentsa typical indication of the size quantization effect. Hence the formation of particles whose size is less than the size of its exciton1 is concluded. Though this effect has been reported earlier in the case of copper sulfide in the form of a blue shift in the absorption spectrum, to our knowledge, we are for the first time reporting it in the form of a distinct sharp peak, which could be assigned to the S0 to S1 molecular transition. Effect of Concentration of Reactants and w. In order to study the effect of the initial concentration of the reactants on the overall particle formation, the reaction has been carried out at various reactant concentrations (ratio 1:1) at constant w. The results are depicted in Figure 2. It is clear that particle density (or concentration) changes linearly with initial concentration of the reactants (refer to Figure 2 inset). However, neither the peak position nor the half width at full maximum (hwfm) of the peaks shows any dependence on it. This indicates that though an increase in concentration leads to an increase in particle density in the solution, it does not have any effect on size as well as the size distribution of the particles.
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Figure 4. Schematic representation of the adsorption of Cu2S nanoparticles on the inner wall of TX-100 inverse micelles.
Figure 2. Absorption spectra recorded on Q-Cu2S synthesized by reacting equimolar amounts of the copper ammonia complex and thiourea at concentrations (a) 0.72 × 10-3, (b) 1.08 × 10-3, (c) 1.44 × 10-3, (d) 1.8 × 10-3, and (e) 2.16 × 10-3 mol/L in TX-100/cyclohexane (0.2 mol/L, w ) 2) microemulsions. Inset shows a plot of corresponding peak height vs reactant concentration.
Figure 3. Effect of w on the absorption spectra of Q-Cu2S [(a) w ) 2; (b) w ) 3; (c) w ) 4; (d) w ) 8; (e) w ) 16] in TX-100 (0.2 mol/ L)/cyclohexane microemulsions. The reactant concentration was 2.16 × 10-3 mol/L.
As particles are formed in the microemulsion environment that is bound by the micellar ‘cage’, whose size is governed by the value of w,34,41-43 it is expected that the particle size will increase with an increase in w. It will result in a red shift in the absorption spectra. Such a w dependent shift in the absorption spectra has been well established in the case of Q-CdS.44 Figure 3 depicts the absorption spectra recorded on Q-Cu2S prepared at different values of w keeping the net concentration of reactants identical in all the cases. Surprisingly, it does not show any effect on the peak position though the peak height shows a decrease in its value on the increase in w. It suggests that the size of the particles does not reach its limiting value, i.e. the size of the “micellar cage”. This observation is quite unusual and may be interpreted on the basis of the model given below. Model for Nanoparticle Formation. On the basis of the observations made, we propose a model depicted schematically in Figure 4. As stated earlier, the formation of two different forms of copper sulfide, i.e. one in the form of small particles dispersed in the solution and the other in the form of a black precipitate immediately after mixing, reveals that the reaction between the copper ammonia complex and thiourea takes place in two different environments. As both the reactants are freely soluble in water, one of the environments is an aqueous core of microemulsions. It leaves three possible environments, namely (1) organic, the cyclohexane/2-methylpropan-1-ol phase,
(2) the hydrophobic tail of TX-100, and (3) the hydrophilic POE moiety directed toward the aqueous core. The first possibility has been ruled out, as both the reactants are found to be insoluble in it. The second possibility of the presence of the reactants in the hydrophobic tail region is also less likely due to the same reason. It leaves only one possibility of the presence of the reactants in the vicinity of the hydrophilic POE periphery. Therefore, it is reasonable to assume that the reactants are adsorbed there and are in dynamic equilibrium with the aqueous core. The relative concentration of these reactants in this environment would be decided by their solubility, i.e. the hydrophilicity of the reactants. Though the negatively charged acetate part of the copper ammonia acetate complex is likely to get adsorbed on an inner micellar wall, the labile Cu2+ ion will still be free to move. Thus, an adsorption of thiourea on the POE chain appears to be most probable. In this, NH2 groups are more likely to get tailored through hydrogen bonding with the ether linkage of POE, having a dangling polar >CdS¨ : site free for adsorption. On mixing these microemulsions, droplets will communicate through collision and exchange of their contents. The nucleation takes place simultaneously in the aqueous core, away from the periphery as well as near the periphery. In the aqueous core, being a homogeneous reaction, the rate of the reaction would be faster and leads to immediate formation of the particles. The collision of micelles facilitates an exchange of the particles, leading to the agglomeration and flocculation in the form of a black precipitate. At the same time the nuclei which are forming just near the periphery would get entrapped by thiourea through the dangling >CdS¨ : group. This interpretation is based on a previous study of the adsorption of thiourea on the Cu electrode45 and Cu2O46 in the form of a Cu(I) thiourea complex. This is analogous to the reports on stabilization of Q-CdS and Q-CdSe by coating (‘capped’) particles with selenophenol and cysteine, respectively.47,48 It leads to a localization of Q-Cu2S at the periphery. As most of the thiourea near the periphery is involved in the adsorption, it leads to a scarcity of S2- ions at that instant49 and, hence, the formation of a sulfur deficient phase of copper sulfide, i.e. Cu2S, though the formation of CuS is expected on the basis of the reaction mechanism. The nuclei so formed immediately get trapped with adsorbed thiourea (refer to Figure 4), and hence further growth of the particles ceases. Due to a decrease in the degree of freedom of the particles from three dimensions to two, it reduces the probability of agglomeration and hence enhances the stability. An increase in w by increasing in the net water content at a constant surfactant concentration will lead to a small number of big micelles.43 This will increase the droplet size, which will facilitate the homogeneous reaction taking place, away from the periphery of the micelles. At the same time, it will reduce the surface area of the micelles available for adsorption. This would lead to a decrease in the number of adsorbed particles responsible for the sharp peak. Thus, it shows the decrease in the peak height with an increase in w. As the particles are
Copper Sulfide Nanoparticles in Microemulsions
Figure 5. Absorption spectra of Q-Cu2S particles prepared by reacting 0.36 × 10-3 mol/L copper ammonia complex with (a) 0.36 × 10-3 mol/L Na2S and (b) H2S in TX-100/cyclohexane (0.2 mol/L, w ) 2) microemulsions.
Figure 6. Absorption spectra of Q-Cu2S particles in TX-100/ cyclohexane (0.2 mol/L, w ) 2) microemulsions at (a) 0.72 × 10-3, (b) 1.08 × 10-3, and (c) 1.44 × 10-3 mol/L thiourea at constant copper ammonia complex concentration (0.72 × 10-3 mol/L).
adsorbed and its size is not restricted by the size of micelles, the peak position is found to be independent of w. Testing of the Model. Effect of Thiourea on Particle Formation. In order to study the role of thiourea in the tailoring of particles near the periphery, the particles were synthesized using sulfurating agents other than thiourea. Figure 5 depicts the absorption spectra recorded on copper sulfide nanoparticles in TX-100/cyclohexane microemulsions using H2S and Na2S as the sulfurating agents. Though it shows a blue shift in the spectra as expected, the presence of a peak at 475 nm is not observed. The role of thiourea is further confirmed by synthesizing the particles at various thiourea concentrations. The corresponding absorption spectra are depicted in Figure 6. It indeed shows an increase in the peak height with an increase in the concentration of thiourea. The kinetics of agglomeration has also been studied by recording absorption spectra of Q-Cu2S at various thiourea concentrations and stipulated time intervals. The results are depicted in Figure 7 in the form of the reciprocal of absorbance vs time, which leads to a straight line. The agglomeration of particles adequately fits in the second-order kinetics.50 The interpretation is in accordance with the Smoluchowski coagulation mechanism of electroneutral species,51 which is already well tested in the case of agglomeration of Q-CdS.42,52 From the slope, the apparent second-order decay constants K are determined.50 These are plotted as a function of initial thiourea concentration in Figure 7 (inset). It shows a decrease in the value of K with an increase in the thiourea concentration. Hence, a definite role of thiourea in the stabilization of the
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Figure 7. Plot of reciprocal absorbance vs time for Cu2S nanoparticles prepared by (a) 0.72 × 10-3, (b) 1.08 × 10-3, and (c) 1.44 × 10-3 mol/L thiourea in TX-100/cyclohexane (0.2 mol/L, w ) 2) microemulsions. The net initial concentration of copper ammonia complex was 0.72 × 10-3 mol/L. The inset depicts a plot of apparent decay constant K vs thiourea concentration.
Figure 8. Absorbance spectra recorded on Cu2S nanoparticles synthesized in SDS (0.07 mol/L, w ) 10) microemulsions in toluene with (a) 0.36 × 10-3 and (b) 0.72 × 10-3 mol/L initial reactant concentration.
particle is inferred. However, no evidence has been found in the case of stabilization of the particles by the copper complex. Effect of the Nature of the Surfactant on Particle Formation. In the above model, it is also assumed that the POE chain of TX-100 mediates the tailoring of particles. If this is so, then the absence of this important link should give a destabilization of the particles which are assumed to be adsorbed near the periphery. In order to verify this, the reaction has been carried out in the microemulsions of the ionic surfactant sodium dodecyl sulphate (SDS), which does not contain the POE chain. The golden brown transparent dispersion obtained indicates the formation of a chalcocite (Cu2S) phase. It is further confirmed by the absorption spectra depicted in Figure 8, which are in accordance with the spectrum reported for the Cu2S phase.26,31 Though it shows a blue shift in the spectrum as expected, it does not show a sharp peak as displayed by the nanoparticles prepared in TX-100 microemulsions. This observation leads to the conclusion that POE chains are indeed involved in the stabilization of particles near the periphery. The absence of these chains leads to the formation of particles in the ‘water dew’. Though they display a blue shift in the spectrum, they are not small and mono-dispersed enough to give a sharp peak. The involvement of the POE chain in the stabilization of particles, which is responsible for the 475 nm peak, is further confirmed by carrying out the reaction in the (w/o) microemulsions of POE containing surfactants other than TX-100. For that purpose, the series of nonyl phenyl ethers (NP series) has been used. The absorption spectra recorded on such microemulsion stabilized particles are given in Figure 9. In all the
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Haram et al. Conclusions The synthesis of nanoparticles of copper sulfide has been carried out in the (w/o) microemulsions of nonionic surfactants like TX-100, NP-4, NP-7, and NP-9.5 and ionic surfactants like SDS. The formation of Cu2S (chalcocite) is inferred by means of absorption spectroscopy. The role of the POE chain and thiourea in the stabilization of the particles through hydrogen bound thiourea has been confirmed. The sharp maxima in the absorption spectra indicate the presence of a quasi-monodispersed size distribution. The kinetics of agglomeration has been fitted in the second-order rate law. The solution stability of the particles is further enhanced by means of hexametaphosphate.
Figure 9. Absorbance spectra on Cu2S nanoparticles synthesized in various nonyl phenyl ether/cyclohexane (0.2 mol/L, w ) 2) microemulsions: (a) NP-4; (b) NP-7; (c) NP-9.5. The reactant concentration was 1.44 × 10-3 mol/L.
Acknowledgment. One of the authors (S.K.H.) thanks Prof. M. M. Sharma, The Director, U. D. C. T., for his kind provision of a Golden Jubilee Post Doctoral Fellowship. References and Notes
Figure 10. Plot of the reciprocal of absorbance vs time for Q-Cu2S nanoparticles prepared in TX-100/cyclohexane (0.2 mol/L, w ) 2) microemulsions at various HMP concentrations. (a) 0.0; (b) 0.36 × 10-3; (c) 1.08 × 10-3; (d) 2.16 × 10-3 mol/L HMP. The inset shows a plot of apparent decay constant K vs HMP concentration.
cases a peak at 475 nm is observed, though it does not show a specific trend of peak height as a function of POE chain length. These results unambiguously indicate the important mediation of the POE chain in adsorption and hence the stabilization of particles. Stabilization of the Particles. It has been seen in an earlier section that thiourea stabilizes the particles to a certain extent, but it is not sufficient. Often a fourth component has to be present in order to control the rate of flocculation and maintain the particles in the suspension. Use of various stabilizing agents like sodium hexametaphosphate (HMP),14,53-56 Na2S2O3, glycerin, and polyvinyl acetate has been reported so far. In the present study, the enhancement in the solution stability has been achieved by means of HMP. Figure 10 depicts a reciprocal absorbance-time curve for Q-Cu2S particles with different amounts of HMP. A decrease in the slope of the second-order kinetics lines with an increase in the concentration of HMP indicates a dramatic enhancement in the stability against flocculation. The corresponding apparent decay constants K of the flocculation are obtained from the slope,50 and they are plotted as shown in Figure 10 (inset). It indicates an order of magnitude increase in the stability of the particles due to the addition of HMP. It is reasonable to believe that HMP most probably covers the exposed portion of the nanoparticlessa portion facing toward a core which is not able to be covered by a POE tailored thiourea. By doing that, it reduces the probability of agglomeration due to the collisions and enhances the solution stability further.
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