Synthesis and Electrophoretic Concentration of Cadmium Sulfide

Article pubs.acs.org/Langmuir

Synthesis and Electrophoretic Concentration of Cadmium Sulfide Nanoparticles in Reverse Microemulsions of Tergitol NP‑4 in n‑Decane Aleksei N. Kolodin,*,† Vladimir V. Tatarchuk,† Alexander I. Bulavchenko,† and Evgeniia V. Poleeva Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, prosp. Akademika Lavrentieva 3, Novosibirsk 630090, Russia ABSTRACT: The kinetics of thiourea synthesis of CdS nanoparticles (NPs) in reverse microemulsions of Tergitol Np-4/ndecane was studied in the temperature range of 313−333 K by spectrophotometry, photon-correlation spectroscopy, and transmission electron microscopy. The formation of NPs is described by the kinetic model, including two consecutive steps: homogeneous nucleation in a solution as the first step and the autocatalytic growth of NPs due to heterogeneous reaction on a continuously increasing surface as the second step. Effective rate constants of the steps (k1 = 1.52 × 10−2−1.75 × 10−3 s−1 and k2 = 4.9 × 10−1−5.1 × 10−2 M−1 s−1) and effective activation energies (Ea1 = 156 and Ea2 = 149 kJ/ mol) were estimated in the pseudo-first-order reaction with respect to cadmium (cCd = 0.9 mM, cThio = 9 mM). The obtained constants were used to calculate the dependence of nanoparticle diameter on the synthesis time (d3 ∼ t). The calculated values correlate well with experimental data of photon-correlation spectroscopy and transmission electron microscopy.

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INTRODUCTION CdS nanoparticles (NPs)quantum dots(QDs)are widely used in the development of composite materials with unique properties,1−5 in photocatalytic processes, 6 and in the production of devices with semiconductor coatings, for example, solar cells.7,8 Stable hydrosols and organosols of NPs are most useful for the introduction and uniform distribution of QDs in the composite bulk or film.9 According to the literature, different reagents, templates, and nanoreactors can be employed in the synthesis of CdS particles.9,10 Thiourea (Thio) is quite promising as a source of sulfide sulfur because of its low toxicity in comparison to that of widely used H2S and Na2S reagents.11−13 In addition, Thio is a thermoregulated molecular batcher of sulfide ions,14 which makes it possible to finely regulate the synthesis, crystallization, and aggregation of NPs. In our previous work,15,16 the nucleation of CdS in bulk aqueous ammonia solutions of Thio and cadmium chloride and on the surface of polystyrene substrates was studied in detail. In the absence of molecular stabilizers, templates, and nanoreactors, the synthesis was accompanied by the coagulation and sedimentation of NPs; therefore, stable dispersions of NPs could not be obtained. Microemulsion (micellar) synthesis is widely used to produce stable organosols.10,17,18 However, no attempts were made to perform thiourea synthesis in the surfactant reverse micelles. This may be related to the suppression of alkaline hydrolysis of thiourea in AOT micelles, the most popular surfactant employed in microemulsion synthesis. © XXXX American Chemical Society

This work was aimed at elucidateing the kinetic regularities in the synthesis and concentration of stable organosols of CdS NPs in microemulsion systems based on nonionic Tergitol Np-4 and anionic AOT. The study was carried out using Scheme 1, which comprises several steps: thiourea synthesis in Tergitol Np-4 microemulsions at different temperatures; inhibition of the process and charging of CdS NPs at different steps by introducing AOT into microemulsion; electrophoretic separation of a liquid concentrate of NPs from excess surfactant; and multiple dilution of the concentrate in pure solvents. In each step the dispersions were studied by PCS and spectrophotometry.

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EXPERIMENTAL SECTION

General. The study was performed with CdCl2·2.5H2O, thiourea, aqueous ammonia, toluene, acetone, n-decane, malachite green dye, and surfactants Tergitol Np-4 (99%, The Dow Chemical Company) and AOT (99%, Aldrich). Aqueous solutions of 0.09 M CdCl2 and 0.9 M thiourea were prepared using accurately weighed samples of reagents just before the experiments; they also contained 4 M ammonia. Micellar solutions of reagents were obtained by injection solubilization of the aliquot quantities (Vs) of reserve water−ammonia solutions in equal volumes (V0) of a 0.25 M solution of Tergitol Np-4 in decane. The concentrations of Cd and Thio in micellar solutions were 1.8 and 18 mM, respectively, at the same solubilization capacity, Vs/V0 = 0.02 in both cases. Received: March 13, 2017 Revised: June 10, 2017 Published: July 27, 2017 A

DOI: 10.1021/acs.langmuir.7b00690 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Scheme 1. Scheme of the Experiment

Figure 1. Experiments on measuring the ζ-potential and electrophoretic kinetics of CdS particles: (A) dependence of particle velocity on the supplied voltage and (B) measurement of the ζ-potential at 200 V. The formation kinetics of CdS NPs in the micellar synthesis from CdCl2 and Thio was studied by UV−vis and photon-correlation spectroscopy. The experiments were carried out by the following procedure. Micellar solutions of CdCl2 and Thio were thermostated to a specified temperature and rapidly mixed in equal volumes; the resulting reaction mixture was poured into a measuring cuvette of the spectrophotometer or photon-correlation spectrometer to record time-related changes in the extinction spectrum or hydrodynamic diameter. The reaction time was counted off from the moment of mixing the micellar solutions of reagents. Constant parameters of the mixtures in the experiments were Vs/V0 = 0.02 and total concentrations were cCd = 0.9 mM and cThio = 9 mM. The temperature was varied in the range from 313 to 333 K as as 313, 318, 321, 323, 328, and 333 K. To stop the formation of CdS, the microemulsion was cooled to room temperature and diluted 2-fold with a 0.25 M micellar solution of AOT. (The blank run showed that CdS particles are not formed in the system Tergitol Np-4/AOT (1:1) in n-decane). Prior to electrophoretic concentration, ammonia and water were removed from the microemulsion under continuous stirring with a magnetic stirrer in an open vessel for 3 h. The release of ammonia was controlled using a test paper, and the content of water was controlled by IR Fourier spectroscopy. This procedure was shown to exert no effect on the electrophoretic mobility of NPs. Before measuring the sizes, organosols were purified by 8-fold cyclic filtering at room temperature through a PTFE membrane filter with a pore diameter of 0.2 μm (Sartorius, Germany) directly into the measuring cell. Methods. Measurement of the Sizes of CdS Particles. The effective hydrodynamic diameter of the particles was found by dynamic light scattering (photon-correlation spectroscopy) on a NanoBrook Omni (Brookhaven, USA) spectrometer. The autocorrelation function was processed using monomodal analysis by the cumulant method and polymodal analysis with the non-negatively constrained least-squares (NNLS) algorithm. The power of a solid-state laser with a wavelength of 640 nm was 35 mW; photons scattered by the particles were detected at an angle of 90° with respect to the radiation source. Photons for one

measurement were accumulated for 10 s; the hydrodynamic diameter was found by averaging over 25−50 measurements. The diameter of NPs (the diameter of NPs without an adsorption layer) was measured on a JEM-2010 electron microscope with a maximum point-to-point resolution of 0.2 nm. An electrophoretic concentrate of CdS NPs was diluted 500-fold with toluene, and a droplet of solution was then deposited on the carbon and carbon-free substrates and dried at room temperature. The function of the nanoparticle size distribution was plotted from 100−200 measurements at different magnifications. TEM and PCS data were compared using the numberaverage diameter. Measurement of the Electrokinetic Potential. The electrokinetic potential (ξ-potential) was measured using phase-analysis light scattering (PALS) at an angle of 15°. Measurements were made in a special SRR2 cell, which is stable to the action of organic solvents, using plane-parallel palladium electrodes with an interelectrode gap of 3.45 mm and an area of ∼45 mm2. The velocity of NPs was measured manually at a voltage from 75 to 200 V (Figure 1A,B). For each voltage, the average velocity was found from 10−20 measurements. For all of the systems, the dependences of nanoparticle velocity on the field strength were linear, which indicated that the criteria of the true (linear) electrophoresis were met.19 Electrophoretic mobility was determined from the slope, and the ζ-potential was calculated with the Hückel− Onsager formula. Electrophoretic Concentration of NPs. The electrophoretic concentration was employed for different purposes: (1) to separate NPs from excess surfactant, reagents, and side products of the reaction; (2) to increase the concentration of NPs; and (3) to replace the solvent.20 This allowed us to determine correctly the size of the NPs by the indicated methods. To select the optimal mode of electrophoretic concentration of CdS NPs, two types of electrophoretic cells were tested: with a vertical and horizontal orientation of plane-parallel electrodes. The area of each copper electrode was 4.2 cm2, and the interelectrode gap was 0.8 cm. Electrodes were mounted in a 1 cm spectrophotometer cuvette so that they did not shade the diaphragmatic space of the cuvette. Cells with the electrodes were arranged in the B


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Figure 2. Experiments on measuring the ζ-potential and electrophoretic kinetics of CdS particles: (A) evolution of the spectrum during the CdS electrophoresis at 200 V for the systems with horizontal electrodes and (B) time dependence of the optical density at 299 (1), 200 (2), and 150 V (3) for the systems with horizontal electrodes. (Inset) Time dependence of the optical density at 200 V. (The shaded dot corresponds to a change in the electric field polarity.) cuvette section of a UV-1700 (Shimadzu, Japan) spectrophotometer. The constant voltage across the electrodes (150−300 V) was supplied by a B5-50 external power supply (Russia). The optical density dynamics in the interelectrode space was recorded spectrophotometrically at 5 min intervals. The testing of both cells revealed their virtually similar efficiency. The optical density in the interelectrode space continuously decreased under the action of the electric field, and the processes were completely reversible with a change in electrode polarity (Figure 2A,B). However, in the case of horizontal orientation, quite a stable gel-like film of NPs with good adhesion to the copper surface formed on the electrodes during electrophoresis. As a result, NPs were poorly redispersed in solvents after electrophoresis. Thus, cells with vertical orientation at a periodic change of electrode polarity were preferable. The liquid concentrate of NPs gradually drained off of the electrodes to the bottom of the cell, where it was not subjected to the action of the field owing to a 2 mm distance between electrodes and the bottom. The obtained liquid concentrate (100 μL) was taken with a batcher and diluted 50−100-fold in toluene to determine the size of NPs by PCS and TEM at surfactant concentrations lower than the critical micelle concentration. Plotting the Calibration Dependence and Determining the Extinction Coefficient of CdS. Preliminary experiments allowed us to determine the extinction coefficient of CdS in reverse micelles of Tergitol Np-4 in n-decane and to plot the calibration dependence of the optical density on concentration. To this end, CdS was synthesized in Tergitol Np-4/n-decane at 323 K for 5 h. The synthesis was interrupted by a sharp cooling of the reaction mixture to room temperature, and then the mixture was divided into two equal parts: solutions 1 and 2. Solution 1 was used to plot the calibration dependence: the mixture was gradually diluted with the background solution of 0.25 M Tergitol Np-4 in n-decane, and changes in the optical density with respect to the background solution were recorded spectrophotometrically. The obtained dependence was linear over the entire range of concentration (Figure 3).

Solution 2 was used to determine the extinction coefficient of CdS. To precipitate the particles, the solution was supplemented with acetone in a volume ratio of 2:1, and the resulting mixture was centrifuged for 5 min at 1.5 × 103 rpm. The precipitate was washed five times with nheptane with subsequent centrifugation under the same conditions and then washed with water three times and centrifuged for 30 min at 1.5 × 103 rpm. The described procedures make it possible to separate CdS particles from micelles and soluble forms of the cadmium ion. The obtained precipitate and CdS NPs that adhered on the bottom and walls of the vial were dissolved using portions of 4 M HCl; after that, the resulting mixture was diluted 5-fold with water. Thus, the final concentration of HCl was 0.8 M. The produced Cd(II) solution was analyzed using the reference solutions containing 0.50−10.0 μg/mL cadmium and having a background composition identical to that of the sample solution; this is why the sample was diluted 25-fold with a 0.8 M aqueous solution of HCl just before the analysis. The cadmium content was measured on a Z-8000 (Hitachi) atomic absorption spectrophotometer in an air−acetylene flame at an analytical wavelength of 228.8 nm. The known concentrations of calibration solutions and the calibration dependence (Figure 3) were used to find the extinction coefficient of CdS in reverse micelles of Tergitol Np-4 equal to 1090 ± 17 L/(mol· cm). Investigation of the Catalytic Properties of CdS NPs. The catalytic properties of CdS NPs were studied via the photodegradation of malachite green.21 The obtained concentrate of CdS NPs was redispersed in 10 mL of the AOT solution (0.25 M) with subsequent addition of 100 μL of an aqueous solution of the dye (1.1 × 10−4 M) by injection solubilization. A similar system (100 μL of the dye solution in 10 mL of the AOT solution) without CdS NPs served as the reference. Both systems were exposed to a UV lamp with a power of 100 W located 7.5 cm from the cuvette; time-related changes in the optical density were recorded on the spectrophotometer. Diffraction Studies of CdS NPs. X-ray diffraction analysis of polycrystals was carried out on a Shimadzu XRD-7000 diffractometer (Cu Kα radiation, Ni filter, 2Θ range 5−60°, 2Θ step 0.03°, and accumulation time 20 s). Silicon served as the external standard. A sample was prepared for analysis by the following procedure. The isolated powder was ground in an agate mortar in the presence of nheptane; the resulting suspension was deposited on the polished side of a standard quartz cuvette; when heptane dried out, the sample was represented by a thin, smooth layer with a thickness of ∼100 μm. The diffraction pattern was indexed using the PDF database.22

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RESULTS AND DISCUSSION Kinetic Model. In the micellar synthesis of CdS NPs, an interaction occurs in the dispersed aqueous phase that is formed by mixing the micellar solutions of CdCl2 and Thio and is uniformly distributed over the internal cavities of the surfactant reverse micelles. Because of the fast intermicelle exchange, the

Figure 3. Dependence of optical density on Cd(II) concentration (λ = 450 nm, T = 298 K). C


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Figure 4. Spectrophotometry data: (A) evolution of the spectrum of the reaction mixture during the synthesis of CdS NPs (scanning from 4.5 to 553.5 min with a step of 3 min at T = 323 K); (B) kinetic curves at different temperatures (λ = 450 nm, T = 313 (1), 318 (2), 321 (3), 323 (4), 328 (5), and 333 K (6)).

their surfaces, the areas of which are continuously increasing, thus causing the autocatalytic nature of the process. New nuclei do not form in the second step. The approach developed earlier for the growth of Au and Ag NPs was used in our study to process the kinetic data for the growth of CdS NPs.30−32 The kinetic model implied that the rates of nucleation and growth steps were limited by the rate of reaction 2, which occurred homogeneously in solution or heterogeneously on the surface of growing CdS NPs. In the pseudo-first-order reaction with respect to Cd at a ratio of analytical concentrations in the dispersed phase c′Cd < c′Thio, c′NH3, c′OH, reaction 2 can be written schematically as reaction 3 to present the process steps as follows: nucleation:

composition of the dispersed phase is continuously averaged and becomes locally identical in all of the reverse micelles. Because the dispersed phase is the water−ammonia medium and CdII complexes are labile, the initial state of the reaction mixture may include ammonia and hydroxide forms of cadmium complexes, among which Cd(NH3)42+ should dominate at a concentration of 4 M NH3.23 It is known that in aqueous solutions thiourea is hydrolytically decomposed into hydrogen sulfide and cyanamide (ammonia and urea may also form), and in alkaline media, to S2− and CN22− (reaction 1). However, judging from the equilibrium constant, the conversion should not be high upon mixing the fresh micellar solutions of reagents.24 SC(NH 2)2 + 4OH− ↔ S2 − + CN2 2 − + 4H 2O

(1)

Thus, the gross reaction of CdS synthesis in our case can be presented in the general form as Cd(NH3)4

2+

+ SC(NH 2)2 + 4OH

→ CdS + 4NH3 + CN2

2−

−

+ 4H 2O

Cd(NH3)4 2 + → CdS + products, k1*

(3)

nCdS → (CdS)n , fast

(4)

growth of NPs: (CdS)n + Cd(NH3)4 2 + → (CdS)n + 1 + products, k 2*

(2)

(5)

The mechanism of reaction 2 is not quite clear now. Taking into account the processes considered above and the ability of CdII to complex with Thio,25 reaction 2 can proceed by two radically different routes, which may be denoted as “ionic” and “coordination”. By the first route, some CdII speciesthe initial Cd(NH3)42+ or another one related to it via the labile complexation equilibriainteracts with S2− that emerges via the hydrolysis of Thio by reaction 1. According to the second route, CdS is produced via decomposition of the thiourea complex that forms upon interaction of the CdII species directly with Thio. Figure 4 illustrates the typical evolution of the reaction mixture spectrum during the growth of CdS NPs and displays kinetic curves in the coordinates “current extinction of solution (At) at a fixed wavelength (λ)−time (t)” at different temperatures (T). The presence of an induction period and the s-shaped curves suggest that the formation of NPs is controlled by the chemical reaction and proceeds autocatalytically. The autocatalytic formation of cadmium sulfide sol from cadmium salt and thiourea in an aqueous solution of ammonia was reported in one of the first studies on the kinetics of this reaction.26 This observation reveals an analogy with the growth of noble metal NPs, 27−32 where the initial part of the kinetic curve corresponding to the induction period is related to nucleation in solution. The next part of the curve reflects the growth of NPs due to the reaction of precursors (initial forms of reagents) on

Rate constants k1* and k2* were the effective values, functions of the fixed concentrations of Thio, NH3, and OH−, and equilibria constants in the system involving the indicated components and CdII.23 The kinetic equation for Cd(NH3)42+ consumption, by analogy to the formal kinetic model reported in ref 27, had the following form: −

d[Cd(NH3)4 2 + ]′t = k1*[Cd(NH3)4 2 + ]′t dt + k 2*[CdS]′t · [Cd(NH3)4 2 + ]′t

(6)

The measurable quantities in kinetics experiments were the extinctions of CdS in micellar solution: current At and limiting Amax at t = ∞. The other components of the reaction mixture virtually did not absorb light in the visible region. A minor initial extinction A0 at t = 0 did not exceed ∼10−2 and was caused by incomplete compensation of the background absorption upon measuring with respect to decane. Taking into account the material balance for cadmium, c′Cd ≈ [Cd(NH3)42+]′t + [CdS]′t, where [Cd(NH3)42+]′t and [CdS]′t are the current concentrations in the dispersed aqueous phase and relations of At and Amax with the concentrations in micellar solution ([CdS]t, cCd) and the dispersed aqueous phase (c′Cd, [CdS]′t) expressed by the equalities At − A0 = εCdSl[CdS]t = εCdSl[CdS]′t(Vs/V0) and Amax − A0 = εCdSlcCd = εCdSlc′Cd(Vs/V0), where εCdS is the molar D


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Langmuir Table 1. Kinetic Parameters of the Process parameter

T = 333 K

T = 328 K

T = 323 K

T = 321 K

a0 a1103, min−1 a2103, min−1 Amax − A0 k1* × 103, min−1 k2*, M−1·min−1

−0.19 ± 0.04 −33 ± 7 21 ± 13 0.82 ± 0.03 15.2 0.488

−0.26 ± 0.02 −9.57 ± 0.09 6.4 ± 0.4 0.77 ± 0.01 4.73 0.108

−0.26 ± 0.01 −6.34 ± 0.04 5.1 ± 0.1 0.767 ± 0.005 2.46 0.086

−0.22 ± 0.01 −3.59 ± 0.04 2.3 ± 0.1 0.802 ± 0.003 1.75 0.051

process depth over the time of measurement was insufficient for correct processing of the data and the determination of rate constants. Parameters and constants obtained at four higher temperatures are listed in Table 1. The kinetic curves calculated with the constants by the integral in eq 9 satisfactorily described the experimental points denoted by markers (Figure 4B). Effective activation energies of the steps estimated from the constants at 321, 323, 328, and 333 K were Ea1* ≈ 156 kJ/mol for nucleation and Ea2* ≈ 149 kJ/mol for the growth of NPs. Thus, a gain in the activation energy when going from the homogeneous route of reaction in solution to the heterogeneous autocatalytic route on the surface of growing NPs was ca. 7 kJ/mol. Size of NPs. The dependence of the hydrodynamic diameter on the synthesis time was measured at 323 K. Figure 5 illustrates

extinction coefficient of CdS, eq 6 was transformed to eq 7, which gave the linear regression (eq 8) that was employed in processing the kinetic dependences of At on t and had variables x1 = t and x2 = ∫ (Amax − At) dt (the numerical integral between the limits 0 and t) and parameters a0 = ln(Amax − A0), a1 = −{k1* + k2*cCd(V0/Vs)} = −(k1*+k2*c′Cd), and a2 = {k2*/(εCdSl)}(V0/ Vs) = k2*c′Cd/(Amax − A0). The obtained values of parameters were used to calculate the effective rate constants k2* = a2ea0/c′Cd and k1* = −(a1 + a2ea0). ⎧ ⎛ V ⎞⎫ d ln(A max − A t ) = − ⎨k1* + k 2*cCd⎜⎜ o ⎟⎬ dt ⎩ ⎝ Vs ⎠⎭ ⎪

⎪

⎪

⎪

⎧ k * ⎫⎛ V ⎞ + ⎨ 2 ⎬⎜ o ⎟(A max − A t ) ⎩ εCdSl ⎭⎝ Vs ⎠

(7)

ln(A max − A t ) = a0 + a1x1 + a 2x 2

(8)

The kinetics curves were calculated with the constants by integral eq 9. A t = A max − (A max − A 0)(1 + β)/(1 + β eα t)

(9)

where α = k1* + k2*c′Cd and β = k1*/(k2*c′Cd). It is not difficult to show a theoretical relation of the size of a growing nanoparticle with the current extinction of the micellar solution of NPs at λ = const. Upon completion of the growth, the average diameter of NPs reaches its limiting value dmax, which can reliably be found from TEM or PCS data, and [CdS]t = cCd. In this case, the average number of CdS molecules in a single particle will be n = πdmax3/(6VCdS), where VCdS is the volume of the CdS molecule, and the concentration of NPs in micellar solution, which does not change in the process starting from nucleation, is cNP = cCd/n = cCdVCdS/(πdmax3). Current values of [CdS]t and dt are interrelated by the equality [CdS]t = {πdt3/ (6VCdS)}cNP = cCd(dt/dmax)3, which makes it possible to express At − A0 = εCdSl[CdS]t = εCdSlcCd(dt/dmax)3, Amax − A0 = εCdSlcCd, and (At − A0)/(Amax − A0) = (dt/dmax)3 and obtain the constraint (eq 10), where b = dmax3/(Amax − A0). d t 3 = b(A t − A 0)

Figure 5. PCS data. Dependences of the hydrodynamic diameter of particles on the synthesis time at T = 323 K {1, polymodal distribution [(a) mode 1; (b) mode 2]; 2, monomodal distribution; 3, blank run with the Tergitol Np-4/n-decane system in the absence of CdS; 4, the concentrate of CdS diluted in n-decane and supplemented with the calculated amount of Tergitol Np-4}.

the results of mono- and polymodal analysis of the autocorrelation function. In the initial step, changes in the hydrodynamic diameter are virtually inappreciable. Such behavior agrees well with the spectrophotometry data and conclusions based on them: the nuclei of NPs formed in this step are smaller than Tergitol Np-4 micelles. Seventeen to twenty minutes after the beginning of the synthesis, the particles start to grow gradually because of the reaction on the surface of the formed nuclei. Polymodal analysis clearly distinguished two modes: particles with a small size that virtually does not change with time and large particles with a continuously growing diameter that reaches nearly 100 nm. The diameter of the firstmode particles is close to the diameter of empty micelles of Tergitol Np-4 containing 2% of the aqueous pseudophase (∼10 nm). Thus, the first mode can reliably be attributed to the empty micelles. The situation for the second mode is much more complicated. Many studies showed that micelles can be adsorbed on the surface of NPs, thus forming a developed adsorption layer.20,33 As a result, PCS gives strongly overestimated diameters, which

(10)

Thus, based on eqs 9 and 10, the analytical expression for calculating the nanoparticle size at any time in the process with the use of effective rate constants of the steps has the following form: dt 3 =

dmax 3β {eαt − 1} 1 + β e αt

(11)

The obtained dependences (eqs 9 and 11) make it possible to describe the experimentally observed dependences of At and dt with the use of effective constants within the proposed kinetic model of NP formation and growth. Kinetic Constants and Activation Energy. At temperatures of 313 and 318 K, the rate of NP formation was low, so the E


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Figure 6. CdS particles synthesized for 20−90 min at 323 K: (A−D) TEM images and size distributions.

Figure 7. Characterization of the crystal structure of CdS nanoparticles synthesized for 40 min at 323 K: (A) the diffraction pattern of CdS particles and (B) the XRD data of the crystal structure of CdS.

are the sum of the particle diameter and a double thickness of the adsorption layer of micelles. To get a true value of the particle diameter, the obtained dispersion should be diluted to surfactant concentrations lower than the critical micelle concentration (CMC is commonly equal to 10−4−10−3 M).34 Taking into

account that micellar synthesis is usually performed with the surfactant concentrations equal to 0.1−1 M, the dispersion with NPs should be diluted 102−103-fold. However, such a dilution will sharply decrease the concentration of NPs, which makes it difficult to determine their size by PCS. To solve the problem, F


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Langmuir Table 2. Comparison of Calculated Optical and Experimental Size Characteristics of the Growing CdS NPsa dt3, nm3

dt, nm t, min

At − A0

TEM

PCS

TEM

PCS

20 40 60 90

0.038 0.077 0.117 0.177

10.8 13.3 16.2 18.1

7.7 8.5 11.0 12.8

1260 2353 4252 5930 R = 0.999, b = (3.4 ± 0.3) × 104

457 614 1331 2097 R = 0.994, b = (1.1 ± 0.2) × 104

At − A0 values were calculated for λ = 450 nm and T = 323 K, and dt values were estimated by TEM and PCS at T = 323 K. Errors b are represented by the confidence intervals for P = 0.95. a

Figure 8. Growth of the particle diameter (A) and volume (B) by extending the synthesis time according to PCS (1) and TEM (2).

the electrophoretic concentration is employed.20 First, the liquid concentrate with NPs is separated from the reaction mixture: its volume constitutes 10−2−10−3 of the initial microemulsion volume, so the concentration of NPs increases 100−1000-fold whereas the surfactant concentration does not change.20 After that, the concentrate is diluted with a pure solvent to produce a dispersion with the concentration of NPs the same as in the solution after synthesis, but the surfactant concentration is decreased by a factor of 102−103. After the synthesis, CdS NPs in the microemulsion are not charged; therefore, the electrophoretic mobility cannot be measured by PALS. After the introduction of AOT into the microemulsion (Experimental Section), the particles acquired a positive charge (Figure 1A,B). This allowed us to carry out electrophoretic concentration in a cell with vertical plane-parallel electrodes. The resulting concentrate was diluted with toluene, and particle sizes were measured with PCS and TEM. The measurements were made with four systems at different synthesis times at 323 K: 20, 40, 60, and 90 min. PCS revealed a single mode. For 90 min, the initial system was reconstructed: the concentrate diluted in decane was supplemented with Tergitol Np-4 so that its final concentration became ∼0.25 M. In this case, PCS showed the presence of two modes (4.2 and 105.5 nm, Figure 5), which indicates that their initial identification was correct. In the same systems, the diameter of NPs was also estimated by TEM (Figure 6A−D). The particles have a crystal lattice (Figure 7A,B). According to the XRD data, the crystal phase of the sample is represented by the cubic modification of CdS; in addition, there are some low-intensity nonindexed peaks. It was also found that an increase in the synthesis time to 7 h is not accompanied by the further growth of CdS particles. The sizes of the particles synthesized by different methods are compared in Table 2. Note that the average sizes according to TEM exceed those obtained by PCS, although commonly the reverse situation is observed. A possible explanation is that the shape of the produced particles differs from the spherical shape implied in the

Stokes−Einstein equation. In addition, the surface roughness of the particles should be noted. Data on diameters of the particles measured by TEM and PCS clearly demonstrate that the main increase in size from 0 to 8−11 nm took place in the first 20 min. In the period of 20−90 min, the process of autocatalytic growth was close to completion, so the size was increasing much more slowly (Figure 8A). The obtained sizes were compared to those calculated using kinetics constants. Testing corresponded to eq 9: linear correlations with coefficient R > 0.99 (Figure 9, Table 2) were

Figure 9. Correlations of the calculated (At − A0) values with experimental dt3 according to TEM (1) and PCS (2). The conditions are listed in Table 2.

observed between experimental values of dt3 determined from TEM and PCS data at t = 20, 40, 60, and 90 min at room temperature and (At − A0) values calculated for the same time points at T = 323 K and λ = 450 nm with the use of kinetics parameters. The obtained values of parameter b and (Amax − A0) for 323 K (Table 1) allowed us to estimate the limiting size of CdS NPs as dmax = {b(Amax − A0)}1/3: 30 ± 14 (TEM) and 21 ± 12 nm (PCS). Volumes of CdS particles were calculated from experimental values of their diameters; a spherical shape of the particles was assumed in these and further calculations. The dependence of G


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The same values (Table 3) were obtained by calculating the indicated parameters using the data of linear dependence, d3(t). A good fit between calculated and experimental values demonstrates the correctness of the proposed model. Photocatalytic Degradation. The photodegradation of dyes is a classic tool for revealing the photocatalytic properties of various nanoparticle systems, for example, TiO2, TiO2/ZnO, and CdS,35−37 particularly in reverse micelle systems.21 In our work, photocatalytic processes were investigated using basic dye malachite green because it has been successfully employed in similar studies and is a well-understood model system for solving such problems.21,35,36 Because the degradation rate of the dye is affected by the pH of the medium,35 all the experiments were carried out at a constant pH 5, which is quite far from the pH value corresponding to transitions of the dye. This means that in experiments the dye will not change its form. The photodestruction of malachite green was investigated by means of spectrophotometry (Figure 10 A,B). For both systems, the absorption spectra clearly show two absorption bands (424 and 636 nm) assigned to malachite green. A decrease in the optical density for both bands with increasing exposure time testifies to the destruction of the dye. In addition, exposure to UV light for 3 h produced a hypsochromic shift for each of the bands: from 424 to 418 nm for the first band and from 636 to 631 nm for the second band, which also testified to the destabilization of the system. Therefore, the evolution of the spectra for both systems was not accompanied by the appearance of new absorption bands in the UV range (λ > 200 nm). To estimate correctly the rate of dye destruction, we calculated the degree of its decomposition (αt, %) from the second band using the formula

particle volume on the synthesis time is linear over the entire time range (Figure 8B), which testifies to a monotonic increase in the particle size. Therefore, a stepwise increase in the diameter of the aggregates in the initial synthesis step is virtually not observed on the particle volume scale because the volume of CdS nuclei is negligible in comparison to the volumes of growing particles. It should be noted that dependence V(t) obtained by eq 11 describes the experimental results (continuous lines in Figure 8B). PCS and TEM data allow us to obtain additional information on the second step, namely, the autocatalytic growth of particles at T = 323 K. In particular, the rates of gain in CdS particles diameter (dd/dt) and volume (dV/dt) were calculated. In addition, reference data on the molecular weight and density of CdS (ρ = 4.88 g/cm3) as well as its extinction coefficient (ε) and optical density (ACdS) measured in the experiments at T = 323 K were used to estimate the rate of numerical growth of the particles (dN/dt) and calculate their numerical concentration (Nv) by eqs 12 and 13: NV = dN = dt

A CdSMCdS εlρV

(12)

( ddVt )(ρNA) MCdS

(13)

Therewith, the optical path length (l) was 1 cm. Because PCS data somewhat deviate from TEM data, the estimation was made for both methods (Table 3). Table 3. Growth Parameters of CdS Particles during the Synthesis at 323 K parameter

PCS

TEM

dd/dt, nm/s dV/dt, nm3/s dN/dt, s−1 Nv, l−1

(1.3 ± 0.8) ×10−3 (2 ± 1) ×10−1 4±2 1.5 × 1016

(1.8 ± 0.1) × 10−3 (6 ± 1) × 10−1 12 ± 1 4.6 × 1015

αt = 1 −

A0 − At A0

(14)

where A0 is the initial optical density and At is the current optical density (at time t). Figure 11 displays time dependences of the degradation degree. According to the results obtained, the addition of CdS particles increased the rate of dye photodestruction. After 3 h of exposure to UV light, the dye decomposed by 58% in the presence of CdS particles and by 41% in their absence. Thus, it was found that CdS particles exhibit photocatalytic properties and can be employed as photocatalysts.

It is noteworthy that numerical concentrations of CdS particles calculated from diameters by TEM and PCS remained virtually unchanged in the range of 20 to 90 min during the synthesis; their averaged values are listed in Table 3. The constancy of the concentrations indicates that new CdS nuclei are not formed in the second step, which verifies our kinetic model.

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CONCLUSIONS The formation of cadmium sulfide NPs in reverse microemulsions of Tergitol Np-4 was studied. The novelty of our

Figure 10. Experiments on the photodegradation of malachite green dye: (A) evolution of the spectrum for the system without CdS particles and (B) evolution of the spectrum for the system with the addition of CdS particles. H


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(3) Ramprasad, S.; Su, Y.-W.; Chang, C.-H.; Paul, B. K.; Palo, D. R. Continuous microreactor-assisted solution deposition for scalable production of CdS films. ECS J. Solid State Sci. Technol. 2013, 2, P333−P337. (4) Pavel, F. M.; Mackay, R. A. Reverse Micellar Synthesis of a Nanoparticle/Polymer Composite. Langmuir 2000, 16, 8568−8574. (5) Levy, L.; Feltin, N.; Ingert, D.; Pileni, M. P. Isolated Mn2+ in CdS Quantum Dots. Langmuir 1999, 15, 3386−3389. (6) Samadi-Maybodi, A.; Sadeghi-Maleki, M.-R. In-situ synthesis of high stable CdS quantum dots and their application for photocatalytic degradation of dyes. Spectrochim. Acta, Part A 2016, 152, 156−164. (7) Genovese, M. P.; Lightcap, I. V.; Kamat, P. V. Sun-Believable Solar Paint. A Transformative One-Step Approach for Designing Nanocrystalline Solar Cells. ACS Nano 2012, 6, 865−872. (8) Zarazúa, I.; Esparza, D.; López-Luke, T.; Ceja-Fdez, A.; ReyesGomez, J.; Mora-Seró, I.; de la Rosa, E. Effect of the electrophoretic deposition of Au NPs in the performance CdS QDs sensitized solar Cells. Electrochim. Acta 2016, 188, 710−717. (9) Taguchi, M.; Yagi, I.; Nakagawa, M.; Iyoda, T.; Einaga, Y. Photocontrolled Magnetization of CdS-Modified Prussian Blue Nanoparticles. J. Am. Chem. Soc. 2006, 128, 10978−10982. (10) Harruff, B. A.; Bunker, C. E. Spectral Properties of AOTProtected CdS Nanoparticles: Quantum Yield Enhancement by Photolysis. Langmuir 2003, 19, 893−897. (11) Petit, C.; Pileni, M. P. Synthesis of cadmium sulfide in situ in reverse micelles and in hydrocarbon gels. J. Phys. Chem. 1988, 92, 2282− 2286. (12) Robinson, B. H.; Towey, T. F.; Zourab, S.; Visser, A. J. W. G.; van Hoek, A. Characterisation of cadmium sulphide colloids in reverse micelles. Colloids Surf. 1991, 61, 175−188. (13) Modes, S.; Lianos, P. Luminescence probe study of the conditions affecting colloidal semiconductor growth in reverse micelles and waterin-oil microemulsions. J. Phys. Chem. 1989, 93, 5854−5859. (14) Kosareva, L. A.; Lavrenova, L. G.; Zegzhda, T. V.; Shulman, V. M. Decomposition of thiourea in the alkali media. Izv. SO AN SSSR 1968, 6, 57−63. (15) Bulavchenko, A. I.; Kolodin, A. N.; Podlipskaya, T.; Yu; Demidova, M. G.; Maksimovskii, E. A.; Beizel’, N. F.; Larionov, S. V.; Okotrub, A. V. Photon Correlation Spectroscopic and Spectrophotometric Studies of the Formation of Cadmium Sulfide Nanoparticles in Ammonia−Thiourea Solutions. Russ. J. Phys. Chem. A 2016, 90, 1034− 1038. (16) Bulavchenko, A. I.; Kolodin, A. N.; Demidova, M. G.; Podlipskaya, T.; Yu; Maksimovskii, E. A.; Gevko, P. N.; Korol’kov, I. V.; Rakhmanova, M. I.; Larionov, S. V.; Okotrub, A. V. Mechanism of Formation of Cadmium Sulfide Nanoparticles on Polystyrene Supports from Ammonia−Thiourea Solutions. Russ. J. Phys. Chem. A 2016, 90, 827− 832. (17) Romanova, R. G.; Sitnikova, E.; Yu; Berezina, T. N.; Romanov, B. V.; Dresvyannikov, A. F. Micellar synthesis as a promising method of fabricating Nanoparticles with a given morphology. Bulletin of Kazan Technological University [in Russian] 2013, 16, 51−56. (18) Calandra, P.; Longo, A.; Liveri, V. T. Synthesis of Ultra-small ZnS Nanoparticles by Solid−Solid Reaction in the Confined Space of AOT Reversed Micelles. J. Phys. Chem. B 2003, 107, 25−30. (19) Dukhin, S. S.; Deryagin, B. V. Electrophoresis; Nauka: Moscow, 1976; pp 39−42. (20) Bulavchenko, A. I.; Sap’yanik, A. A.; Demidova, M. G. Synthesis and Electrophoretic Concentration of Nanoparticles of CdS in Reversed Micellar Solutions. Russ. J. Phys. Chem. A 2014, 88, 509−514. (21) He, Y.; Wang, P.; Deng, A.-P.; Yang, J.; Huang, Y.-P.; Yang, Y. Preparation of CdS Nanoparticles with Reverse Micelle Method and Photo-degradation of Malachite Green Dye. Wuji Cailiao Xuebao 2010, 25, 1221−1227. (22) Powder Diffraction File, Release 2010; International Centre for Diffraction Data: Newtown Square, PA, USA. (23) Watts, B. E. Solution Synthesis of Chalcogenides. Ph.D. Thesis, CNR IMEM, Parma, Italy, 2011.

Figure 11. Dynamics of dye photodestruction in the presence of CdS particles (1) and in the absence of aggregates (2).

experimental approach consists of the electrophoretic separation of NPs from the reaction mixture to control their size. The indicated method is more tender in comparison to the destruction of microemulsions by more-polar solvents followed by the isolation of NPs as an ultradispersed powder and ultrasonic redispersion in another solvent to prepare samples for TEM. In the case of nonaqueous electrophoresis, a liquid concentrate of NPs is separated from the microemulsion; the size and morphology of NPs remain unchanged in the process. We have developed a simple and clear model for the formation of CdS NPs and predicted the growth dynamics of nanoparticle diameter with the use of spectroscopically determined kinetics constants. The developed methods make it possible to produce stable organosols with CdS NPs of a controllable size. The obtained dispersions of CdS NPs exhibit photocatalytic properties and can be employed as photocatalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aleksei N. Kolodin: 0000-0002-7296-5624 Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Russian Science Foundation (project no. 15-13-00080). ABBREVIATIONS Thio, thiourea; AOT, sodium bis(2-ethylhexyl) sulfosuccinate; PCS, photon-correlation spectroscopy; Vs/V0, solubilization capacity of the micellar solution; NNLS, non-negatively constrained least-squares algorithm; TEM, transmission electron microscopy; PALS, phase analysis of light scattering; CMC, critical micelle concentration; QDs, quantum dots; NPs, nanoparticles

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REFERENCES

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Synthesis and Electrophoretic Concentration of Cadmium Sulfide

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