Stabilization of Ag2S nanoparticles in aqueous solution by MPS

Colloids and Surfaces A: Physicochem. Eng. Aspects 520 (2017) 369–377

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Stabilization of Ag2 S nanoparticles in aqueous solution by MPS Yulia V. Kuznetsova a , Svetlana V. Rempel a,b , Ivan D. Popov a , Evgeny Yu. Gerasimov c , Andrey A. Rempel a,b,∗ a b c

Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620990 Ekaterinburg, Pervomaiskaya Str. 91, Russia Ural Federal University, 620002 Ekaterinburg, Mira Str. 19, Russia Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Lavrentieva 5, Russia

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• We

report a simple one-step synthesis on Ag2 S nanoparticles in an aqueous solution with (3mercaptopropyl)trimethoxysilane (MPS) as the capping molecules. • The concentration of surfactant affects the size and the colloid stability. • A mechanism for the stabilization of Ag2 S nanoparticles in an aqueous solution by MPS was proposed.

a r t i c l e

i n f o

Article history: Received 11 November 2016 Received in revised form 25 January 2017 Accepted 3 February 2017 Available online 6 February 2017 Keywords: Silver sulfide Colloid solution Silanes MPS Surface modification of nanoparticles Stabilization of nanoparticles

a b s t r a c t Silver sulfide (Ag2 S) nanoparticles have been synthesized using an one-pot route which provides the direct metathetical reaction in an aqueous solution at pH = 4 with (3-mercaptopropyl)trimethoxysilane (MPS) as the capping molecule. Exploring of MPS gives rise to control chemical composition of the nanoparticles, nanoparticle size and prevent an agglomeration of the nanoparticles. The MPS-capped Ag2 S nanoparticles synthesized were well dispersed with a particle diameter from 2 to 10 nm. It has been showed that with sulfur in MPS molecule to Ag on nanoparticle surface atomic ratio equal to (1/3. . .3/5):1 the MPS-capped nanoparticles were able to maintain their stability to agglomeration in aqueous solution at room temperature and under laboratory lighting conditions. © 2017 Published by Elsevier B.V.

1. Introduction

∗ Corresponding author at: Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620990 Ekaterinburg, Pervomaiskaya Str. 91, Russia. E-mail addresses: [email protected] (Y.V. Kuznetsova), svetlana [email protected] (S.V. Rempel), [email protected] (I.D. Popov), [email protected] (E.Yu. Gerasimov), [email protected] (A.A. Rempel). http://dx.doi.org/10.1016/j.colsurfa.2017.02.013 0927-7757/© 2017 Published by Elsevier B.V.

In the recent years, nanosized semiconductors have been studied extensively due to their technological importance [1]. Silver sulfide (Ag2 S) nanoparticles have attracted much attention due to their potential applications in photoconductors, solar cells, nearinfrared photo-detectors, etc. (for ref. see review [2]). More impor-

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Fig. 1. Scheme for a typical synthesis of water-soluble MPS-terminated Ag2 S nanoparticles.

tantly, Ag2 S nanoparticles were considered to be much less toxic as compared to other nanoparticles such as PbSe, PbS, and CdHgTe QDs, owing to the absence of toxic metals such as Cd, Pb, and Hg [3–5]. Since the Ag2 S nanoparticles possess so many application advantages, different approaches, such as the hydrothermal route [6,7], the solvothermal method [8], single-source precursor routes [3,9,10], templated synthesis in aqueous surfactant media [11] and in reverse micelles [12] have been developed for synthesis of Ag2 S nanoparticles. Each method has advantages and produces particles with particular properties. However, the synthetic method mentioned above are not always suitable for the synthesis of biocompatible nanoparticles [13]. In this work, we report a simple, efficient, and reproducible route to the synthesis of colloidal Ag2 S capped with (3-mercaptopropyl)trimethoxysilane (MPS) to prevent change of nanoparticles chemical composition, particle growth, and following types of nanoparticles aggregation, coalescence, coagulation, and agglomeration, in aqueous solution without any additional steps usually needed for pre-synthesis of metal precursors. The aim of present study was to investigate the effects of different concentrations of MPS on the synthesis of silver sulfide nanoparticles. Furthermore, Ag2 S nanoparticles capped with biomolecules also provide the opportunity to study interaction between nanoparticles and biomolecules and are promising for applications including biological fluorescence imaging, as shown earlier with CdS nanoparticles [14,15]. 2. Materials and methods 2.1. Chemicals (3-mercaptopropyl)trimethoxysilane (MPS, >95%) HSC3 H6 Si(OCH3 )3 from Sigma Aldrich, ethanol (EtOH), silver nitrate (AgNO3 ) and sodium sulfide (Na2 S) from domestic market were used. All of the chemicals were of analytical grade (A.C.S) and used without further purification. Distilled water was used for the sample preparation. 2.2. Synthesis of Ag2 S nanoparticles The Ag2 S nanoparticles in water solution were synthesized by chemical condensation technique based on an exchange reaction between AgNO3 and Na2 S. The overall molecular equation for the formation of the disperse phase of the Ag2 S colloid solution can be written as follows: 2AgNO3 + Na2 S = Ag2 S ↓ + 2NaNO3 .

Table 1 The optimization of stabilizing agent. Sample

Ag2 S-0

Ag2 S-1

Ag2 S-2

Ag2 S-3

Ag2 S-4

Volume of MPS, ␮l MPS, ␮mol MPS:Ag2 S molar ratio

0 0 0:1

0.425 2.5 0.4:1

0.85 4.4 0.7:1

1.7 9.4 1.5:1

3.4 18.7 3:1

First, 1.25 mM AgNO3 and 0.625 mM Na2 S solutions were prepared by dissolving silver nitrate and sodium sulfide into distilled water, respectively. For a typical synthetic procedure (Fig. 1), AgNO3 and Na2 S solutions were mixed in equal volumes to save a stoihometric Ag to S relation as 2 to 1 under red lightning at ambient temperature (Tat ) with immediate adding of MPS solution in ethanol to prevent the agglomeration and sedimentation of nanoparticles. After sonication in an ultrasonic bath for 10 min to ensure homogeneity and complete the consumption of reagents, the solution became transparent and light brown colored. The red lightning was used to allow working in the darkness that prevent the reduction of silver ions to metal form under visible light. The resulting Ag2 S–MPS solution can be stored without visible changes for up to few months at ambient temperature. In order to evaluate the influence of stabilizing agent in the synthesis of the Ag2 S nanoparticles, the MPS to Ag2 S nanoparticles molar ratio ranging from 0 to 3 was used in the experimental runs. Five reaction baths were used in the experiment. The variation in the concentration of stabilizing agent was done by varying the volume for a given molarity. Thus, various volumes of MPS solution in ethanol were added immediately after AgNO3 and Na2 S solutions were mixed as indicated in Table. 1.

2.3. Dynamic light scattering (DLS) and zetametry The hydrodynamic diameter DH , size distribution and zeta potential of Ag2 S nanoparticles in the solution were measured by dynamic light scattering on Zetasizer Nano ZS (Malvern) at 25 ◦ C. The Nano ZS contains 4 mW He-Ne laser operating at a wavelength of 633 nm. The scattered light is detected at angles of 13 or 173◦ . The samples were placed in standard Malvern zeta potential disposable capillary cells and polystyrene cuvettes for zeta potential and size measurements, respectively. All the measurements were repeated 3 times for a good statistic of the result.

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Fig. 2. HRTEM images of silver sulfide nanoparticles from stable colloid solution (Ag2 S-2 as example) prepared with MPS as capping agent.

2.4. UV–vis–NIR UV–vis–NIR optical characterization of the synthesized Ag2 S–MPS solutions was carried out using Spectrofluorometer FS4 (Edinburgh Instruments) and UV-3600 spectrophotometer (SHIMADZU) at ambient temperature in the wavelength range from 230 to 1250 nm. 2.5. Energy dispersive X-ray spectroscopy (EDX) Chemical composition patterns made by energy dispersive Xray spectroscopy (EDX) were recorded using a JEOL JSM-6390 Scanning Electron Microscope operated at 5–15 kV. Samples were prepared by dropping a dilute Ag2 S-MPS colloidal solution onto a carbon coated holder and allowing it to dry on air. Several places of the sample surface were analyzed for data averaging. 2.6. Transmission electron microscopy (HRTEM) The HRTEM images of the synthesized Ag2 S–MPS solutions were recorded on a JEOL JEM-2010 transmission electron microscope. The colloidal solutions of Ag2 S nanoparticles were dropped on a copper grid and dried on air at room temperature for examination. The identification of phase components was made by Fast Fourier Transformation (FFT) of the HRTEM images. 3. Results 3.1. HRTEM HRTEM has identified the presence of near spherical Ag2 S nanoparticles of 2–10 nm in diameter in the matrix of MPS formed after solution drying on the copper grid (Fig. 2). The essential Ag2 S phase volume is comprised in larger fraction (about 10 nm) that has a chemical composition and structure of monoclinic Ag2 S. The experiment conditions failed to allow obtaining the structure of smallest fraction but one must mention that the existing smallest particles are stable under electron beam irradiation that can be considered as fact that this fraction is not metallic silver in comparing with results of Ag particle formation under irradiation [16,17]. 3.2. EDX EDX was performed to determine the chemical composition of the as-prepared Ag2 S nanoparticles to further confirm the formation of Ag2 S nanoparticles. This verifies the presence of Ag, S, Si in the samples under study. The difficulty of quantitative analysis of

Table 2 Experimental Ag to S and Ag to Si atomic ratios for different MPS:Ag2 S molar ratios. Sample

Ag2 S-0

Ag2 S-1

Ag2 S-2

Ag2 S-3

Ag2 S-4

MPS:Ag2 S molar ratio Ag:S atomic ratio Ag:Si atomic ratio

0:1 2.5 –

0.4:1 1.8 3.1

0.7:1 1.6 1.5

1.5:1 1.2 0.9

3:1 1.1 1.1

the samples due to the inaccurate quantitative determination of light elements such as carbon and oxygen which are also elements of MPS molecule has resulted in calculation of arbitrary atomic ratios Ag:S and Ag:Si for the samples (Table 2). 3.3. DLS Dynamic light scattering (also known as photon correlation spectroscopy (PCS) and quasi-elastic light scattering (QELS)) measures the time-dependent fluctuations in the intensity of scattered light that occurs because the particles are undergoing Brownian motion [18,19]. The velocity of this Brownian motion is measured and called the translational diffusion coefficient D. This diffusion coefficient is converted into hydrodynamic particle diameter DH using the Stokes-Einstein equation. Particle sizing was chosen as a quality response parameter on MPS concentration changing. The stability of sols to agglomeration is largely controlled by two factors: steric effects and electrostatic effects. Both of these factors can be used to control the distance of closest approach of neighboring particles. The zeta potential of nanoparticles is representative of the apparent charge at the slipping plane or the particle interaction boundary, and is routinely used to monitor and predict stability at different synthesis conditions. Fig. 3 shows the change of hydrodynamic size and size distribution along with zeta potential of Ag2 S nanoparticles in aqueous solutions depending on initial MPS to nanoparticle molar ratio. To examine the time stability of the Ag-containing nanoparticles, we left the MPS-capped Ag2 S nanoparticles in aqueous solutions at room temperature and under regular daylight conditions. The sample Ag2 S-0 synthesized without a stabilizer demonstrated a turbidity increasing with time that is the evidence of large agglomerates presence. A dark brown precipitate appeared within an hour. Over time, the initial stable suspensions have demonstrated no significant changes of average DH and zeta-potential (Fig. 4). These results also have indicated that the MPS-capped nanoparticles in samples Ag2 S-1 (1:0.4), Ag2 S-2 (1:0.7) and Ag2 S-3 (1:1.5) had more positive charges (from plus 20 to plus 30 mV) on the surface, which helped disperse the QDs better and ensure the stability.

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1500

DH, nm

1000

100

40 30

80

20

DH, nm

Zeta-potential, mV

372

10 0

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

20

100 80 60 40 20

8

0.4

0.8

1.2

1.6

2.0

2.4

Fig. 3. Hydrodynamic particle size DH and zeta-potential (inset) of MPS stabilized Ag2 S nanoparticles in the aqueous solution depending on MPS/Ag2 S molar ratio.

DH, nm

16

20

24

28

32

36

40

44

48

52

Fig. 5. Stability of average DH of Ag2 S-2 nanoparticles covered by MPS in aqueous solution with heating (filled symbol) and cooling (open symbol).

2.8

MPS : Ag2S

Zeta-potential, mV

12

Temperature, °C 0.0

700 600 500 400 300

Ag : MPS 1:3 1 : 1.5 1 : 0.7 1 : 0.4

120 100 80 60 40 20 0 0

1

2

3

4

5

Time, days

Ag : MPS 1 : 0.4 1 : 0.7 1 : 1.5 1:3

40 30

10 0 0

3

6

9

12

15

out appearing of sediment that is the evidence of deagglomeration and of system stabilization. The average value of zeta-potential increased about twice from 12 to 25 mV within 3 days and then left constant for about 20 days. The average DH of nanoparticles in the sample Ag2 S-2 was about 40 nm. Meanwhile, the solution was stable within the observation time having the zeta potential of about plus 28 mV. The sample Ag2 S-3 demonstrated the average DH of nanoparticles about 15 nm after the synthesis. Minimum size determined for the sample could be related to changing of the ionic strength of solution due to increasing of MPS concentration that influence on the DLS results. The average zeta potential increased from plus 14 to plus 20 mV and remained unchanged within two weeks. The sample Ag2 S-4 with highest MPS concentration has showed the average DH about 400 nm after synthesis with tendency of decreasing and with minimum at 40 nm that is adjusted with the sedimentation observed. The excess of MPS made the stabilization worse. A number of experiments were performed to study the temperature effect on the stability of the colloid solution of Ag2 S-2 nanoparticles at storage (Fig. 4). The temperature was varied in the range of 10–50 ◦ C at a step of 2.5 ◦ C by heating and cooling from 25 ◦ C (Fig. 5). 3.4. Optical absorption

20

b)

40

MPS : Ag2S

500

a)

60

18

Time, days

Fig. 4. The time dependence and stability of average diameter DH (a) and zetapotential (b) of Ag2 S nanoparticles in aqueous solution covered by the different quantity of MPS.

Fig. 6 presents the UV–vis–NIR optical absorption spectrum of the colloidal solution containing Ag2 S nanoparticles covered by MPS. Regardless of the MPS concentration, the colloidal Ag2 S absorption spectra in the 230–1250 nm region look the same and are represented by curves monotonously rising towards the shortwave side. The intensive increase of absorption starting about 400 nm and reaching maximum at 245 nm was detected for all samples. The increase of MPS concentration up to reaching the MPS to Ag2 S ratio of 1.5 leads to increasing of optical absorption at 245 nm while further addition of MPS fails to influence on this. 4. Discussion

The average DH of nanoparticles in the sample Ag2 S-0 without stabilizer after synthesis was about 1 ␮m in diameter. The turbidity sample was unstable and precipitating that makes time measurements impossible. The zeta potential was near 0 mV that agrees with the colloid solution instability. The sample Ag2 S-1 showed after the synthesis the presence of two fractions having average 100 nm and 1 ␮m in diameter, respectively. It could be related to dynamic stabilization proses accompanying by nanoparticle size changing due to the lack of stabilizer. In a day, the signal of large particles disappeared with-

4.1. Synthesis of MPS-caped Ag2 S nanoparticles in aqueous solution MPS monolayers are formed through metal-thiolate bonding of the S of MPS molecule with Ag atom on nanoparticle surface, similar to other alkanethiol chemisorption chemistries [20–22]. MPS molecule consists of two reactive functional groups. The thiol tail (-SH) is able to form a strong covalent bond to a variety of metals such as Ag, Au, Pt, Cu [for reference, see [22]] and is responsible

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Fig. 6. The optical absorption of aqueous solution contained Ag2 S nanoparticles covered by MPS (sample Ag2 S-1 as example). Inset shows the difference of spectra in the 230–400 nm region depending on the MPS to Ag2 S molar ratio: 0.4 (1), 0.7 (2), 1.5 and 3 (3). Gray line presents the optical absorption of MPS water/ethanol solution that indicates the transmittance in UV region as well as the initial solutions.

for protecting the silver sulfide nanoparticles against agglomeration [23–25]. The methoxy headgroup (-Si(OCH3 )3 ) presented at the other extreme of the molecule should be responsible for a stabilization in aqueous media and further functionalization [26–29]. The three methoxy (–OCH3 ) groups of MPS are not soluble in water. MPS starts to become water-soluble when hydrolyzing them to hydroxyl (–OH) groups [30,31]. Thus, firstly, MPS was dissolved in ethanol to suppress early hydrolysis and condensation before the synthesis start along with further increase of the solubility in water. The hydrolysis to form –OH groups also releases methanol (CH3 OH), which acts as a co-solvent to dissolve MPS. Through the controlled hydrolysis of MPS the hydrolyzed MPS is soluble in water and thus easily approaches the surface of the Ag2 S nanoparticle to form strong metal–thiol bonds [32,33]. In addition, the sonication of reaction mixture promotes a uniform distribution of MPS molecules in the solution volume to provide an equal chemisorption on all of the particles. When the −SH groups are bound to the metal surface the −Si(OH)3 groups of the surface-adsorbed hydrolyzed MPS are arranged outward from the surface for the easy stabilization in aqueous solution [32]. In the course of preliminary experiments, it was found that pre-mixing of MPS and AgNO3 before addition of Na2 S does not allow to obtain a stable colloidal solution. Perhaps, this is due to the features of covalent bonds formed between MPS and Ag ions unlike the coordination complexes of cations with sodium citrate (Na3 C6 H5 O7 ) and disodium salt of EDTA (C10 H14 N2 O8 Na2 ) (for example, [34–36]). Furthermore, the addition of MPS after mixing the initial solutions of AgNO3 and Na2 S should eliminate the formation of intermediate Ag complexes and the ability of silver ions reduction to metal form. Thus, first the formation of a water insoluble Ag2 S phase is occurred, and then the stabilization of colloid nanoparticles by forming the MPS shell on the surface is possible. 4.2. Binding of MPS by Ag ions on the Ag2 S nanoparticle surface In the presence of MPS the atomic ratio (Table 2) of Ag:S calculated from the experimental data was in the range from 1 to 2 which is consistent with the assumption that there is an increase in a sulfur content near the surface with respect to silver atoms by attaching of MPS molecules (having sulfur atom in the chain)

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to the nanoparticle surface. The sample without MPS according to EDX data showed the excess of silver ions as compared with initial stoichiometric composition of Ag2 S that may be due to the volatilization of the sulfur in the synthesis process. At the same time, HRTEM has confirmed the formation of Ag2 S nanoparticles. In addition, the presence of additional phases and structures as Ag particle and Ag2 S/Ag heterostructures [17,37–39] or heterodimers [40–43] were not detected in the samples studied. Probably it is connected with a specific stabilizer used and the varying only the stabilizer concentration not the stoichiometric composition of Ag2 S particles synthesized. In comparison with [43] which studies the formation of Ag2 S/Ag heterodimers in the presence of hexamethyldisilazane being not only the capping agent but the reducing one due to the formation of intermediate polymer S-N and S4 N4 compounds [44], there is no evidence of the similar role of MPS molecules used in present work. This confirms the idea that synthesis conditions studied allow to obtain Ag2 S nanoparticles and prevent the reduction of silver ions and formation of Ag particles and heterostructures. To analyze the changes in atomic ratio Ag:Si with the increase of MPS concentration it should be considered that the EDX provides information from the sample surface, and the MPS attaches the surface of nanoparticle through covalent bond S-Ag with surface Ag ions. Thus, the number of MPS on the surface and the atomic ratio of Ag:Si will be determined by not only the concentration of MPS, but also the size of the nanoparticles. The atomic ratio Ag:Si calculated from the experimental data indicated that the presence of MPS into the reaction mixture in a molar ratio of MPS:Ag2 S = 0.4, 0.7, 1.7, and 3 lead to the MPS molecules binding with 1/3, 3/5, 4/5 and all of surface silver atoms, respectively. Since MPS is attached through the connection R-Si-S-Ag-NP (NP surface) it can be assumed that the atomic ratio always satisfies Ag:Si ≥ 1, and MPS excess remains in solution participating in reactions of (poly)condensation [45]. This assumption is confirmed by the experimental results that demonstrated the least stability, turbidity and precipitation of Ag2 S-3 and Ag2 S-4 solutions with time. It should also be noted that the stable solution samples with a molar fraction of MPS of 0.4 and 0.7 possessed characteristic excess of S with respect to Si that may indicate that, firstly, not all of the surface Ag atoms are associated with MPS molecules, and, secondly, the surface layer of nanoparticle contains both silver and sulfur atoms. Tendency of atomic ratio S:Si to 1 with the MPS content increase can be attributed to the fact that full covering of nanoparticle surface by MPS increased the density of these molecules thus leading to intensive (poly)condensation and formation of siloxane layer on the surface. 4.3. Influence of MPS concentration on hydrodynamic size and size distribution When lower concentrations of stabilizing agent were used, the nanoparticles kept growing until the total surface area decreased sufficiently to be well covered by the limited number of MPS molecules. This extended growth process resulted in silver sulfide nanoparticles with larger sizes and a broader size distribution (sample Ag2 S-1) [23]. When higher concentrations of the MPS were used in the system, the growth time of the silver sulfide nanoparticles was reduced. The larger number of MPS molecules cover the larger surface area of the nanoparticles. These covered areas worked as diffusion barriers for the addition of atoms, and also hindered the agglomeration of the growing nanoparticles by steric effects [46]. From the other side, small amounts of MPS lead to full stabilizer adsorption along with not dense placement of molecules on the nanoparticle surface with some distance d1 between adsorbed molecules (Fig. 7, Scheme 1). This decreases the probability of (poly)condensation of adsorbed MPS molecules with

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Fig. 7. Schemes for a typical hydrolysis and condensation reactions of MPS molecules in the solution containing MPS-terminated Ag2 S nanoparticles.

free or neighbor ones after the hydrolysis thus saving the stability (Fig. 7, Scheme 1). From Figs. 3 and 4a one can see that the average DH decreases to 15 nm with an increasing MPS to Ag2 S molar ratio up to 1.5. Conceivably, with a higher MPS molar ratio, more MPS molecules would bind to the Ag ions at the nanoparticle surface, which would impede the growth of the nanoparticles to result in a smaller size. Therefore, Ag2 S nanoparticles with smaller and less dispersed sizes were produced (sample Ag2 S-3). On the other hand, an increasing MPS to Ag2S molar ratio up to 3 (Ag2S-4) fails to continue the trend of nanoparticle size decrease according to the DLS that can be associated with significant effect of MPS concentration, firstly, on the stabilization process, and secondly, on the average DH measurement result. Thus, the further increase of MPS concentration in within certain limits (Ag2S-4) is undesirable due to shortening of the distance (d2) between adsorbed MPS molecules and thus intensification of the (poly)condensation processes which are likely lead to formation of polysylaxane bonds with other free MPS molecules and MPS-caped particles (Fig. 7, Scheme 2, 3) that in the end is able to increase the hydrodynamic diameter and decrease the stability. Meanwhile, according to the study of Peng et al. [47], in the presence of both lighting and oxygen, the thiol groups of

two adjacent capping molecules can react to form a disulfide bond and dissociate from the particle surface, leading to the aggregation of nanoparticles (Fig. 7, Scheme 4). Although this disulfide reaction could happen, the cross linking reaction of the silanes of the MPS could form a network wrapping the nanoparticles, minimizing the dissociation effect due to the disulfide formation. The silica network could also serve as a diffusion barrier to keep oxygen from reaching the nanoparticle surface and consequently suppress the disulfide reaction [48]. In addition, the small concentrations of reagents could decrease the probability of particle interaction during the Brownian motion. The stability of the colloidal Ag2 S nanoparticles produced with different concentrations of MPS was evaluated by measuring the zeta potential. The samples of solutions were analyzed within about 20 days after the reactions to ensure that the colloidal systems reached their stability condition. According to the results shown in Fig. 3 and 4b, the zeta potential value close to the isoelectric point (i.e., 0) for Ag2 S-4 demonstrated that the colloidal nanoparticles produced with the highest concentration of the stabilizing agent (molar ratio MPS:Ag2 S = 3) was unstable. This result is related to intensive hydrolysis and (poly)condensation processes. The good results of the zeta potential of samples Ag2 S-3 and Ag2 S-2 con-

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Fig. 8. Dependences (␣Ehv )2 −Ehv and (␣Ehv )1/2 −Ehv for aqueous colloid solution contained Ag2 S nanoparticles covered by MPS with MPS/Ag2 S molar ratio 0.4 for band gap calculating of direct (left y axis) and indirect optical transition (right y axis).

firmed the good stability of these colloid solutions starting from the moment of their production. Small value of Ag2 S-1 zeta potential about plus 12 mV after the synthesis is related to the insufficient amount of MPS used in the reaction to properly cover and protect the silver nanoparticles against agglomeration at the first moment. Along with this, the stability is reached in three days with the maximum zeta potential of plus 25 mV. According to the results of heating and cooling of stable solution the mean size of MPS–caped Ag2 S nanoparticles in solution did not significantly change when the temperature was varied from 10 to 50 ◦ C. This indicates that the colloid solution is stable at above mentioned temperature interval. So, chemical bonding of MPS to Ag2 S nanoparticle is really strong and no intensive formation of net between MPS-caped nanopartilces takes place. 4.4. Optical properties of Ag2 S nanoparticles The lack of a strong absorption onset characteristic, e.g. edge of the absorption band and sharp exciton absorption peaks on spectral curves for a semiconductor system complicates the extraction of the information about photophysical properties of Ag2 S nanoparticles or their electronic structure directly from the absorption spectra. Consequently, an analysis of the absorption spectra was carried out to elucidate the nature of the primary photoprocesses using current concepts of semiconductor physics developed for massive materials [49]. According to the theory of optical transitions in semiconductors light absorption can lead both to direct and indirect interband electronic transitions, while the last one occurring with the participation of phonons. The linearity of spectral curve in (␣Ehv )2 −Ehv and (␣Ehv )1/2 −Ehv coordinates according to Tauc theory [50] indicates the direct and indirect optical transition, respectively. This principle was used to determine the type of transitions and their energies (Fig. 8) by analyzing the absorption spectra of the Ag2S colloidal solutions. The figure shows that silver sulfide nanoparticles have both direct and indirect interband transitions. Consequently, these nanoparticles are indirect semiconductors with direct transitions in the high energy spectral region that is in accordance with the earlier investigation of [51–53]. For the measurement performed we estimate an indirect band gap energy of 1.81 eV that is greater than the width of the forbidden zone of bulk Ag2 S (1240 nm, Eg = 0.9 eV) [51,54,55]. The high energy indirect transition attests the nanoparticles are character-

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ized by quantum size effects. In accordance with the calibration curve proposed in [53] for dependence of Ag2S nanoparticle diameter on the indirect band gap energy, it is possible to estimate an average size of nanoparticles equal to about 7 nm that coincides with HRTEM results prepared within the present study. Apart from the indirect transition in nanoparticles with dimensions close to the average, which usually predominate in colloidal solutions, analysis of the absorption spectra shows the presence other indirect transition with lower energy about 0.9 eV. We suggest that these transitions occur in Ag2 S particles of an extreme fraction of larger size. The absence of detectable fluorescence at room temperature would seem to be consistent with an indirect band gap transition in these Ag2 S nanoparticles along with very low concentration [51]. From analysis of absorption in the UV region, it was established that there is a direct interband electron transition about 3.4 eV. Apart from this transition, analysis of the spectra showed the presence of transitions with energy more than 3.6 eV which could occur with nanoparticles smaller than the average ones. HRTEM results confirm the distribution of nanoparticles in the sample from 2 to 10 nm due to the nucleation and growth process are possibly not well separated in time. The results obtained within the study are in accordance with the data received earlier by Kryukov et al. [52,56] within the research of stabilizer effect on the optical properties of Ag2 S nanoparticles and also supplement its. Thus, the exploring of MPS as capping agent allows to synthesis Ag2 S nanoparticles of Eg ∼ 1.8 eV that exceeds the values of nanoparticles stabilized by PVA or gelatin and, in this way, the nanoparticles of smaller size. In addition, an energy gap between the direct and indirect transitions in Ag2 S particles of average size is about 1.6 eV, coincides with results in [56] and thus confirms the idea that these transitions of both types correspond to the same nanoparticles with size close to the average. The intensive increase of absorption (Fig. 6) starting about 400 nm and reaching maximum at 245 nm is related to silverthiolate complexes on the Ag2 S nanoparticle surface [45,57,58]. Increasing of the MPS concentration leads to the increase of optical absorption at 245 nm to a certain value until it becomes constant with MPS/Ag2 S molar ratio more than 1.5. In comparison with [45] the absence of pronounced band pattern like at mercaptoethanol (C2 H6 OS)-silver complexes having picks at 368, 306, 266 is probably related to the fact that the silver-thiolate complexes with MPS have the side chains held the charge. The electrons fail to move effectively through the sulfur-silver chain and thus fail to give rise to electron distributions corresponding to well defined band patterns. Thus, the presence of charges on the side chains is likely to disturb this electron distribution and thus make the band pattern less evident. The absorbance spectra of the silver nanoparticles immersed in a solution were measured at 1, 7 and 20 days after the reactions to evaluate the stability of the produced colloidal solution (results not shown). The absorbance spectral measurement of these samples in the range from 500 to 1250 nm remained unchanged for a week, indicating that the Ag2 S nanoparticles proper reached a stable state. However, the contribution of silver-thiolate complexes on the Ag2 S nanoparticle surface decreased with time indicating the completion of MPS to surface bonding and stabilizing. Thus, in some cases it could lead to (poly)condensation possesses on the surface and transformation silver-thiolate complexes to Ag2 S-like complexes [21,59–61]. After 20 days the decrease of absorption of Ag2 S-3 and Ag2 S-4 in all wave range was detected. The decrease of the absorption can be related to an excessive growth of some nanoparticles followed by their precipitation that is confirmed by sediments in the solutions.

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5. Conclusion The synthesis method of Ag2 S nanoparticle of 2–10 nm in diameter in aqueous media was suggested using MPS molecules as stabilizing agent. The terms of colloid solution stabilization by placing layer of MPS molecules on Ag2 S nanoparticle surface were determined. It is found that the attaching of MPS molecules to the 1/3 part of silver atoms at the Ag2 S nanoparticle surface is insufficient for long-term stabilization of the nanoparticles due to deficient reduction of excess energy at the interface nanoparticlewater medium that leads to coagulation and agglomeration. From the other side the excess of MPS also fails to improve the stability of the nanoparticles in aqueous solution. Thus, the best can be considered as forming bonds between sulfur in MPS and 3/5 . . . 4/5 of silver atoms on the Ag2 S nanoparticle surface, thus achieving the steric and electrostatic stability of nanoparticles in the colloidal solution. Ag2 S nanoparticles capped with MPS by bonding of 3/5 Ag atoms on the surface were more positively charged and better dispersed due to the silanol functionality. The increase of MPS on the surface up to 4/5 could impede the growth of the nanoparticles to result in a smaller size. Therefore, in present study not only new method of synthesis of a stable colloidal solution of Ag2 S nanoparticles has been suggested but also new concept of nanoparticle size regulation has been developed. The method and the concept exceeds the findings made earlier by other workers [29,32,51,61,62]. In future works further specifying of the stabilization mechanism would allow to set the condition of synthesis of non-heavy metal Ag2 S nanoparticles with high luminescence and usable for biology applications. In addition, further functionalization of MPS capped Ag2 S nanoparticles with biomolecules also will provide the opportunity to study interaction between them, that is promising for applications including biological fluorescence imaging.

Acknowledgment This work was financially supported by the Russian Science Foundation (Grant 14-23-00025) through the Institute of Solid State Chemistry of the UB of the RAS.

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