S₄N₄ as an intermediate in Ag₂S nanoparticle synthesis

Abstract

In the present work, hexamethyldisilazane assisted synthesis of Ag₂S nanoparticles is demonstrated. Classical chemical investigations and nano investigations were utilized to explain the formation of nanoparticles. Controlled reactions were performed to explain the reaction mechanism. While establishing the reaction mechanism, the formation of Ag nanoparticles and tetrasulfur tetranitride (S₄N₄) was identified. Observations of a controlled reaction explained that sulfur reduction occurred through a S–N polymeric intermediate. Then, the polymeric intermediate was decomposed to tetrasulfur tetranitride. The formation of S₄N₄ was unambiguously confirmed by single crystal X-ray diffraction measurements. The formation of S₄N₄ also confirmed the role of hexamethyldisilazane as a reductant. Obtained nanoparticles were characterised by PXRD, EDAX, FTIR, FESEM, TEM, HRTEM and UV measurements.

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Introduction

Chemical syntheses of nanoparticles (NPs) are attractive because of the ability to manipulate the properties of NPs by simple modification of reagents, capping agents and synthetic conditions.^1–6 To achieve a generalized synthetic approach for any material, a thorough understanding regarding the nature of intermediate species involved in the synthetic procedure is required. Even though potential benefits of colloidal synthesis are well explored,^7–12 establishing a generalized procedure for the synthesis of NPs has hardly been realized. The reason may be the lack of understanding of the chemistry behind the formation of NPs. This situation has made colloidal synthesis a combinatorial play of starting materials and capping agents rather than a scientific approach.

Ag₂S is a typical functional material with a band gap of 1.1 eV.^13a Applications such as solar cell,^13b photocatalysis,^13c water splitting,^13d nanoprobes,^13e hydrogen production,^13f transistors,^13g bioimaging,^13h have been demonstrated with Ag₂S nanoparticles. Wet chemical synthesis of Ag₂S nanoparticles is preferred because of its tunability and favourable manipulation of required properties. Various synthetic procedures were established to synthesise high quality Ag₂S nanoparticles, and made Ag₂S a competitive functional material among other chalcogenides.¹⁴

In the present work, Ag₂S NPs were synthesized by a hexamethyldisilazane assisted synthetic method.^15,16 Nano characterizations revealed that Ag₂S NPs were synthesized with a bulk crystal structure. While elucidating the reaction pathway of Ag₂S formation, we identified Ag NPs and two intermediates viz.; S–N polymer and S₄N₄. The polymeric intermediate was earlier evidenced using GPC (M_w = 2908 and M_n = 2684; PDI = 1.08) and NMR data^15,16 and now confirmed by HRMS. Isolated red-orange crystalline S₄N₄ was confirmed by single crystal X-ray diffraction study. The S–N polymer might have decomposed to yield S₄N₄. Isolation and characterization of two intermediates from this reaction concluded the steps involved in the hexamethyldisilazane assisted synthesis of metal sulfides.

Results and discussion

Ag₂S synthesis

Ag₂S NPs were synthesized using a stoichiometric ratio of AgNO₃ : S = 2 : 1 in hexamethyldisilazane (Scheme S1, ESI†). Formation of Ag₂S was primarily confirmed by PXRD measurements (Fig. 1a). Obtained patterns were matched with the standard bulk phase Ag₂S monoclinic structure (JCPDS # 14-0072). Relative intensities of peaks and positions were matched with the standard pattern. The pattern clearly explained the phase purity of Ag₂S and the absence of other stoichiometric ratios. Peaks were broadened and merged with adjacent peaks, which can be attributed to the smaller size of the synthesised NPs. The contribution of organic moieties to line broadening can be excluded due to the absence of HMDS in the EDAX spectrum (Fig. 1b). The EDAX spectrum confirmed the atomic ratio Ag : S = 2 : 1 of the samples. This ratio was very uniform throughout the analytical sample. Since there was no signal of oxygen (for AgNO₃), silicon (HMDS) or carbonaceous materials, the phase purity of Ag₂S NPs was self-evident. Furthermore, the non-appearance of silicon in the EDAX spectrum clearly revealed the absence of HMDS (capping agent) on the surfaces of the NPs. An FTIR spectrum of Ag₂S NPs did not show any signature of amines or organic moieties (Fig. S1, ESI†). This observation clearly indicated that the surface of NPs was clean and with no capping agent on the surface.

Fig. 1 (a) PXRD pattern of synthesised Ag₂S nanoparticles. Obtained pattern was plotted with a standard pattern for visual comparison. Vertical lines represent standard monoclinic Ag₂S diffraction pattern from JCPDS library. Peaks were broadened due to smaller size of nanoparticles. (b) EDAX spectrum of Ag₂S nanoparticles: Peaks are labelled according to energy values. Relative atomic ratio of Ag and S is 2 and 1, respectively. Absence of Silicon signal in the spectrum indicates complete removal of capping agent (HMDS).

Microscopic studies

The morphology of Ag₂S NPs determined by scanning electron microscopy (Fig. S2, ESI†) showed a spherical nature. Because of the limitation of FESEM resolution, NPs appeared as agglomerated with each other. However, the particles were well separated as seen by TEM. The particle abundance was high and the distribution of particles was observed even at the micron level. The particle distribution, crystallinity and phase of Ag₂S nanocrystals have been determined by transmission electron microscopy (Fig. 2).

Fig. 2 (a) Representative TEM micrograph of Ag₂S NPs. Inset: HRTEM image of dominant (1 0 3) plane. (b) SAED pattern of a few NPs. (c) Particles distribution diagram. Particles ranged from 4 to 19 nm with an average diameter of 10 ± 2 nm. (d) High resolution TEM image of single Ag₂S nanocrystal. Clear lattice points confirmed the crystalline nature of NPs. (e) Corresponding FFT image of the nanocrystal.

TEM analysis confirmed the spherical shape of NPs (Fig. 2a). NPs were monodispersed and abundant. Particle size ranged from 4 to 19 nm with an average size of 10 ± 2 nm (Fig. 2c). The uniformity in size may be attributed to the excess availability of hexamethyldisilazane (capping agent) in the reaction. Selected area electron diffraction (SAED) analysis of NPs showed a clear dotted pattern (Fig. 2b). This observation clearly revealed the single crystalline nature of the obtained NPs. The obtained planes of the SAED pattern matched with the PXRD pattern of NPs. For a detailed analysis, a single NP was focused on to the atomic level and the corresponding two-dimensional fast Fourier transformations (FFT) were obtained (Fig. 2d and e). HRTEM analysis of NPs showed an interplanar distance of 2.04 Å, which indicates that one of the dominant d_{(1 0 3)} planes of silver(I) sulfide is in PXRD pattern. The UV-vis spectra of the nano-particles showed characteristic broad absorption of Ag₂S (Fig. S3, ESI†).¹⁷

Controlled reactions

In numerous reactions involving sulfur and nitrogen, despite isolating pure products, the nature of intermediate steps was unclear. In our earlier reports on hexamethyldisilazane assisted syntheses of metal sulfide NPs, isolation of an S–N polymer as intermediate species was described.^15,16 However, the nature of reduced sulfur was not clear. To comprehend the reaction pathway of Ag₂S formation, four controlled reactions were conducted (Table 1). While the reaction of AgNO₃ with S (reaction 1) did not form any product, the reaction with HMDS (reaction 2) interestingly yielded pure phase Ag NPs. This reaction also explained the twin roles of HMDS as reducing and capping agent. The reaction between HMDS and S yielding smelly S–N polymer (reaction 3) was well-established earlier using GPC and ²⁹Si NMR spectral data.^15,16 In the present study, HRMS clearly showed repeating units of SNSi (∼74.0202) polymer chains (Fig. 5 and S4, ESI†). Since sulfur was soluble in HMDS, it interacted with HMDS yielding an S–N polymer. This polymer was formed in the absence of any metal ions (In³⁺, Cu²⁺ and Ag⁺). However its formation was faster in the presence of metal ions.^15,16 This observation indicated catalysis or facilitation by metal ions for S–N polymer formation.

Table 1 Details of controlled reactions performed to establish mechanism

S. No	Reactions	Products
1	AgNO3 + S	No reaction
2	AgNO3 + HMDS	Ag nanoparticles
3	S + HMDS
4	2AgNO3 + S + HMDS	Ag2S nanoparticles + S4N4

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Ag nanoparticles

Formation of Ag NPs (reaction 2) was primarily confirmed by PXRD measurements (Fig. 3a). The obtained pattern showed a bulk phase Ag cubic structure (JCPDS # 04-0783). The relative intensities and positions of peaks matched with standard diffraction patterns. Peaks were broadened due to the smaller size of NPs. The contribution of organic moieties in broadening can be excluded due to the absence of HMDS in EDAX spectra. The EDAX spectrum confirmed the purity of silver NPs (Fig. 3b). Silver NPs were very small and were suitably followed by TEM analysis (Fig. 4a).

Fig. 3 (a) PXRD pattern of the obtained Ag nanoparticles. The pattern was plotted with a standard pattern for visual comparison. Vertical lines represent standard cubic Ag diffraction pattern from JCPDS library. (b) EDAX spectrum of Ag nanoparticles. Peaks are labelled according to energy values (inset: silver deposition on beaker).

Fig. 4 (a) Large area TEM Micrograph of Ag NPs. (b) SAED pattern of a few NPs and dominant (1 1 1) plane shown. (c) Particles distribution diagram. The particles were smaller and ranged from 29 to 75 nm with an average size of 49 ± 8 nm. (d) High resolution TEM image of single Ag nanocrystal. Clear lattice points confirmed crystalline nature of NPs. (e) Corresponding FFT of the nanocrystal and representative (2 0 0) plane were indexed.

TEM images showed that the Ag NPs were spherical and their abundance was high (Fig. 4a). Since Ag NPs were very small, agglomeration of NPs was observed. The size of NPs ranged from 29 to 75 nm with an average size of 49 ± 8 nm (Fig. 4c). Some of the smaller sized nanoparticles were also observed in TEM micrographs. The selected area electron diffraction (SAED) analysis of NPs revealed a cluttered spot pattern (Fig. 4b). This may be due to smaller size and wide distribution among NPs. The obtained planes of the SAED pattern matched with the obtained PXRD pattern. To further clarify the orientation, a single NP was focused on to atomic level and their corresponding two-dimensional fast Fourier transformations (FFT) were obtained (Fig. 4d and e). All the interplanar distances matched with the obtained PXRD pattern. Particles were highly crystalline and no amorphous layer of capping agent was observed on the surface. This observation clearly showed complete removal of the capping agent from the surface of NPs.

S₄N₄ isolation

The reaction of stoichiometric amounts of AgNO₃ and sulfur in HMDS (reaction 4) led to phase pure Ag₂S NP formation. From this reaction tetrasulfur tetranitride (S₄N₄) was also isolated (Fig. 5 inset) in a small quantity. S₄N₄ was unambiguously confirmed by single crystal X-ray diffraction analysis of crystals obtained from acetonitrile. Crystal structure (Fig. S5, ESI†) (CCDC 961232) reconfirmed the beautiful cradle structure of S₄N₄. The TGA/DTA analysis (Fig. S6, ESI†) of the crystals showed that it was decomposing at near melting temperature (178.3 °C), thus reconfirming S₄N₄ formation. S₄N₄ yield was good in higher sulfur ratios (AgNO₃ : S = 2 : 2), and the orange red color of the reaction solution was very clear (Fig. S7, ESI†) (caution: S₄N₄ is an explosive and requires care at higher stoichiometry).

Fig. 5 Isolated intermediates during Ag₂S formation. HRMS data of polymeric intermediate with repeating SNSi unit. Inset: ORTEP diagram of the isolated S₄N₄.

Reaction mechanism

An important finding from the controlled reactions was that no reaction occurred between AgNO₃ and sulfur in the absence of HMDS (reaction 1). Reaction 1 revealed that hexamethyldisilazane acted as reducing agent. No S₄N₄ formation was observed in the absence of AgNO₃ in the reaction (reaction 3). Hence, S₄N₄ was not formed by direct reaction between S and HMDS. Reaction 2 clearly explained that S₄N₄ formation was synergistic with Ag₂S formation in HMDS. Controlled reaction conditions also revealed the possibility for two parallel reactions viz. the formation of Ag NPs and the formation of Ag₂S NPs. In the absence of sulfur, Ag NPs were formed. In the presence of sulfur, phase pure Ag₂S NPs were formed. Moreover, no traces of Ag NPs were observed in any analysis of Ag₂S nanoparticles. Therefore, sulfur reduction and Ag₂S formation in reaction 4 was ensued through three steps. Sulfur was activated by HMDS through S–N polymer formation. The polymer was decomposed to S₄N₄. It is worth mentioning here that S₄N₄ is one of the stable products among smaller S–N cyclic systems¹⁸ and is well-known as a sulfide source in metal sulfide synthesis.¹⁹ Then, the S₄N₄ reacted with Ag⁺ ions and promoted the formation of Ag₂S NPs. This can be attributed to the higher activity of sulfide ions towards a metallic Ag⁺ source.

Conclusions

In summary, a complete reaction mechanism of Ag₂S NPs formation was elucidated. Clearly, the hexamethyldisilazane assisted synthesis followed a different reaction pathway compared with the conventional sulfur-amine “black box” system.^20,21 Black box systems were explained based on the generation of H₂S gas from a sulfur-amine system. However, in our case, the sulfur-amine (hexamethyldisilazane) reaction clearly led to S–N polymer and S₄N₄. Results indicated that the present system could be an ideal system for predicting the first principles of nanochemistry and establish generalized nanoparticle synthesis.

Experimental section

All the chemicals used in the syntheses were purchased from Aldrich. Standard air-free conditions were maintained throughout the reactions. AgNO₃ was chosen for HMDS assisted method due to its better solubility and photo stability than silver halides.

Synthesis of Ag₂S nanoparticles

A reaction flask (two-necked, round-bottomed flask) containing AgNO₃ (0.2 g, 1.18 mmol) and sulfur (0.02 g, 0.62 mmol) was degassed and flushed with dry-deoxygenated N₂. Then, HMDS (5 ml, 23.98 mmol; 99.9%, used as received) was introduced into the flask. After purging with nitrogen gas, the reaction mixture was heated to reflux. Even though the formation of Ag₂S was instantaneous, the reaction was extended for 1 h for uniform distribution. A black precipitate of Ag₂S was obtained at the end of the reaction. Volatile side-products and unreacted HMDS were decanted and the residue was washed with methanol to remove S₄N₄ traces. Then, the black residue was washed with methanol (3 × 20 ml) followed by toluene (3 × 20 ml) to remove unreacted AgNO₃ and sulfur, respectively. The product was purified at ambient conditions and then dried at 120 °C for 4 h before analysis. The nanoparticles are stable in dark conditions and were stored in powder form. S₄N₄ and polymer were isolated from decanted reaction mass. S₄N₄ was extracted from dried filtrate using acetonitrile. Caution: S₄N₄ is an explosive and would explode in dry conditions. S₄N₄ yield was good in higher sulfur ratios (Ag : S = 2 : 2) and isolated comfortably. Polymer was isolated from distilled filtrate.

Synthesis of Ag NPs

Ag nanoparticles were obtained in the absence of a sulfur source in the reaction. In a reaction flask containing AgNO₃ (0.20 g, 1.18 mmol), HMDS (5 ml, 23.98 mmol; 99.9%, used as received) was introduced. After purging with nitrogen gas, the reaction mixture was heated to reflux. The reflux reaction was carried out for 24 h for a uniform distribution of NPs. Caution: AgNO₃ should not be heated without sufficient capping agent. Shiny black precipitate was obtained at the end of reaction. Volatile side-products and unreacted HMDS were removed under vacuum. The precipitate was continuously washed with methanol (3 × 20 ml) and a shiny silver mirror was obtained on the walls of the beaker. The product was purified under dark conditions and then dried at 120 °C for 4 h before analysis. Ag NPs were very reactive towards light and transformed to white grey if exposed to light. Most of the microscopic analyses were carried out in the crude form. All the microscopic measurements were immediately carried out after the synthesis to avoid light induced modifications.

Instrumentation

Powder X-ray diffraction measurements were carried out with a Bruker D8 X-ray diffractometer [λ(Cu-K_α) = 1.54 Å] with scan rate of 1° min⁻¹. Powder samples were spread in a PMMA holder and the measurements were performed at ambient conditions. Single crystal diffraction measurements were obtained with a Oxford CCD X-ray diffractometer (λ = 0.71073 Å). Data reduction was performed using CrysAlisPro 171.33.55 software.²² The crystal structure was solved and refined using SHELXS-97 and SHELXS-97, respectively.²³ FESEM images of nanoparticles were acquired with an Ultra 55 Carl Zeiss instrument operated at variable voltages. The NPs were dispersed in isopropyl alcohol and dry cast on glass/ITO plates. Light exposure was avoided at all times while handling samples for microscopic analysis to avoid any light induced modifications. TEM micrographs were obtained with a FEI Technai G² 20 STEM instrument at an acceleration voltage of 200 kV. The NPs were dispersed in isopropyl alcohol and coated on carbon-coated copper/Nickel grids (200 mesh). Copper grids were reacted with ionic silver and formed dendrites at the edge of all meshes. Hence, the nickel grids were utilized for TEM measurements. For all the particle distribution analysis, a diameter of 100 particles (randomly) were measured and plotted. IR spectra were recorded with an Alpha FTIR spectrometer. FTIR spectra were subtracted from the spectrum of the pure substrate. TOF and quadrupole mass analyser types were used for HRMS analysis measurements.

Acknowledgements

Authors thank Centre for Nanotechnology for TEM facility and school of physics for FESEM facility both at University of Hyderabad. B.G.K gratefully acknowledges the Council of Scientific and Industrial Research (CSIR), India for Senior Research Fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: FTIR, UV spectrum, HRMS, single crystal refinement details of S₄N₄ and TGA/DTA measurements. CCDC 961232. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra04339a

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