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Two facile routes for functionalization of WS2 nanotubes with silver nanoparticles
A. Yu. Polyakov1, V.A. Lebedev1, L. Yadgarov2, E. A. Goodilin1
department of Materials Science, Lomonosov Moscow State University, 1-73 Leninskiye gory, Moscow, 119991, Russia 2Tel Aviv University, P.O. Box 39040, Tel Aviv 6997801, Israel a.yu.polyakov@gmail.com, goodilin@gmail.com
PACS 81.10.-h, 88.30.mj DOI 10.17586/2220-8054-2017-8-5-628-634
Silver-coated WS2 nanotubes (NT-WS2) were successfully synthesized via two wet chemistry techniques. The first employs spontaneous silver nanoparticle growth resulting from an interaction of disulfide nanotubes with AgNO3 in aqueous suspensions at 100 °C without any additional reducing agents or stabilizers. The second utilizes [Ag(NH3)2]OH complex to produce silver nanoparticles upon thermal decomposition. Both techniques are capable of producing Ag-NT-WS2 nanocomposites containing 5-60 nm silver nanoparticles tightly attached to the nanotubes' surfaces. The hexagonal arrangement of sulfur atoms of the outer WS2 layer was postulated to facilitate crystallization of silver nanocrystals with hexagonal crystallographic system (4H-Ag). The physical-chemical model for spontaneous AgNP formation is proposed. Keywords: WS2 nanotubes, silver nanoparticles, plasmonic nanoparticles, nanocomposite, spontaneous growth, 4H silver. Received: 30 September 2017 Revised: 6 October 2017
1. Introduction
Currently, much attention is paid to the nanocomposites of noble metals and semiconductors due to novel synergistic properties arising in such materials [1]. Among others, semiconducting nanotubes functionalized with silver nanoparticles (AgNPs) were successfully synthesized and became widely studied multifunctional composites. Due to the presence of bright plasmon resonance in Ag nanostructures, such nanocomposites can be employed in numerous optical applications. For example, carbon nanotubes (CNT) coated with Ag and Au nanoparticles have been found to be broad-band optical limiters [2]. Ag-CNT nanocomposites are widely used as mediators for highly-sensitive surface-enhanced Raman spectroscopy (SERS) [3-5]. AgNP-loaded CNT and TiO2 nanotubes possess high efficiency in photocatalytic applications [6-8] including antibacterial photocatalysis in visible light [9]. Additionally, Ag-CNT nanocomposites are considered promising gas sensors [10,11], catalysts [12], antibacterial agents [13-15] and drug delivery carriers [16].
WS2 nanotubes (NT-WS2) are the analogues of multiwalled CNT and represent folded and nested S-W-S sheets containing six-fold-bonded W atoms sandwiched between three-fold-bonded sulfur atoms [17]. Mass production of NT-WS2 is based on reduction of WO3 nanoparticles using a H2S/H2/N2 gaseous mixture in fluidized-bed reactors [18]. A number of distinctive advantages of NT-WS2 were reported. Contrary to CNT, disulfide nanotubes cannot be bent or entangled easily, therefore they are more easily dispersed in polymer blends and other matrices [19]. Individual NT-WS2's were also shown to be perfect torsional resonators with the highest quality factor (Q) and torsional resonant frequency in comparison to CNT and BN nanotubes [20]. Recently, the unique nonreciprocal superconductive behavior of individual chiral NT-WS2 was also reported [21]. Surface modification of NT-WS2 with inorganic nanoparticles expands application areas of disulfide nanotubes. Co@NT-WS2 [22] and Ni@NT-WS2 [23] nanocomposites effectively catalyze hydrodesulfurization reactions of thiophene and similar compounds. Additionally, Co@NT-WS2 is considered as a promising visible-light photocatalyst [24]. Pd@NT-WS2 nanocomposites were found to be efficient catalysts for cross-coupling (Heck and Suzuki) reactions [25]. FeWO4 nanoparticles were successfully deposited onto NT-WS2 resulting in magnetic nanocomposites [26]. Recently, CNT/NT-WS2 electrically conductive hybrid films were also produced [27].
There are plenty of versatile techniques for CNT surface modification, e.g., generation of chemically active carboxylic groups by etching the CNT surface with HNO3/H2SO4 [13,28]. Similarly, many routes for coating of CNT with AgNPs have been reported, including deposition of pre-synthesized AgNPs [10,11], magnetron sputtering of Ag and high-temperature annealing [3], E-beam coating in vacuum [2], Ag2O reduction upon sonication in presence of CNT [29] and reduction of Ag(I) ions in aqueous CNT suspensions. The latter includes direct photoreduction, thermal decomposition [8], usage of reducing agents (e.g. glucose [14], formaldehyde [28], etc.)
and sacrificial layers [9], as well as spontaneous reduction of Ag+ ions on the surface of CNT [30]. On the contrary, the functionalization of NT-WS2 with plasmonic silver nanoparticles has not been studied yet, despite several attempts having been made to synthesize Au-NT-WS2 nanocomposites [31-34]. Moreover, it was suggested in our previous work [32], which focused on the functionalization of NT-WS2 with AuNPs via spontaneous HAuCl4 reduction on the disulfide surface, that the same technique could be applied using AgNO3 solutions for the production of Ag-NT-WS2 nanocomposites.
Going beyond this suggestion, here, we report two facile techniques for decoration of NT-WS2 with AgNPs: the first is based on a reaction of AgNO3 with aqueous NT-WS2 suspension at 100 °C; the second employs the thermal decomposition of freshly prepared [Ag(NH3)2]OH complex solution in the presence of disulfide nanotubes. The resultant nanocomposites are carefully characterized using transmission electron microscopy (TEM).
2. Experimental
WS2 nanotubes were kindly provided by NanoMaterials Ltd. (Israel) and personally by Prof. Reshef Tenne (Weizmann Institute of Science, Israel). The nanotubes are multiwalled, typically 1-20 ^m long and 30-150 nm in diameter. Before all the syntheses the nanotubes were deagglomerated by sonication in acetone according to the common procedure [35].
Silver nitrate was of chemically pure (c.p.) grade. Sodium hydroxide standard solution was purchased from Merck (Germany). All other chemicals were purchased locally and were of analytical grade. All aqueous solutions and suspensions were prepared using high purity water (Milli-Q RG, 18.2 MO-cm resistivity, Millipore). Glassware and magnetic stirring bars utilized for AgNP synthesis were washed by 63 % HNO3.
Within the first technique towards Ag-NT-WS2 nanocomposites, 14 ml of 114 mM AgNO3 solution were heated up to 100 °C in a foil-wrapped beaker to avoid photoinduced silver salt decomposition. Then, 2 ml of freshly prepared aqueous nanotube suspension (1.3 g/L NT-WS2) were added to the beaker upon vigorous stirring. The reaction mixture was kept at 100 °C for 3 min and then cooled down to room temperature with continued stirring.
For the second NT-WS2 decoration technique, freshly prepared [Ag(NH3)2]OH complex was used. This technique was adopted from the protocol for AgNP-decoration of SiO2 microspheres reported elsewhere [36]. For the complex preparation, 0.1 M NaOH was added to 10 ml of 0.01 M AgNO3 in a dropwise manner until the end of Ag2O precipitation. The resulting brown sediment was centrifuged (4000 rpm, 10 min), redispersed in purified water by 1 min sonication and centrifuged again. This washing procedure was repeated three times. The purified sediment was dissolved in 10 % NH3-H2O to form 15 mM [Ag(NH3)2]OH solution, which was stored at 4 °C and used within 30 min after preparation.
For the nanocomposite synthesis, 2 ml of aqueous nanotube suspension (1.3 g/L NT-WS2) were added to 13.89 ml of purified water pre-heated up to 100 °C under vigorous stirring. Then, 110 ^l of freshly prepared 0.015 M [Ag(NH3)2]OH solution were added. The reaction mixture was kept in foil-wrapped beaker at 100 °C for 3 min and then cooled down to room temperature. In both procedures, the WS2 concentration in reaction mixtures after addition of all the reactants was about 0.66 mM and the Ag concentration was about 0.1 mM.
The nanocomposite morphologies were analyzed using a Carl Zeiss Libra 200 MC transmission electron microscope (TEM) operating at 200 kV. Selected-area electron diffraction (SAED) patterns were registered for representative selection of the particles. For TEM analysis the aqueous composite suspensions were dripped on lacey-carbon copper grids (SPI, USA) and dried. Images were processed using open-source Gwyddion software [37] including calculation and processing of Fast Fourier Transforms.
Scanning electron microscopy (SEM) was performed using Carl Zeiss Leo Supra 55 microscope operating at 15 kV in secondary electron imaging (SE2) regime. For SEM analysis, the composite nanoparticle suspensions were dried on conductive silicon wafers.
3. Results and discussion
The silver-decorated nanotubes were multiwalled, 1-20 ^m long and 30-150 nm in diameter. Both developed techniques resulted in functionalization of the NT-WS2 sidewalls with a layer of silver nanoparticles (Fig. 1,2). If using AgNO3, the size of the nanoparticles varied from 5 nm to 60 nm; the mean size is 30±8 nm (Fig. 1(a-d)). A synthesis with the same concentration of [Ag(NH3)2]OH resulted in smaller nanoparticles: from 5 to 35 nm, the mean size being 20±5 nm. As visualized by TEM, in both the cases the nanoparticles are uniformly distributed on the nanotube surface. A few AgNP agglomerates were also detected, especially near the tips of NT-WS2. The AgNPs covering prepared from [Ag(NH3)2]OH looks more uniform and less agglomerated in comparison with those prepared at the same concentration of AgNO3. SEM also revealed patterning of AgNPs along the
surface defects of disulfide (Fig. 1(c)). This suggestion is supported by AgNPs deposition on step defects of outer WS2 layers observed by HRTEM (Fig. 1(d)). The same behavior was previously observed for gold nanoparticles grown on NT-WS2[32]. The local composition of the Ag-NT-WS2 nanocomposite prepared using AgNO3 was also visualized using STEM-EDX mapping (Fig. 2). The AgNPs have a characteristic non-spherical shape indicating their heterogeneous nucleation and growth directly on the NT-WS2 surface. Such growth results in a tight contact between AgNP and disulfide. Free AgNPs detached from nanotube surface or formed in bulk solution are rare.
Fig. 1. Micrographs of Ag-NT-WS2 nanocomposites synthesized using AgNO3 (a-d) and [Au(NH3)2]OH (e, f) precursors. The arrow on (d) designates the step defect of NT-WS2 surface underlying the grown AgNP
1 Electrons
lOOr-m
Fig. 2. STEM-EDX mapping of Ag-NT-WS2 nanocomposite synthesized employing the AgNO3 precursor
HRTEM analysis revealed that, while some grown particles exhibits "normal" (111) and (200) cubic silver (3C-Ag) plane arrays (Fig. 3(a)) with ca. 2.36 A and ca. 2.00 A spacings, respectively (Fig. 3(a-c)), the other particles have an unusual hexagonal (4H-Ag) structure. Distinct plane arrays with ca. 2.48 A spacing were detected (Fig. 3(d-e)), coinciding well with 2.5 A spacing of (004)-(100) 4H-Ag planes (entry #41-1402, ICDD PDF2 database [38]). To illustrate the difference in nanoparticle structures, Fast Fourier Transforms (FFTs) were calculated from HRTEM images of the selected "nanoparticle 1" (Fig. 3(a)) and "nanoparticle 2" (Fig. 3(d)). The integrated radial profiles of FFTs are shown on Fig. 3(g). It is clearly seen that "particle 1" is composed of 3C-Ag while the "particle 2" has the 4H-Ag structure.
Such a hexagonal structure of silver nanocrystals can be dictated by the arrangement of sulfur atoms on the surface of the outer WS2 layer. Generally speaking, an arrangement of atoms in a sulfur layer in the S-W-S "sandwich" can be a perfect template for the growth of hexagonal close-packed (hcp) Ag atomic layers. indeed, sulfur atoms of the WS2 surface form a hexagonal array with a characteristic distance between the atoms of 3.15 A. At the same time, the inter-atom distance in hcp Ag layer is about 2.9 A (Fig. 3h). These hcp layers can be either
Fig. 3. (a-e) are HRTEM micrographs of AgNPs grown on the surface of NT-WS2. (f) is a suggested crystallographic model for (e) in which (004) 4H-Ag planes continue the "chevrons" of outer WS2 layers. (g) represents integrated radial profiles of FFTs calculated from nanoparticles 1 and 2 marked on (a) and (d), respectively. Data for 3C-Ag and 4H-Ag cells are taken from entries #4-783 and #41-1402 from ICDD PDF2 database [38]. The cif-files for crystallographic visualizations were taken from CrystalMaker Materials Library [43] and from the works [39,44] using Crystallography Open Database [45]. (h) depicts the suggested model of hcp atomic silver layers grown on the surface of WS2. Please, refer to the article text for more details
(111) planes of cubic silver (3C-Ag) or (004) planes of hexagonal silver (4H-Ag). If the silver hcp layers propagate parallel to the surface of NT-WS2, their spacing will not be influenced by the disulfide substrate. However, if the silver hcp planes propagate nearly perpendicularly to the disulfide surface, 4H structure becomes more preferable. Sulfur atomic rows on the WS2 surface are located at 2.73 A distances, so a 4H-Ag with 2.50 A spacing between hcp Ag layers matches better with those rows than the 3C-Ag with a 2.35 A spacing of equivalent atomic planes. In this case, a sulfur atoms arrangement of NT-WS2 can pre-organize the 4H-Ag motifs. This also can explain the observation of (004) 4H-Ag planes continuing the "chevrons" of outer WS2 layer (Fig. 3(e,f)). It should be noted that 4H-polytype was initially observed in natural silver samples from northern USSR [39] and later in a number of synthetic nanostructures: AgNPs prepared using Rumex hymenosepalus extracts [40], silver nanowires [41] and thin films [42].
Of great importance is the fact that Ag-NT-WS2 preparation by the proposed techniques does not require any additional reducing agents or linkers, thus providing nanocomposite synthesis a material efficiency. Within the first technique, the spontaneous reaction of AgNO3 with WS2 takes place, resulting in heterogeneous silver nucleation. Obviously, the driving force of such chemical interaction originates from the energy difference of WS2 Fermi level and the redox potential of dissolved Ag+ ions. The Fermi level of WS2 is known to be of 4.7 range below the vacuum level [46]. The standard redox potential of Ag+ ions is +0.799 V against the standard hydrogen electrode (SHE) [47]. The absolute potential of SHE is, in turn, reported to be within the -4.73 to -4.43 V range [48]. Thus, the Fermi level of tungsten disulfide should be higher by 0.5-0.8 eV than the energy of missed valence electron in Ag+ ions (Fig. 4). This should result in spontaneous electron transfer from the NT-WS2 surface to silver ions resulting in Ag+ ^ Ag0 reduction and nucleation of silver nanocrystals. Obviously, the first Ag atoms became bound by surface sulfur atoms, dictating hexagonal silver crystal symmetry as described above. The electron deficiency in the nanotubes can further be compensated by WS2 oxidation, e.g. at the nearest surface
defects. [Ag(NH3)2]OH has ca. twice lower reduction potential (+0.38 V) [47] then Ag+, so the energy difference with WS2 Fermi level is lower but still negative. However at 100 °C, the diamminesilver(I) complex decomposes rapidly initiating silver nucleation as described in [36]. Previously, it was shown that the presence of excessive ammonia in the reaction mixture can prevent secondary nucleation during citrate-mediated AgNP synthesis, leading to the formation of smaller and quasi-monodisperse nanoparticles [49]. This was explained by the entrapment of residual free Ag+ ions responsible for secondary nucleation. Possibly, the same moderating effect of ammonia leads to the aforementioned formation of smaller AgNPs on NT-WS2 when [Ag(NH3)2]OH is employed instead of AgNO3.
Fig. 4. Proposed energy diagram representing relative energies of WS2 Fermy level and energy of missed valent electron in Ag+ ion. The arrow designates possible electron transfer from NT-WS2 to Ag+ ions resulting in spontaneous AgNP growth on the sidewall of disulfide nan-otubes
4. Conclusions
Reactions of aqueous AgNO3 or [Ag(NH3)2]OH solutions with NT-WS2 resulted in functionalization of the nanotubes with a 5-60 nm layer of AgNPs without requiring any additional reducing agents. The spontaneous formation of AgNPs on NT-WS2 surface is suggested to be driven by electron transfer from disulfide surface to dissolved Ag1 ions. Part of the grown AgNPs exhibited unusual hexagonal crystal structure (4H-Ag). Based on careful HRTEM study, the hexagonal arrangement of sulfur atoms within the WS2 layers, including the outer disulfide layer, is suggested to predict the observed formation of 4H-Ag nanocrystals. The prepared nanocomposites may have applications as sensors or antibacterial low-friction surface coatings.
Acknowledgements
This work is supported by the Russian Science Foundation (grant 14-13-00871), AYP acknowledges partial personal support by the Russian Foundation for Basic Research (project # 16-33-01058 moLa) and Lomonosov Moscow State University Program of Development (the use of Carl Zeiss Libra 200 MC microscope). Authors thank Maria Poliakova (Tsukuba University, Japan) for the proof-reading.
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