Zinc-Salt-Mediated Synthesis, Growth Kinetic, and Shaped Evolution of Silver Nanoparticles

1 Department of Chemistry, University of Agriculture, Abeokuta, Nigeria 2 Center for Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria 3 Nanosciences Laboratories, Materials Research Department, iThemba LABS, Somerset West 7129, South Africa 4 National Agency for Science and Engineering Infrastructure, Idu Industrial Area, Abuja, Nigeria 5 Department of Physics, University of Nigeria, Nsukka, Nigeria


Introduction
Nanostructured materials have received broad attention due to their distinguished performance in electronics, optics, and photonics [1]. With reduction in size, novel electrical, mechanical, chemical, and optical properties are introduced, which are largely believed to be the result of surface and quantum confinement effects. They have been widely exploited for use in photography, catalysis, biological labeling, photonics, optoelectronics, information storage, and formulation of magnetic ferrofluids [2][3][4].
Among the various metal nanostructures, silver nanoparticles (SNs) have been widely investigated because they exhibit unprecedented optical, electronic, and chemical properties, depending on their sizes and shapes, thus opening many possibilities for diverse technological applications [5][6][7].
Generally, silver nanoparticles have been produced by various methods including chemical reduction of silver ions with or without [8,9] stabilizing agents, thermal decomposition of organic solvents, and electrospining [10]. Using these methods [8][9][10][11][12][13][14], silver nanoparticles with spherical, octahedral, tetrahedral, hexagonal, cubic, wire, coaxial cable, triangular prism, disc, and belt shapes have been produced. Apart from metal-silver nanobimetallics [15][16][17][18][19], functionalization or core-shell of silver-metal oxide nanocomposites is limited in the literature. Ag-ZnO electrical contact material was only recently produced using mechanochemical synthesis route [20]. Polymer functionalized silver has been reported [21]. Furthermore, the presence of various ions has been shown to influence the shape and size of metallic nanostructures produced via the polyol method. Xia group [22] has shown that the presence of iron (II) or iron (III) in the polyol synthesis facilitates the growth of silver nanowires or cubes, depending on the concentration of iron ions. Copper salts [23] have been used to affect polyol reduction of silver nitrate.
In this present paper, we report the synthesis of various shaped silver nanoparticles and zinc oxide-wrapped silver  Figure 1: XRD patterns of as-prepared (a) silver core-ZnO shell nanocomposite, (b) silver sample.
(core-shell) nanocomposite using chemical reduction in a polyol process under the influence of zinc chloride salt. Shapes of the silver and silver core zinc oxide shell nanoparticles have been followed by various techniques such as XRD, TEM, EDX, and UV-vis spectroscopy.

Synthesis
3.1. Shaped Silver Nanoparticles. The shaped silver nanoparticles were synthesized by chemical reduction method in a polyol process. The polyols used which served as the reducing agents were ethylene glycol (EG), diethylene glycol (DEG), or glycerol (GL). Molar concentrations of reactants were prepared in the probe solvent, unless otherwise stated. PVP served as protective stabilizing agent, while zinc chloride (salt) is expected to facilitate reduction process and seed formation. The molar ratio PVP/Ag + was set within the range 0.3-2.0. 10%-300% mole of the salt (relative to silver nitrate) was used, unless otherwise stated.
In a typical procedure, 20 mL of the polyol [EG, DEG, TEG, GL] was heated to 100 • C with stirring for 30 min. A solution containing 5.0 mM silver nitrate and 0.5 mM (10%) were added to the stirring polyol (EG, DEG, TEG or GL). Polyol solution of PVP (2.5 mM) was injected over a period of 5 min, and thereafter temperature of 180 • C was maintained. Reflux condition was used for the coreshell synthesis. The colloidal system was cooled to room temperature, and silver nanoparticles were separated from the liquid by centrifugation and then washed with acetone and ethanol.

Results and Discussion
Growth and nucleation of SN was apparently noticed as PVP was introduced. The reactant mixture changes quickly from colorless to yellow after the addition of PVP and ZnCl 2 salt solution in EG. Yellow to light greenish-opaque colloidal color transition, which was maintained for more than 40 minutes, served to indicate a complete reaction. Turbid brown, wispy gray, and wispy gray colors were the final colors marking a complete reaction in the case when reducing solvents were GL, DEG, and TEG, respectively. The color changes in nanomaterial synthesis are due to confinement of electrons and consequent changes in electronic energy levels [9].
The XRD patterns of a typical silver nanoparticle synthesized in this study are almost identical regardless of experimental conditions (Figure 1(b)), which is a reflection of high degree of crystallinity. This indicates that all of the diffraction peaks can be readily indexed to face-centered cubic (fcc) silver with a calculated lattice constant of 4.1Å.

Effects of Polyols, PVP/Silver Nitrate Mole Ratio, and
Salt. It has been established that morphology, sizes, and size distribution of metallic nanoparticles depend on some reaction conditions such as temperature, time, concentration, PVP/Ag + mole ratio and style of reagents addition [8] as it affects whether nucleation is homogeneous or heterogeneous [24]. The synthesis conditions used for producing the various shaped SNs are summarized in Table 1.
Apparently, nature of the polyol, PVP/silver nitrate ratio, and fraction of the salt added affected shapes, sizes, and reaction time significantly. The reaction in the presence of EG where PVP/Ag mole ratio (0.5) and 50 mole% ZnCl 2 salt were adopted produced nanowires whose length and diameter are in the range 5-30 μm and 20-26 nm, respectively (Table 1, Figure 2(h)).
Obviously, Ag nanowires must have been generated following self-seeding process previously explained [25,26]. Relatively lower Ag + precursor concentration accounts for a reduced chemical potential which favours thermodynamically stable multiply-twinned particles (MTPs) ( Table 1, Figures 2(a), and 2(b)). Maintaining that condition with Cl − addition favours oxidative etching of the MTPs and the resultant overriding growth led to nanowires (Figure 2(h)). However, there was a significant change in length and diameter of the nanowires as the PVP/Ag + is reduced to 0.3 while keeping 50% salt addition. This led to increased growth rate of the etched MTPs, leading to longer and thicker nanowires (Figure 2(i)). A report elsewhere [22] indicated contributory effort of Fe 2+ in promoting production of nanowires. Cu 2+ has also aided reduction of AgNO 3 [23]. At PVP/Ag + ratio and 10% Zinc salt intervention, a perfect nanosphere (Table 1, Figure 2(c)) of diameter 48 nm resulted within 75 min. However, a cross-check experiment under the same condition and without salt mediation offered quazi nanosphere in 3 hours reaction time.
Scheme 1 explained the formation of shaped Ag which, however, depends on the level of Cl − mediation vis-à-vis the nature of polyol. Following path 8 and 9 of the scheme, Ag + is reduced to Ag o , nucleation occurs, and seed formation leads to quazi nanosphere whose seed growth proceed in an isotropic manner (OD particles). Since nanosphere belongs to the lowest energy shape, simple reduction of the silver precursor is expected with or without exotic reagent. In this case, synergy effect of the salt has been noted towards formation of a perfect nanosphere within 75 min in hopefully kinetically controlled manner. Hopefully, the synergy efforts of both Zn 2+ and PVP lowered {100} surface energy and brought down reaction time to 75 min from 120 min spent without salt contribution.
PVP has been shown to preferentially adsorb onto the {100} surface of silver particles, where it served to stabilize and protect the small single crystal seeds [27]. Insufficient molar concentration of PVP to lower surface energy usually give room for the production of twin detects via a higher surface energy [24].
The morphology of the seed particles is one important parameter that decides anisotropic growth of noble metal nanoparticles; the process that supports the type of shape obtained is dependent on twinning probability [28]. Following reaction Scheme 1 shaped anisotropic growth, Ag emanated from both AgCl and Ag + (path 2 and 3) due to large excess of AgNO 3 present in the solution. The only few Cl − came from AgCl, which dissolved as the temperature is increased beyond 150 • C. Ag + concentration in the medium is sufficient enough to promote twinning process amidst some oxidative etching effect of Cl − /O 2 of the initially formed MTPs. Under this condition, the growth rate from decahedra to 1D anisotropic products is faster and it competes overwhelmingly with etching rate. In DEG, high Ag + concentration around the growing nuclei pushes in  high twining probability requisite for MTPs formation from where nanorod and nanowires were grown. As the concentration of Ag + is reduced and Cl − in the reaction medium significantly increased while keeping DEG as reducing solvent (Scheme 2), the morphologies of the Ag formed changed beyond scientific expectation and tuned towards 2D anisotropic nanostructures (nanoflakes and nanosheets) depending on the amount of Cl − in the reaction medium as indicated in the Eventually, Cl − supposedly suppresses the crystal growth along the {111} direction [29]. Obviously, the area ratio of {100}/{111} facet varies depending on the relative growth rate of the two surfaces, resulting in differences in particle shapes [29]. Therefore, the formation and structural changes observed for nanoflakes → nanosheets can be explained by the Cl − /O 2 etching vis-à-vis site-specific deposition of AgCl on the surface of Ag nanoparticle seeds [30]. In other words, selective adsorption of Cl − on the {111} plane of Ag seeds and the formation of AgCl, also in contact with the seeds, play a role in the nanosheets and nanoflakes formation as promoted by DEG. Interestingly, reaction in the presence of GL while keeping equal ratio of PVP/Ag + offered what we may name as a fluffy round nanocake (Figure 2(k)). This observation further shows that the type of aldehyde emanated from the polyol, which in this case GL, causing the reduction process is also important in the shape determination of the growing silver seeds. The predominating anisotropic 1D nanorod (78 nm diameter, 352 nm length on the average, Figure 2(j)) obtained is similar to that previously reported [31].
To further clarify, if indeed Zn 2+ Cl − has a significant effect on the morphological changes of the nanoparticles, one may ask then what happens when Cl − is greatly increased. To answer this, mole% of ZnCl 2 was made up to 300 and the reaction was carried out under reflux condition. Representative SEM spectrum in Figure 2(l) shows what we may call ZnO-wrapped Ag nanocomposite, which obviously followed a core-shell model as monitored by UV-vis absorption spectroscopy (spectra not shown for brevity) and XRD.
The XRD pattern (Figure 1(a)) of the as-prepared Ag core-ZnO shell is quite revealing and distinct in terms of crystallinity and peaks attributable to the heterostructure (OD on 1D). This exceptional distinct nature of the crystalline phases peaks rules out the possibility for an alloy formation as there is no evidence of intimate association of ZnO within Ag matrix. The C Ag /C ZnO ratio in the coreshell is ca 6.4, indicating a large core enrichment of the Ag. Structural evolution of Ag-ZnO (Figure 1(b)) from Ag (Figure 1(a)) has been demonstrated with the emanation of peaks (1010), (0002), (1011), (1012), (1120), and (1013) which are due to ZnO nanostructure.
The plausible mechanism of formation of agglomerated Ag-ZnO can be followed from Scheme 2. Obviously, Ag nanorod resulted from dissolving AgCl and Zn(NO 3 ) 2 offererd Zn(OH) 2 precipitate, which was transformed into ZnO nuclei. The ZnO nuclei grew and self-assembled itself on the Ag nanorod surface due to relative surface energy concepts.
The morphologies and structures of some of the shaped silver nanoparticles were investigated by TEM, EDX, and UVvis spectroscopy. Figures 3(a) and 3(b) are the representative TEMs of the anisotropic 1D structures of nanorod and nanowire, respectively, which are crystalline in nature. Electron diffraction pattern recorded from Figure 3 405 nm and corresponding particle sizes are plotted against time (Figure 4(a)). Curve z (dashed line ---) represents the particle sizes evolution as the nanosphere grew. Curve x and curve y are those with and without ZnCl 2 salt, respectively. In both cases, absorbance and particle size increased with increasing time until when equilibrium was attained after which there was no further growth. However, absorbance of Ag nanosphere under the mediating influence of salt was significantly higher (curve x), indicating increased particle density. A plot of log(A − At) against time (Figure 4(b)) is quite revealing and concluded the effect of salt mediation. Calculated rate constant (k = 4.6 × 10 −1 S −1 ) under salt influence has been found to be almost three times higher than that without salt mediation (k = 1.6 × 10 −2 S −1 ). Apparently, nucleation and growth process vis-à-vis particle size of the perfect nanosphere could be kinetically controlled to reach equilibrium and which eventually brought the reaction to completion within 75 min in contrast to that which was not mediated, taking up to 120 min reaction time. Both cation and anion may be responsible for this observation [32]. Certainly, Zn 2+ influenced the rate of reaction and size equilibrium, while Cl − is responsible for the shape of nanoparticles formation.
It has been established that copper, gold, and silver display plasmon absorption in the visible region. However, silver in particular usually has an absorption maximum at 404 nm [33]. The UV-visible spectra of silver nanosphere, nanorod, and nanowires synthesized are showed in Figure 5 with corresponding curves x, y, and z, respectively. Ag nanosphere, which is of the lowest energy, displays absorption maximum at 421 nm, while Ag nanorod and Ag nanowire show red-shifted plasma absorption maximum at 452 nm and 484 nm, respectively. The reason behind the discrepancies in the observed optical properties is attributed to increase in aspect ratio as the morphologies change from spheres to rods and wires; the larger the aspect ratio, the more red-shifted the longitudinal Plasmon band [34,35]. Moreover, the shifting experienced by the nanorod and nanowire is due to enhanced absorption of visible light along their respective length and width [35]. Nanowire is longer and is therefore more red-shifted. It is worth noting that accidentally acquired results obtained on the core-shell Ag/ZnO heterostructure, though agglomerated, seems hopeful if further experimental measures are taken. Study in this direction on full scale basis is underway.
In conclusion, it has been established that ZnCl 2 salt vis-à-vis variations in PVP/AgNO 3 ratio and type of polyol, to a large extent, have influence on the nucleation, growth, and final shapes of silver nanostructure. The mole% of the salt relative to the silver precursor determined whether the Ag seed formation proceeded in an isotropic or anisotropic manner, which in this case was found to depend on twinning probability and amount of Cl − and Ag + in the reaction mixture. Ag shape formation was either initiated via seed growth arising from MTPs leading to nanowires and nanorods or via Cl − /O 2 leading to other shapes (flakes, sheets, cake, etc.) depending on the extent of etching and polyol used. Extremely high mole% of the salt has been found to be contributory factor to the development of Ag core-ZnO shell heterostructure bearing OD on 1D. In particular, the strategies to control the kinetic and formation of desired nuclei with twin planes during nucleation offered option for anisotropically shaped silver nanostructures. The type of polyols adopted has been found to be responsible for that option.