Previous studies have demonstrated that, upon addition of AgNO3 into a poly(vinyl pyrrolidone)/ethylene glycol (PVP/EG) solution, Ag nanoparticles are first produced, followed only significantly later by emergence and dominance of Ag nanowires. Mechanistic details in terms of nucleation and growth of Ag nanowires in competition with initially populated Ag nanoparticles remain unclear. Via in-situ small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM), here we show that, for a molar ratio of Ag/VP = 1, the first 5 min of reaction at 150 °C gave only Ag particles of radius up to RP ≈ 29 nm; after t = 6 min, Ag rods start to appear and become the dominant product after t ≥ 12 min. The rod radius was initially comparable (RR ≈ 26 nm) to coexisting nanoparticles before subsequent increases to RR ≈ 43 nm near the end of reaction at t = 19 min. More importantly, the apparent pH value was observed to decrease quickly from 3.7 to a minimum value of 1.8 at t = 5 min (which corresponds to the time that Ag rods start to appear) before gradually increasing to pH ≈ 3.2. Similar correlation between the change of pH and morphology of Ag nanocrystals was also observed for the case of Ag/VP = 2 with higher reaction rate and halved reaction period. We therefore conjectured that the change in polarizability of the reaction medium with the extent of reaction may affect the surface adsorption behavior of PVP onto different facets of Ag nanocrystals. In support of this conjecture, the pH-dependence of intrinsic viscosity of PVP in EG indeed indicated increased PVP coil size with decreasing pH, presumably due to intrachain ionic repulsion upon protonation of PVP. By adjusting the starting pH value to 1.5 for Ag/VP = 2, the rate of formation of Ag particles was decreased; nevertheless, once the population of Ag nanoparticles reached an adequate level, Ag rods/wires started to emerge as indicated by a rapid increase in UV-visible absorbance and the drastic change in color accompanied by an abrupt jump in apparent pH value within a short period of 1 min; this change in morphology of Ag nanocrystals was further confirmed by SEM and SAXS observations, from which evidences for coalescence of existing Ag nanoparticles were identified. Integrating all observations above, we conclude that the formation of Ag nanowires starts with nucleation from coalescence of Ag particles, followed by 1D growth with partial capping of the lateral (100) surfaces by protonated PVP chains.

Previous studies have demonstrated that, upon addition of AgNO3 into a poly(vinyl pyrrolidone)/ethylene glycol (PVP/EG) solution, Ag nanoparticles are first produced, followed only significantly later by emergence and dominance of Ag nanowires. Mechanistic details in terms of nucleation and growth of Ag nanowires in competition with initially populated Ag nanoparticles remain unclear. Via in-situ small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM), here we show that, for a molar ratio of Ag/VP = 1, the first 5 min of reaction at 150 °C gave only Ag particles of radius up to RP ≈ 29 nm; after t = 6 min, Ag rods start to appear and become the dominant product after t ≥ 12 min. The rod radius was initially comparable (RR ≈ 26 nm) to coexisting nanoparticles before subsequent increases to RR ≈ 43 nm near the end of reaction at t = 19 min. More importantly, the apparent pH value was observed to decrease quickly from 3.7 to a minimum value of 1.8 at t = 5 min (which corresponds to the time that Ag rods start to appear) before gradually increasing to pH ≈ 3.2. Similar correlation between the change of pH and morphology of Ag nanocrystals was also observed for the case of Ag/VP = 2 with higher reaction rate and halved reaction period. We therefore conjectured that the change in polarizability of the reaction medium with the extent of reaction may affect the surface adsorption behavior of PVP onto different facets of Ag nanocrystals. In support of this conjecture, the pH-dependence of intrinsic viscosity of PVP in EG indeed indicated increased PVP coil size with decreasing pH, presumably due to intrachain ionic repulsion upon protonation of PVP. By adjusting the starting pH value to 1.5 for Ag/VP = 2, the rate of formation of Ag particles was decreased; nevertheless, once the population of Ag nanoparticles reached an adequate level, Ag rods/wires started to emerge as indicated by a rapid increase in UV-visible absorbance and the drastic change in color accompanied by an abrupt jump in apparent pH value within a short period of 1 min; this change in morphology of Ag nanocrystals was further confirmed by SEM and SAXS observations, from which evidences for coalescence of existing Ag nanoparticles were identified. Integrating all observations above, we conclude that the formation of Ag nanowires starts with nucleation from coalescence of Ag particles, followed by 1D growth with partial capping of the lateral (100) surfaces by protonated PVP chains.

ABSTRACT: 1
1. Introduction 5
1.1. Background 5
1.2. UV-Visible Absorption Spectroscopy of Ag Nanowires 7
1.3. Objectives and Approach 9
2. Experimental Details 10
2.1. Materials and Specimen Preparation 10
2.2. Instruments 10
2.3. Data Analysis 11
3. Results and Discussion 12
3.1. Morphology Evolution of Ag from SAXS 12
3.2. Variation of pH Value During Reaction Process 20
3.3. Nucleation and Growth of Ag Nanowires 26
4. Conclusions 30
References 31
Appendix A. SAXS Fitting Parameter 32

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