(167d) Role of Mixing and Solution Phase Nitric Oxide Concentration on the Morphology of Silver Nanowires Synthesized By Polyol Process

Role
of Mixing and Solution Phase Nitric Oxide Concentration on the Morphology of Silver
Nanowires Synthesized by Polyol Process

Prachi Kate¹ and Amol A. Kulkarni¹*

¹Chemical Engineering and Process Development Division,
CSIR-National Chemical Laboratory, Pune-411008, India

*Corresponding
author: aa.kulkarni@ncl.res.in

Abstract:

Silver
nanowires (AgNWs) are emerging as a replacement to ITO due to their high
electrical conductivity, transparency, mechanical flexibility, and excellent
optical properties [1, 2]. The performance of AgNWs can be attributed to their
size dependent properties. The length and diameter are the key factors to
achieve excellent electrical conductivity and higher transmission [3]. Silver
nanowires were synthesized by the conventional Polyol method. A
polyol synthesis involves heating a polyol with a salt precursor and a
polymeric capping
agent to produce metal colloids.

In
the present work, effect of Nitric Oxide i.e. the headspace gas was studied on
morphology of Silver nanowires. In order to achieve
better control of the morphology of Silver nanowires, it is necessary to
decrease the generation rate of Silver solids. During the oxidative
etching of Silver solids, Nitric acid (HNO₃) dissolves the Silver solids
into Silver ions at high temperature evolving Nitric Oxide (NO) and H₂O.
The NO gas thus accumulates in the reactor headspace and its excess
concentration affects the decomposition rate of HNO₃ at closed
condition. Based on this, experiments were carried out in 100 ml flask and the
headspace volume was decreased by increasing the reaction volume in the flask. With
the decrease in headspace of the flask, significant decrease in the length of Silver
nanowires was observed.  As the percent headspace
decreases and the reaction volume increases, more amount of NO gas will be
evolved. NO gas reacts with oxygen to form NO₂,which
combines with the refluxing H₂O vapors to form HNO₃ in
reaction mixture. This will decrease the decomposition rate of nitric acid and
thus will increase the etching effect of NO₃^-/Cl^-
ions on the surface of AgNWs and forms more number of small AgNPs which
eventually limits the growth of Silver nanowires. Thus, we studied the
effect of headspace gases on length of Silver nanowires at different headspace
volumes. For 80% headspace volume, long Silver nanowires with length ~ 15.5±5
µm were formed. With the decrease in headspace volume the length decreased to
5.8±1.5 µm. For all the cases, diameter was almost same ~ 70±20 nm.

Further
experiments were carried out to study the effect of mixing on length and
diameter of Silver nanowires at different temperatures.  It was observed that
without stirring, very long AgNWs were formed with diameter ~100 nm, however at
200 rpm the wire diameter reduced to 40±5 nm, beyond which the length as well
as diameter of the AgNWs decreased forming shorter nanowires and trace
particles. From above the
observations, it can be concluded that a desired extent of turbulence in the
reactor is a key regulator of the synthesis of high quality AgNWs, with the
additional merits of high yield, ultrathin diameter, and longer length.

Figure 1:
 SEM images
of Silver
nanowires formed
at different headspace
volume percentage
in the
flask

Figure
2:  SEM
images of
Silver nanowires
formed at
different stirring
rates

Reference:

[1] Kan
Zhan et. al., Nanoscale (2016), 8, 18121-18133

[2] Yang Liu et. al., RSC
Advances (2017), 7, 4891-4895

[3] Amirmostafa Amirjani et. al., Journal
of Experimental Nanoscience (2015), 10 (18), 1387-1400