Deposition of Ag and Ag-Au nanocrystalline films with tunable conductivity at the water-toluene interface

Article abstract of DOI:10.1039/c7ra13370g

Thin films of Au, Ag and Ag-Au alloy nanocrystals extending to areas of several square centimetres are obtained by deposition at the interface of water and toluene. Toluene containing chlorotris(triphenylphosphine)silver(i) and/or chlorotriphenylphosphine

Full text of DOI:10.1039/c7ra13370g

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Deposition of Ag and Ag–Au nanocrystalline films
with tunable conductivity at the water–toluene
interface†
Cite this: RSC Adv., 2018, 8, 6225
b
Gemma L. Stansfield,a Helena M. Johnston,a Sean N. Baxter
b
*
and P. John Thomas
Thin films of Au, Ag and Ag–Au alloy nanocrystals extending to areas of several square centimetres are obtained
by deposition at the interface of water and toluene. Toluene containing chlorotris(triphenylphosphine)silver(I)
and/or chlorotriphenylphosphine gold(I) is reacted with aqueous tetrakishydroxymethylphosphonium
chloride to obtain nanocrystalline films adhered to the interfacial region. Alloying was induced by varying the
composition of the toluene layer. The composition change results in regular and reproducible variation in
the transport characteristics of the films, with the initially metallic deposits turning non-metallic with
increased Au content. The films at the interface were transferred to different substrates and characterised
using atomic force microscopy, UV-visible spectroscopy, X-ray diffraction, X-ray photoelectron
spectroscopy, scanning and transmission electron microscopy.
Received 15th December 2017
Accepted 1st February 2018
DOI: 10.1039/c7ra13370g
rsc.li/rsc-advances
vanguard of a revival interest in using the interface of oil–water
to deposit inorganic nanostructures.15 A myriad of materials of
1 Introduction
contemporary interests have been recently obtained using this
scheme, including: single crystalline scrolls of CuO,16 nano-
scopic pyramids of crystalline PbS with a large proportion of an
highly reactive and open {331} surface17 as well as hierarchical
anisotropic Cd(OH)2 nanowire structures.18
Films of Ag and Au nanocrystals possess unique optical and
electronic properties due to the reduced dimensions of the
crystallites and the dominance of surface plasmons.1 A number
of potential applications have been identied for lms of metal
nanocrystals including the design of the next generation elec-
trical and electrochemical sensors, solar cell devices,2–4 as well
as optoelectronic devices.5,6 Strong local electrical elds present
in nanocrystalline Ag lms make them ideal substrates for
surface enhanced Raman and related spectroscopic tech-
niques.7 Nanocrystalline lms of Ag have previously been shown
to considerably enhance the performance of Si based solar
cells.8,9 In order to be harnessed in different applications
uniform nanocrystalline lms spread over large areas with
strongly interacting nanocrystals acting as robust charge
transport network are desirable. The quest for such lms
dominates current research in this area.10
Interfacial deposition is accomplished by simply layering
a dispersion of organometallic precursor in a solvent such as
toluene atop an aqueous solution containing the anionic
counterpart of the material of interest. When the right
precursors are chosen, the reaction proceeds at the region of
interface between the two liquids and produces a thin lm that
adheres to the interface. The lm can subsequently be trans-
ferred to a wide variety of substrates. Deposition at the water–
oil interface usually features simple chemistry, as the inter-
facial barrier eliminates the need for masking agents. The
method provides a remarkably low-cost, green and scalable
route to complex solids. The deposition scheme requires little
or no energy input. Furthermore, the nature of properties of
interfacial deposits can be varied by controlling parameters
such as deposition temperature, growth time and precursor
concentrations.10,19
We have shown that device quality lms of Au nanocrystals
can be directly obtained from interfacial deposits using a novel
transfer process.20,21 Herein, we have been successful in
obtaining interfacial deposits of Ag nanocrystals with highly
reproducible transport characteristics, indicative of robust
coupling between the constituent nanocrystals. Further, we
show that the transport properties can be systematically varied
In the past few years, the interface between two immiscible
liquids has emerged as a powerful new method to assemble
nanocrystals as well as to deposit lms of nanostructured
material.11,12 The rapid and reversible adsorption of suitable
nanocrystals at the water–oil interface has led to more resilient
assembly schemes.13,14 Rao and co-workers have been at the
aSchool of Chemistry, The University of Manchester, Oxford Road, Manchester, M13
9PL, UK
bSchool of Chemistry, Bangor University, Deniol Road, Bangor, LL57 2UW, UK. E-mail:
john.thomas@bangor.ac.uk
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra13370g
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across a large range by co-reducing Ag and Au precursors were carried out using a home-built system composed of
yielding Au–Ag nanoalloys.
a closed-cycle He cryostat, Keithley 2400 series source-measure
unit and a Lakeshore 325 temperature controller. The samples
were then mounted onto the cold nger of the closed-cycle
cryostat and electrodes contacted using copper wires and
conductive silver paint. Charge transport studies were carried
out on samples held in a vacuum of 10ꢀ6 mbar from room
temperature down to 15 K. FT-IR was carried out using a Bruker
Alpha II spectrometer tted with an ATR attachment. Proton
NMR was carried out using Bruker 400 MHz spectrometer using
a dispersion of the sample in CDCl3. Film thickness were
measured using Dektak stylus prolometer.
2 Experimental
2.1 Chemicals
All chemicals were purchased from Aldrich, with the exception
of chloro(triphenylphosphine)gold(I) (Acros Organics), they
were all used as received with no further purication or
analysis.
2.2 Synthesis of chlorotris(triphenylphosphine)silver(I)
The complex was prepared following a method previously re-
ported.22 Briey, to a stirred dispersion of AgCl (1.43 g, 10 mmol)
in dichloromethane (150 ml), a solution of triphenylphosphine
(TPP) (10.97 g, 4 mmol), in dichloromethane (50 mL), was added
drop wise and the stirring continued for an hour, during which
time the solids almost completely dissolved. The clear liquid
obtained was ltered and, to the ltrate, pentane was added to
precipitate a white crystalline solid. The solid was then washed
with pentane and air dried. Elemental analysis (%) found: C,
69.45; H, 4.84; Cl, 4.23; P, 9.69; Ag, 11.02. Calcd: C, 69.71; H, 4.84;
Cl, 3.82; P, 10.00; Ag, 11.62.
3 Results and discussion
In aqueous solutions, THPC hydrolyses in the presence of
NaOH to yield HCHO and also possibly H2:23–26
PCl(CH2OH)4 + NaOH / P(CH2OH)3 + HCHO + NaCl + H2O
P(CH2OH)3 + NaOH / O]P(CH2OH)3 + H2(g)
(1)
The nascent formaldehyde and hydrogen as well as tetra-
kishydroxymethylphosphine (THP) are all capable of reducing
the bound Ag and Au ions present in toluene.27 Following
reduction, the continual growth of reduced ions into large
crystallites is prevented by the phosphine based ligands:
2.3 Interfacial deposition of nanocrystalline lms
A 10 ml 1.5 mM toluene solution of chlorotris(triphenyl-
phosphine)silver(I) (Ag(PPh3)3Cl) was layered atop 16 ml of
6.25 mM aqueous sodium hydroxide in a 100 ml beaker
(diameter 4.8 cm). To initiate the reaction, 330 mL of 50 mM
aqueous tetrakishydroxymethylphosphonium chloride (THPC)
was injected into the lower layer. Almost immediately a yellow
colouration was observed in the lower aqueous layer. The
beaker was then quickly transferred to an oven set at the desired
temperature and at the end of the reaction; a lm with
a lustrous grey sheen was seen at the interface between toluene
and water. The lm was then transferred onto glass substrates
for further analysis. To obtain alloyed lms, the reaction was
carried out with equimolar solutions of (Ag(PPh3)3Cl) and
Au(PPh3)Cl, mixed in different propositions. For example, to
produce a Au0.7Ag0.3 alloy, a toluene layer with 7 ml 1.5 mM
Au(PPh3)Cl solution and 3 ml 1.5 mM (Ag(PPh3)3Cl) was
employed.
THP,
triphenylphosphine
(TPP)
and
tetrakishydrox-
ymethylphosphine oxide (THPO) bind to the surfaces formed
by Ag(0) and Au(0) species resulting in the formation of
nanocrystals.27 Proton NMR and IR spectra reveal bands char-
acteristics of aliphatic and aromatic species indicating that the
surface of the nanocrystals consists of species such as TPH,
THPO and TPP. X-ray photoelectron spectroscopy on Ag and Au
lms revealed P(2p) peaks consistent with the presence of
phosphinyl based ligands in the lms (see ESI†).28 The partic-
ulates thus obtained in the vicinity of the interface posses high
chemical potential and are adsorbed at the interface. Previous
studies suggest that the room temperature adsorption energies
of such nanoparticles at the water–oil interface are of the order
of a few millielectron volts.29 Following detailed mechanistic
studies of growth of Au nanocrystals at the water–toluene
interface, Dryfe and co-workers suggest that following initial
adsorption of nanoparticulates, further growth into lms takes
2.4 Characterization methods
X-ray diffraction was carried out on lms deposited on glass place with formaldehyde as main reducing agent.30
substrates using a Bruker AXS D8 diffractometer with a Cu Ka Deposits were grown at the interface by changing the mole
source. Atomic force microscopy was carried out using a Nano- ratio of the Au : Ag precursors at the start of the reaction at
scope IIIa microscope operating in tapping mode on interfacial intervals of 10% (AgxAu1ꢀx, 0 # x # 1.0). Energy dispersive X-ray
deposits transferred to mica substrates whose surface was analysis conrmed that the lm compositions were in line with
freshly cleaved. Scanning and transmission electron micros- the feed ratio used. Striking differences were seen in the
copy was carried out on Delong Instruments LVEM5 microscope appearance of the lms. As the Ag content decreased, the colour
with a FEG gun operating at 5 keV. High resolution SEM and of the lm became darker, moving from metallic grey to a dull
energy dispersive X-ray analysis was carried out using a Phillips brown with a low metallic sheen (see Fig. 1). Absorption spectra
XL30 FEG SEM. Samples for TEM were prepared by dispersing revealed plasmon bands whose position shied nearly linearly
the lms in ethanol by sonication, followed by drop casting on from the initial ca. 390 nm for pure Ag lms to 720 nm for pure
holey carbon coated Cu grids. Charge transport measurements Au lms (see ESI†). Such robust colour changes and peak shis
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Fig. 1 Photograph showing the various films studied. The compositions from the left are Ag, Ag0.90Au0.10, Ag0.80Au0.20, Ag0.70Au0.30
,
Ag0.60Au0.40, Ag0.50Au0.50, Ag0.40Au0.60, Ag0.30Au0.70, Ag0.20Au0.80, Ag0.10Au0.90 and Au. The colour changes from a metallic grey to dull brown as
the Ag content falls.
are expected on the basis of changes to plasmon resonance 408.53 pm for Ag to 407.82 pm in the case of Au). Toluene
accompanying composition changes.31,32
dispersions of Ag, Au and alloy lms were obtained by sonica-
SEM images (Fig. 2) reveal highly uniform and continuous tion and used for TEM studies. In the case of Ag lms, TEM
deposits spanning areas extending over several tens of square images reveal rough aggregates with irregular outlines (Fig. 3a).
microns. Folds and cracks are apparent in the deposits. At high Closer inspection reveals that the aggregates contain nano-
resolution, globular clusters with diameters in the range of tens crystals with dimensions of z10 nm. In the case of Au nano-
of nanometers can be seen throughout the deposits. The cluster crystals, sonicating is much more effective in breaking up the
edges are jagged due to the presence of smaller particulates. aggregates. TEM images reveal well resolved spherical nano-
High-resolution images reveal irregular clusters with diameters crystals with diameters of 10 nm (Fig. 3b). Lattice planes cor-
of 70 nm. AFM images (obtained in tapping mode) reveal responding to the separation between h111i planes of Ag0.5Au0.5
another layer of hierarchy in the morphology. The clusters can alloys are apparent in Fig. 3c. The coarse aggregates (in the case
be resolved into spherical particulates with diameters of around of Ag) as well as the clearly separated nanocrystals (Au) suggest
10 nm. Interestingly, change in the composition of the lms that organic ligands are present at the surface of the particu-
produces little change in the dimensions of the particulates. lates. The lms herein consist of nanocrystals coated with
Prolometric measurements reveal the thickness of the various organic ligands aggregating to form spherical clusters with
lms to be in the range of 100–150 nm.
diameters of several tens of nanometres. Previous studies, have
X-ray diffraction patterns of Ag, Au and Ag0.5Au0.5 lms yield conrmed that (at room temperature) nanocrystals with diam-
broad peaks characteristic of nanocrystals (see ESI†). The peak eters of z1.5 nm rapidly form an initial adlayer at the interface
widths appear to mask the expected small change in peak that anchors further growth.30,33 Subsequent reduction happens
positions accompanying the change in lattice parameters (from at the surface of these adsorbed nanocrystals.30 The signi-
cantly larger diameters obtained herein (ca. 10 nm) suggest that
the higher temperature employed here has resulted in extensive
secondary growth at the interface following initial adsorption.
In the bulk, Au and Ag form alloys in the entire range of
compositions.34 More recent studies have suggested that alloying
at the nanoscale follows distinct pathways.35,36 In the specic case
Fig. 2 Scanning microscopic images of films obtained at 50 ꢁC over
180 min. (A) High magnification image of films of Au nanocrystals. The
cracks are produced by solvent evaporation post-deposition. (B) Low Fig. 3 (A) Transmission electron microscopic image of drop cast
magnification image of the same film, showing continuous and dispersions of Ag nanocrystal films. (B) TEM image of a drop cast
uniform deposits spanning many square millimetres (C & D) low and dispersion of Au nanocrystal films. (C) TEM image of the Ag0.5Au0.5
high magnification images of Ag nanocrystal films. (E & F) Low and high alloy film & (D) atomic force microscopic images of a film of Au
magnification images of Ag0.5Au0.5 films.
nanocrystals. The area is 250 nm ꢂ 250 nm.
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of Au–Ag, it has been indicated that compositional inhomoge-
neities are expected in particles synthesized at temperatures close
to room temperature.31 The evidence from HR-TEM suggests that
alloy particles have indeed been obtained. However, we believe
that further studies based on techniques such as X-ray absorption
spectroscopy and related techniques are required to establish the
veracity of alloying in these deposits.37
As alluded to earlier, in all cases, the lms consist of hier-
archical structure whereby nanocrystals with diameters of
z10 nm form higher order aggregates. The persistence of this
organization across the composition range suggests that
structural control lies in factors extrinsic to the reaction. We
believe that the structure adopted by the lms is shaped by the
interfacial medium used to grow the lms. This argument is
further reinforced by the previous observations of similar
hierarchical organizations in lms of Bi2S3,17 Cd(OH)2 (ref. 18)
and Pd38 obtained at the water–toluene interface. Clearly, such
organizations are brought about by the water–oil interface, the
only common factor in all the studies. However, the complex
nature of the interfacial deposition process means that it is
currently impossible to identify specic properties of the
medium that underpin such control.
We nd that the room temperature resistance of Ag nano-
crystal deposits was in the region of about 102 126 sq.ꢀ1 and is
only slightly higher than bulk Ag. The resistance increases
nearly exponentially with increasing Au content (see Fig. 4). A
systematic change of resistance over eight orders of magnitude
is seen. The robust and methodical change herein is particularly
impressive in the context of lms of nanocrystals where
a number of factors (discussed later) conspire to render reliable
measurements across different lms all but impossible.
The electronic properties of lms of metal nanocrystals
differ from those of the bulk metal.39–41 Charge transport is
primarily dependent on the diameter of the crystallites42,43 and
separation distance between the grains.44–47 Other pertinent
factors include structural disorder in the lms48 and mono-
dispersity in the size of grains.47
Fig. 5 Plot showing the temperature dependence of the film resis-
tance for different compositions (labelled). The plot in top left show
resistance vs. temperature data for metallic Ag films, while the top right
plot corresponds to Au (non-metallic). Three distinct temperature
profiles is apparent in this panel.
We studied the dependence of electrical resistance
on temperature in detail using different lms of Au–Ag nano-
crystals. We nd that the AgxAu1ꢀx lms with x # 0.4 exhibit
a negative co-efficient of resistance signalling that deposits are
metallic (Fig. 5). Such behaviour is indicative of strong exchange
coupling between the particulates. Robust coupling extending
over a number of compositions is noteworthy, especially in light
of the disorder inherent to these lms. When the percentage of
Au in the lms is increased, the temperature co-efficient
becomes positive, signalling that the deposits are non-
metallic. The resistance change with fall in temperature
increases with Au percentage. The observed behaviour is
consistent with the hopping of localized electrons from one
nanocrystal to another.21,49–51 Here, the conductivity (s) is:
s f eꢀ2dbeꢀE /kT
(2)
a
where d is the separation distance between the nanocrystals,
b the constant associated with tunnelling between the particles
separated by a dielectric medium, and Ea the activation energy
for electron hopping between the grains. In the case of inter-
facial deposits, the particulates have a well-dened surface layer
and hence, d and b may be assumed to be constant at different
values of temperatures T (hence neglecting thermal expansion)
for a specic lm, resulting in Arrhenius type equation:
s ¼ AeꢀE /kT
(3)
a
Fig. 4 Plot showing the change in the natural log of resistance, ln(1/s)
against the composition of the film of nanocrystals. The resistance is in
units of ohms per square.
The activation energy, Ea corresponds to:
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e2
r
silver lms that have been at the interface between two
immiscible liquids using other precursors.55,56 The charge
transport characteristics of these lms are unknown.
1
Ea ¼
(4)
4p303r
where 30 is the vacuum permittivity, 3r the dielectric constant of
the medium surrounding the particles, and r the radius of
nanocrystals (z5 nm). Films of different compositions are ex-
pected to have different Ea, as 3r is different.52 Similarly, changes
are expected to the pre-exponential factor, as the surface density
and possibly composition of the ligand shell varies with Au : Ag
ratios.
In deposits with non-metallic characteristics, two Arrhenius
regimes are apparent, with changeover centred around z50 K.
The Eas are in the range of a few meV per grain in the high
temperature region (100–250 K). At lower temperatures, Eas falls
lower (<0.1 meV). Hence, charge transport appears to be medi-
ated by a activated hopping in the high temperature region,
while at lower temperatures, tunnelling is dominant. In the case
of pure Au, another high temperature regime, above 225 K with
very low Ea, mirroring the low temperature region is also seen.
Similar multiple domains of activated transport have been
previously reported, through the reasons of this behaviour
remains elusive.21 Rao and co-workers have studied electronic
transport properties of nanocrystalline lms obtained at the
water–oil interface.53 They found that the charge transport can
be metallic when particulate diameters are high (>10 nm) or
when particular ligands capable of bridging the gap between
particulates are used.
4 Conclusions
Highly uniform lms of Ag and Ag–Au nanocrystals with
phosphinyl surface ligands have been obtained at the water–
toluene interface. The lms posses a hierarchical structure
consisting of large aggregates of 10 nm sized nanocrystals. The
ubiquity of this structure across the range of compositions
studied suggests that the medium shapes the lms. Remarkable
differences were seen in the optical and electronic properties of
the lms as the composition is varied. Strong electronic
coupling between Ag nanocrystals and possibly weak interac-
tion with surface ligands yields metallic lms with resistance of
a few ohms. On the other hand, lms comprising of Au nano-
crystals exhibit behaviour akin to semiconductors with resis-
tance values in the range of MUs. The resistance of the lms
scales exponentially with composition of the lms, changing
from metallic to non-metallic behaviour when the percentage of
Ag falls below 70%.
Conflicts of interest
There are no conicts to declare.
A switch from metallic to non-metallic behaviour can be
brought about by factors such as decrease in diameter of the
nanocrystals, increasing disorder and separation distance as
well as by changes in the nature of interaction with surface
ligands. As alluded to earlier, change in composition of the
alloy produces little discernible change to the overall dimen-
sions of nanocrystals and therefore unlikely to account for the
change to non-metallic behaviour. Given the regular trend
with change in composition, we believe that the switch in
behaviour can be attributed to the differences in interaction
between the particulates and the surface ligands. In the case of
Ag rich lms, relatively weak binding to surface ligands results
in physical contact between neighbouring nanocrystals
leading to development of conductive pathways (via partial
sintering) which result in overall metallic behaviour. Sintering
is expected to be extensive, particularly during the secondary
growth phase when the main site of reduction is the surface of
interfacially adsorbed particulates. In the case of Au rich lms,
stronger interactions lead to better dened surface structure
and robust interparticle repulsion, resulting in the collapse of
conductive pathways, leading to semiconducting behaviour.
This argument is supported by TEM images in 3 which show
that following sonication, Au lms are broken down to well
dened spherical nanocrystals while Ag lms yield larger
aggregates.
Acknowledgements
The authors thank Dr Graham Smith and Dr Simon Hodgson of
Chester University for help with X-ray photoelectron spectro-
scopic measurement.
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