copper conductive inks-rsc adv.2015!5!63985

8/16/2019 Copper Conductive Inks-RSC Adv.2015!5!63985

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Copper conductive inks: synthesis and utilization inexible electronics

Venkata Abhinav K,a Venkata Krishna Rao R,a P. S. Karthika and Surya Prakash Singh*ab

Conductive inks are a recent advance in electronics and have promising future applications in exible

electronics and smart applications. In this review we tried to focus on a particular conductive ink that is

based on copper nanoparticles. Although extensive research is being done all over the world, a few

complications are yet to be perfectly solved. We tried to focus on some of the complications involved in

their synthesis and their various applications in the different elds of science. Conductive inks have

promising applications in the present trends of science and technology. The main intention behind this

review is to list some of the best methods to synthesize copper nanoparticles according to the method

of synthesizing them. We chose copper nanoparticle synthesis and the preparation of conductive inks

because copper is a very abundant material, possesses high conductivity (after silver), and it has huge

potential to replace expensive conductive inks made of silver, graphene, CNTs, etc. The other reason

behind focussing on copper is its properties, such as ductility, malleability, thermal dissipation activity,

anti-microbial nature, etc. In this review, we have listed some of the best methods of synthesizing

copper conductive inks and their usage in various printing techniques. Different methods of sintering for

the obtained conductive patterns are also included.

1. Introduction

Nanotechnology is a rapidly developing advance and its prod-

ucts are extremely useful in all elds, in view of their small size

(109 m) and substantial surface range. Nanoparticles off er a

larger surface-to-volume ratio when compared to macro- and

micro-materials. The extraordinary properties of nanoparticles

are because of a solid exchange between versatile, geometric,

and electronic parameters. The consequence of these features

can be tuned by physical and substance properties contrasting

with those of the mass material.1 The examination of nano-

particles has attracted wide interest in the most recent decades

on account of their strange and size-dependent optical,

a Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical

Technology, Uppal Road, Tarnaka, Hyderabad-500007, Indiab Network Institute of Solar Energy (CSIR-NISE), Academy of Scienti c and Innovative

Research (AcSIR), New D elhi, I ndia. E-mail: [email protected]

Dr Surya Prakash Singh is a

Scientist at the CSIR-Indian

Institute of Chemical Tech-

nology, Hyderabad. He studied

chemistry at the University of

Allahabad, India, and obtained his D.Phil. degree in 2005. A er

working at the Nagoya Institute

of Technology, Japan, as a post-

doctoral fellow, he joined Osaka

University, as an Assistant

Professor. He worked as a

researcher at the Photovoltaic

Materials Unit, National Institute for Materials Science (NIMS),

Tsukuba, Japan. He has been involved in the design and synthesis

of materials for organic solar cells and exible devices. He has

published over 100 papers and reviews in peer-reviewed journals.

Venkata Abhinav K is a research

student at the CSIR-Indian


nology, Hyderabad, India, in the

group of Dr Surya Prakash

Singh. He completed his Bache-lor ’ s degree in Electronics and

Communication engineering at

Jawaharlal Nehru Technological

University, Hyderabad, India.

His research interests are

focussed on synthesizing

conductive nanomaterials using

various techniques and applying them in the eld of printed elec-

tronics. He is also interested in self assembly of fullerenes and

fabrication of solar cells using cost e ff ective materials.

Cite this: RSC Adv., 2015, 5, 63985

Received 4th May 2015

Accepted 22nd June 2015

DOI: 10.1039/c5ra08205f

www.rsc.org/advances

This journal is © The Royal Society of Chemistry 2015 RSC Adv., 2015, 5, 63985–64030 | 63985

RSC Advances

REVIEW


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attractive, electronic, and compound properties. To completely

use these properties, the size and shape must be eff ectively

controlled. This technology has gained a lot of importance in

recent years due to its applications in multi-disciplinary elds

(biology, chemistry, electronics, pharmacy, cosmetics, energy,

etc.). This is one of the promising technologies for the future

due to its advantages over the present technologies and their

applications.2

The incorporation of nanotechnology in the eld of elec-tronics was initiated more than a decade ago. Even though vast

research is being conducted in this eld, much more has to be

done to implement them in real-time applications. We tried to

focus on one of the sub-elds of electronics i.e., exible elec-

tronics. Flexible electronics is a contemporary eld which has

applications in energy harvesting, touch screens, solar panels,

microcontrollers, paper electronics, PCBs (printed circuit

boards), etc. The top three metals in terms of conductive

applications are silver, copper and gold. Silver is a widely known

metal as an ornament due to its lustrous character. Its added

advantages are it being the most conductive element (6.30 107

Siemens meter

1

at 20

C), its thermal conductivity that can withstand extreme temperature conditions, very good reec-

tance, anti-bacterial nature, corrosion-free capacity, etc. Apart

from all the advantages silver is considered to be one of the

most expensive metals and it is very much less abundant in the

earth’s crust (68th place with 7.9 106%). The second element

with high conductance a er silver is copper.

Copper is a reddish element with a bright metallic lustre. As

with other metals, copper is also malleable and ductile and it is

the 26th most abundant element in the earth’s crust with

0.0068% availability.3 The name copper was derived from ‘aes

cyprium’ a Latin word which means “from the land of Cyprus”4

and it was later changed to cuprum and to copper in English.

The electron conguration of copper is [Ar].3d10.4s1; it has onefree electron in its outermost shell which contributes to its

conduction. The atomic number of copper is 29 with atomic

weight 63.546. The melting and boiling points are 1084.62 C

and 2562 C, respectively. The structure of copper is FCC with

possible crystal morphologies of cubic {100}, octahedral {111},

dodecahedral {110}, tetrahexahedral {530} and their

combinations.

In this review we have tried to focus on the synthesis of

copper nanoparticles and the preparation of conductive inks

with the synthesized copper particles (Scheme 1). Copper

nanoparticles have shown promising applications in several

technological elds as thermal dissipation agents, anti-microbial and anti-fungal agents, lubricants, metal injection

moulding, catalysts, exible electronics, transparent conduc-

tors, etc. Copper nanoparticles5 have been obtained basically

using three diff erent approaches, physical, chemical and bio-

logical; however, biological synthesis was referred to as a sub-

division of chemical processes. The physical approaches

includes thermal evaporation, laser ablation, spray pyrolysis,

ball milling, etc., whereas the chemical synthesis processes

include electrochemical, chemical reduction, photochemical,

sono-chemical, polyol, etc.

Copper lms are of high interest for their use as inter-

connecting materials in multilevel integrated circuits, becauseof their high conductivity (59.88 106) and excellent electron-

migration resistance.6 Various methods for the preparation of

Cu lms have been reported and the most extensively investi-

gated methodology has been MOCVD (metal–organic chemical

vapor deposition) due to its advantages of uniform step

coverage and selectivity. However, solution deposition also has

potential and, in particular, electro-deposition has proved

capable of eff ective integration in standard complementary

metal oxide semiconductor (CMOS) processes as well as

production of nano-structured layers.

Copper has very few disadvantages when compared to silver,

such as low conductivity and high oxidation tendency when

exposed to the atmosphere.6 Copper has advantages of low cost and high thermal and electrical conductivity (a er silver).

Therefore, it is best to select copper over silver. The synthesis of

pure (oxide free) copper nanoparticles requires very clean and

Venkata Krishna Rao R is a

research student at the CSIR-

Indian Institute of Chemical

Technology, Hyderabad, India,

in the group of Dr Surya Prakash

Singh. He completed his Bache-lor ’ s degree in Electrical and

Electronics Engineering at




focussed on synthesizing various

conductive nanomaterials using

di ff erent techniques and applying them in the eld of exible

electronics. He is also interested in self assembly of fullerenes,

conductive inks and fabrication of photovoltaics using cost e ff ec-

tive materials.

P. S. Karthik is a research

student at the CSIR-Indian


nology, Hyderabad, India, in the

group of Dr Surya Prakash

Singh. He has completed his Bachelor and Master degrees at




focussed on synthesizing carbon

nanomaterials using various

techniques and applying them in

the eld of solar energy. He is also focussed on fabricating solar

cells using di ff erent light absorbing materials. He has published

three research papers in the eld of Nanotechnology.

63986 | RSC Adv., 2015, 5, 63985–64030 This journal is © The Royal Society of Chemistry 2015

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2. Techniques in synthesizing coppernanoparticles

As mentioned above, copper nanoparticles can be synthesized

using many techniques. Chemical reduction is the most widely

used technique for the preparation of copper nanoparticles. The

polyol method, microwave synthesis, photochemical synthesis,

and electrochemical chemical syntheses are very rarely used.

Some of the best methods for synthesizing copper nanoparticles

using the above mentioned techniques are explained in detail

below.

2.1. Chemical reduction

Reduction can be termed as a reaction that involves the gain of

electrons. For example, when iron forms rust, oxygen gets

reduced whereas iron gets oxidized. Using the same concept,

copper nanoparticles can also be synthesized by chemical

reduction, where the copper salt gets reduced to copper and the

corresponding reducing agent gets oxidized. Reduction is one

of the most prominent and widely used processes for synthe-sizing metallic nanoparticles. Many methods have been repor-

ted for synthesizing copper nanoparticles via chemical

reduction in the vicinity of diff erent capping agents. However,

the use of PVP as a stabilizing agent has been the most reported,

although the combination of PVP with CTAB was also reported.

Along with PVP, diff erent stabilizing agents like oleic acid,

carboxylic acids (glycolic acid, lactic acid, acetic acid, etc.),

PAAm, PEG, etc. were also used. Some of the best methods to

synthesize copper nanoparticles using chemical reduction are

explained in detail.

2.1.1 Using PVP. Huang et al.8 reported a method for the

preparation of copper nanoparticles synthesized by the reduc-

tion of 0.01 M copper(II) acetate dispersed in ethanol. Thereaction mixture was made by adding the copper acetate–

ethanol dispersion to 5 ml 2-ethoxyethanol in the vicinity of

poly(n-vinylpyrrolidone) (PVP) (PVP was used in diff erent

concentrations i.e., 0.2, 0.5, 1.0 wt%). In response, Cu2+ ions in

the mixture were reduced to copper metal by the inhibition of a

surplus of hydrazine-monohydrate under reuxing conditions.

The total synthesis process was carried out in a nitrogen envi-

ronment to prevent the particles from oxidizing. The same

process was repeated using water as a replacement for 2-

ethoxyethanol. The same method was used for the preparation

of the nanoparticles through this particular arrangement and

allowed the eff

ective combination of polymer-coated coppernanoparticles. By the use of diff erent amounts of PVP, copper

nanoparticles of diff erent sizes were obtained.

The characterization of the particles was carried out by the

utilization of transmission electron microscopy (TEM) as well as

UV-visible spectroscopy. The non-linear optical properties of the

copper nanoparticles were determined utilizing the Z -scan tech-

nique. UV-vis characterization was carried out for measuring the

absorbance of the obtained copper nanoparticle colloid.

Depending on the concentration of stabilizing agents, the absor-

bance varied, exhibiting a surface plasmon resonance (SPR) in the

range of 570 nm to 582 nm, as shown in Fig. 1 and 2.

The non-linear optical properties were measured using the Z -

scan technique.9,10 It was used to determine the magnitude8 of

the non-linear refractive index (n2) and non-linear absorption

coefficient (a2). The obtained results imply that n2 doesn’t

correspond to the third order non-linear response and hence

the susceptibility totally corresponds to a2, as shown in Fig. 3.

Fig. 1 UV-visible absorption spectra of copper nanoparticles with

water as a function of PVP concentration, acting as a stabilizing agent

(reprinted with permission from ref. 8).

Fig. 2 UV-visible absorption spectra of copper nanoparticles with 2-

ethoxyethanol as a function of PVP concentration, acting as a stabi-

lizing agent (reprinted with permission from ref. 8).

Fig. 3 Z -Scan plot representing the plot of transmittance vs. sample

position to determine the non-linear optical characteristics of the

obtained copper nanoparticles (reprinted with permission from ref. 8).


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Li et al.11 has reported a method to synthesize copper

nanoparticles. The precursors used are copper nitrate trihydrate

(Cu(NO3)2$3H2O), poly(vinylpyrrolidone) (PVP K-30) and

hydrazine hydrate (N2H4). Copper nitrate was reduced to

metallic copper nanoparticles with the hydrazine hydrate in the

presence of PVP, which acts as a stabilizing as well as capping

agent.

The obtained nanoparticles were characterized using SEM,

a UV-Visible spectrophotometer and XRD. The analyzed SEMimage is illustrated in Fig. 4. It can be clearly seen that the

obtained particles are uniform in size and shape with an

average size of 80 nm, as shown in Fig. 4. The UV-visible

spectra show that the copper particles have an absorption

band in the range of 550–600 nm (ref. 12) with the highest

absorption rate at a wavelength of 596.5 nm. The XRD peaks

represent diff raction at 43.6, 50.7 and 74.45, representing the

(111), (200) and (220) planes of the FCC crystal structure of

pure copper without any impurities. The reason for this anti-

oxidizing nature being due to the presence of PVP as a

capping agent.

Takuya et al.

13

synthesized copper nanoparticles using aliquid phase reduction method. The source of copper is copper

acetate. Copper acetate is dissolved in distilled water and

sodium borohydride (0.1 mol dm3) is added to the copper

acetate solution. Here, sodium borohydride acts as a reducing

agent and polyvinyl pyrrolidone (PVP ( M w 10 000)) in varying

quantities (0.5, 0.1 and 2.0 g) was used as a stabilizing agent.

The mixed solution was reuxed at 20 C for 1 h. A er one hour,

a black colloidal dispersion of copper nanoparticles was

obtained. The experiment was carried out in diff erent atmo-

spheres (nitrogen, oxygen and atmospheric air).

Characterization was carried out using TEM and HRTEM,

and simultaneously a SAED pattern was recorded for the

obtained copper nanoparticles. The TEM images of the coppernanoparticles demonstrate the variation of morphology of the

nanoparticles depending upon the ambient conditions. The

particles obtained under a N2 environment are spherical and

elliptical in shape, as illustrated in Fig. 5a, with a size range of

5–30 nm. Nanorods are formed when the synthesis process is

carried out in atmospheric air, which are illustrated in Fig. 5b.

The average aspect ratio of length to breadth was determined to

be 20 : 2. The average size was 5 nm and these nanorods line upin several straight lines which leads to long chains. When the

synthesis of copper nanoparticles is carried out in an oxygen

environment the particle size was comparatively smaller (3 nm),

as shown in Fig. 5c.

The particle morphology is also dependent on the quantity of

PVP used in the synthesis process. When 0.1 g of PVP is used,

the nanoparticles formed are in the shape of a cube, as shown in

Fig. 6a. If 2.0 g of PVP is used, spherical nanoparticles are

formed under atmospheric air conditions, as shown in Fig. 6b.

When a moderate amount (0.5 g) of PVP is used as a capping

agent, a combination of rods and spheres are formed, as shown

in Fig. 6c.Sampath et al. synthesized jasmine bud-shaped copper

nanoparticles14 by selecting copper(II) sulphate pentahydrate,

isonicotinic acid hydrazide, L-ascorbic acid, sodium hydroxide

(NaOH) and poly-vinylpyrrolidone as precursors. Copper

sulphate was dissolved in Milli-Q water and was added to the

solution containing 1% PVP. The solution of NaOH in de-

ionized water was added to adjust the pH (greater than 7) of

the copper salt solution and stirred for 1 h. Ascorbic acid was

dissolved in de-ionized water and added to the copper solution

and stirred for 1 h, maintaining the solution at room temper-

ature. A er 1 h the temperature is raised to 70 C and theFig. 4 SEM image of copper particles (reprinted with permission from

ref. 8).

Fig. 5 TEM image of copper nanoparticles prepared in (a) N2, (b) air

and (c) O2 (reprinted with permission from ref. 13).


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solution stirred for 10 min. The obtained ascorbic acid solution

was added to the reaction mixture dropwise; the colour of the

reaction mixture turns to yellow, indicating the formation of

copper nanoseeds. 0.001 M isoniazid was added gradually to thereaction mixture; the colour of the reaction mixture changes

from yellow to reddish brown, which is an indication of the

formation of the copper nanobuds. The solution was then

centrifuged at 8000 rpm for 30 min and washed with ethanol

and dried under vacuum.

The characterization of the obtained jasmine bud-shaped

copper nanoparticles was carried out using TEM, UV-visiblespectroscopy, XRD and AFM. The TEM image clearly show the

bud-shaped copper nanoparticles with a scale bar of 100 nm;

apparently the particle size is suspected to be less than 10 nm,

as shown in Fig. 7. The XRD peaks of the copper nanobuds

display diff raction at 43.6, 64.5 and 77.7 corresponding to the

(111), (200) and (220) planes, respectively. The diff raction at

64.5 represents an impurity, i.e., cupric oxide (Cu2O). The

obtained peaks have been matched with JCPDS no. 4-0836. It is

calculated that the average crystallite size is about 6.95 nm

(using the Debye–Scherrer equation).

AFM was used to determine the height and structure of the

copper nanobuds and the obtained AFM image is in closeagreement with the obtained TEM results, as shown in Fig. 8.

The AFM measurement reports that the average size of the

nanobuds is 6.41 nm, which is approximately equal to the XRD

calculations. UV-visible spectroscopy shows that the surface

plasmon resonance phenomenon occurs at 573 nm and the

absorption band was located around 560–570 nm, which is

reported to undergo a blue shi 15 with decrease in size.

Yang et al. synthesized copper nanoparticles in an oblate

shape using a one-step large scale synthetic method with a yield

of 91.36%. The average size is calculated to be 80 nm and these

nanoparticles exhibited good anti-oxidation properties. In this

synthesis process,16 the precursors used were copper(II) oxide,

poly-vinyl pyrrolidone (PVP, K-30), hydrazine hydrate and

ethanol. Firstly, an appropriate amount of PVP was dissolved in

ethanol by stirring at a temperature of 40 C until a clear

solution was obtained. To this reaction mixture, copper chlo-

ride was added under vigorous stirring. Subsequently, hydra-

zine hydrate was injected into the reaction mixture and stirred

for 60 min. The change in colour of the solution from green to

henna colour indicates the growth of copper nanoparticles.

These particles were collected by centrifugation and washed

with ethanol and oleic acid. A er washing, the copper nano-

particles were dried at room temperature.

Fig. 6 TEM images with varying concentration of PVP in air: (a) 0.1 g,

(b) 2.0 g and (c) 0.5 g (reprinted with permission from ref. 13).

Fig. 7 TEM image of bud-shaped copper NPs (reprinted with

permission from ref. 14).

Fig. 8 AFM image of copper nanobuds (reprinted with permission

from ref. 14).


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The characterization of the dried copper nanoparticles was

carried out using TEM, HRTEM, UV-visible spectroscopy, XRD,

and FTIR. The TEM results show that the particles are in oblate

shape with an average size of 80 nm. A layer of PVP was also

detected and said to be the main cause of the size restriction of

the nanoparticles and the anti-oxidation property. The HRTEM

image displays the thickness of the PVP coated on the copper

nanoparticles, which is observed to be 8.72 nm as illustrated in

the Fig. 9. The obtained absorption peaks were diff erent for thetwo diff erent concentrations of PVP (0.01 M and 0.015 M)

exhibiting surface plasmonic resonance at 603 nm and 596 nm,

respectively. The XRD peaks suggest that diff raction occurs at

43.2, 50.3, 74.1 and 89.6, which correspond to the (111),

(200), (220) and (311) crystal planes, respectively, which are

clearly FCC structured. Surprisingly, no diff raction of oxides

was detected.

2.1.2 Using PVP & CTAB. Chen et al.17 synthesized air-stable

copper nanoparticles with an average diameter of 6.5 nm. The

precursors used for the preparation of the nanoparticles are

copper sulphate pentahydrate (CuSO4$5H2O), hydrazine

(N2H4$

H2O), di-ethylene glycol (DEG), PVP and CTAB. Thecopper source is copper nitrate, the reducing agent is hydrazine

and PVP and CTAB act as stabilizing agents18,19 with DEG as the

solvent. The reaction was carried out at a temperature of up to

80 C for about 30 minutes to 2 h. The same process was carried

out using a single capping agent, i.e., only PVP or only CTAB,

and the results were compared with the help of characterization

techniques. The synthesized nanoparticles were characterized

using XRD, TEM and FTIR for a clear understanding of the

obtained results.

The XRD analysis show that diff raction peaks at 43.3, 50.4

and 74.08 were obtained, as shown in Fig. 10, representing the

(111), (200) and (220) diff raction planes of the FCC structure

when PVP/CTAB are used, and a minor peak of copper oxide was

observed when using only PVP or CTAB.

TEM analysis was performed for all three diff erent combi-

nations. The importance of the PVP/CTAB combination can be

clearly seen from (Fig. 11a) the PVP-coated Cu NPs, (Fig. 11b)

Fig. 9 TEM image of oblate-shaped copper NPs, HRTEM image dis-

playing a layer of PVP, and SAED pattern indicating the polycrystalline

structure (reprinted with permission from ref. 16).

Fig. 10 XRD pattern recorded for copper nanoparticles stabilized with

(a) PVP, (b) CTAB and (c) a combination of PVP/CTAB (reprinted with


Fig. 11 TEM micrographs of (a) PVP-Cu nano-particles, (b) CTAB-Cu

NPs and (c) PVP/CTAB (reprinted with permission from ref. 17).


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the CTAB-coated Cu NPs and (Fig. 11c) the PVP/CTAB-coated Cu

NPs, reduced by hydrazine. It can be clearly observed from

Fig. 11 that the PVP/CTAB-coated copper NPs were uniform and

the same in size and shape, with a scale bar of 50 nm. The

average diameters of the PVP and CTAB coated Cu NPs are

reported to be equivalent to 14.03 nm and 12.35 nm, respec-tively, and the average diameter of the Cu NPs coated with a

combination of PVP and CTAB is nearly 6.5 nm. The main

reason behind this distinction in their diameters is due to their

relative rate of nucleation & growth and their tendency to

agglomerate. However, the growth of ner nanoparticles may be

due to the fact that PVP or CTAB as sole stabilizers have less

tendency to get adsorbed on the nuclei, therefore resulting in

rapid agglomeration. In the case of assorted capping agents, the

adsorption onto the nuclei is better, which restricts the

agglomeration of the nanoparticles. FTIR spectroscopy was

performed to further examine the coordinative interactions

between the copper nanoparticles.

2.1.3 Using oleic acid. Jing et al.20 have reported a similar

method, using copper acetate as the source of Cu+ ions and

oleic acid as a stabilizing agent, with a reducing agent of

hydrazine hydrate and toluene as a solvent. The reaction

process was carried out in a nitrogen atmosphere for 30 min.The temperature was maintained at 70 C for 3 h.

The size and shape of the copper particles were observed

using TEM. All the particles obtained were uniform in size and

shape, as shown in the Fig. 6. The XRD pattern was obtained

showing maximum diff ractions at 43, 50 and 74, which are

reported to be very close to the JCPDS le no. 4-0836. FTIR

analysis was carried out to understand the role of the organic

molecules used in their study. The TEM image is shown

in Fig. 12, showing the uniformity in size of the copper

nanoparticles.

Zhong et al.21 synthesized size-controlled and potentially

shape-controlled copper nanoparticles in organic solvents inthe vicinity of amine/acid capping agents. The synthesis

procedure involves reducing a copper(II) acetylacetonate

(Cu(acac)2)/octyl-ether solution in 1,2-hexadecanediol under

reuxing conditions at a temperature of 105 C with a constant

stirring rate for 10 min. The synthesis process is carried out in

an argon atmosphere. Oleic acid and oleyl amine were added to

the solution; a er the addition of both the capping agents, the

temperature of the solution was raised to higher temperature

Fig. 12 TEM image with a scale bar of 200 nm displaying particles with

uniform shape and size (reprinted with permission from ref. 20).

Fig. 13 TEM results of copper nanoparticles synthesized at different temperatures: (i) 150 C, (ii) 160 C, (iii) 190 C and (iv)210 C (reprinted with



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(150–210 C). The solution is kept at this temperature for 30 min

then cooled down to room temperature. Finally, the reacted

solution was mixed with ethanol and the solution was kept

aside overnight for the precipitate to settle down. The precipi-tate is washed and dried with a stream of N2 gas. The obtained

nanoparticles were suspended in hexane and were kept ready

for analysis.

TEM, XRD, UV-Vis spectroscopy and TGA were performed on

the obtained copper nanoparticles. The TEM results explain the

change in the morphology with respect to the temperature of

the synthesis of the nanoparticles, as shown in Fig. 13. The XRD

peaks conrm the formation of copper nanoparticles exhibiting

diff raction at 43.5, 50.6 and 74.3 at the [111], [200] and [220]

planes, respectively, which represent cubic symmetry without

any impurities. The UV-visible spectrum of the obtained nano-

particle solution was recorded, which displayed surface plas-mon resonance at 600 nm. Thermogravimetric analysis was

used to test the relative composition of the organic stabilizing

agents, which results in the prediction that 39% of the mass is

due to the capping shell and the remaining 61% is copper.

2.1.4 Using carboxylic acids. Xiao et al.22 utilised a process

where copper acetate was used as the source of copper ions,

with carboxylic acids (lactic acid, acetic acid, glycolic acid,

glycine, alanine and citric acid) as stabilizing agents. Hydrazine

hydrate (50%) was added as a reducing agent for reducing the

Cu ions to metallic copper, which was observed as a function of the change in the colour of the solution from blue to brown and

then to henna colour. The pH of the solution was monitored

using a mixture of ammonia and water. The temperature was

maintained at 40 C for 3 hours in an inert atmosphere.

The characterization was carried out using UV-Visible spec-

troscopy, TEM, FTIR and XRD. The UV-visible spectrum shows

that the absorption is in the range of 550–600 nm. The typical

values of the absorption peaks for acetic acid, glycolic acid,

alanine, lactic acid and citric acid are observed at 616, 610, 601,

600 and 582 nm, respectively.

The TEM results show that the sizes of the copper nano-

particles vary with diff erent concentrations of carboxylic acids.The formation of nanoparticles also depends on the type of

carboxylic acid used for the particle preparation. For example,

in Fig. 14a it is clearly depicted that the sizes of the un-stabilized

copper nanoparticles are larger when compared to the stabi-

lized nanoparticle dispersions (Fig. 15).

Fig. 14 TEM analysis of copper nanoparticles with different size distributions obtained with different concentrations of lactic acid: (a) 0 mol l1,

(b) 2.8 mol l1, (c) 5.6 mol l1, (d) 8.4 mol l1, (e) 11.2 mol l1 and (f) 14 mol l1 (reprinted with permission from ref. 22).

Fig. 15 TEM analysis of copper particles stabilized by different carboxylic acids of concentration 14 mol l1: (a) acetic acid, (b) glycolic acid, (c)

glycine, (d) alanine, (e) citric acid (reprinted with permission from ref. 22).


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The FTIR spectroscopy displays peaks at 1608–1561 and

1395–1375 cm1 which indicate stretching of carboxylates. The

amine peaks were obtained around 3400–3200 cm1

. The strong

absorption at 3400 cm1 is probably from the adsorbed water.

The peaks around 3400 cm1 showed the presence of the

hydroxyl groups of glycolic acid and lactic acid, respectively.

XRD patterns were recorded for the ethanol-washed copper

nanoparticles. The diff raction is mainly observed at 43.2, 50.3

and 74.1 for all the carboxylic acids, with miller indices (111),

(200) and (220), respectively (as shown in Fig. 16), which

conrms that the obtained structure is FCC, and a minor peak

for Cu2O was observed at 36.6

, representing the (111) index.2.1.5 Using PEG. Thi et al.23 synthesized copper nano-

particles using copper(II) sulfate penta-hydrate (CuSO4$5H2O),

which acts as a copper source. The copper salt is dissolved in de-

ionized water to obtain a blue-coloured solution. PEG 6000

(polyethylene glycol) was added to the copper salt solution.

Here, PEG acts as a stabilizing agent. The mixed solution is kept

under vigorous stirring until a clear solution is observed. The

reducing agents used are ascorbic acid (0.02 M) and sodium

borohydride (0.1 M). Firstly, a solution of ascorbic acid and

sodium hydroxide (NaOH) are mixed and added to the copper

salt solution, when a colour change (white to yellow) is

observed. Sodium borohydride solution is added and constant

Fig. 16 XRD patterns of copper nanoparticles synthesized using

different carboxylic acids at a concentration of 14 mol l1: (a) acetic

acid, (b) glycolic acid, (c) glycine, (d) alanine, (e) lactic acid and (f) citric

acid (reprinted with permission from ref. 22).

Fig. 17 TEM images of PEG-stabilized copper nanoparticles with PEG : copper: (a) 6 : 1, (b) 7 : 1 and (c) 9 : 1 and PEG-stabilized copper

nanoparticles (reprinted with permission from ref. 23).


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stirring is maintained. A er a certain amount of time, the

yellow solution changes to black/red, which indicates that the

reduction has started and copper nanoparticles are formed.Characterization was carried out using TEM, FT-IR and UV-

visible spectroscopy. The TEM images explain the importance

of PEG in the particle size; diff erent ratios of PEG : copper (6 : 1,

7 : 1 and 9 : 1) were used and the particle size decreased with

the increase in quantity of PEG used, as illustrated in Fig. 17.

The UV-visible spectra of the same ratios of PEG and copper

nanoparticles exhibit plasmonic resonances at 562 nm and it is

also observed that as the percentage of stabilizing agent

increases, the absorbance decreases. The FT-IR spectra of PEG

and PEG–copper are compared, which explains the interaction

between the PEG and copper nanoparticles. However, two

absorption peaks appear with the copper nanoparticles at 1690cm1 and 1760 cm1.

2.1.6 Using PAAm. The synthesis of copper nanoparticles

stabilized with nitrogen ligands was reported by Alvarez et al.24

In this method, nitrogen ligands like allylamine (AAm) and

poly-allylamine (PAAm) were used as stabilizers. Partially cross-

linked polyallylamine (PAAMc) leads to the formation of nano-

particles with low yields and high stabilization, whereas the use

of linear PAAm leads to the formation of nanoparticles with

high yield and low-coating content. The synthesis process is

carried out by selecting copper sulphate pentahydrate (CuSO4-

$5H2O), distilled water, hydrazine, sodium hydroxide (NaOH)

and PAAm as precursors. The source of copper is copper

sulphate, the reducing agent is hydrazine, PAAm is used for

stabilization of the copper nanoparticles and sodium hydroxide

(NaOH) is used to maintain the pH of the solution. Firstly,

copper sulphate is added to distilled water and stirred for 10

min at a temperature of 60 C. Subsequently, an appropriate

amount of PAAm solution (PAAm dissolved in distilled water)

was added drop-wise to the reaction mixture under vigorous

stirring. In time, sodium hydroxide (NaOH) solution dissolvedin distilled water was added drop-wise and made to react for 30

min followed by addition of hydrazine. This reaction mixture

was monitored at constant stirring at 60 C. The change in

colour (black) of the solution indicates the formation of copper

nanoparticles. The solution was then transferred to a centrifuge

tube and centrifuged at 15 000 rpm and washed with distilled

water and ethanol. The washed copper nanoparticles were dried

at 60 C for two hours. The synthesis of copper nanoparticles

was carried out by selecting diff erent molar ratios of PAAm/Cu.

The characterization of the obtained copper nanoparticles

was accomplished using TEM, XRD and TGA. The XRD infor-

mation shows the diff

raction that is identi

ed for threediff erent samples containing three diff erent molar ratios of

PAAm/Cu (2.00, 0.11 and 0.46). The diff raction is observed at

43.4, 50.5 and 74.0 for all three molar ratios, where the

sample with molar ratio R1 also displays trace peaks at 35.9

and 38.6 indicating that oxidation has started to form cuprous

oxide. The obtained peaks are comparable to the JCPDS number

04-0836 (ref. 25) shown at the bottom of Fig. 18. The average

crystallite size has been calculated using the Debye–Scherrer’s

formula26,27 and was calculated to be 13 nm.

TEM micrographs for all the molar ratios of PAAm/Cu are

shown in Fig. 19. The TEM image of the copper nanoparticles

with PAAm/Cu at a molar ratio of 2.0 is shown in Fig. 19a. In this

case the yield was less and the average diameter was calculatedto be 3.9 nm. Fig. 19b shows the TEM image with the molar ratio

of AAm/Cu of 97.0, with an average particle diameter of 6.0 nm.

Fig. 19c shows the TEM micrograph of PAAm/Cu with a molar

ratio of 0.11; the yield with this particular molar ratio is high.

Fig. 19d shows the TEM image with a molar ratio of 0.46. In this

case, the average particle diameter was 55 nm.

2.2 Polyol synthesis

In polymer chemistry, polyols are compounds with multiple

hydroxyl functional groups. Glycerin, pentaerythritol, ethylene

Fig. 18 XRD peaks displaying copper nanoparticles stabilized in PAAm

or AAm with different molar ratios (reprinted with permission from ref.

24). Index for Fig. 18: PAAm or AAm/Cu molar ratios (R1 ¼ 2.00, R2 ¼

97.0, R3 ¼ 0.11 and R4 ¼ 0.46).

Fig. 19 TEM results displaying the copper nanoparticles stabilized in PAAm/AAm with different molar ratios of PAAm/Cu: (a) 2.00, (b) 97.0{AAm/

Cu}, (c) 0.11 and (d) 0.46 (reprinted with permission from ref. 24).


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glycol, polyesters, polyethylene glycol, polyurethanes and

sucrose are some examples of polyols.28 The polyol process is

described as a novel route for preparing ultra-ne nano-sized

metal particles such as those of copper, gold, palladium,

silver, nickel, cobalt, iron, their alloys, etc. The synthesis

procedure in the polyol method is carried out by suspending the

precursor material in the liquid polyol (nitrates, chlorides and

acetates are more soluble whereas oxides and hydroxides are

slightly soluble). The supernatant is stirred and heated to agiven temperature, which can reach the boiling point of the

polyol for less reducible metals and in the case of easily

reducible metals, the reaction can be carried out even at 0 C.

Copper nanoparticles stabilized in PVP were synthesized by

Moon et al.29 using the polyol method. The synthesized copper

nanoparticles were around 45 nm (approx.) in size and the

shape was observed to be spherical. The important parameters

for controlling the shape and size of the copper nanoparticles

were the concentration of the reducing agent, reaction

temperature and rate of precursor injection. These parameters

are controlled to change the morphology of the copper nano-

particles. CuSO2, PVP, sodium phosphinate monohydrate and

DEG are the precursor materials for this synthesis. Firstly, the

desired amount of PVP was dissolved in DEG until a clear

solution was obtained, then diff erent concentrations of sodium

phosphinate monohydrate were added to the reaction mixture

and heated. An aqueous solution of copper sulphate pentahy-drate was injected into the hot reaction medium using a syringe

pump; the rate of injection was varied from 2 to 8 ml min1 and

the solution was stirred vigorously for 1 h. A er 1 h, the reaction

mixture was cooled to room temperature and le until the

particles settled at the bottom. The precipitated nanoparticles

were later separated by centrifugation, washed and dried.

XRD, XPS, SEM and HR-TEM characterizations were per-

formed. Phase composition and crystallite size were calculated

Fig. 20 XPS spectra of Cu nanoparticles: (a) Cu 2p3/2, (b) C 1 s and (c) O 1 s (reprinted with permission from ref. 29).


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using XRD. The FCC structure of the nano-copper was

conrmed by the XRD data with corresponding planes at (111),

(200) and (220). No traces of characteristic impurities were

found through XRD. The surface of the obtained copper nano-

particles was analyzed through XPS, where a copper peak was

identied at 932.0 eV with very weak CuO peaks at 934.2 eV (ref.

30), as illustrated in Fig. 20a. The interaction that is achieved via

a coordination bond between Cu and PVP31 molecules is dis-

played in Fig. 20b and c. SEM micrographs were recorded when varying all the reaction parameters. Fig. 21 displays the SEM

images of copper nanoparticles showing the size distribution of

the copper nanoparticles as a function of reducing agent

concentration (12.75 mmol, 17.53 mmol and 19.13 mmol). The

particle sizes were found to vary from 54–72 nm depending on

the concentration of reducing agent used. The reason for the

variance in the size of particles can be explained as follows: if

the concentration of the reducing agent is high, there is an

enhancement in the reduction rate, which in turn favors a high

probability of nuclei generation, causing the particle size to

decrease. In this case, the formation of an impurity (Fig. 21c)

takes place due to the formation of an intermediate phase.

However, in the case of low reducing agent concentration, the

rate of reduction is slow and this favors the formation of larger

sized particles. A moderate reducing agent concentration helps

in the formation of small sized particles with less of the

impurity (Fig. 21b). Fig. 22 shows the SEM images of the copper

nanoparticles when varying the reaction temperature (200 C,

170 C and 140 C); the particle size range is 45–53 nm. As the

temperature is high there is a chance of rapid generation of

copper particles resulting in multiple nucleations. In this

particular case, the particles formed are broader in diameter, as

shown in Fig. 22a. In a situation where the temperature of the

solution is low, the nucleation rate is slower and the nuclei

count is enough to reduce the concentration of copper atoms in

the limit of the critical supersaturation level, which further

results in monodispersed particles. Fig. 23 and 24 represent the

SEM images of the copper nanoparticles prepared by varying

rate of precursor injection (2 ml min1, 6 ml min1 and 8 ml

min1) at two diff erent temperatures (200 C and 140 C)

resulting in particle sizes of 47–63 nm. The HR-TEM image

suggests that the synthesized copper particles are mostly single

crystals with some of them possessing twin boundaries (amor-

phous), as illustrated in Fig. 25. The boundary thickness was

Fig. 21 SEM images of Cu NPs synthesized as a function of concen-

tration of reducing agent: (a) 12.75 mmol, (b) 17.53 mmol and (c) 19.13

mmol (reprinted with permission from ref. 29).

Fig. 22 SEM images of Cu NPs synthesized as a function of reaction

temperature: (a) 200 C, (b) 170 C and (c) 140 C (reprinted with



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measured to be 1.5 nm. The SAED pattern also corresponds tothe FCC structure without any traceable impurities.

Baldi et al.32 synthesized copper nanoparticles using a

microwave-assisted polyol method. PVP-stabilized copper

nanoparticles with a diameter range of 45 to 130 nm were

synthesized with very high yield and stability. The synthesis

procedure starts with dissolving the chelating agent (PVP) in

DEG, which was stirred until a clear solution was obtained and

heated using a microwave oven. A er reaching a certain reac-

tion temperature, two diff erent DEG solutions containing

ascorbic acid and copper acetate were added to the PVP solu-

tion. A change in color from green to dark red is observed which

indicates that the nucleation process has started, resulting inthe formation of copper nanoparticles. The temperature of the

microwave oven was varied (60–170 C) to study the eff ect of

heat treatment and it was found that the temperature was one of

the most inuencing factors in the morphology and growth of

the nanoparticles.

The obtained copper nano-suspensions were characterized

using UV-Vis spectroscopy, DLS, XRD and STEM. Fig. 26a shows

the diff erent steps in the copper reduction during the synthesis

process. The copper acetate precursor solution was green and as

the reduction starts the color change is clearly observed, ending

in a dark red colour. Fig. 26b displays a comparison between

reduced copper and cuprous oxide particles, with absorption

peaks at 725 nm and 450 nm, respectively. The UV-vis spectra

were recorded as a comparison of the copper nanoparticles

synthesized at various temperatures with their reaction times is

shown in Fig. 27b and c. Fig. 27a displays the STEM image of thecopper colloidal solution with homogenous particle sizes, with

an average size of 46 nm and a standard deviation of 9 nm.

Fig. 27b illustrates the particle size distribution from the STEM

Fig. 23 SEM images of Cu NPs synthesized at 200 C as a function of

precursor injection rate: (a) 2 ml min1, (b) 6 ml min1 and (c) 8 ml min


Fig. 24 SEM images of Cu NPs synthesized at 140 C as a function of

precursor injection rate: (a) 2 ml min1, (b) 6 ml min1 and (c) 8 ml min


Fig. 25 HR-TEM image of synthesized Cu NP displaying an amor-

phous layer twin boundary, with a scale bar of 5 nm. SAED pattern of a

single particle is inset (reprinted with permission from ref. 29).


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diff

raction patterns were recorded for the copper nanoparticlessynthesized with a Cu(NO3)2 : NaOH molar ratio of 2 : 1,

reuxed at 140 C and observed at diff erent time periods (20, 30,

120 and 240 min) a er heating the solution. The SEM images of

the copper powders were obtained at various NaOH concen-

trations at a reuxing temperature of 140 C. It can be seen that

the sample prepared with low NaOH concentration at a reaction

time of 20 hours was non-uniformly agglomerated. The average

size of the obtained copper powders decreased with increasing

NaOH concentration, as shown in Fig. 30. The SEM images of

copper powders synthesized at various reaction temperatures of (a) 120 C, (b) 140 C and (c) 160 C, with a molar ratio of 3 : 1,

are shown in Fig. 31.

2.3 Photochemical synthesis

Photochemistry is a branch of chemistry dealing with chemical

synthesis upon irradiation with photons. It is the study of

chemical reactions that proceed with the absorption of sunlight

by atoms or molecules.37 One of the best examples of photo-

chemical synthesis is photosynthesis. Degradation of plastics

and the formation of vitamin D with sunlight are also part of

photochemistry. Photochemistry is concerned with the

absorption, excitation and emission of photons by atoms,atomic ions, molecules, molecular ions, etc. In photochemistry,

energy is absorbed or emitted in discrete quanta called photons

and the absorption of light leads38 to an electronic excitation,

where the whole process starts working. An example schematic

explaining the photochemical synthesis of articial oxygen by

RGO45 sheets is shown in Fig. 32, using the principles of

photochemistry. Copper nanoparticles have been synthesized

using many synthesis methods, some of which are brie y

described below.

Copper metal nanoparticles have been synthesized by Kapoor

et al.39 by irradiation with 253.7 nm light, carried out using a low

Fig. 27 (a) STEM image of the copper nanocolloid. (b) Particle size distribution interpreted through STEM analysis. (c) Particle size analysis

through DLS (reprinted with permission from ref. 32).

Fig. 28 TEM images of copper nanoparticles (a) after removing PVP

and (b) after coating with octanethiol (reprinted with permission from

ref. 35).


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pressure mercury arc lamp. A de-aerated aqueous solution of

copper sulphate, PVP (polyvinyl pyrrolidone) and benzophenone

was placed in a rectangular quartz cuvette. A 200 W low pressure

Hg lamp was used as the source of ultraviolet rays for irradiation

at 253.7 nm at ambient temperature. The cuvette was placed in

the reactor for photolysis. The incident number of photons was

determined using a tris(oxalato)ferrate(III) actinometer to be 5.0 1015 cm2 s1. The solution was de-oxygenated by bubbling

nitrogen gas through it for 15 min and was excited with a fourth

harmonic output pulse of 35 ps duration with a laser ash

photolysis at a temperature of 20 1 C. The concentrations of

PVP and benzophenone inuence the particle size proportion-

ately. A similar process was carried out by Giuff rida et al.44 by

using a bis(2,4-pentandionato)copper(II) complex illuminated in

the presence of monochromatic emissions at wavelengths of 254

nm or 300 nm, stabilized in PVP.

The synthesized solution was characterized using a UV-

Visible spectrophotometer. Optical absorption spectra were

recorded, which displayed an intense absorption band exhib-

iting SPR at 565 nm, which is in the prescribed range for copper

particles.40 TEM images of the sample composed of PVP-stabilized copper nanoparticles were captured with a scale bar

of 20 nm, as illustrated in Fig. 33a. The size of the obtained

copper particles was found to be in the range of 15 4 nm. The

SAED pattern conrms that the obtained particles are poly-

crystalline in nature, as shown in Fig. 33b.

Nano-sized copper metallic particles and colloidal copper

nanoparticles were synthesized by Giuff rida et al.41 upon ultraviolet

Fig. 29 Colour change of precursor solution after homogenizing copper nitrate with NaOH with various molar ratios: (a) no NaOH addition, (b)

1 : 1, (c) 2 : 1, (d) 3 : 1, (e) 4 : 1 and (f) 5 : 1 (reprinted with permission from ref. 36).

Fig. 30 Morphology of the copper powders synthesized from various molar ratios of NaOH : Cu(NO3)2 at reux at 140 C (reprinted with



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irradiation of ethanol over bis(2,4-pentanedionate)copper(II)

[Cu(acac)2]. The copper colloid was obtained by irradiating the

solution of Cu(acac)2 in deoxygenated ethanol with 254 nm light in

the range of 106 to 105 Nhv min1.

A UV-vis spectrophotometer was used to monitor the specicabsorptions of the precursor solution at 242 and 294 nm. Also, a

new band was observed at 274 nm due to the chelation by 2,4-

pentanedione (Hacac). The absorption at 575 nm, as shown in

Fig. 34, was observed as a characteristic surface plasmon reso-

nance band of copper in the colloidal state as a result of longer

irradiation. A er aging of the colloidal copper, the X-ray

diff ractometer displayed diff raction of the crystals at 43.3

and 50.4 in the (111) and (200) planes, respectively. SEM

analysis was performed for the investigation of size and

morphology of the nanostructures formed by drying the

colloidal copper solution. Spherical particles with diff erent size

distributions of 20 to 80 nm were conrmed by the SEM images,

as shown in Fig. 35.

Nie et al.42 developed a facile method for the preparation of

copper nanoparticles via ultraviolet irradiation of a solution

Fig. 31 Morphology of copper colloids prepared using a NaOH : Cu(NO2)3 molar ratio of 3 : 1 at various reaction temperatures: (a) 120 C, (b)

140 C and (c) 160 C (reprinted with permission from ref. 36).

Fig. 32 An example schematic of photochemical synthesis (reprinted

with permission from ref. 45).

Fig. 33 (a) TEM image of copper nanoparticles and (b) SAED pattern


Fig. 34 Spectral alterations in the visible region upon irradiation of

Cu(acac)2 solution in ethanol (reprinted with permission from ref. 39).

Fig. 35 SEM imagesof aged copper colloid: (a) copperdried on silicon

and (b) copper lm deposited on quartz substrate (reprinted with



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containing a photo-initiator and a copper-amine compound.

Photoreduction of an ethanol solution of copper chloride using

a photoinitiator for the preparation of a copper nanoparticle

colloid was carried out by mixing CuCl2 in an ethanol solution

with photoinitiator-184 (1-hydroxycyclohexyl phenyl ketone).The solution was irradiated with a xenon lamp from the trans-

parent side of the cuvette for 40 minutes with a radiation

intensity of 45 mW cm2. The intensity of the irradiated light

was measured with the help of a ferric oxalate actinometer. The

total synthesis process was carried out in a vacuum glove box

because the preparation of the colloid is unstable in the pres-

ence of oxygen and hence it should be prepared in an oxygen-

free environment and should also be preserved in a dark envi-

ronment away from sunlight.

The photoreaction was monitored using a UV-vis spectro-

photometer, and the product characterized using XRD for the

determination of the composition of the obtained coppernanoparticles. The size and morphology was obtained using

TEM analysis. The diff raction peaks at 43.55 and 50.66

correspond to the (111) and (200) planes, respectively, which

conrms that the precipitate is copper. Simultaneously, the

TEM images revealing the size and shape of the nanoparticles

showed that the size of the prepared particles synthesized using

the photo-reduction method was less than 100 nm, as shown in

Fig. 36. UV-vis spectra were recorded every 20 s to detect the

changes in absorption and the change in the colour of the

solution from blue to colorless, colorless to black and then

black to colorless (formation of precipitate). SPR was observed

in the 550–600 nm range (approximately 575 nm) which is in

good agreement with the XRD and SEM results.

Colloidal copper was synthesized via laser irradiation of CuO

powder in the presence of 2-propanol by Yeh et al.43 The source

of laser irradiation was a Nd-YAG laser. A laser with a funda-

mental frequency of 1024 nm and second harmonic frequency

of 532 nm was used as a light source. CuO powder was dissolved

in 2-propanol and placed in a Pyrex vial to be irradiated by laserbeams of 1064 nm and 532 nm for the formation of a copper

nanoparticle colloid.

UV-vis spectra of the colloidal copper irradiated at wave-

lengths of 1064 nm and 532 nm were recorded. Peaks were

observed for the colloid prepared with irradiations of 1064 nm

and 532 nm due to the SPR phenomenon at 580 nm, which is

the characteristic peak of copper, with low absorbance. XRD

analysis was performed for the dried copper colloid, exhibiting

diff raction at 43.2, 50.3 and 73.3 corresponding to the

formation of metallic copper nanoparticles. TEM analysis was

performed for the copper colloid synthesized by irradiating at a

wavelength of 1064 nm; the shape of the particles was found to

be spherical with an average diameter of 55.9 nm, as shown inFig. 37. Fig. 38 shows copper nanoparticles synthesized by

irradiating at the wavelength of 532 nm. The average particle

size was found to be around 50 nm. From the TEM images

shown in Fig. 38, it can be clearly understood that the particle

size varies with the photon energy transmitted to sinter the

particles eff ectively.

2.4 Microwave-assisted synthesis

Microwave-assisted synthesis refers to the technique of

applying microwave radiation to promote a chemical reaction.

Fig. 36 TEM analysis of copper nanoparticles irradiated with ultravi-

olet radiation; the obtained particle size is less than 100 nm (reprinted


Fig. 37 TEM image of colloidal copper synthesized with irradiation at

1064 nm with a power of 100 mJ pulse1 for 5 min and 10 min,

respectively (reprinted with permission from ref. 43).

Fig. 38 TEM analysis of copper colloid synthesized by irradiating with

a 532 nm laser,irradiated with a power of: (a) 50 mJ pulse1 for 10 min,

(b) 50 mJ pulse1

for 30 min, (c) 115 mJ pulse1

for 30 min and (d) 300mJ pulse1 for 30 min (reprinted with permission from ref. 43).


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High frequency electric elds generated by microwaves have the

capability to generate controlled heat.46 The energy dissipated

by the microwaves has the ability to heat any material that contains mobile electric charges, such as conducting ions or

polar molecules in a solvent. One of the advantages of

microwave-assisted synthesis is that the time taken for a reac-

tion to reach completion is very much less when compared to

the other synthesis techniques.47 This is due to the fact that the

energy released is controlled and evenly distributed through the

chamber. Microwave-assisted synthesis of copper nanoparticles

has advantages of faster reaction times, rapid optimization,

higher yield and energy efficiency.

A one-step chemical synthesis of copper nano-uids was

developed by Yin et al.48 The precursors used in this synthesis

process are copper sulphate pentahydrate (CuSO4$5H2O),

ethylene glycol, poly-vinylpyrrolidone (PVP) and sodium hypo-phosphite (NaH2PO2$H2O). Firstly, copper sulphate was dis-

solved in a solution of ethylene glycol and PVP and stirred for 30

min. An ethylene glycol solution of sodium hypophosphite was

added to the reaction mixture and stirred for 5 min. A er 5 min,

the reaction mixture was put into a microwave oven and reacted

for 5 min under medium power. The copper nanoparticle

formation was conrmed by observing the change of the color

of the mixture from blue to dark red.

The XRD pattern was recorded, exhibiting diff

raction as perthe JCPDS le no. 04-0838, corresponding to the planes at (111),

(200), (220) and (311). The diff raction peaks can be indexed to

be a pure FCC structure. The TEM image reveals that spherical

copper nanoparticles, shown in Fig. 39 and 40, were obtained

with an average diameter ranging from 10 nm to 20 nm.

Surfactant-free synthesis of air-stable copper nanoparticles

was achieved by Shivashankar et al.49 Cu(acac)2 was dissolved in

benzyl alcohol in a round bottom ask and was exposed to

microwaves at 800 W for a time period of 3 minutes. The change

in the colour of the reaction mixture from blue to green and

further red indicates the formation of copper nanoparticles.

These copper nanoparticles were separated by centrifugation

and thoroughly washed with ethanol twice and diethyl etheronce. A er washing, the particles were dried under vacuum. The

obtained copper nanoparticles were found to be free from

oxides even a er 12 months of exposure to ambient atmo-

spheric conditions.

XRD spectra of fresh copper nanoparticles and copper

nanoparticles exposed to air for 12 months were compared and

found to be in good agreement with JCPDS no. 04-0836. The

average crystal size was determined to be 23 nm using the

Debye–Scherrer equation. The low magnication FESEM image,

as shown in Fig. 41, was used to analyze the copper sample,

revealing that it consists of mono-disperse spherical particles

with an average diameter of 150 nm. The optical absorption of the metal nanostructures was analyzed using UV-vis spectros-

copy, which exhibits SPR at 580 nm with low absorbance. The

TEM images reveal the size, structure and morphology of the

metallic nanostructures, as shown in Fig. 42. The average size

was calculated to be close to 30 nm, which is similar to that

from the XRD analysis. The SAED pattern was also obtained,

which conrms that the obtained nanoparticles have a FCC

structure with d ¼ 2.08 ˚ A, as shown in Fig. 42(c) and (f).

Zhu et al.50 synthesized functionalized copper nanoparticles

for application in glucose sensing. The copper nanoparticles

were functionalized with dimethylglycoxime using a microwave-

Fig. 39 (a) TEM image of copper nanoparticles prepared using stan-

dard procedure. (b) SAED pattern of the obtained copper (reprinted


Fig. 40 TEM images of copper nanoparticles synthesized using

CuSO4 at concentrations of (a) 0.2 M and (b) 0.5 M (reprinted with


Fig. 41 FESEM images of copper particles: (a) low magnication and

(b) high magnication (reprinted with permission from ref. 49).


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assisted synthesis process. Copper acetate hydrate (CuAc2$H2O)

was reacted with dimethylglycoxime (DMG) dissolved in

ethylene glycol. The mixture was placed in a microwave reux system under ambient conditions at a power of 365 W for 30

min. A er 30 min, the reaction mixture was cooled to room

temperature and centrifuged to collect the precipitate. The

obtained precipitate was washed with distilled water, ethanol

and acetone and dried under vacuum.

The TEM image reveals the size, shape and morphology of

the obtained copper nanoparticles. DMG played a signicant

role in the dispersion of the copper nanoparticles, withobtained diameters of 10 nm to 20 nm as shown in Fig. 43. A

comparison between the FTIR spectra of DMG and copper

functionalized with DMG was carried out. A noticeable diff er-

ence in the absorption band at 3419 cm1 was observed for the

DMG-stabilized copper nanoparticles. XRD peaks correspond-

ing to the copper structure (CCID les no. 040836 Cubic),

exhibiting diff raction at 43.3 and 50.4 in the planes of (111)

and (200), respectively, were found to be in close agreement

with the FCC structure of the copper nanoparticles.

2.5 Electrochemical synthesis

Electrochemical synthesis of nano-structured materials is ach-

ieved by passing an electric current between a cathode and

anode separated by an electrolyte. Electrochemical synthesis

has advantages of low cost, simple handling, exibility, low

contamination and no requirement of vacuum.5 Electro-

chemical synthesis has been used for the fabrication of nano-

structured energy harvesting materials, nanosheets, nanorods,

etc.51,52 The electrochemical deposition methods have been

proved to be highly productive and readily adoptable. Electro-

deposition of nanomaterials allows the formation of thin layers

with the added advantage of kinetic control. Some parameters

Fig. 42 (a & d) TEM images of powder and colloid, respectively, (b & e) HR-TEM of powder and colloid, respectively, and (c & f) SAED pattern of

powder and colloid, respectively (reprinted with permission from ref. 49).

Fig. 43 TEM image of copper nanoparticles with an average diameter

in the range of 10 nm to 20 nm (reprinted with permission from ref.

50).


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that are known to aff ect the morphology of the nanomaterial

are:

Current density and pH of the electrolyte are kept constant

with variation of deposition time.

Current density and deposition time are kept constant with variation of the pH.

Varying current density with constant pH and deposition

time.

Apart from the above mentioned parameters, voltage, power

and type of sacricial electrode also have an important role in

the synthesis of nanoparticles. Some methods to synthesize

copper nanoparticles through the electrochemical method are

discussed in brief.

Copper nanoparticles were synthesized by Gupta et al.53 from

copper sulfate, sodium acetate, sodium hydroxide and sulfuric

acid using electrochemical synthesis. ITO was used as a working

electrode and Ag/AgCl as a reference electrode, while fullling

all the condition parameters. Spherical copper nanoparticles

and brous nanoparticles were obtained. The SEM images

reveal the shape and size of the obtained copper nanoparticles.

There is a variation in size and shape of the obtained nano-structures depending upon the parameters, such as pH, current

density, etc., as shown in Fig. 44 and 45. XRD analysis shows

that the copper nano-particles exhibited diff raction in the

ranges of 25 to 40 and 60 to 70 corresponding to (111), (200)

and (220) planes, respectively.

Ahmad et al.54 synthesized template-based copper nanowires

using an electro-deposition technique. In this method, AAO

(anodized aluminium oxide) templates were used as one of the

electrodes (cathode) and a pure copper wire of 1 mm diameter

was used as the anode. The precursors used were copper chlo-

ride, dilute sulphuric acid and boric acid. The bath used

(Fig. 46) for the electro-deposition was controlled by a computer

to record the current density during the process. The copper

ions startmigrating to the pores of the templates, are reduced to

copper metal and nanowires start growing.

FESEM was used to characterize the surface morphology of

the obtained copper nanowires. It can be clearly observed that

Fig. 44 SEM images of copper thin lms obtained by electrodepositing at varying pH: (a) 4.0, (b) 5.0 and (c) 9.0 (reprinted with permission from

ref. 53).

Fig. 45 SEM image obtained by electrodepositing lms with pH ¼ 13


Fig. 46 Schematic representation of bath used for electro-deposition

of Cu nanowires (reprinted with permission from ref. 54).

Fig. 47 FESEM image of Cu nanowires (reprinted with permission

from ref. 54).


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applications in agricultural, industrial and technological elds.

Copper nanoparticles were synthesized in a single step by Tha-

kare et al.,58 where starch was used as a stabilizing agent for

copper nanoparticles formed by the reduction of CuCl with

hydrazine hydrate. The average particle size obtained was in the

range of 20–70 nm, as shown by the TEM analysis (Fig. 55). They

noticed that the size of the copper nanoparticle depends on the

concentration of the copper precursor. Nayak et al.59 synthesized

copper nanoparticles by reducing the copper precursor (CuSO4)

with ginger (Zingiber o fficinale) extract and proved that the

obtained nanoparticles exhibit an anti-microbial eff ect. Gajera

et al. synthesized copper nanoparticles using the extract of nag champa ( Artabotrys odoratissimus)60 leaf broth to reduce copper

sulphate pentahydrate. The size of the obtained nanoparticles

was 135 nm at an average rate.

2.7 Other chemical synthesis methods

Apart from all the above mentioned synthesis techniques,

copper nanoparticles have also been synthesized using hydro-

thermal,61 solvo-thermal, thermal decomposition,62 pulsed wire

discharge,63 alcohol media reduction,64 dual plasma synthesis,65

and sono-chemical synthesis66 techniques, etc. Copper nano-

particles were synthesized using an electrochemical approach67

combined with graphene to enhance their conductivity andimprove the strength of the lms. Copper nanowires were used

to fabricate exible transparent electrodes through the elec-

trochemical68 method, reduction69 and catalytic formation

process.70

Copper nanoparticles synthesized using the aforementioned

techniques have the tendency to possess a conductive property.

This property has been used in the synthesis of conductive inks.

Conductive inks can be synthesized via the addition of binding

agents to the synthesized copper nanoparticles. Some methods

to convert copper nanoparticles to conductive ink are explained

in detail herein.

3. Preparation of copper conductiveinks

Conductive inks were synthesized by Lee et al.,71 using 30%

weight of copper nanoparticles and 2-(2-butoxythoxy)ethanol. 2-

(2-Butoxythoxy)-ethanol was used as a dispersant for the copper

nanoparticles. The copper nanoparticles were dispersed by

thorough mixing in the dispersant for 15 min, followed by

microuidization. 0.4 mm syringes were used to completely

abolish agglomerated large particles. A er ltration using a

micro-lter, the dispersion was used in an iTi industrial inkjet

Fig. 53 TEM images of Cu nanowires (reprinted with permission from

ref. 55).

Fig. 54 SAED pattern indicating Cu nanowires with a crystalline

structure (reprinted with permission from ref. 55).

Fig. 55 TEM image of copper nanoparticles stabilized in starch solu-

tion (reprinted with permission from ref. 58).

Fig. 52 SEM image of Cu nanowires showing uniformity in length and

diameter (reprinted with permission from ref. 55).


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printing system with a nozzle size of 38 mm. The temperature of the substrate needs to be maintained at 85 C for the copper ink

to cure properly. A er printing the copper ink on the substrate

surface, the substrate was sintered at 200 C for 1 h in a furnace

under nitrogen atmosphere. The printed pattern is shown in

Fig. 56.

The resistivity of the copper pattern printed on the polyimide

substrate was tested as a function of sintering time at 200 C.

The results shows that the resistivity was reduced up to 2.2

times when compared to bulk copper, as shown in Fig. 57.

Copper nanoparticle paste was synthesized by Yabuki et al.72

In the synthesis process, copper nanoparticles were dispersed

in a-terpineol (50% by weight) to make a nanoparticle paste.

The synthesized paste was coated on a glass substrate using the

doctor blade technique. It was later dried at 70 C for 15 min

and annealed under a ow of air, nitrogen gas or 5% hydrogen–

argon mixed gas at 300 C for 1 h.

The morphologies of the copper patterns were analyzed

using SEM, as shown in Fig. 58 and 59, and the resistivities of

the copper patterns were tested, as shown in Fig. 60, at oxida-

tion and reduction temperatures.

Kim et al.73 synthesized a copper nanoparticle ink for inkjet

printing by dispersing the organic-coated copper nanoparticles

in non-polar solvent at 40% by weight. The organic coating was

used to prevent the copper nanoparticles from agglomerating.

The synthesized copper nanoparticle ink was patternized using

Fig. 56 Copper ink printed using an inkjet printer on polyimide

substrate (reprinted with permission from ref. 71).

Fig. 57 Resistivity of copper pattern printed with the help of an inkjet

printer as a function of time at a sintering temperature of 200 C


Fig. 58 SEM images of copper patterns by annealing pattern in the

presence of (a) N2, (b) 5% H2–Ar and (c) air & 5% H2–Ar; (d) CuO lm

formed in the presence of air & 5% H2–Ar (reprinted with permission

from ref. 72).

Fig. 59 Cross-sectional view of (a) copper at 300 C under air and 5%

H2–Ar and (b) CuO pattern in 5% H2–Ar (reprinted with permission

from ref. 72).


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a piezoelectric inkjet. A glass fabric/bismaleimide triazine (BT)

composite of 100 mm thickness was used as a substrate. The BT

substrate was heated up to 85

C when the patternization (5, 10and 20 times printed) process was complete. A er the patter-

nization of the copper lm, it was thermally sintered at a

temperature of 200 C for more than an hour.

Morphological images of the printed copper pattern were

obtained using SEM. Images of the printed copper pattern

before and a er sintering were taken, as shown in Fig. 61 and

62. The resistivities of the lms were measured by four-probe

measurement, as shown in Fig. 63, and they were found to be

61.3 nU, 36.7 nU and 98.9 nU for copper lms printed 5, 10 and

20 times, respectively. A prolometer was used to determine the

thickness of the patterned electrodes, and the thickness

measurements were shown to be 1731, 3690 and 11954 nm for

5, 10 and 20 times printed patterns, respectively, as shown in

Fig. 64.

Pulsed wire evaporated copper nanoparticles were used in

synthesizing conductive ink for inkjet printing by Lee et al.63

Octanethiol-stabilized copper nanoparticles synthesized via

pulsed wire evaporation were dispersed in a mixed solvent of

DEG, isopropyl alcohol (IPA) and ethanol (DEG : IPA : ethanol

in the ratio of 6 : 2 : 2% by volume) and sonicated for 1 h. A

piezoelectric nozzle inkjet printer was used to print the

synthesized conductive ink onto a glass substrate. A er printing

the copper patterns, they were sintered at 350 C for 4 h in the

presence of H2 or H2 + Ar (5.18 : 94.81 vol%) at a heating rate of

5 C min1.

The thickness of the copper pattern was measured using a

prolometer and the measurements are shown in Table 1. The

resistivity of the pattern was measured using a four-point probesystem and the measurements are shown in Table 1. SEM

Fig. 60 Resistivity of copper patterns at oxidation and reduction

temperatures (reprinted with permission from ref. 72).

Fig. 61 SEM of printed electrode (a) before sintering and (b) after

sintering (reprinted with permission from ref. 73).

Fig. 62 SEM images of pattern printed (a) 5 times, (b) 10 times and (c) 20 times (reprinted with permission from ref. 73).

Fig. 63 Resistivity measurements of printed copper patterns

compared to bulk copper (reprinted with permission from ref. 73).


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analysis was performed to analyze the surface of the patterned

substrate, as shown in Fig. 65.

Druff el et al.75 synthesized copper ink via IPL (Intense Pulsed

Light) sintering. Tergitol NP-9, anhydrous Cu(NO3)2, ethylene

glycol, BaBH4 and NH4OH were used as the precursors for the

preparation of the copper ink. Firstly, Tergitol was mixed with

an aqueous solution of Cu(NO3)2 in water. The pH of the reac-

tion mixture was maintained from 7 to 11 by drop-wise addition

of NH4OH. Later, aqueous NaBH4 in water was added to the

reaction mixture. The same procedure was later repeated by

replacing water with ethylene glycol. The obtained solution with

ethylene glycol acts as the ink. The synthesized ink was ultra-

sonicated to break up the large agglomerates formed in the

Fig. 64 Surface proles of copper patterns printed (a) 5 times, (b) 10 times and (c) 20 times (reprinted with permission from ref. 73).

Fig. 66 Schematic representation of the fabrication of copper nano-

particle ink and sintering by means of intense light pulses (reprinted


Fig. 67 XRD patterns of copper lms synthesized at pH 7, with EG; (b)

pH 11 with EG and(c) pH 11 without EG (reprinted with permission from

ref. 75).

Fig. 65 SEM images of copper pattern sintered in (a) only hydrogen

and (b) a mixture of hydrogen and argon gases (reprinted with


Table 1 Values obtained from electrical resistivity and thickness

measurements. N.A. – not applicable

Sample Synthesis atmosphere Thickness Resistivity

Sample 1 Hydrogen 1.8 0.3 mm 1.74 107 UmSample 2 Hydrogen + argon 5.3 0.3 mm 9.68 107 UmBulk Cu N.A. N.A. 1.67 108 Um

Fig. 68 XRD patterns of copper lms prepared in presence of EG with

different concentrations of NaBH4 ((a)–(d): 0.05 M, 0.1 M, 0.3 M, 0.6 M

respectively) (reprinted with permission from ref. 75).


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synthesis process. A er sonication, the ink was deposited on

glass substrates, which were preheated to 160 C. The copper

lms deposited on the glass substrate were sintered in an inert

nitrogen atmosphere with the help of light pulses. IPL sintering

was achieved by employing a xenon lamp to generate pulse of

incoherent light with a range of wavelengths varying from 190

nm to 1000 nm. The energy of the light pulses as well as the

energy densities was varied by varying the input voltages. To

sinter a conductive lm with a thickness greater than 5 mm,more than one light pulse was required and hence 10 light

pulses were applied at each energy density. Copper lms were

also deposited on moisture-resistant polyester substrates which

possess a melting point of 150 C. The schematic of the prep-

aration of inks with the IPL sintering process is shown in

Fig. 66.

XRD was employed to characterize the copper lms obtained

from IPL (intense pulsed laser) sintering. Films synthesized in

water as well as in ethylene glycol were analyzed, as shown in

Fig. 67 and 68. TEM and HRTEM images of the conductive ink

synthesized at pH ¼ 11 were recorded, as shown in Fig. 69.

Resistivity measurements are shown in Fig. 70, which werecarried out using a four-probe measuring instrument. Topo-

graphical images of the copper lms were recorded using SEM,

Fig. 70 Sheet resistance vs. energy during the IPL sintering (reprinted


Fig. 69 Imagesof conductive ink prepared at pH¼ 11 in the presence ofEG, reduced using NaBH4 at concentrations of (a) and (c) 0.05 M (TEM),

(b) and (d) 0.6 M (HRTEM) (reprinted with permission from ref. 75).

Fig. 71 Topographical images of copper patterns (a) before sintering, (b) sintered with energy of 576 J cm2 and (c) sintered with energy of 1723

J cm2 (reprinted with permission from ref. 75).

Fig. 72 (a) XRD pattern of IPL-sintered copper lm on exible PET

substrate, (b) SEM image of copper patterned exible substrate and (c)

digital photograph showing exiblesubstrate with copperpattern on it



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exhibiting a rough and porous structure, as shown in Fig. 71.

Applying IPL energy to the copper pattern caused the particles

to coalesce and thereby a change in morphology was observed,

as shown in Fig. 72.

Kim et al.76 synthesized a copper nano/micro-particle ink

which was used for fabrication of printed electronics by means

of ash light sintering. In their typical synthesis technique,

copper nano/micro-particles of 20–50 nm diameter, with very

little oxide of a thickness of >2 nm, were chosen. PVP was

dispersed in DEG solution. To this reaction mixture, copper

nanoparticles were dispersed using a mechanical stirrer and

ultra-sonicator simultaneously for 3 h. The nal step for

preparing the ink is to ball mill the mixture for 12 h. The

synthesized nano/micro-particle ink was printed (at a thickness

of 20 mm) on a polyimide (PI) substrate and dried on a hot plate

at 100 C for 30 min. Flash light sintering is followed by drying,

where a xenon ash lamp was used for the sintering process.

The schematic representation of ash light sintering is shown

in Fig. 74, which compares the copper pattern before and a er

sintering.

The characterization of the copper lms was carried out

using XRD, SEM, four-probe methods and FT-IR. The resistivity

of the copper patterns was tested using the four-probe method

as shown in Fig. 76. In situ temperature and resistance tests

were carried out using a thermo-couple based circuit and

Wheatstone bridge during the sintering process. The XRD

patterns comparing the ink prepared using nanoparticles (sin-

tered and unsintered) and microparticles (sintered and unsin-

tered) are shown in Fig. 75.

As shown in Fig. 74, the nanoparticles w

copper conductive inks-rsc adv.2015!5!63985

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