Copper conductive inks: synthesis and utilization in flexible electronics

Abstract

Conductive inks are a recent advance in electronics and have promising future applications in flexible 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 fields 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.

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Venkata Abhinav K

Venkata Abhinav K 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 Bachelor’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 field of printed electronics. He is also interested in self assembly of fullerenes and fabrication of solar cells using cost effective materials.

Venkata Krishna Rao R

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 Bachelor’s degree in Electrical and Electronics Engineering at Jawaharlal Nehru Technological University, Hyderabad, India. His research interests are focussed on synthesizing various conductive nanomaterials using different techniques and applying them in the field of flexible electronics. He is also interested in self assembly of fullerenes, conductive inks and fabrication of photovoltaics using cost effective materials.

Karthik P. S

P. S. Karthik is a research student at the CSIR-Indian Institute of Chemical Technology, Hyderabad, India, in the group of Dr Surya Prakash Singh. He has completed his Bachelor and Master degrees at Jawaharlal Nehru Technological University, Hyderabad, India. His research interests are focussed on synthesizing carbon nanomaterials using various techniques and applying them in the field of solar energy. He is also focussed on fabricating solar cells using different light absorbing materials. He has published three research papers in the field of Nanotechnology.

Surya Prakash Singh

Dr Surya Prakash Singh is a Scientist at the CSIR-Indian Institute of Chemical Technology, Hyderabad. He studied chemistry at the University of Allahabad, India, and obtained his D.Phil. degree in 2005. After working at the Nagoya Institute of Technology, Japan, as a postdoctoral 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 flexible devices. He has published over 100 papers and reviews in peer-reviewed journals.

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1. Introduction

Nanotechnology is a rapidly developing advance and its products are extremely useful in all fields, in view of their small size (10⁻⁹ m) and substantial surface range. Nanoparticles offer 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 nanoparticles has attracted wide interest in the most recent decades on account of their strange and size-dependent optical, attractive, electronic, and compound properties. To completely use these properties, the size and shape must be effectively controlled. This technology has gained a lot of importance in recent years due to its applications in multi-disciplinary fields (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 field of electronics was initiated more than a decade ago. Even though vast research is being conducted in this field, much more has to be done to implement them in real-time applications. We tried to focus on one of the sub-fields of electronics i.e., flexible electronics. Flexible electronics is a contemporary field 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 reflectance, 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 (68^th place with 7.9 × 10⁻⁶%). The second element with high conductance after 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 26^th 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 configuration of copper is [Ar].3d¹⁰.4s¹; it has one free 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 fields as thermal dissipation agents, anti-microbial and anti-fungal agents, lubricants, metal injection moulding, catalysts, flexible electronics, transparent conductors, etc. Copper nanoparticles⁵ have been obtained basically using three different approaches, physical, chemical and biological; 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.

Scheme 1 Flow chart of synthesis of Cu nanoparticles and its application as ink.

Copper films are of high interest for their use as interconnecting materials in multilevel integrated circuits, because of their high conductivity (59.88 × 10⁶) and excellent electron-migration resistance.⁶ Various methods for the preparation of Cu films have been reported and the most extensively investigated 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 effective 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 (after silver). Therefore, it is best to select copper over silver. The synthesis of pure (oxide free) copper nanoparticles requires very clean and non-corroding conditions because copper posseses an inherent tendency to oxidize at room temperature very rapidly. The basic requirement to synthesize copper nanoparticles is that the nanoparticle growth part should be carried out in inert atmosphere or in the presence of nitrogen. Some methods to reduce the effect of oxidation have also been reported and discussed in this review. These techniques can be used to synthesize conductive copper inks, which are our main interest. To prepare high quality conductive inks, certain procedures and methods have been followed and are explained in this review. Some chemical methods to synthesize copper nanoparticles are explained,⁷ from which some are further processed to make conductive inks that can be used as a subcomponent of flexible electronics. They are:

• Chemical reduction;

• Polyol synthesis;

• Electrochemical synthesis;

• Photochemical synthesis;

• Microwave-assisted synthesis;

• Biological and green synthesis;

• Sonochemical synthesis;

• Hydrothermal and solvo-thermal synthesis.

2. Techniques in synthesizing copper nanoparticles

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 synthesizing metallic nanoparticles. Many methods have been reported for synthesizing copper nanoparticles via chemical reduction in the vicinity of different 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, different 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 reduction of 0.01 M copper(II) acetate dispersed in ethanol. The reaction 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 different 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 refluxing conditions. The total synthesis process was carried out in a nitrogen environment 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 effective combination of polymer-coated copper nanoparticles. By the use of different amounts of PVP, copper nanoparticles of different 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 technique. UV-vis characterization was carried out for measuring the absorbance of the obtained copper nanoparticle colloid. Depending on the concentration of stabilizing agents, the absorbance varied, exhibiting a surface plasmon resonance (SPR) in the range of 570 nm to 582 nm, as shown in Fig. 1 and 2.

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

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 (α₂). The obtained results imply that n₂ doesn’t correspond to the third order non-linear response and hence the susceptibility totally corresponds to α₂, as shown in Fig. 3.

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).

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 SEM image 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 diffraction 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.

Fig. 4 SEM image of copper particles (reprinted with permission from ref. 8).

Takuya et al.¹³ synthesized copper nanoparticles using a liquid phase reduction method. The source of copper is copper acetate. Copper acetate is dissolved in distilled water and sodium borohydride (0.1 mol dm⁻³) 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 refluxed at 20 °C for 1 h. After one hour, a black colloidal dispersion of copper nanoparticles was obtained. The experiment was carried out in different atmospheres (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 copper nanoparticles 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 up in 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.

Fig. 5 TEM image of copper nanoparticles prepared in (a) N₂, (b) air and (c) O₂ (reprinted with permission from ref. 13).

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.

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).

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 temperature. After 1 h the temperature is raised to 70 °C and the 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 the reaction 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-visible spectroscopy, 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 diffraction at 43.6, 64.5 and 77.7 corresponding to the (111), (200) and (220) planes, respectively. The diffraction 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).

Fig. 7 TEM image of bud-shaped copper NPs (reprinted with permission from ref. 14).

AFM was used to determine the height and structure of the copper nanobuds and the obtained AFM image is in close agreement 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 shift¹⁵ with decrease in size.

Fig. 8 AFM image of copper nanobuds (reprinted with permission from ref. 14).

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 chloride was added under vigorous stirring. Subsequently, hydrazine 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. After washing, the copper nanoparticles were dried at room temperature.

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 different for the two different 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 diffraction 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 diffraction of oxides was detected.

Fig. 9 TEM image of oblate-shaped copper NPs, HRTEM image displaying a layer of PVP, and SAED pattern indicating the polycrystalline structure (reprinted with permission from ref. 16).

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. The copper source is copper nitrate, the reducing agent is hydrazine and PVP and CTAB act as stabilizing agents^18,19 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 diffraction peaks at 43.3°, 50.4° and 74.08° were obtained, as shown in Fig. 10, representing the (111), (200) and (220) diffraction 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.

Fig. 10 XRD pattern recorded for copper nanoparticles stabilized with (a) PVP, (b) CTAB and (c) a combination of PVP/CTAB (reprinted with permission from ref. 17).

TEM analysis was performed for all three different combinations. The importance of the PVP/CTAB combination can be clearly seen from (Fig. 11a) the PVP-coated Cu NPs, (Fig. 11b) 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, respectively, 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 finer 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.

Fig. 11 TEM micrographs of (a) PVP-Cu nano-particles, (b) CTAB-Cu NPs and (c) PVP/CTAB (reprinted with permission from ref. 17).

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 diffractions at 43°, 50° and 74°, which are reported to be very close to the JCPDS file 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.

Fig. 12 TEM image with a scale bar of 200 nm displaying particles with uniform shape and size (reprinted with permission from ref. 20).

Zhong et al.²¹ synthesized size-controlled and potentially shape-controlled copper nanoparticles in organic solvents in the 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 refluxing 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; after the addition of both the capping agents, the temperature of the solution was raised to higher temperature (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 precipitate 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 confirm the formation of copper nanoparticles exhibiting diffraction 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 nanoparticle solution was recorded, which displayed surface plasmon 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.

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 permission from ref. 21).

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 spectroscopy, 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 nanoparticles vary with different 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 stabilized 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 l⁻¹, (b) 2.8 mol l⁻¹, (c) 5.6 mol l⁻¹, (d) 8.4 mol l⁻¹, (e) 11.2 mol l⁻¹ and (f) 14 mol l⁻¹ (reprinted with permission from ref. 22).

Fig. 15 TEM analysis of copper particles stabilized by different carboxylic acids of concentration 14 mol l⁻¹: (a) acetic acid, (b) glycolic acid, (c) glycine, (d) alanine, (e) citric acid (reprinted with permission from ref. 22).

The FTIR spectroscopy displays peaks at 1608–1561 and 1395–1375 cm⁻¹ which indicate stretching of carboxylates. The amine peaks were obtained around 3400–3200 cm⁻¹. The strong absorption at 3400 cm⁻¹ is probably from the adsorbed water. The peaks around 3400 cm⁻¹ 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 diffraction 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 confirms that the obtained structure is FCC, and a minor peak for Cu₂O was observed at 36.6°, representing the (111) index.

Fig. 16 XRD patterns of copper nanoparticles synthesized using different carboxylic acids at a concentration of 14 mol l⁻¹: (a) acetic acid, (b) glycolic acid, (c) glycine, (d) alanine, (e) lactic acid and (f) citric acid (reprinted with permission from ref. 22).

2.1.5 Using PEG. Thi et al.²³ synthesized copper nanoparticles 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 stirring is maintained. After 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; different 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 1690 cm⁻¹ and 1760 cm⁻¹.

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).

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 nanoparticles 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 dissolved in 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 different molar ratios of PAAm/Cu.

The characterization of the obtained copper nanoparticles was accomplished using TEM, XRD and TGA. The XRD information shows the diffraction that is identified for three different samples containing three different molar ratios of PAAm/Cu (2.00, 0.11 and 0.46). The diffraction 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 formula^26,27 and was calculated to be 13 nm.

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 = 2.00, R₂ = 97.0, R₃ = 0.11 and R₄ = 0.46).

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 calculated to 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.

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).

2.2 Polyol synthesis

In polymer chemistry, polyols are compounds with multiple hydroxyl functional groups. Glycerin, pentaerythritol, ethylene 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-fine 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 a given 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 nanoparticles. 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 different concentrations of sodium phosphinate monohydrate were added to the reaction mixture and heated. An aqueous solution of copper sulphate pentahydrate was injected into the hot reaction medium using a syringe pump; the rate of injection was varied from 2 to 8 ml min⁻¹ and the solution was stirred vigorously for 1 h. After 1 h, the reaction mixture was cooled to room temperature and left 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 performed. Phase composition and crystallite size were calculated using XRD. The FCC structure of the nano-copper was confirmed 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 nanoparticles was analyzed through XPS, where a copper peak was identified 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 displayed 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 min⁻¹, 6 ml min⁻¹ and 8 ml min⁻¹) at two different 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 (amorphous), as illustrated in Fig. 25. The boundary thickness was measured to be ∼1.5 nm. The SAED pattern also corresponds to the FCC structure without any traceable impurities.

Fig. 20 XPS spectra of Cu nanoparticles: (a) Cu 2p_3/2, (b) C 1 s and (c) O 1 s (reprinted with permission from ref. 29).

Fig. 21 SEM images of Cu NPs synthesized as a function of concentration 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 permission from ref. 29).

Fig. 23 SEM images of Cu NPs synthesized at 200 °C as a function of precursor injection rate: (a) 2 ml min⁻¹, (b) 6 ml min⁻¹ and (c) 8 ml min (reprinted with permission from ref. 29).

Fig. 24 SEM images of Cu NPs synthesized at 140 °C as a function of precursor injection rate: (a) 2 ml min⁻¹, (b) 6 ml min⁻¹ and (c) 8 ml min (reprinted with permission from ref. 29).

Fig. 25 HR-TEM image of synthesized Cu NP displaying an amorphous layer twin boundary, with a scale bar of 5 nm. SAED pattern of a single particle is inset (reprinted with permission from ref. 29).

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. After reaching a certain reaction temperature, two different DEG solutions containing ascorbic acid and copper acetate were added to the PVP solution. A change in color from green to dark red is observed which indicates that the nucleation process has started, resulting in the formation of copper nanoparticles. The temperature of the microwave oven was varied (60–170 °C) to study the effect of heat treatment and it was found that the temperature was one of the most influencing 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 different 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 the copper 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 imaging technique. DLS analysis was performed to show the size distribution of the copper colloidal solution, as illustrated in Fig. 27c. The sizes of the particles that were synthesized at different temperatures (60 °C, 100 °C, 140 °C and 180 °C) were compared. The XRD results in diffraction at 43.3° and 50.5° without any impurities, which is consistent with the FCC shape of metallic copper (JCPDS card no. 4-0836). The colloid was again characterized with XRD after thirty days and no traces of copper oxides were observed. The size, shape and structure of the obtained copper nanocolloids are dependent on certain parameters, namely influence of reducing agent, synthesis temperature, reagent addition temperature, rate of reagent addition, etc.^33,34 By varying the aforementioned parameters, the morphology of the copper nanoparticles can be modified.

Fig. 26 (a) Solutions of copper oxide to pure copper nanoparticles, (b) UV-visible spectra, (c) UV-vis spectrum of Cu₂O sample (reprinted with permission from ref. 32).

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).

Copper nanoparticles were synthesized by Lee et al. using the polyol method with an average diameter of 100 nm.³⁵ A typical experimental procedure includes the preparation of copper nanoparticles stabilized in PVP in ethylene glycol solution using the polyol method.²⁹ After the synthesis process, PVP was removed from the surface by heating it by means of RF plasma arc discharge for 50 seconds. 1-Octanethiol was used to coat the surface of the copper by heating it with separately designed equipment. The heating was carried out in a vacuum under ambient temperature and pressure conditions. Fig. 28 compares the copper nanoparticles before and after coating with 1-octaethiol.

Fig. 28 TEM images of copper nanoparticles (a) after removing PVP and (b) after coating with octanethiol (reprinted with permission from ref. 35).

Chokratan et al. synthesized³⁶ ultra-fine copper powders with a controlled size by the addition of sodium hydroxide. The particle sizes are reported to vary depending upon the molar ratios of the precursor solution. The synthesis procedure starts with dissolving copper nitrate trihydrate in a solution of sodium hydroxide and glycerol with different molar ratios (0 : 1 to 5 : 1), as shown in Fig. 29, while maintaining a constant molar ratio of copper nitrate : glycerol of 0.02 : 1. The reaction mixture was refluxed at temperatures of 120 °C to 160 °C at a constant stirring rate. The stirring was stopped when the reduction process was complete. The obtained copper nanoparticles were separated and washed with ethanol several times.

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).

The obtained copper nanoparticles were characterized using XRD, SEM and FESEM, where SEM and FESEM are employed for particle size distribution and image analysis and XRD is employed to find out the crystallite size and structure. XRD diffraction patterns were recorded for the copper nanoparticles synthesized with a Cu(NO₃)₂ : NaOH molar ratio of 2 : 1, refluxed at 140 °C and observed at different time periods (20, 30, 120 and 240 min) after heating the solution. The SEM images of the copper powders were obtained at various NaOH concentrations at a refluxing 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.

Fig. 30 Morphology of the copper powders synthesized from various molar ratios of NaOH : Cu(NO₃)₂ at reflux at 140 °C (reprinted with permission from ref. 36).

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).

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 photochemical 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 artificial 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 briefly described below.

Fig. 32 An example schematic of photochemical synthesis (reprinted with permission from ref. 45).

Copper metal nanoparticles have been synthesized by Kapoor et al.³⁹ by irradiation with 253.7 nm light, carried out using a low 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 s⁻¹. 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 flash photolysis at a temperature of 20 ± 1 °C. The concentrations of PVP and benzophenone influence the particle size proportionately. A similar process was carried out by Giuffrida 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 exhibiting 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 confirms that the obtained particles are polycrystalline in nature, as shown in Fig. 33b.

Fig. 33 (a) TEM image of copper nanoparticles and (b) SAED pattern (reprinted with permission from ref. 39).

Nano-sized copper metallic particles and colloidal copper nanoparticles were synthesized by Giuffrida et al.⁴¹ upon ultraviolet 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 10⁻⁶ to 10⁻⁵ Nhv min⁻¹.

A UV-vis spectrophotometer was used to monitor the specific absorptions 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 resonance band of copper in the colloidal state as a result of longer irradiation. After aging of the colloidal copper, the X-ray diffractometer displayed diffraction 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 different size distributions of 20 to 80 nm were confirmed by the SEM images, as shown in Fig. 35.

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 images of aged copper colloid: (a) copper dried on silicon and (b) copper film deposited on quartz substrate (reprinted with permission from ref. 39).

Nie et al.⁴² developed a facile method for the preparation of copper nanoparticles via ultraviolet irradiation of a solution 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 transparent side of the cuvette for 40 minutes with a radiation intensity of 45 mW cm⁻². 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 presence of oxygen and hence it should be prepared in an oxygen-free environment and should also be preserved in a dark environment away from sunlight.

The photoreaction was monitored using a UV-vis spectrophotometer, and the product characterized using XRD for the determination of the composition of the obtained copper nanoparticles. The size and morphology was obtained using TEM analysis. The diffraction peaks at 43.55° and 50.66° correspond to the (111) and (200) planes, respectively, which confirms 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.

Fig. 36 TEM analysis of copper nanoparticles irradiated with ultraviolet radiation; the obtained particle size is less than 100 nm (reprinted with permission from ref. 42).

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 fundamental 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 laser beams of 1064 nm and 532 nm for the formation of a copper nanoparticle colloid.

UV-vis spectra of the colloidal copper irradiated at wavelengths 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 diffraction 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 in Fig. 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 effectively.

Fig. 37 TEM image of colloidal copper synthesized with irradiation at 1064 nm with a power of 100 mJ pulse⁻¹ 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 pulse⁻¹ for 10 min, (b) 50 mJ pulse⁻¹ for 30 min, (c) 115 mJ pulse⁻¹ for 30 min and (d) 300 mJ pulse⁻¹ for 30 min (reprinted with permission from ref. 43).

2.4 Microwave-assisted synthesis

Microwave-assisted synthesis refers to the technique of applying microwave radiation to promote a chemical reaction. High frequency electric fields 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 reaction 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-fluids 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 hypophosphite (NaH₂PO₂·H₂O). Firstly, copper sulphate was dissolved 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. After 5 min, the reaction mixture was put into a microwave oven and reacted for 5 min under medium power. The copper nanoparticle formation was confirmed by observing the change of the color of the mixture from blue to dark red.

The XRD pattern was recorded, exhibiting diffraction as per the JCPDS file no. 04-0838, corresponding to the planes at (111), (200), (220) and (311). The diffraction 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.

Fig. 39 (a) TEM image of copper nanoparticles prepared using standard procedure. (b) SAED pattern of the obtained copper (reprinted with permission from ref. 48).

Fig. 40 TEM images of copper nanoparticles synthesized using CuSO₄ at concentrations of (a) 0.2 M and (b) 0.5 M (reprinted with permission from ref. 48).

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 flask 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 ether once. After washing, the particles were dried under vacuum. The obtained copper nanoparticles were found to be free from oxides even after 12 months of exposure to ambient atmospheric 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 magnification 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 spectroscopy, 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 confirms that the obtained nanoparticles have a FCC structure with d = 2.08 Å, as shown in Fig. 42(c) and (f).

Fig. 41 FESEM images of copper particles: (a) low magnification and (b) high magnification (reprinted with permission from ref. 49).

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).

Zhu et al.⁵⁰ synthesized functionalized copper nanoparticles for application in glucose sensing. The copper nanoparticles were functionalized with dimethylglycoxime using a microwave-assisted synthesis process. Copper acetate hydrate (CuAc₂·H₂O) was reacted with dimethylglycoxime (DMG) dissolved in ethylene glycol. The mixture was placed in a microwave reflux system under ambient conditions at a power of 365 W for 30 min. After 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 significant role in the dispersion of the copper nanoparticles, with obtained 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 difference in the absorption band at 3419 cm⁻¹ was observed for the DMG-stabilized copper nanoparticles. XRD peaks corresponding to the copper structure (CCID files no. 040836 Cubic), exhibiting diffraction 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.

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).

2.5 Electrochemical synthesis

Electrochemical synthesis of nano-structured materials is achieved by passing an electric current between a cathode and anode separated by an electrolyte. Electrochemical synthesis has advantages of low cost, simple handling, flexibility, low contamination and no requirement of vacuum.⁵ Electrochemical 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. Electrodeposition of nanomaterials allows the formation of thin layers with the added advantage of kinetic control. Some parameters that are known to affect 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 sacrificial 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 fulfilling all the condition parameters. Spherical copper nanoparticles and fibrous 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 nanostructures 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 diffraction in the ranges of 25° to 40° and 60° to 70° corresponding to (111), (200) and (220) planes, respectively.

Fig. 44 SEM images of copper thin films 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 films with pH = 13 (reprinted with permission from ref. 53).

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 chloride, 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 start migrating to the pores of the templates, are reduced to copper metal and nanowires start growing.

Fig. 46 Schematic representation of bath used for electro-deposition of Cu nanowires (reprinted with permission from ref. 54).

FESEM was used to characterize the surface morphology of the obtained copper nanowires. It can be clearly observed that the copper nanowires are aligned parallel to each other with a high aspect ratio, as shown in Fig. 47. EDAX was used to characterize the composition of the synthesized copper nanowires. In the EDX spectrum, aluminium and oxygen along with copper are present, as shown in Fig. 48; the presence of aluminium and oxygen may be due to the AAO templates. The TEM image suggests that the diameter of the nanowires was around 15 nm, with amorphous as well as crystalline structures, as shown in Fig. 49.

Fig. 47 FESEM image of Cu nanowires (reprinted with permission from ref. 54).

Fig. 48 EDX showing the presence of Al and O along with Cu (reprinted with permission from ref. 54).

Fig. 49 TEM image showing the presence of amorphous and crystalline structures (reprinted with permission from ref. 54).

The potentiostatic electrochemical metal deposition technique was used for synthesizing Cu nanowires by Narayan et al.,⁵⁵ using ITO-coated glass as a working electrode, Pt wire as a counter electrode and Ag/AgCl as a reference electrode in the electrochemical cell. The electrolyte used was a solution of CuSO₄·5H₂O in KCl, and the pH of the solution was maintained at 4.5 by addition of dilute sulfuric acid. The schematic of the cell used in this experiment is illustrated in Fig. 50.

Fig. 50 Schematic of the electrochemical cell used to synthesize Cu nanowires (reprinted with permission from ref. 54).

SEM was performed on the obtained Cu nanowires grown on the ITO glass substrate. The diameters of the obtained nanowires ranged from 110 nm to 140 nm. SEM analysis also showed greater uniformity and higher yield, as shown in Fig. 51 and 52, when compared to the conventional AAO template technique. The TEM images of the Cu nanowires are shown in Fig. 53, with the SAED pattern shown in Fig. 54.

Fig. 51 SEM images of Cu nanowires synthesized using the electrochemical process (reprinted with permission from ref. 55).

Fig. 52 SEM image of Cu nanowires showing uniformity in length and diameter (reprinted with permission from ref. 55).

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).

2.6 Biological and green synthesis

The syntheses of nano-sized particles have gained utmost interest during the past decade because of their peculiar properties, leading to their applications in multiple disciplines in science and technology.⁵⁶ Even though metallic nanoparticles have a wide range of applications, their use is limited due to the involvement of toxic chemicals in their synthesis process.⁵⁷ To overcome this limitation, metal nanoparticles can be synthesized using non-toxic biosynthesis methods. Copper nanoparticles have been synthesized using the biological method owing to their applications in agricultural, industrial and technological fields. Copper nanoparticles were synthesized in a single step by Thakare 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 officinale) extract and proved that the obtained nanoparticles exhibit an anti-microbial effect. 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.

Fig. 55 TEM image of copper nanoparticles stabilized in starch solution (reprinted with permission from ref. 58).

2.7 Other chemical synthesis methods

Apart from all the above mentioned synthesis techniques, copper nanoparticles have also been synthesized using hydrothermal,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 nanoparticles were synthesized using an electrochemical approach⁶⁷ combined with graphene to enhance their conductivity and improve the strength of the films. Copper nanowires were used to fabricate flexible transparent electrodes through the electrochemical⁶⁸ 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 conductive inks

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 microfluidization. 0.4 μm syringes were used to completely abolish agglomerated large particles. After filtration using a micro-filter, the dispersion was used in an iTi industrial inkjet printing system with a nozzle size of 38 μm. The temperature of the substrate needs to be maintained at 85 °C for the copper ink to cure properly. After 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.

Fig. 56 Copper ink printed using an inkjet printer on polyimide substrate (reprinted with permission from ref. 71).

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.

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 (reprinted with permission from ref. 71).

Copper nanoparticle paste was synthesized by Yabuki et al.⁷² In the synthesis process, copper nanoparticles were dispersed in α-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 flow 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 oxidation and reduction temperatures.

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 film 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).

Fig. 60 Resistivity of copper patterns at oxidation and reduction temperatures (reprinted with permission from ref. 72).

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 a piezoelectric inkjet. A glass fabric/bismaleimide triazine (BT) composite of 100 μm thickness was used as a substrate. The BT substrate was heated up to 85 °C when the patternization (5, 10 and 20 times printed) process was complete. After the patternization of the copper film, 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 after sintering were taken, as shown in Fig. 61 and 62. The resistivities of the films were measured by four-probe measurement, as shown in Fig. 63, and they were found to be 61.3 nΩ, 36.7 nΩ and 98.9 nΩ for copper films printed 5, 10 and 20 times, respectively. A profilometer 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.

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).

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

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. After 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 min⁻¹.

The thickness of the copper pattern was measured using a profilometer and the measurements are shown in Table 1. The resistivity of the pattern was measured using a four-point probe system and the measurements are shown in Table 1. SEM analysis was performed to analyze the surface of the patterned substrate, as shown in Fig. 65.

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 μm	1.74 × 10^−7 Ωm
Sample 2	Hydrogen + argon	5.3 ± 0.3 μm	9.68 × 10^−7 Ωm
Bulk Cu	N.A.	N.A.	1.67 × 10^−8 Ωm

---

Fig. 65 SEM images of copper pattern sintered in (a) only hydrogen and (b) a mixture of hydrogen and argon gases (reprinted with permission from ref. 63).

Druffel 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 reaction 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 ultrasonicated to break up the large agglomerates formed in the synthesis process. After sonication, the ink was deposited on glass substrates, which were preheated to 160 °C. The copper films 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 film with a thickness greater than 5 μm, more than one light pulse was required and hence 10 light pulses were applied at each energy density. Copper films were also deposited on moisture-resistant polyester substrates which possess a melting point of 150 °C. The schematic of the preparation of inks with the IPL sintering process is shown in Fig. 66.

Fig. 66 Schematic representation of the fabrication of copper nanoparticle ink and sintering by means of intense light pulses (reprinted with permission from ref. 75).

XRD was employed to characterize the copper films 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 = 11 were recorded, as shown in Fig. 69. Resistivity measurements are shown in Fig. 70, which were carried out using a four-probe measuring instrument. Topographical images of the copper films were recorded using SEM, 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.

Fig. 67 XRD patterns of copper films synthesized at pH 7, with EG; (b) pH 11 with EG and (c) pH 11 without EG (reprinted with permission from ref. 75).

Fig. 68 XRD patterns of copper films 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).

Fig. 69 Images of conductive ink prepared at pH = 11 in the presence of EG, 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. 70 Sheet resistance vs. energy during the IPL sintering (reprinted with permission from ref. 75).

Fig. 71 Topographical images of copper patterns (a) before sintering, (b) sintered with energy of 576 J cm⁻² and (c) sintered with energy of 1723 J cm⁻² (reprinted with permission from ref. 75).

Fig. 72 (a) XRD pattern of IPL-sintered copper film on flexible PET substrate, (b) SEM image of copper patterned flexible substrate and (c) digital photograph showing flexible substrate with copper pattern on it (reprinted with permission from ref. 75).

Kim et al.⁷⁶ synthesized a copper nano/micro-particle ink which was used for fabrication of printed electronics by means of flash 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 final 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 μm) 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 flash lamp was used for the sintering process. The schematic representation of flash light sintering is shown in Fig. 74, which compares the copper pattern before and after sintering.

The characterization of the copper films 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 (sintered and unsintered) and microparticles (sintered and unsintered) are shown in Fig. 75.

As shown in Fig. 74, the nanoparticles were welded between the microparticles, thereby acting as a contact between them. The SEM image shown in Fig. 73 explains this more clearly.

Fig. 73 SEM images of (a) copper nanoparticle ink and (b) copper microparticle ink after sintering (reprinted with permission from ref. 76).

Fig. 74 Schematic representation of copper pattern drawn on the PI substrate before and after sintering (reprinted with permission from ref. 76).

Fig. 75 XRD patterns of the sintered and unsintered copper nano/microparticle inks (reprinted with permission from ref. 76).

Fig. 76 Sheet resistance change analysis of copper films during sintering of (a) copper nanoparticle ink and (b) copper microparticle ink (reprinted with permission from ref. 76).

Kang et al.⁷⁷ synthesized a copper nano-ink by using copper nanoparticles of diameter 100–120 nm. These nanoparticles were dispersed in a mixed solution of IPA (iso-propanol) and PGME (propylene glycol methyl ether) to form a copper nano-ink. The synthesized copper ink was spin-coated on a glass substrate and dried at 50 °C for 5 min. The thickness was measured to be 1.3 μm. The schematic representation of the system employed in laser sintering is shown in Fig. 77. A DPSS (diode-pumped solid-state laser) with an average intensity of 190 W cm⁻² was utilized as the laser source for the sintering process.

Fig. 77 Schematic diagram representing the system employed for laser sintering (reprinted with permission from ref. 77).

The resistivity of the spin-coated conductive ink was measured using a four-point probe, and is shown in Fig. 78. The specific resistance of the film tends to decrease when sintering is carried out with an applied intensity of >100 W cm⁻². The XRD spectrum of the spin coated copper film prepared using laser sintering with varying scan rate was compared to the copper pattern prepared using thermal sintering as a function of time, as shown in Fig. 79. The SEM micrographs are depicted in Fig. 80, showing the surface morphologies of the copper pattern sintered with increasing power. The SEM image clearly shows that the grain size is increasing with the increase in laser intensity.

Fig. 78 Specific resistance of copper film with varying scan rates of 2 mm s⁻¹, 1 mm s⁻¹ and 0.6 mm s (reprinted with permission from ref. 77).

Fig. 79 XRD peaks of copper with scan rates of (a) 2 mm s⁻¹, (b) 1 mm s⁻¹ and (c) 0.6 mm s⁻¹, sintered using a laser of power 190 W cm⁻² or using hotplate with temperature of 300 °C for (d) 120 s, (e) 20 s and (f) 10 s (reprinted with permission from ref. 77).

Fig. 80 SEM micrograph representing the morphology of copper ink sintered by laser with power and scan rate as (a) 69 W cm⁻², 1 mm s⁻¹, (b) 148 W cm⁻², 1 mm s⁻¹ and (c) 190 W cm⁻², 2 mm s⁻¹ respectively (reprinted with permission from ref. 77).

Song et al.²⁰ synthesized a conductive copper ink by mixing copper nanoparticles in a solution of hydroxyl terminated polybutadiene (HTPB) and toluene diisocyanate (TDI) in a three-neck bottle flask. To this reaction mixture, toluene was also added as a viscosity lowering agent. The synthesized copper ink was deposited on a glass substrate and annealed at 120 °C for 2 hours.

The characterization of the patternized ink was carried out using SEM and XRD. SEM was employed for morphological analysis. Morphological analysis enables the conductivity and thin film formation measurements. It is one of the most influencing factors and is useful for determining the quality of the conductive film formation. Fig. 81a shows that connectivity is achieved between the copper nanoparticles and the film is more prone to cracks because of the small grain sizes, whereas Fig. 81b depicts the film formation which is more promising for enhancing the conductivity and is crack-free. XRD peaks are obtained exhibiting diffraction at 43°, 50° and 74°, which correspond to the FCC structure of copper.

Fig. 81 SEM micrograph of sintered ink composite prepared using (a) 90% Cu and (b) 60% Cu (reprinted with permission from ref. 20).

Luhai et al.¹¹ synthesized a copper nanoparticle ink for gravure printing. Copper nanoparticles were added to a dispersion of ethylene glycol and glycerin additives. The obtained copper colloid was re-dispersed by vigorous stirring and ultra-sonication until it formed a paste. The obtained copper paste was used for gravure printing of conductive patterns. Gravure printed copper patterns are shown in Fig. 82.

Fig. 82 Conductive patterns of copper nano-ink fabricated using gravure printing (reprinted with permission from ref. 11).

The resistivity of the fabricated patterns was characterized using a four-probe tester. By thermogravimetric analysis it was found that the content of copper in the ink was 35.15% and that of the stabilizing agent (PVP) was 1.29%, which will be removed completely when the sintering is carried out, as shown in Fig. 83.

Fig. 83 Thermo gravimetric analysis exhibiting residual mass of copper and its mass change (reprinted with permission from ref. 11).

Kim et al.⁷⁸ synthesized a conductive ink by combining copper nanoparticles and copper nanowires. The synthesized conductive ink was employed to increase the reliability of flexible electronics. The synthesis process was carried out by using copper nanowires (150 ± 50 nm diameter and 1–2 μm length) and copper nanoparticles (20–50 nm in diameter) as an additive for attaining the conducting property. The additive mixture was dispersed in a mixed solvent of PVP and DEG by mechanical stirring and ultra-sonication for 2 h followed by ball milling for 24 h. The obtained material was further subjected to three-roll milling three times for successful preparation of the conductive copper ink. The obtained conductive ink was coated on a PI substrate using the doctor blade technique and dried at 100 °C for 30 min. The ratio of Cu nanowires and NPs was varied from 0% to 100% for the mechanical fatigue testing process. The schematic representation of the fatigue tester is shown in Fig. 84a.

Fig. 84 (a) Schematic representation of mechanical fatigue tester, (b) image of sintered ink for the fatigue test and (c) calculation of bending radius of the patterned conductive ink (reprinted with permission from ref. 78).

A xenon flash lamp was employed for the sintering of the fabricated copper patterns. Sintering of the copper patterns was accomplished by irradiating the copper film with an applied energy of 7.5 J cm⁻² to 15 J cm⁻².

The characterization of the Cu NW/NP ink was accomplished using SEM (morphological calculations), XRD (crystalline phase calculations), the four probe-technique (resistivity calculation) and a profilometer (measurement of the thickness).

4. Other types of copper based inks

Although the preparation of conductive inks was initially accomplished by adding copper nanoparticles as additive materials, self-reducing MOD (metal–organic decomposition) inks are being synthesized in the present day. Piao et al.⁷⁹ and Magdassi et al.⁸⁰ synthesized a self-reducible copper MOD ink for use in flexible and printed electronics. The main advantage of MOD inks over conventional nanoparticle inks is that the metal–organic compound allows very low temperature sintering. The ink doesn’t contain any particles and hence can be used in printers. Another advantage of MOD ink is that an even surface is formed after the deposited ink is cured.⁷⁴

Copper crystalline inks were prepared for application in photovoltaic (PV) materials by Korgel et al.⁸¹ In their typical synthesis process, chalcopyrite compounds, such as nano-sized CuInS₂, CuInSe₂, and Cu(In_xGa_1−x)Se₂, were synthesized to fabricate uniform and relatively thick nano-crystalline layers for photovoltaic applications. A schematic representation for the fabrication of PV devices using CuInSe₂ ink is shown in Fig. 85. Fig. 86 shows the I–V characteristics for the fabricated PV device.

Fig. 85 PV device fabricated by using CuInSe₂ (reprinted with permission from ref. 81).

Fig. 86 I–V characteristics measured for the fabricated PV device (reprinted with permission from ref. 81).

The electro-less deposition (ELD) method was used by Wang et al.⁸² for the inkjet printing of metallic patterns. ELD is an auto-catalytic technique employed in depositing metals such as copper, silver, gold, nickel, etc. Through ELD, the metallic patterns can be deposited on substrates like paper, plastic, alumina, etc. In a typical procedure, glossy photo-paper treated with polyelectrolytes possessing quaternary ammonium was used as a substrate. The schematic shown in Fig. 87 explains the detailed process involved in the formation of copper patterns using the ELD technique. The prepared ink was maintained at a viscosity of ∼11 centipoises (cp), which is the optimum recommended range (10–12 cp) for the viscosity of inkjet inks.⁸⁹

Fig. 87 Schematic representation of the synthesis process used for patternizing copper through ELD (reprinted with permission from ref. 82).

Copper complex inks for application in photonic sintering were synthesized by Sugahara et al.⁸³ using copper formate tetrahydrate, copper acetate and copper oleate as metallic precursors. A Cu₂ZnSnS₄ nano-crystal based ink was synthesized by Agrawal et al.⁸⁴ for application in solar cells.

Transparent electrodes have been fabricated using copper nanowires as an alternative for photovoltaics, OLEDs (organic light emitting diodes), touch screens, etc. Traditional transparent electrodes are replaced by these copper nanowire-coated transparent substrates. The optical responses (absorbance, transmittance and reflectance) of these fabricated transparent electrodes were observed to be better than the conventional ITO and FTO.

5. Techniques for patternizing conductive inks

The synthesized copper conductive inks can be patternized by using spin coating and spray pyrolysis. Some advanced printing techniques have been used for the printing of conductive patterns on flexible as well as rigid substrates which can be utilized in PV applications. Various types of printing techniques have been employed in the successful printing of conductive patterns.^85,86 They are:

• Gravure printing⁹⁰

• Screen printing⁹³

• Inkjet printing⁹⁵

• Flexographic printing⁹⁸

• Spray coating¹⁰⁰

• Knife coating

• Slot-die coating

• Double slot-die printing.⁹³

Gravure printing has been used in the field of electronics since the 1980’s. This technique was widely used and considered to be one of the most cost effective techniques employable in everyday life. This type of printing is used in the fabrication of solar cells, polymer field effect transistors (PFETs) and organic light emitting diodes (OLEDs).^91,92 The schematic illustration of gravure printing is shown in Fig. 88.

Fig. 88 Schematic illustration of gravure printing (reprinted with permission from ref. 86).

Screen printing is one of the most versatile printing techniques and was invented in the 20^th century. This technique is very simple to operate, is compatible with a large variety of organic materials and has the ability to print under ambient temperature and pressure conditions.⁹⁴ The basic screen printer consists of only two parts, a squeegee and a mesh (screen), as shown in Fig. 89. There are two types of screen printing, namely flat-bed screen printing and rotary screen printing. Screen printing can be used to fabricate solar cells, fuel cells, organic FETs, and organic thin film transistors (OTFTs).

Fig. 89 Schematic illustration of screen printing: (a) flat-bed and (b) rotary (reprinted with permission from ref. 86).

The key technology for printing in industrial usage is inkjet printing. The advantages of ink-jet printing are accuracy, rapid deposition rate, mask-less printing and non-contact mode, and it can be used for the printing of a broad range of functional materials.⁹⁶ An ink-jet printer, as shown in Fig. 90, can be operated in two modes, namely continuous mode and drop-on-demand mode.⁹⁷ The flexographic printing process has the potential to achieve the required resolution and precision for the printing of flexible electronics and OLED lighting substrates.⁹⁹ Flexographic printing is similar to gravure printing, in which a doctor blade is used to restrict excess ink. The schematic illustration of flexographic printing is shown in Fig. 91.

Fig. 90 Schematic representation of inkjet printing (reprinted with permission from ref. 86).

Fig. 91 Schematic representation of flexographic printing (reprinted with permission from ref. 85).

Spray coating is similar to inkjet printing, as shown in Fig. 92. In spray coating, ink is sprayed continuously on the substrate. The spray is achieved by atomizing the ink, pressurized by air or gas (nitrogen gas), which helps in splitting the liquid into droplets. A shadow mask is used while using spray coating, which leads to low resolution of the printed pattern and wastage of ink. Knife coating is one of the types of one-dimensional coating. In knife coating, the ink is supplied in front of the knife, which is placed near to the substrate, as shown in Fig. 93. Knife coating has applications in fuel cells, photovoltaic devices, etc.

Fig. 92 Schematic comparing (a) inkjet and (b) spray coating (reprinted with permission from ref. 85).

Fig. 93 Schematic representation of knife coating (reprinted with permission from ref. 85).

In slot-die coating,^98,99 a coating head is employed and is kept very close to the printable substrate. A constant supply of ink is used to print on the movable substrate. Using the slot-die coating technique, high precision coating can be achieved. Double slot-die coating was used to print aqueous suspensions of P3HT/PCBM and PEDOT/PSS solutions¹⁰¹ simultaneously in the fabrication of organic photovoltaic cells, with a processing speed of 1 m min⁻¹ (Fig. 94 and 95).

Fig. 94 Schematic of slot-die coating (reprinted with permission from ref. 85).

Fig. 95 Schematic illustration of double slot-die coating (reprinted with permission from ref. 85).

The aforementioned processes for copper conductive coatings have been also used for printing other types of metallic coating, such as silver, gold, platinum, etc. Even non-metallic coatings, such as carbon, graphene, polymer-based inks, carbon nanotube inks, biological active layers, etc., can be fabricated using the aforementioned coating processes.

6. Complications and challenges in synthesizing conductive inks and their remedies

The main disadvantage in using copper nanoparticles in conductive inks is that copper nanoparticles have the inherent tendency to oxidize under atmospheric conditions, which is a problem of great concern, as this oxidation has adverse effects, such as decreasing the conductivity of the copper and increasing the sintering temperature. Several methods have been introduced by the researchers around the world to reduce the effects of oxidation. However, the rate of oxidation depends upon the surrounding atmosphere in which the nanoparticles are synthesized and temperature and pressure have the most influence.

7. Methods to prevent oxidation of copper particles

Magdassi et al.¹⁰² have discussed and determined some methods for controlling oxidation on the surface of copper nanoparticles. They are:

7.1 Polymer coating,

7.2 Carbon/graphene encapsulation,

7.3 Metallic core–shell preparation and

7.4 Silica coating.

All the above mentioned methods are successful in their respective approaches, although some of the approaches have shown enhancement in the conductive properties and some failed to do so. Out of all the methods, the best method has to be chosen for the preparation of oxide free nanoparticles depending on their specific application and the cost of preparation for obtaining a good result with the product.

7.1 Polymer coating

Jiang et al.¹⁰³ devised a method to prepare copper nanoparticles with a polymer as the capping as well as stabilizing agent. LLDPE (maleic anhydride grafted linear low-density polyethylene) and an antioxidant (n-octadecyl-3-(4-hydroxy-3,5-di-tert-butyl-phenyl)-propionate) were used, with which a fine powder of CuO was used as an initial precursor. The characterization results show the purity of the obtained copper nano-particles.

The XRD patterns of the prepared nano-Cu, LLDPE and CL-15 (nano-Cu with 5% mass) are illustrated in Fig. 96. It can be clearly observed that the peaks in Fig. 96 correspond to pure Cu and no impurity or metal oxide is detected by XRD, indicating a pure nano-copper product. The EDS analysis in Fig. 97 shows that the product is composed of copper only. Similarly, the XRD results indicate that no new phase has been found in the Cu-LLDPE nano-composite.

Fig. 96 XRD pattern of copper nanoparticles stabilized with LLDPE. (a) Nano copper (b) LLDPE and (c) CuO (reprinted with permission from ref. 103).

Fig. 97 EDAX pattern of polymer-stabilized copper nanoparticles (reprinted with permission from ref. 103).

The SEM images of CuO, nano-Cu and CL-15 are compared and are illustrated in Fig. 98. From Fig. 98, it is clear that the morphology of nano-Cu is similar to that of CuO. It can be easily observed that the shape of nano-Cu is nearly spherical. The SEM image also shows that the particles obtained are pure copper without any detectable impurity (oxides). The obtained size of the copper particles is in close proximity to the required size (i.e., 100 nm).

Fig. 98 SEM images of (a) initial material CuO, (b) prepared nano-Cu and (c) CL-15 (reprinted with permission from ref. 103).

Lee et al.¹⁰⁶ carried out dry coating of self-assembled monolayer (SAM) alkanethiol-stabilized copper nanoparticles. Unique equipment was designed to dry coat SAM on copper nanoparticles, as shown in Fig. 99. The source of the equipment contains a bottle of octanethiol. Ambient temperature and pressures are maintained to coat the nanoparticles with evaporated octanethiol. Copper nanoparticles stabilized in PVP were chosen for dry coating. PVP has to be removed from the surface of the copper nanoparticles because SAM doesn’t get coated on the oxidized surface. For removal of the PVP, RF plasma treatment was used for 50 s. The TEM images, as shown in Fig. 100, were recorded to confirm whether PVP had been removed. The coating of copper nanoparticles with SAM was carried out by heating the octanethiol liquid to 40 °C under a vacuum of 4 × 10⁻² Torr. The evaporated octanethiol was coated on the surface of the copper nanoparticles, which were placed inside the cylinder and rotated at 25 rpm. To test the suitable coating concentration of alkanethiol, copper nanoparticles were coated by varying the number of coating cycles (6, 12 and 18). After coating with SAM, TEM analysis was carried out, which confirmed that there were no layers of oxide, as shown in Fig. 101(b).

Fig. 99 Schematic of dry coating equipment: (a) cylindrical chamber with three grooves where copper nanoparticles were placed, (b) source bottle in which to place the organic solvent (octanethiol in this case) and (c) joint connecting the small chamber to the main chamber (reprinted with permission from ref. 106).

Fig. 100 TEM image of PVP-coated copper nanoparticles (a) before and (b) after plasma treatment (reprinted with permission from ref. 106).

Fig. 101 TEM images of copper nanoparticles dry-coated with octanethiol (reprinted with permission from ref. 106).

7.2 Carbon/graphene encapsulation

Carbon-encapsulated copper nanoparticles were synthesized to prevent the copper from oxidizing. This method of synthesis was carried out using the plasma arc discharge method by Hao et al. A helium and methane environment is used in the generation of the plasma arc and methane acts as a carbon source.¹⁰⁴ A vacuum of 0.1 MPa to 10⁻³ Pa is utilized, with a discharge current of 80 A.

The size, morphology and surface characterization was carried out using TEM, as shown in Fig. 103. XRD characterization, as shown in Fig. 102, was also carried out for comparison, which clearly shows the presence of pure copper (a) when compared with carbon-coated copper (b).

Fig. 102 (a) XRD of commercial copper and (b) XRD of carbon-encapsulated copper (reprinted with permission from ref. 104).

Fig. 103 TEM images depicting the amorphous carbon coated on metallic nanoparticles (reprinted with permission from ref. 104).

Later, it was found that the carbon coated on the copper is amorphous, which fails in terms of conductivity and hence cannot be used in the preparation of conductive inks. Therefore, an alternative for the conductive carbon material should be used. In this case, a conductive carbon material like graphene¹⁰⁸ can be used.

He et al.¹⁰⁷ successfully prepared a conductive ink using graphene-encapsulated copper to protect the copper from oxidation. In their research they found that amorphous carbon-coated copper nanoparticles have very poor conductivity. To overcome this problem, graphene was used to encapsulate the copper nanoparticles by coalescence. Graphene is a better conductor, and hence the conductivity is not affected and there is a chance of enhancing the conductivity due to the addition of graphene.

The synthesis of graphene-encapsulated copper nanoparticles was carried out by placing Cu(acac)₂ precursor in the presence of hydrogen gas in a closed horizontal tube furnace, as shown in Fig. 104, equipped with a vacuum pump. A temperature of 150–600 °C was maintained inside the furnace to test the thermal stability and the flow rate of hydrogen gas was maintained at 200 standard cubic centimeters per minute with a pressure of 50 Pa.

Fig. 104 Schematic of vacuum-equipped furnace used for the synthesis of graphene-encapsulated copper nanoparticles (reprinted with permission from ref. 107).

The obtained nanoparticles were analyzed using X-ray diffraction, Raman spectroscopy and TEM. The thermal stability of the nanoparticles was also examined using a thermogravimetric analyzer and the results were interesting.

The obtained XRD peaks clearly show that the graphene/copper nanoparticles have the same peaks as conventional commercial copper powder, as shown in Fig. 105a, which clearly shows that copper is obtained with an average crystal size of 17 nm. The Raman absorption spectrum shows the formation of a D-band at 1374 cm⁻¹ and G-band at 1588 cm⁻¹, as shown in Fig. 105b, where the D-band exhibits the defects in the hexagonal sp² carbon network and G-band corresponds to the stretching motion of the sp² carbon pairs in both the rings and chains. In addition to all these first order bands, second order bands such as 2D and D + G bands are obtained, demonstrating the formation of graphene-encapsulated copper particles.

Fig. 105 XRD and Raman analysis: (a) comparison of conventional copper peaks and graphene/copper peaks and (b) Raman spectra of the synthesized particles (reprinted with permission from ref. 107).

TEM images are provided as a comparison, as shown in Fig. 106.The images are taken at the time of synthesis and 60 days after synthesis; in these 60 days the particles were exposed to normal atmospheric conditions.

Fig. 106 TEM images after the synthesis of nanoparticles: (a) TEM of nanoparticles obtained at 500 °C, (b) corresponding HR-TEM image of a single particle, (c) TEM image of particles at 700 °C and (d) HR-TEM image showing the layer of carbon at the outer shell (reprinted with permission from ref. 107).

The comparison of the TEM and HR-TEM images shows that there is no oxide layer formed, even after 60 days. Fig. 107 shows clearly that there are more layers of graphene, its thickness is measured to be 0.21 nm and the orientation of the copper is (111). This clearly shows that oxidation is prevented by deposition or encapsulation of graphene on the copper particles.

Fig. 107 TEM images taken 60 days after synthesizing nanoparticles: (a) low magnification TEM images taken showing the morphology and structure of the particles and the SAED pattern, (b) HR-TEM image showing the graphene layer outside the copper nanoparticles (reprinted with permission from ref. 107).

7.3 Metallic core–shell nanocomposite

Another method proposed by Cazayous et al. describes the synthesis of Cu–Ag core shell nanoparticles, where silver acts as a protective coating against oxidation (corrosion). These metallic core–shells are prepared using high vacuum thermal evaporation equipment with an evaporation rate of 0.25 nm min⁻¹ for copper and silver simultaneously.¹⁰⁵ Structural characterization was carried out with the help of TEM, HAADF-STEM, EFTEM and HRTEM (Fig. 109).

Since there is no possibility for silver to be oxidized at room temperature, these metallic core–shells do not oxidize unless they react with any other acidic gases. There is a possibility of enhancement of conductivity due to the presence of silver, which is more conductive than copper. The preparation of Cu–Ag core–shell metallic particles also increases the cost of production, which is an important factor. The mechanism of formation of core–shells of Cu–Ag is given in Fig. 108.

Fig. 108 Schematic representation of mechanism of formation of metallic core–shells of Cu–Ag (reprinted with permission from ref. 105).

Fig. 109 Electron microscopy images of Ag–Cu metallic core–shells: (a) TEM of obtained core–shells, (b) HAADF-STEM image, (c) EFTEM image depicting the Cu–Ag particles and (d) HR-TEM showing the difference between both the particles by lattice fringes (reprinted with permission from ref. 105).

7.4 Silica coating

A successful attempt has been made by Kobayashi et al.¹⁰⁹ to synthesize silica-coated copper nanoparticles. It is one of the inorganic attempts made to prevent oxidation of copper. The XRD peaks illustrated in Fig. 110 are a comparison of silica-coated copper and un-coated copper. The XRD also shows that the oxide of copper is formed when there is no inorganic coating on the copper.

Fig. 110 Comparison of XRD peaks of silica coated and uncoated copper: (a) uncoated copper nanoparticles, (b) silica-coated copper immediately after its preparation and (c) silica-coated copper after one month (reprinted with permission from ref. 109).

The XRD spectrum clearly shows the presence of copper oxide (Cu₂O) when not coated with silica (a); when coated with silica (b) very little trace of oxide is found, as shown in Fig. 110. Even after one month it is observed that the silica-coated copper particles have not oxidized (Fig. 110c). However, although silica is a very efficient protective agent, it is an insulating material at room temperature and it possess a high melting point, which restricts its use in the preparation of conductive inks.

8. Applications

Conductive copper inks have a wide range of applications in the field of electronics, automation communications, energy harvesting, MEMS/NEMS devices, display devices, computer peripherals, biological sensors, etc. (Fig. 111–113)

Fig. 111 Relative market size for printed and flexible sensors in 2024 (reprinted from ref. 113).

Fig. 112 Conductive ink market by region up to 2018 (reprinted from ref. 110).

Fig. 113 Market value share for conductive inks in all applications 2014–2023 (reprinted from ref. 110).

• Flexible circuits for membrane touch switches are one of the best applications of conductive inks due to the cost of manufacture when compared to the conventional methods in fabricating them.

• Flexible keyboards for desktop and notebook PCs can be fabricated using conductive inks, where the interconnections present inside the keyboard can be replaced via inkjet printing of conductive inks.

• Conductive copper inks can be used in the fabrication of automotive sensors which are used in automobiles for better performance, safety and control. Automotive sensors include air bag sensors, engine failure sensors, fuel sensors, parking sensors, brake sensors, etc.^111,112 MEMS devices are used in the fabrication of these automotive sensors where conductive ink can be applied.

• Biosensors, one of the widely known recent technologies in the field of science, can be fabricated using conductive inks. Presently, gold ink¹¹⁴ is used in the fabrication of biosensors, but in the near future, wearable biosensors may not be too far away.

• Antennas for contact-free smart cards and RFID labels¹¹⁵ used for identification, asset management and security access systems. A RFID inlay consists of an RF chip, antenna and substrate in which the antenna can be fabricated using a conductive ink.

• ITO-coated substrates are used as touch screens in mobiles, tablets, laptops, etc. Transparent conductive electrodes synthesized by using copper nano inks^87,88 can be used as a replacement for expensive ITO electrodes.

• Copper ink has applications in the printing of organic photovoltaic devices (OPVs) which are fabricated using the spray-mist coating method.¹⁰⁰

• The Global Trend & Forecast to 2018 shows the Conductive Ink Market by application (photovoltaics, membrane switches, displays, bio-sensor, automotive) and type (silver flake & nanoparticle, copper nanoparticle, carbon/graphene, CNT, dielectric, polymer).¹¹⁶

9. Recent developments in the field of flexible electronics

Foldable and wearable electronics are the future of the present electronics trend. The potential of these flexible electronics is unlimited. The need for flexible electrodes is to enhance the feasibility of managing bulk electronic devices. For example, imagine a single sheet of plastic replacing all the books in your backpack or a situation where every device is made to be non-breakable. It’s easy to use these flexible devices because of their advantages of light weight, foldable nature, low cost involved in their production and controllability in their optical and electronic properties,¹²³ which make this technology a stand out among others. This technological innovation can be incorporated into display technologies, electronic papers, biomedical implants, etc. Presently, some electronic devices have been fabricated using this technology. OLED televisions,¹¹⁸ flexible phone displays,¹¹⁹ printed solar cells,¹²⁰ flexible microfluidic devices for protein screening (lab on a chip),¹²¹ foldable biosensors for weight tracking,¹²² RFID tags,¹¹⁵ etc., are some examples of today’s modern flexible electronic technologies (Fig. 114).

Fig. 114 Some recent developments in the field of flexible electronics. From left to right and top to bottom: OLED TV, flexible display for mobile, printable photovoltaic cell, flexible microfluidic device, biological sensing tattoo for monitoring weight with the help of sweat (reprinted from ref. 118–122).

10. Future perspectives

In the current trend of the advancement of technology, conductive inks play a vital role in achieving the goals of technological challenges. Synthesizing inexpensive conductive inks is always a challenge for researchers. However, silver ink is being used in many applications, although it is expensive and has a negative impact on the device performance fabricated with it. Conductive ink made of silver is being widely used for the fabrication of conductive as well as transparent electrodes in photovoltaic applications.

Recent advancements made for synthesizing copper inks have the potential to replace silver ink. The main advantages of copper nanoparticle-based inks when compared to silver, graphene and CNT-based inks are its low cost of preparation and abundancy compared to silver, and its excellent electron migration resistance is also an added advantage. Sensing applications are on the way to advancement, which may alter and replace the conventional sensors that are being used today; conductive inks can be utilized in the fabrication of sensors using the latest technological changes. Replacement of expensive flexible devices based on ITO, graphene, silver and CNT electrodes is a tougher job but there is a chance of integrating copper-based conductive patterns in these devices by fabricating them in an inert atmosphere. Research into controlling the oxidation problem in copper-based inks should also be done.

The automotive sector has a huge demand for MEMS devices that are based on physical lithographic techniques as well as conductive inks. Inexpensive copper ink has the capability to replace expensive silver-based, graphene and CNT inks by overcoming its drawbacks. Some of the future technological aspects where copper ink can be integrated are explained in detail below.

Self drawing biosensors:¹¹⁷ University of California researchers developed a biocompatible enzymatic ink-based roller pen which is phenol-sensitive (Fig. 115). To this ink, they added some conductive additives like graphite powder for sensing purposes. They also claim that the strips used for sensing are re-usable. The group is presently working on wireless self drawing sensors and their analysis over various extremes of conditions like temperature, humidity and sunlight.

Fig. 115 Patterning of bio-compatible self drawing sensors on human body (reprinted with permission from ref. 117).

Artificial ophthalmologic implants¹²⁴ are one of the most important aspects for vision defective people. Research into contact lenses for suppressing color-blindness is on the way. Copper-based conductive ink instead of gold can be incorporated for electrical interconnections in these ophthalmologic implants because of its antibacterial and antifungal properties (Fig. 116).

Fig. 116 Electronic contact lens for ophthalmologic implants (reprinted from ref. 124).

Apart from the aforementioned applications, solar-powered wearable trackers, pain relief skin patches, dermatological implants, flexible touch screens, etc. constitute the most anticipated applications.

11. Research groups working in the area of flexible devices^125–136

All over the world many research groups are actively participating in the fabrication of flexible devices. Some of the most prominent research groups are mentioned in Table 2.

Table 2 Active research groups working on flexible devices

Group name	University/Institute	Research areas	Reference
Lewis Lab	Harvard University	Complex fluids, colloidal assembly and printing of functional materials	125
Rogers Research Group	University of Illinois	Bio-inspired/bio-integrated electronics, solid state lighting and photovoltaics, semi-conductor nanomaterials and carbon transistors	126
Zheng Research Group	Stanford University	Transfer printing methods for flexible electronics	127
Shlomo Magdassi Group	Hebrew University of Jerusalem	Conductive inks: metal nanoparticles and carbon nanotubes, coatings, and inkjet printing	128
Flexible and Nanobio Device Lab (FAND)	Korea’s Advanced Institute for Science and Technology	Flexible energy sources, flexible optoelectronics, flexible electronics and laser material interaction	129
Flexible Energy and Electronics Laboratory	University of Toronto	Electrochemical energy storage and printed organic memories	130
Advanced Flexible Electronics Laboratory	Seoul National University	Thin film devices, printed electronics and flexible/stretchable electronics	131
Flexible Electronics Group at MSU	Michigan State University	Integrated circuits, flexible AMOLED displays, interactive electronic skin, printed electronics and high speed flexible RF transistors	132
Flexible Electronics and Energy Lab (FEEL)	University of British Columbia	Wearable e-textiles for wireless health monitoring, flexible solar/battery nano-textile, nanowire growth and device fabrication, flexible organic solar cells, flexible transistors and circuit fabrication technology and modeling of nanomaterials and nanocomposites	133
Electronics Laboratory-Wearable Computing	Swiss Federal Institute of Technology, Zurich	Devices on plastic, sensors, flexible circuits, unusual substrates and smart textiles	134
Arias Research Group	University of California, Berkely	Printed and flexible energy harvesting devices, printed and flexible energy storage, devices, printed large-area antennas, printed large-area electronics, printed large-area optoelectronics, printed sensors, wearable medical devices	135
Flexible Materials Base Team	The University of Tokyo	Exploration of functional materials, innovative printing processes and microscopic evaluations	136

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Apart from the mentioned research groups, many other groups all around the world are working on this technology. Many government-funded laboratories are also actively participating in the research of flexible electronic devices as it is the most awaited technology of the future.

12. Conclusion

In this review, we have tried to bring into light all the possible methods for the synthesis of copper nanoparticles and conductive inks. However, only a few synthesis techniques have been successfully applied in conductive inks. Initially, conductive inks were prepared using copper nanoparticles that were synthesized via chemical reduction. Among the above-discussed chemical reduction methods, all the synthesis techniques discussed in this review are unique in synthesizing copper nanoparticles. However, only a few have been successful in formulating them into conductive inks and conductive patterns. LI Song, LI Luhi, F. Xiao, Chen, W. Yang, etc. have formulated conductive inks. Apart from chemical reduction, pulsed vapor deposition, photochemical synthesis, particle-free conductive ink, graphene–copper nanocomposites, transparent electrodes, nanoparticle/nanowire suspensions, etc. were also used for successful formulation of conductive inks, as discussed above. We hope that this review serves as a good source of knowledge for researches who are interested in conductive inks and their applications. We have discussed in brief the synthesis and challenges of the present technology of flexible electronics via copper inks. The next era of technology depends on flexible electronics and hence we present this review as a stepping stone for the future.

Acknowledgements

Authors are thankful to CSIR-TAPSUN for financial support. We also gratefully acknowledge DST-UK (‘APEX’) for their support. Authors show their gratitude to all the researchers who contributed to the work cited in this article.

Notes and references

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