Conductive silver inks and their applications in printed

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To overcome these drawbacks, silver conductive inks can serve as alternative to the current ..... wanathan et al.62 using this method, employing silver oxalate as.
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Conductive silver inks and their applications in printed and flexible electronics Venkata Krishna Rao R.,a Venkata Abhinav K.,a Karthik P. S.a and Surya Prakash Singh*ab Conductive inks have been widely investigated in recent years due to their popularity in printed electronics (PE) and flexible electronics (FE). They comprise specific and unique applications that belong to a whole new level of future technology. In this context, silver is a keenly researched material for its promising application in conductive inks. In printing technology, silver conductive inks have a major role in electronic applications. The emerging integration of different technologies is in the form of silver nanoinks. In recent years, the printed electronics market has been dominated by expensive materials such as gold, platinum, etc., which result in costly and complex instruments. To overcome these drawbacks, silver conductive inks can serve as alternative to the current

Received 22nd June 2015 Accepted 25th August 2015

technology. Presently, printed circuit boards (PCBs) use complex and expensive techniques to fabricate the circuit boards, which in turn increases the overall cost. Solvent-based silver conductive inks are capable of substituting PCB technology while reducing the cost of manufacturing. Due to

DOI: 10.1039/c5ra12013f

their stellar reputation, investors are looking forward to applying this technology in printed

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electronics industries.

Network Institute of Solar Energy (CSIR-NISE) and Academy of Scientic and Innovative Research (AcSIR), New Delhi, India. E-mail: [email protected]

attention. Silver conductive ink has its own prominence among various conductive inks. This prominence is due to its novelty in being applied to diverse elds of technology. Inherently, silver itself is an incredible material and abundantly available on our planet. Silver conductive inks have silver in nanoform, which is extensively engaging the nanotechnology community. Silver in the nanometer range has predominantly occupied the interest of researchers since the beginning of the nanotechnology era. Silver was and is considered a prominent material; and even historically silver has its own importance.

Dr Surya Prakash Singh is a Scientist at CSIR-Indian Institute of Chemical Technology, Hyderabad. He studied chemistry at the University of Allahabad, India, and obtained his D. Phil. degree in 2005. Aer working at 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 exible devices. He has published over 115 papers and reviews in peer-reviewed journals.

Venkata Krishna Rao R. is a research student at 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 from Jawaharlal Nehru Technological University, Hyderabad, India. His research interests are focused on synthesizing various conductive nanomaterials using different techniques and applying them in the eld of exible electronics. He is also interested in the self-assembly of fullerenes, conductive inks, and the fabrication of photovoltaics using costeffective materials.

1. Introduction Any technology is adopted commercially when it has the potential for sustained, long-term signicance in diverse applications. The development of conductive ink is worthy of such

a

Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad-500007, India

b

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This material has been prospering in its applications ever since researchers focused on its study in the nanoscale, which has led to promising applications in many elds of science and technology. The properties that make silver suitable are that it is rare, strong, corrosion resistant, and unaffected by moisture. Silver is also resonant, moldable, malleable, and possesses the highest thermal and electric conductivity of any substance. Silver is one of the strongest oxidants; it is an essential catalyst for the chemical process industry. Metal nanoparticles act as a key element in conductive inks. In nano range, silver has gained so much prominence that it does not require any special introduction about its enhanced properties possessed in the nanometer range. Silver nanoparticles (Ag NPs) have been synthesized using different types of approaches, and each type of synthesis has its own advantages and disadvantages. Silver has unique and enhanced properties in the nanometer range by possessing the highest electrical conductivity per unit volume when compared to any other metal, so it is expected to be used extensively in printed electronics technology. In this review, we try to highlight the methods that were successful in the synthesis of conductive ink derived from silver nanomaterial. This work is an attempt to understand the signicance of silver-based conductive inks and to motivate researchers who opt to bring about change in future electronics. We focused this review on gathering a brief preface of all the possible methods of synthesis, the chemicals used, the process followed, and the broad applications of silver-based conductive ink. Silver ink has been studied and formulated for the past three decades.1 Since then, many improvements have been applied to bring it to its current advanced state.2–9 Conductive silver ink is being investigated for a wide variety of applications10–16 and also blended with other composites.17–21 1.1 Why is conductive ink a chief priority in the scientic community? Technology is advancing day by day, and in this advancement race we need material that will support and encourage further advancement. Conductive inks mark one of the top places on Venkata Abhinav K. is a research student at 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 from Jawaharlal Nehru Technological University, Hyderabad, India. His research interests are focused on synthesizing conductive nanomaterials using various techniques and applying them in the eld of printed electronics. He is also interested in the self-assembly of fullerenes and the fabrication of solar cells using cost-effective materials.

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Scheme 1 Representation of the process involved in the formulation of conductive ink.

the list. The reach of conductive inks lies in the application of the next level of technological advancement, such as exible and printable electronics, which comprise a major area of application of conductive inks. There is great necessitate in the present market as well as serious competition between technological giants in producing such type of appliances, which not only simplify lifestyle but also fetch major attention. Thus, in this regard, silver ink stands as the best option in hand for researchers worldwide. It should also be noted that a few other conductive inks have also been synthesized using other conductive metals such as copper,22–26 platinum,27 gold,28,29 tin,30 and iron.8 A few carbon allotropes,31 which are known for their outstanding properties, have also been used for formulating inks such as CNT32,33 and graphene.35–38 Other than these polymers,39,40 hybrid inks41–44 are also being successfully used for conductive inks. However, their applications are limited in comparison to silver ink; their applications do not cover as wide an area as silver. The fascinating fact is that not all the conductive metal elements can be easily synthesised to formulate an ink—the consequences in ink formulation are high and have a negative impact on the properties, which in turn makes the products fall behind silver ink. Fundamentally, there are three challenges involved in synthesizing conductive silver ink (Scheme 1). These three challenges are further classied and discussed below, with focus on each aspect and its prominence.

P. S. Karthik is a research student at CSIR-Indian Institute of Chemical Technology, Hyderabad, India, in the group of Dr Surya Prakash Singh. He has completed his Bachelor and Master degree from Jawaharlal Nehru Technological University, Hyderabad, India. His research interests are focused on synthesizing carbon nanomaterials using various techniques and applying them in the eld of solar energy. He is also focused on fabricating solar cells using different light-absorbing materials. He has published three research papers in the eld of nanotechnology. RSC Adv., 2015, 5, 77760–77790 | 77761

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2.

Techniques to synthesise Ag NPs

Many methods have been reported for the synthesis of Ag NPs by using chemical, physical, and electrochemical reduction routes. Each method has its own advantage and disadvantage primarily with cost and particle size. Among the methods, costwise, the chemical reduction method is the best process for the synthesis of Ag NPs. Chemical methods provide an easy way to synthesize Ag NPs in solution. Achieving the best result by following the best method of synthesizing Ag NPs is our second priority, the rst being the best formulation of ink. We have mentioned in brief each method for the synthesis of Ag NPs.

Review

Although not all Ag NPs synthesized by different methods have been used to synthesize silver ink, our interest is to put all the possible methods of synthesis in one paper for a better understanding. In order to fully understand the formulation and challenges faced in synthesizing the conductive silver ink, the rst analysis should be done on the synthesis of Ag NPs. Close observation is needed, as there is a very wide range of synthesis techniques, chemicals, and challenges. Acquiring the perfect Ag NPs will solve the rst and foremost challenge in synthesizing the conductive silver ink. Hence, it is very important to master the synthesis of Ag NPs (Scheme 2). 2.1

Different methods for synthesis of Ag NPs

A wide variety of chemicals and methods have been adopted in the synthesis of Ag NPs in the past decade. Each method and chemical plays its own part in the factors affecting the synthesis of Ag NPs. A brief introduction and comparison on two major types, i.e., physical and chemical, are discussed below: 2.2

Scheme 2

Schematic representation of the review of Ag NP synthesis

routes.

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Physical methods

The physical approach is the top-down approach, and it is also called a high-energy method. Top down is a process in which microsized particles are broken down to nanosized particles, either by mechanical force or evaporation/condensation.48 The evaporation is done by thermal heating or plasma excitation of the solid, microsized particles, powders, plates or wires. The vapour or plasma is transferred onto a substrate by inert gas media. Silver synthesized by physical methods has particle sizes ranging from 4–10 nm.50 The benet of the physical method of synthesis is that it results in dry nanoparticles, i.e., the resultant nanoparticles are not in liquid medium; hence it is suitable for dry applications. The other prominent approach is laser ablation, in which the bulk particles are dispersed in aqueous media and irradiated with powerful lasers. The irradiation causes the reduction in size by providing enough energy to break down the bulk material into smaller particles.51,52 The physical method has less chemicals involved compared to chemical methods, which is an added benet. The drawback of the physical method is that it involves sophisticated instruments, and the total cost involved in synthesis is very high compared to the bottom-up approach. 2.2.1 Brief introduction to the physical synthesis of nanoparticles. The synthesis of Ag NPs by physical approach is accomplished by techniques such as evaporation–condensation, laser ablation, mechano-chemical synthesis, pulsed wire discharge, spray pyrolysis, thermal decomposition, etc.54,55 These techniques fall under the top-down approach. Even though nanoparticles obtained through the physical approach have some advantages, such as uniform particle growth, restriction of solvent contaminants, and the preparation of highly pure nanoparticles with negligible agglomeration, the physical approach is prone to disadvantages such as use of expensive equipment, increased time to synthesize nanoparticles, high energy consumption, etc. Out of the methods listed above, evaporation– condensation and laser ablation are the most widely used. Makitalo et al.56 synthesized Ag NPs by evaporation– condensation. It is also called the inert gas condensation

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technique. In this method, the sacricial metallic source is resistively heated inside a high vacuum chamber. The silver atoms inside the chamber collide with the gas atoms and lose their kinetic energy, thus condensing to form small-sized particles. A liquid nitrogen-lled cold nger is placed inside the chamber, on which metallic particles deposit. This technique can be used to prepare metallic nanoparticle alloys as well as oxides, nitrides, carbides, etc. Laser ablation is the ejection of a particular material from a surface by laser irradiation. It is useful for non-equilibrium vapour/plasma conditions created at the surface by intense laser pulse. The two essential components in a laser ablation instrument are the pulse laser and the ablation chamber. A high-power laser beam is induced on the target material, which rapidly increases in temperature and vaporizes into a laser plume. The vaporized material condensates into clusters and is collected on a glass bre mesh. The collected nanoparticles can be used for further applications.51 The synthesis of silver nanoparticle colloid by laser ablation technique in aqueous medium was attained by Zamiri et al., who provide detailed descriptions.57 The average size of obtained silver particles was found to be less than 100 nm with uniform shape. Mechano-chemistry is an extension of science alluding to the chemical and physico-chemical responses of substances produced from the impact of mechanical force. In mechanochemical synthesis, the mechanical and chemical phenomena are combined in molecular scale.58 The best example of mechano-chemical synthesis is ball milling. Ball milling is one of the most widely used techniques for synthesizing nanosized metals, metallic alloys, oxides, sulphates, etc. In a typical synthesis carried out by Khayati et al.,59 silver nanostructured powders were synthesized by mechano-chemical reduction of silver oxide precursor. Pulsed wire discharge is one of the physicochemical methods for the synthesis of nanoparticles using plasma/vapour. This method has the advantages of high energy conversion efficiency and high production rate of nanoparticle preparation. The factors affecting the nanoparticle growth rate are ambient gas pressure (P) and relative energy (K). In a typical synthesis process, Tokoi et al.60 synthesized Ag NPs by pulsed wire discharge in liquid medium. The size of nanoparticles obtained through this process was below 100 nm. Pulsed wire discharge of nanoparticles in liquid media was chosen due to the fact that the particles formed in liquid media are more highly dispersive than the nanoparticles formed in air. Spray pyrolysis technique is mainly used for thin lm coating; however, nanoparticles can be synthesized through spray pyrolysis. Generally, metal oxide nanoparticles are synthesized by spray pyrolysis. This technique was rst used in 1966 for the growth of cadmium sulphide thin lms for photovoltaic applications. In the recent past, FTO (uorine doped tin oxide) and ITO (indium doped tin oxide) were synthesized by spray pyrolysis for photovoltaic applications. This technique is cost effective, and complex geometries can be coated. The advantage of spray pyrolysis is that uniform and high quality coating can be deposited. Monodisperse Ag NPs were synthesized on a polymer matrix using this method by Kim et al.61

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Generally, thermal decomposition of nanoparticles is an endothermic process. In this technique, the heat generated by the equipment breaks the chemical bonds undergoing decomposition. This technique is very rarely used due to its disadvantages such as high energy consumption and the large amount of time required to initialize the system, etc. Single crystalline silver nanostructure has been synthesized by Viswanathan et al.62 using this method, employing silver oxalate as the source of silver. For bulk production of nanoparticles, physical methods present a good option in hand. But for cost effectiveness, chemical methods prove to be the better alternative.

2.3

Chemical reduction methods

The chemical reduction method has been the most followed and predominantly researched method in the synthesis of any metal nanoparticle in the past few decades. This approach is the bottom-up approach, where the nanoparticles are formed by precursors. The precursor supplies the ions and molecules, which are built up into nanoparticles by the inuence of a suitable reducing agent. This is a very cost-effective method for the synthesis of metal nanoparticles. The nanoparticles synthesized by this method hold good stability and have numerous applications. There are many types of synthesis techniques providing different paths for different applications. Green and biological syntheses of Ag NPs have great demand, as they are eco-friendly and biocompatible.53 2.3.1 Challenges for synthesis of Ag NPs. There are a wide range of chemical methods available for the synthesis of Ag NPs. Fundamentally, the following are the challenges for any chemically synthesized nanoparticle used in conductive ink (Scheme 3). 2.3.2 Effective size reduction. The size of the resultant particles plays a crucial role in the nanoparticle synthesis. Silver conductive ink synthesis primarily depends on the size of the particles due to its application in printed electronics/ MEMS6,63,64 and inkjet printing.4,65–69,71 Table 3 shows the different combinations of silver precursor and reducing agents, which result in different size ranges. 2.3.3 Avoiding oxidation. Oxidation is a very serious and undesired issue that has become a main drawback in a few metal-based inks. Oxidation occurs when the surface of the metal exposed to air reacts with the oxygen in the atmosphere and forms a dense layer of metal oxide. The undesired oxidation of metals not only decreases conductivity, but it also affects the performance of the ink. Eliminating oxidation requires a few set-ups and equipment, which in turn increases the cost of production. To overcome the oxidation of metal atoms, ink

Scheme 3

Factors affecting the conductive ink formulation.

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printing has to be carried out in an inert atmosphere or vacuum, or the ink has to be stabilized by adding suitable blends or stabilizers that restrict oxidation and enhance conductivity. Silver ink benets in this regard, as silver does not get oxidized quickly. Hence, silver ink has good performance and life compared to other metallic conductive inks. 2.3.4 Avoiding agglomeration. Agglomeration is one of the major consequences in nanoparticle synthesis. It affects the life and performance of the application and the applicant. As Ag NPs are widely used in inkjet printing and in conductive ink pens, the agglomerations block the dispenser system, i.e. the nozzle of the ink jet printer cartridge or the nib of the conductive ink pen. Hence, it results in blockage and malfunction of the equipment. This agglomeration can be overcome by adding suitable capping agents. In Table 3, we list different capping agents used in the synthesis of Ag NPs.

2.4

Review

(a) TEM image of silver nanoparticles, (b) AFM image of Ag NPs (reprinted with permission from ref. 73).

Fig. 2

Electrochemical synthesis of silver nanoparticles

Electrochemical synthesis72 is a process where electrical energy is converted to chemical energy. In electrochemical synthesis, electrons act as a source through which the reaction is accomplished. In electrochemical synthesis, the electrolyte acts as the carrier of metal ions coming from the sacricial electrode. Metallic gold nanowires were synthesized electrochemically on a polycarbonate membrane with pore diameter of 100 nm. Rashid et al.73 synthesized Ag NPs by electrochemical method without using any stabilizing agents. The process was performed in three stages. At stage one, oxygen gas is released due to the electrolysis of water; the silver is deposited on the anode. The silver ions are attracted towards the cathode during this process. Formation of Ag NPs occur during nucleation. Fig. 1 shows the comparison of nanoparticle growth with respect to different temperatures and variation in current. In order to obtain stable Ag NPs, the reaction should be run for a long time. To ensure particle size stability, the concentration of solution should be in range of 20–40 mg L1. The reaction time should be restricted to 50 to 70 min at a temperature of 50 to 80  C. At stage two, ltration is done to avoid agglomerated particles and to enhance reduction of silver

Fig. 1 Different temperatures vs. current (reprinted with permission from ref. 73).

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FE-SEM image of silver nanoparticles with different sizes of Ag NPs (reprinted with permission from ref. 73).

Fig. 3

ions. In Fig. 2(a), the TEM image shows that the average particle size is 7 nm. The stability of Ag NPs was tested, and as a result the Ag NPs exhibited the same properties for more than 7 years at atmospheric conditions. AFM measurement shows that the diameter of Ag NPs ranges between 100 and 150 nm (Fig. 2(b)). The Ag NPs that were deposited on the cathode have weak adhesion towards the substrate, and hence they are easy to remove by mechanical exfoliation. Particles obtained with a large diameter and nearly spherical and polyhedral shape can be seen in Fig. 3. In the FE-SEM image, it can be understood that some particles are 40 nm in diameter (Fig. 3). Ibrahim et al.74 synthesized Ag NPs by electrochemical technique, using two pure silver electrodes (99.2%) with a 20 V voltage pass and 0.4 A current pass through the solution. The Ag NPs were characterized by XRD and AFM. The Ag NPs exhibited diffraction at 38 , 44 , and 64 , corresponding to the [111],

Fig. 4 XRD pattern of Ag NPs (reprinted with permission from ref. 74).

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Fig. 5

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AFM structural image (reprinted with permission from ref. 74).

[200], and [220] planes, respectively, conrming the FCC (facecentered cubic) structure, as shown in Fig. 4. The morphology of the Ag NPs was characterized by AFM (atomic force microscopy), and the measured particle size was calculated to be 72.14 nm, in a clustered form shown in Fig. 5. The nanoparticle size was measured at 11.4 nm with high purity. This type of Ag NPs synthesis is less expensive, does not use any type of chemical stabilizing agents, and can be used in various applications such as antibacterial, optoelectronics, printed electronics, and exible electronics. Rold´ an et al.75 prepared Ag NPs successfully by using polyethylene glycol (PEG) as a stabilizing agent and silver nitrate as a silver precursor. The whole process was done in an ultrasonic bath. In a typical synthesis process, 2.5 mM silver nitrate solution was prepared by dissolving it in 2% w/v PEG. The whole setup was carried out in nitrogen atmosphere for 30 min. A current (7 mA, 10 mA, or 3 mA) was used with an adjustable applied potential. The optical response was measured by UV-vis spectroscopy using Ag NPs stabilized by 1% w/v PEG-2000 by applying 10 mA current with a wavelength near 300 nm, and

XRD pattern of Ag NPs exhibiting three different peaks (reprinted with permission from ref. 75).

Fig. 7

another spectrum was observed at 392 nm with the intensity increased up to 425 nm aer 30 min of reaction (Fig. 6). The characterization of particle size and crystal structure was done by XRD. The diffraction was observed at 38.13 , 44.31 , and 64.47 , corresponding to the planes at (111), (200), and (220), respectively, shown in Fig. 7, thus conrming the cubic crystal structure. The average crystalline size was measured to be 45.85 nm from the Debye–Scherrer formula. To determine the size and shape of the silver particle, AFM was used. It can be seen that the shape is near-spherical with an average diameter of 30.36 nm. The average particle size is in

Fig. 8 AFM image of Ag NPs with size distribution analysis (reprinted with permission from ref. 75).

UV-visible spectrum of Ag NPs (reprinted with permission from ref. 75).

Fig. 6

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TEM image of PEG-based Ag NPs at different nanoscales (reprinted with permission from ref. 75).

Fig. 9

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Fig. 10 TEM images of PVP and SBS under mechanical stirring: (a) distribution of PVP and SDS, (b) at high magnification, (c) particle size distribution chart of PVP under mechanical stirring (reprinted with permission from ref. 76).

close relation to XRD calculations. The AFM scale bar of 85 nm is shown in Fig. 8. TEM images show that the spherical-shaped particles are between 10 and 30 nm in size, and some of them are in bigger sizes, with diameters up to approximately 200 nm (Fig. 9). Yin et al.76 reported an electrochemical method to prepare Ag clusters using poly(N-vinylpyrrolidone) (PVP) as a stabilizing agent. KNO3 (0.1 mol dm3) was used as electrolyte and mixed with AgNO3 (5.0  103 mol dm3) to control the size and shape; PVP plays a key role as a stabilizer. Two types of PVPs were used, PVPK30 and PVPK17, to stabilize the length and size of Ag NPs synthesized at room temperature (22  C) under mechanical stirring with an applied potential. UV-vis

SEM image of (a) tin coating and (b) complex coating of tin and Ag NPs (reprinted with permission from ref. 76).

Fig. 11

spectroscopy analysis of the Ag NPs stabilized by the two different PVPs was performed. The maximum absorbance occurred at 420 nm, conrming the presence of Ag NPs (Fig. 10). The average diameter of particles was found to be 16.6 nm, with a standard deviation (SD) of 6.22. The formation of nanoparticles small in size was due to ultrasonic agitation; mechanical stirring can result in signicant prevention of silver clusters and ensure uniform distribution. Here PVP is assisted in the formation of dispersed nanoparticle deposits on the electrode. The SEM image of the Ag NPs doped on tin substrate is shown in Fig. 11(a). The crystal grain structure with different shapes is shown in Fig. 11(b). Sanchez et al.77 have synthesized Ag NPs by using H2SO4 (sulphuric acid), tetrabutylammonium bromide (TBABr), tetrabutylammonium acetate (TBAAcO), and aluminium oxide as precursors. The processing temperature was maintained at 25  0.1  C by a thermostatic bath. In a typical synthesis process, Ag/AgCl was used as a reference electrode, a sacricial silver sheet was used as anode, and the same-sized platinum sheet was used as cathode. The electrodes were placed inside the electrochemical cell, where the platinum electrode was polished with alumina powder. Triangular potential cycling between the range of 1.35 V and 0.15 V at a scan rate of 500 mV s1 was performed for 5 min. The electrolyte consists of TBABr dissolved in acetonitrile and de-aerated with nitrogen for 15 min in an inert atmosphere.

SEM micrograph of Ag NPs deposited on platinum sheet: (A) low current density, (B) high current density, and (C) magnified image of (B) (reprinted with permission from ref. 77).

Fig. 12

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TEM images with the size distribution of silver particles synthesized in TBAACO at various current densities (1.35 mA cm2 and 6.9 mA cm ) (reprinted with permission from ref. 77). Fig. 13 2

Fig. 12 presents SEM images with scale sizes of 1 mm, 2 mm, and 100 mm. SEM micrographs for Ag NPs with different current densities are shown in Fig. 12(A)–(C). Surface plasmon resonance of the silver particles was observed at 420–444 nm, which is dependent on the average particle size. Fig. 13 shows the TEM images of Ag NPs as well as their corresponding size distributions. 2.5

Green synthesis

Similar to other chemical methods, the use of various plant extracts has been taken into consideration to prepare Ag NPs of diverse sizes. Ag NPs are synthesized by using various plant extracts, which act as reducing as well as stabilizing agents. Bar et al.78 have reported a green synthesis method using an ecofriendly process to prepare Ag NPs with the stem of the Jatropha curcas plant. The milky white latex (1 mL) was diluted in 300 mL triple-distilled de-ionized water to make it 0.3%, and

20 mL of latex solution was added to 20 mL of 5  103 M silver nitrate solution. The solution was reuxed constantly at 85  C in an oil bath under constant stirring for 4 h. Fig. 14 shows the HRTEM image of the unevenly shaped nanoparticles with the 50 nm scale, along with the size distribution of the particles, which have a diameter of 20–30 nm. The XRD spectrum shown in Fig. 15 shows peaks representing the diffraction obtained at 38.03 , 46.18 , 63.43 , and 77.18 , corresponding to the planes at (111), (200), (220), and (311), respectively. Erusan et al.79 have reported the preparation of Ag NPs with coconut water (Cocos nucifera) as a reducing agent. The reduction is accomplished by adding 10 mL of coconut water and 90 mL of 1 mM AgNO3 solution to a beaker, maintained at 80  C for 15 to 20 min in a steam bath until the color changes to reddish. Aer observing the color change, the solution was centrifuged at 18 000 rpm for 20 min to obtain nanosized Ag particles.

Fig. 14 (a) HRTEM micrograph of Ag NPs with SAED pattern of polycrystalline particles, (b) histogram of HRTEM showing the particle size distribution of silver nanoparticles (reprinted with permission from ref. 78).

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(Fig. 16(b)). The diffraction peaks were obtained at 38.12 , 44.27 , 64.27 , and 76.23 , corresponding to (111), (200), (220), and (311) planes, respectively (Fig. 17). Pai et al.80 reported the synthesis of Ag NPs with fresh tulasi leaves. 25 g of tulasi leaves were added to 100 mL deionised water, heated 95  C for 1 h, and ltered to collect 15 mL of leaf extract. The extract was added to 85 mL of 0.001 M AgNO3 and heated for 80 seconds using microwave (power of 1.2 kW at the frequency of 2450 MHz). The obtained particles were characterized by SEM and UV-vis spectrometry. The UV analysis showed silver surface plasmon resonance at 441 nm, which lies in the spectrophotometric range as shown in Fig. 18(c). SEM analysis was performed at high and low magnications to conrm the spherical shape and size (20 to 40 nm), as shown in Fig. 19(a) and (b).

X-ray diffraction pattern of Ag NPs obtained by AgNO3 treated with 0.3% latex of Jatropha with 5  103 M AgNO3 solution (reprinted with permission from ref. 78). Fig. 15

Fig. 16(a) displays the UV-vis absorption spectrum of Ag NPs synthesized from coconut water (C. nucifera) at 1 mM concentration of silver nitrate. The green-coloured peak in the UV-vis spectrum represents the absorption of Ag colloidal particles. The XRD peak shows the Ag NPs prepared from C. nucifera

2.6

Microwave irradiation synthesis

Microwave irradiation is an electromagnetic irradiation in the frequency range of 0.3 to 30 GHz. All the ‘kitchen’ microwave ovens as well as the dedicated microwave reactors are suitable for chemical synthesis, operating at a frequency of 2.45 GHz49 with the corresponding wavelength of 12.24 cm to avoid interference with telecommunications and cellular phone frequency. The energy of the microwave (0.0016 eV) is too low to break chemical bonds and is also lower than Brownian motion energy.81 Microwave synthesis is a one-step synthesis method in

Fig. 16 (a) UV-vis absorption spectrum. (b) XRD analysis showing the diffraction at 38.12 , 44.27 , 64.27 , and 76.23 (reprinted with permission from ref. 79).

Fig. 17

(a) SEM image of Ag NPs, (b) EDAX analysis (reprinted with permission from ref. 79).

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Fig. 18 Optical image of silver nitrate and tulasi reaction mixture: (a) yellow, (b) brown. (c) UV-vis spectrum graph showing silver nitrate solution absorption peak at 414 nm (reprinted with permission from ref. 80).

aqueous media with the advantage of energy efficiency. It works using the efficient heating of materials by the “microwave dielectric heating” effect. The heating process is dependent on the type of material (solvent or reagent) that absorbs the microwave energy and converts it to heat. The electrical component83 of electromagnetic eld heating depends on two mechanisms, namely dipole polarization and ionic conduction. The microwave preparation of Ag NPs84 was done using PVPK30, silver nitrate, and PEG. In a typical synthesis process, aqueous silver nitrate was mixed with PVP solution and PEG in a 100 mL round bottom ask and heated under magnetic stirring in a microwave heating instrument. The solution changes its colour from transparent to yellow colloid, indicating the growth of Ag NPs. The same method was carried out by replacing microwave heating with oil heating. PEG and other

Fig. 19 SEM images showing (a) spherical Ag NPs (b) with 20 to 40 nm

impurities were separated by centrifugation of the reaction mixture at 8000 rpm and dispersed in ethanol for further characterization. Fig. 20(a) and (b) show TEM images with various size distributions of Ag NPs synthesized by microwave irradiation. When 700 W of microwave power is applied, the total reaction time is 30 min with a uniform particle size of 50 nm (Fig. 21). Ag NPs were also prepared using starch solution by adding 5 mL of silver nitrate to 1% starch and stirring for 30 min. The transparent solution was heated to a temperature range of 45–75 . The reaction was carried out under microwave irradiation, in which the formation of silver particles was conrmed by the change in colour from yellowish brown to greyish black. The obtained particles have been stored at room temperature for a period of 2 months, which proves that starch is a good reducing agent as well as stabilizing agent under microwave irradiation (Fig. 22). The size of the obtained Ag NPs was analyzed by TEM. The average diameters of the silver particles are recorded to be 12 nm. UV-vis spectra of silver particles synthesized at different temperatures with time variation conrm that Ag NPs are sustainable when synthesized by microwave irradiation (Fig. 23). Fig. 24 shows the UV-vis absorption spectra of starchstabilized Ag NPs prepared at different time intervals (10–60 min) at 80  C. As the temperature increases, the surface plasmon resonance of the silver particles appears at 420 nm.

size (reprinted with permission from ref. 80).

TEM images with size distribution histogram: (a) particle size of 50 nm of Ag NPs obtained by microwave heating and (b) the corresponding particle size distribution; (c) silver particles synthesized by oil heating and (d) the corresponding particle size distribution (reprinted with permission from ref. 84).

Fig. 20

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Fig. 21 (a) UV-visible spectra of silver synthesized by microwave and oil heating methods, (b) UV-visible spectra of silver particles synthesized at different temperatures using microwave heating (reprinted with permission from ref. 84).

Another method to synthesize Ag NPs was reported,86 in which silver nitrate (AgNO3) was used as Ag precursor; hexamine and sodium borohydride (NaBH4) were used as reducing agents. The typical synthesis process starts with pectin powder (extracted from citrus peels) dissolved in 90 mL of hot water. The solution was placed under microwave power of 800 W at a frequency of 2459 MHz for 5 min. Upon microwave irradiation, the colourless solution turned yellowish-brown, indicating Ag particle formation.

The UV-visible absorption spectrum of pectin shows the absorption from 200 to 600 nm. The peak observed at 416 nm aer microwave irradiation (MI) for 3 min is shown in Fig. 25(a). Fig. 25(b) shows the UV-visible spectrum showing two different peaks with increased intensity, 4-nitrophenolate at 317 nm and 4-nitrophenol at 400 nm. The crystalline structure was conrmed by the XRD pattern shown in Fig. 26. Four distant peaks were observed at 37.86 , 43.94 , 64.17 , and 77.12 . These peaks correspond to the

(a) UV-visible absorption spectra at different molar ratios of PVP to silver under 700 W microwave heating, (b) UV-visible extinction spectra at different amounts of microwave power (reprinted with permission from ref. 84).

Fig. 22

Fig. 23 TEM images of Ag NPs (a) with an average diameter of 30 nm, (b) refluxed under 75  C, and (c) with an average diameter of 12 nm. (d) HRTEM image of a single silver nanoparticle. (e) SAED pattern for silver (reprinted with permission from ref. 85).

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Fig. 24

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Absorption spectra of Ag NPs under different heating conditions: (a) 55 , (b) 65 , and (c) 75 (reprinted with permission from ref. 85).

planes at (111), (200), (220), and (311), respectively, conrming that the crystal structure is face-centered cubic (FCC), which is in close relation with JCPDS le no. 04-0783. Using energy dispersive X-ray spectra (EDX), the element description of the sample was obtained, with peaks located at 2 keV and 4 keV directly related to the K and L lines of silver. The smallest peak is observed at 0.3 keV. This is due to carbon, and another peak at 0.5 keV is due to oxygen, as shown in Fig. 27. TEM was employed to determine the size, shape, and morphology of Ag particles. Fig. 28(a) presents less uniformly distributed particles with spherical shape and highly crystalline structure. The particle size was found to be 18.84 nm  0.9 (Fig. 28(b)). The diameters of particles were about 16–20 nm, with size distribution of symmetrical nanoparticles at the range of 14–24 nm. The HR-TEM image in Fig. 28(c) shows the crystalline lattice growth of silver nanoparticles. The SAED in Fig. 28(d) shows four planes, (111), (200), (220), and (311), which are a reection of high-quality FCC structure. Another preparation of Ag NPs by microwave irradiation was accomplished by silver nitrate (AgNO3) and benzo-18-crown-6 as reducing and stabilizing agents.87 1 mL of 0.2 M benzo-18-

Fig. 26

XRD pattern for Ag NPs (reprinted with permission from ref. 86).

Fig. 25 (a) UV-visible absorption (MI (microwave irradiation)) time, (b) UV with two ion peaks at 317 nm and 400 nm (reprinted with permission from ref. 86).

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Fig. 27

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EDX spectrum of Ag NPs (pectin) (reprinted with permission from ref. 86).

crown-6 and 10 mL of 0.0001 M AgNO3 were taken in a conical ask and placed in a microwave oven under 300 W for 3 min. The colorless solution slowly turned to pale yellow color, indicating the formation of Ag NPs.

The presence of Ag NPs was conrmed by TEM image as shown in Fig. 29(a). The particle size distribution shows that the particles ranging from 6–11 nm formed under optimum experimental conditions shown in Fig. 29(b).

Fig. 28 (a) TEM image of Ag NPs, (b) particle size histogram, (c) HRTEM image of Ag NPs, (d) SAED pattern of silver-pectin (reprinted with permission from ref. 86).

Fig. 29 (a) TEM image of Ag NPs, (b) size distribution for Ag NPs (reprinted with permission from ref. 87).

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Fig. 30 (a)–(d) UV-visible spectra of Ag under different time intervals and temperature conditions (reprinted with permission from ref. 87).

Fig. 31 (a) and (b) SEM images of Ag nano-wires, (c) HRTEM image of a particular Ag nanowire, (d) XRD pattern for Ag nanowires (reprinted with permission from ref. 88).

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The UV-visible spectrum of the silver colloidal solution exhibits a strong absorption at 420 nm as shown in Fig. 30(a) and (b). The UV-visible spectra of Ag NPs synthesized with different time intervals and constant power of 300 W are shown. Fig. 30(c) shows the UV-visible spectra of Ag synthesized at a constant time interval (3 min) with variation in microwave power from 450 W to 180 W. The silver particles were kept for 5 months in the refrigerator, and optical absorption was characterized by UV-visible spectra, as shown in Fig. 30(d). 2.7

Synthesis of Ag NPs by polyol process

The polyol process is widely practiced for large-scale synthesis, and hence it is one of the best methods available. Many studies on the polyol process have been reported, and the process has its own impact on nanotechnology. Cheng et al. reported88 the successful one-step synthesis of silver nanowires with a diameter of 40 nm and length > 10 mm, which has been tested for applications in conductive ink. Ag nanowires were prepared by dissolving silver nitrate (100 mg) and PVP (100 mg) in 30 mL ethylene glycol with 1 M HCl. The solution was reuxed at 140  C for 2 h; the colour turned orange; then the temperature was raised to 160  C for 30 min. The solution was centrifuged for some cycles, which resulted in Ag nanowires. SEM, XRD, and HR-TEM were employed in the

Fig. 32 (a) Silver lines patterned on PET substrate, (b) LED glows, (c) PET substrate twisted to check for conductivity, (d) LED glows perfectly after twisting the substrate (reprinted with permission from ref. 88).

Fig. 33

Review

investigation of the Ag nanowires. The TEM images conrmed that the size and shape of the Ag nanowires were about 40 nm in diameter and greater than 10 mm in length. The XRD result estimated the interplanar distances at 0.24 nm for (111) and 0.14 nm for (110) planes, as shown in Fig. 31(d). The obtained Ag nanowires were tested for conductivity and their application in ink by fabricating a circuit on PET substrate and lighting an LED, as shown in Fig. 32. Similarly, Dang et al. synthesized Ag NPs using the same polyol process,11 changing a few parameters, and obtained an average diameter of 10 nm. Firstly, PVP was dissolved in ethylene glycol (20 mL), then silver nitrate was added. Instead of reuxing the solution as in the previously mentioned process, the solution was ultrasonicated for 3 min, and colour change was observed as an indication of the growth of silver particles. The silver particles were formulated into silver ink by dissolving them in organic solvents. The conductive ink was then printed onto different substrates using an inkjet printer. The obtained particles were characterized by TEM, and a size distribution chart was produced. The particle sizes ranged from 4 to 16 nm, which was sufficient for ejection from the nozzle of the inkjet printer (Fig. 33). In the ink formulation, Ag NPs (20% wt) were mixed with ethanol (32% wt), ethylene glycol (32% wt), 11.2% wt of 2-isopopoxyethanol, and 4.8% glycerine and ltered though a syringe of 0.2 mm pore size. The formulated ink was then applied on PET, glass, and silicon wafer. Among the three substrates, PET and glass had less contact angle, but the contact angle was high for silicon wafer. The adhesion was good for silicon wafer substrate, but was wavy and formed droplets in the case of glass and PET substrate. The adhesion was improved by changing the concentrations in the ink formulation to H2O (31% wt), ethanol (15% wt), ethylene glycol (1.5% wt), glycerine (15% wt), 2-isopropoxyethanol (1.5% wt), ethyl acetate (15.6% wt), SDS (0.3% wt), ethyl glycolate (0.05% wt), and ethyl formate (0.05% wt) (Fig. 34). Barkey et al.89 synthesized an Ag nano-wire suspension for printable conductive media. In a typical synthesis process, ethylene glycol was reuxed at 150  C for 1 h under continuous stirring. An aqueous solution of copper(II) chloride was introduced into the ethylene glycol solution; an aqueous solution of PVP was added further, along with aqueous silver nitrate, to the

TEM image of Ag NPs and their size distributions (reprinted with permission from ref. 11).

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Fig. 34

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Optical images of printed patterns on (a) PET, (b) glass, and (c) silicon substrates (reprinted with permission from ref. 11).

Fig. 35 SEM images of nanowires prepared by the polyol process at different volumes of “ethylene glycol” (a) 160 mL, (b) 25 mL (reprinted with permission from ref. 89).

Fig. 36 Silver pattern screen printed using nanowire ink (reprinted with permission from ref. 89).

reaction mixture. During the synthesis process, the reaction ask was purged with argon gas. The change in colour of the solution from yellow to red, then olive green, clear peach, and nally to opaque grey indicates the formation of nanoparticles. The obtained solution was centrifuged for 20 min at 3000 rpm once in the presence of acetone, and three times in the presence of deionised water. Characterization of the synthesized Ag nanoparticles was performed using SEM, which shows the morphology of silver nanowires formed in Fig. 35. The average diameter of the synthesized nanowires was found to be 90–100 nm. Conductive ink was formulated by dispersing silver nanowires in water acting as a solvent, carboxymethyl cellulose (CMC) as binder, and Dispex® Ultra FA 4416 (Old Hydropalat 216, BASF) as a dispersive agent. The synthesized conductive ink was printed with the help of a screen printer, as shown in Fig. 36. The conductive ink pattern was characterized by SEM and rheometer. Fig. 37(a) displays the SEM micrograph of the silver pattern printed on polycarbonate substrate. Fig. 37(b) shows the viscosity versus time plot for the ink prepared using nanowire and nanoparticle suspensions. The rheology of the nano-ink was measured as a function of time, and it is

Fig. 37 (a) SEM image of printed ink at different magnifications, (b) rheology measurement comparing Ag NP ink and Ag NW ink as a function of

time (reprinted with permission from ref. 89).

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Fig. 38 (a) XRD pattern of Ag nanoparticles obtained by reducing AgNO3 with tripropylene glycol and PVP of Mw ¼ 10 000; 55 000; and 1 300 000. (b) UV-vis spectra of Ag stabilized with PVP (reprinted with permission from ref. 90).

found that the nanowire ink was more viscous than the nanoparticle ink. Chiang et al.90 synthesized Ag NPs with silver nitrate, tripropylene glycol, 3-ethyl-3-oxetanemethanol, polycaprolactone triol, and PVP as the precursors. Firstly, various amounts of PVP with different molecular weights (Mw ¼ 10 000; 55 000; 1 300 000) were used in three different solutions. To these solutions, the required amount of AgNO3 was added in the ratio of 1 : 100 (AgNO3 : reducing agent). All the prepared solutions were reuxed at 120  C at a rate of 4  C min1 for 3 to 24 h. Silver nanoparticle growth was observed when 3-ethyl-3-oxetanemethanol was added to the reaction mixture and sonicated for 10 min. The obtained solution was centrifuged to separate the Ag NPs, which were washed 5 times to remove the capping agents. Characterization of the Ag NPs was accomplished by XRD, UV-vis spectroscopy, FTIR, and TEM. XRD pattern of the Ag NPs

produced by reduction with tripropylene glycol is shown in Fig. 38(a). It shows that the produced nanoparticles are FCC in structure, which is in close relation to JCPDS no. 26-0339. UV-vis spectra of nanoparticles synthesized using PVP with molecular weights (Mw ¼ 10 000, 55 000, 1 300 000) are shown in Fig. 38(b). From the gure it can be clearly understood that silver particles stabilized by PVP with Mw ¼ 10 000 shows very good absorption compared to the others, whereas the SPR band of silver particles stabilized with PVP of Mw ¼ 55 000 exhibits a wider band at 470 nm, implying that the prepared nanoparticles are smaller in size. FTIR spectrum of AG NPs reduced by tripropylene glycol was compared with Ag NPs reduced by tripropylene glycol + AgNO3, as shown in Fig. 39(a). The interactions between tripropylene glycol and Ag NPs can be analyzed from the FTIR peaks. TEM micrographs of Ag NPs prepared by reducing it with tripropylene glycol are shown in Fig. 39(b). Ag NPs were also stabilized

Fig. 39 (a) FT-IR spectra comparison of tripropylene glycol nanoparticles and tripropylene glycol + AgNO3, (b) TEM micrographs of silver synthesized by heating at 120  C with tripropylene glycol as a reducing agent and stabilized in PVP of Mw ¼ 55 000 (reprinted with permission from ref. 90).

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Fig. 40 Schematic of processing in the segmented flow tubular reactor (SFTR) system for the multi-gram synthesis of Ag NPs (reprinted with permission from ref. 91).

Fig. 41

TEM micrographs of Ag NPs: (a) G0, (b) G1, (c) G2, and (d) G3 (reprinted with permission from ref. 91).

in PVP of Mw ¼ 10 000 and 1 300 000, but the particles obtained using Mw ¼ 55 000 are preferable in size. In another report, the precursors for synthesizing Ag NPs include AgNO3, PVP of Mw ¼ 10 000 and 40 000, and ethylene glycol (EG).91 Firstly, two separate dispersions of AgNO3 in PVP (Mw ¼ 10 000 and 40 000) were prepared under reux

Fig. 42

conditions at room temperature and 70  C, sonicated, stirred. The reactants were added into the reactor system rate of 1.5 mL min1. The temperature was raised to 120 150  C, respectively, for 12 min. The growth of silver sol observed; it was decanted further (Fig. 40).

and at a and was

TEM micrographs of Ag NPs: (a) G4, (b) G5, and (c) G7 (reprinted with permission from ref. 91).

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Fig. 43

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XRD peaks of G3, G4, G5, and G7 samples (reprinted with permission from ref. 91).

Table 1 Ag NPs prepared by varying the reaction parameters temperature and stabilizing agent (reprinted with permission from ref. 91). The values in the parentheses are standard deviations

Sample name

Temperature ( C)

PVP Mw (g mol1)

TEM size (nm)

Mean diameter (nm)

G0 G1 G2 G3 G5 G5 G7

120 130 140 150 140 140 150

10k 10k 10k 10k 40k 40k 40k

23 (4) 25 (4) 47 (8) 58 (7) 7 (2) 79 (15) 104 (20)

14 (4) 20 (8) 33 (12) 56 (17) 9 (2) 107 (30) 123 (32)

The obtained Ag NPs were characterized by TEM as shown in Fig. 41(a) to (d) for G0, G1, G2, and G3. The average particle size obtained is in the range of 23–58 nm by various samples as shown in (Table 1). Fig. 42 shows the TEM of G4, G5, and G7 samples with mean diameter of 9–123 nm. The XRD patterns of the four samples (G3, G4, G5 and G7) prepared by varying the reaction parameters are shown in Fig. 43(b).

3. Silver ink formulation by chemical reduction The chemical reduction method is one of the common methods used for obtaining Ag NPs. This process requires a reduction agent to reduce the silver nanoparticles. Lewis et al.92 described a typical process to synthesize silver ink; the silver particles were suspended in a viscosifying solution, which were obtained by reducing silver nitrate in the presence of poly acrylic acid (PAA) by stabilizing them in diethanolamine and stirring for 22 h to yield Ag NPs with an average diameter of 5 nm (Scheme 4). The Ag particles were sonicated at the reuxing condition of 65  C for 1.5 h, which results in the formation of

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Scheme 4

Representation of the conversion of nanoparticles to ink.

agglomerated silver particles with a mean size of 400  120 nm, shown in Fig. 44(a). Aer the formation of particles, ethanol was added to the silver particles to remove impurities by centrifugation with a high speed of 9000 rpm. The obtained silver particles were re-dispersed in distilled water to remove excess PAA. The same process was repeated three times to yield pure Ag NPs, which were dispersed in a solution of hydroxy ethyl cellulose (HEC) dissolved in ethanol and water in the ratio of 1 : 1. The mixture was homogenized at 2000 rpm for 3 min and dried until ink was obtained. Fig. 44(b) shows the viscosity measurement of the synthesized ink. The shaded pattern indicates the preferred viscosity range for using the ink in inkjet printing. The acquired viscosity was measured at a shear rate of 1 s1 for silver ink synthesized with HEC. For the synthesized inks with different concentrations of silver, the HEC ratio is stable for months when stored in sealed ink pens. The electrical resistivity of silver ink was measured by heating and drying the ink at room temperature as a function of rise in temperature and time. Silver lms (1 cm  1 cm wide, 12 mm in height) were fabricated by doctor blade technique on glass substrate. The synthesized ink exhibited good conductivity aer drying at room temperature for 30 min, with a resistivity of 1.99  104 U cm. The increased annealing temperature of 110  C shows a reduction in resistivity of the synthesized ink. Fig. 44(c) shows that a further increase in

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Fig. 44 (a) TEM image of the Ag-NPs and optical image of the ink (inset); (b) ink viscosity (h), the shaded part indicating the ink viscosity required for printing; (c) specific resistivity of silver ink measured with respect to time (reprinted with permission from ref. 92).

Fig. 45 (a) Optical images of silver electrodes with an array pattern, flat (top) and bent; (b) SEM images of silver electrodes in straight position; (c) SEM images of silver electrodes after 10 000 bend cycles; (d) electrical resistance as a function of the bend cycle of the printed silver electrodes of various bend radii; and (e) customized machine used to characterize mechanical flexibility (reprinted with permission from ref. 92).

temperature (140  C, 170  C, and 200  C) results in a further decrease in resistivity. To characterize the mechanical exibility of the silver ink printed on paper, an array of 5 lines was printed with a roller ball point pen and dried at room temperature for 24 h. The pattern coated on the xerox paper was actuated between at and bent states at a bending rate of 2 cm s1 using a customized mechanical micropositioning system shown in Fig. 45(e). In Fig. 45(a), the silver electrode is shown in at and bent states. SEM image conrms that no cracks were found in the rst bent stage shown in Fig. 45(b) and (d) shows the bend radius and the bend cycles with respect to electrical resistance (R) with a silver pattern radius of 2.9 mm and 1.6 mm. Fig. 45(c) shows that silver electrodes exhibit good response even aer 10 000 bending cycles. The ink was applied in an electronic art

displaying a surface-mounted LED placed on the roof of a house (Fig. 46(a)). A multi-coloured LED pattern created by the roller pen is displayed in Fig. 46(b). A 3D antenna is patterned using conductive ink as shown in Fig. 46(c). Song et al.67 reported a method for the preparation of Ag NPs and formulation of silver nanoink using silver nitrate as a source of silver. Monoethanolamine (MEA) acts as a reducing agent; PAA acts as stabilizing agent. Firstly, MEA, PAA and AgNO3 were sequentially dissolved in deionized water under vigorous stirring at room temperature for 1 h. The reaction was considered complete when the colour changed rst to yellow and further to a transparent color as shown in Fig. 47(a). The next step is heating the solution to 65  C and stirring for 1 h, by which brown, orange, and dark reddish colours were observed as shown in Fig. 47(b)–(d), depicting that nucleation of Ag NPs has been started. The

Fig. 46 (a) Optical image of conductive electronic art drawn by silver inks with glowing LED, (b) optical image of a flexible paper display with an LED array on paper, (c) optical image of a 3D antenna, with electronic silver ink art patterned on a hemispherical hollow glass substrate (reprinted with permission from ref. 92).

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Fig. 47

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Schematic process of synthesizing Ag NPs (reprinted with permission from ref. 67).

Fig. 48 (a) XRD pattern Ag NPs, (b) SEM micrograph of Ag NPs (reprinted with permission from ref. 67).

supernatant was removed, and the silver particles were washed several times with ethanol. Aer washing, the pure Ag NPs were let dry. Silver ink was prepared by adding Ag NPs to deionized water and ethylene glycol, and then ultrasonicating for 20 min. The processed solution appears to be brown in colour and ready for making conductive patterns. The XRD pattern shows three major peaks, which can be observed at 38.2 , 44.4 , and 64.5 , as shown in Fig. 48(a), with the corresponding planes at (111), (200), and (220), respectively,

which are in close relation to the JCPDS File no. 04-0783. The advantage of making PAA-stabilized silver particles is that they can be stored at room temperature. The particle sizes of silver are in the range of 20–230 nm, and the mean size varied between 30 to 50 nm, as shown in Fig. 49(a). The particle size analysis (Fig. 49(b)) shows the viscosity measurement of the formulated silver ink. The gure also depicts the comparison of surface tension and viscosity. Table 2 describes the parameters of the ink synthesized using ve different percentages of Ag NPs. Physical parameters of the

Fig. 49 (a) Particle size distribution of Ag NPs (PDA) with TEM image, (b) graph of viscosity and surface tension as a function of wt% (reprinted with permission from ref. 67).

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Table 2 Explanation of the physical parameters of the Ag NPs synthesized using various concentrations of silver (reprinted with permission from ref. 67)

Ag NPs

Viscosity (mPa s)

Density (103 kg m3)

Surface tension g (mN m1)

Z (1/Oh)

5 wt% 10 wt% 15 wt% 20 wt% 25 wt%

2.51 2.86 3.32 3.62 4.03

1.056 1.074 1.123 1.158 1.218

42.16 43.37 45.29 45.69 46.65

9.94 8.93 8.04 7.52 7.00

Ag NPs synthesized using various concentrations of silver are described below (Fig. 50). Reactive transparent silver ink was synthesized using Tollen's process by Lewis et al.71 In this method, silver acetate is mixed in aqueous ammonium hydroxide solution. Formic acid is added dropwise to the reaction mixture. The colour change of the reaction mixture from light orange to brown and further to greyish was observed by the rapid reduction of silver ions to silver particles, and they are ltered by a 200 nm syringe lter. The optical image of synthesized silver ink is shown in Fig. 51(a). Conductive lines drawn on silicon substrate by a glass nozzle (100 mm diameter) with a 90  C bend and with

contact on substrate are shown in Fig. 51(b). UV-vis absorption spectrum characterized by a standard reference with a background calibration of deionised water is shown in Fig. 51(c). TGA (thermogravimetric analysis) was performed in 79% : 21% ratio of nitrogen : oxygen environment at a rate of 100 mL min1 under 23  C for 48 h, and another sample was characterized at 10  C min1 to 90  C for 30 min in Fig. 51(d). XRD pattern shows the silver ink's peaks at 39  C, 45  C, 65  C, and 78  C, with the corresponding planes at (111), (200), (220), and (311), respectively, as shown in Fig. 51(e). The conductive ink patterns were annealed at different temperatures (40  C, 80  C, 90  C) for 15 min. The patterned silver being dried at 23  C with a size of 92  12 nm, 84  8 nm, 61  7 nm, and 50  5 nm can be observed in Fig. 52. Using hydrazine hydrate, Guzm´ an93 synthesized Ag NPs using two stabilizing agents, sodium dodecyl sulphate (SDS) and sodium citrate. The combination of silver nitrate solution and SDS with different concentrations is shown in Fig. 53: (a) hydrazine with SDS, (b) with hydrazine and citrate sodium, (c) hydrazine hydrate citrate sodium and SDS used as a metal salt precursor and stabilizing agent, respectively. Hydrazine hydrate concentration is (2.0 mM to 12 mM and 1.0 mM) and sodium citrate concentration is (1.0 mM to 2.0 mM). Firstly, silver nitrate solution was dissolved in an aqueous solution of SDS and sodium citrate. Aer change in colour to pale yellow

(a) Optical images of silver patterns with different widths, (b) optical image of silver pattern with 1.0 pt width, (c) SEM image of printed silver pattern after heat-curing at 50  C, (d) thickness of the printed silver pattern measured by profilometer (reprinted with permission from ref. 67). Fig. 50

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(a) Optical image of transparent silver ink, (b) silver conductive pattern drawn by 100 nm nozzle printed on silicon substrate, (c) UV-visible spectrum, (d) TGA curve measured at 23 to 90  C in air, (e) XRD pattern of two different conditions at 23  C for 24 h (on blue pattern) and annealed at 90  C for 15 min (red patterned graph) (reprinted with permission from ref. 71). Fig. 51

Fig. 52 SEM image of silver ink written through a 100 nm nozzle (reprinted with permission from ref. 71). Fig. 54 TEM images of Ag NPs and the particle size distribution (reprinted with permission from ref. 93).

3.1 Ag NP synthesis with various reducing agents and stabilizing agents in detail As discussed above, Ag NPs have been synthesized through various techniques such as physical, chemical, electrochemical, and microwave synthesis. Among all the synthesis techniques, the chemical reduction technique is the most widely followed and researched method for the synthesis of conductive silver ink. Table 3 shows the detailed approach of chemical reduction of Ag salts with different reducing agents for making conductive inks. Fig. 53 Optical images of obtained Ag NPs: (a) pale brown, (b) pale yellow, and (c) pale red (reprinted with permission from ref. 93).

and to pale red, the obtained particles were washed with the help of a centrifuge at least three times under a stream of nitrogen. The structural characterization of Ag NPs was performed by TEM, which shows the NPs have spherical shape with 100 nm scale. Particle size distribution is 8 to 50 nm, with the mean diameter of 24 nm. UV-vis absorption analysis was performed with surface plasmon resonance (SPR) at 418–420 nm (Fig. 54).

77782 | RSC Adv., 2015, 5, 77760–77790

4. Conductive ink formulation Silver-based conductive ink is highly conductive, and silver possesses a conductive oxide; hence, its permanence is for a longer period compared to copper oxide. Conductive silver inks require elevated temperatures for curing, which opposes its adherence to different types of substrates. Due to the high sintering temperatures, the stabilizing agents, binders, and solvents evaporate, damaging its conductivity and connectivity (Scheme 5). The quality of ink formulation mainly depends on the following factors:

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Table 3

RSC Advances The various reducing agents, stabilizing agents, and the size of the resulting Ag NPs

S. no.

Metal source

Reducing agent

Stabilizing agent

Size

Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Silver nitrate Silver nitrate Silver nitrate Silver nitrate Silver nitrate Silver nitrate Silver nitrate Silver nitrate Silver nitrate Silver nitrate Silver nitrate Silver acetate Silver acetate Silver nitrate

EG (ethylene glycol) Sodium hydroxide Trisodium citrate Ultrasonication DEA MEA Hydrazine hydrate Hydrazine hydrate Sodium borohydrate Ascorbic acid — Formic acid Tin acetate Tripropylene glycol, 3-ethyl-3-oxetanemethonal

PVP PVP PVP PVP PAA PAA DDA SDS and citrate of sodium SDS CTAB Sodium oleate — Oleic acid PVP

40–380 nm 19 nm 3–10 nm 10 nm 200–600 nm 30–50 nm 5 nm 9–30 nm 30–40 nm 4 nm 7–12 nm