Anion-mediated synthesis of monodisperse silver nanoparticles useful for screen printing of high-conductivity patterns on flexible substrates for printed electronics

Abstract

Monodisperse silver nanoparticles (NPs) have been synthesized on a large scale by oxidation–reduction reactions in water by adding triethylamine to the aqueous solutions of AgNO₃, glucose and poly(vinyl pyrrolidone). Different anions, including –SO₄²⁻, –PO₄³⁻, –CO₃²⁻ and –Br⁻, are introduced to the abovementioned mixture to form slightly soluble silver compounds, which are used as the precursor to synthesize silver NPs. The effects of silver nitrate–glucose ratio and reaction temperature are investigated. The electrical performance of the as-obtained Ag NPs has been studied, and the results reveal that the –CO₃²⁻ mediated-synthesized Ag NPs possess the lowest resistivity. Note that silver NPs can be well-dispersed in ethanol and generated as inks, which can be screen printed onto flexible polyester (PET) and paper substrates and form conductive patterns after a low-temperature sintering treatment. An optimal electrical resistivity can be reached at 5.72 μΩ cm, which is much closer to the value of bulk silver (1.6 μΩ cm). Evidently, the synthesized silver NPs could be considered as low-cost and effective materials that have a great potential application for flexible printed electronics.

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1. Introduction

Recently, there has been a shift to develop simple approaches and techniques for fabricating electronic devices, and printing technologies have become a research focus and represent an emerging field. Owing to its ability to bypass traditional expensive and inflexible silicon-based electronics, printed electronics can help in the fabrication of a variety of devices on flexible substrates, which is also being explored for the manufacture of large-area electronic devices by the patterned application of functional inks using high-throughput printing approaches.^1,2 However, the critical issue that needs to be noticed when applying printed electronics to these applications is the development of high-performance and low-cost conductive inks. The development and integration of innovative printable electronic materials is a core activity since many types of materials are required for printed circuits, including high conductivity metals for electrodes and interconnects, high capacitance/low leakage gate dielectrics for capacitors and thin film transistors (TFTs), and high-mobility n-type and p-type semiconductors for complementary, low-power logic gates.

For designing and fabricating an optimal conductive ink, the ink preparation process should be both simple and high-yielding; moreover, the ink should possess good printability, low viscosity and good stability to ensure that it can be stored at room temperature and is compatible with different printing technologies. Furthermore, the patterned features should be highly conductive at room temperature and achieve bulk conductivity on annealing at mild temperatures.³ Though enormous efforts have been made to produce conductive inks using different methods, the fabrication of conductive inks is still a key goal in modern materials chemistry and printed electronics field and has drawn substantial attention in recent years. To date, conductive polymers,^4–6 carbon,^7,8 graphene and graphene oxide,^9–13 and metallic nanoparticle inks^14–18 have been developed and used in the printed electronics field. Recent studies have concentrated on fabricating and using conductive inks for integration on various flexible substrates such as textiles, plastics and papers. Compared with graphene and graphene oxide, metallic NPs are still low-cost and can be produced on a large-scale for industrial application. After the discovery that metallic NPs show reduced melting temperature compared to the bulk material, many studies have been conducted to form stabilized nanoparticle dispersions that can be printed. Note that the use of silver NPs in conductive inks and their direct imaging by inkjet printing technology has been known for years.^19–24 For example, Perelaer and co-workers fabricated silver-based conductive patterns by inkjet printing, which exhibited excellent conductivity values of 5–56% of bulk silver.²⁵ Layani and co-workers have synthesized conductive inks based on silver NPs, which can be used to fabricate the 3D conductive patterns by inkjet printing.²⁶ In fact, with respect to the inkjet printing, screen printing is a more convenient technique for industrial application.

Furthermore, the search for new geometries is an important aspect for noble metal nanomaterials, and understanding the correlation between the sintering temperature, electronic properties of printed patterns/devices and the morphology of nanostructures are prerequisites for widespread applications of printing electronics. Indeed, various experimental parameters, such as anions, temperature, contents of precursor and reducing agent, and reduction time, could result in synthesized metal NPs for conductive inks with different properties.^14,27–31 The synthesis of various Ag nanostructures has achieved great success due to the development of synthetic methods. It is known that precipitates with a fairly low solubility will be generated from the addition of different anions to aqueous solution of Ag⁺ ions; moreover, the different chemical equilibrium (caused by the introduced anions) is an influence on the nucleary of Ag NPs, which will occur in the reduction reaction and lead to change in the morphology of Ag NPs.^32–34 For example, Li and co-workers have synthesized monodisperse, quasi-spherical Ag NPs directly in water by the addition of the aqueous solution of AgNO₃ and sodium citrate, and I⁻ ions were used to tailor the growth of the Ag NPs into a quasi-spherical shape via their preferential adsorption on the {111} facets.³⁵ Moreover, different shapes of silver particles can generate different electrical performance in conductive inks.³⁶ For example, Yang and co-workers have reported the effect on electrical performance of the inkjet-printed tracks of Ag particles, rods and plates in conductive inks by comparing the microstructure of three tracks, and the results reveal that conductive ink filled with a mixture of nanorods and nanoparticles was more favorable to form a random, three-dimensionally interconnected conduction network that exhibits intriguing electrical characteristics.³⁷

Encouraged by these aforementioned progresses, herein the present work was planned to interrogate how the morphology of Ag NPs formed upon addition of different anions, including –SO₄²⁻, –PO₄³⁻, –CO₃²⁻ and –Br⁻ ions, are obtained and the corresponding electronic performance after printed. We report a simple, environmentally friendly and cost-effective process for the preparation of monodisperse silver NPs for flexible printed electronics application. Different morphologies of silver nanoparticles were synthesized, and the effects of introduced anions (–SO₄²⁻, –PO₄³⁻, –CO₃²⁻ and –Br⁻) in reaction process were systematically investigated. The conductivity results of the Ag electrodes reveal that the introduced carbonate ions are best, among the four introduced anions. Furthermore, the other synthesized parameters of anions have been optimized and the corresponding conductivity of Ag electrodes has been studied. The conductivities as a function of sintering temperature and time were considered to achieve high conductive tracks; moreover, we also used FE-SEM to analyze the sintering mechanism of the as-prepared Ag conductive inks. Note that our conductive ink can be easily applied to screen printing, and it could be printed directly onto flexible substrates such as PET and cellulose paper.

2. Experimental sections

2.1 Materials and chemicals

Silver nitrate (AgNO₃, 99%) was purchased from Aladdin Chemistry Co., Ltd. Glucose (C₆H₁₂O₆, AR), poly(N-vinylpyrrolidone) (PVP, GR), triethylamine (TEA, C₆H₁₅N, AR), anhydrous alcohol (C₂H₆O, AR), sodium sulfate (Na₂SO₄, AR), sodium carbonate (Na₂CO₃, AR) sodium phosphate (Na₃PO₄, AR) and sodium bromide (NaBr, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. The thickness of used high-temperature-resistant PET substrate is 0.3 mm, and the maximum bearing temperature is 230 °C. All the materials were used as received without further purification in the entire experimental process. Deionized water was used throughout the experiments.

2.2 Synthesis of silver NPs at different reaction parameters

In our synthesis process, silver nitrite was used as a metal source, and different anions were employed to tailor the morphology of the products. AgNO₃, glucose, PVP and a given number of anions were consecutively added to water with stirring at room temperature, and then heated to the reaction temperature. TEA was added dropwise at a rate of ∼0.6 mL min⁻¹ into the solution using a peristaltic pump, and the color of the reaction solutions changed quickly. The reaction lasted for 20 min, and the resulting dispersion was washed three times by ethanol by centrifuging at 10 000 rpm for 10 min. All the as-synthesized Ag NPs were washed to remove all the impurities before formulating the inks, and then the synthesized silver NPs were re-dispersed in ethanol. The influences of the important synthesis parameters on the final product evolution were investigated through controlled studies on the morphology of Ag NPs, and the detailed reaction conditions are summarized in Table 1.

Table 1 Synthesis conditions for the preparation of silver particles

Sample ID	Reducing agent (M)	PVP (g)	Et3N (μL)	Reaction temperature (°C)	Anion	Silver nitrate (mol)	Reaction time (min)
1	0.0015	1	300	60	SO4^2−	0.003	20
2	0.0015	1	300	60	PO4^3−	0.003	20
3	0.0015	1	300	60	CO3^2−	0.003	20
4	0.0015	1	300	60	Br^−	0.003	20
5	0.0015	1	300	30	CO3^2−	0.003	20
6	0.0015	1	300	90	CO3^2−	0.003	20
7	0.003	1	300	60	CO3^2−	0.003	20
8	0.006	1	300	60	CO3^2−	0.003	20

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2.3 Preparation of silver films on PET substrate using direct deposition

Silver conductive ink was fabricated by re-dispersing the as-synthesized silver NPs into ethanol and adjusting its concentration to 30 wt%. The PET substrates were cleaned using acetone and ethanol, and then the PET substrates were dried at room temperature prior to use. The silver inks were deposited onto the PET substrate by using a metal mask to finalize the design pattern (this pattern also can be screen printed on PET) and dried at room temperature to remove the solvent. The samples were dried at different temperatures in the range of 80–200 °C in an oven for 30 min, and then dried at 160 °C for different times within the range of 10–180 min.

2.4 Screen printing the Ag inks on cellulose paper substrate

Using this silver-based conductive ink, conductive lines, arrays, and printed circuit boards were manufactured on a paper substrate through screen printing, which was conducted using 400 mesh counts' screen to form conductive patterns with different line widths (0.3, 0.4 and 0.5 mm).

2.5 Characterization

Field emission scanning electron microscopy (FE-SEM) images were obtained using a high resolution field emission SEM (FEI Nova-400). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) analysis were performed on a JEOL JEM-2100F. The absorption spectra were recorded via UV-vis spectroscopy using a Shimadzu UV-2550 spectrophotometer. Electrical resistances of the samples were measured with a digital multimeter (victor 8245), and all measurements were performed at room temperature.

3. Results and discussion

3.1 The morphology and product evolution of the as-prepared Ag NPs

Previous studies have reported the synthesis of Ag NPs.^38–42 Herein, we investigate the morphology of Ag products synthesized in the presence of different anions such as SO₄²⁻, PO₄³⁻, CO₃²⁻, and Br⁻ ions. As shown in Fig. 1, Ag NPs were synthesized by the reduction of silver nitrate with glucose using PVP as a capping agent at different experimental parameters. Due to the importance of anions for the reduction, herein, we optimized the concentrations of anions that were used (details are provided in the ESI†). The addition of these anions to the aqueous solution of Ag⁺ is known to generate new silver compounds that are slightly soluble in water, the solubility is an influence on the reaction of Ag⁺ ions are reduced and formed as Ag NPs via glucose/TEA reduction. The mechanism of silver ion reduction by glucose has been described in various reports.⁴³ In addition, the experimental process is easy to scale up: it took us only 20 min to acquire nearly 5.2 g of Ag NPs in a 500 mL reaction mixture with 96% yield.

Fig. 1 Schematic illustration of the synthesis process for Ag NPs in the presence of different anions.

Owing to the fact that different reaction parameters would influence the size and uniformity of the products, TEM was used to investigate the morphology and size of as-synthesized silver NPs. Fig. 2 shows the TEM images of the as-obtained Ag NPs using different anions such as PO₄³⁻, SO₄²⁻, Br⁻ and CO₃²⁻ ions. As shown in Fig. 2a, when the SO₄²⁻ ions were introduced into the reaction mixture (sample S1), most of the Ag NPs were spherical in shape and were well dispersed. The representative SAED pattern (inset, Fig. 2a) agrees with the face centered cubic (fcc) crystalline structure of Ag NPs. When PO₄³⁻ ions were introduced into the reaction mixture (Fig. 2b, sample S2), the as-obtained Ag NPs were mainly polygonal in shape, whereas some Ag NPs were dumbbell-shaped. When CO₃²⁻ ions were introduced into the reaction mixture (Fig. 2d, sample S3), the as-obtained Ag NPs were mainly polygonal in shape, whereas most Ag NPs were hexagonal in shape. When Br⁻ ions were introduced into the reaction mixture (Fig. 2e, sample S3), the as-obtained Ag NPs were mainly dumbbell-shaped. As shown in Fig. 2c and f, the average diameter of S1, S2, S3 and S4 is found to be approximately 49.2 nm, 43.7 nm, 68.4 nm and 53.1 nm, respectively. The different morphology and size of the products suggests that anions can affect the nucleation and the growth process during the formation of silver NPs. The abovementioned results further illustrate that anions can be used to tailor the shape of Ag NPs.

Fig. 2 TEM images of the synthesized Ag NPs in the presence of different anions: (a) SO₄²⁻ (the inset is the representative SAED pattern), (b) PO₄³⁻, (d) CO₃²⁻, and (e) Br⁻. The corresponding histogram of particle size distributions are shown in (c) and (f).

We also used HRTEM and EDX to study the structure and composition of the as-prepared Ag products. Fig. 3a shows the representative HRTEM image of the as-obtained individual Ag nanoparticle (sample S3), which reveals that the Ag nanoparticle possesses good crystallinity with a d-spacing of 0.235 nm, which can be indexed to the (111) facet of the fcc Ag. Fig. 3b shows the EDX spectrum of the single Ag nanoparticle, and the results shows that only Ag can be observed (C and Cu belong to carbon film covered copper grid), and no peaks of other impurities can be observed, which reveals the purity of as-synthesized Ag NPs.

Fig. 3 (a) HRTEM image of Ag NPs (a) (inset is the fast Fourier transformation (FFT) pattern); (b) EDX spectra of individual Ag NPs, (inset table is the composition of the silver particles). The peaks located at the binding energies of 0.28, 0.95, 8.05 and 8.92 keV belong to the C and Cu signal of carbon film covered copper grid.

Owing to the electronic characterization results reveal that the electronic properties of sample S3 is best, the morphology influence of the reaction temperature and molar concentration of the reductant were also studied. When the reaction temperature is decreased to 30 °C (sample S5), as shown in Fig. 4a, the Ag NPs are polygonal in shape; moreover, when the reaction temperature is increased to 90 °C (Fig. 4b, sample S6), the polygonal shape of Ag NPs still remains, and some large Ag NPs can be observed. Fig. 4c shows the size histogram of the diameter of the as-obtained Ag products, and the average diameter of S5 and S6 is 54.0 nm and 56.5 nm, respectively. Compared with the sample S3 (Fig. 3f), the mean diameter is decreased, which demonstrates the size distribution decreases when the temperature was increased. However, the influence on the molar concentration of the reductant is not evident. As shown in Fig. 4d and e, when the concentration of the reducing agent is increased from 0.0015 M (AgNO₃–glucose ratio = 2 : 1, sample S3) to 0.03 M (AgNO₃–glucose ratio = 1 : 1, sample S7) and 0.06 M (AgNO₃–glucose ratio = 1 : 2, sample S8), the polygonal shape of Ag NPs still remains, and the average diameter of S7 and S8 is closer to that of S3. The abovementioned results illustrate that the reaction temperature can be used to tailor the size of Ag NPs.

Fig. 4 TEM images of synthesized silver NPs (synthesized in the presence of CO₃²⁻) at different temperatures: (a) 30 °C, (b) 90 °C and at different ratios of n(AgNO₃)–n(glucose), (d) 1 : 1, and (e) 1 : 2. (c) and (f) are the corresponding histograms of particle size distributions.

Noble metal NPs often have a strong absorption in the visible area, which is due to the surface plasmon resonance (SPR) effect of the metal NPs. The intensity and location of SPR absorption peak can be used to forecast the size and morphology of the synthesized silver nanoparticle.^44,45 In general, the absorption peak of Ag⁺ ions is around 310 nm,⁴⁶ and the absorption bands of Ag NPs are in the range 380–620 nm.⁴⁷ Fig. 5 shows the images and UV-vis absorption spectra of Ag NPs (samples S1–S8) obtained at different reaction parameters. Fig. 5a shows the images and UV-vis absorption spectra of Ag NPs of samples S1–S4 synthesized in the presence of different anions. It is evident that there are changes in the color of the samples and there is no absorption peak at about 310 nm, and their SPR absorption peaks remain shifted and centered in the range of 410–450 nm for the obtained NPs. The shape of SPR absorption peaks is inerratic and symmetrical, which indicates that the Ag⁺ ions were completely reduced to Ag NPs, and the increasing trend of SPR absorption peaks of each samples is consistent with the result of TEM and particle size distribution; moreover, the largest average size is obtained in the presence of –CO₃²⁻ (sample S3). As shown in Fig. 5b, the absorption peak moved and the maximal absorption peak is at 60 °C (sample S3); however, when the AgNO₃–glucose ratio changes from 2 : 1 to 1 : 2, the absorption peak shifts to the left and the color of samples evidently varies, as shown in Fig. 5c.

Fig. 5 Images of the as-obtained Ag NPs solution and the UV-vis absorption spectra at different reaction parameters: (a) in the presence of SO₄²⁻, PO₄³⁻, CO₃²⁻ and Br⁻; (b) at different temperatures: 30 °C, 60 °C and 90 °C; (c) at different ratios of n(AgNO₃)–n(glucose): 2 : 1, 1 : 1 and 1 : 2.

3.2 The electronic properties of Ag tracks

Fig. 6a shows the representative images of deposited Ag patterns (six electrodes) on PET substrates, and the line width is 2.5 mm. The electrodes appear metallic gold yellow (see the inset image), and the substrate can be severely bended and rolled without visible signs of the Ag paint chipping, indicating the flexibility and mechanical robustness of the as-deposited Ag patterns. This feature indicates the possible application of the product in flexible printed electronics. Fig. 6b shows the top-view SEM image of the surface of the electrodes composed of Ag NPs (sample S3), which shows no aggregates and a homogeneous dispersion over the large area. The cross-view SEM image was used to determine the thickness of the as-deposited Ag electrodes (sample S3) (Fig. 6c), and the average thickness is about 715 nm. Subsequently, the electronic properties of Ag electrodes were measured, and the results reveal that the resistivity of S3 (using the CO₃²⁻ mediated-synthesized Ag NPs) is the lowest, and if the SO₄²⁻ mediated-synthesized Ag NPs were used, the resistivity can be up to 15 000 μΩ cm. Moreover, change in the resistance of S3 as a function of bending times has been investigated, and the results demonstrate that the resistance increased 2.5 times and 4 times after 100 times bending at 90° and 180°, respectively (Fig. S1 in the ESI†). The abovementioned results reveal that the durability of the printed electrode is good, and thus the sample S3 was used for further investigation.

Fig. 6 Optical image of silver patterns deposited on the PET substrate and dried at room temperature without sintering-treatment (a); top-view (b) and cross-view SEM image (c) of the silver tracks formed after sintering at 160 °C for 30 min; and electrical resistivity of silver conductive patterns of all samples of S1–S8 (d).

Sintering treatment is an important factor that determines the electronic performance of the deposited patterns, which can facilitate the removal of solvents and induce coalescence between silver NPs.⁴⁸ Fig. 7a shows the electrical resistivity of patterns after heating at different temperatures for 30 min. It can be found that the electrical resistivity will decrease with increase in the sintering temperature. Note that when the sintering temperature is 120 °C, the electrical resistivity is 9.8 μΩ cm. Moreover, it should be noted that the Ag NPs are uniform and independent of each other without heat-treatment. As shown in Fig. 7b, adjacent Ag NPs start melting and fuse together, and the surface still remains smooth. Moreover, when the sintering temperature is elevated to 160 °C, a clear melting and fusing phenomenon of adjacent Ag NPs can be observed, as shown in Fig. 7c. The small agglomerates connect with each other and form large agglomerates, and their resistivity decreases from 9.8 μΩ cm to 8.6 μΩ cm. As shown in Fig. 7d, all the adjacent Ag NPs are completely interconnected with each other when the sintering temperature reaches 200 °C, thus causing their resistivity to continuously decrease to 5.7 μΩ cm, which is very close to the value of bulk silver (1.6 μΩ cm). It should be noted that the Ag NPs-based electrodes display a lower resistivity compared with the referred results because the electrodes were manufactured by relatively large Ag NPs and without using any surfactants.^49–51 Clearly, electrical resistivity is functionally related to the microstructure of Ag NPs-based electrodes. In addition, well-formed necks between the adjacent Ag NPs can be observed after sintering, this necks serve as a percolation path for electricity.

Fig. 7 Electrical resistivity of patterns after sintering at different temperatures for 30 min (a); representative SEM images of silver patterns on PET substrates sintered in an oven at different temperatures: 120 °C (b), 160 °C (c), and 200 °C (d).

Moreover, sintering time is another important factor to tailor the electronic performance of the deposited patterns. Fig. 8a shows the electrical resistivity of patterns after sintering treatment at 160 °C for different times. Evidently, the electrical resistivity decreases with increase in sintering time. Moreover, the morphology of the electrode surface was investigated, and the representative SEM images are shown in Fig. 8b–d. The surface morphology is similar to the SEM images of Ag electrodes as a function of sintering temperature, many necks between the Ag NPs can be well formed after elevate the sintering time. Because of the increase in necks, the electrical resistivity value decreases from 11.4 μΩ cm (sintering time is 10 min) to 6.7 μΩ cm (sintering time is 150 min), and the SEM images of sintering time of 30 min (Fig. 8b), 60 min (Fig. 8c) and 90 min (Fig. 8d) show a clear microstructural change in the Ag patterns. Note that the conductivities of our Ag inks (only loading of 30% Ag NPs) can be up to 25% of that of bulk Ag with line widths as narrow as 2.5 mm with a mild sintering temperature and short sintering time.

Fig. 8 Electrical resistivity of patterns after sintering at 160 °C for different times (a); representative SEM images of silver patterns on PET substrates sintered in an oven at different times: 30 min (b), 60 min (c), and 90 min (d).

Furthermore, we used paper as the substrate and then screen printed the abovementioned Ag inks with various patterns on paper. Like PET, paper-based flexible electronics have also attracts broad interest due to their huge commercial value in the future.^52–54 Fig. 9a shows the images of the screen plate, and the inset is the magnified patterns, which were printed and sintered at 160 °C for 30 min in an oven. Fig. 9b–g shows the images of printed patterns, and the average resistivity is 8.7 μΩ cm. The abovementioned results demonstrate that the as-prepared Ag inks possess good printability; moreover, in future, they can be readily applied to various flexible electronics, such as RFID antenna, solar cells, displays, sensors and smart labels.

Fig. 9 Images of the screen printing plate (a) and the different conductive patterns on the paper (b–g) by screen printing of the as-prepared silver NPs (sample S3).

4. Conclusions

In conclusion, monodisperse Ag NPs were directly synthesized on a large scale by an oxidation–reduction reaction, and the effects of different introduced anions (–SO₄²⁻, –PO₄³⁻, –CO₃²⁻ and –Br⁻) in the reaction process were systematically investigated. The conductive patterns that were obtained, which have excellent electrical properties, on the flexible PET and paper substrate using the as-obtained Ag NPs as conductive inks, as well as the results of the fabricated conductivity of Ag electrodes, reveal that the introduced carbonate ions is best. The optimal electrical resistivity is 5.7 μΩ cm, which is only 3.6 times higher than that of bulk silver. Improved electrical resistivity can be attributed to the changing of microstructure after sintering treatment. Our method illuminates a promising route for directly printing electronic patterns on flexible substrates that will be very useful in a wide variety of practical applications.

Acknowledgements

This work was partially supported by the NSFC (51201115, 51471121, 51171132, 11375134), the Hong Kong Scholars Program, the Young Chenguang Project of Wuhan City (2013070104010011), the China Postdoctoral Science Foundation (2014M550406), the Hubei Provincial Natural Science Foundation (2014CFB261), the Basic Research Plan Program of Shenzhen City (no. JCYJ20130401160028783), and the Fundamental Research Funds for the Central Universities and Wuhan University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13641a

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