materials Article
The Preparation of Ag Nanoparticle and Ink Used for Inkjet Printing of Paper Based Conductive Patterns Lin Cao, Xiaohe Bai, Zhidan Lin *
ID
, Peng Zhang *, Shuling Deng, Xusheng Du and Wei Li
Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University, Guangzhou 510632, China; [email protected] (L.C.); [email protected] (X.B.); [email protected] (S.D.); [email protected] (X.D.); [email protected] (W.L.) * Correspondence: [email protected] (Z.L.); [email protected] (P.Z.); Tel.: +86-20-8522-2151 (Z.L.); +86-20-8522-3562 (P.Z.) Received: 14 June 2017; Accepted: 21 August 2017; Published: 28 August 2017
Abstract: Ag nanoparticles were successfully prepared using a liquid reduction method with a suitable mixture reductant of polyethylene glycol (PEG) and ethylene glycol (EG). OP-10 as a dispersing agent, was used to prepare the conductive Ag ink. Ag nanoparticles with an average particle size of 40 nm were prepared while the ratio of PEG to EG was 1:2. Meanwhile, the Ag particles had a narrow size distribution and great dispersion performance. The effects of paper substrates, sintering temperature, and sintering time on the conductivity of the printed Ag ink pattern were also studied. It was found that Lucky porous high glossy photo paper was a good candidate as the printing substrate. The resistivity of the printed pattern could reach 5.1 × 10−3 Ω·cm after heated at 100 ◦ C for 2 h. Hence, the printed pattern showed good conductivity which led to the LED light being on. Furthermore, the Ag nanoparticle ink could be printed to form any pattern as required that still showed good electrical conductivity after being sintered under low-temperature. This could provide new possibilities for the preparation of flexible electrodes. Keywords: Ag nanoparticles; flexible electrode; Ag ink; inkjet printing
1. Introduction As a new type of functional material, Ag nanoparticles are widely used in many fields such as chemicals, aerospace, medical, electronics, and so on. With the development of large-area and flexibility of electronic products, Ag nanoparticles have broad application prospects, especially in the electronic and information manufacturing industries [1]. The liquid reduction method is the main method of preparing Ag nanoparticles (AgNPs) and presents great advantages including simple production equipment, convenient operation, and the low price of raw materials [2,3]. However, in the production process of silver powders, reaction parameters such as temperature, concentration of reducing agent, and amounts of capping agent are quite complex. As it is really difficult to control the size and shape of the silver powders, the value and application of Ag nanoparticles has been limited. Thus, the preparation of uniform, small sized, and single-shaped Ag nanoparticles rapidly and controllably is a current technology problem worldwide. With the development of electronic printing technology, conductive thin films are widely used in flexible display circuit printing [4], solar photovoltaic cells [5], and light emitting diodes [6]. Paper substrate materials have great potential for use in portable electronic devices as they are flexible and portable. There are several methods for the deposition of conductive materials on a paper surface such as stamping, chemical vapor deposition, and spin coating [7]. However, to provide a smooth layer of conductive materials on solid substrates, these methods require a dust free environment and strictly controlled temperature, which are very expensive and difficult to maintain. As a form of digital
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printing, inkjet printing has developed quickly and has become more extensively applied. Compared Compared traditional printing methods, inkjet hasofamaterials, high utilization of materials,process, simple to traditionalto printing methods, inkjet has a high utilization simple manufacturing manufacturing process, low prices, large area, high precision, and many other advantages. low prices, large area, high precision, and many other advantages. Generally,nano-Ag nano-Agisisthe thekey keyfunctional functional material silver conductive is composed Generally, material of of thethe silver conductive inkink andand is composed of of Ag particles, dispersants, surfactants, solvents, and other additives.Usually Usuallyititisisdifficult difficultto toremove remove Ag particles, dispersants, surfactants, solvents, and other additives. the dispersant dispersant and lower sintering temperatures because of their high boiling point. the and other otheradditives additivesatat lower sintering temperatures because of their high boiling TheseThese additives severely decrease the conductivity of theofprinted pattern formed by the ink. point. additives severely decrease the conductivity the printed pattern formed by silver the silver To remove these additives, the sintering temperature of the conductive ink has to generally be higher ink. To remove these additives, the sintering temperature of the conductive ink has to generally be than 200 [8],◦ Cwhich severely restricts the the useuse of of certain flexible [9], higher than°C200 [8], which severely restricts certain flexiblesubstrates substratessuch such as as paper paper [9], polyethyleneterephthalate terephthalate(PET) (PET)[10], [10],and andso soon. on. polyethylene In our study, Ag nanoparticles were prepared different mixing two reducing In our study, Ag nanoparticles were prepared usingusing different mixing ratios ratios of two of reducing agents agents and various experimental parameters the mechanism. reaction mechanism. Specific preparation and various experimental parameters to study to thestudy reaction Specific preparation schemes schemes were proposed obtain a goodof dispersal of Ag nanoparticles rapidly, had different were proposed to obtain atogood dispersal Ag nanoparticles rapidly, which had which different controlled controlled particle sizes. A series of low boiling point additives were studied and introduced the particle sizes. A series of low boiling point additives were studied and introduced into the ink into system inkvariety systemand for variety andatolow acquire a low sintered temperature ink for ink-jet printing. for to acquire sintered temperature conductiveconductive Ag ink forAg ink-jet printing.
Results 2.2.Results 2.1. 2.1.Characterization CharacterizationofofAgNPs AgNPs The vary under Themorphology, morphology,particle particle size, size, and and size size distribution distribution of of Ag Ag nanoparticles nanoparticles vary under different different temperatures and reaction times so the effects of the volume ratios, temperatures, and time factors temperatures and reaction times so the effects of the volume ratios, temperatures, and timefactors of ofthe thePEG-EG PEG-EGcompound compoundon onreducing reducingAgNPs AgNPswere wereinvestigated. investigated.Figure Figure1a–c 1a–cshows show the theSEM SEMimage image of AgNPs prepared under different temperatures. As shown in Figure 1, the prepared AgNPs of AgNPs prepared under different temperatures. As shown in Figure 1, the prepared AgNPs were were spherical. Figure is the UV-vis extinctionspectra spectraofofthe the products products prepared prepared under spherical. Figure 2 is 2the UV–vis extinction under different different ◦ C, temperatures temperatureswhen whenthe thevolume volumeratio ratioof ofPEG:EG PEG:EGwas was1:2. 1:2.When Whenthe thereaction reactiontemperature temperaturewas was130 130 °C, the theUV-vis UV–visabsorption absorptionband bandfor forAgNPs AgNPswas wasaround around440 440nm. nm.In Inaddition, addition,SEM SEMphotographs photographsindicated indicated ◦ C, the UV-vis absorption that the average size was 100 nm. When the temperature was 140–150 that the average size was 100 nm. When the temperature was 140–150 °C, the UV–vis absorption band band for AgNPs was around 420UV-vis nm. UV-vis diffuse reflection spectra showed that absorb for AgNPs was around 420 nm. diffuse reflection spectra resultsresults showed that absorb bands bands shifted to a shorter wavelength (blue shift), which was induced by the smaller particle size of shifted to a shorter wavelength (blue shift), which was induced by the smaller particle size of AgNPs AgNPs reduced in the solution. 2 shows that the intensity theat peak 420 the samples reduced in the solution. FigureFigure 2 shows that the intensity of the of peak 420 at nm fornm thefor samples aged ◦ ◦ aged at and 140 and C was much stronger than that 130°C,C,mainly mainlybecause because aa complete complete reaction at 140 150 150 °C was much stronger than that atat130 reaction isis difficult anan insufficient temperature. Thus, it was difficult to obtain AgNPs with high difficulttotoachieve achieveunder under insufficient temperature. Thus, it was difficult to obtain AgNPs with ◦ yield, which was also verified by the SEM image. When the reaction temperature was 140 or 150 high yield, which was also verified by the SEM image. When the reaction temperature was 140 C, or the absorption peak had peak the same and the half-peak the same width on thewidth plot 150characteristic °C, the characteristic absorption hadhigh the same high and thehad half-peak had the same graph. that 140 ◦ C was critical temperature the reaction. the temperature on the This plot indicated graph. This indicated thata140 °C was a critical of temperature of When the reaction. When the ◦ was higher than 140 C, the rate of the reaction was almost constant. The morphology and particle temperature was higher than 140 °C, the rate of the reaction was almost constant. The morphology size characterized by SEM remained essentially unchanged, and the photographs indicated that the and particle size characterized by SEM remained essentially unchanged, and the photographs average size of the resources be saved and quality under indicated that the AgNPs averagewas size40ofnm. theTherefore, AgNPs was 40 nm.could Therefore, resources couldassured be saved and ◦ C. the selected temperature of 140 quality assured under the selected temperature of 140 °C.
Figure 1. SEM images of Ag nanoparticles prepared under different temperatures: (a) 130 °C; Figure 1. SEM images of Ag nanoparticles prepared under different temperatures: (a) 130 ◦ C; (b) 140 ◦ C; (b) 140 °C; and (c) 150 °C. and (c) 150 ◦ C.
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Figure 2. UV–vis extinction spectra of the products prepared at different temperatures at PEG:EG Figure 2. UV-vis extinction spectra of the products prepared at different temperatures at PEG:EG ratio of 1:2. ratio of 1:2.
The The results results shown shown in in Figure Figure 33 demonstrate demonstrate that that the the color color of of the the solution solution changed changed obviously. obviously. The color of the solution changed from colorless yellow within 2 Figure 2. gradually UV–vis extinction spectra of the products prepared to at different temperatures at min. PEG:EGFigure 4 shows the The color of the solution gradually changed from colorless to yellow within 2 min. Figure 4 shows ratio of 1:2. TEM imageimage of AgNPs and Figure 4a shows appearance of the condensation nuclei. The solution the TEM of AgNPs and Figure 4a the shows the appearance of the condensation nuclei. The mixture continued to be stirred after 3 min at the same temperature, and the color of the solution The results shown in Figure 3 demonstrate that the color of the solution changed obviously. solution mixture continued to be stirred after 3 min at the same temperature, and the color of the The color of the solution gradually changed from colorless to yellow within 2 min. Figure 4 in shows gradually changed to tea red before turning dark and cloudy, due to the increase scattering degree solution gradually to tea before dark cloudy, due the TEMchanged image of AgNPs andred Figure 4a showsturning the appearance ofand the condensation nuclei. to Thethe increase in Figure 2.growth UV–vis extinction spectra of the products prepared at different temperatures at PEG:EG dark green, and the Ag of the crystal seeds after the of the light. After 12 min, the color became solution mixture continued to be stirred after 3 min atof thethe same temperature, the color the scattering degree of the crystal seeds after the growth light. After and 12 min, theofcolor became dark ratio of 1:2. solutionfrom gradually changed to tea red nucleus before turning dark and cloudy, due to20 themin, increase incolor remained nanoparticles grew the condensation (Figure 4b–d). After the green, and the scattering Ag nanoparticles grew from the condensation nucleus (Figure 4b–d). After 20 min, the The results shown in Figure 3 demonstrate thatof the color of the solution changed obviously. degree of the crystal seeds after the growth the light. After 12 min, the color became dark basically stable green, asbasically brown which indicated that the toreaction was complete the absorption and The color of the solution changed colorless yellow within 2that min. Figure 4reaction shows and the green, Ag nanoparticles grew from thefrom condensation nucleus (Figure 4b–d). 20as min, the color remained stable as gradually brown green, which indicated theAfter was complete as the TEM image of AgNPs Figure 4a showswhich the appearance ofthat the the condensation nuclei. The colorintensity remained basically stableand as brown green, indicated reactiongrowing. was complete as scattering of light are constant when the grain size is no longer the absorption and scattering lighttointensity are constant theandgrain size solution mixtureof continued be stirred after 3 min at the same when temperature, the color of theis no longer growing. the absorption and scattering of light intensity are constant when the grain size is no longer growing. solution gradually changed to tea red before turning dark and cloudy, due to the increase in scattering degree of the crystal seeds after the growth of the light. After 12 min, the color became dark green, and the Ag nanoparticles grew from the condensation nucleus (Figure 4b–d). After 20 min, the color remained basically stable as brown green, which indicated that the reaction was complete as the absorption and scattering of light intensity are constant when the grain size is no longer growing.
Figure 3. The color change during the preparation of Ag nanoparticles.
Figure 3. The color change during the preparation of Ag nanoparticles. Figure 3. The color change during the preparation of Ag nanoparticles.
Figure 3. The color change during the preparation of Ag nanoparticles.
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Figure 4. TEM images of the preparation of Ag nanoparticles. (a) 2 min; (b) 5 min; (c) 12 min; and (d) 20 min. Figure 4. TEM images of the preparation of Ag nanoparticles. (a) 2 min; (b) 5 min; (c) 12 min; Figure 5 show the SEM images and particle size distribution images of Ag nanoparticles and (d) 20 min.
prepared under different volume ratios of PEG to EG where it can be seen that the nano-Ag particles were well spread. Figure 5a–c show the SEM image of the AgNPs with an average size of about 65–100 nm with a wide size distribution. When the ration of PEG:EG was 1:2 (Figure 5b), the average nano-Ag size was about 40 nm, and the particle size distribution was concentrated within 30–80 nm, which was an ideal ratio in this series of experiments. Figure 5e shows the SEM image when the ratio of PEG:EG was 1:0; the corresponding size distribution result is displayed and show a mean diameter of 40 nm. However, this ratio also had some disadvantages, for example, PVP and AgNO3 were barely soluble in PEG, had a low yield, took a long time for reaction processes, and had incomplete reactions. The results showed that the ratio (1:2) of PEG to EG were more suitable. The addition of PEG effectively inhibited the growth rate of Ag particles and also effectively controlled the size of
Materials 2017, 10, 1004 images of the preparation of Ag nanoparticles. (a) 2 min; (b) 5 min; (c) 12 min; and4 of 9 Figure 4. TEM
(d) 20 min.
Figure SEM images and and particle size distribution images images of Ag nanoparticles prepared Figure5 5shows showthe the SEM images particle size distribution of Ag nanoparticles under different volume ratios of PEG to EG where it can be seen that the nano-Ag particles were well prepared under different volume ratios of PEG to EG where it can be seen that the nano-Ag particles spread. Figure 5a–c shows the SEM image of the AgNPs with an average size of about 65–100 nm with were well spread. Figure 5a–c show the SEM image of the AgNPs with an average size of about a wide size distribution. When the ration of PEG:EG was 1:2 (Figure 5b), the average nano-Ag size was 65–100 nm with a wide size distribution. When the ration of PEG:EG was 1:2 (Figure 5b), the average about 40 nm, and the particle size distribution was concentrated within 30–80 nm, which was an ideal nano-Ag size was about 40 nm, and the particle size distribution was concentrated within 30–80 nm, ratio in this series of experiments. Figure 5e shows the SEM image when the ratio of PEG:EG was 1:0; which was an ideal ratio in this series of experiments. Figure 5e shows the SEM image when the ratio the corresponding size distribution result is displayed and show a mean diameter of 40 nm. However, of PEG:EG was 1:0; the corresponding size distribution result is displayed and show a mean diameter this ratio also had some disadvantages, for example, PVP and AgNO3 were barely soluble in PEG, of 40 nm. However, this ratio also had some disadvantages, for example, PVP and AgNO3 were barely had a low yield, took a long time for reaction processes, and had incomplete reactions. The results soluble in PEG, had a low yield, took a long time for reaction processes, and had incomplete reactions. showed that the ratio (1:2) of PEG to EG were more suitable. The addition of PEG effectively inhibited The results showed that the ratio (1:2) of PEG to EG were more suitable. The addition of PEG the growth rate of Ag particles and also effectively controlled the size of nano-Ag particles by adjusting effectively inhibited the growth rate of Ag particles and also effectively controlled the size of the ratio of PEG to EG. PEG also prevented the agglomeration of Ag particles so that the prepared nano-Ag particles by adjusting the ratio of PEG to EG. PEG also prevented the agglomeration of Ag nano-Ag particles had good dispersibility. particles so that the prepared nano-Ag particles had good dispersibility.
. (a)
(b)
(c)
(d)
(e) Figure 5.5. SEM SEM images images and and particle particle size size distribution distribution images images of of Ag Ag nanoparticles nanoparticles prepared prepared under under Figure different volume ratios of PEG to EG. (a) PEG:EG = 0:1; (b) PEG:EG = 1:2; (c) PEG:EG = 1:1; (d) PEG:EG different volume ratios of PEG to EG. (a) PEG:EG = 0:1; (b) PEG:EG = 1:2; (c) PEG:EG = 1:1; = 2:1; and (e) = 1:0. (d) PEG:EG = PEG:EG 2:1; and (e) PEG:EG = 1:0.
The growth mechanism of the spherical Ag nanoparticles in the mixed solution of PEG and EG was proposed as Figure 6I. When EG was used alone as the reductant, the nano-Ag formed very slowly due to the weak reducing power of EG. In addition, selective adsorption of PVP onto nano-Ag was improved, which resulted in the formation of various morphologies of nano-Ag in the reaction system such as nanowires and nanocubes. PEG was employed to tune the morphology of nano-Ag as PEG has a different reducing ability and viscosity from that of EG. On one hand, due to the stronger reducing power of PEG than EG, its addition would accelerate the reaction rate, and the concentration of Ag+ in the solution decrease quickly and reach to a supersaturated status. If the reduction rate was very fast, the newly formed Ag atoms could be quickly deposited onto the nuclei without affecting the formation of new nuclei. According to the conservation of mass, the atomic weight of the whole system was constant. As more Ag atoms were involved in nucleation, less of them would take part in the formation of new nuclei, and consequently the number of nuclei per unit time increased [11]. On the other hand,
reduction rate veryasfast, the formed Ag atoms be quickly deposited onto the nuclei waswas proposed Figure 6I.newly When EG was used alone as thecould reductant, the nano-Ag formed very slowly due the weak reducing power of EG. In addition, selective adsorption of PVP onto without affecting thetoformation of new nuclei. According to the conservation of nanomass, the atomic Ag was improved, which resulted in the formation of various morphologies of nano-Ag in the weight of the whole system was constant. As more Ag atoms were involved in nucleation, less of reaction system such as nanowires and nanocubes. PEG was employed to tune the morphology of them wouldnano-Ag take part in the of new nuclei, and consequently number of to nuclei per unit as PEG has aformation different reducing ability and viscosity from that of EG. the On one hand, due the stronger reducing powerhand, of PEG than EG, its addition would accelerate the reaction rate,of and the reduced the time increased [11]. On the other as shown in Figure 6II, the high viscosity PEG Materials 2017, 10, 1004 5 of 9 concentration of Ag+ in the solution decrease quickly and reach to a supersaturated status. If the movement speed of the grains in the solution and the possibility of collision among the grains reduction rate was very fast, the newly formed Ag atoms could be quickly deposited onto the nuclei decreased. Thus, nucleation and growth rate According of Ag increased with theofincreasing reduction ability withoutthe affecting the formation of new nuclei. to the conservation mass, the atomic of the system was constant. more Ag atomsthe were involvedand in nucleation, of grains as shown inweight Figure 6II,whole the viscosity of As PEG reduced movement speed ofless the in the of PEG, which facilitated thehigh formation of numerous relatively uniform large crystals. In contrast, them would take partof in collision the formation of new nuclei, and consequently the Thus, numberthe of nuclei per unit and growth solution and the possibility among the grains decreased. nucleation if the reduction ability of the reductant was weak, the nucleation and growth rate were slow. time increased [11]. On the other hand, as shown in Figure 6II, the high viscosity of PEG reduced the rate of Ag increased with thetheincreasing of in PEG, facilitated the formation of Therefore, employing the of reductant of PEG and EG the reaction led to nucleation and movement speed grains mixture in thereduction solution andability the possibility of which collision among thefast grains decreased. the nucleation and growth rate ofIn Agcontrast, increased with the increasing reduction ability numerous relatively uniform and large crystals. if the reduction ability of the reductant slow growth rates toThus, obtain a single morphology of nano-sized particles. of PEG, which facilitated the formation numerous relatively uniformemploying and large crystals. contrast, was weak, the nucleation and growth rateofwere slow. Therefore, theInreductant mixture of if the reduction ability of the reductant was weak, the nucleation and growth rate were slow. PEG and EGTherefore, in the reaction fast nucleation andand slow rates obtain a single employingled theto reductant mixture of PEG EG ingrowth the reaction led to to fast nucleation and morphology nucleation and slow growth rates to obtain a single morphology of nano-sized particles. of nano-sized particles. grows
EG + PEG
AgNO3
PVP capping nucleation and grows PVP capping
EG + PEG
PEG mediated
AgNO3
PEG mediated (II)
(I) (I)
Ag atom Ag atom
(II)
Ag seeds Ag seeds
Ag nanoparticles Ag nanoparticles
Figure 6. Schematic illustration of formation of uniformity Ag nanoparticles in mixed solvents of EG Figure 6. Schematic illustration of formation of uniformity Ag nanoparticles in mixed solvents of EG and PEG. and PEG.
XRD analysis was carried out to verify the component of the prepared AgNPs. As shown in Figure 7, the significant diffraction peaks at 2-theta angle of 38.12°, 44.3°, 64.5°, and 77.5° were
XRD analysis was carried out to verify verify the the component component of of the the prepared prepared AgNPs. AgNPs. As shown in assigned to the (111), (200), (220), and (311) planes of face-centered-cubic (fcc) silver (JCPDS file No. ◦ ◦ ◦ , and 77.5 significant diffraction peaks peaks at at 2-theta 2-theta angle angle of of 38.12 38.12°,, 44.3 44.3°,, 64.5 64.5°, 77.5°◦ were Figure 7, the significant diffraction 04-0783), respectively. (311) planes of face-centered-cubic (fcc)(fcc) silver (JCPDS file No. assigned to to the the (111), (111),(200), (200),(220), (220),and and (311) planes of face-centered-cubic silver (JCPDS file 04-0783), respectively. No. 04-0783), respectively.
Figure 7. XRD pattern of Ag powder samples.
Figure 7. 7. XRD pattern of of Ag Ag powder powder samples. samples. Figure XRD pattern
2.2. Characterization of Ag Conductive Patterns The optimum conductivity of AgNPs on different photo paper substrates such as EPSON, Lucky, FANTAC, and Kodak was investigated. The fabrication of conductive patterns and patterned anodes are illustrated in Figure 8. A personal computer was used to design conductive patterns which were then inkjet-printed with a home printer (Canon iP1188, Canon, Tokyo Daejeon, Tokyo, Japan). Ink concentration is important for inkjet printing as a much higher concentration can block the nozzles, and much lower concentration may increase the printing times for acquiring appropriate electrodes. Therefore, 15 wt % Ag was chosen for the printed ink. Figure 8a–d shows the photo images of different paper substrates. The optical images of the photo paper showed a homogenous and thin coating of AgNPs on the photo paper. To verify the homogeneity of the AgNPs thin coating on the photo paper, a series of surface morphology SEM images of different photo papers were taken. As shown in Figure 8a–d. The conductivity of AgNPs on the different paper substrates was tested by measuring
The optimum conductivity of AgNPs on different photo paper substrates such as EPSON, Lucky, FANTAC, and Kodak was investigated. The fabrication of conductive patterns and patterned anodes are illustrated in Figure 8. A personal computer was used to design conductive patterns which were then inkjet-printed with a home printer (Canon iP1188, Canon, Tokyo Daejeon, Tokyo, Japan). Ink concentration is important for inkjet printing as a much higher concentration can block the nozzles, and much lower concentration may increase the printing times for acquiring appropriate electrodes. Materials 2017, 10, 1004 15 wt % Ag was chosen for the printed ink. Figure 8a–d show the photo images of different 6 of 9 Therefore, paper substrates. The optical images of the photo paper showed a homogenous and thin coating of AgNPs on the photo paper. To verify the homogeneity of the AgNPs thin coating on the photo paper, a seriesresistance, of surface morphology SEM images of different photo papers wereThe taken. As shown their respective which all showed a high value of conductivity. Lucky photoinpaper was 8a–d. The conductivity of AgNPs circuit on the different paper substrates by measuring chosen forFigure the preparation of the electronic and electrodes due towas itstested flexibility, smooth surface, their respective resistance, which all showed a high value of conductivity. The Lucky photo paper fine pore size, and moderate absorption of the ink. Other paper substrates were unevenly coated with was chosen for the preparation of the electronic circuit and electrodes due to its flexibility, smooth ink stains as the sized pores of these absorption paper surfaces ledOther the AgNPs to spread and become trapped surface,large fine pore size, and moderate of the ink. paper substrates were unevenly with ink stains as thealarge sized pores ofofthese surfaces track. led the AgNPs to spread among thecoated paper fibers, causing discontinuity the paper conductive Therefore, the and Lucky photo become trapped among the paper fibers, causing a discontinuity of the conductive track. Therefore, paper was selected for electrode preparation. Figure 8e shows the SEM of the Lucky photo paper the Lucky photo paper was selected for electrode preparation. Figure 8e shows the SEM of the Lucky printed Agphoto patterns, and the macroscopic picture is provided in Figure 8d. paper printed Ag patterns, and the macroscopic picture is provided in Figure 8d.
. Figure 8.The SEM of the printed Ag patterns using different photo papers as substrates and the
Figure 8. The SEM of the printed Ag patterns using different photo papers as substrates and the surface surface structure of different photo papers. (a) Epson; (b) Kodak; (c) Fantac; (d) Lucky; and (e) the structure of different photoprinted papers. Epson; (b) Kodak; (c) Fantac; (d) Lucky; and (e) the Lucky photo Lucky photo paper Ag (a) patterns. paper printed Ag patterns. The Ag ink was used to inkjet conductive patterns. The electrical resistance reduced rapidly with the increasing number of overprinting. It was also observed that adding Ag nanoparticles The Ag ink wasimproved used to conductivity, inkjet conductive patterns. Thetoelectrical significantly which was mainly due the good resistance conductivityreduced of Ag rapidly nanoparticlesnumber and the good betweenItthewas Ag and photo paper. After being printedAg 40 times with the increasing of connection overprinting. also observed that adding nanoparticles −3 Ω·cm sheet resistivity. (Figure 9a), the conductive pattern fabricated by the Ag ink formed a 4 × 10 significantly improved conductivity, which was mainly due to the good conductivity of Ag However, the decreasing resistivity was not obvious after being printed 25 times, making 25 the best nanoparticles and the good connection between the Ag and photo paper. After being printed 40 times number of printed times. (Figure 9a), the conductive pattern fabricated by the Ag ink formed a 4 × 10−3 Ω·cm sheet resistivity.
However, the decreasing resistivity was not obvious after being printed 25 times, making 25 the best number of printed times. Figure 9b,c shows the relationship between the resistivity of the printed Ag patterns and the sintering temperature, resistivity, and sintering time, respectively. The variation of sintering temperature and time are the most important parameters that affect the resistivity of AgNPs on the substrate materials. The stability and conductivity of the electronic track was tested by changing the sintering temperatures from 60 to 120 ◦ C. The pattern was printed 25 times and sintered for 120 min. The best conductivity that the thin film AgNPs obtained was when the sintering temperature was 120 ◦ C, as sintering temperatures over this decreased resistivity. The sintering time also impacted on the conductivity of the AgNPs pattern on the photo paper substrate. The effects of sintering time on conductivity were studied by heating the printed track at 120 ◦ C with different interval times of 0.5, 1, 2, 4, and 6 h. The results of the conductivity of AgNPs at different sintering time are given in Figure 9c. A high conductivity was acquired when the sintering time was 2 h; moreover, there was no damage to the paper substrate after heating. It was best to obtain a maximum conductivity of AgNPs on the paper substrate. After the sample was bended 1000 times, conductivity decreased by 29.4% (Figure 9d). Furthermore, Ag conductive patterns were used to demonstrate the lighting of an LED bulb using a 9 V battery, as shown in Figure 10. This result demonstrated that the prepared conductive track could be used as an electronic circuit and also applied in other electronic applications.
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5 Figure 9. The resistivity of the printed Ag patterns with: (a) different printing times; (b) different sintering temperature; (c) different sintering time; and (d) different bending times.
Figures 9b,c show the relationship between the resistivity of the printed Ag patterns and the sintering temperature, resistivity, and sintering time, respectively. The variation of sintering temperature and time are the most important parameters that affect the resistivity of AgNPs on the substrate materials. The stability and conductivity of the electronic track was tested by changing the sintering temperatures from 60 to 120 °C. The pattern was printed 25 times and sintered for 120 min. The best conductivity that the thin film AgNPs obtained was when the sintering temperature was 120 °C, as sintering temperatures over this decreased resistivity. The sintering time also impacted on the conductivity of the AgNPs pattern on the photo paper substrate. The effects of sintering time on conductivity were studied by heating the printed track at 120 °C with different interval times of 0.5, 1, 2, 4, and 6 h. The results of the conductivity of AgNPs at different sintering time are given in Figure 9c. A high conductivity was acquired when the sintering time was 2 h; moreover, there was no damage to the paper substrate after heating. It was best to obtain a maximum conductivity of AgNPs on the paper substrate. After the sample was bended 1000 times, conductivity decreased by 29.4% (Figure 9d). Furthermore, Ag conductive patterns were used to demonstrate the lighting of an LED bulb Figure Theresistivity resistivity of the printed Ag This patterns with: (a) different (b) sintering different The of the printed Ag patterns with: (a) different printingprinting times; (b)times; different usingFigure a 9 V9.9.battery, as shown in Figure 10. result demonstrated that the prepared conductive sintering temperature; (c) different sintering time; and (d) different bending times. (c) different sintering circuit time; and (d)also different bending times. tracktemperature; could be used as an electronic and applied in other electronic applications. Figures 9b,c show the relationship between the resistivity of the printed Ag patterns and the sintering temperature, resistivity, and sintering time, respectively. The variation of sintering temperature and time are the most important parameters that affect the resistivity of AgNPs on the substrate materials. The stability and conductivity of the electronic track was tested by changing the sintering temperatures from 60 to 120 °C. The pattern was printed 25 times and sintered for 120 min. The best conductivity that the thin film AgNPs obtained was when the sintering temperature was 120 °C, as sintering temperatures over this decreased resistivity. The sintering time also impacted on the conductivity of the AgNPs pattern on the photo paper substrate. The effects of sintering time on Figure 10. The simple conductivity test of printed Ag patterns: (a) Square; (b) Rectangle; (c) Spiral. conductivity were by heatingtest theofprinted at 120 °C with different interval times of 0.5, Figure 10. The studied simple conductivity printed track Ag patterns: (a) Square; (b) Rectangle; (c) Spiral. 1, 2, 4, and 6 h. The results of the conductivity of AgNPs at different sintering time are given in Figure 9c. A Materials high conductivity was acquired when the sintering time was 2 h; moreover, there was no damage 3. and Methods to the paper substrate after heating. It was best to obtain a maximum conductivity of AgNPs on the 3.1. Materials paper substrate. After the sample was bended 1000 times, conductivity decreased by 29.4% (Figure 9d). All chemicals used in this study were of analytical reagent grade. Meanwhile, Ag nitrate, Furthermore, Ag conductive patterns were used to demonstrate the lighting of an LED bulb polyvinyl pyrrolidone (PVP), ethylene glycol (EG), polyethylene glycol (PEG), and isopropanol (IPA) using a 9 V battery, as shown in Figure 10. This result demonstrated that the prepared conductive were obtained from the Tianjin Damao Chemical Reagent Factory (Tianjin, China). Ethyl alcohol track could be used as an electronic circuit and also applied in other electronic applications. was provided by the Guangzhou Chemical Reagent Factory (Guangzhou, China). Octylphenol polyoxyethylene ether (10) (OP-10) was provided by the Tianjin Fuyu fine chemical industry (Tianjin, China). 3.2. Synthesis of AgNPs PEG and EG were mixed for use as a reducing agent. The preparation steps of spherical nano-Ag particles were as follows. First, 100 mg of AgNO3 and 150 mg PVP was transferred into a round-bottomed flask containing 20 mL of EG. Next, 20 mL of PEG was added to the stirred mixture, Figure 10. The simple conductivity test of printed Ag patterns: (a) Square; (b) Rectangle; (c) Spiral.
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and combined until well mixed. The mixed solution was then transferred to an oil bath with a constant temperature and 200 r/min constant stirring. At the completion of the reaction, the mixture was washed and diluted with anhydrous ethanol 5 times before concentrated products were achieved after 20 min centrifugation at 8000 rpm. The liquid had obvious stratification, with the lower layer having a higher AgNPs concentration. The precipitated product was cleaned and centrifuged with anhydrous ethanol 5–8 times. The precipitated product was dried at room temperature standby. 3.3. Synthesis of Nano Ag Ink To synthesize the new Ag nano-ink, the recipe used was as follows: 3 wt % of OP-10; 20 wt % of ethylene glycol; 1.5 wt % ethanol; and 1 wt. % isopropyl alcohol was placed into glass bottles containing 59.5 wt % deionized water. The solution mixture was stirred at 200 r/min constant speed. After 10 min, 200 mg of AgNPs was added to the mixed solution which was then ultrasonically cleaned for 30 min, and the mixture formed a uniform system at room temperature when it was stirred for 30 min. Finally, a uniform AgNPs ink was obtained. The AgNPs ink was injected into a clean ink cartridge (Canon iP1188, Canon, Tokyo Daejeon, Tokyo, Japan) and printed using a Canon iP1188 inkjet printer. To deposit a large number of AgNPs, the printer settings were adjusted for the “best print quality” and “photo print mode” to ensure sufficient AgNPs ink was printed on the paper substrate. 3.4. Characterization of AgNPs Morphological observations were carried on using a field-emission scanning electron microscope (FE-SEM, Zeiss, Oberkochen, Germany) and a field-emission transmission electron microscope (TEM, Philips TECNAI-F20, Chur, Switzerland). Samples were also analyzed by X-ray diffraction (XRD; D8, Bruker, Billerica, MA, USA) at room temperature with a scanning angle from 20◦ to 90◦ . The scanning speed was 10◦ /min with 40 kV accelerating voltages and a 15 mA current. A UV-Vis absorption spectroscopy test was conducted using a UV-Vis spectrophotometer (UV-2550, Japan Shimadzu Corporation, Kyoto, Japan). The sample was diluted 1000 times and placed in a 1 × 3 cm quartz cuvette for testing. The resistance of the thin AgNPs pattern was tested using a KDB-1 digital four-probe resistance tester (Guangzhou Kunde Technology Companies (Guangzhou, China)). 4. Conclusions In summary, based on a controlled reaction temperature, ratio of PEG to EG, and reaction time, the Ag nanoparticles were prepared through a liquid reduction method. PEG and EG were mixed as reducing agents to prepare silver nanoparticles with a range of different particle sizes under 140 ◦ C. When the ratio of PEG to EG was 1:2, silver nanoparticles were prepared with an average particle size of 40 nm, a narrow size distribution and good dispersion performance. The conductive AgNPs ink was produced using OP-10 as a dispersing agent. The influence of other parameters was also studied, such as the effects of paper substrates, sintering temperature and time on the conductivity of the printed pattern. It was found that the Lucky porous high glossy photo paper was a good candidate substrate, and the resistivity of the printed pattern after heating at 100 ◦ C for 2 h could reach 5.1 × 10−3 Ω·cm. When the printed pattern was connected to an LED light and supplied the power, the LED lit up. Therefore, it was possible to print any patterns as designed, had good electrical conductivity, and could be sintered at a low-temperature and has provided new ideas for the preparation of flexible electrodes. Acknowledgments: This project was supported by Guangdong provincial science and technology projects. (Item Number 2014B090915003 and 2015B090901052). Author Contributions: Zhidan Lin, Peng Zhang and Lin Cao conceived and designed the experiments; Lin Cao and Xiaohe Bai performed the experiments; Peng Zhang, Shuling Deng, Xusheng Du and Lin Cao analyzed the data; Peng Zhang and Zhidan Lin contributed reagents, materials and analysis tools; Lin Cao wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Cobley, C.M.; Xia, Y. Engineering the Properties of Metal Nanostructures via Galvanic Replacement Reactions. Mater. Sci. Eng. 2011, 70, 44–62. [CrossRef] [PubMed] Leydet, A.; Boyer, B.; Lamaty, G.; Roque, J.P.; Catlin, K.; Menger, F.M. Nitrogen Analogs of AOT. Synthesis and Properties. Langmuir 1994, 10, 1000–1002. [CrossRef] Li, M.; Lebeau, B.; Mann, S. Synthesis of Aragonite Nanofilament Networks by Mesoscale Self-Assembly and Transformation in Reverse Microemulsions. Adv. Mater. 2003, 15, 2032–2035. [CrossRef] Li, R.Z.; Hu, A.; Zhang, T.; Oakes, K.D. Direct writing on paper of foldable capacitive touch pads with silver nanowire inks. ACS Appl. Mater. Interfaces 2014, 6, 21721–21729. [CrossRef] [PubMed] Chew, S.Y.; Ng, S.H.; Wang, J.; Wang, J.; Liu, H.-K.; Novak, P.; Krumeich, F. Flexible free-standing carbon nanotube films for model lithium-ion batteries. Carbon 2009, 47, 2976–2983. [CrossRef] Chong, L.W.; Lee, Y.L.; Wen, T.C. Surface modification of indium, tin oxide anodes by self-assembly monolayers: Effects on interfacial morphology and charge injection in organic light-emitting diodes. Thin Solid Films 2007, 515, 2833–2841. [CrossRef] Russo, A.; Ahn, B.Y.; Adams, J.J.; Duoss, E.B.; Bernhard, J.T.; Lewis, J.A. Pen-on-paper flexible electronics. Adv. Mater. 2011, 23, 3426–3430. [CrossRef] [PubMed] Jung, I.; Shin, K.; Na, R.K.; Lee, H.M. Synthesis of low-temperature-processable and highly conductive Ag ink by a simple ligand modification: The role of adsorption energy. J. Mater. Chem. C 2013, 1, 1855–1862. [CrossRef] Huang, G.W.; Xiao, H.M.; Fu, S.Y. Paper-based silver-nanowire electronic circuits with outstanding electrical conductivity and extreme bending stability. Nanoscale 2014, 6, 8495–8502. [CrossRef] [PubMed] Vo, D.Q.; Shin, E.W.; Kim, J.S.; Kim, S. Low-temperature preparation of highly conductive thin films from acrylic acid-stabilized silver nanoparticles prepared through ligand exchange. Lang. ACS J. Surf. Colloids 2010, 26, 17435–17443. [CrossRef] [PubMed] Watzky, M.A.; Finke, R.G. Transition metal nanocluster formation kinetic and mechanistic studies. A new mechanism when hydrogen is the reductant: Slow, continuous nucleation and fast autocatalytic surface growth. J. Am. Chem. Soc. 1997, 119, 10382–10400. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
The Preparation of Ag Nanoparticle and Ink Used for Inkjet Printing of Paper Based Conductive Patterns Lin Cao, Xiaohe Bai, Zhidan Lin *
ID
, Peng Zhang *, Shuling Deng, Xusheng Du and Wei Li
Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University, Guangzhou 510632, China; [email protected] (L.C.); [email protected] (X.B.); [email protected] (S.D.); [email protected] (X.D.); [email protected] (W.L.) * Correspondence: [email protected] (Z.L.); [email protected] (P.Z.); Tel.: +86-20-8522-2151 (Z.L.); +86-20-8522-3562 (P.Z.) Received: 14 June 2017; Accepted: 21 August 2017; Published: 28 August 2017
Abstract: Ag nanoparticles were successfully prepared using a liquid reduction method with a suitable mixture reductant of polyethylene glycol (PEG) and ethylene glycol (EG). OP-10 as a dispersing agent, was used to prepare the conductive Ag ink. Ag nanoparticles with an average particle size of 40 nm were prepared while the ratio of PEG to EG was 1:2. Meanwhile, the Ag particles had a narrow size distribution and great dispersion performance. The effects of paper substrates, sintering temperature, and sintering time on the conductivity of the printed Ag ink pattern were also studied. It was found that Lucky porous high glossy photo paper was a good candidate as the printing substrate. The resistivity of the printed pattern could reach 5.1 × 10−3 Ω·cm after heated at 100 ◦ C for 2 h. Hence, the printed pattern showed good conductivity which led to the LED light being on. Furthermore, the Ag nanoparticle ink could be printed to form any pattern as required that still showed good electrical conductivity after being sintered under low-temperature. This could provide new possibilities for the preparation of flexible electrodes. Keywords: Ag nanoparticles; flexible electrode; Ag ink; inkjet printing
1. Introduction As a new type of functional material, Ag nanoparticles are widely used in many fields such as chemicals, aerospace, medical, electronics, and so on. With the development of large-area and flexibility of electronic products, Ag nanoparticles have broad application prospects, especially in the electronic and information manufacturing industries [1]. The liquid reduction method is the main method of preparing Ag nanoparticles (AgNPs) and presents great advantages including simple production equipment, convenient operation, and the low price of raw materials [2,3]. However, in the production process of silver powders, reaction parameters such as temperature, concentration of reducing agent, and amounts of capping agent are quite complex. As it is really difficult to control the size and shape of the silver powders, the value and application of Ag nanoparticles has been limited. Thus, the preparation of uniform, small sized, and single-shaped Ag nanoparticles rapidly and controllably is a current technology problem worldwide. With the development of electronic printing technology, conductive thin films are widely used in flexible display circuit printing [4], solar photovoltaic cells [5], and light emitting diodes [6]. Paper substrate materials have great potential for use in portable electronic devices as they are flexible and portable. There are several methods for the deposition of conductive materials on a paper surface such as stamping, chemical vapor deposition, and spin coating [7]. However, to provide a smooth layer of conductive materials on solid substrates, these methods require a dust free environment and strictly controlled temperature, which are very expensive and difficult to maintain. As a form of digital
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printing, inkjet printing has developed quickly and has become more extensively applied. Compared Compared traditional printing methods, inkjet hasofamaterials, high utilization of materials,process, simple to traditionalto printing methods, inkjet has a high utilization simple manufacturing manufacturing process, low prices, large area, high precision, and many other advantages. low prices, large area, high precision, and many other advantages. Generally,nano-Ag nano-Agisisthe thekey keyfunctional functional material silver conductive is composed Generally, material of of thethe silver conductive inkink andand is composed of of Ag particles, dispersants, surfactants, solvents, and other additives.Usually Usuallyititisisdifficult difficultto toremove remove Ag particles, dispersants, surfactants, solvents, and other additives. the dispersant dispersant and lower sintering temperatures because of their high boiling point. the and other otheradditives additivesatat lower sintering temperatures because of their high boiling TheseThese additives severely decrease the conductivity of theofprinted pattern formed by the ink. point. additives severely decrease the conductivity the printed pattern formed by silver the silver To remove these additives, the sintering temperature of the conductive ink has to generally be higher ink. To remove these additives, the sintering temperature of the conductive ink has to generally be than 200 [8],◦ Cwhich severely restricts the the useuse of of certain flexible [9], higher than°C200 [8], which severely restricts certain flexiblesubstrates substratessuch such as as paper paper [9], polyethyleneterephthalate terephthalate(PET) (PET)[10], [10],and andso soon. on. polyethylene In our study, Ag nanoparticles were prepared different mixing two reducing In our study, Ag nanoparticles were prepared usingusing different mixing ratios ratios of two of reducing agents agents and various experimental parameters the mechanism. reaction mechanism. Specific preparation and various experimental parameters to study to thestudy reaction Specific preparation schemes schemes were proposed obtain a goodof dispersal of Ag nanoparticles rapidly, had different were proposed to obtain atogood dispersal Ag nanoparticles rapidly, which had which different controlled controlled particle sizes. A series of low boiling point additives were studied and introduced the particle sizes. A series of low boiling point additives were studied and introduced into the ink into system inkvariety systemand for variety andatolow acquire a low sintered temperature ink for ink-jet printing. for to acquire sintered temperature conductiveconductive Ag ink forAg ink-jet printing.
Results 2.2.Results 2.1. 2.1.Characterization CharacterizationofofAgNPs AgNPs The vary under Themorphology, morphology,particle particle size, size, and and size size distribution distribution of of Ag Ag nanoparticles nanoparticles vary under different different temperatures and reaction times so the effects of the volume ratios, temperatures, and time factors temperatures and reaction times so the effects of the volume ratios, temperatures, and timefactors of ofthe thePEG-EG PEG-EGcompound compoundon onreducing reducingAgNPs AgNPswere wereinvestigated. investigated.Figure Figure1a–c 1a–cshows show the theSEM SEMimage image of AgNPs prepared under different temperatures. As shown in Figure 1, the prepared AgNPs of AgNPs prepared under different temperatures. As shown in Figure 1, the prepared AgNPs were were spherical. Figure is the UV-vis extinctionspectra spectraofofthe the products products prepared prepared under spherical. Figure 2 is 2the UV–vis extinction under different different ◦ C, temperatures temperatureswhen whenthe thevolume volumeratio ratioof ofPEG:EG PEG:EGwas was1:2. 1:2.When Whenthe thereaction reactiontemperature temperaturewas was130 130 °C, the theUV-vis UV–visabsorption absorptionband bandfor forAgNPs AgNPswas wasaround around440 440nm. nm.In Inaddition, addition,SEM SEMphotographs photographsindicated indicated ◦ C, the UV-vis absorption that the average size was 100 nm. When the temperature was 140–150 that the average size was 100 nm. When the temperature was 140–150 °C, the UV–vis absorption band band for AgNPs was around 420UV-vis nm. UV-vis diffuse reflection spectra showed that absorb for AgNPs was around 420 nm. diffuse reflection spectra resultsresults showed that absorb bands bands shifted to a shorter wavelength (blue shift), which was induced by the smaller particle size of shifted to a shorter wavelength (blue shift), which was induced by the smaller particle size of AgNPs AgNPs reduced in the solution. 2 shows that the intensity theat peak 420 the samples reduced in the solution. FigureFigure 2 shows that the intensity of the of peak 420 at nm fornm thefor samples aged ◦ ◦ aged at and 140 and C was much stronger than that 130°C,C,mainly mainlybecause because aa complete complete reaction at 140 150 150 °C was much stronger than that atat130 reaction isis difficult anan insufficient temperature. Thus, it was difficult to obtain AgNPs with high difficulttotoachieve achieveunder under insufficient temperature. Thus, it was difficult to obtain AgNPs with ◦ yield, which was also verified by the SEM image. When the reaction temperature was 140 or 150 high yield, which was also verified by the SEM image. When the reaction temperature was 140 C, or the absorption peak had peak the same and the half-peak the same width on thewidth plot 150characteristic °C, the characteristic absorption hadhigh the same high and thehad half-peak had the same graph. that 140 ◦ C was critical temperature the reaction. the temperature on the This plot indicated graph. This indicated thata140 °C was a critical of temperature of When the reaction. When the ◦ was higher than 140 C, the rate of the reaction was almost constant. The morphology and particle temperature was higher than 140 °C, the rate of the reaction was almost constant. The morphology size characterized by SEM remained essentially unchanged, and the photographs indicated that the and particle size characterized by SEM remained essentially unchanged, and the photographs average size of the resources be saved and quality under indicated that the AgNPs averagewas size40ofnm. theTherefore, AgNPs was 40 nm.could Therefore, resources couldassured be saved and ◦ C. the selected temperature of 140 quality assured under the selected temperature of 140 °C.
Figure 1. SEM images of Ag nanoparticles prepared under different temperatures: (a) 130 °C; Figure 1. SEM images of Ag nanoparticles prepared under different temperatures: (a) 130 ◦ C; (b) 140 ◦ C; (b) 140 °C; and (c) 150 °C. and (c) 150 ◦ C.
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Figure 2. UV–vis extinction spectra of the products prepared at different temperatures at PEG:EG Figure 2. UV-vis extinction spectra of the products prepared at different temperatures at PEG:EG ratio of 1:2. ratio of 1:2.
The The results results shown shown in in Figure Figure 33 demonstrate demonstrate that that the the color color of of the the solution solution changed changed obviously. obviously. The color of the solution changed from colorless yellow within 2 Figure 2. gradually UV–vis extinction spectra of the products prepared to at different temperatures at min. PEG:EGFigure 4 shows the The color of the solution gradually changed from colorless to yellow within 2 min. Figure 4 shows ratio of 1:2. TEM imageimage of AgNPs and Figure 4a shows appearance of the condensation nuclei. The solution the TEM of AgNPs and Figure 4a the shows the appearance of the condensation nuclei. The mixture continued to be stirred after 3 min at the same temperature, and the color of the solution The results shown in Figure 3 demonstrate that the color of the solution changed obviously. solution mixture continued to be stirred after 3 min at the same temperature, and the color of the The color of the solution gradually changed from colorless to yellow within 2 min. Figure 4 in shows gradually changed to tea red before turning dark and cloudy, due to the increase scattering degree solution gradually to tea before dark cloudy, due the TEMchanged image of AgNPs andred Figure 4a showsturning the appearance ofand the condensation nuclei. to Thethe increase in Figure 2.growth UV–vis extinction spectra of the products prepared at different temperatures at PEG:EG dark green, and the Ag of the crystal seeds after the of the light. After 12 min, the color became solution mixture continued to be stirred after 3 min atof thethe same temperature, the color the scattering degree of the crystal seeds after the growth light. After and 12 min, theofcolor became dark ratio of 1:2. solutionfrom gradually changed to tea red nucleus before turning dark and cloudy, due to20 themin, increase incolor remained nanoparticles grew the condensation (Figure 4b–d). After the green, and the scattering Ag nanoparticles grew from the condensation nucleus (Figure 4b–d). After 20 min, the The results shown in Figure 3 demonstrate thatof the color of the solution changed obviously. degree of the crystal seeds after the growth the light. After 12 min, the color became dark basically stable green, asbasically brown which indicated that the toreaction was complete the absorption and The color of the solution changed colorless yellow within 2that min. Figure 4reaction shows and the green, Ag nanoparticles grew from thefrom condensation nucleus (Figure 4b–d). 20as min, the color remained stable as gradually brown green, which indicated theAfter was complete as the TEM image of AgNPs Figure 4a showswhich the appearance ofthat the the condensation nuclei. The colorintensity remained basically stableand as brown green, indicated reactiongrowing. was complete as scattering of light are constant when the grain size is no longer the absorption and scattering lighttointensity are constant theandgrain size solution mixtureof continued be stirred after 3 min at the same when temperature, the color of theis no longer growing. the absorption and scattering of light intensity are constant when the grain size is no longer growing. solution gradually changed to tea red before turning dark and cloudy, due to the increase in scattering degree of the crystal seeds after the growth of the light. After 12 min, the color became dark green, and the Ag nanoparticles grew from the condensation nucleus (Figure 4b–d). After 20 min, the color remained basically stable as brown green, which indicated that the reaction was complete as the absorption and scattering of light intensity are constant when the grain size is no longer growing.
Figure 3. The color change during the preparation of Ag nanoparticles.
Figure 3. The color change during the preparation of Ag nanoparticles. Figure 3. The color change during the preparation of Ag nanoparticles.
Figure 3. The color change during the preparation of Ag nanoparticles.
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Figure 4. TEM images of the preparation of Ag nanoparticles. (a) 2 min; (b) 5 min; (c) 12 min; and (d) 20 min. Figure 4. TEM images of the preparation of Ag nanoparticles. (a) 2 min; (b) 5 min; (c) 12 min; Figure 5 show the SEM images and particle size distribution images of Ag nanoparticles and (d) 20 min.
prepared under different volume ratios of PEG to EG where it can be seen that the nano-Ag particles were well spread. Figure 5a–c show the SEM image of the AgNPs with an average size of about 65–100 nm with a wide size distribution. When the ration of PEG:EG was 1:2 (Figure 5b), the average nano-Ag size was about 40 nm, and the particle size distribution was concentrated within 30–80 nm, which was an ideal ratio in this series of experiments. Figure 5e shows the SEM image when the ratio of PEG:EG was 1:0; the corresponding size distribution result is displayed and show a mean diameter of 40 nm. However, this ratio also had some disadvantages, for example, PVP and AgNO3 were barely soluble in PEG, had a low yield, took a long time for reaction processes, and had incomplete reactions. The results showed that the ratio (1:2) of PEG to EG were more suitable. The addition of PEG effectively inhibited the growth rate of Ag particles and also effectively controlled the size of
Materials 2017, 10, 1004 images of the preparation of Ag nanoparticles. (a) 2 min; (b) 5 min; (c) 12 min; and4 of 9 Figure 4. TEM
(d) 20 min.
Figure SEM images and and particle size distribution images images of Ag nanoparticles prepared Figure5 5shows showthe the SEM images particle size distribution of Ag nanoparticles under different volume ratios of PEG to EG where it can be seen that the nano-Ag particles were well prepared under different volume ratios of PEG to EG where it can be seen that the nano-Ag particles spread. Figure 5a–c shows the SEM image of the AgNPs with an average size of about 65–100 nm with were well spread. Figure 5a–c show the SEM image of the AgNPs with an average size of about a wide size distribution. When the ration of PEG:EG was 1:2 (Figure 5b), the average nano-Ag size was 65–100 nm with a wide size distribution. When the ration of PEG:EG was 1:2 (Figure 5b), the average about 40 nm, and the particle size distribution was concentrated within 30–80 nm, which was an ideal nano-Ag size was about 40 nm, and the particle size distribution was concentrated within 30–80 nm, ratio in this series of experiments. Figure 5e shows the SEM image when the ratio of PEG:EG was 1:0; which was an ideal ratio in this series of experiments. Figure 5e shows the SEM image when the ratio the corresponding size distribution result is displayed and show a mean diameter of 40 nm. However, of PEG:EG was 1:0; the corresponding size distribution result is displayed and show a mean diameter this ratio also had some disadvantages, for example, PVP and AgNO3 were barely soluble in PEG, of 40 nm. However, this ratio also had some disadvantages, for example, PVP and AgNO3 were barely had a low yield, took a long time for reaction processes, and had incomplete reactions. The results soluble in PEG, had a low yield, took a long time for reaction processes, and had incomplete reactions. showed that the ratio (1:2) of PEG to EG were more suitable. The addition of PEG effectively inhibited The results showed that the ratio (1:2) of PEG to EG were more suitable. The addition of PEG the growth rate of Ag particles and also effectively controlled the size of nano-Ag particles by adjusting effectively inhibited the growth rate of Ag particles and also effectively controlled the size of the ratio of PEG to EG. PEG also prevented the agglomeration of Ag particles so that the prepared nano-Ag particles by adjusting the ratio of PEG to EG. PEG also prevented the agglomeration of Ag nano-Ag particles had good dispersibility. particles so that the prepared nano-Ag particles had good dispersibility.
. (a)
(b)
(c)
(d)
(e) Figure 5.5. SEM SEM images images and and particle particle size size distribution distribution images images of of Ag Ag nanoparticles nanoparticles prepared prepared under under Figure different volume ratios of PEG to EG. (a) PEG:EG = 0:1; (b) PEG:EG = 1:2; (c) PEG:EG = 1:1; (d) PEG:EG different volume ratios of PEG to EG. (a) PEG:EG = 0:1; (b) PEG:EG = 1:2; (c) PEG:EG = 1:1; = 2:1; and (e) = 1:0. (d) PEG:EG = PEG:EG 2:1; and (e) PEG:EG = 1:0.
The growth mechanism of the spherical Ag nanoparticles in the mixed solution of PEG and EG was proposed as Figure 6I. When EG was used alone as the reductant, the nano-Ag formed very slowly due to the weak reducing power of EG. In addition, selective adsorption of PVP onto nano-Ag was improved, which resulted in the formation of various morphologies of nano-Ag in the reaction system such as nanowires and nanocubes. PEG was employed to tune the morphology of nano-Ag as PEG has a different reducing ability and viscosity from that of EG. On one hand, due to the stronger reducing power of PEG than EG, its addition would accelerate the reaction rate, and the concentration of Ag+ in the solution decrease quickly and reach to a supersaturated status. If the reduction rate was very fast, the newly formed Ag atoms could be quickly deposited onto the nuclei without affecting the formation of new nuclei. According to the conservation of mass, the atomic weight of the whole system was constant. As more Ag atoms were involved in nucleation, less of them would take part in the formation of new nuclei, and consequently the number of nuclei per unit time increased [11]. On the other hand,
reduction rate veryasfast, the formed Ag atoms be quickly deposited onto the nuclei waswas proposed Figure 6I.newly When EG was used alone as thecould reductant, the nano-Ag formed very slowly due the weak reducing power of EG. In addition, selective adsorption of PVP onto without affecting thetoformation of new nuclei. According to the conservation of nanomass, the atomic Ag was improved, which resulted in the formation of various morphologies of nano-Ag in the weight of the whole system was constant. As more Ag atoms were involved in nucleation, less of reaction system such as nanowires and nanocubes. PEG was employed to tune the morphology of them wouldnano-Ag take part in the of new nuclei, and consequently number of to nuclei per unit as PEG has aformation different reducing ability and viscosity from that of EG. the On one hand, due the stronger reducing powerhand, of PEG than EG, its addition would accelerate the reaction rate,of and the reduced the time increased [11]. On the other as shown in Figure 6II, the high viscosity PEG Materials 2017, 10, 1004 5 of 9 concentration of Ag+ in the solution decrease quickly and reach to a supersaturated status. If the movement speed of the grains in the solution and the possibility of collision among the grains reduction rate was very fast, the newly formed Ag atoms could be quickly deposited onto the nuclei decreased. Thus, nucleation and growth rate According of Ag increased with theofincreasing reduction ability withoutthe affecting the formation of new nuclei. to the conservation mass, the atomic of the system was constant. more Ag atomsthe were involvedand in nucleation, of grains as shown inweight Figure 6II,whole the viscosity of As PEG reduced movement speed ofless the in the of PEG, which facilitated thehigh formation of numerous relatively uniform large crystals. In contrast, them would take partof in collision the formation of new nuclei, and consequently the Thus, numberthe of nuclei per unit and growth solution and the possibility among the grains decreased. nucleation if the reduction ability of the reductant was weak, the nucleation and growth rate were slow. time increased [11]. On the other hand, as shown in Figure 6II, the high viscosity of PEG reduced the rate of Ag increased with thetheincreasing of in PEG, facilitated the formation of Therefore, employing the of reductant of PEG and EG the reaction led to nucleation and movement speed grains mixture in thereduction solution andability the possibility of which collision among thefast grains decreased. the nucleation and growth rate ofIn Agcontrast, increased with the increasing reduction ability numerous relatively uniform and large crystals. if the reduction ability of the reductant slow growth rates toThus, obtain a single morphology of nano-sized particles. of PEG, which facilitated the formation numerous relatively uniformemploying and large crystals. contrast, was weak, the nucleation and growth rateofwere slow. Therefore, theInreductant mixture of if the reduction ability of the reductant was weak, the nucleation and growth rate were slow. PEG and EGTherefore, in the reaction fast nucleation andand slow rates obtain a single employingled theto reductant mixture of PEG EG ingrowth the reaction led to to fast nucleation and morphology nucleation and slow growth rates to obtain a single morphology of nano-sized particles. of nano-sized particles. grows
EG + PEG
AgNO3
PVP capping nucleation and grows PVP capping
EG + PEG
PEG mediated
AgNO3
PEG mediated (II)
(I) (I)
Ag atom Ag atom
(II)
Ag seeds Ag seeds
Ag nanoparticles Ag nanoparticles
Figure 6. Schematic illustration of formation of uniformity Ag nanoparticles in mixed solvents of EG Figure 6. Schematic illustration of formation of uniformity Ag nanoparticles in mixed solvents of EG and PEG. and PEG.
XRD analysis was carried out to verify the component of the prepared AgNPs. As shown in Figure 7, the significant diffraction peaks at 2-theta angle of 38.12°, 44.3°, 64.5°, and 77.5° were
XRD analysis was carried out to verify verify the the component component of of the the prepared prepared AgNPs. AgNPs. As shown in assigned to the (111), (200), (220), and (311) planes of face-centered-cubic (fcc) silver (JCPDS file No. ◦ ◦ ◦ , and 77.5 significant diffraction peaks peaks at at 2-theta 2-theta angle angle of of 38.12 38.12°,, 44.3 44.3°,, 64.5 64.5°, 77.5°◦ were Figure 7, the significant diffraction 04-0783), respectively. (311) planes of face-centered-cubic (fcc)(fcc) silver (JCPDS file No. assigned to to the the (111), (111),(200), (200),(220), (220),and and (311) planes of face-centered-cubic silver (JCPDS file 04-0783), respectively. No. 04-0783), respectively.
Figure 7. XRD pattern of Ag powder samples.
Figure 7. 7. XRD pattern of of Ag Ag powder powder samples. samples. Figure XRD pattern
2.2. Characterization of Ag Conductive Patterns The optimum conductivity of AgNPs on different photo paper substrates such as EPSON, Lucky, FANTAC, and Kodak was investigated. The fabrication of conductive patterns and patterned anodes are illustrated in Figure 8. A personal computer was used to design conductive patterns which were then inkjet-printed with a home printer (Canon iP1188, Canon, Tokyo Daejeon, Tokyo, Japan). Ink concentration is important for inkjet printing as a much higher concentration can block the nozzles, and much lower concentration may increase the printing times for acquiring appropriate electrodes. Therefore, 15 wt % Ag was chosen for the printed ink. Figure 8a–d shows the photo images of different paper substrates. The optical images of the photo paper showed a homogenous and thin coating of AgNPs on the photo paper. To verify the homogeneity of the AgNPs thin coating on the photo paper, a series of surface morphology SEM images of different photo papers were taken. As shown in Figure 8a–d. The conductivity of AgNPs on the different paper substrates was tested by measuring
The optimum conductivity of AgNPs on different photo paper substrates such as EPSON, Lucky, FANTAC, and Kodak was investigated. The fabrication of conductive patterns and patterned anodes are illustrated in Figure 8. A personal computer was used to design conductive patterns which were then inkjet-printed with a home printer (Canon iP1188, Canon, Tokyo Daejeon, Tokyo, Japan). Ink concentration is important for inkjet printing as a much higher concentration can block the nozzles, and much lower concentration may increase the printing times for acquiring appropriate electrodes. Materials 2017, 10, 1004 15 wt % Ag was chosen for the printed ink. Figure 8a–d show the photo images of different 6 of 9 Therefore, paper substrates. The optical images of the photo paper showed a homogenous and thin coating of AgNPs on the photo paper. To verify the homogeneity of the AgNPs thin coating on the photo paper, a seriesresistance, of surface morphology SEM images of different photo papers wereThe taken. As shown their respective which all showed a high value of conductivity. Lucky photoinpaper was 8a–d. The conductivity of AgNPs circuit on the different paper substrates by measuring chosen forFigure the preparation of the electronic and electrodes due towas itstested flexibility, smooth surface, their respective resistance, which all showed a high value of conductivity. The Lucky photo paper fine pore size, and moderate absorption of the ink. Other paper substrates were unevenly coated with was chosen for the preparation of the electronic circuit and electrodes due to its flexibility, smooth ink stains as the sized pores of these absorption paper surfaces ledOther the AgNPs to spread and become trapped surface,large fine pore size, and moderate of the ink. paper substrates were unevenly with ink stains as thealarge sized pores ofofthese surfaces track. led the AgNPs to spread among thecoated paper fibers, causing discontinuity the paper conductive Therefore, the and Lucky photo become trapped among the paper fibers, causing a discontinuity of the conductive track. Therefore, paper was selected for electrode preparation. Figure 8e shows the SEM of the Lucky photo paper the Lucky photo paper was selected for electrode preparation. Figure 8e shows the SEM of the Lucky printed Agphoto patterns, and the macroscopic picture is provided in Figure 8d. paper printed Ag patterns, and the macroscopic picture is provided in Figure 8d.
. Figure 8.The SEM of the printed Ag patterns using different photo papers as substrates and the
Figure 8. The SEM of the printed Ag patterns using different photo papers as substrates and the surface surface structure of different photo papers. (a) Epson; (b) Kodak; (c) Fantac; (d) Lucky; and (e) the structure of different photoprinted papers. Epson; (b) Kodak; (c) Fantac; (d) Lucky; and (e) the Lucky photo Lucky photo paper Ag (a) patterns. paper printed Ag patterns. The Ag ink was used to inkjet conductive patterns. The electrical resistance reduced rapidly with the increasing number of overprinting. It was also observed that adding Ag nanoparticles The Ag ink wasimproved used to conductivity, inkjet conductive patterns. Thetoelectrical significantly which was mainly due the good resistance conductivityreduced of Ag rapidly nanoparticlesnumber and the good betweenItthewas Ag and photo paper. After being printedAg 40 times with the increasing of connection overprinting. also observed that adding nanoparticles −3 Ω·cm sheet resistivity. (Figure 9a), the conductive pattern fabricated by the Ag ink formed a 4 × 10 significantly improved conductivity, which was mainly due to the good conductivity of Ag However, the decreasing resistivity was not obvious after being printed 25 times, making 25 the best nanoparticles and the good connection between the Ag and photo paper. After being printed 40 times number of printed times. (Figure 9a), the conductive pattern fabricated by the Ag ink formed a 4 × 10−3 Ω·cm sheet resistivity.
However, the decreasing resistivity was not obvious after being printed 25 times, making 25 the best number of printed times. Figure 9b,c shows the relationship between the resistivity of the printed Ag patterns and the sintering temperature, resistivity, and sintering time, respectively. The variation of sintering temperature and time are the most important parameters that affect the resistivity of AgNPs on the substrate materials. The stability and conductivity of the electronic track was tested by changing the sintering temperatures from 60 to 120 ◦ C. The pattern was printed 25 times and sintered for 120 min. The best conductivity that the thin film AgNPs obtained was when the sintering temperature was 120 ◦ C, as sintering temperatures over this decreased resistivity. The sintering time also impacted on the conductivity of the AgNPs pattern on the photo paper substrate. The effects of sintering time on conductivity were studied by heating the printed track at 120 ◦ C with different interval times of 0.5, 1, 2, 4, and 6 h. The results of the conductivity of AgNPs at different sintering time are given in Figure 9c. A high conductivity was acquired when the sintering time was 2 h; moreover, there was no damage to the paper substrate after heating. It was best to obtain a maximum conductivity of AgNPs on the paper substrate. After the sample was bended 1000 times, conductivity decreased by 29.4% (Figure 9d). Furthermore, Ag conductive patterns were used to demonstrate the lighting of an LED bulb using a 9 V battery, as shown in Figure 10. This result demonstrated that the prepared conductive track could be used as an electronic circuit and also applied in other electronic applications.
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5 Figure 9. The resistivity of the printed Ag patterns with: (a) different printing times; (b) different sintering temperature; (c) different sintering time; and (d) different bending times.
Figures 9b,c show the relationship between the resistivity of the printed Ag patterns and the sintering temperature, resistivity, and sintering time, respectively. The variation of sintering temperature and time are the most important parameters that affect the resistivity of AgNPs on the substrate materials. The stability and conductivity of the electronic track was tested by changing the sintering temperatures from 60 to 120 °C. The pattern was printed 25 times and sintered for 120 min. The best conductivity that the thin film AgNPs obtained was when the sintering temperature was 120 °C, as sintering temperatures over this decreased resistivity. The sintering time also impacted on the conductivity of the AgNPs pattern on the photo paper substrate. The effects of sintering time on conductivity were studied by heating the printed track at 120 °C with different interval times of 0.5, 1, 2, 4, and 6 h. The results of the conductivity of AgNPs at different sintering time are given in Figure 9c. A high conductivity was acquired when the sintering time was 2 h; moreover, there was no damage to the paper substrate after heating. It was best to obtain a maximum conductivity of AgNPs on the paper substrate. After the sample was bended 1000 times, conductivity decreased by 29.4% (Figure 9d). Furthermore, Ag conductive patterns were used to demonstrate the lighting of an LED bulb Figure Theresistivity resistivity of the printed Ag This patterns with: (a) different (b) sintering different The of the printed Ag patterns with: (a) different printingprinting times; (b)times; different usingFigure a 9 V9.9.battery, as shown in Figure 10. result demonstrated that the prepared conductive sintering temperature; (c) different sintering time; and (d) different bending times. (c) different sintering circuit time; and (d)also different bending times. tracktemperature; could be used as an electronic and applied in other electronic applications. Figures 9b,c show the relationship between the resistivity of the printed Ag patterns and the sintering temperature, resistivity, and sintering time, respectively. The variation of sintering temperature and time are the most important parameters that affect the resistivity of AgNPs on the substrate materials. The stability and conductivity of the electronic track was tested by changing the sintering temperatures from 60 to 120 °C. The pattern was printed 25 times and sintered for 120 min. The best conductivity that the thin film AgNPs obtained was when the sintering temperature was 120 °C, as sintering temperatures over this decreased resistivity. The sintering time also impacted on the conductivity of the AgNPs pattern on the photo paper substrate. The effects of sintering time on Figure 10. The simple conductivity test of printed Ag patterns: (a) Square; (b) Rectangle; (c) Spiral. conductivity were by heatingtest theofprinted at 120 °C with different interval times of 0.5, Figure 10. The studied simple conductivity printed track Ag patterns: (a) Square; (b) Rectangle; (c) Spiral. 1, 2, 4, and 6 h. The results of the conductivity of AgNPs at different sintering time are given in Figure 9c. A Materials high conductivity was acquired when the sintering time was 2 h; moreover, there was no damage 3. and Methods to the paper substrate after heating. It was best to obtain a maximum conductivity of AgNPs on the 3.1. Materials paper substrate. After the sample was bended 1000 times, conductivity decreased by 29.4% (Figure 9d). All chemicals used in this study were of analytical reagent grade. Meanwhile, Ag nitrate, Furthermore, Ag conductive patterns were used to demonstrate the lighting of an LED bulb polyvinyl pyrrolidone (PVP), ethylene glycol (EG), polyethylene glycol (PEG), and isopropanol (IPA) using a 9 V battery, as shown in Figure 10. This result demonstrated that the prepared conductive were obtained from the Tianjin Damao Chemical Reagent Factory (Tianjin, China). Ethyl alcohol track could be used as an electronic circuit and also applied in other electronic applications. was provided by the Guangzhou Chemical Reagent Factory (Guangzhou, China). Octylphenol polyoxyethylene ether (10) (OP-10) was provided by the Tianjin Fuyu fine chemical industry (Tianjin, China). 3.2. Synthesis of AgNPs PEG and EG were mixed for use as a reducing agent. The preparation steps of spherical nano-Ag particles were as follows. First, 100 mg of AgNO3 and 150 mg PVP was transferred into a round-bottomed flask containing 20 mL of EG. Next, 20 mL of PEG was added to the stirred mixture, Figure 10. The simple conductivity test of printed Ag patterns: (a) Square; (b) Rectangle; (c) Spiral.
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and combined until well mixed. The mixed solution was then transferred to an oil bath with a constant temperature and 200 r/min constant stirring. At the completion of the reaction, the mixture was washed and diluted with anhydrous ethanol 5 times before concentrated products were achieved after 20 min centrifugation at 8000 rpm. The liquid had obvious stratification, with the lower layer having a higher AgNPs concentration. The precipitated product was cleaned and centrifuged with anhydrous ethanol 5–8 times. The precipitated product was dried at room temperature standby. 3.3. Synthesis of Nano Ag Ink To synthesize the new Ag nano-ink, the recipe used was as follows: 3 wt % of OP-10; 20 wt % of ethylene glycol; 1.5 wt % ethanol; and 1 wt. % isopropyl alcohol was placed into glass bottles containing 59.5 wt % deionized water. The solution mixture was stirred at 200 r/min constant speed. After 10 min, 200 mg of AgNPs was added to the mixed solution which was then ultrasonically cleaned for 30 min, and the mixture formed a uniform system at room temperature when it was stirred for 30 min. Finally, a uniform AgNPs ink was obtained. The AgNPs ink was injected into a clean ink cartridge (Canon iP1188, Canon, Tokyo Daejeon, Tokyo, Japan) and printed using a Canon iP1188 inkjet printer. To deposit a large number of AgNPs, the printer settings were adjusted for the “best print quality” and “photo print mode” to ensure sufficient AgNPs ink was printed on the paper substrate. 3.4. Characterization of AgNPs Morphological observations were carried on using a field-emission scanning electron microscope (FE-SEM, Zeiss, Oberkochen, Germany) and a field-emission transmission electron microscope (TEM, Philips TECNAI-F20, Chur, Switzerland). Samples were also analyzed by X-ray diffraction (XRD; D8, Bruker, Billerica, MA, USA) at room temperature with a scanning angle from 20◦ to 90◦ . The scanning speed was 10◦ /min with 40 kV accelerating voltages and a 15 mA current. A UV-Vis absorption spectroscopy test was conducted using a UV-Vis spectrophotometer (UV-2550, Japan Shimadzu Corporation, Kyoto, Japan). The sample was diluted 1000 times and placed in a 1 × 3 cm quartz cuvette for testing. The resistance of the thin AgNPs pattern was tested using a KDB-1 digital four-probe resistance tester (Guangzhou Kunde Technology Companies (Guangzhou, China)). 4. Conclusions In summary, based on a controlled reaction temperature, ratio of PEG to EG, and reaction time, the Ag nanoparticles were prepared through a liquid reduction method. PEG and EG were mixed as reducing agents to prepare silver nanoparticles with a range of different particle sizes under 140 ◦ C. When the ratio of PEG to EG was 1:2, silver nanoparticles were prepared with an average particle size of 40 nm, a narrow size distribution and good dispersion performance. The conductive AgNPs ink was produced using OP-10 as a dispersing agent. The influence of other parameters was also studied, such as the effects of paper substrates, sintering temperature and time on the conductivity of the printed pattern. It was found that the Lucky porous high glossy photo paper was a good candidate substrate, and the resistivity of the printed pattern after heating at 100 ◦ C for 2 h could reach 5.1 × 10−3 Ω·cm. When the printed pattern was connected to an LED light and supplied the power, the LED lit up. Therefore, it was possible to print any patterns as designed, had good electrical conductivity, and could be sintered at a low-temperature and has provided new ideas for the preparation of flexible electrodes. Acknowledgments: This project was supported by Guangdong provincial science and technology projects. (Item Number 2014B090915003 and 2015B090901052). Author Contributions: Zhidan Lin, Peng Zhang and Lin Cao conceived and designed the experiments; Lin Cao and Xiaohe Bai performed the experiments; Peng Zhang, Shuling Deng, Xusheng Du and Lin Cao analyzed the data; Peng Zhang and Zhidan Lin contributed reagents, materials and analysis tools; Lin Cao wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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