Zigzag-Shaped Silver Nanoplates: Synthesis via Ostwald Ripening

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Functional Nanostructured Materials (including low-D carbon)

Zigzag-Shaped Silver Nanoplates: Synthesis via Ostwald Ripening and Their Application in Highly Sensitive Strain Sensors Jinwoo Kim, Sang Woo Lee, Mun Ho Kim, and O Ok Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11322 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Materials & Interfaces

Zigzag-Shaped Silver Nanoplates: Synthesis via Ostwald Ripening and Their Application in Highly Sensitive Strain Sensors Jinwoo Kima, ‡, Sang Woo Leeb, ‡, Mun Ho Kima, ‡,* and O Ok Parkb,* aDepartment

of Polymer Engineering, Pukyong National University, 365 Sinseon-ro, Nam-gu, Busan 48547, Republic of Korea

bDepartment

of Chemical & Biomolecular Engineering (BK 21+ Graduate Program), Korea

Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡These

authors contributed equally to the work.

*Corresponding author: M. H. Kim ([email protected]) (Tel.; +82-51-629-6459, Fax; +82-51-629-6429) O O. Park ([email protected]) (Tel.; +82-42-350-3923, Fax; +82-42-350-3910)

ACS Paragon Plus Environment

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ABSTRACT

Zigzag-shaped Ag nanoplates display unique anisotropic planar structures with unusual jagged edges and relatively large lateral dimensions. These characteristics make such nanoplates promising candidates for metal inks in printed electronics, which can be used for realizing stretchable electrodes. In the current work, we used a one-pot coordination-based synthetic strategy to synthesize zigzag-shaped Ag nanoplates. In the synthetic procedure, cyanuric acid was used both as a ligand of the Ag+ ion, hence producing complex structures and controlling the kinetics of the reduction of the cation, and as a capping agent that promoted the lateral growth of the Ag nanoplates. Hence, cyanuric acid played a crucial role in the formation of zigzag-shaped nanoplates. In contrast to previous studies that reported oriented attachment to be the predominant mechanism responsible for the growth of zigzag-shaped nanoplates, Ostwald ripening was the dominant growth mechanism in the current work. Our findings on the particle morphology and crystalline structure of the Ag nanoplates motivated us to use them as conductive materials for stretchable strain sensors. Strain sensors based on nanocomposites of our zigzag-shaped Ag nanoplate and polydimethylsiloxane (PDMS) in the form of a sandwich structure were successfully produced by following a simple, low-cost and solution-processable method. The strain sensors exhibited extremely high sensitivity (gauge factor  2,000), high stretchability with a linear response ( 27%), and high reliability, all of which allowed the sensor to monitor diverse human motions, including joint movement and phonation.

Keywords Silver nanoplates, coordination complex, Ostwald ripening, stretchable electrodes, strain sensors


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1. Introduction Recently, the development of stretchable and wearable strain sensors has attracted great interest due to the ability of such sensors to monitor diverse human motions.1-3 Novel strain sensors with high sensitivity and reliability have been produced using essentially one-dimensional (1D) or twodimensional (2D) conducting materials with high conductivity levels and aspect ratios, such as carbon nanotubes (CNTs), metal nanowires, and graphene.4-11 Surprisingly, no study to date has sought to exploit 2D silver nanostructures (nanoplates), which have large surface areas and tend to form ultrathin sheets, as the basis for stretchable conductive electrodes. To achieve a low percolation threshold for conductivity, 2D nanostructures should have large lateral dimensions.3,12 However, synthesizing large Ag nanoplates in high yield is difficult. To the best of our knowledge, one-pot synthetic routes based on solution-phase methods developed so far have mostly been limited to the preparation of Ag nanoplates with lateral dimensions of less than 500 nm.13-15 Ag nanoplates with lateral dimensions greater than one micrometer have been produced,16-17 via multistep deposition processes in which Ag seed particles were synthesized and then subjected to gradual growth steps; however, these processes are tedious and time consuming, and require continuous attention and effort. Efforts to synthesize 2D Ag nanostructures with large lateral dimensions via a one-pot route have been hampered by a lack of relevant easy-to-use synthetic methods. Zigzag-shaped Ag nanoplates display anisotropic planar structures with unusual jagged edges and relatively large lateral dimensions. These characteristics suggest that these nanoplates should be suitable for use as metal inks in printed electronics, and hence for use in stretchable electrodes. Such 2D Ag nanostructures with asymmetric and irregular shapes have been produced using the oriented attachment method18-20 rather than using the conventional nucleation and growth process


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involving Ostwald ripening. In the oriented attachment method, nanoparticles with common crystallographic orientations were used as building blocks to form more complex nanostructures such as arrow-, tetrapod- and zig-zag-shaped nanostructures.21-24 The formation of zigzag-shaped nanoplates has been shown to be based on a bimodal particle growth mechanism in which an initial formation of small nanoplates is followed by a fusion of these plates along their lateral planes to form large secondary nanoplates.14 In the current work, we developed a facile one-pot synthetic method, based on Ostwald ripening instead of oriented attachment, that yielded zigzag-shaped Ag nanoplates in very high yield. The key to the proposed method was the use of a coordination strategy to manipulate the reduction kinetics of the Ag+ ions, thus promoting the formation of zigzag-shaped nanoplates despite this shape being highly unfavorable from a thermodynamic standpoint. By using this new method, we were able to form zigzag-shaped Ag nanoplates with lateral dimensions reaching several micrometers and thicknesses of approximately 17 nm. The proposed synthetic route reproducibly produced zigzag-shaped Ag nanoplates in nearly 100% yield (defined as the percentage of zigzagshaped plates in the products). Finally, we developed highly sensitive and stretchable strain sensors by using the zigzag-shaped Ag nanoplates as conducting materials and applied these sensors to the detection of various human activities. The Ag nanoplate sensors based on a simple structure consisting of a conductive network of zigzag-shaped Ag nanoplates on the polydimethylsiloxane (PDMS) substrate were easily prepared by using a low-cost and solution-processable method requiring no special equipment. Most of the strain sensors using 1D materials such as metal nanowires showed low sensitivity (~14 of gauge factor) because high aspect ratios of 1D materials are beneficial for formation of the effective percolation networks.1-4,6-7,11 The strain sensors using 2D materials (mostly graphene)


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exhibited excellent sensitivity but nonlinear response due to the crack propagation mechanism.12,5,8

Interestingly, the fabricated sensors based on the zigzag-shaped Ag nanoplates showed strong

piezoresistivity, extremely high sensitivity as indicated by their gauge factors of over 2,000, and high stretchability with a linear response up to 27%. We demonstrated that the high performance strain sensors could be successfully used to detect and measure the magnitudes, in human subjects, of both small-scale motions such as those used for phonation and large-scale motions including the movements of joints.

2. Results and Discussion Since two-dimensional (2D) silver nanostructures (nanoplates) have large surface areas, they have a strong tendency to join together to form large, ultrathin sheets. The electrical connection of conductive materials in a polymer matrix can be determined using the power law relationship 𝜎 = 𝜎0(𝑉𝑓 ― 𝑉𝑐)𝑠

(1)

where 𝜎 is the electrical conductivity of the composite film, 𝜎0 is a scaling factor proportional to the intrinsic conductivity of the filler, s is the critical exponent of the conductivity, 𝑉𝑓 and 𝑉𝑐 are the volumetric fraction of the conductive filler and the percolation threshold, respectively.25 For randomly distributed filler particles without any preferred orientation, 𝑉𝑐 can be calculated in the case of a plate structure using the equation 𝑉𝑐 = 27D2t/4(D+DIP)3

(2)

where D, t, and DIP are the lateral dimension of the nanoplate, the thickness of the plate, and the average inter-particle distance, respectively.3,12 This equation indicates that the percolation threshold decreases with an increase in the lateral dimension of the nanoplate and is inversely


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proportional to the thickness of the nanoplate. Therefore, if such systems are to be applied as conductive ink materials for electrodes, it is important to have the ability to fabricate thin Ag nanoplates with large lateral dimensions. In syntheses of metal nanostructures, the nontoxic, chemically inert, water-soluble polymer polyvinylpyrrolidone (PVP) has been used extensively as a steric stabilizer to maintain a uniform colloidal dispersion without product aggregation.26 PVP has also been used as a shape-controlling agent that promotes attachment of metal atoms onto specific crystal facets, leading to anisotropic crystal growth.27 Recent studies have shown that, due to its hydroxyl (OH) end groups, PVP acts as a new type of reductant with a mild reducing power, instrumental to the kinetically controlled synthesis of metal nanoplates.28-31 Previously, we reported a synthetic method based on a chemical reduction approach to produce Ag nanoplates in high yield.14 During the synthesis, PVP successfully served both as a colloidal stabilizer and as a reductant with weak reducing power to reduce AgNO3 in N,N-dimethylformamide (DMF), leading to the formation of Ag nanostructures with stacking faults and thus to the formation of Ag nanoplates with a high yield. Here, we report a facile method for producing zigzag-shaped Ag nanoplates in high yield. The proposed method is essentially the same as the previous synthetic method except that we now used cyanuric acid as a ligand to form a complex with Ag+ ions. The formation of these complexes was shown to significantly reduce the concentration of free Ag+ ions and thus slowing the rate at which Ag+ ions were reduced to Ag atoms. Because both the nucleation and growth of Ag nanoplates proceeded very slowly under these conditions, most of the synthesized nanostructures could assume the more thermodynamically unfavorable shape. The Ag nanostructures produced in the presence of cyanuric acid were visualized using TEM. In a typical one-pot synthesis, the ratio of the weight of PVP to that of AgNO3 was kept at 19.5:1,


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and the reaction was performed at 100 °C in DMF. As shown in Figures 1A-1C, each single nanoplate obtained after 24 h of reaction in the presence of cyanuric acid displayed a variety of edges and corners. Interestingly, unlike other 2D Ag nanostructures that grow along crystallographically equivalent directions, the nanostructures that grew in the presence of cyanuric acid had one predominant crystal-growth axis, leading to a zigzag morphology. The percentage of the product consisting of nanoplates, i.e., the nanoplate yield, was measured to be nearly 100%, and many of the nanoplates showed each a lateral dimension greater than one micrometer. Figure S1 shows representative AFM images and height profiles of the nanoplates formed after a reaction time of 24 h and then fixed onto a silicon wafer. The nanoplates were measured to have a mean thickness of about 17 nm.


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Figure 1. (A-C) TEM images and (D) XRD pattern of Ag nanoplates grown in the presence of cyanuric acid. In the synthesis, the ratio of the weight of PVP (29 kDa) to that of AgNO3 was 19.5. The concentration of cyanuric acid was 10 mM. The synthesis was carried out at a temperature of 100 °C for a duration of 24 h.

Figure 1D shows the X-ray diffraction (XRD) pattern of the sample shown in Figures 1A-1C. Only one strong peak was observed in this pattern, at a two-theta value of 38.2, corresponding to the (111) reflection of the face-centered cubic (FCC) lattice of Ag. The intensities of peaks at


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44.3, 64.4, and 77.3, which could be indexed, respectively, to the (200), (220), and (311) reflections, were very low. The combination of a very strong (111) peak and very weak other peaks indicated that the sample was almost exclusively comprised of nanoplates oriented parallel to the XRD sample holder.32 These results showed that thin Ag nanoplates with high aspect ratios were formed when cyanuric acid was included in the synthesis. High-resolution TEM (HR TEM) imaging, with images collected perpendicular to the major plane of the nanoplate, was utilized to further investigate the nanoplate structures. The HR TEM image and the corresponding fast Fourier transform (FFT) patterns of a zigzag-shaped Ag nanoplate are shown in Figure S2. The FFT pattern contained diffraction spots related to each other by 6-fold rotational symmetry, indicating that the nanoplate was a single crystal and the zone axis was along the 111 directions of an FCC configuration of Ag atoms. Several nanoplates were analyzed in this manner, and the patterns they produced were nearly identical (see Figure S3). These observations indicated that showed that these zigzag-shaped Ag nanoplates were single crystals without twin defects. To investigate the process by which the Ag nanostructures grew in the presence of the cyanuric acid ligand, we monitored, as a function of time, the shape of nanostructures forming in a reaction mixture containing the ligand. We were able to easily follow the course of the synthesis of the nanostructures by noting the distinctive color changes that occurred.33 When the AgNO3 solution was injected into the PVP solution in DMF without cyanuric acid, the solution became yellow, indicating the formation of silver nanoparticles. Heating the reaction mixture resulted a series of color changes, from yellow to brown to blue to dark green and finally to gray. In the absence of cyanuric acid these changes occurred over 4 h. In the presence of cyanuric acid, by contrast, the dark green and grey colors first appeared only after more than 12 h, indicating that the reaction


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was considerably slower when the ligand was present. Consistent with the color observations, centrifugation (13,000 rpm, 10 min) of the reaction mixture containing cyanuric acid after 4 h of reaction yielded very few particles, suggesting that few if any Ag nanocrystals formed in the first 4 h of reaction. After 5 h of reaction, needle-shaped particles were observed (see Figure S4); these particles were expected to consist of supramolecular complexes of cyanuric acid and AgNO3. It has been reported that molecules such as cyanuric acid and melamine can be connected to each other through AgN bonds to form NAgN, giving rise to the linear morphologies.34-36 To confirm the needle-shape particles to be supramolecular complexes of cyanuric acid and AgNO3, we carried out Fourier transform infrared (FTIR) spectroscopy analysis. FTIR spectrum of the needle-shaped particles is shown in Figure S5A, which showed typical cyanuric acid peaks with the enolic form.37 In solution, cyanuric acid occurs as a mixture of keto and enol tautomers, but the latter form could effectively form NAg bonding with Ag+ ion (see Figure S5B). Elemental mapping analysis by using energy dispersive spectroscopy (EDS) was also performed to reveal the composition of a single needle-shaped particle, which showed that Ag and N were clearly detected over the entire region of the nanostructure (Figure S6). These results demonstrated that the needleshapes particles were comprised of supramolecular complexes of cyanuric acid and Ag+ ion. After a reaction time of 6 h, the needle-shaped complex nanostructures disappeared and multiply twinned particles formed, specifically inside complex structures with irregular shapes (see Figure 2A). A reductant (PVP in the present study) would be expected to reduce the Ag+ ions in the complexes to form Ag atoms, and these atomic species would then aggregate into small nanocrystals. Ag nanocrystals have been shown to catalyze the reduction of the Ag+ via an autocatalytic reduction mechanism by drastically reducing the reduction potential compared to that of free Ag+ ions.38 It has also been reported that N-Ag-N coordination bonds are relatively weak


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and hence that complexes based on such bonds are not stable.34,35 Therefore, as the Ag nanocrystals form, they catalyze the reduction of Ag+ ions, thus promoting the breaking of the N-Ag-N bonds in the complexes. As a result, the needle-shaped particles disappear. We further characterized the multiply twinned particles shown in Figure 2A using HR TEM techniques. Figure S7 shows an HR TEM image of such a particle, which appeared they have a crystalline structure with a lattice length of ~ 0.23 nm. The measured lattice spacing was in agreement with the (111) lattice of the Ag nanocrystals (see Figure S7).39 These results confirmed that multiply twinned Ag nanocrystals began to form after 6 h of reaction. After a reaction time of 7 h, seed particles for the zigzag-shaped nanoplates were observed among the multiply twinned Ag nanoparticles (see Figure 2B). Prolonging the reaction time to 8 h and 9 h (see Figure 2C-2D) resulted in a further increase in the sizes and numbers of the zigzag-shaped nanoplates. These observations suggested that as Ag ions were reduced, the newly formed Ag atoms attached to the surfaces of the seed particles, resulting in the growth of nanoplates.


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Figure 2. TEM images of Ag nanostructures prepared under the same conditions as in Figure 1 except for the reaction time: (A) 6 h, (B) 7 h, (C) 8 h, and (D) 9 h (D) (scale bar  50 nm).

Figure 3 shows TEM images of the Ag nanoplates we produced after reaction times of 10, 12, 16, and 24 h. At the reaction time of 10 h, the product contained complex structures in which multiply twinned Ag nanocrystals were embedded (see Figure 3A; the complex structures are marked by blue arrows). However, as the reaction proceeded, both the complex structures as well as the small plates disappeared and large plates predominated (see Figure 3B-3D). This behavior indicates that the system underwent Ostwald ripening. Previous studies concluded that zigzagshaped Ag nanostructures, including zigzag-shaped Ag nanoplates, formed and grew


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predominantly via oriented attachment.18-21 Under this mechanism, many overall small but different-sized nanoplates are initially formed under kinetic control, and then fuse with each other along their lateral planes to form large secondary nanoplates having jagged edges. However, in the present study, nanoplate assembly via edge-selective fusion was not observed. Instead, the multiply twinned Ag nanocrystals provided the source of elemental silver for the formation of Ag nanoplates. The multiply twinned Ag nanocrystals formed inside the complex structures in the presence of cyanuric acid can be regarded as intermediates; these intermediates formed at the early stages of the reaction and then almost completely disappeared. Collectively, these results indicate that, in the presence of cyanuric acid, zigzag-shaped Ag nanoplates are formed via a process driven by Ostwald ripening. As the reaction time was increased up to 24 h, the lateral dimensions of the nanoplates continuously increased, but their vertical dimensions, determined based on TEM image contrast, hardly changed. Previous reports of cyanuric acid adsorption onto the {111} facets of FCC metals40-41 suggest that cyanuric acid may act as a capping agent that binds to the {111} facets of the Ag nanoplates, restricting growth in the vertical direction while allowing growth in the lateral direction. These results, demonstrating that cyanuric acid served both as a ligand of Ag+ ions and as a capping agent that promotes lateral growth, indicate the crucial role of the ligand in the formation of the zigzag-shaped structure.


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Figure 3. TEM images of the Ag nanoplates we produced after reaction times of (A) 10 h, (B) 12 h, (C) 16 h, and (D) 24 h (scale bar  200 nm). The blue arrows in (A) indicate the complex structures embedding multiply twinned Ag nanocrystals. The Ag nanostructures were made in the presence of 10 mM cyanuric acid. In the synthesis, the ratio of the weight of PVP (29 kDa) to that of AgNO3 was 19.5.

To confirm the role of cyanuric acid in the formation of zigzag-shaped Ag nanoplates, comparative experiments were carried out in which various amounts of cyanuric acid were added to the reaction mixture. In the absence of cyanuric acid, only conventional polygonal nanoplates


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were obtained (Figure 4A). In this case, nucleation and growth were rapid because sufficient Ag atoms were present in the solution from the initial stage of the reaction. As a result, most particles grew into isotropic planar structures as the reaction proceeded. When cyanuric acid was added to the reaction mixture, zigzag-shaped Ag nanoplates formed and the number of such structures increased markedly as the cyanuric acid concentration was increased (Figure 4B-4C), indicating that cyanuric acid promoted the formation of anisotropic planar structures. As shown in Figure 4D, an increase of the cyanuric acid concentration up to 20 mM adversely affected the growth of zigzag-shaped Ag nanoplates.


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Figure 4. TEM images of Ag nanoplates formed (A) in the absence of cyanuric acid and (B-D) in its presence, at concentrations of (B) 5 mM, (C) 10 mM, and (D) 20 mM. All of the syntheses were carried out at a temperature of 100 C for a duration of 24 h and included a 19.5:1 ratio of the weight of 29 kDa PVP to that of AgNO3.

It is worth noting that, in the present study, the lateral dimensions of the anisotropic planar structures were sensitive to the ratio of the weight of PVP to that of AgNO3. Doubling the concentration of PVP relative to that of AgNO3, which increased the PVP-to-AgNO3 weight ratio to 39, yielded small nanoplates and multiply twinned nanoparticles (see Figure 5A). Reduction rates are generally expected to be proportional to the concentration of the reductant, and a higher concentration of PVP in our case indeed increased the reduction rate, and the formation of the thermodynamically favored shape was promoted. Decreasing the concentration of PVP to give PVP:AgNO3 weight ratios of 9.8 and 4.9 yielded large zigzag-shaped Ag nanoplates (see Figure 5C-5D). For the ratio of 4.9:1, most of the nanoplates displayed a lateral dimension greater than one micrometer, and some of the nanoplates reached several micrometers. According to the GibbsThomson equation, low reaction rates would have been expected to promote the addition of newly formed Ag atoms onto the pre-existing crystals, while high reaction rates would have favored selfnucleation.17 In addition, PVP serves not only as a reductant, but also as a colloidal stabilizer. A low concentration of stabilizer would have been expected to have a negative effect on the colloidal stabilization of the resulting Ag nanostructures, leading to the formation of larger nanostructures. These two effects can explain why in our experiments the nanoplate size depended inversely on the PVP-to-AgNO3 weight ratio.


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Figure 5. TEM images of Ag nanostructures grown at 100 C for 24 h in the presence of 10 mM cyanuric acid and various ratios of the weight of 29 kDa PVP to that of AgNO3, specifically ratios of (A) 39:1, (B) 19.5:1, (C) 9.8:1, and (D) 4.9:1. The Ag nanostructures were grown in the presence of cyanuric acid (10 mM). All of the syntheses were carried out at a temperature of 100 C for a duration of 24 h.

Flexible strain sensors that can be attached to human skin as patches have recently attracted considerable attention because of their potential applications in monitoring muscle movements during body motions. To detect large strains on skin near the moving joints of the fingers, as well


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as to detect small strains near the wrist (pulsation) and neck (phonation), it is necessary to develop strain sensors capable of detecting a wide range of strains with high sensitivity.42-43 We tested the suitability of the zigzag-shaped Ag nanoplates with highly anisotropic planar structures as conducting materials for such high-performance strain sensors. Figure 6A shows the fabrication procedure for the strain sensor based on zigzag-shaped Ag nanoplates (the fabrication procedures are described in detail in the Experimental section). To construct conductive networks in three dimensions within an elastomeric matrix, a PDMS substrate with a line-patterned channel was first prepared. Zigzag-shaped Ag nanoplates synthesized in the current work were deposited directly onto the channels of the PDMS substrate. Thermal annealing was carried out at a temperature of 150 C for 3 min. Figure S8 shows SEM images of thin films comprised of zigzag-shaped Ag nanoplates after thermal annealing. The films were found to have very rough surfaces with microcracks that formed during the drying process. After thermal annealing, however, the film surfaces became smoother and the microcracks disappeared, consistent with enhanced stacking of the nanoplates. These results indicated a great increase in the extent of contact between the Ag nanoplates, and a continuous metallic network was formed during thermal annealing. To form contact pads for the measurements taken, two pieces of conductive copper tape were attached to the Ag conductive pattern, one to each end, on the PDMS substrate. A PDMS mixture was then deposited onto the conductive pattern and the sample was degassed and cured at a temperature of 60 C for a duration of 20 minutes. In addition, we were able to directly mount them on the skin and easily attach them to complex surfaces without any damage.


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Figure 6. (A) Schematic illustration of the fabrication of the Ag nanoplate strain sensor. (B) Relative resistance change of each of three tested Ag nanoplate strain sensors as a function of the applied strain. (C) Response curve of the strain sensor fabricated using the large zigzag-shaped nanoplates during repeated applications of a strain of ~27% at a frequency of 1 Hz.

The electrical characteristics of the fabricated sensors under strain were determined by measuring the resistance levels while the sensors were subjected to a strain. To investigate the effect of the shape and lateral dimension of the Ag nanoplates on the electrical properties of the strain sensor, Ag nanoplates grown in the absence of cyanuric acid (shown in Figure 4A) were used as a reference sample. Figure 6B shows plots of the relative resistance change (RRC = R/R0,


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where R and R0 are the change in resistance and the initial resistance, respectively) of each of the tested nanoplate strain sensors as a function of the applied strain. RRC increased with increasing strain, but with different slopes depending on the shape and lateral dimension of the Ag nanoplates. For the strain sensor based on conventional polygonal nanoplates (Figure 4A), R/R0 increased markedly when the strain was increased from 5% to 6% (Figure 6B). In contrast, the strain sensors formed from zigzag-shaped Ag nanoplates were highly stretchable (as tested using various strain levels greater than 10%). In particular, the sensor comprising zigzag-shaped Ag nanoplates with large lateral dimensions (Figure 5D) showed a linear relationship between RRC and applied strain up to relatively large strains,   27% (Figure 6B). Linearity over a wide range of strains is important for stretchable strain sensors because it increases the accuracy with which the strain can be determined from the electrical signal of a sensor under strain.1 In addition to the linear response of the strain sensor based on large zigzag-shaped Ag nanoplates during the application of strain, this sensor also exhibited an almost constant, extremely high value ( 2,000) of the gauge factor (GF = RRC/ε, where ε is an applied strain), as shown in Figure 7. In particular, the sensor comprising large zigzag-shaped Ag nanoplates exhibited outstanding gauge factors over a wide range of strains. A trade-off between sensitivity and stretchability has been generally accepted as being unavoidable for strain sensors involving simple thin film structures.1 However, our strain sensor showed both high stretchability with a linear relationship (to an  of  27%) and extremely high sensitivity (GF  2,000) (see Table S1 & Figure S9).


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Figure 7. Gauge factors of the fabricated Ag nanoplate strain sensors as a function of the applied strain.

The excellent performance of our sensor can be explained by the anisotropic planar structure of the Ag nanoplates. Compared to nanowires with lengths reaching several tens of micrometers, the lateral dimensions of zigzag-shaped nanoplates were quite small and thus significant structural deformations that negatively affected electrical percolation likely arose in the stretching process,. As a result, electrical percolation in the latter nanostructures will be more strongly affected by stretching, leading to higher sensitivity. To confirm this geometric structural effect, we fabricated the strain sensor under the identical conditions by using Ag nanowires as the conductive materials and compared RRC values with those of the strain sensors based on zigzag-shaped Ag nanoplates. The diameter of the Ag nanowires was about 27 nm, and the length was about 22 m. TEM and SEM images of the Ag nanowires are shown in Figure S10A-S10B. RRC of the strain sensors based on Ag nanowires did not change notably with increasing strain up to nearly 5 % (see Figure S10C). Since Ag nanowires have the very high aspect ratio, the sensors would be morphologically


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intact under small stretching44-45 and therefore, the sensor could not exhibit sensitive responses in low strain ranges. Another potential effect of strain on the zigzag-shaped Ag nanoplates is that it may cause the structures to align in the direction in which they are stretched because of their anisotropic shape. Figure S11 shows a top-view SEM image of the Ag nanoplate sensors based on zigzag-shaped Ag nanoplates with large lateral dimensions under an application of 30% strain; in the image, the Ag nanoplates are aligned parallel to the stretching direction. Such alignment of the Ag nanoplates would cause the nanostructures to maintain long-range connectivity at greater strains. The anisotropic shape of the Ag nanoplates presumably has influenced on the formation of cracks on the Ag layer which generally hinders the electrical connection through the sensor. The microstructures of the cracks, which formed on the Ag layers of the strain sensors under an application of 10% strain, were shown in Figure S12. These SEM images in Figure S12 exhibited that more longitudinal cracks (parallel to the stretching direction) have been developed in the strain sensor based on zigzag-shaped Ag nanoplates, than the one based on polygonal Ag nanoplates. In addition, the sizes of transversal cracks (formed in the perpendicular direction of the stretching) and the width of Ag islands (the regions between two adjacent cracks) were small in the strain sensor based on zigzag-shaped Ag nanoplates (see Figure S12E). Formation of the longitudinal cracks and relatively small sizes of the transversal cracks are expected to promote the formation of more conductive pathways for electron flow in the Ag layer because Ag islands are pushed together due to the compressive Poisson stress acting in the perpendicular direction of the stretching,46-47 as shown in Figure S13. Taken together, these results demonstrate that, due to the anisotropic planar structure of the zigzag-shaped Ag nanoplates, the strain sensor maintained


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electrical percolation contacts upon the relatively high levels of stretching, hence leading to the high stretchability and linearity of the sensor. Given that superior performance of the sensor consisting of large zigzag-shaped Ag nanoplates, this sensor was chosen for further experiments. The electrical performance of the strain sensor was monitored during repeated cycles of stretch and release in order to assess its operational stability. The sensor maintained good sensitivity over 200 cycles, with a fast response to each change in strain, indicating good mechanical stability and reliability (see Figure 6C). The high electrical stability levels of the nanoplate strain sensors may have been due to reversible sliding between the Ag nanoplates resulting from good adhesion of the Ag nanoplates to the PDMS substrate, which in turn was due to the nanoplates having relatively large surface areas and thus having a strong tendency to form large, ultrathin sheets. These results demonstrated that, in addition to displaying extremely high sensitivity and a wide sensing range, the strain sensors fabricated from zigzagshaped Ag nanoplates also displayed high durability and stability. In addition, response test was conducted to figure out the response speed of the Ag nanoplate strain sensor. As shown in Figure S14, the sensor was located near the speaker that made a periodic sound of drum beating with frequency of 0.25 Hz, and the response of the sensor was monitored. Analysis on a single peak of the signal revealed that the response time constant of the sensor was found to be approximately 0.09537 s (see Figure S14), indicating that the sensor exhibits a fast response less than 0.2 s to each change in strain. We investigated the ability of the zigzag-shaped Ag nanoplate strain sensor to monitor human motion. To do this, an elastomeric medical patch was attached to the Ag nanoplate sensor in such a way as to make a complete contact between the sensor and a curved surface on the human body. To evaluate the strain-sensing performance of the sensor in large-scale human motion, the sensor


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was mounted on a mobile joint of the index finger. Figure 8A shows the RRC response of the sensor depending on the bending and relaxing of the index finger. The sensor exhibited high sensitivity to the movements of the index finger, and excellent R/R0 responses during many cycles of bending and relaxation (see Figure 8A). These results demonstrated the ability of the Ag nanoplate strain sensor to monitor, with excellent sensitivity and reliability, the bending motion of a finger.

Figure 8. (A) Photographs (left) and R/R0 response (right) of our fabricated Ag nanoplate strain sensor mounted on the index finger of a test subject. (B,C) RRC and photograph (inset) of the sensor attached to a human (B) neck to detect motions of the throat and (C) wrist to monitor the pulse.


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We additionally tested the sensitivity and recognition ability of the Ag nanoplate sensor by using it to monitor subtle human body motions. The ability of the Ag nanoplate strain sensor to detect phonation was tested by attaching it to a human neck to monitor vibration of the larynx during speech. Figure 8B shows the RRC graph of the sensor obtained during repeated articulation of the word “hello” by the test subject. Inspection of the RRC graph indicated that the sensor produced a distinct RRC pattern containing several distinguishable peaks each time the word was spoken. The Ag nanoplate sensor was also attached to a wrist and used to detect pulse signals: the resulting measured RRC data showed obvious peaks corresponding to the heart beating (Figure 8C). These results revealed that the Ag nanoplate sensor displays very high sensitivity and that it can be effectively used to detect and measure the magnitudes of even subtle motions of the human body.

3. Conclusion Zigzag-shaped Ag nanoplates with a high lateral dimension were made by carrying out a onepot coordination-based synthesis in the presence of cyanuric acid. In the synthesis, cyanuric acid acted both as a ligand of Ag+ ions and as a capping agent to promote lateral growth. The formation of thermodynamically unfavorable, anisotropic planar structures with unusual jagged edges was attributed to the reduction in the number of free Ag+ ions due to complexation of these ions, which changed the reduction kinetics of Ag+ ions. Although oriented attachment growth has generally been accepted to be the predominant mechanism by which zigzag-shaped Ag nanostructures are formed, Ostwald ripening was found to be responsible for the growth of the zigzag-shaped Ag nanoplates in the current study. Strain sensors fabricated using nanocomposites of these Ag nanoplates and PDMS exhibited extremely high sensitivity (gauge factor  2,000), high


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stretchability with a linear response ( 27%), and high reliability. This high sensitivity of the strain sensor allowed it to be used to monitor wide ranges of motions of the human body. We expect the results of this study to spur on the development of a practical and meaningful approach for realizing high-performance stretchable electrodes based on 2D Ag nanostructures.


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4. Experimental details Chemicals and Materials: All chemicals were used as received without further purification. Silver nitrate (AgNO3, 209139), polyvinylpyrrolidone (PVP, Mw ≈ 29 kDa, 234257), cyanuric acid (C3H3N3O3, 185809), and gallium-indium eutectic (Ga/In, 495425) were purchased from Aldrich (USA). Dimethyl formamide (DMF, 35770-0380) was purchased from Junsei Chemical (Japan). Copper tape (EMI copper foil shielding tape 1181) was obtained from 3M (USA). Polydimethylsiloxane and catalyst (SH9555) were purchased from Dow Corning Toray (Japan). Ag nanowires (NTC-27) were purchased from Nanopyxis Co. Ltd. (Republic of Korea). Deionized water (AH365-4) was purchased from SK Chemicals (Republic of Korea).

Synthesis of Zigzag-shaped Ag Nanoplates: To synthesize the Ag nanoplates, we typically first mixed 7mL of DMF with 1.87g of PVP and 10 mM cyanuric acid (1 mL of DMF) in a 20 mL vial (a liquid scintillation vial with a polyethylene liner and a white cap, Research Product International Corp.), into which 3 mL of a mixture of DMF and AgNO3 (282 mM) was rapidly added. The vial was placed in an oil bath at 100 °C with magnetic stirring for 24 h. The resulting mixture was subjected to centrifugation at 13,000 rpm for 10 min and was washed three times with DI water to remove any remaining DMF, PVP and cyanuric acid. Finally, the precipitate product was redispersed in DI water and stored in this form for later use in characterization studies.

Fabrication of Strain Sensors: A PDMS mixture (PDMS base and catalyst at a weight ratio of 12 : 1) that had been degassed in a vacuum for 10 min was poured onto a glass substrate with a convex line pattern, degassed again for five minutes to remove air bubbles, and then cured at a temperature of 60 °C for a duration of 20 minutes. The cured PDMS layer was then peeled off the


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glass substrate, and solutions of the Ag nanoplates synthesized in the current work were drop-cast on the linear channel formed in the PDMS substrate. The surfaces of the channel were selectively treated with O2 plasma before the deposition to make them hydrophilic. After the solution was dried, the PDMS films containing Ag nanoplate inks were heated at a temperature of 150 C for a duration of three minutes to sinter the nanoplates and transform them into a continuous metallic network. To form contact pads for electrical measurements, two pieces of conductive copper tape (specifically 3M EMI copper foil shielding tape 1181) were attached to, respectively, the two ends of the conductive pattern consisting of Ag nanoplates on the PDMS substrate. The contact between each piece of copper tape and the conductive pattern was strengthened by applying Ga/In eutectic conductive paste to their interface. A PDMS mixture was then deposited onto the conductive pattern and the sample was degassed and cured at a temperature of 60 C for a duration of 20 minutes. When tested on human subjects, the sensor was affixed with an elastomeric medical patch.

Characterizations: A transmission electron microscopy (TEM) analysis was performed using a Philips Tecnai F30 microscope operated at 300 kV. Samples were prepared by taking the final product, diluting it and placing a drop of the diluted material on a carbon-coated copper grid (Ted Pella, USA). In order to remove the remaining PVP, the grid was washed with DI water. X-ray diffraction (XRD) was recorded using a Philips 1820 diffractometer. AFM images were acquired using a Seiko SPA-400 with an SPI-3800 probe station. The sheet resistance of Ag nanoparticles on the PDMS was determined using a four-probe low-resistivity system (MCP-T610, Mitsubishi Chemical Analytech). The electrical resistance of each strain sensor was measured with a source meter (2400, Keithley). The infrared (IR) spectrum was acquired by using a Fourier transform infrared (JASCO 4100, Japan) equipped with an attenuated total-reflection diamond crystal


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accessory (ATR). Elemental mapping analysis was carried out by using an aberration-corrected scanning TEM (JEM-ARM200F, JEOL, Japan).

Supporting Information The Supporting Information file is available free of charge. (PDF) AFM images of zigzag-shaped Ag nanoplates; HR TEM images of the Ag nanoplates; TEM image of the product (needle-shaped particles) after 5 h of reaction; FT-IR spectrum of needle-shaped particles; Elemental mapping on a single needle-shaped particle; HR TEM images of Ag nanocrystals; SEM images of Ag nanoplate thin films; Gauge factor versus maximum stretchability plot, extracted from recently reported papers; Comparison of sensing performance between zigzag-shaped Ag nanoplates and Ag nanowires; SEM top view image of Ag nanoplate sensor under application of 10 % strain; SEM images of microstructures of the cracks formed on the Ag layers of the strain sensors; Schematic illustrations for percolation network through islands after stretching in Ag layer; Sensor response data; Selected parameters extracted from recently reported papers on strain sensors.

Corresponding Author *Corresponding author: M. H. Kim ([email protected]) (Tel.; +82-51-629-6459, Fax; +82-51-629-6429) O O. Park ([email protected]) (Tel.; +82-42-350-3923, Fax; +82-42-350-3910)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.


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Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2017R1E1A0107444) and KUSTAR-KAIST Institute, KAIST, Korea, the Principal Research Program (PNK5600) in the Korea Institute of Materials Science (KIMS). This work also was supported by the National Research Foundation of Korea (KRF) grants funded by the Ministry of Science,

ICT

&

Future

Planning

(MSIP)

of

Korea

under

contact

no.

NRF-

2015R1C1A1A02036649. The authors appreciate J.M. Do at SnM for assistance of the sensor characterization.

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ToC Figure


37

Zigzag-Shaped Silver Nanoplates: Synthesis via Ostwald Ripening

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