Synthesis of silver nanoparticles embedded with single-walled carbon nanotubes for printable elastic electrodes and sensors with high stability

Soft electronic devices that are bendable and stretchable require stretchable electric or electronic components. Nanostructured conducting materials or soft conducting polymers are one of the most promising fillers to achieve high performance and durability. Here, we report silver nanoparticles (AgNPs) embedded with single-walled carbon nanotubes (SWCNTs) synthesized in aqueous solutions at room temperature, using NaBH4 as a reducing agent in the presence of highly oxidized SWCNTs as efficient nucleation agents. Elastic composite films composed of the AgNPs-embedded SWCNTs, Ag flake, and polydimethylsiloxane are irradiated with radiation from a Xenon flash lamp within a time interval of one second for efficient sintering of conductive fillers. Under high irradiation energy, the stretchable electrodes are created with a maximum conductivity of 4,907 S cm−1 and a highly stretchable stability of over 10,000 cycles under a 20% strain. Moreover, under a low irradiation energy, strain sensors with a gauge factor of 76 under a 20% strain and 5.4 under a 5% strain are fabricated. For practical demonstration, the fabricated stretchable electrode and strain sensor are attached to a human finger for detecting the motions of the finger.


Methods
Materials. Single-walled carbon nanotube (SWCNT) powder (75% purity) synthesized using the chemical vapour deposition method was obtained from OCSiAl, Luxembourg. Fuming nitric acid was purchased from Tokyo Chemical Industry, and NaClO 3 , HCl and H 2 O 2 were purchased from Samchun Chemicals. AgNO 3 powder was obtained from Daejung. NaBH 4 powder was obtained from Sigma-Aldrich and used without any further treatment for the oxidation and synthesis of SWCNT and ox-SWCNT/Ag NP. To synthesize the elastic composite paste, Ag flake and PDMS (Sylgard 184) were purchased from Chang Sung Corp. and Dow Corning, respectively.

Synthesis of oxidized SWCNTs and ox-SWCNT dispersion.
The oxidation of SWCNTs was carried out using the kneading process. Before addition of a strong acid solution, 1 g of SWCNT powder and 7.5 g of sodium chlorate (NaClO 3 ) powder were mixed with a high speed mixer in order to distribute tiny NaClO 3 particles between the SWCNT networks. This promotes the dissolution of NaClO 3 in the acid resulting in an efficient oxidation of the SWCNTs. Then, 20 mL of fuming HNO 3 was poured slowly into the mixed powder while kneading with a Teflon spatula for a few minutes. The mixture was kept at room temperature for 1 h; 1 L of deionized water was then added to the mixture in order to dilute the acid. Subsequently, HCl and H 2 O 2 were added to remove metal ions and terminate the reaction. The resulting mixture was centrifuged to remove residual acid and oxidant molecules, resulting in an Ox-SWCNT paste. To fabricate the Ox-SWCNT dispersion in DI water with 500 mg L −1 , the horn sonicator was used without any dispersant molecules for 5 min.

Synthesis of the SWCNT-embedded AgNPs in the flask. The Ox-SWCNT dispersion was poured
while stirring into a 3-neck round bottom flask and stirred at room temperature. A 0.08 M AgNO 3 solution was prepared using DI water and stirred for 10 min at room temperature to dissolve the AgNO 3 powder. After stirring the AgNO 3 solution was injected into the SWCNT dispersion and stirred for 30 min. In addition, 0.6 M NaBH 4 solution was injected into the dispersion using a feeding pump at a rate of 1 mL min −1 . After the NaBH 4 solution had been totally injected, the solution was stirred overnight. To terminate the reaction, the mixture was washed with DI water using centrifugation.
Synthesis of SWCNT-embedded AgNPs in the Couette-Taylor reactor. The Ox-SWCNT dispersion (500 mg L −1 ) was put in a Couette-Taylor reactor and the inner cylinder rotation rate was set as 1000 rpm. A 0.2 M aqueous AgNO 3 solution was injected into the Couette-Taylor reactor and mixed with the SWCNT dispersion. After 10 min of mixing, the previously prepared 0.6 M NaBH 4 solution was injected into the reactor with a feeding pump at various rates. After injecting all the NaBH 4 solution, the reaction continued for a further 6 h. The resultant Ox-SWCNT/Ag hybrid materials were washed with DI water using centrifugation. This whole process was performed under ambient conditions. Synthesis of elastic composite film. Composite pastes were prepared by mixing the commercial Ag flake (2 µm, Chang Sung Corp.), SWCNT-embedded AgNPs, and PDMS. The contents of the conducting filler with respect to the PDMS elastomer (Sylgard 184, Dow Corning Corp.) was varied from 15 to 23 vol% in order to optimize the performance of the stretchable electrodes and sensors. The weight ratio of Ag flake and SWCNTembedded AgNPs was fixed at 1:0.6. Conducting fillers and as-prepared PDMS (mixing a PDMS rubber and surfactant with a weight ratio of 10:1) were mixed with a planetary mixer (ARE-310, Thinky) for 0.5 h at 2000 rpm. Prepared composite pastes were screen-printed on a PDMS substrate using a patterned screen mask (SUS 325 mesh) with an emulsion mask thickness of 10 µm, then followed by drying at 100 ℃ for 1 h. The embedding structure of SWCNT in AgNPs did not damaged during this mechanical fabrication process of stretchable paste as shown in Figure S7. IPL sintering of the elastic composite films was carried out using a Xenon flash lamp with a broad wavelength range from 300 to 900 nm. Films were irradiated with IPL energy density between 10 to 20 J cm −2 . All methods and experiments were carried out in accordance with the relevant guidelines and regulations. Informed consent was obtained from the participation in the experiment.
Characterization. The surface morphology of Ox-SWCNTs and AgNPs was observed using an atomic force microscope (AFM), (Nanoscope Multimode system, Veeco Instruments) and a field-emission scanning electron microscopy (FE-SEM, Hitachi S4800). A high-resolution transmission electron microscope (HR-TEM, Titan G2 60-300, FEI) was used to observe the formation of AgNPs on the surface of Ox-SWCNT bundles at an accelerating voltage of 80 kV. The chemical structure of Ox-SWCNTs was characterized using an X-ray photoelectron spectroscopy (XPS, K-alpha + system, Thermo Fisher Scientific) with mono-chromated Al K α X-ray radiation as the excitation source. The crystalline structure of SWCNT-embedded AgNPs and AgNPs was characterized www.nature.com/scientificreports/ using an X-ray diffraction (XRD, Philips PW 3830) with Cu, Kα radiation (λ = 1.5418 Å). The structural characteristics of Ox-SWCNTs and SWCNT-embedded AgNPs were identified using Raman spectrometry (NTEGRA SPECTRA, NT-MDT) with an excitation wavelength of 633 nm. The electrical conductivity of the elastic conducting films was measured using a four-point probe method (Loresta, MCP-T610). The electrical conductivity and resistance change for the stretchable electrodes and sensors were measured using a Keithely 2636B source meter connected to a custom-built automatic stretching tester. Before using humans as test subjects, informed consent from the human subjects was obtained appropriately in accordance with the guidelines of the research ethics in science and engineering approved by Korean Federation of Science and Technology Societies.

Results
To enhance the affinity between the Ag precursor and SWCNTs for the nucleation of AgNPs, the SWCNT surface must be highly oxidized thus maintaining the SWCNT dimension. Harsh oxidation conditions from using a strong acid and oxidant cause detrimental effects on the CNT structures resulting in unzipping and cutting 27,28 . Particularly, SWCNTs are vulnerable to be cut in harsh oxidation conditions because of the pyramidalization effect of sp 2 hybridized carbon atoms, which enhances the chemical reactivity of the side wall of the SWCNT. In this study, to counteract this concern, the SWCNTs were efficiently oxidized by using a chlorate-based oxidation method and applying the kneading process as previously reported 25 (Fig. 1a). As illustrated in Fig. 1b, pristine SWCNTs in the core still have their intrinsic structures and the outside SWCNTs are highly oxidized, makes them a nucleation site of AgNPs. Essentially, to enhance the oxidation efficiency, 1 g of the SWCNT and 10 g of NaClO 3 powder were mixed and grinded by a high speed blade mixer, prior to adding 50 mL of fuming HNO 3 for kneading ( Figure S1). The SWCNT/NaClO 3 /fuming HNO 3 mixture was then kneaded using a Teflon spatula for 10 min and finally kept at room temperature for 1 h. Reducing the particle size of NaClO 3 powder increases its solubility in fuming HNO 3 and results in uniformly distributed SWCNT networks, which contributes to a fast oxidation process, as quick as even taking 1 h to complete. The resultant Ox-SWCNT bundles have an average bundle size of 10 nm and a bundle length of more than 5 μm (Fig. 1c). The TEM image in Fig. 1d shows that the surfaces of the SWCNT bundles have limited crystalline structures because of the introduction of oxidative groups. The pristine SWCNTs exist at the core of the bundle as illustrated in Fig. 1b. The XPS spectrum in Fig. 1e shows that the SWCNTs were highly oxidized and consist of a C/O atomic ratio of 2.34; deconvolution of C1s peak shows that the SWCNTs were functionalized with mainly hydroxyl and carboxyl groups (Fig. 1f). This functionalization led to an increase in the D band (D/G intensity ratio = 0.89) of the Raman spectrum (Fig. 1g). The number of functional groups and the extent of the D band intensity can be controlled by varying the ratio of www.nature.com/scientificreports/ NaClO 3 : SWCNTs or by varying the duration of the oxidation process. The Ox-SWCNTs were easily dispersed in water by using a homogenizer without dispersant to synthesize AgNPs. The morphology of AgNPs can be affected by many parameters, such as type of reducing agent and its solvent, concentration, the feeding rate of reducing agents, temperature, and additives such as stabilizers or complexing agents. In this study, as shown in Fig. 2a, an aqueous solution of NaBH 4 was used as the reducing agent for the Ag nanostructure formation and Ox-SWCNTs were utilized as a nucleation template. Before feeding NaBH 4 to the aqueous Ox-SWCNT dispersion, 12.5 mL of AgNO 3 solution (0.08 M) was mixed with 62.5 ml of Ox-SWCNT dispersion (500 mg L −1 ) and then stirred for 30 min to enhance the interaction between the Ag + ions and the functional groups on the Ox-SWCNT surface, as shown in Fig. 2a. In the presence of Ox-SWCNTs, microscopic AgNPs can be nucleated on the surface. To verify the occurrence of this process, after 10 s of feeding NaBH 4 (0.48 M), the nanostructure of Ox-SWCNTs were characterized using high resolution TEM. As shown in Fig. 2b, numerous minuscule particles 2.5-3 nm were decorated on the Ox-SWCNT surface, which were identified as AgNPs from an EDAX analysis (Fig. 2c). The high resolution TEM image in Fig. 2d shows the typical d-spacing of the Ag (111) crystal measuring 2.36 Å. This result indicates that SWCNTs can be decorated with metal nanoparticles without modifying the SWCNT surface with ionic organic materials.
Since the concentration of AgNO 3 and Ox-SWCNTs in the solution is vital in controlling their structure, the crystallization of Ag in the presence of Ox-SWCNT was investigated by varying the concentration of AgNO 3 in different solutions. Figure 2e-h presents representative morphologies of the Ag nanostructures obtained from different AgNO 3 solutions with concentrations ranging from 80 to 20 mM. It is important to note that Ox-SWCNTs were embedded in the synthesized AgNPs like a pearl necklace, and the size of AgNPs gradually decreased from values above 500 nm to values below 100 nm. During synthesis of AgNPs, their shape can be controlled by using the stirring method, which influences the nucleation rate. To control the shear forces during the reduction of Ag + ions in the presence of Ox-SWCNTs, a Couette-Taylor (C-T) reactor was utilized. The C-T reactor has been widely used to synthesize nanoparticles owing to the C-T flow forming in the gap between two rotating cylinders, as presented in Fig. 3a. As expected, using the C-T flow reactor, the smaller AgNPs incorporated with the SWCNTs that were synthesized (Fig. 3b). Furthermore, a faster feed rate of the reducing agent generated smaller www.nature.com/scientificreports/ AgNPs (Fig. 3c,d). Notably, the Ox-SWCNTs were still embedded onto the AgNPs, which means that the oxidative functional groups on the SWCNTs work as nucleation sites even under high shear flow conditions. The X-ray diffraction (XRD) pattern of these SWCNT-embedded AgNPs demonstrates the co-existence of SWCNTs and AgNPs (Fig. 3e). The SWCNT bundle produced a broad peak at 26.2°, which corresponds to a typical intertube spacing of 3. To elucidate the crystalline structure of Ox-SWCNTs after synthesis of AgNPs, Raman spectroscopy was employed using laser at a wavelength of 633 nm; the laser exposure time was 5 s. The Raman spectra of pristineand Ox-SWCNTs was compared with that of the SWCNT-embedded AgNPs as shown in Fig. 3f. The intensity of Raman scattering of pristine SWCNTs was depressed after oxidation due to the reduction of the resonance enhancement effect of the Ox-SWCNTs 30 . However, for the SWCNT-embedded AgNPs, the Raman scattering intensities for the D and G band of the SWCNTs are drastically amplified compared with that of the Ox-SWCNTs. This means that the synthesized SWCNT-embedded AgNPs can play as a surface enhanced Raman scattering (SERS) substrate. On the nanostructured Ag surface, laser excitation of Ag nanostructures, resonantly drives www.nature.com/scientificreports/ the surface charges, creating a highly localized plasmonic light field. Notably, the typical disorder-induced D band at 1343 cm −1 is more enhanced than the G band at 1597 cm −1 for the SWCNT-embedded AgNPs, and the G band shifted to 1594 cm −1 by dedoping with Ag. D/G intensity ratios of Ox-SWCNTs and SWCNT-embedded AgNPs were 0.89 and 1.3, respectively. The D band of the SWCNT is the double resonance intervalley process of elastic scattering caused by a defect of the SWCNT, while the G band originates from the first order resonance Raman scattering process 30 . This result indicates that the double resonance Raman scattering is more sensitive to the SERS substrate than the first order Raman process. Moreover, this result indicates that during the chemical reduction of Ag + ions by NaBH 4 , Ox-SWCNTs were not chemically reduced, implying that most of the oxidative functional groups interact with Ag + ions before chemical reduction. Without Ag + ions, the Ox-SWCNTs were reduced by deoxygenation using NaBH 4 at room temperature ( Figure S2). In order to investigate the electrical properties under mechanical deformation conditions, composite pastes comprising Ag flakes, SWCNT-embedded AgNPs, and polydimethylsiloxane (PDMS) pre-polymer were screenprinted on the PDMS substrate. Ag flakes have been solely used as a stretchable conducting filler due to their large contact area 31 ; their hybridization with AgNPs or carbon nanotubes could enhance electrical and stretchable properties 4,14 . Figure 4a shows intrinsic electrical conductivities of the composite films with varying conducting filler contents, without any mechanical strains. As expected, the electrical conductivity gradually increased as the filler contents increased, reaching a conductivity of 472 S cm −1 at 23 vol%, which is relatively low compared to previously reported literature 4,14 . This is probably because of the limited electrical contact area between the conducting fillers. As shown in Fig. 2b-d of SEM images for the as-synthesized SWCNT-embedded AgNPs, www.nature.com/scientificreports/ most of the AgNPs are spherical shaped and connected through point-to-point contact. The electrical failure was observed at high filler contents of more than 23 vol% because of the cracks or tears in the composite films.
To increase the contact area and thus enhance the electrical conductivity of the composite films, IPL sintering using a Xenon flash lamp with a broad wavelength spectrum was employed. IPL-induced heating of conductive nanomaterials has recently attracted a lot of interest in the field of flexible and stretchable applications, because it allows for the ultrafast heating of nanomaterials without damaging or deforming polymer substrates 32,33 .
Conductive nanomaterials can play a significant role as an efficient and immediate source of heat, because the heating is considerably related to the localized surface plasmon resonance effect which resulted from the coupling of surface plasmons of nanomaterials with light. Thus, when the filler contents and sintering energy were increased, the electrical conductivity also increased, reaching a maximum value of 4907 S cm −1 at 20 J cm −2 for a 23 vol% sample, which is 10.4 times greater than its initial value shown in Fig. 4a. High sintering energy of more than 20 J cm −2 gave rise to extremely high thermal energy in the sample, resulting in the severe damage of the PDMS elastomer.
To investigate the stretchable properties of the composite films, the electrical conductivity was measured under a mechanical strain ranging from 0 to 100%. Figure 4b shows the stretchability at 1% change of the electrical conductivity with respect to the filler contents and sintering energy. In the case of low filler contents (15 and 17 vol%) and sintering energy (10-16 J cm −2 ), the stretchability was less than 20%. This low stretchability could be attributed to the relatively low conducting path upon strain which in turn below percolation threshold. However, high stretchabilities of more than 20% were measured at sintering energies of more than 18 J cm −2 in the case of filler contents (19 and 21) vol%. This could be confirmed with well-distributed and highly dense conducting fillers over entire composite films as shown in Figure S3. SWCNT-embedded AgNPs might play a key role for this high stretchability, as shown in the scheme of Fig. 4c. During the IPL sintering process, AgNPs synthesized on SWCNTs were connected to the Ag flakes as well as the neighbouring NPs; this resulted in the formation of an electron-transfer path. When mechanical strain is applied to the sample, the SWCNTs, as the efficient templates for the sintered NPs, can preserve the electron path due to their high durability upon stretching. A repeatability test (20% stretching) of up to 10,000 cycles for the composite film of 21 vol% filler contents and 18 J cm −2 IPL sintering shows a negligible change in the electrical conductivity (Fig. 4d), which is superior to previously reported results of Ag flake/AgNPs/PDMS and Ag/MWCNT/PDMS 4,14 . Similar results were observed for the composite film applied at higher mechanical strains ( Figure S4). This high durable stretchability was confirmed by the absence of flickering or brightness decrease in the LED light connected with the composite film during the stretching test, as shown on the inset of Fig. 4d and Video S1. For a comparison, we carried out the repeatability test for the composite films composed individually of Ag flake and PDMS as well as Ag flake, AgNPs, SWCNT and PDMS. Even though the initial electrical properties were very similar to that of the AgNPsembedded SWCNT, after IPL sintering, the resistivity was gradually increased as the number cycles increased; electrical failure was observed at 80 and 1,000 cycles, respectively, as shown in Figure S5a and Figure S5b. The SWCNTs could not hold and support the sintered AgNPs, so that the cracks were formed on the surface of the composite film upon stretching as shown on the inset of Figure S5b and Figure S5c.
In addition to its application in a stretchable electrode, the SWCNT-embedded AgNPs can also be used as an active material for a strain sensor. Figure 5a shows the electromechanical properties of the 15 vol% composite film sintered at various IPL energies ranging from 10 to 14 J cm −2 . In the case of the composite film sintered at 14 J cm −2 , the conductivity-strain slope was relatively small, even though the initial electrical conductivity was higher than for other sintering energies. Therefore, the gauge factor (GF) values were measured and found to be below 10, as shown in Fig. 5b. It is known that the sensitivity of a strain sensor can be evaluated using the GF; the equation to obtain GF values is shown below.
∆R/R 0 is the normalized change in the electrical resistance, and ε is the mechanical strain. In contrast, the electrical conductivity of the composite film sintered at 10 J cm −2 was significantly decreased when subjected to mechanical strain. The GF value was calculated to be 76 for a mechanical strain of 20%; this value is much higher than the conductor/elastomer-based stretchable strain sensors reported in previous literature [33][34][35] . This high sensitivity could be attributed to the embedded structure of the SWCNT inside AgNPs, which provides a long-range conductive path without suffering from the high contact resistance between junctions of the AgNPs.
For the feasibility of the stretchable devices in human motion detection, we fabricated the stretchable electrode and strain sensor in the same PDMS substrate as discussed earlier and attached it to a human finger as shown in Fig. 5c. Stretchable electrodes and strain sensors were prepared using 18 and 10 J cm −2 IPL sintering of a composite film with 21 and 15 vol% filler contents, respectively. Both ends were connected to conventional copper wires in order to measure the relative resistance change. Figure 5d shows the relative resistance change of the stretchable electrode and strain sensor when the finger is bent with a strain of 5% and 20%. The resistivity of strain sensor was significantly increased when the bending increased, but the stretchable electrode did not change as expected. The reproducible resistivity change for 5-time bending and releasing cycles indicated that the composite film could be used as a strain sensor with high mechanical stability. This behaviour was also confirmed by long-term durability results up to 500 cycles ( Figure S6) and the flickering of the LED lights connected to the strain sensor in accordance with finger gestures (Video S2).

Conclusion
CNT-embedded metal nanostructures are promising for realization of highly stable and stretchable electrode for soft electronics. In this study, silver nanostructures were systematically modulated by using SWCNTs that were highly oxidized by chlorate-based oxidation. It was demonstrated that this approach allows SWCNT-embedded Ag nanostructures to be synthesized in the presence of Ox-SWCNTs in environmentally friendly aqueous solutions at low temperature without any other additives. The SWCNT-embedded AgNPs as an efficient conductive filler were subsequently used to create highly conductive (4907 S cm −1 ) and highly stable and stretchable PDMS composite electrodes at over 10,000 cycles under a 20% strain. Strain sensors with a gauge factor of 76 under a 20% strain and 5.4 under a 5% strain are also achieved by rational irradiation of a Xenon flash lamp. It is hoped this approach will create new opportunities for development of conformable electrodes and sensors in soft electronics or wearable electronics. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.