Enhanced Electrical Networks of Stretchable Conductors with Small

Jan 19, 2016 - Enhanced Electrical Networks of Stretchable Conductors with Small Fraction of Carbon Nanotube/Graphene Hybrid Fillers. Jae Young Oh†,...
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Enhanced Electrical Networks of Stretchable Conductors with Small Fraction of Carbon Nanotube/Graphene Hybrid Fillers Jae Young Oh,† Gwang Hoon Jun,† Sunghwan Jin,§ Ho Jin Ryu,*,‡ and Soon Hyung Hong*,† †

Department of Material Science and Engineering and ‡Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea § IBS Center for Multidimensional Carbon Materials (CMCM), Ulsan National Institute of Science and Technology (UNIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: Carbon nanotubes (CNTs) and graphene are known to be good conductive fillers due to their favorable electrical properties and high aspect ratios and have been investigated for application as stretchable composite conductors. A stretchable conducting nanocomposite should have a small fraction of conductive filler material to maintain stretchability. Here we demonstrate enhanced electrical networks of nanocomposites via the use of a CNT−graphene hybrid system using a small mass fraction of conductive filler. The CNT−graphene hybrid system exhibits synergistic effects that prevent agglomeration of CNTs and graphene restacking and reduce contact resistance by formation of 1D(CNT)− 2D(graphene) interconnection. These effects resulted in nanocomposite materials formed of multiwalled carbon nanotubes (MWCNTs), thermally reduced graphene (TRG), and polydimethylsiloxane (PDMS), which had a higher electrical conductivity compared with MWCNT/PDMS or TRG/PDMS nanocomposites until specific fraction that is sufficient to form electrical network among conductive fillers. These nanocomposite materials maintained their electrical conductivity when 60% strained. KEYWORDS: carbon nanotubes, graphene, hybrid materials, nanocomposite, stretchable conductor

1. INTRODUCTION Electrical devices are typically rigid, are often heavy, and can be damaged by physical impact. For these reasons, there has been much recent research interest in stretchable conductors, which maintain their conductivity following physical deformation.1 These properties can enable electronic devices including wearable devices,2 flexible organic light-emitting diodes (OLEDs),3 stretchable energy storage devices,4 and portable digital information devices.5 However, it is difficult to satisfy the required mechanical and electrical properties simultaneously.6 Many previous studies have attempted to overcome the limitations of stretchable conductors. The methods used in these studies can be classified into two types.6 The first involves structural development of inorganic conductive materials;7−9 the second involves mixing a conductive filler with an elastomer.6,10,11 The former has limits in terms of the anisotropic stretchable properties, and the fabrication process required for flexible inorganic materials is typically complex.12 The latter provides isotropically stretchable materials, and the fabrication processes are simpler. This method can be classified into two categories: techniques that create patterned stretchable conductors consisting of patterned conductive material on a stretchable material,11,13,14 and techniques that create composites of conductive fillers with a stretchable matrix.10,15,16 Stretchable conductors formed of composites typically have a greater variety of potential applications, as the © XXXX American Chemical Society

stretchable electrodes are fully conductive, unlike patterned stretchable conductors.11,13,14 Carbon-based nanomaterials, including carbon nanotubes (CNTs) and graphene, have particularly promising properties as conductive fillers due to the high electron mobility (∼10 000 cm2 V−1 s−1) and high aspect ratio (>1000).8,17 However, to date, stretchable conductors formed of CNTs and graphene via solution mixing processes have demonstrated poor electrical performance.15 One of the main challenges in the solution mixing of nanocomposites formed using CNTs and graphene is preventing agglomeration or restacking. Most studies have focused on improving the dispersibility of the conductive filler via functionalization of the surface of the CNTs or of the graphene. Chua et al. reported enhanced dispersibility via the use of the diphenyl-carbinol and silanized diphenyl-carbinol functionalization of multiwalled carbon nanotubes (MWCNTs).15 However, nanocomposites consisting of functionalized MWCNTs have a lower conductivity than those formed of pristine MWCNTs due to degradation of the electrical properties of MWCNTs following covalent functionalization. Noncovalent functionalization, however, maintains the electrical properties of the conductive fillers. Hwang et al. Received: November 19, 2015 Accepted: January 19, 2016

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DOI: 10.1021/acsami.5b11205 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic illustration of interaction between CNTs and graphenes. (b) CNT/graphene/PDMS nanocomposites fabrication. then filtered and washed using 10% HCl solution and deionized water. The filtered GO was dried under a vacuum for 4−5 days. The steps for the fabrication of TRG from GO were as follows. First, GO was heated to 900 °C in an N2 atmosphere at a rate of 20 °C/min, which caused it to undergo volume expansion and thermal reduction. The temperature was maintained at 900 °C for 1 h; during the first 30 min, an N2 atmosphere was used, whereas an H2 atmosphere was used for the final 30 min, which induced the chemical reduction of GO. 2.2. Fabrication of MWCNT/PDMS, TRG/PDMS, and MWCNT/ TRG/PDMS Nanocomposites. Nanocomposites formed of PDMS, MWCNTs, and TRG were fabricated via solution mixing. MWCNT or TRG was dispersed in tetrahydrofuran (THF) via sonication for 2 h. The PDMS was dissolved in THF by stirring, and then the MWCNT (or TRG) and the PDMS solutions were mixed. Solution mixing and removal of the solvent proceeded via heat treatment at 70 °C with stirring. To remove the residual solvent from the solution, an additional heat treatment step was carried out at 90 °C for 3 h under a vacuum. A curing agent was mixed with the solution containing PDMS, MWCNT, or TRG with a PDMS/curing agent mass ratio of 10:1. Nanocomposites were also formed with MWCNT/TRG mass ratios of 7:3 and 9:1. The mixtures were poured into dog-bone shaped molds to fabricate samples for electrical conductivity and strain measurements. The curing proceeded at 90 °C for 3 h. 2.3. Microstructural Characterization. The microstructures of the MWCNT, TRG, and MWCNT−TRG samples were observed using scanning electron microscopy (SEM) (Hitachi S-4800), atomic force microscopy (AFM) (SPI 3800N, Seiko), and transmission electron microscopy (TEM) (Tecnai TF30 ST). Samples were prepared via solvent dispersion and spin-coated onto Si wafers to be observed using SEM, and the samples for TEM characterization were drop-dried onto copper grids. The fractured surfaces of the nanocomposites were also observed using SEM following coating with osmium. 2.4. Measurement of the BET Surface Area of the Conductive Fillers. The BET surface area of the MWCNT, TRG, and hybrid MWCNT−TRG (1:1) samples was characterized by measuring N2 adsorption. Variations in surface area due to the interaction between the MWCNTs and TRG were examined by dispersing the samples in THF via sonication and heat-drying to evaporate the solvent. 2.5. Electrical Conductivity Measurements. Electrical conductivity was measured using a four-point probe (Loresta-P MCP-

reported a lower percolation threshold via noncovalent functionalization of MWCNTs using poly(3-hexylthiophene).16 A critical issue when fabricating such nanocomposites is the electrical properties of the functionalizing materials. Most functionalizing materials that result in enhanced dispersibility of the conductive fillers in elastomers are insulators, so the contact resistance between the individual conductive filler particles is large. Accordingly, a key challenge when using CNTs and graphene in stretchable conductors is identifying and incorporating conductive functionalizing materials using a noncovalent functionalization processes. Hybrid CNT−graphene materials are bonded by π−π interactions, which induce functionalization due to the differences in geometry between the graphene and the CNTs. The most important advantage of hybrid CNT−graphene materials is that they offer functionalization without requiring an additional insulating material. Several studies have reported enhanced electrical properties through hybrid CNT−graphene materials. Tang et al. reported that the electrical conductivity of hybrid CNT−graphene films increased due to a decrease in the contact resistance as well as the formation of an efficient percolating network.18 However, no previous studies have focused on fabricating stretchable conductors via solution mixing of CNT−graphene hybrid fillers with an elastomer matrix. Here we describe stretchable nanocomposites formed by solution mixing of hybrid CNT−graphene conductive fillers with polydimethylsiloxane (PDMS) and present the enhanced electrical networks of the resulting materials.

2. EXPERIMENTAL SECTION 2.1. Fabrication of TRG. Thermally reduced graphene (TRG) has a moderate electrical percolation threshold due to less restacking of graphene sheets by its wrinkled structure and the high Brunauer− Emmett−Teller (BET) surface area.19 TRG was fabricated via thermal expansion and reduction of graphene oxide (GO), which was fabricated using the modified Hummers method. Highly ordered pyrolytic graphite (HOPG) was dissolved in H2SO4 and stirred in an ice bath. To oxidize the HOPG, KMnO4 was slowly added to the solution. After stirring at 35 °C for 2 h, H2O2 was added dropwise to the solution, which was maintained in the ice bath. The solution was B

DOI: 10.1021/acsami.5b11205 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Microstructure of MWCNTs by SEM (inset: photograph of MWCNTs). (b) Microstructure of TRG by SEM (inset: photograph of TRG) and structural information on TRG by AFM. (c, d) Interaction of hybrid MWCNT/TRG (1:1). (e) BET surface area of MWCNT, TRG, hybrid MWCNT/TRG. T610), and strain was measured using a universal testing machine (Instron 8848).

ratio (∼320) than that of the TRGs (∼150), and it means that the MWCNTs have structural advantages to form electrical network compared to the TRGs.23 However, high aspect ratio of the MWCNTs induces agglomeration of the MWCNTs by strong van der Waals forces, so a bundled structure was typically observed. When these MWCNTs are used as conductive fillers of nanocomposites, this agglomeration degrades the electrical networks of the nanocomposites.24 To reduce electrical networks degradation by agglomeration of conductive fillers, TRGs were used as hybrid fillers in our research. In the MWCNT/TRG nanocomposites, the TRG was bonded onto the MWCNTs via π−π interactions, which induced steric hindrance and prevented the bundling of MWCNTs.21 The interactions between MWCNTs and TRGs were observed using SEM and TEM analyses of the hybrid MWCNT/TRG materials, as shown in Figure 2, panels c and d. MWCNTs were located between the TRG sheets, which inhibits restacking of the TRG sheets. The BET surface areas of the MWCNTs, TRG, and hybrid MWCNT/TRG (1:1) were characterized to confirm effects through the interaction between MWCNTs and TRGs, as shown in Figure 2, panel e. We found that the hybrid MWCNT/TRG material had a large BET surface area of 481.3 m2/g, which was greater than that of the MWCNTs (165.8 m2/ g) or the TRGs (328.7 m2/g). The increase in the surface area of the hybrid MWCNT/TRG nanocomposite is attributed to the inhibition of restacking of TRG and agglomeration of MWCNTs. Some researchers reported a relationship between the increase of BET surface area of expanded graphite and the enhancement of electrical conductivity of nanocomposites comprising these graphites.25 In this research, effects of surface area increase by hybrid MWCNT/TRG materials were confirmed through various analyses. Additionally, electrical contact between the constituent particles of the fillers was enhanced by introducing the combination of TRG and MWCNTs. Interconnections between MWCNTs (a 1D material) and TRG (a 2D material) in the hybrid MWCNT/TRG nanocomposite material yielded

3. RESULTS AND DISCUSSION Figure 1, panel a presents schematic illustrations of the interactions in the hybrid CNT−graphene system, and Figure 1, panel b shows the fabrication of the CNT/graphene/PDMS nanocomposite through CNT/graphene interaction. CNT and graphene exhibited strong cohesiveness because of the strong van der Waals forces due to the large surface-area-to-mass ratio.20 In single-component dispersions of CNTs or graphene, this leads to agglomeration and restacking, respectively. However, the coexistence of CNTs and graphene inhibits these interactions due to increased steric hindrance via the π−π interactions of CNTs and graphene sheets, and instead CNTs are inserted between graphene sheets.21 This leads to an enhanced electrical network of the conductive filler in the matrix. Another effect of using the hybrid CNT−graphene system is a reduction in the contact resistance. As shown in Figure 1, panel a, when both CNTs and graphene are used as fillers, 1D−2D interconnections are formed, with a large contact area, thus decreasing the electrical resistivity.22 The relationship between CNT/TRG mass ratio and the electrical properties is not clear, however, and it is necessary to investigate the optimal mass ratio to provide optimized electrical properties of the stretchable conductors. Figure 2, panel a presents SEM images of the MWCNTs, and Figure 2, panel b presents SEM images and AFM images of the TRGs. Samples were dissolved in dimethylformamide (DMF) and spin-coated onto Si wafers and dried. MWCNT diameters ranged from 15−20 nm, and lengths are around 5 μm; thus, it has an aspect ratio of around 320. TRGs have wrinkled and exfoliated structures due to thermal expansion. The SEM image of Figure 2, panel b showed stacked TRGs. To confirm accurate structure information on each the TRG, TRGs were analyzed by AFM after solution exfoliation process by sonication and spin coating them onto mica. TRGs have structures of around 4 nm of thickness, 600 nm of lateral size, and thus approximately 150 aspect ratio. The MWCNTs have relatively higher aspect C

DOI: 10.1021/acsami.5b11205 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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than that of the TRG/PDMS nanocomposites at all filler fractions. The electrical conductivity graph of MWCNT/PDMS nanocomposites is divided into two areas as shape of curve in Figure 4, panel a. Area 1 exhibits rapid increase of the electrical conductivity, while gradual increase of electrical conductivity is observed in the area 2. This rapid increase of electrical conductivity in the area 1 is due to weak electrical network by small fraction of conductive filler.27 The two areas showed different variation of electrical conductivity with addition of TRG (see Figure 4b,c). In area 1, 0.4 wt %, 0.6 wt % of MWCNT/TRG nanocomposites show increase of electrical conductivity at some ratios MWCNTs to TRGs unlike the decrease of electrical conductivity with addition of TRG in area 2. These results are different from general predictions. The electrical conductivity (σ) of nanocomposites comprising conductive fillers is predicted by scaling law of the following formula:28

improved electrical contact compared with nanocomposites formed using only MWCNTs. The inclusion of TRG provided a large contact area with the MWCNTs and resulted in an increase in the number of contact sites among the conductive filler particles, and hence a low electrical resistivity.22 MWCNT/TRG/PDMS nanocomposites were fabricated using these MWCNT/TRG hybrid fillers. To confirm dispersion of MWCNT/TRG hybrid fillers and interaction among these conductive fillers in PDMS matrix, same fractions (0.4 wt %) of MWCNT/TRG/PDMS and MWCNT/PDMS nanocomposites were fabricated, and then their microstructures were observed (see Figure 3). MWCNT/TRG/PDMS nano-

σ = σ0 × (Φ − Φc)t

By this formula, the electrical conductivity of nanocomposites is proportional to that of conductive fillers (σ0). As rule of mixture, electrical conductivities of MWCNT/TRG hybrid fillers are lower than single MWCNT fillers due to lower electrical conductivity of TRGs (∼3 S/cm) than that of MWCNTs (∼10 S/cm). Thus, it is expected that the electrical conductivity of MWCNT/TRG/PDMS nanocomposites is lower compared to that of MWCNT/PDMS nanocomposites (see Figure 4d). However, experimental electrical conductivity of 9:1 (MWNCT/TRG) has higher values than that of MWCNT/PDMS nanocomposites at low fraction of conductive fillers (0.4 wt %, 0.6 wt %). These results were caused by reduction of Φc (filler’s fraction to electrical percolation threshold) according to formula. We confirmed reasons for such results by observation of microstructures in MWCNT/ PDMS and MWCNT/TRG/PDMS nanocomposites. Although TRG, which has lower electrical and structural properties (intrinsic conductivity, aspect ratio) compared to those of MWCNT, showed the dispersibility enhancement of conductive filler with formation of 1D−2D interconnection enhanced electrical network among conductive fillers as addition of TRG. These synergistic effects to enhance electrical networks by MWCNT/TRG hybrid fillers are more dominant than intrinsic properties of conductive fillers at some ratios MWCNT to TRG in area 1 comprising small fraction of conductive fillers. In the area, the electrical conductivity curves exhibit different shapes with changing fraction of fillers. Electrical conductivity of 7:3 and 9:1 (MWCNT/TRG)/ PDMS nanocomposites (2.78 × 10−5 S/cm (7:3), 6.54 × 10−5 S/cm (9:1)) is higher in MWCNT/PDMS nanocomposite (1.88 × 10−5 S/cm) with a filler fraction of 0.4 wt %, while electrical conductivity of only 9:1 of (MWCNT/ TRG)/PDMS nanocomposite (6.17 × 10−3 S/cm) is higher than that of MWCNT/PDMS nanocomposite with a filler fraction of 0.6 wt % (1.85 × 10−3 S/cm) (see Figure 4c). These results prove that synergistic effects for improving the electrical network among conductive fillers strengthen as the fraction of fillers reduces. However, the electrical conductivity of MWCNT/TRG/ PDMS nanocomposites did not continuously increase as the ratio of TRG increased. We found that there were optimal TRG/MWCNT mass ratios in our research. As the TRG/ MWCNT mass ratio increased beyond 1:9, the electrical

Figure 3. (a) Microstructure of MWCNT/TRG/PDMS and MWCNT/PDMS nanocomposites. (b) Microstructure of MWCNT (1D)/TRG (2D) interconnection in PDMS matrix.

composites exhibited homogeneous dispersion of MWCNT/ TRG hybrid fillers at their fractures, while relatively much agglomeration of fillers existed at the fractures of MWNCT/ PDMS nanocomposites (see Figure 3a). Additionally, it was observed that 1D−2D interconnections between MWCNTs and TRGs were formed and maintained in the PDMS matrix (see Figure 3b). These dispersibility enhancements of conductivity fillers and formation of 1D−2D interconnection can be attributed to increase the electrical conductivity and reduction of the electrical percolation threshold of nanocomposites. To confirm how these contributions affect electrical properties of nanocomposites, we analyzed changes of electrical properties by controlling fraction of conductive fillers in the matrix and mass ratio MWCNTs to TRGs. Figure 4, panel a shows the electrical conductivity of the MWCNT/PDMS and TRG/PDMS nanocomposites with various filler fractions. The TRG samples typically exhibited less electrical conductivity than the theoretical electrical conductivity of graphene, which can be attributed to residual oxide and defects that formed during the oxidation and reduction processes.26 We found that the TRG films had lower electrical conductivity (∼3 S/cm) compared to that of the MWCNTs films (∼10 S/cm) (see Figure S3) as well as aspect ratio of TRG (∼150) was lower than that of MWCNTs (∼320). Because of the difference of electrical and structural properties between MWCNTs and TRGs, the electrical conductivity of MWCNT/PDMS nanocomposites was higher D

DOI: 10.1021/acsami.5b11205 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Electrical conductivity of MWCNT/PDMS and TRG/PDMS nanocomposites. (b) Electrical conductivity of MWCNT/PDMS, MWCNT/TRG/PDMS nanocomposites with changing filler’s fraction. (c) Electrical conductivity of MWCNT/PDMS, MWCNT/TRG/PDMS nanocomposites with changing ratio MWCNTs to TRGs. (d) Comparison of electrical conductivity in MWCNT/PDMS and MWCNT(9)/ TRG(1)/PDMS nanocomposites (expected electrical conductivity and experimental electrical conductivity).

the help of TRG, although the electrical resistance rapidly increased to out of measurement range owing to loss of electrical networks among fillers after 60% strain. This stability in the electrical properties in response to strain demonstrates that MWCNT/TRG/PDMS nanocomposites can be used as stretchable conductors. Furthermore, these stretchable conductors were fabricated using a simple solution process involving only sonication and stirring, in contrast to the complex chemical processing that is required with covalently functionalized nanocomposite materials.15,16

conductivity of the nanocomposite decreased (see Figure 4c). These results mean that when TRG ratio is above specific values, electrical degradation by lower conductivity, and aspect ratio of the TRG than that of MWCNT, is more dominant than enhancement of electrical networks by addition of TRG. Stretchable conductors were fabricated using MWCNT/ TRG/PDMS nanocomposites with various mass fractions of the conductive fillers for strain and conductivity tests, as shown in Figure 5, panel a. As the filler fraction increased, the electrical conductivity of the nanocomposites also increased; however, the stretchability decreased, as shown in Figure 5, panel b. To develop enhanced stretchable conductors, it is necessary to show enhanced and stable electrical conductivity of the conductor with strain when the mass fraction of conductive filler is small. As shown in Figure 5, panel c, electrical enhanced MWNCT/TRG/PDMS nanocomposites showed stable electrical conductivity with strain. The electrical conductivity of the 0.4-wt % 9:1 (MWCNT/TRG)/PDMS nanocomposites was maintained in the range of 10−5−10−4 S/cm when a strain of 60% was applied, and the conductivity of the 0.6-wt % 9:1 (MWCNT/TRG)/PDMS and 1-wt % 9:1 (MWCNT/TRG)/ PDMS nanocomposites remained in the range of 10−3−10−2 S/ cm (0.6-wt %) and 1−2 × 10−2 S/cm (1-wt %) when a strain of 60% was applied. The stress was caused by tensile strain affects arrangement and networks of fillers by being transferred from the matrix to fillers.29−31 The transferred stress induces the alignment variation of fillers along the strain direction without damage of the electrical network under a low strain range; after then, contact losses among fillers occur under a high strain range.30 Our stretchable conductors containing a small fraction of MWCNT/TRG hybrid fillers show maintenance of stable electrical properties until 60% strain mainly due to the high aspect ratio of MWCNT and its homogeneous dispersion with

4. CONCLUSIONS CNTs and graphene are attractive as conductive fillers for stretchable conductors because of their high electrical conductivity and high aspect ratio. However, agglomeration of CNTs and restacking of graphene sheets present challenges for the fabrication of stretchable composite conductors. We fabricated and characterized stretchable composite materials using CNT−graphene hybrid fillers and a PDMS matrix. The synergistic effects of 1D−2D interconnections inhibit restacking and agglomeration, as confirmed by observations of the interconnected CNT−graphene fillers with the PDMS matrix. The electrical conductivity of nanocomposites formed of MWCNT/PDMS, TRG/PDMS, and MWCNT/TRG/PDMS were investigated with various fractions of filler. We observed enhanced electrical networks and conductivity of the MWCNT/TRG/PDMS nanocomposite compared with the MWCNT/PDMS and TRG/PDMS nanocomposites at small fraction of conductive fillers, which is insufficient to form electrical networks among conductive fillers. However, in nanocomposites that contain high fraction of conductive fillers, intrinsic properties of conductive fillers are more important to E

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the KUSTAR-KAIST Institute, KAIST, Korea, the ICT R&D program of MSIP/IITP, [B010115-0239, Human Friendly Devices (Skin Patch, Multimodal Surface) and Device Social Framework Technology], a grant (10037689) from the Fundamental R&D Program for Technology of World Premier Materials (WPM) funded by the Ministry of Knowledge Economy (MKE, Korea), and the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (2013M3A6A5073173).

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Figure 5. (a) MWCNT/TRG/PDMS composites. (b) Strain at break and electrical conductivities of MWCNT(9)/TRG(1)/PDMS composites. (c) Electrical conductivity variation of 0.4 wt %, 0.6 wt % MWCNT/TRG/PDMS composites during stretching.

enhance electrical conductivity of nanocomposites than these synergistic effects that enhance electrical networks of nanocomposites. Additionally, nanocomposites that contain low fraction of conductive fillers retained their electrical conductivity in the range of 10−5−10−4 S/cm (0.4 wt %) and 10−3−10−2 S/cm (0.6 wt %) when strained up to 60%, which demonstrates their potential for applications as stretchable conductors.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11205. Structural information on MWCNT, XRD data of GO and TRG, and electrical conductivity of MWCNT and TRG films (PDF) F

DOI: 10.1021/acsami.5b11205 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b11205 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX