S-1. Dramatic enhancement of graphene oxide/silk nanocomposite membranes: increasing toughness and strength via annealing of interfacial structures

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1 S-1 Supporting Information Dramatic enhancement of graphene oxide/silk nanocomposite membranes: increasing toughness and strength via annealing of interfacial structures Yaxian Wang, a,b Ruilong Ma, b Kesong Hu, b Sunghan Kim, b Guangqiang Fang, a Zhengzhong Shao, a * Vladimir. V. Tsukruk b * a State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai, , People s Republic of China b School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia Corresponding Author 0245, USA * vladimir@mse.gatech.edu; * zzshao@fudan.edu.cn

2 S-2 The GO concentration in the nanomembrane was calculated by elliposometry (J.A. Woollam). The thickness of SiO 2 layer on silicon wafer was measured beforehand. Then SF solution and GO suspension were alternatively spun on silicon wafer under different concentration (3000 rpm, 30 s). After each deposition of the material, the thickness of the nanomembrane was measured immediately. The light signals at three progressive angles (65, 70 and 75 ) were detected and Cauchy model was used to fit the experimental data. Figure S1. Photograph of GO dispersion in MeOH after 1 month at room temperature Figure S2. The thickness of the nanomembranes under different SF and GO concentrations

3 S-3 By measuring the thickness increase after each deposition, we could tell the volume fraction of GO/SF in the nanomembrane. The final GO concentration was calculated as the average of five bilayers. The data was shown in Figure S2, the concentration of SF and GO were 0.2% and 0.05% (Figure S2, A), 0.1% and 0.05% (Figure S2, B), 0.075% and 0.075% (Figure S2, C), 0.05% and 0.075%(twice) (Figure S2, D), 0.05% and 0.1%(twice) (Figure S2, E), respectively. Bulging Test: Bulging test is widely adapted in engineering mechanical studies for ultrathin films where traditional tensile tests aren t suitable due to the low thickness. It s capable of testing stressstrain curve and thus the tensile Young s modulus, ultimate stress/strain and toughness can be obtained. Detailed experimental setup and data analysis can be referred from our previous publications. 1-3 Briefly, the nanomembranes were caught by a custom made TEM grid with a 300 μm circular aperture in the center. After dried overnight in a vacuum oven at room temperature, the samples were mounted on the homemade interference bulging setup, as shown in Figure S3. 4 It contains two major parts. Figure S3. Scheme of the custom-built interferometry set-up. 4

4 S-4 1. Single-wavelength interferometer was used to monitor the interference pattern and then get the vertical deflection of the apex of the membrane as it was bulged. A beam of nm He-Ne laser was guided and reflected on the nanomembrane and a reference glass slide. The brightness of the reference point in the center of the membrane underwent bright-dark-bright cycles due to the increase of the distance between the membrane and the reference glass slide. The increase deflection in every cycle was half of the wavelength of the laser. The brightness evolution of the reference point due to the negative pressure were recorded by a digital camcorder and analyzed by custom-made software. 2. A syringe vacuum system with a pressure gauge provided the negative pressure to deflect the nanomembrane suspended on the copper aperture. Therefore, the pressure (P) and vertical deflection at the apex of the deformation dome (d) were captured simultaneously and converted to stress and strain by the relationships 5 : =Pr 2 /4hd and =d 2 /3r 2, where h is the thickness of the nanomembrane and r is the radius of the aperture.

5 S-5 Figure S4. (a) FT-IR spectra of GO. Amide I/ II/ III region (band) of SF is labeled by blue/ green/ red region on the image, respectively. Just for readers convenience to compare. The absorbance peak in Amide I region is centered at 1627cm -1, that is the same region of -sheet absorption (centered at 1626 cm -1 ). No peak shows in Amide III region. (b-f) FTIR spectra deconvolution of the corresponding amide III band of GO/SF membrane under GO volume concentration of 9.7% and 26.9% before (b,e) and after annealing (c,f), respectively. (d) SA-LbL SF membrane without methanol treatment (circles, original spectrum; blue curve, deconvoluted -sheet peak; red curve, deconvoluted random coil peak; black curve, simulated spectra from summed peaks)

6 S-6 Figure S5, XRD pattern of silicon wafer from 1 o to 15 o The schematic diagram is made under the same size ratio of GO/SF nanomembrane under GO volume concentration of 26.9%. The structure illustration contains the following 3 main points. 1: Thickness The total thickness of the nanomembrane is measured by ellipsometry. It could also be tuned by

7 S-7 changing the concentration of GO/SF. For the nanomembrane for GO volume concentration of 26.9%, the average thickness increase is about 1.4 nm after GO deposition (h1) and 3.8 nm after SF deposition(h2). 2:D-spacing The d-spacing and the mean size of the crystalline domains can be calculated by XRD data. For graphite, the d-spacing is nm. With the level of oxidation for GO, the (002) spacing increases. The d-spacing of GO we used about 0.83 nm. For GO/SF membranes, with some of the molecular chains inset into GO layer, the d-spacing would further increase. The d-spacing of the membrane before and after annealing is calculated to be 1.46 nm and 1.26 nm. The mean size of the crystalline domains is calculated to be 3.2 nm and 3.6 nm, corresponding to the 2-3 layers of GO. In this way, the structure as marked by 2 represent 2-3 layers of GO with some SF molecular chains inset. 3:Total thickness change The d-spacing of the membrane decrease from 1.46 nm to 1.26 nm after water vapor annealing, some readers may doubt that why the total thickness of the membrane doesn t change accordingly. The (002) diffraction peak is from 2-3 layers of GO packing. The average thickness increase is about 1.4 nm after each layer GO deposition. Roughly estimate, there re 40% area with GO stacking. Let s assume all of them are well aligned to have diffraction peak. Then after annealing, the total thickness for a 10 bilayers membrane would change % = 0.8 nm. However, could the total layer of GO thickness liner decrease with the d-spacing decrease (stacking area)? Could all stacking GO layer be well aligned to have a diffraction peak? If we count these two factors, the total thickness decrease would be much smaller than 0.8 nm. On the other hand, the error of total thickness measurement by AFM is around 2 nm. In this way, the thickness change is negligible to measure.

8 S-8 Reference 1. C. Y. Jiang, X. Y. Wang, R. Gunawidjaja, Y. H. Lin, M. K. Gupta, D. L. Kaplan, R. R. Naik and V. V. Tsukruk, Mechanical Properties of Robust Ultrathin Silk Fibroin Films. Adv Funct. Mater. 2007, 17, E. Kharlampieva, V. Kozlovskaya, B. Wallet, V. V. Shevchenko, R. R. Naik, R. Vaia, D. L. Kaplan and V. V. Tsukruk, Co-cross-linking Silk Matrices with Silica Nanostructures for Robust Ultrathin Nanocomposites. Acs Nano 2010, 4, C. Y. Jiang, S. Markutsya, Y. Pikus and V. V. Tsukruk, Freely Suspended Nanocomposite Membranes as Highly Sensitive Sensors. Nat. Mater. 2004, 3, S. Markutsya, C. Y. Jiang, Y. Pikus and V. V. Tsukruk, Freely Suspended Layer-By-Layer Nanomembranes: Testing Micromechanical Properties. Adv Funct Mater 2005, 15, D. D. Kulkarni, I. Choi, S. Singamaneni and V. V. Tsukruk, Graphene Oxide-Polyelectrolyte Nanomembranes. Acs Nano 2010, 4,