Supplemental Material:

Boron nitride nanosheets as oxygen-atom corrosion protective coatings

MinYi,1,2,ZhigangShen,1,2,3,a)Xiaohu Zhao,2 Shuaishuai Liang,1and LeiLiu1,3

1Beijing Key Laboratory for Powder Technology Research and Development, Beijing University of Aeronautics and Astronautics, Beijing 100191, China.

2Plasma Laboratory, Ministry-of-Education Key Laboratory of Fluid Mechanics, Beijing University of Aeronautics and Astronautics, Beijing 100191, China.

3School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China.

E-mail:

Experimental

Oxygen-atom exposure experiments were carried out in a filament-discharge plasma-type atomic oxygen effect facility in BeijingUniversityofAeronautics andAstronautics,1,2under a condition of pressure of 0.15 Pa, filament discharge voltage of 120 V, and filament discharge current of 140 mA. This facility iswith the filament discharge and bound of magnetic field. The temperature in the test is low, less than 50 oC.The mass loss of Kapton in the atomic oxygen exposure experiments was used as a criterion to calculate the atomic oxygen flux. The calculation formula is Ft=M/(AEy) in whichF is the effective flow rate of atomic oxygen onto the sample surface and M, , A, t, Ey are mass loss, density, surface area, exposure time, and erosion yield, respectively. Ft is the accumulative atomic oxygen flux exerting on the sample surface. For Kapton, Ey is equal to ~3.0×10-24cm3/atom. In this study, The exposure time during oxygen-atom corrosion is about 12 h and the accumulative atomic oxygen flux was about 2.78×1020 atoms/cm2.

UV-Vis absorption spectroscopy was performed with a Purkinje General TU1901 (1cm cuvettes).The concentration after centrifugation was calculated from Lamber-Beer law, A/l=C, where A is the absorbance measured at 300 nm, l is the path length, C is the concentration, and  is taken as 2367 mL/mg/m.3Scanning electron microscopy (SEM) images were collectedby a LEO 1530VP.Atomic force microscopy (AFM) images were captured with a Multimode 8 AFM (Bruker) in ScanAsyst Air mode.Transmission electron microscopy (TEM) images were taken with a Jeol 2100 operating at 200 KV.Themass of samples before and after oxygen-atom exposure was measured using a DT-100 balance with a sensitivity of0.05 mg.X-ray photoelectron spectroscopy (XPS) was obtained by an ESCALAB-250 spectrometer.

Fig. S1 (a) SEM image of the pristine h-BN particles. The inset red number is the measured relative length in which 92 is corresponds to a length of 10 m (i.e. 10/92m/unit). (b) Estimated histogram of frequency vs length based on the measurement of 50 particles in (a).

Fig. S2(a) Two types of force for exfoliating BN particles, i.e. normal force and shear force. (b) Sonication depends its cavitation effect to generate a nomrla force-dominated way for exfoliating BN particles. (c)Ball milling4 and shear fluidic film5 generate shear force-dominated way.

Fig. S3 (a) Computational fluid dynamics (CFD) results for analyzing fluid dynamics events. (b) The fluid dynamic route has multiple events for generating both normal and shear force for high-efficiency exfoliation of BN particles, showing advantages over single normal force-dominated sonication and single shear force-dominated shear fluidic film in Fig. S2b and c.Forsimplification and qualitatively revealing the fluid dynamics events involved in the hydrodynamics apparatus, we used a 2D axisymmetric model to perform CFD calculations (FLUENT 6.3.26).The model is shown in Fig. S3a. We adopted structured meshes to numerically simulate the flow field. The number of total cells was approximately 139 0000 for half of the axisymmetric model.A two phases (liquid andvapor), incompressible, continuous flow with Newtonian viscosity is assumed. The specific values of parameters are set as follow:density 920kg/m3, viscosity 0.00241Pas, vapor density 0.02558kg/m3,viscosity 1.26×10-6 Pa s, vaporization pressure 10000Pa, surface tension coefficient 0.0294 N/m, non-condensable gas mass fraction 1.0×10-4.The inlet and outlet were set as pressure inlet and pressure outlet conditions, respectively. The standard k-ε turbulence model was used, with a segregated solver.In order to obtain a converged solution, we intentionally computed an initial field with 1st-order discretization and small under-relaxation factor.

Fig. S4Illustration of the reason that small flakes are much easier to be exfoliated than large ones.

Fig. S5 Surface SEM images of (a) pristine naked membrane, (b)oxygen-atom-corroded membrane, (c) pristine BNNSs-coated membrane, and (d) oxygen-atom-corroded BNNSs-coated membrane. The BNNSs coating is ~30 nm thick.

Fig. S6Illustration of the vacuum filtration process. The vacuum-induced pressure gradient results in dynamic flow which drives BNNSs to deposit on the nylon membrane. Before BNNSs contact the formed film underneath, they tend to move vertically. And the surfaces of these BNNSs (in-plane direction perpendicular to the thickness direction) are parallel to the flow direction, because this kind of movement is favourable for reducing kinetic resistance in liquid. However, the configuration that BNNSs are perpendicular to the film surface is intrinsically unstable. When the flakes approach the formed film surface, they would be destabilized due to their out-of-plane flexibility and the perturbation from flowing liquid. Once destabilization happens, the flakes would be inclined and further be pushed down by the vertically dynamic flow. Eventually, BNNSs would nearly lie horizontally one by one to form the tight layer-by-layer structure.

References

1X.-H. Zhao, Z.-G. Shen, Y.-S Xing, and S.-L. Ma, J. Phys. D: Appl. Phys. 34 (15), 2308 (2001).

2M. Yi, W. Zhang, Z. Shen, X. Zhang, X. Zhao, Y. Zheng, and S. Ma, J. Nanopart. Res. 15, 1811 (2013).

3J. N. Coleman, M. Lotya, A. O'Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H. Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist, and V. Nicolosi, Science 331 (6017), 568 (2011).

4L. H. Li, Y. Chen, G. Behan, H. L. Zhang, M. Petravic, and A. M. Glushenkov, J. Mater. Chem. 21 (32), 11862 (2011).

5X. Chen, J. F. Dobson, and C. L. Raston, Chem. Commun. 48 (31), 3703 (2012).

S1