Surface Innovations http://dx.doi.org/10.1680/si.12.00007 Research Article Received 13/09/2012 Accepted 18/10/2012 Nonwetting and optical properties of BN Published online 30/10/2012 nanosheet films Keywords: nanostructures/superhydrophobicity/thin film/ vapor deposition Pakdel, Bando, Shtansky and Golberg ice | science ICE Publishing: All rights reserved Nonwetting and optical properties of BN nanosheet films 1 Amir Pakdel PhD* 3 Dmitry Shtansky PhD International Center for Materials Nanoarchitectonics (MANA), National University of Science and Technology (MISIS), Moscow, National Institute for Materials Science (NIMS), Tsukuba, Japan Russia 2 Yoshio Bando PhD 4 Dmitri Golberg PhD* International Center for Materials Nanoarchitectonics (MANA), International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan National Institute for Materials Science (NIMS), Tsukuba, Japan 1 2 3 4 This study begins with a brief discussion on the high-temperature chemical vapor deposition synthesis of transparent boron nitride nanosheet films on silicon/silicon dioxide substrates. The compact nanosheets grew per- pendicular to the substrate surface, and the majority of them had thicknesses of less than 5 nm. Ultraviolet-visible spectroscopy measurements demonstrated a wide optical band gap of ~5·6 eV of nanosheets, and cathodo- luminescence spectroscopy showed their strong luminescence emission in the ultraviolet region. The nanor- ough surface morphology of the films induced nonwetting and self-cleaning features with water-contact angles reaching ~153°. Such transparent superhydrophobic films can be utilized for the preparation of nonwetting ultraviolet light-emitting surfaces for optoelectronics applications, antifouling surfaces on marine vessels or oil–water separation equipments. 1. Introduction binary structures, two-dimensional zinc oxide pore arrays, and car- The control of the surface wettability is a research topic of funda- bon nanofiber and nanosphere arrays.4 The usage of polymeric and mental interest and is essential in a variety of applications, such as organic superhydrophobic surfaces is limited by their short lifetime marine vehicles, stain resistant materials and clothing, and fluid due to mechanical erosion and heat degradation. Strong acids, bases power system components.1 The wettability of a surface can be and UV irradiation from the sunlight accelerate instability and deg- measured by the equilibrium contact angle (CA) of a water droplet radation of these surfaces.5 Therefore, an important breakthrough on it. If the water droplet CA is larger than 150°, the surface is supe- is to fabricate superhydrophobic surfaces from inorganic materials rhydrophobic. When a superhydrophobic surface with a small CA with high chemical and thermal stability. Such durable surfaces can hysteresis is tilted, water droplets can move spontaneously on that also exhibit stable optical and electrical properties. surface.2 It has been documented that the water repellency of a solid surface mainly depends on two factors: its chemical composition Boron nitride (BN) low-dimensional materials are among the most and functionality, as well as its micro/nano morphological features.3 promising inorganic nanosystems explored so far. BN is a chemical However, a lot of questions still remain in this field, and further compound, consisting of equal numbers of boron (B) and nitrogen research is necessary to fully realize the potentials. On the basis (N) atoms, which is not found in nature and is therefore produced of the regarded two factors, many superhydrophobic surfaces have synthetically.6 Hexagonal BN (h-BN) is an analogue of graphite in been fabricated, for instance, organosilane films, mixed inorgan- which alternating B and N atoms substitute for carbon (C) atoms in a ic–organic coatings, gold cluster films, silicon pyramid/nanowire honeycomb network with sp2 bonding. Within each layer of h-BN, B *Corresponding author e-mail addresses: [email protected]; [email protected] 1 Surface Innovations Nonwetting and optical properties of BN nanosheet films Pakdel, Bando, Shtansky and Golberg and N atoms are bound by covalent bonds, whereas the layers are held coating by a microsyringe. A high-resolution Keyence VH-5000 together by van der Waals forces.7 Unlike the popular graphene, mon- optical instrument equipped with a WinROOF V5·03 analysis soft- olayer BN sheets have rarely been observed8,9 due to the peculiar B−N ware was used for measuring the water CA on the films. stacking characteristics. The hexagons of neighboring planes in h-BN are superposed, that is, B and N atoms are in succession along the 3. Results and discussion c-axis, while in graphite, they are shifted by half a hexagon. Moreover, due to the difference in electronegativity of B and N, the B−N bonds 3.1 Growth and structure are partially ionic, in contrast with the purely covalent C−C bonds in Figure 1a and 1b shows typical SEM images of a BN film con- graphitic structures. This can lead to the so-called “lip−lip” interac- sisting of partially aligned nanosheets along the vertical direc- tions between neighboring layers in BN nanosheets, that is, chemical tion. The nanosheets are uniformly distributed over a large area bonds form bridges or “spot-welds” between the atoms of adjacent and display a compact and curly morphology. Compared with the layers. Therefore, formation of multilayers stabilizes the structure.10 BN nanosheets synthesized at lower temperatures,15 the present ones show a branching feature, that is, subnanosheets grow on the BN structures exhibit unique features such as superb thermal surface of the main nanosheets producing a peculiar three-dimen- conductivity, excellent mechanical and chemical stability, and sional nanostructure. The suggested mechanism is illustrated in a stable wide band gap.11,12 After the successful realization of Figure 1c. During the heterogeneous nucleation and quick growth superhydrophobic coatings based on insulating and chemically of preferential crystal planes on the substrate at a high temperature inert BN nanotubes,13,14 the present authors developed a chemical (1300°C), abundant growth vapor (trapped in the combustion boat) vapor deposition (CVD) method to prepare BN nanosheet coatings caused additional growth steps on the pre-existing nanosheets, with controllable water repellency.15 To further investigate the mer- which resulted in the outgrowth of new crystal planes. This repeti- its of such coatings, in this manuscript, the authors describe the tive branching led to the formation of a hierarchical BN nanos- high-temperature CVD formation of nonwetting h-BN films that tructure on the substrate. Another possibility is the intergrowth consist of nanosheets assembled in a perpendicular-to-the-substrate of several nanosheets in different directions, as shown in Figure fashion and their superhydrophobic and optical properties. 1d. In this case, on the initial heterogeneous nucleation of the BN nanosheets on the substrate, continuous supply of growth species could lead to their growth in various directions along the energeti- 2. Experimental cally favorable axis until their collision. As a result, flexible per- The CVD growth of the crystalline BN nanosheets was performed pendicular-to-the-substrate BN nanosheets intermeshed with each in a horizontal tube furnace, as described elsewhere.15 In brief, the other and formed well-aligned and highly dense BN networks. precursor powders were mechanically mixed and positioned in an alumina combustion boat covered with a Si/SiO substrate. The boat 2 A typical TEM image of the nanosheets is illustrated in Figure 2a. was then set into an alumina test tube inside vacuum chamber. The This indicates the compact BN network with very thin nanosheets chamber was evacuated to 1 Torr, and then ammonia gas flow was ∼ that are almost transparent to the electron beam. In addition, intrin- introduced at the rate of 0·4 mL/min. The precursors were heated sic bending and scrolling of the nanosheets can be noticed in Figure to 1300°C, held for 30 min and then cooled to the room tempera- 2a, similar to previously reported BN nanosheets prepared by other ture. The morphology of the films was studied by a field-emission methods.16,17 Typical HRTEM images of the nanosheets in Figures scanning electron microscope (FE-SEM; Hitachi S4800, Japan). 2b and 2c reveal that they are less than 5 nm in thickness. Figure Chemical composition and structural features of the nanosheets 2b depicts highly ordered lattice fringes denoting a well-crystallized were investigated by an X-ray photoelectron spectrometer (XPS; product. The average spacing between adjacent fringes in Figure 2c is Thermo Scientific Theta Probe, USA) and a high-resolution field- ~0·33 nm, which indicates the formation of layered (002) BN planes. emission transmission electron microscope (HRTEM; JEOL JEM- 2100F, Japan) equipped with an electron energy loss spectrometer (EELS, Gatan, USA). Fourier transform infrared (FTIR) spectros- 3.2 Characterization copy (Nicolet 4700, USA), Raman spectroscopy (LabRam HR-800, To establish the elemental composition and structural features of Japan), ultraviolet-visible (UV-Vis) spectroscopy (Jasco V-570, the nanosheets, EELS measurements were carried out, as depicted Japan) and cathodoluminescence (CL) spectroscopy (Gemini elec- in Figure 3a. The EEL spectrum shows two distinct absorption tron gun; Omicron, inside FE-SEM; Hitachi S4600, Japan) were features