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Surface Innovations

Nonwetting and optical properties of BN nanosheet films

Pakdel, Bando, Shtansky and Golberg

http://dx.doi.org/10.1680/si.12.00007Research Article Received 13/09/2012 Accepted 18/10/2012Published online 30/10/2012

Keywords: nanostructures/superhydrophobicity/thin film/vapor deposition

ICE Publishing: All rights reserved

Nonwetting and optical properties of BN nanosheet films

Amir Pakdel PhD*International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan

Yoshio Bando PhD International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan

Dmitry Shtansky PhDNational University of Science and Technology (MISIS), Moscow, Russia

Dmitri Golberg PhD*International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan

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.

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1. IntroductionThe control of the surface wettability is a research topic of funda-mental interest and is essential in a variety of applications, such as marine vehicles, stain resistant materials and clothing, and fluid power system components.1 The wettability of a surface can be measured by the equilibrium contact angle (CA) of a water droplet on it. If the water droplet CA is larger than 150°, the surface is supe-rhydrophobic. When a superhydrophobic surface with a small CA hysteresis is tilted, water droplets can move spontaneously on that surface.2 It has been documented that the water repellency of a solid surface mainly depends on two factors: its chemical composition and functionality, as well as its micro/nano morphological features.3 However, a lot of questions still remain in this field, and further research is necessary to fully realize the potentials. On the basis of the regarded two factors, many superhydrophobic surfaces have been fabricated, for instance, organosilane films, mixed inorgan-ic–organic coatings, gold cluster films, silicon pyramid/nanowire

binary structures, two-dimensional zinc oxide pore arrays, and car-bon nanofiber and nanosphere arrays.4 The usage of polymeric and organic superhydrophobic surfaces is limited by their short lifetime due to mechanical erosion and heat degradation. Strong acids, bases and UV irradiation from the sunlight accelerate instability and deg-radation of these surfaces.5 Therefore, an important breakthrough is to fabricate superhydrophobic surfaces from inorganic materials with high chemical and thermal stability. Such durable surfaces can also exhibit stable optical and electrical properties.

Boron nitride (BN) low-dimensional materials are among the most promising inorganic nanosystems explored so far. BN is a chemical compound, consisting of equal numbers of boron (B) and nitrogen (N) atoms, which is not found in nature and is therefore produced synthetically.6 Hexagonal BN (h-BN) is an analogue of graphite in which alternating B and N atoms substitute for carbon (C) atoms in a honeycomb network with sp2 bonding. Within each layer of h-BN, B

*Corresponding author e-mail addresses: pakdel.amir@nims.go.jp; golberg.dmitri@nims.go.jp

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and N atoms are bound by covalent bonds, whereas the layers are held together by van der Waals forces.7 Unlike the popular graphene, mon-olayer BN sheets have rarely been observed8,9 due to the peculiar B−N stacking characteristics. The hexagons of neighboring planes in h-BN are superposed, that is, B and N atoms are in succession along the 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 are partially ionic, in contrast with the purely covalent C−C bonds in graphitic structures. This can lead to the so-called “lip−lip” interac-tions between neighboring layers in BN nanosheets, that is, chemical bonds form bridges or “spot-welds” between the atoms of adjacent layers. Therefore, formation of multilayers stabilizes the structure.10

BN structures exhibit unique features such as superb thermal conductivity, excellent mechanical and chemical stability, and a stable wide band gap.11,12 After the successful realization of superhydrophobic coatings based on insulating and chemically inert BN nanotubes,13,14 the present authors developed a chemical vapor deposition (CVD) method to prepare BN nanosheet coatings with controllable water repellency.15 To further investigate the mer-its of such coatings, in this manuscript, the authors describe the high-temperature CVD formation of nonwetting h-BN films that consist of nanosheets assembled in a perpendicular-to-the-substrate fashion and their superhydrophobic and optical properties.

2. ExperimentalThe CVD growth of the crystalline BN nanosheets was performed in a horizontal tube furnace, as described elsewhere.15 In brief, the precursor powders were mechanically mixed and positioned in an alumina combustion boat covered with a Si/SiO

2 substrate. The boat

was then set into an alumina test tube inside vacuum chamber. The chamber was evacuated to ∼1 Torr, and then ammonia gas flow was introduced at the rate of 0·4 mL/min. The precursors were heated to 1300°C, held for 30 min and then cooled to the room tempera-ture. The morphology of the films was studied by a field-emission scanning electron microscope (FE-SEM; Hitachi S4800, Japan). Chemical composition and structural features of the nanosheets were investigated by an X-ray photoelectron spectrometer (XPS; Thermo Scientific Theta Probe, USA) and a high-resolution field-emission transmission electron microscope (HRTEM; JEOL JEM-2100F, Japan) equipped with an electron energy loss spectrometer (EELS, Gatan, USA). Fourier transform infrared (FTIR) spectros-copy (Nicolet 4700, USA), Raman spectroscopy (LabRam HR-800, Japan), ultraviolet-visible (UV-Vis) spectroscopy (Jasco V-570, Japan) and cathodoluminescence (CL) spectroscopy (Gemini elec-tron gun; Omicron, inside FE-SEM; Hitachi S4600, Japan) were used to investigate the optical properties of the nanosheets at room temperature. The topographical images of the films were obtained by a JEOL JSPM-5200 scanning probe microscope in the tapping atomic force microscopy (AFM) mode at ambient conditions. The CA measurement was carried out by a sessile drop method using a deionized water droplet of about 10-μL volume positioned on the

coating by a microsyringe. A high-resolution Keyence VH-5000 optical instrument equipped with a WinROOF V5·03 analysis soft-ware was used for measuring the water CA on the films.

3. Results and discussion

3.1 Growth and structureFigure 1a and 1b shows typical SEM images of a BN film con-sisting of partially aligned nanosheets along the vertical direc-tion. The nanosheets are uniformly distributed over a large area and display a compact and curly morphology. Compared with the BN nanosheets synthesized at lower temperatures,15 the present ones show a branching feature, that is, subnanosheets grow on the surface of the main nanosheets producing a peculiar three-dimen-sional nanostructure. The suggested mechanism is illustrated in Figure 1c. During the heterogeneous nucleation and quick growth of preferential crystal planes on the substrate at a high temperature (1300°C), abundant growth vapor (trapped in the combustion boat) caused additional growth steps on the pre-existing nanosheets, which resulted in the outgrowth of new crystal planes. This repeti-tive branching led to the formation of a hierarchical BN nanos-tructure on the substrate. Another possibility is the intergrowth of several nanosheets in different directions, as shown in Figure 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-cally favorable axis until their collision. As a result, flexible per-pendicular-to-the-substrate BN nanosheets intermeshed with each other and formed well-aligned and highly dense BN networks.

A typical TEM image of the nanosheets is illustrated in Figure 2a. This indicates the compact BN network with very thin nanosheets that are almost transparent to the electron beam. In addition, intrin-sic bending and scrolling of the nanosheets can be noticed in Figure 2a, similar to previously reported BN nanosheets prepared by other methods.16,17 Typical HRTEM images of the nanosheets in Figures 2b and 2c reveal that they are less than 5 nm in thickness. Figure 2b depicts highly ordered lattice fringes denoting a well-crystallized product. The average spacing between adjacent fringes in Figure 2c is ~0·33 nm, which indicates the formation of layered (002) BN planes.

3.2 CharacterizationTo establish the elemental composition and structural features of the nanosheets, EELS measurements were carried out, as depicted in Figure 3a. The EEL spectrum shows two distinct absorption features for B and N K-shell ionization edges at 188 and 401 eV, respectively. The authors have observed similar peak positions in BN nanosheets prepared at lower temperatures (1000−1200°C).15 Quantification analysis of the EEL spectrum gives a B/N atomic ratio of ~1·0. The sharp peaks on the left side of B-K and N-K edges correspond to 1s→π* antibonding orbits, and the peaks on the right side of the absorption edges correspond to 1s→σ*

Surface Innovations Nonwetting and optical properties of BN nanosheet filmsPakdel, Bando, Shtansky and Golberg

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antibonding orbits. This type of EELS edge structure is typical of the sp2-hybrizied layered h-BN.

XPS analysis identified the chemical composition and bonding states in the synthesized films. A typical XPS spectrum of the nanosheets is presented in Figure 3b. The photoelectron peaks observed at ~191 and ~399 eV are related to B and N, respectively. A very weak peak related to O can also be seen at around 533 eV, possibly due to a slight oxidation of the nanosheets. The sharp B 1s and N 1s peaks in Figure 3c and 3d are attributed to the B−N bonding in pure BN7,18, and the π plasmon loss peak located at ~9 eV away from the center of the B 1s and N 1s peaks in Figure 3b is characteristic of the hexagonal phase BN.19,20

Raman spectroscopy was used to further characterize the film. Figure 3e shows a typical Raman spectrum of the nanosheets. The

characteristic peak at 1365 cm–1 is attributed to the B–N high-fre-quency vibrational mode (E

2g) within h-BN layers, analogous to the

G peak in graphene.21 The previous measurements show Raman shifts of 1363−1365 cm−1 for BN nanosheets synthesized at lower temperatures15 and 1366 cm−1 for BN nanotubes.22 The reported Raman shift for different BN structures is in the range of 1366–1374 cm–1.9,23 Considering the multilayer nature of the nanosheets, it can be expected that they show the same features as bulk h-BN materi-als (Raman shift at 1366 cm–1). The observed 1 cm–1 red shift can be attributed to the possible local temperature increase caused by the laser9 and generation of stress in nanosheets due to folding and interactions with the substrate.7 Figure 3e also proves the absence of carbon G band trace at ~1600 cm–1, indicating the high purity of the present BN nanosheets. The full width at half maximum (FWHM) of the Raman peaks can be used to evaluate the crystallinity of BN nanomaterials. The FWHM of present BN nanosheets was 16 cm−1,

Figure 1. (a) Typical SEM image of the BN nanosheet films. (b) Higher

magnification SEM image showing branching of the nanosheets. (c

and d) Suggested growth mechanisms for the vertically aligned BN

nanosheets. BN, boron nitride; SEM, scanning electron microscope.

(a) (b)

(c)

(d)

1 µm 200 nm

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smaller than the BNNSs prepared at lower temperatures (19−37 cm−1),15 which indicates the good crystallinity of the product.

Figure 3f is a typical FTIR spectrum of the film, showing an absorp-tion peak at ~815 cm–1 and one broad absorption band in the range of ~1350–1580 cm–1 with its bottom at 1377 cm–1, which can be attributed to the A

2u (B–N–B bending vibration mode parallel to the

c-axis) and E1u

(B–N stretching vibration mode perpendicular to the c-axis) modes of h-BN, respectively.16 The peak at ~1084 cm–1 is related to the Si/SiO

2 substrate, as shown at the onset of Figure 3f.

The FTIR results are in a good agreement with previously reported ones for BN nanocrystals,16,24 which confirm the good crystallinity of the present BN nanosheets.

3.3 Nonwetting propertiesNonwetting properties of the BN films were then measured. A peculiar surface morphology can generate superhydrophobicity in nanostructured films.16,25 Figure 4a displays the round shape of a deionized water droplet on the BN film. The measured CA is 153 ± 1·8° indicating the superhydrophobic feature of the coating. Figure 4b is a typical AFM topography image of the BN nanosheet films. Quantitative AFM measurements showed that the maximum height difference on the film surface, and its average roughness were 188 and ~26 nm, respectively. This implies that the strong nonwetting tendency could be ascribed to the nanoscale surface roughness of the BN films. Cassie-Baxter model considers a liquid droplet sit-ting on a rough surface as partly on the nanostructure and partly on air and describes the CA of the droplet as,26

1. cos = 1 + f 1 + cos CB s eθ − θ( )

where fs is the fraction of solid–liquid contact and θ

e is the CA

in Young’s mode (for an ideally smooth surface). Obviously, the main factors affecting the CA value are the chemical composition

of the surface layer (associated with θe) and the roughness of the

wetted area (portion of the film surface in contact with the liquid droplet, f

s). Therefore, water CA higher than 150° (superhydro-

phobicity) can be achieved on any surface (even an intrinsically hydrophilic material such as BN) provided that it is roughened enough. Moreover, airborne adsorbates on the BN films could be an additional factor to enhance their hydrophobic properties, as has been recently reported for BN nanotube films.5

The practical interest in superhydrophobic films is closely related to the dynamic properties of droplets on them. The first interest-ing property is a very low degree of sticking to the surface. The authors performed an experiment by pushing a suspended water droplet from a syringe on the film and moving it on the sample. The suspended droplet did not attach to the surface and was easily sliding over the film indicating that the adhesion between the film and water was considerably weaker than that between the syringe needle and the water drop.

Self-cleaning behavior is also a very important feature of super-hydrophobic films. The authors observed that on the present BN films most of the dust was picked up under water droplet rolling, and no residue was left behind. It means that the adhesion between water droplets and dust particles is larger than that between the surface and the dust. This results from the conjunction of a very large CA (which reduces the solid/liquid surface area) and a very small hysteresis. A liquid flowing on a solid is conceived not to slip at the interface between the phases, but a microscopic slip may exist if the solid behaves in a hydrophobic manner.27 This effect can become more notable if the hydrophobic solid is textured and air is trapped in the textures (Cassie-Baxter model), as is the case of the present films. The self-cleaning characteristic makes the regarded films suitable for rendering a surface antimicrobial. The ability of water to easily slide on such films opens the door for the reduction of energy required to pump fluids in pipe networks. The extremely

Figure 2. (a) Low-magnification TEM image of the BN nanosheets. (b

and c) Typical HRTEM images of the nanosheets indicating their good

crystallinity. BN, boron nitride; HRTEM, high-resolution field-emission

transmission electron microscope; TEM, transmission electron

microscope.

50 nm 10 nm 2 nm

0·33 nm

(a) (b) (c)

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low density makes them a good candidate for applications in avionics as a moisture barrier or an ice-resistant material.

In another set of experiments, the authors noticed that water droplets falling on the present superhydrophobic films with a decent velocity

can bounce back, after experiencing an almost elastic collision. This is also a reason for these films to remain dry even after coming in contact with some liquid. The rebound becomes possible due to small energy dissipation as the drop impacts the solid, that is, because of the high CA, viscous dissipation close to the moving contact line (which

Figure 3. (a) Typical EEL spectrum from a BN nanosheet. (b) XPS

survey spectrum of the BN film. (c and d) N 1s and B 1s core-level XPS

spectra, respectively. (e and f) Raman and FTIR spectra of the BN film.

BN, boron nitride; EEL, electron energy loss; FTIR, Fourier transform

infrared spectroscopy; XPS, X-ray photoelectron spectrometer.

Inte

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Arb

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200 300 400Energy loss (eV)

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N-K edge

B-K edge

(a) (b)

(c) (d) (e)

(f)

N1s

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Binding energy (eV)

404

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B 1s

B 1s

0 200 400 600 800 1000 1200

900 1200 1500 1800Raman shift (cm−1)

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Binding energy (eV)

186 189 192Binding energy (eV)

195

2400 2000 1600 1200 800

Wavenumber (cm−1)

1500 1000

Si/SiO2 substrate

500

N-K edge

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usually is the primary cause of viscous loss) becomes nearly negli-gible. A drop impacting a superhydrophobic film deforms; however, due to the very large CA it can store its kinetic energy under surface deformation and thus bounces back. The drop therefore behaves as a spring, whose stiffness displays the surface tension of the liquid.28

3.4 Optical propertiesThe optical absorption properties reveal the electronic state of a material and can be used to verify the band gap of semiconduc-tors. UV-Vis absorption spectroscopy was utilized to investigate the optical energy gap (E

g) of the synthesized films. Figure 5a

shows the UV-Vis absorption spectrum of the BN film. Since the Si/SiO

2 substrate is opaque to UV and visible light, the UV-Vis

measurements were performed in the reflection mode. Reflectance (R) was then converted to absorbance (A) automatically by the machine’s software considering zero transmittance (T) through the opaque substrate (A + R + T = 100%). A sharp absorption peak is observed at ~212 nm, corresponding to an optical band

gap of ~5·7 eV (according to Tauc’s calculation method),29 larger than those of the previously reported BN nanoribbons and nanosheets.24,30 However, theoretical studies of band structures of a single-layer h-BN predict a 6-eV band gap.31 The measured smaller gap in the present multilayer BN nanosheets could be attributed to an increase in the electronic band dispersion because of the layer–layer interactions.31 A dip is observed at ~230−300 nm, which could be due to defect transitions. The measurements on various BN nanostructures indicate that some strong lumi-nescence emissions at ~300−330 nm can be noticed in them due to excitation at 240−290 nm. Multiple absorption peaks in this region may also originate from absorption centers associated with both sp2- and sp3-bonded structures, since the peaks may be the reflection of the phonon–electron coupling associated with the infrared vibration modes.32

The optical properties of the nanosheets were further investigated by CL spectroscopy. Figure 5b depicts the CL spectrum of a BN film at room temperature, indicating two luminescence bands at ~334 and ~377 nm. This strong ultraviolet CL emission can be attributed to the deep-level emissions associated with defect-related centers (B or N vacancy-type defect-trapped states).30 Although theoretical calculations and experiments on bulk BN and BN nanotubes have demonstrated that strong Frenkel type excitonic transitions at ~211–234 nm take place at temperatures ≤100 K,33–35 this luminescence emission did not appear in the present BN films at room temperature.30 In fact, the BN nanosheets displayed strong luminescence emission in the ultraviolet range, although the near band edge CL emission was not recognized in their spectrum.

4. ConclusionIn summary, transparent h-BN films were synthesized at 1300°C via a CVD method on Si/SiO

2 substrates. The films were composed

of compactly aligned nanosheets protruding out of the substrate

Figure 4. (a) Typical optical photograph of a water droplet on a

BN film. (b) Atomic force micrograph showing the film surface

topography. BN, boron nitride.

0·0

0

4·1 0·0

4·4

(µm

)

(µm)

(nm)1·88

(b)(a)

Figure 5. (a) Ultraviolet-visible absorption spectrum of a BN film. (b)

Cathodoluminescence spectrum from the hexagonal BN nanosheets.

BN, boron nitride.

200

(a) (b)

400 600

Wavelength

Ab

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(A

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its)

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Wavelength

800 0 200 400 600 800 1000

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surface. The majority of the nanosheets were less than 5-nm thick. Their chemical composition and structural features were studied by EELS, XPS, FTIR, Raman spectroscopy, CL spectroscopy and UV-Vis spectroscopy. The nanosheets displayed a wide band gap and strong CL emission in the ultraviolet region at room tempera-ture, as well as excellent nonwetting behavior due to their rough morphology and nanoscale features. No sticking between water droplets and the films was observed, and the droplet-film collision was almost elastic. The present films are envisaged to be valuable for diverse applications such as self-cleaning, nonfogging displays and protection from acid rain corrosion.

AcknowledgmentsThis work was supported by the WPI Center for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS; Tsukuba, Japan). Amir Pakdel is grateful to Prof. Tomonobu Nakayama, Prof. Takashi Sekiguchi, Dr. Chunyi Zhi and Mr. Xuebin Wang for useful discussions and also to Dr. Yoshihiro Nemoto, Dr. Shinichi Hara and Dr. Kentaro Watanabe for their technical support.

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