br BP nanosheets synthesis and characterization
2.3. BP nanosheets synthesis and characterization
Similar to our previous work (Sun et al., 2015), BP nanosheets were synthesized by the use of liquid ultrasonication-based exfoliation of bulk BP in solvent, which was replaced by isopropanol in this Kainic acid work. As shown in Fig. 2a, the bulk BP was grinded into small pieces then dis-solved by 25 mL isopropanol. Subsequently, the BP dispersion was so-nicated in an ultrasonic bath operating at 25 kHz 1200 W power for 3 h to break the van der Waals stack of BP, where the temperature was maintained below 277 K by the use of ice. After sonication, the as-prepared solution was centrifuged at 5000 rpm for 15 min to remove the residual un-exfoliated BP. In final, the supernatant liquid containing few-layered BP nanosheets was collected for further usage.
Atomic force microscopy (AFM) was used to characterize the liquid-exfoliated BP nanosheets (Fig. 2b). The BP nanosheets owned terraces of diﬀerent thickness features, which were widely found in liquid-ex-foliated 2D materials (Hanlon et al., 2016). Typically, the individual nanosheets were observed to include regions of diﬀerent thickness, often with the thicker at the center and thickness decreasing at the edges of the nanosheet, which were partly due to restacking with small nanosheets aggregating on the surface of large nanosheets. As shown in Fig. 2d, the size of BP nanosheets was from several ten nanometers to several hundred nanometers and the thickness was around 7.3 nm–23.8 nm. As the thickness of BP monolayer was ∼0.9 nm
Fig. 2. BP nanosheets synthesis and characterization. (a) Schematic illustration of BP nanosheets synthesis with liquid ultrasonication-based exfoliation. (b) AFM image of BP nanosheets. (c) TEM image of BP nanosheets. (d)The corresponding height profiles (A, B, C) of three measured BP nanosheets in (b) along the white lines.
(e) Raman spectra of bulk BP and BP nanosheets.
(Kumar et al., 2016), which suggested that the BP nanosheets in this work contained 8–26 atomic layers. In further, the transmission elec-tron microscopy (TEM) was used to characterize the detailed mor-phology of the BP nanosheets obtained. As shown in Fig. 2c, the BP nanosheets are very thin and ca. 200–500 nm in lateral size. Raman spectra (Fig. 2e) of bulk BP and BP nanosheets reveal the bands at 362.0, 439.6, 467.1 cm−1 assigned to A1g , B2g and A2g three characteristic peaks corresponding to BP materials, indicating BP na-nosheets preserve the structure of bulk BP. The A1g corresponds to atoms oscillate out-of-plane while B2g and A2g relate to atoms vibrate within the plane. Due to the ultrathin thickness of the exfoliated nanosheets, the Raman spectrum of BP nanosheets shows slightly shift toward high wavenumber compared with bulk BP (Dhanabalan et al., 2017).
2.4. BP deposition on cylindrical fiber
Transferring nano-film or depositing 2D-layered nanosheets on non-planar substrate is a big challenge. We developed an in-situ layer-by-layer deposition technique based on chemical-bonding associated with physical-adsorption (Liu et al., 2017). In this work, we used the freshly
exfoliated BP nanosheets therefore the BP nanosheets had very good stability and dispersibility.
As illustrated in Fig. 3a, the section of fiber over the grating region was rinsed with acetone for 30 min to remove the organic contaminants and washed with DI water thoroughly and dried. Subsequently, (i) an alkaline treatment was conducted by immersing the fiber device in 1.0 M NaOH solution for 1 h at room temperature then washed with DI water to enrich the number of hydroxyl (-OH) groups on fiber surface;
(ii) a silanization procedure was followed by incubating the fiber into a freshly-prepared 5% (v/v) APTES ethanol solution for 1 h at room temperature, which mainly reacted with hydroxyl groups to form Si-O-Si bonding yielding a positively charged surface, then the fiber was washed with ethanol to remove unbound monomers and baked in an oven at 70 °C for 30 min to enhance the stability of APTES monolayer;
(iii) the APTES-silanized fiber was kept straightly in a custom-made microchannel container where 30 μL BP isopropanol solution was added to immerse the fiber device. Upon solvent evaporation and drying, the negatively charged BP nanosheets were gradually bonded on the positively charged fiber surface. After 30 min when the solvent fully evaporated, the second cycle was implemented by carefully
Fig. 3. BP nanosheets deposition on fiber and surface morphology. (a) Schematic diagram of BP deposition: (i) Alkaline treatment for silica fiber surface, (ii) Silanization by APTES, (iii) BP multi-cycle deposition. (b) 3D view of AFM image of BP-coated cylindrical fiber with step boundary. (c) 2D AFM micrograph with the height profile (inset) extracted along the white line. (d) SEM image of BP overlay on fiber surface (scale bar: 20 μm). (e) Raman spectra of BP-coated fiber and bare fiber.