Tailoring Carbon Nanostructure with Diverse and Tunable Morphology

Jan 24, 2017 - When capped with a layer of SiO2 followed by pyrolysis, the lamellar nanodomains were converted to pod-like, spaghetti-like, or long wo...
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Tailoring Carbon Nanostructure with Diverse and Tunable Morphology by the Pyrolysis of Self-Assembled Lamellar Nanodomains of a Block Copolymer Ya-Sen Sun,* Wei-Hua Huang, Chien-Fu Lin, and Shao-Liang Cheng Department of Chemical and Materials Engineering, National Central University, Taoyuan 32001, Taiwan S Supporting Information *

ABSTRACT: The pyrolysis of a block copolymer thin film, the free surface of which was in contact with air or a capping layer of SiO2, produced four carbon nanostructures. Thin films of a diblock copolymer having perpendicularly oriented lamellar nanodomains served as carbon and nitrogen precursors. Before pyrolysis, the lamellar nanodomains were cross-linked with UV irradiation under nitrogen gas (UVIN). Without a capping layer, pyrolysis caused a structural transformation from lamellar nanodomains to short carbon nanowires or to dropletlike nanocarbons in a row via Rayleigh instability, depending on the duration of pyrolysis. When capped with a layer of SiO2 followed by pyrolysis, the lamellar nanodomains were converted to pod-like, spaghetti-like, or long worm-like carbon nanostructures. These carbon nanostructures were driven by controlling the surface or interface tension and the residual yield of solid carbonaceous species.

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INSTRUCTION Significant effort has been dedicated to the development of new carbon nanomaterials with a tailored nanoscale dimension and well-defined morphology because their new physical, electrochemical, and chemical properties depend on their nanoscale features, i.e., their shapes, sizes, and even spatial organization.1 Because of functional properties, carbon nanomaterials with novel structures have several potential applications as catalysts, sensors, and energy storage materials.1,2 Using a block copolymer (BCP) as a template to produce carbon nanostructures after pyrolysis at elevated temperatures has attracted much interest, as the self-assembly of BCP offers access to ordered nanodomains with tunable dimension and structure through the control of molecular parameters such as volume fraction and molecular mass.3−13 Self-assembled BCP nanodomains can serve as a soft template to guide conversion from small organic molecules to carbon nanostructures or can be pyrolyzed directly into carbon nanostructures without the addition of small organic precursors. In the former method, the BCP template acts as a sacrificial component and involves no carbonization of small organic precursors; the template is hence totally degraded during pyrolysis. In contrast, in the latter method, BCP can itself serve as a source of solid carbon.14−26 For example, the pyrolysis of BCP materials with one block composed of poly(acrynitrile) (PAN) or poly(vinylpyridine) can directly produce carbon nanostructures. For vinyl-based BCP, cross-linking was triggered with UV irradiation (UVI) in an inert environment,21,22,26 whereas for PAN-based BCP, UVI was unnecessary as PAN itself triggered cross-linking during pyrolysis after cyclization.14−20,23−25 Although carbon nanostructures have been prepared from the direct pyrolysis of BCP © 2017 American Chemical Society

nanodomains, important issues remain to be addressed, such as the morphological diversity and thermal stability of carbon nanostructures. For example, the morphology of the resultant carbon nanostuctures completely replicates that of their pristine films after pyrolysis. The spatial order, orientation, shape, and dimension of carbon nanostructures are hence intrinsically controlled by their self-assembled nanodomains in a pristine state. The pyrolysis of nanodomains of one type leads inevitably to the morphology of a carbon nanostructure of only one type. The second issue concerns the thermal stability of nanostructures. Upon pyrolysis, BCP thin films might have mobility sufficient to undergo structural transformations to minimize the Gibbs energy. Our previous work indicated that, after pyrolysis at 430 °C of the UVIN-treated polystyrene-blockpoly(2-vinylpyridine), PS-b-P2VP, thin films initially composed of perpendicularly oriented cylindrical P2VP nanodomains within the PS matrix fragmented into hexagonal arrays of pillarlike carbon nanostructures.26 The structural evolution was driven by the amplification of thermal fluctuations of the film thickness as a result of dispersive forces (i.e., film instability) to attain a minimum Gibbs energy in the thin film geometry.26 Such instability is reported for one- or two- dimensional metallic27 or polymeric nanostructures.28,29 On capping with a layer of SiO2 that is favorable to anchoring of the P2VP block, the structural transformation was totally suppressed.26 This result indicated that the interplay between film instability and Received: December 8, 2016 Revised: January 24, 2017 Published: January 24, 2017 2003

DOI: 10.1021/acs.langmuir.6b04410 Langmuir 2017, 33, 2003−2010

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Langmuir

Figure 1. (a) 5 × 5 μm2 AFM, (b) 2D GISAXS, and (c, d) corresponding 1D GISAXS in-plane profiles with intensity distribution versus q∥ for lamellar nanodomains of PS-b-P2VP. A rod-shaped beamstop was placed before the detector to attenuate the intense direct (TB) and reflected beams (RB). The position of the RB is the origin. The 1D GISAXS in-plane profiles resulted from horizontal scan cuts at (c) q⊥= 0.02 and (d) 0 Å−1. space. As carbonaceous residues are insoluble in HF, removing the capping layer with HF did not damage the carbon nanostructures. Apparatus and Characterization. The morphology of BCP nanodomains and carbon nanostructures was monitored with an atomic-force microscope (AFM, Seiko SPA400, tapping mode), a high-resolution field-emission scanning electron microscope (JEOL: JEM-7600F, field-emission source of 10 kV voltage), and a transmission electron microscope (TEM, Hitachi H-7100, 200 kV). Aluminum-coated silicon cantilevers were used with a force coefficient of 7.4 N/m; the resonance frequency (for length 150 μm, width 26 μm, and thickness 300 μm) was 160 kHz. The images were monitored at a scan rate of 1 Hz and with scanning pixels of 256 lines/frame. A Raman microscope was used to record the Raman inelastic scattering signals excited with a laser (wavelength 532 nm, power 5−30 mW); the duration of acquisition of each Raman spectral curve was 30 s. The Raman data were obtained with a microscope objective (100×) for Raman excitation and collection. The film thickness was characterized with an alpha-step (KLA tencor Alpha-Step IQ). Grazing-incidence small-angle X-ray scattering (GISAXS) was measured using Cu Kα Xrays produced with a rotating copper-anode generator (Nanoviewer, Rigaku, 1.2 kW, in vacuum, 40 kV and 30 mA, equipped with confocal max-flux optics). The scattering vectors in these GISAXS patterns were calibrated with a sample standard of silver behenate. To acquire scattering images with large signal-to-noise ratios, we set the angle of incidence of each X-ray beam to be αi = 0.2°. X-ray scattering images were collected with a 2D areal detector (Pilatus 100 K, 83.8 × 33.5 mm2). The duration of exposure of the GISAXS measurement was 20−30 min for each image. 1D GISAXS in-plane profiles with intensity as a function of q∥ were obtained with horizontal scan cuts at a given q⊥; q∥ and q⊥ are the components of the wave-vector transfer parallel with and perpendicular to the surface, respectively.

phase separation might extend the possible routes for nanoscale patterning of the nanostructures via self-assembly. Pursuing these ideas, we created carbon nanostructures in various patterns that were obtained via the thermally induced instability of a film and self-assembly during pyrolysis. We used thin films of PS-b-P2VP BCP having lamellar nanodomains oriented perpendicularly to the surface. The films were crosslinked with UVI and then pyrolyzed to produce carbon nanostructures, according to which the structural transformation evolved via Rayleigh instability to form chains of droplet-like nanocarbons proceeding through worm-like carbon nanostructures in an intermediate state. When capped with a thin layer of SiO2 and then pyrolyzed, the thin films carbonized into pod-, spaghetti-, and worm-like carbon nanostructures.

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EXPERIMENTS

Sample Preparation. A nearly symmetrical diblock copolymer PSb-P2VP having molar mass Mn = 265 000 g/mol (molar masses of the two blocks: MnPS = 133 000 g/mol and MnP2VP = 132 000 g/mol; molar mass distribution Mw/Mn = 1.15) (Polymer Source, Inc.) was used as received. We prepared films of thickness of approximately 90 nm on spin-coating solutions (1.5 mass %) of PS-b-P2VP in o-xylene at 1000 rpm (60 s) on cleaned Si wafer substrates. To form perpendicularly oriented lamellar nanodomains, we then placed thin films in a sealed jar (110 mL) filled with acetone (liquid, 300 μL) for 4 h at a temperature maintained at 21 ± 1 °C so that the thin films were exposed to saturated acetone vapor. After specimens were treated with solvent-vapor annealing (SVA, 4 h), those films were quickly removed from the solvent vapor and dried near 23 °C. After SVA, the films were directly cross-linked with UV irradiation under nitrogen (UVIN) for 6 h (UV lamp: a germicidal lamp of G20T10 light tube, 20 W, Sankyo Denki). The details of UVIN treatments are reported elsewhere.30 To deposit a capping layer of silicon dioxide (SiO2), we loaded the samples into a high-vacuum system for electron-beam evaporation; the base pressure in the deposition chamber was less than 5 × 10−7 Torr. An amorphous SiO2 (a-SiO2) thin film (purity 99.999%, thickness 40 or 100 nm) was deposited onto the polymer-film-coated Si substrate near 23 °C. The rate of deposition was controlled to be about 0.06−0.1 nm/s. During this deposition, the film thickness and rate of deposition were monitored in situ with a quartz-crystal microbalance. The UVINtreated films with or without a capping layer under gaseous argon were pyrolyzed in a one-zone diffusion furnace at 430 and 900 °C for varying periods to form carbon nanostructures. After pyrolysis, the specimens were dipped in HF to remove the capping layer and then measured with an imaging technique to show the morphology in real

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RESULTS AND DISCUSSION To form perpendicularly oriented lamellae, we annealed PS-bP2VP thin films in saturated acetone vapor for 4 h. As expected from the constituted volume fraction, a lamellar nanostructure with perpendicular orientation was developed for PS-b-P2VP with SVA in acetone. After spin coating, the lamellar nanodomains of the PS-b-P2VP film seemed to exhibit a periodic pattern composed of alternate mesas and trenches (Figure 1a). The interlamellar distance was 132 ± 4.1 nm. According to the constituted volume fractions of the two blocks and the solvent selectivity, the mesa region consisted of PS-rich domains whereas the trench region had P2VP-rich domains. The width of the PS nanodomains was 104 ± 3.5 nm; the width of the P2VP nanodomains was 28 nm (Figure S1a,b, Supporting Information). The height difference between the 2004

DOI: 10.1021/acs.langmuir.6b04410 Langmuir 2017, 33, 2003−2010

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Figure 2. (a) 5 × 5 μm2 AFM topographic image, (b) GISAXS 2D pattern, and (c, d) 1D profiles of the PS-b-P2VP film in plane with perpendicularly oriented lamellar nanodomains after pyrolysis at 430 °C (1 h) without a capping layer of SiO2 on top. The inset shows an SEM image. A rod-shaped beam stop was placed before the detector to attenuate the intense direct (TB) and reflected beams (RB). The position of the RB marks the origin. The 1D GISAXS in-plane profiles were made through horizontal scan cuts at (c) q⊥ = 0.02 and (d) 0 Å−1.

mesa and trench regions randomly fluctuated in the range of 3−6 nm (Figure S1c,d). The height difference was due to the distinct swelling behavior of PS and P2VP chains in the solvent vapors that were selective for one block or another.31 As the non-neutral solvent selectively swelled the chains of one block, the swollen block adopted an expanded conformation, whereas chains of the other block adopted a collapsed conformation. Upon exposure to acetone vapor that was an effective solvent for PS but a poor solvent for P2VP chains, the acetone content in the PS-rich nanodomains became greater than that in the P2VP domains. This nonequal swelling led to the much wider PS nanodomain compared to the P2VP nanodomain even though the constituted volume fractions of the two blocks were similar. After quick drying, the P2VP block with less solubility solidified earlier than the PS block. As the molar masses for both PS and P2VP were much greater than that for Me (the molar mass of the entanglement), the relaxation time for polymer chains to recover an unperturbed state was expected to be much greater. As a result, chains of the two blocks became kinetically trapped in a nonequilibrium state. The surface shape of the nanodomains was determined by the distinct surface tensions of the two phases.32 The PS-rich phase (γ = 32.1 mN/ m)33 with the surface tension smaller than that of P2VP (γ = 46.7 mN/m)34 had a convex curvature whereas the P2VP phase had a concave curvature (Figure S1a). The corresponding GISAXS 2D pattern and 1D profile displayed four diffraction rods and maxima at q∥ = 1:2:3:4 with the first-order signal at q∥ = 0.0052 Å−1 along the in-plane direction (Figure 1b−d). The q∥ ratio along the in-plane direction can be taken as an indication of the formation of lamellar nanodomains with periodic packing and perpendicular orientation. Figure 2a shows an AFM topographic image and a SEM image of the PS-b-P2VP film with perpendicularly oriented nanolamellae after pyrolysis at 430 °C (1 h). After UVIN treatment and pyrolysis, the interlamella distance was slightly altered, to 137 ± 3.1 nm; the width of the mesa base was 100.3 ± 1.1 nm; the height difference between the round mesa and the flat trench was 18.3 ± 1.2 nm (Figure S2a−c). The top of the mesas displayed a round shape; the bottom had a contact angle in the range of 48.2−49.9° (Figure S2d). The contact angle and round top were driven by surface tension. Close scrutiny of the AFM image indicated that some areas of the

mesa regions were present as discrete single granular nanocarbons arrayed in rows, indicating that the stripe-shaped carbon nanostructures were unstable and broke into droplets during pyrolysis. The corresponding GISAXS 2D pattern and 1D profile revealed Bragg diffractions in a series with the same q∥ ratio of 1:2:3:4 with respect to the first-order signal at q∥ = 0.0049 Å−1, indicating that the resultant carbon nanostructures retained their spatial order but exhibited a slightly expanded periodic long period. The Raman spectrum displayed G and D lines centered at 1594.6 and 1382.2 cm−1, indicative of the formation of carbon materials (Figure S3). Upon prolonged pyrolysis for 2, 3, and 7 h, carbon nanostructures of two types, short stripe-shaped nanostructures and rows of hemispherical carbon nanostructures, still coexisted (Figure S4a−c), but with time, rows of droplet-like nanostructures were present in major proportions via fission of the stripe-shaped carbon nanostructures. Scrutiny of the AFM images demonstrated that only the top half of, rather than the entire, stripe-like nanocarbons broke into droplet-like nanocarbons while the bottom retained a striped shape. The droplet-like nanostructures sat on the bottom half of the stripelike nanostructures with a contact angle in the range of 11.4 ± 0.5° (Figure S4d). The fragmentation of the relief stripes upon pyrolysis to chains of such discrete granular nanocarbons can be a result of Rayleigh instability to decrease the total surface energy.35,36 A linear stability revealed the most rapidly growing mode λ (i.e., the wavelength described in terms of the interdroplet spacing) for a harmonic perturbation during the initial stage of fragmentation. For a surface undulation of the stripe-shaped nanostructures, a value of λ ≈ 8x0 was predicted; x0 denotes the half-width of the stripe.35,36 According to the AFM observations, upon pyrolysis at 430 °C the stripe-shaped nanodomains of width L and thickness h were transformed into spherical carbon droplets proceeding through an intermediate stage of stripe-shaped nanocarbons of radius R0 and contact angle θ. According to the two geometries having a constant cross-sectional area, the half-width of the nanocarbons is given by36 x0 = R 0 sin θ =

sin θ θ − sin θ cos θ

Lh

(1)

The wavelength λ is thus 2005

DOI: 10.1021/acs.langmuir.6b04410 Langmuir 2017, 33, 2003−2010

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Figure 3. 2 × 2 μm2 AFM height (a−c) and SEM images (d−f) of the PS-b-P2VP films with perpendicularly oriented nanolamellae after pyrolysis at 430 °C for (a, d) 1, (b, e) 3, and (c, f) 8 h in the presence of a capping layer (thickness 40 nm) of SiO2 on top. After pyrolysis, the capping layer was removed on dipping the specimens in HF for AFM and SEM characterization. The granular nanoparticles were residual SiO2 that were not completely removed with HF. Scale bar: 1 μm.

λ=

8 sin θ θ − sin θ cos θ

Lh

stripe with x0 = 50.15 nm and θ = 49.5°. The discrepancy is ascribed to an effect that, when the original height of the lamellar nanodomains was less than 100 nm, a long-range force must be considered as a role for thermally excited capillary waves. The long-range van der Waals force accounting for the difference in Gibbs energy between the interface and the free surface of a thin film is described with37,38

(2)

The relation between the wavelength and the radius of the droplet was examined. The reason that the radius and wavelength of droplet-like nanocarbons were polydispersive and chaotic is that the structural parameters depended on the height difference between mesas and trenches, as interpreted below. First, we selectively examined one row of droplet-like carbon nanostructures that had an interdroplet distance in the range of 129.3 ± 8.8 nm (Figure S4e). The measured λ is less than the theoretical value (∼400 nm), predicted for decay of a

P(e) = −AH/12πe 2

(3)

in which AH is the Hamaker constant and e is the film thickness. Defining AH as the nonretarded Hamaker constant for media 1 2006

DOI: 10.1021/acs.langmuir.6b04410 Langmuir 2017, 33, 2003−2010

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Langmuir and 2 interacting across medium 3,33 we calculated approximate values of AH on combining relations, given by AH = A132 ≈ ( A11 −

A33 ) − ( A 22 −

A33 )

length of striped nanodomains was small, a complete structural transformation to grow spaghetti-like carbon nanostructures was achievable after pyrolysis at 430 °C for 1 h (Figure S6). Figure 4a,b shows AFM and TEM images of the PS-b-P2VP film with perpendicularly oriented lamellar nanodomains after

(4)

The excess interaction between the free surface and the substrate-contacted interface of a film can be either attractive (AH > 0) or repulsive (AH < 0). The excess interaction with positive AH tends to destabilize the film.37,38 It was predicted that the relaxation time and the initial fluctuations of the film surface for the most rapidly growing mode of a sufficiently thin (