Block-Copolymer-Templated Hierarchical Porous Carbon

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Block-Copolymer-Templated Hierarchical Porous Carbon Nanostructures with Nitrogen-Rich Functional Groups for Molecular Sensing Ya-Sen Sun,*,† Chien-Fu Lin,† Shih-Ting Luo,† and Ching-Yuan Su‡ †

Department of Chemical and Materials Engineering and ‡Graduate Institute of Energy Engineering, National Central University, Taoyuan 32001, Taiwan S Supporting Information *

ABSTRACT: The self-assembly of a block copolymer offers access to micellar nanodomains with tunable dimensions and structural diversity through control of such molecular parameters as the volume fraction and molecular mass. We fabricated hierarchical porous carbon (HPC) nanostructures with bundles of aggregated nanospheres and with nitrogen-rich functional groups through pyrolysis of diblock copolymer micelles in multiple layers. The resultant HPC nanostructures with a considerable specific surface area serve as an excellent substrate for surface-enhanced Raman spectroscopy (SERS), coupled with fluorescence quenching, for molecular sensing of physically adsorbed Rhodamine 6G. The abundant nitrogen atoms terminating on the surface of HPC nanostructures play a critical role in promoting a large Raman enhancement generated via a chemical mechanism. Most importantly, the observed enhancement factors show a clear dependence on the mesoscale porosity within HPC nanostructures, indicating that the chemical enhancement can be steadily tuned with control over the interfacial areas as a function of the nanosphere size. The unique architecture of HPC nanostructures based on the construction of a building block of a well-defined network of core−shell nanospheres provides a new design strategy for fabricating SERS substrates. KEYWORDS: hierarchical porous carbon nanostructure, block copolymer, surface-enhanced Raman scattering, chemical mechanism, thin film

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INTRODUCTION With their ability to self-assemble into ordered nanodomains with dimensional tunability and morphological diversity, block copolymer (BCP) materials have aroused great interest.1−3 The unique features of the self-assembly of BCP materials through control of such molecular parameters as the block volume fraction, molecular mass, and segregation strength create opportunities to obtain access to fabricating carbon nanostructures of zero, one, two, or three dimensions.3−10 There are two approaches to using BCP nanodomains to produce carbon nanostructures upon pyrolysis at elevated temperatures.4−23 According to the first approach, self-assembled BCP nanodomains serve only as a template to guide the conversion from small organic molecules to carbon nanostructures with a welldefined morphology during pyrolysis.3−14 In this approach, the BCP template acts a sacrificed component that is totally degraded, and the carbon yield is from added small organic precursors and not from the BCP during pyrolysis. The second approach involves conversion of BCP nanodomains directly to carbon nanostructures because the BCP itself is an organic material and can be regarded as a source of solid carbon.15−25 Cross-linked BCP nanodomains have been demonstrated to be © XXXX American Chemical Society

pyrolyzed directly into carbon nanostructures with morphological fidelity without the addition of small organic precursors of great thermal stability.15−25 To retain this morphological fidelity and to improve the yield of solid residues, sample pretreatments involving thermal annealing15−19,21,22 or exposure to ultraviolet light20,24,25 are further required to stabilize the BCP nanodomains through cyclization or cross-linking reactions before pyrolysis at a temperature above the decomposition temperature (Td) of the polymer. Recent reports have demonstrated that stabilization and subsequent pyrolysis of BCP materials with one block composed of nitrogen-rich components, such as poly(acrylonitrile) (PAN) or poly(vinylpyridine) (PVP), can directly produce carbon nanostructures with electrochemical properties such as catalytic activity and capacitance.21−25 These unique electrochemical properties were ascribed to the presence of nitrogen species in a large proportion originating Special Issue: Block Copolymers for Nanotechnology Applications Received: November 29, 2016 Accepted: February 28, 2017

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DOI: 10.1021/acsami.6b15317 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces from the PAN or PVP block. Besides the use of PAN and PVP that tend to lead to high nitrogen doping, there are additional routes to obtaining high nitrogen doping in the BCP hybrids with additives.11−14 The current scope of the study of BCPtemplated carbon nanostructures focuses mainly on their fundamental synthesis and primary electrochemical properties.11−14,21−25 No attempt has been devoted to the use of such carbon nanostructures with new phenomena and applications in molecular sensing through Raman spectra. Raman spectra provide an important tool for the detection of trace molecules,26−28 but because Raman scattering has a small cross section, Raman signals always appear with small intensities.29,30 For dye molecules, the incident light from a laser inevitably generates strong fluorescence signals but weak Raman signals under resonance conditions.29,30 As a result, Raman signals become submerged in the fluorescence background of the spectrum, increasing the difficulty of the analysis of Raman data for molecular sensing. This situation calls for the use of a dedicated substrate with rough surfaces or nanoparticles to diminish the fluorescence background and simultaneously to enhance the Raman signals. Various noblemetal,31−34 transition-metal,35 semiconductor-based35,36 and even carbon-based26,37−41 nanostructures are being used as substrates to promote surface-enhanced Raman spectroscopy (SERS) through an electromagnetic mechanism (EM) or a chemical mechanism (CM).27,28,35 In particular for metal nanostructures, the two mechanisms concurrently contribute to the Raman spectrum with disparate degrees of influence of its appearance. Because the EM makes a major contribution to enhancement of the Raman intensity, most Raman studies have focused mainly on the EM. Little is understood about the CM because the EM inevitably entangles with the CM. Carbonbased nanomaterials, such as graphene sheets, graphene quantum dots, and sheets of reduced graphene oxide as well as oxidized graphene, have attracted great interest in the investigation of Raman enhancement because they cannot generate an EM contribution in the visible region.26,37−41 They thus provide an excellent opportunity to study a CM-driven Raman enhancement separately from the EM. In this work, we explore the possibilities of Raman enhancement from BCP-templated HPC nanostructures based on the CM. Herein we propose a robust, efficient, and template-free strategy for the synthesis of HPC nanostructures with a nitrogen-enriched surface area by cross-linked and carbonized polystyrene-block- poly(x-vinylpyridine) (PS-bPxVP, where x = 2 or 4) BCP micelles. The overall fabrication of HPC nanostructures is illustrated in Scheme 1. BCP was first dissolved in a selective solvent with adequate solubility for the PS block but with poor solubility for the PxVP block. In solution, the BCP chains tended to form monodispersive micelles with a core PxVP −shell PS structure at a small concentration;42−44 the micelles served as building blocks for HPC nanostructures. Upon spin coating, the micelles were deposited onto a solid substrate. Before pyrolysis, those micelles were exposed to ultraviolet irradiation (UVI) in vacuum or under gaseous nitrogen to promote intra- and intermicelle cross-linking,24,25,44 which enables the stabilization of BCP micelles and assures the formation of HPC nanostructures not susceptible to collapse upon pyrolysis (step i). Meso- and macroscaled porosities are tuned mainly by the micellar dimension and number density of packing through varied lengths of the PS and PxVP blocks. After UVI, the yield of solid carbon was improved, and its morphological fidelity

Scheme 1. Schematic Illustration of a Template-Free Strategy for the Synthesis of HPC Nanostructures with a Nitrogen-Enriched Surface Area by Cross-Linked and Carbonized PS-b-PxVP (x = 2 or 4) BCP Micelles through UVI under Gaseous N2 (Step i) and Pyrolysis (Step ii) and R6G Molecules Prepared on HPC Nanostructures for Molecular Sensing through the Raman Spectra (Step iii)

was retained during pyrolysis (1 h) at 450 °C for PS-b-P2VP or 500 °C for PS-b-P4VP, in a furnace filled with gaseous argon (step ii). Next, the structural and chemical details of nitrogenenriched HPC nanostructures were identified by means of imaging in real space, small-angle X-ray scattering in reciprocal space, and X-ray photoelectron spectroscopy (XPS). Rhodamine 6G (R6G) molecules were deposited on HPC nanostructures supported on a SiOx/silicon (Si) substrate for molecular sensing through Raman spectra (step iii). The possible CM of the enhancement factors (EFs) for Raman signals of adsorbed R6G molecules on HPC nanostructures is proposed and discussed.

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EXPERIMENTAL SECTION

Preparation of HPC Nanostructures. PS49-b-P2VP70 (MnPS = 48500 g mol−1; MnP2VP = 70000 g mol−1; Mw/Mn = 1.15), PS5-b-P4VP5 (MnPS = 5000 g mol−1; MnP4VP = 5000 g mol−1; Mw/Mn = 1.18), PS10b-P4VP10 (MnPS = 10000 g mol−1; MnP4VP = 10000 g mol−1; Mw/Mn = 1.08), PS21-b-P4VP21 (MnPS = 21000 g mol−1; MnP4VP = 21000 g mol−1; Mw/Mn = 1.15), PS24-b-PEO21 (MnPS = 24000 g mol−1; MnPEO = 21000 g mol−1; Mw/Mn = 1.09), PS10-b-PMMA10 (MnPS = 10000 g mol−1; MnPMMA = 10000 g mol−1; Mw/Mn = 1.05), PS21-b-PMMA21 (MnPS = 21000 g mol−1; MnPMMA = 21000 g mol−1; Mw/Mn = 1.07), and homopolymer polystyrene (hPS290; MnhPS = 290000 g mol−1; Mw/ Mn = 1.08) were purchased from Polymer Source, Inc., Canada, and used as received. Homopolymer poly(2-vinylpyridine) (hP2VP152; MnhP2VP = 152000 g mol−1; Mw/Mn = 1.04) was purchased from Aldrich and used as received. PS-b-P2VP powders were dissolved in oxylene, and PS-b-P4VP powders were dissolved in a binary mixture of toluene/tetrahydrofuran (THF) (7/3) at 70 °C for 2 h and cooled to room temperature (ca. 25 °C) to yield 2 mass % solutions. Before use, the solutions were aged for at least 1 day. Then micellar films were spin-coated at 1000 rpm (60 s) onto cleaned silicon wafer substrates with an oxide layer of 300 nm (SiOx/Si). It has been demonstrated that the solvents used are selectively good for PS but poor for PxVP.42−44 Thus, at low concentrations, the BCPs in the selective solvents tended to form spherical micelles with a core−shell structure. Each spherical micelle generally comprised a compact PxVP core and a swollen PS corona. The substrates were cleaned in a piranha solution B


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Figure 1. (a) TEM image, (b) top-view SEM image, (c) GISAXS 1D in-plane profile (symbols) obtained with a horizontal scan cut, (d) highresolution TEM image, and (e) Raman spectrum of HPC nanostructures prepared from pyrolysis (450 °C) of UVIN-treated PS-b-P2VP micelles. The lines of part c denote the fitted curves based on the models. [30% H2O2/98% H2SO4 = 3/7 (v/v)] for 40 min, rinsed with deionized (DI) water three times, and subsequently dried under a N2 flow. To stabilize micellar nanodomains, the as-spun films were directly cross-linked by UV irradiation in nitrogen (UVIN) for 6 h (UV lamp: a germicidal lamp of G20T10 20 W light tube, Sankyo Denki). The details of UVIN treatments were reported elsewhere.44 The UVIN-treated films were pyrolyzed in a one-zone diffusion furnace at a target temperature for 1 h in argon gas to grow HPC nanostructures. Raman Study. Rhodamine 6G (R6G; Aldrich) of 10−5 M aqueous solution was used as a probe dye molecule for Raman measurements. To avoid variations in the adsorption affinity and deposition rate for R6G onto different types of substrates, we directly deposited R6G molecules by directly dropping a 280−300 μL droplet of the 10−5 M R6G aqueous solution onto the substrates (HPC/SiOx/Si and SiOx/ Si). This drop-cast approach allowed the solution to quickly cover the substrate surface so that the R6G molecules could immediately adsorb on the substrate surface at an identical density. After drop-casting and drying, the specimen with R6G molecules was rinsed with a great quantity of DI water to remove the free dye molecules and then dried under flowing N2. A Raman microscope (UniRAM system) was used to observe the Raman inelastic scattering signals excited by a low-noise DPSS laser with a wavelength of 532 nm, a laser power of ∼1 mW, and a spot size of about 1 μm on the samples. The Raman data were obtained with a 100× (NA = 0.8) objective lens for Raman excitation/ collection. The acquisition time of each Raman spectral curve was 30 s. Apparatus and Characterization. The morphology of HPC nanostructures was monitored by atomic force microscopy (AFM; Seiko SPA400) in tapping mode, high-resolution field-emission scanning electron microscopy (SEM; JEOL JSM-7600F) with a field-emission source of energy 10 kV, and transmission electron microscopy (TEM; Hitachi H-7100) performed at 125 kV. Highresolution transmission electron microscopy (HR-TEM; JEM2100) was performed at 200 kV. The analysis of chemical components was carried out with a XPS system (Thermal VG-Scientific, Sigma Probe) with microfocus monochromator Al anode X-ray. Aluminum-coated silicon cantilevers were used with a force coefficient of 7.4 N/m, and the resonant frequency was 160 kHz (length = 150 μm, width = 26 μm, and thickness = 300 μm). UV−vis absorption of R6G adsorbed on HPC and graphene supported on quartz was measured with a V-670 UV−vis/near-IR spectrophotometer (JASCO Analytical Instruments). Grazing-incidence small-angle X-ray scattering (GISAXS) measure-

ments were implemented with a Nanoviewer (Rigaku) using Cu Kα Xrays produced by a rotating copper-anode generator (Nanoviewer, Rigaku) operated at 1.2 kW in a vacuum (40 kV and 30 mA) equipped with confocal max-flux optics at National Central University. The scattering vectors in these GISAXS patterns were calibrated by a sample standard of silver behenate. To acquire scattering images with high signal-to-noise ratios, the angle of incidence of each X-ray beam was αi = 0.2°. X-ray scattering images were collected with a 2D areal detector (Pilatus 100 K of 83.8 × 33.5 mm2). The exposure duration of the GISAXS measurement was 20−30 min for each image. It has been demonstrated that GISAXS enables structural characterization of nanostructures supported on a surface of a solid or buried in thin films.45−47 For BCP-templated HPC nanostructures buried within the specimens, we further analyzed GISAXS 1D in-plane profiles to reveal their structural details, including pore surface characteristics, pore shape, size distribution, and pore-network structure with an Igor Pro using simulated scattering models.48 GISAXS 1D in-plane profiles were obtained with horizontal scan cuts along the Yoneda stripe. Thermal pyrolysis produced carbon spherical nanostructures with an interconnected mesoporous framework, which are analogous to polydisperive spheres being in contact with the surrounding air. Therefore, the in-plane GISAXS intensity is modeled with I(q ) = (Δρ)2 VpP(q ) S(q )

(1)

where Δρ and Vp are the scattering length density (SLD) contrast between the carbon spheres and the “air” matrix and P(q∥) and S(q∥) are the form factor and structure factor scattering from the spherical carbon nanostructures. The form factor P(q) of spherical nanostructures of radius R is given by49

⎡ sin(q R ) − q R cos(q R ) ⎤2 ⎥ P(q ) = ⎢3 3 ⎢ ⎥ ( ) q R ⎣ ⎦

(2)

The Schultz distribution function was introduced into eq 2 to yield a polydispersity in the radius. In addition, there is an additional intensity scattered by interconnected mesoporous channels of a few nanometers as a result of the packing of carbon nanostructures. For the irregular mesopores or micropores, the scattering intensity decay as a function of q∥ can be C


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ACS Applied Materials & Interfaces described by a Guinier−Porod empirical model to determine the size and dimensionality of porous structures.48,49 This model has the following form, given by

⎡ − q 2R 2 ⎤ g ⎥ I(q ) = I0 exp⎢ ⎢ 3 ⎥ ⎦ ⎣

I(q ) = Aq −Df + B

Table 1. Results of Fitting a GISAXS 1D In-Plane Profile for HPC Nanostructures Fabricated from Pyrolysis at 450 °C of the PS-b-P2VP Film Parameters Used for the Guinier−Porod Model

for q ≤ 1/R g

for q ≥ 1/R g

6.5 Rg/nm Porod exponent 3.5 Parameters Used for Polydispersive Spheres

(3) (4)

mean radius/nm polydispersity carbon sphere SLD/Å−2 pore SLD/Å−2

where I0 is the number density, Rg is the radius of gyration, A is a constant, B is the background, and Df is the power-law exponent. To simplify the curve fitting, we ignored modeling simulations for S(q ∥) by assuming S(q ∥) ∼ 1 because the resultant HPC nanostructures are disordered and the scattered intensity by the form factor almost dominates the GISAXS profiles.

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23.1 0.15 3.27 × 10−5 1.10 × 10−8

1d). The corresponding Raman spectra of the resultant carbon layer show two broad Raman lines centered at shifts of 1361.4 and 1594.4 cm−1 (Figure 1e); these Raman shifts arise from amorphous and graphitic nanostructures of carbon. The broadening of the two Raman shifts is ascribed to the overlapping of several underlying Raman shifts at 1219.5 (assigned as band I), 1355.2 (band D), 1583.3, (band G), 1487 (band D″) and 1624 (band D′) cm−1. Band G corresponds to a E2g vibrational mode, whereas the D, D′, and D″ bands reflect the formation of disordered species and defects.50 The crystallite size of the graphite grain was calculated to be ∼20.2 nm based on the ratio of bands G to D. XPS spectra were recorded to determine the chemical composition of the as-prepared HPC. The high-resolution N 1s spectrum of HPC reveals the presence of four nitrogen functional forms: pyridinic nitrogen (N6 at 399.0 eV), pyrrolic nitrogen (N5 at 400.3 eV), graphitic nitrogen (NQ at 401.4 eV), and nitrogen oxide (NO at 403.0 eV).51,52 Quantitative analysis of the four forms depicted in Figure 2a indicates that, among these nitrogen-containing species, both N6 and NQ exist in major proportions, whereas N5 and NO are present in minor proportions. Figure 2b presents the C 1s spectrum of HPC, in which five lines appear: 284.6 eV (trigonal carbon), 285.4 eV (tetrahedral carbon), 286.2 eV (C−N bonds), 288.2 eV (carbonyl group), and 291.7 eV (π → π* shakeup satellite of aromatic rings).53−55 The atomic ratio of nitrogen to carbon was estimated to be ∼1:11.7 from quantitative analysis of the XPS spectra. A possible structure of carbon- and nitrogencontaining species within HPC based on the above analysis is illustrated schematically in Figure 2c. Raman spectra of R6G dye molecules deposited on HPC/ SiOx/Si and SiOx/Si were recorded with excitation at 532 nm from a laser. To avoid variation of the adsorption affinity and rate of deposition of R6G onto substrates of various types, we deposited molecules by dropping dye molecules in droplets of aqueous solution (10−5 M) directly onto a substrate. This dropcast approach allows the solution to quickly cover the substrate surface so that the R6G molecules become immediately adsorbed onto the substrate surface at an identical density. Excitation at 532 nm from a laser was used to generate Raman spectra of R6G dye molecules. We first compared the Raman spectra of R6G molecules on HPC/SiOx/Si and SiOx/Si before and after rinsing with DI water (see Figure 3a). After natural drying without rinsing with DI water, the substrates became covered with R6G molecules in two conformations: R6G molecules adsorbed onto the underlying substrate and free R6G molecules. Upon rinsing with DI water, the free R6G molecules were removed so that only the adsorbed R6G molecules remained on top of the substrate. As Figure 3a shows, in the absence of HPC, the R6G dye molecules on SiOx/Si fluoresced

RESULTS AND DISCUSSION An amphiphilic PS-b-P2VP BCP was first used as carbon and nitrogen precursors for the synthesis of HPC materials through pyrolysis at 450 °C. Our previous work demonstrated that the PS-b-P2VP BCP exhibited P2VP core radii of 22.3−24.9 nm with PS shell thicknesses of 22.0−26.3 nm and polydispersities of 0.12−0.15 in o-xylene.42 Spin coating from a given concentration (i.e., 2 mass %) led to the formation of randomly packed micelles.24,25 Because at such a large coverage density the PS chains would interpenetrate to form a continuous matrix, the morphology of randomly packed micelles resembled discrete spherical P2VP domains dispersed within the PS matrix.25 Quantitative analysis of the GISAXS 1D in-plane profile based on a model of polydispersive spheres indicates that the dispersive P2VP nanodomains have a mean core radius of 18.3 nm and a polydispersity of 0.17 (not shown). The decreased size of the P2VP nanodomain was due to the removal of solvent after drying. After UVI-induced cross-linking followed by pyrolysis, the structural details of the resultant HPC nanostructures were first characterized by TEM and SEM. The images reveal that the carbon nanomaterial (CN) was composed of individual nanospheres stacked together, forming bundles of aggregated spheres (Figure 1a,b). The resultant HPC contains pores of three types: macropore, mesopore, and micropore. Mesopores and macropores arise from the space unoccupied by close and loose aggregates of nanospheres, micelle shrinkage, and intermicelle fusion during pyrolysis. Micropores form mainly as a result of the loss of mass of volatile gaseous molecules generated through chain scission reactions (see Scheme 1 and Figure 1b). Close scrutiny of the SEM image (Figure 1b) indicates that the micropores are present predominantly at the surface of the bundles of aggregated spheres as a surface fractal-like structure. GISAXS was further utilized to characterize the structural details of HPC nanostructures on a nanometer scale. On the basis of an established model (see the Experimental Section), we further analyzed the GISAXS 1D profile with the intensity distribution as a function of q∥ obtained with a horizontal cut to reveal their structural details, including the characteristics of the pore surface, pore dimension, and size of the bundles of aggregated spheres, in a quantitative manner. Table 1 lists the structural parameters extracted from the curve fitting, which shows that HPC nanostructures have an average radius of 23.1 nm and a polydispersity of 0.15. The mesoporous channels have the size Rg = 6.5 nm and fractal dimension 3.5. The high-resolution image shows graphitic grains with visible crystalline lattice fringes. The spacing of these fringes is about 0.34 nm, which corresponds to graphite planes (002) (Figure D


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Figure 2. XPS spectra of (a) N 1s, (b) C 1s, and (c) possible structure of carbon- and nitrogen-containing species within HPC fabricated with pyrolysis at 450 °C of PS-b-P2VP micelles. The fraction of each component for nitrogen- and carbon-containing species is designated in parentheses.

associated with C−C stretching in the plane of the xanthene ring; the line at 1309 cm−1 is associated with breathing of the xanthene ring; the line at 1185 cm−1 is assigned to deformations in the plane of the xanthene ring. Intense lines at 613 and 773 cm−1 are respectively associated with a deformation vibration out of the plane of the xanthene ring and the C−H bending vibration out of the plane.56,57 The Raman spectrum of R6G adsorbed on the surface of HPC resembles the RRS spectrum of R6G in an ethanol solution with excitation at 488 nm56 and the SERS spectrum of R6G adsorbed on colloidal silver nanoparticles58 and hydrogenterminated Si nanostructures with excitation at 532 nm.36 These Raman shifts and their relative intensities shown in Figure 3c are also generally in agreement with the simulated spectrum of R6G.59 To measure the Raman signals of adsorbed R6G, we prepared also carbon nanosheets on SiOx/Si substrates through pyrolysis of cross-linked homopolymer PS and P2VP layers at 450 °C. As Figure 4 shows, with excitation at 532 nm, R6G molecules showed Raman signals on the carbonized P2VP layer, whereas such Raman signals were absent on the pyrolyzed PS layer. Furthermore, the D and G lines associated with the carbon feature were present with large intensities for the pyrolyzed P2VP layer, whereas those carbon lines were absent for the pyrolyzed PS. This discrepancy indicates that pyrolysis at 450 °C significantly degraded the PS layer, which is further supported by the morphological observation with an AFM that shows a featureless surface (see the inset of Figure 4b). This feature of the surface is associated with the surface of the underlying SiOx/Si substrate because of severe degradation

strongly without showing Raman lines associated with R6G vibrational modes. Even after rinsing with water, only the second-order Raman scattering of silicon was visible (see curve iii of Figure 3b). In contrast, several sharp Raman lines, coupled with a suppression of the fluorescence, emerged for R6G on HPC/SiOx/Si (see curve i of Figure 3b). Before rinsing with water, the spectrum showed sharp R6G Raman lines superimposed upon a broad continuum; upon rinsing with water to remove the free R6G molecules, the broad continuum was absent. This result indicates that the broad continuum was dominated by fluorescent emission produced from free R6G molecules, which was quenched for adsorbed R6G in contact with HPC. After the removal of free R6G molecules, the Raman lines became even sharper, indicating that the Raman signals were dominated mainly by adsorbed R6G. The existence of free R6G molecules hence weakened the Raman intensities of the adsorbed R6G because of the absorption of incident and scattered photons. As curve i of Figure 3b shows, after the removal of the excess free R6G, 13 Raman lines, with an improved resolution, at 613, 639, 660, 772, 1127, 1188, 1311, 1363, 1422, 1507, 1537, 1572, and 1648 cm−1, were readily identified for R6G adsorbed on HPC and were assigned to vibrational modes ν53, ν54, ν55, ν65, ν96, ν103, ν115, ν117, ν127, ν146, ν147, ν151, and ν154, respectively.56 These lines possessed diverse relative intensities. The Raman signals of R6G in the range 1000−1700 cm−1 are overlapped with those of HPC. Upon subtraction of the carbon Raman signals, the Raman spectrum of R6G adsorbed on HPC was consistent with those previously reported36,56−59 (see Figure 3c). The lines at 1648, 1573, 1507, and 1363 cm−1 are E


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in a monolayer on carbonized PS-b-PEO and PS-b-PMMA nanodomains (see Figure S1). As Figure S1 shows, the adsorbed R6G molecules display small Raman signals with weak intensities. The above control experiments demonstrate that the P2VP component plays a critical role in enhancing the Raman signals. Fluorescence quenching can be considered to be the first reason for the resulting Raman spectra of R6G on HPC. In principle, for R6G, the cross section (σFL) for bare fluorescence is typically ∼10−16 cm2sr−1,29,30 the value of which is much greater than the corresponding differential Raman cross section (σRaman ∼ 10−23 cm2sr−1)30,60 by a factor about 107 for excitation at a given wavelength near the absorbance maximum. Despite the fact that the use of carbon materials was demonstrated to suppress fluorescence significantly with decreasing σFL from 10−16 to 10−19 cm2sr−1,29 the quenching of fluorescence induced by the underlying substrate alone is insufficient to compensate for the remaining factor 104 of efficiency between σFL and σRaman; surface-field effects due to an interaction between R6G molecules and CNs thus account for the observable Raman signals of R6G adsorbed on HPC. Graphene and its derivatives were reported to serve as a platform for efficient applications of SERS.26,37−41 Graphene nanomaterials can simultaneously suppress the fluorescence background and enhance the Raman signals via a chargetransfer (CT) mechanism. Such a chemical enhancement, coupled with fluorescence quenching, thus makes the vibrational fingerprints of fluorescence dyes visible in Raman spectra with improved signal-to-noise ratios. For graphene derivatives, local electric fields generated upon adding polar elemental species or creating structural defects within graphene can even induce much more intense Raman signals.38−41,61 For comparison, we measured the Raman signals of R6G in a monolayer adsorbed on top of a monolayer of CVD-grown graphene sheets. As Figure S2 shows, the R6G molecules adsorbed on graphene show Raman intensities much smaller than those adsorbed on HPC. Some Raman signals, such as those at 639, 660, 1127, 1422, 1537, and 1572 cm−1, are not even readily recognizable for R6G on graphene. The second prominent feature is that the observed Raman lines show almost no spectral shift between the two cases; albeit R6G on graphene shows Raman lines with much smaller intensities. No spectral shift is ascribed to similar physical interactions for R6G on these CNs.38 To realize quantitatively the surface enhancement of the Raman signals of R6G on the CNs (HPC and graphene), we

Figure 3. Raman spectra, excited at 532 nm, of R6G (10−5 M) dropcast on HPC/SiOx/Si (navy lines) and SiOx/Si (black lines) (a) before and (b) after rinsing with DI water. In part b, a Raman spectrum, excited at 532 nm, of HPC is shown as a red line for comparison. The position of the second-order silicon Raman line marked with a star served as a reference to normalize the intensity. (c) After that normalization, the Raman signals associated with HPC were subtracted from the Raman spectrum to show only the Raman signals of R6G.

of PS after pyrolysis at 450 °C. In contrast, under the same UVIN treatment for cross-linking and pyrolysis for carbonization, the P2VP layer retained solid carbon in a large yield. The small thermal stability of PS also explains the absence of the Raman signals of R6G on the PS layer pyrolyzed at 450 °C. We also measured Raman spectra of adsorbed R6G molecules

Figure 4. Raman spectra of adsorbed R6G molecules in a monolayer on pyrolyzed (a) P2VP and (b) PS layers supported on SiOx/Si. Raman spectra of pyrolyzed P2VP and PS layers are shown for comparison. The insets are AFM topographic images of carbonized P2VP and PS layers. Scale bar: 1 μm. The position of the second-order Si Raman line marked with a star served as a reference for intensity normalization. F


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613, 772, and 1363 cm−1, in particular, have much greater enhancement. Two further possible factors are discussed to interpret the varied intensity enhancement of separate Raman lines. The vibrational modes at 613 and 772 cm−1 have been demonstrated to gain enhanced intensity greater than other lines in the wavenumber range 1000−1800 cm−1 by a vibronic coupling effect resulting from molecular resonance59,63 or by a CT effect through specific interactions of the dye molecule with the substrate.36,57,64 The vibronic coupling effect given by molecular resonance depends on the wavelength of excitation and modifies the intensities of the two modes at 613 and 772 cm−1 when an excitation wavelength is near the maximum or vibronic shoulder of the UV−vis absorption of R6G.56,59 The second effect in terms of CT could involve either covalent bonding or physical adsorption of dye molecules onto the substrate.36,57,64 For the former case, a surface complex forms through chemical binding between the molecule and substrate. As a result, Raman enhancement given by a CT resonance through the surface complex leads to the emergence of new lines or spectral shifts in the Raman spectra.27,28 Furthermore, the intensity and position of the CT lines upon the presence of the surface complex depend on both the surface potential41,65 and energy of excitation.66 Tuning the surface potentials of the substrate or the energy of the light enable CT to occur in either the substrate-to-molecule or molecule-to-substrate direction, which depends on the relative energies of the Fermi level (or the valence and conduction bands) of the metal (or semiconductor) substrates and the highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the adsorbed molecules.27,28 In contrast, Raman enhancement might occur when the molecule adsorbs physically onto the substrate; in this case, there is no spectral shift in the Raman spectra.38 Furthermore, for physical adsorption, a CT resonance occurs in a ground state, whereas for chemical bonding, it occurs in an excited state.65 As Figure S3 shows, R6G molecules on HPC show a UV−vis absorption spectrum with a maximum at 540.7 nm and a vibronic shoulder at 504.5 nm. We detected the Raman spectra of R6G with excitation at 532 nm, which is near the maximum UV−vis absorption. If the vibronic coupling effect predominantly accounted for the intensity enhancement, the R6G molecules adsorbed on both HPC and graphene should have the same vibronic coupling effect under excitation at 532 nm; the two lines should appear with the same enhancement for R6G adsorbed on both HPC and graphene; the molecular resonance is hence not the main reason for the selective enhancement. A CT resonance with green light at 532 nm from a laser possibly explains the enhanced magnitude at 613 and 772 cm−1. No spectral shift in the Raman spectra (Figure S2) and no difference in the UV−vis absorption spectra (Figure S3) indicate that the R6G molecules are physically adsorbed on both HPC and graphene. In either case, the evidence indicates that a CT resonance occurred through physical adsorption rather than chemical binding and that it resulted in the noticeably enhanced two lines at 613 and 772 cm−1. As characterized by XPS, the nitrogen component within HPC contains four nitrogen configurations (i.e., N6, N5, NQ, and NO). N5, N6, and NO are typically located at edge and defect sites, whereas NQ exists within graphitic domains. Implantation of nitrogen atoms with a strong electron affinity has been demonstrated to result in electron-deficient graphitic domains at positions near nitrogen species.67 Nitrogen-induced charge delocalization could enhance the physical adsorption of

calculated approximately a rough first-order EF of every signal with EF = (IA/CN − ICN)C A/SiO2 /I A/SiO2CA/CN

(5)

in which IA/CN and IA/SiO2 are the Raman intensities of a specific line of analyte (A) adsorbed on CNs and SiOx/Si at number densities CA/CN and CA/SiO2. Because HPC itself has broad Raman signals such as lines G and D overlapping the Raman signals of adsorbed molecules in wavenumber range 1000− 1900 cm−1, the carbon signals should be subtracted from the Raman spectra to obtain the EF. Before background subtraction, all intensities of the Raman signals were normalized based on the intensity of the second-order Raman scattering shift of silicon as a reference signal. Figure 5,

Figure 5. EFs of Raman lines of R6G in a monolayer adsorbed on HPC and graphene.

summarizing the EF, shows the EF obtained for the seven Raman modes of R6G at 613, 772, 1188, 1311, 1363, 1507, 1572, and 1648 cm−1 on the graphene range between 2.5 and 9.4, which are comparable with the reported values.62 In comparison with the monolayer graphene, those lines for R6G on HPC nanostructures gain some additional intensity. The enhancement of the main peaks is greater, and the EFs are in the range 20−47. The vibrational modes reveal varied EFs; the most noticeable enhancement is obtained for the lines at 613 (EF 46.7), 772 (EF 25.9), and 1363 cm−1 (EF 39.7). Mechanisms for the Enhancement of R6G on HPC. Two mechanisms are reported to account for the Raman enhancement due to surface fields: EM and CM.27,28 EM involves localized surface-plasmon resonance (LSPR) resulting from laser-excited collective oscillations of electrons in the conduction band of the nanosize or roughened metallic surface; it therefore results in enhancements of 106−108 times.31−34 The CM is based on a CT effect or dipole−dipole interactions between the underlying substrate and the molecule, accounting for enhancements of 10−100 times.27 Because LSPR occurs at only terahertz frequencies for carbon materials, the EM can be excluded;37 the enhanced Raman signals for R6G on HPC are thus ascribed to CM. Here we propose two sources of the resulting Raman enhancement for R6G on HPC through CM: graphitic domains and nitrogen-induced charge redistribution. The presence of graphitic domains favors a π interaction with the xanthene ring of R6G and creates an effective path for charge conductivity and transition. This effect accounts for the enhanced Raman lines at large wavenumber, reflecting vibrational modes of the xanthene ring stretching for R6G adsorbed on graphene. This effect fails to explain that the R6G molecules adsorbed on HPC show greater EF than those on graphene and that the intensities of the vibrational modes at G


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Figure 6. (a) Raman spectra and (b) EFs of R6G molecules adsorbed on HPC nanostructures fabricated from pyrolysis at 500 °C of cross-linked spherical micelles of PS5-b-P4VP5 (black curve), PS10-b-P4VP10 (red), and PS21-b-P4VP21 (blue). The Raman curves shown in part a are shifted for clarity along the ordinate axis. The inset in part b show EFs as a function of the surface areas of the HPC nanostructures for two selected bands at 613 (black squares) and 772 (red circles) cm−1.

dye molecules through a combination of π and dipole−dipole interactions;68 the electronic structure of R6G could, consequently, be perturbed on nitrogen-enriched HPC. Such a perturbation results in an altered molecular polarizability tensor and leads to a varied Raman efficiency of the vibrational modes. In addition to the CM, active surface areas for Raman enhancement should be considered. Carbon nanostructures fabricated from pyrolysis of cross-linked BCP nanodomains provide a large surface area. The structural tuning of micropores and macropores with control over the micellar dimension can produce varied surface areas. To recognize the influence of the surface area on the Raman enhancement, we prepared symmetric PS-b-P4VP micelles of varied size at a concentration of 2 mass % in a binary mixture of toluene/THF (70/30). Our previous work demonstrated that the PS-b-P4VP micelle in the binary liquid mixture consisted of a hydrophilic P4VP core and a swollen hydrophobic shell of PS.43 The dimensions of the micelles with a core−shell structure were governed by symmetric PS-b-P4VP BCP of three molecular masses. After cross-linking and subsequent pyrolysis at 500 °C for 1 h, the PS-b-P4VP micellar nanodomains were converted to HPC nanostructures, which we characterized by SEM, TEM, and GISAXS (see Figures S4 and S5). Quantitative structural details were provided on the curve fitting of the GISAXS 1D inplane profiles and are summarized in Table S1. According to the structural parameters, the fraction of voids and surface areas at the nanoscale could be approximately estimated (see the surface area analysis in the Supporting Information). As Figures S4 and S5 and Table S1 show, the PS-b-P4VP micelles of small molecular mass grew small HPC nanostructures with a small porous size, whereas an opposite trend was observable for PS-bP4VP micelles of large molecular mass. Decreasing the dimensions of the HPC nanostructure decreased the pore size but increased the number density; as a result, the surface area increased (see Table S1). We recorded Raman spectra of R6G in a monolayer physically adsorbed on these HPC nanostructures and calculated the corresponding EFs. As Figure 6 shows, R6G physically adsorbed on the HPC nanostructures fabricated from pyrolysis of PS-b-P4VP micelles of small molecular mass shows the greatest intensities and EF values, indicating that the Raman enhancement depends on the surface areas of the HPC nanostructures (see the inset of Figure 6b).

nanostructures. It has the ability to tune the size of the pores and the surface areas of HPC nanostructures through control of the molecular mass of the BCP. The Raman characterization has demonstrated that HPC nanostructures serve as a source of chemically enhanced Raman intensity of R6G molecules. The graphitic domains and nitrogen sites within the HPC play an important role in promoting efficient CT and enable the physical adsorption of molecules with enhanced Raman intensity. The observed highly enhanced Raman signals on HPC nanostructures provide a new point of view for the design of carbon-based substrates through pyrolysis of cross-linked BCP micelles. In particular, the self-assembly of a BCP offers access to varied ordered nanodomains with tunable shape and morphology through control of such molecular parameters as the volume fraction, molecular mass, segregation strength, BCP concentration in solution, and even solvent selectivity.69 The unique advantages, morphological diversity, and dimensional tenability of BCP nanodomains might lead to BCP-templated HPC nanostructures with tunable pore size and surface area. These findings also make HPC nanostructures of great interest for light-harvesting applications in the realm of molecular detection through Raman spectra.

CONCLUSION In summary, we have demonstrated that the use of BCP micellar nanodomains is a novel method to fabricate HPC

Ya-Sen Sun: 0000-0002-7480-0380

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15317. Raman spectra (Figure S1) of R6G on carbonized PS24-bPEO21, PS10-b-PMMA10, and PS21-b-PMMA21 nanodomains, Raman (Figure S2) and UV−vis absorption (Figure S3) spectra, SEM and TEM images (Figure S4), GISAXS 1D in-plane profiles (Figure S5), and structural parameters used for the curve fitting (Table S1) of HPC nanostructures fabricated from 500 °C pyrolysis of PS-bP4VP thin films of various MWs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ORCID Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS Financial support from the Ministry of Science and Technology (Grants MOST 103-2221-E-008-084-MY3 and MOST 1042221-E-008-125-MY3) is gratefully acknowledged. We thank the Instrument Center of National Central University and beamline BL23A at the National Synchrotron Radiation Research Center for measurement of the GISAXS data.

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