Synthesis of carbon microrings using polymer blends as templates

Abstract

Carbon microrings were produced using a template based on phase separation of amylose/pentadecyl phenol (PDP)/dimethyl sulfoxide (DMSO) mixtures. The two-step phase separation of the mixture showed a micron-sized porous structure in which the rim of the pores was enclosed by rings of PDP. PDP was converted into a carbon precursor resin via reaction with formaldehyde vapor followed by pyrolysis under nitrogen atmosphere, leading to a carbon microstructure with ring morphology. The products before and after pyrolysis were characterized by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). AFM and SEM confirmed the ring morphology of the samples obtained after pyrolysis. The presence and phase of carbon were confirmed by EDX and XRD, respectively. This study demonstrates a straightforward novel route to prepare carbon microrings from a phenolic precursor using amylose as a matrix material.

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1. Introduction

Since the discovery of carbon nanotubes, carbon micro- and nanostructure materials have attracted significant attention due to their unusual chemical, optical, thermal and electronic properties useful for future innovations.^1,2 Due to their unique properties, carbon materials find potential applications in electronics, catalysts, medical industry, energy storage devices, sensors, textiles, etc.^3–6 Numerous methods have been developed to synthesize carbon-based micro and nanostructures such as arc discharge, laser ablation, chemical vapor deposition, soft templates, etc.^3,4,6,7 Besides carbon nanotubes, nanoparticles, nanowires and nanorods, other complex structures such as ellipsoids and rings also have attracted significant attention due to their distinct physical and chemical properties.^8,9 The ring-shaped morphology demonstrated different features in comparison to tubes, wires, dots, and spherical structures. For example, Shea and co-workers have reported a negative magnetoresistance of carbon nanorings at low temperatures.¹⁰ Furthermore, micro and nanorings can be used as high-finesse optical resonators which can find potential applications in optoelectronics,¹¹ telecommunications,¹² enhanced lithium storage properties,¹³ and bio-sensing.¹⁴

Different solubility of polymers, block copolymers and polymer blend mixtures in common solvents lead to phase separation during evaporation of the solvent, which can generate an organized morphology on micro- or nanoscale. Block copolymers can self-assemble to form various phase separated well-ordered microstructures and nanostructures.^15–17 Recently, phase separation of polymer blend solutions has attracted significant attention for the straightforward fabrication of ordered microscale structures.^18–20 Kim and co-workers have reported a unique microporous structure fabricated via a two-step phase separation upon drying of a ternary polymer solution.²¹ A ring-like morphology was formed during solvent evaporation through two-step phase separation. In this morphology the rim of each pore is surrounded by a low molecular weight polymer. Similarly, unique internal ring structures were fabricated with assistance of surfactants in polymer blend mixtures.²²

In the present study, we fabricate porous ring morphologies using amylose with a phenolic surfactant in dimethylsulfoxide (DMSO) and apply them as a template to fabricate carbon microrings. 3-Pentadecylphenol (PDP) is a substituted phenol and can be easily converted into a phenolic resin.^23,24 Amylose is a natural linear polysaccharide with glycosidic α-(1 → 4) linkages, having around 20–30% fraction of starch.²⁵ The template showing a ring morphology was fabricated via two-step phase separation of an amylose/PDP/DMSO solution followed by transformation of PDP into a precursor for carbon resin. In the final step, the inert pyrolysis was performed to convert the carbon precursor into amorphous carbon. The amylose acts as a matrix to synthesize rings of PDP. Recently, Yu et al. synthesized metal/carbon nanocables by a hydrothermal metal-catalyzed carbonization process from starch and metal salts as starting materials.²⁶ Hu et al. reported the synthesis of various carbon nano-architectures from carbohydrates and bio-waste products.²⁷ In spite of various synthesis methods,^3,12,28,29 the preparation of carbon microrings from biobased materials has been scarcely reported in the literature. Here we present an alternative use of amylose (environmental friendly natural resource) as a cheap and widely abundant building block for the synthesis of carbon micro- and nanostructures.

2. Experimental section

2.1 Materials

Amylose with a molecular weight of ca. 180 kg mol⁻¹ was obtained from Avebe. Silicon wafers of [100] orientation were purchased from Semiconductor Wafer Inc. 3-Pentadecylphenol (PDP) was obtained from Sigma-Aldrich and recrystallized three times from petroleum ether, filtered and dried in vacuo at 40 °C before use. Ammonium hydroxide (NH₄OH, 30 wt%), hydrogen peroxide (H₂O₂), dichloromethane (DCM), dimethylsulfoxide (DMSO), and formaldehyde (CH₂O) were purchased from Sigma-Aldrich and used as received.

2.2 Preparation of ring morphology

Silicon wafers were used as a substrate to prepare thin films of amylose/PDP/DMSO solutions. The wafers were cleaned prior to the deposition of films with DCM in a ultrasonic bath for 20 min followed by immersion in a solution of 40% H₂O, 30% H₂O₂ and 30% NH₄OH at 65 °C for 1 h. Subsequently, the wafers were rinsed with demi water and dried with an air flow. Amylose (200 mg) and PDP (20 mg) were dissolved in 2 ml DMSO at 80 °C. A spin coater was used to deposit the film with a speed of 500 rpm for 60 s followed by 1000 rpm for 60 s. The films were dried under a continuous flow of N₂ for 10 min. PDP was transformed in a phenol–formaldehyde resin, which further acted as a precursor for carbon, by treating the films with formaldehyde vapor at 110 °C for 5 h. Finally, samples were pyrolyzed at 700 °C for 3 h under a N₂ atmosphere to convert the resin into carbon material.

2.3 Characterization

The morphology of the samples was obtained using a Digital Instruments EnviroScope NANO Scope III AFM in tapping mode. Veeco RTESPW silicon cantilevers (f_o = 240–296 kHz and k = 20–80 N m⁻¹ as specified by the manufacturer) were used for the AFM measurements. The SEM images were captured on a JEOL 6320F Field Emission Microscope operating at 1.5 kV with a beam current of 1 × 10⁻¹⁰ A. A powder diffractometer (Bruker D8) using a radiation source of CuKα with a wavelength of 1.54 Å was used for XRD measurements.

3. Results and discussion

Recently, Fan et al. reported the fabrication of unique microring structures from self-assembly of ternary polymer solutions.²² In this paper, we extended the application of this approach to the formation of carbon microrings. The ring morphology was formed in the first step and used as a template to fabricate carbon microrings in the final step. Fig. 1 shows a schematic outline of the fabrication process of those carbon microrings. First, an amylose/PDP mixture was dissolved in DMSO. The thin films composed of amylose/PDP/DMSO were spin-coated on a silicon wafer and dried under nitrogen flow. Upon drying of the sample under a continuous flow of nitrogen, phase separation of the ternary amylose/PDP/DMSO solution had occurred. The phase separation of binary and ternary polymer blends can lead to different structure formations on silicon surfaces.³⁰

Fig. 1 Schematic representation of the preparation of carbon microrings. (a) Amylose and PDP were mixed in DMSO and spin-coated onto the silicon wafer; (b) cross-linking of the amylose/PDP film with formaldehyde vapor at 110 °C; (c) carbon microrings after pyrolysis at 700 °C under nitrogen atmosphere.

In the present study, a ring morphology was observed and the process of ring formation is believed to be caused by the two-step phase separation. PDP and amylose have different solubility in DMSO and after the spin-coating deposition, DMSO started to evaporate from the sample which led to changes in the concentration of PDP and amylose within the film. The high molecular weight and relatively less soluble amylose first solidified on the substrate in the course of primary phase separation and induced a bilayer configuration. This kind of vertical phase separation can be used to fabricate sharp-edged and round protrusions in polymer thin films from polymer blend mixtures.³¹

After solidification of amylose, PDP is still swollen in DMSO and further evaporation of solvent initiated interfacial instabilities between the amylose/PDP-rich phase (or PDP/DMSO) and film/air interface, that led to a break up of the bilayer configuration and induced a secondary phase separation.³⁰ A continuous flow of nitrogen was used to create an up- and down flow motion of the fluid which generated a temperature gradient between the upper (PDP-rich) and lower layer (amylose-rich) of the fluid.³² The temperature gradient led to changes in the surface tension of the interfacial region and the film surface, and initiated a spinodal decomposition of the ternary mixture, which led to the formation of disordered pores on the surface.^33–38

Due to its higher surface tension, PDP was aggregated at the film surface of the pores and created a ring at the rim of the pore as reported previously.^18,21 The size of the pores and rings mainly depends on the chemical composition and reaction conditions. Furthermore, the effect of different solvents, substrates, humidities, solution concentrations, weight ratios and molecular weight of the components are critical to tune the morphology of the microstructure and detailed investigations were reported elsewhere.^{18,20,21,37,39,40}

The obtained ring morphology was cured into a highly cross-linked phenolic resin in the presence of formaldehyde vapor at 110 °C for 5 h.^24,41,42 The cross-linked resin was carbonized under a nitrogen atmosphere at 700 °C for 3 h to obtain the carbon microrings.

The surface morphology of dried spin-coated samples was examined by AFM. Fig. 2(a) and (b) show respectively the height and amplitude image of the amylose/PDP sample after evaporation of DMSO. Both the height and amplitude image reveal a ring topography in the film and this morphology coincides with previously reported structures.^21,22 The average diameter and depth of the ring are ca. 1.3 μm and 80 nm, respectively. The obtained ring size was smaller compared to the reported rings from polymer ternary solutions (5–10 μm).^21,22 This reduced size may be due to the lower molecular weight of PDP that increases the miscibility of the polymer mixture resulting in a change of the width of the air–polymer interface.⁴³ From the above explanation about the ring formation, it can be fairly concluded that the yellow color around the pore in the height image (Fig. 2a) originates from PDP, while the remaining matrix is amylose. The amplitude image (Fig. 2b) confirmed the microstructure of the thin film sample.

Fig. 2 AFM images of a film from amylose/PDP/DMSO after nitrogen annealing: (a) height image and (b) amplitude image. Lateral scale 40 μm × 40 μm.

The obtained ring morphology was utilized as a template to fabricate carbon microrings. However, before the carbonization process, highly cross-linked resin was obtained by exposing the samples to formaldehyde vapor. This crosslinking had minimum perturbation on the ring morphology.⁴¹ The crosslinking reaction rate can be controlled by the reaction time and vapor pressure of the formaldehyde.^42,44

In the final step, the cross-linked samples were introduced into a furnace for pyrolysis under nitrogen atmosphere at 700 °C for 3 h in order to obtain carbon microrings. Fig. 3 shows the AFM height and phase images of the carbon microrings obtained after this treatment. The organic moiety was decomposed during the pyrolysis and pure carbon is expected in the final product. It is clear from the AFM images that the ring morphology remains intact even after pyrolysis. The carbon microrings have an average diameter around 1.2 μm and a height of 45 nm. Pyrolysis at a higher temperature resulted in microrings with a relatively smaller diameter and height.

Fig. 3 AFM images of microrings after pyrolysis at 700 °C for 3 h: (a) height image and (b) phase image. Lateral scale 20 μm × 20 μm.

The rim of each pore is mainly surrounded by a ring of phenolic precursor and therefore a higher amount of carbon was expected in the ring surface compared to the remaining matrix. The relative amount of carbon was confirmed by EDX analysis. Fig. 4 shows the EDX spectra of ring and matrix surfaces of the sample obtained after pyrolysis. Those spectra revealed the characteristic peaks of C, O and Si at 0.28 keV, 0.53 and 1.74 keV, respectively. Silicon and oxygen peaks originate from the substrate (silicon wafer, oxidation of Si) and the carbon peak arises from carbonization of the film. It can be fairly concluded from EDX analysis that the carbon amount is significantly higher in the rings compared to the flat surface of the matrix.

Fig. 4 Elemental distribution of ring and flat surface after pyrolysis. Ring surface (black line) and matrix surface (red line).

The topography of the carbon microrings was further analyzed by SEM. Fig. 5 shows a SEM image of microrings obtained after carbonization of the sample. SEM confirmed the conservation of the ring shape even after high temperature pyrolysis, as shown earlier by AFM characterization. The ring diameter varied from 700 nm to 1.4 μm.

Fig. 5 SEM images of carbon microrings obtained after the pyrolysis of thin films containing amylose/PDP at 700 °C for 3 h.

The phase of carbon obtained after pyrolysis was studied by X-ray diffraction (XRD). Fig. 6 shows the XRD pattern of the carbon microrings. The pattern reveals broad peaks at 22.7° and 44° which are the characteristics peaks of amorphous carbon. Liang and co-workers have reported similar results for the preparation of mesoporous carbon films through carbonization of a nanostructured phenolic resin and block copolymer composites.⁴¹

Fig. 6 XRD diffraction pattern of the sample obtained after pyrolysis.

4. Conclusions

We have demonstrated a novel template-based method for the fabrication of carbon microrings. The template was prepared by a two-step phase separation method. Amylose formed the matrix and the rim of the pores was enclosed by PDP rings during the first and second phase separation of the amylose/PDP/DMSO solution, respectively. Samples were cured and carbonized in order to obtain carbon rings. The morphology was characterized by AFM, SEM, EDX and XRD. The carbon microrings, due to distinct electrical and chemical properties, have potential applications in microelectronics, energy storage, and medical diagnostics.^13,45,46 Further optimization of reaction conditions such as controlled humidity, more precise evaporation of solvents etc. could improve the long-range order of the rings and we are working in this direction.

Acknowledgements

We are grateful to the group of Solid State Materials for Electronics (Zernike Institute for Advanced Materials, University of Groningen, The Netherlands) for access to the X-ray diffraction spectrometer. The authors thank Dr Petr Formanek from Leibniz Institute of Polymer Research Dresden, Germany for his help in EDX analysis. This work was financially supported by a VIDI grant from the Netherlands Organization for Scientific Research (NWO).

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Footnote

† Present address: Functional Organic Materials and Devices, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands.

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