Article pubs.acs.org/JPCC
Mesoporous Thin Films of Nitrogen-Doped Carbon with Electrocatalytic Properties Keun-Young Park,† Ji-Hoon Jang,‡ Jeung-Eun Hong,† and Young-Uk Kwon*,†,‡ †
Department of Chemistry, BK-21 School of Chemical Materials Science, and ‡SAINT/HINTSungkyunkwan University, Suwon, 440-746, Republic of Korea S Supporting Information *
ABSTRACT: Mesoporous carbon thin films (MCTFs) doped with nitrogen was synthesized by replication of mesoporous silica thin films. Nitrogen dopants were introduced by using nitrogencontaining polypyrrole as the carbon precursor through electrodeposition. The electron microscopic data on MCTFs showed replication with fidelity of the pore structure of mesoporous silica films. The nitrogen content of MCTFs is relatively high and a large fraction of them (∼40%) is pyridinic, the most catalytically active form of nitrogen dopants in carbon. Electrochemical characterization data show that MCTFs have high electrocatalytic properties for the iodine/iodide redox reactions and a large capacitance with the geometric capacitance of 3.92 mF cm−2, the largest for porous carbon materials synthesized by pyrolysis of carbon precursors. a carbon layer on a mesoporous silica film to improve the mesopore morphology and mesopore size distribution.12 Heteroatom, especially nitrogen, doped carbon materials such as N-doped graphene, carbon nanotubes, and carbon black are important research subjects in the field of electrocatalysis because they show the potential to replace the prevailing but expensive Pt.13−15 In this regard, it is desirable to develop direct fabrication methods of N-doped porous carbon films. However, none of the above-mentioned examples of direct fabrication has achieved it. Of the various forms of dopant nitrogen atom in carbon, pyridinic nitrogen is known to be the most active for catalysis. However, it seems that synthesis methods with rational designs to increase the content of pyridinic nitrogen is generally lacking. In this study, we report the synthesis of mesoporous carbon thin films (MCTFs) through replicating the pores of mesoporous silica thin films (MSTFs). Mesoporous materials are characterized by their regular and controllable pores with the pore dimension in the range of 2−50 nm.16,17 Because of these features, mesoporous materials, especially mesoporous silica materials, have been used as templates to synthesize porous carbon materials with controlled pore properties.18,19 In this regard, MSTFs can be a promising template for the synthesis of MCTFs. We used MSTF templates to form nanostructured polypyrrole (PPy) thin films inside the pores by electro-
1. INTRODUCTION Porous carbon materials have been the subject of extensive research due to their widespread applications in many fields.1−6 They have many advantageous features such as being lightweight and having chemical durability and electrical conductivity, in addition to the low cost.7 Recently, a new direction of application as electrocatalysts has emerged, raising the hope to replace expensive platinum.8 There are several scientific and technical issues in this field such as the control of the porosity and conductivity. In order for them to be used as electrocatalysts, they need to be dispersed and attached onto the electrodes. Although this is a dependable way for many of the cases, it is desirable to fabricate the porous carbon materials directly into thin films on the electrodes. The direct fabrication does not necessitate the use of binders or additives and, thus, can avoid the problems of resistance of various sources such as interparticle resistance and the contact resistance between the electrode carbon material and the current collector. However, there have been only a few examples of direct fabrication of porous carbon films in the literature. Gogotsi’s group has reported the conversion of a TiC film into a porous carbon film9 and Zhao’s group has reported the synthesis of a mesoporous carbon film through carbonization of selfassembled film between a surfactant and a carbon precursor.10 Composite carbon films with metal oxides have been reported: Vogt’s group introduced metal oxides to the carbon film for improvements in film morphology and observed enhanced electrochemical properties.11 Nishihara et al. synthesized mesoporous films of carbon and silica composites by coating © 2012 American Chemical Society
Received: April 3, 2012 Revised: July 12, 2012 Published: July 13, 2012 16848
dx.doi.org/10.1021/jp3031557 | J. Phys. Chem. C 2012, 116, 16848−16853
The Journal of Physical Chemistry C
Article
Scheme 1. Schematic Drawing of the Preparation of MCTFs
vs NHE) was used as a reference electrode and a Pt mesh as a counter electrode. For carbonization, PPy-deposited MSTFs were placed in a quartz tube furnace, purged with N2 for 30 min, heated to 200 °C for 2 h, then heated to 600 °C at a heating rate of 1 °C min−1, and kept at the final temperature for 2−5 h under a flow (30 cm3 min−1) of N2/H2 (volume ratio of 96:4). Finally, the MSTF tempates were removed by dissolving in a 1 wt % HF or a 2 wt % NaOH aqueous solution to obtain silica-free MCTFs on Pt-coated Si wafers. Electrochemical Study. Cyclovoltammetric (CV) measurements were conducted by a potentiostat (Ivium compactstat) in a three-electrode cell composed of a Ag/AgCl reference electrode, Pt mesh counter electrode, and the MCTF as the working electrode. Electrolyte of HClO4, HCl, or NaCl was used in 0.1 M concentration. For the electrocatalysis study on a I−/I3− redox couple, MCTFs on FTO substrates were cycled at a scan rate of 50 mV s−1 in an electrolyte composed of 10 mM LiI, 1 mM I2, 0.1 M LiClO4, and acetonitrile. 2.3. Characterization. XRD patterns were obtained by a Rigaku D/max 2200 Ultra diffractometer with Cu Kα (λ = 1.54056 Å) radiation. The TEM images were obtained using a JEOL JEM-3011 (300 kV). FESEM images were acquired using a JEOL 7000F (5 kV). Platinum coating (3 nm) was used for cross-sectional SEM images. For the elemental analysis of MCTFs, Fourier transform infrared (FT-IR) spectroscopy (BRUKER, Optics/vertex 70), confocal Raman spectroscopy (WITEC, alpha 300 m), energy-dispersive X-ray spectroscopy (EDS) attatched to the SEM, and X-ray photoelectron spectroscopy (XPS) were used. For XPS a monochromatic Mg Kα source (1253.6 eV) was used and XPS spectra were collected in the fixed pass energy mode using a Concentric Hemispherical Analyzer (CHA).
chemical deposition. Carbonization under an inert atmosphere and removal of the MSTF by a HF or NaOH solution produced MCTFs (see Scheme 1). The MCTFs so synthesized are strongly adherent to the substrate and the carbon material is doped with nitrogen atoms, the feature of which is beneficial for applications such as electrochemical capacitors, catalysts, or catalyst supports.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Mesoporous Silica Thin Films. MSTFs were synthesized by spin coating precursor solutions composed of tetraethylorthosilicate (TEOS, Aldrich), nonionic surfactant F-127 ((EO)106(PO)70(EO)106, EO = ethylene oxide, PO = propylene oxide, Aldrich), absolute ethanol (99.9%, Merck) and HCl (35.0−37.0%, Aldrich) on a substrate, followed by aging and calcination. The precursor solution was stirred at 20−25 °C for 20 h under a controlled relative humidity (RH) of
