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Science of Advanced Materials Vol. 5, pp. 1–7, 2013 (www.aspbs.com/sam)

Microwave-Assisted Synthesis of Nitrogen and Phosphorus Co-Doped Mesoporous Carbon and Their Potential Application in Alkaline Fuel Cells Venu Gopal Bairi1, ∗ , Shawn E. Bourdo2 , Udaya B. Nasini1 , Sunil K. Ramasahayam1 , Fumiya Watanabe2 , Brian C. Berry1 , and Tito Viswanathan1, ∗ 1 2

Department of Chemistry, University of Arkansas at Little Rock, 2801 S. University Ave, Little Rock, AR-72204 Center for Integrative Nanotechnology Sciences, University of Arkansas at Little Rock, 2801 S. University Ave, AR-72204

ABSTRACT

1. INTRODUCTION High surface area heteroatom-doped carbon materials are being rigorously explored due to a host of potential applications including lithium-ion batteries,1–3 supercapacitors,4–6 catalysts,7 8 and electrode materials.9 10 Dual heteroatom modified carbon materials exceed the performance of single heteroatom modified carbons in many touted applications.8 10–14 Heteroatom doped-carbon materials have been synthesized from resorcinol,4 11 15 melamine,5 16 17 formaldehydes,4 15 16 urea,18 dicyandiamide,8 17 graphite oxides,19 boric acid,10 11 20 phosphoric acid,11 ammonia15 19 and triphenylphosphines.21 Synthesis of heteroatom-doped carbons using above materials require prolonged heating (> 6 hours in most cases) in presence of inert gases,5 11 16 21 22 template guided synthesis,7 23 and other processes that are prohibitive to large-scale economical processing. None of the authors have reported yields for these kinds of materials.6–8 11 14–16 20 24 25 This article focuses on the synthesis of dual heteroatom-doped carbons using an efficient, economic and rapid (30 minutes) process of carbonization ∗

Authors to whom correspondence should be addressed. Email: txviswanathaualr.edu Received: xx Xxxx xxxx Accepted: xx Xxxx xxxx

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by microwave heating in ambient conditions with yields in the range of 18–21%. Properties of these carbon materials vary widely depending on their morphology,23 whereas controlling the morphology is very important for potential applications.23 26 Over the past decade extensive work has been performed on the development of solid carbon spheres.14 16 21 23 26 The solid carbon spheres were synthesized by pyrolysis of carbon rich polymer resins (resorcinol-formaldehyde4 15 23 and melamine-formaldehyde,16 23 furfuryl alcohol,23 27 styrene-divinyl benzene block copolymers,23 and carbohydrate sources.23 26 In contrast, this article presents a simple method to synthesize carbon microspheres doped with both phosphorus and nitrogen (PNDC) by carbonizing tannin-melamine-hexamine polymeric precursor in the presence of poly phosphoric acid. The structure of the tannin-melamine-hexamine polymer precursor is shown in Figure 1. The reaction mechanism for the polymerization of tannin and hexamine has been previously established.28 Tannins are environmentally friendly renewable resources that are polyphenolic and have been touted as a replacement for phenol in phenol-formaldehyde resins. Tannin has been previously reported as a precursor for both nanocarbons and heteroatom doped-carbons by few groups.22 29–31 Microwave carbonization of biomass,32 33 tannins30 31 34 has been reported elsewhere by one of the co-authors

1947-2935/2013/5/001/007

doi:10.1166/sam.2013.1583

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A simple, highly efficient, and rapid method for the synthesis of carbon microspheres doped with both phosphorus and nitrogen using microwaves is reported. The microwave assisted carbonization of a tannin-melaminehexamine polymer in the presence of polyphosphoric acid gives yields approaching 20.5% from the carbon precursor. The method uses no inert or reducing gas during the transformation. Elemental analysis from XPS studies confirmed the doping of nitrogen and phosphorus in a sp2 hybridized carbon lattice. The content of phosphorus is 2.98% while that of nitrogen is 1.12%. The spheres are mesoporous with a BET surface area of ∼1000 m2 /g and show significant promise in oxygen reduction reactions and are potential candidates for use in fuel cells. KEYWORDS: Tannin-Melamine-Hexamine Polymer, Microwave Carbonization, P,N-Doped Carbon, Carbon Microspheres and Mesoporous Materials.

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Microwave-Assisted Synthesis of Nitrogen and Phosphorus Co-Doped Mesoporous Carbon

Fig. 1.

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Synthesis of polymeric precursor from tannin, hexamine and melamine.

of this paper. Tannin-based magnetite carbon composites were previously reported for the removal of phosphates and arsenates from contaminated waters31 34 by one of us, and the current materials may also have use in such environmental remediation applications.

2. EXPERIMENTAL DETAILS

a wide evaporating dish and heated at 70 C to 80 C for a period of 12 hours to remove water. PNDC was obtained by microwaving (1.25 kW and 2.4 GHz frequency microwave) a mixture of 1.5 g of brown polymer and 0.5 g of polyphosphoric acid at full power for 30 minutes using alumina crucibles. The yield of the PNDC obtained was 041 ± 004 g, which is about 205 ± 20%. Polyphosphoric acid dehydrates the polymer and helps in

2.1. Synthesis In a typical procedure, 7.56 g of melamine was dissolved in 275 mL of hot distilled water, to which 17.28 g of unmodified tannin was added and allowed to stir, resulting in solution A. Solution B was prepared by dissolving 2.85 g of hexamine in 10 mL of distilled water. Solution B was added to solution A and continued to stir while heating, until the formation of a brown colored polymer was observed. The brown polymer was transferred to Table I. Elemental composition of PNDC and reduced PNDC compounds.

Atomic % PNDC

2

Nitrogen (N1s)

Phosphorus (P2p)

Oxygen (O1s)

Carbon (C1s)

1.12

2.98

10.72

85.19

Fig. 2. Survey scan of PNDC showing the presence of O1s, P2p, N1s and C1s elements. Sci. Adv. Mater., 5, 1–7, 2013

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Table II. Different bonding environments of carbon, nitrogen, oxygen and phosphorous in PNDC and their relative atomic percents. Name C1s C1s C1s C1s C1s

scan scan scan scan scan

A B C D E

Peak BE

At.%

Name

Peak BE

At.%

Name

Peak BE

At.%

Name

Peak BE

At.%

283.98 285.22 285.58 289.24 292.98

43.47 5.60 32.94 13.45 4.55

N1s scan A N1s scan B N1s scan C N1s scan D N1s scan E

394.09 398.13 400.49 403.59 405.95

3.85 17.71 61.77 12.47 4.20

P2p scan A P2p scan B

132.94 136.34

79.12 20.88

O1s scan A O1s scan B O1s scan C O1s scan D O1s scan E

529.95 531.79 533.00 534.39 535.99

19.86 43.00 22.37 6.70 8.06

ARTICLE Fig. 3. (a) Deconvoluted spectrum of N1s of PNDC compound. (b) Deconvoluted spectrum of P2p of PNDC compound. (c) Deconvoluted spectrum of C1s of PNDC compound. (d) Deconvoluted spectrum of O1s of PNDC compound.

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the process of carbonization. The obtained PNDC was characterized using different techniques.

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2.2. Characterization A Thermo K-alpha X-ray photoelectron spectrometer (XPS) was used to analyze the elemental composition of powder samples (Source-Al K radiation, 1486 eV, 12 kV, spot size-200 m, and carbon internal standard at 285.0 eV). Morphology of the PNDC sample was analyzed by JEOL 7000F scanning electron microscope (SEM). Brunauer–Emmett–Teller (BET) surface area and pore size distribution were estimated by nitrogen sorption studies on powder samples at a bath temperature of 77.3 K, by a Micromeritics Surface Area Analyzer ASAP-2020. Raman spectroscopy was performed on powder samples using a Horiba Jobin Yvon LabRam HR800 spectrometer equipped with 633 nm He–Ne laser. Cyclic voltammetry studies were performed separately in oxygen saturated 0.1 M KOH and nitrogen saturated 0.1 M KOH solvent system (Scan rate of 100 mV/S), with a glassy carbon working electrode, platinum counter electrode and Ag/AgCl reference electrode. UV-Visible spectroscopy (PERKIN-ELMER Lambda 19 UV-Vis/Near IR spectrophotometer) was performed on thin films of spray coated P,N-doped carbon (dispersed in dimethylformamide, DMF) and heated at 200 C until dry.

3. RESULTS AND DISCUSSION

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atomic%)5 15 18 19 24 25 35 moieties are the most dominant. The N1s deconvoluted spectra and the different binding energies of nitrogen’s are shown in Figure 3 and Table II, respectively. Pyridinic and pyrrolic nitrogen’s are important for many potential applications like oxygen reduction reactions (ORR),5 8 15 photocatalysis,18 19 and etc. Doping by nitrogen atoms in a carbon lattice increases the spin and charge densities of adjacent carbon atoms making them more reactive.19 35 36 The active regions generated are promising for oxygen reduction reactions and catalysis. Two different configurations for phosphorus have been identified from the deconvoluted P2p spectra, namely, P–O (132.94 eV, 79.12 atomic%)12 21 35 37 and a new high energy bond at 136.34 eV. The P2p deconvoluted spectra and approximate atomic abundance data are shown in Figure 3 and Table II. Owing to the presence of nitrogen, phosphorus and oxygen in the carbon lattice, carbon exhibits five different binding energies of which the graphitic/sp2 (283.98 eV, 43.47 atomic%)8 14 18 19 24 and carbonyl (285.58 eV, 32.54 atomic%)8 14 18 19 24 are the major carbon bonding environments. The C1s deconvoluted spectra and their atomic abundance data are shown in the Figure 3 and Table II. Deconvoluted O1s spectra show the existence of five different binding energies, the major ones being quinone (529.95 eV, 19.86 atomic%),11 18 38 carbonyl (531.79 eV, 43.00 atomic%)11 18 38 and C–O (533.00 eV, 22.37 atomic%).11 18 38 The O1s deconvoluted spectra and their atomic abundance data are shown in the Figure 3 and Table II.

3.1. XPS Characterization Elemental composition of the PNDC is listed in Table I. Doping of carbon by nitrogen and phosphorus is established by the existence of N and P in the XPS survey scans, shown in Figure 2. The deconvoluted N1s spectrum of the PNDC shows the existence of five different nitrogen configurations, out of which the pyridinic (398.13 eV, 17.71 atomic%)5 15 17–19 24 25 35 and pyrrolic (400.49 eV, 61.77

3.2. SEM Studies The as synthesized PNDC have a dense spherical morphology ranging from 0.5 m to 2.6 m in diameter as shown in Figure 4(a). A trivial amount of graphite-like planes/sheets are observed in Figure 4. The cross-section of some PNDC spheres exhibit a layered morphology which is shown as an inset in Figure 4(a), representing the dense nature of these spheres. The surface of the spheres

Fig. 4. (a) SEM image showing the spherical morphology of PNDC along with the graphite like flakes. Inset shows the cross-section of some PNDC spheres. (b) Image showing the formation of PNDC spheres from tannin-melamine-hexamine polymer matrix after carbonizing for 10 minutes. Inset demonstrating the growth pattern of the spheres inside the polymer matrix.

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carbon-precursor matrix, and eventually emerge out of the carbon matrix as a separate moiety shown in Figure 4(b). The inset in a high magnification image of Figure 4(b), demonstrating the formation of spherical structures inside the polymer matrix after microwaving for 10 minutes. After microwaving for 30 minutes, the spheres become separated from each other. 3.3. BET Surface Area

Fig. 5.

Nitrogen sorption linear isotherms of PNDC compound.

PNDC has a predominantly mesoporous nature, as the nitrogen adsorption isotherms were found to be Type II according to IUPAC classification,39 shown in Figure 5. The average pore size and total pore volume of the PNDC were found to be 21.0 Å and 0.54 cm3 /g respectively. BET analysis has found that PNDC is made up of predominantly mesopores (volume 0.360 cm3 /g) and as well as some micropores (volume 0.180 cm3 /g). The BET surface area of the material was found to be very high (ca. ∼ 1033 m2 /g). 3.4. Raman Spectroscopy

Raman spectrum of PNDC, showing D-band and G-band.

is very rough resulting in a high surface area that has been confirmed by, BET analysis. These solid spheres were produced as a result of the microwaving process and the growth pattern of these spheres was determined by performing SEM analysis of a 10 minute carbonized sample (shown in Fig. 4(b)). The spheres grow inside the

3.5. UV-Visible Spectroscopy The PNDC shows broad range of absorption in the UVVisible spectrum as shown in Figure 7(a). The maximum

Fig. 7. (a) UV-visible absorption spectra of spray coated PNDC film. (b) Tauc plot showing the direct band gap of PNDC compound calculated using the absorption coefficient. Sci. Adv. Mater., 5, 1–7, 2013

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Fig. 6.

The Raman spectrum shown in Figure 6 of the powdered samples exhibits two different bands at 1320 cm−1 and 1588 cm−1 , which corresponds to the D-band and G-band of the sp2 carbon atoms.8 19 21 25 37 The G-band is due to the specific vibrations of carbon atoms in the graphite crystal lattice plane and D-band arises due to the defects in the graphite crystal lattice. The D-band in N-doped carbons is due to the presence of N and P atoms in the carbon lattice which results in some sp3 -hybridized carbons. The ratio of intensities of the D-band and G-band (ID /IG ) gives information about defects in the carbon lattice.8 19 37 ID /IG value of the PNDC was found to be 1.39, which is in agreement with the other published values in the literature.


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References and Notes

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Fig. 8. Cyclic voltammograms of PNDC in oxygen saturated 0.1 M KOH solution and nitrogen saturated 0.1 M KOH solution (scan rate of 100 mV/S).

absorbance was found to be max (film)/nm at 314, with the onset of absorption around 600–700 nm. Direct band gaps of PNDC was found to be 2.68 eV as analyzed by the Tauc equation40 41 as shown in the Figure 7(b). 3.6. Cyclic Voltammetry Cyclic voltammograms of PNDC in oxygen saturated 0.1 M KOH and nitrogen saturated 0.1 M KOH is shown in Figure 8. The oxygen reduction peak disappeared, when the cyclic voltammetry was performed in nitrogen purged 0.1 M KOH solution. The ORR potential of the PNDC was found to be at −0407 V, with an onset potential of −0205 V. The oxygen reduction potential of the PNDC is found more towards the positive voltages and is comparable to other heteroatom doped carbon materials synthesized by different methods.14 21 37 38 42 This material could be promising for ORR applications owing to the presence of relatively high amounts of pyridinic and pyrrolic nitrogens.

4. CONCLUSION A facile, rapid and versatile method for the synthesis of heteroatom doped carbon materials employing an environmentally benign renewable carbon resource is reported and is unique in that the method uses no inert or reducing gas during the synthetic process. Due to the very high surface area, low ORR potentials and suitable band gap of PNDC, is must be considered superior to any other heteroatomdoped carbon materials which are currently in use. Visible light photocatalytic dye degradation, use in Li batteries, supercapacitor and fuel cell applications of these PNDC, with varied different doping levels are envisioned as future work. 6

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