Modifying Carbon Nitride through Extreme Phosphorus Substitution

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Modifying Carbon Nitride through Extreme Phosphorus Substitution Qianqian Wang, Huiyang Gou, Li Zhu, Haw-Tyng Huang, Arani Biswas, Brian L. Chaloux, Albert Epshteyn, James P. Yesinowski, Zhenxian Liu, George Cody, Mengdong Ma, Zhisheng Zhao, Yingwei Fei, Clemens Prescher, Eran Greenberg, Vitali Prakapenka, and Timothy A. Strobel ACS Materials Lett., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Materials Letters

Modifying Carbon Nitride through Extreme Phosphorus Substitution Qianqian Wang†*, Huiyang Gou†‡, Li Zhu†, Haw-Tyng Huang , Arani Biswas , Brian L. Chaloux §, Albert Epshteyn§, James P. Yesinowski§, Zhenxian Liu¶, George Cody†, Mengdong Ma , Zhisheng Zhao , Yingwei Fei†, Clemens Prescher#, Eran Greenberg , Vitali Prakapenka , and Timothy A. Strobel†* † Geophysical ‡ Center

Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA

for High Pressure Science and Technology Advanced Research, Beijing 100094, China

Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, USA §

Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, DC 20375, USA



Institute of Materials Science, Department of Civil and Environmental Engineering, The George Washington University, Washington, DC 20052, USA State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China #

Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Straße 49b 50674, Köln Center for Advanced Radiations Sources, University of Chicago, Chicago, Illinois 60637, USA

ABSTRACT: A glassy carbon phosphonitride material with bulk chemical composition roughly approximating C3N3P was synthesized through a high-pressure, high-temperature (HPHT) process using a pure P(CN)3 molecular precursor. The resulting material (hereafter referred to as “HPHT-C3N3P”) was characterized using a variety of techniques, including Xray scattering; pair distribution function (PDF) analysis; 31P, 13C, 15N magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies; X-ray photoelectron spectroscopy (XPS); and Raman and IR spectroscopies. The measurements indicate that HPHT-C3N3P lacks long-range structural order with a local structure composed of a predominantly sp2, s-triazine-like network in which phosphorus atoms substitute for bridging nitrogen sites found in related C3N4 materials. The HPHT-C3N3P sample exhibits semiconducting properties, with electrical transport dominated by variable-range hopping. The high phosphorus content of HPHT-C3N3P (approaching 13 at.%) leads to a major decrease in the optical absorption edge (~0.4 eV) and a ~1010-fold increase in electrical conductivity, as compared to previouslyreported P-doped graphitic g-C3N4 (0.6-3.8 at.% P). The HPHT-C3N3P sample is considerably harder than layered g-C3N4 and exhibits superior thermal stability up to ~700 °C in air. These results demonstrate a remarkable range of tunable properties possible for C3N4-related materials through elemental substitution and provide valuable information to guide the design of new materials.

Carbon nitride materials have received considerable interest for decades from both scientific and technological perspectives. Hypothetical threedimensional C3N4 frameworks predicted from ab initio calculations are promising candidates for superhard materials due to their strong covalent C-N bonds.1-4 Additionally, graphitic C3N4 (g-C3N4) has been investigated as a metal-free, non-toxic semiconductor (Eg 2.7~2.8 eV) that demonstrates photocatalytic activity for water splitting and CO2 reduction.5-7 However, synthesis routes and materials properties for C3N4–based materials require optimization in order to realize specific applications including energy conversion and advanced structural performance. For example, light absorption of g-C3N4 is not optimized in the visible region of the spectrum where sunlight has its maximal intensity, and

crystalline sp3-bonded 3D C3N4 materials have not yet been definitively synthesized. Different methods, such as structural amorphization or the intentional introduction of impurities, have been applied to alter and improve the optical, mechanical and electrical properties of C3N4-based materials. Amorphous C3N4 possesses a reduced optical absorption edge (1.90 eV) compared to the crystalline layered counterpart (2.82 eV), leading to superior activity for hydrogen generation in the visible light region.8 Phosphorus-doped g-C3N4 (0.6-3.8 at. % P) has been synthesized through chemical methods in order to tailor the performance of g-C3N4 for photoelectric applications.9 The band gap decreases with increasing phosphorus content, as observed via optical absorption spectroscopy and the sample color. The Pdoped g-C3N4 samples exhibit increased electrical

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conductivity and enhanced photocurrent generation, while maintaining the layered structure g-C3N4 based on the presence of crystalline Bragg peaks.9-12 Crystalline phosphorus tricyanide P(CN)3 is an ideal molecular precursor to produce heavily P-substituted C3N4-like materials for study. Chaloux et al. reported an amorphous carbon phosphonitride extended material obtained by thermal self-condensation of molecular P(CN)3 powder sealed under vacuum.13 The amorphous structure of this C3N3P material was characterized as a 3D network polymer of disordered, P-bridged s-triazines and s-heptazine-like “phosphohexazines,” that also contains unreacted nitriles. These samples lacked graphitic, longrange order with a measured density of 1.85 g/cm3 and thermal stability to well over 700 °C when heated in air. Recently, a similar material was prepared via solvothermal growth from P(CN)3 in the form of thin films.14 A systematic investigation was also previously conducted on the structural and vibrational behavior of molecular P(CN)3 at ambient temperature as a function of pressure. A recoverable C3N3P material resulting from the self-reactions of P(CN)3 above 10 GPa was obtained; however, no characterization of bulk properties was performed.15 In this work, HPHT-C3N3P samples were produced by pressurizing P(CN)3 to 12 GPa and heating at 1000 °C for 30 minutes using a multianvil press (see Experimental Methods in the Supporting Information). After the HPHT process, samples were recovered to ambient conditions for characterization. The recovered samples displayed irregular conchoidal fractures and a glassy-carbon-like luster, indicating the production of a nominally amorphous and brittle material. The X-ray scattering patterns of the recovered material show broad peaks indicating the lack of long-range order (Supporting Information). Raman spectra show broad peaks similar to the D and G bands of previously-reported amorphous carbon nitride solids (Supporting Information).8,16 Based on comparisons with reports of various amorphous carbons and carbon nitrides, such as a-C3N4,8,16,17 the X-ray and Raman results indicate that the recovered HPHT material has amorphous character with a large fraction of sp2 bonding. Energy- and wavelength dispersive X-ray spectroscopy (EDS/WDS, Supporting Information) indicate that phosphorus, carbon and nitrogen appear homogeneously distributed in the sample on the subnanometer scale, and yield an average bulk composition of C3.0±0.3N2.6±0.2P0.8±0.1. We suggest that extensive P substitution for N via this molecular pathway help stabilize this carbon nitride analog, given that other carbon nitrides decompose into graphite and nitrogen under similar temperature and pressure conditions.18-21 Since amorphous materials lack long-range order, structural information is accessible primarily through spectroscopic techniques that probe local bonding and electronic structure. Fourier transform IR spectra of HPHT-C3N3P (Figure S2), indicate the absence of both amine groups (usually present in C3N4-related materials from melamine-containing precursors) and adsorbed water molecules / surface hydroxyl groups. The HPHT-

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C3N3P sample exhibits several broad IR absorption bands between ca. 800-1600 cm-1, which are different from the vibrational frequencies of the P(CN)3 starting material, but similar to C3N4-related materials including amorphous C3N3PO3S and “triazine-based graphitic carbon nitride” (TGCN).13,16,22-26 The lower-frequency absorptions are also consistent with phenyl-phosphorus stretching modes as in P(C6H5)3–type compounds.27 Based on the observed vibrational modes, the basic local structure of HPHT-C3N3P is inferred to be P-bridged heteroatomic C-N rings. The local structure of HPHT-C3N3P was further characterized in the bulk and at the surface using MAS NMR and XPS (Supporting Information), respectively. The 13C MAS NMR spectra exhibit a broad peak centered near 125 ppm, indicating predominantly sp2 carbon within aromatic or olefinic ring structures (Figure 1a). Compared with other sp2 carbon materials such as graphite,28,29 triazine/heptazine-based structures30 and graphitic carbon nitrides,25 the 13C spectrum of the HPHT material most closely resembles the related carbon phosphonitride material produced at low pressure.13 The HPHT material has a somewhat narrower half-height line width (HHLW) that may indicate it is less disordered, in addition to a small shift to lower frequency compared to the lowpressure phase. Theoretical calculations of the 13C chemical shifts for a variety of carbon nitride materials31,32 are reasonably close to the experimental HPHT sample, which differs by having a 3-coordinate P atom replacing the 3-coordinate N atom. The 15N MAS NMR spectrum (Figure 1b) shows an asymmetric peak whose maximum is at 289.7 ppm, with a shoulder at ca. 294 ppm, as well as a much weaker peak at 71.1 ppm. The main peak (and shoulder) has a larger chemical shift and a narrower linewidth than what has been reported for 2- and 3-coordinate N atoms in amorphous graphitic carbon nitride,33 melon (or “C/H/Ngraphite”),34 as well as for TGCN.25 The 15N MAS NMR linewidths of crystalline melamine and melem are very sharp, but the chemical shifts are at much lower frequency compared to the HPHT sample.30 The TGCN material may represent the closest non-P-containing analogue to the HPHT sample, with the difference in the 15N chemical shifts arising from tricoordinate P replacing tricoordinate N, and the sharper linewidth in the HPHT sample due to greater uniformity, including possibly a more uniform degree of buckling of the graphitic planes when P is the bridging atom. The weak peak at 71 ppm may be due to N atoms bonded to three P atoms, based on the 15N chemical shift assignment in P3N5 35 and the 31P MAS NMR results below. The 31P MAS NMR spectrum (Figure 1c) shows several unresolved peaks between -25 ppm and -56 ppm, plus weak spinning sidebands. The lack of significant bulk oxidation in the HPHT sample makes it plausible to assign these peaks to bridging phosphine (phosphorus) atoms. The absence of a broad peak at +230 ppm assigned to the central P atom in P-substituted heptazine rings,13 along with the 15N results discussed above, suggests that the HPHT sample is based only on s-triazine rings. Note

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ACS Materials Letters synthesis based on novel synthetic pathways and elemental substitution.

ASSOCIATED CONTENT Supporting Information. Experimental details including NMR, XPS, composition measurements, NMR calculation details, FTIR, Raman, X-ray diffraction, force-indentation curve from nanoindentation measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

ACKNOWLEDGMENT This work was supported by the Defense Advanced Research Projects Agency (DARPA) under ARO Contract Number W31P4Q-13-1-0005. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-1634415) and Department of Energy- GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357. We thank Dr. Viktor Struzhkin, Dr. Jianjun Ying, Dr. Zachary Geballe, Dr. Emma Bullock, Dr. Matthew Ward, Dr. Michael Guerette, Dr. Venkata Bhadram and Dr. Gustav Borstad for helpful technical discussions.

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