Tackling the stacking disorder of melon—structure elucidation in a semicrystalline material

Seyfarth, Lena; Seyfarth, Jan; Lotsch, Bettina V.; Schnick, Wolfgang; Senker, Jürgen

doi:10.1039/b919918g

Cited by 62 publications

(70 citation statements)

References 61 publications

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“…It has been suggested that distortions in those PCN samples with the melon structure, such as the different stackings, the shrinking or buckling of the layers, are introduced into the samples by high temperature treatment. [16][17][18]44 These distortions try to minimize the electron cloud repulsion of tri-s-triazine rings induced by reduced interlayer distances. Structurally, the distortions cause the sliding of layers as modeled in our calculations.…”

Section: Discussionmentioning

confidence: 99%

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Shining Light on Carbon Nitrides: Leveraging Temperature To Understand Optical Gap Variations

Melissen

Bahers

et al. 2018

Chem. Mater.

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Polymeric carbon nitride (PCN)-based materials have opened new research avenues in the photocatalytic field. The understanding of parameters underlying the optical properties of PCNs is critical to improve their performance. Herein, the optical properties of PCNs are investigated with a combination of Time-Dependent Density Functional Theory (TD-DFT) calculations and laboratory characterizations, including Variable Temperature (VT)-XRD, VT-UV-Vis and VT-IR techniques, on samples prepared using a broad range of synthesis temperatures. Upon increasing the synthesis temperature from 390 to 600°C, the optical gap E opt g (eV) narrows from 2.92 to 2.67, and on further increasing the synthesis temperature to 700°C, widens to 3.03 and another absorption at ∼2.2 eV is observed. The TD-DFT calculations show that E opt g of PCNs converges rapidly with heptazine oligomer size, indicating localized photophysics behavior and justifying a molecular approach. Calculations exploring geometrical variations upon increasing the temperature at which the optical properties were determined revealed that the net effect of these variations were limited, suggesting a vibrational origin of the observed decrease in the gap. The absorption at ∼2.2 eV is tentatively attributed to fully polymerized graphitic PCN oligomers as suggested by the excitation wavelength-dependent photoluminescence emission behavior. The results provide novel insights into the optical properties of PCNs, providing directions for the development of the next generation of PCN-based photocatalysts with optimal light harvesting abilities.

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Section: Discussionmentioning

confidence: 99%

“…The asprepared PCN sample often has a C/N ratio of around 0.67, and ∼1 wt% H. Therefore, the structure of this carbon nitride is best represented by the melon model. [15][16][17][18][19] . This model has compositional formula C 6 N 9 H 3 .…”

Section: Polymeric Carbon Nitridesmentioning

confidence: 99%

Shining Light on Carbon Nitrides: Leveraging Temperature To Understand Optical Gap Variations

Melissen

Bahers

et al. 2018

Chem. Mater.

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“…Originally inspired by the "harder than diamond" postulate, 9 this family of materials were thought to be a precursor to the predicted β-C 3 N 4 and have been heavily investigated for their synthetic pathway and structure. [10][11][12][13][14][15][16] The main advantages of these materials as photocatalysts are that they can be facilely synthesized by heat treatment of inexpensive precursors (e.g. melamine or dicyandiamide), are active under visible light irradiation, and chemically stable.…”

Section: Introductionmentioning

confidence: 99%

Low-Molecular-Weight Carbon Nitrides for Solar Hydrogen Evolution

et al. 2015

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This work focuses on the control of the polymerization process for melon ("graphitic carbon nitride"), with the aim of improving its photocatalytic activity intrinsically. We demonstrate here that reduction of the synthesis temperature leads to a mixture of the monomer melem and its higher condensates. We show that this mixture can be separated and provide evidence that the higher condensates are isolated oligomers of melem. On evaluating their photocatalytic activity for hydrogen evolution, the oligomers were found to be the most active species, having up to twice the activity of the monomer/oligomer mixture of the as-synthesized material, which in turn has 3 times the activity of the polymer melon, the literature benchmark. These results highlight the role of "defects", i.e., chain terminations, in increasing the catalytic activity of carbon nitrides and at the same time point to the ample potential of intrinsically improving the photocatalytic activity of "carbon nitride", especially through the selective synthesis of the active phase.

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“…[39][40][41] Such electrodes exhibited significant visible-light induced anodic photocurrents. [51][52][53][54][55][56][57] The precise structure of polyheptazine sheets is currently under intense investigations (see particularly the recent work of Schnick et al 58,59 polyheptazine-type powders, designated as ''graphitic carbon nitride'', were recently introduced as visible-light active photocatalysts by X. Wang et al and their photoactivity is now being intensely investigated. [39][40][41] The chemistry behind the modification procedure was elucidated by Mitoraj and Kisch 42,43 who identified the nitrogen and carbon-containing species as poly(tri-s-triazine) (polyheptazine) formed in situ at the surface of titania from urea pyrolysis products (Fig.…”

Section: Introductionmentioning

confidence: 99%

Visible-light photocurrent response of TiO2–polyheptazine hybrids: evidence for interfacial charge-transfer absorption

Bledowski

Wang

Ramakrishnan

et al. 2011

Phys. Chem. Chem. Phys.

173

129

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We investigated photoelectrodes based on TiO(2)-polyheptazine hybrid materials. Since both TiO(2) and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO(2) (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO(2) gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs. NHE, pH 7) allows for photooxidation of water (+0.82 V vs. NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO(2) nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO(2)-polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.

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Tackling the stacking disorder of melon—structure elucidation in a semicrystalline material

Cited by 62 publications

References 61 publications

Shining Light on Carbon Nitrides: Leveraging Temperature To Understand Optical Gap Variations

Shining Light on Carbon Nitrides: Leveraging Temperature To Understand Optical Gap Variations

Low-Molecular-Weight Carbon Nitrides for Solar Hydrogen Evolution

Visible-light photocurrent response of TiO2–polyheptazine hybrids: evidence for interfacial charge-transfer absorption

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