57 research outputs found
Structure and optical properties of polymeric carbon nitrides from atomistic simulations
Detailed understanding of the structural and photophysical properties of
polymeric carbon nitride (PCN) materials is of critical importance to derive
future material optimization strategies towards more desirable optical
properties and more photocatalytically active materials. However, the wide
range of structural motifs found in synthesized PCNs complicates atomistic
simulations that rely on well defined models. Performing hybrid DFT studies, we
systematically investigate formation energy trends and optical properties of
PCNs as a function of dimensionality, going from molecular oligomers over
periodic sheet models to stacked crystals. Thermochemical calculations that
take into account vibrational enthalpy and entropy contributions predict that a
mixture of structural motifs including the melon string structure,
poly(heptazine imide), and g-C3N4 motifs is stable under typical synthetic
conditions. The degree of lateral condensation as well as stacking can reduce
the bandgap while out-of-plane corrugation of the material increases both
stability and the optical gap. The key result of this work is that already
small domains of strongly condensed PCN are calculated to give rise to
favorable optical properties. This result reconciles conflicting literature
reports indicating that the thermodynamically favorable melon motif has a too
large bandgap compared to experiments, while the g-C3N4 structure, for which
bandgap calculations are in better agreement with experiments, does not agree
with measured chemical compositions of PCNs. Finally, we postulate a new
computational model for carbon nitride materials that encompasses the most
important structural motifs and shows a bandgap of ca. 2.9 eV
Structure and Optical Properties of Polymeric Carbon Nitrides from Atomistic Simulations
Detailed understanding of the structural and photophysical properties of polymeric carbon nitride (PCN) materials is of critical importance to derive future material optimization strategies towards more desirable optical properties and more photocatalytically active materials. However, the wide range of structural motifs found in synthesized PCNs complicates atomistic simulations that rely on well defined models. Performing hybrid DFT studies, we systematically investigate formation energy trends and optical properties of PCNs as a function of dimensionality, going from molecular oligomers over periodic sheet models to stacked crystals. Thermochemical calculations that take into account vibrational enthalpy and entropy contributions predict that a mixture of structural motifs including the melon string structure, poly(heptazine imide), and g-C3N4 motifs is stable under typical synthetic conditions. The degree of lateral condensation as well as stacking can reduce the bandgap while out-of-plane corrugation of the material increases both stability and the optical gap. The key result of this work is that already small domains of strongly condensed PCN are calculated to give rise to favorable optical properties. This result reconciles conflicting literature reports indicating that the thermodynamically favorable melon motif has a too large bandgap compared to experiments, while the g-C3N4 structure, for which bandgap calculations are in better agreement with experiments, does not agree with measured chemical compositions of PCNs. Finally, we postulate a new computational model for carbon nitride materials that encompasses the most important structural motifs and shows a bandgap of ca. 2.9 eV
Photodriven charge accumulation and carrier dynamics in a waterâsoluble carbon nitride photocatalyst
Charge accumulation in photoactive molecules and materials holds great promise in solar energy conversion as it allows for decoupling solarâdriven charging from (dark) redox reactions. In this contribution, lightâdriven charge accumulation was investigated for a recently reported novel waterâsoluble carbon nitride [K,Naâpoly(heptazine imide); K,NaâPHI] photocatalyst, which exhibits excellent activity and stability in highly selective photocatalytic oxidation of alcohols and concurrent reduction of dioxygen to H 2 O 2 under quasiâhomogeneous conditions. An excellent charge storage ability of the K,NaâPHI material was demonstrated, showing an optimal density of accumulated electrons (32.2â
ÎŒmol of electrons per gram) in the presence of 10â
volâ% MeOH as a sacrificial electron donor. The longâlived electrons accumulated under anaerobic conditions as K,NaâPHI .â radical ions were utilized in interfacial electron transfer to O 2 or methyl viologen in a subsequent dark reaction. Ultrafast timeâresolved spectroscopy was employed to reveal the kinetics of chargeâcarrier recombination and methanol oxidation. Geminate recombination of electrons and holes within approximately 100â
ps was followed by trapâassisted recombination. The presence of methanol as a sacrificial electron donor accelerated the decay of the transient absorption signal when a static sample was used. This behavior was ascribed to the faster charge recombination in the presence of the radical anions generated after hole extraction. The work suggests that photodriven electron storage in the waterâsoluble carbon nitride is enabled by localized trap states, and highlights the importance of the effective electron donor for creating longâlived photoâgenerated carbon nitride radicals.Taking a (very) quick look : A waterâsoluble carbon nitride reveals excellent storage capacity for photogenerated charges. The photoinduced dynamics in this highâperformance waterâsoluble carbon nitride photocatalyst are investigated by ultrafast timeâresolved spectroscopy
Photodoping and fast charge extraction in ionic carbon nitride photoanodes
Ionic carbon nitrides based on poly(heptazine imides) (PHI) represent a vigorously studied class of materials with possible applications in photocatalysis and energy storage. Herein, for the first time, the photogenerated charge dynamics in highly stable and binderâfree PHI photoanodes using in operando transient photocurrents and spectroelectrochemical photoinduced absorption measurements is studied. It is discovered that lightâinduced accumulation of longâlived trapped electrons within the PHI film leads to effective photodoping of the PHI film, resulting in a significant improvement of photocurrent response due to more efficient electron transport. While photodoping is previously reported for various semiconductors, it has not been shown before for carbon nitride materials. Furthermore, it is found that the extraction kinetics of untrapped electrons are remarkably fast in these PHI photoanodes, with electron extraction times (ms) comparable to those measured for commonly employed metal oxide semiconductors. These results shed light on the excellent performance of PHI photoanodes in alcohol photoreforming, including very negative photocurrent onset, outstanding fill factor, and the possibility to operate under zeroâbias conditions. More generally, the here reported photodoping effect and fast electron extraction in PHI photoanodes establish a strong rationale for the use of PHI films in various applications, such as biasâfree photoelectrochemistry or photobatteries
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Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes
Ionic carbon nitrides based on poly(heptazine imides) (PHI) represent a vigorously studied class of materials with possible applications in photocatalysis and energy storage. Herein, for the first time, the photogenerated charge dynamics in highly stable and binder-free PHI photoanodes using in operando transient photocurrents and spectroelectrochemical photoinduced absorption measurements is studied. It is discovered that light-induced accumulation of long-lived trapped electrons within the PHI film leads to effective photodoping of the PHI film, resulting in a significant improvement of photocurrent response due to more efficient electron transport. While photodoping is previously reported for various semiconductors, it has not been shown before for carbon nitride materials. Furthermore, it is found that the extraction kinetics of untrapped electrons are remarkably fast in these PHI photoanodes, with electron extraction times (ms) comparable to those measured for commonly employed metal oxide semiconductors. These results shed light on the excellent performance of PHI photoanodes in alcohol photoreforming, including very negative photocurrent onset, outstanding fill factor, and the possibility to operate under zero-bias conditions. More generally, the here reported photodoping effect and fast electron extraction in PHI photoanodes establish a strong rationale for the use of PHI films in various applications, such as bias-free photoelectrochemistry or photobatteries. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH Gmb
Solâgel processing of waterâsoluble carbon nitride enables highâperformance photoanodes **
In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of highâquality, binderâfree PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a waterâbased solâgel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The solâgel process capitalizes on the use of a waterâsoluble PHI precursor that allows formation of a nonâcovalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (âŒ100â%) conversions with very high photocurrents (>0.25â
mAâcm â2 under 2â
sun) down to <0â
V vs. RHE. This enables even effective PEC operation under zeroâbias conditions and represents the very first example of a âsoft matterââbased PEC system capable of biasâfree photoreforming. The robust binderâfree films derived from solâgel processing of waterâsoluble PCN thus constitute a new paradigm for highâperformance âsoft matterâ photoelectrocatalytic systems and pave the way for further applications in which highâquality PCN films are required.Completely unbiased : Robust binderâfree films derived from solâgel processing of a waterâsoluble polymeric carbon nitride precursor exhibit unprecedented performance in photoelectrocatalytic reforming of alcohols, including effective operation under biasâfree conditions
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