Neat 3D C3N4 monolithic aerogels embedded with carbon aerogels via ring-opening polymerization with high photoreactivity

doi:10.1016/j.apcatb.2020.118652

Applied Catalysis B: Environmental

Volume 266, 5 June 2020, 118652

https://doi.org/10.1016/j.apcatb.2020.118652 Get rights and content

Highlights

•
Carbon aerogels doped neat 3D C₃N₄ monolithic aerogels (NC/C₃N₄) are prepared depending on ring-opening polymerization.
•
A favorable electron contact between 3D C3N4 monolithic aerogels and carbon aerogels can be achieved by tuning the heat curing behavior of BZ.
•
NC/C₃N₄ demonstrate ultrahigh photoreactivity and high thermal stability.

Abstract

Ring-opening polymerization affords an extremely powerful driver for large-scale delocalization of π-electrons and good charge transport via incorporating aromatic repeat units and rigid molecular skeleton. Herein, we first developed a branched six functional benzoxazine monomer (BZ) through Mannich condensation reaction. The rigid chain segment structure of crosslinked polybenzoxazine (PBZ) penetrated into the π-conjugated planes of C₃N₄ via the room-temperature HCl-catalyzed polymerization of the monomer, leading to the formation of a range of 3D C₃N₄ monolithic aerogels embedded with carbon aerogel. High nitrogen self-doped carbon aerogels were flexibly embedded in situ into the C₃N₄ matrix during the heat curing process, leading to the redshift of the C₃N₄ absorption edge and the increase of electrical conductivity. The special framework of the NC/C₃N₄ monolithic aerogels can tremendously enrich the electron-trapping sites through the generation of delocalized big π bonds. This work provides new insights into the understanding of polymer chemistry and photocatalysis mechanism.

Graphical abstract

Enhanced photocatalytic reaction at high nitrogen self-doped carbon/carbon nitride monolithic aerogels interface for stable visible water splitting.

Introduction

Polybenzoxazine is an intriguing phenolic-formaldehyde resin, which consists of C, N and O atoms in the building block and can provide an idea platform to stack the basic unit and tune the photocatalytic performance of the resultant catalysts. [[1], [2], [3]] Integrating repeat aromatic cells to generate conjugated architectures endows the photocatalysts with large-scale delocalization of π-electrons and good charge transport. The superiority of polybenzoxazine includes near-zero volume shrinkage during the ring-opening polymerization of its monomer, high char yields and good thermal and chemical stability. Besides, the heat curing reaction of branched benzoxazine in suitable reagents can induce gelation and subsequently construct stable 3D macromolecular skeleton. In addition to the curing reaction route, a greater understanding of photocatalyst-monomer interactions is also of great importance.

As a typical metal-free photocatalyst, inexpensive, nontoxic carbon nitride (C₃N₄) with an appropriate band position of ≈ 2.7 eV and high thermal and chemical stability has great commercial might in the production of H₂, O₂, or H₂O₂ [[4], [5], [6], [7], [8], [9]]. Consequently, diverse C₃N₄ derived powdered nanocrystals or nanosheets have been fabricated depending on simple pyrolysis of urea or melamine. [[10], [11], [12], [13]] Nevertheless, the photocatalysis capability of pure C₃N₄ is far from satisfactory as a result of the low specific area and the π–π conjugated electronic system caused inefficient light absorption (λ > 450 nm), as well as the fast recombination of photoinduced carriers. [[14], [15], [16], [17], [18]] Nevertheless, these nanoscale powders are invariably inclined to agglomerate, which results in a number of intricate steps to separate the photocatalysts from the reaction system for recycling. In this case, various modification strategies have been developed to address these issues, for example, the embedding strategies, namely fastening the photocatalysts onto a suitable carrier such as ceramic foam [[19], [20], [21], [22]], film [23], and sponge [24,25]. It often leads to a remarkably decreased surface area and active sites and thereby weakens the photocatalytic performance. These practical obstacles severely limit the large-scale applications of photocatalysis [[26], [27], [28], [29], [30]]. Monolithic aerogels, a new booming photocatalytic material, has stood out in the clean energy production industry, leading to the stimulation of exploring more mechanically stable and multifunctional photocatalysts [[31], [32], [33], [34], [35]]. The particular features of monolithic aerogel photocatalysts, including impressive specific surface area, interconnected open-frameworks as well as highly macroscopic operability and recoverability endow it with tremendous advantage in photochemical synthesis and green energy production. [[36], [37], [38]]

Here, using branched benzoxazine monomer as an initiator, we first proposed a time- and energy-efficient strategy to achieve a range of 3D C₃N₄ monolithic aerogels embedded with carbon aerogels with high specific surface area, low density and excellent electron mobility on a large scale. The drive of acid catalysis curing reaction and gelation of the six functional benzoxazine monomer allowed for direct and tight contact with C₃N₄ and the generation of delocalized big π bonds. The particular electronic structure configuration and increased electrical conductivity of 3D NC/C₃N₄ monolithic aerogels facilitated the electronic mobility and charge transport among the adjacent layers, leading to outstanding photooxidation and photoreduction activities.

Section snippets

Materials

Hexachlorocyclotriphosphazene [N₃P₃Cl₆] was recrystallized using hexane and activated carbon, followed by sublimation three times under the pressure of 0.05 mm Hg at the temperature of 60 °C. Urea, aniline (99 %), p-hydroxy benzaldehyde paraformaldehyde (AR.), paraformaldehyde (95 %), toluene (AR.), dimethylacetamide (AR.), anhydrous ether (AR.), dichloromethane (AR.), ethyl acetate (AR.), p-aminophenol (AR.), benzophenone (AR.), calcium hydride (AR.), absolute ethyl alcohol (AR.), methyl

Results and discussion

We first proposed a highly reactive 3D C₃N₄ monolithic aerogels embedded with carbon aerogels via a room-temperature HCl-catalyzed polymerization route followed by carbonization treatment (see Scheme 1). Three steps were involved in the main approach. Branched six functional benzoxazine monomer [N₃P₃(OC₆H₄{CHN(C₆H₃)(4-O)CH₂CH₂(4-N)C₆H₅}-4)₆] (C3), which possessed six oxazine groups in each benzoxazine monomer was firstly synthesized by the Mannich condensation of cyclotriphosphazene (CP)

Conclusions

In summary, by means of HCl-catalyzed heat-curing reaction, we first developed a ring-opening polymerization- promoted approach to produce a range of 3D monolithic aerogels (NC/C₃N₄) consisted of 2D ultrathin C₃N₄ nanosheets and high-quality nitrogen self-doped carbon aerogels. The intrinsic electronic and band structure of NC/C₃N₄ aerogels can be tuned by the heat-curing reaction. The drive of acid catalyzed curing reaction and gelation of the six functional benzoxazine monomer allowed for

Author contributions

The manuscript was written through contributions of all authors.

Declaration of Competing Interest

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the Natural Science Foundation of Henan province (182300410285), the Henan Key Scientific Research Project (16A430026 and 17A150048) and the Nanhu Scholar Program for Young Scholars of XYNU.

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Neat 3D C3N4 monolithic aerogels embedded with carbon aerogels via ring-opening polymerization with high photoreactivity

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Author contributions

Declaration of Competing Interest

Acknowledgments

Appl. Catal. B: Environ.

Nanoscale Adv.

Appl. Catal. B: Environ.

Electrochemical strategies for C–H functionalization and C–N bond formation

Chem. Soc. Rev.

Nitrogen self-doped carbon aerogels derived from trifunctional benzoxazine monomers as ultralight supercapacitor electrodes

Nanoscale

Polybenzoxazine aerogels. 1. High-yield room-temperature acid-catalyzed synthesis of robust monoliths, oxidative aromatization, and conversion to microporous carbons

Chem. Mater.

Surface engineering for extremely enhanced charge separation and photocatalytic hydrogen evolution on g-C3N4

Adv. Mater.

Carbon self-doping induced high electronic conductivity and photoreactivity of g-C3N4

Chem. Commun.

A tunable and sizable bandgap of a g-C3N4/graphene/g-C3N4 sandwich heterostructure: a van der Waals density functional study

J. Mater. Chem. C

Impact of interfacial electron transfer on electrochemical CO2 reduction on graphitic carbon nitride/doped graphene

Small

Functional carbon nitride materials-design strategies for electrochemical devices

Nat. Rev. Mater.

Metal-free photocatalysts for various applications in energy conversion and environmental purification

Green Chem.

Metal-free carbonaceous electrocatalysts and photocatalysts for water splitting

Chem. Soc. Rev.

Metal-free 2D/2D phosphorene/g-C3N4 Van der Waals heterojunction for highly enhanced visible-light photocatalytic H2 production

Adv. Mater.

Graphitic carbon nitride “reloaded”: emerging applications beyond (photo) catalysis

Chem. Soc. Rev.

High-efficiency broadband C3N4 photocatalysts: synergistic effects from upconversion and plasmons

ACS Catal.

Unprecedented catalytic performance in amine syntheses via Pd/gC3N4catalyst-assisted transfer hydrogenation

Green Chem.

Integrating the g-C3N4 Nanosheet with B-H Bonding Decorated Metal-Organic Framework for CO2 Activation and Photoreduction

ACS Nano

Significant enhancement of visible-light-driven hydrogen evolution by structure regulation of carbon nitrides

ACS Nano

Template-induced high-crystalline g-C3N4 nanosheets for enhanced photocatalytic H2 evolution

ACS Energy Lett.

One-step nickel foam assisted synthesis of Holey G-carbon nitride nanosheets for efficient visible-light photocatalytic H2 evolution

ACS Appl. Mater. Interfaces

3D foam strutted graphene carbon nitride with highly stable optoelectronic properties

Adv. Funct. Mater.

Macroscopic foam-like holey ultrathin g-C3N4 nanosheets for drastic improvement of visible-light photocatalytic activity

Adv. Energy Mater.

Fabrication and enhanced photoelectrochemical performance of MoS2/S-doped g-C3N4 heterojunction film

ACS Appl. Mater. Interfaces

Neat 3D C₃N₄ monolithic aerogels embedded with carbon aerogels via ring-opening polymerization with high photoreactivity