Page 125 - Multifunctional Photocatalytic Materials for Energy

112 Multifunctional Photocatalytic Materials for Energy

carbon nitride in the pore canal of template. After removal of the template, a larger
external surface and ordered nanostructured g-C 3 N 4 was obtained. Application of
this kind of photocatalyst into H 2 evolution enabled the reaction rate to reach as
−1
high as 85 μmol h in the four consecutive runs, which revealed excellent stability
in water splitting. Additionally, to avoid hazardous reagents being involved in the
hard-templating method, a novel soft-templating route for preparation of porous car-
bon nitride was reported by Yan et al. [44]. In this method, a surfactant of Pluronic
P123 was selected as the soft template, and melamine was chosen as the precursor.
Different from hard-templating, which requires an extra reagent/procedure for the
removal of templates, soft-templating synthesis can easily remove the templates by
way of calcination. The obtained mesoporous carbon nitride showed a large surface
2 −1
area of 90 m g . Additionally, an extended light absorption of up to 800 nm of solar
spectrum made it possible to more efficiently utilize solar light. When it comes to the
hydrogen evolution reaction, the rate of as-prepared mesoporous carbon nitride could
−1
reach as high as 148.2 μmol h .
Bulk carbon nitride always faces the challenges of low electron mobility and other
optical properties because of the thickness obstacle. To overcome these challenges and
mimicking the exfoliation of graphite into graphene, researchers fabricated numerous
2D carbon nitride nanosheets. With this method of graphene preparation in mind, re-
searchers developed a now widely used solvent ultrasonic route because the ultrasonic
wave can surmount the Van der Waals force in the interlayer of bulk carbon nitride
2
[45]. Moreover, based on the equation of ΔH Mix /V Mix = 2(δ G − δ sol ) φ/T sheet (ΔH is
mixing enthalpy, φ the volume fraction of nanosheet, δ the square root of the compo-
nent surface energy, and T the thickness of nanosheets), an appropriate solvent with
matchable surface energy was desired. Therefore a variety of solvents were studied in
the exfoliation of bulk carbon nitride. For example, Yang et al. [46] fabricated carbon
nitride nanosheets via exfoliation of bulk carbon nitride in various solvents, such as
isopropyl alcohol, N-methyl-pyrrolidone, water, ethanol, and acetone. After sonication
of the suspension solution for 10 h, carbon nitride nanosheets were obtained. Further
analysis found that isopropyl alcohol is a good solvent for fabricating carbon nitride
with a minimal thickness. The hydrogen evolution ability of as- prepared carbon ni-
tride nanosheets was also investigated. The results showed that layered 2D material
2 −1
with a large surface area (384 m g ) achieved a much higher hydrogen evolution rate
−1
−1
(93 μmol h ) than that of bulk carbon nitride (10 μmol h ) and even that of meso-
porous carbon nitride in the presence of Pt. In addition to organic solvents and water,
acid [47] or alkaline conditions [48] were also employed as the medium in the de-
lamination of carbon nitride. It was found that the photocatalysis efficiencies of the
obtained 2D nanosheets in acidic and basic conditions were both enhanced in compar-
ison with pristine carbon nitride.
A 1D nanostructure of carbon nitride was also constructed for boosting wa-
ter splitting abilities. For example, Liu and coworkers [41] took advantage of the
nano-confinement space in a 1D silica template to synthesize carbon nitride na-
norods (Fig. 6.4B). The nanostructure not only endowed a larger external surface
but also facilitated the electron mobility. When it came to hydrogen generation,
the performance of carbon nitride nanorods was almost 10 times greater than that