Polymeric
carbon nitride (C3N4) has emerged
as the most promising candidate for metal-free photocatalysts but
is plagued by low activity due to the poor quantum efficiency and
low specific surface area. Exfoliation of bulk crystals into ultrathin
nanosheets has proven to be an effective and widely used strategy
for enabling high photocatalytic performances; however, this process
is complicated, time-consuming, and costly. Here, we report a simple
bottom-up method to synthesize porous few-layer C3N4, which involves molecule self-assembly into layered precursors,
alcohol molecules intercalation, and subsequent thermal-induced exfoliation
and polycondensation. The as-prepared few-layer C3N4 expose more active sites and greatly enhance the separation
of charge carriers, thus exhibiting a 26-fold higher hydrogen evolution
activity than bulk counterpart. Furthermore, we find that both the
high activity and selectivity for the oxidative coupling of amines
to imines can be obtained under visible light that surpass those of
other metal-free photocatalysts so far.
Due to the increasingly polluted environment and the gradual depletion of fossil fuel reserves, the development of renewable technologies for environmental remediation and energy production is highly desirable. Over the past decades, oxide-based semiconductor photocatalysis has attracted much attention. On various frontiers for efficient photocatalysis, surface-tuning strategies for synthesis and modification of oxides on the nanometer scale have progressed at a fast pace. Hence, it is of significance to review recent advances in the development of surface tuning for oxide-based nanomaterials as activity-enhanced photocatalysts. In this review, special emphases, especially for recent advances in our group, are given to surface tuning of novel nanocrystallites for high thermal stability, hierarchical structure assembly, heterojunctional nanocomposites and high-energy-facet exposure, along with effective testing tools for photogenerated charge properties at the surfaces and/or interfaces. This is of great significance for fields related to energy and environment from scientific and engineering viewpoints.
Thermally-stable, ordered mesoporous anatase TiO 2 with large pore size and high crystallinity has been successfully synthesized through an evaporationinduced self-assembly technique, combined with encircling ethylenediamine (EN) protectors to maintain the liquid crystal mesophase structure of TiO 2 primary particles, followed by calcination at higher temperature. The structures of the prepared mesoporous TiO 2 are characterized in detail by small-angle and wide-angle X-ray diffraction, Raman spectra, N 2 adsorption/ desorption isotherms, and transmission electron microscopy. Experimental results indicate that the well-ordered mesoporous structure could be maintained up to 700 ° C (M700) and also possesses large pore size (10 nm), high specifi c BET surface area (122 m 2 g − 1 ), and high total pore volumes (0.20 cm 3 g − 1 ), which is attributed to encircling EN protectors for maintaining the mesoporous framework against collapsing, inhibiting undesirable grain growth and phase transformation during the calcination process. A possible formation mechanism for the highly stable large-pore mesoporous anatase TiO 2 is also proposed here, which could be further confi rmed by TG/FT-IR in site analysis and X-ray photoelectron spectroscopy. The obtained mesoporous TiO 2 of M700 exhibit better photocatalytic activity than that of Degussa P25 TiO 2 for degradation of highly toxic 2,4-dichlorophenol under UV irradiation. This enhancement is attributed to the well-ordered large-pore mesoporous structure, which facilitates mass transport, the large surface area offering more active sites, and high crystallinity that favors the separation of photogenerated electron-hole pairs, confi rmed by surface photovoltage spectra.
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