Thereafter, the photocatalytic degrada tion of polychlorobiphenyls [2] and photo electrocatalytic reduction of CO 2 into hydrocarbon compounds [3] in aqueous semiconductor suspensions greatly broad ened the applications of photocatalysis. Although the photocatalytic technology has got worldwide attention for its eco nomic, clean, safe, and renewable charac teristics, the photocatalytic performance of currently known photocatalysts is still far from commercial applications, especially in solartofuel conversion. [4][5][6][7][8][9][10][11] Generally, the photocatalytic reactions can be divided into three basic processes. First, the semiconductor photocatalysts absorb effective photons whose energy (h v ) is equal to or above their bandgap (E g ), resulting in the generation of electronhole pairs. Second, the photogenerated charge carriers separate and transfer to the surface of photocatalysts. Third, the photogenerated electrons and holes partic ipate in reactions of substances adsorbed on the surface of the photocatalysts. [12,13] Thus, the improvements of the three aforementioned processes play impor tant roles in enhancing the photocatalytic performance. Light absorption is the first essential step of photocatalysis process. The traditional anatase phase TiO 2 photocatalyst is active only under UV light with wavelength below 387 nm due to its wide bandgap (3.2 eV). However, solar energy is mainly concentrated in the visible light region, and UV light accounts for less than 4% of the solar spectrum. [7] In order to achieve maximum utilization efficiency of solar energy, the exploration of visiblelightresponsive photo catalysts is an urgent task. Graphitic carbon nitride (gC 3 N 4 ) as a promising visiblelightresponsive photocatalyst has received worldwide attention due to its fascinating merits, such as moderate bandgap (≈2.7 eV), proper electronic band structure, nontoxicity, low cost, good stability, and easy preparation. [14][15][16][17][18] Since the first report on photocatalytic H 2 evolution over gC 3 N 4 by Wang et al. in 2009, [19] research endeavors toward improving the photocatalytic performance of gC 3 N 4 based photocatalysts have formed a forefront of photocatalysis research. [20][21][22][23][24][25] Bulk gC 3 N 4 powder can be prepared by the thermal poly condensation of lowcost nitrogencontaining organic pre cursors, e.g., urea, thiourea, melamine, cyanamide, dicyan diamide, guanidine hydrochloride, and so on. [26][27][28][29][30][31][32] The pure bulk gC 3 N 4 prepared by this method suffers from several shortcomings, including low specific surface area, insufficient visible light utilization, and, particularly, rapid recombination Photocatalysis is considered as one of the promising routes to solve the energy and environmental crises by utilizing solar energy. Graphitic carbon nitride (g-C 3 N 4 ) has attracted worldwide attention due to its visible-light activity, facile synthesis from low-cost materials, chemical stability, and unique layered structure. However, the pure g-C 3 N 4 photocatalys...

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Exploring cheap and efficient cocatalysts for enhancing the performance of photocatalysts is a challenge in the energy conversion field. Herein, 2D ultrathin Ti3C2 nanosheets, a kind of MXenes, are prepared by etching Ti3AlC2 with subsequent ultrasonic exfoliation. A novel 2D/2D heterojunction of ultrathin Ti3C2/Bi2WO6 nanosheets is then successfully prepared by in situ growth of Bi2WO6 ultrathin nanosheets on the surface of these Ti3C2 ultrathin nanosheets. The resultant Ti3C2/Bi2WO6 hybrids exhibit a short charge transport distance and a large interface contact area, assuring excellent bulk‐to‐surface and interfacial charge transfer abilities. Meanwhile, the improved specific surface area and pore structure endow Ti3C2/Bi2WO6 hybrids with an enhanced CO2 adsorption capability. As a result, the 2D/2D heterojunction of ultrathin Ti3C2/Bi2WO6 nanosheets shows significant improvement on the performance of photocatalytic CO2 reduction under simulated solar irradiation. The total yield of CH4 and CH3OH obtained on the optimized Ti3C2/Bi2WO6 hybrid is 4.6 times that obtained on pristine Bi2WO6 ultrathin nanosheets. This work provides a new protocol for constructing 2D/2D photocatalytic systems and demonstrates Ti3C2 as a promising and cheap cocatalyst.

Artificial photosynthesis of hydrocarbon fuels by utilizing solar energy and CO is considered as a potential route for solving ever-increasing energy crisis and greenhouse effect. Herein, hierarchical porous O-doped graphitic carbon nitride (g-C N ) nanotubes (OCN-Tube) are prepared via successive thermal oxidation exfoliation and curling-condensation of bulk g-C N . The as-prepared OCN-Tube exhibits hierarchically porous structures, which consist of interconnected multiwalled nanotubes with uniform diameters of 20-30 nm. The hierarchical OCN-Tube shows excellent photocatalytic CO reduction performance under visible light, with methanol evolution rate of 0.88 µmol g h , which is five times higher than bulk g-C N (0.17 µmol g h ). The enhanced photocatalytic activity of OCN-Tube is ascribed to the hierarchical nanotube structure and O-doping effect. The hierarchical nanotube structure endows OCN-Tube with higher specific surface area, greater light utilization efficiency, and improved molecular diffusion kinetics, due to the more exposed active edges and multiple light reflection/scattering channels. The O-doping optimizes the band structure of g-C N , resulting in narrower bandgap, greater CO affinity, and uptake capacity as well as higher separation efficiency of photogenerated charge carriers. This work provides a novel strategy to design hierarchical g-C N nanostructures, which can be used as promising photocatalyst for solar energy conversion.

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