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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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.

Recently, great attention has been paid to fabricating direct Z‐scheme photocatalysts for solar‐energy conversion due to their effectiveness for spatially separating photogenerated electron–hole pairs and optimizing the reduction and oxidation ability of the photocatalytic system. Here, the historical development of the Z‐scheme photocatalytic system is summarized, from its first generation (liquid‐phase Z‐scheme photocatalytic system) to its current third generation (direct Z‐scheme photocatalyst). The advantages of direct Z‐scheme photocatalysts are also discussed against their predecessors, including conventional heterojunction, liquid‐phase Z‐scheme, and all‐solid‐state (ASS) Z‐scheme photocatalytic systems. Furthermore, characterization methods and applications of direct Z‐scheme photocatalysts are also summarized. Finally, conclusions and perspectives on the challenges of this emerging research direction are presented. Insights and up‐to‐date information are provided to give the scientific community the ability to fully explore the potential of direct Z‐scheme photocatalysts in renewable energy production and environmental remediation.

Inspired by nature, artificial photosynthesis through the construction of direct Z‐scheme photocatalysts is extensively studied for sustainable solar fuel production due to the effectiveness in enhancing photoconversion efficiency. However, there is still a lack of thorough understanding and direct evidence for the direct Z‐scheme charge transfer in these photocatalysts. Herein, a recyclable direct Z‐scheme composite film composed of titanium dioxide and cadmium sulfide (TiO2/CdS) is prepared for high‐efficiency photocatalytic carbon dioxide (CO2) reduction. In situ irradiated X‐ray photoelectron spectroscopy (ISI‐XPS) confirms the direct Z‐scheme charge‐carrier migration pathway in the photocatalytic system. Furthermore, density functional theory simulation identifies the intrinsic cause for the formation of the direct Z‐scheme heterojunction between the TiO2 and the CdS. Thanks to the significantly enhanced redox abilities of the charge carriers in the direct Z‐scheme system, the photocatalytic CO2 reduction performance of the optimized TiO2/CdS is 3.5, 5.4, and 6.3 times higher than that of CdS, TiO2, and commercial TiO2 (P25), respectively, in terms of methane production. This work is a valuable guideline in preparation of highly efficient recyclable nanocomposite for photoconversion applications.

Sunlight‐driven photocatalytic water splitting to generate hydrogen (H2) is a promising approach for utilizing solar energy. Herein, direct Z‐scheme Fe2O3/g‐C3N4 photocatalysts are rationally fabricated for H2 evolution under visible light. The graphitic carbon nitride (g‐C3N4) nanosheets obtained by solvent exfoliation of bulk g‐C3N4 display modest photocatalytic activity. Strikingly, its photocatalytic performance can be greatly improved by electrostatically assembling with hematite α‐Fe2O3 nanoplates. With platinum (Pt) as co‐catalyst and triethanolamine (TEOA) as hole scavenger, the H2 generation rate of optimized Fe2O3/g‐C3N4 composite with Fe2O3 weight percentage of 10% is about 13‐fold that of g‐C3N4. Based on the enhanced photocatalytic performance and slower time‐resolved photoluminescence decay, Z‐scheme charge transfer process is accepted for running this photocatalytic system, which is further evidenced by selective photo‐deposition of Pt nanoparticles on the g‐C3N4 surface. This rationally synthesized Fe2O3/g‐C3N4 composite is expected to have great potentials in solar energy conversation.

Layered manganese oxide, i.e., birnessite was prepared via the reaction of potassium permanganate with ammonium oxalate. The water content in the birnessite was adjusted by drying/calcining the samples at various temperatures (30 °C, 100 °C, 200 °C, 300 °C, and 500 °C). Thermogravimetry-mass spectroscopy showed three types of water released from birnessite, which can be ascribed to physically adsorbed H2O, interlayer H2O and hydroxyl, respectively. The activity of birnessite for formaldehyde oxidation was positively associated with its water content, i.e., the higher the water content, the better activity it has. In-situ DRIFTS and step scanning XRD analysis indicate that adsorbed formaldehyde, which is promoted by bonded water via hydrogen bonding, is transformed into formate and carbonate with the consumption of hydroxyl and bonded water. Both bonded water and water in air can compensate the consumed hydroxyl groups to sustain the mineralization of formaldehyde at room temperature. In addition, water in air stimulates the desorption of carbonate via water competitive adsorption, and accordingly the birnessite recovers its activity. This investigation elucidated the role of water in oxidizing formaldehyde by layered manganese oxides at room temperature, which may be helpful for the development of more efficient materials.

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