The significance of photocatalysts is unquestionable, and scientists are devoted to improving their photocatalytic efficiency. To solve the high recombination rates of photogenerated electron-hole pairs and their low reduction and oxidation abilities in a single photocatalyst, heterojunction manipulation is urgently required. Two mainstream heterojunctions-type-II and Z-scheme heterojunctionshave been widely acknowledged. However, we soberly reflect the charge-transfer mechanism from many perspectives and are finally aware of the fundamental challenges they face. To ensure a correct understanding, it is necessary to share our analysis with others. Moreover, step-scheme (S-scheme) heterojunctions, consisting of a reduction photocatalyst and an oxidation photocatalyst with staggered band structure, are introduced to avoid misinterpretation. The differences in the charge-transfer mechanism between S-scheme, type-II, and Z-scheme heterojunctions are highlighted. Finally, limitations and the future research direction of S-scheme photocatalysts are discussed.

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...

TiO2 thin films were prepared on fused quartz by the liquid-phase deposition (LPD) method from a (NH4)2TiF6 aqueous solution upon addition of boric acid (H3BO3) and calcined at various temperatures. The as-prepared films were characterized with thermogravimetry (TG), Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), UV−Visible spectrophotometry (UV−Vis), scanning electron microscopy (SEM), photoluminescence spectra (PL), and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic activity of the samples was evaluated by photocatalytic decolorization of methyl orange aqueous solution. It was found that the as-prepared TiO2 thin films contained not only Ti and O elements, but also a small amount of F, N, and Si elements. The F and N came from the precursor solution, and the amount of F decreased with increasing calcination temperature. Two sources of Si were identified. One was from the SiF6 2- ions, which were formed by a reaction between the treatment solution and quartz substrate. The other was attributed to the diffusion of Si from the surface of quartz substrate into TiO2 thin film at 700 °C or higher calcination temperatures. With increasing calcination temperature, the photocatalytic activity of the TiO2 thin films gradually increased due to the improvement of crystallization of the anatase TiO2 thin films. At 700 °C, the TiO2 thin film showed the highest photocatalytic activity due to the increasing amount of SiO2 as an adsorbent center and better crystallization of TiO2 in the composite thin film. Moreover, the SiO2/TiO2 composite thin film showed the lowest PL intensity due to a decrease in the recombination rate of photogenerated electrons and holes under UV light irradiation, which further confirms the film with the highest photocatalytic activity at 700 °C. When the calcination temperature is higher than 700 °C, the decrease in photocatalytic activity is due to the formation of rutile and the sintering and growth of TiO2 crystallites resulting in the decrease of surface area.

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.