Solar hydrogen production using photocatalytic water splitting is regarded as a promising strategy for harnessing solar energy to supply hydrogen energy. 1,2 Titanium dioxide (TiO2) is a popular and standard semiconductor used in photocatalysis, and exists in three common crystalline structures, anatase, rutile and brookite, that have been extensively investigated. Generally, anatase TiO2 is recognized as the most active phase in photocatalysts for environmental applications, while rutile and brookite TiO2 are seldom considered. [3][4][5][6] In the past few decades, almost all the researches on TiO2 can only obtain H2 but no O2 was detected during photocatalytic overall water splitting although it has thermodynamic feasible band structure. In the photocatalytic overall water splitting reaction (POWS, 2 2 2 22 H O H O  ), H2 and O2 should be produced simultaneously with H2/O2 stoichiometric ratio of 2.0, which has been achieved in photoelectrochemical (PEC) system using TiO2 photoanode as early as 1972. 7 However, it has seldom been achieved on TiO2-based nanoparticulate photocatalyst. This challenge persists despite the fact that TiO2 has suitable band structure for both proton reduction and water oxidation under UV light irradiation. Besides, a similar phenomenon has been observed for other popular photocatalysts (e.g., Ta3N5, TaON). That is, some photocatalysts have suitable band structures that are thermodynamically feasible for POWS, yet they fail to catalyze POWS reaction.Immense efforts have been made to achieve POWS on TiO2 previously. The introduction of some inorganic ions (e.g., Cl -, CO3 2-) has been reported to somewhat improve the stoichiometric production of H2 and O2, which might be attributed to the intermediates involving the ions (e.g., C2O4 2-, ClO -) that are formed in these systems. 8,9 produce O2 on rutile. However, for anatase and brookite TiO2, the formed · OH radical may be strongly absorbed on the surface and coupled to evolve O2 after saturation of absorption. The different oxygen-containing intermediates formed on anatase and rutile TiO2 can also be demonstrated by the diversity of surface hydroxyl oxygen of TiO2 before and after the reaction 29,30 (Figures S6 and S7). It was found that the proportion of hydroxyl oxygen for anatase TiO2 was obviously increased after UV light irradiation (from 5.0% to 9.7%) but remained almost unchanged for rutile TiO2. Thus, the peroxy species are the most likely oxygen-containing intermediate derived from water oxidation on rutile TiO2 while · OH radical species are preferring to prevail from water oxidation on anatase TiO2. Our EPR results suggest that different intermediates are really formed during the photocatalytic water splitting for three kinds of TiO2 samples (peroxy species for rutile TiO2, · OH radical for anatase and brookite TiO2). The different intermediates resulted in different surface reaction processes, which mainly contribute to the kinetics for POWS on TiO2-based photocatalysts. EPR experiments without electron trapping agent were...

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