Reactive oxygen species (ROS) are key oxidants for the degradation of organic pollutants in sunlight-driven photocatalytic water treatment, but their interaction with the photocatalyst is easily ignored and, hence, is comparatively poorly understood. Here we show that graphitic carbon nitride (CN, a famous visible-light-responsive photocatalyst) is chemically stable toward ozone and superoxide radical; in contrast, hydroxyl radical (OH) can tear the heptazine unit directly from CN to form cyameluric acid and further release nitrates into the aqueous environment. The ratios of released nitrogen from nanosheet-structured CN and bulk CN that finally exists in the form of NO reach 9.5 and 6.8 mol % in initially ultrapure water, respectively, after 10 h treatment by solar photocatalytic ozonation, which can rapidly generate abundant OH to attack CN. On a positive note, in the presence of organic pollutants which compete against CN for OH, the CN decomposition has been completely or partially blocked; therefore, the stability of CN under practical working conditions has been obviously preserved. This work supplements the missing knowledge of the chemical instability of CN toward OH and calls for attention to the potential deactivation and secondary pollution of catalysts inOH-involved water treatment processes.

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Aiming at improving the visible-light photocatalytic activities of TiO 2 (101) surface we make an in-depth study on the TiO 2 (101) doped with 4d transition metal (TM) atoms.It is shown that the 4d TM dopings can not only produce new impurity energy bands in the band gap but also result in the semiconductor-metal phase transition. Consequently, the visible-light absorption is strongly strengthened due to the dopings of Y, Zr, Nb, Mo, and Ag, while it is only weakly improved for Tc, Ru, Rh, Pd, and Cd dopings. The improvement in visible-light absorption can be attributed to the intraband or interband transition of electrons. Moreover, the photocatalytic activities are explored, and we find Y and Ag dopings can effectively enhance the photocatalytic activity of TiO 2 (101) surface. Thus the mechanism of improving photocatalytic activity of TiO 2 (101) has been clearly addressed, which is beneficial to further experimental and theoretical researches on TiO 2 photocatalysts. K E Y W O R D S4d transition metal doping, anatase TiO 2 (101) surface, effective mass, photocatalyst modification, visible-light response 1 | INTRODUCTION TiO 2 , as a promising semiconductor photocatalyst, has attracted extensive attention due to its advantages, such as the biological and chemical inertness, stability to corrosion, nontoxicity, relatively low cost, and high photoactivity [1-3]. In the three phases of TiO 2 , rutile, anatase, and brookite TiO 2 , anatase TiO 2 exhibits the best photocatalytic activity and great potential in the fields of solar energy conversion, environmental purification, and particularly the degradation of harmful organic pollutants in water [4][5][6][7]. However, the popular application of anatase TiO 2 is severely restricted due to the following two reasons [8]: (i) The band gap E g of 3.23 eV is so large that it leads to the low utilization efficiency of visible light; (ii) The quantum yield is very low because of the low electron transfer rate and the high recombination rate of photoexcited electron-hole pairs. Therefore, it is still urgent to improve the photocatalytic activity of TiO 2 .In the past decades, much effort has been devoted to extending the optical response range of TiO 2 to the visible-light region, by means of impurity doping [9-17], noble metal loading [18], semiconductor compounding [19][20][21], and organic sensitizing [22,23]. Among all these methods, impurity doping is considered to be one of the most effective and efficient methods. In particular, transition metal (TM) atoms have been proven to be the ideal dopants to promote the photocatalytic performance of TiO 2 catalysts because of their d electronic configuration and unique characteristics. Quantum-sized TiO 2 doped with 0.5 at.% Fe 3+ , Mo 5+ , Ru 3+ , Os 3+ , Re 5+ , V 4+ , and Rh 3+ are found to induce the red shift of absorption edge and can improve photoactivity for both CHCl 3 oxidation and CCl 4 reduction [24]. Mesoporous TiO 2 impregnated by Ag + , Co 2+ , Cu 2+ , Fe 3+ , and Ni 2+ can accelerate the degradation of acetop...

Aiming at improving the visible-light photocatalytic activities of TiO2(101) surface (TiOS) we make an in-dept study on the TiOS doped with 4d transition metal (TM) atoms. It is shown that the 4d TM dopings can not only produce new impurity energy bands in the bandgap but also result in the semiconductor-metal phase transition. Consequently, the visible-light absorption is strongly strengthened due to the dopings of Y, Zr, Nb, Mo, and Ag, while it is only weakly improved for Tc, Ru, Rh, Pd, and Cd dopings. The improvement in visible-light absorption can be attributed to the intraband or interband transition of electrons. Moreover, the photocatalytic activities are explored, and we find Y and Ag dopings can effectively enhance the photocatalytic activity of TiOS. Thus the mechanism of improving photocatalytic activity of TiOS has been clearly addressed, which is beneficial to further experimental and theoretical researches on TiO2 photocatalysts.

Exploring new types of photocatalysts and modifying the photocatalytic activity have attracted more and more extensive attention in many research fields. Anatase TiO2, a promising photocatalyst widely studied, can only absorb the ultraviolet light and thus only make little use of the power in visible light. Therefore, it is an urgent task to make theoretical and experimental investigations on the photocatalytic mechanism in anatase TiO2 and then improve its visible light response so as to utilize more visible light. Now, in the present paper, we carry out a systematic theoretical investigation on modifying the photocatalytic properties of the anatase TiO2 (101) surface via doping transition metal neutral atoms such as Fe, Ni, Pd, Pt, Cu, Ag, and Au by using the plane wave ultrasoft pseudopotential method of the density functional theory. The dependence of the macroscopic catalytic activity on electronic structure and optoelectronic property is uncovered by making a comparative analysis of the geometric structures, the electronic structures, and the optical properties of the undoped and doped anatase TiO2 (101) surfaces. Our numerical results show that doping certain transition metals can suppress the band gap or induce extra impurity energy levels, which is beneficial to improving the visible light response of the TiO2 (101) surface in different ways. In most cases, the new impurity energy levels will appear in the original band gap, which comes from the contribution of the d electronic states in the transition metal atoms. Moreover, the photocatalytic activity of the TiO2 (101) surface can be changed differently by doping different transition metal atoms, which is closely dependent on the bandgap width, Fermi energy, the impurity energy level, and the electron configuration of the outermost shell of the dopants. This research should be an instructive reference for designing TiO2 (101) photocatalyst and improving its capability, and also helpful for understanding doping transition metal atoms in other materials.