Graphitic carbon nitride (CN) as an environmental photocatalyst works under continuous illumination and stops under the dark condition. Here, we develop a bifunctional CN (4phenoxyphenol-functionalized alkalinized carbon nitride (PP-ACN)) that exhibits not only enhanced photoactivity but also residual catalytic activity in the post-irradiation dark period by grafting 4-phenoxyphenol (PP) moieties onto the polymeric alkalinized CN framework and demonstrate the successful performance of the sequential photocatalysis−dark reaction process. The modified CN catalyst can degrade organic compounds (or inactivate bacteria) and concurrently generate and accumulate H 2 O 2 (up to 40−60 μM) in the solution under visible light illumination. In the following dark period, it decomposes the accumulated H 2 O 2 to generate • OH and continues to work for hours even in the absence of light. PP-ACN in the presence of H 2 O 2 shows both the peroxidase-like activity and the dark Fenton-like activity in a wide range of pH (3−9), which is clearly different from the conventional photocatalysts. It exhibits an intrinsic radical character and a high selectivity for sequential three-electron reduction of O 2 under irradiation and a single-electron reduction of H 2 O 2 in the dark that lead to the generation of • OH radical. The present strategy of combining the photocatalytic activity and the post-illumination (dark) catalysis utilizing in situ H 2 O 2 in series should be a versatile platform of photon utilization in the overall process.

Bromide ion (Br–) can be oxidized to reactive bromine species (RBS; Br•, Br2 •–, and HOBr/OBr–) which can serve as an effective alternative to chlorine disinfectant. This study investigated the generation of RBS in a photoelectrochemical (PEC) system using an electrochromic TiO2 nanotube arrays (Blue-TNTs) electrode under UV light (λ > 320 nm) and demonstrated the effect of RBS on the direct conversion of ammonium (NH4 +) to dinitrogen (N2) with near 100% efficiency. The PEC system utilizing in situ generated RBS not only removed NH4 + more efficiently than photocatalytic (PC) and electrochemical (EC) systems but also prevented the generation of unwanted products (i.e., NO2 – and NO3 –). In addition, compared with the PEC-Cl system, the PEC-Br system exhibited a superior ammonium removal efficiency (16% vs 95% for 120 min of reaction; under air-equilibrated condition). The PEC system also showed higher NH4 + removal efficiency and lower energy consumption when compared to an EC system (using a boron-doped diamond electrode). While bromate ions (BrO3 –) are produced as a toxic byproduct of bromide oxidation in a typical ozonation system, the Blue-TNTs PEC-Br system fully hinders the bromate formation as long as ammonium is present in the solution because RBS rapidly reacts with NH4 + with little chance of further oxidation to bromate.

Fe 3+ is widely used as a conduction band electron acceptor in the photocatalytic and photoelectrochemical (PEC) oxidation of water and various substrates. However, a question of the possible involvement of Fe 3+ as a valence band hole acceptor has been raised. Herein, we demonstrate that the PEC water oxidation using oxide semiconductor (WO 3 , TiO 2 , and BiVO 4 ; primarily WO 3 ) films can proceed via the formation of high-valent iron species in the presence of aqueous Fe 3+ ions at pH 1−3. The presence of Fe 3+ (1−100 mM) enhances the photocurrent generation, O 2 evolution, and the Faradaic efficiency (FE) of ∼90% with a biased WO 3 electrode (1.23−1.88 V RHE ), whereas the formation of Fe 2+ is significantly inhibited. An in situ transient absorption spectroscopic analysis reveals the formation of high-valent iron species. The selective oxidation of dimethyl sulfoxide to dimethyl sulfone using in situ high-valent iron species is achieved with an FE of ∼98% in the PEC reaction with Fe 3+ . The proposed reaction mechanism should call for attention to the conventional role of Fe 3+ in the PEC reaction.