Natural Sunlight-Driven Photocatalytic Overall Water Splitting with 5.5% Quantum Yield Promoted by Porphyrin-Sensitized Zn Poly(heptazine imide)

Meso-tetrakis(4-carboxyphenyl)porphyrin (H4TCPP) has been loaded on a partially exchanged Zn2+ poly(heptazine imide) (PHI), changing the light harvesting properties of the system, without altering the PHI structure. At the optimal loading (20 wt %), the photosensitized (Zn/K)-PHI is able to produce 1.06 mmolH2/g and 0.46 mmolO2/g after 12 h of reaction irradiation of Milli-Q water under visible light by a 100 mW/cm2 white LED. The apparent quantum yield for the overall water splitting reaction was 5.5% at 400 nm and 2% at 700 nm. Outdoor water splitting irradiation with natural sunlight shows the feasibility of the process. The photocatalytic performance of TCPP20%@(Zn/K)-PHI is considerably higher than that of analyzed reference samples such as graphitic carbon nitride, poly(triazine imide), and potassium PHI with H4TCPP photosensitization. These relative photocatalytic activities point out the relevance of the PHI structure and the presence of Zn2+. It is proposed that Zn2+ simultaneously binds PHI and H4TCPP. Transient absorption spectroscopy supports the occurrence of photoinduced electron transfer in which electrons are located at the H4TCPP and holes at the PHI moiety. Transient photocurrent measurements show a higher charge separation efficiency on TCPP20%@-(Zn/K)-PHI compared to (Zn/K)-PHI, and measurement of the frontier orbitals indicates an adequate energy alignment of the HOMO/LUMO levels of TCPP4- with respect to (Zn/K)-PHI. The results show the possibility of developing efficient noble metal-free photocatalytic systems based on PHI dye sensitization.