Natural Sunlight-Driven Photocatalytic Overall Water Splitting with 5.5% Quantum Yield Promoted by Porphyrin-Sensitized Zn Poly(heptazine imide)
Horațiu Szalad, Andrés Uscategui-Linares, Rodrigo García‐Muelas, Alexey Galushchinskiy, Aleksandr Savateev, Markus Antonietti, Josep Albero, Hermenegildo Garcı́a
Abstract
Meso- tetrakis(4-carboxyphenyl)porphyrin (H 4 TCPP) has been loaded on a partially exchanged Zn 2+ 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 mmol H2 /g and 0.46 mmol O2 /g after 12 h of reaction irradiation of Milli-Q water under visible light by a 100 mW/cm 2 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 H 4 TCPP photosensitization. These relative photocatalytic activities point out the relevance of the PHI structure and the presence of Zn 2+ . It is proposed that Zn 2+ simultaneously binds PHI and H 4 TCPP. Transient absorption spectroscopy supports the occurrence of photoinduced electron transfer in which electrons are located at the H 4 TCPP 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 TCPP 4– with respect to (Zn/K)-PHI. The results show the possibility of developing efficient noble metal-free photocatalytic systems based on PHI dye sensitization.