Bio-inspired WO3 based photoanodes for Artificial Photosynthesis

Artificial photosynthesis is a long-term goal for humankind to achieve sustainable development. To realize it, PSII-inpsired Quantasome approach has been attempted, with a 2D lamellar structure formed by surrounding polyoxometalate water-oxidation catalyst (Ru4POM) with 5 perylene-bisimide (PBI) chromophores ({[PBI]5Ru4POM}n), and water channels (Tetraethylene glycol chain modified PBI, PBI-TEG), promising results for oxygenic photosynthesis have been achieved. However, the efficiency of this system is still low due to fast recombination from semiconductor to Ru4POM and poor red light absorption, to slow down recombination, we designed a new WO3 3D interconnected nanoheet (WO3 3D-NS) electrode using ZnO film as auxiliary materials, the resulting film shows a densely packed structure with thickness of 25 μm, made with nanosheets with diameter and thickness of 220 and 30 nm respectively, TEM results shows that the nanosheet has highly exposed {001} facet. Combined with XRD and SEM, the morphology was correlated to the dissociation of 3D-ZnWO4 formed during the impregnation of ZnO with ammonium metatungstate. PBI-TEG and Ru4POM were deposited on WO₃ 3D-NS by co-deposition, forming Quantasomes structure (QS-TEG), yielding an incident photon-to-current conversion efficiency (IPCE) of 0.24% at 500 nm and 0.7 V vs. RHE. This represents a 1.4-fold and 3.8-fold improvement compared to WO₃ inverse opal (IO) and microplate (MP) systems, respectively. Through intensity-modulated photocurrent spectroscopy (IMPS), this enhancement was attributed to a higher electron diffusion coefficient and reduced recombination of the WO₃(e⁻)-Ru₄POM(h⁺) charge-separated state, with a lifetime in the millisecond range. Despite these improvements, the system exhibited a limited charge injection efficiency of 0.19. Further optimization of the deposition method demonstrated that soaking yielded a more uniform distribution of QS-TEG, with a concentration of 4.1 ± 0.6 nmol cm⁻². This approach increased the IPCE to 0.4% at 500 nm, representing an 80% and 270% improvement compared to co-deposition and drop-casting methods, respectively. This enhancement is attributed to better charge injection efficiency from the dye to the semiconductor. Notably, the IPCE is also 80% higher than the state-of-the-art ITO IO/QS-TEGlock system. To improve red light absorption of the system, a PBI analogue: pyrrolidine substituted naphthalene diimide dye (NDI) was applied for Quantasome formation. The resulting materials show good yellow and red light (λ > 600 nm) harvesting with an absorption edge up to 750 nm and an intense absorption peak at 624 nm. Deposited on WO3 3D-NS electrode by codeposition, an IPCE value of 0.14% was reached at 0.91 V vs RHE, in close analogue to meso-ITO/PSII bio-electrodes (0.125% at 680 nm). In the case of HBr splitting, that charge injection of the dye PBI was evaluated. After overnight soaking in a PBI acetonitrile solution, the PBI loading reached 50.8 ± 0.7 nmol cm⁻², which is 3.2 and 4.9 times higher than on WO₃ IO and MP electrodes, respectively. This resulted in a photocurrent of 0.33 mA cm⁻² at 0.85 V vs. RHE (100 mW cm⁻², AM 1.5, λ > 450 nm), representing a 65%, 230%, and 83% improvement compared to WO₃ IO, MP-based photoanodes, and colloidal WO₃ systems reported in the literature. Electrochemical impedance spectroscopy indicated that the improved photocurrent was due to a lower charge-transfer resistance at the photoanode/electrolyte interface, attributed to higher dye loading. Keywords: Artificial Photosynthesis, hydrobromic acid oxidation, PSII-inspired, Quantasomes, WO3 3D interconnected nanosheet, Perylene bisimide, Naphthalene diimide. IMPS.