Self-assembled hole transport engineering and bio-inspired coordination/incoordination ligands synergizing strategy for productive photoelectrochemical water splitting

Charge transport and metal site stability play a critical role on realizing efficient solar water splitting in photoelectrochemical devices. Here, we investigated BiVO₄-based composite photoanodes (labelled as NF@PTA/2PACz/BVO) in which BiVO₄, [2-(9H-carbazol-9-yl) ethyl] phosphonic acid (2PACz) hole transport layers based on self-assembled monolayers (SAMs), and terephthalic acid (PTA)-functionalized NiFeOOH (NF@PTA) oxygen evolution cocatalysts (OECs) structurally similar to the OECs in natural photosystem II, were assembled sequentially. Alignment of energy levels and stabilization of metal sites can be achieved by this layer-designed structure. And the uncoordinated (COOH) carboxylate groups can accelerate the proton transfer. Fundamental investigations reveal that the NF@PTA/2PACz/BVO photoanode exhibits unique properties including passivated surface traps, excellent carrier density and lifetime, enlarged photovoltage, and smoother hole transport band structure. Consequently, the optimum NF@PTA/2PACz/BVO photoanode shows the photoelectrochemical (PEC) performance of 5.43 mA cm⁻² at 1.23 V vs reversible hydrogen electrode with an applied bias photon-to-current efficiency of 1.45 %. The coupled COFe bond between the coordinating carboxylate and the metals not only inhibits the leaching of the metal species but also maintains a steady photocurrent density over 20 h of stability test. Our work paves the way for the development of more efficient PEC cells with superior charge separation and breakthroughs in the stability of metal active sites, thus broadening their potential applications.