Hydrogen is one of the most widely used essential chemicals worldwide, and it is also employed in the production of many other chemicals, especially carbon-free energy fuels produced via photoelectrochemical (PEC) water splitting. At present, gallium arsenide represents the most efficient photoanode material for PEC water oxidation, but it is known to either be anodically photocorroded or photopassivated by native metal oxides in the competitive reaction, limiting efficiency and stability. Here, we report chemically etched GaAs that is decorated with thin titanium dioxide (~30 nm-thick, crystalline) surface passivation layer along with nickel-phosphate (Ni-Pi) cocatalyst as a surface hole-sink layer. The integration of Ni-Pi bifunctional co-catalyst results in a highly efficient GaAs electrode with a ~ 100 mV cathodic shift of the onset potential. In this work, the electrode also has enhanced photostability under 110 h testing for PEC water oxidation at a steady current density J_ph > 25 mA·cm⁻². The Et-GaAs/TiO₂/Ni-Pi║Ni-Pi tandem configuration results in the best unassisted bias-free water splitting device with the highest J_ph (~7.6 mA·cm⁻²) and a stable solar-to-hydrogen conversion efficiency of 9.5%.

The development of solution-processable routes as well as compounds consisting of earth abundant elements is highly desirable to reduce the fabrication cost. Recently, kesterite Cu₂ZnSnS₄ (CZTS) nanocrystals (NCs) have attracted great attention for photoelectrochemical (PEC) water splitting owing to their suitable low-cost, earth-abundancy and suitable band gap energy. However, the environmentally benign synthesis of high-quality CZTS NCs without toxic solvents remains elusive. Here, a green chemistry approach employing vegetable oil as a non-toxic solvent for the synthesis of monodisperse and size-tunable CZTS NCs is introduced for the first time. Additionally, the relationship between the abnormal size behavior of the CZTS NCs and the degree of decomposition in the vegetable oil using electrospray ionization mass spectrometry (ESI-MS) measurements is elucidated for the first time. As a conceptual strategy, a ternary abundant compound based heterojunction nanostructure for efficient solar water splitting by introducing CZTS NCs onto 5 nm Zn(O,S) passivated layer/hydrothermally grown TiO₂ nanorod arrays (TNRs) is designed and developed. Remarkably, this ternary CZTS NCs/Zn(O,S)/TNR photoelectrode shows a photocurrent density as high as 15.05 mA cm⁻² at 1.23 V (vs. the NHE), which is the highest ever for previously reported CZTS NC-based photoelectrodes. The reasons for the enhanced PEC performance are discussed in detail based on different PEC characterizations. More importantly, this work reflects the sophistication of eco-friendly solution phase synthesized CZTS NCs without using any toxic chemicals as an earth abundant sensitizer and constitute a new paradigm towards the enhanced PEC performance with quantum dot based hetero-nanostructures.