Abstract
Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified. In the past decades, various systems have been developed but most of them suffer from low activities, a narrow range of absorption and poor quantum efficiencies (Q.E.) due to fast recombination of charge carriers. Here we report a dramatic suppression of electron-hole pair recombination on the surface of N-doped TiO2 based nanocatalysts under enhanced concentrations of H+ and OH−, and local electric field polarization of a MgO (111) support during photolysis of water at elevated temperatures. Thus, a broad optical absorption is seen, producing O2 and H2 in a 1:2 molar ratio with a H2 evolution rate of over 11,000 μmol g−1 h−1 without any sacrificial reagents at 270 °C. An exceptional range of Q.E. from 81.8% at 437 nm to 3.2% at 1000 nm is also reported.
Highlights
Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified
The TiO2 (P25) powder was treated by temperature ramping in a NH3 flow to a specific temperature T to obtain N-doped TiO2 with different nitrogen-doping concentrations denoted as N-P25-T. This was characterised by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), UV–visible absorption spectroscopy (UV–vis), Raman spectroscopy and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques to gauge structural, surface and spectroscopic changes due to the N inclusions (Supplementary Figs. 1–6)
The two forms of N were undifferentiated by the LEIS, the N peak gradually decreased after sputtering several times with highly energetic Ar+ and disappeared, whereas both the Ti and O peaks grew, indicating that nitrogen must have penetrated from the top surface into a subsurface region
Summary
Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified. In a watersplitting reaction over oxide systems, photoreduction of protons for hydrogen evolution is generally believed to be the kinetic facile process, but oxygen evolution from OH− is a slow fundamental step[1], which means the photogenerated electron–hole (exciton) pair must have a sufficient lifetime to react with both the dissociative H+/OH− species from the water molecule to allow the photocatalysis to happen Different approaches such as shape and facet engineering, heterojunction formation, cocatalyst deposition and internal electric fields to enable charge carrier separation have been explored to suppress the electron–hole recombination[23,24,25,26]. We report a direct photocatalytic water-splitting reaction which can use solar energy efficiently at elevated temperatures, showing greatly enhanced H2 evolution rates and QEs in a broad spectral range over the Au/N-doped TiO2/MgO (111) nanocatalyst due to the prolonged exciton lifetime in this system. We believe that the above work is a major milestone in the quest to harness solar energy via H2 for future energystorage applications
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