Photocatalytic water splitting by N-TiO2 on MgO (111) with exceptional quantum efficiencies at elevated temperatures

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.

Fig. 1

EPR patterns of the N-doped TiO₂ photocatalysts. a N-doped TiO₂ N-P25-550 (freshly prepared) at different times. After being treated in NH₃ at 550 °C for 8 h, the EPR of the freshly made sample was measured immediately. Then more EPR spectra were also collected after the sample was exposed to air for 0.5, 1, 3 and 24 h. b Deactivated N-P25-550 after recalcination in N₂ at different temperatures; N-P25-550 quenched from c liquid water and d water vapour at different temperatures

Fig. 2

Photocatalytic water-splitting reaction activity tests. a Photocatalytic activities of N-P25-620 and Au/N-P25-620 with MgO (111) at different temperatures. b Stable stoichiometric decomposition of water to 2:1 H₂/O₂ with no sacrificial reagent over Au/N-P25-620 with and without MgO (111) at a constant rate for 50 h. Typically, 5 mg of Au/N-P25-620 was added to 10 mL of Milli-Q H₂O in a 25-mL stainless-steel autoclave equipped with quartz windows under vigorous magnetic stirring, and Ar gas was used as the inert gas. Then the autoclave was heated up to designated temperatures. c QE of Au/N-P25-620 with and without MgO (111) by using incident wavelengths of 385, 437, 575, 620, 750 and 1000 nm, respectively (UV–vis spectra of filters are shown in Supplementary Fig. 9). QE measurements were carried out with a similar procedure as that stated before. The reaction system was then irradiated by a 300-W xenon lamp through quartz windows by using the corresponding band-pass filters. Error bars indicate the standard deviation

Fig. 3

Time-resolved photoluminescence (TRPL) measurements of N-doped TiO₂ photocatalysts under different conditions. a N-doped P25 treated by ammonia at different temperatures and promoted by Au. b N-P25-620 after being soaked in acidic solutions with different pH. c N-P25-620 after being soaked in alkaline solutions with different pH. d Au/N-P25-620/MgO(111), Au/N-P25-620/MgO(110) and Au/N-P25-620/MgO(100) with Au/N-P25-620 included as a reference (exciton lifetimes and errors are shown in Supplementary Table 4). e Schematic illustration of the local electric field of polar MgO(111) nanocrystals with negative and positive ion-terminated surfaces can assist the photocatalytic water splitting to H₂/O₂ via H⁺ and OH⁻ surrounding the N-doped TiO₂ catalyst particle

Fig. 4

The photocatalytic overall water-splitting activity tests in the light furnace. a A photographic image of a four-mirror floating-zone light furnace from Crystal Systems Inc. (Supplementary Fig. 18) used to mimic a solar concentrator to focus a light beam to provide both heat and photons to the N-doped TiO₂ without any other energy input from an electrical device. b The reactor temperature of 270 °C is maintained by this light source, and H₂ evolution rates of about 12 and 20 mmol g⁻¹ h⁻¹ were achieved over Au- promoted N-P25-620 and N-P25-620/MgO(111), respectively. Error bars indicate the standard deviation