Nitrogen cycle on N-doped graphene loaded TiO2 for efficient photocatalytic dinitrogen conversion

FTIR: Fourier transform infrared (FTIR) spectra were recorded in the transmittance mode using a FTIR spectrometer (Nicolet 6700, ThermoFisher, USA). First, ca. 5 mg of the sample was ground with KBr and pressed into pellet. Next, the sample pellet was placed into the FTIR system. Then, run a background scan without the sample to account for atmospheric interference and instrument noise. Finally, the FTIR spectra of the samples can be obtained.TEM: The transmission electron microscopy (TEM) images were taken on Hitachi Model H-7700 microscope operated at 100 kV. During the TEM experiment, the obtained sample powder is placed on a copper grid. Once the sample is fixed in place, the test is carried out. Morphological images of the samples at different magnifications were obtained.TOFMS: The photocatalytic nitrogen reduction test was performed in a home-made reactor. Generally, 10 mg sample was first dispersed in 15 mL DI water and poured into the the reactor. The dispersion was dried at 70 oC for 12 h, creating a thin film at the bottom of the reactor. Prior to the photocatalytic reaction, 1 mL DI water was added into the notch in the reactor. After purging the reactor with dinitrogen gas with a flow rate of 30 ml/min for 30 min, the reactor was irradiated by 300 W Xe lamp for 2 h under the magnetic stirring and flow of dinitrogen gas flow. Finally, the reaction gas was syringed out from the reactor for TOFMS characterizations.Ex situ XPS: After the photocatalytic dinitrogen conversion reaction, the used-samples were collected. X-ray photoelectron spectroscopy (XPS) characterization of the prepared samples was carried out on JPS-9010MC (JEOL, Japan) with a hemispherical electron energy analyzer (1486 eV Al Kα radiation). AP-XPS: The ambient-pressure XPS (APXPS) was performed on XPS system connected to the bending magnet beamline 02B at Shanghai synchrotron radiation facility. An energy of 720 eV was applied to collect the surface information of the prepared samples. Generally, this APXPS system is consisted of four parts including load-lock chamber, preparation chamber, μ-metal analysis chamber and photoelectron analyzer. Prior to the APXPS test, 10 mg sample was first dissolved in 1 ml DI, and dropped on an Al foil. Then, the sample was dried, forming a thin film on the Al film. Afterward, the sample was placed into the analysis chamber. To simulate the actual photocatalytic nitrogen fixation condition, the dinitrogen gas and water vapor were flowed into the analysis chamber with a partial pressure of 0.16 mbar. Subsequently, the sample was irradiated with a 365 nm LED light (3 W, Shenzhen Lamplic). The high-resolution N 1s spectra were recorded at each stage for determining the change of the surface status of the prepared samples. DRIFTS: The in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) was collected using Nicolet iS50 infrared Fourier transform spectrometer (TMO, USA). Prior to test, 100 mg sample was placed into the reaction chamber of the DRIFTS. The chamber was then sealed and flowed with the dinitrogen gas and water vapor. The DRIFTS spectra were recorded under dark and light irradiation conditions.