Green sol–gel route for selective growth of 1D rutile N–TiO2: a highly active photocatalyst for H2 generation and environmental remediation under natural sunlight

Abstract

We report selective growth of N–TiO₂ 1D nanorods using a green aqueous sol–gel method followed by hydrothermal treatment. Titanium tetraisopropoxide, diethanolamine, and H₂O₂ were used as precursors for preparing an aqueous gel. Trifluoroacetic acid (TFA) was used as a growth regulator for selective growth of desired structure and morphology. Effects of TFA on the structure and morphology of N–TiO₂ were studied by varying concentration of TFA between 1–10% by volume. Structural characterization using XRD confirmed formation of a specific rutile phase with slight crystal disorder with N-doped TiO₂ samples. FESEM and HRTEM analysis showed formation of 1D rice grain shaped N–TiO₂ in the presence of 1% TFA solution. One directional growth along the (211) plane was confirmed by both HRTEM and XRD. Optical characterization using UV-visible revealed a red shift in the absorption edge which is marginally extended in the visible region. Subsequent Tauc plot analysis showed decrease in the band gap reflecting a decrease in the energy threshold of TiO₂ from N-doping. A FTIR spectrum showed Ti–O and Ti–N vibrations confirming the presence of N in the TiO₂ matrix. XPS analysis was used to examine electronic states of nitrogen incorporation; peaks at 397.1 and 399.5 eV were attributed to substitutional (Ti–N–O) and interstitial (Ti–O–N) site doping, respectively. Quantification of N content in 5% N-doped TiO₂ showed nitrogen concentration of 4.5% which is very close to the doped concentration, suggesting no loss of N in the hydrothermal reaction. The applicability of both undoped and N–TiO₂ was tested for photosplitting of H₂O and degradation of methyl orange (MO) under artificial solar light (xenon lamp) and natural solar light. Results of hydrogen production in aqueous solution of methanol under natural solar light were compared with artificial sunlight; N–TiO₂ showed excellent photocatalytic activity under natural sunlight for H₂O splitting and MO degradation. The rice grain shaped N–TiO₂ sample showed a H₂ generation rate of 7990 μmol g⁻¹ h⁻¹ under natural sunlight but only 4740 μmol g⁻¹ h⁻¹ under a xenon lamp. The superior photoactivity under natural sunlight may be due to the presence of both UV and visible light.