Highly efficient doping of titanium dioxide with sulfur using disulfide-linked macrocycles for hydrogen production under visible light

Abstract

Doping sulfur into TiO₂ can narrow the bandgap of native TiO₂, offering an efficient way to improve its photocatalytic efficiency and extend its absorption from the ultraviolet to the visible light region. However, its low doping efficiency has remained a stumbling block in the advanced development of sulfur-doped TiO₂ (S-TiO₂). Here, we demonstrated disulfide macrocycles (DSMs) as ideal candidates for efficiently engineering TiO₂ with sulfur. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis showed that the sulfur doping content in the entire S-TiO₂ was about 1.5 wt%, while its surface contained about 2.0 wt% sulfur determined by X-ray photoelectron spectroscopy (XPS), using only 5.0 wt% DSM as the dopant. The overall utilisation rate of sulfur was determined to be about 95%, which was much higher than those of reported examples. The XPS analysis of the S-TiO₂ material suggested that the sulfur element was in the S⁶⁺ form on the crystal lattice by replacing Ti⁴⁺ and the surface of TiO₂ with the formation of SO₄²⁻. The energy bandgap of S-TiO₂ was 2.3 eV, a reduction compared to 3.2 eV of native TiO₂, which allowed the extension of the light absorption range of the engineered material to 700 nm. Accordingly, the materials, loaded with platinum nanoparticles, photo-catalytically split water into hydrogen under visible light. The hydrogen production performance was about 573.3 mmol g⁻¹ h⁻¹ (based on the mass of platinum) and remained similar even after 50 h, with a turnover number of 4990. Our findings indicate that DSMs are efficient dopants for modifying TiO₂ with sulfur. The high-efficiency doping of TiO₂ with sulfur from the macrocycles significantly reduced the amounts of dopants needed, presenting a possibility for the green industrial production of S-TiO₂ in the future.