Enhanced visible light response and cytocompatibility of TiO2–TiC shell–core structured S-scheme photocatalyst

Abstract

To enhance the visible light response of titanium dioxide (TiO2), titanium carbide (TiC) nanoparticles (NPs) were thermally treated in carbon powder, effectively overcoming the challenges associated with conventional doping methods. During the treatment, a TiO2 thin shell with oxygen vacancies (OVs) formed around the TiC NPs, creating a shell–core structure S-scheme photocatalyst. Transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) spectroscopy confirmed the successful formation of the TiO2 shell. By optimizing the shell thickness, the TiO2–TiC shell–core structure achieved an ideal shell–core ratio, resulting in strong visible light absorption (400–800 nm), and the degradation rate constant of Rhodamine B (RhB) of sample cHT500 reached 0.0687 min−1, which is 20.8 times higher than that of pristine TiO2 (0.0033 min−1) under visible-light irradiation. In addition, cytocompatibility tests showed that sample cHT500 exhibits favorable cell viability, which is comparable to that of TiO2 nanoparticles, and thus remarkably mitigates the poor biocompatibility inherent to TiC, making them promising candidates for biomedical and photocatalytic applications.

1. Introduction

TiO2-based photocatalysis integrates physical chemistry into biomedicine, offering significant advantages over other inorganic materials due to its stability, low toxicity, and photon-responsive activity within the visible light or even near-infrared (NIR) spectrum [1–3]. Current research is focused on TiO2's potential as a biomedical tool, especially as an effective nanocarrier platform for various photosensitizers, owing to its unique optical and photocatalytic properties [4-5]. When light energy exceeds the band gap of TiO2, electron–hole pairs are generated, enabling strong oxidative reactions. This mechanism supports the use of TiO2 as an injectable agent to target and destroy cancer cells. However, TiO2 nanoparticles (NPs) accumulate in tumor tissues via the enhanced permeability and retention (EPR) effect [5]. Due to the wide band gap of TiO2 (anatase: 3.2 eV; rutile: 3.0 eV), it primarily responds only to ultraviolet (UV) light. This greatly limits its utility in emerging medical applications such as photodynamic therapy (PDT) and photothermal therapy (PTT), which require excitation by longer-wavelength light to achieve sufficient tissue penetration [6–9].

To develop catalysts responsive to visible light, various strategies have been applied to TiO2-based materials, including co-doping with non-metal and metal elements, inducing oxygen vacancies (OVs), and constructing core–shell structures with upconversion NPs (UCNPs) [10–13]. Studies by Sakthivel and Kisch have shown that incorporating transition metal chlorides either into the bulk or on the surface of TiO2 enables visible-light photocatalysis [14]. Although doping TiO2 with rare-earth and metallic elements can enhance visible-light absorption, these dopants may induce dark toxicity and persist in the body after treatment. Moreover, the doping process introduces impurities and defects into the TiO2 crystal lattice, which act as recombination centers for photogenerated carriers, thereby reducing catalytic efficiency [15]. Research by Wang et al. [16] has shown that OVs act as recombination centers for photoinduced electrons and holes, thus diminishing photocatalytic activity. Similarly, Lakshminarasimhan et al. [17] reported that with increased Pt loading, Pt clusters can act as recombination centers, negatively impacting charge transfer from TiO2 to Pt and/or from Pt to the electrolyte.