Fe-Modified TiO2 Photocatalyst for Effective Indoor Bacterial Inactivation Using LED Light: Toward Safer Disinfection Technologies

Abstract

Commercial TiO₂ P25 was modified with iron species using simple, mild and cost-effective synthesis conditions. The novel TiO₂-Fe material exhibits both high disinfectant activity and long stability under visible light irradiation. The bactericidal activity of the synthesized catalyst was assessed by its capability to inactivate Escherichia coli under both simulated solar and LED-visible light. XRD confirmed the structural integrity of the material, while diffuse reflectance spectroscopy evidenced its absorption in the visible region. Mössbauer and FTIR analysis of the TiO₂-Fe catalyst indicate that iron exists predominantly in the Fe³⁺ state in a Ti-O-Fe bond. Electrochemical studies allowed to correlate the reactivity with the conduction and valence band positions, providing insights into the production of reactive oxygen species (ROS) responsible for bacterial inactivation. A first noteworthy aspect of this study is the identified Ti-O-Fe bond in the TiO₂-Fe material, a feature that has not been previously reported in the literature. Another innovative aspect is the simplicity of the synthesis method, which employs a thermally mild and time-efficient process, thereby providing an efficient and accessible approach for enhancing photocatalytic properties. Our findings suggest that the prepared TiO₂-Fe photocatalyst holds potential for future applications in antimicrobial paint coatings designed for hospital environments, aiming to enhance infection control under indoor lighting conditions.

1

X-ray diffractograms for TiO₂ and TiO₂–Fe samples. Above the corresponding legend, a picture of the material is inserted as an inset.

2

E. coli disinfection kinetic profiles induced by TiO₂ (black triangles) and TiO₂–Fe (red squares) using (a) the Suntest Atlas solar simulator or (b) the LED-visible light. The dashed and continuous lines represent experiments under irradiation and darkness, respectively. The insets display the corresponding inactivation percentages. Experimental conditions: 150 mL of bacterial suspension and 15 mg of photocatalyst. The error bars represent standard deviation (SD) from duplicate experiments (n = 2). (c) UV–vis DRS spectra of TiO₂ (black) and TiO₂–Fe (red) with corresponding emission spectra of the photon sources used in this study (solid blue line) Solar simulator (Suntest Atlas) and visible light (LED) (dashed blue line). (d) TEM micrographs of E. coli before phototreatment (left) and (e) after 60 min of phototreatment under visible LED light with TiO₂–Fe (right).

3

(a) E. coli disinfection under visible light during six reuse cycles of TiO₂–Fe. Experimental conditions:1000 mg. L^–1, 4 h each reuse cycle, the initial concentration of bacteria in each cycle was ∼10⁶ CFU/mL. (b) ICP-MS analysis of soluble Fe concentration (mg·L^–1 Fe/mg TiO₂–Fe) on the aqueous phase during runs presented in panel (a). (c) E. coli disinfection profiles under different experimental conditions (Catalyst concentration: 100 mg·L^–1).

4

Electrochemical characterization summary. (a) Photochronoamperometry of the TiO₂ and TiO₂–Fe materials supported on FTO electrodes under irradiation using the Sunset Atlas simulator followed by measurements in the dark. (b) Nyquist plot for TiO₂ (black circles) and TiO₂–Fe (red triangles) along with the fitted circuit measured at −0.3 V. (c) Mott–Schottky plot of the same samples. The continuous lines represent the extrapolation of the linear regions to estimate the flat band potential of the conduction band, as indicated by the numbers (Hankin et al., 2019). (d) Schematic representation of the valence band (E_VB) and conduction band (E_CB) for TiO₂ (black lines) and TiO₂–Fe (red lines) along with selected redox pairs and their standard reduction potential vs NHE electrode.

5

Schematic illustration of the E. coli cell inactivation mechanism by ¹O₂, O₂ ^•– and holes generated on TiO₂–Fe under visible light. Numbers in parentheses correspond to key photophysical and photochemical processes detailed in the main text.