Visible-Light-Active Black TiO2 Nanoparticles with Efficient Photocatalytic Performance for Degradation of Pharmaceuticals

Abstract

Special attention has recently been paid to surface-defective titanium dioxide and black TiO₂ with advanced optical, electrical, and photocatalytic properties. Synthesis of these materials for photodegradation and mineralization of persistent organic pollutants in water, especially under visible radiation, presents interest from scientific and application points of view. Chemical reduction by heating a TiO₂ and NaBH₄ mixture at 350 °C successfully introduced Ti³⁺ defects and oxygen vacancies at the surface of TiO₂, with an increase in the photocatalytic degradation of amoxicillin-an antibiotic that is present in wastewater due to its intense use in human and animal medicine. Three TiO₂ samples were prepared at different annealing temperatures to control the ratio between anatase and rutile and were subjected to chemical reduction. Electron paramagnetic resonance investigations showed that the formation of surface Ti³⁺ defects in a high concentration occurred mainly in the anatase sample annealed at 400 °C, contributing to the bandgap reduction from 3.32 eV to 2.92 eV. The reduced band gap enhances visible light absorption and the efficiency of photocatalysis. The nanoparticles of ~90 m²/g specific surface area and 12 nm average size exhibit ~100% efficiency in the degradation of amoxicillin under simulated solar irradiation compared with pristine TiO₂. Mineralization of amoxicillin and by-products was over 75% after 48 h irradiation for the anatase sample, where the Ti³⁺ defects were present in a higher concentration at the catalyst's surface.

Keywords: Ti3+ states; amoxicillin; black TiO2; chemical reduction; defective TiO2; photocatalysis; wastewater treatment.

Figure 1

XRD patterns of white and black titanium oxide samples.

Figure 2

Low-magnification TEM images and the corresponding electron diffraction patterns for the white samples: (a,d) T400W, (b,e) T550W, (c,f) T800W; for the black samples: (g,j) T400B, (h,k) T550B, (i,l) T800B. The letter “A” from the indexed diffraction patterns stands for “anatase”, while the letter “R” stands for “rutile”.

Figure 3

TEM images at higher magnifications, revealing the samples’ morphology: (a) T400W, (b) T550W, (c) T800W, (d) T400B, (e) T550B, and (f) T800B.

Figure 4

HRTEM images of the (a) T400W, (b) T550W, (c) T800W, (d) T400B, (e) T550B, and (f) T800B samples. Interplanar distances are marked for the anatase (a,b,d,e) and rutile (c,f) phases.

Figure 5

Q-band EPR spectra of the white (a) and black (b) samples at 120 K. The broad line at ~1185 mT in (a) is a background resonator signal.

Figure 6

Experimental (solid black line) spectra of the T400W (a) and T400B (b) samples at 120 K. The simulated spectra (red dotted line) are the sums of the calculated spectra of the various paramagnetic centers represented below. The background resonator Mn²⁺ spectrum (navy blue dashed line) was included for accuracy. The amplitudes of the Ti³⁺_A(II) (a) and A, D (b) spectra were multiplied with the factors from the figure for better observation.

Figure 7

Experimental (solid black line) spectra of the T400B (a), T550B (b), and T800B (c) samples at 120 K. The simulated spectra (red dotted lines) are the sum of the calculated spectra of the different Ti³⁺ centers.

Figure 8

(a) Evaluation of the visible-light-driven photocatalytic activity of the original white TiO₂ and black TiO₂ prepared through NaBH₄ reduction. (b) A plot of ln(c/c₀) versus irradiation time for the photocatalytic degradation of amoxicillin at different catalysts. (c) First-order kinetics rate constant of amoxicillin removal by visible simulated irradiation over original white and black titanium oxide photocatalysts.

Figure 9

(a) The c/c₀ versus irradiation time curves of black TiO₂ for long-term AMX photodegradation. (b) The AMX mineralization efficiency for long-term degradation over black TiO₂ photocatalysts.