Aero-TiO2 three-dimensional nanoarchitecture for photocatalytic degradation of tetracycline

The morphology of the hollow TiO₂ microtetrapods is illustrated in Fig. 1. The dimensions of the arms of aero-TiO₂ microtetrapods vary in the range from 20 to 40 µm in length and 1 to 3 µm in diameter. The wall thickness of the TiO₂ microtubes is around 50 nm.

(a, b) SEM images of the aero-TiO₂ material consisting of hollow microtetrapods, and (c) the schematic route of the aeromaterial preparation, starting with initial ZnO microtetrapods, atomic layer deposition of TiO₂ on their surface (also illustrated in cross section) and, finally, removal of sacrificial ZnO substrate in the presence of HCl and H₂ gases at 800 °C.

The XRD pattern shown in Fig. 2a demonstrates the presence of the rutile phase TiO₂ (JCPDS 00–021-1276) and the ternary compound Zn₂Ti₃O₈ (JCPDS 01–073-0579). All diffraction lines of the TiO₂ and Zn₂Ti₃O₈ were indexed by tetragonal TiO₂ with the space group P42/mnm(136) and cubic Zn₂Ti₃O₈ with the space group Fd-3 m(227).

XRD pattern (a) and Raman spectrum (b) of the fabricated aero-TiO₂ material.

The sizes of crystallites were determined considering the main peaks of the compounds and were found to be 50 nm for rutile TiO₂ and 36 nm for Zn₂Ti₃O₈, which were determined by the Scherrer equation (Eq. 1):

where, λ is the wavelength (Co Kα, λ = 1.7903 Å), β is the full width at the half-maximum (FWHM) and θ is the diffraction angle.

It was found previously that aero-TiO₂ can be obtained in a mixture of anatase–rutile compound with Zn₂TiO₄ inclusions by using the same ALD process with subsequent selective wet chemical etching of ZnO sacrificial template¹³. Higher annealing temperature and the different approach we used for the ZnO removal allowed one to obtain a composite consisting of rutile phase TiO₂ and cubic Zn₂Ti₃O₈.

The XRD data are corroborated by the Raman scattering analysis (Fig. 2b). The rutile structure of TiO₂ belongs to the space group \(D\genfrac0pt144h\) and it has four Raman active vibrations: A_1g + B_1g + B_2g + E_g. The observed peaks at 447 cm⁻¹ and 612 cm⁻¹ are attributed to the E_g, and A_1g modes, respectively. Second order scattering features can also be visible in the spectrum, the most intensive one being at 238 cm^{−1 22}.

By using the Kubelka–Munk equation (Eq. 2), the optical band gap of the sample was determined from the diffuse reflectance spectrum:

where α is the optical absorption coefficient, hν is the photon energy, A is a constant of proportionality, and exponent p is determined by the transition type of the material: p = 2 for direct allowed transitions, p = 2/3 for direct forbidden transition, p = 1/2 for indirect allowed transitions, and p = 1/3 for indirect forbidden transitions.

Since TiO₂ rutile phase is known as a direct transition semiconductor, the function used for plotting was

The optical band gap energy of the material was determined by the extrapolation of the slope to F(R) → 0 from the plot [F(R)·hν]² vs. hν, as shown in Fig. 3.

Optical bandgap determined from UV–visible diffuse reflectance spectrum.

According to the modified Kubelka–Munk function, the UV–visible diffuse reflectance spectra show that TiO₂ hollow microtetrapods have the bandgap of 3.12 eV. For the purpose of comparison one can note that anatase and rutile phases of TiO₂ have a bandgap of around 3.0 eV and 3.2 eV, respectively²³, while Zn₂Ti₃O₈ has a calculated bandgap of 3.55 eV ²⁴.

Photoluminescence (PL) measurements were performed to evaluate the presence of defects in the synthesized samples. The PL spectra from Fig. 4a span a broad energy range from 2.0 to 3.5 eV. Notably, as the temperature rises from 10 K to room temperature, the intensity of the high-energy band decreases more significantly than that of the low-energy band.

PL spectra of aero-TiO₂ at 10 K and 300 K (a) and the deconvoluted spectrum of PL recorded at 10 K (b).

The deconvolution of the PL spectrum presented in Fig. 4b reveals that it comprises three distinct energy bands: one green band and two violet bands. The green band is centered at 2.5–2.6 eV, while the first and second violet bands are located respectively at 2.9–3.0 eV and 3.15 eV.

One can assign the high-energy emission band at 3.15 eV to near-bandgap transitions, while the violet PL band at 2.9 eV may be associated with an unidentified defect. The green band has previously been linked to the recombination of self-trapped excitons formed from carrier polarons²⁵. It is suggested that oxygen vacancies facilitate effective trapping of carriers or polarons and, simultaneously, the efficient charge separation allows for electron and hole accumulation or trapping at distinct sites, potentially on the surface. Given the extensive surface areas of aeromaterials, one may expect surfaces to play a major role in their photoluminescence behavior.

The kinetics of the tetracycline photodegradation was fitted according to the pseudo-first-order model (Eq. 4):

where, C₀ and C_t represent the concentrations of tetracycline in solutions at irradiation time t = 0 min and t, respectively, and k represents the degradation rate (min^-1).

Without the photocatalyst, the tetracycline concentration in the solution is not significantly influenced by irradiation with visible or UV light. When aero-TiO₂ is added to the solution (Fig. 5e), the concentration of tetracycline decreases by about 75% when irradiated with visible light for 180 min. Upon irradiation with UV light, the photocatalysis process is faster, and the tetracycline concentration decreases by about 90% during 150 min (Fig. 5a). The degradation rates of tetracycline were estimated to be about 0.0064 min^-1 and 0.0120 min^-1 upon irradiation with visible and UV light, respectively, as shown in Figs. 5b.

Photocatalysis performance of aero-TiO₂ for the degradation of tetracycline under visible or UV (a) and under UV light irradiation in the continuous solution flow conditions (c) and their degradation rates (b, d); the schematics of the experimental setup for the photocatalysis tests (e, f).

Previously, Wu et al. have demonstrated that TiO₂ P25 nanoparticles with the surface area of about 55 m²/g are able to degrade tetracycline with a ratio of about 0.038 min^-1 under 350 nm irradiation, and the photocatalytic efficiency decreases with the increase of the wavelength irradiation source, down to 0.00055 min^-1 at 850 nm ²⁶. Despite the differences in testing conditions, these results still can be roughly compared with those from our work. The observed enhanced photocatalytic performance of aero-TiO₂ can be attributed to the presence in the nanocomposite structure of Zn₂Ti₃O₈ inclusions decreasing the rate of recombination of photogenerated electron–hole pairs, thus allowing them to reach the photocatalyst surface^27,28,29.

Table 1 provides a summary of the existing knowledge on tetracycline photodegradation using various structures of TiO₂ photocatalysts, emphasizing the performance of these materials and reaction parameters.

There are three main types of active species which mainly contribute to the photocatalytic degradation of tetracycline, namely ^·O₂⁻, ^·OH and h⁺ as was previously observed by other authors³⁷. ^·O₂⁻ and h⁺ species have a major role in the photocatalysis under UV and only ^·O₂⁻ becomes important under visible light irradiation³⁸. The electrons from the valence band are excited to the conduction band, reacting with O_2, leading to the formation of ^·O₂⁻ species, which further react with the adsorbed tetracycline molecules at the material surface, while h⁺ species directly contribute to the oxidation of tetracycline³⁹.

Previous studies have demonstrated that during photocatalytic oxidation reactions, oxygen vacancy defects in ZnO-TiO₂ nanocomposite materials serve as active sites for capturing photoinduced electrons, significantly enhancing photocatalytic efficiency. Additionally, oxygen vacancies facilitate the adsorption of environmental oxygen onto the sample, leading to strong interactions between the photoexcited electrons captured by these vacancies and the adsorbed oxygen molecules⁴⁰.

The ability to fabricate nanocomposites with precise control opens new pathways for bandgap engineering, enabling alignment of the conduction and valence bands of composite materials with the HOMO and LUMO molecular orbitals of organic compounds targeted for photocatalytic degradation. Under these conditions, photogenerated electrons transfer from the conduction band of one component to that of another, while photogenerated holes similarly move between valence bands. Additionally, transition of the excited electrons from organic molecules to the conduction bands of the nanocomposite components generate reactive species that drive chemical reactions⁴¹. Given the substantial specific surface area of the synthesized aeromaterials, it is also likely that surface states play a critical role in modulating the valence band edge, thereby enhancing photocatalytic properties under visible-light irradiation, including those relevant to water splitting.

In the experiment performed under continuous liquid flow conditions using UV light with a density of 3.2 mW/cm² (see Fig. 5f), the concentration of tetracycline decreases by about 65% during seven hours of irradiation with a degradation rate of 0.0022 min^-1 (Fig. 5c and d). After the full degradation of tetracycline, the material was repeatedly used in three more consecutive tests. It was observed that the degradation performance was not influenced, thus demonstrating the reusability of the material. Hence, the 3D shape of our material with 2D features, such as the wall thickness of about 50 nm, makes it suitable for incorporation in active filters for water treatment without the risk of water contamination with the active material.