Facile aqueous synthesis and comparative evaluation of TiO₂-semiconductor and TiO₂-metal nanohybrid photocatalysts in antibiotics degradation under visible light

Abstract

Advanced oxidation processes using TiO₂-based nanomaterials are sustainable technologies that hold great promise for the degradation of many types of pollutants including pharmaceutical residues. A wide variety of heterostructures coupling TiO₂ with visible-light active nanomaterials have been explored to shift its photocatalytic properties to harness sun irradiation but a systematic comparison between them is lacking in the current literature. Furthermore, the high number of proposed nanostructures with different size, morphology, and surface area, and the often complex synthesis processes hamper the transition of these materials into commercial and effective solutions for environmental remediation. Herein, we have designed a facile and cost-effective method to synthesize two heterostructured photocatalysts representative of two main families of novel structures proposed, hybrids of TiO₂ with metal (Au) and semiconductor (CeO₂) nanomaterials. The photocatalysts have been extensively characterized to ensure a good comparability in terms of co-catalyst doping characteristics, morphology and surface area. The photocatalytic degradation of ciprofloxacin and sulfamethoxazole as target pollutants, two antibiotics of high concern polluting water sources, has been evaluated and CeO₂/TiO₂ exhibited the highest activity, achieving complete antibiotic degradation at very low photocatalyst concentrations. Our study provides new insights into the development of inexpensive heterostructured photocatalysts and suggests that the non-stoichiometry and characteristic d and f electronic orbital configuration of CeO₂ have a significantly improved role in the enhancement of the photocatalytic reaction.

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1. Introduction

Water sources are becoming increasingly scarce due to various factors such as climate change, industrialization, and human activity. Antibiotic abuse and misuse are among the activities particularly detrimental to marine and fluvial ecosystems, and contribute to the development of antibiotic resistance. To address this problem, advanced oxidation processes (AOPs) have been identified as a promising approach to remove water pollutants, including pharmaceutical residues.^1–4 One widely used AOP is photocatalytic degradation using titanium dioxide (TiO₂) because of its high photocatalytic activity, fast electron transfer ability to molecular oxygen, high stability within a wide pH range, non-toxicity, and low-cost.^5,6 In the catalytic reaction, when TiO₂ absorbs photons with energies higher or equal to its band gap energy, electrons are promoted from the valence band to the conduction band which generates electron–hole pairs that can oxidize water or hydroxide ions to produce hydroxyl radicals. These radicals act as powerful oxidizing agents.⁷ However, the wide band gap of TiO₂ limits its ability to utilize solar light and the recombination of photo-generated electrons and holes decrease the quantum efficiency.⁸ To overcome these limitations, doping TiO₂ with metal ions such as Cr, V, and Fe was proposed in the 90s, showing a red shift of the band-edge of the TiO₂, and this allowed for the catalytic decomposition of NO into N₂, O₂, and N₂O under visible light.⁹

With the advent of nanotechnology, the development of heterostructured photocatalysts that involve the coupling of TiO₂ nanoparticles (NPs) with noble metal NPs (e.g. Pt, Pd and Au)^10–15 or semiconductor NPs (e.g. ZnO, Cu₂O, SnO₂, ZrO₂, WO₃, and CeO₂)^16–21 in different configurations such as core-shells or nanohybrids is an active field of research. These materials are usually chosen for their co-catalyst properties, the plasmonic effect, in the case of metal NPs, which enhances the absorption of light towards the visible range of sunlight, and because they promote the separation of the photoexcited charge carriers (electrons and holes), thus reducing their recombination which prolong the charge separation time and increase redox capacity.^22–25 In a typical report of photocatalytic degradation of pollutants using such NPs, a photocatalyst nanomaterial is proposed and different parameters are explored. For instance, in the case of Au/TiO₂, one of the most widely studied due to the non-toxicity and localized surface plasmon resonance exhibited by AuNPs^12,26–28 it has been shown, among others, how the photocatalytic properties are affected by the interface between Au and TiO₂,²⁹ the size and shape of Au,^30,31 and the oxidation state of TiO₂.³² In the case of coupling TiO₂NPs with other semiconductor NPs, TiO₂/CeO₂ hybrid nanostructures are attracting more attention recently due to the improved textural and structural properties of TiO₂.^33–36 Different types of CeO₂/TiO₂ with different morphologies and textural parameters have been proposed and compared, but a literature review revealed that the preparation conditions of TiO₂/CeO₂ have not been investigated in detail to achieve a nanostructure prepared in an industrially feasible way,³⁷ and this is still a common hindrance for many advanced nanostructures.³⁸

Herein, two of the proposed nanostructured TiO₂-based photocatalysts have been prepared using simple bench-top chemistry, which are representative of the metal (Au/TiO₂) and semiconductor (CeO₂/TiO₂) NP coupling. Furthermore, the photocatalytic degradation using Au/TiO₂ and CeO₂/TiO₂ of ciprofloxacin (CIP) and sulfamethoxazole (SMX), two of the main antibiotics present in water sources, has been evaluated and compared. CIP is a type of antibiotic belonging to the fluoroquinolone class, commonly used for treating bacterial infections. For instance, in one cluster of Indian pharmaceutical factories consisting of 90 bulk drug manufacturers, the effluent discharge of approximately 1500 m³ of wastewater per day was found to contain high concentrations of ciprofloxacin, measuring 28 000 μg L⁻¹ and 31 000 μg L⁻¹ on two consecutive days, exceeding e.g. the levels toxic to some bacteria by over 1000-fold.³⁹ These concentrations correspond to the release of several kilograms of antibiotics daily and tens of tons annually into the environment. Lakes situated in the vicinity of the cluster also showed elevated levels of antibiotic contamination, with ciprofloxacin concentrations measuring up to 6500 μg L⁻¹,⁴⁰ while the Predicted No Effect Concentration-Minimum Inhibitory Concentration (PNEC-MIC) approach described by Bengtsson-Palme and Larsson⁴¹ and the Antimicrobial Resistance Industry Alliance (2018)⁴² has been estimated to be 0.064 μg L⁻¹. SMX is a sulfonamide antibiotic that was initially introduced in combination with trimethoprim by F. Hoffmann-La Roche in 1969. Despite a decline in sales figures over the past two decades, SMX continues to be widely utilized, and its PNEC has been estimated to be 0.59 μg L⁻¹.^43,44

The photocatalysts used in this work have been extensively characterized by a combination of instrumental techniques including High-Resolution Transmission Electron Microscopy (HR-TEM), X-Ray Diffraction (XRD), UV-VIS spectroscopy, Dynamic Light Scattering (DLS) and BET and X-Ray Photoelectron Spectroscopy (XPS) measurements. Results show that CeO₂/TiO₂ exhibit superior photocatalytic performance, achieving complete CIP degradation within 30 and 180 minutes under UV and visible light. Remarkably, coupling TiO₂NPs with CeO₂NPs also allows the use of the hybrids at lowest reported concentrations, thus reducing the environmental impact of TiO₂NPs. The control of single component CeO₂NPs also exhibit superior photocatalytic performance in CIP degradation compared to the hybrid Au/TiO₂, which suggest that the inherent redox properties of the co-catalyst CeO₂ have an important role in the enhancement of the photocatalytic reaction. These results were further confirmed using SMX as pollutant as similar trends were observed.

2. Materials and methods

2.1. Materials and reagents

All the nanomaterials employed in this work were synthesized in the aqueous phase, using Milli-Q grade water. All reagents were purchased from Sigma-Aldrich (99% purity grade unless indicated otherwise) and used as received. Gold(III) chloride trihydrate (HAuCl₄·3H₂O, CAS No. 16961-25-4) was used as precursor of AuNPs, cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, CAS No. 15878-77-0) as precursor of CeO₂NPs and titanium tetrachloride (TiCl₄, CAS No. 7550-45-0) as precursor of TiO₂NPs. Other reagents utilized for the synthesis of the nanomaterials include sodium borohydride (NaBH₄, CAS No. 16940-66-2), trisodium citrate dihydrate (CAS No. 6132-04-3), potassium carbonate (K₂CO₃, CAS No. 584-08-7) and ammonium hydroxide (NH₄OH, 28% NH₃ in H₂O, ≥99.99%, CAS No. 1336-21-6). The structure of model antibiotic molecules Ciprofloxacin (CIP) and Sulfomethoxazole (SMX) are illustrated in Fig. S1† and their physio-chemical data listed in Table S1.†

2.2. Synthesis of single component Au, CeO₂ and TiO₂ NPs

All the synthesis procedures are based in pre-existed ones available in the scientific literature with few modifications to be adapted for the concentrations needed for this work. AuNPs were synthesized following the procedure based on Jana et al.⁴⁵ which consist on the fast injection of 0.1 M sodium borohydride (ice-cold freshly prepared) into an aqueous solution containing 0.25 mM Gold(III) chloride trihydrate and 0.25 mM trisodium citrate. This solution was kept under stirring for three hours and stored in the fridge for subsequent experiments. CeO₂NPs were synthesized by the chemical precipitation of Ce(NO₃)₃·6H₂O in a basic aqueous solution following a procedure described in Zeng et al.⁴⁶ Briefly, 10 mM of Ce(NO₃)₃·6H₂O was dissolved in 100 mL of Milli-Q water at room temperature. Afterward, 3 mL of 1 M ammonium hydroxide was added slowly at room temperature under vigorous stirring and the mixture was allowed to continue under mild stirring overnight. TiO₂NPs synthesis was based in the method described by Pottier et al.,⁴⁷ the synthesis procedure consists on the decomposition of titanium tetrachloride (TiCl₄) at acidic pH = 5. Subsequently, the nanocrystals were left to in an oven at 60 °C for 12 hours and purified by centrifugation steps.

2.3. Synthesis of CeO₂/TiO₂ and Au/TiO₂ hybrid photocatalysts

The hybrid nanostructures were prepared by facile and entirely hydrophilic and open atmosphere synthesis methods. The preparation of CeO₂/TiO₂ involved the following steps: first, 90 mL of the as-prepared TiO₂NPs (5 mg mL⁻¹) were mixed with 10 mL of NH₄OH at 75 mM and sonicated during 10 minutes to ensure that TiO₂NPs were fully dispersed. Second, 108.5 mg of Ce(NO₃)₃·6H₂O were stirred into the mixture and heated under reflux at 100 °C for 4 hours. Finally, the resulting solution was washed by purification and colloidally stable CeO₂/TiO₂ were redispersed in a solution of 1 mM NH₄OH. The preparation of the Au/TiO₂ involved the following steps. First, 96 mL of the as-prepared AuNPs were adjusted to pH 10 by adding equal amounts of sodium citrate and K₂CO₃ at 150 mM. This solution was heated to 90 °C under stirring. Second, 4 mL of TTIP (10 mM) was added to the solution and heating was continued for 2 hours. Similar as the CeO₂/TiO₂, the resulting solution was washed by purification and colloidally stable Au/TiO₂ were redispersed in a solution of 1 mM NH₄OH.

2.4. Characterization techniques

Nanomaterials were visualized using a high-resolution transmission electron microscopy (HR-TEM, FEI Talos, F200s). 40 μL of the colloidal solutions were drop-casted onto a carbon coated 200 mesh copper grid and left to dry at room temperature. Particle size distribution was measured using Image J Analysis software. Hydrodynamic diameters were determined with Dynamic Light Scattering (DLS, Nanotrac wave II, Macchique, USA) with a light source wavelength of 532 nm and fixed scattering angle of 173°. Measurements were conducted in 1 cm path cell and three independent measures were performed. The study of the crystallinity and structural phase of the nanomaterials was performed with a X-ray diffraction (XRD, Rigaku SmartLab SE diffractometer, Tokyo, Japan) using Cu Kα (λ = 0.15418 nm) in the range of 2θ = 5–90° with an increment of 0.02°. To identify the Ti (2p), O (1s) and Ce (3d) peaks, X-ray photoelectron spectroscopy (XPS) was measured with the Thermo Scientific K-Alpha X-ray photoelectron spectrometer (Thermo Fisher Scientific (China) Co., Ltd.) with Al Kα monochromator as an X-ray source. Nitrogen sorption isotherms were measured with a ASAP2010 analyzer (Micromeritcs, USA). Before measurements, the samples were dried in a vacuum oven at room temperature for 24 h, and outgassed in the instrument at 60 °C for 24 h. The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method and the pore size distributions were derived from the adsorption branches of the isotherms using the Barrett–Joiner–Halenda (BJH) method. The optical properties of the photocatalysts were analyzed with UV-Visible Spectrophotometry (UV-VIS), recorded with a Shimadzu UV-1900 (Japan) spectrophotometer. 1 mL of nanomaterials or the solution containing CIP or SMX mixed with the nanomaterials were added to a quartz cuvette and spectra were achieved with scanning in the wavelength range from 190–700 nm. Table S1† list the characteristics of all nanomaterials employed as described in main text.

2.5. Photocatalytic degradation of CIP and SMX

The hybrid and single component nanomaterials were evaluated for the photocatalytic degradation of CIP and SMX both under UV and sunlight. For the degradation experiments under UV light, the artificial irradiation was provided by a 8 W lamp emitting at 365 nm (UV-A) and positioned in top of a quartz reactor (black box, ZF-IB, Shanghai Hannuo Instruments CO., Ltd. Shanghai, China) and maintained at a fixed distance of 5 cm from the samples. The degradation experiments under visible light were carried out at the Wuyi University (Jiangmen, China) whose GPS coordinates are 22° 35′ 53′′ North and 113° 4′ 49′′ East, from 12 p.m. to 4 p.m., 4 h irradiation time and with an average temperature of 27 °C. First, the solutions were equilibrated by stirring for 30 minutes in dark condition to achieve adsorption–desorption equilibrium before subjecting them to irradiation. The maintenance of absorbance or a slight decrease in absorbance was observed in some instances at the end of this 30 minute equilibration period, which we refer to as time = 0.^48,49 The experiments were carried out with 10 mL of 10 or 50 μg mL⁻¹ of each antibiotic and a optimized concentration of 0.5 mg mL⁻¹ of each photocatalyst. At different time points, the reaction mixtures were centrifuged and the concentration of remaining CIP and SMX in the supernatants were calculated by the maximum absorbance at 272 nm for CIP and 265 for SMX. The concentrations of CIP were calculated from a calibration curve using the UV-vis spectroscopy data at a wavelength of 273 nm.

3. Results and discussion

3.1. Synthesis and characterization of the CeO₂/TiO₂ and Au/TiO₂ photocatalysts

In a first set of experiments, single component Au, CeO₂, and TiO₂ NPs were synthesized and further employed for the preparation of the hybrid photocatalysts CeO₂/TiO₂ and Au/TiO_2, and as controls in the antibiotic degradation experiments. A complete synthesis description and characterization of the single component NPs used in this work can be found in previous publications from the authors⁵⁰ and in the materials and methods section. Briefly, CeO₂NPs were synthesized at room temperature by the precipitation in basic media of Ce(NO₃)₃, TiO₂NPs were prepared following a hydrothermal method based on the decomposition TiCl₄ in acidic media and AuNPs were synthesized by the reduction of HAuCl₄ in the presence of sodium citrate. Fig. S1† shows the Transmission Electron Microscopy (TEM) images of the as-synthesized single-component NPs and their size distributions analysed by TEM and Dynamic Light Scattering (DLS). The mean diameters of CeO₂, TiO₂ and Au NPs as determined by TEM image analysis were 5.4 ± 1.1 nm, 8.6 ± 2.1 nm and 8.6 ± 2.2 nm respectively, and their hydrodynamic diameters was 35.2 nm, 47.7 nm and 14.2 nm with narrow size distributions (polydispersity indexes of 0.1–0.2) as analysed by DLS. The increased hydrodynamic diameter in the case of the metal oxide NPs with respect to the TEM image analysis is attributed to the formation of small agglomerates in solution since the formation of the electrical double layer in metal oxide NPs is less efficient in stabilizing them compared with metal NPs.⁵¹

The hybrid photocatalysts were prepared by simple wet-chemistry methods. The selection of Au and CeO₂ NPs co-catalyst sizes, ranging between 5 and 10 nanometers, and the loading amount has been made based on their consistent demonstration of enhanced catalytic activity.^52,53 In the case of CeO₂/TiO₂, 100 mL of the as-prepared TiO₂NPs (5 mg mL⁻¹) were dispersed in an aqueous ammonia solution of 7.5 mM followed by 10 minutes sonication to disperse the TiO₂NPs. Afterwards, 108.5 mg of Ce(NO₃)₃·6H₂O were added under stirring to obtain a concentration of Ce³⁺ precursor of 2.5 mM. This mixture was heated under reflux and maintained at 100 °C for 4 hours. The synthesis of CeO₂NPs based on the oxidation of a cerium salt (mainly nitrate or chloride) in basic conditions is common in the literature and it is known that yield monodisperse small-sized (4–5 nm) CeO₂NPs at this Ce³⁺ : NH₄OH ratio of 1 : 3.^46,54,55 The resulting solution was washed by purification and colloidally stable CeO₂/TiO₂ were redispersed in a solution of 1 mM TMAOH. Fig. 1a shows TEM and high-angle annular dark field images (HAADF) of the as-synthesized CeO₂/TiO₂, which appear as agglomerated nanocomposites containing both CeO₂ and TiO₂. The HAADF corresponding elemental mapping shows that CeO₂ and TiO₂ NPs are thoroughly distributed in the hybrid structure and the inset reveals the (111) planes of face-centered cubic CeO₂ and (101) of anatase TiO₂. According to ICP-MS analysis, a 5% of CeO₂ is present in the hybrids, which is consistent with the mass of precursors added in the synthesis. For the Au/TiO₂, to ensure a good dispersion of the Au and the TiO₂ organized in a similar fashion to the CeO₂/TiO₂ hybrids, TiO₂NPs were formed onto previously synthesized AuNPs. 50 mL of the as-prepared AuNPs were adjusted to pH 10 by adding equal amounts of sodium citrate and K₂CO₃ at 150 mM. The solution was heated to 90 °C under stirring. Subsequently, 50 mL of TiCl₄ (20 mM) was added and the mixture was heated to boiling for 2 hours. As in the case of the CeO₂/TiO₂ hybrids, the calculated data of 5% of Au in the Au/TiO₂ hybrids is similar than ICP-MS data although slightly increased (7%), probably because not all the TiCl₄ precursor reacted and was washed in the purification step. Fig. 1b shows the TEM and HAADF characterization, where the higher density of metallic Au reveals the dark (bright-field TEM) or white (dark-field TEM) Au nanospheres onto a TiO₂ NPs matrix.

Fig. 1 Morphology of the CeO₂/TiO₂ (a) and Au/TiO₂ (b) hybrid nanostructures. For each hybrid, upper rows show representative TEM images and lower rows the high-angle annular dark field TEM (HAADF-TEM) images revealing the formation of both CeO₂and TiO₂ in the CeO₂/TiO₂ and Au and TiO₂ in the Au/TiO₂. Inset in (a) shows the (111) planes of face-centered cubic CeO₂ phase and (101) of anatase TiO₂ and inset in (b) the (101) of anatase TiO₂.

From the TEM images, the presence of both AuNPs and TiO₂NPs in the Au/TiO₂ hybrids is more clearly evidenced compared with CeO₂/TiO₂ hybrids thanks to the higher atomic number of Au compared with Ti. Thus, to further prove the presence of CeO₂NPs and TiO₂NPs in the CeO₂/TiO₂ hybrids, the XRD diffraction patterns of the as-synthesized single component CeO₂ and TiO₂ NPs and CeO₂/TiO₂ hybrid nanostructures were analysed (Fig. 2a and c). The XRD pattern of single component TiO₂NPs shows the diffraction peaks at 25.3°, 37.9°, 48.0°, 53.9°, 55° and 63°, which can be ascribed to the reflection of (101), (004), (200), (105), (211) and (204) planes of the TiO₂ respectively, consistent with the standard JCPDS values of anatase TiO₂ (JCPDS No. 21-1272). In the case of CeO₂NPs, the peaks at 28.6°, 33.1°, 47.5°, 56.3° can be ascribed to the (111), (200), (220) and (311) planes of fluorite (cubic) CeO₂ phase (JCPDS No 34-0394). The XRD patterns of both single component NPs exhibit well-defined peaks, although broad due to the small particle size, indicating that the materials are present in a good crystallinity phase. The crystalline sizes of CeO₂NPs (4.8 nm) and TiO₂NPs (6.08 nm) calculated from the XRD data using Scherrer's formula (taking the Full Width at Medium Height of the CeO₂ cubic (111) and anatase (101) reflections) are consistent with those obtained by TEM analysis.

Fig. 2 Physicochemical characterization of the CeO₂/TiO₂ hybrid nanostrucutures. (a) XRD patterns of the CeO₂ and TiO₂ NPs and the CeO₂/TiO₂ hybrid. (b) Raman spectra of the CeO₂ and TiO₂ NPs and the CeO₂/TiO₂ hybrid. Inset is the predominant peak of TiO₂ which is blue-shifted in the CeO₂/TiO₂ hybrid. (c) Details of the anatase TiO₂ and CeO₂/TiO₂ hybrid diffraction peaks at around 25° (1), 38° (2) and 48.0° (3). (d–g) XPS analysis of the CeO₂/TiO₂ including survey spectra (d), O 1s core-level spectra (e), Ti 2p core-level spectra (f) and Ce 3d core-level spectra (g).

In the case of CeO₂/TiO₂ the calculated crystallite size was 5.12 nm. As the peaks are rather broad, the anatase (101) reflection interfere/mask the CeO₂ cubic (111) main reflection as well as the other dominant (220) and (311) reflections. Other CeO₂ peaks are barely visible due to the low quantity of CeO₂NPs in the hybrids. Nonetheless, the smaller crystallite size of the hybrid compared with the pure anatase TiO₂ is attributed to the broadening effect due to the incorporation of CeO₂ into the TiO₂ matrix,^36,56 and it can be noted from the lower intensity and broader width of the peaks of the hybrid compared with pure TiO₂ (Fig. 2c). Furthermore, unshifted peaks of the TiO₂NPs compared with the CeO₂/TiO₂ hybrid suggest that the TiO₂ lattices were not disturbed by the presence of Ce ions, probably because of the larger radius of Ce⁴⁺ (0.102 nm) than that of Ti⁴⁺ (0.064 nm) which make it difficult for them to enter into TiO₂ crystal lattice.^57,58 Thus, this suggests that the nanocomposites observed in TEM images are composed of CeO₂ and TiO₂ NPs. In the case of Au/TiO₂ hybrids the presence of the peaks corresponding to Au and TiO₂ can be clearly distinguished and the calculated crystalline size using Scherrer's formula of 6.17 nm was similar to the TiO₂NPs since the (101) reflection of TiO₂ is not affected by the presence of the AuNPs (Fig. S2†).

Additionally, Raman spectra were employed to further confirm the above description in the case of CeO₂/TiO₂ (Fig. 2b). Single component TiO₂NPs and TiO₂/CeO₂ hybrids show the characteristic Raman-active modes of TiO₂ anatase phase (A_1g, B_1g, E_g) which are ascribed to the 145, 196, 397, 517 and 639 cm⁻¹ peaks.⁵⁹ As in the case of XRD, the peak of CeO₂NPs (464 cm⁻¹) is barely noted in the CeO₂/TiO₂, but, it can be noted the broadening and blue shift of the most intense peak in the hybrid respect with TiO₂NPs. This has been attributed to nonstoichiometric defects and non-homogeneity of particle size distribution due to the presence of the CeO₂NPs,^36,60 which is consistent with the XRD results.

Further, X-ray photoelectron spectroscopy (XPS) was employed to analyse the chemical composition and chemical state of elements in the CeO₂/TiO₂ hybrids. Fig. 2d–g shows the high-resolution XPS spectra of Ti 2p, Ce 3d, and O 1s in the CeO₂/TiO₂. In the survey plot (Fig. 2d) the presence of four main elements Ti, O, Ce, and C can be observed. The presence of C is common in XPS analysis due to contamination from environmental air during sample preparation. The peaks at 530 and 532 eV are assigned to O²⁻ ion in the TiO₂ crystal structure and Ti–OH bond (Fig. 2e). The dominant peak located at 530 eV has been described as characteristic of metal oxides arising from the lattice of CeO₂ and TiO₂.⁶¹ The Ti 2p core-level spectrum (Fig. 2f) shows two major peaks at 458.4 and 464.2 eV, corresponding to the Ti 2p_3/2 and Ti 2p_1/2 spin–orbit split photoelectrons of Ti⁴⁺ respectively, which is characteristic of TiO₂ species. In the case of Ce, the 4f group is particularly complex because of its associated peak structure. The Ce3d core-level spectrum (Fig. 2g) shows different groups of peaks. One group of two peaks at 882 eV corresponds to the Ce³⁺ 3d_5/2 spin orbital and another group of two peaks around 900 eV corresponds to the Ce⁴⁺ 3d_5/2 (898 eV) and Ce³⁺ 3d_3/2 (904 eV).^62,63 In both cases, the peaks are asymmetric, which is attributed to the mixture of Ce³⁺ and Ce⁴⁺ in the CeO₂NPs structure.^57,64–66 Furthermore, based on the non-linear least squares fitting of CeO₂ synthesized as in the hybrids, compostion of Ce³⁺ and Ce⁴⁺ were found to be 65.1% and 34.9% respectively. The presence of both Ce⁴⁺ and Ce³⁺ in the CeO₂ from the CeO₂/TiO₂ hybrids is important since it improves the electron transfer characteristics and the separation efficiency of photogenerated electrons and holes, and consequently it is expected it will improve the photocatalytic activity with respect to pure TiO₂.

Finally, nitrogen adsorption/desorption measurements were performed on the CeO₂/TiO₂ and Au/TiO₂ hybrid nanostructures to determine their specific surface area and textural properties. Fig. 3 displays the isotherms, revealing that both photocatalysts exhibited a characteristic type-IV isotherm, as per the IUPAC classification. This observation indicates a well-developed mesoporous structure. The BET surface area and average pore diameter determined from the adsorption branch using the BJH method for CeO₂/TiO₂ were 225.5 m² g⁻¹ and 5.8 nm, respectively. Meanwhile, for Au/TiO₂, these values were 276.9 m² g⁻¹ and 5.6 nm. This suggests that the obtained hybrids share similar characteristics, not only in terms of size and co-catalyst doping but also in morphology and surface area and that the coupling of CeO₂ or Au NP lead to similar hybrid structures.

Fig. 3 Nitrogen sorption isotherms and textural properties of the CeO₂/TiO₂ (left) and the Au/TiO₂ (right) hybrid nanostrucutures.

3.2. Optical properties of the CeO₂/TiO₂ and Au/TiO₂ photocatalysts

The optical properties of both CeO₂/TiO₂ and Au/TiO₂ were investigated by UV/VIS spectroscopy. Fig. 4a shows the UV/VIS spectra for both CeO₂/TiO₂ and Au/TiO₂, along with the single component NPs, which confirm the presence of the single component NPs in both hybrids. Furthermore, it can be observed that compared with pure TiO₂, both hybrid structures display an extension of their UV/VIS absorbance to the visible light region. Next, the band gap energy (E_g) values of the single component and hybrid nanostructures were determined by fitting the UV/VIS data with Tauc's plots^67,68 following the developments by Davis and Mott for amorphous semiconductors,⁶⁹ and later by many others. It is worth noting that the linear extrapolation to zero in the plots of (αhν)² versus photon energy to estimate E_g values, is a common approach for providing a rough approximation of the direct band gap. This is because the Tauc plot method assumes measurements under specific conditions, potentially introducing uncertainties.⁷⁰ Additionally, variations in the data or the choice of the linear fitting range can impact the accuracy of the estimated bandgap. The E_g values for TiO₂, CeO₂, Au/TiO₂ and CeO₂/TiO₂ are 3.52, 3.55, 3.34 and 3.01 eV respectively (Fig. 4b). The E_g is lower in the hybrids compared with the single component NPs which confirms again the incorporation of both Au and CeO₂ in the TiO₂ matrix to form the hybrid nanostructures.

Fig. 4 Optical properties of the CeO₂/TiO₂ and Au/TiO₂ hybrid nanostrucutures. (a) UV/VIS spectra of both hybrids along with the single component NPs. (b) Corresponding Tauc plots which represent the square of the absorption coefficient versus photon energy. The bandgap energy (E_g) of the material can be determined from the x-axis intercept of the linear portion of the curve. (c and d) Schematic diagram of the proposed mechanism for the photocatalysis by CeO₂/TiO₂ (c) and Au/TiO₂ (d) under visible light irradiation. The different band structures of the co-catalysts respect TiO₂ facilitate the transfer of electrons to the TiO₂ conduction band (CB), and subsequently from the CB of TiO₂ to oxygen molecules, resulting in the formation of superoxide radicals (O²˙⁻). Furthermore, the co-catalysts facilitate the regeneration of the CB of TiO₂ and act as a trap for the photo-generated holes, preventing their recombination with electrons and prolonging the lifetime of the charge carriers, thus potentially enhancing the photocatalytic activity. Additionally, CeO₂NPs can undergo oxidation reactions by themselves.

Importantly, the decrease of E_g values in the hybrids compared with TiO₂NPs is an indication of the red shifting from UV to the visible region. Therefore, it can be hypothesized that this will enable the photocatalytic activity under sun irradiation. This can be explained by the electronic band structure in the hybrids. CeO₂ is a p-type semiconducting photocatalyst material with potentially suitable band positions for the promotion of charge carriers when kept in intimate connection with TiO₂. When electrons in the CeO₂ valence band are exposed to lower energy lights (e.g. visible light) they move to the conduction band. As a result of the varied energy barriers of both components in the hybrids, electrons will eventually relocate to the conduction band of TiO₂.^36,71 Further, electrons in the TiO₂ valence band will also undergo excitation when exposed to higher energy light (e.g. UV which accounts for the 4% of the sun irradiation), and the hole in its valence band will shift to the CeO₂ valence band. This carrier transport process efficiently separates photoelectrons and holes, making their recombination more difficult and broadening the response to visible light.⁷¹ Au/TiO₂ is most well-described in the literature. In such nanostructures, due to SPR in the Au surface, electrons from the AuNPs conduction band become excited (known as hot electrons) under visible light irradiation. These charge carriers are relocated from the excited AuNPs to the conduction band of the adjacent TiO₂, since there is close contact between the surfaces of Au and TiO₂ (known as Schottky junction). This movement generates holes in the AuNPs electronic structure and achieves a similar effect as described in the CeO₂/TiO₂, making electron–hole recombination more difficult. Thus, with the efficient supply of hot electrons TiO₂ can undergo the photocatalytic reaction under light illumination.^12,27,28,72 Fig. 4c and d shows a scheme of these carrier transport processes under visible light of CeO₂/TiO₂ and Au/TiO₂ hybrid nanostructures.

3.3. Degradation of CIP and SMX

All material improvements of the Au/TiO₂ and CeO₂/TiO₂ hybrids described in the previous sections compared with the single component NPs were evaluated and compared in the context of CIP and SMX degradation. First, the concentration of the photocatalysts to be used in the degradation experiments was determined. This is important since increasing the concentration of a photocatalyst can increase its photocatalytic activity up to a point where the solution becomes turbid due to a high concentration of particles, which reduces the penetration of the light (effect known as catalyst shielding). In addition, the high catalyst concentration may also block access to the catalytic sites because of the agglomeration of the particles. The effect of catalyst concentration on the degradation of Rhodamine B (RhB) and CIP with TiO₂NPs was studied. First, varying concentration of TiO₂NPs were used for the degradation of RhB at initial concentration of 2.5 mM (Fig. S3†). From these studies, 0.5 mg mL⁻¹ of photocatalyst was selected as optimized concentration, which is a similar result than the previously reported for TiO₂ and Au/TiO₂ for RhB degradation.^72,73 Next, we used 0.5 mg mL⁻¹ of CeO₂/TiO₂ and Au/TiO₂ hybrids (and TiO₂, CeO₂ and Au single component NPs as controls) to corroborate its suitability for the degradation of 10 μg L⁻¹ of CIP.

Fig. 5a shows the kinetic plots of CIP degradation, where CeO₂, TiO₂ and the CeO₂/TiO₂ hybrids showed 100% CIP degradation (although at different time points in each case). This concentration of photocatalysts was used with further experiments under sun irradiation (Fig. 5a) and with an at an increased CIP concentration of 50 μg L⁻¹ (Fig. 5b). The CIP^48,74 and SMX^75,76 degradation pathways and degraded products using TiO₂NPs and hybrid TiO₂-based photocatalysts both under UV and sun irradiation have been reported elsewhere, as well as the toxicity of the resulting degradation products. For instance, in the case of CIP, Durán-Álvarez et al.⁷⁷ reported negligible toxicity towards Vibrio fischeri and the complete removal of antibiotic activity against Escherichia coli when employing TiO₂ modified with metallic NPs at an initial CIP concentration of 30 μg mL⁻¹. This was attributed to the complete mineralization of CIP following 360 minutes of photocatalytic treatment under simulated sunlight irradiation. Similarly, Zheng et al.⁷⁸ obtained comparable results using TiO₂-based nanocomposites at an initial CIP concentration of 30 μg mL⁻¹. Regarding SMX, among others, Borowska et al.⁷⁶ reported the decrease of phytotoxicity of SMX degraded products towards the plant Lepidium sativum when using TiO₂NPs doped with noble metals at an initial SMX concentration of 1 μg mL⁻¹. This low phytotoxicity was attributed to the very low concentrations of remaining antibiotics. Gong et al.⁷⁹ also observed a significant reduction in toxic activity against the green alga Chlorella vulgaris and the brine shrimp Artemia salina after photodegradation of SMX under UV radiation.

Fig. 5 Photocatalytic performance in the degradation of CIP of the hybrid CeO₂/TiO₂, Au/TiO₂ and single component NPs under visible and UV light. (a) CIP initial concentration of 10 μg mL; (b) CIP initial concentration of 50 μg mL⁻¹ (b). Black and grey line correspond to CeO₂/TiO₂ and Au/TiO₂ hybrids respectively. Yellow line is AuNPs, green line is CeO₂NPs, red line is TiO₂ NPs and dashed line correspond to the control (H₂O). All photocatalysts were used at a concentration of 0.5 mg mL⁻¹. The control group (ctrl) consists of the antibiotic dissolved in a water solvent. The molecular structure of CIP is in Fig. S1.†

In all cases, a similar pattern can be observed. CeO ₂/TiO₂ hybrids consistently show the highest CIP degradation rate compared to the other nanomaterials. Specifically, at a CIP concentration of 10 μg L⁻¹, both CeO₂/TiO₂ and CeO₂NPs showed complete degradation under UV and sun irradiation while TiO₂NPs show 100% degradation under UV light and 50% degradation under sun irradiation. In the case of the CeO₂/TiO₂ hybrids, the degradation was found to be constant after 40 min of reaction time, while for other nanomaterials it took around 2 hours to reach the stability, suggesting a more powerful activity of the CeO₂/TiO₂ in all conditions. As expected, Au/TiO₂ hybrids also enhanced the photocatalytic activity of TiO₂ under sun irradiation, although to a lesser extent than CeO₂/TiO₂ (around 75% CIP degradation in the case of Au/TiO₂ while 50% degradation was observed in the case of TiO₂ at an initial concentration of CIP of 10 μg mL⁻¹), and AuNPs showed less than 20% CIP degradation in those conditions. Table 1 shows the percentage of CIP degradation in all cases.

Table 1 Degradation of CIP and SMX (%) using the synthesized photo-catalysts at 0.5 mg mL⁻¹

	CIP degradation (%)				SMX degradation (%)
	Co(CIP)		Co(CIP)		Co(SMX)		Co(SMX)
	10 μg mL^−1		50 μg mL^−1		5 μg mL^−1		10 μg mL^−1
	SUN	UV	SUN	UV	SUN	UV	SUN	UV
AuNPs	27.8	39.4	27.2	65.2	21.7	20.3	25.4	8.9
CeO2NPs	99.9	97.8	99.9	50.9	69.5	40.6	78.8	23.8
TiO2NPs	53.2	97.5	66.8	78.2	34.6	65.1	29.4	66.1
Au/TiO2	73.7	61.9	78.8	60.9	56.7	42.2	49.2	32.8
CeO2/TiO2	99.9	99.9	84.6	81.8	87.7	84.2	67.7	32.5

---

Remarkably, the amount of photocatalyst used was lower than many other studies also aimed to study the degradation of CIP by nanostructured photocatalysts. Among others (see Table 2 for full list), Manasa et al.,⁴⁸ employing 1 mg mL⁻¹ of Ce-doped TiO₂NPs achieved 93% CIP degradation at C₀ = 10 μg mL⁻¹. Pattnaik et al.⁸⁰ employing exfoliated graphitic carbon nitride (g-C₃N₄) found that the same 1 mg mL⁻¹ concentration of photocatalyst degraded up to 78% of a 20 μg mL⁻¹ CIP solution exposed to solar light. Yu et al.,⁸¹ employing Zn-doped Cu₂ONPs at 0.6 mg mL⁻¹ achieved 94.6 CIP degradation at C₀ = 20 μg mL⁻¹. Furthermore, Gad-Allah et al.,⁸² used 1.5 mg mL⁻¹ of TiO₂NPs to efficiently degrade CIP at C₀ = 50 μg mL⁻¹, in this case using simulated sunlight.

Table 2 Degradation of CIP by different nanostructured photocatalysts under visible light

	Concentration of photocatalyst (mg mL^−1)	CIP initial concentration (μg mL^−1)	CIP degradation
TiO2-based photocatalysts
TiO2NPs (under simulated sunlight)^82	1.5 mg mL^−1	50 μg mL^−1	99.9%
P25–TiO2 (ref. 83)	1 mg mL^−1	20 μg mL^−1	31%
P25–TiO2 (ref. 84)	1.5–11 mg mL^−1	30 μg mL^−1	99%
Immobilized TiO2NPs^85	1 mg mL^−1	3 μg mL^−1	92.8%
Immobilized TiO2NPs^86	7.5 mg mL^−1	20 μg mL^−1	95%
Au–TiO2, Ag–TiO2 and Cu–TiO2 (ref. 77)	1.5 mg mL^−1	30 μg mL^−1	99%
Hybrid TiO2:Au nanostars^87	1 mg mL^−1	5 μg mL^−1	89%
Ce-doped TiO2 (ref. 48)	1 mg mL^−1	10 μg mL^−1	93%
g-C3N4/TiO2 powder^49	1 mg mL^−1	10 μg mL^−1	95%
Chlorophyll sensitized TiO2NPs^88	0.75 mg mL^−1	10 μg mL^−1	75%
Other materials
MnO2 (ref. 89)	7 mg mL^−1	10 μg mL^−1	90%
		20 μg mL^−1	60%
Pt/InVO4	3 mg mL^−1	20 μg mL^−1	87%
Pt–BiVO4 (ref. 90)	1.5 mg mL^−1	10 μg mL^−1	92%
Exfoliated g-C3N4 (ref. 80)	1 mg mL^−1	20 μg mL^−1	78%
Zn-doped Cu2O^81	0.6 mg mL^−1	20 μg mL^−1	94.6%
N–ZnO/CdS/GO^91	0.5 mg mL^−1	15 μg mL^−1	85%
CeO2/CdS/RGO^92	0.5 mg mL^−1	40 μg mL^−1	90%
N doped-graphene quantum dots-BiVO4/gC3N4 (ref. 93)	0.5 mg mL^−1	10 μg mL^−1	72%
CeO2/ZnO nanocomposites^94	0.25 mg mL^−1	15 μg mL^−1	60%
Reduced graphene oxide-BiVO4 (ref. 95)	0.2 mg mL^−1	10 μg mL^−1	68.2%
This study
CeO2–TiO2	0.5 mg mL^−1	10 μg mL^−1	99.9%
CeO2–TiO2	0.5 mg mL^−1	50 μg mL^−1	85%

---

These results were further corroborated with the photocatalytic degradation of SMX in the same conditions as the used for the CIP degradation. Fig. 6 shows a similar pattern of SMX degradation as observed in the CIP degradation experiments. CeO₂/TiO₂ exhibited superior and faster catalytic performance under UV and sun irradiation than other photocatalysts. Note that to achieve 100% SMX degradation, a lower concentration than in case of CIP degradation was needed (initial SMX concentrations of 5 μg L⁻¹ and 10 μg L⁻¹), probably due to the different molecular structures of both antibiotics and that the system was optimized for CIP. Specifically, at SMX C₀ = 5 μg mL⁻¹ and under sun irradiation, CeO₂/TiO₂ showed the highest degradation (88%) followed by CeO₂NPs (70%) and Au/TiO₂ (57%) CeO₂ and TiO₂ NPs (c.a. 35%). At SMX C₀ = 10 μg mL⁻¹, suggesting once again the importance of the CeO₂ co-catalyst properties for photocatalytic reactions under visible light. Interestingly, in a specific case, at the highest initial concentration of antibiotics (50 μg mL⁻¹ of CIP and 10 μg mL⁻¹ of SMX), CeO₂NPs exhibited a higher rate of degradation compared to the CeO₂/TiO₂ hybrids. It is important to note here that, even though the hybrids possess a greater surface area at the same mass concentration (Table S1†), in the experimental setup the single-component NPs have higher colloidal stability and, consequently, enhanced mobility in solution. This disparity in colloidal behaviour may account for the observed increase in catalytic activity.

Fig. 6 Photocatalytic performance in the degradation of SMX of the hybrid CeO₂/TiO₂, Au/TiO₂ and single component NPs under visible and UV light. (a) SMX initial concentration of 5 μg mL; (b) SMX initial concentration of 10 μg mL⁻¹ (b). Black and grey line correspond to CeO₂/TiO₂ and Au/TiO₂ hybrids respectively. Yellow line is AuNPs, green line is CeO₂NPs, red line is TiO₂ NPs and dashed line correspond to the control (H₂O). All photocatalysts were used at a concentration of 0.5 mg mL⁻¹. The control group (ctrl) consists of the antibiotic dissolved in a water solvent. The molecular structure of SMX is in Fig. S4.†

In any case, the results altogether and the fact that CeO₂ itself also showed efficient photocatalytic activity in the degradation of CIP, suggest the superior capability of CeO₂ as a co-catalyst which can be attributed to its unique redox properties and its characteristic d and f electronic orbital configurations,^96–99 along with other advantages such as the reported visible active band gap value, large oxygen storage capacity, and its variable oxidation states (Ce³⁺, Ce⁴⁺) which favor electron transfer and improve the electron–hole separation. This non-stoichiometry of CeO₂ has also been related with its capacity to store oxygen and release them in oxidation reactions for e.g. the photocatalytic degradation of organic and pharmaceutical pollutants.^97,100–102 Apart from their inherent ability to initiate photocatalytic reactions by themselves, it has been reported that Ce⁴⁺ ions in the CeO₂ crystal structure can easily capture photogenerated electrons and form Ce³⁺ ions, acting as an electron trapping agent and allowing to decrease the recombination rate of the photogenerated charges in the TiO₂.¹⁰³ In addition, this stored oxygen can also be transferred to TiO₂, which increases both the rate of oxygen reduction and the efficacy of the electron–hole separation of TiO₂NPs (see schema in Fig. 3).¹⁰⁴

3.4. Stability studies of the CeO₂/TiO₂ and Au/TiO₂ photocatalysts

The stability and reusability of newly developed photocatalysts is a key aspect to take into account for practical applications. In our experiments, the stability of the employed CeO₂/TiO₂ and Au/TiO₂ photocatalysts has been evaluated under sunlight for five consecutive cycles at the highest concentration of CIP/SMX used in the antibiotic degradation experiments. For each cycle, freshly prepared antibiotic solutions were used and CeO₂/TiO₂ or Au/TiO₂ were collected by centrifugation and drying the resulting pellet. Fig. 7 shows that the efficiency of CIP/SMX degradation remain stable for both photocatalysts during the first three cycles, but is reduced by c.a. 20% at the fifth cycle. This decrease in the degradation efficiency could be attributed to the consecutive drying and resuspension processes that may agglomerate the NPs resulting in a decrease in the available surface area for the catalytic reaction, since no leaching of metal in the supernatants after the washing cycle has been detected by ICP-MS.

Fig. 7 Stability studies of the CeO₂/TiO₂ and Au/TiO₂ photocatalysts under sun irradiation. (a) Degradation of CIP or SMX at 50 μg mL⁻¹ in different cycles of recovering the hybrids from the solution compared with the degradation showed by the as-synthesized hybrid nanocomposites. (b) UV-VIS absorbance of CIP (dashed line; peak at 272 nm) and with different cycles exposed to CeO₂/TiO₂. (c) UV-VIS absorbance of CIP and under the different cycles exposed to Au/TiO₂. (d) UV-VIS absorbance of SMX (dashed line; peak at 265 nm) and with different cycles exposed to CeO₂/TiO₂.

4. Conclusions

Antibiotic residues in wastewaters are of particular concern due to their impact on marine and fluvial ecosystems and the development of antibiotic resistance. Different approaches have been proposed and used to remove antibiotics from water sources such as electrochemical methods, ozonation, reverse osmosis, membrane filtration, biological treatments and advanced oxidation. Among them, photocatalytic degradation using TiO₂-based nanomaterials has been widely explored in the last decades due to its high photocatalytic activity under UV light among other advantages such as its stability in a large range pH and its low-cost. To expand the TiO₂ photocatalytic activity under sun light, a wide variety of photocatalysts based on doping TiO₂ with metal ions or coupling it with light-active nanomaterials have been proposed, all with excellent results. In a typical study focusing on the photocatalytic degradation of pollutants using such nanomaterials, a photocatalyst is characterized and subsequently different parameters are modified to evaluate its activity. Herein, we aimed to evaluate and compare the photocatalytic degradation of CIP and SMX employing two different types of TiO₂ hybrid nanomaterials, semiconductor CeO₂/TiO₂ and metal Au/TiO₂, as representatives from two main families of novel photocatalysts proposed in previous scientific literature. Indeed, conducting a comprehensive comparison is a challenging task, as it involves numerous parameters related to co-catalyst characteristics, including size, shape, and addition ratio, among others. In this study, co-catalyst sizes ranging from 5 to 10 nanometers and a co-catalyst loading of 5% (at%) were employed. Both photocatalysts were extensively characterized by HR-TEM, XRD, UV-VIS, DLS, BET and XPS. The characterization showed that CeO₂/TiO₂ and Au/TiO₂ consisted of heterostructured nanocomposites containing both single component NPs in the hybrid structure and showing a decrease of the E_g values (bandgap) with respect to pure TiO₂, allowing photocatalytic reaction to occur under sunlight.

The results of the antibiotic degradation showed that although both CeO₂/TiO₂ and Au/TiO₂ show enhanced photocatalytic activity under sun irradiation, CeO₂/TiO₂ consistently exhibited the highest performance. These results suggest that the unique redox properties of CeO₂, as well as its characteristic electronic configuration and large oxygen storage capacity have a crucial role for the enhanced photocatalytic activity. Its particular electronic configuration allows CeO₂ to act as an electron trapping agent, enabling both to participate in oxidation reactions by itself and to decrease the recombination rate of the photogenerated charges in TiO₂. In addition, both TiO₂ and CeO₂ are materials with high redox potential (Ti⁴⁺/Ti³⁺ and Ce⁴⁺/Ce³⁺) and their photocatalytic performance is boosted by their associated defects and oxygen vacancies, thus making CeO₂/TiO₂ hybrid nanomaterials promising candidates for a range of environmental and energetic applications.

Author contributions

M. Z. and E. C. conceptualized and designed the research. M. Z., S. J., J. T., and Y. Z. synthesized the photocatalysts and S. J., J. T., Y. L., X. L., and L. L. characterized them. M. Z., G. C., Z. J., and E. C. collected, processed and interpreted the data. M. Z., Z. J, and E. C. wrote the manuscript. All authors discussed the results, commented and reviewed the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The study was financially supported by the National Natural Science Foundation of China (22005221 to M. Z.), the Wuyi University (2018TP010 to E. C., 2018TP011 and 2020FKZX05 to M. Z.), and the Instituto de Salud Carlos III (PI19/00774 and BA22/00017 to G. C., co-funded by the European Regional Development Fund, European Union, “A way to make Europe”).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra06231g

‡ Both authors contributed equally to this study.

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