TiO2 nanoparticles assembled on kaolinites with different morphologies for efficient photocatalytic performance

Natural kaolinite clays with different dimensionalities (including kaolinite nanoflakes and nanorods) supported TiO2 nanoparticles were successfully prepared via a facile sol-gel method. Moreover, comparisons between FK/TiO2 and RK/TiO2 nanocomposites are conducted in terms of matrix morphology, surface property, energy band structure and interfacial interaction. The effects of kaolinite microstructure, morphology and dimensionality on the interfacial characteristics and photocatalytic properties of the nanocomposites were investigated in detail. The results showed that TiO2 nanoparticles are more easily attached on the kaolinite nanoflakes, and possess more uniform distribution and smaller particle size than that of kaolinite nanorods. In particular, the FK/TiO2 nanocatalysts exhibit higher photocatalytic activity for the degradation of tetracycline hydrochloride than that of RK/TiO2 and bare TiO2, which is attributed to the stronger surface adsorptivity, higher loading efficiency and smaller grain size. Additionally, FK/TiO2 composites show excellent stability, which is ascribed to the intimate interfacial contact between two-dimensional kaolinite nanoflakes and TiO2 nanoparticles. Overall, the enhanced catalytic performance for FK/TiO2 composites is the synergistic effect of two-dimensional morphology, better adsorption capability and more active photocatalysis TiO2 species.

Heterogeneous nanocomposites has attracted increasing attention because of the synergetic properties and potential applications as green methods to solve the energy and environmental problems 1 . In recent years, many technologies are proposed to tailor and promote the properties of nanocomposites, including element doping 2 , surface loading [3][4][5][6] , morphology controlling [7][8][9][10][11] , heterostructure constructing [12][13][14][15][16] , energy-band engineering 17 , and so on. Among them, loading functional nanoparticles on the surface of matrix materials is a promising alternative to control the nanoparticle size, and overcome inherent drawbacks of unsupported nanoparticles in terms of stability, agglomeration and reusability 18 .
As is well known, the performance of nanocomposites depends not only on the chemical composition, but also on microstructure, dimension, size and morphology and so on. At present, much interest has been focused on nanoparticles anchoring on a single support or the morphology-controlled synthesis of functional particles via different strategies, and further studied the comparative catalytic efficiency of the synthesized samples. Yang et al. 19 synthesized a series of graphene-TiO 2 nanocomposites with different TiO 2 dimensionalities (including TiO 2 nanoparticles, nanotubes and nanosheets) via sol-gel method, alkaline hydrothermal process and one-step solvothermal method, respectively. Meshram et al. 20 reported synthesis of CuO nanostructures with different morphologies such as spherical, vesicular, nanosheet and platelet using chemical precipitation and hydrothermal methods. The photocatalytic activities of CuO nanostructures were evaluated by monitoring degradation of methylene blue, and the platelet-like CuO nanostructures were found to have the best catalytic activity. Thuy et al. 21 prepared TiO 2 particles with differnent morphologies via hydrothermal process, and investigated the morphological effect of TiO 2 on photocatalytic degradation of organic dyes. However, none of these reports investigate any particular insights into the morphology of the support which has been reported to be as important as its internal structure. Like synthetic MCM-41, as common matrix materials 22 , have different morphologies, including mesoporous

Results
Morphological and structural characteristics. The general morphologies of different samples were observed by electron microscope. Typical SEM and TEM analysis indicates that kaolinite clays used in the experiments naturally possess different morphologies, and the kaolinite particles all possess smooth surface without contamination (Fig. 1a,b,e,f). Kaolinite nanoflakes exhibit an irregular angular shape and the length of particles are in the range of microns (Fig. 1a,b). Nanorod-like kaolinites with smooth surface (Fig. 1e,f) are mostly 2~5 μm in length and 0.1~0.3 μm in diameter with a length to width ratio of about 20:1. It can be seen from Fig. 2d, bare TiO 2 nanoparticles are obtained via the sol-gel method combined heat treatment. However, the dispersion of bare TiO 2 nanoparticles is poor and exists obvious aggregation. In the presence of clays, the aggregation of TiO 2 nanoparticles is markedly inhibited (Fig. 2a,e) and TiO 2 nanoparticles are uniformly deposited on the surface of clays. After loading TiO 2 nanoparticles, the clay supports show different loading results at the same experiment conditions. The TiO 2 nanoparticles exhibit much more uniform and smaller for FK/TiO 2 (Fig. 2a) and aggregation seriously for RK/TiO 2 (Fig. 2e), which indicates that the morphology of supports have a seriously effect on the particles size. The HRTEM (Fig. 2b,f) results indicate that TiO 2 nanoparticles have high crystallinity with well-defined lattice fringes of 0.189 nm corresponding to (200) plane, and 0.352 nm corresponding to (101) plane, in accordance with XRD results (Fig. 2h). Moreover, obvious interfaces are observed between the TiO 2 and clays. The intimate interaction enables the electron to more easily transfer from TiO 2 nanoparticles to kaolinite nanoflakes during the photoexcitation process. Meanwhile, the mean size of TiO 2 nanoparticles for FK/TiO 2 and RK/TiO 2 measured on TEM images (Fig. 2a,e) are 7.6 and 37.9 nm, respectively, as shown in Fig. 2c,g. It is clear that the grain size of TiO 2 nanoparticles is strongly depended on the morphology of the supports. Meanwhile, the TiO 2 nanoparticles on kaolinite clays are much smaller than that of bare TiO 2 (Fig. 2d), indicating the dispersion effect on TiO 2 clusters on the surface of kaolinite clays. The measured surface zeta potential curves and the surface charge distributions sketches of pristine kaolinites with different morphologies are showed in Fig. 1c,g. It is clear that kaolinites with different morphologies carry a net negative charge, which allows for its good dispersibility in water and provided a strong electrostatic adsorption force to the positively charged molecules on the surfaces. But they possess the different surface charge distributions, thus leading to the different surface natures, which allows the adjustable grain size of TiO 2 nanoparticles and further affects the photocatalytic performances.
As a result, the kaolinite clays with different morphologies can serve as supporting materials for in situ growth of TiO 2 nanoparticles. The TiO 2 nanocrystals attached on kaolinite nanoflakes have better dispersion and much smaller grain size than that of RK/TiO 2 and bare TiO 2 samples, resulting from the intimate interaction and good interfacial contact. Therefore, the TiO 2 particles size can be controlled by clays and form interaction with clays surface, which would lead to exposing more catalytic reaction sites, improving photoelectrons transiting and enhancing the photocatalytic activity. Figure 2h shows the XRD patterns of as-synthesized samples to study the crystal structure and crystalline phase of TiO 2 in the nanocomposites. It can be clearly seen that kaolinite clays possessed the natural different morphologies have the similar crystal structure and their XRD patterns are in good agreement with the standard PDF card of JCPDS 14-0164 for kaolinite-1A 33 . The XRD pattern of bare TiO 2 shows the highly crystalline anatase phase TiO 2 (JCPDS No. 21-1272) 34 . The diffraction peaks of FK/TiO 2 and RK/TiO 2 composites are in good agreement with the anatase phase TiO 2 and kaolinite, showing that the structure of kaolinite is maintained during the introduction of TiO 2 nanoparticles. Meanwhile, the diffraction intensities of kaolintes in FK/TiO 2 and RK/TiO 2 composites are decreased, indicating the successful loading of TiO 2 on the surface of kaolinites. The average grain size of TiO 2 nanoparticles for FK/TiO 2 and RK/TiO 2 estimated by the Scherrer's formula are 9.1 and 32.4 nm, respectively, which is consistent with the results based on TEM images. Moreover, the grain size of bare TiO 2 is 16.8 nm. By comparison to the standard JCPDS diffraction patterns of anatase phase TiO 2 , it can be seen that the diffraction peaks of TiO 2 on FK/TiO 2 are much more obvious than that of RK/TiO 2 , which indicates that FK has relatively higher loading efficiency for TiO 2 nanoparticles at the same experiment conditions. In order to further confirm the results, elemental analysis (Table 1) was employed in the experiment. It is shown that the relative content of titanium in FK/TiO 2 is higher than that of FK/TiO 2 , which is consistent with the XRD results.
The nitrogen adsorption-desorption isotherms of different samples and BJH pore size distribution are shown in Fig. 3, and the textural parameters calculated from the corresponding isotherms are summarized in Table 2. As shown in Fig. 3a, the adsorption-desorption isotherms of bare TiO 2 almost have no hysteresis loop, and the BET specific surface area is only 13 m 2 /g due to the seriously aggregation, while the adsorption-desorption isotherms of clays/TiO 2 exhibit typical characteristic of type-IV with distinct type H3 hysteresis loop, indicating the formation of mesoporous structures 35 . However, the adsorption-desorption isotherms of clays almost have no hysteresis loop, and the specific surface area are only about 27 m 2 /g, indicating clays nanopartiles can easily aggregate and result in the performance decline. After supporting TiO 2 nanoparticles on the surface of clays, the BET specific surface area (S BET ) and pore volume (V pores ) of composites are increased, as summarized in Table 2. It shows that the mesopore structure can be formed after loading TiO 2 nanoparticles by revealing increased surface area from 27 to 114 m 2 /g for FK/TiO 2 , and from 26 to 109 m 2 /g for RK/TiO 2 . The pore volume of FK/TiO 2 and RK/TiO 2 are 0.25 and 0.35 cm 3 /g, respectively. Obviously, FK/TiO 2 has relatively higher specific surface area and lower pore volume, compared with RK/TiO 2 , which might be because the smaller size and uniform distribution of TiO 2 nanoparticles attached on kaolinite nanoflakes. These results are further confirmed by the observation from their pore size distribution calculated by BJH method (Fig. 3b). These characteristics seem to be responsible for enhanced catalytic activity and stability on the clays/TiO 2 in comparison with bare TiO 2 . These properties are also attributed to the formation of small particles through oxide-support interaction to provide more active sites for photocatalysts.
Interfacial characteristics. The surface properties of TiO 2 species play an important role in aqueous phase photocatalytic reaction, and a large surface area of FK/TiO 2 is responsible for the efficient catalytic activity. In addition, the interactions between TiO 2 and clays are also important to suppress the aggregation of TiO 2 nanoparticles during the reaction, and the oxide-support interactions are measured by FTIR and XPS, which can provide additional information on the structure of clays/TiO 2 nanocomposites.
The FTIR spectra of the samples are shown in Fig. 4a to analyze the vibrational bands and the interface interaction. For FK and RK, the peak at 1033 cm −1 corresponds to the stretching vibration of the skeleton Si-O network (Si-O-Si and O-Si-O) 36 . The broad band between 1631 cm −1 is assigned to adsorbed water. The other bands at 3658 and 1116 cm −1 are assigned to inner surface hydroxyl out-of-phase stretching vibration and apical    Si-O stretching vibration, respectively. The absorption bands at 3694, 3620, and 915 cm −1 are ascribed to an inner-surface hydroxyl (Al-OH) stretching vibration, which are rarely influenced by intercalation 29   much more effectively and the binding force is much stronger than that of RK, which is consistent with the TEM (Fig. 2) and XRF results ( Table 1). The interactions between TiO 2 and kaolinite clays in the composites are further investigated by using XPS spectra. Figure 4b shows the XPS survey spectra of FK, RK, FK/TiO 2 and RK/TiO 2 in the range 0-1200 eV. The XPS survey spectra of FK/TiO 2 and RK/TiO 2 clearly indentify the signals of the Al, Si, Ti and O elements. The weak signals of Si and Al elements for clays/TiO 2 indicate that the kaolinites surface are coated with a layer of dense nano-sized TiO 2 particles.
The high-resolution XPS spectra of Ti 2p, O 1 s, Si 2p and Al 2p for samples are exhibited as Fig. 4c-f, respectively. As shown in the high-resolution spectra of Ti 2p electrons (Fig. 4c) 37 . This interaction of chemical bond can immobilize TiO 2 to prevent it from movement and agglomeration. The slight shifts for FK/TiO 2 and RK/TiO 2 can be due to a change in the chemical state or coordination environment of Ti 2p, that is, the interaction between kaolinites with different morphologies and TiO 2 nanoparticles. Moreover, the larger peak area of Ti 2p for FK/TiO 2 indicates that TiO 2 nanoparticles are more easily loaded onto the surface of the flake-like support under the same condition.
The high resolution O 1 s spectra of kaolintes can be deconvoluted into two fitted peaks (Fig. 4d). The peak at around 532.6 eV can be assigned to lattice oxygen in the kaolintes, and the other peak at about 531.7 eV is derived from the hydroxyl group 38 . For clays/TiO 2 , the peaks at 532.6 eV and 530.6 eV are assigned to oxygen from Si-O-Si and Ti-O-Si, and the peak at 531.2 eV is assigned to oxygen from hydroxyl group. The result is in good consistent with that of Ti 2p (Fig. 4c) and confirms the existence of Ti-O-Si and surface hydroxyl. Moreover, the shifts of surface hydroxyl groups and the new formed Ti-O-Si demonstrate the integration between TiO 2 and kaolintes and intense interaction between the two components. Moreover, the larger peak area of Ti-O-Si for FK/TiO 2 further indicates FK/TiO 2 has relatively higher loading efficiencyof TiO 2 nanoparticles than that of RK/TiO 2 .  39 . After supporting TiO 2 nanoparticles on the surface of clays, the reduction of hydroxyl groups and slightly shifts for clays/TiO 2 nanocomposites indicate the interaction between the two components.
Optical spectroscopic study. The UV-vis diffuse reflectance spectra are used to determine the optical properties of the synthesized samples. As shown in Fig. 5a, the FK and RK show clear absorption in the UV region, and a visible light absorption around 500 nm for RK can be observed. It is obvious that bare TiO 2 nanoparticles have no or relatively weak visible light absorption, while clays/TiO 2 nanocomposites exhibit enhanced light absorption capacity in the UV-vis region, indicating that the weak absorption in the visible light region can be attributed to the kaolinites. In comparison to kaolintes, the absorptions attributed to crystalline TiO 2 around 400 nm are present, which further confirms that the crystalline TiO 2 nanoparticles are successfully attached on the surface of kaolinites in clays/TiO 2 nanocomposites. Meanwhile, the positions of adsorption onsets of clays/TiO 2 nanocomposites exhibit a significant shift compared to bare TiO 2 nanoparticles, which is due to the quantum size effect and dispersion effect of kaolintes. The shift of adsorption edge for FK/TiO 2 is stronger than RK/TiO 2 . It might be due to the smaller size of TiO 2 in FK/TiO 2 , which is well consistent with the TEM results (Fig. 2).
The bandgap energy of clays/TiO 2 samples can be confirmed roughly according to the plot of (αhv) 2 versus energy (hv) of absorbed light (Fig. 5b), which is obtained on the basis of the Kubelka-Munk function (F(R∞) = (1 − R) 2 /(2 R)), where α, h, v and R are absorption coefficient, Planck constant, light frequency and reflectance with the reflectance at 1000 nm set at 100%, respectively. As shown in Fig. 5b, the band gap energies of the TiO 2 species in FK/TiO 2 and RK/TiO 2 are 3.21 and 3.18 eV, respectively. For comparison, the bandgap energy of bare TiO 2 nanoparticles, calculated form the corresponding plot of (αhv) 2 vs. photon energy (hv), is 3.2 eV. The results indicate that FK/TiO 2 possesses larger band gap energy, which can be attributed to the smaller grain size and the more intense interaction between TiO 2 and FK. It is conducive to the enhancement of photocatalytic performances.
The photoluminescence (PL) spectra of the samples at the excitation wavelength of 254 nm are shown in Fig. 5c. The PL peaks of clays/TiO 2 are much lower than that of bare TiO 2 , thus suggesting the less recombination of photogenerated electrons and holes, which would lead to improved photocatalytic activity. FK/TiO 2 nanocomposites have the intimate and uniform interfacial contact, in which photogenerated electrons from TiO 2 conduction-band are injected rapidly into FK across the particles-nanosheets heterostructure interface. The smaller particles size and more efficient charge separation is achieved and consequently leads to higher photocatalytic activity, compared with RK/TiO 2 and bare TiO 2 nanoparticles. Photocatalytic properties. The photocatalytic performances of as-prepared photocatalysts were measured to degrade antibiotics under UV light irradiation (Fig. 6). Figure 6a shows the degradation curves of tetracycline hydrochloride using FK, RK, FK/TiO 2 , RK/TiO 2 and bare TiO 2 photocatalysts. For the blank experiment analysis, the result shows that tetracycline hydrochloride can be hardly degraded after 60 min under UV light irradiation without catalysts, excluding the possibility of self-photolysis in this system. The bare TiO 2 , FK and RK exhibit 9, 22 and 19% of tetracycline hydrochloride adsorption after 30 min dark adsorption equilibrium, respectively. Upon UV light irradiation, the tetracycline hydrochloride can be slightly degraded with bare TiO 2 , while there is no degradation by FK and RK. The photodegradation activity is enhanced by supporting TiO 2 nanoparticles on the kaolinites, and FK/TiO 2 composites show the highest photodegradation activity. After irradiation for 60 min, the photodegradation rate of FK/TiO 2 composite is 98%, but only 84% for the RK/TiO 2 . It is reported that photocatalytic decomposition of tetracycline hydrochloride follows the pseudo-first-order reaction kinetics 40 . As shown in Fig. 6b, the rate constant for bare TiO 2 is very small compared to that for clays/TiO 2 composites, and the rate constant of FK/TiO 2 is higher than that of RK/TiO 2 . The enhanced photocatalytic activity for FK/TiO 2 composite can be attributed to the large surface area, decreased particle size and increasing density of active sites.
The stability and recyclability of FK/TiO 2 nanocomposite were evaluated by monitoring the reactivity of FK/TiO 2 during five reaction cycles. As shown in Fig. 6c, the photocatalytic activity of FK/TiO 2 can be easily recovered, and the photodegradation activity has no obvious decrease after five successive cycles. Moreover, the postmortem study shows that there is no significant changes for the supported structure of FK/TiO 2 after five reaction cycles, which indicates FK/TiO 2 has good stability. The good stability of FK/TiO 2 catalyst could be ascribed to the intense interaction between TiO 2 and FK, which can immobilize the active sites in photocatalysis. The well stability would greatly promote their practical application to eliminate the antibiotics.

Discussions
Based on the above results, a possible mechanism to explain the enhancement of the photocatalytic properties of the clays/TiO 2 nanocomposites is depicted in Fig. 6d. The proposed mechanism is the synergetic effects between the kaolinite clays and supported TiO 2 nanoparticles. The TiO 2 clusters act as a light absorber, while the kaolinite clays with different morphology are the physical adsorbents of tetracycline hydrochloride molecules. In the dark, tetracycline hydrochloride molecules could be effectively adsorbed around the clays/TiO 2 nanocomposites and reach adsorption-desorption equilibrium on their surface, which could facilitate the photocatalytic reactions. TiO 2 nanoparticles supported on the clays could be excited to yield electrons (e − ) and holes (h + ) (Eq. (1)). These photo-induced electrons-holes reacted with oxygen molecules (O 2 ), H 2 O or hydroxyl groups on the surface of kaolinites to yield hydroxyl radicals ( • OH) and superoxide radical anions (O 2 •− ) (Eq. (2-11)) 41 . These active species with strong oxidizability directly or indirectly interacted with tetracycline hydrochloride molecules already adsorbed on FK/TiO 2 and RK/TiO 2 in aqueous solutions (Eq. (12-13)) 37 . The complete equations of mechanism reactions are as follows: Above-mentioned photocatalytic mechanism for tetracycline hydrochloride photodegradation, the surface adsorption and light-induced charge transfer are two primary factors affecting the photocatalytic activity. Strong adsorptivity contributes to the concentration of antibiotics from the solvent and consequently improves the photocatalytic activity. By comparison with FK/TiO 2 and RK/TiO 2 nanocomposites, FK/TiO 2 exhibits much stronger surface adsorption, compared to that of RK/TiO 2 , which can originate from the large specific surface area. Moreover, the TiO 2 nanoparticles exhibit much more uniform and smaller for FK/TiO 2 , while aggregation seriously for RK/TiO 2 , and FK/TiO 2 has relatively higher loading efficiencyof TiO 2 nanoparticles than that of RK/TiO 2 , which can result in higher light-induced charge transfer, and finally enhance the photocatalytic activity. It is clear that the efficient photodegradation performance of FK/TiO 2 can be attributed to the synergistic effect of better adsorption capability and more active photocatalysis TiO 2 species. The specific surface area and the grain size of TiO 2 nanoparticles are strongly depended on the morphology of the kaolinite supports. Therefore, it is important to develop advanced technologies to control the particles morphology so that to enhance the catalytic performances.
In summary, this paper proposed a facile sol-gel method to synthesize clays/TiO 2 nanocomposites with high catalytic activity and stability. Based on the natural different morphologies and unique layered structure, the efficient assembly and high-density dispersion of uniform TiO 2 nanoparticles were successfully achieved on the surface of kaolinite clays. Degradation of antibiotics using clays/TiO 2 catalysts was investigated to elucidate the effects of kaolinite microstructure, morphology and dimensionality for a significant suppression of the TiO 2 nanoparticle aggregation during the reaction. FK/TiO 2 exhibited remarkably enhanced photoactivities toward degradation of tetracycline hydrochloride, and the overall degradation rate was up to 98% after light irradiation for 60 min. It could be attributed to the two-dimensional morphology, stronger surface adsorptivity, higher loading efficiency, smaller grain size and intimate interfacial contact. Therefore, our insight into the comparison of different dimensionality of kaolinite clays to photocatalytic performance could be a reference function to the similar investigation, and clays/TiO 2 composites have great potential applications to eliminate effectively antibiotics.  Table 1 and showed that kaolinites with different morphologies possessed the similar chemical composition. Other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were analytical grade and used without further purification.
Preparation. TiO 2 /kaolinite nanocomposite materials were prepared through a facile precipitation method.
In a typical synthesis, 0.2 g kaolinite was added to a mixture constituted by 30.0 mL of ethanol and 0.6 mL of deionized water under dispersing in the ultrasonic bath for 30 min. Subsequently, 3 mL of tetrabutyl titanate (TBOT) dissolved in 5 mL of ethanol were put drop-wise into the kaolinite suspensions. After continuous stirring for 2 h at 80 °C, the precipitates were collected by centrifugation and subsequently washed with deionized water repeatedly. The resultant products were dried overnight at 80 °C. Finally, the samples were calcined under 450 °C for 3 h in air with a heating rate of 5 °C/min. For comparison, the bare TiO 2 nanoparticles were also obtained via a similar process as TiO 2 /kaolinite composite without adding kaolinite.
Photocatalytic degradation experiments. The photocatalytic activity of the catalysts for tetracycline hydrochloride was investigated at ambient temperature. In a typical process, 50 mg of catalyst was dispersed in 50 mL of 30 mg/L tetracycline hydrochloride solution. The suspension was vigorously stirred in the dark for 30 min to reach the adsorption-desorption equilibrium, and then irradiated with ultraviolet light. Afterwards, 2.5 mL aliquots of the reaction mixtures were collected and the catalyst was removed from the solution using a 0.45 μm cellulose acetate membrane filter. The tetracycline hydrochloride concentrations in the filtrates were measured at 380 nm using the UV-vis spectrophotometer. The stability of catalyst was evaluated by the catalytic cycle test. At the end of each cycle, the suspension was filtered and the catalyst was tested in the next cycle.
Characterization. The chemical composition of kaolinite minerals and nanocomposites were determined using X-ray fluorescence (XRF) spectrometer. The structural characteristics of the samples were examined by X-ray diffraction (XRD, Rigaku D/MAX2550VB+) using Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 0.02 °/s with a voltage of 40 kV and 40 mA. The microstructures of the samples were observed using a scanning electron microscopy (SEM, FEI Quanta-200) with an accelerating voltage of 5 kV, a transmission electron microscopy (TEM, JEOL JEM-2100F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-3010) operating at 200 kV. The TiO 2 clusters were identified by X-ray photoelectron spectroscopy and their size distributions were determined by counting the sizes of TiO 2 clusters on TEM images taken from different places. The textural properties of the samples were determined by N 2 porosimetry. The N 2 adsorption-desorption isotherms were recorded at 77 K and analyzed using an ASAP 2020 Surface Area analyzer (Micromeritics Co. Ltd.). The specific surface areas were calculated using the Brunnauer-Emmett-Teller (BET) equation, and estimates of the pore size distributions were deduced by means of Barrett-Joyner-Halenda (BJH) methods. The zeta potential of the samples at different pH levels was measured on a zeta potential analyzer (Zetasizer Nano ZS90, Malvern) at solids content of about 0.1% in distilled water. The interface characteristics and their chemical nature were studied by Fourier transform infrared (FTIR, Nicolet Nexus 670) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Diffuse reflectance ultraviolet-visible (UV-vis) spectra were obtained on a Hitachi (Shimadzu) Model UV-2450 spectrophotometer. The Photoluminescence (PL) spectra were measured on a Hitachi F-4500 fluorescence spectrometer at room temperature using a Xe lamp with a wavelength of 254 nm as the excitation source. For the photocatalytic activity evaluation, a 150 W Xe lamp equipped with an optical cutoff filter (λ < 400 nm) was used as the light source, and the concentration of photodegraded tetracycline hydrochloride solution were recorded by a UV-vis spectroscopy (UV2450).
Data availability statement. The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.