Commercial reagent-grade products were used without the need for added purification. Among the products used were TiO₂ anatase 99.5% from Sensient, sodium chloride (NaCl) 100.1% (MW= 58.44 g/mol) from Fisher Chemical, sodium hydroxide (NaOH) 96% (MW= 40.0 g/L) from Erba, copper (II) sulfate pentahydrate (CuSO₄·5H₂O) 99% (MW= 249.68 g/mol) from EMSURE, rice husks from Venezuelan crops (RH), phosphoric acid (H₃PO₄) 85% (MW= 97.994 g/mol) from Fisher Chemical, potassium hydroxide (KOH) 85% (MW= 56.11 g/mol) from Fisher Chemical, carbon dioxide (CO₂), monosodium phosphate (NaH₂PO₄·H₂O) 99% (MW= 137.99 g/mol) from Fisher Chemical, and paracetamol (C₈H₉NO₂) Lote: BP-03/11/08 from G. Amphray Laboratories.

To synthesize ceramic foams, the following steps were followed: first, TiO₂ NPs were obtained by mechanical grinding of commercial TiO₂ with NaCl as an inorganic dispersant, using a Leegol Electric PG-LG-002 ball mill. Then, the wet impregnation method was used with 1% by weight of CuSO₄·5H₂O as a doping precursor, dissolving the CuSO₄·5H₂O in distilled water and mixing it with the TiO₂ NPs. The mixture was stirred at 50 °C for 2 hours (Faithful SH-2 hot plate), and the pH was adjusted to 8 with 1 M NaOH. Subsequently, it was dried at 90 °C for 24 hours, calcined at 450 °C for 2 hours and finally washed and dried again at 90 °C in a Raypa oven.

Biogenic silica was obtained from rice husk ash (RHA) following the protocol of (Castillo et al., 2022). This silica was dissolved in KOH and mixed with the TiO₂ NPs, subjecting it to ultrasound for 15 minutes with an Elmasonic E30H equipment. The mixture was bubbled with CO₂ and dried at 80 °C for 24 hours. In this way, two types of ceramic foam were produced: EC0 (with TiO₂ NPs) and EC1 (with Cu-TiO₂ NPs).

In the present study, tests were conducted to characterize the properties of the synthesized TiO₂ NPs, as well as those of the ceramic foams. For this purpose, the X-ray diffraction (XRD) technique was used to determine both the crystalline structure and the size of the TiO₂ crystals. The analysis was performed using a D2 PHASER XRD Bruker equipment, belonging to the Institute of Advanced Studies Foundation (IDEA). This equipment operated at 30 kV and 10 mA, using Cu Kα radiation. The sensor was scanned in a 2θ angle range of 10° to 70°, with a step size of 0.02° and a time per step of 1 second. In addition, a wavelength of 1.541840 Å and an incidence window of 0.2 mm were used. These experimental conditions allowed precise data to be obtained for the structural and morphological characterization of the analyzed samples.

From the broadening of the peak corresponding to the anatase phase (101), the average size of the crystals of both the TiO₂ and Cu-TiO₂ NPs was estimated, applying the Scherrer equation (Equation 1) (Moongraksathum et al., 2018):

(1)

In the formula given, L is the average crystallite size, λ is the X-ray wavelength (0.154184 nm), β refers to the full width at half maximum (FWHM) in radians, and θ is the diffraction angle.

On the other hand, the ceramic foams were analyzed by XRD using a LANScientific Fringe Class 65410034 equipment, belonging to the School of Geology and Mines of the Faculty of Engineering of the Central University of Venezuela (UCV). This analysis was conducted under a configuration of 30 kV and 16 mA, using Cu Kα radiation with a wavelength of 1.54060 Å. Additionally, morphological characterization and elemental analysis were conducted by scanning electron microscopy (SEM) and X-ray dispersive spectroscopy (EDS). For this, a JEOL JSM-6390 equipment was used, equipped with an Oxford Instruments model 7582 X-ray detector, belonging to the Simón Bolívar University (USB).

The photocatalysis experiments were conducted under natural solar radiation during sunny days in the months of August, September, and October of the year 2024. The initial total radiation recorded was 924 W/m², measured with a digital radiometer. These tests were conducted at the School of Chemistry of the Faculty of Sciences of the UCV, located at the geographic coordinates 10.487348333689186, -66.8958709428127. The experimental system (Figure 1) consisted of a cylindrical borosilicate reactor operated in batch mode, with a volume of 200 mL of ACP solution and a ceramic foam loading of 1.5 g/L. The reactor operated under constant magnetic stirring and with continuous oxygen injection from a continuous air flow of 8.33×10⁻⁶ m³/s, equivalent to 105 mL/min of oxygen, using a Penn-Plax Airtech 2K0 aquarium air pump. Samples were taken at specific time intervals: 17, 33, 50, and 83 minutes. Control tests were also performed, including photolysis (without photocatalytic material) and adsorption under dark conditions.

For in situ monitoring, the Raman Spectroscopy technique was used using the Eddu Raman TO-ERS-532 system, manufactured by Thunder Optics and belonging to the Nanotechnology and Spectroscopy Laboratory of the UCV. This equipment is equipped with an AvaSpec-Mini spectrometer from the Avantes brand, which incorporates a 2048-pixel CMOS detector and a spectral resolution of 0.09 nm. In addition, the system includes a 532 nm laser with a line width of 0.1 nm, an optical fiber, and a compact Raman probe with a range for Raman shifts up to 175 cm⁻¹. It also has a 20X NA 40 microscope objective with a working distance of 11.8 mm, a fixed slit of 50 µm and AvaSoft 8 software for data acquisition and management. The laser operated with a power of 2.8 mW and a spectral acquisition time of 2 seconds. The data obtained were processed and visualized using SpectraGryph software version 1.2.

The crystalline structure of the nanophotocatalysts (TiO₂ NPs and Cu-TiO₂ NPs) prepared in this investigation was evaluated by the XRD technique. Figure 2 depicts the XRD patterns of the undecorated and 1% Cu-decorated TiO₂ NPs samples, which show several well-defined diffraction peaks. The peaks observed at 2θ = 25.10°, 36.56°, 37.40°, 38.16°, 47.48°, 53.22°, 54.38°, 61.90° and 67.88° correspond to the anatase TiO₂ crystal plains (101), (103), (004), (112), (200), (105), (211), (204) and (116), respectively, coinciding with card number 00-900-8216.

The XRD patterns show no evidence of the presence of copper oxides, suggesting that the Cu atoms are in a uniformly dispersed state. In addition to the characteristic TiO₂ peaks, the absence of impurity peaks demonstrates that no aggregated phases are generated during the Cu-TiO₂ NPs synthesis method used in this investigation. Despite using different synthesis methods, these results are consistent with previous research by (Dorraj et al., 2017), (Lin et al., 2018), (Preda et al., 2022), and (Raheem et al., 2023).

Calculations performed from Equation 1 indicate that the crystallite sizes are 21.44 nm for TiO₂ NPs and 20.36 nm for Cu-TiO₂ NPs. The radiation decrease in particle size might be the result of the interaction between copper and TiO₂. These results are consistent with previous studies reported by (Turkten et al., 2019), (Manga et al., 2019), (Preda et al., 2022), and (Leong et al., 2022).

The results are significant since TiO₂ NPs function as photocatalysts in the ceramic foams developed in this study. These NPs have been found to be in their anatase phase, which is considered the most active in radiation-accelerated catalysis processes (Aguilar et al., 2022). Likewise, the nano-scale particle size of the photocatalytic materials has a crucial impact on the optical behavior of the mixture, as well as on the distribution of radiation within the reactor and the efficiency of the photocatalytic process (Cassano & Alfano, 2000)

An analysis using the XRD technique was performed to determine the structure of the synthesized ceramic foams. The X-ray diffractogram of the EC0 ceramic foams is presented in Figure 3. The XRD results showed a broad peak centered at 2θ between 20° and 30°, without the presence of sharp peaks, showing the amorphous nature of the biogenic silica obtained from rice husk ash. No crystalline silica phases were seen in the XRD patterns, suggesting that the SiO₂ in the ash is amorphous. In addition, diffraction peaks related to the anatase crystalline phase of TiO₂ were found, as mentioned above. These results are consistent with previous studies conducted by (Fernández et al., 2019), (Porrang et al., 2021), (Hoerudin et al., 2022), (Shrestha et al., 2023), and (Pereira et al., 2023).

The photocatalytic ceramic foam EC0 was characterized using the SEM technique, the results of which are presented in Figure 4. A rough morphology was observed, composed of a structure of agglomerates of hemispherical particles with an average size of 0.22 µm. The image reveals that the surfaces have dark pigmentation, which corresponds to porous cavities. These pores have an irregular shape and a size that varies between 0.1 µm and 0.4 µm.

To better understand the interaction between the biogenic silica matrix and the TiO₂ NPs, a SEM-EDS analysis was conducted. When seeing the EDS spectrum of samples EC0 (Figure 5a) and EC1 (Figure 5b), a composition rich in Si and Ti atoms is evident. In both spectra, the high oxygen content (57% and 48%, respectively) is consistent with the presence of the SiO₂ and TiO₂ network. In addition, the presence of K atoms, oriinating from the solvent used in the synthesis, is seen, and in the case of the EC1 foam made up of SiO₂/Cu-TiO₂ NPs, the presence of copper was detected, as expected (See Table 1).

During the in situ monitoring of the photocatalytic reaction, the Raman spectroscopy technique was employed, which has been widely used in drug quantification studies (Borio et al., 2012). Raman spectroscopy is a technique that allows the measurement of the vibration modes of a sample, which provides detailed information about its chemical composition. The resulting spectrum shows a distribution of peaks corresponding to the specific molecular vibrations of the analyzed sample. These peaks can be used to identify and quantify chemical substances, such as drugs, based on their frequency and intensity (Shende et al., 2014).

In pharmaceutical manufacturing and finished product testing, figuring out drug content through high-performance liquid chromatography (HPLC) testing is a time-consuming and potentially destructive process. In contrast, Raman spectroscopy has appeared as a widely used technique for the quantification of active pharmaceutical ingredients (APIs) and excipients in drugs, as well as for the identification and quantification of crystal polymorphism. This technique is rapid, convenient, noninvasive, and nondestructive, allowing for the minimization of sample preparation and providing highly specific information on the chemical composition of dense and highly turbid samples. Furthermore, being a solvent-free technique, Raman spectroscopy is positioned as an attractive and promising tool in the framework of green chemistry ((Bleye et al., 2013); (Mojica et al., 2018); (Zhao et al., 2022)).

ACP is composed of an aromatic ring, a carbonyl group (C=O), an alcohol group and nitrogen (Figure 6). In the specific case of ACP, its Raman spectrum (Figure 7a) is dominated by characteristic peaks such as the amide I (C=O) at 1625 cm^-1, the amide II (C-N stretching, N-H bending) at 1575 cm^-1, the C–H bond at 1225 cm^-1, and the phenyl ring at 800 cm^-1. The peaks at 1175 cm^-1, 1275 cm^-1, and 1325 cm^-1 were separately derived from the C-N-C symmetric stretching vibration, the benzene –OH stretching vibration, and the CH₃ symmetric variant, respectively. Two of these peaks (1225 and 1625 cm^-1) are only observed for ACP and can be used as marker peaks to identify and quantify any sample containing ACP (Mojica et al., 2018) (Zhao et al., 2022).

In this research, the conventional method of measuring Raman peak intensities (area) was used to quantify ACP. The peak at 1600 was chosen to conduct the quantitative analysis of ACP, which corresponds to C=O stretching with the contribution of ring stretching (C=C) (Figure 7b). A calibration curve was performed with ACP solutions in a concentration range from 2 ppm to 14 ppm (Figure 7c), starting from the principle that the amount of active ingredient has a direct effect on the generated signal. The most reported validation criteria are the coefficient of determination (R²) of the calibration curve, which in this study was 0.9849, and the concentration range, to evaluate the relationship between concentration and Raman signal in quantitative detections. In the linear calibration range, the concentration is estimated by linear regression (Cailletaud et al., 2018).

By applying this method, the ACP solutions treated photocatalytically with EC0 and EC1 were analyzed under natural solar irradiation, using Raman spectroscopy as a characterization technique. Figure 8 presents the graph illustrating the ACP degradation efficiency as a function of time. Initially, an adsorption process was identified in the foams that reached an average of 29%, which represents a key stage within the photocatalytic mechanism (Ugarteburi, 2018).

Regarding the efficiency of photocatalytic degradation, the results indicate that the use of 1.5 g/L of the EC1 allowed a 91.0% degradation of the ACP to be achieved in a period of 83 minutes. In comparison, under the same load and time conditions, the maximum efficiency obtained with EC0 was 68.3%.

On the other hand, Figure 9 shows the Raman spectra corresponding to the amide I peak recorded during the in-situ monitoring of the photocatalytic reaction of ACP for the systems with EC0 and EC1. In these spectra, a progressive decrease in the area under the curve is observed as the reaction time progresses. Particularly, in the case of EC1, the peak associated with amide I disappears completely after 33 minutes of reaction, evidencing its greater efficiency in the degradation of the compound.

The results obtained indicate that the combination of the SiO₂/Cu-TiO₂ NPs foam system, called EC1, presents a higher efficiency in the degradation of ACP compared to the control and EC0 experiments. This superiority in efficiency is attributed to the fact that, in the heterogeneous photocatalysis process, the catalyst is activated by high-energy photons. During these reactions, an electron from the valence band is promoted to the conduction band, generating a photohole in the semiconductor due to an external source of energy, in this case, natural solar radiation. The TiO₂ semiconductor, modified with Cu²⁺ metal ions, plays a key role in preventing charge recombination, which optimizes the photocatalytic process. In addition, copper shifts the absorption spectrum towards lower energy regions, expanding the amount of usable radiation in the UV-visible range and contributing significantly to the overall performance of the process (Aguilar et al., 2022) (Adamu et al., 2023).

Not applicable.

The authors declare that there is no competing interest.