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Scientific Reports
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10.1038/s41598-024-71605-x
Article
Impact of LED radiation intensity on gold nanoparticles photodeposition on TiO2 with physicochemical and photocatalytic characterization
Kubiak Adam adam.kubiak@amu.edu.pl

grid.5633.3 0000 0001 2097 3545 Faculty of Chemistry, Adam Mickiewicz University, Poznan, Uniwersytetu Poznanskiego 8, 61614 Poznan, Poland
4 9 2024
4 9 2024
2024
14 205636 6 2024
29 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
This study investigates the influence of LED radiation intensity on the photodeposition of gold nanoparticles onto TiO2 substrates, examining their physicochemical properties and photocatalytic activities. Utilizing a range of radiation intensities and wavelengths, TiO2-Au composites were synthesized and characterized through techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). The deposition process, markedly enhanced by shorter wavelengths and higher intensities, efficiently formed gold nanoparticles. This research distinctly highlights observable morphological changes in the nanoparticles; increased radiation intensity not only augmented the size but also altered their shape from spherical to hexagonal. These morphological transformations significantly improve the composites’ light absorption and catalytic properties due to the surface plasmon resonance of the gold nanoparticles. Photocatalytic assessments, using metronidazole as a model pollutant, demonstrated that composites prepared with higher LED intensities showed significantly enhanced degradation capabilities compared to those synthesized with lower intensities. The findings underscore that manipulating photodeposition parameters can critically influence the structural and functional properties of TiO2-Au composites, potentially advancing their applications in environmental remediation and solar energy utilization.

Keywords

Titanium dioxide
Gold nanoparticles
Photodeposition
LED light
Metronidazole
Subject terms

Materials science
Nanoscale materials
Nanoparticles
http://dx.doi.org/10.13039/501100004281 Narodowe Centrum Nauki 2023/07/X/ST5/00022 Kubiak Adam issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

The field of materials science has increasingly focused on the development and optimization of photocatalysts, particularly for environmental and energy applications. Titanium dioxide (TiO2) stands out in this realm due to its exceptional photoreactivity, stability, and non-toxic nature1. However, the efficiency of TiO2 is inherently limited by its wide bandgap, which restricts its active absorption to the ultraviolet (UV) segment of the solar spectrum—a minor fraction of the total solar irradiance2,3. To extend the absorption capabilities of TiO2 into the visible light range, various strategies have been explored, with the incorporation of noble metal nanoparticles, such as gold (Au), proving to be one of the most effective methods4,5. Gold nanoparticles can introduce localized surface plasmon resonances (LSPRs), which are collective oscillations of conduction band electrons induced by light6,7. These resonances significantly enhance the optical properties of TiO2, facilitating improved light absorption and efficiency in generating charge carriers necessary for photocatalytic reactions8,9. The process of depositing these nanoparticles onto TiO2, known as photodeposition, is crucial for achieving the desired modification of the photocatalyst’s properties.

Historically, the process of photodeposition, which involves reducing metal precursors to their metallic forms on a substrate using light, has predominantly relied on the use of broad-spectrum light sources such as mercury vapor or xenon discharge lamps10,11. These light sources, characterized by their ability to emit a wide range of wavelengths, have been instrumental in driving photocatalytic processes due to their intense and robust emission. However, the broad spectral output of these lamps poses several challenges and limitations in the precise control of photocatalytic reactions12. The lack of specificity in wavelength control inherent to mercury vapor and xenon lamps means that light energy is not utilized with maximum efficiency13. These lamps emit light across a wide spectrum, including wavelengths that do not contribute to the activation of the photocatalyst or the photodeposition process. As a result, a significant portion of the light energy emitted by these lamps is wasted, which is not only inefficient but also increases the energy costs associated with the photodeposition process14. Furthermore, the intense and uncontrolled emission of light from these broad-spectrum sources can lead to overheating of the photocatalytic system. Overheating is a critical concern as it can cause thermal degradation of the photocatalyst materials15,16. For instance, excessive heat can alter the structural and chemical properties of TiO2. This degradation not only diminishes the efficacy of the photocatalyst but can also shorten its usable life, requiring more frequent replacement and thereby increasing the operational costs17. Additionally, the use of mercury vapor and xenon lamps raises environmental and health concerns. Mercury vapor lamps contain mercury, a toxic heavy metal that poses significant health and environmental risks if the lamps are broken or improperly disposed of18,19. On the other hand, while xenon lamps do not contain mercury, they typically require higher energy inputs and can lead to increased operational costs due to their energy consumption20. Given these challenges, there has been a growing interest in exploring alternative light sources that offer better control over the emission spectrum and energy efficiency21. Light-emitting diodes (LEDs) have emerged as a promising solution in this regard. LEDs can be engineered to emit light at specific wavelengths, thereby aligning the emission spectrum with the absorption characteristics of the photocatalyst and the plasmon resonance frequencies of metal nanoparticles like gold22,23. This specificity not only enhances the efficiency of the photodeposition process but also minimizes thermal degradation risks by preventing overheating, making the entire process more sustainable and cost-effective24,25. Furthermore, LEDs do not contain toxic materials like mercury, presenting a safer and more environmentally friendly option compared to traditional lamp-based systems26,27.

This study explores the impact of varying LED radiation intensity and wavelength on the deposition of gold nanoparticles onto TiO2 substrates. By systematically adjusting these parameters, we investigate how they influence the morphology, size, and distribution of gold nanoparticles and assess their effect on the photocatalytic performance of TiO2, particularly focusing on the degradation of the pharmaceutical pollutant metronidazole (MNZ). The broader significance of this research lies in its potential to enhance the design and synthesis of TiO2-based photocatalysts. By optimizing the photodeposition process, it may be possible to tailor the properties of these catalysts for specific environmental and energy applications, such as water purification and solar fuel generation28,29. This introduction sets the stage for a detailed presentation of the methods used in synthesizing the TiO2-Au composites, characterizing their properties, and evaluating their performance in practical applications30,31.

In summary, the use of LED technology in photodeposition represents a transformative approach to the synthesis of enhanced photocatalytic materials. It not only promises increased control over the photocatalytic properties through precise wavelength and intensity adjustments but also aligns with sustainable practices by reducing energy consumption and enhancing process efficiency. The outcomes of this research could provide valuable insights into the scalable production of customized photocatalysts, contributing significantly to the fields of materials science and environmental technology. This study, therefore, addresses both the fundamental aspects of photocatalyst design and the practical implications of their application, aiming to bridge the gap between laboratory research and environmental solutions.

Materials and method

Materials

Anatase (nanopowder, Thermo Scientific, 99,7%), chloroauric acid (Thermo Scientific, 99%), methanol (Sigma-Aldrich, ACS reagent, ≥ 99.8%), metronidazole (MNZ) (Thermo Scientific, analytical standard). All reagents were of analytical grade and used without any further purification. The water used in all experiments was deionized.

Synthesis of TiO2-Au materials

TiO2-Au composites were synthesized via the photodeposition method. Initially, 1 g of TiO2 was suspended in a 1:1 v/v water solution, followed by the addition of 250 µL of chloroauric acid solution (1 g/L) to achieve a 0.5 wt% Au concentration. The reaction vessel was sealed, purged with nitrogen for 30 min, and subsequently exposed to monochromatic LED light at λmax = 365 nm or 395 nm with varying intensities of 5, 10, 20, and 40 mW/cm2. A visible color transition from yellow, indicative of the gold precursor presence, to purple signified successful reduction of Au ions. Post irradiation, the samples were filtered and thoroughly rinsed with water to remove impurities and excess precursor. The materials were then dried at 60 ℃ for 6 h. The samples were labeled according to the deposition parameters using the notation: wavelength_radiation intensity (e.g., 365_5, 365_40, 395_5, 395_40).

Characterization of fabricated materials

X-ray diffraction (XRD) analysis was performed using a D8 Advance diffractometer by Bruker, Germany. This device employed Cu Kα radiation (λ = 1.5418 Å) with nickel filtering. Data acquisition occurred through step-scanning at 0.05° increments across an angular range of 5 to 60°.

Elemental analysis of the samples was conducted using an FEI Quanta 250 FEG scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) spectrometer.

X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using a Specs UHV spectrometer, based in Germany, with charge compensation. The calibration was standardized using the C 1 s peak at 284.8 eV.

To determine the gold concentration on TiO2 surfaces, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was performed. Following protocols detailed in prior studies, 15 mg of the sample was dissolved in aqua regia (3:1 HCl) and left to equilibrate overnight. Analysis was then executed using a Varian 710-ES ICP-OES, after diluting 1 mL of the sample with 13 mL of deionized water.

Transmission Electron Microscopy (TEM) was carried out with an FEI TECNAI G2 F20 electron microscope operating at 200 kV, equipped with a Gatan CCD camera. Sample preparation for high-resolution TEM (HR-TEM) involved sonicating a small sample in 2-propanol and depositing it on a copper grid with a holey carbon film. Imaging was done post-solvent evaporation, scanning comprehensively to map the material uniformly.

The optical properties of the materials were evaluated through diffuse reflectance spectroscopy (DRS) in the 200–800 nm wavelength range. Bandgap energies were calculated using a plot of (F(R)·E)0.5 against photon energy (E), with F(R) denoting the Kubelka–Munk function, indicative of radiation absorption. These measurements utilized a Thermo Scientific Evolution 220 spectrophotometer equipped with a PIN-757 integrating sphere and BaSO4 as a reference.

Photoluminescence properties were analyzed using a Horiba Fluorolog version-3 spectrofluorometer, equipped with a 450 W high-pressure xenon arc lamp. The system recorded emission spectra at room temperature with an excitation wavelength of λexcitation = 320 nm and maintained a spectral resolution of 2 nm with a slit width of 2 mm.

Photocatalytic activity

Light sources

This study utilized a light emission diode (LED) light source. Illumination was provided by 12 power LEDs with wavelengths of 365, 385, and 525, 550 nm, connected in series to deliver a total power output of 36 W. The LEDs were powered by an LED driver (MeanWell, Taiwan), offering an output voltage range of 20–40 V and a maximum current of 350 mA. This setup ensured optimal lighting conditions for photocatalytic activity.

Photooxidation tests

Each 100 cm3 aqueous suspension, exposed to irradiation, maintained a uniform photocatalyst concentration of 1 g dm−3 and an initial metronidazole (MNZ) concentration of 20 mg dm-3. Prior to irradiation, the suspension was placed in a container shielded from light and stirred magnetically (IKA Werke GmbH, Germany) for 30 min to achieve adsorption/desorption equilibrium on the photocatalyst surface. This stirring was continued throughout the photocatalytic testing while the reaction mixture was subjected to the previously described LED irradiation system. During the course of the experiments, 3 cm3 samples were periodically drawn from the photoreactor, filtered using a syringe filter (Macherey–Nagel, Germany), and the clear supernatant was analyzed spectrophotometrically at 276 nm, the peak absorption wavelength for MNZ, using a Thermo Scientific Evolution 220 spectrophotometer. The photocatalytic performance of the materials was assessed by calculating the degradation yield (W) using the established formula:1 W%=1-CtC0·100%

where C0 and Ct represent the initial MNZ amount and the concentration of MNZ determined upon a fixed irradiation time, respectively.

In the photo-oxidation experiments, each measurement was conducted in triplicate to ensure accuracy and reproducibility of the results. The data presented in the results section represent the mean values of these three independent experiments. The standard error of the mean was calculated to provide an estimate of the variability and reliability of the data. This statistical method allows for a better understanding of the precision of the results and the consistency of the photocatalytic activity observed under different experimental conditions.

The identification of intermediate products in the metronidazole degradation process was performed using ESI–MS analysis. ESI–MS spectra were obtained employing a Bruker amaZon SL ion trap instrument (Germany) equipped with an electrospray ion source operating in infusion mode. The sample solution was introduced into the ionization source at a rate of 10 μL/min via a syringe pump. The instrument operated in enhanced resolution mode over a mass range of 50–30000 m/z, with a scanning rate of 8100 m/z per second. The capillary voltage was adjusted to + 4.5 kV with an endplate offset of −500 V. The ion source temperature was maintained at 80 ℃, while the desolvation temperature was set to 250 ℃. Nitrogen was utilized as the cone gas, and helium as the desolvating gas, with flow rates of 800 L/h and 50 L/h, respectively. The mass spectrometer functioned in both positive and negative ionization modes.

Kinetic study

The kinetics of metronidazole photooxidation were assessed using a pseudo-first-order kinetic model. This model postulates that the degradation rate is directly proportional to the surface coverage (θ) of MNZ, expressed as follows:2 r=dCdt=kθ=kKC01+KC0+KsCs

Here, k represents the reaction rate constant, 'θ' denotes the surface coverage by naproxen, K and Ks are the adsorption coefficients for MNZ and water, respectively, C0 stands for the initial concentration of MNZ, and Cs represents the concentration of water. The concentration of water, Cs remains nearly constant and is significantly higher than the concentration of MNZ. Consequently, we can express Eq. (3) in the following form:3 lnCtC0=-k1t

In Eq. (3), k1 signifies the first-order rate constant, and t is the time of irradiation.

Verification of the degradation mechanism using scavengers

The objective of this research was to understand the impact of charge carriers and reactive oxygen species on the photocatalytic degradation processes of organic pollutants using TiO2-Au systems. To assess the photocatalytic performance, the procedure outlined in section “Photooxidation tests” was followed, which included the addition of scavenger solutions in specified amounts. The scavenger concentrations were tailored to maintain a consistent level of 20 mg·dm−3 for MNZ. In this context, ammonium oxalate was used as the scavenger for holes (h+), AgNO3 for electrons (e−), tert-butyl alcohol for free hydroxyl radicals (*OH), and benzoquinone for superoxide radical anions (*O2–).

Results and discussion

Physicochemical characterization

To verify the effect of the LED-induced Au deposition process on the titania surface, an XRD analysis was performed. The collected data is presented in Fig. 1. Additionally, the XRD pattern of reference TiO2 is provided in Figure S1 in the Supplementary Materials. The TiO2 NPs sample show characteristic peaks at 2θ values of 25.2, 36.8, 37.7, 47.8, 53.7, and 55.0 which are strictly related to the anatase phase (card no. 9009086)32. The average crystallite size for the mentioned material was 10.2 nm. No apparent differences were observed in the XRD patterns for materials containing Au nanoparticles. Regardless of the LED light source and its power, the obtained TiO2-Au materials had a similar crystallite size in the range of 10.1–10.3 nm for the anatase phase. The presence of gold was not approved by XRD analysis (no peaks for Au particles) due to their low content (0.5 wt.%) and nanometric size. No other crystalline phases were identified in the patterns, which indicated the crystal purity of the obtained materials. Similar conclusions were also reported by Bielan et al.33, who pointed out that the low content of noble metals was not observed in the XRD patterns.Fig.1 The XRD patterns for samples series: (a) 365 nm and (b) 395 nm.

Table 1 presents the concentrations of gold in the analyzed materials, quantified using EDX and ICP-OES. These methods predominantly offer bulk compositional data. To analyze the surface-specific composition, which is critical for understanding the catalytic properties of the nanoparticles, XPS was utilized. The EDX maps and spectra displaying the distribution of individual elements and selected samples, respectively, are presented in Figures S2-S4 of the Supplementary Materials.Table 1 A cumulative list of gold content determined by the EDX, ICP-OES, and XPS techniques.

Sample	EDX	ICP-OES	XPS	
Au (wt. %)	Au (wt. %)	Au (at. %)	
365_5	0.590	0.475	0.435	
365_40	0.653	0.557	0.529	
395_5	0.351	0.309	0.315	
395_40	0.506	0.489	0.445	

The experimental data demonstrate that shorter wavelengths and higher irradiation intensities notably enhance the gold deposition on TiO2. Specifically, irradiation at 365 nm resulted in higher gold deposition compared to 395 nm, as evidenced by the measurement techniques EDX, ICP-OES, and XPS. For example, under an irradiation intensity of 5 mW/cm2 at 365 nm, the gold content was measured at 0.590 wt.% (EDX), 0.475 wt.% (ICP-OES), and 0.435 at.% (XPS). In contrast, at 395 nm, these values decreased to 0.351 wt.%, 0.309 wt.%, and 0.315 at.% respectively. This indicates that the wavelength closer to the UV range, where TiO2 exhibits optimal light absorption, leads to more effective electron–hole pair generation. Furthermore, increasing the irradiation intensity from 5 mW/cm2 to 40 mW/cm2 consistently led to an increase in gold content for both wavelengths tested. This trend highlights the importance of photon flux in enhancing the photoreduction process, as a higher density of photons increases the likelihood of interacting with gold precursors and TiO2, thereby promoting more extensive reduction and deposition. The interaction between the gold nanoparticles and the TiO2 surface also plays a significant role in this process. The deposited gold can alter the photocatalytic properties of TiO2 by increasing its ability to absorb visible light and modifying its surface reactivity34–36. This synergy not only enhances the efficiency of gold deposition but also potentially extends the photocatalytic applications of the TiO2-gold composite, such as in enhanced solar light absorption and improved sensitivity in sensor applications.

Adjusting the wavelength and irradiation intensity during photodeposition on TiO2 can substantially enhance the efficiency and precision of gold nanoparticle formation. This optimization leverages the inherent properties of TiO2, tailoring the process conditions to maximize light absorption and electron–hole pair generation, which are crucial for effective gold deposition. These findings pave the way for customized photocatalytic material design, expanding the range of applications in photoreduction and environmental remediation.

The XPS analysis depicted in Fig. 2 was utilized to determine the oxidation states of elements within the synthesized materials. Specifically, the XPS spectra targeted the Au 4f. region, where a doublet was observed. This doublet, located at binding energies of approximately 84.0 eV (4f7/2) and 87.7 eV (4f5/2), confirms the presence of metallic Au0 37,38, indicative of unoxidized gold. Additionally, it is crucial to note the absence of peaks in the Cl 2p region39. This absence indicates that the gold precursors do not form chemical adsorption bonds with the titanium dioxide surface, a finding that is consistent across various photodeposition efficiencies40.Fig.2 High resolution XPS spectra of: (a) 365_5, (b) 365_40, (c) 395_5, and (d) 395_40 samples.

The HR-TEM analysis was conducted to comprehensively characterize the morphology and crystal structure of the photodeposited gold nanoparticles. The results are presented in Fig. 3.Fig.3 The HR-TEM and FFT images for selected samples: (a) 365_5, (b) 365_40, (c) 395_5, and (d) 395_40.

Based on detailed high-resolution imaging, this study has confirmed that the gold nanoparticles synthesized in our experiments are crystalline in nature. A notable observation from the analysis is the influence of radiation intensity on the morphology of these nanoparticles. When subjected to lower radiation intensities (samples 365_5 and 395_5), the nanoparticles typically exhibited a spherical shape with a diameter of approximately 8 nm. However, an increase in radiation intensity not only augmented the size of these particles but also induced a morphological change to a hexagonal structure, observed in samples 365_40 and 395_40, with diameters ranging from 30–35 nm. Measurements of interplanar distances further validated the structural integrity of these particles, consistently showing a distance of 2.2 Å corresponding to the (111) plane across all samples41,42. Intriguingly, an interplanar distance of 1.9 Å, correlating to the (200) plane, was exclusively observed in samples illuminated with 365 nm light43,44. This specificity suggests that not only does the intensity of radiation influence the overall size and morphology of the nanoparticles, but the wavelength of the radiation also plays a critical role in defining their crystalline structure45,46.

These findings underscore the substantial impact of both radiation intensity and wavelength on the nucleation and growth dynamics of gold nanoparticles. For samples irradiated at lower radiation intensities (365_5 and 395_5), the gold nanoparticles typically exhibited a spherical shape with an average diameter of approximately 8 nm. In contrast, increasing the radiation intensity resulted in not only an increase in particle size but also a change in morphology. For samples 365_40 and 395_40, the nanoparticles displayed a hexagonal structure with diameters ranging from 30 to 35 nm. These observations indicate that higher radiation intensities promote the formation of larger and structurally distinct nanoparticles. Moreover, higher radiation intensities promote the formation of larger and structurally unique nanoparticles, transitioning from spherical to hexagonal shapes. Moreover, the presence of the (200) plane under 365 nm illumination—a wavelength-specific structural characteristic—suggests that certain light wavelengths can be strategically utilized to tailor nanoparticle properties for specific applications. The ability to manipulate the morphology and structure of nanoparticles through controlled irradiation parameters can significantly enhance the use of photodeposited nanoparticles in catalysis.

Considering the presence of gold nanoparticles in the synthesized materials, which could alter their optical properties, diffuse reflectance spectroscopy (DRS) was performed (Fig. 4).Fig.4 The DRS spectra for sample (a) 365 nm and (b) 395 nm series.

The deposition of gold nanoparticles on TiO2 was confirmed by the color change of the modified oxide powder, which turned from white to varying intensities of purple due to the surface plasmon resonance of nanocrystalline Au0 particles47,48. As shown in Fig. 4, which compares the absorption properties of the TiO2-Au series to bare TiO2, all modified materials exhibit a red-shift in the absorption edge. Gold-containing samples also display the typical plasmon resonance absorption band with a maximum around 550 nm43. This absorption occurs when the wavelength of the incident light is much larger than the nanoparticle size and is caused by light resonating with the surface plasmon oscillation, leading the free electrons in the metal to oscillate in resonance with the light’s frequency49,50. The intensity of the plasmon resonance band does not depend on the radiation intensity and wavelength, showing no appreciable difference in the absorption maximum. According to the available literature, the process of photodeposition of noble metals on the surface of semiconductors, including TiO2, does not lead to the modification of their conduction band energy51,52. Therefore, for all analyzed materials, the conduction band energy ranged between 3.1 and 3.2 eV, which is consistent with the values obtained for anatase.

The photoluminescence emission (Fig. 5) observed in the study predominantly arises from the recombination of excited electron–hole pairs. This phenomenon provides crucial insights into the efficiency of charge carrier separation in photoinduced processes. For all materials investigated, a single broad luminescence band centered near approximately 450 nm was noted. Literature reports that the photoluminescence spectrum of TiO2 features two primary emission peaks at approximately 396 and 462 nm, corresponding to photon energies of 3.13 and 2.68 eV, respectively53. Notably, broader luminescence bands have been observed, which are aggregates of these narrower peaks, as documented by Chang et al.54. These emission peaks are attributed to bandgap transitions, with emitted photon energies approximating the bandgap energy of anatase (387.5 nm) and the charge-transfer transitions from Ti3+ ions to oxygen anions in a TiO68− complex55. It is noteworthy that the synthesized TiO2-Au materials do not exhibit significant luminescence quenching, aligning with established scientific findings51. A reduction in luminescence intensity, indicative of enhanced charge carrier separation, was observed solely in materials synthesized under an LED radiation intensity of 40 mW/cm2. Contrary to platinum nanoparticles, gold nanoparticles do not markedly quench luminescence; instead, they enhance the photocatalytic effect primarily through plasmon resonance mechanisms56.Fig.5 The PL spectra for sample (a) 365 nm and (b) 395 nm series.

Photocatalytic activity

Metronidazole is widely utilized in medical treatments as an antibiotic and antiprotozoal medication, but its residues have been detected in various environmental waters, posing potential risks to aquatic life and human health57,58. The allowable concentration of metronidazole in water bodies is strictly regulated; however, its actual levels in wastewater can significantly exceed these limits. Given its pervasive use and potential environmental impact, metronidazole (initial concentration 20 mg/L) was chosen as a model pharmaceutical contaminant for this study59,60. The results of the photocatalytic degradation experiments are illustrated in Fig. 6.Fig.6 The photo-oxidation results of samples from (a) 365 nm and (b) 395 nm series.

Irrespective of the light source utilized during the photodeposition phase, the TiO2-Au composites exhibited superior metronidazole degradation capabilities compared to the bare TiO2 sample. For both sample sets (365 and 395 nm), the maximum removal efficiency of metronidazole, approximately 99%, was achieved with composites synthesized using maximal LED irradiation for gold photodeposition. A reduction in irradiation power during the photodeposition phase correspondingly led to decreased photo-oxidative capabilities of the resultant materials. For the specimens synthesized under minimal irradiation intensity, the degradation efficiencies were approximately 65% for both experimental groups. This indicates that the irradiation intensity during the photodeposition stage is crucial in determining the degradation efficiency of target pollutants, in this instance, metronidazole.

However, it is important to acknowledge that excessively high irradiation power may have detrimental effects, as elaborated in our previous work61. Notably, excessive irradiation could result in sample overheating, thereby increasing evaporation rates. Given that the photodeposition processes are conducted within sealed containers, heightened evaporation could consequently increase internal pressure, potentially leading to reactor damage in extreme scenarios. Therefore, an irradiation intensity range of 20–40 mW/cm2, as employed in our experiments, proved optimal for synthesizing efficacious photocatalysts while avoiding the adverse consequences of excessive light power62,63. Additionally, our findings indicated no discernible impact of the photodeposition wavelength on metronidazole removal efficiency, with differences between individual composites across both experimental sets being around 5%, likely attributable primarily to measurement errors. It is noteworthy that these findings contrast with our prior observations concerning the photodeposition of platinum, where the wavelength of LED light employed in the synthesis distinctly influenced the resultant photocatalytic activity21,51.

In summary, the photodeposition irradiation intensity critically influences the morphological, optical, and surface-reactive properties of TiO2-gold nanoparticle composites, significantly enhancing their photocatalytic performance. This enhancement is achieved through morphological changes that increase the available active surface area, facilitating various photocatalytic pathways and improving the degradation rates of organic contaminants44,64. Additionally, these morphological changes affect the optical properties of the composites through plasmonic resonance effects, thereby enhancing light absorption and the photocatalytic efficacy of the TiO2 matrix under solar irradiation65,66. The electronic interactions promoted by increased nanoparticle size and altered morphology due to higher irradiation levels improve charge transfer processes and stabilize charge carriers, thereby optimizing the catalyst’s performance for environmental applications such as wastewater treatment67,68.

The revised Langmuir–Hinshelwood model offers a suitable framework for comprehending the degradation mechanism, aligning with the surface reaction concept extensively discussed in prior research69–71. According to Eqs. (2), (3), the apparent values of parameter k1 for each catalyst were derived by analyzing the slope of the ln Ct/C0 versus time plot. These computed values are outlined in Table S1, which is available in the Supplementary Materials for comparative analysis. In accordance with the data previously detailed, the 365_40 catalyst demonstrates the most substantial reaction rate constant, quantified at 0.0579 min−1 among the evaluated series of materials. Conversely, the 395_40 material displayed a consistent reaction rate constant of 0.0554 min−1. It is important to note that variations among the series of materials examined do not lead to significant differences in reaction rate constants. The primary determinant affecting the reaction kinetics is the intensity of irradiation; specifically, the catalysts 365_5 and 395_5 showed reaction rate constants of 0.0108 and 0.0101 min−1, respectively.

The 365_40 and 395_40 photocatalysts were selected for the reusability test. Five consecutive cycles of MNZ photodegradation were conducted to assess their photocatalytic reusability, as shown in Figure S5. At the end of each cycle, the photocatalyst was separated from the reaction suspension using filtration. The separated photocatalyst was then reused without any further treatment. The efficiency of photocatalytic degradation decreased by approximately 5% after the 5th cycle compared to the 1st cycle. This slight decrease in activity after each irradiation cycle could be attributed to photocatalyst losses during the separation process. Additionally, it was confirmed that no changes in the morphology of the tested photocatalysts occurred during the reusability test, as evidenced by the presented TEM images (see Figure S6 in the Supplementary Materials).

Figure S7, presents the outcomes of experiments in which diverse scavengers were incorporated into a metronidazole solution with TiO2-Au photocatalysts. These scavengers were employed to capture photo-induced electrons, holes, and key reactive oxygen species, notably hydroxyl radicals (*OH) and superoxide radicals (*O2–). The introduction of scavengers for electrons and holes, such as ammonium oxalate and silver nitrate, led to an approximate 10% reduction in the photooxidation efficiency of MNZ72,73. This moderate reduction is attributed to the observation that photodeposition of gold onto the TiO2 surface does not effectively quench luminescence, as evidenced in Fig. 5, thereby only marginally enhancing the charge carrier separation process. Furthermore, the introduction of tert-butanol, a scavenger targeting hydroxyl radicals, resulted in a significant reduction in the photodegradation efficiency by approximately 60% for the tested photocatalytic materials, underscoring the pivotal role of *OH radicals in the photooxidation mechanism. Conversely, the targeting of superoxide radical anions (*O2–) markedly impacted the MNZ degradation efficiency, with reductions of 45 and 42% for materials 365_40 and 395_40, respectively. This delineates the substantial contribution of these radicals to the photocatalytic degradation pathway. These results corroborate the hypothesis that the generation of reactive oxygen species (ROS), particularly hydroxyl and superoxide radicals, predominantly governs the photocatalytic processes in these aqueous systems, as indicated by the studies of Lopis et al.74 and Kusiak-Nejman et al.75. The major steps in the photocatalytic mechanism are illustrated in the equations below.4 TiO2-Auphotocatalyst+hv→e-+h+

5 e-+O2→∗O2-

6 h++H2O/OH-→H++OH∗

7 ∗OH+metronidazole→degradationproducts

8 ∗O2-+metronidazole→degradationproducts

In the mass spectrometric analysis presented through mass chromatograms, ions at m/z 60, 71, 83, 114, 124, and 152 were detected, as detailed in Table S2 of the Supplementary Materials. These ion signals align with the intermediate degradation products of metronidazole, according to the degradation pathway outlined by Zia et al.76. The first identified intermediate is (1Z)-2-methyl-5-nitroso-1-(2-oxoethylidene)-1H-imidazol-1-ium, formed by the interaction with reactive superoxide radicals. This is followed by the formation of (4-oxo-4H-3λ5-imidazol-3-ylidene) acetaldehyde through hydroxyl radical attack. A subsequent attack by hydroxyl radicals produces the intermediate (1E)-5-hydroxy-1-(hydroxymethylidene)-1H-imidazole-1,3-diium58. The cleavage of the alkyl chain attached to the nitrogen atom leads to the creation of 3,5-dihydro-4H-imidazole-4-one. The intermediate corresponding to m/z=71, 2,3-dihydro-1H-imidazol-1-ium, forms through ring opening induced by a superoxide radical, followed by the transformation into ethene-1,2-diol (m/z 60) under the attack of hydroxyl radicals. Subsequent photooxidation processes and interactions with reactive oxygen species eventually result in the complete mineralization of the compound into carbon dioxide and water77,78. The entire photodegradation pathway is depicted in Fig. 7.Fig.7 Proposed mechanism of photodegradation of MNZ using TiO2-Au photocatalyst.

Conclusion

The comprehensive study conducted on TiO2-Au composites through systematic manipulation of LED radiation parameters—wavelength and intensity—has substantiated the significant influence of these factors on the physicochemical and photocatalytic characteristics of the synthesized materials. XRD, SEM, and XPS analyses confirmed the successful deposition of gold nanoparticles, highlighting an increase in gold content with higher intensity and shorter wavelength irradiation. The surface morphological studies, particularly through high-resolution transmission electron microscopy (HR-TEM), illustrated that varying the radiation intensity affects not only the size but also the shape of gold nanoparticles. Lower intensities typically produced spherical nanoparticles, while higher intensities resulted in hexagonal shapes, demonstrating that radiation parameters can be finely tuned to control nanoparticle morphology, a key factor in catalytic performance. Furthermore, the DRS results provided insights into the light-absorption characteristics of the composites, showing a red-shift in the absorption edge and the distinctive plasmon resonance absorption band around 550 nm. These optical properties are crucial for applications where light harvesting is essential, such as in solar energy conversion and environmental cleanup. Photocatalytic tests using metronidazole indicated that the TiO2-Au composites, especially those synthesized with higher LED intensities, significantly outperformed pure TiO2 in degrading this contaminant. The enhanced photocatalytic activity is largely due to the increased surface area and modified optical properties resulting from gold nanoparticle deposition. The presence of gold facilitates better light absorption across a broader spectrum, primarily through plasmon resonance, thereby enhancing the photocatalytic efficacy.

This research provides a clear pathway for tailoring the characteristics of photocatalytic materials through controlled photodeposition techniques, thereby expanding their practical applications in environmental remediation. The ability to manipulate the size, shape, and distribution of nanoparticles on TiO2 opens up new avenues for the design of highly efficient photocatalysts, making this approach a valuable addition to the field of materials science and photocatalysis.

Supplementary Information

Supplementary Information.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-71605-x.

Acknowledgements

This work was supported by the National Science Centre, Poland, under grant number 2023/07/X/ST5/00022.

Author contributions

A.K.: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Resource, Visualization, Writing—Original Draft, Writing—Review & Editing, Supervision, Funding acquisition, Project administration.

Data availability

The data that support the findings of this research are available from the corresponding author upon reasonable request.

Competing interests

The author declares no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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