Photocatalytic Activity of Cu₂O-Loaded TiO₂ Heterojunction Composites for the Simultaneous Removal of Organic Pollutants and Bacteria in Indoor Air

Abstract

This research investigates the enhanced photocatalytic activity of cuprous oxide (Cu₂O) nanoparticles (NPs)-titanium dioxide (TiO₂) nanotube (NT) composites for air purification, focusing on the removal of volatile organic compounds (VOCs) and Escherichia coli (E. coli) bacteria under simulated sunny light. Cu₂O-NPs were successfully deposited onto TiO₂-NTs via the successive ionic layer adsorption and reaction method. The resulting p- and n-type semiconductor heterojunction nanocomposites were characterized using various techniques, including scanning electron microscopy, transmission electron microscopy, ultraviolet–visible-light spectroscopy, and chlorinated radicals. The photocatalytic activity was evaluated for different VOCs present in indoor air (butadione, chloroform, and butyraldehyde) in the presence of E. coli bacteria. The results showed that the Cu₂O-NPs/TiO₂-NTs composites exhibited enhanced photocatalytic activity compared to pure TiO₂-NTs. The Langmuir–Hinshelwood model was used to describe the degradation kinetics, revealing that Cu₂O loading and the nature of the target pollutant influence the photocatalytic efficiency. This study has also highlighted the role of chlorinated radicals in the degradation process, especially for chloroform. The degradation process of chloroform generated chlorine radicals, which not only contributed to the degradation of other VOCs, but also enhanced the overall oxidative capacity of the system. This synergistic effect was observed to accelerate pollutant removal and improve the antibacterial efficacy against E. coli. The Cu₂O-NPs/TiO₂-NTs composites demonstrated significant reusability and antibacterial properties, highlighting their potential for sustainable indoor air purification applications.

1. Introduction

Air pollution has emerged as a critical concern for society, the economy, and the scientific community. Its detrimental effects on human health, ecosystems, and the environment pose a significant threat to sustainability [1,2,3]. The European Environment Agency has revealed that Europeans spend 90% of their time indoors [4], while the American and Canadian populations spend 87% indoors and an additional 6% in vehicles [5]. This highlights the importance of addressing indoor air quality [6,7]. Volatile organic compounds (VOCs) and microorganisms, such as bacteria and viruses, are major contributors to indoor air pollution, causing respiratory problems, reduced crop yields, and unpleasant haze, particularly in densely populated urban areas and industrial zones [4,5,8].

To mitigate the adverse effects of air pollution, various techniques have been employed to treat industrial emissions and reduce organic pollutants [9]. These include methods such as condensation, plasma–catalysis combinations, filtration, biological degradation, and adsorption [10,11,12,13,14]. However, these techniques often generate hazardous byproducts or simply transfer the pollutants from one phase or location to another [15].

Heterogeneous photocatalysis has gained prominence as a clean and efficient technology for air purification [16,17,18]. This process utilizes semiconductors, typically titanium dioxide (TiO₂), to decompose VOCs and microorganisms, leading to their complete mineralization into harmless end products, such as CO₂ and H₂O [19,20,21,22]. Representative photocatalysts include metal oxides, such as TiO₂, SrTiO₃, WO₃, ZrO₂, KTaO₃, ZnO, SnO₂, In₂O₃, and Bi₂MoO₆; metal sulfides, such as ZnS, CdS, and MoS₂; the nitride C₃N₄; and other compounds such as BiVO₄, K₄Nb₆O₁₇, and perovskite [23,24,25].

TiO₂, renowned for its chemical stability and photocatalytic activity, has been extensively researched for its applications in air and water purification [26,27]. Recent advancements have focused on nanostructuring TiO₂ into forms such as nanoparticles, nanowires, and nanotubes (NTs) to enhance its surface area and photocatalytic efficiency [16,17]. TiO₂-NTs, fabricated through anodization, offer a large surface area, rapid charge transport, and high chemical stability, making them ideal for photocatalytic applications [17,28,29]. Nanotubular layers of TiO₂ exhibit exceptional photocatalytic capabilities, surpassing those of nanoparticulate layers [30]. These layers can be fabricated over large areas (approximately 50 cm²), making them ideal for photocatalysis in gaseous environments. However, their wide band gap (3.2 eV in its anatase phase) limits their light absorption to the ultraviolet (UV) range, and a high recombination rate of photogenerated charges hinders their performance [31].

To overcome these limitations, researchers have explored sensitizing TiO₂-NTs with various semiconductors possessing suitable band structures for visible-light (Vis) absorption and improved electron transport. Chalcogenides, such as CuInS₂, CdS, and PbS, have been utilized, however, they suffer from stability issues and the release of toxic heavy metal ions [32,33,34]. Cuprous oxide (Cu₂O), a p-type semiconductor with a direct bandgap of 2.0 eV, has emerged as a promising and stable sensitizer for TiO₂-NTs [35]. Studies have demonstrated the enhanced photocatalytic activity of Cu₂O/TiO₂-NTs heterojunctions in degrading organic pollutants [36,37]. Xiang et al. [36] have demonstrated a significant enhancement in photocatalytic efficiency by electrodepositing Cu₂O nanoparticles (NPs) onto TiO₂ nanotubes (TiO₂-NTs). The resulting Cu₂O/TiO₂-NTs photocatalysts achieved a 2.3-fold increase in performance compared to pure TiO₂-NTs. Wang et al. [37] have developed p-Cu₂O/n-TiO₂-NTs heterojunction photoelectrodes that exhibit improved photoelectrochemical activity and superior stability for Rhodamine B degradation.

The combination of TiO₂ with semiconductors possessing lower band gaps has demonstrated significant effectiveness in enhancing photocatalytic activity driven by sunlight. This strategy, known as heterojunction formation, leverages the unique energy band alignments between TiO₂ and the Vis-absorbing semiconductors. Based on these band positions, three distinct types of heterojunctions can be identified, each with specific characteristics and implications for photocatalytic performance. In Type-I heterojunctions, the conduction band (CB) of the sensitizer lies at a lower energy level than the CB of TiO₂. Conversely, in Type-II heterojunctions, the valence band (VB) of the sensitizer is positioned at a higher energy level compared to the VB of TiO₂. Cu₂O, a semiconductor with a band gap in the 1.9–2.3 eV range, exemplifies this Type-II configuration. Finally, in Type-III heterojunctions, the sensitizer’s energy levels are positioned between the CB and VB of TiO₂. Studies have shown that Type-II heterojunctions exhibit significantly higher photocatalytic efficiency than Type-I systems, while Type-III systems show negligible improvement. The enhanced Vis photocatalytic activity of Type-II heterojunctions stems from the efficient transfer of holes between the VB of the sensitizer and the VB of TiO₂ [38]. However, systematic investigations exploring the full potential of these heterojunctions for various photoelectrochemical applications remain limited [34,39].

The present study, which is a continuation of our previous investigations on TiO₂ structuration [17], focuses on the fabrication and characterization of Cu₂O/TiO₂-NTs nanocomposites using the successive ionic layer adsorption and reaction (SILAR) method. This technique allows for the controlled deposition of Cu₂O nanoparticles onto TiO₂-NTs at room temperature, facilitating the formation of p-n heterojunctions with tunable properties. The photocatalytic performance of these nanocomposites is evaluated for the removal of E. coli bacteria and specific VOCs under simulated sunny light.

2. Results and Discussion

2.1. Characterization Results of Cu₂O-NPs/TiO₂-NTs

The scanning electron microscopy (SEM) images confirmed the successful decoration of TiO₂-NTs with Cu₂O-NPs via the SILAR technique. These images effectively demonstrated the influence of increasing SILAR cycles on both the morphology and size distribution of the Cu₂O-NPs. Starting with Figure 1a, which shows the top view of pure TiO₂-NT arrays, and progressing through Figure 1b–e, corresponding to SILAR cycles ranging from 3 to 15, a clear evolution in the development of Cu₂O-NPs on the NP surface is shown.

Untreated TiO₂-NTs exhibit a highly ordered and aligned structure, with the NTs perpendicular to the Ti substrate. The average diameter of the NTs, as determined from the histogram in Figure 1a₁, is approximately 100 nm. The inset in Figure 1a shows a cross-sectional view, highlighting vertically aligned pure TiO₂-NTs. The surface appears homogeneous, and the TiO₂-NTs’ length is approximately 12 μm. The evolution of Cu₂O-NP deposition was observed across different SILAR cycles. At lower cycle numbers (as shown in Figure 1b,c), Cu₂O-NPs exhibited uniform distribution on the TiO₂-NTs surface with consistent particle sizes. As the number of SILAR cycles increased (Figure 1d,e), both particle density and agglomeration increased, resulting in the formation of larger clusters (Figure 1e). The Cu₂O-NPs ranged in size from approximately 50 nm to 150 nm (Figure 1f). An increase in SILAR deposition cycles from 10 to 15 resulted in enhanced coverage of the NTs’ surface by Cu₂O-NPs.

The energy-dispersive X-ray spectroscopy analysis of the 10-cycle deposited Cu₂O-NPs/TiO₂-NTs, shown in Figure 1g, confirmed the presence of the elements Cu, O, and Ti. However, at this stage, it was not possible to definitively confirm the formation of crystalline stochiometric Cu₂O-NPs due to the presence of oxygen in both TiO₂ and Cu₂O. To further investigate the existence of crystalline Cu₂O-NPs, additional characterization techniques were employed, including transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and X-ray diffraction (XRD) analyses.

The TEM images (Figure 2a,b) of Cu₂O-NP-decorated TiO₂-NTs prepared using 15 SILAR cycles revealed two distinct populations of Cu₂O-NP, as follows: smaller particles incorporated inside of the NTs, with a size less than the TiO₂-NTs’ opening diameter, and larger particles with a diameter greater than 100 nm located at the periphery of the NT walls. The growth of copper oxide NPs on the surface of the NTs created a barrier effect, preventing the entry of materials inside the NTs. This blockage mechanism ultimately limited the growth of the inner Cu₂O-NPs, resulting in a significant size difference between the inner and outer NP populations. The HRTEM image (Figure 2c) revealed distinct lattice fringes with measured d-spacing of 0.306 nm and 0.2945 nm, corresponding to the (110) crystallographic planes of the Cu₂O structure. The selected area electron diffraction (SAED) pattern (Figure 2d) further confirmed the crystalline nature of Cu₂O-NPs, showing diffraction rings from various crystal planes, including (111), (011), (002), and (220), indicating the formation of well-crystallized Cu₂O-NPs with a cubic crystal structure.

XRD analysis (Figure 3a) was used to investigate the composition and phase purity of products synthesized across different SILAR cycles. The XRD patterns, in agreement with the SAED results, showed characteristic diffraction peaks corresponding to the (110), (111), (200), and (220) crystallographic planes of the Cu₂O phase. The formation of crystalline copper oxide was confirmed by these specific peaks, which are particularly evident in Figure 3c, where the most prominent peak at 2θ = 37° corresponds to the (111) plane. For the 3-, 5-, and 10-cycle samples, the other Cu₂O-NP XRD peaks were not visible, mainly due to the dominant presence of TiO₂ compared to the small number of dispersed Cu₂O-NPs. The broadening of the Bragg diffraction peaks, caused by lattice strain, was calculated using the Wilson and Stokes formula (Equation (1)) [40]. Multiple peaks from the XRD patterns were analyzed to evaluate the extent of broadening due to strain.

The Wilson and Stokes equation used to calculate the strain from the broadening of the Bragg diffraction peaks is written as follows:

$$\epsilon ={\displaystyle \frac{{\beta }_{\tau }}{4tan\theta }}$$

(1)

where ε is the strain, β_τ is broadening due to strain, and θ is the position peak in radians.

The total broadening (β_τ) is due to the combined effect of crystalline size and lattice strain, which can be written as follows:

$${\beta }_{\tau }={\beta }_D+{\beta }_{\epsilon }$$

(2)

where ${\beta }_D$ is broadening due to crystallite size and ${\beta }_{\epsilon }$ is the broadening due to microstrain.

We know that the crystalline size (D) can be calculated using the Scherrer equation [41], as follows:

$$D={\displaystyle \frac{K\lambda }{{\mathsf{\beta }}_{\mathrm{D}}cos\theta }}$$

(3)

where K = 0.9 is the shape factor, λ = 0.15406 nm is the wavelength of X-ray diffraction, D is the crystallite size, and $\theta$ is the peak position in radians.

Therefore, ${\beta }_D$ is given by the following:

$${\mathsf{\beta }}_{\mathrm{D}}={\displaystyle \frac{K\lambda }{Dcos\theta }}$$

(4)

Similarly, the microstrain is as follows:

$${\beta }_{\epsilon }=4\mathrm{Ɛ}\mathrm{t}\mathrm{a}\mathrm{n}\mathsf{\theta }$$

(5)

Rearranging Equation (2) gives the following:

$${\beta }_{\tau }cos\theta ={\displaystyle \frac{K\lambda }{D}}+4\mathrm{Ɛ}\mathrm{s}\mathrm{i}\mathrm{n}\mathsf{\theta }$$

(6)

To assess the influence of Cu₂O deposition on crystallite size and lattice deformation in the Cu₂O-NPs/TiO₂-NTs samples, we performed a Williamson–Hall (W-H) analysis. The W-H plots, shown in Figure 3c, illustrate the relationship between βcos(θ) and 4sin(θ) at different Cu₂O deposition concentrations, corresponding to 0, 3, 5, 10, and 15 SILAR cycles. The extracted values for the crystallite size (determined from the inverse of the intercept) and lattice strain (derived from the slope of the linear fit) are summarized in Figure 3d. The analysis revealed that lattice strain increases with the number of SILAR cycles. Lattice strain increases with the amount of Cu₂O deposited. This suggests a direct correlation between the Cu₂O doping concentration and the resulting lattice strain in the samples.

The reflectance spectra shown in Figure 4a reveal a clear reduction in reflectance in the visible range as the number of SILAR cycles increases for the Cu₂O-TiO₂ composites. This trend is particularly noticeable with higher Cu₂O loadings (3, 5, and 10 cycles), where the reflectance decreases, indicating greater absorption of Vis. The presence of Cu₂O-NPs, known for their strong absorption in this region, is probably responsible for this effect. It is interesting to note that the sample with 15 SILAR cycles exhibits a slightly higher reflectance in the visible range, suggesting a possible change in the optical properties of Cu₂O with increased deposition. Overall, the spectra demonstrate a clear link between the number of SILAR cycles and the optical behavior of the Cu₂O-TiO₂ composites, showing that controlled deposition can be used to fine-tune the material’s optical properties.

This behavior is consistent with the effective bandgap energy trends shown in Figure 4b. The Tauc plots indicate a reduction in effective bandgap energy with increased Cu₂O deposition, which is consistent with the observed decrease in reflectance. A comparison between pure TiO₂-NTs and Cu₂O-TiO₂ composites highlights this relationship. TiO₂-NTs, with their higher intrinsic bandgap energy (3.08 eV), display higher reflectance in the visible range. In contrast, the Cu₂O-TiO₂ composites, which have a smaller effective bandgap due to Cu₂O’s presence, exhibit lower reflectance, particularly in the visible range. This is because a smaller effective bandgap allows greater absorption of Vis, which reduces reflectance.

After a detailed analysis of the effective bandgap values obtained by the Kubelka–Munk method and the Tauc plot measurement, we observed that samples with 10 and 15 SILAR cycles showed remarkably similar effective bandgap energies. The estimated experimental error for our effective bandgap measurements is around ±0.05 eV, determined by repeated measurements and statistical analysis over several sample regions. This margin of error takes into account the instrumental limitations, the sample heterogeneity, and the subjectivity inherent in determining the linear region of the Tauc plot for extrapolation. Given this margin of error, the small difference between the 10- and 15-cycle samples (around 0.03 eV) may not be statistically significant, suggesting that we may have reached a saturation point in effective bandgap modification beyond 10 SILAR cycles. This observation aligns with the reflectance data shown in Figure 4a, where the 15-cycle sample unexpectedly showed a slightly higher reflectance in the visible range compared to the 10-cycle sample. We attribute this saturation behavior to several possible mechanisms, including the formation of thicker Cu₂O layers that can introduce quantum confinement effects, the potential agglomeration of Cu₂O nanoparticles at higher numbers of cycles leading to reduced effective surface area, and changes in the properties of the Cu₂O-TiO₂ interface as layer thickness increases. These results suggest that there is an optimum threshold for SILAR cycle application (around 10 cycles in our system), beyond which further cycles may not produce significant improvements in optical properties. The controlled deposition of Cu₂O on TiO₂-NTs offers a promising approach to modifying their effective bandgap and optical properties, with potential applications in photocatalysis and solar energy harvesting; however, this requires the optimization of deposition parameters to achieve maximum efficiency.

2.2. Photocatalytic Experiments

2.2.1. Effect of the Successive Ionic Layer Adsorption and Reaction Deposition Cycle

The photocatalytic performance of TiO₂-NTs and Cu₂O-NPs/TiO₂-NTs composites, synthesized with different numbers of SILAR cycles (3, 5, 10, and 15), was evaluated for the degradation of three pollutants of three different indoor airs, as follows: butane-2, 3-dione (C₄H₆O₂), chloroform (trichloromethane: CHCl₃), and butyraldehyde (C₄H₈O). The photocatalytic experiments were conducted under Vis irradiation, using a reactor setup where the catalyst samples (dimensions: 1.2 cm × 2.5 cm) were placed at the bottom. An initial concentration of 34 mmol/m³ was used for each pollutant. Prior to illumination, the reactor was kept in the dark for one hour to establish adsorption–desorption equilibrium between the pollutants and the catalyst surface.

Figure 5 presents the photocatalytic degradation of three different pollutants, BUT (Figure 5a), chloroform (Figure 5b), and butyraldehyde (Figure 5c), using Cu₂O-NPs/TiO₂-NTs composites synthesized with varying numbers of SILAR cycles (3, 5, 10, and 15). The plots revealed distinct trends for each pollutant. The figures present a comparative analysis of the photocatalytic efficiency of Cu₂O-sensitied TiO₂-NTs with the pure NTs under UV–Vis. The results showed that Cu₂O-NPs/TiO₂-NTs exhibit enhanced photocatalytic activity compared with pure TiO₂-NTs, regardless of the number of SILAR cycles. VOC degradation increased with the number of SILAR cycles, indicating greater Cu₂O loading. For example, with 10 SILAR cycles, the degradation of the pollutants (for the three pollutants) reached approximately 100% in 200 min. However, after 15 SILAR cycles, pollutant degradation decreased.

This decrease in photocatalytic activity under more SILAR cycles (15 compared with the others) could be attributed to the aggregation of Cu₂O-NPs, which leads to a reduction in the active sites available for photocatalysis. Aggregation potentially reduces the efficiency of charge carrier transfer and separation, ultimately reducing photocatalytic efficiency. Previous studies have confirmed that smaller NP sizes are generally associated with better charge transfer and photocatalytic performance [42,43]. For BUT, the degradation rate increased with more SILAR cycles, suggesting enhanced photocatalytic activity with increased Cu₂O loading. This trend was also observed for chloroform degradation, where the composites with more SILAR cycles exhibited more efficient removal. However, butyraldehyde degradation showed a different pattern, with a slight decrease in degradation under more SILAR cycles. This suggests that the optimal Cu₂O loading for butyraldehyde degradation might be lower than that observed for the other two pollutants.

Overall, these results highlight the importance of optimizing Cu₂O loading for specific pollutants to achieve maximum photocatalytic efficiency. The distinct degradation patterns observed for the three pollutants indicate that the Cu₂O-NPs/TiO₂-NTs composite’s photocatalytic performance is influenced by the chemical properties of the target pollutant.

To investigate the degradation kinetics, we varied the initial concentration of chloroform using Cu₂O-NPs/TiO₂-NTs deposited over 10 cycles. Figure 6a illustrates the impact of varying chloroform inlet concentrations (1, 3, 5, and 7 g/m³) on the degradation rate as a function of irradiation time. A reduction in the initial concentration indicates enhanced degradation efficiency. This behavior can be attributed to the increased availability of photocatalytic active sites at lower initial concentrations [44,45]. Specifically, the number of pollutant molecules effectively participating in the reaction was proportional to the inlet concentration [46,47,48,49]. Consequently, this led to a decrease in degradation efficiency, probably due to the limited adsorption capacity of the active sites on the catalyst surface [49,50,51,52,53].

To describe the chemical transformations involved in the heterogeneous photocatalytic degradation of chloroform on Cu₂O-NPs/TiO₂-NTs deposited over 10 cycles, the Langmuir–Hinshelwood (L-H) kinetic model was employed to analyze the experimental data [54,55,56,57,58]. The L-H model, expressed by the following reaction rate equation, provides a general model for explaining the adsorption and subsequent degradation of chloroform.

The equation $r_0=-{\displaystyle \frac{d\left[VOC\right]}{dt}}=k_c{\displaystyle \frac{{K\left[VOC\right]}_0}{{1+K\left[VOC\right]}_0}}$ describes the initial photocatalytic degradation rate, where r₀ (mmol/g_cat m³ s) is the initial photocatalytic degradation rate, [VOC]₀ is the initial chloroform concentration (mmol/m³), K is the adsorption constant (m³/mmol), and k_c is the kinetic constant (mmol.m⁻³ s g_TiO2) at maximum coverage for the experimental conditions.

By plotting 1/r₀ against 1/[COV]₀, the values of k_c and K can be determined. As shown in Figure 6b, the kinetic and adsorption constants were estimated (

Table 1. L-H constants (kc and K) based on Cu₂O-NPs/TiO₂-NTs-10C catalyst.

Catalyst	kc: Kinetic Constant of L-H (mmol.m−3.s−1.gTiO2−1)	K: Adsorption Constant of L-H (m3.mmol−1) × 10−3
Cu2O- NPs/TiO2-NTs-10C	1.643	5.915

) using the linearized Langmuir–Hinshelwood equation, as follows:

$${\displaystyle \frac{1}{r_0}}={\displaystyle \frac{1}{k_{c}K}}\times {\displaystyle \frac{1}{{\left[VOC\right]}_0}}+{\displaystyle \frac{1}{k_C}}$$

The kinetic and adsorption constants based on the Langmuir–Hinshelwood (L-H) model are summarized in Table 1. The results demonstrate that Cu₂O-modified TiO₂ nanotubes (NTs) enable faster VOC removal compared to other catalysts (

Table 2. Kinetic constants for the photocatalytic degradation of various VOCs using different TiO₂-based photocatalysts.

Ref.	Catalyst	Deposition Method	VOCs Target	Irradiation Source	Kinetic Constant
[53]	Pt/TiO2-NTs	Electrodeposition	Ethyl Acetate	Visible light	0.245 (mg.m−3.s−1)
[53]	Pt/TiO2-NTs	Photodeposition	Ethyl Acetate	Visible light	0.195 (mg.m−3.s−1)
[17]	Cu2O-NPs/TiO2-NTs	Electrodeposition	Butane-2,3-Dione	UV–Vis light	0.85 mmol.m−3.s−1
[54]	AgxO/Ag/TiO2-PES	HiPIMs	Butane-2,3-Dione	UV–Vis light	54 × 10−3 mmol.m−3.s−1
[55]	Pt-NPs/TiO2-NTs	Electrodeposition	Cyclohexane	Visible light	64 × 10−3 (mg.m−3.min−1)
[44]	TiO2/PES	Hydrothermal method	Butyraldehyde Isovaleraldehyde	UV lamp	0.1513 mmol.m−3.s−1 0.2953 mmol.m−3.s−1
[44]	GFT-TiO2	Provided by Ahlstrom Research/ Services	Butyraldehyde	UV lamp	0.4134 mmol.m−3.s−1.gTiO2−1
Present study		SILAR	Chloroform	UV–Vis lamp	1.643 mmol.m−3.s−1.gTiO2−1

).

Table 2 summarizes the removal efficiency and kinetic constants for the photocatalytic degradation of VOCs using different titanium-dioxide-based photocatalysts.

2.2.2. Simultaneous Oxidation of Volatile Organic Compounds in a Binary Mixture System

The photodegradation of two equimolar (17 mmol/m³) indoor air pollutants (CHCl₃/C₄H₆O₂) was investigated. As presented in Figure 7, the removal profiles of chloroform and butadione were comparable. Figure 7 reveals that butadione (BUT) exhibits slightly faster photocatalytic degradation kinetics compared to chloroform. This difference can be attributed to many factors, such as the chemical composition of the pollutants, their respective affinities for the catalyst surface, and the quantities of intermediate byproducts formed during degradation. These factors likely contribute to a competitive adsorption effect between the pollutants for the available active catalytic sites.

Figure 7 shows that the removal efficiency of chloroform and butadione in the binary mixture (CHCl₃/C₄H₆O₂) is lower than that of the individual compounds. This indicates that VOC removal kinetics are slowed down in the presence of the binary mixture. For instance, in the case of chloroform, this slowdown may partly result from the involvement of other primary and/or intermediate VOCs consuming chlorine-containing species [18]. A similar behavior was observed by Debono et al. [47] in their study of a ternary mixture of toluene, decane, and trichloroethylene (TCE), where the reduced removal kinetics of TCE were linked to chlorine-containing species participating in the degradation of other primary or intermediate VOCs (i.e., the consumption of chlorine-containing species by other VOCs). For binary VOC mixtures, the decrease in removal efficiency can be attributed to competitive adsorption between the two compounds for access to the catalyst surface [1,19,48]. Chen et al. [49] have also reported an inhibitory effect when studying a ternary VOC mixture using equimolar concentrations of toluene, ethanethiol, and ethyl acetate, linking the effect of the mixture on VOC elimination to the respective adsorption capacities of the VOCs on the photocatalyst surface.

2.2.3. Simultaneous Oxidation of Chloroform and Bacteria (E. coli)

After studying the effect of the existence of two kinds of indoor air pollutants on Cu₂O-NPs/TiO₂-NTs performance, the efficient photocatalyst was also tested for its ability to deactivate bacteria in the presence of chloroform compounds. This study provides valuable insights, such as the capability of the developed photocatalyst to oxidize multiple pollutants simultaneously, reflecting real indoor air conditions, the impact of multiple pollutants on photocatalytic performance, and the antibacterial properties of the photocatalyst. To meet these objectives, an initial concentration of approximately ~2 × 10⁸ CFU/mL of the E. coli strain was dispersed on the surface of the photocatalyst.

Figure 8 illustrates the concurrent removal of E. coli and CHCl₃ as a function of irradiation time. The bacterial inactivation is shown with histograms, revealing a decline in E. coli concentrations, which leads to complete bacterial death after 300 min of Vis exposure. The results showed that the Cu₂O-modified TiO₂-NTs lead to faster bacterial inactivation compared to other types of catalysts [54,55,56]. It is also important to note the significant reduction in bacteria and CHCl₃ with pure TiO₂-NTs, particularly during prolonged Vis irradiation. Several studies have confirmed the antibacterial activity of TiO₂ under Vis [50,51]. Previous studies have reported that TiO₂ thin films can damage E. coli cell membranes and kill the cells by releasing endotoxins [52]. The aim of combining target molecules with bacteria is to assess the effectiveness of the advanced oxidation process under conditions that more closely resemble real-life scenarios, where large quantities of pollutants need to be removed simultaneously. This approach also allows for the evaluation of pollutant degradation during the mixing process.

In general, the enhanced antibacterial activity observed with Cu₂O-modified TiO₂-NTs arises from the inherent antibacterial properties of each semiconductor individually [33,57,58], as well as the synergistic effect resulting from their combination. Similarly, visible-light irradiation enhances the intrinsic antibacterial activity of Cu(I), which is consistent with previous reports [58] that have demonstrated the significant bacterial inactivation capabilities of Cu₂O. According to Sunada et al. [59], contact between Cu species and bacterial cells can extract electrons from the cells, leading to protein denaturation.

2.3. Cu₂O-NPs/TiO₂-NTs Catalyst Recyclability

The reusability of the catalyst is a crucial factor to consider, as it impacts its potential application in environments contaminated by toxic gases [20]. The long-term stability of TiO₂ nanotubes is influenced by various factors, including environmental conditions, anodizing parameters, and the presence of specific electrolytes. According to research, TiO₂ nanotubes can retain their structural integrity and functionality under certain conditions, although degradation can occur under other conditions. Wierzbicka et al. demonstrated that the treatment of anodic TiO₂ nanotubes at 70 °C in ethanol for 1 h improved not only their photoelectrochemical activity, but also their morphological stability by reducing both surface cracks and eliminating electrolyte contamination. Remarkably, they observed that the resulting amorphous or quasi-amorphous structure supported the long-term stability of the Ti³⁺-Vo states, even in the presence of oxygen, for up to two months [60]. Complementing these results, Abela et al. studied the aging process of TiO₂ nanotubes over 52 weeks and found that the composition of the electrolytes during anodization significantly affected stability. Their results showed that nanotubes produced in organic electrolytes retained exceptional physical and phase stability, with no observable degradation or change in crystal structure [61].

The catalyst was tested for reuse across four consecutive cycles. The Cu₂O-NPs/TiO₂-NTs-10C composites were thoroughly washed after each photodegradation cycle and reused. Figure 9a demonstrates that there was no loss of photoactivity in the sample after four cycles.

To assess the structural stability of Cu₂O/NTs-TiO₂ composites, X-ray diffraction (XRD) analyses (presented in Figure 9b) were carried out after photocatalytic air treatment tests. A comparison of the two diffractograms reveals that all characteristic diffraction peaks remain unchanged at their respective 2θ angles after photocatalytic testing. Notably, the main peaks, which correspond to the crystalline phases of TiO₂ and Cu₂O, are still present. Although a slight reduction in peak intensity is observed, the absence of new diffraction peaks or significant shifts in peak positions confirms that no additional crystalline phases were formed during the photocatalytic process. The slight reduction in peak intensity observed after the photocatalytic process can be attributed to the change in the Cu₂O/NTs-TiO₂ composite surface. These may include minimal adsorption of reaction byproducts, the formation of thin amorphous layers on active sites, or small lattice distortions caused by the generation of electron–hole pairs during photocatalysis. Importantly, the preservation of all characteristic peak positions confirms that the crystalline structure of the catalyst remains unchanged, explaining the stability of its photocatalytic performance over several cycles, despite these minor surface alterations. This result underlines the structural stability of our photocatalyst.

These results highlight the significant potential of Cu₂O-NPs/TiO₂-NTs-10C as a photocatalyst for air treatment applications. Additionally, the consistent slopes observed in each cycle and in XRD diffractograms indicate that the surface sites of the catalyst remain active and that the catalyst exhibits excellent stability.

2.4. Suggested Mechanism for Volatile Organic Compound and E. coli Removal

TiO₂-NTs exhibit significant antibacterial activity under UV irradiation [57], attributed to the photogeneration of electron–hole pairs, which leads to the formation of reactive oxygen species (ROS), such as hydroxyl radicals (OH^•) and superoxide anions (O₂^−•) [43]. Min Cho et al. have further highlighted that OH^•, both in their free form and those bound to the surface, represent a primary mechanism responsible for the inactivation of E. coli [62]. Conversely, they reported that any disruption to the bacterial cell surface, including damage to respiratory or other active systems, can compromise the bacteria’s metabolic processes, reducing the role of ROS (O₂^−• and ^•OH) in bacterial inactivation.

The Cu₂O-NPs/TiO₂-NTs photocatalyst exhibits enhanced photocatalytic activity compared to TiO₂-NTs. This can be attributed to the favorable energy level difference between their conduction and valence bands. Specifically, the Cu₂O conduction band energy level is higher than that of TiO₂-NTs [17,63]. As a result, under UV–Vis illumination, photoexcited electrons (e⁻) in the Cu₂O-NP conduction band (CB) readily transfer to the TiO₂-NTs CB, promoting charge separation and suppressing electron–hole recombination. These photoexcited electrons then migrate to the TiO₂-NTs CB, where they react with adsorbed oxygen, contributing to the formation of superoxide radicals. Subsequently, various oxidizing radicals, including hydroxyl radicals (^•OH), are generated through the reaction of superoxide radicals with H⁺ ions.

The energetic scheme and charge transfer mechanism proposed are based on the established literature values for Cu₂O and TiO₂ band positions rather than direct experimental measurements. According to previous studies, Cu₂O has a conduction band (CB) at around −1.4 eV and a valence band (VB) at approximately 0.7 eV vs. NHE, while TiO₂ has a CB at −0.6 eV and a VB at 2.5 eV vs. NHE [64]. These reported values provide a reliable framework for understanding the charge transfer process in our Cu₂O-TiO₂ composite. The observed photocatalytic trends in this study support the existence of a Type-II heterojunction, where photogenerated electrons migrate from Cu₂O to the CB of TiO₂, while holes move in the opposite direction, effectively reducing charge recombination and enhancing photocatalytic activity [65,66,67]. Although direct experimental determination of the CB and VB positions using techniques such as ultraviolet photoelectron spectroscopy (UPS) or Mott–Schottky analysis was not performed, the systematic reduction in band gap energy with increasing SILAR cycles and the improved photocatalytic efficiency strongly validate the proposed mechanism [68,69,70]. Future studies will focus on direct experimental verification of band edge positions to further strengthen these findings.

This photocatalytic process can be summarized by the following key equations:

Cu₂O + hν (>400 nm) → Cu₂O (e⁻_cb) + Cu₂O (h⁺_vb)

(7)

e⁻_cb + TiO₂ or (Ti⁴⁺) → TiO₂⁻ or (Ti³⁺)

(8)

Ti³⁺ + O₂ → Ti⁴⁺ + ^•O₂⁻

(9)

TiO₂ + hν (<400 nm) → TiO₂ (h⁺_vb) +TiO₂ (e⁻_cb)

(10)

^•O₂⁻ + H⁺ → HO₂^•

(11)

HO₂^• + H⁺ + e⁻_cb → H₂O₂

(12)

H₂O₂ + e⁻_cb → ^•OH + OH⁻

(13)

h⁺_vb (Cu₂O or TiO₂) + bacteria/COV → CO₂, H₂O, N, S

(14)

In the case of butadione, based on our previous work, the intermediate byproducts are acetone (C₃H₅O), acetaldehyde (C₂H₄O), acetic acid (C₂H₄O₂), formic acid (CH₂O₂), and propionic acid (C₃H₆O₂), with TiO₂ deposed on luminous textiles [71]. In the case of chloroform (CHCl₃) degradation, the mechanism involves additional steps that lead to the formation of chlorinated radicals (Cl^•) [65,66,67,68,69,70]. The process begins with the absorption of UV light by CHCl₃ or its reaction with hydroxyl radicals, as follows:

CHCl₃ + hν → CHCl₂^• + Cl^•

(15)

CHCl₃ + OH^• → CCl₃^• + H₂O

(16)

These initial reactions generate chlorine radicals (Cl^•) and dichloromethyl radicals (CHCl₂^•) [65,67]. The chlorine radicals play a crucial role in the degradation of other VOCs and contribute to E. coli bacteria inactivation. The mechanism proceeds as follows:

Cl^• + CHCl₃ → HCl + CCl₃^•

(17)

CCl₃^• + O₂ → CCl₃O₂^•

(18)

CCl₃O^• → COCl₂ (phosgene) + Cl^•

(19)

The chlorine radicals (Cl^•) generated in this process are highly reactive and can participate in the degradation of other VOCs present in the system [65,67], as follows:

Cl^• + VOCs → HCl + VOCs^•

(20)

This reaction initiates a chain of oxidation processes for other VOCs, enhancing the overall efficiency of the photocatalytic system. Moreover, chlorine radicals contribute to the inactivation of E. coli bacteria through several mechanisms. Firstly, they can cause direct oxidative damage to bacterial cell membranes. Secondly, chlorine radicals can react with water to form hypochlorous acid (HOCl), a potent disinfectant, through the reaction (XV). This hypochlorous acid further contributes to bacterial inactivation due to its strong oxidizing properties.

Cl^• + H₂O → HOCl + H^•

(21)

Lastly, the ongoing reaction generates additional reactive chlorine species (RCS) that can penetrate bacterial cells, causing internal damage to vital cellular components.

The synergistic effect of ROS (primarily OH^• and O₂^−•) and RCS (Cl^• and HOCl) significantly enhances the overall photocatalytic activity of the Cu₂O-NPs/TiO₂-NTs system for both VOC degradation and bacterial inactivation. This combined action leads to more efficient and comprehensive air purification, addressing a wider range of pollutants and pathogens simultaneously.

The plausible mechanism for the simultaneous removal of VOCs and E. coli is presented in Figure 10.

3. Material and Methods

This section describes the preparation of TiO₂-NTs through anodization and the subsequent deposition of Cu₂O nanoparticles using the SILAR method. Photocatalytic and antibacterial tests can also be conducted to evaluate the nanocomposite material’s efficacy in this domain.

3.1. Preparation of Cu₂O-NPs/TiO₂-NTs Photocatalysts

TiO₂-NT photoanodes were synthesized using a double anodization process on metallic titanium (Ti) foils with a purity of 99.99%, a surface area of 2 × 2.5 cm², and a thickness of approximately 0.5 mm. First, the Ti plates were polished with abrasive papers of varying grit sizes (320 to 2000) to activate the surface and prevent adhesion issues. They were then rinsed with acetone, ethanol, and deionized water in an ultrasonic bath for 10 min each to remove impurities and dried with air. During the anodization step, a two-electrode electrochemical cell was used with the Ti foil as the anode and the platinum wire as the cathode. Anodization was performed at room temperature with a constant potential of 60 V and continuous stirring for 2 h. The electrolyte solution consisted of 100 mL ethylene glycol, 1% NH₄F, and 2% H₂O. Finally, the anodized samples were annealed at 400 °C for 1 h, rinsed with water, and air-dried to prepare them for Cu₂O deposition. A total of 0.1 M of CuSO_4.5H₂O was dissolved in 20 mL of deionized water with magnetic stirring for 5 min. Then, 0.1 M of Na₂S₂O₃ was added dropwise until the solution became colorless, forming the cationic precursor. For the anionic precursor solution, 0.2 M of sodium hydroxide was dissolved in deionized water at 70 °C with constant magnetic stirring, forming the anionic precursor. The TiO₂-NTs substrate was immersed in a cationic precursor solution containing [Cu(S₂O₃)]⁻ complex ions for 15 s. During this step, the [Cu(S₂O₃)]⁻ ions were adsorbed onto the surface of the TiO₂-NTs. During the anionic reaction and Cu₂O formation, the substrate was transferred to a hot (70 °C) anionic precursor solution containing OH-ions for 15 s. The elevated temperature facilitated the dissociation of the [Cu(S₂O₃)]⁻ complex, releasing Cu⁺ ions that reacted with the OH⁻ ions to form Cu₂O nuclei on the TiO₂-NTs surface.

To remove any unreacted species or byproducts, the substrate was rinsed with deionized water for 10 s (Figure 11).

Thus, the formation of Cu₂O nanoparticles from the cationic and anionic precursors can be described through the following reactions:

2Cu⁺ + 4S₂O₃²⁻ ↔ 2[Cu(S₂O₃)]⁻ + [S₄O₆]²⁻

(22)

[Cu(S₂O₃)]⁻ ↔ Cu⁺ + S₂O₃²⁻

(23)

2Cu⁺ + 2OH⁻ ↔ Cu₂O + H₂O

(24)

Based on Equation (1), the complexation of Cu⁺ ions is made with the presence of S₂O₃²⁻ to form the [Cu(S₂O₃)]⁻ complex, which is the dominant species in the cationic precursor solution. Additionally, the second reaction can be between the released Cu⁺ ions and OH⁻ ions, leading to the formation of Cu₂O on the TiO₂-NTs surface via SILAR cycles; furthermore, a controlled and uniform layer of Cu₂O nanoparticles can be deposited onto the TiO₂-NTs, creating heterostructures with enhanced photocatalytic properties (Equations (2) and (3)).

3.2. Sample Characterization

The surface morphology of the Cu₂O-NPs/TiO₂-NTs samples was examined using scanning electron microscopy (SEM, Jeol JSM-6300, Peabody, MA, USA) and transmission electron microscopy (TEM, FEI Tecnai G2, Hillsboro, OR, USA) operating at 200 kV with a LaB6 filament. High-resolution transmission electron microscopy (HRTEM) was employed to provide further insights into the structure and morphology of the nanoparticles. The chemical composition of the samples was determined using an energy-dispersive X-ray spectroscopy (EDS) system attached to the MEB. X-ray diffraction (XRD) analyses were performed using a Philips X’PERTMPD X-ray diffractometer with Cu Kα irradiation (λ = 1.5406 Å). The diffraction data were collected over a 2θ range of 20° to 80° with a scanning step of 0.016°. UV–Vis analysis was performed to investigate the optical properties of the samples.

3.3. Target Pollutants and Tested Bacteria

In this study, three distinct organic compounds were utilized, as follows: butadione (C₄H₆O₂), chloroform (CHCl₃), and Glutaraldehyde (C₅H₈O₂), commonly found in different sectors, with butadione being primarily associated with the food industry, chloroform being prevalent in hospital settings, and Glutaraldehyde being employed across various industrial applications. These pollutants, each boasting high purity levels (99%, purity), served as volatile organic compound (VOC) probes. The antibacterial activity was assessed using Escherichia coli (E. coli) (DSM 10198–307-001) as the model microorganism. Previous studies have extensively documented the preparation protocol and testing methodology [16].

3.4. Reactor Design for Volatile Organic Compound Removal and Analytic Tools

The experimental setup consisted of a 250 mL spherical batch reactor equipped with a Sylvania CF-L 24W/840 (Gennevilliers, France) (UV–Vis) source inserted from above. VOC concentration variations and oxidation byproducts were monitored using a GC-500 gas chromatograph with a flame ionized detector (FID) and an apolar capillary DB-MS column (60 m length, 0.25 mm diameter, 0.25 µm film thickness). The analysis used a helium carrier gas at 1 mL/min, while the FID operated with an air and hydrogen mixture. The temperature program started at 50 °C for 3 min, increased to 100 °C at 2 °C/min, and was held for 10 min, with injection and detection temperatures set at 250 °C. The sample analysis involved a 500 µL manual injection using a gas-tight syringe, which was repeated at least three times. The reactor (10 mm upper diameter, 50 mm working height) featured two septa for sample extraction and VOC injection, with a 50 mL solution capacity. The catalytic samples were fixed to the internal reactor wall for optimal contact with VOCs and light radiation, which simulated typical indoor illumination. The reactor was covered with aluminum foil during the experiments to prevent external light interference, and gas phase homogenization was achieved using a magnetic stirrer.

4. Conclusions

The development of Cu₂O-NPs/TiO₂-NTs nanocomposites with enhanced photocatalytic activity and stability offers a promising approach for addressing the challenges of indoor air pollution. This study demonstrates that the controlled deposition of Cu₂O-NPs onto TiO₂-NTs leads to the formation of efficient p-n heterojunctions that enhance Vis absorption and charge separation. This improvement translates to significantly higher photocatalytic degradation rates for certain VOCs and effective bacterial inactivation compared to pure TiO₂-NTs.

A key finding of this research is the crucial role played by chlorinated radicals in the degradation of pollutants, particularly in the presence of chlorinated VOCs, such as chloroform. The photocatalytic degradation of chloroform generates chlorine radicals that act as powerful oxidizing agents, contributing significantly to the degradation of other VOCs and bacterial inactivation. These chlorinated radicals initiate chain reactions that enhance the overall oxidative capacity of the system, leading to more efficient removal of pollutants.

This study also sheds light on the importance of tailoring the Cu₂O loading for optimal performance based on the target pollutants. The synergistic effect between the Cu₂O-NPs/TiO₂-NTs nanocomposites and the generated chlorinated radicals emphasizes the potential of the developed material for sustainable and efficient air purification applications, especially in environments where chlorinated compounds are present. This research highlights the multi-functional nature of these nanocomposites. Not only do they effectively degrade a range of VOCs, but they also demonstrate significant antibacterial properties, largely attributed to the combined action of ROS and reactive chlorine species generated during the photocatalytic process. Further research can focus on optimizing the synthesis parameters and exploring the application of these nanocomposites with real condition (presence of high value of humidity) scenarios to further validate their potential for solving environmental challenges.