Boosting the Photocatalytic Behavior of PbS/TiO₂ Nanocomposites via the Pulsed Laser Deposition of PbS Nanoparticles onto TiO₂ Nanotube Arrays Under Various Helium Background Pressures

Abstract

In this study, highly ordered titanium dioxide nanotubes (TiO₂-NTs) have been synthesized using the electrochemical anodization procedure. Subsequently, the TiO₂-NTs were successfully decorated with PbS nanoparticles (NPs) using the pulsed KrF-laser deposition (PLD) technique under vacuum and under different Helium background pressures (P_He) ranging from 50 to 400 mTorr. The prepared samples (PbS-NPs/TiO₂-NTs) were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and UV–Vis and photoluminescence spectroscopies. XRD analyses confirmed that all TiO₂-NTs crystallized in the anatase phase, while the PbS-NPs crystallized in the cfc lattice. The average crystallite size of the (200) crystallites was found to increase from 21 to 33 nm when the pressure of helium (PHe) was raised from vacuum to 200 mTorr and then dropped back to ~22 nm at P_He = 400 mTorr. Interestingly, the photoluminescence intensity of the PbS-NPs/TiO₂-NTs samples was found to start diminishing for P_He ≥ 200 mTorr, indicating a lesser recombination rate of the photogenerated carriers, which also corresponded to a better photocatalytic degradation of the Amido Black (AB) dye. Indeed, the PbS-NPs/TiO₂-NTs samples processed at P_He = 200 and 300 mTorr were found to exhibit the highest photocatalytic degradation efficiency towards AB with a kinetic constant 130% higher than that of bare TiO₂-NTs. The PbS-NPs/TiO₂-NTs photocatalyst samples processed under P_He = 200 or 300 mTorr were shown to remove 98% of AB within 180 min under UV light illumination.

1. Introduction

The photocatalytic degradation of various pollutants continues to garner significant interest due to its considerable potential in addressing some of our environmental challenges [1,2]. Photocatalysis is a well-known process where a photocatalyst material produces electron–hole pairs upon its absorption of photons (of which energy is equal or greater than the material’s band gap). The photogenerated charges become active players in the oxidation/reduction processes of chemical compounds adsorbed on the surface of the photocatalyst. Several semiconducting materials have been utilized as photocatalysts, including wide-bandgap oxide semiconductors such as TiO₂ [3], ZnO [4], SnO₂ [5], and Fe₂O₃ [6]. Among all these semiconducting oxides, titanium dioxide (TiO₂) is by far the most widely investigated photocatalyst material [7,8,9,10,11], since the seminal work of Fujishima and Honda in 1972 [12]. TiO₂ is an intrinsic n-type semiconductor that crystallizes in anatase, rutile, and brookite phases, exhibiting properties that depend on the crystal structure. Among these, anatase, with a 0.1 eV higher Fermi level than rutile and a 3.2 eV bandgap, has a favorable band-edge configuration that enables efficient water decomposition, making it the preferred phase for photoelectrochemical and photocatalytic water-splitting applications [13]. In addition to its excellent photocatalytic properties [14,15], TiO₂ is also a very interesting material for different applications, such as dye-sensitized solar cells [16] and gas sensors [17,18]. Due to its relatively low cost, excellent chemical/photochemical stability, and environmental friendliness, TiO₂ is the material of choice for a variety of applications. However, anatase-TiO₂ has a wide bandgap (Eg) of 3.2 eV, which allows it to absorb only the UV-photons, and it can utilize a minimal amount of the total energy in the solar spectrum, which accounts for 3–5% of the total energy [14]. This relatively wide bandgap has limited the use of TiO₂ as a catalyst, particularly when utilizing a larger portion of the solar spectrum [15]. The photocatalytic performance of TiO₂ has also been reported to be limited by the high recombination rate of the photogenerated electron–hole pairs [19]. To overcome these limitations, various approaches have been considered. These include deposition of noble metals on the TiO₂ surface [20,21], different doping schemes [22,23,24,25], metallic nanoparticles decoration [26,27,28,29], or coupling with other narrow-bandgap semiconductors [30,31]. As for the aforementioned modification strategies towards TiO₂, the construction of heterojunctions is considered one of the most effective approaches to significantly enhance and improve its photoresponse intensity and promote the generation of photogenerated electrons and charge carriers, thus improving its photocatalytic activity [32].

Since photocatalysis is genuinely a surface process, it is also essential to increase the surface area of the photocatalyst. Thus, various strategies have been developed to artificially increase the effective surface area of the TiO₂ photocatalyst. Indeed, TiO₂ nanostructures, such as nanowires, nanorods, and nanotubes, have been shown to exhibit greater photocatalytic activity than TiO₂ nanoparticles or thin films [33]. In particular, TiO₂ nanotubes (TiO₂-NTs) are highly efficient for the photocatalytic process due to their larger surface-to-volume ratio, high orientation, enhanced light absorption, tunable electron transport properties resulting from quantum confinement effects, and lower recombination rate of photoinduced electron–hole pairs and the higher interaction between their surfaces and the reactive species [34]. These unique features make them highly attractive for a wide range of applications in technological fields, including highly efficient photocatalysis systems [26,35], dye-sensitized solar cells [36], biosensors [37], and photocatalytic bacterial inactivation [27,38]. Nanotubular layers of TiO₂ offer enhanced photocatalytic performance relative to nanoparticulate layers. These layers can be synthesized over large areas of approximately 50 cm² [39]. The TiO₂ nanotubes are typically fabricated using the electrochemical anodization method, which enables the production of relatively large samples of TiO₂ nanotubes [28,36,37]. Compared to other fabrication methods, anodization provides a relatively short processing time, high uniformity, and well-aligned TiO₂ nanotubes. The pore/tube size, length, and wall thickness of anodic TiO₂ nanotubes can be precisely controlled by adjusting the electrochemical conditions, with these morphological features exerting a direct influence on their electrochemical and photocatalytic performance [40].

In this paper, we investigate the coupling of TiO₂-NTs with a tiny bandgap semiconductor, namely, PbS (of which the bulk bandgap is only 0.41 eV), and a high energetic conduction band (CB) [41,42]. By decorating the TiO₂-NTs with PbS nanoparticles (NPs), we aim to create local heterojunctions that will enhance the overall photocatalytic activity of these novel PbS/TiO₂ nanocomposites. Such heterostructures are expected not only to broaden the light response range but also to reduce the recombination rate of hole–electron pairs [26,38]. Because of its very small bandgap and large exciton Bohr radius (~18 nm), the optoelectronic properties of PbS NPs can be tuned in the near-infrared region by varying the size and shape of the PbS NPs [43,44,45]. Among chemical methods for constructing TiO₂ NTS-based heterojunctions, the successive ionic layer adsorption and reaction (SILAR) technique is widely used due to its simplicity, low cost, and ability to produce uniform coatings on high-aspect-ratio structures such as nanotubes. Recent studies have highlighted the versatility of SILAR for fabricating semiconductor/oxide heterostructures and enhancing interfacial charge transfer [46,47]. Building on the above, in our earlier work, we employed SILAR to deposit PbS nanoparticles on TiO₂ nanotubes and, at optimized cycle numbers, the amount of the degraded Amido Black (AB) reached 75% (much higher than that of bare TiO₂-NTs). The enhanced photoelectrocatalytic activity of the PbS-NPs/TiO₂-NTs photoelectrode was attributed to a higher absorption of visible light and a better matching of the band structure between TiO₂-NTs and PbS [48]. Ti/TiO₂ nanotube arrays sensitized with PbS quantum dots via the SILAR method showed enhanced photoelectrocatalytic degradation of ifosfamide, with a rate constant of 0.0148 min⁻¹ for three SILAR cycles, compared to 0.0072 min⁻¹ for bare Ti/TiO₂. Higher SILAR cycles (4 and 6) led to aggregation of PbS nanoparticles and lower rates (0.0043 and 0.0033 min⁻¹, respectively) [49]. Other authors have invoked multiple excitons generation (MEG) in PbS-NPs and reduced recombination rates of the photogenerated charges to explain the observed enhancement of photocatalytic activity of PbS-NPs-decorated TiO₂-NTs [44,45,46,47,48,49,50]. Ratanatawanate et al. demonstrated that decorating TiO₂ nanotubes (90–100 nm in size) with PbS quantum dots (4–5 nm) can enhance their photocatalytic activity. PbS nanoparticles deposited on TiO₂ nanotube films improve their photoelectrochemical response and cathodic protection. Gao and al [51] reported that when PbS was incorporated, the bandgap of PbS/TiO₂ NTs decreased to 1.4 eV, and the photocurrent density increased to 4.14 mA·cm⁻²—about 12.8 times higher than TiO₂—highlighting the strong potential of these hybrid structures for environmental applications. The above summarized results indicate that the deposition of PbS-NPs onto TiO₂-NTs enhances the photocatalytic activity of the TiO₂-based photocatalyst. However, in all these studies, PbS-NPs were synthesized by chemical routes, and the latter are generally found to suffer from low stability in air even when they were wrapped by organic ligands [52]. The presence of such ligands is also known to limit charge transfer from the PbS-NPs to the underlying substrate or solution [53], thereby limiting the exploitation of the full potential of PbS-NPs/TiO₂ nanocomposites in photocatalytic applications.

Alternatively, we propose in the present work to deposit PbS-NPs directly on TiO₂-NTs using an energetic physical vapor deposition method, specifically, pulsed laser deposition (PLD). This technique has been previously shown to deposit not only highly pure PbS nanoparticles on the surface of nanotubes without resorting to any ligand engineering, but also to preserve the stoichiometry of the nanoparticles and their strong anchoring to the nanotubes through direct covalent bonds [43,54,55]. Similarly, PLD has been successfully employed to decorate silicon nanowires (SiNWs) with PbS nanoparticles. In this study, PbS was shown to significantly enhance the photocatalytic performance of the SiNWs, improving their efficiency in wastewater treatment applications [56]. TiO₂ nanotube arrays decorated with PbS nanoparticles by pulsed laser deposition (PLD) have been reported to extend the optical absorption of TiO₂ into the visible range and enhance charge separation, with the best photoelectrochemical performance (photocurrent density ~1.05 mA cm⁻², efficiency 2.5%) achieved [57] at NLP = 2500. PLD involves directing high-energy laser pulses onto a target, vaporizing the material and generating a dense, highly excited plasma composed of ions and neutral species with a broad range of kinetic energies. These species are accelerated toward the substrate, where they condense to form a film. Its simplicity, adaptability, and cost-effectiveness make it well suited for producing dense, uniform coatings over large areas. Among physical deposition techniques, it provides the broadest versatility in material composition and is particularly effective for materials with complex chemistries. This highly flexible approach allows for the variation of different deposition parameters (such as laser intensity, number of laser ablation pulses (NLP), deposition time, substrate temperature, and background gas pressure) independently. By filling the deposition chamber with a controlled gaseous environment at specific partial pressures, both physical and chemical interactions between the ablated species and ambient gas can be promoted. It is also the only method that enables deposition over a wide range of pressures, from ultra-high vacuum to the mbar range. By adjusting the background gas pressure, film density, morphology, and dopant incorporation can be precisely tailored [58]. Here, we varied the He background gas pressure while keeping all other PLD parameters constant. The He pressure has a direct effect on slowing down the ablated species and, hence, on the growth kinetics of the NPs onto the substrate [59,60,61]. Very recently, we have identified a P_He = 300 mTorr as a transitional pressure for the structural and optoelectronic properties of PbS-NPs films deposited onto flat silicon and quartz substrates [62]. Here, we investigate the effect of P_He during the PLD deposition of PbS-NPs films onto high surface area TiO₂ nanotubes on the structural, morphological, optical, and photocatalytic properties of PbS-NPs/TiO₂-NTs nanocomposites.

2. Materials and Methods

2.1. Preparation of TiO₂ Nanotubes

Before the electrochemical etching, titanium substrates (2.5 × 2.5 cm² and 0.5 mm thick) were first polished using abrasive sandpaper (with different grain sizes, ranging from 320 to 2000 µm) before being placed in an ultrasonic bath for 15 min, in acetone, in methanol, and then in water. Finally, the substrates were rinsed and air-dried. The highly ordered TiO₂ nanotubes were synthesized through an anodic oxidation process of titanium foils in two steps. The electrochemical anodization experiment was performed using a two-electrode cell with the titanium foil used as the anode and a platinum wire serving as the counter electrode. The first sacrificial anodization was carried out for 1 h at a constant voltage of 60 V at a temperature of 25–27 °C in a stirred electrolyte bath containing 100 mL of ethylene glycol (EG), 1% vol. ammonium fluoride (NH₄F), and 2% vol. of ultra-pure water. To remove the first-formed TiO₂-NTs layer, the samples were sonicated in water. For the formation of the final TiO₂-NTs arrays, we used the same anodization conditions as in the first step, except that the anodization time was increased to 2 h in the second step. Then, the samples were rinsed with ethanol in an ultrasonic bath for 2 min. Subsequently, the obtained TiO₂-NTs electrodes were annealed at 400 °C for 3 h, in air, at a heating rate of 5 °C·min⁻¹ to form aligned TiO₂ nanotube arrays crystallizing in the anatase phase.

2.2. Decoration of TiO₂ Nanotubes by PbS-NPs

To achieve the PLD decoration of the TiO₂-NTs with PbS-NPs, a KrF excimer laser (wavelength = 248 nm, pulse duration = 14 ns, repetition rate = 20 Hz, and pulse energy = 120 mJ) was focused onto a rotating PbS pellet placed in the PLD chamber, at an incident angle of 45°. The depositions were carried out at room temperature. The Ti/TiO₂NTs substrates were placed on a rotating 3”-diameter substrate holder, placed parallel to the target at a distance of 7.5 cm. Before the deposition, the target surface was cleaned under vacuum for 10 min while isolating the samples from the ablation plume by a mechanical shutter placed close to the target. At a fixed NLP of 3500, PLD depositions of PbS-NPs onto TiO₂-NTs were carried out under vacuum (≈5 × 10⁻⁵ Torr) and under different He background pressures (P_He) ranging from 50 to 400 mTorr. The structural, morphological, optical, and photocatalytic properties of the PbS/TiO₂-NTs nanocomposites were systematically characterized as a function of P_He.

2.3. Materials Characterizations

X-ray diffraction (XRD) analyses were performed with a Rigaku Ultima IV diffractometer (Tokyo, Japan) in the Bragg-Bentano configuration using the CuKα irradiation, λ = 1.5406 Å. The morphology of the TiO₂-NTs decorated with PbS NPs was analyzed using a high-resolution TESCAN VEGA3 scanning electron microscope (SEM, Brno, Czech Republic). The elemental composition of the samples was determined using energy-dispersive X-ray spectroscopy (SEM-EDS); the EDS detector is a Zeiss ULTRA 55 model equipped with an In-Lens SE detector. TEM and HRTEM images were carried out using a JEM-100CX2 transmission electron microscope (Tokyo, Japan). The diffuse reflectance spectra of the samples were recorded using a UV-Vis-IR spectrometer. The photoluminescence (PL) spectra were measured on the PbS/TiO₂-NTs samples using a fluorescence spectrophotometer (PerkinElmer LS55, Waltham, MA, USA) equipped with a xenon lamp at an excitation wavelength of λ = 340 nm.

2.4. Photocatalytic Activity Measurement

The photocatalytic performance of the bare TiO₂-NTs and PbS-NPs/TiO₂-NTs nanocomposites were evaluated through the Amido Black (AB) degradation under UV irradiation. The PbS-NPs/TiO₂-NTs samples were immersed in a cuvette filled with 10 mL of aqueous AB solution using deionized water as the solvent. The initial concentration of Amido Black was fixed at a value of 2 × 10⁻³ M. Before turning on the UV lamp, the assembly was kept in the dark for 15 min to reach the adsorption–desorption balance between the pollutant AB and the photocatalyst. Then, the sample was exposed to UV radiation for different durations (15, 30, 60, 90, 120, 150, and 180 min). After each AB experiment, the photocatalyst samples were subjected to a cleaning procedure consisting of rinsing with deionized water, followed by immersion in a cuvette containing deionized water and UV irradiation for 5 min to degrade any residual (non-degraded) adsorbed AB molecules on the surface of the used PbS-NPs/TiO₂-NTs photocatalyst sample. This is a necessary step to clean the surface of the used sample during the photocatalytic degradation of organic pollutants. The PbS NPs-decorated TiO₂ NTs are exposed to an OSRAM germicidal lamp with an electric power of 15 W and a wavelength of 256 nm. The corresponding excitation energy is 4.85 eV, which is greater than the bandgap width of the anatase phase (3.2 eV). The discoloration of the dye solutions was assessed by measuring the absorbance of AB at various intervals using a UV–Vis PerkinElmer Lambda 950 Spectrophotometer. The spectra of AB can be characterized particularly by an intense peak in the visible region at 618 nm, which is due to the presence of azo chromophore (–N=N–).

3. Results and Discussions

Figure 1a shows the X-ray diffraction (XRD) patterns of the TiO₂-NTs decorated with PbS nanoparticles under vacuum and different He pressures (ranging from 50 to 400 mTorr). All the samples exhibited prominent peaks, which constitute the characteristic fingerprint of the TiO₂ anatase phase. Indeed, the diffraction peaks appearing at 2θ = 25.33°, 37.02°, 38.00°, 38.4°, 48.03°, 53.96°, and 55. 03° match well with the (101).

(103), (004), (112), (200), (105), and (211) planes the TiO₂ anatase phase, respectively [63] (these peaks are represented by the letter “A” in Figure 1). The less prominent XRD peaks located at 2θ = 25.98°, 30.10°, 43.11°, and 51.03°, are attributed to the (111), (200), (220), and (311) crystallographic orientations of the cubic face-centered structure of PbS JCPDS [64] (those peaks are designated by the letter “P” in Figure 1).

Finally, one can also notice much weaker XRD peaks at different 2θ positions (i.e., 20.81°, 23.32°, 24.56°, 26.71°,27.68°,32.35°,33.17°, 41.70°, 43.76°, and 44.63°), which could be due to the (101), (111), (120), (021), (210), (211), (002), (122), (212), and (140) diffraction planes of the PbSO₄ phase (those peaks were marked with the letter “O”). The presence of these weak PbSO₄ peaks suggests partial surface oxidation of PbS upon exposure to ambient air. Since the PLD process was performed under an inert atmosphere, it is likely that this oxidation occurred after deposition during subsequent handling and storage in air. PbS is known to exhibit limited chemical stability in oxidative environments, particularly in nanostructured forms with high surface-to-volume ratios. However, despite this minor surface oxidation, the photocatalytic performance of the PbS/TiO₂-NTs nanocomposites remains high, indicating that the overall functional contribution of PbS is preserved. These observations highlight the importance of considering air sensitivity in PbS-based materials and suggest that protective measures, such as controlled storage or surface passivation, could further enhance long-term stability. By zooming on the XRD window where the PbS diffraction peaks are, one can notice that not only the peak intensities but also their positions are sensitive to P_He. Indeed, while the PbS-NPs are seen to be more (200) oriented when deposited under vacuum, their preferential orientation progressively changes to (111) as P_He is increased. For the intermediate P_He of 200 and 300 mTorr, both crystallographic orientations coexist. On the other hand, one can note that the PbS (200) XRD peak slightly shifts towards higher angles (Figure 1b) when the P_He is increased up to 200 mTorr and 300 mTorr and then comes back to its initial value at P_He = 400 mTorr. This indicates that the lattice parameter of the (200) PbS crystallites changes with P_He, as illustrated in Figure 2. It is clearly seen that the lattice parameter of the main PbS (200) crystallites decreases with P_He to reach its minimum in the 200–300 mTorr range and then recovers its initial value (under vacuum) at 400 mTorr. Furthermore, we calculated the average size of PbS (200) crystallites by using the Debye–Scherrer method [65] and the inter-planar distance d200 using Bragg’s law: 2dkℎl sin(θ) = nλ, where dhkl is the interplanar distance, θ is the Bragg angle (angle of Incidence), n is the diffraction order, and λ is the X-ray wavelength). Figure 2 shows that the (200) crystallite size is found to show exactly the opposite trend of the lattice parameter variation. The PbS crystallite size reaches its maximum value (~33 nm) at 200 mTorr, while it remains in the 20–22 nm range for both vacuum and 400 mTorr. This shows that the lattice contraction observed at 200–300 mTorr coincides with the formation of the largest PbS (200) crystallite, suggesting that the growth of larger PbS grains is accompanied with higher strain in the lattice. Conversely, when the crystallites tend to be smaller, the residual strain is relatively relaxed through additional grain boundaries. A possible explanation to these observations is that at intermediate P_He pressures (200–300 mTorr), the residual energy of the ablated species (resulting from energy loss via multiple collisions with background gas) is favorable for the germination and the growth of PbS crystallites. In contrast, under either vacuum (highly energetic species) or high P_He pressures (slow ablated species), the landing species, for opposite reasons, tend to form a higher density of nucleation sites, which ultimately results in smaller grains with more relaxed strain.

The general tendency in the literature is that an increase in gas pressure leads to an increase in particle size [66,67]. In our case, this phenomenon was observed only for pressures ranging from 0 to 200 mTorr. For higher pressures, it is believed that the mean free path of laser-ablated species and their residual surface diffusion energy are both reduced to contribute to the growing of existing nanoparticles on the substrate. Instead, they end up creating new nucleation sites and/or form a larger number of nanoparticles with smaller diameter.

Figure 3 shows the SEM images of the TiO₂-NTs decorated with different loadings of PbS-NPs. Figure 3a,b compare the TiO₂-NTs decorated with PbS-NPs deposited under vacuum and at P_He = 100 mTorr. It appears that the PbS-NPs are mostly deposited on the nanotubes and tend to form a “donut-like” lid on their top. As NLP is increased to higher values, the PbS-NPs are seen to coalesce and form a stack of platelets of which average size increases from ~70 nm to ~150 nm when P_He is increased from 200 to 400 mTorr (Figure 3c,d).

Figure 4 shows TEM (4-a) and HRTEM (4-b) images of PbS-NPs PLD deposited on TiO₂-NTs at PHe = 200 mTorr. Figure 4a reveals that PbS-NPs are not only deposited on the top surface of the NTs’ apertures but also along their walls, increasing thereby the surface area of the PbS-NPs/TiO₂-NTs photocatalyst. From the TEM images, the individual PbS-NPs were identified and their average size determined. The statistical distribution of Figure 4b shows that the average size of PbS-NPs (deposited at P_He = 200 mTorr) is about 30 nm. The high-resolution-TEM image of Figure 4c reveals the PbS lattice planes with d-spacing of ~0.30 and ~0.34 nm, which match well with the (200) and (111) orientations, respectively, of the PbS cubic structure, in accordance with the above-discussed XRD spectra. The underlying TiO₂ (101) lattice plans were also observed on Figure 4c.

Figure 5 shows the PL emission spectra of the PbS-NPs/TiO₂-NTs nanocomposite samples deposited under vacuum and different He pressures. Firstly, regardless of the background gas PbS-deposition condition, the PL spectra is seen to show the same pattern, which is characteristic of TiO₂. Indeed, the peaks located at 402 nm and 420 nm are due to the electronic transition between the conduction band and the valence band of TiO₂ [35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57]. Another PL peak located around 495 nm is attributed to excitonic relaxation in TiO₂, which mainly results from oxygen vacancies and defects at the surface [68]. No PL emission associated with PbS is observed, at least in the visible range. This is not surprising since the bandgap of PbS is in the infrared (0.41 eV), and even when the PbS-NPs undergo quantum confinement (i.e., their average size smaller than 18 nm), their PL emission occurs at wavelengths ≥900 nm [43]. This means that the presence of PbS-NPs merely slightly modified the positions of valence and conduction band edges of the TiO₂ matrix [69]. On the other hand, the PL intensity is seen to be dependent on the PbS-NPs deposition conditions. The highest PL intensities were observed under vacuum and at P_He = 50 and 100 mTorr, suggesting an enhanced light absorption by the PbS-NPs and subsequent charge transfer to the TiO₂-NTs, boosting thereby their PL. At P_He = 200 mTorr, the PL intensity starts to decrease and becomes similar to that of bare-TiO₂-NTs. However, for higher P_He (300 and 400 mTorr), the PL intensity significantly diminishes. Finally, the rather weak PL peak located around ~800 nm is the second order harmonic of the main bandgap electronic transition of TiO₂.

Since the PL emission results from the radiative recombination of photogenerated e-h pairs, a diminution of the PL intensity can result from a low radiative recombination rate or an increase in non-radiative recombination on defects. Better separation of photogenerated charge carriers (long lifetimes) also leads to PL intensity lowering. Thus, for high PHe (300 and 400 mTorr), one would expect larger surface coverage of the TiO₂-NT by the PbS-NPs (that tend to form platelets), leading to a larger extent of the PbS-TiO₂ interfaces, where better separation of photogenerated charge carriers can occur (local heterojunctions) reducing thereby radiative recombination. A more effective separation of the photogenerated charges extends their lifetime and render them available for photocatalysis processing. Therefore, the PL intensity decrease is a positive sign regarding the sensitization of TiO₂-NTs with PbS-NPs.

The photocatalytic performance of the PbS-NPs/TiO₂-NTs nanocomposite samples was studied towards Amido Black (AB) degradation under UV irradiation, while comparing them to bare TiO₂-NTs photocatalyst. The UV-Vis absorption spectrum (Figure 6) illustrates how the concentration of Amido Black dye changes over time during photocatalysis using a PbS/TiO₂ sample synthesized at 300 mTorr. The characteristic absorption peak of Amido Black, located at approximately 618 nm, serves as the main indicator of the dye’s degradation. At the start of the experiment, the solution exhibits a strong absorbance at this wavelength, indicating a high concentration of dye. As UV irradiation progresses, the absorbance at 618 nm begins to decline noticeably. After 30 min, a significant decrease is already apparent, and the reduction continues steadily through 120, 150, and finally, 180 min, where the absorption approaches a minimum, reflecting near-complete degradation of the dye.

This aforementioned trend clearly demonstrates the photocatalytic efficiency of the PbS/TiO₂ material in breaking down Amido Black molecules. The gradual fading of the absorption peak over time suggests that the catalyst actively promotes the decomposition of the dye under UV exposure. The performance of this sample can be linked to the presence of PbS, which likely improves charge separation and enhances the light-harvesting ability of the TiO₂ nanotubes. The conditions used for deposition, particularly the pressure of 300 mTorr, may have contributed to a favorable structure or surface state that supports these effects. Overall, the results confirm that the PbS/TiO₂ photocatalyst is effective in reducing dye concentration through photodegradation over a relatively short irradiation period.

Figure 7 shows the normalized concentration (C/C₀) versus UV irradiation time for the different PbS-NPs/TiO₂-NTs photocatalysts (deposited at different P_He) and the inset of Figure 7 shows the Linear transform Ln (C₀/C) versus UV light irradiation time for bare TiO₂-NTs. It clearly appears that the photocatalytic activity of PbS-decorated TiO₂-NTs at different He pressures is much higher than that of bare TiO₂-NTs. We found that the photocatalyst prepared at P_He = 300 mTorr exhibits the best AB degradation (98% of the dye was degraded within 180 min). This performance is quite consistent with the PL results, which revealed that the photocatalyst prepared at P_He = 300 mTorr exhibits the lowest PL emission. It is thus argued that for this optimal P_He condition (300 mTorr), the photogenerated charge carriers would have longer lifetimes (less recombination) and participate more actively in the photocatalytic process.

These degradation curves were fitted according to the Langmuir-Hinshelwood (L-H) model equation [70] C/C₀ = ${\mathrm{e}}^{-\mathrm{k}.\mathrm{t}}$, where C₀ is the initial AB concentration, C is the concentration at a given time (t), and k is the kinetic constant (min⁻¹). The best fittings allowed us to determine the k values as a function of P_He (Figure 8). The results show that the addition of PbS-NPs to the TiO₂-NTs significantly improves their photocatalytic capacity, as can be seen from their kinetic constant, which increased by 130% for the optimum P_He conditions of 200 and 300 mTorr. Indeed, the kinetic constant (k) increased from 0.010 to 0.023 min⁻¹ when comparing bare TiO₂-NTs and those decorated by PbS-NPs at P_He = 300 mTorr.

This photocatalytic improvement is thought to be due to the formation of local p-n junctions between PbS and TiO₂, which seem to reach their maximum under the P_He conditions of 200 and 300 mTorr. These local junctions contribute to absorb additional photons in the visible by PbS and effectively transfer the photogenerated charges to TiO₂ (see the inset of Figure 7), fuelling thereby the overall photocatalytic process. The formation of local p-n junctions improves the separation of photogenerated charge carriers, lowering thereby their recombination rate and increasing their chances to contribute to the photocatalytic process.

The material shows promising photocatalytic activity, indicating potential for applications such as wastewater treatment and solar-driven remediation. Its visible light response is especially relevant for degrading pollutants in complex water matrices and for solar-based systems. Nonetheless, the study has limitations. Tests under controlled conditions with model pollutants may not reflect real environments, and long-term stability against photo corrosion, fouling, or structural degradation was not assessed. Scalability and possible catalyst leaching also remain unexamined.

Future work should prioritize durability testing, evaluation under realistic conditions, and strategies to improve recyclability and resistance to degradation to enable practical applications.

4. Conclusions

We have achieved a systematic study on the effect of the He background gas pressure on the structural, morphological, optical, and photocatalytic properties of PbS-NPs/TiO₂-NTs nanocomposites fabricated by the PLD-based PbS decoration of TiO₂-NTs. Our results show that not only the surface coverage of the TiO₂-NTs by PbS-NPs increases as P_He is increased, but also the NPs tend to coalesce and form larger platelets of PbS on the surface of the nanotubes, particularly at the highest He pressure of 400 mTorr. The PbS-NPs films crystallize in the cfc lattice with a preferential (200) orientation. The average size of the (200) crystallites was found to increase and reach its maximum of 33 nm at P_He = 200 mTorr. At the intermediate He pressures of 200 and 300 mTorr, the PL intensity of TiO₂-NTs was found to decrease significantly, indicating a lower recombination rate of the photogenerated charge carriers. Consistently, the photocatalytic activity of the PbS-NPs/TiO₂-NTs nanocomposites also reached their maximum at P_He = 200–300 mTorr, with an increase of ~130% in the corresponding kinetic coefficient (k), in comparison with bare-TiO₂-NTs photocatalysts. It is proposed that this photocatalytic enhancement is due to the formation of PbS/TiO₂ local junctions that contribute to generate more charge carriers (through the extension of the absorption of visible light by PbS-NPs) and ensure their effective separation (with less recombination) to fuel up the photocalytic process. Our results suggest that the density of such local p-n junctions along with ensuring the longest lifetimes for the generated charge carriers, reach their maximum at P_He = 200–300 mTorr. Finally, these results demonstrate the potential of PbS-NPs as an enhancer of the photocatalytic capacity of the standard TiO₂-NTs. They also pave the way for the design and achievement of new PbS/TiO₂ nanostructures with augmented photocatalytic and/or photoelectronic properties.