Aluminum nanoparticle enhanced TiO2 photocatalysis of organic pollutants under solar and UV-B irradiation
Saadia Wasim, Stephanie K. Loeb
TL;DR
A new sustainable water treatment method uses aluminum nanoparticles with titanium dioxide to efficiently break down pollutants using sunlight.
Contribution
A low-cost plasmonic-photocatalytic heterostructure using Al nanoparticles with TiO2 is introduced for efficient organic pollutant degradation under solar light.
Findings
Al/TiO2 heterostructures enhance TiO2 photocatalysis through improved light absorption and plasmon resonance.
Cysteine-modified Al/TiO2 degrades the dye amaranth 60% faster than P25 TiO2 and remains stable over multiple cycles.
Abstract
Photocatalytic water treatment offers a sustainable method for removing organic micropollutants but is often limited by low efficiency and complexity. We report a plasmonic-photocatalytic heterostructure combining aluminum (Al) nanoparticles with titanium dioxide (TiO2) for contaminant degradation under solar light without external oxidants or pH adjustment. Using an organic colloidal Al nanoparticle suspension, this approach enhances TiO2 photocatalysis through improved light absorption, plasmon resonance, and contaminant adsorption. The low-cost Al/TiO2 heterostructure provides light-harvesting benefits comparable to other noble metal heterostructures (Au/TiO2 and Ag/TiO2), offering a sustainable alternative. Synthesized via an organic solvent method and ligand modification, the heterostructures were characterized for charge, size, bandgap, and photocatalytic efficiency. A…
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Figure 7- —https://doi.org/10.13039/501100003151Nature et technologies
- —https://doi.org/10.13039/501100000038Natural Sciences and Engineering Research Council of Canada
- —https://doi.org/10.13039/501100000196Canada Foundation for Innovation
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Taxonomy
TopicsTiO2 Photocatalysis and Solar Cells · Advanced Photocatalysis Techniques · Nanomaterials for catalytic reactions
Introduction
Worldwide, 2.2 billion people are reliant on contaminated drinking water. This results in 27% of the population lacking access to clean drinking water, primarily in developing regions^1^. It is estimated that the urban population facing water scarcity will double from 930 million in 2016 to 2.4 billion in 2050^2^. Providing clean drinking water to a growing population is becoming an important global challenge. Current water treatment infrastructure faces several significant challenges, including limitations in effectively targeting complex organic pollutants, such as dyes, high energy consumption due to the reliance on pumps for water collection and distribution over long distances, and, most critically, the inability to be deployed in resource-limited regions^3^. To address these challenges, the integration of renewable energy sources, such as solar-driven processes, can help alleviate energy-related issues and facilitate the development of low-energy, cost-effective techniques for water treatment applications.
Advanced oxidation processes (AOPs), widely used in water treatment applications, typically require a continuous supply of precursor chemicals that can be expensive and hazardous to handle^4^. Photocatalysis offers a green and sustainable alternative to conventional AOPs. Using semiconductor photocatalysts for advanced oxidation is advantageous because it eliminates the need to supply precursor chemicals continuously. It also generates highly oxidative electrons, holes and reactive oxygen species (ROS), which help in the degradation of more challenging organic compounds^5^. The benchmark photocatalytic nanomaterial for water treatment applications is titanium dioxide (TiO_2_)^6,7^. TiO_2_ possesses a relatively wide bandgap (E_g_~3.2 eV; 400 nm), which restricts its photoresponse to the ultraviolet (UV) region and limits absorption to approximately 4% of incident solar radiation^8^. To enhance the solar utilization of TiO_2_, various strategies have been explored, including band-gap narrowing to enable visible-light absorption. However, reducing the bandgap can compromise the oxidative potential of photogenerated ROS and increase electron-hole recombination due to decreased charge-carrier separation energy^9,10^. Consequently, alternative approaches that improve charge separation and interfacial charge transfer without substantially altering the intrinsic band structure of TiO_2_ have attracted increasing attention.
Small metals with surface plasmon resonance (SPR) termed plasmonic nanoparticles, can be employed to overcome these efficiency limitations. Plasmonic nanoparticles can act as light antennae when incoming resonant light radiation generates an intense electric-field enhancement, leading to absorption cross-sections many times greater than the size of the particle. Essentially, the particle can absorb more light than is incident on it by bending the path length of the light towards the particle^11^. The ability of plasmonic nanoparticles to harness the incoming light and improve absorption presents an opportunity to enhance photocatalytic efficiency without reducing the bandgap. Plasmon-enhanced photocatalysis with TiO_2_ has been widely explored using noble metals like gold, silver, and copper. Noble metals are rare, cost-prohibitive, and highly critical resources. Their filled or nearly filled d-orbitals often limit chemical reactivity under mild conditions, and despite visible-light absorption in some case, their large-scale use in photocatalysis is restricted by cost and scarcity ^12–14^. The proposed enhancement mechanisms for these plasmonic metal nanoparticles are improved light absorption over a wider range of the solar spectrum, and improved quantum yield for charge carrier generation^15^. Noble metals, such as Au and Ag, owing to their high work functions, form Schottky barriers with TiO_2_ that govern interfacial electron transfer. Although plasmonic excitation enables absorption of lower-energy photons, efficient charge transfer is strongly dependent on the barrier height and interfacial electronic structure, which can limit photocatalytic efficiency in some systems^16^.
Aluminum nanoparticles (Al-NPs) have garnered significant attention as a promising alternative to traditional noble plasmonic materials due to their absorption spectrum overlap with TiO_2_, enhanced light absorption properties, environmental abundance, and low metal criticality^16–25^. These characteristics position Al-NPs as a potential substitute in various applications, including photovoltaics^26^, surface-enhanced Raman scattering, and surface-enhanced fluorescence^26–30^. Previous studies considering the combination of Al and TiO_2_ have employed top-down, energy-intensive and expensive fabrication techniques to control the size and shape of the Al deposition. However, despite challenges related due to surface oxidation and size distribution, a solution-based approach is highly desirable for mass-scale fabrication as it offers several advantages, such as scalability, yield, crystallinity and better quality plasmonic responses. Herein, colloidal synthesis of aluminum nanoparticles offers an inexpensive, scalable opportunity which may be preferred over explored techniques like the electrical explosion^31^, oblique angle deposition^32^, direct-current voltage^19^ and electrophoretic deposition^33^. Few studies have explored the use of Al enhanced TiO_2_ in environmental applications, and none have employed colloidal solutions of Al-NPs for the fabrication of the Al/TiO_2_ heterostructures. As such, utilizing Al-NPs for colloidal Al/TiO_2_ heterostructure nanoparticle synthesis is a novel and inexpensive alternative.
Herein, we report the development of a solvent-based method to fabricate Al/TiO_2_ heterostructures for the degradation of anionic organic micropollutants in water under broad-band UV-B and simulated solar irradiation (Fig. 1). We designed and synthesized Al-NPs coupled with TiO_2_ via ligands to overcome challenges of particle stability. The presence or absence of the ligands in coupling the Al/TiO_2_ heterostructure and its role in organic degradation under different light sources was compared to an industrial standard benchmark material, P25 TiO_2_, a ~ 25 nm particle composed of a 4:1 ratio of anatase to rutile crystalline phases^34^. Moving towards a more practical engineering solution, the reusability of the fabricated heterostructure was compared against P25 TiO_2_ under solar irradiation.Fig. 1. Overview of the experimental setup and schematic illustration of synthesized photocatalytic-plasmonic heterostructure nanoparticles.
Results And Discussion
Aluminum Nanoparticles Characterization
Synthesized Al-NPs were found to contain an aluminum metal core and a thin oxide shell, as anticipated based on previous studies^35^. Structural characteristics and oxidation behavior of the Al-NPs were analyzed to understand the plasmonic properties and the role of the alumina shell in preventing further oxidation. Al-NPs generally sustain plasmonic resonances in the UV range^36,37^. Figure 2a shows the absorbance spectrum, which denotes the SPR peak in the near UV region around 305 nm for the synthesized Al-NPs (also refer to SI Fig. S1). This spectrum is desirable for solar and TiO_2_ applications, as it aligns closely with the bandgap of TiO_2_ and the highest energy photons available in the solar spectrum ( ~ 280 nm and longer). To confirm that the NPs were not entirely an oxide of aluminum (Al_2_O_3_) but consisted of an Al(0) core and an Al_2_O_3_ shell, we conducted high-resolution TEM imaging, as shown in Fig. 2b. The presence of aluminum was further confirmed by XRD and XPS analyses (SI Fig. S2). The absence of distinct Al_2_O_3_ diffraction peaks in the XRD pattern indicates that the oxide shell is amorphous, which is consistent with the HR-TEM observation shown in Fig. 2b. This imaging allowed for the measurement of the thickness of the Al(0) core and the Al_2_O_3_ shell. The NPs were synthesized over a month prior to the measurements to demonstrate the stability of the synthesized particles (SI Fig. S3). The diameter of the nanoparticle was found to be 10 nm, with an oxide layer of 3 nm surrounding the particle’s surface. The alumina shell around the nanoparticle surface is formed due to low oxidation resistance, creating a passivation layer that prevents bulk oxidation of the nanoparticles. This small thickness of the oxide layer is sufficient to prevent further oxidation. Figure 2b shows the highly crystalline aluminum core, with a visible atomic plane (111) exhibiting an interplanar distance of 0.235 nm +/- 0.038 nm as determined over 10 Al-NPs, as also indicated in the diffraction pattern shown. EDS mapping in Fig. 2c–f shows that the nanoparticles are composed of a core-shell structure, with a crystalline Al(0) metal core and a thin alumina shell. The size of Al nanoparticles conventionally quoted in studies includes the oxide layer^21,23,38,39^, however, due to the importance of shell thickness in SPR applications, we analyzed the diameter of the core and the shell thickness separately, as shown in Fig. 2g. It was observed that the Al(0) core ranged from 4.5 to 10 nm, with the alumina shell thickness ranging from 1 to 3.5 nm, based on measurements from 60 nanoparticles using high-resolution TEM images (see SI Figs. S4–5). This analysis of the size distribution of 60 NPs provided a more accurate determination of the actual diameter of the Al-NPs and the shell thickness separately.Fig. 2. Structural and optical characterization of synthesized aluminum nanoparticles.Ultraviolet-visible (UV-vis) absorption spectrum of the synthesized Al nanoparticles (Al-NPs) (a), high-resolution transmission electron microscopy (HRTEM) image of Al-NPs (scale bar: 10 nm) (b), and scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) images with energy-dispersive X-ray spectroscopy (EDS) elemental mapping showing the Al core and Al_2_O_3_ shell (c–f; scale bar: 10 nm). Distribution of Al core and Al_2_O_3_ shell thicknesses measured from 60 nanoparticles in suspension (g).
Improved Stability of ligand-modified Al/TiO2
To enhance the stability and performance of Al/TiO_2_ nanocomposites, we fabricated Al/TiO_2_ heterostructures using surface-binding ligands- Cys, GSH, and DMSA. These ligands facilitated the attachment of synthesized Al-NPs and P25 TiO_2_, thereby improving the stability of the Al/TiO_2_ heterostructures in aqueous solution. Two parameters were prioritized in the design: the concentration of Al-NPs and the presence of ligands. The Al-NPs concentration was intentionally low relative to TiO_2_ in order to minimize aggregation. Based on preliminary experiments, excessive Al-NPs loading led to a noticeable decline in photocatalytic activity, likely due to surface coverage that inhibited TiO_2_ surface reaction. Meanwhile, the ligands provide colloidal stability and prevent aggregation. The formation of a stabilizing interfacial layer around TiO_2_ particles was confirmed by the HRTEM technique, as shown in SI Fig. S6. The exceptional stability of the heterostructures during degradation is attributed to the strong interaction between the Al/Al_2_O_3_ NPs and P25 TiO_2_, facilitated by the covalent binding of thiol groups (-SH) and/or hydrogen bonding of amine groups (-NH_3_) to Al/Al_2_O_3_ NPs and carboxyl groups (-COOH) to P25 TiO_2_^40–42^.
Further investigations were undertaken to elucidate the specific attachment between Al/Al_2_O_3_ NPs and P25 TiO_2_. STEM-HAADF with EDS elemental mapping was used to analyze the Al/TiO_2_ Cys heterostructure’s composition and morphology. The core elements were Ti and O, with Al on the nanoparticle surface, as shown in Fig. 3a–d and confirmed by line scan analysis (Fig. 3e). FTIR-ATR analysis (400–2900 cm^−1^) provided insights into functional groups and ligand interactions in the Al/TiO_2_ heterostructures, as shown in Fig. 3f. FTIR spectra of Al/TiO_2_ Cys, Al/TiO_2_ DMSA, and Al/TiO_2_ GSH revealed characteristic bands for both P25 TiO_2_ and Al, indicating successful ligand modification to the surface of the Al-NPs and TiO_2_ resulting in covalent and hydrogen bonding. In Al/TiO_2_ GSH, bands at 2532, 1717, 1600, 1456, 1402, and 1077 cm^−1^ were assigned to S-H, C = O, N-H, CH_3_, CH_2_, O-H, and C-O vibrations^43,44^. The presence of deformed N-H and carboxyl group bands indicated that amine and carboxyl groups participated in bonding, while the thiol group did not. In Al/TiO_2_ DMSA, bands at 2527, 1648, and 1134 cm^−1^ confirmed S-H, C = O, and O-H vibrations, with the absence of the S-H band suggesting deprotonation for Al-S bond formation. In Al/TiO_2_ Cys, bands at 2527, 1650, 1595, and 1050 cm^−1^ suggested S-H, C = O, N-H, and C-N stretch, indicating Al-S and Al-N bond formation with the carboxyl group involvement in bond formation with P25 TiO_2_. These findings confirm the successful synthesis of ligand-modified Al/TiO_2_ heterostructures through covalent interactions. The Cys ligand demonstrates binding via three functional groups, -SH, -NH_2_, and -COOH, compared to the two functional groups (-SH and -COOH) bound in the other ligands, DMSA and GSH. This increased binding versatility suggests that Cys could offer enhanced performance in comparison to the other ligands.Fig. 3. Structural and chemical characterization of ligand-modified Al/TiO_2_ heterostructures.Scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) images with energy-dispersive X-ray spectroscopy (EDS) elemental mapping of cysteine-modified Al/TiO_2_ (Al/TiO_2_ Cys) heterostructures (a–d; scale bar: 50 nm), line-scan elemental analysis of Al/TiO_2_ Cys showing spatial elemental distribution (e; scale bar: 5 nm), and Fourier-transform infrared (FTIR) spectra of P25 TiO_2_, Al-NPs, and ligand-modified Al/TiO₂ heterostructures (f,g).
To elucidate the specific interaction between TiO_2_ and Al-NPs mediated by the Cys ligand, Cys-functionalized TiO_2_ was analyzed (Fig. 3g). Spectroscopic data revealed that the -NH_2_ and -COOH functional groups of Cys were involved in binding to the TiO_2_ surface. Characteristic bands observed at 2566, 1632, 1596, and 1055 cm^-1^ were attributed to vibrational modes of S-H, C = O, N-H, CH_3_, CH_2_, O-H, and C-O groups. The deformation of the N-H and carboxyl bands suggests that the amino and carboxyl groups are primarily responsible for the interaction with TiO_2_, whereas the thiol group remains unbound. These findings imply that, in the Al/TiO_2_ Cys heterostructure, the thiol moiety preferentially coordinates with the Al/Al_2_O_3_ surface rather than TiO_2_.
Influence of Cys Ligand on the Structural and Functional Properties of the Heterostructure
Two key characteristics of the cysteine (Cys)-modified Al/TiO_2_ heterostructure were examined: light absorption and surface properties. First, the light absorption property was evaluated by analyzing the band gap energy. The overlap in the band gap energies of the ligand-modified Al/TiO_2_ and P25 TiO_2_ (Fig. 4a) suggests that there is no significant alteration in the band gap structure of P25 TiO_2_ after coupling with Al-NPs (additional plots for other heterostructures are shown in Fig. S7, Table S1). The Tauc plot (Fig. 4b), derived from the reflectance data presented in Fig. 4a, was used to determine the band gap energy of the heterostructures, identified from the point where the tangent line intersects the x-axis.Fig. 4. Optical and physicochemical characterization of Al/TiO_2_ Cys, Al/TiO_2_, cysteine-modified TiO_2_, and P25 TiO_2_.Diffuse reflectance spectra as a function of wavelength (a), Tauc plots used to estimate bandgap energies (sample concentration: 1000 ppm, 3 mL) (b), and surface charge (zeta potential) and hydrodynamic size of the nanoparticles measured across pH 4-7 at a concentration of 0.003 w/v% (c,d).
The effect of pH on surface charge and colloidal stability of Al/TiO_2_-based heterostructures was systematically evaluated to elucidate the role of surface modification under environmentally relevant conditions. As shown in Fig. 4c, d, zeta potential measurements revealed pronounced pH-dependent variations in surface charge, with ligand-modified systems exhibiting non-monotonic behavior attributable to the presence of ionizable cysteine functional groups (SI Fig. S7). For Al/TiO_2_ Cys, a reduced surface charge magnitude was observed near neutral pH, consistent with a shifted isoelectric point relative to bare P25 TiO_2_. Corresponding hydrodynamic diameter measurements indicated increased aggregation under conditions where the zeta potential approached neutrality, highlighting the importance of electrostatic stabilization in governing colloidal behavior. At lower pH values (pH 4-5), Al/TiO_2_ Cys maintained moderate surface charge and reduced aggregation, suggesting that cysteine ligands contribute steric and electrostatic stabilization under acidic conditions. Variations in absolute zeta potential values are attributed to differences in nanoparticle batch preparation; however, the overall pH-dependent trends remain consistent.
Performance Enhancement by Al-NPs
The incorporation of aluminum nanoparticles (Al-NPs) enhanced the photocatalytic activity of P25 TiO_2_ when conjugated with Cys ligands, in comparison to both unmodified P25 TiO_2_ and Cys-modified P25 TiO_2_ alone, for the degradation of the anionic dye amaranth under solar irradiation (Fig. 5a; p-value = 0.00035). This enhancement is not attributed to the ligand alone, as Cys-functionalized P25 TiO_2_ showed no significant improvement in photocatalytic performance (Fig. 5a–d). Instead, the improvement is primarily attributed to the plasmonic and adsorptive functionalities imparted by the Al/Al_2_O_3_ nanoparticles when coupled with Cys to P25 TiO_2_. Despite the presence of a ~ 5 nm Al_2_O_3_ passivation layer around the Al-NPs, interfacial charge transfer between the Al core and the P25 TiO_2_ via the Cys ligands may remain a possibility as previous studies have demonstrated such charge transfer, even in the presence of an Al_2_O_3_ shell, enhancing photocatalytic degradation through electron tunneling mechanisms between the metal core and the semiconductor via a ligand interface^19,40,45^. The Al-NP core contributes electrons through SPR, concentrating electromagnetic energy at the nanoparticle surface, which likely facilitates photo-induced charge transfer to the TiO_2_ via the Cys ligand. Concurrently, the Al_2_O_3_ shell serves multiple roles: it acts as an electron acceptor, promoting the adsorption of polar organic molecules, such as amaranth near TiO_2_ reactive sites, and potentially functioning as an electron trap that prevents charge recombination within the TiO_2_ photocatalyst system. Overall, the synergistic combination of Al-NPs (core) plasmonic enhancement, contaminant adsorption by Al_2_O_3_ shell, and the interfacial electron transfer mediated by the Cys ligand appears to drive the enhanced photocatalytic degradation observed in Fig. 5a. Nevertheless, further mechanistic studies are warranted to fully elucidate the charge transfer dynamics underpinning this enhanced performance.Fig. 5. Solar and UV-B photocatalytic degradation of amaranth and phenol using Al/TiO_2_ Cys and P25 TiO_2_.Photocatalytic degradation of amaranth (C_0_ = 20 ppm, 20 min) and phenol (C_0_ = 10 ppm, 25 min) under simulated sunlight and broadband UV-B irradiation. Comparison of degradation efficiencies for amaranth under simulated sunlight (a), phenol under simulated sunlight (b), amaranth under broadband UV-B (c), and phenol under broadband UV-B (d) using Al/TiO_2_ Cys, P25 TiO_2_, Al/TiO_2_, and TiO_2_ Cys. Concentrations were recorded at 5 min intervals.
To further understand the role of ligand chemistry in photocatalytic performance, we investigated the degradation efficiency of Al/TiO_2_ heterostructures modified with different ligands in additional to Cys, namely GSH and DMSA (Supporting Information, Fig. S8–12; Text S1, Table S2). The photocatalytic performance of Cys-modified P25 TiO_2_ without the addition of Al-NPs was found to be similar to that of unmodified P25 TiO_2_ in the degradation of the anionic dye amaranth, indicating negligible impact of the ligand alone on the degradation of the contaminant (Fig. 5a, c). However, in the degradation of phenol, a neutral contaminant, Cys-modified P25 TiO_2_ exhibited reduced performance under both UV and solar light sources, a finding that will be elaborated in the following section (Fig. 5b, d). Among the three ligands tested, Cys showed the most effective enhancement in the Al/TiO_2_ system when benchmarked against pristine P25 TiO_2_ (Supporting Information, Fig. S8–12; Text S1, Table S2). Despite Cys having minimal effect on the performance of TiO_2_ alone, its inclusion in the Al/TiO_2_ heterostructure resulted in a notable improvement in photocatalytic activity, particularly for the degradation of amaranth under solar illumination (Fig. 5a). These observations suggest that while the Cys ligand does not intrinsically enhance the photocatalytic properties of TiO_2_ for anionic dye degradation, it plays a critical role when part of the Al/TiO_2_ heterostructure.
Performance Evaluation under Different Light Sources
The photocatalytic performance of Al/TiO_2_ heterostructures was evaluated under two light sources: UVB (280-320 nm, 14.6 W m⁻²) and a solar simulator (280-700 nm, 268.26 W m⁻²) as shown in Fig. 5a–d. Notably, the Al/TiO_2_ Cys heterostructure exhibited superior photocatalytic activity compared to commercial P25 TiO_2_ under solar irradiation (Fig. 5a, b). Under UVB irradiation, the performance enhancement of Al/TiO_2_ Cys over P25 TiO_2_ was modest, with only a ~ 10% increase in degradation efficiency observed for the degradation of amaranth (Fig. 5c). In contrast, under solar illumination, the difference in photocatalytic activity became more pronounced. After 20 minutes of exposure, Al/TiO_2_ Cys achieved 71% degradation, while P25 TiO_2_ reached only 53%, corresponding to a 60% relative increase in degradation efficiency (Fig. 5a). This significant enhancement under solar irradiation, which more closely mimics natural sunlight, highlights the practical applicability and cost-effectiveness of Al/TiO_2_ Cys for real-world environmental remediation. The observed performance improvement is attributed to several synergistic mechanisms: (i) charge transfer mediated by the Cys ligand, (ii) enhanced charge separation due to the Al_2_O_3_ interface, and (iii) SPR-induced energy transfer between Al-NPs and P25 TiO_2_. While these mechanisms offer a plausible explanation for the observed enhancements, isolating the relative importance and overall contribution of each mechanism remains a significant challenge.
Selective Reactivity of the Heterostructure Toward Anionic Contaminants
The Al/TiO_2_ Cys heterostructure exhibited superior photocatalytic degradation of the anionic dye amaranth compared to the neutral molecule phenol under both solar and UVB irradiation, as shown in Fig. 5a–d. In contrast, the Al/TiO_2_ system lacking the Cys ligand demonstrated degradation performance for amaranth that was nearly identical to that of pristine P25 TiO_2_ (Fig. 5a, c), suggesting that in the absence of ligand-mediated coupling, Al-NPs do not obstruct the surface accessibility of TiO_2_ for anionic species. However, for phenol degradation, the Al/TiO_2_ system (without Cys) showed markedly reduced activity, with performance comparable to that observed for photolysis-degradation induced by light in the absence of a photocatalyst-under both UVB and solar irradiation (Fig. 5b, d). To investigate the lower photocatalytic degradation efficiency of phenol, zeta potential and hydrodynamic diameter measurements were conducted following phenol exposure (SI Fig. S13 a,b). The increase in positive surface charge with decreasing pH likely influenced phenol adsorption behavior and, consequently, its photocatalytic degradation.
These results imply that the charge characteristics of the target contaminant play a critical role in determining the efficacy of the Al/TiO_2_ system. For the anionic dye amaranth, efficient adsorption and interaction with TiO_2_ active sites are preserved, even without ligand coupling. In contrast, neutral phenol appears to induce unfavorable surface interactions that impair performance. Control experiments confirmed the photostability of both amaranth and phenol in the absence of photocatalysts under solar and UVB irradiation (Fig. 5a–d). Furthermore, as evidenced by Supplementary Fige. S14 and S15, all three ligand-modified Al/TiO_2_ heterostructures showed negligible adsorption of either contaminant in the absence of irradiation, reinforcing that photocatalytic degradation is primarily light-activated and not governed by passive adsorption processes.
Reactive species trapping analysis
Reactive species trapping experiments were conducted to elucidate the dominant oxidative pathways involved in amaranth dye degradation over Al/TiO_2_ Cys (SI Fig. S13c). The addition of EDTA, a hole scavenger, resulted in a pronounced suppression of photocatalytic activity, indicating that photogenerated holes play a major role in the degradation process. In contrast, the presence of isopropanol (IPA), a hydroxyl radical scavenger, led to a partial decrease in the degradation rate, suggesting that •OH radicals contribute to the reaction but are not the primary active species. The highest degradation rate was observed in the absence of scavengers, implying the synergistic involvement of multiple reactive species. These results highlight the importance of hole-driven oxidation pathways in the Al/TiO_2_ Cys system.
Photocatalyst Reusability
To promote technological applications in photocatalysis, the catalyst must be stable throughout the reaction. SI Figure. S16 and S17 show the crystal structure of Al-NP along with P25 TiO_2_ before and after the amaranth degradation experiment for 20 min. The interplanar distance between Al-NPs and P25 TiO_2_ remained essentially unchanged from the beginning to the end of the experiment. This demonstrates that the fabricated Al/TiO_2_ Cys undergoes negligible structural change during the 20-minute photodegradation experiments, indicating good photostability and aqueous stability; therefore, the Al-NPs retain their plasmonic properties. In addition, the oxidation of the ligand in the heterostructure is theoretically possible (a limitation), but it was not observed in our experiments. To further demonstrate the stability of the catalyst, reusability tests were performed to measure the change in degradation performance after repeated testing.
Reusability of photocatalysts is a key consideration for their practical application in water treatment. The reusability of the ligand-modified Al/TiO_2_ Cys heterostructure was evaluated over six consecutive cycles under simulated solar irradiation (Fig. 6a). For all cycles, the reactor volume and amaranth dye concentration (20 ppm) were kept constant; and the amount of Al/TiO_2_ Cys was normalized based on Brunauer-Emmett-Teller (BET) surface area to account for differences in available active sites relative to P25 TiO_2_ to enable direct comparison of performance. Each run consisted of a 30 min dark adsorption phase followed by 60 min of irradiation. At the end of each cycle, aliquots of the suspension were centrifuged, and the recovered catalyst was resuspended in an equal volume of deionized water and returned to the reactor without additional washing or drying steps.Fig. 6. Reusability and structural stability of Al/TiO_2_ Cys during amaranth degradation under simulated solar irradiation.Reusability of Al/TiO_2_ Cys evaluated over six consecutive cycles for the photocatalytic degradation of amaranth (C_0_ = 20 ppm) under simulated solar irradiation (with a UVC filter blocking UV light < 280 nm; irradiance: 286.26 W m⁻^2^). Comparison of degradation performance of Al/TiO_2_ Cys and P_2_5 TiO_2_ over six cycles (a), photographs showing color changes at the end of each cycle for Al/TiO_2_ Cys and P_2_5 TiO_2_ (b), high-resolution transmission electron microscopy (HRTEM) image of Al/TiO_2_ Cys indicating FFT#1 for TiO_2_ and FFT#2 for Al-NP after the sixth cycle (scale bar: 20 nm) (c), and photograph of amaranth color change during the first cycle using Al/TiO_2_ Cys (d).
The Al/TiO_2_ Cys photocatalyst achieved ~94% conversion compared to 82% by P25 TiO_2_ in the first cycle. Throughout the reusability tests, Al/TiO_2_ Cys consistently exhibited higher apparent first-order reaction rate constants than P25 TiO_2_ (Fig. 6a, b, d). A decrease in reaction rate with increasing cycle number was observed for both catalysts, which may be associated with partial surface coverage by adsorbed dye molecules and a reduction in accessible active sites.
Moreover, the reuse of conventional powder catalysts typically requires recovery by centrifugation followed by washing and oven drying, which renders the process inefficient and cost-prohibitive for practical applications. In contrast, the fabricated Al/TiO_2_ Cys do not require washing or drying steps during reuse, as demonstrated in the reusability experiments. Combined with their improved photocatalytic efficiency compared to P25 TiO_2_, the sustained performance over multiple reuse cycles indicates that Al/TiO_2_ Cys has acceptable structural stability (Fig. 6c) and potential for reuse. Further studies are required to assess their long-term performance and robustness under realistic environmental conditions.
Methods
Materials
All reagents for nanoparticle synthesis were purchased from Sigma-Aldrich. Dimethyl ethyl alane alanine (DMEAA, 99.999%) served as the precursor, trioctylphosphine as the solvent, titanium isopropoxide as the catalyst, and dibutylphosphate as the capping agent. Phenol and amaranth dye were also sourced from Sigma-Aldrich. Ligands Glutathione (GSH), dimercaptosuccinic acid (DMSA), and L-Cysteine (Cys), selected for their -COOH and -SH groups to bind with P25 TiO_2_ and Al-NPs, were used to promote charge transfer^45,46^. Aeroxide P25 TiO_2_ (100 mg), a widely accepted industrial standard material known for its photocatalytic properties, low cost, and broadband UV absorption (SI Figs. S18–19), was supplied by Evonik Industries.
Synthesis of Photocatalytic Nanomaterials
Al-NPs were fabricated via thermal decomposition of DMEAA using a modified Schlenck line procedure adapted from Renard et al^35^, as further detailed in SI Text S2. The nanoparticle supernatant was diluted using IPA to 67-70 µg/L, and 5 mL of the diluted solution was used for heterostructure fabrication. Ligands (Cys, GSH, and DMSA) were added, sonicated and mixed with the synthesized Al-NPs in IPA suspension for 30 minutes (SI Text S2 & Fig. S20). Then, 20 mg of P25 TiO_2_ was added, and the suspension was sonicated and mixed for another 30 min. The resulting suspension was centrifuged at 5000 rpm for 15 min to separate the nanoparticles as pellets and supernatant. The supernatant was discarded, and the wash step was repeated with the addition of IPA to remove unbonded ligands. The supernatant was again discarded, and the remaining IPA was allowed to evaporate in the fume hood, following which the nanoparticles were sealed and refrigerated at -80 °C for lyophilization.
Material Characterizations
Transmission electron microscopy (TEM) with a Thermo Scientific Talos F200X G2 S/TEM at 200 kV and Energy-dispersive X-ray spectroscopy (EDS) were used to examine the surface morphology, crystallinity, and composition of the nanoparticles. Velox software estimated the shell thickness and particle size distribution of Al-NPs, while ImageJ was used to calculate the interplanar distance of Al-NPs and P25 TiO_2_. X-ray diffraction (XRD) was employed to confirm the crystalline structure. UV-vis spectroscopy with a Genesys 50 spectrophotometer was used to determine the absorption spectra, while the bandgap of the Al/TiO_2_ heterostructures was analyzed using a PerkinElmer Lambda 850 Plus Spectrophotometer with a 150 mm Integrating Sphere (UV-DRS). Bandgap energies were calculated from reflectance spectra with UV WinLab Software. Hydrodynamic diameter and ζ potential were measured by dynamic and electrophoretic light scattering (Anton Paar Litesizer DLS 501) at pH 4-7. Concentration of Al-NPs used in Al/TiO_2_ heterostructures were determined using Perkin Elmer ICP-OES Optima 8300 after acid digestion, yielding 67–70 µg/L (details in SI Fig. S21).
Concentration Determination of Al-NPs by ICP-OES
Al-NPs samples were prepared for ICP-OES analysis by using a modified hot plate-assisted digestion protocol^47^. 1 mL of Al-NPs in IPA was left in the fumehood to evaporate the solvent. The sample was then dissolved in 1 mL 67–70% HNO_3_. The sample was digested by heating at 80 °C and refluxing for 30 minutes without boiling, followed by allowing the sample to cool for 10 s. Another 1 mL of HNO_3_ was added and heated to 80 °C until no brown fumes appeared. The sample was heated for 2.5 h until ~100 µL volume was remaining and diluted 10x with filtered deionized water. Following this, the sample was filtered with a 0.45 µm syringe filter, diluted 12x, 14x, 16x, 18x with filtered deionized water, producing concentrations in the range of 10–1000 µg/L, and pH < 1. Samples were run on a Perkin Elmer ICPOES Optima 8300 using QC 4 as Al standard solution prepared in deionized water. Once samples with known concentrations were determined, results were used to create a UV-vis calibration with absorption in a range of 315–325 nm, where the peak SPR wavelengths, as shown in Fig. S5a, b allowing for facile and accurate determination of sample concentrations using UV-vis measurements prior to further experimentation. Standard calibration results for Al dilution standards recorded at 394.401 nm emission line are shown in Fig. S21 b^48^.
Degradation Test
The performance of ligand-modified Al/TiO_2_ heterostructures was compared to industry-standard P25 TiO_2_ by evaluating phenol and amaranth dye degradation rates using UV-vis. An overview of the experimental set-up is shown in Fig. 1. In each experiment, 3 mg of catalyst was dispersed in 30 mL of solution (20 ppm dye, 10 ppm phenol), sonicated for 10 minutes, and stirred in the dark at 240 rpm for 30 minutes to reach adsorption equilibrium. For the degradation study, the solutions were exposed to broadband UV B (UV-B, 280-320 nm) irradiation from a SoIR 100-Series Phototherapy lamp with 14.6 W m^-2^ intensity and simulated solar irradiation from an Oriel LCS-100 Solar Simulator with a solar AM 1.5 G filter and 286.26 W m^-2^ intensity (with a filter to block UV light <280 nm). The intensity of light at the reaction level was measured using an ILT960-Series Spectrometer, as shown in SI Fig. S22. The distance between the light source and the reaction medium was 3.5 cm (UV-B lamp) and 17 cm (solar simulator). Samples were taken every 5 minutes, syringe filtered using a 0.45 μm PTFE filter, centrifuged, and analyzed via UV-vis to measure absorbance at 270 nm (phenol) and 520 nm (amaranth). pH was recorded at the start and end of each experiment.
Reusability Study
Reusability of the ligand-modified Al/TiO_2_ Cys photocatalyst was examined through six consecutive photocatalytic degradation cycles of amaranth dye (C_o_=20 ppm) under simulated solar irradiation. Each cycle consisted of a 30 min dark adsorption period followed by 60 min of irradiation. At the end of each cycle, aliquots were centrifuged, and the supernatant was analyzed by UV-Vis spectroscopy. The recovered photocatalyst was then resuspended in an equal volume of deionized water and reused without washing or drying.
Supplementary information
SI_Paper_2025-01-26
The reference list from the paper itself. Each links out to its DOI / PubMed record.
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