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Article

Eco-Friendly TiO2 Nanoparticles: Harnessing Aloe Vera for Superior Photocatalytic Degradation of Methylene Blue

by
Agnese De Luca
1,
Angelantonio De Benedetto
1,
Valeria De Matteis
1,2,*,†,
Mariafrancesca Cascione
1,2,*,†,
Riccardo Di Corato
2,3,
Chiara Ingrosso
4,
Massimo Corrado
1 and
Rosaria Rinaldi
1,2
1
Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Via Arnesano, 73100 Lecce, Italy
2
Institute for Microelectronics and Microsystems (IMM), CNR, Via Monteroni, 73100 Lecce, Italy
3
Center for Biomolecular Nanotechnologies, Istituto Italiano di Tecnologia (IIT), 73010 Arnesano, Italy
4
CNR-IPCF S.S. Bari, c/o Department of Chemistry, Università degli Studi di Bari, Via Orabona 4, 70126 Bari, Italy
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2024, 14(11), 820; https://doi.org/10.3390/catal14110820
Submission received: 1 October 2024 / Revised: 11 November 2024 / Accepted: 13 November 2024 / Published: 13 November 2024
(This article belongs to the Special Issue Recent Developments in Photocatalytic Water Treatment Technology)
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Versions Notes

Abstract

:
In recent years, the contamination of aquatic environments by organic chemicals, including dyes such as methylene blue (MB), Congo red, and crystal violet, has become an increasing concern, as has their treatment. In this work, titanium dioxide nanoparticles (TiO2 NPs) were studied for their photocatalytic performance by measuring the degradation of MB under UV light. TiO2 NPs were synthesized using two synthetic processes optimized in this study: a green method, namely leveraging the natural properties of Aloe vera leaf extract; and a conventional approach. The resulting NPs were thoroughly characterized using X-rays Diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Brunauer–Emmett–Teller (BET), UV–Vis and ζ-potential analysis. The TiO2 NPs synthesized by the green method demonstrated a degradation efficiency of (50 ± 3)% after 180 min, which was significantly higher than the (16 ± 3)% achieved by NPs synthesized through the conventional route. Moreover, the reaction rate constant for the green-synthesized TiO2 NPs was found to be approximately five times greater than that of the conventionally synthesized NPs. These results open up a new scenario in the pollution removal strategy research, using resources accessible in nature to synthesize NPs with high photocatalytic activity, which could also be useful for other applications, such as hydrogen production.

1. Introduction

In recent years, the aquatic environment has been found to be contaminated by organic chemicals such as industrial chemicals, pharmaceuticals and consumer products [1]. These substances are only partially removed through conventional physical and biological wastewater treatment, necessitating the use of additional technologies to eliminate toxic pollutants and biologically recalcitrant compounds [2]. For instance, synthetic dyes, such as congo red, toluidine blue, crystal violet and methylene blue, do not biodegrade easily, so they remain in the environment [3]. Advanced oxidation processes (AOPs) are widely employed for treating various types of wastewaters, often in conjunction with biological treatments [4]. AOPs facilitate the in-situ generation of strong oxidants that enable the oxidation of organic compounds [5]. Different AOPs utilize different mechanisms for organic destruction and can be classified into ozone-based, UV-based, catalytic (cAOP), physical (pAOP) and electrochemical (eAOP) AOPs [6]. Heterogeneous photocatalysis is the one of the most investigated AOPs [7,8]. It involves the creation of electron–hole pairs through the absorption of photons with energy equal to or greater than the semiconductor bandgap [9]. If electron–hole recombination does not occur, these charge carriers can induce the redox reaction with adsorbed species on the semiconductor surface, producing radical species such as hydroxyl radical, superoxide radical anions, and other reactive oxygen species that facilitate the degradation of pollutants [10].
Among several semiconductors, titanium dioxide (TiO2) is the most thoroughly investigated due to its properties, such as its chemical and thermal stability, high photoactivity, cost-effectiveness, and low toxicity [11]. TiO2 exhibits exceptional photocatalytic activity when exposed to UV light. Its wide bandgap allows it to generate electron–hole pairs upon irradiation, which are essential for initiating the oxidation and reduction reactions necessary to degrade organic compounds like methylene blue [12,13]. Most recently, due to the necessity of using UV light for TiO2 irradiation, much research has been focused on different kinds of photocatalysts in order to develop photocatalytic processes that efficiently utilize the visible light [14,15,16]. For instance, graphitic carbon nitride (g-C3N4) is a material with promising properties to be used as a catalyst for treating contaminants in aqueous solution [17]. However, bare g-C3N4 was found to have a low and unsatisfactory photocatalytic efficiency, requiring the use of strategies to increase its photocatalytic activity [18]. Another kind of innovative photocatalyst are MXenes and MXene-based nanomaterials, which exhibit fascinating properties such as the possibility of adjusting morphology and bandgap configuration, large specific surface area and thermal strength [19]. However, current studies have only been limited to Ti3C2Tx MXene-based nanocomposites, mostly covering the treatment of only a few dyes, and in addition, an ecotoxicological assessment is still lacking [20]. TiO2 exhibits many advantages over the broad range of other nanomaterials [21], among which there are the following: the more powerful oxidative power of valence band holes compared to the reducibility of photoinduced electrons; the possibility of producing TiO2 at an industrial level; the fact that it is easy to obtain, inexpensive, and it can be easily synthesized in the laboratory; and being chemically and photochemically stable, safe and non-toxic [22]. TiO2 occurs in nature in four polymorphisms: anatase; rutile—both with a tetragonal crystal structure—brookite, with an orthorhombic geometry; and TiO2(B), the monoclinic phase of titanium dioxide [23]. Anatase and rutile are the most commonly occurred forms [24], finding numerous applications including photocatalysis. Anatase exhibits considerably a higher photocatalytic activity than rutile [25], but the photocatalytic activity of TiO2 depends on several factors, including phase structure, crystallite size, specific surface area and pore structure [26].
Several strategies can be employed to obtain TiO2 NPs [27], including sol–gel, hydrothermal, and solvothermal methods. For instance, Collazzo et al. used the hydrothermal method with titanium tetraisopropoxide (TTIP) as a precursor in order to obtain TiO2 nanopowders with a crystallite size ranging from 9 to 17 nm, depending on the synthesis conditions such as temperature and reaction time [28]. Li et al. prepared nano-TiO2 powders by sol–gel method using tetra-n-butyl-titanate as precursor, investigating different synthesis parameters such as calcination temperature and pH value, to control the grain size and microstructure of nano-TiO2 powders [29]. Xu et al. synthesized nano-TiO2 from Tetrabutyl titanate (n-TBT) [30], obtaining NPs with a homogeneous microstructure and a size of around 10–15 nm through a sol–gel process mediated in reverse microemulsion combined with a solvent thermal technique. Sadek et al. used TTIP to prepare TiO2 nanopowders with a crystal size of 49.3 nm through the sol–gel method [31]. Currently, the growing use of NPs in various applications has stimulated the development of a more inexpensive and sustainable synthesis approach. In particular, the green approach [32], based on the use of natural source materials, allows the elimination or reduction in the use of chemical reagents and the generation of hazardous substances [33]. Green synthesis can be carried out by means of the use of plants and their extracts as well as the microbes, although the former is considered more stable [34]. Several parts of the plant, such as flowers, roots, seeds, and leaves can be employed to prepare plant extracts, though leaves are more commonly used. Leaves are rich in biomolecules such as proteins, amino acids, terpenoids, flavonoids, and saponins. These molecules are key elements in the synthesis of NPs because they act as reducing agents and capping agents as stabilizer and redox mediators. Santhoshkumar et al. prepared TiO2 NPs using TiO(OH)2 as a precursor and the aqueous extract of Psidium guajava leaves [35]. Ahmad et al. synthesized spherical TiO2 NPs ranging from 20 to 70 nm using TTIP and Mentha arvensis leaves extract as a precursor and reducing agent, respectively [36]. Saini et Kumar achieved the green synthesis of TiO2 NPs, with an average crystallite size of 15.02 nm, by mixing the Tinospora cordifolia leaves extract to the precursor, TTIP [37]. Aloe Vera leaves are considered a waste product because they are the residues of the pharmaceutical and cosmetic industries, as the result of processes in which only the Aloe Vera gel is used, after the mechanical separation from the leaf. Several studies have shown the use of Aloe Vera leaves for different applications ranging from catalyst support material [38], energy field [39,40] to nanomedicine [41,42]. However, most works in which Aloe Vera leaves extract was used exploit the entire leaf and thus also the gel content [43,44], whereas in the present study, only the outer part of the leaf was used, leaving the gel for other applications.
Indeed, in this context, TiO2 NPs have been synthesized via both a green route using Aloe Vera leaves extract and a conventional route. The properties of the obtained NPs were characterized by different techniques, specifically TEM, SEM, XRD, BET, UV–vis and ζ-potential measurements. Moreover, the photocatalytic activity of NPs was evaluated by the degradation of methylene blue (MB) under UV light, and the effect of the calcination temperature for TiO2 NPs synthesized by the green route was investigated. MB was chosen because it is a popular cationic dye frequently used for dyeing (clothes, paper, and leathers) and in the textile industry [3] and it is harmful to human health above a certain concentration [45]. Additionally, methylene blue is resistant to biodegradation, meaning it can persist in the environment for extended periods, leading to the long-term contamination of water bodies [46]. It is highly toxic to aquatic life, even at low concentrations, disrupting ecosystems by affecting the health and reproduction of various species [47]. It also absorbs strongly in the visible spectrum, which can block sunlight from penetrating the water, inhibiting photosynthesis in aquatic plants and algae, further disturbing the balance of aquatic ecosystems [3,48]. Moreover, methylene blue can enter the human food chain through contaminated water sources, posing health risks to humans and animals [49,50]. These characteristics make it a significant concern for environmental pollution. The importance of using green synthesis for environmental purposes, as in the case of wastewater treatment, lies in the fact that the problem that has to be solved is an environmental issue, so it is better to reduce the use of chemical reagents for NP synthesis because their use and disposal could also become an environmental threat.

2. Results and Discussion

The findings of this study underscore the potential of green synthesis methods for TiO2 NP production, namely by enhancing the efficiency of photocatalytic processes for environmental remediation. In particular, we focused on MB, which is considered a potent pollutant. TiO2 NPs are crucial in the photocatalysis of methylene blue due to their unique properties and effectiveness in breaking down organic pollutants [51,52]. However, synthetic processes require the use of toxic substances and time-consuming equipment. Therefore, the development of environmentally friendly methods to obtain highly efficient TiO2 NPs is necessary in order to make it a safe option for treating wastewater without introducing additional harmful substances into the environment [32,53]. Green-synthesized TiO2 NPs are generally considered non-toxic and eco-friendly, as highlighted in recent studies. For instance, several works reported that TiO2 NPs produced through green synthesis methods show significantly reduced toxicity compared to conventionally synthesized TiO2 [54,55]. The use of biocompatible capping agents from plant extracts in green synthesis often results in nanoparticles with lower reactivity and minimal ROS generation, which are primary contributors to toxicity in aquatic environments. Furthermore, the green-synthesized TiO2 NPs demonstrate negligible bioaccumulation in various organisms, supporting their potential as a safer, eco-friendly alternative [56]. In our work, we obtained TiO2 NPs from a conventional route and green route using, in the latter case, Aloe Vera leaf extract. This plant is widely distributed in the Mediterranean region and is extensively used in the agricultural, biomedical, and cosmetic fields. In the latter case, in particular, the gel found in the leaves is a valuable material often used to produce skin and beauty lotions. On the other hand, the leaf epidermis, which is considered a waste product, contains high concentrations of vitamins, proteins, and polyphenols, and its extract can be used to produce TiO2 NPs. The biomolecules enriched the extracts acting as reducing and capping agent [44,57]. The possible principle involves the phytochemicals present in Aloe vera that reduce titanium salts (i.e., titanium(IV) isopropoxide) to TiO2 NPs under mild conditions, avoiding the need for harsh chemicals. These biomolecules also stabilize the nanoparticles, controlling their size and morphology. The green synthesis method is eco-friendly, and the Aloe vera extract helps produce TiO2 NPs with enhanced photocatalytic properties due to the presence of these bioactive compounds [58,59].
Then, we proceed to achieve TiO2 NPs from the two different routes described in the Materials section following the characterization of their physico-chemical properties. Firstly, we investigated TEM analysis (Figure 1) showing that synthesized TiO2 NPs showed an irregular shape, as also reported in the literature [60,61,62]. It was observed that the synthesized NPs had a similar size. An average size of (12 ± 3) nm for the TiO2-chem (Figure 1a,c,e) NPs and an average size of (10 ± 2) nm for the TiO2-green NPs were estimated (Figure 1b,d,f). However, aggregation phenomena were also observed, which were more evident in the case of TiO2-green NPs, probably due to the high concentration of organic compounds enriching the leaves extract.
In order to better analyze the morphologic characteristics of the two types of NPs, SEM analysis was performed. In Figure 2a,b TiO2-chem NPs and TiO2-green NPs were reported, respectively. The SEM images derived from chemical approach showed a spherical shaped morphology confirming the TEM acquisition. The TiO2-green NPs were more aggregated for the presence of organic materials. The EDS spectra confirmed the presence of Ti and O (Figure 2c,d).
Both synthesized TiO2 NPs showed a negative surface charge in ultrapure water at neutral pH. ζ-potential values of (−18.2 ± 0.2) mV and (−28.4 ± 0.9) mV were observed for TiO2-chem and TiO2-green NPs, respectively. Therefore, the adsorption of MB is expected to be favored on the surface of the synthesized NPs, since MB is a cationic dye [63,64].
In addition, since the photocatalytic activity is strictly correlated to the surface area, we performed this measure in order to add more information in evaluating the efficiency of photocatalytic TiO2-chem and TiO2-green NPs.
The values of the specific surface area from both types of NPs calculated from the BET method are reported in Table 1. The BET specific surface area of TiO2-chem NPs was estimated to be 63.2 m2/g. This value increased in TiO2-green NPs (70.2 m2/g).
A larger surface area, typically associated with smaller particles or a porous structure, provides more active sites available for the adsorption of pollutant or reagent molecules. This increase in active sites tends to enhance photocatalytic efficiency, as more molecules can interact with the photogenerated charges (electrons and holes) produced by irradiation.
The UV characterization showed a pronounced absorption peak at a wavelength of 335 nm from both types of TiO2 NPs (Figure 3 left). The optical behavior of NPs was investigated using the diffuse reflectance spectrum (Figure 3 right). The spectrum at 385 nm indicates the coordinated electronic transition between the O 2p state and the Ti 3d state. During the green approach, the colloidal solution changed color from white to yellowish-gray, indicating the formation of NPs. The white solution indicated the formation of TiO2 during the process. The absorption peak was associated with changes in the crystalline phase and the average crystalline size. Consequently, the investigated NP is suitable for catalytic applications. The absorption peak around the 385–400 nm region confirmed the formation of TiO2 NPs, while the reflectance spectra were consistent with previous studies [59,65].
The optical band gap can be calculated from the Kubelka–Munk method using Tauc relation on reflectance spectra according to [66] (Table 2).
These results were interesting, since the lower band gap for TiO2-green NPs means a good photocatalytic behavior with respect to TiO2-chem NPs.
The XRD pattern of the synthesized TiO2 NPs (Figure 4) showed characteristic peaks of anatase for both TiO2-chem and TiO2-green NPs around the following 2θ values: 25.4° (101), 37.9° (004), 48.1° (200), 62.9° (204). Two small characteristic peaks of rutile phase, around 27.5° (110) and 36.2° (110), were only observed for the TiO2-green NPs [67,68]. In addition, a small peak at around 30.8° (121), which can be related to the brookite phase [69], was also observed for both TiO2 NPs. The anatase phase generally has a higher photocatalytic performance than that of rutile due to a higher density of localized states and consequent surface-adsorbed hydroxyl radicals and lower recombination of photogenerated electrons and holes in anatase than in rutile [70].
After TiO2 characterization, we performed the assessment of their photocatalytic activity. The photocatalytic activity of the synthesized TiO2 NPs was evaluated by the degradation of MB in aqueous solution under UV light (Figure 5, upper image). As shown in a lower image of Figure 5, it was found that the kinetics of the photocatalytic degradation follow a pseudo-first-order model (Equation (1)):
l n C t C 0 = −   k · t
where Ct is the concentration of MB at time t, C0 is the concentration of MB at time t = 0 and k is the reaction rate constant. The reaction rate constant for TiO2-green NPs was about five times higher than that for TiO2-chem NPs. As shown in Table 1, the reaction rate constants were 0.004 min−1 and 0.0008 min−1 for TiO2-green and TiO2-chem NPs, respectively. The degradation efficiency was calculated using the following Equation (2):
D e g r a d a t i o n   e f f i c i e n c y % = C 0 − C t C 0 · 100
where C0 is the concentration of MB at time t = 0 and Ct is the concentration of MB at time t.
The general mechanism of degradation is that the generated reactive oxygen species (•OH, O2•−, and others) are highly oxidative and can attack the MB molecules, breaking down their chromophore structures and ultimately mineralizing them into CO2, H2O, and other less harmful by-products.
This degradation efficiency was obtained using a TiO2 NPs concentration of 0.4 g/L that is lower than the concentration used in other works, such as in the study carried out by Arvind et al., in which 100 mg of TiO2 NPs in 100 mL MB solution were used [65]. In addition, only 2 UV lamps of 8 W were used in the irradiation, differently from many other studies [71,72,73].
As shown in Table 3, the TiO2-green NPs showed higher efficiency for MB degradation than the TiO2-chem NPs. In fact, the degradation efficiency values obtained after 180 min were (50 ± 3)% and (16 ± 3)% for TiO2-green and TiO2-chem NPs, respectively.
The higher negative ζ-potential value of TiO2-green NPs, compared with TiO2-chem NPs, may suggest an improvement in the adsorption of cationic organic pollutants leading to an enhancement of photocatalytic efficiency [74]. Moreover, the small rutile content found for TiO2-green NPs may indicate that the enhancement of the photocatalytic activity may also be due to charge transfer effects in mixed-phase TiO2 (anatase/rutile) that improve the charge separation of photogenerated carriers [75,76].
The effect of calcination temperature on TiO2-green NPs was also analyzed by the evaluation of the photocatalytic activity of the synthesized NPs. As shown in Figure 6, it was observed that the TiO2-green NPs calcined at 500 °C showed a higher degradation efficiency than the TiO2 NPs calcined at 400° C. This can be due to the residual organic material on the surface of NPs calcined at 400 °C. The degradation efficiency values obtained after 180 min were (50 ± 3)% for the TiO2 NPs calcined at 500 °C and (10 ± 4)% for the TiO2 NPs calcined at 400 °C. A fine white powder was obtained by calcination process at 500 °C, whereas a grey powder with large grains was observed for TiO2 NPs calcined at 400° C, which may be due to residual carbon from the organic compounds of the leaves extract. The excess residual carbon material on the surface of NPs calcined at 400 °C can cover part of the photocatalyst, leading to an impediment to light access and reactants’ access to the photocatalyst surface [77], resulting in a decreased photocatalytic efficiency.

3. Materials and Methods

3.1. Reagents

Titanium (IV) isopropoxide (TTIP, 97%), ethanol absolute (≥99.8%), 2-propanol (≥99.8%), nitric acid (HNO3 65%), ultrapure water (produced by Barnstead Smart2Pure water purification system Thermo Scientific), and methylene blue hydrate.

3.2. Synthesis of TiO2 NPs

3.2.1. Conventional Route for Synthesis of TiO2 NPs (TiO2-Chem NPs)

TiO2 NPs were synthesized following the method described in [78] by sol–gel route. Herein, 1 mL of TTIP was slowly added to 5 mL of 25% (v/v) ethanol aqueous solution. The mixture was kept under stirring for 60 min, then the pH was adjusted to 2–3 by adding nitric acid and it was kept under stirring for 60 min. Afterwards, the mixture was dried in an oven using two steps: 120 °C for 120 min and then at 270 °C for 24 h. Finally, the obtained TiO2 NPs were ground with a mortar. In Figure 7 (on the left), the flow chart of the synthesis process is shown.

3.2.2. Green Synthesis of TiO2 NPs (TiO2-Green NPs)

Preparation of the Leaves Extract

Aloe vera leaves were separated from the gel and washed with ultrapure water. After drying at room temperature, they were cut into small pieces. Then, 25 g of leaves were transferred into a glass flask containing 250 mL of ultrapure water and the mixture was boiled at 100 °C for 20 min. After cooling, the mixture was filtered by a Whatman filter.

Synthetic Procedure

Here, 0.140 mL of TTIP was slowly added to 1 mL of 2-propanol and the solution was kept under stirring for 120 min. Then, 5 mL of the Aloe vera leaves extract were slowly added to the solution and the obtained mixture was kept under stirring for 120 min. Afterwards, TiO2 NPs were collected by centrifugation at 3900 rpm for 30 min. The collected NPs were washed 4–5 times with 50% (v/v) ethanol aqueous solution, then the TiO2 NPs were calcined at 500 °C (or at 400 °C) for 150 min. Lastly, the TiO2 NPs were ground with a mortar. The flow chart the synthesis process is shown in Figure 7 (on the right).

3.3. Characterization of TiO2 NPs

TEM characterizations were performed by a JEOL JEM-1011 transmission electron microscope operating at 100 kV. For both synthesized TiO2 NPs, samples were prepared by dropping a dilute suspension of TiO2 NPs in ultrapure water on TEM grid and drying overnight at room temperature. ζ-potential analysis was performed at 25 °C by Zetasizer Nano-ZS (Model ZEN3600, Malver Instruments Ltd., Malvern, UK) equipped with a HeNe laser working at 663 nm. Each suspension of TiO2 NPs (0.4 mg/mL) in ultrapure water was prepared for the measurements. The field emission scanning electron microscopy (FE-SEM) analyses of the samples were performed by using a Zeiss Sigma microscope (Carl Zeiss Co., Oberkochen, Germany) operating at 2–5 kV and equipped with an in-lens secondary electron detector and an INCA Energy Dispersive Spectroscopy (EDS) detector. All the specimens were fixed onto silicon slides that were mounted onto stainless-steel sample holders by using a double-side carbon. For the surface area of the synthesized nanoparticles, a Smart Sorb 92/93 was used. The samples were degassed at 100 °C under nitrogen flow for 90 min before the determination of their surface area. UV–vis analysis spanning the spectral range of 300 ÷ 800 nm was conducted at room temperature using a Varian Cary 5 spectrophotometer (ZEN3600, Malvern Instruments Ltd., Malvern, UK) equipped with a quartz cuvette having a 10 mm path length. Photoluminescence (PL) spectra were measured using FLS1000 photoluminescence spectrometer.
X-ray diffraction analysis was performed in Bragg–Brentano reflection geometry using filtered Cu-Ka radiation. The X-ray diffraction data were collected at a scanning rate of 0.02 degrees per second in 2θ ranging from 20° to 80° by step scanning.

3.4. Assessment of Photocatalytic Activity

The photocatalytic activity of the synthesized TiO2 NPs was evaluated by the degradation of MB in aqueous solution. With this aim, an UV irradiator (Figure 8), equipped with 2 UV lamps (8 W, λ = 365 nm) and a magnetic stirrer, was used. A suspension of the TiO2 NPs with a concentration of 0.4 g/L was prepared in 5 mg/L MB solution and it was homogenized by sonication for 60 s. The suspension was kept under stirring and under dark conditions for 30 min to achieve the adsorption–desorption equilibrium, then it was exposed to UV light under stirring condition at a distance from the UV source of about 12.5 cm and samples were taken at different times: 0 min, 30 min, 60 min, 90 min, 120 min, and 180 min. Afterwards, the collected samples were centrifuged in order to remove the nanoparticles and the absorption spectra of the samples were acquired by BioTek Synergy Mx multi-mode microplate reader. The absorption maximum at 664 nm was used to monitor the dye degradation over time.

4. Conclusions

In this work, the photocatalytic activity of TiO2 NPs synthesized through two synthesis methods, conventional and green, was evaluated by the degradation of MB in aqueous solution, under UV light. Both types of TiO2 NPs, that appear aggregated from TEM images, showed characteristic peaks of anatase and rutile phase, and TiO2-green NPs also showed a small peak that can be related to the brookite phase. ζ-potential analysis showed that both synthesized TiO2 NPs had a negative surface charge at a neutral pH, with a higher negative value of TiO2-green NPs compared with TiO2-chem NPs. Concerning the evaluation of the photocatalytic activity by the degradation of MB in aqueous solution, the reaction rate constant for TiO2-green NPs was about five times higher than that for TiO2-chem NPs. In fact, the degradation efficiency values obtained after 180 min were (50 ± 3)% for TiO2-green NPs and (16 ± 3)% for TiO2-chem NPs. The higher photocatalytic activity exhibited by TiO2-green NPs compared with TiO2-chem NPs may be due to the presence of both anatase and rutile phases, which may lead to charge transfer effects that improve the charge separation of photogenerated carriers. Also, the likely introduction of defects or dopants into the TiO2 lattice by the green synthesis method may create localized energy states that facilitate charge separation and reduce electron–hole recombination. In addition, the higher negative surface charge of TiO2-green NPs in comparison with TiO2-chem NPs may lead to the better adsorption of cationic organic pollutants, leading to further enhancement in photocatalytic activity.

Author Contributions

V.D.M. and M.C. (Mariafrancesca Cascione), conceptualization, methodology, supervision, data analysis, editing draft of manuscript; A.D.L., synthesized the nanomaterials; A.D.B. and A.D.L., methodology, writing—original manuscript preparation; R.D.C. and C.I., methodology and data analysis; M.C. (Massimo Corrado), methodology; R.R., funding and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

V.D.M. kindly acknowledges Programma Operativo Nazionale (PON) Ricerca e Innovazione 2014–2020 Azione IV.6 “Contratti su tematiche green”-DM 1062/2021 for sponsoring her salary and work.

Data Availability Statement

The data presented in this study are available in this article.

Conflicts of Interest

There are no conflicts to declare.

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Catalysts 14 00820 g001
Figure 1. Representative TEM images of TiO2-chem NPs on the left (a,c,e), TiO2-green NPs on the right (b,d,f).
Figure 1. Representative TEM images of TiO2-chem NPs on the left (a,c,e), TiO2-green NPs on the right (b,d,f).
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Catalysts 14 00820 g002
Figure 2. SEM images of TiO2NPS chem (a) and green (b). EDS spectra of TiO2NPS-chem (c) and TiO2NPS-green (d).
Figure 2. SEM images of TiO2NPS chem (a) and green (b). EDS spectra of TiO2NPS-chem (c) and TiO2NPS-green (d).
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Catalysts 14 00820 g003
Figure 3. UV–Vis absorbance (left) and reflectance spectrum (right) of TiO2-chem NPs (red line) and TiO2-green NPs (green line).
Figure 3. UV–Vis absorbance (left) and reflectance spectrum (right) of TiO2-chem NPs (red line) and TiO2-green NPs (green line).
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Catalysts 14 00820 g004
Figure 4. XRD pattern of TiO2-chem NPs (on the left) and TiO2-green NPs (on the right).
Figure 4. XRD pattern of TiO2-chem NPs (on the left) and TiO2-green NPs (on the right).
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Catalysts 14 00820 g005
Figure 5. Photocatalytic degradation of MB as a function of time under UV light (upper image) and pseudo first-order kinetics (lower image). The slope of the fitted lines gave the values of the reaction rate constants: 0.004 min−1 (R2 = 0.97) for TiO2-green NPs (green circles) and 0.0008 min−1 (R2 = 0.95) for TiO2-chem NPs (red triangles).
Figure 5. Photocatalytic degradation of MB as a function of time under UV light (upper image) and pseudo first-order kinetics (lower image). The slope of the fitted lines gave the values of the reaction rate constants: 0.004 min−1 (R2 = 0.97) for TiO2-green NPs (green circles) and 0.0008 min−1 (R2 = 0.95) for TiO2-chem NPs (red triangles).
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Catalysts 14 00820 g006
Figure 6. Photocatalytic degradation of MB under UV light: degradation efficiency as a function of time using TiO2-green NPs calcined at 500 °C (black squares) and calcined at 400 °C (blue circles).
Figure 6. Photocatalytic degradation of MB under UV light: degradation efficiency as a function of time using TiO2-green NPs calcined at 500 °C (black squares) and calcined at 400 °C (blue circles).
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Catalysts 14 00820 g007
Figure 7. Flow charts concerning the synthesis of TiO2 NPs: conventional route (on the left) and green route (on the right).
Figure 7. Flow charts concerning the synthesis of TiO2 NPs: conventional route (on the left) and green route (on the right).
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Catalysts 14 00820 g008
Figure 8. Pictures of the UV irradiator used for photocatalysis experiments.
Figure 8. Pictures of the UV irradiator used for photocatalysis experiments.
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Table 1. BET surface area of TiO2-chem and TiO2-green NPs.
Table 1. BET surface area of TiO2-chem and TiO2-green NPs.
CatalystBET Surface Area (m2/g)
TiO2-chem NPs63.2
TiO2-green NPs70.2
Table 2. Band gap values.
Table 2. Band gap values.
CatalystBand Gap (eV)
TiO2-green NPs3.1
TiO2-chem NPs3.3
Table 3. Degradation values after 180 min and reaction rate constants for TiO2 NPs synthesized by green route (TiO2-green NPs) and by conventional route (TiO2-chem NPs).
Table 3. Degradation values after 180 min and reaction rate constants for TiO2 NPs synthesized by green route (TiO2-green NPs) and by conventional route (TiO2-chem NPs).
CatalystDegradation After 180 mink
TiO2-green NPs(50 ± 3)%0.004 min−1
TiO2-chem NPs(16 ± 3)%0.0008 min−1
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De Luca, A.; De Benedetto, A.; De Matteis, V.; Cascione, M.; Di Corato, R.; Ingrosso, C.; Corrado, M.; Rinaldi, R. Eco-Friendly TiO2 Nanoparticles: Harnessing Aloe Vera for Superior Photocatalytic Degradation of Methylene Blue. Catalysts 2024, 14, 820. https://doi.org/10.3390/catal14110820

AMA Style

De Luca A, De Benedetto A, De Matteis V, Cascione M, Di Corato R, Ingrosso C, Corrado M, Rinaldi R. Eco-Friendly TiO2 Nanoparticles: Harnessing Aloe Vera for Superior Photocatalytic Degradation of Methylene Blue. Catalysts. 2024; 14(11):820. https://doi.org/10.3390/catal14110820

Chicago/Turabian Style

De Luca, Agnese, Angelantonio De Benedetto, Valeria De Matteis, Mariafrancesca Cascione, Riccardo Di Corato, Chiara Ingrosso, Massimo Corrado, and Rosaria Rinaldi. 2024. "Eco-Friendly TiO2 Nanoparticles: Harnessing Aloe Vera for Superior Photocatalytic Degradation of Methylene Blue" Catalysts 14, no. 11: 820. https://doi.org/10.3390/catal14110820

APA Style

De Luca, A., De Benedetto, A., De Matteis, V., Cascione, M., Di Corato, R., Ingrosso, C., Corrado, M., & Rinaldi, R. (2024). Eco-Friendly TiO2 Nanoparticles: Harnessing Aloe Vera for Superior Photocatalytic Degradation of Methylene Blue. Catalysts, 14(11), 820. https://doi.org/10.3390/catal14110820

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