Layered Perovskite La₂Ti₂O₇ Obtained by Sol–Gel Method with Photocatalytic Activity

Abstract

This paper presents the synthesis of La₂Ti₂O₇ nanoparticles by the sol–gel method starting from lanthanum nitrate and titanium alkoxide (noted as LTA). Subsequently, the lanthanum titanium oxide nanoparticles are modified with noble metals (platinum) using the chemical impregnation method, followed by a reduction process with NaBH₄. The comparative analysis of the structure and surface characteristics of the nanopowders subjected to thermal treatment at 900 °C is conducted using Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), ultraviolet-visible (UV–Vis) spectroscopy, as well as specific surface area and porosity measurements. The photocatalytic activity is evaluated in the oxidative photodegradation of ethanol (CH₃CH₂OH) under simulated solar irradiation. The modified sample shows higher specific surfaces areas and improved photocatalytic properties, proving the better conversion of CH₃CH₂OH than the pure sample. The highest conversion of ethanol (29.75%) is obtained in the case of LTA-Pt after 3 h of simulated solar light irradiation.

1. Introduction

Over decades of investigation, the production of innovative photocatalysts has been an important subject in the scientific community for their various applications, including the degradation of organic pollutants [1,2,3], mineralization of toxic organic compounds [4], degradation of polymer fibers [5], degradation of pharmaceutical waste [6], industrial wastewater treatment [7] and treatment of agricultural wastewater [8], or the splitting water to generate hydrogen [9,10].

La₂Ti₂O₇ belongs to the family of layered perovskite-type materials with the general formula A_nTi_nO_3n+2 [11]. For n = 4, the layered perovskite phase is stable under ambient conditions when the cationic radius ratio is greater than or equal to 1.78; in this case, the La³⁺/Ti⁴⁺ ratio is 1.92 [12,13].

The La₂Ti₂O₇ phase crystallizes in the monoclinic system (space group P2₁) and contains four formula units per unit cell. Its structure is composed of four distorted perovskite units aligned perpendicularly on infinite layers of TiO₆ octahedra [14]. This structural anisotropy results in anisotropic dielectric, electrical, and mechanical properties [12].

La₂Ti₂O₇ is a multifunctional oxide recognized for several notable attributes:

• Piezoelectric/ferroelectric properties: exhibits a high Curie temperature [15,16].
• Superior thermal stability: possesses good electron mobility due to ions on A sites and high conductivity [16].
• Wide bandgap (~3.8 eV): capable of generating photoelectrons with a high reduction potential [17,18].
• Good resistance to ionizing radiation [19].
• Layered structure: favorable for the separation and transfer of photogenerated electron–hole pairs, as well as separating oxidation from reduction reaction sites [18].
• Host matrix: can host other lanthanides to create new phosphors with conventional luminescence under UV excitation [15].
• Photocatalytic capabilities: particularly for hydrogen generation via water splitting [20,21] and for the degradation of volatile organic compounds (VOCs) [22].
• However, its large band gap limits photocatalytic efficiency under visible light [16], and its low solar light utilization and easy recombination of photogenerated charge carriers hinder achieving the commercial application target of 5–10% solar hydrogen conversion efficiency (STH) [9].

La₂Ti₂O₇ is a material with increasing applications in a variety of domains, from catalysis and photocatalysis [23,24,25] to dielectrics [26], superconductors [27], and luminescent materials [19]. Numerous environmental issues could be resolved by creating stable, inexpensive, and highly efficient photocatalysts that utilize solar energy [28]. Applications of La₂Ti₂O₇ oxide include water-splitting [29]; co-catalyzation with metals [30,31]; the photocatalytic degradation of organic pollutants and colorants [13,15,16,32,33]; the oxidation of methanol [34,35], ethanol [36,37], and phenol [14,38]; and the photodegradation of antibiotics [21,39].

However, the degree of crystallinity and the material’s exchange surface have a significant impact on the photocatalytic efficiency. In recent years, scientific research has seen a great deal of interest regarding the optimization of synthetic parameters to control the size of crystallites and their crystallinity, specifically through the following: (i) the use of polymer precursors for controlling porosity [40], (ii) the type of precursors used [41,42], and (iii) the diversification of synthesis processes. Various techniques have been used to synthesize La₂Ti₂O₇, including solid-state processing [43], co-precipitation of hydroxides [44], urea precipitation [45], hydrothermal synthesis [46], high-temperature decomposition of metal–organic precursors [47], thermal decomposition of nitrates [48], liquid mix techniques [49], the hydrothermal route [50,51], microwave-assisted synthesis [52], and the sol–gel route [53,54,55]. While these methods offer the advantage of smaller particle sizes over the solid-state approach, most of them typically come with the drawbacks of extended heating times and multi-step synthetic procedures.

The sol–gel process has attracted attention as an interesting method for creating nanocomposites [56,57]. Better control of stoichiometry and homogeneity, lower reaction temperatures, reduced contamination, and being an easy way of fabricating ultrafine powders, thin films, and fibers are some of the benefits of the sol–gel method over many techniques. The hydrolysis and subsequent polycondensation reactions of the component alkoxides represent a major part of the sol–gel synthesis [58,59]. In contrast to conventional solid-state reactions, which typically require calcination temperatures above 1000 °C and often produce inhomogeneous, aggregated particles with low surface areas, the sol–gel method facilitates the mixing of precursors at the molecular level. This allows for better control over stoichiometry, particle size, and morphology. Compared to hydrothermal synthesis, which involves prolonged reaction times under high-pressure autoclave conditions that are difficult to scale up [51], the sol–gel method offers a more scalable and reproducible approach. Its ability to operate at lower synthesis temperatures and to fine-tune textural properties makes it particularly attractive for tailoring the photocatalytic activity of La₂Ti₂O₇.

The aim of the present work is to study the synthesis of lanthanum and titanium perovskite through the sol–gel method and their photocatalytic activity for ethanol oxidative degradation under simulated solar light. The photocatalytic activity enhancement by impregnating La₂Ti₂O₇ with platinum (Pt), the evolution of reaction products in time, and the relationship between catalytic performances, morphology, and the structure of catalysts are also presented.

2. Materials and Methods

2.1. Gel and Powder Preparation

Nanopowders of La₂Ti₂O₇ were prepared by the sol–gel method (see Figure 1), while the La₂Ti₂O₇ modified with platinum was obtained by post-synthesis impregnation.

In the synthesis, the reaction started from lanthanum (III) nitrate hexahydrated [La(NO₃)₃ × 6H₂O] (Merck, Darmstadt, Germany) as the La precursor and titanium (IV) isopropoxide [Ti[OCH(CH₃)₂]₄] (Merck, Hohenbrunn, Germany) as the precursor of Ti. The molar ratio of La/Ti was 1:1. The lanthanum precursor was dissolved in glacial acetic acid (CH₃COOH) (Sigma-Aldrich, Steinheim, Germany) in a first solution, while the titanium precursor was mixed with isopropanol (i-C₃H₇OH) (Merck, Hohenbrunn, Germany) in a second solution, both with a mass ratio of 1:5. The two freshly prepared solutions were mixed and stirred for 30 min. The resulting translucent solution was placed in a water bath at 60 °C for 60 min until a whitish powder was formed. This powder was dried at 80 °C (~3 h), then thermally treated at 900 °C for 2 h to obtain the white La₂Ti₂O₇ nanopowder, noted as LTA.

The choice of the La(NO₃)₃ × 6H₂O and Ti[OCH(CH₃)₂]₄ precursors for obtaining high-quality La₂Ti₂O₇ by the sol–gel method allows for good control over the stoichiometry and leads to a homogeneous distribution of La and Ti ions at the molecular level. Lanthanum nitrate provides La³⁺ ions without introducing undesired anions (such as Cl⁻, which could remain as impurities or create by-products). Titanium alkoxides are the preferred titanium precursors in sol–gel chemistry due to their tunable reactivity. The alkoxide groups readily undergo hydrolysis and condensation, forming the Ti–O–Ti and La–O–Ti structures. Another advantage of these precursors is that they decompose cleanly upon calcination. This clean decomposition leads to fewer impurities and better control of the crystallinity and phase purity in the final La₂Ti₂O₇ powder [15,54,56,60,61].

The metal-modified catalysts were prepared as follows: First, the previously obtained nanopowder was dispersed in distilled water. Then, an aqueous solution containing the dissolved 1 mol% noble metal precursor, PtCl₄ (Fluka, Buchs, Switzerland) was slowly added to La₂Ti₂O₇ suspended previously in water and left under continuous stirring at room temperature for 24 h. The metal reduction process was completed by slowly adding 5 mL of aqueous solution with 0.04 g of freshly dissolved NaBH₄ (TCI, Tokyo, Japan) to the La₂Ti₂O₇ suspension. The solid material was recovered by filtration, washed with distilled water, and dried at 80 °C for 5 h [38].

2.2. Methods

The study and characterization of La₂Ti₂O₇-based oxide nanopowders obtained by the sol–gel method was approached according to the main factors influencing their structure and morphology.

The following methods of characterization were used: infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) coupled with Energy Dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), the determination of the BET-specific surface area and the pore distribution, and UV-Vis spectroscopy.

FT-IR spectra of the thermally treated powders were recorded over the range of 4000–400 cm⁻¹ on a Jasco FT-IR-4700 spectrophotometer (Jasco, Tokyo, Japan) using the KBr disk method.

The Renishaw inVia Qontor Spectrometer System (Renishaw PLC, Wotton-under-Edge, Gloucestershire, UK) for confocal Raman spectral imaging using the visible laser wavelength (532 nm) was used to examine the synthesized samples. The acquisition was performed in the spectral range 100 to 1000 cm⁻¹, using an exposure time of 10 s, a power lower than 10% (5 mW) with different number of accumulations (usually 5 accumulations), and a grating of 1800 l/mm. The microscope objective was a Leica N PLAN X100 magnification (Leica Camera AG, Wetzlar, Germany) with a focal distance of 0.33 mm and numerical aperture of 0.85. The detector was a Master Renishaw Centrus (Renishaw, Wotton-under-Edge, UK).

XPS measurements were performed using an XPS spectrometer (SPECS Surface Nano Analysis GmbH, Berlin, Germany) equipped with a PHOIBOS 150 mm hemispherical electron energy analyzer, a multichanneltron detector, and a monochromated Al Kα radiation (1486.7 eV) anode as the excitation source. The operating power was 250 W (12.5 kV × 20 mA) and the spectra were recorded with 99 eV pass energy for general surveys and 30 eV pass energy for high-resolution, core-level measurements, with partial charge compensation provided by a neutralizer flood gun (1 eV × 100 μA). The base pressure in the analysis chamber before measurements was 10⁻¹⁰ mbar.

The particle morphology and elemental composition of the samples were examined using a Verios G4 UC scanning electron microscope (Thermo Scientific, Brno, Czech Republic) equipped with an Energy Dispersive X-ray (EDX) spectroscopy analyzer (Octane Elect Super SDD detector, Octane, New York, NY, USA). For SEM analysis, the specimens were mounted on aluminum stubs using double-sided carbon adhesive tape and subsequently coated with a 6 nm platinum layer via a Leica EM ACE200 sputter coater (Leica, Vienna, Austria) to enhance electrical conductivity and mitigate charging effects under electron beam irradiation. Morphological characterization was performed utilizing a secondary electron detector (TLD—Through the Lens Detector) to accurately delineate particle shape and size. SEM micrographs were acquired at an accelerating voltage of 10 kV and a beam current of 0.4 nA. For EDX elemental analysis, the samples were examined at an accelerating voltage of 20 kV and a beam current of 6.4 nA.

The TEM studies were performed using a transmission electron microscope Hitachi HT-7700 (Hitachi, Tokyo, Japan) at an accelerating voltage of 100 kV. For TEM analysis the nanoparticles were dispersed in ethanol, ultrasonicated, placed on carbon-coated copper grids with a 300-mesh size, and dried in an oven until the solvent was removed.

The X-ray diffraction (XRD) patterns of the powders were obtained using a Rigaku Ultima IV diffractometer (Rigaku Corp., Tokyo, Japan). The instrument was configured in a parallel beam geometry with Cu Kα radiation (λ = 1.5406 Å) and CBO optics, and operated at 40 kV and 30 mA. Data were collected with a step size of 0.02° and a scan speed of 2° min⁻¹. Phase identification was carried out with a Search–Match algorithm, connected to the PDF database.

Elemental analysis was conducted via X-ray fluorescence (XRF) using a Rigaku ZSX Primus II spectrometer (Rigaku Corp., Tokyo, Japan) equipped with a 4.0 kW X-ray Rh tube. Data processing was performed using EZscan combined with Rigaku SQX fundamental parameters software v.5.18, operating in a standardless mode.

The textural analysis of the powders was performed through nitrogen adsorption–desorption analysis at −196 °C using a ASAP 2020 automated gas adsorption analyzer (Micromeritics, Norcross, GA, USA). Before analysis, the samples were degassed under vacuum at 200 °C for 5 h. The specific surface area was determined using the Brunauer–Emmett–Teller (BET) equation, while pore size distribution curves were generated from the adsorption branch based on the Barrett–Joyner–Halenda (BJH) model. The total pore volume was calculated from the amount adsorbed at a relative pressure of 0.99.

The UV–VIS spectra of the samples were recorded using a Perkin Elmer Lambda 35 spectrophotometer (PerkinElmer, Norwalk, CT, USA), equipped with an integrating sphere, in the wavelength range of 200–1100 nm [ultraviolet (200–400 nm), visible (400–780 nm), and near-infrared (780–2500 nm)], using a white reflective standard Spectralon.

ROS monitoring was performed according to previously reported works [62,63]:

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·O₂⁻ photogeneration. UV–Vis spectra of aqueous mixtures containing 50 μM XTT sodium salt 9([2, 3-bis(2-methoxi-4-nitro-5sulfophenyl)-2H-tetrazolium-5-carboxanilide]) (AlfaAesar, Karlsruhe, Germany) and LTA, LTA-Pt powders (2 mg) were measured with an Specord 200 Plus Spectrophotometer (Analytik Jena, Jena, Germany). The reaction was carried out under simulated solar light of AM1.5 (1000 W m², Peccel-L 01) (Peccell, Kawasaki, Japan).

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·OH radical trapping experiments have also been performed by PL spectroscopy using the Cary Eclipse G9800A fluorescence spectrometer (Agilent Technologies, Penang, Malaysia) with coumarin (Merck, Darmstadt, Germany), checking for the presence of umbelliferone (a fluorescent product which peaked at 470 nm) and proving that hydroxyl radicals were missing.

The photoluminescence spectroscopy measurements were performed at room temperature using a Carry Eclipse G9800A fluorescence spectrometer (Agilent Technologies, Penang, Malaysia) with excitation/emission slit widths of 10/10 nm. The powders of interest (2 mg) were suspended in 3 mL of water containing 30 μL absolute ethanol.

The catalytic activity of the prepared materials was tested for the oxidative degradation of ethanol in the gaseous phase under simulated solar light irradiation. As part of the experimental process, 0.05 g of photocatalyst powder was deposited as a uniform layer (surface area of about 3.6 cm² and 1 mm thicknesses) on a substrate. The surface was then exposed to simulated solar light. Ethanol (7.2 μL) (Merck, Darmstadt, Germany) was introduced into the photoreactor and evaporated into a volume of 120 cm³ air and then left in the dark for 30 min to balance the experimental system. A cryostat was used to regulate and maintain the photoreactor’s internal temperature at 25 °C. A Peccell L01 solar simulator (Peccell, Kawasaki, Japan) provided the AM1.5 (1000 W/m²) solar light. A gas chromatograph (Agilent, Model 7890A, Wilmington, DE, USA) equipped with the Flame Ionization Detector (FID) was used to analyze 200 μL of gas sample every 30 min, and a gas chromatograph (Buck Scientific, East Norwalk, CT, USA) equipped with the Thermal Conductivity Detector (TCD) was used to analyze 500 μL of gas sample every 60 min. The photocatalytic activity of the investigated catalysts was monitored for 180 min.

3. Results and Discussion

The samples obtained in the conditions presented in the experimental part were yellowish white powders. Based on the TG/DTA results of the La₂Ti₂O₇ [15], the dried powders were thermally treated at 900 °C for 2 h, obtaining white crystallized powders. The thermally treated powders were investigated for their morphology, structure, and photocatalytic properties.

3.1. Fourier Transform Infrared Spectroscopy (FT-IR)

The FT-IR spectra of the thermally treated powders are shown in Figure 2 and the corresponding vibration bands are presented comparatively in

Table 1. Assignment of vibration bands in FT-IR spectra of thermally treated samples.

Wavenumbers (cm−1)		Assignments and Vibration Mode
LTA	LTA-Pt
806	806	Ti–O–Ti bridges in TiO6 octahedra from titanate
776	772	Ti–O stretching in TiO6 octahedra from titanate
620	621	La–O stretching and Ti–O vibrations
547	549	La–O stretching and Ti–O vibrations
491	490	La–O stretching vibrations and Ti–O–Ti bridges in TiO6 octahedra
464	464	La–O stretching vibrations and Ti–O bending modes

. In Figure 2 the spectrum in the case of the LTA sample is shown. The vibration band at 806 cm⁻¹ is attributed to the stretching vibrations of the Ti–O–Ti bridges, which are characteristic of the TiO₆ octahedra in the titanate structures and 776 cm⁻¹ is assigned to the Ti–O stretching vibrations, specifically related to the TiO₆ octahedra framework within La₂Ti₂O₇ [64]. The vibration bands at 620 cm⁻¹ and 547 cm⁻¹ represent La–O stretching and Ti–O vibrations within the La₂Ti₂O₇ lattice, which contribute to the stability of the titanate framework. The vibration bands at 491 cm⁻¹ and 464 cm⁻¹ are assigned to La–O stretching vibrations and Ti–O bending modes, which are fundamental vibrations in La₂Ti₂O₇. The presence of these bands confirms the formation of a well-crystallized La₂Ti₂O₇ phase. The vibration bands between 800 and 400 cm⁻¹ are assigned to metal–oxygen (M–O) stretching vibrations, suggesting the formation of La–O and Ti–O networks [65].

Comparing the FT-IR spectra in Figure 2a,b for the Pt-modified sample, no changes are observed due to the incorporation of the noble metal into the perovskite structure.

3.2. Raman Spectroscopy

Table 2. Assignment of Raman shifts in thermally treated samples.

No.	Synthesized Samples		Literature for La–O and Ti–O Bonds [64,66]	Assignment to Raman Vibrations
	LTA-Pt	LTA
1	109	111	112	La–O stretching vibration
2	129	131	131	Ti–O bond stretching vibration for the six-fold coordinated Ti in rutile TiO2 (B1g mode)
3	153	154	151	La–O stretching vibration
4	179	181	180	La–O stretching vibration
5	213	213	214	La–O stretching vibration
6	228	227	227	La–O stretching vibration
7	237	240	238	La–O stretching vibration
8	260	262	261	La–O stretching vibration
9	275	274	275	La–O stretching vibration
10	284	285	284	La–O stretching vibration
11	317	316	317	La–O stretching vibration
12	337	342	336	La–O stretching vibration
13	348	352	350	La–O stretching vibration
14	367	367	367	La–O stretching vibration
15	401	402	400	La–O stretching vibration
16	424	426	425	La–O stretching vibration
17	445	446	446	Stretching vibrations of Ti–O–Ti octahedra (Eg mode)
18	-	488	-	-
19	517	517	517	-
20	536	538	533	-
21	555	557	556	La–O stretching vibration
22	605	605	606	Stretching vibrations of Ti–O–Ti octahedra (A1g mode)
23	-	668	-	-
24	794	792	790	Stretching vibrations of Ti–O–Ti octahedra
25	809	808	808	Ti–O bond stretching vibration for the six-fold coordinated Ti in rutile TiO2 (B2g mode)

lists the Raman shifts in both LTA-Pt and LTA, according to [9,66] and our work. According to the literature [9], the wavenumbers of the rare-earth oxygen-stretching modes (La–O) generally lie in the region of 300–500 cm⁻¹. Some of the bands in this range are due to Ti–O vibrations or, more probably, to complex motions involving the participation of both La and Ti cations.

There are no new chemical bonds forming in the Pt-modified La₂Ti₂O₇ samples, suggesting that the dopant (Pt) might be found in the metallic form. A newly formed chemical bond would have given a vibration in the Raman spectra of the LTA-Pt. It could be the case that there is the intercalation of the Pt between the layers of the La₂Ti₂O₇ perovskite.

The typical crystalline structure of monoclinic layered perovskite La₂Ti₂O₇ is observed with Raman analysis. Except the peaks at 488 and 668 cm⁻¹ from the LTA sample, the peaks at 342, 371, 405, 430, 448, 520, 541, 559, 609, and 793 cm⁻¹ correspond to the typical monoclinic layered perovskite crystalline structure [13,67]. The increased Raman intensity at 446 cm⁻¹ in the LTA without Pt is more considerable when compared to the LTA-Pt. For LTA the 446 cm⁻¹ peak has similar intensity with the 402 cm⁻¹ peak, as in LTA-Pt, where it is less intense. As the peak appearing at 446 cm⁻¹ is related to stretching vibrations of Ti–O–Ti in the distorted [TiO₆], the small disruption in crystallinity and further distortion of the TiO₆ may suggest the introduction of defects by Pt [9]. This tendency was already observed in the literature for the doping Na-doped LTA [9], as the intercalation of Pt between the LTA layers disrupts the crystallinity of the pure LTA. The appearance of the 668 cm⁻¹ peak in varying intensities according to the probed spot on the surface of pure LTA (LTA without Pt) is the most notable difference in the Raman spectra. This additional Raman mode at 668 cm⁻¹ marked in Figure 3 could indicate local lattice imperfections. This additional mode at 668 cm⁻¹ is close to the assigned Ti–O–Ti vibration (661 cm⁻¹) from two-dimensional lepidocrocite-type TiO₆ layers [64]. It could be related to the vibrations of the [TiO₆] distorted octahedron, as the other peaks are higher than 500 nm, and it could be assigned to the slightly increased asymmetry of the layered structure when compared to the other structural symmetries of perovskites. The Raman bands of the layered perovskite structure are complex due to this asymmetry. The crystalline facet which is exposed to the Raman laser plays an important role. If present at low concentrations (<1%), then the effect on the host lattice vibrational modes may not be perceptible.

3.3. X-Ray Photoelectron Spectroscopy (XPS)

XPS analysis was employed to determine the chemical composition and valence states of the ions in the samples. C 1s was used as the reference for the calibrations of the spectra of the other elements. The two survey spectra are shown in Figure 4. The main difference between the samples is the presence of the Pt core level and Auger electron peaks in the spectral shapes of La, Ti, and O.

In the Ti 2p spectrum of the LTA sample (Figure 5a), two components are identified. The peak at approximately 458 eV is characteristic of Ti⁴⁺ [9,26,27,40]. A second peak at a higher binding energy, around 460 eV, may be attributed either to a different TiO₂ phase than that present in the La₂Ti₂O₇ structure or to a non-stoichiometric TiO_x phase [68]; however, it is unlikely to correspond to Ti³⁺, since Ti₂O₃ is typically detected at lower binding energies. Moreover, the peak at the lower binding energy could be assigned to a La–O–Ti component, while the higher energy peak would correspond to TiO₂; alternatively, as attributed by Lv et al. [21] for La₂Ti₂O₇ powders forming rectangular nanosheets, the additional peaks observed at higher binding energies around 460 eV and 465 eV could be from the unsaturated Ti of TiO₅ motifs on the Ti-terminated facet of the La₂Ti₂O₇ nanostructures. The spin–orbit splitting (SOS) is 5.7 eV, consistent with the Ti⁴⁺ oxidation state. Furthermore, the presence of this additional component may reflect differences between Ti⁴⁺ species on the surface and in the bulk of the sample.

The O 1s region of the LTA sample (Figure 5b) can be deconvoluted into four peaks, corresponding to La–O/Ti–O bonds at 529.34 eV [9,27,40], TiO₂ (or TiO_x) at 530.72 eV, C=O and/or OH groups at 531.94 eV [16,39], and C–O bonds at 533.48 eV, respectively [16,21].

The La 3d spectrum (Figure 5c) of the LTA sample exhibits a spin–orbit splitting of 16.9 eV between the La 3d_5/2 and La 3d_3/2 peaks, confirming the presence of La³⁺ [21,40]. Component c2 (green line in Figure 5c), observed at approximately 835–836 eV, corresponds to the “antibonding” orbitals. Component c4 (orange line in Figure 5c), centered around 848 eV, exhibits a broad profile and is attributed to plasmonic loss [69]. Furthermore, the “multiplet splitting” of the La chemical state in this sample (6.6 eV) is larger than the typical values reported for La₂O₃ (4.6 eV) and La₂Ti₂O₇ (4.3 eV) [26].

The C 1s region is deconvoluted into three distinct peaks (Figure 5d). The peak at 284.6 eV is assigned to C–C bonds, typically arising from carbon contamination on the sample surface. The peak at 286.27 eV corresponds to C=O functional groups, while the peak at 289.07 eV is attributed to O–C=O bonds, indicative of carbonate species. This is consistent with the tendency of lanthanum compounds to form surface carbonates under ambient conditions.

The Ti 2p XPS spectrum for the LTA-Pt sample is presented in Figure 6a. The spectrum of the doped sample exhibits a single chemical state of Ti⁴⁺, characterized by a Ti 2p_3/2 peak at 458.32 eV. The spin–orbit splitting (SOS) of 5.7 eV between the Ti 2p_3/2 and Ti 2p_1/2 peaks further confirms the exclusive presence of Ti⁴⁺. It is worth mentioning that the additional peak observed around 460 eV is no longer detectable. One reason can be the presence of platinum on the surface of La₂Ti₂O₇ crystals that fulfill the unsaturated Ti in TiO₅ motifs on the Ti-terminated facets.

The O 1s region of the LTA-Pt sample (Figure 6b) can be deconvoluted into three peaks, corresponding to La–O/Ti–O bonds at 529.34 eV, C=O and/or OH groups at 531.94 eV, and C–O bonds at 533.48 eV, respectively. The peak at around 531 eV is no longer detectable in the doped sample, which would suggest that this peak is actually given by the presence of a further Ti–O bond in the unmodified case (not an unsaturated Ti termination).

The XPS spectrum of the La 3d region exhibits a typical profile for lanthanum-based compounds, with a similar number of components appearing at comparable binding energies, but in different relative proportions compared to the unmodified sample. This indicates that platinum doping also affects the lanthanum bonding environment. Components 1 and 3 (blue and gray lines in Figure 6c), which correspond to multiplet splitting, are separated by approximately 4.5 eV—significantly less than the splitting observed in the unmodified LTA sample, but closer to the typical values reported for La₂O₃ and La₂Ti₂O₇, as noted above.

The binding energies of 70.36 eV and 71.55 eV are attributed to Pt 4f_7/2. The binding energy at 70.36 eV is attributed to the platinum metal, while the binding energy at 71.55 eV can be attributed to platinum oxide.

The C 1s region has the same behavior as the unmodified sample, with small shifting of the peak values. The spectrum is deconvoluted into three distinct peaks (Figure 6e). The peak at 284.6 eV is assigned to C–C bonds. The peak at 285.84 eV corresponds to C=O functional groups, while the peak at 288.34 eV is attributed to O–C=O bonds, indicative of carbonate species.

Table 3. Compositional table—Binding energies with the attributions of the different components with each component’s relative intensity and percentage of total integral intensity obtained from XPS data and deconvolutions.

Sample	Element Type	Element Component	Relative Intensity	Binding Energy	Attribution	Atomic Percentage
			(%)	(eV)		(%)
LTA	C 1s	1	62.2	284.6	C–C	7.2
		2	29.5	286.27	C=O
		3	8.3	289.07	O–C=O, La carbonates
	O 1s	1	45.8	529.34	TiO2 (surf.)/La–O–Ti	53.8
		2	26.7	530.72	TiOx,TiO2 bulk/TiO2
		3	23.3	531.94	C=O, OH
		4	4.2	533.48	C-O
	La 3d	1	2.4	833.82	La3+ multiplet peak 1	25.0
		2	49.9	836.46	antibonding
		3	25.2	840.44	La3+ multiplet peak 2
		4	22.5	847.5	satellite
	Ti 2p	1	66.3	458.06	TiO2 (surf.)/La–O–Ti	14.0
		2	33.7	459.99	TiOx, TiO2-bulk/TiO2
LTA-Pt	C 1s	1	76.2	284.6	C–C	9.6
		2	17.4	285.84	C–O
		3	6.4	288.34	O–C=O, La carbonates
	O 1s	1	67.6	529.95	TiO2/La–O–Ti	67
		2	27.3	531.73	C=O, OH
		3	5.1	532.8	C–O, Pt oxide
	La 3d	1	31.1	833.78	La3+ multiplet peak 1	12.4
		2	20.2	835.48	antibonding
		3	34.8	838.31	La3+ multiplet peak 1
		4	13.9	847.90	satellite
	Ti 2p	1		458.32	TiO2/La–O–Ti	9.8
	Pt 4f	1	77.4	70.36	Pt	1.2
		2	22.6	71.55	PtO

lists binding energy (BE) and atomic percentage obtained from the analysis of these spectra. The “deconvolutions” were performed using Voigt profiles as detailed by Teodorescu et al. [70].

3.4. Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy (SEM and EDX)

Figure 7 shows the results of the SEM morphology analysis for La₂Ti₂O₇ and La₂Ti₂O₇-Pt nanoparticles. The SEM micrographs highlight the influence of the impregnation of the noble metal on the morphology of the obtained pure La₂Ti₂O₇ nanoparticles. The LTA sample shows irregular particles with a size ranging between 50 and 100 nm. The particles are between 50 and 100 nm in size and appear as clusters of nanoparticles (Figure 7a). In the case of the LTA-Pt sample (Figure 7b), a better individualization of the particles can be observed, as they are less aggregated, with uniformly distributed Pt.

To confirm the elemental composition, EDX measurements were performed. The elemental composition obtained by EDX is presented in

Table 4. Elemental composition in the weight and atomic percentage of the samples.

LTA			LTA-Pt
Element	Weight (%)	Atomic (%)	Element	Weight (%)	Atomic (%)
OK	13.24	46.65	OK	14.29	50.24
TiK	23.51	27.68	TiK	19.80	23.27
LaL	63.24	25.67	LaL	64.07	25.96
			PtL	1.84	0.53

. Qualitative EDX analysis confirms the presence of elements La, Ti, O, and Pt.

3.5. Transmission Electron Microscopy (TEM)

The TEM micrographs of the samples obtained by the sol–gel route are shown in Figure 8 and indicate agglomerated particles that form clusters. Crystals in unmodified samples aggregate into large particle sizes, where crystals can merge. Apparently, the LTA-Pt sample does not change its morphology after impregnation with platinum chloride.

There is no apparent porous structure in the large particles. In Figure 9, a uniform distribution of platinum particles across the entire surface of the La₂Ti₂O₇ particle agglomerates (sample LTA-Pt) can be observed.

Particle size distributions were determined from TEM micrographs for both samples. The mean particle sizes were comparable, measuring approximately 70 nm (69.54 ± 19 nm for the LTA sample and 68.64 ± 22 nm for the LTA-Pt sample). The corresponding particle size distribution histograms are presented in Figure 10.

3.6. X-Ray Diffraction (XRD)

X-ray diffraction (XRD) analysis was performed to determine the crystalline structure of the thermally treated samples. Figure 11a,b display the XRD patterns of titanate nanopowders after heating at 900 °C for 2 h. As shown in Figure 11a, a single phase of La₂Ti₂O₇ was obtained, according to PDF #00-070-0903.

La₂Ti₂O₇ is a layered perovskite [9,15,27,34,39] with a monoclinic structure and space group P2₁(4) [14,15,16,21,64].

Table 5. Phase identification, lattice parameters, and crystallite sizes of the thermally treated samples.

Sample	Crystalline Phase	Lattice Parameters (Å)			Crystallite Size (nm)
		a	b	c
LTA	La2Ti2O7	7.816	13.022	5.539	14
LTA-Pt	La2Ti2O7	7.829	13.034	5.553	15

summarizes the lattice parameters and crystallite sizes. The lattice constants of the sample LTA closely matched those in the PDF card #00-070-0903.

According to Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) data, the LTA sample contains a titanium oxide-based phase. However, no secondary phases were detected in the X-ray diffraction (XRD) pattern of the LTA sample. This absence may be attributed to the secondary phase being present in an amorphous state, to the possible overlap of its reflections with those of the La₂Ti₂O₇ phase, or to its concentration being below the detection limit of the instrument.

The crystalline structure of the LTA-Pt sample was also analyzed. All the reflections in the XRD diffractograms of the doped sample became sharper and higher after the impregnation with PtCl₄ aqueous solution for 24 h and the subsequent drying at 80 °C for 5 h. The modified sample (LTA-Pt) also contained single-phase La₂Ti₂O₇. No platinum-based compounds were detected. X-ray photoelectron spectroscopy (XPS) data indicated that platinum was present in the sample either in its metallic form or as an oxide. If platinum compounds were to crystallize, their diffraction line would completely overlap with those of the La₂Ti₂O₇ phase in the X-ray diffraction (XRD) pattern. The crystallite size slightly increased to 15 nm after platinum impregnation.

The unit cell parameters of the LTA-Pt sample (listed in Table 5) are larger than those of the LTA sample. This increase can be attributed to the incorporation of platinum, which likely partially entered the interlayer spaces between the perovskite slabs.

3.7. X-Ray Fluorescence (XRF)

To analyze the elemental composition of the modified sample, X-ray fluorescence analysis was conducted to determine the presence of the dopant element. The results, presented in

Table 6. Elemental composition of the analyzed samples.

Sample	Composition	Values (Mass %)	Line
LTA-Pt	Ti	26.46	Ti-Kα
	La	72.50	La-Lα
	Pt	1.03	Pt-Lα

, indicate that platinum (Pt) was detected in quantities closely matching the initially calculated composition.

3.8. Textural Characterization (BET)

To assess the texture of the heat-treated powders, N₂ adsorption–desorption isotherms were obtained and are displayed in Figure 12. The textural properties (BET surface area, total pore volume, and average pore size) are provided in

Table 7. The specific surface areas (S_BET), total pore volumes (V_total), and average pore diameters (d_BJH) of the samples.

Sample	SBET (m2/g)	Vtotal (cm3/g)	dBJH (nm)
LTA	10.1	0.106	29.6
LTA-Pt	8.8	0.087	30.2

.

Both samples exhibit type IV isotherms with H3 hysteresis loops, according to the IUPAC classification [71]. The small hysteresis loop area and its appearance at relatively high pressures (P/P₀ > 0.8) in all samples suggest the presence of relatively large, well-defined mesopores, with some contribution from macropores. This observation is further supported by the pore size distribution (PSD) graphs (shown as inserts in the figures) which confirm broad distributions spanning almost the entire mesopore range and extending into the macropore domain. As expected for materials of this type, the specific surface areas and total pore volumes are relatively low (see Table 7).

3.9. Ultraviolet-Visible Spectroscopy (UV-Vis)

UV–Vis absorption spectra of the thermally treated samples are illustrated in Figure 13. For pure LTA samples, a strong absorption is observed in the UV region that can be assigned to the direct bandgap transition in La₂Ti₂O₇ [17].

For LTA-Pt powder, the characteristic UV absorption of the support is preserved. However, the platinum addition increases this UV absorption band’s intensity and shifts the absorption edge to higher wavelengths. At the same time, this sample exhibits a strong light absorption in the visible region due to the d-d transitions in Pt-doped species of platinum nanoparticles [72,73].

The direct optical bandgap of the samples was calculated using the Tauc method. The obtained experimental values were 3.78 eV for LTA and 3.83 eV for LTA-Pt, being close to the literature data reported for La₂Ti₂O₇ samples [74].

3.10. ROS Monitoring

Figure 14a,b reveal the time-course of the XTT-Formazan presence characterized by a broad absorbance band centered at 470 nm (UV–Vis spectra). This compound results from the XTT sodium salt interaction with the photogenerated superoxide anion radical (·O₂⁻) in the presence of the investigated samples (LTA, LTA-Pt) under simulated solar light irradiation. The first 30 min of light exposure is insignificant relative to the ·O₂⁻ generation, which clearly increases after 45 min of irradiation time for both samples.

Figure 14c illustrates the relative amount of ·O₂⁻ (I_max = 470 nm) produced by the two powders during the irradiation time (120 min), with a higher amount of ·O₂⁻ being photogenerated by LTA-Pt.

3.11. Photoluminescence Measurements

The photoluminescence measurements are meant to describe the radiative phenomena induced by the recombination of the photogenerated charges. The almost quenched photoluminescence of the LTA-Pt sample relative to the LTA one in the ethanol/water mixture indicates the better use of the photogenerated charges for a chemical reaction on the catalyst surface than for the recombination process of electrons and holes (Figure 15).

3.12. Photocatalytic Tests

Simulated solar light photo-oxidation processes of organic matter are important for several practical reasons: (i) they require low material and operational costs, (ii) they have the potential to clean water and air by mineralizing organic pollutants to CO₂ [35,37,75,76], and (iii) they are attractive alternative routes for the selective synthesis of high-added-value oxygenated products [38,77].

Ethanol is a common compound obtained from biomass, with its photodegradation being important for hydrogen production and depollution. But it can also be present as a contaminant in air and industrial wastewater [37,78].

The procedure can be depicted from Figure 16. The representative samples, namely LTA and LTA-Pt, were selected for photocatalytic tests. The photoactive surface was exposed to simulated solar light (AM1.5). The photocatalytic test lasted three hours.

Photocatalytic tests are conducted at a 20% oxygen concentration. The main intermediate in the ethanol photogeneration process is acetaldehyde, which can be further oxidized in CO₂ and H₂O.

Table 8. Ethanol degradation in gaseous phase measured after 3 h of reaction.

Catalyst	CIN (μmoles)	COUT (μmoles)				Conversion of CH3CH2OH (%)
	Ethanol (CH3CH2OH)	Ethanol (CH3CH2OH)	Acetaldehyde (CH3CHO)	Formic Acid (HCOOH)	Carbon Dioxide (CO2)
LTA	243.15	208.39	11.18	0.29	5.13	14.29
LTA-Pt	243.45	171.02	36.47	1.06	13.67	29.75

shows the experimental data for reactant and product distribution, and ethanol conversion over bare and platinum-modified powder in the gaseous phase after 3 h of reaction time.

The conversion of ethanol is 14.29% (for LTA) and 29.75% (for LTA-Pt), meaning that platinum modification influences catalytic activity. At the end of the reaction time, both catalysts show the formation of mild oxidation products of ethanol (acetaldehyde and formic acid), as well as carbon dioxide, with the highest value obtained for LTA-Pt.

Figure 17 shows the comparative results for the oxidative degradation of ethanol on LTA and LTA-Pt catalysts exposed to simulated solar light. CH₃CHO and HCOOH are formed successively in the complex process of the mineralization of CH₃CH₂OH to CO₂. The blank test has been used to correct the results.

The photodegradation of ethanol is presented in Figure 17, showing reaction sequences to create intermediates.

Figure 17g,h show the photo-oxidative degradation of ethanol to CO₂ (and water) under simulated solar light irradiation. The materials investigated show activity in terms of ethanol mineralization in the gaseous phase. The higher production is recorded for LTA-Pt, which produces around 14 μmoles CO₂ after 3 h. Both catalysts of interest prove their activity in ethanol photodegradation. Following the first hour of irradiation, there is an increase in the rate of CO₂ generation, which is most likely due to the mineralization of previously produced intermediates. In terms of ethanol mineralization, the La₂Ti₂O₇ catalyst is less active than the platinum-modified catalyst by post-synthesis impregnation.

As can be seen in Figure 18, the amount of H₂ generated was 0.48 μmoles for the bare LTA sample and 1 μmole for the Pt-modified sample after a 3 h reaction.

The photocatalytic reaction follows first-order kinetics according to the equation “ln(C/C₀) = −kt”, where C and C₀ designate the ethanol concentration at time 0 and time t, and “k” is the apparent rate constant. Figure 19 shows the graph ln(C₀/C) versus the catalyst tested for the photocatalytic degradation of ethanol in the gas phase under simulated solar light irradiation. After three hours of reaction time, the value of the apparent speed k follows the following sequence: LTA (0.0008 min⁻¹) > LTA-Pt (0.0016 min⁻¹). These results show the increasing rate of reaction after the addition of the noble metal, which is in line with Figure 15 showing the decrease in the PL signal for the LTA-Pt sample (relative to LTA) in the water/ethanol solution, meaning a slower recombination of the photogenerated charges and their use for the catalytic process (a higher photoactivity of the LTA-Pt sample).

3.13. Proposed Mechanism of Oxidative Degradation of Ethanol Under Simulated Solar Light

Light-activated photocatalysts generate electrons and holes that, after reaching the catalyst surface, can display a variety of photoredox reactions [35,37,38,79,80,81].

In our case, the photocatalytic oxidative degradation of ethanol produces intermediates (acetaldehyde and formic acid) and end-products (carbon dioxide). The ability of the investigated materials to produce reactive oxygen species (specifically hydroxyl radical ·OH and superoxide anion radical ·O₂⁻) was checked, with only the presence of superoxide anion radical being confirmed (Figure 14).

A significant number of photocatalysts, including those based on TiO₂, are active, especially under UV or near-UV light irradiation [37,38,82], with their efficiency also being decreased by charge recombination [83]. In order to increase the photocatalyst activity and extend the absorption wavelength range, noble metal doping was performed.

In the present experiment, the oxidative degradation of ethanol (CH₃CH₂OH) under AM1.5 irradiation on pure and Pt-modified La₂Ti₂O₇ follows a photocatalytic mechanism involving the generation of electron–hole pairs. The reaction pathway includes the oxidation of ethanol to acetaldehyde (CH₃CHO) followed by further oxidation to carbon dioxide (CO₂), with formic acid (HCOOH) detected as a minor intermediate. Below is the detailed mechanism:

(a)

The process of photon absorption and charge separation (Equation (1)), according to the literature [18,84], can be described as follows:

$${\mathrm{L}\mathrm{a}}_2{\mathrm{T}\mathrm{i}}_2{\mathrm{O}}_7+\mathrm{h}\mathrm{\nu }\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}$$

(1)

(b)

Pt-modifying role—Platinum (Pt) enhances charge separation by trapping electrons and reducing recombination (Equation (2)), leading to more efficient ethanol oxidation and CO₂ formation [35,85,86] and promoting the adsorption and reduction of the oxygen (Equation (3)).

$$\mathrm{P}\mathrm{t}+{\mathrm{e}}^{-}\to {\mathrm{P}\mathrm{t}}^{-}$$

(2)

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}\to \left[\mathrm{O}\right]\left(\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{g}\mathrm{e}\mathrm{n}\right)$$

(3)

(c)

Oxidative degradation of ethanol by photogenerated holes—ethanol is oxidized by holes (h⁺) to form the ethoxy radical (CH₃CH₂O·) (Equation (4)) and this undergoes further oxidation, yielding acetaldehyde (CH₃CHO) (Equations (4)–(6)). Acetaldehyde is also oxidized by holes to generate formic acid (HCOOH) in trace amounts, finally leading to CO₂ (Equations (7) and (8)) [37,87,88,89,90,91].

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{H}+{\mathrm{h}}^{+}\to \mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}·+{\mathrm{H}}^{+}$$

(4)

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}·+\left[\mathrm{O}\right]\to \mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{O}·$$

(5)

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{O}·+{\mathrm{h}}^{+}+{\mathrm{e}}^{-}\to \mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(6)

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+\left[\mathrm{O}\right]\to \mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}$$

(7)

$$\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+\left[\mathrm{O}\right]\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(8)

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{H}\stackrel{\mathrm{s}\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{l}\mathrm{y}}{\to }\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}\to \mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\stackrel{\mathrm{r}\mathrm{a}\mathrm{p}\mathrm{i}\mathrm{d}\mathrm{l}\mathrm{y}}{\to }\mathrm{C}{\mathrm{O}}_2$$

(d)

Photogenerated electrons (e⁻) can reduce protons (H⁺) to form molecular hydrogen (H₂) (Equation (9)) [92].

$$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$$

(9)

This mechanism demonstrates how ethanol undergoes oxidative degradation in the gaseous phase when exposed to simulated solar light.

The literature data present the use of La₂Ti₂O₇ mostly for a liquid phase reaction targeting the hydrogen evolution and water depollution by organic compound degradation, especially for dyes [93,94].

Li et al. [46] obtained, by hydrothermal synthesis, La₂Ti₂O₇ two-dimensional (2D) nanosheets and used them as photocatalysts for the decolorization of methyl orange solution and the evolution of H₂ from water–ethanol solution, registering rates of H₂ evolution of about 750 μmol g⁻¹ h⁻¹ at 160 min.

In conclusion, this study presents the photocatalytic degradation of ethanol vapor under simulated solar light with 20% oxygen in the presence of pure or platinum-modified La₂Ti₂O₇ nanoparticles. The main products are acetaldehyde and CO₂. Two samples were tested in ethanol photodegradation experiments, and showed photocatalytic activity for ethanol mineralization. The photodegradation of ethanol in simulated solar light brings new data concerning the use of double lanthanum titanium oxide obtained by the sol–gel method as an efficient depollution photocatalyst.

4. Conclusions

Layered perovskite La₂Ti₂O₇ was prepared by the sol–gel method. The effect of noble metal addition on the properties of the resulting Pt-modified La₂Ti₂O₇ was evaluated.

The morphology of the samples showed aggregates of nanoparticles with well-defined mesopores.

XRD analysis indicated the formation of single-phase La₂Ti₂O₇ and the TEM micrographs indicated a uniform distribution of platinum across the surface of the La₂Ti₂O₇.

Oxidative degradation of the ethanol in the gaseous phase under simulated solar light irradiation followed a photocatalytic mechanism involving the generation of electron–hole pairs and superoxide anion radical leading to active oxygen.

The photocatalytic activity in terms of ethanol conversion and selectivity to CO₂ was increased by the addition of platinum to the La₂Ti₂O₇ catalyst, with platinum enhancing the charge separation but also the adsorption and reduction of the oxygen. Accordingly, these powders showed the potential for application in air depollution technologies.