MgAlTi-LDH/gCN heterocomposites analyzed by x-ray photoelectron spectroscopy

The exponential growth of water and air pollution caused by the increasing industrialization represents a main threat for human health and for a sustainable development (Refs. 1–5). In this regard, over the last two decades, heterogeneous photocatalysis has emerged as an extremely promising avenue for the decomposition of aqueous and gaseous pollutants into nontoxic products, thanks to its efficiency, economic viability, and inherently green character (Refs. 1 and 5–8). Accordingly, considerable efforts worldwide have been dedicated by the scientific community to the design and fabrication of active photocatalysts effectively activated by solar energy, enabling the removal of pollutants from water and of nitrogen oxides from the outer atmosphere (Refs. 6 and 9–11). Among the possible alternatives, layered double hydroxides (LDHs) stand as very attractive functional platforms, thanks to the uniform metal dispersion, high structural flexibility, wide range of chemical compositions, as well as tunable photoresponse range and surface chemistry (Refs. 1, 3, 4, 7, 9, and 12). Nonetheless, these advantages are at least partially eclipsed by the rapid recombination of photogenerated electrons and holes, limiting their efficiency (Refs. 1 and 2). In order to circumvent this drawback, a proficient option is offered by the combination of LDHs with other 2D materials, resulting in heterojunctions promoting the separation of electron-hole pairs and broadening the sunlight absorption spectrum (Refs. 1, 9, 13, and 14). To this aim, graphitic carbon nitride (gCN), a low-cost 2D semiconductor, is a very valuable candidate thanks to a plethora of favorable characteristics, encompassing nontoxicity, chemical stability, matching band structure, and excellent visible light response (Refs. 2, 4, 11, and 15–19).

In this general context, the present work is part of a comprehensive investigation aimed at the preparation and characterization of LDH-based heterogeneous photocatalysts for the removal of NO_x gases (x = 1 and 2) from urban environments (De-NO_x processes) (Refs. 3, 6, 9, 20, and 21). In particular, we focus on the study of LDH/gCN heterocomposites that, to our knowledge, have never been proposed for similar applications so far, at variance with bare gCN and LDHs (Refs. 7, 8, and 22). Since the system functional performances are directly affected by their structure, morphology, and chemical composition, we propose herein the investigation of MgAlTi-LDH/gCN systems prepared by a simple mixing procedure using x-ray photoelectron spectroscopy. Specifically, the attention is focused on the analysis of the main core levels (C 1s, Mg 1s, Mg 2s, Al 2p, Ti 2p, O 1s, and N 1s). The related spectral features reveal the formation of heterocomposites characterized by a direct electronic interplay between the system components, occurring through an interfacial gCN → MgAlTi-LDH charge transfer. These results, which anticipate an improved photocatalytic activity in comparison to the pristine LDH, may be of general interest for the scientific community working on the implementation of green photocatalysts for environmental remediation.

Specimen: MgAlTi-LDH

CAS Registry #: Unknown

Specimen Characteristics: Homogeneous; powder; polycrystalline; semiconductor; inorganic compound; other

Chemical Name: Magnesium aluminum(III) titanium(IV) layered double hydroxide

Source: Specimen prepared by a coprecipitation method

Composition: C, Mg, Al, Ti, and O

Form: Supported nanocomposite

Structure: X-ray diffraction (XRD) analysis revealed the presence of signals located at 2θ = 11.2°, 22.5°, 34.5°, 38.7°; 45.8°, 60.2°, and 61.4°, attributable, respectively, to the (003), (006), (009), (015), (018), (110), and (113) crystallographic planes of Mg₃Al_0.8Ti_0.2 layered double hydroxide (Ref. 23).

History and Significance: Mg₃Al_0.8Ti_0.2 layered double hydroxide was prepared via coprecipitation. In particular, titanium(IV) tetraisopropoxide (TiOⁱPr₄) was introduced dropwise for 45 min in a vessel containing concentrated HCl under a nitrogen atmosphere. The obtained TiCl₄ was subsequently introduced into a solution containing MgCl₂•6H₂O and AlCl₃•6H₂O. The 100 ml resulting solution of metal salts with the suitable proportion [Mg(II)/(Al(III) + Ti(IV)) = 3 and Ti(IV)/Al(III) = 0.25] was gradually dropped into a Na₂CO₃•10H₂O solution (100 ml) under stirring at room temperature, maintaining pH = 10 by the introduction of NaOH 4 M through an autotitrator. The resulting suspension was stirred for 16 h, washed with de-ionized water, and filtered. The obtained material was dried at 60 °C overnight. The delamination process was carried out following the AMOST (aqueous miscible organic solvent treatment) method (Refs. 3 and 24). In particular, the LDH was washed with de-ionized water until pH = 7 and subsequently with acetone (500 ml). The wet powders were then dispersed in 300 ml of acetone and stirred for 6 h at room temperature. The resulting product was then filtered, washed with 200 ml of acetone, and finally dried overnight at 60 °C.

As Received Condition: As grown

Analyzed Region: Same as the host material

Ex Situ Preparation/Mounting: The powder was fixed to the sample holder using a copper double-sided adhesive tape.

In Situ Preparation: Prior to analysis, the specimen was allowed to degas under vacuum conditions (×10⁻⁷ mbar) at 298 K for 12 h.

Charge Control: Charge compensation was obtained by means of a dual-beam low energy electron and ion coaxial flood source (0.1 V, 175 μA, and gas cell at 20 V).

Temp. During Analysis: 298 K

Pressure During Analysis: 10⁻⁷ Pa

Pre-analysis Beam Exposure: 130 s

Specimen: MgAlTi-LDH/gCN

CAS Registry #: Unknown

Specimen Characteristics: Homogeneous; powder; polycrystalline; semiconductor; composite; other

Chemical Name: Magnesium aluminum(III) titanium(IV) layered double hydroxide/graphitic carbon nitride

Source: Specimen prepared by mixing of single constituents

Composition: C, Mg, Al, Ti, and O

Form: Supported nanocomposite

Structure: XRD analysis yielded a diffraction pattern very similar to the one of bare MgAlTi-LDH (see accession #01965). The only difference was the presence of a weak signal at 2θ ≈ 28.2°, attributed to the (002) diffraction plane corresponding to the interplanar stacking of graphitic carbon nitride sheets (Refs. 15, 16, and 25).

History and Significance: The preparation of gCN powders was performed according to a previously reported synthetic route (Refs. 17 and 18). The target heterocomposite specimen was fabricated using a two-stage procedure. A mixture of Mg₃Al_0.8Ti_0.2 and gCN (285 and 15 mg, respectively) was suspended in 10 ml of deionized water. The slurry was stirred bar and heated to 40 °C. Subsequently, 10 ml of acetone was added to the mixture, and the stirring was continued at room temperature until complete solvent evaporation.

As Received Condition: As grown

Analyzed Region: Same as the host material

Ex Situ Preparation/Mounting: The powder was fixed to the sample holder using a copper double-sided adhesive tape.

In Situ Preparation: Prior to analysis, the specimen was allowed to degas under vacuum conditions (≈10⁻⁷ mbar) at 298 K for 12 h.

Charge Control: Charge compensation was obtained by means of a dual-beam low energy electron and ion coaxial flood source (0.1 V, 175 μA, and gas cell at 20 V).

Temp. During Analysis: 298 K

Pressure During Analysis: 10⁻⁷ Pa

Pre-analysis Beam Exposure: 130 s

Manufacturer and Model: ThermoFisher Scientific EscalabTM QXi

Analyzer Type: Spherical sector

Detector: Channeltron

Number of Detector Elements: 6

Analyzer Mode: Constant pass energy

Throughput (T = E^N): Calculated from a cubic polynomial fit to a plot of log[peak area/(PE × RSF)] (y) versus log(KE/PE) (x): y = a + bx + cx² + dx³, where PE is the pass energy, KE is the kinetic energy, and RSF is the relative sensitivity factor (Ref. 26). The coefficients corresponding to the adopted operating conditions are as follows: a = 3.867 64; b = − 0.075 012 2; c = 0.003 690 77; d = − 0.045 752 4.

Excitation Source Window: None

Excitation Source: Al K_α monochromatic

Source Energy: 1486.6 eV

Source Strength: 200 W

Source Beam Size: 500 × 500 μm²

Signal Mode: Single channel direct

Incident Angle: 58°

Source-to-Analyzer Angle: 58°

Emission Angle: 0°

Specimen Azimuthal Angle: 90°

Acceptance Angle from Analyzer Axis: 45°

Analyzer Angular Acceptance Width: 22.5° × 22.5°

Manufacturer and Model: ThermoFisher Scientific MAGCIS Dual Beam Ion Source

Energy: 4000 eV

Current: 7 mA

Current Measurement Method: Biased stage

Sputtering Species and Charge: Ar⁺

Spot Size (unrastered): 500 μm

Raster Size: 4500 × 4500 μm²

Incident Angle: 40°

Polar Angle: 40°

Azimuthal Angle: 270°

Comment: Differentially pumped ion gun

Energy Scale Correction: The reported binding energies were corrected for charging phenomena by assigning a BE of 284.8 eV to the adventitious C 1s signal (Ref. 27).

Recommended Energy Scale Shift: +0.7 eV for accession #01965 and +0.4 eV for accession #01966.

Peak Shape and Background Method: After performing a Shirley-type background subtraction (Ref. 28), least-squares fitting was performed adopting Gaussian/Lorentzian sum functions (% Lorentzian = 30%).

Quantitation Method: Atomic concentrations were calculated by peak area integration, using sensitivity factors provided by Thermo Scientific Avantage software (version 6.6.0, Build 00114).

This work was financially supported by CNR (Progetti di Ricerca @CNR—avviso 2020-ASSIST), Padova University (Nos. P-DiSC#04BIRD2020-UNIPD EUREKA, P-DiSC#02BIRD2023-UNIPD RIGENERA, and DOR 2022-2024), INSTM21PDBARMAC-ATENA, PRIN No. 2022474YE8 (SCI-TROPHY project), finanziato dall’Unione Europea—Next Generation EU—Bando PRIN 2022—M4.C2.1.1, and Agencia Estatal de Investigación (Spain; No. MCIN PID2020-117516GB-l00/AEI/10.13039/501100011033). The used instrumentation was funded by “Sviluppo delle infrastrutture e programma biennale degli interventi del Consiglio Nazionale delle Ricerche (2019).”

The authors have no conflicts to disclose.

Mattia Benedet: Data curation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (lead). Gian Andrea Rizzi: Data curation (equal); Funding acquisition (equal); Investigation (equal); Visualization (equal); Writing – review & editing (equal). Laura Marín: Investigation (equal); Software (equal); Validation (equal); Writing – review & editing (equal). Ivana Pavlovic: Data curation (equal); Investigation (equal); Validation (equal); Visualization (supporting). Luis Sánchez: Data curation (lead); Formal analysis (lead); Funding acquisition (equal); Investigation (equal); Methodology (equal); Writing – original draft (lead). Davide Barreca: Conceptualization (lead); Formal analysis (equal); Funding acquisition (equal); Supervision (equal); Writing – review & editing (lead). Chiara Maccato: Formal analysis (equal); Funding acquisition (equal); Methodology (equal); Supervision (lead); Visualization (lead); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.