ZnO and ZnFe₂O₄-functionalized graphitic carbon nitride: An XPS study

Over the past decade, graphitic carbon nitride (gCN)-based systems have received significant attention as versatile, metal-free multifunctional (photo)electrocatalytic platforms for environmental remediation and clean energy production (Refs. 1–4). This widespread interest has been fueled by gCN chemical-physical properties, in particular its ability to absorb Vis light (E_g ≈ 2.7 eV), its facile synthesis from readily available and nontoxic precursors, chemical stability, along with the tunable defectivity and electronic structure (Refs. 5–7). These benefits are partially hindered by some intrinsic limitations, such as a restricted active surface area and the rapid recombination of photogenerated electrons and holes (Refs. 2, 8, and 9). Among the various possible approaches to address these challenges (Refs. 7 and 10–12), the modulation of material nano-organization (Refs. 13 and 14) and the combination with nano-dispersed metal or metal oxide co-catalysts (Refs. 1, 15, and 16) hold a considerable promise. In particular, thanks to the presence of electron-rich sites on its surface, gCN can be effectively employed as an active support for anchoring and stabilizing metal and metal oxide nanostructures (Refs. 17–19). The intimate contact between the introduced functionalizing agents and the underlying gCN can lead to the formation of heterojunctions, which can favorably extend the lifetime of electron/hole pairs and suppress detrimental recombination losses, thereby increasing the ultimate system performances (Refs. 9, 15, 16, and 20).

Building on these insights, the present research work explores the development of gCN-ZnO (zincite) and gCN-ZnFe₂O₄ (zinc ferrite) composite (photo)anodes to be employed for ethanol oxidation reaction (EOR), an appealing process for clean energy generation (Refs. 21 and 22). Previous works have reported on the successful combination of graphitic carbon nitride with ZnO and ZnFe₂O₄, in particular for the photocatalytic remediation of water from persistent pollutants (Refs. 23–27), whereas the functional application of these materials as EOR electrocatalysts is still substantially unexplored up to date.

In this work, the synthesis of the target composites is performed by a multistep approach. First, gCN was prepared by thermal polymerization of thiourea, mixed with acetylacetone (Ref. 12). Subsequently, carbon nitride was deposited onto carbon cloth (CC) substrates through an electrophoretic deposition (EPD) procedure, already optimized by our group (Ref. 28). ZnO or ZnFe₂O₄ nanoparticles were then introduced employing plasma-assisted radio frequency (RF)-sputtering at low temperatures. Finally, the prepared materials underwent a thermal treatment in air, for their final stabilization before chemico-physical characterization and functional tests.

In the present contribution, an insightful XPS investigation of representative gCN-ZnO and gCN-ZnFe₂O₄ specimens is reported, providing a systematic examination of the elemental chemical states by the analysis of their main peaks (C 1s, N 1s, O 1s, Zn 2p, Zn LMM, and Fe 2p). The present data could serve as a useful comparison in the XPS investigation of similar composite materials for (photo)electrocatalytic and energy-related applications.

Specimen: gCN-ZnO

CAS Registry #: Unknown

Specimen Characteristics: Homogeneous; solid; polycrystalline; semiconductor; composite

Chemical Name: Graphitic carbon nitride-zinc (II) oxide

Source: Specimen prepared by gCN electrophoretic deposition on carbon cloth, followed by functionalization with ZnO by RF-sputtering for 30 min, and final thermal treatment in air at 350 °C for 90 min.

Composition: C, N, O, Zn

Form: Supported nanocomposite

Structure: X-ray diffraction (XRD) investigation evidenced the presence of one intense signal at 2θ ≈ 25.6° and minor peaks at ≈43.5° and 52.9° ascribed, respectively, to the (002), (100), and (404) reflections from the CC support (Ref. 29). No signal related to graphitic carbon nitride or ZnO could be clearly observed. Fourier-transform infrared (FT-IR) spectroscopy analysis showed the characteristic vibrational modes of the gCN network in the 1200–1700 cm⁻¹ range, and a peak at ≈820 cm⁻¹ due to out-of-plane bending of heptazine rings (Refs. 30 and 31). The broad band at 3200–3300 cm⁻¹ was ascribed to the presence of uncondensed −NH_x groups (x = 1, 2) and chemisorbed –OH (Refs. 20 and 32). Scanning electron microscopy (SEM) analyses revealed that gCN deposits were characterized by an exfoliated, sheet-like morphology, covering the underlying CC. No ZnO-containing particles could be clearly observed. Transmission electron microscopy (TEM) confirmed the presence of nano-dispersed ZnO onto carbon nitride. The latter feature is expected to result in an enhanced contact between the system constituents, favoring their synergistic interplay and boosting thus the ultimate electrocatalytic performances.

History and Significance: gCN was prepared by thermal condensation of thiourea (TU) powders, mixed with acetylacetone (AcAc), in air (550 °C for 2 h, heating rate = 3 °C/min) (Ref. 12). The obtained powders were deposited onto a precleaned carbon cloth substrate (Quintech E35; 150 μm thickness, ≈2 × 1 cm² area) via EPD. Deposition was carried out under previously optimized experimental conditions (applied potential = 10 V; duration = 10 min) (Ref. 28). The resulting sample was annealed in air at 300 °C for 1 h. The subsequent functionalization with ZnO was performed by RF-sputtering (ν = 13.56 MHz) from an Ar plasma, in a custom-built two-electrode reactor. To this aim, a ZnO target (Neyco, purity = 99%, thickness = 0.1 mm) was fixed on the RF-electrode, whereas CC-supported gCN was mounted on the grounded one. Sputtering was performed using the following experimental settings: Ar flow rate = 10 standard cubic centimeters per minute (SCCM); total pressure = 0.30 mbar; growth temperature = 60 °C; RF-power = 20 W; duration = 30 min. After sputtering, the obtained composite material underwent final annealing in air at 350 °C for 90 min.

As Received Condition: As grown.

Analyzed Region: Same as specimen.

Ex Situ Preparation/Mounting: The specimen was mounted on a grounded sample holder by metallic clips and introduced into the chamber through a fast entry system.

In Situ Preparation: The sample was analyzed as received. Core-level spectra recorded for the received sample, and at the end of the first round of analyses, did not show any significant variations, enabling thus to exclude the occurrence of appreciable analysis-induced damages arising from x-ray exposure.

Charge Control: None

Temp. During Analysis: 298 K

Pressure During Analysis: 10⁻⁷ Pa

Pre-analysis Beam Exposure: 150 s

Specimen: gCN-ZnFe₂O₄

CAS Registry #: Unknown

Specimen Characteristics: Homogeneous; solid; polycrystalline; semiconductor; composite

Chemical Name: Graphitic carbon nitride-zinc ferrite

Source: Specimen prepared by gCN electrophoretic deposition on carbon cloth, followed by functionalization with ZnFe₂O₄ by RF-sputtering for 200 min and final thermal treatment in air at 350 °C for 90 min.

Composition: C, N, O, Zn, Fe

Form: Supported nanocomposite

Structure: XRD and FT-IR analyses yielded results analogous to the material described in accession # 01996. SEM analyses evidenced a similar gCN morphology, whereas TEM measurements highlighted the presence of ultradispersed ZnFe₂O₄.

History and Significance: gCN-ZnFe₂O₄ synthesis conditions were the same adopted for gCN-ZnO; the only differences were the use of a ZnFe₂O₄ target (Neyco, purity = 99%, thickness = 0.1 mm) and the duration of the sputtering process (200 min).

As Received Condition: As grown.

Analyzed Region: Same as specimen.

Ex Situ Preparation/Mounting: The specimen was mounted on a grounded sample holder by metallic clips and introduced into the chamber through a fast entry system.

In Situ Preparation: The sample was analyzed as received. As for accession # 01996, the occurrence of x-ray beam damages was ruled out by reacquiring core level spectra at the end of the analysis.

Charge Control: None

Temp. During Analysis: 298 K

Pressure During Analysis: 10⁻⁷ Pa

Pre-analysis Beam Exposure: 150 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): The transmission function is calculated from a cubic polynomial fit to a plot of log[peak area/(PE × XSF)] vs log(KE/PE), where PE is the pass energy, KE is the kinetic energy, and XSF is the relative sensitivity factor (Refs. 33–35).

Excitation Source Window: 1.5-μm Al window

Excitation Source: Al K_α

Source Energy: 1486.6 eV

Source Strength: 200 W

Source Beam Size: 500 × 200 μm²

Signal Mode: Single channel direct

Incident Angle: 55°

Source-to-Analyzer Angle: 135°

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: Binding energy values were corrected for charging by setting the adventitious C 1s signal to 284.8 eV (Ref. 36). The validity of the obtained results is supported by a detailed comparison of the contributing band positions with previous literature results on homologous systems (Refs. 9, 20, 28, 33, and 37).

Recommended Energy Scale Shift: −1.08 eV for accession # 01996 and −1.10 eV for accession # 01997.

Peak Shape and Background Method: In the present work, after a Shirley-type background subtraction, peak fitting was carried out with the least-squares fitting method (Ref. 38), employing the xpspeak software (version 4.1) (Ref. 39) and adopting Gaussian/Lorentzian sum functions (typical mixing parameter = 0.2–0.3) (Ref. 40). We verified that the fitting results did not yield significant differences and were satisfactory, with small variations of the mixing parameter in this range. No constraints on the relative binding energy positions and FWHM values of the contributing components were ever imposed. The reliability of the obtained data is supported by their consistency with literature references, as well as with the outcomes of our previous results on different composite materials based on graphitic carbon nitride (Refs. 33, 37, and 41–46).

Quantitation Method: Quantification was accomplished by normalizing peak areas for the respective sensitivity factors (Ref. 47), 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 (P-DiSC#02BIRD2023-UNIPD RIGENERA, DOR 2021–2024), INSTM Consortium (INSTM21PDGASPAROTTO-NANO^MAT, INSTM21PDBARMAC-ATENA), and PRIN 2022474YE8 SCI-TROPHY project (financed by the European Union - Next Generation EU—Bando PRIN 2022–M4.C2.1.1). The instrumental apparatus used in this work was funded by “Sviluppo delle infrastrutture e programma biennale degli interventi del Consiglio Nazionale delle Ricerche (2019).”

The authors have no conflicts to disclose.

Giacomo Marchiori: Investigation (lead); Software (equal); Validation (equal); Writing – review & editing (lead). Mattia Brugia: Investigation (equal); Software (equal); Validation (equal); Writing – review & editing (equal). Tommaso Sturaro: Data curation (equal); Methodology (equal); Validation (equal); Visualization (equal). Mattia Benedet: Data curation (equal); Methodology (equal); Validation (equal); Visualization (equal). Davide Barreca: Conceptualization (lead); Formal analysis (lead); Funding acquisition (lead); Supervision (equal); Writing – review & editing (lead). Alberto Gasparotto: Data curation (lead); Formal analysis (lead); Funding acquisition (equal); Investigation (equal); Methodology (equal); Writing – original draft (lead). Gian Andrea Rizzi: Data curation (equal); Funding acquisition (equal); Investigation (equal); Visualization (equal); Writing – review & editing (equal). Chiara Maccato: Formal analysis (equal); Funding acquisition (lead); Methodology (equal); Supervision (equal); Visualization (equal); Writing – review & editing (equal).

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