XPS investigation of Cu_xO-functionalized amorphous carbon nitride

The development of efficient and clean energy production and conversion technologies represents an open challenge of utmost importance for sustainable development (Refs. 1–4). In this context, (photo)electrochemical water splitting yielding green H₂ is regarded as the “holy grail” of the future energy infrastructure (Refs. 5–7). However, the overall process is strongly limited by the oxygen evolution reaction (OER) that, due to its high overpotential and intrinsic kinetic limitations, severely hampers H₂ generation yield (Refs. 3, 8, and 9).

In the present work, we report on the synthesis and characterization of a novel nanocomposite material, specifically designed to be employed as OER (photo)electrocatalyst. The specimen was prepared by a two-step, low-temperature plasma-assisted route, which has never been adopted so far for the fabrication of analogous systems. First, amorphous carbon nitride (aCN) was deposited onto conductive fluorine-doped tin oxide (FTO) via reactive magnetron sputtering, using a graphite target in Ar/N₂ atmosphere. Subsequently, highly dispersed copper oxide (Cu_xO, x = 1, 2) nanoparticles (NPs) were grown on top of the aCN deposit by means of RF-sputtering. Amorphous carbon nitride systems, characterized by an N/C atomic ratio lower than stoichiometric C₃N₄ (Refs. 10 and 11), display a remarkable chemical stability and favorable physical and mechanical features, thanks to their covalently bonded network (Refs. 12 and 13). Whereas some papers have reported on the use of powdered aCN for photocatalytic evolution of H₂ (Refs. 14–16), the investigation of the material as supported OER (photo)electrocatalyst is still in its preliminary stages, particularly if compared to crystalline graphitic carbon nitride (g-C₃N₄) (Refs. 15 and 17). On the other hand, nanostructured copper oxides (Cu₂O and CuO, both p-type semiconductors with a band gap of ≈2.0 and 1.2/1.5 eV, respectively) have already been extensively studied for (photo)electrocatalytic applications due to their large availability, low cost, nontoxicity, and significant vis-light absorption (Refs. 5 and 18). In a different way, the synthesis and characterization of aCN-Cu_xO nanocomposites undoubtedly deserves further attention for both fundamental and applicative purposes.

In this work, the use of plasma sputtering for the synthesis of the composite material allowed to achieve an intimate contact between aCN and Cu_xO. This feature results in the formation of heterojunctions between p-type copper oxides and n-type carbon nitride, improving charge separation and boosting catalytic activity toward OER under illumination (Refs. 2, 3, 19, and 20). In addition, the carbon nitride matrix also promoted the ultradispersion of copper oxides NPs, stabilizing them against aggregation (Refs. 9 and 21).

In the following, a detailed XPS investigation of a representative aCN-Cu_xO sample is reported, focusing on the analysis of C 1s, N 1s, O 1s, Cu 2p, and Cu LMM spectral regions, providing information on the elemental chemical states. The collected data, unavailable in the literature up to date, could be useful to researchers focusing on the XPS investigation of amorphous carbon nitride-based nanomaterials.

Specimen: aCN-Cu_xO

CAS Registry #: Unknown

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

Chemical Name: Amorphous carbon nitride-copper (I)/copper (II) oxides

Source: Sample prepared by aCN magnetron sputtering deposition on FTO and subsequent annealing in Ar at 500 °C for 2.5 h, followed by functionalization with Cu_xO NPs by RF-sputtering for 15 min and final thermal treatment in Ar at 450 °C for 2.5 h.

Composition: C, N, O, and Cu

Form: Supported nanocomposite

Structure: X-ray diffraction (XRD) analyses only revealed reflections from the FTO substrate, whereas no signals originating from the aCN-Cu_xO deposit were detected. Such a result was traced back to the amorphous nature of the carbon nitride layer and to the low amount and high dispersion of copper oxide NPs. Fourier-transform infrared (FT-IR) spectroscopy measurements showed a broad band in the 1100–1700 cm⁻¹ region, associated to characteristic vibrational modes of the nitride network (Refs. 22 and 23). Moreover, a small but well distinguishable peak at ≈2220 cm⁻¹ was observed, pointing out to the presence of —C≡N groups (Ref. 24). The broad and multicomponent band between 2600 and 3600 cm⁻¹ was associated to the stretching modes of chemisorbed O—H moieties, as well as to C—H and N—H terminal groups (Ref. 8). Scanning electron microscopy (SEM) evidenced that the aCN deposit consisted of a relatively compact and well-adherent film with a thickness of ≈450 nm. Transmission electron microscopy (TEM) revealed an ultradispersion of low-sized Cu-containing NPs over the aCN matrix.

History and Significance: The aCN film was synthesized in a custom built multiple-electrode magnetron sputtering apparatus, equipped with a 13.56 MHz RF power source. A pyrolytic graphite target (Nanovision Srl, thickness = 3 mm, diameter= 50 mm, and purity = 99.999%) was fixed to the magnetron source, while a precleaned FTO substrate (2 × 1 cm²; Aldrich®; ≈7 Ω × sq⁻¹; thickness = 600 nm) was mounted on the ground electrode. The synthesis procedure was performed at room temperature under the following conditions: RF power = 100 W; duration = 30 min; total pressure = 5.6 × 10⁻³ mbar; Ar flow rate = 21 standard cubic centimeters per minute (SCCM); and N₂ flow rate = 42 SCCM. After deposition, the specimen underwent a thermal annealing at 500 °C for 2.5 h in Ar atmosphere.

Functionalization with copper oxides was performed by RF-sputtering using a different custom built two-electrode apparatus (ν = 13.56 MHz). A copper target (Alfa Aesar®; thickness = 0.3 mm; and purity = 99.95%) was fixed on the RF electrode, whereas the FTO-supported aCN deposit was mounted on the grounded one. NPs deposition was carried out using the following settings: Ar flow rate = 10 SCCM; total pressure = 0.30 mbar; growth temperature = 60 °C; RF power = 10 W; duration = 15 min; and target-to-substrate distance = 60 mm. Lastly, the material was subjected to a final thermal treatment at 450 °C for 2.5 h in Ar atmosphere.

As-Received Condition: As grown.

Analyzed Region: Same as host material.

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 specimen was analyzed as-received.

Charge Control: None

Temp. During Analysis: 298 K

Pressure During Analysis: <10⁻⁸ Pa

Preanalysis 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) vs log(KE/PE) (x): y = a+ bx + cx² + dx³, where PE and KE are the pass energy and the kinetic energy, and RSF is the relative sensitivity factor. The coefficients corresponding to the adopted operating conditions are a = 3.867 64; b = − 0.075 012 2; c = 0.003 690 77; and 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: None.

The analyzer calibration procedure performed in this work was the one proposed by Seah (Ref. 25).

Recommended Energy Scale Shift: 0 eV

Peak Shape and Background Method: In the present work, peak analysis was performed employing the XPSpeak software (version 4.1) (Ref. 26). A Shirley-type background was used. Fitting of C 1s, N 1s, and O 1s peaks was carried out with least-squares fitting method (Ref. 27), adopting Gaussian/Lorentzian sum functions (typical mixing parameter = 0.2–0.3) (Ref. 28) and a number of average points at endpoints equal to 7. No constraints were imposed.

Recommended background start and end points (±0.4 eV) are given below:

C 1s: 280–296 eV

N 1s: 395–407 eV

O 1s: 527–537.5 eV

Quantitation Method: Quantification was accomplished using Thermo Scientific Avantage software (version 6.6.0, Build 00114) by normalizing peak areas for the respective sensitivity factors (Ref. 29). The used sensitivity factors were provided by the same software (peak library: ALTHERMO1; energy correction: TPP-2M IMFF).

This work was financially supported by CNR (Progetti di Ricerca @CNR—avviso 2020—ASSIST), Padova University (P-DiSC#04BIRD2020-UNIPD EUREKA, P-DiSC#02BIRD2023-UNIPD RIGENERA, DOR 2021–2023), INSTM Consortium (INSTM21PDGASPAROTTO-NANOMAT and 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).” Thanks are due to Dr. S. M. Deambrosis, Dr. V. Zin, and Dr. F. Montagner (CNR-ICMATE, Padova, Italy) for valuable support and discussions.

The authors have no conflicts to disclose.

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

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