X-ray Photoelectron Spectroscopy (XPS) Analysis of Carbon Nanotube Oxidation

X-ray Photoelectron Spectroscopy (XPS) is the most widely used surface analytical technique for quantifying oxidation and identifying oxygen-containing functional groups on carbon nanotubes (CNTs). By deconvoluting the C1s core-level spectrum and measuring the O/C atomic ratio, XPS provides direct, quantitative evidence of the degree and chemistry of CNT surface oxidation. This article presents a comprehensive technical guide covering sample types, testing items, measurement protocols, data interpretation, and typical applications of XPS in carbon nanotube oxidation analysis.

Technical Overview

X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), is a surface-sensitive quantitative technique that probes the elemental composition and chemical bonding states of the top 5–10 nm of a material. For carbon nanotubes, oxidation is a critical functionalization step—typically achieved through acid treatment, plasma exposure, ozone treatment, or thermal oxidation—that introduces oxygen-containing functional groups such as hydroxyl (C–OH), carbonyl (C=O), carboxyl (O–C=O), and epoxy/ether (C–O–C) groups onto the tube walls, defects, and end caps.

XPS detects these groups because each chemical environment produces a characteristic chemical shift in the C1s and O1s core-level binding energies. Through high-resolution spectral acquisition and curve fitting, XPS can determine not only the total oxygen content but also the relative abundance of each distinct functional group. This makes XPS the benchmark method for evaluating oxidation efficiency, comparing treatment protocols, and verifying surface chemistry before downstream processing such as composite fabrication, catalyst loading, or bioconjugation.

The key spectroscopic signatures used in CNT oxidation analysis include:

• C1s sp2 graphitic carbon (C=C) at approximately 284.4–284.6 eV, the dominant component of pristine nanotubes.
• C1s sp3 carbon (C–C) at approximately 285.0–285.2 eV, associated with defects and amorphous content.
• C1s C–O component (epoxy and hydroxyl groups) at approximately 286.1–286.6 eV.
• C1s C=O component (carbonyl groups) at approximately 287.2–287.8 eV.
• C1s O–C=O component (carboxyl and ester groups) at approximately 288.5–289.2 eV.
• C1s pi–pi* shake-up satellite at approximately 290.5–291.5 eV, an indicator of delocalized sp2 conjugation.
• O1s signal near 531.5–533.5 eV, with the C=O contribution at lower binding energy and the C–O contribution at higher binding energy.
• The O/C atomic ratio, the single most quoted parameter for expressing the overall degree of CNT oxidation.

Testing Samples

Pristine Carbon Nanotubes

Untreated single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) serve as reference samples. Pristine CNTs typically show low oxygen content (O/C below 0.03), which originates from adventitious contamination, residual catalyst oxidation, or inherent edge defects. Acquiring a baseline spectrum allows the intrinsic oxidation introduced by subsequent treatment to be accurately calculated.

Acid-Oxidized Carbon Nanotubes

The most common oxidized CNT samples are those treated with concentrated nitric acid, sulfuric/nitric acid mixtures (3:1 H2SO4/HNO3), or piranha solutions. These aggressive liquid-phase oxidations attack defect sites and tube ends, generating abundant carboxyl groups. XPS of such samples typically shows elevated O/C ratios of 0.05–0.20 and a pronounced O–C=O component in the fitted C1s envelope.

Plasma-Treated Carbon Nanotubes

Gas-phase oxidation using oxygen, air, or CO2 plasma produces a more homogeneous distribution of functional groups with less structural damage than liquid acid treatment. XPS is used to correlate plasma power, exposure time, and gas composition with the resulting O/C ratio and functional group distribution.

Ozone- and UV/Ozone-Treated Carbon Nanotubes

Mild ozonation selectively functionalizes the nanotube surface while largely preserving the sp2 lattice. XPS distinguishes these samples by their relatively high C–O fraction and moderate total oxygen content, confirming gentler oxidation chemistry.

Thermally Oxidized and Irradiated Carbon Nanotubes

CNTs oxidized in air at elevated temperature or modified by gamma irradiation, electron beams, or ion beams can also be characterized. XPS tracks progressive oxidation as a function of annealing temperature or irradiation dose.

Carbon Nanotubes Extracted from Composite Materials

When CNTs are recovered from polymer matrices, ceramic composites, or catalyst supports, XPS verifies whether interfacial oxidation or covalent bonding occurred during processing. Care must be taken to account for matrix-derived carbon and oxygen signals during data interpretation.

Testing Items

A complete XPS characterization of CNT oxidation typically includes the following deliverables:

• Wide survey scan (0–1100 eV) identifying all elements present (C, O, and possible residues such as N, S, Cl, Fe, Co, Ni, Al from catalysts or treatment reagents).
• High-resolution C1s spectrum with full peak deconvolution into sp2 C=C, sp3 C–C, C–O, C=O, O–C=O, and pi–pi* satellite components, each with binding energy, full width at half maximum (FWHM), and relative area percentage.
• High-resolution O1s spectrum fitted into C=O and C–O contributions for cross-validation of the C1s results.
• Quantitative elemental composition in atomic percent, including the O/C atomic ratio as the primary oxidation metric.
• sp2/sp3 carbon ratio, reflecting the integrity of the graphitic structure after oxidation.
• Functional group distribution tables and comparison charts before and after oxidation treatment.
• Optional: valence band spectra, work function measurements, and angle-resolved XPS for depth distribution of oxidation.

Testing Methods

Sample Preparation

CNT powder is typically mounted by pressing a thin, uniform layer onto conductive double-sided carbon tape, an indium foil, or a gold substrate. Loose particles must be removed by gentle tapping or compressed nitrogen blowing to avoid contamination of the analyzer lens. Alternatively, dilute CNT dispersions can be drop-cast onto clean gold or silicon substrates and dried under vacuum. All samples should be dried at 60–80 °C for several hours and stored in a desiccator, since CNT powders readily adsorb atmospheric moisture and hydrocarbons that distort the oxygen signal. For composite samples, a freshly cut or polished cross-section is preferred to expose representative material.

Instrumentation and Acquisition Parameters

Measurements are performed on instruments equipped with a monochromatic Al Kα X-ray source (hv = 1486.6 eV) under ultrahigh vacuum (approximately 10⁻⁹ mbar or better). Typical parameters are: survey scan pass energy of 100–160 eV with a step size of 1 eV; high-resolution C1s and O1s scans with a pass energy of 20–30 eV and step size of 0.05–0.1 eV. Because CNTs are sufficiently conductive in powder form, charging is generally modest; however, isolated or composite-embedded tubes may require a low-energy electron flood gun for charge neutralization. Spectra are charge-corrected by setting the adventitious/graphitic C1s sp2 component to 284.6 eV (or 284.8 eV depending on laboratory convention).

Data Processing and Peak Fitting Protocol

• Background subtraction using the Shirley or Tougaard method applied over an appropriate energy window.
• Peak synthesis using mixed Gaussian–Lorentzian (pseudo-Voigt) line shapes in software such as CasaXPS, Thermo Avantage, or PHI MultiPak.
• Constraint of component binding energies within established literature ranges (sp2 at 284.4–284.6 eV, sp3 at 285.0–285.2 eV, C–O at 286.1–286.6 eV, C=O at 287.2–287.8 eV, O–C=O at 288.5–289.2 eV, satellite at 290.5–291.5 eV).
• Keeping FWHM values physically reasonable, typically 0.8–1.6 eV for carbon components on monochromatic instruments.
• Application of appropriate relative sensitivity factors (RSF) for quantification, with sensitivity factors consistent with the instrument database (e.g., Scofield cross-sections).
• Reporting of peak areas, atomic percentages, and the calculated O/C ratio with clear documentation of all fitting constraints.

Quality Assurance Considerations

Adventitious oxygen and hydrocarbon contamination can artificially inflate the measured oxidation level. Best practice includes comparing the O1s intensity with the fitted oxygen-containing C1s components for internal consistency; if the O1s-derived oxygen greatly exceeds the sum of the C1s oxidized components, contributions from adsorbed water or inorganic residues should be suspected. Gentle in-situ heating or vacuum desorption prior to measurement can reduce adsorbed species. Replicate measurements at multiple positions across the sample verify homogeneity.

Applications

XPS oxidation analysis of carbon nanotubes underpins research and quality control across numerous fields:

• Polymer composites: verifying that CNT oxidation improved interfacial adhesion and load transfer with epoxy, polyimide, polypropylene, and natural fiber matrices, correlating O/C ratio with measured mechanical property enhancement.
• Catalysis: quantifying surface oxygen anchor sites for dispersing metal nanoparticles (Pt, Pd, Ru, Fe) on CNT supports in fuel cell electrodes and electrocatalytic reactors.
• Biosensing and biomedical engineering: confirming carboxyl group density for EDC/NHS-mediated antibody or drug immobilization on CNT electrodes and drug delivery carriers.
• Energy storage: assessing functionalization of CNT anodes and conductive additives in lithium-ion batteries and supercapacitors, where excessive oxidation degrades conductivity while moderate oxidation improves wettability.
• Field emission and electronics: monitoring the sp2/sp3 ratio and satellite intensity, since oxidation-induced defects influence emission stability and transport behavior.
• Environmental remediation: characterizing oxidized CNT adsorbents for heavy metal ion and organic pollutant capture, linking carboxyl density with uptake capacity.
• Process optimization: statistically comparing acid concentration, reflux time, plasma power, and ozone exposure to identify the mildest treatment that achieves a target oxidation level while preserving tube structure.

FAQ

How does XPS quantify the degree of CNT oxidation?

The overall oxidation degree is expressed as the O/C atomic ratio derived from survey or region scans using sensitivity-factor-corrected peak areas. Additionally, the summed relative areas of the C–O, C=O, and O–C=O components in the fitted C1s spectrum quantify the fraction of carbon atoms bonded to oxygen, giving a chemically resolved oxidation profile.

What is the detection depth of XPS for CNT samples?

XPS samples the top 5–10 nm of material, corresponding to roughly 10–20 atomic layers. For CNTs with diameters of 1–50 nm, this means the measured signal represents essentially the entire tube wall for small-diameter tubes and the outermost region for large multi-walled tubes. XPS therefore reports surface oxidation, which is precisely the region governing interfacial behavior.

Can XPS distinguish between hydroxyl, carbonyl, and carboxyl groups?

Yes, with appropriate curve fitting. Hydroxyl and epoxy groups both contribute to the C–O peak near 286.3 eV, carbonyl groups appear near 287.5 eV, and carboxyl groups produce the distinctive O–C=O peak near 288.8 eV. Carboxyl quantification is the most reliable because its chemical shift is well separated, whereas resolving hydroxyl from epoxy carbon requires complementary FTIR or chemical derivatization-XPS methods.

How does XPS compare with TGA and FTIR for evaluating CNT oxidation?

Thermogravimetric analysis measures the mass loss of functional groups upon heating and provides bulk-averaged information but cannot identify specific bonding configurations. FTIR identifies functional group types qualitatively but struggles with quantitative analysis of dark, strongly absorbing CNT samples. XPS uniquely combines quantitative elemental analysis with chemical-state resolution at the surface, making it the preferred technique, while TGA and FTIR serve as valuable complementary methods.

How much sample is required for XPS analysis?

Typically 5–20 mg of CNT powder is sufficient to prepare a uniform film on the sample holder. The analyzed spot is usually a few hundred micrometers in diameter, so uniform deposition across at least a 5 mm × 5 mm area is recommended.

What O/C ratio indicates successful oxidation?

Values depend on the treatment method. Pristine CNTs usually show O/C below 0.03. Mild ozone or short plasma treatment raises it to approximately 0.05–0.10, while aggressive nitric/sulfuric acid reflux can reach 0.15–0.25. Excessively high ratios often indicate structural degradation or residue contamination and should be cross-checked with Raman spectroscopy (D/G band ratio) and TEM.

Why do reported C1s binding energies vary slightly between publications?

Differences arise from instrument calibration conventions (284.6 versus 284.8 eV for the C–C/C=C reference), monochromatic versus non-monochromatic sources, charge compensation settings, and the conductive state of the sample. Consistency within a single data set matters more than absolute agreement across laboratories, which is why all comparative studies should be conducted under identical conditions.