Carbon-Doped Cantor Alloy Amorphous HEA NPs

• The paper demonstrates that precise carbon doping via pulsed laser ablation produces metastable amorphous HEA nanoparticles with controllable carbon shell thicknesses.
• Characterization confirms near-equiatomic metal distribution, tunable carbon contents (5–20 at.%), and structural stability up to 350°C for enhanced OER performance.
• Kinetic control through solvent choice balances carbon uptake and metal coalescence, enabling adjustable nanoparticle morphology and improved electrocatalytic behavior.

Carbon-doped Cantor alloy-based amorphous high-entropy alloy (HEA) nanoparticles (NPs) are a class of multicomponent nanomaterials comprised of equiatomic Cr, Mn, Fe, Co, and Ni, synthesized with high levels of carbon incorporation and a disordered atomic arrangement. These NPs exhibit a metastable amorphous phase stabilized by supersaturated carbon, possess a graphitic carbon shell, and display tunable surface and bulk properties governed by synthesis conditions. Their structural, compositional, and functional characteristics enable enhanced electrocatalytic activity, notably for the oxygen evolution reaction (OER), and offer a versatile framework for surface and phase engineering in multicomponent nanoalloy systems.

1. Synthesis Protocol and Carbon Doping Mechanism

Carbon-doped Cantor alloy-based amorphous HEA NPs are fabricated by nanosecond-pulsed laser ablation in organic solvents, using a bulk CrMnFeCoNi HEA target and no added stabilizer (Nallathambi et al., 12 Nov 2025, Nallathambi et al., 22 May 2025). The irradiation conditions involve high pulse energy (35 mJ, 8 ns duration, 5 kHz repetition rate) focused onto a ∼100 µm spot, inducing local temperatures >5,000 K and pressures >GPa. The organic solvent—acetonitrile, acetone, or ethanol—flows at ≈50 mL/min under N₂ purge to exclude O₂.

Laser-induced plasma and cavitation events first ablate metal fragments (M_l), then decompose solvent molecules via homolytic cleavage. For acetonitrile, pyrolysis reactions liberate C* (reactive atomic carbon):

$$\mathrm{CH_3CN} + h\nu \rightarrow \cdot\mathrm{CH_3} + \cdot\mathrm{CN} \rightarrow \mathrm{C_{doped}} + \text{volatile fragments}$$

Carbon atoms are kinetically trapped in the quasi-liquid alloy droplets and migrate toward interfaces, initiating rapid supersaturation. Subsequent graphitization at the NP–liquid interface forms carbon shells of varying thickness, dependent on the carbon fraction $X_C = \frac{C}{C+O}$ in the solvent.

$$\mathrm{M_{(l)}} + x\,\mathrm{C} \rightarrow \mathrm{M-C}_{(\text{amorphous HEA NP})}$$

Three characteristic timescales—metallic coalescence $(\tau_\text{coal})$, carbon uptake, and carbon shell formation $(\tau_\text{shell})$—compete to define final NP morphology: thick shells and full amorphization are favored when carbon shell formation is faster than coalescence (acetonitrile), while thinner shells and partial crystallization result if coalescence dominates (ethanol). This kinetic regime is controllable via solvent choice (Nallathambi et al., 22 May 2025).

2. Structural and Compositional Characterization

Amorphous HEA NPs exhibit characteristic morphological and structural features (Nallathambi et al., 12 Nov 2025, Nallathambi et al., 22 May 2025):

• Composition (STEM–EDS, APT, XPS): Near-equiatomic metallic distribution (Co, Cr, Fe, Mn, Ni ≈ 19 at.% each), with carbon content tunable from 5–20 at.% (solvent-dependent; acetonitrile yields highest).
• Particle Size/Morphology (TEM/STEM): Spherical to slightly rugged particles with average diameter 15–19 nm, unimodal distribution, and full encapsulation by a continuous carbon shell.
• Carbon Shell Thickness: Uniform shell thickness of 2–3 nm (as-synthesized, Figure 1a), up to 10–15 graphitic layers in acetonitrile, decreasing with lower $X_C$ (see table).
• Amorphous Phase: XRD/SAED show broad halos (2θ ≈ 20–43°, centered at 42°), absence of fcc reflections pre-annealing.
• Thermal Stability: Retention of the amorphous structure up to 350 °C; crystallization onset at 400 °C for smallest NPs; complete transition to fcc metal alloy with elemental partitioning at 600 °C (volume shrinkage ∼5%).
• Surface Enrichment: Mn preferentially partitions to the NP surface in acetonitrile-derived samples (up to 42 at.%), with Cr and Mn tending toward subsurface and shell upon annealing.

Solvent	C Shell Thickness	Structure	Mn Surface at.%
Acetonitrile	~10 layers	Fully amorphous	42
Acetone	~5 layers	Fully amorphous	34
Ethanol	~2 layers	Amorphous + fcc	32

3. Phase Metastability, Annealing, and Heterostructure Formation

The thermodynamic basis for metastability involves a competition between high configurational entropy ($$\Delta S_\text{config} = R \ln 5 \approx 13.4\,\text{J\,mol}^{-1}\text{K}^{-1}$$) and enthalpic preference for crystallization. Supersaturated carbon elevates the Gibbs free energy $\Delta G$ of the amorphous phase:

$\Delta G = \Delta H - T \Delta S_{\text{total}}$

Upon annealing (400–600 °C), the enthalpic stabilization overcomes entropic effects, and NPs undergo a core-volume contraction, full crystallization, and elemental partitioning. EDS maps after annealing reveal formation of Cr-rich and Ni-rich domains, with Ni and Co co-partitioned in the core, Fe accumulating centrally, and Mn mobilized toward the shell. The resulting structure is a heterostructured, carbon-encapsulated fcc nanoalloy with chemically distinct crystalline interfaces.

A plausible implication is that the kinetically engineered amorphous-to-crystalline transition enables deliberate tailoring of compositional gradients and interface densities, which could serve as active sites for catalysis.

4. Electrocatalytic Properties and Activity Enhancement

The functional performance of these NPs is highlighted in OER catalysis (Nallathambi et al., 12 Nov 2025). In alkaline KOH, annealed samples show a marked increase in current density ($j_\text{geo}$ at 1.65 V vs. RHE):

• Acetonitrile-HT: $j_\text{geo} = 21.15\,\text{mA\,cm}^{-2}$
• Acetone-HT: $j_\text{geo} = 47.32\,\text{mA\,cm}^{-2}$
• Ethanol-HT: $j_\text{geo} = 70.83\,\text{mA\,cm}^{-2}$

Overpotential ($\eta$) at 10 mA cm$^{-2}$ decreased by 60–80 mV post-annealing (to 358–388 mV). ECSA (from double-layer capacitance) increased by factors of 1.5–2.3, yet ECSA-normalized activity confirms a genuine intrinsic enhancement.

Impedance spectroscopy reveals a lower charge transfer resistance ($R_{ct}$) for annealed NPs (reduction by 21–82%, solvent-dependent), and a single small semicircle characteristic of facile OER kinetics.

Long-term OER cycling (flow cell–ICP-MS dissolution) demonstrates periodic dissolution/regeneration of active sites, with STEM–EDS confirming the persistent core–shell heterostructure and carbon shell post-operation. Stable surface Fe–Co–Ni oxyhydroxide phases underpin the observed high durability.

5. Surface Engineering via Solvent Choice and Kinetic Control

Solvent selection orchestrates carbon supply rate, shell formation kinetics, and compositional outcomes (Nallathambi et al., 22 May 2025):

• Acetonitrile (highest carbon fraction, $X_C=1.00$) leads to pronounced Mn enrichment at the surface, rugged NP morphologies, and thick shells; amorphization is locked before metallic coalescence.
• Acetone offers intermediate shell thickness and Mn content; particle shapes are mixed.
• Ethanol provides thinnest shells, lowest Mn surface content, and partial crystallization.

Shell formation impedes further aggregation and metallic crystallization when rapid ($\tau_\text{shell} \ll \tau_\text{coal}$), yielding high local atomic disorder. Conversely, slower carbon shell kinetics allow partial coalescence and formation of crystalline inclusions. This solvent-driven regime establishes a practical method for fine-tuning electronic structure, surface atom distribution, and phase stability.

6. Mechanistic Insights: Carbon’s Multifunctional Role and Metastability Utilization

In situ-formed carbon shells serve several mechanistic functions:

• Provide electronic conductivity across the shell–core interface, lowering series resistance ($R_s$).
• Act as permeable barriers, regulating metal (notably Cr, Co) dissolution during catalysis.
• Template the nucleation of active oxyhydroxide clusters under OER conditions by adsorbing OH$^-$ species.

Exploration of metastability enables controlled phase separations via post-synthesis annealing. Cr-Fe and Ni-Co-Fe interfaces formed upon annealing generate high densities of crystallographically and compositionally distinct catalytic sites.

During OER operation, dissolution of subsurface Mn and Cr exposes stable Fe–Ni–Co oxyhydroxide regions, renewing catalytically active sites through periodic subsurface migration and dissolution/redeposition cycles.

7. Applications and Broader Implications

The combination of surfactant-free synthesis, compositionally fine-tuned carbon shells, and metastability-driven heterostructuring yields catalysts displaying 5–7-fold OER activity improvement compared to as-synthesized amorphous counterparts (Nallathambi et al., 12 Nov 2025). Thermal stability up to 350 °C and compositional flexibility render these materials suitable for use in high-temperature catalysis, battery electrode fabrication, and magnetic applications.

Cantor-based carbon-doped HEA NPs exemplify a versatile nanoalloy platform where phase, morphology, and surface functionality are controlled via kinetic parameters and solvent environment. A plausible implication is that similar strategies could be extended to other multi-elemental alloy systems, broadening materials exploration for energy conversion and storage.