Unveiling Temperature‑Dependent Electrical Transport in Individual NiCo₂O₄ Nanowires for Energy‑Storage Applications

Abstract

Electrical transport in single‑nanostructures dictates the performance of next‑generation nanodevices. NiCo₂O₄ nanowires, widely used as electrodes in electrocatalysis, supercapacitors, and lithium‑ion batteries, have yet to have their intrinsic conduction mechanisms fully clarified. Here we report the successful synthesis of individual NiCo₂O₄ nanowires via thermal conversion of CoNi‑hydroxide precursors, followed by a detailed investigation of their temperature‑dependent electrical transport. Current–voltage measurements reveal an ohmic regime at low electric fields (<1.0×10³ V cm⁻¹), Schottky‑type emission at intermediate fields (1.0–3.0×10³ V cm⁻¹), and Poole–Frenkel conduction at high fields (>3.0×10³ V cm⁻¹). Temperature‑dependent conductivity displays semiconducting behavior; below 100 K the data fit Mott’s variable‑range hopping (VRH) model, while above 100 K both VRH and nearest‑neighbor hopping (NNH) mechanisms contribute. These insights provide a fundamental framework for optimizing NiCo₂O₄‑based energy‑storage devices.

Introduction

High‑performance energy‑storage systems are pivotal for electric vehicles, grid‑scale storage, and micro‑electronics. Conventional carbon‑based electrodes in lithium batteries and supercapacitors suffer from limited first‑cycle efficiency, lack of discharge plateaus, and poor cycling stability. Transition‑metal oxides, especially mixed‑metal spinels such as NiCo₂O₄, offer high theoretical capacities, low cost, and superior conductivity compared to their monometallic counterparts. Nanostructuring these oxides—particularly into nanowires or nanorods—enhances active surface area, shortens ion‑transport pathways, and mitigates strain during cycling. Despite extensive performance reports, the intrinsic electrical transport of a single NiCo₂O₄ nanowire remains elusive, hindering rational design of high‑efficiency devices. Moreover, temperature profoundly influences ion diffusion and carrier dynamics in electrode materials, making temperature‑dependent studies essential.

Methods/Experimental

Synthesis of NiCo₂O₄ Nanowires

CoNi‑hydroxide precursors were prepared by dissolving 1.19 g CoCl₂·6H₂O, 0.595 g NiCl₂·6H₂O, 0.728 g hexadecyltrimethylammonium bromide, and 0.54 g urea in 50 mL DI water, stirring for 30 min. The solution was transferred to a Teflon‑lined autoclave containing a pre‑cleaned carbon cloth and heated at 100 °C for 12 h. Post‑hydrolysis, the precipitate was annealed in air at 300–380 °C for 3 h, yielding crystalline NiCo₂O₄ nanowires.

Fabrication of Individual Nanowire Devices

Individual nanowires were dispersed in ethanol, sonicated for 3 min, and deposited onto Si/SiO₂ substrates. A 250‑nm PMMA layer was spin‑coated and patterned by electron‑beam lithography to define Cr/Au contacts (5–10 nm Cr, 70 nm Au). Lift‑off in acetone left two electrode pads at the nanowire termini.

Characterization

SEM (Nova Nano SEM 450), TEM (JEM 2010), and AFM (Dimension Icon) provided morphological data. UV–Vis spectra were recorded with a PE Lambda 950 spectrophotometer. Electrical measurements were performed in a Lakeshore CCR‑VF cryostat with a Keithley 4200 semiconductor analyzer.

Results and Discussion

Nanowire Morphology and Band Structure

SEM images show smooth precursor nanowires (Fig. 1a,b). Post‑annealing at 300–380 °C produced nanowires with grain sizes increasing from ~20 nm to ~90 nm (Fig. 1c–f). TEM confirms a mesoporous, polycrystalline structure (Fig. 1g), with selected‑area diffraction indicating spinel NiCo₂O₄ (Fig. 1h). UV–Vis absorption of 300 °C‑annealed samples yields two band‑gap energies (1.1 eV and 2.3 eV) attributable to high‑spin Co²⁺/low‑spin Co³⁺ states (Fig. 2).

Electrical Transport Mechanisms

The device geometry (length ≈1.55 µm, diameter ≈188 nm) yields a conductivity of ~0.48 S cm⁻¹, comparable to polycrystalline NiCo₂O₄ and lower than single‑crystal nanoplates. I–V curves at room temperature (Fig. 4a) are linear below 0.15 V (ohmic behavior). Above this bias, the current rises exponentially, and analysis of ln J vs E¹ᐟ² (Fig. 4c) indicates Schottky emission between 1.0–3.0×10³ V cm⁻¹. At higher fields, ln(J/E) vs E¹ᐟ² (Fig. 4d) follows the Poole–Frenkel model, revealing a relative dielectric constant of ~55.

Temperature‑Dependent Conductivity

I–V characteristics measured from 10–300 K (Fig. 5a) show exponential increases in current and corresponding exponential decreases in resistance (Fig. 5b), confirming semiconducting behavior. Below 100 K, the conductivity follows Mott’s VRH relation: σ = 0.016 exp[−(1840/T)¹ᐟ⁴] (Fig. 5c). Above 100 K, the data fit a combined VRH/NNH model with an activation energy of 0.0235 eV (Eq. 7, Fig. 5d). The lower activation energy compared to bulk NiCo₂O₄ (0.03 eV) underscores the influence of nanostructuring and defect states.

Conclusions

We have demonstrated that individual NiCo₂O₄ nanowires exhibit a clear hierarchy of electrical conduction: ohmic at low fields, Schottky emission at intermediate fields, and Poole–Frenkel behavior at high fields. Temperature‑dependent studies reveal VRH domination below 100 K and a transition to combined VRH/NNH conduction above 100 K. These findings provide a mechanistic foundation for designing NiCo₂O₄‑based electrodes with optimized performance in batteries and supercapacitors.

Abbreviations

AFM:

Atomic force microscope

EBL:

Electron beam lithography

I‑V:

Current‑voltage

NNH:

Nearest neighbor hopping

P‑F:

Poole–Frenkel

PMMA:

Polymethylmethacrylate

SEM:

Scanning electron microscope

TEM:

Transmission electron microscope

VRH:

Variable range hopping