TiO2 Nanowire Rotation in Electric Fields - Sharif University, 2018

Farain, K., Esfandiar, A., & Moshfegh, A. Z. (2018). Universal rotation of nanowires in static uniform electric fields in viscous dielectric liquids. *Applied Physics Letters*. https://doi.org/10.1063/1.5034406

Department of Physics, Sharif University of Technology 1 , P.O. Box 11155–9161, Tehran, Iran · Applied Physics Letters · 2018

Sharif University researchers used ACS Material TiO2 nanowires to show that non-metallic high-aspect-ratio nanowires rotate identically to metallic Ag nanowires.

About this research

Researchers at the Department of Physics, Sharif University of Technology used TiO2 nanowires obtained from ACS Material to demonstrate a counterintuitive universal rotation behavior of nanowires suspended in viscous dielectric liquids under static uniform electric fields. The headline finding is that semiconducting TiO2 nanowires rotate with essentially the same angular dynamics as metallic silver nanowires, despite their bulk conductivity being roughly eight orders of magnitude lower. Published in Applied Physics Letters in 2018, the study establishes that for high-aspect-ratio geometries the rotation depends almost exclusively on the dielectric constant and viscosity of the surrounding liquid, not on the nanowire material or precise dimensions. This opens a path to synchronous, controllable alignment of heterogeneous nanowire ensembles.

Controlling the orientation of one-dimensional nanostructures in liquid media is a long-standing requirement for assembling nanowire-based electronics, photonic devices, and biosensors. Most prior work relied on non-uniform or alternating electric fields, dielectrophoresis, or rotating fields to manipulate suspended wires. The more fundamental question of how a simple static, uniform electric field reorients metallic versus non-metallic nanowires in a viscous dielectric had remained unresolved. Resolving it matters for liquid-crystal-like field-induced optical anisotropy, for transferring wires from suspension to surfaces with defined alignment, and for any scheme that needs deterministic orientation across a mixed population of wires with different chemistries and aspect ratios.

The TiO2 nanowires purchased from ACS Material were used without modification or surface treatment, ensuring that the reported physics reflects the as-supplied material. Scanning electron microscopy of eight independent images gave a mean diameter of 72 ± 16 nm, closely matching the silver nanowire comparison sample (74 ± 14 nm). The TiO2 nanowires were first dispersed in isopropanol at 785 µg/mL, then diluted into Zeiss Immersol 518 F microscope immersion oil to a final concentration of 0.1 µg/mL. A droplet of this suspension was placed between two photolithographically patterned 100 nm gold electrodes separated by 3 mm on a glass slide, with a 1 mm PDMS confinement ring preventing evaporation-driven flow. Constant voltages from 1 to 150 V were applied while individual nanowires were tracked by inverted dark-field microscopy and angles were extracted with a custom MATLAB routine. XPS analysis was also performed on the TiO2 nanowires before and after field application to verify changes in titanium oxidation state and oxygen vacancy content.

In a representative experiment, a 33 µm TiO2 nanowire and a 28 µm Ag nanowire were rotated from perpendicular to parallel to the field under an identical 3.0 × 10^4 V/m bias. Fitting the angle-versus-time trajectories to the derived prolate-spheroid model yielded a dimensionless geometric constant α = 1.3 for the TiO2 wire and α = 1.2 for the Ag wire, in excellent agreement with the theoretical α_metal = 1.0 but three orders of magnitude larger than the dielectric prediction α_dielectric = 1.1 × 10^-3. The rotational speed at θ = π/4 scaled linearly with E^2 across the tested field range, confirming the analytical relation ω = ε1·α·E^2/(4η) using viscosity η = 0.435 Pa·s and ε1 = 2.30·ε0 for the oil. Translational dielectrophoretic drift was kept below 0.2 µm/s by tracking only wires in the central uniform-field region. Most strikingly, TiO2 nanowires with lengths spanning 4 to 14 µm rotated by the same 22° in 8 s under 6.3 × 10^4 V/m, confirming the length-insensitivity predicted for metallic-like rotation. The authors attribute the metal-like response to a few-orders-of-magnitude increase in free-carrier density driven by oxygen-vacancy formation under the applied bias, which screens the axial field component within a sub-micron Debye-Hückel length on the wire tips.

These findings enable simultaneous, deterministic alignment of mixed metallic and semiconducting nanowire populations using nothing more complex than a DC bias across two electrodes. Applications include nanowire-based transparent conductors, photonic and plasmonic device fabrication, hybrid sensor architectures that require both conductive and photoactive wires in registered orientations, and liquid-crystal-analog optical elements. The same equation also provides a route to measuring the ratio of permittivity to viscosity (ε/η) of dielectric liquids at the nanoscale by tracking suspended wires, which is valuable for microrheology and lubricant characterization. The authors point toward extension to slowly rotating fields for synchronous ensemble rotation of heterogeneous wire mixtures.

For researchers building nanowire assembly platforms, photocatalytic systems, or 1D semiconductor devices, the ACS Material TiO2 Nanowire used in this study is available through the Nanowire Series catalog. The work illustrates that high-aspect-ratio TiO2 nanowires from this supplier behave reproducibly under electric-field manipulation and exhibit the field-induced carrier modulation needed for metallic-like electromechanical response. This combination of geometric uniformity and dopant-vacancy responsiveness makes the product a practical building block for studies that require predictable orientation control in viscous dielectric media.

How ACS Material products were used

• TiO2 Nanowire (Nanowire Series)  — “Ag and TiO2 NWs were purchased from Sigma-Aldrich and ACS Material, respectively, and were used without any modification and treatment.”

Product Performance in this Study

The ACS Material TiO2 nanowires were the central non-metallic test specimen. With a mean diameter of 72 ± 16 nm, they unexpectedly exhibited the same electromechanical rotational torque as metallic Ag nanowires under identical static uniform electric fields, validating the high-aspect-ratio metallic-like rotation model.

Related product categories

• Nanowire Series

Frequently asked questions

Why do semiconducting TiO2 nanowires rotate like metallic nanowires in a static electric field?

Under an applied DC bias, oxygen vacancies are introduced into TiO2 nanowires, increasing free-carrier density by several orders of magnitude. According to Debye-Hückel theory, this raises the screening capability so that the axial electric-field component is screened within sub-micron length scales on the wire tips. Because the high aspect ratio makes the parallel field component dominant, screened semiconducting nanowires behave electromechanically the same as metallic ones.

How does nanowire length affect rotation speed in a uniform electric field?

For high-aspect-ratio metallic and metal-like nanowires, the rotation speed is essentially independent of length. The governing geometric factor [ln(2l/d) − 0.5] / [ln(2l/d) − 1] is extremely insensitive to aspect ratio; doubling l/d from 500 changes it by under one percent. Experimentally, 4 µm and 14 µm TiO2 nanowires rotated by the same 22° in 8 s under 6.3 × 10^4 V/m, confirming this prediction.

What is the rotational speed equation for nanowires in a viscous dielectric liquid?

The angular velocity follows dθ/dt = −(ε1·α·E^2)/(4η) · sin(2θ), where ε1 is the liquid permittivity, η its viscosity, E the field strength, and α a dimensionless geometric constant near unity for high-aspect-ratio metallic wires. At θ = π/4 the speed scales linearly with E^2, which means a single measurement of rotation can yield the ε/η ratio of the dielectric liquid at the nanoscale.