Elliptical Au–TiO₂ Nanopillars

• Elliptical Au–TiO₂ nanopillars are anisotropic plasmonic-semiconductor nanostructures engineered to boost visible light absorption and photocatalytic activity.
• They utilize controlled synthesis methods, including seed-mediated growth and template-assisted anodization, to fine-tune morphology and plasmonic resonance for applications like dye-sensitized solar cells.
• Their polarization-sensitive plasmonic responses and compositional gradients enable precise modulation of photocatalytic reactions and enhanced SERS performance.

Elliptical Au–TiO₂ nanopillars are anisotropic plasmonic-semiconductor nanostructures engineered to combine compositional and morphological control for enhanced visible light absorption, photocatalytic activity, and spectroscopic sensitivity. Their unique elliptical geometry, synergistic interface between gold (Au) and titanium dioxide (TiO₂), and tailored resonance properties enable precise optical and catalytic modulation, with applications spanning photocatalysis, dye-sensitized solar cells, and surface-enhanced Raman scattering (SERS).

1. Electronic and Optical Properties of Au-Doped TiO₂

Substitutional Au doping of anatase TiO₂ introduces localized midgap states primarily derived from Au 5d and O 2p orbitals (Guo et al., 2012). In a modeled 72-atom TiO₂ supercell (Ti₂₃AuO₄₈, ~4.17% doping), the band structure exhibits discrete energy levels $E_{Au}$ within the fundamental gap, such that $E_V < E_{Au} < E_C$ (valence band maximum $E_V$, conduction band minimum $E_C$). The calculated band gap is slightly narrowed and a new optical absorption feature emerges at 2.19 eV, corresponding to O 2p $\rightarrow$ Au 5d transitions. The imaginary part of the dielectric function, $\epsilon_2(\omega)$, confirms these transitions:

$$\epsilon_2(\omega) \propto \sum_{v,c} \int_{BZ} |\langle\psi_c(\mathbf{k})|p|\psi_v(\mathbf{k})\rangle|^2 \delta(E_c(\mathbf{k})-E_v(\mathbf{k})-\hbar\omega) d^3k$$

Visible light absorption is enhanced, with the absorption edge shifted from the UV into 400–1000 nm. While intensity is lower compared to Cu-doped TiO₂, the midgap states facilitate photocatalytic activity under visible irradiation, a prerequisite for solar-driven environmental applications.

2. Morphological Control, Plasmonic Enhancement, and Shell Effects

In core-shell Au@TiO₂ architectures, plasmonic enhancement depends critically on shell geometry and thickness (Liu et al., 2013). Localized surface plasmon resonance (LSPR) behavior is dictated by the aspect ratio and shell thickness, influencing both electromagnetic field distribution and effective refractive index ($n_{eff}$, ranging from 1.61 for 5 nm shells to ~1.69 for 37 nm). Near-field intensity decays exponentially with distance:

$E(d) \propto E_0 \exp(-d/\delta)$

Thin shells maximize dye excitation rates by maintaining minimal dye-metal separation. Conversely, thicker shells promote voltage gains through semiconducting behavior modification and Fermi level shifts. Power conversion efficiency (PCE) in dye-sensitized solar cells (DSSCs) is expressed as:

$$\mathrm{PCE} = \frac{J_{sc} \cdot V_{oc} \cdot FF}{I_s}$$

where $J_{sc}$ is short-circuit current, $V_{oc}$ open-circuit voltage, $FF$ fill factor, $I_s$ incident light power density. Morphological optimization involves balancing near-field excitonic enhancement with charge equilibrium effects. Synthesis strategies such as controlled solvent exchange and template-assisted growth allow precision in creating elliptical nanopillars with tailored aspect ratios and shell gradients, enabling resonance tuning and maximizing overlap with dye absorption bands.

3. Compositional Gradients and Site-Selective Decoration for Photocatalysis

Co-sputtering Ti and Au with engineered power profiles produces alloy layers with compositional gradients (Hejazi et al., 2020). Subsequent anodization in fluoride-containing electrolyte at 60 V yields vertically aligned TiO₂ nanotubes with expansion factors (~2.7) and site-selective Au nanoparticle decoration. For gradient layers (e.g., Gtopₓ), Au nanoparticles agglomerate at the top of the tubes, leaving lower portions pristine. Homogenously decorated tubes (Hₓ) manifest uniform nanoparticle placement. H₂ evolution rates scale with both Au content and placement; top-selective decoration maximizes rates ($r_{H_2} \approx 34\, \mu L\,h^{-1}\,cm^{-2}$ for H0.9, with lower loading achievable for gradient substrates). Spatial placement allows reduction in noble metal usage without sacrificing activity by optimizing electron transfer co-catalysis at illuminated regions. Compared to conventional decoration, gradient methods offer precise spatial control, tunable loading, and versatile integration in multicomponent architectures.

4. Photonic and Plasmonic Synergy in Light-Harvesting and SERS

Multi-leg TiO₂ nanotubes (MLNTs), with and without Au nanoparticle coatings, demonstrate enhanced SERS sensitivity (s et al., 2023). MLNT fabrication employs single-step electrochemical anodization, followed by Au sputtering and annealing (causing dewetting and nanoparticle formation). MLNTs exhibit branched, multi-leg morphology and refractive index gradients along the tube (e.g., $n_{eff}$ from 1.06 top to 2.49 bottom), supporting multiple internal reflections and photonic absorption edge near 535 nm, closely matching a 532 nm incident laser. Enhancement factor (E.F.) for SERS is calculated as:

$$\mathrm{E.F.} = \frac{I_{\text{SERS}}}{I_{\text{ore}}} \times \frac{N_{\text{ore}}}{N_{\text{SERS}}}$$

Au-coated MLNTs realize $E.F. \sim 10^5$ for nM Methylene Blue, compared to $E.F. \sim 10^4$ for bare MLNTs, due to synergistic plasmonic (LSPR-induced "hot spots") and photonic mode coupling. The principle of geometrically-enhanced light harvesting, as demonstrated in MLNTs, provides direct guidance for elliptical nanopillar design, where elliptical aspect ratio and refractive index gradients support multiple scattering and plasmon-photonic mode interactions, further boosting light-matter coupling in spectroscopic and photocatalytic systems.

5. Polarization-Sensitive Metasurfaces and Resonance-Driven Photocatalytic Modulation

Anisotropic metasurfaces composed of elliptical Au–TiO₂ nanopillars capped with gold nanodisks enable active control of resonance-driven photocatalytic reactions (Lyu et al., 26 Sep 2025). Each nanopillar (Rx ≈ 35 nm, Ry ≈ 60 nm) exhibits polarization-dependent plasmonic resonance: TM (short axis) yields strong resonance near 630–650 nm, TE (long axis) redshifts the peak (~820–850 nm). Measured absorption at 633 nm is 89% for TM versus 58% for TE, with >60% differential. Photocatalytic activity for MB N-demethylation correlates with polarization; Raman product yield—tracked by the 480 cm⁻¹ peak area—increases from 4.7 (TE) to 9.98 (TM) over 10 s, matching absorption increase. Continuous tuning via polarization state provides real-time modulation of reactivity, permitting selectivity in multi-pathway reactions.

Configuration	Absorption at 633 nm	Raman Yield (10 s)
TE	58%	4.7
TM	89%	9.98

This methodology offers direct selectivity for multibranch reactions, extending application to solar fuels and environmental remediation, contingent on further optimization of geometry and plasmon–molecule coupling.

6. Synthesis Methodologies and Structural Engineering

Deterministic synthesis approaches for elliptical Au–TiO₂ nanopillars include seed-mediated growth, template-assisted methods, and controlled solvent exchange processes (Liu et al., 2013). Surfactant-assisted anisotropic growth and hard template arrays can produce elliptical nanopillar arrays with uniform shell coverage and controlled aspect ratios. For site-selective decoration, compositional gradient sputtering and self-ordering anodization yield precise placement of Au nanoparticles (Hejazi et al., 2020). Nanopillar tapering and aspect ratio variation influence plasmonic coupling and polarization sensitivity (Lyu et al., 26 Sep 2025); improved lithography and template definition are future directions for enhancing spectral selectivity and reactivity.

7. Applications, Optimization Strategies, and Implications

Elliptical Au–TiO₂ nanopillars constitute a platform for resonance-tuned solar energy conversion, photocatalytic hydrogen evolution, SERS, and selective chemical catalysis. Optimizing visible absorption, maximizing overlap with molecular electronic transitions, modulating shell thickness or nanoparticle placement, and leveraging polarization-dependent LSPR are critical strategies for maximizing performance. Reduced noble metal loading and tailored site-selectivity offer substantial material cost and functional advantages (Hejazi et al., 2020). Advances in metasurface engineering enable real-time selectivity in complex reactions, with ongoing work targeting sharper resonances, multiresonant arrays, and integration into renewable energy devices (Lyu et al., 26 Sep 2025).

A plausible implication is that future elliptical Au–TiO₂ nanopillar architectures may incorporate multiple selective resonances and compositional gradients to achieve high-yield, pathway-selective catalysis, effective light harvesting, and sensitive spectroscopic interfaces for broad scientific and technological utility.