Au@TiO₂ Yolk–Shell Nanostructures: Tailored Synthesis and Their Superior Visible‑Light Photocatalytic Degradation and SERS Detection of Methylene Blue

Abstract

We present a facile, scalable route to Au@TiO₂ yolk–shell nanostructures that combines ion sputtering of gold onto carbon nanocoils (CNCs) with atomic‑layer deposition (ALD) of TiO₂, followed by calcination. By tuning the sputtering time, the size and loading of Au nanoparticles are precisely controlled, yielding yolk–shell composites that absorb visible light (LSPR peaks 550–590 nm) and act as efficient photocatalysts for methylene blue (MB) degradation and as high‑performance SERS substrates. Compared with pure TiO₂ nanotubes, Au@TiO₂ shows markedly enhanced photocatalytic activity and excellent recyclability, while the Au-30 s sample delivers the best SERS sensitivity, detecting MB down to 10⁻⁶ M.

Background

Heterogeneous metal/semiconductor composites are central to solar‑energy conversion, biomedicine, SERS, LEDs, and environmental remediation. In particular, Au/TiO₂ hybrids combine TiO₂’s robust photocatalytic properties with Au’s strong localized surface plasmon resonance (LSPR) in the visible region, enabling wide‑bandgap light harvesting and electron‑hole pair separation. However, conventional Au/TiO₂ suffers from Au sintering and corrosion under reaction conditions, limiting practical deployment. Core–shell and yolk–shell architectures, which confine Au within a TiO₂ shell, have emerged as a promising solution to enhance stability while preserving plasmonic benefits. Yet, scalable synthesis of well‑defined Au@TiO₂ yolk–shell structures remains challenging.

In this work we overcome these limitations by integrating ion sputtering and ALD, enabling precise control over Au loading and TiO₂ shell thickness, and producing helical, yolk–shell composites that exhibit superior photocatalytic and SERS performance.

Experimental

Synthesis of Au@TiO₂

Carbon nanocoils (CNCs) were grown by CVD from acetylene on copper catalysts, yielding helically wound fibers (~80 nm diameter). CNCs were dispersed in ethanol, sonicated, and drop‑cast onto glass slides. Au layers were deposited by ion sputtering (10 mA) for 30–120 s (CNCs@Au‑x). The sputtering time dictates Au particle size (≈4.5–20.5 nm). Samples were then dispersed in ethanol, spread onto quartz wafers, and coated with TiO₂ via ALD (TTIP/H₂O) at 145 °C for 200 cycles (~8 nm thick). Finally, the composite was calcined at 450 °C for 2 h in air, removing the carbon core and yielding Au@TiO₂ yolk–shell structures.

Characterization

• XRD (Bruker D8) for phase analysis.
• SEM (Hitachi S‑4800) and TEM (JEOL JEM‑2100) for morphology.
• HRTEM, SAED for lattice structure.
• XPS (PHI5000) for chemical states.
• UV–Vis diffuse reflectance for optical absorption.
• Raman (Renishaw Invia) for SERS.

Photocatalytic Activity

Photodegradation of 0.01 mg mL⁻¹ MB was monitored under 420–780 nm visible light (300 W Xe lamp, ~154 mW cm⁻²). A 2 mg catalyst was dispersed in 20 mL MB solution, stirred in the dark for 30 min, then irradiated for 90 min with periodic sampling.

Results and Discussion

Morphology & Phase

Sequential TEM images confirm the evolution from CNC → Au‑coated CNC → TiO₂‑coated Au‑CNC → Au@TiO₂ yolk–shell. Au nanoparticles increase from 4.5 nm (30 s) to 20.5 nm (120 s). TiO₂ shell remains ~8 nm for sputtering times ≤80 s; for 120 s it thins to ~5 nm due to Au agglomeration.

XRD shows anatase TiO₂ (JCPDS 21‑1272) and FCC Au (JCPDS 01‑1174). A weak γ‑Ti₃O₅ peak at 35.5° indicates slight oxygen vacancies induced during calcination.

Optical Properties

UV–Vis spectra reveal a broad LSPR band centered at ~580 nm, shifting slightly with Au size. All Au@TiO₂ samples exhibit enhanced visible‑light absorption (400–800 nm) compared with pure TiO₂.

Photocatalytic Degradation

TiO₂ alone degrades ~60 % MB in 90 min under visible light. Au‑x@TiO₂ composites outperform TiO₂, with Au‑80@TiO₂ achieving ~90 % degradation. The optimum arises from a balanced Au size (10.5 nm) and TiO₂ shell thickness (8 nm), maximizing LSPR‑induced hot‑electron transfer while preserving active surface area.

Recycling tests over three cycles show negligible activity loss for Au‑80@TiO₂, confirming structural stability due to the yolk–shell confinement.

SERS Performance

Au‑30@TiO₂ delivers the strongest SERS signal for 10⁻⁵ M MB, attributed to dense Au–TiO₂ hot‑spot formation. The detection limit reaches 10⁻⁶ M, demonstrating its suitability for trace pollutant monitoring.

Conclusions

We have developed a robust, scalable synthesis of Au@TiO₂ yolk–shell nanostructures with helical morphology. By tuning Au loading, the composites achieve superior visible‑light photocatalysis (up to 90 % MB degradation) and sensitive SERS detection (down to 10⁻⁶ M). The yolk–shell architecture provides excellent stability, enabling practical applications in water purification, solar‑driven catalysis, and analytical sensing. This strategy can be extended to other metal/semiconductor systems (e.g., Ag@TiO₂, Au@ZnO) for diverse energy and environmental technologies.