Facile Thermal Decomposition Loading of CeO₂ Nanoparticles onto Anodic TiO₂ Nanotube Arrays

Abstract

This study presents a straightforward, low‑cost method for decorating anodic TiO₂ nanotube (NT) arrays with cerium oxide (CeO₂) nanoparticles (NPs). By exploiting the TiO₂ NTs as nano‑containers, a dilute Ce(NO₃)₃ solution is introduced into the tubes, followed by a 450 °C anneal that thermally decomposes the precursor into crystalline cubic CeO₂ NPs. X‑ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and energy‑dispersive X‑ray spectroscopy (EDS) confirm the formation of a well‑ordered CeO₂/TiO₂ heterojunction. The resulting composite offers enhanced photocatalytic activity and holds promise for environmental remediation and energy applications.

Background

Titanium dioxide, especially its anatase polymorph, remains a benchmark photocatalyst due to its wide band gap (~3.2 eV), chemical stability, and non‑toxic nature. However, the rapid recombination of photogenerated electron–hole pairs limits its practical efficiency. Loading TiO₂ with a narrower‑band semiconductor such as cubic CeO₂ (band gap ~2.92 eV) can create a heterojunction that facilitates charge separation, boosts radical production, and improves photocatalytic degradation of organic pollutants and gas‑sensing performance.

Conventional synthesis routes (sol‑gel, hydrothermal) often produce CeO₂/TiO₂ powders with complex procedures. In contrast, our method directly deposits CeO₂ onto self‑organized TiO₂ NT arrays, simplifying fabrication while ensuring intimate contact between the two phases.

Experimental Section

Synthesis of TiO₂ Nanotube Arrays

Commercial Ti sheets (5 × 1.5 cm) were ultrasonically cleaned, then anodized in a glycol electrolyte (10 mL H₂O, 1.66 g NH₄F) at 60 V for 2 h. The resulting amorphous TiO₂ films were annealed at 450 °C for 3 h to yield dense anatase NTs with ~110 nm diameter.

Loading of Ce(NO₃)₃ and Formation of CeO₂/TiO₂ Heterojunctions

Each TiO₂ NT film was briefly immersed (3 s) in Ce(NO₃)₃ solutions of varying concentration (0.05–1 mol L⁻¹). Excess solution was removed by filtration and gentle tilting to promote uniform filling. The films were then dried at 70 °C for 1 h, followed by annealing at 450 °C for 2 h to decompose Ce(NO₃)₃ into CeO₂ NPs within the nanotube walls.

Characterization

• Crystalline phases: XRD (D/max 2400 X Series).
• Morphology: SEM (JSM‑7000F, JEOL).
• Elemental mapping: EDS.
• Vibrational analysis: Raman (inVia, Renishaw).

Results and Discussion

Crystallinity

XRD patterns reveal anatase TiO₂ peaks (e.g., 25.28°, 36.80°) alongside cubic CeO₂ reflections (111 at 28.55°, 200 at 33.08°). Increasing Ce(NO₃)₃ concentration enhances the intensity of CeO₂ peaks, confirming successful incorporation.

Surface Morphology

SEM images show densely packed, open‑mouth TiO₂ NTs (~110 nm diameter). After loading, CeO₂ NPs decorate tube mouths and interior walls, with particle density increasing with precursor concentration. The nanotube wall thickness also grows slightly due to NP deposition.

Elemental Composition

EDS spectra for pristine TiO₂ NTs display only Ti and O. After loading (0.1 mol L⁻¹ Ce(NO₃)₃), Ce is detectable (~12 at %), confirming successful NP deposition.

Raman Spectroscopy

Characteristic anatase peaks at ~400, 530, and 645 cm⁻¹ persist, while a new peak at ~460 cm⁻¹ emerges, attributable to cubic CeO₂. This corroborates the dual‑phase structure.

Mechanism

The thermal decomposition of Ce(NO₃)₃ within the TiO₂ NTs follows:

Ce(NO₃)₃·6H₂O → Ce(NO₃)₃ + 6H₂O
Ce(NO₃)₃ → CeO₂ + NO↑ + O₂↑

Thus, CeO₂ NPs form in situ, tightly anchored to the TiO₂ walls.

Potential Applications

By mitigating charge recombination and enhancing surface adsorption, the CeO₂/TiO₂ heterojunctions exhibit improved photocatalytic degradation of pollutants, potential for hydrogen evolution, and low‑temperature oxygen sensing.

Conclusions

We have demonstrated a simple, scalable route to produce CeO₂/TiO₂ heterojunction films by filling anodic TiO₂ NT arrays with Ce(NO₃)₃ and thermally decomposing the precursor. The resulting films combine the high surface area of TiO₂ NTs with the favorable electronic properties of CeO₂, offering a promising platform for energy and environmental technologies.

Abbreviations

• EDS – Energy‑dispersive spectrometry
• NT – Nanotube
• SEM – Scanning electron microscopy
• XRD – X‑ray diffraction