Simple Fabrication and Performance of Polyaniline/CeO₂‑Co‑Decorated TiO₂ Nanotube Arrays for Efficient Photoelectrocatalytic Degradation of TBBPA

Abstract

Polyaniline (PANI) and cerium dioxide (CeO₂) were co‑decorated onto TiO₂ nanotube arrays by a facile electrochemical method. SEM, XRD, and EDS confirmed the uniform distribution of CeO₂ nanoparticles and the porous PANI coating. As a photoelectrode under simulated solar light, the composite achieved >96 % degradation of 10 mg L⁻¹ tetrabromobisphenol A (TBBPA) in 120 min. The high activity arises from band‑gap narrowing, enhanced visible‑light absorption, increased reactive oxygen species, and suppressed electron–hole recombination via the Ce³⁺/Ce⁴⁺ cycle and conductive PANI network. Degradation followed first‑order kinetics (k = 0.0283 min⁻¹) and maintained >92 % efficiency over ten consecutive runs, confirming excellent stability.

Introduction

The rapid expansion of industry has released a spectrum of toxic pollutants—organic and inorganic—that threaten ecosystems and human health. Efficient, clean degradation technologies are therefore critical. Photocatalysis, especially with heterogeneous catalysts like TiO₂, offers a cost‑effective, non‑toxic solution for removing organic contaminants from water and air.

TiO₂’s wide band‑gap (~3.20 eV) limits activation to UV light, hindering practical use. Numerous strategies—sensitization, metal/metal‑oxide doping, and semiconductor coupling—have been explored to extend TiO₂ activity into the visible range. Noble metals (Au, Ag, Pt, Pd) and metal oxides such as CeO₂ (band‑gap ≈ 2.92 eV) can trap photogenerated electrons, reduce recombination, and generate reactive oxygen species, thereby enhancing photocatalytic performance. CeO₂’s Ce³⁺/Ce⁴⁺ redox couple is particularly effective in electron transfer and radical generation.

Polyaniline (PANI) is a conductive polymer that absorbs visible light, facilitates electron transfer, and improves charge separation when combined with TiO₂. Although PANI/TiO₂ composites have shown promise, the simultaneous incorporation of CeO₂ and PANI onto TiO₂ nanotube arrays has not been thoroughly investigated for TBBPA removal.

Here we report a one‑step electrochemical fabrication of PANI/CeO₂/TiO₂ NTAs and evaluate their photoelectrocatalytic (PEC) efficiency against TBBPA, a prevalent brominated flame retardant that is an endocrine disruptor. We also dissect the role of each component, optimize synthesis parameters, and elucidate the degradation mechanism.

Materials and Methods

Materials

Analytical‑grade reagents were used directly. Titanium foils (99.6 % purity) were obtained from the Northwest Institute for Non‑ferrous Metal Research, China. Aniline, CeCl₃·7H₂O, and TBBPA were purchased from Aladdin Chemistry Co., Ltd. All solutions were prepared with deionized water.

Preparation of TiO₂ NTAs

Ti foils were polished, cleaned with acetone, isopropanol, and methanol, and anodized in 0.5 M H₃PO₄/0.14 M NaF at 20 V for 30 min. After rinsing, the films were calcined at 500 °C for 2 h to form anatase‑phase TiO₂ nanotube arrays (NTAs). The resulting NTAs (20 × 25 × 0.2 mm) were used directly.

Electrochemical Co‑Decoration with CeO₂ and PANI

CeO₂ deposition: a three‑electrode setup (working: TiO₂ NTA, counter: Pt foil, reference: SCE) with 0.025 M CeCl₃ solution was used. After 1 h pre‑soak, a galvanostatic current of 0.3 mA was applied for 15–45 min to control Ce loading. The films were rinsed and annealed at 200–500 °C to convert Ce⁺³ to CeO₂ and form anatase TiO₂.

PANI coating: the CeO₂/TiO₂ NTA was immersed in 0.5 M Na₂SO₄/0.2 M aniline under the same galvanostatic conditions (0.3 mA). Deposition time (10–30 min) dictated PANI loading. The composite was cleaned and dried.

Characterization

SEM (SU8000, 5 kV) revealed morphology; EDS quantified elemental composition; XRD (Bruker D8 Advance) identified crystal phases.

Photoelectrocatalytic Activity

PEC degradation of 10 mg L⁻¹ TBBPA was performed in a quartz cell under a 500‑W xenon lamp (120 mW cm⁻²) with 0.05 M Na₂SO₄ as electrolyte. Samples were collected every 15 min and analyzed by HPLC (Agilent SB‑C18, 85 % MeOH/15 % H₂O). A bias of 9 V was applied.

Results and Discussion

Morphology and Composition

SEM images (Fig. 1) show well‑ordered TiO₂ NTAs with 90–110 nm pores and 5 nm walls. CeO₂ nanoparticles (~10 nm) decorate the tube tops, while a porous, laminar PANI film (50–70 nm pores, 40 nm thickness) coats the composite. EDS confirms Ti, O, Ce, C, and N presence; N and Ce atomic percentages are 2.11 % and 1.01 % respectively. XRD patterns (Fig. 1e) confirm anatase TiO₂ and CeO₂ peaks; PANI is amorphous and undetectable by XRD.

PEC Degradation Performance

After 120 min irradiation, degradation efficiencies were: TiO₂ NTAs = 85.34 %, CeO₂/TiO₂ = 90.33 %, PANI/TiO₂ = 86.78 %, PANI/CeO₂/TiO₂ = 93.98 % (Fig. 2). The composite outperformed all controls, confirming the synergistic role of CeO₂ and PANI in enhancing PEC activity.

Optimization of Synthesis Parameters

CeO₂ loading (15–45 min deposition): 15 min gave the highest activity; excessive CeO₂ obstructs active sites and increases recombination (Fig. 3a). PANI loading: 15 min deposition maximized performance; longer times reduced light absorption (Fig. 3b). Annealing temperature: 200–500 °C gradually converted amorphous TiO₂ to anatase, improving activity; temperatures above 600 °C caused rutile formation and a slight drop (Fig. 3c).

Effect of Solution pH and Hole Scavengers

Optimal pH was 3; alkaline conditions suppressed •OH and HO₂• formation, reducing degradation (Fig. 4). Among hole scavengers, ethanol (10 mmol L⁻¹) yielded the highest efficiency (96.32 %) by quenching holes and preventing recombination (Fig. 5a). Higher ethanol concentrations decreased activity, indicating an optimal scavenger level (Fig. 5b).

Kinetics and Stability

First‑order kinetics (k ≈ 0.0283 min⁻¹) were observed with R² = 0.996–0.998 (Fig. 5). The composite retained >92 % efficiency over ten consecutive runs (Fig. 6), demonstrating robust stability.

Proposed Mechanism

Under simulated sunlight, TiO₂ and CeO₂ generate e⁻/h⁺ pairs. Electrons transfer to PANI and CeO₂, reducing Ce⁴⁺ to Ce³⁺ and producing superoxide (O₂⁻•). •O₂⁻ reacts with H⁺ to form HO₂• and with 4H⁺ to generate •OH. Holes oxidize H₂O/ OH⁻ to •OH, and all radicals (HO₂•, •OH) along with h⁺ attack TBBPA, leading to its mineralization (Fig. 7). This cooperative electron‑hole separation and radical generation underpins the superior PEC performance.

Conclusions

The PANI/CeO₂/TiO₂ NTAs were fabricated via a simple electrochemical route and exhibited exceptional PEC activity for TBBPA removal, achieving >92 % degradation in 120 min under simulated solar light and with ethanol as a hole scavenger. The synergy between CeO₂’s redox cycle and PANI’s conductive network enhances visible‑light absorption, radical generation, and suppresses recombination. First‑order kinetics and excellent reusability underscore the catalyst’s practicality for environmental remediation of brominated flame retardants and other organic pollutants.

Abbreviations

BFRs

Brominated flame retardants

CB

Conduction band

EDS

Energy‑dispersive X‑ray spectroscopy

HO₂•

Hydroperoxy radical

HPLC

High‑performance liquid chromatography

PANI/CeO₂/TiO₂ NTAs

Polyaniline and CeO₂ co‑decorated TiO₂ nanotube arrays

PEC

Photoelectrocatalytic

SEM

Scanning electron microscopy

TBBPA

Tetrabromobisphenol A