Enhanced piezoelectricity and spectral absorption in Nd-doped bismuth titanate hierarchical microspheres for efficient piezo-photocatalytic H2 production and pollutant degradation

Abstract

Hydrothermally synthesized Nd-doped bismuth titanate (Bi_3.15Nd_0.85Ti₃O₁₂, BIT-Nd) hierarchical microspheres with assembled nanosheets were developed for water splitting and wastewater purification utilizing the piezo-phototronic effect for the first time. Undoped Bi₄Ti₃O₁₂ (BIT) microspheres were simultaneously studied for comparison to reveal the role of Nd doping. Under the combined light and ultrasound excitation, the degradation efficiency of RhB (C₀ = 10 mg L⁻¹) solution reached 97.5% in 8 minutes in the presence of the BIT-Nd microspheres, leading to an ultrahigh first-order rate constant k of 0.407 min⁻¹, which was 4.9 times higher than that of the BIT microspheres. More impressively, superior piezo-photocatalytic activity in water splitting compared to that of other reported piezo-photocatalysts was achieved in the BIT-Nd microspheres, which showed a H₂ evolution rate of 1330.3 μmol g⁻¹ h⁻¹ without any cocatalyst or scavenger. To understand the origins of the excellent piezo-photocatalytic performance, microstructure analysis and piezoelectric and photoelectric characterization, as well as density functional theory calculations were carried out. After Nd doping, the atomic framework of the Ti–O octahedron underwent an apparent distortion, resulting in a stronger ferroelectric polarization, and thus a larger piezoelectric potential output under pressure. Moreover, the thinner nanosheets in the BIT-Nd microspheres were more susceptible to deformation under the ultrasonic effect, which was also beneficial to generate a higher piezoelectric field that inhibited the charge carrier recombination. Additionally, the presence of Nd 4f orbitals in BIT-Nd effectively reduced the band gap and promoted the spectral absorption and production of photogenerated carriers. These merits enabled the BIT-Nd microspheres to exhibit strong piezo-photocatalytic activity.