Photocatalysis is an effective technology to solve environmental pollution. Ferroelectric ceramics, as a new type of catalyst, have been received widespread attention mainly attributed to the ability to reduce the recombination rate of photo^- induced carriers and to improve catalytic performance by building a built-in electric field. In order to avoid the decrease of remanent polarization in ceramic bulks during the traditional poling technology and simplify the poling process, corona poling technology has been used for the poling of powders. Several researches indicated that corona poling could significantly improve the catalytic performance, while the relationship among the particle size, phase structure, and piezo-photocatalytic performance was still not so clear. Therefore, BaTiO₃ (BT) nanoparticles with different particle sizes were prepared via sol-gel method and then polarized these powders for 1 h under a voltage of 6.56 kV. The phase structures and the lattice distortion before and after poling were carefully explored. In addition, the effect of corona poling on the piezo-photocatalytic performance of BT powders was investigated, Rhodamine B (RhB) was utilized as the target pollutant.

Barium acetate (C₄H₆O₄Ba) and tetrabutyl titanate (C₁₆H₃₆O₄Ti) were used as raw materials, both were dissolved in acetic acid and ethanol, respectively. Subsequently, these two solutions were mixed and then stirred for 3 h, after that, the solution was left to stand for 16 h, and then dried in an oven at 70 ℃ for 24 h. The mixtures were calcined at 800 ℃ for 2, 5 h and 12 h, respectively, in order to obtain BT powders with different particle sizes (the corresponding BT powders were respectively referred to as 2, 5 h and 12 h).

A SmartLab type X-ray diffractometer (XRD) was used to analyze the crystal structure of the obtained powders. The obtained powders were measured using an Invia Reflex model laser micro Raman spectrometer (Raman). In addition, the Helios G4 UC type scanning electron microscopy (SEM) was used to observe the surface morphology of the obtained powders. A Mapada UV-6300 UV-Vis spectrophotometer (UV-Vis) was used to characterize the transmittance of the obtained powders, thereby evaluating the degradation rate of powders. A home-made corona poling set-up at 6.56 kV for 1 h was used to polarize the BT powders. The unpoled/poled BT powders were mixed in RhB and stirred for 30 min to complete the dark adsorption. A Xe lamp (300 W) was used to simulate the sunlight irradiation, and an ultrasonic machine (30 kHz, 100 W) was used to assist in completing the piezo-photocatalytic test.

Particle sizes of the 2, 5 h and 12 h BT powders were 55, 110 nm, and 130 nm, respectively. XRD patterns showed that the cubic and tetragonal phases coexisted for all kinds of samples, and Raman spectroscopy confirmed the presence of tetragonal phases. The c/a values of tetragonal phase (c_T/a_T) in these BT powders, evaluated by Rietveld refinement analysis, were 1.007 85 (2 h-unpoled), 1.008 00 (5 h-unpoled), and 1.009 10 (12 h-unpoled), respectively, these were closely related to the particle size. The larger size corresponded to the higher c_T/a_T, illustrating that the ratio of the tetragonal phase would be increased in 12 h BT nanoparticles. After corona poling process, the lattice distortion further strengthened and the c_T/a_T values were 1.008 89 (2 h-poled), 1.008 44 (5 h-poled), and 1.009 32 (12 h-poled), respectively. The difference of c_T/a_T between unpoled and poled BT powders (Δ(c_T/a_T)) was significantly decreased with the increase of particle size, demonstrating the higher poling effectiveness in nanoparticles with smaller size.

Moreover, effect of poling on the piezo-photocatalytic performance of samples was evaluated via observing the degradation of RhB. It was found that the piezocatalysis existed a "bottleneck period", being related to the pressure pulse generated by cavitation bubbles and the free charges generated by particles through surface friction. The reaction rate constants k for unpoled BT powders were 0.003 28 min^-1 (2 h-unpoled), 0.004 39 min^-1 (5 h-unpoled), and 0.003 82 min^-1 (12 h-unpoled), respectively. After poling, the k values were 0.016 20 min^-1 (2 h-poled), 0.009 69 min^-1 (5 h-poled), and 0.006 20 min^-1 (12 h-poled), 4.94, 2.21 and 1.62 times higher than before poling, respectively, mainly attributed to the enhancement of the built-in electric field caused by corona poling, which could increase the driving force for the separation of photo-induced carriers and prolong the lifetime of carriers. Among them, the 2 h-poled powder exhibited the highest rate constant and a degradation efficiency of 96.81%, as that particle size of 2 h-poled samples was smaller than that of 5 h powders and 12 h powders, resulted in a high degree of lattice distortion and enhanced the tetragonality. Moreover, the small particle size of samples was considered to be favorable for the sufficient contact with RhB. Therefore, highest reaction rate constant and degradation efficiency were achieved for the 2 h-poled sample.

BT powders with different particle sizes were prepared and poled by corona poling technology. Effect of the poling process on the degree of lattice distortion, phase structure, and piezo-photocatalytic performance for degrading RhB of BT powders was systematically investigated. Results indicated that the corona poling could deepen the crystal lattice distortion and increase the tetragonality of BT nanoparticles. In addition, poling process could enhance the built-in electric field, reduce the recombination rate of photo-induced carriers, and significantly improve catalytic performance. The rate constants k of the poled powders were 4.94 times (2 h-poled), 2.21 times (5 h-poled), and 1.62 times (12 h-poled) higher than those of unpoled BT powders. Among all samples without and with poling process, the 2 h-poled BT powder achieved the highest degradation efficiency of 96.81% within 180 min, indicated the optimal piezo-photocatalytic performance. This work could provide a simple and rational method to design ferroelectric materials for achieving the high catalytic performance.