a X-ray diffraction pattern of the BTO nanoparticles. The inset shows the close-view around (002)C diffraction peaks, revealing the BTO nanoparticles have a tetragonal structure. b Scanning electron microscope image of BTO nanoparticles, and the inset shows the particle size distribution of BTO nanoparticles. c Transmission electron microscope, d high-resolution transmission electron microscope images and e selected area electron diffraction patterns of the BTO nanoparticles. The out-of-plane piezoresponse force microscope images of BTO nanoparticles for f amplitude, g phase and h piezoelectric hysteresis loop at “off” state. Scale bar: b is 200 nm, c is 100 nm, d is 2 nm, e is 5 1/nm, f, g is 200 nm.