Two-Dimensional Amorphous TiO2 Nanosheets Enabling High-Efficiency Photoinduced Charge Transfer for Excellent SERS Activity.

Substrate-molecule vibronic coupling enhancement, especially the efficient photoinduced charge transfer (PICT), is pivotal to the performance of nonmetal surface-enhanced Raman scattering (SERS) technology. Here, through developing novel two-dimensional (2D) amorphous TiO2 nanosheets (a-TiO2 NSs), we successfully obtained an ultrahigh enhancement factor of 1.86 × 106. Utilizing the Kelvin probe force microscopy (KPFM) technology, we found that these 2D a-TiO2 NSs possessed more positive surface potential than their 2D crystalline counterpart (c-TiO2 NSs). First-principles density functional theory (DFT) was used to further reveal that the low coordination number of surface Ti atoms and the large amount of surface oxygen defects endowed the 2D a-TiO2 with high electrostatic potential, which allowed significant charge transfer from the adsorbed molecule to the 2D a-TiO2 and facilitated the formation of a stable surface charge-transfer (CT) complex. Significantly, comparing with the 2D c-TiO2, the smaller band gap and higher electronic density of states (DOS) of the 2D a-TiO2 effectively enhanced the vibronic coupling of resonances in the substrate-molecule system. The strong vibronic coupling within the CT complex obviously enhanced the PICT resonance and lead to the remarkable SERS activity of a-TiO2 NSs. To the best of our knowledge, this is the first report on the remarkable SERS activity of 2D amorphous semiconductor nanomaterials, which may bring the cutting edge of development of stable and highly sensitive nonmetal SERS technology.