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

Abstract

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 TiO 2 nanosheets (a-TiO 2 NSs), we successfully obtained an ultrahigh enhancement factor of 1.86 × 10 6 . Utilizing the Kelvin probe force microscopy (KPFM) technology, we found that these 2D a-TiO 2 NSs possessed more positive surface potential than their 2D crystalline counterpart (c-TiO 2 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-TiO 2 with high electrostatic potential, which allowed significant charge transfer from the adsorbed molecule to the 2D a-TiO 2 and facilitated the formation of a stable surface charge-transfer (CT) complex. Significantly, comparing with the 2D c-TiO 2 , the smaller band gap and higher electronic density of states (DOS) of the 2D a-TiO 2 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-TiO 2 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.

Irreversible accumulated SERS behavior of the molecule-linked silver and silver-doped titanium dioxide hybrid system

“…And the lower levels are denoted as surface-state level E ss1 and E ss2 , respectively. According to the literatures 54,55 , the conduction band (CB) and valence band (VB) levels of TiO 2 are −3.63 and −6.83 eV, and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of 4MBA are −6.43 and −2.32 eV, respectively. As illustrated in Fig.…”

High SERS Sensitivity Enabled by Synergistically Enhanced Photoinduced Charge Transfer in Amorphous Nonstoichiometric Semiconducting Films

“…While the photoinduced charge transfer resonance (PICT) between substrates and molecules is regarded as another active Raman‐enhancing process, which significantly amplifies the scattering cross‐sections of the molecules, thereby leading to a stronger Raman enhancement . This PICT‐involved Raman enhancement endows abundant possibilities to adjust the inherent features of bandgap, stoichiometry, crystal structure, n‐/p‐doping, and stable exciton productions in semiconductors for better SERS performance . Motivated by the SERS promotion in hybrid plasmonic–photonic structures, we speculate that the inherent features of semiconductors can be well manipulated to synergistically enhance the PICT resonance processes between the substrates and the analytes.…”

SERS Activity of Semiconductors: Crystalline and Amorphous Nanomaterials

“…Typically, the contribution of PICT transitions to the molecular polarizability tensor strongly depend on the efficiency of the vibronic coupling. When the energy levels of the semiconductor substrate and molecules match well with each other, PICT transitions from |I⟩ to |C⟩ via transition dipole moment μ IC , or from |V⟩ to |K⟩ via transition dipole moment μ VK could “borrow” intensity either from intramolecular transitions ( μ IK ) or exciton transitions ( μ VC ) due to the vibronic coupling of various resonances in the surface complex through the Herzberg–Teller coupling term ( h VI or h CK ), as shown in Figure …”

“…The enriched defect states effectively promote charge‐transfer pathways as well as vibrational couplings, leading to the strong SERS effect. These achievements paved the way for us to reveal the SERS effect of amorphous semiconductors …”

“…Very recently, we proposed that making 2D TiO 2 nanosheets amorphous can narrow the band gap and significantly change the electronic DOS . By developing novel amorphous 2D TiO 2 nanosheets (a‐TiO 2 NSs), we successfully obtained an ultrahigh EF of 1.86×10 6 .…”