Increased O 2p State Density Enabling Significant Photoinduced Charge Transfer for Surface-Enhanced Raman Scattering of Amorphous Zn(OH)₂

Enriching the electronic density of states (DOS) of semiconductors is the key to promoting charge transfer (CT) and achieving a large surface-enhanced Raman scattering (SERS) enhancement. Metal hydroxide semiconductors are anticipated to exhibit DOS that are higher than those of metal oxide because of their abundant O atoms; however, their SERS activity has not been verified. Here, combining density functional theory and experiments, we report a SERS sensitivity of amorphous Zn(OH)₂ [a-Zn(OH)₂] that is much higher than that of amorphous ZnO (a-ZnO), ascribed to the abundant O atoms and hence enriched O 2p state density near the Fermi level in a-Zn(OH)₂, which gives rise to higher CT probabilities. Moreover, we find a-Zn(OH)₂ exhibits significant advantages in energy-level matching over a-ZnO for efficient photoinduced CT via strong vibronic coupling, ascribed to the upshifted valence band maximum and the narrower band gap of a-Zn(OH)₂. Via the synthesis of a-Zn(OH)₂ nanocages, an ultrahigh enhancement factor of 1.29 × 10⁶ is obtained in semiconductor-based SERS.