Insights into Chemical Bonds for Eliminating the Depletion Region and Accelerating the Photo-Induced Charge Efficient Separation toward Ultrasensitive Photoelectrochemical Sensing

The empty-space-induced depletion region in photoelectrodes severely exacerbates the recombination of electron–hole pairs, thereby reducing the photoelectrochemical (PEC) analytical performance. Herein, the chemical bond that can suppress the potential barrier and overcome the high energy barrier of out-of-plane Ohmic or Schottky contact is introduced into the PEC sensor to eliminate the depletion region and dramatically promote the separation of electron–hole pairs. Specifically, three-dimensional (3D) hierarchically wheatear-like TiO2 (HWT) nanostructures featuring a large surface area to absorb incident light are crafted as the substrate. The facile carbonized strategy is further employed to engineer the Ti-C chemical bond, serving as the touchstone. The average PL lifetime of HWT-C (4.14 ns) is much shorter than that of the 3D HWT (8.57 ns) due to the promoting effect of the chemically bonded structure on carrier separation. Consequently, the 3D HWT-C covalent photoelectrode (600 μA/cm2) exhibits a 3.6-fold increase in photocurrent density compared with the 3D HWT (167 μA/cm2). Ultimately, the model analyte of the tumor marker is detected, and the linear range is 0.02 ng/mL–100 ng/mL with a detection limitation of 0.007 ng/mL. This work provides a basic understanding of chemical bonds in tuning charge separation and insights on strategies for designing high-performance PEC sensors.