Overcoming Charge Collection Limitation at Solid/Liquid Interface by a Controllable Crystal Deficient Overlayer

Abstract

Bulk and surface charge recombination of photoelectrode are two key processes that significantly hinder solar‐to‐fuel conversion of photoelectrochemical cell (PEC). In this study, the function of a “crystal‐deficient” overlayer is unveiled, which outperforms a traditionally used amorphous or crystalline overlayer in PEC water splitting by exhibiting a high conductivity and large electron diffusion length to enable unlimited electron collection. The optimized ≈2.5 nm thickness of the “crystal‐deficient” shell results in a depletion layer with a width of 3 nm, which overcomes the flat band limitation of the photovoltage and increases the light absorptivity in the wavelength range from 300 to 420 nm. In addition, a 50‐fold increase in the conductivity yields a one‐order‐of‐magnitude increase in the diffusion length of an electron (Ln)(≈20 μm), allowing for unlimited electron collection in the 1.9 μm TiO2 nanowire array with the “crystal‐deficient” shell. The controllable “crystal‐deficient” overlayer in rutile TiO2 nanowires photoanode achieves a photocurrent density greater than 2.0 mA cm−2 at 1.23 V versus reversible hydrogen electrode (RHE), a 1.18% applied bias photon‐to‐current efficiency at 0.49 V versus RHE, a faradaic efficiency greater than 93.5% at 0.6 V versus Pt under air mass 1.5G simulated solar light illumination (100 mW cm−2).

Facile and Efficient Atomic Hydrogenation Enabled Black TiO2 with Enhanced Photo‐Electrochemical Activity via a Favorably Low‐Energy‐Barrier Pathway

“…The IPCE of H–TiO 2 for T wire = 1700 °C shows the best redshift of band edge (bandgap narrowing) and the best improvement of visible photoactivity. Recently, two effective and practical hydrogenation approaches using either lithium reduction or Pd‐catalyzed instant hydrogenation could achieve the maximum 2 mA cm −2 photocurrent density at 1.23 V versus RHE. In comparison to the photocurrent density of these superior hydrogenation methods, our approach achieved 2.5 mA cm −2 photocurrent density, 25% enhancement.…”

“…The Al‐reduced amorphous shell is proved to be responsible for the improved visible‐light absorption. Zhang et al revealed that the disordered shell trapped photogenerated holes and promoted the charge separation . Therefore, developing novel approaches to modify black TiO 2 with the disordered shell is critical to achieve enhanced PEC activity.…”

“…Additionally, the underlying mechanism of atomic hydrogenation is still unclear. A recent study also suggested that black TiO 2 with controllable crystal‐deficient disordered shell attributed to the enhanced PEC activity . Currently, in order to achieve high PEC activity, the most commonly adopted hydrogenation approaches rely on molecular hydrogen gas or hydrogen plasma to treat TiO 2 .…”

Black Titanium Dioxide Nanomaterials in Photocatalysis

“…The introduction of the TiO 2 -MoS 2 heterojunction and the Ti 3+ species narrowed the band gap of TiO 2 , leading to the excellent activity in photocatalytic degradation of methyl orange and water splitting for H 2 evolution under visible-light irradiation [85]. The H 2 production rates were 0, 0.13, 0.32, and 0.56 mmol·h [86,87]. Typically, 14 mg of metallic Li foils were dissolved in 20 mL of ethanediamine under dry conditions; then, 200 mg of TiO 2 nanocrystals (Degussa P25) was immersed into the ethanediamine solution for 6 h with continuous stirring; 1 M HCl was used to consume the excess Li or electrons when the reaction was complete; finally, the product was rinsed with deionized water several times and dried in vacuum oven at room temperature [86].…”

“…Furthermore, Zhang et al found that the "crystal-deficient" layers on the surface of the rutile TiO 2 nanowires increased the conductivity by 50 times, which increased the electron diffusion length to~20 μm and overcame the charge collection limitation at the solid/liquid interface for efficient conversion of solar energy to chemical energy [87]. These studies highlight the importance of controlling the surface localization of defects and the solid/liquid interface towards enhanced photoactivity over TiO 2 photocatalysts [86][87][88]. 9 International Journal of Photoenergy [30].…”