Photoelectrochemical Electrochemical Impedance Spectroscopy Research Articles

Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO2 Photoelectrodes.

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO2 as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO2 surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO2. One of the complexities that have recently been discovered is the size-dependent hole-transfer mechanism. In this work, we reveal another anomalous behavior in the hole-transfer process that the hole scavenging ability of the electrolyte also plays a role in determining the hole-transfer mechanism in the NC-TiO2 system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na2SO3), the hole transfer in Au18-TiO2 occurs directly through the highest occupied molecular orbital (HOMO) of Au18 NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au18-TiO2 does not occur through the HOMO and shifts to surface state-assisted hole transfer. Due to surface state charging, this surface state-assisted hole-transfer mechanism results in delayed photocurrent response in Au18-TiO2. Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed.

A Ni2P nanocrystal cocatalyst enhanced TiO2 photoanode towards highly efficient photoelectrochemical water splitting

Developing low-cost oxygen evolution reaction (OER) cocatalysts with high conductivity and excellent catalytic activity is highly desirable for mitigating interfacial charge recombination and accelerating the OER reaction kinetics of the semiconductor photoanode for photoelectrochemical (PEC) water splitting. Herein, a hybrid photoanode integrating a high-quality light absorber, a bilayer TiO2 nanorod-inverse opal (NR-IO) semiconductor and an efficient OER cocatalyst consisting of colloidal Ni2P nanocrystals is constructed via a spin-coating process. The resulting TiO2/Ni2P NR-IO photoanode yields an impressive photocurrent density of 1.93 mA cm−2 at 1.23 V vs. the reversible hydrogen electrode (RHE) under simulated solar illumination (AM 1.5G, 100 mW cm−2), corresponding to a more than 2-fold increase compared to the pristine TiO2 NR-IO. The photoelectrochemical electrochemical impedance spectroscopy (PEIS) and photoluminescence (PL) results reveal that the Ni2P cocatalyst not only accelerates the interfacial charge transfer from TiO2 NR-IO to Ni2P but also efficiently transmits the holes to participate in aqueous OER. Moreover, the Ni2P cocatalyst effectively prevents the substantial dark current observed with fully covered transition-metal-phosphide shell modification. This work is expected to spur more insights into integrating semiconductors with proper OER cocatalysts for highly efficient and stable solar water spitting.