Jijie Zhang

Enhanced Surface Reaction Kinetics and Charge Separation of p–n Heterojunction Co3O4/BiVO4 Photoanodes

Surface reaction kinetics and bulk charge separation are both critical to the efficiency of solar water splitting. In addition to the well-documented surface catalytic effect, the promotion of bulk charge separation upon loading of cocatalysts has rarely been reported. This paper describes the synergetic enhancement of surface reaction kinetics and bulk charge separation by introducing discrete nanoisland p-type Co3O4 cocatalysts onto n-type BiVO4, forming a p-n Co3O4/BiVO4 heterojunction with an internal electric field to facilitate charge transport. Being highly dispersed on the surface of photoanodes, the nanoisland cocatalysts could suppress the formation of recombination centers at the photoanode/cocatalyst interface. This cocatalyst-loading method achieved a charge separation efficiency of up to 77% in the bulk and 47% on the surface of catalysts. An AM 1.5G photocurrent of 2.71 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode for water oxidation was obtained, which is the highest photocurrent yet reported for Co-catalyzed undoped BiVO4 photoanodes, with a photoconversion efficiency of 0.659%.

Dendritic Hematite Nanoarray Photoanode Modified with a Conformal Titanium Dioxide Interlayer for Effective Charge Collection

This paper describes the introduction of a thin titanium dioxide interlayer that serves as passivation layer and dopant source for hematite (α-Fe2 O3 ) nanoarray photoanodes. This interlayer is demonstrated to boost the photocurrent by suppressing the substrate/hematite interfacial charge recombination, and to increase the electrical conductivity by enabling Ti4+ incorporation. The dendritic nanostructure of this photoanode with an increased solid-liquid junction area further improves the surface charge collection efficiency, generating a photocurrent of about 2.5 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (vs. RHE) under air mass 1.5G illumination. A photocurrent of approximately 3.1 mA cm-2 at 1.23 V vs. RHE could be achieved by addition of an iron oxide hydroxide cocatalyst.

Surviving High-Temperature Calcination: ZrO2-Induced Hematite Nanotubes for Photoelectrochemical Water Oxidation

Nanotubular Fe2 O3 is a promising photoanode material, and producing morphologies that withstand high-temperature calcination (HTC) is urgently needed to enhance the photoelectrochemical (PEC) performance. This work describes the design and fabrication of Fe2 O3 nanotube arrays that survive HTC for the first time. By introducing a ZrO2 shell on hydrothermal FeOOH nanorods by atomic layer deposition, subsequent high-temperature solid-state reaction converts FeOOH-ZrO2 nanorods to ZrO2 -induced Fe2 O3 nanotubes (Zr-Fe2 O3 NTs). The structural evolution of the hematite nanotubes is systematically explored. As a result of the nanostructuring and shortened charge collection distance, the nanotube photoanode shows a greatly improved PEC water oxidation activity, exhibiting a photocurrent density of 1.5 mA cm-2 at 1.23 V (vs. reversible hydrogen electrode, RHE), which is the highest among hematite nanotube photoanodes without co-catalysts. Furthermore, a Co-Pi decorated Zr-Fe2 O3 NT photoanode reveals an enhanced onset potential of 0.65 V (vs. RHE) and a photocurrent of 1.87 mA cm-2 (at 1.23 V vs. RHE).

WO3 photoanodes with controllable bulk and surface oxygen vacancies for photoelectrochemical water oxidation

Introduction of oxygen vacancies for semiconductor photoanodes is an effective method to accelerate charge carrier transfer and improve the photoelectrochemical performance. However, excessive surface oxygen vacancies are always created in this process and the surface recombination will be aggravated. This paper describes an ozone treatment method that could effectively heal surface oxygen vacancies and suppress the recombination of photogenerated electron–hole pairs on the surface of two-dimensional WO3 nanoflakes. The ozone treatment method can oxidize the W5+ on the surface of photoanodes to decrease the surface oxygen vacancies, which results in a significant cathodic shift of the onset potential of ∼0.15 V in comparison with the pristine WO3 photoanode. The hydrogen and ozone-treated WO3 sample exhibits a photocurrent of 2.25 mA cm−2 at 1.23 V vs. reversible hydrogen electrode and excellent stability over 10 hours and its overall faradaic efficiency for the water splitting reaction is ∼90%.