Chengcheng Li

Thin Heterojunctions and Spatially Separated Cocatalysts To Simultaneously Reduce Bulk and Surface Recombination in Photocatalysts

Efficient charge separation and light absorption are crucial for solar energy conversion over solid photocatalysts. This paper describes the construction of Pt@TiO2 @In2 O3 @MnOx mesoporous hollow spheres (PTIM-MSs) for highly efficient photocatalytic oxidation. TiO2 -In2 O3 double-layered shells were selectively decorated with Pt nanoparticles and MnOx on the inner and outer surfaces, respectively. The spatially separated cocatalysts drive electrons and holes near the surface to flow in opposite directions, while the thin heterogeneous shell separates the charges generated in the bulk phase. The synergy between the thin heterojunctions and the spatially separated cocatalysts can simultaneously reduce bulk and surface/subsurface recombination. In2 O3 also serves as a sensitizer to enhance light absorption. The PTIM-MSs exhibit high photocatalytic activity for both water and alcohol oxidation.

Enhanced Charge Separation through ALD‐Modified Fe2O3/Fe2TiO5 Nanorod Heterojunction for Photoelectrochemical Water Oxidation

Hematite suffers from poor charge transport and separation properties for solar water splitting. This paper describes the design and fabrication of a 3D Fe2 O3 /Fe2 TiO5 heterojunction photoanode with improved charge separation, via a facile hydrothermal method followed by atomic layer deposition and air annealing. A highly crystallized Fe2 TiO5 phase forms with a distinct interface with the underlying Fe2 O3 core, where a 4 nm Fe2 TiO5 overlayer leads to the best photoelectrochemical performance. The favorable band offset between Fe2 O3 and Fe2 TiO5 establishes a type-II heterojunction at the Fe2 O3 /Fe2 TiO5 interface, which drives electron-hole separation effectively. The Fe2 O3 /Fe2 TiO5 composite electrode exhibits a dramatically improved photocurrent of 1.63 mA cm(-2) at 1.23 V versus reversible hydrogen electrode (RHE) under simulated 1 sun illumination (100 mW cm(-2) ), which is 3.5 times that of the bare Fe2 O3 electrode. Decorating the Fe2 O3 /Fe2 TiO5 heterojunction photoanode with earth-abundant FeNiOx cocatalyst further expedites surface reaction kinetics, leading to an onset potential of 0.8 V versus RHE with a photocurrent of 2.7 mA cm(-2) at 1.23 V and 4.6 mA cm(-2) at 1.6 V versus RHE. This sandwich photoanode shows an excellent stability for 5 h and achieves an overall Faradaic efficiency of 95% for O2 generation. This is the best performance ever reported for Fe2 O3 /Fe2 TiO5 photoanodes.

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).

Spatial control of cocatalysts and elimination of interfacial defects towards efficient and robust CIGS photocathodes for solar water splitting

Chalcopyrite thin film absorbers such as Cu(In,Ga)Se2 (CIGS) exhibit excellent solar energy conversion efficiency, particularly when coupled with CdS to form an excellent p–n junction. To advance CIGS towards an efficient photoelectrochemical (PEC) hydrogen evolution reaction (HER), a protective overlayer (typically TiO2) is needed to prevent the corrosion of CIGS and CdS in the electrolyte, and a HER catalyst (typically Pt) is required to promote the surface reaction. However, it is a great challenge to realize delicate spatial control of the HER catalyst on the surface of the protective overlayer using the traditional deposition method. The charge transport through the CdS/overlayer interface is also of vital importance but is rarely considered. Herein, through a new two-step platinization strategy, the dispersion and particle size of Pt nanoparticles are independently controlled to realize high HER catalytic activity. Moreover, defects at the CdS/TiO2 interface are passivated via an ultrathin Al2O3 insertion layer. Consequently, we have obtained a robust CIGS/CdS/Al2O3/TiO2/Pt photocathode for PEC hydrogen evolution, which yields an applied bias photon-to-current efficiency (ABPE) of 6.6% in neutral electrolyte with a long-term stability up to 8 h (4.5% drop), and an unprecedented ABPE of 9.3% in acidic electrolyte that is the highest among chalcopyrite-based photocathodes. When paired with a BiVO4 photoanode to form a PEC tandem cell, an unbiased solar-to-hydrogen conversion efficiency of 1.01% is achieved.

A Low-Cost NiO Hole Transfer Layer for Ohmic Back Contact to Cu2O for Photoelectrochemical Water Splitting

Cuprous oxide (Cu2 O) photocathode is reported as a promising candidate for photoelectrochemical water splitting. The p-type Cu2 O usually forms a Schottky junction with the conductive substrate due to its large work function, which blocks the collection of photogenerated holes. NiO is considered as one of the most promising hole transfer layers (HTL) for its high hole mobility, good stability, and easy processability to form a film by spin coating. The utilization of NiO HTL to form an Ohmic back contact to Cu2 O is described, thus achieving a positive onset potential of 0.61 V versus reversible hydrogen electrode and a twofold increase of solar conversion efficiency.

Surface, Bulk, and Interface: Rational Design of Hematite Architecture toward Efficient Photo‐Electrochemical Water Splitting

Collecting and storing solar energy to hydrogen fuel through a photo-electrochemical (PEC) cell provides a clean and renewable pathway for future energy demands. Having earth-abundance, low biotoxicity, robustness, and an ideal n-type band position, hematite (α-Fe2 O3 ), the most common natural form of iron oxide, has occupied the research hotspot for decades. Here, a close look into recent progress of hematite photoanodes for PEC water splitting is provided. Effective approaches are introduced, such as cocatalysts loading and surface passivation layer deposition, to improve the hematite surface reaction in thermodynamics and kinetics. Second, typical methods for enhancing light absorption and accelerating charge transport in hematite bulk are reviewed, concentrating upon doping and nanostructuring. Third, the back contact between hematite and substrate, which affects interface states and electron transfer, is deliberated. In addition, perspectives on the key challenges and future prospects for the development of hematite photoelectrodes for PEC water splitting are given.

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%.

Nanostructured NiFe (oxy)hydroxide with easily oxidized Ni towards efficient oxygen evolution reactions

Electrochemical water splitting into hydrogen and oxygen is one of the most promising strategies for the utilization and storage of solar energy. However, the sluggish kinetics of the oxygen evolution reaction (OER) hinder its larger-scale application. NiFe-based catalysts are some of the most efficient oxygen evolution catalysts (OECs). High-valence Ni is considered as an active site or conductive framework in NiFe-based OECs. However, the oxidation of Ni is usually hindered in the presence of Fe. This paper describes an effective strategy to produce NiFe (oxy)hydroxide with easily oxidized Ni and a hierarchical nanosheet structure as the most efficient OEC. High valence Ni may provide frameworks with good conductivity and facilitate OERs in Fe active sites. In addition, the hierarchical structure can increase the electrochemical surface area, resulting in more active sites for the OER. The as-prepared Fe–O–Ni(OH)2 supported on nickel foam needs overpotentials of 185, 220 and 261 mV to drive current densities of 10, 100 and 500 mA cm−2, respectively. The catalyst also shows excellent stability at the current densities of 100 and 500 mA cm−2 for 50 hours.