3D Branched Ca-Fe(2) O(3) /Fe(2) O(3) Decorated with Pt and Co-Pi: Improved Charge-Separation Dynamics and Photoelectrochemical Performance.

The construction of junctions on hematite is an effective way to overcome the problems of slow charge separation and transfer kinetics, but constructing the junction is a significant challenge in photoelectrochemical (PEC) water splitting. Herein, a considerable improvement in PEC performance for α-Fe2 O3 was achieved following the introduction of a p-n homojunction between n-type α-Fe2 O3 and p-type Ca-doped α-Fe2 O3 through a facile hydrothermal method. The resultant 3D branched Ca-Fe2 O3 /Fe2 O3 enhanced the absorption intensity and reached a photocurrent density of 2.14 mA cm-2 at 1.23 V vs. reversible hydrogen electrode (RHE). The merit of the desired lattice matching of the buried p-n homojunction structure built an internal electric field, which led to appropriate band alignment. These results were supported by a series of photoelectrochemical measurements, in particular, surface photovoltage (SPV) measurements. For further improvement of the charge-separation efficiency, a combination of separated cocatalysts was established on the homojunction structure, in which Pt acted as the electron collector and was deposited on the bottom, and Co-Pi as the hole-extraction cocatalyst was inserted to accelerate hole transfer on the surface of the photoanode. The resulting Co-Pi/Ca-Fe2 O3 /Fe2 O3 /Pt branched nanorods showed a significant improvement in charge-separation efficiency and photocurrent density (2.94 mA cm-2 at 1.23 V vs. RHE). The present strategy, both the construction of the p-n homojunction and the coupling electron- and hole-transfer cocatalyst, could be expanded to many unstable or low-efficiency semiconductors for the design and fabrication of cost-effective photoanodes in PEC water splitting.

Chen D ，Liu Z ，Guo Z ，Ruan M ，Yan W ... -  《-》

Enhanced Photoelectrochemical Water Oxidation Performance in Bilayer TiO(2) /α-Fe(2) O(3) Nanorod Arrays Photoanode with Cu : NiO(x) as Hole Transport Layer and Co-Pi as Cocatalyst.

Efficient charge transfer and excellent surface water oxidation kinetics are key factors in determining the photoelectrochemical (PEC) water splitting performance in photoelectrodes. Herein, a bilayer TiO2 /α-Fe2 O3 nanorod (NR) arrays photoanode was prepared with deposited Cu-doped NiOx (Cu : NiOx ) hole transport layer (HTL) and Co-Pi oxygen evolution reaction (OER) cocatalyst for PEC water oxidation. The hierarchical TiO2 /α-Fe2 O3 composite obtained by a secondary hydrothermal process exhibited an inapparent bilayer structure by embedding the underlayer TiO2 NR arrays at the bottom part of the post-grown α-Fe2 O3 NR arrays. The underlayer TiO2 NRs acted as an effective shuttling pathway for transferring photoelectrons generated in the upper hematite light absorber layer. A p-type inter-Cu : NiOx HTL was introduced to form a build-in p-n electric field between Cu : NiOx and α-Fe2 O3 NRs, which improved the hole extraction from α-Fe2 O3 to Co-Pi OER catalyst. As expected, the as-engineered TiO2 /α-Fe2 O3 /Cu : NiOx /Co-Pi photoanode displayed an excellent photocurrent density of 2.43 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (VRHE ), up to 4.05 and 2.23 times greater than those of the bare α-Fe2 O3 (0.60 mA cm-2 ) and TiO2 /α-Fe2 O3 , respectively. The results demonstrate that the bottom-up engineering of electron-hole transport channels and cocatalyst modification is an attractive maneuver to enhance the PEC water oxidation activity in hematite and other photoanodes.

Li H ，Yin M ，Li X ，Mo R ... -  《-》

Enhanced Charge Separation through ALD-Modified Fe2 O3 /Fe2 TiO5 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.

Li C ，Wang T ，Luo Z ，Liu S ，Gong J ... -  《-》

NiO Nanoparticles Anchored on Phosphorus-Doped α-Fe(2) O(3) Nanoarrays: An Efficient Hole Extraction p-n Heterojunction Photoanode for Water Oxidation.

The photoelectrochemical (PEC) water-splitting efficiency of a hematite-based photoanode is still far from the theoretical value due to its poor surface reaction kinetics and high density of surface trapping states. To solve these drawbacks, a photoanode consisting of NiO nanoparticles anchored on a gradient phosphorus-doped α-Fe2 O3 nanorod (NR) array (NiO/P-α-Fe2 O3 ) was fabricated to achieve optimal light absorption and charge separation, as well as rapid surface reaction kinetics. Specifically, a photoanode with the NR array structure allowed a high mass-transport rate to be achieved, while phosphorus doping effectively decreased the number of surface trapping sites and improved the electrical conductivity of α-Fe2 O3 . Furthermore, the p-n junction that forms between NiO and P-α-Fe2 O3 can further improve the PEC performance due to efficient hole extraction and the water oxidization catalytic activity of NiO. Consequently, the NiO/P-α-Fe2 O3 NR photoanode produced a high photocurrent density of 2.08 mA cm-2 at 1.23 V versus a reversible hydrogen electrode and a 110 mV cathodic shift of the onset potential. This rational design of structure offers a new perspective in exploring high-performance PEC photoanodes.

Li F ，Li J ，Zhang J ，Gao L ，Long X ，Hu Y ，Li S ，Jin J ，Ma J ... -  《-》

Dual-Axial Gradient Doping (Zr and Sn) on Hematite for Promoting Charge Separation in Photoelectrochemical Water Splitting.

One of the crucial challenges to enhance the photoelectrochemical water-splitting performance of hematite (α-Fe2 O3 ) is to resolve its very fast charge recombination in bulk. Herein, we describe the design and fabrication of dual-axial gradient-doping on 1D Fe2 O3 nanorod arrays with Zr doping for x-axial and Sn doping for y-axial directions to promote the charge separation. This dual-axial gradient-doping structure fulfills the requirements of a greater electron-carrier concentration for increasing conductivity as well as a higher charge-separation efficiency across the dual-axial direction of Fe2 O3 nanorods, ultimately showing an excellent photocurrent density of 1.64 mA cm-2 at 1.23 V vs. RHE, which is 26.3 times more than that of the bare Fe2 O3 . Furthermore, the remarkably improved photocurrent density, when comparing the uniform Zr-doped Fe2 O3 nanorod arrays (1.0 mA cm-2 at 1.23 V vs. RHE) with dual-axial gradient-doped (Zr and Sn) Fe2 O3 nanorod arrays, highlights the additional charge-separation effect resulting from gradient codoping of Zr and Sn. Hence, this promising design may provide guidelines for dual-axial gradient doping into photoelectrodes to realize efficient PEC water splitting.

Chen D ，Liu Z 《-》