Nanoporous hematite structures to overcome short diffusion lengths in water splitting

Abstract

In this report, we show that by creating a nanoporous haematite (aFe 2 O 3 ) structure using boric acid (H 3 BO 3 ) treatment, the chronic issue of the short diffusion length of carriers in a-Fe 2 O 3 for photo- Photoelectrochemical (PEC) water splitting cells which use solar energy to produce hydrogen or oxygen gas are a promising technology for solving the energy crisis and environmental problems. 1-6 Since Fujishima and Honda rst reported a PEC water splitting system using a titanium dioxide semiconductor, n-type semiconductors have been considered to be promising photoanode materials for PEC water splitting cells.7,8 Hematite (a-Fe 2 O 3 ) is a very attractive material among n-type semiconductors for PEC water splitting because of its relatively narrow band gap (2.1-2.2 eV) and superior chemical stability in electrolytes.9,10 More importantly, compared to rare semiconductor materials (As, Ga, In, Se, and so on), which are commonly used in monolithic photovoltaic-PEC cells, a-Fe 2 O 3 is very cheap and abundant, and therefore, able to achieve high solar-to-hydrogen (STH) efficiency economically. The elemental doping of a-Fe 2 O 3 uses a relatively simple procedure and has attracted much attention as a way to improve a-Fe 2 O 3 performance by increasing its electronic conductivity and carrier life time. 16,[20][21][22] However, the use of rare metal dopants such as Pt, Si, and Ti and so on increases the overall price of the PEC cell. Alternatively, it has been reported that forming oxygen vacancies in semiconductors such as TiO 2 and a-Fe 2 O 3 by treatment with reducing agents could enhance the PEC performance by increasing the donor density. In particular, Fe 2+ ions in a-Fe 2 O 3 with oxygen vacancies could dramatically increase the conductivity of the a-Fe 2 O 3 via a polaron hopping mechanism. 23Controlling the nanostructure morphology of a-Fe 2 O 3 has been another efficient way to overcome the disadvantages of aFe 2 O 3 by increasing its surface area and reducing the path length of hole transport. For example, diverse morphologies of a-Fe 2 O 3 such as mesoporous nanotubes/nanowires and threedimensional nano-structures have exhibited much improved PEC performance compared to normal lm type a-Fe 2 O 3 . 24-26Although the control of a-Fe 2 O 3 morphology has produced highly enhanced performance, the fabrication process involves complex steps and the resulting particle sizes are above 50-80 nm which are still signicantly larger than the 2-4 nm hole diffusion length. 10,27 In this work, the fabrication of boric acid (H 3 BO 3 ) treated a-Fe 2 O 3 with optimized sizes using a cheap and broadly applicable hydrothermal growth method is reported. The dissociation constant of the [FeB(OH) 4 ] 2+ complex controls the rate of formation of b-FeOOH, and accounts for the nal nanoporous morphology of a-Fe 2 O 3 nanorods with $15 nm

Enhanced Charge Transfer Process in Morphology Restructured TiO2 Nanotubes via Hydrochloric Acid Assisted One Step In‐Situ Hydrothermal Approach

“…Even though, the acid treatment of TNT changes the shape of the nanotube into elliptical granular particles but still provides stronger adhesion on Ti foil. This result indicates that the surface states of the existing TNT is affected in acid treated process, and new structure in TH‐40 might facilitate the short diffusion path to support easy electron transfer in photocatalytic reaction . Figure S1 (A,B) of the Supplementary Information (SI) shows the corresponding FESEM energy dispersive spectrometry (EDS) spectra of the TNT and TH‐40 photoelectrode samples that were measured.…”

Influence of anodization time on the surface modifications on α-Fe2O3photoanode upon anodization

“…Although a significant improvement has been made on conductivity of a-Fe 2 O 3 by elemental doping 5,[22][23][24][25] and nanostructuring, [26][27][28][29] the influence of the anodizing process on the photocatalytic performance of a-Fe 2 O 3 photoanode in a PEC cell has not been extensively studied. However, a detailed research on the surface molecular structure of anodized a-Fe 2 O 3 photoanode has been conducted and reported transformation of Fe 21 to Fe 31 states at the surface upon subject to anodizing conditions and the findings were attributed to filling of oxygen vacancies due to increasing oxygen content at the surface.…”

Study on Dynamic Recrystallization in the Plastic Deformation Zone of Vibration Cast‐Rolling

“…On the one hand, the vertical reciprocating vibration of the single-side cast-rolling roll can promote the dendrite fusing in DOI: 10.1002/crat.201900044 the solidification stage and increase the nucleation rate, thereby refining the crystal grains and increasing the equiaxed grain ratio in the solidified structure. On the other hand, it can apply bidirectional shear force to the high temperature strip to change the stress state of the plastic deformation zone, thereby enhancing the effect of dynamic recrystallization and further refining the crystal grains.…”

“…Using hot compression test, Mirzadeh and Parsa studied the thermal deformation and dynamic recrystallization of Ni-Ti metal composites, discussed the influence of Z parameters on critical flow stress, determined the critical stress and strain of initial dynamic recrystallization, and established the constitutive equation of Ni-Ti metal. [10] Bobbili and Madhu studied the dynamic recrystallization of medical alloy Ti-13Nb-13Zr. It was found that the alloy exhibited different microstructures at different temperatures, and its dynamic recrystallization was related to the initial grain size.…”