Morphology and Doping Engineering of Sn-Doped Hematite Nanowire Photoanodes

Abstract

High-temperature activation has been commonly used to boost the photoelectrochemical (PEC) performance of hematite nanowires for water oxidation, by inducing Sn diffusion from fluorine-doped tin oxide (FTO) substrate into hematite. Yet, hematite nanowires thermally annealed at high temperature suffer from two major drawbacks that negatively affect their performance. First, the structural deformation reduces light absorption capability of nanowire. Second, this "passive" doping method leads to nonuniform distribution of Sn dopant in nanowire and limits the Sn doping concentration. Both factors impair the electrochemical properties of hematite nanowire. Here we demonstrate a silica encapsulation method that is able to simultaneously retain the hematite nanowire morphology even after high-temperature calcination at 800 °C and improve the concentration and uniformity of dopant distribution along the nanowire growth axis. The capability of retaining nanowire morphology allows tuning the nanowire length for optimal light absorption. Uniform distribution of Sn doping enhances the donor density and charge transport of hematite nanowire. The morphology and doping engineered hematite nanowire photoanode decorated with a cobalt oxide-based oxygen evolution reaction (OER) catalyst achieves an outstanding photocurrent density of 2.2 mA cm at 0.23 V vs Ag/AgCl. This work provides important insights on how the morphology and doping uniformity of hematite photoanodes affect their PEC performance.

Progress in Developing Metal Oxide Nanomaterials for Photoelectrochemical Water Splitting

“…[116] To achieve controlled doping in nanomaterials is known to be extremely difficult. Although many metal oxide materials hold great potentials as photoelectrodes or catalysts, there are a number of challenges are remained to be addressed.…”

A novel CoOOH/(Ti, C)-Fe2O3 nanorod photoanode for photoelectrochemical water splitting

“…IPCE can be expressed as [16]: IPCE=(1240×I)/(λ×J light ), where I is the photocurrent density, λ is the incident light wavelength, and J light is the measured irradiance. Yield of the surface reaching holes or the named transfer efficiency (Φ OX ) can be expressed as [29]:…”

“…The J max of α-Fe 2 O 3 is more than 12 mA cm -2 , but the reported water oxidation photocurrent densities of α-Fe 2 O 3 are within the range of 1-5 mA cm −2 [20,23], due to the poor electrical conductivity [3,16,24], which results in inefficient migration rate and poor separation of photo-generated electrons and holes [25], resulting in solar energy degradation [22,23]. We believe that future enhancement of the PEC performance of α-Fe 2 O 3 photoanode will focus on improving the electrical conductivity of α-Fe 2 O 3 .…”

“…Hematite (α-Fe 2 O 3 ) is one of the most promising earthabundant catalysts for photoanode, which can absorb a substantial portion of the solar spectrum (300-600 nm) [16][17][18][19]. This feature makes α-Fe 2 O 3 possess a remarkable maximum water oxidation photocurrent density (J max ) for solar water splitting [20].…”

“…For example, Li et al [16] indicated that a novel Sn-Fe 2 O 3 photoanode achieved the excellent photocurrent density of ∼1.36 mA cm −2 at 0.23 V vs. Ag/ AgCl. The Sn element doping can effectively enhance its electrical conductivity.…”