A new hematite photoanode doping strategy for solar water splitting: oxygen vacancy generation

Yang, Tae‐Youl; Kang, Hyungsook; Sim, Uk; Lee, Young‐Joo; Lee, Jihoon; Koo, Byungjin; Nam, Ki Tae; Joo, Young‐Chang

doi:10.1039/c2cp44352j

Cited by 130 publications

(98 citation statements)

References 29 publications

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“…31,34,35,42 Additional information about the phase of the nanostructures forming the sample top surface is obtained using X-ray diffraction (XRD) patterns performed with a grazing incidence angle of 0.5°. The XRD spectra presented in Figure 2 (a) show that all samples contain hematite, magnetite, and iron, a result consistent with the mixed iron oxides formed during thermal oxidization of iron foils.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

Zhu

Zheng

et al. 2015

ACS Appl. Mater. Interfaces

170

111

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The incorporation of oxygen vacancies in hematite has been investigated as a promising route to improve oxygen evolution reaction activity of hematite photoanodes used in photoelectrochemical water oxidation. However, introducing oxygen vacancies intentionally in α-Fe2O3 for active solar water splitting through facile and effective methods remains a challenge. Herein, air plasma treatment is shown to produce oxygen vacancies in α-Fe2O3, and ultrathin α-Fe2O3 nanoflakes are used to investigate the effect of oxygen vacancies on the performance of photoelectrochemical oxygen oxidation. Increasing the plasma treatment duration and power is found to increase the density of oxygen vacancies and leads to a significant enhancement of the photocurrent response. The nanoflake photoanode with the optimized plasma treatment yields an incident photo-to-current conversion efficiency of 35.4% at 350 nm under 1.6 V vs RHE without resorting to any other cocatalysts, an efficiency far exceeding that of the pristine α-Fe2O3 nanoflakes (∼2.2%). Evidence for the presence of high density of oxygen vacancies confined in nanoflakes is clarified by X-ray photoelectron spectroscopy. The increased number of oxygen vacancies after plasma treatment resulting in an increased carrier density is interpreted as the main cause for the enhanced oxygen evolution reaction activity.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

Zhu

Zheng

et al. 2015

ACS Appl. Mater. Interfaces

170

111

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“…This oxygen‐deficient thermal decomposition method has further been extended to a two‐step low temperature (350 °C) activation process . More importantly, by coupling the inducing oxygen vacancy and element‐doping (such as Sn, and Ti), the photoactivity of hematite nanostructure was further increased. In spite of the recent progress toward enhancing the photocurrent density, an outstanding challenge for hematite photoanode is the large onset voltage, due to the slow water oxidation kinetics and low lying conduction band .…”

Section: Discussionmentioning

confidence: 99%

Review of Sn‐Doped Hematite Nanostructures for Photoelectrochemical Water Splitting

Ling

2014

Part & Part Syst Charact

106

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Hematite (α-Fe 2 O 3 ) nanostructures have been extensively studied as photoanodes for photoelectrochemical (PEC) water splitting. However, the photoactivity of pristine hematite nanostructures is limited by a number of factors, including poor electrical conductiviy and slow oxygen evolution reaction kinetics. Previous studies have shown that using tin (Sn) as an n-type dopant can substantially enhance the photoactivity of hematite photoanodes by modifying their optical and electrical properties. Here, the recent accomplishments in using Sn-doped hematite photoanodes for solar water splitting are highlighted.1.9-2.2 eV [ 59 ] for harvesting solar light, which corresponding to a high theoretical STH conversion effi ciency of 14%-17%. [ 60 ] Iron is the fourth most abundant element in the Earth's crust (6.3% by weight), [ 5 ] and hematite is a low-cost material. In addition, hematite is chemically stable in solutions of different pH. [ 61 ] However, the STH conversion effi ciencies of reported hematite photoanodes are considerably lower than the theoretical value, owing to its very short-excited state lifetime (<10 ps), [ 62 ] short hole diffusion length (≈2-4 nm), [ 5,61 ] poor surface oxygen evolution reaction kinetics, [ 63 ] and poor electrical conductivity (10 −6 Ω −1 cm −1 ). [ 5 ] The recent development of hematite nanostructures, including nanoparticles, [ 59,64,65 ] nanowires [ 66,67 ] and nanonets, [ 56,58 ] opens up new opportunities in addressing the abovementioned limitations. Nanostructured photoanode offers increased semiconductor/electrolyte interfacial area for water oxidation, as well as substantially reduced diffusion length for minority carriers. Therefore, nanostructured hematite photoelectrodes are expected to be more effi cient in charge carrier collection than their bulk counterparts. [ 60 ] For example, Lin et al. reported the growth of ultrathin hematite fi lm (25 nm) on a conductive nanonet structure. The nanonet-based hematite photoanode achieved an excellent photocurrent of 1.6 mA cm −2 at 1.23 V versus RHE, which is four times higher than the value obtained from the planar sample with the same thickness. The photocurrent enhancement was due to the increased activation sites for water oxidation ( Figure 2 ). [ 56 ] To further improve the photoactivity of hematite photoanode, element doping has been extensively studied to improve the structural, electronic and optical properties of hematite. The role of dopants, such as Ti, [ 65,[68][69][70][71][72][73][74][75] Si, [ 68,[76][77][78][79] Al, [ 71,80 ] Mg, [ 81,82 ] Zn, [ 71,78 ] Be, [ 80 ] Mo, [ 83 ] and Sn, [ 4,61,69,80,81,[84][85][86][87][88][89][90] on the PEC performance of hematite have been investigated. Recently, there is an increasing interest of developing Sn-doped hematite nanostructured photoanode due to the signifi cant effect of Sn doping on the photoactivity of hematite. Sn 4+ is a tetravalent dopant that can be substitutionally doped into hematite at the Fe 3+ sites. [ 69,71,89 ] The electrical conductivity of Sn-doped hema...

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“…Yang et al recently demonstrated the intentional introduction of oxygen vacancies within the crystal structure of hematite thin films through controlled annealing under an oxygen-deficient atmosphere. [15] As the partial pressure of oxygen was decreased during the annealing step, the photocurrent response was found to increase concurrently. This was correlated to a higher number of Fe 2 + sites resulting from an increase in the number of oxygen vacancies.…”

Section: Introductionmentioning

confidence: 97%

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H₂ Treatment

et al. 2015

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Thermal treatment of ultrathin films of hematite (α-Fe2 O3 ) under an atmosphere of 5 % H2 in Ar is presented as a means of activating α-Fe2 O3 towards the photoelectrochemical splitting of water. Spin-coated films annealed in air exhibited no photoactivity, whereas films treated in hydrogen exhibited a photocurrent response. X-ray photoelectron spectroscopy and UV/Vis absorption spectroscopy results showed that the H2 -treated films contain oxygen vacancies, which suggests improved charge transport. However, Tafel slopes, scan-rate dependent measurements, and kinetic analyses performed by using H2 O2 as a hole scavenger suggested that surface modification may also contribute to their induced photoactivity. Electrochemical impedance spectroscopy results revealed the buildup of a surface trap capacitance at the point of photocurrent onset for the hydrogen-treated films under illumination. A decrease in charge trapping resistance was also observed, which suggests improved transport of charges away from the surface.

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A new hematite photoanode doping strategy for solar water splitting: oxygen vacancy generation

Cited by 130 publications

References 29 publications

Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

Review of Sn‐Doped Hematite Nanostructures for Photoelectrochemical Water Splitting

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H₂ Treatment

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A new hematite photoanode doping strategy for solar water splitting: oxygen vacancy generation

Cited by 130 publications

References 29 publications

Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

Review of Sn‐Doped Hematite Nanostructures for Photoelectrochemical Water Splitting

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H2 Treatment

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Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H₂ Treatment