Performance enhancement of hematite photoanode with oxygen defects for water splitting

doi:10.1016/j.cej.2020.126163

Chemical Engineering Journal

Volume 402, 15 December 2020, 126163

https://doi.org/10.1016/j.cej.2020.126163 Get rights and content

Highlights

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Methanol solvothermal treatment induces a hydroxylated surface on Fe₂O₃:Ti.
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HCl hydrothermal treatment induces more chemisorbed oxygen on Fe₂O₃:Ti.
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Surface hydroxylation enhances the photocurrent and chemisorbed oxygen reduces the onset potential of Fe₂O₃:Ti.
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A synergistic effect is achieved by oxhydryl and chemisorbed oxygen.
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The photocurrent density of Fe₂O₃:Ti is optimized to 3.0 mA cm⁻² at 1.23 V_RHE.

Abstract

Two types of oxygen defects are created on the Fe₂O₃:Ti photoanode and their acting roles on prompting the photoelectrochemical performance are clarified. Methanol solvothermal treatment is discovered to expediently import hydroxyl oxygen on Fe₂O₃:Ti. Accordingly, the charge carrier density is multiplied and surface hydrophilicity is dramatically improved, resulting in inhibition of surface transfer recombination and enhancement of the photocurrent density. In addition, HCl hydrothermal treatment can induce surface defects with chemisorbed oxygen, which barely affects the surface hydrophobicity or the carrier density. HCl hydrothermal treatment behaves on promoting the charge transfer in surface states as well as suppressing the bulk charge recombination, thus moderately enhancing the photocurrent density and lowering the onset potential. By combining two treatment processes, both defects are formed simultaneously with synergistic effectiveness. In addition, the FeCoW oxy–hydroxide electrocatalyst is incorporated for further accelerating the surface charge transfer. As a result, by compositing the strategies of methanol solvothermal, HCl hydrothermal, and FeCoW deposition, the photocurrent density of Fe₂O₃:Ti photoanode is significantly enhanced to 3.0 mA cm⁻² at 1.23 V_RHE with an onset potential of as low as 0.67 V_RHE.

Graphical abstract

Introduction

On the concern of the energy demand and environmental crises, photoelectrochemical (PEC) water splitting is an inspirational route to efficiently convert solar light to hydrogen, thus it has been endowed as “artificial photosynthesis” [1]. This uphill reaction is challenging in thermodynamics with the necessity to overcome the large positive Gibbs free energy change (ΔG° = 237 kJ/mol) [2]. What is more, the anodic water oxidation (H₂O + 2 h⁺ → 1/2O₂↑ + 2H⁺) comprises multi-electron transfer processes and is accompanied by significant kinetics difficulty. The PEC water oxidation is more efficiently accomplished with the assistance of external bias voltage, which is compromising with electric energy loss. Whereas, the overall light conversion efficiency is still limited by the simultaneous light-harvesting, photoinduced charge separation and surface charge transfer efficiencies relevant to the photoanode materials.

Hematite (α-Fe₂O₃) is one of the most attractive and promising candidates as photoanode materials with a broad absorption of photons up to 560–650 nm, not to mention its high durability, good economic utility, and nontoxicity [3]. Given that the flat potential of hematite is more positive than water reduction reaction, a bias potential is indispensable to achieve overall water splitting. Worse still, the surface states existing in the forbidden band of hematite induce the Fermi-level pinning effect, and the associated sluggish surface kinetics [4]. Surface modifications such as surface treatment [5], [6], [7] by passivating surface states and electrocatalyst deposition [8], [9], [10], [11] by accelerating the surface kinetics were used to cut dependency on the impressed voltage. Yet, the PEC practicability of hematite is also overshadowed by the extraordinarily short lifetime of photoexcited carriers due to the poor electron conductivity with low carrier mobility and the associated short hole diffusion length (less than 5 nm) [12]. A concerted effort has been made to realize progress on the photocurrent density by ion-doping with cationic (including Ti⁴⁺, Si⁴⁺, Sn⁴⁺, Ag⁺, Ta⁵⁺, Ca²⁺, Pt²⁺and Zn²⁺) [13], [14], [15], [16], [17] and certain non-metallic elements (F, P, S, etc.) [18], [19] to increase conductivity as well as by deliberate manipulation of nano architectures for shortening the holes-transport distance to suppress bulk recombination and enlargement of surface area to extend contact between photoanode and water molecules [20], [21], [22].

Creating crystal defects in the lattice of hematite has been reported to manipulate the PEC process. Previous reports suggested that lattice defects in hematite lead to a negative shift of the flat-band, therefore not only enhancing the photocurrent but also reducing the onset potential [23]. Experimental evidence indicated that oxygen vacancy promotes the plateau photocurrent by increasing the electrical conductivity, while it also acts as a recombination center so that it is deleterious to surface charge transfer and increases onset potential [24], [25], [26]. The surface hydroxyl groups on hematite can serve as a hole collection layer, resulting in a considerably enhanced photocurrent [27]. Acid corrosion was also found to influence the charge separation in Fe₂O₃ [6] or the back recombination in Ti:Fe₂O₃ [5] that leads to either increase of plateau photocurrent or cathodic shift of the onset potential.

Although intensive work has been done prompted by these concepts, there still is much scope to improve the charge separation/transfer efficiencies and boost the onset potential reduction of hematite photoanodes. To seek for further improvement of the PEC performance by hematite, one direct strategy is to integrate advantages of different techniques and reach synergistic effect. Herein, we report a three-step post-processing route that simultaneously increases the photovoltage as well as suppresses bulk and surface charge recombination, as depicted in Fig. 1. At First, methanol solvothermal process is discovered to expediently import surface hydroxyl on Fe₂O₃:Ti and increase charge carrier density, so that improving surface hole transfer and enlarging the photovoltage, and thereby significantly inhibits surface charge recombination and increases the photocurrent density. Second, HCl hydrothermal treatment creates lattice defects that chemisorbed water molecules by surface corrosion and accordingly enhances both the charge separation and injection efficiencies. The third approach is to use the FeCoW oxy–hydroxide electrocatalyst to further accelerate the charge transfer. As a result, the optimized photocurrent density reaches 3.0 mA cm⁻² at 1.23 V_RHE and the onset potential is reduced to 0.67 V_RHE on the basis of high durability.

Section snippets

Materials

The chemicals of iron (III) chloride hexahydrate (FeCl₃·6H₂O, 99.0%), titanium (IV) butoxide (C₁₆H₃₆O₄Ti, 99%), methanol (CH₃OH, 99.9%), acetonitrile (CH₃CN, AR), N,N-Dimethylformamide (DMF, 99.5%) were purchased from Shanghai Titan Co., Ltd. Hydrochloric acid (HCl, 37%), anhydrous iron chloride (FeCl₃), cobalt chloride (CoCl₂) and tungsten hexachloride (WCl₆) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium acetate trihydrate (CH₃COONa·3H₂O, 98.0%) was purchased from Tianjin

Characterizations of the electrodes

The Fe₂O₃:Ti electrodes were prepared through a hydrothermal method with ex-situ Ti-doping, as detailed in the experimental section. The Fe₂O₃:Ti electrodes were further treated through solvothermal and hydrothermal processes in methanol and HCl solution, respectively. The crystal structure of α-Fe₂O₃ (JCPDS No. 33-0664) with a dominant orientation of (1 1 0) plane was identified with X-ray powder diffraction (Fig. S1). There was no noticeable change in the phase structure of the electrodes after

Conclusion

This work concerns the acting roles of different kinds of surface defects in Fe₂O₃:Ti and more importantly on the integration of multiple modification strategies to maximize the PEC water splitting performance of the Fe₂O₃:Ti photoanode. First, two types of oxygen vacancy defects were created on the surface of Fe₂O₃:Ti. Solvothermal treatment was discovered to expediently import oxygen vacancies in Fe₂O₃:Ti which are occupied by surface hydroxyl. Accordingly, charge carrier density was

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Start-Up Scientific Research Funds for Newly Recruited Talents of Huaqiao University (No. 605-50Y19013) and the Key Laboratory of Fujian Province in Huaqiao University.

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Performance enhancement of hematite photoanode with oxygen defects for water splitting

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Characterizations of the electrodes

Conclusion

Declaration of Competing Interest

Acknowledgments

J. Catal.

Int. J. Hydrogen Energy

J. Catal.

Electrochem. Commun.

Int. J. Hydrogen Energy

Electrochim. Acta

J. Catal.

Appl. Catal. B: Environ.

Chem. Eng. J.

Chem. Eng. J.

J. Alloys Compd.

Nano Energy

Nature

ACS Energy Lett.

ChemSusChem

J. Mater. Chem. A

Energy Environ. Sci.

Angew. Chem.

Chem. Commun.

Angew. Chem. Int. Ed.

J. Am. Chem. Soc.

Nat. Commun.

Adv. Energy Mater.

Chem. Mater.