Elsevier

Applied Catalysis A: General

Volume 498, 5 June 2015, Pages 126-141
Applied Catalysis A: General

Review
α-Fe2O3 as a photocatalytic material: A review

https://doi.org/10.1016/j.apcata.2015.03.023Get rights and content

Highlights

  • α-Fe2O3 is useful in photocatalytic water treatment & water splitting applications.

  • α-Fe2O3 has advantage of low band gap, chemical stability, low cost & nontoxicity.

  • The property is restricted by high e–h+ recombination rate & low diffusion length.

  • The article summarizes different attempts to enhance the photocatalytic activity.

  • The surface area & e–h+ recombination rate control the photocatalytic property.

Abstract

Photocatalysis has been attracting much research interest because of its wide applications in renewable energy and environmental remediation. There are many materials that are found to show good photocatalytic activity in the presence of ultraviolet (UV) and visible light. However, the applications of these materials are limited to the UV portion of sunlight. α-Fe2O3 has an advantage over the other conventional materials like TiO2, ZnO, etc. in using solar energy for photocatalytic applications due to its lower band gap ∼2.2 eV value. As a result of which Fe2O3 is capable of absorbing a large portion of the visible solar spectrum (absorbance edge ∼600 nm). Also its good chemical stability in aqueous medium, low cost, abundance and nontoxic nature makes it a promising material for photocatalytic water treatment and water splitting applications. Except these advantages the usage of Fe2O3 has been restricted by many anomalies such as higher e–h recombination effect, low diffusion length and VB positioning (VB is positive with respect to H+/H2 potential). This article reviews the research that has been carried out to overcome these basic limitations and to enhance the photocatalytic activity of α-Fe2O3.

Introduction

The sun is the principal energy source for earth and sustains life on earth. The most important natural method for utilization of solar energy is the photosynthesis process by plants to produce carbohydrates and oxygen gas (O2) by utilizing carbon dioxide (CO2) and water (H2O). Most of our day-to-day energy requirements are being met by fossil fuels. However, the limited availability of these non-renewable sources of energy has motivated researchers to find different techniques to use solar energy as an alternative for future energy needs. The most common method to directly convert solar energy into electric energy is photovoltaic (PV), and this process utilizes semiconductors which generate electron–hole pairs upon illumination with visible light, thereby producing electric power in solar cells. However, the utility of photovoltaic cells is limited by poor conversion efficiency. To overcome these problems, researchers have tried to find suitable methods to produce hydrogen (H2) from photocatalysis of H2O using sunlight, which can be used in fuel cells for power generation.

Another important current problem before scientists is environmental pollution, such as water contamination, from industrial hazardous waste. Common pollutants include toxic organic compounds like chlorinated and non-chlorinated aliphatic and aromatic compounds, organic dyes, surfactants, detergents, insecticides, pesticides, herbicides, disinfection byproducts, volatile organic compounds, plastics, heavy metals, NOx, SOx, CO, NH3, and pathogens (bacteria, fungi and viruses) [1]. Although there are many methods that have been followed to remove such wastes from water, none are able to fully purify the water. Further, the processes are very slow, and need high temperature along with strict pH control. These problems are overcome by the advanced oxidation process (AOP), which involves the production of highly reactive species (OH, O2radical dot, O3) generated by a light source [2]. These species react with contaminants and form simple, non-hazardous byproducts.

Photocatalytic ability in materials is one of the most interesting research topics due to its usefulness in various fields such as H2 generation [3], [4], [5], artificial photosynthesis [6], [7], waste water treatment [8], [9], [10], removal of toxic gases from air [11], [12], [13] and dye sensitized solar cells (DSSC) [14], [15].

A catalyst is a material whose presence enhances the rate of a chemical reaction but itself remains unchanged at the end of the reaction. Some common examples of catalysts are ammonia formation in the presence of iron, hydrogenation of Cdouble bondC bonds in the presence of nickel, enzymes for biological reactions, and many more. Photocatalytic activity refers to the catalytic properties of materials in the presence of light, and photocatalysis is the acceleration of the rate of a chemical reaction by activating a catalyst by ultraviolet (UV) or visible light. The catalysts in photocatalytic reactions are normally semiconductor materials, which can form electrons and holes when exposed to light, and the reactions are normally either oxidation or reduction. Photocatalysts are generally divided into two categories (i) homogeneous catalysts and (ii) heterogeneous catalysts. A catalyst is defined as homogeneous if it is present in the same phase as that of the reagents, which means that the catalysts are present as solutes in a homogeneous catalytic reaction system. In contrast, a catalyst is said to be heterogeneous if it is present in a different phase than the reaction mixture. The advantages of heterogeneous catalysts are that they can be easily separated from the reaction mixture and can be reused [16]. An ideal photocatalyst should have the following properties: (i) it should have good photoactivity, (ii) it must be biologically and chemically inert, (iii) it must be suitable for absorbing visible or near UV light, (iv) it also must be stable to photocorrosion, (v) cheap, and (vi) non-toxic [17].

The steps of a photocatalytic reaction involve the absorption of light by a semiconductor material followed by an electron transfer from the valence band (VB) to the conduction band (CB) creating a hole in the VB. The VB and CB are separated by some energy barrier called the band gap (Eg). Typical band gaps of some semiconductors are listed in Table 1. The interface transfer process, which involves electrons and holes and their deactivation by recombination, determines the efficiency of the photocatalyst. The detailed process regarding photocatalytic mechanism can be obtained from [2], [18]. The hydroperoxyl radical, if formed, also serves as a scavenger, thus doubly prolonging the lifetime of photo-holes. Both the oxidation and reduction can take place at the surface of the photoexcited semiconductor photocatalyst. Recombination between electrons and hole occurs unless oxygen is available to scavenge the electrons to form superoxides (O2radical dot), followed by its protonated form, the hydroperoxyl radical (HO2radical dot), and subsequently to hydrogen peroxide (H2O2). Each subsequent ion formed reacts to form intermediates and final products. As a consequence of this series of reactions, the photonic excitation of the catalyst serves as the initial step of the activation of the whole catalytic system.

Solar water splitting is another important application of photocatalysis. The electrons and holes generated by the excitation of semiconductors simultaneously initiate oxidation/reduction reactions, which is similar to that of electrolysis. The photo-generated e moves toward the cathode by application of external potential and reduce H+ ions to H2. However the photogenerated h+ moves toward the anode to oxidize the H2O molecules to form O2. The reactions are as follows.Cathode: H2O + 2 h+ = 1/2O2 + 2H+Anode: 2H+ + 2e = H2

The electrons and holes react with water molecules to form H2 and O2, respectively. The potential involves in a water splitting reaction is 1.23 eV, which concludes a photocatalyst that can be considered for water splitting applications should have a band gap value greater than 1.23 eV and also suitable positioning of the CB and VB levels. The bottom level of the conduction band of the semiconductor has to be more negative than the redox potential of H+/H2 (0 V vs. normal hydrogen electrode (NHE)), whereas the valence band of the semiconductor has to be more positive than the redox potential of O2/H2O (1.23 V) [19]. The positioning of the VB and CB depends on the d-orbital distribution of a particular semiconductor. In TiO2 the VB is more negative in comparison to the H+/H2 potential making it a suitable material for H2 production. Although the band gap of Fe2O3 suggests utilization of larger range of solar spectrum, the application is limited by the VB position of Fe2O3 which is more positive in comparison to the H+/H2 electrode making it unsuitable for H2 production. Several other semiconductors who have negative VB in reference to the H2 electrode are suitable for usage because of their instability in aqueous medium. Several attempts have been made to shift the VB of Fe2O3 to the more negative position of VB of H2 potential. The efficiency of photon to H2 conversion has been described by several terms such as Quantum Efficiency (QE) and Applied Bias Photon-to-Current Efficiency (ABPE). The details regarding the water splitting mechanisms and efficiency can be read from the review articles by Liao et al. [20] and Walter et al. [21].

The photocatalytic properties and efficiencies are influenced by the surface structures of the photocatalysts including surface area, porosity, defects, nature of the exposed planes, concentration of pollutants, amount of dye, pH of the reaction medium, calcination temperature, reaction temperature, light intensity, and presence of oxidizing agents [22]. All the parameters are listed in Fig. 1. There are many techniques that have been followed to enhance the photocatalytic activity of a material such as surface modification, doping with other materials, formation of composites, and formation of heterostructures (Fig. 2).

The photocatalytic activity of materials can utilize solar energy, which makes this an important alternative to present energy sources. Further, photocatalysts can be used for environmental remediation, which involves the degradation of organic and inorganic pollutants present in water as well as in air. Further, applications have been found in disinfecting water by reduction of microorganisms in water. Self-cleaning applications of photocatalytic materials can be used in exterior tiles, kitchen, and bathroom components, interior furnishings, plastic surfaces, aluminum siding, building stone, and curtains and window blinds. Photocatalytic activity can be used for H2 production, which can further be used in fuel cells. The possibility of reducing CO2 using sunlight in a similar manner to that occurring in plants, better known as artificial photosynthesis, is another important application of photocatalysis.

Section snippets

Structure and properties

Iron oxide is a transition metal oxide which has different stoichiometric and crystalline structures, including wüstite (FeO), hematite (α-Fe2O3), maghemite (ν-Fe2O3), and magnetite (Fe3O4). Out of all the phases, hematite (α-Fe2O3) is the most stable state of iron oxide at ambient conditions. The structure of α-Fe2O3 is a hexagonal crystal system consisting of iron atoms surrounded by six oxygen atoms. Hematite's strong absorption of yellow to UV photons in the visible region and transmission

Research on the photocatalytic property of α-Fe2O3

Having discussed some of the applications of hematite catalysts above, we focus here on three forms of hematite that have been studied. Work associated with hematite powders is addressed first, followed by studies related to modified and doped hematite. This section concludes with a discussion on thin solid hematite films.

Conclusions and future proposals

This paper summarizes the basic mechanism of the photocatalytic properties of oxide semiconductors. Among all the oxide semiconductors, α-Fe2O3 has the advantage of low band gap and using the visible light for its photocatalytic reactions. Also, its non-toxic nature makes it readily usable without significant safety concerns. Many attempts have been made to overcome the basic problems like short hole diffusion length and high recombination rate. Many authors have reported a good result

Acknowledgement

This work was supported by the 2014 Research Fund of University of Ulsan.

References (154)

  • M. Ni et al.

    Renew. Sustain. Energy Rev.

    (2007)
  • D.K. Bora et al.

    J. Electron Spectrosc.

    (2013)
  • S. Sun et al.

    J. Hazard. Mater.

    (2010)
  • I.M. Arabatzis et al.

    J. Catal.

    (2003)
  • R.S. Sonawane et al.

    J. Mol. Catal. A: Chem.

    (2006)
  • J. Mo et al.

    Atmos. Environ.

    (2009)
  • S. Pavasupree et al.

    Mater. Res. Bull.

    (2008)
  • U.I. Gaya et al.

    J. Photochem. Photobiol. C

    (2008)
  • L.A. Marusak et al.

    J. Phys. Chem. Solids

    (1980)
  • M.J. Katz et al.

    Coord. Chem. Rev.

    (2012)
  • C. Karunakaran et al.

    Electrochem. Commun.

    (2006)
  • A.L. Stroyuk et al.

    J. Photochem. Photobiol. A

    (2007)
  • S.N. Dang et al.

    J. Non-Cryst. Solids

    (2008)
  • M.A. Valenzuela et al.

    J. Photochem. Photobiol. A

    (2002)
  • G. Zhang et al.

    Mater. Res. Bull.

    (2012)
  • S.K. Maji et al.

    Polyhedron

    (2012)
  • C.T. Seip et al.

    Nanostruct. Mater.

    (1999)
  • Y. Xu et al.

    Mater. Lett.

    (2013)
  • J. Bandara et al.

    Appl. Catal. B

    (2007)
  • S. Cao et al.

    J. Phys. Chem. Solids

    (2010)
  • W. Du et al.

    Catal. Commun.

    (2009)
  • F.B. Li et al.

    J. Hazard. Mater.

    (2007)
  • D. Bi et al.

    J. Mol. Catal. A: Chem.

    (2013)
  • E. Subramanian et al.

    Int. J. Hydrogen Energy

    (2009)
  • W. Zhou et al.

    Mater. Sci. Eng. C

    (2009)
  • S. Guo et al.

    Carbon

    (2013)
  • L. Song et al.

    Colloids Surf. A: Physicochem. Eng. Aspects

    (2009)
  • R. Vinu et al.

    J. Indian Inst. Sci.

    (2010)
  • W.V. Atul et al.

    Res. J. Chem. Environ.

    (2013)
  • M. Barroso et al.

    PNAS

    (2012)
  • K. Maeda et al.

    J. Phys. Chem. C

    (2007)
  • J.J. Concepcion et al.

    PNAS

    (2012)
  • L. Lin et al.

    Open J. Inorg. Chem.

    (2013)
  • W. Wang et al.

    Aerosol Air Qual. Res.

    (2014)
  • Y. Kondo et al.

    Langmuir

    (2008)
  • R. Abe et al.

    Chem. Mater.

    (1997)
  • D.S. Bhatkhande et al.

    J. Chem. Technol. Biotechnol.

    (2001)
  • A. Kudo et al.

    Chem. Soc. Rev.

    (2009)
  • C.H. Liao et al.

    Catalysts

    (2012)
  • M.G. Walter et al.

    Chem. Rev.

    (2010)
  • X.L. Fang et al.

    J. Mater. Chem.

    (2009)
  • L.X. Chen et al.

    J. Phys. Chem. B

    (2002)
  • R.M. Cornell et al.

    The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses

    (2003)
  • A.I. Galuza et al.

    Low Temp. Phys.

    (1998)
  • J.H. Kennedy et al.

    J. Electrochem. Soc.

    (1978)
  • N.C. Debnath et al.

    J. Electrochem. Soc.

    (1982)
  • B. Wang et al.

    J. Am. Chem. Soc.

    (2011)
  • J. Chen et al.

    Adv. Mater.

    (2005)
  • Z. Sun et al.

    Adv. Mater.

    (2005)
  • X. Hu et al.

    Adv. Mater.

    (2007)
  • Cited by (0)

    View full text