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Iron based photoanodes for solar fuel production Cite this: Phys. Chem. Chem. Phys., 2014, 16, 11834
Prince Saurabh Bassi,a Gurudayal,a Lydia Helena Wong*a and James Barber*abc In natural photosynthesis, the water splitting reaction of photosystem II is the source of the electrons/ reducing equivalents for the reduction of carbon dioxide to carbohydrate while oxygen is formed as the by-product. Similarly, for artificial photosynthesis where the end product is a solar fuel such as hydrogen, a water splitting-oxygen evolving system is required to supply high energy electrons to drive the reductive reactions. Very attractive candidates for this purpose are iron based semiconductors which have band gaps corresponding to visible light and valence band energies sufficient to oxidise water. The
Received 9th December 2013, Accepted 21st January 2014 DOI: 10.1039/c3cp55174a
most studied system is hematite (Fe2O3) which is highly abundant with many attributes for incorporation into photoelectrochemical (PEC) cells. We review the recent progress in manipulating hematite for this purpose through nanostructuring, doping and surface modifications. We also consider several hybrid iron-based semiconducting systems like ferrites and iron titanates as alternatives to hematite for light driven water splitting emphasizing their advantages with respect to their band levels and charge
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transport properties.
1. Introduction The continuous increasing demand for energy coupled with the environmental impact of atmospheric CO2 produced by the combustion of fossil fuels, has motivated the urgent need to explore and develop clean energy sources. Currently, the rate of energy consumption by the global population, averaged over a year, is around 16 Terawatts while the sun provides energy at an average annual rate of 120 000 Terawatts at the Earth’s surface. Even though solar energy is available far in excess of the needs of humankind, the challenge is to convert and store solar energy by efficient, cost effective and convenient methods on a large scale. In natural photosynthesis, solar energy is captured and stored in the chemical bonds of organic molecules which then serves as food and fuel, and is the origin of fossil fuels. Given the desire to move away from relying on fossil fuels and to reduce CO2 emissions, there is an increasing interest to develop technologies which can mimic photosynthesis and produce solar fuels, the simplest being H2 derived from light driven water splitting. We will start this article by giving a very brief overview of the energy conversion process in natural photosynthesis and then consider artificial photosynthesis. For the
a
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. E-mail: [email protected] b Applied Science and Technology Department – BioSolar Lab, Politecnico di Torino, Viale T. Michel 5, 15121 Alessandria, Italy c Department of Life Sciences, Imperial College London, London, SW7 2AZ, UK. E-mail: [email protected]
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latter, we will focus on semiconductor materials with particular emphasis on iron-based semiconductors.
2. Natural photosynthesis Natural photosynthesis occurs through a series of light driven electron transfer processes to produce sufficient energy to convert CO2 to carbohydrates using reducing equivalents (high energy electrons) derived from water splitting.1,2 There are two photosystems involved, photosystem I (PSI) and photosystem II (PSII).1,2 PS I and PS II absorb light through an assembly of light harvesting chlorophylls and carotenoids, and excite electrons to higher energy states inside a reaction center.1,2 These photosystems are connected in series by an electron transfer chain arranged as a Z-scheme.1,2 The main step of natural photosynthesis involves the absorption of sunlight and creation of electron–hole pairs.1,2 The photogenerated holes in PSII are used by the oxygen evolving complex (OEC) to oxidize water to dioxygen3 aided by manganese–calcium oxide cluster located in the PSII complex.4,5 In contrast, PSI is not involved directly in the water oxidizing chemistry but acts to supply extra energy to the reducing equivalents produced by PSII so that they possess sufficient redox potential to reduce CO2. This requirement for two photons to drive one electron from water to CO2 comes about because the energy gap between water oxidation and CO2 reduction is sufficiently large that one photon of visible light is energetically only marginally sufficient and takes no account for the need for overpotentials. This is particularly true for the
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red end of the spectrum where photosynthetic organisms utilize photons. Other important features of natural photosynthesis are extensive light harvesting systems per reaction center and the arrangements of electron carriers to minimize recombination reactions.1,2 These features underline the high quantum efficiency of the primary reactions of photosynthesis and provide a guide for the design and development of artificial photosynthetic systems.
3. Artificial photosynthesis The water oxidation reaction is fundamentally important for the development of any type of artificial photosynthetic system. It is a thermodynamically and kinetically difficult (4 electron process) reaction, which requires a minimum energy corresponding to a potential of 1.23 V vs. Normal Hydrogen Electrode (NHE) at pH 0.6 However, this multi-step oxygen evolution reaction involves various energy intermediates, which requires a significant overpotential.6–8 Just as in natural photosynthesis, artificial photosynthesis requires a light driven water splitting catalyst, or photoelectrode, to provide high energy electrons which are required to reduce protons to hydrogen or even reduce CO2 to carbon based high energy molecules. These cathodic reduction reactions can also be facilitated by suitable catalysts.8 One attractive approach to replicate a photosynthetic light harvesting system coupled to a reaction center is to employ semiconductors for absorbing light energy and generating charge separation. Here, photon absorption leads to the formation of charge carriers as electrons in the conduction band and positively charged holes in the valence band of the semiconductor. For solar hydrogen generation, the water oxidation is driven by photogenerated holes located in the valence band while the energized electrons in the conduction band reduce protons to hydrogen as represented by the following;9–11 2H2O + light - 4H + O2 + 4e
(1a)
4H + 4e - 2H2
(1b)
+
+
Thus, photocatalytic water splitting involves four steps: (1) absorption of photons, (2) creation of electron–hole pairs, (3) separation and stabilization of electron–hole pairs, (4) utilization of the charged separated states for hydrogen and oxygen production.8,12 A single material which can carry out all these steps at high efficiency is the holy grail of this field.11 In fact, it has been a challenge to find a material and a system as efficient as natural photosynthesis. Such photocatalytic systems must have a good stability in aqueous solution, appropriate band energies with respect to required redox levels, have high light absorption in visible range, good thermodynamic and chemical stability, good conductivity, be robust, abundant and ideally non-toxic. Among the semiconductor metal oxides, TiO2 has attracted much attention owing to its band levels, but due to its high band gap and its inefficient absorption of visible light, its application for solar energy capture is limited.13 To tackle this problem dyes are frequently used to coat TiO2 in devices like
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dye sensitized solar cells (DSSC).14 The electricity generated by DSSC could be used for water splitting and hydrogen production via electrolysis.14 But there are drawbacks associated with using a two reactor system which would have non-affordable significant losses of energy. Another inexpensive stable oxide, WO3 has been considered as a photocatalyst for DSSC. Recently, Yong et al.15 presented one dimensional WO3 nanorods based DSSC device with an efficiency of around 1.51% which is higher than WO3 nanoparticles based DSSC reported earlier. But it should be noted that WO3 needs much improvement since its efficiency is still lower than TiO2 based DSSC devices. There have also been major developments in preparation of oxynitride materials which have an optimum band gap and can undergo both water oxidation and proton reduction reactions but with relatively low quantum efficiencies of B5–6%.16 In many such types of photocatalytic systems, charge recombination can be very high which limits the photon-toelectron quantum yield. Hence, photoelectrochemical (PEC) cells with semiconductor–electrolyte junctions or semiconductor homojunctions–heterojunctions are more efficient due to presence of electric field or band levels alignment which improves charge separation and hence limits the probability of recombination.10 Various photocatalytic and other hybrid material systems have been extensively studied for water splitting over the past decades and many good review papers are available.8,13,16–23 Of all the candidates studied as a photoanode in PEC, the most attractive and well researched is Fe2O3 (hematite).22–25 In the following sections therefore, we will highlight the contribution of hematite and other promising iron-based semiconductors for the light driven oxidation of water.
4. Hematite as a photoanode Hematite is an iron ore which is abundant in the Earth’s crust. It is a low cost, non-toxic and stable material in aqueous solutions with a band gap energy which makes it a promising photocatalyst for solar water splitting.25 Fe2O3 is an n-type semiconductor with a crystal structure similar to that of corundum, Al2O3, which is trigonal (hexagonal scalenohedral, 3% 2/m) with space group R3c with lattice parameters a = 5.0356 Å, c = 13.7489 Å, and six molecules per unit cell.25 It has a band gap of 2.2 eV and has conduction and valence band energy levels as shown in Fig. 1. It can be seen that although the valence band is very favourable for oxidizing water, the conduction band is not favourable for hydrogen evolution. Hence, an external bias is needed to overcome this potential (B0.4 V) for both hydrogen and oxygen evolution. Unassisted water splitting without external bias has been reported by surface modification of Ti-doped hematite with CoF3 even though the Incident Photon-to-Current efficiency (IPCE) of this system is still too low for practical application.26 Tandem cells of hematite and an ideal photocathode with an appropriate onset potential is one possible solution to improve the IPCE.
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To increase the plateau photocurrent, advancements in nanostructuring and doping is required, while to lower the onset potential, the surface properties of semiconducting active layer needs to be improved. To improve the light absorption and photon-to-electron conversion efficiency, different approaches in nanostructuring and doping have been employed, which will be discussed in detail in the following section. Another limitation of hematite is the sluggish hole transport from semiconductor surface to the electrolyte which increases the required overpotential. Strategies to reduce the overpotential by surface passivation with metal oxide and surface treatment with co-catalysts will also be reviewed below. 4.1
Fig. 1 Schematic representation of band levels for hematite and for an ideal photocatalyst for water oxidation to hydrogen and oxygen.
Most of the reported works on hematite used it as a working photoanode in PEC systems with an applied external bias generally comprising of a counter electrode, Pt and reference electrode, Ag/AgCl immersed in an electrolyte at basic pH. A typical current–voltage (I–V) curve for untreated hematite would have a relatively high onset potential and low current (black curve of Fig. 2). On the other hand, a highly efficient hematite-based PEC device should have a high plateau photocurrent and low onset potential (red curve). The emphasis of research on hematite as a photoanode for water splitting is to close the gap between the two extremes and emulate the red curve as much as possible. To do this various treatments have been explored and reported as emphasized in Fig. 2 and discussed below.
Fig. 2 cell.
Typical (black) and ideal (red) I–V curve of a hematite based PEC
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Nanostructuring
The need for nanostructuring of hematite arises because of the trade-off between the relatively low absorption coefficient which requires 400–500 nm thick films for complete light absorption and the short hole diffusion length (2–4 nm) which requires 5–10 nm film for efficient hole transport to the electrolyte. The ideal nanostructure to fulfil these criteria is therefore a nanowire array with diameter of 5–10 nm and length of 400–500 nm as shown schematically in Fig. 2. It is worth mentioning that the interest in hematite was renewed after the discovery of the effectiveness of cauliflower-like ¨tzel’s group.27 Since then, a significant nanostructures by Gra number of efforts in nanostructuring have been reported and well documented in the review paper by Sivula et al.25 In this section, we will give a brief update on the recent developments in hematite nanostructuring for PEC applications. An important development in producing hematite on large area substrates has been the introduction of Fe2O3 nanorods prepared by an inexpensive solution based hydrothermal technique.28,29 The perpendicular alignment of 1-D arrays of nanorods on fluorine doped tin oxide (FTO) substrates in [110] direction is beneficial as the anisotropic conductivity along this direction is four fold higher than in the orthogonal direction which enhances the electron transport and hence the photocurrent.27 Other reports of hematite on large area substrates include plasma/thermal oxidation of Fe foils to hematite30,31 and the facile solution synthesis of a-FeF33H2O nanowires and their conversion to a-Fe2O3 nanowires.32 Another important strategy in nanostructuring is the enhancement of hematite nanorods or nanowire surface area by introducing branched/dendritic structures. Ideally, the enhanced surface area should increase the rate of photogenerated holes to the electrolyte. Zheng et al. reported dendritic Fe2O3 nanowire arrays formed by thermal oxidation of the electrodeposited dendritic Fe nanowires.33 Although the dendritic structure had a larger surface area than the nanowires, it showed lower photocurrent than the other reported nanowires and nanotubes which may be because of higher electron–hole recombination probability or the presence of higher amount of surface defects. These results emphasize the trade-off associated with increasing surface area, i.e. on one hand increasing the probability of charge separation while on the other hand increasing the density of surface defects which increases the
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surface recombination probability. Yang et al.34 demonstrated a unique hematite photoanode which is composed of b-FeOOH nano-branches on a Ti-doped hematite nanorods grown on an undoped hematite underlayer by hydrothermal method. The nanorods structure and the underlayer enhanced the charge separation and improved the onset potential. After the introduction of b-FeOOH nano-branches, the photocurrent further improved but the onset potential shifted positively due to increased flat band potential as confirmed by impedance spectroscopy. Deng et al. has reported Ti-doped hematite nanostructures with an urchin-like morphology, which enhanced the effective surface area compared to untreated nanostructures.35 With this system, a remarkable plateau photocurrent density value of 3.76 mA cm 2 was observed at 1.74 V vs. Reversible Hydrogen Electrode (RHE) under 1 Sun AM 1.5G condition in 1 M NaOH electrolyte, which was 2.5 times higher than that for the pristine nanostructures (1.48 mA cm 2). Finally, most recently, Kim et al. presented a worm-like morphology of hematite doped with Pt which resulted in photocurrent density 4.32 mA cm 2 at 1.23 V vs. RHE.36 More about this work will be reviewed in Section 4.2 below.
4.2
Doping
The process of doping is well established and has been proven to be beneficial to many semiconductor systems for enhancing the bulk carrier concentration and hence the conductivity. Pure hematite has very low electrical conductivity (ca. 10 14 O 1 cm 1),37 carrier concentration (1018 cm 3) and electron mobility (10 2 cm2 V 1 s 1).38 Since the conductivity of a material is given by s = neme + pemp, increasing carrier concentration (i.e. n or p) by doping is often used to compensate for the low electron (me) and hole (mp) mobilities. Hematite is a n-type semiconductor due to naturally occurring oxygen vacancies in the bulk. When divalent metal dopants, such as Mg, Cu and Ni, replace Fe3+ in the lattice, hole carriers are formed and hematite becomes p-type.7,39,40 When tetravalent metal dopants, such as Sn, Ti, Zr and Si, replace Fe3+, extra electrons will be generated and hematite will remain as n-type with increased electron concentration.27,35,41–43 Ling et al. showed the dependence of high temperature annealing on the photocurrent for hematite nanowires.41 The improvement in photoactivity was due to the presence of Sn leading to an increase in carrier concentration from 1.89 1019 to 5.38 1019 cm 3 for high temperature annealing of 650 and 800 1C respectively. The Sn4+ ions diffused into the Fe2O3 nanostructure from the FTO substrate during high temperature annealing which acted as electron donor and increased the ¨tzel et al. reported Si-doped carrier density of hematite. Gra hematite cauliflower-like nanostructures with highly rough surface and a feature size of 10–20 nm which showed appreciable photocurrent (2.2 mA cm 2 @ 1.23 V versus RHE). The enhanced performance was attributed to the increased donor density of Si-doped Fe2O3, which was found to be 1.7 1020 cm 3 by Mott–Schottky analysis. It should be noted that
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in this specific case, Si is also instrumental in creating the cauliflower nanostructure with small feature size.27 A theoretical study on Cu and Ti doped hematite was reported by Meng et al.40 They showed by using density function theory (DFT), that the conduction band minima (CBM) of Cu doped hematite was sufficient to facilitate hydrogen production which is not the case for untreated hematite as emphasized in Fig. 1. Thus, the energy levels of the valence and conduction bands would be able to drive both photoinduced oxygen and hydrogen production from water without the application of a voltage bias. In the case of Ti doping, their DFT calculations indicated an improvement in electronic conductivity and a slight change in band gap due to creating shallow donor levels which encourages the Fermi level to shift towards the conduction band of hematite.40 Ti doped hematite has been investigated experimentally by Shen et al. in which photocurrent after Ti doping increased 14 times compared with that of pristine hematite.44 This and similar other works35,45,46 strengthen the importance of Ti doping which seems to enhance the charge carrier density in hematite and increases its performance. Recently, a photoanode consisting of hematite nanorods doped with Pt and surface treated with Co–Pi, yielded a recordbreaking photocurrent of 4.32 mA cm 2 at 1.23 V vs. RHE under simulated 1-sun (100 mW cm 2).36 The hematite nanorods had a unique ‘‘wormlike’’ morphology formed by two steps annealing at 550 1C and 800 1C. Platinum doping was used to improve the charge transfer characteristics in the bulk of hematite while the Co–Pi acted as an oxygen evolution cocatalyst on the surface. The Pt doping resulted in an improvement of the electrical conductivity of hematite by increasing its donor density from 3.27 1017 cm 3 for untreated Fe2O3 to 2.77 1018 cm 3 and 3.91 1018 cm 3 for Pt-doped Fe2O3, and Pt-doped Fe2O3/Co–Pi, respectively.36 Recently, in our laboratory, we have developed a solution based method to synthesize Mn doped hematite nanorods with improved water oxidation activity compared to non-treated hematite nanorods.47 We found that 5 mol% Mn precursor doping was optimum to enhance the photocurrent densities by 3 times at 1.23 V vs. RHE and also in reducing the onset potential by 30 mV compared to the pristine hematite nanorods (Fig. 3a). The enhancement in current density was attributed to increase in the donor densities as estimated by Mott–Schottky analysis (Fig. 3b). Chronoamperometry studies also show that there is a decrease in transient currents for doped Mn samples compared to pristine hematite, indicating the beneficial effects of Mn doping which seems to be due to a suppression in electron–hole recombination and a reduction in the energy barrier for hole transport. This study confirms that Mn doping enhances the charge carriers and reduces the recombination losses and emphasizes that this treatment results in better catalytic performance of Fe2O3 nanostructured photoanodes. The enhancement after Mn doping is possibly due to the multivalent oxidation states of manganese which leads to a low (O–Mn–O) energy barrier for hole transport. Another kind of doping strategy is the creation of oxygen vacancies which act as shallow donors and increases the donor
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Fig. 3 (a) Photocurrent–potential curve of pristine hematite nanorods and 5% Mn doped hematite nanorods measured under AM 1.5G 100 mW cm 2 in 1 M NaOH electrolyte solution, (b) Mott Schottky plots of pristine (inset) and 5% Mn incorporated hematite nanorods extracted from electrochemical impedance data measured at 1 KHz in the dark. Calculated donor densities (Nd) and flat band potential (Vo) are shown in the figure.
density. For example, the incorporation of oxygen vacancies in hematite has been reported recently by Ling et al.48 They produced hematite nanowires with increased oxygen vacancy concentration which showed an enhanced photoactivity of water oxidation. This was attributed to an increase in the donor density as supported by Mott–Schottky analysis. The study was also the first demonstration of highly photoactive hematite nanowires at relatively low post-annealing temperature (550 1C) without the addition of other elemental doping. Subsequently, a systematic study to decouple the interaction between oxygen vacancies and extrinsic Sn dopants was performed by Yang et al.49 They demonstrated that both oxygen vacancy and dopants complement each other to enhance the electrical conductance and in turn increase the photocurrent and reduce the onset potential. Recently, Zandi et al. reported a different role of Ti doping in enhancing the photoactivity of hematite.50 Instead of the conventional role of increasing charge carrier density, Ti was suspected to increase the efficiency of hole transport properties across the hematite/electrolyte interface. Even though some preliminary data was presented, further experimental evidence is still needed to support this suggestion. To summarize, both the nanostructuring and the doping strategies have been shown to be beneficial in altering the electronic properties of pristine hematite which in turn enhancing the plateau photocurrent of PEC cells. 4.3
Surface treatment
Another important drawback of untreated hematite is the requirement for a bias to overcome the overpotential necessary for water splitting. This is attributed to: (1) the position of the conduction band of hematite, which is 0.4–0.5 V too positive to allow the reduction of protons to hydrogen (Fig. 1) and; (2) the sluggish hole transport across the semiconductor/ electrolyte interface The sluggish hole transport can be improved by surface treatment by another metal oxide which either causes surface passivation or act as co-catalysts. These two phenomena have fundamentally different mechanisms in assisting hole transport.
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Surface passivation implies the reduction of hematite surface defects which reduces the hole–electron recombination rate, while co-catalyst implies the oxidation of the metallic element of the co-catalyst which assist hole transport from hematite surface to the electrolyte. It should be noted however, that for a few types of surface treatments using Co- or Ir-based materials, more conclusive experimental evidence which can clearly determine whether they act as surface passivation layer or as co-catalyst is still needed. A few surface passivation strategies such as CoF3,26 Al2O3,51 Ga2O3,52 SnO253 and ZnO54 have been reported so far. Hu et al. showed a favourable flat band shift in Ti-doped iron oxide thin films after surface modification with CoF3 aqueous solution.26 The photogenerated electrons, due to this shift, were able to reduce protons and generate hydrogen without any applied bias, emphasising the importance of surface modification in shifting band level positioning. Formal et al. were able to reduce the overpotential by 100 mV by using an ultra-thin coating of Al2O3 deposited by Atomic Layer Deposition (ALD) on to hematite photoanodes.51 This alumina overlayer enhanced photocurrent to a value of 0.85 mA cm 2 at 1.0 V vs. RHE as compared to 0.24 mA cm 2 for untreated hematite. Through electrochemical studies and optical spectroscopy, the role of the alumina was confirmed as a surface passivating agent. Another report by Hisatomi et al. on surface passivation of hematite photoanodes highlights the use of corundum-type overlayers which decreases the lattice strain and hence the density of surface states. They showed a negative shift of 200 mV in overpotential using Al2O3 and Ga2O3 overlayers.52 In our laboratory, we found that ZnAc treatment of spray pyrolyzed hematite mesoporous thin films resulted in an enhanced PEC performance. Here, a very thin ZnO overlayer was deposited on surface of a thin film of hematite resulting in an increased photocurrent accompanied by reduction in onset potential as compared to untreated hematite. After 3 cycles of ZnAc treatment, the photocurrent increased more than 40% to 1.08 mA cm 2 at 0.23 V vs. Ag/AgCl and onset potential for water oxidation shifted by 170 mV (Fig. 4a). Based on impedance spectroscopy and ideality factor measurements (Fig. 4b and c) it was proposed that the ZnO overlayer changes the flat band potential of hematite and reduces the recombination reactions caused by surface defects.54 Various oxygen evolution catalysts like Co–Pi,55,56 IrO2,57 cobalt oxides27,58 have also been used with hematite photoanodes because of the fast kinetics they exhibit in charge transfer for water oxidation. A catalyst present on the surface can act as a mediator to ease hole transfer from the interface owing to a reduced kinetic barrier and its ability to oxidize water and decreases the overpotential. Indeed, Tilley et al. showed that the surface modification of nanostructured hematite photoanode by IrO2 nanoparticles resulted in an overpotential shift of 0.2 V and a photocurrent as high as 3.75 mA cm 2.57 Being one of the best co-catalyst in terms of reducing the overpotential, IrO2 nanoparticles can transfer photogenerated holes to a lower overpotential and thus a higher photocurrent is achieved due to
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Fig. 4 (a) Photocurrent–potential curves before and after different cycles of ZnAc treatment of hematite mesoporous thin films (b) Mott–Schottky plots of hematite films in a 1 M NaOH electrolyte in the dark (c) local ideality factor extracted from the dark I–V curves (inset) before and after surface treatment.
less charge carrier recombination. Even though IrO2 is one of the best water oxidation catalysts, it consists of an expensive non-abundant element. In this context, recent developments have been made on earth abundant Co and Mn based nanoclusters which could achieve the target of high turnover frequency (TOF) and are stable inorganic based oxide catalysts which could be further used on a large scale. It would be interesting to integrate these nanoclusters with nanostructured photoanodes to design a durable water oxidation system.59 Nocera et al. reported the oxygen evolving catalyst, Co–Pi, which can be deposited and coupled with a photoanode using electrodeposition.60 Since its discovery it has gained a lot of interest because of a low overpotential of only 0.41 V is required to oxidize water at pH = 7 with a current density of 1 mA cm 2. Zhong et al. reported 0.35 V cathodic shift in overpotential for Co–Pi–a-Fe2O3 composite photoanodes.55 McDonald et al. photochemically deposited Co–Pi OEC on electrodeposited iron oxide films and demonstrated an enhancement in photocurrent as well as in O2 generation which implied an improvement in O2 evolution kinetics.61 ¨tzel et al. performed surface treatment of cauliflower Gra nanostructured hematite with 10 mM Co(NO3)2 solution as a precursor of cobalt oxide.27 The cobalt treatment resulted in an 80 mV cathodic shift in onset potential, and a slightly higher photocurrent 2.7 mA cm 2 at 1.23 V versus RHE compared with pristine hematite (2.2 mA cm 2 at 1.23 V versus RHE). The explanation for the observed enhancement of the cobaltmodified hematite was that it involved surface holes trapped by Co(II) sites as Co(III), which in turn could be oxidized by a second hole to Co(IV) which then drives the water oxidation reaction. This explanation suggests that cobalt oxide behaves as a cocatalyst on the surface of hematite but alternative explanations have been explored. Recently, for example, Peter et al. calculated the photogenerated charge transfer and recombination constants of pristine hematite and Co(II) oxide treated hematite thin film by intensity modulated photocurrent spectroscopy (IMPS).62 They showed that the charge transfer rate of Co(II) treated hematite was similar to the untreated hematite, indicating that the photocurrent improvement was
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not due to catalytic behaviour of Co. On the other hand, the recombination rate constant was found to decrease for treated hematite as compared to the untreated control. This seems to suggests that the improvement in photocurrent in Co(II) treated hematite electrodes was due to suppression of surface recombination and not due to the catalysis of hole transfer (i.e. an increase in charge transfer rate) as suggested previously.27 There have also been other developments to understand the role of surface passivation layers or cocatalysts through transient spectroscopic measurements. Ultrafast studies for Fe2O3 phase was earliest reported by Cherepy et al. which found the short charge carrier lifetime of 1 ps for hematite nanoparticles.63 Subsequently, Joly et al. probed carrier dynamics of a-Fe2O3 thin films by femtosecond transient absorption spectroscopy and found the electrons recombining with holes/traps in order of 3 ps and claimed carrier trapping by midgap Fe d–d states to be the dominant trapping mechanism.64 Since carrier dynamics is very important in fundamental understanding of intrinsic carrier trapping and charge transport properties to design more efficient solar harvesting devices, spectroscopic studies have been pursued in the past few years. Huang et al. reported in situ Transient Absorption (TA) studies for hematite thin film electrodes which concluded that the ultrafast electron–hole recombination is the limiting factor for efficient water oxidation and the minimum hole lifetime for hematite to be catalytically active is 6 ms.65 Pendlebury et al. presented a correlation between photocurrent and the population of higher lifetime holes under external bias.66 Through transient photocurrent (TPC) and TA measurements, it was shown that the long-lived holes drive the single hole oxidation under bias as opposed to four-hole oxidation process. To understand the role of Co–Pi in the enhancement of photocurrent for hematite, TA measurements were performed by Barroso et al. on a-Fe2O3–CoOx nanocomposite photo electrodes.56 It was proposed that the reduction in recombination losses improved the photoelectrochemical activity. Another report by the same group have shown the reduction in electron–hole recombination is responsible for cathodic onset potential shift for surface treated hematite with overlayer (Ga2O3) and cocatalyst (CoOx).58
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Fig. 5 (a) Photocurrent potential curve for hematite nanorods for in situ added different amount of Co2+ (b) SEM image of hematite nanorods annealed at 550 1C for 2 h. (Inset-top: cross-section view of hematite nanorods on FTO substrate; bottom: TEM image of in situ Co2+ added hematite nanorods.)
Since the application of Co catalysts to the surface of hematite has been shown to significantly reduce the over potential for water oxidation and improve the passivation of defects, we recently investigated the role of Co by integrating Co3O4 using in situ and ex situ methods during hydrothermal growth of hematite nanorod arrays.29 It was found that the photocurrent density increases from 0.72 mA cm 2 for the pristine Fe2O3 nanorod to 1.20 mA cm 2 at 1.23 V versus RHE (i.e., 67% improvement) with 5 mol% Co2+ treatment (Fig. 5a). Concomitant with this change was a shift in the onset potential by B40 mV and improvements in the IPCE and oxygen evolution. Hematite photoanodes with in situ deposition of Co3O4 nanoparticles showed better performance than those prepared by ex situ procedures because of high surface roughness, larger Co3O4/hematite interfacial area, and smaller Co3O4 particle size (Fig. 5b).
5. Other iron based semiconductors As diagrammatically shown in Fig. 1, the challenge is to strive for a semiconductor material which has band levels straddling the water oxidation/proton reduction redox levels and have good charge transport properties to inhibit charge carriers recombination. Despite the many attractive features of treated and nano-structured hematite as a photoanode for water oxidation, it is important to explore different classes of iron-based materials which possess more favourable intrinsic properties than hematite. Theoretically, this improvement could be achieved by changing the band structures of the material by solid solution or composites of two or more systems. Experimentally, hybrid structures have been developed but have had little impact to date owing to the difficulty in obtaining pure phases and making porous electrodes for enhanced photoactive behaviour. Recent report of combinatorial methods like inkjet printing of array of multicomponent metal oxides as oxygen evolution catalysts is a promising tool to speed up the process of discovering efficient photocatalysts utilizing the properties of different metals in the formulation of hybrid systems.67 Iron based ferrites are interesting class of materials to study as an alternative or for integration with hematite. Boumaza et al.
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considered ZnFe2O4–SrTiO3 system for hydrogen production as ZnFe2O4 has a conduction band higher than water reduction level.68 Utilizing such favourable conduction band energy for hydrogen evolution, McDonald et al. integrated ZnFe2O4 with Fe2O3 as a photoanode.69 It was shown that ZnFe2O4 has a band gap of around 2 eV and stable in aqueous solutions. Its conduction and valence band edges are shifted in negative direction by ca. 200 mV as compared to hematite resulting in better photocurrent response in Fe2O3–ZnFe2O4 composite system as compared to pure hematite system due to efficient electron–holes separation. A similar approach has been reported by Miao et al.70 where heterojunction of Ti-doped Fe2O3 and ZnFe2O4 has been shown to enhance photoelectrochemical performance owing to better charge separation. Recently, Rekhila et al. reported that NiFe2O4 can be used as a photocatalyst for visible light driven hydrogen production in the presence of a sacrificial electron donor.71 The physical characterization of the material showed the p-type behaviour with a band gap of 1.56 eV which suggests that it can be integrated with hematite as a p–n junction in future. Another group of materials, the titanates, have also been shown to possess promising qualities for water splitting catalysis.72,73 Kudo et al. has described a number of diverse heterogeneous photocatalysts, including both titanates and tantalates, for solar water splitting.8 They showed how the solid solution of two materials with not so favourable band levels can be converted to efficient photocatalytic water splitting materials. A similar idea was proposed by Thimsen et al. where the conditions to prepare mixed transition metal oxides with optimum band gaps was presented.74 Even though they focussed on Fe–Ti–O system, it was claimed that it could be extrapolated to Ti–V–O or Ti–Ni–O systems as well. Stoichiometric changes in TiO2 through incorporation of Fe were shown to effectively tune band gaps. They showed theoretically and experimentally how iron titanates have better band gap for visible light excitation as compared to TiO2. It was claimed that with valence band level already being favourable for water oxidation, the shift in conduction band level towards hydrogen generation would in turn result in the reduction of the overpotential. Ginley et al. synthesized crystals for different iron titanates FeTiO3, Fe2TiO4 and Fe2TiO5 to be used as anodes for photoelectrolysis of water.75 Among these, FeTiO3 with a band gap of 2.5–2.9 eV has been studied mainly with TiO2 for photocatalytic applications.76 Kim et al. reported nanodiscs of FeTiO3 prepared by hydrothermal treatment which were then exposed to TiO2 P25 nanoparticles so as to integrate them.77 They reported FeTiO3–TiO2 as water oxidizing visible light photocatalyst where FeTiO3 acts as the light absorber and TiO2 as electron transporter. Similar work has been conducted where FeTiO3 is utilized as an absorption enhancing layer on TiO2 for photochemical processes where it is also shown that valence band matching between them increases the hole transport across the semiconductor/electrolyte interface and hence enhances the photocatalytic activity.76,78 It is the theoretical and experimental studies by diverse groups in tuning the band gap through solid solutions of two
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oxides which has recently motivated us to work on Fe–Ti–O system. We have investigated FeTiO3 as a photoanode for solar driven water oxidation. But owing to the band gap of FeTiO3 (B2.5–2.9 eV), using it by itself as a photoanode results in an incomplete light absorption in the visible range. Also, Fe2+ oxidation state present in the material was difficult to process as it may result in mixed phases with Fe3+ ions after high temperature annealing. Due to these difficulties we switched to a more stable phase Fe2TiO5 which is a hybrid of Fe2O3 and TiO2. With a favourable band gap of around 2.0 eV, it has a good solar spectrum absorbing qualities and is also stable thermodynamically as well as in aqueous solutions.79–81 Being a titanate, it is likely that its conduction band and valence band levels would be as favourably for water splitting as compared to hematite.11,81 The idea behind using hybrid of Fe2O3 and TiO2 is to utilize good charge transport properties of TiO2 and the better band gap of Fe2O3 for absorption of solar spectrum to be used as a photoanode in PEC. We have prepared Fe2TiO5 nanoporous thin films by solvothermal technique on FTO substrates and have investigated its potential as a photocatalyst for overall water splitting. It is worth noting that a lower bias was needed for Fe2TiO5 to produce photo-current as compared to Fe2O3 which makes it an interesting alternative system (unpublished data).
6. Conclusions Iron based semiconductors are attractive for developing cheap and robust photoanodes for PEC cells to split water and generate hydrogen as a solar fuel. Some of their disadvantages like non-optimal conduction band levels, high hole–electron recombination probability, incomplete solar light absorption, poor hole diffusion length and low catalytic kinetics at the surface can be partially overcome by various approaches. Some of the approaches like nanostructuring, doping and treatments with surface passivation overlayers or co-catalysts have been addressed in this review. It is evident that significant improvements have taken place in past decade but there is still much scope to explore these systems completely in terms of their internal charge dynamics. Several hybrid iron-based systems like ferrites and iron titanates are also emerging as alternatives to hematite having significantly advantageous with respect to their band levels and charge transport properties. Hence, we can expect to see new and striking developments emerging in the future in the area of hybrid systems. We believe that these inorganic semiconducting catalysts are more likely to be used in a functional and realistic artificial photosynthesis technology than organic based systems due to their cheapness, robustness, light absorbing and charge separation properties.
Acknowledgements We would like to acknowledge financial supports from the Centre of Artificial Photosynthesis, NTU.
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Iron based photoanodes for solar fuel production Cite this: Phys. Chem. Chem. Phys., 2014, 16, 11834
Prince Saurabh Bassi,a Gurudayal,a Lydia Helena Wong*a and James Barber*abc In natural photosynthesis, the water splitting reaction of photosystem II is the source of the electrons/ reducing equivalents for the reduction of carbon dioxide to carbohydrate while oxygen is formed as the by-product. Similarly, for artificial photosynthesis where the end product is a solar fuel such as hydrogen, a water splitting-oxygen evolving system is required to supply high energy electrons to drive the reductive reactions. Very attractive candidates for this purpose are iron based semiconductors which have band gaps corresponding to visible light and valence band energies sufficient to oxidise water. The
Received 9th December 2013, Accepted 21st January 2014 DOI: 10.1039/c3cp55174a
most studied system is hematite (Fe2O3) which is highly abundant with many attributes for incorporation into photoelectrochemical (PEC) cells. We review the recent progress in manipulating hematite for this purpose through nanostructuring, doping and surface modifications. We also consider several hybrid iron-based semiconducting systems like ferrites and iron titanates as alternatives to hematite for light driven water splitting emphasizing their advantages with respect to their band levels and charge
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transport properties.
1. Introduction The continuous increasing demand for energy coupled with the environmental impact of atmospheric CO2 produced by the combustion of fossil fuels, has motivated the urgent need to explore and develop clean energy sources. Currently, the rate of energy consumption by the global population, averaged over a year, is around 16 Terawatts while the sun provides energy at an average annual rate of 120 000 Terawatts at the Earth’s surface. Even though solar energy is available far in excess of the needs of humankind, the challenge is to convert and store solar energy by efficient, cost effective and convenient methods on a large scale. In natural photosynthesis, solar energy is captured and stored in the chemical bonds of organic molecules which then serves as food and fuel, and is the origin of fossil fuels. Given the desire to move away from relying on fossil fuels and to reduce CO2 emissions, there is an increasing interest to develop technologies which can mimic photosynthesis and produce solar fuels, the simplest being H2 derived from light driven water splitting. We will start this article by giving a very brief overview of the energy conversion process in natural photosynthesis and then consider artificial photosynthesis. For the
a
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. E-mail: [email protected] b Applied Science and Technology Department – BioSolar Lab, Politecnico di Torino, Viale T. Michel 5, 15121 Alessandria, Italy c Department of Life Sciences, Imperial College London, London, SW7 2AZ, UK. E-mail: [email protected]
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latter, we will focus on semiconductor materials with particular emphasis on iron-based semiconductors.
2. Natural photosynthesis Natural photosynthesis occurs through a series of light driven electron transfer processes to produce sufficient energy to convert CO2 to carbohydrates using reducing equivalents (high energy electrons) derived from water splitting.1,2 There are two photosystems involved, photosystem I (PSI) and photosystem II (PSII).1,2 PS I and PS II absorb light through an assembly of light harvesting chlorophylls and carotenoids, and excite electrons to higher energy states inside a reaction center.1,2 These photosystems are connected in series by an electron transfer chain arranged as a Z-scheme.1,2 The main step of natural photosynthesis involves the absorption of sunlight and creation of electron–hole pairs.1,2 The photogenerated holes in PSII are used by the oxygen evolving complex (OEC) to oxidize water to dioxygen3 aided by manganese–calcium oxide cluster located in the PSII complex.4,5 In contrast, PSI is not involved directly in the water oxidizing chemistry but acts to supply extra energy to the reducing equivalents produced by PSII so that they possess sufficient redox potential to reduce CO2. This requirement for two photons to drive one electron from water to CO2 comes about because the energy gap between water oxidation and CO2 reduction is sufficiently large that one photon of visible light is energetically only marginally sufficient and takes no account for the need for overpotentials. This is particularly true for the
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red end of the spectrum where photosynthetic organisms utilize photons. Other important features of natural photosynthesis are extensive light harvesting systems per reaction center and the arrangements of electron carriers to minimize recombination reactions.1,2 These features underline the high quantum efficiency of the primary reactions of photosynthesis and provide a guide for the design and development of artificial photosynthetic systems.
3. Artificial photosynthesis The water oxidation reaction is fundamentally important for the development of any type of artificial photosynthetic system. It is a thermodynamically and kinetically difficult (4 electron process) reaction, which requires a minimum energy corresponding to a potential of 1.23 V vs. Normal Hydrogen Electrode (NHE) at pH 0.6 However, this multi-step oxygen evolution reaction involves various energy intermediates, which requires a significant overpotential.6–8 Just as in natural photosynthesis, artificial photosynthesis requires a light driven water splitting catalyst, or photoelectrode, to provide high energy electrons which are required to reduce protons to hydrogen or even reduce CO2 to carbon based high energy molecules. These cathodic reduction reactions can also be facilitated by suitable catalysts.8 One attractive approach to replicate a photosynthetic light harvesting system coupled to a reaction center is to employ semiconductors for absorbing light energy and generating charge separation. Here, photon absorption leads to the formation of charge carriers as electrons in the conduction band and positively charged holes in the valence band of the semiconductor. For solar hydrogen generation, the water oxidation is driven by photogenerated holes located in the valence band while the energized electrons in the conduction band reduce protons to hydrogen as represented by the following;9–11 2H2O + light - 4H + O2 + 4e
(1a)
4H + 4e - 2H2
(1b)
+
+
Thus, photocatalytic water splitting involves four steps: (1) absorption of photons, (2) creation of electron–hole pairs, (3) separation and stabilization of electron–hole pairs, (4) utilization of the charged separated states for hydrogen and oxygen production.8,12 A single material which can carry out all these steps at high efficiency is the holy grail of this field.11 In fact, it has been a challenge to find a material and a system as efficient as natural photosynthesis. Such photocatalytic systems must have a good stability in aqueous solution, appropriate band energies with respect to required redox levels, have high light absorption in visible range, good thermodynamic and chemical stability, good conductivity, be robust, abundant and ideally non-toxic. Among the semiconductor metal oxides, TiO2 has attracted much attention owing to its band levels, but due to its high band gap and its inefficient absorption of visible light, its application for solar energy capture is limited.13 To tackle this problem dyes are frequently used to coat TiO2 in devices like
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dye sensitized solar cells (DSSC).14 The electricity generated by DSSC could be used for water splitting and hydrogen production via electrolysis.14 But there are drawbacks associated with using a two reactor system which would have non-affordable significant losses of energy. Another inexpensive stable oxide, WO3 has been considered as a photocatalyst for DSSC. Recently, Yong et al.15 presented one dimensional WO3 nanorods based DSSC device with an efficiency of around 1.51% which is higher than WO3 nanoparticles based DSSC reported earlier. But it should be noted that WO3 needs much improvement since its efficiency is still lower than TiO2 based DSSC devices. There have also been major developments in preparation of oxynitride materials which have an optimum band gap and can undergo both water oxidation and proton reduction reactions but with relatively low quantum efficiencies of B5–6%.16 In many such types of photocatalytic systems, charge recombination can be very high which limits the photon-toelectron quantum yield. Hence, photoelectrochemical (PEC) cells with semiconductor–electrolyte junctions or semiconductor homojunctions–heterojunctions are more efficient due to presence of electric field or band levels alignment which improves charge separation and hence limits the probability of recombination.10 Various photocatalytic and other hybrid material systems have been extensively studied for water splitting over the past decades and many good review papers are available.8,13,16–23 Of all the candidates studied as a photoanode in PEC, the most attractive and well researched is Fe2O3 (hematite).22–25 In the following sections therefore, we will highlight the contribution of hematite and other promising iron-based semiconductors for the light driven oxidation of water.
4. Hematite as a photoanode Hematite is an iron ore which is abundant in the Earth’s crust. It is a low cost, non-toxic and stable material in aqueous solutions with a band gap energy which makes it a promising photocatalyst for solar water splitting.25 Fe2O3 is an n-type semiconductor with a crystal structure similar to that of corundum, Al2O3, which is trigonal (hexagonal scalenohedral, 3% 2/m) with space group R3c with lattice parameters a = 5.0356 Å, c = 13.7489 Å, and six molecules per unit cell.25 It has a band gap of 2.2 eV and has conduction and valence band energy levels as shown in Fig. 1. It can be seen that although the valence band is very favourable for oxidizing water, the conduction band is not favourable for hydrogen evolution. Hence, an external bias is needed to overcome this potential (B0.4 V) for both hydrogen and oxygen evolution. Unassisted water splitting without external bias has been reported by surface modification of Ti-doped hematite with CoF3 even though the Incident Photon-to-Current efficiency (IPCE) of this system is still too low for practical application.26 Tandem cells of hematite and an ideal photocathode with an appropriate onset potential is one possible solution to improve the IPCE.
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To increase the plateau photocurrent, advancements in nanostructuring and doping is required, while to lower the onset potential, the surface properties of semiconducting active layer needs to be improved. To improve the light absorption and photon-to-electron conversion efficiency, different approaches in nanostructuring and doping have been employed, which will be discussed in detail in the following section. Another limitation of hematite is the sluggish hole transport from semiconductor surface to the electrolyte which increases the required overpotential. Strategies to reduce the overpotential by surface passivation with metal oxide and surface treatment with co-catalysts will also be reviewed below. 4.1
Fig. 1 Schematic representation of band levels for hematite and for an ideal photocatalyst for water oxidation to hydrogen and oxygen.
Most of the reported works on hematite used it as a working photoanode in PEC systems with an applied external bias generally comprising of a counter electrode, Pt and reference electrode, Ag/AgCl immersed in an electrolyte at basic pH. A typical current–voltage (I–V) curve for untreated hematite would have a relatively high onset potential and low current (black curve of Fig. 2). On the other hand, a highly efficient hematite-based PEC device should have a high plateau photocurrent and low onset potential (red curve). The emphasis of research on hematite as a photoanode for water splitting is to close the gap between the two extremes and emulate the red curve as much as possible. To do this various treatments have been explored and reported as emphasized in Fig. 2 and discussed below.
Fig. 2 cell.
Typical (black) and ideal (red) I–V curve of a hematite based PEC
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Nanostructuring
The need for nanostructuring of hematite arises because of the trade-off between the relatively low absorption coefficient which requires 400–500 nm thick films for complete light absorption and the short hole diffusion length (2–4 nm) which requires 5–10 nm film for efficient hole transport to the electrolyte. The ideal nanostructure to fulfil these criteria is therefore a nanowire array with diameter of 5–10 nm and length of 400–500 nm as shown schematically in Fig. 2. It is worth mentioning that the interest in hematite was renewed after the discovery of the effectiveness of cauliflower-like ¨tzel’s group.27 Since then, a significant nanostructures by Gra number of efforts in nanostructuring have been reported and well documented in the review paper by Sivula et al.25 In this section, we will give a brief update on the recent developments in hematite nanostructuring for PEC applications. An important development in producing hematite on large area substrates has been the introduction of Fe2O3 nanorods prepared by an inexpensive solution based hydrothermal technique.28,29 The perpendicular alignment of 1-D arrays of nanorods on fluorine doped tin oxide (FTO) substrates in [110] direction is beneficial as the anisotropic conductivity along this direction is four fold higher than in the orthogonal direction which enhances the electron transport and hence the photocurrent.27 Other reports of hematite on large area substrates include plasma/thermal oxidation of Fe foils to hematite30,31 and the facile solution synthesis of a-FeF33H2O nanowires and their conversion to a-Fe2O3 nanowires.32 Another important strategy in nanostructuring is the enhancement of hematite nanorods or nanowire surface area by introducing branched/dendritic structures. Ideally, the enhanced surface area should increase the rate of photogenerated holes to the electrolyte. Zheng et al. reported dendritic Fe2O3 nanowire arrays formed by thermal oxidation of the electrodeposited dendritic Fe nanowires.33 Although the dendritic structure had a larger surface area than the nanowires, it showed lower photocurrent than the other reported nanowires and nanotubes which may be because of higher electron–hole recombination probability or the presence of higher amount of surface defects. These results emphasize the trade-off associated with increasing surface area, i.e. on one hand increasing the probability of charge separation while on the other hand increasing the density of surface defects which increases the
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surface recombination probability. Yang et al.34 demonstrated a unique hematite photoanode which is composed of b-FeOOH nano-branches on a Ti-doped hematite nanorods grown on an undoped hematite underlayer by hydrothermal method. The nanorods structure and the underlayer enhanced the charge separation and improved the onset potential. After the introduction of b-FeOOH nano-branches, the photocurrent further improved but the onset potential shifted positively due to increased flat band potential as confirmed by impedance spectroscopy. Deng et al. has reported Ti-doped hematite nanostructures with an urchin-like morphology, which enhanced the effective surface area compared to untreated nanostructures.35 With this system, a remarkable plateau photocurrent density value of 3.76 mA cm 2 was observed at 1.74 V vs. Reversible Hydrogen Electrode (RHE) under 1 Sun AM 1.5G condition in 1 M NaOH electrolyte, which was 2.5 times higher than that for the pristine nanostructures (1.48 mA cm 2). Finally, most recently, Kim et al. presented a worm-like morphology of hematite doped with Pt which resulted in photocurrent density 4.32 mA cm 2 at 1.23 V vs. RHE.36 More about this work will be reviewed in Section 4.2 below.
4.2
Doping
The process of doping is well established and has been proven to be beneficial to many semiconductor systems for enhancing the bulk carrier concentration and hence the conductivity. Pure hematite has very low electrical conductivity (ca. 10 14 O 1 cm 1),37 carrier concentration (1018 cm 3) and electron mobility (10 2 cm2 V 1 s 1).38 Since the conductivity of a material is given by s = neme + pemp, increasing carrier concentration (i.e. n or p) by doping is often used to compensate for the low electron (me) and hole (mp) mobilities. Hematite is a n-type semiconductor due to naturally occurring oxygen vacancies in the bulk. When divalent metal dopants, such as Mg, Cu and Ni, replace Fe3+ in the lattice, hole carriers are formed and hematite becomes p-type.7,39,40 When tetravalent metal dopants, such as Sn, Ti, Zr and Si, replace Fe3+, extra electrons will be generated and hematite will remain as n-type with increased electron concentration.27,35,41–43 Ling et al. showed the dependence of high temperature annealing on the photocurrent for hematite nanowires.41 The improvement in photoactivity was due to the presence of Sn leading to an increase in carrier concentration from 1.89 1019 to 5.38 1019 cm 3 for high temperature annealing of 650 and 800 1C respectively. The Sn4+ ions diffused into the Fe2O3 nanostructure from the FTO substrate during high temperature annealing which acted as electron donor and increased the ¨tzel et al. reported Si-doped carrier density of hematite. Gra hematite cauliflower-like nanostructures with highly rough surface and a feature size of 10–20 nm which showed appreciable photocurrent (2.2 mA cm 2 @ 1.23 V versus RHE). The enhanced performance was attributed to the increased donor density of Si-doped Fe2O3, which was found to be 1.7 1020 cm 3 by Mott–Schottky analysis. It should be noted that
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in this specific case, Si is also instrumental in creating the cauliflower nanostructure with small feature size.27 A theoretical study on Cu and Ti doped hematite was reported by Meng et al.40 They showed by using density function theory (DFT), that the conduction band minima (CBM) of Cu doped hematite was sufficient to facilitate hydrogen production which is not the case for untreated hematite as emphasized in Fig. 1. Thus, the energy levels of the valence and conduction bands would be able to drive both photoinduced oxygen and hydrogen production from water without the application of a voltage bias. In the case of Ti doping, their DFT calculations indicated an improvement in electronic conductivity and a slight change in band gap due to creating shallow donor levels which encourages the Fermi level to shift towards the conduction band of hematite.40 Ti doped hematite has been investigated experimentally by Shen et al. in which photocurrent after Ti doping increased 14 times compared with that of pristine hematite.44 This and similar other works35,45,46 strengthen the importance of Ti doping which seems to enhance the charge carrier density in hematite and increases its performance. Recently, a photoanode consisting of hematite nanorods doped with Pt and surface treated with Co–Pi, yielded a recordbreaking photocurrent of 4.32 mA cm 2 at 1.23 V vs. RHE under simulated 1-sun (100 mW cm 2).36 The hematite nanorods had a unique ‘‘wormlike’’ morphology formed by two steps annealing at 550 1C and 800 1C. Platinum doping was used to improve the charge transfer characteristics in the bulk of hematite while the Co–Pi acted as an oxygen evolution cocatalyst on the surface. The Pt doping resulted in an improvement of the electrical conductivity of hematite by increasing its donor density from 3.27 1017 cm 3 for untreated Fe2O3 to 2.77 1018 cm 3 and 3.91 1018 cm 3 for Pt-doped Fe2O3, and Pt-doped Fe2O3/Co–Pi, respectively.36 Recently, in our laboratory, we have developed a solution based method to synthesize Mn doped hematite nanorods with improved water oxidation activity compared to non-treated hematite nanorods.47 We found that 5 mol% Mn precursor doping was optimum to enhance the photocurrent densities by 3 times at 1.23 V vs. RHE and also in reducing the onset potential by 30 mV compared to the pristine hematite nanorods (Fig. 3a). The enhancement in current density was attributed to increase in the donor densities as estimated by Mott–Schottky analysis (Fig. 3b). Chronoamperometry studies also show that there is a decrease in transient currents for doped Mn samples compared to pristine hematite, indicating the beneficial effects of Mn doping which seems to be due to a suppression in electron–hole recombination and a reduction in the energy barrier for hole transport. This study confirms that Mn doping enhances the charge carriers and reduces the recombination losses and emphasizes that this treatment results in better catalytic performance of Fe2O3 nanostructured photoanodes. The enhancement after Mn doping is possibly due to the multivalent oxidation states of manganese which leads to a low (O–Mn–O) energy barrier for hole transport. Another kind of doping strategy is the creation of oxygen vacancies which act as shallow donors and increases the donor
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Fig. 3 (a) Photocurrent–potential curve of pristine hematite nanorods and 5% Mn doped hematite nanorods measured under AM 1.5G 100 mW cm 2 in 1 M NaOH electrolyte solution, (b) Mott Schottky plots of pristine (inset) and 5% Mn incorporated hematite nanorods extracted from electrochemical impedance data measured at 1 KHz in the dark. Calculated donor densities (Nd) and flat band potential (Vo) are shown in the figure.
density. For example, the incorporation of oxygen vacancies in hematite has been reported recently by Ling et al.48 They produced hematite nanowires with increased oxygen vacancy concentration which showed an enhanced photoactivity of water oxidation. This was attributed to an increase in the donor density as supported by Mott–Schottky analysis. The study was also the first demonstration of highly photoactive hematite nanowires at relatively low post-annealing temperature (550 1C) without the addition of other elemental doping. Subsequently, a systematic study to decouple the interaction between oxygen vacancies and extrinsic Sn dopants was performed by Yang et al.49 They demonstrated that both oxygen vacancy and dopants complement each other to enhance the electrical conductance and in turn increase the photocurrent and reduce the onset potential. Recently, Zandi et al. reported a different role of Ti doping in enhancing the photoactivity of hematite.50 Instead of the conventional role of increasing charge carrier density, Ti was suspected to increase the efficiency of hole transport properties across the hematite/electrolyte interface. Even though some preliminary data was presented, further experimental evidence is still needed to support this suggestion. To summarize, both the nanostructuring and the doping strategies have been shown to be beneficial in altering the electronic properties of pristine hematite which in turn enhancing the plateau photocurrent of PEC cells. 4.3
Surface treatment
Another important drawback of untreated hematite is the requirement for a bias to overcome the overpotential necessary for water splitting. This is attributed to: (1) the position of the conduction band of hematite, which is 0.4–0.5 V too positive to allow the reduction of protons to hydrogen (Fig. 1) and; (2) the sluggish hole transport across the semiconductor/ electrolyte interface The sluggish hole transport can be improved by surface treatment by another metal oxide which either causes surface passivation or act as co-catalysts. These two phenomena have fundamentally different mechanisms in assisting hole transport.
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Surface passivation implies the reduction of hematite surface defects which reduces the hole–electron recombination rate, while co-catalyst implies the oxidation of the metallic element of the co-catalyst which assist hole transport from hematite surface to the electrolyte. It should be noted however, that for a few types of surface treatments using Co- or Ir-based materials, more conclusive experimental evidence which can clearly determine whether they act as surface passivation layer or as co-catalyst is still needed. A few surface passivation strategies such as CoF3,26 Al2O3,51 Ga2O3,52 SnO253 and ZnO54 have been reported so far. Hu et al. showed a favourable flat band shift in Ti-doped iron oxide thin films after surface modification with CoF3 aqueous solution.26 The photogenerated electrons, due to this shift, were able to reduce protons and generate hydrogen without any applied bias, emphasising the importance of surface modification in shifting band level positioning. Formal et al. were able to reduce the overpotential by 100 mV by using an ultra-thin coating of Al2O3 deposited by Atomic Layer Deposition (ALD) on to hematite photoanodes.51 This alumina overlayer enhanced photocurrent to a value of 0.85 mA cm 2 at 1.0 V vs. RHE as compared to 0.24 mA cm 2 for untreated hematite. Through electrochemical studies and optical spectroscopy, the role of the alumina was confirmed as a surface passivating agent. Another report by Hisatomi et al. on surface passivation of hematite photoanodes highlights the use of corundum-type overlayers which decreases the lattice strain and hence the density of surface states. They showed a negative shift of 200 mV in overpotential using Al2O3 and Ga2O3 overlayers.52 In our laboratory, we found that ZnAc treatment of spray pyrolyzed hematite mesoporous thin films resulted in an enhanced PEC performance. Here, a very thin ZnO overlayer was deposited on surface of a thin film of hematite resulting in an increased photocurrent accompanied by reduction in onset potential as compared to untreated hematite. After 3 cycles of ZnAc treatment, the photocurrent increased more than 40% to 1.08 mA cm 2 at 0.23 V vs. Ag/AgCl and onset potential for water oxidation shifted by 170 mV (Fig. 4a). Based on impedance spectroscopy and ideality factor measurements (Fig. 4b and c) it was proposed that the ZnO overlayer changes the flat band potential of hematite and reduces the recombination reactions caused by surface defects.54 Various oxygen evolution catalysts like Co–Pi,55,56 IrO2,57 cobalt oxides27,58 have also been used with hematite photoanodes because of the fast kinetics they exhibit in charge transfer for water oxidation. A catalyst present on the surface can act as a mediator to ease hole transfer from the interface owing to a reduced kinetic barrier and its ability to oxidize water and decreases the overpotential. Indeed, Tilley et al. showed that the surface modification of nanostructured hematite photoanode by IrO2 nanoparticles resulted in an overpotential shift of 0.2 V and a photocurrent as high as 3.75 mA cm 2.57 Being one of the best co-catalyst in terms of reducing the overpotential, IrO2 nanoparticles can transfer photogenerated holes to a lower overpotential and thus a higher photocurrent is achieved due to
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Fig. 4 (a) Photocurrent–potential curves before and after different cycles of ZnAc treatment of hematite mesoporous thin films (b) Mott–Schottky plots of hematite films in a 1 M NaOH electrolyte in the dark (c) local ideality factor extracted from the dark I–V curves (inset) before and after surface treatment.
less charge carrier recombination. Even though IrO2 is one of the best water oxidation catalysts, it consists of an expensive non-abundant element. In this context, recent developments have been made on earth abundant Co and Mn based nanoclusters which could achieve the target of high turnover frequency (TOF) and are stable inorganic based oxide catalysts which could be further used on a large scale. It would be interesting to integrate these nanoclusters with nanostructured photoanodes to design a durable water oxidation system.59 Nocera et al. reported the oxygen evolving catalyst, Co–Pi, which can be deposited and coupled with a photoanode using electrodeposition.60 Since its discovery it has gained a lot of interest because of a low overpotential of only 0.41 V is required to oxidize water at pH = 7 with a current density of 1 mA cm 2. Zhong et al. reported 0.35 V cathodic shift in overpotential for Co–Pi–a-Fe2O3 composite photoanodes.55 McDonald et al. photochemically deposited Co–Pi OEC on electrodeposited iron oxide films and demonstrated an enhancement in photocurrent as well as in O2 generation which implied an improvement in O2 evolution kinetics.61 ¨tzel et al. performed surface treatment of cauliflower Gra nanostructured hematite with 10 mM Co(NO3)2 solution as a precursor of cobalt oxide.27 The cobalt treatment resulted in an 80 mV cathodic shift in onset potential, and a slightly higher photocurrent 2.7 mA cm 2 at 1.23 V versus RHE compared with pristine hematite (2.2 mA cm 2 at 1.23 V versus RHE). The explanation for the observed enhancement of the cobaltmodified hematite was that it involved surface holes trapped by Co(II) sites as Co(III), which in turn could be oxidized by a second hole to Co(IV) which then drives the water oxidation reaction. This explanation suggests that cobalt oxide behaves as a cocatalyst on the surface of hematite but alternative explanations have been explored. Recently, for example, Peter et al. calculated the photogenerated charge transfer and recombination constants of pristine hematite and Co(II) oxide treated hematite thin film by intensity modulated photocurrent spectroscopy (IMPS).62 They showed that the charge transfer rate of Co(II) treated hematite was similar to the untreated hematite, indicating that the photocurrent improvement was
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not due to catalytic behaviour of Co. On the other hand, the recombination rate constant was found to decrease for treated hematite as compared to the untreated control. This seems to suggests that the improvement in photocurrent in Co(II) treated hematite electrodes was due to suppression of surface recombination and not due to the catalysis of hole transfer (i.e. an increase in charge transfer rate) as suggested previously.27 There have also been other developments to understand the role of surface passivation layers or cocatalysts through transient spectroscopic measurements. Ultrafast studies for Fe2O3 phase was earliest reported by Cherepy et al. which found the short charge carrier lifetime of 1 ps for hematite nanoparticles.63 Subsequently, Joly et al. probed carrier dynamics of a-Fe2O3 thin films by femtosecond transient absorption spectroscopy and found the electrons recombining with holes/traps in order of 3 ps and claimed carrier trapping by midgap Fe d–d states to be the dominant trapping mechanism.64 Since carrier dynamics is very important in fundamental understanding of intrinsic carrier trapping and charge transport properties to design more efficient solar harvesting devices, spectroscopic studies have been pursued in the past few years. Huang et al. reported in situ Transient Absorption (TA) studies for hematite thin film electrodes which concluded that the ultrafast electron–hole recombination is the limiting factor for efficient water oxidation and the minimum hole lifetime for hematite to be catalytically active is 6 ms.65 Pendlebury et al. presented a correlation between photocurrent and the population of higher lifetime holes under external bias.66 Through transient photocurrent (TPC) and TA measurements, it was shown that the long-lived holes drive the single hole oxidation under bias as opposed to four-hole oxidation process. To understand the role of Co–Pi in the enhancement of photocurrent for hematite, TA measurements were performed by Barroso et al. on a-Fe2O3–CoOx nanocomposite photo electrodes.56 It was proposed that the reduction in recombination losses improved the photoelectrochemical activity. Another report by the same group have shown the reduction in electron–hole recombination is responsible for cathodic onset potential shift for surface treated hematite with overlayer (Ga2O3) and cocatalyst (CoOx).58
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Fig. 5 (a) Photocurrent potential curve for hematite nanorods for in situ added different amount of Co2+ (b) SEM image of hematite nanorods annealed at 550 1C for 2 h. (Inset-top: cross-section view of hematite nanorods on FTO substrate; bottom: TEM image of in situ Co2+ added hematite nanorods.)
Since the application of Co catalysts to the surface of hematite has been shown to significantly reduce the over potential for water oxidation and improve the passivation of defects, we recently investigated the role of Co by integrating Co3O4 using in situ and ex situ methods during hydrothermal growth of hematite nanorod arrays.29 It was found that the photocurrent density increases from 0.72 mA cm 2 for the pristine Fe2O3 nanorod to 1.20 mA cm 2 at 1.23 V versus RHE (i.e., 67% improvement) with 5 mol% Co2+ treatment (Fig. 5a). Concomitant with this change was a shift in the onset potential by B40 mV and improvements in the IPCE and oxygen evolution. Hematite photoanodes with in situ deposition of Co3O4 nanoparticles showed better performance than those prepared by ex situ procedures because of high surface roughness, larger Co3O4/hematite interfacial area, and smaller Co3O4 particle size (Fig. 5b).
5. Other iron based semiconductors As diagrammatically shown in Fig. 1, the challenge is to strive for a semiconductor material which has band levels straddling the water oxidation/proton reduction redox levels and have good charge transport properties to inhibit charge carriers recombination. Despite the many attractive features of treated and nano-structured hematite as a photoanode for water oxidation, it is important to explore different classes of iron-based materials which possess more favourable intrinsic properties than hematite. Theoretically, this improvement could be achieved by changing the band structures of the material by solid solution or composites of two or more systems. Experimentally, hybrid structures have been developed but have had little impact to date owing to the difficulty in obtaining pure phases and making porous electrodes for enhanced photoactive behaviour. Recent report of combinatorial methods like inkjet printing of array of multicomponent metal oxides as oxygen evolution catalysts is a promising tool to speed up the process of discovering efficient photocatalysts utilizing the properties of different metals in the formulation of hybrid systems.67 Iron based ferrites are interesting class of materials to study as an alternative or for integration with hematite. Boumaza et al.
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considered ZnFe2O4–SrTiO3 system for hydrogen production as ZnFe2O4 has a conduction band higher than water reduction level.68 Utilizing such favourable conduction band energy for hydrogen evolution, McDonald et al. integrated ZnFe2O4 with Fe2O3 as a photoanode.69 It was shown that ZnFe2O4 has a band gap of around 2 eV and stable in aqueous solutions. Its conduction and valence band edges are shifted in negative direction by ca. 200 mV as compared to hematite resulting in better photocurrent response in Fe2O3–ZnFe2O4 composite system as compared to pure hematite system due to efficient electron–holes separation. A similar approach has been reported by Miao et al.70 where heterojunction of Ti-doped Fe2O3 and ZnFe2O4 has been shown to enhance photoelectrochemical performance owing to better charge separation. Recently, Rekhila et al. reported that NiFe2O4 can be used as a photocatalyst for visible light driven hydrogen production in the presence of a sacrificial electron donor.71 The physical characterization of the material showed the p-type behaviour with a band gap of 1.56 eV which suggests that it can be integrated with hematite as a p–n junction in future. Another group of materials, the titanates, have also been shown to possess promising qualities for water splitting catalysis.72,73 Kudo et al. has described a number of diverse heterogeneous photocatalysts, including both titanates and tantalates, for solar water splitting.8 They showed how the solid solution of two materials with not so favourable band levels can be converted to efficient photocatalytic water splitting materials. A similar idea was proposed by Thimsen et al. where the conditions to prepare mixed transition metal oxides with optimum band gaps was presented.74 Even though they focussed on Fe–Ti–O system, it was claimed that it could be extrapolated to Ti–V–O or Ti–Ni–O systems as well. Stoichiometric changes in TiO2 through incorporation of Fe were shown to effectively tune band gaps. They showed theoretically and experimentally how iron titanates have better band gap for visible light excitation as compared to TiO2. It was claimed that with valence band level already being favourable for water oxidation, the shift in conduction band level towards hydrogen generation would in turn result in the reduction of the overpotential. Ginley et al. synthesized crystals for different iron titanates FeTiO3, Fe2TiO4 and Fe2TiO5 to be used as anodes for photoelectrolysis of water.75 Among these, FeTiO3 with a band gap of 2.5–2.9 eV has been studied mainly with TiO2 for photocatalytic applications.76 Kim et al. reported nanodiscs of FeTiO3 prepared by hydrothermal treatment which were then exposed to TiO2 P25 nanoparticles so as to integrate them.77 They reported FeTiO3–TiO2 as water oxidizing visible light photocatalyst where FeTiO3 acts as the light absorber and TiO2 as electron transporter. Similar work has been conducted where FeTiO3 is utilized as an absorption enhancing layer on TiO2 for photochemical processes where it is also shown that valence band matching between them increases the hole transport across the semiconductor/electrolyte interface and hence enhances the photocatalytic activity.76,78 It is the theoretical and experimental studies by diverse groups in tuning the band gap through solid solutions of two
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oxides which has recently motivated us to work on Fe–Ti–O system. We have investigated FeTiO3 as a photoanode for solar driven water oxidation. But owing to the band gap of FeTiO3 (B2.5–2.9 eV), using it by itself as a photoanode results in an incomplete light absorption in the visible range. Also, Fe2+ oxidation state present in the material was difficult to process as it may result in mixed phases with Fe3+ ions after high temperature annealing. Due to these difficulties we switched to a more stable phase Fe2TiO5 which is a hybrid of Fe2O3 and TiO2. With a favourable band gap of around 2.0 eV, it has a good solar spectrum absorbing qualities and is also stable thermodynamically as well as in aqueous solutions.79–81 Being a titanate, it is likely that its conduction band and valence band levels would be as favourably for water splitting as compared to hematite.11,81 The idea behind using hybrid of Fe2O3 and TiO2 is to utilize good charge transport properties of TiO2 and the better band gap of Fe2O3 for absorption of solar spectrum to be used as a photoanode in PEC. We have prepared Fe2TiO5 nanoporous thin films by solvothermal technique on FTO substrates and have investigated its potential as a photocatalyst for overall water splitting. It is worth noting that a lower bias was needed for Fe2TiO5 to produce photo-current as compared to Fe2O3 which makes it an interesting alternative system (unpublished data).
6. Conclusions Iron based semiconductors are attractive for developing cheap and robust photoanodes for PEC cells to split water and generate hydrogen as a solar fuel. Some of their disadvantages like non-optimal conduction band levels, high hole–electron recombination probability, incomplete solar light absorption, poor hole diffusion length and low catalytic kinetics at the surface can be partially overcome by various approaches. Some of the approaches like nanostructuring, doping and treatments with surface passivation overlayers or co-catalysts have been addressed in this review. It is evident that significant improvements have taken place in past decade but there is still much scope to explore these systems completely in terms of their internal charge dynamics. Several hybrid iron-based systems like ferrites and iron titanates are also emerging as alternatives to hematite having significantly advantageous with respect to their band levels and charge transport properties. Hence, we can expect to see new and striking developments emerging in the future in the area of hybrid systems. We believe that these inorganic semiconducting catalysts are more likely to be used in a functional and realistic artificial photosynthesis technology than organic based systems due to their cheapness, robustness, light absorbing and charge separation properties.
Acknowledgements We would like to acknowledge financial supports from the Centre of Artificial Photosynthesis, NTU.
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