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Hydrogen generation by water splitting on hematite (0001) surfaces: first-principles calculations Haijun Pan,ab Xiangying Mengab and Gaowu Qin*a The surface chemical activity is a critical factor affecting the photocatalytic efficiency of hematite. In this study, we investigate systematically the reaction kinetics of water heterolytic dissociation (H2O–OH
+ H+) and
hydrogen generation by water splitting on four kinds of hematite (0001) surfaces, namely perfect and defective O- and Fe-terminated surfaces, at the electronic level based on first-principles calculations. The simulation results illustrate that the chemical reaction rate for the dissociation and hydrogen generation is sensitive to the morphology of the hematite (0001) surface. For water heterolytic dissociation, the hydrogen atom is apt to Received 20th July 2014, Accepted 15th October 2014
drop from water molecules on the perfect O-terminated (0001) surface without energy consumption.
DOI: 10.1039/c4cp03209h
on which the whole photoelectrochemical process needs to overcome a rate determined barrier of
However, the Fe-terminated (0001) perfect surface is a preferable candidate for hydrogen generation, 2.77 eV. Our investigation shows that O- or Fe-vacancy on hematite (0001) surfaces is not conductive
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to hydrogen generation by water splitting.
Introduction Since the discovery of the Fujishima–Honda effect,1 many metal oxide semiconductors have been investigated to solve environmental and energetic problems. Hematite (a-Fe2O3) has been found to meet stringent requirements for photocatalytic hydrogen production by water splitting. Hematite is cheap, nontoxic, abundant, and chemically stable to photo-corrosion; its desired band gap of B2.1 eV can capture roughly 40% of the incident solar spectrum.2–5 With respect to the water splitting redox potential, the valence band edge is appropriately located to oxidize water, but the position of the conduction band edge is just below the reduction level of H+ to drive hydrogen evolution.6,7 This mismatch of conduction band edge could be modified by elemental doping and surface treatment.5,8–11 Hematite has been considered a promising photocatalytic material, especially when used for hydrogen generation by water splitting. Experimental exploration of water splitting as well as hydrogen generation show that the interface structure, electronic structure, local atomic coordination, surface functional groups, and the composition and topology of the surface are intrinsic factors affecting the chemical activities of semiconductor surfaces.
a
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, People’s Republic of China. E-mail: [email protected] b College of Sciences, Northeastern University, Shenyang 110819, People’s Republic of China
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In the case of hematite, the low redox kinetics on its surface have been an obvious obstacle that hindered the development of efficient photocatalytic devices.2,12 Thus, it is of great significance to understand the chemical reaction mechanism of water molecules on different hematite surface morphologies, from both thermodynamic and kinetic aspects. It has been experimentally reported that the hematite (corundum) (0001) surface is energy preferable under different conditions. The different morphologies of the hematite (0001) surface were first reported experimentally by Wang et al.13 Their SEM results verified that the surface of the hematite sample prepared in ambient oxygen atmosphere were terminated with two kinds of monophonic configurations: a single Fe-layer (Fe–O3–Fe–R) and an O-layer (O3–Fe–Fe–R), in which R represented the corresponding repeated unit in the bulk structure. Subsequently, Shaikhutdinov et al. explored the surface structures of hematite under oxygen partial pressure in a span of 10 6–1 mbar at the temperature of 850 1C.14 The co-existence of these two surface structures was discovered at an interval of 10 4–10 1 mbar. A similar surface structure is also replicated by chemical vapor transport, and characterized using atomic force microscopy (AFM).15 According to the present experimental results, it can be seen that the (0001) surface termination of the hematite sample is basically composed of a single Fe-layer (Fe–O3–Fe–R) and an O-layer (O3–Fe–Fe–R), and we construct water-contacted surfaces based on the two Fe- and O-terminations in this work. For potential technological application as a photocatalyst, it is essential to understand comprehensively the surface activity
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of water splitting, and make appropriate modifications. Previously, the thermodynamic behaviour of water molecules on pure or doped hematite (0001) surfaces were investigated by a series of experimental and theoretical works.16–20 These explorations provided the static free energies of some isolated reaction states, but to what extent these state-transfers can be realized are still unclear. Moreover, hydrogen generation is not mentioned in earlier studies because of the unfavourable conduction band edge with respect to the reduction potential of water. In this regard, the reaction barriers and intrinsic electronic mechanism of hydrogen generation by water-splitting on hematite perfect- and defective-(0001) surfaces are calculated and discussed. In this paper, we found some exothermic reactions during the water-splitting actually cannot really spontaneously become realized as the thermodynamics predicts, thus some of our kinetic results are unanticipated by previous works.
Computational method and modelling All calculations were performed in the framework of the spinpolarized density functional theory with the Vienna ab initio simulation package VASP.21 The pseudopotentials and wave functions were generated within the projector-augmented wave method.22 The electronic wave functions were expanded in a plane-wave basis set, and a kinetic-energy cutoff of 550 eV was adopted. For the exchange correlation functional, the generalized gradient approximation of Perdew, Burke and Ernzerhof was employed.23 Because of the strong on-site coulomb repulsion between the localized d-shell electrons, the effects of electron correlation were taken into account within the framework of GGA + U according to Dudarev et al.24 The value of U–J was set to 4.3 eV, which produced a series of data consistent with experiments25 and previous theoretical study.26 Along the c-axis of the hexagonal unit cell, hematite is alternately stacked by iron and oxygen atom layers, as shown in Fig. 1a. Thus, we could construct three possible surfaces: the single Fe-layer (Fe–O3–Fe–R), the double Fe-layers (Fe–Fe–O3–R), and the O-layer (O3–Fe–Fe–R), in which R represented the corresponding repeated unit in the bulk structure.16 However, based on experimentally measured stable surface terminations,
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the O-terminated and single Fe-terminated hematite (0001) surfaces were modelled by periodical slabs with seven atomic layers in simulation. In order to study the reaction pathway of a single water molecule on the hematite (0001) surface, a (2 2) supercell was used to avoid spurious interactions between water molecules in neighbouring cells in the low-coverage regime. A vacuum space with a vertical distance of 15 Å was used to separate the electronic interaction between the periodic atom layers. In Fig. 1b and c, the top views of perfect O-terminated and Fe-terminated surface are displayed, and the possible adsorption sites of water molecules are indicated. Non-stoichiometric surfaces were obtained by artificially removing one oxygen and iron atom from the corresponding perfect-surface models. The resulting concentration of O- and Fe-vacancies was 1/12 ML and 1/4 ML, respectively. The top six layers were allowed to relax, whereas the atomic layer at the bottom was fixed at bulk truncated lattice constants (a = b = 4.82 Å and c = 13.06 Å), which were calculated by the GGA + U method. The Brillouin-zone integration was performed using Monkhorst–Pack grids of 2 2 1.27 The structure was relaxed until the force is less than 1 meV Å 1 per atom. The global energy minimum adsorption structure of water molecule on each surface was deemed as the initial state, and the corresponding structure of hydrogen generation was set as the final state. Between the initial state and final state, a series of intermediate images along the reaction path way were linearly interpolated. Our designed reaction pathway has been commonly accepted and adopted by similar investigations.28–30 Reaction pathways and energy barriers were calculated using the climbing image nudged elastic band (cNEB) method, in which the apex of the minimum energy paths (MEP) were assigned to be the saddle points.31
Results and discussion Stable structure and adsorption energy of the isolated water molecule on (0001) surfaces We began with searching the stable adsorption structure of the single water molecule on the hematite (0001) surface in this study. From the top view of the O-terminated hematite (0001) surface
Fig. 1 (a) The hexagonal unit cell of hematite. (b) Top views of the O-terminated hematite (0001) (2 2) surface supercells. (c) Top views of the Fe-terminated hematite (0001) (2 2) surface supercells. The possible adsorption sites of water molecule are represented by top, hollow and bridge on the surface. Oxygen and iron atoms appear as red and golden yellow spheres, respectively. The defective surface is obtained by manually removing one oxygen and iron atom from the topmost layer in the above mentioned surface, respectively.
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Fig. 2 Geometries and relative energies obtained from theoretical calculations of hydrogen generation from water splitting on the O-terminated (a) and defective O-terminated hematite (0001) surface (b). In each process, the energy of the initial state is set to zero. Transition states are marked by green font.
(shown in Fig. 1b), there are three possible adsorption positions: top site, bridge site, and hollow site. In order to obtain a stable structure of the adsorbate–surface system, the isolated water molecule is placed at a distance of 2 Å above three possible adsorption sites. The final optimized configuration shown in Fig. 2a (noted as IS) is in global energy minimum, in which the water molecule locates on the top of the O atom, and the plane formed by H–O–H is parallel to the surface. The distance between the O atom in the adsorbate and surface is 2.49 Å, and the O–H bond lengths of the water molecule are 0.98 Å. Compared to the structure of water adsorption on well ordered O-terminated (0001) surface, the water molecule is prone to adsorb on the defective site, as shown in Fig. 2b. The O–H bond lengths in the adsorbed water molecule are 0.98 Å, and the O–H bonds point to the oxygen atoms in the neighbourhood. The distance between the O atom of the adsorbate and Fe atoms in the sub-layer is 2.18 Å and 2.48 Å, respectively. Fig. 3a shows the structure of the isolated water molecule adsorption on an Fe-terminated (0001) surface. The oxygen atom of the adsorbate interacts directly with the coordinately unsaturated Fe atom, and the Fe–O bond length is 2.16 Å. One of the O–H bonds stretches into the subsurface, pointing to the nearest O atom, and the bond length is 0.01 Å longer than that
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in the water molecule. The other O–H bond points toward the vacuum with the same bond length as the water molecule. The structure of water adsorption on the defective Fe-terminated surface is shown in Fig. 3c, which is similar to the geometry on the perfect surface. Except for the reduction of Fe–O bond length (2.15 Å), there is little variation for the geometry of the O–H bond. The adsorption energy is used in order to evaluate the stability of the isolated water molecule–surface structure, which is defined as Eads = EH2O/sur
Esur
EH2O
where EH2O/sur is the total energy of the combined system (H2O bound to surfaces), Esur is the energy of the surface alone, and EH2O refers to the energy of a single H2O molecule. The adsorption energy is merely 0.49 eV for the H2O/perfect O-terminated surface system, which is considered as physisorption. The adsorption energy is enhanced to 1.08 eV when the surface contains O-defects. When the water molecule bonds to the perfect and defective Fe terminated surfaces, the system will release 2.33 eV and 1.49 eV, respectively. These results indicate that the stability of the adsorption structure is gradually
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Fig. 3 The variation of configurations and relative energies in the theoretically derived reaction pathway of hydrogen atom splitting from a water molecule on an Fe-terminated hematite (0001) surface (a) and a defective Fe-terminated hematite (0001) surface (b), and the variation of configurations and relative energies in the theoretically calculated reaction steps of hydrogen generation on an Fe-terminated hematite (0001) surface (c) and a defective Fe-terminated hematite (0001) surface (d). In each process, the energy of initial state is set to zero. Transition states are marked by green font.
improved with the increased concentration of iron atoms in the surface. The reactive kinetics of the single water molecule on (0001) surfaces The stable configurations of water molecules adsorbed on O-terminated (0001) surfaces are set as the initial states (IS) in each reactive pathway. The potential energy profiles and local structures of reactants in the reactive pathway are shown in Fig. 2a and b. On the O-terminated stoichiometric (0001)
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surface, there is no energy barrier for the first hydrogen atom splitting from water molecule, which indicates that spontaneous decomposition is achievable on the low coverage O-terminated surface. The subsequent intermediate state (MS1) is still inclined to dissociate due to its energetic instability. In the transformation pathway toward the second middle state (MS2), an energy barrier of 0.19 eV is encountered. However, the required energy can be supplied easily from the initial dissociation energy of 1.78 eV to overcome this activation barrier. Two detached hydrogen atoms adsorb on the different O atoms on the surface,
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and then achieve a more stable geometry through a transition state (TS2: 0.62 eV). The generation of hydrogen from the Oterminated (0001) surface is reached directly by receiving an amount of energy larger than 4.93 eV. The generated hydrogens are 3.12 Å away from the surface, and there is no interaction between them. On the whole, this process is endothermic, and needs to absorb at least 1.73 eV from the external environment. On the oxygen defective-terminated surface, the transition state (TS1) will be reached if at least 0.35 eV is absorbed in the system, in which the O–H bond will substituted by two hydrogen bonds as shown in Fig. 2b. Even at room temperature, this step can work if a small amount of energy is provided. During the generation of hydrogen, the H atom in the O–H bond and the other H get closely with each other, and then form directly if the energy barrier of 5.18 eV (TS2) is surmounted. Finally, the distance between the molecular H2 and the surface increases to 2.55 Å, at which point it can easily desorb from the surface. In total, the system harvests at least 4.51 eV in this reactive pathway. We next display the microscopic mechanism of water dissociation on the perfect Fe-terminated surface in Fig. 3a. The splitting of hydrogen atom in molecules is thought to slide into the nearest oxygen atom in the sub-layer, which goes through a larger energy barrier of 0.69 eV. Taking account of this large energy barrier as well as the higher energy in the final state (FS), it is hard for this process to occur. Thus, the reaction route of the hydrogen generation is designed to get away directly from the water molecule (see Fig. 3c). In this process, the adsorbed water molecule raises its head at the beginning, and needs to absorb 1.81 eV to achieve the first transition state (TS1). Then, the water molecule continues to move towards the vacuum region, leading to an intermediate state (MS1). From the IS to MS1, the Fe–O bond keeps on increasing, from 2.16 Å to 2.24 Å. After the second energy barrier of 2.77 eV (TS2), these two hydrogen atoms are linked by the hydrogen bond, and restricted to the surface. To reach the final state, they must obtain at least 0.44 eV to get away from surface. We also calculate the process of hydrogen atom splitting from a water molecule on the Fe-defective surface in Fig. 3b. The large barrier as well as energy difference suggests this process is difficult to realize. The reactive pathway shown in Fig. 3d needs to go through two transition states (TS1 and TS2) and an intermediate state (MS) from initial state to final state. The first energy barrier is associated with the breaking of the O–H bond and the formation of a hydrogen bond. After climbing this barrier (4.9 eV), the system relaxes to the MS state, in which molecular hydrogen has been generated. This intermediate configuration will transform into the final state by overcoming the second barrier of 0.37 eV, which is related to the breaking of weak electronic interaction between the hydrogen and the surface. Due to the improper position of the conduction band edge, a majority of studies mainly concentrated on the chemical behaviour of the H2O molecule on the Fe2O3 (0001) surface under photoelectrochemical conditions as a photoanode.32–34 In the framework of the one electron transfer mechanism, the hydrogen ion is supposed to break away directly from the water molecule in each step. Thus, the energy barrier is neglected in
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the reaction pathway leaving the reaction rate unanticipated. In this study, we described the basic microscopic picture of the kinetics of elementary reactions for a quasi-isolated water molecule on the hematite surface, namely, dissociation and hydrogen generation. Recently, the similar behaviour of a water molecule has also been investigated on the surface of other oxide semiconductors by other researchers.28–30 For example, on the Ga-terminated (0001) surface of GaN, a transition state with a barrier energy of 1.42 eV has been encountered in the minimum reaction path of hydrogen generation.28 On ZnS wurtzite (0001) surfaces, this minimum barrier has been predicted to be 2.24 eV by our group.29 Compared to the previous works, the reaction rate is a little lower, with an energy barrier of 2.77 eV. The photoelectrochemical properties of Fe2O3 have been investigated extensively by our group in the past few years,35–37 from both experimental and theoretical aspects. We find that although the band structures of modified hematite meet the chemical potential of oxidation–reduction reactions, hydrogen generation by water-splitting cannot occur spontaneously unless a bias voltage is applied to the system. The reaction thermodynamics can hardly account for this phenomenon, even taking into the band-bending at the solid–liquid interfaces and experimental errors. For example, Hu et al. introduce a surface treatment technique to decompose directly molecular water by hematite, but the photocurrent ultimately disappears without bias.8 In this work, our simulation results indicate that the hydrogen generation on the hematite (0001) surface is a crucial step in the overall process, and the extra bias is needed to overcome the kinetic energy barrier. Furthermore, the perfect Fe-terminated surface is more appropriate for hydrogen generation. Our simulation results might be of great value in further developing modified hematite with higher photocatalytic activity by proper surface treatment. Up to now, the O-terminated and Fe-terminated (0001) surfaces can appear alone at an oxygen partial pressure of 1 mbar and 10 5 mbar, respectively, at the temperature of 850 1C.14 Beside, Fe-terminated surface has been realized using the method of molecular beam epitaxy in the ultra-high vacuum environment by some groups.38,39 Electronic structure In order to gain insight into the chemical reaction mechanism in these pathways, we calculate systematically the electronic density of states for the reactants. This paper is mainly concerned with the chemical bond mechanism between the adsorbate and surface, and evaluates the ability of photon-excited electrons to transfer from the surface to a hydrogen atom. Furthermore, it is of importance to determine whether the absorbed solar energy can be converted to chemical energy, and stored in hydrogen bond.28,29 As can be seen in Fig. 4a, there is a slight interaction between a water molecule and the surface in the initial state (IS), which can be indicated by the isolated molecular orbital of adsorbate. The PDOSs of initial state (IS) in the other three models shown in Fig. 3b–d show that the surface region containing Fe atoms with dangling bonds is active toward the water molecule. The interaction between surface and water molecules is mainly through the oxygen atom in the adsorbate
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Fig. 4 Projected density of states (DOS) for water molecules on the hematite (0001) surface, and the total DOS for surface in the reactive pathway. The electronic structures for key points in the reactive pathway of water molecule on (a) the O-terminated surface, (b) the O-defective terminated surface, (c) the Fe-terminated surface, and (d) the Fe-defective surface, respectively. The top panels show the PDOS for the hydrogen and oxygen atoms, and the bottom panels show total DOS for a clean surface. The position of the Fermi energy is indicated by the vertical dashed line, and set to zero.
as well as the Fe atom in the surface. Combined with the adsorption energy, it is reasonable to explain why the sputtered surface containing Fe atoms can chemisorb the water molecule strongly.40 The MS3 in Fig. 4a indicates the electronic structure in the middle state of the water molecule on an O-terminated surface. The highest occupied state of hydrogen atoms is just below the Fermi level, and the photo-excited electrons are easy to transfer from surface to adsorbate in this situation. However, from the middle state to the final state, the hydrogen production energy needs at least 4.93 eV, which is larger than the band gap of hydrogen atom (1.61 eV). This could imply that the electron needs to absorb photons with higher energy. Otherwise, this process will not be realized. Fig. 4b shows that the reactive process encounters this phenomenon on an O-defective surface. Next, we analyse the electronic structure of water on the defect-free Fe-terminated surface. The system needs to absorb energy from outside to deform in the process of transformation from the initial state (IS) to the first middle state (MS). The electronic transfer is not involved, but the PDOS of hydrogen atoms is broadened and delocalized due to the variation of molecular structure. In the first middle state, the band gap of
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hydrogen atoms is 3.39 eV. If photons with an energy larger than 3.39 eV can be captured by the electron of hydrogen atom, the electron in the deep energy will be excited to the conduction band. Then, the electron can transfer to the hydrogen atom, and the hydrogen atom will split from the oxygen atom in the second transition state. In the above-mentioned process, photoassistance is needed to overcome the energy barrier of 2.77 eV. Hydrogen can then be produced via changes in internal electronic conversion. Finally, electron coupling occurs only between the O atom and the surface in the final state (FS), while molecular hydrogen has no action with the O atom and desorbs from the surface. For the water molecule on the Fe-defective terminated surface, the electronic structure in the middle state localizes around 4 eV, and it is hard for electron to transfer between the surface and hydrogen atoms.
Conclusion In summary, we applied transition state theory based on DFT to investigate the reactive pathway of chemical behaviour of a
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single water molecule on four kinds of hematite (0001) surfaces at an electronic level. The calculated free energy profiles show that the heterolytic dissociation of the water molecule will meet an energy barrier of less than 0.93 eV on the hematite (0001) surface. The hydrogen generation on the hematite (0001) surface is a crucial step in the whole process, rather than water dissociation. For hydrogen generation, the perfect Fe-terminated surface is preferable due to the small energy barrier of 2.77 eV in this reaction pathway, which can be overcome by harvesting the phonons in visible light and ultraviolet light. The existence of Oor Fe-vacancies on hematite (0001) surfaces are not conductive to the photoelectrochemical reaction of water splitting, because the energy barrier is not decreased effectively. Our results are conductive to a comprehensive understanding of the reaction mechanism of water splitting and hydrogen production by hematite, and helpful for the morphology design of hematite (0001) surfaces for photoelectrochemical applications.
Acknowledgements This work is financial supported by the Fundamental Research Funds for the Central Universities (N130405003 and N110810001), National Natural Science Foundation of China (No. 51001025), and National High Technology Research and Development Program of China (Grant No. 2013AA031601).
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Hydrogen generation by water splitting on hematite (0001) surfaces: first-principles calculations Haijun Pan,ab Xiangying Mengab and Gaowu Qin*a The surface chemical activity is a critical factor affecting the photocatalytic efficiency of hematite. In this study, we investigate systematically the reaction kinetics of water heterolytic dissociation (H2O–OH
+ H+) and
hydrogen generation by water splitting on four kinds of hematite (0001) surfaces, namely perfect and defective O- and Fe-terminated surfaces, at the electronic level based on first-principles calculations. The simulation results illustrate that the chemical reaction rate for the dissociation and hydrogen generation is sensitive to the morphology of the hematite (0001) surface. For water heterolytic dissociation, the hydrogen atom is apt to Received 20th July 2014, Accepted 15th October 2014
drop from water molecules on the perfect O-terminated (0001) surface without energy consumption.
DOI: 10.1039/c4cp03209h
on which the whole photoelectrochemical process needs to overcome a rate determined barrier of
However, the Fe-terminated (0001) perfect surface is a preferable candidate for hydrogen generation, 2.77 eV. Our investigation shows that O- or Fe-vacancy on hematite (0001) surfaces is not conductive
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to hydrogen generation by water splitting.
Introduction Since the discovery of the Fujishima–Honda effect,1 many metal oxide semiconductors have been investigated to solve environmental and energetic problems. Hematite (a-Fe2O3) has been found to meet stringent requirements for photocatalytic hydrogen production by water splitting. Hematite is cheap, nontoxic, abundant, and chemically stable to photo-corrosion; its desired band gap of B2.1 eV can capture roughly 40% of the incident solar spectrum.2–5 With respect to the water splitting redox potential, the valence band edge is appropriately located to oxidize water, but the position of the conduction band edge is just below the reduction level of H+ to drive hydrogen evolution.6,7 This mismatch of conduction band edge could be modified by elemental doping and surface treatment.5,8–11 Hematite has been considered a promising photocatalytic material, especially when used for hydrogen generation by water splitting. Experimental exploration of water splitting as well as hydrogen generation show that the interface structure, electronic structure, local atomic coordination, surface functional groups, and the composition and topology of the surface are intrinsic factors affecting the chemical activities of semiconductor surfaces.
a
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, People’s Republic of China. E-mail: [email protected] b College of Sciences, Northeastern University, Shenyang 110819, People’s Republic of China
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In the case of hematite, the low redox kinetics on its surface have been an obvious obstacle that hindered the development of efficient photocatalytic devices.2,12 Thus, it is of great significance to understand the chemical reaction mechanism of water molecules on different hematite surface morphologies, from both thermodynamic and kinetic aspects. It has been experimentally reported that the hematite (corundum) (0001) surface is energy preferable under different conditions. The different morphologies of the hematite (0001) surface were first reported experimentally by Wang et al.13 Their SEM results verified that the surface of the hematite sample prepared in ambient oxygen atmosphere were terminated with two kinds of monophonic configurations: a single Fe-layer (Fe–O3–Fe–R) and an O-layer (O3–Fe–Fe–R), in which R represented the corresponding repeated unit in the bulk structure. Subsequently, Shaikhutdinov et al. explored the surface structures of hematite under oxygen partial pressure in a span of 10 6–1 mbar at the temperature of 850 1C.14 The co-existence of these two surface structures was discovered at an interval of 10 4–10 1 mbar. A similar surface structure is also replicated by chemical vapor transport, and characterized using atomic force microscopy (AFM).15 According to the present experimental results, it can be seen that the (0001) surface termination of the hematite sample is basically composed of a single Fe-layer (Fe–O3–Fe–R) and an O-layer (O3–Fe–Fe–R), and we construct water-contacted surfaces based on the two Fe- and O-terminations in this work. For potential technological application as a photocatalyst, it is essential to understand comprehensively the surface activity
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of water splitting, and make appropriate modifications. Previously, the thermodynamic behaviour of water molecules on pure or doped hematite (0001) surfaces were investigated by a series of experimental and theoretical works.16–20 These explorations provided the static free energies of some isolated reaction states, but to what extent these state-transfers can be realized are still unclear. Moreover, hydrogen generation is not mentioned in earlier studies because of the unfavourable conduction band edge with respect to the reduction potential of water. In this regard, the reaction barriers and intrinsic electronic mechanism of hydrogen generation by water-splitting on hematite perfect- and defective-(0001) surfaces are calculated and discussed. In this paper, we found some exothermic reactions during the water-splitting actually cannot really spontaneously become realized as the thermodynamics predicts, thus some of our kinetic results are unanticipated by previous works.
Computational method and modelling All calculations were performed in the framework of the spinpolarized density functional theory with the Vienna ab initio simulation package VASP.21 The pseudopotentials and wave functions were generated within the projector-augmented wave method.22 The electronic wave functions were expanded in a plane-wave basis set, and a kinetic-energy cutoff of 550 eV was adopted. For the exchange correlation functional, the generalized gradient approximation of Perdew, Burke and Ernzerhof was employed.23 Because of the strong on-site coulomb repulsion between the localized d-shell electrons, the effects of electron correlation were taken into account within the framework of GGA + U according to Dudarev et al.24 The value of U–J was set to 4.3 eV, which produced a series of data consistent with experiments25 and previous theoretical study.26 Along the c-axis of the hexagonal unit cell, hematite is alternately stacked by iron and oxygen atom layers, as shown in Fig. 1a. Thus, we could construct three possible surfaces: the single Fe-layer (Fe–O3–Fe–R), the double Fe-layers (Fe–Fe–O3–R), and the O-layer (O3–Fe–Fe–R), in which R represented the corresponding repeated unit in the bulk structure.16 However, based on experimentally measured stable surface terminations,
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the O-terminated and single Fe-terminated hematite (0001) surfaces were modelled by periodical slabs with seven atomic layers in simulation. In order to study the reaction pathway of a single water molecule on the hematite (0001) surface, a (2 2) supercell was used to avoid spurious interactions between water molecules in neighbouring cells in the low-coverage regime. A vacuum space with a vertical distance of 15 Å was used to separate the electronic interaction between the periodic atom layers. In Fig. 1b and c, the top views of perfect O-terminated and Fe-terminated surface are displayed, and the possible adsorption sites of water molecules are indicated. Non-stoichiometric surfaces were obtained by artificially removing one oxygen and iron atom from the corresponding perfect-surface models. The resulting concentration of O- and Fe-vacancies was 1/12 ML and 1/4 ML, respectively. The top six layers were allowed to relax, whereas the atomic layer at the bottom was fixed at bulk truncated lattice constants (a = b = 4.82 Å and c = 13.06 Å), which were calculated by the GGA + U method. The Brillouin-zone integration was performed using Monkhorst–Pack grids of 2 2 1.27 The structure was relaxed until the force is less than 1 meV Å 1 per atom. The global energy minimum adsorption structure of water molecule on each surface was deemed as the initial state, and the corresponding structure of hydrogen generation was set as the final state. Between the initial state and final state, a series of intermediate images along the reaction path way were linearly interpolated. Our designed reaction pathway has been commonly accepted and adopted by similar investigations.28–30 Reaction pathways and energy barriers were calculated using the climbing image nudged elastic band (cNEB) method, in which the apex of the minimum energy paths (MEP) were assigned to be the saddle points.31
Results and discussion Stable structure and adsorption energy of the isolated water molecule on (0001) surfaces We began with searching the stable adsorption structure of the single water molecule on the hematite (0001) surface in this study. From the top view of the O-terminated hematite (0001) surface
Fig. 1 (a) The hexagonal unit cell of hematite. (b) Top views of the O-terminated hematite (0001) (2 2) surface supercells. (c) Top views of the Fe-terminated hematite (0001) (2 2) surface supercells. The possible adsorption sites of water molecule are represented by top, hollow and bridge on the surface. Oxygen and iron atoms appear as red and golden yellow spheres, respectively. The defective surface is obtained by manually removing one oxygen and iron atom from the topmost layer in the above mentioned surface, respectively.
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Fig. 2 Geometries and relative energies obtained from theoretical calculations of hydrogen generation from water splitting on the O-terminated (a) and defective O-terminated hematite (0001) surface (b). In each process, the energy of the initial state is set to zero. Transition states are marked by green font.
(shown in Fig. 1b), there are three possible adsorption positions: top site, bridge site, and hollow site. In order to obtain a stable structure of the adsorbate–surface system, the isolated water molecule is placed at a distance of 2 Å above three possible adsorption sites. The final optimized configuration shown in Fig. 2a (noted as IS) is in global energy minimum, in which the water molecule locates on the top of the O atom, and the plane formed by H–O–H is parallel to the surface. The distance between the O atom in the adsorbate and surface is 2.49 Å, and the O–H bond lengths of the water molecule are 0.98 Å. Compared to the structure of water adsorption on well ordered O-terminated (0001) surface, the water molecule is prone to adsorb on the defective site, as shown in Fig. 2b. The O–H bond lengths in the adsorbed water molecule are 0.98 Å, and the O–H bonds point to the oxygen atoms in the neighbourhood. The distance between the O atom of the adsorbate and Fe atoms in the sub-layer is 2.18 Å and 2.48 Å, respectively. Fig. 3a shows the structure of the isolated water molecule adsorption on an Fe-terminated (0001) surface. The oxygen atom of the adsorbate interacts directly with the coordinately unsaturated Fe atom, and the Fe–O bond length is 2.16 Å. One of the O–H bonds stretches into the subsurface, pointing to the nearest O atom, and the bond length is 0.01 Å longer than that
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in the water molecule. The other O–H bond points toward the vacuum with the same bond length as the water molecule. The structure of water adsorption on the defective Fe-terminated surface is shown in Fig. 3c, which is similar to the geometry on the perfect surface. Except for the reduction of Fe–O bond length (2.15 Å), there is little variation for the geometry of the O–H bond. The adsorption energy is used in order to evaluate the stability of the isolated water molecule–surface structure, which is defined as Eads = EH2O/sur
Esur
EH2O
where EH2O/sur is the total energy of the combined system (H2O bound to surfaces), Esur is the energy of the surface alone, and EH2O refers to the energy of a single H2O molecule. The adsorption energy is merely 0.49 eV for the H2O/perfect O-terminated surface system, which is considered as physisorption. The adsorption energy is enhanced to 1.08 eV when the surface contains O-defects. When the water molecule bonds to the perfect and defective Fe terminated surfaces, the system will release 2.33 eV and 1.49 eV, respectively. These results indicate that the stability of the adsorption structure is gradually
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Fig. 3 The variation of configurations and relative energies in the theoretically derived reaction pathway of hydrogen atom splitting from a water molecule on an Fe-terminated hematite (0001) surface (a) and a defective Fe-terminated hematite (0001) surface (b), and the variation of configurations and relative energies in the theoretically calculated reaction steps of hydrogen generation on an Fe-terminated hematite (0001) surface (c) and a defective Fe-terminated hematite (0001) surface (d). In each process, the energy of initial state is set to zero. Transition states are marked by green font.
improved with the increased concentration of iron atoms in the surface. The reactive kinetics of the single water molecule on (0001) surfaces The stable configurations of water molecules adsorbed on O-terminated (0001) surfaces are set as the initial states (IS) in each reactive pathway. The potential energy profiles and local structures of reactants in the reactive pathway are shown in Fig. 2a and b. On the O-terminated stoichiometric (0001)
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surface, there is no energy barrier for the first hydrogen atom splitting from water molecule, which indicates that spontaneous decomposition is achievable on the low coverage O-terminated surface. The subsequent intermediate state (MS1) is still inclined to dissociate due to its energetic instability. In the transformation pathway toward the second middle state (MS2), an energy barrier of 0.19 eV is encountered. However, the required energy can be supplied easily from the initial dissociation energy of 1.78 eV to overcome this activation barrier. Two detached hydrogen atoms adsorb on the different O atoms on the surface,
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and then achieve a more stable geometry through a transition state (TS2: 0.62 eV). The generation of hydrogen from the Oterminated (0001) surface is reached directly by receiving an amount of energy larger than 4.93 eV. The generated hydrogens are 3.12 Å away from the surface, and there is no interaction between them. On the whole, this process is endothermic, and needs to absorb at least 1.73 eV from the external environment. On the oxygen defective-terminated surface, the transition state (TS1) will be reached if at least 0.35 eV is absorbed in the system, in which the O–H bond will substituted by two hydrogen bonds as shown in Fig. 2b. Even at room temperature, this step can work if a small amount of energy is provided. During the generation of hydrogen, the H atom in the O–H bond and the other H get closely with each other, and then form directly if the energy barrier of 5.18 eV (TS2) is surmounted. Finally, the distance between the molecular H2 and the surface increases to 2.55 Å, at which point it can easily desorb from the surface. In total, the system harvests at least 4.51 eV in this reactive pathway. We next display the microscopic mechanism of water dissociation on the perfect Fe-terminated surface in Fig. 3a. The splitting of hydrogen atom in molecules is thought to slide into the nearest oxygen atom in the sub-layer, which goes through a larger energy barrier of 0.69 eV. Taking account of this large energy barrier as well as the higher energy in the final state (FS), it is hard for this process to occur. Thus, the reaction route of the hydrogen generation is designed to get away directly from the water molecule (see Fig. 3c). In this process, the adsorbed water molecule raises its head at the beginning, and needs to absorb 1.81 eV to achieve the first transition state (TS1). Then, the water molecule continues to move towards the vacuum region, leading to an intermediate state (MS1). From the IS to MS1, the Fe–O bond keeps on increasing, from 2.16 Å to 2.24 Å. After the second energy barrier of 2.77 eV (TS2), these two hydrogen atoms are linked by the hydrogen bond, and restricted to the surface. To reach the final state, they must obtain at least 0.44 eV to get away from surface. We also calculate the process of hydrogen atom splitting from a water molecule on the Fe-defective surface in Fig. 3b. The large barrier as well as energy difference suggests this process is difficult to realize. The reactive pathway shown in Fig. 3d needs to go through two transition states (TS1 and TS2) and an intermediate state (MS) from initial state to final state. The first energy barrier is associated with the breaking of the O–H bond and the formation of a hydrogen bond. After climbing this barrier (4.9 eV), the system relaxes to the MS state, in which molecular hydrogen has been generated. This intermediate configuration will transform into the final state by overcoming the second barrier of 0.37 eV, which is related to the breaking of weak electronic interaction between the hydrogen and the surface. Due to the improper position of the conduction band edge, a majority of studies mainly concentrated on the chemical behaviour of the H2O molecule on the Fe2O3 (0001) surface under photoelectrochemical conditions as a photoanode.32–34 In the framework of the one electron transfer mechanism, the hydrogen ion is supposed to break away directly from the water molecule in each step. Thus, the energy barrier is neglected in
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the reaction pathway leaving the reaction rate unanticipated. In this study, we described the basic microscopic picture of the kinetics of elementary reactions for a quasi-isolated water molecule on the hematite surface, namely, dissociation and hydrogen generation. Recently, the similar behaviour of a water molecule has also been investigated on the surface of other oxide semiconductors by other researchers.28–30 For example, on the Ga-terminated (0001) surface of GaN, a transition state with a barrier energy of 1.42 eV has been encountered in the minimum reaction path of hydrogen generation.28 On ZnS wurtzite (0001) surfaces, this minimum barrier has been predicted to be 2.24 eV by our group.29 Compared to the previous works, the reaction rate is a little lower, with an energy barrier of 2.77 eV. The photoelectrochemical properties of Fe2O3 have been investigated extensively by our group in the past few years,35–37 from both experimental and theoretical aspects. We find that although the band structures of modified hematite meet the chemical potential of oxidation–reduction reactions, hydrogen generation by water-splitting cannot occur spontaneously unless a bias voltage is applied to the system. The reaction thermodynamics can hardly account for this phenomenon, even taking into the band-bending at the solid–liquid interfaces and experimental errors. For example, Hu et al. introduce a surface treatment technique to decompose directly molecular water by hematite, but the photocurrent ultimately disappears without bias.8 In this work, our simulation results indicate that the hydrogen generation on the hematite (0001) surface is a crucial step in the overall process, and the extra bias is needed to overcome the kinetic energy barrier. Furthermore, the perfect Fe-terminated surface is more appropriate for hydrogen generation. Our simulation results might be of great value in further developing modified hematite with higher photocatalytic activity by proper surface treatment. Up to now, the O-terminated and Fe-terminated (0001) surfaces can appear alone at an oxygen partial pressure of 1 mbar and 10 5 mbar, respectively, at the temperature of 850 1C.14 Beside, Fe-terminated surface has been realized using the method of molecular beam epitaxy in the ultra-high vacuum environment by some groups.38,39 Electronic structure In order to gain insight into the chemical reaction mechanism in these pathways, we calculate systematically the electronic density of states for the reactants. This paper is mainly concerned with the chemical bond mechanism between the adsorbate and surface, and evaluates the ability of photon-excited electrons to transfer from the surface to a hydrogen atom. Furthermore, it is of importance to determine whether the absorbed solar energy can be converted to chemical energy, and stored in hydrogen bond.28,29 As can be seen in Fig. 4a, there is a slight interaction between a water molecule and the surface in the initial state (IS), which can be indicated by the isolated molecular orbital of adsorbate. The PDOSs of initial state (IS) in the other three models shown in Fig. 3b–d show that the surface region containing Fe atoms with dangling bonds is active toward the water molecule. The interaction between surface and water molecules is mainly through the oxygen atom in the adsorbate
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Fig. 4 Projected density of states (DOS) for water molecules on the hematite (0001) surface, and the total DOS for surface in the reactive pathway. The electronic structures for key points in the reactive pathway of water molecule on (a) the O-terminated surface, (b) the O-defective terminated surface, (c) the Fe-terminated surface, and (d) the Fe-defective surface, respectively. The top panels show the PDOS for the hydrogen and oxygen atoms, and the bottom panels show total DOS for a clean surface. The position of the Fermi energy is indicated by the vertical dashed line, and set to zero.
as well as the Fe atom in the surface. Combined with the adsorption energy, it is reasonable to explain why the sputtered surface containing Fe atoms can chemisorb the water molecule strongly.40 The MS3 in Fig. 4a indicates the electronic structure in the middle state of the water molecule on an O-terminated surface. The highest occupied state of hydrogen atoms is just below the Fermi level, and the photo-excited electrons are easy to transfer from surface to adsorbate in this situation. However, from the middle state to the final state, the hydrogen production energy needs at least 4.93 eV, which is larger than the band gap of hydrogen atom (1.61 eV). This could imply that the electron needs to absorb photons with higher energy. Otherwise, this process will not be realized. Fig. 4b shows that the reactive process encounters this phenomenon on an O-defective surface. Next, we analyse the electronic structure of water on the defect-free Fe-terminated surface. The system needs to absorb energy from outside to deform in the process of transformation from the initial state (IS) to the first middle state (MS). The electronic transfer is not involved, but the PDOS of hydrogen atoms is broadened and delocalized due to the variation of molecular structure. In the first middle state, the band gap of
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hydrogen atoms is 3.39 eV. If photons with an energy larger than 3.39 eV can be captured by the electron of hydrogen atom, the electron in the deep energy will be excited to the conduction band. Then, the electron can transfer to the hydrogen atom, and the hydrogen atom will split from the oxygen atom in the second transition state. In the above-mentioned process, photoassistance is needed to overcome the energy barrier of 2.77 eV. Hydrogen can then be produced via changes in internal electronic conversion. Finally, electron coupling occurs only between the O atom and the surface in the final state (FS), while molecular hydrogen has no action with the O atom and desorbs from the surface. For the water molecule on the Fe-defective terminated surface, the electronic structure in the middle state localizes around 4 eV, and it is hard for electron to transfer between the surface and hydrogen atoms.
Conclusion In summary, we applied transition state theory based on DFT to investigate the reactive pathway of chemical behaviour of a
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single water molecule on four kinds of hematite (0001) surfaces at an electronic level. The calculated free energy profiles show that the heterolytic dissociation of the water molecule will meet an energy barrier of less than 0.93 eV on the hematite (0001) surface. The hydrogen generation on the hematite (0001) surface is a crucial step in the whole process, rather than water dissociation. For hydrogen generation, the perfect Fe-terminated surface is preferable due to the small energy barrier of 2.77 eV in this reaction pathway, which can be overcome by harvesting the phonons in visible light and ultraviolet light. The existence of Oor Fe-vacancies on hematite (0001) surfaces are not conductive to the photoelectrochemical reaction of water splitting, because the energy barrier is not decreased effectively. Our results are conductive to a comprehensive understanding of the reaction mechanism of water splitting and hydrogen production by hematite, and helpful for the morphology design of hematite (0001) surfaces for photoelectrochemical applications.
Acknowledgements This work is financial supported by the Fundamental Research Funds for the Central Universities (N130405003 and N110810001), National Natural Science Foundation of China (No. 51001025), and National High Technology Research and Development Program of China (Grant No. 2013AA031601).
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