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Pt-Doped #-FeO for Enhanced Water Splitting Efficiency: A DFT+U Study Ofer Neufeld, and Maytal Caspary Toroker J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512002f • Publication Date (Web): 27 Feb 2015 Downloaded from http://pubs.acs.org on March 3, 2015

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Pt-Doped α-Fe2O3 for Enhanced Water Splitting Efficiency: A DFT+U Study Ofer Neufeld1 and Maytal Caspary Toroker2,* 1

The Nancy and Stephen Grand Technion Energy Program and 2Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel Keywords: Water splitting, Density Functional Theory, DFT+U, Iron oxides, Pt-doping. Abstract Hematite (α-Fe2O3) is commonly considered for converting solar energy into hydrogen fuel through water splitting. Recent experiments performed in 2013 reached a maximum efficiency in Fe2O3 photoelectrochemical cells, while using platinum-doped Fe2O3. In order to understand how platinum increases efficiency we use density functional theory +U (DFT+U) method to model the bulk and the (0001) surface of platinum-doped Fe2O3. We also give a unique ligand field theory combined with Bader charge analysis to explain changes resulting from symmetry breaking by the dopant. First, we find that although platinum has a lower oxidation state than usual n-type dopants, platinum donates electrons. We find a theoretical ideal doping range of 0.64-2.96%at for enhanced electron conductivity, which is within the optimal range obtained by previous experiments. Second, we find that the energy gap decreases upon doping, improving solar energy absorption. Third, in agreement with previous experiments, we calculate an unfavorable increase in overpotential for oxidizing water upon platinum doping. Since platinum has both good and bad affects, we recommend bypassing this duality by platinum doping with a gradient-based strategy: high doping in the bulk for enhanced conductivity, and low doping at the surface to not interfere with catalysis. We anticipate that experimentally testing our proposed strategy will advance the development of better electrodes for photoelectrochemistry. *Corresponding author: E-mail: [email protected] , Tel.: +972 4 8294298. 1 ACS Paragon Plus Environment

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I.


Introduction

A promising route in the search for renewable energy sources is using solar energy to split water into oxygen and hydrogen.1-3 This process is environmentally “clean”, i.e., does not produce greenhouse gases. Other advantages include earth’s large water reservoir and the high energy density of hydrogen fuel.

Solar energy conversion into hydrogen can be accomplished in a photoelectrochemical (PEC) cell (see Figure 1), where solar energy is absorbed at the semiconductor photoanode. As a result of absorbing photons, excited electrons transport through the circuit and arrive at the opposing cathode where they take part in the half-cell reaction of hydrogen reduction. Holes transport in the opposite direction toward the anode surface and complete the other half-cell reaction of water oxidation. Overall, this process splits water to produce hydrogen and oxygen gas 4.

Figure 1 - Diagram for photoelectrochemical water splitting with a Fe 2O3 photoanode performing oxygen evolution (oxidation reaction) and a cathode performing hydrogen evolution (reduction reaction).𝚽 is the externally applied bias,

is the solar photon frequency, Wd is the width of the depletion layer, and Eg is the

band gap.

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Conversion efficiencies of PEC cells are limited by the absorbing electrode's intrinsic qualities. The electrode should be a good conductor, a good solar light absorber, a good catalyst, and have an appropriate band edge alignment for water splitting. Furthermore, the electrode should be relatively inexpensive, abundant, nontoxic, and maintain stability in operating conditions. Most of these qualities can be found in hematite (α-Fe2O3, α will be dropped from this point on),4-5 and therefore this material has been widely studied for PEC cells.6-9 However, Fe2O3 also has disadvantages, including low charge mobility,10-11 high electron-hole recombination rate, and a large overpotential of 0.5-0.6[V] that is required for water oxidation.4 The efficiency of a PEC cell containing Fe2O3 can be amplified through doping.12-16 In particular, measurements show a significant increase in efficiency and photo-current in platinum (Pt)-doped cells.16-21 In these measurements, Pt increases electron conductivity and therefore was regarded as an n-type dopant. Pt also changes the electrode's morphology, causing smaller grain size, larger surface area, and a more uniform and dense Fe2O3 film, which are thought to aid in charge transport throughout the electrode.19-21 In fact, Jae Young Kim et al.20, using Pt-doped Fe2O3 with a single-crystalline “wormlike” morphology and a cobalt phosphate co-catalyst manufactured the world's highest record for a Fe2O3 PEC cell current density in 2013. All of these studies report an optimum in Pt-doping in the 0.1-4%at range, yet no study has fully explained why this is the optimal range. In addition, no study has given a complete explanation to why Pt is a successful dopant.

Therefore, the goal of this research is to use electronic structure theory to shed light on the influences Pt-doping has on the properties of Fe2O3 which are important for solar energy conversion. We go beyond previous work on first principles calculations of bulk Pt-doped


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Fe2O316 by extending the analysis to account for atomic Bader charges. We give a unique assignment to atoms according to their site symmetry relative to the Pt dopant position and provide a novel explanation for routes of electron transport and for the observed ideal doping range. Furthermore, we calculate the free energies for water oxidation on Pt-doped Fe2O3(0001) and find that Pt reduces the catalytic activity of the surface.

The paper includes a computational details section with details on a Density Functional Theory (DFT) method that includes an exchange term (DFT+U) needed for calculating the electronic structure of Fe2O3. This section also includes details about calculations of the free energies required for intermediate steps in the water oxidation reaction. The results and discussion section covers our electronic structure analysis of the bulk and the (0001) surface of Pt-doped Fe2O3 and our explanation for the ideal doping concentration range. Finally, the conclusion section contains our proposed gradient doping scheme.

II.

Computational Methods

All calculations were performed using the Vienna Ab-initio Simulation Package (VASP).22-24 We chose spin polarized DFT+U formalism of Dudarev et al.25, since Fe2O3 is a first row transition metal oxide containing highly correlated 3d electrons, whose electronelectron interactions are ill described by regular exchange-correlation (XC) approximations of DFT.26-28 A U value of 4.3[eV] that was derived ab-initio was chosen for iron (Fe) atoms in order to best describe the ground state properties of Fe2O3.29-31 A Perdue-Burke-Ernzerhof (PBE)32-33 XC functional was chosen due to previous results correctly describing similar systems containing Fe2O3 and Pt metal.12,


16, 34-36

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augmented wave (PAW) potentials37-38 were used to represent the frozen core electrons and nuclei of each atom. For all bulk calculations the 6s, 5p and the 5d orbitals of Pt, 4s and 3d of Fe and 2s and 2p of O were used as valence shells and all other inner electrons replaced by appropriate PAW potentials. In surface slab calculations the 3p Fe electrons were also used in the Fe valence shell since surface properties required the addition of Fe p orbitals and since we wanted to compare to previous work on doped Fe2O3(0001) that included explicit Fe p orbitals.12

The Kohn-Sham equations were solved with a plane wave basis set in three dimensional periodic boundary conditions to self-consistency with a tolerance of 10-5[eV] in total energy. Symmetry was not imposed to allow an accurate description of distortions within the lattice. Kspace integration was performed using the tetrahedron method with Blöchl corrections.39-40 Ionic charges were calculated using the Bader charge scheme.41 The Bader Fast Fourier Transform (FFT) grids were converged for ionic charges tolerances of 0.05e.

Our calculations involved three main structures, including pure Fe2O3 in the rhombohedral primitive cell, Pt-doped Fe2O3 in the rhombohedral 2x2x2 supercell, and a (0001) (1x1) surface slab for both pure and Pt-doped Fe2O3. For the slab structure, we calculated the free energies required for intermediate reaction steps of water oxidation. Unit cells can be found in supplementary information. The various details of each computation are presented below.

1. Pure Fe2O3: The pure Fe2O3 structure was used for comparison to Pt-doped Fe2O3. Pure Fe2O3 has a 10-atom primitive rhombohedral cell (Figure 2a). The full long range antiferromagnetic ordering of Fe2O3 was taken into account.42-43 A 7x7x7 gamma-centered k-space grid and a plane wave energy cutoff of 700[eV] converged the cell energy to a tolerance of

A DFT+U Study - ACS Publications - American Chemical Society

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