Chemical Dynamics of the First Proton Coupled Electron Transfer of Water Oxidation on TiO 2 Anatase

Transcription

1 Supplementary Information Chemical Dynamics of the First Proton Coupled Electron Transfer of Water Oxidation on TiO 2 Anatase Jia Chen, Ye-Fei Li, Patrick Sit, and Annabella Selloni Department of Chemistry, Princeton University, Princeton, NJ S1. Methods and Calculation Details First principles Molecular dynamics 1 (FPMD) simulations and geometry relaxations were performed using the gradient-corrected Perdew-Burke Ernzerhof (PBE) 2 and hybrid PBE0 3 functional, respectively, as implemented in the Quantum-Espresso package. 4 Plane wave basis set and norm-conserving pseudopotentials 5 were used, with the semicore 3s, 3p states of Ti treated explicitly. The plane wave kinetic energy cutoff for the electronic wavefuntions was 70 Ry and its convergence was checked by studying the adsorption energy of a water molecule on the TiO 2 surface. FPMD simulations were carried out using a time step of fs and a Nosѐ-Hoover thermostat 6 set at 330K. PBE0 hybrid functional calculations used an order-n implementation 7 based on Maximally Localized Wannier Functions (MLWFs). 8 Damped molecular dynamics was employed for geometry relaxations until the total electron kinetic energy was less than Hartree. The anatase (101) surface was modeled using a periodic slab with two TiO 2 layers of 24 TiO 2 units and a rectangular surface cell with dimensions Å 2 ; the vacuum layer (12.6Å) between two adjacent TiO 2 slabs was filled by 48 water molecules at a density of g/cm 3 (Figure 1). Due to the large cell size, k-sampling was restricted to the Γ-point. The effect of the thickness of the water layer between adjacent slabs had been checked in a previous comparative study 9 for models with 39 and 78 water molecules. The results showed that the smaller system was sufficient for studying the electronic properties of a single water-tio 2 interface. S2. Study of the Effect of a Uniform Compensating Background of Charge To model the PT in the presence of a surface trapped hole at the TiO 2 /water interface, a charged slab (48H 2 O+TiO 2 +h + ) was considered with a uniform compensating background of charge. To estimate the influence of this background charge, we repeated the calculations for a system (47H 2 O+OH +TiO 2 +h + ) in which the charge of the hole was compensated by the removal of a proton from an adsorbed water molecule on the bottom surface of thetio 2 slab. The potential energy surfaces of proton transfer for the two systems (one S1

2 with background charge and the other with a proton removed) were compared by performing calculations at the PBE level. As shown in Figure S4, we found that the differences between the two potential energy surfaces are less than 0.015eV. We can thus conclude that for our model system the influence of the background charge is negligible. Figure S1. Total potential energies along 10ps FPMD simulations for: a TiO 2 slab in contact with liquid water (48H 2 O+TiO 2 ); a TiO 2 slab with a photo-hole and an adsorbed OH in liquid water (47H 2 O+OH+TiO 2 ). The large initial fluctuations in the MD simulation for (47H 2 O+OH+TiO 2 ) are due to the relaxation and equilibration of the assumed initial liquid water structure configuration. The simulation for (48H 2 O+TiO 2 ) was started from a liquid water configuration taken from the equilibrated (47H 2 O+OH+TiO 2 ) system, and therefore shows much smaller fluctuations throughout. Red circles indicate the snapshots used for the hybrid functional calculations. All snapshots were chosen in the second half of their respective runs and at large time interval one from the other, so as to correspond to well equilibrated and uncorrelated configurations. S2

3 Figure S2. Schematic illustration of the first proton coupled electron transfer and reaction coordinate, Δd O-H = d(o a -H) - d(o b -H), for the description of the PT. Red: O; White: H; Gray: Ti; Blue: surface trapped hole. Figure S3. Energy profiles for proton transfer in the water-hole and surface-hole states for selected configurations along the FPMD trajectory in the upper panel of Figure S1(snapshots II and III). The fact that the two potential energy surfaces do not intersect at all indicates that the ET does not occur during the PT. Since the water hole state (red curve) is less stable than the surface hole state (blue curve), we conclude that the ET occurs preferentially after the PT. S3

4 Figure S4. Potential energy surfaces of proton transfer, H 2 O* HO *+H +, for 48H 2 O+TiO 2 +h + with background charge (blue line), and 47H 2 O+OH +TiO 2 +h + without background charge (red line). d O-H is the bond length of O a -H in Figure S2. Figure S5. ET energy profiles as a function of the distance between the oxygen atom of the adsorbed hydroxyl and the surface hole trapping site (snapshots V and VI in Figure S1) S4

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