Point of Anchor: Impacts on Interfacial Charge Transfer of Metal Oxide Nanoparticles

Transcription

1 Supporting Information Point of Anchor: Impacts on Interfacial Charge Transfer of Metal Oxide Nanoparticles Yi Peng, a,# Bingzhang Lu, a,# Feng Wu, a Fengqi Zhang, b Jia En Lu, a Xiongwu Kang, b Yuan Ping, a, * and Shaowei Chen a,b, * a Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California 95060, USA b New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong , China # These authors contributed equally to the work Table S1. Bader charge of TiO 2-alkyne and TiO 2-carboxy calculated by PBE and HSE06 functionals Exchange Charge of alkyne (electrons) Charge of carboxylic acid (electrons) collection functional on TiO2 isolated difference on TiO2 isolated difference PBE HSE A B Figure S1. Interfacial structures of alkyne-modified TiO2: (A) when the ligands are initially placed on Ti atoms, and (B) the corresponding structure after relaxation, where the ligands moved to the O atoms spontaneously. This suggests that the O atom, rather than the Ti atom, in TiO2 is the anchoring site, forming a Ti-O-C C-, instead of Ti-C C-, interfacial bond. SI-1

2 Figure S2. (A) UV-vis spectra of TiO2-OA. In the inset, α is the absorbance and h is the photon energy. Extrapolation to the x-axis yields a bandgap of ca. 3.2 ev. (B) SSPL spectra of TiO2-OA at various excitation wavelengths ( ex, shown in figure legends). At ex = 305 nm, the emission reaches the maximum (green-shaded). A B Figure S3. Total density of states (TDOS) calculated by HSE06 (top) and PBE (bottom) of (A) TiO2-alkyne and (B) TiO2-carboxy structures.. SI-2

3 DOS (states/cell/ev) Figure S4. Fixed bottom models and corresponding TDOS profiles of TiO2 slabs modified with two ligands. (A) Three-layer fixed bottom and partially-relaxed model with two ligands on one side of the slab surface, and (B) 3-, (C) 5-, (D) 9-layer fully-relaxed surface model with two sides of the slab surface modified by alkyne ligands. (E) The corresponding TDOS profiles of the various models. The isolated interfacial states in the gap can be found for (B), (C) and (D) surface models. Figure S5. Charge transfer between (A, B) alkyne or (C-D) carboxy and TiO2 slab by different exchange correlation functionals: (A, C) PBE and (B, D) HSE06. The isovalue is e/au 3. SI-3

4 (A) (B) Figure S6. Optimized geometrical structures by CDFT calculation of (A) Ti(OH)3-O-C C-CH=CH2, and (B) Ti(OH)3-OOC-CH=CH2. The extra electron is placed and localized on -CH=CH2 and -Ti(OH)3, respectively, for the calculations of electronic coupling (Hab) between the two sites. Within CDFT, an external potential is added to the self-consistent potential entering the Kohn-Sham equations, and its strength is varied self-consistently in order to localize a desired integer number of charges N0 on a specified site. Inset shows the potential energy surface of charge transfer from TiO2 to ligand (e.g., EPy) through an anchor group (i.e., -COO- or -C C-). Hab is the electronic coupling between TiO2 and ligand EPy, which is larger with -C C- than with -COO- and results in a lower barrier WH for excited electron or hole transfer from TiO2 to Epy. Note that PL and photocatalysis involve excited states. In the present study, since we are mainly interested in charge (electron or hole) transfer between TiO2 and ligands (e.g., EPy) bridged by different anchor groups (instead of an electron-hole recombination process), we can still use a Marcus theory type of picture for simplicity, as shown in Figure S6 inset (the exact shape and relative position of potential energy surfaces can be different but does not affect the conclusion here). The physical justification for this picture is the following: after an electron is excited from the valence to conduction bands leaving a hole in the valence band, electron transfer occurs in the conduction band and hole transfer in the valence band. In principle, the system is still neutral. If we consider that electron and hole transfers are relatively uncorrelated (i.e., neglect electron hole interaction), we can add an electron to the system which will occupy the lowest conduction band/unoccupied state and compute the charge transfer rate between TiO2 and ligand; or we can add a hole to the system which will stay at the valence band for hole conduction. The charge transfer direction is determined by the relative band SI-4

5 alignment between TiO2 and the specific ligand (e.g., EPy). In our CDFT calculation (Figure S6), we added an excess electron to the system to simulate the process of excited electron transfer between TiO2 and ligand bridged by anchor groups. When we change the anchor group from -COO- to -C C- (Ranchor in Figure S6 inset), the electrons become delocalized due to the formation of conjugated interfacial bonds that causes a large electronic coupling (HAB in Figure S6 inset) between TiO2 and EPy. If the overall potential energy surfaces of TiO2 and Epy are not strongly modified by the anchor group (i.e., λ and G 0 are unchanged when we change the anchor groups, which is a reasonable assumption considering the relatively small size of the anchor group compared to TiO2 and ligands), the charge transfer barrier (W H ) will diminish, because W H = (λ+δg 0) 2 4λ ( H AB + λ+δg 0 2 (λ+δg 0 )2 4 + H AB 2 ) (S1) which is based on a generalized Landau-Zener theory that correctly provides both the small coupling H AB limit (nonadiabatic charge transfer in the Marcus theory) and the large coupling limit (adiabatic charge transfer in the transition state theory). 1,2 In the CDFT calculation (Figure S6), we found the electronic coupling between TiO2 and Epy (HAB, which is computed with one extra electron to represent the excited electron transfer process) is five times larger with the -C C- anchor group than with -COO-. Based on the Eq. (S1), WH will be much smaller, and the charge transfer rate (k) will be much larger due to the exponential dependence of k on W H (Eq. S2), 1,3 k = κ el ν n exp ( W H k B T ) In other words, the results that we obtained in ground state charge transfer demonstrate that conjugated interfacial bonding interactions indeed improve charge delocalization across the oxide-ligand interface, which is essentially related to the enhanced electronic coupling between TiO2 and EPy bridged by -C C-, in comparison to -COO-. (1) Wu, F.; Ping, Y. Combining Landau-Zener Theory and Kinetic Monet Carlo Sampling for Small Polaron Mobility of Doped BiVO4 from First-principles. J Mater Chem A 2018, in press. (2) Blumberger, J.; McKenna, K. Constrained density functional theory applied to electron tunnelling between defects in MgO. Phys. Chem. Chem. Phys. 2013, 15, (3) Oberhofer, H.; Reuter, K.; Blumberger, J. Charge Transport in Molecular Materials: An Assessment of Computational Methods. Chem Rev 2017, 117, (S2) SI-5

6 Figure S7. Charge transfer between ligands and metal oxide slabs: (A) ZnO-alkyne, (B) RuO2-alkyne, (C) ZnO-carboxy, and (D) RuO2-carboxy. The cyan area indicates electron loss, and yellow area indicates electron gain. The isosurface value is e/au 3. SI-6

7 Figure S8. SSPL spectra of (A) monomeric EPy and (B) TiO2-EPy at various excitation wavelengths (shown in figure legends). SI-7

8 Figure S9. (A) UV-vis and (C) excitation and emission spectra of TiO2-PyCA and monomeric PyCA. Steady-state excitation and emission spectra of (B) TiO2-PyCA and (D) monomeric PyCA at various excitation wavelengths. (E) UV-vis spectra of monomeric EPy and a mixture of EPy and TiO2-HC8. Inset is the corresponding steady-state excitation and emission spectra. (F) TRPL decay plots of monomeric PyCA and TiO2-PyCA. Symbols are experimental data and solid curves are single-exponential fits. SI-8

9 Figure S10. (A) Interfacial configuration and (B) DOS of a phenylacetylene-modified TiO2 slab. B Figure S11. (A) UV-vis spectra of TiO2-EPy and TiO2-PyCA at the same concentration of 0.4 mg/l. The absorbance at 365 nm is slightly higher with TiO2-EPy (0.062) than with TiO2-PyCA ( ). (B) Photocatalytic performance of TiO2-EPy (sphere) and TiO2-HC8 (star) manifested by the variation of MB peak absorbance with photoirradiation time. Symbols are experimental data and lines are linear regressions. SI-9