Significantly Enhanced Visible Light Photoelectrochemical Activity in ...
Supporting Information for
Significantly Enhanced Visible Light Photoelectrochemical Activity in TiO2 Nanowire Arrays by Nitrogen-Implantation Gongming Wang,1,2† Xiangheng Xiao,1,3† Wenqing Li,3 Zhaoyang Lin,1 Zipeng Zhao,4 Chi Chen,5 Chen Wang,2,4 Yongjia Li,4 Xiaoqing Huang,4 Ling Miao,5 Changzhong Jiang,3,* Yu Huang,2,4 and Xiangfeng Duan1,2* 1
Department of Chemistry and Biochemistry, University of California, Los Angeles,
California 90095, USA. 2
California Nanosystems Institute, University of California, Los Angeles, California
90095, USA. 3
Department of Physics, Hubei Nuclear Solid Physics Key Laboratory and Center for
Ion Beam Application, Wuhan University, Wuhan, 430072, People’s Republic of China. 4
Department of Materials Science and Engineering, University of California, Los
Angeles, California 90095, USA. 5
School of Optical and Electronic Information, Huazhong University of Science and
Technology, Wuhan 430074, People’s Republic of China. *Corresponding email: [email protected] and [email protected] † These authors contribute equally to this work.
Methods: Fabrication of TiO2 and N-TiO2 NW arrays: Rutile TiO2 NW arrays on FTO glass were synthesized using a well-developed hydrothermal method.1, 2 The obtained TiO2 sample was annealed in air at 550 °C for 2 hours, before nitrogen implantation. Nitrogen was implanted into TiO2 NW arrays at room temperature at 30 kV with different ion dosage, using a metal vapor vacuum arc (MEVVA) ion source implanter. The implanted sample was then annealed in air at 350 °C for 10 minutes. Anatase TiO2 NW: Anatase TiO2 NWs were prepared on a titanium foil by using a hydrothermal method. A clean Ti foil was placed in a Teflon-lined stainless-steel autoclave filled with an aqueous 1.0 M NaOH solution and was then heated at 180 oC for 2 hour. Then, the Ti foil was taken out and washed with DI water and further immersed in an aqueous 1 M HCl solution for 10 min to exchange sodium ions. Finally, the Ti foil with NWs was annealed in air at 500 oC for 2h. P25 thin film: 1 g Commercial Degussa P25 TiO2 powder was added into a mixture solution with 0.3 g polyethylene glycol (PEG) (20,000), 1 mL DI water, and 50 µL acetylacetone. Then, the solution was then sonicated for 1 hour to let TiO2 powders well disperse and form a homogenous paste. P25 films were fabricated by spin-coating the prepared TiO2 paste on FTO glass at 1000 rmp. Finally, the P25 samples were annealed in air at 550 oC for 2 h to remove the organic binders. (Photo)electrochemical
characterization:
Linear
sweeps,
I-t
scans
and
electrochemical impedance spectra were collected in 1.0 M NaOH aqueous solution
(pH = 13.6)
by a Versastudio electrochemical workstation (Princeton Applied
Research), with Ag/AgCl as the reference electrode and a Pt wire as the counter electrode, under a 300 W xenon lamp (Thermo Oriel instruments, Model: SP66905-3513).
Incident-photon-to-current-conversion-efficiencies (IPCE) were
collected by Versastudio electrochemical workstation with the xenon lamp coupled with a monochromator (Thermal Oriel Instruments, Model 74100). Charge injection efficiency is estimated by measuring the photocurrent density in 1.0 M NaOH solution with/without H2O2. Charge transfer efficiency is measured by the photocurrent densities under 370 and 450 nm light illumination in 1.0 M NaOH solution with H2O2. Material and electrical characterization: SEM images were collected on JEOL 6700 and TEM was carried out on FEI CM 120. X-ray diffraction was collected on a Panalytical X’pert Pro X-ray powder diffractometer with Cu-Kα radiation. TiO2 single nanowire devices were fabricated using electron beam lithography (EBL) followed by electron beam deposition of Ti/Au (180/50 nm) thin films as the contact electrodes. Electrical properties measurements were conducted in ambient conditions in a Lakeshore probe station, coupled with Agilent B2902A precision source/voltage units. Theoretical calculation: Calculations are performed by using SIESTA package based on density-functional theory (DFT).5,
6
3, 4
The exchange-correlation energy
function is Perdew-Burke-Ernzerhof (PBE) based on general gradient approximation (GGA) 7. The double-zeta basis set with polarization functions and the energy cutoff
of 150 Ry are used. The structure optimization is carried out by relaxing the forces on all the atoms until a 0.05 eV/Å force tolerance is reached, using a conjugate gradient method. The energy convergence is 10-4 eV. The escape energy is defined as ΔE = Eesc + Edopant – Einitial, where Einitial, Eesc and Edopant are the calculated total energies of initial doped TiO2 structure, dopant escaped TiO2 structure and escaped dopant. The positiveΔE means that the system needs to gain energy for the dopant escaping.
Figure S1. XRD patterns of pristine TiO2, as prepared N-TiO2 and post-annealed N-TiO2 NWs. The * indicates the peaks from FTO substrate.
Figure S2. The digital picture of PEC measurement, with N-TiO2 as working electrode (middle), Pt as counter electrode (left) and Ag/AgCl as reference electrode (right).
Figure S3. (a) Photocurrent densities of N-TiO2 annealed at 350 oC at 0.5 V vs. Ag/AgCl as a function of nitrogen implanted doses. (b) Photocurrent densities of N-TiO2 as a function of annealing temperatures at 0.5 V vs. Ag/AgCl.
Figure S4. (a) SEM image of anatase TiO2 NWs on titanium foil. (b) Photocurrent densities of anatase TiO2 and N-TiO2 NWs versus time curves collected at 0.5 V vs. Ag/AgCl in 1.0 M NaOH solution. (c) SEM image of commercial P25 thin film on FTO glass. (d) Photocurrent densities of P25 and N-P25 versus time curves collected at 0.5 V vs. Ag/AgCl in 1.0 M NaOH solution.
Figure S5. The schematic diagram shows the charge separation and charge injection process.
Figure S6. Charge separation (C-S) efficiency of N-TiO2 NWs under the wavelength of 450 nm.
Figure S7. Linear sweep voltammograms of N-TiO2 and OER/N-TiO2 under visible light illumination with a scan rate of 20 mV/s.
Figure S8. The comparison of average lifetime distribution of photoexcited carriers in TiO2, N-TiO2 and OER/N-TiO2 NWs.
Figure S9. Photocurrent density of N-TiO2 and OER/N-TiO2 at 0.5 V vs. Ag/AgCl for a week under visible light illumination. The dark color in OER/N-TiO2 after PEC measurement is due to the oxidation of OER catalyst of Co(OH)2 to higher oxidation state.
* Estimated values based on the results in the literature.
Table S1. Comparison of the IPCE values at 450 and 500 nm of this work with literature results.
Reference 1.
Wang, G. M.; Wang, H. Y.; Ling, Y. C.; Tang, Y. C.; Yang, X. Y.; Fitzmorris, R.
C.; Wang, C. C.; Zhang, J. Z.; Li, Y. Nano Lett. 2011, 11, (7), 3026-3033. 2.
Liu, B.; Aydil, E. S. J. Am. Chem. Soc. 2009, 131, (11), 3985-3990.
3.
Roman-Perez, G.; Soler, J. M. Phys. Rev. Lett. 2009, 103, (9), 4.
4.
Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.;
Sanchez-Portal, D. J. Phys.-Condes. Matter 2002, 14, (11), 2745-2779. 5.
Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, (3B), B864-871.
6.
Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, (4A), 1133-1138.
7.
Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, (18),
3865-3868.
Significantly Enhanced Visible Light Photoelectrochemical Activity in TiO2 Nanowire Arrays by Nitrogen-Implantation Gongming Wang,1,2† Xiangheng Xiao,1,3† Wenqing Li,3 Zhaoyang Lin,1 Zipeng Zhao,4 Chi Chen,5 Chen Wang,2,4 Yongjia Li,4 Xiaoqing Huang,4 Ling Miao,5 Changzhong Jiang,3,* Yu Huang,2,4 and Xiangfeng Duan1,2* 1
Department of Chemistry and Biochemistry, University of California, Los Angeles,
California 90095, USA. 2
California Nanosystems Institute, University of California, Los Angeles, California
90095, USA. 3
Department of Physics, Hubei Nuclear Solid Physics Key Laboratory and Center for
Ion Beam Application, Wuhan University, Wuhan, 430072, People’s Republic of China. 4
Department of Materials Science and Engineering, University of California, Los
Angeles, California 90095, USA. 5
School of Optical and Electronic Information, Huazhong University of Science and
Technology, Wuhan 430074, People’s Republic of China. *Corresponding email: [email protected] and [email protected] † These authors contribute equally to this work.
Methods: Fabrication of TiO2 and N-TiO2 NW arrays: Rutile TiO2 NW arrays on FTO glass were synthesized using a well-developed hydrothermal method.1, 2 The obtained TiO2 sample was annealed in air at 550 °C for 2 hours, before nitrogen implantation. Nitrogen was implanted into TiO2 NW arrays at room temperature at 30 kV with different ion dosage, using a metal vapor vacuum arc (MEVVA) ion source implanter. The implanted sample was then annealed in air at 350 °C for 10 minutes. Anatase TiO2 NW: Anatase TiO2 NWs were prepared on a titanium foil by using a hydrothermal method. A clean Ti foil was placed in a Teflon-lined stainless-steel autoclave filled with an aqueous 1.0 M NaOH solution and was then heated at 180 oC for 2 hour. Then, the Ti foil was taken out and washed with DI water and further immersed in an aqueous 1 M HCl solution for 10 min to exchange sodium ions. Finally, the Ti foil with NWs was annealed in air at 500 oC for 2h. P25 thin film: 1 g Commercial Degussa P25 TiO2 powder was added into a mixture solution with 0.3 g polyethylene glycol (PEG) (20,000), 1 mL DI water, and 50 µL acetylacetone. Then, the solution was then sonicated for 1 hour to let TiO2 powders well disperse and form a homogenous paste. P25 films were fabricated by spin-coating the prepared TiO2 paste on FTO glass at 1000 rmp. Finally, the P25 samples were annealed in air at 550 oC for 2 h to remove the organic binders. (Photo)electrochemical
characterization:
Linear
sweeps,
I-t
scans
and
electrochemical impedance spectra were collected in 1.0 M NaOH aqueous solution
(pH = 13.6)
by a Versastudio electrochemical workstation (Princeton Applied
Research), with Ag/AgCl as the reference electrode and a Pt wire as the counter electrode, under a 300 W xenon lamp (Thermo Oriel instruments, Model: SP66905-3513).
Incident-photon-to-current-conversion-efficiencies (IPCE) were
collected by Versastudio electrochemical workstation with the xenon lamp coupled with a monochromator (Thermal Oriel Instruments, Model 74100). Charge injection efficiency is estimated by measuring the photocurrent density in 1.0 M NaOH solution with/without H2O2. Charge transfer efficiency is measured by the photocurrent densities under 370 and 450 nm light illumination in 1.0 M NaOH solution with H2O2. Material and electrical characterization: SEM images were collected on JEOL 6700 and TEM was carried out on FEI CM 120. X-ray diffraction was collected on a Panalytical X’pert Pro X-ray powder diffractometer with Cu-Kα radiation. TiO2 single nanowire devices were fabricated using electron beam lithography (EBL) followed by electron beam deposition of Ti/Au (180/50 nm) thin films as the contact electrodes. Electrical properties measurements were conducted in ambient conditions in a Lakeshore probe station, coupled with Agilent B2902A precision source/voltage units. Theoretical calculation: Calculations are performed by using SIESTA package based on density-functional theory (DFT).5,
6
3, 4
The exchange-correlation energy
function is Perdew-Burke-Ernzerhof (PBE) based on general gradient approximation (GGA) 7. The double-zeta basis set with polarization functions and the energy cutoff
of 150 Ry are used. The structure optimization is carried out by relaxing the forces on all the atoms until a 0.05 eV/Å force tolerance is reached, using a conjugate gradient method. The energy convergence is 10-4 eV. The escape energy is defined as ΔE = Eesc + Edopant – Einitial, where Einitial, Eesc and Edopant are the calculated total energies of initial doped TiO2 structure, dopant escaped TiO2 structure and escaped dopant. The positiveΔE means that the system needs to gain energy for the dopant escaping.
Figure S1. XRD patterns of pristine TiO2, as prepared N-TiO2 and post-annealed N-TiO2 NWs. The * indicates the peaks from FTO substrate.
Figure S2. The digital picture of PEC measurement, with N-TiO2 as working electrode (middle), Pt as counter electrode (left) and Ag/AgCl as reference electrode (right).
Figure S3. (a) Photocurrent densities of N-TiO2 annealed at 350 oC at 0.5 V vs. Ag/AgCl as a function of nitrogen implanted doses. (b) Photocurrent densities of N-TiO2 as a function of annealing temperatures at 0.5 V vs. Ag/AgCl.
Figure S4. (a) SEM image of anatase TiO2 NWs on titanium foil. (b) Photocurrent densities of anatase TiO2 and N-TiO2 NWs versus time curves collected at 0.5 V vs. Ag/AgCl in 1.0 M NaOH solution. (c) SEM image of commercial P25 thin film on FTO glass. (d) Photocurrent densities of P25 and N-P25 versus time curves collected at 0.5 V vs. Ag/AgCl in 1.0 M NaOH solution.
Figure S5. The schematic diagram shows the charge separation and charge injection process.
Figure S6. Charge separation (C-S) efficiency of N-TiO2 NWs under the wavelength of 450 nm.
Figure S7. Linear sweep voltammograms of N-TiO2 and OER/N-TiO2 under visible light illumination with a scan rate of 20 mV/s.
Figure S8. The comparison of average lifetime distribution of photoexcited carriers in TiO2, N-TiO2 and OER/N-TiO2 NWs.
Figure S9. Photocurrent density of N-TiO2 and OER/N-TiO2 at 0.5 V vs. Ag/AgCl for a week under visible light illumination. The dark color in OER/N-TiO2 after PEC measurement is due to the oxidation of OER catalyst of Co(OH)2 to higher oxidation state.
* Estimated values based on the results in the literature.
Table S1. Comparison of the IPCE values at 450 and 500 nm of this work with literature results.
Reference 1.
Wang, G. M.; Wang, H. Y.; Ling, Y. C.; Tang, Y. C.; Yang, X. Y.; Fitzmorris, R.
C.; Wang, C. C.; Zhang, J. Z.; Li, Y. Nano Lett. 2011, 11, (7), 3026-3033. 2.
Liu, B.; Aydil, E. S. J. Am. Chem. Soc. 2009, 131, (11), 3985-3990.
3.
Roman-Perez, G.; Soler, J. M. Phys. Rev. Lett. 2009, 103, (9), 4.
4.
Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.;
Sanchez-Portal, D. J. Phys.-Condes. Matter 2002, 14, (11), 2745-2779. 5.
Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, (3B), B864-871.
6.
Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, (4A), 1133-1138.
7.
Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, (18),
3865-3868.
