Large-Scale Synthesis of Transition-metal Doped ... - Semantic Scholar
Large-Scale Synthesis of Transition-metal Doped TiO2 Nanowires with Controllable Overpotential Bin Liu1‡, Hao Ming Chen,1‡ Chong Liu1,3, Sean C. Andrews1,3, Chris Hahn1, Peidong Yang1,2,3,* 1
Department of Chemistry, University of California, Berkeley, California 94720, USA
2
Department of Materials Science and Engineering, University of California, Berkeley, California
94720, USA 3
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720,
USA
Experimental section TiO2 nanowire synthesis. Rutile TiO2 nanowires were synthesized using a molten salt flux method. In a typical synthesis, 1 part (by weight) of P25 nanoparticles (Degussa), 4 parts of NaCl, and 1 part of Na2HPO4 were ground with mortar and pestle to form a fine mixture. The mixture was transferred to a crucible and calcined inside a box furnace at 825 oC for 8 hours. After calcination, the crucible was cooled to room temperature, and the calcined mixture was washed in boiling deionized water extensively to remove all soluble salts. In some control experiments, P25 nanoparticles were replaced with rutile or anatase TiO2 particles to study the effect of TiO2 precursor (as mentioned above). The following chemicals were added (2% by atomic percentage) to make transition metal doped TiO2 nanowires: V2O5, Cr(NO3)3, Mn(NO3)2, FeCl3, Co(NO3)2, Nb2O5, Rh(NO3)3, (NH4)6Mo7O24. Characterizations. The crystal structure of nanowires was examined by X-ray diffraction (XRD). The XRD patterns were recorded at the National Synchrotron Radiation Research Center (01C beam S1
line) while the incident X-ray energy in this work was 16 KeV ( λ = 0.7749 Å). Morphological and structural information were examined with field-emission scanning electron microscopy (FESEM, JSM-6340F), transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy (TEM/SAED/EDX, Hitachi H-7650), and electron energy loss spectroscopy (HRTEM/EELS, Tecnai FEI F20). The optical absorption spectra were recorded using a UV-vis-NIR scanning spectrophotometer equipped with an integration sphere (Shimadzu UV-3101PC). Electrochemical measurements were carried out using a Gamry potentiostat (Model 600) in a three electrode electrochemical cell using a TiO2 nanowire coated FTO substrate as a working electrode, a coiled Pt wire as a counter electrode, and an Ag/AgCl electrode as a reference. The electrolyte was an aqueous solution of 1M KOH (pH = 13.61). X-ray absorption characterization. A series of EXAFS measurements of the synthesized samples were made using synchrotron radiation at room temperature. Measurements were made at the Ti K-edge (4966 eV) with the sample held at room temperature. The backscattering amplitude and phase shift functions for specific atom pairs were calculated ab initio using the FEFF code. X-ray absorption data were analyzed using standard procedures, including pre-edge and post-edge background subtraction, normalization with respect to edge height, Fourier transformation, and nonlinear least-squares curve fitting. The normalized k2-weighted EXAFS spectra, k2x(k), were Fourier transformed in a k range from 2 to 13.8 Å-1, to evaluate the contribution of each bond pair to the Fourier transform (FT) peak. The experimental Fourier-filtered spectra were obtained by performing an inverse Fourier transformation with a Hanning window function with r between 2.2 and 3.1 Å. The S02 (amplitude reduction factor) values of the Ti-Ti were fixed at 0.91, to determine the structural parameters of each bond pair. The experiments were conducted at the 01C and 17C beamline of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. S2
Figure S1.. XRD specctra were collected c too examine the t phase evolution e du uring the reeaction. (●)) anatase TiO O2, (▲) rutiile TiO2, annd () Na4TiP T 2O9. Thee formation of Na4TiP2O9 intermeddiate occurss in two stepps: First, thee instabilityy of Na2HPO O4 above 215 oC causees it to condense into tetrasodium m pyrophosphhate (Na4P2O7), and fuurther heatinng of the mixture m driv ves the nex xt reaction between b thee anatase andd Na4P2O7 to t generate Na4TiP2O9. The XRD patterns were recorded in a Brukker Advancee diffractomeeter (model D8) with Cu C K radiaation ( = 1..5406 Å). S3
Figure S2. (a) TEM im mage of P225 nanopartiicles. (b) TE EM image of o starting materials, m shhowing thatt P25 nanopparticles, NaaCl and Naa2HPO4 still keep theiir initial moorphologiess after blennding. TEM M o images of starting maaterials afterr 1 min(c), 2 min(d), 4 min(e) an nd 8 min(f)) of reactionn at 825 C, showing thhe morphoology of TiO T 2 nanopaarticles staarted to ch hange oncee the mixtuure of P255 o nanoparticlles, NaCl annd Na2HPO4 were heated at 825 C for more than t 2 minu utes. S4
Figure S3. SEM imag ges of the prroducts from m (a) NaCl m medium and d (b) Na2HP PO4 medium m at 825 oC..
Figure S4.. SEM imaages of TiO O2 sample prepared p usiing (a) rutiile microparrticles and (b) anatasee nanoparticlles as the so ource materiial of titaniuum dioxide at 825 oC S5
Figure S5. SEM imag ges of variouus transitionn-metal dopped TiO2 nannowires. S6
Figure S6. Mott-Schottky plot off various Nb b doped-TiO O2 nanowiree electrodes measured at a 500 Hz.
Table S1. Structurall parametters of tran nsition‐m metal dopa ants in ruttile TiO2 n nanowiress from EXA AFS studiess. Sample
patth
C CN
R (Å)
DW W(Å2)
ΔE(eV))
TiO2
Ti‐T Ti Ti‐T Ti
1.93(3) 1.78(4)
3.01(3) 3.00(6 6)
0.0 0088(5) 0.0 0094(8)
0.9(4) 3.3(5)
Ti‐V V Ti‐T Ti Ti‐C Cr Ti‐T Ti
0.05(2) 1.81(5) 0.03(1) 1.86(3)
2.93(55) 3.01(5) 2.99(33) 2.99(77)
0.0 0108(7) 0.0 0087(6) 0.0 0112(8) 0.0 0069(9)
‐6.4(7)) 2.4(5) 3.5(7) 6.2(6)
Ti‐M Mn Ti‐T Ti Ti‐F Fe Ti‐T Ti
0.04(2) 1.82(4) 0.04(1) 1.83(2)
3.04(55) 2.98(6 6) 2.94(6 6) 2.98(33)
0.0 0094(5) 0.0 0077(6) 0.0 0092(7) 0.0 0083(7)
4.5(5) 3.1(8) 5.3(6) ‐1.8(7)
Ti‐C Co Ti‐T Ti Ti‐N Nb Ti‐T Ti
0.03(2) 1.79(3) 0.05(2) 1.80(3)
3.03(55) 2.98(55) 3.05(4 4) 3.00(33)
0.0 0095(8) 0.0 0075(5) 0.0 0103(9) 0.0 0089(5)
7.7(5) 3.2(4) ‐8.2(6)) ‐2.6(9))
Ti‐M Mo Ti‐T Ti Ti‐R Rh
0.04(1) 1.84(3) 0.05(2)
3.01(6 6) 2.99(77) 3.06(6 6)
0.0 0109(6) 0.0 0092(5) 0.0 0101(6)
7.9(8) 5.7(3) 3.9(4)
V‐TiO2 Cr‐TiO2 Mn‐TiO2 Fe‐TiO2 Co‐TiO2 Nb‐TiO2 Mo‐TiO2 Rh‐TiO2
S7
CN, coordination number; R, interatomic distance between absorber and backscatter atoms; DW, Debye–Waller factor; ΔE, inner potential shift; S02 (amplitude reduction factor) fitting from TiO2 powders defined as 0.91.
S8
Department of Chemistry, University of California, Berkeley, California 94720, USA
2
Department of Materials Science and Engineering, University of California, Berkeley, California
94720, USA 3
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720,
USA
Experimental section TiO2 nanowire synthesis. Rutile TiO2 nanowires were synthesized using a molten salt flux method. In a typical synthesis, 1 part (by weight) of P25 nanoparticles (Degussa), 4 parts of NaCl, and 1 part of Na2HPO4 were ground with mortar and pestle to form a fine mixture. The mixture was transferred to a crucible and calcined inside a box furnace at 825 oC for 8 hours. After calcination, the crucible was cooled to room temperature, and the calcined mixture was washed in boiling deionized water extensively to remove all soluble salts. In some control experiments, P25 nanoparticles were replaced with rutile or anatase TiO2 particles to study the effect of TiO2 precursor (as mentioned above). The following chemicals were added (2% by atomic percentage) to make transition metal doped TiO2 nanowires: V2O5, Cr(NO3)3, Mn(NO3)2, FeCl3, Co(NO3)2, Nb2O5, Rh(NO3)3, (NH4)6Mo7O24. Characterizations. The crystal structure of nanowires was examined by X-ray diffraction (XRD). The XRD patterns were recorded at the National Synchrotron Radiation Research Center (01C beam S1
line) while the incident X-ray energy in this work was 16 KeV ( λ = 0.7749 Å). Morphological and structural information were examined with field-emission scanning electron microscopy (FESEM, JSM-6340F), transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy (TEM/SAED/EDX, Hitachi H-7650), and electron energy loss spectroscopy (HRTEM/EELS, Tecnai FEI F20). The optical absorption spectra were recorded using a UV-vis-NIR scanning spectrophotometer equipped with an integration sphere (Shimadzu UV-3101PC). Electrochemical measurements were carried out using a Gamry potentiostat (Model 600) in a three electrode electrochemical cell using a TiO2 nanowire coated FTO substrate as a working electrode, a coiled Pt wire as a counter electrode, and an Ag/AgCl electrode as a reference. The electrolyte was an aqueous solution of 1M KOH (pH = 13.61). X-ray absorption characterization. A series of EXAFS measurements of the synthesized samples were made using synchrotron radiation at room temperature. Measurements were made at the Ti K-edge (4966 eV) with the sample held at room temperature. The backscattering amplitude and phase shift functions for specific atom pairs were calculated ab initio using the FEFF code. X-ray absorption data were analyzed using standard procedures, including pre-edge and post-edge background subtraction, normalization with respect to edge height, Fourier transformation, and nonlinear least-squares curve fitting. The normalized k2-weighted EXAFS spectra, k2x(k), were Fourier transformed in a k range from 2 to 13.8 Å-1, to evaluate the contribution of each bond pair to the Fourier transform (FT) peak. The experimental Fourier-filtered spectra were obtained by performing an inverse Fourier transformation with a Hanning window function with r between 2.2 and 3.1 Å. The S02 (amplitude reduction factor) values of the Ti-Ti were fixed at 0.91, to determine the structural parameters of each bond pair. The experiments were conducted at the 01C and 17C beamline of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. S2
Figure S1.. XRD specctra were collected c too examine the t phase evolution e du uring the reeaction. (●)) anatase TiO O2, (▲) rutiile TiO2, annd () Na4TiP T 2O9. Thee formation of Na4TiP2O9 intermeddiate occurss in two stepps: First, thee instabilityy of Na2HPO O4 above 215 oC causees it to condense into tetrasodium m pyrophosphhate (Na4P2O7), and fuurther heatinng of the mixture m driv ves the nex xt reaction between b thee anatase andd Na4P2O7 to t generate Na4TiP2O9. The XRD patterns were recorded in a Brukker Advancee diffractomeeter (model D8) with Cu C K radiaation ( = 1..5406 Å). S3
Figure S2. (a) TEM im mage of P225 nanopartiicles. (b) TE EM image of o starting materials, m shhowing thatt P25 nanopparticles, NaaCl and Naa2HPO4 still keep theiir initial moorphologiess after blennding. TEM M o images of starting maaterials afterr 1 min(c), 2 min(d), 4 min(e) an nd 8 min(f)) of reactionn at 825 C, showing thhe morphoology of TiO T 2 nanopaarticles staarted to ch hange oncee the mixtuure of P255 o nanoparticlles, NaCl annd Na2HPO4 were heated at 825 C for more than t 2 minu utes. S4
Figure S3. SEM imag ges of the prroducts from m (a) NaCl m medium and d (b) Na2HP PO4 medium m at 825 oC..
Figure S4.. SEM imaages of TiO O2 sample prepared p usiing (a) rutiile microparrticles and (b) anatasee nanoparticlles as the so ource materiial of titaniuum dioxide at 825 oC S5
Figure S5. SEM imag ges of variouus transitionn-metal dopped TiO2 nannowires. S6
Figure S6. Mott-Schottky plot off various Nb b doped-TiO O2 nanowiree electrodes measured at a 500 Hz.
Table S1. Structurall parametters of tran nsition‐m metal dopa ants in ruttile TiO2 n nanowiress from EXA AFS studiess. Sample
patth
C CN
R (Å)
DW W(Å2)
ΔE(eV))
TiO2
Ti‐T Ti Ti‐T Ti
1.93(3) 1.78(4)
3.01(3) 3.00(6 6)
0.0 0088(5) 0.0 0094(8)
0.9(4) 3.3(5)
Ti‐V V Ti‐T Ti Ti‐C Cr Ti‐T Ti
0.05(2) 1.81(5) 0.03(1) 1.86(3)
2.93(55) 3.01(5) 2.99(33) 2.99(77)
0.0 0108(7) 0.0 0087(6) 0.0 0112(8) 0.0 0069(9)
‐6.4(7)) 2.4(5) 3.5(7) 6.2(6)
Ti‐M Mn Ti‐T Ti Ti‐F Fe Ti‐T Ti
0.04(2) 1.82(4) 0.04(1) 1.83(2)
3.04(55) 2.98(6 6) 2.94(6 6) 2.98(33)
0.0 0094(5) 0.0 0077(6) 0.0 0092(7) 0.0 0083(7)
4.5(5) 3.1(8) 5.3(6) ‐1.8(7)
Ti‐C Co Ti‐T Ti Ti‐N Nb Ti‐T Ti
0.03(2) 1.79(3) 0.05(2) 1.80(3)
3.03(55) 2.98(55) 3.05(4 4) 3.00(33)
0.0 0095(8) 0.0 0075(5) 0.0 0103(9) 0.0 0089(5)
7.7(5) 3.2(4) ‐8.2(6)) ‐2.6(9))
Ti‐M Mo Ti‐T Ti Ti‐R Rh
0.04(1) 1.84(3) 0.05(2)
3.01(6 6) 2.99(77) 3.06(6 6)
0.0 0109(6) 0.0 0092(5) 0.0 0101(6)
7.9(8) 5.7(3) 3.9(4)
V‐TiO2 Cr‐TiO2 Mn‐TiO2 Fe‐TiO2 Co‐TiO2 Nb‐TiO2 Mo‐TiO2 Rh‐TiO2
S7
CN, coordination number; R, interatomic distance between absorber and backscatter atoms; DW, Debye–Waller factor; ΔE, inner potential shift; S02 (amplitude reduction factor) fitting from TiO2 powders defined as 0.91.
S8
