Large-Scale Synthesis of Transition-metal Doped TiO 2 Nanowires with Controllable Overpotential Bin Liu 1, Hao Ming Chen, 1 Chong Liu 1,3, Sean C. Andrews 1,3, Chris Hahn 1, Peidong Yang 1,2,3,* 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 TiO 2 nanowire synthesis. Rutile TiO 2 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 Na 2 HPO 4 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 o C 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 TiO 2 particles to study the effect of TiO 2 precursor (as mentioned above). The following chemicals were added (2% by atomic percentage) to make transition metal doped TiO 2 nanowires: V 2 O 5, Cr(NO 3 ) 3, Mn(NO 3 ) 2, FeCl 3, Co(NO 3 ) 2, Nb 2 O 5, Rh(NO 3 ) 3, (NH 4 ) 6 Mo 7 O 24. 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 TiO 2 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 k 2 -weighted EXAFS spectra, k 2 x(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 S 2 0 (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 spectra were collected to examine the phase evolution during the reaction. ( ) anatase TiO 2, ( ) rutile TiO 2, and ( ) Na 4 TiP 2 O 9. The formation of Na 4 TiP 2 2O 9 intermediate occurs in two steps: First, the instability of Na 2 HPO 4 above 215 o C causes it to condense into tetrasodium pyrophosphate (Na 4 P 2 2O 7 ), and further heating of the mixture drives the next reaction between the anatase and Na 4 P 2 O 7 to generate Na 4 TiP 2 O 9. The XRD patterns were recorded in a Bruker Advance diffractometer (model D8) with Cu K radiation ( = 1..5406 Å). S3
Figure S2. (a) TEM image of P25 nanoparticles. (b) TEM image of starting materials, showing thatt P25 nanoparticles, NaCl and Na 2 HPO 4 still keep their initial morphologies after blending. TEM images of starting materials after 1 min(c), 2 min(d), 4 min(e) and 8 min(f) of reaction at 825 o C, showing the morphology of TiO 2 nanoparticles started to change once the mixture of P25 nanoparticles, NaCl and Na 2 HPO4 were heated at 825 o C for more than 2 minutes. S4
Figure S3. SEM images of the products from (a) NaCl medium and (b) Na 2 HPO 4 mediumm at 825 o C. Figure S4. SEM images of TiO 2 sample prepared using (a) rutile microparticles and nanoparticles as the source material of titanium dioxide at 825 o C (b) anatase S5
Figure S5. SEM images of various transition-metal doped TiO 2 nanowires. S6
Figure S6. Mott-Schottky plot of various Nb doped-tio 2 nanowire electrodes measured at 500 Hz. Table S1. Structural parameters of transition metal dopants in rutile TiO2 nanowires from EXAFS studies. Sample TiO 2 path CN 1. 93(3) V TiO 2 Cr TiO 2 Mn TiO 2 Fe TiO 2 Co TiO 2 Nb TiO 2 Mo TiO 2 Rh TiO 2 1. 78(4) 1. 81(5) 1. 86(3) 1. 82(4) 1. 83(2) 1. 79(3) 1. 80(3) 1. 84(3) Ti V Ti Cr Ti Mn Ti Fe Ti Co Ti Nb Ti Mo Ti Rh 0.05(2) 0.03(1) 0.04(2) 0.04(1) 0.03(2) 0.05(2) 0.04(1) 0.05(2) R (Å) DW(Å ) ΔE(eV) ) 3.01(3) 0.0088(5) 0.9(4) 3.00(6) 0.0094(8) 3.3(5) 2.93(5) 0.0108(7) 3.01(5) 0.0087(6) 2.99(3) 0.0112(8) 2.99(7) 0.0069(9) 3.04(5) 0.0094(5) 2.98(6) 0.0077(6) 6.4(7)) 2.4(5) 3.5(7) 6.2(6) 4.5(5) 3.1(8) 2.94(6) 2.98(3) 3.03(5) 2.98(5) 3.05(4) 3.00(3) 3.01(6) 2.99(7) 3.06(6) 0.0092(7) 0.0083(7) 0.0095(8) 0.0075(5) 0.0103(9) 0.0089(5) 0.0109(6) 0.0092(5) 0.0101(6) 5.3(6) 1.8(7) 7.7(5) 3.2(4) 8.2(6)) 2.6(9)) 7.9(8) 5.7(3) 3.9(4) S7
CN, coordination number; R, interatomic distance between absorber and backscatter atoms; DW, Debye Waller factor; ΔE, inner potential shift; S 2 0 (amplitude reduction factor) fitting from TiO 2 powders defined as 0.91. S8