Supporting Information: High Efficiency Photoelectrocatalytic Hydrogen Generation Enabled by Palladium Quantum Dots Sensitized TiO 2 Nanotube Arrays Meidan Ye, Jiaojiao Gong, Yuekun Lai, Changjian Lin,*, and Zhiqun Lin*, State Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA Experimental Methods Fabrication of highly ordered TiO 2 nanotube arrays (TNTAs). Highly ordered TiO 2 nanotubes were fabricated by electrochemical anodization of Ti foils (0.1 mm thick, 99.5% purity) in ethylene glycol solution containing 0.3 wt% NH 4 F and 2 vol% H 2 O using Ti foil as a working electrode and platinum as a counter electrode at room temperature. The Ti foil was first anodized at 50 V for 4 h. The anodized Ti foil was then ultrasonically rinsed in acetone to separate the formed TiO 2 nanotubular layer from the Ti substrate. Subsequently, a second anodization of Ti substrate was performed at 50 V for 2h and ultrasonicated again to remove the resulting TiO 2 nanotubes. Finally, a third anodization was conducted for 5 min, yielding 1.2 µm-thick TiO 2 nanotube arrays situated on Ti foil. Meanwhile, another type of 1.2 μm-thick TiO 2 nanotube arrays, which was extensively used in the hydrothermal process, was fabricated in the glycerol/h 2 O (v/v=2/1) electrolyte containing 0.5 wt% NaF and 0.2 mol/lna 2 SO 4 at 25 V for 1.5h. These nanotubes were then utilized to explore the effects of temperature, PVP concentration, and reaction time on the size and loading of Pd QDs (Figures S4-S6) as the time to yield nanotube arrays in the glycerol electrolyte was much shorter than that in the ethylene glycol electrolyte. The as-prepared TiO 2 nanotube arrays were amorphous; they were annealed at 450 o C in air for 2 h to convert into crystalline photoactive anatase phase. Synthesis of Pd@TNTAs by modified hydrothermal reaction. Briefly, crystallineanatasetio 2 nanotube arrays and a 13 ml solution consisting of 800 mg polyvinylpyrrolidone (PVP), 9 mg palladium chloride (PdCl 2 ), and 300 mg NaI were transferred to a 23 ml Teflon-lined stainless-steel autoclave. The sealed vessel was then heated at 180 C for 1.5 h. After that, the TiO 2 nanotube arrays were washed with ethanol for several times and blow-dried with N 2. The initial concentration of PVP in the solution, reaction time and temperature were found to be critical in controlling the Pd QD size and the amount of Pd loading on TiO 2 nanotubes, which both increased S1
with the reaction time, while keeping other experimental parameters constant. For the preparation of Pd@TNTAs nanocomposites with the same Pd QD size but different loading amount, the same amount of PdCl 2 and NaI were used; as the addition of PVP increased, a higher Pd loading amount on the TiO 2 substrate was resulted in. Characterizations. The morphologies of prepared samples were examined by field-emission scanning electron microscope (FESEM, S4800, Hitachi) and transmission electron microscopy (TEM, JEM2100) equipped with an energy-dispersive X-ray (EDX) spectrometer. The crystalline structure of samples was confirmed by x-ray diffraction (XRD, Philips, Panalytical X pert, Cu KR radiation). The elemental composition of nanotube arrays was analyzed by x-ray photoelectron spectroscopy (XPS, VG, Physical Electrons Quantum 2000 Scanning Esca Microprob, Al K α radiation). The photoelectrochemical measurements were performed in a three-electrode photoelectrochemical cell in 0.5 M KOH solution with a quartz window for light incidence. To evaluate the photocatalytic performance, the water splitting experiment was carried out in a gas-closed circulation system equipped with a three-electrode photoelectrochemical (PCE) cell (Scheme S1) and a volumetric device with a vacuum line. The volume of electrolyte consisting of 2M Na 2 CO 3 and 0.5 M ethylene glycol was 150 ml. The electrolyte was purged with N 2 for 30 min to remove O 2 prior to the experiment. A 300 W Xe lamp (PLS-SXE300, Beijing Bofeilai Technology Co, Ltd.) was used as a solar light source with a light intensity of 320 mw cm -2. Impedance measurments were performed in dark and under illumination (320 mw cm -2 ) in 0.05M Na 2 SO 4 solution at open circuit voltage over a frequency range from 10 5 to 10 1 Hz with an AC voltage at 10 mv. The Mott-Schottky plots were obtained at a fixed frequency of 1 KHz to determine the flat-band potential and carrier density. The impedance data were analyzed by Potentiostat/.Galvanostat Model 263A equipment (Princeton). Room-temperature photoluminescence (PL) spectra were recorded using a fluorescence spectrophotometer (Hitachi High-Tech, F-7000) equipped with a Xenon lamp as an excitation source (excited at 325 nm). S2
Characterizations 1. HRSEM. Figure S1 shows the HRTEM image of an individual TiO 2 nanotube loaded with Pd QDs on the surface, from which homogeneous distribution of Pd QDs throughout the entire nanotube was clearly evident. The Pd QDs were mainly spherical in shape (D = 3.3±0.7 nm). 10 nm Figure S1. Left: HRTEM image of an individual TiO 2 nanotube after the deposition of Pd QDs by hydrothermal reaction. Right: The plot of the particle size distribution. S3
2. XPS. Figure S2.(a) XPS spectrum of Pd@TNTAs nanocomposites prepared by the hydrothermal synthesis at 200 o C for 1.5h (Pd% = 2.15 wt%, D Pd = 3.3 ± 0.7 nm). (b-c) The close-up XPS spectra, showing the existence of Pd and O. S4
3. XRD. The as-prepared TiO 2 nanotube arrays supported on the Ti foil were amorphous; they were annealed at 450 o C in air for 2 h to convert into crystalline photoactive anatase phase as revealed by XRD (Figure S3). Quantitative analysis showed that all peaks in the XRD profile can be indexed to anatase phase of TiO 2 (JCPDS file No. 21-1272) and metallic Ti substrate (JCPDS file No. 44-1294), marked with A and T, respectively (Figure S3b). No additional peaks attributed to metallic Pd can be found, suggesting a small particle size of Pd inside and outside of nanotubes. Clearly, the intensity of anatase peaks (e.g., at 2θ = 25.3 o ) for the annealed Pd@TNTAs (Figure S3c) was not higher than that of annealed TiO 2 nanotubes (Figure S3b), suggesting that the hydrothermal reaction did not improve the TiO 2 crystallization and the enhanced hydrogen generation was a direct consequence of the decoration of Pd QDs on TiO 2 nanotube arrays (i.e., capitalizing on Pd@TNTAs nanocomposites). Figure S3. XRD profiles of (a) as-prepared TiO 2 nanotubes arrays (TNTAs), (b) TNTAs after annealed at 450 o C, and (c) annealed TNTAs loaded with Pd QDs (Pd% = 2.15 wt% and D =3.3±0.7 nm). S5
4. FESEM. Figures S4-S6 present the FESEM images of Pd@TNTAs obtained by hydrothermal reaction (see Experiemntal Methods) at different temperature, polyvinylpyrrolidone (PVP) concentration, and reaction time in which 9 mg PdCl 2 and 300 mg NaI were added. Obviously, with increased reaction temperature from 150 o C to 200 o C, the Pd loading was increased and the QD size was also slightly increased (Figure S4). As a stabilizer for the reaction, PVP also played a critical role on the size and amount of Pd QDs. Figure S5 clearly shows that the coverage of Pd QDs was greatly increased with increased PVP concentration. At high PVP concentration, Pd QDs aggregated into large particles (Figure S5d). Furthermore, longer growth time (from 1.5 h to 4.0 h) led to the formation of a large number of Pd particles with bigger size (Figure S6). Thus, hydrothermal reaction using 800 mg PVP at 200 o C for 1.5h was chosen in the study, from which homogenously dispersed Pd QDs were successfully obtained. (a) (b) (c) (d) Figure S4. FESEM images of Pd@TNTAs obtained by hydrothermal reaction at different temperature. (a) 150 o C, (b) 160 o C, (c)180 o C, and (d) 200 o C for 1.5 h. S6
(a) (b) (c) (d) Figure S5. FESEM images of Pd@TNTAs obtained by hydrothermal reaction at different PVP concentration. (a) 400 mg, (b) 800 mg, (c) 1000 mg, and (d) 1200 mg at 200 o C for 1.5h. (a) (b) (c) (d) Figure S6. FESEM images of Pd@TNTAs otbained by hydrothermal reaction at different time. (a and b) 1.5 h, and (c and d) 4.0 h at 200 o C. S7
5. Current Density. The photocurrent measurement revealed that the amount of Pd QD loading in the resulting Pd@TiO 2 NTAs nanocomposites was extremely crucial to photoelectrochemical activities. The photocurrent densities first increased as the Pd loading increased, and decreased with further increase of Pd loading (beyond 2.15 wt%) as shown in Figure S7. The improved photoelectric response at the low Pd loading (i.e., below 2.15 wt%) was due primarily to the better charge separation as compared to that of pure TiO 2. 1 On the other hand, Pd QDs may also act as charge carrier recombination centers, which was due to the electrostatic attraction of negatively charged Pd resulted from the charge transfer from TiO 2 to Pd and positively charged holes, thereby reducing the photocurrent after the Pd loading exceeded an optimal value. 2 Furthermore, the higher surface coverage of Pd QDs may decrease the accessibility of active sites on TiO 2 nanotubes and induce the recombination of photogenerated charges, which also led to low photoactivity. 2 Figure S7. Photocurrent density of Pd@TNTAs as a fuction of the amount of Pd QDs loaded. Pd@TNTAs electrodes were used as cathode and the measurement was performed at 0.9 V SCE in 0.5 M KOH (Pd% = 2.15 wt%) under 320 mw cm -2 irradiation. S8
6. Schemes. Scheme S1. Schematic representation of a photoelectrochemical cell for hydrogen generation via water splitting by capitalizing on Pd@TNTAs(or TiO 2 ) as photoanode, Pd@TNTAs(or Pt) as cathode, and SCE as the reference electrode, respectively. As an example, the close-ups show that both photoanode and cathode are Pd@TNTAs. Scheme S2. Schematic illustration of TiO 2 nanotube arrays deposited with Pd QDs and the charge transfer process from TiO 2 to Pd (lower right panel). References (1) Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. Renew. Sustain. Energ. Rev. 2007, 11, 401-425. (2) Chang, Y.; Xu, J.; Zhang, Y.; Ma, S.; Xin, L.; Zhu, L.; Xu, C. J. Phys. Chem. C 2009, 113, 18761-18767. S9