Towards Visible Light Hydrogen Generation: Quantum Dot-Sensitization via Efficient Light Harvesting of Hybrid-TiO 2

Transcription

1 Supporting Information Towards Visible Light Hydrogen Generation: Quantum Dot-Sensitization via Efficient Light Harvesting of Hybrid-TiO 2 Kwanghyun Kim, Myeong-Jong Kim, Sun-I Kim, and Ji-Hyun Jang * Interdisciplinary School of Green Energy and Low Dimensional Carbon Materials Center, UNIST, Korea Experimental Procedure Chemicals used Titanium(IV) Chloride ( 98%), the triblock copolymer, poly(ethylene glycol)-block-poly(- propylene glycol)-block-poly(ethylene glycol) (EO 20 PO 70 EO 20, M = 5,800), titanium(iv) isopropoxide, and Ethanol. Cadmium acetate dihydrate (reagent grade, 98%) and Selenium (powder, 99.99%) were used as the Cd + and Se - precursors, respectively. All chemicals were purchased from Sigma-Aldrich, and all chemicals were used without further purification. Characterization The surface area was determined by Brunauer-Emmett-Teller (BET) methods using an ASAP 2000 surface area analyzer (Micrometerics Instrucment Corp.). The morphology of the structures was examined by SEM (NOVA NANOSEM 230 FESEM, 15kV) and TEM (JEM- 2100, 200kV). The powder diffraction data were obtained using a Rigaku Co. High Power X- Ray Diffractometer D/MAZX 2500V/PC from 20 o to 80 o. Diffractometer D/MAZX 2500V/PC from 20 o to 80 o. Open circuit potentials were investigated at the same condition as the ones in the PEC measurements. The XPS spectra were obtaineded with a VG Scientific ESCALAB 250 spectrometer (Beverly, MA, USA) with a monochloromatized AL Kα X-ray source ( ev) 1

2 Fabrication of st-tio After the scotch tape was applied on the FTO substrate to make a window with an area of 5 mm x 5 mm to determine the size of the sample, the substrate was treated with O 2 plasma for 10 min. The diluted precursor gel solution 1 was spin-coated and annealed at 75 o C for 30 minutes on the hot plate followed by spin coating of polystyrene (PS) nanospheres and further annealing at 85 o C for 1 hour to ensure good contact between assembled PS nanospheres and the substrate. The height of the assembled PS layer was determined by the number of spin coating iterations. Then, 0.9 M of TiCl 4 solution (water:ethanol=1:1) was infiltrated into the assembled PS layer. The TiCl 4 /PS composite was heated at 90 o C for 10 min. to evaporate the solvent and at 500 o C for 1 hour, providing a solution-gelation reaction and calcination of PS nanospheres. The sample was dip-coated using a 40 mm TiCl 4 aqueous solution at 70 o C for 30 min to render it fully connected and thus provide efficient electron transport networks. The TIO structure was dip-coated in a diluted solution of triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (EO 20 PO 70 EO 20, M = 5,800). The solution was aged at 55 o C for 2 hours for self-assembly of triblock copolymer, and heated at 90 o C for 10 hours. After the yellow liquid turned into a bright yellow forming powder, the sample was removed from the vial. Finally, the samples were washed with D.I. water and ethanol followed by heat treatment at 500 o C for 30 minutes to calcinate the block copolymer domains. 2

3 Calculation of Effective Refractive Index of Antireflection (AR) Layer The volume ratio of TiO 2 : air = 0.46 : 0.54 from the SEM image. On the basis of the effective medium theory, 2, 3 the effective refractive index n can be calculated as by the following equation n 0.46 n 0.54 n, q 2/3 Here, n is for anatase and n is 1. Figure S1. BET analysis of H-TiO 2. The inset is pore-size distribution curve. The specific values are summarized in the table. 3

4 Figure S2. X-ray diffraction (XRD) patterns and TEM image of as-prepared TiO 2 nanostructures. (a) XRD patterns of st-tio and p-p25. (b)-(d) TEM images of mesoporous st-tio structure. The close-up image in the inset (b) confirms the single crystalline phase of as-prepared st-tio with interplanar spacing of nm for anatase. Figure S2a shows the X-ray diffraction (XRD) pattern of hydrothermally prepared st-tio and p-p25 after sintering in air for 1 hour at 500 o C. The two as-synthesized types of mesoporous TiO 2 structures are anatase with enhancement of the (101), (004), (200), (105), and (204) planes, as confirmed by # JCPDS Figures S2b-2d show TEM images of the st-tio confirming that the TiO 2 structures are crystalline with interplanar d-spacing of nm, corresponding to the (100) plane of anatase TiO 2. Additionally, it can be seen that a thin layer of mesoporous TiO 2 structures are tightly connected with a thick layer of TIO. 4

5 Figure S3. TEM images of (a) CdSe and (b) CdSe/H-TiO 2. Figure S4. EDX analysis CdSe/H-TiO 2 : (a) Comparison of the composition ratio. (b) Top surface SEM image of CdSe/H-TiO 2 and EDX mapping of Ti, O, Cd, and Se confirming uniform dispersion of Quantum dots. 5

6 Figure S5. Transmittance spectrum of the UV cutoff filter Figure S6. Experimental setup for PEC measurements showing the presence of a large bubble created from the aggregation of a large amount of H 2 bubbles near the Pt counter electrode. The movie clip is included. 6

7 Figure S7. Photocurrent vs. time profile of CdSe/H-TiO 2 under UV-visible illumination at a fixed bias of 0.5 V vs. Ag/AgCl. Proposed Energy-Level Diagram for the Electron Transfer 7

8 Figure S8. (a) Valence band XPS spectra of the P25 and H-TiO 2. (b) Tauc plot of P25 and H-TiO 2. (c) Proposed energy-level diagram for the electron transfer at the interface of CdSe/P25 and CdSe/H-TiO 2. In order to verify that the electron transfer mechanism in H-TiO 2 is faster (more efficient) than that in P25, the valence band energy and band gap energy of the samples were obtained by XPS spectra and tauc plot, respectively. 4 Figure S8a shows the valence band XPS spectra of the P25 and H-TiO 2, through which the valence band energy of 3.3 ev and 3.5 ev is respectively acquired for P25 and H-TiO 2. In addition, the band gap energy of 3.33 ev and 2.92 ev was respectively obtained for P25 and H-TiO 2 (Figure S8b). The energy-level diagram for the electron transfer in CdSe/P25 and CdSe/H-TiO 2 can be illustrated by combining two corresponding values, which indicates the presence of more facile pathways between the conduction band edge of CdSe QDs and H-TiO 2 nanostructures in CdSe/H-TiO 2, as shown in Figure S8c. Electron Recombination Kinetics Figure S9. (a) Open circuit potential vs. time profile of P25 and H-TiO 2. (b) The corresponding electron lifetime measurements determined from the V oc decay. 8

9 To get a clearer understanding of the charge transport properties and the electron recombination kinetics of P25 and H-TiO 2, We monitored the photovoltage and its decay upon termination of illumination. The open circuit photovoltage decay under dark conditions allows the evaluation of photoelectron lifetime by measuring V oc with time. 5, 6 Under irradiation, electrons excited into the conduction band of the CdSe are transferred to TiO 2 nanostructures and are accumulated within the TiO 2 nanostructures. When the light is turned off, the electron density in the conduction band of TiO 2 decays rapidly due to recombination, showing V oc decay. Figure S9a plots the V oc decay as a function of time for P25 and H-TiO 2. It is evident that H-TiO 2 presents a significantly slower V oc decay rate than P25, suggesting slower recombination kinetics in H-TiO 2. The average lifetime (τ ) of photoexcited electrons before recombination can be calculated from the V oc decay with the following equation τ k T e dv dt, where k B is Boltzmann's constant, T is temperature and e is the elementary charge, and V oc is the open-circuit voltage at time t. The calculated τ is plotted in Figure S9b as a function of V oc for P25 and H-TiO 2, which confirms the longer lifetime of a photoexcited electron in H-TiO 2 than in P25. This can be attributed to the conduction band energy of H-TiO 2 which is lower than P25, and which drives facile electron transport and thus suppresses the charge recombination. 9

10 Photoconversion Efficiency Calculations The chronoamperometric I-t studies on CdSe/H-TiO 2 as a function of applied over potential at 100 mw/cm 2 was measured, from which the photocurrent values have been taken for the efficiency measurements. The efficiency of the device was calculated using the following equation η % = 100 E E E E where J is the measured photocurrent in ma/cm 2, E is constant value as 1.23 V for the Ag/AgCl reference electrode, E is the term related to the applied potential calculated through upper equation and I is the intensity of incident light in mw/cm 2. E is V for Ag/AgCl reference electrode and E is V for CdSe/H-TiO 2. Figure S10. Photoconversion efficiency of the PEC cell with CdSe/H-TiO 2 electrode as a function of applied potential. 10

11 Photocatalytic Activity under Visible Light Illumination Figure S11. Comparision of photocatalytic activity of CdSe/P25 and CdSe/H-TiO2 for the photodegradation of methylene blue under visible light (λ > 420 nm) illumination. We confirmed the excellent performance of our system as a catalyst by comparing the photodegradation efficiency of methylene blue (MB). Photodegradation of organic materials utilizing TiO 2 has been extensively researched during the last decade. The rate of photodegradation caused by absorption of photons from solar light is directly related to the number of photoexited electrons used to dissociate organic materials. The photodegradation of MB was measured with the absorption spectroscopic technique. An aqueous solution of MB dyes (0.01 g/l, i.e., 2.7 x 10-5 M, 5 ml) and photocatalysts (CdSe/P25 and CdSe/H-TiO 2 ) were placed in a 20 ml vial. The samples containing the photocatalysts and MB solution were irradiated under UV filtered (λ > 420 nm) exposure conditions. After the given exposure period during which all other lights were insulated and other external conditions, such as temperature and ph, were kept same, the absorbance change was tracked by a UV-visible spectrophotometer. Photocatalytic activity of CdSe/H-TiO 2 for degradation of MB obtained by the absorption value under visible light was compared with that of the control sample. The 11

12 CdSe/H-TiO 2 system shows 3 times higher photocatlytic activity at 60 min, which is attributable to the reduced charge recombination achieved by the effective charge transfer. This result also confirms the great performance of CdSe/H-TiO 2. References 1. Zui, R., Jiang, C., Liu, X., Liu, B., Kumar, A. & Ramakrishna, S. Improved adhesion of interconnected TiO 2 nanofiber network on conductive substrate and its application in polymer photovoltaic devices. App. Phys. Lett. 93, (2008). 2. Li, H., Jiang, B., Schaller, R., Wu, J. & Jiao, J. Antireflective Photoanode Made of TiO 2 Nanobelts and a ZnO Nanowire Array. J. Phys. Chem. C 114, (2010). 3. Stavenga, D. G., Foletti, S., Palasantzas, G. & Arikawa, K. Light on the moth-eye corneal nipple array of butterflies. Proc. R. Soc. B-Biol. Sci. 273, (2006). 4. Tauc, J., Grigorovici, R. & Vancu, A., Optical Properties and Electronic Structure of Amorphous Germanium. Physica Status Solidi. 15, (1966). 5. Kang, Q., Cao, J., Zhang, Y., Liu, L., Xu, H. & Ye, J., Reduced TiO 2 Nanotube Arrays for Photoelectrochemical Water Splitting. Journal of Materials Chemistry A. 1, (2013). 6. Zhang, Z. & Wang, P. Optimization of photoelectrochemical water splitting performance on hierarchical TiO 2 Nanotube arrays. Energy & Environmental Science. 5, (2012). 12