Supporting Information

Transcription

1 Supporting Information Wiley-VCH Weinheim, Germany Aqueous-Solution Route to Zinc Telluride Films for Application to CO 2 Reduction** Ji-Wook Jang, Seungho Cho, Ganesan Magesh, Youn Jeong Jang, Jae Young Kim, Won Yong Kim, Jeong Kon Seo, Sungjee Kim, Kun-Hong Lee,* and Jae Sung Lee* ange_ _sm_miscellaneous_information.pdf

2 Supporting Information Experimental Section Synthesis of ZnO nanowire arrays : All chemicals used in this study were of analytical grade and used without further purification. Zinc acetate dihydrate (0.11 g, Zn(CH 3COO) 2 2H 2O, 99%, Aldrich) and 2 ml ammonia water (NH 3, wt%, Samchun) were added to 50 ml deionized (DI) water. The solution was transferred to a glass bottle. A Zn sheet (Nilaco, 99.5%, feature size 1 2 cm 2 ) was transferred into the solution and was leaned against the inner wall of the bottle. The substrate and solution-containing bottle were put into an oven and maintained at 95 C for 1 h 30 min. After the reaction, the substrate was washed several times with de-ionized water and dried in an oven at 60 C for 12 h. Synthesis of ZnO/ZnTe structure : An aqueous solution containing 3.6 mm sodium tellurite (Na 2TeO 3, 99%, Aldrich) and 0.33 M hydrazine monohydrate (N 2H 4 H 2O, Samchun) was prepared at room temperature. The ZnO nanowire array substrate and the 25 ml solution were loaded into a microwave reactor (Discover SP, CEM Corporation). The microwave reactor automatically controlled reaction temperatures as programmed. The solution was heated to 180 C and was maintained at 195 C for 2 h. The solution was then permitted to cool to 80 C. The substrate was washed several times with de-ionized water and dried in an oven at 60 C for 12 h. The dried substrate was annealed at 300 C for 2 h under Ar flow (200 sccm). Photoelectrochemical reduction of CO 2 : A two compartment acrylic reactor with the Nafion115 proton exchange membrane between the two chambers was used for the reaction. The cathode compartment of the reactor was connected to a closed gas circulation system with an on-line gas chromatography sampling loop. The total volume of gas in the recirculation system is 79 ml. The ZnTe electrode, Pt wire and silver-silver chloride (Ag/AgCl) electrode served as the cathode, anode and the reference electrode respectively. 40 ml of 0.5 M potassium bicarbonate (Wako, 99.9%) solution with a ph of 7.5 was used as the catholyte and anolyte. The visible light irradiation was carried out using a 500 W Hg lamp (Newport, Model 66902) with 420 nm cut-off filter and water for blocking the infra-red radiation. The intensity of the irradiation was measured using a Newport Calibrated

3 Reference Cell and Meter (Model 91150) (Figure S7). And solar simulator (Oriel 91160, 100mW/cm 2 ) was used as another light source for linear sweep voltammograms of ZT. Before the reaction, the electrolyte and the recirculation system were purged with CO 2 for 40 min and 10 min respectively. The recirculation system was filled with CO 2 before the reaction and the gas was continuously recirculated through the catholyte during the whole reaction period. The CO 2 reduction experiments were carried out under potentiostatic conditions using Princeton Applied Research Potentiostat/Galvanostat Model 263A. Mott-Schottky analysis was carried out at a DC potential range V vs. Ag/AgCl with the AC potential frequency 5 khz and the amplitude of AC potential V under dark condition in a 0.5M potassium bicarbonate 0.35 M Na 2SO 3 solution (ph 8.5). The incident-photon-tocurrent-conversion efficiency (IPCE) was measured using a Hg lamp (450 W, Oriel) and a monochromator with a bandwidth of 5 nm. The IPCE was measured at 0.7 V vs. RHE in a 0.5M CO 2 saturated potassium bicarbonate solution (ph 7.5). CO 2 reduction product analysis: The gaseous samples were tested for products of CO 2 reduction such as CO, methane, ethylene and ethane by injecting samples through the on-line sampling loop onto the gas chromatography. An Agilent Model 6890 gas chromatography with a Carboxen 1000 packed column and thermal conductivity detector (TCD) was used for the analysis. Agilent poraplot-q capillary column was employed for testing the liquid products for methanol and ethanol with an FID detector. Attempts to measure the liquid products such as HCOOH, CH 3COOH were made using HPLC (Agilent, VWD detector), NMR (Agilent, 600MHz) and Mass spectroscopy (Bruker, ESI). Characterization : The morphology, crystallinity, crystalline nature and optical properties of the samples were characterized by field-emission scanning electron microscopy (FESEM, JEOL JMS-7401F and Philips Electron Optics B. V. XL30S FEG, operated at 10 kev), X-ray diffraction spectrometry (XRD, Mac Science, M18XHF using Cu Kα radiation, λ = nm), high-resolution scanning transmission electron microscopy (HR-STEM, Cs-corrected, JEOL JEM-2200FS and UV-Vis diffuse reflectance spectroscopy (Shimadzu, UV2501PC). Chemical states of the ZT were elucidated by X-ray photoelectron microscopy (XPS, ESCALAB 250Xi).

4 Figure S1. a) Mott-Schottky plots of the Zn/ZnO/ZnTe structure (ZT) based on the Mott Schottky relation: 1/C 2 = (2 / e ε ε 0 N)[V a V fb kt/e], where C = space charge layers capacitance, e = electron charge, ε = dielectric constant, ε 0 = permittivity of vacuum, N = the charge carrier density, V a = applied potential, and V fb = flat band potential. The temperature term is generally small and can be neglected. b) Tauc plot of ZT using the following relationship: α hυ = A (hυ-e g) n/2 where α, hυ, A, and E g are absorption coefficient, the photon energy, constant and the optical band gap energy, respectively. The value of n depends on whether the transition is direct (n = 1) or indirect (n = 4), and it was determined to be 1 from extrapolating the linear part of (αhυ) 2 vs. hυ plot to the energy axis at (αhυ) 2 = 0.

5 Current density (ma cm -2 ) Solar simulator (100mW/cm 2 ) V (vs. RHE) Figure S2. Photocurrent-potential curves for ZT using solar simulator (100mW/cm2) in 0.5M KHCO3 solution (ph 7.5). Table S1. Faradaic efficiencies of CO and H 2 at -0.7, -0.5, -0.4 and -0.2V vs. RHE for 3 hours.

6 0 Current (ma cm -2 ) vs. RHE -0.5 vs. RHE -0.4 vs. RHE -0.2 vs. RHE Time (minutes) Figure S3. Chronoamperometry data of ZT electrode at -0.7, -0.5, -0.4 and -0.2 V vs. RHE in a CO 2 -saturated 0.5 M KHCO 3 (ph 7.5) under irradiation from a 500 W Hg lamp with >420 nm filter. H 2 (micromoles cm -2 ) V -0.4 V -0.5 V -0.7 V Time (minutes) Figure S4. Quantity of H 2 evolved vs. time at different potentials in a ZT electrode in CO 2 saturated 0.5 M KHCO 3 solution (ph 7.5) under irradiation from a 500 W Hg lamp with >420 nm filter.

7 Table S2. Photoelectrochemical reduction products in 0.5M KHCO 3 0.5M NaHCO 3 and tetrabutylammonium bromide (TBABr) solution at -0.7 vs. RHE. [a] Electrolyte Faradaic efficiency (%) CO H 2 KHCO NaHCO TBABr [a] Using Zn/ZnO/ZnTe photocathode and Pt anode after 1hr of irradiation with 500W Hg lamp with a >420nm cut-off filter, 0 Current (ma cm -2 ) KHCO 3 NaHCO 3 TBABr Time (minutes) Figure S5. Chronoamperometry data of ZT electrode in 0.5M KHCO3, 0.5M NaHCO3 and tetrabutylammonium bromide (TBABr) solution at -0.7 vs. RHE.

8 Intensity (a. u.) Te 2-3d 5/2 (ZnTe) -0.2 vs. RHE -0.7 vs. RHE Te 4+ 3d 5/2 (TeO 2 ) Te 4+ 3d 3/2 Te 2-3d (TeO 2 ) 3/2 (ZnTe) Before reaction Bindinding Energy (ev) Figure S6. X-ray photoelectron spectroscopy (XPS) of Te 3d peaks for the bare ZT and two used electrode for 3hours at -0.2 and -0.7V vs. RHE respectively. Table S3. Atomic ratio of Zn / Te and peak ratio of Te 3d5/2(ZnTe) / Te 3d5/2(TeO2) of bare and used two electrodes (at -0.2, -0.7V vs. RHE) Ratio Atomic Zn % Te 3d Electrodes 5/2 peak of ZnTe / / Atomic Te % Te 3d 5/2 peak of TeO 2 Bare electrode Used electrode (-0.2 vs. RHE) Used electrode (-0.7 vs. RHE)

9 (A) Irradance at 0.5m (mw m -2 nm -1 ) (B) Wavelength (nm) Figure S7. The output spectrum of the Newport 500 W Hg lamp (Model 66902): (A) Whole range. (B) From 420 nm to 800 nm. Table S4: The lowest potentials reported for the photoelectrochemical reduction of CO2 to various products using p-type semiconductors. Electrode Potential (V vs. RHE) Products Ref. p-si -0.62, CO, HCOOH Au/p-Si CO 39, 40 Ag/p-Si CO Cu/p-Si CH 4, CO,HCOOH, C 2H 4 p-cu 2O/CuO CH 3OH 41 P-GaP HCOOH 42 p-inp, p-cdte CO, HCOOH 43 p-znte CO present study * Reported potentials were converted to RHE values using the ph of the electrolyte V RHE = V + (0.059 X ph) + V Ref Where V Ref = and for SCE and Ag/AgCl respectively