Supporting Information for: Proton Reduction using a Hydrogenase-Modified Nanoporous Black Silicon Photoelectrode

Transcription

1 Supporting Information for: Proton Reduction using a Hydrogenase-Modified Nanoporous Black Silicon Photoelectrode Yixin Zhao,, Nicholas C. Anderson, Michael W. Ratzloff, David W. Mulder, Kai Zhu, John A. Turner, Nathan R. Neale, * Paul W. King, * and Howard M. Branz * 1513 Denver West Parkway, National Renewable Energy Laboratory, Golden, Colorado 841 USA These authors contributed equally to this work. * To whom reprint requests should be addressed. nathan.neale@nrel.gov; paul.king@nrel.gov; howard@branztechnologypartners.com S-1

2 Experimental Procedures Black Si Photoelectrode: 3 µm thick p-type (1) float-zone Si wafers with resistivities of 5 Ω cm are processed into a nanoporous Si photoelectrodes by the following procedure. The Si wafer was first spin-coated with photoresist S-1818 on the back-side. The front-side Si electrode surface is then immersed in 1 mm AgNO3 and 1 wt% HF solution for 9 s, and then transferred into an etching solution composed of 5 wt% HF, 3 wt% H2O2, and deionized H2O with volume ratio of 25:6:37 for 6 minutes, followed by rinsing with de-ionized water. The remaining Ag NPs at the bottom of pores from the etching process are removed by soaking in 35% wt HNO3 solution for 6 minutes, resulting in clean nanoporous Si. The photoresist on the back-side was removed by rinsing with acetone and isopropanol and the substrate was subsequently etched with 5 wt% HF for 3 seconds, then rinsed and dried. An ohmic contact is formed on back-side of the Si wafer by applying a commercial Ga-In eutectic. The backside Ga-In eutectic of the Si is contacted with a silver paint and connected to the Cu wires encapsulated in a glass tube. The exposed back-side Si surface, edges, and a portion of the front side of the Si electrodes were sealed with an industrial epoxy (Loctite 9462). The exposed the front-side Si electrode area was around.5 cm2. Hydrogenase preparation: Recombinant Clostridium acetobutylicum [FeFe]-hydrogenase I ([FeFe]-H2ase) was expressed in Escherichia coli BL21 (DE3) from petduet based plasmids as described previously.43,44 Constructs were confirmed by DNA sequencing before transformation into Escherichia coli. [FeFe]-H2ase purification was carried out under strict anaerobic conditions in either a MBRAUN glove box with a 1% N2 atmosphere. In the final Strep-Tactin purification step, the enzyme was eluted in 5 mm Tris ph 8., 3 mm NaCl, 5% glycerol, 5 mm sodium dithionite. The purity of the enzyme was verified by SDS-PAGE and the concentration was determined by Bradford assay. Typical yields were 1 2 mg/l of culture. The H2 evolution activity of [FeFe]-H2ase was measured as the rate of H2 evolution in the presence of 5 mm methyl viologen and 1 mm sodium dithionite in 5 mm Tris-HCl, ph 8.1 Hydrogenase/b-Si photoelectrode: The b-si photoelectrode was etched by 5% HF for 3 sec then immediately transferred to a dry box within 5 minutes. To form a typical hydrogenase-modified b-si photoelectrodes, a small droplet (5. µl) of a stock solution of 15 µm hydrogenase solution in buffer (5 mm tris(hydroxymethyl)aminomethane, 2 mm NaCl, 5% glycerol, and 5 mm sodium dithionite) was spread on a b-si photoelectrode surface by micropipette and soaked for 5 min, at which point the electrode appeared dry by eye. Given the electrode surface area of ~.5 cm2, a hydrogenase concentration of 15 pmol/cm2 typically was applied. For determining the turnover frequency (TOF) and turnover number (TON), the hydrogenase stock solution was diluted 5-fold with additional buffer, and a small droplet (.5 4. µl) of this diluted S-2

3 hydrogenase solution was similarly applied to the b-si electrode surface. These volumes of the diluted (3. µm) hydrogenase concentration solution result in pmol hydrogenase being applied to the electrode surface. Given the electrode surface area of ~.5 cm2, hydrogenase concentrations of 3, 6, 9, 12, 18, and 24 pmol/cm2 were applied for.5, 1., 2., 2.5, 3., and 4. µl, respectively, diluted hydrogenase solution. Photoelectrochemical measurements: The photoelectrochemical measurement were taken in an oxygen-free drybox with 3% H2 in N2 atmosphere. A custom-built Pyrex glass vessel with flat quartz windows was used as the PEC cell for all experiments except the oxygen-free photocurrent measurement, where a custom-built Pyrex glass vessel with a glass frit separating the reference and counter electrodes from the working electrode was used. A tungsten lamp (1 W, Oriel) with a fiber optic was the light source used in the dry box. The.1 M,.2 M,.5 M and 1 M phosphate buffer solutions with ph 6.8 were used as electrolyte for the photoelectrochemical measurement. Ag/AgCl (3M NaCl) reference and Pt foil counter electrodes were used throughout. Electrochemical measurements used a CHI 6C for instrument control and data collection. The photoelectrochemical I V curves are measured at a scan rate of 2 mv/s. H2 Production Measurements: A 5 µl aliquot of hydrogenase (15 µm) in buffer was deposited on b-si in a N2 glovebox (no hydrogen atmosphere) and left to dry for 5 min. The [FeFe]-H2ase/b-Si substrate was immersed in 61 ml of a 6.8 ph phosphate buffer solution in a custom-made, 1 ml, 3-neck photoelectrochemical flask equipped with a platinum mesh counter electrode. The flask was sealed with rubber stoppers and septa and removed from the glove box. A Ag/AgCl (3M NaCl) reference electrode was inserted through one of the septa and then all electrodes were connected to a potentiostat. The cell was illuminated with 1 mw/cm2 white light and coulombs were measured under potentiostatic (.5 V vs Ag/AgCl) conditions for 1 h. Using a 5 µl syringe, 4 µl aliquots were removed from the head space (39 ml) and injected into an Agilent 789A gas chromotograph fitted with 5 Å molecular sieve column (Superlco). S-3

4 Figure S1. I V response of buffer/b-si (blue curves) photoelectrodes measured in 1. M ph 6.8 phosphate buffer solution under 1 mw/cm 2 irradiation. Similar to the application of hydrogenase solution, 5 µl of buffer solution (5 mm tris(hydroxymethyl)aminomethane, 2 mm NaCl, 5% glycerol, and 5 mm sodium dithionite) was applied to the b-si electrode surface and was left on the electrode for 5 min before immersing in phosphate buffer. Data was acquired every 15 min for 3 min. Bare b-si (black curve) and [FeFe]-H 2 ase/b-si (red curve) are shown for comparison. S-4

5 M 15 mw/cm 2 8 mw/cm 2 5 mw/cm 2 2 mw/cm 2 1 mw/cm M 15 mw/cm 2 8 mw/cm 2 5 mw/cm 2 2 mw/cm 2 1 mw/cm M 15 mw/cm 2 8 mw/cm 2 5 mw/cm 2 2 mw/cm 2 1 mw/cm M 15 mw/cm 2 8 mw/cm 2 5 mw/cm 2 2 mw/cm 2 1 mw/cm Figure S2. The irradiation-dependent photocurrent densities versus voltage at 1.,.5,.2 and.1 M phosphate buffer solution. Figure S3. J V responses for pl-si (black) and [FeFe]-H 2 ase/pl-si (red). All photoelectrodes were measured in 1. M ph 6.8 phosphate buffer solution under 1 mw/cm 2 irradiation. S-5

6 Figure S4. Hydrogen gas evolved by [FeFe]-H 2 ase/b-si at.5 V vs Ag/AgCl (3M NaCl) under 1 mw/cm 2 irradiation. Black points display theoretical yield of H 2 predicted from coulombs passed during potentiostatic measurement. The red points show the H 2 measured by gas chromatography. Error bars are estimated as ±5% for the coulombs measured and ±35% for the moles of H 2 based on the integrated chromographic peaks. Figure S5. The photocurrent over time at.5 V versus Ag/AgCl under 1 mw/cm 2 irradiation for hydrogenase on black silicon ([FeFe]-H 2 ase/b-si) where the working electrode and counter electrode are kept separate by a frit to avoid oxidation of the hydrogenase (purple). The photocurrent decays at the same rate when oxygen from the counter electrode is able to reach the working electrode (red). S-6

7 Figure S6. The photocurrent over time at.5 V versus Ag/AgCl under 1 vs. 5 mw/cm 2 irradiation for hydrogenase on black silicon ([FeFe]-H 2 ase/b-si) showing greater rate of decay for the higher illumination intensity. Acknowledgements Present address: 8 Dongchuan RD, Shanghai Jiao Tong University, Shanghai 224, China References 1. Brown, K. A.; Dayal, S.; Ai, X.; Rumbles, G.; King, P. W., Controlled Assembly of Hydrogenase-CdTe Nanocrystal Hybrids for Solar Hydrogen Production. J. Am. Chem. Soc. 21, 132 (28), S-7