High-Performance Bioassisted Nanophotocatalyst for Hydrogen Production Shankar Balasubramanian, †,§ Peng Wang, †,§ Richard D. Schaller, †,‡ Tijana Rajh, † and Elena A. Rozhkova* ,† † Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States * S Supporting Information ABSTRACT: Nanophotocatalysis is one of the potentially efficient ways of capturing and storing solar energy. Biological energy systems that are intrinsically nanoscaled can be employed as building blocks for engineering nanobio- photocatalysts with tunable properties. Here, we report upon the application of light harvesting proton pump bacteriorhodopsin (bR) assembled on Pt/TiO 2 nanocatalyst for visible light-driven hydrogen generation. The hybrid system produces 5275 μmole of H 2 (μmole protein) -1 h -1 at pH 7 in the presence of methanol as a sacrificial electron donor under white light. Photoelectrochemical and transient absorption studies indicate efficient charge transfer between bR protein molecules and TiO 2 nanoparticles. KEYWORDS: Nanobiohybrid, energy, protons photoreduction, Pt/TiO 2 , bacteriorhodopsin D irect conversion of solar energy to chemical fuels such as hydrogen promises technology for providing clean energy in the near future. 1-3 Semiconductor photocatalysts such as TiO 2 play a crucial role in the generation of hydrogen via water splitting using solar energy. Since the first demonstration of the water splitting to oxygen and hydrogen on TiO 2 electrodes under UV light irradiation nowadays known as the Honda- Fujishima effect 4 there is a great continuous interest in extending the visible light reactivity of TiO 2 photocatalyst. 5 Various synthetic and natural dyes have been utilized to sensitize TiO 2 nanoparticles for visible light capture and to produce H 2 either with Pt catalyst or hydrogenase enzyme. 6-10 For example, Armstrong and co-workers 8 showed that Ru (bpy) 2 dye-sensitized TiO 2 nanoparticles when coupled with Fe-Ni hydrogenase efficiently produce H 2 under visible light irradiation with a turnover frequency of 50 (mole H 2 )s -1 (mole enzyme) -1 using triethanolamine (TEOA) as a sacrificial electron donor. Analogous construct was shown to photo- reduce CO 2 to CO when carbon monoxide dehydrogenase (CODH) enzyme was used instead of hydrogenase thereby demonstrating the versatility of visible light TiO 2 photo- catalysis. 11 In addition to dye photosynthesizers, natural protein frameworks such as photosystem I (PSI) were exploited for solar H 2 production when combined with Pt nanoparticles, cobaloxime (bis(dimethylglyoximato)cobalt(III)) complexes or hydrogenase enzymes as reduction cocatalysts. 12-15 Other naturally occurring light-harvesting proteins phycocyanins, accessory pigments to chlorophyll, were recently exploited to enhance the photocurrent density on hematite thin-film photoelectrode. 16 To date, a great progress has been made in visible light-driven H 2 generation. However, systems employing dyes or biomolecules isolated from their natural system generally have limited stability to variations in environmental factors. Natural phototrophic systems utilize the light energy to produce and store it in a form of chemical compounds via two main evolutionary-distinct mechanisms. First multistep elec- tron-shuttling mechanism known as photosynthesis that utilizes chlorophylls is found in plants, algae, and cyanobacteria. Another “simple” pathway is based on sunlight-driven proton transfer across the membrane by a proton pump bacterio- rhodopsin (bR) that is found in Archaea, for example, Halobacteria. 17 The resultant electrochemical gradient is further converted into chemical energy in the form of ATP that powers the cell. bR proton pumps are biologically occurring nano- devices capable of transporting ions against an electrochemical potential up to 250 millivolts, which translates into a 10 000- fold difference in proton concentration on either side of the membrane. 18 The bR pumps are relatively small 26 kDa prototype molecular membrane transporters. They are neatly arranged as a two dimentional (2D) nanocrystal lattice integrated into the bacterial cell membrane with a uniform orientation that is known as the purple membrane (PM). The transmembrane protein consists of seven α-helices burying a Received: May 7, 2013 Revised: June 18, 2013 Published: June 19, 2013 Letter pubs.acs.org/NanoLett © 2013 American Chemical Society 3365 dx.doi.org/10.1021/nl4016655 | Nano Lett. 2013, 13, 3365-3371