Light-induced charge transport within a single asymmetric nanowire.

Artificial photosynthetic systems using semiconductor materials have been explored for more than three decades in order to store solar energy in chemical fuels such as hydrogen. By mimicking biological photosynthesis with two light-absorbing centers that relay excited electrons in a nanoscopic space, a dual-band gap photoelectrochemical (PEC) system is expected to have higher theoretical energy conversion efficiency than a single band gap system. This work demonstrates the vectorial charge transport of photogenerated electrons and holes within a single asymmetric Si/TiO(2) nanowire using Kelvin probe force microscopy. Under UV illumination, higher surface potential was observed on the n-TiO(2) side, relative to the potential of the p-Si side, as a result of majority carriers' recombination at the Si/TiO(2) interface. These results demonstrate a new approach to investigate charge separation and transport in a PEC system. This asymmetric nanowire heterostructure with a dual band gap configuration and simultaneously exposed anode and cathode surfaces represents an ideal platform for the development of technologies for the generation of solar fuels, although better photoanode materials remain to be discovered.

[1]  Charles M. Lieber,et al.  Coaxial silicon nanowires as solar cells and nanoelectronic power sources , 2007, Nature.

[2]  Alexander J. Cowan,et al.  Water Splitting by Nanocrystalline TiO2 in a Complete Photoelectrochemical Cell Exhibits Efficiencies Limited by Charge Recombination , 2010 .

[3]  R. Guckenberger,et al.  STM on Wet Insulators: Electrochemistry or Tunneling? , 1995, Science.

[4]  D. Ginger,et al.  Time-resolved electrostatic force microscopy of polymer solar cells , 2006, Nature materials.

[5]  Yun Jeong Hwang,et al.  High density n-Si/n-TiO2 core/shell nanowire arrays with enhanced photoactivity. , 2009, Nano letters.

[6]  V. Palermo,et al.  Nanoscale quantitative measurement of the potential of charged nanostructures by electrostatic and Kelvin probe force microscopy: unraveling electronic processes in complex materials. , 2010, Accounts of chemical research.

[7]  Nathan S Lewis,et al.  Photoelectrochemical hydrogen evolution using Si microwire arrays. , 2011, Journal of the American Chemical Society.

[8]  M. Tatarkhanov,et al.  Water growth on metals and oxides: binding, dissociation and role of hydroxyl groups. , 2009, Faraday discussions.

[9]  Robert A. Martienssen,et al.  The Biology of Plants , 2012 .

[10]  M. Grätzel.  Photoelectrochemical cells : Materials for clean energy , 2001 .

[11]  Peidong Yang,et al.  Nanowire dye-sensitized solar cells , 2005, Nature materials.

[12]  Detlef W. Bahnemann,et al.  Charge Carrier Dynamics at TiO2 Particles: Reactivity of Free and Trapped Holes , 1997 .

[13]  Arthur J. Nozik,et al.  p‐n photoelectrolysis cells , 1976 .

[14]  R. Rosenfeld.  Nature , 2009, Otolaryngology--head and neck surgery : official journal of American Academy of Otolaryngology-Head and Neck Surgery.

[15]  Kazuhiro Sayama,et al.  Development of new photocatalytic water splitting into H2 and O2 using two different semiconductor photocatalysts and a shuttle redox mediator IO3-/I-. , 2005, The journal of physical chemistry. B.

[16]  A. Fujishima,et al.  Electrochemical Photolysis of Water at a Semiconductor Electrode , 1972, Nature.

[17]  James R. Bolton,et al.  Limiting and realizable efficiencies of solar photolysis of water , 1985, Nature.

[18]  William R. Silveira,et al.  Direct measurement of the electric-field distribution in a light-emitting electrochemical cell. , 2007, Nature materials.

[19]  W. Ingler,et al.  Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2 , 2002, Science.

[20]  Peidong Yang,et al.  Controlled growth of Si nanowire arrays for device integration. , 2005, Nano letters.

[21]  N. Serpone,et al.  Subnanosecond Relaxation Dynamics in TiO2 Colloidal Sols (Particle Sizes Rp = 1.0-13.4 nm). Relevance to Heterogeneous Photocatalysis , 1995 .

[22]  Michael Grätzel,et al.  Light-induced water splitting with hematite: improved nanostructure and iridium oxide catalysis. , 2010, Angewandte Chemie.

[23]  K. Domen,et al.  Photocatalyst releasing hydrogen from water , 2006, Nature.

[24]  Nathan S. Lewis,et al.  Energy-Conversion Properties of Vapor-Liquid-Solid–Grown Silicon Wire-Array Photocathodes , 2010, Science.

[25]  Miquel Salmeron,et al.  Growth and structure of water on SiO2 films on Si Investigated by Kelvin probe microscopy and in Situ X-ray spectroscopies. , 2007, Langmuir : the ACS journal of surfaces and colloids.

[26]  Tomoki Akita,et al.  All-solid-state Z-scheme in CdS–Au–TiO2 three-component nanojunction system , 2006, Nature materials.

[27]  Xiaobo Chen,et al.  Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals , 2011, Science.

[28]  James R. McKone,et al.  Solar water splitting cells. , 2010, Chemical reviews.