Researchers develop free-standing nanowire mesh for direct solar water-splitting to produce H2; new design for “artificial leaf”

3 December 2014

The mesh with BiVO₄ nanowire photoanode for water oxidation and Rh-SrTiO₃ nanowire photocathode for water reduction produces hydrogen gas without an electron mediator. Credit: ACS, Liu et al. Click to enlarge.

Researchers from UC Berkeley, Lawrence Berkeley National Laboratory and Nanyang Technological University, Singapore have developed a new technology for direct solar water-splitting—i.e., an “artificial leaf” to produce hydrogen—based on a nanowire mesh that lends itself to large-scale, low-cost production. A paper describing their work is published in the journal ACS Nano.

In the design, semiconductor photocatalysts are synthesized as one-dimensional nanowires, which are assembled into a free-standing, paper-like mesh using a vacuum filtration process from the paper industry. When immersed in water with visible light irradiation (λ ≥ 400 nm), the mesh produces hydrogen gas. Although boosting efficiency remains a challenge, their approach—unlike other artificial leaf systems—is free-standing and doesn’t require any additional wires or other external devices that would add to the environmental footprint.

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To develop an efficient solar water splitting system, it is necessary: (1) to develop semiconductor materials which absorb in the visible region of solar spectrum; (2) to design architectures for effective capture and conversion of sunlight, at the same time, allowing easy transport of protons and gas products; (3) to develop robust ion-conducting membranes, which are impermeable to the gas products; and (4) to integrate each individual component into a complete and functioning system.

In the present study, we developed a new architecture for direct solar water-splitting. In this design, semiconductor photocatalysts were synthesized as one-dimensional nanowires, which were assembled into free-standing, paper-like mesh for solar water-splitting. The large aspect ratio of semiconductor nanowires allows for the formation of intertwined and porous nanowire networks. The porous structure of nanowire mesh networks can benefit photochemical reactions by decoupling directions for light absorption and charge carrier extraction as well as providing a large area of catalytic surfaces. Furthermore, the porous structure can also facilitate proton transport and gas evolution.

As a proof-of-concept, they used BiVO₄ and Rh-SrTiO₃ nanowires for overall water-splitting. The BiVO₄ nanowires act as a photoanode for water oxidation and the Rh-SrTiO₃ nanowires work as a photocathode for water reduction.

Photoelectrochemical overall water splitting over linked Rh-SrTiO₃ and BiVO₄ photoelectrodes without applying any external bias under visible light irradiation. Dashed line: half amount of electrons which had passed through the external circuit of linked photoelectrochemical cell; (■) hydrogen evolution rate and (●) oxygen evolution rate.

Insets show photocurrent versus time of externally short-circuited Rh-SrTiO₃ and BiVO₄ photoelectrodes (left) and schematic of a Rh-SrTiO₃ and BiVO₄ photoelectrolysis cell system for overall solar-driven water splitting (right). Credit: ACS, Liu et al. Click to enlarge.

In their experiments, they first made two photoelectrodes of BiVO₄ and Rh-SrTiO₃, then loaded the co-catalysts (CoO_x for BiVO₄ and 1 wt % Ru for Rh-SrTiO3) on the surface.

The team selected Ru as the co-catalyst instead of Pt because Ru is an effective co-catalyst for hydrogen evolution that does not enhance back-reaction for water formation from evolved H₂ and O₂.

They assembled two types of nanowire mesh films, including mixed Ru/Rh-SrTiO₃ and BiVO₄ nanowire mesh film and bilayer Ru/Rh-SrTiO₃ and BiVO₄ nanowire mesh film. Prior to photoelectrochemical testing, the nanowire mesh films were annealed at 500–800 °C in argon to promote good contact between the nanowires.

Under testing, the resulting cells split water into H₂ and O₂ in a stoichiometric ratio without using any electron mediator.

The total evolved H₂ and O₂ was ∼4.5 μmol, which corresponds to an overall solar-to-fuel conversion efficiency of 0.0017%. Photoactivity depended on the relative amount of Ru/Rh-SrTiO₃ to BiVO4. The highest photoactivity was obtained using mixed nanowire mesh film assembled from equal amounts of Ru/Rh-SrTiO₃ and BiVO₄ nanowires.

The study was supported by the US Department of Energy’s (DOE) Office of Basic Energy Sciences and the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE).

Resources

• Bin Liu, Cheng-Hao Wu, Jianwei Miao, and Peidong Yang (2014) “All Inorganic Semiconductor Nanowire Mesh for Direct Solar Water Splitting”
  ACS Nano 8 (11), 11739-11744 doi: 10.1021/nn5051954