In Situ Magnetic Alignment of a Slurry of Tandem Semiconductor Microwires Using a Ni Catalyst

Slurries of semiconductor particles individually capable of unassisted light-driven water-splitting are modeled to have a promising path to low-cost solar hydrogen generation, but they have had poor efficiencies. Tandem microparticle systems are a clear direction to pursue to increase efficiency. However, light absorption must be carefully managed in a tandem to prevent current mismatch in the subcells, which presents a possible challenge for tandem microwire particles suspended in a liquid. In this work, a Ni-catalyzed Si/TiO₂ tandem microwire slurry is used as a stand-in for an ideal bandgap combination to demonstrate proof-of-concept in situ alignment of unassisted water-splitting microwires with an external magnetic field. The Ni hydrogen evolution catalyst is selectively photodeposited at the exposed Si microwire core to serve as the cathode site as well as a handle for magnetic orientation. The frequency distribution of the suspended microwire orientation angles is determined as a function of magnetic field strength under dispersion with and without uplifting microbubbles. After magnetizing the Ni bulb, tandem microwires can be highly aligned in water under a magnetic field despite active dispersion from bubbling or convection.