Micro bimorph coils driven by a metalinsulator phase transition in VO2 function as powerful torsional muscles. Reversible torsional motion over one million cycles without degradation is demonstrated, with a superior rotational speed up to ca. 200,000 rpm, an amplitude of 500° per mm length, and a power density up to ca. 39 kW kg⁻¹.

We synthesized by atomic layer deposition (ALD) TiO2–IrO x alloys that enable high photovoltages and catalyze water oxidation on silicon metal–insulator–semiconductor (MIS) photoanodes. The ratio of TiO2 to IrO x was precisely controlled by varying the number of ALD cycles for each precursor. Silicon with a 2 nm surface SiO2 layer was coated with TiO2–IrO x alloys ranging in composition from 18 to 35% iridium relative to the sum of titanium and iridium concentrations. IrO x catalyzed oxygen evolution and imparted a high work function to the TiO2–IrO x alloys, enabling photovoltages during water oxidation that exceeded 600 mV. TiO2 imparted stability and inhibited corrosion of the underlying silicon light absorber. After annealing in forming gas (5% H2/95% N2), TiO2–IrO x alloys were stable for 12 h of continuous water oxidation in 1 M H2SO4. Key properties of the MIS junction affecting electrochemical operation were also extracted by electrochemical impedance spectroscopy. This work demonstrates that alloying by ALD is a promising approach for designing corrosion resistant Schottky contacts with optimized electronic and materials properties for catalyzed, solar driven water oxidation.

Atomic layer deposited titanium dioxide (ALD-TiO2) has emerged as an effective protection layer for highly efficient semiconductor anodes which are normally unstable under the potential and pH conditions used to oxidize water in a photoelectrochemical cell. The failure modes of silicon anodes coated with an Ir/IrO x oxygen evolution catalyst layer are investigated, and poor catalyst/substrate adhesion is found to be a key factor in failed anodes. Quantitative measurements of interfacial adhesion energy show that the addition of TiO2 significantly improves reliability of anodes, yielding an adhesion energy of 6.02 ± 0.5 J/m2, more than double the adhesion energy measured in the absence of an ALD-TiO2 protection layer. These results indicate the importance of catalyst adhesion to an interposed protection layer in promoting operational stability of high efficiency semiconducting anodes during solar-driven water splitting.

Iridium has long been recognized as one of the best oxygen evolution catalysts in terms of activity and stability over a wide range of pH. Despite exhibiting initially high activity for the oxygen evolution reaction (OER), a rapid and reversible activity decay is observed during continuous operation. The potential dependence of recovery of the activity is explored, and 0.0 V vs NHE is found to be an effective potential to recover the initial water oxidation performance. Iridium thin films on rotating disk electrodes are used to show that this OER activity decay is neither explained by progressive oxidation of the iridium nor by reduced mass transport to/from the electrode surface. Careful examination of the time dependence of the activity decay reveals that it is well described by a t −1/4 functional dependence across multiple electrochemical cell geometries. Tafel behavior is analyzed by normal pulse voltammetry, suggesting that, after 10 minutes of activity decay, the catalyst exhibits a five-fold decrease in active site density while the mechanism of water oxidation is not altered. We hypothesize that this decay may result from a loss of active sites capable of forming the Ir(V) = O species, possibly via progressive cross-linking of iridium sites by bridging mu-oxides.

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