Surfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields, including renewable energy and catalysis. Typically, these structures are prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO 3 stoichiometry. This non-stoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (that is, metallic, oxides or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be influenced strongly by surface reorganization characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application. AbstractSurfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields including renewable energy and catalysis. These structures are typically prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO 3 stoichiometry. This nonstoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (i.e. metallic, oxides, or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be strongly influenced 2 by surface reorganisation characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application.

B-site doped, A-site deficient perovskite oxide titanates with formula La 0.4 Sr 0.4 M n+x Ti 1Àx O 3ÀgÀd (M ¼ Fe 3+ or Ni 2+ ; x ¼ 0.06; g ¼ (4 À n)x/2) were employed as solid oxide electrolysis cell (SOEC) cathodes for hydrogen production via high temperature steam electrolysis at 900 C. A-site deficiency provided additional driving force for the exsolution of a proportion of B-site dopants at the surface in the form of metallic nanoparticles under reducing SOEC cathode operating conditions. In the case of La 0.4 Sr 0.4 Fe 0.06 Ti 0.94 O 2.97 , this represents the first time that Fe 0 has been exsolved from a perovskite in such a way. Exsolution was due in part to the inability of the host lattice to accommodate vacancies (introduced (d) oxygen vacancies (V $$ O ) and fixed A-site (V 00 Sr ) and inherent (g) oxygen vacancies) beyond a certain limit. The presence of electrocatalytically active Fe 0 or Ni 0 nanoparticles and higher V $$ O concentrations dramatically lowered the activation barrier to steam electrolysis compared to the parent material (x ¼ 0). The use of defect chemistry to drive the exsolution of less reducible dopant cations could conceivably be extended to produce new catalytically active perovskites with unique properties. Broader contextThe intermittency of renewable power sources signicantly restricts clean energy production viability, which is improved by a system capable of storing excess energy in a carrier and converting this carrier back to electricity as required. However, existing grid energy storage systems utilising pumped hydro, battery, compressed air, or ywheel technologies can suffer from certain limitations. Reversible solid oxide electrolysis cells (SOECs) utilising hydrogen as the energy carrier may offer an efficient and distributable alternative for grid energy storage, although the state-of-the-art Ni/yttria-stabilised zirconia (YSZ) SOEC hydrogen electrode (cathode) material exhibits inherent redox instability. In contrast, ABO 3 perovskite oxides are inherently redox stable and may be optimised for reducing SOEC cathode operating conditions via prudent selection of dopant and defect chemistries. Here a step-change in the high temperature steam electrolysis performance of A-site decient, B-site doped titanate cathodes was achieved with exsolution of B-site dopants in the form of electrocatalytically active metallic nanoparticles from the host lattice during SOEC operation. The design of perovskite oxides that exsolve dopants under operating conditions is a promising strategy for raising SOEC cathode performance, while scope for improvement is expected with further optimisation of dopant and defect chemistries.

Zn-Zn porphyrin dimers have been incorporated into thin dye-sensitized solar cells (DSSCs) to boost their light harvesting efficiency. The photoexcited dimers show efficient and fast electron injection into TiO(2) indicating that both photoexcited chromophores contribute to current generation. The improved light harvesting ability coupled to enhanced DSSC performance demonstrates the potential of 3-D light harvesting arrays as next generation light harvesters for artificial solar energy conversion systems.