The natural process of photosynthesis captures and uses solar energy to convert carbon dioxide and water into sugars, supporting the survival and growth of the organism. In plant photosynthesis, the leaves are primarily responsible for harvesting the light and carrying out key steps in this conversion, and their hierarchical structure has evolved to maximize the efficiency of the process. For instance, the lens-like epidermal cells focus the incident sunlight, and variations in the cellular shape inside the leaf guide and enhance the light.

Fig. 1: A field-emission scanning electron microscopy image of a cross-section through the artificial inorganic leaf.

Now, Tongxiang Fan at Shanghai Jiaotong University, China, and colleagues1 have taken inspiration from nature in the synthesis of a photochemical catalyst for the splitting of water into oxygen and hydrogen. The titanium oxide (TiO2) catalyst is made by a two-step infiltration method using a leaf from the Anemone vitifolia Buch. plant as a template. The hierarchical structure of the resultant material replicates the leaf — for example, the porous framework is inherited from the leaf's veins (Fig. 1). The catalyst is doped with nitrogen originating from the plant, and finally coated with platinum nanoparticles to produce composites of nitrogen, titanium oxide and platinum.

Compared to TiO2 nanoparticles synthesized without templating, these artificial inorganic leaves (AILs) show enhanced absorbance of visible light and a red-shift in the onset of absorbance. Fan attributes these effects respectively to the structural biomimicry of the catalyst and the self-doping of nitrogen. Hydrogen evolution studies under ultraviolet/visible light and in the presence of aqueous methanol as a sacrificial agent show activity of about eight times higher than the control TiO2 nanoparticles.

“We are now considering extending the method to other active photocatalysts, such as niobates, tantalates and metal nitrides, to attain higher photocatalytic efficiency,” says Fan. “Also, artificial polymeric leaves could absorb more visible light and hence improve the activity of the catalyst in this region.”

More generally, a possible application of this strategy could be the production of cost-effective solar cells, photovoltaics and photoelectrochemical cells based on the leaf model. “This could afford an effective pathway for sustainable energy,” says Fan.