The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO(2) nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO(2) nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications.

Black TiO2 nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO2, it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites is still elusive, as well as the fundamental structure–activity relationships that govern formation of crystal defects, increased light absorption, charge separation, and photocatalytic activity. In this Review, we discuss the basic concepts that underlie an effective design of reduced TiO2 photocatalysts for hydrogen production such as (i) defects formation in reduced TiO2, (ii) analysis of structure deformation and presence of unpaired electrons through electron paramagnetic resonance spectroscopy, (iii) insights from surface science on electronic singularities due to defects, and (iv) the key differences between black and gray titania, that is, photocatalysts that require Pt-modification and cocatalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO2 photocatalysts are outlined.

absorption. Although photocatalysis is already used in small-scale abatement of both indoor and outdoor pollution, the low efficiency (5%-10%) of solar-to-chemical energy conversion limits further breakthroughs in strategic applications such as hydrogen production and photovoltaics (PVs). [1,2] The engineering of composite interfaces for optimized operation is a key step to increase the solar absorption, electron-hole separation, and collection, and finally to provide high reaction yield. [3,4] The use of plasmonic nanostructures in energy conversion applications has been recently demonstrated with the enhanced performance of solar-to-fuels and solar-to-electricity devices. [5][6][7][8][9][10][11] Surface plasmons concentrate light into nanoscale volumes at the interface of a dielectric or a semiconductor providing intense electromagnetic field localization and improved scattering. The possibility to harness hot electrons generated from the surface plasmon decay [12,13] has shown to be a promising route toward selective nanocatalysis, [14][15][16][17][18][19][20] full-spectrum solar water splitting, [21][22][23] and ultrafast photodetection. [24][25][26][27] The nonradiative decay of surface plasmons, occurring in the 40-150 fs timescale, [12] produces a population of highly energetic (hot) electrons that are stabilized through injection into the semiconductor conduction band across a Schottky barrier. [28] Plasmonic nanoparticles (NPs) enable the efficient conversion of solar light into electrons with an energy well below the band gap of the semiconductor but limited to energies higher than the potential barrier (φ B ). As of yet, these demonstrations have been limited to NPs made of plasmonic noble metals such as Ag and Au. Despite their good optical performance, several challenges remain for noble metal NPs including high cost, poor chemical (Ag) and thermal stability (Au, Ag), diffusion into surrounding structures, and CMOS incompatibility, which hinders the practical implementation of conventional plasmonic structures.Aluminum nanocrystals are alternative plasmonic photocatalysts that provide efficient hot electron generation at low cost (i.e., high abundance of Al on the earth crust). [29][30][31][32][33][34] Nevertheless, issues for large-scale applications are related to Al NPs preparation, due to explosive reactivity of Al molecular The use of hot electrons generated from the decay of surface plasmons is a novel concept that promises to increase the conversion yield in solar energy technologies. Titanium nitride (TiN) is an emerging plasmonic material that offers compatibility with complementary metal-oxide-semiconductor (CMOS) technology, corrosion resistance, as well as mechanical strength and durability, thus outperforming noble metals in terms of cost, mechanical, chemical, and thermal stability. Here, it is shown that plasmonic TiN can inject into TiO 2 twice as many hot electrons as Au nanoparticles. TiO 2 nanowires decorated with TiN nanoparticles show higher photocurrent enhancement than decorat...