Photodoping of metal oxide nanocrystals for multi-charge accumulation and light-driven energy storage

Abstract

Light-driven multi-charge accumulation (i.e., photodoping) of doped metal oxide nanocrystals opens the way to innovative solutions for the direct conversion and storage of the solar energy.

Control of electronic band profiles through depletion layer engineering in core–shell nanocrystals

“…2c (dotted curves). The spectra are governed by intense resonances in the near-infrared (NIR) that are assigned to localized surface plasmon resonances (LSPRs) as a result of free electrons in the highly doped semiconductor (typically in the range of 10 27 m −3 ) 2 , 4 , 25 – 27 . The LSPR peak position and its peak profile are correlated to several factors, such as the NC geometrical features (e.g., ), , the depletion layer width ( ), the dielectric constant of the surrounding medium ( ), as well as the structural defects and dopant concentration, providing a unique spectral signature of such parameters 21 , 28 – 30 .…”

“…To further study the electronic structure of core–shell NCs out of the equilibrium conditions, we post-synthetically alter the number of free carriers via photodoping 4 , 11 , 21 , 32 . Photodoping consists of introducing multiple free charge carriers via light absorption and suppressing carrier recombination by the quenching of the holes with hole scavengers 11 , 21 , 33 .…”

“…Doped metal oxide (MO) nanocrystals (NCs) are gaining the attention of the scientific community thanks to their unique properties, such as high electron mobility 1 , the tuneability of their carrier density over several orders of magnitude 2 , chemical stability 3 , and low toxicity 3 , as well as suitable operating temperature 1 , which makes them appropriate for a large plethora of applications, ranging from nanoelectronics and plasmonics to the next-generation energy storage 3 – 11 . In doped MO NCs, surface states, such as surface trap states, defects, vacancies, as well as surface ligands and other bound molecules induce Fermi level pinning causing an upward bending of the energetic bands 2 , 4 , 12 – 18 . The spatially varying conduction band translates into a gradient in the carrier density ( ), sufficient to suppress entirely the metallic behavior of carriers close to the nanocrystal surface.…”

Multi-charge transfer from photodoped ITO nanocrystals

“…Photodoping, a light-driven charge accumulation of electrons induced by multiple absorption events of high-energy photons (beyond the bandgap of MO semiconductors), emerged as a contactless and promising tool to promote the photo-conversion process. 7,11–16 The photodoping of ITO NCs in the presence of a sacrificial hole scavenger, such as ethanol, under anaerobic conditions, results in the reversible accumulation of tens to hundreds of electrons per single NC. 16 The reversibility of the photodoping process (and therefore the removal of photo-chemically excited electrons), already well demonstrated, was also analysed via potentiometric titration with the addition of a molecular oxidant to the colloidal suspension of MO NCs.…”

“… 11,12,14,16,17 Due to the proportionality between the plasmonic peak energy ω LSPR and the free carrier density, 18 the storage and depletion of photodoped electrons upon photodoping and oxidative titration can also be performed by monitoring the spectral evolution of the LSPR. 7,14–16,19–21 In this context, the opportunity to access multiple charge transfer events of photodoped electrons would be enormously relevant in terms of efficiency, not only for photovoltaic energy conversion purposes, but also in applications such as batteries, or implementations in photoelectrochemistry and photocatalysis. 22–24 The transfer of more than one electron per unit of active component ( i.e.…”