The influence of nanoparticle surface facets on electrochromic properties remains largely unexplored in nanostructured "smart" materials. Here, we explore how surface facets influence the coloration efficiency (CE) and long-term optical density (OD) stability of hexagonal WO 3 nanorod (h-WO 3 NR) thin films. We synthesized two h-WO 3 NR samples with distinct surface facet orientations and studied how the electrochemical, electrochromic, electrical, and surface chemistry properties change after long-term cycling. The sample with unique {120} facets exhibited reversible optical switching after 500 cycles and negligible variation in interfacial charge transfer resistance. The (120) surface features an open network of square window channels that may enable reversible ion transport and reduced ion trapping, enhancing the optical switching stability. However, the {120}-dominant sample exhibited lower CE than the {100}-dominant sample. The reduced optical density changes in the {120}-dominant sample could be due to a greater fraction of optically inactive trigonal cavity sites on the {001} end-caps. The results indicate surface facet and particle morphology engineering are viable strategies to enhance the CE and long-term stability/lifetime in electrochromic thin films for smart window applications.

Hot carrier-based energy conversion systems could double the efficiency of conventional solar energy technology or drive photochemical reactions that would not be possible using fully thermalized, “cool” carriers, but current strategies require expensive multijunction architectures. Using an unprecedented combination of photoelectrochemical and in situ transient absorption spectroscopy measurements, we demonstrate ultrafast (<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical solar cell made from earth-abundant and potentially inexpensive monolayer (ML) MoS 2 . Our approach facilitates ultrathin 7 Å charge transport distances over 1 cm 2 areas by intimately coupling ML-MoS 2 to an electron-selective solid contact and a hole-selective electrolyte contact. Our theoretical investigations of the spatial distribution of exciton states suggest greater electronic coupling between hot exciton states located on peripheral S atoms and neighboring contacts likely facilitates ultrafast charge transfer. Our work delineates future two-dimensional (2D) semiconductor design strategies for practical implementation in ultrathin photovoltaic and solar fuel applications.

Hot carrier extraction occurs in 2D semiconductor photoelectrochemical cells [Austin et. al., PNAS, 2023, 120, e2220333120]. Boosting the energy efficiency of hot carrier-based photoelectrochemical cells requires maximizing the hot carrier...

Monolayer transition-metal dichalcogenides (ML-TMDs) have the potential to unlock novel photonic and chemical technologies if their optoelectronic properties can be understood and controlled. Yet, recent work has offered contradictory explanations for how TMD absorption spectra change with carrier concentration, fluence, and time. Here, we test our hypothesis that the large broadening and shifting of the strong band-edge features observed in optical spectra arise from the formation of negative trions. We do this by fitting an ab initio based, many-body model to our experimental electrochemical data. Our approach provides an excellent, global description of the potential-dependent linear absorption data. We further leverage our model to demonstrate that trion formation explains the nonmonotonic potential dependence of the transient absorption spectra, including through photoinduced derivative line shapes for the trion peak. Our results motivate the continued development of theoretical methods to describe cutting-edge experiments in a physically transparent way.

The fundamental problem that limits the solar energy conversion efficiency of semiconductors such as CdTe and Si is that all excess solar photon energy above the band gap is lost as heat. Avoiding thermalization energy losses is of paramount significance for solar energy conversion because hot-carrier-based systems theoretically achieve 66% efficiency, which breaks the detailed balance limit of 33%.Of all the candidate materials, 2D semiconductors such as monolayer (ML) MoS2 have unique physical and photophysical properties that could make hot-carrier energy conversion possible. The knowledge gap in the field is that the electronic states of 2D materials move with carrier density, due to either light absorption or an applied electrochemical potential. The energy level movements are significant because the real fundamental driving force for charge transfer (ΔG 0´) is unclear for a given reaction and applied potential. In principle, quantifying ΔG 0´ under working conditions opens up the possibility to tune the hot carrier extraction rate relative to the cooling rate. Our research team has employed photocurrent spectroscopy, steady-state absorption spectroscopy, and in situ femtosecond transient absorption spectroscopy as a function of applied potential to characterize underlying steps in a ML MoS2 photoelectrochemical cell. The rich data set informs us on the timescales for hot-carrier generation/cooling and exciton formation/recombination, as well as the magnitudes of changes in exciton energy levels, exciton binding energies, and the electronic band gap. These findings open the possibility of tuning the hot-carrier extraction rate relative to the cooling rate to ultimately utilize hot-carriers for solar energy conversion applications.