Ultrafast Transient Absorption Studies of Hematite Nanoparticles: The Effect of Particle Shape on Exciton Dynamics

Abstract

Much progress has been made in using hematite (α‐Fe2O3) as a potentially practical and sustainable material for applications such as solar‐energy conversion and photoelectrochemical (PEC) water splitting; however, recent studies have shown that the performance can be limited by a very short charge‐carrier diffusion length or exciton lifetime. In this study, we performed ultrafast studies on hematite nanoparticles of different shapes to determine the possible influence of particle shape on the exciton dynamics. Nanorice, multifaceted spheroidal nanoparticles, faceted nanocubes, and faceted nanorhombohedra were synthesized and characterized by using SEM and XRD techniques. Their exciton dynamics were investigated by using femtosecond transient absorption (TA) spectroscopy. Although the TA spectral features differ for the four samples studied, their decay profiles are similar, which can be fitted with time constants of 1–3 ps, approximately 25 ps, and a slow nanosecond component extending beyond the experimental time window that was measured (2 ns). The results indicate that the overall exciton lifetime is weakly dependent on the shape of the hematite nanoparticles, even though the overall optical absorption and scattering are influenced by the particle shape. This study suggests that other strategies need to be developed to increase the exciton lifetime or to lengthen the exciton diffusion length in hematite nanostructures.

Fe2O3–TiO2 Nano‐heterostructure Photoanodes for Highly Efficient Solar Water Oxidation

“…To further elucidate the role of Fe 2 O 3 /TiO 2 heterojunctions in promoting the PEC water splitting and investigate charge‐carrier dynamics in the target photoelectrodes, transient absorption spectroscopy (TAS) analyses were carried out. TAS measurements allow the monitoring of photogenerated electron and hole dynamics from a picosecond to a second timescale, and are a powerful tool for the study of the interplay between nanostructure and PEC response in Fe 2 O 3 and TiO 2 based photoanodes . In this work, the evolution of charge carriers was examined by using band‐gap excitation of both Fe 2 O 3 and TiO 2 at λ = 355 nm, simulating the solar illumination conditions in which light is absorbed by both components.…”

Trapped State Sensitive Kinetics in LaTiO2N Solid Photocatalyst with and without Cocatalyst Loading

“…14-17 A variety of other photocatalytic systems such as GaN:ZnO, LaAlO 3 /SrTiO 3 heterostructures, α-Fe 2 O 3 etc. [18][19][20][21][22][23] and also some very innovative photocatalytic materials such as three dimensional CdS-Titanate nanocomposites 24 and Ag@AgCl cubic cages 25 have also been successfully investigated using TAS. In general, the complicated carrier dynamics needs monitoring of a wide spectral and temporal window to gain a better insight into the whole process.…”

“…13 Thus, understanding the carrier dynamics in the early stages is crucial to the understanding of the overall performance of the photocatalytic material. 13,14,[21][22][23][24][25] For materials, which are translucent or opaque, instead of transmitted light as done in case of TAS, the diffuse reflected light from the sample can be effectively collected and analysed to obtain similar information on the kinetics of transient species. [26][27][28] In this work, we use femtosecond and sub-nanosecond Time-Resolved Diffuse Reflectance (fs-TRDR and sub-ns TRDR) spectroscopy to elucidate the complicated carrier dynamics in n-type 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 perovskite semiconductor photocatalyst LTON.…”

Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3

“…In particular, haematite (α-Fe 2 O 3 ) is an Earth-abundant semiconductor that has a visible-light bandgap in the optimal 2 eV range and is photostable, making it an ideal candidate [1][2][3] . However, the lifetime of photoexcited carriers in haematite is picoseconds, not the milliseconds needed to drive water oxidation [4][5][6][7][8][9][10] . Short carrier recombination lifetimes in haematite are usually assigned to surface and mid-gap trap states [4][5][6][7][8][9][10] .…”