Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes

Abstract

The effects that Ta2O5-overlayer and Ta-doping have on the photoelectrochemical performance and surface state capacitance of hematite photoanodes.

Phenomenological Understanding of Hematite Photoanode Performance

“…35 In general, the TA data of the studied hematite samples (see Tables 2−5) present: (i) a first fast decay transient, τ 1 , with values ranging from 0.43 to 1.6 ps; (ii) a second one, τ 2 , with values in the 2.8−41 ps range; (iii) a third one, τ 3 , with values ranging from 160 to 400 ps; (iv) in the microsecond time range, transient lifetimes in the 0.5−49 μs range, τ 4 ; and (v) long-lived transients ranging from 5 to 806 μs, τ 5 . These lifetimes are in good agreement with the values reported for 50 nm thick hematite photoelectrodes made by atomic layer deposition (TAS data obtained in air), 40 in which: (i) the fast τ 1 transient lifetime was assigned to the initial relaxation of hot electrons; (ii) the τ 2 and τ 3 values to recombination of free conduction electrons trapped by mid-gap states; and (iii) the τ 4 to τ 5 lifetimes to the slower recombination dynamics of the remaining free and mid-gap trapped electrons. 29 It is worth mentioning that from Tables 2−5, it can be seen that, going from the dry hematite thin films to the samples immersed in 1 M NaOH and applying an anodic bias potential, a significant decrease of the longest lifetimes (mainly in τ 3 and in the microsecond range lifetimes, τ 4 and τ 5 ) is observed, thus showing that contrary to what was expected, the charge recombination of trapped holes and electrons is faster in operando conditions.…”

“…In general, the TA data of the studied hematite samples (see Tables –) present: (i) a first fast decay transient, τ 1 , with values ranging from 0.43 to 1.6 ps; (ii) a second one, τ 2 , with values in the 2.8–41 ps range; (iii) a third one, τ 3 , with values ranging from 160 to 400 ps; (iv) in the microsecond time range, transient lifetimes in the 0.5–49 μs range, τ 4 ; and (v) long-lived transients ranging from 5 to 806 μs, τ 5 . These lifetimes are in good agreement with the values reported for 50 nm thick hematite photoelectrodes made by atomic layer deposition (TAS data obtained in air), in which: (i) the fast τ 1 transient lifetime was assigned to the initial relaxation of hot electrons; (ii) the τ 2 and τ 3 values to recombination of free conduction electrons trapped by mid-gap states; and (iii) the τ 4 to τ 5 lifetimes to the slower recombination dynamics of the remaining free and mid-gap trapped electrons …”

Tin and Oxygen-Vacancy Co-doping into Hematite Photoanode for Improved Photoelectrochemical Performances

“…For now, numerous metal oxides (e.g., WO 3 [5], BiVO 4 [6], and TiO 2 [7]) have been constructed into photoanodes for water oxidation due to the excellent chemical stability and the maximum of valence band positive to the potential of H 2 O/O 2 . Among them, hematite (α-Fe 2 O 3 ) is particularly promising benefited from the suitable bandgap (~2.1 eV) for the absorption of visible light, vast abundance of the consisting elements, non-toxicity, low-cost preparation, and so on [8][9][10]. However, its practical efficiency is far less than the theoretical limit mainly due to the poor conductivity, short hole diffusion length, and slow hole kinetics [11][12][13].…”

“…The methods for resolving these problems include element doping (to improve the photoactive-material conductivity) and surface modification of the photoelectrode surface (to enhance the surface reaction kinetics or to suppress the surface carrier recombination) [14][15][16][17]. Doping of α-Fe 2 O 3 with moderate additives such as Zr 4+ [18], Ti 4+ [8], Sn 4+ [19], and Al 3+ [20] can improve the conductivity and then reduce the obstruction of the carrier collection. Moreover, the short hole diffusion length makes it difficult for the extraction of the photogenerated holes to the photoanode surface for water oxidation.…”

Nanocarbon-Enhanced 2D Photoelectrodes: A New Paradigm in Photoelectrochemical Water Splitting

“…Among them, EIS spectroscopy is widely utilized to explore photogenerated charge dynamics and recombination kinetics in the photoelectrodes, where the impedance of a system can be measured over a range of frequencies, thereby facilitating to reveal the frequency response (such as dissipation properties) of the PEC system [73]. Besides, transient absorption spectroscopy (TAS) is an effective tool to obtain absorption and concentration data of photoinduced charge carriers in the PEC system, which is helpful to measure the lifetime of photogenerated electrons and holes, and thereby analyze transport and recombination dynamics in photoelectrodes on the picosecond to microsecond time scales [74][75][76]. Moreover, time-resolved photoluminescence (TRPL) spectroscopy is also a powerful technique for charge carrier dynamic measurements, which can provide valuable information on photogenerated charge carrier lifetime and electron-hole diffusion length for investigating the size-, surface-and interface-dependent effects [77].…”