Size-Dependent Ultrafast Charge Carrier Dynamics of WO₃ for Photoelectrochemical Cells

Abstract

Time-resolved terahertz (THz) spectroscopy and open-circuit photovoltage measurements were employed to examine the size-dependent charge carrier dynamics of tungsten (VI) oxide (WO 3 ) particles for their use as the photoanode in photoelectrochemical cells. Specifically, films of commercially available WO 3 nanoparticles (NPs) and granular particles (GPs) with diameters of 77 ± 34 nm and 390 ± 260 nm, respectively, were examined in air and while immersed in 0.1 M Na 2 SO 4 electrolyte (pH = 2). Examination of the frequency-dependent transient photoconductivity at short and long timescales indicates the presence of both photoinduced high net transport charge carriers at early times, and in some cases low net transport charge carriers at later times. The high net transport charge carriers dominate the photoconductivity signal for ~100 ps after photoexcitation. Depletion of the short-lived high net transport carriers due to trapping leads to the detection of longer-lived low net transport photoinduced charge carriers that likely contribute to surface chemistry. Photoelectrochemical cells (PECs) convert solar energy into either electrical power or a fuel, and their optimization is a promising pathway in the development of renewable energy sources. 1-3 Metal oxide semiconductors have been widely studied for their application in water splitting cells due to their favorable structural and electronic properties. 4-7 Tungsten (VI) oxide (WO 3 ) is of particular interest because it has a relatively small band gap energy of 2.6 eV (corresponding to a wavelength of 475 nm), enabling it to absorb light in the visible spectrum without the need for sensitizer dyes. 8,9 Its electrochemical properties 10,11 and spectroscopic properties, including transient absorption 12-14 and time-resolved microwave spectroscopy, 15 have been studied previously. However, ultrafast photoinduced charge carrier dynamics in WO 3 have not yet been investigated on the picosecond timescale. Our interests lie in the characterization and dynamics of the nascent photoinduced carriers that are required for efficient oxidation at the metal oxide/ electrolyte interface.We examine for the first time ultrafast photoinduced carrier dynamics in WO 3 particles as a function of particle size utilizing optical pump -THz probe (OPTP) spectroscopy. THz measurements were performed under open-circuit conditions on as-prepared dry films in air, as well as in the presence of the aqueous electrolyte employed during photocatalysis in order to gain insights into the ultrafast dynamics under near-operating conditions. The spectroscopic results, including measurements of photoinduced charge carrier lifetimes and the frequencydependent complex transient photoconductivity, are also correlated with open-circuit photoelectrochemical measurements of two WO 3 particle sizes.

Effect of oxygen deficiency on the excited state kinetics of WO3 and implications for photocatalysis

“…5b). This bleach undergoes a stretched exponential decay (DA(t) f exp(À(t/s)) b with b varying 0.19-0.34) which implies that photogenerated carriers are characterized by a distribution of different lifetimes, 36,53 as can be expected for a distribution of trap states at different energies within the bandgap.…”

“…35 In a recent transient photoconductivity study, mobile carriers were observed up to 600 ps for mesoporous WO 3 lms with average lifetimes of around 10 ps. 36 We chose WO 3 as a model system because it adopts substoichiometric compositions more readily than other metal oxides. WO 3Àd is obtained through the release of oxygen from the crystal lattice, which causes the oxide to become reduced.…”

Unveiling the Stimulated Robust Carrier Lifetime of Surface‐Bound Excitons and Their Photoresponse in InSe

“…Consequently, a new batch of 2D materials that have adjustable bandgaps upon variation of the material thicknesses, such as transition metal dichalcogenides (TMDs), black phosphorus (BP), and transition metal carbides/nitrides (MXenes), have steadily gained increasing interest in recent years. Their adjustable bandgap and extraordinary electronic transmission properties have been applied in fields such as electronics, physics, chemistry, and biology …”

Balancing Light Absorption and Charge Transport in Vertical SnS2 Nanoflake Photoanodes with Stepped Layers and Large Intrinsic Mobility

“…[43][44][45][46][47][48] CSSgrown SnS 2 vertical nanoflakes were optically excited with 400 nm laser pulses with 100 fs duration and with 28 µJ cm −2 fluence, and transient photoconductivity (Figure 5a) was measured using a terahertz probe in the 0.25-1.70 THz frequency range, which corresponds to 1-7 meV energy range. [43][44][45][46][47][48] CSSgrown SnS 2 vertical nanoflakes were optically excited with 400 nm laser pulses with 100 fs duration and with 28 µJ cm −2 fluence, and transient photoconductivity (Figure 5a) was measured using a terahertz probe in the 0.25-1.70 THz frequency range, which corresponds to 1-7 meV energy range.…”

“…Similarly, a contactfree timeresolved terahertz spectroscopy (TRTS) technique was used to study the dynamics of photo excited free charge carriers on picosecond time scales, and estimate intrinsic mobility of SnS 2 nanoflakes. [43][44][45][46][47][48] CSSgrown SnS 2 vertical nanoflakes were optically excited with 400 nm laser pulses with 100 fs duration and with 28 µJ cm −2 fluence, and transient photoconductivity (Figure 5a) was measured using a terahertz probe in the 0.25-1.70 THz frequency range, which corresponds to 1-7 meV energy range. With energy in the meV range, terahertz pulses are absorbed by the free car riers, and transient change in transmission of the terahertz probe pulse peak is a measure of photoconductivity ( Figure 5a).…”