CdS–CNT–CoPi Heterostructures for Simultaneous Exciton Separation: Ultrafast and Photoelectrochemical Studies

Abstract

CdS is a well-known semiconductor for photoelectrochemical (PEC) water splitting applications, but the efficiency of this semiconductor is limited due to the high recombination rate of electron–hole pairs. Here, we have adopted a strategy to modify the CdS photoanode by the incorporation of a hole scavenger, cobalt phosphate (CoPi), and an electron scavenger, multi-walled carbon nanotubes (CNTs), which would lead to the simultaneous extraction of electron–hole pairs generated in CdS. An improved suppression of the charge-carrier recombination and effective charge transfer has been attained in case of the CdS–CNT–CoPi composite and is demonstrated using the PEC studies. The CdS–CNT–CoPi composite shows a 90-fold enhancement in photocurrent when compared to CdS at 1.23 V versus the reversible hydrogen electrode. The ternary composite also showed much higher PEC activity compared to the binary composites of CdS–CNT and CdS–CoPi. The fundamental mechanism behind the enhanced PEC activity was understood by the ultrafast transient absorption spectroscopic studies, which provides insights into the kinetics of charge transfer. Such mechanistic studies would be helpful for the optimization of efficiency and design of such photoanodes.

Understanding the charge transfer dynamics of the Cu2WS4–CNT–FeOOH ternary composite for photo-electrochemical studies

“…Here, the growth time can be related to the trapping of shallow and deep trapped electrons, while the multi-exponential decay can be attributed to the recombination of electrons and holes at different depths. 30 The bleach recovery kinetics for Cu 2 WS 4 –FeOOH (Fig. 7A, b) at 475 nm suggest a single exponential growth and multi-exponential recovery with time constants τ g 1 ≤ 100 fs (100%), τ 1 = 0.13 ps (−38%), τ 2 = 470 ps (−6%), and τ 3 ≥ 1 ns (−56%).…”

Photo-electrochemical Effect in the Amorphous Cobalt Oxide Water Oxidation Catalyst Cobalt–Phosphate (CoPi)

“…We demonstrate photo-electrochemical effects in CoPi that involve electron trapping at Co-edge sites under blue light illumination, while red light induces net oxidation and hole trapping at Co-edge sites. Such approaches in elucidating relaxation pathways and energy optimization through light-driven reactions are of urgent and great importance for incorporating CoPi and other similar amorphous cobalt oxides into renewable energy sources. ,− Electrochemical conductivity analyses of CoPi show the nanoporous films to exhibit distinct contributions from molecular-like redox sites and a dual phase material response from the cobaltate domains that switches between insulator and semiconductor states upon oxidation. , For example, Figure a shows a representative cyclic voltammogram (CV) for a CoPi film. An anodic wave is seen about 1.0 V (vs NHE) that is understood to arise from a Co­(II) → Co­(III) redox transition and is associated with a switch from insulator to semiconductor states of the film. , A second anodic redox wave associated with a Co­(III) → Co­(IV) redox transition occurs above 1.2 V and is coupled to the onset of catalysis. ,, The catalytic mechanisms leading to the O–O bond formation in CoPi are proposed to involve the pairing of di-μ-oxo-linked Co­(IV) ions with terminally coordinated oxygen ions. ,,− …”

Virtual Issue on Ultrafast Charge Separation in Energy Research

“…Ultrafast charge carrier dynamics has been probed in transition metal-based chalcogenides/CdS heterojunctions, where electrons are transferred from the chalcogenide to CdS and holes move in the other direction . CdS photoanodes have been encapsulated by electron and hole scavengers, leading to simultaneous extraction of electron–hole pairs in CdS and an increased photocurrent density for the composite …”

“…32 CdS photoanodes have been encapsulated by electron and hole scavengers, leading to simultaneous extraction of electron−hole pairs in CdS and an increased photocurrent density for the composite. 33 Scientific reports on the photophysics of nanomaterials include the study by Patra and co-workers, who showed that varying the halogen atoms in CdSe nanoplate−CsPbX 3 perovskite composites led to improved interfacial charge transfer dynamics and solar cell efficiency (Figure 1d). 34 Hot electron transfer from CdTe QDs to tetrakis(4carboxyphenyl)porphyrin (TCPP) molecules (CdTe QD− TCPP NCs) was also examined using state selective excitation experiments by Mandal et al 35 Numerical modeling and fs-TAS measurements were used to study photogenerated charge carrier dynamics in BiVO 4 .…”