Time-Resolved Spectroscopy of ZnTe Photocathodes for Solar Fuel Production

Abstract

The negative conduction band potential and small bandgap of ZnTe make the material a promising photoelectrode for solar fuels production, photocatalyst, and solar cell component. However, the factors controlling the underlying efficiencies of the light-driven processes on ZnTe are not well understood. Here we report a combined spectroelectrochemical and transient absorption (TA) spectroscopic investigation of ZnTe photoelectrodes for CO2 reduction. In the visible region TA spectra are dominated by a broad positive photoinduced absorption at 540 nm following initial charge carrier relaxation (<540 nm). The 540 nm spectral feature is shown to be related to deeply trapped photoelectrons with charge carrier recombination occurring via a trapping–detrapping model on the microsecond time scale. Significantly these deeply trapped electrons are insensitive to the presence of electron acceptors and to the applied potential of the ZnTe electrode. Trapping at such states is proposed to be a significant factor limiting the photoelectrochemical activity of ZnTe. Near-IR spectral features associated with shallow trapped/conduction band electrons exist at >1150 nm. Shallow trapped electrons are generated and accumulate at potentials where photoelectrochemical H2 evolution and CO2 reduction occur, and we show these charges are able to undergo interfacial electron transfer to an acceptor molecule. The passivation of sites related to deep traps is proposed to be the key to optimize the photocatalytic and photoelectrochemical performance of ZnTe.

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“…In situ transient absorption (TA) spectroscopy is a useful tool to understand interfacial charge transfer processes for PEC water oxidation and reduction 59 . However, there are few TA studies of photocathodes for PEC CO 2 reduction due to the timescale mismatch between charge transfer and the surface catalysis process 60 . Figure 5b shows the in situ TA spectra of as-prepared photocathodes immersed in CO 2 -saturated 0.5 M KHCO 3 solution at OCP.…”

In situ charge carrier dynamics of semiconductor nanostructures for advanced photoelectrochemical and photocatalytic applications

“…Charge carrier dynamics of semiconductor nanostructures in PEC systems have been studied and reported with the help of in situ TA spectroscopy. Several common semiconductor materials have been used as photoelectrodes when studying in situ TA, including TiO 2 photoanodes [37][38][39][40], BiVO 4 photoanodes [41,42], WO 3 photoanodes [43][44][45], α-Fe 2 O 3 photoanodes [46][47][48][49][50], and ZnTe photocathodes [51]. In this section, we introduce several representative works to highlight the importance of using in situ TA spectroscopy toward the realization of interfacial charge dynamics for photoelectrodes under operating conditions.…”

“…For example, the majority of the studied photoelectrodes is of the photoanode type made of n-type semiconductors. Few studies are available on realtime observations of charge dynamics for p-type semiconductor photocathodes [51]. Compared to the plentiful examples of the studied photoelectrodes, research is relatively scarce on the topic of exploring the in situ charge dynamics of photocatalysts.…”

Spectroelectrochemical Sensing: Current Trends and Challenges

“…SEC joins the best of electrochemistry and spectroscopy together during an electron‐transfer process. As can be inferred, this multiresponse and “teamwork” technique has been used in a large number of fields, including, for example, electron transfer processes , chemical reaction mechanisms , side reactions , electrocatalysis , corrosion , conducting polymers , solar cells , memory devices , supercapacitors , synthesis and characterization of nanoparticles , carbon nanomaterials , characterization of metal complexes , liquid/liquid interfaces , electrochromic materials or study of compounds of biological interest , among others.…”