Charge Carrier Dynamics in Metal-Oxide/Nanocarbon Hybrid Photoelectrodes

Abstract

One of the most critical challenges of the 21st century is the shift in energy use from fossil fuels to renewable sources. Photoelectrochemical solar energy conversion, as a potentially attractive candidate, has gained an increased momentum during the recent years. Although both water-splitting (hydrogen evolution) and carbon dioxide reduction are promising avenues, they both have substantial shortcomings to be addressed. A common virtue of these two processes is that a good photoelectrode has to concurrently fulfill many attributes to drive these reactions efficiently. One of the most important is the efficient charge carrier transport, which ensures that most of the photogenerated charge carriers can be extracted both toward the electrode/electrolyte junction and toward the back contact. Numerous promising photoelectrode material showed poor performance due to extensive charge carrier recombination both in the bulk and at the surface. A well-known example is α-Fe2O3 (hematite), in which the hole-diffusion length is only 2-4 nm.1 One possible way to improve charge carrier transport properties is to deposit the metal-oxide on an interconnected nanocarbon network (e.g., carbon nanotubes or graphene). These highly conductive nanoscaffolds can improve the transport of the photogenerated charge carriers, which in turn, results in an enhanced photoelectrochemical performance.2 In my presentation, I am going to discuss how charge carrier dynamics of various hematite and hematite/nanocarbon composites is affected by the composition of the nanohybrids and by the applied bias. Hematite/nanocarbon photoelectrodes were synthesized by a two-step method: first the nanocarbon layer was spray-coated on the surface of the substrate electrode, and the nanostructured metal-oxide component was electrodeposited in a subsequent step. The metal-oxide/nanocarbon ratio was precisely controlled by the deposition charge density. To investigate charge carrier dynamics on a timescale ranging from seconds to femtoseconds three different techniques were employed: (i) transient absorption spectroscopy, employing a fs pump-probe setup, (ii) intensity-modulated photocurrent spectroscopy, and finally (iii) fast chronoamperometry. By covering this broad timescale, it was possible to study charge carrier transport, trapping and recombination processes, which were greatly affected by both the applied potential and the presence of the nanocarbons. 1 Energy Environ. Sci., 2016, 9, 2744—2775. 2 J. Am. Chem. Soc. 2017, 139, 6682−6692.