Strong Potential Gradients and Electron Confinement in ZnO Nanoparticle Films: Implications for Charge-Carrier Transport and Photocatalysis

Abstract

Zinc oxide (ZnO) nanomaterials are promising components for chemical and biological sensors, and operate as electron collectors in photovoltaic technologies. Many of these applications involve nanostructures in contact with liquids or exposed to ambient atmosphere. Under these conditions, single crystal ZnO surfaces are known to form narrow electron accumulation layers with few-nm spatial penetration into the bulk. A key question is to what extent such pronounced surface potential gradients can develop in the nanophases of ZnO, where they are expected to dominate the catalytic activity by modulating charge carrier mobility and lifetimes. Here, we follow the temperature-dependent surface electronic structure of nanoporous ZnO with photoemission spectroscopy to reveal a sizable, spatially averaged downward band bending for the hydroxylated state, and a conservative upper bound of <6 nm for the spatial extent of the associated potential gradient, which implies strong confinement of the conduction-band electrons to the surface of the nanostructure.

Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces

“…1# based on a planar sample surface model, while an isolated single sphere model would yield an even smaller value. 32 The actual surface topography of the ZnO films likely lies in between these two limiting cases.…”

“…that the decomposition of XPS signal contributions from different depths can be used to reconstruct interfacial potential energy surfaces. [32][33][34][35] As described in the following, we translate this approach into the time domain to monitor the temporal evolution of the near-surface potentials in the ZnO substrate as a result of the photoexcitation of the interface (Fig. 3(b)).…”

“…We note that the significant band bending determined here and in ref. 32 is not necessarily expected for nanostructured semiconductors, 38 and makes surface properties (rather than those of the bulk) highly relevant for electron transport and photocatalytic activity of nanoporous ZnO. 29 The pronounced band bending 𝑉 !!…”

“…However, as outlined in ref. 32, the pronounced band bending in the ground state ZnO substrate is the result of electron transfer doping from surface hydroxyl groups, formed upon contact of background water molecules with the ZnO surface. The study also strongly suggests that under the conditions of the trXPS experiment, the surface is fully hydroxylated.…”

“…Generally, XPS spectra of condensed phase materials contain contributions that emerge, most intensely, from the surface and, with decreasing intensity, from various depths within the material (Figure (a)). Previous steady-state studies have demonstrated that the decomposition of XPS signal contributions from different depths can be used to reconstruct interfacial potential energy surfaces. − As described in the following, we translate this approach into the time domain to monitor the temporal evolution of the near-surface potentials in the ZnO substrate as a result of the photoexcitation of the interface (Figure (b)). The technique is based on modeling the space-charge-induced fields and corresponding electronic structure of the hybrid junction in both the ground and excited states.…”