Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers.
Conduction and valence band energies (ECB, EVB) for CdSe nanorods (NRs) functionalized with Au nanoparticle (NP) tips are reported here, referenced to the vacuum scale. We use (a) UV photoemission spectroscopy (UPS) to measure EVB for NR films, utilizing advanced approaches to secondary electron background correction, satellite removal to enhance spectral contrast, and correction for shifts in local vacuum levels; and (b) waveguide-based spectroelectrochemistry to measure ECB from onset potentials for electron injection into NR films tethered to ITO. For untipped CdSe NRs, both approaches show EVB = 5.9-6.1 eV and ECB = 4.1-4.3 eV. Addition of Au tips alters the NR band edge energies and introduces midgap states, in ways that are predicted to influence the efficiency of these nanomaterials as photoelectrocatalysts. UPS results show that Au tipping shifts EVB closer to vacuum by up to 0.4 eV, shifts the apparent Fermi energy toward the middle of the band gap, and introduces additional states above EVB. Spectroelectrochemical results confirm these trends: Au tipping shifts ECB closer to vacuum, by 0.4-0.6 eV, and introduces midgap states below ECB, which are assigned as metal-semiconductor interface (MSI) states. Characterization of these band edge energies and understanding the origins of MSI states is needed to design energy conversion systems with proper band alignment between the light absorbing NR, the NP catalyst, and solution electron donors and acceptors. The complementary characterization protocols presented here should be applicable to a wide variety of thin films of heterogeneous photoactive nanomaterials, aiding in the identification of the most promising material combinations for photoelectrochemical energy conversion.
Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
Ramanan
R
Ehamparam
Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
Nicholas G
NG
Pavlopoulos
Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
Michael W
MW
Liao
Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
Lawrence J
LJ
Hill
Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
Neal R
NR
Armstrong
Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.|World Class University Program for Chemical Convergence for Energy and Environment, School of Chemical and Biological Engineering, Seoul National University , Seoul 151-744, Korea.
Jeffrey
J
Pyun
Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
S Scott
SS
Saavedra
UV photoemission; band edge energies; electron transfer; metal nanoparticle; semiconductor nanorods; spectroelectrochemistry