Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers

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• 作者：Ramanan Ehamparam ; Nicholas G. Pavlopoulos ; Michael W. Liao ; Lawrence J. Hill ; Neal R. Armstrong ; Jeffrey Pyun ; S. Scott Saavedra
• 刊名：ACS Nano
• 出版年：2015
• 出版时间：September 22, 2015
• 年：2015
• 卷：9
• 期：9
• 页码：8786-8800
• 全文大小：808K
• ISSN：1936-086X

文摘

Conduction and valence band energies (E_CB, E_VB) 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 E_VB 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 E_CB from onset potentials for electron injection into NR films tethered to ITO. For untipped CdSe NRs, both approaches show E_VB = 5.9鈥?.1 eV and E_CB = 4.1鈥?.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 E_VB 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 E_VB. Spectroelectrochemical results confirm these trends: Au tipping shifts E_CB closer to vacuum, by 0.4鈥?.6 eV, and introduces midgap states below E_CB, which are assigned as metal鈥搒emiconductor 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.