Probing the Redox States at the Surface of Electroactive Nanoporous NiO Thin Films Andrea G. Marrani,* ,† Vittoria Novelli, † Stephen Sheehan, ‡ Denis P. Dowling, ‡ and Danilo Dini † † Department of Chemistry, “Sapienza” University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy ‡ School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland * S Supporting Information ABSTRACT: Nanoporous NiO thin film electrodes were obtained via plasma-assisted microwave sintering and charac- terized by means of a combination of electrochemical techniques and X-ray photoelectron spectroscopy (XPS). The aim of this study is the elucidation of the nature of the surface changes introduced by the redox processes of this nanostructured material. NiO undergoes two distinct electro- chemical processes of oxidation in aqueous electrolyte with the progress of NiO anodic polarization. These findings are consistent with the sequential formation of oxyhydroxide species at the surface, the chemical nature of which was assessed by XPS. Electronic relaxation effects in the Ni 2p spectra clearly indicated that the superficial oxyhydroxide species resulted to be β-NiOOH and γ-NiOOH. We also show for the first time spectral evidence of an electrochemically generated Ni(IV) species. This study has direct relevance for those applications in which NiO electrodes are utilized in aqueous electrolyte, namely catalytic water splitting or electrochromism, and may constitute a starting point for the comprehension of electronic phenomena at the NiO/organic electrolyte interface of cathodic dye-sensitized solar cells (p-DSCs). KEYWORDS: nickel oxide, X-ray photoelectron spectroscopy, nanoporous electrode, cyclic voltammetry, dye-sensitized solar cell ■ INTRODUCTION Nickel oxide (NiO) presents p-type semiconducting features with the presence of a wide band gap (E g > 3.50 eV) which can be modulated upon chemical and electrochemical doping. 1 Because of that, NiO is transparent in the visible spectrum when used in the configuration of a thin film with a thickness l <3 μm. 2 The high chemical stability associated with its electrical/optical properties renders NiO a particularly intriguing material for those applications and technologies that require controllable changes of electrical conductivity and optoelectronic properties. In particular, NiO has been considered for energy storage applications, 3 electrochromic windows, 4,5 optoelectronic devices, 6 cathodic dye-sensitized solar cells (p-DSCs), 7-10 and, in more recent times, photo- electrochemical water splitting. 11,12 The electronic structure of NiO has been long debated in the past 13 with no definitive understanding of its actual complexity. The most recent experimental results from electron spectros- copies provided a deeper insight into NiO electronic structure, taking advantage of the giant improvement of computational capabilities for the analysis of large sets of data. For a long time, NiO was considered a prototypical Mott-Hubbard insulator with an insulating gap caused by on-site Coulomb repulsion between 3d electrons, this energy gap being represented by the Hubbard U. In 1985, Zaanen, Sawatzky, and Allen proposed a new definition of NiO, in terms of electrical properties, as an intermediate compound between Mott-Hubbard insulators and charge-transfer (CT) semiconductors. 14 This is because they discovered that other states with a predominant ligand character fall inside the d-d correlation gap, thus leading to the formation of a much smaller CT gap (4.3 eV) between these states and the upper Hubbard d band. 15,16 NiO ground state can be described by the interaction among the 3d 8 , 3d 9 L, and 3d 10 L 2 configurations ( L represents a hole in a ligand orbital), 15,17-20 where considerable electron transfer from the ligands to the central Ni atom occurs, leading to a Ni ground state charge of less than 2 in stoichiometric NiO. 21 The significant overlap between Ni 3d and O 2p orbitals in NiO gives rise to complex XP spectra, especially in the region of Ni 2p photoionization. The photoemission-induced formation of a core-hole in the Ni atom, d 8 → cd 8 (where c denotes a hole in the 2p level), is accompanied by a screening mechanism where electrons are transferred to the Ni site from the neighboring ligands. Therefore, in the final core-hole ionized state the lowest energy is of cd 9 L character, followed by the doubly screened cd 10 L 2 and unscreened cd 8 states. 22,23 A matter of long debate has been the interpretation of a shoulder signal in the Ni 2p 3/2 XP spectrum located at ∼1.5 eV at the high binding Received: August 29, 2013 Accepted: December 10, 2013 Research Article www.acsami.org © XXXX American Chemical Society A dx.doi.org/10.1021/am403671h | ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX