Water splitting using ultrafine nickel(II) oxide/titanium dioxide nanosheet heterostructures

Solar-driven photoelectrochemical water splitting is considered a promising method of energy generation for future sustainable development. One major challenge to its realization, however, is the need to overcome the large water-splitting overpotentials that are typically required to produce hydrogen and oxygen. To date, developers have focused on producing low-cost and efficient electrocatalysts for water splitting, such as nickel(II) oxide, which is earth-abundant, highly active, and therefore the most widely used oxygen evolution reaction (OER) catalyst. However, the catalytic performance of nickel(II) oxide nanoparticles remains suboptimal due to their large particle size, which leads to modest specific surface areas and limited exposure of their active/reactive edge and corner sites (which are coordinatively unsaturated and therefore necessary for the catalytic process). Recently, faceted nickel(II) oxide nanoparticles exposing high surface-energy planes have been the focus of intense research due to their significantly enhanced catalytic performance.¹ The surface energy of nickel(II) oxide facets follows the order: {110}≈{101}>{113}>{100}.² Since surface chemical reactivity generally increases with surface energy,³ it would be highly desirable to synthesize ultrafine nickel(II) oxide nanosheets (diameter <10nm, thickness ∼1nm) that expose a high proportion of {110} facets to achieve more efficient OER performance.

Much attention has focused on heterostructured catalysts that have abundant interfaces that facilitate charge separation, since these catalysts can be used in photo/electrocatalysis for solar fuels and chemicals.^{4, 5} We distributed nickel(II) oxide within an inorganic matrix to control nickel(II) oxide morphology and to improve charge separation and electrocatalytic performance. Layered double hydroxides (LDHs) are a family of 2D layered clays that have been widely studied as catalyst supports or precursors due to the tunability of their composition and morphologies.^{6, 7} We postulated that, by calcinating a nanosized ultrathin nickel titanium LDH nanosheet precursor, it should be possible to obtain ultrathin nickel(II) oxide nanosheets that expose reactive nickel(II) oxide {110} facets with controllable size. In this process, titanium dioxide formed during the calcination step has a stabilizing influence on the nanosheet structure.

In our work, we report the first ultrafine nickel(II) oxide nanosheets containing defective-Ni³⁺ active sites. To achieve this, we increased the density of oxygen vacancies (V_o) in the nanosheets by reducing the nanoplatelet thickness from 13 to 1.1nm. We then further decreased the lateral size of the nanosheets from 100 to 4nm by adding titanium dioxide to prevent particle growth (see Figure 1).⁸ We obtained an ultrafine nickel(II) oxide/titanium dioxide nanosheet that exhibited extraordinarily high electrocatalysis for oxygen evolution with low overpotential of 320mV at 10mA cm⁻² (which is superior to most reported nickel oxide-based electrocatalysts). We used transmission electron microscopy and x-ray absorption to image the near-edge structure, and made density functional theory calculations. From the images and calculations, we established that the atomic-thick nickel(II) oxide nanosheets expose a high percentage of reactive {110} facets that contain active sites and abundant interfaces (see Figure 2). These sites promote water adsorption and increase the charge-transfer efficiency, which results in significantly improved water oxidation activity.

Figure 1. Schematic illustration of the synthesis of ultrafine and ultrathin nickel(II) oxide (NiO) nanosheets, stabilized by titanium dioxide (TiO₂), from monolayer nickel-titanium layered double hydroxide (LDH) nanosheet precursors.

Figure 2. (A) Transmission electron microscope (TEM) image of monolayer nickel-titanium LDH nanosheets. (B, C) Images of the NiO/TiO₂ heterostructures taken using TEM and high-resolution TEM, respectively. (D) Fast Fourier transform pattern of the corresponding NiO area. (E) Geometrical model of the hexagonal NiO nanocrystal. (F) Atomic force microscope image of the NiO/TiO₂ heterostructures (marked 1, 2, 3). (G) The corresponding height profiles of the heterostructures.

In summary, our work demonstrates the potential of novel ultrafine and ultrathin oxide nanosheets with controllably exposed high-energy facets and surface-defective active sites for electrocatalytic applications. In future work we will consider using this same synthetic strategy to obtain alternative metallic oxide nanosheet electrocatalysts that have controllable exposed high-energy facets and active sites, such as iron, copper, and manganese oxides.