Origin of Visible Light Photoactivity of the CeO₂/ZnO Heterojunction

Abstract

The synthesis of a mixed CeO 2 −ZnO oxide results in a photocatalyst active under visible light. The characterization of the new material shows that Ce does not enter as a dopant in ZnO but rather forms isolated CeO 2 nanoparticles supported on the surface of larger particles of the more abundant zinc oxide phase. The as obtained material exhibits a band gap corresponding to UV light (∼3.3 eV), but nevertheless. it shows a relevant photoactivity under irradiation with photons with λ > 420 nm (visible light). The working hypothesis is that visible light irradiation leads to a charge separation and stabilization of a fraction of the carriers connected with the formation of the CeO 2 /ZnO interface. This phenomenon has been investigated by means of several methods. A specific EPR-based approach allowed to monitor and quantify the charge separation following the formation of holes in the valence band (VB) of the two materials. More complex is detecting the nature of the excited electrons, as this involves the formation of EPR invisible Ce 3+ ions by trapping the electrons into localized 4f states of Ce ions at the interface between the two oxides. DFT calculations provide a rational for some of the observed phenomena and a basis for the discussion of the band alignment of the two systems as a consequence of the formation of a heterojunction. The theoretical results show that indeed electrons can be excited at the interface from the VB of the two oxides into the Ce 4f states with photons of 2.3 eV, thus justifying the occurrence of a visible-light activity despite the higher band gap of the two materials.

Charge Carriers Cascade in a Ternary TiO2/TiO2/ZnS Heterojunction: A DFT Study

“…We now proceed to the analysis of the electronic properties. In Figure , the calculated plane‐averaged electrostatic potential is reported against the non‐crystallographic direction c , showing the alignment of the band edges of each component in the heterojunction . When looking at fully independent surfaces, the Valence Band Offset (VBO) can be simply calculated as the difference between the Valence Band Maximum (VBM) of the units.…”

“…Similarly, the Conduction Band Offset (CBO) can be obtained from the Conduction Band Minimum (CBM) of the units. When looking at the explicit heterojunction model VBO and CBO are calculated from those of the independent materials by referring the band edges to the same reference quantity, as for instance the stationary points of the electrostatic potential calculated in the fully independent slabs (E TOP ) and in the heterojunction system (E TOP ,Het ), in a spatial region that does not correspond to the interface …”

“…In the case of ZnS, the atomic all electron basis set for Sulphur is 86‐311G*. Zn atoms are described by the all‐electron basis set 86‐411d31G already used in the past . We started from the experimental cubic zinc blend crystal structure, and we designed slab models of various thickness exposing the (110) surface.…”

“…These materials are also called type‐II heterojunctions. Several examples of these composites were presented recently, also including titania‐based materials . More specifically, titania can be combined with a metal, graphene, or another oxide .…”

“…Several examples of these composites were presented recently, also including titania-based materials. [24][25][26][27] More specifically, titania can be combined with a metal, [28][29][30][31][32][33] graphene, [34,35] or another oxide. [19,20,36,37] Composites can be also created by interfacing two different polymorphs of TiO 2 , [38][39][40][41][42][43] or even two different faces of the same polymorph.…”