Photochemical Reaction Patterns on Heterostructures of ZnO on Periodically Poled Lithium Niobate

Abstract

The internal electric field in LiNbO provides a driving force for heterogeneous photocatalytic reactions, where photoexcited holes or electrons can participate in redox reactions on positive (+c) and negative (-c) domain surfaces and at the domain boundaries. One method to characterize the surface chemical reactivity is to measure photoinduced Ag deposition by immersing the LiNbO in an aqueous AgNO solution and illuminating with above bandgap light. Reduction of Ag ions leads to the formation of Ag nanoparticles at the surface, and a high density of Ag nanoparticles indicates enhanced surface photochemical reactions. In this study, an n-type semiconducting ZnO layer is deposited on periodically poled LiNbO (PPLN) to modulate the surface electronic properties and impact the surface redox reactions. After plasma enhanced atomic layer deposition (PEALD) of 1, 2, 4, and 10 nm ZnO thin films on PPLN substrates, the substrates were immersed in aqueous AgNO and illuminated with above band gap UV light. The Ag nanoparticle density increased for 1 and 2 nm ZnO/PPLN heterostructures, indicating an enhanced electron density at the ZnO/PPLN surface. However, increasing the ZnO thickness beyond 2 nm resulted in a decrease in the Ag nanoparticle density. The increase in nanoparticle density is related to the photoexcited charge density at the ZnO/PPLN interface and the presence of a weakly adsorbed Stern layer at the ZnO surface. The decrease in the nanoparticle density for thicker ZnO is attributed to photoexcited electron screening in the ZnO layer that suppresses electron flow from the LiNbO to ZnO surface.

Catalytic Applications of Non‐Centrosymmetric Oxide Nanomaterials

“…After 240 min, 97 % degradation efficiency was achieved and this was attributed to the SPR effect arising from anchored Ag nanoparticles (Figure 7c). In addition to the nanostructure engineering of LiNbO 3 , the pursuit of higher activity has also led to the fabrication of LiNbO 3 heterostructures [44, 118] . The main idea was to exploit the microscopic electric field across the domain's crystal polarity in order to keep the photogenerated charge carriers separated sufficiently long to migrate to the surface.…”

“…Among the niobate perovskites, LiNbO 3 has been shown to exhibit the presence of bulk photovoltaic effect, a mechanism for the separation of photoinduced charge carriers in photochemical processes. [41,42,44,45,116] It has an optical energy bandgap of 4.0 eV, implying its photoabsorption in the UV region. While this has often constrained its visible-light photocatalytic activity, doping (defect chemistry) has typically been adopted to extend its photoresponse.…”

“…[44,118] The main idea was to exploit the microscopic electric field across the domain's crystal polarity in order to keep the photogenerated charge carriers separated sufficiently long to migrate to the surface. Kaur et al [44] reported a heterostructure of a plasma-enhanced atomic-layer-deposited ZnO semiconductor with periodically poled LiNbO 3 , which was used to drive a photochemical reaction by reducing Ag + ions to create patterns of Ag nanoparticles on the surface of the heterostructure. It was found that on application of a positive bias, there was a spatial nucleation of Ag nanoparticles on the positive domain of the poled perovskite.…”

“…Among the niobate perovskites, LiNbO 3 has been shown to exhibit the presence of bulk photovoltaic effect, a mechanism for the separation of photoinduced charge carriers in photochemical processes [41, 42, 44, 45, 116] . It has an optical energy bandgap of 4.0 eV, implying its photoabsorption in the UV region.…”

“…d) Size of Ag nanoparticles across the domains on ZnO/LiNbO 3 heterostructure. Reproduced with permission from ref [44]…”