An Open-Cell Environmental Transmission Electron Microscopy Technique for In Situ Characterization of Samples in Aqueous Liquid Solutions.

The desire to image specimens in liquids has led to the development of open-cell and closed-cell techniques in transmission electron microscopy (TEM). The closed-cell approach is currently more common in TEM and has yielded new insights into a number of biological and materials processes in liquid environments. The open-cell approach, which requires an environmental TEM (ETEM), is technically challenging but may be advantageous in certain circumstances due to fewer restrictions on specimen and detector geometry. Here, we demonstrate a novel approach to open-cell liquid TEM, in which we use salt particles to facilitate the in situ formation of droplets of aqueous solution that envelope specimen particles coloaded with the salt. This is achieved by controlling sample temperature between 1 and 10°C and introducing water vapor to the ETEM chamber above the critical pressure for the formation of liquid water on the salt particles. Our use of in situ hydration enables specimens to be loaded into a microscope in a dry state using standard 3 mm TEM grids, allowing specimens to be prepared using trivial sample preparation techniques. Our future aim will be to combine this technique with an in situ light source to study photocorrosion in aqueous environments.

Al2O3 and SiO2 Atomic Layer Deposition Layers on ZnO Photoanodes and Degradation Mechanisms.

Strategies for protecting unstable semiconductors include the utilization of surface layers composed of thin films deposited using atomic layer deposition (ALD). The protective layer is expected to (1) be stable against reaction with photogenerated holes, (2) prevent direct contact of the unstable semiconductor with the electrolyte, and (3) prevent the migration of ions through the semiconductor/electrolyte interface, while still allowing photogenerated carriers to transport to the interface and participate in the desired redox reactions. Zinc oxide (ZnO) is an attractive photocatalyst material due to its high absorption coefficient and high carrier mobilities. However, ZnO is chemically unstable and undergoes photocorrosion, which limits its use in applications such as in photoelectrochemical cells for water splitting or photocatalytic water purification. This article describes an investigation of the band alignment, electrochemical properties, and interfacial structure of ZnO coated with Al2O3 and SiO2 ALD layers. The interface electronic properties were determined using in situ X-ray and UV photoemission spectroscopy, and the photochemical response and stability under voltage bias were determined using linear sweep voltammetry and chronoamperometry. The resulting surface structure and degradation processes were identified using atomic force, scanning electron, and transmission electron microscopy. The suite of characterization tools enable the failure mechanisms to be more clearly discerned. The results show that the rapid photocorrosion of ZnO thin films is only slightly slowed by use of an Al2O3 ALD coating. A 4 nm SiO2 layer proved to be more effective, but its protection capability could be affected by the diffusion of ions from the electrolyte.

Nanoscale probing of bandgap states on oxide particles using electron energy-loss spectroscopy.

Surface and near-surface electronic states were probed with nanometer spatial resolution in MgO and TiO2 anatase nanoparticles using ultra-high energy resolution electron energy-loss spectroscopy (EELS) coupled to a scanning transmission electron microscope (STEM). This combination allows the surface electronic structure determined with spectroscopy to be correlated with nanoparticle size, morphology, facet etc. By acquiring the spectra in aloof beam mode, radiation damage to the surface can be significantly reduced while maintaining the nanometer spatial resolution. MgO and TiO2 showed very different bandgap features associated with the surface/sub-surface layer of the nanoparticles. Spectral simulations based on dielectric theory and density of states models showed that a plateau feature found in the pre-bandgap region in the spectra from (100) surfaces of 60nm MgO nanocubes is consistent with a thin hydroxide surface layer. The spectroscopy shows that this hydroxide species gives rise to a broad filled surface state at 1.1eV above the MgO valence band. At the surfaces of TiO2 nanoparticles, pronounced peaks were observed in the bandgap region, which could not be well fitted to defect states. In this case, the high refractive index and large particle size may make Cherenkov or guided light modes the likely causes of the peaks.

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

The internal electric field in LiNbO3 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 LiNbO3 in an aqueous AgNO3 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 LiNbO3 (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 AgNO3 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 LiNbO3 to ZnO surface.

Detection of water and its derivatives on individual nanoparticles using vibrational electron energy-loss spectroscopy.

Understanding the role of water, hydrate and hydroxyl species on nanoparticle surfaces and interfaces is very important in both physical and life sciences. Detecting the presence of oxygen-hydrogen species with nanometer resolution is extremely challenging at present. Here we show that the recently developed vibrational electron energy-loss spectroscopy using subnanometer focused electron beams can be employed to spectroscopically identify the local presence and variation of OH species on nanoscale surfaces. The hydrogen-oxygen fingerprint can be correlated with highly localized structural and morphological information obtained from electron imaging. Moreover, the current approach exploits the aloof beam mode of spectral acquisition which does not require direct electron irradiation of the sample thus greatly reducing beam damage to the OH bond. These findings open the door for using electron microscopy to probe local hydroxyl and hydrate species on nanoscale organic and inorganic structures.