Qianlang Liu

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.

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.

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.

Detection and Characterization of OH Vibrational Modes using High Energy Resolution EELS

The recent detection of vibrational excitations in monochromated electron energy-loss spectroscopy recorded from scanning transmission electron microscopes has opened up new opportunities for nanoscale materials characterization [1]. The enhanced energy resolution has the greatest impact on the low-loss EELS and it is now possible to probe vibrational and electronic excitations at the nanometer level. For example, localized bandgap mapping and detection of interband states is now possible providing a new tool to correlate optical properties with atomic structure [2,3]. Vibrational spectroscopy allows hydrogen containing species to be identified and correlated with materials structure. Detection of water and OH species on nanoparticle surfaces is important for developing a fundamental understanding of solar water splitting catalysts. The delocalized nature of the low-loss spectrum also makes it possible to use the aloof beam spectral acquisition mode (i.e. with the electron probe positioned outside the sample) dramatically reducing electron beam damage. To investigate the feasibility of OH detection, a series of hydroxide and hydrates have been investigated.

Design and Application of an In Situ Illumination System for an Aberration-corrected Environmental Transmission Electron Microscope

Photocatalytic water splitting has been considered a promising technology for generation of clean, sustainable, and carbon-neutral fuels. Essentially, the photocatalytic materials enable the process of converting and storing the inexhaustible solar energy in the form of H2 molecules. It is now recognized that atomic level in situ observations are critical for understanding fundamental functionalities of catalytic materials. The active catalyst structures under reaction conditions are not necessarily the same as the initial structures. The detailed structure-reactivity relationships and deactivation behaviors of the catalysts can be developed by following the structural evolutions in situ. For photocatalysts, this requires that the system be observed in the presence of light, gas and thermal stimuli. There are several ways to introduce light illumination capability while observing the materials in an environmental transmission electron microscope (ETEM). Specimen holders that allow light illumination have been developed [1]. However, these designs do not allow heating/cooling of the catalysts. In this study, an optical fiber based in situ illumination system was designed and built for an aberration-corrected ETEM, FEI Titan. The ETEM was equipped with a monochromator and an aberration corrector providing sub-Angstrom image resolution. TEM hot stages can still be employed for in situ thermal processing of catalysts. This is critical for many fundamental studies on catalytic materials because thermal oxidizing or reducing treatments are often essential to create well-defined initial reference states of the materials. In this work, design considerations and applications of this illumination system will be discussed.

Understanding guided light modes in oxide nanoparticles with monochromated EELS

The ultra-high energy resolution and significantly suppressed zero loss peak tail from monochromated electron energy-loss spectroscopy (EELS) offer opportunities to extract information on semiconductors and insulators from features in the bandgap regions of the spectrum. Intensity variations observed in the bandgap region can be caused by: (1) bandgap states induced by defects or surface adsorbates, and/or (2) phenomena such as Cherenkov radiation and guided light or cavity modes [1]. While bandgap states are of great interest in the field of catalysis and solid state physics, it is also important to understand the origin and behavior of cavity modes in oxide nanoparticles, to differentiate the two thus better interpret the spectra. TiO2 nanoparticles act as an ultraviolet (UV) light photocatalyst for water remediation or solar water splitting, and the surface electronic structure controls the thermodynamics and kinetics of the catalytic reactions. By positioning the focused electron beam in a scanning transmission electron microscope (STEM) a few nanometers outside the particle surface (aloof beam geometry), the electron beam damage can be reduced while obtaining an EELS signal from the surfaces. Interestingly, cavity modes show up in these aloof spectra as intensity maxima in the bandgap region of TiO2 anatase. To further investigate the size and refractive index dependence of the cavity modes in oxide nanoparticles, spectra were also recorded from CeO2 and MgO nanocubes with simpler more well-defined morphologies. Classical electrodynamic modeling was applied to interpret the bandgap features.

Investigating the Spatial Resolution of Vibrational Electron Energy-Loss Spectroscopy

Recent work on monochromated electron energy-loss spectroscopy (EELS) has pushed the energy resolution achievable in a scanning transmission electron microscope (STEM) to around 10 meV [1]. This has enabled the detection of vibrational excitations with high spatial resolution. This high spatial and energy resolution can be leveraged to investigate local surface chemistry and analyze chemical bonding arrangements, which can in turn be used to characterize composition, structure, and chemical reactions. A fundamental understanding of this technique will have a tremendous impact on areas like catalysis. To develop this technique comprehensively, we need to perform experiments on relatively simple model systems and corroborate our experimental results with theoretical models. Here we explore the spatial variation in the SiO2 optical phonon when the electron beam is scanned across a SiO2/Si interface. The experimental profiles are interpreted in terms of models based on dielectric theory.

Surface Dynamics Associated with Redox Processes on TiO2 Nanoparticles

TiO2 anatase nanoparticles have shown interesting properties as ultraviolet (UV) light photocatalysts for water and air remediation as well as solar water splitting. The surface of the nanoparticle plays a vital role in controlling the catalytic reactivity. For example, reactant adsorption, breaking/forming of chemical bonds, and product desorption all take place on the surface. In situ observations of such catalysts at the atomic level is required to follow structural transformations under near-reaction conditions thus elucidating the reaction and deactivation mechanisms. An optical fiber based in situ illumination system has been built and installed on an aberration-corrected environmental transmission electron microscope (TEM), allowing the sample to be illuminated with a broadband light source (200-800 nm in wavelength) while exposing to heat and gases such as water vapor. Negative spherical aberration imaging (NCSI) was used to enhance the contrast from oxygen and titanium columns to allow observation of structural dynamics induced by the light and water [1].

Probing Interfacial and Surface Effects with Vibrational Electron Energy Loss Spectroscopy

The novel ability to detect vibrational excitations with monochromated electron energy-loss spectroscopy (EELS) has great potential in high spatial resolution characterization of materials [1]. Such instrumentation capabilities can be leveraged to investigate delocalized behavior of the bulk as well as localized behavior of surfaces and interfaces. For a comprehensive understanding of this technique, experiments need to be performed on simple model systems and the results need to be compared with theory. In this paper, we explore the experimental spatial variation in the vibrational stretch signals from the bulk, surfaces and interfaces when an electron beam is scanned across a SiO2/Si interface. The resultant profiles are interpreted in terms of non-relativistic and relativistic dielectric theories [2,3].

Exploring Vibrational and Valence Loss Spectra from Oxide Nanoparticles

Monochromated electron energy-loss spectroscopy (EELS) now offers energy resolutions of ~ 10 meV allowing unprecedented probing of the visible and infrared regions of the spectrum [1]. Features associated with electronic excitations such as plasmons, bandgap measurement, bandgap states and surface states can potentially be probed with spatial resolutions on the order of a few nanometers [2]. Vibrational fingerprints associated with surface layers and molecular adsorbates, as well as phonon modes can now be observed [1,3,4]. However, there are many fundamental questions regarding the nature and origin of spectral features observed below the bandgap, which must be understood in order to extract useful information on materials’ chemical, optical, and electronic properties. For example, spatial resolution, sensitivity, guided light modes and relativistic effects must be carefully considered in the interpretation of the spectra. Comparison with optical spectroscopies such as infrared/visible/UV absorption and Raman spectroscopy is also helpful.