Liuxian Zhang

Atomic Level In Situ Observation of Surface Amorphization in Anatase Nanocrystals During Light Irradiation in Water Vapor

An in situ atomic level investigation of the surface structure of anatase nanocrystals has been conducted under conditions relevant to gas phase photocatalytic splitting of water. The experiments were carried out in a modified environmental transmission electron microscope fitted with a high intensity broadband light source with an illumination intensity of 1430 mW/cm(2) close to 10 suns. When the titania is exposed to light and water vapor, the initially crystalline surface converts to an amorphous phase one to two monolayers thick. Spectroscopic analyses show that the amorphous layer contains titanium in a +3 oxidation state. The amorphous layer is stable and does not increase in thickness with time and is heavily hydroxylated. This disorder layer will be present on the anatase surface under reaction conditions relevant to photocatalytic splitting of water.

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.

Advanced and In Situ Analytical Methods for Solar Fuel Materials.

In situ and operando techniques can play important roles in the development of better performing photoelectrodes, photocatalysts, and electrocatalysts by helping to elucidate crucial intermediates and mechanistic steps. The development of high throughput screening methods has also accelerated the evaluation of relevant photoelectrochemical and electrochemical properties for new solar fuel materials. In this chapter, several in situ and high throughput characterization tools are discussed in detail along with their impact on our understanding of solar fuel materials.

Nano-level Structure-Reactivity Relationships of Ni-NiO Core-shell Co-catalysts on Ta2O5 for Solar Hydrogen Production

Tantalum oxide and many tantalate-based systems have been reported to show extraordinarily high activities and quantum yields when decomposing water under ultraviolet (UV) illumination [1]. Although pure tantalum oxide shows some photocatalytic activity, loading with a nickel-based cocatalyst improves the initial H2 production rate by 3 orders of magnitude and results in stoichiometric decomposition of pure water into H2 and O2 [2]. Interestingly, this co-catalyst needs to have a particular microstructure, Ni core-NiO shell, to show high activity. However a detailed atomic-level understanding of the relationship between the catalyst microstructures and the photocatalytic reactivities has not yet been fully explored. Aberration-corrected TEM provides an efficient way to observe fine structures at this level and here we investigate the structure-reactivity relations in different co-catalysts to study both reaction and deactivation mechanisms [3].

Opportunities and Challenges for In-Situ Characterization of Photocatalysts in Environmental TEM

Photocatalytic water splitting has been considered a promising technology for generating sustainable clean energy. Essentially, photocatalytic materials enable the process of converting and storing solar energy in the form of H2 molecules. It is now recognized that atomic level in-situ observations of catalytic materials are critical for understanding structure-reactivity relationships and deactivation processes such as photocorrosion. For photocatalysts, this requires that the system be observed not only in presence of reactant and product species but also during in-situ light illumination. Here opportunities and challenges associated with building a “photo-reactor” inside an environmental TEM (ETEM) are discussed.

Bandgaps and Surface Inter-Band States in Photocatalysts with High Energy Resolution EELS

Photocatalysts have potential applications for solar fuel generation through water splitting [1]. The bandgap and inter-band states of the semiconductors significantly affects the performance and efficiency of the catalysts. Recent advances in STEM EELS monochromation now allow for routine ultra-high energy resolution of 15 meV or better in the low-loss region [2]. The ability to now correlate atomic structure and electronic structure in the low-loss and bandgap region of the energy-loss spectrum represents a powerful tool for characterization of electronic and optical properties of nanomaterial such as the high surface area, particulate systems that are generally used as catalysts. The band structure can vary among different nanoparticles depending on particle sizes, facets and also at the surfaces/interfaces of the semiconductors where charge transfer and photocatalytic reactions take place. With the innovation of high energy resolution EELS, it is possible to tackle the issues mentioned above by investigating the bandgap and the fine electronic structures inside the gap at the nano-level.

Atomic level in-situ characterization of metal/TiO 2 photocatalysts under light irradiation in water vapor

Photocatalysts are important for environmental cleanup of undesirable organic compounds and have potential applications for solar fuel generation either through water splitting or CO2 reduction [1]. It is now recognized that atomic level in-situ observations of catalytic materials are critical for understanding the structure-reactivity in catalysts. For photocatalysts, this requires that the system be observed not only in the presence of reactant and product species but also during in-situ light illumination. NiO loaded semiconductor photocatalyst with Ni first reduced and then partially re-oxidized at the surface has been reported to have good photocatalytic properties by forming a metallic Ni Ohmic contact between NiO and the semiconductors. NiO-Ni-SrTiO3 performs better than NiO/SrTiO3 while Ni/SrTiO3 does not have high photocatalytic activity [2]. TiO2 is a promising photocatalyst which has attracted intense research interest for decades since photo-decomposition of water by TiO2 was discovered [3]. The TiO2 photocatalysts are either anatase or rutile which has been well known. Herein we use TiO2 as a model material to develop in situ photocatalytic experimental methodology and explore structure changes of NiO/semiconductor photocatalysts. In-stiu heat treatment in H2 or O2 is applied to prepare initially Ni/TiO2, NiO/TiO2 or NiO-Ni-TiO2 materials in an environmental transmission electron microscope (ETEM). Then, without exposure to air, analysis can be performed in the same modified ETEM under in situ conditions in the presence of light and reactants to explore oxidation/reduction or interface changes under photocatalytic reactions. Insights from these experiments can help in the design of photocatalysts with better performance and stability.