Benjamin K. Miller

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.

System for in situ UV-visible illumination of environmental transmission electron microscopy samples.

A system for illuminating a sample in situ with visible and ultraviolet light inside a transmission electron microscope was devised to study photocatalysts. There are many mechanical and optical factors that must be considered when designing and building such a system. Some of the restrictions posed by the electron microscope column are significant, and care must be taken not to degrade the microscopes electron-optical performance or to unduly restrict the other capabilities of the microscope. We discuss the nature of the design considerations, as well as the practical implementation and characterization of a solution. The system that has been added to an environmental transmission electron microscope includes a high brightness broadband light source with optical filters, a fiber to guide the light to the sample, and a mechanism for precisely aligning the fiber tip.

Analysis of catalytic gas products using electron energy-loss spectroscopy and residual gas analysis for operando transmission electron microscopy

Operando transmission electron microscopy (TEM) of catalytic reactions requires that the gas composition inside the TEM be known during the in situ reaction. Two techniques for measuring gas composition inside the environmental TEM are described and compared here. First, electron energy-loss spectroscopy, both in the low-loss and core-loss regions of the spectrum was utilized. The data were quantified using a linear combination of reference spectra from individual gasses to fit a mixture spectrum. Mass spectrometry using a residual gas analyzer was also used to quantify the gas inside the environmental cell. Both electron energy-loss spectroscopy and residual gas analysis were applied simultaneously to a known 50/50 mixture of CO and CO2, so the results from the two techniques could be compared and evaluated. An operando TEM experiment was performed using a Ru catalyst supported on silica spheres and loaded into the TEM on a specially developed porous pellet TEM sample. Both techniques were used to monitor the conversion of CO to CO2 over the catalyst, while simultaneous atomic resolution imaging of the catalyst was performed.

Novel sample preparation for operando TEM of catalysts

A new TEM sample preparation method is developed to facilitate operando TEM of gas phase catalysis. A porous Pyrex-fiber pellet TEM sample was produced, allowing a comparatively large amount of catalyst to be loaded into a standard Gatan furnace-type tantalum heating holder. The increased amount of catalyst present inside the environmental TEM allows quantitative determination of the gas phase products of a catalytic reaction performed in-situ at elevated temperatures. The product gas concentration was monitored using both electron energy loss spectroscopy (EELS) and residual gas analysis (RGA). Imaging of catalyst particles dispersed over the pellet at atomic resolution is challenging, due to charging of the insulating glass fibers. To overcome this limitation, a metal grid is placed into the holder in addition to the pellet, allowing catalyst particles dispersed over the grid to be imaged, while particles in the pellet, which are assumed to experience identical conditions, contribute to the overall catalytic conversion inside the environmental TEM cell. The gas within the cell is determined to be well-mixed, making this assumption reasonable.

Spectroscopy of Solids, Gases, and Liquids in the ETEM

The use of various nanospectroscopies for determining the nanoscale composition and electronic structure of materials in the environmental transmission electron microscope (ETEM) is described and illustrated. ETEM is a powerful approach for determining the atomic level changes taking place in materials as a result of interactions with ambient gas or liquid environments. A variety of different composition changes may occur with the details depending on the sample composition, gas composition, temperature, and other stimuli. Changes in the elemental and bonding information associated with gas–solid interactions can be detected and quantified using electron energy-loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX). When these spectroscopies are used in combination with the sub-nanometer electron probes associated with scanning transmission electron microscopy (STEM), they provide a powerful tool for determining composition and bonding information at the atomic level. It may also be important to monitor the surrounding gas or liquid composition during in situ experiments. The use of EELS and mass spectrometry, usually a residual gas analyzer (RGA), for gas measurement during ETEM experiments is described and compared. Examples of the application of nanospectroscopies to in situ analysis of compositional changes in a number of metal and oxide systems are given. Combining gas analysis with structural determination opens the door to an operando approach to TEM. Several examples of operando TEM applied to catalytic CO oxidation reactions are given. The impact of current and future instrument development on the analytical capability of ETEM is briefly described.

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.

Operando TEM of Ru\RuO 2 catalyst performing CO oxidation

CO oxidation is a model reaction that is ideally suited for performing operando TEM. Several catalysts for oxidizing CO to CO2 have been extensively studied, including ruthenium and its oxide. Despite these efforts, there has still been considerable debate regarding the most active structure for this system [1,2,3]. In fact, many of the fundamental studies performed on this ruthenium catalyst were surface science studies focused on individual crystal facets. An industrial catalyst will be composed of supported nanoparticles, and it is important to study the structure of these particles at the atomic scale, in a gas environment identical to that used in real-world applications, like hydrogen fuel cells. Environmental TEM (ETEM) can be used to study nanoparticles in a gaseous environment. Operando TEM goes beyond standard in-situ techniques, because the activity of the catalyst is monitored concurrently with the structure [4]. This allows unambiguous determination of active structures, since the activity of the exact structure observed under some experimental condition is known.

Operando Electron Microscopy of Catalysts

Linking catalyst structure with activity is a primary goal of much catalysis research; despite extensive study, these structure-activity relationships are often poorly understood. Observation of a catalyst at the atomic scale using environmental transmission electron microscopy (ETEM) during a catalytic reaction is a powerful technique for correlating structures with activity; to do this well, it is essential to measure the activity of the catalyst while it is being observed. This is known as operando TEM [1]. The activity of the catalyst can be determined by measuring the gas composition using electron energy loss spectroscopy (EELS), and/or mass spectrometry [2].

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.

Resolving Surface Structures of Catalytic Ru Nanoparticles during Catalysis

In situ studies of catalysts in the environmental transmission electron microscope (ETEM) are of great value to elucidate the structure of a catalyst under reaction conditions. This is important, since the structure of many catalysts change at elevated temperature in the presence of reactive gases. One way to more directly relate structure to activity is to measure the activity of the catalyst while it is being observed in the TEM. This is called operando TEM. The relative catalytic activity can be measured by monitoring the amount of product gas produced by the catalytic reaction using electron energy-loss spectroscopy (EELS) and mass spectrometry [1]. This measurement is made possible through the use of a unique sample preparation, shown schematically in Figure 1.