Wenhui Zhu

In Situ Atomic-Scale Probing of the Reduction Dynamics of Two-Dimensional Fe2O3 Nanostructures

Atomic-scale structural dynamics and phase transformation pathways were probed, in situ, during the hydrogen-induced reduction of Fe2O3 nanostructure bicrystals using an environmental transmission electron microscope. Reduction commenced with the α-Fe2O3 → γ-Fe2O3 phase transformation of one part of the bicrystal, resulting in the formation of a two-phase structure of α-Fe2O3 and γ-Fe2O3. The progression of the phase transformation into the other half of the bicrystalline Fe2O3 across the bicrystalline boundary led to the formation of a single-crystal phase of γ-Fe2O3 with concomitant oxygen-vacancy ordering on every third {422} plane, followed by transformation into Fe3O4. Further reduction resulted in the coexistence of Fe3O4, FeO, and Fe via the transformation pathway Fe3O4 → FeO → Fe. The series of phase transformations was accompanied by the formation of a Swiss-cheese-like structure, induced by the significant volume shrinkage occurring upon reduction. These results elucidated the atomistic mechanism of the reduction of Fe oxides and demonstrated formation of hybrid structures of Fe oxides via tuning the phase transformation pathway.

Reduction of CuO nanowires confined by a nano test tube

Using in situ transmission electron microscopy observations of the thermally induced reduction of CuO nanowires sheathed by a carbon shell, we show that a confined nanoscale geometry leads to changes in the oxide reduction mechanism from a surface dominated process to the bulk dominated process. It is shown that the reduction of carbon-confined CuO nanowires occurs via oxygen vacancy clustering in the bulk that results in the nanowire fragmentation into Cu2O segments encapsulated by the carbon shell while the reduction of un-confined CuO nanowires proceeds via the nucleation and growth of Cu2O islands on the nanowire surface. The comparative in situ TEM observations demonstrate that the surface coating layer reduces the thermal stability of the oxide nanowires, which is in contrast to the commonly anticipated effect of enhancing the nanostructure stability by developing a surface protective coating layer. Our density functional theory analyses reveal that the effects of oxygen vacancy ordering at the surface and in the bulk of CuO are comparable in energy, which support the alternative reduction process observed in the bulk of the sheathed CuO nanowires.

Defect-induced enhanced photocatalytic activities of reduced α-Fe2O3 nanoblades

Bicrystalline α-Fe2O3 nanoblades (NBs) synthesized by thermal oxidation of iron foils were reduced in vacuum, to study the effect of reduction treatment on microstructural changes and photocatalytic properties. After the vacuum reduction, most bicrystalline α-Fe2O3 NBs transform into single-layered NBs, which contain more defects such as oxygen vacancies, perfect dislocations and dense pores. By comparing the photodegradation capability of non-reduced and reduced α-Fe2O3 NBs over model dye rhodamine B (RhB) in the presence of hydrogen peroxide, we find that vacuum-reduction induced microstructural defects can significantly enhance the photocatalytic efficiency. Even after 10 cycles, the reduced α-Fe2O3 NBs still show a very high photocatalytic activity. Our results demonstrate that defect engineering is a powerful tool to enhance the photocatalytic performance of nanomaterials.

The Growth of Catalyst-free NiO Nanowires

NiO is a stable p-type semiconductor with wide band gap (3.74 eV). Nanostructured NiO has drawn much attention as a low-cost material for several applications including electrochromic devices, electrode materials in battery systems, and electrochemical supercapacitors. It is also one of the most promising materials for resistive-switching memory devices. There have been reports of different methods to prepare NiO nanocrystals, including evaporation, sputtering, sol-gel techniques and electrochemical deposition using anodic alumina membranes (AAM). To the best of our knowledge, there have been no reports on thermal oxidation-driven NiO nanowire growth. Thermal oxidation is a proven, low-cost, easy-to-control approach for growing oxide nanowires such as CuO [1], α-Fe2O3 [2, 3] and ZnO [4]. Here we present the in situ study of NiO nanowire growth in an environmental transmission electron microscope (ETEM) and elucidate the atomic structure, morphology, and growth mechanism of NiO nanowires from the oxidation of Ni.

In situ Atomic-Scale Visualization of CuO Nanowire Growth

CuO has received much interest owing to its myriad technologically important applications in solar energy conversion, photocatalysts, lithium ion batteries, and gas sensors. Nanostructured CuO is expected to possess improved or unique properties compared to its bulk form and therefore much effort, the majority using chemical synthesis techniques, has been devoted to the production of CuO nanostructures. Among them, thermal oxidation has recently been employed to generate CuO nanostructures due to its technical simplicity and the ease of applying the method to different metals [13]. However, the mechanism of thermal oxidation-driven oxide nanowire formation has widely been debated and is poorly understood [4]. Here we report dynamic, in situ TEM observations of the growth of CuO nanowires during the oxidation of Cu, which provide key insight into the atomic processes for the growth of CuO nanowires.

Formation of Swiss-cheese Nanostructure of α-Fe2O3 by Reduction

Swiss-cheese nanostructures have unique properties and a wide range of applications due to their large surface-to-volume ratios compared to traditional nanostructures, and have attracted intense research focus [1]. By understanding the formation mechanism of the Swiss-cheese structure, researchers can develop biological strategies to tune the properties of single-crystals extrinsically. However, there are technical difficulties in preparing this complicated Swiss-cheese nanostructured metal oxides. The limited understanding of the microstructural evolution makes the fabrication even more challenging. Here we introduce a new approach to prepare these nanostructures of metal oxides and articulate the development mechanism of the structures at atomic resolution.

Automated Image Processing Scheme to Measure Atomic-Scale Structural Fluctuations

The interaction of gases with a solid catalyst nanoparticle during catalysis is a non-equilibrium process that requires high spatial and temporal resolution measurements to elucidate underlying mechanisms. State-of-the-art environmental transmission electron microscopy (ETEM) enables in situ measurements of the dynamic changes occurring under reaction conditions [1,2]. These changes usually take place rapidly at the nanometer scale. Recently, direct electron detection cameras, have enabled us to record atomic-resolution images with ns time resolution, but generate videos with large amount of data (≈ GB s). It is laborious to manually analyze such large-size videos, frame by frame, to extract the events of interest at the required time resolution. Automated analysis would be preferable, but is complicated by (a) noise in individual frames due to rapid readout times and (b) sample drift that occurs in a single video recording period. In order to overcome these issues, we have developed an automated image processing scheme (AIPS), to obtain structural information from the images extracted from videos. AIPS uses a combination of algorithms publically available and developed at NIST that perform noise reduction, drift correction, template matching, atom-position location, and triangulation to accurately determine the positions of atomic columns. We tested our method by quantitatively relating the crystal structure fluctuations in a catalyst nanoparticle to the growth of single-walled carbon nanotube (SWCNT) as a function of time.