Observing Solid-State Formation of Oriented Porous Functional Oxide

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Observing Solid-state Formation of Oriented Porous Functional Oxide Nanowire Heterostructures by in situ TEM Jo-Hsuan Ho, Yi-Hsin Ting, Jui-Yuan Chen, Chun-Wei Huang, Tsung-Chun Tsai, Ting Yi Lin, Chih-Yang Huang, and Wen-Wei Wu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03021 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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Observing Solid-state Formation of Oriented Porous

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Functional Oxide Nanowire Heterostructures by in

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situ TEM

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Jo-Hsuan Ho†,+, Yi-Hsin Ting,† ,+, Jui-Yuan Chen†,+, Chun-Wei Huang‡, Tsung-Chun Tsai†, Ting-

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Yi Lin†, Chih-Yang Huang†, and Wen-Wei Wu*,†,§,∥

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† Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu

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30010, Taiwan, ROC.

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‡ Material and Chemical Research Laboratories, Industrial Technology Research Institute,

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Hsinchu31040, Taiwan, ROC

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§ Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing

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Hua University, Hsinchu 30013, Taiwan.

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∥Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu

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30010, Taiwan.

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+ These authors contributed equally to this work

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corresponding author:[email protected]

ACS Paragon Plus Environment

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KEYWORDS : Oriented porous-Fe3O4, ZnO nanowires, heterostructures, in situ TEM, plate-like

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voids, zigzag-like hollow voids

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ABSTRACT

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Transition metal oxide nanowires have attracted extensive attention because of their

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physical characteristics. Among them, ZnO nanowires have great potential. Due to the

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multifunctional properties of ZnO, devices built using ZnO-based heterostructures always

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perform well. In this study, interesting diffusion behavior between ZnO nanowires and Fe metal

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was observed by using in situ transmission electron microscopy. ZnO nanowires and Fe metal

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were annealed under ultrahigh vacuum (UHV) conditions at 800 K. By controlling the annealing

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time for the solid-state diffusion, porous Fe3O4 and unique ZnO/porous Fe3O4 nanowire

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heterostructures were formed. As-formed porous Fe3O4 nanowires with voids can be divided into

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two types by appearance: plate-like voids and zigzag-like hollow voids. From high-resolution

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transmission electron microscopy (HRTEM) images and fast Fourier transform (FFT) diffraction

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patterns, we found that plate-like voids formed along the {111} plane, which was the close-

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packed plane of Fe3O4, and that zigzag-like hollow voids formed along the {111}/{022} planes.

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Moreover, a transition region existed during diffusion, with a parallel relationship found between

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the Fe3O4 crystal with plate-like voids and the ZnO crystal. A sharp interface was determined to

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exist between the Fe3O4 crystal with zigzag-like hollow voids and ZnO. These oriented porous

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Fe3O4/ZnO axial nanowire heterostructures exhibited a unique appearance and interesting

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formation behavior. Furthermore, the structures had a high surface-area-to-volume ratio, which is

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promising for sensing applications.

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Introduction

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Recently, one-dimensional (1D) nanostructures, which show a diversity of morphologies, such

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as nanowires, nanorods, nanobelts, and nanotubes, have received increasing attention. Due to

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their unique physical properties, which differ from bulk materials, they have potential for

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applications in optical, electrical, piezoelectric, opto-electrical and electrochemical devices, such

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as ultraviolet (UV) sensors,1 photodetectors,2 light-emitting diodes (LED),3 solar cells,4

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photosensors,5 resistive-switching random access memory (RRAM),6-7 nanogenerators,8-9

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nanocapacitors,10 and gas sensors11-12. Among all 1D nanomaterials, 1D nanoporous materials

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are currently the most popular research targets because of their relatively high surface-area-to-

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volume ratios compared to conventional 1D nanostructure materials13-17. There are several

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reported methods for synthesizing 1D nanoporous materials, including the chemical etching

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method18-19, hydrothermal method20-21 and template-assisted method22-23.

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On the other hand, nanowire heterostructures, which present special physical properties, have

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also garnered tremendous interest because of their unique band alignment and structural effects.

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Two main types of nanowire heterostructure exist, namely, radial heterostructures (similar to

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core-shell structures) and axial heterostructures (similar to segmented structures), which are

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usually synthesized by a catalyst-assisted vapor-liquid-solid method. If the reactant vapor is

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absorbed onto the surface of the nanowires, the radial heterostructures can form; in contrast, if

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the reactant vapor is absorbed onto the surface of the catalyst, axial heterostructures are

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produced. However, a method for fabricating both nanopores and heterostructures in the same

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nanowire has not been developed.

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In this study, the diffusional reaction behavior that transforms solid piezoelectric ZnO into

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porous magnetic Fe3O4 is investigated. The structure and physical properties of ZnO and Fe3O4


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are shown in Table S1-S3. We fabricated Fe3O4/ZnO axial nanowire (NW) heterostructures with

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oriented voids through a solid-state diffusion reaction. Furthermore, direct and real-time

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observation was additionally performed by in situ TEM to understand the diffusion dynamics for

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the Fe atoms and ZnO crystal. We also investigated the diffusion phenomenon and formation

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mechanism for the oriented voids, which provided a facile and controllable method for

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synthesizing porous axial nanowire heterostructures.

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Experimental Methods

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Single-crystalline ZnO NWs were synthesized by carbothermic reduction with a vapor–liquid–

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solid (VLS) mechanism. Powders of zinc oxide and carbon were mixed together and placed in

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the upstream of gas flow. The Au-coated silicon substrates were placed in the downstream. The

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temperatures of the upstream and downstream were elevated to 950 °C and 750 °C for 90 min

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using a heating rate of 10 °C per min. Then, the furnace was cooled to 25 °C. ZnO NWs with

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diameters of between 80 nm and 200 nm were detached from the Si substrate into ethanol

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solution. We placed these ZnO NWs onto Si3N4 membranes of a special TEM heating specimen,

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as shown in Figure S1. Positive tone photoresists (methyl methacrylate (MMA) and poly methyl

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methacrylate (PMMA)) were coated onto the samples, followed by baking at 180 °C for 1 min

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and 2 min (Figure S1(b)). After e-beam lithography, iron metal pads with a thickness of 300 nm

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and width of 1 µm were deposited onto the ZnO nanowires by an e-gun evaporation system. An

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additional 30-nm-thick gold film was deposited onto the iron metal pads to protect the iron metal

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from oxidation (Figure S1(c) and S1(d), S2(b), S3). However, when the ZnO nanowires were

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placed onto a Si3N4 membrane, the nanowires usually tended to attract each other due to the Van

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der Waals force. During the fabrication process, nanowire contact led to the formation of a

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branched nanowire following the heat treatment, as shown in Figure 1(a).


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Then, a real-time in situ observation of the solid-state diffusion reaction was carried out in

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using a TEM holder (Protochips Aduro300 TEMTM) and subsequently loaded into a JEOL F200

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field emission scanning transmission electron microscope (STEM) for observation. Iron atoms

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started to diffuse from the metal pads into the ZnO lattice at approximately 800 K, as shown in

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Figure S2(c). As the diffusion continued, Zn sites were replaced by iron atoms. The replaced Zn

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diffused into the surface of the nanowires and evaporated into the vacuum chamber. ZnO

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nanowires finally transformed into heterostructured Fe3O4 and ZnO (Figure S2(d)). EDS

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mapping of the void structures with plate-like and zigzag-like features (Figure S4(a) and S4(b))

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revealed the elemental distribution for Fe, Zn, O and Au after the cation exchange reaction. The

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gold layer did not diffuse into the nanowires, but instead only protected the Fe pad from

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oxidation. The void structures with plate-like and zigzag-like features revealed the same results.

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A Cs-corrected scanning transmission electron microscope (STEM, JEOL-ARM 200F)

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equipped with an energy-dispersive spectrometer (EDS) was utilized to analyze the structure and

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composition of the nanowire heterostructures.

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Results and Discussion

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Figure 1(a) shows the low-magnification TEM image of a Fe3O4/ZnO axial nanowire

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heterostructure with plate-like voids, which can be divided into three parts: unreacted ZnO (blue

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lines), porous Fe3O4with plate-like voids (red lines) and a transition region (orange lines). Many

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plate-like voids with light contrast can be seen in the porous Fe3O4 region. A mixed contrast and

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growing voids can be found in the transition region. To verify the composition of the transition

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region, EDS mapping was analyzed, as shown in Figure 1(f-h). The red dotted line denoting the

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remaining Zn and the blue dotted line denoting the diffused Fe in Figure 1(b) show the transition

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region. Additionally, the EDS line scan in Figure 1(e) demonstrates the declining signal for Fe


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(red line) and the increasing signal for Zn (blue line) (from right to left) in the transition region,

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confirming that Fe atoms diffused into the ZnO lattice and formed the transition region.

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According to the HRTEM images in Figure 1(c-d), the crystals were identified as wurtzite ZnO

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(space group P63mc) with a [1-100] zone axis and cubic inverse spinel Fe3O4 (space group Fd-

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3m) with a [211] zone axis. They also revealed the parallel relationships in this system, which

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were ZnO (0002) // Fe3O4 (1-1-1) and ZnO [1-100] // Fe3O4 [211] with a lattice mismatch of

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8.6%.

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The appearance and structural analysis of the porous Fe3O4 region is shown in Figure 2. Figure

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2(a) presents data for another ZnO nanowire and metal pads before annealing. Figure 2(b) and

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2(c) show the bright-field and dark-field STEM images for this as-formed Fe3O4 region. The

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bright contrast in the bright-field image and the dark contrast in the dark-field image indicate the

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existence of plate-like voids in the Fe3O4 region. The EDS line scan in Figure 2(e) demonstrated

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that the element signals decreased in the void regions. The element signals were not zero since

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the void regions were relatively empty. Appearance of plate-like voids can be clearly seen in the

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TEM image shown in Figure 2(d). Voids in the 2D images look like stripes, but in the 3D real

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case, they appear as plates. Sharp interfaces that were vertical to the radial direction were

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observed between the voids and Fe3O4 crystal.

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From the HRTEM image of Fe3O4 in Figure 2(f), we identified this sharp interface as {111},

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which is the close-packed plane of Fe3O4. Almost all voids were formed along this plane because

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the system was under an ultrahigh vacuum condition, which means it was lacking oxygen.

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During the exchange reaction, oxygen atoms offered a limited source for bonding with Fe atoms.

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The voids would appear resulting from the insufficient amount of oxygen in ZnO. Then, the void

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in the Fe3O4 region tended to form close-packed planes due to having the lowest energy in the


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crystal structure. From the bright-field STEM image in Figure 2(g), the atomic arrangements in

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Fe3O4 can be easily understood.

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In addition, EELS analysis was carried out to confirm the Fe3O4 structure in the nanowire

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heterostructure. The colored points in Figure S5(a) show the analysis area, with the

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corresponding results shown in Figure S5(b). The results confirmed that the crystal in this

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nanowire heterostructure is Fe3O4 according to the reference materials24.

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To understand the void formation mechanism, in situ observation of the solid-state diffusion

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reaction is important. Figure 3(a-f) exhibits the in situ TEM images recorded from an in situ

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video (Movie S1). In the plate-like type region, a transitional region provided the area for the

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diffusion and interaction of Fe atoms into the ZnO lattice. Due to the diffusion of Fe atoms prior

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to the exchange reaction, there was enough time for cation exchange reaction to induce the

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parallel relationship between ZnO (0002) // Fe3O4 (11ത1ത). In each image, the arrows with the

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same color denote the same voids. Plate-like voids with bright contrast appeared randomly in the

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nanowire and grew from the surface to the core of the nanowire. Due to the 2D projection, the

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void in the core (indicated by the yellow arrow) grew from the surface rather than the interior of

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the nanowire. When the Fe atoms diffused into the ZnO nanowires, they preferred to diffuse

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through the surfaces of the nanowires, resulting in growth of the void from the top or bottom

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surface of the ZnO nanowires to the core. More in situ TEM images are shown in Figure S6.

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There was a thinner ZnO nanowire connected to a thicker one that contacted the Fe metal pad

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(the same one as shown in Figure 1). When the Fe atoms diffused to the junction, two interfaces

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with dark contrast were separated as upward and downward, showing that the diffusion of the Fe

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atoms in the ZnO nanowire occurs via the surface.


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Schematic illustrations for the plate-like void formation are shown in Figure 3(g-i). The ZnO

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nanowires are not thermally stable, and, therefore, our assumption that plate-like voids originated

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from the surface is based on the surface containing more dangling bonds and defects, resulting in

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the exchange reaction from the surface to the core.25-26 When the Fe atoms diffused to these

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surface defects, they accumulated there to reduce the surface energy. As the concentration of Fe

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increased, Fe atoms started diffusing into the ZnO lattice to form the transition region. The

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cation exchange reaction for Zn and Fe progressed in the ZnO lattice simultaneously while the

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Fe atoms were diffusing, transforming ZnO into Fe3O4. Since the Fe metal pads can be

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considered an unlimited source compared to the ZnO nanowires in this system, this reaction

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could continue until all the ZnO is transformed into Fe3O4. The transitional region could be

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totally consumed so that the Fe3O4 with plate-like voids would extend the entire length of the

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nanowires. The cation exchange reaction can be described by the following equation:

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4ZnO(s) + 3Fe(s) → Fe3O4(s) + 4Zn(v)

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From the equation above, we know that four ZnO can transform into one Fe3O4; therefore,

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voids will form in the Fe3O4 nanowire after annealing. To realize the formation of voids in the

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Fe3O4 region, detailed calculation of the volume and packing density was carried out in Figure

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S7. These results are also shown in Table S2. The experimental results show that the diameter

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after annealing is not obviously decreased compared to the diameter before annealing (Figure

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S8). Therefore, on the same scale, the density of the nanowire increased from 55.4% to 69%,

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meaning voids formed to retain the size of the nanowire without shrinking.

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Figure 4 shows another type of void in the Fe3O4 region called the zigzag-like hollow void.

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From the bright-field and dark-field images in Figure 4(a) and 4(b), many voids with opposite

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contrast are located in the core of the nanowire. These voids connected with each other and


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formed a zigzag interface with the shell, causing the nanowire to become hollow. An EDS line

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scan analysis in Figure 4(c) shows that the signal for the Fe element decreased in the void region

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and increased in the shell region. Colored points in Figure 4(d) show the area of the EDS point

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analysis, with the corresponding results shown in Figure 4(e). Point A (red) in the void region is

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composed of 54.77% oxygen and 45.17% iron. Since point A only reflects the composition of the

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shell, the composition will be the closest to Fe3O4. Point B (orange) and C (green) in the shell

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region show higher iron atomic percentages of 54.28% and 58.61%, respectively, compared to

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point A. Point D (blue) in the edge of the nanowire shows the highest iron atomic percentages of

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64.68%. This result shows that no matter what kind of void formed in Fe3O4, the Fe atoms all

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diffused via the surface of the ZnO nanowires.

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The interface region of the Fe3O4/ZnO axial nanowire heterostructure with zigzag-like hollow

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voids is shown in Figure 5. Figure 5(a) and 5(b) show the bright-field and dark-field STEM

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images of the interface, respectively. The sharp interface separates the porous Fe3O4 and solid

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ZnO and forms unique nanowire heterostructures. The EDS mapping analysis in Figure 5(c-e)

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shows the distribution of Zn, Fe and O, confirming that no overlap exists for the Fe and Zn and

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that there is no transition region in this nanowire heterostructure. HRTEM images and the

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corresponding FFT of Fe3O4 and ZnO crystals show that there is no parallel relationship between

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the two crystals (Figure 5(f) and 5(g)). The ex situ experiments for the region were also

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conducted without beam exposure because the overall specimen was annealed at 800 K to induce

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the exchange reaction between Zn atoms and Fe atoms. The temperature, which plays the role of

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a driving force, should be high enough to promote the reaction, but a higher temperature can

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accelerate the reaction rate. TEM images were taken before and after the zigzag-like hollow

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formation, as shown in Figure S9(a) and S9(b). In situ TEM images captured from the real-time


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video are shown in Figure S9(c) to S9(f). The ledge diffusion for a few atomic layers (3~4 nm)

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can be observed in the void region. The diffusion mechanism in the system with zigzag-like

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hollow voids, which is shown in Figure S10, is very different from the system with plate-like

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voids. An atomic layer of Fe can also diffuse from the surface through the core to the other

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surface. Once all the ZnO transformed to Fe3O4, the next ledge would start to diffuse. Similarly,

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the difference in the packing densities caused the formation of the voids, but the voids formed in

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the core.

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From the TEM image of the zigzag-like hollow voids in Figure S11(a), it seems that the voids

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also formed along specific planes. Therefore, the plane analyses are shown in Figure S11(b-e).

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Figure S11(b) shows the TEM image for the zigzag-like hollow voids with colored dotted lines

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symbolizing the specific planes with different orientations. From the high-magnification TEM

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image in Figure S11(c), the existence of an amorphous phase confirms that the voids are hollow

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in this nanowire heterostructure. An HRTEM image and its corresponding FFT taken for this

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region are shown in Figure S11(d) and S11(e), respectively. The structure is Fe3O4 with a [211]

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zone axis. The specific planes are {111} (green) and {022} (orange). Due to the ledge

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mechanism, the exchange of cations reacted immediately with insufficient time for crystal

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reorganization. Hence, Fe3O4 tended to form close-packed {111} planes and energetically

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favored {022} planes in the zigzag-like hollow structure.

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Conclusions

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We successfully fabricated an oriented porous Fe3O4/ZnO axial nanowire heterostructure by

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solid-state diffusion reaction. Two types of voids were identified in the as-formed Fe3O4/ZnO

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nanowire heterostructures: plate-like voids and zigzag-like hollow voids. The plate-like voids in

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the porous Fe3O4 formed along the close-packed {111} plane because of the lower surface


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energy. The plate-like void structure was formed by surface diffusion. Fe atoms were found to

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diffuse into ZnO to form a transitional region. Then, Fe atoms substituted for Zn atoms at the

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surface due to surface defects. Finally, the heterostructure with a parallel relationship between

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ZnO (0002) // Fe3O4 (11ത1ത) and ZnO [1-100] // Fe3O4 [211] was formed. However, no parallel

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relationship was found between ZnO and porous Fe3O4 with zigzag-like hollow voids. Although

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there is no transition region, a sharp interface exists between the two crystals. The zigzag-like

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void structure was formed with energetically favored planes by ledge diffusion. Fe atoms directly

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replaced the Zn atoms without a transitional region, resulting in the sharp interface at ZnO

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remaining intact. Zigzag-like voids in the porous Fe3O4 formed along the close-packed {111}

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plane and energy-feasible {022} plane. Moreover, ledge diffusion was observed in this system

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by in situ TEM. Due to the difference in the packing densities between ZnO and Fe3O4 as well as

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the restriction of the energy, the voids formed with a special orientation in the Fe3O4 crystal.

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From their reaction mechanisms, the plate-like or zigzag-like structure may tend to generate by a

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coating or doping process, respectively. These as-fabricated Fe3O4/ZnO axial nanowire

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heterostructures with plate-like voids and zigzag-like voids possess a high surface-area-to-

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volume ratio, which is promising for sensing applications.

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Figure 1. TEM images, HRTEM images, EDS line scan and mapping analysis for the

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ZnO/Fe3O4 nanowire heterostructures. (a) Low-magnification TEM image of the as-fabricated

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ZnO/Fe3O4 nanowire heterostructure. Red, orange and blue lines respectively indicate the porous

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Fe3O4, transition and unreacted ZnO regions. (b) TEM image of the transition region. The blue

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and red dotted lines respectively symbolize the remaining Zn and the diffused Fe signals from

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the EDS mapping. The black dotted line shows the scanning region for the EDS mapping. (c)

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HRTEM image of ZnO; the inset shows the corresponding FFT diffraction pattern with the [1-

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100] zone axis. (d) HRTEM image of Fe3O4; the inset shows the corresponding FFT diffraction


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pattern with the [211] zone axis. (e) The EDS line scan analyses at the transition region. (f-h)

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EDS mapping analyses showing the distribution of Zn, Fe and O in the transition region.

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Figure 2. TEM, HRTEM and STEM images of the porous Fe3O4 region with EDS scan analysis

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(a) TEM image of a ZnO nanowire and Fe metal pad before annealing. (b) Bright-field and (c)

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dark-field STEM images for a porous Fe3O4 region. (d) High-magnification TEM image of a

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porous Fe3O4 region. (e) High-magnification TEM image of a porous Fe3O4 region and EDS line

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scans. The green, blue and red lines represent Fe, O and Zn, respectively. (f) HRTEM image and

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(g) bright-field STEM image for the as-formed Fe3O4 region.

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Figure 3. A series of in situ TEM images for plate-like void formation recorded from video, with

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schematic illustrations for the formation mechanism. (a-f) In situ TEM images of the void

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formation process in the ZnO/Fe3O4 nanowire heterostructure. The colored arrows denote the

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same voids in different images. (g-i) Schematic illustrations of the transformation process from a

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single-crystalline ZnO nanowire to a Fe3O4 nanowire with voids via solid-state diffusion

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reaction.

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Figure 4. TEM and STEM images of the zigzag-like hollow Fe3O4 region with EDS line scan

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and point analyses. (a) Bright-field and (b) dark-field STEM images for the Fe3O4 region with

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voids. (c) EDS line scan analysis. (d) High-magnification TEM image of the Fe3O4 region with

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zigzag-like hollow voids. The colored points (A to D) denote the scanning areas used for the

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EDS point analyses, with the results shown in (e).

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Figure 5. STEM, HRTEM and EDS mapping analysis for the interface region in a ZnO/Fe3O4

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nanowire heterostructure with zigzag-like hollow voids. (a) Bright-field and (b) dark-field STEM

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images for the interface region in the ZnO/Fe3O4 nanowire heterostructure with zigzag-like

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hollow voids. No transition region exists in this hollow void system; however, a sharp interface

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between two crystals can be clearly seen in the images. (c-e) EDS mapping analyses for the

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interface region. (f) HRTEM image of Fe3O4, with the inset showing the corresponding FFT

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diffraction pattern with the [012] zone axis. (g) HRTEM image of ZnO, with the inset showing

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the corresponding FFT diffraction pattern with the [1-100] zone axis.


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ASSOCIATED CONTENT

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Supporting Information

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The Supporting Information is available free of charge on the ACS Publication website at

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http://pubs.acs.org.

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Proof that solid ZnO nanowires transform into oriented porous Fe3O4/ZnO nanowire

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heterostructures through solid-state diffusion and in situ TEM videos that provide evidence for

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the direct observation of the fabrication process.

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AUTHOR INFORMATION

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Corresponding Author

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*E-mail: [email protected]

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Author Contributions

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J.H.H. and C.W.H. fabricated the sample and performed in-situ experiments. J.H.H., Y.H.T.,

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and C.W.H. performed the TEM experiments. J.H.H., T.C.T., T.Y.L, and C.Y.H. analyzed the

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diffraction data and atomic structure. W.W.W. conceived the study and designed the research.

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J.H.H., J.Y.C. and W.W.W. wrote this paper.

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Notes

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The author declares no competing financial interest.

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ACKNOWLEDGMENT

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The authors acknowledge the support by Ministry of Science and Technology through grants

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103-2221-E-009-222-MY3, 106-2628-E-009-002-MY3, 106-2119-M-009-008, 107-3017-F009-


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003), and Ministry of Education, Taiwan (SPROUT Project- Frontier Research Center on

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Fundamental and Applied Sciences of Matters of National Tsing Hua University; Center for

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Emergent Functional Matter Science of National Chiao Tung University).

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ABBREVIATIONS

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HRTEM, high resolution transmission electron microscopy; FFT, fast Fourier transform; EDS,

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energy-dispersive spectrometer.

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There are two types of voids in the as-formed Fe3O4/ZnO nanowire heterostructures: plate-

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like voids and zigzag-like hollow voids. Plate-like voids formed along the {111} plane, which

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was the close-packed plane of Fe3O4, and zigzag-like hollow voids formed along the

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{111}/{022} planes. These oriented porous-Fe3O4/ZnO axial nanowire heterostructures exhibited


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a unique appearance and interesting formation behavior. Furthermore, the structures provide a

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high surface-area-to-volume ratio, which is promising for sensing applications.


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Figure 1.TEM images, HRTEM images, EDS line scan and mapping analysis of ZnO/Fe3O4 nanowire heterostructures. (a) Low-magnification TEM image of the as-fabricated ZnO/Fe3O4 nanowire heterostructure. Red, orange and blue lines respectively indicate the porous-Fe3O4, transition and unreacted ZnO regions. (b) TEM image of the transition region. Blue and red dotted lines respectively symbolize the remaining Zn and the diffused Fe signals from the EDS mapping. Black dotted line shows the scanning region of the EDS mapping. (c) HRTEM image of ZnO and the inset shows the corresponding FFT diffraction pattern with the [1-100] zone axis. (d) HRTEM image of Fe3O4 and the inset shows the corresponding FFT diffraction pattern with the [211] zone axis. (e) The EDS line scan analyses at the transition region. (f-h) EDS mapping analyses show the distributions of Zn, Fe and O in the transition region. 233x237mm (300 x 300 DPI)


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Figure 2. TEM, HRTEM and STEM images of the porous-Fe3O4 region with EDS scan analysis (a) TEM image of aZnO nanowire and Fe metal pad before annealing. (b) Bright-field and (c) dark-field STEM images of a porous-Fe3O4 region. (d) High-magnification TEM image of aporous-Fe3O4 region. (e) High-magnification TEM image of a porous-Fe3O4 region and EDS line scans. Green, blue and red lines represent Fe, O and Zn, respectively. (f) HRTEM image and (g) bright-field STEM image of the as-formed Fe3O4 region. 213x162mm (300 x 300 DPI)


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Figure 3. A series of in situ TEM images for plate-like void formation recorded from video, with schematic illustrations for the formation mechanism. (a-f) In situ TEM images of the void formation process in the ZnO/Fe3O4 nanowire heterostructure. The colored arrows denote the same voids in different images. (g-i) Schematic illustrations of the transformation process from a single-crystalline ZnO nanowire to a Fe3O4 nanowire with voids via solid-state diffusion reaction. 241x167mm (300 x 300 DPI)


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Figure 4. TEM and STEM images of the zigzag-like hollow Fe3O4 region with EDS line scan and point analyses. (a) Bright-field and (b) dark-field STEM images of the Fe3O4 region with voids. (c) EDS line scan analysis. (d) High-magnification TEM image of Fe3O4 region with zigzag-like hollow voids. Colored points (A to D) are the scanning areas of the EDS point analyses, and the results are shown in (e). 184x129mm (300 x 300 DPI)


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Figure 5. STEM, HRTEM and EDS mapping analysis of interface region in ZnO/Fe3O4 nanowire heterostructure with zigzag-like hollow voids. (a) Bright-field and (b) dark-field STEM images of the interface region in the ZnO/Fe3O4 nanowire heterostructure with zigzag-like hollow voids. There is no transition region in this hollow void system but a sharp interface between two crystals that can be seen clearly in the images. (c-e) EDS mapping analyses of interface region. (f) HRTEM image of Fe3O4, and the inset shows the corresponding FFT diffraction pattern with the [012] zone axis. (g) HRTEM image of ZnO and the inset shows the corresponding FFT diffraction pattern with the [1-100] zone axis. 139x172mm (300 x 300 DPI)


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TOC only 518x443mm (300 x 300 DPI)


Observing Solid-State Formation of Oriented Porous Functional Oxide

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