To probe the nature of metal-catalysed processes and to design better metal-based catalysts, atomic scale understanding of catalytic processes is highly desirable. Here we use aberration-corrected environmental transmission electron microscopy to investigate the atomic scale processes of silver-based nanoparticles, which catalyse the oxidation of multi-wall carbon nanotubes. A direct semi-quantitative estimate of the oxidized carbon atoms by silver-based nanoparticles is achieved. A mechanism similar to the Mars–van Krevelen process is invoked to explain the catalytic oxidation process. Theoretical calculations, together with the experimental data, suggest that the oxygen molecules dissociate on the surface of silver nanoparticles and diffuse through the silver nanoparticles to reach the silver/carbon interfaces and subsequently oxidize the carbon. The lattice distortion caused by oxygen concentration gradient within the silver nanoparticles provides the direct evidence for oxygen diffusion. Such direct observation of atomic scale dynamics provides an important general methodology for investigations of catalytic processes.

Zinc oxide nanomaterials with a wurtzite structure are used for various applications including catalysis [1]. ZnO nanowires are of interest due to its high surface-to-volume ratio and thermodynamic stability of non-polar { } surfaces. Metal particles have been deposited onto the prismatic planes of ZnO nanowires for sensing and photocatalytic applications [2]. The interfacial structure between ZnO { } surfaces and various metals have been investigated [3]. Kra evec et al. reported that an oriented overgrowth of the spinel layers was observed between ZnO surfaces and a thin Sb-rich film [4]. Uniform antimony coating onto the surfaces of ZnO nanowires may provide new functions for applications in sensing, catalysis, energy conversion or nanoelectronics. We report here our recent study of the adsorption of Sb onto the { } facets of ZnO nanowires.The Sb/ZnO nanowires were synthesized in a high temperature tube furnace by a standard vapor phase transport process. Mixed ZnO and carbon powders were heated to about 1100°C and powders were placed down the stream of the tube furnace and were heated to generate Sb vapor inside the furnace. ZnO nanowires, formed in the high temperature zone, were carried to the low temperature zone where Sb atoms adsorbed onto the surfaces of the ZnO nanowires. By controlling the Sb partial vapor pressure we can control the thickness of the deposited Sb layers. The JEOL JEM-ARM200F aberrationcorrected scanning transmission electron microscope (STEM), with a nominal image resolution of 0.08 nm in the high-angle annular dark-field (HAADF) imaging mode, was used to investigate the atomic structure of the deposited Sb and its relationship with the facets of the ZnO nanowires. Figure 1a shows a HAADF image of an as-synthesized Sb/ZnO nanowire. The ZnO nanowire is atomically smooth with a diameter of about 20 nm. Analyses of HAADF images revealed that the ZnO nanowire grew along the [0001] axis and was enclosed by the six { } surfaces. Detailed examination of the HAADF images revealed that the surface atoms of the ZnO nanowire ( Fig. 1a and 1b) yielded a brighter image contrast. Furthermore, the brighter atoms only decorated on every other column of the surface Zn atoms. Since such brighter surface atoms have not been observed in ZnO nanowires that have been synthesized without the addition of Sb, we assigned these brighter dots to columns of antimony atoms adsorbed onto the { } surfaces of the ZnO nanowire. It should be noted that Fig. 1a shows both the ( ) and ( ) surfaces of the ZnO nanowire and that Sb only adsorbs { } surfaces.The proposed structure of the Sb-ZnO ( ) surface is shown in Fig 2. Fig 2a was constructed according to the HAADF image (Fig 1b): Sb atoms occupy the positions of the Zn1 atoms on the ( ) surface. The slightly larger spacing from the next Zn layer may be due to the larger size of Sb and modification of its bonding with the oxygen atoms. Four different Zn and four different O are labeled as 1, 2, 3 and 4. The coordination number of the inner oxygen atoms (e.g. O2) is 4...

Carbon nanotubes (CNTs) [1], with their unique physicochemical properties, have attracted broad attention due to their potential applications [2]. A recent in situ study on the oxidation of different types of CNTs has revealed that the initiation temperature of CNT oxidation depends on the wall thickness of the CNTs [3]. Single-and double-wall CNTs can oxidize at lower temperatures and multi-wall CNTs can be stable in oxygen even at temperatures as high as 520 ºC [3]. With the use of catalyst nanoparticles to facilitate the oxidation processes the initiation temperature and the degree of carbon oxidation can be drastically changed [4]. Understanding the behavior of catalyzed oxidation processes is critical to developing better catalysts for combustion of particulates including oxidation of soot particles [5]. We report here the in situ investigation of Ag catalyzed oxidation of multi-wall CNTs inside an aberration-corrected environmental TEM (AC-ETEM) with the goal of probing the nature of the catalyzed oxidation processes and thus developing better understanding of combustion catalysts.The CNTs, which were prepared via a CVD method, were suspended in ethyl alcohol and a drop of the solution was then transferred to a holey-carbon coated TEM grid. The FEI Titan G2 AC-ETEM, operating at 80 KV, was used to carry out the in situ oxidation experiment. The electron dose rate was kept low enough so that the electron beam exerts minimum influence on the integrity of the experimental results. A Gatan Inconel heating holder was used and the images were recorded by a standard CCD camera. To clean the CNTs, the TEM grid was heated to 450ºC in 2 mbar O2 for 45 min inside the ETEM. Repeated experiments confirmed that such treatment did not oxidize the multi-wall CNTs. The CNTs were then loaded with AgNO3 aqueous solution and was then reduced inside the ETEM in 2 mbar H2 at 250ºC for 2 hours, resulting in the formation of crystalline Ag nanoparticles. The electron beam was blanked during the sample treatment processes. Figure 1 shows AC-ETEM images of the CNTs after the sample treatment processes. Ag nanoparticles, with an average diameter of approximately 5 nm, decorated on the surfaces of the CNTs. After this initial quick examination the electron beam was blanked and the sample was oxidized in 2 mbar O2 at 250ºC for 25 minutes. To avoid any potential ionization effects, the oxygen inside the ETEM chamber was completely purged after the oxidation reaction and before the electron beam was un-blanked [3]. The same sample region, before and after the oxidation reaction, was carefully examined and images were recorded. Fig. 1b shows the same area as that of Fig. 1a but after the oxidation reaction. The CNTs were clearly oxidized wherever they were in contact with the Ag nanoparticles. Figure 2 shows another pair of images of a single CNT before and after the oxidation reaction. Based on analysis of these and many other similar images we concluded that the Ag nanoparticles clearly catalyzed the oxidation of multi-wall CNTs at tem...

Semiconducting ZnO is a promising material for various optical and electrical applications. Due to the ultraviolet light-emitting characteristics of ZnO nanowires, their luminescence properties are an attractive subject of investigation [1]. While Photoluminescence (PL) refers to emission from materials excited by photons, cathodoluminescence (CL) measures the degree of luminescence originated from the decay of electron-hole pairs caused by high energy electron bombardment [2]. CL can be used as a valuable tool to evaluate material properties and high resolution CL imaging can provide spatial information on the electronic properties of nanostructures. Here we report our recent investigation of CL imaging of ZnO, Sb/ZnO and Bi/ZnO nanostructures.ZnO nanowires were fabricated via a vapor deposition process inside a horizontal quartz tube furnace. Bi/ZnO and Sb/ZnO nanostructures were prepared by introducing metal vapor and deposit them on ZnO nanostructures via the vapor phase transport process. JEOL JXA-8530F hyper probe, consisting of a field-emission scanning electron microscope equipped with five wavelength dispersive spectrometers and a CL detector of panchromatic type, collecting signals in the wavelength range of 200 nm to 900 nm. This instrument can provide a spatial resolution of 3 nm in the secondary electron and backscattered electron imaging mode. Fig.1 shows high magnification backscattered electron image (a) and the corresponding CL image (b) of ZnO nanowires. Analyses of many CL images showed that when the diameter of ZnO nanowires is below about 30 nm the CL signals were hardly detectable. Preliminary investigation suggests that the signal reduction may not just originate from the smaller emission volume. Further study of this phenomenon is under way. Fig. 2 (a) is a BSE image of the synthesized Bi/ZnO nanostructure; the brighter contrast represents the bismuth oxide phase. Fig. 2 (b) is the corresponding CL image, revealing that the two oxides provide a similar integrated CL signal. Figures 2(c) and 2(d) show a BSE image and a CL image of the Sb/ZnO nanostructures, respectively. The relatively low CL intensity of the antimony oxide microwires (compared to ZnO nanowires) suggests that CL imaging can be used to investigate the CL emission properties of mixed (Sb)ZnO nanoscale structures. Detailed analyses of the local CL emission from various nanostructures will be discussed [3].