Catalyst-free Chemical Vapor Deposition Process Research Articles

Optimizing growth of ZnO nanowire networks for high-performance UV detection

Abstract The response speed and photo-to-dark current ratio are critical indicators of ZnO based UV photodetectors. Here we found the response speed and photo-to-dark current ratio of ZnO nanowire UV photodetectors could be well optimized by tuning the entanglement and density of ZnO nanowires grown on SiO 2 pillars through a catalyst-free chemical vapor deposition process. The best performance of ZnO nanowire based UV photodetectors could be achieved at a growth time of 3 min. After the optimized period of growth, ZnO nanowire UV photodetector presents a rise time of 0.45 s and a decay time of 0.06 s, with a high photocurrent to dark current ratio of 10 2 . The fast tuning of nanowire-nanowire junction barrier height contributes to the enhanced response speed of the ZnO nanowire UV photodetectors, while the high photo-to-dark current ratio is attributed to the sparse ZnO distribution and also multi-channel structures. We believe this work would greatly boost the application of ZnO NWs in nanoelectronics and optoelectronics.

Carbon Nanotubes–Nanoporous Anodic Alumina Composite Membranes: Influence of Template on Structural, Chemical, and Transport Properties

This work presents the synthesis of carbon nanotubes–nanoporous anodic alumina composite membranes (CNTs–NAAMs) with controllable geometric features by a template-assisted catalyst-free chemical vapor deposition (CVD) approach using a mixture of toluene and ethanol as a carbon precursor. NAAMs templates were prepared by anodization of aluminum substrates in different electrolytes containing sulfuric, oxalic, and phosphoric acids with the aim of establishing the template effect on the CNTs growth. The deposition time during the CVD process was systematically modified to determine the formation mechanism of CNTs inside the pores of NAAMs without using metal catalysts. The structural features, chemical composition, and graphitic structures of the resulting CNTs–NAAMs composites were characterized by different techniques to provide a comprehensive understanding of the effect of the template on the formation of these carbon-based nanostructures. CNTs–NAAMs with inner pore diameters ranging from 15 to 180 nm we...

Growth and photoluminescence of Si-SiOx nanowires by catalyst-free chemical vapor deposition technique

We developed a one-step catalyst-free chemical vapor deposition process to synthesize Si-SiOx nanowires using tetraethoxysilane as the precursor. Observations using scanning electron microscopy showed that the Si-SiOx nanowires were 20–50nm in diameter and tens of microns in length. The high-resolution transmission electron microscope analysis and X-ray diffraction demonstrated that the nanowires consisted of crystal silicon and amorphous SiOx. The Si and O with an atomic ratio of the Si-SiOx NWs were 1:1.2 according to the energy dispersion X-ray spectroscope. A systematic study on the effect of the growth conditions, such as reaction temperature, the reaction time, and the TEOS vapor flow rate was performed. The formation of Si-SiOx nanowires was implemented by the non-classical crystallization mechanism. The charged nanoparticles acting as building blocks self-assembled into nanowires. The photoluminescence measurements were carried out and showed that the Si-SiOx nanowires emitted stable ultraviolet and green luminescence excited by ultraviolet light.

Synthesis of Carbon Nanotube-Nanotubular Titania Composites by Catalyst-Free CVD Process: Insights into the Formation Mechanism and Photocatalytic Properties.

This work presents the synthesis of carbon nanotubes (CNTs) inside titania nanotube (TNTs) templates by a catalyst-free chemical vapor deposition (CVD) approach as composite platforms for photocatalytic applications. The nanotubular structure of TNTs prepared by electrochemical anodization provides a unique platform to grow CNTs with precisely controlled geometric features. The formation mechanism of carbon nanotubes inside nanotubular titania without using metal catalysts is explored and explained. The structural features, crystalline structures, and chemical composition of the resulting CNTs-TNTs composites were systematically characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The deposition time during CVD process was used to determine the formation mechanism of CNTs inside TNTs template. The photocatalytic properties of CNTs-TNTs composites were evaluated via the degradation of rhodamine B, an organic model molecule, in aqueous solution under mercury-xenon Hg (Xe) lamp irradiation monitored by UV-visible spectroscopy. The obtained results reveal that CNTs induces a synergestic effect on the photocatalytic activity of TNTs for rhodamine B degradation, opening new opportunities to develop advanced photocatalysts for environmental and energy applications.

Fabrication of vertically aligned single-crystalline lanthanum hexaboride nanowire arrays and investigation of their field emission

High-quality, uniform, one-dimensional (1D) lanthanum hexaboride (LaB6) nanostructures with different morphologies (for example, sparse or dense nanoneedles, or nanorods and nanowire arrays) were fabricated through an effective, easily controlled, one-step, catalyst-free chemical vapor deposition process. The morphologies, structures and temperature-dependent field emission (FE) properties were systematically investigated. FE measurements at room temperature (RT) showed that LaB6 nanowire arrays possess the best FE characteristics among all 1D LaB6 nanostructures, with a low turn-on electric field (Eto, 1.82 V μm−1), a low threshold electric field (Ethr, 2.48 V μm−1), a high current (5.66 mA cm−2 at 2.92 V μm−1) and good stability (at a testing time of 1000 min, fluctuations were <6.0%). Temperature-dependent FE showed that the turn-on and threshold electric fields decreased from 1.82 to 1.06 and 2.48 to 1.62 V μm−1, respectively, whereas the emission current density increased significantly from 0.20 to 9.05 mA cm−2 at 2.20 V μm−1 when the temperature was increased from RT to 723 K. The emission current density and the dependence of the effective work function on temperature were also investigated. We attribute the significant reduction of the turn-on and threshold fields and the remarkable increase of emission current to a decrease in the effective work function with temperature

Controlled CVD growth of Cu–Sb alloy nanostructures

Sb based alloy nanostructures have attracted much attention due to their many promisingapplications, e.g. as battery electrodes, thermoelectric materials and magneticsemiconductors. In many cases, these applications require controlled growth of Sb basedalloys with desired sizes and shapes to achieve enhanced performance. Here, we report aflexible catalyst-free chemical vapor deposition (CVD) process to prepare Cu–Sbnanostructures with tunable shapes (e.g. nanowires and nanoparticles) by transporting Sbvapor to react with copper foils, which also serve as the substrate. By simply controllingthe substrate temperature and distance, various Sb–Cu alloy nanostructures, e.g. Cu11Sb3 nanowires(NWs), Cu2Sb nanoparticles (NPs), or pure Sb nanoplates, were obtained. We also found that the growth ofCu11Sb3 NWs in such a catalyst-free CVD process was dependent on the substratesurface roughness. For example, smooth Cu foils could not lead to the growth ofCu11Sb3 nanowires while roughening these smooth Cu foils with rough sand papers could result in the growthof Cu11Sb3 nanowires. The effects of gas flow rate on the size and morphology of the Cu–Sb alloynanostructures were also investigated. Such a flexible growth strategy could be of practicalinterest as the growth of some Sb based alloy nanostructures by CVD may not be easy dueto the large difference between the condensation temperature of Sb and the other element,e.g. Cu or Co.

High-Yield Synthesis of Boron Nitride Nanosheets with Strong Ultraviolet Cathodoluminescence Emission

Bulk quantities of hexagonal boron nitride (h-BN) nanosheets have been synthesized via a simple template- and catalyst-free chemical vapor deposition process at 1100−1300 °C. Adjusting the synthesis and chemical reaction parameters, the thickness of the BN nanosheets can be tuned in a range of 25−50 nm. Fourier transform infrared spectra and electron energy loss spectra reveal the typical nature of sp2-hybridization for the BN nanosheets. It shows an onset oxidation temperature of 850 °C for BN nanosheets compared with only about 400 °C for that of carbon nanotubes under the same conditions. It reveals a strong and narrow cathodoluminescence emission in the ultraviolet range from the h-BN nanosheets, displaying strong ultraviolet lasing behavior. The fact that this luminescence response would be rather insensitive to size makes the BN nanosheets ideal candidates for lasing optical devices in the UV regime. The h-BN nanosheets are also better candidates for composite materials in high-temperature and hazardous environments.

Selective Growth of α‐Al2O3 Nanowires and Nanobelts

We report the selective growth of α‐Al2O3 nanowires and nanobelts via a catalyst‐free chemical vapor deposition process under ambient pressure. By controlling the flow rates of the carrier gas, high‐yield production of uniform alumina nanowires with diameter distribution (100 nm–200 nm) was achieved at a high growth rate over 200 μm/hour. Alumina nanobelts with variable width were also synthesized by modulating the carrier gas purge process. Further, the effects of temperatures and carrier gas flow rates on the growth of alumina nanostructures were also investigated. Oxygen partial pressure and supersaturation level of the aluminum suboxide are thought to be important factors in the formation process of the alumina nanowires or nanobelts. The typical growth of the alumina nanowires and nanobelts can be ascribed to vapor‐solid (VS) mechanism.

Toward the Innovative Synthesis of Columnar CeO2 Nanostructures

We report on the preparation of supported columnar CeO(2) nanostructures by a simple catalyst-free chemical vapor deposition process at temperatures as low as 623 K. A suitable choice of experimental parameters enables us to control the structural and morphological features of the resulting ceria nanosystems.