Yang-Chih Hsueh

Fabrication of Ag-loaded multi-walled TiO2 nanotube arrays and their photocatalytic activity

TiO2 multi-walled nanotube arrays (MWNTAs) were synthesized by atomic layer deposition (ALD) using anodic aluminum oxide (AAO) as the template. TiO2 and Al2O3 layers were alternately deposited into the AAO pores by ALD. The tube thickness and the gap span were controlled by the ALD cycle numbers of TiO2 and Al2O3, respectively. A MWNT composed of two or three concentric tubes with different diameters was formed after removal of Al2O3. Silver nanoparticles were then deposited on both inner and outer surfaces of the tubes by photochemical reduction. Degradation of methylene blue was carried out to evaluate the photocatalytic activity of bare and silver-loaded TiO2 MWNTAs. It was found that the amount of TiO2, the reaction surface area, the crystalline phase, and silver modification were the factors to determine the photocatalytic activity. The triple-walled MWNTAs showed the best performance, and Ag loading further enhanced the activity.

Fabrication of tin dioxide nanowires with ultrahigh gas sensitivity by atomic layer deposition of platinum

Gas-sensing properties of SnO2 nanowires were investigated before and after their surface functionalization by the atomic layer deposition (ALD) of Pt nanoparticles. The morphology, size, and concentration of Pt particles on SnO2 nanowires can be controlled by varying the number of ALD reaction cycles, and therefore, the gas-sensing properties of the nanowires can be altered via the Pt catalyst effect and the modification of Schottky barrier junctions on the nanowire surface in the vicinity of Pt nanoparticles. The Pt-decorated SnO2 nanowires obtained after 200 ALD reaction cycles exhibited an ultrahigh gas sensitivity (S = Ig/Ia) of ∼8400 to 500 ppm ethanol vapor at 200 °C. This provides an efficient route for strongly enhancing the gas sensitivity of semiconducting nanostructures and fabricating gas sensors that are highly sensitive and responsive.

Deposition of platinum on oxygen plasma treated carbon nanotubes by atomic layer deposition

Platinum nanoparticles were deposited on oxygen plasma treated carbon nanotubes (CNTs) by atomic layer deposition (ALD). The treatment time with oxygen plasma generated by microwaves under a power of 600 W varied from 5 to 20 s. The number of ALD cycles was controlled at 5-125. X-ray photoelectron spectroscopic analysis indicated that oxygen plasma can graft oxygen-containing functional groups to the CNT surface to act as nucleation sites for growth of Pt nanoparticles. Formation of very uniform and well distributed Pt nanoparticles of a size of 1.60-4.80 nm was achieved. The growth rate of Pt nanoparticles could be controlled by the number of ALD cycles and oxygen plasma treatment time. This offers a dry process to deposit well-dispersed metallic nanoparticles on selected support materials.

Atomic Layer Deposition of Zinc Oxide on Multiwalled Carbon Nanotubes for UV Photodetector Applications

A novel process for the fabrication of ZnO-carbon nanotubes (CNT) nanocomposites with high uniformity by atomic layer deposition (ALD) of ZnO on multiwalled carbon nanotubes was reported, and their applications in UV photodetectors were investigated. Two types of photodetectors, p- and n-type, were developed by alternating the ALD reaction cycles. In addition, a schematic model was proposed to explain the p- to n-type conversion of the ZnO-CNTs, which accounts for the amount and surface coverage of ZnO on CNTs.

Deposition of uniform Pt nanoparticles with controllable size on TiO2-based nanowires by atomic layer deposition and their photocatalytic properties.

TiO2-based nanowires were prepared by a hydrothermal method, and Pt nanoparticles were deposited on the nanowires by atomic layer deposition (ALD). The size and loading of Pt nanoparticles with very good uniformity could be controlled by the ALD cycle number, along with acid or water treatment of the nanowires. The lower growth rate and the loading of Pt nanoparticles were obtained with water treatment. With acid treatment, the growth rate and loading were increased, and this increase was attributed to the formation of higher density of hydroxide groups on the nanowires. The photocatalytic activities of Pt-deposited nanowires were investigated. The deposition of Pt on the nanowires resulted in both enhanced light absorption in the visible region and electron–hole separation efficiency. The water-treated nanowires exhibited the highest degradation rate of rhodamine B and the highest hydrogen evolution rate. A maximum amount of hydrogen evolution, 13.8 mmol g−1 in 6 h, was achieved.

Enhancing the Photon-Sensing Properties of ZnO Nanowires by Atomic Layer Deposition of Platinum

Platinum nanoparticles were uniformily deposited on ZnO nanowires for photon-sensing applications. The morphology, size, and concentration of Pt particles on the ZnO nanowires can be controlled by varying the number of atomic layer deposition reaction cycles. The Pt-decorated ZnO nanowires exhibit much faster photon response and recovery speeds than the pristine ZnO nanowires. This provides an efficient route for strongly enhancing the photon-sensing properties of nanostructured metal oxides and fabricating photodetectors with fast response and recovery speeds.

Mesoporous TiO2/WO3 hollow fibers with interior interconnected nanotubes for photocatalytic application

Mesoporous TiO2/WO3 hollow fibers with interconnected nanotubes were prepared by a sol–gel method using polysulfone hollow fibers as a framework template and a block copolymer as an internal template. Polysulfone fibers were immersed in a sol–gel solution and the oxides were deposited on the fibers by vacuum impregnation. The polysulfone and copolymer were removed by thermal treatment at 500 °C for 3 h to form the mesoporous hollow fibers. They preserved the shape and morphology of the polysulfone template after heat treatment. The structure and optical properties of the fibers were characterized, and the performance of the fibers for photocatalytic reaction of methylene blue was investigated. The mesoporous TiO2/WO3 hollow fibers possessed higher surface area (∼130 m2 g−1) and lower band gap energy than pure TiO2 fibers. They exhibited better photodegradation efficiency of methylene blue than TiO2 and WO3 fibers and P25 powder.

Preparation of Pt / SnO2 Core–Shell Nanowires with Enhanced Ethanol Gas- and Photon-Sensing Properties

Pt/SnO 2 core-shell nanowires were prepared by a process that involves thermal evaporation of SnO 2 nanowires and subsequent atomic layer deposition of Pt, and their gas- and photon-sensing properties were investigated systematically. Transmission electron microscopy analysis showed that the Pt shell with a thickness of 8.8 nm is composed of numerous Pt nanoparticles with average diameters of ∼5.3 nm. The Pt/SnO 2 core-shell nanowires with high electrical conductance exhibit remarkably enhanced ethanol gas- and photon-sensing properties due to the surface functionalization resulting from the effective Pt catalyst and the formation of hetero-nanostructures fabricated by the SnO 2 core and Pt shell.

TiO2 hollow fibers with internal interconnected nanotubes prepared by atomic layer deposition for improved photocatalytic activity

TiO2 hollow fibers with internal interconnected nanotubes were prepared by atomic layer deposition using a polysulfone–polyvinylpyrrolidone hollow fiber membrane as the template. The interconnected nanotubes form a titanium oxide network containing the dual characteristics of nano-sized pores and channels with adjustable spaces by the cycle number of the atomic layer deposition, and the titanium oxide network further constructs a centimeter-long hollow fiber. This unique macro–nano composite structure enables us to construct a large scale nanoreactor, which has been demonstrated to possess a higher performance (∼33%) for photo-degrading the organic water contaminants in a continuous flow than conventional P25 TiO2 nanopowder under the same test conditions.

Growth of silica nanowires in vacuum

Silica nanowires were grown on a Pt-coated Si substrate in flowing Ar, dynamic vacuum, and sealed vacuum tubes. The amount of oxygen in the reaction chamber or tubes directly influenced the composition and morphology of the nanowires. The formation of nanowires assisted by Pt can be explained by the VLS mechanism and active oxidation of silicon. In flowing Ar and dynamic vacuum, the dimensions of the nanowires increased with the partial pressure of oxygen. For nanowires grown in sealed tubes, the nanowires became much shorter which can be ascribed to the limited amount of residual oxygen in the tube. Therefore, the growth of silica nanowires is determined by a critical concentration of oxygen, below which the growth of nanowires is suspended.