Water-assisted growth of aligned carbon nanotube-ZnO heterojunction arrays

1D heterojunction structures based on nanomaterials are of significance to both scientific fundamentals in nanoscience and potential applications in nanoscale systems, including various new electronic and photonic nanodevices. Consequently, considerable effort has been made in recent years to devise and characterize various heterojunctions between different low-dimensional nanomaterials. Examples include the syntheses of doped GaN-based core/shell/shell (n-GaN/InGaN/p-GaN) nanowire heterostructures through metal-organic chemical vapor deposition (MOCVD); aligned ZnO heterojunction arrays on GaN, Al0.5Ga0.5N, and AlN substrates by a vapor–liquid–solid phase process using gold as a catalyst; multiwalled CNT–zinc sulfide (CNT: carbon nanotube) heterojunctions by a combination of ultrasonic and heat treatments; CNT–silicon nanowire heterojunctions by localizing a suitable metal catalyst at the end of a preformed nanotube or nanowire; and SWNT–gold (SWNT: singlewalled carbon nanotube) nanorod heterojunctions through the selective solution growth of Au nanorods on an SWNT structure. Among them, CNT-based 1D heterojunctions are of particular interest because of the unique molecular geometry as well as excellent electronic, thermal, and mechanical properties intrinsically associated with CNTs. On the other hand, ZnO nanostructures are shown to possess superior optoelectronic properties useful in nanoscale transistors, sensors, electron emitters, and many other systems. The large-scale formation of heterojunctions of ZnO with other nanomaterials (particularly CNTs) should extend the scope of ZnO nanomaterials for potential applications. Although recent work has demonstrated the large-scale growth of aligned ZnO nanowire arrays, the formation of large-area ordered heterojunctions of ZnO nanostructures with other semiconducting materials has been much less discussed in the literature. In particular, the growth of aligned CNT–ZnO heterojunction arrays remains a big challenge. Here, we report the first synthesis of large-scale aligned CNT–ZnO heterojunctions simply by water-assisted chemical vapor deposition (WACVD) of carbon on a zinc foil, which acts as both the substrate for the CNT growth and zinc source for the formation of the ZnO nanostructures. Water, as a weak oxidizer, provides the oxygen source for the formation of ZnO nanostructures and also enhances CNT growth. This cost-effective and efficient approach requires no other catalytic reagent and allows the large-scale formation of heterojunctions between the perpendicularly aligned CNTs and ZnO nanostructures through a natural contact. As we shall see later, the intimately connected heterojunctions between the aligned CNTs and ZnO nanostructures thus prepared possess interesting optoelectronic properties attractive for many potential applications, including their use as electro-optic sensors and field emitters. Briefly, the formation of the aligned CNT and ZnO heterojunctions was achieved by carrying out pyrolysis of C2H2 on a zinc foil supported by a quartz glass plate in a tube furnace at 850 °C under a combined flow of Ar and H2, with a portion of the carrier gas passing through a water bubbler, followed by annealing with a flow of Ar via the water bubbler passing through the reaction system. Figure 1a shows a low-magnification scanning electron microscopy (SEM) image of the assynthesized CNT–ZnO sample, revealing a large-scale aligned array. Although the corresponding SEM images under higher magnification in Figure 1b and c show some misalignment at the top of the nanotube array, Figure 1b clearly shows the CNT–ZnO heterojunction with a ZnO crystal (bright spot) attached on the tip of each of the constituent aligned CNTs. Figure 1c further reveals that approximately 40 % of the ZnO nanoparticles are of hexagonal morphology. The corresponding transmission electron microscopy (TEM) image in Figure 1d shows that the CNT thus prepared is multiwalled, and that the ZnO nanoparticle is intimately connected to the aligned CNT structure. The multiwalled CNT with an inner diameter of approximately 30 nm and outer diameter of approximately 60 nm possesses a bamboolike structured hollow core, similar to those prepared by solvothermal reduction of ethanol in the presence of Mg or pyrolysis of FePc. Figure 1e shows the selected-area electron diffraction (SAED) pattern of the ZnO nanoparticle, which indicates that the asprepared ZnO nanoparticle is single crystalline and can be indexed as hexagonal wurtzite ZnO. The aligned CNT–ZnO heterojunctions thus prepared were further investigated using energy-dispersive X-ray spectroscopy (EDS) in a scanning electron microscope. As expected, the line-scanning EDS profiles for Zn and C clearly show a dramatic decrease in the Zn (and corresponding increase in C) content along the scanning C O M M U N IC A TI O N S