oa Morphology, Photoluminescence and Photovoltaic Properties of Laser Processed ZnO/carbon Nanotube Nanohybrids

One-dimensional nanoscale materials continue to attract much attention, not only for a better understanding of the physical properties at low dimensionality, but also for their potential in nanodevice applications. Carbon nanotubes (CNT) are of particular interest because of their unique molecular geometry and of their excellent electronic, thermal, and mechanical properties. Various carbon nanostructures have been successfully used as templates for the growth of novel hybrid nanomaterials, exhibiting highly interesting and unprecedented properties. These nanohybrids mainly consist of carbon nanostructures (mostly nanotubes) decorated by nanostructures of either metallic or semiconductor materials such as Au, Pt, TiO2, ZnO or SnO2. They are often obtained via various conventional chemical processing or through chemical functionalization approaches. In particular, nanohybrids consisting of carbon nanostructures decorated with ZnO nanoparticles have been shown to be promising for applications such as photocatalysts, field emitters, solar cells, and electro-photonic nanodevices. Here we report the successful growth of zinc oxide (ZnO)/single walled carbon-nanotubes (SWCNTs) nanohybrids using a two-step laser process. First, an ultraviolet (UV) excimer laser (ArF, λ = 193 nm) was used to grow SWCNTs using the UV-laser ablation method. Second, ZnO nanostructures were grown onto the SWCNTs by means of the CO2 laser-induced chemical liquid deposition technique (LICLD). High resolution transmission electron microscopy (HRTEM) revealed that the SWCNTs mainly consist of nanotubes featuring a high aspect ratio (diameter around 1.2 nm and length of up to several microns), while the ZnO nanostructures consisted of various morphologies, including nanorods, polypods, and nanoparticles sometimes with a size as small as 2 nm. On the other hand, the x-ray photoelectron spectroscopy (XPS) spectrum of the as-prepared ZnO/SWCNT sample showed clearly core level peaks of Zn, O and C, while the high-resolution XPS C 1s peak at 284.5 eV was attributed to the graphitic carbon C–C bonds abundantly present in the SWCNTs. The O 1s peak at 531 eV was attributed to O2– in the ZnO crystal lattice (i.e., O–Zn bonds), and the strong peak at 1022 eV could be attributed to Zn²⁺ (i.e., Zn–O bonds in the ZnO crystal). The observed peaks at 286.2 eV and 290 eV are considered to originate from the C–OH and O–C–O groups, respectively, and the one at about 533 eV was attributed to surface O–C groups. The presence of oxygen components in the high-resolution XPS C 1s spectrum and the presence of carbon components in the O 1s spectrum suggest that oxygen is directly bonded to the SWCNTs structure through the formation of strong covalent bonds between carbon and oxygen atoms, especially when no Zn–C bonding has been detected. However, these XPS data, along with the microscopy results, highly suggest that the growth of ZnO nanocrystals takes place directly on the walls of the SWCNTs through the formation of –Zn–O–C– bonds, as also reported in the case of other metal oxide/CNT composite materials [1, 2]. The ZnO/SWCNTs nanohybrids were found to exhibit a polychromatic photoluminescent (PL) emission, at room temperature, comprising a narrow near-UV band centered around 390 nm, a broad visible to near infrared band (500–900 nm), and a relatively weak emission band centered around 1000 nm. These PL results are compared to those of individual components (SWCNT and ZnO) and discussed in terms of carbon defect density and charge transfer between the ZnO nanocrystals and the carbon nanotubes. In fact, visible PL of the SWCNT is believed to originate from the combination (or at least one) of the three following phenomena: (i) the presence of short single-walled nanotubes which are known to generate a visible PL, (ii) multiple radiative transitions between energy states of the Van Hove singularities (VHS) of the carbon nanotubes, and (iii) the presence of amorphous and/or disordered sp2 carbon which is known to exhibit broad band visible PL. The PL spectrum of the nanohybrids basically combines the emission features of the separate components (i.e. SWCNT and ZnO) with, however, three main differences. Firstly, the PL peak due to the ZnO nanostructures slightly red-shifts to ∼410 nm, while exhibiting some peak broadening (FWHM of ∼55 nm). Secondly, its intensity was drastically diminished by a factor of about 100 (in comparison with that of ZnO alone). Finally, the relatively broad PL peak centered at ∼1000 nm is more defined and slightly increased in intensity. Thus, the observed quenching of the ZnO PL emission when laser-deposited onto the SWCNTs is believed to be due to charge transfer of photoexcited electrons from ZnO nanocrystals to the empty electronic states of the SWCNTs and/or to partial re-absorption into the complex hybrid structures (i.e., lattice distortions and defective nanostructures acting as non-radiative recombination centers). Quenching and red-shifting of the ZnO near band edge (NBE) in UV-excited PL has already been observed but to a lesser extent in electrochemically grown ZnO/CNT deposits [3]. Such charge exchange is highly promising for photovoltaic PV applications, where the photocurrent generated into such hybrid nanomaterials can be collected whenever the underlying SWCNTs network is appropriately deposited and electrically connected. As a matter of fact, the possibility of photocurrent generation by ZnO nanoparticles anchored to chemically functionalized carbon nanotubes in a photo electrochemical cell has been recently reported [4]. In sum, this clear indication of charge transfer occurring between ZnO nanostructures and SWCNTs is paving the way towards the development of novel ZnO/SWCNTs nanohybrids-based photovoltaic devices.

References:

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[2] M. Liu, Y. Yang, T. Zhu and Z. Liu: Chemical modification of single-walled carbon nanotubes with peroxytrifluoroacetic acid. Carbon 43, 1470 (2005).

[3] R. Zhang, L. Fan, Y. Fang, and S. Yang: Electrochemical route to the preparation of highly dispersed composites of ZnO/carbon nanotubes with significantly enhanced electrochemiluminescence from ZnO. J. Mater. Chem. 18, 4964 (2008).

[4] F. Vietmeyer, B. Seger, P. V. Kamat: Anchoring ZnO Particles on Functionalized Single Wall Carbon Nanotubes. Excited State Interactions and Charge Collection. Adv. Mater. 19, 2935 (2007).