Elsevier

Carbon

Volume 50, Issue 15, December 2012, Pages 5472-5480
Carbon

Controlled synthesis of carbon nanostructures using aligned ZnO nanorods as templates

https://doi.org/10.1016/j.carbon.2012.07.034Get rights and content

Abstract

By combining in situ X-ray photoemission spectroscopy, ex situ high resolution transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy, we show that chemical vapor deposition (CVD) on vertically aligned ZnO nanorods can synthesize different carbon nanostructures (CNs), whose morphology is driven by the ZnO nanorods and whose dimensions and structures change as a function of the process temperature. The CNs range from amorphous carbon cups, completely covering the nanorods, to high density one-dimensional carbon nano-dendrites (CNDs), which start to appear like short hairs on the ZnO nanorods. The nanorods are partially etched when the process is done at 630–800 °C, while they are completely etched at temperatures higher than 800 °C. In the latter case, CNDs emerge from a porous carbon sponge formed at the substrate interface but they are preferentially aligned along the location of the pristine ZnO nanorods. When used as a chemiresisitor the CND–ZnO structures have a higher sensitivity to ammonia compared to chemiresistors made by bare ZnO nanorods, to other one-dimensional CNs, like carbon nanotubes or other metal/metal-oxides hybrid CNs.

Introduction

Novel synthesis techniques enable to control the architecture of many materials at the nanoscale level allowing the growth of different hybrid nanostructures suitable for various technological applications, which extend from sensors to field emission electrodes, from fuel cells up to supercapacitors. In fact, the combination of different materials within hybrid nanostructures is able to improve the device performances and may provide a new way for the modulation of electronic, chemical and structural properties [1], [2], [3]. Nanostructures of ZnO and carbon nanostructures, including carbon nanotubes (CNTs), which typically show exceptional qualities in themselves [4], [5], [6], [7], could achieve better performances when combined in CN/ZnO hybrid structures, further extending their possible practical applications [8], [9], [10], [11]. Actually, the growth of these materials is not yet well controlled and understood, and only few works are reported, mainly regarding ZnO grown on CNTs [8], [9], [10], [12]. On the other hand, aligned CNTs were grown without the use of catalysts on a ZnO foil by means of water assisted chemical vapor deposition (CVD) [13]. Recently amorphous tubular carbon caps were synthesized on ZnO nanorods (NRs) using a deposition–etching–evaporation process, and the resulting hybrid exhibited enhanced photosensing properties as compared to pristine ZnO NRs [11]. The carbon caps were due to the growth of a continuous film of amorphous carbon (3–10 nm thick) covering ZnO NRs. These carbon structures are preserved, with negligible changes, also after the complete etching at high temperature (∼700 °C) of ZnO NRs. These last results have opened up new research opportunities to find simple and cheap procedures able to fabricate different architectures of pure carbon materials with high control, possibly showing hierarchical arrangements by dint of ZnO nanostructures employed as a building template that can be removed. This is still an open and critical issue in the technological application field of carbon based materials that, if solved, would open new strategies for the production of electrodes for solar, fuel and electrochemical cells as well as biosensors [14].

Here we report on a simple, scalable, and inexpensive template-based synthesis process, which can be employed to produce hierarchical ZnO–C hybrids and nanostructured carbon. By combining in situ X-ray photoemission spectroscopy (XPS), ex situ high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and Raman spectroscopy, our data show that C2H2 CVD, done on vertically aligned ZnO NRs, can synthesize different carbon nanostructures (CNs), whose morphology is driven by the ZnO NRs and whose dimensions and structures change as a function of the CVD temperature. The CNs span from amorphous carbon cups, completely covering the ZnO NRs, to high-density one-dimensional carbon nano-dendrites (CNDs), which start to appear like short hairs on the pristine ZnO NRs. The NRs are partially etched when the process is performed at 630–800 °C, while they are completely etched at temperatures higher than 800 °C. In the latter case, high density CNDs preferentially aligned along the location of the pristine NRs are observed, which emerge from a high surface/volume ratio porous carbon sponge formed at the support interface. To support the effectiveness of the method and the practical importance of this particular system, we show that when used as chemiresistor the CND/ZnO structures have an higher sensitivity to ammonia (down to 2.3 ppm at room temperature and in atmospheric conditions), compared to chemiresistors made by bare ZnO NRs or one-dimensional CNs, like CNTs, and in general to other metal/metal-oxides hybrid carbon nanostructures [15], [16], [17], [18], [19], [20], [21], [22], [23]. It is rather interesting to observe that such low detection limits for ammonia are virtually unexplored, and sensitivities to low ammonia concentrations are usually achieved only by functionalization [22], leaving potential improvement perspectives for our pristine nanostructured carbon architectures.

Section snippets

ZnO nanorods synthesis

The ZnO NRs were grown on Si(1 1 0) wafers, where a ZnO film was deposited as seed for the nucleation of the NRs. The ZnO films were prepared by direct current magnetron sputtering at the University of Zululand, using an Orion-5 Sputtering System. The sputtering was done using a Zn target and a mixture of O and Ar (O–Ar gas ratio: 4/8) at 3 × 10−3 Torr. After the deposition the films were annealed at 400 °C for 2 h in air to further oxidize the film and to form nanoparticles. The ZnO NRs were then

CVD growth and sample characterization

The CVD was carried on vertically aligned ZnO NRs, which were synthesized using a hydrothermal process (see experimental section), which is a simple and inexpensive process enabling to have homogeneous samples [5]. The typical morphology of the NRs is shown in the SEM image of Fig. 1a.

The NRs are about 1 μm in length and are preferentially vertically aligned on the substrate. They are characterized by a pointed hexagonal shape at the tips, and have a large diameters distribution falling between

Conclusions

We have shown that CVD on nanostructured ZnO is able to synthesize different carbon nanostructures, whose morphology is driven by the ZnO morphology and whose dimensions and structures change as a function of the CVD temperature. The carbon nanostructures range from amorphous carbon cups, covering the ZnO NRs, to nanostructured porous one-dimensional CNDs, which are preferentially aligned along the directions of the pristine ZnO NRs, as schematically shown in Fig. 5. These systems, having

Acknowledgements

This work was supported by the European projects COST ACTION EuNetAir (n.:TD1105), the FVG regional project LR 47/78 (Prat.n. 1953), the National research foundation, South Africa and by the ICTP/IAEA STEP program. S.P. and L.S. wishes to acknowledge P. Galinetto (University of Pavia, Italy) for the access to Raman facilities.

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