Controlled synthesis of carbon nanostructures using aligned ZnO nanorods as templates
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.
References (41)
- et al.
Room temperature ammonia sensors based on zinc oxide and functionalized graphite and multi-walled carbon nanotubes
Sens Actuators B Chem
(2011) - et al.
Detection of O3 and NH3 using tin dioxide/carbon nanotubes based sensors: influence of carbon nanotubes properties onto sensor’s sensitivity
Procedia Engineering
(2010) - et al.
Pt-modified carbon nanotube networked layers for enhanced gas microsensors
Thin Solid Films
(2011) - et al.
Functional characterization of carbon nanotube networked films functionalized with tuned loading of Au nanoclusters for gas sensing applications
Sens Actuators B Chem
(2009) - et al.
ZnO decorated luminescent graphene as a potential gas sensor at room temperature
Carbon
(2012) - et al.
A novel, substrate independent three-step process for the growth of uniform ZnO nanorod arrays
Thin Solid Films
(2010) - et al.
XPS characterisation of plasma treated and zinc oxide coated PET
Appl Surf Sci
(2009) - et al.
The microstructure and cathodoluminescence characteristics of sputtered Zn2SiO4:Ti phosphor thin films
Thin Solid Films
(2007) - et al.
X-ray photoelectron spectroscopy and Auger electron spectroscopy studies of Al-doped ZnO films
Appl Surf Sci
(2000) - et al.
Structural, electrical and optical properties of zinc nitride thin films prepared by reactive rf magnetron sputtering
Thin Solid Films
(1998)