Seed/catalyst-free vertical growth of high-density electrodeposited zinc oxide nanostructures on a single-layer graphene
© Aziz et al.; licensee Springer. 2014
Received: 29 December 2013
Accepted: 10 February 2014
Published: 26 February 2014
We report the seed/catalyst-free vertical growth of high-density electrodeposited ZnO nanostructures on a single-layer graphene. The absence of hexamethylenetetramine (HMTA) and heat has resulted in the formation of nanoflake-like ZnO structure. The results show that HMTA and heat are needed to promote the formation of hexagonal ZnO nanostructures. The applied current density plays important role in inducing the growth of ZnO on graphene as well as in controlling the shape, size, and density of ZnO nanostructures. High density of vertically aligned ZnO nanorods comparable to other methods was obtained. The quality of the ZnO nanostructures also depended strongly on the applied current density. The growth mechanism was proposed. According to the growth timing chart, the growth seems to involve two stages which are the formation of ZnO nucleation and the enhancement of the vertical growth of nanorods. ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics such as photovoltaic devices, sensing devices, optical devices, and photodetectors.
In recent years, graphene/semiconductor nanocrystal hybrid structure is particularly interesting because nanostructures, such as nanowires, nanorods, nanoneedles, nanosheets and nanowalls, can offer additional functionality to graphene for realizing advanced nanoscale electronics and optoelectronic applications in photovoltaics, nanogenerators, field emission devices, sensitive biological and chemical sensors, and efficient energy conversion and storage devices [1–5]. This is due to their high aspect ratio, high thermal and mechanical stability, extremely large surface-to-volume ratio, and high porosity [6–9]. Graphene has a great potential for novel electronic devices because of their extraordinary electrical, thermal, and mechanical properties, including a carrier mobility exceeding 104 cm2/Vs and a thermal conductivity of 103 W/mK [10–13]. Therefore, with the excellent electrical and thermal characteristics of graphene layers, growing semiconductor nanostructures and thin films on graphene layers would enable their novel physical properties to be exploited in diverse sophisticated device applications. Recently, several graphene/semiconductor nanocrystals have been successfully synthesized that show desirable combinations of these properties not found in the individual components. One-dimensional zinc oxide (ZnO) semiconducting nanostructures are considered to be important multifunctional building blocks for fabricating various nanodevices [14, 15]. Since graphene is an excellent conductor and a transparent material, the hybrid structure of ZnO/graphene shall lead to several device applications not only on silicon (Si) substrate but also on other insulating substrates such as glass and flexible plastic. Owing to the unique electronic and optical properties of ZnO nanostructures, such hybrid structure can be used for sensing devices [16, 17], ultraviolet (UV) photodetectors , solar cells , and light-emitting diodes (LED) .
There are several potential methods to grow ZnO on graphene which can be categorized into vapor-phase and liquid-phase methods. The vapor phase method is likely to involve high-temperature process and is also considered as a high-cost method [2, 21]. Also, since the process requires oxygen (O2), the possibility of graphene to be oxidized or etched out during the growth is high since the oxidation of graphene is likely to occur at temperature as low as 450°C . The liquid-phase method seems to be a promising method to grow graphene at low temperature with good controllability in terms of growth rates and structure dimensions.
Up to date, only two methods have been reported on the growth of seed/catalyst-free ZnO nanostructure on graphene via low-temperature liquid-phase method. Kim et al. reported the growth of ZnO nanorods on graphene without any seed layer by hydrothermal method, but the obtained results show low density of nanostructures . Xu et al. reported the seedless growth of ZnO nanotubes and nanorods on graphene by electrochemical deposition [24, 25]. They reported the growth of highly dense ZnO nanostructures by using solely zinc nitrate as the electrolyte with the introduction of oxidation process of graphene prior to the actual growth. In this paper, we report the seed/catalyst-free vertical growth of ZnO nanostructures on graphene by a single-step cathodic electrochemical deposition method. The term ‘seed/catalyst-free’ refers to the omission of predeposition of ZnO seed layer and any kind of catalyst by other processes. A highly dense vertically aligned ZnO nanostructure on a single-layer (SL) graphene was successfully grown.
The surface morphology, elemental composition, crystallinity, and optical properties of the grown ZnO nanostructures were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffractometer (XRD), and photoluminescence (PL) spectroscopy with excitation at 325 nm of He-Cd laser, respectively.
Results and discussion
Density, diameter, length, and average aspect ratio of the grown ZnO nanorods
Current density (mA/cm2)
Diameter of nanorods (nm)
Length of nanorods (nm)
Average aspect ratio
1.84 × 107
190 to 450
450 to 1,160
1.37 × 109
260 to 480
840 to 1,160
1.24 × 108
660 to 1,000
150 to 340
3.42 × 107
950 to 1,330
200 to 560
2.32 × 107
570 to 2,030
1,160 to 2,220
370 to 780
The optical characteristics of the ZnO nanostructures were investigated using RT PL spectroscopy. Figure 4b shows the PL spectra of the ZnO nanostructures deposited on the graphene layers at different current densities. Each RT PL spectrum shows one distinct near-band-edge (NBE) emission peak at 3.210, 3.210, 3.200, 3.200, and 3.080 eV for samples grown at current densities of -0.1, -0.5, -1.0, -1.5, and -2.0 mA/cm2, respectively. The full width at half maximum (FWHM) value was estimated to be around 0.20 to 0.37 eV. The strong, sharp NBE emission indicates the high optical quality of the ZnO nanostructures on the graphene layers. It was reported that the PL spectrum at 17 K typically shows five distinct NBE emission peaks with FWHM value of several milli-electron volt . However, only one of these emission peaks which is equal to 3.240 eV was observed in our room-temperature measurement. The other four peaks which tentatively attributed to neutral-donor bound exciton peaks and free exciton peak were not able to be observed. From the PL spectra, no additional exciton peak associated with carbon impurities in carbon-doped ZnO films  was observed at 3.356 eV. This suggests that the carbon atoms in the graphene were not incorporated into the ZnO nanorods during their growth. The PL characteristics of the ZnO nanostructures on the graphene layers were almost the same to those of the ZnO nanostructures on single-crystalline substrates such as Si [29, 30]. The second band appears in the green region of the visible spectrum at approximately 2.25 to 2.30 eV for the grown samples. The sample at the current density of -2.0 mA/cm2 shows the highest green emission than other samples which indicates that there are more defects such as large fraction of O vacancies that have been introduced during the growth process [27, 31–33]. The defects are speculated to exist in the seed layer which is formed during the initial growth stage. The observation of the NBE emission peak and weak green emission related to defects suggest high optical quality of the ZnO nanorods grown on the graphene layers. It can be said that the samples grown at −0.5 to −1.5 mA/cm2 seem to produce relatively high quality ZnO structures. The control of initial seed layer and further modification of growth procedure may improve the overall structure of ZnO.
Chemical reaction and growth mechanism
When HMTA was added into Zn (NO3)2 · 6H2O, no precipitation occurred as they are just mixed together initially. With the introduction of temperature, HMTA begins to decompose into ammonia and then Zn(OH)2 is produced. The complete decomposition is achieved by continuous heating [34, 35]. Finally, it produces ZnO and H2O with the presence of OH− and e−. HMTA acts as a weak base, slowly hydrolyzing in water and gradually releasing OH− ions . OH− ions are produced during the chemical reaction of HMTA with water as shown in Equations 5 and 6, while e− is obtained from the chemical reaction occurred at the anode as shown in Equation 7. The hydrolyzation of HMTA can be accelerated by increasing the pH of the electrolyte .
The vertically aligned nanorods are produced with the help of HMTA. HMTA is a long-chain polymer and a non-polar chelating agent . It will preferably attach to the non-polar facets of the zincite crystal, by cutting off the access of Zn2+ ions to the sides of the structure, leaving only the polar  face exposed to the Zn2+ ions for further nucleation and growth. Hence, HMTA acts as a non-ionic ligand chelate on the non-polar surface of ZnO nanocrystals on the six prismatic side planes of the wurtzite crystal and induces the growth in the c-axis . Therefore, HMTA acts more like a shape-inducing polymer surfactant rather than just a buffer .
In conclusion, high density vertically aligned ZnO nanorods has successfully been grown on a single-layer graphene by electrochemical deposition method using heated zinc nitrate hexahydrate and HMTA as the electrolyte. HMTA and heat play a significant role in promoting the formation of hexagonal ZnO nanostructures. The applied current in the electrochemical process plays an important role in inducing the growth of the ZnO nanostructures on the SL graphene as well as in controlling the shape, diameter, and density of the nanostructures. The control of the initial structures and further modification of growth procedure may improve the overall structure of ZnO.
NSAA thanks the Malaysia-Japan International Institute of Technology for the scholarship. This work was funded by the Nippon Sheet Glass Corp., Hitachi Foundation, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Malaysia Ministry of Science, Technology and Innovation, and the Malaysia Ministry of Education.
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