Seedless growth of zinc oxide flower-shaped structures on multilayer graphene by electrochemical deposition
© Aziz et al.; licensee Springer. 2014
Received: 7 March 2014
Accepted: 23 June 2014
Published: 5 July 2014
A seedless growth of zinc oxide (ZnO) structures on multilayer (ML) graphene by electrochemical deposition without any pre-deposited ZnO seed layer or metal catalyst was studied. A high density of a mixture of vertically aligned/non-aligned ZnO rods and flower-shaped structures was obtained. ML graphene seems to generate the formation of flower-shaped structures due to the stacking boundaries. The nucleation of ZnO seems to be promoted at the stacking edges of ML graphene with the increase of applied current density, resulting in the formation of flower-shaped structures. The diameters of the rods/flower-shaped structures also increase with the applied current density. ZnO rods/flower-shaped structures with high aspect ratio over 5.0 and good crystallinity were obtained at the applied current densities of −0.5 and −1.0 mA/cm2. The growth mechanism was proposed. The growth involves the formation of ZnO nucleation below 80°C and the enhancement of the growth of vertically non-aligned rods and flower-shaped structures at 80°C. Such ZnO/graphene hybrid structure provides several potential applications in sensing devices.
In recent years, the concept of advanced heterogeneous integration on silicon (Si) platform has attracted much attention towards the realization of a ‘More than Moore’ technology . To realize such technology, the growth of high-quality elements (i.e., germanium (Ge) ) compound semiconductors (i.e., gallium arsenide (GaAs) , gallium nitride (GaN) , silicon carbide (SiC) ), metal oxides (i.e., zinc oxide (ZnO) ), and carbon-based materials (i.e., graphene , carbon nanotube (CNT) ) on Si platform is highly required. The co-integration of these materials enables the present ultra-large-scale integrated circuits (ULSIs) to be facilitated not only with ultra-high speed complementary metal-oxide semiconductor (CMOS) transistors and novel transistors  but also with various kinds of functional devices, such as optical devices , photodetectors , solar batteries , and sensors [13, 14]. Such intelligent system-on-chip (i-SoC) on Si is considered as a promising and practical direction.
ZnO is a promising candidate for the fabrication of several kinds of devices due to its unique properties such as wide bandgap and large exciton energy. In order to fabricate ZnO-based devices on Si substrate, it is necessary to electronically isolate both materials using an insulator such as silicon dioxide (SiO2). Therefore, a breakthrough on the growth technology is strongly required to realize a high-quality ZnO-on-insulator structure with excellent crystallinity since the insulator is amorphous and the lattice mismatch is relatively large. There are several reports on the growth of ZnO nanostructures on insulators such as SiO2[15, 16], but the densities of the grown ZnO nanostructures were very low. Therefore, the ZnO seed layer is commonly used as the nucleation site to enable the subsequent growth of ZnO nanostructures on insulators [17–20].
Graphene is a two-dimensional hexagonal network of carbon atoms which is formed by making strong triangular σ-bonds of the sp2 hybridized orbitals. Since the bonding structure of graphene is similar to the C plane of the hexagonal crystalline structure of ZnO, it seems to be feasible for graphene to serve as an excellent template layer for the growth of high-density ZnO nanostructures on the insulator. In addition, since graphene is an excellent conductor and transparent material, the hybrid structure of a ZnO nanostructure and graphene shall lead to several device applications not only on Si substrate but also on other insulating substrates such as glass and flexible plastic. For examples, such hybrid structure can be used for sensing devices , ultraviolet (UV) photodetectors , solar cells , hybrid electrodes for GaN light-emitting diodes (LEDs) , etc.
There are several potential methods to grow ZnO on graphene which can be categorized into vapor phase and liquid phase methods. Vapor phase method is likely to involve a high-temperature process and is also considered as a high-cost method . 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 temperatures as low as 450°C [26, 27]. 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.
To our knowledge, only two methods have been reported on the growth of seedless ZnO nanostructures on graphene via low-temperature liquid phase method. The term ‘seedless’ refers to the omission of pre-deposition of the ZnO seed layer by other processes and metal catalysts. 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 [28, 29]. 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 actual growth. They also reported that the diameter, length, and morphology of the nanostructures showed significant dependencies on the growth parameters such as current density, precursor concentration, and growth time. Several other reports also indicated that current density plays an important role in inducing the growth of ZnO nanostructures on the seedless substrate [30, 31]. Recently, Aziz et al. reported the electrodeposition of highly dense ZnO nanorods on single-layer (SL) graphene . Furthermore, the distance between the electrodes and the molarity of electrolyte are also able to give significant effects on the properties of the resulting nanostructures . Generally, a change in distance between the two electrodes can change the rate of the electrolysis reaction due to the change in the level of current density. The shorter the distance between the electrodes, the higher the electric field and thus the higher current density will be applied . In this paper, we report the seedless growth of highly dense ZnO flower-shaped structures on multilayer (ML) graphene by a single-step cathodic electrochemical deposition method.
The growth of ZnO structures on graphene/SiO2/Si was carried out by a cathodic electrochemical deposition in a mixture of 50 mM of zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O; Sigma-Aldrich, St. Louis, MO, USA; ≥99.0% purity) and hexamethylenetetramine (HMTA, C6H12N4, Sigma-Aldrich, ≥99.0% purity). As shown in Figure 1d, platinum (Pt) wire acted as an anode (counter electrode) while graphene acted as a cathode. Both anode and cathode were connected to the external direct current (DC) power supply. In this experiment, the electrodeposition was operated under galvanostatic control where the current density was fixed during the deposition. It is noted here that the distance between the two electrodes was fixed at 4 cm for all experiments in order to avoid the other possible effects apart from the current density. The current densities of −0.1, −0.5, −1.0, −1.5, and −2.0 mA/cm2 were applied. All experiments were done by inserting the sample into the electrolyte from the beginning of the process or before the electrolyte was heated up from room temperature (RT) to 80°C. The actual growth was done for 1 h, counted when the electrolyte temperature reached 80°C or the set temperature (ST). Such temperature was chosen since the effective reaction of zinc nitrate and HMTA takes place at temperatures above 80°C. As reported by Kim et al., the activation energy to start the nucleation of ZnO cannot be achieved at temperatures below 50°C in such electrolyte . After 1 h, the sample was removed immediately from the electrolyte and quickly rinsed with deionized (DI) water to remove any residue from the surface. The time chart of the growth is shown in Figure 1e.
The surface morphology, elemental composition, crystallinity, and optical properties of the grown ZnO structures were characterized using field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffractometer (XRD), and photoluminescence (PL) spectroscopy with excitation at 325 nm of a He-Cd laser, respectively.
Results and discussion
It can be seen that the morphology of the grown ZnO at −0.1 mA/cm2 shows the formation of ZnO clusters. As the current density is changed from −0.5 to 2.0 mA/cm2, the morphology shows the mixture of vertically aligned/non-aligned ZnO rods and flower-shaped structures and their diameters or sizes increase with the current density. The formation of flower-shaped structures seems to be promoted by the stacking structures of ML graphene. This formation of flower-shaped structures was not observed for the growth of ZnO nanorods on oxidized bilayer graphene and SL graphene as reported by Xu et al. and Aziz et al., respectively [29, 30]. The proposed growth mechanism is described in the next section.
Density, diameter, length and aspect ratio of the grown ZnO rods
Current density (mA/cm2)
Diameter of rods (nm)
Length of rods (nm)
7.95 × 108
170 to 240
810 to 1,220
7.11 × 108
240 to 360
1,120 to 1,990
1.67 × 108
900 to 1,160
400 to 840
4.18 × 107
1,470 to 1,940
520 to 1,020
3.00 × 107
5.83 × 108
370 to 780
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
Figure 3b shows the RT PL spectra of ZnO structures grown at different current densities. Here, two distinct emission bands were observed. The first band located in the UV region was estimated to be around 379, 385, 392, 395, and 389 nm for samples at current densities of −0.1, −0.5, −1.0, −1.5, and −2.0 mA/cm2, respectively. This band is claimed to be due to the near-band edge (NBE) emission or the recombinations of free excitons through an exciton-exciton collision process [6, 29]. The second band appears in the green region of the visible spectrum at approximately 576, 574, 569, 563, and 569 nm, respectively. This band is commonly referred to as a deep-level or trap-state emission. Some researchers suggested that it could be attributed to the recombination of photogenerated holes with single ionized charge states of specific defects such as O vacancies or Zn interstitials [6, 31, 35]. However, Kang et al. reported that the singly ionized oxygen vacancy is responsible for the green emission and not the ionized Zn interstitials . It is needed to be proved by post-annealing process of samples. Besides, the intensity of the peak also indicates the level of defects in the samples . Surface state has also been identified as a possible cause of the visible emission in ZnO nanomaterials .
There are several reports discussing the relationship of these emission peaks with the quality of the grown structures. As been reported by Djurišić and Leung, the intensity of UV emission is dependent on the nanostructure size . Below a certain size, the luminescence properties of the ZnO nanostructure should be dominated by the properties of the surface. The samples grown at current densities of −0.5 and −1.0 mA/cm2 show highly intense UV emission with the highest aspect ratio (Table 1) compared to other samples. Highly intense UV emission seems to show higher crystallinity and more perfection in surface states as reported by Park et al. . Chen et al. suggested that it may imply a good crystal surface . The enhancement of UV emission is attributed to a larger surface area and fewer defects . Furthermore, the narrow peak with high intensity of NBE emission as well as a decrease in the peak density of green emission may indicate a high crystallinity of the grown structure [31, 42]. It is also due to the vertical growth of ZnO rods and their high surface areas as suggested by Xu et al. . The calculated ratio of the intensity of UV emission to the intensity of green emission, IUV/IVisible, obtained in this work is shown in Figure 3b (inset). As a comparison, the results obtained for the electrodeposition on SL graphene  were also plotted in the same figure. It can be seen that both spectra show a similar tendency. It can be seen that the sample grown on ML graphene at a current density of −1.0 mA/cm2 shows the highest value of 1.6 which seems to indicate the optimum current density for this work. The sample grown at a current density of −2.0 mA/cm2 shows the highest green emission compared to the other samples or the lowest IUV/IVisible value, which indicates that there may be more defects induced during the growth such as O vacancies . Ahn et al. reported that the sensitivity of gas sensing increases linearly with the sample having high green emission intensity or, in other words, with the structure having large defect density . Therefore, it seems to suggest that the sample with large structural defect also has several interesting applications.
In summary, the growth processes involve two main stages which are the formation of seed structure for nucleation sites of rods and flower-shaped structures below the ST point and the effective growth of non-aligned/aligned rods and flower-shaped structures after the ST point. These structures start to grow according to the shape of initial seed structures. Again, as proved by the FESEM images, the vertically aligned/non-aligned rods and flower-shaped structures are not growing directly on the graphene, but they are growing on the nucleation sites formed during the preheated process, i.e., below the ST point.
In conclusion, seedless growth of highly dense vertically aligned/non-aligned ZnO rods and flower-shaped structures on ML graphene by electrochemical deposition was obtained. The applied current in the electrochemical system plays an important role in inducing the growth of ZnO structures on ML graphene as well as in controlling the shape, diameter, and density of structures. ML graphene seems to generate the formation of flower-shaped structures due to the multistacking structures. Such ZnO/graphene hybrid structures seem to provide several potential applications in sensing devices, etc.
NSAA thanks Malaysia-Japan International Institute of Technology for the scholarship. This work was funded by Nippon Sheet Glass Corp., Hitachi Foundation, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Malaysia Ministry of Science, Technology and Innovation, and Malaysia Ministry of Education.
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