Synthesis, optical and electrochemical properties of ZnO nanowires/graphene oxide heterostructures
© Zeng et al.; licensee Springer. 2013
Received: 1 February 2013
Accepted: 15 March 2013
Published: 22 March 2013
Large-scale vertically aligned ZnO nanowires with high crystal qualities were fabricated on thin graphene oxide films via a low temperature hydrothermal method. Room temperature photoluminescence results show that the ultraviolet emission of nanowires grown on graphene oxide films was greatly enhanced and the defect-related visible emission was suppressed, which can be attributed to the improved crystal quality and possible electron transfer between ZnO and graphene oxide. Electrochemical property measurement results demonstrated that the ZnO nanowires/graphene oxide have large integral area of cyclic voltammetry loop, indicating that such heterostructure is promising for application in supercapacitors.
ZnO nanowires (NWs) and graphene are two of the most widely studied nanomaterials; both of them are good candidates for the electrode materials of supercapacitors. Due to the extremely large surface areas and the pseudocapacitance rising from charge transfer between the electrolyte and electrode through fast Faraday redox reaction, ZnO NWs are promising in the application of supercapacitors [1, 2]. On the other hand, graphene has extremely high electron mobility, excellent rate capability, reversibility, and high chemical stability; it has improved electrochemical performance compared with other carbon family materials such as activated carbon, carbon nanotubes, etc. . Moreover, graphene oxide (GO) is considered to be a better choice for the electrodes of supercapacitors than graphene . However, both ZnO NWs and GO suffered from limitations in the real applications. For ZnO NWs, it exhibits low abundance and exhibit poor rate capability and reversibility during the charge/discharge process. For the GO, it is still limited by the low capacitance. Therefore, it is highly desirable for integrating these two materials together because both the double-layer capacitance of GO and pseudocapacitance of ZnO NWs can contribute to the total capacitive performances. Though a few reports have been found on the electrochemical properties of ZnO nanostructures/GO nanocomposites [5–8], however, research on the performance of vertically aligned ZnO NWs/GO heterostructures are very limited although much progress in the controllable synthesis of vertically aligned ZnO nanorods on GO or graphene has been made [9–12].
In this letter, vertically aligned ZnO NWs were grown on GO films using low-temperature hydrothermal method. The optical properties and electrochemical properties of the ZnO NWs/GO heterostructures were studied. Our results showed that the oxygen-containing groups on the surface of GO films can act as the nucleation sites and facilitate the vertical growth of ZnO NWs. Photoluminescence (PL) spectra demonstrated that the deep-level light emission of ZnO NWs grown on GO films were greatly suppressed. Electrochemical property measurement proved that the capacitance of the ZnO NWs/GO heterostructures were much larger than that of the single GO films or ZnO NWs, indicating that such a structure can indeed improve the performance of supercapacitors. Since ZnO NWs are widely studied as sensors, nanogenerators, etc. [13–15] and reduced GO is a good transparent electrode material, we believe that such ZnO NWs/GO heterostructures presented here will also have many other potential applications in all kinds of nanodevices.
GO film was synthesized via a modified Hummers method. The product was dispersed in deionized water by a Branson Digital Sonifier (S450D, 200W, 40%; Branson Ultrasonics Corporation, Danbury, CT, USA). A dialysis process was used to completely remove residual salts and acids. The purified GO were then dispersed in deionized water to form a homogenous suspension (weight percent: 0.05 wt.%). Subsequently, the GO suspension was drop-casted on the clean copper mesh. After drying, the GO films was used as the substrate for the subsequent hydrothermal growth of ZnO NWs. Equimolar solutions of hexamethylenetetramine (99.9%, Sigma-Aldrich, St. Louis, MO, USA) and zinc nitrate (Zn (NO3)2 · 6H2O) (99.9%, Sigma-Aldrich, St. Louis, MO, USA) were mixed thoroughly and transferred to polymer autoclaves to serve as the precursors. The hydrothermal reaction was carried out at 90°C for 6 h for growing ZnO NWs. After NW growth, the substrate was cleaned with deionized water and then dried at 60°C for 1 h. Finally, the ZnO NWs/GO heterostructure was peeled off from the copper mesh for characterization.
The microstructures of ZnO NWs were characterized by transmission electron microscopy (TEM, Tecnai G2, FEI, Hillsboro, OR, USA), X-ray diffraction (XRD, D8-ADVANCE, Bruker AXS, Inc., Madison, WI, USA) with 0.154 nm Cu Kα radiation, and Raman spectroscopy (laser wavelength 514 nm, via Reflex spectrometer, Renishaw, Wotton-under-Edge, UK). The morphologies of ZnO NWs were examined using a scanning electron microscope (SEM, Quanta FEG, FEI, Hillsboro, OR, USA). Room temperature PL spectra were obtained with a HORIBA Jobin Yvon Fluorolog-3 fluorescence spectrometer (HORIBA Process and Environmental, Les Ulis, France) with an excitation wavelength of 325 nm. A typical three-electrode experimental cell equipped with a working electrode, a platinum foil counter electrode, and a standard calomel reference electrode was used to measure the electrochemical properties. All electrochemical measurements were carried out in 0.10 M Na2SO4 electrolyte. The cyclic voltammetry (CV) curves were recorded on a CHI660B electrochemical working station (CH Instruments, Austin, TX, USA).
Results and discussions
The Raman spectra of the samples before and after ZnO NW growth are revealed in Figure 3b. Four peaks at 334, 438, 579, and 1143 cm−1 are observed in the spectra of ZnO NWs/GO heterostructure. The peak at 438 cm−1 corresponds to the finger signal of the characteristic E2 mode of ZnO wurtzite structure, while the peaks at 334 and 579 cm−1 are attributed to the transversal optical modes with A1 symmetry and the longitudinal optical (LO) modes. The peak at 1143 cm−1 belongs to the Raman 2LO mode of ZnO.
Two characteristic peaks (D and G bands) of GO can be seen in both curves (Figure 3b). The D-band at 1345 cm−1 is due to the A1g mode breathing vibrations of six-membered sp2 carbon rings and requires a defect for its activation, and the G-peak at 1598 cm−1 corresponds to the E2g vibrational mode of sp2 carbon pairs in both rings and chains. In general, the ID/IG ratio is a measure of the degree of disorder and average size of the sp2 domains in graphene materials23: the increased ID/IG intensity ratio generally suggests a decrease in the average size of the sp2 domains upon the reduction of the GO and the removal of the oxygen functional groups in GO films. The values of ID/IG in GO and ZnO NWs/GO heterostructure are calculated to be 0.871 and 1.006, respectively. The increased ID/IG ratio in NWs/GO heterostructure suggests that there is a nanostructure change of GO and the average size of the sp2 domains decrease. Such structure changes can be attributed to the variation of oxygen functional groups. It was reported that at the initial stage of the reaction, zinc ions are adsorbed on GO films through coordination interactions of the C-O-C and -OH or ion-exchange with H+ from carboxyl. In our case, zinc ions are expected to be attached on and reacted with oxygen functional groups to form zinc oxide nuclei, resulting in partial removal of oxygen functional groups from GO films, which is consistent with the observation that the growth of ZnO NWs via hydrothermal method can induce the nanostructure change in GO films.
In comparison, the CV curves of GO films and ZnO NWs arrays are shown in Figure 5b,c, respectively. In Figure 5b, the shape of the CV loop of GO films is close to a rectangle, indicating good charge propagation at the electrode surface. In contrast, due to the internal resistance of the composite electrode, the curve shape of the ZnO NWs arrays is distorted (Figure 5c). In addition, the curve shape of ZnO NWs/GO heterostructure is neither a rectangle (Figure 5a). The CV loops result from the superposition of the electric double-layer capacitance and pseudocapacitance due to the reaction between ZnO and electrolyte, which is mainly governed by the intercalation and deintercalation of Na+ from electrolyte into ZnO: ZnO + Na+ + e− ← → ZnO Na. Figure 5d shows the cyclic CV curves of ZnO NWs/GO films at different sweep rates. The distorted regular shape of the CV curves reveals double-layer capacitive and pseudocapacitance behaviors, which were due to the large internal resistance of the composite and the redox reaction of ZnO, as aforementioned. It can be seen that the CV curves retain a similar shape for the entire sweep. This indicates that the materials have excellent stability, and the electrolyte ions can diffuse into the GO network.
In summary, ZnO NWs/GO heterostructures have been successfully prepared via a simple solution approach at low temperature. The results showed that the GO layer can facilitate the vertical growth of ZnO NWs and improve their crystal quality. Visible emission quenching was observed in the PL spectra of ZnO NWs/GO heterostructures. The UV emission was greatly enhanced, and the defect-related visible light emission was suppressed. The heterostructures exhibited reversible electrochemical behavior. The combination of the GO and ZnO NWs enabled such composites to possess positive electrochemical behaviors that are promising as electrode material for supercapacitors. In addition, the prepared materials are expected to have potential applications as catalysts, absorbents, and electrodes for other electronic devices.
We acknowledge the financial support of the NSFC (51072119, 51102168, 51272157), Innovation Program of Shanghai Municipal Education Commission (12ZZ139), Shanghai Leading Academic Discipline Project (B502) and the Key Project of Chinese Ministry of Education (12057).
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