Catalyst–free growth of well–aligned zno nanowires on graphene/si substrateby thermal evaporation - Tran Van Khai

Journal of Science and Technology 55 (1B) (2017) 174–184 CATALYST–FREE GROWTH OF WELL–ALIGNED ZnO NANOWIRES ON GRAPHENE/Si SUBSTRATEBY THERMAL EVAPORATION Tran Van Khai*, Cao Xuan Viet, La Thi Thai Ha Faculty of Materials Technology, Ho Chi Minh City University of Technology–VNUHCM 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, Vietnam *Email: tvkhai1509@gmail.com Received: 30 December 2016; Accepted for publication: 3 March 2017 ABSTRACT Vertically well–aligned ZnO nanowire (NW) arrays with high density were directly synthesized on graphene/Si substrate by thermal evaporation of zinc powder without catalysts or additives. The ZnO NWs were characterized by field emission scanning electron microscopy (FE–SEM), high resolution transmission electron microscopy (HRTEM), X–ray diffraction (XRD), photoluminescence (PL), and Raman spectroscopy. The results showed that the obtained ZnO NWs have diameters in the range of 300–350 nm with lengths of several tens micrometers. The prepared ZnO NWs are of a single crystal, which have a hexagonal wurtzite crystal structure with c–axis (002) orientation growth perpendicular to the substrate surface. The NW arrays had a good crystal quality with excellent optical properties, indicating a sharp and strong ultraviolet emission at 380 nm, and a weak visible emission at around 516 nm. Keywords: graphene, graphene oxide, ZnO nanostructures. 1. INTRODUCTION Two–demensional graphene, a single layer of sp2–bonded carbon atoms arranged in a two– dimensional hexagonal lattice, has attracted tremendous attention in both fundamental studies and practical applications due to its novel structural and exceptional physical, chemical, and mechanical properties [1–2]. Graphene–based sheets have been shown to be very promising for high–performance nanoelectronics, transparent conductors, polymer composites, and microscopy support, etc. Currently, various methods have been developed for production of graphene, including chemical vapor deposition (CVD) [3], micromechanical exfoliation of graphite [4], epitaxial growth on electrically insulating surfaces such as SiC [5], physical method [6], and chemical processing [7]. Among them, the chemical approach is the most suitable method for economically producing graphene sheets on a large scale. In recent years, there have been significant research efforts to synthesize one–dimensional (1D) nanostructures, such as NWs [8], nanorods [9], nanobelts, and nanotubes [10] due to their various potential applications in electronics, optics, photonics, and sensing devices by high surface/volume ratio [11]. Zinc oxide (ZnO), a wide band gap semiconductor material (Eg = 3.37 Tran Van Khai, Cao Xuan Viet, La Thi Thai Ha 175 eV) with a large exciton bonding energy (60 meV), which is much larger than that of GaN (25 meV), and the thermal energy (26 meV) at room temperature, it ensures an efficient excitonic emission up to room temperature. In addition, ZnO has a high mechanical strength, high conductivity, and thermal stabilities. All these features make the ZnO material an outstanding candidate for visible, ultraviolet light–emitting diodes (LEDs) and lasers, and thus promise to compete with GaN–based LEDs for next generation data–storage lasers, etc. Recently, many kinds of different–shaped ZnO nanostructures such as nanorods, NWs, nanotubes, nanobridges, nanowalls, nanobelts, nanoflowers etc., have been successfully synthesized by various methods, namely thermal evaporation [12], chemical vapor deposition (CVD)[13] or physical vapor deposition[14], pulsed laser deposition and laser ablation[15], and solution phase methods[16]. Synthesis of vertically well–aligned ZnO NW arrays not only has many advantages in practical applications, such as, light–emitting diodes and laser diodes, but also may be enhanced efficiency due to the homogeneous behavior of vertically well–aligned nanostructure. More recently, well–aligned ZnO NW arrays, which have the well–faceted hexagonal structure, were synthesized on several substrates, such as GaN [17], sapphire and ZnO [18, 19], which showed good field emission and lasing action due to their high aspect ratios, negative electron affinity, high conductivity, and chemical stability. These substrates show good lattice match with ZnO material, were expected to improve the crystal quality and to reduce the existence of structural defects; however, most studies often use catalysts such as Sn [10], Au [20], Cu[21]or other additives such as Ga [19], and NiO [22] to assist and control the growth process of ZnO NWs. Hence, the remains of catalysts or additives may be a significant source of contamination that will unavoidably influence the purity of the final products. Thus, for many applications of ZnO NWs in various areas, it is necessary to remove the catalysts nanoparticles from the NWs synthesized via a vapour–liquid–solid (VLS) mechanism; that is, catalysts–free growth of NWs is favorable for subsequent applications. However, until now it seems to be very difficult to synthesize the well–aligned NW arrays by catalyst–free methods, most these methods are commonly used to synthesize disarrayed nanostructures [23]. In this work, we will demonstrate that vertically well–aligned ZnO NW arrays can be synthesized on the graphene/Si substrate by simple thermal evaporation method. Graphene sheets firstly were prepared via chemical reduction of graphene oxide (GO) and then graphene film was deposited on Si using spray deposition process. On the prepared graphene/Si substrate, a thermal evaporation synthesis was conducted to produce ZnONWs. The obtained ZnO NWs were characterized by FE–SEM, HRTEM, XRD, PL, and Raman spectroscopy. 2. MATERIALS AND METHODS 2.1. Synthesis of graphene Graphite powder (99.9 %, Qingdao Yanshou Graphite Co., Ltd. Qingdao Brand, China), H2SO4 (98 %), H3PO4 (98 %), KMnO4 (98 %) and H2O2 (30 wt%), hydrazine monohydrate (98 %, Otsuka Chemical Co., Ltd, Japan), N,N–dimethylformamide (DMF, 99.8 %, Otsuka Chemical Co., Ltd, Japan) were obtained from commercial resources and used as received. GO nanosheets were prepared from graphite powder via a modification of Hummers’s method [24]. The aqueous GO suspension was subsequently reduced to graphene colloid using DMF in the presence of hydrazine monohydrate. At first, 25 mL of the obtained GO suspension (~ 6 mg GO/mL) was dispersed in 125 mL of DMF, followed by a mild sonication for 1 h (in a sonic bath) to achieve a homogenous aqueous GO solution, and then 2.5 mL of hydrazine Catalyst–free growth of well–aligned ZnO nanowires on graphene/Si substrate by thermal 176 monohydrate was added. The mixtures were heated at 90 ± 5 °C using a water bath for 24 h; a black solid precipitated from the reaction mixtures. Next, to generate a homogenous colloidal suspension of graphene in DMF solvent, the obtained graphene was diluted 6 times with DMF (resulting concentration ~ 0.2 mg/mL), and then the mild sonication was applied for 60 minutes to order to obtain stable and homogenous graphene dispersion. It was observed that the graphene was stable at room temperature for a few weeks. This stable period allowed sufficient time for sample preparation and characterization steps. 2.2. Fabrication of graphene films Si substrates with 15 × 15 mm square were used for graphene film formation, which were washed in an acetone bath by sonication to remove any organic contamination, and then cleaned with distilled water, followed by drying in oven. The clean substrates were stored an oven at 80 °C before being used in the next step.Air–brush spraying technique was employed to produce graphene films on the substrate. In brief, an air–brush was used to spray the as–prepared graphene suspension onto the Si substrates. The air–brush is connected to an Ar tank and gas pressure is controlled by a control valve. During film formation, the air–brush is held at a distance of 10 cm from the substrate surface; the graphene suspension streams are kept perpendicular to the substrate surface. The substrate is put on a hot–plate at about 120 °C. The thickness of film was carefully controlled by adjusting the volume of colloidal suspension. 2.3. Fabrication of well–aligned ZnO nanowire arrays on the graphene/Si substrate The synthesis of ZnO NWs was carried out using a conventional horizontal tube furnace with inner diameter of 20 mm and a heating zone of 300 mm. The substrates were put on the top of an alumina boat with the length and diameter of 4 and 1.5 cm, respectively, containing the high purity metallic zinc powder (75 µm, 99.99 %, Sigma–Aldrich) about 0.25 g, and inserted into a horizontal quartz tube furnace. The vertical distance between the zinc source and the substrate was about 5 mm, with a downstream separation of 7 mm. The furnace was heated to the reaction temperature of 620 °C in 30 min under Ar at a flow rate of 350 standard cubic centimeter per minute (sccm). The zinc source was thermally vaporized to synthesize ZnO NWs at atmospheres pressure under Ar (99.99 %) at a flow rate of 350 sccm for 90 min at 620 °C. After reaction, the quart tube was cooled to room temperature under Ar at a flow rate of 350 sccm. The substrate surface appeared to be a layer of white wax–like material. 2.4. Characterization The morphology and structures of as–synthesized products were characterized by using a field–emission scanning electron microscope (FE–SEM, JSM–6700, JEOL Ltd., Tokyo, Japan) operated at an accelerating voltage of 12 kV, at Hanyang University, Seoul, Korea. High resolution transmission electron microscope (HRTEM) images were obtained on a JEOL JEM– 2010 TEM (JEOL Ltd., Tokyo, Japan) with an accelerating voltage of 200 kV, at Hanyang University, Seoul, Korea. Atomic force microscope (AFM) images were obtained on an AFMXE–100 (Park system) equipment, at Hanyang University, Seoul, Korea. X–ray diffraction (XRD) characterization was obtained using a D/MAX Rint 2000 diffractometer model (Rigaku, Tokyo, Japan) with CuKα radiation (λ = 1.54178 Å, 40 kV, 200 mA), at INOMAR–VNUHCM. 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The cross–sectional FE–SEM images of the NW arrays are presented in Figure 2c, d. This shows that vertically well–aligned NWs have an average length of about 25–30 μm, the NWs synthesized show uniform diameters over their entire lengths. Jo et al.[25]and Zhu et al.[26] showed that good field emission properties were caused by the small geometry of ZnO NWs. Besides, it is believed that field emission also strongly depends on tip morphology and density of the NWs. Figure 2.(a & b) Tilt–view and cross–view (c & d) FE–SEM images of well–aligned ZnO NWs arrays grown on graphene/Si substrate. The structure and morphology of the ZnO NWs were further characterized by using TEM, HRTEM, and the SAED pattern. Figure 3a shows the TEM image of an individual ZnO NW. It indicates that the obtained NWs have a relatively straight shape and uniform diameters along their lengths, and no catalyst particles at the tips. The typical HRTEM image is shown in Figure 3b. It can be clearly observed that the ZnO crystal lattice is well–oriented with no observable structural defects over the whole region. This result suggests that our ZnO NWs are structurally homogeneous and defect–free. However, it does not show that there are no other types of defects in the synthesized ZnO NW arrays, since defects such as vacancy or interstice may not be visible in the HRTEM observation. From the HRTEM image, the lattice spacing of ZnO NW is found to be approximately 0.26 nm, corresponding to distance between two (002) crystal planes, confirming that the ZnO NWs are single crystalline and it is referentially grown along the [001] direction. Figure 3c indicates the corresponding SAED pattern, which can be indexed to the reflection of hexagonal ZnO single crystals along the c–axis [001] direction. Tran Van Khai, Cao Xuan Viet, La Thi Thai Ha 179 Figure 3.TEM, HRTEM and selected area electron diffraction (SAED) images of an individual ZnO NW. (a) TEM, (b) HRTEM images and (c) the corresponding SAED pattern. In order to further examine the structural properties of the produced ZnO NWs, XRD measurements were performed, and the results were shown in Figure 4. Figure 4a shows the XRD pattern of the graphene film. As can be seen, there is a broad peak approximately 21.25°, corresponding to an interlayer distance (d(002)–spacing) of about 0.392 nm, which is the typical interlayer spacing of stacked graphene sheets in pristine graphite. Figure 4b displays XRD pattern of ZnO NWs grown on graphene/Si substrate. From the curve, we can see there is only one strong peak at 33.76°, corresponding to the (002) diffraction peak of the hexagonal wurtzite structure of ZnO. This suggests that the ZnO NWs are preferentially oriented in c–axis direction and normal to the substrate surface. No trace of zinc, impurities, or substrate is detected from the spectrum, showing that the ZnO NWs samples are of the pure hexagonal ZnO crystalline phase. Figure 4. XRD patterns of (a) the as–synthesized graphene film/Si and (b) ZnO NWs fabricated on graphene/Si substrate. Catalyst–free growth of well–aligned ZnO nanowires on graphene/Si substrate by thermal 180 Figure 5a & b show the Raman spectra of graphene and ZnO NWs grown on graphene/Si substrate, respectively. It is well–known that Raman spectroscopy is sensitive to crystallization, structural disorder and defects in micro– and nanostructures. As shown in Figure 5a, the Raman spectrum of graphene displays a prominent G band at 1603 cm–1, which is commonly ascribed to the first–order scattering of the E2g mode, being observed for sp2–carbon domains at the Brillouin zone center. Also, it has a very weak D band at 1311 cm–1, being related to sp3– hybridzed, structural defects, grain boundaries, carbon amorphous or edge planes that can break the symmetry and selection rule [27]. ZnO has a wurtzite hexagonal phase belonging to space group of ܥ଺ఔସ (Hermann–Mauguin symbol P63 mc); having two formula units per primitive cell and all atoms occupy the sites of symmetry C3v [28]. According to the Group theory, the single crystalline ZnO has eight sets of optical phonon modes near the center (Г) of the Brillouin zone, and hence is classified as [29]: Г୭୮୲ ൌ Aଵ ൅ 2Bଵ ൅ Eଵ ൅ 2Eଶ. Among these, the A1 and E1 modes are polar and can split into transverse optical (TO) and longitudinal optical (LO) phonons [30], both being active in Raman and infrared spectroscopy, whereas the B1 modes are neither Raman nor infrared active (silent modes). The non–polar E2 modes are Raman active only, have two wave numbers, namely; low–frequency E2L and high–frequency E2H modes, associated with vibration of the heavy Zn sub-lattice and oxygen atoms, respectively. All described phonon modes have been reported in the Raman–scattering spectra of bulk ZnO [31]. In the backscattering configuration, only the E2H, E2L, and A1LO modes are allowed, while the other modes are forbidden according to Raman selection rules [30]. As shown in Figure 5b, the appearance of a high–intensity, dominant, sharp and strong peak at 439 cm–1 is attributed to the Raman active optical phonon E2 mode of ZnO, confirming that the synthesized ZnO NWs have the wurtzite hexagonal crystal structure. The peak at 334 cm–1 could be assigned to the E2H–E2L mode (multiple–phonon scattering processes). Also, there is one peak at about 581 cm–1. This peak is ascribed to the E1LO mode, and this might be due the misalignment of NWs or the formation of defects such as Zn interstitials, oxygen vacancies or their complexes. On the other hand, the peaks observed at higher wave frequencies (> 1200 cm–1), i.e., 1370 and 1600 cm–1, which is attributed to the D– and G–band of graphene, respectively. Therefore, the presence of a high–intensity, sharp and dominant E2 mode in the Raman spectrum indicated that the hexagonal ZnO NWs formed are highly crystalline with the wurtzite hexagonal phase, and with significantly less structural defects. This is an additional indication of the wurtzite structure of the NWs with a main orientation along the c–axis, as previously observed by the XRD and SAED patterns. PL spectroscopy is widely being used technique to examine the optical properties of nanosized materials. Therefore, we have measured PL spectrum of ZnO NW arrays, and the result is shown in Figure 6. Usually, the PL of pure ZnO NWs exhibits two distinct peaks: one narrow peak in the UV region and one broad green band in the visible region [32]. In our prepared ZnO NWs, a sharp and strong band in the UV region at 380 nm anda weak broad green band peaking at around 516 nm. Comparable results were reported in the literatures [8, 33]. The UV emission is well–known as near–band–edge (NBE) emission of the wide band gap ZnO, which originated from a recombination of free–excitons through an exiciton–exiciton collision process [14, 19]. The green band emission is commonly attributed to a deep level emission caused by the impurities and the structural defects such as zinc interstitials (Zni), zinc vacancies (VZn), oxygen interstitials (Oi) or antisite oxygen (OZn), and oxygen vacancies (VO) in the surface and sub–surface lattices of ZnO materials [34]. It is widely accepted that the surface states play a very important role in PL spectra of ZnO nanomaterials [35]. Many studies showed that the slight emergence of green–yellow band emission in the visible region is due to the radial recombination of the photo–generated holes with the singly ionized charged state of the defects Tran Van Khai, Cao Xuan Viet, La Thi Thai Ha 181 in the ZnO [36]. More singly ionized oxygen vacancy defects will result in higher green PL intensity [37]. Dijken et al. have proposed that the origination of green emission is due to the transition from the conduction band to the deeply trapped holes [38]. In addition, it is also demonstrated that the NBE emission peak in the PL spectra is dependent on the crystallinity of the fabricated products, i.e., if the crystal quality of the grown nanostructures is improved (decrease of impurity, and structural defects such as zinc interstitials and oxygen defects etc.), the intensity of the UV peak is also increased [39]. In our work, no metal catalyst or any other type of additive was used to synthesize these ZnO NWs, and after growth no metal particles or any other type of impurities were found on the products, as seen from SEM images. In the HRTEM images, we did not find any dislocations or stacking faults, and additionally the XRD and SEAD patterns showed that the obtained ZnO NWs are single crystalline. These results are in good agreement with the PL observation. Furthermore, Ji et al. and Dai et al. have reported that if the concentration of the oxygen vacancies and interstitial oxygen ions is reduced in the synthesized products, a sharp and strong intensity NBE, and a short or suppressed green emission appear [40]. Therefore, the intensity ratio of UV to green emission is larger for the NWs in our study than those obtained from ZnO nanocrystals grown by hydrothermal [41], vapor–liquid–solid [42], CVD [43], chemical vapor transport and condensation [44], and electrochemical methods [45], suggesting that the ZnO NWs prepared in our work are good in crystallinity with very less structural defects. Figure 5. Raman spectra of (a) the synthesized graphene film/Si and (b) ZnO NWs grown on graphene/Si substrate. Figure 6. PL spectrum of ZnO NW arrays grown on graphene/Si substrate. Catalyst–free growth of well–aligned ZnO nanowires on graphene/Si substrate by thermal 182 4. CONCLUSIONS In conclusion, vertically well–aligned ZnO NW arrays have been successfully synthesized on graphene/Si substrate without catalyst or additives by a simple thermal evaporation method. The as–synthesized ZnO NWs had diameters in the range of 300–350 nm, lengths over several tens micrometers. The SEM, HRTEM, XRD and Raman measurements revealed that the ZnO NWs have a single–crystalline hexagonal structure and c–axis orientation with good crystal quality. The PL spectrum at room temperature showed that ZnO NW arrays have excellent optical properties. ZnO–graphene NW structures produced in our experiments are expected to be promising for photonic and optoelectronic devices. Acknowledgement. This work was supported by the National Foundation for Science and Technology Development of Viet Nam (NAFOSTED, code 104.03.2013.39) and by Ho Chi Minh City University of Technology–VNUHCM, under grant number “T–CNVL–2016–104”. REFERENCES 1. Geim A. K., Novoselov K. 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