Synthesis and Characterization of ZnO Nanowire–CdO Composite Nanostructures
ZnO nanowire–CdO composite nanostructures were fabricated by a simple two-step process involving ammonia solution method and thermal evaporation. First, ZnO nanowires (NWs) were grown on Si substrate by aqueous ammonia solution method and then CdO was deposited on these ZnO NWs by thermal evaporation of cadmium chloride powder. The surface morphology and structure of the synthesized composite structures were analyzed by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The optical absorbance spectrum showed that ZnO NW–CdO composites can absorb light up to 550 nm. The photoluminescence spectrum of the composite structure does not show any CdO-related emission peak and also there was no band gap modification of ZnO due to CdO. The photocurrent measurements showed that ZnO NW–CdO composite structures have better photocurrent when compared with the bare ZnO NWs.
KeywordsZinc oxide Cadmium oxide Nanowires Composites Optical absorbance
Zinc oxide (ZnO) is one of the most important materials for the optoelectronic applications because of its wide band gap (3.37 eV) and high-exciton binding energy (60 meV) that is much larger than other semiconductor materials such as ZnSe (22 meV) and GaN (25 meV). ZnO nanostructures have been extensively investigated in the past decade due to their interesting optical [1, 2] and electrical properties [3–6]. These nanostructures have potential applications as UV light sources, photodetectors, sensors, photocatalysts, solar cells, field effect transistors, field emission devices and piezoelectric devices [5–12]. Among the various ZnO nanostructures, ZnO nanowires have attracted much attention because of their unique material properties and well-developed synthesis methods. Various methods have been employed to fabricate ZnO nanowires including gas-phase methods such as metal-organic chemical vapor deposition (MOCVD) , evaporation , pulsed-laser deposition , solution-phase methods such as chemical bath deposition (CBD) , electrochemical deposition  and hydro-thermal method . Especially, solution-phase methods are appealing because of the low growth temperatures, potentials for scaling up and capability of producing high-density arrays .
Recently, ZnO nanowire arrays have been applied as a transparent electrode in the solar energy devices due to their high surface area and good vertically aligned electrical pathways, which are expected to increase the efficiency of these photoelectric devices [11, 20, 21]. However, ZnO can only absorb a small portion of the solar spectrum in the visible region due to its wide band gap. To further widen the useable wavelength range and improve the efficiency of ZnO-based photodevices, a narrow band gap material should be alloyed or composited with ZnO. In principle, the coupling of ZnO with a narrow band gap material, can reduce its band gap, extend its absorption range to visible-light region, promote electron-hole pair separation under irradiation and consequently achieve a higher efficiency for the ZnO-based photodevices. In recent years, heterostructures of ZnO with metals or semiconductors have attracted much attention because of their enhanced optical and photocatalytic properties [22–30].
CdO, an n-type II–VI semiconductor, has attracted considerable attention for various optoelectronic devices due to its high electrical conductivity (even without doping), high carrier concentration and high optical transmittance in the visible region of the solar spectrum. By alloying with CdO, which has a cubic structure and a narrower direct band gap of 2.2–2.5 eV, the band gap of ZnO can be red-shifted to a blue or even a green spectral range. Wang et al.  have shown that UV near-band-edge emission was red-shifted to 407 nm (3.04 eV) from 386 nm (3.21 eV) with the increasing Cd content for their quasi-aligned ZnCdO nanorods. Up to our knowledge, there are no reports available on the heterostructures of ZnO nanostructures with CdO. In the present study, we report the synthesis and characterization of ZnO nanowire–CdO composite structures by a two-step process involving chemical solution method and thermal evaporation. The synthesized ZnO NW–CdO composite structures showed enhanced optical absorbance in the visible region.
ZnO NW–CdO composite structures were fabricated on silicon substrates by using a two-step process. First, ZnO NWs were grown on Si substrates using the previously reported ammonia solution method [32, 33]. A 25 nm ZnO buffer film was coated on the Si substrate by sputtering a ZnO target at room temperature and then was air-annealed at 800 °C for 1 h. After cooling to room temperature, the substrates were immersed in a 10 mM Zn(NO3)2·6H2O (98%, Aldrich) aqueous solution where pH was adjusted to 11 by adding the ammonia solution [28 wt% of NH3 (Aldrich) in water], and the solution was heated at 95 °C for 10 h. After the growth, the substrate was removed from the solution, rinsed with the deionized water and then dried by nitrogen blow. Then ZnO NW–CdO composite structures were grown by thermal evaporation of CdCl2 powder in argon atmosphere using a conventional horizontal tube furnace. Pure CdCl2 powder was deposited in the middle of the alumina boat and the ZnO NW substrate was placed on the top of the boat with the ZnO nanowire surface facing the powder. The alumina boat was then placed at the uniform-temperature zone of the furnace and heated to 500–550 °C (ramp rate ~12 °C/min) with a constant argon flow of 100 sccm. The temperature was maintained at 500–550 °C for about 1 h and then the furnace was allowed to cool normally to room temperature before taking the sample out for characterization. When CdCl2 is evaporated, CdO is formed on ZnO NWs by taking the residual oxygen present in the furnace.
The surface morphology, structure and composition of the as-grown ZnO NW and ZnO NW–CdO composites were characterized by field emission scanning electron microscopy (FE-SEM; JEOL JSM 330F), X-ray diffraction (XRD; Rigaku D-Max1400, Cu Kα radiation λ = 1.5406 Å), Raman spectroscopy (SENTERRA dispersive Raman microscope, 532 nm laser wavelength), high-resolution transmission electron microscopy (HR-TEM; JEOL 2100F) and energy-dispersive X-ray spectroscopy (EDX) measurements. The optical absorbance (diffuse reflectance spectroscopy—DRS) measurements were carried out using a UV-visible spectrophotometer. The photoluminescence measurements were carried out at room temperature using He–Cd laser (325 nm) as the excitation source. The photocurrent measurements were carried out in a typical three-electrode cell (Potentiostat/Galvanostat, Model 263A) that included a Pt counter electrode, a saturated calomel reference electrode and a working electrode made from ZnO NW or ZnO NW–CdO composites on the ITO substrate. A 1 M Na2S solution was used as the electrolyte. The working electrode was illuminated from front side with a solar-stimulated light source (AM1.5G filtered, 100 mW/cm2, 91160, Oriel).
Results and Discussion
We synthesized ZnO NW–CdO composite structures using a simple two-step process involving ammonia solution method followed by thermal evaporation. SEM and TEM analysis indicated that CdO was deposited mainly on the tip of the ZnO nanowires. XRD analysis of the composite structures showed additional diffraction peaks corresponding to cubic CdO, apart from the signals from the hexagonal ZnO. The ZnO NW–CdO composite structures showed enhanced optical absorption extending to about 550 nm in the visible region. PL measurements do not show any band gap modification for the composite structures. The higher visible-light absorption capability of these composite structures can be applied to enhance their photoelectrochemical and photocatalytic properties. Systematic studies are now in progress to explore these properties.
This work was supported by grant no. R01-2006-000-10230-0 (2006) from the Korea Science and Engineering Foundation, grant no. RTI04-01-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE) and the Korean Research Foundation Grants funded by the Korean Government (MOEHRD; KRF-2005-005-J13101) and grant no. KRF-2007-521-D00118.