Fabrication and sensing behavior of one-dimensional ZnO-Zn2GeO4 heterostructures
© Liang and Lin; licensee Springer. 2014
Received: 22 May 2014
Accepted: 27 June 2014
Published: 9 July 2014
Well-crystalline one-dimensional ZnO-Zn2GeO4 (ZGO) heterostructures were successfully synthesized using a high-temperature solid-state reaction between the ZnO and Ge layers of ZnO-Ge core-shell nanostructures. The polycrystalline ZGO crystallites had a thickness in the range of 17 to 26 nm. The high-temperature solid-state reaction induced grooves and crystal defects on the surfaces of the ZGO crystallites. The sensors made from the ZnO-ZGO heterostructures exhibited a marked photocurrent response to UV light at room temperature and a gas sensing response to acetone gas at 325°C. The observed sensing properties are attributed to the rugged surface of the ZGO heterointerfaces between ZnO and ZGO, surface crystal defects of ZGO, and cross-linked contact regions of ZnO-ZGO.
Binary wide-bandgap oxides are promising materials for optoelectronic, catalyst, and sensor applications [1, 2]. To satisfy the different requirements of device applications, binary oxides doped with various dopants were studied to improve the intrinsic characteristics and increase the functionality of the oxides [3–5]. Binary oxides with a one-dimensional (1D) morphology show particular potential for nanodevice applications because of their high surface-to-volume ratios. Among various binary oxides, 1D ZnO is one of the most commonly used materials for nanodevices because the quality of its synthetic processes is satisfactory [4, 6].
In addition to controlling the composition of binary oxides by doping, construction of an oxide heterostructure enhances their functionality . Several proposed ZnO-based binary heterostructures exhibit satisfactory physical and chemical properties. The one-step or two-step processes involving chemical solutions and/or thermal evaporation methodologies have been adopted for fabricating binary oxide heterostructures [8, 9]. However, research on an oxide heterostructure consisting of a ternary oxide is still lacking. This is because synthesis of an oxide heterostructure with a 1D ternary oxide counterpart is technologically challengeable [10–12]. A high-temperature solid-state reaction is a feasible methodology to form a ternary oxide by using constituent binary oxides [11, 12]. A small ionic radius difference between Ge and Zn ions increases the probability of the Ge ion replacing the Zn ion. Incorporating Ge into a ZnO crystal changes the optical properties of ZnO through modification of the electronic structure around the band edge . Moreover, Zn2GeO4 (ZGO) is a ternary wide-bandgap semiconductor and a native defect phosphor exhibiting white luminescence under UV light excitation . Lin et al. showed that hydrothermally synthesized ZGO rods annealed at 1,000°C exhibit satisfactory photocatalytic hydrogen generation . Solvothermally synthesized ZGO nanostructures have been studied for the photocatalytic reduction of CO2 to CH4. In addition to photocatalytic applications, research on structure-dependent sensing characteristics of a single 1D ZGO or ZnO-ZGO heterostructure has been limited . In this study, a 1D ZnO-ZGO heterostructure was synthesized using a high-temperature solid-state reaction of ZnO-Ge core-shell nanostructures. The correlation between the structural properties and potential application of such structures in UV photodetectors and gas sensors was investigated.
Cross-linked ZnO nanostructures were used as the substrate for the growth of Ge nanofilms onto ZnO nanostructures to form ZnO-Ge core-shell nanostructures. The experimental setup for the preparation of cross-linked ZnO nanostructures has been published elsewhere . Deposition of Ge nanofilms was performed using a radio-frequency magnetron-sputtering system. During deposition, the substrate temperature was maintained at room temperature and the deposition gas pressure was fixed at 20 mTorr, with pure Ar ambient. The as-synthesized ZnO-Ge samples were further annealed in air at 800°C for 30 min to form ZnO-ZGO heterostructures.
Crystal structures of the samples were investigated by X-ray diffraction (XRD) using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) analysis was used to determine the chemical binding states of the constituent elements. The morphologies of the as-synthesized samples were characterized by scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) was used to investigate the detailed microstructures of the samples. Room temperature-dependent photoluminescence (PL) spectra were obtained using the 325-nm line of a He-Cd laser. The UV photoresponse of the samples was measured at a fixed external voltage of 5 V with and without UV irradiation. To measure gas sensing properties, heterostructure samples were placed in a closed vacuum chamber and various concentrations of acetone gas were introduced into the chamber, using dry air as the carrier gas. Silver glues were laid on the surfaces of the samples to form two contact electrodes, and the samples were fixed at 325°C during gas sensing test. Sensor response to test gases was defined as Ig/Ia, where Ia is the current in air and Ig is the current in the test gas.
Results and discussion
We successfully prepared ZnO-ZGO heterostructures for UV light photoresponse and acetone gas sensing applications by the sputter deposition of Ge ultrathin films onto ZnO nanowire templates after a high-temperature solid-state reaction. The ZGO crystallites were homogeneously formed on the surface of the residual ZnO underlayer, exhibiting a rugged morphology. The XPS spectra and PL spectrum of the ZnO-ZGO heterostructures indicated the existence of surface crystal defects. The ZnO-ZGO heterostructures exhibited clear photocurrent sensitivity to UV light at room temperature and a gas sensing response to acetone in a concentration range of 50 to 750 ppm at 325°C. The detailed structural analyses in this study accounted for the observed UV light photoresponse and acetone gas sensing properties of the ZnO-ZGO heterostructures.
YCL is a professor of the Institute of Materials Engineering at National Taiwan Ocean University (Taiwan). TYL is a graduate student of the Institute of Materials Engineering at National Taiwan Ocean University (Taiwan).
This work is supported by the National Science Council of Taiwan (Grant No. NSC 102-2221-E-019-006-MY3) and National Taiwan Ocean University (Grant No. NTOU-RD-AA-2012-104012).
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