Physical synthesis methodology and enhanced gas sensing and photoelectrochemical performance of 1D serrated zinc oxide–zinc ferrite nanocomposites
© Liang et al. 2015
Received: 24 June 2015
Accepted: 26 August 2015
Published: 3 September 2015
We successfully prepared one-dimensional ZnO–ZnFe2O4 (ZFO) heterostructures for acetone gas-sensing and photoelectrochemical applications, by using sputter deposition of ZFO crystallites on ZnO nanostructure templates. The nanoscale ZFO crystallites were homogeneously coated on the surfaces of the ZnO nanostructures. Electron microscope images revealed that the ZnO–ZFO heterostructures exhibited a serrated surface morphology. Coating the ZnO nanostructures with a ZFO aggregated layer appreciably enhanced their acetone gas-sensing capability at 250 °C in comparison with pure ZnO nanostructures. The presence of many depleted nanoscale ZFO crystallites, the rugged surface of the heterostructures, and electron depletion at the ZnO/ZFO interface might contribute to the enhanced acetone gas-sensing response. Furthermore, the larger surface area and higher light absorption of ZnO–ZFO relative to the surface area and light absorption of ZnO were correlated with a substantial enhancement of the photocurrent value of ZnO–ZFO in photoelectrochemical tests produced by the simulated solar light irradiation.
Spinel oxides have a wide range of technical applications and are described as AB2O4. In these oxides, the anions (O2− ions) form a face-centered, cubic, and close-packed structure, and cations occupy four-coordinated and six-coordinated sites [1, 2]. ZnFe2O4 (ZFO) is a promising spinel oxide that has fascinating electrochemical, optical, and magnetic properties [3, 4]. This ferrite compound shows visible reducing gas-sensing properties . Moreover, ZFO has a bandgap in the visible light wavelength range and is widely used for photocatalytically degrading pollutants . Various methods, including the aspartic-acid-assisted combustion method and sputtering techniques, are used to synthesize ZFO in powder and thin-film forms for scientific applications [1, 3]. Because of its high surface-to-volume ratio, ZFO demonstrates unique physical and chemical properties. These functions have motivated recent studies on the synthesis and characterization of one-dimensional (1D) ZFO. For example, carbon-decorated ZFO nanowires have been prepared through the calcination of glucose-coated ZnFe2(C2O4)3 nanowires synthesized from glucose-containing microemulsion solutions, and used as a highly reversible lithium-ion anode material . Preferentially oriented ZFO nanowire arrays were fabricated through the postannealing of ZnFe2 nanowires, and the magnetic properties of the as-synthesized ZFO were investigated . Floriated ZFO with porous nanorod structures was synthesized using a hydrothermal method, and its use as a photocatalyst in hydrogen production under visible light was investigated . Highly ordered ZFO nanotube arrays were prepared using a sol–gel AAO template method, and they were found to display high sensitivity to organic gases . Although several chemical methodologies for preparing ternary ZFO nanostructures have been demonstrated, a mature physical method for synthesizing 1D ZFO remains technologically challenging because of the complex composition of this compound.
Several 1D hybrids or heterostructures consisting of binary semiconductors have been shown to enhance gas-sensing properties and demonstrate higher photoelectrochemical performance compared with their single counterpart [10, 11]. However, in contrast to the numerous studies on binary semiconductor hybrids, reports on 1D hybrids or heterostructures integrated with ternary spinel semiconductor compounds are considerably low in number [12, 13]. In this study, ZFO crystallites with high crystal quality were prepared through radio-frequency (RF) sputtering. They were homogeneously decorated on ZnO nanowires to form 1D ZnO–ZFO heterostructures. In a previous study, spinel ZFO-nanoparticle-coated rod-like ZnO nanostructures were successfully synthesized using a low-temperature hydrothermal strategy, and they exhibited high selectivity for n-butanol . In another study, efficient visible-light photoelectrochemical oxidation of water was realized in nanostructured ZnO–ZFO heterojunctions . According to relevant studies, a 1D hybrid obtained by combining ZnO and ZFO has potential applications in gas detection and high-efficiency photoelectrochemical sensing devices because of the large difference between bandgaps. However, the morphology and crystal quality of hybrids or heterostructures are known to affect their sensing performance [16, 17]. Moreover, for a given material system, different preparation methodologies yield nanostructures that exhibit different crystal features. Understanding the correlation between microstructure and sensing performance is crucial for designing hybrids or heterostructures that demonstrate the required device performance. In the current study, serrated ZFO-crystallite-decorated ZnO nanostructures were synthesized using physical methodologies. Subsequently, the gas and photoelectrochemical sensing performance of sensors fabricated from 1D ZnO–ZFO heterostructures was correlated with that of their microstructures to examine the possible use of this heterostructure in small sensing devices.
In this study, ZFO crystallites were fabricated using RF magnetron sputtering in an Ar/O2 (Ar:O2 = 3:1) mixed ambient. The growth temperature of the ZFO was maintained at 350 °C. The gas pressure during deposition was fixed at 20 mTorr and sputtering power was fixed at 80 W. Cross-linked ZnO nanowires were employed as templates to fabricated ZFO crystallite-coated ZnO nanostructures. ZFO nanofilms with tens of nanometers were grown onto ZnO nanowire templates according to the aforementioned thin-film deposition parameters to form ZnO–ZFO heterostructures. The cross-linked ZnO nanowires were synthesized through thermal vapor evaporation in a horizontal quartz tube furnace and detailed experimental setup was described elsewhere . Sample crystal structures were investigated by X-ray diffraction (XRD; Panalytical X’Pert Pro MPD) using Cu Kα radiation. The surface morphology of the samples was investigated by scanning electron microscopy (SEM; Hitachi S-4800). The detailed microstructures of the as-synthesized samples were characterized by high-resolution transmission electron microscopy (HRTEM; Philips Tecnai F20 G2). Silver glue was used to fabricate two metal electrodes onto the samples for electric measurements. To measure acetone gas-sensing properties, samples were placed in a closed vacuum chamber, and various concentrations (50–750 ppm) of acetone gas were introduced into the chamber, using dry air as the carrier gas. The sensor response to acetone gas is defined as the ratio (Ra/Rg). Ra is the electrical resistance of the sensor in the absence of acetone gas. Rg is the electrical resistance of the sensor in acetone gas. The photoelectrochemical (PEC) properties were measured in a convenient three electrodes electrochemical system (SP-50 Potentiostat/Galvanostat). Na2SO4 aqueous solution (0.1 M) was used as electrolyte. Work electrodes were made of ZnO nanowires and ZnO–ZFO heterostructures on conductive F-doped SnO2 glasses. Ag/AgCl (1 M KCl) electrode was used as a reference electrode, and a platinum wire was used as a counter electrode. A 100 W Xe arc lamp was used as the illumination source for photocurrent measurement.
Results and Discussion
Approaches involving thermal evaporation and RF sputtering were developed for the synthesis of heterostructures consisting of a crystalline ZFO-aggregated shell layer and core ZnO nanostructures. When such ZnO–ZFO heterostructures were used as acetone-sensing materials, the gas sensor fabricated from the ZnO–ZFO heterostructures showed considerably higher acetone gas-sensing response in comparison with the sensor fabricated from pure ZnO nanostructures. The markedly enhanced acetone gas-sensing response is ascribed to the large surface area of the heterostructures and the formation of heterojunctions between ZnO and ZFO. The ZnO–ZFO heterostructures exhibited high photocurrent response under sunlight illumination during PEC tests. The increased area of the rugged surfaces of the heterostructures and the relatively small bandgap of the ZFO crystallites explain the superior PEC performance of the heterostructures compared with the PEC performance of pure ZnO.
This work is supported by the Ministry of Science and Technology of Taiwan (Grant No. NSC 102-2221-E-019-006-MY3).
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