Haiyong Gao

Structure and magnetic properties of three-dimensional (La,Sr)MnO3 nanofilms on ZnO nanorod arrays

Three-dimensional (3D) cubic perovskite (La,Sr)MnO3 (LSMO) nanofilms have been deposited on ZnO nanorod arrays with controlled dimensionality and crystallinity by radio frequency (rf) magnetron sputtering and post thermal annealing. Compared to the two-dimensional (2D) LSMO nanofilm on flat Si, the structure and magnetic properties of 3D LSMO nanofilms on ZnO nanorod arrays have a strong anisotropic morphology and thickness dependence. Ferromagnetic property has been observed in both 2D and 3D LSMO nanofilms while a ferromagnetic–superparamagnetic transition was revaled in 3D LSMO nanofilms on ZnO nanorod array with decreasing nanofilm thickness, due to a large surface dispersion effect. The LSMO/ZnO nanofilm/nanorod structures could open up new avenues for intriguing magnetic properties studies and applications of nanoscale perovskites.Three-dimensional (3D) cubic perovskite (La,Sr)MnO3 (LSMO) nanofilms have been deposited on ZnO nanorod arrays with controlled dimensionality and crystallinity by radio frequency (rf) magnetron sputtering and post thermal annealing. Compared to the two-dimensional (2D) LSMO nanofilm on flat Si, the structure and magnetic properties of 3D LSMO nanofilms on ZnO nanorod arrays have a strong anisotropic morphology and thickness dependence. Ferromagnetic property has been observed in both 2D and 3D LSMO nanofilms while a ferromagnetic–superparamagnetic transition was revaled in 3D LSMO nanofilms on ZnO nanorod array with decreasing nanofilm thickness, due to a large surface dispersion effect. The LSMO/ZnO nanofilm/nanorod structures could open up new avenues for intriguing magnetic properties studies and applications of nanoscale perovskites.

Three dimensional koosh ball nanoarchitecture with a tunable magnetic core, fluorescent nanowire shell and enhanced photocatalytic property

A mild, wet chemical strategy has been developed to synthesize three-dimensional (3D) multicomponent koosh ball nanoarchitectures comprised of a tunable magnetic iron oxide core and a 3D fluorescent ZnO nanowire shell, which demonstrate enhanced photocatalytic property towards dye degradation under UV irradiation. The phase transition of the magnetic iron oxide core and the native defects induced fluorescence of the nanowire shell can be simultaneously manipulated by post-hydrogen annealing to produce various koosh balls with retained morphology but different magnetic and photocatalytic properties. This unique koosh ball architecture initiates the three-dimensional nanowire growth on the micro-scale spherical substrate and enables the rational combination of multiple desired functionalities originating from dissimilar constituents.

Perovskite Nanoparticle-Sensitized Ga2O3 Nanorod Arrays for CO Detection at High Temperature

Noble metal nanoparticles are extensively used for sensitizing metal oxide chemical sensors through the catalytic spillover mechanism. However, due to earth-scarcity and high cost of noble metals, finding replacements presents a great economic benefit. Besides, high temperature and harsh environment sensor applications demand material stability under conditions approaching thermal and chemical stability limits of noble metals. In this study, we employed thermally stable perovskite-type La(0.8)Sr(0.2)FeO3 (LSFO) nanoparticle surface decoration on Ga2O3 nanorod array gas sensors and discovered an order of magnitude enhanced sensitivity to carbon monoxide at 500 °C. The LSFO nanoparticle catalysts was of comparable performance to that achieved by Pt nanoparticles, with a much lower weight loading than Pt. Detailed electron microscopy and X-ray photoelectron spectroscopy studies suggested the LSFO nanoparticle sensitization effect is attributed to a spillover-like effect associated with the gas-LSFO-Ga2O3 triple-interfaces that spread the negatively charged surface oxygen ions from LSFO nanoparticles surfaces over to β-Ga2O3 nanorod surfaces with faster surface CO oxidation reactions.

(La,Sr)CoO3/ZnO nanofilm–nanorod diode arrays for photo-responsive moisture and humidity detection

Large scale (La,Sr)CoO3 (LSCO)/ZnO nanofilm–nanorod diode arrays have been successfully fabricated using a combination of hydrothermal synthesis and colloidal deposition. With well-controlled dimensionality, crystallinity, crystal structures and device structures, LSCO/ZnO nanofilm–nanorod diode arrays display an excellent rectifying current–voltage (I–V) characteristic under ±1 V bias with negligible leakage current upon reverse bias. These nanostructured diode arrays have been found to be sensitive to UV illumination and different relative humidities at room temperature upon forward bias. A negative photoconductivity response is revealed upon UV illumination on the diode arrays as a result of the desorption process of nanofilm–nanorod surface moisture. The forward current of LSCO/ZnO nanofilm–nanorod diodes increases significantly with increasing relative humidity. These unique nanostructured diode arrays could be useful as photo-responsive moisture and humidity detectors.

La0.67Sr0.33MnO3 nanofibers for in situ, real-time, and stable high temperature oxygen sensing

By calcining electrospun La(NO3)3–Sr(NO3)2–Mn(NO3)2–PVP precursory nanofibers, La0.67Sr0.33MnO3 (LSMO) nanofibers have been successfully fabricated on a large scale. The as-prepared LSMO nanofibers with an average diameter of ∼126 nm displayed a homogenous perovskite phase with a rough surface and porous structure. The LSMO nanofibers were thermally stable in terms of composition and crystalline structure after multiple heating/cooling cycles between room temperature and 1000 °C, accompanied by a slightly increased grain size. The LSMO nanofibers were applied for in situ real-time oxygen sensing at 800 °C and showed sensitive, reversible and reproducible response towards oxygen with a detection limit as low as 3.5 ppb (S/N = 3). These results suggest that electrospun LSMO nanofibers are a promising nanomaterial for the design and fabrication of stable high temperature gas sensors.

In situ TPR removal: a generic method for fabricating tubular array devices with mechanical and structural soundness, and functional robustness on various substrates

Wire array templates have been utilized for fabricating various three-dimensional tubular structure devices, such as solar cells, batteries and supercapacitors, as well as electronic and photonic devices. It is necessary to remove the templates by using post-treatments such as wet chemical etching, decomposition, or Kirkendall approaches. However, it remains a challenge to ensure tubular structure integrity, mechanical soundness, and chemical purity during the template removal process, which affects the functional robustness of enabled tubular structure array devices. In this work, by utilizing ZnO nanorod array devices as templates and temperature programmed reduction (TPR) as the removal method, nanotube array devices made of various functional oxides have been directly converted with well-retained uniformity, structure and mechanical soundness, and chemical homogeneity on both two-dimensional (2D) planar and three-dimensional (3D) monolith device substrates. The successful examples range from binary metal oxides such as fluorite CeO2 to complex oxides like perovskite La0.8Sr0.2CoO3 (LSCO). This TPR removal method is generic, simple and rationally controllable, and can be easily expanded to the preparation of other oxides and non-oxide tubular structure devices regardless of the interfaced device substrate geometry. As a demonstration, the enabled CeO2 nanotube array devices could function as good high-temperature O2 sensors and active photocatalytic devices with good robustness.

UV-enhanced CO sensing using Ga2O3-based nanorod arrays at elevated temperature

Monitoring and control of the gaseous combustion process are critically important in advanced energy systems such as power plants, gas turbines, and automotive engines. However, very limited gas sensing solutions are available in the market for such applications due to the inherent high temperature of the combustion gaseous atmosphere. In this study, we fabricated and demonstrated high-performance metal oxide based nanorod array sensors assisted with ultra-violet (UV) illumination for in situ and real-time high-temperature gas detection. Without UV-illumination, it was found that surface decoration of either 5 nm LSFO or 1 nm Pt nanoparticles can enhance the sensitivity over CO at 500 °C by an order of magnitude. Under the 254 nm UV illumination, the CO gas-sensing performance of Ga2O3-based nanorod array sensors was further enhanced with the sensitivity boosted by 125% and the response time reduced by 30% for the La0.8Sr0.2FeO3(LSFO)-decorated sample. The UV-enhanced detection of CO might be due to the i...

Bimodular high temperature planar oxygen gas sensor.

A bimodular planar O2 sensor was fabricated using NiO nanoparticles (NPs) thin film coated yttria-stabilized zirconia (YSZ) substrate. The thin film was prepared by radio frequency (r.f.) magnetron sputtering of NiO on YSZ substrate, followed by high temperature sintering. The surface morphology of NiO NPs film was characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). X-ray diffraction (XRD) patterns of NiO NPs thin film before and after high temperature O2 sensing demonstrated that the sensing material possesses a good chemical and structure stability. The oxygen detection experiments were performed at 500, 600, and 800°C using the as-prepared bimodular O2 sensor under both potentiometric and resistance modules. For the potentiometric module, a linear relationship between electromotive force (EMF) output of the sensor and the logarithm of O2 concentration was observed at each operating temperature, following the Nernst law. For the resistance module, the logarithm of electrical conductivity was proportional to the logarithm of oxygen concentration at each operating temperature, in good agreement with literature report. In addition, this bimodular sensor shows sensitive, reproducible and reversible response to oxygen under both sensing modules. Integration of two sensing modules into one sensor could greatly enrich the information output and would open a new venue in the development of high temperature gas sensors.

Hierarchical oxide-based composite nanostructures for energy, environmental, and sensing applications

Self-assembled composite nanostructures integrate various basic nano-elements such as nanoparticles, nanofilms and nanowires toward realizing multifunctional characteristics, which promises an important route with potentially high reward for the fast evolving nanoscience and nanotechnology. A broad array of hierarchical metal oxide based nanostructures have been designed and fabricated in our research group, involving semiconductor metal oxides, ternary functional oxides such as perovskites and spinels and quaternary dielectric hydroxyl metal oxides with diverse applications in efficient energy harvesting/saving/utilization, environmental protection/control, chemical sensing and thus impacting major grand challenges in the area of materials and nanotechnology. Two of our latest research activities have been highlighted specifically in semiconductor oxide alloy nanowires and metal oxide/perovskite composite nanowires, which could impact the application sectors in ultraviolet/blue lighting, visible solar absorption, vehicle and industry emission control, chemical sensing and control for vehicle combustors and power plants.