Yihui Sun

A self-powered ultraviolet photodetector based on solution-processed p-NiO/n-ZnO nanorod array heterojunction

We report fabrication of an all inorganic, self-powered and rapid-response ultraviolet (UV) photodetector using solution-processed p-NiO/ZnO-nanorod array heterojunction. The device exhibited a fast binary-response with a rise time of 0.23 s and decay time of 0.21 s. A large responsivity of 0.44 mA W−1 was achieved for a 0.4 mW cm−2 UV light irradiation at a zero-bias voltage. The self-powered performance could be attributed to the proper built-in electric field between ZnO and NiO arising from the well-aligned energy-band structure of the device, which gives rise to a photovoltaic effect.

Design of sandwich-structured ZnO/ZnS/Au photoanode for enhanced efficiency of photoelectrochemical water splitting

We developed and demonstrated a ZnO/ZnS/Au composite photoanode with significantly enhanced photoelectrochemical water-splitting performance, containing a ZnS interlayer and Au nanoparticles. The solar-to-hydrogen conversion efficiency of this ZnO/ZnS/Au heterostructure reached 0.21%, 3.5 times that of pristine ZnO. The comparison of the incident photon-to-current efficiency (IPCE) and the photoresponse in the white and visible light regions further verified that the enhancement resulted from contributions of both UV and visible light. The modification of the Au NPs was shown to improve the photoelectrochemical (PEC) performance to both UV and visible light, as modification encouraged effective surface passivation and surface-plasmonresonance effects. The ZnS interlayer favored the movement of photogenerated electrons under UV light and hot electrons under visible light, causing their injection into ZnO; this simultaneously suppressed the electron-hole recombination at the photoanode-electrolyte interface. The optimized design of the interlayer within plasmonic metal/semiconductor composite systems, as reported here, provided a facile and compatible photoelectrode configuration, enhancing the utilization efficiency of incident light for photoelectrochemical applications.

High On–Off Ratio Improvement of ZnO-Based Forming-Free Memristor by Surface Hydrogen Annealing

In this work, a high-performance, forming-free memristor based on Au/ZnO nanorods/AZO (Al-doped ZnO conductive glass) sandwich structure has been developed by rapid hydrogen annealing treatment. The Ron/Roff rate is dramatically increased from ∼10 to ∼10(4) after the surface treatment. Such an enhanced performance is attributed to the introduced oxygen vacancies layer at the top of ZnO nanorods. The device also exhibits excellent switching and retention stability. In addition, the carrier migration behavior can be well interpreted by classical trap-controlled space charge limited conduction, which verifies the forming of conductive filamentary in low resistive state. On this basis, Arrhenius activation theory is adopted to explain the drifting of oxygen vacancies, which is further confirmed by the time pertinence of resistive switching behavior under different sweep speed. This fabrication approach offers a useful approach to enhance the switching properties for next-generation memory applications.

Temperature-dependent electrochemical capacitive performance of the α-Fe2O3 hollow nanoshuttles as supercapacitor electrodes

The design and optimization of supercapacitors electrodes nanostructures are critically important since the properties of supercapacitors can be dramatically enhanced by tunable ion transport channels. Herein, we demonstrate high-performance supercapacitor electrodes materials based on α-Fe2O3 by rationally designing the electrode microstructure. The large solid-liquid reaction interfaces induced by hollow nanoshuttle-like structures not only provide more active sites for faradic reactions but also facilitate the diffusion of the electrolyte into electrodes. These result in the optimized electrodes with high capacitance of 249 F g(-1) at a discharging current density of 0.5 A g(-1) as well as good cycle stability. In addition, the relationship between charge storage and the operating temperature has been researched. The specific capacitance has no significant change when the working temperature increased from 20 °C to 60 °C (e.g. 203 F g(-1) and 234 F g(-1) at 20 °C and 60 °C, respectively), manifesting the electrodes can work stably in a wide temperature range. These findings here elucidate the α-Fe2O3 hollow nanoshuttles can be applied as a promising supercapacitor electrode material for the efficient energy storage at various potential temperatures.

Fiber-shaped asymmetric supercapacitors with ultrahigh energy density for flexible/wearable energy storage

Fiber-shaped supercapacitors (FSCs) have attracted significant interest owing to their unique advantages of small size, light weight, high flexibility, and capability of being integrated into wearable electronics and smart textiles. Their main limitation, however, is their low energy density when compared with batteries. Here a fiber-shaped asymmetric supercapacitor (FASC) with high energy density has been developed successfully using CNT@ZnO-NWs@MnO2 fibers as the positive electrode and CNT fibers as the negative electrode. Due to the high capacitances and excellent rate performances of CNT@ZnO-NWs@MnO2 fibers and CNT fibers, such an asymmetric cell exhibits superior electrochemical performances. An optimized FASC can be cycled reversibly in the voltage range of 0–1.8 V, and exhibits a maximum energy density of 13.25 μW h cm−2, which is much higher than those reported for fiber-shaped supercapacitors. Owing to the rational structure design, the all-solid-state FASCs demonstrate excellent mechanical and electrochemical stability. Over 1000 bending cycles, 96.7% of the initial capacitance can still be retained.

Synergistic Effect of Surface Plasmonic particles and Surface Passivation layer on ZnO Nanorods Array for Improved Photoelectrochemical Water Splitting

One-dimensional zinc oxide nanorods array exhibit excellent electron mobility and thus hold great potential as photoanode for photoelelctrochemical water splitting. However, the poor absorption of visible light and the prominent surface recombination hider the performance improvement. In this work, Au nanoparticles and aluminium oxide were deposited onto the surface of ZnO nanorods to improve the PEC performance. The localized surface plasmon resonance of Au NPs could expand the absorption spectrum to visible region. Simultaneously, the surface of passivation with Au NPs and Al2O3 largely suppressed the photogenerated electron-hole recombination. As a result, the optimal solar-to-hydrogen efficiency of ZnO/Au/Al2O3 with 5 cycles was 6.7 times that of pristine ZnO, ascribed to the synergistic effect of SPR and surface passivation. This research reveals that the synergistic effect could be used as an important method to design efficient photoanodes for photoelectrochemical devices.

Reduced Graphene Oxide Functionalized with Cobalt Ferrite Nanocomposites for Enhanced Efficient and Lightweight Electromagnetic Wave Absorption

In this paper, reduced graphene oxide functionalized with cobalt ferrite nanocomposites (CoFe@rGO) as a novel type of electromagnetic wave (EW) absorbing materials was successfully prepared by a three-step chemical method including hydrothermal synthesis, annealing process and mixing with paraffin. The effect of the sample thickness and the amount of paraffin on the EW absorption properties of the composites was studied, revealing that the absorption peaks shifted toward the low frequency regions with the increasing thickness while other conditions had little or no effect. It is found that the CoFe@rGO enhanced both dielectric losses and magnetic losses and had the best EW absorption properties and the wide wavelength coverage of the hole Ku-Band when adding only 5wt% composites to paraffin. Therefore, CoFe@rGO could be used as an efficient and lightweight EW absorber. Compared with the research into traditional absorbing materials, this figures of merit are typically of the same order of magnitude, but given the lightweight nature of the material and the high level of compatibility with mass production standards, making use of CoFe@rGO as an electromagnetic absorber material shows great potential for real product applications.

Influence of carrier concentration on the resistive switching characteristics of a ZnO-based memristor

Sandwich-style memristor devices were synthesized by electrochemical deposition with a ZnO film serving as the active layer between Al-doped ZnO (AZO) and Au electrodes. The carrier concentration of the ZnO films is controlled by adding HNO3 during the growth process. A resulting increase in carrier concentration from 1017 to 1019 cm–3 was observed, along with a corresponding drop in the on–off ratio from 6,437% to 100%. The resistive switching characteristics completely disappeared when the carrier concentration was above 1019 cm–3, making it unsuitable for a memory device. The decreasing switching ratio is attributed to a reduction in the driving force for oxygen vacancy drift. Systematic analysis of the migration of oxygen vacancies is presented, including the concentration gradient and electrical potential gradient. Such oxygen vacancy migration dynamics provide insight into the mechanisms of the oxygen vacancy drift and provide valuable information for industrial production of memristor devices.

Band alignment engineering for high-energy-density solid-state asymmetric supercapacitors with TiO2 insertion at the ZnO/Ni(OH)2 interface

In this paper, an adaptive interface electronic band structure was proposed for improving the capacitance of nano-architectured Ni(OH)2 by introducing a TiO2 embedding layer at the ZnO/Ni(OH)2 interface. A stair-like band alignment was designed to reduce the electron interface transport barrier and induce efficient electron-injection through the interface to the reaction region when it is charging. Consequently, the activation energy of reduction dropped, which further brought about a decreased equilibrium potential (Eeq) depending on the Butler–Volmer model of electrode kinetics. As expected, a superior capacitance of 1981 F g−1 at 2 A g−1 was triggered. After that, this advanced electrode was assembled in an asymmetric cell with a ZnO@Fe2O3 based negative electrode; the as-fabricated device delivered a high energy density of 52.2 W h kg−1 at a power density of 1.3 kW kg−1 within the voltage range of 0–1.6 V as well as a good cycling performance (96.6% capacity retention after 5000 cycles). These features demonstrate that suitable interface engineering may open up new opportunities in the development of high-performance supercapacitors.

High carrier concentration ZnO nanowire arrays for binder-free conductive support of supercapacitors electrodes by Al doping

Doping semiconductor nanowires (NWs) for altering their electrical and optical properties is a critical strategy for tailoring the performance of nanodevices. Here, we prepared in situ Al-doped ZnO nanowire arrays by using continuous flow injection (CFI) hydrothermal method to promote the conductivity. This reasonable method offers highly stable precursor concentration for doping that effectively avoid the appearance of the low conductivity ZnO nanosheets. Benefit from this, three orders of magnitude rise of the carrier concentration from 1016cm-3 to 1019cm-3 can be achieved compared with the common hydrothermal (CH) mothed in Mott-Schottky measurement. Possible effect of Al-doping was discussed by first-principle theory. On this basis, Al-doped ZnO nanowire arrays was developed as a binder-free conductive support for supercapacitor electrodes and high capacitance was triggered. It is owing to the dramatically decreased transfer resistance induced by the growing free-moving electrons and holes. Our results have a profound significance not merely in the controlled synthesis of other doping nanomaterials by co-precipitation method but also in the application of binder-free energy materials or other materials.