Pengyi Liu

Hydrogenated ZnO Core–Shell Nanocables for Flexible Supercapacitors and Self-Powered Systems

Although MnO2 is a promising material for supercapacitors (SCs) due to its excellent electrochemical performance and natural abundance, its wide application is limited by poor electrical conductivity. Inspired by our results that the electrochemical activity and electrical conductivity of ZnO nanowires were greatly improved after hydrogenation, we designed and fabricated hydrogenated single-crystal ZnO@amorphous ZnO-doped MnO2 core-shell nanocables (HZM) on carbon cloth as SC electrodes, showing excellent performance such as areal capacitance of 138.7 mF/cm(2) and specific capacitance of 1260.9 F/g. Highly flexible all-solid-state SCs were subsequently assembled with these novel HZM electrodes using polyvinyl alcohol/LiCl electrolyte. The working devices achieved very high total areal capacitance of 26 mF/cm(2) and retained 87.5% of the original capacitance even after 10 000 charge/discharge cycles. An integrated power pack incorporating series-wound SCs and dye-sensitized solar cells was demonstrated for stand-alone self-powered systems.

Flexible supercapacitors based on carbon nanotube/MnO2 nanotube hybrid porous films for wearable electronic devices

To meet the requirement of clean and efficient energy storage system for practical applications, supercapacitors (SCs) to be promising candidates for the next-generation energy storage devices have received tremendous attentions. In fact, the SCs can have broader potential if they are designed as flexible power supplies for wearable electronic devices. Herein, we develop the first flexible, low-cost and high-performance hybrid electrode based on MnO2 nanotubes (NTs) and carbon nanotubes (CNTs) by employing a facile vacuum-filtering method. The superior mechanical and electrochemical performance of these flexible electrodes is attributed to: (1) the ultra-long one-dimensional nanotube morphology, (2) the synergistic effects between pseudocapacitive MnO2 NTs and conductive CNTs, (3) hierarchical porous structure of the freestanding film, and (4) high mass loading of MnO2 (4 mg cm−2). Subsequently, the as-synthesized freestanding CNT/MnO2 NT hybrid electrodes are assembled in the form of solid-state SC devices using polyvinyl alcohol (PVA)/LiCl as gel electrolyte. The device exhibits an excellent volumetric capacitance of 5.1 F cm−3 and a high energy density of 0.45 mW h cm−3 for the entire SC volume. Finally, these SCs are integrated in a flexible power band that can drive watches and LEDs. The solid-state SC devices with outstanding flexibility and stability clearly demonstrate their broad applications as flexible, low-cost, high-performance power supplies in wearable electronic devices.

TiO2 nanowires for potential facile integration of solar cells and electrochromic devices

Self-powered systems usually consist of energy-acquisition components, energy-storage components and functional components. The development of nanoscience and nanotechnology has greatly improved the performance of all the components of self-powered systems. However, huge differences in the materials and configurations in the components cause large difficulties for integration and miniaturization of self-powered systems. Design and fabrication of different components in a self-powered system with the same or similar materials/configurations should be able to make the above goal easier. In this work, a proof-of-concept experiment involving an integrated self-powered color-changing system consisting of TiO2 nanowire based sandwich dye-sensitized solar cells (DSSCs) and electrochromic devices (ECDs) is designed and demonstrated. When sunlight illuminates the entire system, the DSSCs generate electrical power and turn the ECD to a darker color, dimming the light; by switching the connection polarity of the DSSCs, the lighter color can be regained, implying the potential application of this self-powered color-changing system for next generation sun glasses and smart windows.

Nanoscale Insights into the Hydrogenation Process of Layered α-MoO3

The hydrogenation process of the layered α-MoO3 crystal was investigated on a nanoscale. At low hydrogen concentration, the hydrogenation can lead to formation of HxMoO3 without breaking the MoO3 atomic flat surface. For hydrogenation with high hydrogen concentration, hydrogen atoms accumulated along the <101> direction on the MoO3, which induced the formation of oxygen vacancy line defects. The injected hydrogen atoms acted as electron donors to increase electrical conductivity of the MoO3. Near-field optical measurements indicated that both of the HxMoO3 and oxygen vacancies were responsible for the coloration of the hydrogenated MoO3, with the latter contributing dominantly. On the other hand, diffusion of hydrogen atoms from the surface into the body of the MoO3 will encounter a surface diffusion energy barrier, which was for the first time measured to be around 80 meV. The energy barrier also sets an upper limit for the amount of hydrogen atoms that can be bound locally inside the MoO3 via hydrogenation. We believe that our findings has provided a clear picture of the hydrogenation mechanisms in layered transition-metal oxides, which will be helpful for control of their optoelectronic properties via hydrogenation.

Characteristics of a silicon nanowires/PEDOT:PSS heterojunction and its effect on the solar cell performance.

The interfacial energy-level alignment of a silicon nanowires (SiNWs)/PEDOT:PSS heterojunction is investigated using Kelvin probe force microscopy. The potential difference and electrical distribution in the junction are systematically revealed. When the PEDOT:PSS layer is covered at the bottom of the SiNW array, an abrupt junction is formed at the interface whose characteristics are mainly determined by the uniformly doped Si bulk. When the PEDOT:PSS layer is covered on the top, a hyperabrupt junction localized at the top of the SiNWs forms, and this characteristic depends on the surface properties of the SiNWs. Because the calculation shows that the absorption of light from the SiNWs and the Si bulk are equally important, the bottom-coverage structure leads to better position matching between the depletion and absorption area and therefore shows better photovoltaic performance. The dependence of JSC and VOC on the junction characteristic is discussed.

Three-level hierarchical TiO2 nanostructure based high efficiency dye-sensitized solar cells

Hierarchical structures are believed to be a viable approach to achieve higher conversion efficiency of dye-sensitized solar cells (DSSCs). In this report, a novel three-level hierarchical TiO2 nanostructure with the first level made up of nanourchins (NUs), the second level composed of nanowires (NWs), and the third level made up of nanoparticles (NPs) was synthesized using an ion-modified hydrothermal method and subsequent TiCl4 treatment. The hierarchical structure based DSSCs have achieved an overall conversion efficiency of 3.25%, which is 103.1% higher than that of their counterpart, pure NW based DSSCs. This improvement is attributed to the larger surface area and stronger light scattering effect of this new three-level hierarchical nanostructure, which reveals the significance and broad applications of hierarchical nanostructures.

Self-Powered, High-Speed and Visible–Near Infrared Response of MoO3–x/n-Si Heterojunction Photodetector with Enhanced Performance by Interfacial Engineering

Photodetectors with a wide spectrum response are important components for sensing, imaging, and other optoelectronic applications. A molybdenum oxide (MoO(3-x))/Si heterojunction has been applied as solar cells with great success, but its potential in photodetectors has not been explored yet. Herein, a self-powered, high-speed heterojunction photodetector fabricated by coating an n-type Si hierarchical structure with an ultrathin hole-selective layer of molybdenum oxide (MoO(3-x)) is first investigated. Excellent and stable photoresponse performance is obtained by using a methyl group passivated interface. The heterojunction photodetector demonstrated high sensitivity to a wide spectrum from 300 to 1100 nm. The self-powered photodetector shows a high detectivity of (∼6.29 × 10(12) cmHz(1/2) W(-1)) and fast response time (1.0 μs). The excellent photodetecting performance is attributed to the enhanced interfacial barrier height and three-dimensional geometry of Si nanostructures, which is beneficial for efficient photocarrier collection and transportation. Finally, our devices show excellent long-term stability in air for 6 months with negligible performance degradation. The thermal evaporation method for large-scale fabrication of MoO(3-x)/n-Si photodetectors makes it suitable for self-powered, multispectral, and high-speed response photodetecting applications.

Interface Engineering To Boost Photoresponse Performance of Self-Powered, Broad-Bandwidth PEDOT:PSS/Si Heterojunction Photodetector

UNLABELLED Organic-inorganic hybrid heterojunctions are poised to push toward novel optoelectronics applications, such as photodetectors, but significant challenges complicating practical use remain. Although all organic based photodetectors have been reported with great success, their potential in high-speed, broadband, self-powered photodetectors have not been fully explored. Herein, a self-powered, broad bandwidth of photodetector based on PEDOT PSS/Si heterojunction is built by a facial low temperature spin-coating method. By interface engineering of heterojunction with optimal band alignment and heteromicrostructures, enhanced photoresponse performances are obtained. The bandwidth of the hybrid photodetector could be broadened by 10 kHz after interfacial passivation by a methyl group. Further manipulating the geometrical structure of the hybrid heterojunction with silicon nanowire, a broad spectrum response from 300 to 1100 nm, with bandwidth as high as 40.6 kHz, fast response speed of 2.03 μs and high detection of 4.1 × 10(11) Jones under zero bias was achieved. Meanwhile, the close dependence between the photoresponse performance of heterojunctions and Si nanowire length is observed in the top-coverage configuration. Finally, a coverage effects model is proposed based on the competition of Si bulk and surface recombination, which is also confirmed by the designed bottom-coverage experiment. The mechanisms behind the enhanced photoresponse of the hybrid photodetector is attributed to the optimum band alignment, as well as the optimum balance of carrier dissociation and recombination of heterojunction. The scalable and low temperature method would be of great convenience for large-scale fabrication of the PEDOT PSS/Si hybrid photodetector.

Morphology-controllable ZnO nanotubes and nanowires: synthesis, growth mechanism and hydrophobic property

Controlled growth of nanostructures has long been regarded as one of the biggest obstacles to their commercial applications. Herein we demonstrate the feasibility of controlling growth of ZnO nanotubes and nanowires by floating substrates on the surfaces of reacting solutions with different concentrations, a key factor for differential growth. These slim ZnO nanotubes with outer diameters of 26.6 ± 12.2 nm and inner diameters of 8.3 ± 6.2 nm (confidence level of 95%) are a recently discovered new class of nanostructures whose growth is driven by screw dislocations (S. A. Morin et al., Science2010, 328, 476–480). Concentrations of precursors below 0.5 mmol L−1 adequately allow the slow relocation of solute clusters and spontaneous formation of the single crystal tube while the concentrations above 15 mmol L−1 induce high-speed deposition of newly forming solute clusters and fast growth of nanowires; therefore the concentration plays a crucial role in determining the morphology of one dimensional (1D) ZnO nanostructures. Furthermore, the ZnO nanotube surface, which apparently displays smaller dimension and lower density of nanostructures, exhibits improved hydrophobic property compared to the ZnO nanowire surface. The large scale controlled growth of arrays with different 1D ZnO nanostructures should be essential for fabricating novel devices and exploring advanced applications.

Insights into the interfacial properties of low-voltage CuPc field-effect transistor.

The interfacial transport properties and density of states (DOS) of CuPc near the dielectric surface in an operating organic field-effect transistor (OFET) are investigated using Kelvin probe force microscopy. We find that the carrier mobility of CuPc on high-k Al2Oy/TiOx (ATO) dielectrics under a channel electrical field of 4.3 × 10(2) V/cm reaches 20 times as large as that of CuPc on SiO2. The DOS of the highest occupied molecular orbital (HOMO) of CuPc on the ATO substrate has a Gaussian width of 0.33 ± 0.02 eV, and the traps DOS in the gap of CuPc on the ATO substrate is as small as 7 × 10(17) cm(-3). A gap state near the HOMO edge is observed and assigned to the doping level of oxygen. The measured HOMO DOS of CuPc on SiO2 decreases abruptly near E(V(GS) = V(T)), and the pinning of DOS is observed, suggesting a higher trap DOS of 10(19)-10(20) cm(-3) at the interface. The relationships between DOS and the structural, chemical, as well as electrical properties at the interface are discussed. The superior performance of CuPc/ATO OFET is attributed to the low trap DOS and doping effect.