Yongming Hu

Highly Responsive Room-Temperature Hydrogen Sensing of α-MoO3 Nanoribbon Membranes

[001]-Oriented α-MoO3 nanoribbons were synthesized via hydrothermal method at temperature from 120 to 200 °C and following assembled a membrane on interdigital electrodes to form sensors. The sensitivity, response speed, and recovery speed of the sensor improve with the increasing hydrothermal temperature. Among them, the sample obtained at 200 °C exhibits a room-temperature response time of 14.1 s toward 1000 ppm of H2. The nanoribbons also show good selectivity against CO, ethanol, and acetone, as well as high sensitivity to H2 with a concentration as low as 500 ppb. The hydrogen sensing behavior is dependent on the redox reaction between the H2 and chemisorbed oxygen species. Higher hydrothermal temperature creates larger specific surface area and higher Mo(5+) content, leading to increased chemisorbed oxygen species on the nanoribbon surface.

Hydrothermal growth and optical properties of Nb2O5 nanorod arrays

Nb2O5 nanorod arrays were grown on Nb foil through an in situ hydrothermal treatment process using NH4F as the mineralizing agent and H2O2 as the oxidant. The as-prepared Nb2O5 nanorod arrays were well crystallized with a hexagonal structure and a c-axis orientation. The effects of hydrothermal temperature and concentration of NH4F on the growth of the nanorods were investigated. Nb2O5 nanorod arrays are formed by crystal nucleation, oriented growth, followed by orientation attachment. A higher concentration of NH4F accelerates the generation of Nb2O5 nanorods as a result of corroding Nb foil and releasing Nb ions and promotes the oriented growth of Nb2O5 nanorods. The band gap of Nb2O5 nanorod arrays is measured to be about 3.3 eV with a blue light emission located at 456 nm (2.719 eV) and a cyan light emission located at 490 nm (2.53 eV), respectively. The 2.53 eV peak can well be attributed to the donor–acceptor pair (DAP) emission, and the 2.719 eV peak be related to the conduction-band-to-acceptor transitions. There is only one quenching channel for 2.719 eV peak with increasing temperature, which corresponds to the activation energy of about 16.9 meV, according to the theoretical fitting.

Piezoelectric Nanowires in Energy Harvesting Applications

Recently, the nanogenerators which can convert the mechanical energy into electricity by using piezoelectric one-dimensional nanomaterials have exhibited great potential in microscale power supply and sensor systems. In this paper, we provided a comprehensive review of the research progress in the last eight years concerning the piezoelectric nanogenerators with different structures. The fundamental piezoelectric theory and typical piezoelectric materials are firstly reviewed. After that, the working mechanism, modeling, and structure design of piezoelectric nanogenerators were discussed. Then the recent progress of nanogenerators was reviewed in the structure point of views. Finally, we also discussed the potential application and future development of the piezoelectric nanogenerators.

Electrospun Bismuth Ferrite Nanofibers for Potential Applications in Ferroelectric Photovoltaic Devices

Bismuth ferrite (BFO) nanofibers were synthesized via a sol-gel-based electrospinning process followed by thermal treatment. The influences of processing conditions on the final structure of the samples were investigated. Nanofibers prepared under optimized conditions were found to have a perovskite structure with good quality of crystallization and free of impurity phase. Ferroelectric and piezoelectric responses were obtained from individual nanofiber measured on a piezoelectric force microscope. A prototype photovoltaic device using laterally aligned BFO nanofibers and interdigital electrodes was developed and its performance was examined on a standard photovoltaic system. The BFO nanofibers were found to exhibit an excellent ferroelectric photovoltaic property with the photocurrent several times larger than the literature data obtained on BFO thin films.

Fast and highly sensitive humidity sensors based on NaNbO3 nanofibers

A humidity sensor based on NaNbO3 nanofiber networks was fabricated through the electrospinning process. The as-synthesized NaNbO3 nanofibers with monoclinic perovskite structure are uniformly distributed and integrated by interdigital Pt/Ti electrodes on alumina substrates. The sensor exhibits fast and ultra-sensitive resistance type response to the variation of environmental humidity at room temperature. The sensor resistance is in logarithmic dependence on the relative humidity. The highest sensitivity is up to 105 for the humidity change from 20 to 80% RH. Moreover, the response time for the humidification process is less than 3 s. The response time for the dehumidification process is slower due to the slower desorption of water molecules. In addition, the sensor exhibits outstanding selectivity against hydrogen, ethanol, and acetone steam. Among them, the sensitivity to ethanol steam is 5 orders of magnitude smaller than that to humidity, while no sensing response is found for hydrogen and acetone. According to the fully recovered performance and non-sensing behavior to hydrogen, the sensing behavior of the nanofibers could be attributed to the electrical-field-driven transfer of proton between H3O+ induced by the physisorption of water molecules.

(K,Na)NbO3 Nanofiber-based Self-Powered Sensors for Accurate Detection of Dynamic Strain

A self-powered active strain sensor based on well-aligned (K,Na)NbO3 piezoelectric nanofibers is successfully fabricated through the electrospinning and polymer packaging process. The device exhibits a fast, active response to dynamic strain by generating impulsive voltage signal that is dependent on the amplitude of the dynamic strains and the vibration frequency. When the frequency is fixed at 1 Hz, the peak to peak value of the voltage increases from ∼1 to ∼40 mV, and the strain changes from 1 to 6%. Furthermore, the output voltage is linearly increased by an order of magnitude with the frequency changing from 0.2 to 5 Hz under the same strain amplitude. The influence of frequency on the output voltage can be further enhanced at higher strain amplitude. This phenomenon is attributed to the increased generating rate of piezoelectric charges under higher strain rate of the nanofibers. By counting the pulse separation of the voltage peaks, the vibration frequency is synchronously measured during the sensing process. The accuracy of the sensing results can be improved by calibration according to the frequency-dependent sensing behavior.

Self-Powered Viscosity and Pressure Sensing in Microfluidic Systems Based on the Piezoelectric Energy Harvesting of Flowing Droplets

The rapid development of microscaled piezoelectric energy harvesters has provided a simple and highly efficient way for building self-powered sensor systems through harvesting the mechanical energy from the ambient environment. In this work, a self-powered microfluidic sensor that can harvest the mechanical energy of the fluid and simultaneously monitor their characteristics was fabricated by integrating the flexible piezoelectric poly(vinylidene fluoride) (PVDF) nanofibers with the well-designed microfluidic chips. Those devices could generate open-circuit high output voltage up to 1.8 V when a droplet of water is flowing past the suspended PVDF nanofibers and result in their periodical deformations. The impulsive output voltage signal allowed them to be utilized for droplets or bubbles counting in the microfluidic systems. Furthermore, the devices also exhibited self-powered sensing behavior due to the decreased voltage amplitude with increasing input pressure and liquid viscosity. The drop of output voltage could be attributed to the variation of flow condition and velocity of the droplets, leading to the reduced deformation of the piezoelectric PVDF layer and the decrease of the generated piezoelectric potential.

Evidencing the structural conversion of hydrothermally synthesized titanate nanorods by in situ electron microscopy

Despite the high potential of using hydrothermal methods in the industrial production of TiO2 one-dimensional nanostructures (nanotubes, nanowires, etc.) for either environmental or energy applications, there has been much debate on the crystalline structure of the resultant hydrothermal product due to its weak crystallinity and small size. As a result, a range of titanates have been proposed as possible structures for the as-synthesized product. Herein, by using in situ transmission electron microscopy and a highly stable heating system, we microscopically explore the hydrothermally synthesized TiO2-derived single-crystal nanorods and their subsequent structural conversion upon annealing. In our case, a full set of evidence obtained from chemical, crystallographic, and valence state analyses suggests that the as-synthesized product possesses a Na0.8Ti4O8 structure, and undergoes a monoclinic-to-monoclinic transition (topochemical transformation) towards TiO2 (B) via desodiation and reorientation of TiO6 octahedra under constant heating. The as-observed kinetic restructuring process is further examined through density functional theory calculations, revealing the energy-favourable nature of the process. Such an observation at high spatial resolution demonstrates itself as an effective complement to conventional characterization methods, and may pave the way for large-scale synthesis of TiO2-based nanostructures.