Zuxun Xu

Hierarchical ZnO microarchitectures assembled by ultrathin nanosheets: hydrothermal synthesis and enhanced photocatalytic activity

A simple and economical citrate-mediated hydrothermal route has been developed to fabricate three-dimensional hierarchical ZnO microarchitectures with high surface-to-volume ratio and large population of unconventional (0001) surface planes. This complex architecture with flowerlike morphology is assembled by many interleaving nanosheets which have ultrathin thickness of about 5 nm. According to the experimental results, a growth mechanism which involves the assembly of the nanosheets from nanoparticles into flowerlike morphology is proposed. Importantly, this type of hierarchically-structured ZnO displays a strong structure-induced enhancement of photocatalytic performance and exhibits a significantly improved photocatalytic activity in the photodegradation of methyl orange than that of other monomorphological ZnO, such as ZnO nanoparticles, nanorods, and nanosheets. In addition to the large specific surface areas due to ultrathin size of the nanosheet building blocks, the enhanced photocatalytic activity can mainly be ascribed to the special structural feature with good stability and high proportion of active (0001) planes. This work provides an efficient route for the structure-induced enhancement of photocatalytic performance by designing a desirable micro/nanoarchitecture, which could also be extended to synthesize other metal-oxide microarchitectures with superior photocatalytic performance.

A novel photoanode architecture of dye-sensitized solar cells based on TiO2 hollow sphere/nanorod array double-layer film.

A novel TiO(2) double-layer (DL) film consisting of TiO(2) hollow spheres (HSs) as overlayer and single-crystalline TiO(2) nanorod arrays (RAs) as underlayer was designed as the photoanode of dye-sensitized solar cells (DSSCs). This new-typed TiO(2) HS/RA DL film could significantly improve the efficiency of DSSCs owing to its synergic effects, i.e. the relatively large specific surface area of TiO(2) HSs for effective dye adsorption, enhanced light harvesting capability originated from TiO(2) RA film, and rapid interfacial electron transport in one-dimensional TiO(2) nanorod arrays. The overall energy-conversion efficiency of 4.57% was achieved by the formation of TiO(2) DL film, which is 16% higher than that formed by TiO(2) HS film and far larger than that formed by TiO(2) RA film (η=0.99%). The light absorption and interfacial electron transport, which play important roles in the efficiency of DSSCs, were investigated by UV-vis absorption spectra and electrochemical impedance spectra.

Surfactant-assisted hydrothermal synthesis of Bi2O3 nano/microstructures with tunable size

A novel and simple citrate-assisted solution approach has been developed for the shape-selective synthesis of Bi2O3 nanostructures with controllable bandgaps and morphologies at a relatively low temperature of 40 °C. Different distinctive morphologies, including nanorods, nanoplates, plate-built cylinders, nanoplates with holes, and nanorings, are created due to the selective adsorption of the citrate molecules on certain faces during crystal growth. The bandgaps and aspect ratios of the Bi2O3 nanostructures are easily tuned by modifying the product morphologies by adjusting the amount of trisodium citrate. More novel and complex Bi2O3 nanostructures with controllable morphologies and sizes can be manufactured with our method by optimizing the experimental parameters. The distinctive nanostructures presented here extend the family of Bi2O3 nanostructures, and they also provide new opportunities for exploring the potential applications of Bi2O3 in a number of fields including photocatalysis, gas sensors, and photoelectrochemistry.

Atomic Layer Deposition of TiO2 for a High-Efficiency Hole-Blocking Layer in Hole-Conductor-Free Perovskite Solar Cells Processed in Ambient Air

In this study we design and construct high-efficiency, low-cost, highly stable, hole-conductor-free, solid-state perovskite solar cells, with TiO2 as the electron transport layer (ETL) and carbon as the hole collection layer, in ambient air. First, uniform, pinhole-free TiO2 films of various thicknesses were deposited on fluorine-doped tin oxide (FTO) electrodes by atomic layer deposition (ALD) technology. Based on these TiO2 films, a series of hole-conductor-free perovskite solar cells (PSCs) with carbon as the counter electrode were fabricated in ambient air, and the effect of thickness of TiO2 compact film on the device performance was investigated in detail. It was found that the performance of PSCs depends on the thickness of the compact layer due to the difference in surface roughness, transmittance, charge transport resistance, electron-hole recombination rate, and the charge lifetime. The best-performance devices based on optimized TiO2 compact film (by 2000 cycles ALD) can achieve power conversion efficiencies (PCEs) of as high as 7.82%. Furthermore, they can maintain over 96% of their initial PCE after 651 h (about 1 month) storage in ambient air, thus exhibiting excellent long-term stability.

One-Step Hydrothermal Formation of Bi2O3 Nanourchins with Radially Ultrathin Nanotubes

A novel Bi 2 O 3 urchin-like microarchitecture with high-density radially oriented Bi 2 O 3 nanotubes (NTs) was fabricated by a one-step hydrothermal approach without any template or surfactant at a relative low temperature of 90 °C. The radial Bi 2 O 3 NTs on the surface of nanourchins (NUs) were 4–7 nm in diameter and ∼1.7 µm in length. The mean wall thickness of the NTs was as thin as 1.5 nm. Based on the time-dependent evolutions of morphology examined by scanning electron microscope (SEM), a “rolling-up mechanism” was demonstrated to explain the formation of the ultrathin Bi 2 O 3 NTs. The ultrathin and long NTs bring on the high surface-to-volume ratios of the Bi 2 O 3 urchin-like microarchitectures, which makes them great potential applications in gas sensors, photocatalysis, environmental purification, and solar energy conversion.

Application of mixed-organic-cation for high performance hole-conductor-free perovskite solar cells

ABX3-type organic lead halide perovskites have gained increasing attention as light harvester for solar cells due to their high power conversion efficiency (PCE). Recently, it has become a trend to avoid the use of expensive hole-transport materials (HTMs) and precious metals, such as Au, to be competitive in future commercial development. In this study, we fabricated mixed-cation perovskite-based solar cells through one-step spin-coating using methylammonium (CH3NH3+) and formamidinium (HN=CHNH3+) cations to extend the optical absorption range into the red region and enhance the utilization of solar light. The synthesized hole-conductor-free cells with carbon electrode and mixed cations exhibited increased short-circuit current, outperforming the cells prepared with pure methylammonium, and PCE of 10.55%. This paper proposes an efficient approach for fabricating high-performance and low-cost perovskite solar cells.

Synergic Effects of Upconversion Nanoparticles NaYbF4: Ho3+ and ZrO2 Enhanced the Efficiency in Hole-conductor-free Perovskite Solar Cells

Extending the spectral absorption of organic-inorganic mixed-cation perovskite solar cells (PSCs) from visible light to the near-infrared (NIR) range minimizes the nonabsorption loss of solar photons. Few studies have focused on the application of high-fluorescence NaYbF4:Ho3+ upconversion nanoparticles (UCNPs) in solar cells. In this study, NaYbF4:Ho3+ UCNPs were successfully prepared using a solvothermal method. We incorporated the NaYbF4:Ho3+ UCNPs into ZrO2 as the scaffold layer for FA0.4MA0.6PbI3 perovskite solar cells. This design enabled the dual-functional effects, that is, the harvesting of NIR light and its conversion to visible light and the reduction of the electron-hole recombination rate. The effect of different amounts of NaYbF4:Ho3+ UCNPs on the device performance was investigated in detail. The best-performance devices based on optimized addition of UCNPs (40 wt%) could achieve a power conversion efficiency as high as 14.32%, an increase of 28.8% compared with conventional ZrO2 nanoparticle-based PSCs (11.12%). The design of the synergetic operation of UCNPs and ZrO2 in the mesoporous structure of PSCs enhanced the photocurrent and photovoltage. The results offered the flexibility for the device architecture and broaden the solar spectral use of UCNP-based devices.

A two-step reduction method for synthesizingwell-dispersed, low-Pt loaded Pt/graphene nanocomposites for use as counterelectrodes in DSSCs

A new method was proposed for constructing total variation diminishing (TVD) upwind schemes in conservation forms. Two limiters were used to prevent non-physical oscillations across discontinuity. Both limiters can ensure the nonlinear compact schemes TVD property. Two compact TVD (CTVD) schemes were tested, one is third-order accuracy, and the other is fifth-order. The performance of the numerical algorithms was assessed by one-dimensional complex waves and Riemann problems, as well as a two-dimensional shock-vortex interaction and a shock-boundary flow interaction. Numerical results show their high-order accuracy and high resolution, and low oscillations across discontinuities.更多还原The globed solution for a coupled nonlinear Klein-Gordon system in two-dimensional space was studied. First, a sharp threshold of blowup and global existence for the system was obtained by constructing a type of cross-constrained variational problem and establishing so-called cross-invariant manifolds of the evolution flow. Then the result of how small the initial data for which the solution exists globally was proved by using the scaling argument.Both a real time optical interferometric experiment and a numerical simulation of two-dimension non-steady state model were employed to study the growth process of aqueous sodium chlorate crystals. The parameters such as solution concentration distribution, crystal dimensions, growth rate and velocity field were obtained by both experiment and numerical simulation. The influence of earth gravity during crystal growth process was analyzed. A reasonable theory model corresponding to the present experiment is advanced. The thickness of concentration boundary layer was investigated especially. The results from the experiment and numerical simulation match well.Based on the large deflection dynamic equations of axisymmetric shallow shells of revolution, the nonlinear forced vibration of a corrugated shallow shell under uniform load is investigated. The nonlinear partial differential equations of shallow shell are reduced to the nonlinear integral-differential equations by the method of Green’s function. To solve the integral-differential equations, expansion method is used to obtain Green’s function. Then the integral-differential equations are reduced to the form with degenerate core by expanding Green’s function as series of characteristic function. Therefore, the integral-differential equations become nonlinear ordinary differential equations with regard to time. The amplitude-frequency response under harmonic force is obtained by considering single mode vibration. As a numerical example, forced vibration phenomena of shallow spherical shells with sinusoidal corrugation are studied. The obtained solutions are available for reference to design of corrugated shells更多还原As vibration-based structural damage detection methods are easily affected by environmental noise, a new statistic-based noise analysis method is proposed together with the Monte Carlo technique to investigate the influence of experimental noise of modal data on sensitivity-based damage detection methods. Different from the commonly used random perturbation technique, the proposed technique is deduced directly by Moore-Penrose generalized inverse of the sensitivity matrix, which does not only make the analysis process more efficient but also can analyze the influence of noise on both frequencies and mode shapes for three commonly used sensitivity-based damage detection methods in a similar way. A one-story portal frame is adopted to evaluate the efficiency of the proposed noise analysis technique.更多还原A new class of generalized mixed implicit quasi-equilibrium problems (GMIQEP) with four-functions is introduced and studied. The new class of equilibrium problems includes many known generalized equilibrium problems and generalized mixed implicit quasi-variational inequality problems as many special cases. By employing the auxiliary principle technique, some predictor-corrector iterative algorithms for solving the GMIQEP are suggested and analyzed. The convergence of the suggested algorithm only requires the continuity and the partially relaxed implicit strong monotonicity of the mappings.A nonlinear Galerkin mixed element (NGME) method for the stationary incompressible magnetohydrodynamics equations is presented. And the existence and error estimates of the NGME solution are derived.The Newtonian method is employed to obtain nonlinear mathematical model of motion of a horizontally cantilevered and inflexible pipe conveying fluid. The order magnitudes of relevant physical parameters are analyzed qualitatively to establish a foundation on the further study of the model. The method of multiple scales is used to obtain eigenfunctions of the linear free-vibration modes of the pipe. The boundary conditions yield the characteristic equations from which eigenvalues can be derived. It is found that flow velocity in the pipe may induced the 3:1, 2:1 and 1:1 internal resonances between the first and second modes such that the mechanism of flow-induced internal resonances in the pipe under consideration is explained theoretically. The 3:1 internal resonance first occurs in the system and is, thus, the most important since it corresponds to the minimum critical velocity.更多还原The possible intermittent impacts of a two-stage isolation system with rigid limiters have been investigated. The isolation system is under periodic external excitation disturbed by small stationary Gaussian white noise after shock. The maximal impact Poincare map is proposed based on the multi-body dynamics with unilateral constrains. Then in the period after shock, the zero order approximate stochastic discrete model and the first order approximate stochastic model are developed. The real isolation system of an MTU diesel engine is used to evaluate the established model. After calculating of the numerical example, the effects of noise excitation on the isolation system are discussed. The results show that the property of the system is complicated due to intermittent impact. The difference between zero order model and the first order model may be great. The effect of small noise is obvious. The results may be expected useful to the naval designers.更多还原In a vertically oscillating circular cylindrical container, singular perturbation theory of two-time scale expansions is developed in weakly viscous fluids to investigate the motion of single free surface standing wave by linearizing the Navier-Stokes equation. The fluid field is divided into an outer potential flow region and an inner boundary layer region. The solutions of both two regions are obtained and a linear amplitude equation incorporating damping term and external excitation is derived. The condition to appear stable surface wave is obtained and the critical curve is determined. In addition, an analytical expression of damping coefficient is determined. Finally, the dispersion relation, which has been derived from the inviscid fluid approximation, is modified by adding linear damping. It is found that the modified results are reasonably closer to experimental results than former theory. Result shows that when forcing frequency is low, the viscosity of the fluid is prominent for the mode selection. However, when forcing frequency is high, the surface tension of the fluid is prominent.The reduction approaches are presented for vibration control of symmetric, cyclic periodic and linking structures. The condensation of generalized coordinates, the locations of sensors and actuators, and the relation between system inputs and control forces are assumed to be set in a symmetric way so that the control system posses the same repetition as the structure considered. By employing proper transformations of condensed generalized coordinates and the system inputs, the vibration control of an entire system can be implemented by carrying out the control of a number of sub-structures, and thus the dimension of the control problem can be significantly reduced.A class of Hopfield neural network with time-varying delays and impulsive effects is concerned. By applying the piecewise continuous vector Lyapunov function some sufficient conditions were obtained to ensure the global exponential stability of impulsive delay neural networks. An example and its simulation are given to illustrate the effectiveness of the results.It is demonstrated that when tension leg platform (TLP) moves with finite amplitude in waves, the inertia force, the drag force and the buoyancy acting on the platform are nonlinear functions of the response of TLP. The tensions of the tethers are also nonlinear functions of the displacement of TLP. Then the displacement, the velocity and the acceleration of TLP should be taken into account when loads are calculated. In addition, equations of motions should be set up on the instantaneous position. A theoretical model for analyzing the nonlinear behavior of a TLP with finite displacement is developed, in which multifold nonlinearities are taken into account, i.e., finite displacement, coupling of the six degrees of freedom, instantaneous position, instantaneous wet surface, free surface effects and viscous drag force. Based on the theoretical model, the comprehensive nonlinear differential equations are deduced. Then the nonlinear dynamic analysis of ISSC TLP in regular waves is performed in the time domain. The degenerative linear solution of the proposed nonlinear model is verified with existing published one. Furthermore, numerical results are presented, which illustrate that nonlinearities exert a significant influence on the dynamic responses of the TLP.