A Trend-Switching Financial Time Series Model with Level-Duration Dependence

The financial time series model that can capture the nonlinearity and asymmetry of stochastic process has been paid close attention for a long time. However, it is still open to completely overcome the difficult problem that motivates our researches in this paper. An asymmetric and nonlinear model with the change of local trend depending on local high-low turning point process is first proposed in this paper. As the point process can be decomposed into the two different processes, a high-low level process and an up-down duration process, we then establish the so-called trend-switching model which depends on both level and duration (Trend-LD). The proposed model can predict efficiently the direction and magnitude of the local trend of a time series by incorporating the local high-low turning point information. The numerical results on six indices in world stock markets show that the proposed Trend-LD model is suitable for fitting the market data and able to outperform the traditional random walk model

Novel p-n heterojunction of BiVO4/Cu2O decorated with rGO for low concentration of NO2 detection

NO2 is a toxic gas that can cause photochemical smog, acid rain and ozone layer depletion. The reduced graphene oxide (rGO) decorated nanocomposites of semiconductor metal oxides (MOS) as sensing materials of NO2 sensors have received much attention. Here, BiVO4/Cu2O/rGO nanocomposite was successfully prepared by a facile solvothermal method and thermal reduction method. The gas-sensing test indicates that the response of optimum nanocomposite to 1 ppm NO2 is 8 times higher than the pure BiVO4 and 5 times higher than the p-n heterojunction of BiVO4/Cu2O, respectively at 60 degrees C operating temperature. The response time and recovery time also shortened compared to pure BiVO4 and BiVO4/Cu2O composite, and this composite has better stability for detection of NO2, which benefits from the superimposed effect of p-n heterojunction formation and rGO decoration. This work can provide reference for improving the sensing properties of metal oxide semiconductor composites for detection of NO2

Controllable Synthesis and Gas-Sensing Properties of Zinc Oxide Nanocrystals With Exposed Different Percentage of Facets

A facile and controllable one-step hydrothermal method to synthesize nanodisks and nanorods of zinc oxide with exposed different crystal facets of (0001), (1 (1) over bar 00), (01 (1) over bar0), and (10 (1) over bar0) has been realized. It was found that the average percentage of exposed (0001) crystal facets follows the order of nanodisks > nanorods-1 > nanorods-2 > nanorods-3, while the average percentage of exposed (1 (1) over bar 00), (01 (1) over bar0), and (10 (1) over bar0) crystal facets decreases monotonically. The sensing tests to NO2 indicate that the nanodisks exhibit higher sensitivity. It has also been validated that the (0001) crystal facet has the highest surface energy and the lowest binding energy, which is considered to be the key factor of high gas-sensing by the calculations of density functional theory. This finding will open a new window to explore the relationship between gas-sensing and exposed facets and will be useful for designing gas-sensing materials with specifically exposed surfaces

Hybridization of ZnSnO3 and rGO for improvement of formaldehyde sensing properties

The rGO/ZnSnO3 composites have been elaborately synthesized by a facile solution-based self-assembly synthesis method at low temperature. The structure, morphology, thermal stability, composition and specific surface area of composite were characterized by XRD, SEM, TG, XPS and BET analysis, respectively. The sensing experiments show that the sensor based on 3 wt% rGO composite not only exhibits high sensitivity (low limit of detection), excellent selectivity and linearity relationship between responses and formaldehyde concentration from 1 to 10 ppm, but also exhibits fast response and recovery at operating temperature of 103 degrees C. The enhanced sensing mechanism of the hybrid to formaldehyde was discussed in detail, which can mainly be ascribed to the large specific surface area for gas adsorption, porous channels for gas diffusion, the fast carrier transport benefited from rGO and the formation of heterojunction at the interface between the rGO and ZnSnO3. (C) 2017 Elsevier B.V. All rights reserved.</p

A novel rGO-decorated ZnO/BiVO4 heterojunction for the enhancement of NO2 sensing properties

A ZnO/BiVO4 heterojunction has been first prepared by the co-precipitation and hydrothermal methods; then, rGO nanosheets are decorated onto the heterojunction by simple hydrazine hydrate reduction to construct a novel ZnO/BiVO4/rGO composite-based NO2 sensor. The structure, morphology and gas sensing performance of the composite were characterized and measured using various spectroscopies and gas sensing tests. Compared to the same kinds of gas sensors reported previously, this sensor exhibits competitive sensing performance with a maximum response of 126.64 to 1 ppm NO2 at an operating temperature of 95 degrees C, which is 5.7 times and 2.4 times higher than those of the ZnO and ZnO/BiVO4 junctions, respectively. Also, the sensor shows a rapid response and long-term stability to NO2 compared with pure ZnO. The good sensing properties are ascribed to the formation of the n-n heterojunction between ZnO and BiVO4 and the decoration with rGO

rGO decorated CdS/CdO composite for detection of low concentration NO2

The novel sensing materials of rGO decorated CdS/CdO heterojunction have been synthesized successfully for the first time by a facile one-step hydrothermal method combining with calcination treatment. The structure, morphology, composition, and specific surface of the composite were characterized by XRD, SEM, XPS spectra and BET analysis, respectively. The gas sensing properties to 2 ppm of NO2 were examined. The results show that the 4.0 wt rGO/CdS/CdO composite not only exhibits 9 times higher response and 4.75 times lower detection limit than that of pristine CdS, but also owns rapid response and recovery, indicating the composite is a promising novel sensing material for detecting of low-concentration NO2. The enhancement mechanism was also discussed in detail, which benefits from the formation of the heterojunctions between CdS and CdO and the decoration of rGO. rGO also as an efficient electron mediator increases gas adsorption and accelerates electron transfer of the composite.</p

Synthesis of novel BiVO4/Cu2O heterojunctions for improving BiVO4 towards NO2 sensing properties

To develop a high sensitive and low temperature NO2 gas sensor, the novel BiVO4/Cu2O heterojunctions were synthesized by a modified metal organic decomposition method to decorate BiVO4 nanoplates using Cu2O nanoparticles for enhancement of BiVO4 sensing performance to NO2. The structure and morphology of BiVO4, Cu2O and BiVO4/Cu2O composites were characterized by XRD, SEM and TEM spectra. The results indicate that the BiVO4/Cu2O heterojunctions are composed of monoclinic BiVO4 nanoplates with the thickness about 1.0-1.2 mu m and 30-40 nm diameters of cubic Cu2O nanoparticles. The gas-sensing tests display that the composite exhibits rapid and linear responses to low concentration NO2 (from 100 ppb to 8.0 ppm), the highest response reaches 4.2 towards 4 ppm NO2 at 60 degrees C and relative humidity of 28.3%, which is more than 2 times of pure BiVO4 at the same condition. The enhanced sensing properties benefit from the novel p-n heterojunction between BiVO4 and Cu2O, which forms a depletion layer at the interface, leading to resistance increase of composites in NO2. The work demonstrates the as-synthesized BiVO4/Cu2O is a promising sensing material to detect NO2 gas. (C) 2020 Elsevier Inc. All rights reserved

rGO decorated W doped BiVO4 novel material for sensing detection of trimethylamine

The development of new materials forever is research front for various application fields, otherwise becomes making bricks without straw. A novel sensing material of rGO decorated W-doped BiVO4 was synthesized for the first time by metal organic decomposition combined with hydrothermal methods for detection of trimethylamine (TEA) vapor. The structure and morphology of material were characterized by spectroscopy techniques. The sensing properties of sensor to TEA were measured. The results showed that the 6WBiVO(4)/rGO5 composite exhibits response of 12.8 and response time of 16 s, which is 5.12 times higher and 2.75 times lower than that of BiVO4, respectively. Moreover, the sensor shows excellent selectivity and stability to 20 ppm TEA at 135 degrees C. The enhancement is ascribed to the increase of electron density, enhancement of specific surface and accelerating of electron transfer for BiVO4 due to W doping, rGO decoration and formation of heterojunctions

Cu2O and rGO Hybridizing for Enhancement of Low-Concentration NO2 Sensing at Room Temperature

The spherelike Cu2O and the hybrids with reduced graphene oxide (rGO) have been synthesized successfully by a facile solution-based self-assembly method. The structure, morphology, composition, and specific surface area of composite were characterized by XRD, SEM, FTIR, XPS, and BET analysis, respectively. The gassensing properties to 1 ppm of NO2 at room temperature were also examined. The results indicated that the 1 wt % rGO/Cu2O composite not only exhibits 2.8 times higher response than that of pristine Cu2O and excellent selectivity but also owns rapid response and recovery at room temperature, indicating the composite is a promising sensing material for room temperature detection of low-concentration NO2 . The enhanced sensing properties were discussed in detail, which can mainly be attributed to increased gas adsorption active sites and the fast carriers transport due to the incorporating of rGO