No abstract

Many studies have analysed the nonstationarity in single hydrological variables due to changing environments. Yet, few researches have been done to investigate how the dependence structure between different individual hydrological variables is affected by changing environments. To investigate how the reservoirs have altered the dependence structure between river flows at different locations on the Hanjiang River, a time‐varying copula model, which takes the nonstationarity in the marginal distribution and/or the time variation in dependence structure between different hydrological series into consideration, is presented in this paper to perform a bivariate frequency analysis for the low‐flow series from two neighbouring hydrological gauges. The time‐varying moments model with either time or reservoir index as explanatory variables is applied to build the time‐varying marginal distributions of the two low‐flow series. It's found that both marginal distributions are nonstationary, and the reservoir index yields better performance than the time index in describing the nonstationarities in the marginal distributions. Then, the copula with the dependence parameter expressed as a function of either time or reservoir index is applied to model the variable dependence between the two low‐flow series. The copula with reservoir index as the explanatory variable of the dependence parameter has a better fitting performance than the copula with the constant or the time‐trend dependence parameter. Finally, the effect of the time variation in the joint distribution on three different types of joint return periods (i.e. AND, OR and Kendall) of low flows at two neighbouring hydrological gauges is presented. Copyright © 2014 John Wiley & Sons, Ltd.

Despite the fact that many strategies have been developed to improve the efficiency of the oxygen evolution reaction (OER), the precise modulation of the surface electronic properties of catalysts to improve their catalytic activity is still challenging. Herein, we demonstrate that the surface active electron density of Co3O4 can be effectively regulated by an argon‐ion irradiation method. X‐ray photoelectron and synchrotron x‐ray absorption spectroscopy, UV photoelectron spectrometry, and DFT calculations show that the surface active electron density band center of Co3O4 has been upshifted, leading to a significantly enhanced absorption capability of the oxo group. The optimized Co3O4‐based catalysts exhibit an excellent overpotential of 260 mV at 10 mA cm−2 and Tafel slope of 54 mV dec−1, superior to the capability of the benchmark RuO2, representing one of the best Co‐based OER catalysts. This approach could guide the future rational design and discovery of ideal electrocatalysts.

which is composed of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). [1] Among them, OER displays a high thermodynamic voltage (1.23 V), because it needs more energy to overcome the sluggish kinetics of the four-electron reaction in comparison with the simple HER system (two-electron reaction). [2] Moreover, these two halfreactions typically undergo the different determining steps and chemisorption of the water-splitting intermediates, which often require different electrolytes to support them. [3] In addition, most catalysts can only be used for HER or OER, and employing diverse catalysts for an integrated electrolyzer requires sophisticated processes, resulting in increased costs. [4] Thus, it is indispensable to construct efficient bifunctional electrocatalysts and realize both efficient OER and HER. Initially, the precious metal catalysts were applied for water splitting, such as RuO 2 , IrO 2 for OER, and Pt/C for HER, but the scarcity, high cost, and poor stability hindered their promotion in industry application. [5] Ni metal is cheap, plentiful, and optimally positioned on the Volcano Plot, which means the moderate adsorption/desorption ability to the intermediate species, can be used to catalyze OER and HER simultaneously, usually owing superior performance to the noble metal catalysts. [6] Moreover, urea oxidation reaction (UOR, CO(NH 2 ) 2 + 6OH − → N 2 + CO 2 + 5H 2 O +6e − ) exhibits an intrinsic lower thermodynamic potential of 0.37 V, which can replace the OER as the anodic for electrolyzer to realize energy-saving H 2 production and urea wastewater treatment simultaneously. [7] In addition, UOR is the dominating reaction of direct urea fuel cells. [7b,8] Similar to OER, Ni metal and its derivatives have been reported to be the most promising in UOR catalysis. [9] Although Ni metal and its oxides or (oxy)hydroxides have been unremittingly designed for overall water and urea splitting, these catalysts suffer poor electrical conductivity, resulting in unsatisfactory catalytic performance and durability. Fortunately, Ni 3 S 2 , a metal chalcogenide occurs naturally as the pyrite and mineral heazlewoodite, has intrinsic metallic behavior because of Ni−Ni bonds throughout its structure, which exhibits better electrical conduction and corrosion resistance than nickel oxides or (oxy)hydroxides. [10] Moreover, Exploring earth-abundant, highly effective, and stable electrocatalysts for overall water and urea electrolysis is urgent and essential for developing hydrogen energy technology. Herein, a simple self-derivation method is used to fabricate a Fe-doped Ni 3 S 2 electrode. The electrode exhibits an impressive trifunctional catalyst, with low overpotentials of 290, 198, and 254 mV at 100 mA cm −2 for the oxygen evolution reaction (OER), urea oxidation reaction (UOR), and hydrogen evolution reaction (HER). The durability is higher than 3500 h (146 days) at 100 mA cm −2 for the OER without obvious change. In situ Raman spectra reveal the incorporation of Fe inhibited S dissol...

In recent years, In(2)O(3) nanowires (NWs) have been widely explored in many technological areas due to their excellent electrical and optical properties; however, most of these devices are based on In(2)O(3) NW field-effect transistors (FETs) operating in the depletion mode, which induces relatively higher power consumption and fancier circuit integration design. Here, n-type enhancement-mode In(2)O(3) NW FETs are successfully fabricated by doping different metal elements (Mg, Al, and Ga) in the NW channels. Importantly, the resulting threshold voltage can be effectively modulated through varying the metal (Mg, Ga, and Al) content in the NWs. A series of scaling effects in the mobility, transconductance, threshold voltage, and source-drain current with respect to the device channel length are also observed. Specifically, a small gate delay time (0.01 ns) and high on-current density (0.9 mA/μm) are obtained at 300 nm channel length. Furthermore, Mg-doped In(2)O(3) NWs are then employed to fabricate NW parallel array FETs with a high saturation current (0.5 mA), on/off ratio (>10(9)), and field-effect mobility (110 cm(2)/V·s), while the subthreshold slope and threshold voltage do not show any significant changes. All of these results indicate the great potency for metal-doped In(2)O(3) NWs used in the low-power, high-performance thin-film transistors.

Unraveling the essence of hydrogen adsorption and desorption behaviors can fundamentally guide catalyst design and promote catalytic performance. Herein, the regulation of hydrogen adsorption is systematically investigated by d–d orbital interaction of metallic tungsten dioxide (WO2). Theoretical simulations show that the incorporation of post‐transition metal atoms including Fe, Co, Ni, and Cu can gradually reduce the bond order of W—M sites, consequently weakening the hydrogen adsorption and accelerating the hydrogen evolution reaction (HER) process. Under that theoretical guidance, various 3d metal doped WO2 electrocatalysts are systematically screened for HER catalysis. Among them, the Ni‐WO2/nickel foam exhibits an overpotential of 41 mV (−10 mA cm−2) and Tafel slope down to 47 mV dec−1 representing the best tungsten‐based HER catalysts so far. This work demonstrates that optimizing hydrogen adsorption via d–d orbital modulation is an effective approach to developing efficient and robust catalysts.

Under changing environments, not only univariate but also multivariate hydrological series might become nonstationary. Nonstationarity, in forms of change‐point or trend, has been widely studied for univariate hydrological series, while it attracts attention only recently for multivariate hydrological series. For multivariate series, two types of change‐point need to be distinguished, i.e., change‐point in marginal distributions and change‐point in the dependence structure among individual variables. In this paper, a three‐step framework is proposed to separately detect two types of change‐point in multivariate hydrological series, i.e., change‐point detection for individual univariate series, estimation of marginal distributions, and change‐point detection for dependence structure. The last step is implemented using both the Cramér‐von Mises statistic (CvM) method and the copula‐based likelihood‐ratio test (CLR) method. For CLR, three kinds of copula model (symmetric, asymmetric, and pair‐copula) are employed to construct the dependence structure of multivariate series. Monte Carlo experiments indicate that CLR is far more powerful than CvM in detecting the change‐point of dependence structure. This framework is applied to the trivariate flood series composed of annual maxima daily discharge (AMDD), annual maxima 3 day flood volume, and annual maxima 15 day flood volume of the Upper Hanjiang River, China. It is found that each individual univariate flood series has a significant change‐point; and the trivariate series presents a significant change‐point in dependence structure due to the abrupt change in the dependence structure between AMDD and annual maxima 3 day flood volume. All these changes are caused by the construction of the Ankang Reservoir.

Metal/semiconductor contact is a significant constraint in short-channel field effect transistors (FETs) comprising black phosphorus (BP) and other 2D semiconductors. Due to the pinning effect at metal/2D semiconductor interface, the Schottky barrier usually does not follow the Schottky-Mott rule, resulting in thickness-dependent FET performance. In this work, the Schottky barrier in BP FETs is investigated via theory calculation and electrical measurement. A simple metal/BP contact model is presented based upon thickness-dependent electrical characteristics of BP FETs. The model considers the Schottky barrier as a combined effect of the Schottky-Mott rule and the pinning effect and provides a feasibility to track the conducting behavior of other 2D semiconductor FETs.