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We have conducted a flow-through experiment using a Flechtingen sandstone sample containing a single macroscopic fracture. Based on this experiment, we obtained range of various intrinsic rock parameters, such as permeability and specific stiffness of the combined matrix-fracture system under hydrostatic loading. In order to quantify the processes behind the laboratory observations, we carried out coupled hydro-mechanical simulations of the matrix-fracture system. Navier-Stokes flow was solved in the 3-dimensional open rough fracture domain, and back-coupled to Darcy flow and mechanical deformation of the rock matrix.To capture the volumetric shape of the fracture, the two fracture surfaces were scanned using a 3D-profilometer (Keyence VR-3200) before and after the experiment. The resulting fracture surfaces were aligned using a grid-search algorithm and subsequently offset to mimic the shear displacement as applied during the laboratory experiment. Based on the obtained 3D representation of the fracture volume embedded in a porous media, the stress path of the laboratory experiment was simulated numerically. By means of the simulation results, values of fracture closure, increase of contact area, fracture permeability and fracture stiffness due to normal load on the fracture surface were obtained.The results demonstrate that the numerical simulation could capture the elastic and inelastic behaviour as well as the related permeability alteration of the fracture domain. Both, the laboratory experiments as well as the numerical simulation indicate an inelastic deformation of the single fracture even at low normal stress. The inelastic deformation is expressed by an increase of the fracture contact area and therefore fracture stiffness with increasing stress. The increase in the contact area is due to a reduction in mean aperture and is therefore accompanied by a reduction in the fracture permeability. The development of the contact area is irreversible and thus indicates the maximum stress that the sample previously experienced. We call this behaviour "stress-memory effect".We present the workflow to obtain the numerical results and a comparison with the laboratory experiment to show that the dominant processes were captured by the simulation.

The poor electronic conductivity and large volume expansion effect of MoS2 limit its application in potassium-ion batteries (PIBs). In addition to exploring effective modification methods, it is also necessary to build a new self-standing electrode system to improve its energy density. In this work, based on the potassium storage advantages and disadvantages of MoS2 and carbon nanofibers, we have successfully prepared a self-standing soft carbon-coated MoS2 nanofiber film electrode without any additives or metal collectors. As for the application in PIBs, it exhibits excellent rate performances (about 93 mA h g−1 at the current density of 10 A g−1), and superior long-term cycling stability performances (a high-capacity retention of ~75% after 1800 cycles at the current density of 1 A g−1). The enhanced potassium storage performance can be attributed to the unique self-standing nanofiber film architectures.

Intercalation-type Nb2O5, based on its inherent structural advantages in energy storage, shows excellent energy storage characteristics in sodium-ion batteries (SIBs). The rapid pseudocapacitive Na-ion insertion/extraction dynamic mechanisms result in its outstanding rate performance. However, the inherent low electronic conductivity hinders its application and development in SIBs. Though various modification projects can effectively ameliorate these shortcomings, there are also some basic research problems that need to be clarified and solved. This review summarizes the latest research progress of Nb2O5 in SIBs. The structural advantages and pseudocapacitive characteristics of sodium storage are emphasized. The recent advanced modification strategies are summarized comprehensively, including carbon modification, structural optimization, defect engineering, increased mass loading, flexible electrodes, synergistic effect electrodes, etc. In addition, this review summarizes and prospects the key research strategies and future development directions of Nb2O5 in future practical applications.

In deep tight reservoirs like Enhanced Geothermal Systems (EGS), the fracture flow often plays a dominant role. The hydraulic and mechanical behaviors of the fracture are affected by a couple of factors such as the sealing deposits owing to mineral cementation. Here we aimed to investigate the impact of the sealing material on the hydro-mechanical properties of a rough fracture using a well-established self-affine rough fracture model. We developed finite element model based on the MOOSE/GOLEM framework dedicated to modeling coupled Hydraulic-Mechanical (HM) process of the rock-fracture system. We conducted numerical flow through a granite reservoir hosting one single large and partly sealed fracture of size 512x512 m2. Navier-Stokes flow and Darcy flow are solved in the 3-dimensional rough aperture and in the embedding poro-elastic matrix, respectively. In order to mimic the impact of the fracture sealing material on the physical properties of the rock-fracture system, we sequentially increased the amount of the fracture-filling material in the rough fracture by changing the thickness of the sealing deposits. The evolution of the contact area, fracture permeability, fracture diffusivity and normal fracture stiffness, is monitored up to the percolation threshold of the fluid flow. We show that sealing induces strong permeability anisotropy, significant decrease of hydraulic diffusivity and increase of fracture stiffness. The results have strong implications for optimizing the stimulation strategy like chemical stimulation of fractured reservoirs, as well as understanding the fluid-induced seismicity.

Energy problems restrict the sustainable development of human society [...]

We develop a fully coupled hydro-mechanical model to simulate fault slip due to fluid injection. We consider the interaction between a hydraulic fracture and pre-existing faults as well as the fluid exchange between the fracture/fault and the porous matrix. In order to consider a pressure diffusion mechanism, we set a relatively high permeability around the stimulated path. Our parametric study shows that a couple of factors affect the fault activation and its slip behavior such as fault properties, friction properties and injection scenario. We observe that pore pressure diffusion induces poroelastic stress change, which are able to produce shut-in events with a time and space lag. This mechanism also affects the slip behavior during injection in particular when the surrounding permeability is high (e.g., up to 1e-13 m 2 /s), and provides a new insight into understanding the occurrence of stronger seismic events after shut-in compared to the injection phase. In addition, we show that small perturbations may trigger large seismic fault slip which highlights the key role of the initial fault stress state. The results have profound implications for deep fluid injection related engineering as well as for soft cyclic injection strategies aiming to mitigate the risk of large earthquakes.

We investigate the impact of closing a fracture with rough surfaces on the fracture hydraulic diffusivity, which controls the spatiotemporal evolution of pore-pressure perturbations in geological formations, particularly those composed of an impermeable matrix and highly permeable natural fractures. We build distributions of synthetic fracture apertures at a reservoir scale ( $$\sim$$ 500 m) from a self-affine model with isotropic Hurst exponents derived from field observations of fault surfaces. To quantify the hydraulic diffusivity of rough fractures, we conduct finite element simulations of transient fluid flow in a single fracture. We use a surface representation of the fracture aperture following the Reynolds lubrication approximation. We verify that our approximation is valid for a steady-state flow and a low Reynolds number (Re $$\ll$$ 1) from the comparison with a volume-represented fracture aperture model solved by the Navier–Stokes equations for incompressible fluids (INS). Subsequently, the effective hydraulic diffusivity of the rough fracture is estimated by fitting the computed pressure field with the solution of an equivalent parallel plate model. The results show that the long-range correlation aperture field (up to the fault scale) due to self-affinity significantly affects hydraulic pressure diffusion, which is manifested as a strong variability in the pressure distribution with the orientation of the imposed pressure drop. Based on a rigid-plastic rheology, when closing the fracture stepwise from the initial contact to the flow percolation threshold, a decrease in the hydraulic diffusivity over seven orders of magnitude in one direction along the fracture but over four orders of magnitude in the perpendicular direction is obtained. Our results have strong implications for the interpretation of some measured hydraulic diffusivity data as well as for the use of hydraulic diffusivity in interpreting the spatial distribution of fluid-induced seismic events in faulted reservoirs.

Fluid flow along fractures or in fractured rock is of great importance in Enhanced Geothermal System, since natural fracture networks generally affect the permeability of the reservoir rocks and therefore the hydraulic performance. The cubic law commonly estimates the permeability of a single fracture, which is only valid for the flow through two smooth parallel plates. In fact, the flow performance is strongly influenced by the aperture fluctuations, which are related to the fracture surface roughness, the fluid-rock interaction process, and the amount of flow exchange between the matrix and the fracture itself, etc.To quantify the hydraulic performance and get the better knowledge of the more real fracture flow, we conduct numerical simulations of fluid flow in a fracture-rock system hosting one single rough fracture from laboratory to field scales. As an example, a 2D self-affine rough surface is synthetically generated (Candela et al, 2012), with two anisotropic roughness exponents H// = 0.6 along the slip direction, Hperp = 0.8 in the perpendicular direction and a RMS amplitude of 0.1m at the 512m scale. Based on this surface generation, the opening geometry of a rough fracture is obtained as an input structure for finite element mesh generation. On one hand, we apply a lubrication approximation and limit the fracture opening to spatially variable 2D features with lower-dimensional element embedded in a saturated porous. On the other hand, we consider the full 3D features of the fracture opening as the space between two surfaces symmetrical about the mean fracture plane. The simulations are performed in the framework of the Mutiphysics Object Oriented Simulation Environment (MOOSE) combined with a MOOSE-based application GOLEM dedicated to modeling coupled Thermal-Hydraulic-Mechanical (THM) process in fractured geothermal reservoirs.For the lubrication case, the mass balance equation for a saturated porous medium is described in terms of volumetric averaged mass conservation equations for the fluid phase, with Darcy’s law governing the momentum conservation equation. For the 3D fracture case, the incompressible Navier-Stokes equation is solved for the dynamic pressure and the velocity field inside the fracture only.We compare the 2D and 3D cases and assess the effects of the nonlinear inertial term (u•∇)u in 3D case especially when the Reynolds number is high. The objective is to evaluate the large-scale hydraulic diffusivity of the fractured domain and its anisotropy owing to the strong contrast between the fluctuating fracture opening, and the homogeneous bulk porosity. The results show that the long-range aperture variations significantly affect the fluid flow, like the channeling effect and the hydraulic diffusivity anisotropy (i.e., along and perpendicular to the fault), which may have strong implications on the spatial distribution of fluid-induced seismic events in faulted reservoir.

Ameliorating the conductivity and terrible phase aggregation are the primary tasks of tin-based anodes for practical applications in lithium storage. Inspired by this, we have adopted an in-situ gas reduction strategy for fine SnS-SnO2 nanoparticles anchoring uniformly on N-doped graphene (C@SnS-SnO2@NGr) to realize superior rate performance in lithium-ion batteries (LIBs) applications. Especially, the better electric contact between SnS and SnO2 can avoid localized reaction of SnMx (M signifies O/S) and retard serious aggregation of Sn/LixSn. As a result, a higher initial Coulombic efficiency (ICE) (78%) was achieved with almost reversible conversion reaction of Sn/Li2M. The capacity retention reaches around 85% at the current density of 0.1 A g−1 for 500 cycles (1120 mA h g−1). Besides, the N-doped graphene as the skeleton benefits the well-distribution of p-n SnS-SnO2 nanoparticles and the conductivity of hybrids. Through high-rate and longest evaluation of 2.0 A g−1, the unique anode still keeps a high capacity of 630 mA h g−1 above 1000 cycles, which accordingly reveals a dominated surface-controlled redox reaction. Correspondingly, the evolution of electrode indicates that the ameliorate conductivity by N-doped graphene and the in-situ gas reduction procedure indeed enhance the charge-transfer kinetics and contribute to a durable high-rate performance.

Lead-free ferroelectric Bi0.5Na0.5TiO3 (BNT) has attracted considerable attention taking into account environment issues and applications in micro electro mechanical systems. The effects of manganese (Mn) substitution on microstructure, lattice dynamics, and optical properties of BNT films have been investigated by X-ray diffraction, Raman scattering and ellipsometric spectra. The phase transitions as a function of temperature have been systemically explored by temperature-dependent Raman and ellipsometric spectra. The anomalous temperature-dependent behavior of Raman-active modes suggests that there is an intermediate phase between ferroelectric rhombohedral and paraelectric tetragonal phase, which is confirmed by the temperature-dependent optical band gap ( E g ) and extinction coefficient (κ) extracted from ellipsometric spectra analysis. And then, a phase diagram as a function of Mn composition has been proposed. Finally, we demonstrate that Mn-dopant is an effective approach to enhance ferroelectric properties, improve energy storage ability, and increase permittivity as well as suppress leakage current. Electrical dielectric responses indicate that there is a critical frequency and polarization relaxation behavior in the films. The present results will be helpful for the application of BNT-based lead-free multifunctional devices.

Although the photochemical and ion exchange properties of layered KNb3O8 (KN) have been extensively studied, its potential lithium storage applications were ignored. Unlike typical acid hydrothermal method, this work demonstrate that interlayer-controlled KN nanoflakes can be prepared based on alkaline hydrothermal conditions. Pristine KN performs a high first-discharge capacity and superior cyclic stability. Moreover, polydopamine derived carbon as a conductive coating shell was firstly applied to modify KN for enhancing its lithium insertion ability. It performs outstanding rate character (310, 255, 110 mA h g−1 at the current density of 0.2, 1, 10 A g−1, respectively), as compared with pristine KN (96, 51, 13 mA h g−1). It also shows excellent long-term cycling feature (209 mA h g−1 after 3000 cycles, corresponds to 95% capacity retention). In addition, relevant energy storage mechanisms have been expounded. The present work could be helpful in developing potential multifunctional applications of KN-based and other similar niobates.

Boosting the lifespan of MnO-based materials for future lithium ion batteries is one of the primary challenges due to the intrinsic low ionic conductivity and volume expansion during the conversion process. Herein, superior lithium storage in a new quaternary MnO-Cu-CNT/graphene composite has been demonstrated, which is boosted by pseudocapacitance benefitting from the three-dimensional CNT/graphene and nanosized Cu additives. Such architecture offers highly interpenetrated porous conductive networks in intimate contact with MnO-Cu grains and abundant stress buffer space for effective charge transport upon cycling. The ternary MnO-Cu-graphene electrode contributes an ever-increasing reversible capacity of 938.3 mA h g-1 after 800 cycles at 0.8 A g-1. In particular, the quaternary MnO-Cu-CNT/graphene electrode demonstrates a high specific capacity of 1334 mA h g-1 at 0.8 A g-1 after 800 cycles and long lifetimes of more than 3500 cycles at 5 A g-1 with a capacity of 557.9 mA h g-1 and close-to-100% Coulombic efficiency. The boosted pseudocapacitive lithium storage together with the simple material fabrication method in a MnO-Cu-CNT/graphene hybrid could pave the way for the development of high-capacity and long-life energy storage devices.

Improving the conductivity and charge transfer kinetics is favourable for innovation of sustainable energy devices such as metal oxide/sulfide-based electrodes. Herein, with an intercalation pseudocapacitance effect, an in situ polymerization-carbonization process for novel carbon-sealed vertical MoS2-SnO2 anchored on graphene aerogel (C@MoS2-SnO2@Gr) has enabled excellent rate performance and durability of the anode of lithium ion batteries to be achieved. The integrated carbon layer and graphene matrix provide a bicontinuous conductive network for efficient electron/ion diffusion pathways. The charge transfer kinetics could be enhanced by the synergistic effects between vertical MoS2 nanosheets and well-dispersed SnO2 particles. Based on the crystal surface matching, the ameliorated electric contact between MoS2 and SnO2 can promote the extraction of Li+ from Li2O and restrain the serious aggregation of LixSn. As a result, the improved reversibility leads to a higher initial coulombic efficiency (ICE) of 80% (0.1 A g-1 current density) compared to that of other materials. In particular, with the dominating surface capacitive process, the C@MoS2-SnO2@Gr electrode delivers a stable capacity of 680 mA h g-1 at 2.5 A g-1 for 2000 cycles. Quantitative insight into the origin of the boosted kinetics demonstrated the high pseudocapacitance contribution (above 90%) which leads to the durable high rate Li ion storage.

The structure and optical properties of the CuGa1−xCrxO2 (CGCOx) system with 0 ≤ x ≤ 1 have been investigated by combining theoretical calculations and optical experiments. Density functional theory within the generalized gradient approximation (GGA) was utilized to calculate the electronic structure of the CGCOx system. It reveals strong hybridization between the 3d states of the transition metal ions and 2p states of the O element, which has an important effect on the electronic transitions of CGCOx materials. Moreover, to confirm the theoretical results, CGCOx films with different Cr compositions were deposited via a sol–gel method and the optical properties were measured directly by temperature dependent UV-Vis transmittance and infrared reflectance spectroscopy. The frequency of two acoustic modes (Eu and A2u) gradually increases, whereas the values of the electronic band gap decrease linearly with increasing Cr composition, which can be attributed to the stronger Cr–O covalent interaction. Remarkably, an additional direct electronic band gap has been observed for the CuGa0.75Cr0.25O2 film, which shows an abnormal behavior in a low temperature region. It can be assigned to the p–d electron hybridization at the top of the valence band. These results show that the first-principles calculations agree well with the experimental data and can be used to explain the microscopic origin of the interband transitions for CGCOx films. The present work further improves the potential applications of delafossite-type oxides in the field of optoelectronic devices.

Two-dimensional layered K4Nb6O17 (KN) possesses two different types of interlayer regions, which is of great interest for applications in energy conversion, environmental purification, etc. Although the photocatalytic properties of KN have been extensively studied, there still remain some pivotal problems that need to be clarified for future applications. Here we demonstrate that the KN nano-family (including KN nanolaminas and nano hollow spheres) can be derived from the same Nb2O5-based hydrothermal reaction. Different morphologies of KN show unique microstructures and optoelectronic properties. Remarkably, the initial pH of a dye solution has been proven to play a vital role in affecting the adsorption and photocatalytic performances of KN. Due to the effects of dye sensitization, KN shows superior photodegradation performance under both ultraviolet and visible light. Based on these crucial results, a highly-efficient and feasible scheme has been proposed to deal with dye wastewater. In addition, KN as a potential anode material for lithium ion batteries has been investigated for the first time. The present work could be helpful in broadening the multifunctional applications of KN and other layered niobate materials.

Developing high power-high energy electrochemical energy storage systems is an ultimate goal in the energy storage field, which is even more difficult but significant for low-cost sodium ion batteries. Here, fluoride is successfully prepared by the electrostatic spray deposition (ESD) technique, which greatly expands the application scope of ESD. A two-step strategy (solvothermal plus ESD method) is proposed to construct a bicontinuous ordered network of 3D porous Na

Due to the intrinsic low electrical conductivity and large volume expansion of the CoFe

To accommodate huge volume change and boost the inferior electrochemical reaction kinetics of manganous oxide anodes for lithium-ion batteries, a unique 3D porous CNT/graphene-MnO architecture has been synthesized, with MnO nanoparticles homogeneously decorated on 3D interconnected CNT/graphene (3DCG) conductive networks. This porous 3DCG matrix with its abundant open pores and large surface area can provide efficient channels for fast charge transport and allow full contact between the electrode and electrolyte, leading to improved electrochemical activity. The robust 3D architecture offers abundant stress buffer space to tolerate volume expansion and ensures robust structural stability during the electrochemical processes. The synergistic effect between components endows the 3DCG/MnO electrodes with excellent electrochemical performance, retaining a high specific capacity of 526.7 mA h g-1 at 2.0 A g-1 with 98% capacity retention over 1400 cycles. This work provides a promising route for the practical application of fast and durable lithium-ion batteries and suggests insights for rational structural designs with other transition metal oxides.

Despite the extensive study of two-dimensional layered KNb3O8 (KN), there still remains some vital problems need to be clarified for future applications in environmental purification. Here we demonstrated the successful preparation of interlayer-controlled KN nanoflakes using alkaline hydrothermal conditions by adjusting the amount of thiourea in the reaction. This process resulted in KN nanoflakes with a larger specific surface area than previously reported. Moreover, the initial pH of dye solution and discrepant preferential orientation of interlayer peak have been proved to significantly influence the adsorption and photocatalysis performances of KN. In addition, relevant photocatalysis mechanisms have been expounded, by combined the first-principles calculation. The present work could be helpful in revealing the intrinsic adsorption-photocatalysis features of KN and other similar niobates.

Orthorhombic Nb

Optical properties and metal–insulator transition (MIT) of vanadium dioxide (VO2) films grown by pulsed laser deposition with different oxygen pressures (5 to 50 mTorr) have been investigated by temperature dependent transmittance spectra. Three interband critical points (E1, E2 and E3) can be obtained via fitting transmittance spectra and the hysteresis behavior of the center transition energies E1 and E2 is presented. The VO2 film grown at optimized oxygen pressure exhibits the well-defined resistivity drop (∼103 Ω cm) across the MIT process. It is found that the metal–insulator transition temperature (TMIT) increases with the oxygen pressure and the complex dielectric functions are drastically affected by oxygen pressure. It is believed that the oxygen pressure can lead to lattice defects, which introduce the donor level and the acceptor level in the forbidden gap produced by oxygen vacancies and vanadium vacancies, respectively. The donor level provides electrons for higher empty π* bands, which can make the energy barrier lower and decrease critical temperature. On the contrary, electrons jumping from the d∥ band can be recombined by holes on the acceptor, impeding the MIT occurrence. It is claimed that the electronic orbital occupancy is closely related to oxygen pressure, which changes the energy barrier and manipulates the phase transition temperature. The present results are helpful to understand the fundamental mechanism of VO2 films and practical applications for VO2-based optoelectronic devices.

High-quality lead-free piezoelectric K0.5Na0.5Nb1−xMnxO3−δ (KNNMx, 0 ≤ x ≤ 0.10) films have been successfully deposited on Pt(111)/Ti/SiO2/Si(100) substrates by a modified sol–gel method. The effects of Mn substitution on the microstructure, morphology, lattice vibrations, and optical and ferroelectric properties of the KNNMx films have been investigated in detail. All films are polycrystalline, crack-free and show a pseudo-cubic (pc) structure with a thickness of about 215 nm. Raman analysis indicates that the characteristic frequency of ν1, ν5 and ν1 + ν5 modes shifts towards lower wavenumbers with increasing Mn concentration. The optimal ferroelectric properties were obtained in the film doped with x = 0.06, whose remnant polarization (2Pr) and coercive field (2Ec) values at the applied electric field of 1000 kV cm−1 are 51 μC cm−2 and 265 kV cm−1, respectively. The increased valence of Mn2+, which is substituted at the Nb5+ site as Mn3+, plays an important role in reducing the amount of both oxygen vacancies and holes. In addition, the dielectric functions of the KNNMx films have been uniquely extracted by fitting ellipsometric spectra with the Adachi dielectric function model and a four-phase layered model (air/surface rough layer/KNNMx/Pt) in the photon energy range of 1.5–5.5 eV. The optical band gap (Eg) slightly decreases, while the high-frequency dielectric constant (e∞) linearly increases with increasing Mn concentration. Moreover, temperature dependent optical dispersion behavior of the KNNM0.06 film has been investigated from 300 K to 800 K. The analysis of Eg and the extinction coefficient (κ) reveals the correlation between optical properties and structural phase transition. Furthermore, a distinct in-plane (180°) polar nano-domain pattern with a well-defined rectangular phase hysteresis loop has been observed in the KNNM0.06 film from piezoresponse force microscopy (PFM) experiments. The present results could be crucial for potential multifunctional KNN-based device applications.

Improving the conductivity and charge transfer kinetics is favourable for innovation of sustainable energy devices such as metal oxide/sulfide-based electrodes. Herein, with an intercalation pseudocapacitance effect, an in situ polymerization-carbonization process for novel carbon-sealed vertical MoS

Two-dimensional layered K4Nb6O17 (KN) was easily formed as a secondary phase caused by the volatilization of alkali metal ions, when preparing ferroelectric K x Na1−x NbO3 based ceramics and films. In this work, it was believed that KN film is with weak ferroelectricity and has a little effect on the ferroelectric properties of K x Na1−x NbO3 based films. Moreover, temperature dependent (77–500 K) dielectric functions of KN film have been firstly extracted by fitting ellipsometric spectra with the Adachi dielectric function model and a four-phase layered model. The high-frequency dielectric constant linearly increases and optical band gap slightly decreases with increasing the temperature. We also research its photoelectrochemical properties and its application in high-efficient light-induced H2 evolution. In addition, X-ray photoelectron spectroscopy, Raman scattering, temperature dependent transmittance and infrared reflectance spectra, and first-principles calculation were conjointly performed to further reveal the intrinsic optoelectronic features and relevant mechanisms of KN.

The enhanced light-harvesting capacity and effective separation of photogenerated carriers in fantastic hierarchical heterostructures enjoy striking attention for potential applications in the field of solar cells and photocatalysis. A three-dimensional (3D) dandelion-shaped hierarchical Bi2S3 microsphere compactly decorated with wing-shaped few layered MoS2 lamella (D-BM) was fabricated via a facile hydrothermal self-assembly process. Especially, polyethylene glycol (PEG) has been proven as the vital template to form D-BM microsphere. Importantly, the as-prepared D-BM microsphere presents pH-dependent superior adsorption behavior and remarkable visible light photocatalytic activity for degradation of organic dyestuffs (Rhodamine B/RhB and Methylene blue/MB), far exceeding those for the pure Bi2S3 and MoS2. It is understandable that D-BM with high surface area possesses more active sites and promotes light utilization due to the unique porous structure with outspread wings. Besides, based on the experiments and theoretical calculations, the staggered type II band alignment of D-BM permits the charge injection from Bi2S3 to MoS2, subsequently accelerates the separation and restrains the recombination of carriers, leading to excellent photocatalytic activity, as well as the photoconductance and photoresponse performance (with Ilight/Idark ratio 567).

The enhanced light-harvesting capacity and effective separation of photogenerated carriers in fantastic hierarchical heterostructures enjoy striking attention for potential applications in the field of solar cells and photocatalysis. A three-dimensional (3D) dandelion-shaped hierarchical Bi2S3 microsphere compactly decorated with wing-shaped few layered MoS2 lamella (D-BM) was fabricated via a facile hydrothermal self-assembly process. Especially, polyethylene glycol (PEG) has been proven as the vital template to form D-BM microsphere. Importantly, the as-prepared D-BM microsphere presents pH-dependent superior adsorption behavior and remarkable visible light photocatalytic activity for degradation of organic dyestuffs (Rhodamine B/RhB and Methylene blue/MB), far exceeding those for the pure Bi2S3 and MoS2. It is understandable that D-BM with high surface area possesses more active sites and promotes light utilization due to the unique porous structure with outspread wings. Besides, based on the experiments and theoretical calculations, the staggered type II band alignment of D-BM permits the charge injection from Bi2S3 to MoS2, subsequently accelerates the separation and restrains the recombination of carriers, leading to excellent photocatalytic activity, as well as the photoconductance and photoresponse performance (with Ilight/Idark ratio 567).

Niobium-based oxides including Nb2 O5 , TiNbx O2+2.5x compounds, M-Nb-O (M = Cr, Ga, Fe, Zr, Mg, etc.) family, etc., as the unique structural merit (e.g., quasi-2D network for Li-ion incorporation, open and stable Wadsley- Roth shear crystal structure), are of great interest for applications in energy storage systems such as Li/Na-ion batteries and hybrid supercapacitors. Most of these Nb-based oxides show high operating voltage (>1.0 V vs Li+ /Li) that can suppress the formation of solid electrolyte interface film and lithium dendrites, ensuring the safety of working batteries. Outstanding rate capability is impressive, which can be derived from their fast intercalation pseudocapacitive kinetics. However, the intrinsic poor electrical conductivity hinders their energy storage applications. Various strategies including structure optimization, surface engineering, and carbon modification are effectively used to overcome the issues. This review provides a comprehensive summary on the latest progress of Nb-based oxides for advanced electrochemical energy storage applications. Major impactful work is outlined, promising research directions, and various performance-optimizing strategies, as well as the energy storage mechanisms investigated by combining theoretical calculations and various electrochemical characterization techniques. In addition, challenges and perspectives for future research and commercial applications are also presented.

Due to the intrinsic low electrical conductivity and large volume expansion of the CoFe2O4 based active materials, designing more novel structures is still one of the most important challenges for its lithium ion battery application. In this work, the CoFe2O4/reduced graphene oxide/carbon (CFO/rGO/C) composite with integrated multi-layer structure has been synthesized through a facial two-step hydrothermal method. Benefiting from the introduction of the graphene network and amorphous carbon coating layer, as well as the accompanying synergistic effect, this composite can exhibit fast and reversible lithium intercalation/deintercalation reactions. With the aid of a surface-induced capacitive process, the CFO/rGO/C composite delivers a superior specific capacity (945 mA h g-1 at 0.1 A g-1) and excellent long-term cyclic stability (421 mA h g-1 at 4 A g-1 with closely 100% Coulombic efficiency after 2000 cycles). Significantly, at a high current density of 1 A g-1, the reversible capacity exhibits a rapid increasing after 100 cycles and finally shows an ultra-high-capacity of 1430 mA h g-1 over 500 cycles. This method could be generalized to the preparation of other similar transition metal oxide-based materials for the development of high-performance energy storage systems.

To overcome inferior rate capability and cycle stability of MnO-based anode materials for lithium-ion batteries (LIBs), we reported a novel 3D porous MnO@GS/CNT composite, consisting of MnO nanoparticles homogeneously distributed on the conductive interconnected framework based on 2D graphene sheets (GS) and 1D carbon nanotubes (CNTs). The distinctive architecture offers highly interpenetrated network along with efficient porous channels for fast electron transfer and ionic diffusion as well as abundant stress buffer space to accommodate the volume expansion of the MnO nanoparticles. The MnO@GS/CNT anode exhibits an ultrahigh capacity of 1115 mAh g-1 at 0.2 A g-1 after 150 cycles and outstanding rate capacity of 306 mAh g-1 at 10.0 A g-1. Moreover, a stable capacity of 405 mAh g-1 after 3200 cycles can still be achieved, even at a large current density of 5.0 A g-1. When coupled with LiMn2O4 (LMO) cathode, the LMO [Formula: see text] MnO@GS/CNT full cell characterizes an excellent cycling stability and rate capability, indicating the promising application of MnO@GS/CNT anode in the next-generation LIBs.

The metal-insulator transition (MIT) is of key importance for understanding the fundamental electronic interaction that determines the physical properties of vanadium dioxide (VO2) film. Here, the spectral slopes of transmittance and reflectance in the infrared absorption region (about 0.62-1.63 eV) and the interband electronic transitions for VO2 films with thicknesses of 27, 40 and 63 nm have been investigated. The potential applications of the spectral slopes were presented in detail. It is found that the variation of resistivity and transmittance increases with the spectral slopes of transmittance and reflectance. It is surprising that the resistivity of the VO2 film with a thickness of 27 nm is larger than that of the VO2 film with a thickness of 40 nm in the metal state. In addition, an anomalous counterclockwise thermal hysteresis with higher energy from the interband electronic transition was also found during the MIT process for the thinnest film. It is believed that this remarkable phenomenon could be related to the correlation effects in the rutile phase, which could lead to the splitting of the a1g band into Hubbard bands. The lower Hubbard band would result in an electronic transition blue-shift with the empty e band, which can explain the origin of the counterclockwise thermal hysteresis and the abnormal resistivity in the metal state.

Post-annealing has been approved to effectively relax the out-of-plane strain in thin films. Epitaxial EuTiO3 (ETO) thin films, with and without strain, have been fabricated on (001) LaAlO3 substrates by pulsed laser deposition. The absorption and electronic transitions of the ETO thin films are investigated by means of temperature dependent transmittance spectra. The antiferrodistortive phase transition can be found at about 260–280 K. The first-principles calculations indicate there are two interband electronic transitions in ETO films. Remarkably, the direct optical band gap and higher interband transition for ETO films show variation in trends with different strains and temperatures. The strain leads to a band gap shrinkage of about 240 meV while the higher interband transition an expansion of about 140 meV. The hardening of the interband transition energies in ETO films with increasing temperature can be attributed to the Frohlich electron–phonon interaction. The behavior can be linked to the strain and low temperature modified valence electronic structure, which is associated with rotations of the TiO6 octahedra.