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Local strain, as a small degree and single direction strain method, can effectively regulate the structures and electronic properties of armchair Janus MoSSe nanoribbon, so that the system can be transformed from the original 0.467 eV indirect band gap into 0.259 eV (3-zig), 0.117 eV (3-arm), 0.080 eV (6-arm) and 0.139 eV (9-zig) direct band gap semiconductor according to the different strain degrees and directions. Compared with traditional MoS2 and MoSe2 nanoribbons, Janus MoSSe nanoribbon shows relatively stable band structure under local strain. The structures and electronic properties of Janus MoSSe nanoribbon are anisotropic when the local strain along different directions. Due to the broken mirror symmetry of Janus system and the appearance of in-plane local polarization, the spin polarization effect of Janus nanoribbon under local strain is more remarkable. When the local strain degree C = 0.167 along the zigzag direction and the local strain C ≥ 0.056 along the armchair direction, the Janus nanoribbon exhibits half-metallic property and surprisingly induces magnetic moment. For the local strain along the direction of armchair, the total magnetic moment of the system can be up to 2.05 μB when C = 0.111. To sum up, a local strain method is applied to the nanoribbon system, which can effectively regulate the geometric configuration and electronic structure without external doping, and introduce magnetism, providing the possibility for expanding nanoribbons as potential nanoelectronics and spintronic material.

Electrocatalytic hydrogen evolution is an exercisable way to achieve large-scale application of hydrogen energy. It is of great significance to develop an effect, stable, and cost-effective electrocatalyst. Here, we applied the two-dimensional (2D) bismuth (Bi) to the electrocatalytic hydrogen evolution, and proposed the strategies to enhance the catalytic performance of the catalyst. Due to more active sites located along the edges of 2D structure, Bi nanosheets revealed a higher electrocatalytic activity (overpotential of −958 mV vs RHE at 10 mA cm−2, Tafel slope of 122 mV/dec) than the bulk counterpart. To further evaluate the electrocatalytic performance of Bi nanosheets, the typical parameters measured in different H+ concentration (C[H+]) are carried out. The improved catalytic activity obtained in 0.5 M H2SO4 is attributed to enhancing the hydrogen adsorption and accelerating the charge transport on the surface of catalyst. Moreover, the durability of Bi nanosheets has been texted, where the current is not evident fluctuation during the 40,000 s electrolysis measurement indicating its excellent stability. The present work expands the application of Bi in the catalysis and provides the simple strategies to improve its hydrogen evolution performance.

Two-dimensional (2D) molybdenum disulfide (MoS2) nanomaterials have emerged as promising candidates for constructing excellent supercapacitors, but the lack of large-scale, efficient and low-cost methods for preparing MoS2 nanosheets severely hinders its practical application. This study demonstrates an accessible and efficient approach for electrochemical exfoliating bulk MoS2 into high-quality MoS2 nanosheets with size distribution in the range of 1–3 μm and a thickness of several nanometers in an easily available inorganic salt solution. Furthermore, we construct symmetric all-solid-state supercapacitors based on exfoliated MoS2 nanosheets. The 2D structure will provide stable channels to facilitate the intercalation/desorption of ions during charge and discharge, and to a certain extent can prevent deposition and agglomeration. Therefore, compared with the unexfoliated MoS2, as-prepared MoS2 nanosheets show great improvement in supercapacitor performance (the specific capacitance increases from the original 130 F g−1 to 215 F g−1 at 5 A g−1, for instance). In detail, the electrode possesses a specific capacitance of 285 F g−1 at a current density of 2 A g−1 and maintains the great capacitance retention of 83.8% at 8 A g−1. Moreover, the supercapacitor exhibits a high energy density of 136.8 Wh kg−1 at a power density of 2550 Wh kg−1. This work provides basic research on the preparation of 2D nanomaterials by electrochemical exfoliation.

Two-dimensional (2D) MoTe2 nanomaterials have emerged as a promising candidate for constructing excellent supercapacitors due to their high capacitive performance. Recent studies have revealed that MoTe2 has broad prospects as an active material in supercapacitors, although its strict requirements in terms of the external environment and complex preparation process remain serious obstacles to its practical application. Toward this end, MoTe2 nanosheets (NSs) were prepared by a facile and inexpensive liquid-phase exfoliation approach for the fabrication of MoTe2 electrodes. The results show that the MoTe2 NSs exhibit significantly improved electrochemical performance, with a specific capacitance (859 F g−1) more than three times that of bulk MoTe2 (271 F g−1) at a current density of 1 A g−1. In addition, the MoTe2-based electrode showed excellent cycle stability and maintained excellent specific capacitance (92.7% of the initial value) after 1000 cycles under the working condition of a current density of 10 A g−1. This study describes a fundamental investigation on the energy storage and conversion properties of 2D layered materials.

Van der Waals (vdW) heterostructure, vertically assembled by two kinds of two-dimensional layered materials with extraordinary electronic and optical properties, has emerged as an interesting candidate in the applications of electronics and optoelectronics. It is known that vacancies are crucial in determining the physical properties of materials, and always exist in the materials during the process of actual experimental preparations and can be artificially introduced. In the present work, the structures and electronic properties of a black phosphorus/molybdenum disulfide (BP/MoS2) vdW heterostructure with P atom vacancies are investigated via first principles calculations. Based on the structural symmetry, two types of heterostructures with single-atom vacancy and three types of heterostructures with diatomic vacancies are constructed. It is found that the presence of a single P atom vacancy leads to the transformation from semiconductor to metal in the BP/MoS2 heterostructure, which is mainly due to the contribution of the remaining P atomic p orbitals based on analyses of the partial density of states (PDOS). On the other hand, the heterostructures with diatomic P atom vacancies still maintain their pristine semiconductor features, along with a decrease in their bandgap value. In addition, the plane electrostatic potential indicates that the introduction of P atom vacancy defects causes a change in the electrostatic potential of the BP atomic layer, resulting in asymmetric electrostatic potential on both sides of the BP layer. Finally, it is interesting to note that the accumulation-depletion of electrons occurs around the vacancy, but also partially emerges at the MoS2 layer. Our results provide the possibility for the application of 2D-vacancy-defect heterostructure systems.

Inspired by potential application prospects of spintronics and valleytronics, a novel heterobilayer Janus structure is designed by replacing the chalcogenide atomic layers in the original bilayer MoS2. Based on first-principles calculations, it is found that the SMoS/SeMoS structure exhibits a direct band-gap semiconductor and a typical type-II band alignment with longer carrier lifetime. The transition metal (TM) atom represented by V/Cr/Mn can be stably adsorbed on the heterobilayer Janus SMoS/SeMoS sheet and effectively introduce magnetic moments (m). The calculation results demonstrate that the most stable adsorption site of the TM atom is CX(A), and the TM (V/Cr/Mn) adatom modified SMoS/SeMoS system is converted into metal (V-) or half-metal (Cr/Mn-), respectively. Under the coupling of different indirect exchange interactions, the structure exhibits stable intrinsic anti-ferromagnetic interactions for V-SMoS/SeMoS and ferromagnetic ground state for Cr/Mn-SMoS/SeMoS, respectively, and the magnetic transition temperature (T c) reaches a high temperature or even room temperature. Moreover, the robust out-of-plane magnetocrystalline anisotropy energy ensures stable long-range magnetic order. Specifically, the combination of spin injection and strong spin–orbit coupling interaction effectively breaks the time-reversal symmetry, which leads to valley polarization of the system. Based on this, the biaxial strain can effectively regulate the electronic structure, magnetic properties and valley polarization of TM-SMoS/SeMoS nanosheets with double breaking of spatial-inversion and time-reversal symmetry.

Aiming to find an excellent anode material for Li-ion batteries, we report the two-dimensional Zr2B2 monolayer combining the density functional theory and crystal structure prediction techniques. We find that the monolayer Zr2B2 has initial metallicity and good stability, and keeps excellent electrical conductivity during the whole process of lithiation. The adsorption energy of lithium is −0.628 eV, which is enough to make sure the stability of the processing of lithiation. Furthermore, the ultralow diffusion energy barrier of Li ion on the surface of the monolayer Zr2B2 (17 meV) indicates an excellent charge-discharge rate. The theoretical specific capacity of 526 mA h g−1 is larger than that of the commercial graphite electrode. All these results propose the hexagonal monolayer Zr2B2 as an excellent anode material for Li-ion batteries.

In this report, WS2-reduced graphene oxide (WS2-rGO) hybrid was successfully synthesized through a hydrothermal reduction process in which WS2 nanosheets (WS2 NSs) were prepared by a lithium-ion intercalation and exfoliation technique. The photodetection properties of as-synthesized samples have been systematically investigated using a photoelectrochemical (PEC) system, which presents a reproducible photoresponse of WS2-rGO under zero bias, indicating that such hybrid is a promising photoanode for self-powered photodetectors. At a bias of 0.8 V, the photocurrent density of WS2-rGO hybrid was 4.32 μA/cm2, which is approximately 800% of that of the bare WS2 photodetector. Meanwhile, the photoresponsivity of as-prepared WS2-rGO hybrid increases linearly with the irradiation intensity, up to 9.3 μA/W at power density of 30 mW/cm2. In addition, the stability test exhibits no conspicuous degradation after 100 cycles. The enhanced performance of the WS2-rGO hybrid benefits from the high carrier mobility of rGO and the efficient separation of photogenerated carriers at the WS2-rGO interface. This research demonstrates that the WS2-rGO hybrid has a good prospect in PEC-type photodetectors.

Tin disulfide/graphene composites were successfully synthesized by one-step hydrothermal method. A novel SnS2/graphene solid-state photoelectrochemical photodetector with the advantages of small size, light weight, easy portability, and easy storage was successfully constructed based on solid-state electrolytes. The SnS2/graphene composites with pleated flower-like structure were characterized by scanning electron microscope characterization. At the same time, X-ray diffraction and Raman spectroscopy were carried out to confirm the composition and inherent physical properties of SnS2/graphene. Photoelectrochemical tests show that the SnS2/graphene solid-state photoelectrochemical photodetector has excellent photoresponse characteristics, and its photocurrent density is about 9 nA/cm2 under sunlight irradiation without additional power. In addition, the SnS2/graphene solid-state photoelectrochemical photodetector exhibits a good stability and the photocurrent density is only slightly attenuated (77% of the initial value) after 2000 s (50 cycles). Experimental results that SnS2/graphene solid-state photoelectrochemical photodetector is a potential new type self-powered photodetector. We believe that solid-state electrolytes with the advantages of small size, light weight, easy portability, and easy storage can be extended to other fields, such as solar cells, and supercapacitors.

Tin disulfide (SnS2) has been attracted intensive attention in the field of photoelectric conversion due to its appropriate band gap and glorious electronic mobility. The hexagon SnS2 nanosheets has been successfully integrated through a facile one-pot hydrothermal method. SEM images, Raman spectra, atomic force microscope and X-ray diffraction patterns are measured to carry out to investigate the morphologies and microstructures of SnS2 nanosheetsm, confirming a good crystallized SnS2. Then, the photochemical activity of as-prepared SnS2 nanosheets were tested in the electrolyte of Na2SO4. Photoelectrochemical tests demonstrate that the photocurrent density of as-prepared hexagon SnS2 nanosheets (1.66 µA/cm2 at a light intensity of 140 mW/cm2) is hugely increased with increasing light intensity. Furthermore, after 50 cycles, the photocurrent density does not change significantly, indicating that the as-prepared SnS2 nanosheets possesses superior stabilities. The outstanding photocatalytic performances of SnS2 nanosheets are not only resulted from its huge specific surface area, which can harvest more light and provide more active sites, but also attributed to its superior charge mobility, which can facilitate the separation photogenerated electron–hole pairs and the charge transfer between SnS2 nanosheets and the electrode. The most important is that our work reveals the hexagonal SnS2 nanosheets not only possess superior photoelectrochemical properties, but also have great potential applications in energy conversion and photodetector fields.

Two-dimensional C3N has attracted various attention owing to its excellent electrical and mechanical properties. However, a critical issue is that the trap of Li, introduced by the vacancy defect during experimental preparation, will weaken the performance of the batteries. In this work, atom-doped regulation is proposed to repair the vacancy and further improve the performance of pristine C3N. The calculated formation energy indicates that C and B atoms are more favorable to appear as compared to other atoms such as Si, P, and N atom. Moreover, the Li ionic conductivity and the binding energy of Li on doped C3N are greatly improved compared to pristine C3N without the sacrifice of mechanical properties, and migration capability. These studies could provide theoretical guidance for the realization of high-performance C3N anode materials.

Molybdenum Disulfide (MoS2), as a typical two-dimensional (2D) material, has abundant reserves and unique electronic structures. However, the limited number of active sites severely hinders its application in electrocatalytic oxygen evolution reaction (OER). Black phosphorus quantum dots (BP QDs), with large exposed surface areas and a mass of active edge sites, are suitable for loading on MoS2 nanosheets to form type-II heterojunctions, resulting in additional active sites and higher mobility, thereby improving the OER performance of MoS2. In addition, photo-assistance was applied to reduce high energy barriers and the energy loss, thus improving the overall efficiency of energy conversion. Here, BP QDs and MoS2 nanosheets were prepared by liquid phase exfoliation and mixed-dimensional BP QDs/MoS2 heterojunctions were constructed. The experimental results show that the BP QDs/MoS2 heterojunctions have been successfully synthesized and the OER performance of the heterojunctions outperform pure MoS2 and BP QDs, with lower onset potentials and Tafel slope. Meanwhile, both photo-assistance and changing KOH concentration can adjust the OER performance of BP QDs/MoS2 heterojunction. Furthermore, the as-prepared heterojunctions demonstrated strong stability in KOH aqueous solution. This work identifies that constructing BP QDs/MoS2 heterojunctions and photo-assistance are effective strategies to facilitate the electrocatalytic OER performance.

A novel P25/Black phosphorus/Graphene hybrid has been successfully prepared by loading two components [P25 and Black phosphorus (BP)] on graphene nanosheets via a simple one-step hydrothermal method. The P25/BP/Graphene hybrids are characterized by scanning electron microscopy (SEM),Raman spectra and X-ray diffraction patterns, which confirm a good crystallized P25 and BP hybridization with graphene. Photoelectrochemical tests verify that the photocurrent density of as-prepared P25/BP/Graphene ternary hybrid (9.32 μA/cm2) is greatly improved at 1 V, which is nearly 34 times higher than that of sole P25 and 4.8 times as much as that of P25/Graphene. The improved photocatalytic activity is proposed to be benefited from the higher carrier mobility and additional accessible sites derive from the special configurations of ternary hybrid, as well as the introduction of the visible and near-infrared-activated BP photocatalyst. More importantly, this work demonstrates that the as-prepared P25/BP/Graphene hybrid would be an attractive candidate as high-performance photocatalyst, and provide positive proof of concept for developing the practical applications of graphene and black phosphorus based composites.

Bismuth as an element of group VA (P, As, Sb, Bi), which is considered as beyond graphene materials, has attracted much attention because of its stable topological insulator properties. Herein, photoelectrochemical (PEC)-type photodetector based on two-dimensional bismuth nanosheets was designed and fabricated. By using a solid electrolyte to replace the traditional electrolyte, we have achieved a novel structure to make the device more portable. Besides that, due to the solid/liquid heterojunction, which generates an built-in electric field that can separate electron hole pairs effectively, this type of photodetector possess self-powered ability, and it exhibited preferable photoresponsivity which is about 0.5 μA W−1 without external bias potential; meanwhile, the rise time of the photodetector is about 3.3 s and decay time is about 1.8 s. Moreover, we used the flexible indium-tin oxide (ITO) as the substrate; the prepared device can work under the bending states. The novel structure and the excellent properties of the fabricated self-powered flexible PEC-type photodetector make it more promising in the optical detection field.

Metal atom adsorption is recently considered to be an efficient approach to achieve stability of black phosphorus required by practical devices with no damage to its pristine excellent properties. Using first-principles calculations, we studied the structures and electronic properties of monolayer black phosphorus with Fe atom adsorption under different biaxial strain. Monolayer BP becomes an indirect bandgap semiconductor due to the contribution of adsorbed Fe atom. Strain is considered as a promising way to turn the electronic structures of the adsorption systems. The compressive strain reduces the band gap value of the systems, and eventually results in a semiconductor-metal transition when the strain reaches −8%. The tensile strain would turn the value of band gap ranging from 0.96 eV to 0.60 eV, and an indirect-direct band gap transition would occur with the tensile strain of 4%. In addition, it is found that the charge density difference and charge transfer nearly unchanged under different strain, means that the Fe–P covalent bonds are very strong. The combination of adjustable electronic properties with stability makes black phosphorus an attractive material for fundamental physics studies.

Antimony phosphorus in the alpha phase (α-SbP) is a novel two-dimensional binary material with suitable band gap and environmental stability, it has attracted attention for its promising applications in the field of optoelectronics. In this work, the electronic and optical properties of α-SbP monolayer under biaxial strain have been studied by first principles calculation method based on density functional theory. It is found that when the tensile strain is applied, the band gap value of monolayer SbP increases firstly and then decreases under tensile strain, but the whole band gap value fluctuated is controlled within 0.08 eV. However, in the process of applying compressive strain, the band gap value decreases linearly with the strain value, and semiconductor-metal transition occurs at about −5% strain value. This is due to the difference in coupling between Sb and P atoms. Its optical properties under different strains are analyzed in detail, and it is found that the optical properties of SbP changed little under tensile strain, the light absorption peak is mainly concentrated in the ultraviolet region. In contrast, the compressive strain significantly adjust the optical properties of SbP. The innovation of this work is that the optoelectronic properties of SbP are studied by means of biaxial strain regulation. We find that the optoelectronic properties of SbP are robust to tensile strain. On the other hand, compressive strain has a significant effect on the photoelectric properties of SbP, and semiconductor-metal phase transition occurs. The research work in this paper provides theoretical support for the applications of monolayer α-SbP as a robust tensile and flexible compression material in optoelectronic nanodevices.

Two dimensional Tin Selenide (SnSe) nanosheets (NSs) have been prepared via a facile hydrothermal intercalation and exfoliation route. Morphological test verifies high yield of SnSe NSs with good quality. Additional X-ray diffraction pattern and Raman spectra are carried out and confirm the exfoliated SnSe nanosheet is pure and well crystalized. AFM measurement, along with the SEM images and Raman shifts, reveals few-layers SnSe nanosheet has been successfully obtained after hydrothermal intercalation and exfoliation route. Photoelectrochemical tests also demonstrate the photocurrent density of SnSe NSs is greatly improved compare to that of bulk SnSe. Photocurrent density of exfoliated SnSe NSs can achieve 16 μA/cm2 when the applied potential is 0.8 V, which is nearly four times higher than that of bulk SnSe. This work demonstrates that the two-dimensional SnSe NSs may have a great potential application in photovoltaic devices.

In this work, CuO nanoneedles have been in situ grown on Cu foam through simple thermal oxidation in the air. The morphologies and microstructures of as-prepared sample were characterized by using X-ray diffraction, Raman spectrum, scanning electron microscopy, transmission electron microscopes. It was found that the one-dimensional CuO nanoneedles with single crystalline were uniform distributed upon the layer of Cu foam’s skeleton. Directly using the as-prepared sample as the anode material for lithium-ion battery, the electrochemical performances of CuO nanoneedles on Cu foam were investigated by galvanostatic discharge–charge tests, cyclic voltammetry, as well as impedance spectroscopy measurement. The in situ growth electrode without binder realized an initial capacity as high as 940 and 446 mAh g−1 still retained even after 100 cycles of charging and discharging. The superior lithium storage performance may be attributed to the unique hierarchical architectured of nanoneedles on skeleton of porous foam, which offers excellent electro-conductivity for the intimate contact between CuO active material and Cu foam current collector, and provides a large surface to volume ratio that favors lithium-storage. The results propose that the as-prepared CuO nanoneedles on Cu foam would be a potential anode material for lithium-ion battery applications.

Novel TiO2/MoSe2 catalyst has been synthesized via a hydrothermal method. The morphologies, microstructures and optical properties were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, UV–Vis absorption spectroscopy, the result show that a large quantity of MoSe2 nanosheets (NSs) were obtained from MoSe2 bulks by lithium intercalatation way and TiO2 nanoparticles has been grown on the NSs successfully. According to the photoelectrochemical measurements, the TiO2/MoSe2 hybrid nanostructures show the photocurrent density of 15 μA/cm2, which is much higher than bare TiO2 of 8 μA/cm2 and bare MoSe2 of 10 μA/cm2. It is proposed that type-II heteroshtucture of TiO2/MoSe2 can efficiently promote photocatalytic activity.

Two-dimensional NiSe2 nanosheets arrange on three-dimensional carbon cloths (CC) have been synthesized through a facile two-step hydrothermal process. The as-prepared NiSe2/CC photoelectrode demonstrates outstanding photoelectrochamical (PEC) performance. Subsequently, a series of characterization techniques including X-ray diffraction and Raman spectra are carried out and confirm a well crystalline NiSe2/CC. Additional morphological and microstructural tests depict large-scale NiSe2 nanosheets are uniformly aligned on the basement of conductive carbon cloth. The PEC measurements show the steady photocurrent density can be effectively raised from 10 to 600 μA/cm2 when bias potential increases from 0 to 0.5 V, which is proposed to be attributed to the adequate exposure of active sites and reduction of surface contact resistance provided by carbon cloth. The structure and performances of hydrothermal synthesized NiSe2/CC suggest promising applications in the field of PEC hydrogen production.

We demonstrated that titanium dioxide (TiO2) significantly enhances the reversible capacity of silicon nanoparticles (SiNPs) used as the anode material for lithium ion batteries. The novel silicon and titanium dioxide core–shell nanoparticles (Si/TiO2) was prepared via a facile sol–gel method with the following heat-treatment process. The as-prepared materials were confirmed by X-ray diffraction, Raman spectra and transmission electron microscope. As an anode material for battery, the electrode was investigated by a series of electrochemical measurements. The as-prepared Si/TiO2 electrode exhibits superior electrochemical performance in comparison with pure SiNPs, which realizes an initial capacity as high as 2636.9 mAh/g and still retains 1010.7 mAh/g even after 100 cycles of charging and discharging. The excellent cycle stability and capacity retention of the as-prepared electrode would be attributed to the existence of TiO2 layers to accommodate the large volume changes of silicon and enhancement ability to minimizes loss of pulverization during the lithium insertion/desertion cycles. Therefore, the as-prepared Si/TiO2 is believed to be an potential anode material for lithium-ion battery applications.

Electrocatalytic water splitting and photocatalytic water splitting have been widely researched in the progress of hydrogen generation. Among them, The former is of great significance to produce high-purity hydrogen industrially, while The latter is considered to be a efficient, low energy consumption and a green way of hydrogen production. Herein, we design a simple hydrothermal method to synthesize VSe2 with superior photocatalytic activity and HER/OER Electrochemical experiments reveal the samples with overpotential of 0.796 V in 0.5 M H2SO4 and 1.011 V in 1 M KOH for HER at current density of −10 mA cm−2, respectively. The VSe2 also exhibited optimal electrocatalytic properties for hydrogen evolution reaction (HER) with a low Tafel slope of 134.4 mV dec−1 (in 0.5 M H2SO4) and 242.44 mV dec−1 (in 1 M KOH), respectively. Furthermore, VSe2 has an extremely overpotential of 2.467 V at 10 mA cm−2 and Tafel slope of 247.43 mV dec−1 in 1 M KOH for OER. The electrochemical properties of VSe2 are mainly attributed to the high conductivity and the atomic composition way of each layer. Photoelectrochemical tests prove that the photocurrent density of VSe2 exhibits 107 μA/cm2 in 1 M KOH at 0.8 V. The superior photocatalytic activity is benefited from its intrinsic electronic structure.

A novel ternary CdS/ZnO/graphene composite has been successfully prepared by loading ZnO and CdS nanoparticles in graphene nanosheets via a facile one-step hydrothermal method. The microstructures and properties have been examined by X-ray diffraction (XRD), scanning electron microscopy with an energy dispersive spectroscope (EDS), transmission electron microscopy, Raman and UV–vis diffuse reflectance spectra (DRS). The characterization results reveal that the crystalline of the composite is very well, the graphene sheets were tightly coated with ZnO and CdS nanoparticles, and the light-harvesting was effectively strengthened. Taking photoelectrochemical test, the ternary CdS/ZnO/graphene composite exhibits enhanced photocatalytic activity compared with its foundation matrix binary composites and pure ZnO and CdS. The improved photocatalytic performance can be attributed to the enhanced light absorption, the extremely efficient charge separation, as well as superior durability of the ternary composite. It is proposed that graphene-based composites by coupling graphene to suitable, multiple semiconductors can not only greatly improve the capacity for photocatalytic, but also expand the exploration and utilization of graphene-based nanocomposites for energy conversion.

Using the first-principles calculation, we systematically studied the electronic properties of the bilayer transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se, Te) with replacing one, two, three or four layers of X atoms as Y atoms (X ≠ Y = S, Se, Te). By comparison, it is found that when the inner two layers of chalcogenide atoms are different, the system has both valence band offset (VBO) and conduction band offset (CBO). Among them, values of the band offsets reach maxima when the inner one layer of X atoms is replaced by Y atoms, namely forming the asymmetric Janus bilayer XMX/YMX. We take SMoS/SeMoS as an example to analyze the formation of the band offsets and the improvement of optoelectronic properties. Importantly, it is also found that both external electric field and biaxial strain can regulate electronic structures of asymmetric Janus bilayer TMDs with noticeable modulation of the values of band offsets. When the external electric field changes from negative to positive continually, CBO decreases and VBO increases. While when the biaxial strain changes from compression to stretch continually, CBO increases and VBO decreases. These findings enrich the study of bilayer TMDs that can be used as optoelectronic, nanoelectronic and valleytronic devices.

The photoresponse properties of solution processed graphene oxide (GO) ranged from ultraviolet–visible–near infrared regime are studied. Photoelectrochemical results indicate that the as-prepared GO devices have excellent sensitivity, high-speed and superior reproducibility as a visible and near infrared photodetector. Especially, the responsivity and external quantum efficiency are found to be about 95.8 mA/W and 26.2%, respectively, for incident wavelength of 455 nm at 0.8 V bias voltage. Resulting from the degradation of GO under UV irradiation, the GO devices exhibit unstable photoresponse performance, as well as high UV photosensitivity.

Ultrathin high-quality Bi2Te3 and Bi2Se3 topological insulator nanoplates are prepared on n-type and p-type silicon (111) substrates via vapor phase deposition method forming topological insulator–silicon heterostructures. The quantitative results obtained by Kelvin probe force microscopy indicate that the work function and Fermi level of both Bi2Te3 and Bi2Se3 nanoplates depend on the substrates, which are ascribed to the charge exchange that exists at the interface between nanoplates and silicon substrates with different doping. Our results provide an effective strategy to tune electronic properties of TI nanostructures through topological insulator–silicon heterostructures.

Controllable preparation of different nanoscale-shaped titania materials was realized by hydrothermal treatment of anatase TiO2 in an alkaline solution. The gradually changing morphologies and microstructures of titanium dioxide were investigated by powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. And a top-down path is illuminated to have an insight to the morphological evolution from nanoparticle to nanosheet by adjusting the concentrations of Na ion in the aqueous solution. The results of photocatalytic experiments indicated that the TiO2 nanobelts exhibited enhanced photocatalytic performance, due to their lower electron-hole recombination rate confirmed from the photoluminescence spectra. This study suggests that the photocatalysis efficiency of nanocrystals can be significantly improved by the shape-dependent morphological transformation.

We studied the structures and electronic properties of Janus transition-metal dichalcogenide monolayers MXY (M = Mo, W; X ≠ Y = S, Se, Te) by first-principles calculations. The results of the electronic band structures and the density of states reveal that all of the MXY monolayers show semiconducting characteristics. Particular attention has been focused on the bandgap engineering by applying in-plane biaxial compressive and tensile strain. It is observed that the bandgap values of the MXY monolayers decrease with the increase of strain degree under the tension and compression biaxial strain, and a semiconductor-to-metal transition can be undergone at a critical value of strain. The possibility of the tunable energy gap over a wide range makes MXY monolayers potential candidates for nanoelectronics and optoelectronics.

Large-size 2D black phosphorus (BP) nanosheets have been successfully synthesized by a facile liquid exfoliation method. The as-prepared BP nanosheets are used to fabricate electrodes for a self-powered photodetector and exhibit preferable photoresponse activity as well as environmental robustness. Photoelectrochemical (PEC) tests demonstrate that the current density of BP nanosheets can reach up to 265 nA cm−2 under light irradiation, while the dark current densities fluctuate near 1 nA cm−2 in 0.1 M KOH. UV–vis and Raman spectra are carried out and confirm the inherent optical and physical properties of BP nanosheets. In addition, the cycle stability measurement exhibits no detectable distinction after processing 50 and 100 cycles, while an excellent on/off behavior is still preserved even after one month. Furthermore, the PEC performance of BP nanosheets-based photodetector is evaluated in various KOH concentrations, which demonstrates that the as-prepared BP nanosheets may have a great potential application in self-powered photodetector. It is anticipated that the present work can provide fundamental acknowledgement of the performance of a PEC-type BP nanosheets-based photodetector, offering extendable availabilities for 2D BP-based heterostructures to construct high-performance PEC devices.

2D photocatalytic TiO2/MoS2 hybrid nanosheets (HNs) have been prepared via a facile hydrothermal process. X-ray diffraction patterns and Raman spectra are carried out and confirm a well crystalized anatase and 2H-MoS2 hybridization. Additional morphological and microstructural tests verify a distinct MoS2 framework, indicating the relatively stability of the MoS2 nanosheet platform with a high specific surface area. UV–vis spectra and electrochemical impedance spectra exhibit an enhanced light absorption ability and conductivity of TiO2/MoS2 compared to that of just TiO2. Photoelectrochemical (PEC) tests also demonstrate the photocurrent of 20 : 1 TiO2/MoS2 HNs is greatly improved compared to that of as-prepared TiO2. The saturation current density is about 33 µA cm−2 when the applied potential is 0.2 V, which is nearly twice that of pure TiO2 and four times as high as 5 : 1 TiO2/MoS2 HNs and 1 : 1 TiO2/MoS2 HNs. Besides that, the duration test exhibits no detectable distinction after processing 25 cycles. The improved photocatalytic activities are perhaps derived from the high conductivity and the increased active sites for the introduction of co-catalytic MoS2 nanosheets as well as the positive synergetic effect between the TiO2 and MoS2. This work demonstrates that the as-prepared TiO2/MoS2 HNs may have a great potential application in PEC hydrogen production.

We have fabricated a novel sunlight photo-detector based on a MoS2/graphene heterostructure. The MoS2/graphene heterostructure was prepared by a facile hydrothermal method along with a subsequent annealing process followed by a substrate-induced high selective nucleation and growth mechanism. The microstructures and morphologies of the two-dimensional MoS2/graphene heterostructure can be experimentally confirmed by x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a UV–vis absorption spectrometer. Photoresponse investigations performed by a photoelectrochemical (PEC) measurement system indicate that the synthesized MoS2/graphene heterostructure shows superior photoresponse activities under the illumination of sunlight in contrast with bare MoS2 and graphene. The improved photoresponsivity can be attributed to the enhanced light absorption, strong light–matter interaction and the extremely efficient charge separation of the heterostructure. The structure and performances of the MoS2/graphene heterostructure suggest promising applications in the field of photonics and optoelectronics.

Based on first-principles calculations in the framework of van der Waals density functional theory, we investigate the structural, electronic properties and band-gap tuning of the h-BN/MoS2/h-BN sandwich heterostructure under an external electric field. We find that, different from the suspended monolayer MoS2 with a direct band-gap, h-BN/MoS2/h-BN has an indirect band-gap. Particular attention has been focused on the engineering of the band-gap of the h-BN/MoS2/h-BN heterostructure via application of an external electric field. With the increase of electric field, the band-gap of the h-BN/MoS2/h-BN heterostructure undergoes an indirect-to-direct band-gap transition. Once the electric field intensity is larger than 0.1 V A−1, the gap value of direct band-gap shrinks almost linearly with the field-strength, which indicates that the h-BN/MoS2/h-BN heterostructure is a viable candidate for optoelectronic applications.

We have carried out first-principles calculations and theoretical analysis to explore the structural and electronic properties of MoS2/n-h-BN heterostructures consisting of monolayer MoS2 on top of h-BN substrates with one to five layers. We find that the MoS2/n-h-BN heterostructures show indirect bandgap features with both of CBM (in the K point) and VBM (in the ? point) localized on the monolayer MoS2. Difference charge density and surface bands indicate there is no obvious charge exchange in the heterostructure systems. We show that the changes from a direct bandgap in monolayer free-stranding MoS2 to an indirect bandgap in MoS2/n-h-BN heterostructure is induced by the strain. Moreover, we find that the bandgaps of MoS2/n-h-BN heterostructures decrease with increasing number of h-BN layers, which is proposed to result from the different strain distributions in MoS2 due to the varieties of lattice mismatch rates between MoS2 and h-BN layers. Our results suggest that the MoS2/n-h-BN heterostructure could serve as a prototypical example for band structure engineering of 2D crystals with atomic layer precision.

By using a first-principles method within the framework of density functional theory, we study the formation energies of Cd and Ca dopants in Bi2Se3. The calculation results show that CdBi and CaBi are the most stable defects for Cd and Ca dopants in Bi2Se3, respectively. Additionally, both the CdBi and CaBi defects in Bi2Se3 are p-type dopants, with accepter levels at e(0/−1) = 0.030 eV and e(0/−1) = 0.028 eV, respectively. Comparing their formation energies with that of the native defects under a different growth condition, CdBi and CaBi can effectively compensate the n-type native defects, VSe and SeBi, under Se-rich growth conditions. It is interesting to note that such compensation effects become weaker, gradually, with the growth conditions changing from Se-rich to Bi-rich. Therefore, a p-to-n or n-to-p type conversion for Bi2Se3 can be achieved by changing the growth conditions. Simultaneously, the Fermi level, which is determined by the charge neutrality condition, can be effectively tuned, which is in agreement with previous experimental reports.

Functionalization of MoS2 sheet (monolayer and bilayer) by the adsorption of transition metal Fe adatom to its surface and interlayer has been investigated computationally using first-principles calculations based on the density functional theory. We found that the systems with absorption of Fe adatoms on the surfaces of both monolayer and bilayer MoS2 sheets are still semiconductors, without spin polarization at the Fermi level. However, for the system with absorption of Fe adatom in the interlayer of bilayer MoS2 sheet, its electronic structure exhibits half-metal behavior, with 100% spin polarization at the Femi level, which provides a promising material for spintronic devices.

We have carried out first-principles calculations and theoretical analysis to explore the structural, spin-polarized electronic and magnetic properties of bi-layered MoS2 with transition-metal (TM) atoms (Cr, Mn, Fe, Co and Ni) doped in the interlayer. The charge density distribution indicates that the doping TM atoms and the nearest S atoms in the lower and upper planes display a clear covalent-bonding feature. The local moments of the doping TM atoms are smaller than the magnetic moments of their free states. Also, the spin polarization is found to be 100% at the Fermi level or HOMO level for interlayer doping with Cr, Mn, Fe and Co.

We report the electrostatic properties of few-layer Bi2Te3 and Bi2Se3 nanoplates (NPs) grown on 300 nm SiO2/Si substrate. Electrons and holes are locally injected in Bi2Te3 and Bi2Se3 nanoplates by the apex of an atomic force microscope tip. Both carriers are delocalized uniformly over the whole nanoplate. The electrostatic property of topological insulator Bi2Te3 and Bi2Se3 nanoplates after charge injection is characterized by Kelvin probe force microscopy under ambient environment and exhibits an ambipolar surface potential behavior. These results provide insight into the electronic properties of topological insulators at the nanometer scale.