Synthesis of two-dimensional MoO2 nanoplatelets and its multistep sulfurization into MoS2.

To control the growth of layered two-dimensional structures, such as transition metal dichalcogenide materials or heterostructures, understanding the growth mechanism is crucial. Here, we report the synthesis of ultra-thin MoO2 nanoplatelets through the sublimation of MoO3. Rhombus MoO2 nanoplatelets with the P21/c space group were characterized using various microscopic and spectroscopic techniques. Introducing sulfur sources into the chemical vapor deposition system also leads to the formation of monoclinic MoO2 nanoflakes due to the incomplete sulfurization of MoO3. With a gradual increase in the vapor concentration of sulfur, MoO3 undergoes stepwise reduction into MoS2/MoO2 and eventually into MoS2. Additionally, utilizing MoO2 as a precursor for Mo sources enables the formation of monolayer MoS2 single crystals. This work provides an effective approach for growing MoO2 nanoplatelets and elucidates the mechanism behind the stepwise sulfurization of MoO3.

Single-Atom Phosphorus Defects Decorated CoP Cocatalyst Boosts Photocatalytic Hydrogen Generation Performance of Cd0.5 Zn0.5 S by Directed Separating the Photogenerated Carriers.

Design and preparation of an efficient and nonprecious cocatalysts, with structural features and functionality necessary for improving photocatalytic performance of semiconductors, remain a formidable challenge until now. Herein, for the first time, a novel CoP cocatalyst with single-atom phosphorus vacancies defects (CoP-Vp ) is synthesized and coupled with Cd0.5 Zn0.5 S to build CoP-Vp @Cd0.5 Zn0.5 S (CoP-Vp @CZS) heterojunctions photocatalysts via a liquid phase corrosion method following by an in suit growth process. The nanohybrids deliver an attractive photocatalytic hydrogen production activity of 2.05 mmol h-1 30 mg-1 under visible-light irradiation, which is 14.66 times higher than that of the pristine ZCS samples. As expected, CoP-Vp further enhances the charge-separation efficiency of ZCS, in addition to the improvement of the electron transfer efficiency, which is confirmed by the ultrafast spectroscopies. Mechanism studies based on density functional theory calculations verify that Co atoms adjacent with single-atom Vp play the key role in translation, rotation, and transformation of electrons for H2 O reduction. This scalable strategy focusing defect engineering provides a new insight into designing the highly active cocatalysts to boost the photocatalytic application.

Synthesis and Optimization of Cs2B'B″X6 Double Perovskite for Efficient and Sustainable Solar Cells.

Hybrid perovskite materials with high light absorption coefficients, long diffusion lengths, and high mobility have attracted much attention, but their commercial development has been seriously hindered by two major problems: instability and lead toxicity. This has led to lead-free halide double perovskite becoming a prominent competitor in the photovoltaic field. For lead-free double perovskites, Pb2+ can be heterovalent, substituted by non-toxic metal cations as a double perovskite structure, which promotes the flexibility of the composition. However, the four component elements and low solubility in the solvent result in synthesis difficulties and phase impurity problems. And material phase purity and film quality are closely related to the number of defects, which can limit the photoelectric performance of solar cells. Therefore, based on this point, we summarize the synthesis methods of Cs2B'B″X6 double perovskite crystals and thin films. Moreover, in the application of solar cells, the existing research mainly focuses on the formation process of thin films, band gap adjustment, and surface engineering to improve the quality of films and optimize the performance of devices. Finally, we propose that Cs2B'B″X6 lead-free perovskites offer a promising pathway toward developing highly efficient and stable perovskite solar cells.

Ferroelectric polarization effect on the photocatalytic activity of Bi0.9Ca0.1FeO3/CdS S-scheme nanocomposites.

BiFeO3 (BFO), as a kind of narrow band-gap semiconductor material, has gradually emerged advantages in the application of photocatalysis. In this paper, Ca doped BFO nanoparticles Bi0.9Ca0.1FeO3 (BCFO) were prepared by sol-gel method. And BCFO and CdS nanocomposites with two morphologies were obtained by controlling the time of loading CdS under a low temperature liquid phase process. It is found that the band gap becomes narrower after doping Ca into BFO, which is conducive to the absorption of visible light. Among all the samples, the composite of CdS nanowires and BCFO nanoparticles obtained by reaction time of 10 min has the best photocatalytic performance. The degradation rate of Methyl Orange solution was 94% after 90 min under visible light irradiation, which was much higher than that of pure BCFO and CdS. Furthermore, significant enhancement in the degradation rate (100% degradation in 60 min) can be achieved in poled samples after electric polarization process. The highest degradation rate is due to the promoted separation of photogenerated carriers induced by the internal polarization field and the formation of S-scheme heterostructure between BCFO and CdS. Such BCFO-CdS nanocomposites may bring new insights into designing highly efficient photocatalyst.

Spatial Relation Controllable Di-Defects Synergy Boosts Electrocatalytic Hydrogen Evolution Reaction over VSe2 Nanoflakes in All pH Electrolytes.

Defect engineering of transition metal dichalcogenides (TMDCs) is important for improving electrocatalytic hydrogen evolution reaction (HER) performance. Herein, a facile and scalable atomic-level di-defect strategy over thermodynamically stable VSe2 nanoflakes, yielding attractive improvements in the electrocatalytic HER performance over a wide electrolyte pH range is reported. The di-defect configuration with controllable spatial relation between single-atom (SA) V defects and single Se vacancy defects effectively triggers the electrocatalytic HER activity of the inert VSe2 basal plane. When employed as a cathode, this di-defects decorated VSe2 electrocatalyst requires overpotentials of 67.2, 72.3, and 122.3 mV to reach a HER current density of 10 mA cm-2 under acidic, alkaline, and neutral conditions, respectively, which are superior to most previously reported non-noble metal HER electrocatalysts. Theoretical calculations reveal that the reactive microenvironment consists of two adjacent SA Mo atoms with two surrounding symmetric Se vacancies, yielding optimal water dissociation and hydrogen desorption kinetics. This study provides a scalable strategy for improving the electrocatalytic activity of other TMDCs with inert atoms in the basal plane.

Two-dimensional IV-VA3 monolayers with enhanced charge mobility for high-performance solar cells.

High-performance photovoltaics (PVs) constitute a subject of extensive research efforts, in which silicon (Si)-based solar cells (SCs) have been widely commercialized. However, the low carrier mobility of Si-based SCs can limit the effective charge separation, thereby negatively impacting the device performance. Here, via calculating the physicochemical and PV performance based on density functional theory, we demonstrate SCs based on two-dimensional (2D) group IV and V compounds with an AX3 configuration. Firstly, the cleavage energies of AX3 (A = Si, Ge; X = P, As, and Sb) are calculated to be less than 1 J m-2, providing an experimental feasibility to be exfoliated from the corresponding bulk. Secondly, electronic and optical properties have been systematically investigated. To be specific, the band gap of monolayer AX3 falls in the range of 1.11-1.27 eV, which is comparable with that of Si. Significantly, the electron mobility of monolayer AX3 can reach as high as ∼30 000 cm2 V-1 s-1, which is one order of magnitude higher than that of Si. Furthermore, the optical absorbance of monolayer SiAs3, SiP3 and GeAs3 exhibits high coefficients in visible light. Therefore, we believe that our designed AX3-based PV systems with power conversion efficiency of 20% can offer great potential in the application of high-performance two-dimension-based PVs.

Tunable magnetoelectric coupling and electrical features in an ultrathin Cr2Si2Te6/In2Se3 heterostructure.

Two-dimensional (2D) van der Waals (vdW) heterostructures based on multiferroic materials have potential applications in novel low-dimensional spintronic devices. In this work, we have investigated a strong magnetoelectric coupling and electrical dependence between single layer (1L) Cr2Si2Te6 and In2Se3. By switching the direction of ferroelectric polarization in In2Se3, we observed a significant magneto-crystalline anisotropy energy (MAE) enhancement of Cr2Si2Te6. The analysis of the spin-resolved orbital-decomposed band structure shows stronger magnetoelectric coupling between the In2Se3 and Cr2Si2Te6 layers. The modulation of the electrical features could also be achieved in the switching of the ferroelectric polarization. Furthermore, the switching of Ohmic-Schottky contacts in the heterojunction with different polarization states was successfully achieved under the effect of strain engineering. Based on these findings, we design a novel 2D ferroelectric-ferromagnetic heterojunction that exploits the controllability and nonvolatility of ferroelectrics to modulate the electrical properties of the device. These findings indicate the high application potential of Cr2Si2Te6/In2Se3 multiferroic heterojunctions in spintronics.

Cation-Modulated HER and OER Activities of Hierarchical VOOH Hollow Architectures for High-Efficiency and Stable Overall Water Splitting.

Atom-scale modulation of electronic regulation in nonprecious-based electrocatalysts is promising for efficient catalytic activities. Here, hierarchically hollow VOOH nanostructures are rationally constructed by partial iron substitution and systematically investigated for electrocatalytic water splitting. Benefiting from the hierarchically stable scaffold configuration, highly electrochemically active surface area, the synergistic effect of the active metal atoms, and optimal adsorption energies, the 3% Fe (mole ratio) substituted electrocatalyst (VOOH-3Fe) exhibits a low overpotential of 90 and 195 mV at 10 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media, respectively, superior than the other samples with a different substituted ratio. To the best of current knowledge, 195 mV overpotential at 10 mA cm-2 is the best value reported for V or Fe (oxy)hydroxide-based OER catalysts. Moreover, the electrolytic cell employing the VOOH-3Fe electrode as both the cathode and anode exhibits a cell voltage of 0.30 V at 10 mA cm-2 with a remarkable stability over 60 h. This work heralds a new pathway to design efficient bifunctional catalysts toward overall water splitting.

Enhanced insulating behavior in the Ir-vacant Sr2Ir1-xO4 system dominated by the local structure distortion.

Sr2IrO4, known as the Jeff = 1/2 Mott insulator, was predicted to be an unconventional superconductor upon doping since it highly resembles the high-temperature cuprates. However, recent work pointed out an enhanced insulating behavior in the Ir-vacant Sr2Ir1-xO4 system. In this contribution, to investigate the microscopic mechanism of its enhanced insulating behavior, X-ray absorption spectroscopy was applied to study the electronic structure and local structure distortion of Sr2Ir1-xO4. Due to the presence of Ir5+ ions, the preconceived holes are barely doped in the Ir-vacant system. Nevertheless, Ir vacancies finely modulate the local atomic structure, i.e. the topology of IrO6 octahedra and the in-plane Ir-O1-Ir bond angle. Combined with theoretical calculations, it is demonstrated that both the more distorted IrO6 octahedra and decreased Ir-O1-Ir angle contribute to the increment of the band gap, and then result in the enhanced insulating state for Sr2Ir1-xO4.

Pressure-induced anomalies and structural instability in compressed ß-Sb2O3.

Here, we report a high-pressure study of orthorhombic structured ß-Sb2O3 (valentinite) by the combination of synchrotron in situ X-ray diffraction and first-principles theoretical calculations at pressures up to 40.5 GPa. Our results reveal that the metastable ß-Sb2O3 undergoes an isostructural phase transition at high pressure, yielding a distorted ß phase at 7-15 GPa through symmetry breaking and structural distortion as inferred from our XRD analyses and DFT theoretical calculations where pressure-induced elasticity softening is observed at pressures of 7-15 GPa. At pressures higher than 15 GPa, a new high-pressure monoclinic phase is discovered from the current synchrotron X-ray diffraction data. Upon further compression up to ∼33 GPa, the monoclinic Sb2O3 starts to lose its long-range order and forms an amorphous component coexisting with the monoclinic one. To further explore the structural instability and understand the origin of pressure-induced phase transitions in ß-Sb2O3 upon compression, we have performed first-principles calculations to track the evolution of its phonon velocities, density of states and phonon dispersion curves under high pressure. Our results may play an important role in determining the local structures as well as their structural relationship among sesquioxides.

Compact broadband spectrometer based on upconversion and downconversion luminescence.

In this Letter, a compact spectrometer based on upconversion and downconversion luminescence for operation in the infrared, visible, and ultraviolet bands is presented. The proposed spectrometer has three components that are used for dispersion, frequency conversion, and detection. The conversion component converts the incident signal beam into a spectral window appropriate for the detection component. The detection component images the speckle pattern generated by scattering or diffraction in the random structure of the dispersion component. With the two-dimensional intensity data captured from both the speckle pattern and a calibration measurement process, one can reconstruct the spectra of the signal beam by solving a matrix equation. A smoothing simulated annealing algorithm has been implemented to improve the accuracy of the spectral reconstruction. We have analyzed possible sources of error in the algorithm and the corresponding limits of operation. The reported broadband, compact, high-resolution, luminescence-based spectrometer is well suited for portable spectroscopy applications.

Simultaneous enhancement of magnetic and ferroelectric properties in Dy and Cr co-doped BiFeO3 nanoparticles.

Multiferroic BiFeO3 (BFO), Bi0.95Dy0.05FeO3 and Bi0.95Dy0.05Fe0.95Cr0.05O3 samples were successfully synthesized by a carbon microsphere-assisted sol-gel (CSG) method. X-ray diffraction analysis confirmed a lattice distortion from a rhombohedral structure to a tetragonal structure upon doping Dy and Cr in BFO. The morphology of BFO and doped BFO could be effectively controlled to form nanoparticles, due to the nucleation sites of the carbon microspheres. The co-doping of Dy and Cr in BFO had a significant improvement effect on the magnetic properties, with the remnant magnetization being 0.557 emu g(-1), due to the structural phase transition, size effects and the strong ferromagnetic interaction between Fe(3+)-O-Cr(3+) ions arising from Cr substitution. Meanwhile, the doping of Dy into BFO effectively reduced the leakage current and enhanced the ferroelectric properties. The simultaneous enhancement of magnetic and ferroelectric properties shows the great potential application of Dy- and Cr-co-doped BFO in future multifunctional devices.

Miniature spectrometer based on diffraction in a dispersive hole array.

We present an ultra-compact spectrometer that uses a 10×10 hole array as the dispersive component. Our analysis shows that the two-dimensional intensity distribution can be modeled by a system of simultaneous linear equations when the size of each hole in the dispersive component has been pre-designed appropriately. One can readily recover the spectral contents of the input radiation by solving the linear equation system with regularized procedure. Experimental results show that the reconstruction range is at least within the entire visible band, which can be further extended if a near-infrared CCD is used. One therefore envisions strong potential for many wavelength analysis applications.

Hexylammonium Acetate-Regulated Buried Interface for Efficient and Stable Perovskite Solar Cells.

Due to current issues of energy-level mismatch and low transport efficiency in commonly used electron transport layers (ETLs), such as TiO2 and SnO2, finding a more effective method to passivate the ETL and perovskite interface has become an urgent matter. In this work, we integrated a new material, the ionic liquid (IL) hexylammonium acetate (HAAc), into the SnO2/perovskite interface to improve performance via the improvement of perovskite quality formed by the two-step method. The IL anions fill oxygen vacancy defects in SnO2, while the IL cations interact chemically with Pb2+ within the perovskite structure, reducing defects and optimizing the morphology of the perovskite film such that the energy levels of the ETL and perovskite become better matched. Consequently, the decrease in non-radiative recombination promotes enhanced electron transport efficiency. Utilizing HAAc, we successfully regulated the morphology and defect states of the perovskite layer, resulting in devices surpassing 24% efficiency. This research breakthrough not only introduces a novel material but also propels the utilization of ILs in enhancing the performance of perovskite photovoltaic systems using two-step synthesis.

Synthesis of graphene by chemical vapor deposition: effect of growth conditions.

Graphene has attracted a great deal of attention due to its extraordinary physical and chemical properties. But the control of growth of high-quality, large-area and inexpensive graphene is still the bottleneck for practical applications. Chemical vapor deposition (CVD) has become the most common method for graphene growth due to its high production and large area of product. However, it generally suffers from an uncontrollable carbon precipitation effect that leads to inhomogeneous growth and strongly dependent on to the growth conditions. Until now, scientists have struggled to synthesize higher quality, larger area graphene through changing the experimental conditions. In this review, the progress made in the last few years concerning the exploration of preparation graphene by CVD is summarized in three aspects (catalysts, precursors and experimental parameters) that influence graphene growth.

Multilevel resistive switching in stable all-inorganic n-i-p double perovskite memristor.

Memristors are promising information storage devices for commercial applications because of their long endurance and low power consumption. Particularly, perovskite memristors have revealed excellent resistive switching (RS) properties owing to the fast ion migration and solution fabrication process. Here, an n-i-p type double perovskite memristor with "ITO/SnO2/Cs2AgBiBr6/NiOx/Ag" architecture was developed and demonstrated to reveal three resistance states because of the p-n junction electric field coupled with ion migration. The devices exhibited reliable filamentary with an on/off ratio exceeding 50. The RS characteristics remained unchanged after 1000 s read and 300 switching cycles. The synaptic functions were examined through long-term depression and potentiation measurements. Significantly, the device still worked after one year to reveal long-term stability because of the all-inorganic layers. This work indicates a novel idea for designing a multistate memristor by utilizing the p-n junction unidirectional conductivity during the forward and reverse scanning.

Metal Clusters Effectively Adjust the Local Environment of Polymeric Carbon Nitride for Bifunctional Overall Water Splitting.

Compared with single-atom catalysts, clusters not only possess more metal-loadings and stability but also provide flexible active sites to break the linear scaling relationship of multistep reactions. However, exploring precise structure-activity relationships and the synergistic effect between clusters and nanosheets is still in its infancy. Here, based on first-principles and nonequilibrium Green's function simulation, the C2N-supported Fe and Co tetrahedral clusters exhibit remarkable bifunctional catalytic performance with a very low overpotential of hydrogen (0.12 and 0.07 V) /oxygen (0.20 and 0.55 V) evolution reactions (HER/OER), respectively. The C2N-regulated Fe and Co clusters have suitable d-band centers around the Fermi surface for HER. In turn, the Fe and Co clusters activate the subadjacent dual-carbon sites for OER. Simultaneously, the cluster enhances the electronic conductivity of C2N, and the initial current only needs ultralow bias voltage around 0.1-0.4 V. The desired metal cluster regulation strategy offers cost-effective potential for advancing clean energy technology.

Facile construction of a sulfur vacancy defect-decorated CoSx@In2S3 core/shell heterojunction for efficient visible-light-driven photocatalytic hydrogen evolution.

Photoinduced electron-separation and -transport processes are two independent crucial factors for determining the efficiency of photocatalytic hydrogen production. Herein, a sulfur vacancy defect-decorated CoSx@In2S3 (CoSx@VS-In2S3) core/shell heterojunction photocatalyst was synthesized via an in situ sulfidation method followed by a liquid-phase corrosion process. Photocatalytic hydrogen evolution experiments showed that the CoSx@VS-In2S3 nanohybrids delivered an attractive photocatalytic activity of 4.136 mmol h-1 g-1 under visible-light irradiation, which was 8.23 times higher than that of the pristine In2S3 samples. As expected, VS could enhance the charge-separation efficiency of In2S3 through rearranging the electrons of the In2S3 basal plane, in addition to improving the electron-transfer efficiency, as visually verified by transient absorption spectroscopy. Mechanism studies based on density functional theory calculations confirmed that the In atoms adjacent to VS played a key role in the translation, rotation, and transformation of electrons for water reduction. This scalable strategy focused on defect engineering paves a new avenue for the design and assembly of 2D core/shell heterostructures for efficient and robust water-splitting photocatalysts.

Two-dimensional InSb/GaAs- and InSb/InP-based tandem photovoltaic device with matched bandgap.

The past several years have witnessed remarkable research efforts to develop high-performance photovoltaics (PVs), to curtail the energy crisis by avoiding dependence on traditional fossil fuels. In this regard, there is an urgent need to accelerate research progress on new low-dimensional semiconductors with superior electronic and optical properties. Herein, combining abundant related PV experimental data in the literature and our systematic theoretical calculations, we propose two-dimensional (2D) InSb/GaAs and InSb/InP-based tandem PVs with high solar-to-electric efficiency up to near 30.0%. Firstly, according to first-principles calculations, the stability, electronic and optical properties of single-layer group-III-V materials (XY, X = Ga and In, Y = N, P, As, Sb, and Bi) are systematically introduced. Next, due to the high bandgap (Eg) of GaAs and InP being a perfect match with the low Eg of InSb, InSb/GaAs- and InSb/InP-based tandem PVs are constructed. In addition, the complementary absorption spectra of these two subcells can facilitate the achievement of high tandem power conversion efficiency. Furthermore, we have analyzed in detail the influencing factors for PCE and the physical mechanism of the optimized match between the top and bottom subcells in the tandem configurations. Our designed 2D-semiconductor-based PVs can be expected to bring a new perspective for future commercialized high-efficiency energy devices.

Recent Advances in g-C3N4 for the Application of Perovskite Solar Cells.

In this study, graphitic carbon nitride (g-C3N4) was extensively utilized as an electron transport layer or interfacial buffer layer for simultaneously realizing photoelectric performance and stability improvement of perovskite solar cells (PSCs). This review covers the different g-C3N4 nanostructures used as additive and surface modifier layers applied to PSCs. In addition, the mechanism of reducing the defect state in PSCs, including improving the crystalline quality of perovskite, passivating the grain boundaries, and tuning the energy level alignment, were also highlighted in this review. Currently, the power conversion efficiency of PSCs based on modified g-C3N4 has been increased up to 22.13%, and its unique two-dimensional (2D) package structure has enhanced the stability of PSCs, which can remain stable in the dark for over 1500 h. Finally, the potential challenges and perspectives of g-C3N4 incorporated into perovskite-based optoelectronic devices are also included in this review.