•Single- and double-shelled Cu2−xSe and Cu7S4 nanocages are synthesized using Cu2O nanocubes as sacrificial templates.•The synthesis is based on an ion-exchange route coupled with Kirkendall and chemical etching effects.•The copper chalcogenide nanocages are demonstrated as efficient counter electrodes of QDSSCs.•Influences of shell numbers on photovoltaic performance of QDSSCs have been investigated.•The QDSSC presents 19.6% increase in PEC using the double-shelled Cu2−xSe instead of the single-shelled counterparts.Double-shelled copper chalcogenide (Cu2−xSe and Cu7S4) nanocages of about 250 nm sizes are synthesized respectively by using Cu2O nanocubes as sacrificial precursor via Kirkendall diffusion and etching. Both the double-shelled Cu2−xSe and Cu7S4 nanocages are demonstrated to be excellent counter electrode (CE) materials in quantum dot-sensitized solar cells (QDSSCs) and exhibit high electrocatalytic activities for polysulfide electrolyte regeneration. The QDSSCs using the double-shelled Cu2−xSe and Cu7S4 nanocages as CEs show power conversion efficiencies (PECs) of 4.76 and 4.53%, respectively, which are 19.6% and 15.3% higher than the corresponding devices using the single-shelled Cu2−xSe and Cu7S4 nanocages as CEs. The overall enhancement of photovoltaic performance including the fill factor, short-circuit current density and PEC is attributed to the larger CE-electrolyte interface provided by the double-shelled nanocages facilitating fast electron transfer in the QDSSCs.Download high-res image (188KB)Download full-size image

•Arrays of ZnO/MoS2 nanocables and MoS2 nanotubes on FTO substrates have been prepared.•The as-prepared MoS2 nanotube array shows expanded (0 0 2) interlayer spacings and mixed 1T-2H phases.•The ZnO/MoS2 nanocable arrays are demonstrated as promising photoanodes for PEC water splitting.•Nanoarray structure and phase engineering of MoS2 nanotubes play key roles in active HER.The increasing exploration of clean energy sources has driven the development of advanced materials and nanostructures as electrodes for water splitting to generate hydrogen. In this work, arrays of ZnO/MoS2 nanocables and MoS2 nanotubes on F-doped SnO2 (FTO) glass substrates are synthesized and demonstrated as promising electrodes for photoelectrochemical (PEC) water splitting and electrochemical water splitting, respectively. Few-layered MoS2 nanosheets are deposited on surface of a ZnO nanorod array on FTO glass substrate by a solvothermal method to form a ZnO/MoS2 nanocable array. The ZnO/MoS2 nanocable array exhibits expanded light absorption up to 960 nm and a stepwise energy band alignment for fast charge carrier separation, consequently resulting in improved performance of PEC water splitting. The array of MoS2 nanotubes with mixed 1T and 2H phases is demonstrated as an efficient binder-free electrode for electrochemical water splitting, exhibiting a small Tafel slop of 45 mV per decade and a low onset potential of 103 mV. The superior electrochemical hydrogen evolution performance is considered to be attributed to synergistic effects of the 1T phase, the interlayer expansion, and the nanoarray structure.Arrays of ZnO/MoS2 nanocables and MoS2 nanotubes on FTO substrates are synthesized and demonstrated as efficient electrodes for photoelectrochemical water splitting and electrochemical water splitting, respectively.Download high-res image (68KB)Download full-size image

S-Doped 2H-MoSe2 (i.e., 2H-MoS2xSe2−2x) mesoporous nanospheres assembled from several-layered nanosheets are synthesized by sulfurizing freshly-prepared 1T-MoSe2 nanospheres, and they serve as a robust host material for sodium storage. The sulfuration treatment is found to be beneficial for removing surface/interface insulating organic contaminants and converting the 1T phase to the 2H phase with improved crystallinity and electrical conductivity. These result in significantly enhanced sodium storage performance, including charge/discharge capacity, first Coulombic efficiency, cycling stability, and rate capability. Coupled with benefits from in situ carbon modification and its mesoporous morphology, the 2H-MoS2xSe2−2x (x = 0.22) nanosphere anode can maintain a reversible capacity of 407 mA h g−1 after 100 cycles with no observable capacity fading at a high current density of 2.0 A g−1. This value is much higher than those of the anode fabricated with the freshly-prepared 1T-MoSe2 (95 mA h g−1) and the annealed 2H-MoSe2 (144 mA h g−1) samples. As the current density rises from 0.05 to 5.0 A g−1 (100-fold increase), the discharge capacity retention is significantly increased from 39% before sulfuration to 65% after sulfuration. This superior electrochemical performance of the 2H-MoS2xSe2−2x electrode suggests a promising way to design advanced sodium host materials by surface/interface engineering.

Sodium-ion batteries (SIBs) have emerged as a promising candidate for large-scale stationary energy storage in light of possible concerns over cost and abundance. Advanced sodium host materials are required for developing SIBs with high energy density, long cycling stability and high safety. Layered metal dichalcogenides (MX2, M = Mo, W, Sn, V, Ti; X = S, Se, Te) have become a hot spot for anode materials in SIBs due to their merits of high conductivity, mechanical and thermal stability and structural stability. In this review, we first present a comprehensive overview of the progress of layered MX2 anodes for SIBs. Detailed discussion on the advantages of MX2 as SIB anodes is then made. Emphasis is placed on enhancing the electrochemical performance through nanostructure engineering, crystal structure modulation, doping/alloying and composite design. We conclude with a perspective on the further development of SIBs in view of their applications.

Sodium-ion batteries (SIBs) are considered as a promising energy storage device, but suffer from poor cycling performance. In this work, hierarchical MoSe2 nanotubes have been synthesized for the first time and demonstrated as a highly durable electrode material of SIBs. The hierarchical nanotubes consist of few-layered MoSe2 nanosheets with an expanded (002) interlayer spacing of 1.00 nm (54.8% expansion). The growth of hierarchical nanotubes is a result of oriented attachment and Ostwald ripening effects. As a robust sodium host material, the MoSe2 nanotubes featuring hierarchical organization, hollow interiors, and interlayer expansion are beneficial for (i) lowering diffusion energy barriers and facilitating fast Na+ insertion/extraction reaction kinetics, (ii) accommodating volume changes upon sodiation/desodiation, (iii) preserving structural and morphological stability, and (iv) improving electronic conductivity by in situ carbon modification. By controlling the cut-off voltage in the range of 0.5–3.0 V with an intercalation mechanism, the MoSe2 nanotube electrode shows highly stable cycling performance and delivers a reversible discharge capacity of 228 mA h g−1 after 1500 cycles at a high current density of 1000 mA g−1. This work demonstrates the best cycling performance to date of MoSe2-based anodes for SIBs.

Green synthesis of Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S1−xSex)4 (CZTSSe) nanocrystals is highly desirable for low-cost and high-efficiency solar energy conversion devices. In this work, scalable synthesis of multinary CZTS and CZTSSe nanocrystals at room temperature has been achieved by a simple metal complex solution mixing (Metcomix) process. In the Metcomix process, CZTS or CZTSSe nanocrystals are formed by simply mixing aqueous solutions of copper thiourea complex ([Cu(TU)4]2+), zinc ammonium complex ([Zn(NH3)4]2+) and tin chalcogen complex ([Sn2S6]4−) or tin double chalcogen complex ([Sn2S4Se2]4−) at room temperature. The Metcomix process features low-energy-consuming, low-cost, environmentally friendly, high-purity, and scalable-production. The CZTS and CZTSSe nanocrystals have a small size of 4–10 nm and exhibit remarkable room-temperature photoluminescence and optical absorption properties. The CZTS and CZTSSe nanocrystals are also deposited onto ZnO nanorod arrays and demonstrated as efficient photoanodes for photoelectrochemical water splitting. The ZnO/CZTSSe photoanode exhibits a photocurrent density of 9.06 mA cm−2 at 1.23 V (vs. the NHE) and an optimal applied bias photon-to-current efficiency (ABPE) of ∼3.43% at a bias of 0.60 V. The present work demonstrates a new approach for synthesizing eco-friendly multinary chalcogenide nanocrystals at room temperature and their promising applications in solar energy conversion devices.

Green synthesis of metastable wurtzite Cu2ZnSnS4 nanocrystals through a top-down synthetic strategy is presented. Formation mechanisms associated with Kirkendall and etching effects are illustrated in detail. The nanocrystals exhibit remarkable photoluminescence properties at room temperature.

•Hierarchical nanotubes assembled from MoS2 and carbon monolayer sandwiched superstructure nanosheets are synthesized for the first time.•The (002) interlayer spacing of MoS2 are significantly enlarged from 0.615 to 0.986 nm.•The 2D MoS2:C superstructure possesses an ideal interface contact between MoS2 and carbon for improved electrical conductivity.•The MoS2:C hierarchical nanotubes work as a robust anode material and exhibit superior rate and cycling performance for SIBs.Interface engineering on 2D layered nanomaterials plays pivotal roles in achieving novel properties and superior device performance. In this work, hierarchical nanotubes consisting of 2D monolayer MoS2 and carbon (MoS2:C) interoverlapped superstructure nanosheets have been synthesized, in which the MoS2 and carbon layers are alternately sandwiched. The hierarchical architectures assembled from the MoS2:C superstructures are beneficial for: (i) providing substantially expanded (002) interlayer spacing (0.98 nm) of 2H-MoS2 which facilitates fast Na+ insertion/extraction reaction kinetics, (ii) improving electrical conductivity of MoS2 by carbon monolayer insertion with ideal heterointerface contact, (iii) preventing aggregation of MoS2 nanosheets, and (iv) accommodating volume change upon sodiation/desodiation. The superstructure nanotubes are demonstrated as a robust anode material for sodium storage with superior electrochemical performance. They deliver a high rate-capability and maintain discharge capacities of 295 and 187 mAh g−1 at high current densities of 10.0 and 20.0 A g−1, respectively. Furthermore, they show durable cycling life (capacity retention of 101.3%, 108.2% and 107.8% after 200 cycles at current densities of 0.2, 0.5 and 1.0  A g−1, respectively, in comparison to those of the 2nd cycles), and an initial Coulombic efficiency as high as 84%. The MoS2:C superstructure nanotubes perform among the best of current MoS2-based electrode materials.Hierarchical nanotubes consisting of 2D monolayer MoS2 and carbon (MoS2:C) interoverlapped superstructure nanosheets are synthesized. They exhibit superior rate and cycling performance for sodium storage by providing significantly expanded (002) interlayer spacing and improving the electrical conductivity via carbon monolayer intercalation.

In this work, we report a simple and low-temperature approach for the controllable synthesis of ternary Cu–S–Se alloys featuring tunable crystal structures, compositions, morphologies, and optical properties. Hexagonal CuSySe1–y nanoplates and face centered cubic (fcc) Cu2–xSySe1–y single-crystal-like stacked nanoplate assemblies are synthesized, and their phase conversion mechanism is well investigated. It is found that both copper content and chalcogen composition (S/Se atomic ratio) of the Cu–S–Se alloys are tunable during the phase conversion process. Formation of the unique single-crystal-like stacked nanoplate assemblies is resulted from oriented stacking coupled with the Ostwald ripening effect. Remarkably, optical tuning for continuous red shifts of both the band-gap absorption and the near-infrared localized surface plasmon resonance are achieved. Furthermore, the novel Cu–S–Se alloys are utilized for the first time as highly efficient counter electrodes (CEs) in quantum dot sensitized solar cells (QDSSCs), showing outstanding electrocatalytic activity for polysulfide electrolyte regeneration and yielding a 135% enhancement in power conversion efficiency (PCE) as compared to the noble metal Pt counter electrode.Keywords: composition tuning; counter electrodes; Cu2−xSySe1−y; CuSySe1−y; optical properties

In this work, we report for the first time a simple solvothermal method to synthesize assembled 1T-MoSe2 nanosheets, which possess expanded (002) interlayer spacings as large as 1.17 nm with an 81% expansion as compared to that (0.646 nm) of the bulk counterpart. The 1T-MoSe2 nanosheets exhibit striking kinetic metrics for the hydrogen evolution reaction (HER) with a low onset potential of 60 mV and a small Tafel slope of 78 mV dec−1, which are better than those of the 2H-MoSe2 counterpart with a normal interlayer spacing of 0.64 nm. The outstanding electrocatalytic activity is attributed to the high concentration of the metallic 1T phase as well as the expanded interlayer distance, contributing to increasing the number and catalytic activity of the active sites.

Green synthesis of metastable wurtzite Cu2ZnSnS4 nanocrystals through a top-down synthetic strategy is presented. Formation mechanisms associated with Kirkendall and etching effects are illustrated in detail. The nanocrystals exhibit remarkable photoluminescence properties at room temperature.

Sodium-ion batteries (SIBs) have emerged as a promising candidate for large-scale stationary energy storage in light of possible concerns over cost and abundance. Advanced sodium host materials are required for developing SIBs with high energy density, long cycling stability and high safety. Layered metal dichalcogenides (MX2, M = Mo, W, Sn, V, Ti; X = S, Se, Te) have become a hot spot for anode materials in SIBs due to their merits of high conductivity, mechanical and thermal stability and structural stability. In this review, we first present a comprehensive overview of the progress of layered MX2 anodes for SIBs. Detailed discussion on the advantages of MX2 as SIB anodes is then made. Emphasis is placed on enhancing the electrochemical performance through nanostructure engineering, crystal structure modulation, doping/alloying and composite design. We conclude with a perspective on the further development of SIBs in view of their applications.