Synthesis And Properties Of Transition-Metal Tellurides With Diversified Nanostructures By Hydrothermal Method

The transition-metal chalcogenides(TMChs)have attracted much attention due to their unique physio-chemical properties and widespread applications.In particular,nanostructured TMChs present more superior properties.As a member of TMChs,transition-metal tellurides(TMTs)with polymouphous structures and different compositions also possess much novel properties and applications in energy conversion and storage,solar cells,photoelectronic and nanoeletronic devices.However,the research work on most nanostructured TMTs(e.g.,CoTe,CoTe2,NiTe,NiTe2 and FeTe2)are less reported in comparison with other chalcogenides except few tellurides(e.g.CdTe),including synthesis,intrinsic magnetic properties and practical applications.So,it is very important to develop a facile synthesis strategy for tellurides with controlled compositions and morphologies.Herein,we prepared diverse MTe/MTe2/MTex(M=Fe.Co.Ni)nanostructures by one-step hydrothermal route.Their magnetic properties with different characteristics,photocatalytic performance and possible mechanism were reported.The detailed and meaningful results were obtained as follows.1.Synthesis and characterization of FeTe2 nanocrystals The optimized conditions for the synthesis of FeTe2 nanocrystals were conducted by series of experiments with different alkali concentrations,reductants,Fe/Te molar ratios and temperatures.The phase and structure indicate that uniform FeTe2 nanocubes were obtained at Fe/Te molar ratio of 0.5 with 0.5 M of NaOH concentration at 150 ? for 5 h.The FeTe2 nanocubes modified with Fe3O4 nanoparticles were also obtained by changing Fe/Te molar ratio.The refinement of XRD shows that as-obtained FeTe2 nanocubes have orthorhombic structure with cell parameters of a = 5.266 A,b = 6.271 A and c = 3.874 A.The FeTe2 single crystalline nanocube exposed(1 1 0),(0 0 1)and(1 0 1)crystal planes was further determined by HRTEM measurements.The characteristics of Fe3O4 nanoparticle loaded on FeTe2 nanocube were also determined by HRTEM,EDX and XPS.The investigation of optic properties suggests that FeTe2 nanocubes have obvious visible photoresponse and enhanced intensity after loaded Fe3O4 nanoparticles.2.Magnetic properties of CoTe and diversified CoTe2 nanostructures We developed a facile hydrothermal strategy to fabricating of nanostructured cobalt tellurides with desired compositions and controlled morphologies.Uniform CoTe and CoTe2 nanorods with average diameter of 100 nm and high purity were synthesized by a simple hydrothermal process.CoTe2 nanostructures with diversified morphologies at different NaOH concentrations were also obtained using this approach.CoTe nanorods exhibit weak ferromagnetism while CoTe2 nanorods present paramagnetic behavior.The other CoTe2 nanostructures present a first increase and then decrease in ferromagnetism with the increase in NaOH concentration due to Na+ entrance into CoTe2 crystal lattice,which confirmed by the first-principles study on the magnetic characteristics of Na-doped CoTe2.This research provides an effective route to synthesize telluride nanostructures and control the magnetism in Na-doped CoTe2.3.Magnetization treatment enhanced photocatalysis in magnetic Co/CoTe2 nanocomposites The recombinations of photoinduced electron-hole pairs result in a low photocatalytic efficiency during photocatalysis process.Considerable efforts have been devoted to improve the transfer of photogenerated carriers and enhance the photocatalytic activity,including ferroelectric and piezoelectric effects on specific materials.But magnetization treatment on photocatalyst in photocatalysis is less reported.Herein,we prepared a magnetic nanocomposite to enhance its photocatalysis by magnetization treatment.Based on the previous results,the Co/CoTe2 nanocomposites with different concentrations of Co were fabricated by a simple hydrothermal process.The phase and structure of composites were analyzed by XRD.The as-obtained Co/CoTe2 nanocomposites have flower-like morphologies and formed gradually from CoTe2 rods.The element composition is confirmed by EDX analysis.The crystalline characteristic of Co/CoTe2 is determined by HRTEM,indicating combining interfaces between CoTe2 and Co.The ferromagnetisms of CoTe2 and Co/CoTe2 composites were further confirmed by M-H loops.the photocatalytic performance of Co/CoTe2 samples were examined by photodegrading organic dye MB,indicating the composites with magnetization treatment significantly enhance the photocatalytic efficiency in a wide range by improving charge separation.It is proposed that the effect enhanced photocatalysis by magnetoresistance will provide a strategy for magnetic photocatalyst applications.4.Magnetic properties of NiTex nanorods and NiTe2 nanoparticles NiTe and NiTe2 nanorods with high purity and excellent crystallinity were synthesized by using appropriate ascorbic acid and changing the molar ratio of elements via a facile hydrothermal route.The nanorod grows along the[1 0 0]direction in length.NiTe nanorods present paramagnetic property at both high and low temperature while NiTe2 nanorods exhibit a diamagnetic characteristic at high temperature and a paramagnetic behavior below 58 K.Series NiTex nanorods(x = 1.00,1.15,1.33,1.60,1.80,2.00)with average diameter of 150 nm were selectively synthesized for investigating the possible mechanism of magnetic properties.The nanorods with various x share similar hexagonal structure and morphology.The magnetization reduces with the increase in tellurium content.The difference in magnetic behaviors of NiTex nanorods is mainly attributed to the competition between the paramagnetism and diamagnetism in the NiTex nanocrystals based on the experimental observation.More Te atoms enhance the diamagnetism and result in the transition temperature decreases.Furthermore,series NiTe2 nanoparticles were prepared at 0.4 M of NaOH with different Te/Ni molar ratios(2.00,1.91,1.86 and 1.76),possessing similar hexagonal structure and morphology.The NiTe2 nanoparticles exhibit ferromagnetism and the saturation magnetization presents a first increase and then decrease with the Te/Ni molar ratios.The difference in magnetic behavior of NiTe2 nanoparticles is originated of both Na+ and Ni atoms entrance into NiTe2 crystal lattice in a specific range.The experimental observation in this work is helpful for comprehensively understanding the NiTex magnetism.5.One-step synthesis of NiTe2(CoTe2)nanorods coated with few-Layers MoS2 for enhancing photocatalysis A facile one-step hydrothermal process was developed for fabrication of three-dimensional(3D)hierarchical NiTe2@MoS2 heterostructures.A few layers of MoS2 uniformly grew on the NiTe2 nanorods,possessing a higher surface area.The strategy was extended to CoTe2@MoS2 heterostructures with a few layers of MoS2.The photocatalytic activities of the heterostructures were evaluated by the photodegradation of methylene blue(MB).The composites show strong adsorption ability and much better photocatalytic efficiency in comparison with pure MoS2 microflowers and NiTe2 nanorods.Especially,the NiTe2@MoS2 heterostructure with 40 wt%of MoS2 presents the highest performance in photocatalytic degradation of dye molecules.The enhanced photocatalytic activity was mainly attributed to the improved election-hole pair separation by the heterojunction between NiTea nanorods and MoS2 nanosheets.And the possible mechanism of the enhanced photocatalytic activities was discussed.The synthetic strategy has good prospects in extensive applications and provides an effective route to fabricate few layer MoS2-based 3D heterostructures.It is expected that the development of novel photocatalyst with 3D hierarchical heterostructure via compositing 1D and 2D nanostructures is helpful towards the promising applications in photocatalysis.