Yunfeng Qiu

Controlled growth of vertical 3D MoS2(1−x)Se2x nanosheets for an efficient and stable hydrogen evolution reaction

Layered transition metal dichalcogenides (TMDs) are considered as promising hydrogen evolution reaction (HER) candidates due to their exposed active sites at edges and superior electron mobility along sheets, however their inert basal planes and non-ohmic contact with current collectors greatly hamper their application in HER reactions. Exposing active sites, accelerating charge transfer, and manipulating hydrogen adsorption free energy close to thermoneutral are significant to favor the HER process. Herein, component-controllable 3D MoS2(1−x)Se2x alloy nanosheets with a vertically oriented architecture were successfully grown on conductive carbon cloth substrates through a CVD technique. The bigger radius of Se can cause a slight distortion and bring about a polarized electric field in the basal planes, resulting in favorable bond breaking of adsorbed molecules. Among all tested catalysts, Mo(S0.53Se0.47)2 alloy nanosheets exhibit the lowest Tafel slope (55.5 mV dec−1), smallest overpotential (183 mV) at 10 mA cm−2, and highest conductivity. The Mo(S0.53Se0.47)2 alloy maintains its activity after 2000 cycles. Density functional theory calculations manifest adjustment of hydrogen adsorption free-energies and vacancy formation energies in MoS2(1−x)Se2x alloy nanosheets. S and Se vacancies serve as a crucial factor for HER performance. The 3D exposed active sites, adjusted hydrogen adsorption free energy, vacancy formation energies, and ohmic contact with carbon cloth are found to be responsible for the enhanced HER performance.

In-Plane Mosaic Potential Growth of Large-Area 2D Layered Semiconductors MoS2–MoSe2 Lateral Heterostructures and Photodetector Application

Considering the unique layered structure and novel optoelectronic properties of individual MoS2 and MoSe2, as well as the quantum coherence or donor-acceptor coupling effects between these two components, rational design and artificial growth of in-plane mosaic MoS2/MoSe2 lateral heterojunctions film on conventional amorphous SiO2/Si substrate are in high demand. In this article, large-area, uniform, high-quality mosaic MoS2/MoSe2 lateral heterojunctions film was successfully grown on SiO2/Si substrate for the first time by chemical vapor deposition (CVD) technique. MoSe2 film was grown along MoS2 triangle edges and occupied the blanks of the substrate, finally leading to the formation of mosaic MoS2/MoSe2 lateral heterojunctions film. The composition and microstructure of mosaic MoS2/MoSe2 lateral heterojunctions film were characterized by various analytic techniques. Photodetectors based on mosaic MoS2/MoSe2 lateral heterojunctions film, triangular MoS2 monolayer, and multilayer MoSe2 film are systematically investigated. The mosaic MoS2/MoSe2 lateral heterojunctions film photodetector exhibited optimal photoresponse performance, giving rise to responsivity, detectivity, and external quantum efficiency (EQE) up to 1.3 A W-1, 2.6 × 1011 Jones, and 263.1%, respectively, under the bias voltage of 5 V with 0.29 mW cm-2 (610 nm), possibly due to the matched band alignment of MoS2 and MoSe2 and strong donor-acceptor delocalization effect between them. Taking into account the similar edge conditions of transition metal dichalcogenides (TMDCs), such a facile and reliable approach might open up a unique route for preparing other 2D mosaic lateral heterojunctions films in a manipulative manner. Furthermore, the mosaic lateral heterojunctions film like MoS2/MoSe2 in the present work will be a promising candidate for optoelectronic fields.

Effects of Organic Molecules with Different Structures and Absorption Bandwidth on Modulating Photoresponse of MoS2 Photodetector

Organic dye molecules possessing modulated optical absorption bandwidth and molecular structures can be utilized as sensitizing species for the enhancement of photodetector performance of semiconductor via photoinduced charge transfer mechanism. MoS2 photodetector were modified by drop-casting of methyl orange (MO), rhodamine 6G (R6G), and methylene blue (MB) with different molecular structures and extinction coefficients, and enhanced photodetector performance in terms of photocurrent, photoresponsity, photodetectivity, and external quantum efficiency were obtained after modification of MO, R6G, and MB, respectively. Furthermore, dyes showed different modulating abilities for photodetector performance after combination with MoS2, mainly due to the variation of molecular structures and optical absorption bandwidth. Among tested dyes, deposition of MB onto monolayer MoS2 grown by CVD resulted in photocurrent ∼20 times as high as pristine MoS2 due to favorable photoinduced charge transfer of photoexcited electrons from flat MB molecules to the MoS2 layer. Meanwhile, the corresponding photoresponsivity, photodetectivity, and an external quantum efficiency are 9.09 A W(1-), 2.2 × 10(11) Jones, 1729% at 610 nm, respectively. Photoinduced electron-transfer measurements of the pristine MoS2 and dye-modified MoS2 indicated the n-doping effect of dye molecules on the MoS2. Additionally, surface-enhanced Raman measurements also confirmed the direct correlation with charge transfer between organic dyes and MoS2 taking into account the chemically enhanced Raman scattering mechanism. Present work provides a new clue for the manipulation of high-performance of two-dimensional layered semiconductor-based photodetector via the combination of organic dyes.

Tuning electrochemical catalytic activity of defective 2D terrace MoSe2 heterogeneous catalyst via cobalt doping

This study presents the successful growth of defective 2D terrace MoSe2/CoMoSe lateral heterostructures (LH), bilayer and multilayer MoSe2/CoMoSe LH, and vertical heterostructures (VH) nanolayers by doping metal cobalt (Co) element into MoSe2 atomic layers to form a CoMoSe alloy at high temperatures (∼900 °C). After the successful introduction of metal Co heterogeneity in the MoSe2 thin layers, more active sites can be created to enhance hydrogen evolution reaction (HER) activities combining with metal Co catalysis through mechanisms such as (1) atomic arrangement distortion in CoMoSe alloy nanolayers, (2) atomic level coarsening in LH interfaces and terrace edge layer architecture in VH, and (3) formation of defective 2D terrace MoSe2 nanolayers heterogeneous catalyst via metal Co doping. The HER investigations indicated that the obtained products with LH and VH exhibited an improved HER activity in comparison with those from pristine 2D MoSe2 electrocatalyst and LH type MoSe2/CoMoSe. The present work shows a facile yet reliable route to introduce metal ions into ultrathin 2D transition metal dichalcogenides (TMDCS) and produce defective 2D alloy atomic layers for exposing active sites, eventually improving their electrocatalytic performance.

Moisture-responsive films consisting of luminescent polyoxometalates and agarose

Novel switchable and tunable luminescent composite films consisting of Na9[EuW10O36]·32H2O (EuW10) and agarose have been prepared. EuW10 can be converted into a non-luminescent species by photoreduction and the recovery is facilitated by oxidation under moisture air. The luminescence recovery of the UV-reduced films showed dependence on the relative humidity (RH) ranging from 43 to 78% after a certain exposure time. X-ray photoelectron energy spectroscopy (XPS) measurements confirm the generation of the reduced W5+ species in EuW10 polyoxoanions. Further analysis of EuW10 indicates that the photo-generated d1 electron hopping is prohibited in the hybrid films. The absence of recombination of the photogenerated d1 electrons and holes, and the consumption of holes to form ˙OH radicals are responsible for the luminescence quenching rather than luminescence resonance energy transfer (LRET). The moisture-responsive behavior of the EuW10–agarose films is attributed to the kinetically controlled back reaction of W5+O6 with O2. The present study provides new insights into lanthanide-containing POMs as tunable luminescent components in UV and moisture dual-responsive materials.

Electrostatic Assembly Preparation of High-Toughness Zirconium Diboride-Based Ceramic Composites with Enhanced Thermal Shock Resistance Performance

The central problem of using ceramic as a structural material is its brittleness, which associated with rigid covalent or ionic bonds. Whiskers or fibers of strong ceramics such as silicon carbide (SiC) or silicon nitride (Si3N4) are widely embedded in a ceramic matrix to improve the strength and toughness. The incorporation of these insulating fillers can impede the thermal flow in ceramic matrix, thus decrease its thermal shock resistance that is required in some practical applications. Here we demonstrate that the toughness and thermal shock resistance of zirconium diboride (ZrB2)/SiC composites can be improved simultaneously by introducing graphene into composites via electrostatic assembly and subsequent sintering treatment. The incorporated graphene creates weak interfaces of grain boundaries (GBs) and optimal thermal conductance paths inside composites. In comparison to pristine ZrB2-SiC composites, the toughness of (2.0%) ZrB2-SiC/graphene composites exhibited a 61% increasing (from 4.3 to 6.93 MPa·m(1/2)) after spark plasma sintering (SPS); the retained strength after thermal shock increased as high as 74.8% at 400 °C and 304.4% at 500 °C. Present work presents an important guideline for producing high-toughness ceramic-based composites with enhanced thermal shock properties.

Efficiently Synergistic Hydrogen Evolution Realized by Trace Amount of Pt-Decorated Defect-Rich SnS2 Nanosheets

The electrocatalytic hydrogen evolution reaction (HER) has attracted increasing attention in the field of hydrogen-based economy, whereat developing cheap and efficient catalysts to reduce the use of Pt-based catalysts is highly required. Tin disulfide (SnS2) as a new rising star has exhibited intriguing properties in energy storage and conversion applications, while showing slow progress in HER due to the inherent poor activity. Herein, we demonstrate the successful structural engineering and simultaneous integration of trace amount Pt in SnS2 nanosheets via a facile and effective in situ cycling voltammetry activation process, leading to the efficiently synergistic HER. Defect-rich SnS2 nanosheets decorated with a trace amount (0.37 wt %) of Pt exhibit greatly enhanced HER activity due to the synergy between them, revealing low onset potential of 32 mV and overpotential of 117 mV at 10 mA/cm2, small Tafel slope of 69 mV/dec, and large exchange current density of 394.46 μA/cm2. Present work provides an intriguing strategy for developing ultralow loading Pt electrocatalysts with high HER performance.

Modulation of opto-electronic properties of InSe thin layers via phase transformation

Phase engineering of two-dimensional (2D) materials offers unique opportunities for acquiring novel opto-electronic properties and allows for the searching of outstanding candidates for applications in opto-electronic detectors, sensors, catalysis, or phase-change memory devices. Here, we report the phase-transformation from β-InSe to γ-In2Se3, exploiting the thermal annealing route to trigger the process starting at 200 °C that expands the family of phase-change materials. The presence of γ-In2Se3 is solidly confirmed by the characteristic peaks in X-ray diffraction (XRD) and energy dispersive X-ray (EDX), and is quite stable at ambient condition, thus facilitating substantial application in phase-change memory devices. A Raman shift in the A′1 mode from 225 cm−1 to 230 cm−1 further illustrates the phase transformation. Besides the photoluminescence (PL) peak of β-InSe, the ∼2 eV PL peak, ascribed to γ-In2Se3, is observed in the annealed nanosheet. The increased PL band gap of β-InSe as a function of annealing temperature during phase transformation was possibly affected by the suppressed interlayer coupling, as well as the planar quantum confinement of photo-excited carriers by the external surfaces of the sheets. The photodetector performance with respect to photocurrent, mobility, detectivity, responsivity, and external quantum efficiency was subsequently evaluated after thermal annealing, showing deteriorated optical performance. The present work proved that thermal annealing could induce the successful phase transformation, and adjusted the opto-electronic properties in some extent, providing useful information for processing 2D materials based nano-devices.

Chitosan assisted synthesis of 3D graphene@Au nanosheet composites: catalytic reduction of 4-nitrophenol

We report the eco-friendly chitosan assisted synthesis of 3D graphene@chitosan@Au nanosheet (3DG@CS@AuNSs) composites without using any toxic reductants or capping agents. The 3D graphene network consists of few-layer graphene sheets. One the one hand, the interfacial energy between graphene and Au nanostructures is significantly weakened due to the existence of chitosan polymers, serving as “glue” for anchoring Au nanosheets on the surface of 3D graphene. On the other hand, chitosan contains abundant hydroxyl and amine groups acting as reducing and stabilizing agents for the formation and even distribution of Au nanosheets. Mild redox reaction between hydrochloroauric ions and hydroxyl and amine groups occurred under thermal conditions due to the matching electrochemical potentials of the oxidants and reductants. The as-prepared 3DG@AuNSs exhibited much higher activities than that of 3DG@Au nanoaggregates towards the reduction of 4-nitrophenol, as well as more convenient recyclability than other supported Au NPs in terms of a robust free-standing foam structure. Furthermore, high performance liquid chromatography was also applied to monitor the emergence of 4-aminophenol during reaction. The present studies not only build a new route for the design of a robust and free-standing foam catalyst based on the integration of graphene and Au nanostructures by the assistance of a natural polymer, but also shed deep insight on the understanding of the synergistic catalytic activity towards nitrophenols.

Fast growth of graphene on SiO2/Si substrates by atmospheric pressure chemical vapor deposition with floating metal catalysts

Graphene on dielectric substrates is essential for its electronic applications. Graphene is typically synthesized on the surface of metal and then transferred to an appropriate substrate for fabricating device applications. This post growth transfer process is detrimental to the quality and performance of the as-grown graphene. Therefore, direct growth of graphene films on dielectric substrates without any transfer process is highly desirable. However, fast growth of graphene on dielectric substrates remains challenging. Here, we demonstrate a transfer-free chemical vapor deposition (CVD) method to directly grow graphene films on dielectric substrates at fast growth rate using Cu as floating catalyst. A large area (centimeter level) graphene can be grown within 15 min using this CVD method, which is increased by 500 times compared to other direct CVD growth on dielectric substrate in the literatures. This research presents a significant progress in transfer-free growth of graphene and graphene device applications.