Guangbo Liu

Vertically aligned two-dimensional SnS2 nanosheets with a strong photon capturing capability for efficient photoelectrochemical water splitting

Two-dimensional (2D) metal dichalcogenides have emerged as attractive materials for application in photoelectrochemical (PEC) water splitting due to their unique structure and strong interaction with light. To date, deposition of exfoliated 2D nanosheet dispersions onto conductive substrates by a variety of techniques (e.g. casting, spin-coating and self-assembly) has been the most exploited approach to fabricate photoelectrodes from these materials. However, such solution processing strategies do not allow for control over the flake orientation and formation of intimate electrical contacts with conductive substrates. This could negatively affect the PEC efficiency. Herein, we demonstrate, for the first time, vertically aligned 2D SnS2 nanosheets with controllable growth and density on conductive substrates (FTO and carbon cloth (CC)) by a modified chemical vapor deposition (CVD) method. In PEC measurements, these vertically aligned 2D SnS2 nanosheet photoelectrodes exhibit a high incident photon to current conversion efficiency (IPCE) of up to 40.57% for SnS2⊥CC and 36.76% for SnS2⊥FTO at 360 nm, and a high photocurrent density of up to 1.92 ± 0.01 mA cm−2 for SnS2⊥CC and 1.73 ± 0.01 mA cm−2 for SnS2⊥FTO at 1.4 V vs. reversible hydrogen electrode (RHE). These values are two times higher than that of their photoelectrode (SnS2//FTO) counterparts prepared by conventional spin-coating. Our demonstration of this controllable growth strategy offers a versatile framework towards the design and fabrication of high performance PEC photoelectrodes based on 2D metal chalcogenides.

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.

Gate Modulation of Threshold Voltage Instability in Multilayer InSe Field Effect Transistors

We report a modulation of threshold voltage instability of back-gated multilayer InSe FETs by gate bias stress. The performance stability of multilayer InSe FETs is affected by gate bias polar, gate bias stress time and gate bias sweep rate under ambient conditions. The on-current increases and threshold voltage shifts to negative gate bias stress direction with negative bias stress applied, which are opposite to that of positive bias stress. The intensity of gate bias stress effect is influenced by applied gate bias time and the sweep rate of gate bias stress. The behavior can be explained by the surface charge trapping model due to the adsorbing/desorbing oxygen and/or water molecules on the InSe surface. This study offers an opportunity to understand gate bias stress modulation of performance instability of back-gated multilayer InSe FETs and provides a clue for designing desirable InSe nanoelectronic and optoelectronic devices.

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.

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.

A Dual-Band Multilayer InSe Self-Powered Photodetector with High Performance Induced by Surface Plasmon Resonance and Asymmetric Schottky Junction

A dual-band self-powered photodetector (SPPD) with high sensitivity is realized by a facile combination of InSe Schottky diode and Au plasmonic nanoparticle (NP) arrays. Comparing with pristine InSe devices, InSe/Au photodetectors possess an additional capability of photodetection in visible to near-infrared (NIR) region. This intriguing phenomenon is attributed to the wavelength selective enhancement of pristine responsivities by hybridized quadrupole plasmons resonance of Au NPs. It is worth pointing out that the maximum of enhancement ratio in responsivity reaches up to ∼1200% at a wavelength of 685 nm. In addition, owing to a large Schottky barrier difference formed between active layer and two asymmetric electrodes, the responsivities of dual-band InSe/Au photodetector could reach up to 369 and 244 mA/W at the wavelength of 365 and 685 nm under zero bias voltage, respectively. This work would provide an additional opportunity for developing multifunctional photodetectors with high performance based on two-dimensional materials, upgrading their capacity of photodetection in a complex environment.