Kuei-Hsien Chen

The thickness of transition metal dichalcogenides (TMDs) plays a key role in enhancing their photocatalytic CO2 reduction activity. However, the optimum thickness of the layered TMDs that is required to achieve sufficient light absorption and excellent crystallinity has still not been definitively determined. In this work, ultra-thin molybdenum disulfide films (MoS2TF) with 25 nm thickness presented remarkable photocatalytic activity, and the product yield increased by about 2.3 times. The photocatalytic mechanism corresponding to the TMDs’ thickness was also proposed. This work demonstrates that the thickness optimization of TMDs provides a cogent direction for the design of high-performance photocatalysts.

The quest of earth-abundant bifunctional electrocatalysts for highly efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is essential for clean and renewable energy systems. Herein, directed by the experimental analysis, we demonstrate layered nickel lithium phosphosulfide (NiLiP2S6) crystal as a highly efficient water-splitting catalyst in alkaline media. With strained lattice due to stacked layers as observed by TEM and electronic structure analysis performed by XPS showed mixed Ni2+,3+ oxidation states induced by addition of Li as a cation, NiLiP2S6 displays excellent OER (current density of 10 mA cm–2 showed an overpotential of 303 mV vs. RHE and a Tafel slope of 114 mV dec–1) and HER activity (current density of −10 mA cm–2 showed an overpotential of 184 mV vs. RHE and a Tafel slope of 94.5 mV dec–1). Finally, an alkaline media was employed to demonstrate the overall water splitting using NiLiP2S6 as both the anode and the cathode, which attained a 50 mA cm−2 current density at 1.68 V. This bimetallic phosphosulfide, together with long-term stability and enhanced intrinsic activity, shows enormous potential in water splitting applications.

Most chemical vapor deposition methods for transition metal dichalcogenides use an extremely small amount of precursor to render large single-crystal flakes, which usually causes low coverage of the materials on the substrate. In this study, a self-capping vapor-liquid-solid reaction is proposed to fabricate large-grain, continuous MoS2 films. An intermediate liquid phase-Na2Mo2O7 is formed through a eutectic reaction of MoO3 and NaF, followed by being sulfurized into MoS2. The as-formed MoS2 seeds function as a capping layer that reduces the nucleation density and promotes lateral growth. By tuning the driving force of the reaction, large mono/bilayer (1.1 mm/200 μm) flakes or full-coverage films (with a record-high average grain size of 450 μm) can be grown on centimeter-scale substrates. The field-effect transistors fabricated from the full-coverage films show high mobility (33 and 49 cm2 V−1 s−1 for the mono and bilayer regions) and on/off ratio (1 ~ 5 × 108) across a 1.5 cm × 1.5 cm region.

The conversion of CO2 into hydrocarbon fuels via the photocatalytic reaction route is considered a potential strategy to concurrently address serious energy crisis and greenhouse gas emission problems. Nevertheless, the generation of long-chain hydrocarbon products (Cn, n ≥ 2) from the visible-light-reactive photocatalytic CO2 reduction has also been considering a contemporary challenge. Herein, we indicate that Ag nanoparticles (Ag NPs) loaded gC3N4/ZnO nanorods heterojunction (Ag-gC3N4/ZnO NRs abbreviation) has extended photoactive range and enhanced specific surface area. The combination of Ag NPs and gC3N4/ZnO NRs significantly enhances photocatalytic CO2 reduction efficiency to form the acetone product. Detail, the acetone production efficiency of Ag-gC3N4/ZnO NRs is 8.4 and 7.5 times higher than pure ZnO NRs and gC3N4/ZnO NRs at the same condition, respectively. This study represents a potential approach toward higher-energy-value hydrocarbons production and greenhouse gas emission mitigation.

Bi doped Mg2Si0.4Sn0.6 had been synthesised in a high energy ball mill followed by compaction using a sintering hot press. The structural and compositional characterization of sintered mass indicated the formation of a highly densified single-phase product. The microstructure of the hot-pressed samples had been critically assessed. Thermoelectric properties were measured between room temperature and 723 K. A decrease in electrical conductivity was found with the increase in temperature but the Seebeck coefficient showed a reverse trend justifying the attainment of degenerate semiconducting behaviour. Meanwhile, the lattice thermal conductivity was subdued to 1.5 W/mK at 623 K. However, the highest zT value of 0.8 was achieved at 723 K. Moreover, the detailed X-ray photoelectron spectroscopic analysis was carried for the determination of binding energy of the constituent elements in the experimental alloy; it also provided the correct estimation of atomic percentage of the concerned elements. The Raman spectrum revealed a shift in F2g peak with respect to that of Mg2Sn and Mg2Si in correspondence with the composition of the synthesised alloy. The synthesised alloy showed micro and nano hardness of 3.7 and 4.03 GPa respectively, which implies that good mechanical strength could be achieved in the synthesised alloy.

Photocatalytic water splitting is a promising way to produce hydrogen fuel from solar energy. In this regard, the search for new photocatalytic materials that can efficiently split water into hydrogen is essential. Here, using first-principles simulations, we demonstrate that the dual-defective SnS2 (Ni-SnS2-VS), by both single-atom nickel doping and sulfur monovacancies, becomes a promising two-dimensional photocatalyst compared with SnS2. The Ni-SnS2-VS monolayer, in particular, exhibits a suitable band alignment that perfectly overcomes the redox potentials for overall water splitting. The dual-defective monolayer displays remarkable photocatalytic activity, a spatially separated carrier, a broadened optical absorption spectrum, and enhanced adsorption energy of H2O. Therefore, the dual-defective SnS2 monolayer can serve as an efficient photocatalyst for overall water splitting to produce hydrogen fuel. Furthermore, a novel dual-defect method can be an effective strategy to enhance the photocatalytic behavior of 2D materials; it may pave inroads in the development of solar-fuel generation.

Designing highly conducting metal–organic frameworks (MOFs) is currently a subject of great interest for their potential applications in diverse areas encompassing energy storage and generation. Herein, a strategic design in which a metal–sulfur plane is integrated within a MOF to achieve high electrical conductivity, is successfully demonstrated. The MOF {[Cu2(6-Hmna)(6-mn)]·NH4}n (1, 6-Hmna = 6-mercaptonicotinic acid, 6-mn = 6-mercaptonicotinate), consisting of a two dimensional (–Cu–S–)n plane, is synthesized from the reaction of Cu(NO3)2, and 6,6′-dithiodinicotinic acid via the in situ cleavage of an S–S bond under hydrothermal conditions. A single crystal of the MOF is found to have a low activation energy (6 meV), small bandgap (1.34 eV) and a highest electrical conductivity (10.96 S cm−1) among MOFs for single crystal measurements. This approach provides an ideal roadmap for producing highly conductive MOFs with great potential for applications in batteries, thermoelectric, supercapacitors and related areas.

Polybenzimidazoles containing heterocyclic benzo[c]cinnoline structure are synthesized from 3,8-benzo[c]cinnoline dicarboxylic acid, terephthalic acid and 3,3′-diaminobenzidine. Their membranes are prepared by sol-gel process, involving the conversion of polymer solution in polyphosphoric acid to phosphoric acid. The acid doping levels of the as-prepared membranes increase as the contents of benzo[c]cinnoline increase, indicating good interaction between phosphoric acid and benzo[c]cinnoline structure. The as-prepared membranes with high acid doping levels might lead to the dissolution of membranes in phosphoric acid at temperature higher than 120 °C. A new method is proposed to adjust acid doping levels by immersing the as-prepared membranes in diluted phosphoric acid solutions of various concentrations. The adjusted membranes (acid doping levels around 30 PA RU−1) exhibit enhanced mechanical properties with tensile strength in the range of 4.1–5.2 MPa. The proton conductivity of adjusted membranes maintain at 0.15–0.17 S cm−1 at 160 °C under ambient atmosphere without humidification. The single cells based on the adjusted membranes exhibit open circuit voltages and peak power densities from 0.89 to 0.91 V and 691–1253 mW cm−2 at 160 °C, respectively. Compared to other polybenzimidazole membranes prepared by sol-gel process, the adjusted polybenzimidazoles show higher mechanical strength and better single cell performance.

The present investigation mainly addresses the open circuit voltage (Voc) issue in kesterite based Cu2ZnSn(S,Se)4 solar cells by simply introducing alkali metal fluoride nanolayers (~ several nm NaF, or LiF) to lower the work functions of the front ITO contacts without conventional hole-blocking ZnO layers. Kelvin probe measurements confirmed that the work function of the front ITO decreases from 4.82 to 3.39 and 3.65 eV for NaF and LiF, respectively, resulting in beneficial band alignment for electron collection and/or hole blocking on top electrodes. Moreover, a 10.4% power conversion efficiency (~ 11.5% in the cell effective area) CZTSSe cell with improved Voc of up to 90 mV has been attained. This demonstration may provide a new direction of further boosting the performance of copper chalcogenide based solar cells as well.

Abstract Application of lasers is omnipresent in modern-day technology. However, preparation of a lasing device usually requires sophisticated design of the materials and is costly, which may limit the suitable choice of materials and the lasing wavelengths. Random lasers, on the other hand, can circumvent the aforementioned shortcomings with simpler fabrication process, lower processing cost, material flexibility for any lasing wavelengths with lower lasing threshold, providing a roadmap for the design of super-bright lighting, displays, Li-Fi, etc. In this work, ultralow-threshold random laser action from semiconductor nanoparticles assisted by a highly porous vertical-graphene-nanowalls (GNWs) network is demonstrated. The GNWs embedded by the nanomaterials produce a suitable cavity for trapping the optical photons with semiconductor nanomaterials acting as the gain medium. The observed laser action shows ultralow values of threshold energy density ≈10 nJ cm?2 due to the strong photon trapping within the GNWs. The threshold pump fluence can be further lowered to ≈1 nJ cm?2 by coating Ag/SiO2 upon the GNWs due to the combined effect of photon trapping and strong plasmonic enhancement. In view of the growing demand of functional materials and novel technologies, this work provides an important step toward realization of high-performance optoelectronic devices.

Abstract The present investigation mainly addresses the open circuit voltage (Voc) issue in kesterites based Cu2ZnSn(S,Se)4 solar cells by simply introducing alkali metal fluoride nanolayers (  several nm NaF, or LiF) to lower the work functions of the front İTO\} contacts without conventional hole-blocking ZnO layers. Kelvin probe measurements confirmed that the work function of the front İTO\} decreases from 4.82 to 3.39 and 3.65 eV for NaF and LiF, respectively, resulting in beneficial band alignment for electron collection and/or hole blocking on top electrodes. Moreover, a 10.4% power conversion efficiency ( 11.5% in the cell effective area) \{CZTSSe\} cell with improved Voc of up to 90 mV has been attained. This demonstration may provide a new direction of further boosting the performance of copper chalcogenide based solar cells as well.

One of the key challenges in artificial photosynthesis is to design a photocatalyst that can bind and activate the CO2 molecule with the smallest possible activation energy and produce selective hydrocarbon products. In this contribution, a combined experimental and computational study on Ni-nanocluster loaded black TiO2 (Ni/TiO2[Vo]) with built-in dual active sites for selective photocatalytic CO2 conversion is reported. The findings reveal that the synergistic effects of deliberately induced Ni nanoclusters and oxygen vacancies provide (1) energetically stable CO2 binding sites with the lowest activation energy (0.08 eV), (2) highly reactive sites, (3) a fast electron transfer pathway, and (4) enhanced light harvesting by lowering the bandgap. The Ni/TiO2[Vo] photocatalyst has demonstrated highly selective and enhanced photocatalytic activity of more than 18 times higher solar fuel production than the commercial TiO2 (P-25). An insight into the mechanisms of interfacial charge transfer and product formation is explored.

The development of earth-abundant and efficient electrocatalysts for the hydrogen evolution reaction (HER) is one of the keys to success for future green energy systems using hydrogen fuel. Nanostructuring of electrocatalysts is a promising way to enhance their electrocatalytic performance in the HER. In this study, pure pyrite-type beaded stream-like cobalt diselenide (CoSe2) nanoneedles are directly formed on flexible titanium foils through treating a cobalt oxide (Co3O4) nanoneedle array template with selenium vapor. The beaded stream-like CoSe2 nanoneedle electrode can drive the HER at a current density of 20 mA cm−2 with a small overpotential of 125 mV. Moreover, the beaded stream-like CoSe2 nanoneedle electrode remains stable in an acidic electrolyte for 3000 cycles and continuously splits water over a period of 18 hours. The enhanced electrochemical activity is facilitated by the unique three-dimensional hierarchical structure, the highly accessible surface active sites, the improved charge transfer kinetics and the highly attractive force between water and the surface of the nanoneedles that exceeds the surface tension of water.