We will review some of the major results in random graphs and some of the more challenging open problems. We will cover algorithmic and structural questions. We will touch on newer models, including those related to the WWW.

Random graphs

A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

In recent years, gas sensors have been increasingly used in industrial production and daily life. Metal oxide semiconductor gas sensing materials are favoured for their outstanding physical and chemical properties, low cost and simple preparation methods. However, the gas sensing mechanisms of metal oxide semiconductors have not been considered by researchers, resulting in omissions and errors in the interpretation of gas sensing mechanisms in many articles. This review organizes and introduces several common gas sensing mechanisms of metal oxide semiconductors in detail and classifies them into two categories. The scope and relationship of these mechanisms are clarified. In addition, this review selects four strategies for enhancing the gas sensing properties of metal oxide semiconductors and analyses the gas sensing mechanisms to highlight the importance of the gas sensing mechanism. Finally, some perspectives for future investigations on the gas sensing mechanisms of metal oxide semiconductors are discussed as well.

Gas sensing mechanisms of metal oxide semiconductors: a focus review

The global photovoltaic (PV) market is dominated by crystalline silicon (c-Si) based technologies with heavily doped, directly metallized contacts. Recombination of photo-generated electrons and holes at the contact regions is increasingly constraining the power conversion efficiencies of these devices as other performance-limiting energy losses are overcome. To move forward, c-Si PV technologies must implement alternative contacting approaches. Passivating contacts, which incorporate thin films within the contact structure that simultaneously supress recombination and promote charge-carrier selectivity, are a promising next step for the mainstream c-Si PV industry. In this work, we review the fundamental physical processes governing contact formation in c-Si. In doing so we identify the role passivating contacts play in increasing c-Si solar cell efficiencies beyond the limitations imposed by heavy doping and direct metallization. Strategies towards the implementation of passivating contacts in industrial environments are discussed. The development of passivating contacts holds great potential for enhancing the power conversion efficiency of silicon photovoltaics. Here, De Wolf et al. review recent advances in material design and device architecture, and discuss technical challenges to industrial fabrication.

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Passivating contacts for crystalline silicon solar cells

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS2, MoSe2, MoTe2, WS2 and WSe2, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1–2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Numerous efforts in improving the hydrogen evolution reaction (HER) performance of transition metal dichalcogenides mostly focus on active sites exposing, vacancy engineering, and phase engineering...

Magnetic Enhancement for Hydrogen Evolution Reaction on Ferromagnetic MoS2 Catalyst.

Anatase hierarchical TiO2 with innovative designs (hollow microspheres with exposed high-energy {001} crystal facets, hollow microspheres without {001} crystal facets, and solid microspheres without {001} crystal facets) were synthesized via a one-pot hydrothermal method and characterized. Based on these materials, gas sensors were fabricated and used for gas-sensing tests. It was found that the sensor based on hierarchical TiO2 hollow microspheres with exposed high-energy {001} crystal facets exhibited enhanced acetone sensing properties compared to the sensors based on the other two materials due to the exposing of high-energy {001} crystal facets and special hierarchical hollow structure. First-principle calculations were performed to illustrate the sensing mechanism, which suggested that the adsorption process of acetone molecule on TiO2 surface was spontaneous, and the adsorption on high-energy {001} crystal facets would be more stable than that on the normally exposed {101} crystal facets. Further c...

Enhanced Gas-Sensing Properties of the Hierarchical TiO2 Hollow Microspheres with Exposed High-Energy {001} Crystal Facets

Array-based sensors are considered as potential candidates for gas detection due to their low cost and great miniaturization potential. However, the fabrication process of array-based sensors is complex and time-consuming since it usually contains the formation and growth processes of seed layers on the surface of alumina substrates. In addition, the gas-sensing materials for the fabrication of array-based sensors are mainly confined to n-type semiconductors such as ZnO, TiO2 and WO3. In this work, a kind of p-type heterostructures arrays composed of NiMoO4 nanosheets and Co3O4 nanowire (Co3O4@NiMoO4) was fabricated in-situ on flat alumina substrates via a simple hydrothermal method without seed layers. SEM and TEM characterizations revealed that the Co3O4 nanowire arrays were fully covered with NiMoO4 nanosheets. The gas-sensing measurements revealed that the Co3O4@NiMoO4 composite arrays showed the highest response (Rg/Ra = 17.12) towards 100 ppm trimethylamine at its optimal operating temperature of 250 °C. This response value was 3.91 times higher than that of Co3O4 arrays (Rg/Ra = 4.39) and 9.25 times higher than that of NiMoO4 nanosheets (Rg/Ra = 1.85) at their optimal operating temperatures of 250 and 350 °C, respectively. Meanwhile, the enhanced sensing mechanism of the Co3O4@NiMoO4 composite arrays was also discussed. It could be explained by the special heterojunction structure of the Co3O4@NiMoO4 composite arrays, which offered a high surface area and an additional modulation in resistance. Our studies shed a new light to design p-type heterostructure arrays in-situ for fabricating sensing device by a facile method. Moreover, the as-designed Co3O4@NiMoO4 composite arrays are potential candidates in the fabricating of high performance trimethylamine sensors.

In situ growth of Co3O4@NiMoO4 composite arrays on alumina substrate with improved triethylamine sensing performance

Resistive gas sensors based on metal oxides have aroused great interest in the sensing of NO2 gas due to their low cost, good stability, and easy fabrication. However, drawbacks such as low sensitivity and a lack of selectivity, which originate from the limited kinds of intrinsic active centers on the surface of the metal oxides that could be involved in the gas-sensing reaction, remain great challenges to overcome. To solve these problems, surface modification of SnO2 by S-doping was carried out by the sintering of flower-like SnS2. Gas-sensing tests revealed that the S-doped SnO2 showed ultra-high sensitivity to NO2 (Rg/Ra = 600 toward 5 ppm) with low optimal operating temperature (50 °C). The detection limit of the sensor was as low as 50 ppb (Rg/Ra = 11). Notably, the S-doped SnO2 showed negligible cross-responses to alcohol, acetone, HCHO, SO2, H2S, and xylene. The ultra-high sensitivity and selectivity toward NO2 were closely related to the content of the S-dopant. This phenomenon is attributed to the active role of S-dopant during the surface reactions with NO2, which was substantiated by in situ Raman characterization and DFT-based calculations. This study offers an important guide for surface modification by doping to improve the sensitivity and selectivity of metal oxides and sheds new light on material design to develop resistive gas sensors for NO2 detection.

High selectivity of sulfur-doped SnO2 in NO2 detection at lower operating temperatures.

Hierarchical porous (HP) nanostructures of p-type metal oxide have attracted great attention in gas-sensing application due to their less agglomerated configurations and the advantages for gas diffusion. However, the contacts between grain boundaries around pores of HP nanostructures are usually too fragile to resist the shear force caused by ultrasonic treatment during sensor fabrication processes and the thermal stress produced from high operating temperature. To solve this problem, uniform Co3O4 microspheres assembled by single-crystalline porous nanosheets were synthesized by an ethylene glycol (EG) mediated solvothermal method with the co-assistant of water and polyvinyl pyrrolidone (PVP). Due to the structural stability of single-crystalline nanosheets, the as-synthesized HP Co3O4 is well kept during the sensor fabrication processes. Moreover, the Co3O4 shows a high response (Rg/Ra = 74.5–100 ppm) to xylene at the optimized temperature (150 °C), which is almost 4 times larger than that of commercial Co3O4 (19.1). It also exhibits excellent long-term stability with small deviations (less than 6%) for two months, which can be ascribed to the structural stability of single-crystalline nanosheets. Combined with their high selectivity and low detection limit, the as-prepared HP Co3O4 microspheres assembled by single-crystalline porous nanosheets are highly promising gas-sensing materials for xylene detection.

Ting Yu

Papers

Magnetic Enhancement for Hydrogen Evolution Reaction on Ferromagnetic MoS2 Catalyst.

Enhanced Gas-Sensing Properties of the Hierarchical TiO2 Hollow Microspheres with Exposed High-Energy {001} Crystal Facets

In situ growth of Co3O4@NiMoO4 composite arrays on alumina substrate with improved triethylamine sensing performance

High selectivity of sulfur-doped SnO2 in NO2 detection at lower operating temperatures.

Single-crystalline porous nanosheets assembled hierarchical Co3O4 microspheres for enhanced gas-sensing properties to trace xylene