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Recently, atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) have attracted great interest in electronic and opto-electronic devices for high-integration-density applications such as data storage due to their small vertical dimension and high data storage capability. Here, we report a memristor based on free-standing multilayer molybdenum disulfide (MoS2) with a high current on/off ratio of ∼103 and a stable retention for at least 3000 s. Through light modulation of the carrier density in the suspended MoS2 channel, the on/off ratio can be further increased to ∼105. Moreover, the essential photosynaptic functions with short- and long-term memory (STM and LTM) behaviors are successfully mimicked by such devices. These results also indicate that STM can be transferred to LTM by increasing the light stimuli power, pulse duration, and number of pulses. The electrical measurements performed under vacuum and ambient air conditions propose that the observed resistive switching is due to adsorbed oxygen and water molecules on both sides of the MoS2 channel. Thus, our free-standing 2D multilayer MoS2-based memristors propose a simple approach for fabrication of a low-power-consumption and reliable resistive switching device for neuromorphic applications.

In food industries, the detection of different tastes in low level is required to enhance the quality of products. Recently, 2-D materials and metal–organic framework (MOF) have attracted extensive attention owing to their unique properties, and they can be used in various applications, especially chemical and biochemical sensing. In this review, we investigate the recent progress of the 2-D materials and MOF in the taste sensing applications. From the review, we could conclude that these materials would be promising candidates for taste sensing applications, thereby leading to the development of food industry.

Electrochemical carbon dioxide (CO2) reduction is considered to be an efficient strategy to produce usable fuels and overcome the concerns regarding global warming. For this purpose, an efficient, earth abundant, and a low cost catalyst has to be designed. It has been found that graphene‐based materials could be promising candidates for CO2 conversion because of their unique physical, mechanical, and electronic properties. In addition, the surface of graphene‐based materials can be modified by using different strategies, including doping, defect engineering, producing composite structures, and wrapping shapes. In this review, the fundamentals of electrochemical CO2 reduction and recent progress of graphene‐based catalysts are investigated. Furthermore, recent studies on graphene‐based materials for CO2 reduction are summarized.

This study reports an effective method to enhance the performance of photoelectrochemical (PEC) solar water reduction. We design and prepare a SnO2@WS2 NF heterostructure on p-Si that has better vi...

P–N heterostructures based on transition-metal dichelcongenides (TMDs) and a conventional semiconductor, such as p-Si, have been considered a promising structure for next-generation electronic devi...

The use of chemoresistive gas sensors based on metal oxides has expanded to various fields such as medical diagnosis and air quality systems as well as gas leakage detectors with the development of the Internet of Things. Accordingly, sensitivity, selectivity, power consumption, and reproducibility become important factors in the development of gas sensors. Herein, we developed a facile method to fabricate a gas sensor based on porous Au-incorporated tungsten trioxide (WO3) thin films for highly sensitive and selective NO2 sensing. The mixed solution of ammonium tetrathiotungstate [(NH4)2WS4] and gold chloride (AuCl3) was transformed to Au-incorporated WO3 thin films through the spin-coating and annealing process. The gas sensors based on the Au-incorporated WO3 thin films exhibited improved sensitivity, selectivity, and response time upon exposure to NO2 with a significantly low theoretical detection limit of 28 ppt at 150 ℃ compared to gas sensors based on the pristine WO3 thin film. The high sensing properties are attributed to the porous structure and catalytic effects of Au nanoparticles. In addition to these remarkable NO2 sensing properties, the facile and cost-effective fabrication process enlarges the potential of the Au-incorporated WO3 thin films for practical and commercial gas sensing applications.

The issues of global warming and fossil fuel shortage have increased the demand for clean and renewable energy. Many researchers are investigating strategies to produce hydrogen and reduce CO2 by using solar power. Two-dimensional (2D) materials, such as graphene, graphene derivatives, and transition metal dichalcogenides (TMDs), have been extensively used owing to their extraordinary electronic and optical properties. In this review, we investigate the recent developments in 2D materials for photocatalytic applications involving the hydrogen evolution reaction and CO2 reduction. The synthesis methods and the photocatalytic properties of TMDs and graphene-based 2D materials are thoroughly discussed. Moreover, a summary of the recently developed 2D nanostructures and devices for solar hydrogen production and CO2 reduction is presented, and it is revealed that the use of 2D catalyst materials has great potential for commercialization in the near future to help overcome the energy crisis.

In this research, we investigate the effect of metal doping on the electrochromic (EC) performance of tungsten trioxide (WO3) films. These films were prepared by a novel method involving solution processing and thermal annealing. In this procedure, ammonium tetrathiotungstate ((NH4)2WS4) was dissolved in dimethylformamide at a high concentration ratio (200 mg mL−1) to obtain a homogeneous solution and then spin-coated onto the indium thin oxide (ITO) substrate for use as a working electrode. Subsequently, the film was annealed at the different temperatures (200, 300, 400, and 500 °C) to form a crystal structure of WO3. X-ray diffraction, Raman, and X-ray photoelectron spectroscopic results confirm the crystal formation of WO3. Moreover, in order to improve the electrochromic performance, different concentrations (10, 20, 30, and 40 mM) of different metal chlorides such as PtCl4, PdCl2, AuCl3, AgCl, CuCl2, NiCl2, NaCl, and KCl were added into the (NH4)2WS4 precursor and the mixture was coated on the ITO substrate and annealed at 500 °C. The results indicate that the performance of the Au-doped WO3 film was better than those with other dopants, likely as a result of the plasmonic effect. Therefore, the proposed method and Au-doped WO3 films are great candidates for the development of smart windows with high EC performance.

A facile, highly efficient approach to obtain molybdenum trioxide (MoO3)-doped tungsten trioxide (WO3) is reported. An annealing process was used to transform ammonium tetrathiotungstate [(NH4)2WS4] to WO3 in the presence of oxygen. Ammonium tetrathiomolybdate [(NH4)2MoS4] was used as a dopant to improve the film for use in an electrochromic (EC) cell. (NH4)2MoS4 at different concentrations (10, 20, 30, and 40 mM) was added to the (NH4)2WS4 precursor by sonication and the samples were annealed at 500 °C in air. Raman, X-ray diffraction, and X-ray photoelectron spectroscopy measurements confirmed that the (NH4)2WS4 precursor decomposed to WO3 and the (NH4)2MoS4–(NH4)2WS4 precursor was transformed to MoO3-doped WO3 after annealing at 500 °C. It is shown that the MoO3-doped WO3 film is more uniform and porous than pure WO3, confirming the doping quality and the privileges of the proposed method. The optimal MoO3-doped WO3 used as an EC layer exhibited a high coloration efficiency of 128.1 cm2/C, which is larger than that of pure WO3 (74.5 cm2/C). Therefore, MoO3-doped WO3 synthesized by the reported method is a promising candidate for high-efficiency and low-cost smart windows.

Graphene has introduced as a single layer of honeycomb carbon atoms, and has been attracted due to its outstanding physical, optical, and electronic properties. There are various methods to synthesize the graphene, and chemical vapor deposition can offer an excellent approach to prepare large-scale graphene for flexible transparent conducting electrodes with low resistance and high transmittance. These unique properties enable the graphene to be utilized as efficient transparent electrodes for optoelectronic devices, commercially. However, there are some issues that need to be solved. For example, the energy level difference between graphene and the active layer in an optoelectronic device can decrease the efficiency. Therefore, the tuning and modulation of graphene should be considered to enhance device performance. In this chapter, we aim to investigate the optical and electrical properties of the graphene-based transparent electrodes and the effect of doping graphene on properties for optoelectronic applications.

Transition metal dichalcogenides (TMDs) have been considered as one of the most promising electrocatalysts for the hydrogen evolution reaction (HER). Many studies have demonstrated the feasibility of significant HER performance improvement of TMDs by constructing composite materials with Ni-based compounds. In this work, we prepared Ni3Se4@MoSe2 composites as electrocatalysts for the HER by growing in situ MoSe2 on the surface of Ni3Se4 nanosheets. Electrochemical measurements revealed that Ni3Se4@MoSe2 nanohybrids are highly active and durable during the HER process, which exhibits a low onset overpotential (145 mV) and Tafel slope (65 mV/dec), resulting in enhanced HER performance compared to pristine MoSe2 nanosheets. The enhanced HER catalytic activity is ascribed to the high surface area of Ni3Se4 nanosheets, which can both efficiently prevent the agglomeration issue of MoSe2 nanosheets and create more catalytic edge sites, hence accelerate electron transfer between MoSe2 and the working electrode in the HER. This approach provides an effective pathway for catalytic enhancement of MoSe2 electrocatalysts and can be applied for other TMD electrocatalysts.

Transition metal dichalcogenides (TMDs) are promising two-dimensional (2D) materials, and MoS2 has been specifically utilized in electronic devices and integrated circuits. However, the direct synthesis of MoS2 on traditional semiconductors, such as silicon, remains challenging due to the hydrophobic surface of nonoxide wafers (e.g., Si, GaAs, and InP). Herein, a novel, facile, reliable, and one-step method for the direct synthesis of single-crystal MoS2 on a p-Si wafer via hybrid thermolysis is proposed. To demonstrate the applicability of the proposed method, a MoS2/p-Si heterojunction was fabricated and used for solar-driven hydrogen production. The as-fabricated n-MoS2/p-Si heterojunction exhibited a benchmark current density of −13.5 ± 1 mA/cm2 at 0 V and an onset potential of +0.02 V. This method reliably and efficiently produced high-quality MoS2 crystals on a wafer scale and is sufficiently simple to overcome the challenges associated with previous approaches. The method developed herein represents a tremendous advancement in the fabrication of 2D electronic devices. Solar panels that catalyze the splitting of water into hydrogen fuel and oxygen can now be fabricated using a simple deposition process. Interfaces between two-dimensional molybdenum disulfide (2D-MoS2) and electron-poor silicon can split water using light, but only when the naturally random crystallization patterns of 2D-MoS2 are inhibited. Researchers led by Ho Won Jang from Seoul National University and Soo Young Kim at Chung-Ang University, Seoul, have improved uniformity in this ultrathin material by initially coating silicon wafers with a molybdenum oxide precursor that adheres in smooth layers. By depositing a second, sulfur-rich coating and then heating the sample, the team produced uniform 2D-MoS2/silicon junctions down to 10-nanometer scales. The transparent device maintained a water-splitting photocurrent for over forty hours without degradation, thanks to the corrosion-resistant nature of high-quality 2D-MoS2 crystals. A direct synthesis method for high-quality MoS2 thin films on p-Si wafer is reported herein. To increase the hydrophilicity of the p-Si wafer, MoO3 was deposited before spin-coating. The (NH4)2MoS4 precursor was easily coated onto the MoO3/p-Si and was converted to a MoS2/p-Si heterojunction via thermolysis. This method has the potential to be used in 2D electronic device fabrication in the future.

Here we present an innovative flexible ammonia (NH3) sensor based on S and N co-doped graphene quantum dots (S, N: GQDs)/polyaniline (PANI) hybrid loading on flexible polyethylene terephthalate thin film by chemical oxidative polymerization. The sensor exhibited not only excellent response, good selectivity and fast response and recovery time at room temperature but also flexibility, low cost and wearable characteristics. The experimental results reveal that the response of S, N: GQDs/PANI hybrid was about 5 fold higher than that of pure PANI at 100 ppm of NH3 and the attained gas-sensing performance may be attributed to the increased hole like carriers by S, N: GQDs and enhanced interactions between the hybrid sensing film and NH3 molecules via π electron networks. The NH3-sensing mechanisms of the flexible S, N: GQDs/PANI gas sensor based on acid-base doping/de-doping process, carriers mobility, and swelling process are highlighted.

Low dimensional transition metal carbide and nitride (MXenes) have been emerging as frontier materials for energy storage and conversion. Ti3C2Tx was the first MXenes that discovered and soon become the most widely investigated among the MXenes family. Interestingly, Ti3C2Tx exhibits ultrahigh catalytic activity towards the hydrogen evolution reaction. In addition, Ti3C2Tx is electronically conductive, and its optical bandgap is tunable in the visible region, making it become one of the most promising candidates for the photocatalytic hydrogen evolution reaction (HER). In this review, we provide comprehensive strategies for the utilization of Ti3C2Tx as a catalyst for improving solar-driven HER, including surface functional groups engineering, structural modification, and cocatalyst coupling. In addition, the reaming obstacle for using these materials in a practical system is evaluated. Finally, the direction for the future development of these materials featuring high photocatalytic activity toward HER is discussed.

The use of noble-free materials to convert atmospheric CO2 into energy-rich fuels has gained a significant amount of attention in an effort toward decreasing global warming due to high concentrations of CO2. Metallic catalysts, two-dimensional materials (such as graphene and graphene based), metal oxides, and metal-organic frameworks have been used as catalysts in the CO2 reduction reaction and recently recognized as promising platforms due to their excellent electrical and thermal conductivity, outstanding mechanical properties, and good chemical stability. This review summarizes the progress made related to the electrochemical and photoelectrochemical CO2 reduction reaction over the past few decades. In addition, the fundamentals and principles that govern both electrocatalytic and photocatalytic CO2 reduction are discussed. Then, a detailed discussion of the different electrocatalysts, photocatalysts, and strategies used to improve the performance is provided. Keywords: Two dimensional materials, Metal-organic frameworks, CO2 reduction, Electrochemical reduction, Photoelectrochemical reduction

Nanocrystal quantum dots (QDs) provide tunable optoelectronic properties on the basis of their dimension. CdSe QDs, which are size-dependent colloidal nanocrystals, are used for efficient electrochromic devices owing to their unique properties in modulating quantum confinement, resulting in enhanced electron insertion during the electrochromic process. Incorporating a well-known metal oxide electrochromic material such as WO3 into CdSe QDs enhances the redox process. Herein, we propose a facile method for producing and optimizing CdSe QDs doped in WO3. The fabrication of the electrochromic film involves a solution and annealing process. Moreover, the effect of the QD size to optimize the electrochromic layer is studied. As a result, the coloration efficiency of WO3 and optimized CdSe QD-WO3 are obtained as 68.6 and 112.3 cm2/C, respectively. Thus, size-tunable nanocrystal QDs combined with a metal oxide yield high-performance electrochromic devices and are promising candidates for producing smart windows.

There has been considerable research to engineer composites of transition metal dichalcogenides with other materials to improve their catalytic performance. In this work, we present a modified solution-processed method for the formation of molybdenum selenide (MoSe2) nanosheets and a facile method of structuring composites with graphene oxide (GO) or reduced graphene oxide (rGO) at different ratios to prevent aggregation of the MoSe2 nanosheets and hence improve their electrocatalytic hydrogen evolution reaction performance. The prepared GO, rGO, and MoSe2 nanosheets were characterized by X-ray powder diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. The electrocatalytic performance results showed that the pure MoSe2 nanosheets exhibited a somewhat high Tafel slope of 80 mV/dec, whereas the MoSe2-GO and MoSe2-rGO composites showed lower Tafel slopes of 57 and 67 mV/dec at ratios of 6:4 and 4:6, respectively. We attribute the improved catalytic effects to the better contact and faster carrier transfer between the edge of MoSe2 and the electrode due to the addition of GO or rGO.

A highly efficient gas sensor is described based on the use of a nanocomposite fabricated from poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) and ultra-large graphene oxide (UL-GO). The nanocomposite was placed by drop casting in high uniformity on interdigitated gold electrodes over a large area of silicon substrate and investigated for its response to volatile organic compounds (VOCs) at room temperature. Monolayers of UL-GOs were synthesized based on a novel solution-phase method involving pre-exfoliation of graphite flakes. The nanocomposite was optimized in terms of composition, and the resulting vapor sensor (containing 0.04 wt% of UL-GO) exhibits strong response to various VOC vapors. The improved gas-sensing performance is attributed to several effects, viz. (a) an enhanced transport of charge carriers, probably a result of the weakening of columbic attraction between PEDOT and PSS by the functional groups on the UL-GO sheets; (b) the increase in the specific surface area on adding UL-GO sheets; and (c) enhanced interactions between the sensing film and VOC molecules via the network of π-electrons. The sensitivity, response and recovery times of the PEDOT-PSS/UL-GO nanocomposite gas sensor with 0.04 wt% of UL-GO are 11.3 %, 3.2 s, and 16 s, respectively. At a methanol vapor concentration as low as 35 ppm, this is an improvement by factors of 110, 10, and 6 respectively, compared to a PEDOT-PSS reference gas sensor without UL-GO.

Here we present a highly efficient ammonia (NH3) gas sensor made of copper(II) tetrasulfophthalocyanine supported on a three-dimensional nitrogen-doped graphene based framework (CuTSPc@3D-(N)GF)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) nanocomposite sensing film with high uniformity over a large surface area. The NH3 gas sensing performance of the nanocomposite was compared with those of sensors based on pure PEDOT-PSS and a pristine CuTSPc@3D-(N)GF. It was revealed that the synergistic behavior between both of these candidates allowed excellent sensitivity and selectivity for NH3 gas in a low concentration range of 1–1000 ppm at room temperature. The CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor exhibited a much better response (∼5 and 53 times, with a concentration of NH3 gas at 200 ppm) to NH3 gas than those of the pure PEDOT-PSS and pristine CuTSPc@3D-(N)GF gas sensors, respectively. The combination of the CuTSPc@3D-(N)GF and PEDOT-PSS facilitated the enhancement in the sensing properties of the final nanocomposite and paved a new avenue for the application of CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposites in the gas sensing field.

Graphene oxide sheets suspended in ethanol interact with excited CdS nanoparticles and contributed to photocatalytic reduction by accepting electron from nanoparticle. The UV-Vis measurement showed that electrical absorbance of the CdS/graphene oxide sheets increased by decreasing the irradiation time and after 2 h it remained constant which indicates the optimum reduction time. Furthermore, the direct interaction between CdS nanoparticles and graphene sheets hinders the collapse of exfoliated sheets of graphene. The 4-point probe measurement of nanocomposite with different ratios of graphene oxide in CdS solution after irradiation shows that the conductivity of them increased by increasing the amount of GO, but further increasing causes incomplete photo reduction process due to exorbitance increasing GO sheets which contribute to decreasing the conductivity. The CdS/RGO composite material can be used as a gas sensor for CO2based on its electrocatalytic behavior. The low-cost and easy fabrication sensor shows rapid response and high sensitivity. By varying the amount of GO the optimum concentration which shows high sensitivity is found and its good performance compared with other is attributed to its higher conductivity due to complete reduction. Moreover, the effects of thermal annealing on the conductivity of CdS/RGO film and the performance of devices are researched.

A highly efficient and facile sensing system made of N-doped graphene quantum dots (N-GQDs)/poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) was fabricated by drop coating on interdigitated Au electrodes with high uniformity over a large area of silicon substrate, and was used to sense the vapors of volatile organic compounds (VOCs) for detection at room temperature. An innovative method was developed for the synthesis of N-GQDs through hydrothermal processing of citric acid. The N-GQDs obtained have a N to C atomic ratio of 0.058 and a diameter of 2–7 nm. The nanocomposite sensing system exhibits highly sensitive, selective, rapid, and reversible responses for the detection of 1–1000 ppm VOCs. The N-GQDs/PEDOT–PSS nanocomposite gas sensor has an enhanced methanol sensing property of 13 times the value for pristine PEDOT–PSS in a low concentration of 50 ppm methanol. Moreover, the gas sensor based on the nanocomposite has a fast response (ca. 12 s) and recovery (ca. 32 s) behavior, excellent room temperature selectivity and stability. The methanol-sensing mechanisms of the N-GQDs/PEDOT–PSS nanocomposite gas sensor, based on direct charge transfer and a swelling process, are highlighted.