Road Map of Semiconductor Metal-Oxide-Based Sensors: A Review

Identifying disease biomarkers and detecting hazardous, explosive, flammable, and polluting gases and chemicals with extremely sensitive and selective sensor devices remains a challenging and time-consuming research challenge. Due to their exceptional characteristics, semiconducting metal oxides (SMOxs) have received a lot of attention in terms of the development of various types of sensors in recent years. The key performance indicators of SMOx-based sensors are their sensitivity, selectivity, recovery time, and steady response over time. SMOx-based sensors are discussed in this review based on their different properties. Surface properties of the functional material, such as its (nano)structure, morphology, and crystallinity, greatly influence sensor performance. A few examples of the complicated and poorly understood processes involved in SMOx sensing systems are adsorption and chemisorption, charge transfers, and oxygen migration. The future prospects of SMOx-based gas sensors, chemical sensors, and biological sensors are also discussed.

Construction of organic-inorganic "chelate" adsorption sites on metal oxide semiconductor for room temperature NO2 sensing

Metal oxide semiconductors (MOS) have been extensively studied for gas sensing due to their excellent chemical stability and adjustable electronic properties. However, there is still a lack of ingenious design strategies to achieve customizable gas detection in complex environments. Herein, a novel and scalable strategy of constructing organic-inorganic "chelate" adsorption sites is proposed to promote the affinity of MOS sensing materials to target molecules. Specifically, 3-aminopropyltriethoxysilane (APTES)-functionalized reduced graphene oxide (rGO) was decorated on In₂O₃ tubes (AG/In_x), and its NO₂ sensing performance was studied. As a result, the optimal AG/In_x shows boosted room-temperature NO₂ response, and its response to 1 ppm NO₂ is 4.8 times that of In₂O₃. More attractively, the optimal AG/In_x exhibits good selectivity, as well as outstanding detection ability (R_g/R_a = 1.6) for low concentration NO₂ (20 ppb). Experimental results suggest that APTES-rGO not only acts as the electron acceptor to accelerate charge transfer, but also enhances NO₂ adsorption. Further theoretical calculations reveal that NO₂ is simultaneously adsorbed at rGO and APTES via a flexible "chelate" mechanism. The multidentate adsorption configuration remarkably strengthens the NO₂-host interaction, which is conducive to improving sensing performance. This work may inspire the material design of a new generation high-performance gas sensors.

Surface photosensitization of ZnO by ZnS to enhance the photodegradation efficiency for organic pollutants

It is challenging to develop a material which has low cost, high activity, good stability and recyclability under light exposure. Apart from these properties, the photocatalyst should also have good visible region absorbance and low electron-hole pair recombination rate. Keeping all this in view, we have designed a simple scalable synthesis of ZnO–ZnS heterostructures for the photocatalytic treatment of industrial waste (p-nitrophenol and methyl orange). The ZnO–ZnS heterostructures are synthesized via a solvent-free route by thermal annealing of solid-state mixture of ZnO and thiourea (a sulphur source) which results in ZnO–ZnS core shell kind of heterostructure formation. The interface formation between the ZnO–ZnS heterostructure favored the band-gap reduction in comparison to the bare ZnO and ZnS nanoparticles. Further, these ZnO–ZnS heterostructures were utilized as a photocatalyst for the degradation of toxic phenolic molecules (p-nitrophenol) and harmful organic dyes (methyl orange) present in the water under the light exposure (> 390 nm).

Progress in Graphene/Metal Oxide Composite Photocatalysts for Degradation of Organic Pollutants

The sewage discharge of industrial wastewater seriously pollutes the water source and rivers, which is very harmful to the health of humans and wildlife. Among those methods for treating wastewater, photocatalysis is a sustainable and environmental-friendly technique for removing the organic pollutants with no secondary pollution. As a popular photocatalyst, graphene/metal oxide nanocomposites have been widely reported in the photocatalysis field. In this review, the recent progress of graphene/metal oxide composites including binary and ternary composites is summarized in detail. The synthesis, microstructure design, and application performance of graphene/TiO2, graphene/ZnO, graphene/SnO2, graphene/WO3, graphene/Fe2O3, and graphene/Cu2O composites are introduced firstly. Then, the synthesis, the selection of components, and the performance of various ternary composites are summarized specifically, including graphene/TiO2-, graphene/ZnO-, graphene/SnO2-, graphene/Cu2O-, graphene/FexOy-, and graphene/Bi-containing ternary composites. At last, the possible research directions of graphene/metal oxide nanocomposites are put forward. The main purpose is to provide a theoretical guidance for designing high-performance graphene/metal oxide photocatalysts for wastewater treatment.

State of the Art in Alcohol Sensing with 2D Materials

Since the discovery of graphene, the star among new materials, there has been a surge of attention focused on the monatomic and monomolecular sheets which can be obtained by exfoliation of layered compounds. Such materials are known as two-dimensional (2D) materials and offer enormous versatility and potential. The ultimate single atom, or molecule, thickness of the 2D materials sheets provides the highest surface to weight ratio of all the nanomaterials, which opens the door to the design of more sensitive and reliable chemical sensors. The variety of properties and the possibility of tuning the chemical and surface properties of the 2D materials increase their potential as selective sensors, targeting chemical species that were previously difficult to detect. The planar structure and the mechanical flexibility of the sheets allow new sensor designs and put 2D materials at the forefront of all the candidates for wearable applications. When developing sensors for alcohol, the response time is an essential factor for many industrial and forensic applications, particularly when it comes to hand-held devices. Here, we review recent developments in the applications of 2D materials in sensing alcohols along with a study on parameters that affect the sensing capabilities. The review also discusses the strategies used to develop the sensor along with their mechanisms of sensing and provides a critique of the current limitations of 2D materials-based alcohol sensors and an outlook for the future research required to overcome the challenges.

An investigation of the photovoltaic parameters of ZnS grown on ZnO(101̄1)

The selective growth of ZnS on ZnO (zinc nitrate versus acetate precursors) affects the photovoltaic parameters when the material is used as a photoanode in solar cells.

K. N, V. P. N. N, M. K. Understanding the role of alcohols in the growth behaviour of ZnO nanostructures prepared by solution based synthesis and their application in solar cells

We report the successful control of the ZnO nanostructures by a simple solution method using alcohols such as methanol, ethanol, butanol, hexanol, octanol and decanol as solvents.

Facile and scalable production of heterostructured ZnS-ZnO/Graphene nano-photocatalysts for environmental remediation

A facile and eco-friendly strategy is described for the synthesis of ZnS-ZnO/graphene heterostructured nano-photocatalysts for the first time. This solvent-free and technologically scalable method involves solid-state mixing of graphite oxide (GO), Zn salt and surfeit yet non-toxic elemental sulfur using ball-milling followed by thermal annealing. The as-formed hybrids are composed of uniformly distributed in-situ formed ZnS-ZnO nanoparticles simultaneously within the thermally reduced GO (graphene) matrix. A series of hybrid compositions with varying content of ZnS/ZnO and graphene were prepared and thoroughly characterized. Further, the effect of heterostructure composition on the photocatalytic properties was investigated under visible-light illumination. The synergistic ZnS-ZnO/graphene hybridization promoted the band-gap narrowing compared to the pristine ZnS nanoparticles. The ZnS:ZnO composition was controlled by graphite oxide under thermal treatment and observed to be a crucial factor in enhancement of photocatalytic activity. As a proof of concept, the phase optimized and surface enhanced ZnS-ZnO/graphene nano-photocatalysts was tested towards visible light driven photocatalytic degradation of environmentally harmful organic dyes and toxic phenol molecules from aqueous media. The presented cost-effective strategy provides high potential in large-scale production of heterostructured nano-photocatalysts for environmental remediation and photocatalytic greener production of hydrogen.

Synthesis of the ZnO@ZnS Nanorod for Lithium-Ion Batteries

The ZnO@ZnS nanorod is synthesized by solvothermal method as an anode material for lithium ion batteries. ZnS is deposited on ZnO and assembles in nanorod geometry successfully. The nanosized rod structure supports ion diffusion by substantially reducing the ion channel. The close-linking of ZnS and ZnO improves the synergetic effect. ZnS is in the middle of the ZnO core and the external environment, which would greatly relieve the volume change of the ZnO core during the Li+ intercalation/de-intercalation processes; therefore, the ZnO@ZnS nanorod is helpful in maintaining excellent cycle stability. The ZnO@ZnS nanorod shows a high discharge capacity of 513.4 mAh g−1 at a current density of 200 mA g−1 after 100 cycles, while a reversible capacity of 385.6 mAh g−1 is achieved at 1000 mA g−1.

Adaptable surfactant-mediated method for the preparation of anisotropic metal chalcogenide nanomaterials

The hot injection synthesis of nanomaterials is a highly diverse and fundamental field of chemical research, which has shown much success in the bottom up approach to nanomaterial design. Here we report a synthetic strategy for the production of anisotropic metal chalcogenide nanomaterials of different compositions and shapes, using an optimised hot injection approach. Its unique advantage compared to other hot injection routes is that it employs one chemical to act as many agents: high boiling point, viscous solvent, reducing agent, and surface coordinating ligand. It has been employed to produce a range of nanomaterials, such as CuS, Bi2S3, Cu2-xSe, FeSe2, and Bi4Se3, among others, with various structures including nanoplates and nanosheets. Overall, this article will highlight the excellent versatility of the method, which can be tuned to produce many different materials and shapes. In addition, due to the nature of the synthesis, 2D nanomaterial products are produced as monolayers without the need for exfoliation; a significant achievement towards future development of these materials.

Novel ZnO–ZnS nanowire arrays with heterostructures and enhanced photocatalytic properties

The graphical abstract exhibits the SEM images and schematic representation of ZnO–ZnS heterostructure NW arrays, the schematic energy band diagram and the rates of the photocatalytic H₂ at different temperatures.

Fluorescence from graphene oxide and the influence of ionic, π-π interactions and heterointerfaces: electron or energy transfer dynamics

2D crystals such as graphene and its oxide counterpart have sought good research attention for their application as well as fundamental interest. Especially graphene oxide (GO) is quite interesting because of its versatility and diverse application potential. However the mechanism of fluorescence from GO is under severe discussion. To explain the emission in general two interpretations were suggested, viz localization of sp(2) clusters and involvement of oxygeneous functional groups. Despite this disagreement, it should be acknowledged that the heterogeneous atomic structure, synthesis dependent and uncontrollable implantation of oxygen functional groups on the basal plane make such explanations more difficult. Nevertheless, a suitable explanation enhances the applicability of the material which also enables the design of novel materials. At this juncture we believe that given the complexity in understanding the emission mechanism it would be very useful to review the literature. In this perspective we juxtapose various results related to fluorescence and influencing factors so that a conclusive interpretation may be unveiled. Apparently, the existing interpretations have largely ignored the factors such as self-rolling, byproduct formation etc. Vis-a-vis previous reviews did not discuss the interfacial charge transfer across heterostructures and the implication on the optical properties of GO or reduced graphene oxide (rGO). Such analysis would be very insightful to determine the energetic location of sub band gap states. Moreover, ionic and π-π type interactions are also considered for their influence on emission properties. Apart from these, quantum dots, covalent modifications and nonlinear optical properties of GO and rGO were discussed for completeness. Finally we made concluding remarks with outlook.

Highly enhanced photocatalytic properties of ZnS nanowires–graphene nanocomposites

ZnS–graphene nanocomposites exhibit excellent photocatalytic activity for degradation of methylene blue under UV light irradiation.