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 surface, interfaces and grain boundaries of a halide perovskite film carry critical tasks in achieving as well as maintaining high solar cell performance due to the inherently defective nature across their regime. Passivating materials and felicitous process engineering approaches have significant ramifications in the resultant perovskite film, and the solar cell's overall macroscale properties as they dictate structural and optoelectronic properties. Herein, we exploit a vast number of defect engineering approaches aiming to increase the performance and the stability of perovskite solar cells, especially against humidity, continuous illumination, and heat. This review begins with the perovskite materials' fundamental structural properties followed by the advances made to induce higher stabilization in perovskite solar cells by fine-tuning materials chemistry design parameters. We continue by summarizing defect passivation strategies based on molecular entities' application, including suitable functional groups that enable sufficient surface, bulk and grain boundary passivation, morphology, and crystallinity control. We also present methods to control the density of defects through the variation of processing conditions, solvent annealing and solvent engineering approaches, gas-assisted deposition methods, and use of self-assembled monolayers, as well as colloidal engineering and coordination surface chemistry. Finally, we give our perspective on how a combined understanding of materials chemistry aspects and passivation mechanisms will further develop high-efficiency and stability perovskite solar cells.

Passivation and process engineering approaches of halide perovskite films for high efficiency and stability perovskite solar cells

Electrochemical impedance spectroscopy (EIS) is a highly applicable electrochemical, analytical, and non-invasive technique for materials characterization, which allows the user to evaluate the impact, efficiency, and magnitude of different components within an electrical circuit at a higher resolution than other common electrochemical techniques such as cyclic voltammetry (CV) or chronoamperometry. EIS can be used to study mechanisms of surface reactions, evaluate kinetics and mass transport, and study the level of corrosion on conductive materials, just to name a few. Therefore, this review demonstrates the scope of physical properties of the materials that can be studied using EIS, such as for characterization of supercapacitors, dye-sensitized solar cells (DSSCs), conductive coatings, sensors, self-assembled monolayers (SAMs), and other materials. This guide was created to support beginner and intermediate level researchers in EIS studies to inspire a wider application of this technique for materials characterization. In this work, we provide a summary of the essential background theory of EIS, including experimental design, signal responses, and instrumentation. Then, we discuss the main graphical representations for EIS data, including a scope of the foundation principles of Nyquist, Bode phase angle, Bode magnitude, capacitance and Randles plots, followed by detailed step-by-step explanations of the corresponding calculations that evolve from these graphs and direct examples from the literature highlighting practical applications of EIS for characterization of different types of materials. In addition, we discuss various applications of EIS technique for materials research.

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Reducing the resistance for the use of electrochemical impedance spectroscopy analysis in materials chemistry

The introduction of oxygen vacancies into metal oxide semiconductors is an effective way to enhance their gas sensing performance. In this review paper, firstly, the roles of oxygen vacancies on band structure, electrical conductivity, optical absorption and gas adsorption are presented. The presence of highly concentrated oxygen vacancies narrows the bandgap width of semiconductors, thus reducing the energy required for electron transition. It also increases the active sites on the material surface and enhances the chemisorption, thus improving the adsorption performance of the material. In addition, it also improves the electrical conductivity and light absorption ability of the material. Then, this review paper briefly introduced the state of the art of metal oxide semiconductors with highly concentrated oxygen vacancies fabricated by various processes, which are mainly divided into direct and indirect methods. At last, the application of metal oxide semiconductors with highly concentrated oxygen vacancies in the field of gas sensors is reviewed.

Metal oxide semiconductors with highly concentrated oxygen vacancies for gas sensing materials: a review

Quantum Dots for Photovoltaics: A Tale of Two Materials

In this report, we have prepared Cu-doped ZnO nanorods by a simplistic wet chemical method with subsequent annealing. The phase formation, optical, surface morphological studies along with the electrical properties of the Cu doped ZnO thin films were characterized by X-ray diffraction (XRD), photoluminescence (PL), field emission scanning electron microscopy (FESEM) and Hall measurements, respectively. The addition of copper (Cu) in to the matrix of ZnO influenced the surface morphology of the product dramatically. The regular hexagonal nanorods-like surface morphology of ZnO becomes a broken rod-like morphology, after addition of copper. Owing to the complexes surface morphology of Cu doped ZnO, the system was used to detect the toxic gases like NO2. The effect of doping on the gas-sensing properties of ZnO was slightly counterintuitive in that they initially increased up to Cu concentrations of 1% before beginning to decline. Meanwhile, the Cu doped ZnO exhibited the gas response of ˜ 71 for 1 ppm of NO2 gas at low operating temperatures. This higher gas sensing response is due to the defect states observed in Cu doped ZnO films. In addition to above mentioned findings, the transformation of n-type ZnO to p-type Cu doped ZnO was also observed. The p-type conductivity is attributed to the holes created in the merging of Cu on Zn sites.

Construction of Cu doped ZnO nanorods by chemical method for Low temperature detection of NO2 gas

Highly efficient mixed-halide mixed-cation perovskite solar cells based on rGO-TiO2 composite nanofibers

Tuning the analog and digital resistive switching properties of TiO2 by nanocompositing Al-doped ZnO

Quantum dot solar cells (QDSCs) are attractive technology for commercialization, owing to various advantages, such as cost effectiveness, and require relatively simple device fabrication processes. The properties of semiconductor quantum dots (QDs), such as band gap energy, optical absorption, and carrier transport, can be effectively tuned by modulating their size and shape. Two types of architectures of QDSCs have been developed: 1) photoelectric cells (PECs) fabricated from QDs sensitized on nanostructured TiO2 , and 2) photovoltaic cells fabricated from a Schottky junction and heterojunction. Different types of semiconductor QDs, such as a secondary, ternary, quaternary, and perovskite semiconductors, are used for the advancement of QDSCs. The major challenge in QDSCs is the presence of defects in QDs, which lead to recombination reactions and thereby limit the overall performance of the device. To tackle this problem, several strategies, such as the implementation of a passivation layer over the QD layer and the preparation of core-shell structures, have been developed. This review covers aspects of QDSCs that are essential to understand for further improvement in this field and their commercialization.

Akhilesh P. Patil

Papers

Construction of Cu doped ZnO nanorods by chemical method for Low temperature detection of NO2 gas

Highly efficient mixed-halide mixed-cation perovskite solar cells based on rGO-TiO2 composite nanofibers

Tuning the analog and digital resistive switching properties of TiO2 by nanocompositing Al-doped ZnO

Quantum Dot Based Solar Cells: Role of Nanoarchitectures, Perovskite Quantum Dots, and Charge-Transporting Layers.

Study of solvent variation on controlled synthesis of different nanostructured NiCo2O4 thin films for supercapacitive application.