Room-temperature (RT) gas sensing is desirable for battery-powered or self-powered instrumentation that can monitor emissions associated with pollution and industrial processes. This review (with 171 references) discusses recent advances in three types of porous nanostructures that have shown remarkable potential for RT gas sensing. The first group comprises hierarchical oxide nanostructures (mainly oxides of Sn, Ni, Zn, W, In, La, Fe, Co). The second group comprises graphene and its derivatives (graphene, graphene oxides, reduced graphene oxides, and their composites with metal oxides and noble metals). The third group comprises 2D transition metal dichalcogenides (mainly sulfides of Mo, W, Sn, Ni, also in combination with metal oxides). They all have been found to enable RT sensing of gases such as NOx, NH3, H2, SO2, CO, and of vapors such as of acetone, formaldehyde or methanol. Attractive features also include high selectivity and sensitivity, long-term stability and affordable costs. Strengths and limitations of these materials are highlighted, and prospects with respect to the development of new materials to overcome existing limitations are discussed.

A review on chemiresistive room temperature gas sensors based on metal oxide nanostructures, graphene and 2D transition metal dichalcogenides

Photovoltaic devices have become ideal alternatives to common energy sources due to their excellent mechanical robustness and high power conversion efficiency, which can meet the human requirements for green, inexpensive and portable electricity sources. Moreover, due to the rapid development of wearable devices, telecommunication, transportation, advanced sensors, etc., the need for green and accessible power sources for these state-of-the-art devices accompanied with appropriate mechanical stability has become a new challenge. In this regard, flexible–wearable photovoltaic platforms can be easily adapted to any device/substrate and can supply diverse electronic devices with their required energy via harvesting energy from sunlight. Similarly, photovoltaic platforms can be integrated into hybrid platforms and can be used in diverse applications. Herein, we summarize the recent approaches to developing flexible–wearable solar cells as energy sources for supplying self-powered wearable devices. In this regard, first, recent advances in transparent flexible electrodes and their diversities are reported; then, recently developed flexible solar cells and important factors for designing these platforms are summarized. Further, flexible solar cells are categorized into five different sections (i.e., perovskite, dye-sensitized, organic, fiber-shaped and textile solar cells) and their mechanisms, working principles and design criteria along with their recent advances have been discussed. Finally, novel applications of wearable sensors/devices are summarized and reported to highlight the functionality of these practical platforms.

Recent progress in flexible–wearable solar cells for self-powered electronic devices

Flexible Perovskite Solar Cells

Silver nanoparticles (Ag-NPs) readily transform in the environment, which modifies their properties and alters their transport, fate, and toxicity. It is essential to consider such transformations when assessing the potential environmental impact of Ag-NPs. This review discusses the major transformation processes of Ag-NPs in various aqueous environments, particularly transformations of the metallic Ag cores caused by reactions with (in)organic ligands, and the effects of such transformations on physical and chemical stability and toxicity. Thermodynamic arguments are used to predict what forms of oxidized silver will predominate in various environmental scenarios. Silver binds strongly to sulfur (both organic and inorganic) in natural systems (fresh and sea waters) as well as in wastewater treatment plants, where most Ag-NPs are expected to be concentrated and then released. Sulfidation of Ag-NPs results in a significant decrease in their toxicity due to the lower solubility of silver sulfide, potentiall...

Environmental Transformations of Silver Nanoparticles: Impact on Stability and Toxicity

A high-response formaldehyde sensor based on fibrous Ag-ZnO/In2O3 with multi-level heterojunctions.

We report highly bendable and efficient perovskite solar cells (PSCs) that use thermally oxidized layer of Ti metal plate as an electron transport layer (ETL). The power conversion efficiency (PCE) of flexible PSCs reaches 14.9% with a short-circuit current density (Jsc) of 17.9 mA/cm2, open-circuit voltage (Voc) of 1.09, and fill factor (ff) of 0.74. Moreover, the Ti metal-based PSCs exhibit a superior fatigue resistance over indium tin oxide/poly(ethylene terephthalate) substrate. Flexible PSCs maintain 100% of their initial PCE even after PSCs are bent 1000 times at a bending radius of 4 mm. This excellent performance of flexible PSCs is due to high crystalline quality and low oxygen vacancy concentration of TiO2 layer. The concentration of oxygen vacancies in the oxidized Ti metal surface controls the electric function of TiO2 as ETL of PSCs. A decrease in the oxygen vacancy concentration of the TiO2 layer is critical to improving the electron collection efficiency of the ETL. Our results suggest that...

Highly Bendable Flexible Perovskite Solar Cells on a Nanoscale Surface Oxide Layer of Titanium Metal Plates.

The addition of Ag nanoparticles (Ag NPs) with an average size of 30 nm into ZnO increases the electric conductivity up to 1000 times. While a similar increase in the conductivity is observed in a ...

Role of the Interface between Ag and ZnO in the Electric Conductivity of Ag Nanoparticle-Embedded ZnO

Multi-functional transparent electrode for reliable flexible perovskite solar cells

A trade-off between the carrier concentration and carrier mobility is an inherent problem of traditional transparent conducting oxide (TCO) films In this study, we demonstrate that the electron concentration of TCO films can be increased without deteriorating the carrier mobility by embedding Ag nanoparticles (NPs) into Al-doped ZnO (AZO) films An increment of Ag NP content up to 07 vol % in the AZO causes the electron concentration rising to 4 × 1020 cm–3 A dependence of the conductivity on temperature suggests that the energy barrier for the electron donation from Ag NPs at room temperature is similar to the Schottky barrier height at the Ag–AZO interface In spite of an increase in the electron concentration, embedded Ag NPs do not compromise the carrier mobility at room temperature This is evidence showing that this electron donation mechanism by Ag NPs is different from impurity doping, which produces both electrons and ionized scattering centers Instead, an increase in the Fermi energy level o

Novel Carrier Doping Mechanism for Transparent Conductor: Electron Donation from Embedded Ag Nanoparticles to the Oxide Matrix

Silver (Ag)-copper (Cu) bimetallic nanoparticles (NPs) were synthesized by the reduction of silver nitrate and copper (II) acetate monohydrate using ethylene glycol in a microwave (MW) heating system with controlled reaction times ranging from 5 min to 30 min. The molar ratio Ag/Cu was varied from 1:1 to 1:3. The effect of reaction conditions on the bimetallic NPs structures and compositions were characterized by x-ray photoelectron spectroscopy, x-ray diffraction and transmission electron microscopy. The average particle size was approximately 150 nm. The surface plasmon resonance (SPR) of Ag-Cu bimetallic NPs was investigated by monitoring the SPR band peak behavior via UV/Vis spectrophotometry. The resonance peak positions and peak widths varied due to the different structures of the bimetallic NPs created under the synthesis conditions. In the MW heating method, the reduction of Cu was increased and Cu was inhomogeneously deposited over the Ag cores. As the composition of Cu becoming higher in the Ag-Cu bimetallic NPs, the absorption between 400 nm to 600 nm was greatly enhanced.

Fen Qin

Papers

Highly Bendable Flexible Perovskite Solar Cells on a Nanoscale Surface Oxide Layer of Titanium Metal Plates.

Role of the Interface between Ag and ZnO in the Electric Conductivity of Ag Nanoparticle-Embedded ZnO

Multi-functional transparent electrode for reliable flexible perovskite solar cells

Novel Carrier Doping Mechanism for Transparent Conductor: Electron Donation from Embedded Ag Nanoparticles to the Oxide Matrix

Effect of Synthesis Techniques on Crystallization and Optical Properties of Ag-Cu Bimetallic Nanoparticles