An overview of electrochromic devices with electrolytes containing viologens

Viologens-based electrochromic (EC) devices with multiple color changes, rapid response time, and simple all-in-one architecture have aroused much attention, yet suffer from poor redox stability caused by the irreversible aggregation of free radical viologens. Herein, the semi-interpenetrating dual-polymer network (DPN) organogels are introduced to improve the cycling stability of viologens-based EC devices. The primary cross-linked poly(ionic liquid)s (PILs) covalently anchored with viologens can suppress irreversible face-to-face contact between radical viologens. The secondary poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) chains with strong polar groups of -F can not only synergistically confine the viologens by the strong electrostatic effect, but also improve the mechanical performance of the organogels. Consequently, the DPN organogels show excellent cycling stability (87.5% retention after 10 000 cycles) and mechanical flexibility (strength of 3.67 MPa and elongation of 280%). Three types of alkenyl viologens are designed to obtain blue, green, and magenta colors, demonstrating the universality of the DPN strategy. Large-area EC devices (20 × 30 cm) and EC fibers based on organogels are assembled to demonstrate promising applications in green and energy-saving buildings and wearable electronics.

Synergistic Interaction of Dual-Polymer Networks Containing Viologens-Anchored Poly(ionic liquid)s Enabling Long-Life and Large-Area Electrochromic Organogels.

Smart windows, which could save building energy consumption by modulating indoor thermal and light intensity, have attracted considerable attention in recent years. Photochromic (PC) and electrochromic (EC) are two representative working modes of smart windows. Integrating PC and EC on a single smart window is beneficial for adapting to various environments and meeting individual needs. However, it remains challenging to develop PC and EC dual-responsive materials. In this work, we reported two extended viologen derivatives, EV-1 and EV-2, with both PC and EC properties. Photochromic devices (PCDs) and electrochromic devices (ECDs) based on these materials were fabricated successfully. Under UV light irradiation, the PCDs exhibited a significant color change from transparent to green (PCD-1) and blue (PCD-2) color, and the colored state can maintain a long time (2.73 h for PCD-1 and 41.72 h for PCD-2) after removing the UV light. Owing to the extension of the conjugate length of the chromophore as compared with traditional viologens using bipyridine as chromophores, the spectra response range of the devices was extended to the near-infrared (NIR) region. To obtain different light modulation capabilities, we fabricated electrochromic devices using propylene carbonate as solvent. These devices display different colored states (red for ECD-1 and purple for ECD-2) and spectra absorption compared to PCD-1 and PCD-2. Furthermore, a photo/electrochromic device (PECD) smart window system was developed to enhance light and thermal regulation. This system offers four working modes, including off mode, photochromic mode, electrochromic mode, and photo-electrochromic mode, which opens up new possibilities for expanding its applications. 

Novel extended viologen derivatives for photochromic and electrochromic dual-response smart windows

The synergistic effects of the photocatalytic activity of ZnO NPs and the photochromic properties of viologens make the film rapidly respond to UV-irradiation.

A novel ZnO/viologen photochromic composite film with a rapid UV response for rewritable paper, solar UV detection, smart windows and anti-counterfeiting

Constitutional isomerism of covalent organic frameworks (COFs) has recently garnered attention for its potential applications in advanced photoelectrochemical fields. However, there have been no reports of materials in which half of the linkages between building blocks of the COFs are isomerized (so-called semi-isomerism) as a means of modulating the physicochemical properties of COFs. In this work, semi-isomeric pyrene-based COFs with the same topology were synthesized with imine linkages, namely, Py-Py and SI-Py COFs. Both COFs featured different imine orientations in half of the linkages. Tiny atomic-level dislocations of the imine linkages led to different electron orbital contributions and bandgaps (2.39 vs 2.44 eV for the Py-Py COFs and SI-Py COFs, respectively), resulting in distinct electron transfer and photoelectrochemical properties. In comparison with the Py-Py COFs, the SI-Py COFs with imine semi-isomeric linkages exhibited superior electrochromic performance over the optical modulation range (35% vs 24%), response time (0.29/2.1 s vs 0.72/3.8 s), and coloration efficiency (180 vs 144 C cm–2). 

Tuning Electron Transfer in Pyrene-Based Two Dimensional Covalent Organic Frameworks: Unveiling the Impact of Semi-isomerism on Physicochemical Properties

Abstract Layered transition metal hydroxides (LTMHs) with transition metal centers sandwiched between layers of coordinating hydroxide anions have attracted considerable interest for their potential in developing clean energy sources and storage technologies. However, two-dimensional (2D) LTMHs remain largely understudied in terms of physical properties and applications in electronic devices. Here, for the first time we report &gt; 20 μm α-Ni(OH) 2 2D crystals, synthesized from hydrothermal reaction. And an edge-on condensation mechanism assisted with the crystal field geometry is proposed to understand the 2D intra-planar growth of the crystals, which is also testified through series of systematic comparative studies. We also report the successful synthesis of 2D Co(OH) 2 crystals (&gt; 40 μm) with more irregular shape due to the slightly distorted octahedral geometry of the crystal field. Moreover, the detailed structural characterization of synthesized α-Ni(OH) 2 are performed. The optical band gap energy is extrapolated as 2.54 eV from optical absorption measurements and the electronic bandgap is measured as 2.52 eV from reflected electrons energy loss spectroscopy (REELS). We further demonstrate its potential as a wide bandgap (WBG) semiconductor for high voltage operation in 2D electronics with a high breakdown strength, 4.77 MV/cm with 4.9 nm thickness. The successful realization of the 2D LTMHs opens the door for future exploration of more fundamental physical properties and device applications. 

Synthesis of 2D layered transition metal (Ni, Co) hydroxides via edge-on condensation

Abstract Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for trace-level fingerprinting. Recently, layered two-dimensional (2D) materials have gained significant interest as SERS substrates for providing stable, uniform, and reproducible Raman enhancement with the potential for trace-level detection. Yet, the development of effective 2D SERS substrates is still hindered by the lack of fundamental understanding of the coupling mechanism between target molecules and substrates. Here, we report a systematic excitation-dependent Raman spectroscopy investigation on the coupling between 2D materials such as SnS 2 , MoS 2 , WSe 2 , and graphene and small organic molecules like rhodamine 6G (Rh 6G). Strong coupling between SnS 2 and Rh 6G is found due to their degenerate excitons through Raman excitation profiles (REP), leading to the enhancement of Rh 6G vibrational modes that are observable down to 10 −13 M. Our study shows that exciton coupling in the substrate-adsorbate complex plays a vital role in the Raman enhancement effect, opening a new route for designing SERS substrates for high sensitivity. 

Raman enhancement induced by exciton hybridization in molecules and 2D materials


 
 In this work, the conversion of ultrathin 2D‐Ni (OH)
 2
 to NiO is studied using in situ X‐ray absorption near edge spectroscopy at the Ni
 L
 3
 and O
 K
 edges and using variable photon energy X‐ray photoelectron spectroscopy at the Ni 2p
 3/2
 and O 1s edges. The Ni
 L
 3
 X‐ray absorption spectroscopy and corresponding 2p
 3/2
 X‐ray photoelectron spectroscopy show the oxidation state of the Ni atom to be Ni
 2+
 in the course of the reaction; however, the film surface and oxygen defect structure show the presence of a large metallic Ni component before the complete disappearance of the hydroxide phase indicating oxygen vacancy formation before the completion of water desorption. O 1s spectral changes during heat treatment under ultrahigh vacuum, O
 2
 dosing, and H
 2
 O dosing suggest the spectral features are due to water desorption and adsorption. By comparing the relative probe depths of X‐ray photoelectron spectroscopy and X‐ray absorption spectroscopy, it is determined that the surface, at each step in the dehydration process, consists of a combination of Ni, NiO, and Ni (OH)
 2
 phases, which each contain a mixture of adsorbed hydroxyl groups. Repeated dosing with H
 2
 O, studied with O 1s X‐ray photoelectron spectroscopy, suggests that the dissociation of water facilitates the formation of oxygen vacancies.


Dehydration Pathway Study of Ultrathin Ni (OH)
 2
 Probed via In Situ X‐Ray Photoelectron Spectromicroscopy

Photo-Induced Self-Reduction Enabling Ultralow Threshold Voltage Energy-Conservation Electrochromism

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Synthesis of 2D layered transition metal (Ni, Co) hydroxides via edge-on condensation

Raman enhancement induced by exciton hybridization in molecules and 2D materials

Dehydration Pathway Study of Ultrathin Ni (OH) 2 Probed via In Situ X‐Ray Photoelectron Spectromicroscopy

Photo-Induced Self-Reduction Enabling Ultralow Threshold Voltage Energy-Conservation Electrochromism