High Optical Modulation Research Articles

Organic Ligand‐Free Scalable Dual‐Band Electrochromic Smart Windows

AbstractDual‐band electrochromic (DBEC) smart windows are an emerging energy‐saving technology, especially those constructed from single‐component DBEC materials as they offer an easy way to independently regulate visible (VIS) and near‐infrared (NIR) light. However, the current complicated organic ligand approach used to synthesize single‐component DBEC materials necessitates a post‐annealing treatment, which may severely compromise the scalability of DBEC smart windows. Herein, one organic ligand‐free process is developed to produce well‐controlled Nb‐doped anatase TiO2 nanocrystals (NTO NCs) followed by spraying on arbitrary substrates, giving NTO rigid or flexible films with good DBEC properties and a high optical modulation over 92%. A prototype of a 20 cm × 20 cm DBEC smart window is built. The simplicity and scalability of the synthesis and fabrication process make it a scalable approach to constructing large‐area electrochromic smart windows and foils, with promising applications in fields such as buildings and automobiles.

Point-to-multipoint beam-steering terahertz communications using a photonics-based leaky-wave transmit antenna

Abstract We report on the successful implementation of a photonic-based beam steering approach for point-to-multipoint terahertz (THz) communications. The frequency-agile radiation properties of a THz leaky-wave antenna connected to a photodiode are utilized in the remote transmitter for generating multiple individually steerable THz beams. The approach benefits from the ultra-wide frequency tunability of optical heterodyne systems for frequency-agile THz beam steering. Moreover, the approach allows to exploit high performance and mature optical modulation techniques for generating THz beams with a high data capacity. In the THz receiver, a carrier frequency insensitive envelope THz detector based upon a single Schottky-barrier diode is used. In detail, THz communications in the 300 GHz band (WR3.4) with a maximum steering angle up to 90° is reported. Experimentally, for single steerable THz beam operation, a record high data rate of 35 Gbit/s at a wireless distance of 30 cm is achieved using two-level pulse amplitude modulation. Also, longer wireless distances up to 110 cm with 5 Gbit/s are demonstrated. Furthermore, point-to-multipoint THz communications is reported using two individually steerable THz beams carrying 10 Gbit/s and 5 Gbit/s over 70 cm and 50 cm, respectively.

A high-capacity and self-healable electrochromic system based on I0/CuI conversion reaction

Constructing electrochromic devices (ECDs) requires high-performance electrochromic electrodes with not only high color contrasts, but also large charge storage capacities. In this paper, a I0/CuI conversion-type electrochromic system is developed, achieving reversible and profound color switching between colorless transparent and dark brown, while delivering a large charge storage capacity of 246.7 mA h g−1. To improve switching durability, a multifunctional P10 (Polyquaternium-10) binder is introduced into the electrode, which can effectively confine the polyiodide intermediates and suppress the shuttle effect. At the same time, the CuI/Cu redox couple is further activated by controlling the applied voltage. The generated Cu from this couple can spontaneously react with residual I0 species and facilitate the conversion from I0 to CuI, and therefore in-situ rejuvenate the degraded electrochromic performance. Thanks to these rational designs, this electrochromic electrode achieves rapid response speeds (tcoloring: 14 s, tbleaching: 5 s), high optical modulation amplitude (73.4 % at 500 nm), decant coloration efficiency (57.0 cm2 C−1) and impressive cycle stability (without performance degradation after 150 cycles). This work provides a novel electrochromic system for the construction of high-performance electrochromic devices.

Optical and Structural Properties of V2O5 Electrochromic Thin Films

The increase in global temperature has led to a significant surge in energy consumption within the air conditioning industry, resulting in environmental deterioration. Electrochromic (EC) windows have emerged as a promising solution to address these challenges. Vanadium pentoxide (V2O5) stands out among all metal oxide materials due to its remarkable EC properties, including substantial Li+ ion insertion capacity and multicolor capabilities. Despite the potential of V2O5, there remains a lack of comprehensive research on the structural and optical properties of V2O5 films with varying thicknesses. Therefore, this study aims to investigate the structural and optical properties of V2O5 thin films with thicknesses ranging from 46 to 344 nm. By employing the sol-gel spin coating method, V2O5 thin films were fabricated and analyzed using X-ray diffraction (XRD) spectroscopy and ultraviolet-visible (UV-Vis) spectrophotometry. The fabricated V2O5 thin films with thicknesses of 46-274 nm demonstrated an average film transparency of 83 %. XRD analysis further revealed that the V2O5 thin films reached their peak crystallinity at a thickness of 344 nm. Moreover, CV analysis revealed that the V2O5 device, with a thickness of 274 nm, exhibited a cathodic peak current of -1.63 mA, indicating its excellent ability to facilitate Li+ ion diffusion. Additionally, CA measurements displayed a high optical modulation of 37.78 %. Ultimately, this research contributes to the development of energy-efficient solutions for sustainable environmental practices.

Ultralong Bistable, Electrolytic MnO2-Based, Electrochromic Battery Enabled by Porous, Low-Barrier, Hydroxylated TiO2 Interface.

Electrochromic (EC)battery technology shows great potential in future "zero-energy building" by controlling outdoor solar transmission to tune heat gain as well as storing the consumed energy to reuse across other building systems. However, challenges still exist in exploring an electrochemical system to satisfy requirements on both ultra-long optical memory (also called bistability) without continuous power supply and high energy density. Herein, an ECbattery is proposed to demonstrate ultra-long bistability (>760h) based on the reversible deposition and dissolution of manganese oxide (MnO2) without the addition of any mediators. A porous low-barrier hydroxylated titanium dioxide (TiO2) interface is incorporated to synergistically enrich Mn2+-affinity active sites for deposition and effectively reduce the electron transport barrier of MnO2 for dissolution, thereby significantly improving the reversibility, high optical modulation (60.2% at 400nm), and energy density (352mAhm-2). The modification strategy is also verified on the cathode-less button cells with a much higher average coulombic efficiency (99.9%) compared to the batteries without the porous hydroxylated TiO2 interface (74.6%). These achievements lay a foundation for advancements in both electrochromism and Zn-Mn aqueous batteries.

On the enhanced electrochromic performance of nano-structured V2O5 thin films developed by substrate temperature induced orientated Growth

Electrochromic society is keen to develop devices with multicolor displays with high stability, using eco-friendly inorganic materials. In the present research, Influence of substrate temperature on the electrochromic V2O5 active electrodes deposited using automated nebulizer spray pyrolysis was examined to demonstrate V2O5 nanostructured films with good cyclic stability, high color contrast, and a robust memory effect. The structural characterization of the V2O5 thin films was conducted through R-ray diffraction and thermogravimetric analysis to assess material stability. Nanostructure morphology under different conditions was analysed using SEM and TEM. The electrochromic studies revealed that the higher optical modulation (ΔT) was achieved (with 70.8 % at 550 nm) in a very short switching time. Feature studies on V2O5 under different potentials were conducted to understand the capacitive and diffusion effects. The electrochromic response V2O5 thin films to various potentials was analysed using ex-situ XRD, elucidates phase changes from V2O5 to LiVO5 and V4O9, and a 2000-cycle CV measurement comprehended the material stability. The significantly improved electrochromic performance is primarily attributed to the nanosheet structure of the V2O5 thin films, since the structures enhance the material stability during lithium-ion intercalation/deintercalation processes.

Exploring electrochromic performance via layered deposition of tungsten oxide on niobium oxide composite electrode

Previous studies examined the incorporation of several transition metal oxides into niobium oxide (Nb2O5) to enhance the electrochromic (EC) performance. The objective was to improve the durability, color neutrality, coloring efficiency, and optical characteristics because individual metal oxides often exhibit limits that might affect overall functioning. Therefore, an innovative approach, integrating mixed metal oxides, emerges. This novel approach addresses these challenges and unlocks new possibilities to enhance EC efficiency. This paper introduces a new method for synthesizing niobium oxide/tungsten oxide (Nb2O5/WO3) bilayer composite materials using a combination of facile hydrothermal synthesis with electrodeposition. The electrochromic, structural, morphological, and optical characteristics of Nb2O5 thin films were enhanced by conducting thorough investigations into different electrodeposition cycles of WO3. X-ray diffraction confirmed the amorphous nature of WO3 in combination with the crystalline orthorhombic Nb2O5 structure. Raman spectroscopy examined the vibrational modes and structural attributes in the Nb2O5/WO3 bilayer composite thin films. X-ray photoelectron spectroscopy was conducted to examine the chemical composition, elemental state, and surface characteristics. Field emission scanning electron microscopy was used to examine the impact of different WO3 electrodeposition cycles on the morphology of Nb2O5 thin films, revealing the formation of porous, clumped, and dense nanogranules on the film surface. EC investigations highlight the superior performance of the NW-20 thin film, showing efficient Li+ accommodation. The optimized NW-20 sample exhibits excellent EC performance, including high optical modulation (81.48 %), good reversibility (98.11 %), and high coloration efficiency (115.09 cm2/C) with long-term cycling stability. The NW-20 electrode incorporated into a device form exhibited functionality on a real-world scale, delivering remarkable EC performance with substantial optical modulation (78.1 %) and excellent cycling stability. These findings expand the potential applications of Nb2O5/WO3 bilayer composite materials for EC performance.

Interface engineering of SnO2 to enhance the cycle stability of WO3 and Prussian blue for complementary electrochromic smart windows and energy storage

The interface mismatch between electrochromic materials and conductive layer seriously affects the cycle stability of electrochromic smart windows (ESW). However, it is difficult to improve the interface matching at the same time because of the different physical and chemical properties of anode and cathode materials. In this work, SnO2 nanosheets are used as the interface modification layer between electrochromic materials and conductive layer. SnO2 interface engineering significantly can improve the cycle stability of WO3 film and Prussian blue (PB) film. The WO3/SnO2-PB/SnO2 complementary ESW exhibits a high optical modulation of 60.2 % at 633 nm, fast response time (bleached/colored time = 8.5/2.5 s), and coloration efficiency of 91.7 cm2 C−1. The WO3/SnO2-PB/SnO2 device can still maintain an optical modulation of 45.2 % after 2000 cycles, suggesting excellent long cycle stability. And the device shows the energy storage of 5.06 mF cm−2 at 0.1 mA cm−2, which can improve energy efficiency. The excellent electrochromic properties and energy storage suggests that SnO2 interface engineering has a good application prospect in multi-functional ESW with excellent cycle stability.

Unlocking dual-band electrochromism with stacked structure of amorphous tungsten oxide and Prussian blue

Dual-band electrochromic smart windows can selectively regulate the transmittance of visible and near-infrared (NIR) light, thereby significantly reducing energy consumption in buildings. However, the selective regulation for visible and NIR regions often relies on the integration of multiple components, together with meticulous design and fabrication. Herein, we propose the stacking of amorphous tungsten oxide (a-WO3) on the surface of Prussian blue (PB) through a straightforward magnetron sputtering deposition. Benefiting from the numerous voids-containing structures in the microstructure of a-WO3 films, the stacking structures are conducive to the insertion and transport of electrolyte ions, ensuring the electrochemical activity and electrochromic performance of the inner PB layer. The prepared PB/WO3 bilayer thin film can achieve three functionality modes in dual-band electrochromism: “bright”, “warm”, and “dark”, with high optical modulation of 74.9 % at 700 nm and 80.3 % at 1300 nm, respectively. Additionally, they demonstrate fast coloring/bleaching response (2.9/7.6 s at 700 nm and 6.0/6.2 s at 1300 nm) and satisfying cycling stability (retaining 95.3 % of its initial modulation after 500 cycles). The bilayer thin films based on amorphous electrochromic materials with an intrinsic abundance of voids-containing structures introduce a novel approach to the independent dual-band modulation for demanded light/thermal control.

Preparing and investigating the monolithically stacked flexible tandem all-thin-film electrochromic device

A flexible tandem all-thin-film electrochromic device (ATF-ECD) with a device structure of polyethylene terephthalate (PET)/tin doped indium oxide (In2O3:Sn, ITO)/tungsten oxide (WO3)/lithium tantalate (LiTaO3)/nickel oxide (NiOX)/ITO/NiOX/LiTaO3/WO3/ITO has been fabricated by all magnetron sputtering technologies, in which the bottom ATF-ECD with a superstrate-type structure and the top ATF-ECD with a substrate-type structure are physically and monolithically stacked but electrically connected in parallel. They are systematically investigated for the optical and electrochemical properties through chronoamperometry (CA) and cyclic voltammetry (CV) measurements under different test cases. Both sub-devices work well and the flexible tandem ATF-ECD has achieved a high optical modulation of 59.8% at 600 nm. The numerical simulation is carried out for the tandem ATF-ECD and the single ATF-ECDs with different device structures. The details on transmission spectra are deeply discussed. The flexible tandem ATF-ECD with good electrochromic performances under the low driving voltages provides promising applications in the future smart windows.

A Reversible MnO2 Deposition-Enabled Multicolor Electrochromic Device with Efficient Tunability of Ultraviolet-Visible Light.

Electrochromic technology offers exciting opportunities for smart applications such as energy-saving and interactive systems. However, achieving dual-band regulation together with the multicolor function is still an unmet challenge for electrochromic devices. Herein, an ingenious electrochromic strategy based on reversible manganese oxide (MnO2) electrodeposition, different from traditional ion intercalation/deintercalation-type electrochromic materials is proposed. Such a deposition/dissolution-based MnO2 brings an intriguing electrochromic feature of dual-band regulation for the ultraviolet (UV) and visible lights with high optical modulation (93.2% and 93.6% at 400 and 550nm, respectively) and remarkable optical memory. Moreover, a demonstrative smart window assembled by MnO2 and Cu electrodes delivers the electrochromic properties of effective dual-band regulation accompanied by multicolor changes (transparent, yellow, and brown). The robust redox deposition/dissolution process endows the MnO2-based electrochromic device with excellent rate capability and an areal capacity of 570mAhm-2 at 0.1mAcm-2. It is believed that the metal oxide-based reversible electrodeposition strategy would be an attractive and promising electrochromic technology and provide a train of thought for the development of multifunctional electrochromic devices and applications.

Stable WO3 electrochromic system based on NH4+ hydrogen bond chemistry

As the best-known and extensively-studied electrochromic material, tungsten trioxide (WO3) has drawn tremendous interest due to its low material cost, environmental benignity, dual band regulation, high optical modulation, and high chemical stability. However, these films always suffer from ion-trapping-induced degradation in electrochromic performance when cycled in alkali cation (such as Li+, Na+, K+, etc.) electrolytes. Herein, we report a stable WO3 electrochromic system enabled by a NH4CF3SO3-TEP (Triethyl phosphate) organic electrolyte. Based on comparative experiments and systematic characterizations, the prolonged cycling stability is mainly attributed to the formation of hydrogen bonds between the NH4+ ions and the WO3 lattice, which are much weaker than metallic coordination bonds. Furthermore, the bulky CF3SO3− anion and large TEP solvent molecule are also favorable to stabilize WO3, since they are more resistive to be co-inserted into the WO3 lattice along with the shuttling NH4+ cations. Thanks to these merits, this NH4CF3SO3-TEP electrolyte simultaneously enables great electrochromic activity (76.1 % transmittance modulating ability at 633 nm) and impressive cyclic color-switching stability (1000 times color change without any detectable electrochromic performance or mechanical degradation), overwhelmingly outperforming the conventional Li+/K+ electrolytes. Furthermore, as a demonstration of its application, a WO3/Zn electrochromic device is fabricated based on a NH4+/Zn2+-CF3SO3 dual-cation TEP electrolyte. These findings may shed light on the in-depth understanding of WO3′s “ion trapping” effect, as well as the electrolyte design of high-performance WO3-based electrochromic devices.

Ionic Storage Materials for Anodic Discoloration in Electrochromic Devices

The ion storage layer in electrochromic devices (ECDs) stores protons or lithium ions to provide electrochemical stability and extend cycle durability. This paper reports on the performance and stability of ECDs paired with various ion storage layers (NiO, V2O5, and IrO2 films). The complementary ECD using a V2O5 ion storage layer presented the fastest response time, but the lowest optical contrast. In addition, the ECD using an IrO2 ion storage layer proved the most effective as an ion storage layer, due to its high optical modulation ability capability and long-term stability. Chronoamperometry analysis revealed that IrO2-based ECD (glass/IZTO/WO3/liquid electrolyte/IrO2/IZTO/glass) can be highly effective in modulating optical transmittance, as indicated by T = 61.5% (from Tbleaching (69.6%) to Tcoloring (8.1%)) and switching times of 5.3 s for coloring and 7.3 s for bleaching at 633 nm.

Approach to Significantly Enhancing the Electrochromic Performance of PANi by In Situ Electrodeposition of the PANi@MXene Composite Film.

Electrochromic materials (ECMs) are capable of reversibly adjusting their transmittance or reflectance properties in response to changes in the external biasing voltages. In this study, we enhanced the electrochromic and electrochemical properties of polyaniline (PANi) effectively through the incorporation of MXene Ti2CTx using an in situ composite strategy. This improvement in the electrochromic and electrochemical properties observed can be attributed to the intermolecular forces between the aniline group of PANi and the terminal groups of MXene Ti2CTx sheets. The presence of hydrogen bonds between the PANi monomers and the MXene sheets was confirmed through theoretical calculations and photoluminescence results, which effectively improved the composite interfaces. Additionally, the PANi@MXene composite films were successfully prepared through a simple one-step in situ polymerization process, as verified by SEM and XPS characterization. The electrochemical studies revealed enhanced electronic conductivity, a high ion diffusion coefficient, and a narrow energy redox gap, all contributing to the excellent electrochemical properties observed. Overall, our results demonstrate that the MXene Ti2CTx composition effectively enhances the electrochromic performance of PANi. The PANi@MXene composite films exhibited a high optical modulation range, rapid switching response time, good thermal radiation regulation, and excellent operational stability. This composite strategy significantly improves the performance and practical applicability of ECMs.

A salt-free poly(acrylic acid) hydrogel electrolyte with self-released ions for quasi-solid-state electrochromic devices

The ionic conductivity of gel polymer electrolytes (GPEs) is usually provided by the added inorganic salt ions. Here, we report a salt-free poly(acrylic acid) (PAA) hydrogel electrolyte with an ionic conductivity of 3.7 × 10−3 S cm−1. The carboxylic groups of PAA can release free H+ ions to provide ionic conductivity, which not only simplifies the preparation of GPEs, but also benefits the fabrication of GPEs with high transparency (>95.6 % from 400 to 1000 nm). The as-prepared hydrogel electrolyte is assembled into an electrochromic (EC) device (ECD) by using Prussian blue/PAA GPE/hydrated tungsten trioxide. The fabricated ECD achieves a high optical modulation (72.7 %), fast responses (2.6 s for coloration and 2.2 s for bleaching), a high coloration efficiency (172.9 cm2 C−1) and good cycling stability (retaining 96.3 % of the original optical modulation after 1200 cycles). Moreover, the ECD has good energy storage properties, demonstrating a potential application for energy-storage smart windows.

Visible light electrochromism based on reversible dissolution/deposition of MnO2

Abstract Electrochromic (EC) windows based on reversible metal electro-deposition/dissolution (RME) are exciting alternatives to static lighting control using blinds and shades. However, the difficulty of reversibly and uniformly electrodepositing large-area metal layers seriously hinders the development of RME-type EC windows. In this paper, we develop a novel EC system based on reversible oxide electro-deposition/dissolution (ROE) of MnO2, achieving a profound and bistable color change between rust brown and completely transparent. This ROE-MnO2 system exhibits a significantly high optical modulation amplitude (ΔT) without any deterioration within 1000 switching cycles (ΔT 550nm actually increases from 44.8 % up to 46.9 % after the cycling test), which is almost double compared to the traditional cation-insertion/extraction triggered MnO2 (RCI-MnO2) EC system. This work may inspire the exploration of novel ROE-type EC systems.

Surfactant integrated nanoarchitectonics for controlled morphology and enhanced functionality of tungsten oxide thin films in electrochromic supercapacitors

This research focuses on tungsten oxide (WO3) as a promising material, and the study investigates the electrodeposition of WO3 thin films on fluorine-doped tin oxide (FTO) coated glass substrates using three different surfactants: cationic hexamethylenetetramine (HMTA), anionic sodium dodecyl sulfate (SDS), and non-ionic polyethylene glycol (PEG). The X-ray diffraction analysis confirms that the resulting thin films exhibit amorphous structures. Raman analysis supports the identification of the pure WO3 phase based on the stretching and vibrational modes observed in the colored and bleached states. The X-ray photoelectron spectroscopic study examines the influence of cathodic and anodic potentials on the formation of oxidized and reduced tungsten species in surfactant-aided WO3 thin films. Field emission scanning electron microscopy is utilized to investigate the impact of different surfactants on the morphology of WO3 thin films, revealing the formation of porous, clumped, and dense nanogranules on the film surface. Electrochromic energy storage investigations highlight the superior performance of the W-PEG (polyethylene glycol assisted WO3) thin film, which exhibits efficient lithium-ion accommodation and desirable bifunctional characteristics. The optimized W-PEG sample demonstrates excellent electrochromic performance, including high optical modulation (84.39 %), good reversibility (98 %), and high coloration efficiency (117.95 cm2/C). Supercapacitive measurements reveal a high areal capacitance of 44.1 mF/cm2 at a current density of 0.1 mA/cm2, as well as an energy density of 0.024 mWh/cm2 at a power density of 0.1 mW/cm2, with 85 % capacitive retention over 8000 consecutive galvanostatic charge-discharge cycles. Furthermore, the application potential of nanogranular WO3 for electrochromic energy storage is demonstrated through the successful illumination of a red and green light-emitting diode (LED) using fully colored electrochromic supercapacitor devices (4 × 3 cm2), highlighting their energy storage capability.

Mach-Zehnder interferometric engineering in silicon optical modulators: towards extrinsic OMA enhancement.

The realization of a high dynamic extinction ratio (ER) and optical modulation amplitude (OMA) while keeping the optical and radio-frequency (RF) signal losses low is a major issue for carrier-depletion Mach-Zehnder (MZ) silicon optical modulators. However, there is still room to improve modulator performance by applying the information gained from recent advanced testing technology to the modulator design. In this study, the extrinsic OMA (E-OMA) enhancement effect, which was discovered through the evaluation process and by revisiting the physics of the MZ interferometer (MZI), is investigated. First, we raise the issue of a periodic ripple observed on an MZI spectrum that has previously been overlooked but can affect modulator performance and attribute it to optical resonance between the multi-mode interferometers that compose an MZI. We show that, although having the effect of reducing the dynamic ER in the push-pull regime, as demonstrated experimentally, this resonance can take them beyond the realm of modulation efficiency and generate an E-OMA enhancement effect in the single-arm-drive regime without involving any optical and RF signal losses. By comparing two modulator structures that generate resonance internally, we successfully identify the factors that are responsible for increasing the E-OMA enhancement effect. We reveal that theoretically the OMA can easily be increased by 0.45dB or more.

4×100Gb/s PAM4 Multi-Channel Silicon Photonic Chipset With Hybrid Integration of III-V DFB Lasers and Electro-Absorption Modulators

A silicon photonic based transmitter and receiver chipset for 4x106Gb/s 400GBASE-DR4 data rates is presented. Each channel of the transmitter chip reaches high extinction ratio and optical modulation amplitude (OMA) with a low TDECQ penalty in full compliance with the IEEE standard. The receiver chips possess high responsivity with low polarization dependent loss. The use of discrete III-V arrayed components hybridized onto the silicon platform and passive alignment of single-mode fibers provides a low-cost, compact and scalable solution extendable to even higher aggregate rates and channel count.

Niobium and Fluorine Dually Doped Titanium Oxide Nanosheet Arrays for Selective Dual-Band Electrochromic Applications

Dual-band electrochromic (EC) films selectively modulating visible (VIS) and near-infrared (NIR) light promise an attractive option for efficiently utilizing solar energy. Anatase titanium dioxide (TiO2) promises dual-band EC capability; however, it suffers from the difficulty in circumventing the obstacle in the NIR region and sluggish reaction kinetics as well as poor cycling stability. In this work, 2D (Nb, F) co-doped TiO2 nanosheets (NSs) arrays are reported as an effective single-component dual-band EC material. The co-doping can induce an oriented growth of freestanding ultrathin NSs with exposed (001) facets which promote the EC reaction kinetics. The high aspect ratio and orientation of the nanosheets favor the localized surface plasmon resonance (LSPR) effect triggered by electrochemical bias voltages, thus achieving independent modulation of the NIR transmittance. Three application scenarios of the NSs can be presented based on selective, efficient, and high optical modulation in the NIR and VIS regions (79.5% at 700 nm and 78.4% at 1300 nm, respectively), unprecedented fast bleaching/coloring times (3.0/8.4 s), and a long cycle life (500 cycles). Moreover, the films exhibit multi-color EC and energy storage materials. This work may provide an insight into the single-component dual-band EC mechanism of 2D TiO2 nanostructures and present promising potential for energy saving and smart display applications.