RESEARCH ARTICLE www.small-journal.com Brookite TiO2 Nanorods as Promising Electrochromic and Energy Storage Materials for Smart Windows Congcong Xing, Linlin Yang, Ren He, Maria Chiara Spadaro, Yu Zhang,* Jordi Arbiol, Junshan Li, Bed Poudel, Amin Nozariasbmarz, Wenjie Li, Khak Ho Lim, Yu Liu, Jordi Llorca, and Andreu Cabot* 1. Introduction Electrochromic smart windows (ESWs) offer an attractive option for regulating indoor lighting conditions. Electrochromic materials based Electrochromism is a phenomenon where a material’s optical properties, such as color on ion insertion/desertion mechanisms also present the possibility for energy or transparency, can be changed by ap- storage, thereby increasing overall energy efficiency and adding value to the plying an electric current.[1,2] This phe- system. However, current electrochromic electrodes suffer from performance nomenon is generally related to structural degradation, long response time, and low coloration efficiency. This work aims changes, chemical reduction/oxidation pro- to produce defect-engineered brookite titanium dioxide (TiO2 ) nanorods (NRs) cesses, and/or intercalation of extrinsic with different lengths and investigate their electrochromic performance as ions.[3,4] This reversible process makes elec- trochromic materials attract increasing in- potential energy storage materials. The controllable synthesis of TiO2 NRs with terest in various applications, including inherent defects, along with smaller impedance and higher carrier concen- smart windows, displays, and sensors.[5,6] trations, significantly enhances their electrochromic performance, including The importance of electrochromism lies in improved resistance to degradation, shorter response times, and enhanced its ability to provide energy-efficient solu- coloration efficiency. The electrochromic performance of TiO2 NRs, particularly tions for controlling the amount of light and heat that enter a building.[7,8] For in- longer ones, is characterized by fast switching speeds (20 s for coloration stance, smart windows made with elec- and 12 s for bleaching), high coloration efficiency (84.96 cm2 C−1 at a 600 trochromic materials can switch from trans- nm wavelength), and good stability, highlighting their potential for advanced parent to opaque or tinted to reduce the electrochromic smart window applications based on Li+ ion intercalation. amount of solar heat gain, thereby improv- ing the energy efficiency of buildings and C. Xing, Y. Zhang, B. Poudel, A. Nozariasbmarz, W. Li L. Yang, R. He Department of Materials Science and Engineering Departament d’Enginyeria Electronica i Biomedica Pennsylvania State University Universitat de Barcelona University Park, PA 16802, USA Barcelona 08028, Spain E-mail: yvz5897@psu.edu M. C. Spadaro, J. Arbiol C. Xing, L. Yang, R. He, Y. Zhang, A. Cabot Catalan Institute of Nanoscience and Nanotechnology (ICN2) Catalonia Institute for Energy Research (IREC) CSIC and BIST Sant Adrià de Besòs Campus UAB Barcelona 08930, Spain Bellaterra, Barcelona 08193, Spain E-mail: acabot@irec.cat J. Arbiol, A. Cabot C. Xing, J. Llorca ICREA Institute of Energy Technologies Pg. Lluis Companys 23 Department of Chemical Engineering and Barcelona Research Center in Barcelona 08010, Spain Multiscale Science and Engineering J. Li Universitat Politècnica de Catalunya Institute for Advanced Study EEBE, Barcelona 08019, Spain Chengdu University Chengdu, Sichuan 610106, China The ORCID identification number(s) for the author(s) of this article K. H. Lim can be found under https://doi.org/10.1002/smll.202303639 Institute of Zhejiang University-Quzhou © 2023 The Authors. Small published by Wiley-VCH GmbH. This is an Quzhou, Zhejiang 324000, China open access article under the terms of the Creative Commons Y. Liu Attribution-NonCommercial-NoDerivs License, which permits use and School of Chemistry and Chemical Engineering distribution in any medium, provided the original work is properly cited, Hefei University of Technology the use is non-commercial and no modifications or adaptations are Hefei, Anhui 230009, China made. DOI: 10.1002/smll.202303639 Small 2023, 19, 2303639 2303639 (1 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com reducing energy costs. Electrochromic materials also find appli- transparency in ESWs. Similarly, Chen et al.[50] reported that core- cations in automotive rearview mirrors that automatically dim shell NR arrays of crystalline/amorphous TiO2 enhance elec- in response to headlights, aircraft windows that can change trochromic performance. tint to reduce glare and heat, and displays that can change We synthesized TiO2 NRs with tunable lengths using a col- color or brightness based on user preferences or environmental loidal approach and investigated their performance as elec- conditions.[9,10] Thus, electrochromism offers the appealing po- trochromic materials. We analyzed the importance of optimizing tential to provide energy-efficient and cost-effective solutions for the length of TiO2 NRs to achieve optimal electrochromic perfor- a variety of applications that require dynamic control of optical mance. The inherent defects in the NRs contribute to efficient Li+ properties. insertion/extraction and enhance electrochromic response effi- Various metal oxides and conducting polymers have been ciency. The TiO2 NRs demonstrate reduced impedance, facilitat- investigated as promising electrochromic materials.[11–14] Inor- ing faster charge transport and ion diffusion, resulting in shorter ganic electrochromic materials, such as titanium oxide,[15,16] response times and improved color-switching capabilities. Sub- tungsten oxide,[17,18] niobium oxide,[19,20] nickel oxide,[21,22] and sequently, we used them to assemble ESW devices. The elec- molybdenum oxide,[23,24] have better commercial prospects due trochromic properties of the TiO2 NR-based ESW devices were to their excellent thermal and chemical stability, abundance of further evaluated, including coloration and bleaching rates, col- their constituent elements, low cost, safety, and notable perfor- oration efficiency, and stability, which sheds light on the mecha- mance. Among these inorganic electrochromic materials, TiO2 nism of electrochromic performance and provides a pathway to- stands out as an especially promising candidate. Within the TiO2 ward developing more efficient and reliable ESW devices for en- lattice, the Ti4+ ions are surrounded by six O2− ions to create ergy storage applications. an [TiO6 ] octahedron, forming large vacant sites that provide enough space for the accommodation of protons or other guest 2. Results and Discussion ions.[25] Within electrochromic smart windows (ESWs), the col- oration mechanism of TiO2 is generally related to the insertion Colloidal TiO2 NRs were synthesized via a hydrolysis approach or extraction of Li+ from the ion-conducting electrolyte into the in an organic solution (see details in the Experimental section). TiO2 lattice accompanied by the reduction of TiO2. [26,27] In brief, titania nuclei were produced in the presence of oley- lamine (OAm), 1-octadecanol (ODO), oleic acid (OAc), and even- TiO2 + xLi+ ↔ Lix TiO2 (1) tually 1,2-hexadecanediol (HDDOL) or methylamine hydrochlo- ride (MAC), by heating to 290 °C a titanium (IV) chloride (TiCl4 ) Despite the coloration mechanism remaining the same, the precursor stock solution obtained by dissolving TiCl4 in OAc and electrochromic properties of TiO2 have been demonstrated ODO. At this moderate temperature, OAm and OAc slowly react to be influenced by its crystallographic phase,[28] electronic to generate small amounts of water, resulting in slow hydrolysis conductivity,[29] and defect level.[27,30] While considerable re- and condensation of TiCl4 along with the release of HCl. After search efforts have been devoted to the development of TiO2 - nucleation, TiO2 NRs were further grown by slowly adding addi- based ESWs, key parameters such as the TiO2 particle size and tional amounts of the TiCl4 stock solution into the initial nuclei morphology have been rarely investigated. The brookite phase of solution maintained at 290 °C. This two-step synthesis approach TiO2 outperforms the anatase and rutile phases, displaying supe- allows for preventing the nucleation of additional NRs during the rior performance in electrochromism, dielectrics, photoactivity, growth of the preformed ones. As shown in Figure S1, Support- catalytic activity, and electrochemistry,[31–33] attributed to its dis- ing Information, the initially formed nuclei in the absence of HD- tinct structural characteristics, including a distorted lattice and DOL or MAC already presented elongated morphologies with an variable Ti–O bond lengths.[34] average length of 15 ± 5 nm. After the nucleation step, simulta- In addition to composition and crystal structure, the mate- neous and uniform growth of all the nuclei in the ensemble takes rial architecture, including particle shape, morphology, and ar- place and the TiO2 particles preserve the NR morphology, a rel- rangement, plays a pivotal role in determining optical and elec- atively narrow size distribution, and excellent colloidal stability trochemical performance.[35–41] TiO2 nanoparticles, nanoflakes, (Figure 1a–c). and amorphous/crystalline structures exhibit excellent elec- Transmission electron microscopy (TEM) characterization trochromic performance due to their unique properties.[42–46] showed that when adding no HDDOL and no MAC, TiO2 NRs Among them, 1D materials such as nanorods (NRs) and with an average length of 30 ± 12 nm and an average thickness of nanowires offer several advantages in ESWs. First, 1D materials 2.6 ± 0.6 nm were obtained (Figure 1b and Figure S2, Supporting have a large specific surface area that allows for efficient interac- Information). This sample will be referred to as M-TiO2 , standing tion with the electrolyte while simultaneously facilitating charge for medium TiO2 . In the presence of a small amount of HDDOL transport, which reduces charge-transfer resistance.[47] Second, in the initial solvent solution, shorter TiO2 NRs, with an average 1D materials are highly tolerant to volume changes during ion length of 17 ± 5 nm and an average thickness of 2.5 ± 0.7 nm, insertion/extraction, which minimizes damage to the electrode were produced (Figure 1a and Figure S2, Supporting Informa- structure and improves stability.[48] Third, electrodes based on tion). This sample will be referred to as S-TiO2 , standing for short 1D materials promote ion diffusion and enhance contact between TiO2 . In contrast, when adding a small amount of MAC, longer the electrolyte and electrode, thereby shortening the charge and TiO2 NRs, with an average length of 45 ± 16 nm and an aver- discharge time threshold and the distance of electron trans- age thickness of 2.8 ± 0.8 nm, were grown (Figure 1c and Figure port and ion diffusion.[20] For instance, Chen et al.[49] demon- S2, Supporting Information). This sample will be referred to as strated that anatase TiO2 nanowire electrodes improve the optical L-TiO2 , standing for long TiO2 . High-resolution TEM (HRTEM) Small 2023, 19, 2303639 2303639 (2 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 1. a–c) Representative TEM micrographs and optical images of S, M, and L-TiO2 dispersed in hexane solution with a concentration of 10 mg mL−1 . d–f) HRTEM micrographs of TiO2 NRs showing the Brookite orthorhombic crystal phase: d) S-TiO2 NRs oriented along the [011] zone axis; e) M-TiO2 NRs oriented along the [123] zone axis; f) L-TiO2 NRs oriented along the [120] zone axis. g) Schematic illustration of the influence of HDDOL and MAC on the size and aspect ratio of the TiO2 NRs. h) EELS chemical composition maps of L-TiO2 NRs: Ti L edge at 456 eV (green) and O K edge at 532 eV (red). i) Powder XRD patterns (reference pattern for brookite TiO2 JCPDF No. 29–1360), j) Raman spectra, and k) 3D atomic models of S, M, and L-TiO2 NRs. analyses of the different TiO2 NRs (Figure 1d–f) showed them to (EELS) elemental maps of the TiO2 NRs recorded at the Ti L edge be characterized by the brookite orthorhombic crystal structure. at 456 eV and O K edge at 532 eV display a homogeneous distri- Table S1, Supporting Information, summarizes the main geom- bution of Ti and O within the nanostructures. etry characteristics of the synthesized TiO2 NRs with different Both the thermodynamics of the system and the kinetics of the lengths. As shown in Figure 1h, electron energy loss spectroscopy nucleation and growth process can determine the morphology Small 2023, 19, 2303639 2303639 (3 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com of the final structure. Specifically, thermodynamic properties[51] to the presence of abundant defects that can shift upwards the determine that brookite TiO2 only exists as nanocrystalline par- Fermi level (Figure S5, Supporting Information).[58] ticles and that the stability of both anatase and brookite is For the preparation of an optically transparent electrochromic closely related to the particle size, being the brookite phase thin film electrode, a layer of TiO2 NRs with a thickness more stable in the range of 10–40 nm. For brookite TiO2 of ≈2 μm was spin-coated on an indium-doped tin oxide (ITO)- nanocrystals, the [001] crystal facets have high energy.[52,53] coated glass (Figure S6b, Supporting Information). The films Thus, energy is minimized by growing preferentially along were afterward heated in the air at 350 °C for 45 min to remove the [001] direction, especially in the presence of chlorine the organic ligands. The in situ UV–vis optical transmittance ions (Cl− ).[54,55] spectra versus EAg/AgCl potential from −4 to 0 V of S, M, and L- The routes toward TiO2 NRs with different aspect ratio is TiO2 NRs thin films were first investigated in view of their use shown in Figure 1g. While OAm and OAc play a double role as a cathodic electrochromic electrode for ESWs (Figure 2a–c). as coordinating/stabilizing solvents and releasing water, Cl− is XRD analysis confirms that the brookite phase is preserved key in directing the TiO2 asymmetric growth and determining (Figure S7, Supporting Information) after heat treatment. TEM its aspect ratio. MAC and HDDOL were introduced to tune the analysis (Figure S8a–f, Supporting Information) reveals a sig- amount of chlorine binding at the nanoparticle surface. We hy- nificant aggregation of the TiO2 NRs during deposition and an- pothesize that while the introduction of hydrochloride increases nealing but with no impact on their geometry and size. Be- the chlorine concentration, the presence of the diol competes sides, annealing promotes a strong adhesion between TiO2 NRs with chloride for the binding at the TiO2 NR surface, decreasing and ITO substrate and results in enhanced color modulation the surface concentration of chlorine. Since the thickness of all (Figure S8g,h, Supporting Information). the NRs is approximately the same and the volume of TiO2 grown The optical modulation (ΔT) at different wavelengths of S, M, must be similar for all the samples, the difference in lengths must and L-TiO2 NRs thin films was calculated using the following come from a different number of NRs, i.e. from the amount of equation. NR nuclei initially produced within each ensemble. Thus, we ( ) conclude that the presence of HDDOL, reducing the amount ΔT = Tb − Tc ∕Tb × 100% (2) of surface Cl− , promotes the TiO2 nucleation, while the pres- ence of MAC, increasing the amount of capping Cl− , hampers where Tc represents the transmittance of the device in the col- it. ored state, and Tb represents the transmittance in the bleached The formation of the brookite phase was confirmed by state. As shown in Figure 2d, the L-TiO2 electrode exhibited the X-ray diffraction (XRD) and Raman spectroscopy. The sharp highest ΔT values compared to the M and S-TiO2 electrodes, par- XRD diffraction peaks of each sample match well with the ticularly at the wavelengths of 400, 600, and 800 nm. Addition- standard diffraction pattern of brookite (JCPDF No. 29–1360), ally, the L-TiO2 electrode required the lowest applied potential with no additional XRD peak detected (Figure 1i). Besides, to achieve the same transmittance as the other TiO2 NR-based the main characteristic peaks of anatase that do not overlap electrodes. The optical photographs in Figure 2e demonstrate with brookite, at 2𝜃 of 62.57˚ and 75.0˚, are not observed, that increasing the applied potential to the TiO2 NRs on the ITO pointing to a pure brookite phase (Figure S3, Supporting In- glass resulted in a gradual decrease in transmittance intensity. formation). The significant difference in XRD peak intensity To evaluate the reproducibility of the L-TiO2 NRs thin film de- further confirms the asymmetric geometry of the crystalline vice, a series of four samples were fabricated using the same domains.[56] process and underwent in situ UV–vis optical transmittance Raman spectroscopy was further used to confirm the crys- measurements (Figure S9, Supporting Information). The results talline structure of TiO2 (Figure 1j). The Raman spectra of the demonstrated remarkable consistency in performance across all TiO2 NRs display several Raman peaks attributed to the brookite the fabricated samples. Furthermore, additional measurements TiO2 Raman vibration modes at A1g (152 and 246 cm−1 ), B1g were carried out at different temperatures, shedding light on the (327 and 622 cm−1 ), B2g (366 and 460 cm−1 ), and B3g (416 temperature-dependent behavior of the devices. Notably, the data cm−1 ).[57] Thus, the Raman spectra of the TiO2 NRs are also presented in Figure S10, Supporting Information, clearly illus- in good agreement with the brookite crystal phase. Overall, ac- trates that temperatures above 35 °C have a detrimental effect cording to the TEM, HRTEM, XRD, and Raman results shown on both the performance and stability of the devices. These find- above, the 3D models of the TiO2 brookite NRs are displayed ings provide crucial insights into the operational limitations and in Figure 1k. thermal stability considerations associated with TiO2 thin film Figure S4, Supporting Information, depicts the electron para- devices. magnetic resonance (EPR) spectra measured at 77 K of TiO2 NRs Electrochemical impedance spectroscopy (EIS) spectra were with various lengths. EPR spectra exhibit distinct characteristics obtained in 1M LiClO4 within propylene carbonate (PC) elec- associated with lattice and surface Ti3+ vacancy centers for S, M, trolyte to investigate the charge transfer of the TiO2 thin and L-TiO2 NRs. Notably, the EPR signal intensity of L-TiO2 NRs films. The Nyquist plots of the EIS spectra obtained from S, is significantly higher than that of S-TiO2 and M-TiO2 NRs, in- M, and L-TiO2 electrodes are shown in Figure 2f. The se- dicating that longer TiO2 NRs possess a higher defect density. rial resistance (Rs ), charge-transfer resistance (Rct ), and ca- Furthermore, high-resolution X-ray photoelectron spectroscopy pacitance of the double-layer capacitor (Cdl ) were determined (XPS) spectra revealed a shoulder in the Ti 2p spectrum, indi- by fitting the Nyquist plots with an equivalent circuit model cating the presence of Ti3+ within the TiO2 lattice, along with a (see inset in Figure 2f) and are compiled in Table S2, Sup- positive shift in both the Ti 2p and O 1s spectra indirectly related porting Information. Rct decreased with the NR length from Small 2023, 19, 2303639 2303639 (4 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 2. a–c) In situ UV–vis optical transmittance spectra, d) the optical modulation (ΔT) at the wavelength of 400, 600, and 800 nm, and e) the optical photographs of L-TiO2 NRs thin film electrodes with different applied potentials. f) Nyquist plots of S, M, and L-TiO2 thin film electrode at 0.5 V versus EAg/AgCl ; inset is the equivalent circuit model. g) CV curves of L-TiO2 thin film electrode with scan rates of 0.1, 0.25, and 0.5 mV·s−1 ; h) Mott–Schottky curves of S, M, and L-TiO2 NRs. 20.1 kΩ for S-TiO2 to 12.3 kΩ for L-TiO2 , demonstrating a faster CV curve obtained at different scan rates. The absolute ratio of charge-transfer process for the L-TiO2 . In contrast, Cdl remained the cathodic peak current (iPc = −1.16 × 10−3 A) and the an- constant and the serial resistance surprisingly increased with odic peak current (iPa = 0.64 × 10−3 A) is 2.1, implying reversibil- the NR length, which we associate with the differences in the ity. In electrochromic systems, the observed difference in peak film architecture related to the worse piling of the longer NRs. currents, with higher cathodic peak current compared to anodic The smaller impedance in L-TiO2 NRs indicates improved charge peak current, is primarily attributed to factors such as higher con- transport and better ion diffusion due to increased defect density, centration of electroactive species and various kinetic and trans- leading to faster response times and more efficient color change port limitations affecting the reduction and oxidation reactions. in electrochromic processes. Mott–Schottky (M–S) curves (Figure 2h) were used to analyze the The cyclic voltammogram (CV) curves of L-TiO2 films are dis- carrier concentrations of three samples. The M–S curves were played in Figure 2g. A pair of redox peaks are observed in each generated by measuring the capacitance as a function of applied Small 2023, 19, 2303639 2303639 (5 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com voltage, with the linear region representing the semiconductor charging leads to shorter coloration time in electrochromic de- behavior. The carrier concentrations (Nd ) were calculated using vices compared to the bleaching time during discharging. The the following formula.[59] fast response with a low voltage of the L-TiO2 thin films is ex- ( ) pected to mitigate the challenge in large voltage requirements 1 2 KB T across the large-area ESW devices. To evaluate the ESW perfor- = V − Efb − (3) C2 𝜀𝜀0 A2 eNd e mance, coloration efficiency (CE) is calculated according to Equa- tion (4).[60] where A is the interphase area which is calculated from the geo- ( ) metric area of the NRs contained within the 1 × 2 cm2 electrode, CE = ΔOD∕ΔQ = log Tb ∕Tc ∕ΔQ (4) 𝜖 is 48, the dielectric constant of the TiO2 , 𝜖 0 is the permittivity of free space, KB is Boltzmann’s constant, T is the absolute temper- where Tb and Tc are the transmittance intensity at a spe- ature, and e is the electronic charge. The carrier concentrations cial wavelength in bleached and colored states, respectively; were determined to be 1.97 × 1016 cm−3 for L-TiO2 NRs, 1.18 × and ΔQ is the required charge density for the corresponding 1016 cm−3 for M-TiO2 NRs, and 9.50 × 1015 cm−3 for S-TiO2 NRs, changed optical density (ΔOD). Thus, CE is calculated from respectively, based on the analysis of the M-S curves (Figure 2h). the slope of the linear region of the ΔOD versus ΔQ plots The results indicate an increasing trend in carrier concentration, (Figure S12, Supporting Information). The obtained CE is 84.96 with a slightly higher density of charge carriers in L-TiO2 NRs. cm2 C−1 for the L-TiO2 NRs at 600 nm, which is higher than These differences in charge carrier concentration between the other reported TiO2 -based electrochromic electrodes (Table S3, different TiO2 NRs should be attributed to the different defect Supporting Information).[31,50,27,61,62] levels, as observed by EPR and XPS analysis, but may be in part The optical transmittance spectra of the L-TiO2 thin film un- modulated by the underlying ITO. der three applied potentials (0, −2.5, and −4 V) are depicted in The L-TiO2 NRs exhibit a significant defect density on their Figure 3d. The results demonstrate effective control over visible surface, as observed by XPS and EPR analysis. This abundance (vis) and near-infrared (NIR) light transmittance intensity by ad- of defects provides numerous active sites for redox reactions, re- justing the applied potentials. At 0 V, the TiO2 thin film appears sulting in remarkable electrochromic activity and coloration effi- colorless and exhibits complete transparency to both vis and NIR ciency. Moreover, the L-TiO2 NRs demonstrate lower impedance spectra (400–1000 nm), referred to as the transparent mode. This and higher carrier concentration, indicating enhanced charge mode is ideal for applications requiring high visible light trans- transport and Li+ diffusion within the material. As a result, elec- mission and daylighting displays. At −2.5 V, the L-TiO2 thin film trochromic devices incorporating L-TiO2 NRs exhibit faster re- blocks 60.2% of vis and NIR light transmittance intensity, rep- sponse times and more efficient color switching. The combina- resenting the cloudy mode. When the applied potential is set to tion of higher defect density, reduced impedance, and increased −4 V, the TiO2 thin film shows significantly reduced vis and NIR carrier concentration collectively contributes to the exceptional light transmittance intensity (17.5%). The black mode is suitable electrochromic performance of the L-TiO2 NRs, positioning them for applications that prioritize personal privacy protection and re- as highly promising materials for advanced electrochromic appli- duction of solar heat gain in buildings. Digital photos of the TiO2 cations. thin film electrode in the transparent, cloudy, and black modes The architecture of an ESW device is shown in Figure 3a. are presented in Figure 3e. The ≈2 μm thick L-TiO2 thin film on ITO glass outperformed The switching mechanism between these modes is based on TiO2 films of 1 and 4 μm thicknesses, demonstrating the Li+ intercalation within the TiO2 thin film electrode. A schematic most favorable performance under identical applied voltage con- illustration of the ESW device in the three modes is depicted in ditions (Figure S6, Supporting Information), with a ≈2 μm- Figure 3f–h. With no applied potential, the TiO2 thin film just thick NiLi0.83 Ti0.28 Ox anodic electrochromic electrode in 1 M adsorbs Li+ on its surface, with no intercalation, which corre- LiClO4 /PC electrolyte. To evaluate the switching performance sponds to the transparent mode (Figure 3f). At a moderate ap- of the assembled ESW device, the in situ current density and plied potential of −2.5 V, a moderate Li+ is inserted into the transmittance at 600 nm were monitored by combining an elec- TiO2 crystal lattice to induce the partial LiTiOx transformation, trochemical workstation and a UV–vis spectrophotometer, and thus leading to a moderate blocking of the light, which corre- switching the voltage using a square-wave function between 1 sponds to the cloudy mode. (Figure 3g). With a further increase and −3 V versus EAg/AgCl . Figure 3b shows the current density in the applied potential to −4 V, more Li+ is inserted into the and transmittance transients upon switching the applied poten- TiO2 lattice, forming the Lix TiO2 phase which has a high ab- tial. The ESW device shows good switching and electrochromism sorption in the UV–vis, thus it is referred to as the black mode properties and a constant ΔT during these cycles (Figure S11, (Figure 3h) and alters the transmittance of both vis and NIR Supporting Information). Besides, the coloration response time light. was evaluated through double potential-step chronoamperome- To investigate the long-term stability of the assembled ESW try (−4 and 0 V) measurement. The coloration and bleaching re- device, the optical transmittance spectra of the L-TiO2 thin film sponse time of ESWs is defined as the time it takes for a 90% electrode before and after 500 full switching cycles, that is, up change between the colored and bleached states. For ESWs based to −4 V, are shown in Figure 4a. The ΔT of L-TiO2 thin film on L-TiO2 , the coloration time (tc,90% ) was calculated to be 20 electrode at 400, 600, and 800 nm with an applied potential at s, while the bleaching time (tb,90% ) was determined to be 12 s, −2.5 and −4 V is summarized in Figure S13, Supporting Infor- which benefit from the fast charge transfer and low Rct of the L- mation. 96.1– 99.2% of the ΔT is maintained after 500 full cy- TiO2 thin film (Figure 3c). Faster electrochemical reaction during cles of the L-TiO2 thin film electrode, implying good coloration Small 2023, 19, 2303639 2303639 (6 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 3. a) Schematic illustration of the ESW device structure with the sandwich structure of ITO/TiO2 /electrolyte/NiLi0.83 Ti0.28 Ox /ITO. b) Transmit- tance at 600 nm wavelength and current density (J) of the ESW device upon electrochromic switching with a potential of 1 and −3 V with a cycle time of 50 s. c) Change of transmittance of the ESW device at 600 nm upon a switching cycle with a potential of 0 and −4 V with time intervals of 80 s. d) Transmittance spectra from 300–1000 nm of the L-TiO2 thin film under three potential set values of 0, −2.5, and −4 V. e) Optical photographs of the ESW device at the corresponding “transparent”, “cloudy” and “black” modes. f–h) Schematic illustration of the ESW device in three modes during Li+ insertion. stability, attributed to the TiO2 1D NR structure with high toler- vice is observed after the 200th coloration cycle, further proving ance to electrode volume changes during the Li+ intercalation, its good stability. which is higher than other reported TiO2 -based electrochromic TiO2 -based ESW can store the energy used to change their electrodes.[27,62–64] The CV curves of the ESW device before and transmittance mode, which allows for maximizing the en- after the 200 switching cycles are shown in Figure S14, Support- ergy efficiency of the process. ESW could be even potentially ing Information. Just a slight current decrease of the ESW de- used as building integrated structural energy storage devices, Small 2023, 19, 2303639 2303639 (7 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 4. a) Transmittance spectra of L-TiO2 thin film electrode before and after the 500 switching cycles. b) Galvanostatic charge curve (black line) at a current density of −4 V for 60 s and potentiostatic discharge curve (red line) for 60 s. c) A photograph of a LED (2 V) lighted by an ESW device. d) Charge/discharge curves for the ESW device at a current of 0.2 mA with a charge threshold voltage of 2.8 V. The effective area of the ESW device is 2.5 × 4 cm2 . e) Charge-discharge efficiency of the ESW device. f) Charge/discharge curves for the ESW device at the 1st and 3000th cycles. g) Specific capacity for the ESW device at different cycles. contributing to solving the current limitations in energy storage long-time cycling stability and long lifetime. The initial improve- being part of the future energy storage mix.[65,66] As shown in ment in charge-discharge efficiency of the TiO2 NRs-based ESW Figure 4b, during the coloration process, the ESW device was device before 500 cycles is attributed to the activation and op- charged potentiostatically at −4 V for 60 s with an energy con- timization of electrochemical processes and the formation of a sumption of 786 mWh m−2 . In the bleaching process, ESW was stable solid-electrolyte interface. These findings highlight the fa- discharged from −4 V for 60 s recovering 272 mWh m−2 , which vorable electrochemical properties of TiO2 NRs for reliable ESW could light up an LED bulb for more than 30 s (Figure 4c and applications. As the charge and discharge cycles increase, the Video S1, Supporting Information). TiO2 -based ESW devices require less time to reach the threshold Figure 4d shows extended charge/discharge curves, for 35 h voltage, as shown in Figure 4f, implying a decrease in capacity. and ≈3000 cycles, of the ESW device up to 2.8 V at a current of The longer charging time in TiO2 NRs-based electrochromic de- 0.2 mA. The size of the ESW device is 5 × 5 cm2 , and the effec- vices compared to discharging can be attributed to factors such tive area is 2.5 × 4 cm2 . Charge and discharge curves at a higher as slower redox reaction kinetics, limited ion diffusion during rate of 0.5 mA are displayed in Figure S15, Supporting Infor- charging, and favorable material properties.[67,68] The initial spe- mation. As shown in Figure 4e, the charge-discharge efficiency cific capacity of the TiO2 -based ESW device reaches 2.1 mAh m−2 , of the TiO2 ESW device is above 50% at the first cycle and in- which is the maximum charge that the device can store when it creases up to 75% at the 500th cycle when it stabilizes, proving its is fully charged. However, after 500 cycles, the specific capacity Small 2023, 19, 2303639 2303639 (8 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com decreases to 0.5 mAh m−2 , indicating a decrease in the amount Ligand Exchange: Native organic ligands were displaced from the NRs of charge, which could be ascribed to various factors, such as the surface using an NH4 SCN solution. Briefly, 1 mL hexane solution contain- degradation of the electrodes or the loss of active sites. The spe- ing NRs (5 mg mL−1 in hexane) was mixed with 1 mL 0.15 M NH4 SCN solution (in ethanol). The mixture was then shacked for several minutes cific capacity further decreases to 0.3 mAh m−2 at the 1000th cy- with a vortex and finally centrifuged at 4000 rpm for 3 min. The resulting cle, where it stabilizes and reaches a steady-state condition, and product was further washed with 2 mL ethanol twice to remove residual further degradation is unlikely. Figure 4g depicts the change in NH4 SCN. specific capacity over multiple cycles, which reveals the gradual Material Characterization: The crystal structures of the samples were decrease in specific capacity over time and the stabilization of the identified by XRD (a Bruker AXS D8 Advance X-ray diffractometer) using capacity after 1000 cycles. Ni-filtered Cu-K𝛼 irradiation at 45 kV and a diffracted beam monochro- mator at 40 mA. SEM (Zeiss Auriga) was used to obtain the surface mor- phology. Elemental analysis was conducted using an Oxford EDX com- 3. Conclusion bined with the Zeiss Auriga SEM working at 20.0 kV. The lattice structure of TiO2 was visualized by phase-contrast HRTEM carried out with 300 keV In summary, TiO2 NRs with tunable lengths ranging from electrons in an FEI Tecnai F20 Field emission instrument. UV–vis diffuse 17–45 nm were produced using a colloidal synthesis method, reflectance spectra were obtained by a UV–vis spectrometer (Shimazu) us- and their performance as electrochromic materials were investi- ing BaSO4 as the background. XPS experiments were performed using a gated. Optimizing the length of TiO2 NRs is crucial for achiev- Physical Electronics VersaProbe III instrument equipped with a monochro- matic Al k𝛼 x-ray source (h𝜈 = 1,486.6 eV) and a concentric hemispherical ing optimal electrochromic performance. The longest NRs ex- analyzer. Raman spectroscopy was performed on a Renishaw inVia Qon- hibit higher defect density, smaller impedance, and higher car- tor confocal Raman microscope using a laser excitation source of 532 nm. rier concentration, resulting in superior electrochromic perfor- EPR measurements were conducted at 9.39 GHz using a Bruker EMXmi- mance and promising applications. Using these longest NRs, cro6/1 spectrometer. we assembled electrochromic smart ESW devices and evaluated Electrochemical Measurements: Electrochemical characterization was their electrochromic properties. The devices based on the longest performed in a three-electrode system using an electrochemical worksta- TiO2 NRs exhibited excellent electrochromic properties, includ- tion (CHI760e). A Pt mesh and Ag/AgCl were used as the counter and reference electrodes, respectively. To prepare the working electrode, 2 mg ing fast coloration and bleaching rates, high coloration efficiency, of NRs was dispersed in 2 mL ethanol by sonicating for obtaining a ho- and good stability. Furthermore, we demonstrated three optical mogeneous ink. The inks were coated onto ITO (2.5 × 4.8 cm2 ) substrate modes and unveiled the mechanism of the switching of these and then annealed at 200 °C for 1 h in the glovebox. An active area of 2.5 × modes based on the intercalation within the TiO2 lattice of Li+ in 4 cm2 was left for tests. 1 M LiClO4 /PC solution was used as an electrolyte. the electrolyte. These findings offer insights into the TiO2 NRs- The electrolyte was purged with Ar for 30 min before the measurement. For based electrochromic performance mechanism and a pathway to EIS measurements, the frequency was swept from 1 to 10 MHz with a sinu- soidal amplitude of 5 mV. M–S spectra were obtained by scanning the bias design and engineering more efficient and reliable ESW devices potential from −1.2 to 0.2 V versus Ag/AgCl at a scan rate of 0.01 V s−1 , for diverse optical display applications. and 0.5 M Na2 SO4 solution was used as an electrolyte. The geometric area of the nanorods was calculated taking into account their dimensions, the density of brookite TiO2 (4130 kg m−3 ), and the amount of TiO2 deposited on the 1 × 2 cm2 substrate (0.05 mg). 4. Experimental Section Preparation of TiO2 NRs Thin Film Electrodes: A 5 × 5 cm ITO glass Materials: TiCl4 , lithium perchlorate (LiClO4 , 99.99%), PC, anhy- (20 Ω sq−1 ) was first thoroughly cleaned with acetone solution for 30 min drous, 99.7%, nickel acetate tetrahydrate (Ni(CH3 COO)2 4H2 O, 98%), and rinsed with ethanol for another 30 min. 100 μL of the TiO2 NR solution titanium isopropoxide (Ti(OC3 H7 )4 , 97%), lithium acetate (CH3 COOLi, (≈35 mg mL−1 ) was then spun-coated onto the ITO glass at 500 rpm for 60 99.95%), and HDDOL, C16 H34 O2 , 90% were purchased from Fisher Sci- s. After drying on a hot plate at 200 °C for 5 min, a second spin-coating was entific. 1-ODO (99%), OAC (90%), OAm (90%), and MAC (98%) were applied. Spin-coating was repeated four times (total spin-coating times is purchased from Sigma-Aldrich. All chemicals were used as received. ITO- 5). The as-deposited TiO2 NR film on ITO glass was then heated in the air coated glass slides (5 × 5 cm, 2.2 mm-thick, 6–9 Ω sq−1 ) were supplied at 350 °C for 45 min to remove the organic ligands. by Dyesol (TEC8 Glass). Preparation of NiLi0.83 Ti0.28 Ox -Based Counter Electrode: The Preparation of Stock Solutions: Stock solutions were prepared in a NiLi0.83 Ti0.28 Ox -based counter electrode was prepared by the sol-gel glovebox under a nitrogen atmosphere. The TiCl4 stock solution consisted spin coating method. The precursor solution was prepared from a of 0.12 g TiCl4 in 1.5 mL OAc and 3.4 mL 1-ODO. 1-ODO and OAc were mixture of 610 mg Ni(CH3 COO)2 4H2 O, 134 mg CH3 COOLi, and purified by vacuum distillation. The TiCl4 stock solution was stirred on a 200 mg Ti(OC3 H7 )4 in 10 mL ethanol. The freshly prepared precursor hot plate at a temperature of 80 °C to promote the dissolution of TiCl4 . solution was spin-coated on a clean ITO glass at 500 rpm for 60 s. Once dissolved, the TiCl4 stock solution turned to be brown. After drying over a hot plate at 200 °C for 5 min, a second layer was Synthesis of TiO2 NRs: In a 100 mL flask, 15 mL OAm, 5.2 mL 1-ODO, spin-coated and dried, and this procedure was repeated four times. The 0.25 mL OAc, and (10 mmol of HDDOL for S-TiO2 or MAC for L-TiO2 ) NiLi0.83 Ti0.28 Ox -coated ITO glass was then annealed in air at 350 °C for were combined and degassed at 120 °C for 1 h. After degassing, the sys- 45 min. tem was allowed to cool down to 60 °C to add 0.25 mL of the desired Assembly and Measurement of ESW: The ESW device was assembled stock solution (or mixed stock solution). Then, the solution was quickly by using a TiO2 thin film as the cathode, a NiLi0.83 Ti0.28 Ox -based film as heated to 290 °C for 15 min and held for 10 min to allow for the formation the anode, and a 1 M LiClO4 /PC solution as the electrolyte. An active area of seed crystals. A 4 mL stock solution was then pumped into the flask of 2.5 × 4 cm2 was left for tests. The two facing electrodes of the optical cell kept at 290 °C with the injection of 20 min using a New Era Pump Sys- were spaced apart by a ∼1 mm thickness double-sided tape. The electrolyte tems NE-1000 syringe pump. Then, the mixture was allowed to react at was introduced into the cell cavity with a syringe and the cell was sealed 290 °C for 5 min. Afterward, the flask was left to cool naturally to ambient with double-sided tape. For the 2-electrode device measurements, in situ temperature. Finally, NRs were collected by precipitation, adding 15 mL optical transmission spectra were recorded as a function of the applied hexane, and centrifuging the solution at 5500 rpm for 5 min. The super- voltage on an ocean optics machine (USB2000+XR1-ES). The background natant was discarded, and the precipitate was dispersed in 5 mL hexane transmittance of the ESW device without applied voltage was used as the for further use. baseline. Energy input and output (E, Wh m−2 ) were calculated based on Small 2023, 19, 2303639 2303639 (9 of 11) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 49, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202303639 by Readcube (Labtiva Inc.), Wiley Online Library on [22/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com the equation: E = ∫UIdt, where U (V) is the voltage, I is the current density [3] V. K. Thakur, G. Ding, J. Ma, P. S. Lee, X. Lu, Adv. Mater. 2012, 24, (A m−2 ), and t (h) is the time. 4071. [4] S. Zhang, S. Cao, T. Zhang, A. Fisher, J. Y. 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Roghani-Mamaqani, B. Razavi, Prog. Polym. Sci. de Catalunya 2021SGR00457, 2021SGR01061, and 2021SGR01581. This 2019, 98, 101149. study was supported by MCIN with funding from European Union [12] S. Yan, K. P. Abhilash, L. Tang, M. Yang, Y. Ma, Q. Xia, Q. Guo, H. Xia, NextGenerationEU (PRTR-C17.I1). This research is part of the CSIC pro- Small 2019, 15, 1804371. gram for the Spanish Recovery, Transformation and Resilience Plan funded [13] L. Yu, X. Xing, D. Fang, H. Meng, Adv. Opt. Mater. 2022, 10, by the Recovery and Resilience Facility of the European Union, estab- 2201423. lished by the Regulation (EU) 2020/2094. The authors thank the sup- [14] T.-H. Le, Y. Kim, H. Yoon, Polymers 2017, 9, 150. port from the project NANOGEN (PID2020-116093RB-C43), funded by [15] Q. Meng, S. Cao, J. Guo, Q. Wang, K. Wang, T. Yang, R. Zeng, J. Zhao, MCIN/AEI/10.13039/501100011033/ and by “ERDF A way of making Eu- B. Zou, J. Energy Chem. 2023, 77, 137. rope”, by the “European Union”. ICN2 is supported by the Severo Ochoa [16] A. Ghicov, H. Tsuchiya, R. Hahn, J. M. Macak, A. G. Muñoz, P. program from Spanish MCIN / AEI (Grant No.: CEX2021-001214-S) and Schmuki, Electrochem. Commun. 2006, 8, 528. is funded by the CERCA Programme / Generalitat de Catalunya. J.L. is grateful for the Natural Science Foundation of Sichuan project (2022NS- [17] B. Zhang, H. Wang, Y. Xiang, H. Jiang, L. Tang, J. Luo, Y. Tian, Elec- FSC1229). L.Y. thanks the China Scholarship Council for scholarship sup- trochim. Acta 2023, 440, 141749. port. [18] Z. Liu, J. Yang, G. Leftheriotis, H. Huang, Y. Xia, Y. Gan, W. Zhang, J. Zhang, Sustainable Mater. Technol. 2022, 31, 00372. [19] S. Santhosh, K. Balamurugan, M. Mathankumar, K. Shankar, B. Subramanian, Opt. Mater. 2023, 135, 113248. Conflict of Interest [20] G. K. Ong, C. A. Saez Cabezas, M. N. Dominguez, S. L. Skjærvø, S. Heo, D. J. Milliron, Chem. Mater. 2019, 32, 468. The authors declare no conflict of interest. [21] F. Zhao, Z. Cheng, G. 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