Open Access Article. Published on 11 April 2022. Downloaded on 5/19/2022 11:36:06 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Energy Advances View Article Online PAPER Cite this: Energy Adv., 2022, 1, 321 View Journal | View Issue High colouring efficiency, optical density and inserted charge in sol–gel derived electrochromic titania nanostructures Robert C. Pullar, *ab Roberto Giannuzzi,*cd Tania Prontera,c David M. Tobaldi, bc Marco Pugliese,c Luisa De Marco, c Pierluigi Cossari, Giuseppe Giglice and Vincenzo Maioranoc c A pure TiO2 thin film (100–120 nm) was made from a green aqueous sol–gel precursor on FTO glass and calcined at 430 1C. It was a mix of amorphous, anatase, rutile and brookite TiO2 phases, and exhibited very good electrochromic properties over visible and NIR wavelengths with an applied bias of +0.1 V to 1.5 V. It was highly transparent showing excellent coloration with applied voltage, with trans- mittance modulation (DT) = 69.7% at 550 nm, 86% at 700 nm and an overall DT between 400–1650 nm of 60%, giving a very large change in optical density (DOD) of 1.4 at 550 nm and 2.4 at 700 nm. Cyclic voltammograms had typical peaks for TiO2 at 1.3 V for colouration and 0.9 V for bleaching, with a high separation of 0.37 V between peaks, and a charge density after charging for 25 min of Qc = 50 mC cm 2. After only 60 s and 120 s at 1.5 V, inserted charge values of 17.6 and 22 mC cm 2 2 were observed, leading 1 Received 20th January 2022, Accepted 9th April 2022 to a high colouration efficiency (CE) of 55.9 cm C at 550 nm. These DOD, DT, Qc and CE values are superior to any previously reported for crystalline sol–gel TiO2 films. They also possessed rapid switching DOI: 10.1039/d2ya00016d times for bleaching and colouring of tb90% = 10 s and tc90% = 55 s, comparable to the best previously reported sol–gel anatase-based TiO2 films. This makes this nanomaterial an excellent candidate for smart rsc.li/energy-advances windows and other electrochromic devices and applications. Department of Molecular Sciences and Nanosystems (DSMN), Ca’ Foscari University of Venice, Scientific Campus, Via Torino 155, 30172 Venezia Mestre, VE, Italy. E-mail: robertcarlyle.pullar@unive.it b Department of Engineering of Materials and Ceramics/CICECO-Aveiro Institute of Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal c CNR-NANOTEC – Istituto di Nanotecnologia, Strada Provinciale Monteroni, Lecce, Campus Ecotekne, 73100 Lecce, Italy. E-mail: roberto.giannuzzi@nanotec.cnr.it d Department of Mathematics and Physics, University of Salento, Via Monteroni, 73100, Lecce, Italy e Dipartimento di Matematica e Fisica E. de Giorgi, Università Del Salento, Campus Ecotekne, via Monteroni, Lecce, 73100, Italy such materials, their optical properties such as transmittance and reflectance can be reversibly modulated, causing a reversible darkening.1 In 1953, Kraus first described the electrochromism of tungsten oxide in an unpublished laboratory report at Balzers AG in Liechtenstein, describing electrically induced colour changes in thin films of tungsten oxide immersed in sulphuric acid.2,3 However, electrochromism was brought to public attention after Deb’s publications on amorphous and crystalline tungsten oxide films and the first WO3-based electrochromic devices between 1969–1973.4,5 Since then, studies of electrochromism have gained great attention owing to their extensive potential applications, e.g. architecture and smart windows to save energy, variable rear-view mirrors, sun-roofs and spectacles/ eyewear, e-papers and electrochromic displays (which are nonemissive, similar to LCD liquid crystal displays), dye sensitised solar cells for photovoltaics, energy storage materials such as Li-ion batteries, as well as thermal control of spacecraft and military camouflage/shielding against thermal sensors in the infrared between 3–14 mm.6–17 Electrochromic properties can be found in almost all the transition-metal oxides and their properties have been investigated extensively.2 During recent decades, numerous studies © 2022 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2022, 1, 321–330 | 321 1. Introduction Chromism is a change in colour depending on the change of electron states in the molecule or crystal, especially the p electrons and d electrons, the most common forms being thermochromic, photochromic and electrochromic materials. Once a stimulus (heat, light, electrical field) changes the electron states of a material, chromism will occur. Electrochromic materials manifest reversible and visible change in optical properties as the result of electrochemical oxidation or reduction at different electrical potentials. When a voltage is applied to a View Article Online Open Access Article. Published on 11 April 2022. Downloaded on 5/19/2022 11:36:06 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Paper have reported electrochromic materials, including metal oxides, organic molecules and conducting polymers. As result, many electrochromic materials have been developed, such as WO3,18 TiO2,19 NiO,20 supramolecular metallo-polymers,21 polyaniline,22 polythiophene23 and other polymers.24 As a suitable electrochromic material, TiO2 shows great potential in electrochromic research. TiO2 based films can possess a great degree of transparency to visible light, large specific surface area, large dielectric constant and capable charge transport properties. One common characteristic of electrochromic oxide materials is that they are wide-band gap semiconductors with a structure that allows easy electrical field induced proton (H+) or Li+ ion intercalation or insertion. TiO2 is such a material.25 In the case of TiO2, the insertion process is accompanied by a reduction process: TiO2 + xY+ + xe YxTiO2 (in which Y+ is the charged ion (e.g. H+ or Li+) and e is an electron released from the reduction of Ti4+ - Ti3+). Some of the Ti4+ is reduced to Ti3+ to compensate for the cation, and this leads to a colour change of the material (cathodic colouration), typically darkening, as there is a change of visible light absorption, ascribed to the electronic band gap changing from the UV (Eg = 3.0–3.2 eV) to the visible (Eg = 2.2–2.5 eV) range.26 This is due to the formation of a polaronic state, widening the band gap as the conduction band gains electrons.27–29 Being a semiconducting material, TiO2 is also suitable for photocatalytic applications under sunlight, and this combination of photocatalysis and electrochromism makes titania thin films promising candidates for self-cleaning smart window applications.30 Other related applications of transparent electrochromic titania in particular are in hydrogen sensing, water photolysis and photoelectrochemical devices.11 To fabricate a device that is transparent in the unbiased state, the active oxide layer is usually put on a conductive glass, and another conductive electrode is mounted on top. The space between the electrodes is filled with an electrolyte that contains the ions to be inserted.11 The conductor must be transparent, combining excellent electrical conductivity and very low optical absorption, but many oxide conductors such as indium tin oxide (B10% Sn-doped In2O3, ITO) often require a heated substrate for optimum properties. For this reason, fluorinedoped tin oxide (FTO) is often preferred. A typical electrolyte in such devices is LiClO4 in propylene carbonate, enabling the insertion of Li+ ions into the TiO2. The nanoporosity of the electrochromic and counter-electrode films must also be well-controlled, and the electrolyte should not allow leakage currents between the electrochromic and counter-electrode layers. Ideally, devices should also have long-term durability with voltage and current changes during colouring/bleaching.12 Sol–gel was the first process investigated to make thin films of amorphous or crystalline nanoparticles (NPs) of electrochromic TiO2, exhibiting a large optical transmission variation in the UV, visible or infrared range and acceptable kinetics under H+ or Li+ insertion.31 The first reports were in 1989–1992,32,33 in which the electrochromic properties of the coatings were rather poor, with a grey colour. Superior electrochromic TiO2 NP thick films (3.5–4 mm thick) were reported in 1994, made by 322 | Energy Adv., 2022, 1, 321–330 Energy Advances spreading a paste of 15 nm colloidal TiO2 NPs on conducting glass, which after autoclaving at 200 1C and firing at 450 1C/30 in in air formed crystalline and highly porous anatase.34 This showed reversible Li intercalation accompanied by an intense colour change from transparent to dark blue, which contrasted with anatase films prepared by conventional methods which were unable to intercalate Li ions to any significant extent. In 1999 spin-coated sol–gel TiO2 thin films (80–150 nm) were used to produce amorphous and anatase TiO2 films on ITO-coated glass, and the charge capacity and coloration efficiency of the amorphous film was found to be greater than that of the anatase film, a trend which has often been observed, and attributed to the greater porosity of the amorphous TiO2 facilitating Li+ intercalation. However, the electrochromic processes were quite slow, taking tens of seconds to colour or bleach.35 Nanocrystalline-TiO2 anatase thin films on ITO prepared by a sol–gel dipping method and annealed at 480 1C/3 h reported in 2003 exhibited a good reversible coloration and bleaching process,36 but the full coloration response time (typically the time passed until the colouration or bleaching reaches 90% of its maximum) was still found to be large at around 45 min. Another study in 2009 demonstrated that amorphous TiO2 sol–gel derived films annealed at 300 1C, which still contained some OH groups, on Sb-doped SnO2 coated glass, had a poor initial transmittance and showed only moderate colouration, which was even worse for anatase films.37 More recent results on sol–gel TiO2 films still show rather poor results, such as 200–300 nm spin coated anatase films (6 deposited layers, with heating to 250 1C between each coating) which had an initial transmittance of B80% in the visible and NIR region that reduced to B50% when coloured,30 and amorphous TiO2 films switching between B80% transmittance when bleached and 60% transmittance when coloured.38 No switching times were given for either of these materials. Clearly, purely sol–gel derived TiO2 electrochromic materials still need a lot of development and optimisation. In an attempt to improve sol–gel films, amorphous and anatase sol–gel derived TiO2 films (600–332 nm thick) were aged with diethanolamine as a stabilising agent for one week prior to gelation and coating, exhibiting an enhanced diffusion coefficient, ion storage capacity and optical modulation in comparison to films prepared from a freshly prepared sol. The 332 nm thick porous anatase films produced at 400 1C were the best, but still had slow colouration times of several minutes.39 In this paper we report the electrochromic properties of a thin film of TiO2 nanoparticles (NPs) produced via a simple, green, aqueous sol–gel process. This was spin-coated on FTO-glass and heated to 430 1C for 30 min to produce a transparent 100–120 nm thick electrochromic film, which exhibited very good rapid and reversible colouration demonstrated by vis-NIR spectrometry, cyclic voltammetry and chronoamperometric curves. It is superior to any previously reported sol–gel derived electrochromic titania films. 2. Results and discussion The pure titania sol and NPs have been fully characterised and their physical and crystallographic properties discussed © 2022 The Author(s). Published by the Royal Society of Chemistry View Article Online Open Access Article. Published on 11 April 2022. Downloaded on 5/19/2022 11:36:06 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Energy Advances previously.40,41 To summarise, when calcined at 450 1C/2 h the pure TiO2 NPs consisted of 25.1 wt% anatase, 37.7 wt% rutile, 10.3 wt% brookite and 26.9 wt% amorphous phases. All of these have been shown to be useful electrochromic phases, with brookite,42 rutile43 and porous amorphous35,37,38 phases all being shown to have some advantages over anatase, and rutile nanowires decorated with anatase NPs having enhanced optical contrast, coloration efficiency and transient current density.44 Hence, a material with a combination of all of these phases has great potential. The calculated crystallite sizes of these phases in our pure TiO2 NPs were 9.9 nm for anatase, 14.1 nm for rutile and 3.2 nm for brookite,45 although the actual particle size ranged from 10 nm up to 50 nm, and they were heavily clustered.40 There was no significant change in the crystal phases of the titania produced with PAA compared to that without. When spin coated onto the FTO-glass substrates, the TiO2 films produced were opaque due to agglomeration of the NPs, and hence not suitable for electrochromic devices. Therefore, 5 wt% of PAA surfactant was added to the sols and the films produced as before. The PAA prevented agglomeration during coating, but was then burnt out during annealing at 430 1C, leaving a very even, transparent TiO2 film, probably with increased porosity due to the loss of the PAA. The as-prepared transparent film after annealing at 430 1C/30 min is shown in Fig. 1e. SEM images of the as-produced film after annealing at 430 1C/30 min are shown in Fig. 1a. The TiO2 film presented a smooth surface, with a few larger fissures tens of microns in length from the drying process, but as expected, any nanoparticles and mesopores were below the resolution of the SEM instrument (i.e., o30 nm). This was confirmed by TEM analysis carried on the scratched flakes of the film (Fig. 1b), showing that the thin film is made of NPs sized about around 10 nm, partially fused to each other at the edges. The cross-section of the annealed film is shown in Fig. 1c, and the depth profile demonstrating that the thickness is 100–120 nm is shown in Fig. 1d. The high transparency and homogeneity of as-produced thin films can be observed in Fig. 1e. As also seen in Fig. 1e, the TiO2 film exhibited strong electrochromic properties when subjected to a voltage, possessing a high degree of colouration and darkening under an applied voltage, becoming completely opaque as the voltage increased. 2.1 Visible-NIR spectra, transmittance, optical contrast (DT) and optical density (DOD) The transmittance spectra were measured in situ with a threeelectrode set-up, by applying a bias of +0.1 V and 1.5 V for 120 s. The visible-near IR (vis-NIR) spectra of the film between 320–1650 nm in the coloured and bleached states are shown in Fig. 2a. The TiO2 film showed a high degree of transparency with 86–95% transmittance across the entire 400–1650 nm range. As an increasing voltage was applied, the film became progressively darker, and Fig. 2a shows the reduction in transmittance with an application of 1.5 V. A transmittance of below 27% was observed in the entire visible region, falling to as low at 8% at around 700 nm where the greatest coloration occurred. In the NIR region between 800–1650 nm a transmittance © 2022 The Author(s). Published by the Royal Society of Chemistry Paper Fig. 1 (a) SEM image of the sol–gel derived TiO2 thin film, as prepared and annealed at 430 1C/30 min. (b) TEM image of scratched portions of the sol–gel derived TiO2 thin film. (c) SEM image showing cross-section of the film. (d) Depth profile of the film, 100–120 nm thick. (e) Photographs of initial transparency, and increasing colouration with increasing applied voltage, of the sol–gel TiO2 electrochromic thin film (these are not the samples used for electrochromic measurements). of between 9% to 60% was observed, the value increasing with greater wavelengths. To the eye, the electrode appeared black in the coloured state with an applied bias of 1.5 V, able to effectively shield the visible radiation and a significant portion of the initial NIR region. A bias of only +0.1 V was sufficient to bleach the film back to the fully transparent state. The optical contrast of an electrochromic material is given by its transmittance modulation (DT): DT = Tb Tc (1) where Tb = transmittance at bleaching, and Tc = transmittance at colouration, all in % transmittance. The DT at 550 nm was 69.7%, at 700 nm it was a maximum of 86%, and the overall DT between 400–1650 nm was B60%, as shown in the inset in Fig. 2a. The stability of the film over 500 cycles is demonstrated in Fig. 2b, with no significant change in colouration or bleaching behaviour after 500 cycles. Another measure of the colour change between bleached and coloured states is the change in optical density (DOD), which is given by: DOD = ln(Tb/Tc) (2) The DOD at 550 nm was 1.4, and at 700 nm it was 2.4, both with an applied bias of 1.5 V for 120 s. Energy Adv., 2022, 1, 321–330 | 323 View Article Online Paper Energy Advances Open Access Article. Published on 11 April 2022. Downloaded on 5/19/2022 11:36:06 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. coloured state (red curve, 1.5 V). Dashed lines indicate the transmittance range in the bleached state (86–95%). Inset shows the transmittance modulation (DT, optical contrast). (b) Change in transmittance of coloured and bleached states after 500 cycles. (c) Cyclic voltammogramms recorded at a scan rate of 1 mV s 1 between +0.1 and 1.5 V, for 4 cycles. Inset shows change after 500 cycles. (d) Inserted charge vs. time with a constant applied bias of 1.5 V. Many reports on transmittance, DT and DOD of TiO2 electrochromics are very selective in the wavelengths they choose to analyse, reporting those with the optimum values, and giving no details over a wide range of wavelengths. Most reported sol– gel anatase TiO2 films have rather poor transmittance modulation with DT B20–30%, even at the optimum selected wavelengths,30,35,37–39 without the addition of additives such as chromophores or other ions. For example: 80–150 nm thick films had DT at 630 nm of 18% and 10% when annealed at 150 1C and 400 1C, respectively;35 films annealed at 300 1C had DT = 27%, 20% and 10% at wavelengths of 750, 600 and 550 nm, respectively;37 films annealed at 250 and 400 1C had DT values between 550–750 nm of 20% and 10%, respectively;38 a 200 nm thick film annealed at 400 1C had DT = 50% at 550 nm;39 a 230 nm thick film had DT up to B52% at 550 nm when bias was applied for 45 min.36 Aged TiO2 sol–gel films (332 nm thick) proved to be a bit better, with peak transmittance of 80% at 550 nm, 65–80% over the 400–800 nm range, and DT of up to 50% at 550 nm.39 Our films have DT values superior to all previously reported sol–gel films, despite their small thickness of only 100–120 nm, which creates an initial high transparency in the bleached state, but makes their subsequent high degree of colouration even more impressive. In particular, our TiO2 films exhibit high DT values of Z60% over the range of 400–1200 nm. This may be at least partly due to the fact that they consist of a combination of anatase, rutile, brookite and amorphous phases of TiO2, which have all been shown above to have individual advantages and contributions to the electrochromic process (for example, brookite nanoneedle films had a reported DOD = 0.85 at 600 nm, although this value was obtained using a log, and not ln, conversion42). Early sol–gel films were reported with low DOD values of 0.18 for amorphous and 0.12 for anatase TiO2.31,35 A 280 nm sol–gel anatase film had a reported DOD of 0.55 at 550 nm, with a stated DT of 30%,30 and the best DOD values previously reported for sol–gel TiO2 films were 1.7 at 550 nm and 1.57 at 700 nm.36 Our DOD value of 1.4 at 550 nm is comparable to this, and the maximum DOD of 2.4 at 700 nm is greatly superior. Our films are compared to all previously published sol–gel titania films, as well as some of the best titania films produced by other methods, in Table 1. 2.2 Cyclic voltammetry, inserted charge and colouring efficiency (CE) Fig. 2 (a) Vis-NIR transmittance spectra of the sol–gel TiO2 based electrode (thickness = 100–120 nm) in bleached state (blue curve, +0.1 V) and 324 | Energy Adv., 2022, 1, 321–330 The cyclic voltammogram (CV) is shown for the TiO2 film in Fig. 2c, taken at a scan rate of 1 mV s 1 in a 1 M LiClO4 based electrolyte (total charging time = 25 min). The first four CV © 2022 The Author(s). Published by the Royal Society of Chemistry View Article Online Energy Advances Paper Open Access Article. Published on 11 April 2022. Downloaded on 5/19/2022 11:36:06 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Table 1 Comparison of the electrochromic sol–gel titania films reported here with previous sol–gel films and other electrochromic titania films. Tb = transmittance at bleaching, Tc = transmittance at colouration, DT = transmittance modulation, DOD = change in optical density, QC = cathodic charge insertion, CE = colouration efficiency, tc = time required to achieve 90% of full colouration modulation, tb = time to recover by 90% from full modulation via bleaching. Wavelengths at which measurements were made or times for which a potential were applied are given where available Ref. Thin film preparaYear tion technique Film thickness (nm) Tb (%) 35 1999 Sol–gel 80 (heated to 150 1C) Tc (%) DOD QC (ln Tb/Tc) (mC cm 2) DT (%) 90 at 630 nm 72 at 630 nm 18 at 630 nm 0.18 at 630 nm 150 (heated to 85 at 630 nm 75 at 630 nm 10 at 630 nm 0.12 at 450 1C) 630 nm 36 39 2003 Sol–gel (heated to 480 1C) 2005 Sol–gel (heated to 400 1C) 230 200 64 at 550 nm 12 at 550 nm 52 at 550 nm 1.7 at 550 nm 58 at 700 nm 13 at 700 nm 45 at 700 nm 1.57 at 700 nm 70 at 550 nm 20 at 550 nm 50 at 550 nm — 14.8 9.93 at — 630 nm 13.95 at 850 nm 6.7 7.36 a 630 nm 8.07 at 850 nm 56 for 45 min 15 at 525 nm — 2009 Sol–gel (heated to 300 1C) 30 2014 Sol–gel (heated to 280 400 1C) 380 2014 Sol–gel coated on TiO2 nanowire array (heated to 450 1C) 2015 Sol–gel — 44 38 — This 2022 Sol–gel (heated to work 430 1C) 100–120 34 3500–4000 49 50 51 42 52 53 54 1994 Colloidal 15 nm NPs autoclaved at 200 1C, then heated at 450 1C 2011 Doctor-bladed 15 nm TiO2 NPs (heated to 450 1C) 2014 DC-magnetron sputtering 2014 Metal–organic chemical vapour deposition 2016 Hot filament metaloxide vapour deposition 2017 Thermoionic vacuum arc method 2020 One-pot nonaqueous colloidal nanosynthesis 2020 Linear accelerated e-beam evaporation (LAeB) — 70 75 85 84 — at at at at 550 750 550 800 nm nm nm nm 7.68 80 at 550– 750 nm (annealed at 250 1C) 70–75 at 550– 750 nm (annealed at 400 1C) 93 at 550 nm — — — 23.3 17.56 27.6 2.1 20 at 550– 750 nm — 5.13 — — 60–65 at 550– 10 at 550– 750 nm 750 nm — 2.95 nm nm nm nm 60 at 550– 750 nm nm nm nm nm nm nm nm 5 0.55 at 550 nm — 550 750 550 800 550 600 750 550 750 550 800 11.4 at 179 550 nm 19.2 at 750 nm 10 at 550 nm — 50 at 750 nm — at at at at at at at at at at at — — 40 20 65 52 10 20 27 30 55 20 32 — 6 for 5 min 70 at 700 nm 10 at 700 nm 60 at 700 nm 37 CE (cm2 C 1) tc (s) tb (s) — — — 23 at 550 nm 70 at 550 nm 1.4 at 550 nm 94 at 700 nm 8 at 700 nm 86 at 700 nm 2.4 at 700 nm — — — — 50 for 25 min 55.9 at 55 550 nm 22 for 120 s 17.6 for 60 s 100 for 14 s 20 at 780 nm 14 15 600 78 at 550 nm 10 at 550 nm 68 at 550 nm 2.0 at 80 at 750 nm 5 at 750 nm 75 at 750 nm 550 nm 61 for 2 s 33.7 at 550 nm 2 2 300 90 85 90 95 28.3 — — — — — — — 240 at at at at 550 750 550 750 nm nm nm nm 35 15 35 10 at at at at 550 750 550 750 nm nm nm nm 55 70 55 85 at at at at 550 750 550 750 nm — nm nm — nm 120 — 78 at 600 nm 11 at 600 nm 67 at 600 nm 0.85 at 85 at 750 nm 22 at 750 nm 63 at 750 nm 600 nm 3.76 226 at 600 nm 13.15 5.14 110 65 78 95 95 7.3 18.6 at 550 nm 38.2 at 633 nm 1100 120 at at at at 550 800 633 800 nm nm nm nm 57 at 550 nm 70 at 800 nm 1 at 633 nm 1 at 800 nm 8 at 550 nm — 8 at 800 nm 94 at 633 nm — 94 at 800 nm 76 at 550 nm 66 at 550 nm 10 at 550 nm — 70 at 700 nm 65 at 700 nm 5 at 700 nm scans compared in Fig. 2c, and the shape and current values became stable after only 3 cycles, demonstrating the electrochemical stability of these electrodes, and a small reduction of the area under the curve of only about 8% occurred. The inset in © 2022 The Author(s). Published by the Royal Society of Chemistry — 5.41 — — 35.1 9.6 26 at 550 nm — — Fig. 2c demonstrates that this stability continues for 500 cycles. Such a small capacitance loss is usually attributed to irreversible phase changes, ion trapping at the TiO2 defect sites, or an irreversible reaction of Li+ with adsorbed water molecules.46–48 Energy Adv., 2022, 1, 321–330 | 325 View Article Online Open Access Article. Published on 11 April 2022. Downloaded on 5/19/2022 11:36:06 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Paper Energy Advances The TiO2 film showed a cathodic colouration peak of 2.5 mA cm 2 at 1.3 V, and an anodic bleaching peak of +2.5 mA cm 2 at a potential of 0.9 V. These values are around the regions expected for anatase based electrodes (although rutile and brookite are similar), attributable to the intercalation/deintercalation processes of lithium ions into the anatase lattice: TiO2 + xLi+ + xe 2 LixTiO2 where x is the insertion coefficient, usually close to 0.5. The sizeable peak separation between the cathodic and anodic peaks of 0.37 V demonstrates the irreversibility of the electrochemical process with the application of a bias. The insertion charge during the 25 min cathodic colouring process (Qc, the area under the cathodic peak) was 50 mC cm 2. However, even with only 60 s with an applied bias of 1.5 V, a high Qc of 17.6 mC cm 2 was achieved, increasing to high Qc of 22 mC cm 2 after 120 s (Fig. 2d). Such high values are probably due to the small particle size and, hence, large surface area of these films. More porous films can also accommodate more Li+ intercalation, and thus have a greater charge inserted, although that does not necessarily mean improved materials regarding DT, DOD, colouration efficiency or switching times. Typical reported values for cathodic charge insertion in sol– gel TiO2 films are Qc = 6.7 mC cm 2 after 75 s in anatase films up to 150 nm thick (and 14.8 mC cm 2 in amorphous films),35 and Qc = 7.68 mC cm 2 in aged sol–gel anatase films.39 Despite their high DOD, brookite nanoneedles only had a reported Qc of 3.76 mC cm 2.42 Amorphous sol–gel films which still contained organic precursors after heating to 150 1C and had a stated particle size of 7 nm had high Qc values of 29.8 mC cm 2 ( 2.0 V in 40 s), although this had decreased greatly to 5.1 mC cm 2 when heated to 250 1C (still amorphous but organic component lost), and 3.0 mC cm 2 when heated to 400 1C (anatase).38 The colour change was also reported as very poor in the films which still contained the organic phase, heated below 250 1C, and they still had very low DT values (o20%) and poor transparency once heated to 250 1C. Clearly our sol–gel TiO2 films have a very high insertion charge, with 17.6 mC cm 2 being achieved after a bias of 1.5 V was applied for only 60 s, increasing to 50 mC cm 2 after 25 min. The only comparable value for a sol–gel titania film was 56 mC cm 2 after a large bias of 3.5 V had been continuously applied for a long colouration time of 45 min – this fell to 36, 15 and 6 mC cm 2 after colouration times of 30, 15 and 5 min, respectively.36 The coloration efficiency (CE) is another fundamental parameter of an electrochromic device, and it is used to compare performances. CE is given by following equation: CE = DOD/Qc (3) in units of cm2 C 1, at a given wavelength. High CE values correspond to a large optical modulation per amount of charge inserted/extracted, which can be regarded as an advantage in terms of time responsivity of the device and long-term stability with cycling. Calculation of the CE parameter from the slope 326 | Energy Adv., 2022, 1, 321–330 of the linear region of the plot in Fig. 3a gave a CE value of 55.9 cm2 C 1 for our sol–gel TiO2 film at l = 550 nm. CE values such as 15 cm2 C 1 at 525 nm,36 10 cm2 C 1 at 550 nm and 50 cm2 C 1 at 750 nm,37 9.93 cm2 C 1 at 630 nm and 13.95 cm2 C 1 at 850 nm in amorphous films and 7.36 cm2 C 1 at 630 nm and 8.07 cm2 C 1 at 850 nm in anatase films,35 and 11.4 cm2 C 1 at 550 nm and 19.2 cm2 C 1 at 750 nm,39 have been reported previously for anatase-containing sol–gel TiO2 films. Our film has the highest CE value yet reported for a sol–gel TiO2 film (at any wavelength) when measured at 550 nm, and as DOD is much greater at around l = 700 nm, it is expected that CE values will be much greater at this wavelength as well. See Table 1 for comparison with other sol–gel and non-sol–gel titania films. 2.3 Chronoamperometry and electrochromic switching kinetics The chronoamperometric plot is shown in Fig. 3b. Measurements were carried out by applying a square wave voltage of +0.1/ 1.5 V for 120 s, and by measuring the corresponding transmittance value at 550 nm. The switching time is defined as the time required for a system to achieve 90% of its full modulation for colouration (tc90%). Recover from 90% of its full modulation for bleaching (tb90%). For colouration tc90% = 55 s, while for the bleaching process tb90% is much quicker at 10 s. This substantial difference between the colouring and bleaching kinetics is related to an asymmetry between the intercalation and de-intercalation processes. Intercalation in anatase-based titanium oxide occurs mainly at the electrode surface in a ‘‘top-down’’ filling mechanism. This means that Li+ intercalation leads to a build-up of LixTiO2 at the electrode/ electrolyte interface, which slows down the diffusion process. However, in the de-intercalation process Li+ is easily released from the top of the electrode into the electrolyte, enabling the diffusion of Li+ from the ‘‘bottom’’ of the electrode.55 Sol–gel electrochromic anatase TiO2 films typically have bleaching and colouration kinetics in the order of tens of seconds,32 tc90% = 136 s and tb90% = 7 s,39 or several minutes.36 Our sol–gel films clearly compare to the best of these for bleaching and colouration kinetics. This makes our sol–gel TiO2 film an excellent material for rapid electrochromic switching in smart windows. 3. Experimental 3.1 Synthesis of sol–gel titania films The synthesis of these aqueous sols is based on our previously published syntheses for pure titania sols and NPs.40,41 0.2 moles of titanium-iso-propoxide (Merck, 97%) was added to 400 ml of iso-propanol and stirred at 800 rpm. 5.6 ml of concentrated HNO3 (Fluka Puriss PA, 65 wt%) in 20 ml distilled water were added to 50 ml IPA, and this was then added dropwise to the Titanium-iso-propoxide with stirring. This showed no initial precipitation, but after approximately half of the acidic solution was added a thick white precipitate formed, and the stirring © 2022 The Author(s). Published by the Royal Society of Chemistry View Article Online Open Access Article. Published on 11 April 2022. Downloaded on 5/19/2022 11:36:06 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Energy Advances Paper appeared slightly more viscous and paler white after PAA addition. This sol could be dried to a powder at 80 1C for transport/storage, and then redispersed in water. To obtain the nanostructured thin films, 0.05 g ethylcellulose (30–70 mPa s, Sigma Aldrich) dissolved in methanol (10% wt/wt) was added to 5 ml of the TiO2 sol and stirred for 30 minutes at room temperature. Then, the resulting sols were spin coated on 1 cm2 Fluorine-doped tin oxide (FTO, 15 O sq 1, Kintec) coated glass substrates at 1500 rpm/30 s and annealed at 430 1C/30 min in air. The resulting 100–120 nm thick film was highly transparent and homogeneous. 3.2 Physical, electrochemical and spectro-electrochemical characterisation Fig. 3 (a) Plot of optical density variation as a function of charge density monitored at 550 nm (bias = 1.5 V) for the sol–gel TiO2 film, giving a CE of 55.9 cm2 C 1. (b) Chronoamperometric transmittance variation curve monitored at the wavelength of 550 nm, obtained by applying a square wave voltage of 1.5 V (colouration) and +0.1 V (bleaching) 120 s each. tc90% and tb90% are the times required to reach 90% of the maximum transmittance modulation for colouration and bleaching. rate had to be increased to B1200 rpm. After about 34 of the acidic solution had been added, the precipitate became less viscous, resembling milk, and stirring could be reduced to 800 rpm again. Total addition time was about 30 min. This mixture was then placed on a rotary evaporator and the solvent removed at 60 1C, 100 mbar pressure, to produce a thick white gel. This was redispersed in 400 ml of distilled water, and then the water removed by rotary evaporator at 60 1C, 70 mbar pressure to produce a viscous but powdery yellow gel which never fully dried. Another 400 ml of distilled water was added, the gel redispersed, and the water removed at 60 1C, 70 mbar pressure until a viscous yellow gel had formed. Distilled water was added to make a total volume of 200 ml, and the gel redispersed to create a yellowish-white sol, 1 M concentration. To add 5 wt% polyacrylic acid (PAA) as a surfactant (–CH2CHCO(OH)–, Aldrich PAA N1 323667, Av. MW = 1800), 5 wt% was dissolved in water and added to the sol. The sol © 2022 The Author(s). Published by the Royal Society of Chemistry Low-resolution Transmission electron microscope (TEM) investigation of scratched flakes of the nanostructured thin film was performed with a JEM 1400Plus microscope, operating at an acceleration voltage of 120 kV. The film thickness was measured using a Veeco Dektak 150+ Profiler. The morphology of the films was analysed by Scanning Electron Microscopy (SEM) using a Carl Zeiss Auriga40 Crossbeam instrument. Electrochemical measurements of the prepared films were carried out in anhydrous 1 M LiClO4 in propylene carbonate electrolyte solution. A platinum foil served as the counter electrode (area = 4 cm2) and an Ag/AgCl electrode in 1 M LiClO4 in propylene carbonate was used as the reference. All potentials are referred to as the Ag/AgCl reference electrode in 1 M LiClO4 in propylene carbonate. The active area of the electrode was 1 cm  1 cm. Gold contacts, 150 nm thick with a 5 nm chromium adhesion layer, were prepared by thermal evaporation at one edge of the sample.56 All measurements were carried out within a potential window of 0.9 and +1.5 V determined with cyclic voltammetry. In situ optical spectra were recorded at several potentials after allowing for stabilisation of the optical signal, which required several minutes. The path length of the electrolyte was B5 mm during the measurements. Charge measurements between the potential limits were performed via chronopotentiometry, with a potential switched between 1.1 and +1.4 V for a 60 s holding time. Stability tests were performed via chronopotentiometry with 30 s holding time for the bleached and coloured state. Colouration efficiency was calculated from the slope of the optical density as a function of the charge density. All electrochemical measurements were performed with an AUTOLAB PGSTAT302N potentiostat and a Varian Cary 5000 UV-VIS-NIR spectrophotometer. Conclusions A pure TiO2 thin film (100–120 nm), made from a green aqueous sol–gel precursor, was spin coated on FTO coated glass and calcined at 430 1C/30 min. The film consists of a mixture of amorphous, anatase, rutile and brookite TiO2 phases. It exhibited excellent electrochromic properties over visible and NIR wavelengths when tested with an electrolyte of LiClO4 in propylene carbonate, enabling Li+ insertion and Energy Adv., 2022, 1, 321–330 | 327 View Article Online Open Access Article. Published on 11 April 2022. Downloaded on 5/19/2022 11:36:06 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Paper colouration with an applied bias between +0.1 V to 1.5 V. The film showed a high degree of transparency, exhibiting 86–95% transmittance between 400–1650 nm in the visible and NIR region. It underwent excellent coloration when a voltage was applied, with peak colouration around 700 nm, a transmittance of below 27% in the entire visible region, and between 9% to 60% in the NIR region, the value increasing with greater wavelength. It had a transmittance modulation (DT) of 69.7% at 550 nm, 86% at 700 nm and an overall DT between 400– 1650 nm of 60%. This resulted in a very large change in optical density (DOD) of 1.4 at 550 nm and 2.4 at 700 nm. This DOD at 550 nm compares to the best previously reported values for sol–gel TiO2, and the DOD at 700 nm and all DT values are superior to any previously reported sol–gel TiO2 films. Cyclic voltammograms had typical peaks for TiO2 at 1.3 V for colouration and 0.9 V for bleaching, which a high separation of 0.37 V between peaks. The charge density during cathodic colouration was the highest ever reported for anatase based sol–gel films after charging for 25 min at Qc = 50 mC cm 2, and was also extremely high after only 60 s and 120 s with an applied bias of 1.5 V giving Qc = 17.6 and 22 mC cm 2, respectively. Combined with the large DOD this led to a high colouration efficiency (CE) of 55.9 cm2 C 1 at 550 nm ( 1.5 V), the highest CE value yet reported for a sol–gel TiO2 film at any wavelength. They also possessed very rapid switching times for bleaching and colouring of tb90% = 10 s and tc90% = 55 s, comparable to the best previously reported sol–gel anatase-based TiO2 films. As this film has the highest reported DT, DOD and CE values ever reported for a sol–gel anatase-based TiO2 film, along with Qc, DOD550nm and tb90% and tc90% values virtually equivalent to the best reported, this makes it an excellent candidate material for smart windows and other electrochromic devices and applications. The next step will be to test the stability of these films over repeated electrochemical cycles, and explore their properties further into the IR region. Energy Advances Acknowledgements This work was developed within the scope of the FCT (Fundação para a Ciência e a Tecnologia, Portugal) project CICECOAveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. R.C. Pullar wishes to thank FCT grant IF/00681/2015. David Maria Tobaldi is grateful to Portuguese national funds (OE), through FCT, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19. The authors gratefully aknowledge Progetto FISR – C.N.R. ‘‘Tecnopolo di nanotecnologia e otonica per la medicina di precisione’’ – CUP B83B17000010001, Apulia regional project ‘‘MOSAICOS – MOSAici Interattivi eCO-Sostenibili’’, Cod. HOQ3PM3 – CUP B37H17004900007 and Apulia regional project ‘‘FONTANAPULIA – Fotocatalizzatori nanostrutturati e radiazione UV per un’acqua più pulita’’ Cod. WOBV6K5 – CUP B37H17005230007 for funding. References There are no conflicts to declare. 1 C. G. Granqvist, P. C. Lansåker, N. R. Mlyuka, G. A. 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