Steaming‐Assisted Conversion: A New Strategy for the Synthesis of Anatase TiO2, Nb, and W‐doped Anatase TiO2 2D Inverse Opal Films

Steaming‐assisted conversion route, a new strategy, is first adapted for the synthesis of highly crystallized anatase TiO2 2D inverse opal (IO) monolayer films, and then to Nb‐doped TiO2 and W‐doped TiO2 2D IO monolayer films. Pure water, ammonia, or HCl solutions are used as a source of steaming vapor to convert dry films of amorphous TiO2 IO, NbCl5/TiO2, and WCl6/TiO2 composite IOs into anatase TiO2, Nb‐doped TiO2, and W‐doped TiO2 IO films. This new strategy renders possible the doping of metal ions within the framework of the anatase TiO2 IO films under low temperature and liquid‐free conditions. Further, the ordered array structure of the IO films is also effectively retained. The low steaming conversion temperature allows high dopant rates of homogeneously distributed heteroatoms, resulting in Nb doping as high as ≈34%. The thus prepared TiO2, Nb‐doped TiO2, and W‐doped TiO2 anatase IO films are successfully used as active electrodes in the fabrication of electrochromic devices.


Introduction
Inverse opal (IO) of transition metal oxides (TMOs) have received a great deal of attention recently, due to their practical application in a broad variety of fields, including energy conversion/storage, sensors, photonics, and so on. [1,2][5][6] Nevertheless, the intrinsic drawbacks of TiO 2 make its sunlight exploitation efficiency far from satisfactory.The main reason is its wide optical bandgap (3.2 eV), [7] which limits its light-harvesting ability within the UV region.Considering that a great part of sunlight lies in the visible (43%) and NIR (52%) regions, [8] extending the light-harvesting ability of TiO 2 to visible or NIR regions is of fundamental interest.To achieve this goal, various strategies were developed, such as bandgap engineering, [9] hybridization with a narrow optical gap semiconductor, [10] or forming a composite with photosensitizer or plasmonic materials. [11]he Homogeneous introduction of metal ions into the TiO 2 lattice has been proven to be an effective way of tuning the electronic structure as well as the surface reactivity of TiO 2 . [12]By element doping, defects or disorders appear in the lattice, leading to intermediate states and therefore enhancing solar light harnessing.Many efforts have been made to introduce a high amount of dopant into the TiO 2 lattice while maintaining its original network with mitigated success.Most of the synthesis routes used so far were thermal post-treatment, a kind of ceramic route. [13]However, under thermodynamic equilibrium, nanoparticles are prone to annealing out dopants to minimize the total free energy, i.e., so-called "self-purification process", which leads to low dopant content or even exclusion of the dopant. [14,15]In contrast, low-temperature chemical methods, favor metastable compositions with a high amount of dopant. [16]hose low-temperature routes (below 400 °C), usually in solution phase, such as colloidal synthesis, solvo(hydro)-thermal synthesis, and sol-gel method, [17][18][19] make use of kinetics instead of thermodynamics, to achieve the nanocrystalline growth.Among them, solvo(hydro)-thermal synthesis access to high pressure and relatively higher temperature than the normal atmospheric boiling points of the solvent, showing extraordinary advantages in crystallization process. [20]Despite of it, similar to other solution based synthesis, it is important in solvo(hydro)-thermal synthesis to carefully balance dopant reaction kinetics with host nanocrystalline growth: the dopant precursor decomposition rate must be equal to that of the host precursor so dopant atoms uniformly incorporate within the host nanocrystalline lattice. [21,22]Therefore, it is of primary importance to judiciously select the reactive precursors. [20]nspired by our earlier experience in the synthesis of ferrite nanoparticles and zeolite by steaming-assisted conversion route, [23,24] we further extended this strategy to first synthesize highly crystallized anatase TiO 2 2D IO monolayer films and then to fabricate Nb-doped anatase TiO 2 and W-doped anatse TiO 2 2D IO monolayer films.
Differently from the traditional doping process in the solution phase, in this work, vapor from pure water, HCl, or ammonia solutions was used as a steaming reagent.NbCl 5 or WCl 6 solutions were used as dopant precursor, in which amorphous or partially crystallized TiO 2 IO films were dip-coated, resulting in composite precursor films for the fabrication of Nb-doped and W-doped anatase TiO 2 2D IO monolayer films.The composite precursor films were separated from the steaming solution using a Teflon support in a pressure vessel (Figure 1).This setup is sealed and heated in an oven, thereby allowing the vapor to react with the films.
This novel strategy combines the advantages of CVD and hydrothermal methods for the following reasons.First, by employing a vapor as a reactant, the reaction is conducted under a liquidfree environment, rendering flexible the selection of reactive precursors that is always a trouble with the solution-based doping route.Second, the autogenous vapor pressure, usually higher than atmospheric pressure, which is the case in solution-based routes, combined with high temperature, which is beyond the ability of usual solution routes, greatly benefits the crystallization of the metal oxide material.Third, in a solid/gas reaction, the highly ordered framework of the inverse opal precursor film could be retained during the doping reaction and the crystallization process, which is difficult to achieve under a solution-based route.This strategy can further be potentially adapted for the fabrication of other doped crystallized metal oxide materials.
To our knowledge, this is the first report of the synthesis of anatase TiO 2 , Nb-doped TiO 2 , and W-doped TiO 2 2D inverse opal monolayer films using the "steaming-assisted conversion" strategy.Considering that such process was realized simultaneously under a liquid-free environment and at low-temperature (180 °C), similar to the solution-based reaction, this strategy has great advantages by allowing a high amount of doping homogeneously distributed in a highly crystallized anatase phase.

Results and Discussion
In the following text, the synthesized films were named according to composition, steaming solution, processing temperature, and processing duration using letters and digits in the following way: T, NT, and WT refer to TiO 2 IO, Nb-doped TiO 2 IO, and Wdoped TiO 2 IO, respectively, followed by a dash symbol ("-") and a number indicating the steaming temperature in °C.The succeeding letter or letters refer to the steaming solution type.The letter "a" is used for ammonia, "ae" for ethanolic ammonia, "w" for pure water, and "aw" for aqueous ammonia.Finally, a number was added indicating the steaming duration time in "hours".For instance, sample NT-180aw24 is an Nb-doped TiO 2 IO steamed by ammonia aqueous solution (aw) at 180 °C for 24 h.

Fabrication of TiO 2 IO Films
2D TiO 2 /PS opal composite films were synthesized by using a TiOSO 4 aqueous solution as TiO 2 precursor.When dissolved in water, TiOSO 4 is partly hydrolyzed into H 2 TiO 3 or TiO 2 •H 2 O (Equation 1) and further converts into TiO 2 particles depending on the reaction conditions. [25]OSO 4 Such formed H 2 TiO 3 or TiO 2 •H 2 O stably exist in water as Ti(OH) 3 HSO 4 aqueous colloid.This stable aqueous colloid allows the fabrication of 2D TiO 2 /PS opal composite films under our earlier developed "dynamic hard template infiltration" strategy. [26,27]As illustrated in Figure 2, after the formation of PS spheres opal template monolayer obtained from PS spheres with a nominal size of 700 nm in diameter floating on the surface of the water (Figure 2a), the stable aqueous colloid precursor sol was injected underneath the opal monolayer (Figure 2b).This resulted in the formation of a Ti(OH) 3 HSO 4 /PS opal composite monolayer, still floating on the water surface (Figure 2c).This opal composite monolayer was allowed to self-deposit on quartz or ITO substrates by removing the water (Figure 2d).Afterward, a low-temperature steaming-treatment (65 °C for 4 h) by an ammonia ethanolic solution (Figure 2e) allowed the conversion of the Ti(OH) 3 HSO 4 /PS opal composite monolayer into an amorphous TiO 2 /PS opal composite film according to the following reaction: Ti(OH) 3 HSO 4 → Ti(OH) + 3 → TiO 2 H 2 O (Figure 2f).The use of an ammonia ethanolic solution as steaming vapor source is the key to ensure that the IO film structure is preserved during the removal of the PS opal template by THF, since the Ti(OH) 3 HSO 4 in Ti(OH) 3 HSO 4 /PS opal composite film during the first steps of the process is highly soluble because of the presence of sulfate ions still complexed with Ti 4+ as ligand. [25]lso, the use of ethanol as solvent instead of water or a mixture of water and ethanol (ratio of 1:2) in ammonia solutions was adopted to avoid small parts of the film to be peeled off the ITO substrates (Figure S1, Supporting Information).However, this peeling-off was not observed when the films were coated on quartz substrates and treated with ammonia aqueous or ammonia hydro-ethanolic steaming solutions.Only samples steamed by ammonia ethanolic solution demonstrated its advantage in keeping the film intact on either quartz or ITO substrates.Moreover, the steaming strategy established its advantage over the hydrothermal process when quartz substrates were used: after the hydrothermal treatment, a large area of the film peeled off the substrate (Figure S2b, Supporting Information), while after steaming, the film remained intact showing its structural opal iridescent color (Figure S2d, Supporting Information).
Once an amorphous TiO 2 /PS opal composite film was obtained (Figure 2f), a chemical etching was conducted to remove the PS opal template, leaving the amorphous TiO 2 IO film T-65ae4 (Figure 2g).The crystallization of this amorphous film was attempted by using various steaming solutions such as water, hydrochloric (HCl) acid (0.2 and 0.5 mol L −1 ), and ammonia (0.8 and 2.0 mol L −1 ) aqueous solutions (Figure 2j).
Figure 3b-g shows the Raman spectra of the amorphous TiO 2 IO film (T-65ae4) after steaming with various steaming solutions.The Raman spectrum of the amorphous sample (Figure 3a) before steaming does not show any Raman band, indicating its amorphous nature.Also, no residue of the PS opal template is observed in the spectral recorded range, showing that all the PS opal template was removed by chemical etching.After steaming the amorphous film by water at 160 °C for 4 h, the Raman spectrum revealed a weakly crystallized anatase TiO 2 IO film (T-160w4) as evidenced by the broad main anatase Raman peak located at 148 cm −1 (Figure 3b).In order to obtain a complete crystallization of the films, the steaming temperature was raised to 180 °C for 24 h.Under these conditions, after steaming by water, the Raman spectrum shows a well-crystallized anatase TiO 2 IO film (T-180w24) with Eg modes at 146 and 638 cm −1 , B1g modes ≈395 and 515 cm −1 (Figure 3e).Steaming with HCl solutions (0.2 and 0.5 mol L −1 ) also resulted in complete crystallization of the films as evidenced by the sharp main anatase Raman modes appearing at 146 and 145 cm −1 , respectively (Figure 3d,c).However, when a 0.8 mol L −1 ammonia aqueous solution was used, the film showed only very weak crystallization as evidenced by the weak broad anatase Raman peak at 146 cm −1 (Figure 3f), while it remained amorphous when steamed by a 2.0 mol L −1 ammonia aqueous solution (Figure 3g).

Synthesis of Nb-Doped TiO 2 and W-Doped TiO 2 IO Monolayer Films
For doping of TiO 2 IO film with Nb or W ions, dip-coating was first needed using NbCl 5 and WCl 6 ethanolic solutions, respectively (Figure 2h,i).Though amorphous TiO 2 IO film (T-65ae4) as precursor film, was stable after dip-coating in WCl 6 solution, it was destroyed when dipped in NbCl 5 solution (Figure S3, Supporting Information).However, when a partially crystalized TiO 2 IO film (T-160w4) was used as precursor film instead, no peeling off occurred after dipped-coating in NbCl 5 solution.NbCl 5 easily hydrolyzes into Nb(OH) 5 once it contacts with water, such as the aqueous vapor from the atmosphere or the water on the surface of the amorphous TiO 2 IO film.Therefore, the process during the whole dip-coating could be understood as follows: Nb(OH) 5 could dissolve in water or ethanol that is the solvent of the NbCl 5 solution.The formed Nb(OH) 5 changes the interaction between film and substrate, leading to the peeling-off of the film.
Instead, WCl 6 is more stable than NbCl 5 , which only hydrolyzes into hot water or organic solvent.Therefore, under present condition, W 6+ is adsorbed on the film in the original form of WCl 6 .Therefore, the interaction between film and substrate is kept constant without peeling off.
The coated samples were then subjected to a steaming treatment using water, 0.5 mol L −1 HCl, and 0.8 mol L −1 ammonia aqueous solutions at 180 °C for 24 h.Both Nb-doped and Wdoped TiO 2 IO films were found crystallized under the various steaming solutions used (Figures S4 and S5, Supporting Information).Figure 4 shows the Raman spectra of the Nb-doped and W-doped TiO 2 IO films after steaming using 0.8 mol L −1 ammonia aqueous solution (NT-180aw24) and water (WT-180w24), re-spectively.NT-180aw24 and WT-180w24 show only the characteristic modes of a well-crystallized TiO 2 in the anatase phase at 148, 396, 515, 634 cm −1 and 147, 393, 520, 626 cm −1 , respectively.
In contrast to the Raman spectrum of commercial anatase TiO 2 powder that has a main Raman pick position at 142.5 cm −1 and a full width at half maximum (FWHM) of 7.5 cm −1 , the corresponding mode for TiO 2 (T-180w24), Nb-doped TiO 2 (NT-180aw24), and W doped TiO 2 (WT-180w24) IO films is blue shifted at 146, 148, and 147 cm −1 and broadened to 32.7, 28.3, and 27.1 cm −1 , respectively.These features present the main aspect of nanocrystalline TiO 2 .The crystallite sizes were determined independently based on Raman and X-ray diffraction (XRD) data.Figure S6 (Supporting Information) shows the XRD patterns of T-180w24 , NT-180aw24, and WT-180w24 samples after steaming.Anatase and ITO diffraction peaks are identified.Due to a peak at q = 10.7 nm -1 , we assume that the sample T-180w24 may contain TiO 2 (B), a monoclinic variant [Marchand 1980]. [28]n additional, relatively narrow peak in this sample at 6.3 nm -1 has not been identified, but may be a reaction product involving the ITO substrate.A residual amorphous component in the spectrum of T-180w24 originates most likely from the glass substrate.For XRD, the peak widths are used in conjunction with the Scherrer equation.The Raman estimates are based on literature data that link the crystallite to the main peak position and the FWHM of anatase TiO 2 peaks. [29]The results for T-180w24 are 5.6 nm (Raman) and 7.0 ± 0.9 nm (XRD), for NT-180aw24 6.5 nm (Raman) and 5.3 ± 2.0 nm (XRD), and for WT-180w24 6.9 nm (Raman) and 8.4 ± 1.1 nm (XRD).
Figure S7a-f (Supporting Information) shows the optical photos of the TiO 2 IO films and corresponding Nb-doped and Wdoped films before (a,c,e) and after steaming treatment (b,d,f).It is evident from Figure S7 (Supporting Information) that all the films show iridescent colors inherited from the highly ordered pore arrays, revealing that after coating with doping solutions and crystallization by steaming the IO structure of the films is not disturbed.This is also confirmed by the SEM images of TiO 2 , Nb-doped, and W-doped TiO 2 IO films after steaming displaying a large area honeycomb structure with hexagonal symmetry (Figure 5a-c).The inverse opal periods and pore sizes of the three samples were estimated from the SEM images using the Nanomeasure software.The period and pore size are 709.5 and 552.6 nm for TiO 2 (T-180w24), 710.4 and 564.3 nm for Nb-doped TiO 2 (NT-180aw24), 719.6 and 562.1 nm for W-doped TiO 2 (WT-180w24).Such results show that the structure of the inverse opal was perfectly replicated from the traces of the spheres left in the form of spherical air cavities after the removal of PS spheres by THF etching.Based on Nanomeasure software, the size of the PS spheres, and accordingly the period of PS opal, did not correspond to the manufacturer's nominal value of 700 nm for PS sphere's diameter.From Nanomeasure, the period of the PS opal film was found to be 653.3nm (Figure S8, Supporting Information).The lateral shrinkage was estimated from the ratio of the diameter of the IO macropores to the diameter of the template spheres (1-[Φ IO macropore /Φ template ]) × 100%. [30]After crystallization by steaming, the lateral shrinkages were 15% for T-180w24, 14% for NT-180aw24, and 14% for WT-180w24, much lower than 30% obtained by the conventional method of IO fabrication. [30]Conventional route uses calcination at high temperature of 450 °C or higher to convert Ti-containing IO precursor into crystallized TiO 2 and simultaneously remove the PS opal template.It brings about a great shrinkage upon the removal of PS by sintering due to the interaction between PS spheres template and Ti-containing IO precursor.In the present study, PS spheres were first removed under low temperature, which renders unchanged the period (or micropores size) of the composite opal.Further, the crystallization was done under a kind of solid-state route with steaming at low temperature (180 °C).Such low temperature solid-state-like route of steaming assures minimum shrinkage and unchanged IO structure.From the cross-sectional SEM observations, the thickness for these three samples is measured to be ≈592 nm for T-180w24, 617 nm for NT-180aw24, and 513 nm for WT-180w24.
It is also seen from Figure 5 and Figures S8 and S9 (Supporting Information) that with the exception of minor disorders due to the inevitable sporadic presence of PS spheres of different diameters, the steaming-assisted conversion method renders crack-free large area IOs. Figure S10 (Supporting shows the energy disper-X-ray spectroscopy (EDS) spectrum (a) and the EDS mapping (b-d) of the Nb-doped TiO 2 IO film (NT-180aw24).It was found from the EDS spectrum (Figure S10a, Supporting Information) that the Nb:Ti atomic ratio is 0.34:1 (see Table 1).Moreover, it can be seen from the EDS mapping of the sample that the distribution of Nb in the film is homogeneous, without any phase segregation.Further high-resolution X-ray photoemission spec-  Apart from it, a small shoulder at lower energies, normally related to Ti 3+ (456.77eV) and Nb 4+ (205.66 eV) was disclosed, which could be attributed to the charge compensation caused by the incorporation of Nb 5+ ions into the TiO 2 lattice, resulting in the reduction of Ti 4+ into Ti 3+ . [12]Based on XPS results, the atomic ratio of Nb to Ti is 0.36:1, well consistent with that of EDS (0.34:1), indicating a uniform distribution of Nb from the top surface to bottom of the film (see Table 1).EDS and XPS were also used to investigate the doping of anatase TiO 2 by tungsten in the W-doped TiO 2 IO film (WT-180w24).Figure S12 (Supporting Information) shows the EDS spectrum (a) and the high resolution XPS spectra (b and c) of WT-180w24.The EDS and XPS results are presented in Table 1.The sample presents an atomic ratio W:Ti of (0.08:1) from EDS, slightly lower than that from XPS which is 0.07:1.HRTEM was further used to ascertain the doping.The TEM result in Figure 6 agrees with this value and the fitted XRD peak of 0.354 ± 0.003 nm.The interplanar spacing of Nbdoped IO film (NT-180aw24) is expanded to 0.360 nm, in agreement with the XRD results of 0.364 ± 0.003 nm.The lattice of the W-doped IO film (WT-180w24) is expanded less to 0.357 nm (TEM) or 0.353 ± 0.005 nm (XRD).We attribute the lattice expansion to the replacement of Ti 4+ ions (0.060 nm) by the bigger Nb 5+ (0.064 nm) /Nb 4+ (0.068 nm) and W 5+ (0.062 nm) ions. [31,32]ombining the results from HRTEM, EDS, XPS, and Raman, it could be reasonably deduced that Nb has been successfully and homogeneously doped into the TiO 2 lattice.Therefore, the steaming strategy shows its great advantage in high-content doping of Nb, reaching 34%, which is much higher than most literature results. [12,17,30]The advantage of the steaming strategy is also verified by comparing the NbCl 5 /T-160w4 composite film hydrothermally treated in water NT-180w24-hy with its steamed counterpart NT-180w24.The XPS result of the hydrothermally treated film shows an extraordinarily high Nb:Ti ratio (0.88:1), while the body ratio is 0.34:1 based on the EDS result, indicating that most the Nb existed on the surface without effective doping within the TiO 2 lattice (see Table S1, Supporting Information).
Figure S13 (Supporting Information) gives the UV-vis-NIR transmittance spectra of PS opal and TiO 2 /PS opal composite films as well as the amorphous T-65ae4, crystallized anatase T-180w24, Nb-doped NT-180aw24, and W-doped WT-180w24 TiO 2 IO films.Considering a negligible absorption in visible light for anatase TiO 2 , the valleys in the UV-vis-NIR transmittance spectra in Figure S13 (Supporting could be mainly attributed to reflection.The reflection maxima in the 400-900 nm range corresponding to red and blue-green are observed for all samples, which coincide with the optical reflection images of Figure S7 (Supporting Information).Considering that all samples are composed of anatase TiO 2 or doped anatase TiO 2 , such optical color could be attributed to their opal structure. [33]Apart from it, the transmittance of the doped TiO 2 IO films is obviously smaller than that of the crystallized IO film, with such differences being larger in the NIR region, indicating a strong absorption under NIR light.This phenomenon could be attributed to localized surface plasmon resonance absorption (LSPR). [34]he bandgap energy (Eg) of anatse TiO 2 (T-180w24), Nb-doped (NT-180aw24) and W doped (WT-180w24) anatase TiO 2 IO films was evaluated using a Tauc plot (Figure 7).For comparison with these samples, the Ti(OH) 3 HSO 4 /PS opal composite film from step (d) in Figure 2 was calcinated under 550 °C for 1.5 h.It was named "TiO 2 -550C IO" and its Eg is also included in Figure 7.Its crystallite size was estimated to be 11.5 nm. [25]The calculated Eg for TiO 2 -550C IO is 3.20 eV, nearly the same value (3.1-3.2 eV) reported in most literature. [7,35,36]The Eg for T-180w24 (3.02 eV) is red-shifted in contrast to its calcined counterpart (3.20 eV, 11.5 nm).The Eg shift for T-180w24 can partly be explained by its crystallite size.Indeed, it is known that crystallite size influences the bandgap energy. [37,38]Other factors that could also influence the Eg shift are the presence of monoclinic titania appearing at q = 10.07 nm −1 , an unidentified peak at 6.3 nm −1 , and the possible presence of an amorphous phase, since, relative to the NT-180aw24 sample, the T-180w24 sample appears to have a residual amorphous component (Figure S6, Supporting Information).For the W-doped TiO 2 IO film (WT-180w24), Eg is blue-shifted to 3.14 eV, partly due to its bigger crystallite size (6.9 nm) as well as the LSPR that commonly exists in Wdoped TiO 2 . [34]In contrast to Cao et al. result that showed an Eg blue-shift in Nb-doped TiO 2 due to LSPR, [34] in our case the Eg is red-shifted to 2.99 eV for Nb-doped TiO 2 IO film (NT-180aw24), close to the value reported by Wang et al. in which 20% Nb-doped anatase TiO 2 nanosheets showed an Eg of 3.02 eV. [12]Such difference could probably be attributed to the electron compensation effect by intrinsic acceptors (such as interstitial oxygen) in the doped TiO 2 , [39,40] considering that there is a high amount of Nb doping in our present sample.
It is known that metal-doped TiO 2 , whose EC effects depend on the doping efficiency and defects (for instance, O-vacancy), has been regarded as a promising active material for electrochromic (EC) devices. [41]The EC behavior of the steaming-assisted synthesized anatase TiO 2 (T-180w24), Nb-doped (NT-180aw24), and W-doped (WT-180w24) TIO 2 IO films was investigated.Figure 8 shows transmittance spectra of the IO films along with their optical images at open circuit (0 V), colored and bleached states for applied potentials of ±1.5 V.All three samples show good reversibility since the optical response of the device when bleached at +1.5 V is similar to that of the open circuit at 0 V.As seen in Table 2, upon coloration at −1.5 V, T-180w24 shows little coloration with ΔT = 1.1% at 550 nm and 1.4% at 1100 nm.Doping greatly increases the optical response since for the Nb-doped TiO 2 IO sample (NT-180aw24), ΔT = 13.5% in the NIR region  (1100 nm) moving from T = 49.5% at colored state to T = 63% at bleached state.The improved EC behavior for metal-doped TiO 2 could probably be attributed to conductivity and the relation between various defects (O-vacancy, Ti 3+ , Nb 4+ , and so on), which will be described in detail in another manuscript that is currently in preparation.For the visible region at 550 nm, ΔT = 4.4%.In contrast, under a small amount of W dopant in W-doped TiO 2 IO film, WT-180w24 shows an improved optical response in both the visible (ΔT = 18.4% at 550 nm) and NIR regions (ΔT = 23.7% at 1100 nm), which is even much higher than the values reported for WO 3 IO (ΔT = 19.6% at 1100 nm, ±1.5 V). [27] The lower optical response of NT-180aw24 in comparison to WT-180w24, is possibly due to the too-high content of Nb dopant (0.35:1 of Nb:Ti) in the film.Indeed, literature reported that an Nb dopant amount above 10% results in inferior optical contrast in the NIR region due to Burstein-Moss effect. [32,42]

Conclusion
A novel steaming-assisted conversion strategy was successfully developed for the synthesis of anatase TiO 2 , Nb-doped, and Wdoped anatase TiO 2 IO films.By using water, HCl, or ammonia aqueous solution as a steaming vapor source, amorphous TiO 2 IO, NbCl 5 /TiO 2 composite IO, and WCl 6 /TiO 2 composite IO were converted into anatase TiO 2 IO, Nb-doped and W-doped anatase TiO 2 IO films through a liquid-free, low-temperature vapor environment.A liquid-free, ammonia ethanol vapor in the first steaming step converts dissolvable amorphous TiO 2 into a stable film, simultaneously strongly binding the films to the substrates, avoiding peeling off.The successive liquid-free steaming step under an appropriate temperature, converts amorphous TiO 2 , NbCl 5 , or WCl 6 /TiO 2 composite IO into anatase TiO 2 , Nb, or W-doped anatase TiO 2 IOs.In the Nb or W-doped anatase TiO 2 IO films a remarkably homogeneous high amount of doping could be achieved.After Nb-doping, the ratio of Nb to Ti reached as high as 34%.Introducing Nb or W dopant into TiO 2 IO films remarkably improves electrochromic performance due to modification of the electronic structure.It is anticipated that the steaming-assisted conversion method can be considered as a new method for the fabrication of crack-free large-area IO films and that it will provide an ideal strategy for the synthesis of various doped metal oxides.
Fabrication of TiO 2 IO Films: The 2D TiOSO 4 /PS opal composite monolayer building blocks were synthesized by "dynamic hard template infiltration" strategy as developed in the previous works. [25,26]Initially, a large area PS opal floating on the surface of water was obtained by gasliquid-solid interface self-assembly method as follows: a clean hydrophilic glass slide, previously cleaned in a piranha solution (30% H 2 O 2 : concentrated H 2 SO 4 = 3:7 v/v) at 100 °C for 15 min, and then washed with Millipore water, was placed on the bottom of a Petri dish (diameter, 9 cm).Then, Millipore water was added to nearly submerge the slide.The diluted PS suspension was then added drop-by-drop onto the glass slide, to get a self-assembled monolayer of PS spheres on the water surface.Afterward, a few drops of 2 wt.%, SDS aqueous solution were added into the water from a side of the dish to pack the PS monolayer closely, resulting in a 2D PS opal template floating over the water.Once the PS opal monolayer was formed, the glass slide was gently removed (Figure 2a).
After removing most of the water, leaving ≈6 mL of water in the dish, with the PS opal film still floating onto the water surface, 7 mL TiOSO 4 aqueous solution (0.43 mol L −1 ) was injected into the water from the side of the Petri dish where there was no PS opal (Figure 2b).After ≈20 min, a 2D TiOSO 4 /PS opal composite monolayer was self-assembled on the water surface, then ITO coated glass substrate was introduced under the opal composite monolayer (Figure 2c).Next, the water was slowly sucked out to sink the opal composite monolayer film onto the ITO substrate (Figure 2d).After drying in an oven at 50 °C overnight, the substrate coated with the TiOSO 4 /PS opal composite film was inserted vertically into a selfbuilt Teflon support and then placed in a 75 mL commercial hydrothermal synthetic vessel that was composed of a hydrothermal synthetic vessel, including a stainless steel outer vessel and a Teflon liner (Figure 1).A volume of 0.8 mL of ammonia ethanol solution (4 mol L −1 ) was added at the bottom of the liner.The pressure vessel was sealed, and heated to 65 °C for 4 h, thus transforming the TiOSO 4 /PS opal composite film into an amorphous TiO 2 /PS opal composite.
After removal of the PS opal template by THF treatment for 2 h an amorphous TiO 2 IO monolayer film was obtained and named T-65ae4.
To obtain anatase 2D TiO 2 IO films, the amorphous TiO 2 IO monolayer film was further steamed by pure water (0.8 mL) in the Teflon-lined pressure vessel at 180 °C for another 24 h.This film was named T-180w24.T-65ae4 was also steamed in water at 160 °C for 4 h.The resulting TiO 2 IO was named T-160w4.
The TiOSO 4 /PS opal composite film was also calcinated at 550 °C for 1.5 h.The calcined film was named TiO 2 -550C IO.
The obtained composite structure was steamed by ammonia aqueous solution (0.8 mol L −1 , 0.8 mL) at 180 °C for 24 h and named NT-180aw24.
Fabrication of Anatase W-doped TiO 2 IO Films: The amorphous TiO 2 IO monolayer film T-65ae4 was first dip coated using a WCl 6 ethanol solution (0.12 mol L −1 ): dipping speed 75 mm min −1 , withdrawal speed 75 mm min −1 , immersion time 20 s, 1 layer.The obtained composite structure was then steamed in pure water at 180 °C for 24 h and named WT-180w24.
Characterization: The morphology of the films was characterized using a Hitachi S-4800 FE-SEM microscope (Ibaraki-ken, Japan).For phase analysis, Raman Spectroscopy was recorded at room temperature with a Thermo Scientific DXR spectrometer equipped with a motorized xy stage and autofocus.The spectra were generated with ≈0.45 mW, 633 nm He-Ne laser excitation at the sample surface.
For EC studies, the films deposited on ITO substrates were used as working electrodes while a platinum grid served as a counter electrode and a commercial Ag/AgCl 1 m KCl electrode served as reference.A 0.5 mol L −1 LiClO 4 /propylene carbonate solution was used as an electrolyte.
Electrochromic measurements were conducted by combining optical transmittance spectra using a UV-3600 spectrophotometer (Shimadzu, Tokyo, Japan) with electrochemical analysis using CHI600E workstation (Chenhua, Shanghai, China) with an applied voltage from −1.5 to +1.5 V.″ The elemental composition and chemical valence states of the films were characterized by X-ray photoelectron spectroscopy (XPS) using Thermo Scientific K-alpha.
HRTEM images were obtained on a JEM-2010 electron microscope operated at 200 kV.
XRD data were obtained with Cu K  radiation on custom-built diffractometer equipped with pyrolytic graphite monochromator and analyzer crystals.To enhance the signal of the film with respect to that of the substrate, the data were taken with a fixed incident angle of 1.0°, and the scattering angle varied between 1.0 ○ and 64.8°.The data were fitted with pseudo-Voigt profiles to determine the lattice constants and the crystallite sizes.

Figure 1 .
Figure 1.Pressure vessel with setup for the steaming process.
troscopy (XPS) was used to investigate the nature of the extrinsic electrons on the surface in NT-180aw24.The appearance of the Ti2p and Nb3d signals in the XPS spectra (FigureS11, Supporting Information) validates that the TiO 2 IO film has been successfully doped with Nb.The Ti2p and Nb3d spectra were both simple spin-orbit doublets with a Ti2p 3/2 binding energy of 458.47 eV and a Nb3d 5/2 binding energy of 207.06 eV, indicating the highest oxidation states (Ti 4+ and Nb 5+ ) of both elements.

Figure 7 .
Figure 7. Plot of the transformed Kubelka-Munk function versus photon energy for all anatase TiO 2 IO films.

Table 1 .
Atomic ratios of Nb, W, and Ti ions in the films.

Table 2 .
Optical modulation of the films.