Fluoride-Assisted Synthesis of Plasmonic Colloidal ... - ACS Publications

Article Cite This: Chem. Mater. 2018, 30, 4838−4846

pubs.acs.org/cm

Fluoride-Assisted Synthesis of Plasmonic Colloidal Ta-Doped TiO2 Nanocrystals for Near-Infrared and Visible-Light Selective Electrochromic Modulation Sheng Cao,†,‡ Shengliang Zhang,†,‡ Tianran Zhang,†,‡ and Jim Yang Lee*,†,‡ †

Chem. Mater. 2018.30:4838-4846. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/30/18. For personal use only.

Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260, Singapore ‡ Cambridge Centre for Advanced Research and Education in Singapore, 1 Create Way, 138602, Singapore S Supporting Information *

ABSTRACT: Dual-band electrochromic materials are integral to the development of smart windows where visible and near-infrared (NIR) light transmittance may be individually controlled. We present here colloidal Ta-doped TiO2 anatase nanocrystals (NCs) as a promising candidate and their preparation by a fluoride-assisted synthesis method. The dual-band electrochromic performance of these NCs may be credited to their strong localized surface plasmon resonance (LSPR) absoption in the NIR region. The Ta doping of the TiO2 NC host, which has not been attempted before, is made easy in the presence of the fluoride anions. The synthesis produces Ta-doped TiO2 NCs as a highly uniform colloidal solution. Spectroscopic measurements indicate the generation of free carriers in the TiO2 conduction band by the Ta5+ substitution of Ti4+ cations as the origin of the LSPR. Good dual-band electrochromic performance in terms of a high dynamic range for visible and near-infrared light modulation (86.3% at 550 nm and 81.4% at 1600 nm) and good electrochemical stability (the optical modulation at 550 and 1600 nm decreased by 1.3% and 6.7%, respectively, after 2000 cycles) were demonstrated in three-electrode cells to suggest Ta-doped TiO2 NCs as a promising new electrode material for the smart windows.

■

INTRODUCTION

regions to deliver the independent control of NIR and visible light transmittance.11 A simpler alternative would be to use single component electrochromic materials with a dual-band response.11,12 Among them TiO2 with such desirable material features as chemical stability, environmental compatibility, low cost, and earth abundance is deservedly a target of research interest.13−16 It has been found that the intercalation of Li+ into TiO2 is distinctly visible due to the localization of the injected electrons at the Ti4+ sites.17−19 This phenomenon is particularly noticeable at high intercalation levels, with the absorption band associated with the formation of the LixTiO2 phase blue shifting continuously to cover a broad visible light region.17−19 The optoelectronic properties of TiO2 NCs can be easily modified via aliovalent doping. The substitution of Ti4+ by Nb5+ in TiO2 NCs was found to be particularly effective, increasing the electronic conductivity to the level of TiO2 NCs and inducting an LSPR band in the NIR region.20 It is known in the literature that the NIR response is tunable by varying the

Dual-band electrochromic smart windows are gaining considerable attention lately because of their potential to reduce building energy consumption through individual control of near-infrared (NIR) and visible light transmission based on weather conditions and/or personal preferences.1−6 The approach to the dual-band electrochromic electrode was first demonstrated by the Milliron group,1 using a Sn-doped indium oxide (ITO)-NbOx nanocomposite where the plasmonic ITO nanocrystals (NCs) modulate the NIR by a localized surface plasmon resonance (LSPR) absorption while the amorphous NbOx phase modulates the visible light by polaronic absorption. NIR and visible light transmittance modulation was accomplished by applying different electrode potentials. Most dual-band performance is delivered by nanocomposite materials such as WOx-NbOx,4,7 PMe2T2-ITO,8 W18O49-PB,9 and W18O49-polyoxometalates.10 In these nanocomposites, the NIR-selective and visible-modulating components have to be carefully chosen and meticulously integrated into an open network structure with high porosity to facilitate the electrolyte transport.7,9 These nanocomposites also require the NIR- and visible-selective components to operate in different potential © 2018 American Chemical Society

Received: May 24, 2018 Revised: June 25, 2018 Published: June 27, 2018 4838

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846

Chemistry of Materials

Article

■

RESULTS AND DISCUSSION Fluoride-Assisted Synthesis of Colloidal Ta-Doped TiO2 NCs. The one-pot method of our previous study was appropriately modified to enable the fluoride-assisted synthesis of Ta-doped TiO2 colloidal NCs.24 NH4F served as the fluoride source to increase the Ta-doping efficiency. In brief, a predetermined amount of NH4F, together with 1 mmol of titanium and Ta precursor salts (as ethoxides) mixed to yield a nominal Ta content of 13 atom % were added to the reaction flask all at once. Synthesis was repeated by varying the fluoride amounts while keeping all other operating conditions the same. Samples could then be drawn as a function of the NH4F amount and analyzed for doping efficiency. Table 1 shows the chemical compositions of Ta-doped TiO2 NCs synthesized with different amounts of NH4F for the same

dopant type and/or postsynthesis electrochemical modulation methods (e.g., ITO and aluminum-doped zinc oxide (AZO) NCs).19,21 Thus, doped TiO2 NCs can be a good singlecomponent candidate for modulating the visible and NIR spectral response. In practice, dual-band electrochromic materials based on doped-TiO2 single-component NCs are uncommon other than in the Nb-doped system.12,19 The preparation of metal-doped colloidal TiO2 NCs with LSPR properties is nontrivial. This is because doped NCs are notorious for “self-purification”, i.e., the exclusion of dopants from the host lattice because of the more favorable thermodynamics of impurity formation.22,23 Our recent work on Mo-, W-, and Nb-doped colloidal TiO2 NCs also showed that the doping efficiency decreases with the increase in the mismatch between Ti4+and the dopant ionic radius.24 Many of the syntheses in the literature also have not demonstrated good uniformity control, which has an impact on the LSPR performance. Hence there is a need to develop effective methods to improve the doping efficiency and uniformity of LSPR-active metal-doped TiO2 NCs.25,26 It has been known that the presence of fluoride ions in TiO2 synthesis can lead to the better control of the NC morphology.13,18,27,28 Fluoride-assisted synthesis of anatase TiO2 NCs also predisposes the formation of the chemically active {001} facets,13,18,27,28 which may bind the dopant precursor ions more strongly to raise the doping efficiency. Recent research also showed that fluoride anions could cause charge-compensating cationic vacancies to form in the TiO2 lattice,15,29 which are expected to promote cationic doping. However, a successful fluoride-assisted synthesis of metaldoped colloidal TiO2 NCs has yet to be demonstrated, especially one with the aim to optimize the dual-band performance of the synthesized product. This is a report on the development of a fluoride-assisted synthesis to produce aliovalently doped anatase TiO2 NCs with good uniformity control and strong LSPR absorption in the NIR region suitable for dual-band electrochromic applications. We selected Ta-doped TiO2 NCs as the model system because the significant ionic radius mismatch between Ta5+(0.64 Å) and Ti4+ (0.60 Å)30 is a good test of versatility for the fluoride-assisted synthesis. In addition, theoretical studies have projected Ta-doped TiO2 to have an even higher free electron concentration than Nb-doped TiO2.31,32 The experimental results verified the effectiveness of the fluorideassisted synthesisnot only could Ta be facilely incorporated into the TiO2 host NCs, but the as-synthesized Ta-doped TiO2 NCs also demonstrated good NC dispersion uniformity. More importantly the Ta-doped TiO2 NCs exhibited a tunable broad absorption in the NIR region. Spectroscopic investigations indicated that this optical behavior is LSPR-based and may be attributed to the substitution of Ta5+ on Ti4+ sites resulting in the generation of free carriers in the conduction band of the TiO2 NCs. To the best of our knowledge, this should be the first successful demonstration of the synthesis of LSPR-active colloidal Ta-doped TiO2 NCs. These plasmonic NCs also showed a high dynamic range for visible and NIR light modulation (up to 86.3% at 550 nm and 81.4% at 1600 nm) and good electrochemical stability (the transmittance losses at 550 and 1600 nm were 1.3% and 6.7%, respectively, after 2000 cycles) in three-electrode measurements.

Table 1. Chemical Compositions of Ta-Doped TiO2 NCs Synthesized with Different Amounts of NH4F for a Nominal Doping Target of 13 atom % Ta NH4F (mmol)

Ta atom % (EDS)

Ta atom % (ICP-OES)

0 0.1 0.2 0.4 0.8

2.5 8.2 14.8 16.9 16.1

1.5 5.6 14.3 16.6 16.5

nominal dopant content of 13 atom % Ta. Analyses by energydispersive X-ray spectroscopy (EDS) and inductively coupled plasma optical emission spectrometry (ICP-OES) showed generally good concurrence of results. Without NH4F, the amount of Ta that could be doped into TiO2 was limited (the results were similar for other nominal Ta dopant contents; see Table S1 in Supporting Information). The presence of NH4F increased the doping efficiency significantly (a series of control experiments was used to confirm the role of fluorine as the dopant precursor activator; see Table S2 in Supporting Information). The increase was monotonic up to a maximum of 0.4 mmol of NH4F under the experimental conditions. Thus, we standardized the use of 0.4 mmol of NH4F in the syntheses of all Ta-doped TiO2 NCs in this study. The Tadoped TiO2 NC dispersions in hexane synthesized with NH4F were strongly colored (Figure S1 in Supporting Information), contrasting strongly with the colorless Ta-doped TiO2 NC dispersion synthesized without NH4F. The optical property change was the first indication of the LSPR effect (vide infra) and is in general an indication of the successful incorporation of dopant in TiO2 NCs.24 The structures of Ta-doped TiO2 NCs prepared with different amounts of NH4F were analyzed to investigate the role of the NH4F activator. The X-ray diffraction (XRD, Figure 1a) patterns of doped TiO2 NCs are typical of that of anatase TiO2 (JCPDS card no. 21-1272).13,16,18,33 The expanded view of the (101) diffraction in the right panel of Figure 1a shows peak shifts to lower Bragg angles with the increase in the amount of NH4F used in the synthesis. On the contrary, the XRD patterns of Ta-doped TiO2 NCs prepared without the NH4F for different nominal dopant contents (Figure S2, Supporting Information) and the XRD patterns of TiO2 NCs synthesized without a Ta precursor salt in the presence of different NH4F amounts (Figure S3, Supporting Information) show no detectable (101) peak shifts. Hence the crystal structure of the TiO2 NCs could not be altered using only the 4839

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846

Article

Chemistry of Materials

Nb-doped TiO2 NCs, and the doping efficiency was similarly increased (Figure S4, Supporting Information). This is an encouraging indication of the general utility of fluoride-assisted synthesis for the preparation of colloidal metal-doped TiO2 NCs. Morphology, Microstructure, and Optical Properties of Ta-Doped TiO2 NCs. The fluoride-assisted synthesis was used to yield a series of Ta-doped TiO2 NCs with various nominal Ta dopant contents (0, 5, 9, 13, and 17 atom % and denoted as S0, S5, S9, S13, and S17, respectively). Table 2 Table 2. Chemical Compositions of Ta-Doped TiO2 NCs with Different Ta Contents sample

initial dopant atom %

dopant atom % (EDS)

dopant atom % (ICP-OES)

S0 S5 S9 S13 S17

0 5 9 13 17

0 8.8 11.7 16.9 22.1

0 7.1 11.5 16.6 22.5

shows the positive correlation between the nominal and actual dopant contents. The higher actual Ta contents (relative to the nominal Ta contents) could be attributed to the different chemical reactivities of the Ti and Ta precursors. The XRD patterns of all Ta-doped TiO2 NCs in Figure 2 show a well-crystallized anatase TiO2 structure without

Figure 1. Structural evolution of Ta-doped TiO2 NCs with the increasing use of NH4F in the synthesis. (a) XRD patterns, (b) Raman spectra, and (c) F 1s peaks of Ta-doped TiO2 NCs.

Ta dopant or the fluoride activator. The shifts in the (101) diffraction also indicate that a greater extent of Ti substitution by Ta was made possible in the presence of NH4F. The same structural evolution was also witnessed in the Raman spectroscopy of doped TiO2 NCs. While the Raman spectra in Figure 1b are fairly typical of anatase TiO2, there are shifts in the low frequency Eg(1) peak (144 cm−1) to ∼155 cm−1 with the increasing use of NH4F. Since the Eg(1) peak is a characteristic Ti−O stretching mode,13,34,35 the shifts may be taken as more evidence for the formation of substitutional solid solutions when the lattice Ti atoms were replaced by the dopant Ta atoms.20,30 The F 1s peaks in the XPS spectra (Figure 1c) of doped TiO2 NCs are all centered around 684.5 eV. There is however no indication of fluorine presence in the crystal lattice region (688.0−689.0 eV binding energy) to suggest significant fluoride substitution of oxygen anions in the TiO2 lattice.27,36−38 Elemental analyses by both bulk and surface sensitive techniques indicated that fluorine was present mostly on the NC surface (Table S3, Supporting Information). These measurements confirm that NH4F had promoted the Ta doping of TiO2 NCs without itself being incorporated in the TiO2 lattice. Based on the basis of the current understanding of fluorideassisted synthesis, the following promotional effects of the NH4F activator may be proposed for the Ta doping of TiO2 NCs. It is known that NH4F could release hydrofluoric acid (HF) in situ from NH4F to preferentially expose the anatase {001} facets.27,28,38 These chemically more active facets could have improved the energetics of dopant precursor adsorption to result in doping efficiency increases.36,39 Recent research has also shown that fluoride could predispose the formation of charge-compensating titanium vacancies in the TiO2 lattice15,29 which are more accommodative to the inclusion of dopant ions. We have also applied the fluoride-assisted synthesis to

Figure 2. XRD patterns of Ta-doped TiO2 NCs with different nominal Ta dopant contents. The right panel magnifies the (101) diffraction to show the peak shifts.

crystalline impurities. There are notable shifts in the XRD patterns toward lower 2θ (Figure 2 right panel) with the increase in the Ta content. The changes in the lattice parameters were calculated and are summarized in Table S4 (Supporting Information). The deviations in the lattice parameters can be understood in terms of the ionic radius difference between Ta and Ti and the bond length difference between covalent Ta−O (2.17 Å) and Ti−O (2.01 Å) bonds.30,32,40 The Ta doping of TiO2 NCs therefore occurred mainly via the formation of substitutional solid solutions. Figure 3 shows the typical transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of Tadoped TiO2 NCs sampled from S0, S5, and S13 (i.e., with nominal Ta dopant contents of 0, 5, and 13 atom %). All Tadoped TiO2 NCs were pseudospherical, with diameter 4840

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846

Article

Chemistry of Materials

Figure 3. (a1, b1, and c1) TEM images and (a2, b2, and c2) HRTEM images of Ta-doped TiO2 NCs sampled from S0, S5, and S13, respectively. The insets in a1, b1, and c1 show the statistical size distributions, and those in a2, b2, and c2 are the FFT patterns of a sampled single NC.

Figure 4. (a) Ti 2p core-level and (b) Ta 4f core-level spectra of Ta-doped TiO2 NCs sampled from S13 (13 atom % Ta nominal). (c) Dopant contents (as [Ta]/([Ta] + [Ti]) ratios from XPS and ICP-OES analyses as a function of the nominal Ta dopant content.

distributions of 8.7 ± 0.9, 9.9 ± 1.2, and 11.6 ± 1.4 nm for S0, S5, and S13, respectively (Figure 3a1−c1). It is apparent that an increase in the dopant content also increased the NC size, congruent with the sharpness of the diffraction peaks in the XRD patterns (Figure 2). Figure 3a2−c2 show the typical HRTEM images of the Ta-doped TiO2 NCs and the fast Fourier transform (FFT) patterns (insets in Figure 3a2−c2) of a sampled single nanoparticle (NP). The NPs were all singlecrystalline, and the well-resolved lattice fringes with interplanar spacings of 0.35 and 0.47 nm correspond well with the TiO2 (101) and (002) planes, respectively.33,41 The angle between the fringes in the FFT images (68.3°) is also theoretically the angle between the (101) and (002) planes of anatase

TiO2.28,33 These measurements confirmed the basic anatase TiO2 structure of all Ta-doped TiO2 NCs. The metal oxidation states and their distribution in the NCs were analyzed by XPS. Figure 4a,b is respectively the Ti 2p and Ta 4f spectra of a S13 sample (TiO2 with 13 atom % of Ta nominal) which exhibit strong spin−orbit splitting. The binding energies of the Ti 2p3/2 and Ti 2p1/2 peaks at 459.1 and 464.9 eV correspond well with the Ti4+ oxidation state.20,30 Similarly, the spin−orbit split Ta 4f7/2 and Ta 4f5/2 peaks at 26.3 and 28.2 eV are consistent with the Ta5+ oxidation state.30,42 The Ta dopant contents as measured by XPS (a surface analysis) and ICE-OPS (a bulk analysis) versus the nominal Ta content are compared in Figure 4c. The good agreement between the two measurement methods over a wide 4841

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846

Article

Chemistry of Materials

Figure 5. (a) Normalized absorption spectra and (b) optical bandgaps of Ta-doped TiO2 NCs with different nominal Ta dopant contents.

Figure 6. (a) Top view and (b) cross-sectional view of the SEM images of a S13 Ta-doped TiO2 NC thin film on ITO glass. (c)Transmittance and corresponding digital photos of a S13 Ta-doped TiO2 NC thin film electrode at three different applied potentials (4 V (bright), 1.8 V (cool), 1.5 V (dark)).

optical band gap as per the Burstein−Moss effect.20,22,24 The (αhν)1/2 versus hν plots in Figure 5b confirm the increase of the optical bandgap (Eg) of the UV-absorption edge with the Ta content. The increasing trend verifies the substitutional Ta5+ ions as electron donors. The LSPR absorption intensity and optical bandgap however decreased when the nominal Ta dopant level was above 13 atom %. This behavior suggests the formation of interstitial defects in the host lattice at high dopant concentrations.22,43−45 To the best of our knowledge, this is the first demonstration of the plasmonic absorption properties of Ta-doped TiO2 NC dispersions. The strong LSPR absorption as well as excellent solvent dispersibility are well suited for the solution processing of Ta-doped TiO2 NCs in dual-band smart windows applications. NIR and Visible-Light Selective Electrochromic Modulation. The potential of Ta-doped TiO2 NCs as NIR and visible light tunable electrochromic films was evaluated by using 13 atom % (nominal) Ta-doped TiO2 NCs (Figure S13), which showed the strongest LSPR absorption among the Tadoped TiO2 NCs in this study. The doped TiO2 NCs were spin-coated on an ITO glass and underwent a programmed

range of dopant concentrations suggests the uniformity of Ta doping throughout the NCs in general. Deviations were detected at high Ta concentrations (i.e., nominal doping ≥17 atom %) with the higher readings from the surface sensitive XPS suggesting surface enrichment by Ta. These measurements together with the XRD and TEM data demonstrate the viability of doping the anatase TiO2 lattice uniformly with Ta5+ to a relatively high level. The substitutional Ta5+ ions in the TiO2 anatase lattice are expected to release electrons to the conduction band resulting in the creation of free charge carriers and consequently the emergence of plasmonic absorption in the NIR region. The color of Ta-doped TiO2 NC dispersions in hexane was the first indication of the presence of free carriers in Ta-doped TiO2 NCs (Figure S5, Supporting Information).20,22,24 Their corresponding absorption spectra in Figure 5a show broad absorption from the red edge of the visible region to the NIR region with the absorption intensity increasing with the increase in the dopant content. In addition to surface plasmon absorption in the NIR region, the accumulation of free electrons in the conduction band should also increase the 4842

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846

Article

Chemistry of Materials

Figure 7. Optical and electrochemical performance of S13 Ta-doped TiO2 NC thin films. Changes in optical transmittance measured at (a) 550 nm in the 4 V−1.5 V window and (b) 1600 nm in the 4 V−1.8 V window. Optical density changes (ΔOD) as a function of the charge density measured at the wavelength of (c) 550 nm and (d) 1600 nm, with calculated coloration efficiencies (CE) of 33.2 and 124.5 cm2 C−1, respectively. (e) Integrated charge capacity of a film in cyclic voltammetry (normalized to the first cycle) at 20 mV s−1 for 2000 cycles between 4 and 1.5 V. (f) Transmittance spectra of an electrode film after 2000 cycles at 4 V, 1.8 V, and 1.5 V, respectively.

sequence of heat treatment in air up to 400 °C to remove the surfactants to form into a transparent conductive NC thin film. Scanning electron microscopy (SEM, Figure 6a,b) measured the film to be ∼1.2 μm thick with a uniform surface. XRD confirmed the preservation of the TiO2 anatase structure of the starting NCs (Figure S6, Supporting Information). The electrochromic properties of the S13 Ta-doped TiO2 NC thin film was characterized in situ by a three-electrode spectroelectrochemical cell in an argon glovebox (see Experimetal Methods for more details). The optical transmittance spectra show good dual-band electrochromic modulation, with independently controllable NIR and visible light transmittance to support three distinct operating modes (Figure 6c). At an applied electrode potential of +4 V, the film was in the completely discharged state and transparent to both NIR and visible light (the “bright mode”). When the applied electrode potential was lowered to 1.8 V vs Li+/Li, the film transited to the “cool mode” state where it blocked most of the NIR radiation (>780 nm) while allowing most of the visible light (400−780 nm) to pass through. The transmittance decrease in the NIR region, which is similar to the NIRselective plasmonic absorption of ITO and AZO NCs,21 was likely caused by the restoration of free electrons which give rise to LSPR absorption in the NIR region through the capacitive charging of Ta-doped TiO2 (Figures S7 and S8, Supporting

Information). This particular mode which blocks the solar heat gain selectively while maintaining visible transmittance is most useful for daylight. At +1.5 V (“dark mode”), the Ta-doped TiO2 was intercalated with Li+. Both the NIR and visible transmittance were simultaneously blocked due to the phase change from tetragonal anatase TiO2 to orthorhombic LixTiO2.17−19,46 The corresponding digital photos of the composite electrode in Figure 6c show the near colorless, light blue, and black coloration of the film in these three electrochromic operating modes. The independent control of NIR and visible light transmittance as demonstrated here indicates the potential of the fluoride-synthesized Ta-doped TiO2 NCs as a dual-band electrochromic material. Figure 7 shows the dynamics in the electrochemical modulation of the optical response of the S13 thin film electrode. The transmittance modulation in switching the Tadoped TiO2 film was up to 86.3% at 550 nm and 81.4% at 1600 nm (Figure 7a,b), which is comparable to all current dual-band electrochromic materials including the advanced composites (Table S5, Supporting Information). The transmittance change at 1600 nm between 4 and 1.8 V was used to evaluate the switching time between the bright and cool modes. The fast coloration times (τc) of 18.4 s and bleaching time (τb) of 1.1 s were due to capacitive charging.21 By comparison the switching times between the bright and the 4843

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846

Article

Chemistry of Materials

(0.4 mmol) were mixed in a 50 mL three-neck flask and degassed at 120 °C for 20 min. The flask was then purged with nitrogen and kept under a nitrogen atmosphere throughout the synthesis. After heating at 280 °C for 60 min for the NC growth, the reaction mixture was cooled to ∼60 °C and the solid product was recovered by precipitation with acetone and redispersed in hexane. Ta-doped TiO2 with nominal Ta contents from 0 to 17 atom % were produced by varying the amount of tantalum ethoxide in the starting mixture while keeping all other synthesis parameters fixed. Fabrication of Ta-Doped TiO2 NCs Film. Thin films were prepared by spin-coating a concentrated solution of Ta-doped TiO2 NCs in hexane (60 mg mL−1) at 1500 rpm for 45 s on a cleaned ITO glass. After drying on a hot plate at 280 °C for 5 min, a second layer was applied, and the procedure was repeated five times to build up the film thickness. The film was annealed in air at 400 °C for 30 min to decompose the surfactants on the NCs and to form a transparent coated layer. Structure Characterization and Optical Property Measurements. The structure, morphology, and composition of the NCs were characterized by X-ray diffraction (XRD, D8 Advance, Bruker), Raman microscope (Raman, inVia, Renishaw, UK, with a 532 nm Nd:YAG laser exciting source), high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL), field emission scanning electron microscopy (FESEM, JSM 6700F, JEOL) with energy dispersive X-ray spectroscopy (EDX, using an INCA x-act attachment, Oxford), inductively coupled plasma optical emission spectrometry (ICP-OES; Ultima 2, Horiba Jobin Yvon, France), and X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD spectrometer, Kratos). The XPS peaks were curve-fitted by the XPSPeak41 software. Binding energies were corrected by referencing the C 1s peak of adventitious carbon to 284.5 eV. The vendor-supplied VISION software was used for the quantitative analysis of the XPS data. Optical measurements were performed on an ASD Quality Spec Pro visible-NIR spectrometer and AVANTES spectrometer (AvaSpecULS2048CL-EVO + AvaSpec-NIR256-1.7). All electrochemical measurements were carried out in a three-electrode setup using an AUTOLAB PGSTAT204 potentiostat/galvanostat. Electrochemical and Electrochromic Measurements. A custom-made spectroelectrochemical cell was used for electrochemical and optical measurements inside a glovebox. A Ta-doped TiO2 film formed the working electrode of a cell connected to the spectrometer using a fiber optics connected light source. For the standard Li+ charging experiments, a three-electrode setup using two Li foils as the counter electrode and reference electrode and a 0.5 M Li-TFSI in tetraglyme electrolyte were used. For the measurements of capacitive charging, a Pt foil counter electrode, a Ag/Ag+ reference electrode (in 0.1 M TBA-TFSI/PC), and 0.1 M TBA-TFSI in propylene carbonate electrolyte were used. All potentials were quoted with reference to the Li+/Li standard. Chronoamperometry was used for charge and transport measurements between fixed potential limits. Cycling stability was evaluated by cyclic voltammetry between 1.5−4 V at the sweep rate of 20 mV s−1. The background for optical measurements was the electrolyte solution and the ITO/glass substrate of the custom-made quartz electrochemical cell (the transmittance spectrum of ITO glass was also measured, see Figure S9 (Supporting Information)). In situ optical transmittance measurements as a function of the applied potential were recorded by an ASD Quality Spec Pro UV/vis/NIR spectrometer. Data were collected only after the stabilization of the optical response. The time-course measurements of the transmittance at specified wavelengths (550 and 1600 nm), on the other hand, were performed on an AVANTES spectrometer. Switching time was defined as the time required to attain 90% of the full modulation in the specified potential range. Coloration efficiency (CE) was calculated by the formula CE = ΔOD/ΔQ = log(Tb/Tc)/ΔQ, where ΔQ is the injected charge and τb and τc are the transmittances in the bleached and colored states at the wavelength of interest, respectively.

dark modes measured by the transmittance changes at 550 nm were much longerτc of 66.8 s and τb of 6.9 s. The slower switching times in comparison with WO3- based electrodes47,48 could be attributed to the slower phase change kinetics of Li+ intercalation and deintercalation reaction with TiO2.12,19 Coloration efficiency (CE) is another performance indicator for electrochromic materials. Based on Figure 7c,d, a CE of 33.2 cm2 C−1 at 550 nm and 124.5 cm2 C−1 at 1600 nm could be obtained, which is comparable to the advanced compositebased dual-band electrochromic films (Table S5, Supporting Information). The cycle stability of the Ta-doped TiO2 NC thin film was good, retaining 85.2% of its first-cycle (integrated) charge capacity even after 2000 electrochemical cycles (Figure 7e). The high coloration contrasts in both the NIR and the visible regions also persisted very well in electrochemical cycling85.4% at 550 nm and 76% at 1600 nm after 2000 cycles (Figure 7f). Consequently, the decrease in transmittance modulation at 550 and 1600 nm after 2000 cycles was relatively minorat 1.3% and 6.7%, respectively. These measurements further demonstrated the potential of the fluoride-synthesized Ta-doped TiO2 NCs as a dual-band electrochromic material to meet the needs of next-generation smart windows.

■

CONCLUSIONS In summary, we have reported the development of a fluorideassisted synthesis of Ta-doped anatase TiO2 colloidal NCs with good uniformity control and strong LSPR absorption in the NIR region suitable for dual-band smart windows applications. It was found that the presence of NH4F in the synthesis facilitated the Ta dopant incorporation into the anatase TiO2 host lattice. The substitution of lattice Ti4+ by Ta 5+ resulted in free electron injection into the TiO 2 conduction band to activate a LSPR effect. The uniformity and dispersibility of the as-synthesized Ta-doped TiO2 NCs thus showed strong LSPR absorption in the NIR region, with absorption maximums tunable by the Ta dopant content. Dualband electrochromic films could easily be fabricated from these NCs by spin-coating and heat treatment. The fabricated films showed good performance with a large dynamic range for the modulation of visible and NIR light (86.3% at 550 nm and 81.4% at 1600 nm) and good electrochemical cycling stability (1.3% and 6.7% loss of the initial transmittance at 550 and 1600 nm, respectively, after 2000 cycles). This study presented Ta-doped TiO2 NC as a promising single-component dualband electrochromic material and also demonstrated an effective method of preparation for the plasmonic aliovalentdoped colloidal TiO2 NCs.

■

EXPERIMENTAL METHODS

Materials. 1-Octadecene (ODE) 90%, titanium ethoxide (Ti(OEt)2, technical grade), tantalum(V) ethoxide (Ta(OEt)5, trace metals basis), niobium(V) ethoxide (Nb(OEt)5, 99.95% trace metals basis), oleic acid (OA, 90%), oleylamine (OLA, 70%), 1-octadecanol (ODAL 99%), and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI, 99.95 wt %) were supplied by Sigma-Aldrich and used as received. ITO glasses (2 cm × 2 cm, 20 Ω sq−1) were procured from Latech. Synthesis of Ta-Doped TiO2 NCs. Ta-doped TiO2 NCs were synthesized in one pot by heating the metal precursor salts in ODE at 280 °C under a blanketing N2 atmosphere. OLA and OA were used as the cosurfactants and ODAL as the hydroxyl groups provider. In a typical synthesis, Ti(OEt)2 (1 mmol), Ta(OEt)5 (0.1 mmol), ODE (8 mL), OLA (0.5 mL), OA (0.5 mL), ODAL (10 mmol), and NH4F 4844

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846

Article

Chemistry of Materials

■

(11) Zhang, S.; Cao, S.; Zhang, T.; Yao, Q.; Fisher, A.; Lee, J. Y. Monoclinic Oxygen-Deficient Tungsten Oxide Nanowires for Dynamic and Independent Control of Near-Infrared and Visible Light Transmittance. Mater. Horiz. 2018, 5, 291−297. (12) Barawi, M.; De Trizio, L.; Giannuzzi, R.; Veramonti, G.; Manna, L.; Manca, M. Dual Band Electrochromic Devices Based on Nb-Doped TiO2 Nanocrystalline Electrodes. ACS Nano 2017, 11, 3576−3584. (13) Chen, X.; Liu, L.; Huang, F. Black Titanium Dioxide (TiO2) Nanomaterials. Chem. Soc. Rev. 2015, 44, 1861−1885. (14) Sun, Q.; Cortie, D.; Zhang, S.; Frankcombe, T. J.; She, G.; Gao, J.; Sheppard, L. R.; Hu, W.; Chen, H.; Zhuo, S.; Chen, D.; Withers, R. L.; McIntyre, G.; Yu, D.; Shi, W.; Liu, Y. The Formation of DefectPairs for Highly Efficient Visible-Light Catalysts. Adv. Mater. 2017, 29, 1605123. (15) Koketsu, T.; Ma, J.; Morgan, B. J.; Body, M.; Legein, C.; Dachraoui, W.; Giannini, M.; Demortiere, A.; Salanne, M.; Dardoize, F.; Groult, H.; Borkiewicz, O. J.; Chapman, K. W.; Strasser, P.; Dambournet, D. Reversible Magnesium and Aluminium Ions Insertion in Cation-Deficient Anatase TiO2. Nat. Mater. 2017, 16, 1142−1148. (16) Sang, L.; Zhao, Y.; Burda, C. TiO2 Nanoparticles as Functional Building Blocks. Chem. Rev. 2014, 114, 9283−9318. (17) Boschloo, G.; Fitzmaurice, D. Electron Accumulation in Nanostructured TiO2 (anatase) Electrodes. J. Phys. Chem. B 1999, 103, 7860−7868. (18) De Angelis, F.; Di Valentin, C.; Fantacci, S.; Vittadini, A.; Selloni, A. Theoretical Studies on Anatase and Less Common TiO2 Phases: Bulk, Surfaces, and Nanomaterials. Chem. Rev. 2014, 114, 9708−9753. (19) Dahlman, C. J.; Tan, Y.; Marcus, M. A.; Milliron, D. J. Spectroelectrochemical Signatures of Capacitive Charging and Ion Insertion in Doped Anatase Titania Nanocrystals. J. Am. Chem. Soc. 2015, 137, 9160−9166. (20) De Trizio, L.; Buonsanti, R.; Schimpf, A. M.; Llordes, A.; Gamelin, D. R.; Simonutti, R.; Milliron, D. J. Nb-Doped Colloidal TiO2 Nanocrystals with Tunable Infrared Absorption. Chem. Mater. 2013, 25, 3383−3390. (21) Garcia, G.; Buonsanti, R.; Llordes, A.; Runnerstrom, E. L.; Bergerud, A.; Milliron, D. J. Near-Infrared Spectrally Selective Plasmonic Electrochromic Thin Films. Adv. Opt. Mater. 2013, 1, 215−220. (22) Agrawal, A.; Cho, S. H.; Zandi, O.; Ghosh, S.; Johns, R. W.; Milliron, D. J. Localized Surface Plasmon Resonance in Semiconductor Nanocrystals. Chem. Rev. 2018, 118, 3121−3207. (23) Norris, D. J.; Efros, A. L.; Erwin, S. C. Doped Nanocrystals. Science 2008, 319, 1776−1779. (24) Cao, S.; Zhang, S.; Zhang, T.; Fisher, A. C.; Lee, J. Y. MetalDoped TiO2 Colloidal Nanocrystals with Broadly Tunable Plasmon Resonance Absorption. J. Mater. Chem. C 2018, 6, 4007−4014. (25) Sun, Q.; Zheng, C.; Huston, L. Q.; Frankcombe, T. J.; Chen, H.; Zhou, C.; Fu, Z.; Withers, R. L.; Norén, L.; Bradby, J. E.; et al. Bimetallic Ions Codoped Nanocrystals: Doping Mechanism, Defect Formation, and Associated Structural Transition. J. Phys. Chem. Lett. 2017, 8, 3249−3255. (26) Primc, D.; Niederberger, M. Synthesis and Formation Mechanism of Multicomponent Sb−Nb:TiO2 Mesocrystals. Chem. Mater. 2017, 29, 10113−10121. (27) Pan, J.; Liu, G.; Lu, G. Q. M.; Cheng, H. M. On the True PhotoreActivity Order of {001},{010}, and {101} Facets of Anatase TiO2 Crystals. Angew. Chem., Int. Ed. 2011, 50, 2133−2137. (28) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638−641. (29) Li, W.; Corradini, D.; Body, M.; Legein, C.; Salanne, M.; Ma, J.; Chapman, K. W.; Chupas, P. J.; Rollet, A.-L.; Julien, C.; et al. High Substitution Rate in TiO2 Anatase Nanoparticles with Cationic Vacancies for Fast Lithium Storage. Chem. Mater. 2015, 27, 5014− 5019.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02196.

■

Chemical composition, XRD, digital images, calculated lattice parameters, cyclic voltammograms, transmittance spectra, and a table comparing the measured electrochromic performance with the typical current dual-band electrochromic materials (PDF)

AUTHOR INFORMATION

Corresponding Author

*(J.Y.L.) E-mail: [email protected]. ORCID

Sheng Cao: 0000-0002-6203-9088 Shengliang Zhang: 0000-0001-6368-1147 Tianran Zhang: 0000-0003-2837-4971 Jim Yang Lee: 0000-0003-1569-9718 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

■

REFERENCES

(1) Llordes, A.; Garcia, G.; Gazquez, J.; Milliron, D. J. Tunable Near-Infrared and Visible-Light Transmittance in Nanocrystal-inGlass Composites. Nature 2013, 500, 323−326. (2) Cai, G.; Eh, A. L.-S.; Ji, L.; Lee, P. S. Recent Advances in Electrochromic Smart Fenestration. Adv. Sustainable Syst. 2017, 1, 1700074. (3) Wang, Y.; Runnerstrom, E. L.; Milliron, D. J. Switchable Materials for Smart Windows. Annu. Rev. Chem. Biomol. Eng. 2016, 7, 283−304. (4) Heo, S.; Kim, J.; Ong, G. K.; Milliron, D. J. Template-Free Mesoporous Electrochromic Films on Flexible Substrates from Tungsten Oxide Nanorods. Nano Lett. 2017, 17, 5756−5761. (5) Granqvist, C. G. Electrochromics for Smart Windows: OxideBased Thin Films and Devices. Thin Solid Films 2014, 564, 1−38. (6) Li, H.; McRae, L.; Firby, C. J.; Al-Hussein, M.; Elezzabi, A. Y. Nanohybridization of Molybdenum Oxide with Tungsten Molybdenum Oxide Nanowires for Solution-Processed Fully Reversible Switching of Energy Storing Smart Windows. Nano Energy 2018, 47, 130−139. (7) Kim, J.; Ong, G. K.; Wang, Y.; LeBlanc, G.; Williams, T. E.; Mattox, T. M.; Helms, B. A.; Milliron, D. J. Nanocomposite Architecture for Rapid, Spectrally-Selective Electrochromic Modulation of solar Transmittance. Nano Lett. 2015, 15, 5574−5579. (8) Barile, C. J.; Slotcavage, D. J.; McGehee, M. D. Polymer− Nanoparticle Electrochromic Materials That Selectively Modulate Visible and Near-Infrared Light. Chem. Mater. 2016, 28, 1439−1445. (9) Wang, Z.; Zhang, Q.; Cong, S.; Chen, Z.; Zhao, J.; Yang, M.; Zheng, Z.; Zeng, S.; Yang, X.; Geng, F.; Zhao, Z. Using Intrinsic Intracrystalline Tunnels for Near-Infrared and Visible-Light Selective Electrochromic Modulation. Adv. Opt. Mater. 2017, 5, 1700194. (10) Gu, H.; Guo, C.; Zhang, S.; Bi, L.; Li, T.; Sun, T.; Liu, S. Highly Efficient, Near-Infrared and Visible Light Modulated Electrochromic Devices Based on Polyoxometalates and W18O49 Nanowires. ACS Nano 2018, 12, 559−567. 4845

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846

Article

Chemistry of Materials (30) Singh, N.; Prakash, J.; Misra, M.; Sharma, A.; Gupta, R. K. Dual Functional Ta-Doped Electrospun TiO2 Nanofibers with Enhanced Photocatalysis and SERS Detection for Organic Compounds. ACS Appl. Mater. Interfaces 2017, 9, 28495−28507. (31) Osorio-Guillén, J.; Lany, S.; Zunger, A. Atomic Control of Conductivity Versus Ferromagnetism in Wide-Gap Oxides Via Selective Doping: V, Nb, Ta in Anatase TiO2. Phys. Rev. Lett. 2008, 100, 036601. (32) Bawaked, S. M.; Sathasivam, S.; Bhachu, D. S.; Chadwick, N.; Obaid, A. Y.; Al-Thabaiti, S.; Basahel, S. N.; Carmalt, C. J.; Parkin, I. P. Aerosol Assisted Chemical Vapor Deposition of Conductive and Photocatalytically Active Tantalum Doped Titanium Dioxide Films. J. Mater. Chem. A 2014, 2, 12849−12856. (33) Dai, Y.; Cobley, C. M.; Zeng, J.; Sun, Y.; Xia, Y. Synthesis of Anatase TiO2 Nanocrystals with Exposed {001} Facets. Nano Lett. 2009, 9, 2455−2459. (34) Biswas, A.; Chakraborty, A.; Jana, N. R. Nitrogen and Fluorine Codoped, Colloidal TiO2 Nanoparticle: Tunable Doping, Large Red Shifted Band Edge, Visible Light Induced Photocatalysis and Cell Death. ACS Appl. Mater. Interfaces 2018, 10, 1976−1986. (35) Tian, F.; Zhang, Y.; Zhang, J.; Pan, C. Raman Spectroscopy: A New Approach to Measure the Percentage of Anatase TiO2 Exposed (001) Facets. J. Phys. Chem. C 2012, 116, 7515−7519. (36) Foo, G. S.; Hu, G.; Hood, Z. D.; Li, M.; Jiang, D.-e.; Wu, Z. Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes. ACS Catal. 2017, 7, 5345−5356. (37) Gordon, T. R.; Cargnello, M.; Paik, T.; Mangolini, F.; Weber, R. T.; Fornasiero, P.; Murray, C. B. Nonaqueous Synthesis of TiO2 Nanocrystals using TiF4 to Engineer Morphology, Oxygen Vacancy Concentration, and Photocatalytic Activity. J. Am. Chem. Soc. 2012, 134, 6751−6761. (38) Wu, X.; Chen, Z.; Lu, G. Q. M.; Wang, L. Nanosized Anatase TiO2 Single Crystals with Tunable Exposed (001) Facets for Enhanced Energy Conversion Efficiency of Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2011, 21, 4167−4172. (39) Liu, S.; Yu, J.; Jaroniec, M. Anatase TiO2 with Dominant HighEnergy {001} Facets: Synthesis, Properties, and Applications. Chem. Chem. Mater. 2011, 23, 4085−4093. (40) Robertazzi, A.; Platts, J. A. Binding of Transition Metal Complexes to Guanine and Guanine−Cytosine: Hydrogen Bonding and Covalent Effects. JBIC, J. Biol. Inorg. Chem. 2005, 10, 854−866. (41) Zhai, P.; Hsieh, T. Y.; Yeh, C. Y.; Reddy, K. S. K.; Hu, C. C.; Su, J. H.; Wei, T. C.; Feng, S. P. Trifunctional TiO2 Nanoparticles with Exposed {001} Facets as Additives in Cobalt-Based PorphyrinSensitized Solar Cells. Adv. Funct. Mater. 2015, 25, 6093−6100. (42) Gonçalves, R. V.; Wojcieszak, R.; Uberman, P. M.; Teixeira, S. R.; Rossi, L. M. Insights into the Active Surface Species Formed on Ta2O5 Nanotubes in the Catalytic Oxidation of CO. Phys. Chem. Chem. Phys. 2014, 16, 5755−5762. (43) Lee, H.-Y.; Robertson, J. Doping and Compensation in NbDoped Anatase and Rutile TiO2. J. Appl. Phys. 2013, 113, 213706. (44) Runnerstrom, E. L.; Bergerud, A.; Agrawal, A.; Johns, R. W.; Dahlman, C. J.; Singh, A.; Selbach, S. M.; Milliron, D. J. Defect Engineering in Plasmonic Metal Oxide Nanocrystals. Nano Lett. 2016, 16, 3390−3398. (45) Ghosh, S.; Saha, M.; Paul, S.; De, S. K. Shape Controlled Plasmonic Sn Doped CdO Colloidal Nanocrystals: A Synthetic Route to Maximize the Figure of Merit of Transparent Conducting Oxide. Small 2017, 13, 1602469. (46) Wagemaker, M.; Borghols, W. J.; Mulder, F. M. Large Impact of Particle Size on Insertion Reactions. a Case for Anatase Li xTiO2. J. Am. Chem. Soc. 2007, 129, 4323−4327. (47) Li, H.; McRae, L.; Elezzabi, A. Y. Solution-Processed Interfacial PEDOT: PSS Assembly into Porous Tungsten Molybdenum Oxide Nanocomposite Films for Electrochromic Applications. ACS Appl. Mater. Interfaces 2018, 10, 10520−10527. (48) Li, H.; Chen, J.; Cui, M.; Cai, G.; Eh, A. L.-S.; Lee, P. S.; Wang, H.; Zhang, Q.; Li, Y. Spray Coated Ultrathin Films from Aqueous

Tungsten Molybdenum Oxide Nanoparticle Ink for High Contrast Electrochromic Applications. J. Mater. Chem. C 2016, 4, 33−38.

4846

DOI: 10.1021/acs.chemmater.8b02196 Chem. Mater. 2018, 30, 4838−4846