Unraveling the Intercalation Chemistry of Hexagonal Tungsten Bronze

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Unraveling the Intercalation Chemistry of Hexagonal Tungsten Bronze and Its Optical Responses Yonghyuk Lee,† Taehun Lee,† Woosun Jang, and Aloysius Soon* Global E3 Institute and Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea S Supporting Information *

ABSTRACT: In an attempt to promote energy saving through the clever control of varying amounts of visible light and solar energy in modern buildings, there has been a surge of interest in the novel design of multifunctional glass windows otherwise known as “smart windows”. The use of chromogenic materials (e.g., tungsten oxides and their alloys) is widespread in this cooling energy technology, and for the case of hexagonal tungsten oxide (h-WO3)-based systems, the overall efficiency is often hindered by the lack of a systematic and fundamental understanding of the interplay of intrinsic charge transfer between the alkali-metal ions and the host h-WO3. In this work, we present a first-principles hybrid density-functional theory investigation of bulk hexagonal tungsten bronzes (i.e., alkali-metal-intercalated h-WO3) and examine the influence of the intercalation chemistry on their thermodynamic stability as well as optoelectronic properties. We find that the introduction of the alkali-metal ion induces a persistent n-type electronic conductivity, and dramatically reduces the optical transmittance (down to ∼28%) for infrared wavelengths while maintaining fair optical transparency for next-generation electrochromic devices in very energy efficient chromogenic device technology.

I. INTRODUCTION Tungsten oxide has been widely studied with high expectations to be utilized in various applications such as smart windows, chromogenic devices, sensors, dye-sensitized solar cells, and photoelectrochemical water splitting.1−4 However, precedent research conducted on the tungsten trioxide systems is mainly focused on the most stable phase at room temperature, namely, the monoclinic phase (γ-WO3), and its doped systems.5,6 The metastable hexagonal tungsten trioxide (h-WO3), though chemically similar to the monoclinic phase, has a rather unique crystal structure with empty hexagonal and trigonal nanochannels in a framework of vertex-shared WO6 octahedrons.7,8 This phase has been found to be stabilized under specific synthesis conditions, e.g., by controlling the ligands or intercalating different kinds of alkali-metal ions,9 and this stabilization effect has also been investigated theoretically.10 The hexagonal channels in h-WO3 were then suggested as potential binding sites for the intercalated cations such as Li+, Na+, and K+, proving h-WO3 to be a very useful intercalation oxide host material to form tungsten bronzes for various important technological applications.11−14 © XXXX American Chemical Society

It is undeniable that further development of novel functional materials is only possible when a firm fundamental understanding of the microscopic atomic picture and the optoelectronic structure is established. For the more commonly studied monoclinic WO3 phase, recent work on electronic band gap engineering has shown that a decrease in the band gap energy is closely linked to the geometrical distortion induced by the incorporation of alkali-metal cations where the electrons are transferred to the host oxide structure via the easy ionization of the alkali-metal atoms.15 Indeed, it was found that the optical band gap energy of WO3 could be reduced via the distortions of the WO6 octahedrons by intercalating different small molecules, e.g., N2, as in the case of the monoclinic phase.5,16 Given the importance of this structure−property relationship for functional tungsten oxides and their bronze alloys, it is surprising that such microscopic details for the hexagonal phase are not well studied, and there is still much to be learned about Received: October 13, 2015 Revised: June 13, 2016

A

DOI: 10.1021/acs.chemmater.5b03980 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

To calculate the optical properties of h-WO3 and its intercalated structures, we have adopted the independent-particle approximation (IPA), which neglects excitonic and local-field effects.25 The IPA approach has been used in many previous studies26−30 with reasonable success to investigate the optical properties of various materials systems (including oxides). In ref 31, the authors have also compared the influence of both the excitonic and local-field effects on the optical properties of the monoclinic WO3 and have found that the overall shape of the adsorption spectra is not significantly affected. Within the IPA, the frequency-dependent dielectric matrix is first calculated after the determination of the electronic ground state. The imaginary part is then derived by the summation over empty states with the following equation:

this hexagonal tungsten bronze phase. For instance, it will be interesting to study how the intercalation chemistry of alkalimetal-doped h-WO3 actually depends on the intercalation site preference. It would be rather premature to assume that the knowledge gained from that of the monoclinic phase could be directly transferred to the hexagonal system. Given the difference in ionic sizes and chemistries of Na and K cations, our first-principles density-functional theory (DFT) calculations could shed new light as to why certain intercalation site preferences might be found in h-WO3, consequently altering its optoelectronic structure. On a more technical note, previous theoretical calculations10 on h-WO3 and its alkali-metal-doped bronze alloys were studied using the semilocal generalized gradient approximation (GGA) to the exchange-correlation (xc) functional, which is known to fail in providing an adequately accurate description of the electronic structure. Thus, it is the aim of this study to address, using firstprinciples density-functional theory calculations, the intercalation chemistry of Na- and K-intercalated h-WO3 bronze alloys and elucidate the specific intercalation site preference for each metal cation, as well as investigate the optical responses of hWO3 to metal intercalation via an improved theoretical description of the electronic structure of h-WO3 and its bronze alloys. This will aim to provide a more accurate microscopic picture for understanding the less studied (albeit, important) hexagonal phase of WO3 and will in turn help to further development of functional tungsten oxides for chromogenic applications.

(2) εαβ (ω) =

4π 2e 2 1 lim 2 q 0 → Ω q

∑ 2ωk δ(ϵc k − ϵvk − ω)⟨uc k + eαq|uvk⟩ c ,v ,k

⟨uc k + eβq|uv k ⟩*

(1)

where the indices Ω, q, c, v, α, and β denote the volume of the primitive cell, the Bloch vector of the incident wave, the conduction band states, the valence band states, and two Cartesian components, respectively, and uck stands for the cell periodic part of the Kohn− Sham orbitals at the k-point. Then the real part of the dielectric matrix, ε(1) αβ (ω), is acquired from the Kramers−Kronig transformation: (1) εαβ (ω) = 1 +

2 P π

∫0

∞

(2) εαβ (ω′)ω′

ω′2 − ω2

d ω′

(2)

where P denotes the principal value. Detailed information on the optical calculations here can be found in ref 25. To calculate the optical constants, using the DFT-derived frequency-dependent dielectric function, we define the refractive index (n), extinction coefficient (k), and optical transmittance (T) by using the following equations:32−34

II. METHODOLOGY Spin-polarized DFT calculations are performed using the projector augmented wave17 method as implemented in the Vienna Ab initio Simulations Package (VASP).18,19 For this work, we have included the 5p, 5d, and 6s states of tungsten and the 2s and 2p states of oxygen as valence states for all our DFT calculations. Various approximations to the exchange-correlation (xc) functional have been tested and used, namely, the GGA due to Perdew−Burke− Ernzerhof (PBE)20 and its semiempirical van der Waals (vdW) corrected form due to Grimme with zero damping (PBE+D3)21 and the self-consistent optB8822 and vdW-DF223 xc functionals, including the hybrid DFT xc functional due to Heyd−Scuseria−Ernzerhof (HSE06).24 The planewave kinetic energy cutoff is set to 500 eV, and the Brillouin-zone integrations are performed using a Γ-centered 6 × 6 × 9 k-point grid for the unit cell of h-WO3, yielding 35 special k-points in the irreducible Brillouin zone (IBZ) for all GGA xc functionals. To model the various concentrations of alkali metals (i.e., Na and K) in hWO3, a (1 × 1 × 2) supercell is used with a k-point grid of 6 × 6 × 6 (with the corresponding 28 special k-points in the IBZ). For the HSE06 xc functional, a reduced Γ-centered 3 × 3 × 4 k-point grid is used for all structural and energetic calculations. To obtain the optimal lattice parameters for all structures considered in this work, we minimize the stress tensor and all internal degrees of freedom to determine the lattice vectors iteratively by ensuring that an external pressure of no more than 0.5 kbar is met. In addition, all atomic structures are fully relaxed so the total energies and forces do not change by more than 20 meV and 0.02 eV Å−1, respectively. To afford a better description of both the electronic and optical structures of h-WO3 and its alloys, we have employed the hybrid HSE06 xc functional for the projected density-of-states (PDOS), difference electron density, difference electrostatic potential, and linear optical response calculations, with a denser k-point grid yielding 35 special k-points in the IBZ. In passing, we also note that all h-WO3 systems considered in this work do not yield a magnetic moment, in line with previous studies.15

n=

k=

T=1−

ε12 + ε2 2 + ε1 2

(3)

ε12 + ε2 2 − ε1 2 (n − 1)2 + k 2 (n + 1)2 + k 2

(4)

(5)

where ε1 and ε2 are taken as the real and imaginary parts of the dielectric function, respectively.

III. RESULTS AND DISCUSSION A. h-WO3: Structure, Energetics, and Electronic Properties. There have been earlier reports of bulk h-WO3 crystallizing in two crystal symmetries, namely, P6/mmm (often referred to as “H1”) and P63/mcm (usually known as the “H2” structure).7,35 The more commonly identified H1 crystal structure is the so-called “idealized” h-WO3 which contains three W atoms and nine O atoms in the unit cell, where the W atoms are centrally coordinated to six neighboring O atoms in a near-perfect (i.e., slightly distorted) WO6 octahedral coordination, as shown in Figure 1a. On the other hand, the closely related H2 structure (as proposed by Gerand et al.7) contains half-occupied disordered oxygen sites, often resulting in longer W−O bond distances. In a previous theoretical study,10 the relative energy difference between the H1 and H2 structures was calculated by DFT and compared. They concluded that the H2 model was energetically slightly more favorable (by 0.089 eV) than the H1 model. B


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functionals overestimate the lattice a parameter by approximately 2% with respect to the experimental value of 7.30 Å.7,8 In particular, the vdW-DF2 xc functional is found to severely overestimate it by almost 4%, while the HSE06 functional agrees much closer by about 1%. The opposite trend is found for the lattice c parameter, where all xc functionals underestimate it by about 1.5% with respect to the experimental value of 3.90 Å.7,8 In this case, the HSE06 xc functional is found to severely underestimate it by about 2.5%, while the vdW-DF2 xc functional does a much better job by narrowing the difference to only 0.3%. We measure the bond distance between the W and O atoms in the basal plane and find they have fairly similar lengths, ranging from 1.92 Å when calculated with the PBE, PBE+D3, and optB88 xc functionals to 1.95 Å for the vdW-DF2 functional. Using the HSE06 xc functional, we obtain a slightly shorter W−O bond length of 1.90 Å. Upon relaxation, the optimized WO6 octahedrons in the basal plane are slightly distorted from the perfect 90° squareplanar arrangement. We measure the basal O−W−O angle and find it to be close to 87.9° as calculated with the PBE+D3, optB88, and vdW-DF2 xc functionals. We find the PBE and HSE06 xc functionals predict a slightly smaller basal angle of 87.7°. Referring to Table 1, we do notice that the hybrid xc functional HSE06 provides a much better description of the inplane lattice a parameter of the basal plane, whereas the vdWDF2 xc functional accurately predicts the out-of-plane lattice c parameter in the axial direction. This seems to suggest that the in-plane and out-of-plane O−W bonds could exhibit dissimilar bonding characteristics, i.e., higher degree of covalency in the basal O−W bonds versus a larger influence of van der Waalstype weak bonding in the axial direction. Overall, the optB88 xc functional provides the most balanced description, and thus, all results discussed in this work will be based on the optB88 xc functional (unless stated otherwise). Turning to the energetics, we calculate the theoretical enthalpy of formation of h-WO3 per O atom, ΔHfWO3, as

Figure 1. (a) Crystal structure of hexagonal tungsten trioxide, WO3. The tungsten and oxygen atoms are shown as large gray and small red spheres, respectively. (b) Top view of h-WO3 with hexagonal and trigonal channels. Binding sites for intercalated ions in the hexagonal channel are shown in the right panel as top and side views.

However, in a more recent study involving in situ X-ray scattering measurement of solvothermal synthesis of h-WO3,36 it was found that the H1 h-WO3 was the thermodynamic product while the H2 h-WO3, due to a lack of adequate time to reorient the WO6 octahedrons into the ordered H1 structure, was then determined as the kinetic product. Given that the relative energy difference between these two h-WO3 structures is speculated to be small and the recent experiments seem to suggest the H1 structure to be the thermodynamically stable product, the H1 crystal structure will be used and studied in this work. In the P6/mmm h-WO3 structure, there are two distinct types of O atoms in the unit cell where each is shared by two octahedral unitsin either the basal (xy-direction) or axial (zdirection) planes. This octahedron is arranged in layers normal to the hexagonal vertical axis, and they form both “threemembered” and “six-membered” ringlike structures in the (0001) plane. Stacking of such layers along the vertical axis leads to the formation of small trigonal and large hexagonal channels, respectively (as shown in Figure 1b). Interestingly, these channels have been reported to be suitable for the intercalation of metal species, namely, alkali metals such as Li, Na, K, and Cs.37,38 In this work, the optimized atomic bulk structure of h-WO3 is studied using various approximations to the DFT xc functional, and the results are summarized in Table 1. All xc

f ΔH WO = 3

Etot WO3,

xc

a

a (Å) 7.47 7.45 7.46 7.56 7.38 7.30

(2.32) (2.13) (2.22) (3.57) (1.17)

c (Å) 3.84 3.83 3.83 3.89 3.80 3.90

(−1.53) (−1.73) (−1.64) (−0.34) (−2.61)

Etot W,

(6)

Etot O2,

where and NO are the total energies of the bulk h-WO3, bulk metal W, and O2 molecule and the number of oxygen atoms, respectively. The various xc-calculated values are also reported in Table 1. The optB88-calculated ΔHfWO3 for the monoclinic γ-phase is −3.17 eV/O atom, while that of h-WO3 is determined to be −3.10 eV/O atom (cf. HSE06 (− 2.72 eV/ O atom) and vdW-DF2 (−3.26eV/O atom)). Using the HSE06 hybrid xc functional, we calculate the PDOS shown in Figure 2. In Figure 2a, the top view of the atomic structure of the unit cell of h-WO3 is presented to illustrate the two distinct O atoms labeled as O1 (the axialplane oxygen) and O2 (the basal-plane oxygen), with their corresponding PDOS shown in Figure 2b. The valence band of h-WO3 is dominated by the 2p states of O1 and O2, especially near the valence band maximum, while the conduction band is comprised mainly of the empty 5d states of W. In other words, the O 2p levels are well separated from the W 5d levels, and the octahedral field then induces a splitting of the 5d levels into bands commonly denoted as eg and t2g. The HSE06 electronic band gap of h-WO3 is calculated to be 1.64 eV, which is about 1.16 eV smaller than that of γ-phase monoclinic WO3.39

Table 1. Structural Parameters and Enthalpy of Formation of h-WO3 (P6/mmm) per O Atom, ΔHfWO3, Calculated with Different xc Functionalsa

PBE PBE+D3 optB88 vdW-DF2 HSE06 experiment (at T = 300 K)

NO tot⎞ 1 ⎛ tot tot ⎜E EO2 ⎟ WO3 − E W − ⎠ NO ⎝ 2

ΔHfWO3 (eV/O atom) −2.81 −2.75 −3.10 −3.26 −2.72

The experimental values are obtained from refs 7 and 8. C


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Table 2. Calculated Enthalpies of Formation (eV/O atom) and Bronze Formation Energies (eV/fu) for Na0.33WO3 (at Site A) and K0.33WO3 (at Site B) Using Various xc Functionals xc

ΔHfNa0.33WO3

ΔHfK0.33WO3

ΔHbronze Na0.33WO3

ΔHbronze K0.33WO3

PBE PBE+D3 optB88 vdW-DF2 HSE06

−3.08 −3.05 −3.42 −3.56 −3.01

−3.15 −3.14 −3.52 −3.66 −3.09

−0.81 −0.90 −0.95 −0.93 −0.89

−1.04 −1.19 −1.25 −1.20 −1.12

Figure 2. (a) Top view of the (0001) layer of pure h-WO3. Atom numbers are notated according to the geometric symmetry. (b) Projected density-of-states (PDOS) of O and W atoms in h-WO3. The Fermi energy is indicated by the vertical dashed zero line.

B. Alkali-Metal-Intercalated h-WO3: Structure, Energetics, and Electronic Properties. The metastable h-WO3 is known to be stabilized by alloying alkali metals at low concentrations.40 The optimal concentration10 is reported to be close to 33%, beyond which the h-WO3 intercalation alloy becomes relatively unstable due to the large repulsive interaction between the positively charged alkali-metal cations. Following ref 10, two of the most stable concentrations, 17% and 33%, are chosen for this study. First, to determine the optimal intercalation binding site, extensive calculations have been made to search for the lowest energy binding site. The optimized atomic coordinates and lattice constants for Na/hWO3 and K/h-WO3 are fully relaxed and are shown in Figure 1b, with the two distinct binding sites, A and B, clearly shown. To assess their relative energetic stability, we define the enthalpy of formation per oxygen atom, ΔHfMxWO3 as f ΔHM = xWO3

⎛ 1 ⎡ tot bulk bulk ⎢E MxWO3 − ⎜NMEM + NW E W ⎝ NO ⎣ ⎞⎤ N ⎟⎥ + O EOmlc 2 ⎠ ⎦ 2

Figure 3. (a) Calculated energy difference, ΔE, of alkali-metalintercalated (Na, K) h-WO3 and pristine h-WO3 at 17% and 33% intercalation concentration as a function of the volume difference, ΔV (given with respect to the volume of the pristine h-WO3 formula unit). Yellow and blue markers represent Na- and K-intercalated h-WO3. ΔE is the change in energy of h-WO3 per formula unit achieved after the bulk bulk metal intercalation (ΔE = Etot MxWO3 − (NMEM + EWO3)), and negative values indicate that the process is exothermic. (b) Side view of optimized stable structures of Na0.33WO3 and K0.33WO3. Na and K atoms are represented as yellow and blue spheres.

intercalating atoms, the stabilization effect is found to be different. The Na atom prefers to bind at site A, whereas the K atom favors site B. The Na0.33WO3 system shows a fairly small energy difference of about 0.09 eV between the A and B sites in the bronze formation energy. In a previous theoretical study on hexagonal tungsten bronze,38 a continuing trend toward higher stability with increasing intercalated alkali-metal ion size has been reported for 33% intercalation concentration. Our DFT results agree well with their size-dependent findings, and we observe the same tendency for the 17% intercalated systems. Here, for both Na- and K-intercalated models, the 33% concentration is indeed more favorable than that of 17%. This is again in line with previous reports.10 Considering sites A and B, site A has six nearest neighbor O atoms and six next nearest neighbor W atoms in the xy-plane where optB88 xc functional calculated bond distances are about 2.71 and 3.72 Å, respectively. Site B, on the other hand, also has six nearest neighbor O atoms and six next nearest neighbor W atoms in the xy-plane where measured bond distances are about 3.33 and 4.19 Å, respectively. With volume changes, ΔV, at both sites A and B, Naintercalated h-WO3 expands marginally by about 0.02 Å3 when a Na atom binds at site A with a corresponding small contraction of about 0.03 Å3 at site B. In contrast to these very

(7)

where NO and NM are the numbers of oxygen atoms and intercalated alkali-metal atoms in the unit cell, respectively. The total energies of the intercalated MxWO3 system, bulk W, and mlc bulk the O2 molecule are represented by Etot MxWO3, EM , and EO2 , respectively. Furthermore, we calculate the bronze formation energy of the MxWO3 system, ΔHbronze MxWO3, using tot bulk ΔHMbronze = [E M − (NMEMbulk + E WO )] xWO3 xWO3 3

(8)

Here, the total energy of the bulk host oxide h-WO3 is given by Ebulk WO3. The calculated enthalpies of formation and bronze formation energies of Na0.33WO3 (at site A) and K0.33WO3 (at site B) using various xc functionals are summarized in Table 2. In Figure 3a, we calculate and report the bronze formation energy difference, ΔE between the alkali-metal-intercalated (i.e., Na or K) h-WO3 and pristine h-WO3 as a function of the relative volume, ΔV (as taken with respect to the volume of pristine h-WO3). We find that, by intercalating the alkali metal at the hexagonal interstitial site, the alloyed h-WO3 phase is actually stabilized via at least 0.5 eV. However, depending on the type of D


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Chemistry of Materials small ΔV values, Na- and K-intercalated h-WO3 expands by a large amount of 0.72 Å3 at site A, while a smaller expansion of 0.07 Å3 is found at site B. These results can be rationalized by considering the Shannon radius as a function of the coordination number.41 When the intercalated atom has a coordination number of 6, the Na+ ion has a Shannon radius of 1.02 Å, and the K+ ion has a corresponding Shannon radius of 1.38 Å, which is much larger than that of the Na+ ion. This might help to explain the selective binding site preference, as shown in Figure 3b. Now turning to the electronic properties of Na- and Kintercalated h-WO3, we calculate the PDOS of Na0.33WO3 and K0.33WO3 in Figure 4. We see that the 2p and 3s states of Na

Figure 5. (a) Electron density difference and (b) electrostatic potential difference of Na0.33WO3 in the (1̅21̅0) direction. (c) Electron density difference and (d) electrostatic potential difference of K0.33WO3 in the (1̅21̅0) direction. In (a) and (c), the blue line represents the region of charge accumulation and the red line represents the region of charge depletion. The highest negative contour line corresponds to −0.0118 eV/Å, and the contour lines are separated by a factor of 2.5 × 10−3 eV/Å. In (b) and (d), the blue line represents an increase in the electrostatic potential while the red line represents a corresponding decrease. Here, the contour lines change successively by a factor of 0.15 eV/Å.

Likewise, the corresponding expression is deduced for ΔV̅ MxWO3: ΔVM̅ xWO3 = VM̅ xWO3(r) − VM̅ (r) − VWO ̅ 3(r)

The electron redistribution from the alkali-metal (M) atom to the basal-plane O atoms results in a electron depletion along the M−M channel and results in an asymmetric polarization of the O atoms in the basal plane nearest to the M−M channel. This corresponds well with the calculated ΔV̅ MxWO3, as shown in Figure 5b,d. The introduction of both alkali metals clearly results in a great reduction in the electrostatic potential along the M−M channel (as shown by the red contours), with the largest reduction located at sites A and B for K0.33WO3 and Na0.33WO3, respectively. In the case of K0.33WO3, the electron depletion of the M−M channel provides a strong screening effect between O atoms in the basal plane, providing a larger stabilizing effect in K0.33WO3 compared to Na0.33WO3. C. Optical Properties of h-WO3 and Its Alloys. To date, detailed mechanisms for optical activities in electrochromic oxides (e.g., h-WO3) are still poorly understood. Generally, optical excitation and adsorption in these oxides are often thought to be associated with electronic and polaronic charge transfers. As shown in Figure 5, electronic charge transfers in alkali-metal-intercalated h-WO3 is indeed evident and results in a persistent n-type electronic conductivity (see Figure 4). To further understand and investigate the optical properties of pristine h-WO3, Na0.33WO3, and K0.33WO3, we calculate the imaginary part of the dielectric function (ε2), the absorption coefficient (α), the transmittance spectrum, and the reflectance spectrum, as shown in Figures 6 and 7, accordingly. Referring to Figure 6, we see that pristine h-WO3 absorbs near the blue and violet visible light range (between 2.8 and 3.5 eV), and this absorption characteristic implies and matches well with the experimentally observed light yellow coloration.42,43 Likewise, we do notice that, in contrast to the pristine h-WO3 structure, the absorption peaks of Na0.33WO3 and K0.33WO3 are mostly detected in the low-energy infrared region (3.5 eV). With regard to this low absorption energy range, plasmonic contributions to the overall optical spectrum may become important for these heavily intercalated h-WO3 bronze alloys where Na and K contribute to the occupation of the conduction band of h-WO3 (see Figure 4). Within the IPA approach (as implemented in the VASP code),25 the contributions due to indirect and intraband adsorptions are neglected. To estimate the plasmonic contributions to the optical properties of these bronze alloys, using the all-electron Elk code,44 we have also calculated their theoretical electron energy-loss spectra (EELS) where the Drude-like contributions to the dielectric tensor are further compared and examined (see Figure S1). Details of these calculations can be found in the Supporting Information. In Figure S1, we estimate that when intraband Drude-like excitations are explicitly included in our linear optical response calculations, the plasma frequency peak, ωp, is actually slightly shifted to higher energies by ∼0.40 eV. However, for higher photon energies (from 4 to 10 eV), the overall shapes of the EELS (with and without intraband Drude-like excitations) are virtually identical, leading us to argue that the influence of the plasmonic contributions is indeed confined to the low-energy spectrum and will not change the conclusions discussed in this

IV. CONCLUSION In closing, using hybrid density-functional theory calculations, we have studied the fundamental structural and optoelectronic properties of pristine h-WO3 and its alkali-metal-intercalated alloys at selected concentrations and intercalated ion binding sites. We find that the optB88 xc functional gives the most balanced description of their structural properties and their associated energetics, predicting that K0.33WO3 (at B site) is thermodynamically more favorable than Na0.33WO3 (at A site), due to a larger electrostatic screening effect between the O atoms by electron charge depletion along the M−M channel. Using the hybrid HSE06 xc functional, we demonstrate that the intercalation chemistry of Na and K promotes an upshift of the

Figure 7. (a) Calculated optical transmittance spectra and (b) reflectance spectra of h-WO3 (gray dotted line), Na0:33WO3 (orange solid line), and K0:33WO3 (blue solid line). (c) Schematic diagram for application of alkali-metal-intercalated h-WO3 in an electrochromic device. F


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Fermi energy to the conduction band by donating an electron to the host h-WO3, resulting in a persistent n-type electronic conductivity. This regulation of electronic charges to h-WO3 results in a dramatic reduction in the calculated infrared transmittance (down to ∼28%) without disrupting the transmittance of visible light. This selective shielding property against the radiation of thermal energy lays the platform for the rational design of the next-generation “smart windows” for very energy efficient chromogenic device technology.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03980. Computational details of additional linear optical response calculations with (and without) intraband Drude-like plasmonic excitations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

Y.L. and T.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Basic Science Research Program by the National Research Foundation of Korea (NRF) (Grant 2014R1A1A1003415). Computational resources have been provided by the Korea Institute of Science and Technology Information (KISTI) supercomputing center (Grant KSC-2015-C3-009).

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REFERENCES

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