Predictions of electronic structures and optical performance of potential near infrared absorber Sn0.33WO3

For better synthesis and development of novel WO3-based near infrared absorbing materials for smart-window applications, the structural, electronic, and optical properties of hexagonal Sn0.33WO3 were investigated through the first-principles calculation. The optimized crystal structure parameters agree well with experimental values. The electronic structure shows that when Sn ions are doped in the host hexagonal WO3, Sn0.33WO3 displays a typical n-type electronic conductivity, which leads to an upshift of the Fermi energy to the conduction band. It was found that Sn0.33WO3 exhibits low reflectivity and weak absorption in the visible region, while exhibiting strong reflectivity and absorption in the near infrared light region. Therefore, it significantly reduces the optical transmittance of infrared wavelengths (down to 3.9% for the compacted film and 25.3% for the coated film), while maintaining fair optical transparency for visible wavelengths. This research indicates that Sn0.33WO3 is a prospective near infrared absorber and it can be used as near infrared shielding filters for smart windows with high transparency for visible light.For better synthesis and development of novel WO3-based near infrared absorbing materials for smart-window applications, the structural, electronic, and optical properties of hexagonal Sn0.33WO3 were investigated through the first-principles calculation. The optimized crystal structure parameters agree well with experimental values. The electronic structure shows that when Sn ions are doped in the host hexagonal WO3, Sn0.33WO3 displays a typical n-type electronic conductivity, which leads to an upshift of the Fermi energy to the conduction band. It was found that Sn0.33WO3 exhibits low reflectivity and weak absorption in the visible region, while exhibiting strong reflectivity and absorption in the near infrared light region. Therefore, it significantly reduces the optical transmittance of infrared wavelengths (down to 3.9% for the compacted film and 25.3% for the coated film), while maintaining fair optical transparency for visible wavelengths. This research indicates that Sn0.33WO3 is a prospective near...


I. INTRODUCTION
For almost two decades, the alkali-metal-doped hexagonal tungsten bronzes MxWO 3 (M = Na, 1 K, 1 Rb, 2 Cs 3,4 ) have received attention for their remarkable near infrared (NIR) absorption, which can be applied as a solar filter for smart-windows. However, the alkali-metal-intercalated hexagonal WO 3 (h-WO 3 ) typically exhibits chromatic instabilities under high temperature and high humidity, which limits its application range. 5 Therefore, there is a surge of interest to develop novel WO 3based near infrared absorbing materials for solar filter applications to meet the high standards of energy saving and emission reduction in modern buildings and automobiles.
Doping is a very effective way to tune the properties of materials. Recently, it has been found that SnxWO 3 tungsten bronze is a novel image-guided cancer therapy, which has better chemical stability and lower elemental toxicity than CsxWO 3 and RbxWO 3 , 6 and can well absorb excellent near-infrared light. Up to now, although SnxWO 3 tungsten bronze could be hydrothermally synthesized, there are few reports on the physical properties and application prospects of Sn+ doped hexagonal tungsten trioxide. Therefore, the electronic structures and optical properties of Sn-doped h-WO 3 are still of great interest. For a better understanding of SnxWO 3 tungsten bronze, it is essential to study the electronic and optical properties.
Actually, the first-principles calculations based on Density Functional Theory (DFT) have become an important and powerful complementary tool for obtaining and quantifying fundamental properties of solid functional materials. Recently, the first-principles calculations of Solid Oxide Fuel Cell (SOFC) cathode materials have been performed by using the DFT method 7 in conjunction with the supercell model. 8 The on-site Coulombic interaction (DFT+U) ARTICLE scitation.org/journal/adv which can correct the electron self-interaction problem for the B 3d-electrons has been successfully used to describe electronic structures. [9][10][11] Pure DFT-generalized gradient approximation (GGA) significantly underestimates bandgaps of SOFC cathode materials such as perovskite oxides ABO 3 ; however, pure DFT-GGA can lead to a metallic description of SOFC cathode materials. 10,11 To our surprise, hexagonal WO 3 is itself a semiconductor, but alkali-metal atoms doped in the hexagonal window of h-WO 3 can improve the electronic conductivity by changing its electronic structure. 12,13 Yang et al. 14,15 studied the electronic and optical properties of alkali metal monodoped and codoped h-WO 3 using the firstprinciples hybrid density-functional HSE06 theory; however, the ratio (0.083) of alkali metal doped h-WO 3 is significantly lower than the experimentally reported values of 0.15-0.33. Lee et al. 16 calculated optical properties of Na 0.33 WO 3 and K 0.33 WO 3 using the hybrid functional HSE06, but they did not compare their calculation results with experimental values. Yoshio and Adachi 17 elucidated well the mechanism of optical properties of Cs-doped hexagonal tungsten bronzes using the DFT+U method. Xu et al. 18 presented a systematic theoretical investigation on the optical properties of Cs 0.33 WO 3 using the DFT-GGA method, and the theoretical calculations are well carried out and there are good comparisons between the theoretical results and other practical experimental results. On this basis, we conclude that DFT-GGA calculations provide the best model fit for Sn-doped h-WO 3 .
Therefore, we studied the electronic structure and optical properties of hexagonal Sn 0.33 WO 3 by DFT with GGA in this work. The solar radiation shielding performance of hexagonal Sn 0.33 WO 3 films is predicted.

II. COMPUTATIONAL DETAILS
In this work, we adopted the plane-wave pseudopotential method within the density functional theory to calculate the structural, electronic, and optical properties of Sn 0.33 WO 3 . 19 The ionelectron interaction was performed by using the ultrasoft pseudopotential. 20 For the exchange-correlation energy, the GGA-PBE was implemented by the CASTEP code. 21 The electronic valence states of Sn, W, and O were employed as 5s 2 5p 2 , 5p 6 5d 4 6s 2 , and 2s 2 p 4 . The cutoff energy and the k-points of the Brillouin zone were set as 500 eV and 9 × 9 × 15, which was performed to relax the unit cell of h-WO 3 and 2 × 2 × 1 supercell. The force per atom has a tolerance of 5.0 × 10 −6 eV/Å, a maximum displacement of 5.0 × 10 −4 Å, and a maximum stress of 0.02 GPa.

III. RESULTS AND DISCUSSION
A. Geometry structure The optimized geometry structure of the 2 × 2 × 1 supercell of tin-doped h-WO 3 (Sn 0.33 WO 3 ) is shown in Fig. 1. Fully optimized lattice parameters of hexagonal Sn 0.33 WO 3 in comparison with the experimental results are summarized in Table I. The calculated results in this work agree well with the previous experimental data. 22,23

B. Electronic structures
The calculated band structures of undoped h-WO 3 and Sn 0.33 WO 3 are shown in Fig. 2. The dotted lines at zero indicate the Fermi energy level. It can be found that the bandgap for undoped h-WO 3 is 0.62 eV from Fig. 2(a), which is close to the reported value of 0.66 eV, 24 as the DFT simulation generally underestimates this value. 25 From Fig. 2(b), it is interesting to observe that the Fermi level moves up to the conduction band for the insertion of Sn ions, rendering Sn 0.33 WO 3 to have metal-like properties. Figure 3 shows the density of states (DOS) of Sn 0.33 WO 3 . The calculated DOS for h-WO 3 is not shown here but is in accordance with those obtained by Xu et al. As can be seen from Fig. 3, the Fermi level is located at the bottommost conduction band and the W-5d state curve crosses the Fermi level after doping. The Sn-5p state on the conduction band shows that the Sn-5p electrons mainly play the role of free electrons, implying that the Sn atoms are ionized and create charge density to the bottommost conduction band. As a result, Sn 0.33 WO 3 displays a typical n-type electronic conductivity. The intercalation of Sn atoms stimulates the upshift of the Fermi energy to the minimum conduction band by donating free electrons to the pristine h-WO 3 . 15 Figure 4 shows the dielectric function ε(ω) = ε 1 (ω) + iε 2 (ω) of hexagonal Sn 0.33 WO 3 . ε(ω) is related to the interaction of electrons with photons, which comes from the contributions of intraband and interband transitions. From Fig. 4, the critical peaks of imaginary part ε 2 (ω) associated with the electron excitation are as follows. Peak A at 0.05 eV and peak B at 4.07 eV are attributed to the transition from W states of the valence band to Sn 5p states of the conduction band near the Fermi energy. It can be found that the calculated static real part ε 1 (ω) of hexagonal Sn 0.33 WO 3 is 161.28. Figure 5 shows the reflectivity spectra of hexagonal Sn 0.33 WO 3 . Figure 5 shows that the average reflection is over 60% in the NIR region from 1000 to 2500 nm. However, the reflection curve is Vshaped distribution for the visible light from 380 to 780 nm and the reflectivity minimum is 7.5% at about 621 nm (2.0 eV). Figure 6 plots the absorption spectra of hexagonal Sn 0.33 WO 3 . From Fig. 6, we can see that a strong absorption coefficient appears in the ultraviolet and NIR region, and the absorption coefficient maximum for NIR light is 346 255 cm −1 at 1211 nm (1.02 eV).  However, the absorption valleys occur in the visible region and the absorption coefficient minimum for visible light is 75 176 cm −1 at 541 nm (2.29 eV), which is attributed to the plasma oscillation of the metalloid characteristic. The energy-loss spectra L(ω) displayed in Fig. 7 describe the energy loss of a fast electron traversing in the materials. The sharp peak in the low energy range of the L(ω) spectra is associated with the plasma oscillation, and the peak position is corresponding to the relevant plasma frequency (ωp). 26 In a word, the position of the plasma oscillation corresponds to the sharp decrease in the reflectance and absorption spectrum. As illustrated in Fig. 7, we can reasonably deduce that the plasma energy ( ̵ hωp) of Sn 0.33 WO 3 is 1.78 eV. Simultaneously, Sn 0.33 WO 3 exhibits low reflectivity and weak absorption when the incident light energy is close to its plasma energy. Finally, we calculated the transmittance of diverse Sn 0.33 WO 3 films by employing the calculated reflectance and the absorption spectra according to the formula 27

C. Optical properties
where d is the thickness of the material film, and the possibility of multiple reflections between the front and back surface of the film is ignored. The systematic presentation of the theory associated with appropriate models has been reported by Xiao et al. 28 Figure 8 shows the transmittance of the compacted film and the coated film for Sn 0.33 WO 3 . It should be noted that the theoretical transmittance of the coated film for Sn 0.33 WO 3 is only deduced from the absorption coefficient (Fig. 6), whereas the transmittance of the

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scitation.org/journal/adv compacted film is deduced from the absorption coefficient (Fig. 4) and the calculated reflectance (Fig. 5) of hexagonal Sn 0.33 WO 3 . From Fig. 8, we can see that the curves of the transmission of diverse Sn 0.33 WO 3 films form "hanging bell" shape distribution in the visible region. The largest transmission of the visible light for the compacted film and the coated film is 60.3% at 573 nm and 74.0% at 541 nm, respectively. However, the minimum transmittance of NIR for the compacted film and the coated film is 3.9% at 1319 nm and 25.3% at 1210 nm, respectively. The difference between the transmittance maximum and the transmittance minimum for the compacted film and the coated film are 56.4% and 48.7%, respectively. These data indicate that Sn 0.33 WO 3 is a perfect NIR absorber with a high visible transmittance which could serve as an NIR shielding material for transparent windows.

IV. CONCLUSIONS
In closing, we systematically studied the electronic structure and optical properties of Sn 0.33 WO 3 using the first-principles method. The solar radiation shielding performance of diverse Sn 0.33 WO 3 films was predicted. The results of optical properties show that Sn 0.33 WO 3 is a perfect NIR absorber. The theoretical transmittance of different hexagonal Sn 0.33 WO 3 films indicates that Sn 0.33 WO 3 possesses an outstanding high blocking effect for NIR radiation with high transmittance of visible light.