Tunable Transparency and NIR-Shielding Properties of Nanocrystalline Sodium Tungsten Bronzes

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Article Tunable Transparency and NIR-Shielding Properties of Nanocrystalline Sodium Tungsten Bronzes

Luomeng Chao 1 , Changwei Sun 1, Jianyong Dou 1, Jiaxin Li 1, Jia Liu 1, Yonghong Ma 1,* and Lihua Xiao 2,*

1 College of Science, Inner Mongolia University of Science and Technology, Baotou 014010, China; [email protected] (L.C.); [email protected] (C.S.); [email protected] (J.D.); [email protected] (J.L.); [email protected] (J.L.) 2 Guizhou Institute of Technology, Guiyang 550003, China * Correspondence: [email protected] (Y.M.); [email protected] (L.X.)

Abstract: The NaxWO3 nanoparticles with different x were synthesized by a solvothermal method and the absorption behavior in visible and near-infrared light (NIR) region was studied. Well- crystallized nanoparticles with sizes of several tens of nanometers were conﬁrmed by XRD, SEM and TEM methods. The absorption valley in visible region shifted from 555 nm to 514 nm and the corresponding absorption peak in NIR region shifted from 1733 nm to 1498 nm with the increasing x.

In addition, the extinction behavior of NaxWO3 nanoparticles with higher x values were simulated by discrete dipole approximation method and results showed that the changing behavior of optical properties was in good agreement with the experimental results. The experimental and theoretical

data indicate that the transparency and NIR-shielding properties of NaxWO3 nanoparticles in the visible and NIR region can be continuously adjusted by x value in the whole range of 0 < x < 1. These

tunable optical properties of nanocrystalline NaxWO3 will expand its application in the fields of transparent heat-shielding materials or optical filters. Citation: Chao, L.; Sun, C.; Dou, J.; Li, J.; Liu, J.; Ma, Y.; Xiao, L. Tunable Keywords: sodium tungsten bronzes; nanoparticles; tunable optical properties Transparency and NIR-Shielding Properties of Nanocrystalline Sodium Tungsten Bronzes. Nanomaterials 2021, 11, 731. https://doi.org/10.3390/ 1. Introduction nano11030731 Tungsten bronze is a kind of solid solution formed by other cations filling in the lattice structure of WO3. Its chemical formula can be written as MxWO3, in which M is dopant Academic Editor: Matthieu Roussey cation and the x value can vary in a certain range (0 < x < 1, M is typically electropositive metals such as alkali, alkaline earth or rare-earth metals), which are non-stoichiometric Received: 17 February 2021 compounds. The dopant M cation can contribute a number of electrons to reduce part Accepted: 12 March 2021 Published: 14 March 2021 of the hexavalent tungsten to pentavalent, which makes tungsten bronzes have special physical and chemical properties such as superconductivity [1], photochromism [2], elec-

Publisher’s Note: MDPI stays neutral trochromism [3], photothermal conversion [4] and transparent heat-shielding properties [5], with regard to jurisdictional claims in etc. Among these properties, the transparent heat-shielding properties have been studied published maps and institutional afﬁl- extensively in recent years because the tungsten bronze exhibits low absorption of visible iations. light and high absorption of near-infrared light (NIR), which meets the demand of smart windows with high visible transmittance and heat-shielding performance. The free electrons in tungsten bronzes can be regarded as the electron gas moving under the background of uniform positive charge (actually a kind of plasma). The plasma may resonate when it meets the incident light, while the plasma resonance frequency increases with the increase Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of carrier concentration and moves to the short-wave direction. This article is an open access article Tungsten bronzes have three types of cubic, tetragonal and hexagonal phases [5], and distributed under the terms and the tunnels formed by the WO6 octahedra with different phases are also different. The cubic conditions of the Creative Commons phase contains only one type of cubic cavity, while the tetragonal phase contains not only Attribution (CC BY) license (https:// tetragonal, but also tripartite and pentagonal channels. The hexagonal structure, which creativecommons.org/licenses/by/ has been widely studied, contains tripartite and hexagonal channels. The size of dopant + 4.0/). cation M determines the content and position of M in the structure of tungsten bronze. H

Nanomaterials 2021, 11, 731. https://doi.org/10.3390/nano11030731 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 731 2 of 10

and smaller alkali metal ions (such as Li+) can locate in the narrow tripartite channel, while larger alkali metal ions (such as K+, Cs+) or NH4+ can only locate in the hexagonal channel. If the dopant cation occupies all the hexagonal channels, then x = 0.33. Among the various MxWO3, many studies indicate that NaxWO3 is the only compound which is possible to be synthesized in the wide range of 0 < x < 1 [6–8], so it has become the most studied tungsten bronze. The electrical and optical properties of NaxWO3 can substantially change with varying Na content. When x < ∼0.2, NaxWO3 exhibits semiconductor properties, and the electrical conductivity increases with increasing x and thus NaxWO3 shows metallic properties at high x [9]. Moreover, the changing electrical conductivity also leads to changing color in NaxWO3. As the x increases from 0 to 1, the color of NaxWO3 gradually changes from lime green to dark blue, violet, pink, orange and yellow [10]. This change in color can be attributed to increase of bulk plasma frequency (ωp)[6]. Unlike other metals, the ωp of NaxWO3 can be tuned by Na content in WO3 [11,12]. In our previous work, we found that extinction behavior in visible and NIR region of rare-earth hexaboride (RB6) is directly correlated with its ωp, and nanocrystalline RB6 shows tunable optical characteristic as an excellent transparent heat-shielding material [13]. Like RB6, nanocrystalline MxWO3 is also an excellent transparent heat-shielding material, and even has a wider absorption range in the NIR region than LaB6 [14]. Because of the tunable ωp characteristic of NaxWO3, we infer that the extinction in the visible and NIR region of nanocrystalline NaxWO3 is also tunable by different Na content. However, the current research on tungsten bronzes mainly focuses on its preparation methods or co-doping effect [5,15–18]; the tunable absorption behavior of nanocrystalline tungsten bronzes has very rarely been reported in the literature. In this paper, nanocrystalline NaxWO3 powders with different x were synthesized by a solvothermal method and their optical properties were discussed. In order to more systematically study the optical properties of nanocrystalline NaxWO3, the discrete dipole approximation (DDA) method was also used to investigate the inﬂuence of different x, particle size and particle shape on its optical properties.

2. Materials and Methods 2.1. Fabrication

To fabricate NaxWO3 nanopowders, different amounts (0.09 g, 0.27 g, 0.45 g and 0.63 g, respectively) of NaOH particles (Tianjin Kemiou Chemical Reagent Co., Tianjin, China) were fully ground and added gradually into 150 mL benzyl alcohol, then mixed with WCl6 (Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) by magnetic stirring to obtain a dark blue precursor solution. In the precursor solution, the concentration of WCl6 remained at 0.015 M. Subsequently, the precursor solution was transferred into Teﬂon-lined autoclave, and reacted at 200 ◦C for 4 h. After natural cooling, the reactants were washed with pure water and ethanol several times, then centrifuged and dried in vacuum at 40 ◦C for 1 h to obtain blue powder of NaxWO3.

2.2. Characterization The phase identiﬁcation of the samples was analyzed by X-ray diffraction (XRD, Philips PW1830, Hohhot, Inner Mongolia, China) with Cu Kα (λ = 1.5406 Å), at 30 kV voltage with 30 mA current and scanning rate of 1◦/min. The sample morphology was examined by using a ﬁeld emission scanning electron microscope (SEM, Gemini 300, Beijing, China) with an Energy Dispersive Spectrometer (EDS, Oxford X-Max, Beijing, China). The microstructure was further characterized by transmission electron microscopy (TEM, FEI Talos F200X, Beijing, China) with accelerating voltage of 200 kV. The valence state of elements was analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Baotou, Inner Mongolia, China) using a monochromatic Al kα X-ray source. Optical measurements of samples were performed at room temperature by using ultraviolet–visible–near infrared spectrometer (UV-Vis, HITACHI UH4150, Baotou, Inner Mongolia, China). Nanomaterials 2021, 11, 731 3 of 10

2.3. Simulation Method Discrete dipole approximation method (DDA) was used to study the influence of different x, different size and different shape on the extinction behavior of NaxWO3 nanoparticles. DDA is a method to calculate the far-field and near-field optical properties of nanostructures (usually called “target”) with a variety of shapes and sizes, which adopts a finite array of dipoles to approximate the continuous material. The dipoles polarize under the action of light field and interact with each other through electric field. The absorption and scattering of materials can be obtained by calculating the polarization of the dipoles. Our previous work has proved that DDA method has strong advantages in calculation of transparent heat-shielding materials [19]. In the present work, DDSCAT7.3 software was used to simulate all the DDA results. DDSCAT7.3 is an open source fortran-90 program developed by Drain and Flatau of Princeton University to calculate the scattering and absorption of electromagnetic waves by particles with various geometric shapes and complex refractive indexes [20,21]. The extinction efficiency of target can be expressed as

C = ext Qext 2 πae f f

where Cext is the extinction cross section and aeff is the effective radius (a radius of an equal volume sphere) of target. The complex dielectric constant of NaxWO3 measured by Owen et al. was used to calculate the extinction efﬁciency in this work [22].

3. Results and Discussion

To obtain a nanocrystalline NaxWO3 with different x value, we mixed precursor solutions with different molar ratio of Na/W, as shown in Figure1. After magnetic stirring at 30 min, the solution with Na/W molar ratio of 1:1, 3:1 and 5:1 turned dark blue, but the solution with molar ratio of 7:1 was always kept at yellowish. After treatment in the reactor, blue powder was obtained from the solution with Na/W molar ratio of 1:1, 3:1 and 5:1, but no solid precipitate was formed in the solution with Na/W molar ratio of 7:1. We infer Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 11 that tungsten trioxide reacted with NaOH and formed sodium tungstate when NaOH was excessive, so other methods are needed to obtain nanosized NaxWO3 with high x value.

FigureFigure 1. 1. As-preparedAs-prepared precursor precursor solutions solutions with with different different molar molar ratio ratio of of Na/W Na/W after after different different times. times.

FigureFigure 2 shows the XRDXRD resultsresults ofof threethree kindskinds ofof NaNaxxWO3 nanopowdersnanopowders obtained obtained fromfrom the the solvothermalsolvothermal reaction.reaction. The The sharp sharp and and intensive intensive XRD XRD reﬂections reflections indicate indicate the well-the well-crystallizedcrystallized character character of the of samples. the samples. The XRDThe XRD pattern pattern of sample of sample with with Na/W Na/W molar molar ratio ratioof 1:1 of can 1:1 becan indexed be indexed on the on basis the basis of a hexagonalof a hexagonal phase phase of Na-WO of Na-WO3 (JCPDS3 (JCPDS 81-0577). 81–0577). With With increasing Na content, the peaks of (001) and (002) become stronger and all other peaks become weaker. The peaks of (001) and (002) also correspond to a cubic phase of Na-WO3 (JCPDS 28-1156). In addition, it can be seen from the partial enlarged view that the (001) peak slightly moves to higher angle with the increase of Na content, which corresponds to the position of the peak near 23° on the JCPDS 81-0577 and JCPDS 28-1156. Therefore, we infer that the sample exhibits a tendency to change to cubic phase with increasing Na content. According to literature, the NaxWO3 shows a perovskite-type crystal structure with cubic symmetry at x > ~0.4 [6,7], which is consistent with our XRD results.

Figure 2. XRD patterns of the obtained powders (left) and partial enlarged view (right).

The SEM images of the synthesized NaxWO3 powders with different Na/W ratios are shown in Figure 3. It can be seen from Figure 3a–c that all three samples are mainly composed of homogeneous and well-dispersed nanoparticles with sizes of several tens of nanometers. The EDS images in Figure 3d–f gives that the atomic ratios of Na/W for three samples are 0.077, 0.173 and 0.243, respectively. Figure 3g–i show the element mapping

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Figure 1. As-prepared precursor solutions with different molar ratio of Na/W after different times.

Figure 2 shows the XRD results of three kinds of NaxWO3 nanopowders obtained from the solvothermal reaction. The sharp and intensive XRD reflections indicate the well-crystallized character of the samples. The XRD pattern of sample with Na/W molar Nanomaterials 2021, 11, 731 ratio of 1:1 can be indexed on the basis of a hexagonal phase of Na-WO3 (JCPDS 81–0577).4 of 10 With increasing Na content, the peaks of (001) and (002) become stronger and all other peaks become weaker. The peaks of (001) and (002) also correspond to a cubic phase of Na-WO3 (JCPDS 28-1156). In addition, it can be seen from the partial enlarged view that the (001)increasing peak Naslightly content, moves the peaksto higher of (001) angle and with (002) the become increase stronger of Na andcontent, all other which peaks correspondsbecome weaker.to the position The peaks of of the (001) peak and near (002) 23° also on correspond the JCPDS to a81-0577 cubic phase and ofJCPDS Na-WO 3 28-1156.(JCPDS Therefore, 28-1156). we In infer addition, that the it cansample be seen exhibits from a thetendency partial to enlarged change viewto cubic that phase the (001) peak slightly moves to higher angle with the increase of Na content, which corresponds with increasing Na content. According to literature, the NaxWO3 shows a perovskite-type to the position of the peak near 23◦ on the JCPDS 81-0577 and JCPDS 28-1156. Therefore, crystal structure with cubic symmetry at x > ~0.4 [6,7], which is consistent with our XRD we infer that the sample exhibits a tendency to change to cubic phase with increasing Na results. content. According to literature, the NaxWO3 shows a perovskite-type crystal structure with cubic symmetry at x > ~0.4 [6,7], which is consistent with our XRD results.

Figure 2. XRD patterns of the obtained powders (left) and partial enlarged view (right). Figure 2. XRD patterns of the obtained powders (left) and partial enlarged view (right).

The SEM images of the synthesized NaxWO3 powders with different Na/W ratios areThe shown SEM images in Figure of the3. It synthesized can be seen Na fromxWO Figure3 powders3a–c thatwithall different three samples Na/W ratios are mainly are showncomposed in Figure of homogeneous3. It can be seen and from well-dispersed Figure 3a–c nanoparticles that all three with samples sizes of are several mainly tens of composednanometers. of homogeneous The EDS images and well-dispersed in Figure3d–f givesnanoparticles that the atomicwith sizes ratios of ofseveral Na/W tens for of three nanometers.samples The are 0.077,EDS images 0.173 and in Figure 0.243, respectively.3d–f gives that Figure the atomic3g–i show ratios the of element Na/W for mapping three of samplessample are with0.077, Na/W 0.173 ratioand 0.243, of 1:1, respectively. which confirms Figure the presence3g–i show and the uniform element distribution mapping of O, W and Na in selected area. To further study the detailed microstructure of obtained products, TEM was used to observe the grain morphology and crystallinity of the sample with Na/W molar ratio of 1:1, and results are given in Figure4. It can be seen from the TEM image that the sample is composed of nanoparticles with sizes of several tens of nanometers, which is consistent with the results of SEM. Moreover, two kinds of crystalline lattice constant in the HRTEM images were calculated as 0.389 and 0.638 nm, which agrees well with the interplanar spacing (0 0 1) and (100) of the hexagonal phase of Na-WO3 (JCPDS 81–0577), respectively. The chemical state of the NaxWO3 nanoparticles was carefully determined by XPS. Figure5 shows the typical XPS of the tungsten core level (W 4f) in the NaxWO3 nanoparticles with Na/W ratio of 1:1. The spectrum can be fitted to two groups of spin-orbit doublets of W4f7/2 and W4f5/2 with a separation distance of 2.1 eV, indicating two different oxidation states of W element. The peaks at 37.6 and 35.5 eV can be attributed to the W element being in a 6+ oxidation state, while 36.5 and 34.4 eV can be assigned to the W element being in a 5+ oxidation state. It is suggested that the typical nature of non-stoichiometric 6+ 5+ tungsten bronzes can be expressed as the formula of MxW 1-xW xWO3; our XPS results are in good agreement with this reduced feature. Na atoms can contribute a number 6+ of free electrons when they are doped into the structure of WO3, and part of the W will be reduced to W5+. The transparent heat-shielding properties of tungsten bronzes are closely related to the plasmon resonance of free electrons, and the concentration of free electrons has a great influence on the ωp. Therefore, the transparent heat-shielding properties of tungsten bronzes could be tuned by controlling the concentration of free electrons in its microstructure. Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 11

Nanomaterials 2021, 11, 731 5 of 10 of sample with Na/W ratio of 1:1, which confirms the presence and uniform distribution of O, W and Na in selected area.

Nanomaterials 2021Figure, 11Figure, x3. FOR SEM 3.PEER imagesSEM REVIEW images of Na x ofWO Na3 xsynthesizedWO synthesized with the with Na/W the ratio Na/W of ( ratioa) 1:1, of (b (a) )3:1, 1:1, (c (b) 5:1;) 3:1, corresponding (c) 5:1; corresponding EDS images6 of EDS 11 ( d images) 1:1, (e) 3:1, 3 (f) 5:1;(d element) 1:1, (e) mapping 3:1, (f) 5:1; of element sample mappingwith Na/W of ratio sample of 1:1 with (g Na/W) O, (h) ratioW, (i of) Na. 1:1 (g) O, (h) W, (i) Na.

To further study the detailed microstructure of obtained products, TEM was used to observe the grain morphology and crystallinity of the sample with Na/W molar ratio of 1:1, and results are given in Figure 4. It can be seen from the TEM image that the sample is composed of nanoparticles with sizes of several tens of nanometers, which is consistent with the results of SEM. Moreover, two kinds of crystalline lattice constant in the HRTEM images were calculated as 0.389 and 0.638 nm, which agrees well with the interplanar spacing (0 0 1) and (100) of the hexagonal phase of Na-WO3 (JCPDS 81–0577), respectively.

x 3 FigureFigure 4. TEM 4. TEM image image (a) and (a) HRTEM and HRTEM images images (b,c) of(b theand Na c) xofWO the3 nanoparticlesNa WO nanoparticles with Na/W with ratio Na/W of 1:1. ratio of 1:1.

The chemical state of the NaxWO3 nanoparticles was carefully determined by XPS. Figure 5 shows the typical XPS of the tungsten core level (W4f) in the NaxWO3 nanoparticles with Na/W ratio of 1:1. The spectrum can be fitted to two groups of spin-orbit doublets of W4f7/2 and W4f5/2 with a separation distance of 2.1 eV, indicating two different oxidation states of W element. The peaks at 37.6 and 35.5 eV can be attributed to the W element being in a 6+ oxidation state, while 36.5 and 34.4 eV can be assigned to the W element being in a 5+ oxidation state. It is suggested that the typical nature of non-stoichiometric tungsten bronzes can be expressed as the formula of MxW6+1-xW5+xWO3; our XPS results are in good agreement with this reduced feature. Na atoms can contribute a number of free electrons when they are doped into the structure of WO3, and part of the W6+ will be reduced to W5+. The transparent heat-shielding properties of tungsten bronzes are closely related to the plasmon resonance of free electrons, and the concentration of free electrons has a great influence on the ωp. Therefore, the transparent heat-shielding properties of tungsten bronzes could be tuned by controlling the concentration of free electrons in its microstructure.

Figure 5. Tungsten core-level (W4f) XPS spectra of the NaxWO3 nanoparticles with Na/W ratio of 1:1.

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Figure 4. TEM image (a) and HRTEM images (b and c) of the NaxWO3 nanoparticles with Na/W ratio of 1:1.

The chemical state of the NaxWO3 nanoparticles was carefully determined by XPS. Figure 5 shows the typical XPS of the tungsten core level (W4f) in the NaxWO3 nanoparticles with Na/W ratio of 1:1. The spectrum can be fitted to two groups of spin-orbit doublets of W4f7/2 and W4f5/2 with a separation distance of 2.1 eV, indicating two different oxidation states of W element. The peaks at 37.6 and 35.5 eV can be attributed to the W element being in a 6+ oxidation state, while 36.5 and 34.4 eV can be assigned to the W element being in a 5+ oxidation state. It is suggested that the typical nature of non-stoichiometric tungsten bronzes can be expressed as the formula of MxW6+1-xW5+xWO3; our XPS results are in good agreement with this reduced feature. Na atoms can contribute a number of free electrons when they are doped into the structure of WO3, and part of the W6+ will be reduced to W5+. The transparent heat-shielding properties of tungsten bronzes are closely related to the plasmon resonance of free electrons, and the concentration of free electrons has a great influence on the ωp. Therefore, the transparent heat-shielding properties of tungsten bronzes could be tuned by Nanomaterials 2021, 11, 731 controlling the concentration of free electrons in its microstructure. 6 of 10

Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 11

FigureFigureTo 5. Tungsteninvestigate 5. Tungsten core-level the core-level optical (W4f) (WXPS properties4f )spectra XPS spectra of ofthe of NaNa thexWO Na3x3 WO,nanoparticles the3 nanoparticles same amountwith withNa/W of Na/W ratioobtained ratioof of 1:1. 1:1.powder sample was uniformly dispersed and coated on the glass slide, and the absorp- To investigate the optical properties of Na WO , the same amount of obtained powder tion spectra are shown in Figure 6. All three samplesx 3 exhibit low and high absorption sample was uniformly dispersed and coated on the glass slide, and the absorption spectra characteristics in the visible and NIR region, respectively. The absorption valley occurs at are shown in Figure6. All three samples exhibit low and high absorption characteristics 555 nm for the sample with Na/W ratio of 1:1, and shifts to shorter wavelength of 514 nm in the visible and NIR region, respectively. The absorption valley occurs at 555 nm for for the sample with Na/W ratio of 5:1. The corresponding absorption peak in NIR region the sample with Na/W ratio of 1:1, and shifts to shorter wavelength of 514 nm for the shows the same trend as the absorption valley, with shifts from 1733 nm to 1498 nm. sample with Na/W ratio of 5:1. The corresponding absorption peak in NIR region shows These results illustrate that the transparency in visible and NIR region of nanocrystalline the same trend as the absorption valley, with shifts from 1733 nm to 1498 nm. These results NaxWO3 can be effectively tuned by x content. Generally, the low optical absorption in illustrate that the transparency in visible and NIR region of nanocrystalline NaxWO3 can visible region can be ascribed to the ωp, which corresponds to the peak in low energy re- be effectively tuned by x content. Generally, the low optical absorption in visible region gion of energy loss spectra. The electron energy loss spectra for NaxWO3 with different x can be ascribed to the ωp, which corresponds to the peak in low energy region of energy value have been measured by Tegg et al. [12], and results showed that the position of loss spectra. The electron energy loss spectra for NaxWO3 with different x value have peakbeen in low measured energy region by Tegg shifts et al. to [higher12], and energy results as showedx increases, that which the position is consistent of peak with in low our resultsenergy in region Figure shifts 6. to higher energy as x increases, which is consistent with our results in Figure6.

Figure 6. Absorption spectra of NaxWO3 nanoparticles with Na/W ratio of 1:1, 3:1 and 5:1. Inset Figure 6. Absorption spectra of NaxWO3 nanoparticles with Na/W ratio of 1:1, 3:1 and 5:1. Inset showsshows the photograph the photograph of the of thin the film thin samples ﬁlm samples used usedin the in test. the test.

However, the nanoparticulated NaxWO3 with higher x values could not be synthesized However, the nanoparticulated NaxWO3 with higher x values could not be synthe- in our experiment. Therefore, the optical properties of nanosized NaxWO3 with higher sized in our experiment. Therefore, the optical properties of nanosized NaxWO3 with higher x values in the visible and NIR region were calculated theoretically by using the DDA method. The DDA simulation gives the extinction efficiencies (Qext) of single particle. In order to compare simulation results with the experiment, we plotted Qext/aeff [23]. It should be noted that the absorption curve measured by UV-Vis spectrophotometer is converted from the transmittance of the sample, so the absorption spectrum from our measurement corresponds to the extinction curve from DDA calculations. Figure 7 gives the extinction curves of sphere-shaped NaxWO3 particles with size of 50 nm and x value of 0.522, 0.628, 0.695, 0.740, 0.860 and 0.940. For x = 0.522, the extinction valley occurs at 501 nm and gradually shifts to 423 nm for x = 0.94 with increasing x, which is in good agreement with our experimental results. Combining the experimental results with the DDA simulation results, it can be found that the position of the transmission peak of NaxWO3 in the visible region can be continuously adjusted by x value in the whole range of 0 < x < 1. In addition, the extinction peak position shifts from 749 nm to 640 nm when x increases from 0.522 to 0.94. Although the shifting trend with x value is the same as that in Figure 6, the position of extinction peak is much smaller than that in Figure 6. The

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x values in the visible and NIR region were calculated theoretically by using the DDA method. The DDA simulation gives the extinction efﬁciencies (Qext) of single particle. In order to compare simulation results with the experiment, we plotted Qext/aeff [23]. It should be noted that the absorption curve measured by UV-Vis spectrophotometer is converted from the transmittance of the sample, so the absorption spectrum from our measurement corresponds to the extinction curve from DDA calculations. Figure7 gives the extinction curves of sphere-shaped NaxWO3 particles with size of 50 nm and x value of 0.522, 0.628, 0.695, 0.740, 0.860 and 0.940. For x = 0.522, the extinction valley occurs at 501 nm and gradually shifts to 423 nm for x = 0.94 with increasing x, which is in good agreement with Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 11 our experimental results. Combining the experimental results with the DDA simulation results, it can be found that the position of the transmission peak of NaxWO3 in the visible region can be continuously adjusted by x value in the whole range of 0 < x < 1. In addition, reasonthe for extinction this difference peak position should shifts be related from 749 to the nm size to 640 and nm shape when ofx increasesnanoparticles. from 0.522The to experimental0.94. Although sample the is shifting composed trend withof particlesx value isof the various same assizes that inand Figure shapes,6, the and position the of measuredextinction absorption peak is muchspectrum smaller is the than compre that inhensive Figure6. effect The reason of all for particles. this difference However, should whatbe DDA related calculates to the size is particles and shape with of size nanoparticles. of 50 nm and The shape experimental of ideal sphere. sample In isorder composed to of particles of various sizes and shapes, and the measured absorption spectrum is the verify this inference, we also calculated the extinction behavior of NaxWO3 particles of differentcomprehensive sizes and shapes. effect of all particles. However, what DDA calculates is particles with size of 50 nm and shape of ideal sphere. In order to verify this inference, we also calculated the extinction behavior of NaxWO3 particles of different sizes and shapes.

Figure 7. Extinction curves of sphere-shaped NaxWO3 particles with size of 50 nm and different Figure 7. Extinction curves of sphere-shaped NaxWO3 particles with size of 50 nm and different x x value. value.

The calculated extinction curves of spherical-shaped Na0.522WO3 particles with dif- The calculated extinction curves of spherical-shaped Na0.522WO3 particles with different sizes and differently shaped Na0.522WO3 particles with size of 50 nm are given in ferentFigure sizes8 and. It can differently be found shaped that there Na was0.522WO significant3 particles difference with size between of 50 thenm extinctionare given curvesin Figureof different-sized8. It can be found particles. that Thethere extinction was significant valley shifts difference toward between short wavelength the extinction direction curvesand of broadens different-sized with the particles. decreasing The particleextinction size. valley While shifts the extinctiontoward short peak wavelength broadens and directionshifts and to long broadens wavelength with directionthe decreasing with the particle increasing size. particleWhile the size, extinction the peak intensitypeak broadensweakens. and Forshifts different to long shapes, wavelength there is direction little difference with the in theincreasing shape and particle position size, of the the ex- peaktinction intensity valley, weakens. while theFor intensity,different widthshapes, and there position is little of extinctiondifference peak in the are shape very different.and positionGenerally, of the aextinction strong absorption valley, while or scattering the intensity, effect occurswidth whenand position the collective of extinction oscillation peakfrequency are very different. of the conduction Generally, electrons a strong is ab thesorption same as or that scattering of the incident effect occurs photons, when which the collectiveleads to an oscillation enhancement frequency of the of electromagnetic the conduction field electrons in a very is the small same area as that of the of particlethe incidentsurface. photons, On the which other leads hand, to the ancharge enhancement distribution of the produced electromagnetic by collective field in oscillation a very of smallparticles area of the with particle different surface. shapes On is the also ot different;her hand, ourthe charge calculated distribution electric fieldproduced distribution by collectivearound oscillation the variously of particles shaped with NaxWO different3 particles shapes is shown is also in different; Figure9 (all our simulations calculated as- electricsume field that distribution the incident around light propagates the variously along shaped the x-axis.). NaxWO This3 suggestsparticles thatis shown the localized in Figuresurface 9 (all plasmonsimulations resonance assume (LSPR) that the is inci highlydent sensitive light propagates to different along symmetries the x-axis.). and This angles. suggests that the localized surface plasmon resonance (LSPR) is highly sensitive to different symmetries and angles. Therefore, the extinction behavior of particles with different shapes is quite different in the near infrared region. From the simulation results of particles with different sizes and shapes, it can be concluded that the wide range absorption in the NIR region in the experiment is a common effect of various sizes and shapes of NaxWO3 particles.

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Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 11 Therefore, the extinction behavior of particles with different shapes is quite different in Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 11 the near infrared region. From the simulation results of particles with different sizes and shapes, it can be concluded that the wide range absorption in the NIR region in the experiment is a common effect of various sizes and shapes of NaxWO3 particles.

Figure 8. Extinction curves of spherical-shaped Na0.522WO3 particles with different sizes Figure 8. Extinction curves of spherical-shaped Na0.522WO3 particles with different sizes (left) and (left) and differently shaped Na0.522WO3 particles with size of 50 0.522nm (right).3 Figuredifferently 8. Extinction shaped Na curves0.522WO of3 particlesspherical-shaped with size of Na 50 nmWO (right particles). with different sizes (left) and differently shaped Na0.522WO3 particles with size of 50 nm (right).

. Figure 9. Electric field distribution around the variously shaped Na0.522WO3 particles with effective radius of 50 nm at their Figure 9. Electric field distribution around the variously shaped Na0.522WO3 particles with effective . corresponding plasma resonance peaks. radius of 50 nmFigure at their 9. correspElectriconding field distribution plasma resonance around peaks.the variously shaped Na0.522WO3 particles with effective radius4. Conclusions of 50 nm at their corresponding plasma resonance peaks. 4. Conclusions The optical properties of nanoparticulated NaxWO3 with different x have been both The optical4.experimentally Conclusions properties of andnanoparticulated theoretically investigatedNaxWO3 with in different this paper. x have The EDSbeen results both showed that experimentallythe andThe atomic theoretically optical ratios properties of investigated Na/W of for nanoparticulated three in this samples paper. are The Na 0.077, xEDSWO 0.173 3results with and different showed 0.243, indicatingxthat have been that bothx is the atomic ratiosexperimentallygradually of Na/W increasing for and three theoretically samples in the synthesized are investigated 0.077, 0.173 samples. in and this Absorption0.243, paper. indicating The measurement EDS that results x is showed showed that that gradually increasingthethe atomic absorption in theratios synthesized valleyof Na/W in visiblefor samples. three region samples Absorption shifts are from 0.077, measurement 555 0.173 nm and to 514showed 0.243, nm indicating whenthat Na/W that ratiox is the absorptiongraduallychanges valley in fromincreasing visible 1:1 toregion 5:1,in the and shifts synthesized the from corresponding 555 samples. nm to absorption 514Absorption nm when peak measurement Na/W in NIR ratio region showed shifts that from changes fromthe 1:11733 absorptiontonm 5:1, to and 1498 thevalley nm. corresponding Thesein visible show region absorption that the shifts absorption peakfrom in 555 NIR (or nm transmittance) region to 514 shifts nm fromwhen behavior Na/W in visibleratio 1733 nm to 1498changesand nm. NIR Thesefrom region 1:1 show to of 5:1, nanocrystallinethat and the the absorption corresponding Nax (orWO transmittance)3 canabsorption be effectively peak behavior in tuned NIR in regionvisi- by x content. shifts from The ble and NIR region1733subsequent nm of tonanocrystalline 1498 DDA nm. simulation These Na showxWO results3 thatcan showbethe effectively absorption that this tuned tunable (or transmittance) bycharacteristic x content. Thebehavior is also suitablein visi- subsequent DDAblefor and thesimulation NIR nanoparticulated region results of nanocrystalline show Nax thatWO 3thwithis Natunable higherxWO3 characteristiccanx values. be effectively The is continuously also tuned suitable by x tunable content. optical The for the nanoparticulatedsubsequentproperties ofNaDDA nanocrystallinexWO simulation3 with higher results Na xWO values. show3 make Thethat it continuouslyth veryis tunable promising characteristic tunable in the optical fields is ofalso transparent suitable properties of fornanocrystallineheat-shielding the nanoparticulated materialsNaxWO3 Namake orx opticalWO it 3very with filters. promising higher x values. in the fieldsThe continuously of transparent tunable optical heat-shieldingproperties materials of or nanocrystalline optical filters. NaxWO3 make it very promising in the fields of transparent heat-shielding materials or optical filters.

Nanomaterials 2021, 11, 731 9 of 10

Author Contributions: Conceptualization, supervision and writing–original draft, L.C.; investiga- tion, methodology and software, C.S.; data curation, J.D.; visualization, J.L. (Jiaxin Li); validation, J.L. (Jia Liu); resources and formal analysis, Y.M.; writing–review and editing and funding acquisition, L.X. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (Grant No. 51776046), the Natural Science Foundation of Inner Mongolia (Grant No.2019MS05015), the Innovation Fund of Inner Mongolia University of Science and Technology (Grant No. 2019QDL-B35), the Guizhou Provincial Science and Technology Foundation (20191133) and the Youth Science and Technology Talent Development Fund in Guizhou Provincial Department of Education (2018250). Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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