A Study of the Optical Properties and Stability of Cs_0.33WO₃ with Different Particle Sizes for Energy-Efficient Window Films in Building Glazing

Abstract

Cs_0.33WO₃ (CWO) is a widely used inorganic material in window films and glass coatings, known for its excellent near-infrared radiation (NIR) blocking property and high visible light transmittance (T_vis). However, the stability of NIR blocking and the optical properties of CWO in the process of application is an urgent and important problem, because significant changes in optical results can impact the related products, such as window films, glass coatings, and so on. In this paper, the particle sizes and optical properties of CWO are tested to study the light stability and their relative relations. The results indicate that CWO particle sizes between 130 nm and 100 nm (D90, the point where 90% of the particles have a diameter smaller than the specified value) exhibit high stability in terms of NIR blocking and visible light transmittance (Tvis). CWO particles with D90 < 100 nm experience a greater reduction in NIR blocking, though this ability significantly recovers upon exposure to sunlight, making these coatings particularly suitable for use in tropical and subtropical climates.

1. Introduction

Carbon dioxide (CO₂) emissions have resulted in the frequent occurrence of climate anomalies and extreme climate disasters all over the world, making global climate change a common challenge for human beings [1]. Energy saving in residential and commercial buildings has recently become a critical global challenge all over the world, since these building types are responsible for 40% of total energy consumption and 24% of greenhouse emissions [2]. Windows in both buildings and automobiles play a crucial role in the energy saving field. Transparent conductive materials, such as antimony-doped tin oxide (ATO) [3,4,5,6], tin-doped indium oxide (ITO) [5], rare-earth hexaborides (RB₆) [7], metal-doped tungsten oxide (M_xWO₃) [8,9,10,11], and vanadium dioxide (VO₂) [12,13], are widely used for energy saving in sunshading films or coatings. In terms of their ability to block NIR, CWO (800–2000 nm) > ITO (1000–2500 nm) > ATO (1400–2500 nm) > LaB₆ (800–1300 nm) > VO₂ (1000–2500 nm). The main properties of CWO and ITO are high NIR shielding and transmittance of visible light. ITO is most widely used in applications such as light-emitting diodes, liquid crystal devices, touch screens, and photovoltaic cells [14]; however, ITO is less widely used than CWO in NIR shielding because of its high cost.

M_xWO₃ is well known for its NIR-shielding capabilities in energy-saving applications, such as window films and transparent coatings, including Na_xWO₃ [11], K_xWO₃ [9], (NH₄)_xWO₃ [9], and Cs_xWO₃ [8,10,15]. M_xWO₃, as one kind of NIR-shielding material, has attracted much attention in recent years due to its unique optical properties. Improving the NIR-shielding properties of M_xWO₃ has become a major research focus [16,17,18,19,20]. CWO is widely used because of its high NIR-shielding property and high visible light transmittance. However, the NIR-shielding ability of CWO used as a nano coating deteriorates in air, and its visible light transmittance increases at the same time. There have been few investigations into the stability of CWO. Kenji Adachi studied the mechanism of CWO bleaching on heating in air, in high humidity, or in water. Additionally, it has been concluded that the bleaching phenomenon occurs by an oxidation of CWO to produce Cs-deficient WO₃ on the particle surface [21]. Shuhei Nakakura demonstrated that less Cs-deficient Cs_0.32WO₃ nanoparticles can improve the photochromic stability [22]. Wi Hyoung Lee found that applying various protective layers (e.g., TEOS, fluoropolymer (CYTOP), FDS) [23] enhance the environmental stability of tungsten bronze NIR-absorbing coating. Yunxiang Chen studied the novel core–shell structure of CWO@ZnO, which demonstrated high NIR-shielding performance and excellent stability [24].

In this paper, CWO is synthesized and dispersed in a solvent with varying grinding times, resulting in CWO dispersions with different particle sizes. The stability and particle size of CWO are systematically investigated. The particle size of CWO significantly impacts its stability, which is crucial for its effective application due to its stability and optical properties.

2. Materials and Experiments

2.1. Materials

Ammonium paratungstate (APT, (NH₄)₁₀W₁₂O₄₁·5H₂O), cesium carbonate (Cs₂CO₃), and citric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). A pressure-sensitive adhesive (PSA) with a solid content of 45% was used as the resin in the window films. The dispersants were purchased from ALTANA Specialty Chemicals (Wessel, Germany).

2.2. Synthesis of CWO Nanopowders

APT was added to deionized water and stirred with citric acid. Cs₂CO₃ was dissolved in deionized water and then gradually added dropwise to the previously prepared mixture. The atomic ratio of Cs to W was maintained at 0.33. The resulting mixture was transferred to a Teflon-lined autoclave and heated at 200 °C for 12 h. The final product, CWO, was obtained by washing it three times with deionized water and ethanol, respectively, and then drying it at 100 °C. The obtained powder was subsequently heated in a quartz tubular furnace (Model: HTF1700-4/30, Shanghai Haoyue Co., Ltd., Shanghai, China) at 800 °C for 1 h under a 5% H₂/N₂ gas flow.

2.3. Preparation of Nano CWO Dispersions

A total of 90 g of CWO was added to 165 g of methyl isobutyl ketone (MIBK) solvent along with 45 g of dispersant. The mixture was milled using a planetary ball mill (Changsha MITR Instrument Co., Ltd., Changsha, China) with 0.3 mm ϕ ZrO₂ beads for various durations. The milling speed was set at 200 rpm. The resulting nano CWO dispersions were collected from the ZrO₂ agate jars and stored in brown glass bottles, as shown in Figure 1. The white bottle caps were labeled with the corresponding milling durations, such as 16 h, 28 h, and so forth.

2.4. Characterization

The prepared CWO was tested with an energy dispersive spectrometer (EDS, Model: Axis Supra+, Kratos) to analyze the ingredient, as shown in Figure 2. The result shows that the molecular formula of cesium tungsten oxide is Cs_0.32WO₃.

The CWO sample dispersions were obtained after milling for 16, 28, 40, 52, 64, 70, and 88 h, respectively. The particle sizes of CWO in the dispersions were analyzed using a Malvern particle size analyzer (Zetasizer Nano ZS, Malvern, UK). D10, D50, D90, and D99 are defined as the particle sizes below which 10%, 50%, 90%, and 99% of the particles fall, respectively, and were used to characterize the CWO particle size distribution.

To evaluate the optical performance of the CWO nanoparticles, transparent thermal insulating coatings were prepared on a soda-lime glass plate (5 mm thick, 100 mm × 100 mm). The coatings were formulated by mixing 3 g of CWO dispersion, 5 g of PSA resin, and 5 g of MIBK solvent. The mixture was applied onto the glass plate using an applicator with a concave edge, producing a wet film with a thickness of 50 μm. The coating was then dried at 100 °C for 3 min to remove the organic solvent from the resin and covered with a piece of release polyethylene terephthalate (PET) film to protect the sample coatings from air and humidity.

The haze values (Haze) were measured using a haze meter (Model: NDH 5000, Nippon Denshoku Industries Co., Ltd., Tokyo, Japan). Visible light transmittance (Tvis), UV light transmittance (Tuv, with a peak value at 365 nm), and near-infrared radiation blocking (RNIR, with a peak value at 940 nm) were tested using a solar film transmission meter (Model LS101, Shenzhen Linshang Technology Co., Ltd., Shenzhen, China). The T_vis of the transmission meter is in the range of 380 to 760 nm. When testing the coatings on glass, the released PET films were removed and then reapplied after testing to protect the coatings from humidity. The coating samples were stored at room temperature, away from direct sunlight.

3. Results and Discussion

3.1. Particle Size Analysis

The samples of milled CWO dispersions were obtained, after 16 h, 28 h, 40 h, 52 h, 64 h, 70 h, and 88 h of milling. The particle sizes depend on the duration of milling. These samples were analyzed to determine the particle size distribution of the CWO nano particles, as shown in Figure 3. With increased grinding time, the D50 value steadily decreases, along with D10 and D90 (Figure 3a). After 16, 28, and 40 h of milling, the D90 values remained above 140 nm. The D50 values for all samples were below 70 nm, which does not necessarily indicate low haze values in the CWO coatings. The D90 values of almost all samples are more than 100 nm, except for those milled for 64 h, 70 h, and 88 h. Once the grinding time surpassed 64 h, the D90 values fell below 100 nm, as shown in Figure 3a. A small number of larger nanoparticles can significantly affect the transparency of the coatings. Regarding D99, the particle size reached 185 nm in the sample milled for 88 h, which is larger than the D99 observed in the sample milled for 70 h. These larger particle sizes may be attributed to grinding bead contamination due to the extended milling duration.

3.2. Optical Data and Stability of CWO Nanoparticles

The CWO coating samples are from dispersions milled for different durations. The optical properties of the samples are shown in Figure 4. When the particles are grinded into smaller particles, the smaller the particles are, the higher the visible light transmittance and the greater the NIR-blocking capability of the nano coatings. The CWO dispersion, milled for 88 h, was used to form coatings on glass after being stored for 1 day and 30 days at room temperature, respectively. The optical properties are shown in Figure 4a. The NIR-blocking ability of the coating made with dispersion stored for 30 days significantly decreased compared to that of the dispersion stored for 1 day. This means the stability of the CWO dispersion ground for 88 h is somewhat compromised over time. In this paper, the CWO dispersions used for coatings were stored for 30 days in the brown glass bottles, as the optical properties changed among the storage times.

The haze values of the coatings are shown in Figure 4b. The haze values of the CWO coatings become smaller with the increasing grinding time. When the grinding time is 64 h, the lowest haze value appears. The haze values of 70 h and 88 h are bigger than that of 64 h; a small amount of big particles in the impure substance obtained from grinding ZrO₂ beads is because of the long grinding time.

The optical properties of CWO coatings with different storage and grinding time are shown in Figure 4c,d. The larger particle sizes of CWO dispersions with less grinding time, such as 16 h and 28 h, show higher NIR blocking, demonstrating a high stability among the optical performances. However, the transmittance of visible light is low and the haze value is very high (26%). In contrast, the CWO dispersions with smaller particle sizes and with more grinding time, such as 70 h and 88 h, show higher transparency and transmittance of visible light. However, the NIR blocking with more grinding time is lower than that of with less grinding time. When the D90 value of the CWO dispersion is less than 100 nm, high transmittance of visible light will be obtained, while the NIR blocking falls to about 5% to 7% compared with that of D90 > 100 nm.

The optical properties of CWO coatings tend to stabilize after more than 12 days at room temperature. The optical data of 90 d are shown in Figure 5. To investigate the effect of sunlight on the optical properties, the coating samples are placed on a balcony where they received a whole afternoon of sunlight, as shown in Figure 5c. In the morning, there was no direct sunlight. After 91 days, the NIR-blocking performance increased by approximately 2% to 7%, particularly for the samples milled for 70 and 88 h, which saw improvements of about 6% to 7%. This indicates a significant enhancement in NIR blocking. Although the samples milled for 16, 28, and 40 h exhibited high NIR blocking, their visible light transmittance was low, decreasing by around 2% to 5%. After 10 days of exposure on the balcony, the NIR-blocking performance of the 70 h and 88 h samples improved to 92% and 94%, respectively, with minimal changes in the other samples. Following 10 days of balcony exposure, all samples were kept in the dark for 12 h, with results shown in Figure 5. NIR blocking decreased in all samples after this dark period. Although the NIR-blocking performance of the 70 h and 88 h samples was lower than the others without direct sunlight, it was significantly enhanced with direct sunlight exposure. This variation in NIR blocking with and without sunlight is an interesting phenomenon that warrants further investigation.

The investigation results indicate that the chromatic instability of CWO is due to reactions between CWO particles and surrounding O₂ and H₂O molecules [21]. In this study, different CWO particles exhibited varying levels of NIR-blocking stability, with smaller particles showing less stability due to their larger specific surface area compared to larger particles. Smaller particles are more prone to oxidation, leading to instability. Particle size plays a crucial role in the application of CWO, as NIR blocking can be enhanced by direct sunlight exposure on CWO films or coatings. The UV radiation in sunlight particularly boosts NIR blocking in smaller CWO particles, a phenomenon that warrants detailed experimental study to better understand the relationship between NIR blocking and sunlight exposure.

4. Conclusions

CWO is synthesized using a hydrothermal method and annealed in a reduced atmosphere. While CWO is an excellent NIR-blocking material, its instability poses a challenge for commercial applications. To address this, research on enhancing the stability of CWO has been conducted, including adjusting its composition and applying various protective layers. In this study, the effect of particle size on the stability of CWO is investigated. Larger particles offer greater stability but result in higher haze values and lower visible light transmittance. Conversely, smaller particles are less stable but produce lower haze values. Exposure to direct sunlight improves the NIR-blocking efficiency of CWO coatings, particularly those with smaller particle sizes. However, once the direct sunlight is removed, these coatings allow more visible light and NIR to pass through, which can contribute to energy savings in buildings. Further research is needed to develop an optimal NIR-blocking material suitable for use in buildings or vehicles.