Enhanced Photoelectrochromic Performance of WO₃ Through MoS₂ and GO–MoS₂ Quantum Dot Doping for Self-Powered Smart Window Application

Abstract

Photoelectrochromic devices, which combine light-induced color change with energy-efficient optical modulation, have attracted significant attention for applications such as smart windows, displays, and optical sensors. However, achieving high optical modulation, fast switching speeds, and long-term stability remains a major challenge. In this study, we explore the structural and photoelectrochromic enhancements in tungsten oxide (WO₃) films achieved by doping with molybdenum disulfide quantum dots (MoS₂ QDs) and grapheneoxide–molybdenum disulfide quantum dots (GO–MoS₂ QDs) for advanced photoelectrochromic devices. X-ray diffraction (XRD) analysis revealed that doping with MoS₂ QDs and GO–MoS₂ QDs leads to a reduction in the crystallite size of WO₃, as evidenced by the broadening and decrease in peak intensity. Transmission Electron Microscopy (TEM) confirmed the presence of characteristic lattice fringes with interplanar spacings of 0.36 nm, 0.43 nm, and 0.34 nm, corresponding to the planes of WO₃, MoS₂, and graphene. Energy-Dispersive X-ray Spectroscopy (EDS) mapping indicated a uniform distribution of tungsten, oxygen, molybdenum, and sulfur, suggesting homogeneous doping throughout the WO₃ matrix. Scanning Electron Microscopy (SEM) analysis showed a significant decrease in film thickness from 724.3 nm for pure WO₃ to 578.8 nm for MoS₂ QD-doped WO₃ and 588.7 nm for GO–MoS₂ QD-doped WO₃, attributed to enhanced packing density and structural reorganization. These structural modifications are expected to enhance photoelectrochromic performance by improving charge transport and mechanical stability. Photoelectrochromic performance analysis showed a significant improvement in optical modulation upon incorporating MoS₂ QDs and GO–MoS₂ QDs into the WO₃ matrix, achieving a coloration depth of 56.69% and 70.28% at 630 nm, respectively, within 10 min of 1.5 AM sun illumination, with more than 90% recovery of the initial transmittance within 7 h in dark conditions. Additionally, device stability was improved by the incorporation of GO–MoS₂ QDs into the WO₃ layer. The findings demonstrate that incorporating MoS₂ QDs and GO–MoS₂ QDs effectively modifies the structural properties of WO₃, making it a promising material for high-performance photoelectrochromic applications.

1. Introduction

Since Bechinger’s discovery of photoelectrochromism in 1996 [1], the concept has gained popularity as a cost-effective energy-saving technology in buildings [2,3]. Photoelectrochromism is the phenomenon of a reversible change in optical transmittance without the need for an external voltage [4,5,6]. The development of self-switchable photoelectrochromic devices (PECDs), which can autonomously adjust their optical properties in response to light, represents a significant advancement in smart window technology and energy-efficient displays.

A PECD is a self-powered device that combines electrochromic and photovoltaic functions [7], changing color upon exposure to light [8]. For decades, PECDs have undergone architectural restructuring aimed at improving performance. Initially, the electrochromic (EC) and photoactive (PA) layers were coated on separate electrodes connected by wiring [1,9,10]. Later, in 2001, Hauch and his team improved device performance by coating the PA layer directly over the EC layer. This modified structure showed enhanced optical modulation along with improved bleaching kinetics [11].

However, in both structures, transparent conductive oxide (TCO) glass was used as the electrode, which was connected via wiring, complicating the manufacturing process and increasing the overall cost [12,13,14]. Furthermore, the use of TCO glass has been observed to significantly affect the initial transmittance, which may not be ideal for smart window applications. Over the past decades, scientists have investigated alternative PECD architectures, such as glass/EC/dye-sensitized TiO₂/electrolyte/glass structures, eliminating the need for both TCO glass and wiring as a cost-reduction strategy [14,15,16,17].

The electrochromic layer is regarded as one of the most crucial components in PECDs. Recent research has shown significant efforts directed toward improving this layer through the use of organic/inorganic hybrid materials and the incorporation of various transition metals. In particular, tungsten trioxide (WO₃) is the most studied transition metal oxide (TMO) for use as an electrochromic material due to its numerous outstanding properties, including excellent optical modulation and remarkable chemical stability [18,19,20]. A typical reversible color change mechanism of WO₃ is demonstrated in Equation (1) [20,21].

$${\mathrm{W}\mathrm{O}}_3\left(\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{r}\mathrm{l}\mathrm{e}\mathrm{s}\mathrm{s}\right)+{\mathrm{x}\mathrm{M}}^{+}+{\mathrm{x}\mathrm{e}}^{-}\rightleftharpoons {\mathrm{M}}_{\mathrm{x}}{\mathrm{W}}_{1-\mathrm{X}}^{6+}{\mathrm{W}}_{\mathrm{X}}^{5+}{\mathrm{O}}_3\left(\mathrm{b}\mathrm{l}\mathrm{u}\mathrm{e}\right)$$

(1)

Here, M represents ions such as H, Li, K, Zn, Al, and Na, while x denotes the number of intercalated ions [22], varying from 0 to 1. When exposed to light, the partial reduction of W⁶⁺ to W⁵⁺ and the subsequent introduction of a proton for charge compensation in WO₃ result in the formation of tungsten bronze (M_xWO₃). This compound displays a blue color due to the intervalence charge transfer (IVCT) transition, which involves an electron transfer between adjacent W⁵⁺ and W⁶⁺ ions [23,24].

Despite the impressive properties of WO₃, WO₃-based PECD technology suffers from long response times [3,25], which poses a drawback for commercialization. To address this, numerous attempts have been made to fabricate EC materials using WO₃-based composites, such as WO₃/MoO₃, WO₃/Pt, and WO₃/graphene quantum dots (QDs), showing improved response times in PECDs [3,14,17]. According to Dao et al. [12] and Sarwar et al. [13], the introduction of Pt into WO₃ resulted in a tremendous improvement in response time. Additionally, the results from our previous study illustrated that co-doping WO₃ with Al and Pt led to enhanced optical modulation and bleaching kinetics [16].

However, the Pt catalyst has certain drawbacks, such as high cost and changes in its electrocatalytic properties due to the adsorption of iodide species, including iodine [26], which can impact not only the overall cost of the device but also its long-term performance. Therefore, it is essential to search for alternative dopant materials that are both cost-effective and capable of improving the electron transport properties of WO₃.

According to Chang et al. [3] and Ahmad et al. [27], the introduction of two-dimensional (2D), conductive, or layered nanomaterials into the WO₃ matrix offers a large surface area, improved electron transport properties, and enhanced optical transmittance for composite materials. Recently, graphene quantum dots (GQDs) have been used in conjunction with WO₃ for the fabrication of PECDs, and the results revealed an improved response time, transmittance contrast, and outstanding cycle stability of up to 1000 cycles, attributed to the presence of hydrogen bonds formed between the functional groups of GQDs and WO₃ [14].

In addition, two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as MoS₂, CoS₂, and MoSe₂ have recently been explored in combination with WO₃ for the development of ECDs, revealing enhanced coloration efficiency along with good long-term cycle stability [27,28,29,30]. In a study by Sharma et al. [31], it was observed that MoS₂–WO₃ composites exhibit faster response times and higher coloration efficiencies compared to pristine WO₃ films. The introduction of MoS₂ was found to facilitate more efficient charge transfer and provide additional active sites for ion storage, thereby improving the overall performance of the electrochromic device. In another study, Xiao and Zhang utilized ultra-small MoS₂ QDs as surface sensitizers to enhance the photoelectrochemical (PEC) properties of MoS₂/WO₃. They found that incorporating MoS₂ QDs into WO₃ not only expands the photoabsorption range but also offers more active sites for surface reactions and improves the separation of photogenerated electron–hole pairs [32]. However, the contact area between WO₃ and MoS₂ tends to be limited. To address this, Li and his team incorporated MoS₂–rGO into WO₃ to increase the conductivity and contact surface area between WO₃ and MoS₂ [33]. The findings from these studies highlight the impressive role of MoS₂ materials in reducing electron–hole recombination and enhancing light absorption in WO₃, both critical factors for efficient device operation. However, its performance in photoelectrochromic devices has yet to be explored.

Building on this foundation, our study aims to investigate the doping of WO₃ with MoS₂ QDs and GO–MoS₂ QDs specifically for self-switchable photoelectrochromic applications. By leveraging the superior light absorption and charge transport properties of MoS₂ QDs, we seek to develop a self-switchable photoelectrochromic device that offers rapid response, high efficiency, and long-term stability. Additionally, by combining the excellent conductivity of graphene with the large specific surface area of MoS₂ QDs, the incorporation of the GO–MoS₂ QD composite into the WO₃ layer may significantly enhance the device’s performance. Furthermore, the introduction of MoS₂ QDs and GO–MoS₂ QDs may facilitate Li⁺ ion insertion and reduce interface defects, resulting in the enhanced photoelectrochromic properties of WO₃. This work not only contributes to the understanding of the interactions between WO₃, MoS₂ QDs, and GO–MoS₂ QDs but also opens new avenues for the design of advanced smart materials for energy-efficient PECD applications.

2. Experimental Method

2.1. Reagents

All chemicals used in this study were of analytical grade and utilized as received, without further purification. Tungsten sol (ECS-C1) and titanium sol (TSD-030) were obtained from Adchro (Busan, Republic of Korea). 5-Methylsalicylic acid (5MSA, >98%) was obtained from Tokyo Chemical Industry Co. (Tokyo, Japan). Lithium trifluoromethanesulfonate (LiTF), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), tetramethylthiourea (TMTU), tetramethylformaminium disulfide (TMFDS²⁺), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O), chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O), molybdenum disulfide powder (MoS₂, <2 μm diameter), graphene oxide (GO), gamma-butyrolactone (GBL), adiponitrile (ADN), hydrogen peroxide (30 wt% aqueous solution), N-methyl-2-pyrrolidinone (NMP), and other solvents were obtained from Sigma-Aldrich (St. Louis, MI, USA) and used without modification.

2.2. Synthesis of MoS₂ and GO–MoS₂ Quantum Dots

MoS₂ quantum dots (QDs) were synthesized using a process in which a mild oxidant (H₂O₂) initiated exfoliation [31,34]. Briefly, 200 mg of MoS₂ powder was dispersed in 100 mL of a solvent mixture containing 30% H₂O₂ and NMP (1:1 ratio) and stirred at 35 °C for 12 h. The exfoliated MoS₂ solution [34] was then centrifuged at 10,000 rpm for 5 min, and the top quarter of the supernatant was collected. This centrifugation process was repeated several times, resulting in a yellowish transparent solution.

A similar procedure was followed for the synthesis of GO–MoS₂. Here, a mixture of 200 mg of MoS₂ and 40 mg of GO powder was dispersed in 100 mL of the same solvent mixture (30% H₂O₂ and NMP, 1:1 ratio). The subsequent steps were identical to those used in synthesizing the MoS₂ QD solution. After repeated centrifugation, a yellowish transparent solution was obtained, as shown in Figure 1.

2.3. Deposition of WO₃ and TiO₂ Layers

The precursor solutions of WO₃ doped with MoS₂ QDs and WO₃ doped with GO–MoS₂ QDs were prepared by adding 30 µL of MoS₂ QD solution and 60 µL of GO–MoS₂ QD solution, respectively, dropwise into 5 g of a commercial WO₃ solution while magnetically stirring. The mixtures were stirred continuously for 2 h. The films of WO₃ and TiO₂ layers were deposited using the spin-coating technique, following the process described by Chun et al. [35].

Briefly, chromogenic layers were formed on soda–lime glass substrates (5 cm × 5 cm) by spin-coating undoped WO₃ and WO₃ doped with MoS₂ QDs at 5000 rpm for 30 s. The coated films were dried in an oven at 135 °C for 2 min. To optimize the thickness of the chromogenic layer, the procedure was repeated. This was followed by the deposition of a TiO₂ solution on top of the chromogenic layer at 5000 rpm for 30 s, with subsequent annealing in an oven at 400 °C for 1 h (with an acceleration time of 2 h).

2.4. Electrolyte Preparation

The electrolyte was prepared by dissolving 0.002 M LiTf, 0.0005 M LiTFSI, 0.26 M LiBF₄, 0.5 M TMTU, and 0.1 M TMFDS²⁺ in a mixed solvent comprising 1 mL ADN and 1 mL GBL. The solution was magnetically stirred for approximately 4 h.

2.5. PECD Assemblage

The annealed substrates were dipped in a 0.9 mM solution of 5-MSA in pure ethanol and incubated in a warmer at 40 °C for 4 h to allow for the adequate attachment of the ligand to the TiO₂ surface. After dye adsorption, the substrates were removed, rinsed twice with pure ethanol to wash away unattached ligands, and air-dried at room temperature. The coated glass substrates were then covered with pre-drilled, uncoated glass and sandwiched using 60 μm-thick thermoplastic resin (Surlyn). The assembled devices were dried in an oven at 135 °C for 7 min. Finally, the electrolyte, containing TMTU/TMFDS²⁺ as the redox couple, was injected into the device through the pre-drilled uncoated glass (Figure 2).

2.6. Sample Characterization

A scanning electron microscope (SEM) analysis was performed using a Hitachi S-4800 scanning electron microscope (Hitachi High-Technologies Corporation, Tokyo, Japan) to examine the morphology and thickness of the samples. X-ray diffraction (XRD) analysis, with monochromatized Cu-Kα radiation (λ = 1.5406 Å), was conducted using a SmartLab High Resolution (Rigaku Holdings Corporation, Tokyo, Japan) instrument to investigate the crystal structure of WO₃, pure MoS₂ QDs, pure GO–MoS₂ QDs, MoS₂ QD-doped WO₃, and GO–MoS₂ QD-doped WO₃ within a 2θ range of 10° to 90°. The structure of the MoS₂ QD-doped WO₃ and GO–MoS₂ QD-doped WO₃ films was further analyzed using a transmission electron microscope (TEM) Talos F200X (Thermo Fisher Scientific lnc., Seoul, Republic of Korea).

After annealing, the films were scraped off the glass substrate with a stainless-steel blade and dispersed in anhydrous ethanol. The resulting mixture was sonicated, and a drop of the solution was deposited onto a grid for imaging. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Scientific K-alpha (Thermo Fisher Scientific lnc., Seoul, Republic of Korea) instrument to identify the chemical states of the main elements in each layer. The binding energies were calibrated against the C 1s peak at 283.73 eV as the reference.

Photoelectrochromic devices (PECDs) were illuminated using a commercial solar simulator Oriel 91,192 (Oriel Instrument, New York, NY, USA) under one sun (AM 1.5) condition for coloration induction, with the simulator calibrated using a Si reference. The optical properties of the PECDs in their colored and bleached states were measured using a UV–Vis–NIR spectrometer DH-2000-BAL (Ocean Optics, Orlando, FL, USA).

3. Results and Discussion

3.1. XRD Analysis

XRD analysis was conducted to gather information about the crystallinity, purity, and molecular structures of the samples, as these factors significantly influence the photoelectrochromic performance. The diffraction peaks of pristine WO₃, pure MoS₂ QDs, pure G–MoS₂ QDs, MoS₂ QD-doped WO₃, and G–MoS₂ QD-doped WO₃ nanocomposites are shown in Figure 3. The diffraction peaks of pristine WO₃, centered at 2θ = 13.94°, 23.04°, 28.35°, 36.68°, etc., exhibit characteristic XRD peaks corresponding to the (010), (001), (020), (111), (021), (220), (221), (040), and (041) planes [36], associated with a hexagonal structure (JCPDS card no. 98-001-3847). While similar peaks were observed for the MoS₂ QD-doped WO₃ and G–MoS₂ QD-doped WO₃ films, their intensities were reduced, and the peaks indexed to the (111) and (041) planes, previously observed in pristine WO₃, were flattened. This change can be attributed to structural transformation caused by the layered structure of MoS₂ QDs. However, no peaks corresponding to MoS₂ or graphene compounds were observed in the XRD spectrum, likely due to the low doping amount and weak diffraction signals in the as-prepared samples. Figure 3a,b shows a broad XRD peak for MoS₂ and G–MoS₂, centered at 2θ = 23.09°, which can be indexed to the (002) plane of the hexagonal MoS₂ structure (JCPDS 37-1492) [29]. The broadening of this peak is attributed to the small crystallite size, consistent with the nanoscale dimensions of quantum dots. No other peaks indicating impurities, such as MoO₃ or related compounds, were identified, confirming the successful formation of the pure phase of MoS₂ QDs and G–MoS₂ QDs.

The average crystallite size (D) for WO₃, MoS₂ QDs-doped WO₃, and G–MoS₂ QD-doped WO₃ was calculated using the Debye–Scherrer equation [37,38].

$$D={\displaystyle \frac{0.94\lambda }{\beta cos\theta }}$$

(2)

where β is the full width at half maximum (FWHM) of the XRD peaks and λ is the X-ray wavelength. The calculated average crystallite sizes for WO₃, MoS₂ QD-doped WO₃, and G–MoS₂ QD-doped WO₃ were determined to be 26.5 nm, 22.4 nm, and 14.3 nm, respectively. This reduction in crystallite size indicates structural refinement and an increase in the surface area, which could enhance charge transport and optical properties in the MoS₂ QD-doped WO₃ and G–MoS₂ QD-doped WO₃. Furthermore, the additional reduction in crystallite size for the G–MoS₂ QD-doped WO₃ film can be attributed to the presence of graphene, which influences nucleation and growth processes during film formation, resulting in smaller crystallites.

3.2. Morphology Analysis

This section presents the morphological characterization of the as-prepared films. Detailed analysis using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) was conducted to investigate the surface structure, grain size, particle distribution, and interfacial characteristics of pure and doped WO₃ films. Understanding the morphology is crucial, as it directly influences the optical, electrical, and photoelectrochromic performance of the devices.

Figure 4 presents the SEM micrographs of the as-prepared samples, illustrating uniform and firmly adhered films. Figure 4a,b shows SEM images of pure MoS₂ QD and GO–MoS₂ QD film samples, respectively, depicting an even distribution of small, spherically shaped nanoparticles. The SEM image of pure MoS₂ QDs (Figure 4a) reveals a surface covered with grains that are evenly distributed, though some agglomerated granules of spherical shape with varied sizes ranging from 11 nm to 70 nm (average grain size: 32.7 nm) are observed. The grain sizes are larger than the commonly reported sizes of around 10 nm or less, which can be attributed to Ostwald ripening during the annealing process. In contrast, as shown in Figure 4b, the incorporation of graphene into MoS₂ QDs results in a significant reduction in grain size, attributed to the stabilizing effect of graphene. The surface is covered with regularly shaped GO–MoS₂ QDs, with an average grain size of 10.9 nm.

The surface image of neat WO₃ in Figure 4c clearly shows uniform nanoparticles with irregular hexagonal shapes, which are well interconnected—indicating sufficient binding energy to combine with neighboring grains. The morphology of the neat WO₃ film is similar to that of WO₃ decorated with MoS₂ QDs (Figure 4d) and G–MoS₂ QDs (Figure 4e), a feature attributable to the small size of MoS₂ QDs [31]. The surface images also reveal a noticeable reduction in grain size in the samples of WO₃ doped with MoS₂ QDs and WO₃ doped with G–MoS₂ QDs. The grain sizes of neat WO₃ range from 46 nm to 110 nm, with an average grain size of 84.4 nm. In comparison, the grain sizes of MoS₂ QD-doped WO₃ range from 18 nm to 52 nm (average grain size: 33.8 nm), and those of G–MoS₂ QD-doped WO₃ range from 18 nm to 37 nm (average grain size: 26.1 nm). This observation aligns well with the results of the XRD analysis. The reduction in particle sizes upon the incorporation of MoS₂ QDs and G–MoS₂ QDs can be attributed to the creation of nucleation sites and changes in the growth process, which are strongly influenced by the lattice parameters of the dopant atoms [39]. These factors limit the growth of WO₃ particles, resulting in smaller and more uniformly sized particles.

Additionally, the SEM images clearly reveal the presence of pores and grains, which are especially pronounced in the WO₃ samples decorated with MoS₂ QDs and GO–MoS₂ QDs. This feature is crucial, as it suggests potential enhancement in photoelectrochromic performance due to the ease of intercalation and deintercalation of Li⁺ ions during the coloring and bleaching processes [14]. These pores act as channels that facilitate ion intercalation and deintercalation, effectively lowering diffusion barriers. As a result, ions can traverse the material more efficiently and swiftly, improving ion storage and enhancing the kinetics of coloration and bleaching [14]. The cross-sectional SEM images confirm that the films have good adhesion to the glass substrate (Figure 4f–h). The measured thicknesses of the films were 724.3 nm for neat WO₃, 578.8 nm for MoS₂ QD-doped WO₃, and 588.7 nm for GO–MoS₂ QD-doped WO₃, respectively.

The observed changes in film thickness can be explained by the interactions between the WO₃ matrix and the dopants. Doping WO₃ with MoS₂ QDs reduces the thickness to 578.8 nm, a reduction attributed to enhanced packing density and structural reorganization facilitated by the incorporation of MoS₂ QDs. The more compact and dense structure observed in the doped film suggests improved mechanical stability and potentially better photoelectrochromic (PEC) performance, owing to shorter pathways for ion and electron transport. When doped with graphene-modified MoS₂ QDs, the thickness slightly increases to 588.7 nm. This increase is attributed to the presence of graphene, which induces a less dense packing structure compared to pure MoS₂ QDs. By comparing Figure 4c–h, it can be concluded that the gradual decrease in grain size and the changes in morphology enhance the ease of Li⁺ ion transportation through the samples.

To confirm the incorporation of MoS₂ QDs and G–MoS₂ QDs into WO₃, the samples were further analyzed using EDS mapping. Figure 5a,b displays the spatial distribution of W, O, Mo, and S in the MoS₂ QD-doped WO₃ and G–MoS₂ QD-doped WO₃ samples, respectively. The uniform distribution of MoS₂ and G–MoS₂ on the WO₃ nanoparticles confirms the successful inclusion of these dopants into the WO₃ surface. These results demonstrate that the doping process effectively modifies the WO₃ film structure, enhancing its suitability for application in photoelectrochromic devices.

3.3. TEM Analysis

The structures of MoS₂ QD-doped WO₃ and GO–MoS₂ QD-doped WO₃ were further characterized using TEM images (Figure 6a–f), which confirms that MoS₂ QDs and GO–MoS₂ QDs are successfully bonded with WO₃. The TEM images reveal inter-planar ddd-spacings of 0.36 nm and 0.43 nm, corresponding to the (001) and (011) crystal planes of hexagonal WO₃, respectively. These observations are consistent with the XRD results. Additionally, inter-planar ddd-spacings of 0.34 nm and 0.63 nm were identified, corresponding to the (002) crystal planes of graphene and MoS₂, respectively [33,40]. This is in agreement with the XRD results for the pure MoS₂ QDs and GO–MoS₂ QDs reported earlier. These findings confirm the successful doping of WO₃ with MoS₂ QDs and GO–MoS₂ QDs, indicating that the composites retain a high degree of crystallinity, which is likely to enhance their performance in photoelectrochromic devices. The insets in Figure 6c,f display distinct ring patterns from selected area electron diffraction (SAED), further demonstrating the polycrystalline nature of both film samples.

3.4. XPS Analysis

XPS analysis was conducted to verify the surface elemental compositions and the valence states of elements in the sample. The XPS survey of the GO–MoS₂ QD-doped WO₃ film, shown in Figure 7a, confirms the presence of tungsten (W) and oxygen (O). However, due to the small amount of GO–MoS₂ QDs, the peaks for molybdenum (Mo), sulfur (S), and carbon (C) are less intense and not clearly visible in the survey spectrum but are detectable in the high-resolution spectra. In Figure 7b, high-resolution XPS analysis reveals two prominent peaks centered at 35.6 eV and 37.6 eV, corresponding to the 4f orbitals (4f₇/₂ and 4f₅/₂) of W⁶⁺ [41]. Similar peaks were observed in the WO₃ film doped with MoS₂ QDs. Additionally, two lower-intensity peaks at 36.7 eV and 38.7 eV indicate the presence of W⁵⁺ (4f₇/₂ and 4f₅/₂) in the film. The blue coloration observed during the operation of the self-switchable PECD WO₃ film is primarily attributed to the inter-valence charge transfer (IVCT) between W⁶⁺ and W⁵⁺. No peaks corresponding to W⁴⁺ were observed, likely due to the annealing process in air, which promotes higher oxidation states and maximizes contrast in both the bleached and colored states [30].

Figure 7d shows a sharp peak at 231.6 eV, attributed to the 3D orbitals of Mo⁴⁺ ions [42,43]. Figure 7e displays a sharp peak for S 2p at 167.9 eV [44], corresponding to Mo–S bonding. This peak is shifted to a higher binding energy compared to the pure S atom [30], likely due to interaction with WO₃, forming a Mo–S–W linkage. However, these peaks are less intense, possibly due to the partial oxidation of MoS₂ to Mo₂O₅ or MoO₃ in the oxygen-rich WO₃ environment. No direct evidence for these compounds is observed in the XRD results, necessitating further studies to confirm their existence. The XPS peak for C 1s, presented in Figure 7f, is deconvoluted into four components with binding energies of 284.8 eV, 286.4 eV, 288.6 eV, and 293.3 eV, attributed to the functional groups C–C, C–O, and O–C=O [40,45,46], as well as the π–π* transition, indicative of delocalized electrons within the composite [47]. The high-resolution XPS spectra of the O 2p peaks at binding energies of 530.04 eV, 531.80 eV, and 532.57 eV provide valuable insights into the chemical states of oxygen in the WO₃ doped with GO–MoS₂ QDs. The peak at 530.04 eV corresponds to oxygen atoms bonded to tungsten (W–O) in the WO₃ lattice, confirming the preservation of the WO₃ structure after doping. The peak at 531.80 eV is attributed to surface hydroxyl groups (O–H), likely resulting from surface adsorption processes. The peak at 532.57 eV is associated with loosely bound oxygen species such as O⁻ and adsorbed water (H₂O), reflecting their presence on the film surface. These findings highlight the complex chemical environment in the doped WO₃ film, which is critical for understanding its enhanced photoelectrochromic properties [12].

3.5. Optical Performance Analysis

We fabricated a photoelectrochromic device (PECD) for architectural applications, structured as glass/EC layer/TiO₂–5-MSA/electrolyte/glass. This device was used to investigate the effectiveness of doping WO₃ with MoS₂ quantum dots (QD) and GO–MoS₂ QDs by comparing it with the performance of pristine WO₃-based devices (Figure 8a–c). The coloration of the device is driven by the efficiency of the dye-sensitized solar cells under illumination. To induce coloration in the PECD, we exposed it to standard 1 sun solar irradiation, and bleaching was achieved by storing the device in the dark.

The devices under investigation demonstrated an initial transmittance of over 79% in the visible region, highlighting their high transparency in the unaltered state. This high transparency can be attributed to the uniformity of the films and the use of soda–lime glass instead of TCO glass. Upon exposure to one sun solar illumination, the devices rapidly transitioned to a blue coloration, reaching saturation within 4 min of exposure. The exposure time for all devices was standardized at 10 min. The device with pristine WO₃ exhibited a notable change in optical properties, achieving a coloration depth of 36.81% at a wavelength of 550 nm.

In comparison, the WO₃ device doped with MoS₂ QDs showed enhanced optical modulation, reaching a coloration depth of 41.3% at the same wavelength. Remarkably, the incorporation of GO–MoS₂ QDs into WO₃ further improved modulation, resulting in a coloration depth of 51.79% at 550 nm. This wavelength, chosen for its relevance to visible light applications and high sensitivity to the human eye [14], underscores the device’s effectiveness in modulating light within the visible spectrum. Notably, at 630 nm, the GO–MoS₂ QD-doped WO₃ device achieved a remarkably high coloration depth of 70.28%, comparable to established electrochromic devices [48,49,50].

The devices were then placed in the dark for bleaching, and their transmittance was measured periodically. Post-recovery measurements revealed that the device containing WO₃ decorated with GO–MoS₂ QDs regained approximately 95% of its original transmittance within 7 h in the dark. Similarly, the device with WO₃ decorated with MoS₂ QDs recovered about 93.21% of its original transmittance within the same time frame. This recovery rate demonstrates a commendable degree of reversibility and stability in the device’s optical properties, especially considering the level of optical modulation achieved.

Furthermore, the devices exhibited no significant residual coloration, indicating their ability to reliably return to their initial state after use. The impressive performance of the device containing WO₃ doped with GO–MoS₂ QDs can be attributed to the presence of graphene, which facilitates the reconstruction of π–π* conjugation within the nanocomposite, as observed in the earlier C 1s peak analysis. These changes alter the density of states within the composite, causing a downward shift in the conduction band and reducing the bandgap energy. This effectively suppresses electron–hole recombination [25], thereby enhancing the photoelectrochromic performance of the composite.

Figure 8d compares the performance of undoped WO₃-based PECDs with those doped with MoS₂ QDs and GO–MoS₂ QDs. The bleaching half-life period (τ, in minutes) of the composite devices was calculated, where τ is defined as the time required for the film’s transmittance to change from its initial colored state (achieved after 10 min of irradiation) to half of its final value in the bleached state [51]. The undoped WO₃-based PECD exhibited a coloration depth of 36.81% and a half recovery time of 180 min. Upon doping with MoS₂ QDs, the WO₃-based PECD demonstrated an improved coloration depth of 41.3% and a significantly reduced half recovery time of 50 min. The WO₃-based PECD doped with GO–MoS₂ QDs achieved the highest performance, with a coloration depth of 51.79% at 550 nm and a half recovery time of 72 min. The half recovery time (t₁/₂) refers to the time needed for the transmittance of a photoelectrochromic device to return to half of its maximum change when placed in a dark condition. This parameter reflects the device’s efficiency and responsiveness in transitioning from the colored (darkened) state to the bleached (clear) state. The data clearly show that doping WO₃ with MoS₂ QDs and GO–MoS₂ QDs enhances the photoelectrochromic performance by increasing coloration depth and reducing recovery time.

The significant improvements observed in the MoS₂ QDs and GO–MoS₂ QD-doped WO₃-based PECDs can be attributed to several factors: enhanced charge separation, improved electronic properties, increased surface area, and synergistic effects of the dopants. These modifications lead to a shift in the conduction band and reduced bandgap energy, resulting in more effective photon absorption, improved utilization, higher coloration depths, and faster electron transport pathways. Consequently, the doped WO₃-based PECDs outperform the undoped counterparts, achieving higher coloration depths and significantly faster recovery times.

In comparison to our previous work on the effect of co-doping WO₃ with Al and Pt [16], where the optical modulation was 43.61% with a recovery time of 7 h, it is evident that GO–MoS₂ QDs are a superior alternative dopant material to Al and Pt. As highlighted above, the incorporation of GO–MoS₂ QDs into WO₃ enhanced optical modulation, achieving 51.74% while maintaining the ability to recover the original transmittance within 7 h in dark conditions. This notable improvement in optical modulation, combined with a comparable bleaching time, underscores the potential of GO–MoS₂ QDs as an effective dopant for improving the photoelectrochromic performance of WO₃-based devices.

3.6. Working Mechanism of PECD

Figure 9 illustrates a schematic diagram of the proposed working mechanism for our device. The coloration and bleaching processes in the WO₃ doped with a MoS₂ QD-based photoelectrochromic device involve complex interactions between electronic and ionic processes, enhanced by the presence of MoS₂ QDs as reported in earlier studies. Upon illumination, electrons from 5-MSA are donated directly to TiO₂ via ligand-to-metal charge transfer (LMCT) [12,52] and subsequently injected into the WO₃/MoS₂ QDs matrix. The injected electrons reduce W⁶⁺ ions in WO₃ to W⁵⁺. This reduction process is facilitated by the enhanced charge transfer capabilities of MoS₂ QDs, which provide additional pathways for efficient electron transport. To maintain charge neutrality, Li⁺ ions from the electrolyte migrate into the WO₃/MoS₂ QDs matrix, leading to a color change from colorless to deep blue.

The presence of both W⁶⁺ and W⁵⁺ ions enables intervalence charge transfer (IVCT), wherein electrons hop between W⁶⁺ and W⁵⁺ states. This IVCT mechanism is responsible for the blue coloration observed in the device [53]. The enhanced conductivity and charge separation provided by MoS₂ QDs and GO–MoS₂ QDs further improve the efficiency of this process. Simultaneously, the oxidized form of 5-MSA (5-MSA⁺) undergoes regeneration through a redox reaction. During this process, TMTU releases an electron and is oxidized to TMFDS²⁺ [23], ensuring charge balance during the coloration period and stabilizing the coloration state.

The coloring mechanism can be expressed as follows [12,13,23]:

$$5-\mathrm{M}\mathrm{S}\mathrm{A}+{\mathrm{T}\mathrm{i}\mathrm{O}}_2+\mathrm{h}\mathrm{v}\to {5-\mathrm{M}\mathrm{S}\mathrm{A}}^{+}+{\mathrm{T}\mathrm{i}\mathrm{O}}_2\left({\mathrm{e}}^{-}\right)\left(\mathrm{L}\mathrm{M}\mathrm{C}\mathrm{T}\right)$$

(3)

$${\mathrm{W}\mathrm{O}}_3:{\mathrm{M}\mathrm{o}\mathrm{S}}_2\left(\mathrm{s}\right)+\mathrm{x}\left({\mathrm{L}\mathrm{i}}^{+}+{\mathrm{e}}^{-}\right)\to {\mathrm{L}\mathrm{i}}_{\mathrm{x}}{\mathrm{W}\mathrm{O}}_3{\mathrm{M}\mathrm{o}\mathrm{S}}_2(\mathrm{b}\mathrm{l}\mathrm{u}\mathrm{e} \mathrm{c}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{r})$$

(4)

$$2\left({5-\mathrm{M}\mathrm{S}\mathrm{A}}^{+}\right)+\mathrm{T}\mathrm{M}\mathrm{T}\mathrm{U}\to {\mathrm{T}\mathrm{M}\mathrm{F}\mathrm{D}\mathrm{S}}^{2+}$$

(5)

When the light is withdrawn, electrons are extracted from the WO₃/MoS₂ QDs layer. This extraction process is facilitated by the high conductivity and efficient electron pathways provided by the MoS₂ QDs. The extracted electrons reoxidize W⁵⁺ ions back to W⁶⁺, reversing the reduction process and contributing to the bleaching of the device. Simultaneously, Li⁺ ions migrate out of the WO₃ layer and return to the electrolyte to maintain charge neutrality. This migration also facilitates the reduction of TMFDS²⁺ back to TMTU [54]. This de-intercalation process is essential for returning the material to its original bleached state. The removal of intercalated ions and the reoxidation of tungsten ions interrupt the intervalence charge transfer (IVCT), restoring the transparency of the WO₃ layer.

The bleaching process can be expressed as follows [14,54]:

$${\mathrm{L}\mathrm{i}}_{\mathrm{x}}{\mathrm{W}\mathrm{O}}_3{\mathrm{M}\mathrm{o}\mathrm{S}}_2\to {\mathrm{W}\mathrm{O}}_3{\mathrm{M}\mathrm{o}\mathrm{S}}_2+{\mathrm{x}\mathrm{L}\mathrm{i}}^{+}+{\mathrm{x}\mathrm{e}}^{-}\left(\mathrm{bleaching}\right)$$

(6)

$${\mathrm{T}\mathrm{M}\mathrm{F}\mathrm{D}\mathrm{S}}^{2+}+{2\mathrm{e}}^{-}\to \mathrm{T}\mathrm{M}\mathrm{T}\mathrm{U}(\mathrm{T}\mathrm{M}\mathrm{T}\mathrm{U} \mathrm{r}\mathrm{e}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n})$$

(7)

Long-term cycle stability testing is a critical parameter for evaluating the practical applicability of PECDs, as it represents the device’s service life. This stability is measured by the change in optical transmittance after repeated coloring and bleaching cycles. To assess the repeatability and long-term stability of our device, cycle stability tests were conducted under a solar simulator for coloration and in dark conditions for bleaching, all at room temperature. The durability test involved 500 cycles, with slight adjustments made to the bleaching and coloring durations due to the lengthy cycle times depicted in Figure 8. The coloration time was set to 1 min, while the bleaching time was extended to 1 h.

The transmittance changes under colored and bleached conditions were recorded at 550 nm throughout the durability test, as shown in Figure 10. The optical contrast (∆T) of the MoS₂ QD-doped WO₃ PECD gradually decreased from the start, retaining approximately 67% of its ∆T after 500 cycles. In contrast, the GO–MoS₂ QD-doped WO₃ PECD exhibited a gradual decline in ∆T during the first 200 cycles but showed no significant change from 200 to 500 cycles, maintaining a stable ∆T of around 18%. After 500 cycles, the GO–MoS₂ QD-doped WO₃ PECD demonstrated an excellent ∆T retention of approximately 90%.

These results highlight the promising stability and performance of GO–MoS₂ QD-doped WO₃ PECDs, which could be attributed to their unique quantum morphologies and sizes [29]. Furthermore, the strong binding of the 5-MSA dye to the TiO₂ film and the simple yet effective electrolyte system likely contributed to the observed durability and stability [4].

4. Conclusions

In this study, we investigated the effects of doping WO₃ films with MoS₂ quantum dots (QDs) and G–MoS₂ QDs for applications in photoelectrochromic devices. An X-ray diffraction (XRD) analysis revealed that incorporating MoS₂ QDs and GO–MoS₂ QDs reduced the crystallite size, as evidenced by peak broadening and intensity reduction. Transmission electron microscopy (TEM) images displayed lattice fringes with interplanar spacings of 0.36 nm, 0.43 nm, and 0.34 nm, corresponding to the characteristic planes of WO₃, MoS₂, and graphene, respectively, confirming the successful integration of GO–MoS₂ QDs into the WO₃ matrix. TEM–EDS mapping showed a uniform distribution of tungsten, oxygen, molybdenum, and sulfur, indicating homogeneous doping.

A scanning electron microscopy (SEM) analysis demonstrated a decrease in film thickness from 724.3 nm for pure WO₃ to 578.8 nm for WO₃ doped with MoS₂ QDs and 588.7 nm for WO₃ doped with GO–MoS₂ QDs, likely due to enhanced packing density and structural reorganization. These structural modifications are expected to improve electrochromic performance by enhancing charge transport and mechanical stability.

The photoelectrochromic performance of undoped WO₃, MoS₂ QD-doped WO₃, and GO–MoS₂ QD-doped WO₃ was systematically evaluated. Undoped WO₃-based PECD exhibited a coloration depth of 36.81% and a half recovery time of 180 min. Doping WO₃ with MoS₂ QDs significantly improved the performance, achieving a coloration depth of 41.3% and a reduced half recovery time of 50 min. The highest performance was observed with GO–MoS₂ QD-doped WO₃, which demonstrated a coloration depth of 51.79% at 550 nm and a half recovery time of 72 min.

Additionally, GO–MoS₂ QD-doped WO₃ showed superior stability over 500 cycles compared to MoS₂ QD-doped WO₃, likely due to the presence of grapheneoxide, which aids in maintaining the structural integrity of the MoS₂. These enhancements can be attributed to the improved charge separation, increased surface area, and synergistic effects of the dopants, resulting in more efficient and stable photoelectrochromic processes.