WO₃ is a highly promising material that has been extensively studied for various scientific and industrial applications owing to its intrinsic physical and chemical attributes [1–3]. Among its potential applications, WO₃ has been investigated as a catalyst for chemical synthesis reactions [4]. Their properties have been proven to be highly applicable in the field of catalytic reactions, including the synthesis of fine chemicals, pharmaceuticals, and polymers. Additionally, owing to its potential use in solar-driven water splitting, photoelectrochromic, and other reactions [5], WO₃ has been investigated as a potential photocatalyst that can absorb light and utilize energy to catalyze chemical reactions. Moreover, WO₃ has been studied for potential use in various sensing applications, such as gas, humidity, and temperature sensing [6]. Owing to its exceptional physical and chemical properties, WO₃ has the potential to revolutionize many fields.

Recently, WO₃ have been highlighted for its potential use as an electrochromic material, which is a material that can change color when an electric current is applied. It has many useful applications in electrochromic windows and displays [2,3]. WO₃ has also been studied as a potential candidate for energy applications in supercapacitors and secondary batteries [7]. However, the potential applications of WO₃ are still being explored, and extensive research is required to better understand its scientific capabilities and industrial limitations. Nevertheless, its unique physical and chemical properties enable WO₃ to be a promising candidate for use in the fields of physical chemistry, materials science, and engineering.

Doping is a widely utilized process that involves the introduction of impurities or dopants into a material to modify its physical and chemical properties [4]. This versatile approach has been applied to materials engineering using metals, semiconductors, and ceramics. In the case of WO₃, dopants can be introduced into its crystal structure to modify its properties to suit specific applications. Consequently, doping has resulted in the development of various WO₃-based materials with enhanced performance characteristics for industrial engineering applications such as electronics, catalysis, and energy. The properties of WO₃ can be significantly altered based on the type and concentration of the dopants used [8,9]. This implies that careful selection and control of the dopants is crucial to achieve the desired properties. Different dopants can have vastly different effects on a material, from altering its electrical conductivity to enhancing its catalytic activity. For example, the introduction of Zr ions as dopants into WO₃ has been demonstrated to improve its optical properties, making it more suitable for applications such as smart windows and optical filters [10]. Conversely, the introduction of Fe ions enhances the magnetic properties of WO₃, making it an ideal material for magnetic storage and sensing applications [11].

Monovalent ions can be incorporated into the crystal structure of WO₃ via physical and chemical approaches. Substitutional approaches can be used to replace the W⁶⁺ ions in the crystal structure with dopant ions [12]. This replacement can be achieved via solid-state synthesis methods, such as solid-state reactions or high-temperature solution growth. Monovalent ions can also be incorporated into the crystal structure of WO₃ through interstitial incorporation, where the ions are inserted into the spaces between the WO6 octahedra in the crystal structure. This can be achieved via hydrothermal synthesis methods. The location of monovalent ions in the WO₃ crystal structure affects the chemical and physical properties of the pristine crystal structure. For example, the concentration and distribution of monovalent ions in a crystal structure can affect the magnetic and catalytic properties of a compound [13]. As a general structure of M_xWO₃, WO₃-based materials have been used to develop solar filters, near-infrared absorption, and shielding [14–16]. However, extensive work is required to comprehensively understand the applicable role of dopants in WO₃ and the potential applications of doped WO₃ materials.

In this study, we investigate the impact of Cs ions on the crystal structure of WO₃ and report a novel bifunctional property induced by Cs doping using an acidic media-coprecipitation method. Our investigation focuses on the characterization of cesium tungsten oxide particles, their photocatalytic properties for pollutant degradation, and their magnetic properties. The Cs-doping strategy adopted in this study aims to enable photochromic applications with simultaneous magnetic properties.

2.1. Materials

Ammonium paratungstate (NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O, cesium carbonate (Cs₂CO₃), tricarboxylic acid, and nitric acid (70 %) were purchased from Sigma-Aldrich (Korea). Methylene blue (MB) dye was purchased from Ducksan.

2.2. Preparation of Cs_xWO₃ nanoparticle

Nanoparticle samples were synthesized using an acidic media coprecipitation method. In a typical preparation, ammonium paratungstate (6.5 mmol) and cesium carbonate (38 mmol) was dissolved in distilled water (300 mL). Following the addition of tricarboxylic acid and nitric acid, the precursor solution was reacted at 80 °C for 1 h. The initial molar ratios of Cs/W were set to 0.3, 0.4, 0.5, 0.6, and 0.7. Following the reaction, the precipitate was filtered, centrifuged, and washed with distilled water and alcohol. The washed precipitate was loaded into a tube furnace under nitrogen gas flow and then heat-treated at 600 °C for 3 h at a heating rate of 5 °C/min.

2.3. Characterization

The crystallinity and crystal structure were obtained using a Rigaku Raxis Spider diffractometer employing Cu Kμ radiation (λ = 1.5406 Å). Raman analysis was performed using a JASCO spectrometer equipped with a 532 nm laser. Morphological characterization was performed using a MIRA3 TESCAN scanning electron microscope equipped with an energy dispersive X-ray spectroscopy (EDS) detector. The UV-vis transmission spectra for photocatalytic dye degradation and adsorption were recorded using a V-670 (JASCO) spectrophotometer. The magnetic hysteresis and susceptibility were measured using a superconducting quantum interference device vibrating sample magnetometer (SQUID-VSM, Quantum Design MPMS 3). The measurements were performed at the Korea Basic Science Institute (Daegu Center, Korea).

2.4. Photocatalytic experiments

The photocatalytic performance of the Cs-doped WO₃ samples with different molar ratios was observed for the degradation of MB under dark and light conditions using a solar light simulator. To conduct the experiment, MB (40 mL) solution was mixed with each sample (30 mg) in distilled water (100 mL). The reactor was covered with aluminum foil and aged overnight in the dark. The characteristic absorption of MB at 664 nm was used to evaluate the photocatalytic degradation process. The MB degradation was calculated as follows:

where C₀ denotes the initial absorbance of the MB dye solution and C_t indicates the absorbance of the MB dye solution at time, t [1,17].

3.1. Structural properties

In a typical acid precipitation method, a precipitating agent such as ammonium metatungstate is used to precipitate the material from an acidic solution. The resulting precipitate is then washed and dried before calcination to obtain a crystal structure. Previous studies have reported the maximum molar ratio between Cs and W to be approximately 0.3 in the Cs-doped tungsten bronze structure [18]. However, in this study, the ratio of the two precursors exceeds this limit under acidic conditions. Owing to the presence of a dominant amorphous phase in the precipitated samples, a heat treatment was performed under N₂ flow at 600 °C for 3 h to obtain their crystallization. The crystal structure of the prepared Cs-doped WO₃ was characterized using XRD, and the results are shown in Fig. 1. The effect of Cs doping with different molar ratios ranging from 0.3 to 0.7 on the crystal structure was compared. The prepared samples were labeled with the following Cs/W molar ratios: 0.3 (black), 0.4 (red), 0.5 (blue), 0.6 (green), and 0.7 (purple), as shown in Fig. 1. The diffraction peaks of the prepared samples were observed at 23.3, 27.2, 27.8, 33.7, 36.5, 44.3, 47.7 and 49.3 of 2 theta deg in Fig. 1(a). These correspond to the (002), (102), (200), (112), (202), (212), (004), and (220) planes of pure cesium tungsten bronze (JCPSD No. 831334) [19,20]. In enlarged XRD ranging from at 26 to at 61 of 2 theta deg [Fig. 1(b)], distinct peaks were observed at 28.6, 29.9, 45.6 and 59.2 of 2 theta deg. This indicates that the observed peaks correspond to the (311), (222), (511), and (622) planes of the CsW₂O₆ crystal phase (JCPSD No. 480950) [21]. In summary, the Cs/W ratios of 0.6 and 0.7 exhibit the presence of a second phase, CsW₂O₆, in the main tungsten bronze phase. The prepared sample with the Cs/W ratio between 0.3 and 0.7 is formed as Cs_xWO₃ of the main phase (JCPDS #831334) and CsW₂O₆ crystal structure of the second phase (JCPDS #480950) is formed as the Cs content increases.

Figure 1. Typical XRD result of Cs-doped WO₃ with different Cs/W molar ratios, 0.3 (i, black), 0.4 (ii, red), 0.5 (iii, blue), 0.6 (ix, green), and 0.7 (x, purple). (a) Wide range and (b) narrow range.

Raman spectroscopy is a powerful characterization tool for probing the vibrational modes of materials. To gain insights into the bonding and structure of the material, the Raman spectra of the Cs-doped WO₃ samples were observed in the wave number range 100–1200 cm⁻¹ in Fig. 2. Typically, WO₃ exhibits a band between 250 and 350 cm⁻¹, which is related to the O-W-O bending modes by oxygen bridging [22]. Other Raman modes between 680 and 820 cm⁻¹ correspond to the O-W-O stretching mode in the WO₃ monoclinic phase [23]. All the prepared samples include the Raman-active modes observed at 200–350, 700–800, 900 and 960 cm⁻¹ in Fig. 2. The observed broad bands at around 276 and 740 cm⁻¹ correspond to the tungsten-oxygen octahedra. Peaks at 900 and 960 cm⁻¹ are matched to that of WO_x [23,24]. The Raman spectrum of Cs-doped WO₃ exhibits several peaks corresponding to different vibrational modes. The most intense peaks in the Raman spectrum indicate the symmetric and antisymmetric stretching modes of the W-O bonds, which appear at around 700 and 800 cm⁻¹, respectively. Cs doping can shift the frequency of these modes as well as change their intensity owing to changes in the local structure of the material. In addition to the stretching modes, the Raman spectrum of Cs-doped WO₃ also shows peaks corresponding to the bending modes of the W-O bonds, as well as modes involving Cs ions. The bending modes appear at lower frequencies around 400 cm⁻¹, whereas the Cs modes appear at higher frequencies above 1,000 cm⁻¹, depending on the specific doping concentration and local environment of the Cs ions [23,24]. The results of the microstructural analysis from the Raman spectra conforms with the XRD results.

Figure 2. Typical Raman spectra of the Cs-doped WO₃ with different Cs/W molar ratios, 0.3 (i, black), 0.4 (ii, red), 0.5 (iii, blue), 0.6 (ix, green), and 0.7 (x, purple).

The morphologies and compositions of samples prepared with different Cs/W ratios were determined. Representative samples with different Cs and W molar ratios of 0.3, 0.5, and 0.7 were analyzed for their morphology and composition, as shown in Figs. 3(a)–(c). All the samples exhibited aggregated particles of size 200 nm, which were composed of particulates measuring only a few nanometers. However, 200 nm nanorods were also observed in the sample with a Cs/W ratio of 0.7. The atomic fractions of Cs, W, and O in the prepared sample with a Cs/W ratio of 0.3 (black) are 7.79, 21.84, and 50.48 %, respectively. In the samples with a Cs/W ratio of 0.5 (blue), the atomic fractions of Cs, W, and O are 8.95, 21.61, and 49.65 %, respectively. For the sample with a Cs/W ratio of 0.7 (purple), the atomic fractions of Cs, W, and O are 10.17, 22.35, and 49.86 %, respectively (Table I). Together with the XRD results shown in Fig. 1, it can be concluded that an excessive amount of Cs results in the formation of Cs oxide with nanorods.

Table 1 . Element ratio of typical Cs doped WO₃ shown in Fig. 3..

Element	Wt % (0.3Cs)	Atomic % (0.3Cs)	Wt% (0.5Cs)	Atomic % (0.5Cs)	Wt% (0.7Cs)	Atomic % (0.7Cs)
C	3.92	19.88	3.84	19.79	3.27	17.62
O	13.24	50.48	12.82	49.65	12.33	49.86
Cs	16.99	7.79	19.21	8.95	20.90	10.17
W	65.85	21.84	64.13	21.61	63.50	22.35

Figure 3. Typical FESEM images with EDS results of Cs-doped WO₃ with Cs/W molar ratios, (a) 0.3, (b) 0.5, and (c) 0.7.

The physical and chemical properties were evaluated by measuring the changes between the adsorption and degradation of MB dye. The effect of the Cs dopant on the WO₃ crystal structure was compared with its photocatalytic performance, as shown in Fig. 4. Three representative samples were prepared with varying Cs and W molar ratios of 0.3, 0.5, and 0.7. To assess their performances, the samples were aged overnight in an MB solution under dark conditions, and the concentrations of the MB solutions are compared in Fig. 4(a). The pristine MB solution (gray) exhibits an absorption peak for the MB molecules at 664 nm, which served as a reference for comparison. The peak intensity of the MB concentration in each sample decreases after aging, indicating a reduction in the concentration of MB molecules in the solution. The sample with a Cs/W ratio of 0.5 (blue) exhibits the largest amount of absorbed MB molecules compared to the samples with Cs/W ratios of 0.3 (black) and 0.7 (purple). This suggests that the adsorption ability of each sample for MB molecules is different. More particularly, the samples with a higher concentration of Cs (0.5 and 0.7) exhibits a lower adsorption ability for MB molecules than the sample with a lower concentration of Cs (0.3). The photocatalytic performances of three representative samples are compared in Fig. 4(b). While all samples exhibited photocatalytic dye degradation ability, the sample with a Cs/W ratio of 0.5 exhibited the highest performance. This result suggests that the Cs concentration plays a key role in the photocatalytic activity of WO₃.

Figure 4. (a) Adsorption and (b) degradation graph of Cs-doped WO₃ with different Cs/W molar ratios, original concentration (i, gray), 0.3 (ii, black), 0.5 (iii, blue), and 0.7 (ix, purple).

3.2. Magnetic measurements

The magnetic properties of materials play a crucial role in various technological applications, such as information storage, energy conversion, and sensing. In particular, WO₃ has attracted considerable attention owing to its unique magnetic properties. The magnetic behavior of WO₃ originates from the unpaired electrons in the d orbitals of the tungsten atoms, which can be reversely aligned with an external magnetic field, resulting in weak magnetization [11,25]. However, the magnetic properties of WO₃ are not solely determined by its intrinsic electronic structure but are also affected by various factors, such as temperature, impurities, and dopants. In this study, the magnetic properties of Cs-doped WO₃ samples with different Cs/W molar ratios were investigated at room temperature using a SQUID-VSM.

The magnetic hysteresis loops of the samples were measured ranging from −10 KOe to 10 KOe, as shown in Fig. 5(a) and ranging from −2 KOe to 2 KOe, as shown in Fig. 5(b). The results show that Csdoped WO₃ samples with Cs/W ratios ranging from 0.3 to 0.6 exhibit typical diamagnetic behavior, which can be attributed to the empty 5d band of W⁶⁺. As the amount of dopant increases from 0.3 to 0.6 of the Cs/W ratio, the diamagnetic properties decrease. Interestingly, the sample with 0.7 Cs/W ratio exhibits a mixed phase of diamagnetic and paramagnetic behavior, which shows a remanent magnetization of 200 Oe. This magnetic behavior originates from the unpaired magnetic spins of the Cs ions, which exhibit paramagnetic behavior. These results suggest that the magnetic properties of the Cs-doped WO₃ samples can be modified by adjusting the Cs/W molar ratio. Moreover, the mixed-phase behavior observed in the sample with 0.7 Cs/W ratio highlights the importance of carefully optimizing the doping conditions to achieve the desired magnetic properties. Furthermore, the magnetic susceptibility versus temperature curve shows that the magnetic properties of the Cs-doped WO₃ samples are highly sensitive to temperature, as shown in Fig. 6. Similar to the tendency of the magnetic behavior in the hysteresis loop shown in Fig. 5, Cs-doped WO₃ with Cs/W ratio ranging from 0.3 to 0.6 exhibit negative magnetic susceptibility. Diamagnetic materials are not attracted to magnetic fields and exhibit negative magnetic susceptibility. When materials are exposed to a magnetic field, their electrons move in a manner that generates a magnetic field that opposes the applied field. This leads to a reduction in the overall magnetic moment of the material. As the amount of Cs dopant in WO₃ increases from 0.3 to 0.6, the negative magnetic susceptibility decreases. However, in the case of Cs-doped WO₃ with 0.7 Cs/W ratio, the coexistence of diamagnetic and paramagnetic phases could be observed, as shown in Fig. 5. This magnetic behavior originates from the presence of the Cs dopant, which exhibits paramagnetic behavior because of its unpaired electrons. This coexistence of the diamagnetic and paramagnetic phases is consistent with the magnetic behavior observed in the hysteresis loop seen in Fig. 5, which also shows a mixed behavior in the 0.7 Cs-doped WO₃ sample.

Figure 5. Magnetic hysteresis loop of Cs-doped WO₃ with different Cs/W molar ratios, 0.3 (i, black), 0.4 (ii, red), 0.5 (iii, blue), 0.6 (ix, green), and 0.7 (x, purple). (a) Wide range and (b) narrow range.

Figure 6. Magnetic susceptibility of Cs-doped WO₃ with different Cs/W molar ratios, 0.3 (I, black), 0.4 (ii, red), 0.5 (iii, blue), 0.6 (ix, green), and 0.7 (x, purple).

In conclusion, this study successfully demonstrated the bifunctional properties of Cs-doped WO₃, in which the amount of dopant was controlled by a reducing agent in an acid media-coprecipitation process. Significant changes in the adsorption and photocatalytic properties were observed based on the amount of dopant, with 0.5 Cs doped WO₃ displaying the highest absorption and photocatalytic efficiencies. Additionally, the magnetic properties of the Cs-doped WO₃ were characterized by a magnetic hysteresis loop and the susceptibility was investigated using SQUID-VSM. The results reveal that Cs doping induced a paramagnetic phase owing to the electron–electron interactions of the Cs dopants, resulting in increased magnetic moments. The addition of Cs to the WO₃ nanostructures resulted in increased photoreactivity, leading to enhanced photochromic properties. This study highlights the potential of Cs-doped WO₃ as a promising material for various applications in energy, environmental, and biomedical engineering. Further research should explore the potential of these materials in greater detail, which may lead to the development of novel and exciting technologies.

This work was supported by a Research Grant of Pukyong National University (2021).

The authors declare no conflicts of interest.