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Article

A Study of Intermediate for Synthesis of Cs0.3WO3 with Near-Infrared Photothermal Response

by
Yue Zhang
and
Ruixing Li
*
School of Materials Science and Engineering, Beihang University, Beijing 100191, China
*
Author to whom correspondence should be addressed.
Energies 2022, 15(22), 8542; https://doi.org/10.3390/en15228542
Submission received: 19 October 2022 / Revised: 10 November 2022 / Accepted: 12 November 2022 / Published: 15 November 2022
(This article belongs to the Special Issue Materials for the Renewable Technologies: Challenges, Opportunities and Recent Advances)
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Abstract

:
Nanoscale tungsten bronze can convert near-infrared light into thermal energy. For a chemical synthesis, intermediate products and processes are potentially positive or negative to an end product. In this study, (NH4)2SO4 was added into the hydrothermal system of WO3, CsCl, CH3CH2CHO, and NH3·H2O to increase the Cs/W ratio in solution. An intermediate, Cs1.1W1.65O5.5, was isolated. Subsequently, it was confirmed via a kinetics experiment conducted for different times. The results of the NH3·H2O and NH4COOCH3 system indicate there are two influence factors that influence the formation of Cs1.1W1.65O5.5: the leaching out of W and imine reactions. A low leaching out rate induces a high Cs/W ratio and low-concentration W in solution, leading to the formation of Cs1.1W1.65O5.5. The imine reaction decreasing CH3CH2CHO restrains the reduction in Cs1.1W1.65O5.5. The increase in CH3CH2COO− with the reduction reaction results in both the micron-size sticks and nano-size equiaxial particles in powder.

1. Introduction

Tungsten bronze (AxWO3, A = Li+, Na+, K+, Rb+, Cs+, NH4+, 0 < x < 1) is a near-infrared absorption material that stems from a collective resonance of free carriers, and converts the absorbed light energy into thermal energy. This property gives this material promising applications in the field of advanced materials science such as photothermal therapy [1,2,3,4] and solar-heat shielding [5,6,7,8]. In contrast with surgery and chemotherapy, photothermal therapy is noninvasive as the heat produced by tungsten bronze can kill the cancer cells only when near-infrared light radiates both of them. Additionally, tungsten bronze also transmits visible light, thus it can be used as a heat-shielding coating on windows of buildings and automobiles to depress the indoor temperature increment caused by near-infrared radiation to reduce cooling energy consumption. For these synthesis methods of tungsten bronze, wet chemistry methods are preferable to solid-state [9], spray pyrolysis [10], fused salt electrolysis [11], hydrogen reduction [12], vaper deposition methods [13] because of the lower temperature, nano-scale particles, and various morphologies [14,15,16,17]. Moreover, among the wet chemical methods, the hydrothermal method is used more and more to synthesize tungsten bronze due to the elevated temperature and pressure driving the dissolution and crystallization of almost all inorganic substances [18].
For many chemical syntheses, regardless of the method used, an intermediate product possibly forms during the reaction process. In some cases, the intermediate product does not influence the final material. However, in other cases, the composition, morphology, and particle size of the final material can be controlled via the intermediate such as controlling the nucleation, growth, and transport of the intermediate; employing a specific method, for example, nanocasting, to tune the particle size of intermediate; changing the phase composition of intermediate [19,20,21,22]. In another case, the appearance of the intermediate is not conducive for the final material. For the direct conversion from the precursor to superconducting phase YBa2Cu3O6.5, a reaction pathway that avoids the production of intermediate BaCO3 is conducive [23]. Therefore, it is significant to investigate the formation mechanism of the intermediate during the reaction for material synthesis. However, it is still a challenge since numerous chemical reactions are black boxes.
Intermediates are formed during the synthesis of tungsten bronze in some cases. For example, in a synthesis of (NH4)0.25WO3, due to the change in the reaction route, the structure and composition of corresponding intermediate products also change, leading to the difference in the atomic structure and particle size of the final product [24]. For the solid-state synthesis of CsxWO3 from WO3, a cubic-structure CsW1.6O6 forms with the insertion of Cs at lower temperature. Then, the content of CsW1.6O6 generally decreases and disappears with the increase in temperature, leaving only Cs0.32WO3 [25]. In addition, a one-dimensional tungsten bronze can be synthesized via the formation and dissolution of an intermediate H+EDA–WO42− of an organic–inorganic structure [26]. However, in this paper, there was an impurity that the authors did not analyze further, which might be possible cubic-structure Cs1.1W1.65O5.5. In the previous study by the Li Group [27], a hydrothermal method based on commercial WO3 was developed to synthesize Cs0.3WO3, which is superior to other methods based on WO3 colloids pre-prepared through acidifying the solution of tungstate [28,29]. However, this study only explored whether cesium tungsten bronze could even be synthesized using commercial tungsten oxide with a micron-scale particle size, without further study on the reaction mechanism. What is interesting is that besides Cs0.3WO3, a cubic-structure Cs1.1W1.65O5.5 was also detected. In the previous study, two conclusions were obtained. (1) A strong base CsOH could not achieve the single-phase Cs0.3WO3 as an extra phase of Cs1.1W1.65O5.5 formed. The content of Cs1.1W1.65O5.5 increased with the concentration of CsOH, since the extra Cs+ introduced with the increase in the CsOH results in excessive doping. (2) A single-phase Cs0.3WO3 could be synthesized using NH3·H2O, without an extra phase of Cs1.1W1.65O5.5. However, it takes time to leach W out from WO3, which results in a higher Cs/W ratio. The low-concentration W in solution might lead to the formation of Cs1.1W1.65O5.5 during the reaction. Therefore, it was assumed that the Cs1.1W1.65O5.5 intermediate was formed. To catch the possible intermediate, a low-concentration W caused by a slow leaching out rate should be created to drive the formation of Cs1.1W1.65O5.5. A salt of strong acid and weak base such as (NH4)2SO4 could be added into the system of NH3·H2O to decrease the pH level and provide a low leaching out rate of W to increase the Cs/W ratio. Thus, the purpose of this study was to explore the intermediate process for the synthesis of Cs0.3WO3.

2. Experimental

2.1. Materials

Propanal (CH3CH2CHO, 99+%) was supplied by Acros. Cesium chloride (CsCl, 99.9%) was purchased from Innochem. Tungsten oxide (WO3 (s), 99.9%), ammonium acetate (NH4COOCH3 (s), 99.99%), ammonium sulfate ((NH4)2SO4 (s), 99.99%), ammonium solution (NH3·H2O (aq), AR, 25–28%), and ammonium oxalate ((NH4)2C2O4 (s), 99.99%) were purchased from Aladdin A.

2.2. Preparation of Cesium Tungsten Bronze

A total of 0.0928 g (0.4 mmol) of WO3, 0.0303 g (0.18 mmol) of CsCl, 0.5163 g (8.8 mmol) of CH3CH2CHO, 0.1850 g (1.4 mmol) of (NH4)2SO4, and 1, 1.5, 2, and 2.5 mL of the NH3·H2O aqueous solution (1.4 mol/L) was mixed in deionized water, respectively. The total volume of each solution was kept at 25 mL. Then, the amount of (NH4)2SO4 was changed to 0.6608 g (5.0 mmol), 0.9911 g (7.5 mmol), and 1.3215 g (10 mmol), respectively, and the above steps were repeated. Afterward, each aqueous suspension was transferred into Teflon-lined stainless-steel autoclave of 50 mL and sealed, and then maintained in an oven at 200 °C for 24 h.

2.3. Characterization

The SEM and TEM images were acquired with a JSM-7500F scanning electron microscope and a JEM-2100F transmission electron microscope, respectively. The X-ray diffraction (XRD) patterns of the samples were obtained on a Rigaku D/max 2200 PC with graphite monochromatized Cu Kα radiation. The infrared absorption spectrum was achieved on an ultraviolet-visible-near infrared spectrophotometer (UV–Vis–NIR, Shimadzu UV-3600) with the samples prepared through painting powder on the surface of a flat sample stage with a certain thickness.

3. Results and Discussion

A series of hydrothermal reactions were conducted at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and NH3·H2O, (NH4)2SO4 with different amounts. The crystalline phases, starting pH values, and mass percentages of Cs1.1W1.65O5.5 are summarized in Table 1. According to Table 1, compared to NH3·H2O, the pH value decreased with an increasing amount of (NH4)2SO4. Single-phase Cs0.3WO3 was synthesized and the cubic-structure Cs1.1W1.65O5.5 was isolated. The results in Table 1 indicate that the appearance of Cs1.1W1.65O5.5 is determined by the amounts of NH3·H2O and (NH4)2SO4. Under the same amount of NH3·H2O, the content of Cs1.1W1.65O5.5 first decreases and then increases with an increase in the amount of (NH4)2SO4. Moreover, under the same amount of (NH4)2SO4, the increase in the amount of NH3·H2O leads to the appearance of Cs1.1W1.65O5.5. The XRD patterns of typical cases of various amounts of NH3·H2O and 5.0 mmol (NH4)2SO4 are shown in Figure 1. In the case of 2.1 mmol NH3·H2O, WO3 and Cs0.3WO3 (JCPDS No. 81-1224) were detected, as shown in Figure 1a. A single-phase Cs0.3WO3 was synthesized when the amount of NH3·H2O increased to 2.8 mmol, as shown in Figure 1b. The cubic-structure Cs1.1W1.65O5.5 (JCPDS No. 44-0017) appeared as the amount of NH3·H2O increased to 3.5 mmol, as shown in Figure 1c.
There is a relationship between the near-infrared response property and morphology of Cs0.3WO3 [16], thus the morphology of Cs0.3WO3 synthesized using NH3·H2O and (NH4)2SO4 was investigated (see Figure 2). It can be seen in Figure 2 that the particle size of Cs0.3WO3 changes with the amounts of NH3·H2O and (NH4)2SO4. Figure 2a shows that Cs0.3WO3 synthesized using 2.1 mmol NH3·H2O and 1.4 mmol (NH4)2SO4 included numerous equiaxial particles and non-uniform sticks. The equiaxial particles had a size range of 0.08–0.30 μm in diameter, with an average size of 0.17 μm. The sticks had a size range of 0.04–0.39 μm in diameter and 0.65–10.09 μm in length, covering an average diameter and length of 0.11 and 2.00 μm, respectively. The powder showed a similar morphology when the amount of NH3·H2O increased to 2.8 mmol (as shown in Figure 2b). The Cs0.3WO3 powder still contained both nano-sticks and equiaxial nanoparticles when the amount of (NH4)2SO4 increased to 5 mmol, however, the particle size changed (as shown in Figure 2c). The length of the nano-sticks increased and covered a size range of 2.79–18.54 μm in length and 0.10–0.34 μm in diameter, having an average size of 8.86 and 0.18 μm, respectively. When the amount of (NH4)2SO4 increased to 7.5 mmol, the length of nano-sticks decreased significantly, and the powder consisted of numerous nano-sticks and equiaxial nanoparticles, as shown in Figure 2d. The nano-sticks had a size range of 0.03–0.13 μm in diameter, and 0.20–1.87 μm in length, with an average size of 0.07 and 0.65 μm, respectively. This result suggests that there is a relationship between the amounts of NH3·H2O and (NH4)2SO4 and the particle size of Cs0.3WO3.
The near-infrared response property of Cs0.3WO3 synthesized using NH3·H2O and (NH4)2SO4 with different amounts were measured employing UV–Vis–NIR spectrophotometers, as shown in Figure 3. The as-synthesized Cs0.3WO3 was transparent in the visible region and absorptive in the partial visible and near-infrared region. The spectrum of Cs0.3WO3 changed with an increase in the amounts of NH3·H2O and (NH4)2SO4. The Cs0.3WO3 synthesized using 2.1 and 2.8 mmol NH3·H2O, respectively, and 1.4 mmol (NH4)2SO4 showed the same transmission range of 380–570 nm. With an increase in the amount of (NH4)2SO4 to 5 and 7.5 mmol, the transmission range broadened to about 380–620 and 380–660 nm. The as-synthesized Cs0.3WO3 using 2.8 mmol NH3·H2O and 5 and 7.5 mmol (NH4)2SO4, respectively, showed a similar near-infrared absorbance. The increase in the amount of (NH4)2SO4 caused the further improvement in the absorbance, accompanied by a red shift.
X-ray photoelectron spectroscopy (XPS) was employed to examine the composition and valence state of elements of Cs0.3WO3. For Cs0.3WO3 synthesized using 2.8 mmol NH3·H2O and 5.0 mmol (NH4)2SO4, the result showed the existence of carbon, tungsten, cesium, and oxygen, as shown in Figure 4a. The C 1s peak of 284.8 eV was induced by the contamination of carbon that was used as a reference, correcting other peaks of elements in the sample (see Figure 4a). More detailed information on the valence state of tungsten is shown in Figure 4b. The high-resolution spectrum of W 4f can be fitted as two doublets of spin orbits, with the interval of 2.1 eV. These are the peaks at 37.8 and 35.7 eV assigned to W6+, and peaks at 36.8 and 34.7 eV assigned to W5+, respectively. The results of the XPS measurement demonstrate the existence of W5+.
According to the results above-mentioned, the possible intermediate Cs1.1W1.65O5.5 was isolated, and it was necessary to confirm it from the perspective of kinetics. Therefore, a hydrothermal reaction of 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 2.8 mmol NH3·H2O, and 5.0 mmol (NH4)2SO4 was conducted at different reaction times to investigate the intermediate process. The XRD patterns of the samples are shown in Figure 5. The composition of the crystalline phases changes with the reaction time, transforming from a multi-phase to a single-phase product. When the reaction progressed to 4 h, there were three phases including WO3, Cs0.3WO3, and Cs1.1W1.65W5.5. As the reaction proceeded, the contents of Cs1.1W1.65O5.5 and WO3 decreased, while the content of Cs0.3WO3 increased. Then, WO3 disappeared at the time of 16 h, and the powder comprised Cs0.3WO3 and Cs1.1W1.65W5.5. Finally, a single-phase Cs0.3WO3 of hexagonal structure was achieved when the reaction time increased to 24 h. These results confirm that the cubic-structure Cs1.1W1.65O5.5 is the intermediate product for the synthesis of Cs0.3WO3.
According to the results shown in Figure 2, all Cs0.3WO3 synthesized using NH3·H2O and (NH4)2SO4 consisted of two morphologies of micron-size sticks and nano-size equiaxial particles. To investigate the morphology evolution, the change in morphology with the reaction time was observed by SEM. When the reaction progressed to 4 h, only equiaxial nanoparticles were observed, as shown in Figure 6a. When the time increased to 12 h, the nano-sticks appeared, aside from numerous equiaxial nanoparticles, as shown in Figure 6b. The content of nano-sticks increased as the reaction proceeded, and more sticks formed when the reaction time increased to 20 h, as shown in Figure 6c.
Subsequently, TEM was further used to research the evolution of the morphology. The TEM and SAED images of samples synthesized at 200 °C for 4 h using 2.8 mmol NH3·H2O and 5.0 mmol (NH4)2SO4 are shown in Figure 7a–d. A square-shape particle of 60 nm and a particle of 100 nm size comprised of several square-shape particles below 10 nm can be observed in Figure 7a and Figure 7d, respectively. Meanwhile, their corresponding SAED images are shown in Figure 7b and Figure 7d, respectively. The diffraction pattern of the square-shape particle shown in Figure 7a indicates that the particle had a cubic structure. Combined with the results in Figure 5, the crystalline phase of this particle is cubic-structure Cs1.1W1.65O5.5. Meanwhile, the diffraction pattern of Figure 7d demonstrates that the aggregated particles of Figure 7c have the same orientation and hexagonal structure. Therefore, the crystalline phase of this aggregated particle is Cs0.3WO3, combined with the results of Figure 5. These results suggest that the aggregated particle comprising of square-shape Cs1.1W1.65O5.5 particles might be transformed into hexagonal-structure Cs0.3WO3 in situ, under the reduction of CH3CH2CHO. The TEM and SAED images of Cs0.3WO3 synthesized at 200 °C for 24 h using 2.8 mmol NH3·H2O and 1.4 mmol (NH4)2SO4 are shown in Figure 7e–g. As observed, aside from micron-size sticks, there were still numerous nanoparticles in the size of 100 nm (see Figure 7e,f). As analyzed, the stick was preferential to growth along the C-axis, namely, the (002) direction, as shown in Figure 7g. Additionally, according to the data of the EDS, there was a difference between the sticks and nanoparticles in the Cs/W ratio, namely, 0.337 and 0.196, respectively.
According to Table 1, under the same amount of NH3·H2O, the pH values of the systems decreased with the increasing amount of (NH2)2SO4, however, the content of Cs1.1W1.65O5.5 decreased first and then increased. This indicates that it is not a single factor that determines the formation of Cs1.1W1.65O5.5. As above-mentioned, to catch the intermediate Cs1.1W1.65O5.5, acidic (NH4)2SO4 was used to decrease the pH value of the NH3·H2O system. Thus, a neutral NH4COOCH3 was selected, instead of (NH4)2SO4, to try to find other influencing factors. A hydrothermal reaction was performed at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 4.0 mmol NH4COOCH3, and NH3·H2O with different amounts. The XRD patterns of the samples are shown in Figure 8. With the amount of NH3·H2O increasing, the change tendency of the content of Cs1.1W1.65O5.5 was different from the system of NH3·H2O and (NH4)2SO4, as shown in Figure 8. In the case of 4 mmol NH4COOCH3 and 0.7 mmol NH3·H2O, there was WO3, Cs1.1W1.65O5.5, and Cs0.3WO3, as shown in Figure 8a. Subsequently, with the increase in the amount of NH3·H2O to 1.4 and 2.1 mmol, the contents of WO3 and Cs1.1W1.65O5.5 decreased, as shown in Figure 8b,c. Then, the continuously increasing amount of NH3·H2O to 2.8 and 3.5 mmol led to the increase of Cs1.1W1.65O5.5 and the disappearance of WO3, as shown in Figure 8d,e.
The increase in NH3·H2O could promote the leaching out of tungsten to decrease the ratio of Cs/W in solution, leading to the decrease in Cs1.1W1.65O5.5. Therefore, the content of Cs1.1W1.65O5.5 declined when the amount of NH3·H2O was below 2.1 mmol. However, the Cs/W still dropped off with an increase in NH3·H2O, and the intermediate Cs1.1W1.65O5.5 increased when the amount of NH3·H2O was more than 2.1 mmol. It is possible that the reduction in intermediate Cs1.1W1.65O5.5 into Cs0.3WO3 is suppressed. Considering that the Cs/W ratio of Cs1.1W1.65O5.5 is 0.67, thus W in solution is another reactant besides CH3CH2CHO, which could drive the rearrangement of the structure of Cs1.1W1.65O5.5 to form a hexagonal-structure Cs0.3WO3 whose Cs/W ratio decreased to 0.3. The reduction is related to the concentrations of CH3CH2CHO and W in solution. In addition, the increase in the leaching out rate could increase the concentration of W in solution. In conclusion, the increase in Cs1.1W1.65O5.5 content might be due to the decrease in the CH3CH2CHO concentration. Aside from the leaching out of W, another influence factor is possibly the imine reaction between NH3 and CH3CH2CHO, which could decrease the concentration of CH3CH2CHO [30,31].
According to the results, it can be concluded that the improvement in the leaching out rate of tungsten reduces the formation of Cs1.1W1.65O5.5. In contrast, the imine reaction between NH3 and CH3CH2CHO provides the reverse effect on the reaction for forming Cs0.3WO3 from Cs1.1W1.65O5.5. Hence, ammonium salts, which promote the leaching out of W while inducing a lower-concentration of NH3, could drive the formation of single-phase Cs0.3WO3. To clarify, (NH4)2C2O4 was selected. The reason is that the chelation effect of C2O42− could contribute to the leaching out of tungsten. Therefore, a hydrothermal reaction of 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and (NH4)2C2O4 with different amounts was carried out at 200 °C for 24 h. The result suggests that in the case of 1 mmol (NH4)2C2O4, three phases of WO3, Cs1.1W1.65O5.5, and Cs0.3WO3 were formed. With an increase in the amount of (NH4)2C2O4 from 1 mmol to 1.5 and 2 mmol, WO3 disappeared and the content of Cs1.1W1.65O5.5 decreased, and the powder consisted of Cs1.1W1.65O5.5 and Cs0.3WO3. Ultimately, a single-phase of Cs0.3WO3 was achieved at 2.5 mmol of (NH4)2C2O4.
The contents of Cs1.1W1.65O5.5 versus various ammonium sources are summarized in Figure 9. As above-mentioned, there are two influencing factors that affect the leaching out of W and the imine reaction. Since it involves the imine reaction, the relationship between the total quantity of NH4+ and the content of Cs1.1W1.65O5.5 was investigated. The trend in the content of intermediate with NH4+ did not change. For the part that Cs1.1W1.65O5.5 decreased, as shown in Figure 9, the improvement in the leaching out of W decreased the content of Cs1.1W1.65O5.5. However, for the three ammoniums, the reasons might be different from each other. For (NH4)2C2O4, it is the chelate effect that promotes the leaching out of W, and a single-phase Cs0.3WO3 is achieved at a lower concentration of NH4+. In the case of (NH4)2SO4, the pH value declines, thus it might be the increase in NH4+ that increases the leaching out rate of W. For the NH4COOCH3 system, it might be the increase in both the pH value and NH4+ concentration that enhances the leaching out of W. Due to the improvement in the leaching out rate, on one hand, the resulting Cs/W ratio drops off to restrain the formation of Cs1.1W1.65O5.5. On the other hand, the concentration of W is increased to promote the reduction of Cs1.1W1.65O5.5. As a result, imine is enhanced by increasing ammonium, however, the content of Cs1.1W1.65O5.5 decreased due to the rise in the leaching out of W, which dominates the reaction. For the part where Cs1.1W1.65O5.5 increased in Figure 9, the reason was different. With the continuous increase in NH4+ concentration, the equilibrium in the imine reaction shifted toward the decrease in the CH3CH2CHO concentration significantly. In the case of the NH4COOCH3 system, the reaction was controlled by the reduction of Cs1.1W1.65O5.5 via CH3CH2CHO, thus despite the leaching out rate being raised, it still led again to the increase in the intermediate. In the case of the (NH4)2SO4 system, besides enhancing the imine reaction, the further increase of (NH4)2SO4 could lower the pH value to suppress the leaching out rate of W, leading to the WO3 residue (see Table 1). Hence, the resulting lower-concentration W in solution enhances the Cs/W ratio, while it restrains the reduction of Cs1.1W1.65O5.5. Consequently, the intermediate Cs1.1W1.65O5.5 improves again.
The morphology of Cs0.3WO3 synthesized using (NH4)2C2O4 was investigated by SEM. As observed in Figure 10, the particle size and morphology of Cs0.3WO3 changed due to the use of (NH4)2C2O4. When 1.5 mmol (NH4)2C2O4 was used, there were numerous equiaxial nanoparticles and a few dumbbell-shaped particles, as shown in Figure 10a. With the amount of (NH4)2C2O4 increasing to 2 mmol, the content of equiaxial particles decreased and the aspect ratio of the Cs0.3WO3 particles changed compared to the Cs0.3WO3 sticks in Figure 2. Ultimately, when the amount of (NH4)2C2O4 increased to 2.5 mmol, the equiaxial nanoparticles disappeared and only rod-like particles were left, as shown in Figure 10c. The rod-like particles covered a size range of 87.44–445.43 nm in length, and 29.82–151.17 nm in diameter, having an average size of 166.43 and 65.70 nm, respectively.
According to the results in Figure 10, the dumbbell-shaped particles might be an intermediate morphology. To clarify, the TEM was used to investigate the particles of powder synthesized using 2 mmol (NH4)2C2O4. The TEM and SAED images of the sample synthesized using 2 mmol (NH4)2C2O4 are shown in Figure 11a and Figure 11b, respectively. As observed, the morphology was consistent with the results shown in Figure 10a,b, which were dumbbell and equiaxial nanoparticles, as shown in Figure 11a. The diffraction pattern of the red square in Figure 11a demonstrates that the rod and equiaxial parts had different structures, namely, hexagonal and cubic structures, respectively, as shown in Figure 11b. Combined with the results of Table 1 and Figure 10, the equiaxial part was the cubic-structure Cs1.1W1.65O5.5, and the rod was the hexagonal-structure Cs0.3WO3. These results suggest that Cs0.3WO3 particles grow from the Cs1.1W1.65O5.5 equiaxial nanoparticles.
According to the results above-mentioned, the morphology evolution of Cs0.3WO3 synthesized using various ammonium salts could be explained as follows. The leaching out rate of WO3 was influenced by the concentrations of ammonium salts. When the leaching out rate was low, the slow rate of releasing tungsten into the solution led to a low concentration of tungsten and corresponded to the low driving force for nucleation. In this case, the product tended to grow on the wall of the reactor, since the interface could reduce the nucleation energy, which was observed during the reaction. Therefore, at the beginning, the Cs/W ratio in solution was high, and the intermediate cubic-structure Cs1.1W1.65O5.5 nucleuses mainly formed on the interface. As the reaction proceeded, the W leaching out continued to nucleate and grow on the surface of the existing nucleuses due to the low nucleation driving force. Meanwhile, with the reduction of CH3CH2CHO and the supplement of tungsten, the cubic-structure intermediate Cs1.1W1.65O5.5 transformed to hexagonal-structure Cs0.3WO3. Therefore, at the beginning of the reaction, the size of the particles was relatively uniform, presenting equiaxial nanoparticles (see Figure 6a).
With the reduction of Cs1.1W1.65O5.5 as CH3CH2CHO proceeds, the concentration of CH3CH2COO− increases. Furthermore, carboxylate ions can be absorbed on the surface of nucleuses [32,33]. Thus, the dangling bonds are saturated, leading to the decrease in nucleation energy, with the coverage increasing. This could promote the formation of nucleuses rather than the growth of existing particles after reaching a certain coverage. Therefore, as the reaction proceeded, fresh nucleuses were formed. Furthermore, the adsorbed CH3CH2COO− on the nucleuses hindered the agglomeration and growth. As a result, aside from micron-sized one-dimensional sticks, nano-sized equiaxial particles were formed. This confirmed that the Cs/W ratio of equiaxial nanoparticles was lower than that of the micron-sized sticks, as above-mentioned.
When the leaching out rate increased, the tungsten in solution improved, which promoted the nucleation. In this case, the nucleus density increased significantly, thus there was more surface to consume the tungsten in solution, leading to the decrease in the particle size. However, due to the same reason, the increase of CH3CH2COO− at the later stage of reaction ultimately led to both sticks and equiaxial nanoparticles of different sizes in the powder.
For the change in the aspect ratio of Cs0.3WO3 synthesized using (NH4)2C2O4, the reason might be related to the chelate that formed between (NH4)2C2O4 and WO3.

4. Conclusions

In this study, the mechanism of the intermediate process of Cs0.3WO3 synthesis from commercial WO3 is found, which could provide a reference for the synthesis of heterogeneous systems for tungsten bronze. A hydrothermal reaction was carried out at 200 °C for 24 h using CsCl, WO3, CH3CH2CHO, and NH3·H2O, with the addition of (NH4)2SO4 decreasing the pH value to increase the Cs/W ratio in solution. The result is that a single-phase Cs0.3WO3 was synthesized and the possible intermediate Cs1.1W1.65O5.5 was isolated. Subsequently, from the perspective of kinetics, a hydrothermal reaction was conducted at 200 °C for different reaction times. It confirms the existence of intermediate Cs1.1W1.65O5.5. Under the same amount of NH3·H2O, the content of Cs1.1W1.65O5.5 decreased first and then increased with increasing (NH4)2SO4, suggesting that there might be multiple influencing factors on the formation of Cs1.1W1.65O5.5. To further investigate this, a hydrothermal reaction was conducted under the same conditions using the systems of NH4COOCH3 and NH3·H2O. The results suggest that aside from the leaching out of W, the imine reaction between NH3 and CH3CH2CHO is another influencing factor. The ratio of Cs/W and W concentration in solution depends on the leaching out rate of W. The concentration of CH3CH2CHO depends on the imine reaction between NH3 and CH3CH2CHO. Together, these influencing factors determine the formation of Cs1.1W1.65O5.5. High Cs/W ratio, low-concentration W, and high-concentration NH4+ in solution could improve the formation of Cs1.1W1.65O5.5, which is not conducive for Cs0.3WO3. Additionally, the chelate effect of C2O42− could be conductive to the leaching out of W to promote the formation of Cs0.3WO3. Low W concentration could form equiaxial particles with the diameter of 0.08–0.30 μm and sticks with the size range of 0.65–10.09 μm in length, 0.04–0.39 μm in diameter. A high W concentration could lead to the large decrease in the particle size of sticks. The reason that the two particles have different sizes is possibly due to the improvement of CH3CH2COO− as the reducing reaction proceeds, which could decrease the nucleation energy to promote the formation of fresh nucleuses and inhibit their growth.

Author Contributions

Conceptualization, R.L.; Validation, Y.Z.; Formal analysis, Y.Z.; Investigation, Y.Z.; Resources, R.L.; Data curation, Y.Z.; Writing—original draft, Y.Z.; Supervision, R.L.; Project administration, R.L.; Funding acquisition, R.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Foundation of China (NSFC 51772013 and 92163111). And the APC was funded by Ruixing Li.

Data Availability Statement

Not applicable.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC 51772013 and 92163111).

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 15 08542 g001 550
Figure 1. XRD patterns of the samples synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 5.0 mmol (NH4)2SO4, and (a) 2.1; (b) 2.8; (c) 3.5 mmol NH3·H2O.
Figure 1. XRD patterns of the samples synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 5.0 mmol (NH4)2SO4, and (a) 2.1; (b) 2.8; (c) 3.5 mmol NH3·H2O.
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Energies 15 08542 g002a 550Energies 15 08542 g002b 550
Figure 2. SEM images of Cs0.3WO3 synthesized hydrothermally at 200 °C using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and (a) 2.1 mmol NH3·H2O, 1.4 mmol (NH4)2SO4; (b) 2.8 mmol NH3·H2O, 1.4 mmol (NH4)2SO4; (c) 2.8 mmol NH3·H2O, 5 mmol (NH4)2SO4; (d) 2.8 mmol NH3·H2O, 7.5 mmol (NH4)2SO4.
Figure 2. SEM images of Cs0.3WO3 synthesized hydrothermally at 200 °C using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and (a) 2.1 mmol NH3·H2O, 1.4 mmol (NH4)2SO4; (b) 2.8 mmol NH3·H2O, 1.4 mmol (NH4)2SO4; (c) 2.8 mmol NH3·H2O, 5 mmol (NH4)2SO4; (d) 2.8 mmol NH3·H2O, 7.5 mmol (NH4)2SO4.
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Figure 3. Absorbance spectra of UV–Vis–NIR of Cs0.3WO3 synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO and NH3·H2O and (NH4)2SO4 of 2.1, 1.4, 2.8 and 1.4, 2.8, and 5.0, 2.8, and 7.5 mmol.
Figure 3. Absorbance spectra of UV–Vis–NIR of Cs0.3WO3 synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO and NH3·H2O and (NH4)2SO4 of 2.1, 1.4, 2.8 and 1.4, 2.8, and 5.0, 2.8, and 7.5 mmol.
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Figure 4. XPS spectra of Cs0.3WO3 synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 2.8 mmol NH3·H2O and 5.0 mmol (NH4)2SO4: (a) survey spectrum; (b) high-resolution spectrum of W 4f. The black and red lines are the experimental data and sum of fits, respectively; the green and purple lines are the fits of W 4f5/2 and W 4f7/2 for W6+, respectively; the yellow and blue lines are the fits of W 4f7/2 and W 4f5/2 for W5+, respectively.
Figure 4. XPS spectra of Cs0.3WO3 synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 2.8 mmol NH3·H2O and 5.0 mmol (NH4)2SO4: (a) survey spectrum; (b) high-resolution spectrum of W 4f. The black and red lines are the experimental data and sum of fits, respectively; the green and purple lines are the fits of W 4f5/2 and W 4f7/2 for W6+, respectively; the yellow and blue lines are the fits of W 4f7/2 and W 4f5/2 for W5+, respectively.
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Figure 5. XRD patterns of the samples synthesized hydrothermally at 200 °C using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 2.8 mmol NH3·H2O, and 5 mmol (NH4)2SO4 at the time of (a) 4 h; (b) 8 h; (c) 12 h; (d) 16 h; (e) 20 h, and (f) 24 h.
Figure 5. XRD patterns of the samples synthesized hydrothermally at 200 °C using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 2.8 mmol NH3·H2O, and 5 mmol (NH4)2SO4 at the time of (a) 4 h; (b) 8 h; (c) 12 h; (d) 16 h; (e) 20 h, and (f) 24 h.
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Figure 6. The SEM images of the samples synthesized hydrothermally at 200 °C using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 2.8 mmol NH3·H2O and 5 mmol (NH4)2SO4 and for (a) 4 h; (b) 12 h; (c) 20 h.
Figure 6. The SEM images of the samples synthesized hydrothermally at 200 °C using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 2.8 mmol NH3·H2O and 5 mmol (NH4)2SO4 and for (a) 4 h; (b) 12 h; (c) 20 h.
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Energies 15 08542 g007a 550Energies 15 08542 g007b 550
Figure 7. The TEM and SEAD images of samples synthesized hydrothermally at 200 °C using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and (a–d) 2.8 mmol NH3·H2O and 5.0 mmol (NH4)2SO4 with a reaction time of 4 h; (e–g) 2.8 mmol NH3·H2O and 1.4 mmol (NH4)2SO4 with a reaction time of 24 h.
Figure 7. The TEM and SEAD images of samples synthesized hydrothermally at 200 °C using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and (a–d) 2.8 mmol NH3·H2O and 5.0 mmol (NH4)2SO4 with a reaction time of 4 h; (e–g) 2.8 mmol NH3·H2O and 1.4 mmol (NH4)2SO4 with a reaction time of 24 h.
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Figure 8. The XRD patterns of the samples synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 4.0 mmol NH4COOCH3, and (a) 0.7; (b) 1.4; (c) 2.1; (d) 2.8; (e) 3.5 mmol NH3·H2O.
Figure 8. The XRD patterns of the samples synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, 4.0 mmol NH4COOCH3, and (a) 0.7; (b) 1.4; (c) 2.1; (d) 2.8; (e) 3.5 mmol NH3·H2O.
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Figure 9. Contents of Cs1.1W1.65O5.5 versus various ammonium sources in the system of WO3, CsCl, CH3CH2CHO and NH3·H2O and NH4COOCH3; NH3·H2O and (NH4)2SO4; (NH4)2C2O4. Note: Cs 1.1 W 1.65 O 5.5   Content = Cs 1.1 W 1.65 O 5.5 WO 3 +   Cs 0.3 WO 3 +   Cs 1.1 W 1.65 O 5.5 ; n N H 4 + = n N H 3 + n N H 4 + .
Figure 9. Contents of Cs1.1W1.65O5.5 versus various ammonium sources in the system of WO3, CsCl, CH3CH2CHO and NH3·H2O and NH4COOCH3; NH3·H2O and (NH4)2SO4; (NH4)2C2O4. Note: Cs 1.1 W 1.65 O 5.5   Content = Cs 1.1 W 1.65 O 5.5 WO 3 +   Cs 0.3 WO 3 +   Cs 1.1 W 1.65 O 5.5 ; n N H 4 + = n N H 3 + n N H 4 + .
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Figure 10. SEM images of Cs0.3WO3 synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and (a) 1.5; (b) 2.0; (c) 2.5 mmol (NH4)2C2O4.
Figure 10. SEM images of Cs0.3WO3 synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and (a) 1.5; (b) 2.0; (c) 2.5 mmol (NH4)2C2O4.
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Figure 11. The (a) TEM and (b) SAED images of samples synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and 2.0 mmol (NH4)2C2O4. The red square is the diffraction region of SAED.
Figure 11. The (a) TEM and (b) SAED images of samples synthesized hydrothermally at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and 2.0 mmol (NH4)2C2O4. The red square is the diffraction region of SAED.
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Table
Table 1. The crystalline phases, starting pH values, and the mass percentages of Cs1.1W1.65O5.5 of the systems conducted at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and different amounts of NH3·H2O and (NH4)2SO4.
Table 1. The crystalline phases, starting pH values, and the mass percentages of Cs1.1W1.65O5.5 of the systems conducted at 200 °C for 24 h using 0.18 mmol CsCl, 0.4 mmol WO3, 8.8 mmol CH3CH2CHO, and different amounts of NH3·H2O and (NH4)2SO4.
NH3·H2O (mmol)(NH4)2SO4 (mmol)Crystalline PhaseCs1.1W1.65O5.5
(wt%)		pH
1.40WO3, Cs0.3WO3010.13
1.41.4WO3, Cs0.3WO308.50
2.1Cs0.3WO308.66
2.808.81
3.5Cs0.3WO3, Cs1.1W1.65O5.56.7 ± 0.358.91
2.85.0Cs0.3WO308.30
3.5Cs0.3WO3, Cs1.1W1.65O5.55.43 ± 0.478.41
2.87.5Cs0.3WO308.19
3.5Cs0.3WO3, Cs1.1W1.65O5.53.18 ± 0.428.29
3.510.0WO3, Cs0.3WO3, Cs1.1W1.65O5.56.00 ± 1.128.18
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Zhang, Y.; Li, R. A Study of Intermediate for Synthesis of Cs0.3WO3 with Near-Infrared Photothermal Response. Energies 2022, 15, 8542. https://doi.org/10.3390/en15228542

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Zhang Y, Li R. A Study of Intermediate for Synthesis of Cs0.3WO3 with Near-Infrared Photothermal Response. Energies. 2022; 15(22):8542. https://doi.org/10.3390/en15228542

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Zhang, Yue, and Ruixing Li. 2022. "A Study of Intermediate for Synthesis of Cs0.3WO3 with Near-Infrared Photothermal Response" Energies 15, no. 22: 8542. https://doi.org/10.3390/en15228542

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