WO3·nH2O Crystals with Controllable Morphology/Phase and Their Optical Absorption Properties.

In this work, a one-step hydrothermal route is developed to prepare WO3·nH2O crystals with various morphology/phases, for which any surfactants, templates, or structure-directing agents are not used. Five types of WO3·nH2O crystals, including o-WO3·H2O nanoplates, rectangular m-WO3 nanosheets, o-WO3·0.33H2O microspheres, h-WO3 nanorods, and bundle-like h-WO3 hierarchical structures, are successfully obtained by adjusting the amount of H2SO4 and reaction temperature. According to the experimental results, the formation mechanism for various WO3·nH2O species is proposed. In addition, the optical absorption properties of these WO3·nH2O crystals are also investigated by UV-vis absorption spectra.

Introduction

The controllable preparation of crystals is a challenging issue in the field of chemistry and materials.[1−8] Particularly, the synthesis of transition-metal oxides (TMOs) with controllable morphology and crystal phase has gotten widespread international attention because of the strongly determined physical and chemical properties of TMOs.[9−13] Among these TMOs, tungsten trioxide and its hydrates (WO3·nH2O) have been widely investigated owing to their distinct optical and electrical properties and the functional applications in photocatalytic degradation,[14] gas sensors,[15] water splitting,[16,17] supercapacitors,[18] lithium ion batteries,[19,20] and electro/opto/gasochromic devices.[21]

Over the past two decades, many approaches, such as spray pyrolysis,[22] sol–gel,[23] anodization,[24] thermal decomposition,[25] microemulsion technique,[26] chemical vapor deposition,[27] hydro/solvothermal reactions,[28−30] etc., have been used to prepare WO3·nH2O crystals with various morphologies/phases. However, most of them have more or less shortcomings, such as expensive equipment/reagents, complicated operation, and time-consuming processes, which makes large-scale economic production difficult. In this situation, a one-step hydrothermal method may be a viable candidate for preparing WO3·nH2O crystals due to the simple operation, lower energy consumption, good repeatability, and mild environment. For example, Shi et al. produced WO3·nH2O crystals via a hydrothermal route with Na2SO4 additive, and three types of crystals (cubic octahedron-shaped WO3·0.5H2O, hexagonal WO3 nanorods, and orthorhombic WO3·0.33H2O nanoplates) can be produced.[14] Zhang et al. prepared orthorhombic WO3·H2O hierarchical structures by a hydrothermal route with the assistance of cetrimonium bromide (CTAB) surfactant.[28] Mohamed et al. employed the hydrothermal method to prepare hexagonal WO3 with various morphologies by using metal salts as structure-directing agents.[17] Although great progress has been made, there remains a huge challenge in the preparation of WO3·nH2O crystals with controllable morphology/phase, which usually require surfactants, templates, or structure-directing agents.

Herein, a one-step hydrothermal method was developed for preparing controllable WO3·nH2O crystals by using Na2WO4·2H2O and H2SO4 as the starting materials. No surfactants, templates, or structure-directing agents were used in this hydrothermal system. Five types of WO3·nH2O crystals, including orthorhombic WO3·H2O (o-WO3·H2O) nanoplates, rectangular monoclinic WO3 (m-WO3) nanosheets, orthorhombic WO3·0.33H2O (o-WO3·0.33H2O) microspheres, hexagonal WO3 (h-WO3) nanorods, and bundle-like h-WO3 hierarchical structures, were successfully synthesized by adjusting the amount of H2SO4 and the reaction temperature. The formation mechanism of these WO3·nH2O crystals was proposed. In addition, the optical absorption properties and band gaps of them were also evaluated.

Experimental Procedure

All chemicals were used without further purification; they are of analytical grade. In a typical preparation, 4.6 g of Na2WO4·2H2O and a certain amount of H2SO4 solution (96–98 wt %) were added to 40 mL of ultrapure water under vigorous stirring. The obtained solution was transferred into a 50 mL stainless steel Teflon-lined autoclave, which was heated under different temperatures (100–200 °C) for 3 h. The different hydrothermal conditions are listed in Table . The final products were obtained by centrifugation. Each experiment was repeated three times. The WO3·nH2O crystals obtained by this hydrothermal method have high yield, good repeatability, and stability.

Table 1

Summary of the As-Prepared Products under Different Hydrothermal Conditions

mass of Na2WO4·2H2O	amount of H2SO4 solution	reaction temperature/time	morphology	crystal phase
4.6 g	3 mL	100 °C/3 h	nanoplates	o-WO3·H2O
4.6 g	3 mL	150 °C/3 h	nanoplates	Much o-WO3·H2O little o-WO3·0.33H2O
4.6 g	3 mL	200 °C/3 h	rectangular nanosheets	m-WO3
4.6 g	2 mL	100 °C/3 h	nanoplates nanoparticles	Much o-WO3·H2O, few o-WO3·0.33H2O
4.6 g	2 mL	150 °C/3 h	nanoparticles microspheres	o-WO3·H2O, o-WO3·0.33H2O
4.6 g	2 mL	200 °C/3 h	microspheres	o-WO3·0.33H2O
4.6 g	1 mL	100 °C/3 h	nanorods	h-WO3
4.6 g	1 mL	150 °C/3 h	bundle-like hierarchical structures	h-WO3
4.6 g	1 mL	200 °C/3 h	nanorods	h-WO3

The phase was analyzed by X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation at λ = 1.5418 Å. The microstructures were characterized by scanning electron microscope (SEM, Zeiss Σigma) and transmission electron microscope (TEM, FEI Tecnai G2 20). The optical adsorption spectra of the products were measured by a PerkinElmer Lambda 35 UV–vis spectrometer.

Results and Discussion

The morphology/phase of the obtained products under different hydrothermal conditions was characterized by SEM and XRD, respectively. Figure a displays the as-prepared nanoplates (∼150 nm in edge length, ∼20 nm in thickness), which were prepared by adding 3 mL of H2SO4 at 100 °C. All the diffraction peaks (Figure a at 100 °C) of these nanoplates could be well-indexed to the orthorhombic phase of WO3·H2O (JCPDS No. 84-0886, a = 0.5249 nm, b = 1.0711 nm, and c = 0.5133 nm). At 150 °C, the morphology of products is almost unchanged and mainly remains nanoplate (Figure b). Nevertheless, the XRD pattern (Figure a at 150 °C) shows that some weak diffraction peaks appear and can be identified as the orthorhombic phase of WO3·0.33H2O (JCPDS No. 72-0199), suggesting a little amount of WO3·H2O has been dehydrated to form WO3·0.33H2O. At 200 °C, the rectangular sheet-like products (70 to 240 nm) were obtained (Figure c), which can be assigned to the monoclinic phase of WO3 (JCPDS No. 72-0677, a = 0.7306 nm, b = 0.7540 nm, and c = 0.7692 nm) (Figure a at 200 °C).

Figure 1

SEM images of the products prepared by adding 3 mL of H2SO4 at (a) 100, (b) 150, and (c) 200 °C, 2 mL of H2SO4 at (d) 100, (e) 150, and (f) 200 °C, and 1 mL of H2SO4 at (g) 100, (h) 150, and (i) 200 °C.

Figure 2

XRD patterns of the products prepared by adding (a) 3, (b) 2, and (c) 1 mL of H2SO4 at different reaction temperatures.

As shown in Figure d–f, the products were prepared by adding 2 mL of H2SO4 at different reaction temperatures. At 100 °C, a large number of nanoplates coexist with a few nanoparticles (Figure d). The XRD pattern confirms that a small amount of WO3·H2O has been dehydrated to form WO3·0.33H2O at this reaction temperature (Figure b at 100 °C). At 150 °C (Figure e), more WO3·H2O nanoplates dissolved, and the species with a microsphere-like appearance began to form. The XRD pattern (Figure b at 150 °C) also confirms that WO3·H2O nanoplates were further converted into WO3·0.33H2O, for which the diffraction peaks of WO3·0.33H2O intensified, while those of WO3·H2O decreased. As the reaction temperature further increases to 200 °C, lots of microspheres (∼6 μm in diameter) can be obtained (Figure f), and these microspheres are a pure orthorhombic phase of WO3·0.33H2O (JCPDS No. 72-0199, a = 0.7359 nm, b = 1.2513 nm, and c = 0.7704 nm) (Figure b at 200 °C). From the magnified SEM image (inset in Figure f), it can be seen that the microsphere is made up of many primary WO3·0.33H2O nanoflakes.

As shown in Figure g–i, the products were prepared by adding 1 mL of H2SO4 at different reaction temperatures. At 100 °C, the nanorods (∼12 nm in diameter, ∼60 nm in length) were obtained (Figure g). All the diffraction peaks (Figure c at 100 °C) of these nanorods match well with the hexagonal phase of WO3 (JCPDS No. 75-2187, a = 0.7298 nm, c = 0.3899 nm). At 150 °C, the product shows a bundle-like hierarchical structure, which is composed of many primary nanorods with different sizes (Figure h). The XRD pattern confirms that these bundle-like hierarchical structures are the hexagonal phase of WO3 (Figure c at 150 °C). As the reaction temperature further increased to 200 °C, the nanorods (∼20 nm in diameter, ∼130 nm length) were obtained again (Figure i) with the hexagonal phase (Figure c at 200 °C). It is evident that the average size of nanorods obtained at 200 °C is larger than that of nanorods obtained at 100 °C.

The morphology/phase of as-prepared products under different hydrothermal conditions are listed in Table . It can be seen that five types of WO3·nH2O crystals, including o-WO3·H2O nanoplates, rectangular m-WO3 nanosheets, o-WO3·0.33H2O microspheres, h-WO3 nanorods (with two different sizes obtained at 100 and 200 °C, respectively), and bundle-like h-WO3 hierarchical structures, were successfully synthesized by adjusting the amount of H2SO4 and the reaction temperature in our experiments.

Furthermore, TEM characterizations were further performed to comprehend the growth rule of these WO3·nH2O crystals (Figure ). The TEM image shows o-WO3·H2O nanoplates in Figure a. The clear and ordered lattice fringes were observed in a high-resolution TEM (HRTEM) image taken along the [010] zone axis (Figure d), confirming that these nanoplates are single crystals. The crystal interplanar distances of 0.261 and 0.256 nm correspond to the (200) and (002) facets of orthorhombic WO3·H2O. The single-crystalline nature was also demonstrated by the corresponding fast Fourier transform (FFT) diffractogram (inset in Figure d). Combining the HRTEM image and FFT diffractogram, it can be known that the top and bottom facets of the o-WO3·H2O nanoplate are (010) facets. Figure b shows a TEM image of rectangular m-WO3 nanosheets. The HRTEM image (Figure e) taken along the [001] zone axis and the corresponding FFT diffractogram (inset in Figure e) reveal the single-crystalline nature of these nanosheets, and the surface of them consists of a major top (002) and minor lateral (020) and (200) facets. The TEM image of as-prepared o-WO3·0.33H2O microspheres was shown in Figure c, and the HRTEM image (Figure f) displays that the blocks (o-WO3·0.33H2O nanoflakes) of these microspheres are single crystals.

Figure 3

TEM and HRTEM images of the (a, d) o-WO3·H2O nanoplates, (b, e) rectangular m-WO3 nanosheets, (c, f) o-WO3·0.33H2O microspheres, (g, j) h-WO3 nanorods obtained at 100 °C, (h, k) bundle-like h-WO3 hierarchical structures, and (i, l) h-WO3 nanorods obtained at 200 °C.

Figure g shows a TEM image of h-WO3 nanorods obtained at 100 °C, and the HRTEM image (Figure j) of an individual nanorod shows a crystal interplanar distance of 0.383 nm for the (001) facet, confirming its single-crystalline nature and preferential growth along the c-axis of a hexagonal structure. Figure h displays a TEM image of bundle-like h-WO3 hierarchical structures. The HRTEM image (Figure k) indicates that the blocks of this hierarchical structure, that is, the nanorods, are single crystals with the same growth direction (the c-axis of the hexagonal structure). The TEM results of h-WO3 nanorods obtained at 200 °C are shown in Figure i,l, which are similar to the h-WO3 nanorods obtained at 100 °C.

Generally, the external reaction conditions and intrinsic crystal structures determine the crystal growth behavior.[31] The amount of H2SO4 and the reaction temperature play key roles in the preparation of various WO3·nH2O crystals. According to the above experimental results, the suggested reaction routes in this hydrothermal system could be described as follows.

The o-WO3·H2O was obtained by acidic precipitation according to eq ,[32] when the H2SO4 solution was added to the Na2WO4 solution during the hydrothermal process. This o-WO3·H2O has a layered perovskite-like ReO3 structure,[33] which in turn governs its growth and facilitates growth along the a-axis and c-axis to form a plate-like morphology. It is believed that a higher concentration of H+ ions can inhibit the dehydration of tungsten trioxide hydrates, while a higher reaction temperature can promote the dehydration of them during a hydrothermal process. When the hydrothermal reaction occurs at a high concentration of H+ ions (by adding 3 mL of H2SO4) and low reaction temperature (100 °C), the dehydration of o-WO3·H2O was obviously inhibited, so pure-phase o-WO3·H2O nanoplates can be obtained. Under a higher reaction temperature (150 °C), the dehydration trend of o-WO3·H2O becomes stronger, and a small amount of o-WO3·H2O has been dehydrated to form o-WO3·0.33H2O, as shown in eq . Further increasing the reaction temperature to 200 °C, o-WO3·0.33H2O can be dehydrated to obtain h-WO3 as shown in eq . However, h-WO3 belongs to a metastable phase, which can be transformed into more stable m-WO3 under an appropriate hydrothermal condition,[34] as shown in eq . From the TEM result of Figure e, the exposed facets of the m-WO3 crystals were identified as (002), (020), and (200). Because the order of surface energy of these exposed facets is (002) > (020) > (200),[35] this will result in the formation of rectangular m-WO3 nanosheets with a larger area of (200) exposed facet and lower the system energy.

When the concentration of H+ ions decreased (by adding 2 mL of H2SO4), the dehydration trend of o-WO3·H2O was enhanced. Even at a low reaction temperature (100 °C), some o-WO3·H2O was dehydrated to form o-WO3·0.33H2O. As the reaction temperature became higher (150 or 200 °C), the dehydration trend of o-WO3·H2O became stronger, resulting in the increase of the proportion of o-WO3·0.33H2O. Under 200 °C, o-WO3·H2O was completely dehydrated to obtain pure-phase o-WO3·0.33H2O. The morphology of these o-WO3·0.33H2O is microsphere, which is composed of many primary o-WO3·0.33H2O nanoflakes. Without the introduction of any surfactant during the hydrothermal process, the o-WO3·0.33H2O microsphere may be generated by the assembly of primary nanoflakes through a van der Waals interaction to decrease the system energy.[36] This formation process of o-WO3·0.33H2O microsphere is similar to that of the o-WO3·H2O microsphere reported by Huang et al.[37]

When the concentration of H+ ions further decreased (by adding 1 mL of H2SO4), the dehydration trend of o-WO3·H2O was also further enhanced. The pure phase of h-WO3 nanorods was obtained at the low reaction temperature (100 °C) by eqs –(3). The h-WO3 crystal can be regarded as a polar crystal with ±(001) polar facets,[38] which tends to grow preferentially along its polar direction (the c-axis of hexagonal structure), favoring the formation of a one-dimensional structure.[39] At 150 °C, the bundle-like h-WO3 hierarchical structures were obtained through the assembly of primary nanorods in an oriented mode. The lateral capillary force provides the energy for the assembly of the nanorod to decrease the system surface energy.[40] When the reaction temperature was further increased to 200 °C, the smaller h-WO3 nanorods dissolved and the larger h-WO3 nanorods formed due to the strongly Ostwald ripening process.[41] Of course, the possible bundle-like hierarchical structures would be destroyed at this high reaction temperature (200 °C) during the hydrothermal process.

The optical absorption properties of WO3·nH2O crystals with various morphologies/phases were evaluated by UV–vis absorption spectra. The UV–vis absorption spectra of various WO3·nH2O crystals were shown in Figure a. The values of absorption edges of these WO3·nH2O crystals can be obtained from Figure a, as listed in Table . It can be seen that the absorption edges of these crystals are located at visible light region that is in the wavelength range from 438 to 592 nm. The band gap Eg can be determined by the Tauc equation (αhv) = B(hv – Eg) for a semiconductor,[42] where α is the absorption coefficient, hv is the photon energy, B is the edge width parameter, and n is a constant characterizing the nature of the band transition. Considering that WO3·nH2O crystals are indirect band gap semiconductors,[16,43−45] the value of n can be taken as 0.5. As in the method described by Tauc, the band gap is evaluated simply by fitting a straight line to the linear portion of the absorption spectrum (Tauc linear fit). However, this method usually brings large errors to the value of band gap, and some researchers point out that an intersection of the Tauc linear fit and the baseline will give a more reasonable band gap estimation.[46−48] Accordingly, the values of band gaps for WO3·nH2O crystals can be obtained from Figure b in light of this improved method, and these values are also listed in Table . The obtained values of band gaps for WO3·nH2O crystals range from 2.33 to 2.91 eV, which is in the visible light region, suggesting some possible applications of them, such as visible light photocatalysts, etc. Obviously, the different values of absorption edges and band gaps depend on the different morphologies and crystal phases of WO3·nH2O crystals.

Figure 4

(a) UV–Vis absorption spectra and (b) corresponding Tauc plots of the as-prepared WO3·nH2O crystals. Black line: o-WO3·H2O nanoplates; Red line: rectangular m-WO3 nanosheets; Blue line: o-WO3·0.33H2O microspheres; Green line: h-WO3 nanorods (obtained at 100 °C); Pink line: bundle-like h-WO3 hierarchical structures; Purple line: h-WO3 nanorods (obtained at 200 °C).

Table 2

Experimental Values of Absorption Edges and Band Gaps for the As-Prepared Products

product	absorption edge (nm)	band gap (eV)
o-WO3·H2O nanoplates	554	2.33
rectangular m-WO3 nanosheets	488	2.58
o-WO3·0.33H2O microspheres	438	2.76
h-WO3 nanorods (obtained at 100 °C)	592	2.81
bundle-like h-WO3 hierarchical structures	468	2.91
h-WO3 nanorods (obtained at 200 °C)	452	2.83

Conclusions

In summary, WO3·nH2O crystals with controllable morphology/phase have been prepared by a one-step hydrothermal method without employing any surfactants, templates, or structure-directing agents. It was found that five types of WO3·nH2O crystals, including o-WO3·H2O nanoplates, rectangular m-WO3 nanosheets, o-WO3·0.33H2O microspheres, h-WO3 nanorods, and bundle-like h-WO3 hierarchical structures, can be obtained by adjusting the amount of H2SO4 and reaction temperature. The formation mechanism of these WO3·nH2O crystals was proposed. Moreover, the absorption edges and band gaps of as-prepared WO3·nH2O crystals were measured to be in the ranges of 438–592 nm and 2.33–∼2.91 eV, respectively. The current study puts forward a promising method for preparing WO3·nH2O crystals, which may fulfill the requirements of potential applications and industrial production.