Controlled Growth of WO3Nanostructures with Three Different Morphologies and Their Structural, Optical, and Photodecomposition Studies

Tungsten trioxide (WO3) nanostructures were synthesized by hydrothermal method using sodium tungstate (Na2WO4·2H2O) alone as starting material, and sodium tungstate in presence of ferrous ammonium sulfate [(NH4)2Fe(SO4)2·6H2O] or cobalt chloride (CoCl2·6H2O) as structure-directing agents. Orthorhombic WO3having a rectangular slab-like morphology was obtained when Na2WO4·2H2O was used alone. When ferrous ammonium sulfate and cobalt chloride were added to sodium tungstate, hexagonal WO3nanowire clusters and hexagonal WO3nanorods were obtained, respectively. The crystal structure and orientation of the synthesized products were studied by X-ray diffraction (XRD), micro-Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM), and their chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS). The optical properties of the synthesized products were verified by UV–Vis and photoluminescence studies. A photodegradation study on Procion Red MX 5B was also carried out, showing that the hexagonal WO3nanowire clusters had the highest photodegradation efficiency.


Introduction
One-dimensional (1-D) nanostructures have attracted much attention because of their distinct properties and wider applications. Self-assembled growth of one-dimensional nanostructures is a simple and spontaneous process. However, understanding the reaction chemistry and growth mechanism of the process is necessary to obtain these structures. Repeated experiments of this synthetic method are needed to obtain uniform growth of 1-D nanostructures.
Semiconductor metal oxide nanostructures are highly attractive, so more attention has been paid, because of their obvious optical and electronic applications. Tungsten trioxide (WO 3 ) is one of the n type indirect wide band gap materials [1,2]. It is a fundamental functional material having interesting physical properties and wide range of applications. Due to its high work function, it was used as a charge injection layer [3]. Because of its higher catalytic activity, it can be used in photocatalytic and electrocatalytic applications [4,5]. It serves as a good host for ions, so it can be used successfully in electrochemical Li ion batteries, electrochromic, thermochromic, and photochromic devices [6][7][8][9]. Many methods have been developed to synthesize 1-D WO 3 nanostructures, such as templateassisted growth [4], anodization [10], conventional thermal evaporation [1], hot wall chemical vapor deposition [11], arc discharge [12], pulsed laser deposition [13], and hydrothermal method [14]. Among the various methods, hydrothermal method is a facile, dominant tool for the synthesis of anisotropic nanoscale materials. Significant advantages of this method are controllable size, low temperature growth, cost-effectiveness, and less complicated. Number of attempts were paid to synthesize controlled WO 3 nanostructures by hydrothermal method with the help of structure-directing chemicals like Na 2 SO 4 , Rb 2 SO 4 , K 2 SO 4 , Li 2 SO 4 , and Na 2 S [6,[14][15][16][17]. All attempts suggest that the reason behind the controlled growth was due to the presence of sulfate ions in the reaction. The data reported in Refs. [6,[14][15][16][17] show that I-group compounds such as Li 2 SO 4 , Na 2 SO 4 , Na 2 S, K 2 SO 4 , and Rb 2 SO 4 were used as structure-directing chemicals for the synthesis of WO 3 nanostructures. In the present work, we choose VIII-group metal complexes, such as ferrous ammonium sulfate, as structure-directing chemical for the synthesis of WO 3 nanostructures. Since chlorine is next to sulfur in periodic table, we expect that it may have similar tendency to sulfur. Therefore, cobalt chloride was also used as another structure-directing chemical to realize the change in morphology of WO 3 products. It is interesting that we have obtained wire-and rod-shaped WO 3 nanostructures, respectively, from ferrous ammonium sulfate and cobalt chloride as structure-directing agents. Their structural, optical, and photodegradation properties were also studied in this paper.

Synthesis of Tungsten Oxide Nanostructures
All the chemicals were of analytical grade and taken without further purification or modification. Three different reactions were made for the preparation of WO 3 products. In the first experiment, sodium tungstate was dissolved in 100 mL double distilled water (DDW) (6.6 g, 0.2 mole) and acidification was done by adding HCl to get a pH of 1. A white precipitate was obtained, and it was dissolved by adding oxalic acid (0.4 g in 30 mL DDW, 0.1 mole). As a result, a transparent solution was obtained, and it is the final solution for this experiment. In the second reaction, sodium tungstate (6.6 g, 0.2 mole) and ferrous ammonium sulfate (0.4 g, 0.1 mole, 10 mL DDW) were dissolved in DDW separately and mixed under vigorous stirring. A dark brown color mixture was obtained, and it was dissolved by adding oxalic acid. In this case, a transparent yellow color solution was obtained. It is the final solution of this reaction and its pH was 1. In the third experiment, sodium tungstate (6.6 g, 0.2 mole) and cobalt chloride (1.786 g, 0.5 mole, 15 mL in DDW) solutions were prepared separately and mixed with constant stirring. A violet mixture appeared, and it was dissolved by nitric acid. Now, a transparent red solution was obtained and its pH was 1. These three solutions were transferred separately into 40 mL Teflon-lined stainless steel autoclave and maintained at 180°C for 24 h to get the final product. The as-obtained products were washed several times both in water and ethanol and finally dried at 100°C for 2 h.

Characterization of Surface Morphology and Structural Properties
Figure 1a-c shows the scanning electron microscope (SEM) images of the as-prepared WO 3 products synthesized using three different hydrothermal reactions. Rectangular slab-like WO 3 crystals were obtained by direct hydrothermal synthesis. These rectangular slab-like structures show their sharp corners and smooth surfaces as indicated in the Fig. 1a. When ferrous ammonium sulfate was used, WO 3 nanowire clusters were obtained, and their SEM image is shown in Fig. 1b. The average length and diameter of the nanowires were 1 lm (1,000 nm) and 30 nm, respectively. Cobalt chloride-reacted WO 3 products, shown in Fig. 1c, look like rod-shaped nanostructures. Their average length and diameter were only about 100 and 30 nm, respectively. There is no change in the crosssectional dimension between nanowires and nanorods, but a decrease in the length was observed with nanorod morphology.
Usually, when there is no structure-directing chemical, the morphology will be rectangular in shape. When structure-directing chemicals such as sulfate and chlorine ions were added, then one can see the formation of onedimensional nanowire/nanorod-like structures. Though the exact reason is not reported, it is believed that sulfate ions play a major role in giving one-dimensional shape, i.e., sulfate ions adsorb to the surface of the seed crystals of WO 3 and thereby decrease the surface energy of the WO 3 seed crystals in all directions except one direction. In this particular undisturbed direction, further growth takes place by means of agglomeration/attachment with other seed layers. This process continues to give one-dimensional wire/rod-like structures.
Figure 1d-f also shows that the corresponding highresolution transmission electron microscope (HRTEM) images. Rectangular slab structures have shown wellcleared sharp boundaries. Resolved lattice fringes of both directions have been seen in rectangular slab structure. The calculated spacing of the lattice fringes of two perpendicular directions was about 3.14 and 3.63 Å , respectively. Figure 2a-c shows the X-ray diffraction (XRD) patterns for three as-prepared WO 3 products. XRD pattern for rectangular slab structure, with orthorhombic crystal phase,  [18]. HRTEM image of a nanowire shows (Fig. 1e) clear lattice fringes along the growth direction. Here, the spacing of the lattice fringes was 3.63 Å , which was well indexed with the 'd' spacing of the plane (200) of hexagonal WO 3 (Fig. 2b). These products were prepared by using ferrous ammonium sulfate as structure-directing chemical. They follow hexagonal crystal structure. Lattice parameters of the synthesized h-WO 3 are a = 7.324 Å and c = 7.668 Å (JCPDS card No: 85-2460) [14]. From the HRTEM image of WO 3 nanorod, it was observed that the spacing of lattice fringes was about 3.87 Å , which is indexed with the (002) plane in XRD pattern for the h-WO 3 products (Fig. 2c). These were prepared by using cobalt chloride as structure-directing chemical, and their lattice parameters are the same as that of WO 3 nanowire clusters.
Orthorhombic WO 3 structures are the usual product from the direct hydrothermal synthesis. However, when reacted with ferrous ammonium sulfate, it was observed that one of its lattice parameters 'b' is reducing from 12.513 to 7.324 Å . The remaining lattice parameters 'a' and 'c' are almost unchanged in such a case. So, it is clear that the formation of hexagonal crystal structure from the orthorhombic structure is taking place by reducing 'b' lattice significantly. The interlayer spacing for the direct synthesized WO 3 and ferrous ammonium sulfate used WO 3 products, respectively, at d220 (3.143 Å ) and d200 (3.156 Å ) are almost the same, which proves that the lattice parameters 'a' and 'c' are similar to each other. When reacted with cobalt chloride, a reduction in the 'b' value close to 'a' value was noted and that could be the reason for the change in crystal structure from orthorhombic to hexagonal. Though the exact reason behind the onedimensional nanorod growth is not clear, we believe that chlorine ions also play a similar role as that of sulfate ions and help to grow a one-dimensional nanostructure.
In hydrothermal growth condition, nanoparticles of WO 3 are formed and subsequently aggregated to give morphologies like rectangular slab, wire, and rod structures. When there are no structure-directing chemicals, the as-formed nanoparticles are of orthorhombic nature and these orthorhombic nanoparticles aggregated to form a three-dimensional rectangular slab-like structure. When structure-directing chemicals, such as ferrous ammonium sulfate or cobalt chloride, were used, the as-formed WO 3 particles are of hexagonal nature and these hexagonal particles aggregated to give wire-or rod-like structures. Three-dimensional rectangular slabs-like structure is prohibited by the presence of sulfate or chlorine ions. The role played by the sulfate/chlorine ions in getting one-dimensional structure has been explained earlier. Figure 3a-c shows the Raman spectra recorded from the three WO 3 samples. Most of the vibrational modes are similar to one another, but a very few modes differentiate whether it is orthorhombic or hexagonal WO 3 crystal system [19]. Crystalline WO 3 has three main spectral regions, one is between 900 and 600 cm -1 , the second one is between 400 and 200 cm -1 , and the third one is below 200 cm -1 . The three regions, respectively, correspond to stretching, bending, and lattice modes. These peaks are characteristics of tungsten oxide material as well. We observed Raman peaks for orthorhombic rectangular slablike structure at 805, 690, 325, and 255 cm -1 . Here, the In the case of the hexagonal nanowire structure, we observed almost similar modes as that of orthorhombic structure. The modes are 817 and 690 cm -1 and which were due to stretching vibrations. Upon comparing the stretching modes of orthorhombic (805 cm -1 ) and hexagonal (817 cm -1 ) structures, in the case of hexagonal WO 3 , it was found that there is a shift in wavenumber toward higher side (Fig. 3b, c). This shift could be due to a local structural change in the crystal system, upon conversion from orthorhombic to hexagonal crystal structure. In addition to this, we also noticed a characteristic mode of hexagonal WO 3 at 651 cm -1 [20], which again confirmed the crystal structures of nanowire and nanorods. No impurity peaks other than WO 3 were observed, which shows the purity of the as-prepared samples.
Characterization of Chemical States X-ray photoelectron spectroscopy (XPS) analysis was carried out to further confirm the purity of the samples and  Figure 4a-c shows the XPS W 4f core-level spectra for all the three as-prepared WO 3 samples. From these results, we observed that impurities like Fe, Co, Cl, N, and S are not present in all the three samples. The XPS W 4f core-level spectra have revealed that in all the samples under study tungsten atoms are in the formal valence state ?6 [21]. Their 4f peaks binding energies were observed at 38.0 and 36.0 eV, which corresponds to spin-orbit splitting of the W 4f 7/2 and W 4f 5/2 components, respectively, in tungsten oxides [21]. The XPS O 1 s core-level binding energies of the tungsten oxide samples under consideration correspond to 531.0 eV (Fig. 5a-c). From the relative intensities of the XPS spectra, we have calculated the compositional stoichiometry between 'W' and 'O' and it is found to be about 1:3 in the every sample studied.

Characterization of Optical Properties
To study the photoluminescence as well as photodegradation performance of the as-prepared WO 3 samples, it is necessary to know the absorption edge and number of photons absorbed by the material. So, UV-Vis absorption spectra were taken for all the three WO 3 samples. Figure 6a-c shows the absorption spectra with a peak maximum at 275 nm wavelength for all the three samples. A very clear, high intense absorption peak was obtained for nanowire cluster structures, whereas for nanorod and rectangular slab structures less intense absorption was seen. Using this absorption maximum, photoluminescence property of the as-prepared WO 3 nanostructures was studied and their emission results are shown in Fig. 7a-c. We have seen emission peaks at 325 and 420 nm for all the three WO 3 samples. The emission at 325 nm corresponds to edge emission [22][23][24][25]. Only nanowire clusters have shown relatively high intense another emission peak at 420 nm and which could originate from the presence of oxygen vacancies or defect states [23]. Though XPS results have shown a stoichiometry in composition, our PL results have shown the presence of defect states by means of an emission at 420 nm. This could be due to more sensitivity of PL than XPS. Increase in surface to volume ratio results in increase of defect states in the case of WO 3 nanowire clusters and that could be the reason for the observed   Fig. 8a-c for all the three morphologies. Absorption value a, b, and c in Fig. 8, respectively, correspond to pure as-prepared dye, dye in dark for 24 h, and dye kept in UV light for 24 h, but without WO 3 dispersion. From the observed results, we come to a conclusion that there is no any noticeable change when maintained for 24 h in dark, but a slight decrease in absorption was observed after 24 h exposure in UV illumination. With this initial study, the as-prepared WO 3 nanostructures were dispersed in the dye solution and illuminated with UV light for 3, 6, 9, 12, and 24 h and their absorptions were recorded. The spectra in Fig. 8d-h correspond to respective absorption spectra from the overall experimental results. Upon comparison of all the results, it was found that the WO 3 nanowire cluster dispersed medium has responded quickly in decomposing the dye. UV-Vis absorption spectra have shown that, only this particular morphology has high cross-section for absorption of photons, and this could be the reason for the quick response in decomposition.
To estimate the rate of the reaction in the present photodegradation experiment, the following equation was used: is reaction time. The rate of the reaction k was calculated from all the three photodecomposition experiments by drawing a graph between ln(A o /A t ) and time. Figure 9a-c shows the relation between ln(A o /A t ) and time. Rate of the reaction k was obtained from the slope of these graphs and the as-obtained values are 0.1003, 0.1032, and 0.0355 h -1 , which, respectively, correspond to photodecomposition reaction with WO 3 rectangular slab, nanowire clusters and nanorod structures. Comparatively a high photodecomposition was noted from WO 3 nanowire cluster samples.

Conclusion
Self-assembled WO 3 nanostructures were synthesized with new structure-directing chemicals by using hydrothermal route and its structural, optical, and photodecomposition activity were studied. We have obtained different WO 3 morphologies like rectangular slab, nanowire clusters, and nanorods by introducing new structure-directing chemicals (ferrous ammonium sulfate, cobalt chloride) in this hydrothermal reaction. The possible growth mechanism of various shaped WO 3 nanostructures was also discussed. From optical absorption maxima, quantum confinement effect was realized for all the three morphologies. From photodecomposition experiment, a relatively high photodecomposition activity was observed from nanowire clusters sample due to their high absorption. Further study is under research to investigate the concentration-dependent morphologies and their role in photodecomposition.