Controlled Growth of WO3Nanostructures with Three Different Morphologies and Their Structural, Optical, and Photodecomposition Studies
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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.
KeywordsTungsten trioxide Hydrothermal Structure-directing chemicals Nanowires Nanorods Photodecomposition
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 (WO3) 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 . 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 [6–9]. Many methods have been developed to synthesize 1-D WO3 nanostructures, such as template-assisted growth , anodization , conventional thermal evaporation , hot wall chemical vapor deposition , arc discharge , pulsed laser deposition , and hydrothermal method . 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 WO3 nanostructures by hydrothermal method with the help of structure-directing chemicals like Na2SO4, Rb2SO4, K2SO4, Li2SO4, and Na2S [6, 14–17]. 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, 14–17] show that I-group compounds such as Li2SO4, Na2SO4, Na2S, K2SO4, and Rb2SO4 were used as structure-directing chemicals for the synthesis of WO3 nanostructures. In the present work, we choose VIII-group metal complexes, such as ferrous ammonium sulfate, as structure-directing chemical for the synthesis of WO3 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 WO3 products. It is interesting that we have obtained wire- and rod-shaped WO3 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. Sodium tungstate (Na2WO4·2H2O) is the starting material. Hydrochloric acid (HCl), oxalic acid (C2H2O4·2H2O), nitric acid (HNO3), ferrous ammonium sulfate [(NH4)2Fe(SO4)2·6H2O], and cobalt chloride (CoCl2·6H2O) were the other chemicals used for the growth.
Three different reactions were made for the preparation of WO3products. 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.
The surface morphology, structural, and chemical states of the formed nanostructures were characterized by using scanning electron microscopy (SEM-5600 JEOL JSM), transmission electron microscopy (TEM-2011 JEOL STEM), X-ray diffraction (PANalytical X-ray diffractometer—XPERT PRO), Raman spectroscopy (Horiba Jobin Raman spectrometer, reflection mode, wavelength of 532 nm, 2mW), and X-ray photoelectron spectroscopy (XPS—ion-pumped chamber of an ES-2401 spectrometer Mg Kα radiation, photon energy 1,253.6 eV) techniques. Optical measurements were carried out in UV–Vis spectrophotometer (SHIMADZU 3600 UV–Vis–NIR spectrophotometer), and photoluminescence properties were analyzed by using Horiba Jobin Yvon spectrofluoromax spectrometer.
Results and Discussion
Characterization of Surface Morphology and Structural Properties
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 one-dimensional 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 WO3and thereby decrease the surface energy of the WO3seed 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.
Orthorhombic WO3structures 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 WO3and ferrous ammonium sulfate used WO3products, 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 one-dimensional 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 WO3are 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 WO3particles 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.
Characterization of Chemical States
Characterization of Optical Properties
Self-assembled WO3nanostructures were synthesized with new structure-directing chemicals by using hydrothermal route and its structural, optical, and photodecomposition activity were studied. We have obtained different WO3morphologies 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 WO3nanostructures 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.
The author would like to thank Dr. K. Swaminathan, Professor and Head, Department of Microbial Biotechnology, Bharathiar University for his support in utilizing UV–Vis absorption spectrophotometer. One of the authors S. Rajagopal would like to thank Bharathiar University for awarding University Research Fellowship to carry out this work.