Synthesis and characterization of WO3 spherical nanoparticles and nanorods

Accepted Manuscript Title: Synthesis and characterization of WO3 spherical nanoparticles and nanorods Author: Sangeeta Adhikari Debasish Sarkar Himadri Sekhar Maiti PII: DOI: Reference:

S0025-5408(13)00694-6 http://dx.doi.org/doi:10.1016/j.materresbull.2013.08.028 MRB 6977

To appear in:

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Received date: Revised date: Accepted date:

9-3-2013 21-5-2013 18-8-2013

Please cite this article as: S. Adhikari, D. Sarkar, H.S. Maiti, Synthesis and characterization of WO3 spherical nanoparticles and nanorods, Materials Research Bulletin (2013), http://dx.doi.org/10.1016/j.materresbull.2013.08.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and characterization of WO3 spherical nanoparticles and nanorods Sangeeta Adhikari, Debasish Sarkar and Himadri Sekhar Maiti^ Department of Ceramic Engineering, National Institute of Technology, Rourkela-769008, India

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Abstract

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Simple and new wet chemical routes are adopted for the synthesis of tungsten trioxide (WO3) nanopowders having two different morphologies such as spherical and rod-like. Acid catalyzed

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exothermic reaction and a structure directing reagent have been used to control the formation of spherical and rod shaped nanoparticles, respectively. Thermal analysis and FTIR spectral data

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have been used to confirm the formation of the intermediate and the ultimate reaction products. X-ray and Raman spectroscopic data indicate the monoclinic structure of both forms of the

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particles. Rod shaped WO3 particles exhibit better crystallinity and low specific surface area

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irrespective of the morphology.

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compared to those exhibited by spherical particles. Band gaps are found to be nearly identical

Key words: A. nanostructures, A. semiconductors, B. chemical synthesis, C. electron

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micrsocopy, C. infrared spectroscopy.

1. Introduction

Technological interest on defect perovskite tungsten trioxide (WO3) has been renewed in

recent years, particularly for photo-catalytic splitting of water to generate hydrogen and also for fabrication of different useful devices e.g. display panels, smart windows and mirrors [1]. In addition, nano-structured materials find potential applications in several other devices e.g. light emitting diodes, biological labeling and sunroofs which rely on the control of morphology and #

Corresponding Author: Debasish Sarkar ([email protected]), Phone: +91-0661-2462207;

^Himadri Sekhar Maiti, INAE Distinguished Professor ([email protected]). 1 Page 1 of 22

size distribution of the nanoparticles [2, 3]. Focused attention has also been paid at preparing the high surface area, mesoporous and transparent films for their use in solar cells and photocatalysis [4]. Nanostructured WO3 films and particles have direct use in the photodecomposition of several harmful and environmentally polluting dyes like Procion Red

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MX-5B, Alizarin yellow GG (5-(M-Nitrophenylazo) salicylic Acid) and methylene blue

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(methylthioninium chloride) [5-7]. Different schools have proposed multi-directional techniques for synthesis of nano-structured WO3 e.g. hot wall chemical vapor deposition, thermal

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evaporation, hydrothermal synthesis and precipitation [8-10]. Out of these, hydrothermal method has been studied in more details in order to obtain different morphologies starting with

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Na2WO4.2H2O or H2WO4 together with various structure directing agents like sulphides, chlorides, sulphates etc. One dimensional WO3 nanostructure has been hydrothermally grown

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using structure directing agents like M2SO4 (M= Na, K, Rb) [5, 11]. Jiao et al. have synthesized three different morphologies i.e. plate, wedge and sheet-like orthorhombic WO3 nanostructures

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using capping agents like Na2SO4, (NH4)2SO4 and CH3COONH4 [12]. They have used them as

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photo-catalysts particularly for oxidation of methanol and also for splitting of water and

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observed that the sheet like morphology was having the highest photo-conversion efficiency. However, peroxo-tungstic acid is found to be the most useful precursor for synthesis of WO3 films via dissolution of tungsten metal in H2O2 followed by the precipitation as reported by Kudo et al. [13]. Porous spherical network like morphology has been prepared from aqueous solution of peroxopolytunsgtic acid (PPTA), prepared from tungstic acid (H2WO4) and perchloric (HClO4) acid at a pH close to 0.2 [14]. The photo-electrochemical activity of the materials is also evaluated. Either Cetyl-tri-methyl-ammonium-bromide, C19H42BrN (CTAB) or Tetrabutylammonium bromide (TBABr) has been used as the directing agent during a sol-gel technique to prepare films with plate like morphology [15, 16]. It may be noted that most of the wet chemical techniques have been used to prepare direct photoanode instead of nanostructured particles for photo-catalytic applications. In this back drop, we have developed simple but new 2 Page 2 of 22

wet chemical routes for synthesis of WO3 nanoparticles with two different morphologies identified the reaction mechanisms through TG-DSC, X-ray, IR and Raman spectroscopy. In addition, the band-gap energy has been estimated from the UV-visible spectral data.

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2. Materials and methods

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2.1. Preparation of WO3 nanoparticles

All the reagents used were of analytical grade and no further purification was done

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before use. Synthesis of spherical WO3 nanoparticles (designated as SW) was carried out through acid catalyzed reaction between solid tungstic acid and hydrogen peroxide solution. For

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this, the analytical grade H2WO4 powder was taken in a dry glass vessel attached with a Pt – temperature sensor and heated to 90±5oC. This was followed by simultaneous addition of

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hydrogen peroxide solution (30% W/W) and concentrated nitric acid to conduct an acid catalyzed reaction. The pH of the solution was ~1. This exothermic reaction led to the formation

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of a pale greenish yellow precipitate, which was later found to be amorphous spherical WO3 and

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accordingly designated as ASW. The precipitate was washed twice in a centrifuge at 14000 rpm

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followed by freeze drying at -52oC with a vacuum of 20 torr. The dried powder was flash heated at 500oC for 5 minutes. By flash heating, we mean direct insertion of the sample inside a preheated furnace followed by isothermal heating for a requisite length of time. Subsequently, the sample is taken out for normal cooling under the ambient condition [17]. The crystallization temperature was confirmed by thermal analysis prior to flash heating of as-synthesized nanoparticles.

On the other hand, rod-shaped WO3 nanoparticles (designated as RW) were prepared by a similar acid precipitation method from Na2WO4.2H2O with addition of a structure directing agent CTAB (Cetyl-trimethyl-ammonium-bromide, C19H42BrN) at a pH of ~ 3. This reaction led to the formation of a white precipitate designated as ARW. The precipitate was finally flash 3 Page 3 of 22

heated at 500oC for 5 mins. Starting precursors and solution pH were the prime parameters to control the morphology of the prepared powders. The reaction mechanisms are discussed later.

2.2. Characterization of WO3 nanoparticles

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Thermogravimetry (TG) and differential scanning calorimetric (DSC) analyses of the Amorphous Spherical WO3 (ASW) and Amorphous Rod WO3 (ARW) powders were carried out

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up to 600oC in a Netzsch (Germany) model STA449C/4/MFC/G Thermal Analyzer with a

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heating rate of 10oC/min. X-ray diffraction (XRD) patterns for all powders were obtained in a Philips X-Ray diffractometer with Ni filtered Cu-Kα radiation (λ= 1.5418 Å) employing a scan

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rate of 3o/min. Specific surface area of the powders was measured using nitrogen as the adsorbate in a BET apparatus (Quantachrome Autosorb, USA). Morphology of both the WO3

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nanoparticles was identified by transmission electron microscope (JEOL JEM-2100). The physicochemical properties of the samples were determined at room temperature using FTIR

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spectra taken in a Perkin Elmer, RX-I FTIR spectrophotometer using KBr pellet. The Raman

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measurements were carried out in backscattering geometry with a triple-grating spectrometer equipped with a cooled charge coupled device detector. For excitation, the 488 nm line of an

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Ar+/Kr+ mixed-gas laser was used. Diffuse reflectance measurements were carried out in a Shimadzu spectrophotometer (UV-2450) to evaluate band gap energy for both the forms of WO3 nanoparticles. Room temperature diffuse reflection percentage was measured in the wavelength region 200 – 700nm. Barium sulphate was used as the reference for this measurement.

3. Results and Discussion

3.1. Thermal analysis of ASW and ARW The results of thermo-gravimetric analysis of ASW and ARW samples are presented in Figures 1a & 1b. The thermogram of ASW in Figure 1a depicts high instability of the compound due to the presence of both physisorbed and chemisorbed water [18]. Thermo-gravimetric weight loss at 100oC is close to 3wt% associated with loss of physisorbed water and additional 4 Page 4 of 22

~7wt% weight loss is due to removal of chemisorbed water molecule from WO3.H2O. Beyond 300oC no further weight loss has been measured. In DSC, two endothermic peaks are attributed to the loss of physisorbed and chemisorbed water. The exothermic peak at 450oC represented the conversion of amorphous to crystalline WO3 under a dynamic condition. These results suggest

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that the amorphous WO3 is formed at around 300oC with the removal of both the forms of water

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molecules. The same is crystallized at a temperature of 450oC with a distinct exothermic peak. This analysis confirms that the flash calcination at a temperature of more than 450oC favors

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formation of crystalline and phase-pure WO3.

The thermogram of ARW in Figure 1b shows greater stability of the compound till

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300oC, indicating that there is no presence of either physisorbed or chemisorbed water. However, the overall weight loss is much larger (~ 27%) compared to that in

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ASW (< 10%). The DSC plot indicates that there are two exothermic peaks, a sharp one at 380oC and a broad peak at 460oC corresponding to weight losses of ~15% and ~12%

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respectively. Even though there is negligible quantity of water molecules associated with this

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sample, considerable quantity of organic species (resulting from CTAB, the structure directing

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agent) must have been associated with this sample. The weight loss of ~15% at a relatively low temperature is presumed to be due to partial removal of sterically hindered methyl groups. The presence of the organic compounds in ARW has been further discussed through FTIR spectral analysis in a latter part of this paper. The second weight loss of ~12% which occurs between 380 and 500oC can be assigned to desorption and/or decomposition of the remaining organic molecules, presumably the surfactant species, associated with the WO3 particles [19]. Comparing the sharpness of the DSC peaks between the Figures 1a and 1b, amorphous to crystalline transition for the ARW specimen appears to take place at a slightly lower temperature of 380oC (the first peak in Figure 1b) and not at 450oC observed for the ASW specimen (Figure 1a).

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3.2. Phase and morphology analysis of WO3 Nanoparticles Phase analysis is carried out to understand the crystallinity and phase purity of the nanoparticles. Figure 2 represents the XRD patterns of four different specimens namely ASW,

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SW, ARW and RW. As mentioned earlier, ASW and ARW correspond to the powders obtained immediately after freeze drying and are observed to be amorphous in nature (Figure 2). SW and

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RW are the specimens obtained after flash calcination of the ASW and ARW specimens, respectively. XRD patterns of these specimens (Figure 2) confirm their crystallinity with

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apparently monoclinic structure, which has been further confirmed from Raman spectroscopic data presented later [14]. XRD pattern and the peak intensities depict that rod-shaped WO3

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(RW) is better crystalline than the spherical WO3 nanoparticles. The crystallite sizes of both SW and RW specimens have been determined using the Scherrer’s equation: D = 0.9λ / B cosθ;

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where D is the crystallite size (nm), λ is the wavelength of the X-ray radiation, θ is the Bragg’s

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angle and B is the full width at half maximum (FWHM) of the peak at 2θ. The crystallite sizes for SW and RW specimens have been estimated as 42 nm and 55 nm, respectively. The

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measured specific BET surface areas for SW and RW are found to be 9.91m2/g and 5.16m2/g,

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respectively. Both crystallite size and surface area data reconfirm better crystal growth for the RW specimen.

Figure 3 represents the TEM micrographs with SAED pattern and HRTEM images for

both SW and RW nanoparticles. Figure 3a shows the distinct spherical morphology of WO3 nanoparticles with an average particle size of around 50nm. Although the particles are partially agglomerated, there is no sign of the formation of hard agglomerates or neck formation. Figure 3b shows the rod – shaped morphology of the WO3 nanoparticles with an average length of 140nm and width of 40nm. In spite of the fact that spherical and rod shaped WO3 nanoparticles have different shapes; they possess the same crystal structure. The inset represents the selected area electron diffraction (SAED) patterns and HRTEM images of the WO3 particles, which 6 Page 6 of 22

clearly indicates relatively high crystallinity of the rod shaped WO3 nanoparticles than the spherical WO3 nanoparticles. The spacing of the lattice fringes has been found to be about 0.33nm of (002) plane for SW and 0.31nm for RW which confirms that the rod prefers to grow along [002] direction. The above d-spacing also compares well with the d–spacing calculated

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from X-ray diffraction (XRD) patterns.

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3.3. FTIR Spectra of WO3 Nanoparticles

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The infrared transmittance spectra have been recorded once again for the same four specimens namely ASW, ARW, SW and RW to understand the chemical purity and the nature

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of ligands associated with these specimens. The spectra of ASW and SW specimens are shown in Figure 4a. ASW exhibits a small absorption peak at 1611.29cm-1, corresponding to O-H

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stretching mode vibration of the water molecules adsorbed on the surface of the particles, the evidence of which also came from thermal analysis (Figure 1). In conformity with the thermal

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analysis data, this absorption peak disappears on calcination at 500oC (specimen SW) together

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with the formation of crystalline spherical particles. The peaks observed at 890.49cm-1 in ASW

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and at 844.90cm-1 in SW can be assigned respectively to W-O-O-W stretching vibration [20]. Similar spectra for ARW and RW specimens are presented in Figure 4b. Unlike that of

ASW the spectra of the ARW exhibits a number of absorption peaks throughout the range of wave number, most of which arise from the organic ligands present in this specimen. The peaks at 2921.12cm-1 and 2950.31cm-1 are identified with the anti-symmetric stretching vibrations of CH2- and –CH3 group present in the carbon chain of the CTAB [21]. Similar to ASW, ARW shows less hydrated sharp O-H stretching and bending peaks at 1626.90cm-1 and 1466.69cm-1, respectively. Further, peaks at low wave numbers have been found to be at 958.84cm-1 for W=O terminal stretching, 892.99cm-1 for O-O stretching and 797.93cm-1 for W-O-W asymmetric intra-bridge stretching indicating the amorphous nature of the powders [4, 22]. Both SW and RW reveal nearly similar spectra in both low as well as high wave number regions. Similar W7 Page 7 of 22

O-W stretching peak is also observed at 850.92cm-1 for RW. In general, tungsten atom is known to have six-fold coordination in crystalline WO3, whereas in amorphous form it becomes five or four fold coordination due to deficiency of oxygen ions giving rise to distortion in the structure and leading to amorphous nature of the particles [23]. The distinct feature in crystallography and

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long range periodicity of regular planes attributes to the difference in stretching vibration

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behavior of amorphous and crystalline nanoparticles.

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3.4. Raman Spectra of WO3 Nanoparticles

Figure 5 represents the Raman spectra of spherical and rod-shaped WO3 nanopowders to

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better understand the phase and bond characteristics of WO3 nanoparticles. Raman spectrum of both SW and RW supports the formation of crystalline WO3 monoclinic phase as supported by

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available literature [24]. Both the powders reveal very similar wave – number in the wide spectra zone. The W – W bond is identified near to 187cm-1 wave number, however, the

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intensity varies because of crystallinity of WO3 nanoparticles. Raman peaks observed at low

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wave numbers 268 and 327cm-1 relates to the bending vibration of W – O – W bond. The higher

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wave number peaks at 716 cm-1 is due to the O – W – O vibration and 808 cm-1 corresponding to the crystalline WO3 stretching vibration of the bridging oxygen of W – O – W. These two peaks clearly attributes to the monoclinic structure of WO3.

3.5. Proposed reaction mechanism For synthesis of spherical particles, the two most important parameters are found to be

temperature and pH of the reaction media in presence of hydrogen peroxide solution. The exothermic nature of the reaction could possibly be due to spontaneous decomposition of H2O2 leading to the formation of water vapor and nascent oxygen above the critical temperature of 95oC and pH > 5 [25, 26]. On the other hand, a temperature less than 95oC together with a low solution pH of <5 prevent spontaneous loss of H2O2 and therefore are expected to cause the 8 Page 8 of 22

stoichiometric reaction with tungstic acid. However, in this investigation, the preheated pot temperature of 90 ± 5oC initiates the decomposition of H2O2 and accelerates the reaction between nascent oxygen atom and H2WO4 to form peroxotungstic acid (WO3.xH2O2). The value of x varies in the range of 1 to 2, whereas for hydrated WO3, the value of x is in the range of 0

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to2 depending on the reaction conditions [20, 27, 28]. Simultaneous addition of nitric acid favors

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the dehydroxylation by the acid catalyst oxidation reaction and WO3.xH2O2 transforms to insoluble pale yellowish precipitate (ASW). Thus, the overall reaction sequence can be

2H2O + 2Ö

H2WO4 +2Ö

WO3.xH2O2 (peroxo-tungstic acid, PTA)

WO3.xH2O2 + 2HNO3 500oC

WO3.xH2O + 2NO2 + O2 WO3 + xH2O

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WO3.xH2O

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H2O2

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summarized as follows:

(1) (2) (3) (4)

data presented in Figure. 1a.

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The value of ‘x’ in equations 3 and 4 is found to be 1 as confirmed from the thermogravimetric

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On the other hand, synthesis of WO3 with rod like morphology (RW) follows

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acidification of Na2WO4.2H2O forming anionic WO42- and cationic H+ under acidic condition and finally to form hydrated WO3. An optimum amount of CTAB (H3C – (CH2)15 – N+(CH3)3 Br-) provides encapsulation to form a confined reaction zone where the nuclei prefers an anisotropic growth leading to the development of a rod like structure as reported by Brust et al. [29]. The probable reaction inside the reverse micelle formed by CTAB encapsulation is found similar to the nanorod formation as reported by Shen et al. [30]. Thus, the details of the various reactions inside the confined reaction zone arising from the presence of the structure directing agent CTAB may be summarized as follows: Na2WO4.2H2O +2HNO3 +H2O WO3.xH2O

500oC

WO3+ H2O

WO3.xH2O + 2NaNO3

(5) (6) 9 Page 9 of 22

The value of ‘x’ in the intermediate compound WO3.xH2O is reported to vary in the range 1 to 2 when CTAB surfactant is used for the synthesis of WO3 nanoparticles or other morphologies. The degree of ionization of the parent solute such as sodium tungstate and the degree of aggregation of the surfactant depends on the cationic and anionic interaction, which in

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turn controls the growth phenomena of the particular compound [15, 31]. DSC analysis

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presented earlier indicates the decomposition of the organic component through a couple of distinctly different exothermic reactions together with the conversion of amorphous to

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crystalline WO3. Aforementioned analysis reveals the presence of organic groups from CTAB

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3.6. Band gap of WO3 Nanoparticles

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with the hydrated WO3 and needs further work for their quantification.

The UV-Vis diffuse reflectance spectra have been used to determine the band gap

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energy for both of the samples. The measured diffuse reflectance spectra (Figure 6) have been

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used for Tauc plot, in which square root of Kubelka – Munk function multiplied by the photon energy is plotted against the photon energy (Ephoton = hλ) as shown in the inset of Figure 6. The

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Kubelka – Munk unit of absorption is calculated from the equation; KMU = (1 – R)2/2R, where, R = reflectance [14,32]. The band gap energy is calculated as 2.82eV and 2.75eV for SW and RW specimens respectively. Similar band gap energy of 2.62eV has been reported for monoclinic WO3 films prepared by spray pyrolysis but higher value of 3.4eV has been observed for orthorhombic WO3 films prepared by sol gel method [33, 34].

4. Conclusions Crystalline WO3 nanoparticles having monoclinic crystal structure but with two distinctly different morphologies such as spherical and rod shaped could be prepared starting with two different chemicals namely H2WO4 and Na2WO4.2H2O respectively. For the first case the reaction was an acid catalyzed one, while for the second a structure directing surfactant had 10 Page 10 of 22

to be used. Different types of materials characterization techniques have been adopted to determine various characteristics of the nano-particles and also to get an insight into the mechanisms of the chemical reactions involved. For both the reactions, the particles are initially amorphous but become crystalline on calcination at a temperature of 500oC. In the first reaction,

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the resultant particle contains only small amount of hydroxyl ion, which gets removed by

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heating to a temperature of ~ 300oC. Second reaction has amorphous phase with a considerable amount of organic ligands, which gets decomposed in two distinct stages upto a temperature of

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500oC. Both reactions are exothermic in nature. Approximate particle size of spherical nanoparticle is 50nm, whereas, the average dimensions of the nanorods are 140nm/40nm

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(length/width). The nano-particles are partially agglomerated but do not get sintered at the calcination temperature of 500oC. Further work is necessary for evaluation of their physico-

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Figure Captions:

Figure 1: TG-DSC of ASW (a) and ARW (b) specimens.

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Figure 2: XRD patterns for ASW (a), SW (b), ARW (c) and ARW (d) specimens. Figure3: TEM image for spherical (a) and rod shaped (b) WO3 nanoparticles. Inset represents

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the HRTEM images and SAED patterns.

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Figure 4: FTIR spectra of the samples prepared by two different techniques immediately after freeze drying and after calcination: (a) for spherical (ASW and SW) and (b) for rod shaped

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morphologies (ARW and RW)

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Figure 5: Raman Spectra of spherical and rod shaped WO3 Nanoparticles (SW and RW specimens)

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Figure 6: Diffuse reflectance spectra for spherical and rod shaped WO3 nanoparticles (SW and

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calculation.

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RW specimens). Inset shows the Taut plot for the two specimens for the purpose of band gap

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List of Figures:

Figure 1.

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Figure 2.

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Figure 5.

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Figure 6.

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Graphical Abstract Two different morphology WO3 nanoparticles are synthesized by a simple and new wet chemical route through control over pH, temperature and structure directing agents. Reaction

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mechanism has been proposed for the formation of different morphologies. Nanorod WO3 has better crystallinity with less specific surface area compared to the spherical nanoparticles.

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Comparable band gaps are obtained for both the nanoparticles.

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Highlights

Spherical WO3 nanoparticle and nanorod synthesis mechanisms proposed.

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Thermal analysis and spectroscopy confirms the intermediate reactions Both of the morphology has pure and monoclinic phase with identical band gap energy

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