Journal of the Iranian Chemical Society https://doi.org/10.1007/s13738-018-1342-y

ORIGINAL PAPER

Rapid synthesis of novel Cr‑doped ­WO3 nanorods: an efficient electrochemical and photocatalytic performance M. Parthibavarman2   · M. Karthik1 · P. Sathishkumar3 · R. Poonguzhali4 Received: 27 August 2017 / Accepted: 9 March 2018 © Iranian Chemical Society 2018

Abstract Novel Cr-doped ­WO3 nanorods were successfully synthesized by a facile one-step microwave irradiation method for the first time without using any surfactants. The role of Cr on structural, morphological and optical properties was systematically analyzed by using powder X-ray diffraction (PXRD), Raman spectra, electron paramagnetic resonance, scanning electron microscope (SEM), transmission electron microscopy (TEM), UV–Vis diffusion reflectance spectra (DRS) and photoluminescence spectra analysis. XRD analyses confirm that both pure and Cr-doped ­WO3 were monoclinic structure of nanocrystalline ­WO3, whereas the SEM and TEM images exhibit spherical and nanorods-like morphology of the as-synthesized pure and Cr-doped ­WO3, respectively. The diameter and length of the nanorods are found to be 55–65 and 140–160 nm, respectively. A noticeable red shift in the absorbance edge and decreasing the band gap from 3.01 to 2.72 eV for Cr-doped samples were observed by using UV–DRS analysis. The electrochemical properties were significantly improved by Cr-doped ­WO3; this might be due to high surface area that facilitates the contact and transport of electrolyte, providing longer electron pathways and therefore giving rise to highest capacitance in nanorods morphology. The samples were further evaluated by the degradation of methylene blue under visible light irradiation. The photocatalytic activity and reusability of Cr 10 sample were much higher than that of the pure ­WO3. The improvement mechanism by Cr doping was also discussed. Keywords  Cr-doped ­WO3 · Nanorods · Microwave irradiation · Super capacitor · Catalyst

Introduction In recent years, the field of supercapacitors has received much attention due to its importance in energy efficient, eco-friendly, high-power density, storage capacity and high-energy devices. This builds supercapacitors a superior alternative energy source to fossil fuels as one of the most promising candidates for next-generation power devices [1]. Supercapacitor has also very important role in alternative or replacing batteries in the energy storage field from * M. Parthibavarman [email protected] 1



Research and Development Centre, Bharathiar University, Coimbatore, Tamilnadu 641 046, India

2



PG and Research Department of Physics, Chikkaiah Naicker College, Erode, Tamilnadu 638 004, India

3

School of Advanced Sciences, VIT University, Vellore, Tamilnadu 632 014, India

4

Department of Physics, Mahendra Arts and Science College, Kalippatti, Tamilnadu 637 501, India



transportable electronics to hybrid electric vehicles [2]. Recently, metal oxides and conductive polymers have been received enormous attention in the energy storage devices such as super capacitor, batteries, which can provide much higher specific capacitance and higher energy density than that of double-layer capacitors and attract researcher widespread attentions [3]. Compared to other transition metal oxides like, ­RuOx [4], ­CoOx [5], ­NiOx [6], ­FeOx [7], nanostructured tungsten oxide ­(WO3) materials have traditional growing attention owing to their huge potential applications in gas sensors, photocatalysts, field-emission devices, and electrochromic and energy storage devices [8–10]. Nanostructured tungsten oxides as electrode materials are expected to display enhanced electrochemical performance because of their large surface area and low charge transport resistance. Among the various W ­ O3 nanostructures, onedimensional (1D) structures such as nanorods, nanowires and nanotubes are particularly attractive for dimensionality and size, which have been regarded as important factors that may bring novel and excellent properties [11–13]. In pure ­WO3 electrode, one of the main issues leads to high-power

13

Vol.:(0123456789)



consumption, poor charge capacity, reversibility and stability. It has been noted that metal dopants could modify the electronic structure ­WO3, which results in increasing the electrochemical performance [14]. Chromium (Cr) is a suitable dopant into W ­ O3 crystal site and Cr also a member of transition metals and exhibits a wide range of possible oxidation states; this may find promising potential candidate for efficient doping materials in supercapacitor applications. Most of the literature focuses on gas sensing properties of Cr-doped ­WO3 nanostructure [15, 16]. There are a few reports available in the literature on Crdoped ­WO3 nanostructures for electrochemical performance. But to the best of our knowledge, this is the first report on the structural, optical electrochemical and photocatalytic behavior of pure and Cr-doped W ­ O3 nanorods assembled by nanoparticles in large scale by microwave irradiation method. The electrochemical behavior of the obtained Crdoped ­WO3 samples is systematically investigated through cyclic voltammetry, impedance spectra and charge–discharge curve analysis. In addition, Cr-doped catalysts show better photocatalytic activity compared with pristine ­WO3. The possible mechanism of the increase in photocatalytic performance of Cr-doped ­WO3 is also proposed.

Experimental procedure Materials Pure analytical grade tungstic acid ­(H2WO4), chromium nitrate nonahydrate (Cr (­ NO3)3·9H2O), methylene blue (MB) and HCl were purchased from Merck, India (purity 99.98%). ­H2WO4 and Cr ­(NO3)3·9H2O was taken as the precursor for synthesizing pure and chromium-doped ­WO3 nanorods.

Preparation of Cr‑doped ­WO3 nanorods In the preparation method, 2.49 g of ­H2WO4 was dissolved in 10 ml of deionized water to produce 0.1 M solution of tungsten hydroxyl group. Cr ­(NO3)3·9H2O was dissolved with various concentrations (0.01, 0.03, 0.05, and 0.10 wt%) of Cr (­ NO3)3·9H2O in above-mentioned solution stirred for 40 min. Then, HCl is used as precipitate agent and added dropwise under the strong magnetic stirring until the pH value is maintained at 2. The chemical homogeneity is assured during this stirring process. After this reaction, the yellow color precipitate was obtained. This sol was centrifuged and washed with ethanol and deionized water repeatedly until get the clear gel. This product was transferred into Teflon lined household microwave oven (2.45 GHz) with power 900 W (120 °C) and irradiated for 5 min. Then, the light green color Cr-doped W ­ O3 nanopowders were obtained. The preparation of pure ­WO3 was followed by a

13

Journal of the Iranian Chemical Society

same procedure except Cr source. The pure W ­ O3 was light yellow in color. The samples with pure and 0.01, 0.03, 0.05 and 0.10 wt% Cr named as W, Cr 1, Cr 3, Cu 5 and Cu 10, respectively.

Characterization details The powder X-ray diffraction patterns of the samples were recorded at room temperature on a Bruker D8 Advance diffractometer using Cu Kα (k = 1.5406 Å) radiation to identify the crystal phase. The diffraction data were collected in the 2θ range of 10–90° in step scan mode at a rate of 3°/ min. The structural morphology and particle size of the prepared nanoparticles was measured using SEM (JEOL Model JSM-6390LV) and TEM technique recorded with JEM2100 model. Besides, high-resolution electron microscope was recorded with accelerating voltage of 200 kV. The elemental composition and impurity of the samples was calibrated by EDS spectra (JEOL Model JED-2300). The Raman spectra of the nanoparticles were performed using BRUKER RFS 27: Stand-alone FT-Raman spectrometer at a resolution of 0.2 cm−1.The optical properties were studied by using UV–Vis diffusion reflectance spectroscopy (CARY 5E UV–Vis–NIR spectrophotometer) in the wavelength range of 200–900 nm. Perkin-Elmer LS 55 spectrometer was carried out to investigate the photoluminescence spectra of the prepared samples with a He–Cd laser source, Excitation length used was 285 nm. Electron paramagnetic resonance (EPR) spectra were calculated using Bruker TMX spectrometer.

Results and discussion XRD analysis The crystalline structure and grain size of the as-synthesized pure and Cr-doped ­WO3 has been investigated by XRD analyses, and the corresponding XRD profiles are shown in Fig. 1. It can be seen that both pure and Cr-doped ­ O3, and the ­WO3 sample showed monoclinic structure of W consequent pattern is in good agreement with the standard JCPDS card No. 83-0951. A predominant triplet (002), (020) and (200) peaks in the pattern also confirm the monoclinic phase of W ­ O3 [17]. There is no peak related to chromium or any chromium compound was detected, which suggests that purity and well crystalline nature of the sample. In addition to that, the intensity of triplet (200), (020) and (002) peaks improved and shifted toward the higher angle side with increasing Cr dopant concentration (Fig. 1b). As the dopant concentration increases, the full width at half maximum decreases (Fig. 1c), this reveals that crystallinity of ­WO3 improved by Cr doping, which can be attributed to due to the formation of less nucleation centers at the time of

Journal of the Iranian Chemical Society

Fig. 1  a Powder XRD pattern of pure and Cr-doped ­WO3 nanoparticles, b magnified and high-resolution XRD pattern of the triplet peaks, c variation of FWHM as a function of Cr concentrations, d change in average grain size as a function of Cr concentrations Table 1  The average grain size, lattice parameters values of pure and Cr-doped ­WO3 Samples

W Cr 1 Cr 3 Cr 5 Cr 10

Grain size (nm)

Lattice parameters a (Å)

b (Å)

c (Å)

17 20 25 29 34

7.3712 7.3756 7.3798 7.3812 7.3898

7.5855 7.5867 7.5894 7.5902 7.5987

7.7793 7.7802 7.7827 7.7856 7.7889

crystallite growth. Moreover, the calculated lattice parameter values are slightly increased upon Cr doping (Table 1). The changes in the lattice parameters values are due to different ionic radius of ­W6+ (0.067 nm) and ­Cr3+ (0.064 nm) [18], respectively. Figure 1d shows the increase in grain size as a function of Cr concentrations. The average grain size was calculated from Debye–Scherrer’s formula [19] and found to increases from 17 nm to 34 nm with the increase of Cr

concentrations (0.01–0.10 wt%). The increase in grain size due to grain growth of pure W ­ O3 is improved by Cr dopant and confirms the substitution of ­Cr3+ in regular lattice site of ­WO3.

SEM analysis The surface morphology of the nanomaterials is playing an important role in the super capacitor applications. The morphology and average particle sized of the pure and Cr-doped ­WO3 nanoparticles were determined by SEM micrographs. Figure 2a–d shows the SEM picture of pure and Cr-doped ­WO3 nanoparticles (resolution 1 μm). It is obvious from the figures that the pure ­WO3 samples showed mainly sphericalshaped morphology with aggregated particles observed. It was clearly evidenced that the spherical-shaped morphology was changed to rod-shaped morphology (combination of particles with rod shaped) after doping of Cr. The rod shaped was significantly improved (clear and uniform) upon Cr doping sample.

13



Journal of the Iranian Chemical Society

Fig. 2  SEM images of a W, b Cr 3, c Cr 5, d Cr 10

TEM–EDS analysis Figure 3 shows the TEM images of pure and Cr-doped ­WO 3 samples. It was also confirmed that the nanorods are formed with Cr doping. It can be seen that pure W ­ O3 was spherical-shaped morphology with average diameter of around 25–30 nm (Fig. 3a). Figure 3b shows the typical TEM image of Cr-doped W ­ O3 nanorods (Cr 10). The usual length of as-synthesized single nanorod is around 140–160 nm and the width is around 55–65 nm. HRTEM and selected area electron diffraction (SAED) results show that the Cr–WO 3 nanorod is a single crystalline nature (Fig. 3c). The corresponding SAED pattern indicates the growth direction of nanorod along the [020] zone axis (inset in Fig. 3c). The observed lattice spacings are 0.383 and 0.373 nm along the two orthogonal directions, which correspond to the (002) and (020) planes of monoclinic ­WO3, respectively. This result is consistent with the XRD results. In order to verify the compositional elements present in the samples, EDS analysis was carried out for Cr 10 sample and the spectra were presented as shown in Fig. 3d. The sample mainly composed of W, O and Cr elements. The presence of Cr element confirms that Cr doping into ­WO3 structure. The calculated atomic percentage

13

is nearly equal to their nominal stoichiometry within the experimental error.

Raman spectra analysis Raman spectroscopy is a professional tool to investigate the fundamental vibration mode and mismatching of the compounds [20]. Figure 4 shows the Raman spectra of pure and Cr-doped W ­ O 3 samples. It was noted that the strongest wavenumbers are located at 131, 278, 721, and 814  cm −1, which are in good accordance with monoclinic phase of ­WO 3 and are good agreement with the XRD results. The vibrational band appeared at 721 and 814 cm−1 are related to the W–O stretching modes [21]. The peak broadening and peak shifting in higher wave number at 728 and 821 cm−1 is observed; this is due to change in the electronic structure when Cr concentration increase. Raman band intensity is reduced because of stoichiometric defects, quantum confinement effects, particle sizes, crystallinity and modification in morphology [22]. Hence, our Raman spectra again prove that substitution of ­Cr3+ ions in the ­WO3 lattice site.

Journal of the Iranian Chemical Society Fig. 3  TEM images of a W, b typical single nanorod of Cr 10 sample, c corresponding HRTEM and SAED pattern (inset), d EDS spectra of Cr 10 sample

UV–DRS spectra analysis

Fig. 4  Raman spectra of pure and Cr-doped ­WO3

In order to further investigate the optical property and band gap energy, UV–Vis DRS was carried out. Figure 5 shows the UV–DRS spectra of pure and Cr-doped W ­ O3 nanoparticles. It can be seen that the absorption edge of pure ­WO3 was found to be 335 nm; further, it was shifted to higher wavelength region upon doping by Cr. Generally, the incorporation of impurity will result in the change of band gap or a shift of the CB edge. A considerable red shift in the absorption edge indicates the decrease in the band gap energy and surface Plasmon resonance effect. In order to further confirm the relation between doping and band gap (Eg), we have used Kubelka–Munk (K–M) model [23] as described below. The K–M model at any wavelength is given by F (Rα) = (1 − R)2/2R, where R is the percentage of reflectance. A graph is plotted between [F (Rα) hυ]2 versus hυ, and the intercept value is the direct band gap energy [24]. The band gap energy was found to be 3.01, 2.95, 2.91, 2.86 and 2.72 eV for W, Cr 1, Cr 3, Cr 5 and Cr 10 samples, respectively. The decrease in band gap energy might be due

13



Journal of the Iranian Chemical Society

Fig. 5  UV–DRS spectra of pure and Cr-doped W ­ O3 a reflectance spectra, b K–M model

These are associated with the structural defects in the ­WO3 nanocrystals and resulted in decreasing the band gap energy [27]. The observation of blue emission peaks at 442 and 495 nm are owing to the recombination of free excitons. Generally, it is called as near band edge emission (NBE) [28]. On the other hand, the green emission at 525 nm is due to defect energy levels which are corresponded to the oxygen vacancies. These vacancies are formed during the microwave irradiation process, and hence, predominant oxygen vacancies related to V0O, VO+ and V++ O exist in the as-prepared ­WO3 nanostructures. After Cr doping, all the peak intensity was increased with the increase in Cr concentrations, and it was probably due to the fact that Cr dopant offers competitive pathways for recombination, which results in quenching of the emission intensity. So far as optical properties are afraid with increasing Cr doping, it has been observed that more and more defect has been created in the ­WO3 system. Fig. 6  Photoluminescence spectra of pure and Cr-doped ­WO3

to change in electronic structure, morphology and particle size by Cr doping. Similar findings were observed for other metal ions doped ­WO3 nanostructures, such as Ag [25] and Mo [26] doped ­WO3.

Photoluminescence spectra analysis The PL emission spectra of undoped and Cr-doped ­WO3 nanoparticles were recorded at room temperature in the wavelength range 300–600 nm when the material was excited by the photon of wavelength 285 nm using He–Cd laser source. Photoluminescence spectra of pure and Cr-doped ­WO3 nanoparticles are shown in Fig. 6. The UV emissions of doped sample are found to be 359 and 375 nm, respectively.

13

Electron paramagnetic resonance spectra (EPR) analysis Electron paramagnetic resonance spectroscopy is an effective technique to determine the local structural environment and surroundings of paramagnetic ions present in the host lattice. EPR spectra of pure and Cr-doped ­WO3 nanoparticles have been recorded at room temperature, using Varian E 112, operating at X-band frequency 8.5–9.5 GHz and are shown in Fig. 7. In case of EPR spectra, pure W ­ O3 shows weak, less intensive and hyperfine signal. But the Cr-doped ­WO3 samples exhibit broad resonance signals due to the presence of paramagnetic centers (­ Cr3+ ion) in the samples. Further, the broad resonance signals was increased with the increase in Cr concentrations, which is probably due to the shallow deep defects associated with dipolar interactions of

Journal of the Iranian Chemical Society

Cr in higher concentrations [29]. The parameter “g” is determined according to the equation g = hν/µBHr, where h is the Planck’s constant, v is the microwave frequency, and µB is the Bohr magnetron. The value of ‘g’ is calculated from this powder spectrum 1.98, which indicates the purely belongs to exchange coupling between C ­ r3+ and C ­ r3+ ions [30]. 3+ This similar finding was observed in ­Cr doped ­SrMoO4 scheelite compound by co-precipitation method [31]. The above information conforms that the ­Cr3+ ion doped substitutionally in monoclinic phase of ­WO3.

XPS analysis

Fig. 7  EPR spectra of pure and Cr-doped ­WO3

In order to know the further information about chemical state of these elements, the XPS survey spectrum is carried out and depicted in Fig. 8a along with the core level spectra of W4f, Cr2p and O 1s peaks shown in Fig. 8b–d, respectively. The peak appeared at 35.2 eV can be assigned to ­W4f7/2 (Fig. 8b), which indicates that the W atoms are in ­6+ state [32]. The binding energy (BE) of Cr 2p3/2 is

Fig. 8  XPS spectra of Cr 10 doped ­WO3 samples

13



around at 574.2 eV, and the O 1s peak showed at 530.2 eV in pristine WO3. The XPS result confirms that Cr ion can successfully doped with W ­ O3 lattice site with the oxidation state of ­Cr3+ [33]. The surface area and pore size of assynthesized pure and Cr-doped W ­ O3 samples were analyzed through nitrogen adsorption–desorption measurements. As shown in Fig. 9a, it was clearly evident that hysteresis loop is observed, suggesting the subsistence of abundant structures in the nanorods. The BET surface areas of the pure ­WO3 and Cr-doped W ­ O3 (3 and 10 wt%) nanorods are calculated to be 12.50, 25.34 and 19.49 m2 g−1, respectively. Figure 9b shows the corresponding pore-size distribution curves of the pure and Cr-doped W ­ O3 samples. The pure W ­ O3 nanorods have a pore-size distribution centered at 16.76 nm, while the 3 wt% Cr–WO3 nanorods exhibit an increased pore-size distribution at 25.26 nm. These results matched well with the TEM analysis indicate that the 3 wt% Cr–WO3 possesses higher specific surface area and more porous structure, which are beneficial to the electrochemical and photocatalytic applications.

Electrochemical measurements Cyclic voltammetry analysis The cyclic voltammetry (CV) and impedance technique was performed by using software-controlled conventional three electrode electrochemical cell (CHI 660C electrochemical workstation) to investigate the electrochemical properties of the samples. Figure 10a illustrates the CV curves of pure and different concentrations of Cr-doped W ­ O3 nanoparticles at a scan rate of 50 mV s−1, in the voltage window − 0.2 to 1 V. As shown in the figure, the CV curve of ­WO3 is close to an ideal rectangular shape, whereas the CV curves of Cr-doped

Journal of the Iranian Chemical Society

­ O3 show perturbed rectangular shape, indicating the pseuW docapacitance nature of the doped products. Furthermore, it is apparent that the 3 wt% Cr-doped ­WO3 electrode shows large area surrounded by CV curve, indicating its higher electrochemical capacitance. Here, the pseudocapacitance behavior comes mainly from the faradic redox reaction of ­WO3 and the role of Cr is to enhance the electrical conductivity of the Cr-doped ­WO3 nanoparticles. The value of specific capacitance (Cs, F ­g−1) was calculated from the CV curves according to Eq. (1)

Cs =

Q mΔv

where Q is the average charge during anodic and cathodic scan, m is the mass of the active material (g), and ∆ν is the scan rate (mV/s). The specific capacitance value obtained from CV curves for W ­ O3 is 93 F g−1, whereas it is 95, 397, −1 37, and 35 F g , respectively, for 1, 3, 5 and 10% of Crdoped ­WO3 electrodes. A highest capacitance of 397 F/g is obtained for 3% of Cr-doped ­WO3 electrodes hybrids at a scan rate of 50 mV/s. The 3% of Cr-doped ­WO3 nanoparticles exhibited the higher capacitance compared to ­WO3, which is mainly due to their higher surface area. To evaluate the relationship between the scan rate and specific capacitance performance of 3% of Cr-doped ­WO3 electrode, the CV studies were performed at different scan rates (50 and 100 mV s−1). We have seen that the obvious increase in peak current while increasing the scan rate indicates a good rate capability of nanoparticle (Fig. 10b). From Fig. 10b, it was observed that the positions of the anodic and cathodic peaks shifted to higher and lower potentials, which might be due to the ionic diffusion rate as it was not fast enough to keep pace with electronic neutralization in the redox reaction. Even though the specific capacitance of the products drops as

Fig. 9  a Nitrogen absorption–desorption isotherms curves and b pore-size distribution curves of pure ­WO3 and Cr-doped ­WO3 samples

13

(1)

Journal of the Iranian Chemical Society

Fig. 10  a The typical CV curve of pure and Cr-doped W ­ O3 at constant scan rate (50 mv/s), b the CV curve of Cr 3 sample with different scan rate (50 and 100 mv/s), c specific capacitance values of pure

and Cr-doped W ­ O3 samples at constant scan rate (50 mv/s), d specific capacitance values of pure and Cr 3 doped ­WO3 samples

the scan rate increases, the specific capacitance of Cr–WO3 (3%) is still about 262 F/g at the scan rate of 100 mV/s. For the clear understanding, we have plotted the graph of specific capacitance values as function of Cr concentrations (Fig. 10c, d). We can observe that the specific capacitance value decreases with the increase in the scan rate which might be due to that some electroactive surface areas became inaccessible (diffusion limited), leading to less availability of electrode surface area for ion diffusion and adsorption due to inadequate time. Among all the samples, the 3% of Cr-doped ­WO3 electrode displays a larger CV curve area than other counterparts, suggesting it has the highest energy storage capacity. It is notable that the electrochemical property of ­WO3 is much less than Cr–WO3 electrode. Considering that the electrochemical performance of ­WO3 material largely depends on its surface microstructure, we believed that both the composition change (structural water removal and residual organic matter decomposition) and the surface area

reduction, contributed to the significant loss in the specific capacitance of high crystalline Cr 10, and thus, amorphous Cr 3, is more suitable to be applied as electrode material for supercapacitors. Charge–discharge curve Figure 11a shows the typical charge/discharge curves of the electrodes at constant current density of 0.5 A/g. The constant current charge discharge curves all the devices are nearly triangular, with reduced internal resistance at the beginning of the discharge curve. The excellent microstructure and surface morphology of the electrode with uniform distribution of nanocrystalline ­WO3 over mesoporous Cr nanorods presents maximum electrolyte accessible surface area for this sample. Hence, it could be assume that the specific capacitance of the Cr-doped W ­ O3 samples resulted from the faradic charge-transfer reaction of the Cr dopant.

13



Journal of the Iranian Chemical Society

Fig. 11  a The typical charge/discharge curves of pure and Cr-doped W ­ O3 electrodes at constant current density of 1 A/g, b specific capacitance values of pure and Cr-doped ­WO3 samples

A rapid drop in current at start of the discharge is due to the internal resistance of the electrode called “IR drop.” The IR drop of the Cr 3 electrode samples was much lower than that of pure W ­ O3 and other Cr-doped electrodes. To further confirm the capacitance, the same was calculated from charge discharge curves by using the formula as below:

Cs =

iΔt mΔV

(2)

where Cs is the specific capacitance (F/g), I is the specific current (A), ∆t is the discharge time (s), m is the mass of the active material (g), and ∆V is the potential window (V). The highest specific capacitance of 426 F/g was obtained for Cr 3 sample at current density of 1 A/g. For pure and Cr (1, 5 and 10%) doped ­WO3, the specific capacitances of 19, 25, 57 and 24 F/g were obtained at the same current density (Fig. 11b). Cr 3 electrode suits to high-rate charge/discharge, as it has a lower electron hopping resistance which results in a lower IR at a high-rate charge–discharge. Electrochemical impedance spectroscopy (EIS) Electrochemical impedance spectroscopy (EIS) measurements can be applied to investigate electrical conductivity and ion transfer properties of the different electrode materials. Figure  12 presents the Nyquist impedance plots measured in standard 3 electrode configuration in the frequency range from 100 kHz to 10 mHz at open circuit voltage by applying a 5 mV signal for pure W ­ O3 and Cr doped by 3, 5 and 10 wt% of ­WO3. The impedance spectra of the W ­ O 3 and Cr doped by Cr 3, Cr 5 and Cr 10 electrodes are similar in form with a quasi-semicircle

13

Fig. 12  Nyquist impedance plots for of pure and Cr-doped ­WO3 electrodes in the frequency range from 100 kHz to 10 mHz

at a higher frequency region and a linear part at lower frequency. The linear part corresponds to the Warburg impedance (W), which is described as a diffusive impendence of the OH-ion within the electrode. The chargetransfer impendence of the Cr 3 electrode is smaller than that of the pure one. The results above demonstrate that the combination of fast ion diffusion as well as low electron-transfer resistance is also responsible for the enhanced electrochemical performance of the Cr 3–WO3 electrode.

Journal of the Iranian Chemical Society

Fig. 13  a, b shows the UV-light absorbance spectra of MB (λmax = 464 nm) using pure and Cr 10% doped ­WO3 catalyst, c temporal degradation profile of MB, d possible photocatalytic mechanism of pure and Cr-doped W ­ O3 catalyst

Photocatalytic activity set‑up and measurements Photocatalytic activity has been studied for the degradation of methylene blue (MB) on pure and Cr-doped catalysts under visible light irradiation. In order to activate the photocatalytic reaction, a 500 W halogen lamb was taken as a visible light source. Further, quartz glass cylinder was designed exclusively for photo reactor. 10 milligrams of catalyst powders (pure and doped W ­ O3) was disseminated in 10 ml deionised water. In the photoreactor, 25 µM dye solution was dropped and systematically blended and set aside for 30 min in the dark in order to achieve equilibrium condition. The absorption value is found to be 464 nm at maximum wavelength by using the UV–Vis–NIR technique (Perkin-Elmer lambda 25). Based on the ratio of initial (C0) and final (C) concentration, the degradation of MB was measured.

Figure 13a, b shows the UV-light absorbance spectra of ­ O3 MB (λmax = 464 nm) using pure and Cr 10% doped W catalyst as a function of irradiation time during the degradation. It was clearly seen that the absorption was drastically decreased with the increase in irradiation time (0–120 min). Hence, the material is observed to be photocatalytically active. Figure 13c shows the temporal degradation profile of MB using pure and Cr-doped W ­ O3 with different Cr concentrations under visible light irradiation. It was noticed that Cr 10 samples showed better photocatalytic activities compared to those of W, Cr 1, Cr 3 and Cr 5 samples. The degradation efficiency was found to be 56, 61, 65, 73, 88% for pure and Cr-doped (Cr1, Cr 3, Cr 5 and Cr 10) ­WO3, respectively. The significant enhancement in the photocatalytic activity of Crdoped ­WO3 is due to high surface area and reduces the band gap to 2.72 eV, which red shift the ­WO3 absorption edge to 455.4 nm in visible range, maintain the reducing capacity

13



of CBM and simultaneously raise the mobility of the charge carrier. On the basis of above results, the possible photocatalytic mechanism is presented in Fig. 13d. Path (i) the C ­ r3+ ions could create a donor level below the conduction band of ­WO3 to increase the absorption intensity of visible light. In addition, Cr doping reducing the band gap of pure W ­ O3, which results more electrons could be excited under visible light region. Path (ii) ­Cr3+ doped into the lattice of ­WO3 could catch electrons to concealed recombination between electrons and holes. The synergy between the two effects will definitely enhance efficient photocatalytic activity toward MB degradation over the doped ­WO3 photocatalyst.

Conclusions In summary, we have developed a simple approach to synthesize Cr-doped W ­ O3 nanorods by one-step microwave irradiation method for the first time. XRD and TEM results suggest that ­WO3 single crystalline with monoclinic phase and the morphology of the Cr-doped W ­ O3 was changed to nanorod-like shaped. The UV–Vis DRS analysis of pure and Cr-doped ­WO3 exhibits the absorption edge at 335 nm, and the addition of dopant leads to red shift which enumerates the decrease in optical band gap values and confirms the excellent optical behavior of all the Cr-doped compounds. Multicolored emission originated from defective states of sample was investigated by PL spectral studies. EPR spectra confirm the presence of paramagnetic centers caused by Cr ions. XPS result reveals that the presence of chromium and tungsten as C ­ r3+ and W ­ 6+ in state, respectively. Electrochemical studies confirm that the amorphous Cr 3 is more suitable to be applied as electrode material for supercapacitors. The Cr-doped W ­ O3 exhibited superior photocatalytic performance compared to pure W ­ O3 over the degradation of MB under visible light irradiation. Hence, this would be promising potential materials in high-performance photocatalyst and waste water treatment.

References 1. T. Kwon, H. Nishihara, H. Itoi, Q.H. Yang, T. Kyotani, Langmuir 25, 11961 (2009) 2. X.-h. Xia, T. Jiang-ping, Y.-q. Zhang, Y.-j. Mai, X.-l. Wang, G. Chang-dong, X.-b. Zhao, RSC Adv. 2, 1835 (2012) 3. Z.B. Lei, N. Christov, X.S. Zhao, Energy Environ. Sci. 4, 1866 (2011) 4. C.-C. Hu, K.-H. Chang, M.-C. Lin, Y.-T. Wu, Nano Lett. 6, 2690 (2006) 5. C.-W. Kung, H.-W. Chen, C.-Y. Lin, R. Vittal, K.-C. Ho, J. Power Sources 214, 91 (2012)

13

Journal of the Iranian Chemical Society 6. B. Wang, J.S. Chen, Z. Wang, S. Madhavi, X.W. Lou, Energy Mater. 2, 1188 (2012) 7. Q. Qu, S. Yang, X. Feng, Adv. Mater. 23, 5574 (2011) 8. D. Meng, N.M. Shaalan, T. Yamazaki, T. Kikuta, Sens. Actuators B Chem. 169, 113 (2012) 9. F. Han, H. Li, L. Fu, J. Yang, Z. Liu, Chem. Phys. Lett. 651, 183 (2016) 10. C. Guo, S. Yin, M. Yan, M. Kobayashi, M. Kakihana, T. Sato, Inorg. Chem. 51, 4763 (2012) 11. S. Shendage, V.L. Patil, S.A. Vanalakar, P.S. Patil, Sens. Actuators B Chem. 240, 426 (2017) 12. N.V. Hieu, V.V. Quang, N.D. Hoa, D. Kim, Curr. Appl. Phys. 11, 657 (2011) 13. A. Phuruangrat, D.J. Ham, S.J. Hong, S. Thongtem, J.S. Lee, J. Mater. Chem. 20, 1683 (2010) 14. J.M.O.R. de León, D.R. Acosta, U. Pal, L. Castaned, Electrochim. Acta 56, 2599 (2011) 15. Y. Wang, B. Liu, S. Xiao, X. Wang, L. Sun, H. Li, W. Xie, Q. Li, Q. Zhang, T. Wang, Appl. Mater. Interfaces 8, 9674 (2016) 16. M. Yao, Q. Li, G. Hou, L. Chen, B. Cheng, W. Kechen, X. Gang, F. Yuan, F. Ding, Y. Chen, Appl. Mater. Interfaces 7, 2856 (2015) 17. R. Sivakumar, A.M.E. Raj, B. Subramanian, M. Jayachandran, D.C. Trivedi, C. Sanjeeviraja, Mater. Res. Bull. 39, 1479 (2004) 18. R.H. Kadam, A. Karim, A.B. Kadam, A.S. Gaikwad, S.E. Shirsath, Int. Nano Lett. 28, 2 (2012) 19. M. Parthibavarman, K. Vallalperuman, S. Sathishkumar, M. Durairaj, K. Thavamani, J. Mater. Sci. Mater. Electron. 25, 730 (2014) 20. C. Ng, C. Ye, Y.H. Ng, R. Amal, Cryst. Growth Des. 10, 3794 (2010) 21. H.C. Chen, D.J. Jun, C.H. Chen, K.T. Huang, Electrochim. Acta 93, 307 (2013) 22. A. Li Bassi, D. Cattaneo, V. Russo, C.E. Bottani, E. Barborini, T. Mazza, P. Piseri, P. Milani, F.O. Ernst, K. Wegner, J. Appl. Phys. 98, 1 (2005) 23. M. Parthibavarman, B. Renganathanb, D. Sastikumar, Curr. Appl. Phys. 13, 1537 (2013) 24. V. Hariharan, S. Radhakrishnan, M. Parthibavarman, R. Dhilipkumar, C. Sekar, Talanta 85, 2166 (2011) 25. S. Mohammed Harshulkhan, K. Janaki, G. Velraj, R. Sakthi Ganapthy, M. Nagarajan, J. Mater. Sci. Mater. Electron. 27, 4744 (2016) 26. N. Li, H. Teng, L. Zhang, J. Zhou, M. Liu, RSC Adv. 5, 95394 (2015) 27. M. Karthik, M. Parthibavarman, A. Kumaresan, S. Prabhakaran, V. Hariharan, R. Poonguzhali, S. Sathishkumar, J. Mater. Sci. Mater. Electron. 28, 6635 (2017) 28. F. Mehmood, J. Iqbal, T. Jan, W. Ahmed, W. Ahmed, A. Arshad, Q. Mansoor, S.Z. Ilyas, M. Ismail, I. Ahmad, Ceram. Int. 142, 14334 (2016) 29. D.V. Azamat, A. Dejneka, J. Lancock, V.A. Trepakov, L. Jastrabik, J. Appl. Phys. 113, 174106 (2013) 30. H. Li, X.Y. Kuang, A.J. Mao, Z.H. Wang, Solid State Commun. 189, 47 (2014) 31. M. Muralidharan, V. Anbarasu, A. Elaya Perumal, K. Sivakumar, J. Mater. Sci. Mater. Electron. 27, 2545 (2016) 32. A. Shpak, A. Korduban, M. Medvedskij, V. Kandyba, J. Electron Spectrosc. Relat. Phenom. 156, 172 (2007) 33. W. Xiao, K. Xie, Q. Guo, E. Wang, J. Phys.: Condens. Matter 15, 1155 (2003)