J Mater Sci: Mater Electron DOI 10.1007/s10854-016-5373-9

Enhanced visible light photocatalytic activity of pristine and silver (Ag) doped WO3 nanostructured thin films S. Ramkumar1 • G. Rajarajan2

Received: 7 May 2016 / Accepted: 12 July 2016 Ó Springer Science+Business Media New York 2016

Abstract In this report, pristine and silver (Ag) doped tungsten trioxide (WO3) thin films were deposited on FTOcoated glass substrate by using simple chemical bath deposition method for the first time. The structural, morphological, and optical properties of pure and Ag-doped WO3 thin films have been systematically studied. The asdeposited films were annealed at 600 °C for 2 h in the ambient atmosphere in order to improve the crystallinity. The structural properties revealed that both pure and Agdoped WO3 films exhibited the monoclinic phase of WO3. The surface morphology revealed that pure films showed the dense surface and films contained agglomerated grains which were uniformly distributed on the surface of the substrate. The optical transmittance decreased from 95 to 85 % for pure and Ag-doped WO3 films, respectively. The photocatalytic activities of the films were evaluated by degradation of methyl orange and Phenol in an aqueous solution under visible light irradiation. The photocatalytic activity of WO3 nanostructures could be extremely improved by doping with Ag. The improved photocatalytic mechanism by Ag also discussed. The films were further characterized by Fourier Transform Infrared (FTIR) spectra and photoluminescence spectra analysis.

& S. Ramkumar [email protected]; [email protected] 1

Department of Physics, Bannari Amman Institute of Technology, Sathyamangalam, Erode District, Tamil Nadu 638 401, India

2

Department of Physics, Vidhya Mandhir Institute of Technology, Erode, Tamilnadu 638 052, India

1 Introduction Photocatalysts are able to convert solar energy into chemical energy which can be used for numerous purposes like a generation of hydrogen fuel through water splitting; purification of different aqueous media; green synthesis of various chemicals [1]. Organic dyes found in textile effluents are a serious ecological problem since even a small amount of them in water can cause a significant coloring and they can be toxic to aquatic as well as human life [2]. Hence, their removal from wastewater is of fundamental importance for the environment [3]. Metal oxide semiconductors with small and medium band gaps show lower light-harvesting ability in visible light. Therefore, the coupling of semiconductors with different band gaps is a good approach to preparing photocatalysts with high activity and good stability. Among the different types of metal oxide semiconductor photocatalysts (ZnO, TiO2, SnO2 and In2O3), tungsten oxide (WO3) is one of the best photocatalysts which have outstanding stability, good electron transport properties, no-photo-corrosion and high photoactivity [4]. WO3 playing much attention recently, is owing to its wide application prospective and advantageous chemical and electrical properties [5]. WO3 is a good candidate for the fabrication of solar response photocatalyst with absorption in the visible range [6]. The photocatalytic activity depends on many factors and numerous attempts were made in order to control the size distribution and the dimensionality of the nanocatalysts because these are considered as key parameters in affecting their photocatalytic performance. WO3 thin films were prepared by various methods such as solvothermal [7], hydrothermal reaction [8, 9], chemical vapor deposition [10–13], sputtering [14, 15], PLD [16, 17], sol–gel [18] and spray Pyrolysis Deposition

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[19, 20]. Among them, we employed a chemical bath deposition (CBD) technique for a thin film preparation method. Compared with the above methods, the chemical bath deposition method (CBD) is one of the suitable methods for preparing highly efficient thin films in a simple manner. Chemical bath deposition yields stable, adherent, uniform and hard films with good reproducibility by a relatively simple process [21]. Several dopants such as Fe, Cr, Al, Cu, Co, Mn, Mg, S, P, N etc. can lead to an increase in the surface area of the WO3. This property may be helpful to increase the photocatalytic performance. Among these, silver (Ag) doped WO3 has received considerable attention. Moreover, only a few reports are available for synthesis and photocatalytic properties of Ag-doped WO3. A sufficient degree of crystallinity is required to attain the desired electronic properties of the films in order to increase the photocatalytic properties. This can be achieved through annealing the films at various temperatures and atmospheres. The asdeposited Ag-WO3 films were annealed at 600 °C for 2 h in the ambient atmosphere in order to improve the structural perfection and crystallinity. The metallic nature of ‘Ag’ is the best candidate for improve the photocatalytic performance because of its high solubility, small ionic size, and minimum orbital energy. Moreover the ionic radius of Ag is smaller (138 pm) as compared to W (142 pm), thus Ag ion can easily substitute in the W lattice site, which leads to increase the photocatalytic efficiency. So, in the present work, we have chosen silver (Ag) metal dopant, in order to improve the photocatalytic efficiency. The roles of Ag dopants on structural, optical and photocatalytic activity properties have been investigated. The improved photocatalytic mechanism of WO3 by Ag-doping is also discussed. To the best of the authors’ knowledge, this is the first report of photocatalytic activity of pure and Ag-doped WO3 films by a simple chemical bath deposition method.

2 Experimental procedure 2.1 Preparation of Ag–WO3 thin films All the chemicals were of analytical grade without the need of further purification. The raw materials of the tungsten and silver sources were chosen as Tungstic acid (H2WO4) and silver chloride (AgCl22H2O) respectively. Initially 2.49 gm of tungstic acid is mixed with 10 ml of sodium hydroxide (NaOH) solution. The concentration of NaOH is 1 mol. It resulted in a yellow colour solution due to proton exchange protocol process. Appropriate amount of (AgCl22H2O) in (0, 5 and 10 wt%) deionized water was added to the above solution. pH of the solution was adjusted to 2 with the addition of 2 ml of

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HCL. The dropping rate must be well controlled for the chemical homogeneity. Films were grown on 76 mm 9 26 mm 9 1 mm glass microscope slides which were used as depositing substrates. The glass slides as the substrates for depositing Ag–WO3 films were prepared by ultrasonically cleaned by acetone, followed by ethanol and finally, deionized water for 10 min respectively and allowed to air dry. The substrate was suspended vertically in the reaction bath after stirring the solution properly for homogeneity. The as-deposited thin films were annealed at 600 °C for 2 h in a muffle furnace, in order to improve the crystallinity. Pure WO3 thin films were prepared in a similar manner without use of the silver source. 2.2 Characterization techniques The prepared samples (annealed) were successfully characterized by the following techniques. Structural properties of the Ag–WO3 thin films were analyzed by using X-ray diffraction (XRD, JEOL diffractometer) with monochro˚ ) in the range of matized Cu Ka radiation (k = 1.54056 A 20°–80° with the step size of 0.1°. Fourier transform infrared measurements were recorded by using Technos instruments (Seki technotron Corp, Japan) in the range of 200–1000 cm-1. The optical transmittance of the thin films was recorded at room temperature by a Perkin Elmer UV/ VIS/NIR Lambda 19 spectrophotometer in the wavelength range of 300–900 nm. The morphology of the Ag–WO3 thin films is observed by Atomic force microscope (ParkXE100 AFM non-contact mode). Photoluminescence spectra of the samples were recorded using Perkin Elmer LS 55 Spectrometer equipped with a 40 W Xenon lamp and excitation length used was 325 nm. 2.3 Photocatalytic activity set up The photocatalytic activity experimental setup was constructed and studied by Vadivel et al. [22]. The detailed experimental set up is described below. Photocatalytic experiment on the prepared samples for the photodegradation of dyes was performed at ambient temperature. The photocatalytic activities of Ag-doped WO3 films were evaluated by the degradation of two types of dyes, including methyl orange (MO) and Phenol solution, in visible light irradiation. For the photocatalytic process, two pieces of 25 mm 9 75 mm glass plate coated with films were settled into 25 ml of dye solutions (MO and Phenol) with a concentration of 15 mg/l in a 100 ml cylindrical glass reactor. The 125 W high-pressure mercury lamp was used as a light source. The coated glass/dye solution was irradiated in the horizontal direction and the distance between the UV lamp and the glass/dye solution was kept within 25 cm. Then the solution has to be kept in the dark

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room and well stirred with the magnetic stirrer for more than 30 min to attain the equilibrium condition throughout the solution. The concentration of the aqueous suspensions (MO and Phenol) in each sample was analyzed using UV– Vis spectrophotometer at a wavelength of 664 nm. The photocatalytic efficiency has been calculated from the expression g=(1 - C/C0), where C0 is the concentration of dyes (MO and Phenol) before illumination and C is the concentration of dyes after a certain irradiation time.

3 Results and discussion 3.1 XRD analysis The XRD spectra of the pure WO3 and Ag-doped WO3 thin films were shown in Fig. 1. From the fig, it is clearly seen that all the diffraction peaks can be indexed to the monoclinic phase of WO3, and the results are in good agreement with standard value (JCPDS # 83-0950). The diffraction peaks related to the Ag or AgO were not detected in Agdoped WO3 films since the silver substituted into the tungsten in WO3. Moreover, the peaks were shifted to lower angle side and decreased the intensity for Ag doped WO3 samples. The calculated lattice parameters of pure ˚ , b = 7.4889 A ˚ and c = 7.5789 A ˚] WO3 [a = 7.2852 A decreased after doping of Ag-doped WO3 (see Table 1). The observed decrease in the lattice parameters could be attributed to the smaller ionic radius of Ag (138 pm) as compared to W (142 pm). The average crystalline sizes of the pure and Ag doped WO3 thin films were calculated by using Scherrer’s equation [23].

Fig. 1 XRD pattern of WO3 thin film with different Ag content

Table 1 Shows the lattice parameters and average crystallite size of WO3 thin films with different Ag concentrations Ag concentrations (Wt%)

Crystallite size (nm)

Lattice parameter values ˚) a (A

˚) b (A

˚) C (A

0

39

7.3842

7.2889

7.4589

5

32

7.2985

7.3456

7.5467

10

27

7.2854

7.4261

7.5821

d¼

Kk b cos h

where d is the mean crystallite size, K is the shape factor taken as 0.89, k is the wavelength of the incident beam, b is the full width at half maximum and h is the Bragg angle. The average crystalline size was found to be 21 nm, 18 nm and 13 nm for pristine and Ag (5 and 10 wt%) doped WO3 respectively. These results clearly show that Ag- incorporated in WO3 host lattice site.

3.2 Atomic force microscope analysis Atomic force microscopy (AFM) is a useful technique to determine the surface morphology and particle size of the samples. Figure 2 shows the 2D and 3D AFM image of pure and Ag-doped WO3 thin films. It was seen that the pure WO3 films surface seem to be dense with fine grain surface, whereas Ag-doped films contains small nanograins compared to pure WO3. The coarseness of morphology is observed as shown in Fig. 2b). These types of morphological films are very useful for photocatalytic properties. For an optical surface, roughness is normally considered as an important parameter. Surface roughness is not only the light scattering but also gives an idea about the quality of the surface under investigation, in addition to providing some insight on the growth morphology. A systematic description of the various analytical method used for roughness characterization can be found in reference [24]. Root mean square roughness (Rrms) which is defined as standard deviation of the surface height profile from the mean height, is the most commonly reported measurement of the surface roughness and is given by, " # n 1X 2 Rrms ¼ ðhi  ðhÞÞ N i where N is the number of pixels in the image, hi is the height of ith pixel and \h[ is the mean height. The Rrms values of pristine and Ag (5 and 10 wt%) doped WO3 were found to be 22, 19 and 16 nm respectively.

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J Mater Sci: Mater Electron Fig. 2 AFM micrograph (2D image) of WO3 thin film with different Ag content a pure WO3 b Ag 10 wt%: 3D image of WO3 thin films c pure WO3 d Ag 10 wt%

3.3 UV–Vis transmission spectra analysis The optical transmission and band gap energy of Ag-doped WO3 films were characterized by UV–Vis transmission spectra analysis. Figure 3a shows the UV–Vis transmission spectra analysis of pure and Ag-doped WO3 films. The optical absorption of the Ag–WO3 samples increases in the visible spectral region with the increase of the Ag concentration. This behaviour can be attributed to the surface plasmon resonance (SPR) absorption, characteristic to the metallic Ag [25]. In all the doped samples, the absorption range was found to be red-shifted as compared with undoped WO3. The red shift for UV–Vis absorption spectra of the samples is regarded as an indication of Ag incorporating into the WO3 host lattice site and also decreasing the band gap energy of pristine WO3. The change of the band gap was attributed to charge transfer from Ag to WO3 causing downward shifting of the conduction band and upward shifting of the valence band. In order to determine the optical band gap of the films, it is assumed that the direct transition takes place in these films and the absorption coefficient was fitted to the Tauc’s relation: The absorption coefficient (a) was calculated from the transmission spectra using equation [26], a ¼ 1=t lnð1=TÞ where T is the optical transmission and t is the film thickness. The indirect band gap of thin films was calculated from Figs. 3(b) using the formula [27],

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 m ahm ¼ A hm  Eg where a is the absorption coefficient, h is the Planck’s constant, m is the frequency of incident light, Eg is the energy band gap of material and m is the factor governing the indirect transition of the electron from the valence band to the conduction band. The band gap energy was calculated as 3.01, 2.88 and 2.68 eV for pure and Ag (5 and 10 wt%) doped WO3 films (Fig. 3b). The reduction bandgap is caused by the valence band of the Ag atoms in WO3, which forms an acceptor level lead to a red shift. 3.4 Photoluminescence spectra analysis PL emission spectrum has been widely used to study the surface defect, impurity energy level, band gap and oxygen deficiency in the semiconducting oxide materials. Figure 4 shows the PL emission spectra of both pure and Ag-doped WO3 films measured from 300 to 600 nm using a 325 nm He–Cd laser. In both pristine and doped samples, there are series of peaks that were observed at 362, 412, 492 and 523 nm. The UV emission at 362 nm and blue emission at 412 nm is observed on the prepared nanostructured crystalline thin films. This emission peak can be attributed to band–band transition and localized states induced by the presence of oxygen vacancies or defects in the nanostructure [28]. The green emission peak (521 nm) is related to Voþ oxygen vacancies. This emission may be credited to the different luminescent centers such as defect energy levels

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(decrease the band gap energy) arising due to tungsten interstitials and oxygen vacancies as well as dangling bonds into nanocrystals. The UV intensity peaks decrease with the increase of the Ag content. This result suggests that decreasing the band gap energy for Ag doped WO3 is also confirmed by UV–Vis spectra. 3.5 FTIR spectra analysis The functional groups were analyzed by using Fourier transform infrared spectra. Figure 5 shows the FTIR spectra of pristine and Ag-doped WO3 films. The band appeared at 3436.69 cm-1 is related to stretching vibration of a surface hydroxyl group, which is probably due to the fact that re-adsorption of water from the ambient atmosphere has occurred [29]. The band appeared at 1627.90 cm-1 is related to stretching vibration of tungstenhydroxyl (W–OH) bond. The spectrum shows a strong band at 800–600 cm-1 range for W–O–W bridging mode [30]. No other peak was detected for AgO. So FTIR results confirm that formation of WO3 and also Ag has substituted in the regular lattice site of WO3. 3.6 Photocatalytic activity measurements

Fig. 3 UV–Vis spectra of WO3 thin film with different Ag content a transmittance spectra b band gap energy determination

Fig. 4 Photoluminescence spectra of pure and Ag-doped WO3 thin films

The photocatalytic activities of pure and Ag-doped WO3 samples were evaluated by degradation of various dyes such as methyl orange (MO) and phenol under visible light irradiation. Figures 6 and 7 show the MO and Phenol degradation profile of WO3 thin films with Ag different concentrations. In Fig’s, C0 is the initial concentration of dyes, and C is the concentration of dyes after visible light irradiation. The concentration of the MO solution decreases in the presence of the pure WO3 catalyst under visible light

Fig. 5 FTIR spectra of WO3 thin film with different Ag-content

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Fig. 6 Photocatalytic degradation of methyl orange (MO) using Ag– WO3 thin films under visible-light irradiation

change in their particle size and a considerable increase in their photocatalytic activities. This may be attributed to the effect of producing intermediate energy levels in the energy gap of a pure WO3 sample. The reusability of the Ag (10 wt%) doped WO3 samples as a photocatalyst is also studied by collecting and reusing the same photocatalyst for multiple cycles. As shown in Fig. 8, after 7 runs of photodegradation of phenol, the photocatalytic activity of the Ag–WO3 samples shows a slight deterioration due to incomplete remembrance and loss during the washing. The photocatalytic mechanism of the Ag–WO3 catalyst is shown in Fig. 9. Madhan et al. [32] have investigated that photocatalytic properties of Zn doped WO3 nanoparticles by microwave irradiation method. They also reported that the MB degradation efficiencies of pure and Zn (5 and 10 wt%) doped WO3 samples are about 51, 55, and 72 %, respectively. Similarly the RHB degradation efficiencies of pure and Zn (5 and 10 wt%) doped WO3 samples are about 58, 63 and 93 %, respectively. Our previous work also reported that photocatalytic properties Fe doped WO3 thin films [21]. The MO degradation efficiencies of pure and Fe (5 and 10 wt%) doped WO3 samples are about 67, 71, and 82 %, respectively. Similarly the Phenol degradation efficiencies of pure and Fe (5 and 10 wt%) doped WO3 samples are about 73, 79 and 92 %, respectively. Ze-Da Meng et al. [33] have investigated that, the MO decolorization efficiencies of WO3-fullerene, fullerene-TiO2, and WO3fullerene/TiO2composites were 45.17, 32.12, and 23.41 %, respectively. Compared with the above literature values, our prepared Ag–WO3 catalyst shows better photocatalytic performance. The photodegradation results show that the Ag-doped catalysts exhibited a significant increase in the MO &

Fig. 7 Photocatalytic degradation of phenol using Ag–WO3 thin films under visible-light irradiation

irradiation, but the degradation efficiency is very low. Only 55 % of the MO was degraded after 150 min irradiation. After Ag doping, the degradation efficiency was significantly improved. The MO degradation percentage was found to be 81 and 91 % for Ag (5 and 10 wt%) doped WO3 respectively. Similarly, the Phenol degradation efficiencies of pure and Ag (5 and 10 wt%) doped WO3 catalysts are about 57, 86, and 98 %, respectively. This result constitutes a photocatalytic performance of pure WO3 improved by Ag doping. Generally, the activity of photocatalysts depends on several parameters including phase structure, specific surface area and crystalline size [31]. In our case, Ag doping ions into the pure WO3 did not show any change in the crystal structure but caused a slight

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Fig. 8 7-Cycles of degradation of phenol using Ag–WO3 thin film as the photocatalyst

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which is confirmed by photoluminescence spectra analysis. The photocatalytic activity of the WO3 catalyst was extremely enhanced by doping the Ag impurity. This can be attributed to the fact that the incorporation of the Ag doping metal ions is lead to diminish in the electron–hole recombination that improved the photocatalytic activity under visible light irradiation. The Ag-WO3 photocatalysis was proved to be an attractive method to remove pollutants from industrial wastewaters which convert to nontoxic compounds.

References Fig. 9 Schematic representation of Photocatalytic mechanism of Agdoped WO3 catalyst

Phenol photodegradation as compared to pristine WO3. This may be attributed to the introduction of doping ions into the pure oxide matrix can act as electron–hole separation centers [34]. In the presence of visible light, AgWO3 sample gets excited by photons with energy higher than the gap energy (Eg) and a large number of electrons are promoted from valence band (VB) to the conduction band (CB) of Ag and WO3, leading to the generation of electron/hole (e-/h?) pairs. The electrons transfer from the CB of WO3 to the CB of Ag, and conversely the holes transfer from the VB of Ag to the VB of WO3 decreasing the pairs’ recombination rate. Moreover, decreasing the band gap energy is a key role for improving the photocatalytic activity. We also confirm the decrease of the band gap energy by Ag- doping in UV–Vis spectra. In addition decreasing the particle size or increasing the surface area for Ag-doped WO3 catalyst, leads to improve the photocatalytic efficiency as compared to pure WO3.

4 Conclusions In summary, nanostructured thin films of pristine and Agdoped WO3 were prepared by simple chemical bath deposition method. XRD results could be indexed to the monoclinic phase of WO3 and the results were in good agreement with standard value (JCPDS # 83-0950). The films surface is seen to be dense with fine grain surface was confirmed by AFM images. The red shift for UV–Vis absorption spectra of the samples is regarded as an indication of Ag incorporating into the WO3 host lattice site and also decreasing of the band gap energy of pristine WO3. The functional groups were confirmed by FTIR spectra. The defect in samples such as oxygen vacancies and decreasing the band gap energy play a crucial role,

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