Photoelectrochemical, photocatalytic and photochromic performance of rGO-TiO2WO3 composites

Accepted Manuscript Photoelectrochemical, photocatalytic and photochromic performance of rGOTiO2 WO3 composites S. Prabhu, L. Cindrella, Oh Joong Kwon, K. Mohanraju PII:

S0254-0584(18)31068-X

DOI:

https://doi.org/10.1016/j.matchemphys.2018.12.030

Reference:

MAC 21193

To appear in:

Materials Chemistry and Physics

Received Date: 20 July 2018 Revised Date:

26 October 2018

Accepted Date: 12 December 2018

Please cite this article as: S. Prabhu, L. Cindrella, O.J. Kwon, K. Mohanraju, Photoelectrochemical, photocatalytic and photochromic performance of rGO-TiO2 WO3 composites, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/j.matchemphys.2018.12.030. 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.

ACCEPTED MANUSCRIPT Photoelectrochemical, photocatalytic and photochromic performance of rGO-TiO2WO3 composites S. Prabhu1, L. Cindrella1*, Oh Joong Kwon2 and K. Mohanraju2 Fuel Cell, Energy Materials & Physical Chemistry Lab., Department of Chemistry,

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National Institute of Technology, Tiruchirappalli-620015, India. 2

Department of Energy and Chemical Engineering, Incheon National University, 12-1,

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Songdo-dong, Yeonsu-gu, Incheon 406-840, Republic of Korea.

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*Author for correspondence E-mail: [email protected]; Tel: +91 431 2503634; Fax: +91 431 2500133

Highlights

Enhancement in the indoor and outdoor photocatalytic properties of TiO2-WO3 by rGO.

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Higher photon-to-hydrogen conversion efficiency of 6.5 % at 0 V of the composites. Efficient removal of methylene blue in the aqueous solution under visible light.

solar light.

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Abstract

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Transparent coatings of rGO-TiO2-WO3 with excellent photochromic property under

The synthesis of rGO-TiO2-WO3 composites by simple solvothermal method

followed by calcination process is reported. The prepared composite samples were characterized by various techniques such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy and UV-vis diffuse reflectance spectroscopy. The photoelectrochemical (PEC), photocatalytic and photochromic properties of the prepared composites were investigated. The higher photon-to-hydrogen

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ACCEPTED MANUSCRIPT conversion efficiency of 6.5 % was achieved by the rGO-TiO2-WO3 composite. The prepared composites show efficient removal of MB in aqueous solution by adsorption and visible light photocatalytic processes. Under solar light irradiation, the photochromic property of the transparent rGO-TiO2-WO3 composite coatings was high. The enhanced efficiency of the

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rGO-TiO2-WO3 composites is attributed to the presence of reduced graphene oxide (rGO). The mechanism of the PEC, photocatalytic and photochromic properties is also illustrated. This study shows that rGO-TiO2-WO3 composites can be used for indoor and outdoor

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photovoltaic as well as smart window applications.

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Keywords: rGO-TiO2-WO3, Photoelectrochemical, Photocatalysis, Photochromism, Solar

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energy conversion.

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ACCEPTED MANUSCRIPT 1. INTRODUCTION Efficient conversion and utilization of solar energy is an ideal green technology to fulfil the demands of the world facing energy and environmental problems [1].

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Semiconductor photocatalysis is a promising technique to be applied in several applications such as direct conversion of solar energy into clean hydrogen (H2) fuel, degradation of the organic pollutants in air and water, and development of self-cleaning surfaces [2-4]. H2 generation from photoelectrochemical (PEC) cell which involves water electrolysis using

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solar energy is the most simple and attractive eco-friendly method [5-8]. However, H2

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generation efficiency of the PEC cell is determined principally by the semiconductor properties used as photoelectrode (i.e. working electrodes) [6, 9, 10]. Semiconductor photocatalysis is one of the most valid and promising technique to degrade the pollutants present in air and water using solar energy as well as artificial indoor illumination [11]. The efficiency of photocatalytic degradation of pollutants again depends upon the properties of

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the semiconductor catalyst used [12]. The higher PEC and photocatalytic efficiency of a semiconductor can be achieved by efficient generation, collection and utilization of electron

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(e-)-hole (h+) pairs. For these purposes, photocatalysts that are more stable, non-toxic and with high solar light absorption capacity and the band structure that can reduce the

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recombination rate of e--h+ pairs are highly desirable. Among all the semiconductor materials, TiO2 has attracted much attention due to its

favorable chemical and physical properties, low cost, high stability and high photoactivity [5]. However, two major issues of wide band gap of about 3.2 eV and fast recombination of photogenerated e--h+ pairs limit the widespread utilization of TiO2 [13]. These crucial drawbacks can be overcome by TiO2 coupled with other metal oxides, to construct heterojunction photocatalysts [14]. Recently, TiO2 coupled with tungsten oxide (WO3) have been reported as solar or visible light active photocatalyst with reduced recombination rate of 3

ACCEPTED MANUSCRIPT photogenerated electron (e-)-hole (h+) e--h+ pairs [14, 15]. WO3 is another important semiconductor material having narrow band gap in the range of 2.4–2.8 eV which shows catalytic activity under solar or visible light [16, 17]. WO3 is a cubic or hexagonal symmetrical structured n-type semiconductor having excellent electrochromic and

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photochromic properties [18, 19]. The photochromic property of WO3 has gained additional advantages because of its potential applications in information display devices, highsensitivity optical storage materials and smart window for regulating solar input to buildings

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[20-25].

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In the TiO2-WO3 photocatalyst, the electron transfer from TiO2 to WO3 is highly viable since the conduction band (CB) position of WO3 is lower than that of TiO2. Consequently, the holes transfer in opposite direction and hence better charge separation may be achieved. However, the CB position of WO3 is slightly negative than the single-electron reduction potential of O2 which results in accumulation of photogenerated electrons (e-s) on

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the WO3 surface which can then recombine with holes (h+s) in the valence band (VB) of TiO2 [26]. Therefore, the efficient separation of photogenerated e--h+ pairs and their migration to

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the surface reaction sites should be tuned to improve the efficiency of TiO2-WO3 photocatalyst. The superior properties of graphene and reduced graphene oxide (rGO) such as

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high surface area, high electrical conductivity, high carrier mobility and, efficient e¯ collection and transportation have attracted tremendous interest which can improve the efficiency of TiO2-WO3 photocatalyst [27-29]. There are very few reports available on graphene or rGO-TiO2-WO3 composite for photocatalytic degradation of organic pollutants [27-29]. Wang et al. reported the enhanced photoinduced electron storage and two electron reduction of oxygen by rGO/TiO2/WO3 composites [30]. To the best of our knowledge, the PEC and photochromic properties of rGO-TiO2-WO3 composite and their mechanism has not been reported. Therefore, it is very important to study the photoelectrochemical,

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ACCEPTED MANUSCRIPT photocatalytic and photochromic properties and their mechanism of rGO-TiO2-WO3 composite photocatalyst. In this study, the photoelectrochemical (PEC), photocatalytic and photochromic

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properties of rGO-TiO2-WO3 composites prepared by a simple solvothermal method followed by calcination process, have been demonstrated. The various techniques such as X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM),

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scanning transmission electron microscopy (STEM) and UV-vis diffuse reflectance (UV-Vis-

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DRS) spectroscopy were used to characterize the prepared composite samples. The photonto-hydrogen conversion efficiency of rGO-TiO2-WO3 composite enhanced to 6.5 % by varying the titanium isopropoxide and tungstic acid during the synthesis. The composites showed good adsorption and visible light photocatalytic properties for efficient removal of MB in aqueous solution. For the first time, the higher photochromic property of the

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transparent rGO-TiO2-WO3 composite coatings under solar light irradiation was explored. The enhanced PEC, photocatalytic and photochromic efficiency of the rGO-TiO2-WO3

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composites are attributed to the presence of rGO. Also, the mechanism of the PEC, photocatalytic and photochromic processes of the rGO-TiO2-WO3 composites have been

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discussed and illustrated. Thus, the prepared rGO-TiO2-WO3 composites can be used not only for indoor and outdoor photovoltaic applications but also for the smart window applications. 2. Experimental 2.1. Materials Graphite flake, natural (10 mesh) was purchased from Alfa Aser, USA. Sodium nitrate (NaNO3), sodium sulphate (Na2SO4), sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30% ), hydrochloric acid (HCl), methylene blue (MB)

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ACCEPTED MANUSCRIPT and isopropyl alcohol (IPA) were purchased from Merck Ltd., India. Titanium isopropoxide (TTIP) was acquired from Aldrich, India. Tungstic acid (H2WO4) was purchased from Loba, India. ITO-coated glass substrates (<10 Ω, 1.1 mm thickness, >90% transmittance) were procured from Shilpa Enterprises, India. The glass substrates (1.1 mm thickness) were

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purchased from Polar Industrial Co., India. All chemicals and reagents were of analytical grade and were used as such without further purification, and double distilled water was used throughout this work.

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2.2. Synthesis of rGO-TiO2-WO3 composites

WO3 composites.

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Table 1. Sample code and its corresponding components used for the synthesis of rGO-TiO2-

Sample name GO (ml) TTIP (mmol) H2WO4 (mmol) 2.8

GW2T

3.5

GW3T

4.3

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GW1T

GW4T

5.0

4

1

3

2

2

3

1

4

The modified Hummers method was adopted to prepare graphene oxide (GO) [31],

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and it was reported elsewhere [32]. The aqueous GO dispersion of 1mg/mL was prepared by sonication for 1h. The GO dispersion of 0.5 wt% with respect to the weight of TiO2-WO3

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mixture was sonicated for 10 min in 60 ml of ethanol. Then, to maintain the total 5 mmol of TTIP and H2WO4, the appropriate amount of TTIP followed by H2WO4 were added and stirred. To the reaction mixture, 15 ml of HCl (1M) was added dropwise and stirred for 30 min. The mixture was transferred to teflon-lined autoclave and maintained at 150 oC for 6 h. The product was collected by centrifuge, washed with water several times, then ethanol and dried. The resulting product was calcinated at 300 oC for 1 h. The different compositions of

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ACCEPTED MANUSCRIPT rGO-TiO2-WO3 composites were prepared by varying the amount of TTIP and H2WO4, and the corresponding products were named as given in Table 1. 2.3. Coating procedure

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2.3.1. For PEC application The working electrode for PEC study was fabricated by drop-casting method as follows: 5 mg of the composite catalysts (GW1T, GW2T, GW3T and GW4T) was dispersed

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in 0.5 ml of IPA under sonication for 5 min. 50 µl of the dispersion was dropped over 1 cm2

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area of ITO coated glass substrate, and air-dried. The coating procedure was repeated again, and then the substrate was vacuum dried at 65 oC for 6 h. 2.3.2. For photochromic application

The transparent coating of the composite catalysts on glass substrate was prepared by

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spin coating technique. A dispersion was prepared by sonicating 5 mg of the composite catalysts (GW1T, GW2T, GW3T and GW4T) in 0.5 ml of IPA under sonication for 5 min. 50 µl of the dispersion was spin-coated on the glass substrate in 2 x 2 cm area at 1000 rpm for

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30 sec and then air-dried. The coating process was repeated one more time. The coated

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substrates were vacuum dried at 65 oC for 6 h. 2.4. Characterization

The XRD spectra was recorded to study the crystalline phase of the samples using

Rigaku Ultima III X-ray diffractometer (Cu Kα radiation, λ=1.5406 Å) operating at 40 kV and 30 mA. The Raman spectral characterization was performed using high resolution “LabRAM Hr800” Raman-LTPL spectrometer with Ar Laser wavelength of 632.8 nm. The FTIR spectrum of the samples was recorded using “Thermo Scientific (Nicolet iS5)”

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ACCEPTED MANUSCRIPT spectrometer. The TEM analysis was performed using “ALOS F200X” instrument with 200 kV accelerating voltage. The STEM technique was used to map the elements present in the catalysts. The XPS analysis was carried out by “PHI 5000 Versa Probe II” instrument, and the binding energies were analysed using the C 1s peak at 284 eV as an internal reference.

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The UV-Vis-DRS spectra of the catalysts were recorded by “Shimadzu UV-2600” UVvisible spectrophotometer equipped with an integrating sphere assembly, and BaSO4 was used as the reflectance reference. The spin coating was carried out by “spin NXG-p1” apex

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equipment. The water contact angle (WCA) was measured by Holmarc contact angle meter “Model HO-IAD-CAM-01” by dropping 2 µL of water droplet. The “CHI608B”

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electrochemical analyzer was used to perform electrochemical and photoelectrochemical (PEC) studies. A homemade PEC reactor as shown in Fig. S1 was used in this study. The solar simulator (AM 1.5G, 100 mW.cm-2), “Oriel® LCS-100™” provided by Newport Corporation was used as an irradiation source for PEC, photochromic and hydrophilic

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studies. The removal of MB in aqueous solution by the composites was carried out using “Heber Annular Type Photo-reactor” (Fig. S2) under visible light (500W) irradiation. The

spectrometer”.

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UV-Vis absorption spectrum of the MB solution was recorded using “Aventes UV-Vis

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3. Results and discussion 3.1. Characterization

The XRD patterns of rGO-TiO2-WO3 composites are shown in Fig. 1(a) which are

indexed according to the JCPDS card no. 21-1272 and 89-4476 of anatase TiO2 and monoclinic WO3 respectively. The anatase TiO2 planes of (101), (004), (112) and (200) are observed at 25.3 o, 37.8 o, 38.5 o and 47.9 o respectively in GW1T. Whereas, other peaks at 53.9 o, 55.1 o, 62.2

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and 63.1

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corresponding to (105), (211), (213) and (204) planes of

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ACCEPTED MANUSCRIPT anatase TiO2 are merged with the peaks of WO3 monoclinic phase in GW1T. On increasing the WO3 content in the composites, the TiO2 peaks did not appeare in GW3T and GW4T. This may be due to the low content of TiO2 and also the low intensity of the peaks. The planes (002), (020), (200), (120), (112), (022), (202), (220), (222), (140), (410), (402) and

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(422) corresponding to WO3 monoclinic phase are observed at 23.1 o, 23.6 o, 24.1 o, 26.5 o, 28.1 o, 33.1 o, 34.0 o, 36.5 o, 41.6 o, 49.8 o, 54.0 o, 55.4 o and 62.1 o respectively in all the rGOTiO2-WO3 composites. The XRD peaks corresponding to rGO at around 25.0

o

was not

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observed because of the possible overlap with TiO2 peak of plane (101) or low content of

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rGO used.

The peaks observed in the Raman spectra of rGO-TiO2-WO3 composites as shown in Fig. 1(b) are matched with the TiO2 anatase phase and WO3 monoclinic phase [33-35]. The peaks observed at 150, 399, 520 and 630 cm-1 originated from the TiO2 component [33]. The peaks at 150 and 630 cm-1 were attributed to stretching vibrations of O-Ti-O bonds, whereas

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the peaks at 399 and 520 cm-1 were ascribed to the symmetric and asymmetric bending vibrations of O-Ti-O bonds respectively. The peaks observed at 268, 325, 706 and 807 cm-1 originated from the WO3 component [34, 35]. The peaks at 268 and 325 cm-1 were due to the

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bending modes of W-O-W bridged oxide. The peaks at 706 and 807 cm-1 were due to the W–

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O–W stretching mode in the WO3 network. The intensity of WO3 peaks increased in the rGOTiO2-WO3 composites since the synthesis involved increasing the WO3 content and decreasing TiO2 content. Simultaneously, the intensities of TiO2 peaks decreased and became broad. Moreover, the peak at 399 cm-1 was up-shifted to 436 cm-1 in GW3T and GW4T which revealed the strong interaction between TiO2 and WO3. In the Raman spectra of rGO and rGO-TiO2-WO3 composites, the D band corresponding to the edge and disordered sp3 carbon was observed at around 1326 cm-1, and the G band corresponding to the ordered sp2 carbon network was observed at around 1590 cm-1 [36]. The G band in the rGO-TiO2-WO3

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formation of a real composite rather than simple mixture of rGO, TiO2 and WO3 [35].

Fig. 1. (a) XRD and (b) Raman spectra of rGO-TiO2-WO3 composites.

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ACCEPTED MANUSCRIPT The chemical composition of the surface and electronic state of rGO-TiO2-WO3 composites was investigated using XPS analysis as shown in Fig. 2. The peaks in the survey spectra (Fig. 2(a)) were indexed which confirmed the presence of elements, Ti, W and O in GW2T and GW3T samples. The high resolution O 1s XPS spectrum can be fitted with three

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peaks as shown in Fig. 2(b). The peak at 529 and 530 eV for GW2T and GW3T respectively are attributed to the presence of both Ti-O and W-O bonds in the rGO-TiO2-WO3 composites. A peak at around 530 and 531 eV in GW2T and GW3T respectively was ascribed to the

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surface hydroxyl groups present in the samples. The peak for oxygen in C-O or C=O appeared at around 531.5 and 532.5 eV in GW2T and GW3T respectively [11, 37]. The peak

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at 529 eV in GW2T was shifted to 530 eV in GW3T which suggest more W-O bond characteristics in the composite since the preparation of the composites involved increasing WO3 precursor and decreasing TiO2 precursor. The high resolution XPS spectrum of Ti 2p is shown in Fig. 2(c). The Ti 2p1/2 and Ti 2p3/2 corresponding to mixed Ti-O-Ti and Ti-O-W

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bond peaks were observed at 457.8 and 463.6 eV respectively in GW2T, whereas these two peaks were up-shifted to 458.9 and 464.7 eV respectively in GW3T [11]. The energy gap between Ti 2p1/2 and Ti 2p3/2 peaks in GW2T and GW3T is 5.8 eV, suggesting the oxidation

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state of Ti as 4+ [11, 37]. The intensity of Ti 2p peaks of GW3T was decreased than that of

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GW2T which clearly suggest that relatively more WO3 species are present in GW3T than GW2T [37]. It is difficult to fit the different peak positions of Ti 3p with W 4f peaks because of the similar binding energy of the peaks. However, the high resolution XPS spectrum of W 4f can be fitted with Ti 3p peak as shown in Fig. 2(d). The characteristic W 4f5/2 and W 4f7/2 peaks corresponding to the WO3 components appeared at 36.6 and 34.5 eV respectively in GW2T whereas it was up-shifted to 37.6 and 35.5 eV respectively in GW3T. The energy gap between W 4f5/2 and W 4f7/2 peaks in GW2T and GW3T is 2.1 eV which suggested the oxidation state of W to be 6+ [26, 38, 39].

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Fig. 2. (a) XPS survey and high resolution XPS spectra of (b) O 1s, (c) Ti 2p and (d) W 4f of rGO-TiO2-WO3 composites.

The TEM images (Fig. 3 (a and c)) reveal that the TiO2-WO3 composites are

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distributed well on the surface of rGO in GW2T and GW3T respectively. The magnified view of sections indicated earlier in Fig. 3 (a and c) are shown in Fig. 3 (b and d). The

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HRTEM image of GW3T is shown in Fig. 3 (e). The interplanar distance of 0.38 and 0.35 nm matched well with the lattice spacing of (002) and (101) planes of monoclinic WO3 and

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anatase TiO2 respectively. The rGO component in the composite was also mapped in the HRTEM image. The indexed selected area electron diffraction (SAED) pattern of GW3T as shown in Fig. 3 (f) suggests the polycrystalline nature of WO3 and TiO2 particles and the existence of interaction among them. Thus, the TEM results confirm the existence of interaction among TiO2, WO3 and rGO in rGO-TiO2-WO3 composites. The STEM and their corresponding elemental mapping images of GW2T and GW3T are presented in Fig. 4 which further confirms that the elements, Ti, W and O are present in the rGO-TiO2-WO3 composites.

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Fig. 3. TEM images of (a) GW2T, (c) GW3T, (b and d) magnified view of sections indicated in (a and c) respectively, (e) HRTEM image and (f) selected area electron diffraction (SAED) pattern of GW3T.

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Fig. 4. STEM images of (a) GW2T, (f) GW3T and their corresponding elemental mapping of (b, g) carbon, (c, h) oxygen, (d, i) titanium and (e, j) tungsten respectively. The UV-Vis-DRS spectra of the rGO-TiO2-WO3 composites are shown in Fig. 5(a). On successive increment of WO3 content in the composite, the decreased % reflectance and

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ACCEPTED MANUSCRIPT red-shift in the absorption edge by GW3T and GW4T than the other composites was observed. This decreased % reflectance and red-shift in the absorption edge were attributed to the increased absorption of the incident light and the absorption of longer wavelength light respectively. To calculate the indirect band gap of the composites, Tauc plot was plotted

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using Kubelka-Munk function [F(R)hν]1/2 (F(R)= (1-R)2/2R and R is the reflectance) against the energy of light, hν, as shown in Fig. 5(b). The band gap values are obtained from the intercept of the tangent drawn at absorption function to the energy axis at zero absorption

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[40], and are 2.30, 2.46, 2.56 and 2.63 for GW1T, GW2T, GW3T and GW4T respectively. The increasing band bap value of the composites on increasing the WO3 content may be due

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matrix surrounding with TiO2 [41-43].

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to the quantum well confinement effect as well as some dielectric contribution from the

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Fig. 5. (a) UV-Vis-DRS spectra and (b) Tauc plot of rGO-TiO2-WO3 composites.

3.2. Photoelectrochemical (PEC) activity The PEC property of the rGO-TiO2-WO3 composites were studied using three-

electrode electrochemical cell containing the working, reference and counter electrodes of catalyst coated ITO substrate, Ag/AgCl and a platinum wire respectively. 0.1 M Na2SO4 was used as the supporting electrolyte. The linear sweep voltammetry (LSV) results under illumination and dark conditions are shown in Fig. 6(a). The composites show different

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ACCEPTED MANUSCRIPT current density under dark which suggested that the electrical property can be varied by varying the amount of WO3 and TiO2 in the rGO-TiO2-WO3 composites. Moreover, all the composites showed increased current density under illumination than in dark which reveal the generation of photoelectrons under the solar light irradiation. In the LSV curve, a peak

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appeared at around 0.1V corresponding to the nanostructure of WO3 present in the composites [44, 45]. The peak at about 0.2 V to 0.4 V corresponds to the formation of tungsten bronzes (HxWO3) by reduction of W(VI) to W(V) [46].

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The LSV results as shown in Fig. 6(a) were applied to estimate the photon-to-hydrogen

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conversion efficiency using the following equation [9, 10, 47, 48]. I 1.23 Vbias Jlight

(1)

Where I is the photocurrent density (=Ilight - Idark) in mA.cm-2, 1.23 is the potential required

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light (100 mW.cm-2).

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for splitting of water, Vbias is the applied potential, and Jlight is the intensity of the incident

Fig. 6. (a) Linear sweep voltammetry curve and (b) photon-to-hydrogen conversion efficiency of rGO-TiO2-WO3 composites. The calculated photon-to-hydrogen conversion efficiency at the applied potential is presented in Fig. 6(b). The GW3T shows higher efficiency of 6.5 and 5.0 % at 0 and 0.1 V respectively.

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ACCEPTED MANUSCRIPT The GW3T also show peaks of 3.0 and 0.7 % at 0.34 and 0.82 V respectively. The second higher efficiency of 5.3 % was observed for GW2T at 0.01 V. The GW2T also shows peaks of 3.2, 4.1, 3.4 and 1.5 % at 0.09, 0.19, 0.42 and 0.94 V respectively. The GW4T shows peaks of 2.7, 2.3 and 1.4 % at 0.02, 0.19 and 0.71 V respectively. The GW1T shows peaks of

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0.8, 3.3, 2.3 and 1.3 % at 0.06, 0.19, 0.39 and 0.83 V respectively. The enhanced efficiency of GW3T is attributed to the efficient collection and utilization of photocurrent by rGO. Moreover, the peaks at various applied potentials in the rGO-TiO2-WO3 composites suggest

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the presence of multiple energy levels which help in efficient separation of e--h+ pairs.

Fig. 7. (a) Transient photocurrent response and (b) Nyquist plot of rGO-TiO2-WO3

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composites.

The transient photocurrent response of the working electrodes was recorded to

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investigate the stability of the rGO-TiO2-WO3 composites. The chronoamperometry curve at 1.23 V bias potential with the light on/off cycles of 50 s is shown in Fig. 7(a). The higher photocurrent of about 9.5 µA.cm-2 was observed for GW1T but it decreased to about 5.1 µA.cm-2 after two cycles and stabilized. The second higher photocurrent of about 8.0 µA.cm-2 was observed for GW4T and it decreased to about 4.8 µA.cm-2 after two cycles and stabilized. The GW2T and GW3T showed photocurrent response of about 4.7 µA.cm-2 and it decreased to 3.2 and 2.5 µA.cm-2 respectively after two cycles and stabilized thereafter. The

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understanding of the efficient electron transfer through the interface of rGO-TiO2-WO3 composites. The Niquist plots of the composites at 1.23 V bias potential are presented in Fig. 7(b). The diameter of the semicircle curve obtained in the Nequist plots correspond to the charge transfer resistance (Rct) of the composites. The estimated Rct values of GW1T, GW2T,

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GW3T and GW4T in dark are 58.5, 22.6, 195.2 and 65.9 kΩ respectively. Under

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illumination, the Rct values decreased to 30.6, 20.1, 102.4 and 30.0 kΩ for GW1T, GW2T, GW3T and GW4T respectively. The decreased Rct values under illumination than under dark are attributed to the generation and efficient separation of e--h+ pairs in the composites. The difference in the Rct values under dark and illumination for GW1T, GW2T, GW3T and GW4T are 27.9, 2.5, 92.8 and 35.9 kΩ respectively. The large difference in the Rct value by

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92.8 kΩ for GW3T clearly shows the efficient transfer of photogenerated electrons through the interface of rGO-TiO2-WO3.

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3.3. Photocatalytic activity

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The photocatalytic degradation of MB by rGO-TiO2-WO3 composites was carried out to investigate the photocatalytic activity of the catalysts. In a typical experiment, 50 ml of MB solution (10 mg L-1) was added with 15 mg of the catalyst. The mixture was aerated during the experiment for thorough mixing. To reach adsorption-desorption equilibrium, the mixture was first kept in the dark for 30 min and then irradiated. At an interval of 15 min during irradiation, 3 ml of the solution was collected, the catalyst was removed by centrifugation and then the change in the UV-Vis absorption spectrum of MB during irradiation was recorded. The UV-Vis absorption spectrum of MB during irradiation in the

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ACCEPTED MANUSCRIPT presence of GW4T catalyst is shown in Fig. 8(a). The spectrum reveals that the absorption peaks decreased progressively over the period of the irradiation time at all wavelength range. Moreover no shift in the absorption peaks was observed. This decreased absorption without any shift is attributed to the absolute degradation of MB in the presence of the composites.

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The absorbance at 665 nm was used to estimate the % degradation of MB using the following equation and is shown in Fig. 8(b). Ao -At Ao

×100

(2)

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% degradation =

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Where Ao and At are the initial absorbance and the absorbance at time t respectively. The MB degradation of 72% was observed in the presence of GW1T. The % degradation increased to 86% by GW2T and GW3T and slightly decreased to 83% in the presence of GW4T. The MB degradation in the absence of any catalyst is insignificant (< 10 %) which

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shows the stability of MB under visible light irradiation. The increased degradation of MB in the presence of GW2T and GW3T may be due to the high adsorption of MB on the surface of the composites. The GW2T and GW3T show 64 and 60 % of adsorption respectively, while

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GW1T and GW4T show 43% of adsorption under dark for 30 min. This high adsorption capacity of rGO-TiO2-WO3 composites reveal that the composites can also be used as good

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adsorbent towards organic compounds. The rate constant for the degradation of MB can be calculated using the Langmuir-

Hinshelwood kinetic equation [49, 50] as given below,

ln

Co C

= kapp t

(3)

Where, Co is the initial concentration (mol.L-1) of MB and C is the concentration (mol.L-1) of MB at time t (min), and kapp is the apparent rate constant (min-1).

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Fig. 8. (a) Changes in the UV-Vis absorption spectrum of MB by GW4T, (b) % degradation

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and (c) plot of ln(Co/C) versus time for the degradation of MB. The rate constant was obtained from the slope of the plot of ln(Co/C) against irradiation time (Fig. 8(c)). The rate constant for the degradation of MB in the presence of

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GW1T was 1.409 x 10-2 min-1. The rate constant increased to 2.127 x 10-2 and 2.164 x 10-2 min-1 for GW2T and GW3T respectively and then slightly decreased to 2.033 x 10-2 min-1 for

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GW4T.

3.4. Photochromic property The % transmittance spectrum of rGO-TiO2-WO3 composites as (coatings) on glass

substrate are shown in Fig. 9(a). The glass substrate without any coating showed around 90 % transmittance and the rGO-TiO2-WO3 coatings decreased the transmittance of the substrate to around 66 %. At 600 nm, the GW1T coating showed 72.6 % while, the transmittance decreased to 67.5 and 65.5 % for GW2T and GW3T coatings respectively. It slightly

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ACCEPTED MANUSCRIPT increased to 68.7 % for GW4T coating. However, all the coatings were observed to be highly transparent as shown in Fig. 9(b). The alphabet through the coatings can be seen clearly for differentiation purpose, a line was drawn at the borders of the coated area. The photochromic property of the transparent coatings was studied under solar light irradiation. The changes in

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the % transmittance of the coatings on irradiation are shown in Fig. 9(c-f) and the corresponding % transmittance values at 600 nm are given in Table 2. All the coatings show photochromism as the transmittance of the coatings decreased on irradiation. The GW3T

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coating shows a decrease in the transmittance by 1.3 % in 60 min, but the GW1T, GW2T and GW4T coatings show decrease in the transmittance by 0.7, 0.5 and 0.2 % respectively. The

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higher photochromic effect of GW3T than the other composites is attributed to the formation of more HxWO3 [51]. The shape of water droplet on the surface of the coatings before (initial) and after irradiation for 30 and 60 min is shown in Fig. 10 and the corresponding WCA values are given in the Table 2. The uncoated glass substrate shows a WCA of 63.8 o. The o

are observed for the coatings which reveal the

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initial WCA in the range of 26 to 41

hydrophilic nature of the coatings. On irradiation, the hydrophilicity of the coatings increased and the WCA values registered decreases (Table 2). The WCA of GW3T coating decreased

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drastically from 33.5 o to 11.5 o while other coatings showed relatively small decrease in the

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WCA value. The increased hydrophilicity may be due to high interaction of water molecules with the tungsten bronzes formed on the surface of the coatings. The result of enhanced hydrophilicity of GW3T coating than other coatings under solar irradiation is consistent with the enhanced PEC, photocatalytic and photochromic activity of GW3T. Therefore, these studies open-up a new avenue on rGO-TiO2-WO3 composites which can be used for smart window as well as indoor and outdoor photovoltaic applications.

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ACCEPTED MANUSCRIPT Table 2. % Transmittance at 600 nm and water contact angle (WCA) of rGO-TiO2-WO3 coatings under solar light irradiation.

Coatings

Water contact angle (o)

Transmittance (%) 30 min

60 min

Initial

30 min

60 min

GW1T

72.6

72.4

71.9

41.0

32.7

29.7

GW2T

67.5

67.4

67.0

27.7

23.2

22.5

GW3T

65.5

64.6

64.2

33.5

24.1

11.5

GW4T

68.7

68.6

68.5

26.7

20.8

18.8

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Initial

Fig. 9. (a)% Transmittance spectra, (b) photograph and (c-f) change in % transmittance spectra under solar light irradiation of rGO-TiO2-WO3 composite coatings.

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Fig. 10. Shape of water drops (a, d, g and j) before and after (b, e, h and k) 30 min and (c, f, i

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and l) 60 min irradiation on GW1T, GW2T, GW3T and GW4T coatings respectively. 3.5. Mechanism of PEC, photocatalytic and photochromic processes The PEC, photocatalytic degradation of MB and the photochromic reactions are

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driven by the electrons (e-s) and holes (h+s) generated on illumination of the composites. The generation of e-s and h+s by rGO-TiO2-WO3 composites on illumination from a light source is illustrated in the Fig. 11(a). Since both the TiO2 and WO3 are active under solar or visible

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light radiation, the e-s are excited to the conduction bands (CBs) leaving h+s in the valence bands (VBs) of the respective metal oxides. The e-s in the CB of TiO2 are transferred to the

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CB of WO3 because of its lower CB level than TiO2, and transfer of holes occurs in the VBs of the metal oxides in opposite direction. Consequently, the e-s in the CBs are transferred to the rGO. Thus the efficient separation and reduced recombination rate of e--h+ pairs enhance the photocatalytic efficiency of the composites. The evolution of hydrogen (H2) and oxygen (O2) by rGO-TiO2-WO3 composites coated on ITO substrate in the PEC cell is schematically illustrated in Fig. 11(b). The photogenerated e-s are collected through the external circuit to the Pt electrode. Then, the e-s

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ACCEPTED MANUSCRIPT are reacting with the H+ ions present in the electrolyte solution, and H2 is produced. At the same time, O2 is produced on the surface of working electrode by reacting h+s with water molecules.

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The schematic illustration of the photocatalytic degradation of MB by rGO-TiO2-WO3 composites is shown in Fig. 11(c). The photocatalytic degradation of MB occurs by reacting MB with radicals produced by e--h+ pairs. The photogenerated e-s in the CBs of the composites react with O2 molecules and produce •O2-, HOO•, H2O2 and OH• radicals. On the

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other hand, the h+s in the VBs of the composites react with the adsorbed water molecules or

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OH- ions and produce OH• radicals [52, 53]. Then, the produced active radicals react with the MB molecules resulting in degraded products such as CO2 and weak mineral acids [54, 55]. The schematic illustration of the photochromic property of rGO-TiO2-WO3 composites is shown in Fig. 11(d). The photochromic property of the composites can be

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elucidated based on the model of double insertion/extraction of ions and e-s which was developed to explain the electrochromism of WO3 [22]. The photochromism of rGO-TiO2WO3 composites can be represented as follows, (4)

W6 + e → W5

(5)

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rGO−TiO2 − WO3 + hν → e + h

1

1

h + 2 H2 O → H + 4 O2 ↑

(6)

WO3 + xe- + xH+ → Hx WO3

(7)

On illumination, the e--h+ pairs will be generated (Eq. (4)). The photogenerated e- will reduce the W6+ oxidation state of WO3 to W5+ present in the rGO-TiO2-WO3 composites (Eq. (5)). Whereas, the photogenerated h+s react with the adsorbed water molecules, produces protons

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ACCEPTED MANUSCRIPT (Eq. (6)). Subsequently, the produced protons can diffuse into the WO3 lattice resulting in the formation of HxWO3 (Eq. (6)). Then transition of e-s from the VB of W5+ to the CB of W6+ makes the surface blue coloured [51]. The photochromism of rGO-TiO2-WO3 coatings by formation of HxWO3 was well matched with peak appearing at about 0.2 V to 0.4 V in the

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LSV curve (Fig.6(a)) of PEC study. The higher PEC, photocatalytic and photochromic activity of GW3T attributed to the efficient utilization of photogenerated e-s. This was confirmed from the high interaction of water molecules with HxWO3 formed on the surface of

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GW3T by efficient utilization of photogenerated e-s to reduce W6+ to W5+ present in the

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composite.

Fig. 11. Mechanism of (a) general photocatalysis, (b) photoelectrochemical, (c) photocatalytic and (d) photochromic processes.

4. CONCLUSION The rGO-TiO2-WO3 composites were prepared by simple solvothermal method followed by calcination process for application in phoelectrochemical (PEC), photocatalytic

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ACCEPTED MANUSCRIPT and photochromic systems. The anatase and monoclinic phase of TiO2 and WO3 respectively were revealed by XRD analysis. The existence of interaction among TiO2, WO3 and rGO was confirmed by Raman spectral and TEM results. The GW3T showed higher photon-tohydrogen conversion efficiency of 6.5 % at 0 V. The efficient removal of MB in aqueous

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solution using adsorption and visible light photocatalytic processes was observed for GW2T and GW3T catalysts. The photochromic property of the transparent coatings was higher for GW3T than the other composites under solar light irradiation which is attributed to the

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formation of more HxWO3 (tungsten bronzes). The WCA of GW3T coating decreased drastically to 11.5 o from 33.5 o on solar light irradiation for 60 min. The enhanced efficiency

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of the rGO-TiO2-WO3 composites is attributed to the efficient separation, collection and utilization of photoelectrons by rGO. The mechanism of the PEC, photocatalytic and photochromic properties of the rGO-TiO2-WO3 composites was also illustrated. This study shows that rGO-TiO2-WO3 composites can be used for smart window as well as indoor and

Acknowledgment

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outdoor photovoltaic applications.

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Financial support by CSIR, India through the project No.02 (0193)/14/EMR-II to LC

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is thankfully acknowledged. References

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