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Charge transfer and intrinsic electronic properties of rGO-WO3 nanostructures for efficient photoelectrochemical and photocatalytic applications
Materials Science in Semiconductor Processing 74 (2018) 136–146
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Charge transfer and intrinsic electronic properties of rGO-WO3 nanostructures for efficient photoelectrochemical and photocatalytic applications
MARK
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S. Prabhua, S. Manikumara, L. Cindrellaa, , O.J. Kwonb a b
Fuel Cell, Energy Materials & Physical Chemistry Lab., Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India Department of Energy and Chemical Engineering, Incheon National University, 12-1, Songdo-dong, Yeonsu-gu, Incheon 406-840, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: rGO-WO3 nanostructures Photoelectrochemical Photocatalysis Hydrogen generation Solar energy harvesting
The synthesis of WO3 plate-like and rGO-WO3 nanostructured catalysts by a new and simple wet chemistry followed by thermal decomposition method is reported. The prepared catalysts were characterized by X-ray diffraction, Fourier transformed infrared spectroscopy, Raman spectroscopy, UV–vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning transmission electron microscopy techniques and Brunauer-Emmett-Teller surface area measurement. The photoelectrochemical properties and photocatalytic degradation of methylene blue by WO3 and rGO-WO3 nanostructured catalysts under simulated solar light, and visible light respectively was investigated. The incorporation of rGO in WO3 decreased the band gap energy from 2.54 to 2.45 eV which also hindered the recombination rate of photogenerated electron-hole pairs and improved the electron transport properties. The plate-like structure of WO3 and rGO-WO3 nanostructured catalysts was observed from FESEM and TEM techniques. 5.3 and 4.2 folds higher photon-to-hydrogen conversion efficiency by rGO-WO3 photoanode at 0.08 and 0.30 V respectively than WO3 photoanode was demonstrated. The photocatalytic activity of WO3 for the degradation of MB was also improved by forming a composite with rGO. The mechanism of the photoelectrochemical and photocatalytic process was discussed. This study provides a simple and scalable pathway to produce highly efficient rGO-WO3 nanostructured photocatalyst for harvesting solar energy efficiently.
1. Introduction The semiconductor photocatalysis is an excellent technique intended for solar energy conversion and environmental remediation [1]. To achieve clean and renewable energy, the direct conversion of solar energy to chemical energy by water splitting using photocatalysis is one of the best ways. The water splitting by solar energy in the presence of a semiconductor catalyst is an artificial photosynthesis process which converts the solar energy into hydrogen (H2) and oxygen (O2). The photoelectrochemical (PEC) cell is one of the best and widely used water splitting devices which involves solar energy collection with water electrolysis [2–4]. The PEC water splitting is a challenging technique, but it has the convenience of easy adoption and in-situ storage facilities. The TiO2 has been broadly used as semiconductor photocatalyst for decontamination of organic pollutants and water splitting applications [2]. However, the demerits such as fast recombination of electron-hole (eˉ-h+) pairs and lack of visible light absorption of TiO2 limit on its efficient utilization [5].
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The tungsten oxide (WO3) is an n-type semiconductor material and has attracted much attention as visible-light-driven photocatalyst because of merits, such as narrow band gap energy (2.4–2.8 eV), deeper valence band (+ 3.1 eV) and superior physicochemical and thermal stability [6–8]. The efficiency of the pure WO3 for the photocatalytic applications can be improved by inhibiting the recombination of eˉ-h+ pairs. The enhancement of the photocatalytic activity of WO3 has been achieved by adopting several methods which include, morphology control [9], semiconductor coupling [10], noble metal deposition [11], metal ion doping [12] and, formation of composites with graphene and/or reduced graphene oxide (rGO) [13]. Among these methods, the formation of composites with graphene and rGO has attracted tremendous interest because of the special properties such as high surface area, high electrical conductivity (106 S cm−1), high carrier mobility (200,000 cm2 V−1 S−1) and, efficient electron (eˉ) collection and transportation [14–16]. The composites formed with graphene such as TiO2-graphene, CdS-graphene, ZnO-graphene, Sr2Ta2O7-graphene and BiOBr-graphene oxide have been reported for photocatalytic
Corresponding author. E-mail address: [email protected] (L. Cindrella).
http://dx.doi.org/10.1016/j.mssp.2017.10.041 Received 18 August 2017; Received in revised form 20 October 2017; Accepted 23 October 2017 1369-8001/ © 2017 Published by Elsevier Ltd.
Materials Science in Semiconductor Processing 74 (2018) 136–146
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2.3. Characterization
applications [17–23]. But, there are few reports available on graphene and rGO-WO3 composites for PEC application [24–26] and photocatalytic decontamination of environmental pollutants [13,27–29] and the composites have some drawbacks like the weak interaction with rGO and involve tedious synthesis procedure. Hence it is very urgent to develop a simple and scalable method for producing rGO-WO3 composites for efficient utilization of solar energy for various applications. To the best of our knowledge, for the first time, we developed a new and simple wet chemistry followed by thermal decomposition method for the synthesis of WO3 plate-like and rGO-WO3 nanostructured catalysts. The prepared catalysts were characterized by various techniques such as X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), Raman spectroscopy, UV–vis diffuse reflectance spectroscopy (UV–vis–DRS), photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and Brunauer-EmmettTeller (BET) surface area measurement. The efficiency of the synthesized catalysts was investigated for photoelectrochemical and photocatalytic degradation of methylene blue (MB) under simulated solar light and visible light respectively. The reduction in the band gap from 2.54 to 2.45 eV was achieved by incorporation of rGO in WO3 which also hindered the recombination of photogenerated eˉ-h+ pairs and improved the electron transport properties. The photon-to-hydrogen conversion efficiency by rGO-WO3 photoanode was improved to 5.3 fold higher than WO3 photoanode. The photocatalytic activity for the degradation of MB was also improved by forming a composite with rGO and WO3. The mechanism of the photoelectrochemical and photocatalytic process was discussed. We demonstrate a simple and scalable pathway to fabricate highly efficient rGO-WO3 nanostructured photocatalyst for efficient conversion of solar energy.
XRD spectra were recorded in the 2θ range of 5° to 80° with a step size of 0.05° and a scan speed of 1° min−1 using Cu Kα radiation (Rigaku Ultima III X-ray diffractometer 40 kV, 30 mA). Raman spectral analysis was carried out using High Resolution “LabRAM Hr800” Raman-LTPL Spectrometer with Ar Laser wavelength of 632.8 nm. FTIR spectra were recorded by “Thermo Scientific (Nicolet iS5)” spectrometer. The FESEM images were captured using “JSM-6700F, JEOL” instrument. The TEM images were captured using “ALOS F200X” instrument with an acceleration voltage of 200 kV. The elements present were mapped using STEM technique. XPS analysis was carried out using “PHI 5000 Versa Probe II” instrument, UV–vis–DRS spectrum was recorded by “Shimadzu UV-2600” UV–visible spectrophotometer equipped with an integrated sphere assembly and BaSO4 as the reflectance reference. The photoluminescence (PL) spectra were recorded using “HORIBA Fluoromax 4CP-TCSPC” spectrophotometer with an excitation wavelength of 325 nm. The specific surface area of the catalysts was calculated using the Brunauer-Emmett-Teller (BET) model by measuring the N2 adsorption-desorption isotherms with “Autosorb-iQ (Quantachrome Instruments, Boynton Beach, Florida, USA)” at 77 K. Before measurement, the samples were activated at 120 °C for 12 h. The Zeta potential of the samples was measured as a function of pH using “Zetasizer Nano ZSP, Malvern”. The pH of the solution was adjusted with diluted HCl solution and the pH measurement was carried out by digital micro controller pH meter (Roy instruments). 2.4. Photoelectrochemical activity evaluation A homemade three-electrode electrochemical cell (Fig. S1) was used to investigate the photoelectrochemical activity of the catalysts. The catalysts coated on ITO substrates, Ag/AgCl electrode and a platinum wire were used as working electrode, reference electrode and counter electrode respectively. The fabrication of working electrode involves, 5 mg of catalysts (WO3 and rGO-WO3) dispersed in 0.5 mL of IPA under sonication for 5 min 50 µl of the dispersion was dropped on ITO substrate in 1 cm2 area and then air-dried. The coating process was repeated once again. The coated substrates were vacuum dried at 65 °C for 6 h. The electrochemical and photoelectrochemical measurements were carried out in 0.1 M Na2SO4 supporting electrolyte solution. The “Oriel® LCS-100™” solar simulator (Newport Corporation, USA, AM 1.5G, 100 mW cm−2) was used as the irradiation source. Electrochemical and photoelectrochemical studies were performed using “CHI608B” electrochemical analyzer.
2. Experimental 2.1. Materials Graphite flake, natural (10 mesh) was procured 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) and isopropyl alcohol (IPA) were procured from Merck Ltd., India. Tungstic acid (H2WO4) was procured from Loba, India. ITO glass substrates (< 10 Ω, 1.1 mm thickness, > 90% transmittance) were procured from Shilpa Enterprises, 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.
2.5. Photocatalytic activity evaluation The photocatalytic activity of the catalysts was investigated for the photocatalytic degradation of MB under visible light irradiation in “Heber Annular Type photo reactor” (Fig. S2). A 500 W tungsten halogen lamp was held in borosilicate immersion jacketed tube, the IR radiation from the light source was cancelled by water circulation in outer jacket of the immersion tube. Highly polished and anodized aluminium reflector was fitted in the inner surface of the reactor hood. In a typical experiment, 15 mg of the catalyst was suspended in 50 mL of 10 mg L−1 MB solution. The suspension was first aerated in the dark for 30 min to reach adsorption-desorption equilibrium and then irradiated under aeration for thorough mixing. At regular time interval of 15 min on irradiation, 3.5 mL of the suspension was collected and then the catalyst was removed by centrifugation. The degradation of MB in the solution was analyzed by measuring the absorbance at 665 nm wavelength with “Aventes UV–vis spectrometer”. Total organic carbon (TOC) content of the MB solution before and after the photocatalytic reaction was measured using “TOC-TN (SHIMADZU CORP., India)” analyzer. For the TOC analysis, 1 mL of the MB solution was diluted to 20 mL with double distilled water.
2.2. Synthesis of rGO-WO3 nanostructures The graphene oxide (GO) was prepared by Hummers method with some modifications [30], and it was reported elsewhere [20]. The GO dispersion in water (1 mg mL−1) was prepared by sonication for 1 h. In a typical procedure, appropriate amount of the GO dispersion was added with 50 mL of water and 0.5 g of H2WO4. The mixture was sonicated for 2 min and stirred for 4 h at 30 °C ( ± 1 °C). Then, the solid product obtained was centrifuged and dried at 70 °C under vacuum. The product was calcinated at 300 °C for 1 h which resulted in rGO-WO3 nanostructures. Without using any special atmosphere or chemical reagents, the GO could be reduced to rGO by heating it at 300 °C for 1 h [31]. The amount of GO dispersion was varied as 0.46, 2.31 and 4.60 mL to get 0.1, 0.5 and 1.0 wt% with respect to 2 mmol of WO3 (equal to ~ 0.46 g) and the resulted rGO-WO3 nanostructures are named as 0.1 GW, 0.5 GW and 1.0 GW respectively. The pure WO3 was prepared by following the above procedure without adding GO dispersion. 137
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Fig. 1. (a) XRD spectra and (b) FT-IR spectra of GO, WO3 and rGO-WO3 nanostructures.
3. Results and discussion
The W–O–W stretching mode in the WO3 network was observed at 710 and 806 cm−1. The WO3 peak intensities in the rGO-WO3 composite are decreased which may be due to the reduction in the particle size of WO3 on rGO. The D and G bands in the spectra of rGO and rGO-WO3 nanostructures appeared at around 1326 and 1590 cm−1 respectively. The D band represents the A1g symmetry of κ-point phonons breathing mode which correspond to the edge and disordered sp3 carbon, and the G band represents the first-order scattering of E2g phonons which are assigned to the ordered sp2 carbon network [35]. It is worth noting that the G band in the rGO-WO3 nanostructures was up-shifted from 1590 to 1605 cm−1 compared to rGO as shown in Fig. 2(b), which confirms the chemical doping of carbon materials in WO3 [28]. This chemical doping suggests the formation of a real composite rather than simple mixing of rGO and WO3. The structure of the WO3 was analyzed by FESEM and TEM techniques as shown in Figs. 3 and 4 respectively. The plate-like structure of WO3 was irregular, but rGO-WO3 nanostructures showed well ordered plate-like structure, which revealed that the rGO favored the growth of crystals. The plate-like structure of WO3 was observed in the TEM image (Fig. 4(a)) and it was distributed on rGO in 1.0 GW (Fig. 4(b)). The possible components of rGO and WO3 in 1.0 GW are indicated in Fig. 4(b). The average length and width of the plates were calculated from the TEM images and found to be 390 and 150 nm respectively for WO3, and 230 and 110 nm respectively for rGO-WO3 nanostructures. The elements of W, O and C present in 1.0 GW investigated by EDAX technique are shown in Fig. S3. The SEAD pattern shown in Fig. S4 suggest the crystalline nature of 1.0 GW catalyst. The STEM and corresponding elemental mapping images of rGO-WO3 nanostructures further confirm the presence of the elements, Ti, O and C in the composite as shown in Fig. 4(c–f). The elemental composition of WO3 and rGO-WO3 catalysts and their corresponding valence state present on the surface were studied using XPS technique, and the results are shown in Fig. 5(a–d). The elements are indexed in the survey scan spectra of the catalysts (Fig. 5(a)). The survey spectrum confirms the presence of the elements, C, O and W in the catalysts. No other element was observed implying the high purity of the samples. The high-resolution spectrum of W 4f shows two peaks at 35.8 and 37.9 eV (Fig. 5(b)) corresponding to the two characteristic W 4f7/2 and W 4f5/2 peaks of W6+ oxidation state of WO3 [14]. The high-resolution O 1 s spectrum can be fitted with two peaks as shown in Fig. 5(c). The peaks at 530.5 eV are attributed to oxygen bound to tungsten, while the peaks at 531.7 eV are related to the hydroxyl group [36]. The peak at 284.8 eV in the high-resolution spectrum of C 1s is assigned to the graphitic C–C bond (Fig. 5(d)), and the other peaks corresponding to the C-O, C˭O and O-C˭O bonds respectively at about 286, 287 and 289 eV were not observed [13]. The absence of these C-O, C˭O and O-C˭O bonds may be due to the very low amount of rGO
3.1. Characterization The XRD patterns of GO, WO3 and rGO-WO3 nanostructure catalysts are shown in Fig. 1(a). For GO, the peak of (002) plane appears at 11.1o which clearly suggests the oxidation of graphite to GO thereby increasing the interlayer (d-spacing) distance from 0.342 to 0.796 nm [32,33]. The XRD peaks of WO3 are well matched with the monoclinic WO3 phase structure, which are indexed according to the JCPDS card no. 89-4476, indicating the high crystallinity of WO3. The XRD pattern of rGO-WO3 nanostructures was similar to that of WO3. Any peak related to rGO was not observed which may be due to the low content or relatively low diffraction intensity of rGO. The crystallite size of WO3 and rGO-WO3 nanostructures was calculated by the Scherrer formula using full width at half maximum intensity (FWHM) of the (200) plane of WO3 at 2θ = 24.4° and the values are given in Table 1. The FT-IR spectra of the GO, WO3 and rGO-WO3 catalysts are shown in Fig. 1(b). The characteristic functional groups of C˭O stretching vibration, water molecular OH bending vibration, C-OH bending and stretching vibrations and epoxy C-O-C or C-O stretching vibrations corresponding to GO are observed at 1725, 1630, 1390, 1227 and 1054 cm−1 respectively [31]. The broad peak in the range of 500–1000 cm−1 is attributed to the ν(O–W–O) stretching mode in the WO3 [34]. In the FT-IR spectra of rGO-WO3 nanostructures, the functional group of C˭O and C-OH, corresponding to GO are not observed which indicated the reduction of GO to rGO. The peaks at 3430 cm−1 in all the FT-IR spectra are corresponding to the stretching vibrations of OH groups of adsorbed water molecules. The Raman spectral analysis was used to investigate the interaction between the rGO and WO3 as given in Fig. 2. The peaks obtained in the Raman spectrum as shown in Fig. 2(a) confirms the monoclinic phase of WO3 and are well matched with the literature reports [28]. The peaks at 135 and 184 cm−1 correspond to the (W2O2)n chains. The bending modes of W-O-W bridged oxide were observed at 270 and 325 cm−1.
Table 1 Crystallite size, band gaps, Rct values and rate constant for the degradation of MB of WO3 and rGO-WO3 nanostructures. Catalyst
WO3 0.1 GW 0.5 GW 1.0 GW
Crystallite size (nm)
16 11 13 18
Band gap (eV)
2.54 2.64 2.51 2.45
Rct (kΩ) Dark
Light
20.1 27.4 23.1 52.8
19.2 25.7 21.3 32.9
Rate constant (× 10-3 min-1)
8.30 9.15 11.24 11.29
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Fig. 2. (a) Raman spectra of rGO, WO3 and rGO-WO3 nanostructure and (b) expanded Raman spectra of section indicated in (a).
absorption function and energy axis at zero absorption [37]. The calculated band gap of the bare WO3 was found to be 2.54 eV. On introduction of rGO by 0.1, 0.5 and 1.0 wt%, the band gap of WO3 varied from 2.64 to 2.45 eV (Table 1) which suggests the strong interaction between WO3 and rGO affecting the band gap of WO3. The BET specific surface area of the WO3 and rGO-WO3 nanostructure was measured using N2 adsorption-desorption isotherms as shown in Fig. 7(a). The isotherm curves exhibit small hysteresis loops which can be ascribed to type IV isotherm, and indicating the presence of mesopores (2–50 nm) [38]. The pore size distribution curve (Fig. 7(b)) indicates the pore size of ~ 3.5 nm which further confirms the presence of mesopores in the catalysts. The BET surface area of WO3 and 1.0 GW was calculated to be 20.66 and 175.62 m2/g respectively. The drastic increase in the BET surface area of 1.0 GW than WO3 is attributed to the rGO in the rGO-WO3 nanostructure. Thus, the high surface area of 1.0 GW can provide more photocatalytic reaction sites for the reactant to be adsorbed on the surface of the catalyst and also the efficient separation of eˉ-h+ pairs can be achieved. The zeta potentials of WO3 and rGO-WO3 nanostructure at different pH are shown in Fig. 8. The isoelectric point (IEP), defined as the pH at which zeta
present in the 1.0 GW catalyst. In the high-resolution XPS spectra, the peak intensities of O 1s and C 1s of 1.0 GW were higher compared than pure WO3 catalyst which confirms the presence of rGO in the rGO-WO3 nanostructure catalyst. On the other hand, the high-resolution W 4f, C 1s and O 1s XPS spectra of 1.0 GW show a slight shift in lower binding energy than that of the pure WO3 catalyst which may be due to the interaction of rGO with WO3. The light absorption property and the band gap of the WO3 and rGO-WO3 catalysts were investigated by UV–vis–DRS spectral technique, and the spectra are shown in Fig. 6(a). The % reflectance of rGOWO3 nanostructures decreases compared to that of pure WO3 which suggests the increased absorption of incident light. The absorption edge of 0.1 GW catalyst was shifted to lower wavelength region than that of pure WO3. However, the red-shift in the absorption band-edge was observed in 0.5 GW and 1.0 GW catalysts which facilitate the absorption of longer wavelength by narrowing the band gap of the composite. The Tauc plot of [F(R)hν]1/2 (transformed Kubelka-Munk function) vs hν (energy of light) is shown in Fig. 6(b) where F(R) = (1-R)2/2R and R is the reflectance. The band gap of the WO3 and rGO-WO3 nanostructures were calculated from the intercepts of the tangent drawn at
Fig. 3. FESEM images of (a) WO3, (b) 0.1 GW, (c) 0.5 GW and 1.0 GW catalysts.
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Fig. 4. TEM images of (a) WO3 and (b) 1.0 GW respectively, (c) STEM image of 1.0 GW and its corresponding elemental mapping of (d) carbon, (e) tungsten and (f) oxygen.
surface defects [42,43]. The dark current of rGO-WO3 photoanodes (Fig. 9(a)) are very high compared to WO3 photoanode and increased on increasing the GO content which clearly indicates that the electrical property of the WO3 can be improved by incorporating the rGO. The photocurrent by the WO3 photoanode was not observed up to 0.25 V bias potential. On further increase in the bias potential, the photocurrent increased significantly. In contrast, the increased photocurrent was observed in the case of rGO-WO3 photoanodes even from the 0 V bias potential. These results show that rGO not only collects the photoelectrons efficiently but also transfers the collected photoelectrons to the external circuit. The photon-to-hydrogen conversion efficiency versus bias potential was evaluated using the following equation [44,45],
potential is zero, appeared at pH of 0.80 and 0.85 for WO3 and 1.0 GW respectively. These IEP values are comparable with the literature value reported to be in the range of 0.4–1.0 [39].
3.2. Photoelectrochemical property The photoelectrochemical property of the photoanode i.e. WO3 and rGO-WO3 nanostructures coated on ITO substrate was studied by linear sweep voltammetry (LSV) under dark and illumination conditions (Fig. 9(a)). The dark LSV curve of WO3 and rGO-WO3 photoanodes show a broad oxidation peak at about 0.2 V which corresponds to the reduction of W(VI) to W(V) in WO3 and thus resulting in the formation of HxWO3 (tungsten bronzes) [40]. The rGO-WO3 photoanodes show the oxidation peak also at 0.1 V which may be due to the plate-like structure of WO3 on rGO which is well matched with the literature reports [29,41]. There are some minor peaks at higher potential regions which suggest the intrinsic property of the semiconductor related to the
η=
I (1. 23 − Vbias) Jlight
(1)
Where I (= Ilight - Idark) is the photocurrent density in mA cm−2, 1.23 is 140
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Fig. 5. (a) XPS survey and high resolution XPS spectra of (b) W 4f, (c) O 1s and (d) C 1s of WO3 and rGO-WO3 nanostructure.
the efficiency of 1.0 GW compared to WO3 photoanode was about 5.3 and 4.2 times at the bias potentials of 0.08 and 0.30 V respectively. This increased efficiency is indicative of the efficient collection and utilization of photocurrent by rGO. The photocurrent response of the WO3 and rGO-WO3 photoanodes over time was recorded by chronoamperometry (with the light on/off cycles of 50 s) at 1.23 V bias potential technique as shown in Fig. 9(c). The 0.1 GW and 0.5 GW photoanodes show almost same and increased photocurrent than that of WO3 photoanode. However, the 1.0 GW photoanode shows higher photocurrent than all other photoanodes. The photocurrent by rGO-WO3 photoanodes was slightly decreased till two light on/off cycles which may be due to the slight leaching of active materials at the surface of the electrode. On the successive light on/off
the theoretical potential required for water decomposition, Vbias is the applied external bias potential, and Jlight is the light intensity (100 mW cm−2). The calculated photon-to-hydrogen conversion efficiency by the WO3 and rGO-WO3 photoanodes was shown in Fig. 9(b). The efficiency of WO3 photoanode increased beyond 0.30 V bias potential and reached a maximum of 1.4% at 0.85 V and then decreased. The efficiency of 0.1 GW photoanode increased after 0.34 V and reached around 1.5% at 0.72 V and the 0.5 GW photoanode showed peaks of 2.1%, 2.3% and 1.4% efficiency at 0.08, 0.38 and 0.90 V respectively. For 1.0 GW photoanode, the photon-to-hydrogen conversion efficiency was very high, of about 5.3% at a low bias potential of 0.08 V, and the second maximum efficiency of 4.2% at 0.30 V bias potential. The increase in
Fig. 6. (a) UV–vis–DRS spectra and (b) Tauc plot of WO3 and rGO-WO3 nanostructures.
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Fig. 7. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of WO3 and rGO-WO3 nanostructure.
spectrum of MB were decreased without any shift in the peak position. Moreover, no new peak was observed in the absorption spectrum of MB. The same trend of changes in the absorption peaks of MB was also observed in the presence of other catalysts which clearly indicate the degradation of MB. The TOC of MB solution before irradiation was found to be 4.451 ppm. The TOC value was drastically decreased to 0.963, 0.788, 0.657 and 0.658 ppm in the presence of WO3, 0.1 GW, 0.5 GW and 1.0 GW respectively under visible light irradiation for 60 min. These results suggest that the degradation of MB into CO2 and weak mineral acids and thereby decreasing the toxicity of the solution [46,47]. The percentage degradation of MB was calculated using the equation as given below,
%degradation = Fig. 8. Zeta potential of WO3 and rGO-WO3 nanostructure as a function of pH.
(Ao − At ) × 100 Ao
(2)
Where Ao is the initial absorbance of MB, At is the absorbance of MB at time t. The % degradation of MB as a function of irradiation time by various catalysts is shown in Fig. 10(b). The % degradation of MB observed in the presence of rGO-WO3 catalysts within 60 min was higher than that of WO3 which is attributed to the high efficiency of the catalysts. For comparison, the blank experiment without any catalyst was conducted to investigate the photolysis of MB. The result shows insignificant degradation of less than 10% under the given experimental condition. The WO3 shows 35% adsorption of MB at 30 min in dark. The adsorption of MB was increased in the presence of rGO-WO3 nanostructures and reached maximum of around 54% in the presence of 1.0 GW. The increased adsorption capacity of rGO-WO3 nanostructures is attributed to the high surface area of the catalysts. Also, the IEP of the rGO-WO3 nanostructure favors more electrostatic attraction between MB and the surface of the catalyst. The negative charge on the surface of the catalysts, i.e. the low IEP of 0.80 and 0.85 is the ideal characteristic for the adsorption of cationic dyes such as MB [48–50]. Therefore, the combination of more active sites from increased surface area and high adsorption capacity enhances the photocatalytic activity of the rGO-WO3 nanostructures. The pseudo-first order kinetics model will usually fit the photocatalytic degradation of organic compounds. The rate constant for the degradation of MB can be derived using the Langmuir–Hinshelwood kinetic equation [51] which is given below,
cycles, the photocurrent was very stable. The charge transfer and intrinsic electronic properties of the WO3 and rGO-WO3 photoanodes were investigated by electrochemical impedance spectroscopy (EIS). The Nyquist plots of WO3 and rGO-WO3 photoanodes at 1.23 V bias potential are shown in Fig. 9(d) and the charge transfer resistance (Rct) can be deduced from the diameter of the semicircle in the plot. The experimental data were fitted to an equivalent electrical circuit which is shown in the inset of Fig. 9(d). The calculated charge transfer resistance (Rct) in dark and light of WO3 and rGO-WO3 photoanodes are given in Table 1. The Rct value of rGO-WO3 photoanodes in the dark was higher than that of WO3 photoanode which reveals the presence of rGO in rGO-WO3 nanostructures, and hinders the charge transfer. The Rct values of WO3 and rGO-WO3 photoanodes under illumination decreased than under dark which shows that both WO3 and rGO-WO3 nanostructures are active under the solar light illumination. The decrease in the Rct value of WO3, 0.1 GW, 0.5 GW and 1.0 GW photoanodes under illumination than under dark are 0.9, 1.7, 1.8 and 19.9 kΩ respectively. The huge decrease in the Rct value of 1.0 GW photoanode clearly confirms the efficient separation of photogenerated eˉ/h+ pairs and effortless interface charge transfer in this case.
3.3. Photocatalytic property
ln The photocatalytic activity of the WO3 and rGO-WO3 nanostructured catalysts was investigated for the degradation of MB under visible light irradiation. The temporal UV–vis absorption spectral changes of MB under irradiation in the presence of the 1.0 GW catalyst are shown in Fig. 10(a). On irradiation, the peaks in the absorption
Co = kapp t C
(3) −1
Where, Co and C are the concentration of MB (mol L ) at initial and at time t respectively, and kapp is the apparent rate constant (min−1). The rate constant for the degradation of MB was calculated from the slope of the plot, ln(Co/C) versus time as shown in Fig. 10(c) and found 142
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Fig. 9. (a) Linear sweep voltammetry curve, (b) Photon-to-hydrogen conversion efficiency, (c) Transient photocurrent response and (d) Nyquist plot of WO3 and rGO-WO3 photoanodes.
to be in the range of 8.30 and 11.29 × 10−3 min−1 and the values are given in the Table 1. The reusability of the 1.0 GW catalyst for the degradation of MB was studied and the results are shown in Fig. 10(d). The photocatalytic efficiency was not decreased significantly even at fourth cycle which suggests high photocatalytic stability of the rGOWO3 nanostructure.
photoanode was very rapid as shown in Fig. 9(c). The delay time to reach the minimum current after light off condition was calculated and found to be around 25.3 s which further proves that rGO can efficiently hinder the recombination of e--h+ pairs and act as a good collector of the photoelectrons. The photoelectrochemical hydrogen and oxygen evolution is schematically represented in Fig. 12(a). The conduction band potential of WO3 is lower than the potential of H+/H2, and hence the hydrogen cannot be generated. In the photoelectrochemical cell, the photo-generated electrons are collected from the conduction band of WO3 through the substrate to the external circuit, and thus the hydrogen can be produced at the Pt counter electrode while O2 produced at the WO3 working electrode by reacting h+ with the water molecules. When the rGO-WO3 composite is formed, the Schottky barrier is created, and the electron transfer occurs from rGO to the interface state and attains equilibrium, and thus results in the equalization of Fermi levels of rGO and interface states [24]. As the Fermi level in WO3 rises on illumination [53], the photo-generated electron can be transferred from the conduction band of WO3 to rGO. The Schottky barrier formed in rGO and WO3 would play a detrimental role in the transfer of electrons from rGO to WO3, resulting in lowering the recombination rate of e--h+ pairs. As stated earlier, rGO can favor efficient collection and transport of the photo-generated electrons because of thermal reduction of GO to rGO. The thermal reduction of GO to rGO involves the transition of the insulator to semiconductor and finally to semimetal and the band gap of the rGO approaches to zero with a work function of 4.61–4.71 eV [54,55]. The photocatalytic decomposition of MB is schematically illustrated in Fig. 12(b) and the reactions can be represented as follows.
3.4. Mechanism of photoelectrochemical and photocatalytic conversion In general, the photocatalytic process initiated by illumination from a light source involves generation, separation and consumption of electron (e-) - hole (h+) pairs. The very important step to achieving high efficiency is the efficient separation of e--h+ pairs by inhibiting the recombination of e--h+ pairs. The PL spectra were recorded to study the recombination of e--h+ pairs (Fig. 11(a)). The PL intensity of the WO3 significantly decreased on incorporating the rGO which reveals the reduction in the recombination rate of e--h+ pairs. The PL intensity of WO3 at around 420 nm decreases whereas a new peak at around 445 nm emerges in rGO-WO3 nanostructures which may be due to the transfer of excited e- from the conduction band of WO3 to rGO [29]. Also, there are several minor peaks in the PL spectra which are attributed to the surface defects or oxygen vacancy present in the catalysts [27,52]. The PL response shows that efficient separation of photogenerated e--h+ pairs by surface defects or oxygen vacancies occurs followed by efficient transfer of excited e- to the rGO. The photoelectrochemical process involves three main steps as shown in Fig. 11(b). On illumination, (i) e--h+ pairs are generated, (ii) they attributed a constant current by attaining the equilibrium state of generation and recombination of e--h+ pairs and (iii) exhibited recombination of e--h+ pairs rapidly when the light is off. On turning off the light, the photocurrent of the 1.0 GW photoanode showed very slow decay (Fig. 11(b)) whereas, the decay of photocurrent by WO3
WO3 + hν → WO3 (e− + h+)
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(4)
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Fig. 10. (a) Temporal UV–vis absorption spectral changes for the degradation of MB by 1.0 GW, (b) % degradation, (c) plot of ln(Co/C) versus time and (d) reusability of 1.0 GW for the degradation of MB.
e− + rGO → rGO (e−)
(5)
rGO (e−)+O2 → rGO+•
O2−
•O2− + H+ → HOO • HOO
•+ e−
H2 O2 +
h+
e−
+
H+
+ H2 O → OH
(6) (7)
→ H2 O2
(8)
•+ OH−
(9)
→ OH
•+ H+
(10) (11)
h+ + OH− → OH •
OH •,HOO •,H2 O2 & •
As shown in Fig. 12(b), the electrons are excited to the conduction band leaving holes in the valence band of WO3 under visible light irradiation (Eq. (4)). The electrons will be transferred to the rGO and react with adsorbed oxygen (O2) molecules resulting in the •O2- radicals [56] (Eqs. (5) and (6)). The •O2- radicals will first generate HOO• and secondly H2O2 by reacting with electrons and finally, produce OH• radicals [57] (Eqs. (7)–(9)). On the other side, the holes in the valence band of WO3 will react with adsorbed water or OH- molecules and generate OH• radicals (Eqs. (10) and (11)). The active radicals formed such as •O2-, HOO•, H2O2, and OH• attacked the MB molecules and resulted in degraded products (Eq. (12)).
O2−
+ MB → Degradedproducts
(12)
Fig. 11. (a) Photoluminescence spectra of WO3 and rGO-WO3 nanostructures and (b) representative transient photocurrent response of 1.0 GW photoanode.
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4. Conclusion In summary, the WO3 plate-like and rGO-WO3 nanostructures were prepared using a new and simple wet chemistry followed by thermal decomposition method for photoelectrochemical and photocatalytic applications. The crystallite size of WO3 and rGO-WO3 nanostructures was found to be in the range of 11 and 18 nm. The reduction of GO to rGO was confirmed by the absence of C˭O and C-OH peaks in FTIR spectra of rGO-WO3 nanostructures. The chemical doping of carbon materials in WO3 and formation of the composite were revealed by upshifting of G band in Raman spectra. The plate-like structure of WO3 was revealed by FESEM and TEM techniques. The XPS analysis confirmed the interaction of rGO with WO3, and the presence of C, O and W elements. On incorporation of rGO, the band gap of WO3 reduced from 2.54 to 2.45 eV which hindered the recombination rate of photogenerated eˉ/h+ pairs and improved the electron transport properties. The increased photon-to-hydrogen conversion efficiency of 5.3 and 4.2 folds at the lower bias potential of 0.08 and 0.30 V respectively by 1.0 GW photoanode compared to WO3 photoanode was demonstrated. The increased and stable photocurrent by rGO-WO3 photoanode was also verified. The drastic decrease in the Rct values under illumination attributed to the effortless interface charge transfer and efficient separation of photogenerated eˉ/h+ pairs in rGO-WO3 photoanodes. The efficient photocatalytic degradation of MB by the prepared catalysts was achieved due to the combined effect of the active sites from increased surface area and high adsorption capacity of the rGO-WO3 nanostructures. The mechanisms of the photoelectrochemical and photocatalytic processes were illustrated. This study provides a simple pathway to produce scalable and highly efficient rGO-WO3 nanostructured photocatalyst for harvesting solar energy efficiently. Acknowledgment Financial support by CSIR, India through the Project no. 02 (0193)/ 14/EMR-II is thankfully acknowledged. The authors thank NITT for the instrumental facilities. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2017.10.041. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Charge transfer and intrinsic electronic properties of rGO-WO3 nanostructures for efficient photoelectrochemical and photocatalytic applications
MARK
⁎
S. Prabhua, S. Manikumara, L. Cindrellaa, , O.J. Kwonb a b
Fuel Cell, Energy Materials & Physical Chemistry Lab., Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India Department of Energy and Chemical Engineering, Incheon National University, 12-1, Songdo-dong, Yeonsu-gu, Incheon 406-840, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: rGO-WO3 nanostructures Photoelectrochemical Photocatalysis Hydrogen generation Solar energy harvesting
The synthesis of WO3 plate-like and rGO-WO3 nanostructured catalysts by a new and simple wet chemistry followed by thermal decomposition method is reported. The prepared catalysts were characterized by X-ray diffraction, Fourier transformed infrared spectroscopy, Raman spectroscopy, UV–vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning transmission electron microscopy techniques and Brunauer-Emmett-Teller surface area measurement. The photoelectrochemical properties and photocatalytic degradation of methylene blue by WO3 and rGO-WO3 nanostructured catalysts under simulated solar light, and visible light respectively was investigated. The incorporation of rGO in WO3 decreased the band gap energy from 2.54 to 2.45 eV which also hindered the recombination rate of photogenerated electron-hole pairs and improved the electron transport properties. The plate-like structure of WO3 and rGO-WO3 nanostructured catalysts was observed from FESEM and TEM techniques. 5.3 and 4.2 folds higher photon-to-hydrogen conversion efficiency by rGO-WO3 photoanode at 0.08 and 0.30 V respectively than WO3 photoanode was demonstrated. The photocatalytic activity of WO3 for the degradation of MB was also improved by forming a composite with rGO. The mechanism of the photoelectrochemical and photocatalytic process was discussed. This study provides a simple and scalable pathway to produce highly efficient rGO-WO3 nanostructured photocatalyst for harvesting solar energy efficiently.
1. Introduction The semiconductor photocatalysis is an excellent technique intended for solar energy conversion and environmental remediation [1]. To achieve clean and renewable energy, the direct conversion of solar energy to chemical energy by water splitting using photocatalysis is one of the best ways. The water splitting by solar energy in the presence of a semiconductor catalyst is an artificial photosynthesis process which converts the solar energy into hydrogen (H2) and oxygen (O2). The photoelectrochemical (PEC) cell is one of the best and widely used water splitting devices which involves solar energy collection with water electrolysis [2–4]. The PEC water splitting is a challenging technique, but it has the convenience of easy adoption and in-situ storage facilities. The TiO2 has been broadly used as semiconductor photocatalyst for decontamination of organic pollutants and water splitting applications [2]. However, the demerits such as fast recombination of electron-hole (eˉ-h+) pairs and lack of visible light absorption of TiO2 limit on its efficient utilization [5].
⁎
The tungsten oxide (WO3) is an n-type semiconductor material and has attracted much attention as visible-light-driven photocatalyst because of merits, such as narrow band gap energy (2.4–2.8 eV), deeper valence band (+ 3.1 eV) and superior physicochemical and thermal stability [6–8]. The efficiency of the pure WO3 for the photocatalytic applications can be improved by inhibiting the recombination of eˉ-h+ pairs. The enhancement of the photocatalytic activity of WO3 has been achieved by adopting several methods which include, morphology control [9], semiconductor coupling [10], noble metal deposition [11], metal ion doping [12] and, formation of composites with graphene and/or reduced graphene oxide (rGO) [13]. Among these methods, the formation of composites with graphene and rGO has attracted tremendous interest because of the special properties such as high surface area, high electrical conductivity (106 S cm−1), high carrier mobility (200,000 cm2 V−1 S−1) and, efficient electron (eˉ) collection and transportation [14–16]. The composites formed with graphene such as TiO2-graphene, CdS-graphene, ZnO-graphene, Sr2Ta2O7-graphene and BiOBr-graphene oxide have been reported for photocatalytic
Corresponding author. E-mail address: [email protected] (L. Cindrella).
http://dx.doi.org/10.1016/j.mssp.2017.10.041 Received 18 August 2017; Received in revised form 20 October 2017; Accepted 23 October 2017 1369-8001/ © 2017 Published by Elsevier Ltd.
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2.3. Characterization
applications [17–23]. But, there are few reports available on graphene and rGO-WO3 composites for PEC application [24–26] and photocatalytic decontamination of environmental pollutants [13,27–29] and the composites have some drawbacks like the weak interaction with rGO and involve tedious synthesis procedure. Hence it is very urgent to develop a simple and scalable method for producing rGO-WO3 composites for efficient utilization of solar energy for various applications. To the best of our knowledge, for the first time, we developed a new and simple wet chemistry followed by thermal decomposition method for the synthesis of WO3 plate-like and rGO-WO3 nanostructured catalysts. The prepared catalysts were characterized by various techniques such as X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), Raman spectroscopy, UV–vis diffuse reflectance spectroscopy (UV–vis–DRS), photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and Brunauer-EmmettTeller (BET) surface area measurement. The efficiency of the synthesized catalysts was investigated for photoelectrochemical and photocatalytic degradation of methylene blue (MB) under simulated solar light and visible light respectively. The reduction in the band gap from 2.54 to 2.45 eV was achieved by incorporation of rGO in WO3 which also hindered the recombination of photogenerated eˉ-h+ pairs and improved the electron transport properties. The photon-to-hydrogen conversion efficiency by rGO-WO3 photoanode was improved to 5.3 fold higher than WO3 photoanode. The photocatalytic activity for the degradation of MB was also improved by forming a composite with rGO and WO3. The mechanism of the photoelectrochemical and photocatalytic process was discussed. We demonstrate a simple and scalable pathway to fabricate highly efficient rGO-WO3 nanostructured photocatalyst for efficient conversion of solar energy.
XRD spectra were recorded in the 2θ range of 5° to 80° with a step size of 0.05° and a scan speed of 1° min−1 using Cu Kα radiation (Rigaku Ultima III X-ray diffractometer 40 kV, 30 mA). Raman spectral analysis was carried out using High Resolution “LabRAM Hr800” Raman-LTPL Spectrometer with Ar Laser wavelength of 632.8 nm. FTIR spectra were recorded by “Thermo Scientific (Nicolet iS5)” spectrometer. The FESEM images were captured using “JSM-6700F, JEOL” instrument. The TEM images were captured using “ALOS F200X” instrument with an acceleration voltage of 200 kV. The elements present were mapped using STEM technique. XPS analysis was carried out using “PHI 5000 Versa Probe II” instrument, UV–vis–DRS spectrum was recorded by “Shimadzu UV-2600” UV–visible spectrophotometer equipped with an integrated sphere assembly and BaSO4 as the reflectance reference. The photoluminescence (PL) spectra were recorded using “HORIBA Fluoromax 4CP-TCSPC” spectrophotometer with an excitation wavelength of 325 nm. The specific surface area of the catalysts was calculated using the Brunauer-Emmett-Teller (BET) model by measuring the N2 adsorption-desorption isotherms with “Autosorb-iQ (Quantachrome Instruments, Boynton Beach, Florida, USA)” at 77 K. Before measurement, the samples were activated at 120 °C for 12 h. The Zeta potential of the samples was measured as a function of pH using “Zetasizer Nano ZSP, Malvern”. The pH of the solution was adjusted with diluted HCl solution and the pH measurement was carried out by digital micro controller pH meter (Roy instruments). 2.4. Photoelectrochemical activity evaluation A homemade three-electrode electrochemical cell (Fig. S1) was used to investigate the photoelectrochemical activity of the catalysts. The catalysts coated on ITO substrates, Ag/AgCl electrode and a platinum wire were used as working electrode, reference electrode and counter electrode respectively. The fabrication of working electrode involves, 5 mg of catalysts (WO3 and rGO-WO3) dispersed in 0.5 mL of IPA under sonication for 5 min 50 µl of the dispersion was dropped on ITO substrate in 1 cm2 area and then air-dried. The coating process was repeated once again. The coated substrates were vacuum dried at 65 °C for 6 h. The electrochemical and photoelectrochemical measurements were carried out in 0.1 M Na2SO4 supporting electrolyte solution. The “Oriel® LCS-100™” solar simulator (Newport Corporation, USA, AM 1.5G, 100 mW cm−2) was used as the irradiation source. Electrochemical and photoelectrochemical studies were performed using “CHI608B” electrochemical analyzer.
2. Experimental 2.1. Materials Graphite flake, natural (10 mesh) was procured 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) and isopropyl alcohol (IPA) were procured from Merck Ltd., India. Tungstic acid (H2WO4) was procured from Loba, India. ITO glass substrates (< 10 Ω, 1.1 mm thickness, > 90% transmittance) were procured from Shilpa Enterprises, 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.
2.5. Photocatalytic activity evaluation The photocatalytic activity of the catalysts was investigated for the photocatalytic degradation of MB under visible light irradiation in “Heber Annular Type photo reactor” (Fig. S2). A 500 W tungsten halogen lamp was held in borosilicate immersion jacketed tube, the IR radiation from the light source was cancelled by water circulation in outer jacket of the immersion tube. Highly polished and anodized aluminium reflector was fitted in the inner surface of the reactor hood. In a typical experiment, 15 mg of the catalyst was suspended in 50 mL of 10 mg L−1 MB solution. The suspension was first aerated in the dark for 30 min to reach adsorption-desorption equilibrium and then irradiated under aeration for thorough mixing. At regular time interval of 15 min on irradiation, 3.5 mL of the suspension was collected and then the catalyst was removed by centrifugation. The degradation of MB in the solution was analyzed by measuring the absorbance at 665 nm wavelength with “Aventes UV–vis spectrometer”. Total organic carbon (TOC) content of the MB solution before and after the photocatalytic reaction was measured using “TOC-TN (SHIMADZU CORP., India)” analyzer. For the TOC analysis, 1 mL of the MB solution was diluted to 20 mL with double distilled water.
2.2. Synthesis of rGO-WO3 nanostructures The graphene oxide (GO) was prepared by Hummers method with some modifications [30], and it was reported elsewhere [20]. The GO dispersion in water (1 mg mL−1) was prepared by sonication for 1 h. In a typical procedure, appropriate amount of the GO dispersion was added with 50 mL of water and 0.5 g of H2WO4. The mixture was sonicated for 2 min and stirred for 4 h at 30 °C ( ± 1 °C). Then, the solid product obtained was centrifuged and dried at 70 °C under vacuum. The product was calcinated at 300 °C for 1 h which resulted in rGO-WO3 nanostructures. Without using any special atmosphere or chemical reagents, the GO could be reduced to rGO by heating it at 300 °C for 1 h [31]. The amount of GO dispersion was varied as 0.46, 2.31 and 4.60 mL to get 0.1, 0.5 and 1.0 wt% with respect to 2 mmol of WO3 (equal to ~ 0.46 g) and the resulted rGO-WO3 nanostructures are named as 0.1 GW, 0.5 GW and 1.0 GW respectively. The pure WO3 was prepared by following the above procedure without adding GO dispersion. 137
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Fig. 1. (a) XRD spectra and (b) FT-IR spectra of GO, WO3 and rGO-WO3 nanostructures.
3. Results and discussion
The W–O–W stretching mode in the WO3 network was observed at 710 and 806 cm−1. The WO3 peak intensities in the rGO-WO3 composite are decreased which may be due to the reduction in the particle size of WO3 on rGO. The D and G bands in the spectra of rGO and rGO-WO3 nanostructures appeared at around 1326 and 1590 cm−1 respectively. The D band represents the A1g symmetry of κ-point phonons breathing mode which correspond to the edge and disordered sp3 carbon, and the G band represents the first-order scattering of E2g phonons which are assigned to the ordered sp2 carbon network [35]. It is worth noting that the G band in the rGO-WO3 nanostructures was up-shifted from 1590 to 1605 cm−1 compared to rGO as shown in Fig. 2(b), which confirms the chemical doping of carbon materials in WO3 [28]. This chemical doping suggests the formation of a real composite rather than simple mixing of rGO and WO3. The structure of the WO3 was analyzed by FESEM and TEM techniques as shown in Figs. 3 and 4 respectively. The plate-like structure of WO3 was irregular, but rGO-WO3 nanostructures showed well ordered plate-like structure, which revealed that the rGO favored the growth of crystals. The plate-like structure of WO3 was observed in the TEM image (Fig. 4(a)) and it was distributed on rGO in 1.0 GW (Fig. 4(b)). The possible components of rGO and WO3 in 1.0 GW are indicated in Fig. 4(b). The average length and width of the plates were calculated from the TEM images and found to be 390 and 150 nm respectively for WO3, and 230 and 110 nm respectively for rGO-WO3 nanostructures. The elements of W, O and C present in 1.0 GW investigated by EDAX technique are shown in Fig. S3. The SEAD pattern shown in Fig. S4 suggest the crystalline nature of 1.0 GW catalyst. The STEM and corresponding elemental mapping images of rGO-WO3 nanostructures further confirm the presence of the elements, Ti, O and C in the composite as shown in Fig. 4(c–f). The elemental composition of WO3 and rGO-WO3 catalysts and their corresponding valence state present on the surface were studied using XPS technique, and the results are shown in Fig. 5(a–d). The elements are indexed in the survey scan spectra of the catalysts (Fig. 5(a)). The survey spectrum confirms the presence of the elements, C, O and W in the catalysts. No other element was observed implying the high purity of the samples. The high-resolution spectrum of W 4f shows two peaks at 35.8 and 37.9 eV (Fig. 5(b)) corresponding to the two characteristic W 4f7/2 and W 4f5/2 peaks of W6+ oxidation state of WO3 [14]. The high-resolution O 1 s spectrum can be fitted with two peaks as shown in Fig. 5(c). The peaks at 530.5 eV are attributed to oxygen bound to tungsten, while the peaks at 531.7 eV are related to the hydroxyl group [36]. The peak at 284.8 eV in the high-resolution spectrum of C 1s is assigned to the graphitic C–C bond (Fig. 5(d)), and the other peaks corresponding to the C-O, C˭O and O-C˭O bonds respectively at about 286, 287 and 289 eV were not observed [13]. The absence of these C-O, C˭O and O-C˭O bonds may be due to the very low amount of rGO
3.1. Characterization The XRD patterns of GO, WO3 and rGO-WO3 nanostructure catalysts are shown in Fig. 1(a). For GO, the peak of (002) plane appears at 11.1o which clearly suggests the oxidation of graphite to GO thereby increasing the interlayer (d-spacing) distance from 0.342 to 0.796 nm [32,33]. The XRD peaks of WO3 are well matched with the monoclinic WO3 phase structure, which are indexed according to the JCPDS card no. 89-4476, indicating the high crystallinity of WO3. The XRD pattern of rGO-WO3 nanostructures was similar to that of WO3. Any peak related to rGO was not observed which may be due to the low content or relatively low diffraction intensity of rGO. The crystallite size of WO3 and rGO-WO3 nanostructures was calculated by the Scherrer formula using full width at half maximum intensity (FWHM) of the (200) plane of WO3 at 2θ = 24.4° and the values are given in Table 1. The FT-IR spectra of the GO, WO3 and rGO-WO3 catalysts are shown in Fig. 1(b). The characteristic functional groups of C˭O stretching vibration, water molecular OH bending vibration, C-OH bending and stretching vibrations and epoxy C-O-C or C-O stretching vibrations corresponding to GO are observed at 1725, 1630, 1390, 1227 and 1054 cm−1 respectively [31]. The broad peak in the range of 500–1000 cm−1 is attributed to the ν(O–W–O) stretching mode in the WO3 [34]. In the FT-IR spectra of rGO-WO3 nanostructures, the functional group of C˭O and C-OH, corresponding to GO are not observed which indicated the reduction of GO to rGO. The peaks at 3430 cm−1 in all the FT-IR spectra are corresponding to the stretching vibrations of OH groups of adsorbed water molecules. The Raman spectral analysis was used to investigate the interaction between the rGO and WO3 as given in Fig. 2. The peaks obtained in the Raman spectrum as shown in Fig. 2(a) confirms the monoclinic phase of WO3 and are well matched with the literature reports [28]. The peaks at 135 and 184 cm−1 correspond to the (W2O2)n chains. The bending modes of W-O-W bridged oxide were observed at 270 and 325 cm−1.
Table 1 Crystallite size, band gaps, Rct values and rate constant for the degradation of MB of WO3 and rGO-WO3 nanostructures. Catalyst
WO3 0.1 GW 0.5 GW 1.0 GW
Crystallite size (nm)
16 11 13 18
Band gap (eV)
2.54 2.64 2.51 2.45
Rct (kΩ) Dark
Light
20.1 27.4 23.1 52.8
19.2 25.7 21.3 32.9
Rate constant (× 10-3 min-1)
8.30 9.15 11.24 11.29
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Fig. 2. (a) Raman spectra of rGO, WO3 and rGO-WO3 nanostructure and (b) expanded Raman spectra of section indicated in (a).
absorption function and energy axis at zero absorption [37]. The calculated band gap of the bare WO3 was found to be 2.54 eV. On introduction of rGO by 0.1, 0.5 and 1.0 wt%, the band gap of WO3 varied from 2.64 to 2.45 eV (Table 1) which suggests the strong interaction between WO3 and rGO affecting the band gap of WO3. The BET specific surface area of the WO3 and rGO-WO3 nanostructure was measured using N2 adsorption-desorption isotherms as shown in Fig. 7(a). The isotherm curves exhibit small hysteresis loops which can be ascribed to type IV isotherm, and indicating the presence of mesopores (2–50 nm) [38]. The pore size distribution curve (Fig. 7(b)) indicates the pore size of ~ 3.5 nm which further confirms the presence of mesopores in the catalysts. The BET surface area of WO3 and 1.0 GW was calculated to be 20.66 and 175.62 m2/g respectively. The drastic increase in the BET surface area of 1.0 GW than WO3 is attributed to the rGO in the rGO-WO3 nanostructure. Thus, the high surface area of 1.0 GW can provide more photocatalytic reaction sites for the reactant to be adsorbed on the surface of the catalyst and also the efficient separation of eˉ-h+ pairs can be achieved. The zeta potentials of WO3 and rGO-WO3 nanostructure at different pH are shown in Fig. 8. The isoelectric point (IEP), defined as the pH at which zeta
present in the 1.0 GW catalyst. In the high-resolution XPS spectra, the peak intensities of O 1s and C 1s of 1.0 GW were higher compared than pure WO3 catalyst which confirms the presence of rGO in the rGO-WO3 nanostructure catalyst. On the other hand, the high-resolution W 4f, C 1s and O 1s XPS spectra of 1.0 GW show a slight shift in lower binding energy than that of the pure WO3 catalyst which may be due to the interaction of rGO with WO3. The light absorption property and the band gap of the WO3 and rGO-WO3 catalysts were investigated by UV–vis–DRS spectral technique, and the spectra are shown in Fig. 6(a). The % reflectance of rGOWO3 nanostructures decreases compared to that of pure WO3 which suggests the increased absorption of incident light. The absorption edge of 0.1 GW catalyst was shifted to lower wavelength region than that of pure WO3. However, the red-shift in the absorption band-edge was observed in 0.5 GW and 1.0 GW catalysts which facilitate the absorption of longer wavelength by narrowing the band gap of the composite. The Tauc plot of [F(R)hν]1/2 (transformed Kubelka-Munk function) vs hν (energy of light) is shown in Fig. 6(b) where F(R) = (1-R)2/2R and R is the reflectance. The band gap of the WO3 and rGO-WO3 nanostructures were calculated from the intercepts of the tangent drawn at
Fig. 3. FESEM images of (a) WO3, (b) 0.1 GW, (c) 0.5 GW and 1.0 GW catalysts.
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Fig. 4. TEM images of (a) WO3 and (b) 1.0 GW respectively, (c) STEM image of 1.0 GW and its corresponding elemental mapping of (d) carbon, (e) tungsten and (f) oxygen.
surface defects [42,43]. The dark current of rGO-WO3 photoanodes (Fig. 9(a)) are very high compared to WO3 photoanode and increased on increasing the GO content which clearly indicates that the electrical property of the WO3 can be improved by incorporating the rGO. The photocurrent by the WO3 photoanode was not observed up to 0.25 V bias potential. On further increase in the bias potential, the photocurrent increased significantly. In contrast, the increased photocurrent was observed in the case of rGO-WO3 photoanodes even from the 0 V bias potential. These results show that rGO not only collects the photoelectrons efficiently but also transfers the collected photoelectrons to the external circuit. The photon-to-hydrogen conversion efficiency versus bias potential was evaluated using the following equation [44,45],
potential is zero, appeared at pH of 0.80 and 0.85 for WO3 and 1.0 GW respectively. These IEP values are comparable with the literature value reported to be in the range of 0.4–1.0 [39].
3.2. Photoelectrochemical property The photoelectrochemical property of the photoanode i.e. WO3 and rGO-WO3 nanostructures coated on ITO substrate was studied by linear sweep voltammetry (LSV) under dark and illumination conditions (Fig. 9(a)). The dark LSV curve of WO3 and rGO-WO3 photoanodes show a broad oxidation peak at about 0.2 V which corresponds to the reduction of W(VI) to W(V) in WO3 and thus resulting in the formation of HxWO3 (tungsten bronzes) [40]. The rGO-WO3 photoanodes show the oxidation peak also at 0.1 V which may be due to the plate-like structure of WO3 on rGO which is well matched with the literature reports [29,41]. There are some minor peaks at higher potential regions which suggest the intrinsic property of the semiconductor related to the
η=
I (1. 23 − Vbias) Jlight
(1)
Where I (= Ilight - Idark) is the photocurrent density in mA cm−2, 1.23 is 140
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Fig. 5. (a) XPS survey and high resolution XPS spectra of (b) W 4f, (c) O 1s and (d) C 1s of WO3 and rGO-WO3 nanostructure.
the efficiency of 1.0 GW compared to WO3 photoanode was about 5.3 and 4.2 times at the bias potentials of 0.08 and 0.30 V respectively. This increased efficiency is indicative of the efficient collection and utilization of photocurrent by rGO. The photocurrent response of the WO3 and rGO-WO3 photoanodes over time was recorded by chronoamperometry (with the light on/off cycles of 50 s) at 1.23 V bias potential technique as shown in Fig. 9(c). The 0.1 GW and 0.5 GW photoanodes show almost same and increased photocurrent than that of WO3 photoanode. However, the 1.0 GW photoanode shows higher photocurrent than all other photoanodes. The photocurrent by rGO-WO3 photoanodes was slightly decreased till two light on/off cycles which may be due to the slight leaching of active materials at the surface of the electrode. On the successive light on/off
the theoretical potential required for water decomposition, Vbias is the applied external bias potential, and Jlight is the light intensity (100 mW cm−2). The calculated photon-to-hydrogen conversion efficiency by the WO3 and rGO-WO3 photoanodes was shown in Fig. 9(b). The efficiency of WO3 photoanode increased beyond 0.30 V bias potential and reached a maximum of 1.4% at 0.85 V and then decreased. The efficiency of 0.1 GW photoanode increased after 0.34 V and reached around 1.5% at 0.72 V and the 0.5 GW photoanode showed peaks of 2.1%, 2.3% and 1.4% efficiency at 0.08, 0.38 and 0.90 V respectively. For 1.0 GW photoanode, the photon-to-hydrogen conversion efficiency was very high, of about 5.3% at a low bias potential of 0.08 V, and the second maximum efficiency of 4.2% at 0.30 V bias potential. The increase in
Fig. 6. (a) UV–vis–DRS spectra and (b) Tauc plot of WO3 and rGO-WO3 nanostructures.
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Fig. 7. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of WO3 and rGO-WO3 nanostructure.
spectrum of MB were decreased without any shift in the peak position. Moreover, no new peak was observed in the absorption spectrum of MB. The same trend of changes in the absorption peaks of MB was also observed in the presence of other catalysts which clearly indicate the degradation of MB. The TOC of MB solution before irradiation was found to be 4.451 ppm. The TOC value was drastically decreased to 0.963, 0.788, 0.657 and 0.658 ppm in the presence of WO3, 0.1 GW, 0.5 GW and 1.0 GW respectively under visible light irradiation for 60 min. These results suggest that the degradation of MB into CO2 and weak mineral acids and thereby decreasing the toxicity of the solution [46,47]. The percentage degradation of MB was calculated using the equation as given below,
%degradation = Fig. 8. Zeta potential of WO3 and rGO-WO3 nanostructure as a function of pH.
(Ao − At ) × 100 Ao
(2)
Where Ao is the initial absorbance of MB, At is the absorbance of MB at time t. The % degradation of MB as a function of irradiation time by various catalysts is shown in Fig. 10(b). The % degradation of MB observed in the presence of rGO-WO3 catalysts within 60 min was higher than that of WO3 which is attributed to the high efficiency of the catalysts. For comparison, the blank experiment without any catalyst was conducted to investigate the photolysis of MB. The result shows insignificant degradation of less than 10% under the given experimental condition. The WO3 shows 35% adsorption of MB at 30 min in dark. The adsorption of MB was increased in the presence of rGO-WO3 nanostructures and reached maximum of around 54% in the presence of 1.0 GW. The increased adsorption capacity of rGO-WO3 nanostructures is attributed to the high surface area of the catalysts. Also, the IEP of the rGO-WO3 nanostructure favors more electrostatic attraction between MB and the surface of the catalyst. The negative charge on the surface of the catalysts, i.e. the low IEP of 0.80 and 0.85 is the ideal characteristic for the adsorption of cationic dyes such as MB [48–50]. Therefore, the combination of more active sites from increased surface area and high adsorption capacity enhances the photocatalytic activity of the rGO-WO3 nanostructures. The pseudo-first order kinetics model will usually fit the photocatalytic degradation of organic compounds. The rate constant for the degradation of MB can be derived using the Langmuir–Hinshelwood kinetic equation [51] which is given below,
cycles, the photocurrent was very stable. The charge transfer and intrinsic electronic properties of the WO3 and rGO-WO3 photoanodes were investigated by electrochemical impedance spectroscopy (EIS). The Nyquist plots of WO3 and rGO-WO3 photoanodes at 1.23 V bias potential are shown in Fig. 9(d) and the charge transfer resistance (Rct) can be deduced from the diameter of the semicircle in the plot. The experimental data were fitted to an equivalent electrical circuit which is shown in the inset of Fig. 9(d). The calculated charge transfer resistance (Rct) in dark and light of WO3 and rGO-WO3 photoanodes are given in Table 1. The Rct value of rGO-WO3 photoanodes in the dark was higher than that of WO3 photoanode which reveals the presence of rGO in rGO-WO3 nanostructures, and hinders the charge transfer. The Rct values of WO3 and rGO-WO3 photoanodes under illumination decreased than under dark which shows that both WO3 and rGO-WO3 nanostructures are active under the solar light illumination. The decrease in the Rct value of WO3, 0.1 GW, 0.5 GW and 1.0 GW photoanodes under illumination than under dark are 0.9, 1.7, 1.8 and 19.9 kΩ respectively. The huge decrease in the Rct value of 1.0 GW photoanode clearly confirms the efficient separation of photogenerated eˉ/h+ pairs and effortless interface charge transfer in this case.
3.3. Photocatalytic property
ln The photocatalytic activity of the WO3 and rGO-WO3 nanostructured catalysts was investigated for the degradation of MB under visible light irradiation. The temporal UV–vis absorption spectral changes of MB under irradiation in the presence of the 1.0 GW catalyst are shown in Fig. 10(a). On irradiation, the peaks in the absorption
Co = kapp t C
(3) −1
Where, Co and C are the concentration of MB (mol L ) at initial and at time t respectively, and kapp is the apparent rate constant (min−1). The rate constant for the degradation of MB was calculated from the slope of the plot, ln(Co/C) versus time as shown in Fig. 10(c) and found 142
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Fig. 9. (a) Linear sweep voltammetry curve, (b) Photon-to-hydrogen conversion efficiency, (c) Transient photocurrent response and (d) Nyquist plot of WO3 and rGO-WO3 photoanodes.
to be in the range of 8.30 and 11.29 × 10−3 min−1 and the values are given in the Table 1. The reusability of the 1.0 GW catalyst for the degradation of MB was studied and the results are shown in Fig. 10(d). The photocatalytic efficiency was not decreased significantly even at fourth cycle which suggests high photocatalytic stability of the rGOWO3 nanostructure.
photoanode was very rapid as shown in Fig. 9(c). The delay time to reach the minimum current after light off condition was calculated and found to be around 25.3 s which further proves that rGO can efficiently hinder the recombination of e--h+ pairs and act as a good collector of the photoelectrons. The photoelectrochemical hydrogen and oxygen evolution is schematically represented in Fig. 12(a). The conduction band potential of WO3 is lower than the potential of H+/H2, and hence the hydrogen cannot be generated. In the photoelectrochemical cell, the photo-generated electrons are collected from the conduction band of WO3 through the substrate to the external circuit, and thus the hydrogen can be produced at the Pt counter electrode while O2 produced at the WO3 working electrode by reacting h+ with the water molecules. When the rGO-WO3 composite is formed, the Schottky barrier is created, and the electron transfer occurs from rGO to the interface state and attains equilibrium, and thus results in the equalization of Fermi levels of rGO and interface states [24]. As the Fermi level in WO3 rises on illumination [53], the photo-generated electron can be transferred from the conduction band of WO3 to rGO. The Schottky barrier formed in rGO and WO3 would play a detrimental role in the transfer of electrons from rGO to WO3, resulting in lowering the recombination rate of e--h+ pairs. As stated earlier, rGO can favor efficient collection and transport of the photo-generated electrons because of thermal reduction of GO to rGO. The thermal reduction of GO to rGO involves the transition of the insulator to semiconductor and finally to semimetal and the band gap of the rGO approaches to zero with a work function of 4.61–4.71 eV [54,55]. The photocatalytic decomposition of MB is schematically illustrated in Fig. 12(b) and the reactions can be represented as follows.
3.4. Mechanism of photoelectrochemical and photocatalytic conversion In general, the photocatalytic process initiated by illumination from a light source involves generation, separation and consumption of electron (e-) - hole (h+) pairs. The very important step to achieving high efficiency is the efficient separation of e--h+ pairs by inhibiting the recombination of e--h+ pairs. The PL spectra were recorded to study the recombination of e--h+ pairs (Fig. 11(a)). The PL intensity of the WO3 significantly decreased on incorporating the rGO which reveals the reduction in the recombination rate of e--h+ pairs. The PL intensity of WO3 at around 420 nm decreases whereas a new peak at around 445 nm emerges in rGO-WO3 nanostructures which may be due to the transfer of excited e- from the conduction band of WO3 to rGO [29]. Also, there are several minor peaks in the PL spectra which are attributed to the surface defects or oxygen vacancy present in the catalysts [27,52]. The PL response shows that efficient separation of photogenerated e--h+ pairs by surface defects or oxygen vacancies occurs followed by efficient transfer of excited e- to the rGO. The photoelectrochemical process involves three main steps as shown in Fig. 11(b). On illumination, (i) e--h+ pairs are generated, (ii) they attributed a constant current by attaining the equilibrium state of generation and recombination of e--h+ pairs and (iii) exhibited recombination of e--h+ pairs rapidly when the light is off. On turning off the light, the photocurrent of the 1.0 GW photoanode showed very slow decay (Fig. 11(b)) whereas, the decay of photocurrent by WO3
WO3 + hν → WO3 (e− + h+)
143
(4)
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Fig. 10. (a) Temporal UV–vis absorption spectral changes for the degradation of MB by 1.0 GW, (b) % degradation, (c) plot of ln(Co/C) versus time and (d) reusability of 1.0 GW for the degradation of MB.
e− + rGO → rGO (e−)
(5)
rGO (e−)+O2 → rGO+•
O2−
•O2− + H+ → HOO • HOO
•+ e−
H2 O2 +
h+
e−
+
H+
+ H2 O → OH
(6) (7)
→ H2 O2
(8)
•+ OH−
(9)
→ OH
•+ H+
(10) (11)
h+ + OH− → OH •
OH •,HOO •,H2 O2 & •
As shown in Fig. 12(b), the electrons are excited to the conduction band leaving holes in the valence band of WO3 under visible light irradiation (Eq. (4)). The electrons will be transferred to the rGO and react with adsorbed oxygen (O2) molecules resulting in the •O2- radicals [56] (Eqs. (5) and (6)). The •O2- radicals will first generate HOO• and secondly H2O2 by reacting with electrons and finally, produce OH• radicals [57] (Eqs. (7)–(9)). On the other side, the holes in the valence band of WO3 will react with adsorbed water or OH- molecules and generate OH• radicals (Eqs. (10) and (11)). The active radicals formed such as •O2-, HOO•, H2O2, and OH• attacked the MB molecules and resulted in degraded products (Eq. (12)).
O2−
+ MB → Degradedproducts
(12)
Fig. 11. (a) Photoluminescence spectra of WO3 and rGO-WO3 nanostructures and (b) representative transient photocurrent response of 1.0 GW photoanode.
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4. Conclusion In summary, the WO3 plate-like and rGO-WO3 nanostructures were prepared using a new and simple wet chemistry followed by thermal decomposition method for photoelectrochemical and photocatalytic applications. The crystallite size of WO3 and rGO-WO3 nanostructures was found to be in the range of 11 and 18 nm. The reduction of GO to rGO was confirmed by the absence of C˭O and C-OH peaks in FTIR spectra of rGO-WO3 nanostructures. The chemical doping of carbon materials in WO3 and formation of the composite were revealed by upshifting of G band in Raman spectra. The plate-like structure of WO3 was revealed by FESEM and TEM techniques. The XPS analysis confirmed the interaction of rGO with WO3, and the presence of C, O and W elements. On incorporation of rGO, the band gap of WO3 reduced from 2.54 to 2.45 eV which hindered the recombination rate of photogenerated eˉ/h+ pairs and improved the electron transport properties. The increased photon-to-hydrogen conversion efficiency of 5.3 and 4.2 folds at the lower bias potential of 0.08 and 0.30 V respectively by 1.0 GW photoanode compared to WO3 photoanode was demonstrated. The increased and stable photocurrent by rGO-WO3 photoanode was also verified. The drastic decrease in the Rct values under illumination attributed to the effortless interface charge transfer and efficient separation of photogenerated eˉ/h+ pairs in rGO-WO3 photoanodes. The efficient photocatalytic degradation of MB by the prepared catalysts was achieved due to the combined effect of the active sites from increased surface area and high adsorption capacity of the rGO-WO3 nanostructures. The mechanisms of the photoelectrochemical and photocatalytic processes were illustrated. This study provides a simple pathway to produce scalable and highly efficient rGO-WO3 nanostructured photocatalyst for harvesting solar energy efficiently. Acknowledgment Financial support by CSIR, India through the Project no. 02 (0193)/ 14/EMR-II is thankfully acknowledged. The authors thank NITT for the instrumental facilities. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2017.10.041. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental
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