Journal of Materials Science & Technology  2020 , 38 (0): 7-18 https://doi.org/10.1016/j.jmst.2019.08.020

Research Article

Investigation of solar-induced photoelectrochemical water splitting and photocatalytic dye removal activities of camphor sulfonic acid doped polyaniline -WO3- MWCNT ternary nanocomposite

Mir Ghasem Hosseiniab*, Pariya Yardani Sefidia, Ahmet Musap Mertc, Solen Kinayyigitc

aDepartment of Physical Chemistry, Electrochemistry Research Laboratory, University of Tabriz, Tabriz, Iran
bEngineering Faculty, Department of Materials Science and Nanotechnology, Near East University, 99138, Nicosia, North Cyprus, Mersin 10, Turkey
cLaboratory of Nanocatalysis and Clean Energy Technologies, Institute of Nanotechnology, Gebze Technical University, 41400, Kocaeli, Turkey

Corresponding authors:   ∗Corresponding author at: Department of Physical Chemistry, Electrochemistry Research Laboratory, University of Tabriz, Tabriz, Iran. E-mail address: mg-hosseini@tabrizu.ac.ir (M.G. Hosseini).

Received: 2019-03-30

Revised:  2019-07-20

Accepted:  2019-08-7

Online:  2020-02-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

The camphor sulfonic acid doped polyaniline-WO3-multiwall carbon nanotube (CSA PANI-WO3-CNT) ternary nanocomposite was synthesized during in-situ oxidative polymerization and characterized by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM), and Energy-dispersive X-ray spectroscopy (EDS). The application of CSA PANI-WO3-CNT ternary nanocomposite was investigated as the photocatalyst in the degradation of methylene blue dye (MB) and as the noble metal-free photoanode in photoelectrochemical water splitting under solar light irradiation. The degradation percentage of MB dye after 60 min illumination by CSA PANI-WO3-CNT ternary nanocomposite reached 91.40% which was higher than that of pure WO3 (43.45%), pure CSA PANI (48.4%) and CSA PANI-WO3 binary nanocomposite (85.15%). The photocurrent density of indium tin oxide (ITO)/CSA PANI-WO3-CNT photoanode obtained 0.81 mA/cm2 at 1.23 V vs. reversible hydrogen electrode under illumination which was 1.27, 2.13, and 4.26 times higher than that of the ITO/CSA PANI-WO3 (0.64 mA/cm2), ITO/pure CSA PANI (0.38 mA/cm2), and ITO/pure WO3 (0.19 mA/cm2). Also, the applied bias photon-to-current efficiency (ABPE) of ITO/CSA PANI-WO3-CNT was obtained 0.11% which showed two-fold, four-fold, and five-fold enhancements compared to the ITO/CSA PANI-WO3, ITO/CSA PANI, and ITO/WO3, respectively. The electrochemical impedance spectroscopy, as well as the Mott-Schottky results, confirmed the better photoelectrocatalytic activity of ITO/CSA PANI-WO3-CNT in comparison with ITO/WO3, ITO/CSA PANI, and ITO/CSA PANI-WO3. The observed improvement in the photocatalytic and photoelectrocatalytic performances of WO3 in the presence of CSA PANI is due to the formation of type -II heterojunction between WO3 and CSA PANI which allows the separation of charge carriers easier and faster. On the other hand, MWCNT addition to the CSA PANI-WO3 nanocomposite provided the conducting substrate for efficient interfacial charge separation as well as transferring.

Keywords: Camphor sulfonic acid ; Polyaniline ; WO3 ; Photocatalyst ; Water splitting

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Mir Ghasem Hosseini, Pariya Yardani Sefidi, Ahmet Musap Mert, Solen Kinayyigit. Investigation of solar-induced photoelectrochemical water splitting and photocatalytic dye removal activities of camphor sulfonic acid doped polyaniline -WO3- MWCNT ternary nanocomposite[J]. Journal of Materials Science & Technology, 2020, 38(0): 7-18 https://doi.org/10.1016/j.jmst.2019.08.020

1. Introduction

The undesirable influences of hazardous pollutants on the environment and the global demand for using renewable and environmentally clean energy sources necessitate the development of appropriate ways for controlling environmental pollution and also energy challenges [[1], [2], [3], [4], [5], [6]]. Heterogeneous solar photocatalytic removal of various contaminants is one of the beneficial methods that belong to the advanced oxidation processes (AOPs) [[6], [7], [8]]. In this process, the semiconductor photocatalyst material excited during the irradiation of solar light and produced electrons and holes which can degrade the toxic compounds. There are a lot of advantages for solar-induced photocatalytic degradation such as the utilization of cost-effective and eco-friendly solar energy, complete elimination of substances via photogenerated charge carriers and reusability of catalysts [[9], [10], [11]]. On the other side, applying solar photoelectrochemical cells (PECs) for producing chemical fuels such as hydrogen via water splitting using photoactive materials under sunlight irradiation attracted considerable attention from both academic and industrial points of view [[12], [13], [14], [15], [16]]. In this regard, nanostructured semiconductors have been extensively used for photoassisted degradation of organic dyes and solar-driven photoelectrochemical water splitting. Several metal oxides such as TiO2, ZnO, Fe2O3, and WO3 have been widely considered as efficient semiconductors for photocatalytic and photoelectrocatalytic applications [[17], [18], [19], [20], [21]]. Among various metal oxide semiconductors, WO3 is an appropriate candidate for energy conversion and purification of wastewaters due to its unique electrical and chemical features including stability, low toxicity and good capability for photogenerated electrons and holes separation. Nevertheless, the large bandgap (2.5-2.8 eV) and low conduction band level of WO3 limit its usage [[22], [23], [24]]. One of the promising strategies to overcome the mentioned disadvantages is the formation of WO3-based nanocomposites. In this way, the wide range of light can be absorbed and the separation of produced photoexcitons facilitated which is prolonging the recombination of electrons and holes improving the photocatalytic as well as photoelectrocatalytic efficiencies. In recent years, researchers focused on conductive polymer-metal oxide nanocomposites as efficient photoelectrocatalysts. The conductive polymers with conjugated electrons have been used for sensitization of metal oxides due to their high absorption efficiency, the high mobility of charge carriers and good stability [[25], [26], [27]]. Besides conductive polymers, carbon-based materials such as single-wall carbon nanotubes (SWCNTs), multiwall carbon nanotubes (MWCNTs), graphene oxide (GO), reduced graphene oxide (rGO) and carbon quantum dots (CQDs) are suitable options for hybridization of semiconductors which accelerate the transferring of carriers and promotion of the nanocomposites’ performances [[28], [29], [30], [31]]. To the best of our knowledge, there are few investigations on the application of organic acid doped conductive polymer nanocomposites with WO3 and carbon-based compounds for the photocatalytic and photoelectrocatalytic purposes. In this study, the camphor sulfonic acid doped polyaniline- WO3- multiwall carbon nanotubes (CSA PANI-WO3-CNT) ternary nanocomposite was synthesized via in situ chemical oxidative polymerization. The performance of as-prepared nanocomposite was evaluated as a photocatalyst for degradation of methylene blue (MB) and also as the metal-free photoanode for photoelectrochemical water splitting under solar irradiation. The pure WO3 NPs, pure CSA PANI, and CSA PANI-WO3 binary nanocomposite were used for evaluating the influence of CSA PANI and CNT on the optical, photocatalytic and photoelectrocatalytic performances of WO3.

2. Experimental

2.1. Synthesis of camphor sulfonic acid doped polyaniline-WO3-CNT ternary nanocomposite

The camphor sulfonic acid doped polyaniline nanocomposite with WO3 and CNT (CSA PANI-WO3-CNT) was prepared by in situ oxidative polymerization. At first, 0.25 g camphor sulfonic acid as dopant and surfactant (CSA, Fluka purity 90%) and 0.15 g WO3 NPs (US Research Nanomaterials, Inc., 99.95%) were dispersed uniformly in 50 mL distilled water during 60 min stirring at the ambient temperature. At the end of the first stage, 0.15 g of multiwall carbon nanotube with COOH group (COOH-MWCNT, Neutrino, purity > 95%) was added to the homogenous dispersion of camphor sulfonic acid and WO3 and stirred for 30 min before adding aniline. Then, 1.2 mL aniline monomer (Fluka purity 99.5%) was added and stirred for 2 h in an ice bath (0-4°C). Afterward, 12.5 mL of ammonium persulfate as the initiator (2.25 g, APS, Merck) was added drop by drop and stirred for 4 h. Finally, the obtained nanocomposite was filtered and rinsed with distilled water. The synthesis process has been schematically illustrated in Fig. 1. The synthesis procedure of camphor sulfonic acid doped polyaniline-WO3 (CSA PANI-WO3) binary nanocomposite is similar to ternary nanocomposite without adding MWCNT.

Fig. 1.   Schematic illustration of CSA PANI-WO3-CNT synthesis process.

2.2. Photocatalytic studies

The photocatalytic activities of the pure WO3 NPs, pure CSA PANI, CSA PANI-WO3 binary nanocomposite, and CSA PANI-WO3-CNT ternary nanocomposite were investigated by measuring the degradation of methylene blue (MB) dye at room temperature. 50 mg of each photocatalyst was dispersed in an aqueous solution of MB (10 ppm), separately. At the first step, the reaction solutions were magnetically stirred in the dark for 30 min to obtain absorption/desorption equilibrium. Then, the solutions were irradiated under 300 W Xenon lamp (AM1.5 G filter for simulated solar light) with continuous magnetic stirring. At different time intervals, the UV-Vis spectra were recorded using a T80+ UV-vis spectrophotometer (PG Instruments, Ltd) in the 500-800 nm range.

2.3. Photoelectrochemical water splitting studies

Photoelectrochemical measurements were conducted with Origa Flex-OGA 01A Potentiostat/Galvanostat using the thin films of pure WO3, pure CSA PANI, CSA PANI-WO3 and CSA PANI-WO3-CNT on indium tin oxide conductive glasses (ITO, resistance < 15 Ω/sq, thickness: 1.1 mm, transmittance > 84%) as the photoanode, saturated calomel electrode (SCE) as the reference electrode and Pt plate as the counter electrode, respectively, in 0.1 M Na2SO4 solution. Before the coating, ITO substrates were sonicated for 5 min in detergent solution, acetone, and isopropanol, respectively and dried for 24 h. For preparing the photoanodes, 0.05 g of pure WO3, pure CSA PANI, CSA PANI-WO3, and CSA PANI-WO3-CNT nanocomposites were dispersed in 2.5 mL isopropanol and sonicated for 1 h and spin coated on ITO glasses at 2000 rpm for 30 s. Finally, the prepared photoanodes were dried at 60°C for 24 h. The photoelectrochemical tests were carried out in the dark and under the illumination of Xenon lamp (300 W with AM1.5 G filter). The power density was calibrated at 100 mW/cm2. The illumination area was 1 cm2. The linear sweep voltammetry (LSV) was measured at -0.25 to 0.95 V with 0.01 V/s scan rate. The values of potentials were converted to reversible hydrogen electrode (RHE) using (ERHE = ESCE + 0.059 pH + 0.241) equation. The electrochemical impedance spectroscopy (EIS) test was done in the frequency range of 100 kHz-100 mHz at 0.6 V vs. SCE with an AC amplitude of 10 mV. The Mott Schottky (M-S) analysis was performed in the -0.8 - 0.8 V vs. SCE at the fixed frequency of 1 kHz.

2.4. Characterization

The Fourier transform infrared (FTIR) spectroscopy was done with Perkin-Elmer spectrometer in KBr medium (4000-450 cm-1). The Raman spectroscopy was performed on Teksan, Takram P50COR10 Raman spectrophotometer in the 200-3500 cm-1 with excitation wavenumber of 532 nm. The X-ray diffraction (XRD) was carried out by Philips PW1730 with Cu radiation in the 2θ range of 10°-60°. The X-ray photoelectron spectroscopy (XPS) analysis was carried out by Thermo Scientific K-α X-ray Photoelectron Spectrometer. Monochromatized Al-Kα radiation was used. The morphology of compounds was evaluated by field emission scanning electron microscopy (FESEM, TESCAN MIRA 3) and Transmission electron microscopy (TEM, LEO 906 E (100 kV)). The optical absorption spectra were obtained by double-beam UV-vis spectrophotometer (Specord 250, Analytik Jena).

3. Results and discussion

3.1. Characterization

The FTIR spectra of pure WO3 NPs, CSA PANI-WO3 binary nanocomposite and CSA PANI-WO3-CNT ternary nanocomposite were illustrated in Fig. 2. For the pure WO3 NPs, the bands at 732 cm-1 and 815 cm-1 are related to O—W—O stretching mode [32]. It can be seen from Fig. 2(b) that the characteristic peaks of CSA PANI display at 3213 cm-1 (N—H stretching of secondary amine), 2920 cm-1 and 2850 cm-1 (C—H stretching of aromatic rings), 1577 cm-1 (C═N quinoid ring), 1503 cm-1 (C═C benzenoid ring), 1300 cm-1 (C—N stretching) and 1041 cm-1 (SO3- of camphor sulfonic acid dopant) indicating the formation of CSA PANI-WO3 nanocomposite [33]. It is noted that the vibrations of WO3 NPs shifted to higher wavenumbers (732 cm-1 to 750 cm-1 and 815 cm-1 to 823 cm-1) due to polaron - polaron interactions between the WO3 nanoparticles and CSA PANI polymer network.

Fig. 2.   FTIR spectra of (a) pure WO3, (b) CSA PANI-WO3 and (c) CSA PANI-WO3-CNT.

According to Fig. 2(c), besides the characteristic peaks of WO3 NPs and CSA PANI, the peaks at 3440 cm-1 and 1742 cm-1 are ascribed to O—H and C═O stretching vibrations of COOH-MWCNT, respectively, which confirmed the formation of the ternary nanocomposite of multiwall carbon nanotubes with CSA PANI and WO3 NPs.

The pure WO3 NPs, binary and ternary nanocomposites were further characterized by Raman spectroscopy (Fig. 3(a-c)). The peaks at 268 cm-1 and 318 cm-1 are attributed to the O—W—O bending vibrations. Also, the O—W—O stretching vibration modes can be observed at 710 cm-1 and 804 cm-1 [34]. The characteristic peaks of WO3 NPs are observed in all spectra with shifting to higher wavenumbers. The interactions of WO3 NPs with CSA PANI affect the electron densities which resulted in shifting to higher wavenumbers. Besides the peaks of WO3 NPs, the peaks of CSA PANI can be seen at 1382 cm-1 and 1571 cm-1 which are assigned to the C═N and C═C stretching vibrations, respectively [35]. In the case of CSA PANI-WO3-CNT ternary nanocomposite (Fig. 3(c)), two characteristic peaks of the multiwall carbon nanotubes are observed at 1398 cm-1 (D-band, sp3 carbon) and 1576 cm-1 (G-band, sp2 carbon) which overlapped with CSA PANI peaks. Also, the peak at 2750 cm-1 (Gʹ band) is attributed to the overtone of the D band [36].

Fig. 3.   Raman spectra of (a) pure WO3, (b) CSA PANI-WO3 and (c) CSA PANI-WO3-CNT.

Fig. 4 presents the XRD patterns of pure WO3, pure CSA PANI, CSA PANI-WO3, and CSA PANI-WO3-CNT nanocomposites. The diffraction peaks of pure WO3 NPs are viewed at 2θ values of 23.47°, 24.07°, 24.67°, 27.04°, 29.07°, 34.02°, 42.12°, 47.67°, 48.72°, 50.27°, and 56.22° which correspond to (002), (020), (200), (120), (112), (202), (222), (002), (400), (140) and (420) crystal planes of WO3 [37]. The XRD pattern of WO3 NPs matches well with monoclinic WO3 (JCPDS data card No. 43-1035). The peaks at 21° and 26° are related to (010) and (200) planes of CSA PANI [38]. The peaks of CSA PANI and WO3 NPs are seen at the XRD pattern of CSA PANI-WO3 binary composite (Fig. 4(c)). The presence of the small peak at 2θ = 26.2° (002) plane confirmed the successful formation of CSA PANI-WO3-CNT nanocomposite. For better identification the peak of CNT, the 26°-26.4° range were magnified in Fig. 4. It can be seen that the presence of CSA PANI and MWCNT does not influence the crystal structure of WO3 NPs.

Fig. 4.   XRD patterns of (a) pure WO3, (b) pure CSA PANI, (c) CSA PANI-WO3 and (d) CSA PANI-WO3-CNT.

The XPS analysis was used for further characterization of CSA PANI-WO3-CNT ternary nanocomposite. The XPS survey spectrum and core level scans of C 1s, O 1s, N 1s, and W 4f were shown in Fig. 5. The chemical binding energies at 37.08, 168, 285, 399, and 531 eV belonged to W 4f, S 2p, C 1s, N 1s, and O 1s, respectively (Fig. 5(a)). According to Fig. 5(b), five peaks can be seen at 284.04, 284.68, 285.65, 286.24 and 286.76 eV which are related to C = C (sp2), C-C/C-H (sp3), C-N/C═N, C-O, and C═O, respectively. Also, the N 1s main peak was dissociated into four peaks: ═N— (imine) at 398.43 eV, —NH (amine) at 399.62 eV, ═NH+ (protonated imine) at 401.13 eV and -NH2+ (protonated amine) at 402.16 eV (Fig. 5(c)). As shown in Fig. 5(d), the W 4f7/2 and W 4f5/2 peaks were located at 35.56 eV and 38.7 eV, respectively. The XPS results confirmed the successful formation of the composite.

Fig. 5.   XPS analysis of CSA PANI-WO3-CNT (a), survey spectrum (b), C 1s (c) N 1s and (d) W 4f core levels.

The morphology of pure WO3 NPs, pure CSA PANI and synthesized binary and ternary nanocomposites were evaluated by FESEM (Fig. 6) with two magnifications. According to Fig. 6(a1, a2), WO3 NPs have been observed with rectangular nanosized platelet-like morphology. The interconnected network of CSA PANI can be seen in the case of CSA PANI-WO3 nanocomposite in which the WO3 NPs dispersed in CSA PANI matrix. With introducing the multiwall carbon nanotubes into the CSA PANI-WO3 structure, the nanotubes can be observed along with WO3 NPs in the Fig. 6(d1, d2) for CSA PANI-WO3-CNT ternary nanocomposite.

Fig. 6.   SEM images of (a1, a2) pure WO3 NPs, (b1, b2) pure CSA PANI, (c1, c2) CSA PANI-WO3 and (d1, d2) CSA PANI-WO3-CNT.

The synthesis of CSA PANI-WO3-CNT is further characterized by transmission electron microscopy. The related TEM micrograph is depicted in Fig. 7, indicating that the WO3 NPs and also MWCNT distribute on the CSA PANI network.

Fig. 7.   TEM images of CSA PANI-WO3-CNT ternary nanocomposite.

To investigate the elemental compositions of synthesized compounds, the EDS analysis was carried out and the results were presented in Fig. 8(a-d). It can be seen that the corresponding peaks of each sample are distinguished and confirmed the formation of nanocomposites.

Fig. 8.   EDS analysis of (a) pristine WO3, (b) pure CSA PANI, (c) CSA PANI-WO3 and (d) CSA PANI-WO3-CNT.

Fig. 9 illustrates the UV-vis absorbance spectra and energy band gaps of WO3 NPs, CSA PANI -WO3 and CSA PANI-WO3-CNT nanocomposites. The absorbance peaks of pure WO3 NPs display at 258 nm and 350 nm indicating no visible light absorption. After modification of WO3 NPs with CSA PANI, the absorption response range increased in both UV and visible light. The absorption peaks of CSA PANI are observed at 392 nm, 470 nm and 682 nm which are attributed to benzenoid π-π*, polaron- π* and π-polaron transitions, respectively [39]. Also, the peaks of WO3 NPs are blue-shifted from 258 nm to 254 nm and 350 nm to 310 nm due to the interaction of WO3 NPs with CSA PANI. The spectrum of CSA PANI-WO3-CNT ternary nanocomposite is similar to CSA PANI-WO3 binary nanocomposite with slightly red shifting towards high wavelengths (392 nm to 409 nm, 470 nm to 485 nm and 682 nm to 685 nm) that is related to increasing the π conjugations and interactions of MWCNT with polyaniline which facilitates the transfer of charge carriers [40].

Fig. 9.   Absorbance spectra and band gap energies of (a, b) WO3 NPs, (c, d) CSA PANI-WO3 and (e, f) CSA PANI-WO3-CNT.

The bandgap energies (Eg) of compounds determined from the extrapolation of the linear parts of the (αhν)2 results versus hν according to (αhν) = C ( - Eg)n equation where α is the absorption coefficient, is the photon energy, C is the proportional constant, Eg is the bandgap energy and n is a parameter that depends on the nature of semiconductor [41]. The bandgap of WO3 NPs, CSA PANI -WO3 and CSA PANI-WO3-CNT nanocomposites are obtained 2.98 eV, 2.03 eV, and 1.88 eV, respectively. It can be seen that the bandgap of CSA PANI-WO3 binary nanocomposite is lower than pure WO3 NPs confirming the enhancement of the photosensitization ability of composite in the presence of CSA PANI. Also, the optical response and the light absorption capability of CSA PANI-WO3 composite improve with the incorporation of MWCNT which in turn promotes the photocatalytic and photoelectrocatalytic performances.

3.2. Photocatalytic degradation of MB

The photocatalytic performances of pure WO3, pure CSA PANI, CSA PANI-WO3, and CSA PANI-WO3-CNT were investigated by the degradation of MB dye in aqueous solution. The absorption spectra of MB solution at various irradiation times for each photocatalyst were depicted in Fig. 10(a-d). The intensity of MB characteristic absorbance peaks (614 nm and 664 nm) was decreased gradually with irradiation time indicating photocatalytic decomposition of MB. The photodegradation ratios of all photocatalysts (C/C0) versus irradiation time were illustrated in Fig. 10(e). The degradation percentages of MB after 60 min light irradiation by pure WO3, pure CSA PANI, CSA PANI-WO3, and CSA PANI-WO3-CNT photocatalysts were obtained 43.45%, 48.4%, 85.15%, and 91.40%, respectively.

Fig. 10.   Absorption spectra of MB in the presence of (a) WO3 NPs, (b) CSA PANI, (c) CSA PANI-WO3, (d) CSA PANI-WO3-CNT; (e) the photodegradation efficiency versus irradiation time, (f) calculated pseudo-first-order rate constants of photocatalysts.

For the further photocatalytic investigation, the kinetics of the MB degradation process was also studied. The photocatalytic degradation rate was calculated by pseudo-first-order kinetics as follows [42]:

ln(C0/C) = kt (1)

where C0 is the MB concentration at the initial time, C is the concentration of MB at time t and k is the rate constant (min-1). The rate constants of photodegradation processes were determined from the slope of the Ln (C0/C) versus the irradiation time and presented in Fig. 10(f). The calculated rate constant values of CSA PANI-WO3-CNT is obtained 0.044 min-1 which is 1.38, 4, and 4.4 times higher than that of CSA PANI-WO3 (0.032 min-1), pure CSA PANI (0.011 min-1) and pure WO3 NPs (0.01 min-1), respectively. The photocatalytic performance of CSA PANI-WO3 photocatalyst is higher than pure WO3 and CSA PANI. It can be said that the combination of the inorganic semiconductors with conductive polymers has a synergetic effect on the degradation of MB. The composite of CSA PANI -WO3 with MWCNT accelerates the elimination reaction of MB due to efficient trapping and rapid transferring of electrons via MWCNT. The schematic illustration of MB photo degradation by CSA PANI-WO3-CNT was shown in Fig. 11.

Fig. 11.   Schematic presentation of photocatalytic mechanism for MB degradation in the presence of CSA PANI-WO3-CNT.

The electrons of both CSA PANI and WO3 were excited at the same time during light irradiation. The appropriate band alignment between WO3 as the inorganic semiconductor and the CSA PANI as the polymeric semiconductor caused the transferring of the electrons from the LUMO of CSA PANI (-1.63 V vs. NHE) to the CB of WO3 (0.6 V vs. NHE) and the holes from the VB of WO3 (3.58 V vs. NHE) to the HOMO of CSA PANI (0.8 V vs. NHE). Therefore, the recombination of electrons and holes was decreased. The transferred holes in HOMO of CSA PANI reacted with H2O and generated hydroxyl radicals ($OH^.$). The electrons in the CB of WO3 reacted with the oxygen molecules and H+ producing H2O2. The hydrogen peroxide was further produced $OH^.$ radicals. Finally, these $OH^.$ radicals, in turn, reacted with MB molecules and produced CO2 and H2O. The CB of WO3 is not favorable with the standard redox potential of O2/O2.- (—0.046 V vs. NHE). Therefore, the electrons of CB in the WO3 can’t produce superoxide anions ($O_2^.-$) [43,44]. The decomposition percentage of MB is further increased in the presence of COOH-MWCNT. The energy position of CNT (-0.1 V vs. NHE) is more negative than the standard redox potential of O2/$O_2^.-$ [45]. So, in the presence of MWCNT more $OH^.$ radicals generated and increased degradation efficiency.

3.3. Photoelectrocatalytic water splitting

For studying the photoelectrochemical water splitting performance, the linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were carried out in 0.1 M Na2SO4 in dark and under illumination. The linear sweep voltammograms (LSVs) of the prepared photoanodes were shown in Fig. 12(a). There is a noticeable difference between the photocurrent densities of all photoelectrodes in dark and under illumination. The photocurrent density of CSA PANI-WO3-CNT is 0.81 mA/cm2 which is remarkably higher than that of CSA PANI-WO3 (0.64 mA/cm2), pure CSA PANI (0.38 mA/cm2) and pure WO3 (0.19 mA/cm2) under illumination.

Fig. 12.   (a) Linear sweep voltammograms, (b) variation of the photoconversion efficiency vs. potential of the ITO/WO3, ITO/CSA PANI, ITO/CSA PANI-WO3, and ITO/CSA PANI-WO3-CNT photoanodes in dark and under illumination.

Also, the onset potential of water oxidation on the CSA PANI-WO3-CNT shifted towards lower values in comparison with other photoelectrodes. The applied bias photo-to-current efficiencies (ABPE) are calculated with the following equation (Eq. 2).

ABPE =$\frac{ (1.23- |V|)× J}{I_0} ×100$ (2)

where V (V) is the applied potential (vs. RHE), J (mAcm-2) is the photocurrent density and I0 (mW cm-2) is the light power density. The variation of ABPE as a function of the applied potential was depicted for all samples in Fig. 12(b). The photoconversion efficiency of ITO/CSA PANI-WO3-CNT is reached to 0.11% which illustrates two-fold (0.07%), four-fold (0.03%) and five-fold (0.02%) enhancements compared to the ITO/CSA PANI-WO3, ITO/CSA PANI and ITO/WO3, respectively. The enhancements in photocurrent density and ABPE confirm that the presence of MWCNT synergistically improved the interfacial charge transfer and also photoelectrochemical activity toward splitting of water.

The Nyquist and Bode plots for ITO/WO3, ITO/CSA PANI, ITO/CSA PANI-WO3, and ITO/CSA PANI-WO3-CNT photoelectrocatalysts were illustrated in Fig. 13. The EIS data were fitted by Zview software (Fig. 13(c)). The fitted charge transfer resistances were presented in Table 1. The charge transfer resistance (Rct) for ITO/WO3, ITO/CSA PANI, ITO/CSA PANI-WO3, and ITO/CSA PANI-WO3-CNT at electrode/electrolyte interface under illumination were obtained 7720 Ω cm2, 5530 Ω cm2, 2441 Ω cm2, and 1165 Ω cm2, respectively. The lower charge transfer resistance of binary and ternary nanocomposites shows the higher photoelectrocatalytic properties.

Fig. 13.   (a1, a2) Nyquist, (b1, b2) Bode plots of ITO/WO3, ITO/CSA PANI, ITO/CSA PANI-WO3 and ITO/CSA PANI-WO3-CNT photoelectrodes and (c) equivalent circuit model for fitting EIS data.

Table 1   Obtained charge transfer resistance values for WO3, CSA PANI, CSA PANI-WO3 and CSA PANI-WO3-CNT coatings on ITO electrode in dark and under illumination.

CompoundRct dark (Ω cm2)Rct illumination (Ω cm2)
Pure WO3112107720
Pure CSA PANI95015530
CSA PANI-WO390952441
CSA PANI-WO3-CNT26411165

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The combination of WO3 and CSA PANI reduced the charge transfer resistance compared to pure WO3 and CSA PANI which has the synergistic effect on the interfacial separation of electron and holes in the electrode/electrolyte interface. The charge transfer resistance was further decreased with incorporating of MWCNT as a conductive pathway for charge transport and separation.

Under illumination, the electrons of CSA PANI and WO3 are excited simultaneously. The generated electrons can transfer from the LUMO of CSA PANI (-1.63 V vs. NHE) to the MWCNT (-0.1 V vs. NHE) and finally to the conduction band (CB) of WO3 (0.6 V vs. NHE) and the holes transferred from the valence band (VB) of WO3 (3.58 V vs. NHE) to the HOMO of CSA PANI (0.8 V vs. NHE). The electrons are transferred to the Pt electrode and took part in the hydrogen production (2H+ + 2e- →H2). The produced holes on the ITO based photoanodes participated in the oxygen generation reaction. The HOMO level of CSA PANI is at the unsuitable position to the redox potential of O2/H2O. So, the holes in the HOMO of the CSA PANI participated indirectly in the water splitting reaction. In this regard, the holes of CSA PANI reacted with water molecules and produced hydroxyl radicals (OH·) which can react with other hydroxyl radicals to generate hydrogen peroxide (H2O2) which in turn split into O2 and H2O (2H2O2→2H2O + O2). Also, the holes in the VB of WO3 can directly react with H2O and produced O2 at the electrode/electrolyte interface (2h+VB + H2O → 1/2 O2 + 2H+) [44]. The observed improvement in the photoelectrochemical oxidation of water at the ITO/CSA PANI-WO3 electrode compared to ITO/WO3 is related to the formation of type -II heterojunction between WO3 and CSA PANI which allows the separation of charge carriers easier and faster. On the other hand, the MWCNT addition to CSA PANI-WO3 provided the conducting pathway for efficient interfacial charge transfer improving the photoelectrochemical water oxidation performance.

Fig. 14 depicts the Mott Schottky (MS) plots (1/C2 versus voltage) for ITO/WO3, ITO/CSA PANI-WO3, and ITO/CSA PANI-WO3-CNT. It can be seen that the slope of all curves is positive. So, the electrons are considered as the dominant charge carrier. The Mott Schottky equation (Eq. (3)) was used for calculating the electron density (ND) and flat band potential (VFB) [44].(3)1C2=2eε0 εrND(V-VFB - KB Te )where C is the depletion layer capacitance, e is the electron charge, ε0 is the vacuum permittivity, εr is the dielectric constant of semiconductor, ND is the donor density, V is the applied potential, VFB is the flat band potential, KB is the Boltzmann constant and T is the absolute temperature.

Fig. 14.   Mott Schottky plots of WO3 NPs, CSA PANI-WO3 and CSA PANI-WO3-CNT thin films on ITO.

The VFB can be determined by extrapolating the linear part of MS plots toward the horizontal axis (1/C2 →0). The obtained values of VFB for ITO/WO3, ITO/CSA PANI-WO3, and ITO/CSA PANI-WO3-CNT are 0.06 V, 0.16 V, and 0.24 V, respectively. The positive shifting in the VFB values can be attributed to the convenient electron transfer in the binary and ternary nanocomposites. The ND values calculated from the slope of curves according to Eq. (4).

ND=$\frac{2}{eε_0ε_r[\frac{d(\frac{1}{C^2})}{dV}]}$ (4)

The ND values for ITO/WO3, ITO/CSA PANI-WO3, and ITO/CSA PANI-WO3-CNT are 2.29 × 1020 cm-3, 3.33 × 1020 cm-3, and 4.42 × 1020 cm-3, respectively. The increase of ND values is due to the enhancement of charge carrier density in the presence of CSA PANI and MWCNT.

4. Conclusion

In conclusion, the camphor sulfonic acid doped polyaniline-WO3-multiwall carbon nanotube (CSA PANI-WO3-CNT) ternary nanocomposite has been prepared and its photocatalytic and photoelectrocatalytic performances were investigated in the degradation of methylene blue dye and photoelectrochemical water splitting, respectively. The degradation ratio (C/C0) of methylene blue after 60 min illumination was 0.57, 0.59, 0.16 and 0.065 for pure WO3 NPs, pure CSA PANI, CSA PANI-WO3, and CSA PANI-WO3-CNT, respectively. The photocurrent density of CSA PANI-WO3-CNT (0.81 mA/cm2) is 1.27, 2.13, and 4.26 times higher than CSA PANI-WO3 (0.64 mA/cm2), CSA PANI (0.38 mA/cm2) and WO3 NPs (0.19 mA/cm2), respectively. This is in agreement with the electrochemical impedance spectroscopy results in which the lowest charge transfer resistance was obtained for CSA PANI-WO3-CNT ternary nanocomposite (1165 Ω cm2). The improvement in photocatalytic and photoelectrocatalytic properties of the nanocomposite in the presence of MWCNT can be attributed to the decrease in band gap values resulting in the extension of the absorbance range of irradiated light. Also, the incorporation of MWCNT increases the electrons-holes separation and accelerates the interfacial charge transfer causing efficient dye degradation and photoelectrochemical activity toward water splitting. Consequently, the synthesized ternary nanocomposite can be considered as an appropriate candidate for photocatalytic and photoelectrocatalytic usages.

Acknowledgments

This project was supported by The Iran Nanotechnology Innovation Council (INIC) in Ministry of Science, Research and Technology and the Office of Vice-Chancellor in Charge of Research of the University of Tabriz.


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