Ionics https://doi.org/10.1007/s11581-018-2473-y

ORIGINAL PAPERS

Facile synthesis of α-Fe2O3/WO3 composite with an enhanced photocatalytic and photo-electrochemical performance R. A. Senthil 1,2 & A. Priya 1 & J. Theerthagiri 1,3 & A. Selvi 4 & P. Nithyadharseni 5,6 & J. Madhavan 1 Received: 1 November 2017 / Revised: 14 January 2018 / Accepted: 26 January 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract The influence of hematite iron oxide (α-Fe2O3) nanoparticles in tungsten oxide (WO3) nanorods photocatalyst on photodegradation of organic pollutant was investigated in the present work. The spherical-shaped α-Fe2O3 nanoparticles and WO3 nanorods were synthesized from citrate precursor and hydrothermal routes respectively. The different weight percentage (wt%) ratios (1, 2, and 3 wt%) of α-Fe2O3 added heterostructured α-Fe2O3/WO3 composite photocatalysts by a simple physical mixing process. The photocatalytic activities of as-synthesized photocatalysts were evaluated by photodegradation of methylene blue (MB) under visible-light irradiation. It showed that the 2% α-Fe2O3/WO3 composite exhibited excellent photocatalytic activity than the others. This enhancement could be attributed to its strong absorption in the visible region and the low recombination rate of electron-hole pairs. In addition, the photo-electrochemical measurements of the 2% α-Fe2O3/WO3 composite revealed the faster migration of the photo-excited charge-carriers. Hence, this study demonstrates the heterostructured α-Fe2O3/ WO3 composite as a promising candidate for environmental remediation. Keywords Photocatalysis . FT-IR . XRD . Photo-electrochemical characterizations . Visible-light

Introduction Large amounts of toxic and non-biodegradable organic dyes containing wastewaters are discharged from many industries such as textile, leather, plastics, cosmetics, and consumer

electronics, during the dyeing process causing a major threat to the environment [1, 2]. Therefore, the degradation of these organic dyes is an utmost essential requirement for a cleaner environment. Recently, visible-light active semiconductor photocatalysis has received immense interest because of their

Highlights • Sherical-shaped α-Fe2O3 nanoparticles were synthesized by citrate precursor route. • The WO3 nanorods were synthesized by hydrothermal route. • The α-Fe2O3/WO3 composites were synthesized via simple wetimpregnation method. • The composite showed a best photocatalytic activity than the pure αFe2O3 and WO3. * J. Madhavan [email protected] 1

Solar Energy Laboratory, Department of Chemistry, Thiruvalluvar University, Vellore 632115, India

2

State Key Laboratory of Chemical Resource Engineering, Beijing Engineering Center for Hierarchical Catalysts, Beijing University of Chemical Technology, Beijing 100029, China

3

Centre of Excellence for Energy Research, Centre for Nanoscience and Nanotechnology, Sathyabama Institute of Science and Technology (Deemed to be University), Chennai 600119, India

4

Environmental Molecular Microbiology Research Laboratory, Department of Biotechnology, Thiruvalluvar University, Vellore 632115, India

5

Department of Physics, National University of Singapore, Singapore 117542, Singapore

6

Energy Materials, Materials Science and Manufacturing (MSM), Council for Scientific and Industrial Research (CSIR), Pretoria 0001, South Africa

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potential applications in the degradation of these organic pollutants from industrial wastewaters [3–5]. Till date, titanium dioxide (TiO 2 ) is a most widely used semiconductor photocatalyst due to its outstanding properties of high photocatalytic activity, good stability, non-toxicity, safeness, availability in high abundance, low cost, etc. [6, 7]. However, TiO2 has certain practical drawbacks such as low solar energy conversion efficiency (out of 45% light, only 4% is absorbed) and the high recombination of photo-excited electron-hole pairs [8–10]. This has hampered the potential application of TiO2 towards environmental remediation. In this regard, many efforts have been focused on the development of new visiblelight-active photocatalytic materials. More recently, a nano-structured tungsten oxide (WO3) semiconductor material has attracted great scientific interest among the researchers due to its lower bandgap value (~ 2.6 eV) than TiO2, which showed a significant improvement in its photocatalytic activity under visible-light irradiation [11–13]. Unfortunately, the photocatalytic activity of pure WO3 is still far from acceptable that is due to the quick recombination of photo-excited electron-hole pairs and more positive conduction band level than that of reduction potential of O2/ O2− which clearly indicates that it cannot be efficiently reduced O2 during the degradation process [14, 15]. Similarly, various other efforts such as metallic or non-metallic element-doping [16], coupling with other semiconductors [17, 18], and modifying with carbon-based materials [19] have also been explored by researchers towards the improvement the photocatalytic activity of the pure WO 3 . Among these, a composite photocatalyst formed by the WO3 coupling with other low bandgap semiconductor catalysts is one of the most promising way to reduce the recombination of photo-excited electron-hole pairs which may contribute to increase its degradation efficiency [20, 21]. Selvarajan et al. synthesized the BiVO4/WO3 nanocomposite photocatalyst, and it showed an enhanced photocatalytic activity for the degradation of 2-chlorophenol (2-CP) under visible-light irradiation [22]. Wang et al. prepared the Ag3PO4/WO3 hybrid photocatalyst and they observed that the Ag3PO4/WO3 hybrid displayed a higher visible-light photocatalytic activity towards the degradation of methylene blue (MB) when compared with pure Ag3PO4 and WO3 [23]. In another study, Wang et al. fabricated the WO3/g-C3N4 composite photocatalyst with an improved visible-light photocatalytic activity for the photodegradation of rhodamine B (RhB) [24]. Liu et al. also prepared the WO3/g-C3N4 composite with an outstanding photocatalytic activity for the degradation of MB under visible-light illumination [25]. Rong et al. synthesized the Bi2S3/WO3 composite with an improved visible-light photocatalytic activity towards the degradation of RhB [26]. Recently, Cao et al. prepared AgIO3/WO3 composite and achieved a good visible-light photocatalytic performance for the degradation of RhB [27]. From these reports, it can be clearly inferred that the WO3-based semiconductor composites are potential

photocatalysts that encourages the charge separation reaction and also reduce the recombination of photogenerated charge carriers. On the other hand, the hematite phase iron oxide (αFe2O3) is also considered to be one of the most important semiconductor photocatalysts in recent times due to its excellent properties of low cost, simple production, environmental friendliness, non-toxicity, high resistance to corrosion, and excellent chemical stability [28–30]. Furthermore, the α-Fe2O3 possesses the capability of absorbing most of the visible-light because of its narrow bandgap value of about 2.2 eV and thus has become a promising visible-light-driven photocatalyst [31]. Apart from that, the suitable band positions of α-Fe2O3 are also well matched with WO3 to reduce the recombination of photo-excited electron-hole charge carriers during the photodegradation which may lead to achieve an efficient charge separation and higher photocatalytic performance [32, 33]. In addition, a series of α-Fe2O3-based composite photocatalysts like Ag/AgBr/Fe 2 O 3 [34], α-Fe 2 O 3 / graphene oxide [35], α-Fe 2 O 3 /ZnO [36], CeO 2 /Fe 2 O 3 [37], and α-Fe2 O 3 /reduced graphene oxide [38] were found to show an outstanding photocatalytic activity for the visible-light photodegradation of organic pollutants. Based on these interesting reports, the α-Fe2O3 has been chosen as a capable co-catalyst to enhance the visiblelight photocatalytic activity of the WO3 photocatalyst. So, in the present investigation, we have synthesized the α-Fe2O3 loaded heterostructured α-Fe2O3/WO3 composite photocatalysts by a simple physical mixing method to achieve an efficient photocatalytic performance. Among the various organic pollutants, the MB dye is one of the common organic water pollutants which is used in almost all industries for dyeing operations [39, 40]. Therefore, the MB has been chosen as an organic pollutant to study the photocatalytic performance of the as-synthesized materials with a proposed mechanism of photocatalytic reaction.

Experimental section Materials Tungstic acid, cetyl trimethylammonium bromide (CTAB), Triton X-100, sodium sulphate (Na2SO4), and methanol were received from SDFCL, India. Methlene blue (MB) was obtained from Sigma-Aldrich, India. Ferric nitrate nonahydrate was purchased from Qualigens, India. Deionized water was obtained from Nice chemicals, India. Citric acid was obtained from Rankem, India. All chemicals were of analytical grade and were used without any further purification.

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Synthesis of α-Fe2O3 nanoparticles The α-Fe2O3 particles were synthesized by dissolving 1:3 mol ratio of ferric nitrate nonahydrate and citric acid in deionized water. Then, the solution was continuously stirred and subsequently dried at 95 °C. The obtained precipitate was heated in air atmosphere at 450 °C for 2 h. Finally, the resulting bright red color spherical-shaped α-Fe2O3 nanoparticles were ground well into a powder.

Synthesis of WO3 nanorods Initially, 2 g of tungstic acid and 0.895 g of CTAB were dissolved in an appropriate amount of distilled water and the obtained solution was transferred into a well-cleaned autoclave. Then, the hydrothermal reaction was carried out in a muffle furnace at 140 °C for 12 h and then allowed to cool to room temperature. The obtained precipitate was collected and washed several times with deionized water and ethanol to remove the impurities. Then, it was dried in a hot air oven at 60 °C for 6 h. The resulting yellow color WO3 nanorods were ground well into a powder.

Synthesis of α-Fe2O3/WO3 composites The different wt% of α-Fe2O3 loaded α-Fe2O3/WO3 composites were synthesized by a simple physical mixing process as follows: 1, 2, and 3 wt% of α-Fe2O3 nanoparticles (with respect to WO3 weight) were added separately to 0.5 g of WO3 nanorods and the obtained mixture was dissolved in 40 mL of methanol in a beaker. Then, the beaker was placed in an ultrasonic bath for 30 min. The resulting solution was continuously stirred and subsequently heated at 60 °C to remove the solvent. Then, the obtained α-Fe2O3/WO3 composites were dried in a hot air oven at 80 °C for 12 h. The resultant α-Fe2O3/WO3 composites were ground well to a powder form for characterization studies. Also, the as-synthesized 1, 2, and 3 wt% αFe2O3/WO3 composites were named herein as WF-1, WF-2, and WF-3 respectively.

Characterization techniques The FT-IR spectra of the as-synthesized photocatalyts were performed with a PerkinElmer FT-IR spectrometer in the wave number ranging from 4000 to 400 cm−1. The XRD patterns of the as-synthesized photocatalyts were recorded in a Bruker-Advance D8 X-ray diffractometer in the 2θ ranging from 10–80° with Cu-Kα radiation. The UV-vis diffuse reflectance spectra of the as-synthesized photocatalyts were carried out using a Varian Cary 5000 model UV-vis-NIR spectrophotometer in the wavelength ranging from 240 to 1200 nm. The scanning electron microscopy (SEM) images of the assynthesized photocatalyts were obtained using a JEOL-JSM

7500F scanning electron microscope equipped with an EDX EDS analyzer. The transmission electron microscopy (TEM) images of the as-synthesized photocatalyts were taken from a JEOL-Jem 2100 transmission electron microscope.

Photocatalytic degradation studies The photocatalytic activities of the as-synthesized photocatalysts were evaluated by the photodegradation of MB under visiblelight irradiation using a photocatalysis chamber designed by our group. A 150-W tungsten-halogen lamp was employed as a source of visible-light irradiation. The distance between the sample and both the lamps was fixed at 20 cm, and the wavelength range of light source was ~ 380–800 nm. In each experiment, 75 mg of photocatalyst was dispersed in 75 mL of the MB (5 × 10−5 M) dye solution to achieve a catalyst concentration of 1.0 g/ L. Prior to light irradiation, the suspension was magnetically stirred in the dark for 90 min to reach an adsorption-desorption equilibrium at room temperature. During irradiation, 4 mL of aliquots was collected at regular time intervals and then the photocatalyst was removed by filtration through a 0.45-μm membrane filter (Pall Corporation). The concentrations of MB at different time intervals were recorded by its characteristic absorption wavelength of 664 nm using a Jasco V630 UV-vis spectrophotometer (Japan).

Photo-electrochemical measurements The photo-electrochemical properties were investigated with a CHI608E electrochemical workstation in a conventional three-electrode system of Pt-wire as a counter electrode, Ag/ AgCl (in saturated KCl) as a reference electrode, and synthesized catalyst coated on FTO conducting glass as a working electrode. 0.1 M Na2SO4 solution was used as an electrolyte. In a typical working electrode preparation, 5 mg of the catalyst was ground well with 10 μL of Triton X-100 and 20 μL of deionized water to make a slurry. The obtained slurry was coated with the conducting side of FTO glass surface by the doctor blade method. The active area of the electrode was fixed as 0.25 cm2, and scotch tape was used as a spacer for uniform coating. Then, the coated FTO plate was dried in a hot air oven at 100 °C for about 5 h.

Results and discussion XRD studies The XRD studies were performed in order to determine the structure and formed phases of the as-synthesized photocatalyts. Figure 1 shows the XRD patterns of (a) αFe2O3, (b) WO3, and (c–e) different wt% of α-Fe2O3/WO3 composites like WF-1, WF-2, and WF-3. The XRD pattern

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Fig. 1 XRD patterns of α-Fe2O3, WO3, and α-Fe2O3/WO3 composite photocatalysts

Fig. 2 FT-IR spectra of α-Fe2O3, WO3, and α-Fe2O3/WO3 composite photocatalysts

of as-synthesized α-Fe2O3 nanoparticles exhibited several diffraction peaks at 2θ = 24.1°, 33.0°, 35.6°, 40.6°, 49.4°, 54.0°, 57.4°, 62.3°, and 63.9° that corresponds to (012), (104), (110), (113), (024), (116), (112), (214), and (300) diffraction planes of the hematite phase of α-Fe2O3 (JCPDS card no. 80-2377) [41, 42]. Similarly, the XRD pattern of WO3 also showed several diffraction peaks at 2θ = 23.1°, 23.7°, 24.4°, 26.6°, 28.8°, 33.6°, 34.2°, 41.9°, 47.4°, 48.3°, 49.9°, and 55.7° that matched with (002), (020), (200), (120), (112), (022), (202), (222), (004), (400), (140), and (420) diffraction planes of the monoclinic crystal structure of WO3 (JCPDS no. 43-1035) [14, 21]. Whereas, in the case of α-Fe2O3/WO3 composites, the diffraction peaks corresponding to WO3 were only observed as shown in Fig. 1c–e. This may be due to the presence of undetectable amount of α-Fe2O3 nanoparticles in the composites or an indication of a well-dispersed α-Fe2O3 nanoparticles with WO3 nanorods. However, the peak intensity of WO3 was found to be reduced after the addition of α-Fe2O3 content in the composites which confirm the formation of αFe2O3/WO3 heterojunction.

observed at 799 cm−1 is due to the stretching vibration of O–W–O bond of the WO3 [15, 18]. The absorption peaks appeared at 1626 and 1469 cm−1 correspond to the asymmetric and stretching vibrational modes of CH3–N+ bond of CTAB in the sample [14]. The characteristic vibrational peak appeared at 3418 cm−1 is due to the –OH stretching vibration of the weakly adsorbed water molecules on the surface of WO3 nanorods [45]. A sharp peak located at 1613 cm−1 may be indexed to the –OH bending modes of the adsorbed water [21]. In case of all the α-Fe2O3/WO3 composites, characteristic vibrational patterns of both α-Fe2O3 and WO3 are observed as shown in Fig. 3c–e. This confirmed the existence of α-Fe2O3 nanoparticles in the composites by the formation of an interfacial connection with WO3. However, the vibrational peak intensities of WO3 were found to be reduced with an increased α-Fe2O3 nanoparticle content in the composites. Hence, the FT-IR results clearly support the formation of heterojunction between α-Fe2O3 and WO3.

FT-IR spectroscopy studies The formation of α-Fe2O3, WO3, and α-Fe2O3/WO3 composite photocatalysts has been observed further through FT-IR spectroscopy. The FT-IR spectra of the (a) α-Fe2O3, (b) WO3, and (c–e) different wt% of α-Fe2O3/WO3 composites like WF-1, WF-2, and WF-3 are shown in Fig. 2. The vibrational peaks of the synthesized α-Fe2O3 at 461 and 532 cm−1 corresponds to the Fe-O stretching and bending vibration of α-Fe2O3, respectively [41]. The vibrational peaks observed at 3429, 2929, and 1623 cm−1 are related to the asymmetrical stretching and deformation vibrations of the physically adsorbed water molecules on the surface of the α-Fe2O3 nanoparticles [43, 44]. In the case of WO3, a very broad peak

Fig. 3 UV-vis diffuse reflection spectra of α-Fe2O3, WO3, and α-Fe2O3/ WO3 composite photocatalysts

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Optical absorption studies The effect of α-Fe2O3 on optical properties of WO3 was studied by UV-vis diffuse reflectance spectral analysis. The UVvis diffuse reflectance spectra of (a) α-Fe2O3, (b) WO3, and (c–e) different wt% of α-Fe2O3/WO3 composite like WF-1, WF-2, and WF-3 are displayed in the Fig. 3. As seen from Fig. 3, the absorption sharp edges corresponding to α-Fe2O3, WO3, WF-1, WF-2, and WF-3 photocatalysts are found to be around 676, 485, 619, 626, and 622 nm, respectively. In addition, the α-Fe2O3/WO3 composites exhibited an enhanced absorption edge in the visible region on comparison with pure WO3. This enhancement revealed more efficient light utilization for the photocatalytic degradation by the composite photocatalysts than pure form of WO3 [46, 47]. The bandgap energies (Eg) of the α-Fe2O3, WO3, WF-1, WF-2, and WF-3 photocatalysts were also estimated and found to be 1.83, 2.55, 2.00, 1.98, and 1.99 eV respectively. The calculated values of α-Fe2O3 and WO3 in the present study are found to be in good agreement with those of the previous reports by researchers [21, 41]. However, the bandgap of the WO3 showed a decreased value due to the addition of α-Fe2O3 in the case of composites. This decrease is expected to promote the generation of more photo-excited charge carriers in α-Fe2O3/WO3 composite under the visiblelight illumination which may inturn improve the phtotocatalytic activity [14].

SEM and EDX analysis In order to investigate the shape and surface morphologies of the as-synthesized photocatalysts, the SEM analysis was carried out for (a) WO3, (b) α-Fe2O3, and (c) WF-2 composite and the corresponding SEM images are shown in Fig. 4. The pure WO3 particles mainly showed a nanorod-like morphology (4(a)), whereas the α-Fe2O3 particles displayed an aggregated nanoparticles. On the other hand, the morphology of the optimized WF-2 composite showed mostly aggregated αFe2O3 nanoparticles that are attached on the surface of the WO3 nanorods. The morphology of the materials is reported to play a significant role in determining their photocatalytic activity [48, 49]. On this context, the formation of the WO3 nanorods and the aggregated α-Fe2O3 nanoparticles observed in the present study were found to be more favorable for the diffusion and separation of photo-excited electron-hole pairs in the composite photocatalysts [14]. To support further, the elemental composition of the assynthesized photocatalysts was analyzed using the EDX spectroscopy. The typical EDX spectra of (a) WO3, (b) α-Fe2O3, and (c) WF-2 composite photocatalysts are presented in Fig. 5. It can be seen that the elements of W, O, and C were detected for WO3 nanorods and the elements of Fe, O, Al, and C were detected for α-Fe2O3 nanoparticles. Whereas, the WF-2

Fig. 4 SEM images of WO3 (a), α-Fe2O3 (b), and WF-2 (c) composite photocatalysts

composite photocatalyst was found to consist of W, Fe, O, and C elements. The presence of C and Al elements is due to the carbon tape and alumina sample holder that are used during the EDX measurements. Apart from that, the weight and atomic percentages of the as-synthesized photocatalysts were also observed by EDX analysis and are shown in the insert table of Fig. 5. It showed the weight percentage of Fe element is low in the case WF-2 composite which is in good agreement with the optimized weight of α-Fe2O3 to synthesize the composite photocatalyst. In addition, the elemental mapping images of (a) WO3, (b) α-Fe2O3, and (c) WF-2 composite photocatalysts are displayed in Fig. 6. The presence of W, Fe, and O atoms is clearly seen in the composite photocatalyst which is also evident from the obtained EDX spectra. Therefore, the observed results of EDX and elemental mapping images further demonstrated the successful synthesis of α-Fe2O3/WO3 composite in a right proportion and also

Ionics Fig. 5 EDX spectra of WO3 (a), α-Fe2O3 (b), and WF-2 (c) composite photocatalysts

confirmed the formation of interfacial connections between the α-Fe2O3 and WO3.

composite photocatalyst, thus supporting our previous characterization studies.

Photocatalytic degradation studies TEM analysis The morphology of the as-synthesized composite photocatalyst is further investigated by TEM analysis. The TEM images and selected area (electron) diffraction (SAED) pattern of WF-2 composite photocatalyst are shown in Fig. 7a–d. The TEM images (Fig. 7a–c of the photocatalyst exhibited a nanorod-like structure with a well distributed αFe2O3 nanoparticles on the surface of the WO3 nanorods. These TEM images were found supporting the results obtained by SEM analysis. In addition, the observed SAED pattern (Fig. 7d) of WF-2 composite photocatalyst also showed the occurrence of a proper crystalline nature of α-Fe2O3/WO3

The photocatalytic activities of the as-synthesized α-Fe2O3/ WO3 composite photocatalysts were investigated for the degradation of MB under visible-light irradiation. The blank test without any photocatalyst was performed for the dye alone (5 × 10−5 M) under the irradiation of visible-light for about 2 h. No considerable change was noted in the absorbance of MB which indicated the stability of the dye. Additionally, in order to identify the required time to reach the adsorptiondesorption equilibrium, the adsorption studies of the photocatalyst on MB were studied in the dark condition. The results showed that the equilibrium between the catalyst and dye was established after 90 min of stirring in dark condition.

Ionics Fig. 6 EDX elemental mapping images of WO3 (a), α-Fe2O3 (b), and WF-2 (c) composite photocatalysts

The visible-light photodegradation curves of MB using αFe2O3, WO3, and α-Fe2O3/WO3 composite photocatalysts are shown in Fig. 8a, and their corresponding degradation efficiencies are shown in Fig. 8b respectively. As seen from the figure, the pure α-Fe2O3 and WO3 exhibited a limited photocatalytic performance towards degradation of MB with an efficiency of 17.2 and 46.2% after 60-min irradiation, respectively. However, the as-synthesized α-Fe2O3/WO3 composite photocatalysts were found to exhibit a significantly high photocatalytic activity than that of individual α-Fe2O3 and WO3. This clearly indicates that the added α-Fe2O3 had a significant influence on the photocatalytic performance of α-Fe2O3/WO3 composite photocatalysts [31, 50]. Among three composites, the best photocatalytic performance was observed for WF-2 composite with a degradation efficiency of 79.5% when compared with 55.6% for WF-1 and 64.7% for WF-3. These results indicated that, when the α-Fe2O3 content was increased beyond 2 wt%, a decrease in the photocatalytic performance

was noted and thus proving WF-2 as an optimized composite photocatalyst for the degradation of MB. The enhancement in photocatalaytic performance of the composite photocatalyst can be contributed due to an improved visible-light absorption ability as well as efficient electron-hole transfer occurred at the interfaces of α-Fe2O3/WO3 composite [51, 52]. Therefore, the degradation studies proved that the α-Fe2O3/WO3 composite is an effective photocatalyst material for degradation of organic pollutants from the wastewaters.

Effect of catalyst concentration To investigate the influence of catalyst concentration on the degradation efficiency of MB, a series of photodegradation experiments were performed with different concentrations of an optimized WF-2 composite by maintaining other parameters constant. The photodegradation curves of MB over different amounts (0.5, 1, 1.5, and 2 g/L) of WF-2 composite

Ionics Fig. 7 a–c TEM images WF-2 composite photocatalyst. d SAED pattern of the WF-2 composite photocatalyst

photocatalyst and their corresponding degradation efficiencies are presented in Fig. 9a, b respectively. From the figure, it was

observed that the photocatalytic degradation of MB was increased with an increasing catalyst concentration. This may be

Fig. 8 a Photocatalytic degradation of MB over α-Fe2O3, WO3, and αFe 2 O 3 /WO 3 composite photocatalysts. b Their corresponding degradation efficiency under visible-light irradiation

Fig. 9 a Photocatalytic degradation of MB over WF-2 composite photocatalysts at different amounts; 0.5, 1.0, 1.5, and 2.0 g/L. b Their corresponding degradation efficiency under visible-light irradiation

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attributed to the enhancement in the number of available catalyst sites for visible-light absorption, which, enables to produce more electron-hole pairs for the dye degradation and improved photocatalytic activity [41]. Finally, the complete degradation (around 100%) of MB was found to be achieved within 60 min when the concentration WF-2 composite was at 2 g/L. Hence, these results well supported the application studies of α-Fe2O3/WO3 composite towards the degradation of organic pollutants.

Photo-electrochemical studies The photo-electrochemical measurement is a powerful method to investigate the charge separation efficiency of electrons and holes. To study the charge-transfer properties of the photocatalyts, the transient-photocurrent responses were carried out for pure α-Fe2O3, pure WO3, and optimized WF-2 composite photocatalysts with typical three on-off cycles of irregular visible-light irradiation and the obtained results are given in the Fig. 10. The photostability of all the photocatalysts was also confirmed from a reproducible photocurrent response for few on-off cycles under visible-light irradiation. Figure 10 clearly indicates that the photocurrent response of WF-2 composite is much higher than that of pure α-Fe2O3 and pure WO3. This suggests that the α-Fe2O3/WO3 composite is more efficient in electron-hole separation and possesses a long lifetime of photoexcited electron-hole pairs than the other catalysts [46, 51]. Further, this study supports that the composite photocatalyst can generate more photo-induced charge carriers, which improve the photocatalytic performance of the catalyst towards the degradation of MB. Additionally, the electrochemical impedance spectroscopy (EIS) measurement was also performed in order to study the charge-transfer resistance of the photocatalysts. Figure 11

Fig. 11 Nyquist plot of the α-Fe2O3, WO3, and WF-2 composite photocatalysts and inset indicates the zoomed view of selected region of Nyquist plot

shows the Nyquist plot of the pure α-Fe2O3, pure WO3, and WF-2 composite photocatalysts. As seen from Fig. 11, the arc radius of WF-2 composite is smaller than that of pure α-Fe2O3 and pure WO3 indicating that the composite has a lower charge-transfer resistance [53, 54]. Whereas, in the case of α-Fe2O3/WO3 composite, the photo-excited electron-hole pairs can be easily separated and transferred to the surface of the material and thus the charge recombination can be reduced which favors an enhancement in their photocatalytic activities [17, 55]. The observed results inferred the action of α-Fe2O3 as an efficient co-catalyst in α-Fe2O3/WO3 composite by forming an interfacial interaction with WO3.

Photostability and reusability studies In order to study the photostability and reusability of the assynthesized photocatalyts, the recycling test was performed to an optimized WF-2 composite via photodegradation of MB dye for four cycles under the same conditions. At the end of each cycle, the recovered sample was centrifuged and dried at 60 °C for 4 h. Then, the obtained sample was weighed and lost portion was added to degrade fresh MB dye, and recycling experiment was carried out for four times and the obtained results are given in Fig. 12. It can be seen that the photocatalytic activity of WF-2 composite remained even at the end of four cycling runs, which clearly proved the good photostability and reusability properties of the as-synthesized α-Fe2O3/WO3 composite.

Photocatalytic degradation mechanism Fig. 10 Transient photocurrent curves for the α-Fe2O3, WO3, and WF-2 composite photocatalysts with typical on-off cycles of irregular visiblelight irradiation

A proposed reaction mechanism for the photo-excited electron-hole separation and transport processes of photocatalytic degradation of MB over α-Fe2O3/WO3 composite under

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Fig. 12 Cycling runs for the photocatalytic degradation of MB over WF2 composite under visible-light irradiation

visible-light irradiation is presented in Fig. 13. The valence band (VB) and conduction band (CB) edge potentials of the as-synthesized α-Fe2O3 and WO3 were determined using the following equations. E VB ¼ X −Ee þ 0:5E g

ð1Þ

E CB ¼ E VB −Eg

ð2Þ

where, EVB, ECB, X, and Eg correspond to the VB potential, CB potential, electronegativity, and the bandgap energy of the semiconductor respectively. Ee corresponds to the energy of free electrons on the hydrogen scale (~ 4.5 eV). The absolute electronegativity of the α-Fe2O3 and WO3 is 5.83 and 6.59 eV, respectively, as reported on other studies too [14, 31]. The VB and CB edge potentials of the α-Fe2O3 are estimated to be + 2.24 and + 0.41 eV, respectively. Similarly, the VB and CB edge potentials of the WO3 were estimated to be + 3.36 and +

Fig. 13 A possible mechanism for the photo-excited electron-hole separation and transport processes of MB over α-Fe 2 O 3 /WO 3 composite photocatalysts under visible-light irradiation

0.81 eV, respectively, and these values are mentioned in Fig. 13. It noted that the CB edge potential of α-Fe2O3 is more lower than the that of WO3 [56, 57]. Under visible-light irradiation, the ground state α-Fe2O3 and WO3 goes to an excited state to produce some electron-hole pairs because of their narrow bandgaps. Therefore, the photo-excited electrons on the CB of α-Fe2O3 can be freely transferred to the CB of WO3 through a well-developed α-Fe2O3/WO3 interface. Additionally, the VB edge potential of WO3 is found to be more larger than that of the α-Fe2O3 which helps to the photo-excited holes on the VB of WO3 to transfer more freely to the VB of α-Fe2O3 through a well-developed heterojunction [41]. In this regard, the recombination rate of charge carriers is remarkably reduced, which favors the increase in the interfacial charge-transfer reactions towards the degradation of the adsorbed dye molecules. Thus, an improved photocatalytic performance was observed for αFe2O3/WO3 composite compared with the pure WO3 [18, 21].

Summary In summary, the influence of spherical-shaped α-Fe2O3 nanoparticles loaded in WO3 nanorods photocatalyst on visible-light photodegradation of organic pollutant was studied in the present work. The α-Fe2O3 nanoparticles and WO3 nanorods were synthesized from citrate precursor and hydrothermal route, respectively, which are further used as precursors to synthesize a series of α-Fe2O3/ WO3 composite photocatalysts with different wt% (1, 2, and 3%) of α-Fe2O3 by a simple physical mixing process. The photocatalytic activities of the as-synthesized pure αFe2O3, pure WO3, and α-Fe2O3/WO3 composites towards the degradation of MB under visible-light irradiation showed a highest photocatalytic activity found to be observed for 2% α-Fe2O3/WO3 composite photocatalyst. This enhancement is due to their efficient visible-light absorption ability and higher charge carrier separation efficiency of photo-excited electron-hole pairs. The photoelectrochemical measurements exposed that the recombination of the photo-excited electron-hole charge carriers was significantly reduced in the case of α-Fe2O3/WO3 composite photocatalyst than that of pure WO3. A possible reaction mechanism has also been proposed to explain the improved photocatalytic performance of α-Fe2O3/WO3 composite photocatalyst. Thus, the obtained results of this study clearly proved the α-Fe2O3/WO3 as a potential material for the removal of organic contaminants from the polluted waters. Funding information The authors Mr. R. A. Senthil and Dr. J. Madhavan are grateful to the authorities of the Thiruvalluvar University for their support for this study.

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