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SnO2 nanoheterostructure for photocatalytic degradation of malachite green
Catalysis Today 224 (2014) 171–179
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Facile fabrication of mesoporous ␣-Fe2 O3 /SnO2 nanoheterostructure for photocatalytic degradation of malachite green Gajendra Kumar Pradhan, K. Hemalata Reddy, K.M. Parida ∗ Colloids and Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar 751013, Odisha, India
a r t i c l e
i n f o
Article history: Received 27 July 2013 Received in revised form 6 October 2013 Accepted 11 October 2013 Available online 8 November 2013 Keywords: Nanoheterostructure Mesoporous Photocatalysis.
a b s t r a c t A series of ␣-Fe2 O3 /SnO2 semiconductor nanoheterostructure photocatalysts have been fabricated using a facile co-precipitation method. Precipitation of both SnO2 and ␣-Fe2 O3 were achieved with the slow hydrolysis of urea followed by addition of 1% NH3 and the products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), DR UV–vis spectroscopy, and N2 adsorption–desorption. Analytical results explain the synthesized materials are crystalline, mesoporous, visible light active (except SnO2 ) and also presents in the nano-order form. The as-prepared ␣-Fe2 O3 /SnO2 photocatalysts possessed extended light absorption in the visible range and proficient charge separation properties simultaneously. The significant enhancement in the reaction rate was observed in ␣-Fe2 O3 /SnO2 , compared to the bare SnO2 towards photodegradation of malachite green. The highest degradation percentage was found to be 86% with the 2 mol % ␣-Fe2 O3 modified SnO2 catalyst. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor heterojunction is the building block of two or more different semiconductor having dissimilar physical properties. In recent years several heterostructure systems such as TiO2 /V2 O5 , FeOOH/TiO2 , TiO2 /SnO2 , ZnO/SnO2 , NiO/SnO2 , ZnO/CdS, ␣-Fe2 O3 /ZnO, WO3 /ZnO, CdS/ZnO, CaFe2 O4 /MgFe2 O4 have been reported for different applications [1–8]. Electron transport at the interface between two semiconductors is one of the important aspects for the design of this novel material. Thus, the photogenerated charge carriers, i.e. electrons and holes are easily separated and are utilized for redox reaction. The new hybrid structure has several positive aspects for photocatalytic application which includes dye degradation and water splitting. SnO2 is an n-type UV light absorptive semiconductor possessing band gap energy of 3.6 eV [1,9]. So modification is necessary to shift its optical absorptive property towards higher wavelength. On the other hand, ␣-Fe2 O3 is a visible light active photocatalyst having band gap energy 2.2 eV [10–12]. SnO2 and ␣-Fe2 O3 are two opposite character photocatalyst having optical absorptivity in the UV and visible region. Therefore, the heterojunction between these two semiconductors would be more fruitful to use as a visible light active photocatalyst. ␣-Fe2 O3 /SnO2 system has special interest and has been studied by many research groups for
∗ Corresponding author. Fax: +91674 2581637. E-mail address: [email protected] (K.M. Parida). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.10.038
various photocatalytic application [11,13–15]. Niu et al. synthesize the ␣-Fe2 O3 /SnO2 semiconductor nanoheterostructures by hydrothermal technique and studied for the degradation of methylene blue [11]. Kang and co-workers designed hierarchical ␣Fe2 O3 /SnO2 heterostructure by chemical vapour deposition (CVD) method and also tested their photocatalytic activity for the degradation of same dye, i.e. methylene blue[13]. Xu et al. fabricated SnO2 /␣-Fe2 O3 nanoheterostructures and studied their photocatalytic degradation of methylene blue [14]. All groups claimed that photogenerated electrons and holes were separated at the ␣-Fe2 O3 /SnO2 interfaces and resulting in the enhancement of photocatalytic activity. Another important aspect of the catalyst is mesoporous nature of the material because of its high surface area. In the last three decades, mesoporous materials exhibited superior performances in different catalytic reactions whereas the mesoporous structure is highly required in the photodegradation of organic pollutants, which can make the macromolecules quickly diffuse into the active sites in a photocatalyst, leading to photooxidation. So, the semiconducting material having mesoporosity would be more preferable and suitable for the photocatalysis reactions[16,17]. Thus, our target is to prepare mesoporous semiconducting material which is suitable for photocatalytic dye degradation. The first attempt to synthesize mesoporous tin oxide was reported by Rao and Ulagappan using anionic surfactant [18]. There are several methods reported in the literature for the synthesis of meso-SnO2 such as sol gel, sonochemical, hydrothermal, microwave hydrothermal and template-assisted methods [19–23]. Although most synthesis
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processes involve the use of templates and hydrothermal treatment. Here, the preparation of meso-SnO2 can be achieved by a simple co-precipitation method without using any template. Although many SnO2 /Fe2 O3 heterostructures have been reported for photocatalytic reactions, we have fabricated a mesoporous SnO2 /Fe2 O3 heterostructures for the first time by co-precipitation method to improve the photocatalytic performance. Moreover, the introducing of Fe2 O3 species into the mesoporous SnO2 framework can effectively extend the photo response of the SnO2 to the visible region and improved the photocatalytic activity of SnO2 . The fabricated material possesses larger surface area, mesoporous nature and visible light absorption capacity, which are the important factors for the improvement of photocatalytic activities. Previously, we have studied various systems such as ␣-Fe2 O3 , Fe2 O3 -ZnO, Fe2 O3 -CeO2 , S,N doped ␣-Fe2 O3 , ␣-Fe2 O3 /RGO composite for degradation of organic pollutants [24–28]. With the purpose of efficient electron–hole separation and enhancement of photocatalytic performance of SnO2 in the visible region, our principal approach is to design a heterostructure between ␣-Fe2 O3 and SnO2 along with mesoporous character for photocatalytic application. In this context, we have synthesized a series of visible light active ␣-Fe2 O3 /SnO2 nanoheterostructures by a simple coprecipitation method and tested their photocatalytic activity for the degradation of malachite green under natural sunlight. 2. Experimental 2.1. Materials All the chemicals and reagents are of analytical grade and used without further purification. Iron(III) nitrate nonahydrate (Fe (NO3 )3 ·9H2 O) (Across), tin(IV) chloride pentahydrate (SnCl4 ·5H2 O) (Sarabhai chemicals), urea (NH2 CONH2 ) (S.d. fine chemicals), ethanol and deionised water were used for the sample preparation. Malachite green (MG) dye was used for testing for photocatalysis. 2.2. Methods
Scheme 1. Schematic diagram of fabrication of mesoporous ␣-Fe2 O3 /SnO2 nanoheterostructure.
were calibrated by using the contaminant carbon (C1s = 284.9 eV) as a reference. Charge neutralization of 2 eV was used to balance the charge of the sample. The surface area and pore diameter were measured by N2 adsorption–desorption method at −196 ◦ C in Automated Surface area and Porosity Analyser (ASAP 2020, Micromeritics, USA). Prior to the analysis, samples were degassed under vacuum (10−5 torr) at 300 ◦ C for 4 h. Accordingly, specific surface area, pore volume and pore diameter of all the synthesized photocatalysts were calculated by BET and BJH method. 2.4. Photocatalytic dye degradation
In a typical experiment, stoichiometric amount of urea, Fe (NO3 )3 .9H2 O, SnCl4 .5H2 O, water and ethanol were taken for the synthesis of photocatalysts. Keeping the tin(IV) chloride amount fixed, we have varied iron(III) nitrate nonahydrate. The molar ratio of iron(III) nitrate nonahydrate to tin(IV) chloride were controlled to be 0, 0.5, 1, 1.5 and 2 mol% and were represented as S1, S2, S3, S4, and S5, respectively. The above solutions were allowed to stir for complete hydrolysis under ice-cold condition. After 3 h of stirring, the pH of the solution was maintained to 7 with 1% NH3 for complete precipitation. Again it was stirring for 1 h, filtered, and washed with water and ethanol. Then it was dried at 110 ◦ C for overnight and calcined at 500 ◦ C for 4 h. The calcined samples were tested for photocatalytic dye degradation.
The photodegradation of all the synthesized photocatalysts were tested towards degradation of malachite green (MG) under solar radiation. In a typical experiment, 40 mg photocatalyst with 20 ml of 100 ppm MG solution was taken in a 100 ml closed Pyrex flask. The solutions were exposed to sunlight with constant stirring for 4 h. After irradiation, the suspension was centrifuged and the concentration of the supernatants were analyzed quantitatively at 616 nm (max for MG) using a Cary-100 (Varian, Australia) spectrophotometer. The intensity of sun light was measured using Digital Illuminance Meter (Model-TES-1332A, Taiwan). During the measurement, the sensor was set in such a position where the intensity is high. The average light intensity was around 100,000 lux, which was nearly constant (10AM-2PM) during the experiments.
2.3. Characterization
3. Results and discussion
Powder X-ray diffraction patterns of the samples were recorded in a PANalytical X-ray diffractometer using Ni filtered Cu K␣ radi˚ SAX in the 2 range from 0 to 10o was also ation (= 1.5418 A). carried out to know the mesoporous nature of the material by using a Rigaku X-ray diffractometer. Transmission electron microscope (TEM) of the catalysts were recorded with a Phillips, TECNAI G2 20 operating at an acceleration voltage of 200 kV. Optical absorbance spectra of the catalysts were recorded using a Cary 100 spectrophotometer (Varian) in the spectral range of 200–800 nm. Electronic states of Fe and Sn were examined by X-ray photoelectron spectroscopy (XPS, Kratos Axis 165 with a dual anode (Mg and Al) apparatus) using the MgK␣ source. All the binding energy values
3.1. Fabrication of mesoporous SnO2 and ˛-Fe2 O3 /SnO2 nanoheterostructure The synthetic procedure for ␣-Fe2 O3 /SnO2 nanoparticles is presented in Scheme 1. Urea precipitation allows the easy control of the rate of supersaturation and permits the synthesis of well-defined particles, once the appropriate conditions are maintained [29]. Previously, various metal oxides are prepared by precipitation method in the presence of urea followed by heating at higher temperature (80–100 ◦ C) [30–33]. In our case, the reaction was designed at room temperature to control the rate of particle growth and surface properties. The ammonia generates under this condition is not
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Fig. 3. SAX pattern of the 2 mol% Fe2 O3 /SnO2 .
Fig. 1. Typical PXRD patterns of S1, S2, S3, S4 and S5 photocatalysts.
as 2.69 is identified as ␣-Fe2 O3 . Fig. 3 represents the small angle ◦ (2 = 0–10 ) X-ray diffraction (SAX) of the 2 mol% Fe2 O3 /SnO2 . It was observed that 2 mol% Fe2 O3 /SnO2 shows the diffraction peaks ◦ at 2 = 0.48 and 0.69 , respectively. This confirms the mesoporous nature of the material. 3.3. TEM
Fig. 2. Deconvoluted [1 0 1] plane in the XRD of S5 photocatalyst.
sufficient enough to precipitate both SnO2 and ␣-Fe2 O3 . Thus a small amount of dilute NH3 was added for complete precipitation. During the addition of dilute NH3 , aggregation of nuclei leads to growth of ␣-Fe2 O3 /SnO2 particle. The aggregation of small particles leading to voids in between the particle which escorts the formation of interparticle pore. 3.2. XRD Powder X-ray diffraction pattern of the synthesized photocatalyst is presented in Fig. 1. All the diffraction patterns obtained were indexed to the characteristic peaks of tetragonal SnO2 in the S1 (JCPDS 13-534). However no marginal shifting of peaks was observed in the S2, S3, S4 and S5 samples. As the percentage of Fe2 O3 is very small, so no peaks were found for it in the XRD pattern. At lower concentration of Fe2 O3 (0.5–1.5 mol%), no crystalline phase of Fe2 O3 was observed in the XRD pattern. This may be possible due to very low content of Fe2 O3 which can barely be detected by XRD or the Fe2 O3 species was highly dispersed on the mesoporous SnO2 framework structure [34,35]. When the Fe2 O3 content was 2.0 mol%, the characteristic XRD peaks became broad in comparison to low iron containing samples. This broadening of the peak may be due to the overlapping of the Fe2 O3 and SnO2 peaks which was later confirmed by the 110 peak deconvolution. Taking into the account, to identify the unknown moiety in case of 2 mol% Fe2 O3 /SnO2 sample, we have deconvoluted the [101] peak into two and is shown in Fig. 2. The d-value with 2.64 is for SnO2 where
The morphology, particle size and structure of the products were characterized by TEM (Fig. 4). The particles are well developed having definite size and shape. It can be seen that the diameters of the particles are in the range of 6–14 nm for all the photocatalysts (Table 1). S1 sample exhibits particle size 13.27 nm which gradually decreases with incorporation of iron oxide. This is also strongly supported by the peak broadening in the XRD figure. No marginal changes observed on the surface and morphology of the particle in all the samples. The selected area electron diffraction (SAED) pattern of 2 mol% ␣-Fe2 O3 /SnO2 sample was presented in Fig. 5(a). The SAED pattern revealed several bright continuous concentric rings attributed to the diffraction from the (1 1 0), (1 0 1), (2 0 0), (2 1 1) and (3 0 1) planes of SnO2 , respectively, consistent well with the XRD data. The SAED pattern indicates the polycrystalline nature of the prepared photocatalysts. The HRTEM image of 2 mol% ␣-Fe2 O3 /SnO2 was presented in Fig. 5(b). The HRTEM image shows clear lattice fringes that demonstrate the highly crystallinity nature of the sample. The HRTEM image of the 2 mol% ␣-Fe2 O3 /SnO2 shows lattice planes in regular interval and the plane intervals of 0.335 nm and 0.264 nm corresponding to interplanner spacing of (1 1 0) and (1 0 1) planes of SnO2. This data is well consistent with the XRD pattern of the sample. The HRTEM image describes the wormwhole type mesoporosity in case of ␣Fe2 O3 /SnO2 nanoheterostructure (Fig. 6). This type of mesoporosity arises due to intergrowth of small particles and the same leads to aggregates with significant extra framework void space. This adds extra advantage for the fast diffusion of charge carriers in the photocatalytic process [36–38] (Fig. 6).. 3.4. DRS The photo absorption properties of neat SnO2 and ␣Fe2 O3 /SnO2 nanoheterostructures were characterized by UV–vis diffuse reflectance spectrometry (Fig. 7). The absorption edges of ␣Fe2 O3 /SnO2 are located at longer wavelengths than those of SnO2 but gradually shifted to longer wavelengths with raising the concentration of ␣-Fe2 O3 . The results indicate that the ␣-Fe2 O3 /SnO2 nanoheterostructure have a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation. The band gap energy of the present nanoheterostructures is roughly estimated to be 3.57–2.81 eV based on the onsets of the diffuse reflectance spectra, which is substantially smaller than
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Fig. 4. Bright-field TEM images of S1, S2, S3, S4 and S5 photocatalysts.
Table 1 Textural properties and degradation results of all the synthesized photocatalysts. Catalysts
Particle size (nm)
Band gap energy (eV)
Surface area (m2 /g)
Pore volume (cm3 /g)
Pore diameter (nm)
Degradation (%)
S1 S2 S3 S4 S5
13.27 9.15 8.5 6.75 6.2
3.57 3.09 3.0 2.91 2.81
23.6 26.6 35.5 38.6 46.6
0.105 0.097 0.087 0.110 0.168
17.9 14.4 14.2 11.4 9.8
66 71 75 80 86
that of SnO2 (3.57 eV). The shifts in the position of the absorption edges from 347 to 450 nm justify the range of white-pale yellow colours observed for the powders. The red shift of ␣-Fe2 O3 /SnO2 nanoheterostructure is due to the contribution of ␣-Fe2 O3 in SnO2 . The ␣-Fe2 O3 is a visible light active n-type semiconductor with bandgap energy 2.2 eV [39–41]. When it couples with n-type SnO2 semiconductor, the electrons from the conduction band of ␣-Fe2 O3 migrate to the conduction band of SnO2 and photogenerated holes move in the opposite direction, which leads to an increase in charge
± ± ± ± ±
4 2 3 4 3
separation efficiency and extends the photoresponding range to visible light [42]. 3.5. XPS X-ray photoelectron spectroscopy was carried out to know the oxidation states, electronic structure and electronic environment of the elements present in the S5 photocatalyst. Fig. 8 (a)–(c) presents the high-resolution XPS spectrum of O 1s, Fe 2p and Sn 3d in
Fig. 5. (a) SAED and (b) HRTEM of S5 photocatalyst.
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Fig. 6. HRTEM images of S5 photocatalyst presenting the mesopore.
the S5 sample. The binding energy (BE) component observed at 530 eV is attributed to the O2− forming oxide with metal [43]. The obtained binding energy (BE) peaks of Sn 3d5/2 and Sn 3d3/2 were 486.2 and 494.6, respectively, which is due to +4 oxidation states of SnO2 [44]. The peak position of Fe 2p3/2 for ␣-Fe2 O3 has been investigated by many researchers previously and reported that the binding energy (BE) value lies between 710.6 and 711.2 eV. However in our system, the Fe 2p3/2 and Fe 2p1/2 peak positions were 711.1 and 724.6 eV which could be assigned to +3 oxidation state of iron in ␣-Fe2 O3 [45–48]. The difference between Fe 2p3/2 and Fe 2p1/2 peak is 13.5 eV which ensures the Fe is in oxide form and is ␣-Fe2 O3 . XPS also support the presence of both SnO2 and ␣-Fe2 O3 in the composite sample. Fig. 7. DRS of S1, S2, S3, S4 and S5 photocatalysts.
Fig. 8. (a) High-energy resolution O 1s core-level XPS spectrum of S5 photocatalyst. (b) High-energy resolution Fe 2p core-level XPS spectrum of S5 photocatalyst. (c): High-energy resolution Sn 3d core-level XPS spectrum of S5 photocatalyst.
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Fig. 9. N2 adsorption-desorption isotherm of (a) S1, (b) S2, (c) S3, (d) S4 and (e) S5 photocatalysts.
3.6. N2 adsorption–desorption A typical nitrogen adsorption–desorption isotherm of all the photocatalysts are shown in Fig. 9. All the samples exhibit type IV nitrogen sorption isotherm with a capillary condensation step in the relative pressure (P/P0 ) range 0.6–0.9 (type H2 hysteresis loop), indicating a well-developed mesoporosity [49]. The surface area, pore diameter and pore volume of all the photocatalysts were presented in Table. 1. It was found that the specific surface area is gradually increasing with the increase of ␣-Fe2 O3 content in the SnO2 moiety and lies in the range of 23.6–46.6 m2 /g. Therefore, we conclude that the incorporation of Fe2 O3 in to the SnO2 framework is responsible for the decrease in particle size and increase in surface area [50,51]. In addition to that the pore-size distribution is calculated using the Barrett–Joyner–Halenda (BJH) from the desorption isotherm (Fig. 10). It indicates that all the photocatalysts have pronounced mesoporosity of a narrow pore-size distribution with an average pore size from 17.9 to 9.8 nm. The pore diameter and pore volume have also the same trend, i.e. S1< S2 < S3 < S4 < S5. The mesoporous structure and surface area play pivotal role in
Fig. 10. BJH isotherm of S1, S2, S3, S4 and S5 photocatalysts.
catalyst design for its ability to improve the molecular transport of reactants. 3.7. Photocatalytic reaction The performance of all the photocatalysts were discussed in terms of particle size, band gap energy and surface area. The percentage of degradation of malachite green was examined as a function of catalyst (Fig. 11). Previously various groups have reported the MG degradation by taking different photocatalysts which is presented in Table 2. Chen et al. studied the degradation of malachite green by BiWO6 photocatalyst. They claimed 100% degradation of malachite green (20 ppm) with a time period of 75 min and 1 g L−1 of catalyst [52]. Wei groups reported the degradation of 10 ppm malachite green with 99% degradation in 30 min [53]. Here we have made an attempt to degrade the same MG (100 ppm) with a new system, i.e. ␣-Fe2 O3 /SnO2 nanoheterostructure and tested the efficiency of our synthesized photocatalysts. Prior to our study,
Fig. 11. Photocatalytic degradation of MG over S1, S2, S3, S4 and S5 photocatalysts in sunlight.
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Table 2 Comparison of dye degradation result of previously reported ␣-Fe2 O3 /SnO2 photocatalysts with our synthesized photocatalyst. ␣-Fe2 O3 /SnO2
Dye concentration
Amount of catalyst
MB MB MG
1 × 10−5 M (150 ml) 1 × 10−3 M (10.3 ml) 1 × 10−4 M (20 ml)
50 mg 3 mg 40 mg
Time period (h) 4h 2.5 4h
Light
Degradation (%)
Ref.
Visible Visible Sun light
70 100 86
Ref. [11], Ref. [13], Our system (S5)
Table 3 Comparison of previously reported MG degradation with our synthesized sample. Malachite Green (ppm)
Catalyst amount ( g L−1 )
Photocatalysts
Time period (min)
Degradation (%)
Reference
20 10 100
1 1 2
BiWO6 TiO2 /WO3 Our system
75 30 360
100 99 86
[47] [48] Our system (S5)
␣-Fe2 O3 /SnO2 semiconductor heterostructure has been studied for methylene blue degradation and was presented in Table 3 [11,13]. They investigate the superior property of heterostructure which explores the easy charge separation in the interface of ␣-Fe2 O3 and SnO2 . But we made an attempt to degrade malachite green dye over mesoporous ␣-Fe2 O3 /SnO2 nanoheterostructure. Aqueous MG solution with the photocatalyst was directly exposed to the sunlight. Also the dye was tested in presence of sunlight without catalyst. But the decolourization was negligible in absence of the catalyst. Before the illumination to sunlight, the MG solution with the photocatalyst was kept in dark condition with constant stirring for 30 min, but the adsorption was only 2–3%. However with the exposure of sunlight, the performances of the photocatalysts were extremely improved. The photocatalytic degradation of MG has the trend S1< S2 < S3 < S4 < S5. With the increase of ␣-Fe2 O3 percentage, the degradation percentage is linearly improved. To optimize the reaction period, time variation plot has been carried out with highest obtained photocatalyst S5 (Fig. 12). It has been found that the percentage of degradation is increasing with time. Thus we have carried out all the photocatalytic reaction with the optimized time 4 h. The kinetic study for the degradation of MG solution under sunlight irradiation was carried out for a better comparison of the photocatalytic efficiency of all the prepared samples. The kinetics of MG degradation under sunlight irradiation over all the photocatalysts was investigated by applying the Langmuir–Hinshelwood model. [54] ln(C0 /C) = kt
Fig. 12. Effect of time over the photodegradation of MG.
Where k is the pseudo-first-order rate constant, C0 is the initial concentration of the dye and C is the concentration of the dye in the reaction time. The plots ln(C0 /C) vs. irradiation time is almost linear (as shown in Fig. 13.),which indicates the photocatalytic degradation of MG solution using the photocatalysts follows the first-order kinetics model. The apparent rate constants (k) for the photocatalytic degradation of MG were evaluated from experimental data using a linear regression. In all the cases, the R2 (correlation coefficient) value was greater than 0.95, which confirmed the proposed rate law for MG degradation. The apparent rate constants for S1, S2, S3, S4 and S5 samples were determined as 0.0034, 0.0052, 0.006, 0.0068 and 0.0075 min−1 , respectively. 3.8. Factors affecting the photocatalytic activity The significant improvement after ␣-Fe2 O3 modification could be discussed in three parts. Firstly, average particle size of a material could vary the catalytic activity of a reaction. Decreasing particle size means large percentage of atoms remains on the surface and can directly interact with the substrate [55]. Thus, particles with small size have more surface reactive site and will get enough adsorbed sites compared to its bulk counter parts which improves the catalytic activity. In addition to that, decrease of particle size increases the efficiency of the charge separation resulting in the participation of electron and hole in the redox process, according to Harada and co-workers [56]. We have observed the particle sizes are gradually decreases from S1 to S5 as calculated from TEM micrographs. Our degradations results also follow the same trend and S5 sample with 6.2 nm gives the highest activity. So our observation justifies that average particle size of our synthesized heterostructures are directly proportional to the photodegradation of MG.
Fig. 13. Kinetic analysis for the degradation of MG under sunlight using S1, S2, S3, S4 and S5 photocatalysts.
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Fig. 14. Stability test over 2 mol% ␣-Fe2 O3 /SnO2 heterojunction photocatalyst. Scheme 2. Schematic representation of photocatalytic dye degradation.
As the ␣-Fe2 O3 /SnO2 particles are interconnected, so there rapid interparticle, vectorial transport of photogenerated charge carriers (electrons and holes) is likely to occur through the grain boundaries which reduces the electron–hole recombination and hence enhances the photocatalytic activity [57]. Secondly, from a catalysis point of view, a high surface area is an advantage in terms of a greater concentration of active sites per square metre, and hence generally leads to higher reactivity. Specific surface area also plays pivotal role in the photocatalysis. Sakulkhaemaruethai et al. claims that the improvement of photocatalytic activity of TiO2 is due to the large surface area based on the existence of mesopores [58]. Here S5 sample with the highest surface area (46.6) shows highest degradation result. Interparticle mesoporous is very advantageous in enhancing the adsorption of reactants and desorption of products and satisfies as a good platform for the photocatalytic activity [57]. Correlation with this point, S5 photocatalyst with least pore diameter shows the highest activity. Thirdly band gap energy of a semiconductor determines the absorbance of type of light from the sunlight. Accordingly with the increase of visible light active ␣Fe2 O3 content in the SnO2 matrix, our photocatalysts were shifted its absorbance towards visible light as confirmed from DRS study. Therefore, the photocatalyst having the lowest band gap energy (2.81 eV) shows the highest activity for the degradation of malachite green. In ␣-Fe2 O3 /SnO2 nanoheterostructure, the conduction band and valence band edges of ␣-Fe2 O3 and SnO2 semiconductor at the point of zero charge (pHzpc) can be predicted by the following equation [59]: ECB = X − E c − 1/2 Eg
(1)
where X is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. Ec is the energy of free electrons on the hydrogen scale (ca. 4.5 eV) and Eg is the band gap of the semiconductor. The valence band edge (EVB ) can be obtained by EVB = ECB + Eg . The absolute electronegativity (X) values for ␣-Fe2 O3 and SnO2 were determined to be 5.85 and 6.205 eV, respectively. Therefore, the ECB values of ␣-Fe2 O3 and SnO2 were calculated to be 0.25 and −0.085 eV, respectively and the EVB values of ␣-Fe2 O3 and SnO2 were estimated to be 2.45 and 3.48 eV, respectively. Both SnO2 and ␣-Fe2 O3 is typical n-type semiconductor whose Fermi energy level lies just below the conduction band [60]. When the Fermi levels of ␣-Fe2 O3 and SnO2 achieved equilibration, the energy bands of SnO2 goes downward direction along with the Fermi lever whereas the energy bands of ␣-Fe2 O3 moves upward direction. This leads both
conduction band and valence band of ␣-Fe2 O3 lie above the SnO2 as shown in Scheme 2. The results describe the formation of staggered gap (type II) heterojunctions at the interface of ␣-Fe2 O3 /SnO2 [11,13]. This staggered gap (type II) heterojunctions facilitate the transformation of photoinduced electrons on the ␣-Fe2 O3 surface to SnO2 surface; similarly, photoinduced holes on the SnO2 surface migrate to ␣-Fe2 O3. Subsequently, the photogenerated charge carriers were separated at the ␣-Fe2 O3 /SnO2 interfaces, which retard the electron-hole recombination and improved photocatalytic performance.
3.9. Stability study The stability of a photocatalyst during the photocatalytic reaction is an important factor for a possible industrial application. To analyse the stability of the photocatalyst, prolonged recycling studies were performed by taking the highest result giving photocatalyst, i.e. 2 mol% Fe2 O3 /SnO2 . Fig. 14 shows the results of the repeated runs of MG degradation using the same photocatalyst, i.e. 2 mol% Fe2 O3 /SnO2 . After each cycle, the photocatalyst was collected by centrifugation and then washed with distilled water until the complete removal of dye from the catalyst. Then, the catalyst was dried at 100 ◦ C for overnight and used for another cycle. The photocatalytic activity was found to be nearly same up to four cycles which determined the stability of the photocatalyst.
4. Conclusions In summary, we have successfully fabricated visible light active mesoporous ␣-Fe2 O3 /SnO2 heterostructure photocatalysts by simple co-precipitation method. Various characterization tools such as XRD, XPS strongly evidences the formation of ␣-Fe2 O3 /SnO2 nanoheterostructure. Heterostructure with mesoporous structure of the materials tunes the property and bring changes to the activity of the photocatalysts. The observations concludes that the performance of heterostructure photocatalysts were superior to the neat SnO2 in the photocatalytic dye degradation. Particle size, surface area and band gap energy well-supported and justified the outcome of all the synthesized photocatalysts. The highest result giving photocatalyst was reused up to four cycles without significant change in the photocatalytic activity which indicated the stability of the photocatalyst.
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Acknowledgment We acknowledge Prof. B. K. Mishra, Director, IMMT, Bhubaneswar for his constant encouragement and permission to publish the paper. One of the authors, Gajendra Kumar Pradhan (CSIR-SRF), is extremely thankful to CSIR New Delhi, India, for SRF fellowship. References [1] Z. Zhang, C. Shao, X. Li, L. Zhang, H. Xue, C. Wang, Y. Liu, J. Phys. Chem. C 114 (2010) 7920–7925. [2] L. Xu, E.M.P. Steinmiller, S. ara, E. Skrabalak, J. Phys. Chem. C 116 (2012) 871–877. [3] Y. Wang, Y.R. Su, L. Qiao, L. XLiu, Q. Su, C.Q. Zhu, X.Q. Liu, Nanotechnology 22 (2011) 225702–225710. [4] S.B. Rawal, A.K. Chakraborty, W.I. Lee, Bull. Korean Chem. Soc. 30 (2009) 2613–2616. [5] Z. Wang, Z. Li, J. Sun, H. Zhang, W. Wang, W. Zheng, C. Wang, J. Phys. Chem. C 114 (2010) 6100–6105. [6] P. Kundu, P.A. Deshpande, G. Madras, N. Ravishankar, J. Mater. Chem. 21 (2011) 4209–4216. [7] S. Sakthivel, S.U. Geissen, D.W. Bahnemann, V. Murugesan, A. Vogelpohl, J. Photochem. Photobio. A: Chem. 148 (2002) 283–293. [8] H.G. Kim, P.H. Borse, J.S. Jang, E.D. Jeong, O.S. Jung, Y.J. Suhd, J.S. Lee, Chem. Commun. (2009) 5889–5891. [9] H. Zhu, D. Yang, G. Yu, H. Zhang, K. Yao, Nanotechnology 17 (2006) 2386–2389. [10] Z. Zhang, M.F. Hossain, T. Miyazak, T. Takahashi, Environ. Sci. Technol. 44 (2010) 4741–4746. [11] M. Niu, F. Huang, L. Cui, P. Huang, Y. Yu, Yuansheng Wang, ACS Nano 4 (2010) 681–688. [12] D.K. Zhong, J. Sun, H. Inumaru, D.R. Gamelin, J. Am. Chem. Soc. 131 (2009) 6086–6087. [13] J. Kang, Q. Kuang, Z.X. Xie, L.S. Zheng, J. Phys. Chem. C 115 (2011) 7874–7879. [14] J. Xu, F. Huang, Y. Yu, A. Yang, Y. Wang, CrystEngComm 13 (2011) 4873. [15] S. Zhang, F. Ren, W. Wu, J. Zhou, X. Xiao, L. Sun, Y. Liu, C. Jiang, Phys. Chem. Chem. Phys. 15 (2013) 8228–8236. [16] A.A. Ismail, D.W. Bahnemann, J. Mater. Chem. 21 (2011) 11686–11707. [17] Q.Q. Wang, B.Z. Lin, B.H. Xu, X.L. Li, Z.J. Chen, X.T. Pian, Micro. Meso. Mater. 130 (2010) 344–351. [18] N. Ulagappan, C.N.R. Rao, Chem. Commun. 14 (1996) 1685–1686. [19] N. Lakshminarasimhan, E. Bae, W. Choi, J. Phys. Chem. C 111 (2007) 15244–15250. [20] D.M. Antonelli, Y.J. Ying, Angew. Chem. Int. Ed. 34 (1995) 2014. [21] L.Z. Zhang, J.C. Yu, Chem. Commun. (2003) 2078. [22] Y. Wang, H. Xu, X. Wang, X. Zhang, H. Jia, L. Zhang, J. Qiu, J. Phys. Chem. B 110 (2006) 13835. [23] G.J. Wilson, G.D. Will, R.L. Frost, S.A.J. Montgomery, Mater. Chem. 12 (2002) 787.
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Facile fabrication of mesoporous ␣-Fe2 O3 /SnO2 nanoheterostructure for photocatalytic degradation of malachite green Gajendra Kumar Pradhan, K. Hemalata Reddy, K.M. Parida ∗ Colloids and Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar 751013, Odisha, India
a r t i c l e
i n f o
Article history: Received 27 July 2013 Received in revised form 6 October 2013 Accepted 11 October 2013 Available online 8 November 2013 Keywords: Nanoheterostructure Mesoporous Photocatalysis.
a b s t r a c t A series of ␣-Fe2 O3 /SnO2 semiconductor nanoheterostructure photocatalysts have been fabricated using a facile co-precipitation method. Precipitation of both SnO2 and ␣-Fe2 O3 were achieved with the slow hydrolysis of urea followed by addition of 1% NH3 and the products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), DR UV–vis spectroscopy, and N2 adsorption–desorption. Analytical results explain the synthesized materials are crystalline, mesoporous, visible light active (except SnO2 ) and also presents in the nano-order form. The as-prepared ␣-Fe2 O3 /SnO2 photocatalysts possessed extended light absorption in the visible range and proficient charge separation properties simultaneously. The significant enhancement in the reaction rate was observed in ␣-Fe2 O3 /SnO2 , compared to the bare SnO2 towards photodegradation of malachite green. The highest degradation percentage was found to be 86% with the 2 mol % ␣-Fe2 O3 modified SnO2 catalyst. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor heterojunction is the building block of two or more different semiconductor having dissimilar physical properties. In recent years several heterostructure systems such as TiO2 /V2 O5 , FeOOH/TiO2 , TiO2 /SnO2 , ZnO/SnO2 , NiO/SnO2 , ZnO/CdS, ␣-Fe2 O3 /ZnO, WO3 /ZnO, CdS/ZnO, CaFe2 O4 /MgFe2 O4 have been reported for different applications [1–8]. Electron transport at the interface between two semiconductors is one of the important aspects for the design of this novel material. Thus, the photogenerated charge carriers, i.e. electrons and holes are easily separated and are utilized for redox reaction. The new hybrid structure has several positive aspects for photocatalytic application which includes dye degradation and water splitting. SnO2 is an n-type UV light absorptive semiconductor possessing band gap energy of 3.6 eV [1,9]. So modification is necessary to shift its optical absorptive property towards higher wavelength. On the other hand, ␣-Fe2 O3 is a visible light active photocatalyst having band gap energy 2.2 eV [10–12]. SnO2 and ␣-Fe2 O3 are two opposite character photocatalyst having optical absorptivity in the UV and visible region. Therefore, the heterojunction between these two semiconductors would be more fruitful to use as a visible light active photocatalyst. ␣-Fe2 O3 /SnO2 system has special interest and has been studied by many research groups for
∗ Corresponding author. Fax: +91674 2581637. E-mail address: [email protected] (K.M. Parida). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.10.038
various photocatalytic application [11,13–15]. Niu et al. synthesize the ␣-Fe2 O3 /SnO2 semiconductor nanoheterostructures by hydrothermal technique and studied for the degradation of methylene blue [11]. Kang and co-workers designed hierarchical ␣Fe2 O3 /SnO2 heterostructure by chemical vapour deposition (CVD) method and also tested their photocatalytic activity for the degradation of same dye, i.e. methylene blue[13]. Xu et al. fabricated SnO2 /␣-Fe2 O3 nanoheterostructures and studied their photocatalytic degradation of methylene blue [14]. All groups claimed that photogenerated electrons and holes were separated at the ␣-Fe2 O3 /SnO2 interfaces and resulting in the enhancement of photocatalytic activity. Another important aspect of the catalyst is mesoporous nature of the material because of its high surface area. In the last three decades, mesoporous materials exhibited superior performances in different catalytic reactions whereas the mesoporous structure is highly required in the photodegradation of organic pollutants, which can make the macromolecules quickly diffuse into the active sites in a photocatalyst, leading to photooxidation. So, the semiconducting material having mesoporosity would be more preferable and suitable for the photocatalysis reactions[16,17]. Thus, our target is to prepare mesoporous semiconducting material which is suitable for photocatalytic dye degradation. The first attempt to synthesize mesoporous tin oxide was reported by Rao and Ulagappan using anionic surfactant [18]. There are several methods reported in the literature for the synthesis of meso-SnO2 such as sol gel, sonochemical, hydrothermal, microwave hydrothermal and template-assisted methods [19–23]. Although most synthesis
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processes involve the use of templates and hydrothermal treatment. Here, the preparation of meso-SnO2 can be achieved by a simple co-precipitation method without using any template. Although many SnO2 /Fe2 O3 heterostructures have been reported for photocatalytic reactions, we have fabricated a mesoporous SnO2 /Fe2 O3 heterostructures for the first time by co-precipitation method to improve the photocatalytic performance. Moreover, the introducing of Fe2 O3 species into the mesoporous SnO2 framework can effectively extend the photo response of the SnO2 to the visible region and improved the photocatalytic activity of SnO2 . The fabricated material possesses larger surface area, mesoporous nature and visible light absorption capacity, which are the important factors for the improvement of photocatalytic activities. Previously, we have studied various systems such as ␣-Fe2 O3 , Fe2 O3 -ZnO, Fe2 O3 -CeO2 , S,N doped ␣-Fe2 O3 , ␣-Fe2 O3 /RGO composite for degradation of organic pollutants [24–28]. With the purpose of efficient electron–hole separation and enhancement of photocatalytic performance of SnO2 in the visible region, our principal approach is to design a heterostructure between ␣-Fe2 O3 and SnO2 along with mesoporous character for photocatalytic application. In this context, we have synthesized a series of visible light active ␣-Fe2 O3 /SnO2 nanoheterostructures by a simple coprecipitation method and tested their photocatalytic activity for the degradation of malachite green under natural sunlight. 2. Experimental 2.1. Materials All the chemicals and reagents are of analytical grade and used without further purification. Iron(III) nitrate nonahydrate (Fe (NO3 )3 ·9H2 O) (Across), tin(IV) chloride pentahydrate (SnCl4 ·5H2 O) (Sarabhai chemicals), urea (NH2 CONH2 ) (S.d. fine chemicals), ethanol and deionised water were used for the sample preparation. Malachite green (MG) dye was used for testing for photocatalysis. 2.2. Methods
Scheme 1. Schematic diagram of fabrication of mesoporous ␣-Fe2 O3 /SnO2 nanoheterostructure.
were calibrated by using the contaminant carbon (C1s = 284.9 eV) as a reference. Charge neutralization of 2 eV was used to balance the charge of the sample. The surface area and pore diameter were measured by N2 adsorption–desorption method at −196 ◦ C in Automated Surface area and Porosity Analyser (ASAP 2020, Micromeritics, USA). Prior to the analysis, samples were degassed under vacuum (10−5 torr) at 300 ◦ C for 4 h. Accordingly, specific surface area, pore volume and pore diameter of all the synthesized photocatalysts were calculated by BET and BJH method. 2.4. Photocatalytic dye degradation
In a typical experiment, stoichiometric amount of urea, Fe (NO3 )3 .9H2 O, SnCl4 .5H2 O, water and ethanol were taken for the synthesis of photocatalysts. Keeping the tin(IV) chloride amount fixed, we have varied iron(III) nitrate nonahydrate. The molar ratio of iron(III) nitrate nonahydrate to tin(IV) chloride were controlled to be 0, 0.5, 1, 1.5 and 2 mol% and were represented as S1, S2, S3, S4, and S5, respectively. The above solutions were allowed to stir for complete hydrolysis under ice-cold condition. After 3 h of stirring, the pH of the solution was maintained to 7 with 1% NH3 for complete precipitation. Again it was stirring for 1 h, filtered, and washed with water and ethanol. Then it was dried at 110 ◦ C for overnight and calcined at 500 ◦ C for 4 h. The calcined samples were tested for photocatalytic dye degradation.
The photodegradation of all the synthesized photocatalysts were tested towards degradation of malachite green (MG) under solar radiation. In a typical experiment, 40 mg photocatalyst with 20 ml of 100 ppm MG solution was taken in a 100 ml closed Pyrex flask. The solutions were exposed to sunlight with constant stirring for 4 h. After irradiation, the suspension was centrifuged and the concentration of the supernatants were analyzed quantitatively at 616 nm (max for MG) using a Cary-100 (Varian, Australia) spectrophotometer. The intensity of sun light was measured using Digital Illuminance Meter (Model-TES-1332A, Taiwan). During the measurement, the sensor was set in such a position where the intensity is high. The average light intensity was around 100,000 lux, which was nearly constant (10AM-2PM) during the experiments.
2.3. Characterization
3. Results and discussion
Powder X-ray diffraction patterns of the samples were recorded in a PANalytical X-ray diffractometer using Ni filtered Cu K␣ radi˚ SAX in the 2 range from 0 to 10o was also ation (= 1.5418 A). carried out to know the mesoporous nature of the material by using a Rigaku X-ray diffractometer. Transmission electron microscope (TEM) of the catalysts were recorded with a Phillips, TECNAI G2 20 operating at an acceleration voltage of 200 kV. Optical absorbance spectra of the catalysts were recorded using a Cary 100 spectrophotometer (Varian) in the spectral range of 200–800 nm. Electronic states of Fe and Sn were examined by X-ray photoelectron spectroscopy (XPS, Kratos Axis 165 with a dual anode (Mg and Al) apparatus) using the MgK␣ source. All the binding energy values
3.1. Fabrication of mesoporous SnO2 and ˛-Fe2 O3 /SnO2 nanoheterostructure The synthetic procedure for ␣-Fe2 O3 /SnO2 nanoparticles is presented in Scheme 1. Urea precipitation allows the easy control of the rate of supersaturation and permits the synthesis of well-defined particles, once the appropriate conditions are maintained [29]. Previously, various metal oxides are prepared by precipitation method in the presence of urea followed by heating at higher temperature (80–100 ◦ C) [30–33]. In our case, the reaction was designed at room temperature to control the rate of particle growth and surface properties. The ammonia generates under this condition is not
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Fig. 3. SAX pattern of the 2 mol% Fe2 O3 /SnO2 .
Fig. 1. Typical PXRD patterns of S1, S2, S3, S4 and S5 photocatalysts.
as 2.69 is identified as ␣-Fe2 O3 . Fig. 3 represents the small angle ◦ (2 = 0–10 ) X-ray diffraction (SAX) of the 2 mol% Fe2 O3 /SnO2 . It was observed that 2 mol% Fe2 O3 /SnO2 shows the diffraction peaks ◦ at 2 = 0.48 and 0.69 , respectively. This confirms the mesoporous nature of the material. 3.3. TEM
Fig. 2. Deconvoluted [1 0 1] plane in the XRD of S5 photocatalyst.
sufficient enough to precipitate both SnO2 and ␣-Fe2 O3 . Thus a small amount of dilute NH3 was added for complete precipitation. During the addition of dilute NH3 , aggregation of nuclei leads to growth of ␣-Fe2 O3 /SnO2 particle. The aggregation of small particles leading to voids in between the particle which escorts the formation of interparticle pore. 3.2. XRD Powder X-ray diffraction pattern of the synthesized photocatalyst is presented in Fig. 1. All the diffraction patterns obtained were indexed to the characteristic peaks of tetragonal SnO2 in the S1 (JCPDS 13-534). However no marginal shifting of peaks was observed in the S2, S3, S4 and S5 samples. As the percentage of Fe2 O3 is very small, so no peaks were found for it in the XRD pattern. At lower concentration of Fe2 O3 (0.5–1.5 mol%), no crystalline phase of Fe2 O3 was observed in the XRD pattern. This may be possible due to very low content of Fe2 O3 which can barely be detected by XRD or the Fe2 O3 species was highly dispersed on the mesoporous SnO2 framework structure [34,35]. When the Fe2 O3 content was 2.0 mol%, the characteristic XRD peaks became broad in comparison to low iron containing samples. This broadening of the peak may be due to the overlapping of the Fe2 O3 and SnO2 peaks which was later confirmed by the 110 peak deconvolution. Taking into the account, to identify the unknown moiety in case of 2 mol% Fe2 O3 /SnO2 sample, we have deconvoluted the [101] peak into two and is shown in Fig. 2. The d-value with 2.64 is for SnO2 where
The morphology, particle size and structure of the products were characterized by TEM (Fig. 4). The particles are well developed having definite size and shape. It can be seen that the diameters of the particles are in the range of 6–14 nm for all the photocatalysts (Table 1). S1 sample exhibits particle size 13.27 nm which gradually decreases with incorporation of iron oxide. This is also strongly supported by the peak broadening in the XRD figure. No marginal changes observed on the surface and morphology of the particle in all the samples. The selected area electron diffraction (SAED) pattern of 2 mol% ␣-Fe2 O3 /SnO2 sample was presented in Fig. 5(a). The SAED pattern revealed several bright continuous concentric rings attributed to the diffraction from the (1 1 0), (1 0 1), (2 0 0), (2 1 1) and (3 0 1) planes of SnO2 , respectively, consistent well with the XRD data. The SAED pattern indicates the polycrystalline nature of the prepared photocatalysts. The HRTEM image of 2 mol% ␣-Fe2 O3 /SnO2 was presented in Fig. 5(b). The HRTEM image shows clear lattice fringes that demonstrate the highly crystallinity nature of the sample. The HRTEM image of the 2 mol% ␣-Fe2 O3 /SnO2 shows lattice planes in regular interval and the plane intervals of 0.335 nm and 0.264 nm corresponding to interplanner spacing of (1 1 0) and (1 0 1) planes of SnO2. This data is well consistent with the XRD pattern of the sample. The HRTEM image describes the wormwhole type mesoporosity in case of ␣Fe2 O3 /SnO2 nanoheterostructure (Fig. 6). This type of mesoporosity arises due to intergrowth of small particles and the same leads to aggregates with significant extra framework void space. This adds extra advantage for the fast diffusion of charge carriers in the photocatalytic process [36–38] (Fig. 6).. 3.4. DRS The photo absorption properties of neat SnO2 and ␣Fe2 O3 /SnO2 nanoheterostructures were characterized by UV–vis diffuse reflectance spectrometry (Fig. 7). The absorption edges of ␣Fe2 O3 /SnO2 are located at longer wavelengths than those of SnO2 but gradually shifted to longer wavelengths with raising the concentration of ␣-Fe2 O3 . The results indicate that the ␣-Fe2 O3 /SnO2 nanoheterostructure have a suitable band gap for photocatalytic decomposition of organic contaminants under visible light irradiation. The band gap energy of the present nanoheterostructures is roughly estimated to be 3.57–2.81 eV based on the onsets of the diffuse reflectance spectra, which is substantially smaller than
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Fig. 4. Bright-field TEM images of S1, S2, S3, S4 and S5 photocatalysts.
Table 1 Textural properties and degradation results of all the synthesized photocatalysts. Catalysts
Particle size (nm)
Band gap energy (eV)
Surface area (m2 /g)
Pore volume (cm3 /g)
Pore diameter (nm)
Degradation (%)
S1 S2 S3 S4 S5
13.27 9.15 8.5 6.75 6.2
3.57 3.09 3.0 2.91 2.81
23.6 26.6 35.5 38.6 46.6
0.105 0.097 0.087 0.110 0.168
17.9 14.4 14.2 11.4 9.8
66 71 75 80 86
that of SnO2 (3.57 eV). The shifts in the position of the absorption edges from 347 to 450 nm justify the range of white-pale yellow colours observed for the powders. The red shift of ␣-Fe2 O3 /SnO2 nanoheterostructure is due to the contribution of ␣-Fe2 O3 in SnO2 . The ␣-Fe2 O3 is a visible light active n-type semiconductor with bandgap energy 2.2 eV [39–41]. When it couples with n-type SnO2 semiconductor, the electrons from the conduction band of ␣-Fe2 O3 migrate to the conduction band of SnO2 and photogenerated holes move in the opposite direction, which leads to an increase in charge
± ± ± ± ±
4 2 3 4 3
separation efficiency and extends the photoresponding range to visible light [42]. 3.5. XPS X-ray photoelectron spectroscopy was carried out to know the oxidation states, electronic structure and electronic environment of the elements present in the S5 photocatalyst. Fig. 8 (a)–(c) presents the high-resolution XPS spectrum of O 1s, Fe 2p and Sn 3d in
Fig. 5. (a) SAED and (b) HRTEM of S5 photocatalyst.
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Fig. 6. HRTEM images of S5 photocatalyst presenting the mesopore.
the S5 sample. The binding energy (BE) component observed at 530 eV is attributed to the O2− forming oxide with metal [43]. The obtained binding energy (BE) peaks of Sn 3d5/2 and Sn 3d3/2 were 486.2 and 494.6, respectively, which is due to +4 oxidation states of SnO2 [44]. The peak position of Fe 2p3/2 for ␣-Fe2 O3 has been investigated by many researchers previously and reported that the binding energy (BE) value lies between 710.6 and 711.2 eV. However in our system, the Fe 2p3/2 and Fe 2p1/2 peak positions were 711.1 and 724.6 eV which could be assigned to +3 oxidation state of iron in ␣-Fe2 O3 [45–48]. The difference between Fe 2p3/2 and Fe 2p1/2 peak is 13.5 eV which ensures the Fe is in oxide form and is ␣-Fe2 O3 . XPS also support the presence of both SnO2 and ␣-Fe2 O3 in the composite sample. Fig. 7. DRS of S1, S2, S3, S4 and S5 photocatalysts.
Fig. 8. (a) High-energy resolution O 1s core-level XPS spectrum of S5 photocatalyst. (b) High-energy resolution Fe 2p core-level XPS spectrum of S5 photocatalyst. (c): High-energy resolution Sn 3d core-level XPS spectrum of S5 photocatalyst.
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Fig. 9. N2 adsorption-desorption isotherm of (a) S1, (b) S2, (c) S3, (d) S4 and (e) S5 photocatalysts.
3.6. N2 adsorption–desorption A typical nitrogen adsorption–desorption isotherm of all the photocatalysts are shown in Fig. 9. All the samples exhibit type IV nitrogen sorption isotherm with a capillary condensation step in the relative pressure (P/P0 ) range 0.6–0.9 (type H2 hysteresis loop), indicating a well-developed mesoporosity [49]. The surface area, pore diameter and pore volume of all the photocatalysts were presented in Table. 1. It was found that the specific surface area is gradually increasing with the increase of ␣-Fe2 O3 content in the SnO2 moiety and lies in the range of 23.6–46.6 m2 /g. Therefore, we conclude that the incorporation of Fe2 O3 in to the SnO2 framework is responsible for the decrease in particle size and increase in surface area [50,51]. In addition to that the pore-size distribution is calculated using the Barrett–Joyner–Halenda (BJH) from the desorption isotherm (Fig. 10). It indicates that all the photocatalysts have pronounced mesoporosity of a narrow pore-size distribution with an average pore size from 17.9 to 9.8 nm. The pore diameter and pore volume have also the same trend, i.e. S1< S2 < S3 < S4 < S5. The mesoporous structure and surface area play pivotal role in
Fig. 10. BJH isotherm of S1, S2, S3, S4 and S5 photocatalysts.
catalyst design for its ability to improve the molecular transport of reactants. 3.7. Photocatalytic reaction The performance of all the photocatalysts were discussed in terms of particle size, band gap energy and surface area. The percentage of degradation of malachite green was examined as a function of catalyst (Fig. 11). Previously various groups have reported the MG degradation by taking different photocatalysts which is presented in Table 2. Chen et al. studied the degradation of malachite green by BiWO6 photocatalyst. They claimed 100% degradation of malachite green (20 ppm) with a time period of 75 min and 1 g L−1 of catalyst [52]. Wei groups reported the degradation of 10 ppm malachite green with 99% degradation in 30 min [53]. Here we have made an attempt to degrade the same MG (100 ppm) with a new system, i.e. ␣-Fe2 O3 /SnO2 nanoheterostructure and tested the efficiency of our synthesized photocatalysts. Prior to our study,
Fig. 11. Photocatalytic degradation of MG over S1, S2, S3, S4 and S5 photocatalysts in sunlight.
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Table 2 Comparison of dye degradation result of previously reported ␣-Fe2 O3 /SnO2 photocatalysts with our synthesized photocatalyst. ␣-Fe2 O3 /SnO2
Dye concentration
Amount of catalyst
MB MB MG
1 × 10−5 M (150 ml) 1 × 10−3 M (10.3 ml) 1 × 10−4 M (20 ml)
50 mg 3 mg 40 mg
Time period (h) 4h 2.5 4h
Light
Degradation (%)
Ref.
Visible Visible Sun light
70 100 86
Ref. [11], Ref. [13], Our system (S5)
Table 3 Comparison of previously reported MG degradation with our synthesized sample. Malachite Green (ppm)
Catalyst amount ( g L−1 )
Photocatalysts
Time period (min)
Degradation (%)
Reference
20 10 100
1 1 2
BiWO6 TiO2 /WO3 Our system
75 30 360
100 99 86
[47] [48] Our system (S5)
␣-Fe2 O3 /SnO2 semiconductor heterostructure has been studied for methylene blue degradation and was presented in Table 3 [11,13]. They investigate the superior property of heterostructure which explores the easy charge separation in the interface of ␣-Fe2 O3 and SnO2 . But we made an attempt to degrade malachite green dye over mesoporous ␣-Fe2 O3 /SnO2 nanoheterostructure. Aqueous MG solution with the photocatalyst was directly exposed to the sunlight. Also the dye was tested in presence of sunlight without catalyst. But the decolourization was negligible in absence of the catalyst. Before the illumination to sunlight, the MG solution with the photocatalyst was kept in dark condition with constant stirring for 30 min, but the adsorption was only 2–3%. However with the exposure of sunlight, the performances of the photocatalysts were extremely improved. The photocatalytic degradation of MG has the trend S1< S2 < S3 < S4 < S5. With the increase of ␣-Fe2 O3 percentage, the degradation percentage is linearly improved. To optimize the reaction period, time variation plot has been carried out with highest obtained photocatalyst S5 (Fig. 12). It has been found that the percentage of degradation is increasing with time. Thus we have carried out all the photocatalytic reaction with the optimized time 4 h. The kinetic study for the degradation of MG solution under sunlight irradiation was carried out for a better comparison of the photocatalytic efficiency of all the prepared samples. The kinetics of MG degradation under sunlight irradiation over all the photocatalysts was investigated by applying the Langmuir–Hinshelwood model. [54] ln(C0 /C) = kt
Fig. 12. Effect of time over the photodegradation of MG.
Where k is the pseudo-first-order rate constant, C0 is the initial concentration of the dye and C is the concentration of the dye in the reaction time. The plots ln(C0 /C) vs. irradiation time is almost linear (as shown in Fig. 13.),which indicates the photocatalytic degradation of MG solution using the photocatalysts follows the first-order kinetics model. The apparent rate constants (k) for the photocatalytic degradation of MG were evaluated from experimental data using a linear regression. In all the cases, the R2 (correlation coefficient) value was greater than 0.95, which confirmed the proposed rate law for MG degradation. The apparent rate constants for S1, S2, S3, S4 and S5 samples were determined as 0.0034, 0.0052, 0.006, 0.0068 and 0.0075 min−1 , respectively. 3.8. Factors affecting the photocatalytic activity The significant improvement after ␣-Fe2 O3 modification could be discussed in three parts. Firstly, average particle size of a material could vary the catalytic activity of a reaction. Decreasing particle size means large percentage of atoms remains on the surface and can directly interact with the substrate [55]. Thus, particles with small size have more surface reactive site and will get enough adsorbed sites compared to its bulk counter parts which improves the catalytic activity. In addition to that, decrease of particle size increases the efficiency of the charge separation resulting in the participation of electron and hole in the redox process, according to Harada and co-workers [56]. We have observed the particle sizes are gradually decreases from S1 to S5 as calculated from TEM micrographs. Our degradations results also follow the same trend and S5 sample with 6.2 nm gives the highest activity. So our observation justifies that average particle size of our synthesized heterostructures are directly proportional to the photodegradation of MG.
Fig. 13. Kinetic analysis for the degradation of MG under sunlight using S1, S2, S3, S4 and S5 photocatalysts.
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Fig. 14. Stability test over 2 mol% ␣-Fe2 O3 /SnO2 heterojunction photocatalyst. Scheme 2. Schematic representation of photocatalytic dye degradation.
As the ␣-Fe2 O3 /SnO2 particles are interconnected, so there rapid interparticle, vectorial transport of photogenerated charge carriers (electrons and holes) is likely to occur through the grain boundaries which reduces the electron–hole recombination and hence enhances the photocatalytic activity [57]. Secondly, from a catalysis point of view, a high surface area is an advantage in terms of a greater concentration of active sites per square metre, and hence generally leads to higher reactivity. Specific surface area also plays pivotal role in the photocatalysis. Sakulkhaemaruethai et al. claims that the improvement of photocatalytic activity of TiO2 is due to the large surface area based on the existence of mesopores [58]. Here S5 sample with the highest surface area (46.6) shows highest degradation result. Interparticle mesoporous is very advantageous in enhancing the adsorption of reactants and desorption of products and satisfies as a good platform for the photocatalytic activity [57]. Correlation with this point, S5 photocatalyst with least pore diameter shows the highest activity. Thirdly band gap energy of a semiconductor determines the absorbance of type of light from the sunlight. Accordingly with the increase of visible light active ␣Fe2 O3 content in the SnO2 matrix, our photocatalysts were shifted its absorbance towards visible light as confirmed from DRS study. Therefore, the photocatalyst having the lowest band gap energy (2.81 eV) shows the highest activity for the degradation of malachite green. In ␣-Fe2 O3 /SnO2 nanoheterostructure, the conduction band and valence band edges of ␣-Fe2 O3 and SnO2 semiconductor at the point of zero charge (pHzpc) can be predicted by the following equation [59]: ECB = X − E c − 1/2 Eg
(1)
where X is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. Ec is the energy of free electrons on the hydrogen scale (ca. 4.5 eV) and Eg is the band gap of the semiconductor. The valence band edge (EVB ) can be obtained by EVB = ECB + Eg . The absolute electronegativity (X) values for ␣-Fe2 O3 and SnO2 were determined to be 5.85 and 6.205 eV, respectively. Therefore, the ECB values of ␣-Fe2 O3 and SnO2 were calculated to be 0.25 and −0.085 eV, respectively and the EVB values of ␣-Fe2 O3 and SnO2 were estimated to be 2.45 and 3.48 eV, respectively. Both SnO2 and ␣-Fe2 O3 is typical n-type semiconductor whose Fermi energy level lies just below the conduction band [60]. When the Fermi levels of ␣-Fe2 O3 and SnO2 achieved equilibration, the energy bands of SnO2 goes downward direction along with the Fermi lever whereas the energy bands of ␣-Fe2 O3 moves upward direction. This leads both
conduction band and valence band of ␣-Fe2 O3 lie above the SnO2 as shown in Scheme 2. The results describe the formation of staggered gap (type II) heterojunctions at the interface of ␣-Fe2 O3 /SnO2 [11,13]. This staggered gap (type II) heterojunctions facilitate the transformation of photoinduced electrons on the ␣-Fe2 O3 surface to SnO2 surface; similarly, photoinduced holes on the SnO2 surface migrate to ␣-Fe2 O3. Subsequently, the photogenerated charge carriers were separated at the ␣-Fe2 O3 /SnO2 interfaces, which retard the electron-hole recombination and improved photocatalytic performance.
3.9. Stability study The stability of a photocatalyst during the photocatalytic reaction is an important factor for a possible industrial application. To analyse the stability of the photocatalyst, prolonged recycling studies were performed by taking the highest result giving photocatalyst, i.e. 2 mol% Fe2 O3 /SnO2 . Fig. 14 shows the results of the repeated runs of MG degradation using the same photocatalyst, i.e. 2 mol% Fe2 O3 /SnO2 . After each cycle, the photocatalyst was collected by centrifugation and then washed with distilled water until the complete removal of dye from the catalyst. Then, the catalyst was dried at 100 ◦ C for overnight and used for another cycle. The photocatalytic activity was found to be nearly same up to four cycles which determined the stability of the photocatalyst.
4. Conclusions In summary, we have successfully fabricated visible light active mesoporous ␣-Fe2 O3 /SnO2 heterostructure photocatalysts by simple co-precipitation method. Various characterization tools such as XRD, XPS strongly evidences the formation of ␣-Fe2 O3 /SnO2 nanoheterostructure. Heterostructure with mesoporous structure of the materials tunes the property and bring changes to the activity of the photocatalysts. The observations concludes that the performance of heterostructure photocatalysts were superior to the neat SnO2 in the photocatalytic dye degradation. Particle size, surface area and band gap energy well-supported and justified the outcome of all the synthesized photocatalysts. The highest result giving photocatalyst was reused up to four cycles without significant change in the photocatalytic activity which indicated the stability of the photocatalyst.
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Acknowledgment We acknowledge Prof. B. K. Mishra, Director, IMMT, Bhubaneswar for his constant encouragement and permission to publish the paper. One of the authors, Gajendra Kumar Pradhan (CSIR-SRF), is extremely thankful to CSIR New Delhi, India, for SRF fellowship. References [1] Z. Zhang, C. Shao, X. Li, L. Zhang, H. Xue, C. Wang, Y. Liu, J. Phys. Chem. C 114 (2010) 7920–7925. [2] L. Xu, E.M.P. Steinmiller, S. ara, E. Skrabalak, J. Phys. Chem. C 116 (2012) 871–877. [3] Y. Wang, Y.R. Su, L. Qiao, L. XLiu, Q. Su, C.Q. Zhu, X.Q. Liu, Nanotechnology 22 (2011) 225702–225710. [4] S.B. Rawal, A.K. Chakraborty, W.I. Lee, Bull. Korean Chem. Soc. 30 (2009) 2613–2616. [5] Z. Wang, Z. Li, J. Sun, H. Zhang, W. Wang, W. Zheng, C. Wang, J. Phys. Chem. C 114 (2010) 6100–6105. [6] P. Kundu, P.A. Deshpande, G. Madras, N. Ravishankar, J. Mater. Chem. 21 (2011) 4209–4216. [7] S. Sakthivel, S.U. Geissen, D.W. Bahnemann, V. Murugesan, A. Vogelpohl, J. Photochem. Photobio. A: Chem. 148 (2002) 283–293. [8] H.G. Kim, P.H. Borse, J.S. Jang, E.D. Jeong, O.S. Jung, Y.J. Suhd, J.S. Lee, Chem. Commun. (2009) 5889–5891. [9] H. Zhu, D. Yang, G. Yu, H. Zhang, K. Yao, Nanotechnology 17 (2006) 2386–2389. [10] Z. Zhang, M.F. Hossain, T. Miyazak, T. Takahashi, Environ. Sci. Technol. 44 (2010) 4741–4746. [11] M. Niu, F. Huang, L. Cui, P. Huang, Y. Yu, Yuansheng Wang, ACS Nano 4 (2010) 681–688. [12] D.K. Zhong, J. Sun, H. Inumaru, D.R. Gamelin, J. Am. Chem. Soc. 131 (2009) 6086–6087. [13] J. Kang, Q. Kuang, Z.X. Xie, L.S. Zheng, J. Phys. Chem. C 115 (2011) 7874–7879. [14] J. Xu, F. Huang, Y. Yu, A. Yang, Y. Wang, CrystEngComm 13 (2011) 4873. [15] S. Zhang, F. Ren, W. Wu, J. Zhou, X. Xiao, L. Sun, Y. Liu, C. Jiang, Phys. Chem. Chem. Phys. 15 (2013) 8228–8236. [16] A.A. Ismail, D.W. Bahnemann, J. Mater. Chem. 21 (2011) 11686–11707. [17] Q.Q. Wang, B.Z. Lin, B.H. Xu, X.L. Li, Z.J. Chen, X.T. Pian, Micro. Meso. Mater. 130 (2010) 344–351. [18] N. Ulagappan, C.N.R. Rao, Chem. Commun. 14 (1996) 1685–1686. [19] N. Lakshminarasimhan, E. Bae, W. Choi, J. Phys. Chem. C 111 (2007) 15244–15250. [20] D.M. Antonelli, Y.J. Ying, Angew. Chem. Int. Ed. 34 (1995) 2014. [21] L.Z. Zhang, J.C. Yu, Chem. Commun. (2003) 2078. [22] Y. Wang, H. Xu, X. Wang, X. Zhang, H. Jia, L. Zhang, J. Qiu, J. Phys. Chem. B 110 (2006) 13835. [23] G.J. Wilson, G.D. Will, R.L. Frost, S.A.J. Montgomery, Mater. Chem. 12 (2002) 787.
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