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degradation of organic pollutants over highly active hetero junction based vanadium phosphate catalyst
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Unexpected rapid photo-catalytic decolourisation/degradation of organic pollutants over highly active hetero junction based vanadium phosphate catalyst Gobinda Chandra Behera a,∗ , Niranjan Biswal b , Kulamani Parida c,∗ a
Department of Chemistry, Kendrapara Autonomous College, Kendrapara 754211,Odisha, India Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 3200003, Israel c Centre for Nano Science & Nano Technology, Siksha ‘O’Anusandhan University, Bhubaneswar 751030, Odisha, India b
a r t i c l e
i n f o
Article history: Received 24 May 2016 Received in revised form 21 September 2016 Accepted 23 October 2016 Available online xxx Keywords: Coupled oxide (WO3 -VPO) Photocatalytic decolourisation/degradation Organic pollutants Vanadium phosphate Visible light
a b s t r a c t VPO materials are extensively used the in gas phase oxidation of n-butane to maleic anhydride. Herein we explored its catalytic activity towards decolourisation/degradation of organic pollutants. This investigation reports the remarkable photo catalytic activity of tungstate promoted vanadium phosphate (WO3 -VPO) toward photocatalyic decolourisation/degradation of pollutants. The catalyst i.e. 10 wt% WO3 -VPO showed unexpected efficacy i.e. 100% decolourisation of Rhodamine B (RhB) and 60% degradation of phenol under visible light irradiation with a short span of time i.e. 10 min. Several factors involving the significant results of the composite material than the neat sample have been studied in detail. The analysis of photoluminescence (PL) emission spectra, photoelectrochemical measurement and the cyclic voltammeter (CV) measurement of the composite materials greatly support the outstanding results of the catalyst. The remarkable efficiency of VPO materials in photocatalytic decolourisation/degradation would become another trademark in the photocatalysis society. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Environmental problems are normally recounted to waste and toxic organic with inorganic pollutants released in water bodies. The majority of the coloured and uncoloured effluents originated in contaminated waters contain organic dyes and phenolic compounds from textiles, dyestuff, dyeing industries, fertilizers and chemical production industries [1–3]. Several works on decolourisation/degradation of organic pollutants under visible light irradiation have been reported [4–11]. Our group and numerous researchers have evaluated the visible light driven photocatalytic decomposition of Rhodamine B (RhB) and phenolic compounds [12–17]. Very few literatures are reported on phosphate systems and WO3 promoted phosphate catalysts for the oxidation of organic contaminants [18–22]. Herein, we report another phosphate system i.e. vanadium phosphate (VPO), a well known material which have extensively
been used industrially in the manufacture of maleic anhydride from n-butane [23]. However there is relatively little research on VPO materials in liquid phase reactions [24–31]. To tackle this tough issue, we put effort to explore the catalytic potential of VPO material towards some liquid phase reactions [24–26,29–31]. The results of our preceding works encouraged us to move into another catalytic world, photocatalysis. Photocatalytic materials have now gained much attention toward pollutant removal. In this concern, we put effort to explore the photocatalytic activity of WO3 -VPO catalysts for decolourisation/degradation of pollutants. The material has been well characterized and studied for the decolourisation of RhB and degradation of phenol. An outstanding result (100% decolourisation of RhB and 60% degradation of Phenol) has been achieved with a short span of time (10 min) with this system. Furthermore its reusability and stability makes it an efficient material for dye degradation. 2. Experimental section
∗ Corresponding authors. E-mail addresses: [email protected] (G.C. Behera), [email protected], [email protected], [email protected] (K. Parida).
2.1. Synthesis of vanadium phosphate (VPO) The VPO precursor was prepared according to the procedure as follows V2 O5 (5.0 g, SBMC, 98.5%) and o-H3 PO4 (30 mL, 85%
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Aldrich) were refluxed in deionised water (120 mL) for 24 h. The yellow solid was recovered by vacuum filtration, washed with cold water (100 mL) and acetone (100 mL) and dried in air (110 ◦ C, 24 h). Powder X-ray diffraction studies confirmed that the solid was the dihydrate, VOPO4 .2H2 O. The dihydrate (4 g) was refluxed with isobutanol (80 mL, 99%, Spectrochem) for 21 h, and the resulting hemi hydrate was recovered by filtration, dried in air (110 ◦ C, 16 h), refluxed in deionised water (9 mL H2 O/solid(g)) for 2 h, filtered hot, and dried in air (110 ◦ C, 16 h). 2.1.1. Synthesis of tungstate promoted vanadium phosphate (WO3 -VPO) Different wt% WO3 -promoted VPO catalysts were prepared by wetness impregnation method using water as solvent. Requisite amount of promoter (WO3 as ammonium metatungstate) was dissolved in 30 mL of deionised water. Then desired amount of vanadium phosphate precursor in powder form was added. The resulting solution was vigorously stirred at room temperature till dryness. After impregnation; all materials were dried at 60 ◦ C. The present promoted VPO final catalysts consist of 5, 10 and 15 wt% of WO3 and are designated as xWO3 -VPO (x = 5, 10 and 15). The study on different WO3 loadings was carried out after thermal activation at 700 ◦ C. 2.2. Preparation of working electrode For the photo electrochemical study, Fluorine doped tin oxide (FTO) is taken as support because it does not show any photoresponse in the solution. The working electrodes (as prepared materials-film electrodes) were prepared by electrophoretic deposition process. In this process, 30 mg of powder catalyst, 30 mL of acetone, and 20 mg of iodine powder were kept in 100 mL beaker followed by sonication for 10–15 min. Two parallel FTO coated electrodes were immersed in the solution with a 10–15 mm separation between them. Then 50A current and 60 V bias was applied to it for 0.03 min under potentiostat control to produce working electrode with coated area of 1 cm × 1.5 cm. The working electrodes are dried and used for electrochemical measurement. 2.3. Characterizations The VPO catalysts were unambiguously characterized by XRD, UV–vis DRS, NH3 -TPD, SEM, TEM, XPS, (these are explained elsewhere by the same authors [24]), PL, and Photocurrent measurement. Powder X-ray diffraction was performed using an ENRAF Nonius FR590 X-ray generator with a Cu-K␣ source fitted with an Inel CPS 120 hemispherical detector. UV–vis investigation in diffuse reflectance mode was recorded in a UV–vis spectrophotometer (Varian, Australia). The spectrum was recorded in the range of 200–800 nm using boric acid as the reflectance standard. Surface morphology was observed via Scanning Electron Microscope (SEM, Hitachi S-3400N) by the help of gold sputtering. The transmission electron microscopy images were recorded using FEI, TECNAI G2 20, TWIN and the images were recorded by using a Gatan CCD camera. The samples for electron microscopy were prepared by dispersing the powder in ethanol and dropping a very dilute suspension on carbon coated Cu grids. XPS measurements were made on a Kratos Axis Ultra DLD spectrometer using monochromatic Al-K␣ radiation. Samples were mounted using double-sided adhesive tape, and binding energy referenced to the C (1s) of adventitious carbon contamination is taken to be 284.7 eV. The acid character of the catalysts was studied using a TPD-NH3 AutoChem 2920 (micromeritics, USA) chemisorption analyzer equipped with a thermal conductivity detector (TCD). About 0.1 g of the sample was housed in a quartz U-tube. Prior to analysis, the sample was
degassed at 110 ◦ C for 2 h under N2 flow (50 mL min−1). It was then cooled down to 40 ◦ C. The sample was saturated with NH3 by the flow of 20% NH3 -balanced He for 30–40 min (25 mL min−1 ). The N2 gas was flowed over the catalyst (50 mL min−1 ) for 30 min to remove the physisorbed NH3 . The temperature was increased from 40 to 800 ◦ C with the flow of N2 (50 mL min−1 ) to get a TPD profile. The amount of NH3 consumed was determined by the TCD detector, which gives the amount of acid sites in the sample. Before the experiment, the above equipment was calibrated using 20% NH3 -balanced He to know the exact amount of gas consumed during the adsorption. Photoluminescence spectra were recorded with a LS 55 fluorescence spectrometer (Perkin Elmer) with excitation at 300 nm at room temperature. Current voltage was evaluated using a conventional pyrex electrochemical cell consisted of a prepared electrode as working electrode, a platinum wire and a Ag/AgCl electrode were used as counter and reference electrodes, respectively. The potential of the working electrode was measured by a potentiostat. The cell was filled up with an aqueous solution of 0.1 M Na2 SO4 and the pH of the solution was kept at 6. Nitrogen gas was purged to deoxygenate the electrolyte in the cell which is applied for electrochemical measurements. The photoelectrochemical measurement was carried out using a potentiostat/galvanostat (Versastat 3, Princeton Applied Research) under illumination conditions ( ≥ 400 nm). Irradiation fell on the conducting glass using a 300 W Xe lamp with a cold mirror and cut-off filters as necessary.
2.4. Photo-catalytic degradation studies Concentrations of Rhodamine B (RhB) and phenol were resolved by evaluating the absorbance representative wavelengths with Varian Cary IE UV–vis spectrophotometer (Model EL 96043181). A standard solution of the dye and phenol was taken and the absorbance was analyzed at different wavelengths to attain a plot of absorbance versus wavelength. The wavelength resultant to maximum absorbance (max ) was calculated from this plot. Correspondingly, the max for RhB and phenol were instituted to be 553 and 503 nm. Calibration curves were established between absorbance and concentration of the dye and phenol solution. Decolourisation of RhB and degradation of phenol was studied using a batch technique under UV and visible light irradiation in an irradiation chamber (BS 02, Germany) at room temperature. 20 mL of 100 ppm of RhB and 0.03 g of catalyst was taken in a 100 mL stoppered conical flask in every experiment. The pH of the solution remains constant throughout the reaction i.e. 3.79 which was calculated by an Elico digital pH meter (Model LI-612) using a combined glass electrode (Model CL 51B). The solution was centrifuged by Research Centrifuge (Remi scientific works Mumbai) at 6200 rpm for 30 min and the clear residual dye concentration was determined by a Varian Cary IE UV–vis spectrophotometer (Model EL 96043181) fitted with Cary 100 software using 10mm matched quartz cells. In the phenol analysis, the above stated condition was same but before spectrophotometric analysis, the color was developed by the addition of 2.5 mL of 0.5 M ammonium hydroxide solutions, followed by phosphate buffer to maintain the pH in the range of 7.7–7.9. After the pH adjustment, 1 mL of 4aminoantipyrene and 1 mL of potassium ferricyanide were added to develop the red colour. Various parameters involving time, amount of catalyst have been studied in detail. The percentage of removal of RhB and phenol was calculated using the following formula: Percentageremoval(%) = (C0 − Ct)/C0 ∗ 100 Ct is RhB and phenol concentration at time “t”, and Co is the initial concentration of the RhB and phenol under study.
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Fig. 1. UV–vis diffuse reflectance spectra of the neat VPO and xWO3 -VPO (x = 5 and 10 wt%) composite.
3. Results and discussions 3.1. Physical properties 3.1.1. UV–vis spectroscopy The most part of the textural and surface characterizations of the catalysts were explained elsewhere by the same authors [24]. The optical band gap energies of the composites were evaluated from the absorbance data of the composite materials obtained from DRS technique. The comparative UV–visible diffuse reflectance spectrum of VPO with 5 wt% WO3 -VPO and 10 wt% WO3 -VPO composites recorded in the range of 200–800 nm is shown in Fig. 1. Addition of WO3 to the neat material slightly changed the absorption band and thus follow-on change in band gap energy. The absorption edges of all the samples were largely shifted towards red light region compared with that of neat VPO. The band gap energies of the WO3 -VPO materials can be derived by using the following equation [32,33]: ␣h = A(h − Eg )n
(1)
where ␣,, A, and Eg are the absorption coefficient, light frequency, proportionality constant and band gap respectively. In the above equation, n determines the nature of the transition in a semiconductor i.e. n = 1/2 for direct transition and n = 2 for indirect transition. The method to calculate the value of n was reported somewhere else [34,35]. From the Tauc plot of (␣h)n vs. (h), the band gap was determined by extrapolating the straightest line to the energy axis intercept with the best fit values of the absorption coefficient [36]. The indirect band gap (n = 2) were found to be 2.17 eV, 1.93 eV and 1.79 eV for the neat VPO, 5 wt% WO3 -VPO and 10 wt% WO3 -VPO which is shown in Fig. 2(a–c). It is reported elsewhere that the band gap energy of WO3 is 2.4 eV [37]. 3.1.2. PL studies Fig. 3 illustrates the photoluminescence (PL) emission spectra of as-prepared neat VPO, 5 wt% WO3 -VPO and 10 wt% WO3 -VPO composites with excitation wavelength 320 nm at room temperature. It is recognized that optical absorption and emission of photocatalysts appreciably affects the activity of the photocatalysts. All the samples display two significant broad PL signals at around 409 and 601 nm except VPO. VPO shows the maximum intensity amongst all the composites. The first emission peak was contributed to the near band-edge emission of the materials due to the direct recombination of excitons through an exciton–exciton collision method [38]. The detached peaks might be produced by the defects in the
Fig. 2. (a–c) Estimated band gap of (a) VPO, (b) 5 wt% WO3 -VPO, (c)10 wt% WO3 VPO.
interfacial area owing to electronic transitions [39]. The existence of these peaks in visible range is possibly because of oxygen vacancies, defects, surface states, and other structural impurities [40–43]. Oxygen vacancies and defects can easily bind photo-induced electrons through the photoluminescence process. As a result, the fluorescence signals can simply take place. Also, the spectra showed a broad emission band in the range of 370–504 and 563–850 nm, which was assigned to luminescence from localized surface states,
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Fig. 3. PL spectra of VPO and xWO3 -VPO (x = 5 and 10 wt%) composite.
caused by recombination of photogenerated electron–hole pairs [44]. The PL intensity reduces with raising the WO3 content in the composite material. When the light of adequate energy falls on a material, photons are absorbed and photoexcited electrons and holes are created. Ultimately, these excitations relax and the electrons come back to the ground state. If radiative relaxation happens, the emitted light is termed PL emission. This light can be gathered and analyzed to give up valuable information about the photoexcited material. The PL intensity provides the relative rates of recombination. It is well known that PL intensity is directly proportional to the rates of recombination of electron-hole pairs. Hence, the lower PL intensity indicates the lower recombination rate of electrons and holes under light illumination. VPO has the utmost PL intensity among all of the samples signifying a high probability recombination of electrons and holes. The excitonic PL emission intensity considerably decreases with the increase in WO3 content, showing that the recombination of photogenerated carriers was successfully repressed. Among all the samples, the lowest emission intensity was detected for 10 wt% WO3 -VPO composite, proposing that the recombination of photogenerated carriers is decreased due to the effect of WO3 . 3.1.3. Analysis of hydroxyl radical (• OH) The formation of hydroxyl radical (OH) on the surface of photoirradiated WO3 -VPO is analyzed by photoluminescence (PL) technique using terephthalic acid (TPA) as a probe molecule. The PL spectra were studied by a Perkin Elemer LS-55 fluorescence spectrometer with an excitation at 315 nm light. The width of excitation slit and emission slit were 1.0 nm. It is known that TPA readily reacts with OH to generate highly fluorescent product, 2hydroxyterephthalic acid (TAOH) [5,45]. The concentration of TPA solution was prepared as 5 × 10−4 M in a dil. NaOH aqueous solution of 2 × 10−3 M. 0.03 g each of the prepared catalysts was added into the said TPA solution followed by centrifuging for 15 min to separate the catalyst particles and was then used for the PL measurements. The peak position is found to be at 425 nm of all the spectra with an identical shape. There is no PL spectra observed in the absence of visible-light or samples. This proposes that the signal of PL is only caused by the reaction of terephthalic acid with OH formed on the surface of the materials [5,45]. The formation rate of OH radicals is directly proportional to the separation efficiency of electron-hole pairs which indicate that the intensity is linearly related to the number of hydroxyl radicals formed by the composite. From Fig. 4, 10 wt% WO3 -VPO composite shows the highest intensity which implies the formation of a higher number of hydroxyl radicals compared to the other catalysts.
Fig. 4. PL spectral changes with the visible-light irradiation time for all the catalysts in a 5 × 10− 4 M basic solution of terephthalic acid.
3.1.4. Photoelectrochemical measurement The photocurrent of the neat VPO, 5 wt% WO3 -VPO and 10 wt% WO3 -VPO composites were measured by I–V graphs in dark and light condition which are shown in Figs. Fig. 5(a–c) and S1(a–c) , respectively. All the catalysts produce photocurrent in anodic direction which gradually increases with the applied potential under both dark and light condition ( ≥ 400 nm). The observation clearly indicates VPO as an n-type semiconductor. From Fig. 5(a–c), it is found that VPO, 5 wt% WO3 -VPO, and 10 wt% WO3 -VPO indicates photocurrent i.e. 0.37 mA, 17.80 mA, and 22.187 mA, respectively under applied potential 3.99 V. Also, From Fig. S1 (a–c), it has been examined that VPO, 5 wt% WO3 -VPO, and 10 wt% WO3 -VPO displays photocurrent i.e. 0.86 mA, 17.94 mA and 23.38 mA, respectively under light irradiation and same applied potential. Here we have found that the dark current is less than the light current. An increase in separation of charge carriers depends on increase of photocurrent of the materials [46]. The increased in photocurrent in the catalysts with positive applied bias reveal the existence of n-type semiconducting property [47]. It is known that the photocurrent is directly proportional to the concentration of electrons for n-type semiconductor. The photo current density of 10 wt% WO3 -VPO and 5 wt% WO3 -VPO is nearly 27 and 21 times more than that of neat VPO which illustrates that more number of free electrons are available in 10 wt% WO3 -VPO photoelectrode. Therefore, the more photogenerated electrons may be due to inhibition of the recombination rate of electron-hole pairs. Furthermore, the strong interaction between WO3 with VPO played a vital role in enhancing the photocurrent of the 10 wt% WO3 -VPO composite. The I–V characteristic of 10 wt% WO3 -VPO composite is in well match with PL study. 3.1.5. Cyclic voltammetry (CV) study The cyclic voltammetry (CV) of the composite materials i.e. 5 wt% WO3 -VPO and 10 wt% WO3 -VPO were studied to know the stability of the material. There are no peaks observed in the cyclic voltammograms (Fig. S2a and b). The cyclic photocurrent of the materials was also found nearly equal to each I–V (currentpotential) measurement. Therefore the catalysts are stable under light irradiation. 3.2. Photocatalytic activity Decolourisation of Rhodamine B (RhB) was studied varying different parameters such as catalyst dose, time, etc. The photocatalysis was also performed in dark condition and without catalyst.
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Fig. 6. Effect of different catalysts on the decolourisation of RhB; RhB = 100 ppm, catalyst dose = 0.03 g, time = 4 h, 2 h, 30 min and 10 min, pH = 3.79, room temperature = 25 ◦ C.
tion for pH, catalyst dose and concentration of dye solution i.e. 3.79, 0.03 g and 100 ppm, respectively were maintained. A decolourisation of 30%, 20% and 98% was observed over neat VPO, WO3 and 5 wt% WO3 -VPO respectively. However this value gets increased to 100% in case of 10 wt% and 15 wt% WO3 -VPO. This may presumably be due to the availability of more active sites with WO3 loadings. As the catalyst showed 100% decolourisation with short span of time (10 min), the effect of time has not affected the dye and this remain constant over 4 h. In a model study, we used 10 wt% WO3 VPO catalyst as a standard to carry out decolourisation/degradation efficiency. 3.2.2. Effect of 10 wt% WO3 -VPO with different dose From the above study, we found that 10 wt% WO3 -VPO and 15 wt% WO3 -VPO showed highest activity towards photocatalytic decolourisation. So, 10 wt% WO3 -VPO catalyst was taken for further studies of dye decolourisation with different dose i.e. 0.01, 0.02, 0.03 and 0.04 g under similar conditions. It is revealed that 0.01, 0.02, 0.03 and 0.04 gm of the catalyst showed 67, 83, 100 and 100% decolourisation, respectively (Fig. S3(a)). Therefore, 0.03 g catalyst was used for other purposes. Our observation agrees with the observation stated by Sauer et al. [48] for degradation of Safira dye using Degussa P-25 and Das et al. [49] for degradation of organic pollutants under solar radiation. Also, some authors reported the negative effect on degradation efficiency of reactive dyes at higher catalyst concentration [50].
Fig. 5. (a–c) Dark current measurement of VPO and xWO3 -VPO (x = 5 and 10 wt%) composite.
There is no adsorption observed without catalyst but in the presence of catalyst 10% adsorption was observed in dark condition. To compare the activity of the catalysts toward the decolourisation of Rh B, phenol degradation was also carried out as a representative reaction. There is no phenol adsorption obtained in dark condition and without catalyst. However in the presence of catalyst, 5% adsorption was obtained. 3.2.1. Effect of different catalysts with different time intervals Photocatalytic decolourisation of RhB under different time period i.e. 4 h, 2 h, 30 min and 10 min at room temperature (25 ◦ C) was studied and the results are shown in Fig. 6. The optimum condi-
3.2.3. Effect of 10 wt% WO3 -VPO with different RhB concentration Photocatalytic decolourisation of RhB with various concentrations i.e. 300, 500, 700 and 1000 ppm was studied keeping other parameters constant. A decolourisation of 100% was achieved with 100 ppm solution. However the result was found decreased with higher concentrations. The efficiency of 71% decolourisation was obtained with 300 ppm solution while in case of 500, 700 and 1000 ppm, the decolourisation efficiency was 15, 4 and 0% respectively (Fig. S3(b)). This might be due to the light absorbed by the substrate (RhB) is more as compared to the catalyst with increase in the concentration [5]. So, at higher concentration, the photo decolourisation efficiency is decreasing. 3.2.4. Effect of 1 wt% WO3 -VPO and 3 wt% WO3 -VPO on photo decolourisation In order to study the effect of the less WO3 content catalysts, we have prepared 1 wt% WO3 -VPO and 3 wt% WO3 -VPO. The catalysts i.e. 1 wt% WO3 -VPO and 3 wt% WO3 -VPO shows 51 and 79% decolourisation towards RhB under the same condition (Fig. S3(c)).
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Scheme 1. Diagram for energy band levels of WO3 -VPO composites and the possible charge separation process with mechanism of dye degradation.
3.3. Proposed mechanism for the enhancement of the activity The mechanism of the photocatalytic activity of WO3 -VPO heterojunction is explained by calculating their relative band edge positions which shows migration direction of the photogenerated charge carriers. At the point of zero charge, the conduction band edge of a semiconductor can be theoretically predicted by Mulliken electronegativity theory [51]. ECB = X − Ee − 0.5 Eg
Fig. 7. Effect of different catalysts on the degradation of Phenol; Phenol = 100 ppm, catalyst dose = 0.03 g, time = 10 min, pH = 3.79, room temperature = 25 ◦ C.
3.2.5. Effect of catalysts on photodegradation of phenol Photocatalytic degradation of Phenol over neat and promoted VPO catalysts was studied with the same conditions as RhB decolourisation. The results of the phenol degradation were shown in Fig. 7. The degradation was found increased with the WO3 loadings. The neat VPO and WO3 showed a degradation of 10% and 5% while 5 wt%, 10 wt% and 15 wt% WO3 -VPO showed 58, 60 and 60% respectively towards phenol under the same conditions. The catalysts can also be used for the degradation of phenolic compounds under visible light irradiation. There are so many catalysts which have been used for the decolourisation of RhB and Phenol. A comparison activity of our catalyst with those catalysts has been illustrated in Table S1. The present catalyst WO3 -VPO showed remarkable photocatalytic activity with respect to other photocatalytic systems. 3.2.6. Reusability test Figs. S5 and S6 display the reusability performance of 10 wt% WO3 -VPO catalyst for 4 successive cycles. The solid catalyst was separated from reaction mixture by centrifugation after completion of the reaction. It was thoroughly washed with acetone for several times and then reused as catalyst in the degradation process under the same condition as the original. There was no significant change found in the activity of catalyst after 4 cycles. The decolourisation was found to be stabilized in a range between 99 and 100% in case of RhB and degradation of 59–60% for phenol. The synergetic effect between WO3 and VPO made the active phase more stable.
where X is the absolute electronegativity of the semiconductor, ECB and Eg is the position of the conduction band and the band gap of the semiconductor, respectively. Ee is the energy of the free electrons on the hydrogen scale i.e. constant value (∼4.5 eV). The X value and the band gap energy of neat VPO are calculated to be 6.51 and 2.17 eV, respectively. By applying the above equation, we have predicted that the conduction band (CB) edge position of pure VPO is 0.92 eV. It is reported that the Eg and CB of neat WO3 are 2.4 eV and 0.5 eV, respectively [37,52]. The VB can be obtained by EVB = ECB + Eg. The corresponding valence band (VB) positions of pure WO3 and VPO are 2.9 and 3.09 eV, respectively. So, the conduction band bottom and valence band top of WO3 is higher than that of VPO which is illustrated in Scheme 1. The derived results show the composite materials are useful for the separation and transportation of electron-hole pairs charge carriers. When it is excited under visible light irradiation, the photoexcited electrons and holes are generated on the surfaces of the material. The photoexcited electrons on the conduction band of WO3 can easily be transferred to the conduction band edge of VPO due to their close link through interface and simultaneously, holes on the valence band of the VPO can be quickly migrated to the valence band of WO3 under the difference in potential of band gap energy. In this approach, the photogenerated electrons and holes are present on the different surfaces, because of which the recombinations of charge carriers get suppressed which can be correlated with PL spectra. So, the photocatalytic activity of the composite materials towards dye decolourisation/degradation is increased. The neat catalyst WO3 and VPO shows less degradation due to the fast recombination between photogenerated holes and electrons. The photodegradability of the organic pollutants over the heterojunction composite materials greatly enhances owing to the collection of electrons and holes on different surfaces. The electrons present on the surface of VPO in heterojunction composite reacts with absorbed O2 to give rise O2− and OH which can further oxidize dye molecules [14]. Simultaneously, collected holes on the surface of WO3 traps OH−
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have suitable band gaps for photo decolourisation/degradation of organic pollutants under visible light irradiation. • PL intensity It is well known that photoluminescence (PL) signals and their intensity are indirectly proportional to its photocatalytic activity which is discussed in detail in PL studies. From the PL spectra, we have found that the intensity is in order VPO > 5 wt% WO3 VPO > 10 wt% WO3 -VPO (Fig. 3). So, 10 wt% WO3 -VPO showed the highest activity. • Formation of • OH
Scheme 2. Schematic diagram illustrating transfer of an electron injected into WO3 VPO by an excited dye molecule adsorbed at the WO3 -VPO.
radical obtained from H2 O to form OH which oxidizes the absorbed RhB dye [14]. A dye-sensitized mechanism can play a role for RhB decolourisation. For this, we studied the decolourisation of RhB by 365 nm UV light (IUV = 1.381 × 10−6 Einstein L−1 s−1 ) and visible light under the same conditions and found that 70% and 100% decolourisation in 10 min, respectively. The higher activity under visible light shows the presence of a dye-sensitized mechanism in addition to WO3 VPO sensitization due to the adsorption of more dye molecules on the semiconductor surface. The adsorption of dye on WO3 -VPO (10.0%) is higher comparison to WO3 (6.5%) and VPO (4.5%) under the dark condition. It implies that the photoexcited electron transfer from visible light-sensitized RhB molecule to the conduction band of WO3 which produces superoxide radicals (O2− ) (Scheme 2). Here, O2− degrade the RhB by the dye sensitization mechanism which is given below (Eqs. (2)–(4)). Further, to prove it, we also carried out the degradation of phenol by WO3 -VPO under the same above conditions and found that UV induced phenol degradation (87.4%) was more efficient than visible induced (60%), shows the presence of only a catalyst-sensitized mechanism in the phenol degradation. This confirms the presence of a dye-sensitized mechanism for RhB decolourisation [53,54].
Greater generation of OH radicals confirms to a higher photocatalytic activity [47]. Therefore, the 10 wt% WO3 -VPO composite material has higher photocatalytic activity than the pristine VPO. It is observed that the order of the formation rate of OH radicals on the surface of the catalyst is as follows: 10 wt% WO3 -VPO > 5 wt% WO3 -VPO > VPO (Fig. 4). Therefore, 10 wt% WO3 -VPO exhibits the highest activity. • Photocurrent measurement It is known that the photocurrent is directly proportional to the concentration of electrons for n-type semiconductor. We have found that our catalysts are n-type semiconductor and the photocurrent trend is as follows 10 wt% WO3 -VPO > 5 wt% WO3 VPO > VPO (Fig. 5(a–c)). Also, there is some extent of dark current present in the samples. So, the catalytic activity of 10 wt% WO3 -VPO showed remarkable degradation efficiency. It may be the doubling effect due to the presence of dark and light current. • Heterojunction Phenomena
There are six factors which affect the photocatalytic activity of the WO3 -VPO photocatalyst.
From the mechanism part (Section 3.4), we have found the heterojunction property of the catalyst (Scheme 1). In this heterojunction approach, the photogenerated electrons and holes are present on different surfaces i.e. WO3 and VPO, so of which the recombination of charge carriers decrease which can be correlated with PL spectra and photocurrent measurement. So, the photocatalytic activity of the composite materials towards dye degradation is increased. It is known that the redox potential e of OH− /OH and O2 to O2− are 1.89 V/NHE and −0.13 V/NHE. Our catalysts have more positive VB potential i.e. 2.9 and 3.09 eV than OH− /OH which informs the photogenerated holes have strong oxidative ability and they can oxide OH− into OH. Also, the CB potential of our catalyst has more positive potential i.e. 0.5 and 0.92 eV to reduce O2 to O2− . Therefore, the charge separation of e− –h+ pairs are more effective than individual one and tends to increase in the availability of h+ which helps to improve the photocatalytic activity of the WO3 -VPO.
(I) Band Gap Energy (Eg)
• Photosensitization and synergistic property of WO3
From UV–vis DRS, the indirect band gap (n = 2) is found to be 2.17 eV, 1.93 eV and 1.79 eV for the neat VPO, 5 wt% WO3 -VPO and 10 wt% WO3 -VPO respectively (Fig. 2(a–c)). These findings are in well agreement with the observed photocatalytic activity because the band gap energy is in order VPO > 5 wt% WO3 -VPO > 10 wt% WO3 -VPO i.e. 10 wt% WO3 -VPO is more active in visible light region. So, 10 wt% WO3 -VPO showed highest activity i.e. 100 and 60% towards RhB decolourisation and phenol degradation respectively, where as VPO showed some lower value i.e. 30 and 10% respectively. So, the above results imply that the prepared photocatalyst
In our case, WO3 acts as photosensitizer [55] and has synergistic property [56]. Being a photosensitizer, it was quickly excited under light irradiation and provided the electrons to the conduction band of VPO to enrich the amount of electrons. Simultaneously, the holes present on the VB of VPO easily transferred to the VB of WO3 [53,54]. Therefore, the recombination of charge pairs is reduced in the case of composite system than neat sample. The synergetic effect between WO3 and VPO inhibited the recombination of electron-hole leading to an increased charge carrier separation. So the composite materials showed the highest activity.
RhB + h → RhB+• + ecb −
(2)
ecb − + O2 → O2−
(3)
RhB+• + O2 /O2− → degradationproducts
(4)
3.4. Overall reasons for the increase in photocatalytic degradation of RhB and phenol
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4. Conclusions In summary, we have successfully tested the catalytic efficacy of WO3 -VPO in photocatalytic decolourisation of RhB and degradation of phenol. WO3 -VPO was found to be a n-type semiconductor where the photogenerated electrons inhibited the recombination rate of electron–hole pairs. The synergetic interaction between WO3 and VPO, the formation of a higher number of hydroxyl radicals and inhibition of recombination factor play a major role for obtaining such unexpected results. Again the catalysts exhibit a heterojunction phenomenon where the charge separation of e− –h+ pairs are more effective than individual one and tends to increase the availability of h+ which helps to enhance the photocatalytic activity of the WO3 -VPO. The instantaneous reaction with unexpected results paves the way to utilize VPO materials in dye decolourisation and phenol degradation. Our findings will encourage the researchers to work more in this field and will prove that VPO materials can not only be used in the gas phase oxidation but also can be used in the liquid phase reactions. Acknowledgement The authors wish to express their deepest and sincerest recognition of Prof. András Dombi a key figure in the topic of photocatalytic materials for the degradation of contaminants of environmental concern. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.10. 017. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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Unexpected rapid photo-catalytic decolourisation/degradation of organic pollutants over highly active hetero junction based vanadium phosphate catalyst Gobinda Chandra Behera a,∗ , Niranjan Biswal b , Kulamani Parida c,∗ a
Department of Chemistry, Kendrapara Autonomous College, Kendrapara 754211,Odisha, India Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 3200003, Israel c Centre for Nano Science & Nano Technology, Siksha ‘O’Anusandhan University, Bhubaneswar 751030, Odisha, India b
a r t i c l e
i n f o
Article history: Received 24 May 2016 Received in revised form 21 September 2016 Accepted 23 October 2016 Available online xxx Keywords: Coupled oxide (WO3 -VPO) Photocatalytic decolourisation/degradation Organic pollutants Vanadium phosphate Visible light
a b s t r a c t VPO materials are extensively used the in gas phase oxidation of n-butane to maleic anhydride. Herein we explored its catalytic activity towards decolourisation/degradation of organic pollutants. This investigation reports the remarkable photo catalytic activity of tungstate promoted vanadium phosphate (WO3 -VPO) toward photocatalyic decolourisation/degradation of pollutants. The catalyst i.e. 10 wt% WO3 -VPO showed unexpected efficacy i.e. 100% decolourisation of Rhodamine B (RhB) and 60% degradation of phenol under visible light irradiation with a short span of time i.e. 10 min. Several factors involving the significant results of the composite material than the neat sample have been studied in detail. The analysis of photoluminescence (PL) emission spectra, photoelectrochemical measurement and the cyclic voltammeter (CV) measurement of the composite materials greatly support the outstanding results of the catalyst. The remarkable efficiency of VPO materials in photocatalytic decolourisation/degradation would become another trademark in the photocatalysis society. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Environmental problems are normally recounted to waste and toxic organic with inorganic pollutants released in water bodies. The majority of the coloured and uncoloured effluents originated in contaminated waters contain organic dyes and phenolic compounds from textiles, dyestuff, dyeing industries, fertilizers and chemical production industries [1–3]. Several works on decolourisation/degradation of organic pollutants under visible light irradiation have been reported [4–11]. Our group and numerous researchers have evaluated the visible light driven photocatalytic decomposition of Rhodamine B (RhB) and phenolic compounds [12–17]. Very few literatures are reported on phosphate systems and WO3 promoted phosphate catalysts for the oxidation of organic contaminants [18–22]. Herein, we report another phosphate system i.e. vanadium phosphate (VPO), a well known material which have extensively
been used industrially in the manufacture of maleic anhydride from n-butane [23]. However there is relatively little research on VPO materials in liquid phase reactions [24–31]. To tackle this tough issue, we put effort to explore the catalytic potential of VPO material towards some liquid phase reactions [24–26,29–31]. The results of our preceding works encouraged us to move into another catalytic world, photocatalysis. Photocatalytic materials have now gained much attention toward pollutant removal. In this concern, we put effort to explore the photocatalytic activity of WO3 -VPO catalysts for decolourisation/degradation of pollutants. The material has been well characterized and studied for the decolourisation of RhB and degradation of phenol. An outstanding result (100% decolourisation of RhB and 60% degradation of Phenol) has been achieved with a short span of time (10 min) with this system. Furthermore its reusability and stability makes it an efficient material for dye degradation. 2. Experimental section
∗ Corresponding authors. E-mail addresses: [email protected] (G.C. Behera), [email protected], [email protected], [email protected] (K. Parida).
2.1. Synthesis of vanadium phosphate (VPO) The VPO precursor was prepared according to the procedure as follows V2 O5 (5.0 g, SBMC, 98.5%) and o-H3 PO4 (30 mL, 85%
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Aldrich) were refluxed in deionised water (120 mL) for 24 h. The yellow solid was recovered by vacuum filtration, washed with cold water (100 mL) and acetone (100 mL) and dried in air (110 ◦ C, 24 h). Powder X-ray diffraction studies confirmed that the solid was the dihydrate, VOPO4 .2H2 O. The dihydrate (4 g) was refluxed with isobutanol (80 mL, 99%, Spectrochem) for 21 h, and the resulting hemi hydrate was recovered by filtration, dried in air (110 ◦ C, 16 h), refluxed in deionised water (9 mL H2 O/solid(g)) for 2 h, filtered hot, and dried in air (110 ◦ C, 16 h). 2.1.1. Synthesis of tungstate promoted vanadium phosphate (WO3 -VPO) Different wt% WO3 -promoted VPO catalysts were prepared by wetness impregnation method using water as solvent. Requisite amount of promoter (WO3 as ammonium metatungstate) was dissolved in 30 mL of deionised water. Then desired amount of vanadium phosphate precursor in powder form was added. The resulting solution was vigorously stirred at room temperature till dryness. After impregnation; all materials were dried at 60 ◦ C. The present promoted VPO final catalysts consist of 5, 10 and 15 wt% of WO3 and are designated as xWO3 -VPO (x = 5, 10 and 15). The study on different WO3 loadings was carried out after thermal activation at 700 ◦ C. 2.2. Preparation of working electrode For the photo electrochemical study, Fluorine doped tin oxide (FTO) is taken as support because it does not show any photoresponse in the solution. The working electrodes (as prepared materials-film electrodes) were prepared by electrophoretic deposition process. In this process, 30 mg of powder catalyst, 30 mL of acetone, and 20 mg of iodine powder were kept in 100 mL beaker followed by sonication for 10–15 min. Two parallel FTO coated electrodes were immersed in the solution with a 10–15 mm separation between them. Then 50A current and 60 V bias was applied to it for 0.03 min under potentiostat control to produce working electrode with coated area of 1 cm × 1.5 cm. The working electrodes are dried and used for electrochemical measurement. 2.3. Characterizations The VPO catalysts were unambiguously characterized by XRD, UV–vis DRS, NH3 -TPD, SEM, TEM, XPS, (these are explained elsewhere by the same authors [24]), PL, and Photocurrent measurement. Powder X-ray diffraction was performed using an ENRAF Nonius FR590 X-ray generator with a Cu-K␣ source fitted with an Inel CPS 120 hemispherical detector. UV–vis investigation in diffuse reflectance mode was recorded in a UV–vis spectrophotometer (Varian, Australia). The spectrum was recorded in the range of 200–800 nm using boric acid as the reflectance standard. Surface morphology was observed via Scanning Electron Microscope (SEM, Hitachi S-3400N) by the help of gold sputtering. The transmission electron microscopy images were recorded using FEI, TECNAI G2 20, TWIN and the images were recorded by using a Gatan CCD camera. The samples for electron microscopy were prepared by dispersing the powder in ethanol and dropping a very dilute suspension on carbon coated Cu grids. XPS measurements were made on a Kratos Axis Ultra DLD spectrometer using monochromatic Al-K␣ radiation. Samples were mounted using double-sided adhesive tape, and binding energy referenced to the C (1s) of adventitious carbon contamination is taken to be 284.7 eV. The acid character of the catalysts was studied using a TPD-NH3 AutoChem 2920 (micromeritics, USA) chemisorption analyzer equipped with a thermal conductivity detector (TCD). About 0.1 g of the sample was housed in a quartz U-tube. Prior to analysis, the sample was
degassed at 110 ◦ C for 2 h under N2 flow (50 mL min−1). It was then cooled down to 40 ◦ C. The sample was saturated with NH3 by the flow of 20% NH3 -balanced He for 30–40 min (25 mL min−1 ). The N2 gas was flowed over the catalyst (50 mL min−1 ) for 30 min to remove the physisorbed NH3 . The temperature was increased from 40 to 800 ◦ C with the flow of N2 (50 mL min−1 ) to get a TPD profile. The amount of NH3 consumed was determined by the TCD detector, which gives the amount of acid sites in the sample. Before the experiment, the above equipment was calibrated using 20% NH3 -balanced He to know the exact amount of gas consumed during the adsorption. Photoluminescence spectra were recorded with a LS 55 fluorescence spectrometer (Perkin Elmer) with excitation at 300 nm at room temperature. Current voltage was evaluated using a conventional pyrex electrochemical cell consisted of a prepared electrode as working electrode, a platinum wire and a Ag/AgCl electrode were used as counter and reference electrodes, respectively. The potential of the working electrode was measured by a potentiostat. The cell was filled up with an aqueous solution of 0.1 M Na2 SO4 and the pH of the solution was kept at 6. Nitrogen gas was purged to deoxygenate the electrolyte in the cell which is applied for electrochemical measurements. The photoelectrochemical measurement was carried out using a potentiostat/galvanostat (Versastat 3, Princeton Applied Research) under illumination conditions ( ≥ 400 nm). Irradiation fell on the conducting glass using a 300 W Xe lamp with a cold mirror and cut-off filters as necessary.
2.4. Photo-catalytic degradation studies Concentrations of Rhodamine B (RhB) and phenol were resolved by evaluating the absorbance representative wavelengths with Varian Cary IE UV–vis spectrophotometer (Model EL 96043181). A standard solution of the dye and phenol was taken and the absorbance was analyzed at different wavelengths to attain a plot of absorbance versus wavelength. The wavelength resultant to maximum absorbance (max ) was calculated from this plot. Correspondingly, the max for RhB and phenol were instituted to be 553 and 503 nm. Calibration curves were established between absorbance and concentration of the dye and phenol solution. Decolourisation of RhB and degradation of phenol was studied using a batch technique under UV and visible light irradiation in an irradiation chamber (BS 02, Germany) at room temperature. 20 mL of 100 ppm of RhB and 0.03 g of catalyst was taken in a 100 mL stoppered conical flask in every experiment. The pH of the solution remains constant throughout the reaction i.e. 3.79 which was calculated by an Elico digital pH meter (Model LI-612) using a combined glass electrode (Model CL 51B). The solution was centrifuged by Research Centrifuge (Remi scientific works Mumbai) at 6200 rpm for 30 min and the clear residual dye concentration was determined by a Varian Cary IE UV–vis spectrophotometer (Model EL 96043181) fitted with Cary 100 software using 10mm matched quartz cells. In the phenol analysis, the above stated condition was same but before spectrophotometric analysis, the color was developed by the addition of 2.5 mL of 0.5 M ammonium hydroxide solutions, followed by phosphate buffer to maintain the pH in the range of 7.7–7.9. After the pH adjustment, 1 mL of 4aminoantipyrene and 1 mL of potassium ferricyanide were added to develop the red colour. Various parameters involving time, amount of catalyst have been studied in detail. The percentage of removal of RhB and phenol was calculated using the following formula: Percentageremoval(%) = (C0 − Ct)/C0 ∗ 100 Ct is RhB and phenol concentration at time “t”, and Co is the initial concentration of the RhB and phenol under study.
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Fig. 1. UV–vis diffuse reflectance spectra of the neat VPO and xWO3 -VPO (x = 5 and 10 wt%) composite.
3. Results and discussions 3.1. Physical properties 3.1.1. UV–vis spectroscopy The most part of the textural and surface characterizations of the catalysts were explained elsewhere by the same authors [24]. The optical band gap energies of the composites were evaluated from the absorbance data of the composite materials obtained from DRS technique. The comparative UV–visible diffuse reflectance spectrum of VPO with 5 wt% WO3 -VPO and 10 wt% WO3 -VPO composites recorded in the range of 200–800 nm is shown in Fig. 1. Addition of WO3 to the neat material slightly changed the absorption band and thus follow-on change in band gap energy. The absorption edges of all the samples were largely shifted towards red light region compared with that of neat VPO. The band gap energies of the WO3 -VPO materials can be derived by using the following equation [32,33]: ␣h = A(h − Eg )n
(1)
where ␣,, A, and Eg are the absorption coefficient, light frequency, proportionality constant and band gap respectively. In the above equation, n determines the nature of the transition in a semiconductor i.e. n = 1/2 for direct transition and n = 2 for indirect transition. The method to calculate the value of n was reported somewhere else [34,35]. From the Tauc plot of (␣h)n vs. (h), the band gap was determined by extrapolating the straightest line to the energy axis intercept with the best fit values of the absorption coefficient [36]. The indirect band gap (n = 2) were found to be 2.17 eV, 1.93 eV and 1.79 eV for the neat VPO, 5 wt% WO3 -VPO and 10 wt% WO3 -VPO which is shown in Fig. 2(a–c). It is reported elsewhere that the band gap energy of WO3 is 2.4 eV [37]. 3.1.2. PL studies Fig. 3 illustrates the photoluminescence (PL) emission spectra of as-prepared neat VPO, 5 wt% WO3 -VPO and 10 wt% WO3 -VPO composites with excitation wavelength 320 nm at room temperature. It is recognized that optical absorption and emission of photocatalysts appreciably affects the activity of the photocatalysts. All the samples display two significant broad PL signals at around 409 and 601 nm except VPO. VPO shows the maximum intensity amongst all the composites. The first emission peak was contributed to the near band-edge emission of the materials due to the direct recombination of excitons through an exciton–exciton collision method [38]. The detached peaks might be produced by the defects in the
Fig. 2. (a–c) Estimated band gap of (a) VPO, (b) 5 wt% WO3 -VPO, (c)10 wt% WO3 VPO.
interfacial area owing to electronic transitions [39]. The existence of these peaks in visible range is possibly because of oxygen vacancies, defects, surface states, and other structural impurities [40–43]. Oxygen vacancies and defects can easily bind photo-induced electrons through the photoluminescence process. As a result, the fluorescence signals can simply take place. Also, the spectra showed a broad emission band in the range of 370–504 and 563–850 nm, which was assigned to luminescence from localized surface states,
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Fig. 3. PL spectra of VPO and xWO3 -VPO (x = 5 and 10 wt%) composite.
caused by recombination of photogenerated electron–hole pairs [44]. The PL intensity reduces with raising the WO3 content in the composite material. When the light of adequate energy falls on a material, photons are absorbed and photoexcited electrons and holes are created. Ultimately, these excitations relax and the electrons come back to the ground state. If radiative relaxation happens, the emitted light is termed PL emission. This light can be gathered and analyzed to give up valuable information about the photoexcited material. The PL intensity provides the relative rates of recombination. It is well known that PL intensity is directly proportional to the rates of recombination of electron-hole pairs. Hence, the lower PL intensity indicates the lower recombination rate of electrons and holes under light illumination. VPO has the utmost PL intensity among all of the samples signifying a high probability recombination of electrons and holes. The excitonic PL emission intensity considerably decreases with the increase in WO3 content, showing that the recombination of photogenerated carriers was successfully repressed. Among all the samples, the lowest emission intensity was detected for 10 wt% WO3 -VPO composite, proposing that the recombination of photogenerated carriers is decreased due to the effect of WO3 . 3.1.3. Analysis of hydroxyl radical (• OH) The formation of hydroxyl radical (OH) on the surface of photoirradiated WO3 -VPO is analyzed by photoluminescence (PL) technique using terephthalic acid (TPA) as a probe molecule. The PL spectra were studied by a Perkin Elemer LS-55 fluorescence spectrometer with an excitation at 315 nm light. The width of excitation slit and emission slit were 1.0 nm. It is known that TPA readily reacts with OH to generate highly fluorescent product, 2hydroxyterephthalic acid (TAOH) [5,45]. The concentration of TPA solution was prepared as 5 × 10−4 M in a dil. NaOH aqueous solution of 2 × 10−3 M. 0.03 g each of the prepared catalysts was added into the said TPA solution followed by centrifuging for 15 min to separate the catalyst particles and was then used for the PL measurements. The peak position is found to be at 425 nm of all the spectra with an identical shape. There is no PL spectra observed in the absence of visible-light or samples. This proposes that the signal of PL is only caused by the reaction of terephthalic acid with OH formed on the surface of the materials [5,45]. The formation rate of OH radicals is directly proportional to the separation efficiency of electron-hole pairs which indicate that the intensity is linearly related to the number of hydroxyl radicals formed by the composite. From Fig. 4, 10 wt% WO3 -VPO composite shows the highest intensity which implies the formation of a higher number of hydroxyl radicals compared to the other catalysts.
Fig. 4. PL spectral changes with the visible-light irradiation time for all the catalysts in a 5 × 10− 4 M basic solution of terephthalic acid.
3.1.4. Photoelectrochemical measurement The photocurrent of the neat VPO, 5 wt% WO3 -VPO and 10 wt% WO3 -VPO composites were measured by I–V graphs in dark and light condition which are shown in Figs. Fig. 5(a–c) and S1(a–c) , respectively. All the catalysts produce photocurrent in anodic direction which gradually increases with the applied potential under both dark and light condition ( ≥ 400 nm). The observation clearly indicates VPO as an n-type semiconductor. From Fig. 5(a–c), it is found that VPO, 5 wt% WO3 -VPO, and 10 wt% WO3 -VPO indicates photocurrent i.e. 0.37 mA, 17.80 mA, and 22.187 mA, respectively under applied potential 3.99 V. Also, From Fig. S1 (a–c), it has been examined that VPO, 5 wt% WO3 -VPO, and 10 wt% WO3 -VPO displays photocurrent i.e. 0.86 mA, 17.94 mA and 23.38 mA, respectively under light irradiation and same applied potential. Here we have found that the dark current is less than the light current. An increase in separation of charge carriers depends on increase of photocurrent of the materials [46]. The increased in photocurrent in the catalysts with positive applied bias reveal the existence of n-type semiconducting property [47]. It is known that the photocurrent is directly proportional to the concentration of electrons for n-type semiconductor. The photo current density of 10 wt% WO3 -VPO and 5 wt% WO3 -VPO is nearly 27 and 21 times more than that of neat VPO which illustrates that more number of free electrons are available in 10 wt% WO3 -VPO photoelectrode. Therefore, the more photogenerated electrons may be due to inhibition of the recombination rate of electron-hole pairs. Furthermore, the strong interaction between WO3 with VPO played a vital role in enhancing the photocurrent of the 10 wt% WO3 -VPO composite. The I–V characteristic of 10 wt% WO3 -VPO composite is in well match with PL study. 3.1.5. Cyclic voltammetry (CV) study The cyclic voltammetry (CV) of the composite materials i.e. 5 wt% WO3 -VPO and 10 wt% WO3 -VPO were studied to know the stability of the material. There are no peaks observed in the cyclic voltammograms (Fig. S2a and b). The cyclic photocurrent of the materials was also found nearly equal to each I–V (currentpotential) measurement. Therefore the catalysts are stable under light irradiation. 3.2. Photocatalytic activity Decolourisation of Rhodamine B (RhB) was studied varying different parameters such as catalyst dose, time, etc. The photocatalysis was also performed in dark condition and without catalyst.
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Fig. 6. Effect of different catalysts on the decolourisation of RhB; RhB = 100 ppm, catalyst dose = 0.03 g, time = 4 h, 2 h, 30 min and 10 min, pH = 3.79, room temperature = 25 ◦ C.
tion for pH, catalyst dose and concentration of dye solution i.e. 3.79, 0.03 g and 100 ppm, respectively were maintained. A decolourisation of 30%, 20% and 98% was observed over neat VPO, WO3 and 5 wt% WO3 -VPO respectively. However this value gets increased to 100% in case of 10 wt% and 15 wt% WO3 -VPO. This may presumably be due to the availability of more active sites with WO3 loadings. As the catalyst showed 100% decolourisation with short span of time (10 min), the effect of time has not affected the dye and this remain constant over 4 h. In a model study, we used 10 wt% WO3 VPO catalyst as a standard to carry out decolourisation/degradation efficiency. 3.2.2. Effect of 10 wt% WO3 -VPO with different dose From the above study, we found that 10 wt% WO3 -VPO and 15 wt% WO3 -VPO showed highest activity towards photocatalytic decolourisation. So, 10 wt% WO3 -VPO catalyst was taken for further studies of dye decolourisation with different dose i.e. 0.01, 0.02, 0.03 and 0.04 g under similar conditions. It is revealed that 0.01, 0.02, 0.03 and 0.04 gm of the catalyst showed 67, 83, 100 and 100% decolourisation, respectively (Fig. S3(a)). Therefore, 0.03 g catalyst was used for other purposes. Our observation agrees with the observation stated by Sauer et al. [48] for degradation of Safira dye using Degussa P-25 and Das et al. [49] for degradation of organic pollutants under solar radiation. Also, some authors reported the negative effect on degradation efficiency of reactive dyes at higher catalyst concentration [50].
Fig. 5. (a–c) Dark current measurement of VPO and xWO3 -VPO (x = 5 and 10 wt%) composite.
There is no adsorption observed without catalyst but in the presence of catalyst 10% adsorption was observed in dark condition. To compare the activity of the catalysts toward the decolourisation of Rh B, phenol degradation was also carried out as a representative reaction. There is no phenol adsorption obtained in dark condition and without catalyst. However in the presence of catalyst, 5% adsorption was obtained. 3.2.1. Effect of different catalysts with different time intervals Photocatalytic decolourisation of RhB under different time period i.e. 4 h, 2 h, 30 min and 10 min at room temperature (25 ◦ C) was studied and the results are shown in Fig. 6. The optimum condi-
3.2.3. Effect of 10 wt% WO3 -VPO with different RhB concentration Photocatalytic decolourisation of RhB with various concentrations i.e. 300, 500, 700 and 1000 ppm was studied keeping other parameters constant. A decolourisation of 100% was achieved with 100 ppm solution. However the result was found decreased with higher concentrations. The efficiency of 71% decolourisation was obtained with 300 ppm solution while in case of 500, 700 and 1000 ppm, the decolourisation efficiency was 15, 4 and 0% respectively (Fig. S3(b)). This might be due to the light absorbed by the substrate (RhB) is more as compared to the catalyst with increase in the concentration [5]. So, at higher concentration, the photo decolourisation efficiency is decreasing. 3.2.4. Effect of 1 wt% WO3 -VPO and 3 wt% WO3 -VPO on photo decolourisation In order to study the effect of the less WO3 content catalysts, we have prepared 1 wt% WO3 -VPO and 3 wt% WO3 -VPO. The catalysts i.e. 1 wt% WO3 -VPO and 3 wt% WO3 -VPO shows 51 and 79% decolourisation towards RhB under the same condition (Fig. S3(c)).
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Scheme 1. Diagram for energy band levels of WO3 -VPO composites and the possible charge separation process with mechanism of dye degradation.
3.3. Proposed mechanism for the enhancement of the activity The mechanism of the photocatalytic activity of WO3 -VPO heterojunction is explained by calculating their relative band edge positions which shows migration direction of the photogenerated charge carriers. At the point of zero charge, the conduction band edge of a semiconductor can be theoretically predicted by Mulliken electronegativity theory [51]. ECB = X − Ee − 0.5 Eg
Fig. 7. Effect of different catalysts on the degradation of Phenol; Phenol = 100 ppm, catalyst dose = 0.03 g, time = 10 min, pH = 3.79, room temperature = 25 ◦ C.
3.2.5. Effect of catalysts on photodegradation of phenol Photocatalytic degradation of Phenol over neat and promoted VPO catalysts was studied with the same conditions as RhB decolourisation. The results of the phenol degradation were shown in Fig. 7. The degradation was found increased with the WO3 loadings. The neat VPO and WO3 showed a degradation of 10% and 5% while 5 wt%, 10 wt% and 15 wt% WO3 -VPO showed 58, 60 and 60% respectively towards phenol under the same conditions. The catalysts can also be used for the degradation of phenolic compounds under visible light irradiation. There are so many catalysts which have been used for the decolourisation of RhB and Phenol. A comparison activity of our catalyst with those catalysts has been illustrated in Table S1. The present catalyst WO3 -VPO showed remarkable photocatalytic activity with respect to other photocatalytic systems. 3.2.6. Reusability test Figs. S5 and S6 display the reusability performance of 10 wt% WO3 -VPO catalyst for 4 successive cycles. The solid catalyst was separated from reaction mixture by centrifugation after completion of the reaction. It was thoroughly washed with acetone for several times and then reused as catalyst in the degradation process under the same condition as the original. There was no significant change found in the activity of catalyst after 4 cycles. The decolourisation was found to be stabilized in a range between 99 and 100% in case of RhB and degradation of 59–60% for phenol. The synergetic effect between WO3 and VPO made the active phase more stable.
where X is the absolute electronegativity of the semiconductor, ECB and Eg is the position of the conduction band and the band gap of the semiconductor, respectively. Ee is the energy of the free electrons on the hydrogen scale i.e. constant value (∼4.5 eV). The X value and the band gap energy of neat VPO are calculated to be 6.51 and 2.17 eV, respectively. By applying the above equation, we have predicted that the conduction band (CB) edge position of pure VPO is 0.92 eV. It is reported that the Eg and CB of neat WO3 are 2.4 eV and 0.5 eV, respectively [37,52]. The VB can be obtained by EVB = ECB + Eg. The corresponding valence band (VB) positions of pure WO3 and VPO are 2.9 and 3.09 eV, respectively. So, the conduction band bottom and valence band top of WO3 is higher than that of VPO which is illustrated in Scheme 1. The derived results show the composite materials are useful for the separation and transportation of electron-hole pairs charge carriers. When it is excited under visible light irradiation, the photoexcited electrons and holes are generated on the surfaces of the material. The photoexcited electrons on the conduction band of WO3 can easily be transferred to the conduction band edge of VPO due to their close link through interface and simultaneously, holes on the valence band of the VPO can be quickly migrated to the valence band of WO3 under the difference in potential of band gap energy. In this approach, the photogenerated electrons and holes are present on the different surfaces, because of which the recombinations of charge carriers get suppressed which can be correlated with PL spectra. So, the photocatalytic activity of the composite materials towards dye decolourisation/degradation is increased. The neat catalyst WO3 and VPO shows less degradation due to the fast recombination between photogenerated holes and electrons. The photodegradability of the organic pollutants over the heterojunction composite materials greatly enhances owing to the collection of electrons and holes on different surfaces. The electrons present on the surface of VPO in heterojunction composite reacts with absorbed O2 to give rise O2− and OH which can further oxidize dye molecules [14]. Simultaneously, collected holes on the surface of WO3 traps OH−
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have suitable band gaps for photo decolourisation/degradation of organic pollutants under visible light irradiation. • PL intensity It is well known that photoluminescence (PL) signals and their intensity are indirectly proportional to its photocatalytic activity which is discussed in detail in PL studies. From the PL spectra, we have found that the intensity is in order VPO > 5 wt% WO3 VPO > 10 wt% WO3 -VPO (Fig. 3). So, 10 wt% WO3 -VPO showed the highest activity. • Formation of • OH
Scheme 2. Schematic diagram illustrating transfer of an electron injected into WO3 VPO by an excited dye molecule adsorbed at the WO3 -VPO.
radical obtained from H2 O to form OH which oxidizes the absorbed RhB dye [14]. A dye-sensitized mechanism can play a role for RhB decolourisation. For this, we studied the decolourisation of RhB by 365 nm UV light (IUV = 1.381 × 10−6 Einstein L−1 s−1 ) and visible light under the same conditions and found that 70% and 100% decolourisation in 10 min, respectively. The higher activity under visible light shows the presence of a dye-sensitized mechanism in addition to WO3 VPO sensitization due to the adsorption of more dye molecules on the semiconductor surface. The adsorption of dye on WO3 -VPO (10.0%) is higher comparison to WO3 (6.5%) and VPO (4.5%) under the dark condition. It implies that the photoexcited electron transfer from visible light-sensitized RhB molecule to the conduction band of WO3 which produces superoxide radicals (O2− ) (Scheme 2). Here, O2− degrade the RhB by the dye sensitization mechanism which is given below (Eqs. (2)–(4)). Further, to prove it, we also carried out the degradation of phenol by WO3 -VPO under the same above conditions and found that UV induced phenol degradation (87.4%) was more efficient than visible induced (60%), shows the presence of only a catalyst-sensitized mechanism in the phenol degradation. This confirms the presence of a dye-sensitized mechanism for RhB decolourisation [53,54].
Greater generation of OH radicals confirms to a higher photocatalytic activity [47]. Therefore, the 10 wt% WO3 -VPO composite material has higher photocatalytic activity than the pristine VPO. It is observed that the order of the formation rate of OH radicals on the surface of the catalyst is as follows: 10 wt% WO3 -VPO > 5 wt% WO3 -VPO > VPO (Fig. 4). Therefore, 10 wt% WO3 -VPO exhibits the highest activity. • Photocurrent measurement It is known that the photocurrent is directly proportional to the concentration of electrons for n-type semiconductor. We have found that our catalysts are n-type semiconductor and the photocurrent trend is as follows 10 wt% WO3 -VPO > 5 wt% WO3 VPO > VPO (Fig. 5(a–c)). Also, there is some extent of dark current present in the samples. So, the catalytic activity of 10 wt% WO3 -VPO showed remarkable degradation efficiency. It may be the doubling effect due to the presence of dark and light current. • Heterojunction Phenomena
There are six factors which affect the photocatalytic activity of the WO3 -VPO photocatalyst.
From the mechanism part (Section 3.4), we have found the heterojunction property of the catalyst (Scheme 1). In this heterojunction approach, the photogenerated electrons and holes are present on different surfaces i.e. WO3 and VPO, so of which the recombination of charge carriers decrease which can be correlated with PL spectra and photocurrent measurement. So, the photocatalytic activity of the composite materials towards dye degradation is increased. It is known that the redox potential e of OH− /OH and O2 to O2− are 1.89 V/NHE and −0.13 V/NHE. Our catalysts have more positive VB potential i.e. 2.9 and 3.09 eV than OH− /OH which informs the photogenerated holes have strong oxidative ability and they can oxide OH− into OH. Also, the CB potential of our catalyst has more positive potential i.e. 0.5 and 0.92 eV to reduce O2 to O2− . Therefore, the charge separation of e− –h+ pairs are more effective than individual one and tends to increase in the availability of h+ which helps to improve the photocatalytic activity of the WO3 -VPO.
(I) Band Gap Energy (Eg)
• Photosensitization and synergistic property of WO3
From UV–vis DRS, the indirect band gap (n = 2) is found to be 2.17 eV, 1.93 eV and 1.79 eV for the neat VPO, 5 wt% WO3 -VPO and 10 wt% WO3 -VPO respectively (Fig. 2(a–c)). These findings are in well agreement with the observed photocatalytic activity because the band gap energy is in order VPO > 5 wt% WO3 -VPO > 10 wt% WO3 -VPO i.e. 10 wt% WO3 -VPO is more active in visible light region. So, 10 wt% WO3 -VPO showed highest activity i.e. 100 and 60% towards RhB decolourisation and phenol degradation respectively, where as VPO showed some lower value i.e. 30 and 10% respectively. So, the above results imply that the prepared photocatalyst
In our case, WO3 acts as photosensitizer [55] and has synergistic property [56]. Being a photosensitizer, it was quickly excited under light irradiation and provided the electrons to the conduction band of VPO to enrich the amount of electrons. Simultaneously, the holes present on the VB of VPO easily transferred to the VB of WO3 [53,54]. Therefore, the recombination of charge pairs is reduced in the case of composite system than neat sample. The synergetic effect between WO3 and VPO inhibited the recombination of electron-hole leading to an increased charge carrier separation. So the composite materials showed the highest activity.
RhB + h → RhB+• + ecb −
(2)
ecb − + O2 → O2−
(3)
RhB+• + O2 /O2− → degradationproducts
(4)
3.4. Overall reasons for the increase in photocatalytic degradation of RhB and phenol
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4. Conclusions In summary, we have successfully tested the catalytic efficacy of WO3 -VPO in photocatalytic decolourisation of RhB and degradation of phenol. WO3 -VPO was found to be a n-type semiconductor where the photogenerated electrons inhibited the recombination rate of electron–hole pairs. The synergetic interaction between WO3 and VPO, the formation of a higher number of hydroxyl radicals and inhibition of recombination factor play a major role for obtaining such unexpected results. Again the catalysts exhibit a heterojunction phenomenon where the charge separation of e− –h+ pairs are more effective than individual one and tends to increase the availability of h+ which helps to enhance the photocatalytic activity of the WO3 -VPO. The instantaneous reaction with unexpected results paves the way to utilize VPO materials in dye decolourisation and phenol degradation. Our findings will encourage the researchers to work more in this field and will prove that VPO materials can not only be used in the gas phase oxidation but also can be used in the liquid phase reactions. Acknowledgement The authors wish to express their deepest and sincerest recognition of Prof. András Dombi a key figure in the topic of photocatalytic materials for the degradation of contaminants of environmental concern. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.10. 017. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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