Enhanced photocatalytic and electrochemical performance of TiO2-Fe2O3 nanocomposite: Its applications in dye decolorization and as supercapacitors

This work reveals a green combustion route for the synthesis of TiO2, Fe2O3 and TiO2-Fe2O3 nanocomposites as photocatalysts for decolorization of Titan Yellow (TY) and Methyl Orange (MO) dyes at room temperature in aqueous solution concentration of 20 ppm under UV-light irradiation. We observed that the TiO2-Fe2O3 nanocomposite shows superior photocatalytic activity for TY dye compared to pure TiO2 and Fe2O3. Rate constant (k) values of TiO2, Fe2O3 and TiO2–Fe2O3 for TY and MO are 0.0194, 0.0159, 0.04396 and 0.00931, 0.00772 0.0119 kmin−1 respectively. The surface area and pore volume of TiO2-Fe2O3 nanocomposite were found to be 71.56 m2/g and 0.076 cm3/g, respectively as revealed by BET studies. From the Barrett–Joyner–Halenda (BJH) plot, the mean pore diameter of TiO2-Fe2O3 nanoparticles was found to be 2.43 nm. Further, the TiO2-Fe2O3 nanocomposite showed good electrochemical behavior as an electrode material for supercapacitors when compared to pure TiO2 and Fe2O3 nanoparticles resulted in stable electrochemical performance with nearly 100% coulombic efficiency at a scan rate of 10 mV/s for 1000 cycles. Interestingly, the novelty of this work is that the designed supercapacitors showed stable electrochemical performance even at 1000th cycle, which might be useful for rechargeable supercapacitor applications. The electrochemical properties of the nanocomposites were compared by the data obtained by cyclic voltammograms, charge-discharge tests and electrochemical impedance spectroscopic studies. These results demonstrated that the TiO2-Fe2O3 nanocomposite showed stable performance compared to TiO2 and Fe2O3 nanoparticles at current density of 5 Ag−1.

The photocatalytic effectiveness of bare TiO 2 is quiet less under direct solar light irradiation, despite of its superior physicochemical properties. This is possibly due to lack of visible light absorption as it exhibits high band gap energy (3.0-3.2 eV) and rapid electron-hole (e − /h + ) recombination 6 . To reduce these drawbacks and improve structural stability, in the past several methods such as doping of hetero-junction with equivalent and/ or different bandgap materials 7 , dyes sensitization 8 , noble and non-noble metal deposition 9 were tried. Among those different methods explored, researchers tried to combine two or more semiconductor metal oxides having different band gaps [10][11][12][13][14][15] .
The present work was conducted on synthesis of TiO 2 , Fe 2 O 3 and TiO 2 -Fe 2 O 3 nanocomposites from Aloe Vera gel assisted green combustion method. The synthesized nanocomposites were characterized for morphological nature, structural feature, surface area, pore size, surface composition, particle size and band gap energies. The TiO 2 -Fe 2 O 3 composites offered improved photocatalytic efficiency for the decolorization of TY and MO dyes under the UV light irradiation compared with that of base TiO 2 and Fe 2 O 3 nano materials. In addition, TiO 2 -Fe 2 O 3 nanocomposites have been investigated in a preliminary way for potential use as an electrode material for supercapacitor applications.

Materials and Methods
Synthesis of tio 2 and fe 2 o 3 nanocomposites (Green method). TiO 2 and Fe 2 O 3 nanocomposites were synthesized by applying Aloe Vera gel 10 as a fuel via solution combustion method. Freshly collected 20 ml of Aloe Vera gel was added to 80 ml of deionized water. The resulting solution was filtered to get gel. This gel was used as a fuel for the synthesis of TiO 2 and Fe 2 O 3 nanocomposites. The metal precursor salts of Titanium (IV) isopropoxide (Sigma Aldrich) and Ferric nitrate (Fe(NO 3 ) 3 ·9H 2 O) (Sigma Aldrich) were placed in two separate silica crucibles containing 5 ml of Aloe Vera gel. These mixtures were stirred using magnetic stirrer. The blends were placed in a pre-heated muffle furnace maintained at 380 ± 10 °C. The arrangement was bubbled to yield a transparent gel. The gel then formed a white froth, which extended to fill the vessel. From that point, the surface of the froth started burning and continued quickly all through the volume, leaving a white powder with a great degree of porous structure. The energy discharged from the response raised the temperature to 1200 °C which aided to shape TiO 2 and Fe 2 O 3 NPs.
Synthesis of tio 2 -fe 2 o 3 nanocomposite. To load iron oxides on titanium dioxide impregnation method was used with small modification like; replacement of water in place of ethanol and using simple stirrer in place of ultra-sonication 16 , which decreases cost and toxicities towards organic solvent. Having made those important modifications, 0.1 M of TiO 2 -Fe 2 O 3 oxide materials were synthesized by adding appropriate amount of TiO 2 and-Fe 2 O 3 powder in aqueous solution, with continuous stirring at 80 °C. At this point, homogenous woody colored solution was observed. In addition, on drop wise addition of ammonia (NH 3 ) to shift the pH of the solution to basic, the same color, but, a heterogeneous (fall apart) type solution was observed. After drying and grinding the obtained product was subjected to calcination at 500 °C for 3 hours. characterization. The structural details of the nanocomposites were explored by using Shimadzu x-ray diffractometer (PXRD-7000) with CuKα (λ = 1.541 Ǻ). The vibrational spectra of the samples were recorded with a Shimadzu's FTIR spectrophotometer (IR Affinity 1 S) using KBr pellets (400-4000 cm −1 ). The UV-Visible spectra of the samples were recorded in the range 200-800 nm using Shimadzu's UV-2600, Uv-visible spectrophotometer. The morphological features and EDS analysis of all the samples were evaluated by using Field emission scanning electron microscope (FESEM/FIB-model Neon-40), at Nano manufacturing technology center (NMTC), CMTI, India. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas of the samples in relative pressure (P/P o ) range of 0.05-0.25. The pore-size distributions of the samples were determined by Barrett-Joyner-Halenda (BJH) method. The total pore volume of nanocomposites was accumulated at a relative pressure of P/P o = 0.99. Sorption measurements of all the nanomaterials were carried out at −196 °C using a Quanta chrome instrument. The cyclic voltammetric studies were made using CHI604E potentiostat (tri-electrode system-sample as working electrode, platinum wire as counter electrode, and Ag/AgCl as reference electrode with 6.0 M KOH as electrolyte. The potential range utilized during these studies is ranging between −1.5 V and 0.9 V. The scanning has been carried out from 10 mV/s to50 mV/s. The potential range for recording the galvanostatic charge-discharge cycles is ranging between 0 and 0.7 V at a current density of 5 Ag −1 . AC impedance measurements were carried out in the frequency range between 1 Hz and 1 MHz, with AC amplitude of 5 mV.

Results and Discussions
As shown in Fig. 1(a) the composition and structure of synthesized materials were identified by PXRD spectra and is in agreement with tetragonal (anatase) and rhombohedral phases of These groups are possibly believed to influence the phase formation as well as in phase stabilization of all the nanocomposites [23][24][25] . The peak observed at460 cm −1 corresponding to Ti-O bond of TiO 2 , was found to be shifted to 525 cm −1 after the incorporation with Fe 2 O 3 where Fe-O stretching mode was clearly observed in the spectra. The peak at 1074 cm −1 is due to the C-O stretching vibration. The strong band below 700 cm −1 is assigned to Fe-O stretching mode. The band corresponding to Fe-O stretching mode of Fe 2 O 3 is observed at 560 cm −1 . Watchful observation of the SEM image of TiO 2 -Fe 2 O 3 nanoparticles (Fig. 3) will reveal the presence of small Fe 2 O 3 particles (circled in red color in Fig. 3) impregnated throughout the surface of TiO 2 there by concluding that the larger particle is TiO 2 and the smaller one is Fe 2 O 3 .
The SEM images of synthesized composites presented in Fig. 4(a-c) show that the morphology of the sample is somewhat porous and agglomerated, which is believed to be advantageous to enhance the properties. In addition, large and small particles of nearly equal sizes were observed in the images. The elemental confirmation in composites was done by using EDX measurements. The results revealed that the particle size of Fe 2 O 3 is smaller than that of the TiO 2 as observed in Fig. 4b. It is clear from the results that, increasing the calcination temperature causes the increase in the crystallinity of the material and helped to remove the impurities. Nevertheless,  www.nature.com/scientificreports www.nature.com/scientificreports/ increasing the calcination temperature may also increase the crystal grain size 26,27 . In addition, the grain size which increases with surface roughness is believed to be advantageous in enhancing the contact surface area among electrode materials used in supercapacitors 28 . Figure 5(a-c) shows the UV-Visible diffused reflectance spectra of TiO 2 , Fe 2 O 3 and TiO 2 -Fe 2 O 3 nanomaterials. As shown in the spectra, TiO 2 exhibit a strong light absorbance edge close to 380 nm due to its inherent band gap (~3.21 eV) presented in the inset of Fig. 5a, which matches with the band gap of TiO 2 -Fe 2 O 3 showing absorbance edges close to 500 nm (Eg = 2.0 eV), which is in agreement with the band-gap energy of Fe 2 O 3 presented in the inset of Fig. 5b,while, TiO 2 -Fe 2 O 3 edge found between 490 nm and 700 nm can be attributed to TiO 2 and Fe 2 O 3 , with band gap energy (E g = 2.08 eV) as presented in the inset of Fig. 5c. This shows that, significant improvement of visible light reflection and thus improvement in electrochemical reactions by means of visible light in addition to ultra-violet 28,29 . As suggested by 30 , owing to unique half-filed electronic configuration, Fe has the capacity to form new energy levels within the band gap range of TiO 2 and reduces the gap between valence band and conduction band (red-shift of the absorption threshold).
The Kubelka -Munk function F(R) is utilized for examining the powders as given by Eq. (1): where R: the reflectance, F(R): Kubelka-Munk function. The optical energy gap can be calculated by using Tauc relation; the band gap (E g ) of semiconductor material can be calculated from the following equation: n where n = 1/2 and 2 for direct and indirect transitions respectively, for the optical energy gap was calculated using Tauc relation as in Eq. (2) gives the direct band 31 .
Bet study. The optimal porosity and high surface area (low particle size) are the two crucial things for the synthesized TiO 2 -Fe 2 O 3 nanocomposite to be efficient catalyst. From Brunauer-Emmett-Teller (BET) study, N 2 adsorption-desorption measurement carried out at 77 K (Fig. 6), the synthesized TiO 2 -Fe 2 O 3 nanomaterial was found to exhibit type IV isotherm with a sharp increase of the adsorbed volume starting from P/P 0 = 0.97. This confirms the presence of well-developed mesoporous nanostructured nature of nanomaterial. The shifting of the hysteresis loop to the higher as the relative pressure (p/p o ) approaching to 1 indicates the presence of the micro porous particles with size greater than 50 nm. Furthermore, it also confirmed by the presence of mesoporous peaks around ~21, 44, 79, and 124 nm on the pore size distribution curve present as an inset in Fig. 6. The obtained BET surface area and pore volume value were found to be 71.56 m 2 /g and 0.076 cm 3 /g, respectively.
The surface area of TiO 2 -Fe 2 O 3 was found to be much higher relative to TiO 2 , which confirms the role of impregnated Fe 2 O 3 in the enhancement of the specific surface area. From the equivalent, Barrett-Joyner-Halenda (BJH) pore-size, the obtained mean pore diameter for TiO 2 -Fe 2 O 3 was found to be 2.43 nm. www.nature.com/scientificreports www.nature.com/scientificreports/ Photocatalytic studies of dye under UV light irradiation. A circular glass reactor with surface area of 176.6 cm 2 was utilized for the dye decolorization studies. A 125 W medium pressure mercury vapor lamp was used as UV light source. The samples were irradiated directly by focusing light into the reaction mixture at a distance of 23 cm. In a distinctive experiment ~60 mg of photocatalyst was dispersed in 250 ml of 20 ppm different dye solutions. The reaction mixture was subjected to vigorous stirring using magnetic stirrer for the entire period of experiment. The extent of dye decolorization was calculated by using the equation, where 'Q' is the extent of dye decolorization, C o and C are the concentrations of dye before and after decolorization, Vis the volume of the reaction mixture and W is the mass of photo-catalyst present in grams. The unit of Q is ppm ml mg −1 . 6 ml aliquots of solution were drawn from the suspensions at definite time intervals and centrifuged straight away and filtered through a what men filter paper. Photo-decolorization of TY dye for synthesized nanoparticles is shown in the Fig. 7(a-c) with evidence of UV-Visible absorption spectra. The TiO 2 -Fe 2 O 3 nanocomposite was found to exhibitsuperior photo-decolorization results for TY dye solution when compared to TiO 2 and Fe 2 O 3 nanoparticles respectively. www.nature.com/scientificreports www.nature.com/scientificreports/ In the case of TY dye solution, the TiO 2 -Fe 2 O 3 nanocomposite exhibited 92.98% dye decolorization as shown in Fig. 7c, which is superior performance compared to 71.67 and 62.89% for TiO 2 and Fe 2 O 3 , respectively as revealed in Fig. 7a,b at 405 nm. Photocatalytic activity as presented in Fig. 7(a-c) revealed that the photo-decolorization effects in the presence of the TiO 2 -Fe 2 O 3 nanocomposite were found to be more efficient and reached the maximum adsorption capability around 53.58%. Up to 60 min of UV light irradiation during the photocatalytic activity, the synthesized composite continued to exhibit the best decolorization capability with 53.58% decolorization of the MO dye solution as shown in Fig. 7c. TiO 2 nanoparticles also exhibited good decolorization ability for the TY dye solution with a final removal of organic dye around 45.02% at 460 nm compared to decolorization  www.nature.com/scientificreports www.nature.com/scientificreports/ ability of Fe 2 O 3 nanoparticles. Fe 2 O 3 nanoparticles ability to remove MO dye was found to be 38.67% as shown in Fig. 8 [33][34][35][36] .

Decolorization of titan Yellow (tY) and
In addition to that, TiO 2 -Fe 2 O 3 nanocomposite exhibited superior photocatalytic efficiency of 92.98% for the azo dye group under UV light irradiation for a period of 60 min. In the case of MO dye solutions, the TiO 2 -Fe 2 O 3 composite exhibited very less photocatalytic activity of 53.58% after 60 minutes of irradiation which is found to bellower than that of TiO 2 and Fe 2 O 3 for MO organic dye after 60 minutes under UV light irradiation. From these results, it can be concluded that TiO 2 -Fe 2 O 3 nanocomposite is an excellent photocatalyst for the decolorization of TY organic dye with high photo-decolorization activity which can be attributed to e − h + recombination, generation of hydroxyl and superoxide radicals.  Tables 1 and 2. High separation and transportation rate of the e − h + pairs, and reduction of band gap are responsible for higher photocatalytic activity of TiO 2 -Fe 2 O 3 nanocomposite which could be attributed to synthetic method and modified surface morphology of synthesized nanoparticles 37 .
Tables 1 and 2 presents kinetics parameters of TY and MO at various time periods (from 0 to 60 min) at 405 and 460 nm respectively, it evidently shows that the degradation of dye increased with time in the presence of synthesized nanoparticles under UV light irradiation 38 . TY dye decolorization of 92.98% for TiO 2 -Fe 2 O 3 nanocomposite is very significant. From the log C/C o values, it is also evident for decolorization of dyes under UV light irradiation time, exhibiting linear relationship based on the following equation: where C o is concentration of dye at time t = 0 min, C is a concentration of dye at particular time t and k is first order rate constant. This follows first order rate kinetics, photocatalytic decolorization efficiency of dye is calculated by using the following equation as efficiency (%) of photo-decolorization. www.nature.com/scientificreports www.nature.com/scientificreports/ where C o is the initial concentration of the dye solution, C is a residual concentration of the dye in solution after degradation in equilibrium.
The photo-decolorization of dye in the presence of photocatalyst occurs as evidenced by little change in the absorption peak after UV light irradiated for 60 min 39 . Figure 9(a,b) showed plots of C/C 0 for the decolorization of TY and MO, where C o is initial concentration of dye and C is the concentrations of the dye at time t, respectively. It is undisputed that the decolorization rate of TY and MO followed by the order of   www.nature.com/scientificreports www.nature.com/scientificreports/ The electronic arrangement of the semiconductor heterojunctions is categorized into straddling gap, staggered gap, and broken gap. Among these, the staggered band structure has high probability in the direction of electron-hole (e − h + ) separation and improving capacity of the photocatalytic reaction. This happens because of the creation of a depleted layer at the interface of the two (which produces an electric field) and then the diffusion of the photogenerated e − h + out of the depletion layer takes places. Thus matching the band gaps of TiO 2 and Fe 2 O 3 play a significant role in the mechanism. During heterojunction, as shown in Fig. 10, when the Fermi level (FL) of one n-type semiconductor core in contact with the FL of the other n-type, the depletion layer that produces electric field was created.
The activated electric field drives the diffusion of the photogenerated e-h+ out of the depletion layer and results in activation of the photocatalysis efficiency. In the higher energy region, some of the photogenerated e − s with a higher energy level than the CB position of TiO 2 could thermodynamically transfer to the CB of TiO 2 (Fig. 11), while photogenerated holes build up in the valence band of Fe 2 O 3 . The negatively charged e − s in the conduction band of TiO 2 will further transfer to TiO 2 -Fe 2 O 3 nanocomposite via Fe 2 O 3 40 . These negatively charged electrons, which are believed to react with O 2 to form the superoxide anion (O − 2 ) in the dye solution and hydrogen peroxide (H 2 O 2 ), while positively charged holes are accompanied in the TiO 2 -Fe 2 O 3 nanocomposite of the valence band, will respond with OH − groups present on the surface of the catalyst, produces reactive hydroxyl radicals (OH . ). The reactions can be described as follows:   Thus, the enhanced photocatalytic decolorization effectiveness seen in the synthesized nano materials is basically attributed to more effective separation of photogenerated e − and h + pairs 41,42 . electrochemical Studies electrochemical behavior of the synthesized tio 2 , fe 2 o 3 nanoparticles and tio 2 -fe 2 o 3 nanocomposite. The appearance of oxidation peak during the charging and reduction peak during discharge process is usual for CV plots. CV was first performed in a range of −1.6 V and 0.9 at a scan rate of 10 mV/s as shown in Fig. 12.
The CV plot of TiO 2 NPs shows one oxidation and reduction peak due to interconversion of Ti 2+ and Ti 4+ , similarly same behavior can be seen in Fe 2 O 3 NPs due to Fe 2+ and Fe 4+ . But in the TiO 2 -Fe 2 O 3 nanocomposite pair of oxidation-reduction peaks appears because of red-ox process of Ti-Fe composite. This behavior indicates that the capacity is mainly from the pseudo capacitance, which is based on the red-ox mechanism 43 .
For the nickel electrodes, the specific capacitance can be calculated from the CV curves, according to the following Eq. (4): www.nature.com/scientificreports www.nature.com/scientificreports/ = . Δ Δ C i t/m V (6) where, i is the applied current, ΔV is the potential range, Δt is the time of a discharge cycle and m is the mass of the  (Fig. 13) measured at a current density of 5 Ag −1 for 1000 cycles within the potential window of 0-0.6 V (for TiO 2 ), 0-0.5 V (for Fe 2 O 3 ) and 0-0.7 V (for TiO 2 -Fe 2 O 3 ) vs. Ag/AgCl. TiO 2 -Fe 2 O 3 electrode exhibited very good stability after 1000 cycles.
Galvanostatic charge-discharge curves of the TiO 2 , Fe 2 O 3 and TiO 2 -Fe 2 O 3 at various constant current densities are shown in Fig. 14. An ideal capacitive behavior is confirmed due to the appearance of triangular shaped curves in charge-discharge diagrams. The better storage rate ability of the synthesized materials was confirmed with an increase in integrated area on the current-potential axis 45 . The TiO 2 -Fe 2 O 3 nanocomposite has the longest charge and discharge time, informative its maximum specific capacitance, which is in agreement with the consequences of Fig. 13, it confirms the voltage as a function of cycle number respectively revealing an opposite performance amongst them. www.nature.com/scientificreports www.nature.com/scientificreports/ EIS measurements for TiO 2 , Fe 2 O 3 and TiO 2 -Fe 2 O 3 electrodes were carried out with three electrode assembly in 1 M KOH within the frequency range between 1 MHz and 100. The EIS spectra can be seen in Fig. 15. The real component Z real reveals the ohmic properties while the imaginary part (Z img ) relates to the capacitive properties 46 . Typically, semicircles with larger radii refer to higher charge transfer resistance of the electrodes 47 . Therefore, EIS result concludes that the charge transfer resistance R ct of TiO 2 -Fe 2 O 3 electrode is much smaller than that of TiO 2 and Fe 2 O 3 , indicating more effective incorporation of TiO 2 -Fe 2 O 3 nanostructures of this sample in the charge transfer process. Thus EIS results confirmed lower charge transfer resistance for TiO 2 -Fe 2 O 3 electrode as compared to TiO 2 and Fe 2 O 3 electrode, and hence possess more capacitive properties 48 .

conclusions
We efficiently synthesized TiO 2 , Fe 2 O 3 NPs and TiO 2 -Fe 2 O 3 nanocomposites via green combustion route with Aloe Vera gel as a fuel. The diffraction peaks of TiO 2 and Fe 2 O 3 nanoparticles were well matched with tetragonal (anatase) and rhombohedral phasesas investigated by PXRD patterns. Furthermore, FTIR spectroscopy confirmed the formation of TiO 2 and Fe 2 O 3 nanoparticles. The presence of the porous and agglomerated surfaces of the TiO 2 as well as Fe 2 O 3 and spherical for TiO 2 -Fe 2 O 3 nanocomposites were observed through SEM and band gap energy of TiO 2 , Fe 2 O 3 nanoparticles and TiO 2 -Fe 2 O 3 nanocomposites were found to be 3.21, 2.0 and 2.08 eV respectively as confirmed by using DRS spectrum. The significant increase in the surface area of the nanocomposite as revealed by BET study confirmed the enhancement of adsorption capacity of pollutants and photocatalytic efficiency by impregnated TiO 2 -Fe 2 O 3 . The photo-decolorization studies revealed that TiO 2 -Fe 2 O 3 nanocomposite is a good photocatalyst for the decolorization of Titan Yellow (TY) and Methyl Orange (MO)organic dyes with high photo degradation activity compared to TiO 2 and Fe 2 O 3 nanoparticles. The optimal conditions for this study included at room temperature in aqueous solution concentration of 20 ppm and catalyst dosage 60 mg under UV-light irradiation. The rate constants can be ranked in the order of Fe 2 O 3 (MO) < TiO 2 (MO) < TiO 2 -Fe 2 O 3 (MO) < Fe 2 O 3 (TY) < TiO 2 (TY) < TiO 2 -Fe 2 O 3 (TY). The synthesized TiO 2 -Fe 2 O 3 nanocomposites showed excellent electrochemical behavior as electrode material for supercapacitance applications. A superior electrochemical response which includes enhanced charge/discharge capacity and cycling stability when compared to pure TiO 2 and Fe 2 O 3 nanoparticles resulted in stable electrochemical performance with nearly 100% coulombic efficiency at a high current density of 5 A/g for 1000 cycles. Interestingly, the novelty of this work is that the designed supercapacitors showed stable electrochemical performance even at 1000 cycles, which can be beneficial for rechargeable supercapacitor applications.