Fe doped TiO2 catalyst synthesis using ultrasound and subsequent application for photocatalytic oxidation

Photocatalytic oxidation has shown promise as alternative technique to conventional techniques though lack of ecient catalyst is a major problem in commercial application. In the current work, a transition metal, Fe (iron), is doped into the TiO 2 catalyst by coprecipitation method assisted by ultrasound with an objective to achieve reduced particle size and better characteristics. Optimization study for the parameters associated with the synthesis was performed resulting in the best conditions of ultrasonic power (120 W), irradiation time (30 min), temperature (35ºC) and Fe doping (0.4 mol%). Using characterization techniques of Particle size analysis, XRD, FTIR, UV-Vis bandgap analysis and BET analysis, the comparison of the catalysts obtained using ultrasound assisted and conventional approach was performed. The particle size obtained by ultrasound assisted process (US catalyst) was 348 nm which is much lesser than size of catalyst prepared conventionally (CV catalyst) as 1069 nm. XRD and FTIR analysis demonstrated the superiority of the US catalyst over the CV catalyst. The UV-Vis bandgap analysis established the reduction of bandgap from 3.2 eV to 2.8 eV due to doping of Fe. BET analysis showed the improvement of surface area from 19.413 m 2 /g to 40.603 m 2 /g due to the use of ultrasound. The photocatalytic oxidation of Acid Scarlet 3R dye was performed, where US catalyst showed 48.2% degradation, higher than the CV catalyst where degradation was 34%. Overall the work has shown the methodology to obtain ecient catalyst based on doping of Fe and the effect of ultrasound also demonstrating better photocatalytic activity.


Introduction
Advances in technology have resulted in higher demands for water in the industries which also means that higher amount of wastewater is generated from the industries. An increase in the demand for effective water treatment has resulted in research into various oxidation processes for obtaining high e ciency. Wastewater generated by the industries has a huge amount of inorganic and organic soluble substances demonstrating high chemical oxygen demand (COD) and typically offering low degradability for biological oxidation (Jo and Tayade, 2014;Priyaragini et al., 2014;Forgacs et al. 2004;Mahvi et al. 2009;Galindo et al. 2001;Vautier et al. 2001). Adsorption, extraction and chemical oxidation are some of the other conventional methods of wastewater treatment that also have limitations such as speci c applicability, lower e ciency and secondary pollutant formation (Gogate and Patil, 2015). The e cacy of the conventional processes is lower when it comes to presence of molecules having complex structure and also there is always need for developing advanced processes. Advanced oxidation process, which rely on the generation of hydroxyl radicals and its attack on the pollutants offer promise for treatment. The hydroxyl radicals (OH) have a high oxidation potential of 2.81 V which is the strongest oxidant and hence can oxidize any compound that comes in contact with it in the wastewater. Photocatalysis, ozonebased oxidation, Fenton chemistry and cavitation are some of the AOPs (Musmarra et al. 2016) used for wastewater treatment. The hydroxyl radical generation can be enhanced based on the use of the strong oxidizing reagents like hydrogen peroxide (H 2 O 2 ), ozone (O 3 ), molecular oxygen (O 2 ), etc., either in the process itself or as a suitable intensi cation strategy (Divyapriya et al. 2016, Sood et al. 2015. The attack of hydroxyl radicals (OH) on the recalcitrant organic pollutants can lead to the formation of smaller intermediates or complete mineralization to water and CO 2 . Among the different AOPs, photocatalytic oxidation have shown promise for commercial application mainly based on the requirement of ambient conditions and possibility of using solar light which can give an economical operation.
In the case of photocatalytic oxidation, any photocatalyst gets activated when it absorbs light and gives expected and e cient conversion of reactants into products based on the interactions with hydroxyl radicals and/or electrons offering high yield and reaction rate. Many recent studies propose TiO 2 to be one of the most applicable and satisfactory photocatalysts for water treatment which offers noticeable advantages as being non-toxic, stable and chemically inert (Hsien et al., 2001;Ranjit and Viswanathan, 1997). The bandgap energy of TiO 2 is 3.2 eV and it requires a wavelength of 388 nm for excitation of an electron from the valence band to the conduction band which falls in the ultraviolet region (Gu et al. 2010;Huang et al. 2009). TiO 2 is reported as a perfect catalyst for photocatalytic degradation of the organic contaminants in water, decolorization of dyestuff, etc. (Reddy et al. 2016;Bethi et al. 2016). However, due to the huge energy crisis, the use of natural solar light having a visible wavelength over arti cial UV light is crucial. Some modi cation is required to reach the excitation at the wavelength above 388 nm, and one such method of modi cation is doping of catalyst. Doping of catalyst also helps in avoiding the recombination of photo-generated electrons and holes which can provide higher effectiveness for enhanced rates of degradation (Peng et al. 2012;Rauf et al. 2011).
Transition metals doping is predominantly used to improve photocatalytic activity and reduce the recombination hole and electron (Kočí et al. 2012;Barakat et al. 2005). Iron (Fe + 3 ) has been noticed to be a good dopant for TiO 2 as it has an atomic radius of 0.69 Å which is almost equal to that of Ti + 4 as 0.745 Å which makes it convenient to integrate Fe + 3 into the TiO 2 matrix and also due to its stable halflled d 5 con guration (Sood et al. 2015). When a small amount of metal doping is present, photogenerated holes and electrons are well separated which therefore increases the e cacy. Fe 3+ traps the photogenerated holes and it forms Fe 4+ reacting with OH radical on the surface, generating hydroxyl radicals and O 2− in the surface lattice of doped TiO 2 . Also, Fe 4+ itself can react with photogenerated electrons and affect photocatalytic activity (Tong et al. 2008). A study on using Fe-modi ed TiO 2 for the degradation of Reactive Brilliant X-3B azo dye reported that 0.1 wt% of iron doping showed a good photocatalytic degradation (Qi et al. 2005). It was also reported that excessive deposition of Fe on TiO 2 lead to the formation of Fe(OH) 2+ .
Many research work on ultrasound-assisted synthesis reported that it has a signi cant potential in improved nanomaterials synthesis (Sivakumar and Gedanken, 2004;Anandan and Ashokkumar, 2009).
The employment of ultrasound waves through the liquid-based medium (solution for catalyst preparation) creates an alternate high and low-pressure zone in the liquid medium and thus shows different stages of cavitation. The cavity bubbles collapse leads to the formation of hot spots with a temperature of about 5000 K and a pressure of about 1000 bar. The cavitation energy released due to the ultrasound intensi es the micro-mixing which also improves mass transfer i.e., solute transfer rate and nucleation rate. The major reason for obtaining a lower size of the particle is due to the generation of cavitation bubbles and consecutive collapse which leads to fragmentation of the particle through microjets (Chen et al. 2011).
We now present brief literature review on the use of ultrasound for improved photocatalyst synthesis. Li et al. (2010) synthesized Fe doped TiO 2 based on the ultrasound-assisted hydrothermal method for the degradation of methyl orange. Zhou et al. (2006) synthesized Fe doped TiO 2 using the ultrasonicallyinduced hydrolysis method. Shirsath et al. (2013) studied the effect of ultrasound operating parameters affecting the particle size of the catalyst prepared based on the sol-gel synthesis process assisted by the ultrasonic horn. Huang et al. (2009) reported that the catalytic performance of a doped TiO 2 photocatalyst depends on the amount of the dopant and the method by which the doping is carried out.
Analysis of the literature revealed that there has been no study reporting the use of co-precipitation method and the effect of ultrasound on the same for the synthesis of Fe doped TiO 2 . Considering this, the importance of the present work is established.
In the current work, Fe doped TiO 2 catalyst is synthesized based on the co-precipitation method. The irradiation system of the ultrasonic bath is chosen for the study so that there is no probability of tip erosion based on the indirect contact and hence there would not be any adverse effect on the quality of the synthesized catalyst . The main objective of the work is to investigate the effect of ultrasound in the synthesis of Fe doped TiO 2 photocatalyst by the co-precipitation method so as to establish the improvement in the morphology and characteristics of the Fe doped TiO 2 . The work also focuses on the optimization of the sonication parameters so as to obtain conditions for the minimum particle size and optimum Fe doping. Characterization study of prepared photocatalyst was also performed to establish the effects of ultrasound and subsequently photocatalytic effect of the Fe doped TiO 2 catalyst has been studied for dye decolorization application. Initially, 0.4g of urea was dissolved in 40ml of distilled water in which the calculated amount of Ferric Nitrate (as per the required loading of 0.2-1.2 mol% of Fe) was added. Subsequently, 10 ml of Titanium (IV) tetra-isopropoxide (TTIP) was added dropwise to the prepared urea and Ferric Nitrate solution under the stirring at 500 rpm. Soon after the addition of TTIP, the mixture was kept in the ultrasonic bath. After the reaction, the prepared solution was dried in a water bath at 80-90 ºC and calcined at 500 ºC for 3 h in a mu e furnace. The obtained sample (referred as US catalyst) was then subjected to grinding before actual use. Figure 1 depicts the scheme of catalyst synthesis.

Materials And
Experiments with the ultrasound were performed using a low-power ultrasonic cleaning bath (Dakshin Ltd., Mumbai, India). The ultrasonic bath used for the experiments had a stainless steel rectangular tank.
The tank dimensions were 300× 150 ×150 mm. The surface area of the bath is 0.045 m 2 . At the bottom of the tank, there are ve transducers arranged in a zigzag position. Ultrasonic bath operates at 22 kHz and total power of 250 W (Moholkar et al. 2000;Rao and Rathod, 2015). The glass reactor ( at base with a diameter of 7 cm) was placed at the center of the bath and the bath was lled with 4 liters of water used as coupling uid. The schematic setup is shown in Fig. 2. In the case of ultrasound assisted approach, the effect of different parameters as the power of sonication, time of sonication, the temperature of the water bath and % Fe doping on the catalyst size has been studied.

Fe doped TiO 2 synthesis by the conventional approach
In this procedure, the Fe doped TiO 2 was synthesized by the co-precipitation method. 0.4g of urea was taken and dissolved in 40ml of distilled water in which the calculated amount of Ferric Nitrate was added for obtaining loading of 0.4 mol% Fe. 10 ml of TTIP (Titanium (IV) isopropoxide) was added dropwise to the prepared urea and Ferric Nitrate solution under condition of the stirring at 500 rpm. After the reaction, the prepared solution was dried in a water bath at 80-90 ºC and calcined at 500 ºC for 3 hours inside the mu e furnace. The obtained sample was subjected to grinding and referred to as the conventionally prepared catalyst (CV catalyst).

Fe doped TiO 2 catalyst Characterization
The particle size of the iron-doped TiO 2 catalyst samples was analyzed using Malvern Zetasizer Nano S model. X-ray diffraction analysis of the catalyst was carried out using a powder X-ray diffractometer (Philips PW 1800). The diffraction patterns of the sample were obtained using radiation source of Niltered Cu-Kα radiation at wavelength λ = 1.5418 Å. The test was performed at room temperature. The X-Ray Diffraction patterns of the catalyst were recorded between the angles of 20º to 80º at a 2º/min scanning rate. Match software was used to interpret the peaks and characterize the synthesized Fe doped TiO 2 catalyst. FTIR spectra for the catalysts were also recorded using Perkin Elmer FTIR spectrometer (Paragon 1000 PC) with the wavenumber ranging from 500 to 4000 cm − 1 . The UV-Visible band gap spectra were observed using Agilent UV-Vis Cary 100 spectrophotometers between the range of 200 to 800 nm. The BET surface analysis of the samples was performed using SMART SORB 92193 surface area analyzer. The adsorption and desorption study of the catalysts was also carried out using liquid nitrogen to nd the speci c surface area.

Photocatalytic degradation of Acid Scarlet 3R dye
To nd the photocatalytic activity of the catalysts obtained with different amounts of Fe and using ultrasound-assisted and conventional approaches, photocatalytic oxidation experiments were performed.
Experiments were carried out inside a UV chamber with two UV lamps (Phillips 8W TUV) operating at a dominant wavelength of 254 nm. A Quartz beaker of 250 ml capacity having a diameter of 68mm and a height of 95 mm was used throughout the experiment. Two UV lamps were placed inside the chamber with one at the top and one on the side so that the light intensity is evenly distributed to the Quartz reactor. Figure 3 shows the sketch of the photocatalytic reactor. All the experiments were performed using a xed volume of dye solution as 200 ml, 50 ppm of dye concentration with 0.4 g/L.
The degradation of dye was monitored in terms of the color change monitored using a UV spectrophotometer (VARIAN Cary 50 Conc) unit obtained from Agilent Technologies, USA. For the experiments involving catalyst, the withdrawn samples were centrifuged before the spectroscopic analysis and the supernatant was taken for the analysis. Centrifugation was performed for the effective separation of catalyst so as to avoid any interference during the spectroscopic analysis. The calibration curve of the Acid Scarlet 3R dye was used to calculate the concentration of the dye from the recorded absorbance of the dye at wavelength of 505 nm.

Effect of operating conditions of ultrasound on particle size
The effect of the different operating conditions of the ultrasound such as the power of sonication, sonication time, the temperature of the bath and also the mol% of Fe on the particle size of the catalyst was investigated. The aim of optimizing these operating conditions is to obtain the best catalyst with a smaller particle size and good particle size distribution. Also, the characterization of the catalysts synthesized under optimum conditions using both conventional approach and ultrasound-assisted approach was performed to compare their e ciency. It is expected that a smaller size of the particle could be achieved by the assistance of the ultrasound which can be an effective parameter in deciding the catalyst activity (Shirsath et al. 2013;Neppolian et al. 2008).

Effect of Power of sonication
To investigate the effect of the power of sonication on the particle size, the synthesis of the catalyst was carried out at different power of sonication i.e. 80 W, 100 W, 110 W and 120 W. The sonication experiment was performed for 30 min at temperature of 35ºC and frequency of 22 kHz. The percentage of Fe to be doped was considered xed as 0.4 mol%. The resultant particle size obtained at different power dissipation values is shown in Fig. 4. It is noticed that as the power of irradiation is increased, the particle size of the catalyst decreases from 608 nm to 348 nm. The trend shows that high power favors the fragmentation of the particles based on a higher degree of turbulence that tends to inhibit agglomeration or formation of larger particles. Marin et al. (2018) reported similar trend in particle size distribution in silica suspensions. Sivakumar and Gedankan (2004) also reported that an increase in power density increased the extent of particle size reduction of Fe(CO) 5 . The comparison of results presented in the current work and literature showed that the variation in the power of sonication has a noticeable effect on the particle size of the catalyst and the trends are speci c to the system. Considering the results obtained in the present work, the power of 120 W was xed as optimum and used for further studies. The smaller size of particles achieved by the ultrasound-assisted process at higher powers could be associated with the fact that high-intensity ultrasound aids in fragmentation of the particle which leads to smaller particles (Sumari et al. 2013;Jordens et al. 2016;Kusters et al. 1993;Bang and Suslick, 2010), though it is noteworthy to understand that there exists an optimum power speci c to the system in most cases and establishing the trends for speci c system is always important.

Effect of irradiation time
Experiments were performed to understand the irradiation time effect on the particle size of Fe-TiO 2 over the range of 15-60 min keeping the power of sonication constant as 120 W at a bath temperature of 35ºC. The results obtained are plotted in Fig. 5 where it is seen that there is lowering of the particle size up to 30 min of irradiation and later with an increase in time, the particle size starts increasing. This may be because of the agglomeration of the particles to form bigger particles due to extensive processing beyond the optimum. It is again important that the optimum value will be dependent on the speci c system. For example, Neppolian et al. (2008) reported a reduction in crystalline size and also surface area increase for TiO 2 when the ultrasound irradiation time was increased from 30 min to 90 min. Shirsath et al. (2013) also reported that a long time of ultrasound irradiation leads to smaller particle size. Pradhan et al. (2016) reported an increase in particle size at much higher treatment times while synthesizing Mn nanoparticles where lesser particle size was achieved for 15 min of ultrasound irradiation established as optimum. Ambati and Gogate (2017) also reported a similar trend of existence of optimum irradiation time for the case of 0.8 mol% Fe doped on TiO 2 . Results on the particle size obtained in the current work suggested that prolonged sonication has bene cial effect on particle size reduction only till 30 min of sonication and hence this was considered as the optimum sonication time.

Effect of initial temperature
To nd the effect of the temperature on the catalyst's particle size, the catalyst was synthesized at the optimum power of 120 W and irradiation time of 30 min at different temperatures of 35, 45, 55 and 65 ºC. Though the temperature of the sonicator bath was xed at the desired value there was a slight elevation in the temperature of 2 to 5 ºC due to the cavitation effect. From Fig. 6, it was observed that there is an increase of particle size from 348 nm to 516.1 nm when the initial temperature increased from 35 ºC to 45 ºC. When the temperature increases the vapor pressure increases which eventually leads to the formation of a high number of nuclei with low cavitation intensity (Ambati and Gogate, 2017). Patel et al. (2013) also reported the negative effect of the temperature on the particle size reductions attributed to lower cavitational intensity at a higher temperature during their study on cerium zinc molybdate synthesis. It was clearly demonstrated that as the temperature increased from 40 to 80°C, the particle size increased from 26.3 nm to 174 nm. It is crucial to notice that the effect of the increase in temperature on the size reduction is dependent on the system. In the current work, no continuous increase in particle size was seen with an increase in the temperature and the particle size reduced marginally from 516.1 nm at 45 ºC to 502.4 nm and 478.4 nm, with a subsequent increase in temperature to 55 ºC and 65 ºC respectively. Considering the results obtained in the present work, an initial bath temperature of 35°C was considered as the best considering the minimum size obtained that is desirable for better catalytic activity.

Effect of loading of Fe doped on TiO 2
Experiments were carried out to synthesize Fe doped TiO 2 catalyst with different extents of doping based on the mole percentage of Fe as 0.2, 0.4, 0.6, 0.8 and 1 mol% under the optimum conditions as power of 120 W, 35 ºC as temperature and 30 min of the irradiation time. The results for particle size depicted in Fig. 7 established that the particle size of the catalyst increased from 319 for 0.2 mol% to 575 nm for 1 mol%. It was also seen that when the amount of catalyst increased from 0.8 to 1 mol%, the particle size increase was marginal from 544 nm to 575 nm. It is expected that there will be a small increase in the size when the metal is doped to TiO 2 , as the metal controls the in ation in grain size and thereby affects the crystal size (Popa et al. 2010). At higher loadings there is a marginal change, which can be credited to the microstrain effect which results in the formation of a fewer number of recombination spots and active sites eventually leading to a minor difference in particle size (Murashkina et al. 2015). A similar outcome was also reported for Fe doped TiO 2 catalyst synthesized by an ultrasound-assisted sol-gel method where the particle size increased from 185.3 nm to 351.2 nm on increasing the Fe load from 0.4 to 2 mol% (Ambati and Gogate, 2017). Also in Mo doped TiO 2 catalyst, the crystallite size was reported to increase from 14 to 24 nm when the Mo loading increased from 0.0625 to 0.25 mol% (Khan and Berk, 2014).
As the particle size cannot be the only distinguishing factor for the different doping of Fe on TiO 2 it was decided to use the degradation studies to establish the best performing catalyst. The degradation of 50 ppm Acid Scarlet 3R dye was studied in presence of 16 W UV light. Figure 8 shows the degradation results for catalysts with different doping % of Fe. It was seen that Fe-TiO 2 catalyst with 0.4 mole% doping resulted in maximum degradation of 48.39% with a corresponding COD reduction of 46.2%. A subsequent increase in the Fe doping amount resulted in lower extents of degradation. The kinetic analysis of the degradation by the catalyst with different Fe doping is shown in Fig. 9 which con rmed the governance of rst order kinetic mechaisms. From the graph showing the data for rate constants (also tabulated in Table 1), it is clear that 0.4 mol% of Fe doping has more e cacy in dye degradation re ected by the maximum rate constant. The results depicted in Table 1 in terms of observing best extent of degradation, COD reduction and the kinetic rate constants at optimum doping quantum can be explained by the fact that introducing a small quantity of Fe to TiO 2 below the optimum aids in the lowering of the photogenerated hole-electron recombination rate. When the loading of Fe gets higher, these Fe ions can act as the recombination centers due to which the activity declines slowly. Some of the earlier investigations also showed similar trends, for example, Zhou et al. (2006) showed that a limited amount of Fe doping to TiO 2 with an atomic ratio of Fe to Ti as 0.25 demonstrated increased photocatalytic activity. Asiltürk et al. (2009) in their investigation on photocatalytic degradation of Malachite green elucidated that the catalyst with 0.19 wt% of Fe doping to TiO 2 showed improved photocatalytic activity in comparison to undoped catalyst and catalyst with 4.89 wt% Fe doping on TiO 2 . Nahar et al. (2007) researched the photocatalytic degradation of phenol present in the domestic water and reported that 0.5% Fe doped TiO 2 as the optimum had a higher degradation capacity. The conventionally prepared catalyst (without ultrasound) under the optimized conditions had 1069 nm of average particle size, with the polydispersity index of 0.251 and the catalyst which is prepared by ultrasound-assisted approach had 348 nm of average particle size, with a polydispersity index of 0.130. The graph of intensity versus size obtained for the conventionally and the ultrasonically synthesized catalysts is shown in Fig. 10 (a) and Fig. 10 (b) respectively. From the graphs, we can get that the average particle size of the Fe doped TiO 2 catalyst reduces when the catalyst synthesis process is assisted by ultrasound. The observed clear reduction is mainly due to the cavitation effects in terms of the turbulence and micro streaming that tend to avoid the agglomeration of the particles as well as drive some extent of particle size reduction.

X-ray Diffraction Analysis
To nd the phase structure of the prepared Fe doped TiO 2 catalysts, wide-angle XRD diffraction pattern analysis was used. The powder samples of the catalysts prepared using the conventional approach (Fig. 11a) and ultrasound assisted approach (Fig. 11b) and were tested. The prominent peaks or the main peaks at 2ϴ = 25.1º, 38.4º, 47.9º, 54.9º and 62.65º show the presence of TiO 2 in the anatase phase which is predominantly crystalline. These peaks also show that the Fe doping has not affected the anatase phase of TiO 2 and hence it can be con rmed that the crystalline structure is not affected by the doping of metal iron. The peak present at 2ϴ = 36.7º, plane (004) in the XRD pattern can be attributed to the existence of iron in the form of hematite in the catalysts (Shirsath et al. 2013;Shon et al. 2007, Ganesh et al. 2012. When comparing the gures, intensities of the peaks in the case catalyst prepared by ultrasound are slightly higher and sharper when compared to a catalyst prepared by the conventional approach. These results can be associated with the fact that ultrasound leads to the improved formation of crystalline particles with anatase phase. The crystalline size of the catalyst was also calculated by using the Debye-Scherrer formula using Full Width Half maximal (FWHM) data as per the following equation: According to the equation, D represents Average Crystal size (Å), K is the shape factor as 0.9, λ is the wavelength of incident x-ray radiation, β represents line width at half maximum intensity, and θ represents Braggs angle.
The average crystalline size obtained for the Fe doped TiO 2 catalyst synthesized by ultrasound-assisted co-precipitation was 98.108 nm and for the catalyst obtained using the conventional co-precipitation method, it was 170.21 nm. The crystalline size is smaller for the catalyst prepared using the ultrasound approach, which shows the importance and impact of ultrasound in crystalline re nement.

FTIR Analysis
FTIR spectral analysis of Fe doped TiO 2 catalyst is shown in Fig. 12 (a) for conventionally prepared catalyst and Fig. 12 (b) for the ultrasonically prepared catalyst. In the FTIR spectra, the major absorption band peaks are in the range of 3400cm − 1 − 3450cm − 1 , 1625cm − 1 − 1630cm − 1 , 811cm − 1 − 640cm − 1 and 580cm − 1 − 480cm − 1 . In the presented spectra obtained, the absorption band between 3420 cm − 1 to 3450 cm − 1 is assigned to stretched vibrations. Speci cally the absorption peak in the region 3400 cm − 1 − 3450 cm − 1 is attributed to a weak transmittance band i.e., peaks of 3429.85cm − 1 (US) and 3405.10cm − 1 (CV) that are associated with the stretching vibrations of the O-H groups. The absorption band between 1620 D = λ β c o s θ cm − 1 to 1640 cm − 1 is aligned to bending vibrations whereas that between 520 cm − 1 to 580 cm − 1 is attributed to stretched vibrations of Ti-O (Shon et al. 2007;Wang et al. 2010;Li et al. 2008 (Ganesh et al. 2012;Wang et al. 1999;Litter and Navfo, 1996;Hendrickx and Boardman, 1995;Bally et al. 1998). The band below 800 cm − 1 can be due to the Ti-O stretching vibration. The broad band in the region of 500 cm − 1 can be due to Ti-O-Ti vibrations in the lattice. The anatase phase of TiO 2 shows a strong FTIR absorption band at 800 cm − 1 to 650 cm − 1 regions (Reddy et al. 2002). It is important to note that the results of the present work matches with the literature works on Fe doped TiO 2 (Shirsath et al. 2013;Ganesh et al. 2012;Wang et al. 1999;Litter and Navfo, 1996;Bally et al. 1998).

UV-Visible spectroscopy for band gap analysis
UV-Visible bad gap analysis was also performed for the catalysts of TiO 2 and the Fe doped TiO 2 synthesized by the ultrasound-assisted method. The Tauc plot in Fig. 13 demonstrates the estimations of the band gap of both the catalyst. The band gap of the TiO 2 catalyst was found to be 3.2 eV whereas the band gap of Fe doped TiO 2 was 2.8 eV, which clearly shows that the doping of Fe to TiO 2 has reduced the band gap energy. The band gap of Fe is known to be 2.6 eV, so when this is doped to TiO 2 of 3.2 eV, the width of the overall band gap of the doped catalyst reduces as compared to the native TiO 2 . Ganesh et al. (2012) and Ambati and Gogate (2017) also demonstrated similar reports that doping of Fe into TiO 2 matrix reduces the band gap.

BET Analysis
BET surface analysis for the synthesized catalysts was also carried out employing the nitrogen adsorption technique. Table 2 shows the results for the speci c surface area, pore volume and average pore diameter of the catalyst prepared by ultrasound approach and conventional approach. The speci c surface area of the Fe doped TiO 2 catalyst synthesized by ultrasound-assisted approach was 40.603 m 2 /g and for the conventional approach, it was found to be much lower at 19.413 m 2 /g. This trend con rmed that the ultrasound aids in the increase in the surface area.

Comparison of Photocatalytic activity with other catalysts
Experiments were performed to compare the photocatalytic oxidation e ciencies of the Fe doped TiO 2 catalyst with other catalysts such as undoped TiO 2 and Fe doped TiO 2 prepared by conventional approach (i.e. without ultrasound) using the system of the degradation of the Acid Scarlet 3R dye. The results are displayed in Table 3 where it could be noticed that the photocatalytic degradation achieved by using US Fe-TiO 2 was 48.2% with a COD reduction of 46.2%, whereas the that achieved using the CV catalyst was 34% with a COD reduction of 23.8%. The undoped TiO 2 showed degradation of 28.2% with COD reduction of 16% con rming that the Fe doped catalyst had best e ciency. An alike trend was also reported by Sancheti et al. (2018), in their investigation on using NiO 2 -CeO 2 catalyst prepared ultrasonically and conventionally for the Brilliant green dye degradation, where it was elucidated that the NiO 2 -CeO 2 catalyst prepared ultrasonically showed degradation of 82% whereas conventionally prepared catalyst gave only 40% degradation. Overall, Fe doped TiO 2 catalyst prepared with ultrasound assistance demonstrated the best photocatalytic oxidation.

Conclusions
The Fe doped TiO 2 catalyst was synthesized using the co-precipitation method based on the ultrasoundassisted co-precipitation approach and conventional co-precipitation approach. Experimental investigations showed that the catalyst prepared with ultrasonic cavitation using bath-type sonicator had minimum particle size and high speci c surface area compared with the catalyst prepared by the conventional approach. The best particle size of the Fe doped TiO 2 catalyst was achieved at the condition of 120 W power of sonication applied for 30 min at 35 C as temperature. Among different doping content of Fe, 0.4 mol% of Fe doped TiO 2 showed best photocatalytic activity quanti ed based on the degradation of the selected dye of Acid Scarlet 3R. Fe doped TiO 2 catalyst with 0.4 mol% of Fe doping had a particle size of 348 nm at the optimized condition, whereas the conventionally synthesized catalyst had a particle size of 1069 nm. XRD analysis established the crystallite size of conventionally synthesized catalyst as 170.21 nm, which is about 70% higher compared to the crystalline size of the catalyst synthesized with ultrasound assistance as 98.108 nm. FTIR spectra showed that the Fe was e ciently doped into the TiO 2 matrix. The bandgap energy analysis using UV-Visible spectroscopy proved that the doping of Fe shifted the bandgap energy from 3.2 eV to 2.8 eV indicating better photocatalytic e ciency in the near-UV or visible region. BET analysis of the catalyst showed that the speci c surface area was 40.603 m 2 /g for ultrasonically synthesized catalyst whereas, the catalyst prepared by conventional approach showed the lower speci c surface area as 19.413 m 2 /g which evidently shows the vital role of ultrasound over increasing the catalyst surface area. From the photocatalytic oxidation experiments based on the degradation of acid Scarlet 3R dye, the catalyst prepared with ultrasound assistance showed 48.2% degradation whereas the conventionally prepared catalyst showed lower degradation of 34%. The present work clearly establishes that the doping of Fe improves the photocatalytic activity of the catalyst and the ultrasonically synthesized catalyst was more e cient and also yielding better morphological characteristics when compared to the catalyst synthesized by the conventional approach.

Declarations
Ethics approval and consent to participate: Not applicable Figure 1 Flow chart for the catalyst synthesis process Effect of power of ultrasound on particle size of catalyst Effect of Fe doping on particle size of catalyst Kinetics of Acid Scarlet 3R degradation at different Fe doping content on TiO2 Figure 10 The Tauc plot of UV-Visible spectra of TiO2 and Fe doped TiO2 catalyst prepared using ultrasound-assisted synthesis with 0.4 mol% doping of Fe at optimum conditions