Received: 15 April 2018 Revised: 19 May 2018 Accepted: 2 July 2018 Cite as: David Dodoo-Arhin, Frederick Paakwah Buabeng, Julius M. Mwabora, Prince Nana Amaniampong, Henry Agbe, Emmanuel Nyankson, David Olubiyi Obada, Nana Yaw Asiedu. The effect of titanium dioxide synthesis technique and its photocatalytic degradation of organic dye pollutants. Heliyon 4 (2018) e00681. doi: 10.1016/j.heliyon.2018. e00681 https://doi.org/10.1016/j.heliyon.2018 2405-8440/ 2018 The Authors. Pub (http://creativecommons.org/licenses/bThe effect of titanium dioxide synthesis technique and its photocatalytic degradation of organic dye pollutants David Dodoo-Arhin a,b,d,∗, Frederick Paakwah Buabeng a,b, Julius M. Mwabora b,c, Prince Nana Amaniampong d,∗, Henry Agbe e, Emmanuel Nyankson a, David Olubiyi Obada f, Nana Yaw Asiedu g aDepartment of Materials Science and Engineering, University of Ghana, P.O. Box Lg 77, Legon, Accra, Ghana bAfrican Materials Science and Engineering Network (A Carnegie-IAS RISE Network), Ghana cDepartment of Physics, University of Nairobi, P.O. Box 30197-00100, Nairobi, Kenya d INCREASE (FR CNRS 3707), ENSIP, 1 rue Marcel Dore, TSA41105, 86073 Poitiers Cedex 9, France eCentre Universitaire de Recherche sur l’Aluminium, Universite du Quebec a Chicoutimi, Quebec G7H 2B1, Canada fDepartment of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria gDepartment of Chemical Engineering, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana ∗Corresponding authors. E-mail addresses: ddodooo-arhin@ug.edu.gh (D. Dodoo-Arhin), prince.nana.amaniampong@univ-poitiers.fr (P.N. Amaniampong).Abstract Nanostructured mesoporous titanium dioxide (TiO2) particles with high specific surface area and average crystallite domain sizes within 2 nm and 30 nm have been prepared via the sol-gel and hydrothermal procedures. The characteristics of produced nanoparticles have been tested using X-Ray Diffraction (XRD), BrunauereEmmetteTeller (BET) surface area analysis, Scanning Electron Microscopy (SEM), Fourier Transform Infra-Red (FTIR), and Raman Spectroscopy as a function of temperature for their microstructural, porosity, morphological, structural and absorption properties. The as-synthesized TiO2 nanostructures were attempted as catalysts in Rhodamine B and Sudan III dyes’ photocatalytic decomposition in a batch reactor with the assistance of Ultra Violet (UV) light. The results show that for catalysts calcined at 300 C, w100 %.e00681 lished by Elsevier Ltd. This is an open access article under the CC BY license y/4.0/). Article Nowe00681 2 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/lidecomposition of Sudan III dye was observed when Hydrothermal based catalyst was used whilesw94 % decomposition of Rhodamine B dye was observed using the sol- gel based catalysts. These synthesized TiO2 nanoparticles have promising potential applications in the light aided decomposition of a wide range of dye pollutants. Keyword: Materials science 1. Introduction With the rapid growth in industrialization, a considerable amount of waste keeps be- ing discharged into the environment. These on-going waste disposal processes tend to compromise the quality of water for daily use despite the great importance of clean water to society [1, 2, 3]. Dye pollutants from the textile, dye, food, leather and paper industries, among others, contaminate the environment when they are improperly disposed-of [4, 5, 6, 7, 8]. Yet these synthetic polymeric dyes have a wide spectrum of applications, which makes their use inevitable. The only way to deal with these dye pollutants is to develop safe disposal mechanisms and methodologies so that they can co-exist with the ecosystem to provide a green environment. To this end, a plethora of waste water treatment strategies such as neutralization by acid and basic liquors, chemical oxidation and flocculation, activated and effluent with loaded organic carbon, biological degradation means and photocatalysis have all been investigated [9]. Among the aforementioned strategies, photocatalysis appears most appropriate, as it is energetically sustainable, eco-compatible and gives a total or partial decomposition of dye pollutants. In particular, different classes of dyes have been utilized in modelling organic contaminants in effluents of wastewater. Recently conducted surveys of literature have shown that out of 12,400 papers re- corded in the Web of Science that are devoted to photocatalysis, just a small percent- age of dyes have been investigated; with Methyl Orange (36 %) and Methylene Blue (47 %) receiving the most attention [10]. Though the remaining 17 % may arguably be Azo dyes, it appears Sudan III (a neutral class of azo dye) is rarely reported. Azo dyes are characterised by their N]N chromophoric unit in their molecular structures [11]. Since they constitute approximately 60e70 % of all available commercial dyes products used in the textile industry [12], they are worthy of investigation. Aarthi, T. et al [13], have reported the effect of functional groups in Sudan (III and IV) and Azure (A and B) Azo dyes on the structure and degradation rate using TiO2 nano- photocatalysts. They demonstrated that the degradation rate of these dyes was affected by their molecular structures. The same grouped also provided an excellent work on molecular structure-degradation rate relations for Xanthene class of dye such as Rhodamine [14]. TiO2 nanostructures represent a promising semiconductor for several applications such as Solar cells [15], batteries [16], sensors [17] and catalytic degradation ofon.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 3 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/lidye pollutants owing to their chemical stability, strong oxidizing ability, non- toxicity, mechanical robustness, low cost and efficient photocatalytic activity [18]. It is well documented in literature that, the photocatalytic effectiveness of TiO2 is intensely related to its structural, morphological and textural properties. This has led to the development and synthesis of a myriad of TiO2 possessing different struc- tures and morphologies. The degradation of dye pollutants using TiO2 as catalyst in- volves the exposure of the catalytic material to ultra violet light, which is often linked with the creation of holes in the valence and electrons in the conduction bands responsible for oxidizing and reduce the dye pollutants. Nonetheless, critical issues that often limit the photocatalytic activity of TiO2 materials are the wide band gap energy (3e3.2 eV), low mass transport rates and low quantum yield. To succumb these critical challenges, a plethora of scientific strategies have been adopted by re- searchers, such as reducing grain size of TiO2 and increasing its specific surface area and also by employing different synthesis techniques such as solvothermal method [19], precipitation method [20], sol-gel method [21], thermal decomposition of alkoxide [22], chemical vapor deposition [23]. Among these techniques, solegel technique, is a simple, economical, convenient and accomplished technique for pro- ducing nanoparticles which have different morphologies such as sheets, tubes, quan- tum dots, wires, rods and aerogels. The hydrothermal technique also produces mono- dispersed and highly homogeneous nanoparticles due to the highly controlled diffu- sion in the crystallization medium and phase transformations at relatively low tem- perature [24]. To the best of our knowledge, a comparative investigation on sol-gel synthesis and hydrothermal synthesis methods on degradation of Azo dyes (Sudan III) and Xanthene dye (Rhodamine B) is yet to be reported in the literature. In this research work, we investigate the influence of synthesis method (sol-gel and hydrothermally synthesis) of TiO2 nanoparticles on the light-aided catalytic decom- position of Rhodamine B and Sudan III dyes under varied durations of UV illumi- nation. The influence of annealing temperatures on the surface area of the synthesized TiO2 samples was also investigated. Several characterization techniques were further used to investigate the structural, thermal, optical and crystallinity of the synthesized TiO2 samples.2. Experimental In this research, we used analytically rated chemicals reagents which were not further purified. These chemical reagents used were Rhodamine B dye (C28H31ClN2O3), Sudan III dye (C22H16N4O), distilled water, Titanium tetrachloride (TiCl4), Titanium isopropoxide (TTIP), isopropyl alcohol (IPA), ethanol. Sol gel and hydrothermal methods were adopted to synthesize TiO2 nanoparticles by using different metal alkoxide precursors.on.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 4 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liIn the typical sol-gel synthesis process, the precursor solution consisting of (TiCl4) and absolute ethanol in the ratio of 1: 10 were vigorously stirring at 1200 rpm for 4 hrs at room temperature to enhance the homogeneity and stability of the slurry. The sol formed was allowed to age under room temperature conditions for 18 hrs, son- icated for 30 min and dried in an oven at 120 C for 7 hrs. The prepared powder was then subjected to elevated temperatures of calcination at 200 C, 300 C, 400 C, 500 C, and 600 C for 2 hrs to obtain the TiO2 nanoparticles for analysis. In the hydrothermal technique, 10 mL of TTIP was added to 100 mL of isopropyl alcohol under constant magnetic stirring at 1200 rpm for about 10 min for homoge- neity. 10 mL of distilled water was added slowly at a rate of 2 mL/min and the so- lution was subsequently stirred at 1200 rpm for 10 min. The resulting solution was then aged at room temperature conditions for 24 hrs. After sonicating the sol for 10 min it was moved to a teflon-lined steel autoclave under 15psi, 120 C for 8 hrs. Af- ter cooling down to room temperature, the as-prepared TiO2 nanoparticles were calcined at 200 C, 300 C, 400 C, 500 C and 600 C for 2 hrs to obtain the TiO2 nanoparticles for further characterization and application. The obtained TiO2 nanoparticles are then tested using the Fourier Transform Infra- Red (FT-IR) spectroscopy, X-Ray Diffraction (XRD), BrunauereEmmetteTeller (BET) surface area analysis and Scanning Electron Microscopy (SEM) to determine the crystal structural, microstructural, surface and porosity properties. For the qual- itative phases determination as well as the microstructure of the produced TiO2 powders, powder x-ray diffraction (PXRD) profiles were collected using a Panalyt- ical Empyrean diffractometer having a theta-theta geometry and equipped with a copper (wavelength ¼ 1.5418 A) radiation tube operated at a voltage of 40 kV and current of 45 mA. The collection of the diffraction profiles of all the powder samples were done within 20e70 at a step size of 0.017 and 14 seconds per step. The qualitative phase evaluation of diffraction profiles conducted using the X’Pert Highscore plus search match software (Panalytical, Netherlands) and matched with the ICSD’s powder diffraction files database. To aid the microstruc- ture profile fitting analysis, the resolution contribution function for the diffractom- eter was determined using the NIST SRM 640d (Si) standard [25] after which and the resultant peak profiles were concurrently fitted by applying shape and width restricted symmetrical pseudo-Voigt functions as prescribed by the Caglioti formulae [26]. Subsequently, the microstructural determination was accomplished via the Whole Powder Pattern Modelling (WPPM) technique [27], using the soft- ware called PM2K [28]. For the morphological investigations, the nanoparticles were sputtered coated with carbon to make the particles conductive and allow for higher magnifications in a high resolution Zeiss Ultra plus 55 field emission scanning electron microscope (FESEM) which was operated at 2.0 kV. The structural properties where determinedon.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 5 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/lion a Jobin Yvon Horiba TX 6400 micro-Raman spectrometer possessing a triple monochromator system and a LabSpec (Ver. 5.78.24) analytical software. All the powder samples were analysed using an Argon excitation laser (514 nm) through anX 50 objective piece with a resolution of 2 cm1 and an acquisition time of 120 seconds. Prior to the BET surface area and porosity evaluations, about 0.2 g the powder samples were degassed around a temperature of 300 C under vacuum conditions for 24 hrs. The information from the adsorption-desorption isotherms were used for the Brunauer-Emmett-Teller (BET) Surface Area evaluation and Barrett-Joyner-Halenda (BJH) Pore Size-Volume Investigation respectively [29]. Transmission FT-IR spectroscopic data were collected within a range of 4000e400 cm1 and a resolution of 4 cm1 using a Bruker Vertex 70v FTIR spec- trometer equipped with an evacuatable sample compartment, a diamond ATR crystal (2 mm sample-crystal contact diameter) and an Opus acquisition software for analysis. The photocatalytic activity of the titanium dioxide photocatalysts were evaluated by determining the decomposition of Rhodamine B and Sudan III dyes under the irra- diation of Ultra Violet light (UV). The experiments were carried out in a 100 mL capacity double-walled quartz glass immersion wells photochemical reactor (tech- nistro) equipped with a chiller, reaction flask to lamp distance of 20 mm, and a 330 Watt medium pressure mercury lamp (UV lamp, 365 nm). In a typical experi- mental procedure, Rhodamine B dye (0.01 g) was mixed with 200 mL of distilled water to obtain the dye solution. 0.1 g of the as-prepared TiO2 nanoparticles was mixed with 50 mL of distilled water in a different beaker to obtain the TiO2 suspen- sion. 30 ml of the dye solution was then added to the prepared TiO2 suspension. Prior to UV irradiation, the suspension was stirred in the dark room at 1000 rpm for 30 minutes for optimum adsorption. After the 30 min stirring in the dark room, it was then exposed to a 330 W UV light source. While this suspension was exposed to the UV light, 5 mL was withdrawn with syringe every 30 min. The aliquot was centrifuged at 5000 rpm for 10 min. The supernatant was transferred into a cuvette and analyzed using a Pelkin-Elmer Lambda 850 UV-vis spectrophotometer to deter- mine the absorption over a wavelength range of 200 nme800 nm. The various expo- sure times observed in this work were 0, 30, 60, 90, 120 and 150 min. The concentration of the Rhodamine B dye in solution was decreased and the photoca- talytic procedure repeated as described above. Here, a mixture was obtained by mix- ing 0.01 g of the Rhodamine B dye with distilled water (400 mL) to obtain the dye solution. 0.1 g of the powder was also mixed with about 50 mL of distilled water in a different beaker to obtain the TiO2 suspension. This was followed by the addition of 10 mL of the Rhodamine B dye solution to the 50 mL TiO2 suspension. Prior to UV irradiation, the suspension was stirred in the dark room at 1000 rpm for 30 min. After the 30 min of stirring in the dark room, it was then exposed to a UV light operating at 330 W power. Optical UV-Vis spectra were taken at different exposure times. Theon.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Table 1. Rhodamine B dye degradation parameters. Concentration of Rhodamine pH Volume of Rhodamine Mass of TiO2 Method of catalyst B dye solution B dye solution catalyst preparation (mol/dm3) (ppm) (ml) (g) 3.8  105 18.2 3.5 80 0.1 Sol gel 8.5  106 4.1 4.0 60 0.1 Sol gel 3.8  105 18.2 3.5 80 0.1 Hydrothermal 8.5  106 4.1 4.0 60 0.1 Hydrothermal 6 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liparameters involved in the photocatalytic degradation of Rhodamine B dye solution are summarized in Table 1. In the Sudan III dye degradation experiment, a stock solution was prepared by dis- solving 0.8 g Sudan III dye in 400 mL Isopropyl alcohol under sonication for about 30 min. The solution was filtered and the filtrate used as the dye solution for the photodegradation experiment. 50 mL of the as-prepared Sudan III dye so- lution was mixed with 0.1 g of TiO2 catalyst to obtain Sudan III e TiO2 suspen- sion. Prior to UV irradiation, the mixture was then stirred in the dark at 1000 rpm for about 30 min to ensure uniform mixture. It was then exposed to a 330 W UV light irradiation. While this suspension was exposed to the UV light, 5 mL aliquot was withdrawn with a syringe every 30 min over a 150 min exposure period. Each aliquot was centrifuged at 5000 rpm for a period of 10 min and the supernatant transferred into a cuvette for absorption analysis on a Pelkin-Elmer Lambda 850 UV-Vis spectrophotometer over a wavelength range of 200 nme800 nm. Batch formulations with reduced dye concentrations were analyzed under varying expo- sure times following the above discussed procedure. The parameters involved in the photocatalytic degradation of Sudan III dye solution are summarized in Table 2.Table 2. Sudan III dye degradation parameters. Concentration of Sudan pH Volume of Sudan Volume of Mass of TiO2 Method of catalyst III dye solution III dye solution Isopropanol catalyst preparation (mol/dm3) (ppm) (ml) (ml) (g) 5.7  103 2008.7 8.3 50 0 0.1 Sol gel 2.3  103 810.5 4.5 20 30 0.1 Sol gel 5.2  104 183.3 4.0 5 50 0.1 Sol gel 5.7  103 2008.7 8.3 50 0 0.1 Hydrothermal 2.3  103 810.5 4.5 20 30 0.1 Hydrothermal 5.2  104 183.3 4.0 5 50 0.1 Hydrothermal on.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 7 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/li3. Results and discussion In the synthesis of TiO2 nanoparticles by hydrothermal techniques, alkoxides were hydrolysed in the presence of water and subsequently polymerized to obtain a three- dimensional oxide network. A colourless solution was obtained when the Titanium tetraisopropoxide (TTIP, Ti(OCH(CH3)2)4) solution was reacted with isopropanol (IPA), but a white precipitate was formed when water was added dropwise. The re- actions are presented as follows in Eqs. (1), (2), and (3), starting from the reactions between the Titanium alkoxide (TTIP and IPA) and water. Eqs. (1) and (2) show the hydrolysis and condensation reactions respectively. Calcining the samples at various temperatures gave the resultant TiO2 nanoparticles as illustrated by Eq. (3). In the Sol gel processing of TiO2 nanoparticles, the reaction kinetics are described in Eqs. (4) and (5). The hydrolysis reaction was initiated when the Titanium tetrachlo- ride precursor was reacted with absolute ethanol to form the sol. In this process hy- drochloric acid gas was seen expelled from the reaction mixture and a yellow suspension was seen after 2 hours. After the Titanium organic compound obtained was heat treated, white powdered TiO2 nanoparticles were obtained as the hydrocar- bon components were expelled through the heat treatment process.   Ti OCHðCH3Þ2 4 þ 4H2O/TiðOHÞ4 þ 4ðCH3Þ2CHOH ð1Þ TiðOHÞ4/TiO2$xH2Oþ ð2 xÞH2O ð2Þ heat TiO2$xH2O/ TiO2 ð3Þ TiCl4 þ 4C2H5OH /TiðC2H5OÞ4 þ 4HCl ð4Þ Tið heatC2H5OÞ4/ TiO2 þ hydrocarbons ð5Þ The titania nanoparticles were heated at different temperatures and analyzed by X- ray diffraction (XRD). Whole Powder Profile Modelling using PM2K software was used to analyze the TiO2 nanoparticles prepared. Grain sizes and crystalline phases of the catalyst samples were derived from analysis of the XRD patterns [30]. The XRD patterns (Fig. 1a) of the TiO2 powders synthesized using the sol-gel tech- nique, showed that the anatase phase of TiO2 nanoparticles was produced. Hence, calcination tends to improve the crystallinity of the nanoparticles which conse- quently transforms amorphous titanium dioxide (TiO2) into its anatase phase, which upon further increase in the calcination temperature, transforms into the rutile phase. This phenomenon is consistent with available literature which tend to indicate that, anatase-rutile phase transitions typically occurs between 450 C and 600 C: the variation in the transition temperatures is influenced by the type of precursorson.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Fig. 1. XRD Pattern (a) TiO2 nanoparticles prepared by sol gel method (b) TiO2 nanoparticles prepared by hydrothermal method. 8 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liused, the synthesis conditions and the characteristics of the produced particles [31]. Fig. 1a shows that when the heating temperature increases from 200 C to 600 C, there is an increase in the peak intensities as well as narrowing of the peak width, suggesting the transformation of the amorphous as-prepared TiO2 catalyst to a high- ly crystalline TiO2 samples. It’s noteworthy that, for the sol-gel produced particles, no anatase-rutile phase transition was observed, no matter the calcination tempera- ture used within our investigated temperature range. This demonstrated that, the ti- tanium dioxide powders prepared using the sol-gel technique had relatively high thermal phase stability, which tends to subdue the phase transition from anatase to rutile. Contrary to the XRD results of the titania samples prepared using the sol- gel method, the XRD analysis of the titania samples prepared by hydrothermal method and calcined at 600 C exhibited a mixture of rutile and anatase phase. When the calcination temperature was increased to 600 C, a rutile phase, confirmed by the diffraction peak in [110] direction, which is a more stable phase at high tem- peratures, was seen (Fig. 2). This was observed as the peaks labelled R, whereas the Anatase phase direction is labelled A. It was expected that the rutile phase peaks would increase in intensity with increasing annealing temperature since temperature increases the stability of the rutile phases of TiO2. The amount of anatase and rutile phases of TiO2 nanoparticles prepared by hydro- thermal route and calcined at 600 C were calculated using Eq. (1): 100 XA ¼ 1þ ð ð6Þ1:265IR=IAÞ Where XA is the weight fraction of the anatase phase, and IA and IR are intensities of the anatase (101) and rutile (110) diffraction respectively [32]. The TiO2 samples prepared via hydrothermal route and calcined at 600 C resulted in 85 % and 15 % of anatase and rutile phase, respectively, according to Eq. (6). Nonetheless, aton.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Fig. 2. XRD TiO2 catalyst (synthesized by hydrothermal method) calcined at 600 C. 9 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/li200 Ce500 C annealing temperatures, pure anatase phase of TiO2 was formed (Fig. 1). When the experimental XRD profiles were modelled using the Whole Powder Pattern Modelling (WPPM) technique incorporated in the PM2K software, a very low goodness of fit was obtained: an indication of a better fit between experimental and calculated data. The results of modelled XRD profiles are shown in Figs. 3 and 4, respectively. The grain sizes of the TiO2 nanoparticles synthesized by both hydrothermal and sol- gel techniques were investigated as a function of annealing temperature (Fig. 5a and b) respectively. An average grain size of 2 nm was observed for the as-synthesized TiO2 nanopar- ticles synthesized using the sol-gel technique. Annealing the as-prepared TiO2 re- sulted in an increased grain size. More speci cally, at 200 fi C an approximate grain size of 5 nm was obtained, which further steadily increased upon elevation in calcination temperature. At 600 C, a dramatic improvement in grain size (w23 nm) was obtained possible as a result of the enhanced grain growth due to the high temperature treatment. In the case of the TiO2 nanoparticles synthesized using the hydrothermal method, the as-prepared nanoparticles domain sizes were approximately 5 nm. The grain size increased gradually, from w6 nm at 200 C to w15 nm at 500 C. However, a two-fold increase in grain size (31 nm) was seen when the calcination temperatureon.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Fig. 3. Modelled XRD of TiO2 nanoparticles synthesized by hydrothermal method (a) as-prepared (b) calcined at 200 C (c) calcined at 300 C (d) calcined at 400 C (e) calcined at 500 C (f) calcined at 600 C. 10 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liwas raised to 600 C. This observation could be attributed to grain growth as a result of increase in annealing temperature as well as the phase transition from the anatase phase to the rutile phase at 600 C. Fig. 6 depicts the lognormal size distribution of TiO2 nanoparticles prepared via the hydrothermal technique. It shows clearly a narrow frequency peak around a 5 nm domain size for the as-prepared TiO2 nanoparticles.on.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Fig. 4. Modelled XRD of TiO2 nanoparticles synthesized by Sol gel method (a) as-prepared (b) calcined at 200 C (c) calcined at 300 C (d) calcined at 400 C (e) calcined at 500 C (f) calcined at 600 C. 11 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liThe domain size increased and broadened slightly as the calcination temperature was raised up to 400 C, with the domain size distribution revealed to be between 1 nm and 15 nm. However, the domain size increased and broadened drastically over a wide size distribution when the calcination temperatures were increased to 500 C and 600 C, respectively. It was therefore proven that there was a wide crystallite size distribution as well as a general increase in crystallite domain size as the tem- perature was increased. The Lognormal size distribution of the nanoparticles synthesized using sol gel as shown in Fig. 6b also demonstrates a general widening of grain size distributionon.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Fig. 5. Average grain size as a function of temperature of TiO2 catalyst prepared by (a) Sol gel route and (b) Hydrothermal routes. Fig. 6. Lognormal size distribution of (a) Hydrothermal and (b) Sol-gel synthesized TiO2 nanoparticles. 12 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liwith increasing temperature. The peak of each distribution normally gives the average grain size of the synthesized TiO2 nanoparticles. It was also clearly demon- strated that the average grain size was increased as the heating temperature was increased. The as-prepared TiO2 nanoparticles which exhibited a highly amorphous nature showed an average grain size of 2 nm. The grain size increased gradually, and the size distribution broadened with increasing calcination temperature. SEM analysis of the as-synthesized TiO2 nanoparticles demonstrated that nearly spherical nanoparticles were produced (Fig. 7). FT-IR spectroscopy was also conducted on the synthesized nanoparticles to estimate the absorption of the incident light rays with respect to the wavelength of the incident beam. A representative FT-IR spectrum on the prepared TiO2 nanoparticles (Fig. 8) show a broad band around 3228 cm1 which could be attributed to the O-H stretch- ing mode of the surface and adsorbed water molecules. The peaks centered at 1635 cm1 were also attributed to the stretching of titanium carboxylate, which was formed from TiCl4 and ethanol precursors. The band be- tween 800 cm1 and 414 cm1 was a result of the Ti-O bond stretching mode of the anatase polymorph of the TiO2 [32, 33].on.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Fig. 7. SEM images of calcined (300 C) TiO2 nanoparticles prepared by (a) hydrothermal and (b) Sol- gel methods. Fig. 8. FTIR spectra of the prepared TiO2 nanoparticles. 13 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liFurther insight into the synthesized nanostructured TiO2 was gained by Raman spec- troscopy. The TiO2 nanoparticles synthesized using the sol gel method showed intense Raman peaks at 395 cm1, 515 cm1 and 638 cm1 (Fig. 9a). The observed Raman spectra indicates that the TiO2 nanoparticles prepared and calcined at 200 C, 300 C, 400 C, 500 C and 600 C were of anatase phase and in perfect agreement with the XRD results [34]. The Raman spectra shown in Fig. 9b revealed Raman peaks around 380 cm1, 510 cm1, and 640 cm1 when the samples were prepared by the hydrothermal technique. Temperature treatment played an important role as it can be seen that the Raman peaks generally became more distinct and intense with increasing temperature. However, there was not a very significant difference in the Raman peaks as they increased gradually when the samples were calcined up to 500 C. The adsorption isotherms of nitrogen on TiO2 nanoparticles at an increasing relative pressure were analyzed to determine the BET surface area, pore size and poreon.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Fig. 9. Raman Spectra of TiO2 nanoparticles synthesized by (a) sol gel method and (b) Hydrothermal method. 14 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/livolume. The adsorption-desorption isotherms did not seem to follow the same path for a definite range of relative pressures. This type of hysteresis loop is commonly exhibited in adsorbent materials which are mesoporous. From Fig. 10, the Linear Isotherm plot of the as-prepared TiO2 nanoparticles synthe- sized by sol gel method as well as samples calcined at 200 C, 300 C and 400 C showed a stepwise adsorption-desorption branch at a wide range of pressure (P/Po): Type IV isotherm classification as prescribed by the IUPAC classification system [35]. This showed that mesoporous TiO2 nanoparticles were produced at these tem- perature treatments. At 500 C and 600 C calcination temperatures, a stepwise adsorption-desorption branch was not observed and the samples were classified as Type III isotherms according to IUPAC isotherm classifications. The linear isotherm plots of TiO2 nanoparticles prepared by hydrothermal method are presented in Fig. 10b. Isotherms exhibiting a characteristic type IV form and hysteresis loop, was observed for our as-prepared TiO2 nanoparticles as well as the samples heated at temperatures of 200 C, 300 C, 400 C, and 500 C: characteristic feature for ma- terials with mesoporosity in accordance with the isotherm taxonomy of IUPAC. There was also an abrupt rise in volume of N2 adsorption that was observed andFig. 10. Isotherm linear plot of TiO2 nanoparticles synthesized by (a) Sol gel method and (b) Hydrother- mal method. on.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 15 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/lilocated in the P/Po range of 0.6e0.8. This steady increase can be imputed to the capillary condensation, which indicates a good homogeneity of the sample and fairly small pore size since the P/Po position of the inflection point is related to the pore dimension [36]. Sample calcined at 600 C exhibited a Type III isotherm and in this case hysteresis loop with a stepwise adsorption-desorption branch was not observed [35]. A plot of BET surface area against the different heating temperatures (Fig. 11) inves- tigated for both the catalyst samples prepared via sol-gel and hydrothermal methods showed that, an increased in annealing temperature resulted in a decrease in TiO2 surface area. This trend was similar for both catalysts prepared under the different preparation methods investigated. However, the decrease in surface area for catalysts samples prepared via sol-gel was observed to be drastic from 200 C (w182 m2/g) to 400 C (w62 m2/g). Comparably, the observed decrease in surface area as a function of annealing temperature was gradual for the catalyst samples prepared by hydro- thermal method. More specifically, the as-synthesized TiO2 nanoparticles by hydro- thermal method exhibited a surface area of 207 m2/g, which further decreased to w182 m2/g after annealing at 200 C. A further rise in the calcining temperature from 200 C to 400 C, yielded a surface area of w126 m2/g, representing a two- fold increase in the surface area obtained over the TiO2 samples annealed at the same temperature. The rapid decrease in surface area of titania prepared by sol- gel as compared to that prepared using the hydrothermal method when the calcina- tion temperature was increased might be attributed to the enhanced dehydroxylation of the titania samples prepared by sol-gel upon calcination possible due to the weak covalent Ti-O bonds. At a characteristic wavelength of 554 nm, the absorbance peak was recorded at various exposure times for Rhodamine B dye solutions. Prior to the photocatalytic experiments, a control experiment with Rhodamine B dye solution without TiO2 catalyst showed negligible or no degradation when illuminated by ultraviolet light over 150 min exposure. Also, Rhodamine B dye solution with TiO2 catalyst withoutFig. 11. BET Surface area of TiO2 nanoparticles synthesized by (b) Sol gel method and (b) Hydrother- mal method. on.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 16 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liUltraviolet light illumination also showed negligible or no degradation over a 150 min. This showed that both the TiO2 catalyst and ultraviolet light are required to initiate the photocatalytic reaction to cause the degradation of the dye solution. At an initial Rhodamine B dye concentration of 4 ppm the absorption spectrum was studied and reported as seen in Fig. 12. As can be seen from Fig. 12, it is obvious that the photodegradation rate of Rhodamine B by sol-gel approach was efficient compared to TiO2 synthesized by hydrothermal approach. The difference in effi- ciency may be attributed to the better activity between the cationic Rhodamine B dye and titania. Since only anatase phase was produced by sol-gel approach, photo- catalysis was better than the hydrothermal method, that produced both rutile and anatase phases (Figs. 1 and 2). It is well known that anatase phase of TiO2 is most active and in terms of photo activity, anatase is more reactive than rutile because it has a larger surface area, higher electron-hole pair mobility, and improved surface hydroxyl density [37]. At a characteristic wavelength of 507 nm, the absorbance of Sudan III dye was moni- tored and evaluated as it was degraded by the prepared TiO2 catalyst. Sudan III dye exposed to UV light without any TiO2 catalyst showed no or negligible degradation over a 150 minutes’ period. Also, the Sudan III dye with catalyst but not exposed to UV light showed no or negligible degradation. At initial Sudan III dye concentration of 183.3 ppm the absorption spectrum was studied and reported as shown in Fig. 13. Surprisingly, Sudan III dye degradation by TiO2 prepared by hydrothermal method was rather higher than for sol-gel approach. After 150 minutes of irradiation, titania synthesized by hydrothermal approach gave 100 % degradation whilst that by sol-gel approach gave 83 % degra- dation. This can be attributed to presumably, the large specific surface area (Fig. 11) of the hydrothermally synthesized TiO2, which produced mainly anatase, phase titania between 450 Ce500 C (Fig. 1). Again, crystallite domain size of TiO2Fig. 12. Absorption Spectrum of Rhodamine B dye degraded by (a) sol gel prepared catalyst and (b) Hydrothermal prepared catalyst. on.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 Fig. 13. Absorption Spectrum of Sudan III dye degraded by (a) Hydrothermal prepared TiO2 catalyst, and (b) Sol gel prepared TiO2 catalyst. 17 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/lisynthesized by hydrothermal approach was 15 nm for temperature range of 450 Ce500 C. This therefore means a large surface area was available for reaction when compared to the TiO2 synthesized by sol-gel approach. Even though, anatase phase is reactive than rutile phase, it appears the synergistic effect between anatase and rutile phases, and the neutral surface charge of Sudan III, effectively result in higher photodegradation of Sudan III by hydrothermally synthesis method compared to the sol-gel approach. At initial Sudan III dye concentration of 183.3 ppm, there was a negligible degrada- tion of the Sudan III dye solution when the dye solution concentration was increased to 2008.7 ppm while keeping the catalyst loading constant. This is because the active sites on the catalyst which is available for the catalysis reaction is very vital for the decomposition to take place, however, as the dye concentration increases while the catalyst amount is kept constant, less active sites become available for the effective photodecomposition reactions. As the concentration of the dye were raised, the colour of the solution turns out to be deeply coloured; decreasing the path length of light traversing the solution and thereby allowing fewer photons to reach the sur- face of the catalyst. As a result, the generation of degradation species as well as lim- itation of the radicals reduced further the light path length leading to negligible photo-degradation [38]. In addition, pH affects the rate of adsorption and subsequent degradation. The influ- ence of pH on dye photo degradation has extensively been studied elsewhere [9, 12, 39, 40]. It is well known that TiO2 has a point of zero charge (PZC) of 6.8. For acidic media (pH < PZC), TiO2 exists as positively charged catalyst. Conversely, titanium dioxide possesses a negatively charged surface in alkaline media (PH > PZC). Consequently, for a cationic dye like Rhodamine B, TiO2 photocatalysis would be much efficient because of the presence of hydroxyl radicals at the TiO2 active sites. In alkaline media, there is high surface hydroxyl anion density and therefore these hydroxyl anions can attract photo-excited electron from the valence band to produceon.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 18 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/limore hydroxyl radicals. Hydroxyl radicals are effective for photocatalytic degrada- tion of adsorbed species (such as of cationic dye) on TiO2 active sites. Therefore, TiO2 effectively degrades cationic dyes in basic media compared to neutral dyes. In this work, pH range of 3.5e4.0 was observed for the photocatalysis of Rhodamine B. This implies that positively charged holes are the radicals that mediated the photo- catalytic process and not hydroxyl radicals. In the case of Sudan III experiments, pH range of 8e4 was observed. Since the former is a neutral dye, there is presumably less of its adsorption on the titania active sites, hence degradation would be less compared to cationic dye. This explains the relatively high photocatalysis of Rhoda- mine B compared to Sudan III, particularly for TiO2 synthesized by sol-gel method. Generally, increasing degradation time leads to the formation of more radicals [38]. Since the photocatalytic decomposition of the dye molecules happens on the surface of photocatalyst where radicals (OH, O 2 , h þ) are available for photocatalytic degradation, the former are capable of initiating bond cleavages in the adsorbed dye molecules on the surface of the catalyst. For the photocatalytic decomposition of Rhodamine B Dye over TiO2 nanoparticles, the main products of decomposition are CO2 and H2O [41, 42].4. Conclusions Nanostructured TiO2 powders were efficaciously produced via the sol-gel and hy- drothermal routes and characterized by X-ray Diffraction, BET, Fourier Transform Infrared spectroscopy, Raman Spectroscopy and Scanning Electron Microscopy methods. Analysis from the XRD and Raman confirmed that the synthesized par- ticles were pure anatase phase TiO2 except the sample prepared by hydrothermal method and calcined at 600 C which showed 15 % rutile phase and 85 % anatase phase. The synthesized catalysts were used in the photocatalytic degradation of Sudan III and Rhodamine B dyes at various durations of exposure to the Ultra Vi- olet light. The photocatalytic activities of these nanostructures were estimated by determining the decrease in concentration of the respective dye solutions over time when TiO2 was used as the catalyst. It was demonstrated that Rhodamine dye and Sudan III dye were degraded within a short time. The sol gel prepared catalyst showed a higher photocatalytic activity in Rhodamine B dye solution and the hy- drothermal prepared catalyst showed a higher photocatalytic activity in Sudan III dye solution. Typically, 94 % decomposition of Rhodamine B dye was observed when the sol-gel synthesized catalysts were used and Ultra Violet (UV) light irra- diated for 150 minutes. However, for the same period of irradiation, 100 % decom- position of Sudan III dye was observed when the hydrothermally prepared catalysts were used. From these investigation, these produced nanostructured TiO2 powders have potential applications in the decomposition of a wide variety of dye contaminants.on.2018.e00681 ors. Published by Elsevier Ltd. This is an open access article under the CC BY license censes/by/4.0/). Article Nowe00681 19 https://doi.org/10.1016/j.heliy 2405-8440/ 2018 The Auth (http://creativecommons.org/liDeclarations Author contribution statement David Dodoo-Arhin: Conceived and designed the experiments; Analyzed and inter- preted the data; Wrote the paper. Frederick Paakwah Buabeng: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data. Julius M. Mwabora: Performed the experiments; Analyzed and interpreted the data. Prince Nana Amaniampong, Henry Agbe, Emmanuel Nyankson, David Olubiyi Obada, Nana Yaw Asiedu: Analyzed and interpreted the data.Funding statement This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.Competing interest statement The authors declare no conflict of interest.Additional information No additional information is available for this paper.Acknowledgements The African Materials Science and Engineering Network (AMSEN); Regional Initiative on Science and Education (RISE); University of Ghana BANGA-Africa programme and University of Ghana Biotechnology Centre are duly acknowledged for their support.References [1] M. Lazar, S. Varghese, S. Nair, Photocatalytic water treatment by titanium di- oxide: recent updates, Catalysts 2 (4) (2012) 572e601. [2] J. Yang, C. Chen, H. Ji, W. Ma, J. 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