Green synthesis of S-doped rod shaped anatase TiO2 microstructures

Materials Letters 183 (2016) 211–214 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet Green synthesis of S-doped rod shaped anatase TiO2 microstructures K. Mohan Kumar a, S. Godavarthi a,n, T.V.K. Karthik b, M. Mahendhiran c, A. Hernandez-Eligio c, N. Hernandez-Como d, V. Agarwal b, L. Martinez Gomez a a Universidad Nacional Autonoma de Mexico, Instituto de Ciencias Fisicas, Avenida Universidad s/n, Cuernavaca, MOR, 62210 Mexico CIICAp, UAEM, Av. Universidad 1001 Col. Chamilpa, Cuernavaca, Morelos, 62210 Mexico c Departamento de Bioingenieria Celular y Biocatalisis, Instituto de Biotecnologia UNAM, Col. Chamilpa, CP.2001, Cuernavaca, Morelos, Mexico d Centro de Nanociencias y Micro y Nanotecnologías, Instituto Politécnico Nacional, Mexico b art ic l e i nf o a b s t r a c t Article history: Received 22 June 2016 Received in revised form 20 July 2016 Accepted 21 July 2016 Available online 22 July 2016 A simple eco-friendly, direct self assembly process for synthesizing sulphur doped titanium dioxide microrod structures (S-TMS) is presented. Without any specific precursor for sulphur doping, ultrasonicating TiOSO4 in aqueous media followed by refluxing results in the formation of S-TMS. Sulphate ions produced during the reaction act as a source for sulphur doping. In spite of the acidic pH during the reaction, pure anatase phase was favoured due to the increased availability of sulphate ions acting as phase specific directing agent. Pure anatase phase was confirmed using powder X-ray diffraction studies (XRD). Formation of S-TMS with a microrod containing self-assembled nanorods was evidenced from field emission scanning electron microscopy and atomic force microscopy. The obtained S-TMS revealed multiple band light trapping property in visible, near infrared and ultraviolet region which was evidence from UV–vis-NIR spectroscopy. & 2016 Elsevier B.V. All rights reserved. Keywords: Microstructure Self-assembly AFM Electron microscopy and multiple light trapping 1. Introduction Anatase titanium dioxide (TiO2) is one of the most important wide band gap semiconductors due to its intrinsic properties and promising use in photocatalytic, solar cells, gas sensors and white pigment materials [1–4]. Self assembling of TiO2 nanostructures with unique properties became new focus in recent years. Compared to nanoparticle films, the TiO2 morphologies like nanorods can provide direct electrical path ways for photo-generated electrons [5]. Moreover, the decreased inter-crystalline contact of TiO2 rods can suppress the charge recombination characteristic which is a major problem with nanoparticle film in the application of dye sensitized solar cells [6]. In recent years, non-metal doping (such as N, C and S) into TiO2 for narrowing band gap was found to be an efficient way for enhancing the photocatalytic activity in visible region [7]. Among the doped TiO2, S-doped TiO2 received attention due to its excellent photocatalytic activity, band gap manipulation ability and structural stability [8]. Various methods to dope sulphur into TiO2 were studied, but most of the S-doped was produced by TiS2 oxidative annealing or high temperature treatment of titanium precursor in hydrogen sulphide atmosphere [9]. These methods of producing S-doped TiO2 not only involve high energy consumption and n Corresponding author. E-mail address: godavarthi.srinivas@gmail.com (S. Godavarthi). http://dx.doi.org/10.1016/j.matlet.2016.07.100 0167-577X/& 2016 Elsevier B.V. All rights reserved. difficult setups but also suffer low surface areas due to high temperatures. Apart from these methods for S-doped TiO2, methods like solvothermal [10], co-precipitation [11], sol-gel [12] and super critical fluid assisted [13] were presented in the literature. All these methods uses different source of sulphur in order to produce S-doped TiO2 nanomaterials. TiO2 is a key material in solar device fabrication due to its excellent optical and chemical properties, favourable band edge position and low cost. Unique Surfaces of TiO2 material play major role in enhancing light trapping property, and key to enhance solar energy conversion efficiency [14]. So, in the present work, we report a simple, template free, direct self-assembly of S-TMS with unique surfaces in aqueous solution using TiOSO4 as precursor source for both titanium and sulphur was reported. No other structure directing agents, templates, surfactant and high boiling solvents were used. There are reports using TiOSO4 as precursor to prepare TiO2 Nanoparticles [15–17]. Among those reports we find only one group reported the formation of 1D nanostructures (rod in tube structure) by solvothermal treatment of TiOSO4 in the presence of glycerol, ethanol and ethyl ether [17]. Here, ultrasound mediated controlled methodology which was followed by simple thermal treatment results in one step self assembly of S-TMS with unique surfaces. This method of preparing S-TMS may useful for easy industrial bulk synthesis of micro structures which can be beneficial in solar energy conversion applications. 212 K. Mohan Kumar et al. / Materials Letters 183 (2016) 211–214 2. Experimental 2.1. Synthesis of S-TMS 50 mg of TiOSO4 (Sigma Aldrich) was added in to 100 mL (deionised) water and sonicated for 30 min in an ultrasonic bath. The obtained solution was then refluxed for about 15 min. Then the solution was cooled down to room temperature naturally. White powder settled at bottom was separated by removing supernatant after centrifugation at 5000 rpm for 10 min. The obtained powder was washed thoroughly with distilled water followed by ethanol and dried in a hot air oven for five hours at 60° C. 2.2. Material characterization The absorbance spectrum was monitored from 200 nm to 2400 nm by Perkin-Elmer Lambda 950 UV–vis double beam spectrophotometer. X-ray diffraction analysis was done using Bruker D8 advance eco diffractometer. All the surface topographic images were recorded in vibrating mode with a scan rate of 0.1 lines/s using AFM workshop (model. 10-4172-10). A long n þ silicon cantilever supplied by NANOSENSORS was used for measurements. The images were processed using WSXM software [18]. Scanning electron microscopy analysis coupled with EDAX (SEM– EDAX) was done using a Hitachi SU5000 Schottky Field-Emission SEM. EDAX spectrum was recorded at selected areas on the solid surface. FT-IR measurement was carried using Varian 660-IR FT-IR spectrophotometer in order to confirm the presence of sulphur in TiO2. S-TMS induced bactericidal activity tests were conducted with using gram negative and positive bacteria E. coli X L Blue and B. subtilis 168 strains subsequently (see ESI 3 for details). 3. Results and discussion The XRD pattern of as prepared uncalcined TiO2 samples was shown in Fig. 1(a). Formation of anatase phase was very clear from the diffractogram. The peaks at 25.25°, 37.78°, 47.74°, 54.18° and 62.56° corresponds to the planes (101), (004), (200), (105) and (204) respectively and these diffraction peaks are in agreement with JCPDS. No. 00-021-1271 of anatase TiO2, space group I41/amd. No other diffraction peaks of impurities or other phase like rutile and brookite was observed. Compare with JCPDS of anatase TiO2, there is very little orderly peak shift towards lower diffraction angles is explicitly detected. This can be attributed to the occurrence of lattice expansion by the doping of sulphur in TMS. The calculated lattice parameters of S-TMS were found to be a¼ b¼ 3.786 Å, c ¼9.620 Å and V ¼137.89 Å3 which are in good agreement with earlier reports [19], whereas, the cell parameters of undoped anatase TiO2 are a ¼b ¼ 3.7852 Å, c ¼9.5139 Å and V ¼136.31 Å3 as per JCPDS. No. 00-021-1271. In order to obtain the information on the presence of sulphur and its role in crystallization, FT-IR spectra of prepared TiO2 sample was done and shown in Fig. 1(b). Absorption bands at 1652.2, 1120.4 and 587.9 cm 1, was observed in FT-IR Spectra. The strong absorption band at 587.6 cm 1 corresponds to Ti-O stretching. The absorption intensity at 1652.2 cm 1 corresponds to hydroxyl group bending that was adsorbed on TiO2 surface. This phenomenon is due to that anatase surface is more active in adsorbing the hydroxyl groups compared to rutile surface [20]. The less intense band at 1120.4 cm 1 is an indicative of presence of surface-anchored SO42 [20]. This observation indicates that SO42 was bidentately bonded on the surface of the TiO2 surface. The presence of sulphate ions plays a key role to keep the TiO2 in anatase phase even the solution pH turns to acidic during the reaction, as discussed in the below mechanism. In the present study, anatase S-TMS was prepared by ultrasonicating the aqueous TiOSO4 solution for half an hour, which results in a turbid solution. It was noticed that pH of the solution increases to 2.5 because of sulphuric acid produced during the reaction as shown in the Eq. (1). TiOSO4 + 2H2 O ))), RT, 1/2h → TiO(OH)2+ 2H+ + SO42 − …… (1) The obtained turbid solution was refluxed for about 15 min on a hot plate. During heating turbid solution slowly turns into transparent with a white S-TMS powder settled at the bottom as shown in Eq. (2). TiO(OH)2 → TiO2 + H2 O …… (2) Additionally, the presence of the acidic medium favours the formation of rutile phase [21]. Although in the present work the formation of sulphuric acid leads to increased acidic pH the characterizations show the formation of anatase phase. This phenomenon is due to the fact that the presence of sulphate ions in the aqueous solution acts as phase specific directing agent. On sonicating the titanium precursor, with the help of available vacant d-orbital's, titanium ions increases the co-ordination to Fig. 1. (a) XRD pattern of as prepared S-TMS and (b) FT-IR spectra of S-TMS. K. Mohan Kumar et al. / Materials Letters 183 (2016) 211–214 accept electron pair from nucleophilic ligands (-OH groups) forming TiO62 octahedral structure. Every TiO2 crystal structure contains TiO62 octahedral and these octahedral share edges and corners in a different manner which results in rutile and anatase phase nucleation. A zig zag patterned with octahedral four edge sharing can be observed in anatase, but in case of rutile only two edges shared octahedral linear chains can be seen [20]. The placement of third octahedral is very important to judge the phases of titania. In the present case the orientation of third octahedral was influenced by SO42 ions generated in the aqueous solution after sonication. These ions interact with octahedral hydroxyl group by electrostatic interaction and make the third octahedral condense in converse direction resulting in the formation of anatase phase. Similar predictions of formation of anatase phase in the presence of sulphate ion were demonstrated by Yan et al. [20]. So even if the entire solution turns to acidic after sonication, no rutile phase was observed and ends up with pure anatase phase. Once the anatase nucleus was formed, on thermal treatment, this nucleus self assembles and forms rod shaped microstructures as evidenced from AFM and SEM. Optical properties of TiO2 are extremely sensitive to surface morphology of the material [22]. For example, surface features increase the total internal reflectance when compared to smooth surface. In order to understand the topography, AFM analysis implemented for understanding the microstructures and surface features of the developed material. The analysis revealed the formation of rod shaped microstructures with length of the rods around 3–6 mm and shown in the Fig. 1S of supplementary material. Zooming into the surface of the rod gave an impression that each rod contains several small rods of approximately same length with anisotropic arrangement and shown in the Fig. 1S. To 213 understand the surface features further, FE-SEM analysis was carried out. FE-SEM images at different magnifications of the synthesised S-TMS were shown in Fig. 2. Rod shaped microstructures were clear from this analysis. Images at higher magnifications confirmed that the formation of microstructure consists of many tiny nano rods shown in Fig. 2b and c. Even though, size dispersion of S-TMS varies from 3 to 10 mm, homogenous surface arrangements were found throughout the material. Almost no aggregations of S-TMS were found with microscopic analysis. The formation mechanism is also based on SEM observations that the initial formation spheres like seeds and their self assembly in to form of tiny rods and then assemble in to rod shaped microstructures as shown in graphical abstract. Simultaneous elemental analysis through EDAX spectrum reveals the presence of elements Ti, O, S and C (Fig. S2). The elemental analysis showed 2.32 wt% of sulphur which is a clear evidence for doping of sulphur into TiO2 microstructures. The presence of carbon in spectrum is due to the carbon tape used for the SEM-EDAX analysis. The UV–vis-NIR absorption spectra of S-TMS were recorded and shown in Fig. 3. A band gap of 3.06 eV was observed by plotting hυ vs (αhυ)2 for S-TMS (Fig. S5 of ESI). The observed band gap of S-TMS is less than that of general TiO2, and is attributed to doping of sulphur. Multiple absorption peaks were clearly observed around 270 nm in UV region, 417.0 and 585 nm in visible region and 840, 914 nm and multiple peaks in the 1000–2400 nm in near infrared region due to the photonic structure within the obtained S-TMS. The intense band below 400 nm can be attributed to their excitation all the way through a band gap. The absorption peaks in the visible and near-infrared regions can be attributed to anisotropic texture as evidenced from FESEM. In specific, the finite Fig. 2. FE-SEM micrographs of S-TMS at various magnifications. 214 K. Mohan Kumar et al. / Materials Letters 183 (2016) 211–214 Fig. 3. UV–vis NIR absorption spectra of S-TMS (a) 250–970 nm (b)1000–2400 nm. space between internal microrods resembles Fabry-Perot resonator structure which helps in increased multiple internal reflections of light and also interaction time between sample and light [14]. Thus we assume that homogeneous anisotropical texture of S-TMS is very important and crucial factor in enhancing the light trapping activity over a wide wavelength region. The obtained micro rod structures showed antibacterial activity towards gram positive B. Sublitis 168 and showed no activity against gram negative E. Coli XL blue strains (see ESI 3 for details). [2] [3] [4] [5] [6] [7] 4. Conclusion We have presented a simple method to synthesize TiO2 rod shaped microstructure using ultrasound followed by hydrothermal treatment. Even in acidic media pure anatase phase of TiO2 was possible due to phase specific structural direction assisted by larger availability of sulphate ions generated during the reaction. Rod shaped microstructures were confirmed by electron microscopy which shows the anisotropic surface on the rods. This anisotropic surface leads to the multiple band light trapping property as confirmed by UV–vis NIR. This material with unique structure would be a beneficial for enhancing solar energy exchange efficiency due to its exclusive light trapping property. [8] [9] [10] [11] [12] [13] [14] Acknowledgement K. Mohan Kumar, S. Godavarthi and M. Muhilan were thankful to the postdoctoral scholarship from Direccion General de Asuntos del Personal Academico - Universidad Nacional Autonoma de Mexico (DGAPA-UNAM). [15] [16] [17] [18] Appendix A. 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