Aloe-vera flower shaped rutile TiO2 for selective

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Accepted Manuscript Original article Aloe-vera flower shaped rutile TiO2 for selective hydrogenation of nitroaromatics under direct sunlight irradiation Satnam Singh, Roopchand Prajapat, Rayees Ahmad Rather, Bonamali Pal PII: DOI: Reference:

S1878-5352(18)30082-0 https://doi.org/10.1016/j.arabjc.2018.04.002 ARABJC 2293

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Arabian Journal of Chemistry

Received Date: Accepted Date:

2 January 2018 2 April 2018

Please cite this article as: S. Singh, R. Prajapat, R. Ahmad Rather, B. Pal, Aloe-vera flower shaped rutile TiO2 for selective hydrogenation of nitroaromatics under direct sunlight irradiation, Arabian Journal of Chemistry (2018), doi: https://doi.org/10.1016/j.arabjc.2018.04.002

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Aloe-vera flower shaped rutile TiO2 for selective hydrogenation of nitroaromatics under direct sunlight irradiation Satnam Singh*, Roopchand Prajapat, Rayees Ahmad Rather and Bonamali Pal School of Chemistry and Biochemistry, Thapar University, Patiala (Punjab) India-147004

Address for correspondence *Corresponding author: Prof. Satnam Singh School of Chemistry and Biochemistry, Thapar University, Patiala, Punjab, India -147004 Tel: 91-175-2393443, Fax: 91-175-2364498 Email: [email protected]

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ABSTRACT This study reveals the fabrication of a sunlight receptive flower shaped rutile TiO2 microstructure (FTiO2) for the selective photoreduction of nitroaromatics. The crystalline F-TiO2 possesses small band gap (~ 2.8 eV) and large specific surface area (193 m2g-1). Moreover, the F-TiO2 exhibited higher relaxation time (120 µs) for the electron-hole pairs due to its brilliant multi dimensional morphology that enables shorter diffusion path and multiple scattering of active sites. The experimental results revealed the superior photocatalytic activity of the F-TiO2 microstructure in contrast to active P25 and rutile TiO2 (obtained from thermally treated P25 at 800 ˚C for 4h) for the reduction of nitrobenzene, mdinitrobenzene and 2,2-dinitrobiphenyl in 50% aqueous isopropanol (hole scavenger) to aniline (42-72 %), m-nitroaniline (37-42 %), m-phenylenediamine (88-100 %) and benzo[c]cinnoline (80-94 %) respectively under UV and direct sunlight irradiation. The quantitative estimation of byproducts like acetone and hydrogen (H2) produced from iso-propanol oxidation and water splitting during instant reduction of nitroaromatics to aromatic amines is well correlated and explained on the basis of its beneficial surface structural and electronic properties.

KEYWORDS Flower shaped rutile TiO2; Low temperature synthesis; Lower recombination rate of charge carriers; selective photocatalytic nitroaromatic reduction, Sunlight irradiation

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1. INTRODUCTION Semiconductor based heterogenous photocatalysis has been widely reported for the photocatalytic nitro aromatic reduction(Makarova et al., 2000; N. Zhang, Yang, Liu, Sun, & Xu, 2015; N. Zhang, Yang, Tang, & Xu, 2013) and TiO2 is one of the most explored photocatalyst for this reaction. The TiO 2 constitutes several polymorphs as anatase, rutile and brookite. Rutile phase is usually over looked due to its poor photocatalytic performances compared to the mostly explored anatase phase. The main drawbacks of the rutile TiO2 are the higher recombination rate of photogenerated electron–hole pairs, low density of superficial hydroxyl groups at the surface, high temperature for the synthesis, poor O2 adsorption than anatase and lack of visible light response due to its band gap, these limitations hinders its use in numerous photocatalytic applications (Buatong, Tang, & Pon-On, 2015; Patil et al., 2014; Peng et al., 2014; Sinthiya, Ramamurthi, Sethuraman, & Babu, 2017). Besides these detracting features the rutile polymorph possess some good properties such as high chemical and thermal stability, redox potential, non-toxicity, high dielectric constant, refractive index and transparency in visible region enhancing its light scattering activity(Berger, Monllor‐ Satoca, Jankulovska, Lana‐ Villarreal, & Gómez, 2012; Joshi & Schneider, 2012; Li et al., 2009; L. Liu & Chen, 2014; Lv et al., 2013; Roy, Sohn, & Pradhan, 2013; Sun et al., 2011; Wang, Xiao, Zeng, & Xie, 2015). Several studies have been performed with conventional rutile TiO2 in photocatalytically under UV light, Hakki et al.(Hakki, Dillert, & Bahnemann, 2013) have reported the selective reduction of m-nitrotoluene to m-aminotoluene on rutile TiO2 obtained by the thermal treatment of P25-TiO2. Shiraishi et al. (Shiraishi, Togawa, Tsukamoto, Tanaka, & Hirai, 2012) have also reported the hydrogenation of various nitro aromatic compounds to amines by rutile TiO2. The higher photoactivity of rutile phase is usually ascribed to the presence of oxygen vacancies at the surface containing Ti3+ atoms which are favorable defective sites for increasing the rate of any photocatalytic reaction(Batzill, Morales, & Diebold, 2006; S. Liu, Qu, Han, & Sun, 2004). Kaur et al. (Kaur & Pal, 2014) have reported 100 % yield of m-nitoaniline from m-DNB photoreduction based on UV exposure time and the percentage of rutile phase. Same group also (Kaur & Pal, 2015) demonstrated 3

the selective formation (95%) of benzo[c]cinnoline (BC) from m-dinitrobenzophenone reduction by P25TiO2 under 125 mW UV irradiation whereas rutile TiO2 does not exhibited any significant activity for this kind of photoreaction. Earlier Pal et al. (Pal, Torimoto, Okazaki, & Ohtani, 2007) described the photocatalytic redox-combined synthesis of L-pipecolinic acid from L-lysine by suspended titania particles under UV irradiation and selective reduction of nitrobenzene to azoxybenzene by a core-shell morphology of rhodium (Rh) loaded SiO2 @CdS nanocatalyst under 436 nm visible light irradiation. Similarly aromatic nitro compounds are selectively reduced to the corresponding anilines (Mahdavi, Bruton, & Li, 1993) by TiO2 under sunlight irradiation. Ken Tsutsumi et al. have also reported the photoreduction (Tsutsumi, Uchikawa, Sakai, & Tabata, 2016) of nitroarenes by transition-metal-loaded silicon semiconductor under visible light irradiation. The surface property of rutile TiO2 contributes significantly in various photocatalytic processes due to the presence of specifically exposed facets for the reduction and oxidation processes. Ohno et al. (Ohno, Sarukawa, & Matsumura, 2002) studied the redox response of specifically exposed crystal facets on the surface of rutile that played an important role in the separation of electron and holes for the oxidation of water in the presence of UV light. The branched structures provide a diffusion path for surface hydroxyl groups and help them in linking the reactants to the surface of the catalyst(Du et al., 2010; Huo et al., 2012; Li, Chu, Gong, & Diebold, 2010; Pang, Lindsay, & Thornton, 2008; Z. Zhang et al., 2009). Likewise various synthetic strategies have been employed for the preparation of different morphology of rutile such as nanorods, nanotubes, nanobelts, nanowires, nanosheets and Z-schemes with facet oriented geometry and it has been revealed that these types of anisotropic rutile nanostructures possess higher photocatalytic efficiency than the spherical ones for degradation of organic pollutants(Bavykin, Friedrich, & Walsh, 2006; Bian et al., 2010; F. Chen, Yang, Li, et al., 2017; F. Chen, Yang, Wang, et al., 2017; X. Chen & Mao, 2007; Hongo & Yamazaki, 2010; Kang & Chen, 2010; Kaur, Singh, & Pal, 2015; Tanaka, Nogami, Tsuda, & Miyake, 2009; Yogi, Kojima, Takai, & Wada, 2009; Yu, Zhou, Yang, & Rong, 2010). Moreover, tuning the morphology of rutile TiO2 has always resulted in higher photoconversion efficiency 4

and it is possible to make rutile phase active under sunlight by altering its morphology. Previously Zand et al .(Zand, Kazemi, & Safari) synthesized sunlight active TiO2 (mixture of anatase and brookite) by ultrasonic treatment and found that these nanostructures significantly improved (>90 % yield) various nitroaromatic photoreductions under direct sunlight, Lin et al. (Lin et al., 2014) have observed the 5.5% conversion efficiency of dye-sensitized solar cells for 3D rutile TiO2 which was higher than commercially available P25, the study revealed that higher conversion efficiency was due to better structural and electronic properties of rutile. When employed as a photocatalyst in the hydrogenation of nitrocompounds this type of rutile nanostructures can provide larger surface area for adsorption of reactant species and allow the multidirectional electron transport thereby improving (Tétreault & Grätzel, 2012; Q. Zhang & Cao, 2011) the photoproduct formation. Though large numbers of reports are available on the nitroaromatic reduction yet the application is limited because of several reasons like the high temperature synthesis of rutile phase which lead to aggregation of particles resulting in loss of activity, the application of this material in direct sunlight is restricted by its week response and week interfacial charge transfer. Moreover, the selectivity of the material towards the reaction is most essential criteria in aromatic reductions and to make these reactions practically applicable. Taking into consideration these drawbacks, low temperature synthesis of anisotropic rutile phase could be a potential candidate for many specific catalytic reactions than conventional rutile (obtained by high temperature treatment of P25) as it may preserve its nanoscale surface structural properties at low temperature that could be beneficial for various photocatalytic redox reactions. On the basis of available literature the photoactive pure rutile nanostructures prepared under controlled synthetic method for improvement in the surface structural and band energetics could be suitable for hydrogenation of various nitroaromatics under sunlight exposure. In this regard current research report reveals a rational design and fabrication of Aloe-vera flower shaped rutile TiO2 possessing a crystalline leaf-like nanostructures and its efficient application for the hydrogenation of aromatic compounds compared to mostly 5

photoactive P25 and R-TiO2 nanoparticles under both UV and sunlight exposure. This photocatalytic activity of F-TiO2 is first of its type for the selective reduction of nitrobenzene (NB), m-dinitrobenzene (m-DNB) and 2,2-dinitrobiphenyl (DNBP) to aniline (AN), m-nitroaniline (m-NA), m-phenylenediamine (m-PDA) and benzo[c]cinnoline (BC) respectively, exhibiting the highest percentage conversion and yield both under UV and direct sunlight exposure. This type of photocatalytic activity shown by F-TiO2 rutile phase stimulated by direct solar irradiation offers a great potential in the greener synthesis of fine chemicals. 2. EXPERIMENTAL SECTION 2.1 Chemicals and reagents Titanium tetrachloride (TiCl4), tetra-butyl titanate (TBT), ethanol, toluene, iso-propyl alcohol (IPA), nitrobenzene, m-dinitrobenzene, m-nitroaniline and m-phenylenediamine and 2,2-dinitrobiphenyl, benzo[c]cinnoline were purchased from Loba Chemicals Ltd. Ultra-filtered de ionized water was obtained from Millipore- Milli-Q (Conductivity 35 mho cm-1 at 25 ºC. Commercial P25-TiO2 was procured from Degussa Corporation, Germany. All the chemicals were used without further purification. 2.2 Synthesis of Aloe-vera shaped rutile TiO2 nanostructures (F-TiO2) and P25 rutile TiO2 (R-TiO2) The Aloe-vera flower shaped rutile TiO2 was synthesized by surfactant free hydrothermal procedure (Xiang, Zhao, Yin, & Fan, 2012). In a typical procedure the TiCl4 aqueous solution (50 wt %, solution A) was prepared by dropwise addition of TiCl4 to the ice-cooled de ionized water under vigorous stirring. Solution B was prepared by drop wise addition of TBT (4 mL) to ice-cooled 30 mL toluene. After 1h of stirring, 4 mL of solution A was added to it, and further stirred for another 1 h. The contents were shifted to a teflon lined stainless steel autoclave (50 mL) and kept at 150 ˚C for 24 h. The powdered product was filtered, washed several times using ethanol and dried at 80 ˚C. The R-TiO2 was prepared (Kaur & Pal, 2014) by thermally treated P25 at 800 ˚C for 4 h in a high temperature muffle furnace. The obtained powder was cooled at the room temperature for further analysis and application. 2.3 Characterization techniques 6

The optical absorption spectrum of as-synthesized photocatalysts dispersed in ethanol was recorded on a UV-Visible spectrophotometer (Analytic-Jena Specod 205). The band gap energies were calculated with the help of ‘Tauc relation’ (αhν = A (hν - Eg)n) where α is absorption coefficient, A is a constant, Eg is bandgap energy, hν is photon energy and exponent n is the transition type (n = 2 for indirect and ½ for direct band gap). The crystallographic studies were performed on X-ray diffractometer (PANalytical X’ Pert PRO using Cu-Kα radiation (operated at 45kV, λ= 1.54 Å) with 2θ set in the range of 20-80º. Timeresolved decay profile was measured by time correlated single photon counting (TCSPC) Edinburgh FL920 model with excitation diode lasers. The sample (suspended in ethanol) was exposed with a diode laser at 390 nm wavelength. Photoluminescence (PL) was monitored at 370 nm laser wavelength. Surface area (SBET) studies were performed by pre treating 50 mg of the catalyst at 150 ˚C in vacuum for 2h and then analyzed on BET (Brunauer Emmett Teller) surface area analyzer (BEL Sorp-max) at cryogenic temperature. The scanning electron microscope (SEM) images were taken on a 2 Å resolution Hitachi 7500 instrument, operating at 120 kV and carbon coated copper grid was used for the sample preparations. 2.4. Photocatalytic activity study As synthesized F-TiO2, R-TiO2 and P25 catalysts (20 mg) were analyzed for the photoreduction study of nitroaromatic compounds (NB, m-DNB and DNBP; Scheme-1). The nitroaromatic solution (5 mM) was prepared as an aqueous mixture (1:1) in iso-propanol and deionized water by sonication (30 min). In a typical process 5 mL solution of nitroaromatic compound (25 µmol) and 20 mg of the catalyst were charged in a test tube and purged with argon (15 minutes) to remove dissolved oxygen. The test tube was covered with gas tight rubber septum and kept under UV (125 W Hg arc, 10.4 mW/cm2; λ >300 nm), the sunlight based experiment was performed during the month of September 2016 (15-20 and 23-27 September, 11.00 hrs. to 16.00 hrs. IST) with average solar flux ~ 40 mW/cm2 at ~ 36.4 ˚C temperature. The reaction mixture was continuously stirred at 700 rpm. The samples for the analysis were withdrawn at different time intervals and the contents were centrifuged, filtered through cellulose micro filter (0.22 7

µm) and 20 µL of the sample was injected into the HPLC column using methanol: water (7:3) solvent system. The quantification of the samples were done on a C-18 column (250 mm × 4.6 mm, 5 µm, at room temperature with flow rate of 1 mL/min) installed on Agilent 1120 Compact LC HPLC, against the standard samples of the products. Furthermore, the identification of the reaction products was done on a gas chromatography mass spectrometer (GC-MS, Shimadzu, GC-MS-QP 2010 plus and GC-2010 with RTX-5 Sil-MS capillary column (30 mm × 0.25 mm × 0.25 µm) using Helium as carrier gas with the flow rate of 1 mL/min. The injector temperature was maintained at 240 ˚C and the oven was programmed from 50 ˚C to 260 ˚C with the ramp of 10 ˚C per minute. The quantification of acetone was done with the help of standard acetone (99.9 %) as external standard keeping the injection volume constant at 2 μL. The hydrogen (H2) produced was quantified by manually injecting 1 mL of gas evolved during the reaction to gas chromatography (GC, Nucon Ltd, India) with thermal conductivity detector (TCD) and molecular sieve column (5XA, 1.5 m). The temperatures of the injector, column oven and detector were maintained at 35 ˚C. The amount of H2 produced was measured by comparing it with standard 180 PPM H2 (Sigma Gasses Ltd, India). 3. RESULTS AND DISCUSSION The structural characteristic of the as-synthesized photocatalysts were comparatively studied by X- ray diffraction (XRD) analysis (Fig.1). It was observed that the P25 comprises of mixed anatase and rutile phases of TiO2 (JCPDS card, 21-1272) with the dominance of anatase phase (anatase : rutile, 70:30) while as R-TiO2 primarily constitutes 100 % pure and crystalline rutile with strong peak intensity complying with JCPDS card,01-076-0318 pattern. The rutile peaks in case of P25 are less intense due to the mixed and little amorphous nature which turns into strong crystalline rutile (R-TiO2) after heat treatment. The F-TiO2 showed a similar pattern with appearance of some additional peaks at (101), (111), (220), (211), (301) and (112) at 2θ = 41˚, 44˚, 56.5˚ , 61.8˚, 69.1o and 70.2o respectively (JCPDS card, 01-076-1940), the shift of peaks towards higher 2θ is due to change in morphology resulting in the change of lattice parameters, and it is well reported(Vorontsov & Tsybulya, 2018) that change in lattice 8

parameters cause the shift of diffraction angle. The unit cell structure in case of F-TiO2was a tetragonal crystal system with calculated lattice parameters as a = 4.6160 Å, b = 4.6160 Å and c = 2.9770Å. The field emission scanning electron microscopy (FESEM) micrographs presented in Fig.2 (a-d) of the as-synthesized F-TiO2 confirm the Aloe-vera flower type morphology with size varying from 280 x 450 nm (Length ×Width) and a number of leaf-like nanoarchitectures ranging in between 100-200 nm 8-13 nm (Length ×Width). Moreover, due to the heat treatment the particle size and crystallinity of R-TiO2 increased (Choi, Termin, & Hoffmann, 1994). The TEM micrographs of R-TiO2 and that of P25 shows the variable shaped nanoparticles with size ranging from 50-80 nm and 30-50 nm respectively (Fig-ESI1, electronic supporting information). The nitrogen adsorption desorption studies revealed the mesoporous nature of F-TiO2 with calculated specific surface area (SBET) of 193 m2g-1 which is 2-3 folds higher than P25 (69 m2g-1) and R-TiO2 (45 m2g-1) respectively. The higher surface area is due to the formation of branched like nanostructure and uniform distribution of the nanoflowers (Suh et al., 2007). The N2 adsorption desorption curves (Fig.3) of F-TiO2 are possessing the characteristics of type IV isotherms which is specialty of mesoporous and multilayer materials(Rather, Singh, & Pal, 2017), furthermore the Barret-Joyner-Halenda (BJH, Fig-ESI2) analysis showed a uniform and narrow pore size distribution for both F-TiO2 and P25 (~2.6-4.6 nm) while as in case of R-TiO2 the pore size increased to ~5.3 nm ascribed to the aggregation of nanoparticles due to heat treatment. Moreover, due to its branched structure and mesoporous nature the F-TiO2 proves to be effective for the adsorption of reactant species and simultaneous interfacial charge transfer greatly enhances the rate of photoreduction. The optical properties of the photocatalysts were recorded by UV-Vis diffuse reflectance spectra and presented in Fig-ESI-3 showed that the F-TiO2 exhibited an absorption edge at 420 nm and peak at 385 nm, thus exhibiting higher absorption in both visible and UV light with an Einstein shift of 15 nm. The two absorption regions in the F-TiO2 indicate the presence of unique and ordered symmetrical structure 9

corresponding due to the quantum confinement [38] of electrons in assembled leaf-like nanostructures. The second derivative of the absorption spectra and corresponding band gap energy calculated from Tauc’s plot, revealed that absorption onset gradually increasing as 370 nm (P25), 387 nm (R-TiO2) and 420 nm (F-TiO2) as seen in Fig.4 (a-b). As a result band gap energies decreased from 3.4-3.2 eV to 2.8 eV indicating the visible light active nature of as-prepared F-TiO2rutile phase. Upon photoexcitation, the process of electron-hole migration possess towards crystal surface resulting into either respective chemical transformation or recombination, this recombination rate of photogenerated charge carriers is closely dependent on the crystal structure. Herein the photoluminescence (PL) spectra of F-TiO2 (Fig.5a) showed several emission signals due to the occurrence of numerous surface defect sites in the visible region from 400 to 550 nm. The emission peaks at 423 and 446 nm resemble to the direct band emission and self-trapped excitons in rutile material similarly the emission peaks at 460 and 485 nm correspond to defects created during the emission process of oxygen vacancies and charge-transfer from Ti3+ to oxygen anion in a TiO68-complex (Abazović et al., 2009). Furthermore, it was observed that the strong PL signal quenching occurred in the F-TiO2 as compared to R-TiO2 and P25 indicating higher surface defects and surface modification. The strong quenching of PL signals at 530 nm attributed to the anion vacancies at the surface of TiO2 due to decrease in recombination rate of photogenerated charge carriers and its multidimensional anisotropic structure. As shown in Fig.5b, the average lifetime of photogenerated charge carriers was found to be 120 µs (F-TiO2), 34 µs (R-TiO2) and 45 µs (P25) respectively. The higher relaxation time for photogenerated charge carriers upholds the migration of electrons from valance band to conduction band and further to the reactants that helped faster photoreaction. 3.1 Photocatalytic Studies The photocatalytic reduction of every nitroaromatic compounds NB, m-DNB and DNBP (25 µmol in 50% isopropanol solution) to their respective amino derivatives (AN, m-NA and m-PDA, and BC) was carried out using 20 mg TiO2 dosage and studied both under UV and sunlight irradiation as per Scheme10

1. The HPLC chromatographs of DNBP reduced to BC by different TiO2 catalysts under both UV and Sunlight irradiation for 7h are comparatively displayed in Fig. ESI- 4. The peak intensity of DNBP (t R = 5.6 min) is reduced while as intensity of peak at tR = 5.2 min increased corresponding to the BC formation. It was disclosed that maximum amount DNBP reduction to BC formation took place by FTiO2 under sunlight and P25 under UV light irradiation for 7h and comparatively R-TiO2 showed no or least catalytic activity under similar conditions. Similarly the GC analysis (using authentic samples, Fig.6) further confirm the highest amount of BC formation (as evident from the difference in GC peak intensity at t R = 12.9 min) from DNBP reduction (GC peak tR =15 min) by F-TiO2 under 7h sunlight relative to lower activity of P25 under UV light exposure and the characteristic mass spectra of photo produced BC is also complying with the authentic commercial sample. Similarly, the reduction of NB and m-DNB to AN, m-NA and m-PDA respectively, under both UV and sunlight irradiation was also quantitatively calculated by HPLC and GC-MS analysis in comparison to respective authentic samples (Fig-ESI-5, Fig-ESI-6 and Fig. ESI-7). During photoreduction the concentration of all nitroaromatics decreased linearly with increasing amount of amino derivatives as a function of light irradiation time, moreover the reaction kinetics study showed -kt

that the reaction obeys 1st order (C= C0e , k = rate constants) kinetics as shown in the time course graphs (Fig.7), and it was also observed that the maximum yield of amino compounds is achieved after 7h light irradiation. However, m-NA formation is gradually increased upto 3-4 h irradiation and thereafter starts decreasing due to reduction of second –NO2 group to –NH2 group resulting into maximum amount of mPDA formation. Similar trend is also seen in case of NB reduction to AN as shown in Fig-ESI-8. It was also revealed that the photoreduction rate (Fig.7b) of m-DNB and DNBP by F-TiO2 is drastically enhanced (4-5 h) relative to P25 and R-TiO2under (Fig. 7c-d) direct sunlight irradiation as compared to their photoactivity (Fig.7a) under UV radiation.

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The higher rate of reduction for DNBP by F-TiO2 (k = 4.7 × 10-1 min-1) was observed as compared to P25 (k = 3.9 × 10-2 min-1) under direct sunlight and UV light exposure (F-TiO2 (k = 2.2 × 10-1 min-1), P25 (k = 1.2 × 10-1 min-1)) suggested the overall excellent photoreduction efficiency of F-TiO2 (Fig.7c-d) under sunlight irradiation. The comparative and quantitative photoactivity (yield and % conversion) of the different photocatalysts for the photoreduction of nitro organics to respective aromatic amines under 7h continuous UV and sunlight exposure is presented in Fig.8 and ESI-Table-1. The results disclose that compared to R-TiO2 and F-TiO2

the percentage yield gradually increases in case of F-TiO2 for

production of AN (20-72%),m-NA (15-42%), m-PDA (30-100%) and BC (10-100%) that was better (Fig.8a) in comparison to P25 (m-PDA, 100% and BC, 65%) under UV light irradiation. Despite having rutile phase F-TiO2 microstructure imparted the best photoreduction activity (10.5 µmol/42% AN, 22 µmol/88%m-PDA and 23.5 µmol/94% BC) under sunlight in contrast to P25 (22-60% yield) and R-TiO2 giving 10-30 % yield under UV light. As seen in Fig.8b. one key feature of this anisotropic F-TiO2 rutile phase reveals that it could simultaneously be capable of exhibiting the highest photoactivity both upon UV and sunlight irradiation which is hardly found in the reported photocatalysts. Along with the surface structural properties of rutile, the selection of hole scavenger also plays an important role in the photoreduction processes. Several solvents like ethanol, methanol and isopropanol have been used as hole scavenger for various nitroaromatic reduction reactions. It was observed that the iso-propanol is a best medium for the photocatalytic reactions this is also supported by earlier findings (Kaur & Pal, 2014; Zhu, Ke, Yang, Sarina, & Liu, 2010). Due to smaller bandgap (2.8 eV = 420 nm) and upon excitation of F-TiO2, electron-hole (e--h+) pairs are photogenerated in the conduction and valance band, respectively, that eventually diffuses to the surface for redox reactions of the adsorbed substrates as shown in Scheme-2. It is reported (Tamaki et al., 2006) that the photoexcited energetic holes in the valence band causes the oxidation of an hole scavenger alcohol to corresponding carbonyl compound and hydrogen as bi products, and the reduction of a nitrocompound occurs by the excited electron and the in process use of hydrogen produced during oxidation of the solvent. As the average lifetime (120 µs) of the 12

photo-formed charge species over F-TiO2 is significantly much higher than R-TiO2 (34 µs) and P25 (45 µs), the photocatalytic redox reaction occurs in faster rate for reduction of all nitroaromatics (e.g., DNBP to BC) and oxidation of isopropanol to acetone and H2 molecules. Moreover, larger specific surface area and more surface exposed reduction sites (110 and 101 crystal planes) present over F-TiO2 are also responsible for better adsorption and photoreductive activity than R-TiO2 and P25 catalysts, previously it has been reported that (110) and (101) crystal planes of rutile TiO 2 possess large number of reductive sites, Ohno et al.(Ohno et al., 2002) have reported the selective reduction of metal nanoparticles on (110) plane, similarly Farneth et al.(Farneth, McLean, Bolt, Dokou, & Barteau, 1999) have also reported that the selective reduction of Ag+1 takes place on 110 plane followed its deposition. Theoretically, one molecule of DNBP reduction requires 8e- for a molecule of BC formation and simultaneously, 8h+ in the valance band initiates the oxidation of IPA to 4 molecules of acetone and 4 molecules of H2 (requires 2h+ for each IPA oxidation) molecules. Similarly, 12 e- are needed for hydrogenation of two –NO2 group (6eper nitro group) of m-DNB to m-PDA and simultaneously 12 h+ will oxidize IPA into 6 acetone and 6 H2 molecules. In a typical a stoichiometric calculation (BC/m-PDA: H2 = 1: 2 ratio) it found that for 23.5 µmol BC formation required 47 µmol H2 obtained during IPA oxidation to 47 µmol acetone, and for 22 µmol m-PDA formation needs 44 µmol H2 during IPA oxidation to 44 µmol acetone after DNBP and mDNB reduction. Quantitative analysis by GC revealed that amount of acetone (chromatogram Fig-ESI10) and H2 (chromatogram Fig-ESI-11 and 12) are gradually increasing with increased amount of BC and m-PDA production with instantaneous reduction of substrates (DNBP and m-DNB) concentration with sunlight irradiation time as in Fig 9. After 4-6 h irradiation complete reduction of both the substrates occurs and the highest amount of amino compounds (22-24 µmol) and acetone (40-45 µmol) formation (Fig.9a) is observed after 7 h photoreaction. It could be seen in Fig.9b that H2 production increases to a maximum of 156-180 µmol after 5h irradiation followed by a decrease to a minimum value (36-60 µmol) that remained after complete reduction of nitro to amino derivatives (highest yield) after 7h irradiation. This decrease (156-180 to 36-60 µmol) in H2 concentration with irradiation time (5 to 7h) strongly 13

evidencing its utilization for hydrogenation of -NO2 moiety of nitro-organics (Zhu et al., 2010) to-NH2 groups during reduction process. It also evident that the amount of H2 formed thus measured is relatively higher than required stoichiometric amount 40-47 µmol needed for nitroaromatic reduction which could be attributed to the possibility of over oxidation of IPA and water oxidation. Moreover, the amount (4045 µmol) of acetone thus experimentally determined after 7h reaction is in good accordance with stoichiometric amount (44-47 µmol) needed for BC and m-PDA production as discussed above. Numerous reports (S. Chen, Zhang, Yu, & Liu, 2011; Kaur & Pal, 2014) have been published for the selective reduction of various nitroaromatic compounds using conventional rutile and P25-TiO2 catalysts. However, selective reduction of m-dinitrobenzene to m-phenylenediamine, azoxybenzeneand 2,2dinitrobiphenyl to benzo[c]cinnoline using as prepared anisotropic rutile TiO2 sensitive to both UV and direct sunlight irradiation is rarely found in the literature. Hence, this finding of F-TiO2 catalyst consisting of rutile phase prepared at low temperature demonstrating the drastic improvement in the photoreductive activity than P25 and rutile TiO2 under sun light illumination is quite significant for nitroaromatics photoreduction. This photocatalyst appreciably enhanced the hydrogenation activity of DNBP to which undergoes intra-molecular reductive cyclization for BC formation because of the close spatial proximity of the interacting NO2 groups in two different benzene rings. Generally R-TiO2 prepared by high temperature sintering of P25 titania leads to bigger aggregated particles size, hence loss the nanoscale properties and catalytic activity [40]. As F-TiO2 possess exposed surface facets, appropriate band energetics (absorb visible light; λ =420 nm) and beneficial surface structural morphology in the nanoscale dimensions could be ascribed for superior photoactivity under sun light exposure. The reason for higher selectivity of this material is attributed to higher surface area that provides large number of active sites and easy diffusion of ionic species at the surface of F-TiO2 as compared to R-TiO2 and P25 which directly affected the photocatalytic efficiency. Surface facets (110) and (101) with dominant reductive species as reported previously provide diffusion path for surface -OH groups and helped in 14

attaching of reactant species (Huo et al., 2012) with the surface for catalytic reaction by Aloe-vera shaped rutile phase under direct sunlight exposure. Moreover, the electronic energy levels of the (110) and (101) facet helps in the quick separation of photoexcited electrons and holes for the rutile [30, 31] than anatase particles as supported by PL and time resolved spectroscopy. This variation in the surface energy of the conduction and valence band of different crystal faces and their atomic arrangements thus affect the TiO 2 photoreactivity; hence facilitating the nitro to amine conversion by the surface trapped electrons, enabling superior photoreduction of nitroaromatics. In F-TiO2, reactive sites are the Ti3+ species which are positioned at the oxygen vacancies on the surface behaving as the adsorption sites for –NO2 group reduction and trapping sites for the electrons in conduction band(Abazović et al., 2009; Shiraishi et al., 2013). Furthermore, these tuned morphological rutile TiO2 nanostructures allow the multidirectional flow of photo generated charge carriers, leading to improved sunlight absorption. Typically, reduction products of nitroaromatic compounds have been used for the synthesis of polymers, herbicides, food additives, pharmaceutical and agrochemicals etc., which are commonly synthesized at high pressure and temperature using toxic solvents and strong reducing agents [42, 43] e.g., NaBH4 and metal catalysts. Hence the development of visible light sensitive TiO2 catalysts would be beneficial for the hydrogenation of nitro aromatic compounds as they could utilize natural sunlight, inexpensive, nontoxic, and chemically stable. Therefore, such redox combined photocatalytic reduction of nitroaromatics by different anisotropic shape of nanoscale rutile TiO2under direct solar irradiation could be extended to produce many industrially important amino derivatives in more eco-friendly approach. 4. CONCLUSION In summary, rationally designed flower shaped rutile TiO2 microstructure prepared by low temperature hydrothermal process was successfully used for the selective nitroaromatic reduction and this microstructure showed better activity than conventional rutile and commercial P25 titania. This study disclosed that the higher activity of the flower structure is attributed to its well defined and uniform facet structure, large surface area and better interfacial charge transfer. The selectivity, intramolecular 15

cyclizatio and in process utilization of hydrogen are some key outcomes of this study in terms of industrial importance. Hence, present finding offers a loss cost technique to explore photoconversion of fine chemicals by nanoscale rutile TiO2 photocatalyst.

5. ACKNOWLEDGEMENT Authors are thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India for the financial support under sponsored research project (No. 01(2814)/14/EMR-II). Dr. Susheel Kumar Mittal from Air Quality and Pollution Monitoring Lab (Thapar University) for the solar radiation records.

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Figures and Captions

Fig.1 X-ray diffraction patterns of F-TiO2,R-TiO2 and P25

Fig.2 (a-d) FESEM micrographs of F-TiO2. 21

Fig.3 Nitrogen adsorption- desorption isotherms of F-TiO2, R-TiO2 and P25

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Fig.4 (a) Absorbance spectra (2nd derivative) and (b) bandgap energies of F-TiO2,R-TiO2 and P25

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Fig.5 (a) Photoluminescence spectra and (b) time-resolved decay profile of F-TiO2,R-TiO2 and P25

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Fig.6 GC chromatogram and mass spectra of authentic DNBP and BC and their respective reduction products under 7h UV and sunlight illumination

Fig.7 Time course graphs for photoreduction of m-DNB to m-NA andm-PDA, and DNBP to BC under (a and b) UV light and (c and d) direct sunlight exposure for 7 h 25

Fig.8 Histogram showing the yield (%) of aromatic amines formed under 7h (a) UV light and (b) direct sunlight exposure

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Fig.9 Time course graph of (a) acetone formation and (b) hydrogen production during DNBP and mDNB photoreduction upto 7h sunlight exposure 27

Scheme 1. Photoreduction of various nitroaromatics to their corresponding aromatic amines byF-TiO2,RTiO2 and P25 under direct sunlight and UV light exposure

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Scheme-2. Schematic illustration of reaction set-up and photoreduction process of DNBP under direct sunlight exposure

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GRAPHICAL ABSTRACT

Sunlight active Aloe-vera flower shaped crystalline rutile TiO2 microstructures displayed good photocatalytic reduction ability for various nitro aromatics to their respective aromatic amines. Despite having rutile lattice structure this photocatalyst showed dominance of (110) and (101) facets as exposed reduction sites. Moreover, narrow band gap, higher specific surface area and lower rate of recombination for photogenerated charge carriers are responsible for better selectivity and higher yield in nitro aromatic reduction.

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RESEARCH HIGHLIGHTS  Fabrication of highly photoactive flower shaped rutile F-TiO2  The F-TiO2 possesses small band gap (~ 2.8 eV) and large specific surface area (193 m2g-1).  Higher lifetime of photogenerated charge carriers was found in F-TiO2 (120 µs).  Selective photocatalytic reduction of nitroaromatics to aromatic amines with higher yield.  Rutile TiO2 exhibits 4 times higher photoactivity compared to commercial P25-TiO2.

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