Carbon-Doped Mesoporous Anatase TiO2 Multi-Tubes

Aug 7, 2017 - Synopsis. Carbon-doped anatase mesoporous TiO2 multi-tubes were synthesized from a banana stem fiber template for enhanced photocatalyti...
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Carbon-Doped Mesoporous Anatase TiO2 Multi-Tubes Nanostructures for Highly Improved Visible Light Photocatalytic Activity Rahul Purbia, Rituraj Borah, and Santanu Paria* Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela-769008, India S Supporting Information *

ABSTRACT: Development of a high surface area and efficient visible light induced photocatalyst on a large scale is a promising task from the practical perspective. In this study, visible light active C-doped anatase TiO2 multitubes were synthesized using banana (Musa acuminata) stem fiber as a sacrificial template, removed by calcination at 450 °C. During the calcination process, the lattice of anatase TiO2 phase was doped with C, and obtained multi-tubes showed high surface area (∼99 m2/g) with a mesoporous structure made of ∼15 ± 3 nm nanoparticles. The synthesized TiO2 multi-tubes showed an enhanced light absorption property in the whole visible light region and good thermal stability of the anatase phase up to 750 °C. The synthesized C-doped TiO2 multi-tubes manifest an excellent photocatalytic activity for the reduction of Cr (VI) to Cr(III) under the visible light exposure. This process may have lots of practical importance as the method of synthesis of the catalyst is novel and the multi-tubes structure can be synthesized on a large scale through a quick and economical way with excellent photocatalytic activity. This novel multi-tubes structure may also be useful for photovoltaics, antimicrobial, and Li-batteries applications in the future.

1. INTRODUCTION The emergence of different nanostructured TiO2 is becoming a most popular photocatalyst in recent years for the application of solar energy conversion and environmental remediation. Among several semiconductor photocatalysts, TiO2 is the most widely used photocatalyst in various applications because of its high photocatalytic activity, low cost, nontoxic nature, chemical inertness, and biocompatibility. However, development of a highly efficient and low-cost TiO2 nanocatalyst is still a challenging task because of the problems of fast charge-carrier recombination, less surface area, large band gap, etc., while synthesized by conventional methods. For the enhancement of photocatalytic activity by means of physicochemical changes, significant efforts have been made by developing different morphologies for better structural properties with high surface area and doping with different metallic or nonmetallic elements for better performance in visible light.1−3 Regarding the morphological aspect, it has been reported that the hollow hierarchical TiO2 nanostructures morphologies are important for photocatalytic applications because of improved mass transfer at the interface,4,5 reflection6 and scattering6 of incident light in interior cavities, structural stability,7 less aggregation, easy to separation,8 and faster charge transfer9 when compared with other morphologies. In general, uses of hollow nanostructures are a very common practice in catalysis because of the higher specific surface area. It has also been proven that the doping of different elements to a TiO2 © 2017 American Chemical Society

nanostructure helps in narrowing the band gap which in turn to improve the visible light absorption property to achieve better photocatalytic performance. Therefore, significant efforts have been made to develop a doped hollow morphology with different metallic and nonmetallic elements. Generally, the metallic doping such as Cr, V, Fe, Ag, etc., exhibited a lack of thermal stability, atomic diffusion, and increased exciton recombination in defect sites, which in turn shows low photocatalytic activity. On the other hand, the doping of nonmetallic elements such as B, S, F, C, and N shows comparatively better performance compared to that of metallic. Among nonmetallic elements, the carbon doping has received more attraction owing to its excellent properties such as narrowing of the band gap through the introduction of localized electronic states of C into the TiO2 lattice, visible light photosensitization, the large electron storage capacity to avoid charge recombination, and thermal stability.1,10 For the synthesis of hollow nanostructures, template-assisted and template-free approaches have been majorly used, where the disadvantages of template-free approaches are controlling of size/shape, low yield, reproducibility, use of additional surfactant/stabilizer, expensive,9,11,12 etc. On the other hand, the template-assisted methods are the most useful techniques for the synthesis of hollow nanostructured materials with Received: July 21, 2017 Published: August 7, 2017 10107

DOI: 10.1021/acs.inorgchem.7b01864 Inorg. Chem. 2017, 56, 10107−10116

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anatase phase, multiple reflections in hollow spaces, and higher visible light absorption. This easy synthesis process may also be applicable to synthesize other metal oxide nanostructures. In general, hollow structures are made to enhance the catalytic activity by enhancing active sites; additionally, in photocatalytic applications, the light harvesting property in terms of absorbance of hollow structures increases because of the multiple reflection of light inside the hollow structure. However, synthesis of a hollow nanostructure by a chemical sacrificial template method is expensive and large scale production for practical application is difficult. In chemical or green sacrificial template methods, hollow spheres or tubes are reported, but the synthesis of multi-tubes has not been reported before. Because of the simplicity of the method with higher production ability as well other above-mentioned advantages, this study is novel.

unique morphologies and properties because of better control over uniform shell thickness, dimensions, and morphology of the desired structure compared to the template-free methods, in spite of the requirement of an additional sacrificial template removal step in the process.13 As the green route synthesis of nanomaterials is drawing significant attention in recent years over the chemical routes, so, the uses of naturally occurring biotemplates for the synthesis of hollow structures are also not an exception. These natural templates are generally preferred over the chemical templates because of their advantages such as cheap, easily available, abundant, green and renewable, precise dimensions, elimination of core synthesis step, etc.14−22 Several naturally available biotemplates such as cotton,17,23−26 silk,27,28 filter paper, egg shell,21,29,30 cellulose,18,19,31,32 keratin,33 pollen grain,34−36 DNA,16 virus,15 proteins,14 insects,20,21,29 etc., have been reported for the synthesis of several micro/nanostructures which are eventually useful for different applications. Recently, the tubular nanostructure morphology of TiO2 has promising scope in catalysis because of the high specific surface area as well as directed electron transport.37,38 The tubular nanostructure morphology has been utilized with improved performance in photocatalysis, solar cells, electrochromic devices, gas sensors, and biomedical coatings applications.39 As per as reported studies, tubular TiO2 morphologies are mostly fabricated by time-consuming and expensive chemical methods such as electrochemical oxidation,39 hydro/solvothermal process,40 atomic layer deposition,41 sol−gel,42 and templateassisted approach.43 In spite of several literatures on the development of doped hollow TiO2 nanospheres after removing different sacrificial cores (C,44 AgBr,45 S,46 etc.), the development of a high surface area doped tubular structure with pure anatase phase stability at high temperature is still a challenging task to utilize the broad spectrum of solar light for the photocatalysis. On the basis of the available literature, it is very clear that the development of a doped tubular TiO2 nanostructure through an easy, scalable, sustainable, and cheap route for the catalytic or other applications has great scientific as well as practical importance. To the best of our knowledge, this is the first time we report a facile, low-cost, novel, green approach for the synthesis of visible light active and high surface area C-doped TiO2 multitubes nanostructures where the sacrificial template removal, crystallinity, and doping take place during a single calcination step. Herein, banana stem fiber, an agricultural waste product that comprises the unique multi-tubular structure, has been used as a template and dopant to fabricate a visible light responsive pure anatase C-doped TiO2 nanostructure for photocatalytic application. The as-synthesized TiO2 multitubes reported here possess high-surface-area, high crystallinity, low density, mesoporous nature, absorbing visible light, and high thermal stability of anatase phase. The main advantages of this process are green and sustainable, economical, short processing time, scalable, and anatase phase stability. Since the presence of the anatase phase is very important for the photocatalysis applications, the developed material may be useful for this application. Further, the photocatalytic reduction of Cr(VI) was carried out under irradiation of different light sources (LED and high-pressure Hg lamps), and the results indicate that the C-doped TiO2 multi-tubes have better photocatalytic activity compared to that of commercial P25 TiO2 powder. The better photocatalytic activity of the developed catalyst for Cr(VI) reduction can be attributed to the synergistic effects of large surface area, the presence of the

2. EXPERIMENTAL SECTION 2.1. Materials. The required chemicals were purchased from the following companies: Tetrabutyl orthotitanate (TBOT, SigmaAldrich), anhydrous ethanol (Merck), and TiO2 (Aeroxide P25, Sigma-Aldrich). All the reagents were analytical grade and used without any further purification. All the experiments were conducted using ultrapure water of 18.2 MΩ·cm resistivity (Millipore, Elix). A banana stem was picked from the freshly cut matured banana plant. The stem was then cut into thin slices using a knife, and the fibers were collected from the stem during slicing. The fibers were immediately dipped into deionized water and then in alcohol to remove the sap. 2.2. Multi-tubes Synthesis. The fiber was washed with ethanol and dried in an oven at 50 °C. The dried fibers were then immersed into an alcoholic solution of titanium precursor (100 mM TBOT in anhydrous ethanol solution) and sonicated for 10 min. After sonication, the treated fibers were dried in an oven (50 °C for 3 h) and then calcined in a muffle furnace at 450, 550, 650, and 750 °C for 2 h to form TiO2 multi-tubes. 2.3. Characterization. The phase, structure, and average crystalline size of nanostructures were analyzed using a powder Xray diffractometer (Rigaku Japan/Ultima-IV) with Cu Kα radiation (1.5406 Å) operating at a 40 kV and 30 mA. The morphology, structure, and dimensions of the nanostructures were analyzed by using a field-emission scanning electron microscope (FEI, NOVA) and transmission electron microscope (FEI Tecnai, G20 F30). The optical properties of the material were evaluated using a UV−vis−NIR spectrophotometer (Shimadzu-3600) in the UV−visible range. The Raman spectroscopy was performed using a Triple Raman spectrometer (Horiba, T64000) equipped with 1800 grooves/mm gratings, and Ar+ ion. Thermal gravimetric analysis (TGA) was conducted (Shimadzu, DTG − 60H) at a heating rate of 20 °C min−1. Nitrogen adsorption measurements were performed using an automatic adsorption unit, Autosorb-1 (Quantachrome), at 77 K, after degassing of samples for 3 h. The X-ray photoelectron spectroscopy (XPS) was carried out (K-Alpha instrument, Thermo Scientific) to know the chemical bonding states. The zeta (ζ) potentials were measured using a Malvern zeta size analyzer (Nano ZS, U.K.). Fluorescence measurement was performed using a spectrofluorometer (Hitachi, F-7000) with a slit width of 5 nm for the excitation and emission. Fourier transform infrared spectroscopy (FTIR) spectra were carried out using an FT-IR spectrometer (Thermo Fisher, Nicolet, iS-10). 2.4. Analysis of Hydroxyl Radicals (·OH). Hydroxyl radicals generated by the as-synthesized photocatalyst under visible light irradiation were measured by the fluorescence method. The terephthalic acid (TA) was used as a probe molecule to detect ·OH radicals. The 5 mg TiO2 multi-tubes were dispersed in the mixture of TA (2 mM) and NaOH (0.5 mM) aqueous solution to make up 30 mL. The resulting solutions were separately irradiated by white LED (2 × 12 W, 900 lm, COB, Phillips,) and UV−visible (125 W high10108

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Figure 1. FE-SEM images of pure banana fiber: (a) the morphology of banana stem fiber and (b) the closer look at the fiber and in the inset is an enlarged image of the fiber end. (c) TGA graph of banana stem fiber. (d) XRD spectra of banana stem fiber and TBOT coated banana stem fiber. pressure mercury lamp) lights. Then, 1 mL of the suspension was collected in a regular interval of 15 min and centrifuged for further fluorescence measurements with an excitation wavelength of 315 nm. The fluorescence emission intensity was measured at 425 nm wavelength. 2.5. Measurement of Photocatalytic Activity. The photocatalytic activity of the synthesized TiO2 multi-tubes was indiviually evaluated for the reduction of K2Cr2O7 in aqueous solution under visible (2 × 12 W LED lamps, energy flux 160 W/m2) and UV−visible light (125 W high-pressure mercury lamp, energy flux 210 W/m2) source. The Hg lamp was used to see the effect of the combination UV and visible light. During photocatalytic reaction, a UV filter was used for LED light to eliminate the UV wavelengths, if any. The 20 mg of TiO2 powder was added into 50 mL of K2Cr2O7 (50 ppm) solution containing 10 mM of oxalic acid as a hole scavenger. In order to exclude the adsorption−desorption equilibrium, the solution was kept under the dark condition for 1 h before irradiation of light. After, the solution was exposed to the light which was located vertically at 15 cm from the sample. At every 15 min interval, aliquots were taken and centrifuged to separate the photocatalyst. The concentration of K2Cr2O7 was monitored by the absorbance at the maximum peak (365 nm) using UV−vis spectroscopy. Photocurrent measurements were performed using a Metrohm Autolab, PGSTAT204 electrochemical workstation with a Pt counter electrode and Ag/AgCl (3 M KCl) reference electrode.

presented in (a) and (b) parts of Figure 1 show the uniform diameter of the fibers (∼2−3 μm) throughout the lengths and multiple fibers are attached together like a flat ribbon. An enlarged image of a single fiber end presented in the inset of Figure 1b shows that it has a hollow structure with a very thin wall (∼0.5 μm). As these fibers were used a sacrificial template to get the TiO2 multi-tubes, the complete burning temperature was found from the TGA. The TGA plot in Figure 1c depicts that the weight loss within 100 °C is because of unbound moisture, then up to 200 °C because of bound moisture and volatile matter, and finally the cellulosic material burns between 200 and 350 °C. Further, the surface characteristics of the fibers were characterized using FT-IR spectroscopy. The FT-IR spectral analysis of the banana fibers exhibits the vibrational stretching peak of O−H (3310 cm−1), −CH2 (2898 cm−1), and CC (1634 cm−1) bonds. Additionally, vibrational stretching peaks of ester and carboxyl groups are also found at 1433−1115 and 1038 cm−1, respectively, as shown in Figure S1 (Supporting Information). The FT-IR spectra of banana fibers confirm the presence of cellulosic oxygen rich groups on the surface, which makes the surface hydrophilic. The measurement of zeta potential (ζ) was also done and found to be −33 ± 0.6 mV; the negative charge of the fiber is because of the presence of carboxyl and other hydrophilic functional groups. 3.2. Synthesis of TiO2 Multi-tubes. The banana fiber’s multi-tube morphology was replicated by TiO2 through a twostep synthesis methodology. First, in the coating step, fibers were immersed in the alcoholic TBOT solution for adsorption or thin layer coating of TBOT on the fiber surface. The negative surface charge and the presence of oxygen rich polar groups on the fiber surface facilitate the favorable adsorption of

3. RESULTS AND DISCUSSION 3.1. Morphology of Template Fiber. When a banana plant is mature, the corm stops producing new leaves and begins to form a flower spike or inflorescence. The banana stem is the thick flower stalk of the banana plant, grows up from the ground, and forms the backbone of the herbaceous plant. The fibers are available abundantly in a bunch inside the stem when cut into two pieces. The FE-SEM images of the banana fiber 10109

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Figure 2. (a−d) FE-SEM images of banana fiber templated TiO2 multi-tubes at different magnifications.

TEM images of multi-tubes calcined at 450 °C temperature. From the TEM image (Figure 3a,b), the contrast difference between the edges (dark) and central region (light) clearly indicates the presence of a hollow structure of multi-tubes. The high-resolution TEM image (Figure 3c) reveals that the tubes consist of ∼15−18 nm size particles, which is consistent with the FE-SEM image (Figure 2d). Lattice fringes with an interplanar spacing (d) of 0.35 nm (d101) are clearly visible in Figure 3d, which corresponds to the (101) planes of tetragonal anatase phase of TiO2. The small pores of 5 ± 0.7 nm in size between interconnected nanoparticles along the tube are also visible from the TEM image (Figure 3c). The high-angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) images also confirm the tube structure and interconnected NPs with small pores as shown in Figure 3e,f. The ring patterns of the selected area electron diffraction (SAED) pattern as shown in the inset of Figure 3f are indexed to (101), (004), (200), (105), (204), (220), and (215) planes of anatase phase. Further, the HAADF-STEM elemental analysis and mapping of the TiO2 multi-tubes indicate the existence of Ti, O, and C as shown in Figure 3g−j. The point EDX mapping on TiO2 multi-tubes shows that the doped carbon content is ∼6.12 (weight)% and 11.15 (atomic)%, as shown in Figure S3 (Supporting Information); these results are close to the FE-SEM analysis. 3.4. Crystallographic and Raman Spectroscopic Analysis. The XRD characterization was performed to analyze the crystallographic properties as illustrated in Figure 4a. The XRD analysis of the banana fiber and precursor coated banana fiber revealed both are amorphous in nature and show only a

TBOT. In the second step, the multi-tubes were produced after calcination of the TBOT adsorbed fiber at 450 °C in the presence of air. In this step, TBOT was directly converted to TiO2 because of pyrolytic transformation according to the following reaction. 450 °C

Ti(OC4 H 9)4 ⎯⎯⎯⎯⎯⎯→ TiO2 + 2(C4 H 9OC4 H 9)

(1)

During the calcination process, the TiO2 nanostructure was doped by the C atoms that originate from the cellulosic fiber by replacing the O atom and formed Ti−O−C bonds in the nanostructures. Therefore, the C-doped multi-tube structured TiO2 was obtained. 3.3. Morphology and Structure of TiO2 Multi-tubes. The FE-SEM images of TiO2 multi-tubes after calcination at 450 °C are shown in Figure 2. The low-magnification FE-SEM image shows that the multi-tubular morphology of prepared TiO2 is retained similar to that of raw banana fiber (Figure 2b− d). The resulting TiO2 multi-tubes have a diameter of 2 ± 0.5 μm and thickness of 170 ± 10 nm, respectively. From Figure 2, it can be seen that there is a slight shrinkage of the tubes after burning the template at high temperature. The highmagnification FE-SEM image shows that the tubes are composed of interconnected nanoparticles with a size of ∼15−21 nm as shown in Figure 2d. In addition, the FE-SEM elemental mapping of the TiO2 multi-tubes indicates the existence of Ti, O, and C as shown in Figure S2 (Supporting Information). Further, the morphology of the multi-tubes was also analyzed by the transmission electron microscope. Figure 3 shows the 10110

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Figure 3. HR-TEM images of synthesized TiO2 multi-tubes: (a) multi-tube structure, (b) single tube structure, (c) high magnification of single tubes surface (yellow circles indicate pores), and (d) lattice fringes of TiO2 NPs. (e) HAADF-STEM image of tube. (f) High-magnification HAADFSTEM image of tube surface. Inset of (f) shows SAED ring pattern of TiO2 multi-tubes. HAADF-STEM elemental mapping of TiO2 multi-tubes: (g) selected area of elemental mapping with overlay of all elements, (h) mapping of Ti element (yellow), (i) mapping of O element (green), (j) mapping of C element (red).

Figure 4. (a) XRD spectra of TiO2 multi-tubes calcined at different temperatures for 2 h (450, 550, 650, 750 °C). (b) Raman spectra of as-prepared TiO2 annealed at 750 °C. (c) Nitrogen adsorption/desorption isotherm of as-prepared TiO2 annealed at 450 °C (inset pore size distribution graph).

cellulose diffraction peak at 2θ of 22.47° (200) as shown in Figure 1d. However, the TiO2 multi-tubes are crystalline when calcined between 450 and 750 °C for 2 h. Figure 4a shows the XRD patterns of TiO2 multi-tubes after calcination, where the diffraction peaks (2θ) of 25.4, 37.8, 48.1, 54.1, 55.2, and 62.8° are indexed to only the tetragonal anatase phase of TiO2 (JCPDS card no. 83-2243). The crystallite sizes measured using the Scherrer equation from the full-width half-maximum of the

(101) peak are 15.57, 18.98, 21.32, and 24.31 nm at 450, 550, 650, and 750 °C calcination temperatures, respectively. It can be concluded that the crystallite size of anatase phase increases with the increase in calcination temperature without changing the phase. The anatase TiO2 is known to exhibit six Raman active peaks (three Eg, two B1g, and one A1g).47 From the Raman spectra of the TiO2 sample calcined at 750 °C, the peaks of Eg (144 cm−1), Eg (198 cm−1), B1g (399 cm−1), 10111

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Figure 5. Deconvolution XPS spectra of (a) carbon, (b) oxygen, (c) titanium element.

overlapped A1g-B1g (516 cm−1), and an overtone of Eg (639 cm−1) were observed under the active vibration mode as displayed in Figure 4b. Thus, Raman analysis also confirms the presence of pure anatase phase of TiO2 at high temperature, which is consistent with the results of XRD. As the pure anatase phase is important for catalytic applications, the developed material may be useful for the better photocatalytic application. 3.5. Specific Surface Area and Porosity. The specific surface area and porosity of as-prepared multi-tubes were also determined by nitrogen adsorption/desorption isotherms (Brunauer−Emmett−Teller or BET). The experimental results exhibit a type IV isotherm with H3 hysteresis loop (IUPAC classification) which is characteristic for mesoporous material with slit-shaped pores as shown in Figure 4c. The pore size distribution of the material was calculated by the Barrett− Joyner−Halenda (BJH) method and showed a relatively narrow distribution ranging from ∼2 to 8 nm (inset of Figure 4c). The presence of mesopores is attributed to the voids between the deposited TiO2 nanoparticles of the multi-tubes surface as shown in Figure 3c. The BET surface area of mesoporous TiO2 multi-tubes calcined at 450 °C is 99 m2/g, and the value decreases gradually when calcined at higher temperatures because of increasing crystallite size (91, 80, 77 m2/g for 550, 650, 750 °C, respectively). This value is high enough and comparable to those reported mesoporous carbon-doped TiO2 as shown in Table S1 (Supporting Information). 3.6. Surface Elemental Composition and Thermogravimetric analysis. The surface elemental composition and chemical states of as-synthesized TiO2 samples were examined by XPS. The presence of Ti 2p, O 1s, and C 1s elements can be clearly seen in the XPS spectra of the TiO2 sample calcined at 450 °C. It can be seen from the Figure 5a that, after

deconvolution, the carbon 1s spectra show three peaks at binding energies of 284.4, 286.4, and 288.1 eV. The first peak at 284.4 eV is attributed to the presence of adventitious elemental carbon on the surface of NPs, whereas the other two peaks are from C−O (C−O−C) and CO bonds of carbonate-like species because of oxidized carbon species.10,44,48−55 These carbonate-like species may substitute in the lattice of the Ti site or interstitial site. In addition, the absence of a peak around 282.0 eV indicates that the substitution of the oxygen in the lattice of anatase TiO2 as Ti-C was not formed by the anionic carbon species.44,48 On the basis of the above observations, it is confirmed that carbon doping of carbonate-like species occurs at the interstitial position of the TiO2 lattice during the calcination of organic material of banana fibers. As depicted in Figure 5b, the large peak at 529.1 eV is assigned to O2− ions in the Ti−O bonds, and another peak at 531.8 eV is because of chemically adsorbed surface oxygen species, such as OH− and carbonates. The high-resolution Ti 2p spectrum of TiO2 shows two principle peaks at binding energies 457.78 and 463.35 eV corresponding to Ti4+ 2p1/2 and 2p3/2 oxidation states in Figure 5c. To get the carbon content, we investigated the thermal behavior of as-synthesized TiO2 multi-tubes at 450 °C by thermogravimetric (TG) analysis as shown in Figure S4 (Supporting Information). The first weight loss in the range 29−200 °C was because of the removal of physically adsorbed water molecules. The second weight loss observed between 200−800 °C can be attributed to the removal of carbon content on TiO2 multi-tubes (94.7 to 89.6%), which was ∼5.17%. The obtained value is also close to that with the HAADF-STEM EDS mapping analysis (∼6.12%). 10112

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Figure 6. (a) UV−vis spectra of as-synthesized carbon-doped TiO2 and commercial P25. (b) The photoluminescence spectra of P25 and carbondoped TiO2 tubes excited at 426 nm. (c) Fluorescence spectra of TiO2 multi-tube suspension containing 2 mM terephthalic acid at various irradiation periods under Hg lamp irradiation. (d) The time dependence graph of the fluorescence intensity at 425 nm under under LED and Hg lamps irradiation; the value of regression coefficient (R2) is 0.99 for both conditions.

3.9. Hydroxyl Radical Analysis. The generation of ·OH on the surface of TiO2 photocatalyst was analyzed by the fluorescence method using TA as a probe molecule, which reacts with ·OH radicals and forms highly fluorescence active 2hydroxyterephthalic acid (TAOH). The fluorescence intensity of TAOH is proportional to the amount of ·OH produced on the surface of photocatalysts. The maximum emission intensity in fluorescence spectra was recorded at 425 nm when excited at 315 nm wavelength. The fluorescence intensities of the suspension in the presence of TA under the LED and Hg lamps irradiations were measured, and the results are shown in Figure 6c,d. It can be seen that the fluorescence intensity increases gradually with increasing irradiation time because of formation of ·OH during the photocatalytic process under visible and UV−visible light irradiation. Figure 6d shows a linear increase in fluorescence intensity of TiO2 multi-tubes at 425 nm under LED and Hg lamps irradiation against the illumination time. Consequently, we can conclude that ·OH radicals formed at the TiO2 interface are proportional to the light illumination time and obeying first-order reaction rate kinetics. The formation rate of the OH· radicals could be expressed by the slope of the plot shown in Figure 6b. However, no fluorescence intensity increase was observed in the absence of any light. This suggests that the fluorescence signal is only because of the reaction of TA with ·OH formed on the interface of the TiO2 during the light irradiation. Generally, the greater the formation rate of OH· radicals, the greater is the separation efficiency of electron−hole pairs. A little lower efficiency for formation of ·OH in multi-tubes was observed under LED light compared to the Hg lamp because of low intensity of light and elimination of UV components. As the photocatalytic activity has a positive correlation with the

3.7. Optical Property. To see the optical property of carbon-doped TiO2, the UV−vis absorption of the assynthesized TiO2 multi-tubes and P25 was carried out and is shown in Figure 6a. It is observed that the P25 suspension shows an absorbance peak at a wavelength of 320 nm, which is very close to that of pure TiO2 nanoparticles. In contrast, the UV−vis absorption spectrum of the TiO2 multi-tubes shows shifting of the peak toward the visible region with a peak at 350 nm wavelength because of carbon doping in TiO2 and more interaction of incident light by multiple reflections in hollow tubes. The change in light absorption characteristics of TiO2 after the doping should also show the change in the band gap of TiO2. The Tauc plot was used to estimate the band gap energy of the as-synthesized TiO2 and P25. The results indicate that the band gap energies of TiO2 multi-tubes and P25 are 2.83 and 3.3 eV, respectively, which suggests that C-doped TiO2 should exhibit better visible light photocatalytic activity compared to P25 because of the lower band gap. 3.8. Photoluminescence Study. The PL spectrum is commonly used to evaluate the separation and recombination behavior of photoinduced charge carriers as well as the trapping of charge carriers. Thus, the PL spectra were measured to investigate the recombination process of photoexcited electrons and holes using an excitation wavelength of 426 nm at room temperature as depicted in Figure 6b. It can be seen that the P25 TiO2 NPs have high electron−hole recombination intensity, while the recombination decreases with carbondoped TiO2. This suppression in the recombination of photoinduced electron−hole pairs probably suggests that the carbon doping can improve the surface state of TiO2, which could result in improving photocatalytic capability due to the generation of more active species for photocatalytic reaction. 10113

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Figure 7. (a) The change in concentration of Cr(VI) with time during photocatalytic reduction by TiO2 multi-tubes and P25 nanoparticles under LED and Hg lamps. (b) Fitting of experimental data with first-order rate equation; the regression coefficient (R2) values are 0.98, 0.99 and 0.99, 0.98 for TiO2 multi-tubes and P25 NPs under LED and Hg lamps, respectively. (c) Schematic illustration of the photocatalytic reaction mechanism of Cdoped TiO2 multi-tubes.

formation rate of ·OH radicals, it can be expected that this material may be useful as a good photocatalyst. 3.10. Study of Photocatalytic Activity for Cr(VI) Reduction. Photocatalytic activities of TiO2 multi-tubes were investigated for the reduction of aqueous Cr(VI) solution with oxalic acid as a hole scavanger. The white LED and Hg lamps were used as a source of visible and UV−visible lights, respectively, to see the efficiency of the developed catalyst. Figure 7a,b illustrates the photoreduction of Cr(VI) in the presence of TiO2 multi-tubes and P25 nanoparticles under LED and Hg lamps light exposure. The Figure 7a shows that the TiO2 multi-tubes exhibit enhanced photocatalytic activity compared to that with pure P25 TiO2 nanoparticles under both light sources. Under the Hg lamp, the TiO2 multi-tubes are able to reduce 78% of Cr(VI) within 120 min exposure, whereas that of P25 is only 46%. On the other hand, the TiO2 multi-tubes and P25 are able to reduce 66% and 19% of Cr(VI) within 120 min exposure to LED light, respectively. Further, the kinetics of the photocatalytic reduction of Cr(VI) was studied and is depicted in Figure 7b. A linear relationship between Cr(VI) concentration and the irradiation time was obtained in both cases, when fitted with the first-order rate equation ln(C0/Ct) = −kt, where C0 represents the initial Cr(VI) concentration and Ct refers to the Cr(VI) concentration

at irradiation time (t). The rate constant values were found to be 0.0038 and 0.0007 min−1 for TiO2 multi-tubes and P25 NPs under the LED light irradiation, respectively, while the rate constant values were found to be 0.0052 and 0.0023 min−1 for TiO2 multi-tubes and P25 NPs under the Hg lamp light irradiation, respectively. These results indicate that the enhancement of reduction efficiencies of C-doped multi-tubes compared to P25 is ∼69% and 247% under the exposure of high pressure Hg and LED lights, respectively. A little lower efficiency of multi-tubes under LED light is because of low intensity of light and lower content of UV. Finally, these results indicate the developed catalyst is promising for photocatalytic activity under visible light as well as UV−visible mixed light sources. During the photocatalytic process, the light absorption by the catalyst and charge separation properties play a vital role for higher activity. The increase in visible light absorption property obtained from the UV−vis spectrum (Figure 6b) of TiO2 multitubes can also be attributed because of band gap reduction after carbon doping, which eventually enhances the photocatalytic activity. Figure 7c schematically shows the photocatalytic mechanism of C-doped TiO2 multi-tubes under visible light. The improved photocatalytic activity can be attributed to, first, the carbonate-like species on TiO2 act as a photosensitizer, 10114

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Inorganic Chemistry Notes

which absorbs visible light and transfers photogenerated electrons into the conduction band of TiO2. Second, the doping of C in the TiO2 lattice leads to the formation of new mid-gap energy states of 2p orbitals of carbon above the valence band of O 2p orbitals of TiO2. Therefore, the band gap of TiO2 decreases and optical absorptions shift toward the visible region. Thus, the photogenerated electrons of C-TiO2 can easily migrate to the conduction band under the visible light irradiation to improve the photocatalytic efficiency. Finally, these photogenerated electrons reduce the Cr(VI) to Cr(III). In addition, the photogenerated holes produced hydroxyl radicals, which was suppressed by oxalic acid as a hole scavenger. To support the higher photogenerated electrons for C-doped multi-tubes, the photocurrent measurements of the samples were analyzed and found to be much higher efficiency than that of P25 (Supporting Information, Figure S6).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the DST-SERB (ref. No. EMR/ 2016/000810) for this project is gratefully acknowledged. The authors are grateful to the CeNTAB, SASTRA University, Thanjavur, Tamilnadu, India, for giving the opportunity to access their XPS facility. The authors also acknowledge Mr. Subhabrata Chakraborty, NIT Rourkela, for helping in FE-SEM and HR-TEM characterizations.

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4. CONCLUSIONS In summary, a new and facile biomimetic template approach is reported for the synthesis of visible light active, high surface area (99 m2/g), and mesoporous (pore sizes ∼ 2−8 nm) Cdoped TiO2 multi-tubes. The significant advantages of this biomimetic approach are a single step process, low-cost, scalability, thermal stability of the anatase phase up to 750 °C, and multi-tubular morphology. The diameter of a single tube is 2 ± 0.5 μm with a wall thickness of 170 ± 10 nm which is made of ∼15 ± 3 nm nanoparticles. The C doping was taking place during the calcination step at the interstitial and surface positions of the anatase TiO2 lattice. The TG and TEM-EDX analyses show ∼6 wt % C doping in TiO2 multi-tubes. The C doping in TiO2 induces visible light absorption through the introduction of a localized electronic state, lowering the band gap, and improves visible photosensitization. The generation of ·OH radicals on the surface of TiO2 multi-tubes was found to be proportional to the visible light illumination time and obeying first-order reaction rate kinetics. The maximum photoreduction efficiencies of Cr(VI) were 66% and 78% for TiO2 multi-tubes within 120 min exposure under visible and UV−visible light sources, respectively, whereas those of P25 were only 19% and 46%. The enhancement of photocatalytic activity of TiO2 multi-tubes under visible light can be explained by two important factors: (i) visible light absorption and better charge separation due to carbon doping, and (ii) the excellent structural properties including anatase phase, hollow structure, high surface area, and mesoporous structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01864. Sample photographs, FT-IR spectra, TG graph, EDX analysis, photocurrent measurement, and comparison table (PDF)

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Corresponding Author

*E-mail: [email protected] or [email protected]. Fax: +91 661 246 2999. ORCID

Santanu Paria: 0000-0002-3053-7277 10115

DOI: 10.1021/acs.inorgchem.7b01864 Inorg. Chem. 2017, 56, 10107−10116

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01864 Inorg. Chem. 2017, 56, 10107−10116