Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 3 (2016) 1351 1358 www.materialstoday.com/proceedings Recent Advances In Nano Science And Technology 2015 (RAINSAT2015) Highly Efficient Hydrogen Production using Bi 2 O 3 /TiO 2 Nanostructured Photocatalysts Under Led Light Irradiation N. Lakshmana Reddy, G. Krishna Reddy, K. Mahaboob Basha, P. Krishna Mounika, M.V. Shankar* Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science and Nanotechnology, Yogi Vemana University, Kadapa - 516 003, Andhra Pradesh, India Abstract Photocatalytic water splitting (PWS) is one of the cleaner processes for molecular hydrogen (H 2 ) and oxygen (O 2 ) production using semiconductor nanocomposites as efficient photocatalysts. Herein, we report hydrothermal synthesis and modification of Bi 2 O 3 deposited TiO 2 nanostructures (Bi-TNS), its water splitting performance under UV-A LED (λ= 365 nm, 20 W) light irradiation. Characterization data reveals that calcined photocatalyst (TNS) have single crystalline anatase TiO 2 with mixed morphologies viz., nanotubes, nanorods and nanoparticles. The TEM and DR UV-Vis spectra of Bi2O3 deposited TiO2 composite photocatalysts confirmed anatase phase TiO 2 besides mixed morphologies. A dedicated off-line gas chromatograph equipped with molecular sieve column and TCD detector utilized for analysis of H 2 and O 2 gases. Under optimized conditions, Bi 0.5 TNS composite showed enhanced rate of H 2 production 3,601 μmol.h -1 g -1 cat which is much higher than standard TiO 2 P-25 nanoparticles. These results illustrated the advantage of nano-size effect of TiO 2 and Bi 2 O 3 for enhanced H 2 production. Stability of catalyst under light irradiation (100 h) shows that Bi-TNS is a promising photocatalyst for enhanced H 2 production using industrial by-product glycerol as scavenger. 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords: Water splitting; Photocatalysis; Hydrogen Production; Nanocomposites, UV-LED light. * Corresponding author. E-mail address: shankar@yogivemanauniversity.ac.in 2214-7853 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].
1352 N. Lakshmana Reddy, et al./ Materials Today: Proceedings 3 (2016) 1351 1358 1. Introduction Production of cleaner energy using renewable resources and non-energy intensive process are in great demand to cater the present and future energy supply. Hydrogen (H 2 ) gas served as low density and high energy content cleaner fuel, demonstrated as best alternative to fossil fuels for sustainable energy developments [1-2]. Industrial production of H 2 production involve one of the following methods such as steam reforming of methane, gasification of coal, pyrolysis of oil and reforming of bio-mass. It is well known that these methods are non-renewable in nature and environmentally taxing. Among the cleaner routes of H 2 production techniques such as electrolysis, electrocatalysis, photoelectrolysis and photocatalysis are most promising avenues. Photocatalytic water splitting is an energy efficient method to produce H2 fuel in which semiconductor photocatalyst harvests renewable solar energy and utilizes natural water resources [3]. Many semiconducting materials (ZnO, NaTaO 3, KTaO 3, Fe 3 O 4 ) have been researched for photocatalytic applications [4-7], among the oxide-based photocatalysts, TiO 2 is promising in terms of stability, non-toxicity and red-ox potential [8-9]. In recent years, intensive research has been reported using TiO 2 nanostructures especially nanotubes based composite materials to improve the rate of photocatalytic H2 production [10-12]. Xu et al [13] studied the role of Ni as efficient co-catalyst in photocatalytic process for enhanced rate of H 2 production. In these directions, we have succeeded in preparation and tested photocatalytic efficiency of 1-D TiO 2 composite nano structures like nano tubes, nano rods and nano belts [14-16]. Recently Bi2O3 based TiO2 composite material is found to be very good photocatalyst for environmental and photocatalytic water splitting application. These catalysts posses unique opto-electrical and catalytic properties, with a band gap of 2.8 ev can oxidize water and generate highly reactive species for initiating re-ox reactions [17-19]. Incorporation of Bi3+ into TiO2 lattice enhances the spectral absorption range of TiO 2 and inhibits electron-hole recombination, thus facilitating photo catalyzed reactions [19]. Bi-ion shows strong chemical signals with higher post treatment temperatures which has important effects on the photocatalytic activity for H 2 evolution and de-colorization of RhB [20]. For Bi2O3 based TiO2 composite materials more research is made for degradation of dyes, for example NiO-Bi 2 O 3 [21], ZnO-Bi 2 O 3 [22], Bi 2 O 3 -TiO 2 [23] BiO 1.8 4H 0.08 [24] and Bi 5 O 7 I [25], but seldom studied for water splitting. Hence our present work can fulfill the gap of Bi2O3-TiO2 nanostructures for water splitting applications under LED light irradiation. Due to the anti-bonding states between the lone electron pairs of Bi3+ and electrons in O2p orbital, the bismuth containing photocatalysts usually have a narrow band gap and promising photocatalytic efficiency, which are good candidates for photocatalytic water splitting [26]. Most of the photocatalytic experiments on Bi2O3-TiO2 based photo catalysts carried out under UV Lamps (Mercury, Xenon, Halogen, Tungsten etc) which are harmful and consumes more electric power. In the present work we have performed all the photocatalytic water splitting experiments under Light Emitting Diodes (LED) light irradiation. The LED lamps are more advantageous than conventional Mercury, Xenon based UV lamps in such a way that LED lamps consumes much less electrical energy, emits light in narrow range, use of cut-off / band pass filters are not required and have long-lasting for several years. The disadvantages associated with conventional mercury lamp are its fragility, hazardous mercury content, and problems relating to its disposal after use. Additionally, mercury lamps have relatively short working life span (500-2,000 h) compared to LEDs which lost for long life span of more than 50,000 h. Another disadvantage of conventional UV lamps are prone to gas leakage from the tube and some time lamp gets explosion, in addition to that the medium and high pressure mercury lamps operate at very high temperatures (600-900⁰ C) and hence, need cooling during reaction, thus consumes more energy. The advantages of LEDs are robust, safe, compact, cool, do not contain mercury, have relatively low costs of production and operation, and hence, are environmentally friendly, with a long life span of approximately 100,000 h. Ultraviolet LED (UV-A LEDs) are mainly employed for the photocatalytic degradation of organic pollutants present in air and water [27-30]. To the best of our knowledge, there is no report exist on photocatalytic water splitting for H2 production under UV-A LED irradiation with Bi2O3-deposited TiO2 composites. Hence, the present work deals with Bi-deposited TiO2 photocatalysts with different wt% of Bi for enhanced H2 generation via water splitting. The photocatalysts were thoroughly examined for its crystal structure, optical and morphological properties responsible for enhancement of photocatalytic activity. Stability of the photocatalyst explained with time on stream experiments for 100 h.
N. Lakshmana Reddy, et al./ Materials Today: Proceedings 3 (2016) 1351 1358 1353 2. Experimental 2.1. Materials and Reagents All the chemicals used for the present work are of analytical reagent grade and details are given below. Commercial TiO 2 LAB used for Nanotube synthesis was purchased from Merck, India. Sodium Hydroxide, Hydrochloric acid and Glycerol were received from Merck, India. Bi(NO 3 ) 3.5H 2 O, was received from SDFCL India. Commercial TiO 2 P-25 powder catalyst (anatase 80%, rutile 20%, surface area = 50 m 2 /g and particle size 27 nm) was used as received from Degussa Corporation, Germany. Double distilled water was used for materials synthesis and battery grade water used for photocatalytic experiments. 2.2. Hydrothermal synthesis of TiO 2 nanotubes Hydrogen trititanate nanotubes were synthesized by hydrothermal method as detailed in our earlier report [12]. In a typical synthesis, TiO 2 (Merck) particles (2.5 g) was dispersed into 10 M NaOH aqueous solution (200 ml) in Teflon-lined autoclave (capacity 250 ml) and heated at 130 C for 20 h. The white precipitate was washed twice in each stage with distilled H 2 O, dil.hcl and ethanol solution, dried at 80 C for 12 h and bright white powder ( 2.3 g) was obtained. The same was uniformly spread into ceramic boat subjected to calcination in tubular furnace under air flow at 350 C for 5 h at 2 C /min (denoted as TNS). 2.3. Preparation of Bi 2 O 3 -TNS composites Bi 2 O 3 -TNS composite was prepared by wet impregnation method. In a typical method varying amount of Bi precursor Bi(NO 3 ) 3.5H 2 O (0.3 to 0.7 wt%) dissolved in 10 ml distilled water placed in 50 ml beaker. The requisite amount of calcined TNS powder was suspended into Bi-containing solution and stirred magnetically under hot condition till the excess water gets evaporated. Thus obtained powder was dried at 80⁰ C for 16 h and calcined in tubular furnace under air flow at 350⁰ C for 5 h @ 2⁰ C.min -1. The Bi-containing nanocomposites are labeled as BT- 1, BT-2, BT-3 BT-4 and BT-5 for Bi = 0.3, 0.4, 0.5, 0.6, 0.7 respectively. 2.4. Catalyst Characterization The prepared and modified photocatalysts were thoroughly examined by using different characterization techniques. Powder wide-angle X-ray diffraction (WAXRD) patterns of all the catalysts were recorded using a D8 ADVANCE X-ray diffractometer (Bruker) equipped with Ni-filtered Cu Kα (k=1.5418å) radiation (30 kv, 50 ma).the diffractogram of all the catalysts was recorded in the 2θ range of 5-80⁰. The optical properties of the prepared photocatalysts were carried out by UV Visible diffuse reflectance/absorption spectroscopy (Perkin Elmer, UV Lambda-600, USA). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) measurements were carried out by using a FEI Tecnai F20ST electron microscope operated at 200 kev and equipped with high angle annular dark field (HAADF) detector and energy dispersive X-ray (EDX) spectrometer. 2.5. UV-A LED Photocatalytic Experiments The photocatalytic reactions were carried out in a Quartz reactor (capacity: 185 ml) under UV-A LED (λ=365nm) irradiation under ambient conditions (Fig.1). Photocatalyst powder was suspended into 5 vol.% glycerolwater mixtures at concentration of 100 mg.l -1. Dark experiments were carried out to ensure homogeneous system. In a typical process, reaction suspension was evacuated and purged with N 2 gas for at-least 30 min to remove dissolved gases. The reactor was irradiated to UV-A LED under magnetic stirring and gaseous sample was collected and analyzed at regular intervals. The volume of H 2 gas produced via photocatalytic reaction was quantified using an offline gas chromatography equipped with TCD detector (Shimadzu GC-2014 with Molecular Sieve/5A).
1354 N. Lakshmana Reddy, et al./ Materials Today: Proceedings 3 (2016) 1351 1358 The time on stream experiments were conducted continuously for 100 h under UV LED lamp to evaluate the stability of the prepared photocatalysts and activity test was carried out at every 5 h intervals. Fig. 1. Photocatalytic experimental setup for H2 production under UV-A LED irradiation. 3. Results and discussions 3.1. Photocatalytic Hydrogen Production Experimental parameters were chosen based on our earlier report on CuO/TiO 2 nanotube photocatalyst for highly efficient hydrogen production under solar light irradiation [14]. In the present study photocatalyst (5 mg) was suspended in 5 vol.% of glycerol-water mixture (50 ml) and irradiated under UV-A LED (λ=365 nm, 20W) light source. First, the light experiments carried out with BT catalysts. Figure 2(a) displays progress of photocatalytic reaction for H 2 evolution using BT photocatalysts under UV-A LED light. The amount of Bi (0 to 0.7 wt.%) in Bi 2 O 3 -TNS nanocomposite photocatalyst is optimized for enhanced H 2 production. It is obvious that presence of bismuth in the catalyst triggered higher rate of H 2 production and increases linearly with amount of Bi-loading, the rate of H 2 production over BT-3 (0.5 wt. % of bismuth) catalyst was found to be 3, 601 μmol. h -1. g. -1 cat and decreased beyond the optimum level. The high activity at optimum level is attributed to fine dispersion of Bi 2 O 3 nanoparticles in TNS catalyst. The photoexcited charge carriers on both anatase TiO 2 nanotubes and Bi 2 O 3 nanoparticles have the suitable band edge potential for oxidation of glycerol and reduction of H + to H 2 gas. Both catalytic active sites and life-time of photogenerated charge carriers are responsible for H 2 production. At lower Bi 2 O 3 -loading lesser amount of catalytically active sites, above optimal loading surface and photochemical properties of the photocatalysts leads to recombination of charge carriers. Fig. 2. (a) Photocatalytic hydrogen production under UV-LED light irradiation for different amount of Bismuth loading on TNS.
N. Lakshmana Reddy, et al./ Materials Today: Proceedings 3 (2016) 1351 1358 1355 In order to assess the efficiency of BT-3, the H 2 production rate was compared with TNS and standard TiO 2 P- 25 photocatalysts. For this purpose, experiments were carried out under optimal conditions in UV-A LED light irradiation. Fig. 2(b) displays rate of H 2 production in the following order: Bi 0.5 TNS > TNP> TNS. It is evident that Bi 0.5 TNS (BT-3) showed 5 folds of enhanced H 2 production than TNP. The enhanced photocatalytic activity of Bi 2 O 3 -TNS is due to synergetic properties of TNS and visible light sensitization effect of dispersed Bi 2 O 3 nanoparticles. Fig. 2. (b) Comparision of photocatalytic activity in TNP, TNS and BT-3 photocatalysts in glycerol-water mixture under UV-A LED light irradiation. Time on stream experiments were carried out to evaluate the recyclability of the optimized catalyst (BT-3) for 100 h under UV-A LED irradiation, the photocatalytic activity was tested for each 5 h. The optimized catalyst showed sufficient recyclability with increasing H 2 generation for more than 70 h, after that the H 2 generation for each hour calculation was found to be decreased, it may be ascribed to glycerol intermediates formed during photocatalytic reactions. Figure 2(c) depicts stability of optimized photocatalyst under UV-A LED irradiation. Fig. 2. (c) Time on stream activity of optimized photocatalyst under UV-A LED irradiation. 3.2. X-Ray Diffraction The crystalline structures of prepared catalysts (TNS, BT-3) were characterized by X-ray diffractometer. Fig. 3 shows XRD patterns of calcined TiO 2 nanotubes (TNS) and 0.5wt% Bi 2 O 3 -TNS (BT-3) material calcined at 350⁰ C for 5 h. The XRD pattern of TNS indicates the formation of well crystalline anatase TiO 2. The high intense peak at 2θ = 25.3, 37.7, 48.0, 53.8, 54.8 and 62.6⁰ indicates (101), (004), (200), (105), (211), (204) planes of anatase respectively. These results well matches with values of JCPDS card no: 21-1272 of anatase TiO 2. Bi 2 O 3 -TNS XRD patterns also displayed similar structure of TNS which indicating that Bi 2 O 3 deposition does not affect the changes in the crystal phases of TNS. Further there is no characteristic peaks of Bi was observed for BT-3 photocatalyst, this may be due to lower quantity of Bi and it is well dispersed on lattice of the TNS. Wu et al [20] reported Bi loaded anatase TiO 2 synthesized by hydrothermal method, they also observed that with lower concentrations (<5.00 mol%) of Bi deposition results significant anatase phase and with higher concentrations (10.00 mol%) of Bi deposition results transformation of Bi into Bi 2 O 3. These results well matches with our results, since we have loaded very low
1356 N. Lakshmana Reddy, et al./ Materials Today: Proceedings 3 (2016) 1351 1358 wt % (0.5 wt% of Bi), we observed only anatase phase. We did not observe any characteristic peak of Bi 2 O 3 at 2θ = 30⁰. This is because the radius of Bi 3+ ions (0.103nm) is larger than that of Ti 4+ ions (0.061nm) such results demonstrated that Bi cations have been deposited into the crystal lattice of TiO 2. Similar observations were reported in the literature [23]. Fig. 3. X-ray diffraction patterns of calcined TiO2 nanostructures (TNS), 0.5wt% Bi2O3 deposited TiO2 Nanostructures (BT-3). 3.3. DR UV-Vis Spectroscopy Optical properties of the materials were investigated by DR UV-Vis spectroscopy. Fig.4 displays TNS and Bi 2 O 3 -TiO 2 composite with minor shift in absorption edge towards visible region compared to standard TNP particle. The threshold wavelength of TNP is 398 nm, whereas BT-3 showed absorption at 404 nm. This shift is due to Bi deposition and nano size effects of the materials. The corresponding band gap values for TNP and BT-3 was calculated by using formula, E bg =1239.8λ (1) as 3.11 ev and 3.06 ev respectively. Similar type of shift was observed in Bi-TiO 2 particles reported in the literature [19]. Sood et.al., studied Bi-TiO 2 particles; they observed significant red shift from TiO 2 particles at 392 nm with band gap of 3.16 ev to 417 nm with band gap reduction to 2.97 ev for 1wt.% Bi incorporated TiO 2. Li et al [18] also studied β-bi 2 O 3 /TNTs and observed red shift from 385 nm for TNTs to 495 nm for β-bi 2 O 3 /TNTs. The observed results are quite interesting and showed red shift for mixed structures [31]. Though the BT-3 showed minor shift in absorption edge towards higher wave length, the photocatalytic activity has increased under UV-A LED irradiation, this may be due to multiple reflections of light with mixed nanostructures which increases the incident time to utilize more photons. Fig. 4. DR UV-Vis spectra of TiO 2 nanoparticles (TNP), calcined TiO 2 nanostructures (TNS), 0.5wt% Bi 2O 3-TiO 2 composite nanostructures (BT-3). 3.4. Transmission Electron Microscopy Detailed morphology and structure of calcined TiO 2 nano structures (TNS) were studied by high resolution TEM analysis. Fig. 5(a) depicts calcined TiO 2 nanostructures with mixed structures of nanotubes, nanorods and
N. Lakshmana Reddy, et al./ Materials Today: Proceedings 3 (2016) 1351 1358 1357 nanoparticles. The tubes are hollow inside and opened both sides, whereas the nanorods are filled inside with uniform diameter. The diameter of the rods and tubes varies from 3 to 5 nm and length varies from 50 to 100 nm. Fig. 5(b) clearly indicates the nanotubes with hollow inside and well crystalline walls and particles with irregular shape. Fig. 5(c) reveals HRTEM image of TiO 2 nanotubes, the single TiO 2 nano tube with diameter 3 nm and highly crystalline structure and well-defined lattice fringes. Fig. 5(d) shows SAED pattern of TiO 2 nanostructures, which confirms crystalline nature of TiO 2 and [101] crystal plane of the anatase TiO 2. These results are in good agreement with XRD data (vide supra). The well crystalline nature of the nanostructures was attributed by calcination. Fig. 5. TEM images of calcined TNS (a-c) and SAED pattern (d) 4. Conclusions We have prepared TiO 2 mixed nanostructures by simple and eco-friendly hydrothermal method, Bi 2 O 3 -TNS composite materials by impregnation method. The XRD analysis confirms anatase TiO 2 structures with high crystallanity, DR UV Vis results confirms the UV absorption of the Bi 2 O 3 -TNS composite and TEM reveals formation of mixed nano structures with TiO 2 nanotubes, nanorods and nanoparticles. Solar photocatalytic experiments were conducted with varying Bi-loading from 0.3 to 0.7 wt%. in quartz batch reactor using glycerol:water mixture. The efficiency of synthesized photocatalysts TNP, TNS and Bi 2 O 3 -TNS photocatalysts are compared. It is interesting to note that Bi 2 O 3 -TNS showed enhanced photocatalytic H 2 production than standard TiO 2 P-25 particles under UV-A LED light irradiation. Based on the experimental results and literature survey, the enhanced H 2 generation rates were due to efficient utilization of excited charge carriers by mixed nano structures of the composite materials, the synergetic effects of 1-D nano structured properties and unique physic-chemical properties of bismuth. The time on stream experiments reveals the stability of the Bi 2 O 3 -TNS photocatalysts for long hours. Acknowledgements One of the authors N. Lakshmana Reddy is gratefully acknowledge the DST-INSPIRE, New Delhi for financial support through INSPIRE fellowship to carry out the PhD program.
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