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Gelatin-assisted sol–gel derived TiO2 microspheres for hydrogen storage
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage Bin Liu a,b, Jingzhong Xiao b,*, Lan Xu b, Yijun Yao a,b, Benilde F.O. Costa b, Valdemar F. Domingos b, Edivagner S. Ribeiro b, Fa-Nian Shi c, Kai Zhou a, ~ o b, Joa ~ o M. Gil b Jing Su a, Hongyan Wu a, Kun Zhong a, Jose A. Paixa a
School of Physics and Optoelectronic Engineering, Nanjing University of Information & Technology, Nanjing 210044, China b CEMDRX, Department of Physics, University of Coimbra, Coimbra 3004-516, Portugal c CICECO, Department of Chemistry, University of Aveiro, Aveiro 3810-193, Portugal
article info
abstract
Article history:
TiO2 is an important photocatalyst candidate for solar energy or hydrogen energy har-
Received 24 September 2014
vesting. Creation of porous structures or high surface area within TiO2 microspheres may
Received in revised form
potentially address the challenge to improve their efficiency. In present work, we report
13 January 2015
the solegel fabrication of mesoporous TiO2 microspheres that assembled from nano-
Accepted 19 January 2015
particles with the assistance of gelatin template. The phase structure, morphology, and
Available online xxx
mesoporous characteristics were analyzed by X-ray diffraction, transmission electron
Keywords:
nanoparticles (~10e20 nm) were achieved to assemble TiO2 microspheres with diameters
Anatase TiO2 nanoparticle
of 0.2e0.5 mm, which yielded a typical type-IV BET isotherm curve (N2 hysteresis loop) with
microscopy, and BET measurements. Particularly, the gelatin-assisted fabricated TiO2
Microspheres
a large surface area of 98.3 m2/g and a small pore size of 11.9 nm. A simplified model was
Gelatin
proposed to investigate the effect mechanism of gelatin on the formation of TiO2 meso-
Solegel
porous microspheres. The room temperature pressure-dependent hydrogen evolution of
Hydrogen storage
the gelatin-assisted fabricated TiO2 nanostructures has also been investigated, suggesting that gelatin favors high surface area and improves the hydrogen storage performance, and the achieved TiO2 microspheres could be potential candidates to be utilized as the photocatalyst for hydrogen evolution. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction TiO2 has been widely investigated as a model metal oxide, with its intriguing chemical and physical properties, in both fundamental science and technological applications.
Commercially, TiO2 is used in numerous applications, such as toxic materials conversion, solar cells, air purifying, and reusability for environmental applications [1,2], etc. Currently, nanostructured TiO2 is also considered to be a promising photocatalyst for hydrogen production, since it exhibits superior photocatalytic activity compared to traditional bulk
* Corresponding author. E-mail address: [email protected] (J. Xiao). http://dx.doi.org/10.1016/j.ijhydene.2015.01.102 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e6
materials [3,4]. There is a growing trend to integrate specialized nanostructures into photocatalyst, such as mesoporous microspheres with a submicrometer size, nanotube/nanowire arrays, in order to enhance energy harvesting and storage, and to address new challenges in hydrogen production or other energy applications [5e7]. Submicrometer mesoporous microspheres possess high surface area, low density, delivering ability, surface permeability and high harvesting capacities, which enhances energy conversion efficiency and photocatalytic activity of TiO2 [8,9]. For hydrogen energy application, an important factor influencing hydrogen evolution in porous materials is the value of grain surface area; the larger the surface area, the higher the hydrogen adsorption rate. Therefore, facile ways for the formation of mesoporous TiO2 microspheres are quite attractive, since these microspheres had significantly superior specific activity per surface area in compared to a commercial reference sample [10]. In past decades, great progress has been made to fabricate TiO2 mesoporous microspheres with varied structural features [10e13]; among which, a template assisted approach is most commonly used, where soft materials such as ionic or nonionic surfactants, polymers, or organic ligands have been utilized as structural guiding matters [14,15]. In the present work, we propose a gelatin template assisted solegel approach for the synthesis of TiO2 mesoporous microspheres. Gelatin nowadays has been widely used as a surfactant [16,17], for providing a high surface density of anionic functional groups when it derived from collagen by thermal, physical or chemical degradation, to control the grain growth in the fabrication of advanced functional materials. The influence of gelatin on the formation of TiO2 mesoporous microspheres and the improved hydrogen storage properties will also be explored.
Experimental procedure Materials and reagents The chemicals reagents and raw materials used in this work included tetra-n-butyl titanate (C16H36O4Ti, Chengdu Kelong Chemical Reagent Factory, China), ethylalcohol (CH3CH2OH, Shanghai Ling Feng Chemical Reagent Co., Ltd., China), glacial acetic acid (CH3COOH, Nanjing Chemical Reagent Factory, China), gelatin (C102H151O39N31, Sinopharm Chemical Reagent Co., Ltd., China). All the reagents were of analytic grade and used without further purification.
Preparation of TiO2 mesoporous microspheres In a typical synthesis, the titania precursor solution was prepared by dissolving 10 ml tetra-n-butyl titanate into 50 ml absolute alcohol. The gelatin solution was prepared by dissolving 5 g gelatin in 100 ml absolute ethylalcohol. A uniform sol was formed by adding the gelatin solution into the precursor solution under vigorous mixing until pH was around 3. After being aged for at least 24 h, the sol was dried at 50 C in order to vaporize water and gain the gel. The dried gels were then calcined at 450 C in air for 3 h for crystallization. For
comparison, non-gelatin-aids TiO2 sample preparations have also been carried out via similar route.
Characterization for the structure and morphology Structural features of the prepared titanium dioxide microspheres have been determined by X-ray Diffraction (XRD) on a Shimadzu XD-3A diffractometer operating at 40 kV and 30 mA ˚ ). All data was using a Cu Ka radiation source (l ¼ 1.5418 A worked out using Jade X-ray analysis software package. The morphology of the microspheres was observed by using a Tecnai G2 transmission electron microscope (TEM) operating at 200 kV. Their thermal stability was also investigated by the thermogravimetric analysis (TGA, mode of PE Diamond USA).
Investigation for the potential hydrogen storage performance Specific surface area determinations of samples were carried out by N2 adsorption at the boiling point of liquid nitrogen using a BELSORP II instrument. The surface area was calculated using the BrunauereEmmetteTeller (BET) equation. Pore-size distributions were calculated by the BarretteJoyereHalenda (BJH) method using the adsorption branch of the isotherm curves. For primarily disclosing the effect of gelatin additive (respectively gelatin additive of 1%, 2%, and 5%) on the hydrogen evolution behavior of TiO2 microspheres, the hydrogen adsorption measurements were performed at room temperature with the volumetric technique [18], using an inhouse built Sieverts-type apparatus. Before the adsorption measurement, each sample was subjected to at least 10 h of a dynamic vacuum of 102 mbar at room temperature. The sample chamber void volume was measured by expansions of helium gas, also at room temperature. The adsorption isotherms were obtained by cumulative expansions from the calibrated volume up to an equilibrium pressure of about 100 bar, using the BenedicteWebbeRubin equation of state parameters of each gas in the calculations.
Results and discussion XRD analysis Fig. 1 shows the XRD patterns of the prepared TiO2 samples calcined at 450 C for 2 h. The X-ray diffraction peaks around 25.6 , 37.9 , 48.0 , 54.7 , and 63.1 are in good agreement with the standard spectrum of anatase TiO2 (JCPDS, card no: 21e1272), which indicates that the anatase TiO2 with superior crystallinity and high phase purity can be solegel derived with or without the assistance of gelatin. However, the gelatinassisted sample exhibits broader diffractions peaks that being related to smaller crystalline sizes. Table 1 shows the estimated average crystallite sizes of TiO2 based on the (101) peaks via the Scherer's equation, also indicating the gelatinassisted strategy significantly decreases the crystallite sizes. Fig. 2 shows the TG curves of TiO2 microspheres with different additive of Gelatin. It can be seen that all the materials exhibit a nearly linear thermal gravimetric behavior, and there is no
Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e6
Fig. 1 e XRD patterns of samples calcinated at 450 C for 2 h (a) non-gelatin-assisted TiO2 samples and (b) gelatinassisted TiO2 samples.
evident peak of weight loss on the TGA curves, which indicate these microspheres present good thermal stability.
TEM analysis Fig. 3 shows the TEM morphology of TiO2 samples. The TiO2 particles with a size of about 50e100 nm can be observed in non-gelatin-assisted samples (Fig. 3a), which show smooth surfaces and octahedrite anatase single crystallites (magnification in Fig. 3b) that are similar to the observation of Mills and Le Hunte [19]. Fig. 3c shows the TEM morphology of the gelatin-assisted synthesized TiO2 spherical structures with a size of 200e500 nm. The magnification images shown in Fig. 3d clearly indicate the TiO2 microspheres are formed by aggregation of individual fine nanosize crystallites (~10e20 nm in size), which is consistent with the results estimated from the peak broadening of the XRD spectra. Thus, a two-level self-assembled hierarchical structure in the gelatin-assisted produced TiO2 materials can be evidently observed, in which large-scale 0.2e0.5 mm spherical structures were grouped by the 10e20 nm nano-scale TiO2 particles.
The formation mechanism of TiO2 microspheres A mechanism schematically illustrated in Fig. 4 is proposed to describe the possible formation process of the TEM observed hierarchical TiO2 microsphere structures [12,20]. Firstly, although gelatin chains may act as seeds for promoting titania nucleation in the alcoholysis process of tetrabutoxytitanium [8], gelatin provides plenty of anionic functional groups being easy to coordinate Ti4þ via hydrogen bonds, which retards
Table 1 e The crystallite sizes of TiO2 nanoparticles. Sample Non-gelatin-aids TiO2 Gelatin-aids TiO2
Phase
Crystallite size (nm)
Anatase Anatase
74.9 ± 0.6 14.3 ± 0.9
3
Fig. 2 e TGA curves of the 450 C-calcinated TiO2 microspheres with different additive of gelatin, with a measurement temperature range from room temperature to 450 C.
alcoholysis process and restrains growth of crystals [16,21]. Thus as shown in Fig. 4(a), titania crystallite nanoparticles grow with a small rate and gradually assembly on the gelatin chains, forming a Ti/gelatin composite structure [22]. During drying process, these composites may form secondary spherical particles through the volatilization of dissolvent. However, the nanoparticles do not aggregated closely because the gelatin can be assumed to lead to a coating of nanoparticle surfaces (Fig. 4b). Finally, a stable microsphere structure is eventually formed by the compaction and annealing of TiO2 nanoparticles with pyrolysis of gelatin during the calcination process (Fig. 4c). As a result, the introduction of gelatin is crucial to obtain a micro-spherical group of nanoparticles.
Nitrogen adsorption study The microstructure characteristics of the TiO2 samples were further confirmed by BrunauereEmmetteTeller (BET) measurements. Fig. 5 shows the N2 adsorptionedesorption isotherm for the gelatin-aids nanocrystalline TiO2. The N2 hysteresis loop is a typical type-IV isotherm curve in the range of 0.6e0.85 P/P0, which indicates the presence of mesoporous materials according to IUPAC classification [23]. The BarretteJoynereHalenda (BJH) analysis also shows that the surface area of non-gelatin-aids sample is 19.7 m2/g; while the gelatin-assisted TiO2 microspheres have a higher surface area (98.3 m2/g) and pores with the diameter about 11.9 nm (as shown in the insert figure of Fig. 5, the BJH pore size distribution). Wang [24] also proposed a simple model to calculate the surface area for the microspheres. Assuming particles to be spherical for simplicity, the surface area (S) can be also calculated based on the following equation: S¼
6 rD
where r ¼ 3.89 g/cm3 is the density of anatase TiO2, and D is the particle size. As TEM measurements determined the
Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e6
Fig. 3 e TEM images of titania dried gel calcinated at 450 C for 2 h: (a) and (b) non-gelatin assisted TiO2 samples; (c), (d) gelatin assisted TiO2 samples.
average particle size of two samples is 74.9 nm and 14.3 nm respectively, one can calculate the surface areas of them are 21 m2/g and 109 m2/g, which are reasonably consistent with the results of BET surface areas measurements. Combined with these, we conclude that the gelatin assistance favors TiO2 nanoparticles to form a mesoporous microsphere and leads to a large specific surface area [25e27] for exhibiting much higher photocatalytic activity.
Hydrogen adsorption study High-pressure H2 adsorption measurements were performed at room temperature [18,28]. Fig. 6 summarizes the hydrogen adsorption results of 3 gelatin-aids TiO2 microsphere samples (respectively with the additive of gelatin of 1%, 2%, and 5%). As
shown in Fig. 5, 1% gelatin-assisted TiO2 microspheres had a H2 uptake or adsorption of about 3 mol % at room temperature and 90 bars. With the increase of gelatin additive, TiO2 microspheres exhibit the markedly increased H2 storage capacities, especially at high pressures; with a 5% gelatin additive, the sample exhibits a much higher hydrogen uptake of 6 mol % up to 90 bars. These results indicated that the gelatin assistance was crucially important for achieving a high hydrogen adsorption capacity for TiO2 microspheres. We could certainly and simply ascribe these adsorption isotherms to the different grain surface areas, since the adsorbed quantity might be related primarily to the available free surface area; furthermore, the fact that additive of gelatin promoting the high surface areas of TiO2 microspheres, may possibly modify the oxidation state and control the porosity
Fig. 4 e Schematic view of proposed formation mechanism of porous TiO2 microspheres. Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e6
Fig. 5 e N2 adsorptionedesorption isotherm curve and BJH pore size distribution curve (insert; bottom right) of the TiO2 spheres prepared by using gelatin.
5
mesoporous microspheres. The XRD and TEM results reveal that the assistance of gelatin altered the morphology of products instead of their phase composition and purity. The resulting products exhibited a hierarchical structure, i.e., TiO2 nanoparticles with an average crystal size of 10e20 nm can assemble into mesoporous microspheres with diameters of 0.2e0.5 mm. BarretteJoynereHalenda analysis proved the mesoporous characteristics of the product, with a surface area and pore size of 98.3 m2/g and 11.9 nm, respectively. The high pressure hydrogen adsorption evolution measured at room temperature discloses that gelatin template can significantly enhance the hydrogen adsorption capacity and hydrogen storage performance of TiO2 microspheres, which could be prospective candidates for utilization in hydrogen storage and generation. The current work may possibly provide an inspiration for further developing novel advanced nanomaterials within the topic of hydrogen energy harvesting.
Acknowledgments
feature, which is important to realize adsorption of hydrogen (H2 molecules) by chemical pathways (such as the Kubas interaction), although anatase or rutile are generally weak in hydrogen storage [29]. Through the XRD and TEM observation, we also noticed that with the assistance of gelatin, the fabricated TiO2 microspheres show smaller grain size, larger BET surface area and much higher photocatalytic activity, confidently contributing to the substantially increase of H2 adsorption. The mechanism of this adsorption is still under further investigation and optimization.
Conclusions In summary, we have proposed a simple and facile solegel route using gelatin as a template to fabricate anatase TiO2
Fig. 6 e High-pressure hydrogen isotherms at room temperature for gelatin-aids TiO2 microsphere samples (respectively with the additive of gelatin of 1%, 2%, and 5%).
This work was supported by the projects of National Natural Science Foundation of China (No. 51245010, No. 51002079, and No. 51472123), the Portugal FCT project of PEst-C/FIC/UI0036/ 2014 and PTDC/FIS/116146/2009.
references
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Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
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[21] Liu X, Wang H, Chen D, Wang Y, Lu L, Wang X. Study of nanocrystalline TiO2 prepared with raw and modified gelatin dispersants [J]. J Appl Polym Sci 1999;73(13):2569e74. [22] Sangeetha K, Thamizhavel A, Girija EK. Effect of gelatin on the in situ formation of alginate/hydroxyapatitenanocomposite [J]. Mater Lett 2013;91(15):27e30. [23] Sing KSW. Reporting physisorption data for gas/solid system [M]. Pure Appl.Chem. 1982;54(11):2201e18. [24] Wang J, Zhou Y, Hu Y, Hayre R, Shao Z. Facile synthesis of nanocrystalline TiO2 mesoporous microspheres for Lithiumion batteries [J]. J Phys Chem C 2011;115(5):2529e36. [25] Jing J, Feng J, Li W, Wang Y, Lu L, Wang X. Low-temperature synthesis of water-dispersible anatase titanium dioxide nanoparticles for photocatalysis [J]. J Colloid Interface Sci 2013;396(15):90e4. [26] Chen Y, Dionysiou DD. Bimodal mesoporous TiO2-P25 composite thick films with high photocatalytic activity and improved structural integrity [J]. Appl Catal B: Environ 2008;80(1e2):147e55. [27] Chen Y, Dionysiou DD. A comparative study on physicochemical properties and photocatalytic behavior of macroporous TiO2-P25 composite films and macroporous TiO2 films coated on stainless steel substrate [J]. Appl Catal A General 2007;317(1):129e37. [28] Krishna Rahul, Titus Elby, Salimian Maryam, Okhay Olena, Rajendran Sivakumar, Rajkumar Ananth, et al. Hydrogen storage for energy application. Hydrogen Storage INTECH 2012:243e66 [Chapter 10]. [29] Hoang Tuan KA, Antonelli David M. Exploiting the Kubas interaction in the design of hydrogen storage materials. Adv Mater 2009;21:1787e800.
Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage Bin Liu a,b, Jingzhong Xiao b,*, Lan Xu b, Yijun Yao a,b, Benilde F.O. Costa b, Valdemar F. Domingos b, Edivagner S. Ribeiro b, Fa-Nian Shi c, Kai Zhou a, ~ o b, Joa ~ o M. Gil b Jing Su a, Hongyan Wu a, Kun Zhong a, Jose A. Paixa a
School of Physics and Optoelectronic Engineering, Nanjing University of Information & Technology, Nanjing 210044, China b CEMDRX, Department of Physics, University of Coimbra, Coimbra 3004-516, Portugal c CICECO, Department of Chemistry, University of Aveiro, Aveiro 3810-193, Portugal
article info
abstract
Article history:
TiO2 is an important photocatalyst candidate for solar energy or hydrogen energy har-
Received 24 September 2014
vesting. Creation of porous structures or high surface area within TiO2 microspheres may
Received in revised form
potentially address the challenge to improve their efficiency. In present work, we report
13 January 2015
the solegel fabrication of mesoporous TiO2 microspheres that assembled from nano-
Accepted 19 January 2015
particles with the assistance of gelatin template. The phase structure, morphology, and
Available online xxx
mesoporous characteristics were analyzed by X-ray diffraction, transmission electron
Keywords:
nanoparticles (~10e20 nm) were achieved to assemble TiO2 microspheres with diameters
Anatase TiO2 nanoparticle
of 0.2e0.5 mm, which yielded a typical type-IV BET isotherm curve (N2 hysteresis loop) with
microscopy, and BET measurements. Particularly, the gelatin-assisted fabricated TiO2
Microspheres
a large surface area of 98.3 m2/g and a small pore size of 11.9 nm. A simplified model was
Gelatin
proposed to investigate the effect mechanism of gelatin on the formation of TiO2 meso-
Solegel
porous microspheres. The room temperature pressure-dependent hydrogen evolution of
Hydrogen storage
the gelatin-assisted fabricated TiO2 nanostructures has also been investigated, suggesting that gelatin favors high surface area and improves the hydrogen storage performance, and the achieved TiO2 microspheres could be potential candidates to be utilized as the photocatalyst for hydrogen evolution. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction TiO2 has been widely investigated as a model metal oxide, with its intriguing chemical and physical properties, in both fundamental science and technological applications.
Commercially, TiO2 is used in numerous applications, such as toxic materials conversion, solar cells, air purifying, and reusability for environmental applications [1,2], etc. Currently, nanostructured TiO2 is also considered to be a promising photocatalyst for hydrogen production, since it exhibits superior photocatalytic activity compared to traditional bulk
* Corresponding author. E-mail address: [email protected] (J. Xiao). http://dx.doi.org/10.1016/j.ijhydene.2015.01.102 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e6
materials [3,4]. There is a growing trend to integrate specialized nanostructures into photocatalyst, such as mesoporous microspheres with a submicrometer size, nanotube/nanowire arrays, in order to enhance energy harvesting and storage, and to address new challenges in hydrogen production or other energy applications [5e7]. Submicrometer mesoporous microspheres possess high surface area, low density, delivering ability, surface permeability and high harvesting capacities, which enhances energy conversion efficiency and photocatalytic activity of TiO2 [8,9]. For hydrogen energy application, an important factor influencing hydrogen evolution in porous materials is the value of grain surface area; the larger the surface area, the higher the hydrogen adsorption rate. Therefore, facile ways for the formation of mesoporous TiO2 microspheres are quite attractive, since these microspheres had significantly superior specific activity per surface area in compared to a commercial reference sample [10]. In past decades, great progress has been made to fabricate TiO2 mesoporous microspheres with varied structural features [10e13]; among which, a template assisted approach is most commonly used, where soft materials such as ionic or nonionic surfactants, polymers, or organic ligands have been utilized as structural guiding matters [14,15]. In the present work, we propose a gelatin template assisted solegel approach for the synthesis of TiO2 mesoporous microspheres. Gelatin nowadays has been widely used as a surfactant [16,17], for providing a high surface density of anionic functional groups when it derived from collagen by thermal, physical or chemical degradation, to control the grain growth in the fabrication of advanced functional materials. The influence of gelatin on the formation of TiO2 mesoporous microspheres and the improved hydrogen storage properties will also be explored.
Experimental procedure Materials and reagents The chemicals reagents and raw materials used in this work included tetra-n-butyl titanate (C16H36O4Ti, Chengdu Kelong Chemical Reagent Factory, China), ethylalcohol (CH3CH2OH, Shanghai Ling Feng Chemical Reagent Co., Ltd., China), glacial acetic acid (CH3COOH, Nanjing Chemical Reagent Factory, China), gelatin (C102H151O39N31, Sinopharm Chemical Reagent Co., Ltd., China). All the reagents were of analytic grade and used without further purification.
Preparation of TiO2 mesoporous microspheres In a typical synthesis, the titania precursor solution was prepared by dissolving 10 ml tetra-n-butyl titanate into 50 ml absolute alcohol. The gelatin solution was prepared by dissolving 5 g gelatin in 100 ml absolute ethylalcohol. A uniform sol was formed by adding the gelatin solution into the precursor solution under vigorous mixing until pH was around 3. After being aged for at least 24 h, the sol was dried at 50 C in order to vaporize water and gain the gel. The dried gels were then calcined at 450 C in air for 3 h for crystallization. For
comparison, non-gelatin-aids TiO2 sample preparations have also been carried out via similar route.
Characterization for the structure and morphology Structural features of the prepared titanium dioxide microspheres have been determined by X-ray Diffraction (XRD) on a Shimadzu XD-3A diffractometer operating at 40 kV and 30 mA ˚ ). All data was using a Cu Ka radiation source (l ¼ 1.5418 A worked out using Jade X-ray analysis software package. The morphology of the microspheres was observed by using a Tecnai G2 transmission electron microscope (TEM) operating at 200 kV. Their thermal stability was also investigated by the thermogravimetric analysis (TGA, mode of PE Diamond USA).
Investigation for the potential hydrogen storage performance Specific surface area determinations of samples were carried out by N2 adsorption at the boiling point of liquid nitrogen using a BELSORP II instrument. The surface area was calculated using the BrunauereEmmetteTeller (BET) equation. Pore-size distributions were calculated by the BarretteJoyereHalenda (BJH) method using the adsorption branch of the isotherm curves. For primarily disclosing the effect of gelatin additive (respectively gelatin additive of 1%, 2%, and 5%) on the hydrogen evolution behavior of TiO2 microspheres, the hydrogen adsorption measurements were performed at room temperature with the volumetric technique [18], using an inhouse built Sieverts-type apparatus. Before the adsorption measurement, each sample was subjected to at least 10 h of a dynamic vacuum of 102 mbar at room temperature. The sample chamber void volume was measured by expansions of helium gas, also at room temperature. The adsorption isotherms were obtained by cumulative expansions from the calibrated volume up to an equilibrium pressure of about 100 bar, using the BenedicteWebbeRubin equation of state parameters of each gas in the calculations.
Results and discussion XRD analysis Fig. 1 shows the XRD patterns of the prepared TiO2 samples calcined at 450 C for 2 h. The X-ray diffraction peaks around 25.6 , 37.9 , 48.0 , 54.7 , and 63.1 are in good agreement with the standard spectrum of anatase TiO2 (JCPDS, card no: 21e1272), which indicates that the anatase TiO2 with superior crystallinity and high phase purity can be solegel derived with or without the assistance of gelatin. However, the gelatinassisted sample exhibits broader diffractions peaks that being related to smaller crystalline sizes. Table 1 shows the estimated average crystallite sizes of TiO2 based on the (101) peaks via the Scherer's equation, also indicating the gelatinassisted strategy significantly decreases the crystallite sizes. Fig. 2 shows the TG curves of TiO2 microspheres with different additive of Gelatin. It can be seen that all the materials exhibit a nearly linear thermal gravimetric behavior, and there is no
Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e6
Fig. 1 e XRD patterns of samples calcinated at 450 C for 2 h (a) non-gelatin-assisted TiO2 samples and (b) gelatinassisted TiO2 samples.
evident peak of weight loss on the TGA curves, which indicate these microspheres present good thermal stability.
TEM analysis Fig. 3 shows the TEM morphology of TiO2 samples. The TiO2 particles with a size of about 50e100 nm can be observed in non-gelatin-assisted samples (Fig. 3a), which show smooth surfaces and octahedrite anatase single crystallites (magnification in Fig. 3b) that are similar to the observation of Mills and Le Hunte [19]. Fig. 3c shows the TEM morphology of the gelatin-assisted synthesized TiO2 spherical structures with a size of 200e500 nm. The magnification images shown in Fig. 3d clearly indicate the TiO2 microspheres are formed by aggregation of individual fine nanosize crystallites (~10e20 nm in size), which is consistent with the results estimated from the peak broadening of the XRD spectra. Thus, a two-level self-assembled hierarchical structure in the gelatin-assisted produced TiO2 materials can be evidently observed, in which large-scale 0.2e0.5 mm spherical structures were grouped by the 10e20 nm nano-scale TiO2 particles.
The formation mechanism of TiO2 microspheres A mechanism schematically illustrated in Fig. 4 is proposed to describe the possible formation process of the TEM observed hierarchical TiO2 microsphere structures [12,20]. Firstly, although gelatin chains may act as seeds for promoting titania nucleation in the alcoholysis process of tetrabutoxytitanium [8], gelatin provides plenty of anionic functional groups being easy to coordinate Ti4þ via hydrogen bonds, which retards
Table 1 e The crystallite sizes of TiO2 nanoparticles. Sample Non-gelatin-aids TiO2 Gelatin-aids TiO2
Phase
Crystallite size (nm)
Anatase Anatase
74.9 ± 0.6 14.3 ± 0.9
3
Fig. 2 e TGA curves of the 450 C-calcinated TiO2 microspheres with different additive of gelatin, with a measurement temperature range from room temperature to 450 C.
alcoholysis process and restrains growth of crystals [16,21]. Thus as shown in Fig. 4(a), titania crystallite nanoparticles grow with a small rate and gradually assembly on the gelatin chains, forming a Ti/gelatin composite structure [22]. During drying process, these composites may form secondary spherical particles through the volatilization of dissolvent. However, the nanoparticles do not aggregated closely because the gelatin can be assumed to lead to a coating of nanoparticle surfaces (Fig. 4b). Finally, a stable microsphere structure is eventually formed by the compaction and annealing of TiO2 nanoparticles with pyrolysis of gelatin during the calcination process (Fig. 4c). As a result, the introduction of gelatin is crucial to obtain a micro-spherical group of nanoparticles.
Nitrogen adsorption study The microstructure characteristics of the TiO2 samples were further confirmed by BrunauereEmmetteTeller (BET) measurements. Fig. 5 shows the N2 adsorptionedesorption isotherm for the gelatin-aids nanocrystalline TiO2. The N2 hysteresis loop is a typical type-IV isotherm curve in the range of 0.6e0.85 P/P0, which indicates the presence of mesoporous materials according to IUPAC classification [23]. The BarretteJoynereHalenda (BJH) analysis also shows that the surface area of non-gelatin-aids sample is 19.7 m2/g; while the gelatin-assisted TiO2 microspheres have a higher surface area (98.3 m2/g) and pores with the diameter about 11.9 nm (as shown in the insert figure of Fig. 5, the BJH pore size distribution). Wang [24] also proposed a simple model to calculate the surface area for the microspheres. Assuming particles to be spherical for simplicity, the surface area (S) can be also calculated based on the following equation: S¼
6 rD
where r ¼ 3.89 g/cm3 is the density of anatase TiO2, and D is the particle size. As TEM measurements determined the
Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
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Fig. 3 e TEM images of titania dried gel calcinated at 450 C for 2 h: (a) and (b) non-gelatin assisted TiO2 samples; (c), (d) gelatin assisted TiO2 samples.
average particle size of two samples is 74.9 nm and 14.3 nm respectively, one can calculate the surface areas of them are 21 m2/g and 109 m2/g, which are reasonably consistent with the results of BET surface areas measurements. Combined with these, we conclude that the gelatin assistance favors TiO2 nanoparticles to form a mesoporous microsphere and leads to a large specific surface area [25e27] for exhibiting much higher photocatalytic activity.
Hydrogen adsorption study High-pressure H2 adsorption measurements were performed at room temperature [18,28]. Fig. 6 summarizes the hydrogen adsorption results of 3 gelatin-aids TiO2 microsphere samples (respectively with the additive of gelatin of 1%, 2%, and 5%). As
shown in Fig. 5, 1% gelatin-assisted TiO2 microspheres had a H2 uptake or adsorption of about 3 mol % at room temperature and 90 bars. With the increase of gelatin additive, TiO2 microspheres exhibit the markedly increased H2 storage capacities, especially at high pressures; with a 5% gelatin additive, the sample exhibits a much higher hydrogen uptake of 6 mol % up to 90 bars. These results indicated that the gelatin assistance was crucially important for achieving a high hydrogen adsorption capacity for TiO2 microspheres. We could certainly and simply ascribe these adsorption isotherms to the different grain surface areas, since the adsorbed quantity might be related primarily to the available free surface area; furthermore, the fact that additive of gelatin promoting the high surface areas of TiO2 microspheres, may possibly modify the oxidation state and control the porosity
Fig. 4 e Schematic view of proposed formation mechanism of porous TiO2 microspheres. Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e6
Fig. 5 e N2 adsorptionedesorption isotherm curve and BJH pore size distribution curve (insert; bottom right) of the TiO2 spheres prepared by using gelatin.
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mesoporous microspheres. The XRD and TEM results reveal that the assistance of gelatin altered the morphology of products instead of their phase composition and purity. The resulting products exhibited a hierarchical structure, i.e., TiO2 nanoparticles with an average crystal size of 10e20 nm can assemble into mesoporous microspheres with diameters of 0.2e0.5 mm. BarretteJoynereHalenda analysis proved the mesoporous characteristics of the product, with a surface area and pore size of 98.3 m2/g and 11.9 nm, respectively. The high pressure hydrogen adsorption evolution measured at room temperature discloses that gelatin template can significantly enhance the hydrogen adsorption capacity and hydrogen storage performance of TiO2 microspheres, which could be prospective candidates for utilization in hydrogen storage and generation. The current work may possibly provide an inspiration for further developing novel advanced nanomaterials within the topic of hydrogen energy harvesting.
Acknowledgments
feature, which is important to realize adsorption of hydrogen (H2 molecules) by chemical pathways (such as the Kubas interaction), although anatase or rutile are generally weak in hydrogen storage [29]. Through the XRD and TEM observation, we also noticed that with the assistance of gelatin, the fabricated TiO2 microspheres show smaller grain size, larger BET surface area and much higher photocatalytic activity, confidently contributing to the substantially increase of H2 adsorption. The mechanism of this adsorption is still under further investigation and optimization.
Conclusions In summary, we have proposed a simple and facile solegel route using gelatin as a template to fabricate anatase TiO2
Fig. 6 e High-pressure hydrogen isotherms at room temperature for gelatin-aids TiO2 microsphere samples (respectively with the additive of gelatin of 1%, 2%, and 5%).
This work was supported by the projects of National Natural Science Foundation of China (No. 51245010, No. 51002079, and No. 51472123), the Portugal FCT project of PEst-C/FIC/UI0036/ 2014 and PTDC/FIS/116146/2009.
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Please cite this article in press as: Liu B, et al., Gelatin-assisted solegel derived TiO2 microspheres for hydrogen storage, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.102