View Online / Journal Homepage / Table of Contents for this issue Dynamic Article Links CrystEngComm Cite this: CrystEngComm, 2012, 14, 4274–4278 COMMUNICATION www.rsc.org/crystengcomm Fluoride-free self-templated synthesis of hollow TiO2 nanostructures for hydrogen evolution{ Downloaded by Duke University on 19 September 2012 Published on 04 May 2012 on http://pubs.rsc.org | doi:10.1039/C2CE25375E Quang Duc Truong,*ab Thanh Son Lec and Huu Thu Hoac Received 16th March 2012, Accepted 26th March 2012 DOI: 10.1039/c2ce25375e Hollow TiO2 nanostructures have been fabricated by a facile self-templated hydrothermal synthesis without the assistance of any fluoride compound The hollow nanostructures with diameters of approximately mm are composed of mesoporous shell walls of 200 nm in thickness On the basis of the investigation result, it was found that the inside-out Ostwald ripening was responsible for the hollowing process in which the evacuation of solid cores was mainly driven by the minimization of overall surface energy The hollow nanostructures exhibited enhanced photocatalytic activity by means of hydrogen evolution from methanol solution which is attributed to their unique structural features Hollow inorganic nano- and microstructures are particularly important owing to their versatile applications including photocatalysis, sensing, lithium-ion batteries, drug-delivery carrier and solar energy conversion.1 Due to its unique mechanical, optical and chemical properties such as low density, high surface area, effective light-harvesting property, hollow nanostructures usually exhibit a novel or optimal functional performance.2 Therefore, efforts have been devoted for the fabrication of hollow structures with controllable size, shell thickness, wall porosity, and structural features, etc.1–3 Up to now, hollow nanostructures have been successfully synthesized using removable hard templates including polystyrene latex spheres,4,5 silica spheres,6 carbon spheres,7 etc The soft and sacrificial templates such as emulsion droplets,8 micelles/vesicles,9,10 and even gas bubbles,11,12 can also serve as scaffolds to fabricate hollow structures However, the limitations of templating methods such as low yields, labour consuming, lack of structural robustness upon heat treatment, may prevent them from being used for large-scale production Thus, the challenge to explore a facile, rapid, efficient approach for the synthesis of hollow structures still remains a Department of Chemistry, National Tsing Hua University, Hsinchu, 30013, Taiwan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan E-mail: tqduc@mail.tagen.tohoku.ac.jp; Fax: +81-22-217-5651; Tel: +81-22-217-5651 c Department of Chemistry, Vietnam National University, Hanoi, Vietnam { Electronic Supplementary Information (ESI) available See DOI: 10.1039/c2ce25375e/ 4274 | CrystEngComm, 2012, 14, 4274–4278 Recently, interest in the synthesis of hollow nanostructures by self-templated methods has increased dramatically due to their cost-effectiveness and simple implementation In particular, efforts have been devoted to the development of a fabrication method for hollow nanostructures based on unique physical mechanisms such as the Kirkendall effect,13–15 or Ostwald ripening.16 A templatefree synthesis approach based on Ostwald ripening has proved to be an advanced method for practical application due to its efficiency for the construction of various hollow nano-/microstructures covering a wide range of inorganic materials such as metal,16 metal oxide,17–24 metal sulfide,25–27 and other.28,29 Over the past two decades, TiO2 has attracted extraordinary research interest due to its unique intrinsic properties such as strong oxidative ability, stability, efficiency, long-term durability as well as a wide range of industrial applications.30–39 The synthesis of hollow TiO2 nanostructures by a self-templated method is mainly based on Ostwald ripening with a sophisticated solid evacuation process.30–39 This technique, so-called inside-out Ostwald ripening, based on the self-etching or self-transformation of the inner solid core is mediated by a corrosive solution One critical condition of this method is to use fluoride compounds such as TiF4 and NH4F which are able to create HF under hydrothermal reaction for the etching of solid cores.30–39 Therefore, this approach still has many limitations related to the corrosion, toxicity accompanied with low yields, fluoride adsorbed surface, etc Thus, it is remains a challenge to discover a green fluoride-free method for the controlled synthesis of hollow TiO2 structures by a self-templated route Herein, we propose a novel fluoride-free self-templated approach to construct titania hollow nanostructures The hollowing process was directed by inside-out Ostwald ripening without the assistance of any fluoride compound The hollow nanostructures with mesoporous shell features are expected to be superior photocatalysts Herein, we demonstrate an enhancement of the photocatalytic activity of the hollow nanostructures by means of hydrogen evolution from methanol solution which is attributed to their integrated structural properties Hollow TiO2 nanostructures were synthesized by hydrothermal treatment of titanium disulfate (Ti(SO4)2) in the presence of a small amount of hydrogen peroxide (H2O2) Typically, Ti(SO4)2 solution (1.37 cm3, mmol, 24%, J.T Baker, Germany) was diluted in 19 cm3 distilled water One cm3 solution consisting of hydrogen peroxide (30%, 10 mmol, J.T Baker, Germany) was dropped slowly into the above solution with continuous stirring to form a This journal is ß The Royal Society of Chemistry 2012 Downloaded by Duke University on 19 September 2012 Published on 04 May 2012 on http://pubs.rsc.org | doi:10.1039/C2CE25375E View Online characteristic red solution The obtained solution at pH 1.0 was transferred to a Teflon-lined stainless-steel autoclave The autoclave was then heated at 473 K for 24 h (unless otherwise stated) for hydrothermal treatment, after which it was allowed to cool to room temperature The resultant powder was separated by centrifugation and washed with distilled water until the pH of the solution became neutral Finally, the obtained specimen was dried at 353 K for day The crystalline phase was characterized using powder X-ray diffraction (XRD; Rigaku D/MAX-IIB, 40 kV and 30 mA) with ˚ ) The morphology was examined Cu-Ka radiation (l = 1.5406 A using scanning/transmission electron microscopy (SEM Hitachi S4700, TEM Jeol-2010) N2 adsorption and desorption isotherms were measured at 77 K (Micromeritics ASAP 2010) to yield Brunauer–Emmett–Teller (BET) specific surface area Photocatalytic hydrogen evolution was carried out in a closed gas circulation system Each sample contained 150 cm3 of aqueous methanol solution of 10/1 water/methanol (v/v) Typically, 0.1 g TiO2 particles were sonicated in 135 ml distilled water for min, then a solution containing mmol H2PtCl6 (1 wt% Pt/TiO2) was injected directly to solution and finally methanol was used to fill to 150 ml Prior to photocatalytic H2 evolution, a mixture of methanol solution, TiO2 and H2PtCl6 solution was degassed by circulation of Ar gas and evacuated for 30 A 300 W Xe lamp employed as the light source was delivered from the top of the cell through a Pyrex window The amount of hydrogen evolved was measured using an online gas chromatograph (GC-8A, Shimadzu, TCD) The experimental setup for photocatalytic production of hydrogen has been described in detail by Kudo et al.40 Fig shows the SEM and TEM images of the synthesized particles From the SEM image in Fig 1a, it is evident that the synthesized particles comprise interconnected self-assembled spheres with an average diameter of mm Fig 1a also displays the hollow nanostructures which are composed of many sphere particles with broken shell walls This image clearly shows the presence of a hollow core structure The high magnification image in Fig 1b reveals that the shell wall with a thickness of ca 200 nm Fig SEM images (a, b, d) and TEM image (c) of TiO2 nanostructures synthesized by hydrothermal treatment of Ti(SO4)2 in the presence of H2O2 Black arrows indicate large crystallites on the outer surface White arrows indicate small crystallites in the inner cores This journal is ß The Royal Society of Chemistry 2012 is composed of assembled spherical particles with porous hierarchical networks The TEM image (Fig 1c) indicates the presence of interconnected hollow nanostructures which are constructed from primary nanoparticles with diameters of about 20 nm From the TEM image in Fig 1c, the bright areas in the center of each sphere can be observed, which is a typical feature of hollow nanostructures The SAED pattern of a sphere shown in the inset of Fig 1c reveals the presence of a polycrystalline anatase phase The crystalline structure of the product was characterized by powder XRD Fig shows the XRD pattern of the synthesized particles which indicates the presence of tetragonal anatase TiO2 (JCPDS 21-1272) as the major phase The diffraction peaks can be indexed to {101}, {004}, {200} and {105} of anatase structure The high porosity of the synthesized nanostructures was further confirmed by N2 gas adsorption analysis and the results are shown in Fig The BET specific surface area is 60 m2 g21 The isotherm curve can be categorized as an IUPAC type III with a hysteresis loop in the range of 0.5–1 P/P0, indicating the presence of mesopores This was also confirmed by the pore-size distribution with a peak mesopore diameter of about 20 nm In order to investigate the formation mechanism of the hollow nanostructures, the time-dependent experiments were performed Fig displays the TEM images of products obtained by hydrothermal treatment for various ageing times The sample collected after h consists of solid particles (Fig 4a) which were transformed into hollow structures after 10 h treatment (Fig 4b) Extending the ageing time to 24 h, the hollow particles with a uniform interconnected cavity were obtained (Fig 4c), suggesting that the hollowing process has occurred In the present system, titanium disulfate reacted initially with hydrogen peroxide via a ligand-exchange reaction with O222 ion generated under acidic solution.12 The formation of intermediate complex was evident from the characteristic red color of the solution Under hydrothermal conditions, the complex decomposed and crystallized into the anatase form It is well-known that anatase is formed under acidic solutions of sulfuric acid.41 It should be noted that a similar strategy using H2O2 as a coordination agent has been reported for synthesis of TiO2 hollow structures.12 According to their report, the H2O2 amount is about 75 times with respect to titanium which is large enough to produce O2 bubbles to serve as soft templates for the formation of hollow Fig XRD pattern of TiO2 nanostructures synthesized with reaction times of 2, 6, 10, and 24 h, respectively CrystEngComm, 2012, 14, 4274–4278 | 4275 Downloaded by Duke University on 19 September 2012 Published on 04 May 2012 on http://pubs.rsc.org | doi:10.1039/C2CE25375E View Online Fig Nitrogen adsorption–desorption isotherm of TiO2 hollow nanostructures and the BJH pore-size distribution curve (inset) structures In the present case, the H2O2 amount is only times compared to that of titanium which is not sufficient for the production of gas templates To confirm this speculation, thermal treatment at 353 K for 24 h was applied to the mixture of titanium sulfate and hydrogen peroxide to remove excess reagent and O2 bubbles before hydrothermal treatment The obtained red complex solution was then hydrothermally treated as for the typical experiment The resulting powder was found to be composed of hollow nanostructures (Fig S1{) This control experiment indicates that generated O2 is not involved in the formation of the hollow structures We, therefore, reached the conclusion that the hollow nanostructures were formed following an Ostwald ripening mechanism rather than gathering on a soft template According to the above results, a possible mechanism was proposed to illustrate the formation of microspheres and the hollowing process as follows It is well-known that the primary particles preferentially organize on spherical structures to minimize their overall surface energy.30–39 Thus, the preformed primary particles are likely to aggregate to each other to minimize their total surface energy, resulting in the growth of microspheres Besides acting as a coordination agent, H2O2 also played another important role, namely, stabilizing the particle’s interfaces through coordination on them Thus, when O2 was released, the particle interface became vacant and unstable due to the presence of many dangling bonds Consequently, particles have a strong tendency to aggregate into assembled structures for the elimination of dangling interfaces The control experiment also supports this speculation In the absence of H2O2, the monodispersed nanoparticles were obtained under the same hydrothermal treatment (Fig 4, inset) Therefore, it can be concluded that H2O2 is crucial for the aggregation of primary particles into hierarchical structures On the basis of the above investigation, an inside-out Ostwald ripening mechanism is proposed to account for the hollowing process As solid spheres have been formed by the assembly of the primary particles, the surface crystallites have grown into larger and less-soluble crystalline phase due to its full contact with supersaturated solution in the surrounding environment.42 The microscopic observation shows that the crystallites on the outer shells are significantly larger than ones in the inner cores as shown 4276 | CrystEngComm, 2012, 14, 4274–4278 Fig TEM images of particles obtained by hydrothermal treatment with increasing reaction time: (a) h, (b) 10 h, and (c) 24 h in Fig 1d, giving a direct evidence for such growth It is wellknown that smaller particles have a higher surface energy than larger ones Consequently, the particles in the inner sphere possess a strong tendency to dissolve and diffuse out through the shell.30,31 In the next stage, recrystallization on the outer surface may occur because the supersaturation increases in the solution surrounding them.32 Finally, the shell wall increase in the thickness and the cores became depleted to produce hollow interconnected structures The XRD investigation shown in Fig indicates that the crystallinity of the obtained particles is gradually increased with the ageing time The crystallite sizes of the products calculated following the Scherrer equation are also increased These evidences imply that Oswald ripening is the mechanism responsible for the growth of nanostructures in this system In addition, as mentioned above, generated O2 itself could not serve as a scaffold for microsphere formation In contrast, O2 bubbles may act as soft templates to form a narrow pore channel for crystallization of titania.43 As a result, a uniform mesoporous shell wall has been produced together with the hollow nanostructures To further investigate the role of H2O2, a series of control experiments were carried out systematically with tailoring the amount of additive It was found that the amount of hydrogen peroxide strongly affects the formation of the hollow particles The desired crystal shape can be achieved by adding an appropriate amount of hydrogen peroxide (10 mmol) The tailoring of the additive amount has a crucial effect on the morphology of the synthesized particles Particularly, with the addition of a smaller amount of hydrogen peroxide (4–6 mmol), the obtained particles comprise mainly regular agglomerations along with a few hollow nanostructures (Fig S2{) In the present case, H2O2 may not be enough for saturated chelation with Ti4+, thus the growth of microspheres is restrained On the other hand, large amounts of additive were introduced (20–50 mmol), resulting in the formation of regular nanoparticles without any assembly of hierarchical This journal is ß The Royal Society of Chemistry 2012 Downloaded by Duke University on 19 September 2012 Published on 04 May 2012 on http://pubs.rsc.org | doi:10.1039/C2CE25375E View Online structures (Fig S3{) Therefore, it can be concluded that H2O2 plays versatile roles for the self-assembly of microspheres as well as its transformation into hollow nanostructures It should be mentioned that very few fluoride-free self-templated methods for the synthesis of hollow TiO2 nanostructures have been reported.44 The present approach, on the basis of its efficiency and simple implementation, would be of significant advantage in terms of environmental harmony and cost effectiveness for the production of TiO2 hollow structures on a large-scale The photocatalytic activity of the synthesized hollow nanostructures was evaluated by means of the hydrogen evolution reaction The photocatalytic activity of the usual anatase nanoparticles synthesized without H2O2 by the same hydrothermal method was also evaluated as a reference The data for the catalysts is shown in Table and Fig S4.{ Fig shows the hydrogen evolution rate on the synthesized particles It is obvious that the hollow particles exhibit significant improvement of photocatalytic production of hydrogen with a hydrogen evolution rate of 525 mmol h21 which is higher than that on an anatase-type TiO2 ST-21 (Ishihara Sangyo, Co., Ltd., Japan).45 The hydrogen evolution rate on the hollow nanostructures is also two times higher than that on regular nanoparticles despite the lower surface area of the former Recently, Ohtani et al found that large secondary particles composed of large primary particles showed enhanced photocatalytic H2 evolution from aqueous methanol solution.45 In the present study, the primary particle size of anatase hollow structures is larger than anatase nanoparticles as concluded from the measured crystallite sizes (Table 1, Fig S4{) At the same time, the secondary particle size, defined as a volume-average particle size, of interconnected hollow nanostructures is expected to be higher than that of individual monodispersed nanoparticles Thus, the increased photocatalytic activity of hollow nanostructures relative to that of anatase nanoparticles and ST-21 can be explained either by the primary particle size or by its assembled structures Owing to the unique structural features, the hollow porous nanostructures provide hierarchical channel network which may accelerate the reaction velocity Besides that, the presence of an interconnected structure with a continuous conducting pathway could also enhance the photogenerated electron-hole separation and charge transport, which is an important factor for the remarkable photocatalytic activity of the hollow nanostructures Furthermore, a small percentage of rutile was found on the synthesized sample which may lead to a better charge carrier separation and consequently improve the photocatalytic activity In addition, the strong adsorption of sulfate ions on the surface of Table Properties and photocatalytic performances of the TiO2 catalysts Sample Crystallite sizea (nm) BET (m2 g21) Evolution rate (mmol h21) ST 01 ST 21 Nanoparticles Hollow structures — — 10.3 15.8 298 67 83 60 704b 465b 250 525 a Estimated according to the Scherrer equation: D(hkl) = (K l)/(b cosh) where K is the shape factor, l the wavelength of the Cu-Ka radiation, b the full width at half-maximum (fwhm) of the (hkl) peak, and h the diffraction angle b Reference 45 This journal is ß The Royal Society of Chemistry 2012 Fig Hydrogen evolution rate on the synthesized particles: (1) nanoparticles obtained without H2O2, (2) hollow particles prepared with H2O2 the nanoparticles synthesized without H2O2 might also be a reason for their low photocatalytic performance.46 The hollow nanostructures may avoid this effect due to the complete ligandexchange reaction in the early stage of the reaction, thus, their photocatalytic activity is retained Regarding photocatalytic hydrogen evolution, current research has been focused on the effect of the crystal facet and the crystalline phase on the evolution rate.47–50 The effect of architectures has not been considered appreciably.51 The present work suggests that the hierarchical structures with interconnected features could also enhance the hydrogen production capacity Conclusions In summary, we have developed a facile green fluoride-free approach for the fabrication of hollow TiO2 nanostructures Coordination agents associated with the minimization of surface energy lead to the self-assembly of the primary nanoparticles into microsphere structures, while inside-out Ostwald ripening is responsible for the hollowing process The hollow nanostructures exhibited enhanced photocatalytic activity in terms of hydrogen evolution which is attributed to their unique structural features Because of its efficiency and simple implementation, the proposed route could be significantly advantageous in terms of environmental harmony and cost effectiveness and its potential for largescale production Acknowledgements Authors thank Prof Masato Kakihana and Dr Hideki Kato at Tohoku University for providing experimental facilities of hydrogen evolution reaction Author Truong is thankful to MEXT, Japan and National Science Council, Taiwan for CrystEngComm, 2012, 14, 4274–4278 | 4277 View Online financial support for this research We appreciate Dr Sudeshna Ray at Tohoku University for reading the proof Downloaded by Duke University on 19 September 2012 Published on 04 May 2012 on http://pubs.rsc.org | doi:10.1039/C2CE25375E References X W Lou, L A Archer and Z C Yang, Adv Mater., 2008, 20, 3987 Q Zhang, W S Wang, J Goebl and Y D Yin, Nano Today, 2009, 4, 494 J Liu, F Liu, K Gao, J S Wu and D.F Xue, 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12, 14, 427 4– 427 8 | 427 5 Downloaded... improvement of photocatalytic production of hydrogen with a hydrogen evolution rate of 525 mmol h21 which is higher than that on an anatase-type TiO2 ST -21 (Ishihara Sangyo, Co., Ltd., Japan).45 The hydrogen. .. Therefore, it can be concluded that H2O2 plays versatile roles for the self-assembly of microspheres as well as its transformation into hollow nanostructures It should be mentioned that very few fluoride-free