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Hierarchical structures of rutile exposing high-index facets
Journal of Crystal Growth 418 (2015) 86–91
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Hierarchical structures of rutile exposing high-index facets Quang Duc Truong, Hideki Kato, Makoto Kobayashi, Masato Kakihana n Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 6 October 2014 Received in revised form 23 December 2014 Accepted 15 February 2015 Communicated by: D.P. Norton Available online 24 February 2015
Recently, shape-controlled synthesis of crystals exposing high-index facets has attracted much research interest due to their importance for both fundamental studies and technological applications. Herein, crystals of rutile-type TiO2 with hierarchical structures exposing high-index facets have been synthesized by a facile hydrothermal method using water-soluble titanium complex as a precursor and picolinic acid as structure-directing and shape-controlling agents. The synthesized particles were composed of several branches of pyramidal crystals with relatively smooth surface. On the basis of investigation results, it was speculated that the mutual π-stacking and selective adsorption of picolinic acid on specific {111} facets resulted in the formation of rutile crystals bound by high-index surfaces such as {331}. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Interfaces A1. Nanostructure A1. Surface structure A2. Hydrothermal crystal growth B1. Oxides
1. Introduction Recently, a special effort has been devoted to the shape-controlled synthesis of crystals with exposed high-index facets due to their importance for both fundamental studies and technological applications [1–3]. Owing to their high densities of atom, steps and kinks, high-index facets usually exhibit much higher chemical activity than low-index ones. The high-index facets are gradually diminished during the crystal growth due to their high surface energy. Therefore, it is always challenging to control the growth of nanocrystals bound by high-index facets. The controllable synthesis of titanium dioxide (TiO2) is of particular importance due to its versatile industrial applications including photocatalysis, sensors, lithium-ion batteries, drug-delivery carriers, and dye-sensitized solar cells [4]. Anatase TiO2 single crystals have attracted extraordinary research interest since anatase crystals with dominant high-energy facets were successfully synthesized by Lu et al. [5–12]. The synthesis of anatase TiO2 with exposed {105} [13], {103} [14], {401}[15], {201} [16], and {111} [17] high-index facets has also been reported. In contrast to anatase TiO2, there is very few report on rutile TiO2 crystals with exposed such high-energy facets [18]. Rutile is a thermodynamically stable phase with a smaller band gap than anatase phase. In some cases, rutile has been found to be more effective for photocatalysis than anatase [19– 21]. Therefore, rutile nanocrystals with exposed high-index facets are highly desirable for catalytic application. Shi and Wang have recently succeeded in growth of the rutile nanowires with exposed high-index
n
Corresponding author. Tel./fax: þ 81 22 217 5649. E-mail address: [email protected] (M. Kakihana).
http://dx.doi.org/10.1016/j.jcrysgro.2015.02.056 0022-0248/& 2015 Elsevier B.V. All rights reserved.
surfaces by a surface reaction-limited pulsed chemical vapor deposition technique [18]. The study on rutile crystals with well-faceted structures is still in the early stage and facile controlled synthesis of rutile nanocrystals bound by high-index faces remains a challenge. Noncovalent interactions such as Van der Waals forces provide a powerful tool for the morphological control of nanomaterials [22]. The interaction or orientation of benzene derivatives has been proven to be an excellent structure-directing agent for the creation of wellcontrolled and ordered nanostructures [23–25]. Picolinic acid with a pyridine ring may provide a sophisticated π–π interaction for selfassembly of desired nanostructures [26]. Picolinic acid with a nitrogen atom in the pyridine ring and a carboxyl group can also act as a chelating ligand providing selective adsorption and metal ion complexation which are crucial for the morphology-controlled synthesis. In this paper, we report the synthesis of rutile crystals exposing high-index facets, together with the assembly into hierarchical structures by the hydrothermal treatment of a titanium–glycolate complex in the presence of picolinic acid as structure-directing and shape-controlling agents.
2. Experimental section TiO2 was synthesized using a water-soluble titanium–glycolate complex of a precursor [27,28]. Briefly, a transparent yellow peroxotitanic acid solution was prepared by the addition of an ammonia solution (2 cm3, 28%, Kanto Chemicals Co., Inc.) and a hydrogen peroxide solution (10 cm3, 30%, Santoku Chemical Industry Co. Ltd.) to titanium metal powder (2 mmol, Furuuchi Chemical Co.). Glycolic acid (3 mmol, Kanto Chemicals Co., Inc.) was added to the yellow
Q.D. Truong et al. / Journal of Crystal Growth 418 (2015) 86–91
solution of peroxo-titanic acid, as a result of which the solution color changed from yellow to pale-red. Heat treatment at 353 K was carried out to remove excess amounts of ammonia and hydrogen peroxide. Finally, stable titanium–glycolate complex solution was obtained [27]. Then, picolinic acid (0–8 mmol, Kanto Chemicals Co., Inc.) was added to the complex solution as an additive. The solution was diluted to 20 cm3 using distilled water and sealed in a Teflon-lined stainless steel autoclave. The autoclave was heated at 473 K for 24 h for hydrothermal treatment of the complex. After that the autoclave was allowed to cool down to room temperature. The resultant powders were separated by centrifugation and washed with distilled water until the obtained solution was neutral pH. Finally, the obtained specimen was dried at 353 K for 1 day. The crystalline phase of the samples was characterized using powder X-ray diffraction (XRD; Rigaku RINV-2200, 40 kV and 30 mA) with Cu Kα radiation (λ ¼1.5406 Å). Data were collected in the 2θ–θ scanning mode with a scan speed of 41 min 1 and a step size of 0.021. The morphology of particles was observed using a fieldemission scanning electron microscope (FE-SEM, Hitachi S-4800) at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM Hitachi H-7650, 100 kV), high resolution TEM and selectedarea electron diffraction (HR-TEM and SAED, Hitachi HF-2000, 200 kV) were conducted using specimens dispersed in ethanol and then dropped onto a Cu microgrid coated with a holey carbon film, followed by the evaporation of the ethanol. IR spectra were recorded with a Jasco spectrometer within the range of 400–4000 cm 1. Samples in the solid state were measured in KBr matrix pellets were
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obtained with hydraulic press under 40 kN. DRS spectra of solid samples were evaluated in the range of 200–800 nm with a spectrophotometer instrument Shimadzu, UV 2450.
3. Results and discussion 3.1. Characterization of the synthesized nanostructures To investigate the effect of picolinic acid on the crystal growth, a series of control experiments were carried out systematically with a tailoring amount of the additive. It was found that picolinic acid had a critical role in the formation of the flower-like particles bound by high-index facets. The crystals exposing high-index facets could be obtained by adding an appropriate amount of picolinic acid, that is 3–4 mmol. The tailoring of additive amount had a crucial effect on the morphology of the synthesized particles. Rutile particles obtained without picolinic acid or with the addition of 2 mmol picolinic acid were comprised of mainly agglomerated nanorods along with a few star-like particles (Fig. 1a and b). In these cases, the growth of rutile exposing {110} facets is predominant resulting in formation of rodlike particles. Increasing the amount of picolinic acid to 4 mmol caused the increase in the number of branches on each particle. It resulted in flower-like particles as shown in Fig. 1c and d, and was discussed in detail later. When large amount of the additive (5–8 mmol) was introduced to the solution, the growth of the large number of branches resulted in the formation of microspheres as
Fig. 1. SEM images of TiO2 synthesized by hydrothermal treatment titanium–glycolate complex in the presence of different amounts of picolinic acid (a) 0 mmol, (b) 2 mmol, (c, d) 4 mmol, (e) 5 mmol, and (f) 8 mmol.
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Fig. 2. (a) TEM image of a pyramidal branch of a flower-like particle obtained in the presence of 4 mmol of picolinic acid; (b) schematic presentation of {331} planes in rutile structure, (c) HR-TEM image of top apex of a pyramidal branch and (d) SAED pattern of pyramidal branches of a flower-like particle.
shown in Fig. 1e and f. It should be noted that the TiO2 microsphere morphology is similar to the nanorod-based microspheres grown from a titanium–picolinato precursor in our previous report [26]. Thus, when a large amount of the additive was introduced, the picolinic acid might cause the ligand-exchange reaction to form titanium–picolinato and further provide π–π interaction to promote the assembly of the microsphere structures [26]. Fig. 1c and d shows FE-SEM images of typical flower-like particles obtained by the hydrothermal treatment of the titanium complex with addition of 4 mmol picolinic acid. The low magnification SEM image indicates the presence of highly uniform and monodispersed flower-like particles of several hundred nanometers in diameter. The high magnification image reveals that each particle is composed of several pyramidal crystals interconnecting through the common center. It can be seen that the pyramidal branches display relatively smooth surfaces, indicating the presence of flat exposed facets. Each branch is a tetragonal pyramid composed of four similar facets. Fig. 2a shows a TEM image of a representative flower-like particle obtained in the presence of 4 mmol of picolinic acid. This image clearly indicates the presence of the hierarchical structures comprising of multiple pyramidal branches. The branches are of 100–200 nm in length which have grown homogeneously from the common center of the particles. The XRD pattern (Fig. S1, Supporting information) indicates that the obtained sample is composed of mainly rutile phase and a small amount of anatase, brookite phases. All the diffraction peaks can be assigned to the tetragonal rutile TiO2 phase with lattice constants of a¼4.5933 Å and c¼ 2.9592 Å (ICPDS no. 211276). Fig. 2c and d show a HR-TEM image of a typical branch and a SAED pattern, respectively. SAED pattern taken from the branch can be indexed to the [1–10] zone axis of single-crystal tetragonal TiO2, indicating that each branch of flower-like particles is single crystalline
Table 1 The theoretical interfacial angles of different facets with {110} calculated based on tetragonal rutile TiO2 ICPSD no. 21-1276. {hkl}
na
mb
Angle with {110} (deg)
{111} {221} {331} {441} {332} {442} {552}
0 1 2 3 1 1 1
1 1 1 1 2 3 4
47.72 28.88 20.19 15.37 36.28 39.55 41.38
a b
The number of (110) terraces. The number of (111) steps.
in nature. The SAED pattern also illustrates that rutile crystals have grown along the [001]. The HR-TEM image in Fig. 2c clearly shows the interplanar spacing of (110) atomic planes. A corresponding model of atomic arrangement of rutile crystal projected along [1–10] is displayed in Fig. 2b (created by VESTA [29]). According to the nature of rutile, crystal grows along [001] direction enclosing primarily by {110} and {111} facets [30]. The secondary plane can be formed by periodical combination of (110) terraces and (111) steps. The theoretical interface angles θ of the secondary planes produced by periodical combination of n times (110) terraces and m times (111) steps with (110) plane can be calculated as following equation: tan(θ)¼ (m d110)/[(nþm) d001] (Equation 1, Fig. S2). Where d110 and d001 are the d spacing of {110} and {001} planes, respectively. The theoretical interfacial angles of different facets are listed in Table 1. The exposed facets observed by TEM can be indexed using the theoretical interfacial angles as follows. The parts of facets located
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Fig. 3. TEM images of TiO2 synthesized by the hydrothermal treatment of the titanium–glycolate complex in the presence of picolinic acid for (a) 3 h, (b) 4 h, (c) 8 h, (d) 16 h, and (e) 24 h.
Scheme 1. (a) Schematic illustrations of the effect of picolinic acid; (b) atomic model of rutile {111} and (c) the possible model of picolinic acid adsorption on the {111} face of rutile. H atoms are omitted for clarify.
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on the top apex of pyramidal branch with an interfacial angle of ca. 47.51 are assigned to {111} facets while the side planes with an interfacial angle of 20.51 can be assigned to high-index {331} facets. A schematic illustration of atomic arrangement in Fig. 2b shows the presence of a (331) plane constituted of periodical (110) terraces and (111) steps. Thus, the observation using electron microscopes coupled with electron diffraction clarifies the formation of the flower-like rutile particles exposing {331} facets. 3.2. Crystal growth It should be noted that we recently succeeded in the growth of Ti– picolinate hybrids by the hydrothermal reaction of a titanium picolinato solution. The characterization showed that the hybrids were composed of an ordered lamellar structure with a layer spacing of 1.48 nm. Based on the elemental analysis and other measurements including TG-DTA, we determined the most likely chemical formula of the obtained hybrid was TiO(OH)(C5H4NCOO)∙H2O [31]. In order to explore the clear insight into the morphology evolution of the rutile structures in this study; the time-dependent experiments were performed. Fig. 3 displays the TEM image of the products obtained by the hydrothermal treatment of titanium–glycolate in the presence of picolinic acid for various ageing times. The XRD measurement was also taken to confirm the structure of the obtained particles at different reaction times (Fig. S3). Sample collected after 1 h consists of crystalline Ti–picolinate hybrids as mentioned above [31], which were transformed into rutile hierarchical structures after 3 h treatment. Extending the treatment time, the well-faceted particles with uniform pyramidal branches were obtained (Fig. 3c–e). The results indicated that the crystal growth took place through several steps: the ligand-exchange reaction to form titanium–picolinato complexes; the formation of Ti–picolinate hybrids and further production of rutile by the decomposition of picolinate components. The rutile bundles were further grown into single branch with single crystalline nature and well-defined exposed facets with the aid of picolinic acid in the solution (Scheme S1). Hydrothermal treatment of the Ti–glycolate complex without the additive produced typical nanorod particles of rutile without any specific morphology (Fig. 1a) [27,28]. Thus, the growth of the unique nanocrystals critically depends on the presence of picolinic acid. The growth of flower-like particles with exposing specific facets was driven by capping effect in which selective adsorption of picolinic acid on the nanoparticle interface resulted in the preferential growth of high-index planes. The formation of crystals with exposing highindex facets is indeed a result of the competitive growth of {111} facets and {110} facets (Scheme 1a, created by VESTA [30]). Particularly, {111} facets can be stabilized and grown due to the selective adsorption of picolinic acid on them. On the other hand, rutile crystals have strong tendency to growth along [001] accompanied by the appearance of {110} facets. Consequently, rutile crystals have been formed with the presence of both {111} facets (steps) and {110} facets (terraces) as shown in Fig. 2b. The preferential adsorption of picolinic acid on the specific {111} facets can be understood based on the nature of rutile and a unique structure of the additive. In the previous report, it was suggested that rutile has {110} and {111} facets displaying a titanium metal rich, thus, are preferential sites for adsorption of a chelating ligand [31]. For example, glycolic acid may provide the capping effect through the chelation of carboxyl and hydroxyl coordination groups on the {110} facets. Such a bidentate chelation configuration is suitable on {110} facets where short distances of bridging-O atoms or Ti–Ti atoms distance are present. In the present case, picolinic acid possesses a pyridine ring with a mutual π-stacking. The distance between the aromatic rings is of 0.6–0.7 nm depending on the direction [32]. This space can be properly provided on the {111} facets where the distance of Ti–Ti atoms (0.546–0.649 nm) is the largest among low-index
planes of rutile (Scheme 1a). Therefore, picolinic acid preferentially adsorbs on such a facet to form a tight coverage layer on {111} planes (Scheme 1b). As a result, {111} facets were partially grown together with {110} facets to form pyramidal shape crystals bound by highindex surfaces. To clarify the interaction of the organic additive with TiO2 surface, the FT-IR spectrum of synthesized particles was taken as shown in Fig. 4. FT-IR spectra of picolinic acid and TiO2 (ST01, Ishihara Sangyo Kaisha, Ltd.) were also recorded as references. FT-IR absorption peaks owing to picolinates were observed for the synthesized particles while they were not observed for ST01. The absorption peaks at 1615, 1370, 1309 and 1180 cm 1 were assigned with COO asymmetric stretching vibration, COO symmetric stretching vibration, C–N stretching vibration and C–C stretching vibration, respectively. The vs(COO ) stretching frequency was observed in the FT-IR of the synthesized particles, indicated that the carboxyl group was deprotonated due to the chelation to surface titanium ions. The absence of a band at around 1710 cm 1 corresponding to the stretching vibration of CQO in COOH group implied that all the carboxylic group has been involved the adsorption on TiO2. The chelation mode was also identified based on the wavenumber separation. The vibration bands at 1615 and 1370 cm 1 correspond to the stretching vibration of COO group (vas and vs). The wavenumber separation 245 cm 1, in the present case,
Fig. 4. FT-IR spectra of (a) TiO2-ST01, (b) flower-like rutile nanostructures and (c) picolinic acid.
Fig. 5. (a, c) SEM and (b) TEM images of rutile nanostructures synthesized from (a, b) the titanium–lactate complex and (c) the titanium–citrate complex in the presence of picolinic acid.
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suggested that carboxyl groups in picolinic acids coordinated to the titanium atoms with a monodentate configuration [33–35]. The electron-rich nitrogen atom in a pyridine ring may also bind to Ti atom, over all, forming a bidentate configuration. The possible configuration of oxide interface-orgainic molecular interaction was proposed on the basis of above infrared spectra study as shown in Scheme 1c. The proposed chelating bidentate adsorption is most likely since it has been reported for a synthetic titanium–dipicolinato complex [36,37]. When benzoic acid possessing no nitrogen atoms in an aromatic ring or nicotinic acid of an isomer of picolinic acid was used as the additive, no specific structures were observed in the synthesized particles. These control experiment also supports this proposed configuration. A different titanium complex was also used for the synthesis of titania and its effect on the morphology of the synthesized particles was investigated. Fig. 5 shows the microscopic images of TiO2 particles obtained by the hydrothermal treatment of titanium–lactate complex [38] or titanium–citrate complex [39] in the presence of picolinic acid. Rutile nanostructures with reduced number of branches and significant decrease in the size were obtained. As mention above, the ligand-exchange reaction to form titanium– picolinato in the beginning stage of reaction is critical for the formation of the hierarchical structures. Therefore, the kinds of Ticomplexes precursor do not apparently affect the morphologies of the obtained particles. 4. Conclusions In conclusion, hierarchical TiO2 structures composed of rutile crystals exposing high-index facets such as {331} have been synthesized. The obtained particles are composed of several branches of pyramidal crystals with relatively smooth surfaces. It was hypothesized that the mutual π-stacking and selective adsorption of picolinic acid on specific {111} facets as well as the competitive growth of {110} facet and {111} facet resulted in the formation of such unique crystals exposing high-index facets. With their rich edges and corner atoms structure, the synthesized rutile nanostructures are expected to be highly efficient for catalytic applications and the concepts demonstrated in this study may be extended for shape-controlled synthesis of other metal and metal oxide nanostructures. Acknowledgment This work was partially supported by Grant-in-Aid for Scientific Research on Innovative Areas of “Fusion materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 2206). The authors thank Dr. T. Miyazaki (Tohoku University) for his help in the TEM and SAED measurements and analysis. Q. D. Truong would like to thank MEXT for awarding Monbusho Scholarship (No. 093037). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jcrysgro.2015.02.056.
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Hierarchical structures of rutile exposing high-index facets Quang Duc Truong, Hideki Kato, Makoto Kobayashi, Masato Kakihana n Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 6 October 2014 Received in revised form 23 December 2014 Accepted 15 February 2015 Communicated by: D.P. Norton Available online 24 February 2015
Recently, shape-controlled synthesis of crystals exposing high-index facets has attracted much research interest due to their importance for both fundamental studies and technological applications. Herein, crystals of rutile-type TiO2 with hierarchical structures exposing high-index facets have been synthesized by a facile hydrothermal method using water-soluble titanium complex as a precursor and picolinic acid as structure-directing and shape-controlling agents. The synthesized particles were composed of several branches of pyramidal crystals with relatively smooth surface. On the basis of investigation results, it was speculated that the mutual π-stacking and selective adsorption of picolinic acid on specific {111} facets resulted in the formation of rutile crystals bound by high-index surfaces such as {331}. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Interfaces A1. Nanostructure A1. Surface structure A2. Hydrothermal crystal growth B1. Oxides
1. Introduction Recently, a special effort has been devoted to the shape-controlled synthesis of crystals with exposed high-index facets due to their importance for both fundamental studies and technological applications [1–3]. Owing to their high densities of atom, steps and kinks, high-index facets usually exhibit much higher chemical activity than low-index ones. The high-index facets are gradually diminished during the crystal growth due to their high surface energy. Therefore, it is always challenging to control the growth of nanocrystals bound by high-index facets. The controllable synthesis of titanium dioxide (TiO2) is of particular importance due to its versatile industrial applications including photocatalysis, sensors, lithium-ion batteries, drug-delivery carriers, and dye-sensitized solar cells [4]. Anatase TiO2 single crystals have attracted extraordinary research interest since anatase crystals with dominant high-energy facets were successfully synthesized by Lu et al. [5–12]. The synthesis of anatase TiO2 with exposed {105} [13], {103} [14], {401}[15], {201} [16], and {111} [17] high-index facets has also been reported. In contrast to anatase TiO2, there is very few report on rutile TiO2 crystals with exposed such high-energy facets [18]. Rutile is a thermodynamically stable phase with a smaller band gap than anatase phase. In some cases, rutile has been found to be more effective for photocatalysis than anatase [19– 21]. Therefore, rutile nanocrystals with exposed high-index facets are highly desirable for catalytic application. Shi and Wang have recently succeeded in growth of the rutile nanowires with exposed high-index
n
Corresponding author. Tel./fax: þ 81 22 217 5649. E-mail address: [email protected] (M. Kakihana).
http://dx.doi.org/10.1016/j.jcrysgro.2015.02.056 0022-0248/& 2015 Elsevier B.V. All rights reserved.
surfaces by a surface reaction-limited pulsed chemical vapor deposition technique [18]. The study on rutile crystals with well-faceted structures is still in the early stage and facile controlled synthesis of rutile nanocrystals bound by high-index faces remains a challenge. Noncovalent interactions such as Van der Waals forces provide a powerful tool for the morphological control of nanomaterials [22]. The interaction or orientation of benzene derivatives has been proven to be an excellent structure-directing agent for the creation of wellcontrolled and ordered nanostructures [23–25]. Picolinic acid with a pyridine ring may provide a sophisticated π–π interaction for selfassembly of desired nanostructures [26]. Picolinic acid with a nitrogen atom in the pyridine ring and a carboxyl group can also act as a chelating ligand providing selective adsorption and metal ion complexation which are crucial for the morphology-controlled synthesis. In this paper, we report the synthesis of rutile crystals exposing high-index facets, together with the assembly into hierarchical structures by the hydrothermal treatment of a titanium–glycolate complex in the presence of picolinic acid as structure-directing and shape-controlling agents.
2. Experimental section TiO2 was synthesized using a water-soluble titanium–glycolate complex of a precursor [27,28]. Briefly, a transparent yellow peroxotitanic acid solution was prepared by the addition of an ammonia solution (2 cm3, 28%, Kanto Chemicals Co., Inc.) and a hydrogen peroxide solution (10 cm3, 30%, Santoku Chemical Industry Co. Ltd.) to titanium metal powder (2 mmol, Furuuchi Chemical Co.). Glycolic acid (3 mmol, Kanto Chemicals Co., Inc.) was added to the yellow
Q.D. Truong et al. / Journal of Crystal Growth 418 (2015) 86–91
solution of peroxo-titanic acid, as a result of which the solution color changed from yellow to pale-red. Heat treatment at 353 K was carried out to remove excess amounts of ammonia and hydrogen peroxide. Finally, stable titanium–glycolate complex solution was obtained [27]. Then, picolinic acid (0–8 mmol, Kanto Chemicals Co., Inc.) was added to the complex solution as an additive. The solution was diluted to 20 cm3 using distilled water and sealed in a Teflon-lined stainless steel autoclave. The autoclave was heated at 473 K for 24 h for hydrothermal treatment of the complex. After that the autoclave was allowed to cool down to room temperature. The resultant powders were separated by centrifugation and washed with distilled water until the obtained solution was neutral pH. Finally, the obtained specimen was dried at 353 K for 1 day. The crystalline phase of the samples was characterized using powder X-ray diffraction (XRD; Rigaku RINV-2200, 40 kV and 30 mA) with Cu Kα radiation (λ ¼1.5406 Å). Data were collected in the 2θ–θ scanning mode with a scan speed of 41 min 1 and a step size of 0.021. The morphology of particles was observed using a fieldemission scanning electron microscope (FE-SEM, Hitachi S-4800) at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM Hitachi H-7650, 100 kV), high resolution TEM and selectedarea electron diffraction (HR-TEM and SAED, Hitachi HF-2000, 200 kV) were conducted using specimens dispersed in ethanol and then dropped onto a Cu microgrid coated with a holey carbon film, followed by the evaporation of the ethanol. IR spectra were recorded with a Jasco spectrometer within the range of 400–4000 cm 1. Samples in the solid state were measured in KBr matrix pellets were
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obtained with hydraulic press under 40 kN. DRS spectra of solid samples were evaluated in the range of 200–800 nm with a spectrophotometer instrument Shimadzu, UV 2450.
3. Results and discussion 3.1. Characterization of the synthesized nanostructures To investigate the effect of picolinic acid on the crystal growth, a series of control experiments were carried out systematically with a tailoring amount of the additive. It was found that picolinic acid had a critical role in the formation of the flower-like particles bound by high-index facets. The crystals exposing high-index facets could be obtained by adding an appropriate amount of picolinic acid, that is 3–4 mmol. The tailoring of additive amount had a crucial effect on the morphology of the synthesized particles. Rutile particles obtained without picolinic acid or with the addition of 2 mmol picolinic acid were comprised of mainly agglomerated nanorods along with a few star-like particles (Fig. 1a and b). In these cases, the growth of rutile exposing {110} facets is predominant resulting in formation of rodlike particles. Increasing the amount of picolinic acid to 4 mmol caused the increase in the number of branches on each particle. It resulted in flower-like particles as shown in Fig. 1c and d, and was discussed in detail later. When large amount of the additive (5–8 mmol) was introduced to the solution, the growth of the large number of branches resulted in the formation of microspheres as
Fig. 1. SEM images of TiO2 synthesized by hydrothermal treatment titanium–glycolate complex in the presence of different amounts of picolinic acid (a) 0 mmol, (b) 2 mmol, (c, d) 4 mmol, (e) 5 mmol, and (f) 8 mmol.
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Fig. 2. (a) TEM image of a pyramidal branch of a flower-like particle obtained in the presence of 4 mmol of picolinic acid; (b) schematic presentation of {331} planes in rutile structure, (c) HR-TEM image of top apex of a pyramidal branch and (d) SAED pattern of pyramidal branches of a flower-like particle.
shown in Fig. 1e and f. It should be noted that the TiO2 microsphere morphology is similar to the nanorod-based microspheres grown from a titanium–picolinato precursor in our previous report [26]. Thus, when a large amount of the additive was introduced, the picolinic acid might cause the ligand-exchange reaction to form titanium–picolinato and further provide π–π interaction to promote the assembly of the microsphere structures [26]. Fig. 1c and d shows FE-SEM images of typical flower-like particles obtained by the hydrothermal treatment of the titanium complex with addition of 4 mmol picolinic acid. The low magnification SEM image indicates the presence of highly uniform and monodispersed flower-like particles of several hundred nanometers in diameter. The high magnification image reveals that each particle is composed of several pyramidal crystals interconnecting through the common center. It can be seen that the pyramidal branches display relatively smooth surfaces, indicating the presence of flat exposed facets. Each branch is a tetragonal pyramid composed of four similar facets. Fig. 2a shows a TEM image of a representative flower-like particle obtained in the presence of 4 mmol of picolinic acid. This image clearly indicates the presence of the hierarchical structures comprising of multiple pyramidal branches. The branches are of 100–200 nm in length which have grown homogeneously from the common center of the particles. The XRD pattern (Fig. S1, Supporting information) indicates that the obtained sample is composed of mainly rutile phase and a small amount of anatase, brookite phases. All the diffraction peaks can be assigned to the tetragonal rutile TiO2 phase with lattice constants of a¼4.5933 Å and c¼ 2.9592 Å (ICPDS no. 211276). Fig. 2c and d show a HR-TEM image of a typical branch and a SAED pattern, respectively. SAED pattern taken from the branch can be indexed to the [1–10] zone axis of single-crystal tetragonal TiO2, indicating that each branch of flower-like particles is single crystalline
Table 1 The theoretical interfacial angles of different facets with {110} calculated based on tetragonal rutile TiO2 ICPSD no. 21-1276. {hkl}
na
mb
Angle with {110} (deg)
{111} {221} {331} {441} {332} {442} {552}
0 1 2 3 1 1 1
1 1 1 1 2 3 4
47.72 28.88 20.19 15.37 36.28 39.55 41.38
a b
The number of (110) terraces. The number of (111) steps.
in nature. The SAED pattern also illustrates that rutile crystals have grown along the [001]. The HR-TEM image in Fig. 2c clearly shows the interplanar spacing of (110) atomic planes. A corresponding model of atomic arrangement of rutile crystal projected along [1–10] is displayed in Fig. 2b (created by VESTA [29]). According to the nature of rutile, crystal grows along [001] direction enclosing primarily by {110} and {111} facets [30]. The secondary plane can be formed by periodical combination of (110) terraces and (111) steps. The theoretical interface angles θ of the secondary planes produced by periodical combination of n times (110) terraces and m times (111) steps with (110) plane can be calculated as following equation: tan(θ)¼ (m d110)/[(nþm) d001] (Equation 1, Fig. S2). Where d110 and d001 are the d spacing of {110} and {001} planes, respectively. The theoretical interfacial angles of different facets are listed in Table 1. The exposed facets observed by TEM can be indexed using the theoretical interfacial angles as follows. The parts of facets located
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Fig. 3. TEM images of TiO2 synthesized by the hydrothermal treatment of the titanium–glycolate complex in the presence of picolinic acid for (a) 3 h, (b) 4 h, (c) 8 h, (d) 16 h, and (e) 24 h.
Scheme 1. (a) Schematic illustrations of the effect of picolinic acid; (b) atomic model of rutile {111} and (c) the possible model of picolinic acid adsorption on the {111} face of rutile. H atoms are omitted for clarify.
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on the top apex of pyramidal branch with an interfacial angle of ca. 47.51 are assigned to {111} facets while the side planes with an interfacial angle of 20.51 can be assigned to high-index {331} facets. A schematic illustration of atomic arrangement in Fig. 2b shows the presence of a (331) plane constituted of periodical (110) terraces and (111) steps. Thus, the observation using electron microscopes coupled with electron diffraction clarifies the formation of the flower-like rutile particles exposing {331} facets. 3.2. Crystal growth It should be noted that we recently succeeded in the growth of Ti– picolinate hybrids by the hydrothermal reaction of a titanium picolinato solution. The characterization showed that the hybrids were composed of an ordered lamellar structure with a layer spacing of 1.48 nm. Based on the elemental analysis and other measurements including TG-DTA, we determined the most likely chemical formula of the obtained hybrid was TiO(OH)(C5H4NCOO)∙H2O [31]. In order to explore the clear insight into the morphology evolution of the rutile structures in this study; the time-dependent experiments were performed. Fig. 3 displays the TEM image of the products obtained by the hydrothermal treatment of titanium–glycolate in the presence of picolinic acid for various ageing times. The XRD measurement was also taken to confirm the structure of the obtained particles at different reaction times (Fig. S3). Sample collected after 1 h consists of crystalline Ti–picolinate hybrids as mentioned above [31], which were transformed into rutile hierarchical structures after 3 h treatment. Extending the treatment time, the well-faceted particles with uniform pyramidal branches were obtained (Fig. 3c–e). The results indicated that the crystal growth took place through several steps: the ligand-exchange reaction to form titanium–picolinato complexes; the formation of Ti–picolinate hybrids and further production of rutile by the decomposition of picolinate components. The rutile bundles were further grown into single branch with single crystalline nature and well-defined exposed facets with the aid of picolinic acid in the solution (Scheme S1). Hydrothermal treatment of the Ti–glycolate complex without the additive produced typical nanorod particles of rutile without any specific morphology (Fig. 1a) [27,28]. Thus, the growth of the unique nanocrystals critically depends on the presence of picolinic acid. The growth of flower-like particles with exposing specific facets was driven by capping effect in which selective adsorption of picolinic acid on the nanoparticle interface resulted in the preferential growth of high-index planes. The formation of crystals with exposing highindex facets is indeed a result of the competitive growth of {111} facets and {110} facets (Scheme 1a, created by VESTA [30]). Particularly, {111} facets can be stabilized and grown due to the selective adsorption of picolinic acid on them. On the other hand, rutile crystals have strong tendency to growth along [001] accompanied by the appearance of {110} facets. Consequently, rutile crystals have been formed with the presence of both {111} facets (steps) and {110} facets (terraces) as shown in Fig. 2b. The preferential adsorption of picolinic acid on the specific {111} facets can be understood based on the nature of rutile and a unique structure of the additive. In the previous report, it was suggested that rutile has {110} and {111} facets displaying a titanium metal rich, thus, are preferential sites for adsorption of a chelating ligand [31]. For example, glycolic acid may provide the capping effect through the chelation of carboxyl and hydroxyl coordination groups on the {110} facets. Such a bidentate chelation configuration is suitable on {110} facets where short distances of bridging-O atoms or Ti–Ti atoms distance are present. In the present case, picolinic acid possesses a pyridine ring with a mutual π-stacking. The distance between the aromatic rings is of 0.6–0.7 nm depending on the direction [32]. This space can be properly provided on the {111} facets where the distance of Ti–Ti atoms (0.546–0.649 nm) is the largest among low-index
planes of rutile (Scheme 1a). Therefore, picolinic acid preferentially adsorbs on such a facet to form a tight coverage layer on {111} planes (Scheme 1b). As a result, {111} facets were partially grown together with {110} facets to form pyramidal shape crystals bound by highindex surfaces. To clarify the interaction of the organic additive with TiO2 surface, the FT-IR spectrum of synthesized particles was taken as shown in Fig. 4. FT-IR spectra of picolinic acid and TiO2 (ST01, Ishihara Sangyo Kaisha, Ltd.) were also recorded as references. FT-IR absorption peaks owing to picolinates were observed for the synthesized particles while they were not observed for ST01. The absorption peaks at 1615, 1370, 1309 and 1180 cm 1 were assigned with COO asymmetric stretching vibration, COO symmetric stretching vibration, C–N stretching vibration and C–C stretching vibration, respectively. The vs(COO ) stretching frequency was observed in the FT-IR of the synthesized particles, indicated that the carboxyl group was deprotonated due to the chelation to surface titanium ions. The absence of a band at around 1710 cm 1 corresponding to the stretching vibration of CQO in COOH group implied that all the carboxylic group has been involved the adsorption on TiO2. The chelation mode was also identified based on the wavenumber separation. The vibration bands at 1615 and 1370 cm 1 correspond to the stretching vibration of COO group (vas and vs). The wavenumber separation 245 cm 1, in the present case,
Fig. 4. FT-IR spectra of (a) TiO2-ST01, (b) flower-like rutile nanostructures and (c) picolinic acid.
Fig. 5. (a, c) SEM and (b) TEM images of rutile nanostructures synthesized from (a, b) the titanium–lactate complex and (c) the titanium–citrate complex in the presence of picolinic acid.
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suggested that carboxyl groups in picolinic acids coordinated to the titanium atoms with a monodentate configuration [33–35]. The electron-rich nitrogen atom in a pyridine ring may also bind to Ti atom, over all, forming a bidentate configuration. The possible configuration of oxide interface-orgainic molecular interaction was proposed on the basis of above infrared spectra study as shown in Scheme 1c. The proposed chelating bidentate adsorption is most likely since it has been reported for a synthetic titanium–dipicolinato complex [36,37]. When benzoic acid possessing no nitrogen atoms in an aromatic ring or nicotinic acid of an isomer of picolinic acid was used as the additive, no specific structures were observed in the synthesized particles. These control experiment also supports this proposed configuration. A different titanium complex was also used for the synthesis of titania and its effect on the morphology of the synthesized particles was investigated. Fig. 5 shows the microscopic images of TiO2 particles obtained by the hydrothermal treatment of titanium–lactate complex [38] or titanium–citrate complex [39] in the presence of picolinic acid. Rutile nanostructures with reduced number of branches and significant decrease in the size were obtained. As mention above, the ligand-exchange reaction to form titanium– picolinato in the beginning stage of reaction is critical for the formation of the hierarchical structures. Therefore, the kinds of Ticomplexes precursor do not apparently affect the morphologies of the obtained particles. 4. Conclusions In conclusion, hierarchical TiO2 structures composed of rutile crystals exposing high-index facets such as {331} have been synthesized. The obtained particles are composed of several branches of pyramidal crystals with relatively smooth surfaces. It was hypothesized that the mutual π-stacking and selective adsorption of picolinic acid on specific {111} facets as well as the competitive growth of {110} facet and {111} facet resulted in the formation of such unique crystals exposing high-index facets. With their rich edges and corner atoms structure, the synthesized rutile nanostructures are expected to be highly efficient for catalytic applications and the concepts demonstrated in this study may be extended for shape-controlled synthesis of other metal and metal oxide nanostructures. Acknowledgment This work was partially supported by Grant-in-Aid for Scientific Research on Innovative Areas of “Fusion materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 2206). The authors thank Dr. T. Miyazaki (Tohoku University) for his help in the TEM and SAED measurements and analysis. Q. D. Truong would like to thank MEXT for awarding Monbusho Scholarship (No. 093037). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jcrysgro.2015.02.056.
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