Metastable Nature of Titanate Nanotubes in an Alkaline Environment

Aug 25, 2010 - 4421–4427. Metastable Nature of Titanate Nanotubes in an Alkaline Environment. Dmitry V. Bavykin,* Alexander N. Kulak, and Frank C. W...
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DOI: 10.1021/cg100529y

Metastable Nature of Titanate Nanotubes in an Alkaline Environment Dmitry V. Bavykin,* Alexander N. Kulak, and Frank C. Walsh

2010, Vol. 10 4421–4427

Materials Engineering and Energy Technology Research Groups, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, United Kingdom Received April 21, 2010; Revised Manuscript Received July 8, 2010

ABSTRACT: A systematic analysis of the effect of the composition and temperature of a NaOH/KOH binary aqueous mixture on the morphological properties of titanate nanostructures formed during alkaline hydrothermal transformation of TiO2 under atmospheric conditions has been performed using high-resolution transmission electron microscopy techniques. All observed nanostructures, including nanosheets, nanotubes, nanofibers, and nanoparticles, have been mapped over a wide range of compositions (from pure NaOH to pure KOH) and temperatures (from 50 to 110 °C). Attempts to intensify the TiO2 transformation via addition of titanate nanotube seeds or agitation of the reaction mixture have resulted in formation of thermodynamically stable nanofibers rather than nanotubes. The effects of kinetic and thermodynamic control of the reaction are discussed with regard to the transformation of TiO2 to nanosheets, nanotubes, and nanofibers.

Introduction Solid polytitanic acid consisting of single crystals in a nanotubular shape, also known as titanate nanotubes1 (HTiNT), which can be produced by alkaline hydrothermal treatment of TiO2 followed by substitution of alkaline cations with protons, has become the target of many studies in recent years because of its unique combination of physicochemical2,3 and structural4 properties. Nanotubular titanates can be potentially utilized in a wide range of applications,5 including catalysis,6,7 photocatalysis,8,9 hydrogen storage,10,11 lithium batteries,12,13 and solar cells.14,15 The open, mesoporous morphology and high aspect ratio combined with good ionexchange properties16,17 and semiconductor characteristics render nanostructured titanates useful in nanoscale design. Since their discovery by Kasuga et al.18 in 1997, much effort has been spent to understand the mechanism3 of nanotubular titanate formation, allowing control of the morphology of nanotubes via the adjustment of the synthesis conditions, which include temperature, mass/liquid ratio, duration of the experiment, composition of the alkaline solution, and the presence of additives. It is now generally agreed that the reaction proceeds through stages, including (i) slow dissolution of raw TiO2 accompanied by epitaxial growth and exfoliation of layered nanosheets of sodium titanates, (ii) folding of the nanosheets into tubular structures, and (iii) growth of nanotubes along their axis.1 The peculiarities of the processes occurring during folding of multilayer nanosheets determine whether nanotubes (TiNT) or nanofibers (TiNF) form, and the synthesis condition can be critical. For example, a 24 h treatment of TiO2 with a 10 mol/dm3 solution of NaOH results in formation of TiNT over a range of temperatures from 110 to 150 °C and TiNF at >170 °C. A recent approach to reduction of the temperature of TiNT synthesis utilized the binary NaOH/KOH aqueous mixture as a solvent.19 The rationale for such a choice of solvent was based on the idea that the concentration of dissolved Ti(IV) can determine the rate of crystallization of titanate nanosheets (TiNS), which in

turn control the morphology of the final nanostructures. The concentration of Ti(IV) at a given temperature depends on composition of the NaOH/KOH mixture. In general, the addition of KOH to the NaOH solution increases the solubility of TiO2, leading, for example, to formation of fibrous product at low temperatures in pure KOH.20,21 However, no systematic studies for a wide range of compositions of the NaOH/KOH mixture are currently available. In this work, the transformation of TiO2 (anatase) into nanostructured titanates in an aqueous binary NaOH/KOH mixture under atmospheric conditions has been systematically studied, from 50 to 110 °C, for controlled compositions of the solvent. Our aim is to improve our understanding of the mapping of product morphology under a wide range of reaction conditions. Morphological analysis of the products allows us to formulate the range of conditions (zones) that are favorable for formation of selected structures, including nanotubes, nanosheets, and nanofibers (under atmospheric pressures, avoiding the use of an autoclave). The effect of acceleration of the hydrothermal reaction on the position of these zones has also been studied. According to recent reports, acceleration of the overall process of nanostructured titanate formation can be achieved by intensification of mass transfer,22 microwave heating,23 or ultrasonication24 of the reaction mixture. The method of acceleration of crystal growth by addition of the products (seeding) to the reaction mixture has not previously been reported for the studied process. Here, we report the effect of the addition of TiNT to the reacting slurry on the distribution of morphologies of titanates. It was found that such overacceleration of the process results in formation of more thermodynamically stable nanofibers under conditions where nanotubes tend to form without process intensification. Such observations reveal additional insights into the mechanism of nanostructured titanate growth. Experimental Procedures

*To whom correspondence should be addressed. Telephone: þ44 2380598358. Fax: þ44 2380598754. E-mail: [email protected].

Sodium hydroxide (NaOH), potassium hydroxide (KOH), hydrochloric acid (HCl), sulfuric acid (H2SO4), and hydrogen peroxide (H2O2), pure grade, were all obtained from Aldrich and were used

r 2010 American Chemical Society

Published on Web 08/25/2010

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Figure 1. TEM images of the typical morphologies observed during alkaline hydrothermal reaction of TiO2: (a) nanosheets, (b) nanotubes, (c) nanofibers, and (d) nanoparticles. The insets in panels a and c correspond to magnification of the indicated square areas. The inset in panel b is the fragment of another area showing a cross section of a nanotube. without further purification. Titanium dioxide (P25, TiO2) was obtained from Degussa. The preparation of titanate nanotubes was based on the alkaline hydrothermal method proposed by Kasuga et al.18 and further developed toward a reflux synthesis using NaOH/KOH aqueous mixtures.19 Twenty grams of TiO2 was mixed with 250 - x cm3 of 10 mol/dm3 NaOH and x cm3 (x = 0, 5, 10, 25, 50, 125, or 250) of 10 mol/dm3 KOH aqueous solutions and then placed in a PFA (perfluoroalkoxy polymer) round-bottom flask (Bohlender GmbH) equipped with a thermometer and water-jacketed condenser. The mixture was refluxed for 4 days at a controlled temperature in the range of 50-110 °C, with or without stirring using a PTFE-coated magnetic bar. The composition of the binary NaOH/KOH mixture is characterized by the NaOH molar fraction X, which can be calculated using the formula X = (250 - x)/250. After reaction, the white powdery titanates were separated and thoroughly washed with distilled water until the washing solution reached pH 7. To convert titanate nanotubes into their protonated form, the powder was washed with an excess of 0.1 mol/dm3 HCl for more than 30 min until a stable pH value of 2 was reached, followed by water washing to pH 5. The sample was dried overnight in air at 140 °C. The concentration of Ti(IV) in an alkaline solution was measured using an analytical method described elsewhere.19 A 1 cm3 aliquot from the reflux flask was taken, hot filtered, and slowly acidified with 1 cm3 of concentrated H2SO4 followed by addition of 2 cm3 of water and 1 cm3 of 30 wt % H2O2. The concentration of Ti(IV) was determined using a UV-vis spectrophotometer (Scinco Neosys 2000) by measuring the characteristic band of the titanium peroxide complex at 420 nm. Transmission electron microscopy (TEM) images were obtained using a JEOL 3010 transmission electron microscope operating at 300 kV; the powder sample was “dry” deposited onto a copper grid covered with a perforated carbon film. XRD patterns have been recorded using a Bruker AXS D8 Discoverer X-ray diffractometer, with Ni-filtered Cu KR radiation (λ = 0.154 nm) and a graphite monochromator spectrometer with an aluminum sample holder. Typically, data was collected over a 6 h period.

Results and Discussion The alkaline hydrothermal transformation of TiO2 is a complex process involving formation of several intermediate structures, with distinctive morphologies participating in sequential chemical reactions. Figure 1 shows typical nanostructures observed in the reaction mixture. Titanate nanosheets (Figure 1a) are planar or curved thin structures consisting of several layers of (100) planes of titanates. The number of layers in TiNS can typically vary from 2 to 10. The size of nanosheets can exceed several hundreds of nanometers. Usually, TiNS are randomly agglomerated into a larger structure. During alkaline hydrothermal recrystallization, titanate nanosheets can fold into nanotubes (Figure 1b) characterized by a multiwall structure and a length of several hundreds nanometers. The aspect ratio (i.e., the length divided by the diameter) of TiNT is usually >10 and can reach values of several thousand. Another possible product of alkaline reaction is titanate nanofibers (Figure 1c), which are long, solid, parallelepipeds consisting of (100) layers of titanates. The length of the TiNF can be several tens of micrometers, while the width is typically in the range of 10-100 nm. The fourth, less typical, nanostructure found in the reaction mixture is a small unreacted anatase nanoparticle (TiO2 NP) with a typical size of approximately 10 nm (Figure 1d). The particles are characterized by developed crystalline structure and probably originated from initial large particles of TiO2 during their dissolution. The crystal structure of TiNS, TiNT, and TiNF corresponds to the hydrated form of layered polytitanic acid or its salt (see Figure 2). Although the exact crystal structure of these titanates is still disputed, it can be approximated by

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Figure 2. XRD pattern of the typical morphologies observed during alkaline hydrothermal reaction of TiO2: (a) nanotubes obtained at X = 0.96 and t = 110 °C, (b) nanofibers obtained at X = 0 and t = 110 °C, and (c) nanosheets obtained at X = 1 and t = 70 °C. Nanosheets and nanofibers have impurities of the initial anatase. X is the NaOH molar fraction.

monoclinic trititanic acid (H2Ti3O7).4 All three morphologies have a characteristic reflection at ca. 10°, which corresponds to the spacing between layers. The single-layer nanosheets usually do not show such a reflection;25 therefore, its appearance in the XRD pattern of TiNS obtained by alkaline hydrothermal treatment confirms their multilayer structure. Because of the incomplete conversion of initial TiO2 to TiNF and TiNS, their XRD patterns show impurities of the anatase phase. TiNS, TiNT, TiNF, and TiO2 NP can be found in the reaction slurry at specific temperatures and compositions of the NaOH/KOH binary mixture. Figure 3 shows the representative TEM images of the nanostructures obtained after treatment of TiO2 P25 (Degussa) for 4 days with a 10 mol/dm3 NaOH/KOH mixture at temperatures ranging from 50 to 110 °C and over the wide range of solvent compositions. Such TEM mapping allows identification of the region of operational conditions under which preferable formation of each of the nanostructures occurs. Because of the small conversion at 50 °C, all samples have impurities of initial P25 particles (>50 nm spheroidal shape) that disappear at elevated temperatures due to facilitation of the reaction rate. In a pure NaOH solution (X = 1) at temperatures below 90 °C, the main product of the reaction is titanate nanosheets (see Table 1). An increase of the temperature results in the appearance of TiNT and TiNP and the disappearance of TiNS. The yield of nanotubes is poor, and their crystal structure is also not welldeveloped compared to that of TiNT obtained using the standard temperature at 140 °C.18 On the other hand, addition of small quantities of KOH (X = 0.96) improves the quality of TiNT at lower temperatures. Further addition of KOH to the binary mixture results in the appearance of TiNF. Recently, it was suggested that direction of transformation of TiNS to either TiNT or TiNF can be correlated to the concentration of dissolved Ti(IV) in alkaline solution.19 TiNF

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tend to be formed at higher concentrations, whereas TiNT are likely to be obtained at a lower apparent concentration of Ti(IV). Our investigations have shown that the measured bulk concentration of Ti(IV) in alkaline solution is the same at different times during the hydrothermal reaction. It also does not depend on the amount of solid precursor (TiO2) in the reaction mixture, confirming that Ti(IV) is in equilibrium between solid and liquid phases during hydrothermal transformations. This equilibrium concentration, however, depends on the temperature and composition of the NaOH/ KOH mixture (see Figure 4). An increase of reaction mixture temperature results in an increased Ti(IV) concentration. The functional dependence is best described using the Arrhenius equation. The apparent heat of dissolution determined from Figure 4 depends on the composition of the binary mixture and is 6.1 and 26.3 kJ/mol in pure KOH and NaOH, respectively. By simulating the dependence of the heat of dissolution on the molar fraction of NaOH X (Figure S1 of the Supporting Information), we are able to plot curves having a constant concentration of Ti(IV) in the composition-temperature coordinates. Figure 5 shows two iso-concentration curves corresponding to concentrations of Ti(IV) in solution equal to 0.001 and 0.002 mol/dm3. Addition of even a small amount of KOH to the NaOH solution results in a sharp increase in the Ti(IV) concentration. Such an increase is more prominent at lower temperatures. The iso-concentration curves separate several regions in the temperaturecomposition diagram. For better visualization, the data shown in Figure 3 can be summarized in the temperature-composition diagram in the form of a distribution of observed morphologies in the products of the reaction under given conditions. The principle is similar to the construction of morphological phase diagrams obtained for pure NaOH solutions.26 In our case, for example, treatment of P25 in an X = 0.9 NaOH/KOH mixture at 110 °C results in the formation of a product having two major morphologies, namely, tubular and fibrous forms (see Figure 5). The ranges of conditions under which only a single morphology is produced are marked as the zones for each of typical morphologies, including TiNS, TiNT, and TiNF. TiNS generally forms at low temperatures and large values of X, whereas TiNF are usually obtained at higher temperatures over the wide range of X. TiNT can be obtained in the intermediate region with X varied in range from 0.8 to 1. It is remarkable that the shape of the zones in the temperaturecomposition diagram resembles the shape of the areas confined by the iso-concentration curves. This could imply that the distribution of the morphologies of nanostructured titanates obtained via an alkaline hydrothermal process is correlated with the equilibrium concentration of Ti(IV) in a binary solution under a wide range of operational conditions. These zones in the composition-temperature diagram have useful practical applications by indicating conditions for preparations with the desired morphology. However, it is important to know whether the position of these zones is governed by thermodynamic or kinetic factors. In other words, it is important to establish whether the zones are related to the global minimum of the Gibbs free energy for each morphology (thermodynamic control) or to morphologies indicating intermediate metastable states (kinetic control). If it is the first case, then formation of morphologies under given conditions should be a path-independent phenomenon, and any acceleration of the process should not affect the shape of the obtained titanates.

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Figure 3. TEM images of titania and titanate nanostructures obtained during alkaline hydrothermal treatment of TiO2 (P25, Degussa) for 4 days at controlled temperatures and compositions of binary aqueous NaOH/KOH mixtures. Table 1. Distribution of Morphologies in the Products of Reaction after Reflux Treatment of P25 in a NaOH/KOH Mixture for 4 Days molar fraction of NaOH in binary mixture (X)

50 °C

70 °C

90 °C

110 °C

0 0.5 0.9 0.96 1

TiNF, low conversion debris, low conversion TiNT traces TiNT, low conversion TiNS, low conversion

TiNF and TiNT, low conversion TiNT, low conversion TiNP and TiNT TiNP and TiNT TiNS and TiNT

TiNF TiNT and debris TiNT TiNT TiNP and TiNT

TiNF TiNF and debris TiNT and TiNF TiNT TiNP and TiNT

Intensification of the reaction without any change in the temperature or composition of the solvent can be achieved via, for example, improvement of reagent mass transfer. Earlier studies20 have demonstrated that treatment of TiO2 with 10 mol/ dm3 NaOH at 130 °C for 24 h in a revolving autoclave results in the formation of TiNF. According to the diagram in Figure 5, these conditions favor formation of TiNT, which usually form without intensive stirring. A similar behavior was also observed for the NaOH/KOH binary mixture. The 4 day treatment of P25 at 110 °C with a 10 mol/dm3 NaOH/KOH binary mixture

(X = 0.96) results in the formation of TiNF and TiNT with and without stirring, respectively (see panels b and a of Figure 6). It is remarkable that even without reaction intensification by improved mass transfer, TiNF can be obtained under similar conditions (10 mol/dm3 NaOH at 150 °C) after treatment for 120 h.27 In other words, Figure 5 shows the transient distributions of morphologies observed under specific conditions, reaction for 4 days and no stirring of the reacting masses. Such behavior suggests that not only nanosheets but also nanotubes are intermediate species during the transformation

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Figure 4. Concentration of dissolved titanium(IV) [cTi(IV)] as a function of temperature for four different compositions (X is the NaOH molar fraction) of the binary NaOH/KOH mixture.

Figure 5. Distribution of morphologies found in the products of the reaction of P25 with the NaOH/KOH binary mixture as a function of mixture composition and temperature. Dashed lines show equiconcentration curves for an equilibrium concentration of Ti(IV) in the mixture. Three zones identify the range of conditions favorable for the formation of TiNS, TiNT, and TiNF.

of TiO2 to titanates of alkaline metals. The metastable nature of TiNT and TiNS can be associated with their higher chemical potential relative to that of TiNF caused by a larger specific surface area, which can significantly increase the free energy. For example, the specific surface area typically varies in the range from 200 to 300 m2/g for nanotubes and from 20 to 50 m2/g for nanofibers. The corresponding gain in free energy during the transformation of TiNT to TiNF can be estimated to lie between 12σ and 22.4σ kJ/mol, where σ is the excess surface energy per unit area at the solution-particle interface (in joules per square meter). For a solid-liquid interface, the typical value of σ can be approximated1 as 1 J/m2, indicating that the change in the Gibbs energy of reaction is within 20 kJ/mol. Such a significant difference should drive the process toward formation of TiNF unless there are activation barriers, which can stabilize metastable TiNT.

Figure 6. Nanostructured titanates obtained by treatment of P25 with a 10 mol/dm3 NaOH/KOH binary mixture (X = 0.96) at 110 °C for 4 days (a) without stirring, (b) with 100 rpm stirring, and (c) without stirring and with addition of 1 wt % TiNT to the initial mixture.

The existence of such barriers can be illustrated using possible change in Gibbs free energy (ΔG) as a function of the chemical extent (ξ) of the reaction transformation of nanotubes to nanofibers as shown in Figure 7. The possible

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Figure 7. Schematic energy diagram for transformation of TiNT (point A) to TiNF (point C) along a reaction coordinate by unfolding of nanotubes via an intermediate state (point B).

mechanism of this transformation should involve unfolding nanotubes (point A) to a planar multilayer structure similar to a nanosheet (point B). Such planar structures, however, should have a difference in width between different layers because the circumference of the internal layer is always smaller than that of the external layer in nanotube walls. The free energy of such imbalanced multilayer nanosheets is higher than that of corresponding curved structures, which probably drives the process of nanotube formation in the first place1,28 (reverse process from B to A in Figure 7). However, compensation of the width in imbalanced nanosheets under alkaline hydrothermal conditions can occur via crystallization on short layers or dissolution on long layers of TiO6 octahetra rather then nanosheet folding. This process can lead to formation of nanofibers instead of nanotubes (process form point B to C). The activation barriers, which stabilize nanotubes, can be overcome not only by intensification of mass transfer but also by providing an alternative path. Our attempts to facilitate formation of TiNT by addition of small quantities (1 wt %) of the nanotubes in the initial reaction mixture (the method also frequently termed seeding in crystallization process) have resulted in formation of TiNF under the conditions that favor formation of the nanotubes (see Figure 6c). To the best of our knowledge, such an effect of seeding on the morphology of nanostructured titanates has not been reported. It is possible that the occurrence of nanotubes at the early stages of the transformation dramatically changes the route of the reaction. Because nanotubes are added in the beginning of the process, their apparent contact time with alkaline solvent under hydrothermal conditions is greater that that for nanotubes produced during reaction. As a result, these added nanotubes can be converted to a more thermodynamically stable nanofibrous form within the time of the experiment. Such early conversion could also catalyze the process of TiNF formation by providing the alternative routes to point C, avoiding point A in Figure 7. The average mass of a single nanofiber having the characteristic dimensions shown in Figure 6c (20 nm  20 nm  1000 nm) is approximately 100 times larger than the average mass of a single nanotube from Figure 6a (the external diameter is ca. 10 nm, the internal diameter ca. 4 nm, and the length ca. 100 nm). Because the amount of added TiNT is

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100 times smaller than the total amount of the product, it is possible that each added nanotube catalyzes the growth of nanofibers on their surface during their unfolding. In other words, each obtained nanofiber might have been grown from a single nanotube seed. The metastable nature of titanate nanotubes has recently also been shown by their observation as an intermediate product in the reaction of TiNF formation at elevated temperatures (240 °C, 10 mol/dm3 NaOH, no stirring, duration of 2 h), which favor formation of nanofibers.29 It is remarkable that increasing the temperature further and shortening the duration of the reaction (450 °C, duration of ca. 0.5 min in a continuous flow reactor) results in formation of intermediate TiNS.30 As with most of the metastable states, they can only be achieved via a specific synthetic path. In the case of titanate nanotubes, this route should start from thermodynamically less stable species in alkaline solvent states, either TiO2 or TiNS. The use of either bulk sodium titanate crystals or TiNF never results in the formation of nanotubes31 in identical alkaline solvents. Only the use of an acidic32 or neutral33 environment can reverse the process to yield TiNT or TiNS.34,35 Conclusions Systematic studies of the effect of synthetic conditions, including the temperature and composition of the binary NaOH/KOH aqueous mixture, on the morphology of nanostructured titanates, formed during hydrothermal treatment, have allowed identification of the zones of synthetic conditions under which formation of particular nanostructures, such as nanosheets, nanotubes, and nanofibers, is favorable. The shape of these zones in temperature-composition diagrams coincides with the shape of the zones confined by curves having a constant equilibrium concentration of Ti(IV) in binary NaOH/KOH solutions, which confirms the importance of the crystallization and dissolution in the mechanism of nanostructured titanate growth. The apparent distribution map of morphologies of nanostructured titanates as a function of synthetic conditions, having significant practical application, cannot be considered via a simple phase diagram because the acceleration of the reaction rate was found to distort it. The intensification of the reaction rate has been achieved both by stirring and by adding titanate nanotubes to the initial reaction mixture. In both cases, nanofibers rather than nanotubes were formed at low temperatures. The general findings suggest that not only the intermediate titanate nanosheets but also the nanotubes are metastable. Furthermore, such behavior could be generalized over a wider range of multiwall nanotubes (spontaneously formed in templateless hydrothermal conditions), allowing the prediction or improvement of synthetic conditions for future and existing nanotubes. Acknowledgment. We gratefully acknowledge financial support from the EPSRC, U.K. (Grant EP/F044445/1, “A hydrothermal route to metal oxide nanotubes: Synthesis and energy conversion applications”). Supporting Information Available: An additional equation and ΔH/R and A as functions of the molar fraction of NaOH in a binary NaOH/KOH mixture (Figure S2) . This material is available free of charge via the Internet at http:// pubs.acs.org.

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