Non-solvolytic synthesis of aqueous soluble TiO2 nanoparticles and real-time dynamic measurements of the nanoparticle formation
© Chen et al.; licensee Springer. 2012
Received: 26 January 2012
Accepted: 7 June 2012
Published: 7 June 2012
Highly aqueously dispersible (soluble) TiO2 nanoparticles are usually synthesized by a solution-based sol–gel (solvolysis/condensation) process, and no direct precipitation of titania has been reported. This paper proposes a new approach to synthesize stable TiO2 nanoparticles by a non-solvolytic method - direct liquid phase precipitation at room temperature. Ligand-capped TiO2 nanoparticles are more readily solubilized compared to uncapped TiO2 nanoparticles, and these capped materials show distinct optical absorbance/emission behaviors. The influence of ligands, way of reactant feeding, and post-treatment on the shape, size, crystalline structure, and surface chemistry of the TiO2 nanoparticles has been thoroughly investigated by the combined use of X-ray diffraction, transmission electron microscopy, UV-visible (UV–vis) spectroscopy, and photoluminescence (PL). It is found that all above variables have significant effects on the size, shape, and dispersivity of the final TiO2 nanoparticles. For the first time, real-time UV–vis spectroscopy and PL are used to dynamically detect the formation and growth of TiO2 nanoparticles in solution. These real-time measurements show that the precipitation process begins to nucleate after an initial inhibition period of about 1 h, thereafter a particle growth occurs and reaches the maximum point after 2 h. The synthesis reaction is essentially completed after 4 h.
KeywordsSynthesis Nanoparticles TiO2 Aqueously soluble Direct liquid phase precipitation Dynamic real-time measurement
Titania (TiO2) is an important oxide with commercially exploitable physical and chemical properties. There are a variety of applications in, e.g., gas sensing [1–3], catalysis [4, 5], photocatalysis [6–8], optics [9–14], photovoltaics [15–17], and pigmentation [18, 19]. TiO2 nanoparticles (NPs) have been intensively studied over the past two decades. Several different methods have been developed for the synthesis of TiO2 NPs, e.g., sol–gel, reverse micelle method , and non-hydrolytic method . Among them, the solution-based sol–gel synthetic route is the most widely used approach and consists of two continuous steps: solvolytic reaction (usually hydrolysis) of a titanium salt and a further condensation reaction. Based on the solvent used as the reactant in the first step, it can be described as hydrolysis (reaction with water) [22–25], alcoholysis (reaction with alcohols) [26–28], and ammonolysis (reaction with ammonium ions) [29, 30]. The synthesis reaction and subsequent treatments are carried out under ambient [31–33] or solvothermal conditions [11, 34–38]. Recently, a novel solution-based method to synthesize nanostructured metal oxides - the direct liquid phase precipitation (DLPP), has been developed by us . In this approach, a great variety of metal oxide NPs can be easily prepared by exchange of anions of a corresponding metal salt (e.g., CuCl, ZnCl2, FeCl3, and SnCl4) with alkali metal oxides in non-aqueous conditions. Here, the method is used to make aqueously soluble TiO2 NPs from their titanium salt and lithium oxide in the presence of some amphiphilic capping ligands, i.e., gallic acid and dopamine, where the surface functionalization of the TiO2 NPs is made in situ (within the synthesis mixture) so as to define NP dimension and dispersivity.
General procedure for bare TiO2 NP synthesis
In a typical synthesis (stoichiometrical non-synchronous addition), 0.5 to 1.0 mmol of titanium(IV) chloride tetrahydrofuran complex was dissolved into 10 mL of anhydrous ethanol, forming yellow-like clear solution A. Lithium oxide of 1.0 to 2.0 mmol was dispersed into 10 mL of anhydrous ethanol by sonication for 30 min. This solution was filtered using a PTFE filter membrane (0.45 μm pore size) to form cloudy solution B. Then, solutions A and B were mixed either in a simple way by pouring B into A within 1 s or in a slow way by feeding B into A using a syringe pump within 1 h (non-synchronous) under rigorous stirring at room temperature (RT). After 24 h, a white precipitate was formed upon slow ethanol evaporation, where about two-thirds of ethanol was evaporated in the case of pouring addition or formed directly in the case of syringe pump feeding. Then, the white precipitate was collected by filtration using Nylon filter membranes (0.22 μm pore size) and washed with absolute ethanol three times before drying overnight in air at 60 °C. In order to check the dynamic process of the precipitation reaction, solutions A and B were added at the same time with the same rate (synchronous) or different rate (non-synchronous) using a syringe pump.
For a synchronous addition, 10 mL of solutions A and B was simultaneously and stoichiometrically added to a reactor pre-filled with 20 mL of anhydrous ethanol using a syringe pump (model: KDS–270, KD Scientific Inc., Holliston, MA, USA) at a feeding rate of 2 mL·h−1 under rigorous stirring at RT. The resultant mixture was divided into two equal portions. One portion of the solution was constantly stirred at RT under exposure to ambient until a white precipitate was formed. The other portion of the solution was transferred into 30 mL of deionized water in a Teflon-lined autoclave which was then constantly heated at 150 °C for 24 h until a white precipitate was harvested. The white precipitates of both reactions were collected by filtration and washed with absolute ethanol three times before drying overnight in air at 60 °C. The non-synchronous feeding was carried out in a non-stoichiometric way, where 1.0 mL of solution B was transferred to a reactor and the remaining 9 mL of solution B and 10 mL of solution A were then fed synchronously (1 mL·h−1) into the reactor under rigorous stirring at RT. The precipitated products were collected by centrifugation at a speed of 10,000 rpm for 20 min and washed with absolute ethanol three times and dried overnight under vacuum at RT.
Gallic acid-capped TiO2 NP synthesis
Gallic acid (GalA) of 0.25 mmol was added into 10 mL of 0.05 M titanium(IV) chloride tetrahydrofuran complex (i.e., solution A above) prior to the addition of 10 mL of 0.1 M lithium oxide (solution B) as described in the general procedure. The resultant mixture was stirred (in ambient as previously described) until a yale precipitate was formed. The precipitate was harvested, washed, and dried as described above.
Dopamine-capped TiO2 NP synthesis
The process is similar to the previous process except that an equivalent mole (0.25 mmol) of dopamine hydrochloride (Dpa) was used instead of gallic acid.
Powder X-ray diffraction (XRD) patterns were recorded on a Philips X’pert MPD diffractometer (Amsterdam, The Netherlands) using Cu Kα radiation and a working voltage of 40 kV. Transmission electron micrographs (TEM) were taken on a JEM-2011 (Jeol Ltd., Akishima-shi, Japan) electron microscope operating at 200 kV. TiO2 NPs were dispersed into ethanol before use, and one or two drops of the above solution were transferred onto a holey carbon film on copper grids under dry ambient atmosphere at RT and dried overnight. Photoluminescence (PL) and UV-visible (UV–vis) absorbance spectra were obtained using a PerkinElmer LS50B fluorescence spectrometer (Waltham, MA, USA) and a Cary 50 UV-visible spectrophotometer (Agilent Technologies, Inc., Santa Clara, CA, USA), respectively, where 3 mL of clear aqueous solutions containing TiO2 nanoparticles were placed in quartz cuvette cells for the optical analysis. The real-time UV–vis absorbance spectra were collected at intervals of 1 min up to 5 h.
Results and discussion
Hydrothermal treatment of these non-stoichiometric preparations produces very different materials from those in Figure 2b. TEM (Figure 3b) shows that the as-synthesized NPs aged under the hydrothermal conditions have almost similar size as those in the simple stoichiometric preparation (Figure 2a). The small-sized NPs tend to aggregate on the TEM grid (Figure 3b) due to the strong binding interaction caused by the large specific surface area. These small sphere-like particles sharply contrast the rod-like product synthesized by the stoichiometric reaction (Figure 2b). The shape difference of the hydrothermally aged stoichiometric and non-stoichiometric samples probably derives the Ostwald ripening to a different extent due to the different particle size distribution. In the stoichiometric preparation, the as-synthesized products are polydisperse particles with size distribution in a quite broader range. The larger particle aggregates have a slower dissolution rate than the smaller particles under hydrothermal conditions and need a longer time to reach the dissolution-recrystallization equilibrium. Therefore, it favors NRs to be grown under this condition. For the non-stoichiometric reaction, the particles have largely uniform/monodisperse NP aggregates (Figure 3a) and the thermodynamic driving force for the Ostwald ripening is, therefore, minimized.
Surface functionalization of TiO2 NPs
Formation mechanism of TiO2 NPs
The DLPP method used here has several advantages in terms of a direct method of synthesizing nanoparticles in a highly controlled environment. A reaction mechanism can be proposed as described below.
As seen in the above equations, TiCl4·2THF molecule dissociates and reacts with ethanol to form a new complex (1). This complex ion further reacts with alkali metal oxide (Li2O) to form TiO2(THF)2 cluster which is an unstable oxide precursor (2). Through cluster condensation reaction, the small TiO2(THF)2 clusters will grow up. The particle growth and precipitation do not occur unless the cluster size reaches to a critical point (3). It is clearly seen from Equation 2 that either the in situ increase of the Li2O concentration, for example by pouring addition, or the evaporation of ethanol can result in a rapid formation of the clusters (nucleus) and precipitation. However, the solvent evaporation also favors the formation of the final product as depicted in Equation 3. These are consistent with the facts that a non-stoichiometrical addition of TiCl4·2THF to Li2O solution results in a faster particle precipitation than the synchronous stoichiometrical mixing of the two solutions as shown in Figures 3 and 4.
Real-time dynamic measurements of the TiO2 NP formation
3D PL spectra provide some complimentary information (Figure 7b) on the range where the 3D UV–vis spectra do not work effectively. The 3D emission data show that the nucleation starts on the 33rd minute and reaches the most intense point on the 79th minute which is significantly earlier than that of the corresponding 3D UV–vis absorbance data (67th and 97th minutes). The shorter inhibition time detected in 3D PL emission indicates that large numbers of unstable clusters formed in the inhibition and/or nucleation stage might contribute more to the fluorescence emission rather than the UV–vis absorbance since these tiny clusters show almost no absorbance in the UV–vis spectra (Figure 7a) but stronger emission in the PL spectra (Figure 7b) from the 33rd to the 67th minute. The emission photon wavelength redshift along the emission intensity vs. reaction time isoline is consistent with the particle growth tendency. This agreement might be explained by the understanding on the ‘quantum size effect’ observed in small particles. Larger particles have smaller specific surface area and less strain imposed by the external stress compared with smaller particles. The small particles with higher surface stresses are manifested as a widened energy level gap. Thus, for a given energy level, the emitted photon wavelength (energy) caused by a fixed excitation wavelength, e.g., 350 nm, varies with particle size. The bigger the particle, the longer the wavelength of the emitted photons is, and it is, therefore, reasonable to conclude that the increase of the wavelength of the emitted photon with reaction time is a reflection of the particle growth tendency.
Aqueously soluble sub-10-nm TiO2 NPs have been successfully synthesized by a non-solvolytic method at room temperature. The as-synthesized TiO2 NPs are largely amorphous but can be crystallized by mildly higher temperature or hydrothermal treatment. However, the hydrothermally treated sample shows different morphologies where instead of spherical NPs, a higher aspect ratio product - nanorods - are formed. Both gallic acid- and dopamine-capped TiO2 NPs have excellent solubility and stability in water (or other polar solvents) and show distinct UV–vis absorbance and photoluminescent emission properties compared with the uncapped TiO2 NPs in aqueous solution. The dispersivity of the TiO2 NPs prepared in the presence of gallic acid or dopamine is improved dramatically, and both the carboxylic (GalA)- and amine (Dpa)-capped TiO2 NPs tend to form larger secondary particles and have better monodispersivity on them. The different surface chemistries for bare, GalA-, and Dpa-TiO2 NPs are clearly revealed by the FT-IR vibrating/stretching features. By varying the feeding procedure, the influence of the (non-) stoichiometric chemistry has been investigated. The stoichiometric feeding favors the formation of polydispersed TiO2 NPs while the non-stoichiometric feeding prefers the formation of uniform NP aggregates.
3D real-time measurements show abundant information on the precipitation of TiO2 NPs, where a series of progressive reactions involving inhibition, nucleation, growth, and sedimentation have been investigated. Free Ti4+ ions react with the alkaline oxide initially to form unstable and stable clusters in the inhibition and the nucleation stage; eventually, thermodynamically stable NPs are formed and settled down in the growth and the sedimentation stage. Both 3D UV–vis and PL spectra confirm a linear growth tendency with reaction time for TiO2 NPs.
LC (Ph.D.) is a Marie-Curie Intra-European Fellow. NKHS (Ph.D.) is a professor at the Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, UK. KR (Ph.D.) is an associate professor at the Faculty of Natural and Applied Sciences, Department of Sciences, Notre Dame University, Louaize, Lebanon. MAM (Ph.D.) and JDH (Ph.D.) are professors at the Materials Section and Supercritical Centre, Department of Chemistry, University College Cork, Cork, Ireland.
We thank the Science Foundation of Ireland (SFI) and the European Commission FP7 People Programme for the funding supports through the BIONANOINTERACT Strategic Research Cluster grant (07/SRC/B1155) and Marie Curie Intra-European Fellowships grant.
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