Near Room Temperature Synthesis of Monodisperse TiO2

Article pubs.acs.org/JPCC

Near Room Temperature Synthesis of Monodisperse TiO2 Nanoparticles: Growth Mechanism Jenny Perez Holmberg,† Ann-Cathrin Johnson,†,‡ Johan Bergenholtz,† Zareen Abbas,† and Elisabet Ahlberg*,† †

Department of Chemistry and Molecular Biology, University of Gothenburg, SE-412 96 Gothenburg, Sweden Höganäs AB, Bruksgatan 35, SE-263 83 Höganäs, Sweden

‡

S Supporting Information *

ABSTRACT: Hydrolysis of TiCl4 was used to form monodisperse nanoparticles of TiO2 with clean surfaces. The solid fraction and solution composition during synthesis were simulated using equilibrium data, and formation and growth was followed with two complementary techniques, an electrospray-scanning mobility particle sizer (ES-SMPS) and dynamic light scattering (DLS). In ES-SMPS the number density of particles is measured. Droplets formed in the spraying step mainly contain electrolyte, giving rise to residue particles that are detected together with the nanoparticles of interest. Discrimination between the two kinds of particles can be made by changing the flow conditions and applicability of the method for in situ measurements of particle size during growth is demonstrated. In DLS the hydrodynamic mobility is measured, and further insight into the initial growth mechanism was revealed by observation of slow, sustained oscillations in the scattered intensity, indicating a dissolution− precipitation mechanism at the lowest pH values. The size of the particles formed in the dissolution−precipitation step is most likely determined by the surface charge, and larger particles are formed by aggregation.

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INTRODUCTION Oxide nanoparticles have shown increasing use as catalysts due to the possibility to tune the properties for specific applications. In particular, the effect of size and morphology has been studied, and fairly simple synthesis methods are used to control the properties.1,2 Oxide nanoparticles also find interest in fundamental research, for example, in studies of transport properties in aqueous solution,3 the solid−liquid interphase,4,5 solid−solid interactions,6 etc. In solution, parameters such as surface charge, as determined by pH, electrolyte composition, and strength are important for the underlying mechanism of these phenomena and also decisive for the outcome of the synthesis, since they influence both the size and the morphology of the nanoparticles.7−10 For many applications it is important to have clean surfaces, and hydrolysis of TiCl4 in aqueous solution offers a convenient route for synthesis.10−14 The solubility of titanium dioxide is very low, and it is believed that an amorphous titanium oxide phase is formed initially once the critical supersaturation level is obtained.15 This phase is primarily transformed into nanoparticles of the least thermodynamically stable anatase phase which has the lowest interfacial tension.7 The size of the nanoparticles is determined by the surface charge and depends strongly on pH. Under acidic conditions where the solubility is higher a dissolution− precipitation reaction can take place, favoring the more © 2013 American Chemical Society

thermodynamically stable rutile phase. The three polymorphs of TiO2 can be synthesized by control of pH, temperature, and electrolyte anions. For conditions where chloride complexes are present in solution, brookite is favored due to site blocking in the initial olation reaction.16 In strong chloride media brookite is stable also after heat treatment but transforms into rutile in the absence of chloride.8 Nucleation and growth of titanium dioxide have been studied at elevated temperatures and in relation to hydrothermal synthesis.6,15,17 A three-stage process is proposed composed by an induction period before stable particles are formed followed by initial and saturation growth.15 The growth mechanism depends on the solubility of TiO2 in solution. A dissolution− precipitation process will favor growth by Ostwald ripening, while for conditions where the concentration of soluble species is low, a solid state transformation is dominating. The aim of the present study is to follow the evolution of nanoparticles during synthesis and aging. For this purpose two complementary techniques are used, an electrospray-scanning mobility particle sizer (ES-SMPS) and dynamic light scattering (DLS). While the former measures the number density of Received: January 5, 2013 Revised: February 15, 2013 Published: February 19, 2013 5453

dx.doi.org/10.1021/jp4001434 | J. Phys. Chem. C 2013, 117, 5453−5461

The Journal of Physical Chemistry C

Article

In the electrospray apparatus, samples containing particles suspended in liquid pass through a capillary with an internal diameter of 20−40 μm. The tip of this capillary acts as an electrode; a metal plate, the counter electrode, is located at a small distance from the tip. It is in this region that an electric field is generated, which results in production of aerosols. The particle-containing aerosols are then transported, in a particlefree carrier gas, to the scanning mobility particle sizer (SMPS), which consists of a differential mobility analyzer (DMA) and a condensation particle counter (CPC). Particles are size classified according to their electrical mobility in the DMA. The number of particles contained in each resulting size fraction is then determined by the CPC.23,24 Dynamic Light Scattering. Hydrodynamic size determination was conducted using a Malvern Zetasizer Nano ZS (ZEN 3600) instrument with a red He−Ne laser (λ = 633 nm). Measurements were done at 25 °C and a fixed scattering angle (θ = 173°). Samples were taken directly from the reaction flask and dialysis bags without further dilution. Intensity data were collected over the duration of 180 s for each measurement. An apparent hydrodynamic diameter was extracted from a secondorder cumulant analysis of the intensity correlation function assuming dilute-limiting conditions where the Stokes−Einstein expression for the diffusion coefficient for spherical particles applies.25 Thermodynamic Calculations. Thermodynamic calculations were made using the software SolGasWater (SGW).26 Equilibrium constants for the different reactions were extracted from the literature and are given in Table 1. Since an

particles, it is hampered by simultaneous detection of electrolyte residues. The influence of solution residues on the applicability of the method for in situ detection of nanoparticles during synthesis is reported for two systems. It is demonstrated that under favorable conditions discrimination is possible since the size of the salt residues depends on the flow rate while the nanoparticle size is unaffected. However, for small nanoparticles discrimination between salt deposition during electrospraying and stable nanoparticles might be difficult. The ES-SMPS technique is therefore complemented with DLS, where the hydrodynamic mobility of the nanoparticles is measured and recalculated to size. Formation of nanoparticles is discussed in relation to solution composition during formation of the precursor solution and during dialysis, where both the pH and the chloride concentration changes. The optimum size observed experimentally is explained in the context of sizedependent surface charging.4

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EXPERIMENTAL METHODS Materials. Colloidal suspensions of TiO2 were prepared from hydrolysis of titanium tetrachloride (99% synthesis grade, Merck). Cellulose dialysis membranes (MWCO 1000 Da, Spectrapor) were used as part of the synthesis protocol. Bindzil 40/130 silica particles were supplied by Eka Chemicals AB. The size of the particles was determined using DLS (Supporting Information Figure S1). Stock solutions of the electrolyte FeCl3 (pro analysi, Merck), the buffer solution for the ES system (ammonium acetate, 20 mM, pH 8.0), and sucrose (99.5%, Sigma) were prepared using water purified by a Milli-Q purification system (18.2 MΩ cm, Synergy 185, Millipore). The sodium chloride (supra pure, Merck), ammonium acetate (pro analysi, Merck), and ammonia (pro analysi, Scharlau) were used as received. Synthesis of TiO2. Titanium dioxide colloids were synthesized from TiCl4 using a controlled hydrolysis route described elsewhere.11 TiCl4 was used as received without further purification. For the experiments presented in the present investigation, 5.50 mL of precooled TiCl4 was added dropwise to 200 mL of purified deionized water kept at 0 °C. TiCl4 was added in 0.5 mL increments with 5 min intervals. HCl gas produced in the reaction was evacuated by flowing clean air into the reaction flask and out through a bubbler containing 5 M NaOH for 3 h. Precursor solution was then transferred to dialysis membranes, about 100 mL per bag, and dialyzed against 3 L of deionized water at room temperature. Dialysis water was changed every 30 min during the first 3 h; hourly water changes were done afterward. Conductivity and pH were monitored during dialysis. Dialysis proceeded until the pH of the resulting suspension was around pH 2.5. Grazing angle X-ray diffraction on dried particles confirmed formation of TiO2 particles with preferential anatase structure (Supporting Information Figure S2). Size Distribution Measurements. Electrospray-Scanning Mobility Particle Sizer. An electrospray unit (TSI Inc., ES model 3480) and a scanning mobility particle sizer (TSI Inc., SMPS model 3936) were used to measure the size distribution of the crystallites and TiO2 particles during synthesis. This combination has been shown to be capable of sizing colloidal nanoparticles, such as silica, gold, and TiO2, with high accuracy and reproducibility.18,19 The method has been described in detail elsewhere,18−22 and a complete description of the experimental setup is given in Johnson et al.23,24 Therefore, only a short description is given here.

Table 1. Reactions and Equilibrium Constants Used for Simulating Hydrolysis of TiCl4 reaction TiOH3+ TiOH3+ TiOH3+ TiOH3+ TiOH3+ TiOH3+ TiOH3+ TiOH3+

+ + + + + + + +

H2O ⇄ Ti(OH)22+ + H+ 2H2O ⇄ Ti(OH)3+ + 2H+ 3H2O ⇄ Ti(OH)4 + 3H+ 2H2O ⇄ TiO(OH)2 (s) + 3H+ Cl− ⇄ TiOCl+ + H+ 2Cl− ⇄ TiOCl2 + H+ 3Cl− ⇄ TiOCl3− + H+ 4Cl− ⇄ TiOCl4− + H+

log β

ref

−1.80 −4.20 −6.30 −1.3 −1.25 −1.65 −2.68 −3.75

36 36 36 27 38 38 38 38

amorphous phase is believed to form initially10 the equilibrium constant for “active” titanium dioxide as defined by Baes and Messmer27 (written as TiO(OH)2(s)) is used. No corrections were made for the ionic strength, and the temperature was set to 25 °C. In the simulations, TiOH3+, H+, and Cl− were used as components.

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RESULTS AND DISCUSSION Analysis of Aerosols Generated by Electrospray. Prior to size classification, aerosols containing the particles to be analyzed are produced by the electrospray. In order to achieve an accurate particle size determination, the process of aerosol generation necessitates close examination. A number of influential factors have to be taken into account, such as the size of the aerosol droplets as well as the presence of nonvolatile species other than the particles being studied. Evaporation residues can occur as a byproduct of the electrospray process. It is thus worthwhile to determine the extent to which these residues influence the particle size information obtained in the later stages of size analysis. 5454



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The size of electrosprayed droplets scales with the liquid flow rate (QL) and the electrical conductivity (κ) of the solution according to eq 1,19 where dd is the droplet size while ε and ε0 are the relative dielectric permittivity for the solution and vacuum permittivity, respectively. The droplet size can be related to the size of the residual particles (dm)19,20 according to eq 2, where c is the concentration of the nonvolatile species.

⎛ Q Lεε0 ⎞1/3 dd ∝ ⎜ ⎟ ⎝ κ ⎠ dd ∝

Differentiation of Particles from Evaporation Residues in SiO2/Sucrose Suspensions. Given that evaporation residues are transported along with particulate matter from the electrospray apparatus to the SMPS, the next step was to investigate if it is possible to differentiate between these two types of measured sizes. Silica particles were dispersed in a sucrose/buffer solution and tested at different flow rates. Results of these experiments are presented in Figure 2.

(1)

1

c

d 1/3 m

(2)

The residue particle size will therefore scale with the cube root of the flow rate at constant concentration and with the inverse cube root of concentration at constant flow rate. To examine the effect of sample conductivity and flow rate on droplet generation, solutions of different composition were tested on the ES-SMPS system. First, sucrose in water was used to measure the size of the residues as a function of concentration and flow rate in order to confirm the dependencies listed in eqs 1 and 2. Second, monodispersed silica particles were added and measurements made as a function of flow rate. Third, similar measurements were made in an electrolyte, with and without well-defined TiO 2 nanoparticles. The capability to distinguish between particulate matter and evaporation residues was determined by means of size distribution peak analysis over a range of flow rates. Concentration Dependence of Evaporation Residues from Sucrose. Results from ES-SMPS analysis of sucrose solutions of different concentrations are presented in Figure 1. At a constant

Figure 2. Size distribution of a 0.01 v/v % sucrose/buffer solution containing 36.6 ± 0.2 nm SiO2 nanoparticles at different sample flow rates. (Inset) Diameter with respect to flow rate. Black triangles show measured SiO2 sizes, while filled and unfilled circles, respectively, illustrate the flow rate dependence of the sucrose solutions with and without SiO2: 1 PSI = 37 nL/min.

Regardless of the presence of SiO2 particles, the sucrose peak follows a nearly 1/3 power-law dependence on the sample flow rate in accordance with eq 1. This is in contrast to the SiO2 peak, which remains fixed at around 36 nm. The constant size determined for the SiO2 nanoparticle shows that only one nanoparticle per droplet is transported in the electrospray for the range of flow rates used. The shifting peak is a distinct indicator of evaporation residues, and it is clearly demonstrated that particulate matter can be discriminated from solution residues. However, for an unknown system with nonvolatile solutes and particulate matter care must be taken to separate the two. Differentiation of Particles from Evaporation Residues in TiO2/FeCl3 Suspensions. Given the large size difference between particulate and residues in the SiO2/sucrose system, a similar approach was designed with the TiO2/HCl/FeCl3 system to mimic the ionic conditions during synthesis of TiO2 nanoparticles. The particle size distribution of a FeCl3/HCl solution containing TiO2 nanoparticles is shown in Figure 3. At the lowest flow rates, the peaks corresponding to TiO2 nanoparticles and FeCl3 residues are easily distinguished. However, when the flow rate increases, overlaps between the two peaks become increasingly significant. The position of the peak related to electrolyte residues shifts to larger values as the flow rate increases. The diameter of the residue particles follows the expected cube root dependence on flow rate, dd = 1.23QL0.36, where QL is given in nL/min. The peak associated with TiO2 nanoparticles is initially constant, but at a flow rate larger than 45 nL/min, a slight drop in the measured diameter

Figure 1. Size distribution data of evaporation residues from different concentrations of sucrose. (Inset) Sucrose peak sizes: black squares and red circles show the concentration dependence of the primary and secondary peaks, respectively. Both peak sizes follow a concentration dependence of dm ∝ c1/3.

flow rate, the detected number average diameter increases correspondingly with sucrose concentration, following the c1/3 dependence as described in eq 2. Sucrose has been previously used to test the electrospraying conditions20 and discriminate between residual particles and, for example, proteins in solution.18 The second peak appears at larger sizes but is smaller in magnitude. The reason for a second peak is not clear, but the size indicates that two droplets are transferred, giving rise to a larger residue. 5455



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Figure 4. Particle size distributions obtained from ES-SMPS for the precursor solution and suspension during dialysis.

Figure 3. Size distribution of a suspension of TiO2 nanoparticles in a 0.96 mM FeCl3/HCl solution at different sample flow rates. (Inset) Peak sizes with respect to flow rate. Black squares show the measured TiO2 sizes, while filled and unfilled circles, respectively, illustrate the flow rate dependence of the FeCl3 solutions with and without TiO2.

is observed, after which the size is again independent of flow rate. Furthermore, an intermediate peak in between the TiO2 and the FeCl3 peaks is observed. This behavior can be accounted for if the TiO2 material is composed of loosely bound aggregates of smaller primary particles. The intermediate peak then arises from the separation of loose material from the initially larger aggregate. The reason for the shift of the peak with changes in the flow rate is probably that salt from the electrolyte is deposited on the small particle fragments. Even though overlap between residues and nanoparticles occurs to some extent it is clearly demonstrated that the residue particles can be separated from the nanoparticles by changing the flow rate. The conductivity of the test solution was in this case 5 mS/cm, and the data can be used to estimate the proportionality constant between the initial drop size determined from eq 1 and the size of the measured residue particles. A value of about 0.05 was obtained and will be used to calculate the expected diameter of the residue particles during TiO2 formation. TiO2 Formation. ES-SMPS Analysis during Dialysis of Precursor Solution. Synthesis of TiO2 nanoparticles was briefly described in Experimental Methods. ES-SMPS was used to follow the evolution of nanoparticles starting with measurements on the precursor solution followed by measurements during dialysis. The corresponding size distributions are shown in Figure 4 and will be discussed below. The limiting factor in the ES-SMPS setup is the highest particle concentration that can be analyzed by the CPC in order for the sample to be counted as individual particles. Thus, samples extracted from the synthesis solution were accordingly diluted. In order to minimize effects on particle formation, the dilution medium was made with 10 mM HCl. Both pH and conductivity of the solutions were measured during dialysis and are given in Figure 5 together with the size distributions as measured by ES-SMPS from Figure 4. Flow rate was set to 37 nL/min (1 PSI) to be in the region where the TiO2 particles are stable, see Figure 3. In the precursor solution the pH is below 1 and the conductivity is high. Dialysis against deionized water is started in two dialysis bags. Dialysis results in a roughly linear rise in pH as a function of time and is stopped when the pH in the dispersion reaches 2.5, which occurs 6 h after the start of

Figure 5. Size, pH, and conductivity data obtained during synthesis of TiO2 and the first 6 days of aging. First panel shows results from ESSMPS and DLS measurements, presented as squares and circles, respectively. Median between results from two different dialysis bags is shown by the dotted curves. Lower panel shows pH (triangles) and conductivity (stars) as a function of time.

dialysis. pH decreases to 2.2 with time, showing that the initially formed precipitate is partly dissolved, Figure 5. The diameter of residue particles can be estimated from the conductivity of the solution after dilution. Additional HCl will not add to the particle size because HCl is volatile. Using the values obtained in the FeCl3 system, the residue particles should range from 10 to 40 nm when the conductivity of the solution decreases. Much smaller sizes are obtained, indicating that nanoparticles are detected, Figure 4. In the precursor solution (time = 0) several low-magnitude peaks are observed, showing the existence of both nanoparticles and residues. Also, after the first dialysis step the peak is small and broad. As the pH 5456



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on complete conversion of reactants yields 0.5 vol %, which, for strongly interacting particles, may invalidate the use of the Stokes−Einstein relation. In addition, for low pH at the start of the dialysis ionic strengths are high, about 200 mM, leading to short-range interactions. Around 3 h into dialysis, the ionic strength has been reduced by an order of magnitude to about 30 mM, which causes electrostatic repulsions to become significantly longer range. This leads to a reduction in the collective diffusion coefficient,29 which translates into a smaller apparent diameter and a maximum in the DLS-determined size. Indeed, DLS measurements on diluted samples yielded a concentration-dependent collective diffusion coefficient, confirming that the nanoparticle concentration is too high in most of the undiluted dialyzed samples for determining a true hydrodynamic diameter. At the earliest times during dialysis, which brings the pH from 0.7 to 1.2, the static intensity, which generally fluctuates around a stable mean value, was discovered to exhibit sustained oscillations. These are shown as insets to Figure 6 for the first three DLS-measured apparent sizes. As seen, the oscillations in the intensity occur with a period of about 15 s and superimposed on them are the much faster, low-amplitude fluctuations which are the basis for measurement of diffusion coefficients in standard applications of DLS. The oscillations present in the intensity scattered from these samples typically cause no severe problems in analysis due to the extreme separation in time scales between the oscillations and the faster fluctuations. As seen in Figure 6, the oscillations change character as time progresses. Initially, very fast, small-amplitude fluctuations are superimposed on the oscillations, leading to an apparent hydrodynamic diameter of only 2 nm, considerably smaller than the X-ray crystallite diameter of 4.2 ± 0.1 nm.11 Possibly owing to the ionic strength being high, which leads to strong screening of electrostatic interactions and weaker interactions among particles, the first few hydrodynamic diameters extracted from the DLS measurements in Figure 6 may accurately reflect the particle size. In that case the initial jump (a to b in Figure 6), going from an apparent diameter of 2 to about 5 nm, seems to mark the point where solid, compact particles first appear. As seen from the insets to Figure 6, fluctuations in the intensity are quite different between the two. After some time, the fluctuations become somewhat slower and more pronounced. Finally, all that remains are the fluctuations reflecting diffusion of nanoparticles. Oscillations in scattered intensity have been reported before in connection with growth of micrometer-size particles30 but are observed here for the first time in connection with nucleation of nanoparticles. Sustained oscillations are well known to occur in chemical reactions when systems are maintained far from equilibrium,31 such as the famous Belousov−Zhabotinsky reaction,32 giving rise to chemical oscillations in stirred systems and exotic patterns in unstirred systems. Indeed, the pH is known to drive chemical oscillations,33 and it is well worth noting that the pH varies, increasing from below 1 to about 2.5 in the time interval shown in Figure 6. However, nanoparticles are not one of the products in such reactions. Conceivably, the oscillatory intensity in Figure 6 is more closely related to precipitation reactions, yielding colloidal species and patterns such as Liesegang rings in gel media,34,35 but the samples studied here are fluids of low viscosity. Hydrolysis of TiCl4 and Formation of TiO2. From experimental data it is generally agreed that at low pH values

increases, the peaks become higher and better defined. The diameter obtained increases slightly from about 6 to 9 nm during dialysis and stays constant for several days. On day 3, the size of the particles has increased and two peaks are observed in the distribution diagram (not shown). For longer times only one peak appears and the nanoparticle diameter stays constant at about 16 nm. Previous characterization11 of the solid material shows that the particles in solution are aggregates of smaller particles with a diameter of 4−5 nm with lattice fringes that indicate formation of anatase. The fact that the particle size determined is larger than the initial size shows that small aggregates can form in the dilution medium used. Real-Time Monitoring of Particle Growth by DLS. In contrast to the ES-SMPS measurements, which required diluting the dialyzed samples, DLS was conducted directly on undiluted samples during and after dialysis. For particles much smaller than the laser wavelength, the initial decay of the intensity correlation function yields the collective diffusion coefficient, which was converted into an apparent hydrodynamic diameter using the Stokes−Einstein relation.28 The apparent hydrodynamic diameters resulting from sampling from two dialysis bags are shown in Figure 6 as a function of time.

Figure 6. ES-SMPS diameter as a function of time, obtained from diluted samples from two dialysis bags, and apparent hydrodynamic diameter as a function of time from DLS on undiluted samples from the same two dialysis bags. Shown also, as insets, are intensity traces as a function of time (in seconds), as labeled, from which the apparent hydrodynamic diameters were obtained.

As with the ES-SMPS measurements, there are slight systematic differences between the results from the two dialysis bags, which we attach no special significance to. For each data point three measurements are made, and the standard deviation is commonly smaller than the size of the data point markers, Figure 6. The result from the two different dialysis bags merely shows the complexity of the system, and in particular, the short time behavior is decisive for the growth process. Since dialysis is interrupted at different times for the two dialysis bags the results will deviate somewhat. Whereas the ES-SMPS size increases monotonically with time, the hydrodynamic diameters exhibit a maximum roughly 2 h into dialysis, after which the apparent particle size decreases. This difference is rationalized by the fact that the DLS measurements are conducted on nondilute samples; estimating a particle volume fraction based 5457



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and chloride-free oxide is formed.16 The fact that chloride complexes are present during the initial precipitation explains the small amount of brookite found in the final product.11 Conductivity was calculated after each addition of TiCl4 and increases from 26 to 132 mS/cm (Supporting Information). It is assumed that only protons and chloride ions contribute to the conductivity. This leads to a slight underestimation of the conductivity under the most acidic conditions where most of the titanium is soluble. On the other hand, the mobility of protons is at least a factor of 10 larger than the mobility of the titanium complexes, and the proton concentration in this pH range is four times the total titanium concentration. A plot of conductivity, κ, as a function of pH is given in Figure 8 together with experimental data measured during dialysis.

soluble mononuclear hydrolysis complexes are formed in noncomplexing electrolytes, such as TiOH3+, Ti(OH)22+, Ti(OH)3+, and Ti(OH)4.27,36 A multinuclear complex has been proposed but not verified by other investigators,37 and thermodynamic calculations for conditions used in the present paper show that the multinuclear complex is not formed. Formation of chloride complexes has been reported38 but commonly ignored in thermodynamic modeling of the system.39 Since the presence of chloride complexes is considered important for formation of brookite and the synthesis protocol used11 produces a small amount of brookite, chloride complexes were also considered. Hydrolysis of Ti(IV) is difficult to study due to the low solubility of titanium dioxide also at low pH values. However, hydrolysis of titanium tetrachloride has been studied theoretically for reaction in the gas phase.40,41 These studies show that oxychloride or hydroxychloride complexes are active as precursors for formation of a solid phase. Since no temperature data are available for the chloride complexes, all calculations were made at room temperature, 25 °C. This means that calculations for the precursor solution underestimate the fraction of soluble complexes. In Table 1, reactions for the thermodynamic (SGW) calculations are listed together with formation constants extracted from literature. Figure 7 shows the distribution of complexes and the fraction of solid material during preparation of the precursor solution.

Figure 8. Conductivity as a function of pH for three separate dialysis runs.

Experimental data are somewhat scattered but fit quite well to the thermodynamic prediction. On the basis of these data, the concentration of chloride ions was calculated during dialysis. Thus, the changes in distribution of soluble complexes and the fraction of solid material could be calculated for the dialysis solution, Figure 9. The solid fraction increases drastically as pH and pCl increases, and we see an overshooting in pH before it stabilizes at 2.2, where the aging takes place. At this pH, the total concentration of soluble complexes is 30 μM, Figure 7. Distribution of hydrolysis complexes and solid material during formation of the precursor solution.

Pure water is used from the start, but after the first addition pH is lowered to 1.26 due to hydrolysis. At this pH about 80% of the added titanium ends up in solid matter. After each addition, the solid fraction decreases and is less than 20% at the final pH. Note that during preparation of the precursor solution the total concentration of titanium and chloride increases. TiOH3+ is dominating in the entire pH region, while the fraction of oxo− chloride complexes increases with decreasing pH and increasing chloride concentration. At pH = 0.72 (the final pH for a total concentration of 0.24 M Ti and 0.96 M Cl) the fraction of chloride-containing complexes is about 25% but diminishes significantly as the pH and pCl increases. The neutral oxy− chloride complexes have been suggested as a precursor for brookite formation by blocking certain directions of attachment in the initial olation reaction and thereby directing growth. The chloride ligands are later on removed in the oxolation process,

Figure 9. Distribution of titanium species during dialysis from pH 0.7 to just above pH 2.5. Black circles represent the amount of solid titanium in suspension. 5458



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assuming that the solid phase remains TiO(OH)2(s). If the first precipitate is converted to anatase, the soluble content will be lower. Thermodynamic calculations indicate that solid matter might already be present in the precursor solution. ES-SMPS measurements show considerable scatter between experiments with a low-intensity broad distribution, Figure 4. Similarly, in DLS replicate measurements show some differences in the hydrodynamic size of particles in solution, Figures 5 and 6. Also, during early dialysis when the pH of the solution is still less than 1, oscillations in the DLS spectra are observed. These oscillations can be explained by a slow dissolution− precipitation reaction where changes in local pH close to the particles changes the direction of the reaction. During early dialysis, solution complexes are dominating according to the thermodynamic calculations and growth of solid matter takes place by incorporation of mononuclear complexes to existing particles. The incorporation process results in a lowering of the pH close to the solid−liquid interphase, and at some point the dissolution reaction becomes dominating and the pH close to the surface again increases. These subtle changes at the solid− liquid interphase are dependent also on the bulk pH and the availability of titanium complexes in solution. When the solid− solution fraction is larger than 1, the oscillations are no longer detectable and stable particles are formed. The process can be illustrated by the following reaction.

goethite system.4,42 Using the same approach for titanium dioxide, the surface charge density has been calculated for different particles sizes at conditions during dialysis, Figure 10. Three different conditions were considered, two in the early stages of particle formation and one at aging conditions.

Figure 10. Surface charge density calculated as a function of particle size using CDH-SC theory: (■) pH = 0.8 and [Cl−] = 0.5 M, (●) pH = 1.0 and [Cl−] = 0.3 M, (▲) pH = 2.5 and [Cl−] = 0.01 M.

In the nucleation situation at low pH the high ionic strength will lead to enhanced screening of the small highly positively charged particles, neutralizing them. Since the dominating hydrolysis complexes are also positively charged, see Figure 9, neutralization is a requirement for nucleophilic attack from the complexes and for particle growth. The increase in particle size will in turn lower the surface charge density. At a particular pH and pCl the surface charge density decreases rapidly and becomes fairly constant for sizes larger than 4 nm. This fits well with the observed initial size of the particles and supports formation of aggregates as the nanoparticles become larger. The main portion of the particles is formed in the pH interval between 0.7 and 1.3. In this interval the chloride concentration is lowered and the screening of the small particles diminishes. The fact that the shape of the surface charge curve is similar in the entire pH/pCl range of dialysis also supports the agglomeration of small particles. Keeping the pH constant at low values will result in larger particles due to the enhanced screening.

− x) + [TiO(OH)2 ]n + z Ti(OH)(4 x k1

⇄ [TiO(OH)2 ]n + z + z(4 − x)H+ k2

(3)

The frequency of the oscillations depends on the rate of the forward and backward reactions. This mechanism is further supported by the overshooting of pH in the dialysis process. When the concentration of solution complexes becomes low, the growth mechanism changes and transformation to more crystalline matter occurs. The solubility given in the thermodynamic calculations refer to an “active” titanium oxide as defined by Baes and Messmer.27 With time the crystalline phases are formed with a further decrease in the complex concentration and more aggregation is observed, giving rise to stable aggregated particles with about 16 nm size, Figure 5. Particle Size and Surface Charge. The correlation between particle size and surface charge density found for many systems7 suggests that the stability is thermodynamically driven by the variation of the surface tension and primarily depends on pH.7,9 Besides the influence of pH the surface charge density of oxide nanoparticles has also been found to be highly size dependent with increasing charge density as the size gets smaller. On a microscopic scale this can be understood as a larger accumulation of counterions close to the particle surface as the particle radius is decreased.4 Inclusion of both the variation of surface tension with the charge and the charge with the particle size may offer a better understanding and prediction of nanoparticle growth. In the corrected Debye−Hückel theory of surface complexation (CDH-SC)4,42 the oxide nanoparticles are considered as spheres with only one type of surface sites distributed over the surface. Proton binding is described by a 3D harmonic oscillator in which the proton has a specific frequency and interaction energy. These parameters are oxide dependent, and more details about the model have been described for the

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CONCLUSIONS The thermodynamic calculations indicate that solid material may form already in the precursor solution. This is supported by ES-SMPS measurements in the precursor solution, where a low intensity and broad distribution of solid material is observed, related to nanoparticles and residues. The growth mechanism of TiO2 from hydrolysis of TiCl4 involves initially dissolution−precipitation at low pH values where the solubility of the “active” titanium dioxide is still high. Growth of nanoparticles takes place by incorporation of mononuclear complexes to already existing solid material. This is supported by oscillations in the scattered intensity in DLS and can be explained by counteracting slow dissolution and precipitation reactions, where the rate is dependent on the local pH. At larger pH values when the solid fraction is >99% stable particles are formed with a size mainly determined by the surface charge. Since the chloride concentration decreases 5459



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during dialysis screening of the particles diminishes and aggregation can take place. At this stage in the synthesis procedure stable agglomerates of nanoparticles are formed. Applicability of the ES-SMPS method for in situ detection of nanoparticle size during growth was demonstrated. This technique is particularly valuable since it provides a direct measure of size.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +46 31 7869002. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the University of Gothenburg Strategic Research Platform “Nanoparticles in Interactive Environments” and the Swedish Research Council (2005-21028-35344-27) is gratefully acknowledged. Silicon oxide particles (Bindzil 40/ 130) were kindly provided by Eka Chemicals AB.

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REFERENCES

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Near Room Temperature Synthesis of Monodisperse TiO2

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