Role of Surface Area, Primary Particle Size, and Crystal Phase on Titanium Dioxide Nanoparticle Dispersion Properties
- Komkrit Suttiponparnit†1, 2,
- Jingkun Jiang†1,
- Manoranjan Sahu1,
- Sirikalaya Suvachittanont2,
- Tawatchai Charinpanitkul3 and
- Pratim Biswas1Email author
© Suttiponparnit et al. 2010
Received: 30 June 2010
Accepted: 18 August 2010
Published: 3 September 2010
Characterizing nanoparticle dispersions and understanding the effect of parameters that alter dispersion properties are important for both environmental applications and toxicity investigations. The role of particle surface area, primary particle size, and crystal phase on TiO2 nanoparticle dispersion properties is reported. Hydrodynamic size, zeta potential, and isoelectric point (IEP) of ten laboratory synthesized TiO2 samples, and one commercial Degussa TiO2 sample (P25) dispersed in different solutions were characterized. Solution ionic strength and pH affect titania dispersion properties. The effect of monovalent (NaCl) and divalent (MgCl2) inert electrolytes on dispersion properties was quantified through their contribution to ionic strength. Increasing titania particle surface area resulted in a decrease in solution pH. At fixed pH, increasing the particle surface area enhanced the collision frequency between particles and led to a higher degree of agglomeration. In addition to the synthesis method, TiO2 isoelectric point was found to be dependent on particle size. As anatase TiO2 primary particle size increased from 6 nm to 104 nm, its IEP decreased from 6.0 to 3.8 that also results in changes in dispersion zeta potential and hydrodynamic size. In contrast to particle size, TiO2 nanoparticle IEP was found to be insensitive to particle crystal structure.
Nanotechnology is finding applicability in the field of environmental protection and has great potential in improving air, water, and soil quality . For example, engineered nanoparticles can efficiently reduce toxic metal emissions from combustion systems and improve air quality by suppressing metal vapor nucleation and promoting metal nanoparticle condensation and coagulation [2, 3]. Many nanomaterials, such as TiO2, carbon nanotubes, and dendrimers, have been designed to degrade or absorb pollutants in water and soil systems [4–7]. These applications are often determined by the properties of nanomaterials, such as size, surface properties, crystal structures, and morphologies [8, 9]. Although nanotechnology has the potential to improve the quality of the environment, there are also concerns that it can generate a new class of hazards upon release to the environment followed by exposure of either the ecosystem or human beings that may result in potential adverse effects [1, 10, 11]. Toxicological studies with certain engineered nanoparticles (e.g., fullerenes, quantum dots, and metal oxides) have confirmed that they can be potentially harmful due to their high surface molecule/atom fraction and unique physicochemical properties [12, 13]. The emerging discipline of nanotoxicology is aiming to establish the relationship between nanoparticle properties (e.g., size, surface properties, and crystal phase) and their toxic potential [14–16].
Titanium dioxide has been widely used in environmental photocatalysis, sunscreen, and coating industry [17–19]. However, a variety of detrimental pulmonary effects in rodents and antibacterial effects have also been associated with nanosized TiO2 particle exposure [20–22]. Both the functionalities and biological effects of titania nanoparticles are controlled by its physicochemical properties. Nanomaterials that are tested are often dispersed in aqueous systems; this can potentially result in physicochemical property changes, e.g., agglomeration state and surface charge variation [15, 23, 24].
The agglomeration behavior and surface charge variation of nanoparticle dispersions can have a dramatic effect on both the reactivity of nanomaterials and their efficiency in contamination treatment [7, 25, 26]. It also affects the response of organisms upon exposure [27–30]. Therefore, accurate characterization of nanoparticle dispersions becomes very important for its environmental applications and nanotoxicology investigations. Jiang et al.  characterized the state (such as the hydrodynamic size, surface charge, and the degree of agglomeration) of titania and other nanoparticle suspensions and tested the effect of solution pH and ionic strength (IS) on dispersion properties. However, this study involved only a single value of surface area, primary particle size, and crystal phase for examined dispersion state. It has been reported that these properties of TiO2 nanoparticle can affect its photocatalytic activity [19, 31] and toxicity [16, 32–34]; however, little is known about their effect on the dispersion state and agglomeration behavior. There is evidence suggesting that the point of zero charge of hematite nanoparticle dispersion might change with varying particle size . However, systematic investigations for titania nanoparticle dispersions have not been done.
Recent developments in aerosol route synthesis of TiO2-based nanomaterials allow for greater and independent control of their physicochemical properties, such as size, crystal phase, and specific surface area [4, 36–38]. In this study, the influence of particle surface area, primary particle size, and crystal phase on titania nanoparticle dispersion properties is investigated. TiO2 samples with well-controlled properties are synthesized using flame aerosol reactors (FLAR). Six anatase TiO2 samples with different sizes (6–104 nm) are used to study the size effect. TiO2 nanoparticles of different crystal phases with the same size are used to examine the crystal phase effect. Commercially available Degussa TiO2 (P25) sample is also tested. The effect of monovalent and divalent electrolytes is examined using sodium chloride (NaCl) and magnesium chloride (MgCl2).
Materials and Methods
Several types of titania nanoparticles were used in this study. TiO2 (P25) nanoparticle with a primary particle size of 27 nm, specific surface area of 57.4 m2/g, and the phase composition of 80% anatase and 20% rutile was purchased from Degussa Chemicals (Hanau, Germany). Anatase TiO2 nanoparticles of 6, 16, 26, 38, 53, and 104 nm with specific surface areas of 253.9, 102.1, 61.5, 41.2, 29.7, and 15.0 m2/g, respectively, were synthesized using a flame aerosol reactor [16, 36, 39]. TiO2 nanoparticles of 38 nm with different crystal structures (100% anatase, 49% anatase/51% rutile, and 36% anatase/63% rutile) and a specific surface area of 41.2 m2/g were also synthesized in the flame aerosol reactor. The properties of these samples have been characterized using different techniques, including X-ray diffraction, transmission electron microscopy, and BET adsorption. They are reported in our previous studies [14, 34] and are not repeated here. The precursor used to synthesize TiO2 particles was titanium tetra-isopropoxide (Sigma–Aldrich, St. Louis, Missouri). Rutile TiO2 particle with the primary particle size of 102 nm and a specific surface area of 13.8 m2/g was prepared by annealing flame-synthesized anatase TiO2 at size 53 nm in a furnace . Other chemicals used in this study including sodium chloride (NaCl), magnesium chloride (MgCl2), sodium hydroxide (NaOH), and hydrogen chloride (HCl) were obtained from Sigma–Aldrich (St. Louis, Missouri).
The hydrodynamic size and surface charge (zeta potential) of nanoparticle dispersions were characterized using the ZetaSizer Nano ZS (Malvern Instruments Inc., UK), utilizing dynamic light scattering (DLS) and electrophoretic light scattering (ELS), respectively . DLS measures the intensity of the laser light that is scattered from dissolved macromolecules or suspended particles. The dispersion hydrodynamic diameter is derived from the temporal evolution of the scattered light intensity using the Stokes–Einstein equation . ELS measures the frequency or phase shift of an incident laser beam caused by electric field driven particle migration, reported as the electrophoretic mobility. Particle zeta potential is calculated from the measured electrophoretic mobility using the Smoluchowski equation [15, 41].
Summary of experiments performed
Particle concentration: 50 μg/ml; Three ionic strengths (0.001, 0.01, and 0.1 M) and varying pH (3–11) by adding HCl, NaCl, and NaOH.
Determine the effect of solution IS and pH on dispersion characteristics
Particle concentration: 50 μg/ml; NaCl and MgCl2 with the same IS and with the same molar concentrations.
Examine the effect of electrolyte type (monovalent vs. divalent) on dispersion characteristics
Particle concentration: 15, 25, 50, 150, and 500 μg/ml; DI H2O; Solutions with pH of 4 and IS of 0.001–0.1 M by adding HCl and NaCl.
Test the effect of nanoparticle surface area (mass concentration) on the dispersion properties
Anatase TiO2 (6–104 nm)
Particle concentration: 50 μg/ml; DI H2O; Solutions with IS of 0.001 M and varying pH (3–11) by adding HCl, NaCl, and NaOH.
Study the effect of primary particle size on dispersion properties
TiO2 (varying crystal phases)
Particle concentration: 50 μg/ml; Solutions with IS of 0.001 M and varying pH (3–11) by adding HCl, NaCl, and NaOH.
Investigate the effect of crystal phase on dispersion isoelectric point (IEP)
Results and Discussion
The pH at which the surface of titania is neutral is point of zero charge or isoelectric point. If no specific adsorption of the ions presented in the solution takes place on the particle surface, the pH at PZC and IEP would be the same. When pH is less than pHPZC (pHIEP), Eq. 2 results in creation of the positive surface charge and positive zeta potential. When pH is larger than pHPZC (pHIEP), Eq. 3 results in creation of the negative surface charge and negative zeta potential [42, 43, 45]. The dispersion hydrodynamic diameter is controlled by nanoparticle agglomeration in the aqueous system. In the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, the agglomeration of nanoparticles is determined by the sum of the repulsive electrostatic force (the interaction of electrical double layer surrounding each nanoparticle) and the attractive van der Walls force [46, 47]. Increase in particle surface charge (zeta potential) can enhance the electrostatic repulsive force, suppress the agglomeration, and subsequently reduce dispersion hydrodynamic size.
The effects of solution pH and ionic strength (IS) and electrolyte type on titania dispersion properties are presented first, followed by discussion about the influence of particle surface area. Finally, both primary particle size and crystal phase effect on dispersion characteristics are examined.
pH and IS Effect
Particle Surface Area Effect
Primary Particle Size Effect
The size effect on dispersion isoelectric point might originate from size-related properties of nanoparticles. Several other activities of titania nanoparticles had been found to be size dependent. The photocatalytic activity of TiO2 nanoparticle was reported to be a function of particle size when the same total particle surface area was used [19, 56]. Both in vitro and in vivo toxicities of anatase TiO2 (after normalized by surface area) were reported to be a function of particle size [16, 32]. The adsorption affinity of metal (e.g., lead and cadmium) by TiO2 appeared to be size dependent [57, 58]. As nanoparticle size decreases, the percentage of surface atom/molecule increases significantly. Particle electronic structure, surface defect density, and surface sorption sites also vary [7, 59]. Consequently, both nanoparticle IEP and surface reactivity can become dependent on particle size. For instance, it has been observed that variations in the nanoparticle surface coordination environment lead to changes in the surface acidity constants [60, 61].
Nanoparticle Crystal Phase Effect
The effect of particle surface area, primary particle size, and crystal phase on TiO2 nanoparticle dispersion properties was tested. Solution pH and ionic strength play important roles in dispersion zeta potential and hydrodynamic size. Increasing titania particle surface area results in a decrease in solution pH. At fixed pH, an increase in titania mass concentration enhances the collision frequency between particles and leads to higher degree of agglomeration. In addition to synthesis method, TiO2 isoelectric point was found to be dependent on particle size. As anatase TiO2 primary particle size decreases, its IEP increases that also causes changes in dispersion zeta potential and hydrodynamic size. In contrast to particle size, it was demonstrated that TiO2 nanoparticle IEP is insensitive to crystal structure. These results have important implications both in developing nanomaterials for environmental applications and in performing nanotoxicological studies, because nanoparticle dispersion properties affect delivery and transport efficiency for both contamination remediation and for in vitro and in vivo toxicity tests.
The authors acknowledge the Thailand Research Fund (TRF) for Komkrit Suttiponparnit under the Royal Golden Jubilee Ph.D. Program (Grant No.PHD/0237/2004) accompanied with KU-ChE and Affiliation: Center of Excellence for Petroleum, Petrochemicals, and Advance Materials, PERDO, Thailand for partial funding. This work was partially supported by a grant from the US. Department of Defense (AFOSR) MURI Grant, FA9550-04-1-0430. TC also acknowledges the partial support from CU centennial fund.
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