When Size Really Matters: Size-Dependent Properties and Surface

Figure 1. Microscopic and macroscopic behaviors of nanoparticles depend on a ... as the shape of the particle changes from a sphere to cylinder to cub...
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J. Phys. Chem. C 2008, 112, 18303–18313

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CENTENNIAL FEATURE ARTICLE When Size Really Matters: Size-Dependent Properties and Surface Chemistry of Metal and Metal Oxide Nanoparticles in Gas and Liquid Phase Environments† Vicki H. Grassian Departments of Chemistry and Chemical and Biochemical Engineering, and The Nanoscience and Nanotechnology Institute, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed: July 09, 2008; ReVised Manuscript ReceiVed: August 25, 2008

It is clear if one peruses the pages of The Journal of Physical Chemistry and other journals of the American Chemical Society that in the years around the beginning of the twenty first century, and in particular the year two thousand eight, there is a great deal of interest in the physical chemistry of nanoparticles. In this article, the focus is on some of the interesting and often not well understood size-dependent properties and surface chemistry of metal and metal oxide nanoparticles in gas and liquid phase environments. Challenges that remain and suggestions for future research needs are also presented at the end of this article. I. Introduction Gerhard Ertl was awarded the 2007 Nobel Prize in Chemistry “for his studies of chemical processes on solid surfaces”.1 On December 8, 2007, Ertl gave his Nobel lecture “Reactions at solid surfaces: From atoms to complexity” at Stockholm University. The physical chemistry of increasing complex interfaces continues on as an active and challenging area that is expected to continue for years to come and perhaps even for the next 100 years. Complex interfaces include: nanoparticle surfaces with edge and corner sites contributing to nearly 50% of the atoms present on the surface; catalyst surfaces under operando conditions; environmental interfaces under conditions of ambient temperature and relative humidity; liquid interfaces and biological interfaces such as biofilms and lipid membranes. Although Ertl received the Nobel Prize in Chemistry in 2007 for chemical processes on surfaces, much of the groundbreaking work on surface chemistry and the physical chemistry of surfaces began with Irving Langmuir. Langmuir, who received the Nobel Prize in Chemistry in 1932 “for his discoveries and investigations in surface chemistry”,1 was interested in a wide range of surface phenomena. His Nobel lecture delivered in 1932 touched upon many topics including surface adsorption models. The well-known Langmuir adsorption isotherm model continues to be applied in the analysis of adsorbates onto solid surfaces from gas phase and liquid environments. The physical chemistry of surfaces continued on from Langmuir and, as already noted above, The Nobel Prize in Chemistry was awarded an unprecedented second time to research in this area in 2007. In this article, some of the size-dependent properties and surface chemistry of metal and metal oxide nanoparticles in gas and liquid phase environments are discussed. Considering that † This year marks the Centennial of the American Chemical Society’s Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem. Senior Editors.

for particles on the order of 4 nm in diameter, ca. 50% of the atoms are at the surface, it can be stated with great certainty that understanding the surface properties and surface chemistry of nanoparticles is essential if nanoparticle behavior is to be fully understood. Furthermore, as the ability to synthesize nanoparticles/nanostructures grows, there is a need to fully and completely characterize these materials in complex environments. This article does not represent a critical review of the topic but instead introduces some of the interesting and often poorly understood size-dependent properties and surface chemistry of metal and metal oxide nanoparticles in gas and liquid phase environments. This article contains six sections, besides the introduction, they include: a discussion of some general considerations of the physical and chemical properties of metal and metal oxide nanoparticles; several examples of some experimental techniques and methods used to investigate sizedependent and surface properties of metal and metal oxide nanoparticles; specific examples of metal and metal oxide nanoparticle surface chemistry; a brief discussion of the environmental, health, and safety concerns of metal and metal oxide nanoparticles; and future research needs and directions. II. Physical and Chemical Properties of Nanoparticles General Considerations with a Focus on Metal and Metal Oxides In his 1936 Nobel laureate banquet address, Irving Langmuir stated that a career in science that addresses both fundamental knowledge with potential for application to better mankind can be a very fulfilling career for a scientist.1 In fact much of the interest in nanoscale materials arises from both an understanding of the novel physical, chemical, and size-dependent phenomena on the nanometer length scale and the development and beneficial uses of these materials in a wide-range of applications from environmental remediation and green chemistry to telecommunications and medicine. Figure 1 shows some of the important and, in some cases, interrelated physicochemical properties of nanoparticles that dictate their microscopic as well as their macroscopic behaviors.2 These properties include size,

10.1021/jp806073t CCC: $40.75  2008 American Chemical Society Published on Web 11/04/2008

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Eg ≈

Vicki H. Grassian received her B.S. degree in Chemistry from the State University of New York at Albany. From there, she did her graduate studies at Rensselaer Polytechnic Institute (M.S., 1982) and the University of California-Berkeley (Ph.D., 1987) with George C. Pimentel. Following postdoctoral positions, she began her academic career at the University of Iowa in 1990. Professor Grassian is currently the Director of the Nanoscience and Nanotechnology Institute at UI and a professor in the Department of Chemistry with appointments in the Departments of Chemical and Biochemical Engineering and Occupational and Environmental Health. In 2003, Professor Grassian received a National Science Foundation Creativity Award, and in 2005, she was elected as a Fellow of the American Association for the Advancement of Science. Recently, she was named a Collegiate Fellow in the College of Liberal Arts and Sciences and also received the Outstanding Mentoring Award from the Graduate College. She currently serves on the executive committee of the Physical Chemistry Division of the American Chemical Society, the editorial boards of several journals, and the science advisory boards for several nanoscience centers and nanotech startup companies. Her research interests are in the areas of heterogeneous atmospheric chemistry, climate impact of atmospheric aerosols, environmental surface science, and environmental and health aspects of nanoscience and nanotechnology. She has over 150 peer-reviewed publications and is the editor of three books including a recently published book by John Wiley and Sons entitled Nanoscience and Nanotechnology: EnVironmental and Health Impacts.

shape, surface composition, aggregation, concentration, and there ability to be active, i.e., to have changing properties as a function of time or some other variable. These properties impact and dictate the most fundamental characteristics of nanomaterials including their ability to get into cells. Size. For a science that is all about size, one of the most interesting aspects of nanoscience is that properties of nanoparticles change with size. It has been shown that many fundamental properties are size dependent on the nanoscale. For example, the most stable crystalline phase of a material is sizedependent. From thermodynamic considerations, the total free energy is a sum of the free energy of the bulk and the surface of the nanoparticle.

Gnanoparticle ) Gsurface + Gbulk

( )

π2h2 1 1.8e2 εR 2R2 µ

(2)

where R is the particle radius, µ is the reduced mass of the exciton or the electron-hole pair,  is the dielectric constant of the semiconductor and h is Planck’s constant. These changes in the bandgap will influence many properties of nanoparticles including surface reactivity. For noble metals, the localized surface plasmon resonance (LSPR) is both metal and size dependent. For the two most common metals used, Ag and Au, LPSR can occur in throughout the visible region of the spectrum depending on the size of the nanoparticles. The wavelength dependence of the LSPR can be modeled using Mie theory and the wavelength dependence of the extinction spectrum is given by

E(λ) )

(

24NAa3εm3/2 εi λ ln(10) (ε + 2ε )2 + ε 2 r m i

)

(3)

where NA is the density of nanoparticles, a is the radius of the nanoparticle, εm is the dielectric of the medium, λ is the wavelength, and εr and εi are the real and imaginary parts of the metal dielectric function.9 For copper, the LSPR of substrate-deposited Cu nanoparticles is significantly affected by the presence of copper oxides and the removal of the oxide species yields a dramatic difference in the observed LSPR.10 For pure nanocrystalline metal oxide powders, the ability to reflect near-infrared light (NIR) (750-2500 nm) was found to be much higher (+20%) relative to larger common macrocrystalline powders and minerals due to the smaller crystallite sizes and smaller mean aggregate sizes in accordance with Kubelka-Munk theory.11 Shape. Perhaps even more interesting than the size dependence of the extinction spectra for spherical noble metal nanoparticles is the shape dependence. For nanorods, theoretical consideration of the surface plasmon resonance showed there are two controlling factors: the bulk plasma wavelength, a property dependent on the metal itself, and the aspect ratio of the nanorods, a geometrical parameter.12 By changing the shape of Au and Ag nanoparticles, the LSPR can be red-shifted into the near-infrared region of the electromagnetic spectrum. As shown by Haes et al., the LSPR for Ag nanoparticles shifts across the electromagnetic spectrum as the shape of the particle changes from a sphere to cylinder to cube to prism to pyramid.9

(1)

For nanoparticles, Gsurface is no longer a minor component but in fact becomes a large component of the total free energy. Surface free energies and surface stress are important components to the overall phase stability of nanoparticles.3-7 Titanium dioxide is interesting as anatase becomes more stable than rutile for a particle size below 14 nm.3 However, it has been recently shown that the stability of rutile nanoparticles increases relative to anatase and brookite at low pH due to surface charges.5 Electronic properties are size dependent on the nanoscale. The electronic band gap, Eg, for semiconductor nanoparticles can be estimated for a spherical particle as:8

Figure 1. Microscopic and macroscopic behaviors of nanoparticles depend on a number of a number of important characteristics and properties (e.g., shape, concentration (dose/response), surface composition, and aggregation for passive and active (changing) nanostructures. (Modified and adapted from Tinke et al., Am. Pharmaceut. ReV., 2006, September/October, 1.)

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Figure 2. Nanoparticles can be present as isolated particles or they can form aggregates up to microns in size or they can dissolve into ions in solution. For a science that is all about size, these processes result in very different sizes and size regimes that impact environmental transport, lung deposition and cellular interactions. Reprinted with permission of John Wiley & Sons, Inc. (V. H. Grassian, Nanoscience and Nanotechnology: EnVironmental and Health Impacts, Copyright 2008, John Wiley & Sons Inc.)

For these shapes, Mie theory cannot be applied but instead numerical methods are needed to solve Maxwell’s equations. Surface Properties: Composition, Termination, Charge and Functionalization. Although there is growing evidence to suggest that the surface properties of nanoparticles, including surface reactivity, are distinctly different from larger particles, often it is difficult to get a quantitative understanding of surface composition, termination, charge, and functionalization for nanonparticles. Surface functionalization will impact everything from secondary size (through aggregation) to water solubility and the ability of nanomaterials to get into cells. Thus, there is a great deal of interest in gaining a more quantitative understanding of the surface properties of nanoparticles. As discussed in section III, structural disorder and unusual surface relaxation are two fundamental differences between the surface structure of nanoparticles and larger, bulk materials. Aggregation of Nanoparticles. In aqueous environments13 and as aerosol,14 there is a tendency for nanoparticles to form aggregates that are much larger than the primary size of the nanoparticle. The tendency for nanoparticles to aggregate depends on a number of factors including surface functionalization, nanoparticle concentration, pH, and ionic strength. Furthermore, as shown in Figure 2, the size regime that needs to be modeled, for example in transport or lung deposition models, will be vary depending on the state of the nanoparticles (as dissolved ions, isolated nanoparticles, or nanoparticle aggregates).4 Thus, for a science that is all about size, these important size issues beyond just primary particle size need to be fully understood as it impacts not only the electronic and optical properties of nanoparticles but also environmental and biological interactions including transport in water systems and nanoparticle-cellular interactions. Concentration. Because of the small mass of individual nanoparticles, an important consideration is how to standardize nanoparticle concentrations. The important metric whether mass density, number density or some other unit, such as surface area,15,16 will in part depend on the application under consideration. Consider spherical particles suspended in a volume, V, a comparison of mass and number density for particles ranging from 1-1000 nm is shown in Figure 3A. Specifically, it is shown that, for constant particle number density concentrations (number of particles/cm3), the concentration in mass density (µg/m3) changes by orders of magnitude. In particular a comparison between 5, 50, and 500 nm particles, the mass concentration changes by a million-fold over this range. Typically for particulate matter in air it is mass concentrations that are reported and for air pollution regulations of particulate matter, it is mass concentrations that are currently regulated.16 If mass concentrations are kept constant at 1 µg/m3, it can be

Figure 3. Concentration metrics are an important consideration in nanoscience. Shown above is a comparison of mass density versus particle density for particles from 1-1000 nm with 5, 50, and 500 nm sizes specifically compared. The number density of particles is kept constant in A and the mass concentration is kept constant in B. The density of the nanomaterial is taken as 3 g cm-3.

seen that there are orders of magnitude more 5 nm nanoparticles relative to larger particles (e.g., 50 and 500 nm) as shown in Figure 3B. Active Nanostructures. As new nanomaterials are developed, there is interest in nanomaterials that are active and change over time or some other variable. Even greater challenges exist for understanding the fundamental properties of active nanostructures. These challenges are greatest for biomedical applications of active nanostructures as there is interest in following timedependent (or pH-dependent) changes in these properties in vivo. III. Examples of Some of the Experimental Methods Used to Investigate the Size-Dependent Properties and Surface Chemistry of Metal and Metal Oxide Nanoparticles in Gas and Liquid Phase Environments In 2008, nanoscience and nanotechnology continue to grow as fields of scientific research and commercial development. At this time, it is often difficult to make a sample of nanoparticles that has exactly the same number of atoms unlike the synthesis of molecules with exact formulas. Although the goal is to produce samples of monodispersed, single-sized particles, more often than not there is a range of nanoparticle sizes each slightly different than the other that is produced. Inhomogeneous samples are particularly true for a number of commercially available nanomaterials as it is especially difficult to make a monodispersed nanoparticle sample in large quantities and many companies do not have the suite of characterization methods needed to analyze the products that they manufacture. Additionally, it is difficult to quantify surface properties such as surface coverage and the number of functionalized groups at the interface of nanoparticles. Because of these difficulties, the use of a suite of techniques to characterize nanomaterials remains an important component of nanoscience17 and the development of new methods and techniques remains an important priority.18 Some examples of routine and more

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Figure 4. Transmission electron micrographs of synthesized a-FeOOH particles in two different size regimes referred to as nano- and microrods.

specialized methods are discussed below for nanoparticle bulk and surface characterization. The capabilities of several methods will be mentioned and in a few cases a more detailed description of the application and use of the method will be provided. X-Ray Diffraction and Electron Microscopy. Powder X-ray diffraction (XRD) is used routinely when metal and metal oxide nanoparticles are synthesized in powder form to determine whether these particles are crystalline or amorphous in nature. The most stable phase of a material on the nanoscale may differ for the same bulk material. Therefore, it is important to determine the dominant crystalline phase in the material as the thermodynamically most stable phase for a given material may not be the same on the nanoscale (vide infra). Interestingly, very small nanoparticles, below 5 nm, have been shown to have unusual structural disorder that can substantially modify the properties of nanoparticles.19,20 Using a combination of small-angle and high energy wide-angle X-ray scattering measurements, Gilbert et al. investigated interior strain and disorder of nanoparticles directly using real-space pair distribution function analysis.19 Analysis of ca. 3 nm ZnS particles showed that the strength of surface-ligand interactions impacted the crystallinity of the interior of the nanoparticles. For smaller nanoparticles, surface-ligand interactions are quite important and in a recent high resolution X-ray study the structure of a thiol monolayer-protected gold nanoparticle revealed a highly unusual structure of the surface functional groups interacting strongly with each other and the first layer of gold atoms.21 Even for surfaces that are not functionalized with terminating ligands, differences in measured surface plane relaxations in gold nanocrystals further underscores how little is understood about the surface structure of nanocrystals and the structural dynamics of nanocrystals relative to bulk crystalline surfaces.22 Electron microscopy, scanning electron and transmission electron microscopy (SEM and TEM), are used routinely for the analysis of nanoparticle shape and size of substrate-deposited particles. For moderate resolution instruments, characterizing the size of nanoparticles is typically done. An example of these data and analyses is given for R-FeOOH nanoparticles.23 Goethite forms rod-like structures as seen in the TEM images shown in Figure 4. The rods shown in the two images are of different sizes and are denoted as nanorod and microrods. The average dimensions determined from analyzing hundreds of particles show that the nanorods (length × width) are 80 ((30) nm by 7 ((2) nm, and microrods are 670 ((370) nm by 25 ( 9 nm (uncertainties represent one standard deviation). In addition, higher specific surface areas were measured for the

Grassian smaller nanorods. Using N2-BET isotherm method, nanorods and microrods are determined to have surface areas of 110 ((7) and 40 ((3) m2/g, respectively.23 There is growing interest in the use of high-resolution instruments to investigate the details of nanoparticle crystal structures and lattice spacings. HR-TEM has been used as a tool to better understand nanoparticles defects and nanoparticle growth mechanisms from solution phase synthesis as well as biomineralization processes.24-26 As these instruments become more widely available, they will continue to be used to unravel the detailed structure of nanomaterials.27 Nanoparticle Characterization in Air and Water Using Particle Sizing Instruments. There is a great need to analyze the size and composition of nanoparticles and nanoparticle aggregates in air and aqueous phase environments. For size measurements, there are a number of nanoparticle sizing instruments. However, often these instruments operate under very different fundamental principles and this is important to remember when analyzing results and comparing data from different instruments. For spherical particles, the measured diameters from the different instruments mentioned can be related because no corrections need to be made for shape and volume, but for nonspherical particles or agglomerates and aggregates that are irregularly shaped, these diameters are not equivalent (vide infra). The size and size distribution of nanoparticles and nanoparticle aggregates in air can be measured using a scanning mobility particle sizer (SMPS). The SMPS can measure particle sizes ranging from 2-700 nm. For nanoparticles that occur in the atmosphere (e.g., nucleation mode particles that form initial from the condensation of gases and soot particles), these nanoparticles can be drawn into the SMPS for analysis. For nanoparticles in powder form or suspended in a liquid, the nanoparticles must be aerosolized using an aerosol generator (e.g., electrospray or atomizer). The SMPS itself consists of two main parts: a differential mobility analyzer (DMA) and a condensation particle counter (CPC). The DMA is used for size classification and the CPC is used to count the particles. The SMPS instrument is based on the principal of the mobility of a charged particle in an electric field.28 Particles are first charged using a radioactive source and then enter a DMA where the aerosol is classified according to electrical mobility, with only particles of a narrow range of mobility exiting through the output slit. This monodisperse distribution then goes to a CPC. Depending on the voltage placed on the center rod, which determines the trajectories of the particles, only particles of a certain mobility diameter, Dm, will follow the trajectory to make it out of the DMA to be counted by the CPC. For spherical particles, the measured diameters from different instruments can be related because no corrections need to be made for shape, but for nonspherical particles or agglomerates and aggregates that are irregularly shaped the difference in measurements creates a need to define a volume equivalent diameter, Dve, which is defined as the volume of sphere with the same volume as a particle with an irregular shape. The relationship between Dve and Dm is given by

Dve ) Dm

Cs(Dve) χCs(Dm)

(4)

where χ is the dynamic shape factor, and Cs(Dm) and Cs(Dve) are the Cunningham slip factors for the mobility and volume equivalent diameters, respectively. For spherical particles, the dynamic shape factor, χ, is equal to one, and the volume equivalent diameter (Dve) is equal to the measured mobility

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Figure 5. Scanning mobility particle size distribution of gold nanoparticles before and after forming a shell of silicon dioxide around the gold particles for enhanced solution phase stability. The silica shell more than doubles the mobility diameter of the particle, Dm, as seen by the change in the SMPS distribution.

diameter (Dm). The shape factor is typically determined from the relationship between aerodynamic diameter, Da, and mobility diameter

Dm ) Daχ3/2



Fo Cs(Dm)√Cs(Da) Fp C (D )3/2 s

(5)

ve

where Fo is the reference density (1 g cm-3), Fp is the density of the particle, and Cs(Da) is the Cunningham slip factor for aerodynamic diameter and other quantities as defined above. For spherical particles the dynamic shape factor, χ, is equal to one, and the volume equivalent diameter (Dve) is equal to the measured mobility diameter (Dm) and the Da is related to Dm through density. These relationships between Dve, Dm, and Da have been outlined in detail in previous publications and the reader is referred to these papers for additional discussion.29-32 One rather new and very attractive application of the SMPS is in using these instruments to better characterize surface coatings and surface chemistry of nanoparticles. Often nanoparticles do consist of a solid inorganic core with a polymer or organic-based surface coating. These coatings are often designed to stablize the nanoparticles with respect to degradation and aggregation. The coatings also can increase the solubility of the nanoparticle in aqueous solution. The coatings can often be on the size of the inorganic core. Since the interaction of nanoparticles with biological systems depends on its size, it is the overall size of the particle, inorganic core and coating, which is important in these initial interactions yet very few studies report this “true” nanoparticle size. It is more common to report the size of inorganic core of the nanoparticle as the inorganic core, which is usually composed, of a metal or semiconductor that can be easily measured using transmission electron microscopy. The benefit of use of SMPS is the ability to measure the size distribution of many nanoparticles and the ability to measure thicknesses of functionalized or coated nanoparticles. Figure 5 shows just one example of the kind of sizing data and changes in size that can be determined with the SMPS. A sample of Au nanoparticles in aqueous suspension is sent

through an electrospray capillary to an SMPS equipped with a nano-DMA to more accurately measure sizes below 80 nm.15 These Au nanoparticles gives a peak around 20 nm in the distribution which is plotted as dNd(log Dm) versus log Dm for reasons that have been discussed previously.28-32 The Ag nanoparticles were then coated with silica to increase their stability in water yet retain as close as possible their optical properties. Upon coating, the nanoparticle diameter increases by more than a factor of 2 as seen in the SMPS distribution. Besides inorganic coatings, organic coatings may also be analyzed using SMPS. For example, Pease et al. determine the surface coverage of biological molecules conjugated to nanoparticle surfaces.33 In particular, the surface coverage of thiolmodified single-stranded DNA (ss-DNA) coated on the surface of gold nanoparticles was determined. A salt solution containing the ss-DNA was used to coat the gold particles, and a particle size distribution was collected before and after surface coating. Based on changes in the particle size distribution, the coating thickness, which is dependent on the spatial configuration of the ss-DNA, was determined. From the measured coating thickness, it was concluded that the strands exist in a random coil configuration on the gold nanoparticle surface. Identifying the spatial configuration allowed total surface coverage to be calculated using the known size of a ss-DNA molecule. Such characterization of biological surface coatings may have applications in designing and controlling targeting agents for cancer treatments. Particle sizing instruments have recently been used in novel ways to investigate Si nanoparticle surfaces. Roberts and coworkers have shown that the surface chemistry of Si nanoparticles can be investigated.34-37 Surface chemistry ranging from measuring the desorption of a monolayer of hydrogen atoms from Si nanoparticles34 to oxidation kinetics of Si nanoparticles,35 chemical vapor deposition,36 and self-assembly of organic monolayers was investigated.37 Dynamic light scattering (DLS) is a sizing instrument that measures the size of the nanoparticles in aqueous environments including biological fluids. DLS is a size measurement technique that uses scattered light to measure diffusion rates (Brownian motion) of particles in stable suspensions to determine a size based on Stokes-Einstein equation

DH )

kT kT ) f 3πηD

(6)

where DH is the hydrodynamic diameter, k is the Boltzmann constant, T is absolute temperature, f is the particle frictional coefficient which consists of solvent viscosity, η, and the diffusion constant, D. Equation 6 was developed for hard spheres which may not represent many nanoparticles; therefore, the size of nonspherical, hydrated, or solvated particles is related to the hypothetical hard sphere that would behave the same as the measured particles. Using this technique, the hydrodynamic diameter of nanoparticles and nanoparticle aggregates can be measured.21 Nanoparticle Size Measurements Using Spectroscopic Methods. For noble metals, nanoparticle size can be determined from the maximum in the UV absorption spectrum and the width of the extinction band is a measure of the sample heterogeneity with broader peaks appearing for samples that are more heterogeneous.9 Nanoparticle Surface Properties Using Surface Sensitive, Surface Specific and Surface Informative Techniques. There are a number of techniques to monitor the surface properties of nanoparticles. As noted above, particle-sizing instruments are

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Figure 6. Intensity normalized aggregate size distributions measured using DLS for 5 and 32 nm particles. These data show that for pH 2 suspension TiO2 aggregates grow in size as a function of A nanoparticle mass concentration and B with the addition of oxalic acid as measured by an increase in the hydrodynamic diameter, DH. Adapted and reproduced with permission from Langmuir 2008, 24, 6659. Copyright 2008 American Chemical Society.

now being used to investigate surface chemistry. Spectroscopic approaches are more typically used to investigate surfaces. For noble metals, surface plasmon resonance and surface enhanced Raman spectroscopies are important tools for monitoring surface chemistry and for speciation of adsorbates on noble metal nanoparticles in solution. For nanoparticles deposited on substrates or present in powder form, surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS) can be used to characterize surface functional groups. Synchrotronbased techniques show promise in probing nanoparticle surfaces.19 In addition, transmission and ATR-FTIR spectroscopy are tools that have been successfully applied to investigate nanoparticle surfaces and nanoparticle surface chemistry in gas and liquid phase environments.38 DLS instruments can also measure the zeta potential of nanoparticles, which describes the charge on a particle. The zeta potential will also affect the distribution of the nanoparticles in solution as well as influence surface reactive properties which have been shown to affect biological response.39 A common way of measuring the zeta potential of particles is applying an electric field to the particle suspension. The movement of the charged particles relative to the liquid suspension is electrophoresis. The velocity of the particle in the medium when the electric field is applied can be measured and a zeta potential can be calculated according to eq 7

UE )

2εzf(ka) 3η

(7)

where UE is electrophoretic mobility, z is the zeta potential, ε is the dielectric constant, η is the viscosity of the suspension, and f(ka) is Henry’s function which is approximated as either 1.0 or 1.5 depending on particle size and ionic strength. The effect of applied electric fields can also be measured on liquids moving over stationary charged surfaces, electroosmosis, the forced flow of liquid past a charged surface generating an electric field, streaming potential, or when charged particles move relative to a stationary liquid generating an electric field, sedimentation potential.

IV. Examples of Metal Oxide Size-Dependent Nanoparticle Surface Chemistry Titanium Dioxide Nanoparticle Surface Chemistry. Titanium dioxide is a manufactured nanomaterial that is used in many different applications including photocatalysts, solar cells, biomaterials, memory devices, and as environmental catalysts.40-51 Although some studies have investigated size effects in the interactions of molecules and ions on TiO2 particle surfaces, there is no clear consensus on the impact of particle size on surface adsorption and surface chemistry. From thermodynamic considerations, smaller nanoparticles are predicted to show enhanced adsorption due to an increase in interfacial tension and surface free energy with decreasing particle size.44,51 Solution phase adsorption studies with a series of organic acids onto TiO2 nanoparticles seemed to confirm this prediction.44 However in another study, the surface adsorption of Cd2+ on TiO2 was found to decrease with decreasing particle size.52 Using sum frequency generation,6 Shultz and co-workers showed that, for substrate-deposited nanoparticles, smaller TiO2 nanoparticles were more reactive than larger ones as determined by the relative amounts of dissociative versus molecular adsorption of methanol from the gas phase. Smaller nanoparticles were shown to enhance dissociative adsorption.45 From the above examples, it remains unclear as to the expected size-dependent trends in the adsorption and surface of TiO2 nanoparticles. Although it is clear that nanoparticle aggregation is important in aqueous suspensions of TiO2,53,54 one factor that has not been given much consideration in aqueous phase surface adsorption and surface chemistry studies is the role that nanoparticle aggregation can play in the actual amount of surface area available for adsorption and reaction. Figure 6 shows dynamic light scattering results for TiO2 (anatase) in aqueous suspensions. The data in Figure 6A shows that as the mass concentrations of 5 and 32 nm TiO2 particles increase so does the aggregate size. It can be seen that, under the conditions shown in Figure 6A, there is extensive nanoparticle aggregation and that the smaller nanoparticles form the

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Figure 7. Adsorption isotherms for oxalic acid adsorbed on to 5 and 32 nm TiO2 particles from aqueous solution held at pH 6.5. The lines through the points are Langmuir model fits to the data as discussed in the text. Adapted and reproduced with permission from Langmuir 2008, 24, 6659. Copyright 2008 American Chemical Society.

largest aggregates at a given mass concentration and in the presence of oxalic acid there is an increase in aggregate size with the volume of the aggregate (assuming spherical aggregates) increasing nearly 1000 fold. To better understand the adsorption of oxalic acid on 5 and 32 nm TiO2 particles, both macroscopic solution phase adsorption isotherms and spectroscopic measurements were done to quantify coverages and to probe the molecular structure of adsorbed oxalic acid on these two different-sized nanoparticles. Figure 7 shows the solution phase adsorption data collected at pH 6.5. These data show that, when normalized to BET surface area, there are some but small differences in the adsorption of oxalic acid on 5 versus 32 nm particles. A Langmuir adsorption model fit to the data

θ)

CKads N ) Ns 1 + CKads

(8)

where θ is the fractional surface coverage, N is the number of adsorbed molecules or ions, Ns is the maximum number of adsorbed molecules or ions, C is the solution phase concentration, and Kads is the Langmuir adsorption equilibrium constant, yields the following adsorption parameters: Kads of 4900 ( 400 and 2900 ( 400 M-1 for 5 and 32 nm particles, respectively, and saturation surface coverages, Ns, of 7.2 ( 0.2 compared to 6.6 ( 0.6 × 1013 molecules cm-2 for 5 and 32 nm particles, respectively, where Ns has been normalized to BET surface area.54 Within error, the maximum surface coverage is the same for the different-sized nanoparticles, however, Kads differs by nearly a factor of 2 indicating some differences in the adsorption of oxalic acid on the smaller nanoparticles. Spectroscopy can be used to probe molecular-level differences in the adsorption of oxalic acid on TiO2 nanparticles. ATRFTIR spectra for oxalic acid adsorbed on 5 and 32 nm TiO2 particles, as well as solution phase oxalic acid, at pH 6.5 are shown Figure 8. These spectra shows that speciation in solution phase differs from that of the adsorbate. In particular, at pH 6.5, the oxalic acid is completely deprotonated in solution to yield oxalate, C2O42-, as seen by the characteristic absorption bands at 1577 and 1309 cm-1, consistent with previous results.55-60 However, the spectrum for adsorbed oxalate clearly shows that these absorption bands are not present. Instead there are characteristic oxalic acid peaks around 1287 and 1431 cm-1, seen in the 32 nm particle spectrum which have been previously assigned in the literature to the symmetric stretch, νs(CO2) and

Figure 8. ATR-FTIR spectra for oxalic acid adsorption onto 5 and 32 nm TiO2 particles at pH 6.5 compared to the solution phase spectrum shown in the inset. Adapted and reproduced with permission from Langmuir 2008, 24, 6659. Copyright 2008 American Chemical Society.

a combination band, respectively, whereas the bands at 1695 and 1719 cm-1 are assigned to an asymmetric stretch, νas(CO2).55,58 These same absorption bands are present for the 5 nm particle spectrum, however, some are shifted in frequency (for example the band at 1703 cm-1 is now centered at 1695 and the band at 1287 is now at 1300 cm-1). A comparison of the spectra for oxalic acid adsorption on 5 versus 32 nm particles show that besides slight differences in the frequencies of some of the absorption bands, there is an additional absorption band present in the spectrum for oxalic acid adsorbed on 5 nm TiO2 particles. A band near 1630 cm-1 which is either completely absent in the spectrum for oxalic acid adsorption on 32 nm particles or relatively weak compared to the other absorption bands in the spectrum. This peak is assigned to a red-shifted asymmetric stretching carboxyl region, νas(CO2) for oxalic acid adsorption and it is proposed to be associated with a unique adsorbed species on 5 nm particle surfaces associated with edge or corner sites which will be in greater abundance for the 5 nm particles compared to the larger 32 nm particles. Although edge and corner sites have been proposed to increase reactivity in nanoparticles,61,62 it has been difficult to attribute unique adsorption complexes to these sites as seen for the adsorption of oxalic acid on the smaller 5 nm TiO2 particles. Surface Chemistry of r-FeOOH Nanorods: Versus Microrods. Iron oxides are ubiquitous in nature and provide reactive surfaces in air, water, and soil. In natural samples, iron oxides and (oxyhydr)oxides (collectively referred to as iron oxides) often display a broad distribution of particle sizes. In particular, there is evidence that nanometer size regime particles are present and sometimes a predominant form of goethite in some natural aquatic systems.63 Although nanoparticulate iron oxide reactivity is sometimes attributed to very high specific surface areas, recent experimental evidence indicates that iron oxide nanoparticles may display reactive properties that cannot be extrapolated to the behavior of larger materials simply on the basis of differences in surface area. In particular, several studies have reported enhanced nanoparticle reactivity for cation adsorption,64 electron transfer,65 and oxide dissolution.66 Such behavior could result from a greater density of reactive sites per unit surface area on nanoparticle surfaces, or greater inherent reactivity of nanoparticle surface sites.67 Factors that could impart such unique reactivity to iron oxide nanoparticles were recently detailed in a review by Waychunas et al.,68 and include surface restructuring and curvature, as well as quantum confinement effects, that are important for particles on the nanoscale. Recently, a comparison of the extent of adsorption on goethite nanorods and microrods for two different adsorbates, Fe(II) and

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Grassian

TABLE 1: Comparison of Maximum Surface Adsorption Coverages:a r-FeOOH Nanorods and Microrods R-FeOOH adsorbate b

Fe(II) oxalic acidc

nanorods

microrods

2.8 ( 0.5 × 1.3 ( 0.1 × 1014

5.4 ( 0.3 × 1014 2.0 ( 0.1 × 1014

1014

a Maximum surface adsorption coverages, Ns, are normalized to BET-measured surface areas. b pH 7.5. c pH 3.0.

oxalic acid, have been quantitatively measured in aqueous suspensions.23,69 It is important to note that similar to titanium dioxide particles, in solution phase, there is the tendency of iron oxide particles to aggregate under many environmentally relevant conditions and this will influence surface adsorption and surface chemistry.69-71 For both adsorbates, there were differences within experimental error of the extent of adsorption on nanorods as compared to the larger microrods. In general, there is less adsorption capacity, by approximately a factor of 2 when normalized to surface area, on the goethite nanorods as shown by the data given in Table 1. These data could be rationalized as a result of an intrinsically lower number of surface sites on the nanorods compared to the microrods or a decrease in surface activity of comparable amounts of surface sites. For example, the distorted surface octahedra on nanoscale goethite particles observed by Waychunas et al.68 could be interpreted to produce a lower intrinsic adsorption and reactivity capacity of the nanorod surface whereas the observation of less proton and carbonate uptake on goethite nanoparticles was modeled by invoking a decrease in surface site density with decreasing particle size.72 Both represent viable explanations for the decreased adsorption capacity of goethite nanorods with respect to adsorption that has been normalized to BET surface area. Studies comparing SO2 uptake and conversion to adsorbed sulfite showed that this conversion was slower on smaller substrate-deposited, isolated ferrihydrite nanoparticles indicating size-dependent effects are important in gas-phase as well as solution phase iron oxide processes.73 Many questions still remain regarding surface adsorption and surface reactivity of nanoparticulate iron oxides. A combination of experimental techniques to probe these adsorption sites and complexes will be useful in understanding the inherent reactivity of iron oxide nanoparticle surfaces. Additionally, theoretical studies could provide insights into these naturally occurring complex interfaces. V. Environmental Health and Safety Concerns of Metal Oxide and Metal Nanoparticles This article ends with the growing discussion of the implications of man made or engineered materials such as nanoparticles and the potential harm that nanoscale materials may impart to the environment and human health. Although not as much of a consideration in Irving Langmuir’s time but certainly one in Gerhard Ertl’s, there are now questions and issues not only related to the benefit of mankind in the applications of novel chemicals and materials but also its implications. In the case of nanomaterials, the implications of these on human health and the environment are becoming an important consideration as is the potential applications and benefits to mankind. Unlike the development of any other technologies, a discussion on the implications of particles on the nanoscale has been ongoing for several years now concurrent with the increased development and commercialization of nanotechnologies. Furthermore, characterization of nanomaterials, using methods discussed here, is

Figure 9. Number of neutrophils in BAL fluid in controls and animals exposed to 5 or 21 nm TiO2 nanoparticles by instillation, according to surface area. Asterisks represent significant increase (*p < 0.05, **p < 0.01, and ***p < 0.001) in parameter measured, compared to controls. Reprinted by permission from ref 77 by Taylor&Francis Ltd., http://www.tandf.co.uk/journals; “Inflammatory response of mice to manufactured titanium dioxide nanoparticles: Comparison of size effects through different exposure routes”, 2007, Informa Healthcare.

a critical component of these studies and in determining underlying sources of toxicity and biological response. Titanium Dioxide Nanoparticle Toxicity. Several earlier studies have suggested that for a given mass of particles nanoparticle toxicity increased with a decrease in size of the primary particle size due to the greater surface area of these smaller particles. It was shown that for titanium dioxide, the increase in the number of neutrophils, a measure of inflammation and toxicity, was directly correlated with particle surface area for two different sized particles, 20 and 200 nm.74,75 Specifically it was shown that the data collected for these two different sized particles fell on the same dose-response curve when plotted as a function of surface area. Thus, these earlier studies suggested that the most relevant and important dose metric in nanoparticle toxicity was nanoparticle surface area determined from geometric consideration or BET measurements. More recently there have been additional studies that suggest that TiO2 nanoparticle toxicity in particular, and potentially nanparticle toxicity in general, may in fact be more complex. For example, Sayes et al. have suggested the phase of the nanoparticle plays an important role in nanoparticle toxicity.76 Grassian et al. have shown that for smaller TiO2 nanoparticles below 10 nm, the expected increase in toxicity relative to 21 nm particles did not occur.77 Specifically, when the number of neutrophils are plotted as a function of total surface area, two different sized nanoparticles, 5 and 21 nm, did not fall on the same dose-response curve as expected from the earlier studies for larger particles (20 and 200 nm). These data are shown in Figure 9. A possible cause for this unexpected result comes from a very recent study that investigated the formation of reactive oxygen species (ROS), often the cause of nanoparticle toxicity toward cells, as a function of TiO2 particle size.78 It was determined that ROS formation is a maximum at a particle size near 20 nm and that for smaller nanoparticles ROS formation actually decreases. Although for the most part TiO2 nanoparticles show moderate if very limited toxicity, these more recent studies indicate that nanoparticle toxicity is complex and will not be just a function of surface area but will depend more specifically on the nanoparticle physicochemical properties which are size-dependent.76-78

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Figure 10. (A) Powder XRD pattern of copper nanoparticles is shown along with reference patterns for metallic Cu, Cu2O, and CuO. (B) XPS in the Cu 2p region is shown for copper nanoparticles along with reference spectra for of CuO and Cu2O. Based on these data, a pictorial representations of the nanoparticle is shown with a metallic core and a gradient of oxidized phases with the most oxidized phase at the surface. (C) SMPS data show that Cu nanoparticles decrease in size as they undergo dissolution in acid solution (pH 2), as measured by a decrease in mobility diameter, Dm. SMPS data are shown as lognormal fits to the size distribution.

Metal Nanoparticles in Aqueous and Biologically Relevant Media. There is great interest in metal nanoparticles from a number of perspectives ranging from biomedical applications such as imaging to heterogeneous catalysts and sensors in the case of noble metals. Copper and iron nanoparticles are used as catalyst and adsorbents. One question is related to the stability of these nanoparticles in the environment and in biological systems.79,80 In the case of metal nanoparticles, there is the possibility that these will undergo dissolution in acidic aqueous environments to yield ions in solution while concomitantly forming smaller nanoparticles as it undergoes dissolution.80-82 An example of this is studies that have shown that copper nanoparticles can cause extensive inflammation once inhaled and can undergo translocation to various organs.83 Copper nanoparticles typically have an oxidized surface layer that can play a role in the properties of these nanomaterials.10 Characterization of copper and iron nanoparticles have shown that metal nanoparticles can contain multiple phases, a metallic core and an oxidized layer that is complex with at least two oxide phases present.82 Representative XRD and XPS data for copper nanoparticles are shown in Figure 10. The XRD pattern along with standard reference data clearly shows the presence of multiple phases whereas the XPS data shows that the surface contains the most oxidized phase. Using these characterization data, a picture emerges where the Cu nanoparticle contains a metallic core with a gradient of oxidation states in the near surface region with a surface termination of the most oxidized phase. Thus an important component to understanding the fate of metal nanoparticles may be related to the surface chemistry of the oxidized phase.

In acidic media, where low pH environments in biological fluids or environmental systems, metal nanoparticles may dissolve into ions. As ions are released from the particle, the nanoparticle will change size becoming smaller and as it dissolves the physicochemical properties (redox, optical etc.) will be simultaneously change. An example of this is shown in Figure 10C where a change in nanoparticle size is measured with an SMPS as the particle undergoes dissolution in an acidic environment, pH 2. A future goal of these studies will be to correlate toxicity with these size-dependent changes that occur in different environmental and biological mileu. VI. Future Research Needs and Directions The physical chemistry of nanoparticles will continue to be a major theme of many research articles published in The Journal of Physical Chemistry for years to come. A major thrust of much of the research will be toward obtaining a quantitative understanding of size-dependent properties and surface chemistry of metal and metal oxide nanoparticles in gas and liquid phase environments. The research will be motivated by two desires. One to gain a fundamental understanding of sizedependent properties of matter on in the nanometer size-regime and to develop nanoscale materials into useful devices to benefit society by, for example, providing cleaner energy sources and new biomedical applications. Based on the studies shown here as well as discussions presented in some of the recent literature (see ref 18), several research needs and directions emerge. These include: 1. Better methods for sample preparation of monodispersed materials in large quantities. This is especially important for

18312 J. Phys. Chem. C, Vol. 112, No. 47, 2008 commercially available nanomaterials that are often heterogeneous. Reproducible syntheses of nanomaterials in some cases remains a challenge. 2. DeVelopment of nanoparticle characterization methods. This continues to be of great importance from many perspectives from an engineered materials science focus to measurement of atmospheric aerosols to monitoring nanomaterials in living systems for biomedical applications. In situ characterization remains a challenge. There needs to be continued methods development in measuring single particles below 10 nm in size. It is important to link nanoparticle structure to function and reactivity. 3. DeVelopment of experimental methods to quantitatiVely inVestigate nanoparticle surfaces and nanoparticle surface chemistry. It is clear that new tools are needed to study nanoparticle surfaces. Some of the more insightful results are based on new, state-of-the-art instruments that can provide molecular level insights. With many of the atoms located at corner and edge sites, there is the need to further our efforts in obtaining quantitative information on the structure of these more complex interfaces. Furthermore there is evidence to suggest that nanoparticle surfaces are disordered and undergo fluctuations. These observations show that there are large challenges in how to define nanoparticle surfaces and their properties. The role of aggregation and reactive surface area in the surface chemistry of nanoparticle suspensions needs further investigation as well as shown in some of the examples presented here. 4. Theoretical studies into preferential adsorption and reactiVe sites present on nanoparticle surfaces under a widerange of conditions. Theoretical studies can provide enormous insights into nanomaterials including surface structure (including surface fluctuations and disorder), surface reactivity,84,85 nature of active sites, particle-particle interaction and potentially understanding interactions of nanomaterials with cells and living organisms. Acknowledgment. This article discusses a broad range of research as related to the size-dependent properties and surface chemistry of metal and metal oxide nanoparticles. Many of these studies done in collaboration with my colleagues at the University of Iowa and represent the work of a number of students and postdoctoral scientists. I want to specifically acknowledge the contributions of Professor Amanda J. Haes and Dr. Maryuri Roca for preparation of Au nanoparticles for SMPS measurements; Professors Michelle M. Scherer and David Cwiertny for the iron oxide nanoparticle investigations; Professors Patrick T. O’Shaughnessy and Peter S. Thorne and Dr. Andrea Adamacova-Dodd for the nanoparticle toxicity studies. I would also like to thank my graduate students John Pettibone, Sherrie Elzey, and Gordon Hunter who have contributed to the research discussed here. Financial support of the research from the Environmental Protection Agency and the National Science Foundation is gratefully acknowledged. Although the research described in this article has been funded wholly or in part by the Environmental Protection Agency through Grant No. EPA RD-83171701-0 to V.H.G., it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. This material is based upon work partially supported by the National Science Foundation under Grant Nos. EAR0506679 and CHE0639096. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

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