INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 14 pdf

The central mechanism involves the photoinduced generation of charge carriers at the surface of a semiconductor, followed by in-terfacial charge transfer reactions with absorbed molecule

TiO2 Nanoparticles for Photocatalysis

University, East Lansing, Michigan, U.S.A

I INTRODUCTION

Transition metal oxides exhibit a wide range of physical, chemical, and structural properties One of the most widely studied metal oxides in the semiconducting oxide titanium dioxide (TiO2) Titanium dioxide first attracted significant atten-tion when in 1972 Fujishima and Honda discovered that TiO2can act as a catalyst for the photocleavage of water, producing H2 and O2 [1] In the presence of a TiO2 electrode, they observed that water was dissociated using photons with

λ ⱕ 410 nm, whereas direct photodissociation of water requires photons with

λ ⱕ 185 nm This discovery sparked interest in the photocatalytic activity of TiO2and other metal oxide semiconductors as a possible approach to inexpen-sively convert solar radiation to chemical energy [2,3] Subsequent research ef-forts have focused on understanding the fundamental processes that drive these photoelectrochemical cells and in their application to energy storage applications [4,5]

The ability to oxidatively decompose organic molecules present as pollutants

in the environment has recently refocused research attention toward utilizing semiconducting oxides for remediation applications For example, the TiO2 sur-face can participate in a wide range of redox chemistries for many types of ad-sorbed organic molecules, including aromatic, halogenated organic, and commer-cial dye molecules [6] The central mechanism involves the photoinduced generation of charge carriers at the surface of a semiconductor, followed by in-terfacial charge transfer reactions with absorbed molecules The mechanistic as-pects of these reactions have been reviewed [6,7], but in many cases the identities

of intermediates have not been firmly established

The photocatalytic activity of TiO2in many redox reactions is limited by the

relatively large bandgap (E g⫽ 3.0 ⫺ 3.2 eV) of the material, which limits absorp-tion to the UV region of the solar spectrum, below about 350 nm This limitaabsorp-tion

has led to the development of chemically modified TiO2 surfaces with better spectral absorption accomplished by dye-sensitization [8,9] Alternative semicon-ducting oxide material with smaller bandgaps, such as MoS2[10], and composite semiconductors, such as ZnO/ZnS [11] and CdS/PbS [12], have also been investi-gated

Recent scientific interest in TiO2photocatalysts has been motivated by obser-vations that aqueous solutions of colloidal TiO2nanoparticles exhibit significantly enhanced chemical and photochemical reactivity due to so-called quantum size effects (QSE) [13,14] The chemical and electronic properties of semiconductor nanoparticles are distinct from either extended solids or single molecules and thus represent an exciting new class of materials [15] It is known that the properties of such nanoparticles vary strongly as a function of particle size (as well as shape) and consequently have “tunable” optical, electronic, and chemical properties [15– 17] In most cases, the TiO2nanoparticle surfaces and their role in chemical and photochemical reactivity are still poorly understood

In this chapter we will outline the basic mechanism of TiO2 photocatalysis and describe how particle size can influence the photoreactivity of TiO2 We will also examine current methods being utilized to synthesize TiO2nanoparticles and introduce a novel synthetic methodology to grow supported crystalline nanopar-ticles of TiO2

II BACKGROUND

A Fundamental Mechanisms of Semiconductor

Photocatalysis

The central mechanism of photocatalytic activity in semiconductors relies on absorption of a photon of energy greater than or equal to the semiconductor

band-gap energy, E g Since solar radiation is a natural and abundant energy source, most photocatalytic strategies have been directed towards exploiting this energy

by choosing materials with bandgaps within the range of terrestrial sunlight (ap-proximately 4.1 to ⬍0.5 eV) Many semiconductors have bandgap energies within this desired range and so are potential materials for promoting or photocat-alyzing a wide variety of chemical reactions [18]

When a semiconductor absorbs a photon of energyⱖE g, excitation creates an

electron (e⫺) in the conduction band (CB) and leaves a hole (h⫹) in the valence band (VB) [19] In TiO2the CB is composed of empty Ti 3d states and the VB

is composed of filled O 2p states The e⫺/h⫹pair may spontaneously recombine, with thermal or luminescent energy release, or may migrate toward the surface and react with adsorbed acceptor or donor species in reduction or oxidation

reac-FIG 1 Approximate band edge positions for rutile TiO2at pH⫽ 1 (From Ref 4.)

tions, respectively In order for redox reactions to occur, the energy of the ad-sorbate orbitals acting as electron acceptors or those acting as electron donors must lie within the bandgap region of the photocatalyst, as shown in Figure 1 Hence, the position of these adsorbate energy levels relative to those of the semi-conductor surface is crucial In the absence of redox active surface species,

spon-taneous e⫺/h⫹recombination occurs within a few nanoseconds [20]

The reactivity of a photocatalyst is dependent on the rate of e⫺/h⫹recombination

in the bulk or at the surface In order to have an efficient photocatalyst, the photo-generated holes and electrons must have a long lifetime, since recombination is

in direct competition with surface charge transfer to adsorbed species Therefore,

the recombination of the photoexcited e⫺/h⫹ pair must be minimized Surface and bulk defects can generate electronic states that serve as charge carrier traps The presence of these charge carrier trapping sites, such as Ti3 ⫹or surface TiOH sites in TiO2, extend the effective lifetime of the photoexcited e⫺/h⫹pair, increas-ing the probability of an electron transfer process to an adsorbed molecule

It is generally believed that non-negligible recombination rates limit the over-all quantum yield of current photocatalytic systems based on TiO2 Various strate-gies, such as doping [23,24] and creating Schottky barrier traps [25–27], have been attempted to extend the lifetime of surface charge carriers and thus improve overall efficiency For example, the photocatalytic degradation of rhodamine B

by TiO2 was significantly enhanced when doped with lanthanide metals: Eu3 ⫹,

La3 ⫹, Nd3 ⫹, and Pr3 ⫹[24] These dopants create a potential gradient at the surface,

separating the photogenerated e⫺/h⫹pairs

3 Band Bending and the Schottky Barrier

When a semiconductor is in contact with another phase, such as a liquid or gas, there is a redistribution of charge within the semiconductor As mobile charge carriers are transferred between the semiconductor and contact phase or carriers are trapped at intrinsic or adsorbate-induced surface states, a space charge layer develops and there is no longer a uniform distribution of charge within the semi-conductor The electronic band potentials of the semiconductor are distorted, de-pending upon whether there is an accumulation or a depletion of charge in the near-surface region As a consequence, bands may bend upward (n-type semicon-ductors) or downward (p-type semiconsemicon-ductors) close to the surface For example, naturally occurring oxygen surface vacancies on TiO2create five-coordinate Ti3⫹ sites The Ti3⫹sites serve as strong electron traps, causing the surface region to become negatively charged with respect to the bulk of the semiconductor To compensate for this effect, a positive space charge layer develops within the semiconductor, causing a shift in the electrostatic potential and the upward bend-ing of bands TiO2is therefore considered an n-type semiconductor

Following bandgap excitation, photogenerated electrons move away from the surface while the holes move toward the surface, due to the potential gradient that has formed from band bending (Fig 2) This band-bending phenomenon

assists in separating the e⫺/h⫹ pairs and in reducing recombination rates For TiO2, the surface holes oxidize adsorbed molecules by electron transfer from the adsorbate into a hole However, the photocatalytic oxidation of many organic molecules is believed to be mediated by electron transfer from coadsorbed species

FIG 2 Diagram showing the surface band bending and Schottky barrier that serve to

separate h⫹and e⫺following bandgap excitation in an n-type semiconductor

on the surface [6], such as surface hydroxyl groups Ti4 ⫹–OH [21,22], which form surface radicals that can directly oxidize the adsorbed molecule

By placing a noble metal on the TiO2surface, the separation rate of the e⫺/h⫹

pair can be further increased if the metal creates a favorable potential gradient (Schottky barrier) to act as a sink for photogenerated electrons The metal surface then becomes the site of reduction reactions Based on this phenomenon, discrete electrochemical cells incorporting small metal islands deposited onto TiO2 nano-particles have been prepared [28] For example, Dawson and Kamat determined that the photocatalytic oxidation of thiocyanate ions was increased by 40% using gold-capped TiO2nanoparticles [25] The amount of noble metal required to pro-duce an effective Schottky barrier can correspond to less than a few percent of the surface covered

Photocatalytic activity is also affected by particle size When the physical dimen-sions of a semiconductor particle fall within the range of 5–20 nm, the diameter

of the particle becomes comparable to the wavelength of the charge carriers

(e⫺/h⫹) and quantum size effects (QSE) occur [17,29] The electronic structure

of the semiconductor can no longer be described as an extended solid, with over-lapping wavefunctions from each atom giving rise to continuous and delocalized electronic valence and conduction bands Instead, the charge carriers become localized in the effective potential well of the nanoparticle, and discrete quantized energy states are produced (Fig 3) that give rise to the strongly size-dependent optical and electronic phenomena Absorption intensities are perturbed, and the effective bandgap of a semiconductor particle is thought to increase as the particle

FIG 3 Density of states for a semiconductor as a function of particular size

size decreases, corresponding to a blue-shift of the absorption band [15,16] These phenomena can influence the photocatalytic properties of small semiconductor particles For example, in the decomposition of 1-butene by SnO2, 5-nm particles were photoactive, whereas 22-nm particles were not [30] Similarly, Gao and Zhang discovered that 7.2-nm rutile TiO2particles has a much higher photocata-lytic activity in the oxidation of phenol compared to 18.5- and 40.8-nm particles [31]

In addition to changes in the electronic structure of the material, other phenom-ena can occur as particle dimensions are reduced Smaller particles present more surface adsorption/reaction sites per unit volume and are therefore expected to show increased catalytic activity Additionally, the formation of unique electronic surface states or reactive defects may become favored The high curvature of the particle surface creates a large number of low-coordination surface atoms of unique local geometry and bonding, which may also lead to substantial surface relaxation, reconstruction, or faceting In TiO2single crystals, these low-coordi-nation sites have been shown to markedly influence the adsorption and reactivity

of small molecules [32,33] As the volume of the semiconductor becomes very

small, the band-bending phenomenon that spatially separates the e⫺ and h⫹ is reduced Band bending typically operates on the 0.5 to 5-nm distance scale and becomes weak as particle diameters approach these dimensions [4] A small parti-cle is almost completely depleted of charge carriers, so its Fermi potential is located approximately in the middle of the bandgap This implies that there is

an optimum-size semiconductor nanoparticle for surface photoreactivity, which

is dependent upon the material

B TiO2 Photocatalysis

Despite a wide range of materials with suitable bandgaps, titanium dioxide re-mains a primary candidate as a photocatalyst for environmental remediation ap-plications due to its thermodynamic stability, high abundance, low cost, and non-toxicity Titanium dioxide exists as three natural crystalline forms (rutile, anatase, and brookite), with rutile being thermodynamically the most stable [19,34] Most

photocatalytic studies have focused on the rutile (E g ⫽ 3.0 eV) and anatase (E g

⫽ 3.2 eV) forms of TiO2 Bulk-powder, single-crystal, and thin-film studies of anatase and rutile have helped to elucidate the photocatalytic mechanisms of TiO2

as well as the application of this semiconductor to technologies of interest [6,7] Anatase appears to be slightly more photoactive than rutile TiO2, which is thought to be due to its larger charge carrier diffusion rates [5] and lower recombi-nation rates compared to rutile [35,36] The photoreactivity for anatase and rutile

is highly variable, depending on the exact surface preparation methods In many cases, the rutile TiO2(110) surface is seen as “the model system” for surface

studies of TiO2 Such single-crystal studies in ultrahigh vacuum (UHV) have complemented ambient powder and film studies and have contributed to the de-velopment of a fundamental understanding of the role of the surface in the overall photocatalytic activity of TiO2 These studies have determined the influence of parameters such as surface geometric structure, defect nature, and concentration and the identity of reactive intermediates [7,37,38] Unfortunately, single-crystal studies of anatase are rare [39,40] due to the difficulty of preparation, but bulk measurements of dispersed particles have been performed [41]

The surface chemistry of TiO2is significantly influenced by the concentration of oxygen defects A stoichiometric rutile TiO2(110) surface is quite unreactive, since a fully oxidized surface contains no occupied surface states in the bandgap [42] However, defects increase the reactivity of the surface, particularly oxygen defects that produce low-coordination Ti3 ⫹sites As mentioned earlier, these Ti3 ⫹

atoms create surface trap states in the bandgap of TiO2 and ultimately lead to enhanced chemical reactivity [43–47]

Several different types of O atom vacancies have been directly observed by scanning probe microscopies on single crystals [38,48–50], some of which are shown schematically in Figure 4 Chemisorption studies on TiO2surfaces using various probe molecules (such as H2, CO, O2and SO2) indicate that adsorption

is dependent on oxygen defect sites on the surface [33,46,51,52] This depen-dence can influence the nature of the photocatalytic reactions that can take place

on the surface For example, Yates and co-workers have determined that molecu-larly adsorbed oxygen is essential for photoxidation of methyl chloride [53] In their study they discovered that substrate-mediated excitation of adsorbed oxygen

FIG 4 Oxygen atom vacancies (defect sites) on rutile TiO2(110) Ti⫽ 䊉, O ⫽ 䊊

generates an ionic species, probably O2 ⫺

2 , which directly oxidizes coadsorbed

CH3Cl They speculate that at the gas–solid interface, adsorbed oxygen may play

a more important role in the oxidation of certain organic molecules, such as

CH3Cl, than photocatalytically generated⋅ OH radicals Defect concentration and adsorbed oxygen has been shown to play a similar role in the photocatalytic dehydrogenation of 2-propanol [54]

Almost all of the studies of particle size–reactivity relationships for TiO2have been performed using solutions of colloidal nanoparticles Anatase and rutile TiO2

colloid nanoparticles are commonly synthesized by hydrolysis of titanium com-pounds such as titanium tetrachloride, TiCl4[31,55–57], and titanium alkoxides, Ti(OR)4[14,23,58–63], followed by a calcination process Hydrolysis of TiCl4

in HCl produces both anatase and rutile phases of TiO2, and the anatase/rutile ratio can be varied [64] by controlling the pH and temperature of calcination It has also recently been reported by Pottier and co-workers that brookite nanopar-ticles can be synthesized by carefully controlling the molar ratio of Cl : Ti in solution [57] In very acidic solutions of TiCl4, Cl⫺ ions stabilize the brookite nanoparticles These particles vary in size, with a mean diameter of 5.2 nm Using titanium alkoxide precursors such as tetraisopropoxide, Ti(-OCH (CH3)2)4 [14,23,59–63], or tetrabutyl titanate, Ti(OC4H9)4 [58], TiO2 colloidal nanoparticles can be generated By varying the temperature of hydrolysis, the size of the colloid particles can be controlled For example, Martini has shown that hydrolysis at 1°C produces ⬃2-nm-size anatase particles and that at 20°C

⬃20 to 30-nm-size particles are synthesized [63]

During calcination, both the degree of crystallinity and the size of the particles increase, and the colloid composition generally transforms from anatase to rutile This limits the size of anatase nanoparticles that can be made by such hydrother-mal methods Alternative approaches, such as solvotherhydrother-mal synthesis in organic media instead of water, may be more effective in producing smaller nanoparticles with high crystallinity and large surface areas [65,66]

Despite the ability to synthesize TiO2nanoparticles of various size and compo-sition, little is known of the detailed composition or morphology of TiO2 nanopar-ticulate surfaces The particles produced by solution methodologies tend to ag-glomerate, are nonuniform in size and shape and are generally not amenable to surface characterization by experimental techniques such as electron spectros-copy and scanning probe microsspectros-copy However, a complete understanding of the photocatalytic activity of TiO2 nanoparticles on a fundamental atomic level is clearly desirable In the next section, we present a novel approach to growing monodisperse, controlled-size nanoparticles of TiO2 supported on a substrate This will allow for a fundamental spectroscopic investigation of the chemical

and photoreactivity of TiO2 nanoparticles on both the microscopic and macro-scopic scales

III NANOSPHERE LITHOGRAPHY APPROACH TO

CREATING QUANTUM SIZE TiO2PARTICLES

A Background

Our approach to producing ordered arrays of TiO2nanoparticles is based on the formation of a physical mask using close-packed polystyrene microspheres The

technique, termed nanosphere lithography, has been developed and successfully

applied by Van Duyne and co-workers [67–70] to create various supported noble metal nanoparticle arrays, but to our knowledge it has not been applied to TiO2

or other oxides The methodology is simple, intrinsically parallel, relatively inex-pensive, and highly precise, producing particles with uniform size and shape In contrast, standard lithographic methods used to create nanoparticles with con-trolled size and spacing, such as photolithography [71] and electron beam lithog-raphy [72], are limited to the minimum size of features they can produce and are very complex and expensive The precision of nanostructure fabrication by the nanosphere lithography technique is, in principle, comparable to or better than other nanofabrication approaches [70]

In the nanosphere lithography method, an aqueous, solution of polystrene mi-crospheres (⬃50–500 nm in diameter) is drop-coated onto a suitable substrate, and the solvent is allowed to evaporate under controlled conditions The nano-spheres spontaneously order into a hexagonal close-packed array on the surface Depending upon the initial sphere concentration, different periodic particle array (PPA) masks can be produced: a close-packed single-layer periodic particle array (SL-PPA) or a double-layer periodic particle array (DL-PPA) from a monolayer

or bilayer of spheres, respectively [68] The desired material of interest is then evaporated through the nanosphere mask, coating the exposed surface between the spheres The microsphere mask is subsequently removed by an organic sol-vent such as ethanol or methylene chloride These solsol-vents dissolve the polysty-rene spheres, leaving the material deposited through the mask on the substrate (Fig 5).Complete mask removal becomes difficult if the height of the islands exceeds the radius of the microspheres used to make the mask

The diameter of the nanoparticle islands is approximately 15% of the diameter

of the polystyrene microspheres Therefore, by changing the size of the micro-spheres, various-diameter nanoparticles can be made using this technique Poly-styrene spheres with diameters down to 50 nm are commercially available, allowing nanoparticles on the order of 7-nm minimum diameter (i.e., quantum size particles) to be generated Changing the incidence angle of the evaporative

FIG 5 (a) A close-packed array of polystyrene microspheres and (b) an array of nano-particles produced by evaporation through mask and subsequent mask removal

source with respect to the substrate normal can alter the shape of the supported nanoparticles somewhat [70], but to date, nanosphere lithography has been lim-ited to triangular and circular particle shapes Unfortunately, the particle arrays contain up to 1% point and line defects, due to polydispersity in the spheres used to form the mask These disadvantags may be overcome with continued development