INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 14 pdf

14 TiO 2 Nanoparticles for Photocatalysis HEATHER A. BULLEN and SIMON J. GARRETT Michigan State 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 (TiO 2 ). Titanium dioxide first attracted significant atten- tion when in 1972 Fujishima and Honda discovered that TiO 2 can act as a catalyst for the photocleavage of water, producing H 2 and O 2 [1]. In the presence of a TiO 2 electrode, they observed that water was dissociated using photons with λ Յ 410 nm, whereas direct photodissociation of water requires photons with λ Յ 185 nm. This discovery sparked interest in the photocatalytic activity of TiO 2 and 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 TiO 2 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 TiO 2 in many redox reactions is limited by the relatively large bandgap (E g ϭ 3.0 Ϫ 3.2 eV) of the material, which limits absorp- tion to the UV region of the solar spectrum, below about 350 nm. This limitation TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 256 Bullen and Garrett has led to the development of chemically modified TiO 2 surfaces with better spectral absorption accomplished by dye-sensitization [8,9]. Alternative semicon- ducting oxide material with smaller bandgaps, such as MoS 2 [10], and composite semiconductors, such as ZnO/ZnS [11] and CdS/PbS [12], have also been investi- gated. Recent scientific interest in TiO 2 photocatalysts has been motivated by obser- vations that aqueous solutions of colloidal TiO 2 nanoparticles 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 TiO 2 nanoparticle surfaces and their role in chemical and photochemical reactivity are still poorly understood. In this chapter we will outline the basic mechanism of TiO 2 photocatalysis and describe how particle size can influence the photoreactivity of TiO 2 . We will also examine current methods being utilized to synthesize TiO 2 nanoparticles and introduce a novel synthetic methodology to grow supported crystalline nanopar- ticles of TiO 2 . II. BACKGROUND A. Fundamental Mechanisms of Semiconductor Photocatalysis 1. Bandgap Excitation 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 Ͻ0.5 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 energy ՆE g , excitation creates an electron (e Ϫ ) in the conduction band (CB) and leaves a hole (h ϩ ) in the valence band (VB) [19]. In TiO 2 the CB is composed of empty Ti 3d states and the VB is composed of filled O 2p states. The e Ϫ /h ϩ 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- TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TiO 2 Nanoparticles for Photocatalysis 257 FIG. 1 Approximate band edge positions for rutile TiO 2 at pH ϭ 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 Ϫ /h ϩ recombination occurs within a few nanoseconds [20]. 2. Electron/Hole Recombination The reactivity of a photocatalyst is dependent on the rate of e Ϫ /h ϩ 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 Ϫ /h ϩ 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 Ti 3ϩ or surface TiOH sites in TiO 2 , extend the effective lifetime of the photoexcited e Ϫ /h ϩ 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 TiO 2 . 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 TiO 2 was significantly enhanced when doped with lanthanide metals: Eu 3ϩ , La 3ϩ ,Nd 3ϩ , and Pr 3ϩ [24]. These dopants create a potential gradient at the surface, separating the photogenerated e Ϫ /h ϩ pairs. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 258 Bullen and Garrett 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 semiconductors) close to the surface. For example, naturally occurring oxygen surface vacancies on TiO 2 create five-coordinate Ti 3ϩ sites. The Ti 3ϩ 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. TiO 2 is 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 Ϫ /h ϩ pairs and in reducing recombination rates. For TiO 2 , 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 ϩ and e Ϫ following bandgap excitation in an n-type semiconductor. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TiO 2 Nanoparticles for Photocatalysis 259 on the surface [6], such as surface hydroxyl groups Ti 4ϩ –OH [21,22], which form surface radicals that can directly oxidize the adsorbed molecule. By placing a noble metal on the TiO 2 surface, the separation rate of the e Ϫ /h ϩ 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 TiO 2 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 TiO 2 nanoparticles [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. 4. Quantum Size Effects (QSE) 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 Ϫ /h ϩ ) 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. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 260 Bullen and Garrett 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 SnO 2 , 5-nm particles were photoactive, whereas 22-nm particles were not [30]. Similarly, Gao and Zhang discovered that 7.2-nm rutile TiO 2 particles 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 TiO 2 single 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 Ϫ and h ϩ 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. TiO 2 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 ϭ 3.0 eV) and anatase (E g ϭ 3.2 eV) forms of TiO 2 . Bulk-powder, single-crystal, and thin-film studies of anatase and rutile have helped to elucidate the photocatalytic mechanisms of TiO 2 as well as the application of this semiconductor to technologies of interest [6,7]. Anatase appears to be slightly more photoactive than rutile TiO 2 , 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 TiO 2 (110) surface is seen as “the model system” for surface TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TiO 2 Nanoparticles for Photocatalysis 261 studies of TiO 2 . 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 TiO 2 . 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]. 1. Surface Chemistry of TiO 2 : The Role of Defect Sites The surface chemistry of TiO 2 is significantly influenced by the concentration of oxygen defects. A stoichiometric rutile TiO 2 (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 Ti 3ϩ sites. As mentioned earlier, these Ti 3ϩ atoms create surface trap states in the bandgap of TiO 2 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 TiO 2 surfaces using various probe molecules (such as H 2 , CO, O 2 and SO 2 ) 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 TiO 2 (110). Ti ϭ ᭹,Oϭ ᭺. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 262 Bullen and Garrett generates an ionic species, probably O 2Ϫ 2 , which directly oxidizes coadsorbed CH 3 Cl. 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 CH 3 Cl, 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]. 2. Nanoparticles: Current Synthetic Methods Almost all of the studies of particle size–reactivity relationships for TiO 2 have been performed using solutions of colloidal nanoparticles. Anatase and rutile TiO 2 colloid nanoparticles are commonly synthesized by hydrolysis of titanium com- pounds such as titanium tetrachloride, TiCl 4 [31,55–57], and titanium alkoxides, Ti(OR) 4 [14,23,58–63], followed by a calcination process. Hydrolysis of TiCl 4 in HCl produces both anatase and rutile phases of TiO 2 , 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 TiCl 4 ,Cl Ϫ 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 (CH 3 ) 2 ) 4 [14,23,59–63], or tetrabutyl titanate, Ti(OC 4 H 9 ) 4 [58], TiO 2 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 ϳ2-nm-size anatase particles and that at 20°C ϳ20 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 solvothermal 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 TiO 2 nanoparticles of various size and compo- sition, little is known of the detailed composition or morphology of TiO 2 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 microscopy. However, a complete understanding of the photocatalytic activity of TiO 2 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 TiO 2 supported on a substrate. This will allow for a fundamental spectroscopic investigation of the chemical TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TiO 2 Nanoparticles for Photocatalysis 263 and photoreactivity of TiO 2 nanoparticles on both the microscopic and macro- scopic scales. III. NANOSPHERE LITHOGRAPHY APPROACH TO CREATING QUANTUM SIZE TiO 2 PARTICLES A. Background Our approach to producing ordered arrays of TiO 2 nanoparticles 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 TiO 2 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 (ϳ50–500 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 solvents 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 TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 264 Bullen and Garrett 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. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... formed from a SL- Copyright n 2003 by Marcel Dekker, Inc All Rights Reserved TiO2 Nanoparticles for Photocatalysis 269 Arrays can be made from monolayer or bilayer masks, with the resulting arrays being distinguisable by different-size particles and different interparticular spacing In Figure 8, each island is approximately 8 nm high and about 140 nm in diameter, consistent with an SL-PPA mask The diameter... Inc All Rights Reserved 266 Bullen and Garrett FIG 6 AFM image (40 ϫ 40 µm) showing poor packing of 420-nm polystyrene spheres on glass, when the evaporation rate is uncontrolled Images are similar for untreated TiO2 can in uence the packing order [76–78] If the evaporation rate is too high, disordered layers do not have a chance to anneal sufficiently, and masks containing agglomerates, poor packing... close-packing A second important factor in controlling the mask deposition is the hydrophilicity of the support surface In order to achieve wetting of the aqueous suspension of polystyrene spheres on the substrate, the substrate must be treated to produce a hydrophilic surface In many previous nanosphere lithography studies, glass substrates have been used, since these are the simplest to prepare Cleaning... projection arguments considering the packing of the mask Each island appears to be approximately circular in profile, not the expected triangular features observed in work by Van Duyne et al [67–70] This suggests that the surface of the TiO2 nanoparticle undergoes substantial reconstruction or faceting during or following deposition Arrays grown on glass and on TiO2(110) single-crystal substrates are similar... Preparation In order to create TiO2 nanoparticles of uniform size and shape, the packing of the polystyrene spheres on the support surface must be controlled to maximize the size of defect-free domains For the deposition of the polystyrene microsphere mask, it is necessary that the spheres be able to freely diffuse in solution across the substrate, seeking their lowest-energy (close packed) configuration during... Slight tilting of the substrate from the horizontal creates a well-defined liquid front that moves from the higher to the lower portions of the substrate during evaporation of the solvent, further improving the quality of the masks produced Once the masks have been prepared, they are examined by atomic force microscopy (AFM) to verify that they contain large ordered domains of spheres, as shown in Figure... 459.0 eV binding energy (BE), in Ti2O3 is at approximately 457.6 eV BE, and in TiO is at approximately 455.3 eV BE Metallic Ti exhibits a 2p3/2 peak at 453.8 eV The shape of the Ti 2p photoemission envelope is characteristic of the oxide Examination of Figure 9 confirms that the dominant species present on the nanoparticle surface is TiO2 There is also some evidence for Ti3ϩ species, the low-coordination... implicated in catalytic and photocatalytic processes for this material It should be noted that the ˚ sampling depth of the XPS technique in the present context is about 30–60 A, meaning that a substantial fraction of the interior of each nanoparticle is not measured 4 Outlook At present we have no information on the crystallinity of the TiO2 nanoparticles produced, but we are actively engaged in studies... techniques such as scanning tunneling spectroscopy (STM) Unlike solution-phase synthesis of nanoparticles, the TiO2 island arrays produced by the nanosphere lithography approach are nearly identical in size and shape and are expected to have well-defined crystallography and surface structure In addition, the nanoparticle array films have a large surface area, allowing for macroscopic-scale experiments to... photochemical reactivity In this way, it is believed that a fundamental and microscopic view of the role of surface structure in nanoparticle photocatalysis can be achieved IV SUMMARY This chapter provides a general background into photocatalytic properties of TiO2 and highlights some interesting examples of its application to environmental reTM Copyright n 2003 by Marcel Dekker, Inc All Rights Reserved . resulting arrays being distinguisable by different-size particles and different interparticular spac- ing. In Figure 8, each island is approximately 8 nm high and about 140 nm in diameter, consistent. examined by atomic force mi- croscopy (AFM) to verify that they contain large ordered domains of spheres, as shown in Figure 7. Typical domain sizes range from 1 to 10 µm using the method- ology. potential gradient that has formed from band bending (Fig. 2). This band-bending phenomenon assists in separating the e Ϫ /h ϩ pairs and in reducing recombination rates. For TiO 2 , the surface holes